1-s2.0-S0048969712007103-main

nab

yanuRes

gpure Department of Geological Sciences, Stockholm University, SE-106 91 Stockholm, Sweden

a r t i c l e i n f o

Article history:Received 19 April 2012Received in revised form 9 May 2012Accepted 10 May 2012

Science of the Total Environment 431 (2012) 402412

Contents lists available at SciVerse ScienceDirect

Science of the Tot

j ourna l homepage: www.e lse1. Introduction

The extent of human exposure due to naturally occurring dissolvedarsenic (As) in the Holocene sedimentary aquifers of Bengal Basin hasbeen recognized as world's largest mass poisoning in human history(Smith et al., 2000). The presence of elevated As (>10 g/L) in ground-water was rst reported from West Bengal (currently Paschim Banga),India in early 1980s (Saha, 1984). In the subsequent years, such enrich-ment was also accounted from the aquifers of Bangladesh (Dhar et al.,1997; Roy Chowdhury et al., 1999). Currently about 60 million of thepeople from these regions are at risk of chronic As toxicity due to expo-

Since after rst reporting of As in groundwater, signicant pro-gresses have been made in the context of its source characterization,identication of mobilization and immobilization processes, spatialand vertical distributions, etc. (Bhattacharya et al., 1997; Nickson etal., 1998; BGS and DPHE, 1999, 2001; Harvey et al., 2002; van Geen etal., 2003; Islam et al., 2004; Ravenscroft et al., 2005; Polizzotto et al.,2008; Nath et al., 2009; Polya and Charlet, 2009; Mukherjee et al.,2011; Biswas et al., 2012). These developments have led to the govern-ments of West Bengal, India and Bangladesh to introduce various miti-gation strategies together with international aid agencies to ensure safedrinking water supply among the affected population. The undertakensure from drinking water as well as oth(Chakraborti et al., 2008; Chatterjee et al., 201

Corresponding author at: KTH-International GroundDepartment of Land andWater Resources Engineering, KTSE-100 44 Stockholm, Sweden. Tel.: +46 8790 7967; fax:

E-mail address: [email protected] (A. Biswas).

0048-9697/$ see front matter 2012 Elsevier B.V. Alldoi:10.1016/j.scitotenv.2012.05.031tation of the BSA for possible sustainable drinking water supply. 2012 Elsevier B.V. All rights reserved.Available online xxxx

Keywords:Bengal BasinArsenicManganeseRedox conditionSustainable drinking water supplya b s t r a c t

Delineation of safe aquifer(s) that can be targeted by cheap drilling technology for tubewell (TW) installationbecomes highly imperative to ensure access to safe and sustainable drinking water sources for the arsenic(As) affected population in Bengal Basin. This study investigates the potentiality of brown sand aquifers(BSA) as a safe drinking water source by characterizing its hydrogeochemical contrast to grey sand aquifers(GSA) within shallow depth (b70 m) over an area of 100 km2 in Chakdaha Block of Nadia district, WestBengal, India. The results indicate that despite close similarity in major ion composition, the redox conditionis markedly different in groundwater of the two studied aquifers. The redox condition in the BSA is delineatedto be Mn oxy-hydroxide reducing, not sufciently lowered for As mobilization into groundwater. In contrast,the enrichments of NH4+, PO43, Fe and As along with lower Eh in groundwater of GSA reect reductive dis-solution of Fe oxy-hydroxide coupled to microbially mediated oxidation of organic matter as the prevailingredox process causing As mobilization into groundwater of this aquifer type. In some portions of GSA theredox status even has reached to the stage of SO42 reduction, which to some extent might sequester dis-solved As from groundwater by co-precipitation with authigenic pyrite. Despite having low concentrationof As in groundwater of the BSA the concentration of Mn often exceeds the drinking water guidelines,which warrants rigorous assessment of attendant health risk for Mn prior to considering mass scale exploi-Hydrogeochemical contrast between browof Bengal Basin: Consequences for sustain

Ashis Biswas a,b,, Bibhash Nath c, Prosun BhattacharAbhijit Mukherjee d, Debashis Chatterjee b, Carl-Maga KTH-International Groundwater Arsenic Research Group, Department of Land and Waterb Department of Chemistry, University of Kalyani, Kalyani, 741235, West Bengal, Indiac School of Geosciences, The University of Sydney, Sydney, NSW 2006, Australiad Department of Geology and Geophysics, Indian Institute of Technology-Kharagpur, Kharaer dietary components0; Halder et al., 2012).

water Arsenic Research Group,H Royal Institute of Technology,+46 8790 6857.

rights reserved.and grey sand aquifers in shallow depthle drinking water supplya, Dipti Halder a,b, Amit K. Kundu b, Ujjal Mandal b,s Mrth e, Gunnar Jacks a

ources Engineering, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden

, 721302, West Bengal, India

al Environment

v ie r .com/ locate /sc i totenvstrategies include treatment of surface water (pond as well as riverwater), rain water harvesting, installation of dug wells, tubewell (TW)switching (changing of drinking water source from high As TWs tonearest low As TWs), installation of deep TWs (depth>150 m) and dis-tribution of households and community lters (Jakariya et al., 2003,2007). Nevertheless, the community acceptance of most of these strat-egies is not very promising because of technical, economical and socialconstraints (Jakariya et al., 2003, 2007; Ahmed et al., 2006; Nath et al.,

403A. Biswas et al. / Science of the Total Environment 431 (2012) 4024122008a; Johnston et al., 2010). People living in the rural villages do notprefer alternative drinking water sources other than TW water locatedwithin their premises. Ironically, the use of TWwater for domestic pur-pose is deeply embedded into the daily life of the rural villagers(Johnston et al., 2010). Thus TW switching and deep TW installationonly gained popularity and effectively reduced the number of exposedpopulation (Ahmed et al., 2006). However, the installation of deepTWs requires advanced drilling technology, which increases the instal-lation cost (5001200 USD) several times as compared to shallow TWsinstallation (depth b70 m, cost 50120 USD) (Hug et al., 2011). Ruralvillagers cannot afford the cost to install household deep TWs. Instead,they need to depend on the community TWs, which are also few innumber or on the deep TWs installed by wealthier people in the com-munity. Additionally villagers are also reluctant to collect drinkingwater fromTWowned by someone else, a powerful social burden to ac-cess safe drinkingwater bymeans of TW switching and deep TW instal-lation (Johnston et al., 2010). Consequently 54% of the exposedpopulation in Bangladesh still have no other options except to drinkAs contaminatedwater (Ahmed et al., 2006). Thus, to prioritize sustain-able As mitigation management and achieve the United Nations (UN)Millennium Development Goals (MDGs) of halving the proportion ofpopulation having no access to sustainable safe drinking water world-wide by 2015 (Target 7C), it becomes extremely imperative to delineatesafe aquifer(s) within shallow depth, which can be targeted by locallyavailable cheap drilling technology.

