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0journal of Hydrology: Regional Studies xxx (2015) xxx-xxx
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Hydrogeochemical controls on mobilization of arsenic in groundwater of a part of Brahmaputra river floodplain, India
Chandan Mahanta a'**, Gustav Enmarkb c, Daniel Nordborgb c, Ondra Sracekd, Bibhash Nathe, Ross T. Nicksonf g, Roger Herbertc, Gunnar Jacksb, Abhijit Mukherjeeh, A.L. Ramanathan1, Runti Choudhurya, Prosun Bhattacharyab *
a Department of Civil Engineering, Indian Institute of Technology, Guwahati 781 039, Assam, India b KTH-International Groundwater Arsenic Research Group, Department of Sustainable Development, Environmental Science and Engineering, KTH Royal Institute of Technology, Teknikringen 76, SE-100 44 Stockholm, Sweden
c Department of Earth Sciences - Air and Water Science, Uppsala University, Villavägen 16, SE-752 36 Uppsala, Sweden
d Department of Geology, Faculty of Science, Palacky University, 17. listopadu 12, 771 46 Olomouc, Czech Republic
e School of Geosciences, University of Sydney, Sydney, NSW2006, Australia f WA Fairhurst and Partners, Spademill Studio, Spademill Lane, Aberdeen AB15 4EZ, UK g United Nations Children's Fund, 219/2 A.J.C. Bose Road, Kolkata 700 017, India h Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur 721302, India i School of Environmental Sciences, Jawaharlal Nehru University, New Delhi 110067, India
ABSTRACT
Study region: Arsenic enriched groundwater regime within low-industrialized Brahmaputra floodplains in Assam, NE India. Study focus: We examined the origin, distribution and processes of As release by investigating the salient groundwater chemistry and subsurface sedimentological characteristics. Besides collection of
ARTICLE INFO
Article history:
Received 18 March 2014
Received in revised form 14 February 2015
Accepted 8 March 2015
Available online xxx
* Corresponding author. Tel.: +46 8 790 7399; mobile: +46 70 6974241. ** Corresponding author. Tel.: +91 9435119090.
E-mail addresses: mahanta_iit@yahoo.com (C. Mahanta), prosun@kth.se (P. Bhattacharya).
http://dx.doi.org/10.1016Zj.ejrh.2015.03.002
2214-5818/© 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4X>/).
2 C. Mahanta et al. / Journal of Hydrology: Regional Studies xxx (2015)xxx-xxx
groundwater samples from domestic and public water supply wells, sediment samples from boreholes were investigated for textural and colour linkages.
New hydrological insights for the region: Arsenic concentrations above the WHO guideline value of 10 |g/L were present in 33 wells and above the previous Indian national drinking standard of 50 |g/L were present in 15 wells. The green-olive colour sediments were more likely to yield As-enriched groundwater. The supersaturation of groundwater with respect to Fe(II) minerals, such as siderite and vivianite, explained the poor correlation between dissolved As and Fe. The result reinforced the phenomenon of reductive dissolution of Fe(III) oxyhydroxides releasing As to groundwater. This study throws light on the processes and mechanisms involved with As release in groundwater. The homogenous floodplain terrain makes the hydrological As imprint unambiguous and the hydrogeological signatures untarnished. Considering the absence of anthropogenic sources in the study area, the conclusions on the nature and causes for As release to groundwater looked dependable although the final contamination at specific subsurface sites would be influenced by advection-dispersion of groundwater flow accompanied by retardation, ion exchange, surface complexation and possible biodegradation.
© 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction
The grim problem of arsenic (As) enrichment of groundwater has received global attention over the past three decades. Groundwater of several river basins of South Asia has been found to be contaminated by As above the WHO drinking water guideline of 10 |ig/L (Ahmed et al., 2004; Bhattacharya et al., 1997,2002a,b, 2011, 2014; Nickson et al., 1998; Smedley and Kinniburgh, 2002; Chatterjee et al., 2003; Hasan etal., 2007, 2009a,b; McArthur et al., 2001; Ahamed et al., 2006; Nath et al., 2008a, 2009; Mukherjee et al., 2008; Kumar et al., 2010; Naidu and Bhattacharya, 2009). Although the source of As is mostly geogenic and its release in groundwater is a result of natural processes, several studies in recent years have shown that As release has been accentuated by human activities. Besides being toxic, the consumption of As-laced water over extended periods renders individuals consuming groundwater vulnerable to chronic As poisoning (Chakraborti et al., 2004; Kapaj et al., 2006). By now, there have been many instances when prolonged ingestion of As-contaminated water over the years has resulted in skin, lung or liver cancer as well as cardiovascular diseases and damage to internal organs (Kapaj et al., 2006).
