Hydrological and geochemical constraints on the mechanism of formation of arsenic contaminated groundwater in Sonargaon, Bangladesh

Applied Geochemistry 23 (2008) 3155–3176

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Applied Geochemistry

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Hydrological and geochemical constraints on the mechanism offormation of arsenic contaminated groundwater in Sonargaon, Bangladesh

Takaaki Itai a,*,1, Harue Masuda b, Ashraf A. Seddique b,c, Muneki Mitamura b, Teruyuki Maruoka d,Xiaodong Li b,2, Minoru Kusakabe a, Biswas K. Dipak c, Abida Farooqi b, Toshiro Yamanaka e,Shinji Nakaya f, Jun-ichi Matsuda g, Kazi Matin Ahmed c

a Institute for Study of the Earth’s Interior, Okayama University, Misasa, Tottori 682-0193, Japanb Department of Geosciences, Osaka-City University, Sugimoto-tyo, Sumiyoshi, Osaka 558-8585, Japanc Department of Geology, University of Dhaka, Dhaka 1000, Bangladeshd Department of Integrative Environmental Science, University of Tsukuba, Tsukuba, Ibaraki 305-8572, Japane Department of Earth Systems Science, Okayama University, 3-1-1 Tsushima-naka Okayama 700-8530, Japanf Department of Civil Engineering, Shinshu University, Wakazato, Nagano 380-8553, Japang Department of Earth and Space Science, Osaka University, 1-1 Machikaneyama-tyo, Toyonaka-shi, Osaka 560-0043, Japan

a r t i c l e i n f o

Article history:Available online 4 July 2008

0883-2927/$ - see front matter � 2008 Elsevier Ltddoi:10.1016/j.apgeochem.2008.06.017

* Corresponding author. Tel.: +81 824 24 7469; faE-mail address: [email protected] (T. I

1 Present address: Department of Earth and PlaneHiroshima University, 1-3-1, Kagamiyama, Higashii-H739-8526, Japan.

2 Present address: State Key Laboratory of EnvironInstitute of Geochemistry, Chinese Academy of ScienRoad, Guiyang 550002, China.

a b s t r a c t

The geochemical characteristics and hydrological constraints of high As groundwater inSonargaon, in mid-eastern Bangladesh were investigated in order to ascertain the mecha-nism of As release into the groundwaters from host sediments in the Ganges–Brahmaputradelta. Samples of groundwater were collected from ca. 230 tube wells in both the rainy anddry seasons. Similar to previous studies, high As groundwater was found in the Holoceneunconfined aquifer but not in the Pleistocene aquifer. Groundwaters in the Holocene aqui-fer were of the Ca–Mg–HCO3 type with major solutes derived from chemical weathering ofdetrital minerals such as plagioclase and biotite. Groundwater with high As was generallycharacterized by high NHþ4 , possibly derived from the agricultural application of fertilizeras suggested by the small variation of d15NNH4 (mostly 2–4‰). Concentrations of Fe chan-ged between the rainy and dry seasons by precipitation/dissolution of Fe oxyhydroxide andsiderite, whilst there was not an apparent concomitant change in As. Inhomogeneous spa-tial distribution of d18O in the Holocene unconfined aquifer indicates poor mixing ofgroundwater in the horizontal direction. Spatial variation of redox conditions is associatedwith localized variations in subsurface permeability and the recharge/discharge cycle ofgroundwater. Hydrogeochemical data presented in this paper suggest that reduction ofFe oxyhydroxide is not the only mechanism of As mobilization, and chemical weatheringof biotite and/or other basic minerals in the Holocene aquifer could also be important asa primary cause of As mobilization.

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1. Introduction

Poor groundwater quality has attracted worldwideattention particularly because of increasing dependenceon groundwater as a source of water for securing the qualityof life. Arsenic has been of special focus because of its hightoxicity. Arsenic poisoning associated with the consump-tion of local groundwater and the consequent serious healthoutcomes have been reported from various parts of the

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3156 T. Itai et al. / Applied Geochemistry 23 (2008) 3155–3176

world, particularly from Asian countries since the late1980s, e.g., Bangladesh, and West Bengal in India (Garaiet al., 1984; Chakraborty and Saha, 1987; Nickson et al.,1998; Chowdhury et al., 1999; Acharyya et al., 2000; Harveyet al., 2002; van Geen et al., 2003), China (Smedley et al.,2003; Gong et al., 2006), Pakistan (Nickson et al., 2005), Ne-pal (Gurung et al., 2005), Cambodia (Polya et al., 2004, 2005;Rowland et al., 2008) and Vietnam (Berg et al., 2001; Agusaet al., 2006). The affected areas in these countries are vast;e.g., in Bangladesh, ca. 46% of wells have As in excess ofthe WHO provisional guide value for drinking water(10 lg/L), and 27% contain more than the national standardof Bangladesh (50 lg/L) (BGS and DPHE, 2001). Arsenic hasgenerally been considered to be of geogenic origin, derivedfrom host sediments, although anthropogenic influenceshave been proposed, viz. pumping for irrigation (Harveyet al., 2002, 2005; Klump et al., 2006), and application of fer-tilizers (Acharyya et al., 2000). The quantitative contribu-tion of anthropogenic activities and the controlling factorsof As release into the groundwater are still not fully known.

There have been many hydrogeochemical studies carriedout in Bangladesh and West Bengal. Early research on theoccurrences of local scale high As groundwater were carriedout in the mid to late 1990s (summarized by Smedley andKinniburgh, 2002). A nationwide field survey was first carriedout by BGS/DPHE/MMI (1999) to document the spatial distri-bution of As-enriched groundwater; i.e., 3534 well waterswere collected with a sampling density of approximatelyone per 37 km2 or an average of about 6 km distance betweeneach well for the entire landmass of the country. The resultsshowed that the high As groundwater existed in the aquiferswithin the Holocene delta and the flood plains of the Ganges,Brahmaputra and Meghna (GBM) rivers. Aquifers in the Pleis-tocene terrace deposits did not contain such elevated concen-trations of As. Arsenic concentrations vary considerably on alocal scale; e.g., van Geen et al. (2003) analyzed about 6000tube wells within a 25 km2 area, and found high- and low-As groundwaters in adjacent wells only 10 m apart in theHolocene aquifers. Many other authors have noticed the sim-ilar occurrences of high- and low-As groundwaters in the dif-ferent areas of that country (Harvey et al., 2002; McArthuret al., 2004; Zheng et al., 2005).

Reductive dissolution of Fe oxyhydroxide is the consen-sus model to explain the mechanism of mobilization of Asinto groundwater in the GBM delta (Nickson et al., 1998;Harvey et al., 2002; McArthur et al., 2004). According tothis hypothesis, As is released via reduction and decompo-sition of Fe oxyhydroxides associated with biodegradationof organic matter in the aquifer. However, this process hasnot been fully supported in terms of several controversialpoints; e.g., source of organic matter (Harvey et al., 2002;McArthur et al., 2004; Rowland et al., 2006, 2007; van Don-gen et al., in press), chemical form of reduced Fe oxyhy-droxides/oxides (Horneman et al., 2004; van Geen et al.,2004), and transportation process of As (Harvey et al.,2005; Polizzotto et al., 2005).

In order to document the formation mechanism of thehigh As groundwaters in the Holocene aquifer, the Holo-cene and underlying Pleistocene aquifers in Sonargaon, lo-cated at the edge of Pleistocene terrace in centralBangladesh were drilled. Lithology and mineralogy in the

study area are reported in Mitamura et al. (2008) and Sed-dique et al. (2008). Prior to the drilling, well waters weresampled for geochemical analysis and in particular to doc-ument the spatial distribution of groundwater As. In thispaper, first, chemical and stable isotopic characteristics ofthe groundwaters are described in detail. Then, the pro-cesses of As release into the aquifer are discussed in rela-tion to the hydrogeology and mineralogy of the studiedarea, and a principal mechanism for the formation of thehigh As groundwaters in the GBM delta proposed.

