*Article* **Dynamic Water Balance Modelling for Risk Assessment and Decision Support on MAR Potential in Botswana**

#### **Andreas Lindhe 1,\*, Lars Rosén 1, Per-Olof Johansson <sup>2</sup> and Tommy Norberg <sup>3</sup>**


Received: 20 November 2019; Accepted: 3 March 2020; Published: 6 March 2020

**Abstract:** Botswana experiences a water stressed situation due to the climate and a continuously increasing water demand. Managed Aquifer Recharge (MAR) is considered, among other measures, to improve the situation. To evaluate the possibility for increased water supply security, a probabilistic and dynamic water supply security model was developed. Statistically generated time series of source water availability are used in combination with the dynamic storages in dams and aquifers, and the possible supply is compared with the demand to simulate the magnitude and probability of water supply shortages. The model simulates the system and possible mitigation measures from 2013 to 2035 (23 years), using one-month time steps. The original system is not able to meet the demand, and the estimated volumetric supply reliability in the year 2035 is 0.51. An additional surface water dam (now implemented) will increase the reliability to 0.88 but there will still be a significant water shortage problem. Implementing large-scale MAR can further improve the reliability to at least 0.95. System properties limiting the effect of MAR are identified using the model and show how to further improve the effect of MAR. The case study results illustrate the importance and benefit of using an integrated approach, including time-dependence and future scenarios, when evaluating the need and potential of MAR.

**Keywords:** water supply security model; risk assessment; decision support; dynamic; probabilistic; managed aquifer recharge; integrated water resource management

#### **1. Introduction**

Access to potable water is essential to human health and economic development. Water scarcity and drought are, however, major challenges on all continents [1] and must thus be managed to enable safe and secure access to clean water. Climate change, increased water demand and other factors will increase the problem of water scarcity, making this a key issue to reach the UN Sustainable Development Goals [2]. This calls for integrated water resources management, including measures to avoid water losses, the efficient use of water, the introduction of water saving technologies, as well as of water re-use and recycling.

Botswana is an example where the hydrological conditions and a continuously increasing water demand result in a water stressed situation. The arid to semi-arid climate provides a situation with low rainfall and high rates of potential evapotranspiration, resulting in low rates of surface runoff and low rates of natural groundwater recharge. To enable a reliable, safe, and sustainable water supply, it is

widely accepted that a systems approach is needed, encompassing several water resources as well as technical and other mitigation measures. Managed Aquifer Recharge (MAR) to enhance groundwater recharge and store surface water (e.g., natural water, urban storm water, treated sewage, or desalinated water) [3,4] has, in several international studies, been pointed out as an important measure to mitigate water drought and scarcity situations [5–7]. MAR has also been identified as a possible measure to be implemented in Botswana [8,9]. However, to properly evaluate if MAR is a suitable option and the benefits it may provide, a holistic system analysis considering both current and future conditions is needed. Not only the potential storage and other aquifer specific properties should be determined, but also the reliability and access to water over time must be analysed in detail, considering risks and variations in water demand and supply. Hence, a risk-based approach is needed to provide decision support on water supply security and the effect of MAR. Water security is defined in different ways in the literature [10–12] but typically includes the water quantity aspect, which is the focus of this paper.

Different methods for selecting suitable sites for MAR exist and are typically based on GIS [13–15]. When designing the final MAR scheme and determining operational strategies etc., a detailed groundwater model is commonly used [16]. No generically accepted method exists, however, for assessing the overall effect of MAR on the water supply security. Several examples exist where system dynamic modelling [17] has been used to analyse the conjunctive use of surface water and groundwater. This approach is common when evaluating policy options [18,19] but examples including aquifer storage and recover exist [20]. Another type of modelling approach was applied by Clark et al. [21] to analyse the reliability of water supply from stormwater harvesting and MAR. A model combining stormwater hydrology with subsurface storage and demand was used and repeated simulations were performed to estimate the volumetric reliability. Gao et al. [22] analysed the reliability of an urban water supply using Monte Carlo analysis to model variations in surface water availability. Aquifer injection and banking was analysed with the aim to identify the most cost-effective way to meet specified criteria for supply reliability. These examples show the importance of creating models that include the entire system to be analysed and to compare the possible supply and water demand so that mitigation measures' effect on water security can be estimated.

An integrated and holistic approach is important to avoid overlooking interactions between subsystems and events, and to minimise the sub-optimisation of mitigation measures [23]. The importance of considering the entire supply system when assessing drinking water risks is emphasised by, for example, the World Health Organization (WHO) as part of a framework including Water Safety Plans (WSPs) [24]. A WSP is typically focused on water quality but the integrated approach is equally applicable to water supply security.

In this paper, a Water Supply Security Model (WSSM) is presented and applied to assess risks in the main drinking water supply in Botswana and to evaluate and provide decision support on the potential effects of MAR. The overall aim of the work was to enable a holistic assessment of the potential for using large-scale MAR to improve the water supply security in Botswana. The specific objectives were to (i) develop a comprehensive and dynamic water balance model, (ii) simulate and show the predicted water shortage over time, and (iii) model the potential of alternative mitigation measures and identify limiting factors.

#### **2. Study Area**

Botswana is located in south-central Africa and occupies an area of approximately 582,000 km2. The total population of the nation is a little over 2 million, making Botswana one of the most sparsely populated countries in the world. The country is predominantly flat with some parts having a slightly rolling landscape. Botswana is dominated by the Kalahari Desert, covering about 70% of the total area. The north-east part of Botswana has an annual precipitation of about 600 mm, whereas the drier south-west receives, on average, only 200 mm per year. Due to the arid to semi-arid climate, potential evapotranspiration rates exceed the total rainfall at all times of the year [8]. There are no perennial streams originating in Botswana. It is estimated that the mean annual rates of surface runoff do not

exceed 50 mm anywhere except in small steep rocky catchments. The annual recharge to aquifers from rainfall reaches a maximum of about 40 mm in small areas in the Chobe District in the north. For most of the Kalahari region, the natural groundwater recharge is less than 1 mm/year [8]. The largest groundwater resources are in the Kalahari sediments (including the Okavango Delta), the Ntane Sandstone, the Ecca Sandstones, and the Damaran and Ghanzi rock formations.

In eastern and southern Botswana, with its relatively high density of population and substantial water demand, several surface water dams have been constructed to collect and store ephemeral river flow. The largest dams are the Shashe, Dikgatlhong, Letsibogo, and Gaborone dams. The storages of the dams are very variable due to the highly seasonal, occasional, and variable river flows. In addition, the need to store water for drought periods and the flat topography in most areas result in large losses of water to evaporation from these dams. The surface water dams in eastern Botswana have been connected through a nearly 400-km long pipeline transfer system denoted the North–South Carrier (NSC), providing possibilities to transfer water to urban centres.

The NSC water supply system is the focus of this study and the included demand centres, surface water dams, aquifers and additional components are shown in Figure 1. The study was performed in the year 2013 and the descriptions of the system, planned measures, etc., are thus based on the situation at that time. The Dikgatlhong Dam was being constructed when the study was performed and thus included in one of the modelled scenarios to represent the coming system structure. In addition to the surface water dams, a few groundwater wellfields are connected or are planned to be connected to demand centres supplied with water from the NSC, e.g., Palla Road, Chepete, Masama, Makhujwane, Malotwane, and Palapye Wellfields.

**Figure 1.** Schematic illustration of the water supply system linked to the North–South Carrier (NSC), according to the situation when the study was performed in the year 2013.

Due to the highly variable storage in surface water dams, the groundwater aquifers have a potential to support the NSC demand centres during drought periods. Because of the very limited natural recharge to these aquifers, their long-term sustainable capacity could be improved by managed recharge with surface water. Managed recharge (injection) with collected and treated surface water from dams may also reduce the total loss of water to evaporation. The focus of the case study is thus to evaluate the possibility and effects on the water supply security of MAR scenarios including the Palla Road/Chepete and Masama/Makhujwane Wellfields.

#### **3. Materials and Methods**

#### *3.1. Water Supply Security Model (WSSM)*

The WSSM is a dynamic water balance model where statistically generated time series of the availability of source water are used, together with dynamic storages in dams and aquifers, as well as water demands, to simulate the magnitude and probability of water supply shortages. Models have been previously developed for the water supply in Botswana but they have not considered MAR scenarios [8]. The WSSM is developed as a spreadsheet model in Excel since one of the goals of this study was to provide an easily accessible model that can be run without expert knowledge. To enable statistical analysis considering uncertainties in input data and results, an add-in software (Oracle ®Crystal Ball) is used to run Monte Carlo simulations.

The WSSM simulates the NSC system and connected components from 2013 to 2035 (23 years). The period is selected to match water demand forecast in the National Water Master Plan Review [8]. The simulations are performed with a time step of one month and for each month the demand, the available storage in dams and aquifers, as well as treatment capacities, water losses, etc., are considered.

The schematic illustration in Figure 2 shows the parameters considered in the model and the link to the model components. Based on historical data on inflow to the dams (see Section 3.2), a set of possible time series are generated and used to sample from when running the model. The generated time series consider the correlation between the dams and each generated data set includes all five dams. The annual inflow data is transformed into monthly data based on the closest historical annual inflow and the monthly distribution that year. Since the dams are spatially correlated, the historical data for the Gaborone Dam is used when transforming the simulated annual data for the Bokaa and Gaborone Dams. In the same way, the historical data for the Dikgatlhong Dam is used for the Letsibogo, Shashe, and Dikgatlhong dams.

For each dam, water balance calculations are performed for each month, considering initial storage, inflow, abstraction, evaporation, seepage, spill over, and additional parameters presented in Figure 2. The input data is based on [8] and information from personnel at the Water Utility Corporation (WUC) and the Department of Water Affairs (DWA). The evaporation is calculated based on the area–storage relationship and data from previous studies in the area [8,25]. Key inputs for modelling the dams in the WSSM are presented in Table 1.


**Table 1.** Storage properties and environmental flows for the dams included in the Water Supply Security Model (WSSM).

\* %; \*\* Mm3, provided inflow > 0.

**Figure 2.** Overview of the parameters considered in the model and the link between them.

The water balance calculations for MAR wellfields are performed considering initial storage, natural recharge, inflow, injection, outflow, and abstraction. Necessary input data for the aquifers are based on [26–29], and the key figures used in the scenarios modelled here (see Section 3.3) are presented in Table 2. The wellfields are recharged by injecting water if the maximum storage is not reached and provided that water is available in the dams and is at a capacity to abstract, treat, transfer, and inject the water. A critical dam storage level (20%) was defined by the WUC and DWA and is used as an operational rule in the model stating that water for injection may only be abstracted when the dam storage is above this level. The abstraction of water from the MAR wellfields starts when the water demand cannot be met by the supply of treated surface water and the non-MAR wellfields. The abstraction is only limited by the abstraction rate and capacity to treat and distribute the water.

**Table 2.** Input data on Managed Aquifer Recharge (MAR) wellfields to the WSSM.


For the wellfields not considered relevant for MAR, an estimated sustainable yield is used as a maximum abstraction rate to not cause groundwater mining. The sustainable yield is estimated on an annual basis and defined in the model as a monthly maximum abstraction that may not be exceeded. The total sustainable yield for the non-MAR wellfields is 7.2 Mm3, and 11.7 Mm3 when also including the Masama/Makhujuwane and Palla Road/Chepete Wellfields as non-MAR wellfields.

The water demand forecast from [8] is used to determine how much water must be abstracted from the dams and wellfields. The estimated change in water demand is based on a population forecast and an increase in specific water demand, including assumptions of changes from standpipe to yard and yard to house connections. The assumed industrial, commercial, and institutional annual growth rate is 3%. Unaccounted-for water, including technical losses and non-technical losses (unmetered consumption and illegal connections), is also considered in the forecast. Since the reported water consumption in 2012 was 4 Mm3/year lower than the forecast for the same year, the original demand forecast was reduced by this volume. The total annual demand for the demand centres included in the WSSM is 81 Mm<sup>3</sup> in 2013 and 148 Mm<sup>3</sup> in 2035.

In addition to the capacities and other parameters presented above, a set of operational rules are used to determine, for example, when different sources are used and to what demand centres and the extent to which water is supplied. In all calculation steps, available abstraction rates, treatment capacities, water losses during treatment etc., are considered. The model is thus not used to optimize the supply from different sources but to estimate the performance based on the actual operational rules used to manage the system. The schematic illustration in Figure 1 shows how the different components of the system are connected and how water can be transferred. The actual supplied amount of water is compared with the demand for each demand centre and possible shortage etc., is calculated in each time step.

#### *3.2. Dam Inflow Time Series*

The time series of monthly inflows to the five dams (Gaborone, Bokaa, Letsibogo, Dikgatlhong and Shashe) are available based on measurements and hydrological modelling for the 80-year period of 1925 to 2004 [8] (vol. 11). The dams are grouped based on their spatial correlation and the annual inflows are presented in Figure 3. An analysis of the annual inflows is made to generate 96,000 future annual inflow time series (23 years) that are used to sample from when running the WSSM. This five-dimensional time series is modelled as a first-order stationary Gaussian Auto-Regressive, AR(1) sequence:

$$y\_t - \mu = \Phi(y\_{t-1} - \mu) + \varepsilon\_t \tag{1}$$

where the column vector *yt* is the annual inflow and *t* = −79, −78, ... ,0(*t* = 0 corresponds to year 2004). The column vectors μ and ε*<sup>t</sup>* denote the long-time yearly mean and white noise, respectively, the latter with covariance matrix Σ. To carry out a standard least squares (LS) estimation, the model is rewritten as follows:

$$y\_t = \Phi y\_{t-1} + b + \varepsilon\_t \tag{2}$$

where *b* = (*I* − Φ)μ (*I* denotes the identity). The model parameters Φ, *b*, and Σ are estimated by the method of least squares. Φ is a 5 by 5 coefficient matrix and *y*, *b* and ε*<sup>t</sup>* are 5-dimensional column vectors. Also estimated is the spatial covariance matrix:

$$\gamma = E(y\_t - \mu)(y\_t - \mu)'\tag{3}$$

where the prime denotes transpose. Future dam inflow values *y*1, *y*2, ... , *y*<sup>23</sup> are then repeatedly simulated from the estimated model, taking the uncertainty of the LS estimates into account.

**Figure 3.** Historic inflow time series (80 years, 1925–2004) for the dam sites: (**a**) Letsibogo, Dikgatlhong and Shashe; (**b**) Gaborone and Bokaa [8].

#### *3.3. Scenarios*

The system included in the analysis and implemented in the WSSM includes: 6 surface water dams, 8 wellfields, 7 water works, and 18 demand centres. In addition to five dams previously presented, the Molatedi Dam in South Africa is also included in the model. A constant supply (80% of the maximum agreed transfer) from the dam is assumed since no historical time series are available. The same assumption has previously been used when evaluating the supply system [8]. The selection of wellfields for MAR scenarios was based on [9,26–33] and workshops including representatives from the DWA, WUC, and the authors. Several scenarios including MAR and non-MAR wellfields are possible, but we here focus on the three scenarios listed below as a basis for evaluating the potential of using large scale MAR in Botswana. The system structure in 2013 (Scenario A), i.e., when the study was performed, is used as a reference to illustrate the need and potential effects of mitigation measures (Scenarios B and C) on the water supply security. As mentioned above, the Dikgatlhong Dam was under construction when the study was performed and is now in operation. Hence, the dam is included in Scenario B and combined with MAR in Scenario C. The purpose is not to evaluate MAR as an alternative measure to the Dikgatlhong Dam but to see how MAR can further improve the system. To facilitate a relative comparison of the mitigation measures, i.e., the Dikgatlhong Dam and the MAR wellfields, they are included from the start of the simulated period (i.e., year 1).


#### **4. Results**

The results from the WSSM show that the supply system in Scenario A is clearly insufficient to meet the water demand within the simulated period of 23 years. Water shortage is likely to be a problem early in the simulated period and is expected in around 70% (mean value) of the months for most demand centres. Given a month with water shortage, the deficit varies between c. 20–60% of the demand. The water supply security can be assessed based on the volumetric reliability, i.e., the volume of water supplied divided by the demand in a given year. The results are presented in Figure 4 and show that the reliability is dramatically reduced for Scenario A over the simulated period. There is no reliability target level defined in Botswana but at the end of the simulated period the level is only 0.51 (mean value). As a comparison, case studies in Australia [21,22] have applied a 0.995 volumetric reliability target for potable supplies and a 0.95 target level for non-potable use.

**Figure 4.** Annual volumetric reliability (mean value) of the supply to Great Gaborone over the simulated period (23 years).

In Figure 5, the expected (mean) probability of annual water shortage of different magnitudes is presented for Gaborone and the demand centres connected to the capital city, here referred to as Great Gaborone. The results are similar for most of the demand centres, and, for evaluating the potential effect of MAR, we focus on Great Gaborone. The total demand for Great Gaborone will increase over the analysed period from 44.8 (year 2013) to 82.4 Mm3/year (year 2035). The water demand for Great Gaborone constitutes 62% of the total demand in the NSC system.

The connection of the Dikgatlhong Dam to the NSC and the related system upgrades (Scenario B) will have a large positive effect on the supply security, see Figure 6, and reduce the expected total water shortage (summed over the 23 years) by approximately 90%. The supply reliability will increase (Figure 4) and be >0.99 for approximately 10 years. However, in 2035, the reliability is estimated to be 0.88 and the results thus show that there still will be a significant risk for water shortage for Great Gaborone during the late part of the simulation period.

