*Editorial* **Water Resource Management through the Lens of Planetary Health Approach**

**Pankaj Kumar 1,\* and Ram Avtar <sup>2</sup>**


For eternity, water resources have proven to be the key to inclusive social development and human well-being. However, the spatio-temporal variation of this finite resource over various landscapes makes it prone to misuse and mismanagement. Rapid global changes such as urbanization, population growth, socio-economic change, change in lifestyle, evolving energy needs, and climate change have put unprecedented pressure on this finite freshwater resource. Keeping this in mind, it is argued that achieving water security throughout the world is the key to achieving sustainable development in a comprehensive manner. However, scientific studies with a holistic point of view considering persistently changing dimensions are still in their embryonic stage. Broadly, water security evolves from ensuring reliable access to enough safe water for every person (at an affordable price where market mechanisms are involved) to lead a healthy and productive life, including future generations. Moreover, there is a need to transition from water scarcity towards water security for a water-secure present and future. This transition requires a look at this complex issue and interdependencies between water, environment, human health, and governance/institutions regulating it to be more inclusive. Despite recent progress in developing new strategies, practices, and technologies for water resource management, their dissemination and implementation have been limited. The nexus approach encompasses these interdependencies, and, to promote this idea, different global frameworks are there to address global health in holistic and comprehensive ways, like one earth, one health, eco-health, and planetary health. Planetary health is the most recent one advocated by the scientific communities as well as policymakers; however, very little has been conducted to present empirical scientific evidence from the ground.

Considering the above-mentioned information gap, this special issue aimed to capture the persistently changing dimensions and new paradigms of water security, providing a holistic view, including a wide range of sustainable solutions to address water security. It discussed gaps, opportunities, challenges, and lessons learned from past experiences for achieving water security in any particular landscape. It also highlighted how recent scientific innovations in the research methodologies had made progress in realizing a planetary health framework to address the water-food-energy-health-biodiversity nexus, urban-rural nexus, regional-circular-ecological-sphere approach, etc., and address the complex issue of water security. Finally, what are the ways forward for a better science-policy interface through the inclusion of every relevant stakeholder to codesign and codelivery of various adaptation and mitigation strategies needed to achieve global goals, e.g., SDGs at a local level in a timely manner? All the above-mentioned issues were reflected through ten articles included in this issue.

The first two articles (Pradhan et al. [1] and Behera et al. [2]) discussed the status of water resources and processing governing the evolution of its quality and quantity in the data-scarce region of India. Considering water as a limiting factor for socio-economic development, especially in arid/semi-arid regions, both scientific communities and policymakers are interested in groundwater recharge-related data. Therefore, Pradhan et al. [1] used an

**Citation:** Kumar, P.; Avtar, R. Water Resource Management through the Lens of Planetary Health Approach. *Water* **2022**, *14*, 3490. https:// doi.org/10.3390/w14213490

Received: 14 October 2022 Accepted: 17 October 2022 Published: 1 November 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 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 (https:// creativecommons.org/licenses/by/ 4.0/).

integrated approach of environmental isotopes and hydrogeochemical studies to understand the recharge processes and geochemical evolution of groundwater in the fractured basement terranes of Gujarat, NW India. Based on the major ionic distribution, results suggest that the chemical weathering of silicate minerals influences the groundwater chemistry in the aquifer system. Furthermore, the chemical composition of groundwater also reflects that the groundwater has interacted with distinct rock types (granites/granulites). The isotopic signature of the groundwater reveals that the local precipitation is the main source of recharge; however, it is affected by the evaporation process due to different geological conditions, irrespective of topographical differences in the study area.

Despite being a biodiversity hotspot, the Mahanadi delta in eastern India is facing groundwater salinization as one of the main environmental threats in the recent past. Hence, Behera et al. [2] attempt to understand the dynamics of groundwater and its sustainable management options through numerical simulation (MODFLOW) in the Jagatsinghpur deltaic region. The result shows that groundwater in the study area is extensively abstracted for agricultural activities, which also causes the depletion of groundwater levels. The hydraulic head value varies from 0.7 to 15 m above mean sea level (MSL), with an average head of 6 m in this low-lying coastal region. The horizontal hydraulic conductivity and the specific yield values in the area are found to vary from 40 to 45 m/day and 0.05 to 0.07, respectively. The interaction between the river and coastal unconfined aquifer system responds differently in different seasons. The net groundwater recharge to the coastal aquifer has been estimated and varies from 247.89 to 262.63 million cubic meters (MCM) in the years 2006–2007. The model further indicates a net outflow of 8.92–9.64 MCM of groundwater into the Bay of Bengal. Further, the outflow to the sea prevents the seawater from ingress into the shallow coastal aquifer system. The findings of both of these studies provided vital information for the decision-makers or policymakers to take appropriate measures to design water budgets as well as water management plans more sustainably.

The next paper by Kadir et al. [3] aims to assess the impacts of one of the direct drivers, i.e., land use and land cover (LULC) changes on the water quality of the Surma river in Bangladesh. For this, seasonal water quality (physico-chemical parameters) changes were assessed in comparison to the LULC changes recorded from 2010 to 2019. The obtained results from this study indicated that there is a significant seasonal pattern in the water quality changes, with relatively higher concentration found in the dry season. On the other hand, analysis of LULC revealed that agricultural and vegetation classes decreased, while built-up, waterbody, and barren lands increased. The correlation between LULC and water quality parameters showed a significant relationship between them. Built-up areas and waterbodies appeared to have the strongest effect on different water quality parameters. Scientific findings from this study will be vital for decision-makers in developing a more robust land use management plan at the local level.

The next three papers by Hung et al. [4], Acharjee et al. [5], and Kumar et al. [6] discussed the effect of water pollution and its impacts on different tropic levels, human health, and the ecosystem. Excessive nutrient enrichment or eutrophication is an environmental pollution problem that occurs in natural water bodies and causes a lot of socio-economic issues. Hung et al. [4] investigated the factors responsible for eutrophication in three reservoirs in Taiwan using regression analyses. The results indicate that the main factor influencing these reservoirs is total phosphorus, and the influence of total phosphorus when interacting with other factors on water quality trophic state is more serious than that of total phosphorus per se. This implies that the actual influence of total phosphorus on the eutrophic condition could be underestimated. Furthermore, there was no deterministic causality between climate and water quality variables. Moreover, it is found that the influencing patterns for the three reservoirs are different because the type, size, and background environment of each reservoir are different, which means it is difficult to predict eutrophication in reservoirs with a universal index or equation. However, the multiple linear regression model used in this study could be a suitable quick-to-use, case-by-case model option for this problem.

Acharjee et al. [5] examined river sediment as an environmental indicator to measure the pollution level in Surma River, Bangladesh. Further, it compares potential ecological risk index values using Hakanson Risk Index (RI) and Monte Carlo Simulation (MCS) approach to evaluate the environmental risks caused by these heavy metals. The obtained results using risk index values from RI and MCS provided valuable insights into the contamination profile of the river, indicating that the studied river is currently under low ecological risk for the studied heavy metals. This study can be utilized to assess the susceptibility of the river sediment to heavy metal pollution near an urban core and to have a better understanding of the contamination profile of a river.

