**Preface to "Managed Aquifer Recharge for Water Resilience"**

Managed aquifer recharge (MAR) is part of the palette of solutions to water shortage, water security, water quality decline, falling water tables, and endangered groundwater-dependent ecosystems. It can be the most economic, most benign, most resilient, and most socially acceptable solution, but it has frequently not been implemented due to a lack of awareness, the inadequate knowledge of aquifers, the immature perception of risk, and incomplete policies for integrated water management, including linking MAR with demand management. MAR can achieve much towards solving the myriad local water problems that have collectively been termed "the global water crisis". This Special Issue strives to elucidate the effectiveness, benefits, constraints, limitations, and applicability of MAR, together with its scientific advances, to a wide variety of situations that have global relevance. This Special Issue was initiated by the International Association of Hydrogeologists Commission on Managing Aquifer Recharge to capture and extend from selected papers at the 10th International Symposium on Managed Aquifer Recharge (ISMAR10) held in Madrid, Spain, 20–24 May 2019.

> **Peter Dillon, Enrique Fernández Escalante, Sharon B. Megdal, Gudrun Massmann** *Editors*

### *Editorial* **Managed Aquifer Recharge for Water Resilience**

#### **Peter Dillon 1,2,\*, Enrique Fernández Escalante 3, Sharon B. Megdal <sup>4</sup> and Gudrun Massmann <sup>5</sup>**


Received: 4 June 2020; Accepted: 8 June 2020; Published: 28 June 2020

**Abstract:** Managed aquifer recharge (MAR) is part of the palette of solutions to water shortage, water security, water quality decline, falling water tables, and endangered groundwater-dependent ecosystems. It can be the most economic, most benign, most resilient, and most socially acceptable solution, but frequently has not been implemented due to lack of awareness, inadequate knowledge of aquifers, immature perception of risk, and incomplete policies for integrated water management, including linking MAR with demand management. MAR can achieve much towards solving the myriad local water problems that have collectively been termed "the global water crisis". This special issue strives to elucidate the effectiveness, benefits, constraints, limitations, and applicability of MAR, together with its scientific advances, to a wide variety of situations that have global relevance. This special issue was initiated by the International Association of Hydrogeologists Commission on Managing Aquifer Recharge to capture and extend from selected papers at the 10th International Symposium on Managed Aquifer Recharge (ISMAR10) held in Madrid, Spain, 20–24 May 2019.

**Keywords:** groundwater recharge; water quality; water banking; managed aquifer recharge; water crisis

#### **1. Introduction**

The papers in this special issue explain how managed aquifer recharge (MAR) addresses water resilience challenges across the globe. A key water management objective is increasing the security of water supplies in droughts and emergencies. Another is improving water quality so that sources of water are able to supply drinking water or buffer against water quality decline due to ingress of saline or polluted waters. MAR is also used for ecological restoration of wetlands and stream habitats that have been impacted by surface water and groundwater extraction. Well-conceived and executed MAR projects therefore offer water managers the opportunity to realize water resilience benefits.

This collection of papers goes beyond enumerating these benefits in various climatic, geological and social settings. It also addresses the supportive measures to enhance the ability of MAR to proceed sustainably and effectively to achieve these benefits. Identifying suitable sites for MAR is one fundamental prerequisite. In recent years, a systematic way of doing this has been by overlaying layers of relevant variables within a geographic information system and taking combinations of these with predetermined weights and criteria for likelihood of success (multi-criteria decision analysis). Examples and a synthesis of this approach are presented in this special issue. In addition to aquifer suitability mapping, there is also a need to know where sources of water are available for recharge and where there are existing or projected demands for recovered water. The composite is known as opportunity assessment and examples are given. Time series modelling of water availability is also

used in one paper to determine when recharge is possible and when recovery is needed to help with integrating MAR into a national water supply system.

Creating awareness of MAR, especially where it is an underutilised tool in water management, is an important step to increase its effective deployment and impacts. Hence, overviews of MAR practices at the national and continental scales help develop understanding of the relevant conditions where MAR has proven effective. Awareness of the policies and guidelines relating to MAR at the national and state scales, at which water is commonly managed, also helps water regulators determine the regulations warranted for effective implementation of MAR. Examples are presented where policies have had positive and unintended negative impacts on the usefulness of MAR.

Concerns by operators over chronic operational issues, such as clogging, must be addressed to avoid MAR projects becoming unsustainable and therefore not producing the water resilience intended over time. The largest cause of failure of MAR systems is that methods to manage clogging have been insufficient at some sites. Two papers focus on clogging—one in infiltration basins and one in injection wells. They show how well-constructed investigations and research can provide necessary information for the long-term successful operation of projects where recycled water is recharged.

Finally, the future of MAR is enhanced through innovation in MAR methods and monitoring. Several papers reveal highly innovative MAR methods. One paper describes a variety of ways to harness surface water irrigation canals to recharge aquifers where irrigation can draw from canals and aquifers. Another paper initiates an exploration of a method to simplify monitoring of microbiota in aquifers used for bank filtration, which has implications for pathogen removal.

Table 1 maps each paper to water resilience themes and the discussion of this introductory paper. The thematic categories include water security improvement, water quality improvement and environmental protection and restoration. Following these are some cross-lapping supportive themes referenced above: mapping of suitable MAR sites and identifying opportunities; continental-scale and national overviews of MAR practices and policies; operational issues including management of clogging; and innovation in MAR methods and monitoring. Table 1 shows the papers in order of mention. It highlights the section of this introductory paper where each paper is featured and also includes information on the type of source of water used; type of target aquifer involved; type of recharge method; end use of recovered water, and represented geographic area.

**Table 1.** Directory to the matters addressed and the characteristics of managed aquifer recharge (MAR) sites for each paper; Highlightingpapersectionassignment.


*Water* **2020**, *12*, 1846

 shows the

introductory

#### **2. Synopsis of Contents of This Special Issue**

#### *2.1. Water Security Improvement*

Most papers reported on water supply security improvements, with three of the papers providing an assessment of benefits. The broadest range of benefits is reported for a diversity of MAR projects in Spain. Fernández et al. [1] explains how additional storage enables adaptation to climate change by buffering water availability during reduced rainfall and extended droughts. For these cases, the additional storage has been quantified. In Los Arenales aquifer, Santiuste Basin, this is sufficient to supply farmers for three years with no rainfall. Another benefit is the quantified reduced energy demand for the pumping of groundwater, which itself is a step to reduce carbon emissions and mitigate climate change. Furthermore, the aquifer acts as a reticulation system to deliver water without pumping to farmers wells. The integration of treated wastewater in several projects enhanced groundwater recharge and its reliability and further increased storage.

