Arsenic distribution along different hydrogeomorphic zones in parts of the Brahmaputra River Valley, Assam (India)

Hydrogeology Journal, Apr 2017

The spatial distribution of arsenic (As) concentrations along three classified hydrogeomorphological zones in the Brahmaputra River Valley in Assam (India) have been investigated: zone I, comprising the piedmont and alluvial fans; zone II, comprising the runoff areas; and zone III, comprising the discharge zones. Groundwater (150 samples) from shallow hand-pumped and public water supply wells (2–60 m in depth) was analysed for chemical composition to examine the geochemical processes controlling As mobilization. As concentrations up to 0.134 mg/L were recorded, with concentrations below the World Health Organization and the Bureau of Indian Standards drinking-water limits of 0.01 mg/L being found mainly in the proximal recharge areas. Eh and other redox indicators (i.e., dissolved oxygen, Fe, Mn and As) indicate that, except for samples taken in the recharge zone, groundwater is reducing and exhibits a systematic decrease in redox conditions along the runoff and discharge zones. Hydrogeochemical evaluation indicated that zone I, located along the proximal recharge areas, is characterized by low As concentration, while zones II and III are areas with high and moderate concentrations, respectively. Systematic changes in As concentrations along the three zones support the view that areas of active recharge with high hydraulic gradient are potential areas hosting low-As aquifers.

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Arsenic distribution along different hydrogeomorphic zones in parts of the Brahmaputra River Valley, Assam (India)

Hydrogeol J Arsenic distribution along different hydrogeomorphic zones in parts of the Brahmaputra River Valley, Assam (India) Runti Choudhury 0 1 Chandan Mahanta 0 1 Swati Verma 0 1 Abhijit Mukherjee 0 1 0 Department of Geology and Geophysics, Indian Institute of Technology (IIT) , Kharagpur, WB 721302 , India 1 Department of Civil Engineering, Indian Institute of Technology (IIT) , Guwahati, Assam 781039 , India 2 Runti Choudhury The spatial distribution of arsenic (As) concentrations along three classified hydrogeomorphological zones in the Brahmaputra River Valley in Assam (India) have been investigated: zone I, comprising the piedmont and alluvial fans; zone II, comprising the runoff areas; and zone III, comprising the discharge zones. Groundwater (150 samples) from shallow hand-pumped and public water supply wells (2-60 m in depth) was analysed for chemical composition to examine the geochemical processes controlling As mobilization. As concentrations up to 0.134 mg/L were recorded, with concentrations below the World Health Organization and the Bureau of Indian Standards drinking-water limits of 0.01 mg/L being found mainly in the proximal recharge areas. Eh and other redox indicators (i.e., dissolved oxygen, Fe, Mn and As) indicate that, except for samples taken in the recharge zone, groundwater is reducing and exhibits a systematic decrease in redox conditions along the runoff and discharge zones. Hydrogeochemical evaluation indicated that zone I, located along the proximal recharge areas, is characterized by low As concentration, while zones II and III are areas with high and moderate concentrations, respectively. Systematic changes in As concentrations along the three zones support the view that areas of active recharge with high hydraulic gradient are potential areas hosting low-As aquifers. Arsenic; Health; India; Hydrochemistry; Groundwater management Introduction Arsenic (As) rich groundwater in alluvial aquifers is a worldwide problem (Nriagu et al. 2007; van Geen et al. 2008; Bundschuh et al. 2012; Mahanta et al. 2015) . Tens of millions of people around the world are currently exposed to dangerous levels of As (Bhattacharya et al. 1997; Smith et al. 2000; Ravenscroft et al. 2009; Brammer and Ravenscroft 2009, Berg et al. 2006; Buschmann et al. 2008; Guo et al. 2008; Polya and Charlet 2009) . Elevated As concentrations have long been detected in Southeast Asia (eg. West Bengal, Bangladesh, Thailand, Vietnam, Cambodia, Taiwan, China, Mongolia, Nepal and Pakistan (Nickson et al. 1998; Berg et al. 2008; van Geen et al. 2008) with more new areas (Myanmar, Datong Basin, China etc.) reported