Reductive dissolution of iron-oxyhydroxides directs groundwater arsenic mobilization in the upstream of Ganges River basin, Nepal

Journal of Geochemical Exploration 148 (2015) 150–160

Contents lists available at ScienceDirect

Journal of Geochemical Exploration journal homepage: www.elsevier.com/locate/jgeoexp

Reductive dissolution of iron-oxyhydroxides directs groundwater arsenic mobilization in the upstream of Ganges River basin, Nepal Ishwar Chandra Yadav a,⁎, Ningombam Linthoingambi Devi b, Surendra Singh a a b

Centre of Advanced Study in Botany, Banaras Hindu University, Varanasi 221005, Uttar Pradesh, India Centre for Environmental Science, School of Earth, Biological and Environmental Sciences, Central University of Bihar, BIT Campus, Patna 800014, Bihar, India

a r t i c l e

i n f o

Article history: Received 24 May 2014 Accepted 2 September 2014 Available online 16 September 2014 Keywords: Sequential leaching Iron-oxyhydroxides Leachable arsenic Groundwater

a b s t r a c t Mobilization of arsenic (As) in groundwater has become a focus of attention in recent years. A few years back, groundwater As is found as a geogenic contaminant in the upper stream Ganges River basin immediately adjoining to the Himalayan foothills (the Terai Region of Nepal). The source is said to be leached due to weathering of As-bearing minerals. In this study, geologic, hydrogeologic, and geochemical data were used to characterize the As source and the geochemical process controlling As-mobilization in the groundwater aquifers of Terai Region. The findings suggest that the reductive dissolution of iron-oxyhydroxide is the main mechanism of As-mobilization in Terai aquifers. This mechanism is supported by a number of evidences including, 1) presence of arsenolite and scorodite As-bearing minerals; 2) presence of high concentration of As, Fe and low concentra− − tion of SO2− 4 and NO3 ; 3) presence of strong linkage between As and HCO3 and negative correlation between As and oxidation reduction potential; 4) presence of abundant percentage of Fe and Mn-oxides in the sediment samples; 5) presence of similar depth profile trend of As with those of Fe and Mn-oxides; 6) presence of Fe, O and Mn confirmed by energy dispersive X-ray spectroscopy analysis; and 6) presence of predominant form of As adsorbed on organic matter and Fe-oxyhydroxide phases of sequential leaching. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Groundwater is a significant source of drinking water in many parts of the world (Jousma and Roelofsen, 2004). Historically, groundwater supplies are thought to be safer in terms of pathogenic microbes than water from open dug wells and surface water bodies like rivers, streams, lakes, and ponds (World Bank, 2005). This is because of the natural filtering ability of the subsurface water and the distance a microbe would have to travel in order to reach the groundwater source. However, groundwater is more prone to chemical and other such contaminants, derived either from natural sources or anthropogenic activities (EPA, 1993). Arsenic (As) contamination in groundwater is one of several chemical contaminants. The most important route of exposure of As in human is due to drinking of As-contaminated groundwater. Today, majority of the world's chronic As-related health problems are because of consumption of As-contaminated groundwater (WHO, 2009). Groundwater extracted from majority of As-affected region showed that high concentrations of As are associated with young alluvial sediments. Elevated level of As is also characterized by high dissolved iron 2− (Fe) together with low nitrate (NO− 3 ) and sulfate (SO4 ) under reducing conditions of groundwater (Nordstrom, 2002; Smedley et al.,

⁎ Corresponding author. E-mail address: [email protected] (I.C. Yadav).

http://dx.doi.org/10.1016/j.gexplo.2014.09.002 0375-6742/© 2014 Elsevier B.V. All rights reserved.

2003). Water–rock interactions and favorable physical and geochemical conditions (by mobilization and accumulation of As bearing inorganic compounds) in groundwater are the principal reason behind substantial concentration of As. As is quite mobile at pH values of 6.5–8.5, mostly found in groundwater under both oxidizing (aerobic) and reducing (anaerobic) conditions. Naturally, As content in soil generally ranges below 10 ppm; however, it can cause major chaos once it gets into groundwater. Historically, it was believed that As is released in the soil as a result of weathering of arsenopyrite or other primary sulfide minerals (Ahuja, 2008). However, at present, it is well-known that As-contamination of groundwater is a by-product of several biogeochemical process such as oxidative dissolution of As-rich pyrite minerals (Chowdhury et al., 1999; Das et al., 1996), reductive dissolution of Fe(III) oxide/oxyhydroxides and the consequent release of sorbed and/or co-precipitated As (Chauhan et al., 2009; Dowling et al., 2002; Guo et al., 2008, 2011; Harvey et al., 2002; Larsen et al., 2007; McArthur et al., 2001; Mladenov et al., 2010; Pedersen et al., 2006; Reza et al., 2010a,b; Smedley and Kinniburgh, 2002; van Geen et al., 2006), displacement of adsorbed As by phosphate by the agricultural application of fertilizers (Acharyya et al., 1999), by carbonate (Appelo, 2002) produced through microbial metabolism (Harvey et al., 2002), or by changes in the sorptive capacity of ferric oxyhydroxides (Smedley and Kinniburgh, 2002), and anthropogenic activities and combustion of fossil fuels via precipitation from the atmosphere (Ferguson and Gavis, 1972) that metabolize and release As in

I.C. Yadav et al. / Journal of Geochemical Exploration 148 (2015) 150–160

groundwater. Because of complication in understanding of these biogeochemical processes, the identification of mechanisms responsible for a particular case of As contamination is a difficult task. For example, there has been considerable controversy over the cause of Ascontamination of groundwater in Bangladesh. Some researchers have proposed sulfide oxidation as the predominant mechanism for Asrelease (Bagla and Kaiser, 1996; Dipankar et al., 1994; Mandal et al., 1996) while others contend that the reduction of Fe-oxyhydroxides is the cause of high dissolved As-concentrations in groundwater (Bhattacharya et al., 1997; McArthur et al., 2001; Mladenov et al., 2010; Pedersen et al., 2006; Reza et al., 2010a, b; Reza et al., 2011; van Geen et al., 2006). The Terai region is placed in the bottoms of the Himalayas. It is the source of sediment for several Indian River located in North India. Terai Region inhabits about 47% of the country's population (Rahman et al., 2006). The unconfined quaternary aquifers, which are on the order of 300m thick (Khadka, 1993), are tapped by around 400000 (Panthi et al., 2006) to 800000 wells (Smedley, 2005). They supply water to 11 million people, or about 90% of the residents of Terai (Rahman et al., 2006). Elevated As level was first discovered in 1999 in the Jhapa, Morang, and Sunsari districts of eastern Terai region of Nepal, which border the Indian state of West Bengal. About 8% of the 268 groundwater samples had As N10 ppb with two samples exceeding 50 ppb (Panthi et al., 2006). Subsequent surveys of groundwater quality in Terai have revealed the presence of As in some tube wells with depths b50 m. However, most of the samples had b10 ppb of As. It is likely to note that Nawalparasi district is the most As-hit among 20 Ascontaminated Terai districts of Nepal (Gurung et al., 2005; Thakur et al., 2011; Yadav et al., 2011, 2012). Nearly, 50% of the country's rural populations inhabiting in Terai region rely on groundwater for its drinking water supply and are prone to As exposure (NASC/UNICEF, 2008; Yadav et al., 2012, 2014). The biogeochemical processes mainly responsible for As-release and its mobilization are not well understood in Terai aquifers of Nepal. Therefore, the present study aims to identify the different geochemical processes responsible for As release in groundwater of upstream Ganges river basin.