The presence of groundwater As in the state of Assam in northeastern India, was first recognized in 2004 following the studies of Singh (2004), Chakraborti et al. (2004), and later by Nickson et al. (2007). The study conducted by Singh (2004) at the North Eastern Regional Institute of Water and Land Management (NERIWALM) reported drinking water sources in 20 of the 30 districts of Assam having As concentrations exceeding 50 |ig/L. Chakraborti et al. (2004) based on their work in two districts, Karimganj and Dhemaji, reported, 19% of the groundwater samples contained As concentrations >50 |g/L, while 2% contained >300 |g/L. Based on these studies, the Public Health Engineering Department (PHED) of Assam carried out a state-wide blanket survey in 2005 (JOPA, 2005). In total 5,729 water samples collected from 22 of the 30 districts in Assam were analysed for As. The JOPA results revealed that the water samples collected from 18 districts had As concentrations >50 |g/L. Chetia et al. (2011) reported As concentration ranged between BDL (below detection limit) and 128 |g/L in six blocks of the Golaghat district located on the southern bank of the Brahmaputra River in Assam. However, none of these studies attempted detailed hydrogeochemical investigations to understand
Keywords:
Arsenic
Groundwater
Hydrogeochemistry
Brahmaputra river
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Fig. 1. Map showing the location of the study area. Inset (a) map of India where state Assam is marked, (b) study area within state of Assam with arrow head.
the nature of source, distribution, and the mechanism of As release in groundwater. Even a very recent study by Goswami et al. (2013) while drawing attention to magnitude of human exposure in sporadic locations, has not thrown light on As sources and release processes.
This study is focused on hydrogeochemical investigations in As contaminated areas of two districts (Bongaigaon and Darrang) located on the northern bank of the Brahmaputra River in Assam (Fig. 1). The study area bears significance being located in a homogeneous alluvial terrain which is the direct pathway of sediment transfer from the Himalayas to the floodplain of the Brahmaputra river. The spatial distribution of groundwater As in the study area together with the subsurface sedi-mentological characteristics (colour and texture) allowed us to discern the likely source locations of As in groundwater within the fairly homogeneous terrain of the study area. Attempts were made to evaluate the interrelationship between the key water quality parameters and As concentrations to examine the mechanism that possibly control the mobility of As in groundwater. Relating low As aquifers with specific sediment types that produced safe water (As< 10 |g/L) is expected to be useful for local drillers and/or villagers to identify safe drinking water sources.
2. The study area
2.1. Location, physiography and climate
The state of Assam is located between 89°42'-95°16' E longitudes and 24°08'-28°09' N latitudes (Fig. 1). The total geographic area is 78,438 km2 which is 2.4% of the total geographical area of India and local population is 26.64 million. Assam is bordered by six other neighbouring states as well as by Bhutan and Bangladesh. Greater part of Assam falls within the Brahmaputra valley, while the southernmost part lies in the Barak valley, separated from the Brahmaputra valley by the Central Assam Range. The inselbergs (isolated hills, knobs, ridge, or small mountains that rise abruptly from a plain or gently sloping surrounding), situated in the central districts south of the Brahmaputra River, are the distinct features of Assam. The Brahmaputra valley is about 800 km long and 130 km wide
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within Assam and the entire alluvial plain is an As suspect terrain which warrants systemic screening, although distribution has been patchy (Mahanta et al., 2009).
The Brahmaputra valley is characterized by humid sub-tropical climate with four distinct climatic seasons, pre-monsoon, monsoon, post-monsoon and winter. The region is influenced by the southwest monsoon, which accounts for 90% of the annual precipitation in the range between 2500 and 3200 mm causing severe flooding during the rainy season. Average temperatures vary between 29 ° C during summer (month of July-August) and 16 °C during winter (month of December-January).
The two study areas are situated on the northern bank of Brahmaputra River, located in the districts of Bongaigaon (between 26°28'-26°54' N latitudes and 89°-90°96' E longitudes) and Darrang (between 20°9'-26°95'N latitudes and 91°45'-92°22' E longitudes) (Fig. 2a and b). The tributaries flowing through the Bongaigaon district originate in the crystalline rocks of the trans-Himalayan range while the rivers flowing through the Darrang district are sub-Himalayan. Flat topography and large sediment loads make the tributaries to regularly change their course.
2.2. Geology and hydrogeology of the Brahmaputra plain
2.2.1. Geology
The Brahmaputra plain, with a few exceptions, is covered by young alluvial terrain, deposited from the large sediment load carried by the Brahmaputra River and its tributaries. The geological formations range in age from Palaeo-Proterozoic to Recent. The Palaeo-Proterozoic group comprises a metamorphic complex of gneiss to schist. The Neo-Proterozoic group consists of quartzite and phyllite, overlain by the lower Tertiary continental shelf sediments of Eocene age, the upper Tertiary Oligo-Mio-Pliocene constitutes the shelf sediments and unclassified older and newer alluvium of Quaternary age. The areas in the northern side of the Brahmaputra River are fed with sediments carried by the northern tributaries draining the geologically younger Himalayan mountain ranges of unconsolidated sedimentary rocks while the sediments deposited in the southern part of the basin are derived from the tributaries draining the hill ranges that are geologically much older (Datta and Singh, 2004). The topographic gradient of the northern tributaries is steeper and the flow rates are generally larger than in the southern tributaries. The steeper gradients, together with more easily eroded bedrock and larger river discharge, result in a higher sediment load from this part of the drainage area.