In monsoonal North India, imbalance between supply and demand is an annual and interannual problem. MAR has been proposed by Alam et al. [2] as a possible solution to both. They conducted the first systematic, multi-year assessment of the performance of pilot-scale MAR designed to harness village ponds to replenish alluvial aquifers in an intensively groundwater-irrigated, flood-prone area of the Indo-Gangetic Plain. In Ramganga Basin, adjacent to an irrigation canal, an unused village pond in clay soil was equipped with 10 recharge wells, and volumes and levels were measured over each wet season for three years. Recharge averaged 44,000 m3 year−<sup>1</sup> at a rate of 580 m<sup>3</sup> day−<sup>1</sup> (221 mm day<sup>−</sup>1) during up to 3 months each year, enough to irrigate 8–18 ha dry season crop. This was up to 9 times the recharge without wells. Significant reductions in recharge rates occurred during each wet season due to clogging of the annular sand filters surrounding recharge wells and due to hydraulic connection with the aquifer. Authors conclude that the pilot has a beneficial impact on water security for village supplies but would need widespread replication to have an observable impact on flooding.

Another multi-year pilot-scale trial, also in India but using gravel filters to filter field runoff before recharging farmers open dug wells in hard-rock terrain in Rajasthan, was undertaken by Soni et al. [3]. A total of 11 wells were recharged between 1 and 3 years, and depth to water level was monitored weekly for 5 years for all recharge wells and for two control wells near each. In this case, volumes of water recharged were too small to produce sufficient additional crop to justify the cost of recharge infrastructure. This is unlike check dams on streams in the same catchment that have a benefit to cost ratio greater than 4. Water sampling suggested lowered salinity and fluoride in recharged wells but increased turbidity and *Escherichia coli*. An unexpected finding of this study was that no sampled open dug well met drinking water standards. Hence, wellhead water quality protection measures, including parapet walls and covers and prevention of direct recharge, were recommended for wells used for drinking water supplies. Testing of larger-scale field infiltration pits is now planned.

#### *2.2. Water Quality Improvement*

Improving the quality of drinking water supplies through bank filtration was the focus of three papers. Kru´c et al. [4] studied the fate of 25 pharmaceuticals in the Warta River at a bank filtration site in Poland. Thirteen compounds were detected in bank filtrate and removal increased with distance from the stream. Some chemicals were completely removed at distances less than 38 m, while a few known persistent chemicals were still present but at greatly reduced concentrations for wells up to 250 m from the river. At the most distant well, only carbamazepine and sulfamethoxazole were detected. Average removal of most parameters was 70–80% even at less than 100 m distance from the river, demonstrating the additional value of bank filtration in the drinking water treatment train.

Masse-Dufresne et al. [5] studied the quality of water at a bank filtration site near Montreal, Canada, where two lakes contributed to the supply, and the mixing ratios were dynamic depending on relative lake levels and the pumping regime for wells. Salinity contrasts between lakes and seasonal

differences in iron and manganese concentrations allowed an understanding of how to modify pumping to improve the quality of water pumped.

In the same area of south east Canada that contains many streams and lakes and a huge number of municipal water supply wells, Patenaude et al. [6] posed the question "which of these are in fact induced river bank filtration wells that may require greater protection from potential surface water pollution?" They used a GIS with multi-criteria decision analysis (MCDA) to categorise the likelihood of wells inducing infiltration from surface water. Minimum distance of wells from lakes or streams and type of aquifer were the variables selected for categorising wells. It was found that almost one million people are supplied from wells within 500 m of either streams or lakes. The method is seen by authors as a starting point for a risk-based analysis that takes account of water quality, environmental tracers and contaminants in source waters.

Water quality improvement is also an objective of soil aquifer treatment systems that intermittently infiltrate recycled water. Valhondo et al. [7] tested the use of several types of organic-rich reactive layers placed at the bottom of infiltration basins to enhance water quality improvement during soil passage. Field tests were performed at two sites in Spain. Results showed that the reactive layers in most cases enhanced the removal of the selected organic chemicals analysed (pharmaceuticals and personal care products). Candidate mechanisms for removal were proposed but not evaluated, so further research is needed to discuss persistence and resilience. The reactive layer did not increase the removal of *E.coli* (a bacterial pathogen indicator) beyond the 2–4 log10 removals observed in controls.

An aquifer affected by seawater intrusion in Barcelona (Spain) has been preserved by a hydraulic barrier created by MAR, in a study by Fernández et al. [1], which demonstrated improved water quality by mitigating and preventing further water quality deterioration.

#### *2.3. Environmental Protection and Restoration*

In a novel case study in the Snake River catchment of Idaho, USA, Van Kirk et al. [8] used a groundwater model and stream and aquifer water temperature data to assess potential benefits of MAR to protect a trout fishery. Winter and spring MAR operations 8 km from the river supplement recharge incidental to irrigation and were calculated to increase streamflow in 2019 by 4–7% during the driest and warmest time of year by increasing cool groundwater discharge, rather than by reducing stream losses. This lowered the stream temperature from approximately 19 ◦C, where trout are under heat stress, to give cool refuges adjacent to springs at 14 ◦C, which is optimal for trout. This habitat improvement is an additional benefit of MAR that also supports agricultural irrigation. Well-developed water rights and water transaction systems in Idaho and other western states enable MAR. However, the authors note that there remain legal and administrative hurdles to using MAR for cold-water fisheries conservation in Idaho, where conservation groups so far are unable to engage directly in water transactions.

In Spain, wetland restoration has also been achieved through MAR in Castilla y León to restore water levels and maintain a geochemical equilibrium vital for bacteria, vegetation and refuge for aquatic birds (Fernández et al. [1]). Since 1995, a deep recharge well in a karstic aquifer capable of accepting 1000 L/s has been used in Lliria (Valencia) for flood mitigation while also enhancing irrigation water security [1]. In Neila, Burgos, Spain, 15–40% of flow in streams is directed via constructed channels into contour bunds in forested areas to enhance diffuse source recharge while also increasing forest production [1].

#### *2.4. Mapping of Suitable MAR Sites and Identifying Opportunities*

A number of papers made use of geographic information systems (GIS) with multi-criteria decision analysis (MCDA) to identify suitable locations for MAR operations. Sallwey et al. [9] undertook a review of such studies and out of this developed two open-source web-based tools, a query tool and a tool to help standardise weight assignment and criteria. These will help users to make mapping of MAR site suitability more structured and assist in collaboration among multiple partners. Site suitability focuses

on the presence of an aquifer capable of storage and recovery of water, as well as information on the unsaturated zone characteristics to indicate viability of infiltration type methods. Data availability and quality are important in the mapping process and the tools still depend on the assessor's expertise in choosing relevant datasets for each specific study.

Although not discussed in any of the GIS-MCDA papers, modern remote sensing methods, particularly those that are satellite-based provide a dense raster of data relevant to site selection. Spatial correlation ranges can be determined using geostatistics to suggest more robust predictors than possible from sparse point-scale measurements, such as aquifer parameters from pumping tests, although these are valuable to help ground-truth predicted aquifer suitability. It is hoped that in future, greater effort will be put into parameter selection for parsimonious and robust mapping of MAR suitability, and into validation of predictions.