1.1. Geological setting of Terai Region Tectonically, Nepal can be classified into four different zones from north to south followed by Terai Plain bordering them on the south: a) Tibetan (Tethys) Himalayas, b) Higher Himalayas, c) Lesser Himalayas, d) Sub-Himalayas or Siwalik group and, e) Terai Plain. The Tethysian zone which lies in the northern part of western central Nepal consists of high mountain ranges crowned by cretaceous fossiliferous sediments. The higher Himalayan zone consists of crystalline rocks (schist and gneiss). Main control thrust (MCT) marks the contact of crystalline zone to that of lesser Himalayan sedimentary pile. The contact of crystalline with overlying Tethys sediments is gradational. To the south of higher Himalayas, the lesser Himalaya zones lie which consists of meta-sediments with different grades of metamorphism. The sub-Himalayan zone, which lies to south of the lesser Himalayas and separated by main boundary thrust (MBT), consists of sedimentary rocks containing silt, sand, shale and pebble and boulder beds. The pebbles and boulders are quartzite, sandstone, phyllites etc. These formations form the low lying hill ranges that rise from the Indo-Gangetic plain on the south. These hills are called Churia hills and are the Nepalese designation of Siwalik. The Terai plain lies in the southern part of Nepal and is separated from the Churia formations by the Himalayan Frontal thrust. It is the continuation of Indo-Gangetic trough. It gently slopes towards south and covered by recent and older alluvium having the thickness up to 1500 m. The recent alluvium is deposited by the rivers coming from the mountains and hills and hence the different Himalayan ranges lying north of the Terai plain are its provenance.

151

Geomorphologically, Terai plain can also be divided into two parallel zones i.e. Bhabhar zone and Gangetic alluvial plain (GAP) (Sharma, 1990). The Bhabhar zone lies along the foot of Churia hills. It is composed of boulders, cobbles, pebbles, gravels and sand which are extremely poorly sorted. GAP lies to south of the Bhabhar zone. The Gangetic alluvium underlay, interfinger with and boarder the Bhabhar zone deposits. The sediments in this part consist of silt, clay, sand, gravel and pebble beds intercalating and sometimes intermixing with each other. The sediments are generally finer towards South. 1.2. Study area Nawalparasi district, a part of Lumbini zone (one of the 75 districts of Nepal), is located in the western Terai region of Nepal. It lies 147 km. west of the capital city Kathmandu, and surrounded by Chitwan district in the east, Tanahun district in north, Palpa and Rupandehi districts in the west, and the Indian state of Uttar Pradesh in the south. The present study was conducted in three villages (ThuloKunwar, Kasia and Panchgawa) in the eastern Parasi Bazar of Nawalaparasi district (Fig. 1) where several cases of As-poisoning in groundwater have been documented (Gurung et al., 2005; Thakur et al., 2011; Yadav et al., 2011; 2012; 2014). 2. Materials and methods Geological, hydrogeological, and geochemical data are utilized to identify the source of As-contamination and the principal geochemical processes responsible for As-mobilization in the upstream of Ganges river basin. 2.1. Sample collection and analysis 2.1.1. Water samples A total of 24 groundwater samples were collected from selected tubewells (depth 8–50 m) in 2010. Groundwater samples (250 ml) were collected in meticulously cleaned polyethylene bottle (TARSON, India) for analysis. Tubewells were flushed well before the collection of sample to remove the stagnant water. Two sets of samples were collected from each location. One set of sample was collected with 1 ml of conc. HNO3 for cation and heavy metal analysis including As. The other set of sample was collected without any preservatives for the analysis of anions and other parameters. The water samples were not filtered because the residents of the areas use it for drinking unfiltered. The samples were preserved in ice box and brought to the laboratory and stored at 4 °C until analysis. The pH and oxidation reduction potential (ORP) were measured onsite using portable kits (Hanna, HI 98121 waterproof pH/ORP/ Temp) and the values are reported as the electrical potential of water sample relative to the reference electrode. Water quality parameters that were analyzed in the laboratory included major anions (SO2− 4 , − PO34 −, HCO− 3 , and NO3 ) and major cations (Ca, Mg, Na and K) along with Fe, Mn, Ni, Pb, Zn and As. SO24 −, PO34 − and NO− 3 were analyzed spectrophotometerically (Systronics Visiscan 167) while HCO− 3 titrimetrically. Na, K and Ca were analyzed by Flame photometer (Systronics 128; Compressor 126). Water samples were digested with conc. HNO3 following standard methods (APHA, 1998) prior to heavy metal analysis. Fe, Mg, Mn, Ni, Pb, and Zn contents in the groundwater samples were quantified by Flame Atomic Absorption Spectrometer (AAnalyst 800; Perkin Elmer). Total As was analyzed using Atomic Absorption Spectrometer coupled with Hydride Generator (AAS-HG). The detail about AAS-HG has been described elsewhere (Yadav et al., 2014). 2.1.2. Sediment samples Core sediment sample was collected using manual drilling percussion method, locally known as ‘Dhikuli’. The details about drilling

152

I.C. Yadav et al. / Journal of Geochemical Exploration 148 (2015) 150–160

a

b

Fig. 1. (a) Map of Nepal showing study area (b) Subsurface lithology of Thulokunwar, Kasiya and Panchgawa villages.