Microplastics (MPs) are considered an emerging pollutant in the aquatic environment; however, there is a scientific knowledge gap in this regard in several Asian regions. Considering this aforementioned information, Kumar et al. [6] carried out a systematic review to provide an insightful understanding of the spatial distribution of scientific studies on MPs in freshwater conducted across the Asian region, utilized sampling methods, and a detailed assessment of the effects of MPs on different biotic components in freshwater ecosystems, with special focus on its potential risks on human health. The results of this review indicate that research on microplastics in Asia has gained attention since 2014, with a significant increase in the number in 2021 might be because of excessive plastic pollution during the COVID-19 situation. Moreover, these research works are concentrated in China, followed by India and South Korea. When talking about the type of research works, it was also found that most of the studies focused primarily on reporting the occurrence levels of MPs in freshwater systems, such as water and sediments, and aquatic organisms, with a lack of studies investigating the human intake of MPs and their potential risks to human health. Notably, comparing the results among different countries is a challenge because diverse sampling, separation, and identification methods were applied to estimate MPs. This review study suggests that further research on the dynamics and transport of microplastics in biota and humans is needed, as Asia is a major consumer of seafood products and contributes significantly to the generation of plastic litter in the marine environment. Moreover, there is a need for further research on policy and governance frameworks to address this emerging water pollutant more holistically.

The next four papers were based on different management methodologies for water resources and their interlinkages with other socio-ecological components. Considering the water-sensitive socio-economic growth in the Vietnamese Mekong Delta, Minh et al. [7] utilized MIKE 11 to quantify the spatio-temporal dynamics of water quality parameters. Results show that locations near cage culture areas exhibited higher BOD5 values than sites close to pond/lagoon culture areas due to the effects of numerous point sources of pollution, including upstream wastewater and out-fluxes from residential and tourism activities in the surrounding areas, all of which had a direct impact on the quality of the surface water used for aquaculture. Moreover, as aquacultural effluents have intensified and dispersed over time, water quality in the surrounding water bodies has degraded. The findings suggest that the effective planning, assessment, and management of rapidly expanding aquaculture sites should be improved, including more rigorous water quality monitoring, to ensure the long-term sustainable expansion and development of the aquacultural sector in the Long Xuyen Quadrangle in particular and the Vietnamese Mekong Delta as a whole.

Considering the water resources as a limiting factor for socio-economic development, especially in arid/semi-arid regions, Shyam et al. [8] carried out the assessment of groundwater reserves, which was carried out in the Udaipur district, Aravalli range, India using an integrated approach of remote sensing, GIS, and field-based spatial modeling. Results show that the principal aquifer for the availability of groundwater in the studied area is quartzite, phyllite, gneisses, schist, and dolomitic marble, which occur in unconfined to semi-confined zones. Furthermore, all primary chemical ingredients were found within the permissible limit, including granum. We also found that the dynamic GW reserves of the area are 637.42 mcm/annum, and the total groundwater draft is 639.67 mcm/annum. The deficit GW reserves are 2.25 mcm/annum from an average rainfall of 627 mm; hence, the stage of

groundwater development is 100.67% and categorized as over-exploited. However, as per the relationship between reserves and rainfall events, surplus reserves are available when rainfall exceeds 700 mm. It was concluded that enough static GW reserves are available in the studied area to sustain the requirements of the drought period. For the long-term sustainability of groundwater use, controlling groundwater abstraction by optimizing its use, managing it properly through techniques such as sprinkler and drip irrigation, and achieving more crop-per-drop schemes, will go a long way to conserving this essential reserve and creating maximum groundwater recharge structures.

Considering the emergence of different nexus approaches to understanding the dynamics and co-evolution of water and human systems, Nguyen et al. [9] developed a social-hydrological approach to enhance the water supply resilience in Con Dao Island, Vietnam. It used a water-balance model involving the Water Evaluation and Planning (WEAP) tool to conduct a scenario-based evaluation of water demands. In doing so, we assessed the impacts of socio-economic development, such as population growth and climate change, on increasing water demand. The modeling results showed that the existing reservoirs—the main sources to recharge the groundwater—play a critical role in enhancing water supply resilience on the island, particularly during the dry season. In addition, future water shortages can be solved by investment in water supply infrastructures in combination with the use of alternative water sources, such as rainwater and desalinated seawater. The findings further indicate that while the local actors have a high awareness of the role of natural resources, they seem to neglect the impacts of climate change. To meet the future water demands, this paper also gave some potential suggestions like upgrading and constructing new reservoirs, mobilizing resources for freshwater alternatives, and investing in water supply facilities as among the most suitable roadmaps for the island. In addition, strengthening adaptive capacity, raising awareness, and building professional capacity for both local people and officials are strongly recommended. The research concludes with a roadmap that envisages the integration of social capacity to address the complex interaction and co-evolution of the human–water system to foster water-supply resilience in the study area.

Inhabitants of low-lying islands face increased threats due to climate change as a result of their higher exposure and lesser adaptive capacity. Sagar Island, the largest inhabited estuarine island of Sundarbans, is experiencing severe coastal erosion, frequent cyclones, flooding, storm surges, and breaching of embankments, resulting in land, livelihood, and property loss, and the displacement of people at a huge scale. Hence, Bera et al. [10] assessed climate change-induced vulnerability and risk for Sagar Island, India, using an integrated geostatistical and geoinformatics-based approach. Based on the IPCC AR5 framework, the proportion of variance of 26 exposure, hazard, sensitivity, and adaptive capacity parameters was measured and analyzed. The results showed that 19.5% of mouzas (administrative units of the island), with 15.33% of the population in the southern part of the island, i.e., Sibpur–Dhablat, Bankimnagar–Sumatinagar, and Beguakhali–Mahismari, are at high risk (0.70–0.80). It has been concluded that the island has undergone tremendous land system transformations and changes in climatic patterns. Therefore, there is a need to formulate comprehensive adaptation strategies at the policy- and decision-making levels to help the communities of this island deal with the adverse impacts of climate change. The findings of this study will help adaptation strategies based on site-specific information and sustainable management for the marginalized populations living on similar islands worldwide.

**Author Contributions:** Conceptualization, P.K.; writing—original draft preparation, P.K. and R.A. All authors have read and agreed to the published version of the manuscript.

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

**Data Availability Statement:** Not applicable.

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

### **References**


### *Article* **Recharge and Geochemical Evolution of Groundwater in Fractured Basement Aquifers (NW India): Insights from Environmental Isotopes (**δ **<sup>18</sup>O,** δ **<sup>2</sup>H, and <sup>3</sup>H) and Hydrogeochemical Studies**

**Rudra Mohan Pradhan 1,\* , Ajit Kumar Behera <sup>2</sup> , Sudhir Kumar <sup>3</sup> , Pankaj Kumar 4,\* and Tapas Kumar Biswal <sup>1</sup>**


**Abstract:** Considering water as a limiting factor for socio-economic development, especially in arid/semi-arid regions, both scientific communities and policymakers are interested in groundwater recharge-related data. India is fast moving toward a crisis of groundwater due to intense abstraction and contamination. There is a lack of understanding regarding the occurrence, movement, and behaviors of groundwater in a fractured basement terrane. Therefore, integrated environmental isotopes (δ <sup>18</sup>O, δ <sup>2</sup>H, and <sup>3</sup>H) and hydrogeochemical studies have been used to understand the recharge processes and geochemical evolution of groundwater in the fractured basement terranes of Gujarat, NW India. Our results show that the relative abundance of major cations and anions in the study basin are Ca2+ > Na<sup>+</sup> > Mg2+ > K<sup>+</sup> and HCO<sup>3</sup> <sup>−</sup> > Cl<sup>−</sup> > SO<sup>4</sup> <sup>2</sup><sup>−</sup> > NO<sup>3</sup> −, respectively. This suggests that the chemical weathering of silicate minerals influences the groundwater chemistry in the aquifer system. A change in hydrochemical facies from Ca-HCO<sup>3</sup> to Na-Mg-Ca-Cl. HCO<sup>3</sup> has been identified from the recharge to discharge areas. Along the groundwater flow direction, the presence of chemical constituents with different concentrations demonstrates that the various geochemical mechanisms are responsible for this geochemical evolution. Furthermore, the chemical composition of groundwater also reflects that the groundwater has interacted with distinct rock types (granites/granulites). The stable isotopes (δ <sup>18</sup>O and δ <sup>2</sup>H) of groundwater reveal that the local precipitation is the main source of recharge. However, the groundwater recharge is affected by the evaporation process due to different geological conditions irrespective of topographical differences in the study area. The tritium (3H) content of groundwater suggests that the aquifer is mainly recharged by modern rainfall events. Thus, in semi-arid regions, the geology, weathering, and geologic structures have a significant role in bringing chemical changes in groundwater and smoothening the recharge process. The findings of this study will prove vital for the decision-makers or policymakers to take appropriate measures to design water budgets as well as water management plans more sustainably.