MAR site suitability mapping is a foundational layer in assessing MAR opportunity, where the proximity of such aquifers to sources of water such as streams, dams and water recycling plants is also considered. One example is the Island of Gottland, Sweden, where Dahlqvist et al. [10] determined the role for MAR to contribute to future water supplies. They found that 7.5% of the area of Gotland was suitable for MAR compared with 3.3% suitable for surface water supplies through new dams. Although lacking detailed site-specific studies, which they recommend, they claim MAR to be a viable option. They estimated that the unit cost of MAR was four times that of expansion of conventional groundwater supplies where this was possible. However, MAR was comparable in unit cost and yield of expanded surface water supplies and approximately one-quarter of the unit cost of seawater desalination.

Knapton et al. [11] studied MAR options using a partially calibrated groundwater model for the Darwin rural area of northern Australia. The unconfined aquifer is characterised as a lateritic aquifer that refills each wet season and was previously presumed unsuitable for MAR. However, in specific areas, some wet season storage capacity remains, with potential for up to 1.2 Mm3/year recharge. A confined part of this aquifer was identified to have up to 5 Mm3 storage opportunity for water banking for Darwin's water security if a 20 m head increase is acceptable in the aquifer.

Maréchal et al. [12] aim to advance GIS-MCDA mapping approaches by adding an economic evaluation for siting a MAR facility anywhere on an aquifer. They assess the levelised unit cost of recharge from an infiltration basin, including capital and operating costs, implementing a GIS-tool in order to build maps of levelised costs at the aquifer scale. The method was tested in simplified form, with assumptions declared and dependent sensitivity analysis, for an alluvial aquifer in Southern France. Authors propose that this approach be integrated into a broader analysis of soil and aquifer parameters that would influence costs and refine the consequent maps.

GIS-MCDA was also used to map zones suitable for different types of innovative recharge operations on the North China Plain (as mentioned later by Liu et al. [20]).

A different type of opportunity assessment is not based on mapping, but instead uses time series analysis of water supply and demand to determine the need for MAR and the extent to which it can contribute to security of national water supplies. Such an analysis is performed by Lindhe et al. [13] for the north–south water carrier in Botswana. This combines large shallow dams that only irregularly fill, well fields that have small and reliable supplies but only low rates of natural replenishment, and possible future MAR systems of different capabilities. The water supply security model uses monthly time steps over 23 years to relate supply with demand and simulate the magnitude and probability of water supply shortages. Implementing large-scale MAR can be shown to improve the supply reliability from 88% to 95%. The model reveals system properties that constrain the effectiveness of MAR and suggest how to further improve its benefits for an integrated system.

#### *2.5. Continental-Scale and National Overviews of MAR Practices and Policies*

Awareness of existing, relevant MAR practices alerts water managers to the possibilities and is reassuring to those contemplating undertaking a MAR project. This special issue contains a summary of MAR practice in the African continent and at the national level in Brazil and Mexico for both practice and policies. These cover a wealth of experience that is, to date, underreported in international literature. A decade of experience in Australia with MAR guidelines for health and environment protection is also reported. These accounts each have unique and highly advanced elements that will be of interest not only to these geographic areas but also globally.

Ebrahim et al. [14] review and synthesize MAR experience in Africa from 52 reported cases in 9 countries, dating back to the 1960s and covering all main types of MAR. Cases were classified under 13 characteristics including objective of the MAR, hydrogeology and climate. It was found that MAR occurred most commonly in areas of high interannual variability in water availability. The most common objective for projects is to secure and augment water supply and balance variability in supply and demand, in both urban and rural areas. Results revealed a wide diversity of applications including reservoir releases (Morocco), surface spreading/infiltration (Algeria, Tunisia, Egypt, South Africa and Nigeria), riverbank filtration (Egypt), in-channel modifications (Kenya, Tunisia and Ethiopia) and recharge wells (South Africa). Africa also contains several of the world's most sophisticated MAR projects, including aquifer injection of highly treated recycled water into crystalline rock to secure city drinking water supplies (Windhoek in Namibia) and recycling of stormwater and treated sewage via infiltration basins for town water supplies (Atlantis in South Africa). In total, the estimated annual recharge volume is 158 Mm3/year or 0.4% of the continent's annual groundwater extraction. Advancing MAR in Africa requires fostering awareness of existing MAR projects, mapping suitability of aquifers for MAR (as performed in South Africa) and informing account of MAR in water allocation and water quality protection policies.

A study of national advance in the practice and governance of MAR in Brazil is reported by Shubo et al. [15]. Community level and government-level programs have been implemented at many sites to address dry season and drought supplies. The Barraginhas Project alone has seen construction of more than 500,000 infiltration ponds in north east Brazil up to 2013. Another Brazilian MAR design, Caixa Seca (or 'dry box') is widely used to recharge road runoff and would also have international application. More than 90 in-channel modifications for MAR have been recorded. Urban drainage public policies have stimulated urban aquifer recharge initiatives mostly aimed to reduce runoff peak flows. Concerning MAR policies, Brazil has been progressive at the federal level since 2001, when the Water Resources National Council Resolution nº 15 encouraged municipalities to adopt MAR. By 2008, its Resolution nº 92 made prior authorization and mandatory monitoring a condition of aquifer recharge. At the subnational level, regulations in all states mention MAR ('artificial recharge') and two, Pernambuco and Ceará, give incentives and prescriptions for community- and company-established MAR projects. The authors also note where improvements could be made in the reporting, monitoring, and systematic appraisal of opportunities and water quality risk management aspects.

Cruz-Ayala and Megdal [16] reviewed the occurrence and legal framework for MAR in Mexico. They found seven documented operational projects, five pilot projects and five research activities since the 1950s involving natural waters, recycled water and stormwater. Their combined recharge restores depleted aquifers, reduces land subsidence, increases water availability and mitigates floods. There are also very significant opportunities to expand MAR. Regulations are discussed that involve at least three levels of governance from national to basin and user level. There are also Mexican National Standards (NOMs) that create a specific regulatory framework for water allocation and water quality standards that MAR projects must fulfill. These specify the information needed to obtain a permit. Some gaps in regulations are identified, such as on entitlements to recover recharged water, that, if addressed, would help to motivate new MAR projects to address critical needs.

Dillon et al. [17] reviewed the consequences of the Australian MAR guidelines for health and environment protection after 10 years of implementation. They found that, in those states where MAR is progressing, the guidelines are welcomed as giving certainty and objectivity to approvals and fitting broader risk management approaches to water quality. In the other states, there has been no progress, although the need for MAR is just as great. Only minor adjustments are suggested to the guidelines, such as taking specific account of temperature change as a hazard in geothermal settings, referencing advances in environmental genomics, and accounting more explicitly for cumulative impacts of multiple MAR projects. In the entry level section of the guidelines, more explicit water entitlement arrangements for sourcing water, recharging aquifers, recovering from aquifers and end uses are suggested for basins where groundwater management policies need to be strengthened to be effective in securing MAR entitlements. Its relevance for application in other countries depends on capabilities to monitor, sample and analyse water quality. If such capabilities are scant, other forms of guideline are more appropriate, and India's is given as an example.

