Field and Reactive Transport Modeling Study of Arsenic

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Chemodynamics of an arsenic hotspot in a West Bengalaquifer: A field and reactive transport modeling study

Laurent Charlet a,*, S. Chakraborty a,b, C.A.J. Appelo c, G. Roman-Ross d,B. Nath b,e, A.A. Ansari a, M. Lanson a, D. Chatterjee b, S. Basu Mallik e

a LGIT CNRS/UJF, University of Grenoble-I, P.O. Box 53, F 38041 Grenoble Cedex 9, Franceb Department of Chemistry, University of Kalyani, Kalyani, West Bengal, India

c Hydrochemical Consultant, Valeriusstraat 11, NL1071MB Amsterdam, The Netherlands

d Faculty of Sciences, University of Girona, Campus de Montilivi, 17071 Girona, Spaine Department of Geological Sciences, Jadavpur University, Calcutta, West Bengal, India

Received 30 March 2005; accepted 17 December 2006Editorial handling by R.B. Wanty

Available online 20 March 2007

Abstract

Extremely high As concentrations in drinking water of the Ganges Delta (West Bengal and Bangladesh) has emerged asan issue of great concern in the past decade because of its serious impact on the health of millions of people. The distri-

bution pattern of As concentrations in the Ganges Delta region is patchy and there are numerous As hotspots. The pres-ent study is perhaps the first attempt in West Bengal to characterize such a hotspot by geophysical and geochemicalmethods, and to model the transport of the enrichment plume using a 1D reactive transport model (PHREEQC). Thestudy site is located along the Hooghly River, 60 km north of Kolkota City, near the city of Chakdaha. Total As concen-trations in the groundwater range from 0.5 to more than 6 lmol L1; the WHO recommended maximum drinking waterconcentration is 0.13 lmol L1 (i.e. 10 lg L1). Results show groundwater is in chemical equilibrium with siderite and cal-cite, a mineral phase previously shown to be an efficient trap for As(III). Groundwater redox potential is controlled by theFe(OH)3(am)/Fe

2+ couple. The As(III) versus As(V) distribution (42% As(III) and 58% As(V), on average) is not at equi-librium with measured Eh values. No evidence of sulfide solid phases, such as As rich pyrite or arsenopyrite, was found.Although amorphous Fe dissolution is confirmed to play an important role in the release of As, selective dissolution extrac-tions indicate that adsorption of As on carbonates and micas may also be an important component of As cycling in thesediment. Modelling results demonstrate the role of PO34 ;HCO

3 and Fe(II) in mobilizing the As plume, thereby increasing

the threat to the 75,000 inhabitants of Chakdaha.

2007 Elsevier Ltd. All rights reserved.

1. Introduction

Arsenic enrichment in groundwater has been anissue of serious concern worldwide in recent years,particularly in the Bengal Delta Plain (BDP; West

0883-2927/$ - see front matter 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.apgeochem.2006.12.022

* Corresponding author. Tel.: +33 4 7682 8020; fax: +33 4 76828101.

E-mail address: [email protected] (L. Char-let).

Applied Geochemistry 22 (2007) 12731292

www.elsevier.com/locate/apgeochem

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Bengal and Bangladesh) where a large populationrelies on As-rich groundwater for drinking and agri-culture purposes (Acharyya et al., 1999; Das et al.,1994; Bhattacharya et al., 1997; Dhar et al., 1997;Nickson et al., 1998; Ravenscroft et al., 2001; McAr-

thur et al., 2001; Smedley and Kinniburgh, 2002;Charlet and Polya, 2006). Several million people inparts of the Ganges River Delta region are estimatedto be at risk and exhibit symptoms of chronic Asexposure (e.g., keratosis, melanosis, and fatal can-cer, Das et al., 1994; Charlet and Polya, 2006).

Arsenic is both highly toxic and carcinogenic(Morton and Dunnette, 1994). Assessing the risksassociated with groundwater, As enrichmentdepends heavily on the ability to understand andpredict the reactive transport of As. Not only doesthe reactivity and transport of As species depend

on their chemical speciation, so does their toxicity.Reduced As(III) species are more toxic, and adsorbmore weakly than As(V) species, which behave inmany respects similarly to phosphate (Mok andWai, 1994). Present in most waters as H2AsO

4 or

HAsO24 (pK2 = 7.0), As(V) bonds strongly to Fe(and to a lesser extent Al) hydrous oxide surfacesand is therefore highly retarded. The extent ofadsorption strongly depends on pH but also onthe presence of background species that competefor sorption sites, and As(V) can be desorbed by

PO3

4 and HCO

3 anions, as well as by Fe(II) cations(Appelo et al., 2002). In contrast, As(III) is presentin most natural waters as a neutral aqueousAsOH

03 species (pK1 = 9.2), and its adsorption

on hydrous oxides depends little on pH in the neu-tral pH range and is rarely complete. Both ions canadsorb on other substrates as well, such as clays,micas, mackinawite or calcite (Manning and Gold-berg, 1997; Chakraborty et al., in press; Roman-Ross et al., 2006).