**Keywords:** groundwater; fractured rock; hydrogeochemistry; geochemical evolution; environmental isotopes (δ <sup>18</sup>O; δ <sup>2</sup>H; and <sup>3</sup>H); Ambaji Basin; NW India; socio-economic development; water resource management

**Citation:** Pradhan, R.M.; Behera, A.K.; Kumar, S.; Kumar, P.; Biswal, T.K. Recharge and Geochemical Evolution of Groundwater in Fractured Basement Aquifers (NW India): Insights from Environmental Isotopes (δ <sup>18</sup>O, δ <sup>2</sup>H, and <sup>3</sup>H) and Hydrogeochemical Studies. *Water* **2022**, *14*, 315. https://doi.org/ 10.3390/w14030315

Academic Editors: Pankaj Kumar, Ram Avtar and Domenico Cicchella

Received: 7 December 2021 Accepted: 18 January 2022 Published: 21 January 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 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 (https:// creativecommons.org/licenses/by/ 4.0/).

### **1. Introduction**

Globally, groundwater resources serve one-third of freshwater supplies, accounting for nearly 36% for domestic purposes, 42% for agricultural use, and 27% for industrial activities [1]. About 70% of the land area in India is underlain by crystalline basement rocks and approximately 30% of them are merely covered with Precambrian basement or hard rocks [2]. In the last few decades, the demand for groundwater and overexploitation of groundwater resources has significantly amplified, particularly in semi-arid and arid areas due to an increase in population, urbanization, and growth of the worldwide economy.

In semi-arid environments, understanding the groundwater evolution, flow path, and recharge processes mechanisms are very much needed for the optimal usage of groundwater resources [3,4]. Furthermore, groundwater resources in basement hard rock terranes are often restricted to the top weathered fractured zones that usually spread up to 50 m depth, and below that, groundwater movement is mainly controlled by deeply fractured zones [2,5–8]. In these terranes, the geochemical properties of groundwater are essentially a function of the mineral composition of the rock through which it flows [9,10]. In addition, the concentration of chemical species varies along its flow path [9,11–14], and several hydrogeochemical processes impact groundwater geochemistry viz. topography, precipitation intensity, water–rock interaction, mixing, dissolution, ion exchange, and oxidation–reduction process [15–17]. These processes are mainly depending on the physicochemical and biological properties of bedrocks with climatic conditions. Furthermore, hydrogeochemical properties are also used as a proxy for determining the recharge areas and sources [18]. Additionally, physical methods are also used to assess the recharge process. However, the interpretation remains equivocal due to variation in weathered fractured zones as well as seasonal variability of rainfall and groundwater level [19]. Hence, integrating the hydrogeochemical datasets with stable isotopes (δ <sup>18</sup>O and δ <sup>2</sup>H) and radiogenic isotopes (hydrogen-3, i.e., <sup>3</sup>H) can aid significantly in tracing the hydrogeochemical processes and in identifying the recharge environments of groundwater systems [20].

Generally, the stable isotopic compositions are preserved in the groundwater body until it mixed with other water with altered stable isotopic signatures [21,22]. Several studies have used stable isotopes in tracing the sources of groundwater around the world viz. groundwater–surface water interaction studies [23,24], groundwater recharge assessment along different flow paths [22,25–27], isotope tracing of paleo groundwater [28], and evaporation effects on groundwater resources [27,29]. Among the other environmental isotopes, tritium (3H) is a unique isotope used in hydrogeological studies to understand the flow direction and groundwater age, and it has been applied in several regions (Northern China [30], South Florida [31], California Basin [32], Banana Plain [14], Punjab state [33]).

Identifying the recharge processes in semi-arid regions has gained substantial attention in recent years. However, most of the work focused mainly on sedimentary or alluvial terranes and less on crystalline fractured rock terranes with varied climatic zones [4,34–37]. Especially in NW India, no such research has been carried out, although the communities are heavily dependent on groundwater resources for domestic and agricultural activities.

Therefore, the present study is aimed at understanding the complex fractured basement aquifers using hydrogeochemical, stable isotopes (δ <sup>18</sup>O and δ <sup>2</sup>H), as well as radiogenic isotopes (3H). The specific goal is to identify the geochemical evolution and groundwater recharge processes that occurred within the fractured crystalline basement aquifers of Ambaji basin (Gujarat), NW India. The outcome of the work will lead to better groundwater management and practices for sustainable water use especially in fractured basement terrane.

### **2. Description of the Study Area**

### *2.1. General Characteristics*

The study area (Ambaji basin) lies in the northern end part of the Banaskantha district (North Gujarat), NW India. The area is bounded by latitude 24◦100–24◦220 N and longitude <sup>72</sup>◦300–72◦50<sup>0</sup> E (Figure 1). The areal extent is <sup>≈</sup>450 km<sup>2</sup> and is divided into two Talukas (Amirgarh and Danta taluka). The area is mostly dominated by hilly terrane and a smaller portion of the low-lying flat terrane. The altitude varies from 250 to 650 m a.m.s.l. The area comes under a semi-arid climatic type and is characterized by extreme temperature in the summer months (May–July), erratic rainfall, and high evapotranspiration rates [10,38]. The average annual rainfall is ≈771 mm and is typically received through the southwest monsoon, and the temperature varies between 15 and 42 ◦C in this area.

**Figure 1.** Geological map of the study area (Ambaji Basin, NW India) and groundwater sampling locations (Pre-monsoon, December 2017). Figure 1 inset showing the major geological terranes of Aravalli Delhi Mobile Belt (ADMB), NW India after GSI, [38].

### *2.2. Geological and Geomorphological Settings*

Geologically, the Ambaji basin belongs to the South Delhi Terrane (SDT) of Aravalli-Delhi Mobile Belt (ADMB), NW India (Figure 1). The ADMB comprises several geological terranes including SDT (Figure 1 inset) [39]. Furthermore, several granitic intrusions have occurred in the ADMB i.e., the Berach, the Jasrapur, the Sendra, the Ambaji (study area), the mount Abu Granite, and the Erinpura granites. The different geological units in the study area were demarcated through extensive fieldwork and the existing geological map of the Geological Survey of India. As the study basin comes under the Meso-proterozoic

age of SDT, the rocks present in this terrane were mainly of pelitic-, calcareous-, and basic granulites, where three phases of granite intruded namely G1 (gneissic), G2 (medium to coarse-grained, highly fractured), and G3 granites (fine-grained or micro-granite) [38,39]. Figure 1 shows that G2 granite is mostly dominated in this area, which is followed by basic granulite, alluviums, G1 granite, and G3 granites. Furthermore, the G2 granite is highly weathered in different regions. The area is predominantly comprised of different erosional and depositional hydrogeomorphic units viz. structural hills, denudational hills, residual hills, shallow to deep buried pediments, and valley fills. The eastern part mainly consists of hilly terranes with highly undulating surfaces, whereas the southwestern end areas are of gently undulating surfaces. The upper part of the basin mainly consists of structural hills, whereas the lower part of the basin is covered with alluviums [38].