#### *2.6. Operational Issues Including Management of Clogging*

Two papers focus on operational procedures to manage clogging in MAR projects utilising recycled water. The first of these describes changes to tilling operations in intermittent infiltration basins (soil aquifer treatment) at the Dan Region Reclamation Project (Shafdan) near Tel Aviv, Israel (Negev et al. [18]). After 20 years of stable operation, infiltration rates declined over a 3 year period due to changing water quality, reduction in drying periods, and seasonal effects. Tillage changes introduced in a replicated full-scale trial increased recharge capacity up to 95% for deep ploughing and 15% for chisel knife cultivator treatments, both with improved tractor power and depth control systems. Measurements included infiltration rates and soil compaction depths. Minimising compaction by allowing complete drainage before tillage is important for sustaining higher infiltration rates.

Stuyfzand and Osma [19] evaluated clogging at a pilot-scale recycled water aquifer storage and recovery (ASR) well in a confined siliclastic aquifer near Melbourne Australia. They recorded head build up during an injection and recovery trial, analysed water quality and purged solids, and developed some novel tests to predict clogging by suspended particles and biofouling. These revealed that additional water treatment would be needed and reduced rates of injection, requiring more wells to achieve the injection volume target.

#### *2.7. Innovation in MAR Methods and Monitoring*

Two novel recharge systems called a "well–canal combination mode" and an "open channel–underground perforated pipe–shaft–water saving irrigation system" as practiced in the Yellow River irrigation district on the north China Plan are described by Liu et al. [20]. These are among the described numerous types of agricultural MAR practiced since the 1970s in the North China Plain. Adaptive measures to compensate for diversion of more Yellow River water to cities, to sustain conjunctive use during irrigation efficiency improvement, and to prevent clogging by fine sediment are described. Further, a GIS system using a multi-criteria decision analysis (MCDA) was developed to identify zones for sustainable development of MAR projects in the Yellow River Irrigation District of Shandong Province. Mapping revealed highest opportunities for MAR systems in the western part of Liaocheng City irrigation area, where deeper water table and greater sand thickness gave more storage potential.

Non-conventional methods for recharge enhancement are proposed by Narantsogt and Mohrlok [21] to secure the depleting water supply for Ulaanbaatar, the capital city of Mongolia. They modelled several configurations for ice storage and melting in the dry season, when groundwater levels are low and there is no river flow in this cold semi-arid area. Combining these with recharge releases from a dam is predicted to meet the ongoing water needs for the city.

Advances in monitoring methods are important to the efficient operation of MAR and protection of public health and the environment. Microbially-mediated processes in porous media are important for the removal of viruses, bacteria, and protozoa that are pathogenic to humans. Measuring bacterial biomass concentration and enzymatic activity is an innovative way of improving understanding of these processes. Adomat et al. [22] did this using flow cytometry and two precise enzymatic detection methods to monitor dynamic fluctuations in bacterial biomass at three riverbank filtration sites. They also performed online flow cytometry in an ultrafiltration pilot plant. The method showed promise as a rapid, easy and sensitive future alternative to traditional labour-intensive methods for assessing the microbial quality of RBF water, but this is still an early stage of development. Findings of bacterial regrowth on membranes reinforce the value of river bank filtration as a pretreatment for ultrafiltration for drinking water supplies.

#### **3. Summary**

The water security improvement is, perhaps, the biggest target and advantage of MAR, understood as an integrated water resources management (IWRM) technique. Examples from more than ten countries demonstrate MAR as a climate change adaptation mechanism (no regret technique) towards securing long-term groundwater availability. It also facilitates the transmission of water throughout an aquifer to users' wells; a reduction in pumping energy costs due to higher groundwater levels; and, in karst areas, can assist flood mitigation by infiltrating water to the aquifer, thereby increasing the concentration time of the storm and reducing erosion.

Another major motivation to conduct MAR is the fact that the water quality of the source water generally improves during subsurface passage. Bank filtration for drinking water production is discussed in several papers in this special issue. These cover aspects from studies of passive removal of organic chemicals, modifying mixing of waters by adjusting pumping rates, and efforts to differentiate unintentional bank filtration from other groundwater supplies for managing water quality risk. Several studies mentioned MAR in the form of hydraulic barriers to prevent saltwater intrusion, and one used reactive organic-rich layers placed at the bottom of intermittent infiltration basins to accelerate degradation of constituents of recycled water.

To date, less in the spotlight are the benefits of MAR for environmental protection and restoration. Examples herein show that MAR may increase streamflow by increased groundwater discharge to adjacent rivers, hence decreasing water temperatures in summer to benefit a trout fishery. Moreover, wetland restoration can be achieved by increasing water levels following MAR, as shown for cases in Spain.

Mapping of aquifer suitability for MAR has used geographic information systems (GIS) with multi-criteria decision analysis (MCDA) in several areas and web-based tools are emerging aiming to simplify the process. Overlaying aquifer suitability maps with maps of water demand and water sources has allowed MAR opportunity maps to be produced. A pioneering attempt to extend these to levelised cost mapping for MAR has been included. Modelling of the historical operation of a national water grid with and without MAR has helped to define how to integrate MAR with the existing infrastructure for maximum benefit.

The enormous diversity of experience in MAR globally means that many different countries have undertaken initiatives to advance aspects of MAR. Two examples are Brazil's 500,000 Barraginhas (infiltration ponds) and the injection of highly treated recycled water into crystalline rock in Namibia for supplying drinking water. Countries with emerging and growing economies see real opportunities for making precious water supplies resilient and are moving ahead to do so at a faster pace than some countries with more established economies. A number of countries, including Mexico, Brazil and Australia, have for some time recognised the need for MAR regulations to protect aquifers. In Australia, regulations have assured sustainable operations and assisted uptake of MAR. There remain some challenges. An example is in Mexico, where existing water allocation policies give no incentive for investing in MAR to increase water supply.

Clogging, the most frequently encountered threat to successful MAR, has been addressed by a range of methods to maintain the infiltration rates and to improve water quality. In this special issue, field studies of infiltration basins and injection wells have been combined with innovative measurement methods and solutions to trial or recommend strategies. These start with problem avoidance, by improving the quality of source water, monitoring to detect and reject water with key parameters exceeded, removing residual drilling fluids and preventing air entrainment. Then small cycle improvements become the focus, such as improved practices with valves, metering, drying times

and flow rates. Finally, longer-term maintenance and remedial measures such as changing the method of purging of injection wells or tillage in infiltration basins are tested and adopted.