**Figure 5.** Expected probability of annual water shortages of different magnitudes in Great Gaborone for each year in Scenario A.

**Figure 6.** Expected probability of annual water shortages of different magnitudes in Great Gaborone for each year in Scenario B, including the Dikgatlhong Dam.

In Figure 7, the results are presented for Scenario C, i.e., including the MAR wellfields. The probability of shortage is further reduced compared to Scenario B. For example, the probability of having a 10% (8 Mm3) water shortage in Great Gaborone in 2035 is reduced from 40% (Scenario B) to 10%. For Scenario C, the supply reliability is >0.99 in approximately 15 years and is estimated to be 0.95 in 2035. The effects of implementing MAR are, however, limited due to the capacity of different system components. As an example, Figure 8 shows what limiting the injection of water from the dams at the Palla Road Wellfield. Injection is expected to be needed in 41% of the months. In 34% of these months, injection up to the full storage or maximum injection rate is obtained. However, the dam storage (abstraction of water for injection only allowed if dam storage is >20%), the abstraction rate from dams, and the capacity of the treatment plants (only treated water is injected) are limiting the injection in 45%, 17%, and 4% of the cases, respectively. If the most critical technical system properties causing the limitations are eliminated, the positive effects of MAR in Scenario C further increase. For example, the supply reliability in 2035 will increase to 0.97 and the probability of an annual water deficit of 2 Mm3 (2.5% of the demand) will reduce from 32% to 10%. This effect can be obtained without any substantial risk of mining the wellfields. The maximum active storage for the Masama/Makhujwane and Palla Road/Chepete Wellfields are 40 and 42.8 Mm3, respectively (Table 2). The probability of having full storage at the end of the simulated period is 0.8 for both wellfields in Scenario C when the key limiting factors have been eliminated. If the limitations are included, the probability of full storage is 0.2 and 0.5, respectively.

**Figure 7.** Expected probability of annual water shortages of different magnitudes in Great Gaborone for each year in Scenario C, including Masama/Makhujwane and Palla Road/Chepete, MAR.

**Figure 8.** Factors limiting the injection at Palla Road Wellfield, Scenario C.

Provided that no water is abstracted and the maximum injection can be applied, it would take 5–6 years to recharge the MAR wellfield from 0 Mm<sup>3</sup> to full storage. If dependent on natural groundwater recharge only, the wellfields will only be recharged up to approximate half of the maximum storage after the simulated 23 years.

#### **5. Discussion and Conclusions.**

The performed case study addresses the possible effects of implementing MAR in the NSC system in Botswana. It is concluded that the NSC system including the Dikgatlhong Dam (Scenario B) but without MAR is not likely to be able to provide a safe water supply over the entire time period. When the water demand increases, the reliability of the system is reduced. Implementing MAR at the Palla Road/Chepete and Masama/Makhujwane Wellfields (Scenario C) will further improve the system, although not eliminate the risk of future water shortage. However, implementation of MAR may be of great importance in managing the water supply situation in eastern Botswana.

By including the Dikgatlhong Dam in the system (Scenario B), an additional water source is added and the potential volume of water that may be accessible is increased. Due to the spatial correlation between the dams, periods of no or limited inflow may affect several dams at the same time and thus cause a severe water shortage. By including the MAR wellfields, we add components that are dependent on the surface water dams to be recharged but the water stored in the aquifers can be used independently of the dams. The latter part is one of the key reasons for the increased reliability in Scenario C. The results show that over time there is enough water in the dams to be able to recharge the MAR wellfields. However, the possibility to inject water is partly limited in Scenario C due to both the capacity of the system components and access to the surface water. If these limitations are reduced or eliminated, the positive effect of implementing MAR will further increase.

The non-MAR wellfields are operated using the estimated sustainable yields as a maximum monthly abstraction rate. If this criterion would have been defined on an annual basis and allowed a varying abstraction over the year, it is possible that some smaller shortage events could have been avoided. Another criterion for sustainable yield could be implemented into the WSSM but would, however, not have a significant effect on the more severe shortage events. By allowing the same abstraction rates for the Palla Road/Chepete and Masama/Makhujwane Wellfields, as in Scenario C, but without MAR, the system would improve but only for a limited time. Due to the limited natural groundwater recharge, the aquifers would gradually be emptied. MAR is thus needed in order to provide for a long-term sustainable solution.

The results from the modelled scenarios do not show any cases where the implementation of MAR reduces the supply system security in Botswana. This could be the case if, for example, there are large losses of water or the abstraction rate is too low compared to the surface water dams.

The developed WSSM enables a thorough analysis and evaluation of the original system as well as the effect of both MAR implementation and other system changes. A key advantage of the model is the ability to not only model the possible enhanced groundwater recharge over time but to compare the possible supply and demand for all system components. The predicted water shortage over time in combination with dam and groundwater levels provides a comprehensive picture of the system performance. This makes it possible to evaluate system reliability with consideration to the entire system and compare MAR scenarios with other possible measures to avoid the sub-optimisation of risk mitigation measures.

Furthermore, system properties limiting the effects of MAR can be identified as shown in the case study. This result can guide further analysis and improvements to enhance the effect of implementing MAR. Hence, the WSSM can provide results to support decisions on both larger system changes and minor upgrading to improve water supply security.

The main conclusions of this study are:


**Author Contributions:** The study was initiated by all authors. L.R. was the project leader and P.-O.J. analysed and estimated input data related to aquifers and water demand. A.L. created the water supply security model with support from the other authors. T.N. analysed and modelled time series of inflows to surface water dams. A.L. performed the calculations and was the main author of the paper. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Swedish International Development Cooperation Agency (Sida) and the Botswana Department of Water Affairs (DWA).

**Acknowledgments:** The authors gratefully acknowledge the contributions of the DWA and the Water Utility Corporation (WUC) in Botswana.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## *Article* **Managed Aquifer Recharge in Africa: Taking Stock and Looking Forward**

**Girma Y Ebrahim 1,\*, Jonathan F. Lautze <sup>2</sup> and Karen G. Villholth <sup>2</sup>**


Received: 20 November 2019; Accepted: 8 June 2020; Published: 27 June 2020

**Abstract:** Climatic variability and change result in unreliable and uncertain water availability and contribute to water insecurity in Africa, particularly in arid and semi-arid areas and where water storage infrastructure is limited. Managed aquifer recharge (MAR), which comprises purposeful recharge and storage of surface runoff and treated wastewater in aquifers, serves various purposes, of which a prominent one is to provide a means to mitigate adverse impact of climate variability. Despite clear scope for this technology in Africa, the prevalence and range of MAR experiences in Africa have not been extensively examined. The objective of this article is provide an overview of MAR progress in Africa and to inform the potential for future use of this approach in the continent. Information on MAR from 52 cases in Africa listed in the Global MAR Portal and collated from relevant literature was analyzed. Cases were classified according to 13 key characteristics including objective of the MAR project, technology applied, biophysical conditions, and technical and management challenges. Results of the review indicate that: (i) the extent of MAR practice in Africa is relatively limited, (ii) the main objective of MAR in Africa is to secure and augment water supply and balance variability in supply and demand, (iii) the surface spreading/infiltration method is the most common MAR method, (iv) surface water is the main water source for MAR, and (v) the total annual recharge volume is about 158 Mm3/year. MAR schemes exist in both urban and rural Africa, which exemplify the advancement of MAR implementation as well as its out scaling potential. Further, MAR schemes are most commonly found in areas of high inter-annual variability in water availability. If properly planned, implemented, managed, maintained and adapted to local conditions, MAR has large potential in securing water and increasing resilience in Africa. Ultimately, realizing the full potential of MAR in Africa will require undertaking hydrogeological and hydrological studies to determine feasibility of MAR, especially in geographic regions of high inter-annual climate variability and growing water demand. This, supported by increased research to gauge success of existing MAR projects and to address challenges, would help with future siting, design and implementation of MAR in Africa.

**Keywords:** managed aquifer recharge; water security; climate change; Africa

#### **1. Introduction**

Challenges posed by climate variability and change in Africa are widely recognized [1]. Sadoff et al. [2] highlight how rainfall variability disrupts productivity of rain-fed agriculture, contributes to disasters associated with floods and droughts, and stalls economic growth. An African Ministers' Council on Water (AMCOW) report [3] state that recurrent droughts in sub-Saharan Africa are a dominant climate risk that compromises livelihoods, water and food security and exerts a major negative effect on Gross Domestic Product (GDP) growth in one third of the continent's countries.

Groundwater currently contributes water supply to increasing populations in Africa, while also presenting a resource with significant potential for further development for multiple uses in parts of the continent [4]. Particularly in arid and rural areas, groundwater plays a key role in enhancing resilience. However, with increasing climate variability, frequency of extreme events, and population growth, water development in Africa needs to go forward in ways that consider all water sources in conjunction to enhance sustainable, reliable, climate-smart and equitable water availability and access [5].

Managed aquifer recharge (MAR) is a water management option that provides a means of intentionally recharging and storing water underground for subsequent recovery and beneficial use. It provides an important buffer against the impacts of climate variability and change, especially when additional water is recharged during wet or flooding periods for subsequent abstraction during dry or drought periods [6]. MAR has been identified to hold particular potential in arid and semi-arid areas where the control and storage of increasingly irregular surface runoff is challenging [7]. According to Tuinhof et al. [8], MAR is a significant adaptation option for coping with climate change and hydrological variability. Importantly, MAR provides water storage that is generally better protected against loss from evaporation [9,10]. MAR may take advantage of sources other than surface water runoff, e.g., by using treated wastewater, which often provides a more reliable source, while requiring stricter control of water quality [11].

Despite the large scope for MAR in Africa, there has been scant effort to take stock of MAR implementation and experience in the continent and to assess further potential with the exception of project report by Ebrahim et al. [12]. Dillon et al. [13] provided an overview of MAR in the Southern Africa, while Murray [14] provided an introductory guide to MAR in the Southern African Development Community (SADC) for the Groundwater Management Institute (SADC-GMI) and assessed the MAR case in the city of Windhoek, Namibia [15]. Similarly, Bugan et al. [16] and Jovanovic et al. [17] examined the Atlantis MAR scheme in South Africa [18]. Previous regional syntheses have also been undertaken in Europe [19] and Latin America and the Caribbean [20]. Recently, the research group 'Innovative Web-Based Decision Support System for Water Sustainability under a Changing Climate' (INOWAS) and the International Groundwater Resources Assessment Centre (IGRAC) produced the first global inventory of MAR sites. It contains key information and parameters related to implementation and bio-physical conditions of about 1200 MAR schemes in 62 countries worldwide [21]. No work has specifically synthesized past MAR experience in Africa.

As such, the objective of this article is to compile and synthesize experience on MAR across Africa from documented schemes in order to inform the feasibility and modalities of future MAR implementation in the continent.

#### **2. Methods**

#### *2.1. Case Collation*

Collation of cases for this article is primarily based on a review of the Global MAR Portal [22]. Case information in the portal generally includes country, site name, coordinates, MAR type, MAR key objective, operation start year, source of water, and final use. The portal contains 44 cases from Africa. Furthermore, a literature search was undertaken to expand the database with additional cases, as well as obtain Supplementary Information on existing cases. Eight additional cases were identified through this literature search. Hence, in total 52 cases were reviewed. All the cases were originally compiled from sources including: technical governmental documents (35%), peer reviewed publications (28%), conference presentations and proceedings (22%) and Master of Science theses (15%). The compiled set of MAR cases likely does not reflect a complete inventory of MAR practice in Africa, as documentation is sometimes contained in less easily accessible technical governmental reports (e.g., Wipplinger [23]) or in non-English language (e.g., French, Portuguese, and Arabic), which were not considered in this analysis (experts from non-English speaking countries were consulted to inquire about additional

cases). For example, Gijsbertsen and Groen [24] reported that in the Kitui District in Kenya, more than 500 sand dams were constructed since 1994 by collaboration of a Non-Governmental Organization (NGO) called SASOL (Sahelian Solution Foundation) and local communities. However, only eight sand dam cases in Kenya are found in the Global MAR Portal and included in this article. Still, the database applied represents the current best available dataset. It is assumed that the cases included in the analysis are representative of the types of existing MAR in Africa. Both full scale operation and pilot cases were considered (Annex S1 of the Electronic Supplementary Material (ESM) specify which schemes belong to full scale and pilot scale). The focus of this review is on single MAR schemes with individual documentation, disregarding landscape type approaches to enhancing recharge.

#### *2.2. Case Classification*

Classification of MAR cases for this study was based on various scheme characteristics (Table 1) and derived from the classification in the Global MAR Portal [22] and additional sources. In total, 13 parameters were used for classification. Some parameters, related to the biophysical and environmental conditions at the MAR sites, were determined from separate sources, e.g., rainfall, inter-annual variability of water availability, soil type, and geology (Table 1). Other factors, related to socio-economics, costs (capital and operational) and detailed water quality aspects, are important, but scarcity of information on these parameters precluded their inclusion in the analytical framework.



Information on MAR schemes in Table 1 was mostly obtained from the Global MAR Portal, and missing information was obtained from the literature. Likewise, recharge volume, key MAR objective and challenges in MAR implementation were obtained through additional literature review. The key MAR objective classification adopted in this study are more specific than the three main MAR objective classes of the Global MAR portal; (i.e., maximizing natural storage, maximizing natural storage and physical aquifer management, and water quality improvement). Challenges reported associated with MAR applications are broad and may include technical, biophysical, managerial, socio-economic, regulatory, institutional issues, availability of MAR water, water quality and degradation, etc. These were broadly divided into challenges in site selection and design on the one hand, and challenges in operation and maintenance on the other.

Four biophysical parameters were considered. First, long-term average annual rainfall was determined for the 52 cases. Data were extracted from the Climate Hazards Group Infrared Precipitation with Stations version 2 (CHIRPS) [31]. CHIRPS combines 0.05◦ × 0.05◦ resolution satellite imagery with in-situ station measurements. CHIRPS data are available from 1981–present (ftp://ftp.chg.ucsb. edu/pub/org/chg/products/CHIRPS-2.0).

Second, inter-annual variability of water availability were determined using geospatial data from Aqueduct Global Maps 2.1 [33]. Inter-annual variability of water availability is calculated as the standard deviation of the annual available "blue" water divided by the mean of annual available "blue" water (1950–2010). Water availability here is defined as the surface water available after accounting for sanctioned diversions upstream. Hence, "blue" water hereafter is referred to as surface water.

A third biophysical parameter considered was whether the aquifer or aquifer system for a particular MAR scheme is transboundary, i.e., whether the aquifer traverses a national border, as defined from present delineation or not. A map of the transboundary aquifers of the world [34] was used to classify the aquifers. MAR implementation in a transboundary aquifer might be a coincidence rather than planned.

Finally, the fourth biophysical parameter, surficial geology of the MAR sites, was classified using the Global Lithological Map (GLiM) developed by Hartmann and Moosdorf [35], which has an average resolution of 1:3,750,000 and consists of 16 lithological classes. These were lumped into two major rock types for the purpose of this study: sedimentary and hard rock (The geology represents the general surficial geology of the MAR sites. Whether this is also representing the actual geologic formation used to store MAR recharge, which could be the case for many sites, has not been assessed).

It should be noted that additional MAR site information, such as soil infiltration rates, unsaturated zone thickness, and aquifer hydraulic conductivities/transmissivity, were obtainable for a few cases from the literature review. However, these classifications are not included in the present study due to limited data. Detailed data sets of parameters in Table 1 for the MAR cases are provided in Annex S1 (MAR scheme information) and Annex S2 (MAR site information) of the ESM.

#### **3. Results**

The MAR cases are concentrated in nine countries in Africa. The vast majority of African countries have not implemented MAR. MAR is practiced in greatest abundance in South Africa with 17 reported cases, followed by Tunisia with 11 cases, Kenya with eight cases, and Algeria with five cases (Figure 1). Five additional countries also present evidence of MAR implementation (Egypt, Ethiopia, Morocco, Namibia and Nigeria), with up to four cases per country. It appears that MAR is more often practiced in Africa's wealthier, yet drier countries or where a certain MAR technology has gained popularity (like sand dams in Kenya, see below).

**Figure 1.** Number of MAR cases in Africa per country (n = 52).

The main MAR type is the surface spreading/infiltration method. The most common MAR type is the surface spreading/infiltration method, the second most common is open well, shaft and borehole injection, and the third is in-channel modification (Figure 2). In-channel-modification using sand dams is the most practiced MAR type in Kenya, while in Tunisia, it is the spreading/infiltration method, and in South Africa, the open well, shaft and borehole injection method (Table 2). The Sidfa Riverbank Filtration case in Egypt [36] is the only reported induced river bank filtration case in Africa. There are no reported cases of rainwater and runoff harvesting.

**Figure 2.** Number of MAR cases in Africa per main MAR type (n = 52).



Implementation of MAR started in the 1960s and has increased over time. The first MAR projects in Africa were launched in 1965, in Soukra, Tunisia [29] and Polokwane, South Africa [13], two followed in the 1970s, and three in the 1980s. The pace of MAR implementation increased substantially in the 1990s with seven new projects undertaken, 16 more projects in the 2000s, and seven projects in the 2010s (Figure 3). One particular example of MAR proliferation is sand dam implementation in Kenya in the 2000s. The apparently illogical decline in the number of MAR cases in the 2010s could be due to lag in reporting.

**Figure 3.** Historical development of MAR cases in Africa where starting date is known. For each decade, the number of new projects is given (n = 37).