2. Geological settings

The Bengal basin, located downstream of the GBM riversystem, is one of the most active deltaic plains (GBM delta)in the world. A large volume of detritus eroded from theHimalayas has been transported to form the GBM delta.Goodbred and Kuehl (1999) estimated long-term a totalannual sediment load of 1012 kg/a, of which approximately1/3 has been deposited in the subaerial delta and floodplain. Erosion of the ultramafic rocks of the northern partsof the Himalayas and granitic and high grade metamorphicrocks of the central and southern parts, have producedthese sediments and provided the dominant mineralassemblage of quartz, biotite and feldspar (BGS and DPHE,2001).

Fig. 1 shows the surface geology of Bangladesh (Good-bred et al., 2003). The country is geologically divided into4 parts, i.e., modern delta plain in the south, alluvial floodplain in the central part, Pleistocene terraces in the NW (Bar-ind Tract) and central part (Madhupur Tract), and a subsidedbasin (Sylhet Basin) in the upstream Meghna river.

The study area, located at the southeastern edge of thePleistocene terrace deposit, Madhupur Tract, is approxi-mately 3 � 3 km2 in the Sonargaon thana (sub-district). Ageographic map of the study area, in which the meander-ing Old Brahmaputra river is running, is presented inFig. 2a. This river was the main channel of the Brahmapu-tra river presently flowing several hundred km westwardas the Meghna river (Goodbred and Kuehl, 2000). Dissectedterraces, where reddish brown oxidized mud and sand lay-ers are exposed, occur in the western part of the study areaseparated (Fig. 2a, Line A) from the alluvial floodplain inthe eastern part. Natural banks, where settlements com-monly exist, are located along the present and abandonedchannels. Except for such banks, the flood plain is mostlycovered with water during the rainy season (June–Septem-ber). In the dry season, the channel of the Old Brahmaputrariver is almost dry, and the land is used for cultivation.

Observation of the drill core revealed the underlyinggeology and aquifer structure in this area (Mitamuraet al., 2008). A simplified geological cross section is shownin Fig. 2b. The results can be summarized as below:

(i) Quaternary strata in the study area above 100 mdepth are divided into 3 formations; lower sand,middle mud and upper sand layers.

(ii) The age of the middle mud formation was deter-mined to be >54 ka by 14C dating. The sedimentationages of the 3 formations are estimated to be

NarsinghdiNarsinghdi

SonargaonSonargaon

NarayanganjiNarayanganji

DhakaDhaka

Old

Brahm

aput

raRiv

er

23.6 N

23.8 N

90.7

E

0 10 km

MeghnaRiver

90.5

E

Study area

N

Flood plain

Pleistocene terrace

Paludal basin

Delta

Bedrock

Alluvial fan

Madhupur TractMadhupur Tract

Bangladesh

Sita

lakh

yaR

iver

Study area

Fig. 1. (a) Map of Bangladesh. (b) Location map of the study area.

T. Itai et al. / Applied Geochemistry 23 (2008) 3155–3176 3157

Plio-Pleistocene (the lower sand layer), Middle-Upper Pleistocene (the middle mud layer), and Holo-cene (the upper sand layer).

(iii) At the western terrace, which was uplifted due tofault movement, the lower sand layer is exposedon the ground surface.

(iv) The middle mud layer works as an aquitard dividingthe upper and lower aquifers. The surface of thislayer is mostly flat in an E–W direction, while undu-lating irregularly in a N–S direction with depressionsof various sizes.

(v) Two aquifers can be identified in the study area; anunconfined aquifer shallower than ca. 30 m hostedby an upper Holocene sand layer, and a confinedaquifer beyond the 40 m depth hosted by a lower

Pleistocene sand layer. In the northeastern part ofthe study area, in which Pleistocene sediment isdirectly exposed, groundwater in the Pleistoceneaquifer exists at even shallower (<30 m) depth.

(vi) The lower half of the upper Holocene sand layer gen-erally consists of middle to coarse grained sand, sug-gesting high permeability. The majority of the tubewells draw groundwater from this layer.

3. Methods

3.1. Sampling and in situ analyses

Tube wells installed in shallow aquifers of�30 m depth,locally called ‘‘hand tube well”, are densely distributed in

Fig. 2. (a) Enlarged topographic map of the study area. Plotted symbols correspond to the location of sampling wells. The symbols A, B, C, D and E representsamples collected from 8 to 20 m, 20 to 28 m, 28 to 36 m (Holocene aquifer), 36 to 60 m (unspecified groundwater), and below 60 m (confinedgroundwater), respectively. The location of drill sites are represented by a star symbol. Italicised names are the specific names of the wells where 3H wasanalyzed. Line A is the approximate boundary that separates Madhupur Tract (west) from the alluvial plain (east). ‘‘R” represents the sampling point of riverwater. ‘‘M” represents the location of monitoring wells. (b) Schematic geological cross section of study area.


the settlements to satisfy daily water requirements.Groundwaters from 232 tube wells were collected at theend of the rainy season in September 2003 along withwaters from 228 wells previously sampled at the end ofthe dry season in February 2004. Because hand tube wellsare generally not fully screened, and only 3–6 m lengthscreens are attached at the end of the installed pipe, thelength of well pipes can be regarded as the depth of sam-pled groundwater. Locations of the sampling wells weredetermined using a portable GPS receiver (eTrex Vista,GARMIN) with an error limit of <10 m.

Oxidation reduction potential (ORP), pH and electricalconductivity (EC) were measured in situ using portableinstruments (D-55, Horiba Techno Service, Ltd. and SC82,Yokogawa electric co.). The pH was measured by glass elec-trode. ORP was measured as the potential difference be-tween a Pt electrode and a Ag/AgCl reference electrode,and the measured ORP values were converted to Eh (vs.

standard hydrogen electrode) using the following equationrecommended by Horiba Techno Service, Ltd.

Eh ðVÞ ¼ ORP ðVÞ þ 0:206� 0:007� ðT � 25Þ

where T is the water temperature in �C.Groundwater was sampled by continuous pumping

after the readings of pH, ORP, EC and water temperaturebecame stable. Alkalinity was determined in situ by titra-tion with 0.1 N (or 0.16 N) HCl using the mixture of bromc-resol green and methyl red (BCG-MR) as the indicator atpH 4.8.

Each groundwater sample was split and stored in 4 bot-tles and treated in the field as follows. (i) Acidified to0.06 N HCl for determination of total As including particu-late and dissolved As; (ii) filtered through a 0.45 lm mem-brane-filter for analysis of major anions, dissolved organicacids (formate, acetate and oxalate ions), dD and d18O; (iii)


filtered through a 0.45 lm membrane-filter and acidifiedto 0.06 N HCl for analysis of major cations and NHþ4 , dis-solved As and Fe, and d34SSO4 and dissolved organic C(DOC); and (iv) filtered through a 0.45 lm membrane-filterand acidified to 0.09 N H2SO4 solution for analysis of AsIII

and d15NNH4. McCleskey et al. (2004) suggest that acidifi-cation by H2SO4 is a suitable way to avoid AsIII oxidationduring preservation. Samples (i), (ii) and (iii) were pre-served in polyethylene bottles, and samples (iv) were keptin air-tight glass vials. In addition to those samples, 1 L ofgroundwater was also collected from each of the 7 wells,i.e., [LDD3 (60 m; Ledemi), BKB6 (75 m; Bara Khater Bhul-ua), NKD5 (90 m; Darikandi), GLG3 (26 m; Gulnagar),DRK35 (18 m Darikandi), and MCC22 (23 m; Mucharchar),DLD6 (21 m; Daulaudi) in December 2004 for 3H analysis.The samples were selected based on the following con-cepts; (i) to collect from both Holocene and Pleistoceneaquifer, and (ii) the samples having typical water chemis-try and d18O within the regions based on the analyticaldata of groundwaters in the rainy season. Riverwater andrainwater were collected monthly from March to Decem-ber 2004. The sampling locations of riverwaters are shownin Fig. 2a. Rainwater was taken after the first shower atDhaka-Mirpur or the sampling site. The sampling proce-dure for rainwater was the same as that for of groundwa-ter. Chemical fertilizers, i.e., urea, Thiobit, ZnS, (NH4)2SO4,and MgSO4 were purchased in Darikandi village to analyzefor N and S isotopes.