Innovations in MAR methods and monitoring are continuously developed and implemented, and a number of very diverse approaches are presented herein. These include non-conventional methods for recharge enhancement, such as melting of ice storage and underground ice dams in semi-arid cold Mongolia, innovative MAR types for agricultural irrigation in the "open channel–underground perforated pipe–shaft–water saving irrigation system" developed for the North China plain, and new methods to monitor dynamic fluctuations in bacterial biomass at riverbank filtration sites applied in Europe.

The analyses in this special issue strongly indicate that MAR innovations will continue, as will the sharing of results, as exemplified by the following 22 papers. With increased project diversity, and changing climate leading to tighter constraints on availability of surface water and increased needs for recovery of stored water, MAR operations will themselves need to become increasingly resilient and efficient. Good monitoring of demonstration projects to improve process understanding, and good site selection, as illustrated in this special issue, will be foundational for achieving reliable MAR systems that produce resilient water supplies.

**Author Contributions:** Each guest editor contributed as an author to the formulation of the concept and structure of this paper, to writing various sections and to revising the text and table. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** This special issue was initiated jointly by the Commission on Managing Aquifer Recharge of the International Association of Hydrogeologists, the organising committee of the 10th International Symposium on Managed Aquifer Recharge (ISMAR10) held in Madrid, Spain, in the period 20–24 May 2019, and MDPI Journal Water. This is the third special issue in *Water* following those for ISMAR8: Policy and Economics of MAR and Water Banking, and ISMAR9: Water Quality Considerations for Managed Aquifer Recharge Systems. The editors thank all those authors who contributed to this special issue and to the many reviewers whose comments have helped to improve the papers it contains. Finally, thank you to Rachel Lu and the staff of MDPI, who expertly and efficiently managed the publication of this special issue.

**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* **Sites and Indicators of MAR as a Successful Tool to Mitigate Climate Change E**ff**ects in Spain**

#### **Enrique Fernández Escalante 1,\*, Jon San Sebastián Sauto <sup>2</sup> and Rodrigo Calero Gil <sup>1</sup>**


Received: 6 August 2019; Accepted: 15 September 2019; Published: 18 September 2019

**Abstract:** In this article, the authors will support Managed Aquifer Recharge (MAR) as a tool to combat Climate Change (CC) adverse impacts on the basis of real sites, indicators, and specific cases located Spain. MAR has been used in Spain in combination with other measures of Integrated Water Resources Management (IWRM) to mitigate and adapt to Climate Change (CC) challenges. The main effects of CC are that the rising of the average atmospheric temperature together with the decreasing average annual precipitation rate cause extreme weather and induce sea level rise. These pattern results in a series of negative impacts reflected in an increase of certain events or parameters, such as evaporation, evapotranspiration, water demand, fire risk, run-off, floods, droughts, and saltwater intrusion; and a decrease of others such as availability of water resources, the wetland area, and the hydro-electrical power production. Solutions include underground storage, lowering the temperature, increasing soil humidity, reclaimed water infiltration, punctual and directed infiltration, self-purification and naturalization, off-river storage, wetland restoration and/or establishment, flow water distribution by gravity, power saving, eventual recharge of extreme flows, multi-annual management and positive barrier wells against saline water intrusion. The main advantages and disadvantages for each MAR solution have been addressed. As success must be measured, some indicators have been designed or adopted and calculated to quantify the actual effect of these solutions and their evolution. They have been expressed in the form of volumes, lengths, areas, percentages, grades, euros, CO2 emissions, and years. Therefore, MAR in Spain demonstrably supports its usefulness in battling CC adverse impacts in a broad variety of environments and circumstances. This situation is comparable to other countries where MAR improvements have also been assessed.

**Keywords:** Managed Aquifer Recharge; MAR; climate change; water management; IWRM; adaptation measures; indicators; Spain

#### **1. Introduction**

In a world of arising concern for the effects of Climate Change (CC), the search for practical solutions to mitigate undesirable consequences implies a global change of mentality in the management of water resources. Beyond overexploitation of water bodies, it is mandatory to build models that take into account the current effects of CC, especially in those countries with arid or a semiarid climate, such as the Mediterranean area, where the annual rain scarcity overlaps with punctual extreme precipitations. These accepted phenomena are indeed heightened according to the prevailing CC models.

The main manifestations of CC shown in this paper on which the Managed Aquifer Recharge (MAR) techniques can incise are an increase in the average temperature, a decrease in the annual precipitation, recurrent extreme weather and a rise in the sea level [1].

The key problems and impacts of CC whose figures are globally rising are the evaporation rate, water demand, fire risk, and run-off. On the other hand, decreasing figures are found, at least, in the water supply, wetland surface and hydro-electric energy production [2].

Managed Aquifer Recharge (MAR) can provide with a large array of technical solutions to mitigate those adverse CC effects by not only managing groundwater, but also showing an integrated vision of water resources and their associated wetlands, following the EU Water Framework Directive approach (2000/60/CE) [3]. This concept has been put into practice all over the world, facing different CC challenges, from building extreme run-off reservoir systems to fighting sea level intrusion. The monitoring of these devices shows a bunch of indicators of successful recharge and simultaneous local CC mitigation effects.

Some out-of-Spain models to assess the potential impact of future climate conditions on groundwater quantity and quality have been performed, e.g., in the Central Huai Luang Basin of Thailand. There, four different cases were developed to study the spreading saline groundwater and saline soils in this basin as a consequence of MAR activities, concluding that for all future climate conditions, the depths of the groundwater water table not only will increase, but also the salinity distribution areas will follow this trend by about 8.08% and 56.92% in the deep and shallow groundwater systems, respectively [4].

On the Spanish Mediterranean coast—an area with severe salinization over the last 40 years due to intensive exploitation of groundwater—piezometric levels and chloride concentrations have been monitored. Dry periods and their associated increases in pumping caused the advance of seawater intrusion. The sharp reduction in groundwater withdrawals over the last decade has pushed the saline wedge backwards, although the ongoing extraction and the climate conditions mean that this retreat is quite slow, and could be enhanced by means of MAR applications [5].

On the Southern Italian Mediterraneas coast, over four models were tested to foresee saline water intrusion threatening fresh groundwater resources. Among all the processes taken in consideration for the simulations, authors remarked the importance of a detailed statigraphic reconstruction and geomorphological settings. The results of the validated model indicated the strong link between surface water bodies (specially affected by CC impacts) and the coastal aquifer, with a slight salinization increase for the horizon 2050 [6].

