Co-occurrence of geogenic and anthropogenic contaminants in groundwater from Rajasthan, India

Science of the Total Environment 688 (2019) 1216–1227

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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Co-occurrence of geogenic and anthropogenic contaminants in groundwater from Rajasthan, India Rachel M. Coyte a, Anjali Singh b, Kirin E. Furst c, William A. Mitch c, Avner Vengosh a,⁎ a b c

Division of Earth and Ocean Sciences, Nicholas School of the Environment, Duke University, Durham, NC 27708, USA Department of Geology, Mohanlal Sukhadia University, Udaipur, Rajasthan 313001, India Department of Civil and Environmental Engineering, Stanford University, Stanford, CA 94305, USA

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• New data from 243 wells in multiple aquifers and climate regions show widespread contamination of Rajasthan’s groundwater. • Both geogenic and anthropogenic processes affect groundwater chemistry and quality. • Contaminants of most concern (fluoride, nitrate, and uranium) often co-occur at concentrations that threaten human health. • Human contamination may feed into geological processes to exacerbate geogenic contaminants’ mobilization from aquifer rocks.

a r t i c l e

i n f o

Article history: Received 18 April 2019 Received in revised form 7 June 2019 Accepted 21 June 2019 Available online 23 June 2019 Editor: José Virgílio Cruz Keywords: Geochemistry Water quality Rajasthan India Geogenic contaminants Isotope tracers Geohealth

⁎ Corresponding author. E-mail address: [email protected] (A. Vengosh).

https://doi.org/10.1016/j.scitotenv.2019.06.334 0048-9697/© 2019 Elsevier B.V. All rights reserved.

a b s t r a c t Northwest India suffers from severe water scarcity issues due to a combination of over-exploitation and climate effects. Along with concerns over water availability, endemic water quality issues are critical and affect the usability of available water and potential human health risks. Here we present data from 243 groundwater wells, representing nine aquifer lithologies in 4 climate regions that were collected from the Northwestern Indian state of Rajasthan. Rajasthan is India's largest state by area, and has a significant groundwater reliant population due to a general lack of surface water accessibility. We show that the groundwater, including water that is used for drinking without any treatment, contains multiple inorganic contaminants in levels that exceed both Indian and World Health Organization (WHO) drinking water guidelines. The most egregious of these violations were for fluoride, nitrate, and uranium; 76% of all water samples in this study had contaminants levels that exceed the WHO guidelines for at least one of these species. In addition, we show that much of the groundwater contains high concentrations of dissolved organic carbon (DOC) and halides, both of which are risk factors for the formation of disinfectant byproducts in waters that are treated with chemical disinfectants such as chlorine. By using geochemical and isotopic (oxygen, hydrogen, carbon, strontium, and boron isotopes) data, we show that the water quality issues derive from both geogenic (evapotranspiration, water-rock interactions) and anthropogenic (agriculture, domestic sewage) sources, though in some cases anthropogenic activities, such as infiltration of organic- and nitrate-rich water, may contribute to the persistence and enhanced mobilization of geogenic contaminants. The processes affecting Rajasthan's groundwater quality are common in many other worldwide arid areas, and the lessons learned from evaluation of the mechanisms that affect the groundwater quality are universal and should be applied for other parts of the world. © 2019 Elsevier B.V. All rights reserved.

R.M. Coyte et al. / Science of the Total Environment 688 (2019) 1216–1227

1. Introduction Pollution, overuse, and climate change have reduced surface water flows and availability, resulting in increased utilization of groundwater resources (Famiglietti, 2014; Hoekstra et al., 2012; Konikow and Kendy, 2005; Siebert et al., 2010; Wada et al., 2010). India extracts over 75 billion m3 of groundwater annually, which constitutes about 30% of all groundwater extractions globally (Dalin et al., 2017). Most of the extraction occurs along the Indo-Ganga basin in Northern and Northwestern India, which has resulted in significant drawdown and water table decline in many locations (Gleeson et al., 2012; MacDonald et al., 2016; Rodell et al., 2009). The over-exploitation and depletion of groundwater is further exacerbated due to the apparent decline in precipitation over Northern India associated with global climate change, population increases, and river water diversion, all of which lead to significant water stress (Ashfaq et al., 2009; Asoka et al., 2017; McDonald et al., 2011; Wada et al., 2010). In addition to quantity concerns, Northwestern India suffers from several major groundwater quality issues. Previous studies have identified problems such as high mineralization and salinity, fluoride, uranium, and nitrate contamination in groundwater from different parts of Northwestern India (Agrawal et al., 1999; Coyte et al., 2018; Datta et al., 1999; Gupta et al., 2005; Jacks et al., 2005; Podgorski et al., 2018; Singh et al., 2003; Suthar et al., 2009). Since groundwater is a major drinking water source in many parts of Northwestern India, understanding the occurrence, distribution, and sources of contamination is important for the protection of the health of the local population. In this study, we investigated the occurrence of contaminants in groundwater from several areas in Rajasthan. The objectives of this study were three-fold. First, we evaluated the factors that control groundwater chemistry and salinity. Second, we examined the occurrence and distribution of different contaminants in groundwater from