The main source of water for the MAR schemes is river water. Thirty three out of the 52 cases use river water as a source and 11 cases use treated wastewater (Figure 4). Some schemes use several sources of water. The Atlantis scheme [18] uses both urban stormwater runoff and treated wastewater. The Windhoek scheme [15] uses river water and treated wastewater. Among the 33 cases that use river water, 19 cases use an ephemeral river as their water source, while eight cases rely on perennial rivers. Three of the eight cases that rely on perennial rivers are found in Egypt, using Nile River water. The Toushka and EI Bustan area, with two MAR schemes rely on Nile flood water for their source [37], while the Sidfa river bank filtration project relies on perennial flow of the Nile River [36]. Treated

wastewater is used in countries with limited water resources, such as Atlantis, South Africa [18], Windhoek, Namibia [38], and Korba Cap Bon, Souil Wadi, Nabelul, Soukra, Nabeul-Hammamet, El Hajeb-Sidi Abid, Boumerdes and Mahdia-Ksour Essef, Tunisia [29]. The five MAR cases, which rely on groundwater as source of water are at: Williston, South Africa [39], Sishen Mine (Khai Appel), South Africa [25], Elandsfontein, South Africa [25], Kolomela South Africa [13,26], and Koraro-01, subsurface dam, Ethiopia [40]. The Williston case in South Africa [39], uses groundwater abstracted from one aquifer to recharge another aquifer compartmentalized by dykes while the other three cases in South Africa use dewatering water from mines.

**Figure 4.** Number of MAR cases in Africa per water source (n = 52).

The key objective of the MAR schemes is to secure and augment water supply. Out of the 52 cases, 23 have a key objective focused exclusively on securing and augmenting water supply (Figure 5). Nine cases have key objectives focused on water quality improvement. Six cases have ameliorating groundwater level decline as key objective, while four cases have preventing seawater intrusion as key objective.

Examples of securing and augmenting water supply include purposes of increasing water availability to meet rural domestic demand during dry periods (e.g., sand dams in Kenya), of meeting summer peak demand (e.g., Prince Albert [41], and Plettenberg [42], South Africa). Examples of securing water supply in drought and emergency situations include Calvinia, South Africa [43] and Windhoek, Namibia [43]. Augmenting water supply and preventing seawater intrusion refers to storing water to meet domestic demand while simultaneously preventing seawater intrusion in the coastal aquifer. An example is the Atlantis scheme, South Africa [18], which separates the domestic and industrial wastewater and stormwater runoff based on salinity, recharges the low salinity water into the inland aquifer used for domestic supply, and recharges the more brackish water into the coastal aquifer to prevent seawater intrusion. Other examples of cases with a preventative seawater intrusion objective include El Khairat [44] and Teboulba [30] aquifers, Tunisia. Finally, the objective of improving water quality refers to water quality enhancement during infiltration (SAT) process (e.g., Ben Sergao, Morocco [45]). One case with an exclusive key objective of enhancing environmental flow is Elandsfontein, South Africa [25].

**Figure 5.** Number of MAR cases in Africa per MAR key objective where information is available (n = 46).

Rainfall across the MAR schemes varies widely. The average annual rainfall across the MAR sites ranges from 1–2225 mm/year (Figure 6a). The average annual rainfall across the MAR sites is 461 mm/year. The minimum and maximum average annual rainfall occurs in MAR sites in Toushka [37], Egypt and Michael Okpara University of Agriculture, Nigeria [46], respectively. The number of cases per five average annual rainfall interval classes is shown in Figure 7. While just over 65% of cases are located in the low annual rainfall class (<500 mm/year), concentration of MAR cases in low rainfall geographies is not dominant.

MAR schemes are concentrated in regions of above-average levels of inter-annual variability in water availability. Eighty three percent of the MAR cases are located in regions of above average inter-annual variability in water availability (Figure 6b). Twenty one of the MAR cases are located in regions with 'medium to high' inter-annual variability, and another 17 cases in regions with 'high' variability (Figure 8). Five cases are located in 'extremely high' variability regions. Saaipoort [25,27,28], Sishen Mine (Khai Appel) [25], Smouskolk (Vanwyksvlei) [25], Williston [39], South Africa and Windhoek, Namibia [43] are the five cases in the extremely high inter-annual variability class. Conversely, only nine cases are located in collective regions of 'low' or 'medium to low' variability.

MAR cases are concentrated in highly populated regions. In total, 31 cases (60%) are located in regions with population density greater than 100 inhabitants per square kilometer (ESM, Figure S1). Nonetheless, MAR experience is also found in less populated areas of e.g., Egypt, Namibia, South Africa and Tunisia. Cases with low population density could be explained by primary use for rural and irrigation use (Teboulba, Tunisia [30], El Hajeb-Sidi Abid, Tunisia [29]), or because water from MAR is subsequently transferred to population centers (Omaruru Delta (OMDEL), Namibia [47], Williston, South Africa [39]).

**Figure 6.** Location of MAR schemes in Africa overlaid on (**a**) average annual rainfall from Climate Hazards Group Infrared Precipitation with Stations (CHIRPS) [31] and (**b**) inter-annual variability in available surface water from Aqueduct Global Maps 2.1 [33]. MAR case labels corresponds to MAR cases numbering in Annex S1 and Annex S2 of the ESM.

**Figure 7.** Number of MAR cases in Africa per average annual rainfall class (n = 52).

**Figure 8.** Number of MAR cases in Africa per inter-annual variability in surface water availability class (n = 52).

Few MAR schemes are located in transboundary aquifers. Six cases are found to be located in a transboundary aquifer. Four cases in Egypt are located in the Nubian Sandstone Aquifer system shared among Chad, Egypt, Libya and Sudan, one case in Ethiopia is located in the Mereb Aquifer shared between Ethiopia and Eritrea, and one case in Nigeria is located in the Lake Chad Aquifer shared among six countries: Algeria, Cameroon, Central Africa Republic, Chad, Niger and Nigeria. None of the MAR projects in these transboundary aquifers is believed to have significant transboundary concerns or impacts, as they are not located close to the national borders, and the magnitude of enhanced recharge is relatively small compared to the natural storage of the transboundary aquifers.

The primary geological environment of the MAR cases is sedimentary aquifers. The majority of the MAR sites is situated in sedimentary aquifers (n = 42) and the rest (n = 10) is located in hard rock aquifers. This is expected, as sedimentary formations (e.g., carbonate, gravel, and sand) with various degree of consolidation or cementing are usually targeted for MAR [48]. MAR sites situated in sedimentary rocks are found in Egypt, Ethiopia, Morocco, South Africa and Tunisia. Igneous and metamorphic rocks are often loosely referred as hard rock due to their poor drill-ability [49] and have low primary porosity and most of their porosity comes from secondary porosity (cracks and fractures). Examples of MAR in hard rock are found in Kenya, Nigeria, Namibia and South Africa. The Windhoek case, Namibia [15] is a good example of a large-scale municipal use MAR scheme in fractured rock aquifer setting. Although, half of the sand dams (n = 4) in Kenya are located in hard (metamorphic) rock, the weathering product of the same (sand and gravel) produce eroded sediments, which constitute the actual aquifer used for MAR.

Sectoral use of the MAR schemes is predominantly domestic. The final use of the recovered MAR water across all schemes is presented in Figure 9. The dominant use of MAR is to support domestic water supply (n = 23), followed by agricultural use (n = 17), while only one scheme is solely used for industry (Eland Platinum Mine, South Africa [50]). Multiple purpose schemes have multiple uses of the MAR water (typically agricultural and domestic use, e.g., Loeriesfontein, South Africa [39,51], and domestic and environmental use, e.g., Atlantis, South Africa [18,52]). Despite classifying sand dams into domestic water supply, it is likely that the water is used for multiple purposes, including, livestock and garden agriculture [24]. The Elandsfontein, South Africa [25] is the only case with dominant use of MAR water for environmental flows.

**Figure 9.** Number of MAR cases in Africa per final water use of stored water where information is available (n = 44).

Recharge volume per MAR scheme varies widely. Recharge volume estimates (compiled as recharge volume per year) are available for 23 MAR cases in seven of the nine countries with recorded MAR schemes (Table 3). The recharge volume ranges from 0.001–100 Mm3/year, as estimated mostly from reported single daily recharge rates. The highest recharge volume was recorded for the Souss-Massa case, Morocco [53,54] and the lowest recharge volume was for the Koraro-01 subsurface dam (included in the in-channel modification MAR type), Ethiopia [40]. The high recharge volumes (Souss-Massa, Morocco [53,54]; Sidfa Riverbank Filtration, Egypt [36]; Omaruru Delta, Namibia [47]; and Atlantis, South Africa [18]) give an indication of the potential that MAR provides in terms of

securing water supply (most notably in the dry season). Even the smallest recharge schemes, like Koraro-01, Ethiopia [40], provide a secured domestic water supply for rural communities.

Recharge volume per country is also quite variable (Table 4). The total annual recharge volume for Africa from full scale MAR is 158 Mm3/year. To compare, the value for Latin America and the Caribbean is 340 Mm3/year [20]. The Souss-Massa MAR scheme [53,54], which contributes the largest share of MAR recharge in Africa, makes use of Aoulouz and Imi El Kheng dams located in the upstream part of the Souss-Massa River basin to detain flood water and regulate release of water into the streambed downstream to match infiltration capacity of the streambed. The Souss-Massa River basin is one of the major River basins of Morocco located in the southwestern part of the country. The River basin is drained by two main rivers, the Souss River and the Massa River. The Souss River originates from the High Atlas Mountains in the northern part of the basin. The river is seasonal with occasional high floods between October and February. It is regulated by four big dams. The Massa River drains the Souss-Massa from the southern side of the River basin. Both rivers drain into the Atlantic Ocean. The Aoulouz and the Imi EI Kheng dams, two big dams located in the upstream part of the Souss River, are used for MAR mainly due to their favorable geologic conditions and good water quality [55]. The storage capacity of the Aoulouz and the Imi EI Kheng dams is 103 and 12 Mm3, respectively. Based on monthly dam release data of 1991–2004, Bouchaou [53] estimate an annual recharge of 100 Mm3 (86% of total annual release), which is consistent with other studies [56]. The annual recharge volume is estimated using piezometeric water level fluctuations before and after dam release [54].


**Table 3.** Recharge volume per MAR case where information is available (n = 23).

<sup>1</sup> Recharge volume for the Atlantis scheme, South Africa is around 2.7 Mm3/year, but an additional 1.5 Mm3/year more brackish salinity water is recharged in coastal aquifers for seawater control. <sup>2</sup> Annual recharge volume is obtained from Fanus Fourie, DWS (Department of Water and Sanitation), and South Africa (personal communication).


**Table 4.** Recharge volume per country and recharge as percentage of groundwater use, arranged by recharge volume, where information is available (n = 23).

<sup>3</sup> Groundwater use for 2010 from Margat and van der Gun [7].

Recharge volume as percentage of groundwater use per country is also variable. Recharge volume as percentage of groundwater use per country is calculated using recharge volume for operational MAR schemes and groundwater use data from 2010, as reported by Margat and van der Gun [7] (Table 4). Recharge volume as percentage of groundwater use is highest in Namibia (7.9%) followed by Morocco (3.3%) and South Africa (0.5%).

Recharge volume as percentage of groundwater use is the least for Africa compared to other continents. The total groundwater use in 2010 for countries in Africa reported by Margat and van der Gun [7] is 41,640 Mm3/year. Based on this, MAR as percentage of groundwater use in Africa is 0.4%, which is close to the estimate for South America (0.5%) but low compared to other continents, e.g., Middle East (9.4%), Oceania (8.3%), Europe (6.3%), North America (2.3%) and Asia (1.8%) [13] (See also Table S4 of the ESM).

Summary of MAR cases per country. In Morocco, both surface spreading/infiltration and in-channel modification are practiced in equal share. The main objective of MAR is water quality improvement as well as ameliorating groundwater level decline. River water (Souss-Massa [53,54]) and treated wastewater (Ben Sergao [45]) are used as a main water sources for MAR in Morocco, and the final use of recovered MAR water is for agriculture. In Egypt, the main MAR type is surface spreading/infiltration, the main objective of MAR is to secure and augment water supply, the main water source is river water, and the final use of MAR water is domestic water supply. In South Africa, the main MAR type is open well, shaft and borehole injection, the main objective of MAR is to secure and augment water supply, the main water source for MAR is river water, and the final use of recovered MAR water is domestic water supply. In Namibia, both surface spreading/infiltration (Omaruru Delta [47]) and borehole injection (Windhoek [38,43]) are practiced. The main objective of MAR is to secure water supplies in drought and emergency situations as well as ameliorating groundwater level decline. River water and treated wastewater are used as a main water sources for MAR, and the final use of recovered MAR water is for domestic water supply.

In Tunisia, the main MAR type is surface spreading/infiltration method, the main objective of MAR is water quality improvement, the main source of water for MAR is treated wastewater, and the final use of MAR water is for agriculture. In Algeria, the main MAR type is surface spreading/infiltration methods, the main source of water is river water and the final use of recovered MAR water is for agriculture. In Ethiopia, the only analyzed case is a subsurface dam constructed near the village of Koraro [40]. The Koraro-01 subsurface dam supplies domestic water. In Kenya, the main MAR type is in-channel modification (sand dams), the main objective of MAR is to secure and augment water supply, the main water source is river water, and the final use of MAR water is domestic water supply. In Nigeria, surface spreading/infiltration and open well, shaft and borehole injection are practiced equally prevalently. The main objective of MAR is to ameliorate declining groundwater levels, and the main source of water is river water.

#### **4. Discussion and Conclusions**

This article has compiled information on 52 well-reported MAR cases in Africa in order to develop a baseline in understanding of the breadth and status of MAR in Africa. A classification framework was developed and applied to the set of MAR cases, in order to make a first-order assessment based on existing data. Despite the relatively small sample (n = 52), the analysis has generated six significant findings.

First, MAR implementation is not extensive in Africa and is concentrated in nine of the continent's 54 countries. The extent of MAR implementation in Africa remains low compared to other continents [21]. However, the volume reported here (158 Mm3/year) significantly exceeds the only documented summary of MAR in Africa, which was for Southern Africa, presented by Dillon et al. [13] (10 Mm3/year for 2015). MAR as fraction of groundwater use in the Southern African region is now estimated at 0.6%, which is three times the reported value by Dillon, et al. [13]. This increase is partially explained by new cases coming into operation since 2015 (e.g., Elandsfontein [25]) or additional cases included in this study (e.g., Omaruru Delta [47].

The number of MAR cases in Asia, Europe, North America, Oceania and South America from the Global MAR Portal is281, 282, 308, 95and 113, respectively [22]. Still, there are cases in Southern and Northern Africa that have more than 30 years of MAR practice (e.g., Atlantis, South Africa [18], Teboulba, Tunisia [30]). Overall, the extent of MAR practice in Africa is low considering the high variability in surface water availability. Major factors that limit wider application of MAR in Africa may include lack of: (1) awareness of and experience with MAR, (2) financial resources, (3) human and institutional capacity, (4) enabling policy frameworks, (5) sufficient and sufficiently well-functioning demonstration sites, (6) understanding of aquifer hydrogeology and geochemical properties.

Second, the key objective of the MAR schemes is to secure and augment water supply and balance inter-seasonal variability in supply and demand. This is consistent with the study in Latin America and the Caribbean by Valverde et al. [20]. For example, the Atlantis scheme, South Africa [18] contributes approximately 25–30% of the water supply of Atlantis Town and has recently been proposed for an expansion to support future drought resilience of Cape Town [60]. The Calvinia case, South Africa [43] is reported to have the potential to provide two to three months of the water supply to Calvinia Town. According to Murray [14], when developed in the next phase, the Windhoek case, Namibia [38,43] is expected to provide a drought buffer as the main water resource for the city for up to three years. As such, Africa presents an initial set of cases and experiences, on which to further develop and upscale the benefits of MAR in the continent.

Third, the main MAR type is the surface spreading/infiltration method. This is different from other continents, e.g., in Europe (induced bank filtration is dominant) [19], and Latin America and the Caribbean (in-channel modification) [20]. In Africa, while sand dams are practiced widely in Kenya, open well, shaft and borehole injection methods are relatively more common in Southern Africa.

Fourth, and linking to point two, a central bio-physical factor that appears to explain MAR location is variability in water availability. The majority (83%) of the MAR sites in Africa are located in geographic regions with above average inter-annual variability in water availability. This is expected as one of the main purposes of MAR is to buffer short-term, seasonal and across-year-variability in water supply. Population density, representing water demand (Figure S1, ESM) and technical and financial capacity are additional factors that may help explain MAR uptake in Africa. Surface spreading/infiltration methods and open well, shaft and borehole injection methods are mostly used for larger urban supply systems, whereas sand dams and subsurface dams for smaller rural systems. As such, there is an apparent dichotomy between MAR systems in Africa governed by demand setting, investment environment, water sources to be applied for MAR (with increasing use of treated

wastewater in urban settings) and the management schemes required to address the various issues of financial feasibility and technical maintenance. It is important to note that wastewater reuse should only be encouraged if groundwater quality protection measures are demonstrated effective, which requires guidelines, monitoring and analysis capability. Water from MAR in rural areas also tend to have more variety in use requirements compared with municipal systems, which cater mostly to domestic and industrial uses.

Fifth, while not exhaustive, our study pinpointed some general challenges with MAR in Africa (ESM, Table S4). In total, 13 cases (28%) were reported to have a challenge related to site selection and design or to operation and maintenance. Shallow unsaturated zone thickness was identified as a problem in two surface spreading/infiltration MAR cases (Atlantis case, South Africa [18]) and Polokwane, South Africa [43]. The unsaturated zone of one of the basins of the Atlantis case is reported to be fully saturated during the rainy season, which affects the storage and water quality improvement capacity of the system [18]. In the Polokwane case, South Africa [43], due to insufficient unsaturated zone thickness, there is a concern that some bacteria, viruses or parasites could survive and contaminate groundwater.