Few recent studies have investigated the ongoing indigenous drillingpractice by local drillers in the rural Bengal and attempted to correlateaquifer sediment color with the occurrence of As in groundwater (vanGeen et al., 2002, 2003; Jakariya et al., 2007; von Brmssen et al., 2007,2008; Pal and Mukherjee, 2008, 2009; Bundschuh et al., 2010). Basedon the studies in Bangladesh it is reported that grey sand aquifers(GSA) are mostly contaminated with dissolved As (>10 g/L), whereasbrown sand aquifers (BSA) may be safe (b10 g/L) (e.g. von Brmssenet al., 2007). The underlying reason for the occurrence of contrasting dis-solved As in groundwater of these two aquifers has been hypothesizedas: being sub-aerially oxidized during temporal sea level regression peri-od (Umitsu, 1993) the redox condition of the BSA is quite high, notreaching the stage of Fe oxy-hydroxide reduction (von Brmssen et al.,2007, 2008), which is prevailing at the GSA causing high As groundwater(Bhattacharya et al., 1997; Nickson et al., 1998). In someplaces the BSA isalso overlain by low permeable red clay layer (considered as a Palaeosollayer by Goodbred and Kuehl, 2000), which protects the recharges of Asand/or dissolved organic matter (DOC) rich groundwater from the GSA(Shibasaki et al., 2007; von Brmssen et al., 2008; McArthur et al.,2011). Consequently the BSA within shallow depth has been suggestedto be targeted for safe drinking water supply in Bangladesh (vonBrmssen et al., 2007; Bundschuh et al., 2010). However, so far no at-tempt has been made to hydrogeochemically validate the redox statusof these two aquifers, which could be extremely important for assessingthe long term sustainability of BSA for safe drinking water supply.

In order to explore the groundwater evolution and validate redoxstatus at BSA and GSA the present study has undertaken a detailedhydrogeochemical investigation within shallow depth (b70 m). Thedecisive factors responsible for different redox conditions within theaquifers have been delineated by discriminant analysis. Factor analy-sis has been performed to explain the occurrence of As and Mn interms of similar covariance of the hydrogeochemical parameters. Fi-nally effort has been made to justify the potentiality of BSA for sus-tainable drinking water supply.

2. Materials and methods

2.1. Study area settings

The study area (100 km2; 23.0223.14N, 88.4988.62E) is locat-

ed approximately 60 km north of Kolkata city, in the Chakdaha Blockof Nadia district, West Bengal. The area is bounded on the west byriver Hooghly (distributaries of river Ganges) and on the east byriver Ichamati. Our concurrent lithological study (Biswas et al., man-uscript under preparation) has investigated the distribution of differ-ent aquifers (in terms of aquifer sediment color) within shallowdepth of the study area, based on drilling of 29 boreholes by locallyavailable hand suction drilling technology (Fig. 1). Hydrogeologicallythe aquifers of the study area represent part of the Sonar BanglaAquifer (Mukherjee et al., 2007a). Though local drillers are very ef-cient to classify different colors of the sediment, there is still risk ofmisidentication (von Brmssen et al., 2007). In order to minimizethis risk, aquifers within shallow depth were classied according totwo major sediment color categories: viz grey and brown. In theeld, sediment form each 1.5 m interval was assigned either grey orbrown color by looking into the washed sediment soon after their re-covery from the borehole, prior to atmospheric oxidation.

The BSA was distributed along central (northsouth transect) andsouthwestern region, whereas the distribution of GSA was limited tonorthwest (beside Hooghly river) and eastern region (parallel tonorthsouth transact of BSA) of the study area (Fig. 1). In most ofthe boreholes, drilling revealed the formation of either BSA or GSA.Over the entire study area the thickness of surface aquitard, whichcaps the BSA, was higher than that caps the GSA. Only at centralnorthern part of the study area the GSA was overlying the BSA(Fig. 1). In most of these boreholes, the BSA was separated from theGSA by a red clay layer (Biswas et al., manuscript under preparation).The depth to top of the BSA was highest in southwestern part. Drillingof two boreholes (BH-11 and 12) at the southwestern region wasstopped at 50 m even before reaching any aquifer. However, the drill-er conrmed the presence of BSA at the base of red clay layer aroundthe depth of 7080 m.

2.2. Groundwater sampling and analysis

A total of 57 groundwater samples (35 from GSA and 22 from BSA)were collected from the existing TWs installed in shallow depth(b70 m, except for 3 TWs installed in BSA) close to 29 drilled bore-hole locations (Fig. 1). Prior to groundwater sampling the TWs werepurged continuously until pH, electrical conductivity (EC) tempera-ture (T) and oxidation reduction potential (ORP, latter correctedwith respect to standard hydrogen electrode for Eh) were stabilized.The pH, EC, T, ORP and dissolved oxygen (DO) were measured byHACH multimeter (HQd40) coupled to specic IntelliCAL electrode(pH: PHC10105; EC: CDC40105; ORP: MTC10105; DO: LDO10105).The electrodes for ORP and DO were tted with ow cell to minimizethe contact of water with atmospheric oxygen during measurement.The alkalinity (reported as HCO3) was measured on site by titratingwith 0.02 N H2SO4 before groundwater sampling. From each TWfour sets of groundwater samples were preserved after lteringthrough 0.45 m membrane lters (Axiva) for the measurement ofFe(II), DOC, anions, major cations, trace elements and As(III). Thesample for Fe(II) and DOC was directly ltered into 50 mL prewashedhigh density polyethylene bottles (Tarson), containing 2 mL of con-centrated HCl (12N, Suprapur, Merck). The sample for anions was l-tered into a rubber sealed evacuated vials (VAKU-8, Hindustansyringe &Medical devices Ltd.) by tting a needle with the lter hold-er and was left unacidied. The samples for Fe(II) and anions wererapped with aluminum foil to inhibit photo oxidation of the redoxsensitive parameters. In the eld As(III) and (V) were separated bypassing 20 mL sample through a Disposable Cartridge packed withan ion exchanger, at a ow rate of 5 mL/min (Metal Soft Centre, High-land Park, USA, Meng et al., 2001). The samples for quantication ofAs(III), major cations and trace elements were acidied with HNO3(14N, Suprapur, Merck). In the eld all samples were stored in icebox and nally after return to the laboratory, samples were preserved

in a refrigerator at 4 C until analysis.