to have As contamination levels beyond the World Health Organization (WHO) drinking water limit of 0.01 mg/L (van Geen et al. 2013; Xie et al. 2014) . More than three decades of research has revealed primarily three different mechanisms of As release and mobilization in groundwater systems: the release of As through pyrite oxidation (Das et al. 1995) , release by competitive adsorption of anions (Acharyya et al. 2000; Smedley and Kinniburgh 2002) , and the reductive dissolution of Fe(III) oxyhydroxide minerals and subsequent release of As (Bhattacharya et al. 1997; Nickson et al. 1998, 2000; Ravenscroft et al. 2009) . There is a broad consensus that the reductive dissolution of iron (Fe) oxyhydroxides is the key mechanism leading to naturally elevated As concentrations in anoxic groundwater over large expanses of South and Southeast Asia (McArthur et al. 2004; Nath et al. 2005; Berg et al. 2008; van Geen et al. 2008; Ravenscroft et al. 2009) . Groundwater As contamination in the Brahmaputra Valley in Assam, India, is a relatively recent finding that has exposed a significantly large population to serious health threats, although the actual distribution and extent of the As affected groundwater in the aquifers are yet to be established (Goswami et al. 2013) . Studies on groundwater As in Assam are primarily focused in understanding the geochemical and hydrogeochemical aspects of high As groundwater (Chetia et al. 2011; Goswami et al. 2013; Sailo and Mahanta 2014) . These studies report that the reductive dissolution of iron oxyhydroxides is the possible mechanism of As release. However to understand the exact mechanism controlling As mobilization in groundwater systems, it is essential to develop an in-depth understanding of the role of local hydrogeological conditions in controlling As mobilization and distribution. The present study thus aims to assess the variation and distrib u t i o n o f A s c o n c e n t r a t i o n a l o n g t h r e e d i ff e r e n t hydrogeological zones and the controlling role of hydrological processes in spatial distribution of As. Materials and methods Study area The study area, the Darrang and Udalguri districts, forms a part of the upper Brahmaputra Floodplains. Tectonically, four major tectonic units—the Himalayan folded and Tertiary hills, the Naga Patkai Hill ranges, the Meghalaya Plateau, and the Mikir Hills— dominate the upper Brahmaputra River Valley in Assam (Fig. 1; Bora 2004) . Sampling was carried out in the northern bank of the Brahmaputra River in Assam, consisting of the piedmont deposits dominated by older alluvium along the mountain foothills and alluvial deposits along the river banks (Fig. 2). The Brahmaputra River, along with its numerous northern bank tributaries (Nanoi, Bornadi, Noanadi and Dhansiri) that drain the Eastern Himalayas, bring in sediments which eventually become active aquifer materials in the study area (Fig. 1). Two different aquifer systems characterize the study area: unconfined condition in shallow water-table aquifers, and semi confined to confined condition in deeper aquifers (CGWB 2008). The aquifers in the piedmont and alluvial fan deposits, which are active recharge areas, are mostly covered by rock fragments, boulders, pebbles and ill-sorted sand and minor clay, and groundwater generally occurs under water-table conditions (Purkait 2004; CGWB 2008) . The piedmont and fan deposits generally retain percolating water and ground intake from precipitation and runoff. Aquifers in the younger alluvium present along the course of the Brahmaputra River and its tributaries are marked by sand, silt and clay respectively (Heroy et al. 2003) . Flanked by the Eastern Himalayas on the north and the Brahmaputra River in the southern part of the study area, with a topographic gradient of 4 m/km, the general direction of groundwater flow is from north to south. Sampling and Laboratory analysis One hundred and fifty (150) groundwater samples from the northwestern part of Brahmaputra River basin (Darrang district, Assam) were collected from hand-pumped and public water supply wells following the procedures of Mukherjee and Fryar (2008) during February 2013 and 2014. The groundwater samples were collected from depths ranging from 2 to 60 m below ground level (bgl) but normally <50 m bgl representing the shallow aquifers