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different aquifers in Rajasthan, addressing the roles lithology and permeability play on water chemistry and quality. Third, we evaluated the relationships between groundwater chemistry, isotope tracers, and contaminants, elucidating the mechanisms that could control contaminant occurrence and co-occurrence. Combined, this study presents new and comprehensive high-quality water quality and geochemistry data for groundwater from different aquifers and regions in Rajasthan that can help evaluate the potential risks to local populations that consume groundwater as their major drinking water source. 2. Study area and regional setting Rajasthan is located in Northwestern India and is India's largest state by area (Fig. 1). The climate in the majority of the state is arid to semiarid, though parts of the southeast are more humid. All rivers in Rajasthan are rain fed and ephemeral, with the exception of the Chambal and Mahi, which run through a small area of the southeastern and southern part of the state respectively (Narain et al., 2006). Some parts of the state have access to canal water, used for irrigation and water supply to urban areas. The most important canal is the Indira Gandhi Canal, which runs through seven districts in the center of the state. Water from this canal also feeds Kaylana lake, which is the primary water source for Rajasthan's second largest city, Jodhpur (Central Ground Water Board, 2015). Some areas also have access to reservoir water, mainly for irrigation. One of most important dams in Rajasthan for the provision of drinking water is the Bisalpur dam in Tonk district. Water from this dam is imported, sometimes mixed with local groundwater, and supplied as disinfected tap water to Rajasthan's capital and largest city, Jaipur, as well as some parts of Tonk and Ajmer districts (Amit, 2012). The World Bank estimates that 90% of drinking water and 70% of irrigation water in the state is sourced from groundwater (Hooda, 2017). The majority of rural residents in the arid and semi-

Fig. 1. Map of study area and sampling sites in Rajasthan. Insert map shows location of Rajasthan within India.

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arid parts of the state consume groundwater as their primary source of drinking water, with disinfection not routinely practiced. Drinking water disinfection is practiced by the state's water utilities who supply water to major urban centers like Jaipur and Jodhpur.

series of only two or three observations. These samples are identified in the supplement, along with some of their chemistry data (Table 1S, Fig. 1S). Their full chemistry, as well as full chemistry for all samples can be found in the data appendix.

3. Materials and methods

3.2. Region designations

3.1. Water sampling and analysis

The groundwater samples collected in this study have been divided into four categories based on climate and hydrological factors (Fig. 1). Region 1 are groundwater samples from eastern Barmer district in the southwest. The groundwater samples were collected from alluvial aquifers in an arid climate (200–370 mm of rain per year) (Hussain, 2015); they are characterized by high salinity, and the vast majority of the wells are not used for human consumption without desalination treatment. Thirteen total samples and one drinking water sample were taken from this region. Region 2 are groundwater samples collected from Jodhpur district, primarily in and around the city of Jodhpur. The climate in this area is semi-arid (300–500 mm of rain per year) (Hussain, 2015) and samples were taken from alluvial, rhyolite, and sandstone aquifers. Imported surface water provides drinking water for much of this area, but has also caused waterlogging in some places (Central Ground Water Board, 2015). Twenty-eight total samples and nine drinking water samples were taken from this region. Region 3 are samples from the Jaipur, Ajmer, Tonk, and Dausa districts. This is a semi-arid area (500–700 mm of rain per year) (Hussain, 2015), and groundwater samples were collected from alluvial aquifers and metamorphic hard rock aquifers. Though parts of region 3 have access to reservoir water through Bisalpur Dam, groundwater is still an important source of drinking water, and many rely on private or community wells (Everard et al., 2018). Groundwater is also sometimes mixed with imported water from Bisalpur Dam through the public distribution system. This region represents the largest group of samples with 151 total samples and 115 drinking water samples. Region 4 are samples collected from Kota and Bundi districts in southeastern Rajasthan. This is a humid region (650–1000 mm of rain per year) (Hussain, 2015) with primarily sandstone and other consolidated sedimentary aquifers with some alluvial aquifers. Many in this area have some access to surface water through distribution systems, but groundwater is still consumed, especially in areas outside the city. A summary of selected water quality parameters compared to WHO standards, sorted by region can be found in Table 1.