I.C. Yadav et al. / Journal of Geochemical Exploration 148 (2015) 150–160

method and sample collection have been described elsewhere (Gurung et al., 2005). Around 200 g of core sediments was collected at the depth of 5, 10, 15, 20, 25, 30, 35, 40 and 45 ft and packed in plastic bags. The sediment-samples were air dried in room temperature. Large stones and other objects were removed and the soil was ground to break up aggregates and crumbs. Care was taken not to break actual soil particles. The sediments were then crushed to finest powder with agate mortar and sieved through 2 mm screen prior the analysis. Major oxides such as SiO2, TiO2, Al2O3, Fe2O3, MnO, MgO, CaO, Na2O, K2O and P2O5 were analyzed by XRF spectrometer. As in the sediment sample was quantified by AAS-HG. 2.2. XRF analysis Sediment samples (~60 g) were dried in an oven at 105 °C for 24 h and crushed in a corundum jaw crusher (to 60 mesh size). The samples were powdered to less than 200 mesh size using an agate ring mill and were analyzed for selected major oxides using a XRF-1800 (Shimadzu Sequential X-ray Fluorescence Spectrometer) wavelength scanning X-ray fluorescence spectrometer. The samples were measured using rhodium Kα (Rh, X-ray source) at 40 kV and 95 mA. To measure the major oxides, lithium fluoride (LiF) was used as diffraction crystals, which specifically detects the fluorescence of each of these oxides. Analysis was done in a vacuum at a pressure of 25 Pa, scanning speed of 8°min−1, in steps of 0.1°, and 0.75 s per step. The loss on ignition (LOI) was measured by weighing calcinations at 950 °C before and after 30 min. 2.3. XRD analysis Sediment samples containing high As were ground to fine powder using agate mortar and X-ray diffraction (XRD) of the powder was performed using a RIGAKU XRD instrument using CuKα, to identify the As-bearing minerals and other minerals. The sediments were broken and hydrous mineral–apatite-rich portions were hand-picked and powdered using agate mortar. XRD of powder was conducted at 30 kV (λ = 1.541836 Å) and 15 mA and 1.25° beam slit with MiniFlex2 counter detector. The XRD patterns for selected sample were recorded by continuous scan from 5.00 to 90.00°. The instrument was calibrated with Si standard before and after every analysis. The As-bearing minerals as well as other minerals were identified using graphic software PCPDFWIN 1.30 (JCPDS, 1997) search-match program and the International Centre for Diffraction Data (ICDD) database. 2.4. Micro-chemical analysis Transmission Electron Microscopic coupled with energy dispersive X-ray spectroscopy (TEM-EDAX) attachment can help to identify any As enriched particles together with possible combinations of other elements. Physical characterization was performed by TEM-EDAX (Model FEI, Technai G2 20) to obtain data on As association with other elements present in core sediments sample. 2.5. Sequential extraction analysis Modified five steps Community Bureau of Reference (BCR) sequential extraction procedures as proposed by Anawar et al. (2003) and Stollenwerk et al. (2007) have also been applied for the determination of distribution of As in core sediment samples. Step by step sequential extraction technique for chemical leaching of As to separate major and operationally defined As-bearing solid phases consists following steps. 2.5.1. Exchangeable One gram of dry sediment was treated with 20 ml 0.05 M CaCl2 at room temperature for 30 min to remove exchangeable As and the extract was centrifuged at 10,000 rpm for 60 min. The resulting

153

supernatants were filtered using Millipore filter (0.45 μm) and were analyzed for As using HG-AAS (AAnalyst 800; Perkin Elmer). 2.5.2. As-bound to Mn-oxyhydroxide The sediment residue recovered from first step was extracted with 20 ml 0.1 M NH2OH·HCl in 0.01 M HNO3 in 50 ml centrifuge tubes for 30 min. The liquid–solid separation was done by centrifugation at 10,000 rpm for 60 min. This step extracted As from Mn oxides and carbonate phases. The resulting supernatants were filtered using Millipore filter (0.45 μm) and were analyzed for As using HG-AAS (AAnalyst 800; Perkin Elmer). 2.5.3. As-bound to Fe-oxyhydroxide In this step, the sediment residue (recovered after step B) was further extracted with 20 ml 0.25 M NH2OH·HCl in 0.25 M HCl in 50 ml centrifuge tubes at 50 °C for 30 min. The extractant was then centrifuged at 10,000 rpm for 60 min and the s resulting supernatants were filtered using Millipore filter (0.45 μm). The concentration of As in the supernatant was analyzed using HG-AAS (AAnalyst 800; Perkin Elmer). The stronger HCl solution (applied in this step) could extract As from poorly crystalline Fe oxides. 2.5.4. As-bound to organic matter The sediment residue recovered from step C was rinsed with de-ionized distilled water and was then treated with 20 ml 0.1 M sodium pyrophosphate and 1 M ammonium acetate for 2 h to extract As from the organic fraction. The extract was centrifuged at 10,000 rpm, the supernatant was filtered using 0.45 μm Millipore filters and As was determined using HG-AAS (AAnalyst 800; Perkin Elmer). 2.5.5. As bound to sulfide and silicates The remaining sediment residue recovered after performing all the steps from A to D was rinsed and the sulfide phase was dissolved using a mixture of 10 ml of 11.65 N HCl and 0.5 g potassium chlorate. Liquid–solid separation was done by centrifugation at 10,000 rpm for 60 min. Supernatants were filtered using 0.45 μm Millipore filters and As was analyzed using HG-AAS (AAnalyst 800; Perkin Elmer). 3. Results and discussion 3.1. Groundwater chemistry Descriptive characteristics of the groundwater of Nawalparasi are presented in Table 1. Groundwater analysis results showed that groundwater in Terai Region is slightly alkaline. The pH of the water sample ranged from 7.2 to 7.9 and is well within the limit of Nepalese drinking water quality standards (NDWQS) (Government of Nepal, 2005). Total

Table 1 Descriptive statistics of groundwater parameters. Variables

Mean

Standard deviation

Minimum

Maximum

pH ORP Ca2+ Mg2+ Na+ K+ SO2− 4 PO3− 4 HCO− 3 NO− 3 Ni Mn Pb Fe Zn As

7.48 −81.54 71.37 16.68 51.36 3.72 0.19 0.016 551.0 0.257 0.066 0.11 0.122 3.639 0.097 516.5

0.19 46.95 20.0 7.0 18.5 8.57 0.23 0.006 117.4 0.133 0.01 0.16 0.024 0.168 0.073 282.0

7.17 −129 38.8 5.20 7.36 0.90 0.06 0.01 402 0.14 0.05 0.00 0.08 3.41 0.04 155

7.87 97.0 132.2 37.00 93.49 43.0 1.13 0.032 920 0.575 0.089 0.729 0.160 4.14 0.38 1338