### *2.3. Structural and Hydrogeological Scenarios*

The basin has witnessed several faults, fractures, and shear zones. The fractures mostly show three sets of orientations i.e., NE-SW, NNW-SSE, and NW-SE (Figure 1). The study area comprises several criss-crossed fractures in the northern part and northwestern part due to transtensional settings and is characterized by both extensional and compressional structures [38]. These transtensional settings lead to multiple phases of deformation, which caused several criss-crossed fractures or lineaments. The major percentage of the study basin is covered by crystalline basement rocks. Due to the absence of primary porosity, secondary porosity such as fractures, faults, and shear zones serve as the main source of groundwater resources. Furthermore, groundwater resources in the shallow part were mostly covered with weathered zones/topsoil, while the deeper part mostly consists of fault and fracture zones and shear zones followed by massive rocks. The groundwater water levels in this area range from 2.5 to 49 m below ground level (b.g.l.) in the premonsoon (May 2017) and 0.65–47.0 m b.g.l in the post-monsoon (December 2017). The regional groundwater flows from the northeast to southwest direction and is primarily influenced by the secondary porosities (faults/fractures/joints) and surface topography. The area has largely three types for hydrogeological formations: (1) the top weathered zones range from 1 to 30 m, (2) the fault and fractured zones comprised of granite range from 20 to 150 m b.g.l., and (3) massive granites (Figure 2). The area belongs to a semi-arid climate, and the rivers flowing through it are mostly ephemeral and the major drainage network is constituted by the Banas River flowing in the NE–SW direction and its tributaries i.e., Balaram River and Teliya Nadi.

**Figure 2.** A schematic diagram shows different hydrogeological zones (top-soil, weathered zones, fractured zones, and massive zones) and groundwater circulations in the subsurface.

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

### *3.1. Groundwater Sampling and Geochemical Analysis*

A total of forty (*n* = 40) groundwater samples were collected for this study for stable isotopes (oxygen and hydrogen), radiogenic isotope (tritium), and major cations–anions analyses during December 2017 (post-monsoon) from different tube wells, dug wells, and bore wells with variable depths (<30 m) (Figure 1). Pre-cleaned high-density polyethylene (HDPE) bottles (500 mL) were used to collect the groundwater samples. Water was pumped out for 10–15 min before the sampling. For stable isotopes (δ <sup>18</sup>O and δ <sup>2</sup>H) analyses, the water samples were collected in 10 mL HDPE bottles, and for radiogenic isotopes (3H), water samples are collected in 1 L HDPE bottles and sealed tightly. The groundwater samples collected for major cations and anions were filtered using 0.45 µm Millipore filter paper in the field. In situ parameters such as pH, electrical conductivity (EC), temperature (◦C), and total dissolved solids (TDS) were measured in the field through HANNA USA-made (Model: HI98130) portable meter. Bicarbonate (HCO3) of groundwater samples was measured in the field by the titration method. The collected samples were preserved in a cool place after sampling and then transferred to a refrigerator for preservation until the geochemical analysis. The major cations such as Ca2+, Mg2+, Na<sup>+</sup> , and K<sup>+</sup> and anions Cl−, SO<sup>4</sup> <sup>2</sup>−, and NO<sup>3</sup> − of groundwater samples were measured through a UV visible spectrophotometer at IIT Bombay and ion chromatography (Metrohm 883 Basic IC Plus) at the SEOCS, IIT Bhubaneswar with appropriate standards.

#### *3.2. Stable Isotope Data (δ <sup>18</sup>O and δ <sup>2</sup>H) Analysis*

The refrigerated water samples collected for stable isotopes (δ <sup>18</sup>O and δ <sup>2</sup>H) were analyzed at the Nuclear Hydrology Laboratory, NIH Roorkee. For measuring δ <sup>2</sup>H, a dual inlet stable isotope-ratio mass spectrometer (DISIRMS) was used, and a continuous-flow stable isotope-ratio mass spectrometer (CFSIRMS) was used to quantify the δ <sup>18</sup>O. The water samples were equilibrated with CO<sup>2</sup> and H<sup>2</sup> to quantity δ <sup>18</sup>O and δ <sup>2</sup>H values respectively using the standard method [40]. Then, the instrument was calibrated to determine the δ <sup>18</sup>O and δ <sup>2</sup>H composition by analyzing IAEA standards i.e., Vienna standard mean ocean water (VSMOW) [41] with precision range ±1.0‰ for δ <sup>2</sup>H and <sup>±</sup>0.1‰ for <sup>δ</sup> <sup>18</sup>O. The results of the isotopes are expressed in terms of per mil (‰) relative to VSMOW using the 'δ' notation and Equation (1).

$$\delta(\% \text{o}) = \left(\frac{R\_{\text{sample}} - R\_{\text{standard}}}{R\_{\text{standard}}}\right) \times 1000 \tag{1}$$

Here, *R*sample is the ratio of <sup>18</sup>O/16O and <sup>2</sup>H/H isotopes for the collected groundwater sample, and *R*reference is the ratio of <sup>18</sup>O/16O and <sup>2</sup>H/H isotopes for the standard water sample. The reference standard is usually considered IAEA VSMOW, and the measurement precision is ±0.1‰ and ±1‰ for δ <sup>18</sup>O and δ <sup>2</sup>H, respectively. The isotope data reported in this paper correspond to VSMOW.

### *3.3. Radiogenic Tritium (3H) Analysis*

A total of twenty-two identical groundwater samples were collected for tritium (3H) analysis. Usually, one liter of sample is enough for radiogenic <sup>3</sup>H analysis. The water samples were collected in unfiltered condition, and no preservatives were added. The samples were stored in HDPE bottles with air-tight caps. The samples were analyzed at the Nuclear Hydrology Laboratory, NIH Roorkee. For tritium analysis, three steps were followed: (a) sample distillation, (b) fractionation by electrolytic enrichment (for removing <sup>1</sup>H and <sup>2</sup>H), and (c) measurement of tritium on Ultra Low-Level Liquid Scintillation spectrometry. The WinQ and QuickStart software on the QUANTULUS system was used to process the data. The measured tritium concentrations are stated in tritium units (TU), and the error varies between ±0.12 and ±0.24 TU.

### **4. Results and Discussion**

*4.1. Hydrogeochemical Studies*

4.1.1. General Hydrogeochemistry

The physicochemical parameters and statistical data of the groundwater samples used in this study have been provided in Table 1. Detailed information about the water chemistry data is provided in Supplementary Table S1. The pH range of groundwater varies from 6.60 to 7.38 with an average value of 6.97, indicating that the groundwater is slightly acidic, which may be due to the mixture of carbonic acid in the water of the aquifer system. The electrical conductivity (EC) ranges between 360 and 2980 µS/cm, whereas the TDS varies from 216 to 1788 mg/L. The respective average value of EC and TDS is 1231.50 µS/cm and 738 mg/L, which suggests that most of the groundwater samples can be used for drinking water purposes, as it follows the WHO guidelines. However, few groundwater samples (Table S1) with high NO<sup>3</sup> − (>45 mg/L, WHO) are not suitable for drinking purposes. The order of relative abundance of the major cations are Ca2+ > Na<sup>+</sup> > Mg2+ > K<sup>+</sup> and anions HCO<sup>3</sup> − > Cl− > SO<sup>4</sup> <sup>2</sup><sup>−</sup> > NO<sup>3</sup> − (Figure 3).

**Table 1.** Physicochemical and isotopic results of groundwater samples collected during the postmonsoon season from Ambaji Basin (NW India) (December 2017).