Water samples were collected from 243 groundwater wells in Rajasthan following United States Geological Survey sample collection protocols (U.S. Geological Survey, 2006). Samples for dissolved anions, cations, trace metals, and dissolved organic carbon were filtered using 0.45 μm filters. Cation and trace metal samples were preserved with Optima nitric acid to pH b2. Specific conductivity and pH were measured in the field. Samples were analyzed for dissolved anions by ion chromatography on a Dionex IC DX-2100 system. The IC calibration was verified using a secondary Dionex 7-anion standard at varying concentrations. Major cations were analyzed by direct current plasma optical emission spectrometry (DCP-OES), while trace elements were analyzed by inductively coupled plasma mass spectrometry (ICP-MS) on a VG PlasmaQuad-3 system. The DCP-OES and ICP-MS instruments were calibrated to the National Institute of Standards and Technology 1643f standard, which was used at varying concentrations before, after, and throughout sample runs. Internal standards of In, Th, and Bi were spiked into all samples prior to measurement on the ICP-MS. Nitrate was measured by Flow Injection Analysis on a Hach Lachat at the Duke River Center using QuikChem Method 10-107-04-2-D (Nitrate/Nitrite in Water by Hydrazine Reduction) and is reported as NO− 3 throughout the text. Dissolved organic carbon (DOC) was measured on a TOC analyzer (TOC-V CPH; Shidmadzu, Kyoto, Japan). Charge balances were b10% difference for all samples for which charge balance could be calculated. Total iodine was measured at Stanford University using a Thermo Scientific XSERIES 2 ICP-MS, and iodide was measured using a Dionex DX-500 Ion Chromatograph. Total iodine and iodide were quantified with freshly prepared standard curves, and check standards placed in the middle and end of sample runs were used for quality control. The stable isotopes of water (δ2H and δ18O) were analyzed in the Duke Environmental Isotope Laboratory using a ThermoFinnigan Delta Plus XL ratio mass spectrometer via a Conflo III flow adapter. Raw delta values were normalized offline against known vs. measured isotope values for international reference waters VSMOW, VSLAP and IAEA-OH16. The δ2H and δ18O values are expressed in per mil versus VSMOW. Boron and strontium isotope ratios were measured using Thermal Ionization Mass Spectrometry (TIMS) at Duke University. Samples for B analysis were pretreated with 10% Optima hydrogen peroxide. The 11B/10B ratios were measured on single Re filaments as BO− 2 ions in negative mode and normalized to the NIST SRM 951 standard. Boron isotope ratios are reported as: δ11 B ¼ ½1−

11


B=10 B =ð11 BSRM951 =10 BSRM951Þ   1000

ð1Þ

with a precision of ±0.5‰ on repeated measurements of the NIST SRM 951 standard (mean = 4.0050 ± 0.0020, n = 100). Sr was preconcentrated by evaporation in a HEPA filtered clean hood and redigested in 3.5 N HNO3, then separated using Eichrom Sr-specific ion exchange resin. 87Sr/86Sr ratios were measured on the TIMS in positive mode using single Re filaments with a precision of ±0.000009 on repeated measurements of NIST SRM 987 standard (mean = 0.710259, n = 260). Twenty samples were taken from the same 9 wells during different sampling campaigns to see if there were differences in groundwater chemistry with season or with the passage of time. No particular trends were detected, though it is difficult to test this statistically with a time

4. Results and discussion 4.1. Water sources We used the stable isotopes of oxygen and hydrogen (Fig. 2) to investigate the possible sources of groundwater in the study area. Data were compared to local meteoric water lines (LMWL) from New Delhi and Ahmedabad, which bracket the study region to the northeast and southwest, respectively. The local meteoric water line for New Delhi comes from the International Atomic Energy Agency's (IAEA) database of monthly precipitation between 1960 and 2012 using a precipitation weighted least squares regression (PWLSR) (IAEA/WMO, 2019). The local meteoric water line for Ahmedabad was calculated via PWLSR using daily precipitation data collected in 2007 (Deshpande et al., 2010; Hughes and Crawford, 2012). We distinguish between two main water sources. The first is meteoric water infiltrating through the unsaturated zone. Most groundwater samples lie either on the LMWLs, reflecting isotopically unaltered precipitation, or below them on a line that reflects isotopic enrichment from evaporation. The second water source, represented by a few samples, is the recharge from surface water. Samples from the Kota region sandstone aquifer do not follow the regional meteoric water line, have more enriched δ2H and δ18O values, and are of low salinity, suggesting mixing with surface water. This region is home to multiple surface

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Table 1 Violations of WHO guidelines (in parentheses) of selected water quality parameters for all groundwater samples, sorted by the four regions defined in this study. The number of samples analyzed for fluoride, nitrate and uranium for each region are represented by “n”. The numbers of samples measured for arsenic were lower due to a chloride limitation on arsenic measurements. Region