The regional distribution pattern of As enrich-ment in the Ganges Delta region is extremely patchyand there are numerous groundwater hotspots.Analysis of a large database of aquifers in WestBengal reveals that As concentrations decrease withdepth (Acharyya et al., 1999). Three levels of aquifersystems in the upper delta plain (UDP) have beendelineated within the 150 m-deep enriched horizon,where mid-level aquifers are relatively more arsenif-erous (Bhattacharya et al., 1997). Hotspot areas arelocated in low-lying areas of the young Gangeticfloodplain, with the most enriched aquifers between20 and 60 m depth. Nickson et al. (1998) and Nath

et al. (2005) proposed that As-rich lenses could have

been deposited in association with paleomeanders.Indeed, the Ganges River is now primarily a mean-dering river, having metamorphosed from a braidedto a meandering form during the Holocene (Singh,1996).

The aquifer system of West Bengal is hydro-dynamically and hydro-chemically very activebecause of monsoonal rains and several anthropo-genic activities, such as intense agriculture andgroundwater exploitation for irrigation and drink-ing purposes (McArthur et al., 2001). The upper-level (shallow) aquifers are often unconfined,whereas the lower-level (deeper) aquifers are semi-confined. The deeper aquifers are generallyinterconnected with shallow aquifers either by trun-cation of a confining silty/clayey layer or as a resultof groundwater pumping (Chowdhury et al., 1999).

Various factors are responsible for the chemicalchanges in groundwater chemistry and thus for theincreasing mobility of As in aquifer systems of theGanges Delta: biological and abiotic reductive dis-solution of Fe oxides (Nickson et al., 1998; McAr-thur et al., 2001), oxidation of sulfides(Chowdhury et al., 1999) and anion exchange(Appelo et al., 2002) (see Charlet and Polya, 2006,for a detailed discussion on these various factors).

In this paper the transport of As from an appar-ent point source in a deep silty sand aquifer in

Chakdaha, West Bengal, India, is discussed. Thestudy site is located 65 km north of Kolkata, alongthe Hooghly River, the largest tributary of the Gan-ges in West Bengal, India. The 4 km 5 km site wasfully characterized by geochemical and geophysicalmethods. Electrical resistivity of the aquifer sedi-ment was measured to distinguish the silty clay-richsurface layer from the deeper productive aquifer,where most wells are extracting groundwater. A net-work of 44 groundwater monitoring wells were sam-pled for 2 a. Groundwater was monitored for pH,Eh, conductivity, dissolved O2, alkalinity, DOC,Fe(II), Mn(II), sulfide, NH4 ; NO

3 , As(III),

As(V), and other major dissolved species. Aquifersediments were sampled from different boreholesand characterized for porosity and grain size distri-bution, as well as for the presence of crystallinephases using X-ray diffraction. A series of selectivedissolution extractions was also performed on sedi-ment cores obtained from a single borehole.

Field results show that the As hotspot developsin a HCO3 -rich, Fe(II)-rich anoxic environment.A 1D groundwater model for the reactive transport

of both As(III) and As(V) is proposed, based on

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experimental data and results obtained from thesmall-scale field study. The results have importantimplications for the assessment of future risk tonearby populations, who may be exposed to thissource of As-rich groundwater.

2. Study site and monitoring well installation

Field investigations were conducted on a deep,silty sand aquifer in Chakdaha, West Bengal. Thestudy covers an area of 20 km2 and is situated inthe heart of the BDP along the Hooghly River, amain tributary of the river Ganga. The BDP encom-passes an integral part of the GangaBrahmaputra,the worlds largest deltaic alluvial deposit. The com-bined discharge of the Ganges and BrahmaputraRivers into the Bay of Bengal is approximately

82,000 m3/s during the monsoon season (JulySep-tember). The sediment load is extremely high, reach-ing as much as 13 106 tons per day during themonsoon season (Coleman, 1969).