3.2. Analytical methods

Major anions and dissolved organic acids were deter-mined by ion chromatography (DX-120, Dionex for the an-ion analyses). Calcium and Mg were determined with EDTAtitration, and Na and K were quantified by atomic absorp-tion spectrophotometry (SAS7000, Seiko Instruments). Thedetection limits for these elements were �0.1 mg/L. Dis-solved organic C (DOC) was measured by a TOC analyzer(TOC-5000, SHIMADZU). Since sample waters were evapo-rated during pretreatment, the above dissolved organicacids were excluded from the concentration of DOC. Thesaturation index of secondary minerals, e.g., calcite, dolo-mite, siderite, ferrihydrite, etc., was calculated using MINT-EQA2 ver. 2.30 (Allison et al., 1991) for the groundwaterscollected in February 2004, 14 samples from the Pleisto-cene aquifer and 143 samples from the Holocene aquifer.

Total dissolved As was determined by hydridegeneration atomic absorption spectrophotometry(SAS7000, Seiko Instruments). Arsenic(III) was determinedby cathode stripping voltammetry (Holak, 1980) afterfiltration in the laboratory with a solid-phase extractiondisk (Milli-pore) to remove the dissolved organic matters,which interferes with the accurate determination of AsIII,using a potentiostat/galvanostat (AUTOLAB modelPG-STAT 10, Eco chemie) interfaced with a multi-modeelectrode (VA 663, Metrohm). The AsIII analysis was com-pleted within 3 months of sampling. Ammonium concen-tration was determined by a colorimetric Indo-phenolmethod within 3 weeks of sampling. Dissolved Fe concen-tration was determined by a colorimetric o-phenanthrolinemethod.

Hydrogen and O isotope ratios of water were deter-mined from H2 gas produced by the on-line Cr reductionmethod (Itai and Kusakabe, 2004) and a conventionalH2O–CO2 isotopic equilibration method (Epstein and May-eda, 1953), respectively, using mass spectrometry (SIRA10,VG-Micromass, and PRISM, Fison). The H and O isotopiccompositions are expressed in terms of dD and d18O (‰)relative to VSMOW (Vienna Standard Mean Ocean Water).The precision of the analytical data was within ±0.5‰ (dD)and ±0.1‰ (d18O), respectively (1r). For 3H analysis, 1 L ofsampled groundwater was distilled to remove dissolvedsolutes, and then condensed electrically to 50 mL using So-lid Polymer Electrolysis for 3H enrichment (TriPure, Perm-elec Electrode ltd.), before measuring the 3H content usinga liquid scintillation analyzer (Aloka model LB3).

Isotopic compositions of N in NHþ4 of the groundwatersand commercially distributed synthetic fertilizer urea inthe study area were analyzed using an on-line elementalanalyzer continuous flow isotope ratio mass spectrometer(Delta S IRMS, Finnigan-MAT) at Kyoto University afterpretreatment according to the ammonia diffusion protocoldeveloped by Holmes et al. (1998). Precision of the analyt-ical data was within ±0.20‰ (1r). Isotopic compositions ofSO4–S in groundwater and fertilizers (Thiobit, ZnS,(NH4)2SO4 and MgSO4) were measured using He-gas con-tinuous-flow isotope-ratio mass spectrometry (ANCA-GSL, SerCon) at Osaka University, after pretreatmentaccording to the protocol developed by Carmody et al.(1998). The precision of the analytical data was within±0.2‰ (1r).

3.3. Water table monitoring

Three observation wells (W1, W2 and W3) were drilledat the southern end of Darikandi village (specified as ‘‘M”in Fig. 2a) within a 10 m radius to monitor the seasonalvariation of the groundwater table during January toDecember in 2005. The depths of the screens were 6–12 m for W1, 18–27 m for W2, and 45–54 m for W3.

4. Results

In order to constrain the relationship between geologyand the occurrence of high As groundwater, the investi-gated groundwaters were classified into two groups basedon the locality and the depth of the wells installed. Onegroup comprises most of the groundwaters drawn fromthe wells shallower than 36 m located in the alluvial plain,where the Holocene sediments host the unconfined aqui-fer. The other group comprises groundwater drawn fromdepths greater than 50 m from the wells installed in thePleistocene sediments beneath the alluvial plain and allgroundwaters collected from the western Pleistocene ter-race (Gulnagar village), which is a recharge zone of thestudied groundwater in the Pleistocene aquifer. Accordingto this classification, the number of groundwater samplesin the Holocene and Pleistocene aquifers were 178 and23, respectively. The groundwaters drawn from the wellsof 36–50 m depth are excluded from these two categories,since the aquifer cannot be specified without further


information about the subsurface geology. Thus, thesegroundwaters are tentatively classified into a third groupnamed ‘‘unspecified”. Because groundwaters drawn fromwells >100 m depth are too deep to know the hydrologicalrelationship to those in the shallower aquifers, thosegroundwaters are used only as reference data.

4.1. Major chemical composition of groundwater

All the 178 groundwaters collected from the Holoceneaquifer have a pH within the range 6.5–7.4. The studiedgroundwaters are dominantly Ca–Mg–HCO3 type (Fig. 3),similar to those previously reported for other areas in Ban-gladesh (e.g., Zheng et al., 2004; Ahmed et al., 2004). Thekey diagram of cations shows more than 90% of thegroundwaters plot in the region where Ca, Mg and Na+Kfractions are >0.4, 0.2–0.4 and <0.3, respectively. Withincreasing Ca concentration, the Ca:Mg ratio approaches7:2 to 7:3. Less than 10% of groundwaters from the Holo-cene aquifer, mostly found in Daulaudi and Temdi, have aCa fraction <0.4, and Na+K increases with the decreasingCa fraction. The most dominant anion is HCO�3 , with anequivalent fraction among total anions of up to 0.99 witha mean value of 0.82. Some waters, mostly collected fromMammudi and Daulaudi, contained HCO�3 fractions of<0.60 and concentrations of Cl�up to 5.1 meq/L, perhaps

Fig. 3. Piper diagram plotted for the aqueous components of well waters. Shaderepresents groundwater from Pleistocene aquifer.

derived from anthropogenic sources. The fraction of SO2�4

of total anions is generally <0.1. Among the 17 samplescollected from Mammudi village, however, 7 samples col-lected in the rainy season and 6 samples collected in thedry season showed an exceptionally high (>10% of total an-ionic charge) SO2�

4 fraction. The SO2�4 =Cl� ratio ranged from

0.0 to 580 lM/mM. In Bangladesh, surface waters showSO2�

4 =Cl�ratios ranging from hundreds to 3000 lM/mM(Galy and France-Lanord, 1999). Based on the d34SSO4 anal-ysis, Zheng et al. (2004) suggested that very low SO2�

4 =Cl�

ratios (<10 lM/mM) in some regions of Bangladesh isprimarily a result of SO2�

4 reduction and not due to re-charge of low SO2�

4 water. Jessen et al. (2008) draw a sim-ilar conclusion for low SO2�

4 /Cl� groundwaters in the RedRiver Basin, Vietnam. Thirty-three samples from the pres-ent study, most of them distributed in the southern partof Harihardi, Darikandi and Bara Khater Bhulua, exhibit aSO2�

4 =Cl� ratio <10 lM/mM, consistent with the SO2�4

reduction model of Zheng et al. (2004).Groundwaters of the Pleistocene aquifer were repre-

sented by 10 samples from the shallow wells in the westernterrace, and 13 from wells of >50 m depth beneath the allu-vial plain. The former groundwaters have pH ranging 6.9–7.1, and the latter have a pH range of 6.8–7.0. Despite thedifferent depths of the installed wells, the major chemicalcompositions of these groundwaters are similar and dis-

d circle represents groundwater from Holocene aquifer. Square with cross


tinct from most of the groundwaters from the Holoceneaquifer. The equivalent fraction of HCO�3 in the total anionsof the shallow well waters from the terrace is up to 0.93with a mean value of 0.75, and that of Ca2+ in the total cat-ions was <0.46 with a mean value of 0.40. These fractionsare similar to those of HCO�3 and Ca2+ of the deep ground-waters beneath the alluvial plain, which are 0.71 and0.43, respectively. The Na+K fraction increases with thedecreasing Ca fraction. The SO2�

4 =Cl� ratio of the groundwa-ter in the Pleistocene aquifer ranged from 10 to 453 lM/mM. No sample displayed a lower SO2�

4 =Cl� ratio than10 lM/mM, indicating limited occurrence of SO2�

4

reduction.Groundwater from the Holocene aquifer showed NHþ4

concentrations >0.1 mg/L in 139 samples from the rainyseason and 136 samples from the dry seasons, with themaximum values being 7.3 and 8.6 mg/L, respectively.Ammonium-rich groundwaters mostly occured in thesouthern part of Harihardi, Darikandi and Bara KhaterBhulua. Most of the groundwaters did not contain detect-able NO�3 (<0.1 mg/L), while the groundwaters collectedin the rainy season occasionally contain high amounts ofNO�3 , >2 mg/L with the maximum concentration beingabout 16 mg/L. This observation suggests that oxic watercontaminated with NO�3 infiltrates the aquifer in the rainyseason, while such an infiltration rarely occurs in the dryseason. More than 0.1 mg/L of PO3�

4 was detected in 61groundwaters in the rainy season and 52 in the dry seasonwith the maximum value being 12 and 4 mg/L,respectively.

Analysis of dissolved organic C (DOC) was conducted on40 samples collected in the dry season. Three samples col-lected from Pleistocene aquifer ranged in value from 1.2 to2.2 mg/L, while 37 samples from the Holocene aquifershowed a range of 1.1 to 7.9 mg/L. In the Holocene aquifer,high DOC (>4 mg/L) tended to be observed in the southernparts (e.g., Darikandi and Bara Khater Bhulua), while lowDOC (<2 mg/L) was found in the northern part of the studyarea (e.g., Harihardi and Mammudi). Acetate was ubiqui-tous in both Holocene and Pleistocene aquifers. In theHolocene aquifer, the concentration of acetate varied be-tween 1.4 to 11.4 and 1.4 to 14.4 mg/L in the rainy anddry seasons, respectively. Low-acetate groundwater(<4 mg/L) was often observed in the northern part of Hari-hardi, Temdi, and Mammudi, while acetate was generallyhigh (>8 mg/L) in Darikandi and Bara Khater Bhulua.

4.2. Relationships of major chemistry, As and redox activecomponents

Concentrations of total As, including particulate anddissolved forms, were almost the same as those of dis-solved As within analytical precision for more than 80%of samples, and hereafter the concentration of dissolvedAs is used as a proxy for total As for the sake of discussion.The lateral spatial distribution of As concentration ingroundwaters from wells shallower than 36 m is shownin Fig. 4A. The concentration of As in all groundwaters fromthe western terrace (Gulnagar village) is within the limit ofthe WHO guideline (0.01 lg/L). The As concentration varieswidely in groundwaters from the Holocene aquifer. Highly

As-enriched groundwater occasionally occurs as an islandwhich is a few of tens meters wide and a few hundred me-ters long (sometimes called an As hotspot). Kumarchar, themiddle to southern part of Harihardi, and Darikandi aresuch hotspot areas. On the other hand, groundwaters lar-gely free from or low in As occur in 3 areas, i.e., Daulaudi,the south-eastern part of Mucharchar, and the northernpart of Harihardi. In these areas, As-free groundwater isoccasionally found from the wells which are within a fewtens of meters of highly enriched wells.

In order to trace the process of evolution of As enrichedgroundwater chemistry, Ca, Mg and redox active compo-nents (total Fe, NHþ4 and As) of groundwaters from theHolocene aquifer are plotted against alkalinity (given asHCO�3 ) in Fig. 5. The groundwaters are grouped accordingto each village to observe the local scale variation of thechemistry. Relationships between HCO�3 and Ca2+ andMg2+ show good linear correlation, indicating that thosecomponents are mostly dissolved along the chemicalweathering process of silicates, which is a hydration reac-tion promoted by mainly H+ produced from the dissocia-tion of dissolved soil CO2 due to the respiration of soilorganisms.

Alkalinity also increases in the groundwater in associa-tion with the biodegradation of organic matter. Iron-(>5 mg/L) and NHþ4 (>2 mg/L) rich groundwaters mostlyoccur in the southern part of Harihardi, Darikandi and BaraKhater Bhulua villages, and tend to contain high HCO�3(>3 mM). However, HCO�3 -rich (>3 mM) groundwater doesnot always contain high Fe and NHþ4 . Arsenic-rich ground-waters mainly occur in the northern and southern parts ofHarihardi, Darikandi, and Kumarchar villages. Slightly dif-ferent distributions of As and Fe and/or NHþ4 implies differ-ent mobilization processes of these components viadecomposition of organic matter in the aquifer.

The major chemical composition does not change verymuch in groundwater collected from the same well indifferent seasons. Furthermore, there are no systematic dif-ferences in the As, Fe or NHþ4 contents of the groundwatersbetween the rainy and dry seasons, other than for thesouthern part of Harihardi where significantly high Feand NHþ4 are observed in the dry season. Seasonal changesin As [DAs (%) = (Asdry � Asrainy)/ Asmean � 100] content ofthe groundwaters collected from the same well in differentseasons are mostly in the range �20% to +20% (Fig. 6d),while changes in the concentration of NHþ4 and Fe were sig-nificantly higher, in the range �60% to +60% and �40% to+40%, respectively.

Measured groundwater Eh for samples collected in therainy season exhibited only a narrow range and quite prob-ably represents analytical artifacts. These data are not con-sidered further here. Fig. 6a–c shows the relationshipsbetween redox active components and Eh. In the dry sea-son, concentrations of As, Fe, and NHþ4 generally increasewith decreasing Eh. However, NHþ4 and As concentrationsare occasionally high in the groundwaters with high(0.25–0.35 V) Eh. This suggests that As may be mobilizednot only by processes taking place under reducing condi-tions, but also on occasion by processes occurring undermore oxidizing conditions. That lower Fe concentrationsare strongly correlated with lower Eh suggests that disso-

Fig. 4. Map of the distribution of (A) As and (B) d18O in the dry season. Only data from shallow groundwaters (<36 m) are plotted. Regions of high As (or lowEh) are encircled by double lines, while regions of low As (or high Eh) are encircled by dotted lines.


lution and precipitation of secondary Fe minerals such asFe (oxy)hydroxide, siderite, vivianite or mackinawite maybe important mechanisms in controlling Fe concentrationsin the groundwaters.

Fig. 7 shows the relationships of redox active compo-nents of the groundwaters sampled in the dry season.The As concentration increases with increasing NHþ4 anddecreasing SO2�

4 concentrations, consistent with observa-

0 2 4 6 8 100

1

2

3

4

5

HCO3- / mM

Ca2+

/ m

M

0 2 4 6 8 100

1

2

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4

5

HCO3- / mM

Mg2+

/ m

M

0

5

10

15

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0 2 4 6 8 10

Fe /

mg/

L

HCO3- / mM

0

200

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0 2 4 6 8 10

As /

μg/L

HCO3- / mM

0

2

4

6

8

10

0 2 4 6 8 10

NH

4+ / m

g/L

HCO3- / mM

DLDKMC

MCCHHD N

HHD SDRK

BKB

a b

c d

e

Fig. 5. Relationships between HCO�3 and selected dissolved components (dry season). Each symbol represents an area where well water was collected. DLD:Daulaudi, KMC: Kumarchur, MCC: Mucharchar, HHD N: north Harihardi, Mammudi, and Temdi, HHD S: south Harihardi, DRK: Darikandi, and BKB: BaraKhater Bhulua.