In a recent study performed by the International Association of Hydrogeologists (IAH) considering inputs from a vast variety of countries with special focus on Brazil and South Africa, authors claimed the excellent groundwater drought resilience and how it provided a 'natural solution' for the deployment in CC adaptation, by means of 'strategic rethinking', conjunctive use, and quality protection. These actions should be applied on storage availability, supply productivity, natural quality and pollution vulnerability. They advised that though uncertainty remains over the long-term effects of CC on groundwater recharge, a higher impact on shallow aquifers is still expected. Nevertheless, they also remarked the necessity for more studies to be undertaken due to the current lack of definitive data [7].

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

The main objective of this paper was to collect figures and examples that could illustrate that MAR, as well as in combination with other techniques, could successfully contest the effects of CC by adaptation and even mitigation strategies.

The methodological approach used below pairs the problems and solutions together. Main CC impacts and risks were going to be matched with the available MAR techniques that could be used to mitigate them. Effects, impacts and corresponding MAR solutions were organized in 3 different columns in Figure 1.


**Figure 1.** Relationships between the main manifestations of climate change (CC) and their main problems and impacts, and the technological solutions that can be implemented as adaptation measures.

#### **3. Results**

Results will be explained in the following pages, where some indicators will be proposed to measure the quantitative impact on CC mitigation as related to other usual techniques.

Examples of initiatives to combat climate change have been organised into four groups, as shown in Figure 1:


Spanish examples are located for every group (Figure 2).

**Figure 2.** Map of Spain containing the Managed Aquifer Recharge (MAR) sites to fight CC adverse impacts studied and exposed. Numbers in the map follow in brackets after the header of the corresponding example in the next paragraphs.

#### *3.1. Examples of Technological Solutions to Palliate Rising Temperatures*

The International Panel for Climate Change (IPCC) in the 5th Assessment Report (2014) declared that the global temperature will rise more than 1.5 ◦C during the 21st century in all the possible scenarios and probably 2 ◦C in two of the highest emission sceneries [8]. Evaporation, evapo-transpiration and water demand are expected to follow this trend, but MAR has its own means to counteract those effects.

The indicators for each of the following examples have been gathered in Table 1.


**Table 1.** Selected Spanish MAR sites and indicators to track their relationship with CC adverse impacts.


**Table 1.** *Cont*.

(\*) The positions of these MAR sites have been exposed in the Figure 2.

#### 3.1.1. Underground Water Storage. Canal del Guadiana, Castilla-La Mancha (1 in Figure 2)

The high temperatures over 40 ◦C in August, that can be found in the historic record of the Castilla-La Mancha Region [9], and the shallow streams multiply the evaporation rate in summer. A net of wells close to the canal of Guadiana was built by the Guadiana River Water Authority (CHG) in Castilla-La Mancha (Figure 3), for rural development and mitigation of the overexploitation of the groundwater body (known as aquifer #23). This MAR system can increase the total storage volume by means of intentional recharge in about 48 supplementary hm3 per year [10].

**Figure 3.** General sketch of the MAR system of wells near Canal del Guadiana for irrigation and environmental purposes.

#### 3.1.2. Temperature Reduction. Parc Bit, Palma de Mallorca, I. Balears (2)

The Sustainable Drainage Urban Systems (SDUS) consisted in a group of building items that was integrated into the urban architecture [11], with the goal to increase the city water permeability by means of rising run-off infiltration into the aquifers under the town surface. At the same time, they could also combat Urban Heat Island (UHI) through the development of water stores and green areas within the city landscape.

A good example of this practice can be found in Parc Bit (Palma de Mallorca, Figure 4), where the vegetated roofs, fed by rain collection, were able to reduce the air temperature in the range of 1.5 to 6 ◦C. Thermographs were able to establish a clear and quick difference displayed in the pattern of colours when areas with or without a canopy were compared [12]. A square meter of green cover could evaporate more than half a litre of water per day.

**Figure 4.** Sustainable drainage urban systems (SUDS) to reduce the urban heat island (UHI). Model and development of green roof on the Parc Bit building, Palma de Mallorca, Spain. Example of thermography to track the UHI evolution.

3.1.3. Increase in Soil Humidity. Gomezserracín, Castilla y León (3)

Los Arenales aquifer in Gomezserracín provided an example of increased soil moisture and a rise in the phreatic surface brought about by underground storage through a system of canals and streams (Figure 5). Artificial recharge operations, initiated in 2003, resulted in an average rise in the phreatic surface of more than 2 m, even though it was a passive system since it did not require any electrical power to work. This additional storage in the unsaturated zone increased soil moisture by 15%–20% according to datasets obtained from the MARSOL ZNS-3 station [9–17], equipped with a set of sensors which captured measures in both, the saturated and the unsaturated zones. Humidity evolution has been the main assessed indicator after taking into account the natural precipitation. The costs, appart from the initial investment, were due to cleaning and maintenance, with an average of about 30,000 €/year, contributed by the irrigators´ association.

**Figure 5.** Infiltration ponds in the Los Arenales aquifer: Gomezserracín, soil humidity evolution from the so called MARSOL ZNS-3 station datasets (02/11/2014–30/06/2016) and natural precipitation evolution [9].

#### 3.1.4. Reclaimed Water Infiltration. Alcazarén, Castilla y León (4)

The recharge system (Figure 6) began operating in 2012, with an estimated annual recharge of 0.6 hm3 for the whole working period, with scarce variations, thus the main indicator remained constant [14–23]. In the case of Alcazarén, the recharge water came from an advanced secondary treatment at Pedrajas de San Esteban Waste Water Treatment Plant (WWTP). It was convenient to perform post-treatment actions on the treated water (filter beds, geofabrics, reactive filters, and tests with disinfectants or Disinfection By Products (DBP), thus that its quality was more appropriate to make MAR without causing damage to either the environment or the consumers' health. These waters were subsequently used for irrigation and agro-industry supply.

**Figure 6.** Alcazarén Area and its MAR components, Valladolid (Spain). Photos of some key points (**a**) Pedrajas waste water treatment plant (WWTP); (**b**). Connecting junction of water from WWTP, Pirón River abstraction and urban run-off channel; (**c**). Run-off canal from Pedrajas Village to the connection point; (**d**) General scheme.

#### 3.1.5. Punctual Infiltration. Canal de Isabel II, Madrid (5)

Canal de Isabel II or CYII is the public enterprise in charge of water purification, supply and wastewater management in Madrid. This company has built a system of deep injection in a semi-confined aquifer in the aquifer under the city. Punctual recharge takes advantage of low surface need and high capability to recharge peak flows.

This MAR device was mainly used during drought alerts for potable water supply, increasing resources in the city of Madrid with up to 5 hm<sup>3</sup> per year [11]. Thus, the indicator remained about this figure along that time period.

#### *3.2. Examples of Technological Solutions to Palliate Decreasing Annual Precipitation Rates*

The impact of CC over the last decades has been connected to changes on a large scale in the hydrological cycle. Changes in the precipitation pattern were subjected to a significant variability in space and time. During the 20th century, precipitation had risen in inland areas and northern latitudes, while it had fallen between 10◦ S and 30◦ N from the 70s [16].