**Figure 3.** Major ion concentration variation in the study area.

Bicarbonates (HCO<sup>3</sup> −) represent the alkalinity that ranges from 125 to 535 mg/L with an average of 359.25 mg/L. Such a high concentration of bicarbonates in groundwater is observed due to the chemical weathering of carbonate minerals and calcite dissolution [42,43]. Similarly, chloride (Cl−) with an average concentration of 105.01 mg/L indicates that there is an interaction between freshwater from the recharge area and highly dissolved water from the discharge area [44–46]. Apart from the chloride (Cl−) content, a high concentration of nitrate (NO<sup>3</sup> −) (>45 mg/L) [47] is also observed in 30% of the groundwater samples due to agricultural activities and semi-arid climatic conditions. However, the sulfate (SO<sup>4</sup> <sup>2</sup>−) concentration is between 12.29 and 195.94 mg/L with an average of 40.61 mg/L, indicating that the various sources such as pesticides used for agricultural productivity, evaporate dissolution, and domestic sewages are responsible for the high concentration of sulfate [48,49]. Calcium (Ca2+) is the most abundant ion; it ranges from 21.92 to 183.60 mg/L (average 87.04 mg/L), and it is derived from calc-silicate minerals due to chemical weathering and the dissolution of calcium carbonate minerals [50]. Sodium (Na<sup>+</sup> ) is the second most abundant cation, which varies from 28.70 to 260.15 mg/L. This illustrates that the source of sodium (Na<sup>+</sup> ) in groundwater is mostly plagioclase feldspar, which is the common silicate mineral of granite [9,11] as covered in most of the study regions. The magnesium concentration ranges from 11.94 to 111.28 mg/L with an average of 48.97 mg/L, which indicates that the groundwater has significantly interacted with granulites as it consists of Mg-calcite, biotite, and amphibole minerals [51]. Potassium (K<sup>+</sup> ) is the lowest; it is the most abundant in the groundwater samples ranging from 0.50 to 7.27 mg/L (average 2.69 mg/L), which leached out from the clay-bearing minerals being produced by the chemical weathering of silicate minerals [52].

### 4.1.2. Hydrochemical Facies Variation

A piper trilinear plot is very useful in analyzing and understanding the geochemical evolution and chemical relations of groundwater. This plot is further used to assess the recharge flow paths by plotting the concentrations of major cations and anions [53]. The plot (Figure 4) shows that the groundwater samples are categorized by major water types such as (I) Ca-HCO3, (II) Na-Cl, (III) Mixed Ca-Na-HCO3, (IV) Mixed Ca-Mg-Cl, (V) Ca-Cl, and (VI) Na-HCO3. Most of the samples are collected from granite (G2) and Mafic granulite of the shallow aquifer (depth < 50 m) except for four (04) samples (TW-27, TW-28, TW-29, and TW-30), which are collected from alluvial/sandy zones, as shown in Figure 1. Around 77% of the total groundwater samples show Ca-HCO<sup>3</sup> type, out of which 36% samples are derived from the recharge area, which is supposed to be recharged only by rainfall [2,11]. Furthermore, the low groundwater salinity with the Ca-HCO<sup>3</sup> dominated type of water indicates the rapid recharge of groundwater [18]. This infers that the samples (TW-13, TW-18, TW-38, TW-41) with a TDS value <500 mg/L collected from the recharge area of granite aquifer (Figure 1) are in fresh condition, which is due to the early stage of the rainfall recharge process. However, TW-1 and TW-2 (TDS > 500 mg/L) and TW-43 (TDS > 1000 mg/L) are located in the same recharge region, suggesting that the infiltration through fractures leads to the dilution of groundwater with existing groundwater and subsequently increases the TDS value along its flow direction [54]. Furthermore, the hydrochemical facies shift from Ca-HCO<sup>3</sup> to Ca-Na-HCO<sup>3</sup> due to the release of sodium ions from clay minerals, which are formed due to the chemical weathering of silicate minerals. Thus, ion exchange plays a significant role in this geochemical evolution during the initial stage of groundwater flow and is expressed in Equations (2) and (3).

$$2\text{Na}(AlSi\_3)\text{O}\_8 + 2H^+ + 9H\_2\text{O} \rightarrow Al\_2Si\_2\text{O}\_5(\text{OH})\_4 + 2Na^+ + 4H\_4SiO\_4 \tag{2}$$

$$\rm Ca(Al\_2Si\_2)O\_8 + 2H^+ + H\_2O \to Al\_2Si\_2O\_5(OH)\_4 + Ca^{2+} \tag{3}$$

**Figure 4.** Piper trilinear diagram showing major groundwater types and geochemical evolution.

The samples (TW-3 to TW-6) collected from the granulite shallow aquifer are showing TDS > 500 mg/L, which illustrates that there is a prolonged groundwater interaction with the aquifer matrix [2,11]. The incongruent dissolution of ferromagnesian minerals, hornblende, and pyroxene minerals from mafic granulite rock is attributed to the Mg(Ca)- Na-HCO<sup>3</sup> hydrochemical facies in these samples. The presence of Mg2+ in groundwater is driven by the leaching of pyroxene from the granulite by Equation (4).

#### (*Mg*0.7*CaAl*0.3)(*Al*0.3*Si*1.7)*O*<sup>6</sup> + 3.4*H* <sup>+</sup> <sup>+</sup> 1.1*H*2*<sup>O</sup>* <sup>→</sup> 0.3*Al*2*Si*2*O*5(*OH*)<sup>4</sup> <sup>+</sup> *Ca*2<sup>+</sup> <sup>+</sup> 0.7*Mg*2<sup>+</sup> <sup>+</sup> 1.1*H*4*SiO*<sup>4</sup> (4)

The rest of the groundwater samples have been categorized under type III water i.e., Mixed Ca-Na-HCO3, which are mostly from the granitic body. The hydrochemical facies shown by these samples is Ca-Mg-Na-HCO3. This is due to the groundwater chemistry driven by the weathering of ferromagnesian silicate minerals, as there must be an interaction with granulite rock during its flow. In addition, the samples collected from a low topography area or discharge area with a TDS value >1000 mg/L show the mixed-type of water facies viz. Ca-Na (Mg)-HCO3(Cl), Ca-Mg (Na)-HCO3, Na-Mg-Ca-Cl HCO3, etc. The presence of a high concentration of chloride (Cl−) in groundwater implies the sluggish movement of groundwater with minimal flushing capacity of the aquifer [11].

### 4.1.3. Mechanisms Controlling the Groundwater Chemistry

Gibb's plot of groundwater samples in the Ambaji Basin is plotted in Figure 5. The plot shows that most of the groundwater is confined in the rock dominance area. Around 95% of the groundwater samples show ratios of Na+/(Na<sup>+</sup> + Ca2+) and Cl−/(Cl<sup>−</sup> + HCO<sup>3</sup> −) that are less than 0.5 with a TDS value level <1000 mg/L. This suggests that the groundwater significantly interacted with fractured granite or mafic granulite during its movement. Chemical weathering is the controlling factor for releasing Ca into the groundwater, as calcium silicate and ferromagnesian minerals are the major constituents in these rock types [52,55]. Very few numbers (around 5%) of samples show the ionic ratio of Na+/(Na<sup>+</sup> + Ca2+) and Cl−/(Cl<sup>−</sup> + HCO<sup>3</sup> −) more than 0.5 having a TDS value >1000 mg/L, with an

indication of shifting of samples from rock dominance to evaporation dominance (Figure 5). This may be due to the chemical weathering and anthropogenic activities that are attributed to evaporation [56], which leads to changes in the groundwater chemistry in Ambaji Basin. In this case, the release of Na<sup>+</sup> ions from clay minerals or fertilizers and increase in residence time may be responsible for a higher ratio of Na+/(Na<sup>+</sup> + Ca2+) in groundwater. No samples belong to the precipitation dominance category, illustrating a limited supply of ions from the atmosphere [57].