23.12 GSA GSAa c

23.10GSA

GSA & BSA

Chakdaha

23.08RS

GSAb GSAb

23.06 BSABSABSA

GSA

23.04GSA

SA

88oc

Latit

ude

he lthinte dcqui

404 A. Biswas et al. / Science of the Total Environment 431 (2012) 402412Iron(II), NH4+ and anions were analyzed overnight after sampling

B

23.0288.50

Borehole L

Fig. 1. The study area map: a. India, b. West Bengal, white circle in the map indicates tDistrict. The distribution of brown sand aquifer (BSA) and grey sand aquifer (GSA) wiarea enclosed by white dashed line represents distribution of BSA, the area outside whisents the area, where GSA overlies the BSA. The satellite image of the study area was ato minimize redox alteration. From each bottle of HCl acidied sam-ples, 2 mL was pipette out for the analysis of Fe(II) and total Fe spec-trophotometrically by the O-phenanthroline method (APHA, 1998).The remaining volume of sample was preserved for DOC analysis byShimadzu 5000 TOC analyzer. Anions and NH4+ were analyzed inthe same sample by Metrohm Ion Chromatography (model 761 Com-pact IC) using Metrosep Anion 1 (No. 12007935) and Metrosep Cation12 (No. 00300349) column respectively. The major cations and traceelements were analyzed by inductively coupled plasma optical emis-sion spectrometer (ICP-OES, Varian Vista-PRO). Ten percent of thesamples (n=6) were selected randomly to reanalyze to test the pre-cision of analysis by ICP-OES. For all the elements the precision of the

Table 1Covariance among the hydrogeochemical parameters.

pH Eh EC HCO3 Cl Ca2+ Mg2+ Na+ K+

pH 1.00 0.06 0.47 0.56 0.24 0.37 0.39 0.60 0.04Eh 1.00 0.35 0.19 0.36 0.27 0.26 0.10 0.61EC 1.00 0.89 0.70 0.90 0.81 0.66 0.39HCO3 1.00 0.39 0.84 0.81 0.66 0.18Cl- 1.00 0.63 0.53 0.38 0.48Ca2+ 1.00 0.75 0.47 0.26Mg2+ 1.00 0.35 0.24Na+ 1.00 0.11K+ 1.00Si4+

SO42

PO43

NH4+

DOCAlFeMnAs

Underlined Pearson correlation coefcients are signicant at pb0.001.analysis was >97%. Furthermore, the concentration of As in all sam-

4.0 km

.52 88.54 88.56 88.58 88.60

LongitudeationGroundwaterSampling Point

ocation of Chakdaha Block in West Bengal, c. satellite image of Chakdaha Block, Nadiashallow depth of the study area has been shown by white and blue dashed line. Theashed line represents distribution of GSA and area enclosed by blue dashed line repre-red from Google Earth 6.0.2 (for color picture see the web version of the article).ples [for As(III) as well as total As] were also reanalyzed by hydridegeneration atomic absorption spectrometer (HG-AAS, Varian AA240,detection limit b1 g/L) (APHA, 1998). The mutual agreement be-tween As concentrations measured by ICP-OES and HG-AAS was>99% (pb0.01).

2.3. Multivariate statistical analysis

2.3.1. Discriminant analysisThe software package IBM SPSS Statistics 19 was used to perform

discriminant analysis, in order to characterize the decisivehydrogeochemical parameters responsible for redox zonation within

Si4+ SO42 PO43 NH4+ DOC Al Fe Mn As

0.44 0.09 0.22 0.42 0.45 0.13 0.20 0.09 0.080.01 0.22 0.65 0.52 0.19 0.04 0.70 0.53 0.560.02 0.48 0.46 0.45 0.22 0.06 0.57 0.05 0.240.21 0.23 0.32 0.40 0.24 0.09 0.38 0.22 0.240.22 0.46 0.45 0.35 0.18 0.15 0.55 0.23 0.090.01 0.43 0.33 0.30 0.17 0.08 0.46 0.16 0.160.08 0.44 0.38 0.33 0.09 0.02 0.46 0.02 0.250.30 0.19 0.07 0.21 0.38 0.00 0.12 0.23 0.100.12 0.29 0.59 0.44 0.22 0.06 0.66 0.29 0.331.00 0.40 0.09 0.06 0.32 0.11 0.18 0.22 0.04

1.00 0.06 0.07 0.15 0.13 0.33 0.11 0.021.00 0.88 0.22 0.01 0.86 0.53 0.64

1.00 0.34 0.02 0.76 0.41 0.681.00 0.04 0.18 0.06 0.11

1.00 0.10 0.04 0.051.00 0.51 0.74

1.00 0.411.00

the shallow aquifers as well as to classify groundwater samplesaccording to different redox zones. Eighteen hydrogeochemical pa-rameters listed in Table 1 were included in the discriminant analysis.To eliminate the inuence of different measurement units and makethe data set dimensionless the analytical results were rst normalizedto mean value of zero and standard deviation of 1. The normalizationalso increases the inuence of variable with small variance and viceversa (Liu et al., 2003). A two group (less and strong reducing zone)

interpretation of the factor loadings for each factor. The factor scores(contribution of each factor) for individual well were also computed.The spatial distribution of these factor scores may provide better in-sight of the underlying geochemical processes responsible forhydrogeochemical evolution in an area (Liu et al., 2003).

2.4. Groundwater speciation modeling

For individual groundwater sample speciation modeling was per-formed with the geochemical software package of PHREEQC (version2.8) usingwateq4f database (Parkhurst and Appelo, 1999) to calculatethe value of PCO2 and saturation indices (SI=log [IAPKT1], whereIAP and KT are ion activity product and equilibrium solubility constantat ambient temperature respectively) of major mineral phases thatmay regulate the groundwater chemistry in two aquifers. Further,the values of pe (where pe=16.9Eh at 25 C) corresponding toFe(III)/Fe(II) and As(V)/As(III) redox couples were also calculatedfrom measured concentration of Fe(III), Fe(II) and As(V), As(III) re-spectively by PHREEQC to identify the key redox process that regu-lates redox potential in the aquifers.

3. Results

Table 2Classication results of discriminant analysis.