The geology of both the study areas is characterized by Holocene alluvial formations. In Bon-gaigaon district, a few inselbergs consisting of gneisses and schists from Meso-Proterozoic to Palaeo-Proterozoic age are exposed.
2.2.2. Hydrogeology
Heavy rainfall up to 400 cm annually ensures adequate groundwater recharge in the Brahmaputra floodplains. The groundwater table generally lies within 10m below ground level (bgl) (CGWB, 2005). Groundwater occurs under both confined and unconfined conditions. The aquifer is composed of sand, silt, clay, and gravel, with about 50% aquifer material comprised of medium to coarse sand. In the central part of the Brahmaputra valley, a partly hilly terrain is underlain by the crystalline gneiss-schist complex and limestone (Singh, 2004). Groundwater occurs in the weathered horizon and flows alongjoints, fractures, and fissures. Semi-consolidated sediments with mudstone and occasional bands of sandstone are exposed in the southern part of the Brahmaputra valley.
Majority of the population in the study area rely on groundwater as a source of drinking water. Besides hand pump sources, Piped Water Supply Schemes (PWSS) operated by the Public Health Engineering Department (PHED) tap deeper aquifers with apparently low As concentrations provide drinking water to local inhabitants.
3. Materials and methods
3.1. Selection of groundwater wells and drilling sites
The groundwater wells for sampling were selected based on a prior screening conducted by the PHED, SOES, and NERIWALM. While deciding sampling locations, the geographical coverage
Fig. 2. (a) Map of the study areas within Bongaigaon district in Assam, India showing location of tube wells, and (b) Map of the study areas within Darrang district in Assam, India showing location of tube wells. Brahmaputra River is flowing from east to west at the bottom. The satellite image of the study site acquired from Google Earth 6.0.2.
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(i.e. spatial distribution) was considered. Groundwater samples (n = 50; 34 from Bongaigaon and 16 from Darrang districts) were collected from the household wells during the post-monsoon season (October-November 2005). Field measurements of As and other water quality parameters were made for an additional 9 wells in Bongaigaon and 2 wells in Darrang. The drilling sites (n = 4; 3 in Bongaigaon and 1 in Darrang districts) were selected close to sampled groundwater wells for the investigation of subsurface sediment profiles.
3.2. Water and sediment sampling
Procedure of sample collection and preservation was followed as detailed elsewhere (Bhattacharya et al., 2002b). Samples were collected after purging the wells for several minutes to obtain fresh groundwater from the aquifer. The depth of the wells and the installation details were gathered from the well owners. Geographical locations were recorded using a handheld Garmin-60 Global Positioning System (GPS).
Field parameters (pH, redox potential ORP, and temperature) were measured using a flow-through cell. The pH was measured using a portable pH-meter (Radiometer Copenhagen PHM80) with a combination electrode (C2401-7), and the redox potential (ORP) was measured using a combined platinum electrode (MC408Pt) equipped with a calomel reference cell. The measured redox potential values were later corrected for the standard hydrogen electrode (SHE). Electrical conductivity (EC) was measured using a HANNA conductivity meter (HI8733) with an operating range between 0 and 200 mS/cm. Arsenic concentrations were estimated at the well head using a EZ Arsenic Test Kit with a working range of 10-500 |ig/L following the procedure outlined by Hach (2006).
Water samples collected for analyses included: (i) filtered and un-acidified samples (using Sartorius 0.45 |im filters) for major anion analyses; (ii) filtered and acidified samples with suprapure HNO3 (14M) for the analyses of cations and other trace elements including As (Bhattacharya et al., 2002b). Samples for As speciation were collected using disposable cartridges® (MetalSoft Center, PA) following the methodology described by Meng et al. (2001). The cartridge selectively adsorbs As(V) and the eluents were preserved with 5 drops of 14 M HNO3 for the analysis of As(III). All the samples were subsequently stored in the laboratory at 4°C for later chemical analysis.
Drillings (up to 30 m depth) for recording subsurface sedimentological characteristics were executed using the local hand-operated technique (Ali, 2003). The sediment samples were collected as disturbed samples coming out of the drilling pipe and the depth was measured by knowing the increments of pipes used for drilling. Sediment samples were collected at an interval of 1 m and when there were any visible changes in lithology. Munsell colour chart was used to classify the colour of the moist sediment samples in the field.