**Figure 5.** Gibbs plot showing geochemical evolution and rock–water interaction mechanisms.

4.1.4. Ionic Ratios and Hydrogeochemical Evolution

Groundwater chemistry mainly depends on the geochemical reactions and processes that occur within the groundwater system [58]. Various binary plots (Figure 6) can explain the various controlling mechanisms for the geochemical evolution of groundwater. In Figure 6a, the samples lie in between reference lines 1:1 and 2:1 on the bivariate plot of total cations versus alkali earth metals (Ca + Mg), indicating that the contribution of Ca and Mg ions has a significant role in the derivation of groundwater chemistry. This may be due to carbonate dissolution [59] or the weathering of calc-silicate and ferromagnesian minerals/carbonate minerals (calcite and dolomite) that are likely to be present in fractured rock bodies (both in granite and granulite) [11]. However, the scatter plot (Figure 6b) between total cations versus alkali metals (Na + K) shows that nearly all samples are plotted below the reference line (2:1). This infers that the contribution of alkalis metal is relatively less than alkali earth metals, which can be explained by the enrichment of Ca2+ in groundwater due to the Ca-Na exchange process.

The ion exchange and silicate weathering are the major controlling factors for the geochemical evolution of groundwater in this aquifer system. Furthermore, the presence of fractures or faults may also be responsible for the ion exchange process [60], which can bring additional Ca2+ ions to the aquifer system. The scatter plot between Ca2+ and HCO<sup>3</sup> − as shown in Figure 6d illustrates that most of the samples are lying above the 1:1 ratio reference line, whereas very few numbers of samples fall below the reference line. This indicates that the relative enrichment of HCO<sup>3</sup> <sup>−</sup> with respect to Ca2+ is due to silicate weathering. Furthermore, an excess of Ca2+ (some samples falling below the line) infers that the cation exchange process is one of the driving mechanisms for the chemistry of groundwater. The weathering of ferromagnesian silicate minerals in the groundwater can also be elucidated through the correlation between Ca2+ + Mg2+ and HCO<sup>3</sup> −, as shown in

Figure 6c. In this case, the majority of the samples are plotted very close to the equilibrium line (1:1 ratio reference line) as opposite to Figure 6d. Thus, adding Mg2+ brings the major chemical variation in the groundwater system, which originates from clay minerals. During chemical weathering, the feldspar gets altered to clay minerals that mainly constitute Ca2+ , Mg2+, Na<sup>+</sup> , K<sup>+</sup> , etc. as major ions. As the groundwater interacts with the aquifer matrix, an exchange of ions between Ca2+ or Mg2+ and Na<sup>+</sup> takes place, which is also known as the reverse ion exchange process [59,61] and seemed to be one of the controlling factors.

**Figure 6.** Bivariate plots of (**a**) Ca2+ + Mg2+ vs. Total cation, (**b**) Na<sup>+</sup> + K<sup>+</sup> vs. Total cation, (**c**) HCO<sup>3</sup> − vs. Ca2+ + Mg2+, (**d**) HCO<sup>3</sup> <sup>−</sup> vs. Ca2+, (**e**) Ca2+ vs. Mg2+, (**f**) Ca2+ vs. SO<sup>4</sup> <sup>2</sup>−.

The relationship between Ca2+ and Mg2+ can be derived from the scatter plot, as shown in Figure 6e. This graph shows that the samples collected from granite terrane fall both above the reference line, whereas the samples collected from granulite terrane lies below the reference line. This suggests that the samples above and below the reference line are Ca and Mg-rich, respectively. The presence of calc-silicate and ferromagnesian minerals in different lithology influences the groundwater system and is an attribute of the chemical variation in the groundwater. However, to identify any influence of the carbonate weathering process in the groundwater system, a scatter plot between Ca2+ and SO<sup>4</sup> <sup>2</sup><sup>−</sup> has been plotted (Figure 6f). All the samples lie above the reference line with an excess of Ca2+ relative to SO<sup>4</sup> <sup>2</sup><sup>−</sup> ions inferring the existence of carbonate weathering [42].

### 4.1.5. Major Geochemical Processes

Silicate weathering is the main controlling mechanism for the geochemical evolution of groundwater in the study basin. The study area is predominantly composed of granites and granulites, which consist of silicate minerals such as feldspars, pyroxenes, biotite, quartz, etc. The bivariate plots Ca2+/Na<sup>+</sup> versus Mg2+/Na<sup>+</sup> and Ca2+/Na<sup>+</sup> versus Na+/HCO<sup>3</sup> − are used to classify three important processes that govern the geochemistry of water i.e., carbonate dissolution, evaporite dissolution, and silicate weathering [17,62]. As shown in Figure 7a, Ca/Na > 1 is an indication of Ca-rich groundwater, and HCO3/Na > 1 suggests the dissolution of silicate minerals. In this study area, granites consist of Ca feldspar, which has undergone a weathering process, and the dissolution of feldspar releases Ca2+ to the groundwater system. While the groundwater moves from higher to lower topography (from NE to SW direction) in the investigated area (Figure 1), it shows increasing TDS value with an increase in Ca2+. However, there is a sharp change in groundwater chemistry along its flow path when it interacts with mafic granulite rock, consisting of ferromagnesian minerals. In this particular region, Mg2+ slightly exceeds Ca2+ and gives rise to the Mg-Ca-Na-HCO<sup>3</sup> type of water, as discussed in the earlier section of the paper. Thus, the weathering of Mg-rich silicate minerals releases Mg2+ into the groundwater and replaces Ca2+ on participating in the cation exchange process. The Mg/Na > 1 reveals that Mg2+ concentration is more than that of Na<sup>+</sup> in groundwater (Figure 7b), whereas the Mg/Na < 1 shows the dominancy of Na<sup>+</sup> ion in some groundwater samples, suspecting the normal ion exchange process.

To understand this, a plot, Na<sup>+</sup> + K<sup>+</sup> -(Cl−) versus Ca2+ + Mg2+-(SO<sup>4</sup> <sup>2</sup><sup>−</sup> + HCO<sup>3</sup> −), has been plotted, in which almost all samples fall on the straight line with a slope of –1.31 (Figure 8a). This suggests that the reverse ion exchange process is one of the significant factors for the enrichment of Ca or Mg in deeply fractured aquifer relative to Na and also influences the groundwater chemistry in the semi-arid region [60,61,63], as in the case of Ambaji Basin. During this process, Na+ gets adsorbed in favorable exchange sites and replaced with Ca2+ or Mg2+. This can be explained by Equation (5).

$$\text{Zn}^{+} + \text{Ca}^{2+} \left(\text{Mg}^{2+}\right) - \text{Clay} \leftrightarrow \text{Na}^{+} - \text{Clay} + \text{Ca}^{2+} \left(\text{Mg}^{2+}\right). \tag{5}$$

The scattered plot between Na/Cl and Cl illustrates that the Na concentration in groundwater is due to the ion exchange and dissolution process (Figure 8b). The ratio of Na/Cl > 1 indicates the excess of Na over Cl in groundwater, as driven by the ion exchange process in a continuous groundwater flow system. However, some samples show Na/Cl < 1 with rising in salinity (Cl−) concentration, suggesting the effect of dissolution processes, as observed in groundwater discharge points. The subsequent decrease in groundwater flow is attributed to a high Cl− concentration, which also indicates the influence of dissolution on groundwater chemistry. Additionally, the prevailing semi-arid climatic conditions over this region enhance the evaporation effect and may cause an increase in ionic concentration in groundwater [11].