154

I.C. Yadav et al. / Journal of Geochemical Exploration 148 (2015) 150–160

As content in all the tested tubewells was many fold higher than both WHO (0.01 ppm) and NIS (0.05 ppm) limits of drinking water. The concentration of As in the water sample ranged as low as 0.16 ppm in Kasiya village to as much as 1.34 ppm in Thulokunwar village. Majority of the samples showed negative ORP and ranged from − 129 mV to 97 mV. Negative ORP observed in the groundwater of Terai Region suggests progressive decrease in redox potentials of groundwater (Table 1). The concentration of As-gets readily mobilized under anaerobic condition of groundwater (Chauhan et al., 2009; Kim et al., 2009; − Smedley and Kinniburgh, 2002). The concentrations of SO2− 4 , NO3 and 3− 2− − PO4 were quite low. Low SO4 and NO3 in groundwater are the indication of sulfate reduction and nitrate reduction in groundwater, respectively. Bicarbonate (HCO− 3 ) represents major source of alkalinity in groundwater. High level of HCO− 3 was detected in water sample and it ranged between 402 ppm to 920 ppm. High HCO− 3 and low NO− 3 in water samples indicate the reducing nature of groundwater aquifer (Kinniburgh and Smedley, 2001; Nickson et al., 1998; Sahu 2− et al., 2000). Likewise, high HCO− observed in Terai 3 and low SO4 water samples invoke the possibility of sulfate reduction in groundwater (Sahu et al., 2000). Redox-sensitive elements such as Fe and Mn whose concentration is dependent on redox conditions, were detected high in groundwater. Total Fe and Mn were observed high, (ranged from 3.41 to 4.14 ppm and ND to 0.729 ppm, respectively) in groundwater. Higher concentration of Fe and Mn together with predominance of As in the groundwater of Terai Region also indicates the reducing nature of groundwater aquifer. It is imperative to note that high content of Fe(II) together with elevated level of As in groundwater is indicative of reductive dissolution of Fe(III) oxyhydroxide mechanism. During reductive dissolution mechanism, As get adsorbed onto Fe(III)-oxyhydroxides and release under reduction condition (Islam et al., 2004; Nickson et al., 1998). Pb contents in the water samples ranged between 0.08 to 0.16 ppm which is higher than the WHO limit (0.01 ppm) in drinking water. Also, the concentration of Ni was found above WHO limit (0.07 ppm) in the majority of water samples. Very less concentration of Zn was observed in water sample and was much below NDWQS limit (3.0 ppm) in drinking water. 3.2. Interrelationship The interrelationship of As with hydro-geochemical characteristics of groundwater can also be used to explain As-mobilization mechanism. The concentration of total As in groundwater was positively correlated with Fe (Fig. 2a, r = 0.551, p b 0.05) and HCO− 3 (Fig. 2b, r = 0.458, p b 0.05). Moreover, SO2− 4 was weakly and positively correlated with As (Fig. 2c, r = 0.212, p b 0.05). Strong and positive linkage between As and HCO3 − in Terai groundwater is an indication of reducing environmental condition (Ahmed et al., 2004). Likewise, poorly correlated As, Fe and SO2− in the present study area suggest the absence of 4 pyrite/sulfide oxidation mechanism responsible for As release (Das et al., 1996; Nickson et al., 2000). On contrary, As was moderately but negatively correlated with ORP (Fig. 2d, r = − 0.528, p b 0.001). This means that groundwater with low ORP exhibits high level of As, while As content gets depressed at high ORP. It is imperative to see in Fig. 2h that the maximum concentration of As was dominant at depth of 20 m in Terai groundwater (Fig. 2h). 3.3. As-concentration vs depth of water table As content in groundwater also depends on fluctuation in depth of water table (DWT) and flooding cycles. It is clear from Fig. 3 that the concentration of As varied significantly with the change in DWT in groundwater. This is because an increase in water table would bring the groundwater table closer to the land surface thereby mixing of the groundwater with the agrochemical wastes and other wastes at the surface or near the surface of water. The rise in water table will also cause

dissolution of Fe-oxyhydroxide as flood plain sediment gets buried; hence, favoring release of As under reducing environmental conditions (Bhattacharya et al., 2001; Nickson et al., 1998, 2000). Similarly, when water table goes down in dry season and the local rivers which were mostly fed by the groundwater get dried up, resulting in leaching of As bearing minerals into the groundwater brought by river deposition. This is consistent with earlier findings of Berg et al. (2001) and Tong (2002). Further, on lowering of water table As-rich minerals (pyrite and arsenopyrite) get oxidized because of the invasion of atmospheric oxygen (Das et al., 1996; Mallick and Rajagopal, 1996) and release As in groundwater. If this was true in the case of Terai groundwater, the oxidation of As rich minerals would have also shown changes in pH and 2− HCO− between dry season and wet season. 3 , Fe, Mn, As and SO4 Instead, we found that these constituents remained fairly constant with respect to season (Yadav et al., 2014). Hence, this confirms that oxidative mechanism is not responsible for the elevated level of As in the groundwater of Nawalparasi. Rather, the dissolution of Fe-oxyhydroxide mechanism under reducing environmental condition directly releases As.

3.4. Mineralogy and geochemical composition of sediments Metal oxides such as iron oxides, aluminum oxides and manganese oxides are the major minerals that bind As (arsenite and arsenate) in the sediments. The interactions of As with iron oxides provide the best insight to understand the likely behavior of As-mineral interactions in aquifers. Major oxides of trace metals (SiO2, TiO2, Al2O3, Fe2O3, MnO, MgO, Na2O, K2O and P2O5) analyzed in core sediments samples by XRF analysis are presented in Table 2. The concentration of As detected in core sediment samples was found to be unevenly distributed. Its concentration varied from 0.22 to 0.64 ppm (mean 0.36 ppm) in sediment samples. Comparatively, higher concentration of As was observed mostly in the fine-grained clay sediments (black and yellow) than in coarse-grained sediments. Moreover, the highest Asconcentration was detected in clayey soils at the depth of about 11 m. Vertical distribution of Fe followed similar distribution pattern as that of As (Yadav, 2012), showing its higher and lower concentration in clay and fine-sand, respectively (Yadav, 2012). The % concentration of SiO2 was very high in clay-sediments (64.1%) followed by Al2O3 (13.44%). Likewise, CaO, K2O and MgO were also observed in varying proportions based on nature of lithotypes. Their % contribution was more abundant in clay compared to fine sediments. High concentrations of Al2O3 in the present study area may possibly due to the presence of plagioclase, mica and other clay-bearing minerals, especially chlorite and kaolinite in greater amount. Abundant percentage of CaO (wt % 6 to 13) in the present sediments could be due to presence of shell in greater amount and also due association with feldspar-minerals. Very high percentage of Fe2O3 (4.76%) was detected in the core sediment of Terai Region. This indicates the possible role of iron oxide in reductive dissolution theory proposed for As-release (McArthur et al., 2001; Nickson et al., 2000). The average % weight of Fe2O3 (4.76%) obtained from sediment samples at present site was quite close to the value (4.5% by wt.) found in upper continental crust (Taylor and McLennan, 1985). High Fe was found in fine-grained sediments compared to sand. At present site, high As-yielding aquifers also contained higher percentage of calcium, silica, aluminum, and iron. The dissolution of the calcium-related minerals may raise the pH locally, making the environment more alkaline. Alkaline conditions are favorable for desorption of As from As-bearing oxides (Smedley and Kinniburg, 2002; Williams et al., 2004) and also from organic matter (Torres and Ishiga, 2003). MnO2 was found generally low (0.02–0.11% by wt.) in sediments of Terai Region. Percentage concentration of P2O5 was found low in upper clayey-sediments and gradually increased with an increase in depth.