Aquifer redoxcondition

No. ofTWsbased onsedimentcolor ofthescreeningdepth

No. of TWs assigned by DA %Correct

Strong reducing Less reducing

Strong reducing 35 33 2 94.3Less reducing 22 0 22 100

Total 57 33 24 96.5

405A. Biswas et al. / Science of the Total Environment 431 (2012) 402412component analysis (PCA) was adopted to construct the correlationcoefcient matrix (Table 1) based on which eigenvalues and eigen-vectors were calculated and the data were transformed into factors.The criterion was set to extract factors with eigenvalue only greaterthan 1 (Kaiser, 1958; Davis, 1987; Reyment and Joreskog, 1993; Liuet al., 2003). After extraction of the factors, factor axis was rotated fol-lowing Kaiser's Varimax algorithm (Kaiser, 1958) for easier

0.22

0.34

0.46

0.58

0.70

-0.4

0.2

0.8

1.4

2.0a b

ices

(SI)model was adopted for this analysis. Groundwater samples were ini-tially grouped according to sediment color of the screening depth ofTWs in the aquifers. The canonical discriminant function coefcientsfor each hydrogeochemical parameters were computed to calculatethe individual discriminant function (Lee et al., 2008; Hus et al.,2010), which ultimately separated the groundwater samples intotwo different redox zones.

2.3.2. Factor analysisIn order to explain the evolution of groundwater chemistry, spe-

cically the occurrence of dissolved As and Mn in terms of associationof the hydrogeochemical parameters, factor analysis was also per-formed using software package IBM SPSS Statistics 19. The samedata set of discriminant analysis was used for factor analysis. PrincipalBSA GSA0.10

BSA-1.0

BSA GSA-1.0

-0.2

0.6

1.4

2.2

3.0

BSA-1.0

-0.4

0.2

0.8

1.4

2.0ed

Satu

ratio

n In

d

Fig. 2. Distribution of saturation indices (SI) for mineral phases: a. calcite (CaCO3); b. do(MnHPO4); e. siderite (FeCO3) and f. vivianite [Fe3(PO4)2:8H2O] for groundwater samples c3.1. Groundwater composition

3.1.1. Major ion chemistryThe physical properties and ionic compositions of groundwater

samples collected from BSA and GSA have been summarized in sup-plementary data (SD) Tables 1 and 2 respectively. The results indicatethat in both aquifers groundwater has circum-neutral pH (6.877.48)with similar temperature range (26.328.2 C). The ranges of electri-cal conductivity (BSA: 501935 S/cm, median: 663 S/cm; GSA:3561177 S/cm, median: 715 S/cm) suggest roughly similar ex-tents of water-sediment interactions in both aquifers. The groundwa-ter in both aquifers is predominantly of CaMgHCO3 type. However,other hydrochemical facies such as CaMgHCO3Cl, CaMgNaHCO3 and CaNaMgHCO3 are also sometimes present due to thelocal enrichment of particular ions. There is no signicant differencebetweenmajor ion concentrations in groundwater of two aquifers ex-cept for enrichment of K+ (median: GSA4.42, BSA2.32 mg/L) andCl (median: GSA18.4, BSA6.49 mg/L) in GSA. The enrichment ofCl in GSA is also consistent with relatively higher EC in this aquifer.The ranges of calculated PCO2 indicate that for both BSA (range:

GSA BSA GSA-1.0

-0.4

0.2

0.8

1.4

2.0

GSA BSA GSA-7.0

-4.8

-2.6

-0.4

1.8

4.0

c

f

lomite [CaMg(CO3)2]; c. rhodochrosite (MnCO3); d. manganese hydrogen phosphateollected from BSA and GSA in the study area.

103.42 to 102.9 atm, median: 103.01 atm) and GSA (range:103.63 to 102.61 atm, median: 103.05 atm) groundwater samples(except one sample from GSA) have higher PCO2 than the atmosphericPCO2 (103.5 atm). The calculation of SI further reveals that ground-water samples are nearly at equilibrium with calcite and dolomite(Fig. 2).

3.1.2. Distribution of redox sensitive speciesIn all groundwater samples collected from BSA and GSA the concen-

tration of DO and NO3 are below detection limit (BDL). The comparisonof observed pe values (calculated from measured Eh) with that corre-spond to Fe(III)/Fe(II) andAs(V)/As(III) redox couples reveals that the ob-served pe values for the samples of BSA and GSA fall respectively alongthe upper and lower end of calculated pe range of the Fe(III)/Fe(II)redox couple (Fig. 3). This comparison suggests that though the redoxcondition in both aquifers is controlled by the Fe(III)/Fe(II) redox equilib-rium, the groundwater in GSA ismore reducing than groundwater in BSA.The groundwater in GSA is more enriched with species such as PO43,NH4+ and SO42 compare to groundwater in BSA (Fig. 4). Despite enrich-ment of SO42 in GSA the concentration is not very high (range: BDL26.2 mg/L, median: 0.30 mg/L). Strong pungent odor of hydrogen sul-phide (H2S) was noted in fewwells from GSA during sampling. How-ever this odor does not necessarily correspond with low SO42

concentration in groundwater. Moreover, the odor of H2S was absentin groundwater of BSA. This indicates that SO42 reduction is not en-tirely responsible for low SO42 concentration in groundwater of thestudy area. The scarcity of sulphidic minerals in the aquifer sedimentmight also limit initial concentration of SO42 in groundwater(Mukherjee and Fryar, 2008). Though in Bangladesh, von Brmssenet al. (2007) have reported the enrichments of DOC and HCO3 in

groundwater of GSA; in present study the concentrations are similarin both aquifers (Fig. 4).

In the GSA, 32 (91%) and 19 (54%) of the 35 collected groundwatersamples have As concentration above the WHO safe drinking waterguideline value of 10 g/L and Indian national drinking water stan-dard of 50 g/L respectively (Fig. 4). The median value (54.3 g/L) ishigher than both WHO guideline and national standard. However, inonly 1 of 22 samples collected from BSA, the dissolved As concentra-tion exceeds 10 g/L (Fig. 4). In GSA, As is predominantly present asAs(III) [% of As(III) range: 75100, median: 95.6] (SD Table 2), where-as for samples collected from BSA, when As concentration exceeds in-strumental detection limit is mostly present as As(V) [% of As(V)range: 5.41100, median: 100, excluding BDL values] (SD Table 1).Following the same trend the groundwater of GSA is also moreenriched with dissolved Fe (range: 0.8211.3 mg/L, median:5.31 mg/L) compare to groundwater of BSA (range: 0.242.87 mg/L,median: 0.44 mg/L) (Fig. 4). Though, in both aquifers Fe is predomi-nantly present as Fe(II), the extent of predominance is signicantlyhigher at GSA (range: 84.1100%, median: 94.5%) compared to BSA(range: 36.697.2%, median: 71.2%) (SD Tables 1 and 2). The aquiferwise distribution of dissolved Mn in groundwater follows the oppo-site trends of As and Fe. Only 6 (17%) and 7 (20%) samples out of 35collected from GSA have Mn concentration above previous WHOguideline value of 400 g/L and Indian national drinking water stan-dard of 300 g/L respectively. However out of 22 samples of BSA, in18 (82%) and 21 (95%) samples dissolved Mn concentration exceeds400 g/L and 300 g/L respectively (Fig. 4).