3.3. Laboratory analyses
The water samples were analysed for cations and trace elements (including field separated As species) with Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES, Varian VistaPro AX) at the Stockholm University. Based on the measurement of certified standards, an analytical error of ±4% was observed. As(V) was calculated by subtracting As(III) from total As concentrations in the measured groundwater sample. The filtered, un-acidified samples were used for the determination of alkalinity using the standard method SS-EN ISO 9963-2 (SIS, 1996). The major anions were analysed using a Dionex DX-120 ion chromatograph equipped with IonPac As14 column. The analyses of NO3-N, NH4-N and PO4-P were carried out on the Tecator Aquatec(R) 5400 Spectrophotometer at wavelengths of 540 nm and 690 nm respectively at the Water Chemistry Laboratory of the Department of Land and Water Resources Engineering, KTH Royal Institute of Technology, Stockholm. The method gave readings as NH4-N and PO4-P, which have been recalculated as mg/L of NH4 and PO4. The dissolved organic carbon (DOC) concentrations were determined by a Shimadzu TOC-5000 analyzer equipped with ASI 5000 auto sampler using the NPOC mode with a precision of ±10% (at the detection limit of 0.5 mg/L).
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3.4. Geochemical modelling
PHREEQC geochemical code with database MINTEQwas used to calculate saturation indices of mineral species in groundwater (Parkhurst and Appelo, 1999). The program uses mass balance and mass action (chemical reaction) equations to evaluate the saturation index (SI) of minerals in groundwater (Parkhurst, 1995).
The salient results of hydrogeochemical data based on the in-situ measurements and laboratory analyses are presented as supplementary Tables S1 and S2.
4.1. Field measured parameters 4.1.1. pH, ORP and EC
The pH values of groundwater samples collected from Darrang district ranged from 6.4 to 7.4 (median 6.7) indicating circum-neutral conditions (supplementary Table S1). Field ORP measurements (corrected to Standard Hydrogen Electrode, SHE) showed values ranging from 134 to 304 mV (median 179 mV); while EC values ranging from 100 to 500 |S/cm (median 170 |S/cm).
The pH values of groundwater samples collected from Bongaigaon district ranged from 6.6 to
7.3 (median 6.9) again indicating circum-neutral conditions. Field ORP measurements for samples collected from Bongaigaon district (corrected to Standard Hydrogen Electrode, SHE) showed values ranging from 128 to 379 mV (median 159 mV); while EC values ranged from 170 to 690 |S/cm (median 465 |S/cm). Values of pH and ORP showed no significant differences between the two districts. On the other hand, EC values varied significantly among the two areas. In Darrang district, the median value was 170 |S/cm, while in Bongaigaon district it was 465 |S/cm. No apparent trend of EC values could be established with depth of the wells.
4.2. Water chemistry 4.2.1. Major ions
The major ion composition reveal significant variations in the two investigated areas in Darrang and Bongaingaon (see Supplementary data, Table S1). Alkalinity (reported as HCO3-) was in general low in the Darrang groundwaters, with concentrations ranging from 56 to 382 mg/L (median: 175 mg/L). In contrast, the Bongaigaon groundwaters were characterized by high HCO3- concentrations ranging from 170 to 690 mg/L (median: 465 mg/L). Similarly, the concentrations of Cl- (range: 0.1-23 mg/L; median: 1.9 mg/L), NO3- (range: below detection limit (bdl) to 1.2 mg/L; median: 0.1 mg/L) and SO42-(range: 0.1-11 mg/L; median: 0.4mg/L) were also lower in groundwaters in Darrang as compared to Bongaigaon, where these solutes had respective concentrations ranging from 0.4 to 69 mg/L (median: 3.6 mg/L), bdl to 3.8 mg/L (median: 0.1 mg/L), and 0.1 to 30 mg/L (median: 0.5 mg/L). However, the concentrations of PO43- were higher in the Darrang water samples (range: 0.5-5.3 mg/L; median:
2.4 mg/L) as compared to Bongaingaon where the concentrations ranged from bdl to 3.8 mg/L, with a median of 1 mg/L.