I.C. Yadav et al. / Journal of Geochemical Exploration 148 (2015) 150–160

155

DTW

Arsenic

1.5 1 0.5

0 Th 1 02 Th 0 Th 3 0 Th 4 0 Th 5 0 Th 6 0 Th 7 08 K 01 K 02 K 03 K 04 K 05 K 06 K 07 K 08 P0 1 P0 2 P0 3 P0 4 P0 5 P0 6 P0 7 P0 8

0

Sampling locations Fig. 3. Variation in As concentration with DWT.

As (ppm)

7 6 5 4 3 2 1 0 Th

DWT (m)

Fig. 2. Relationship of As with others geochemical parameters of groundwater.

156

I.C. Yadav et al. / Journal of Geochemical Exploration 148 (2015) 150–160

Table 2 Concentration of major oxides (wt %) and As (ppm) in core sample. Sample ID

KC1 KC2 KC3 KC4 KC5 KC6 KC7 KC8 KC9

Depth (m)

1.52 3.05 4.57 6.07 7.62 9.11 10.67 12.19 13.72

Major oxides (weight %)

As (ppm)

SiO2

TiO2

Al2O3

Fe2O3

MnO

MgO

CaO

Na2O

K2O

P2O5

79.40 77.85 80.21 55.56 56.63 52.74 61.80 59.21 53.59

0.59 0.58 0.58 0.77 0.75 0.72 0.62 0.65 0.42

9.55 10.29 9.37 20.23 17.39 17.15 11.05 14.71 11.24

3.41 3.58 3.19 5.75 6.91 6.60 5.13 4.93 3.35

0.04 0.08 0.06 0.02 0.09 0.11 0.10 0.04 0.07

0.82 0.88 0.76 2.19 2.16 2.99 2.48 2.02 2.36

0.23 0.27 0.22 0.72 2.99 4.30 5.95 5.23 12.67

0.51 0.45 0.47 0.36 0.39 0.34 0.54 1.30 1.28

2.21 2.37 2.17 4.18 3.90 3.94 2.71 2.97 2.41

0.03 0.05 0.06 0.08 0.13 0.11 0.11 0.15 0.10

0.27 0.34 0.22 0.32 0.26 0.39 0.64 0.31 0.48

KC: Kasiya village.

3.5. Depth profile relationship of As with Fe, Mn and Al oxides

3.6. Characterization of As-bearing minerals

The comparison of the depth profile of As with Fe and Mn-oxides in the core sediment samples showed similar distribution pattern (Fig. 4). Similar depth-profile-trends of As with Fe and Mn-oxide suggest strong association between As and the oxides of Fe and Mn. The distribution profile of Al2O3 showed similar distribution trend with As up to 5 m depth.

The XRD analysis of core sediment confirms the presence of major minerals such as quartz, illite, chlorite and feldspar. XRD analysis also identified the presence of of As-bearing minerals (arsenolite As2O3), and scorodite FeAsO4·2H2O) in a very small amount in bulk samples. The constituents of arsenolite and scorodite minerals (Fe, O and As)

Fig. 4. Depth profile relationship of As with Al2O3, Fe2O3 and MnO in fractions of core samples (solid lines indicate — As and broken lines — Al2O3, Fe2O3 and MnO).

I.C. Yadav et al. / Journal of Geochemical Exploration 148 (2015) 150–160

157

were also of confirmed by EDAX analysis (Fig. 5). EDAX spectrum (Fig. 5) showed that a distinct As peak with 0.20 wt.% and Mn, Fe and O with 0.80 wt.%, 22.20 wt.% and 43.90 wt.%, respectively was present in the core sediment. The relative abundance (wt.%) of the minerals detected in core sediment samples can be arranged in a decreasing order as: quartz N illite N chlorite N feldspar N calcite N kaolinite N dolomite. Clay minerals relatively contained more As than the sandy sediments.

Table 3 Descriptive statistics of leachable As solid phase.

3.7. Sequential extraction

SD: standard deviation.

Five different forms of As (exchangeable phase, bound to Mn-oxide phase, bound to Fe-oxide phase, organic matter phase and sulfate/silicate phase), chemically extracted during five steps BCR sequential leaching from the core sediments are presented in Table 3.

gives Fe and SO 2− in 1:2 mole ratios (Anawar et al., 2003). If 4 As were released from the oxidative-dissolution mechanism of pyrite/arsenopyrite, then there would have been positive correlation between As, Fe and SO24 −. Also, SO24 − concentration in groundwater would have shown high value. Instead, we found, very less concentration of SO2− in Terai groundwater. As exhibited in regression analysis, 4 weak interrelationship between As, Fe and SO2− suggests the absence 4 of pyrite/arsenopyrite oxidation mechanism in present site. Further, if pyrite would have been oxidized, it's As would have been sorbed onto the resulting Fe-oxyhydroxide rather than getting released in the groundwater. The high concentration of As in groundwater is itself one of the characteristics of reducing environmental conditions. Under strongly reducing condition, the mobility of As can be related to dissolution of Fe and Mn-oxides. This finding suggests that As is mobilized in Terai groundwater through reductive dissolution mechanism of Fe and Mn-hydroxide only, and not due to oxidation mechanism.

3.8. Role of geochemistry in As-mobility Sequential extraction analysis result showed that sulfide/silicate phase, organic matter phase and Fe-oxyhydroxide phase were the major leachable forms of As in groundwater of Terai Region (Table 3). The average concentration of As ranged from 119.8 to 364 ppb (mean 243.3 ppb), 80.8 to 190 ppb (mean 139.5 ppb) and 80.4 to 328 ppb (mean 133.6 ppb) in sulfide/silicate phase, organic matter phase and Fe-oxyhydroxide phase, respectively (Table 3). The maximum value of extractable As was found at 11 m depth. The sequential leaching data suggest that predominant form of As is more likely to be adsorbed on Fe-oxyhydroxide phase, sulfate/silicate phase and organic matter phase; which may get mobilized under 2− reducing-conditions. Low concentrations of NO− together 3 and SO4 high Fe, as found in the geochemical analysis, also indicate reducing conditions, being prevalent in Terai groundwater. In sequential extraction techniques, chemical leaching by potassium chlorate and HCl releases As from sulfide and silicate phases. However, As exists as a non -leachable part of sulfide/silicate phase in soils i.e. immobile elemental fraction inseparably attached to silicate matrix of soil. As content in oxide phases (both Fe and Mn) increased down to the core, while it gets decreased down to the core in organic phase (Fig. 6). As-rich pyrite and arsenopyrite minerals are also susceptible to oxidative-dissolution in the presence of dissolved oxygen. The mobilization of As in the groundwater of many parts of the world including Bengal delta plain (BDP) showed strong linked with oxidativedissolution mechanism of pyrite minerals (Raessler et al., 2000; Schreiber et al., 2000). However, it has been reported that pyrite is rarely found in sediment-samples of Ganges-delta-plain (Das et al., 1996; Nickson et al., 2000). Stoichiometry of the pyrite oxidation-reaction

Phases of As

Mean

SD

Min.

Max.