The distribution of SI for the major mineral phases, which mayregulate the concentration of Fe(II) and Mn(II) in groundwater re-veals that groundwater in BSA are mostly at equilibrium with respectto rhodochrosite (MnCO3), whereas equilibriums with respect to

e e(II

xic

Fe(te, ros

ere

t d

s (cre scon

406 A. Biswas et al. / Science of the Total Environment 431 (2012) 402412Glauconite, othersilicates, sideri

rhodoch

Siderite, vivianite, rhodochrosite, earlier

formed sulfidic minerals Pyri

te

Fig. 3. Schematic representation of redox zonation in the aquifer. The observed pe valuecompared with theoretical pe values (when the activities of equilibrated redox phases acorrespond to Fe(III)/Fe(II) and As(V)/As(III) redox couples [calculated from measured-10 -5 0

H2O

/H2

C(IV

)/C(-I

V)S(

VI)/S

(-II)

V(IV

)/V(II

I)V

(V)/V

(IV)

Se(IV

)/Se(-

II)U

(VI)/

U(IV

)A

s(V)/A

s(III)

Calculated pFe(III)/FAs(V)/As(III)

Anoxic redox zone

Sulfi

dic Post-OMethanic

Theoritical pe of diff

Expected mineral phases adifferent redox zones are also shown. The gure has been developed from Mukherjee et al5 10 15

Fe(II

I)/Fe

(II)

Mn(

IV)/M

n(II)

N(II

I)/N(

-III)

N(V

)/N(II

I)C

r(VI)/

Cr(II

I)Se

(VI)/

Se(IV

)

O2/H

2O

)

GSA

BSA

Oxic redox zone

Hematite, goethite, Mn-oxides

II) and (III) vivianite, ite

nt redox couples

Max.

75 percentileM

edian25 percentileM

in.

Legend

ifferent redox zones

Obs

erve

d pe

alculated from measured Eh) of groundwater samples collected from BSA and GSA areame or PH2=PO2=1 atm at 25 C) of different redox couples and calculated pe valuescentration of Fe(III), Fe(II) and As(V), As(III) respectively]. Expected mineral phases at

. (2008).

BSA GSA200

280

360

440

520

600

BS0

6

12

18

24

30

0.0

1.4

2.8

4.2

5.6

7.0

BS0.0

1.4

2.8

4.2

5.6

7.0

0

HC

O3-

(mg/L

)

SO42

- (mg/L

)

NH

4+ (m

g/L)

PO43

- (mg/L

)

and

407A. Biswas et al. / Science of the Total Environment 431 (2012) 402412siderite (FeCO3) and vivianite [Fe3(PO4)2:8H2O] are prevailing ingroundwater of GSA (Fig. 2). Saturation Indices values further indi-

BSA GSA

Fig. 4. Distribution of redox species in groundwater of BSABSA GSA

600

1200

1800

2400

3000

Mn

(g/L

)cate that MnHPO4 and Fe(III) mineral phases such as ferric hydroxide[Fe(OH)3], goethite (FeOOH), hematite (Fe2O3) and magnetite(Fe3O4) are stable in both aquifers of the study area.

3.2. Discrimination of groundwater samples

The canonical discriminant function coefcients for each hydrogeo-chemical parameters have been summarized in SDTable 3. The canonicalcorrelation value of 0.91 indicates that two groups model explain 83%variation among the hydrogeochemical parameters. The correspondingdiscriminant loadings of each hydrogeochemical parameter (Fig. 5)suggest that the concentrations of PO43, Fe and NH4+ together with the

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

Disc

rim

inan

t Lo

adin

gs

Eh pHEC Ca2

+

Na+

Mg2

+

K+

Si4+

NH

4+

HC

O3-Cl-

SO42

-

PO43

-

Mn As Al

DO

CFe

Fig. 5. Discriminant loadings of different hydrogeochemical parameters. The higher ab-solute value of a loading represents greater contribution of the corresponding param-eter to discriminate groundwater as less reducing or strong reducing.value of Eh are the main discriminators to predict membership ofgroundwater samples to the less or strong reducing group. The hit

A GSA BSA GSA0.0

1.6

3.2

4.8

6.4

8.0

A GSA BSA GSA0

4

8

12

16

20

BSA GSA0

80

160

240

320

400

DO

C (m

g/L)

Fe (m

g/L)

As (

g/L

)

GSA. The legend of the box whisker plot is same as Fig. 2.ratio (overall predictive accuracy of the discriminant function) value of96.5% (Table 2) strongly supports the successful classication of ground-water samples as less or strong reducing, based on sediment color of thescreening depth of respective TW. All the groundwater samples of BSAare reclassied as less reducing (100% accuracy), whereas only 2 ground-water samples (HG14 andHG22) of GSA,whichwere initially grouped asstrong reducing are classied as less reducing (94.3% accuracy) by dis-criminant analysis (Table 2).

3.3. Delineation of factors responsible for groundwater evolution

Themutual covariance of the hydrogeochemical parameters (Table 1)indicates that the major ions are strongly positively correlated to eachother and also individually positively correlates to EC. Arsenic showsstrong positive correlation with NH4+, Fe and PO43 and negative correla-tion with Eh, whereas the correlations of Mn to these parameters are op-posite to that of As. Dissolved organic carbon does not show anysignicant correlation to other hydrogeochemical parameters.Based on these correlations, factor analysis extracted 4 factors witheigenvalue greater than 1, which cumulatively explain 74% of thetotal hydrogeochemical variations (SD Table 4). Among the four fac-tors, factor 1 (26.8% variance) has strong positive loadings of majorions and EC (Fig. 6), which in turns is responsible for total dissolvedsolids in groundwater. Factor 2 (26.1% variance) has positive loadingsof Fe, NH4+, PO43 and As and negative loadings of Eh and Mn (Fig. 6).This factor can be termed as As mobilization as well as Mn immobiliza-tion factor. The spatial distribution of factor scores of this As mobiliza-tion and Mn immobilization factor for each well closely overlaps withthe distribution of GSA in the study area (Fig. 7a), which indicates thatthis factor is mainly prevailing in GSA. Factor 3 possibly representstwo chemical processes. The positive loadings of DOC and NH4+ andnegative loading of SO42 (Fig. 6) characterize the SO42 reduction

process, whereas the positive loadings of Si4+, Na+ and HCO3 togetherwith negative loading of pH (Fig. 6) suggest the weathering of plagio-