Site selection problems were identified as main reasons for underperformance or failure of some sand dams in Kenya [24,61,62]. Site selection may affect rate of sand deposition behind the dam, hence, the time to reach full storage capacity. As an example, three sand dams located in the upstream part of a catchment were reported to have very low infiltration capacity and storage due to the existence of a silt layer in the accumulated sediment, and it took nine years for the sand dam to reach its full storage capacity [24]. Similarly, siting sand dams in areas with a high depth to impermeable bedrock was found to be a problem giving rise to excessive leakage of stored water underneath the dam and scouring of dam foundation [24]. Site selection was a problem in the Abu Rawash case, Egypt [63], where the infiltration basin was sited in clay soil with low infiltration rate.

Infrastructure design problems were reported in the Kharkams well injection case, South Africa [43] and in the in-channel modification Koraro-01 case, Ethiopia [40]. Filter design was an issue in the Kharkams injection MAR scheme, where sand was entering the injection borehole, resulting in a clogging problem [43], whereas the problem at Koraro-01 was the infrastructural design that resulted in breaching of above ground situated subsurface dam crest due to flooding [40]. For some infiltration schemes, i.e., Atlantis, South Africa [18], the Omaruru Delta, Namibia [47] and Abu Rawash, Egypt [63], clogging of infiltration basins or recovery wells, created problems. For example, in Atlantis, South Africa, elevated iron and sulfate in the groundwater caused biological iron-related clogging in recovery wells [18].

Mixing of recharge water with poor inherent water quality in the aquifer was identified as a concern at two MAR sites, Calvinia [43] and Eland Platinum Mine [50,64], both in South Africa. The Calvinia site uses underground natural breccia pipes as storage of MAR water due to their excellent hydraulic confinement and because of the poor surrounding water quality. According to Murray and Tredoux [43], the breccia pipes used in Calvinia are about 100 m in diameter and are very poorly connected to the Karoo sequence aquifer in which they are located. Since the surrounding aquifer already contains high pH, fluoride and arsenic that exceed drinking water standards, there is a concern that the quality of abstracted water could be compromised due to mixing of recharged water with inherently contaminated aquifer water [43]. Use of abandoned mining and quarry sites may reduce costs associated with infrastructure of MAR storage. However, risk of mixing with poor quality water from mining activities may be a concern. For the Eland Platinum Mine, South Africa [50,64], high nitrate and electrical conductivity levels were a concern. Risk of heavy metal contamination from the mining activity is also possible, but data are not available to determine the level of risk. Some of the measures for addressing the above challenges are presented in Table S5 of the ESM. Existing guidelines developed in Australia and elsewhere provide useful guidance for planning and implementation, monitoring and evaluation of MAR (e.g., [39,65–67]). By following approaches presented in these guidelines, problems can be anticipated and counteracted. Re-siting or re-designing projects, and pro-actively

investigating how the schemes can best be monitored and managed are key to effective operation and sustainable outcomes.

Before concluding this discussion it is worth noting that there are other types of aquifer recharge enhancement in Africa that extend beyond those examined in this article. These include cases of road water harvesting [68,69]; spate irrigation [70], and rainwater harvesting [71–73]. Road water harvesting is a type of water harvesting technique where concentrated runoff generated by or along roads is collected using adjacent ponds or channeled to farm land [69,74], which assists road drainage as well as crops, and may incidentally enhance recharge. Spate irrigation is an irrigation technique, by which seasonal river water is diverted and spread to a larger riparian surface areas, primarily to enhance soil moisture for growing crops, and this would also incidentally enhance recharge.

#### **5. Looking Forward**

This paper is the first to systematically examine reported MAR schemes over the African continent and review experiences. While only 52 cases are reported, they provide about 158 Mm3/year of additional water storage to enhance water supply and/or water quality. This volume represents 0.4% of continental groundwater use, but plays a critical role in locally supplying and securing water for large populations. Examples of technical challenges were revealed that could be a deterrent to future African investment in MAR, unless these are systematically addressed. Observations from MAR practices around the world suggest that guidelines and policies can help to prevent or overcome such problems. South Africa has advanced significantly in this regard with a national map showing the prospects for MAR [51], a manual on how to develop MAR projects [75], and well-documented case studies [58]. Wider adoption of this approach would well serve other African nations.

Furthermore, impact assessment, including pre-project baseline assessment and continuous monitoring are warranted for significant MAR schemes to reveal their performance and long-term outcomes with respect to water security and resilience. Maintenance and financial, institutional and socio-economic aspects are also key elements to sustainability, which needs increased attention. Hence, strong feasibility analysis, performance and impact assessment of existing and new MAR projects should be a primary focus going forward.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4441/12/7/1844/s1. References [76–86] are cited in the supplementary materials. An Electronic Supplementary Material (ESM) summarizing MAR cases information. Figure S1: Location of 52 MAR cases in Africa overlaid on population density map of 2015, Table S1: MAR cases per key objectives per country (arranged by total number of MAR cases), Table S2: MAR case studies per source of recharge water per country, Table S3: MAR cases per final use of recovered water per country, Table S4: Recharge volume, absolute and as percentage of groundwater use in different continents or regions, ordered by MAR in 2015, Table S5: Challenges and possible solutions in MAR implementation, Annex S1: MAR cases in Africa (MAR scheme information), Annex S2: MAR cases in Africa (MAR site information).

**Author Contributions:** G.Y.E. led the development of this work including analysis of cases and write-up and prepared the original draft. J.F.L. contributed in conceptual direction and to draft preparation and in the review. K.G.V. contributed to the conceptual framework and review. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was undertaken in the context of and funded by the Potential Role of the Transboundary Ramotswa Aquifer-2 project, funded by the United States Agency for International Development (USAID) under the terms of Award No. AID-674-IO-17-00003, the CGIAR Research Program on Water, Land and Ecosystems (WLE) supporting the Groundwater Solutions Initiatives for Policy and Practices (GRIPP), and the Conjunctive Management across Borders in SADC project, also funded by USAID.

**Acknowledgments:** The authors would like to thank Fanus Fourie (Department of Water and Sanitation, South Africa), Habib Chaieb, (Directeur chez DHER-CRDA de Ben Arous—MARHP El Mourouj, Ben Arous Governorate, Tunisia), and Lhoussaine Bouchaou (Department of Earth Sciences, University Ibn Zohr Agadir, Morocco), and the International Groundwater Resources Assessment Center (IGRAC) for providing support in terms of case study information, and the two anonymous reviewers and the editor for providing constructive comments, which greatly improved the manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

### *Perspective* **An Overview of Managed Aquifer Recharge in Brazil**

#### **Tatsuo Shubo 1,\*, Lucila Fernandes <sup>2</sup> and Suzana Gico Montenegro <sup>2</sup>**


Received: 1 December 2019; Accepted: 7 April 2020; Published: 9 April 2020

**Abstract:** In order to face the severe climate conditions in semiarid regions, many managed aquifer recharge (MAR) and rainwater storage systems have been implemented by local communities. Governmental programs have helped to propagate the concept of MAR. Based on a systematic review, popular initiatives, current legislation, and research lines and programs were compiled and analyzed. Although the MAR global inventory points to the prevalence of in-channel modifications among ninety MAR sites, the Barraginhas Project alone has been responsible for the construction of more than 500,000 infiltration ponds up to 2013. In urban areas, aquifer recharge initiatives mostly aim to reduce runoff peak flows. In some cases these initiatives have been stimulated by urban drainage public policies. Compared to countries such as the USA and Australia, Brazil is still at an early stage in MAR initiatives and needs to overcome technical, legal, and socio-cultural challenges to adopt MAR approaches, in order to help in facing water security challenges in a future climate change scenario. This article aims to provide an overview of the state of the art concerning technological, scientific, and legal issues around MAR in Brazil and the respective challenges for the adoption of this approach at a national level.

**Keywords:** water security; urban water management; semiarid; Social Technology; Managed Aquifer Recharge; developing countries

#### **1. Introduction**

#### *1.1. Historical Background*

Although Brazil has a huge water availability, about 30,342 m3/inhab./year in 2015 [1], it is not evenly distributed across the country, with 80% of the surface water concentrated in the Amazon region [2]. Besides this, Brazil has been struggling with many water crises since the beginning of its settlement by Europeans. A priest called Fernando Cardin recorded the first drought in 1583. Since then, more than 120 droughts have been recorded in the northeastern semiarid region alone. A seven-year drought recorded in the 18th century (1720–1727) that struck the region currently known as the states of Ceará, Rio Grande do Norte, Paraíba, and Pernambuco has been considered the worst one on record. During that particular event, most livestock perished, rivers and springs dried up, and widespread starvation devastated the region [3]. The time period between the years of 1877 and 1879, recorded as the hottest and driest of the 19th century, imposed severe hardships and suffering on the local populations. During that period, approximately five hundred thousand people starved to death, and crops and cattle suffered devastating losses. This scenario triggered massive waves of migration of people moving towards coastal cities, bringing a demographic explosion to areas that did not yet have the appropriate infrastructure in place to support these migrations. Poor living conditions, such as the lack of proper sanitation systems, have been associated with a smallpox epidemic which contributed to the hardships [4].

From the 1980s up to the present time, Brazil has been experiencing many of its worst droughts on record and struggling with their consequences. As a consequence of yet another seven-year drought period (1979–1985), more than 3.5 million people ended up starving to death, with most of the victims being children who perished from undernourishment. Crops and cattle were lost, forcing desperate farm people to loot local markets seeking food [5]. In 2002, Brazil also faced an energy crisis, known as "the apagão" (The Big Blackout), mostly caused by a series of dry periods. In 2007, the northern part of the state of Minas Gerais suffered a fifteen-month dry spell with virtually no rainfall.

In the Brazilian countryside, especially in the semiarid region, there was a lack of rainfall during the time period of 1981–2019. On the other hand, in the northern Region, high rates of rainfall still occur. Climate models show a trend towards increased frequency and intensity of droughts and length of dry periods in the northeast, as already has occurred in some Brazilian regions [6]. From 2012 to 2017 another major drought affected the semiarid region and 2015 was considered the most critical year of that period. Figure 1a shows the average precipitation for the early dry season in Brazil (April–May), from 1981 to 2010, and the Figure 1b shows the total precipitation in 2015. In the Southern Hemisphere, autumn is the transitional period from the wet to the dry season. As can be seen, in 2015, the total precipitation was far lower than the historical average [7]. Although 2017 has not been the driest year in the northeastern region of Brazil, the rainfall amounts there were far below the historic average, and can be counted as an extension of the 2012 drought. During this period (2012–2017), some of the São Francisco River Hydrographic Region gauging stations recorded zero flow, and the flow released by the reservoirs had to be reduced in order to avoid water supply failures. In 2017, 38 million people were affected by drought and 51% (2.839) of all Brazilian municipalities declared a state of emergency [8]. Brasília, the capital of the nation, and São Paulo, the largest and wealthiest city, endured water rationing during this period.

AC: Acre AL: Alagoas AP: Amapá AM: Amazonas BA: Bahia CE: Ceará DF: Distrito Federal ES: Espírito Santo GO: Goiás MA: Maranhão MT: Mato Grosso MS: Mato Grosso do Sul MG: Minas Gerais PA: Pará PB: Paraíba PR: Paraná PE: Pernambuco PI: Piauí RJ: Rio de Janeiro RN: Rio Grande do Norte RS: Rio Grande do Sul RO: Rondônia RR: Roraima SC: Santa Catarina SP: São Paulo SE: Sergipe TO: Tocantins

**Figure 1.** (**a**) Average rainfall in the early dry season (1981–2010); (**b**) total precipitation in 2015. Source: Adapted from Instituto Nacional de Meteorologia (INMET) [7]. The semiarid region (shaded in the Figure 1a,b) encompasses parts of the States of Alagoas, Bahia, Ceará, Minas Gerais, Paraíba, Pernambuco, Piauí, Sergipe, and the whole State of Rio Grande do Norte.

#### *1.2. Water Availability in Brazil*

According to the ANA [8], close to 90% of all Brazilian rivers rely on a base flow from aquifers that feed these rivers during dry periods, keeping them perennial. The exception occurs in the northeastern region where the ground is composed of a thin layer of soil and fractured rocks known to be crystalline, unable to feed water back to the rivers.

Although surface water dams are the main plan of action against droughts in the northeast, corresponding to 67% of the solutions adopted by the government [8], in actuality, at the national level, 47% of all municipalities have adopted surface water sources, while 39% consume groundwater, and 14% supply their systems with a mix of both [9]. In 2013 there were 225,868 registered tubular wells across the country. It is estimated, however, that this number could be much larger as a consequence of the proliferation of non-registered wells, possibly close to 477,000 wells in 2013 [10].

In 2014, there were around 21 million people living in the Alto Tietê Hydrographic Basin; 97% of the São Paulo Metropolitan Region population. In this basin, the water demand far exceeds the natural water availability, making it a necessity to import from other basins almost half of all water consumed. As a result of a lack of adequate water resources management, during the 2014–2015 water shortage, the government of the São Paulo State was forced to impose water rationing [11].

Traditionally, investments to fulfill water demands in large Brazilian cities are exclusively allocated to the discovery of new water sources, generally without considering important alternatives, such as water reuse [12]. During the 2014–2015 water crisis, the number of private wells for groundwater extraction increased exponentially as a popular response to water access restrictions, ultimately depleting regional underground water sources.

Groundwater sourcing represents, thus, a very important issue in the national and international context of water management both for rural and as well as urban areas. Given this, the introduction of the managed aquifer recharge (MAR) concept is an innovation in integrated water resources management in Brazil. In a climate change scenario and ever-increasing demand, it is considered both as an adaptation measure regarding extreme events, such as droughts, as well as a mitigation strategy for future water crises.

#### *1.3. MAR as Solution*

MAR is defined as "the purposeful recharge of water to aquifers for subsequent recovery or environmental benefit" [13]. MAR can take many forms, including recharge weirs, infiltration basins, riverbank filtration, recharge releases from dams, and recharge wells. The applications of MAR have been implemented since the 1950s for various purposes, such as to increase groundwater storage, improve quality, restore groundwater levels, prevent saline intrusion, and increase ecological benefits [14]. In many arid or semiarid areas, where groundwater is usually already overexploited or saline, recharge has the potential of storing excessive runoff, including in fractured rocks aquifers [15]. The most common MAR applications are for maximizing natural storage, representing 45% of the case studies in Australia, 62% in South America, and 84% in Africa. The main application of MAR in Europe is for water quality management, where approximately 200 riverbank filtration schemes are used for the production of drinking water [14].

In China there are reports of canals dug in the middle of the 5th century BC close to rivers in regions periodically flooded by storm water. These channels were intended to facilitate the infiltration of surface water into groundwater, changing the quality of groundwater and transforming saline land into fertile soil [16]. The USA today stands out in MAR capacity, second only to India [17]. Arizona has implemented MAR facilities that are able to recharge up to 173 M m<sup>3</sup> of the Colorado River, the water of which is considered a renewable resource. In the same state, another project consists of spreading basins through a flood plain producing an annual recharge of up to 37 M m<sup>3</sup> [18]. Arizona has a policy for the specific types of MAR that may be implemented in the state. The Underground Storage and Recovery Act, 1986, and the Underground Water Storage, Savings and Replenishment Program, 1994, provide guidelines for allowing state-supported aquifer recharge. This legislation involves three permissions [17].

This paper aims to provide an overview of the current use of MAR, and the potential and challenges of the adoption of MAR in Brazil, by surveying practices adopted to mitigate the effects of droughts, relevant research, and current policy and legal frameworks.

#### **2. Methodology**

A systematic literature review was carried out with the goal of understanding the challenges and opportunities of MAR adoption to strengthen integrated water resources management (IWRM) to confront climate changes in Brazil. Initially, the main social technologies (ST) to deal with droughts were identified, which were simple, low-cost, and easily applicable technologies. Information was sought about their main uses and relevant constructive features. A free term search was applied aiming to eliminate any misunderstandings about these technologies, as a function of the idiomatic diversity among those who share the knowledge construction base.

Then, further research was carried out, in order to provide a brief overview of stormwater/rainwater infiltration and retention techniques increasingly put to use in Brazilian urban areas. Considering that the sustainable urban drainage systems (SUDS) are solutions used worldwide, we decided that it is not necessary to delve into these technologies with as much detail as in the national rural ST.

Finally, environmental and groundwater legislation at national and state levels were gathered. In possession of these documents, the keywords managed aquifer recharge, artificial recharge, and groundwater were used to find the laws that provide specific guidelines on this subject.

The data were obtained from papers, conference proceedings, academic theses and, mainly, from official reports of state and federal institutions that handle this topic directly, e.g., Embrapa and Articulation for the Semi-arid (ASA).

#### **3. Results**

#### *3.1. Strategies to Deal with Drought in Brazil*

When starting a study in any field of knowledge shared by people whose languages are different, it is essential to define clearly basic concepts and terminology. Thus, in this section, the concepts of MAR are discussed, along with the complementary measures of rainwater and stormwater harvesting techniques found in Brazilian rural areas.

As already defined, MAR is a set of measures that aim to artificially increase the recharge of water in an aquifer. In the Brazilian semiarid region, some of the most common techniques for increasing the natural water reserve are underground dams and infiltration ponds. A detailing of these and other techniques and specific examples applied in Brazil are reported below.