Exchangeable Bound to Mn-oxyhydroxide Bound to Fe-oxyhydroxide Organic matter Sulfide/silicate

118.0 103.0 133.6 139.5 243.3

45.1 47.9 75.9 37.7 73.4

55.2 30.2 80.4 80.8 119.8

184.4 171.6 328.0 190.0 364.0

3.9. Microbial metabolism of organic matters Arsenic in groundwater can also get mobilized by microbial population present in near surface groundwater. The microbes can catalytically decompose organic compound and where As-rich Fe-oxyhydroxide gets dissolved and releases Fe and HCO− 3 under reducing condition (Kar et al., 2010; Liaoa et al., 2011; Nickson et al., 1998; Yan et al., 2000). This dissolution reaction in turn controls the mobility and geochemistry of As in groundwater. The leachable As content was high in organic matter phase next to sulfide/silicate phase as observed in sequential leaching analysis. This indicates the role of microbial population and organic matter in mobility of As under reducing condition. Moreover, the microbial oxidation of organic matter consumes dissolved oxygen present in the groundwater resulting in the formation of HCO− 3 . Thus, the mineralization of relatively small amount of organic matter (through microbial route) in the sediments can result in significant increase in As concentration in groundwater. Hence, it is clear that the reductive dissolution of Fe and Mn-oxyhydroxide by microbial

Fig. 5. TEM image and EDAX spectrum of finest core sediment sample.

158

I.C. Yadav et al. / Journal of Geochemical Exploration 148 (2015) 150–160

Fig. 6. Vertical distribution of As in different phases of sediments.

oxidation of organic matters controls As mobilization in groundwater of Terai Region.

of Fe2O3 in the core sediment sample confirms reductive dissolution mechanism for As release. Acknowledgment

3.10. Sediments characteristics and As-content The distribution of grain size of the sediments in groundwater may also play a vital role in the mobility of As. It is evident from XRF analysis that high As concentration was mostly associated with fine-grained clay minerals. This is because the fine grain-size fractions have larger surface area and thus, adsorb the major part of As on their surface. Since, Fe, Mn and Al oxides and hydroxides are the major components of fine grained particles and thought to retain high As under specific pH conditions (Pierce and Moore, 1982), their abundant percentage in Terai groundwater also indicates toward reductive dissolution mechanism for As release.

4. Conclusions Based on geological, mineralogical, hydrogeological and geochemical data, we conclude that the reductive dissolution of Fe-oxyhydroxide is the main mechanism of As-mobilization in the groundwater of Terai Region. The groundwater aquifer in the study area is a distinctive example of reductive dissolution mechanism. Data from BCR sequential leaching also supports the above contention for As-mobilization in Terai groundwater. Moreover, the presence of As-bearing minerals such as arsenolite and scorodite, together with abundant percentage

IC Yadav thanks to Indian Council for Cultural Relation (ICCR), Ministry of External Affairs, Government of India and Ministry of Foreign Affairs, Government of Nepal (F.N.8-2/09 -10/SAARC/ISD-II) for financial assistance in the form of SAARC Fellowship. References Acharyya, S.K., Chakraborty, P., Lahiri, S., Raymahashay, B.C., Guha, S., Bhowmik, A., Chowdhury, T.R., Basu, G.K., Mandal, B.K., Biswas, B.K., Samanta, G., Chowdhury, K., Chanda, C.R., Lodh, D., Roy, S.L., Saha, K.C., Roy, S., Kabir, S., Quamruzzaman, Q., Chakraborti, D., McArthur, J.M., 1999. Brief communications: arsenic poisoning in the Ganges delta. The natural contamination of drinking water by arsenic needs to be urgently addressed. Nature 401, 545. Ahmed, K.M., Bhattacharya, P., Hasan, M.A., Akhter, S.H., Alam, S.M.M., Bhuyian, M.A.H., Imam, M.B., Khan, A.A., Sracek, O., 2004. Arsenic enrichment in groundwater of the alluvial aquifers in Bangladesh: an overview. Appl. Geochem 19, 181–200. Ahuja, S., 2008. The problem of arsenic contamination of groundwater. In: Ahuja, S. (Ed.), Arsenic Contamination of Groundwater: Mechanism, Analysis and Remediation. John Wiley & Sons, Inc. Hoboken, New Jersey, pp. 1–21. Anawar, H.M., Akai, J., Komaki, K., Terao, H., Yoshioka, T., Ishizuka, T., Safiullah, S., Kato, K., 2003. Geochemical occurrence of arsenic in groundwater of Bangladesh: sources and mobilization processes. J. Geochem. Explor. 77, 109–131. APHA, 1998. Standard methods for the examination of water and wastewater, American Public Health Association, American Water Works Association, Water Pollution Control Federation: Washington DC20th ed. Appelo, C.A.J., Van der Weiden, M.J.J., Tournassat, C., Charlet, L.L., 2002. Surface complexation of ferrous iron and carbonate on ferrihydrite and the mobilization of arsenic. Environ. Sci. Technol. 36, 3096–3103.