system in the study area. All sampled wells were purged for several well volumes prior to sample collection to ensure that the water samples represented fresh groundwater from the aquifer. Sample collection commenced only when the specific conductance, pH and temperature measurements were stabilized. In situ measurements for oxidation-reduction potential (EH), temperature, pH and dissolved oxygen (DO) were done using a multiparameter probe (Hanna 9282), with a temperature accuracy of ±0.15 °C (resolution 0.01 °C), pH accuracy of ±0.02 pH (resolution 0.01), Eh accuracy of ±1.0 mV (resolution 0.1 mV), and a DO accuracy of ±0.10 mg/L (resolution 0.01 mg/L). After collection, all groundwater samples were filtered by 0.45-μm filter paper. Samples for cations (major and trace) were collected and preserved by acidification with 6 N HNO3 in the field to pH ∼2. Major anions were measured by ion chromatography (Thermo Scientific BDionex^) in the Hydrogeology Laboratory of Indian Institute of Technology (IIT) Kharagpur and a subset of the samples (n = 60) were analysed at IIT Guwahati. The major cations and trace metals were measured by an inductively coupled plasma with optical emission spectrophotometer (ICP-OES, Thermo Fisher ICAP 6000). Precision for most of the analyses was better than 3% (Verma et al. 2014) . To derive information on aquifer lithology, five boreholes were drilled along a transect in the study area during February 2013 and 2014 with the help of local drillers provided by the Public health Engineering Department (PHED) Assam (Figs. 2 and 3). The borehole sediment samples were collected approximately at every 3 m depth and lithology, colour and sediment texture of aquifer sediments was noted. Results and discussion General water chemistry Groundwater pH ranges between 6.08 to 7.83, indicating circum neutral groundwater (Table 1). Groundwater Fig. 1 Map showing the major geologic and geomorphic units of Assam temperature was found to be almost uniform at all sampling sites, ranging from 23.30 to 29.50 °C. Calcium (Ca2+) dominated the cation chemistry with values ranging between 1.13 and 60.17 mg/L and mean concentration being 11.73 mg/L, with 6.87 mg/L. Concentrations of sodium (Na+) ranged between 0.81 and 46.81 mg/L with a mean concentration of 12.11 mg/L, standard deviation (SD) 7.35 mg/L, and magnesium (Mg+) concentrations ranged between 0.19 and 24.59 mg/ L, mean 6.59 mg/L, SD 4.42 mg/L (Table 1). Major cations in groundwater (Ca2+, Mg2+, Na+ and K+) are released from silicate and carbonate mineral weathering enhanced by respired CO2 from oxic and anoxic organic matter degradation (Halim et al. 2009) . A major source of Ca2+ in groundwater is often the carbonate minerals such as calcite and dolomite, while the major source minerals of Mg2+ are biotite and chlorite minerals (Ho 2001) . Dissolution of orthoclase and clay minerals supply K+ ions in groundwater (Ho 2001) . The observed positively skewed distribution of different cations along the study area is possibly due to the differential rate of carbonate and silicate mineral weathering in the study area. Anions such as Cl−, SO42− and NO3− are major inorganic components deteriorating the quality of groundwater as drinking water (Halim et al. 2009) . Anions are predominated by bicarbonate (HCO3−), with values ranging from 62.70 to 268.50 mg/L, mean 129.67 mg/L and SD 43.94 mg/L. HCO3− in groundwater could be released by the dissolution of carbonate minerals via biodegradation of organic matter (Ho 2001) . Concentrations of chloride (Cl−) ranged between 0.89 and 30.85 mg/L, mean 5.46 mg/L, SD 4.16 mg/L, and sulphate (SO42−) concentrations ranged between 0.13 and 20.11 mg/L with mean 5.49, SD 4.59 mg/L. With a mean of 2.06 mg/L and SD of 2.92 mg/L, NO3− concentrations were generally low for all groundwater samples, expect for one sample where the concentration was 31.02 mg/L. The high skewness value of 6.86 observed for NO3− could be due to agricultural or other anthropogenic causes, which may potentially contribute to high NO3− concentration in the groundwater sample (Raju et al. 2015) . Further, agricultural application of fertilizer may also possibly contribute to high Cl− concentration in groundwater (Halim et al. 2009). Das et al. (2016) , based on their recent study in Assam, reported that locally used bleaching powder [Ca(ClO)2] could be a probable cause of high Cl− into groundwater. Hydrogeochemical data plotted on a Piper diagram suggest that groundwater is of Ca2+–HCO3 to Ca2+–Na–HCO3 type (Fig. 4). These results are consistent with previous studies (Choudhury et al. 2015; Sailo and Mahanta 2014; Verma et al. 2014; Enmark and Nordborg 2007) which show almost similar results for groundwater composition in Assam. Field oxidation-reduction potential (ORP) measurements—corrected to standard Hydrogen Electrode (SHE)— showed values ranging from 28.1 to 197.8 mV (mean 85.39 mV) and generally exhibit higher values near the recharge zone (near Bhutan foothills) compared to groundwater Fig. 3 Lithological profile of transect (N–S) in the study area, based on lithologs collected from drilled boreholes. The colours indicate the sediment colour as observed during drilling Zone II Zone III Zone I samples collected further down near the Brahmaputra River main channel. Dissolved oxygen (DO) concentrations ranged between 0.13 and 2.81 mg/L (mean 0.80 mg/L; SD 1.38 mg/L), with higher DO values observed near the proximal zones and low bdl below detection level values along the runoff areas and discharge zones indicating reducing conditions (Berner 1981) . Based on the measured DO values it can be inferred that aquifers in zone II and zone III are more reducing than those in zone I. Depth variation of arsenic Plots of As concentration with depth revealed that wells with concentrations above WHO and Bureau of Indian Standards (BIS) drinking-water limits were mostly concentrated within the depth range of 20–50 m, as observed in most As contaminated aquifers in South and Southeast Asia (BGS and DPHE 2001; Ahmed et al. 2004; Zheng et al. 2004; Fig. 5) . Concentration of redox-sensitive parameters Nearly 60% of the groundwater samples have As concentrations above the WHO and the BIS standards of 0.01 mg/L for drinking water (WHO 2003; BIS 2003) , with values ranging between below detection limit (bdl) to 0.13 mg/L (mean 0.018 mg/L). Samples close to the mountain foothills, i.e. in the proximal fans, have As concentrations below 0.01 mg/L. Concentrations of As spatially varied along the entire study area, with concentrations gradually increasing from the foothills to the floodplains (Fig. 2). Mn concentration ranged from 0.05 to 5.87 mg/L with mean concentrations of 0.94 mg/L and SD of 0.80 mg/L. Mn concentrations do not show much spatial variability and it is nearly uniformly distributed from the alluvial fans to the discharge zone. Total dissolved iron ranged between 0.04 and 41.02 mg/L with a mean of 13.94 mg/L. 0.00 As (mg/L) 0.05 0.10 0.15 0 10 20 )m30 ( h t p eD40 50 60 70 As< 0.01 mg/L As ( 0.01- 0.05 mg/L) As> 0.05 mg/L Fig. 5 Depth variation of As (n = 150) in the study area Dissolved iron concentrations shows a gradual increasing trend along the flow path (Fig. 6). Arsenic and other water quality parameters To investigate the interrelation between As and other hydrogeochemical parameters, scatter plots were created. No distinctive trends were observed between As and redox sensitive parameters with respect to Fe and Mn (Fig. 7). A strong correlation of As with Fe would indicate reductive dissolution of Fe oxyhydroxides as a process of arsenic release into groundwater (Nickson et al. 1998; McArthur et al. 2004) . However, the observed weak correlation between As and Fe could be due to the non-conservative nature of Fe in the groundwater, which has been previously suggested (Mahanta et al. 2015) or to the precipitation of dissolved Fe as pyrites and siderite solids (FeCO3), vivianite [Fe3(PO4)28H2O], pyrite (FeS2) and rhodochrosite (MnCO3) under reducing conditions (Reza et al. 2010; Guo et al. 2014; Sracek et al. 2004; Mukherjee et al. 2008; Hasan et al. 2007) . Repeated cycles of oxidation and reduction in the subsurface in response to water table fluctuations or addition of oxygen during well sampling is also reported to lead to poor As-Fe correlation (Reza et al. 2010). Poor correlation between As and Fe thus suggests that simple breakdown of FeOOH may not be Fig. 6 Spatial distribution of iron (Fe) in the study area. The size of the circles represents Fe concentrations Fig. 7 Scatter plots showing behaviour of As (mg//L) with various parameters: a Fe, b Mn, c SO4, d HCO3, and e NO3 a c As (µg/L) 0.05 0.1 