4. Discussion

Eh pH EC Ca2

+

Na+

Mg2

+

K+

Si4+

NH

4+

HC

O3- Cl-

SO42

-

PO43

-

Mn As Al

DO

CFe

Eh pH EC Ca2

+

Na+

Mg2

+

K+

Si4+

NH

4+

HC

O3- Cl-

SO42

-

PO43

-

Mn As Al

DO

CFe Eh pH EC Ca2

+

Na+

Mg2

+

K+

Si4+

NH

4+

HC

O3- Cl-

SO42

-

PO43

-

Mn As Al

DO

CFe

Eh pH EC Ca2

+

Na+

Mg2

+

K+

Si4+

NH

4+

HC

O3- Cl-

SO42

-

PO43

-

Mn As Al

DO

CFe

-1.0

-0.5

0.0

0.5

1.0

-1.0

-0.5

0.0

0.5

1.0

-1.0

-0.5

0.0

0.5

1.0

-1.0

-0.5

0.0

0.5

1.0

Fact

or L

oadi

ngs

Fact

or L

oadi

ngs

Factor 1 Factor 2

Factor 3 Factor 4

Fig. 6. Factor loadings of different hydrogeochemical parameters in groundwater of the study area. The value of loading0.5 represents signicant contribution of thecorresponding parameter.

408 A. Biswas et al. / Science of the Total Environment 431 (2012) 402412clase minerals (NaAlSi3O8). However, the spatial distribution of factor3 scores does not indicate any prevalence to particular aquifer(Fig. 7b), whichmight signify that two chemical processes are occurringseparately in two aquifers. Factor 4 has only positive loadings of Al andCl (Fig. 6).Longitude

23.02

23.04

23.06

23.08

23.10

23.12

Latit

ude

GSA

BSA

GSA

BSA

GSA

GSA

GSA

GSA

GSA

BSA4.0 km

88.50 88.52 88.54 88.56 88.58 88.60

a

Longitude

23.02

23.04

23.06

23.08

23.10

23.12

Latit

ude

GSA

BSA

GSA

BSA

GSA

GSA

GSA

GSA

GSA

BSA4.0 km

88.50 88.52 88.54 88.56 88.58 88.60

a

Fig. 7. Spatial distribution of factor scores of individual TW for: a. factor 2, responsible forduction and weathering of plagioclase minerals (NaAlSi3O8) in groundwater of study area. Ttinuous decrease in prevalence of the factors. The area enclosed by white dashed line represeGSA (for color picture see the web version of the article). The factor score of groundwater sduring plotting of spatial distribution to avoid superimposing of prevalence of the factor in4.1. Evolution of major ion chemistry in shallow aquifers

The geochemical evolution of groundwater is mainly regulated bycarbonatemineral dissolution, silicateweathering and ion exchange pro-cesses with the aquifer materials (Dowling et al., 2003; Mukherjee and88.50 88.52 88.54 88.56 88.58 88.60Longitude

23.02

23.04

23.06

23.08

23.10

23.12

Latit

ude

GSA

BSA

GSA

BSA

GSA

GSA

GSA

GSA

GSA

BSA4.0 km

b

88.50 88.52 88.54 88.56 88.58 88.60Longitude

23.02

23.04

23.06

23.08

23.10

23.12

Latit

ude

GSA

BSA

GSA

BSA

GSA

GSA

GSA

GSA

GSA

BSA4.0 km

b

As mobilization as well as Mn immobilization and b. factor 3, responsible for SO42 re-he change of colors of the contour lines from red to yellow to green represents the con-nts distribution of BSA and the area outside white dashed line represents distribution ofamples collected from GSA of the area, where GSA overlies BSA (n=6), was excludedBSA and GSA in this part of the study area.

slow kinetics (Mukherjee et al., 2007a, 2009 and ref. there in).Mukherjee and Fryar (2008) and Mukherjee et al. (2009, 2011) havedocumented in details about the interplay of these processes for thedeeper part of Sonar Bangla aquifer of western Bengal Basin, which en-compasses the study area. In this study as suggested by Mukherjee etal. (2009), solute mass-balance approaches have also been adopted todemonstrate the extent of these chemical processes occurring in BSAandGSA. On the bivariate plot of Na+-normalized Ca2+ versus Na+-nor-malized HCO3 andMg2+ (Fig. 8), groundwater samples from two aqui-fers are clustered between the zone of global average silicate andcarbonate weathering, which indicates that both carbonate mineral dis-solution and silicate weathering are concurrently occurring in the twoaquifers. However, the trend of the groundwater samples to fall alongthe y=2x line on the bivariate plot of Ca2+ + Mg2+ versus HCO3 to-gether with high ratio of HCO3 to Na+ + K+ in the groundwater ofBSA and GSA (Fig. 9) suggests that carbonate mineral dissolution ismore prevailing in the two aquifers. The relative enrichment of PCO2 inthe aquifers compared to atmospheric PCO2 and equilibriums of ground-water with respect to the calcite and dolomiteminerals are further con-sistentwith the prevailing carbonatemineral dissolution in the aquifers.To explore the extent of active cation-exchange in the aquifers, HCO3

and SO42 corrected Ca2+ + Mg2+ (to exclude the contribution ofCa2+ and Mg2+ from carbonate and silicate weathering) was plottedagainst Cl corrected Na+ (to exclude the Na+ input from atmosphericdeposition) (Fig. 10). Jankowski et al. (1998) have reported that for theaquifer with active cation exchange between Na+ and Ca2+ + Mg2+,

0.01

0.10

1.00

10.00

100.00

0.0 0.1 1.0 10.0 100.0

0.01

0.10

1.00

10.00

EvaporiteDissolution

SilicateWeathering Carbonate

Dissolution

EvaporiteDissolution

SilicateWeathering Carbonate

Dissolution

BSA GSA

HC

O3-

/ Na+

Mg2

+ / N

a+

Ca2+ / Na+

0.0 0.1 1.0 10.0 100.0Ca2+ / Na+

Fig. 8. Bivariate plot of Na+ normalized HCO3 and Mg2+ versus Na+ normalized Ca2+

to identify prevailing minerals weathering in groundwater of study area.