tions of mobilization of As and NHþ4 under reducing con-ditions as reported previously in other regions of Bangla-desh (McArthur et al., 2001; Anawar et al., 2003; Zhenget al., 2004; Ahmed et al., 2004). Arsenic is widely be-lieved to be released from Fe (oxy)hydroxide undergoing

decomposition in reducing conditions (e.g., De Vitreet al., 1991), however, it is known that high As is not al-ways correlated with high Fe (BGS and DPHE, 2001;McArthur et al., 2001, 2004; Ahmed et al., 2004; Islamet al., 2004) and this is also observed in this study, for

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0 300 600 900 1200

Eh /

V

As / μg/L

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0 5 10 15 20 25

Eh /

V

Fe / mg/L

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0 1 2 3 4 5 6 7 8

Eh /

V

NH4+ / mg/L

0

200

400

600

800

1000

1200

0 200 400 600 800 1000 1200

Asdr

y / μ

g/L

As rainy / μg/L

+20%

-20%

0

5

10

15

20

25

0 5 10 15 20 25

Fe dr

y / m

g/L

Fe rainy / mg/L

-60%

+60%

a b

c d

e f

DLDKMC

MCCHHD N

HHD SDRK

BKB

0

2

4

6

8

10

0 2 4 6 8 10

NH

4+dr

y / m

g/L

NH4+

rainy / mg/L

+40%

-40%

Fig. 6. (a)–(c): Relationships between Eh and other dissolved components. Abbreviated village names are identical to those of Fig. 5. Solid and open symbolsrepresent samples collected in the rainy and dry season, respectively. (d)–(f): Variation in the concentration of As, Fe and NHþ4 between the rainy and dryseason.


example in groundwater from the northern part ofHarihardi.

Groundwaters in the Holocene aquifer were often satu-rated with calcite (�1.33 < SIcal < 0.64) and dolomite(�2.66 < SIcal < 1.04) as suggested by previous research

(e.g., Ahmed et al., 2004; McArthur et al., 2004), indicatingthat elevated concentrations of HCO�3 in the Holocenegroundwaters could not be attributed to dissolution of car-bonate minerals. More than half (55%) of the groundwatersincluding all the samples containing >0.1 meq/L Fe are sat-

0

200

400

600

800

1000

1200

0 2 4 6 8 10

As /

μg/L

NH4+ / mg/L

0

200

400

600

800

1000

1200

0 5 10 15 20 25

As /

μg/L

Fe2+ / mg/L

0

200

400

600

800

1000

1200

0 20 40 60 80 100 120

As /

μg/L

SO42- / mg/L

a b

c

DLDKMC

MCCHHD N

HHD SDRK

BKB

Fig. 7. Relationships between As and other dissolved components. The results for both rainy and dry season are shown. Abbreviated village names are asdescribed in Fig. 5.


urated or oversaturated with respect to siderite (Fig. 8),with SIsid as high as 1.3. This oversaturation implies thatsiderite is precipitated in the aquifer when groundwateralkalinity and Fe concentration increases. The Fe mineralsthat are stable under the groundwater geochemistry ofthe study area may be evaluated by Eh–pH diagram(Fig. 8). Although goethite is stable under the groundwaterEh and pH regime, ferrihydrite (Fe(OH)3) is unstable to-ward lower Eh and pH values. Groundwaters collectedfrom Darikandi, which were generally reducing andslightly alkaline, plotted close to the boundary of relativestability fields of Fe(OH)3/FeCO3, while Fe(OH)3 wasapparently the most stable Fe phase in groundwaters fromthe other regions; i.e., the northern part of Harihardi (oxicand slightly alkaline) and Daulaudi (oxic and slightlyacidic).

4.3. H and O isotopic composition of waters

Fig. 9 shows the relationship between dD and d18O ofthe groundwaters. All groundwaters plot around the globalmeteoric water line (MWL: dD = 8d18O + 10, Craig, 1961),

indicating that all groundwaters originate from meteoricwater. Variation of isotopes ranged from �46.3‰ to�5.7‰ in dD and �7.2‰ to �2.0‰ in d18O. This range ofd18O is similar to, and the range of dD slightly higher than,that of groundwaters in Bangladesh reported by Aggarwalet al. (2000) (dD = �50‰ to �12‰, d18O = �7.2‰ to�2.4‰). Lower d18O tends to be found in the northern partof the study area, i.e., Daulaudi, northern Mucharchar,northern Harihardi, and Temdi, while higher d18O is morecommon in the southern part of the study area, i.e.. Kumar-char, southern Mucharchar, Darikandi, and Bara KhaterBhulua (Fig. 4B). The spatially variable distribution ofd18O in the Holocene aquifer indicates the presence of mul-tiple recharge zones and insufficient mixing of groundwa-ters in the aquifer.

The isotopic composition of riverwater reflects most re-cent rainfall (overland-flow) and discharge of unconfinedgroundwater (base-flow) (Criss, 1999). Fig. 10a shows themonthly record of d18O of Old Brahmaputra riverwaterduring the period April to December 2004, when the valuesvaried from �7.6‰ to �1.1‰ (Fig. 10b). d18O gradually in-creases during the dry season due to discharge of uncon-

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

0 5 10 15 20 25

SI S

ider

ite

Fe2+ / mg/L

a b

c

Fig. 8. (a) and (b) Eh–pH diagram for the system Fe–O2–CO2–H2O, assuming total dissolved carbonate of 10�2.2 mol/L, and total dissolved Fe of 10�4 mol/L.The diagram is drawn using Geochemist’s Workbench version 6.0 (Bethke, 2006) with the following constraints: (a) Precipitation of hematite, goethite,magnetite, FeO(c) is suppressed, (b) as for (a) but goethite is allowed to precipitate. The regions framed by the dotted line indicate the Eh–pH range in theHolocene groundwater. Three symbols plotted represent the average of groundwater collected from each village. (c) Relationship between the saturationindex of siderite and the concentration of Fe2+. Symbols are as described in Fig. 5.


fined groundwater to the river (Harvey et al., 2005) andalso due to evaporation. Rainwater was found to vary overa wide range with no systematic variation with the monthof samples, viz �9.1‰ (June), �3.7‰ (July), �10.6‰ (Au-gust), and �3.5‰ (September). It is known that the stableisotope ratios of rain change with the variation of volumeand duration of precipitation and sampling timing duringraining, therefore the values reported are not consideredto be representative of the average rainfall for any of themonths sampled.

The O and H isotopic compositions of the groundwatersof the Pleistocene aquifers show a large variation, d18O be-tween �7.1‰ to �4.2‰ and dD between �47‰ and �25‰.The O isotope ratios are consistent with those of riverwaterduring June to September. Thus, the groundwater in thisaquifer must be recharged during the peak of the rainy sea-son without much contamination of surface water. The iso-tope ratios of groundwaters from the Holocene aquifer

range between �6.2‰ to �2.0‰ for d18O and �40‰ and�8‰ for dD, much higher than those of the Pleistoceneaquifer, implying that the surface water remained in theflood plain (mostly in ponds and channel) is incorporatedinto the aquifer at the end of the rainy season.

Table 1 shows the seasonal variation of d18O values ofgroundwaters collected in September 2003 (end of rainyseason), February 2004 (end of dry season), and December2004 (middle of dry season). The d18O values of groundwa-ters in the Pleistocene aquifers collected at different timesdo not show any change except for 3 groundwaters, whered18O varies by >0.5‰, and those collected from the shallowdepths at the western terrace. This result indicates that theresidence time of these groundwaters is long enough tohomogenize d18O. The d18O of groundwaters from theHolocene aquifers have similar values for samples col-lected in September 2003 and February 2004, except foronly 4 groundwaters that show a change in d18O of

-8-50

-40

-30

-20

-10

0

MWLδD = 8δ18O+10

δ18O / ‰

δD/ ‰

-1-2-3-4-5-6-7

Fig. 9. Relationships between dD and d18O of groundwater (dry season).Symbols are as described in Fig. 3.


>0.2‰. However, 52% of groundwaters from Holoceneaquifers (13 out of 25) show d18O values with a differencegreater than 0.2‰ between samples collected in Februaryand December, 2004. The d18O of the groundwaters takenin February is higher than those in December (10 out of13), indicating incorporation of 18O enriched water intothe aquifer in February.