### 4.1.6. Saturation Index (SI) and Geochemical Modeling

For this study, the PHREEQC code [64] has been performed to compute the saturation indices (SI) of dominant carbonate mineral species such as calcite and dolomite, which are the common fracture-filling minerals. As the concentration of Ca and Mg is observed in groundwater samples, the saturation indices with respect to calcite and dolomite can be used as a proxy for identifying the geochemical processes. In Figure 9, the samples have been classified as saturated (SI = 0), undersaturated (SI < 0 or negative), and oversaturated (SI > 0, positive) [50,65]. Most of the samples collected from granitic rocks are plotted below the equilibrium line (SI = 0) (Figure 9a). This infers that groundwater is undersaturated to calcite and dolomite with an excess of Ca and Mg, which are mainly derived from the dissolution of silicate minerals. The higher dissolution rate may be due to the continuous flow of groundwater through fractured granite. However, few samples (BW-9, DW-19, TW-10, TW-11, TW-36, TW-8, TW-42) are distributed close to the saturated line (Figure 9b), acquiring the saturation or over-saturation condition with the increasing of Ca and Mg. These samples are located near the groundwater discharge locations with an average TDS value >1000 mg/L. The relatively slower movement of the groundwater and the evaporation process may be the driving factors for excess ionic concentration.

**Figure 7.** Ionic ratio plot (**a**) HCO<sup>3</sup> <sup>−</sup>/Na<sup>+</sup> vs. Ca2+/Na<sup>+</sup> , (**b**) Mg2+/Na<sup>+</sup> vs. Ca2+/Na<sup>+</sup> showing the silicate weathering as controlling mechanisms.

In the case of mafic granulite, the majority of the samples are near the equilibrium line (Figure 9b), indicating that groundwater is in a nearly saturated state with regard to calcite and dolomite. Groundwater is oversaturated to dolomite in mafic granulite compared to granite. Thus, the sluggish movement of groundwater may lead to an over-saturation condition of calcite and dolomite in granulite rocks, unlike granitic rocks. The concentration of alkali earth metals (Ca + Mg) increases as the groundwater is subjected to saturated or oversaturated conditions to both calcite and dolomite (Figure 9b) due to an incongruent dissolution of silicates.

**Figure 8.** Plot (**a**) Ca2+Mg2+-(SO<sup>4</sup> 2- + HCO<sup>3</sup> <sup>−</sup> vs. Na++ K<sup>+</sup> -(Cl−), (**b**) Na+/Cl<sup>−</sup> vs. Cl<sup>−</sup> showing the reversible ion exchange and evaporation process as the major contributors of Na ions in groundwater at different stages of groundwater evolution.

#### *4.2. Stable Isotopic (δ <sup>18</sup>O and δ <sup>2</sup>H) Signatures and Recharge Process*

The stable isotopic (δ <sup>18</sup>O and δ <sup>2</sup>H) composition of the groundwater in the study area varies from −6.17‰ to −3.23‰ in δ <sup>18</sup>O and <sup>−</sup>41.90‰ to <sup>−</sup>24.51‰ in <sup>δ</sup> <sup>2</sup>H (Table 1) with the mean value of −4.93‰ for δ <sup>18</sup>O and <sup>−</sup>34.03‰ for <sup>δ</sup> <sup>2</sup>H. The Local Meteoric Water Line (LMWL) was calculated based on the concept of Global Meteoric Water Line (GWML) using Equation (6) as shown below [66,67] to understand the relation between <sup>2</sup>H and <sup>18</sup>O.

$$
\delta^2 \mathbf{H} = 8 \delta^{18} \mathbf{O} + 10 \tag{6}
$$

Around 17% and 83% of the total groundwater samples are plotted slightly below and above the LMWL [68] (δ <sup>2</sup>H = (7.6 <sup>±</sup> 0.6) <sup>×</sup> <sup>δ</sup> <sup>18</sup>O-(2.9 <sup>±</sup> 2.2)) respectively, suggesting that groundwater is of meteoric origin (Figure 10).

**Figure 9.** Plot (**a**) spatial variation of saturation index (SI) with respect to calcite and dolomite, (**b**) saturation index (SI) vs. Ca2+ + Mg2+ showing the contribution of silicate minerals in fractured granite and mafic granulite through geochemical modeling.

Furthermore, there is a variation in isotopic composition in groundwater, which is collected from granite and mafic granulite, suspecting the influence of geology on the recharge process. Figure 10 illustrates that the regression line, δ <sup>2</sup>H = 5.89116 <sup>×</sup> <sup>δ</sup> <sup>18</sup>O- 4.73697, which was plotted for the shallow fractured granite aquifer, has a lower slope (approximately 5.9) and intercept (−4) than LMWL. This is due to the fractionation, which leads to a depletion of isotopic composition in groundwater with a positive and strong correlation (R<sup>2</sup> = 0.9) between δ <sup>18</sup>O and δ <sup>2</sup>H. Moreover, the d-excess value of groundwater samples collected from fractured aquifer systems varies from 1.81 to 8.11‰ with an average value of 5.93‰, which also indicates the evaporation process. Similarly, the regression line, δ <sup>2</sup>H = 5.57 <sup>×</sup> <sup>δ</sup> <sup>18</sup>O-7.0672 fits positively (R<sup>2</sup> = 0.83) between δ <sup>18</sup>O and δ <sup>2</sup>H of groundwater samples extracted from mafic granulite. The slope (5.57) and intercept (−7.06) of the regression line is less than that of LMWL, indicating the evaporation effect prior to recharging the groundwater in the mafic granulite aquifer. This is also supported by the d-excess value ranging from 1.30 to 7.10‰ with the mean value of 3.98‰. The slope (<7.6) and d-excess (<10‰) values of the groundwater from both granitic and mafic granulites suggest that the groundwater is influenced by the evaporation process before recharge.

In order to understand the recharge source, a flow direction map of groundwater has been prepared (Figure 11).

**Figure 11.** Groundwater flow direction map of the study area.

Sampling locations such as TW-1, TW-43, T-13, TW-42, and TW-38 show depleted isotopic compositions (average δ <sup>18</sup>O < <sup>−</sup>5.00‰) with the higher hydraulic heads and are located in the Kengora and Ghoda area. Similarly, groundwater from TW-3, TW-5, TW-4, and TW-6 in the Kanpura area shows slightly enriched isotopic values (average δ <sup>18</sup>O > <sup>−</sup>5.00‰) along the flow direction with a decrease in the hydraulic head (Figure 11), which indicates that a higher topography region is depleted with isotopic composition as compared to a low topography region, although they originate from the same source of recharge i.e., local precipitation. However, the depleted isotopic composition is also observed in TW-7, TW-10, TW-21, TW-22, TW-23, TW-25, TW-27, and TW-29 sampling locations with lower hydraulic heads, which are also falling on granitic rocks of the lower topography region. Remarkably, it reveals that groundwater in fractured granite shows the depleted isotopic composition and higher d-excess value as compared to mafic granulite (Figure 12), although they have the same source of recharge. This may be due to the kinetic evaporation of soil moisture that affects the groundwater more effectively in mafic

granulite than fractured granite before it recharges. Furthermore, the sluggish movement of groundwater due to less permeability in mafic granulite is attributed to the enrichment of isotopic composition as compared to the fractured permeable aquifer.