The concentrations of major solutes show distinct relationship with the aquifer characteristics. The Piper plot (Fig. 3) indicates that the groundwater samples from the Bongaigaon district were predominantly of Ca-HCO3- type, while those of the Darrang district were either Na-Ca-HCO3- or Ca-HCO3-type The Piper plot (Fig. 3) reveal Na-Ca-HCO3- type or Ca-HCO3- water types for groundwaters in Darrang district and thereby the prevalence of more than one hydrogeochemical facies in the aquifers, as a consequence of groundwater-aquifer matrix interactions along flow path (Raychowdhury et al. 2014).The principal hydrochemical facies include Ca-HCO3-, Ca-Na-HCO3- and Na-Ca-HCO3- which are in agreement with the reported facies by a similar work in and around the study area by Verma et al. (2015, this volume). Although, the Ca-HCO3- is the most dominant hydrochemical facies, other type of groundwaters are also widely noticed. Based on observations of Verma et al. (2015, this volume), the groundwater samples from aquifers adjoining recent river channels are mostly Ca-HCO3- type,
4. Results
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CATIONS % meq/L ANIONS
Fig. 3. Piper plots showing overall groundwater types in Darrang and Bongaigaon districts of Assam, India and typical groundwater-aquifer matrix interactions governing the major ion chemistry.
whereas the groundwater samples from older alluvial sediment aquifers are both of Ca-Na-HCO3-and Na-Ca-HCO3- types, but dominated by Na-Ca-HCO3- water types. The aquifers made of younger alluvial sediments are predominantly Ca-Na-HCO3- type, with some samples representing Ca-HCO3-type waters.
4.2.2. Distribution of As in groundwater
Field testing of As using the Hach test kit showed As concentration between BDL and 500 |ig/L. Arsenic concentrations in the groundwater indicated a wide range with 66% of the analysed samples (n = 50) having concentrations >10 |g/L (supplementary Table S2). No geographical trends of As concentrations were apparent within the study areas. There was a high spatial heterogeneity in As distributions between neighbouring wells, often revealing contrasting As concentrations. A good agreement was obtained between As concentration measured by the Hach field test kit and the laboratory measurement using ICP-OES (R2 = 0.89, data not shown). In some cases, the results of the Hach field test kit overestimated the measured As concentrations in the laboratory. However, the general trend was an underestimation of total As concentration.
In Darrang district, the highest As concentration was 79 |g/L (median 44 |g/L), while in Bon-gaigaon district, As concentrations were ranging from BDL to 606 | g/L (only three wells contained As>200 |g/L), with a median value of 24 |g/L (supplementary Table S2). Fig. 4 shows that As concentrations were greatest in the depth range from 20 to 30 m bgl (including three high As wells from
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As (Mg/L)
0 200 400 600 800
0 10 20 30
f. 40 ©
50 60 70 80
Fig. 4. Depth distribution of As concentrations in groundwater wells in Darrang and Bongaigaon districts of Assam, India.
■ + -1—.—1—,—1—.—1
+ ■t, + + +
jiP - +h □ Darrang
_ □ + Bongaigaon
Bongaigoan district). The data further showed the presence of As concentrations >10 |xg/L at a depth down to 60 m bgl.
4.2.3. Dissolved organic carbon (DOC), NH4+, Fe and Mn
The DOC values of the groundwater samples ranged between BDL and 4.2 mg/L, with a median of 1.5 mg/L. These values are considered to be moderately high. Ammonium concentration varied between BDL and 9.3 mg/L, median 0.60 mg/L. The median value of the dissolved Fe concentration in Darrang district was 11 mg/L and in Bongaigaon district it was 6.1 mg/L (supplementary Table S2). The median values for Mn were 0.69 and 1.0 mg/L in Darrang and Bongaigaon district, respectively.
4.3. Relationship between As and other hydrogeochemical parameters
Arsenic showed distinct relationship with other hydrogeochemical parameters. Poor correlation was observed between As and Fe, including As and Mn (Fig. 5). Fig. 6 shows the plots of the relationships between (a) HCO3- and As (no relation for Darrang; R2 for Bongaigaon 0.09), and (b) HCO3- and Fe (R2 for Darrang 0.16; for Bongaigaon 0.20). The lack of correlation between As and HCO3- (Fig. 6a) suggest a complex relation between both parameters, e.g. multiple sources and sinks for HCO3 -, like decomposition of DOC and precipitation of carbonates also indicated by SI values. Similarly, a low correlation between Fe and HCO3- (Fig. 6b) is probably also caused by non-conservative behavior of Fe, i.e. by precipitation of Fe-minerals like siderite. Dissolution of carbonate minerals releases calcium or magnesium ions, and by examining the molar ratios of these ions one can predict the origin of HCO3 -(Wagner et al., 2005). If HCO3-/(Ca2+ + Mg2+) ratio is <1, it is probable that the HCO3- was formed by
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Fig. 5. Relationship between: (a) As and Fe (Darrang: R2 value 0.20; Bongaigaon: R2 value 0.20), and (b) As and Mn (Darrang: R2 value 0.01; Bongaigaon R2 value 0.06) in sampled groundwater of the two study areas.
degradation of organic matter. The groundwater in Darrang district contains low concentrations of Ca and Mg and it is probable that HCO3 - was formed primarily through the degradation of organic matter (HCO3-/Ca2+ + Mg2+ ratios are between 0.74 and 2.4). However, in Bongaigaon district, Ca and Mg concentrations are substantially higher than those in the Darrang district, but the molar ratios of HCO3-/Ca2+ +Mg2+ ratios (0.78 to 1.5) suggest that some HCO3- was formed by the degradation of organic matter (Bhattacharya et al., 2002a,b).