As (µg/L) 0.15 0.05 sufficient to explain high levels of dissolved As (Dowling et al. 2002) . Similar to As and Fe, a weak positive correlation between As and Mn was derived that may be explained by the fact that dissolved manganese concentrations in reducing aquifers are generally controlled by rhodochrosite, MnCO3. Manganese can also be implemented as a minor constituent in siderite (Mukherjee and Bhattacharya 2001; Bhattacharya et al. 2001; Sracek et al. 2004; Ahmed et al. 2004) . Another possible explanation for the lack of correlation with Mn is that the adsorption/desorption from manganese oxides/hydroxides plays a minor role in controlling the mobility of As in the aquifer (von Brömssen et al. 2007; Hasan et al. 2007; Mukherjee et al. 2008) . Weak correlation between As and HCO3− (R2 = 0.14; Fig. 5) is suggestive of a complex relation between both parameters, e.g. multiple sources and sinks for HCO3− like decomposition of DOC and precipitation of carbonates. Weak to no correlation of As with NO3− and SO42− further suggests that sulphate oxidation is not a probable mechanism of As release into groundwater. Groundwater chemistry reveals that As concentration values above WHO and BIS drinking water standards occur in the study area coupled with high dissolved Fe, Mn and HCO3− and low SO42− and NO3− concentrations. Such association of groundwater parameters indicating the dominance of reducing condition in aquifers is indicative of the effects of microbially mediated Fe(III) reduction on As mobilization in the presence of natural organic matter. However, weak to poor correlations of As with other redox-sensitive components (Fe, R2 = 0.09; Mn, R2 = 0.09; HCO3−, R2 = 0.14; SO42−, R2 = 0.06; NO3−, R2 = 0.01) indicates that As mobilization in groundwater is an outcome of possible interplay of mechanisms like pH desorption, SO42− reduction etc. and that the reductive dissolution of Fe(III) oxyhydroxides alone cannot explain the As release and mobilization mechanism. Hydrogeomorphological zonation Based on a hydrogeomorphological map obtained from Central Groundwater Board (CGWB) and the field investigations as part of this project, the study area was divided into three distinct hydrogeological zones: zone I comprising the piedmont, zone II comprising the runoff areas and zone III comprising the active floodplains and the discharge zones (Fig. 2). Arsenic and hydraulic gradient in the piedmont zone area (zone I) Located in the proximal fans, zone I is characterized by active local recharge with relatively high permeability as observed by the dominance of coarse sands and rock fragments in boreholes drilled adjacent to groundwater-sampled wells (Fig. 2). In the piedmont recharge areas groundwater is characterized by circum neutral pH (Table 1). Arsenic concentrations lower than 0.01 mg/L (mean 0.003 mg/L; SD 0.003 mg/L) are observed in groundwater samples collected from this zone. DO concentration in this zone range between 0.80 and 2.8 mg/L with a mean of 1.48 mg/L. Controls of hydraulic gradient in promoting As enrichment have been clearly shown by previous researchers (Nordstrom and Archer 2003, Fendorf et al. 2010 and Smedley et al. 2002) and more recently by Zhang et al. (2013); Guo et al. (2013) ; Jia et al. (2014) ; Zhu et al. (2015) . These studies demonstrated that chemical composition of groundwater changes systematically with flow down gradient. Increases in groundwater concentrations of parameters such as As, Fe and Mn along flow paths reflect increase in groundwater residence times (Haque and Johannesson 2006) . Tributaries flowing through the study area drain the strongly deformed Lesser Himalayan metasediments composed of Precambrian limestone, dolestones, shales, quartzites and schists and the turbidites and Neogene Molasses of the Siwalik Group of Eastern Himalayas (Garzanti et al. 2004; Singh et al. 2006) . Materials carried by these tributaries, which eventually become aquifer materials, under processes of water–rock interactions, facilitate leaching of metals such as As, Fe and Mn into groundwater. In zone I, with active recharge, and shorter residence times, it seems less likely that Fe or Mn could get leached out into groundwater. Further, rapid recharge prevents release of As, possibly by supplying O2− and NO3− to oxidize organic matter rather than Fe-minerals (Neumann et al. 2010; van Geen et al. 2008) . In areas of low relief, the aquifer sediment is usually fine and enriched in organic matter, which under low flow rates favours As accumulation (Fendorf et al. 2010; Aziz et al. 2008) . Flushing history is reported to serve as an important hydrogeological control on regional distribution of As in shallow groundwater of the Bengal basin (van Geen et al. 2008) . Rapid recharge of coarse-grained aquifer materials near the mountain foothills thus facilitates flushing of dissolved As in zone I. Arsenic and hydraulic gradient in the runoff zone (zone II) With a flatter gradient compared to zone I, in zone II, the numerous tributaries (Bornadi, Nanoi, Noanadi, and Dhansiri) draining the Lesser Himalayas bring in sediment load and deposit the same within their respective floodplains. DO values for groundwater samples within this zone range between 00.13 and 1.94 mg/L, (mean 0.60 and median 0.50 mg/L), indicating highly reducing conditions of the groundwater aquifers. As concentration in this zone ranges between bdl and 0.13 mg/L, with a mean concentration of 0.024 mg/L. Lithological information reveals that medium to fine sands with thin lenses of clays dominate the aquifer lithology in this zone (Fig. 2). High As accumulation in this zone compared to zone I and zone III may possibly be attributed to the presence of numerous paleochannels that dot the study area and the frequent avulsions of the north bank tributaries (Lahiri and Sinha 2012) . correlation between As and redox sensitive parameters (Fe, Mn, HCO3−, SO42−, NO3− concentrations) suggest that reductive dissolution of Fe (III) oxyhydroxides alone cannot explain the arsenic release mechanism in the study area. Arsenic and hydraulic gradient in the discharge zone area (zone III) Located close to the Brahmaputra River (Fig. 2), where groundwater is locally discharged, zone III, with a hydraulic gradient much lesser compared to the other two zones, is characterized by highly reducing conditions with Eh values varying between 28.08 and 61.72 mV and a median value of 47.49. DO values ranged from 0.21 to 2.72 mg/L with a mean of 0.79 mg/L and median of 0.70 mg/L. As concentrations in groundwater samples collected from this zone range between bdl and 0.06 mg/L with a mean concentration of 0.021 mg/L. The relatively low As concentrations compared to zone II, is likely to be linked to the presence of comparatively less reducing condition in this zone, possibly influenced by flooding events during which aquifers close to the active floodplains and discharge zone are exposed to repeated cycles of flushing. Further influx of fresh oxic recharge water into aquifers located in areas affected by recurrent flooding, favours stability of hydrated Fe oxides, thereby releasing less As into groundwater. Conclusion Spatial variability of groundwater arsenic in shallow aquifers along a transect in the northern bank of the Brahmaputra River Valley was evaluated. Previous studies have demonstrated the distinct role of hydrological controls on As distribution in the lower Brahmaputra River floodplains in Bangladesh. The present study attempts to investigate the controls of hydrologic factors in As distribution along a transect on the upper Brahmaputra Valley in Assam, India. Based on published hydrogeomorphological maps of the study area and field investigation, three distinct zones are delineated along a transect—zone I, the piedmont zone, located close to the Eastern Himalayas, comprising the recharge zone is characterized by low arsenic concentrations, while zone II and zone III, comprising the runoff areas and the active floodplain areas, are characterized by high to moderate levels of As concentration. The systematic variation in As concentrations along the three zones demonstrates the role of hydrology and aquifer redox conditions in controlling As distribution. A steep gradient and rapid flushing facilitated by coarser aquifer materials in the proximal zone aquifers leads to low As concentration in zone I. 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Runti Choudhury, Chandan Mahanta, Swati Verma, Abhijit Mukherjee. Arsenic distribution along different hydrogeomorphic zones in parts of the Brahmaputra River Valley, Assam (India), Hydrogeology Journal, 2017, 1153-1163, DOI: 10.1007/s10040-017-1584-2