I.C. Yadav et al. / Journal of Geochemical Exploration 148 (2015) 150–160 Bagla, P., Kaiser, J., 1996. Epidemiology -India's spreading health crisis draws global arsenic experts. Science 274, 174–175. Bank, World, 2005. Towards more effective operational response: arsenic contamination of groundwater in South and East Asian countries. Technical Report, water and sanitation program, South Asia Region. Volume II. Berg, M., Tran, H.C., Nguyen, T.C., Pham, H.V., Schertenleib, R., Giger, W., 2001. Arsenic contamination of groundwater and drinking water in Vietnam: a human health threat. Environ. Sci. Technol. 35, 2621–2626. Bhattacharya, P., Chatterjee, D., Jacks, G., 1997. Occurrence of arsenic contaminated groundwater in alluvial aquifers from Delta Plains, Eastern India: options for safe drinking water supply. Int. J. Water Resour. Manag. 13, 79–92. Bhattacharya, P., Jacks, G., Jana, J., Sracek, A., Gustafsson, J.P., Chatterjee, D., 2001. Geochemistry of the Holocene alluvial sediments of Bengal Delta Plain from West Bengal, India: implications on arsenic contamination in groundwater. In: Jacks, G., Bhattacharya, P., Khan, A.A. (Eds.), Groundwater Arsenic Contamination in the Bengal Delta Plain of Bangladesh. KTH Special Publication, TRITA-AMI Report 3084, pp. 21–40. Chauhan, V.S., Nickson, R.T., Chauhan, D., Iyengar, L., Sankararamakrishnan, N., 2009. Groundwater geochemistry of Ballia district Uttar Pradesh, India and mechanism of arsenic release. Chemosphere 75, 83–91. Chowdhury, T.R., Basu, G.K., Mandal, B.K., Biswas, B.K., Samanta, G., Chowdhury, U.K., 1999. Arsenic poisoning in the Ganges Delta. Nature 401, 545–546. Das, D., Samanta, G., Mandal, B.K., 1996. Arsenic in groundwater in six districts of West Bengal, India. Environ. Geochem. Health 18, 5–15. Dipankar, D., Chatterjee, A., Samanta, G., Mandal, B., Chowdhury, T.R., Samanta, G., Chowdhury, P.P., Chanda, C.R., Gautam, B., Dilip, L., Swarup, N., Chakroborty, T., Mandal, S., Bhattacharya, S.M., 1994. Arsenic contamination in groundwater in six districts of West Bengal, India: the biggest arsenic calamity in the world. Analyst 119, 168–170. Dowling, C.B., Poreda, R.J., Basu, A.R., Peters, S.L., 2002. Geochemical study of arsenic release mechanisms in the Bengal Basin groundwater. Water Resour. Res. 38, 12–18. EPA, 1993. Wellhead protection: a guide for small communities. Seminar publication, US Environmental Protection Agency, Chapter 3, EPA/625/R-93/002. Ferguson, J.F., Gavis, J., 1972. A review of the arsenic cycle in natural waters. Water Res. 6, 1259–1274. Government of Nepal, 2005. National Drinking Water Quality Standards 2062 and National Drinking Water Quality Standard Implementation Guideline 2062 Singh Durbar. Ministry of Land Reform Management, Kathmandu, Nepal. Guo, H., Yang, S.Z., Tang, X.H., Li, Y., Shen, Z.L., 2008. Groundwater geochemistry and its implications for arsenic mobilization in shallow aquifers of the Hetao basin, Inner Mongolia. Sci. Total Environ. 393, 131–144. Guo, H., Zhang, B., Li, Y., Berner, Z., Tang, X., Norra, S., Stuben, D., 2011. Hydrogeological and biogeochemical constrains of arsenic mobilization in shallow aquifers from the Hetao basin, Inner Mongolia. Environ. Pollut. 159, 876–883. Gurung, J.K., Hiroaki, I., Khadka, M.S., 2005. Geological and geochemical examination of arsenic contamination in groundwater in the Holocene Terai Basin, Nepal. Environ. Geol. 49, 98–113. Harvey, C.F., Swartz, C.H., Badruzzaman, A.B.M., Keon-Blute, N., Yu, W., Ashraf, A.M., 2002. Arsenic mobility and groundwater extraction in Bangladeshi aquifer. Science 298, 1602–1606. Islam, F.S., Gault, A.G., Boothman, C., Polya, D.A., Charnock, J.M., Chatterjee, D., Lloyd, J.R., 2004. Role of metal reducing bacteria in arsenic release from Bengal delta sediments. Nature 430, 68–71. JCPDS, 1997. Powder diffraction files (International Center for Diffraction Data). PCPDFWIN. , (1, 3.). Jousma, G., Roelofsen, F.J., 2004. World-wide Inventory on Groundwater Monitoring, IGRAC, Utrecht 2004available at http://www.igrac.nl/dynamics/modules/SFIL0100/ view.php?filId=56. Kar, S., Maity, J.P., Jean, J.S., Liu, C.C., Nath, B., Yang, H.J., Bundschuh, J., 2010. Arsenicenriched aquifers: occurrences and mobilization of arsenic in groundwater of Ganges Delta Plain, Barasat, West Bengal, India. Appl. Geochem. 25, 1805–1814. Khadka, M.S., 1993. The groundwater quality situation in alluvial aquifers of the Kathmandu valley, Nepal. AGSO J. Aust. Geol. Geophys. 14, 207–211. Kim, K., Moon, J.T., Kim, S.H., Ko, K.S., 2009. Importance of surface geologic condition in regulating As concentration of groundwater in the alluvial plain. Chemosphere 77, 478–484. Kinniburgh, D.G., Smedley, P.L., 2001. Arsenic contamination of groundwater in Bangladesh. Summary. BGS Technical Report WC/00/19 Volume 1. British Geological Survey & Department of Public Health Engineering, p. 15. Larsen, P.D., Hue, N.T.M., Duc, M.T., Viet, P.H., Nhan, Q., Jessen, S., 2007. Arsenic in groundwater of the Red River floodplain, Vietnam: controlling geochemical processes and reactive transport modeling. Geochim. Cosmochim. Acta 71, 5054–5071. Liaoa, V.H.C., Chua, Y.J., Sua, Y.C., Lina, P.C., Hwangb, Y.H., Liua, C.W., Liaoa, C.M., Changa, F.J., Yua, C.W., 2011. Assessing the mechanisms controlling the mobilization of arsenic in the arsenic contaminated shallow alluvial aquifer in the black foot disease endemic area. J. Hazard. Mater. 197, 397–403. Mallick, S., Rajagopal, N.R., 1996. Groundwater development in the arsenic-affected alluvial belt of West Bengal —some questions. Curr. Sci. 70, 956–958. Mandal, B.K., Chowdhury, T.R., Samanta, G., Basu, G.K., Chowdhury, P.P., Chanda, C.R., Lodh, D., Karan, N.K., Dhar, R.K., Tamili, D.K., Das, D., Saha, K.C., Chakraborti, D., 1996. Arsenic in groundwater in seven districts of West Bengal, India—the biggest arsenic calamity in the world. Curr. Sci. 70, 976–986. McArthur, J.M., Ravenscroft, P., Safiulla, S., Thirwall, M.F., 2001. Arsenic in groundwater: testing pollution mechanisms for sedimentary aquifers in Bangladesh. Water Resour. Res. 37, 109–117.