3.2.1. Self-Purification by Natural Biofilters and Nature Based Solutions. Santiuste, Castilla y León (6)

The Wastewater Treatment Plant (WWTP) of Santiuste pours the treated water into four lagooning purifications ponds, and then in the East MAR canal, with two different stretches: The first section works as a natural filter and as a MAR canal, and occupies more than one kilometre in length; while the second has a scarce filtering section, and extends for 1.5 km up to the mouth of the Sanchón artificial wetlands complex. Natural vegetation is respected in this stretch, as it acts as a biofilter, until it reaches the Sanchón spillway or is sent to the wetlands for post-processing actions. After the third (2b) artificial wetlands (AW), water returns to the East MAR canal with improved quality. Sunlight and plant growth play a crucial role in the purifying processes of the resulting water, combining one part from the Voltoya River and another from the treatment plant. Indicators assess the evolution of the main parameters, e.g., nitrate concentration was reduced by almost 30%, turbidity by 34%, and copper ions by more than 60% [9].

#### 3.2.2. Wetlands Restoration. Santiuste, Castilla y León (6)

La Iglesia Lagoon is an alkaline wetland (salt-lake with basic salts of very high pH), which was rehabilitated by means of a solution specifically designed to take advantage of MAR facilities in the area. The recovery of the mineralization fundamental to maintain the characteristics of this type of water bodies, which was thus unique, was achieved through the interaction between the recharge water interacting with the biological and saline sediments deposited in the beach of the lagoon. This allowed the maintenance of a colony of endemic bacteria and the protection of vegetation of high ecological value. It was also an important refuge for aquatic birds. Finally MAR contributed in the preservation of minerals and biominerals considered "rare", thanks to about 5% of the total MAR volume being diverted to La Iglesia Wetland from the Santiuste West MAR Canal [17–22] by gravity (passive system). The amount of water used for environmental purposes has been adopted as the indicator for wetlands restoration.

#### 3.2.3. Gravity Flow Water Distribution. El Carracillo, Castilla y León (7)

A "passive" MAR system is one that does not require electrical energy to operate. They generally function by gravity. Once the behaviour of the aquifer is known, it is possible to infiltrate the recharge water concentrated in a given area, relying on water resources being reused simply by gravity and the quality improvement by naturalization thanks to the aquifer. This technique makes it possible to reduce pipe layouts, with its consequent environmental benefits and cost savings. An example is the MAR artificial recharge at the head of the Carracillo, with distribution of the recharge waters from the storage area throughout the irrigable area, where most of the wells in the region are scattered. The volume of recharge water in the headwaters (East) is naturally directed through the aquifer to the discharge into the Pirón River and its tributary Malucas (West), and can be intercepted throughout the circuit by the irrigation and agro-industry wells, thus avoiding the laying of pipes.

The gravity distribution system (Figure 7) covers up to 40.7 km in length between canals and pipes from the dam to the final discharge area, serving an area of 3500 ha irrigated within 7586 ha of agricultural area [14]. Consequently, the adopted indicator is the length of the network divided by the number of irrigated hectares.

The system represents an important energy saving, which can be added to the savings involved in pumping water from shallower groundwater levels.

**Figure 7.** El Carracillo Area and its MAR sites. Devices, junctions, and functions, Segovia (Spain).

3.2.4. Energy Efficiency Increase through Managed Aquifer Recharge (El Carracillo, Castilla y León)

The recharge in El Carracillo contributes to the rise of the water table. Monitoring of the pumping costs of 314 extraction wells, with an average pumping of 9957 m3 per well per annum, and the rise in the average phreatic level from 6.30 m to 4.00 m since 2003 to 2015 represented a rise of +2.30 m (Figure 8).

**Figure 8.** Mock-up with the rise in water level resulting from artificial recharge operations in El Carracillo aquifer. Groundwater levels before (**a**) and after MAR operations (**b**).

The next task was to calculate how much a rise in water level of 2.30 m represented in energy terms. The saving for the irrigators' community, over a calculation of about 0.16 kW·h/m<sup>3</sup> as an average for water extractions, was between 12% and 36% depending on the area, the equivalent of 3000 €/annum as a maximum. This situation is very beneficial for irrigation and for shallow water ecosystems [9].

The volume of CO2 emitted annually in the El Carracillo irrigation community had fallen by 10,780 kg, which was proportional to the rise in the phreatic surface, without taking into account upgrading, energy efficiency initiatives, etc.

The indicator adopted was either the cost savings or the reduction of emissions thanks to groundwater level rise caused by MAR actions.

#### *3.3. Examples of Technological Solutions to Manage Extreme Phenomena*

Extreme situations characterised by an abundance of water, such as floods, "cold drop" events, etc., GIAE [12] can be used, to a certain extent, for MAR. For this purpose, it was necessary to create a system to detain the fast-flowing water and channel it towards recharge devices.

#### 3.3.1. Infiltration of Extreme Flows. Lliria, Valencia (8)

Since 1995, the Basic Civil Protection Guidelines for flood risk included safety procedures for preventing and limiting potential damage arising from this risk. An outstanding example was "Arnachos", a 300 m deep borehole drilled in Losa del Obispo (Valencia) with an extremely high recharge capacity. This was located just a few metres from the irrigation pond of the Tarragó Irrigation Community (Figure 9). It enabled the extraction of a signification fraction of clean water from the irrigation pond in times of heavy rain. Therefore, this recharge system acted as a safety system, reducing the water excess during floods with zero electricity consumption.

**Figure 9.** Deep borehole "Arnachos" at Balsa del Campo, Valencia (UTM 685,744/4,391,256) located in the margin of an irrigation pond and used as both, a safety and recharge element. Photos courtesy of J.M. Montes and FEGA.

In 2014, it was used twice to reduce the peak-flow in a flood and to recharge the karstified aquifer with an infiltration rate of almost 1000 L/s for a period of 14 h (0.0504 hm3), a significant amount of water that otherwise would have worsened the devastation caused by the flooding.

#### 3.3.2. Forested watersheds. Neila, Castilla y León (9)

Many examples can be given of mechanical soil preparation for the purpose of increasing the infiltration rate: Channelling of river water to forests conditioned to store the water for a period and facilitate infiltration, as well as forests "organised" to receive "ordered" runoff and facilitate infiltration (Figure 10), etc. According to the DINAMAR project, the main share of artificial recharge in Spain comes from these kinds of devices and is estimated to be 200 hm3 per year.

**Figure 10.** Mechanical initiatives to minimise runoff, facilitate recharge, and subsequent plantation (**a**) and infrastructure to channel and level excess runoff water (**b**) Neila (Castilla y León).

An example of this sort was found in Neila, Burgos, where a canal had been constructed to channel water from a road towards a forest adequately prepared for this purpose. This forest was capable of retaining and channelling 15%–40% of the volume of surface runoff [10–18], therefore the indicator adopted is: Percentage of trapped water out of the total runoff.