One Two Three Four

n

13 28 151 51

Percent over WHO standard F− (1.5 mg/L)

NO− 3 (50 mg/L)

B (2.4 mg/L)

As (10 μg/L)

U (30 μg/L)

46% 23% 41% 10%

54% 57% 54% 10%

54% 7% 6% 2%

n/a 6% (n = 16) 1% (n = 70) 2% (n = 45)

77% 11% 42% 6%

water sources, including the Chambal River, Kota Barrage, and multiple irrigation canals, which could be one of the endmembers in this mixing through intensive irrigation. Similarly, in Jodhpur, a negative correlation between Cl and δ18O in groundwater from the sandstone aquifer suggests recharge mixing between low-saline yet evaporated surface water, probably from Kaylana Lake or the Indira Gandhi Canal, and saline groundwater of meteoric origin (Fig. 2S). The distinction between meteoric water and surface water recharge can be a useful tool in future studies aiming to evaluate the hydrological balance of aquifers, the role of artificial recharge from surface water bodies, and evaporative losses from surface storage (Dogramaci et al., 2012; Skrzypek et al., 2015). 4.2. Water chemistry Groundwater investigated in this study is characterized by high salinity (TDS ranged from 275 to 35,700 mg/L; Fig. 3), with a mixed − Na-HCO− composition (Fig. 3S). This salinity varies somewhat 3 -Cl between aquifers of the same climate region. The sandstone aquifer in region 2 has relatively low chloride concentrations compared to the rhyolite and alluvial aquifers (p-value for Mann-Whitney test of 0.0003 and 0.005 respectively, no difference between the rhyolite and alluvial aquifers), which suggests that aquifer lithology and permeability directly affect the recharge rate/evaporation of the infiltrated water and thus the groundwater salinity (Fig. 4). This is also supported by the stable isotopes, which show a greater influence of surface water on the sandstone aquifer in region 2 (Fig. 2S). The other regions have little or no differences between the chloride concentrations of their aquifers (see Table 2S for p-values), though there was not enough data to make a statistical comparison between the limestone, alluvium, and phyllite aquifers and the other aquifers in region 4. The chloride concentrations of groundwater from the different aquifers in region 4 were lower than the other regions (Fig. 4), most likely reflecting less arid conditions in this region relative to the other regions. Transpiration likely also plays a role in salinization, especially considering the importance of agriculture to the region. Prolonged, intensive use of groundwater

for irrigation in an arid environment where water exits only through evapotranspiration and infiltration increases the rate of salt buildup (Fogg et al., 2016). Differences in TDS can be seen between some aquifers in the same climate region, even where no chloride concentration differences exist (Table 3S). A difference in TDS between aquifers without a difference in chloride concentration indicates a greater role for water rock interactions, as salinization induced mostly from evapotranspiration affects conservative elements like chloride and bromide but water-rock interactions tend to mobilize bicarbonate, sodium, and boron from the aquifer rocks to the aquatic phase (Vengosh et al., 2005). The Br/Cl ratios in the majority of the investigated groundwater were lower than the seawater ratio (1.5 × 10−3; molar ratio) (Fig. 5), which could indicate (1) in-situ halite dissolution; and/or (2) lower Br/Cl ratio in precipitation from atmospheric aerosols. We did not observe changes in the Br/ Cl ratio with salinity, which suggests lower Br/Cl in the recharged meteoric water rather than in-situ halite saturation and dissolution. Though salt lakes and halite deposits do exist in Rajasthan, we are not aware of any major salt deposits in the areas we studied (Kajale and Deotare, 1997; Singh et al., 1972; Sinha and Raymahashay, 2004). Waters impacted by halite dissolution are expected to have Na/Cl ratios close to one, but for most samples in this study the Na/Cl ratio is N1 (Fig. 6). There are multiple potential sources for excess Na in the groundwater system, including silicate weathering, reverse-base exchange reactions, and anthropogenic inputs. Geochemical evidence suggests that all of these played a role. Silicate weathering likely contributed to elevated sodium concentrations. Silicate lithologies dominate most of our study area with some secondary carbonates (caliche, known locally as kankar), especially in the alluvial aquifer (Roy and Jakhar, 2002). Dissolved silica concentrations were high in samples from the alluvium and crystalline basement aquifers, and saturation indices calculated for samples from these aquifers show oversaturation of many SiO2 polymorphs and compounds (Table 4S). This is likely the result of weathering of primary silicate minerals, further suggesting the weathering of silicate minerals is the major

Fig. 2. δ18O and δ2H for groundwater samples sorted by lithology. The gray line represents the LMWL from New Delhi (δ2H = 7.46 × δ18O + 5.6) and the purple line represents the LMWL from Ahmedabad (δ2H = 7.87 × δ18O + 8.12).