The MAR Global Inventory, available on the International Groundwater Resources Assessment Centre (IGRAC) portal, has gathered ninety MAR applications located in Brazil divided in eight specific MAR types: ditch and furrow, dug well–shaft–pit injection, excess irrigation, induced bank filtration, infiltration ponds and basins, rooftop rainwater harvesting, subsurface dam, and trenches [19]. According to this inventory, MAR solutions are mostly concentrated in the northeast region, as can be seen in Figure 2 below. With respect to the specific MAR type, this inventory points to a predominance of subsurface dam technology (64%), mainly located in northeastern Brazil, 100% of which aim to maximize natural storage to be used for agricultural purposes. In reality, what has been called "subsurface dam" technology in Brazil should be named "underground dam", which is divided into two main types: submerged dams (Costa & Mello type) and submersible dams [20,21]. These methods will be described later. The main influent sources used in Brazil are river water, representing 54% of the cases and stormwater, representing 40%. Regarding the final uses of MAR, 60 applications are used for agriculture, 20 cases use water for domestic use, 8 have ecological uses, and only 2 applications are for research purposes [19].

**Figure 2.** Localization of managed aquifer recharge (MAR) solutions in Brazil, sorted by (**a**) MAR final use; (**b**) MAR main objective; (**c**) MAR influent source; (**d**) Specific MAR type. Source: Adapted from International Groundwater Resources Assessment Centre (IGRAC) website [19].

The applications of underground dams in the Brazilian semiarid region have been very important for transforming the reality of the region's farming population, as they are sustainable and easy to apply technologies. In Alexandrina-RN, the construction of an underground dam of approximately 2.0 ha provided a significant increase in the production of maize, beans and rice, allowing an increase in family income through the sale of the surplus [22]. In Paraíba, the Local Development Training Project constructed two underground dams in two communities in the municipality of Texeira. In one of the communities it was possible to harvest fruit even in the dry season. In addition, the owners reported a better quality of the harvested fruits, as bigger and better looking. The participation of the community itself in the construction of these dams is highlighted [23]. Silva et al. [24] analyzed the use of four underground dams, one in Pernambuco, one in Paraíba, and two in Bahia. In all of them, the importance of underground dams was verified for the food security of the families, as well as the food security of their animals.

The effort in water quality monitoring varied from site to site. Samples of water taken from 8 underground dams located in the states of Pernambuco and Bahia were analyzed [25]. Among them, six dams presented low salinity and low sodicity water, which made them suitable for crop irrigation. The other two dams presented water with some risk for use in irrigation, requiring careful monitoring and soil and water management actions.

Infiltration ponds are widespread in Brazil and can be found in thirteen states, and the Federal District as well [26]. Up to 2013, the Barraginhas Project alone has been responsible for implementing around 500,000 small ponds, all around the country [27]. According to Embrapa Generated Technologies Impacts Evaluation Report, the project has also been responsible for ensuring water and food security and income generation for thousands of families in the semiarid region. Social Technology has also promoted environmental benefits such as soil conservation/restoration, headwaters recovery, and groundwater replenishment [28].

The program P1MC—A Million Cisterns ("Um Milhão de Cisternas") is a program promoted by ASA, whose objective is to promote and ensure access to drinking water for communities in the semiarid region. Based on the principle of stocking up in times of plenty to have enough in times of shortage, this project was awarded in the Future Policy Award in 2017. The project started in 2003; and the goal of building 1 million cisterns was achieved in 2014. Another program of great relevance for living with the semiarid region was the P1+2—One Land and Two Waters Program ("Uma Terra Duas Águas"), whose objective was to increase water security, as well as to promote land management, food security, and income generation [29].

Most of the rural makeshift schemes implemented to deal with drought in Brazil are low-cost measures focused on storing rainwater and stormwater in buried or semi-buried tanks, aiming to ensure small farmers' food and water security. A large portion of these methods are recorded in Government Documents, in free magazines distributed by NGOs and their websites, and referenced in scientific papers.

In the next section, there is a description of some examples of MAR technology that are based on water infiltration into the soil. Following this, some technologies are presented that are considered as MAR, but do not involve aquifer recharge, being technologies for storing water in buried or semi-buried tanks.

#### 3.1.1. MAR based Solutions in Brazil

• Infiltration Pond (Small Dam)

Infiltration ponds are well-known in Brazil as small dams (Barraginhas). The main objective in utilizing this ancient technique is soil restoration and conservation. The first experiment with a small dam was performed at Embrapa Milho and Sorgo, in Sete Lagoas—Minas Gerais, 1991. The success of this experiment triggered the widespread use of this technique, extending it to the Brazilian semiarid region with the objective of helping small farming communities to deal with degradation and water shortages [30]. Aragão [31] states that, although widely used in Brazil, there is a lack of studies on the most suitable areas that allow the best performance of this technique.

Small dams are small half-moon-shaped dams. Their dimensions range from 1.5 m to 2.0 m in depth, and 15 m to 20 m in diameter. They are scattered and successively constructed in the main thalwegs of pastureland and degraded fields (Figure 3a), as well as along roadsides, aiming to prevent soil erosion by surface runoff [32]. Figure 3b shows a cross section of a small dam scheme. In steeper and dry thalwegs the dam cross-sectional shapes need to be trapezoidal. On smooth level thalwegs and roadsides, a triangular shape is the format usually utilized [30,33]. The dams are equipped with spillways in both sides to squirt around the excess of runoff, protecting the structure [34].

**Figure 3.** Small Dam scheme: (**a**) example of a scattered located small dam scheme; (**b**) cross section of a small dam scheme.

Environmental features, such as topography, soil type, and land use as well, are fundamental elements that affect its location and functioning. A multicriteria analysis carried out on pre-existing small dams, to check if the locations are the most suitable, concluded that slopes less than 3% are not suitable for the implementation of small dams, slopes between 3% and 8% are moderately adequate, and slopes between 8% and 20% are the most suitable [31]. However, field practice points to the fact that slopes greater than 12% should be avoided [35]. Regarding land use and vegetation cover, anthropized areas with sparse vegetation are the most appropriate [31].

Although the soil features such as infiltration and percolation rates are critical for the analysis of rainwater infiltration systems [36], there is a lack of investigation correlating these points to the efficiency of the small dams. A study carried out on eight small dams built in the north of the state of Minas Gerais has highlighted that the recharge capacity increases with soil porosity. Regarding maintenance, silting processes are the main cause of efficiency loss. Around over eleven years of use, the radiuses of the small dams have decreased 1.26 m on average, and the depth loss was up to 1.32 m. Based on these data, the study recommends that every five years, the owners remove the silting from the small dams until recovering the original dimensions (diameter 6 m, depth 1.5–2.0 m) [37].

In order for this technique to be effective, annual precipitation can be up to 1800 mm. Depending on soil type, each small dam can percolate from 800 m<sup>3</sup> up to 1200 m3 in a wet season [31]. The notion of successive dams actually helping with retention of pollutants, and the soil acting as a filtering medium, and thus improving water quality, is approached as a consequence of the application of this method, not as its intended goal.

#### • Underground Dam

An underground dam is an ST that allows rainwater to be stored under riverbeds during the rainy season, making it available for the dry season. Although simple, its construction has to comply with some technical requirements regarding the local where it is built: the alluvium must be predominantly sandy; the slope has to be as level as possible; the depth of the impermeable layer must be greater than 1.5 m; the construction site must be at the narrowest part of the riverbed; and the river head should be avoided, where there is less water. Regarding water quality, low salinity rates are essential to make its implementation feasible [21].

The core idea of its operation is to restrict the flow of the alluvial aquifer by building an impermeable transverse septum, thus raising the level of the upstream water table. In Brazil, there are two types of underground dams suitable to local features: the submerged type, also known as the Costa & Mello type and submersible type. Both of them make use of a buried impermeable septum to restrict the underground flow and are equipped with an Amazon type well to allow the use of the accumulated water in the saturated zone. To build the septum, a trench is dug down to the impervious layer. Then, a plastic blanket is placed over the septum and covered with the excavated material, to block the groundwater flow [21,38,39].

The first type, known as a Costa & Mello-type underground dam (submerged dam), is suitable for the bed river of temporary creeks where the thickness of the sedimentary layer is greater than 1.5 m. This style of construction uses an impermeable septum that is totally buried, retaining only the groundwater flow, making the water table in the alluvium rise upstream of the barrier. There is no physical constraint to the runoff [21,38,39].

There are records of this type of technology in India, Turkey, and Japan, for both irrigation and saline intrusion containment [40–42]. In the Brazilian semiarid region, mainly in the states of Pernambuco, Ceará, and Rio Grande do Norte, this is one of the most applied techniques to deal with water shortages [39]. Figure 4 shows a schematic ground plan (Figure 4a) and cross-section (Figure 4b) of a Costa & Mello type underground dam.

In the second type of underground dam, the submersible underground dam, apart from the buried septum, there is another one made of rocks, bricks, or clay over the riverbed. This barrier makes the superficial flow spread over the land, creating a water pond that lasts up to two to three months after the end of the wet season [21,38,39]. The process of lake formation generates a gradual accumulation of sediments, increasing the thickness of the soil upstream of the dam, thus providing an increase of the storage capacity over time, as happens in sand dams [20,43]. However, some authors warn that it only happens in some cases [44]. This technique is suitable for small rivers and water pathways. This dam over the riverbed is equipped with a spillway made of concrete to spill over excess water and preserve the barrage above the ground, limiting the water level. Upstream, close to the dam, an Amazon-type well is built to recover water for irrigation and for other uses, such as livestock water supply when the water level falls below to the ground level [21,38,39]. Figure 5a shows a schematic ground plan of the submersible underground dam. Figure 5b shows a schematic cross-section of the submersible underground dam.

**Figure 4.** Costa & Mello type underground dam—submerged dam (**a**) Ground plan; (**b**) Cross section.

To implement an underground dam, a set of several factors affects the costs. Among them, the length of the impermeable septum, the raw material, the depth of the impermeable layer, and the workforce available. Based on an underground dam with a septum length of 100 m, using a plastic blanket, and a maximum depth of up to 3.5 m, a study performed by Semiárid Embrapa (CPATSA)/Farming Brazilian Research Company (EMBRAPA) estimated the costs to be around 1300 USD if using heavy machinery [45].

• Dry Well (Caixa Seca)

This is a rainwater harvesting method used to reduce soil erosion, preventing unpaved road deterioration and the silting of rivers and streams. These structures are normally built in a series connected by trenches dug along the roadsides. Based on the builder's empirical knowledge of runoff speed, they are associated with ditches dug diagonally to the axis of the roads, aiming at reducing the surface runoff speed and to covey the rainwater to the dry wells [46]. For safety reasons in the reduction of erosion risks, Bertoni and Neto [47] have recommended that the spacing between the dry wells should be within specific limits, as shown in the Table 1 below.


**Table 1.** Maximum spacing between the Dry Wells as a function of road slope.

**Figure 5.** Submersible underground dam (**a**) ground plan; (**b**) cross section.

The dry well dimensions are defined in the field, based on the builders' experience, never being less than 1 m deep, 1 m wide, and 1 m long (1 m3). Caixa Seca translates literally to English as dry box. The trenches are 0.30 m in depth, and their width is defined by the width of a hoe. These structures also function as infiltration devices, helping to distribute the water into the ground [48]. Whenever

the silt in a dry well reaches 50% of its volume, the silt must be removed [49]. Figure 6 shows a dry well scheme.

**Figure 6.** Dry well (caixa seca) scheme.

• Bank Filtration

Bank filtration is a simplified water treatment technique developed in Europe more than a hundred years ago [50]. It consists of groundwater abstraction by a constructed well located close to a river or a lake, aiming to induce a groundwater gradient, forcing the infiltration of the surface water towards the well, thereby improving water quality [51]. Depending on the soil, underground, and water source features, bank filtration could be the only water treatment before a final chlorination step, or at least used as a pre-treatment [52]. The use of this technique is not limited to the Brazilian semiarid region. It can also be found in the Southeastern Region, especially in the State of Santa Catarina, in the south of Brazil. The bank filtration scheme is shown in Figure 7.

**Figure 7.** Bank filtration scheme.

#### 3.1.2. Other Solutions

• Rainwater Tank

A rainwater tank is a covered, semi-buried cistern connected to the house gutters and equipped with a first flush device. Its capacity ranges between 16,000 and 21,000 L. The most common storage capacity is the 16,000 L with a 3.45 m diameter and 2.4 m depth. Since it is covered and made from concrete, it is impervious, preventing evaporation and debris contamination [53,54].

#### • Boardwalk Cistern (Cisterna calçadão)

This method was conceived of as a way to help scattered small farming communities not being served by a large-scale hydraulic infrastructure as a way to help them sustain their food production in their backyards.

It is a covered, semi-buried reservoir 6.4 m in diameter and 1.8 m in depth. Connected to a 200 m<sup>2</sup> concrete floor by a 100 mm pipe, this system is able to collect and store around 52 m<sup>3</sup> of rainwater even under yearly rainfall rates around 350 mm. Since it is covered and made of concrete, it is impervious, preventing evaporation, and debris contamination.

The harvesting area known as boardwalk (calçadão) is surrounded by a small concrete fence, and has a slight slope towards the cistern. The linking device between the boardwalk and the cistern is provided with a small sedimentation tank to avoid particles entering the cistern [55].

#### • Stormwater Tank

This tank has a 6.2 m diameter, a depth of 1.8 m, and is a covered, totally buried, concrete-plate made cistern, capable of storing up to 52 m<sup>3</sup> of stormwater. The main difference between this technique and the boardwalk cistern is the catchment area. In this model, there is no specific design for the harvesting area. It must be implemented in a slightly sloped ground (<5%) where a water stream naturally flows. A vegetated catchment should be used to avoid the erosion of coarse sediments. This ST uses two serially-placed sedimentation tanks before the water reaches the cistern, in order to remove sand and small stones from the harvested water [56].

• Retention Trench

The standard retention trench (barreiro trench) is a narrow, deep, inverse-trapezoidal shaped, dug reservoir, capable of storing around 500 m<sup>3</sup> of stormwater. The trapezium's larger base measures approximately 24 m with the smaller base at around 16 m, the width at 5 m, and the depth varying from 3 m to 5 m. It should be implemented in the runoff natural pathway, where the terrain slope is as low as possible, avoiding silting and consequent storage volume losses. Its feasibility must be checked by at least three survey boreholes along to the reservoir axis with the objective of identifying the impermeable layer depth [57].

#### *3.2. Early MAR Initiatives in Brazil: Projects and Researches*

In larger Brazilian cities, the main rainwater and stormwater management concern is the runoff peak flow attenuation and flooding avoidance. Towards this, various sustainable drainage techniques have been encouraged, but only a few of these focus on stormwater infiltration. Among them, the most widely used techniques are rain gardens, infiltration trenches, and permeable pavements [58]. None of these have water quality control concerns before infiltration and nor water recovery methods.

Nevertheless, a few MAR initiatives have been evaluated, mainly in the academic environment, as a way of improving integrated water resource management processes. They contribute to the assessment of the technical feasibility and were not implemented on a large scale. Okpala [59] carried out the first specific MAR academic assessment in Brazil in 2010. The study assessed the soil aquifer treatment (SAT) feasibility at the Governador André Franco Montoro International Airport (State of São Paulo), in order to help the Brazilian National Airport Infrastructure Company—Infraero to discover an alternative water source for the growing water demand. In this pilot experiment, the most suitable area was chosen from a group of previously assessed areas. The undisturbed samples taken at the unsaturated layer of the selected area were characterized and subsequently assembled into special testing columns, through which secondary treated effluent was infiltrated. Another set of undisturbed samples were taken from a second area similar to the first one. Based on the results, the authors concluded that for SAT be feasible, the first layer of the soil should be removed or replaced by coarse sand for adequate treatment.

Rayis [60] has assessed water quality requirements and costs for MAR implementation in the São Paulo Metropolitan Region as a response to the 2015 water crises. The most suitable treatment process, recharge methods, and water-treated sewage quality features were based on international experiences, such as in the cities of Shafdan (Israel), Atlantis (South Africa), Sabadell (Spain), and Adelaide (Australia). The study adopted influent water quality standards based on the legislation from the US and Spain that specify the required sewage treatment plant effluent treatment level, which is the tertiary treatment level for nitrogen removal. The MAR unitary operation cost was estimated at 1.41 USD/m3, almost twice the cost of a water mains unitary operation.

The BRAMAR Project, cooperation between Brazil and Germany, intended to help face water shortage by planning and preparing MAR operation schemes. Thus, the Gramame river coastal basin, in the state of Paraíba, was chosen as a study area, aiming to assess the soil infiltration capability and the necessary treatment efficiency of the effluent from the stabilization pond. For this purpose, a 10 mm h−<sup>1</sup> average input flow was infiltrated into two undisturbed soil columns collected from the study area. Over 72 days, the researchers assessed a set of six physicochemical parameters (BOD5, COD, DOC, TSS, NH3, and NO3). The results showed a reduction in organic matter, suspended soil, and ammoniacal nitrogen greater than 60%. Clogging problems were observed, and the feed procedures were changed. From the 42nd day, wet and dry cycles were implemented aiming to restore soil infiltration capability [61].