**Figure 12.** Plot of d-excess vs. δ <sup>18</sup>O (‰) of groundwater samples.

### *4.3. Radiogenic (3H) Isotopic Signatures*

In this study, tritium (3H) dating is used to measure the age of groundwater. The half-life of <sup>3</sup>H is 12.32 <sup>±</sup> 0.02 years. The <sup>3</sup>H concentration in the hydrosphere can be cosmogenic and anthropogenic, and the concentration of <sup>3</sup>H in rain due to cosmic ray production is about 6–8 tritium units (TU) in Indian rainwaters [69]. Based on the tritium unit (TU) values, the semi-quantitative age of groundwater can be inferred [70,71]. Twenty-two (22) representative groundwater samples were collected from different aquifers during December 2017 and investigated for measuring the <sup>3</sup>H in the study area in (Table 1). The measured range of TU varies from 1.97 ± 0.13 TU to 28.05 ± 0.70 TU with an average of 5.10 TU. Usually, the <sup>3</sup>H content in groundwater starts decreasing from the recharge zone to discharge zones with time. The area also shows similar results, i.e., relatively high TU in the recharge areas and low TU at the discharge areas (southwest part) (Figure 13). The results show that the measured tritium values (TU) in groundwater can be categorized broadly into (1) a mixture of sub-modern and modern water (0.8–4.0 TU, 30–40 years), (2) modern water (5–15 TU, <5–10 years), and (3) recent recharge (>15 TU). The <sup>3</sup>H values show that most of the samples show a mixture between sub-modern and modern recharge followed by modern and recent recharge. Figure 13 shows the semi-quantitative groundwater ages and exemplifies that groundwater is dynamically recharged through weathered zones, secondary fractures, faults, and shear zones. The existence of recent and modern groundwater in the deeper depth specifies a link between shallow and deeper aquifers. Furthermore, it directs that the groundwater level will be shallow even if pumping is more in these areas. Among the entire area, one sample shows 28.05 TU (TW-04, Kanpura area), representing the recent age groundwater, which might be due to a river being close to the well location. In this study basin, the identified recharge locations are Kengora (TW-1), Padaliya (TW-43), Ghoda (TW-13), Nichli Ghoda (TW-18), Surela (TW-34, TW-35), Virampur (TW-21, TW-22), and Yogdadi areas (TW-27) (>5 TU). These areas are also a good pact with the presence of structural components i.e., major faults, fractures, and shear zones that control the groundwater circulations in the study basin.

**Figure 13.** Distribution and variation of environmental tritium (3H) content in Ambaji Basin (NW India).

More than 50% of the samples show the TU values between 0.8 and 4.0, which indicates that the groundwater is mixed type i.e., modern and sub-modern age (30–40 years). This implies that the study area is moderately rechargeable in these areas even if few fracture and lineament traces are present. The area consists of several mafic granulite patches where fractures and weathered zones are relatively less as compared to granitic rock. The mafic granulite is massive in nature, which results in the sluggish behavior of groundwater movement in the subsurface. Therefore, these moderately rechargeable areas are expected to show a depletion of groundwater levels if pumping is more.

### **5. Scientific Outcome and Its Policy Relevance for Sustainable Water Resources Management**

This study has not only given mere value for scientific investigation but also an open door for policymakers' or decision-makers' interventions by achieving different objectives, as shown in Figure 14. One of the dimensions is to hasten the process of SDGs targets in a timely manner. Although there are 16 sustainable development goals (SDGs), all the goals are interlinked, and accessing water management is vital for achieving all the goals. For the second dimension, robust science is necessary for designing for better policy in arid/semi-arid regions; that is why this kind of scientific evidence will open pathways for better science–policy interlinkages. For the third dimension, water always proved to be a limiting factor for socio-economic growth in a holistic manner, especially in water-scarce or water-shortage regions such as arid/semi-arid areas. This study will be proved crucial for

laying a good foundation for management policies. Furthermore, this study also helps to achieve a better bio-diversity management plan or global Aichi target.

**Figure 14.** Interactive link between groundwater recharge process and geochemical evolution with IWRM and SDG.

### **6. Conclusions**

Based on hydrogeochemical parameters and isotopic composition, the study was carried out to understand the various geochemical processes that were subjected to hydrogeochemical evolution in a structurally controlled area. The environmental isotopes (δ <sup>18</sup>O, δ <sup>2</sup>H, and <sup>3</sup>H) of groundwater samples were analyzed to identify the source of groundwater and the hydrological process for groundwater recharge. In groundwater, Ca2+ is the dominated cation followed by Na<sup>+</sup> , Mg2+, and K<sup>+</sup> , whereas from the anionic category, HCO<sup>3</sup> − is dominant with a gradual decrease in the concentration of Cl−, SO<sup>4</sup> <sup>2</sup>−, and NO<sup>3</sup> −. The interaction of groundwater with the aquifer matrix is a common phenomenon for the different ionic distribution in the different aquifer systems. To understand the hydrogeochemical pattern from recharge to discharge areas, hydrochemical facies was plotted. All samples are majorly represented by two types of hydrochemical facies, i.e., Ca-HCO<sup>3</sup> and Mixed Ca-Na-HCO3, which are collected from fractured granite and mafic granulite. The chemical behavior of groundwater is different, as it moves from one point to another. Initially, the groundwater shows Ca-HCO<sup>3</sup> facies in both granites and granulites as it gets recharged by rainfall. As soon as it moves further, mixed Ca-Na-HCO<sup>3</sup> type hydrochemical facies are observed. In fractured granite and mafic granulite, groundwater is represented by Ca-Na-HCO<sup>3</sup> and Mg (Ca)-Na-HCO3, respectively, which is due to the chemical weathering of silicate minerals. It is also supported by Gibb's rock–water interaction plot. Along with chemical weathering, ion exchange is also responsible for controlling the groundwater chemistry, in which the Na releases with the replacement of Ca during the early stage of groundwater movement. Gradually, the Ca and Mg replace Na by a reversible ion exchange process.

The stable isotopic (δ <sup>18</sup>O and δ <sup>2</sup>H) composition of groundwater reveals that rainfall is the primary source of the groundwater recharge and shows more depletion value in the recharge area as compared to discharge. Furthermore, the enriched isotopic composition δ <sup>18</sup>O > 5‰and low d-excess value (<10‰) indicate that the groundwater is subjected to evaporation before it recharges. The radiogenic isotope (3H) concentration shows that the study basin is predominantly modern groundwater (<8–10 years) and mixed-type sub-modern to modern groundwater age (30–40 years). Furthermore, the modern recharge is linked to the presence of different structural components i.e., secondary faults, fractures, and shear zones that circulate the precipitation into the subsurface groundwater system. As a way forward, the study recommends hydrological simulation-related studies or detailed policy-relevant studies to achieve water security in the present study area or areas with similar climatic and geographical characteristics.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/w14030315/s1, Table S1. Physicochemical and isotopic results of groundwater samples collected during post-monsoon season from Ambaji Basin (NW India) (December 2017)

**Author Contributions:** Writing—original draft, R.M.P.; reviewing and editing, R.M.P., A.K.B., S.K., P.K. and T.K.B.; methodology, R.M.P. and A.K.B.; supervision, T.K.B., S.K. and P.K.; data collection and conceptualization, R.M.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the Ministry of Earth Sciences (MoES), Govt. of India [MoES/P.O (Geosci)/50/2015].

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Acknowledgments:** Rudra Mohan Pradhan thanks the IRCC IIT Bombay and Ministry of Earth Sciences (MoES), Govt. of India for the financial support to carry out this work as part of the Ph.D. Thesis. TK Biswal is thankful to the MoES, Govt. of India for the sponsored Aquifer mapping project (Project no. MoES/P.O (Geosci)/50/2015). We are thankful to the Department of Earth Sciences, IIT Bombay and Hydrological Investigation Division, NIH Roorkee for providing the necessary laboratory facilities to carry out the analyses.

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

### **References**