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Fig. 3. Boxplots of TDS values in all groundwater samples. The orange line represents the Indian Bureau of Indian Standard's primary guideline for TDS, and the red line represents the “permissible” guideline. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

source for the dissolved cations and bicarbonate in the investigated groundwater. Most silicate weathering would produce Na-Cl−-HCO− 3 type water, which we observed in many of the groundwater samples we analyzed in this study. We can infer from the radiogenic strontium isotope data (Fig. 7) that silicate weathering is the major source for strontium and thus also calcium. The data show clear variations in groundwater 87Sr/86Sr ratios between the different aquifers. Groundwater samples from the alluvial and crystalline basement aquifers have more radiogenic (high) 87Sr/86Sr (0.710697 to 0.733239) than those from the consolidated sedimentary and rhyolite aquifers (0.70988 to 0.714118), though much of this observation in the alluvial aquifer relies on the large dataset of Sr isotope values from region 3 (Fig. 7). This distinction reinforces the important role of the aquifer lithology on water chemical composition. We suggest that the system is further altered by reverse baseexchange reactions. Reverse base-exchange and desorption processes are most often associated with the aquifer freshening process, during which fresh water infiltrates an aquifer saturated with saline groundwater, causing Na to exchange for Ca and Mg on clay minerals (Appelo, 1994). The resulting aquatic phase is depleted in Ca2+ and

+ 2− Mg2+ with respect to HCO− with re3 and SO4 , and enriched in Na spect to Cl−, similar to much of our study groundwater. Reverse base-exchange is further evidenced by the strong correlation between Na/Cl and B/Cl (Fig. 8), showing that both Na+ and B were desorbed from exchange sites on clay minerals in the aquifer rocks (Vengosh et al., 2005). Boron isotopes were measured in a subset of groundwaters from region 3. The boron isotope ratios (δ11B of 7.7 to 42.7‰, Fig. 9) suggest that the majority of boron in the groundwater was affected by adsorption-desorption processes. Previous studies have demonstrated that during adsorption, 10B would adsorb preferentially onto clay minerals and thus 11B would become enriched in the residual groundwater, while desorption would mobilize boron from the exchangeable sites without isotope discrimination (Spivack et al., 1987). This would correspond to the relatively high δ11B observed in most groundwater, which is not consistent with the expected δ11B values of silicate rocks (b10‰; Trumbull and Slack, 2017). Only a single groundwater sample had such low δ11B, while the majority of the samples had higher δ11B. Yet unlike groundwater from coastal aquifers with distinctive high δ11B (i.e., N40‰; Vengosh et al., 2005) the data from Rajasthan show intermediate values (Fig. 9), implying both adsorption

Fig. 4. Cl− boxplots for aquifers in regions 2 (top), 3 (middle), and 4 (bottom). The orange line represents Bureau of Indian Standard's primary guideline for Chloride and the red line represents the “permissible” guideline. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Fig. 5. Br vs Cl, the black line represents the seawater Br/Cl molar ratio of 1.5 × 10−3.

and desorption, typical of freshening process. Overall, we suggest that the distinctive geochemical characterization of the majority of the investigated groundwater in Rajasthan reflects multiple recharge episodes in which lower salinity water from either monsoon rains or irrigation encounters higher salinity evaporated water in the unsaturated zone, resulting in the freshening process. Consequently, reverse base-exchange reactions become the predominant process that control the water chemistry. 4.3. Anthropogenic processes Much of the groundwater in our study has high concentrations of nitrate and DOC (Fig. 10). Seventy out of 169 drinking water samples (41%) and 45% of all investigated samples had nitrate concentrations