A master thesis developed at Campina Grande Federal University aimed to propose a transition from an existing non-managed aquifer recharge reality to an intended MAR scenario in the semiarid region [62]. The study area was the Surucucu alluvial aquifer, located at Paraíba River Basin, in the Sumé Municipality, Paraíba State. The lithologic characterization of the study area was carried out based on data of 117 intrusive investigation boreholes made by the BRAMAR Project. The water table level was monitored through a set of 40 wells distributed along 12 km of the Surucucu riverside, from April 2016 to October 2017. From May 2016 to October 2017, the study has used the chloride ion as a tracer to indicate aquifer contamination by sewage. The results showed that the chloride concentrations kept in high levels, especially in the urban area due to the lack of sanitation. Although decreasing along with the underground flow, the results showed the negative impacts of the non-managed aquifer recharge, pointing out to the risk of salinization. The study has proposed a set of actions to work from non-managed aquifer recharge towards MAR, such as to infiltrate treated sewage by using infiltration ponds, where there is no restriction of space, and to recharge surface water resources when it is possible by using an aquifer storage transfer and recovery systems (ASTR).

#### *3.3. Groundwater Legal Framework*

This section aims to present the main groundwater legal documents from both federal and state levels. Although not the same, artificial recharge is the closest in meaning to the MAR expression. Therefore, it was used as a keyword in the place of MAR when searching on the internet for documents related to the Brazilian legal framework for MAR. From now on, the acronym "MAR" will be used in place of "artificial recharge".

#### 3.3.1. Federal Level

From a set of seventeen groundwater legal documents at the federal level, four (23.5 %) mention MAR in their content. Up to the early 2000s, there was no legislation clearly addressing MAR. In 2001, the Water Resources National Council Resolution nº 15 encouraged municipalities to adopt MAR. In the following year, Water Resources National Council Resolution nº 22 established that withdraw and recharge estimates should be included in the water resources plans. In 2008, the Environment National Council Resolution nº 396 permitted establishment of MAR to avoid saline intrusion, providing that there were no changes in water quality, and established water quality mandatory monitoring. At the end of the same year, the Water Resources National Council Resolution nº 92 made prior authorization and mandatory monitoring a condition of aquifer recharge (Table 2).


**Table 2.** Legal framework at the federal level.

#### 3.3.2. State Level

Of all state water resources laws that address underground water, around a fifth (20.3%) mention MAR. Most of them have been promulgated by the states located in the semiarid region, and follows the federal level regulations. Concerning treated wastewater, the Tocantins State legal framework prohibits its discharge into groundwater, although it allows MAR under technical, economic and sanitary assessment, and prior authorization by the Tocantins' Nature Institute. The States of Pernambuco, Ceará, and Maranhão define MAR clearly as water injection through underground dams or injection wells. Santa Catarina State's definition of MAR is generic, defining it as any intentional infiltration technique. It should be highlighted that, among several states, only Pernambuco and Ceará encourage MAR adoption by citizens and companies through rebate schemes on sanitation taxes. Both Pernambuco and Ceará States condition water withdraws to natural recharge features maintenance or MAR. Table 3 shows the basic principles of the state laws addressing MAR.


**Table 3.** Legal Framework at the State Level.

Santa Catarina

Rio Grande do Sul Decree nº 42.047, 2002/12/26

Resolution CERH nº 02, 2014/08/14

Defines MAR as any intentional infiltration technique The State Water Resources Council allows MAR under technical, economic, and sanitary assessment, preserving

groundwater

 Conditions MAR to prior authorization

 by State Agencies

 quality

#### **4. Discussion**

Brazilian folk wisdom has been the generator of several initiatives to the fight against droughts. At least two of them have been developed and applied only in Brazil: small dams (Barraginhas) and dry wells (Caixa Seca). However, regarding water quality and volume monitoring, there are no systematic records that allow a scientific MAR approach. Although the MAR global inventory points to the prevalence of in-channel modifications (underground dams) over the infiltration ponds and basins, there is no registration of small dams (Barraginhas) on the platform, resulting in underreporting of this type of technology. However, there are governmental reports that state there have been more than 500,000 small dams constructed. It is possible to estimate the number of schemes implemented and costs by mining grey literature, but the reliability of these information should be checked in field research. Some of these technologies have been implemented into government programs, such as P1MC and P1+2. It must be highlighted that some STs, such as rainwater tanks, stormwater cisterns, boardwalk cisterns, and retention trenches may be misinterpreted as an aquifer recharge practice. Although using buried or semi-buried reservoirs, these STs use the impervious tanks to store, not to infiltrate the harvested rainwater. In a large portion of the northeastern region, the soil features, such as salinity and low infiltration rates, have led to these technologies instead of aquifer recharge [62]. Despite being widely encouraged by legal frameworks at both federal and state levels, and being used all around the country, MAR technologies have no water quality monitoring programs in Brazil due to both the lack of a custom of monitoring and government underfunding.

Although federal and state legislation cites water reuse and MAR, there is no clear link between them [60]. In addition, the use of alternative sources of water is generally viewed with some suspicion, as a result of an intricate set of social and institutional barriers resulting from the perception of risks related to the various possible uses of this water [60]. This lack of information and long-term studies is one of the factors that hinder the development of appropriate legal framework supporting on MAR in Brazil. In view of this, projects such as the international cooperation Brazil-Germany (BRAMAR), which helped to face water shortage by planning and preparing MAR operation schemes, should be encouraged, and their results should be further disseminated to promote the acceptance and encouragement of this type of technology.

The main concern related to rainwater/stormwater urban management is the runoff peak flow attenuation and flood avoidance by sustainable drainage techniques. Most of these techniques focus on rainwater retention close to where it precipitates. None of them have aquifer recharge for further water recovery purposes. Consequently, they cannot be considered MAR applications. This delay in adopting MAR initiatives, as happens also in India, induces the risk of loss of economic and social benefits [63]. However, since 2010, there have been some academic MAR assessment initiatives for both urban and rural areas as well. Regarding economic assessment of MAR in urban areas, preliminary assessment points to the MAR unitary operation cost to be almost twice the unitary water mains operation cost (US\$ 1.41/m3 versus US\$ 0.75/m3) [60]. It should be highlighted that "[t]he averaged costs of water supplies from four desalination plants in eastern and southern Australia built since the 'millennium drought' [64] to secure capital city supplies is more than 10 times the long-run marginal costs of normal supplies in those cities" [65].

#### **5. Conclusions**

The semiarid region of Brazil and big cities have been identified as the main areas that need strategies to combat water scarcity and measures that ensure water security. Identifying aquifers that are suitable for MAR application sand the availability of water sources for recharge are strategic actions to enable the selection of projects that present best cost benefits and that are alternatives for the use of traditional sources.

Although there are some studies being developed in the academic environment, Brazil is still at an early stage in MAR initiatives and needs to overcome technical, legal, and socio-cultural challenges to adopt MAR. Adoption of MAR should be considered a strategy for facing future droughts under a climate change scenario [65]. However, the lack of awareness concerning MAR solutions and non-specific local policies linking MAR schemes to water demand have delayed its development in Brazil. In this sense, it is necessary to identify areas, such as the semiarid region in the northeast, urban settlements, and water-intensive agricultural lands, where the demand for water overcomes the availability, threatening water security. In these areas, the government should encourage the search for aquifers suitable for MAR, on which its adoption could be a more cost-effective alternative than traditional sources, with the perspective of improving the integrated management of water resources.

Most Brazilian MAR scheme records are dispersed in grey literature, such as non-governmental organization (NGO) websites and governmental reports. The MAR Global Inventory is, therefore, a helpful initiative to gather them in a reliable source. However, the majority of the information must be checked in the field by research projects.

As has happened in a number of countries where MAR schemes have been widely adopted, there are no consistent regulations for the implementation of MAR in Brazil. Seeking to establish a MAR regulation framework, to be based on the Australian guidelines, is a sound and less difficult attempt to provide principles for safe implementation of MAR schemes [66]. Taking advantage of state autonomy in groundwater management [60], guidelines on MAR suitable for local features must be developed to help improving integrated water resources management in Brazil based on scientific approaches.

**Author Contributions:** T.S.: Writing—original draft; L.F.: Writing—review & editing; S.G.M.: Writing—review & editing. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was partially funded by CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico) through the DIGIRES project, grant no. 400128/2019-5. Also, the authors would like to acknowledge the financial support from CNPq for the PQ research grant, and from FACEPE (Fundação de Amparo à Ciência e Tecnologia do Estado de Pernambuco) for granting PhD scholarship.

**Acknowledgments:** The authors would like to thank the Vice-Presidency of the Environment, Attention and Health Promotion - Fiocruz Brasil, for supporting the research and Rogério Silva, for his contribution to the English review. Our special thanks to Warish Ahmed for kindly transfer his waiver quote to publish this manuscript. Finally, we thank the reviewers and editors for their valuable comments and suggestions.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **Abbreviations**

The following abbreviations are used in this manuscript:


#### **References**


© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## *Article* **An Overview of Managed Aquifer Recharge in Mexico and Its Legal Framework**

#### **Mary Belle Cruz-Ayala 1,\* and Sharon B. Megdal <sup>2</sup>**


Received: 18 November 2019; Accepted: 4 February 2020; Published: 10 February 2020

**Abstract:** In Mexico, one hundred of the 188 most important aquifers dedicated to agriculture and human consumption are over-exploited and 32 are affected by seawater intrusion in coastal areas. Considering that Mexico relies on groundwater, it is vital to develop a portfolio of alternatives to recover aquifers and examine policies and programs regarding reclaimed water and stormwater. Managed Aquifer Recharge (MAR) may be useful for increasing water availability and adapting to climate change in semi-arid regions of Mexico. In this paper, we present an overview of water recharge projects that have been conducted in Mexico in the last 50 years, their methods for recharge, water sources, geographical distribution, and the main results obtained in each project. We found three types of MAR efforts: (1) exploratory and suitability studies for MAR, (2) pilot projects, and (3) MAR facilities that currently operate. This study includes the examination of the legal framework for MAR to identify some challenges and opportunities that Mexican regulation contains in this regard. We find that beyond the technical issues that MAR projects normally address, the regulatory framework is a barrier to increasing MAR facilities in Mexico.

**Keywords:** MAR; Mexico; legal; regulatory; framework; LAN (Law of the Nation's Waters); reclaimed water; arid; semi-arid

#### **1. Introduction**

Groundwater overdraft in Mexico is a serious problem. One hundred of the 188 most important aquifer dedicated to agriculture and human consumption are overexploited, and 32 are affected by seawater intrusion in coastal areas [1,2]. Overexploitation of groundwater refers to the excessive withdrawal beyond the annual average recharge [1]. Expansion of irrigated agriculture, the growth in population, changes in consumption habits, and urbanization are the primary drivers of the increase in water use, resulting in water scarcity, a condition that is becoming a threat to sustainable development in Mexico [3]. Water scarcity refers to the geographic and temporal mismatch between freshwater demand and availability [3,4]. In Mexico, 58% of its national territory has dry lands—semi-arid, arid, and hyper-arid ecosystems [5]. Hence, natural conditions and overexploitation of groundwater [5] have led to a water-stress state in the country. Water stress refers to the ratio of total annual water withdrawals to the total available annual renewable supply [6].

Climate change is projected to affect rainfall patterns in Mexico; in particular, the Northern and Northwest regions may be facing severe water scarcity all year round by the year 2040. Mexican researchers analyzed weather databases to project rainfall patterns and temperature for the next four decades [7]. The information obtained was merged with data of water availability for overexploited aquifers; the outcomes showed that in the Northwest, Northern, and Northeast regions, water availability would decline [7,8]. For the US–Mexico border region, the results obtained by US and Mexican scientists using several climate models, have shown "a continued high degree of

annual precipitation variability" [9,10]. Regarding the temperature, average annual and seasonal temperatures, also nighttime temperature are projected to increase for the border US–Mexico region, the western Sonoran Desert, and the northern region of the Chihuahuan Desert [7]. The Working Group II Contribution to the IPCC Fifth Assessment Report pointed out that over North America, which includes Northwest Mexico, exhibit very likely increases in mean temperature [11].

Future climatic scenarios might also affect groundwater recharge because higher temperatures will increase potential evapotranspiration [9,12]. Groundwater recharge includes the process of recharge as a natural part of the hydrological cycle, human-induced recharge that is also named artificial recharge, and recharge due to human activities such as irrigation and waste disposal [13]. Considering that groundwater supplies 70% of the water used for industry, agriculture, and human consumption in Mexico and several aquifers are overexploited [1,14], alternatives must be evaluated to recover aquifers.

In Northwest Mexico, the states of Sonora and Baja California Sur are annually impacted by tropical storms that provide them with a large volume of water in a short period. The rainfall generates floods because there is no infrastructure to manage stormwater or store it. However, several cities in the same region lack water during periods of the year. Another example is Mexico City, which imports water from distant basins [15–18]. Nevertheless, this city's precipitation ranges between 500 and 1200 mm per year, and almost every summer it suffers floods [1,19]. Mexico City and its metropolitan area combine reclaimed water, raw water, and stormwater to discharge into streams without evaluating whether this water can be recycled for other activities [20]. Reusing water is particularly relevant in water-stressed countries like Mexico. Therefore, it is crucial to examine policies and programs regarding reclaimed water and stormwater, which might be useful resources for increasing water availability in this country.

Artificial recharge, as it is termed in Mexican law, now called Managed Aquifer Recharge (MAR), where water quality management is also explicitly addressed, is a concept applied to describe diverse methods with the aim of both augmenting groundwater resources during times when water is available and recovering the water from the same aquifer in the future when it is needed [21,22]. There are a number of systems that could be categorized as MAR [21,23,24] and to implement one of them depends on local conditions such as financial budget, water quality, land availability, hydrogeological characteristics of the aquifer, water availability, and type of soil, among other considerations [24–26]. Although MAR is not a cure for overexploited aquifers, it can be useful for restoring groundwater balance [21]. In Mexico, reclaimed water and stormwater are the primary sources for MAR. In several countries, Managed Aquifer Recharge using reclaimed water has been growing as an option to recycle this resource, to replenish aquifers, and to stop seawater intrusion [20,24,27–29]. Globally, metropolises that have successfully used MAR to increase water supply include Adelaide, Australia [30], Los Angeles, United States [21], Tucson, United States [31,32], and Orange County, United States [33].

#### *MAR in Mexico and Legal Framework*

Since the 1950s, researchers, consultants, and local governments have designed and implemented water recharge projects [25]. Even though some cases were not designed specifically for water recharge, it has been pointed out that these experiences revealed the potential of harvesting and storing water to be used in the future [34,35]. In the early 2000s, in the states of Chihuahua, Sonora, and Mexico City, facilities for MAR using reclaimed water were proposed.

The need to increase water supply in semi-arid lands has triggered efforts for MAR. For instance, in 2007, in San Luis Río Colorado, Sonora facilities for MAR began operating [36], although there were no regulations in place. Therefore, this case was used as a model to define national standards for MAR. Although academics have stressed that public policies for MAR are needed [37–39], Mexican federal agencies have made slow progress in this regard. Moreover, the role that national policies have played in promoting or impeding MAR applications in Mexico has not been analyzed.

In Mexico, the water administration is centralized in the federal government and its regulation is based on Article 27 of the Political Constitution of the United States of Mexico [40]. The municipal responsibilities for water are established in Article 115. Regarding MAR, the Law of the Nation's Waters establishes that a permit is required to build a MAR pilot project [41]. The Federal Duties Law includes the charges that users must pay to extract groundwater; this legislation contains financial incentives for artificial recharge, as well. Furthermore, there are two official Mexican standards for MAR [42,43]. These regulations establish guidelines and water quality parameters that MAR projects, using reclaimed water and stormwater, must meet.

Throughout the world, water policies have been grounded in hard-path solutions, creating large scale infrastructure to deliver water, capital-intensive supply sources and centralized management [44]. Mexico is no exception. Projects to manage water demand, recycle water, and build small-scale infrastructure for water recharge with decentralized management (soft-path solutions) have not often been incorporated in governmental water project portfolios. Particularly in water-stressed regions, hard and soft-path solutions can be simultaneously implemented to improve water management [44,45]. In Mexico, there are examples of small-scale facilities for MAR in rural communities and medium-sized cities that are helping to increase water supply. Hence, it is beneficial to evaluate the role that small infrastructure for MAR in Mexico could play to increase water availability in rural and peri-urban areas. In this paper, we are using the definition of water availability provided by Tydwell et al. [46]: water availability is the supply of water in excess of that currently allocated for consumptive use in a particular basin, that is, the amount of water available for new development.

In Mexico, researchers have conducted an inventory of projects and estimated a significant potential for MAR in Mexico [35,47]. This study provides additional and updated information, including operational status. We present an overview of water recharge projects that have been conducted in Mexico in the last 50 years, their methods for recharge, water sources, geographical distribution, and the main results obtained in each project. Previous research has been focused only on assessing the performance of MAR projects or suitability studies. Our study also includes the examination of the legal framework for MAR to identify some challenges and opportunities that the Mexican legislation contains in this regard.

#### **2. Materials and Methods**

We carried out a systematic literature review of peer-reviewed publications in English and Spanish language, using such keywords as MAR, Mexico, artificial recharge, and water recharge. In addition, considering that in Mexico several studies for MAR have been conducted by universities and private consultants, the search included grey literature and official reports. These reports are mainly the products that consultants have delivered to state and municipal authorities. Additionally, we obtained information from publicly available conference proceedings and academic theses and reviewed the Mexican regulatory framework for groundwater and wastewater.

Recharge projects were grouped based on whether reclaimed water or stormwater was the source of water. In this paper, we will refer to reclaimed water to describe water that has been treated to meet quality standards; the term effluent will be used for water that has not been treated. We also consider each project's objectives. Finally, we analyzed the Mexican framework regulating MAR to identify gaps or aspects that could be clarified. The Legislation analyzed included the Political Constitution of the United States of Mexico, the Law of the Nation´s Waters, Federal Duties Law, and Official Mexican Standards.