#### 3.3.3. Multi-Annual Management. Santiuste Basin (CyL) (6)

On certain occasions, conditioned by the potential storage volume of the receiving medium, multi-annual management actions may be performed on the recharge waters. This situation is possible either in areas of high volume available and any demand, or in areas of low potential storage volume and low demand.

In previous sections, situations of inter-annual water management have been described, including nodes of return to aquifers in topological schemes and strategic storage as a preventive measure of adaptation to hypothetical future adverse situations. In this same context, it is worth mentioning the multi-year management of reserves. This is a basic water management technique that considers water as a mining resource, renewable in years of favourable weather conditions, for use in years of prolonged drought.

For example, in the Los Arenales aquifer, Santiuste Basin (Figure 11), the storage of water during several winter periods, in addition to that previously existing when the aquifer was provisionally declared overexploited, could cushion short-term drought situations with almost no repercussions for farmers, as the system was passive too. According to data from the DINAMAR R&D project, the economic activity of the region could be maintained for a period of three years with zero rainfall during all this time, thanks to the reserves stored in the different underground basins that the aquifer presents [10–17].

**Figure 11.** Managed underground water storage for use at annual intervals. Santiuste Basin. MAR devices and functions [15].

#### *3.4. Examples of Technological Solutions to Reduce Sea Level Rise*

Positive Hydraulic Barrier. El Prat de Llobregat, Barcelona (10)

One of the most emblematic examples of a water barrier against sea water intrusion is located in the surroundings of Barcelona city´s airport. It is a system of recharging wells injecting water from El Prat WWTP, a positive hydraulic barrier (Figure 12). According to the mathematical models the recovery of the preoperational state previous to the sea intrusion should take around 30 years [20]. The main disadvantage was the huge electricity consumption, thus the activity was eventually stopped during the global economic crisis affecting Spain from 2008.

**Figure 12.** Intrusion barrier in the Llobregat River delta. Hydraulic barrier at Llobregat River delta. Iso-chlorides evolution graphic model for 2035 horizon: Evolution of seawater intrusion without (**a**) and with (**b**) operative recharges.

Specific Spanish MAR sites and proposal/examples of indicators to monitor and track their relationship with CC adverse impacts are exposed in Table 1.

#### **4. Discussion**

Analysing one by one some of the MAR solutions with a direct connection to CC impacts, some outcomes are obtained, according to the different groups established for disaggregated studies. These groups are underground water storage, temperature reduction, soil humidity increase, reclaimed water Infiltration, punctual infiltration, self-purification, off-river storage, restoration of key elements, ground-water distribution by gravity, savings/Lower emissions, infiltration of a part of extreme flows, forested watersheds techniques, multiannual management and intrusion barrier wells.

Table 2 summarizes the main pros and cons for MAR solutions regarding CC adverse impacts.


**Table 2.** Advantages and disadvantages of MAR technical solutions as mitigation measures of CC negative effects.


**Table 2.** *Cont*.

Advantages and disadvantages should be considered before selecting the best fitted or a combination of techniques.

Regarding results, Figure 13 summarizes the relationship between CC current impacts and MAR solutions with their specific site mention and the assessed indicators of positive achievements against CC. Trends and evolutions of the different indicators are explained in each one of the references, but the figures exposed represent an accurate approach for the present MAR-CC binomial.


**Figure 13.** Relations between CC impacts and MAR solutions with their site locations and indicators of positive achievements.

These results have been compared with the referenced international parallel cases. All the studied demo-sites reflect a homogeneous evolution:

Most of the published articles pay special attention to modelling and saline water intrusion and groundwater salinity evolution, though they have scarce data and results about indicators to monitor most of the identified impacts.

Those sites affected by saline water intrusion and salinity increase do not show the groundwater resilience that the other MAR pilots expose, where indicators are showing a better reaction regarding water storage, soils humidity and extreme water related events response.

Monitoring water quality is being a pendant issue, as there are more indicators facing quantity constraints than quality issues.

Reclaimed water infiltration is a first-row topic under permanent revision. In the future and for the studied sites, this kind of MAR will not be an option but a priority.

Dykes play a key role regarding runoff capture and floods, extending the concentration time and enlarging the volume peak, therefore reducing the flood´s devastation capacity.

Wetlands are under permanent support. Most of the studied cases invest about 5% of available water for environmental purposes. A general regeneration is achieved to a certain extent in both, water availability and biodiversity.

#### **5. Conclusions**

Climate change effects and their associated impacts have been related to 10 successful MAR sites in Spain through a series of indicators (Figure 13), that let us assess the efficacy and efficiency of the MAR technique as a multifunctional technique that can simultaneously achieve several purposes.

The list of climate change effects in Spain has been accompanied by several fruitful cases of MAR. This success is economically sustainable as most of them are passive systems (do not require electricity to work). The data associated with these monitored cases have enabled the establishment of status indicators, whilst demonstrating the proficiency of MAR to face frontally CC adverse impacts, not only within the context of the case-studies in Mediterranean areas (Figure 13), but also in parallel circumstances all around the world.

The exposed examples affirm that management schemes featuring intentional aquifer recharge constitute an important set of climate change adaptation measures, while providing guarantees with respect to future water supply. These examples are aligned with other international cases consulted in the references, where despite isolated actions, the response to CC appears to be collegiated [24]. Some of the exposed technical solutions also serve to palliate the adverse effects of CC as mitigation measures. According to indicators, some progress is achieved in replenished aquifers where pumping costs save electricity due to a higher water level with an attached CO2 emission reduction. The attention paid on water and anergy efficiency is also a general asset found in the whole MAR cases under study.

The exposed examples and their comparable potential may have a high practical value for MAR constructions, specially adapted to combat CC in Mediterranean countries [25] where droughts have dramatic effects.

**Author Contributions:** Investigation, E.F.E.; methodology, J.S.S.S.; project administration, R.C.G.

**Funding:** This research was funded by H2020 Marie Skłodowska-Curie Actions, CA 814,066.

**Acknowledgments:** This paper has been edited by members of the TRAGSA Group in the MARSolut European Project (Managed Aquifer Recharge Solutions Training Network) with the support of the European Commission, H2020 MSCA). The information and views set out in this article are those of the authors and do not necessarily reflect the official opinion of the European Union. Authors wish to express their gratitude for the collaboration and support of TRAGSA Group technical staff, IAH-MAR Commission as a source of knowledge and inspiration, and two anonymous reviewers who kindly helped us to improve the quality of this article. The authors wish also to thank Miren San Sebastián and James Haworth who assisted in the proof-reading of the manuscript.

**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, analyses, or interpretation of data; in the writing of the manuscript and in the decision to publish the results.

*Water* **2019**, *11*, 1943

#### **Abbreviations**

The following abbreviations are used in this manuscript:


#### **References**


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