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exceeding the World Health Organization's (WHO) health standard (50 mg/L as NO− 3 ) (World Health Organization, 2011). High nitrate concentrations in groundwater are known to occur all over India and are likely derived from human and animal wastes, as well as agricultural return flows (Agrawal et al., 1999; Central Ground Water Board, 2013; Suthar et al., 2009). In our study area, region 4 had significantly lower nitrate concentrations compared to the other regions, which may be a result of dilution effects. Additionally, groundwater from region 4 had lower NO− 3 /Cl than those from regions 2 and 3, potentially indicating increased denitrification in the more humid region. DOC concentrations in the range of 0.1 to 4 mg/L are common in unpolluted groundwater, and are affected by factors such as pH, redox state, and soil conditions (Graham et al., 2015; Regan et al., 2017). Across all of our study regions, 62 of the 114 samples (~54%) in which DOC were measured had DOC concentrations higher than 4 mg/L (Fig. 10). In several previous studies, where DOC levels were reported to be above 4 mg/L in unpolluted groundwaters, they were usually associated with known organic deposits in the host aquifer material under reducing conditions (e.g. coal deposits, peat soils) (Anawar et al., 2003; Artinger et al., 2000). However, the DOC-rich shallow groundwater in Rajasthan co-exists with high uranium and thus likely oxidizing conditions (Coyte et al., 2018) in an arid to semi-arid environment, without known major organic deposits. The combined high nitrate and DOC concentrations found in the study area suggest significant anthropogenic sources affecting the quality of groundwater in study areas in Rajasthan. Some of the bicarbonate in the system may also be attributed to anthropogenic effects. In general, the ultimate source of dissolved inorganic carbon (DIC) in groundwater derives from a few possible sources, including carbonate rock dissolution, decay of organic matter, atmospheric carbon, and potentially subsurface CO2 flux. In our study area, δ13C values (−13.9 to −6.3‰) suggest that the ultimate source of DIC in the groundwater is soil carbon originating from the decay of C4 and C3 plants. This is consistent with the documented natural vegetation in arid to semi-arid in Rajasthan (Sankhla et al., 1975), as well as the mixture of crops grown (Government of Rajasthan Department of Agriculture, 2018). Human activities can intensify weathering processes by increasing the supply of weathering reactants. In the context of our study, both agricultural practices and domestic sewage inputs have the potential to increase groundwater DIC. The addition of organic matter, e.g. manure or human sewage, would increase the PCO2 of soil during degradation of organic matter in the soil, which could dissociate to HCO− 3 and/or trigger acidity and water-rock interaction. These processes may be evidenced by the correlation we observed between HCO− 3 and DOC (Fig. 11). Nitrification can also increase weathering rates, though only carbonate weathering increases HCO− 3 concentrations via this pathway (Barnes and Raymond, 2009; Burow et al., 2017). High bicarbonate concentrations in turn can enhance the formation of uranyl carbonates species and thus enhance the solubility of uranium in the groundwater (Alam and Cheng, 2014; Coyte et al., 2018; Dong and Brooks, 2006; Vercouter et al., 2015). 4.4. Water quality

Fig. 6. Na vs Cl; the black line represents a 1:1 ratio, which is typical for halite dissolution.

Of the 243 water samples collected from Rajasthan, 169 were from primary drinking water wells. These wells were predominantly located in regions 3 and 4, as well water consumption is more common in these areas. Table 2 presents the compliance of the drinking water samples with respect to both the WHO drinking water guidelines and the Bureau of Indian Standards drinking water regulations for the inorganic water quality parameters measured in this study. The data show systematic exceedances of primary drinking water standards for several parameters, including uranium (bdl-312 μg/L), fluoride (bdl-11.3 mg/L), and nitrate (bdl-517 mg/L; Figs. 10, 12). Of the 169 drinking water samples evaluated in this study, three met the more rigorous “requirement” standard from the Bureau of Indian

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Fig. 7. Sr Isotope values sorted by aquifer lithology. Large variations between aquifer lithologies reflect the influence of water rock interactions on water chemistry.

Standards, and 42 drinking water samples met the less rigorous “permissible limit in the absence of alternative source” guidelines. A comparison to WHO guidelines showed 103 drinking water samples with violations, mostly for fluoride, uranium, or nitrate. Multiple violations often occurred simultaneously in water samples. Focusing on the WHO's guidelines, 83 total samples and 56 drinking water samples had violations for more than one contaminant, especially for fluoride, uranium, and nitrate. This co-occurrence of contaminants could pose additional human health risks through what are known as mixture, or “cocktail” effects (Bopp et al., 2018; Evans et al., 2016; Kortenkamp and Faust, 2018; Svingen and Vinggaard, 2016). While the present study does not attempt to conduct a mixture risk assessment, such information could prove invaluable in future water management and planning decision-making. While high nitrate concentrations are a result of human activities, fluoride and uranium are primarily geogenic in origin. High uranium concentrations in Indian groundwater have been linked to oxidizing conditions affecting uranium solubility, aquifer lithology, and HCO− 3

concentrations that promote the formation of soluble uranyl carbonates (Adithya et al., 2016; Brindha and Elango, 2013; Coyte et al., 2018; Mittal et al., 2017; Rishi et al., 2017; Saini and Bajwa, 2016; Singh et al., 2003). Previous studies have suggested that fluoride contamination in India's groundwater is geogenic and is tied to aquifer lithology and water chemistry, with fluoride concentrations controlled by fluorite (CaF2) solubility (Handa, 1975; Jacks et al., 2005; Podgorski et al., 2018). Nearly all of the investigated groundwater samples were undersaturated with respect to fluorite, primarily because of Ca sinks, such as calcite precipitation and ion-exchange reactions. Consequently, fluoride is well correlated with species connected to similar lithologies, like uranium, and those associated with ion exchange reactions, such as boron and sodium. It is important to emphasize that the evidence for reverse base-exchange reactions we demonstrate in this study (e.g., Fig. 8) suggests that this process is important for controlling the solubility (e.g., Ca

Fig. 8. Molar ratios of Na/Cl vs B/Cl. The correlation (Spearman's ρ = 0.87) is reflective of reverse base exchange reactions.