High DOC values in the groundwater probably resulted from microbial degradation of buried organic matter in the sediments and/or from local sewerage (Ravenscroft et al., 2001; Routh et al., 2000; Bhattacharya et al., 2002a,b; Islam et al., 2004; Nath et al., 2008b). Degradation of organic matter in the subsurface sediment plays an important role in the mobilization of As in the aquifer. There is a weak correlation between DOC and As in the groundwater (Fig. 7a). Elevated concentrations of phosphate and ammonium are also indicators of the degradation of organic matter (Bhattacharya et al., 2002a,b). Furthermore, a weak correlation between As and NH4+ can be seen (Fig. 7b) and the DOC and NH4+ are also weakly correlated (Fig. 7c). On the other hand, no trends between PO43- and As or NH4+ could be found (Fig. 7d and e). The lack of correlation between PO43- and NH4+ in groundwater might be a result of the application of phosphate fertilizers in agriculture in the study area.
As ftig/L)
Fe (mg/L)
Fig. 6. Bivariate plot showing the interrelationship between: a)HCO3- and As, and b) HCO3- and Fe in the sampled groundwater from Darrang and Bongaigaon study areas.
4.4. Speciation modelling
Results of the geochemical equilibrium modelling are shown in Table 1. In Darrang district, all groundwater samples were undersaturated with respect to calcite and dolomite, while in Bongaigaon district, groundwaters (with exception of one sample) were saturated with respect to these minerals. This is in a good agreement with much higher calcium and bicarbonate concentrations in the Bongaigaon district. Also, in Bongaigaon district, calculated log PCO2 values are higher, reaching an extreme value of -0.57 in B30 sample. This suggests a strong input of CO2 from the degradation of organic matter. Saturation indices for siderite and vivianite were positive in both the districts except for one sample. This supports the hypothesis that precipitation of these mineral phases may account for the poor correlation observed between As and Fe in groundwater (Ahmed et al., 2004; Sracek et al., 2004; Biswas et al., 2012). Saturation indices for Mn(II) mineral such as rhodochrosite are positive only in Bongaigaon district. Saturation indices for well-crystalline Fe(III) phases such as goethite are positive. The redox potential calculated on the basis of As(III)/As(V) redox couple are much lower (close to 0mV) than the measured values. This indicates strong redox disequilibrium and a possible mixing of groundwaters within the wells tapping productive aquifers.
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Fig. 7. Relationship between: (a) As and DOC (Darrang: R2 value 0.05; Bongaigaon: R2 value 0.03), (b) As and NH4, (Darrang: R2 value 0.52; Bongaigaon: R2 value 0.08) (c) NH4 and DOC (Darrang: R2 value 0.15; Bongaigaon: R2 value 0.41), (d) As and PO4 (Darrang: R2 value 0.02; Bongaigaon: R2 value 0.01) and (e) NH4 and PO4 (Darrang: R2 value 0.10; Bongaigaon: R2 value 0.16) in sampled groundwater of two study areas.
4.5. Relationship between sediment colour and As concentration
The sediment samples collected from four exploratory wells had clay layers at the top varying in thicknesses from 6 to 13.5 m. This clay layers might act as a confining units to sandy aquifers underneath (Fig. 8). The colour of the sand in borehole 'B1' was mainly greenish. The colour of the sediments in borehole 'B10' was similar to 'B1'. The clay layer was about 6 m thick. The third borehole (B30) drilled in Bongaigaon district contained sand with greyish olive colour, which was topped by 7.5 m thick grey clay. The borehole (D15) drilled in Darrang district contained mostly of grey sand with 12 m of clay and silt capping, acting as a confining layer.
Table 1
Speciation calculations on selected groundwater samples collected from the alluvial aquifers of Darrang and Bongaigaon districts of Assam, India.
Sample ID Calcite Dolomite Gypsum Pco2 FeOOH FeCOa Vivianite MnCOa ORP (mV) based on As(lll)/As(V) redox couple
Darrang
D1 -1.65 -3.27 -4.64 -1.53 8.62 0.51 2.28 -0.30 54
D3 -1.49 -2.99 -5.20 -2.02 9.20 -0.31 na -0.66 -34
D5 -1.35 -2.58 -3.78 -2.12 8.13 0.70 2.94 -0.20 na
D10 -1.55 -3.04 na -1.83 7.79 0.91 1.44 -0.12 18
D16 -1.67 -3.38 -5.17 -1.71 7.49 0.59 2.71 -0.70 -3
Bongaigaon
B1 0.21 0.23 -3.87 -1.23 6.95 0.85 1.15 0.19 -36
B6 0.31 0.43 -4.63 -1.30 7.03 1.22 2.05 0.45 na
B9 0.16 0.15 -4.07 -1.23 7.15 1.46 0.72 0.63 -41
B22 0.36 0.34 -4.15 -1.32 7.23 1.48 2.74 0.75 -59
B30 -0.51 -1.32 -2.23 -0.57 8.59 0.59 -0.61 0.51 31
Note: na - not available; values in bold indicates supersaturation.