Changes in the water table level clearly responded tothe amount of precipitation (Fig. 10b). Since the ground-water table of the Holocene aquifer is consistent with thelevel of surface water, the lowest water table in May isattributed to the drying up of surface water and also maybe due to excessive drawing of groundwater during thedry season. The sharp rise in the water table from May toJuly reflects recharge of rainwater. Changes in the water ta-ble become small from July to September due to aquifersaturation.

A rough estimation of the residence time of the ground-waters can be made based on the 3H data. Tritium was notdetected (<0.5 TU) in the 4 groundwaters collected fromthe Pleistocene aquifer [60 m depth at Harihardi (LDD 3),75 m Bara Khater Bhulua (BKB 6), 90 m Darikandi (NKD5),and 26 m Gulnagar (GLG3)] and one sample from the Holo-cene aquifer [21 m at Daulaudi (DLD6)]. On the other hand,measurable 3H was detected in the two groundwaters col-lected from the Holocene aquifer; one from a 18 m deepwell in Darikandi (DRK35, 1.49 ± 0.38 TU) and the otherfrom a 23 m deep well in the south of Mucharchar(MCC22, 2.84 ± 0.37 TU). Thus, the groundwaters collectedfrom the Pleistocene aquifer have long residence times ofmore than 50 a, while the Holocene aquifer hosts ground-water recharged after 1953. Those results are consistentwith the fact that there are large contributions of freshlyrecharged surface water in the Holocene aquifer but smal-ler such contributions in the Pleistocene aquifer. Moreover,the results are roughly consistent with the estimation ofresidence time of unconfined groundwaters in the Mun-shiganji district by Klump et al. (2006), who found that

the 3H/3He based residence time of groundwater in theHolocene aquifer gradually increases with depth, and var-ies from ca. 35 a at 10 m depth to �80 a at 40 m.

4.4. Nitrogen isotopic characteristics of ammonium

Fig. 11a shows the relationship between NHþ4 concen-tration and d15NNH4. d15NNH4 was determined for ground-waters collected in the rainy (n = 26) and dry (n = 45)seasons. The samples which were used for d15NNH4 analy-sis were mostly collected from Darikandi and Bara KhaterBhulua where NHþ4 concentrations were sufficient high toenable N isotopic analysis to be carried out. The d15NNH4

values, irrespective of the NHþ4 concentration, are withina narrow range of 1.8–4.1‰, except for 4 samplesobtained in the dry season. The d15NNH4 values for thesamples collected in the rainy season are narrow despitea large variation of NHþ4 concentration. Such a record ofthe d15NNH4 suggests a unique source of NHþ4 . Thed15NNH4 values of the samples collected in the rainy sea-son are generally higher than those for the dry season,e.g., the d15NNH4 of two groundwaters from the southernpart of Harihardi is 3‰ lower in the rainy season thanthe dry season, except for a few samples collected fromDarikandi and Bara Khater Bhulua, where the d15NNH4 ishigher in the groundwater having a lower Eh. Highd15NNH4 values under more oxidizing conditions suggestsnitrification (Kendall and Doctor, 2004). However, such aredox reaction is not as an important factor in controllingthe N isotope ratio as the isotopic composition of thesource material. The N isotopic composition of just oneof the locally consumed fertilizers, urea, was measuredand found to be 0.9‰. The source of NHþ4 is furtherdiscussed in Section 5.2.

4.5. Sulfur isotopic characteristics of sulfate

Fig. 11b shows the relationship between concentrationof SO2�

4 and d34SSO4 of groundwater samples collected inthe rainy (n = 10) and dry (n = 17) seasons. Groundwatercontaining enough SO2�

4 to determine the S isotope ratiohas a limited occurrence in the Holocene aquifer of north-ern Harihardi, Daulaudi and Darikandi. d34SSO4 values ran-ged, from 0.9‰ to 14.3‰ for the groundwaters sampledin the rainy season, and from �3.5‰ to 24.3‰ for thosein the dry season. In both seasons, the d34SSO4 was relativelyconstant (2–5‰) in the northern part of Harihardi exceptfor one rainy sample, which was 10.4‰. Values of d34SSO4

did not change with decreasing concentration of SO2�4 in

the northern part of Harihardi in either season. Six samplesfrom Darikandi have a similar range of d34SSO4 values to thenorthern part of Harihardi, although an increase of d34SSO4

with decreasing concentration of SO2�4 is also observed.

These results indicate that the primary factor controllingd34SSO4 in groundwaters is the variation of source material,whereas isotope fractionation arising from SO4-reductionplays a secondary role (Mitchell et al., 1998). Possiblesources of SO2�

4 in groundwater cannot be inferred by usingisotope variation alone, although it is speculated that a pos-sible source is fertilizer, i.e., Thiobit, ZnS, (NH4)2SO4, andMgSO4 with d34SSO4 values of �0.9‰, +15.7‰, +9.1‰, and

-10

-8

-6

-4

-2

0a

b

4/1/04 6/1/04 8/1/04 10/1/04 12/1/04

Month / day / year

18O

/ ‰

Rainy season Dry season

0

100

200

300

400

500

600

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

0

1

2

3

4

5Av

erag

ed m

onth

ly p

reci

pita

tion

in B

angl

ades

h (m

m)

Gro

undw

ater

tabl

e in

200

5 / G

L-m

Month / 2005

45 to 55 m18 to 27 m6 to 12 m

Fig. 10. (a) Seasonal variation in d18O values from the Old Brahmaputra river during the period April to December 2004. (b) Seasonal variation ingroundwater table and monthly averaged rainfall in Bangladesh during 2005. The water table was monitored from the monitoring well (specified in Fig. 2).Each symbol specifies the depth of the screen. The averaged rainfall is inferred from the data cited on the web site of the Bangladesh MeteorologicalDepartment (http://www.bangladeshonline.com/bmd/).


�5.7‰, respectively, which are in a range mostly similar tothose of the groundwaters.

5. Discussion

5.1. Redox reactions in holocene groundwaters

The groundwaters in the study area have the same geo-chemical characteristics with the previously reported highAs shallow groundwaters; i.e., they contain low SO2�

4 andhigh NHþ4 , implying that As rich groundwater is formed un-der reducing conditions. However, the concentrations of Asare occasionally high in the groundwaters that have less orare even free from dissolved Fe, which is sensitive to ambi-ent redox conditions. A similar relationship between As andFe has been observed by previous researchers who havestudied several areas of Bangladesh; e.g., BGS and DPHE(2001), McArthur et al., 2001 and Ahmed et al. (2004).The dissolved Fe concentration is likely to be controlledby the solubility of Fe bearing minerals under a given redox

environment. High concentration of dissolved Fe is ob-served in reducing groundwater, such as Darikandi village.On the other hand, low concentration of dissolved Fe innorthern Harihardi and Daulaudi indicate that Fe oxyhy-droxide is stable in the groundwater beneath these regions.These facts are basically consistent with thermodynamicprediction (Fig. 8). However, brown patches believed tobe Fe oxyhydroxides were occasionally found in the greyaquifer sediment collected from Darikandi village (Mitam-ura et al., 2008), and goethite has been identified as analteration product of magnetite in the same core samplesby XRD analysis (Seddique et al., 2008). These observationsand thermodynamic prediction indicate that despite non-crystalline Fe hydroxide being unstable, goethite whichpossibly formed in the past can persist even in the mostreducing aquifer in the study area.