#### **3. Results**

#### *3.1. MAR Projects, Types, Objectives, and Geographical Distribution*

On the whole, we found that there are three types of MAR efforts (Table 1).

a. The first group includes exploratory and suitability studies for MAR, mainly proposed by researchers.

b. The second group is composed of pilot projects. In some cases, pilot projects have operated for a short period and later were stopped or canceled, and others are still operating. We use the definition of a pilot project for artificial recharge included in the Mexican regulation [40], where a pilot project is a temporary project that has been built to evaluate a recharge system, to assess its technical feasibility, to monitor hydraulic variables and water quality, and to identify possible impacts generated by artificial recharge in the aquifer and the environment.

c. The third group includes MAR facilities that currently operate.

Overall, MAR pilot projects and facilities have been planned with four objectives: to restore contaminated or depleted aquifers, to reduce land subsidence, to replenish aquifers and increase water availability, and to manage floods. Moreover, the infrastructure is mainly composed of small-scale facilities that are using stormwater or reclaimed water. In addition to the identified MAR projects and facilities, we include information on the most significant case of incidental recharge registered in Mexico, in the State of Hidalgo. Currently, only a portion of the water discharged meets quality standards; however, if this water is adequately treated, it could be used to recharge local aquifers. Finally, we present information regarding a MAR project that was created to recover the aquifer and to maintain environmental services in a riparian ecosystem. This information is presented and discussed in the sections below.

#### 3.1.1. MAR Projects to Restore Contaminated or Depleted Aquifers

In a region between the states of Durango and Coahuila (Figure 1), a MAR pilot project was designed to find alternatives to halt arsenic contamination in aquifers used for human consumption and agriculture [48,49]. Using an infiltration reservoir that had previously been constructed in the basin, several experiments were conducted in 1991 and 2000. In those pilot projects, adequate rates of water infiltration and acceptable quality standards were obtained. However, this pilot project was not transformed into a large-scale installation because local farmers had provided most of the volume of water used for the project, and they were not interested in maintaining this experiment. In addition, the project lacked financial support for monitoring activities.

In Baja California Sur, Cardona et al. [50] analyzed the geological conditions in the municipality of Comondú to reduce seawater intrusion in its aquifers. Based on the information from Cardona [50], Wurl [51] evaluated three alternatives for recovering aquifers in this municipality. The first option is reducing 40 percent of withdrawals for agriculture; under this scenario, the aquifer would be recovered in more than 100 years [51]. The second option is to build small reservoirs to catch stormwater and slowly release it to increase natural recharge. The third option is the construction of storage dams for artificial recharge in areas with an adequate infiltration rate. Based on their hydrological models and maintaining the pumping rates registered in 2007, the last option would restore the aquifer at a faster pace. It would mean that in 40 years, 18% of the depleted volume of groundwater would have been restored [51,52].

#### 3.1.2. MAR to Reduce Land Subsidence

Soil subsidence caused by depletion of groundwater has been widely documented in the United States, Mexico, and other countries [53–56]. In Mexico City, land subsidence has been recorded since 1925 and is one of the most notorious cases in the world [57,58]. Mexico City was built upon a lake over the ruins of the former Aztec city Tenochtitlan [59]. During the period 1991–2006 in some areas of this city, the subsidence rate has ranged 0.10–0.40 m per year [60,61]. Several studies argue that one of the leading causes of subsidence is that groundwater extraction exceeds the natural recharge and due to depressurization of the lacustrine aquitard [34,60–62]. It has been estimated that at least 23% of Mexico's City total surface area is placed on the former lakebed [57,59]. Therefore, to address the subsidence problem in Mexico City, some pilot projects using MAR methods have been proposed [34,63,64]. Since the 1960s, Figueroa Vega [62] has conducted studies and developed pilot projects for water recharge in Mexico City. Recently, Figueroa Vega [34] proposed a MAR project to collect water from local streams and runoff during the rainy season. This proposal may be developed where an ancient lake was located, and it would be linked to the hydraulic facilities for water management that has

been built in the metropolitan area of Mexico City [65]. Water collected could be used to recharge the aquifer and would help to reduce the subsidence rate.

**Figure 1.** States in Mexico with the presence of Managed Aquifer Recharge (MAR) projects.

In Mexico City, another effect associated with over-pumping of groundwater and subsidence is the seismic damage in buildings and urban infrastructure. This city is a hot spot for earthquakes because it is situated between the North American Plate and the Cocos Plate [66]. Carreón-Freyre et al. [57] suggest that there is a relationship between seismic damage and land subsidence. Similarly, other scholars have developed a simulation model to identify urban areas located on top of the ancient lake and found that the most energetic shock waves during an earthquake are recorded in these zones [67]. Although more research is required to characterize specific zones where artificial recharge might be applied to diminish the negative impacts caused by earthquakes, it is a promising line for future research.

#### 3.1.3. To Replenish Aquifers and Increase Water Availability

MAR projects in Mexico have been primarily designed to replenish aquifers and increase water availability using two types of water: stormwater and reclaimed water. Stormwater is used in small-scale facilities, mostly in rural environments, whereas in cities reclaimed water is used. In states located in the northern region, such as Baja California Sur, Chihuahua, and Sonora characterized by having a short but intense rainy season and a long dry season, there are proposals to build small-scale infrastructure to collect stormwater for replenishing aquifers [34,51,68,69]. In Chihuahua, a state located in northern Mexico, researchers from the Autonomous University of Chihuahua developed a pilot project in collaboration with the local government. This project has been ongoing for ten years [69]. In Caborca, Sonora, Minjarez et al. [68] conducted studies to develop a numerical groundwater model using stormwater as a source for MAR. Based on the model, they proposed sites for future MAR implementation. This proposal has not been transformed into a pilot project. In Southern Mexico, in a semi-arid region of the State of Oaxaca, there is a pilot project that uses stormwater for recharge [70].

In Mexico, in cities in which the population ranges from less than a million inhabitants to several million, reclaimed water is the primary source for MAR [15,37,63,71–75]. Based on the results registered by water managers, MAR using reclaimed water is more reliable because the water flow is constant, and its performance could be monitored as a component of a wastewater treatment plant.

In Mexico City, the Water System Agency has pursued a long process to create facilities for MAR using reclaimed water [63]. At the beginning of the 1960s, injection wells were used to infiltrate water from a reservoir. Even though their performance was adequate for more than nine years, these wells were closed because the water in the reservoir became contaminated [34]. In the late 1980s and early 1990s, studies were conducted to identify recharge areas in several areas of the city and the potential impacts of recharging reclaimed water [63,76]. In the eastern side of Mexico City, a suitable site for water recharge was identified. Therefore, a pilot project for MAR was built; it was running from 1992 to 2000 [63]. This installation did not operate for five years, and it was re-opened in 2005. Since 2010, these facilities located in the most populated area of Mexico City recharge reclaimed water using injection wells. This facility includes a program to monitor that the recharged water complies with the national water quality standards [63].



\* International Groundwater Resources Assessment Centre-MAR classification \*\* Facilities/projects functioning.

The Valle de Toluca aquifer in Estado de Mexico (which is the state bordering Mexico City) is exporting water to Mexico City, even though this aquifer is experiencing overdraft. In order to increase recharge in the aquifer and reduce water stress in this basin, the government of the Estado de

Mexico built facilities for MAR using reclaimed water [15]. The effluent is treated, including ultraviolet radiation to reach potable water quality standards and then injected into the ground using injection wells [15,71]. The capacity of this facility is 0.63 million cubic meters per year.

In 2007, in San Luis Río Colorado (SLRC), Sonora MAR facilities were built (Figures 2 and 3). The method used for recharge is infiltration ponds and the source of water is reclaimed water [36,72]. Annually 8.2 million cubic meters are being recharged [36]. The SLRC experience is the most successful example of MAR facilities in Mexico, using reclaimed water with the municipal government managing it.

**Figure 2.** Infiltration basins in San Luis Rio Colorado, Sonora.

**Figure 3.** Infiltration basins in San Luis Rio Colorado, Sonora, when empty.

In the city of Chihuahua, Palma et al. [37] propose a model to recharge the unallocated effluent (about 30 million cubic meters) that is currently being discharged into the river. The water recharged would alleviate water stress in the aquifer and increase water availability for the city. In Hermosillo, Sonora, Palma et al. [74] evaluate different scenarios to replenish the aquifer and balance the growing demand for the city's water supply. The proposal includes modifying the recharge in the irrigation districts and building MAR facilities to reduce both water loss and pollution in the system, providing

additional water supply to urban areas. Based on their models, Palma et al. [74] identify the best option would be to reduce the effluent for irrigation and transfer this water to MAR facilities. This scenario means a reduction in the irrigated area, cultivating with less water, and growing a single crop each year. However, this plan would require a robust communication program with farmers and, perhaps, financial incentives to attract their participation. Another scenario would be to share the effluent between MAR and agriculture. The recharging rate will be lower compared to the other scenarios evaluated, but the recharge will be continuous [74]. The last option is the most viable because it will maintain agriculture activities and increase the groundwater recharge.

A different water source for artificial recharge was evaluated in Los Cabos, a municipality of the State of Baja California Sur. Saval [81] conducted a study using water from the San Lázaro dam. This researcher found that dam facilities were not adequate for monitoring the water recharge rate, and water quality was not satisfactory because there were anthropogenic sources of pollution. However, Saval [81] did not propose any options to eliminate these contaminants. Although, based on the information described, pollution was originated from domestic discharges which could be treated to remove contaminants.

Although there are several benefits of MAR using reclaimed water, this type of water has also generated some concerns because this water could include pollutants that cannot be removed by the soil [28,82]. To guarantee the recharged water will not affect native groundwater, the first step is to characterize the reclaimed water and include a monitoring program in the MAR project. Moreover, water composition and residence time before water is extracted for final use warrant evaluation [27,83,84]. Furthermore, new policies might be created to match water quality with native groundwater quality and the final intended use of the recharged water. For instance, water for the mining industry or farming forage crops does not require high-quality standards.

#### 3.1.4. To manage Floods

Located in a semi-arid region, San Luis Potosí, capital of the state of San Luis Potosí, has faced flooding problems and water scarcity; therefore, a master plan to reduce flooding and increase water recharge was designed. For artificial recharge, the alternatives evaluated were building dams, recharge with reclaimed water, and injection wells. Based on studies conducted previously [80], a system was designed that includes a storm drain to collect water and dispose of it in ponds to infiltrate slowly [79]. This system is using facilities that previously were dedicated to sand mining, which meant a lower cost for the project. This artificial recharge project works very well; however, it does not have a program to monitor water quality.

#### 3.1.5. Unintentional Recharge

Unintentional recharge refers to water discharged into a surface body or infiltrated into an aquifer, not as part of a planned project [85,86]. The Official Mexican Standard (NOM), NOM-014-CONAGUA-2003 [42] suggests that water infiltrated can be reclaimed water or non-treated water. In Mexico, one of the largest examples of unintentional recharge is in the Mezquital Valley, in the State of Hidalgo. This valley is located 80 km north from Mexico City. Since 1896, to avoid floods and to drain the sewage, a system was built for delivering non-treated water from Mexico City into the Mezquital Valley [58,87]. According to Jiménez and Chavez [87], 60 cubic meters per second are discharged daily, while during the rainy season, the peak flows may reach 300 cubic meters per second, for a few hours. Although this water contains a high percentage of contaminants that can impact the aquifer, scholars have reported that the soils have been effective in cleaning wastewater, maintaining an aquifer of acceptable quality [87]. If the water is satisfactorily treated, a portion of this resource can be dedicated to replenishing the aquifer [88]. Hence, this environmental problem can be transformed into a solution to increase the water supply in the region.

#### *3.2. Water Governance and MAR*

In Mexico, the water administration is centralized in the federal government, and its regulation is grounded in Article 27 of the Political Constitution of the United States of Mexico [89]. The governance of groundwater could be summarized as having the following components:


3.2.1. National Regulations: Political Constitution of the United States of Mexico, Law of the Nation's Waters, and Federal Duties Law

Article 27 of the Political Constitution of the United States of Mexico (first paragraph) establishes: "The property of all land and water within national territory is originally owned by the Nation, who has the right to transfer this ownership to particulars" [89]. The sixth paragraph defines "the dominion by the State shall be inalienable and imprescriptible, and the exploitation, use or development of those resources, be that by individuals or by corporations incorporated in accordance with Mexican laws, shall not be carried out but through concessions granted by the Federal Executive in accordance with the rules and requirements so established by the laws" [89].

In 1992, Mexico adopted the Law of the Nation's Water (LAN), which together with constitutional regulations, contains specific provisions for groundwater management and the role of the National Water Commission (CONAGUA) as the federal agency responsible for water administration [41]. Article 3 of the LAN defines the types of water entitlements: appropriation, allocation, and concession.

Appropriation: can be consumptive or non-consumptive (for example for hydropower generation).

Allocation: water rights assigned to municipal and state governments for exploitation, use, and extraction of national water to be used for public services or domestic purposes.

Concession: water title assigned to public or private enterprises and citizens for exploitation, use, and extraction of national waters.

Municipal governments, state governments, and Mexico City have allocations for exploitation, use, and extraction of national water for public services and domestic utilities. Furthermore, the LAN describes that a permit issued by CONAGUA will be required to recharge reclaimed water into aquifers (article 91). Additionally, water recharged must fulfill quality parameters established in national standards.

The Federal Duties Law (LFD) contains most federal taxes and financial incentives. It is analyzed and voted on every year as a part of the national budget. The LFD includes the charges that municipal and state governments must pay to extract water (articles 223 and 223b). These charges vary according to the region where water is extracted. There are four zones, based on water availability, charges are higher where water availability is lower and vice versa. Furthermore, the LFD includes financial incentives for water recharge. Article 224, paragraph V mentions: "(Users) do not have to pay the extraction fees if the water is recharged to its original source. The water should comply with a certificate of quality recognized by CONAGUA and all physical, chemical, and biological parameters listed in Article 225". Consequently, a public or private agency that recharges water in the same aquifer, where the water was obtained, can request this payment exception or its reimbursement. In general, municipal and state governments pay water extraction fees and later demand a refund from CONAGUA equal to the fees paid.

#### 3.2.2. Official Mexican Standards (NOMs)

NOMs are specialized regulations which contain technical details for products, processes, and services. However, they are not legislative ordinances because they were not created by both parliamentary bodies, the chamber of deputies (like the US House of Representatives) and the chamber of

senators [90]. There are two NOMs concerning water quality for MAR: NOM-014-CONAGUA-2003 [42] and NOM-015-CONAGUA-2007 [43]. NOM-014 includes regulations for artificial recharge using treated wastewater. However, the NOM-014 does not include the legal definition of reclaimed water. This NOM establishes that to obtain a permit for a large-scale project, the following information is required: location, hydrogeology (piezometric and stratigraphic profile), physical and chemical characteristics of water (pH, biological oxygen demand, chemical oxygen demand, and total organic carbon) and microbiological analysis. On the other hand, NOM-015-CONAGUA-2007 describes the requirements to carry out infiltration activities using runoff to recharge groundwater. For direct aquifer recharge, the water used should meet drinking water standards.

#### 3.2.3. Managers, CONAGUA and Municipalities

Article 4 of the LAN defines that CONAGUA as the federal agency responsible for water administration [41]. CONAGUA assumes the primary obligation to enforce laws, create a specific regulatory framework for water allocation and water quality standards that MAR projects must fulfill. Constitutional article 115 defines the responsibilities for municipal governments. Municipalities are responsible for the following functions and public services: drinking water, drainage, sewerage system, treatment, and disposal of sewage [89]. The federal government is responsible for enforcing the quality standards for drinking water. There are some participatory bodies included in the LAN, such as committees and councils that collaborate with national and state governments on water-related policies.

After examining the applicable regulations for groundwater and MAR (Table 2), we found that the LAN does not include a definition of reclaimed water and the procedures that must be followed to use it for other activities neither. There is no reference to the legal rights that public agencies have on water recharged when conducting a MAR project using wastewater and how this "new water" would be allocated. Today, municipal water agencies can use, sell, and exchange the wastewater produced in treatment plants managed by them. However, neither the CPEUM nor the LAN defines the rights and responsibilities that municipal agencies have on water recharged using a MAR method. Another omission is concerning stormwater. The LAN does not include a definition of stormwater and lacks information regarding the requirements that should be fulfilled to obtain an authorization for MAR using this type of water. Finally, we want to remark that the regulations do not establish how the stormwater recharged would be allocated or managed.

In summary, LAN lacks specific regulations for MAR. NOMs can establish directions regarding the steps to be followed for building MAR projects and define water quality standards for water recharged. However, NOMs cannot determine the requirements to obtain a permit for a MAR project or how to allocate water recharged. NOMs are complementary regulations of the laws [90], but they cannot regulate water rights.


**2.**RegulationsforGroundwaterandMARinMexico,andproposals.

#### **4. Discussion**

In Mexico, there are several MAR pilot projects with favorable results [35]. However, only a few have been transformed into large scale facilities. In general, MAR projects have been created in arid and semiarid regions of Mexico, which have a short but intense rainy season and a long dry season. On the other hand, there are facilities in Mexico City, which is located in a region with an oceanic climate (Cwb). This city registers precipitation throughout the entire year [91] and on average is 600 mm per year.