Fig. 9. Boron isotopes from region three reflecting multiple adsorption and desorption events.

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Fig. 10. Boxplots of nitrate and DOC concentrations in all groundwater samples, sorted by region. The WHO guideline value for nitrate is shown by the red dashed line, and the gray line represents the typical upper limit for naturally occurring DOC in groundwater. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 11. Bicarbonate vs DOC. The correlation (Spearman's ρ = 0.67) evidences a possible connection between DOC and HCO− 3 .

removal) and thus the occurrence of fluoride, boron, and possibly other contaminants, in the groundwater. Unlike in Bangladesh, Pakistan, and other parts of India where groundwater from the alluvial aquifers contain high As (Anawar et al., 2003; Bacquart et al., 2012; Chowdhury et al., 2000; Farooq et al., 2011; Farooqi et al., 2007; Majumder et al., 2014; Nickson et al., 2005; Rahman et al., 2015), the data in our study indicate that arsenic is not a common contaminant in Rajasthan's groundwater. Only three wells out of the 131 where arsenic was measured violated the WHO's standard of 10 μg/L. We attribute this primarily to the oxidizing conditions dominating much of the study area as demonstrated by the high uranium abundance. Arsenic is more soluble under reducing conditions and thus the coexistence of uranium and arsenic in groundwater is unlikely. In addition to the water quality parameters commonly used to define the suitability of water for the domestic sector, we also investigated additional chemical parameters related to the potential formation of disinfection byproducts (DPBs). DBPs are secondary chemicals formed when a disinfectant, like chlorine, is added to water to remove pathogens and reacts with constituents in the source water, mostly organic matter and halides. Among the numerous DBPs identified, many are known to be toxic and/or carcinogenic. Groundwater used for drinking in the study area contains relatively high concentrations of bromide (bdl to 4.81 mg/L), iodide (0.012 to 3.78 mg/L, n = 10), and dissolved organic carbon (1.95 to 11.5 mg/L, n = 34; Figs. 10 and 11). The presence

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Table 2 Drinking water compliance with WHO and Bureau of Indian Standards drinking water guidelines. All percentages are out of 169 drinking water samples except for TDS and Alkalinity, which are out of 166, As, which is out of 107, and F, which is out of 168. Analyte

pH TDS Al (mg/L) Ba (mg/L) B (mg/L) Ca (mg/L) Cl (mg/L) Cu (mg/L) F (mg/L) Fe (mg/L) Mg (mg/L) Mn (mg/L) Nitrate (mg/L) Se (mg/L) Sb (mg/L) Ag (mg/L) Sulfate (mg/L) Alkalinity as CaCO3 (mg/L) Zn (mg/L) Cd (mg/L) Pb (mg/L) Mo (mg/L) Ni (mg/L) As (mg/L) Cr (mg/L) U (mg/L) a

Percent over WHO standard – – – 0 3 – – 0 31 – – – 41 0 0 – – – – 0 0 – 0 2 0 28

Percent over primary Indian standard 6 92 0 0 40 36 29 1 48 11 50 49 42 0 – 0 7 89 0 0 0 2 0 2 0 12a

Percent over “permissible” Indian standard – 17 0 0 17 2 1 0 32 – 5 9 – – – – 1 27 – – – – – 0 – –

Atomic Energy Regulatory Board guideline.

of these halides and DOC could promote the formation of DBPs upon chlorination of groundwater used as a drinking water source. Additionally, it has been shown that nitrogenous waste, which is reflected by the high nitrate contamination of the study groundwater (Fig. 10), can contribute to the formation of particularly toxic DBPs (Shah and Mitch, 2012). As far as the authors are aware, DBPs are not regularly monitored in chlorinated water in India, especially in rural areas. Therefore, future studies should evaluate the possible occurrence of DBPs in India's groundwater, particularly in areas where groundwater is the major drinking water source. Both natural and human factors control groundwater DOC concentrations, and bromide is mostly related to water source and evapotranspiration. Iodine was measured in a subset of groundwater samples from regions 2, 3, and 4 (n = 5, n = 28, and n = 4, respectively), and unlike bromide, iodine concentrations can only partially be explained by