409A. Biswas et al. / Science of the Total Environment 431 (2012) 402412Fryar, 2008). The relative contribution of these processes to the evolutionof major aqueous chemistry largely depends on the bulk mineralogy ofthe aquifer sediment as well as kinetics of the chemical weathering pro-cesses (Tardy, 1971; Faure, 1998). Possibly the longer groundwater res-idence time in Bengal Basin as a consequence of low hydraulic gradient(1.000.01 m/km from northern to southern part) and extended ow

path is responsible for silicate weathering and ion exchange despite of

0

2000

4000

6000

8000

10000

0 1000 2000 3000 4000 5000

0

2000

4000

6000

8000

10000

12000

0 2000 4000 6000

BSA GSA

y = 2x

y = 2x

y = x

y = x

HC

O3-

(M

)H

CO

3-

(M

)

Ca2+ + Mg2+ (M)

Na+ + K+ (M)Fig. 9. Bivariate plot of HCO3 versus Ca2+ + Mg2+ and Na+ + K+ to compare the ex-tents of carbonate and silicate weathering in groundwater of study area.the slope of this equivalent bivariate plot would be 1 (i.e. y=x).Thus the slope of0.65 and0.86 respectively for BSA and GSA indi-cates that cation-exchange is also to some extent responsible for the ob-served groundwater compositions in both aquifers, while the extent ismore prominent in the GSA. Notwithstanding the higher extent of cat-ion exchange in GSA, the concentration of Na+ is relatively higher (al-though not very signicantly) in the groundwater of BSA (median,BSA: 27.2 mg/L and GSA: 22.4 mg/L). This small enrichment of Na+ inBSAmight be due to the chemical weathering of NaAlSi3O8 as indicatedby factor 3. The higher extent of cation-exchange inGSAmight also con-tribute to the relative enrichment of K+ in the groundwater (Mukherjeeet al., 2009). To trace the origin of Cl enrichment in the groundwater ofGSA, Cl-was plotted against Na+ (Fig. 11). The previous mineralogical

y = -0.86x + 1.04

-2.5

-1.5

-0.5

0.5

1.5

2.5

GSA

[(Ca2

+ +

M

g2+ )

(H

CO3-

+ SO

42- )]

(meq

/L)

[(Ca2

+ +

M

g2+ )

(H

CO3-

+ SO

42- )]

(meq

/L)

Na+ - Cl- (meq/L)

y = -0.65x + 1.05

-2.5

-1.5

-0.5

0.5

1.5

2.5

-2.5 -1.5 -0.5 0.5 1.5 2.5

Na+ - Cl- (meq/L)-2.5 -1.5 -0.5 0.5 1.5 2.5

BSA

y = -x

y = -x

Fig. 10. Bivariate plot of HCO3 and SO42 corrected Ca2+ +Mg2+ versus Cl corrected+Na to determine the extent of cation exchange in the aquifers of study area.

410 A. Biswas et al. / Science of the Total Environment 431 (2012) 402412studies of aquifer sediment from the study area by Gault et al. (2005)Nath et al. (2005, 2008c) and Charlet et al. (2007) did not report thepresence of Cl-minerals in the sediment. In absence of any potentialsource of Cl within the aquifer sediment, the Cl content in ground-water might be because of atmospheric deposition and mixing withconnate sea water. In Bangladesh, von Brmssen et al. (2007) havereported the enrichment of Cl and higher EC in the BSA due to mixingwith connate sea water. However in the present study area, the enrich-ment of Cl in shallow GSA compare to deeper BSA discards the possi-bility of mixing with connate sea water. Furthermore, if Cl content ingroundwater is entirely of meteoric origin, it should be balanced byequivalent Na+. However, the bivariate plot of Na+ versus Cl indicatesthat despite higher extent of cation-exchange in the GSA, in somegroundwater samples the ratio of Cl to Na+ is greater than 1 andthis ratio is always less than 1 for the groundwater of BSA. Previousnest piezometric monitoring studies by Biswas et al. (2011) have alsoreported similar enrichment of Cl in near surface aquifer of thestudy area. Thismight indicate that the enrichment of Cl in groundwa-ter of GSA is of anthropogenic origin such as agricultural return ow,septic tank, domestic waste water etc. (Jacks et al., 1999; Rajmohanand Elango, 2006; Nath et al., 2008a). By doing an isotopic studyLawson et al. (2008) have already reported the recharge of evaporatedsurface water, which might get an input from domestic waste water aswell as agricultural return ow (Charlet et al., 2007; Mukherjee et al.,2007b), to the shallow aquifers of this study area.

4.2. Redox zonation and mobilization of As and Mn in shallow aquifers

The enrichment of As in the groundwater of Bengal Basin does notcoincide with high As concentration in the aquifer sediment (Harveyet al., 2002; Swartz et al., 2004; Nath et al., 2005, 2008c), which hasled to conclude that rather than source, the enrichment of As ingroundwater is an artifact of groundwater evolution governed by in-terplay of various biogeochemical interactions (Bhattacharya et al.,

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.0 0.5 1.0 1.5 2.0 2.5 3.0

BSA GSA

y = x

Cl-

(mM

)

Na+ (mM)

Fig. 11. Bivariate plot of Cl versus Na+ to trace the origin of Cl in groundwater ofstudy area.2007). The most widely accepted mechanism of As mobilization inthe Bengal Basin aquifers is microbially mediated reductive dissolu-tion of Fe and/or Mn oxy-hydroxide, to which As is adsorbed or co-precipitated in the aquifer sediment (Bhattacharya et al., 1996,1997, 2002; Nickson et al., 1998; Acharyya et al., 1999; Islam et al.,2004; Gault et al., 2005; Nath et al., 2008b; Biswas et al., 2011). Inaquifers, as a consequence of microbial respiration by oxidation of or-ganic matter, DO and NO3 are reduced at rst and subsequentlyredox status passes through the stages of Mn and Fe oxy-hydroxidereduction, SO42 reduction to methanogenesis respectively (Fig. 3)(Stumm and Morgan, 1981). Though the origin of organic matter inthe aquifers of Bengal Basin is a lively debate (Harvey et al., 2002,2005; Rowland et al., 2006; Sengupta et al., 2008; Neumann et al.,2010; Datta et al., 2011). The reductive dissolution of Mn oxy-hydroxide possibly releases both Mn as well as As into groundwater.Nevertheless, the released As is readily readsorbed onto Fe oxy-hydroxide in the aquifer sediment (Stben et al., 2003; Hasan et al.,2007). Only when the redox status reaches the stage of Fe oxy-hydroxide reduction As is mobilized into groundwater togetherwith Fe and retained in the aqueous phase (Bhattacharya et al.,1996, 1997; Zheng et al., 2004; Mukherjee et al., 2008). Thus oftenFe rich groundwater is enriched with As as well, whereas the concen-tration of As is low in Mn rich groundwater (Biswas et al., 2012).However in the aquifer in presence of high concentration of HCO3