We found examples of MAR projects in 10 states; these projects have primarily been designed to recover aquifers and to prevent floods [70], and local water agencies are responsible for them [25,34,68,69]. The experience and knowledge that local authorities have acquired are valuable resources and might be integrated by the federal government when designing MAR policies. Local agencies know how to create these types of facilities and are aware of the challenges they face in maintaining them.

Three MAR facilities using reclaimed water were operating. These facilities are located in Toluca, Estado de Mexico, Iztapalapa, Mexico City, and San Luis Río Colorado (SLRC), Sonora. To date, only San Luis Rio Colorado MAR facility is functioning. Another example is a pilot project located in the city of Chihuahua that uses reclaimed water [37,92]. Several cities in the United States of America use effluent as an additional source for water supply [31–33]. For instance, in Tucson, Arizona, Tucson Water (the principal water utility in the city) has integrated reclaimed water as a source for future water supply [32,93,94].

One of the largest facilities for MAR in the world, that uses reclaimed water, is located in Orange County, California. The effluent is treated to reach potable water quality standards and then injected into the aquifer [33]. MAR is helping to recover the aquifer and diminish saline intrusion [95]. In the northwest region of Mexico and other states located in the Yucatan Peninsula, several coastal aquifers are affected by seawater intrusion [1,8]. Cardona et al. [50] and Wurl [51] have studied seawater intrusion in Baja California Sur (located in the northwest region) and proposed that MAR can be a tool to address this problem. It is a promising research field for MAR in Mexico that might be explored in the future.

#### *4.1. MAR in the Metro Area of Mexico City*

Mexico City and its metropolitan area, the so-called Metropolitan Zone of the Valley of Mexico (MZVM), faces floods almost every summer because there is no infrastructure to manage and store stormwater. Since the early 1940s, the MZVM began importing small volumes of water from the Estado de Mexico [1]. This decision was made because of the rapid increase in the population that demanded more water and the subsidence problems caused by overexploitation of groundwater from the local aquifer [1,66,76]. To the present, more than 25 percent of the total volume used for human consumption and productive activities is imported. About 65 percent of the water used is groundwater pumped from the local aquifer [1]. At the MZVM, several researchers and consultants have carried out experiments and designed MAR pilot projects to reduce land subsidence [34,63,76]. Given that the rate of subsidence is high, small-scale infrastructure could be insufficient to solve this problem in the near future. However, even little steps can help to reduce the subsidence problem.

For more than 100 years, in Mexico City and the MZVM, the stormwater reclaimed water and untreated water have been blended to be discharged into streams outside of the area, producing positive and negative impacts [1,59,76,88]. On the one hand, the aquifer has been recharged, and the water table has increased. On the other hand, the soil has been contaminated because a portion of the water is not adequately treated. Jiménez [88] found that the water in the aquifer has better quality compared to the inflow; it means that the soil has functioned as a cleaning system. To date, farmers are entitled to a large percentage of this water. Jiménez [88] mentioned that significant volumes of water are wasted because there is no infrastructure for irrigation. If the water is satisfactorily treated, a portion of this resource can be dedicated for MAR [88]. In the future, this scenario would be

possible because a new facility was built and, now, around 60 percent of the water from the MZVM is being treated [96,97]. The new wastewater treatment plant is designed to treat 35,000 L per second. Considering that the MZVM lacks water, part of the reclaimed water can be delivered to agricultural activities, and another percentage might be utilized for MAR. It would mean an additional source of water for this thirsty metropolis.

#### *4.2. MAR Using Stormwater*

Generally, it has been suggested that, before starting a MAR project using stormwater, the following actions must be conducted: characterize the water, create an inflow map, and carry out pre-treatment procedures [98]. These steps are needed because stormwater, from urban areas, may contain contaminants such as oil, grease, metals, and pesticides [98,99]. Globally, there are successful examples of MAR using stormwater from urban environments. For instance, in Santa Cruz County, California, a pilot project composed of a network of basins for MAR has shown positive results. This program is managed by the Resource Conservation District of Santa Cruz County, and it is designed to recover groundwater and collect excess runoff for mitigating flooding as well [100,101]. For the city of Rome, Italy, La Vigna et al. [102] proposed building infrastructure for MAR using stormwater as source for recharge. In Adelaide, Australia MAR facilities have been constructed using stormwater from peri-urban regions [103]. In Mexico, we found that the infrastructure for MAR using stormwater is composed of small-scale installations constructed by local communities, non-profit organizations, and researchers [64,69,70]. Non-centralized systems like these examples offer several benefits relative to a centralized MAR system because these installations take advantage of natural precipitation and flow pathways, they can be developed and operated at relatively low cost [100].

In the cities of Chihuahua, Oaxaca, Mexico City (rural area), and San Luis Potosi, there are pilot projects and small-scale facilities that use stormwater to recharge aquifers [52,64,70,79]. MAR infrastructure has been constructed outside the urban grid or in rural areas, except in the case of San Luis Potosí, where the objective is to reduce floods in the city. Therefore, it is essential to acknowledge the role that these projects have to increase water availability, alleviate water scarcity, provide environmental services in riparian ecosystems, and improve water use in agriculture.

Regarding the role that reclaimed water can play to provide environmental services, there are limited examples. In Tucson, Arizona, there is an effluent-dependent riparian ecosystem that provides environmental services and generates social benefits such as recreation [104,105]. In Mexico City, in a rural community within this city, researchers from the Mexican Institute for Water Technology built infrastructure to recharge water [64]. Although this recharge is not directly increasing the water availability for human consumption, in the long-term, this water will help to recover the local aquifer. Besides, the recharging process by itself is providing water to maintain environmental services.

Soft-path solutions for water management include the creation of small-scale sources for supply, methods to increase efficiency to meet water demands, and the design of new policies [106,107]. In Mexico, it must be acknowledged that the small-scale facilities can be part of the portfolio for better groundwater management. These facilities are part of the soft-path solutions that can be better integrated into national or regional policies to increase MAR projects.

#### *4.3. Legal Framework and Public Policies for MAR*

Worldwide, scientific knowledge in hydrology and geology has expanded, but the governing institutions responsible for making decisions about groundwater have been slowly improved [108,109]. Moreover, the Organisation for Economic Cooperation and Development (OECD) has expressed that "the current water crisis is not a crisis of scarcity but a crisis of mismanagement, with strong public governance features" [110–112]. It has been remarked that adequate policies and guidelines must be in place to ensure the benefits that MAR can provide [113]. In Mexico, there are more than 50 years of technical experience in MAR research [35,51]. Nevertheless, the information generated during this

period has not been sufficiently incorporated into the national policy for MAR to launch long-term projects or propose public policies to increase its practice.

Mexican researchers have provided evidence to support MAR as a feasible option to increase water supply [114]. Recharging facilities are helping in increasing water availability in arid regions, such as the San Luis Rio Colorado, Sonora, and Chihuahua examples [36,69]. MAR facilities in Mexico City are aiding to diminishing the adverse effects caused by floods and reducing the rate of subsidence [65,80]. Two of the most successful MAR projects utilize reclaimed water, and there are still other MAR proposals that could be implemented [37,73]. However, the regulatory framework needs clarification and improvement to succeed in implementing MAR projects.

In Mexico, all the waters are owned by the nation; therefore, to be used or exploited the LAN includes three types of water entitlements: appropriation, allocation, and concessions [41]. In general, municipal operators have appropriations which cannot be transferred or changed to other uses than those for human provision (LAN). Since the Mexican legal framework lacks definitions for entitlements to recover stored groundwater using MAR methods, some options to protect investment for MAR projects must be evaluated. The security of recovery entitlements for MAR operators is an essential consideration for investment [115].

The Law of the Nation's Waters does not include a definition for reclaimed water and lacks procedures to define how reclaimed water can be managed and allocated. Gilabert-Alarcon et al. [116] highlight that policies and regulations for reclaimed water are limited. Considering that reclaimed water can be used for agricultural activities and MAR projects, the lack of a regulatory framework is hampering the opportunities for reusing this water. The responsible use of reclaimed water would generate economic and environmental benefits such as reducing water stress and preventing seawater intrusion in coastal aquifers [116]. In Mexico, the absence of a legal definition for reclaimed water and how it can be allocated generates uncertainty regarding water rights. This gap must be solved when creating national policies for MAR.

Another absence in the legal framework is that neither the Political Constitution of the United States of Mexico or the Law of the Nation's Water (LAN) defines the rights and responsibilities that municipal agencies have on water recharged, using a MAR method. Usually, municipal and state water agencies responsible for wastewater treatment plants exchange reclaimed water with farmers. Furthermore, these agencies sell reclaimed water to industries, golf courses, and construction companies. Nevertheless, if wastewater is recharged, municipal or state agencies lose their rights. This situation is the same if stormwater is used. A water right is a legal entitlement tied to a user and to a volume allocated by CONAGUA in each basin. Only the National Water Commission (CONAGUA) could define how the recharged water can be apportioned. However, Mexican legislation does not establish the guidelines that CONAGUA should follow in this regard. This lack of definition is limiting the participation of municipal water agencies in MAR projects.

Nowadays, it is recognized that scientific and technical information is not enough to address the complexity of groundwater management. It is vital to collaborate with society to solve real-world problems [117]. In Mexico, a single agency, CONAGUA, defines groundwater policies and guidelines for their implementation. There is a single policy for the whole country. The national government has a central vision for groundwater management and MAR that applies to the entire country, regardless of local conditions. An effective groundwater governance framework acknowledges and incorporates the local and regional socio-cultural values of water [118]. Groundwater resources in Mexico are facing tremendous pressure; 70 percent of the water used for agriculture and human consumption is groundwater [1]. It would be desirable that CONAGUA initiates a dialogue with local governments, users, and researchers to design MAR rules. The success or failure of natural resources management initiatives promoted by central authorities strongly depends on the support provided to local governments [119].

Regarding public policies for MAR and decentralization in Mexico, the challenge is how to incorporate in the decision-making process, the knowledge acquired by local governments, without amending the legal framework for groundwater. As Nagendra & Ostrom [119] described in forest management, the interactions between actors from several agencies such as federal, state, and municipal could be more important than the changes in the formal legal structure. In Mexico, it is not required to amend the legislation and decentralize responsibilities to increase the participation of both local government and citizens in strengthening public policies for MAR.

Mexican scholars investigating groundwater and MAR recognize that a multidisciplinary approach needs to be taken regarding groundwater management and MAR policies [22,120]. Furthermore, scholars suggest that financial incentives and new legislative rules for stimulating the creation of MAR facilities are required [34,69,120]. However, academics do not specify how the legislation might be improved or the type of incentives that are needed. Allegedly, the current legal framework in Mexico is limiting the creation of MAR projects. Therefore, beyond technical experience or knowledge, today, there is a need to evaluate and update the national legal framework to incorporate artificial recharge methods as part of the solutions to increase water availability.

#### *4.4. Legal Framework That Might Be Revised and Proposals*

Allocation of the recharged water. Inadequate institutional arrangements for aquifer storage and extraction can lead to a legal dispute and potentially negatively impact natural environments [115,121]. Allocation of water recharged is an issue that has been addressed using different tools. In the US, for instance, the states of Arizona, California, and Florida [113,122–124] have created a complex legal framework that establishes the permits required to recharge and extract water for MAR projects.

In Mexico, the Law of the Nation's Water (LAN) establishes that groundwater might be assigned based on the availability calculated by CONAGUA [41]. If there is unallocated water, it might be apportioned according to the priority list established in the LAN. The use of groundwater for domestic and urban use has primacy over other uses. However, the regulatory framework does not define how to allocate water that has been recharged using artificial methods.

The priority list set in the LAN would be used to determine water rights for recovering water recharged when using a MAR method. It would be possible to consider incorporating in the Mexican legal framework a general concept proposed by Pyne [125] that "if a water user has a right to the water before water recharge, then the user also has a right to recover that water". It would mean that the recharged water would be assigned using the same priority list that the LAN establishes when it is extracted from a natural source. This regulation would help to protect investors that are recharging water.

The institutional arrangement for MAR in Mexico is in its early stages, while federal regulations include some ordinances for recharge projects, there are no provisions for the recovery of stored water. In addition, considering that all waters are public property, there is limited space to create legal tools to protect water volumes for users who lack concessions or assignations. Under the current legal framework, an option that could be explored is allowing that those water users, entitled with allocations and that are recharging water, to extract a specific volume in addition to their allocation. In South Australia, for example, in some MAR projects carried out in overexploited aquifers, a recovery rate of 80% has been established [115]. In Mexico, the percentage of water that could be extracted would depend on the specific conditions of the aquifer and the type of water right (domestic, agriculture, industry).

Demonstration projects. One option to promote MAR methods could be to use successful experiences as demonstration projects. There is an example of this type of public policy in the United States (US). In 1984, the US Congress enacted the High Plains States Groundwater Demonstration Program Act. Federal agencies conducted studies in 17 states to identify appropriate sites for MAR. A report developed by Rogers [126] pointed out that "aquifer recharge programs are a good example of federal-local partnership that result in long-term investments."

Another example is Arizona because, as part of the legislation enacted in the 1990s, specific provisions for state demonstration projects were included [124,127]. Based on this legislation, MAR demonstration projects were built to show their effectiveness and technical aspects to operate them. Arizona's demonstration project program incorporated both facilities permitting and operation provisions. Furthermore, in Arizona, the State Demonstration Recharge Program was funded through the Central Arizona Water Conservation District [122,124,127,128]. In the state of California [129,130], the legislation includes demonstration MAR projects. Furthermore, the Australian Guidelines for MAR include a commissioning phase for MAR projects as a tool to manage risk for projects in the development stage [86].

In Mexico, it could be feasible to incorporate demonstration projects into national policies. The infrastructure will be publicly owned; such has been proposed in Australia for stormwater harvesting [121]. Local demonstration projects that present technical and cost information are essential to draw the attention of a broad audience and increase the chance that MAR can be adopted [26]. Additionally, these facilities would help to create a link between MAR researchers and water managers. In summary, it would be desirable to include specific provisions in the legislation or create public programs to promote successful experiences, show their effectiveness, and technical aspects to operating them in Mexico.

#### **5. Conclusions**

Undoubtedly, one way to ensure water supply in the future is by reducing groundwater demand [23]. For instance, increasing the efficiency in agriculture and recycling water in industrial activities where it is feasible. Furthermore, programs to improve water replenishment, such as MAR projects and new public policies, are needed. In Mexico, previous research has been focused on assessing the performance of MAR projects or suitability studies. To the best of our knowledge, this paper represents the first academic effort to evaluate the role that governance plays in augmenting water supply sources in arid and semi-arid regions in Mexico, using MAR methods.

In Mexico, since the 1950s, researchers, consultants, and local governments have designed and implemented water recharge projects [25]. Even though some cases were not designed specifically for water recharge, it has been pointed out that these experiences revealed the potential of collecting and storing water to be used in the future [34]. We found that beyond the technical issues that MAR projects normally address, the regulatory framework is a barrier to increasing MAR facilities because there are no provisions for the recovery of stored water.

Two of the most successful MAR projects, in terms of the amount of water recharged, use reclaimed water, and local water agencies are responsible for these facilities. Recharging facilities are helping in recovering water-deficit in arid regions in Mexico, such as the San Luis Rio Colorado, Sonora, and Chihuahua cases. However, the Law of the Nation´s Waters does not include a definition for reclaimed water and lacks procedures to define how reclaimed water can be managed and allocated. Gilabert-Alarcon et al. [116] highlight that policies and regulations for reclaimed water are limited. Considering that reclaimed water can be used for agricultural activities and MAR projects, the lack of a regulatory framework is hampering the opportunities for reusing this water.

MAR facilities in Mexico City using stormwater have been valuable tools for diminishing the adverse effects caused by floods and helping to reduce the subsidence rate [25,35]. Local governments are responsible for these facilities. Hence, the experience of municipal water agencies should be incorporated when designing policies for MAR.

We found several cases in rural communities and in medium-sized cities where small-scale projects for MAR have been built. These facilities have been created to recover aquifers, and local authorities managed them. It is essential to recognize the role of small-scale projects, as part of soft-path solutions, can play to increase water supply [131]; also, how these installations can be incorporated into regional water portfolios.

It is critical for water-stressed countries like Mexico to contemplate MAR as an option to increase water availability. It has been remarked that adequate policies and guidelines must be in place to ensure the benefits that MAR can provide [113]. An effective groundwater entitlement scheme is required to ensure investment in MAR projects [115]. In Mexico, beyond engineering knowledge regarding MAR, specific federal policies addressing gaps in the legislation are needed.

**Author Contributions:** This study was designed by M.B.C.-A., under the supervision of S.B.M. Writing—Original Draft Preparation, M.B.C.-A.; Review & Editing S.B.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** Partial funding for this research was provided by the Inter-American Institute for Global Change Research CRN3056 Project (supported by NSF Grant No. GEO-1128040) via support from the Udall Center for Studies in Public Policy and the Tinker Field Research Grant (Summer 2019) via the Center for Latin American Studies at the University of Arizona.

**Acknowledgments:** M.B.C.A. thanks the Mexican Council for Science and Technology (CONACYT) for the fellowship to M.B.C.A. through a Ph.D. grant, the Graduate & Professional Student Council at the University of Arizona and The Herbert E. Carter Travel Award Program for providing partial funding to present this research at the ISMAR 10 conference. We want to thank M. Wilder for her helpful recommendations and the three anonymous reviewers who helped to improve this paper with their accurate and insightful suggestions.

**Conflicts of Interest:** The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analysis, or interpretation of data; in the writing of the manuscript, and in the decisions to publish the results.

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