evapotranspiration. Concentrations ranged greatly by aquifer lithology and within regions. Groundwater from the rhyolite aquifers of region 2 (n = 4) had the highest I concentrations, though the sample sizes were small. Iodine was significantly correlated with sodium (Spearman's ρ = 0.68), boron (Spearman's ρ = 0.71), and HCO− 3 (Spearman's ρ = 0.67) indicating that iodine is likely also affected by reverse base-exchange reactions. The important species of iodine in natural waters, iodide, iodate, and organic iodine are all known to adsorb to organic matter and a variety of solids such as clay minerals (Jia et al., 2018; Neal and Truesdale, 1976; Sheppard et al., 1995; Strickert et al., 1980; Ticknor and Cho, 1990). Adsorption of iodine generally decreases with increasing salinity, and under more oxidized conditions. Consequently, we propose that cycles of adsorption-desorption from clay minerals, combined with oxidizing conditions (Coyte et al., 2018) also control iodine content in this groundwater. We show that iodine was also highly correlated with I/Cl (Fig. 13), which suggests that I concentrations were also impacted by evapotranspiration-dilution effects, in addition to the water-rock interactions. 5. Conclusions Water quality in groundwater from several regions in Rajasthan is affected by both naturally occurring (geogenic) and man-made contamination processes. We suggest that most of the geogenic contaminants such as uranium and fluoride were mobilized from aquifer rocks during weathering processes, with their concentrations in the groundwater further increased by evapotranspiration during recharge of both meteoric water and recycled groundwater for irrigation, as well as ionexchange reactions with clay minerals in the aquifers. Though these are ordinary geologic processes, many studies have shown that human activities, such as water use for agriculture, and landscape changes can also enhance these processes (Barnes and Raymond, 2009; Burow et al., 2017; Jia et al., 2018). Combined, these processes have led to extensive contamination in Rajasthan's groundwater (i.e., 72% of drinking water samples exceeded the WHO drinking water guidelines), posing risks to human health upon consumption without treatment. While determining the actual human health impacts from consuming water with high levels of multiple contaminants is beyond the scope of this study, some studies have highlighted health issues in our study area. High rates of Chronic Kidney Disease (CKD) in India (Rajapurkar et al., 2012) could be related to uranium, and fluorosis related to fluoride intake has been described in Rajasthan (Agrawal et al., 1997; Choubisa et al., 2001). In light of the evidence for anthropogenic inputs to groundwater and reports of bacterial contamination in groundwater used for drinking in Rajasthan (Suthar et al., 2008), disinfection practices are important to protect human health from pathogens in drinking water in Rajasthan. Water disinfection is already in place for municipal water in some

Fig. 12. Boxplots of U and F− in collected drinking waters, sorted by lithology. The red dashed lines indicate WHO guidelines (provisional in the case of U) for each species. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.scitotenv.2019.06.334. References

Fig. 13. Iodine vs I/Cl. Though the data is limited, the correlation between I/Cl and iodine indicates that evaporation is one of the processes affecting iodine concentration in study groundwater.

large cities, but is rarely practiced on a smaller scale in rural areas. However, the combination of high halide concentrations, specifically Br and I, combined with high DOC reported in this study may pose higher risks for the formation of disinfection byproducts in chlorinated water, and thus health risks to the populations consuming treated (chlorinated) water. The occurrence and distribution of disinfection byproducts in treated water in Rajasthan should therefore be evaluated and monitored. While the combined trihalomethane compounds (chloroform, bromodichloromethane, dibromochloromethane, and bromoform, known as THM4) concentrations are regulated in some municipalities in India, information on their and other DBPs' are not publically available. Given the concentrations of halogens and DOC we found in many of the investigated groundwater in Rajasthan, and the evidence for wastewater contamination reported in this study, DBPs should be an important point of consideration for future water studies and water management in Rajasthan. While this study is focused on specific areas in Rajasthan, the lessons are universal; numerous groundwater resources in the developing world share similar water quality issues to those we have observed in Rajasthan. Other parts of Northern India may experience increasing water-scarcity and become more like Rajasthan in the future, particularly due to continued groundwater overexploitation and a potential decrease in precipitation related to climate change (Ashfaq et al., 2009; Asoka et al., 2017). The combination of arid climate, existing geogenic risks where aquifer lithology generates contaminants, and human activities all may lead to multiple water quality issues and exacerbate the human health risks to the population that utilize groundwater as the major drinking water source in northwest India. Acknowledgments We are thankful for the Duke India Initiative for funding this study. We thank Rich Wanty and an anonymous reviewer for their thorough and constructive review that greatly improved the quality of this study. We also thank the Ground Water Department of the Government of Rajasthan for the assistance in fieldwork and water sampling.

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