Under reducing condition, solubility of secondary min-erals containing Fe(II) should be considered. Among them,the most plausible sink of Fe in the study area is siderite.Iron and HCO�3 rich groundwaters were present in the

http://www.bangladeshonline.com/bmd/

Table 1Chemical composition of groundwaters collected from Holocene aquifer (unit: mg/L, except for Eh, AS, and d18O)

Continued to next page

T.Itaiet

al./Applied

Geochem

istry23

(2008)3155–

31763169

Tabl

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nued

)



0 20 40 60 80 100 120 140-5

0

5

10

15

20

25

SO4

2- / mg/L

34S

SO4 / ‰

15N

NH

4 / ‰

NH4+ / mg/L

-2

0

2

4

6

8

0 2 4 6 8 10

ba

δ δ

Fig. 11. (a) Relationships between the concentration of NHþ4 and d15NNH4 in both the rainy and dry season. (b) Relationships between the concentration ofSO2�

4 and d34SSO4 in both the rainy and dry season. Solid and open symbols represent the rainy and dry season, respectively. Abbreviated village names are asdescribed in Fig. 5.


southern part of Harihardi, Darikandi, Bara Khater Bhuluavillages and were supersaturated with respect to siderite.In Fig. 8, the Fe2+ concentration plateaus when SIsid ap-proaches 1, implying that precipitation of siderite occurredwhen SIsid > 1. Since the rate of inorganic precipitation ofsiderite is 8 orders of magnitude lower than that of calcite(Concepcíon and Christopher, 2004), which is of similarsaturation state, high saturation state needs to precipitatesiderite. High Fe2+ in groundwater despite precipitation ofsiderite being predicted by thermodynamics possibly re-flects slow kinetics of precipitation of siderite. It is as-sumed that the relatively large change in Fe over theseason likely reflects various redox transformation pro-cesses discussed previously in response to water move-ment. In addition to the redox transformation process,release of Fe via chemical weathering of biotite should alsobe considered.

Unlike dissolved Fe, the As concentration does notchange much in the groundwaters between different sea-sons (mostly <20%). Those groundwaters which do showconsiderable change in the concentration of As, e.g.,DLD19, DRK36, GLK3 and TMD8 (Table 1) also show rela-tively large changes in d18O, i.e., �2.7‰ to �4.1‰, �3.9‰

to �3.3‰, �3.5‰ to �3.6‰, and �4.3‰ to �7.2‰ betweenthe rainy and dry seasons, respectively. The observed largevariation of d18O indicates a large variation of influx of sur-face water into the aquifer. The As concentration is obvi-ously higher in the rainy season in DLD19, DRK36, andTMD8, when the Eh is higher. The Fe concentration drasti-cally increased (<0.1 mg/L to 8.6 mg/L) in DRK36 in the dryseason despite a considerable decline in the As concentra-tion (314–40 lg/L).

The oxidation state of As is an important control on itsmobility in groundwater. The bulk of dissolved As ingroundwater is present as inorganic oxyanions, such as tri-valent arsenite and pentavalent arsenate (Smedley andKinniburgh, 2002). Because the mobility of arsenite is high-er than arsenate, reduction of arsenate can be a trigger forAs mobilization (e.g., Zobrist et al., 2000; Takahashi et al.,2004; Islam et al., 2004). The AsIII/total As ratio was deter-

mined only for the groundwaters collected in the dry sea-son. The AsIII /total As ratio does not correspond well tothe Eh, i.e., it tends to be higher in the lower Eh groundwa-ter, while being occasionally comparable in value to thehigher Eh ones. Many researchers have reported that AsIII

is dominant in the As rich groundwaters of the GMB delta,e.g., Bhattacharya et al. (2002) suggested that 67–99% of Aswas present as AsIII in 9 As affected districts in Bangladesh.The long storage time until the analysis (�3 months at themaximum) is not the only reason for this discrepancy. Ar-senic(III) is high in the groundwaters from the northernpart of Harihardi and Mammudi, where the As/Fe ratioand the Eh is relatively high. Some researchers have sug-gested that the reduction of arsenate to arsenite is a domi-nant factor in mobilizing As rather than the reduction of Feoxyhydroxide based on field observations (Mitsunobu et al.,2006, 2008) and laboratory studies (Harbel and Fendorf,2006; Kocar et al., 2006). In the study area, the highest con-centration of As is observed under moderately reducingconditions (northern part of Harihardi) rather than morereducing conditions (e.g., Darikandi and Bara Khater Bhul-ua). The low concentration of dissolved Fe in the northernpart of Harihardi may be due to the precipitation of Fe oxy-hydroxides. The results suggest that As may be mobilizedregardless of the stability of Fe oxyhydroxides.

In order to discuss the process of formation of high Asgroundwater in relation to the subsurface geology, it isvery important to discuss the factors controlling the con-trasting redox conditions in each part of study area. Inthe Daulaudi, Mammudi and northern Harihardi, wherethe Eh of the groundwater is high, contributions of oxicwater seem prominent compared to the Darikandi andBara Khater Bhulua. The lithology of the sediment columnin Daulaudi and Mammudi demonstrates the absence of asilt/fine sand layer in the aquifer sediments, indicatinghigher permeability of the aquifer of those villages incomparison to the other villages in the studied area(Mitamura et al., 2008). Thus, vertical infiltration affectsthe permeable part of the aquifer more than the otherparts. Such spatial variation of permeability related to the


lithology of the subsurface is reported from other studiedsites (e.g., Métral et al., 2008). Variation of d18O can beroughly used as a proxy for permeability. Oxic groundwa-ters in Daulaudi, northern Harihardi tend to have lowerd18O (mostly �6‰ to �4‰), while reducing groundwatersin Kumarchar, Darikandi and Bara Khater Bhulua havehigher d18O (mostly �4‰ to �2‰). Such inhomogeneousdistribution of d18O implies that the recharge/discharge cy-cle of groundwater occurs within a narrow region. Small-scale circulation of groundwater is likely promoted bythe intense discharge of groundwater from irrigation wellsas suggested by Harvey et al. (2005).

5.2. Sources of nitrogen and sulfur

According to the microbial reduction–dissolutionhypothesis, NHþ4 is a product of the degradation of organicmatter (e.g., Harvey et al., 2002; McArthur et al., 2004).However, anthropogenic NHþ4 is also a possible source toconsider. Based on the studies of stable isotopes of N andS, the case for anthropogenic perturbation is argued here.

The d15NNH4 of the groundwaters collected during therainy season mostly ranges within 2–4‰, irrespective ofthe concentration, while the range is larger for those col-lected in the dry season. The d15N values of soil organicmatter vary widely in the broad range of �4.5‰ to +17‰

(Létolle, 1980), however it is usually >5‰ (e.g., Chapelle,2001). If ammonification of soil organic matter, includingpeat, occurs in combination with the degradation of organ-ic matter, the d15N of produced NHþ4 does not change muchor at the most becomes slightly lower compared with thatof soil organic matter (Létolle, 1980). Although the d15Ndata of organic N in the sediment were not obtained,d15NNH4 values of the studied groundwaters are in therange of those derived from the degradation of typical soilorganic matter.

The range of d15NNH4 derived from chemical fertilizers issimilar to that of air (e.g., Mariotti et al., 1988; Chapelle,2001). Li et al. (2007) reported that the d15NNH4 of thegroundwaters from farmland in Sichuan basin (0–5‰),China, reflected the N isotope characteristics of locally ap-plied chemical fertilizers; urea ranges from �2‰ to 0‰

and ammonium bicarbonate ranges from 2‰ to 5‰. TheN isotope ratio of only one locally applied fertilizer, urea,was determined, the d15NNH4 value was �0.9‰, corre-sponding well to the values for urea referred to above.The d15NNH4 values of the studied groundwaters are slightlyhigher than the analyzed urea, but within the range of val-ues reported by Li et al. (2007) for NHþ4 derived from chem-ical fertilizers fractionated after application. In Bangladesh,urea is the dominant N fertilizer, constituting >70% of allthe N fertilizers, while diammonium phosphate is also ap-plied (http://www.bangladeshgov.org/moa/moa.html). Thefertilizers are applied during the months of April, May andDecember in Bangladesh. Since the NHþ4 enriched ground-waters are commonly depleted in Cl� and have d18O valuesof �4‰ to �2‰, they possibly were recharged in the earlypart of the peak of the rainy season (i.e., early July) as notedbefore.