and PO43, Mn and Fe are often precipitated as secondary mineralphases like MnCO3, MnHPO4 and FeCO3, Fe3(PO4)2 respectively (vonBrmssen et al., 2008). Furthermore, in anoxic groundwater Mn canalso be immobilized by adsorption and/or co-precipitation onto sider-ite and calcite by forming metastable solid solutions (FeCO3MnCO3;CaCO3MnCO3), which further recrystallizes to pure mineral phases(Wersin et al., 1989; Saunders and Swann, 1992). The considerableamount of Mn incorporation into the carbonate mineral phases hasalready been reported for the aquifer sediments of Bengal Basin(Bhattacharya et al., 2001; Ahmed et al., 2004; Stollenwerk et al.,2007). The equilibriums of groundwater with respect to MnCO3,MnHPO4, FeCO3 and Fe3(PO4)2 (Fig. 2) support the cycling of Mnand Fe in the aquifers of study area also.

The close overlapping of spatial distribution of factor score of Asmobilization and Mn immobilization factor (factor 2) with the distri-bution of GSA in the study area (Fig. 7a) and the positive loadings ofAs, Fe, NH4+ and PO43 together with negative loadings of Eh andMn for this factor suggest that in GSA the redox status has loweredbeyond the stage of Mn oxy-hydroxide reduction. Currently reductivedissolution of Fe-oxyhydroxide, possibly coupled to microbially me-diated oxidation of organic matter is the prevailing redox processes,which causes high As groundwater in this aquifer. The high concen-tration of NH4+ and PO43 was produced during oxidation of organicmatter (Bhattacharyya et al., 2003; Bhattacharya et al., 2011). Howev-er in BSA, high enrichment of Mn along with low concentration ofNH4+, PO43 and Fe suggests that the redox status is mostly limitedto the stage of Mn oxy-hydroxide reduction. The comparison of ob-served pe values with calculated pe values of Fe(III)/Fe(II) redox cou-ple (Fig. 3) further supports the distinct redox zonation in the twoaquifers. The positive loadings of DOC and NH4+ in combinationwith negative loading of SO42 for factor 3 might represent thateven in some locations of GSA redox status has reached to the stageof SO42 reduction. The notable odor of H2S in some samples of GSAfurther supports the ongoing SO42 reduction process. This SO42 re-duction may sequester dissolved As to some extent from groundwa-ter by co-precipitation with authigenic pyrite as reported in theaquifer sediment of the study area (Nath et al., 2008c).

4.3. Consequences of safe drinking water supply from BSA

The present study indicates that though the concentration of dis-solved As in groundwater of BSA is very low, the concentration of dis-solved Mn is signicantly higher than previous WHO drinking waterguideline as well as Indian national drinking water standard. Recently,the elevated concentration of Mn in drinking water has been also identi-ed as potential threat to human health worldwide (e.g. Buschmann etal., 2007; Ljung and Vahter, 2007; Bundschuh et al., 2010; Nath et al.,2011). Nevertheless the severity of Mn exposure is comparativelylower than As exposure (Hug et al., 2011), prolonged consumption ofdrinking water with elevated Mn may decrease the intellectuality (IQ)among children and also causes neurotoxic effect (Wasserman et al.,2006; Bouchard et al., 2011). It should be mentioned here that recentlyWHO has withdrawn the drinking water guideline for Mn by reasoningthat worldwide commonly observed concentrations of Mn in drinkingwater sources are well below the health based guideline value of400 g/L (WHO, 2011). However, our results contradict with the reasonproposed by WHO and warrant re-evaluation of drinking water guide-

line for Mn in near future. The underlying health risk of Mn in drinking

411A. Biswas et al. / Science of the Total Environment 431 (2012) 402412water needs to be addressed more rigorously before considering formass scale exploitation of BSA as safe drinking water source despite ofsignicantly low As concentration in groundwater. Moreover, the sus-tainability of the BSA in terms of advective ow of groundwater inboth natural and pumping conditions is highly suspected and deservesfuture study to assess the risk of cross contamination (Mukherjee et al.,2011). However, considering the severity of As health risk in rural Bengaldue to limited availability of As safe drinking water sources, the BSA canbe targeted temporarily for As safe drinkingwater with regularmonitor-ing program until alternate As as well as Mn safe drinking water sourcesare explored and made available to the affected population.

Acknowledgements

The authors are thankful to KTH International Groundwater ArsenicResearch Group coordinated by PB. PB acknowledges the nancial sup-port from Swedish International Development Cooperation Agency(Sida) and Swedish Research Council (VR) through the Swedish Re-search Link grant (VR-Sida, dnr: 348-2006-6005) and the Strategic En-vironmental Research Foundation (MISTRA) (Idea Support Grant, dnr:2005-035-137) for targeting safe aquifers in regions with high arsenicin groundwater and options for sustainable drinking water supply.The authors AB and DH are thankful to the Erasmus Mundus ExternalCooperation Window (EMECW-Action II) for providing them doctoralfellowship through EURINDIA Program, coordinated by KTH Royal Insti-tute of Technology. BNwould like to acknowledge University of Sydneyfor Research Support Grant. The support from the UGC-SAP programand Department of Science and Technology (DST), Government ofIndia under the FIST program to Department of Chemistry, Universityof Kalyani is also duly acknowledged. We are thankful to the local vil-lagers for their hospitality during eld campaign. We also deeply ac-knowledge the valuable suggestions of two anonymous reviewers toimprove the earlier version of this manuscript.

Appendix A. Supplementary material

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.scitotenv.2012.05.031.

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Hydrogeochemical contrast between brown and grey sand aquifers in shallow depth of Bengal Basin: Consequences for sustainab...1. Introduction2. Materials and methods2.1. Study area settings2.2. Groundwater sampling and analysis2.3. Multivariate statistical analysis2.3.1. Discriminant analysis2.3.2. Factor analysis

2.4. Groundwater speciation modeling

3. Results3.1. Groundwater composition3.1.1. Major ion chemistry3.1.2. Distribution of redox sensitive species

3.2. Discrimination of groundwater samples3.3. Delineation of factors responsible for groundwater evolution

4. Discussion4.1. Evolution of major ion chemistry in shallow aquifers4.2. Redox zonation and mobilization of As and Mn in shallow aquifers4.3. Consequences of safe drinking water supply from BSA

AcknowledgementsAppendix A. Supplementary materialReferences