von Bromssen et al. (2007) reported As-free aquifer that have a yellowish/reddish colour sediment. This colour can be attributed to the presence of ferric oxides and hydroxides such as goethite (FeOOH). If the environment becomes more reducing, there will be a partial reduction of Fe(lll) phases, releasing As into groundwater. However, re-adsorption of As to residual FeOOH in these sediments often keeps the As concentration reasonably low (von Bromssen et al., 2007; Hossain et al. 2014). If all FeOOH is reduced, the sediments will get a greyish colour (Stuben et al., 2003). Our study
B1 B10 D15 B30
Fig. 8. Lithological logs of the four exploratory wells drilled in Bongaigaon (B1, B10 and B30) and Darrang (D15) district of Assam. The colour of the sediments was described based on the Munsell colour chart. (For interpretation of the references to colour in this figure citation, the reader is referred to the web version of the article.)
C. Mahanta etal./Journal of Hydrology: Regional Studies xxx (2015)xxx-xxx
indicated that sediments with a green-olive colour are more likely to bear As contaminated ground-water. However, sediment samples collected from the exploratory well drilled adjacent to a well free of As contamination in the Darrang district were grey, which is contradictory to those observed in previous studies conducted in West Bengal, India and Bangladesh, where sediment with red or brown colours generally yield safe As (As <10 |g/L) groundwater (von Bromssen et al., 2007; McArthur et al., 2011; Biswas et al., 2012; Hossain et al., 2014).
The dynamics of sediment deposition and flushing rate in the study areas and elsewhere in Bangladesh could perhaps play a significant role in the development of sediment colour. The study area lies in the active Brahmaputra floodplains, where sediments are mainly deposited by the Brahmaputra River and its tributaries flowing through the entire length of the study area. However, when we consider the sediment deposition in Bangladesh, it has to be noted that the deposition of sediments occurs by the combined effects of three rivers, viz. the Ganges, Brahmaputra and Meghna. Moreover, eustatic changes in sea level during the Holocene period played a major role in sub-areal exposure, i.e. oxidation of the sediments, and flushing of the deltaic aquifer in Bangladesh (Smedley and Kinniburgh, 2002). The signatures of sediments carried by the Ganges and Meghna river systems are absent in the Brahmaputra floodplains in Assam including the effect of eustatic changes in sea level on sediment colour. The sediment colour classification and its possible implication on As contamination scenarios in Brahmaputra floodplains in Assam needs more detailed investigations on the nature and color of the sediments and their hydrochemical characteristics similar to the studies made by Hossain et al. (2014) in Matlab, Bangladesh. Although, local drillers knowledge on the type of aquifers that will provide potable water is sometimes valid in terms of As contamination. Yet, due to economical and practical reasons, wells are often installed in locations where preferred conditions are not necessarily found.
5. Arsenic release processes
The redox status of the sedimentary aquifer play an important role in the release of As in groundwater where the presence of organic matter trigger the redox reaction (Nickson et al., 1998, 2000; Mukherjee and Bhattacharya, 2001; Bhattacharya et al., 2002a, b, 2006; McArthur et al., 2004; Saunders et al., 2005, Mukherjee et al., 2008; Nath et al., 2011). Hydrogeochemical data suggests predominantly reducing character of the groundwater along with the presence of high HCO3- and low SO42- concentrations. Elevated HCO3- concentrations may have resulted from organic matter oxidation (Mukherjee and Bhattacharya, 2001) while low SO42- concentrations due to sulphate reduction (Bhattacharya et al., 2009). Elevated phosphate and ammonium concentration is also an indicator of organic matter degradation that drive Fe-oxyhydroxide reduction to completion and releasing high As to groundwater (Ravenscroft et al., 2001).