159

Mladenov, N., Zheng, Y., Miller, M.P., Nemergut, D.R., Legg, T., Simone, B., Hageman, C., Rahman, M.M., Ahmed, K.M., McKnight, D.M., 2010. Dissolved organic matter sources and consequences for iron and arsenic mobilization in Bangladesh aquifers. Environ. Sci. Technol. 44, 123–128. NASC/UNICEF, 2008. Situation of Arsenic in Nepal. Government of Nepal, National Arsenic Steering Committee, Maharajgung: Kathmandu, Nepal. Nickson, R.T., McArthur, J.M., Burgess, W.G., Ahmed, K.M., Ravenscroft, P., Rahman, M., 1998. Arsenic poisoning of Bangladesh groundwater. Nature 395, 338. Nickson, R., McArthur, J.M., Ravenscroft, P., Burgess, W.G., Ahmed, K.M., 2000. Mechanism of arsenic release to groundwater, Bangladesh and West Bengal. Appl. Geochem. 15, 403–413. Nordstrom, D.K., 2002. Worldwide occurrences of arsenic in ground water. Science 296, 2143–2145. Panthi, S.R., Sharma, S., Mishra, A.K., 2006. Recent status of arsenic contamination in groundwater of Nepal—a review. Kathmandu Univ. J. Sci. Eng. Technol. 2, 1–11. Pedersen, H.D., Postma, D., Jakobsen, R., 2006. Release of arsenic associated with the reduction and transformation of iron oxides. Geochim. Cosmochim. Acta 70, 4116–4129. Pierce, M.L., Moore, C.B., 1982. Adsorption of arsenite and arsenate on amorphous iron hydroxide. Water Res. 16, 1247–1253. Raessler, M., Michalke, B., Schulte-Hostede, S., Kettrup, A., 2000. Long-term monitoring of arsenic and selenium species in contaminated groundwater by HPLC and HG-AAS. Sci. Total Environ. 258, 171–181. Rahman, M., Vahter, M., Wahed, M.A., Sohel, N., Yunus, M., Streatfield, P.K., 2006. Prevalence of arsenic exposure and skin lesions, a population-based survey in Matlab, Bangladesh. J. Epidemiol. Comm. Health 60, 242–248. Reza, A.H.M.S., Jean, J.S., Lee, M.K., Yang, H.J., Liu, C.C., 2010a. Arsenic enrichment and mobilization in the Holocene alluvial aquifers of the Chapai–Nawabganj district, Bangladesh: a geochemical and statistical study. Appl. Geochem. 25, 1280–1289. Reza, A.H.M.S., Jean, J.S., Yang, H.J., Lee, M.K., Woodall, B., Liu, C.C., Lee, J.F., Luo, S.D., 2010b. Occurrence of arsenic in core sediments and groundwater in the Chapai–Nawabganj District, northwestern Bangladesh. Water Res. 44, 2021–2037. Reza, A.H.M.S., Jean, J.S., Yang, H.J., Lee, M.K., Hsu, H.F., Liu, C.C., Lee, Y.C., Bundschuh, J., Lin, K.H., Lee, C.Y., 2011. A comparative study on arsenic and humic substances in alluvial aquifers of Bengal delta plain (NW Bangladesh), Chianan plain (SW Taiwan) and Lanyang plain (NE Taiwan): implication of arsenic mobilization mechanisms. Environ. Geochem. Health 33, 235–258. Sahu, S.J., Roy, S., Jana, J., Bhattacharya, R., Chatterjee, D., Dey, S.S., 2000. Evidence of iron in arsenic mobilization groundwater of Bengal Delta Plain. Annual International Conference on heavy metals in the environment, Contribution # 1024 University of Michigan. School of Public Health, Ann Arbor, Mich (CD-ROM). Schreiber, M.E., Simo, J.A., Freiberg, P.G., 2000. Stratigraphic and geochemical controls on naturally occurring arsenic in groundwater, eastern Wisconsin, USA. Hydrogeol. J. 8, 161–176. Sharma, C.K., 1990. Geology of Nepal Himalayas and adjacent Countries. Sangeeta Sharma, Kathmandu (p 479). Smedley, P.L., 2005. Arsenic occurrence in groundwater in South and East Asia—scale, causes and mitigation. Towards a More Effective Operational Response: Arsenic Contamination of Groundwater in South and East Asian Countries, Volume II Technical Report, World Bank Report No. 31303. Smedley, P.L., Kinniburgh, D.G., 2002. A review of the source, behavior and distribution of arsenic in natural waters. Appl. Geochem. 17, 517–568. Smedley, P.L., Zhang, M., Zhang, G., Luo, Z., 2003. Mobilisation of arsenic and other trace elements in fluviolacustrine aquifers of the Huhhot Basin, Inner Mongolia. Appl. Geochem. 18, 1453–1477. Stollenwerk, K.G., Breit, G.N., Welch, A.H., Yount, J.C., Whitney, J.W., Foster, A.L., Uddin, M.N., Majumder, R.K., Ahmed, N., 2007. Arsenic attenuation by oxidized aquifer sediments in Bangladesh. Sci. Total Environ. 379, 133–150. Taylor, S.R., McLennan, S.M., 1985. The Continental Crust: Its Composition and Evolution. Blackwell, Oxford. Thakur, J.K., Thakur, R.K., Ramnathan, A.L., Kumar, M., Singh, S.K., 2011. Arsenic contamination of groundwater in Nepal—an overview. Water 3, 1–20. Tong, N.T., 2002. Arsenic pollution in groundwater in the Red River Delta. Geological Survey of Vietnam, Northern Hydrogeological-Engineering Geological Division. Torres, I.S.I., Ishiga, H., 2003. Assessment of the geochemical conditions for the release of arsenic, iron copper into groundwater in the coastal aquifers at Yumigahama, Western Japan. In: Brebbia, C.A., Almorza, D., Sales, D. (Eds.), Water pollution VII, modeling, measuring and prediction. WIT press, Southampton, pp. 147–157. van Geen, A., Zheng, Y., Cheng, Z., Aziz, Z., Horneman, A., Dhar, R.K., Mailloux, B., Stute, M., Weinman, B., Goodbred, S., Seddique, A.A., Hoque, M.A., Ahmed, K.M., 2006. A transect of groundwater and sediment properties in Araihazar, Bangladesh: further evidence of decoupling between As and Fe mobilization. Chem. Geol. 228, 85–96. WHO, 2009. Arsenic in Drinking Water. World Health Organization, Geneva. Williams, V., Breit, G., Whitney, J., Yount, J., Amatya, S.C., 2004. Preliminary observation on the geology and geochemistry of arsenic bearing sediments in Nawalparasi district, Nepal. Proceeding of the Seminar on Arsenic Study in Groundwater of Terai and Summary Project Report, Arsenic Testing and Finalization of Groundwater Legislation Project, pp. 31–47. Yadav, I.C., 2012. Arsenic contamination and migration in groundwater of Nawalparasi district (Terai Region). PhD Thesis Nepal-an approach to sustainable drinking water supply. Banaras Hindu University, Varanasi India. Yadav, I.C., Dhuldhaj, U.P., Mohan, D., Singh, S., 2011. Current status of groundwater arsenic and its impacts on health and mitigation measures in the Terai basin of Nepal: an overview. Environ. Rev. 19, 56–69.

160

I.C. Yadav et al. / Journal of Geochemical Exploration 148 (2015) 150–160

Yadav, I.C., Singh, S., Devi, N.L., Mohan, D., Pahari, M., Tater, P.S., Shakya, B.M., 2012. Spatial distribution of arsenic in groundwater of Southern Nepal. Rev. Environ. Contam. Toxicol. 218, 125–140. Yadav, I.C., Devi, N.L., Mohan, D., Shihua, Q., Singh, S., 2014. Assessment of groundwater quality with special reference to arsenic in Nawalparasi District, Nepal using

multivariate statistical techniques. Environ. Earth Sci. http://dx.doi.org/10.1007/ s12665-013-2952-4 (in press). Yan, X.P., Kerrich, R., Hendry, M.J., 2000. Distribution of arsenic (III), arsenic (V) and total inorganic arsenic in pore waters from a thick till and clay-rich aquitard sequence, Saskatchewan, Canada. Geochim. Cosmochim. Acta 62, 2637–2648.