Although the range of d15NNH4 of the studied groundwa-ters can be explained by both the results of fractional vol-

atilization of NHþ4 from chemical fertilizers and soil organicmatter, the former is more likely to explain the observedchemistry. Ammonium appears not only in the reducinggroundwater but also in the oxic groundwater, implyingthat the NHþ4 is not derived from biodegradation, whichusually drives conditions to become more reducing, butin association with infiltration of surface water. The N iso-tope ratio of the NHþ4 in the groundwater derived from fer-tilizer rises due to isotope fractionation via volatilization ofNH3 before infiltration into subsurface. Li et al. (2007) re-ported the case of Sichuan Basin in China, at where thegroundwater from farmland exhibited N isotope ranges be-tween �5‰ and +5 ‰, although the isotope ratio of theurea fertilizer was near identical to the urea sample ana-lyzed in the present study. The combination of low Cl�

and SO2�4 concentrations, elevated NHþ4 concentrations

and homogenous values of d15NNH4 indicates that theNHþ4 is derived from fertilizer and likely transported duringthe rainy season.

Sulfur isotopes can be employed as another tracer ofbiogeochemical reactions and anthropogenic contribu-tions. As shown in Fig. 11b, most of the d34SSO4 fall withinthe range 0–15‰. Since S isotope fractionation is sup-pressed when the concentration of SO2�

4 drops below1 mM (Habicht et al., 2002), isotope fractionation due toSO2�

4 reduction would be minimal in this study. Hence,the S isotopic composition of the source material is likelyreflected in the groundwater SO2�

4 . Isotopic variations ofrain in Asian countries, e.g., China (Li et al., 2007), Japan(Ohizumi et al., 1997), and Korea (Yu and Park, 2004), aresimilar to the d34SSO4 in the groundwaters of the study areasuggesting that rain is a possible candidate as a sourcematerial in the low SO2�

4 groundwater. However, high con-centrations of SO2�

4 in some groundwaters (up to 118 mg/L) are difficult to explain by rain only. Therefore, some con-taminants like fertilizer and household waste water maybe important. Groundwaters containing high SO2�

4 aremainly observed in the northern part of Harihardi. Suchgroundwaters also have high concentrations of Cl� and of-ten NO�3 , suggesting that SO2�

4 in these groundwaters mostlikely originates from household waste water. Fertilizer isalso possible, because most of the d34S values in groundwa-ter are within the range of those of fertilizers.

5.3. Chemical weathering as controlling factor ofgroundwater chemistry

The groundwater chemistry of the study area indicatesthe degree of chemical weathering. Ca2+–HCO�3 type majorcompositions of groundwaters from the Holocene aquiferindicate that Ca2+ is dissolved via chemical weathering ofplagioclase, a reaction which is one of the primary control-ling factors of the major chemical compositions of naturalwaters (Drever, 2002). Magnesium has a weak positiverelationship with HCO�3 suggesting that Mg2+arises fromthe chemical weathering of basic minerals such as biotiteand hornblende. Because concentrations of these cationsin groundwater is obviously higher than riverwater, chem-ical weathering of minerals following recharge must be aprimary factor controlling the chemical composition ofthe groundwater. Additionally, Seddique et al. (2008) point

http://www.bangladeshgov.org/moa/moa.html


out that plagioclase and biotite are more concentrated inthe Holocene aquifer than the Pleistocene sediments thathost the confined aquifer and the overlying aquitard mudlayer, based on the mineralogical and chemical composi-tion of the sediments of the aquifer drilled at Darikandi.

If biotite is weathered in the aquifer sediment, the fol-lowing reaction would occur:

2KMgFe2AlSi3O10ðOHÞ2 þ 14CO2 þ 15H2O

¼ Al2Si2O5ðOHÞ4 þ 4Fe2þ þ 2Kþ þ 2Mg2þ þ 14HCO�3þ 4H4SiO4

When this reaction occurs under anoxic conditions, Fe2+

may persist in the groundwater. High Fe and HCO�3 ground-waters occur in the southern part of Harihardi, Darikandiand Bara Khater Bhulua, where reducing groundwatersprevail. If the reaction occurs under oxic conditions, thenFe oxyhydroxides may precipitate:

2Fe2þ þ O2 þ 2H2O ¼ 2FeOOHþ 2Hþ

In a slightly oxic environment, where NO�3 ; SO2�4 are avail-

able, Fe oxyhydroxide can be formed by consuming thedissolved Fe present in the groundwater. Such an environ-ment appears in the Daulaudi, Mammudi, northern part ofHarihardi where the groundwater is more oxic and slightlyacidic. Low Fe but high HCO�3 groundwaters were commonin these areas.

Detrital minerals are the likely primary source of As inGBM delta plain, although the specific source mineral(s)and transport processes of As from the river upstream havenot been well studied in comparison to the mechanisms ofAs release from secondary Fe oxyhydroxides. Sulfide min-erals derived from granitic and metamorphic regions ofthe Himalaya have been suggested as a source of As(Polizzotto et al., 2006). Some researchers have suggestedthat As is highly concentrated in the silicate and/or sulfidephases of the sediments in GBM delta based on the selec-tive chemical extraction (e.g., Anawar et al., 2003; Swartzet al., 2004). In the study area, however, As fixed in sulfideshould be minor according to the low abundance of sulfideminerals (Seddique et al., 2008) and As K-edge XANES data(Itai et al., 2006). The most likely candidate host phase ofAs is biotite. Sengupta et al. (2004) measured As concentra-tions in biotite of up to 9 mg/kg. In the studied area, biotitefrom the lower part of the Holocene aquifer was found tocontain more than 50 mg/kg of As (Seddique et al., 2008).Since the weathering rate of biotite is relatively fast amongdetrital silicate minerals, the release of As from biotite viachemical weathering should be examined further.

Combining the groundwater chemistry and the miner-alogical evidence of the aquifer sediments, detrital biotiteand/or other basic minerals can be a candidate for thesource of the As in Holocene aquifer. Since the rate ofweathering of detrital minerals is controlled by the solubil-ity of SiO2, the rate of weathering must increase with theincreasing the flow rate of the groundwater. The presenceof high As groundwater under oxic to moderately reducingconditions where the subsurface permeability seems highmay possibly be attributed to the chemical weathering ofbasic minerals and concomitant release of As.

6. Conclusions

In this paper, the hydrogeochemical characteristics of As-enriched groundwater in Sonargaon, mid-eastern Bangla-desh are described. High As groundwaters are generallyreducing, but this is not always the case. Although the vari-ation of groundwater As concentrations is very small be-tween the rainy and dry seasons, dissolved Fe exhibitssignificant seasonal changes. The concentration of dissolvedFe, which likely originated from detrital biotite, is controlledby the precipitation/dissolution of Fe (oxy)hydroxide andsiderite. Redox transformation of these minerals plausiblycontribute to the poor correlation between As and Fe.

Small scale circulation of groundwater evidenced by theinhomogeneous spatial distribution of d18O implies an im-pact of excessive groundwater abstraction. If the rate ofvertical infiltration of surface water is accelerated byexcessive pumping, it would lead not only to contamina-tion from surface derived chemicals but also to water–sed-iment interaction in the Holocene aquifer, where relativelyunaltered detrital minerals still exist. Release of As viaweathering of detrital minerals should be the focus of fur-ther research to better understand As mobilization pro-cesses in Holocene aquifers.

Acknowledgements

The authors thank the scientists and graduate studentsof Dhaka University for their help in field surveys. Arsenicconcentration was determined by Ms. K. Okazaki. Nitrogenisotope analyses were supported by Dr. K. Koba (TokyoInstitute of Technology) and Ms. C. Hori (Kyoto University).Trivalent As analyses were supported by Prof. H. Chiba(Okayama University). Tritium concentration was deter-mined by Dr. K. Asai (Chikyu Kagaku Kenkyusyo. Inc.) sup-ported by Prof. H. Satake (Toyama University). Totaldissolved C was determined by Mr. H. Akashi (OkayamaUniversity). This study was financially supported by grant15403017 from the Scientific Research Fund of the JapanSociety for the Promotion of Science. The grant from theCOE-21 program ‘‘ Establishment of International ResearchCenter for Solid Earth Science” headed by Prof. EizoNakamura (ISEI, Okayama University) also supported thisstudy. This work was partially supported by the JSPS Re-search Fellowship for Young Scientists (to T.M.).

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Hydrological and geochemical constraints on the mechanism of formation of arsenic contaminated groundwater in Sonargaon, Bangladesh

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