Previous studies in Ganges delta highlighted that the reductive dissolution of Fe-oxyhydroxides is the main mechanism for As release from sediments to groundwater (e.g. Bhattacharya et al., 1997; Nickson et al., 1998). If this release mechanism is correct, both dissolved As and Fe concentrations are expected to be high and show a strong correlation. However, as observed in several studies (e.g. van Geen et al., 2005; Mukherjee and Fryar, 2008; Mukherjee et al., 2008; Nath et al., 2009), the lack of correlation may be limited by the occurrence of multiple geochemical processes occurring simultaneously in the aquifer. The lack of correlation between As and Fe can further be explained if dissolved Fe does not remain in solution then there should be a formation of secondary solid phases such as siderite (FeCO3) that do not incorporate As from solution (Sracek et al., 2004). In addition to Fe(III)-hydroxides, Mn-oxides/hydroxides have earlier been stated as an alternative phase for As sorption (Stuben et al., 2003; Nath et al., 2005; Hasan et al., 2007). When Mn-oxides/hydroxides are reduced, As would be released in the same manner as for Fe(III)-hydroxides. Concentrations of dissolved Mn in the 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 of As with Mn is that the adsorption/desorption from Mn-oxides/hydroxides play a minor role in controlling the mobility of As in the aquifer (von Bromssen et al., 2007; Hassan et al., 2007; Mukherjee et al., 2008).
The fact that Fe is acting non-conservatively and the moderate correlation between As and HCO3-in groundwaters of the study area supports the reductive dissolution of Fe(III)-hydroxides as the
C. Mahanta et al./Journal of Hydrology: Regional Studies xxx (2015) xxx-xxx
mechanism of As release in groundwater (Bhattacharya et al., 1997; Nickson et al., 1998). Non-conservative behaviour of Fe is probably due to the precipitation of secondary Fe(ll) phases such as siderite, FeCO3, and vivianite, Fe3(PO4)2-8H2O (Sracek et al., 2004; Ahmed et al., 2004). These secondary Fe(ll) phases may also form coatings on the surface of Fe(lll)-oxyhydroxides, thus preventing their further dissolution (Vencelides et al., 2007). Suggested precipitation of secondary phases of both Fe(ll) and Mn(ll) is in a good agreement with positive saturation indices for minerals such as siderite, vivianite (in both districts) and rhodochrosite, MnCO3 (only in Bongaigaon district). Evaluation of the collective role of retardation, adsorption, complexation and biodegradation vis-à-vis advective and dispersive flow and hydraulic conductivity of the aquifer media along prevailing hydraulic gradient have currently been taken up to further consolidate the findings. Recent report on competitive ion exchange and surface complexation (Biswas et al., 2014) in neighbouring Bengal basin calls for similar future investigations in Brahmaputra floodplains.
6. Conclusions
The groundwater in the study area is mostly of Ca-HCO3- and/or Na-Ca-HCO3- type with HCO3-as the dominant anion. Low SO42- concentration coupled with high NH4+ and PO43- concentrations are indicative of reducing conditions. High DOC concentrations in groundwater may have resulted from the degradation of organic matter. lnfluence of microbial processes in the organic rich sediment layer creates a favourable reducing environment facilitating the release of As into the groundwater. Furthermore, Fe and Mn are acting non-conservatively, which is indicated by the low to moderate correlation of dissolved As with dissolved Fe and Mn concentrations. Precipitation of siderite, vivianite and rhodochrosite mineral phases, indicated by the positive saturation indices, suggests potential sink for dissolved Fe and Mn together with co-precipitation of As. This study also suggests that the reductive dissolution of Fe-oxyhydroxides is the main mechanism of As release into groundwater in this study area. However, no distinct regions or specific sedimentological features responsible for As enrichment could be identified for now. Compared to the Bengal Delta aquifers, colour of the sediments appears to be yet not a reliable indicator of As-free groundwater in the Brahmaputra floodplains; cases of high As in groundwater has now started to unfold in the Brahmaputra floodplain; this study is a precursor to the ongoing studies on the investigations leading to in-depth understanding the mechanisms of the dynamics of As in the Brahmaputra flood plain aquifers in Assam and delineating low As aquifers in the region that can provide safe and sustainable drinking water supply to the population of more than 20 million.
Acknowledgements
This study forms a part of the research project funded by Swedish Research Council (VR)-Swedish lnternational Development Cooperation Agency (Sida) Swedish-Asia Research Link Programme "Targeting safe aquifers in regions with high arsenic groundwaters and the options for sustainable drinking water supplies" (dnr: 348-2006-6005; 2007-2012), at the Department of Land and Water Resources Engineering, LWR (presently Sustainable Development, Environmental Science and Engineering, SEED). GE and DN thanks Sida for the Minor Field Study (MFS) grant received from the Committee of Tropical Ecology at Uppsala University for supporting the field study undertaken in collaboration with the Uppsala University. We would like to thank Ann Fylkner, Monica Löwen and Bertil Nilsson at the LWR, SEED, KTH Royal lnstitute of Technology, and Professor Magnus Mörth from the Department of Geological Sciences, Stockholm University, for laboratory analyses. We would also like to thank Orianna Courtney, Partha Pratim Barua, Abhimanyu Paul and other staff members at the Public Health Engineering Department, Guwahati, Assam for their assistance during the study.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.ejrh.2015.03.002.
16 C. Mahanta et al. / Journal of Hydrology: Regional Studies xxx (2015)xxx-xxx
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