The large alluvial plain, spreading southwarddown to the Bay of Bengal, is formed by successions

of a fining upward sequence with occasional claylayers. The Ganga has shifted from time to timeto the west during the Holocene. The area is flatand intersected by small rivers, backwaters, minorstreams and swamps. There is a gentle hydraulic

gradient generally sloping towards the SE. Geomor-phologically the area occurs within the active flood-plain of Hooghly and is characterized by a series ofoxbow lakes and meander scars of varied wave-lengths and amplitudes. Landform features, likenatural levees, separate the paddy-filled floodplainfrom the constructed inhabited area (Fig. 1). Theclimate is tropical, hot and humid: temperaturesrange between 16 C and 42 C; average relativehumidity >65%; and annual rainfall ranges between1295 mm and 3945 mm, mostly concentrated duringthe monsoon (JulyOctober).

Ten 30 m-deep wells were installed along agroundwater flowpath from the enriched zone tothe non-enriched zone close to the school (Fig. 1),i.e. at the As hotspot, a nest of four wells weredrilled within a proximity of 20 m of each other,at depths from 8 to 30 m and undisturbed sediment

Fig. 1. IKONOS satellite image of the study site, Chakdaha Block, Nadia district, 65 km north of Kolkata in West Bengal, India showingvarious types of geomorphic features relevant to As reservoirs and source. The data were processed to emphasize in purple the presence ofwater in streams, ponds, and irrigated fields without vegetation. Images were fused to generate a false colour composite merged map withmultispectral behavior and higher resolution (10 m). Contrasting bright areas of the IKONOS image indicate dry, presumably

uncultivated fields. Green areas show the vegetation of the fields that are cultivated as well as the tree cover that identifies the villages.

L. Charlet et al. / Applied Geochemistry 22 (2007) 12731292 1275


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cores were obtained from one of the four hotspotwells (Borehole B-4, Fig. 2). Fourty-four additionalwells (depth range: 946 m) located throughout thestudy site were also monitored (Fig. 2). This net-work extends from the river shore and nearby ricefields to the center of Chakdaha. Distinctive pale-omeanders are indicated on the satellite photograph(Fig. 1) in the flooded area and particularly at theborder between the floodplain and the city. A sepa-rate study is dedicated to a geochemical and geo-physical study of the floodplain lying along theriver (Metral et al., submitted for publication).

3. Methods

3.1. Aqueous chemical characterization

3.1.1. Sampling

Groundwater was sampled in such a way as toexclude contact with the atmosphere. Whenever

possible, a multiprobe (multilane P4 WTW) fitted

with calibrated Pt redox and dissolved O2 electrodeswas introduced directly into the groundwater.Otherwise, about 50 L of water was pumped witha hand pump before water was sampled. Samplesfor dissolved O2 determination were filtered froma 0.2 L amber glass sampling bottle. Samples formeasurement of Fe(II) and sulfide were immediatelycollected with a syringe and filtered through a0.45 lm cellulose ester filter membrane directly intopre-prepared colorimetric reagent solutions (Lind-say and Baedecker, 1986).

For anion and DOC measurements, a 60-mLsample was filtered into a special untreated amberglass bottle, with specifically designed taps to pre-vent air bubbles entering the bottles. To measuremajor and trace elements, a filtered 60-mL samplewas acidified with 0.5 mL of suprapure concen-trated HNO3 acid in a polypropylene bottle. To pre-vent any organic growth or decay, samples wererefrigerated in an icebox within minutes after collec-

tion and kept cooled (4 C) until analysis in the lab-

Fig. 2. Water level map at the end of the dry season, and well locations located within the center of Chakdaha, West Bengal. During thewet season part of the area is flooded (see Nath et al., 2005, for a detailed description). The railway runs through the middle of the city; the

railway station is located by well #9. Resistivity profiles were recorded along the AA0

transect and reactive transport modelling wasperformed along the BB 0 transect.



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oratory. Sampling, sample filtration, treatment andtitration of O2 and H2S took about 23 h.

3.1.2. Major and trace element analyses

Dissolved metals were determined by ICP-AES

(PerkinElmer 3300 DV Optima) and anions bycapillary electrophoresis (CIA Waters) at LGIT,University of Grenoble, France. Cation and anionconcentrations were also in part measured at theDepartment of Chemistry, University of Kalyaniwith a Methrohm ion chromatograph except in afew cases, results of replicated water analyses inboth laboratories were within 10% of each other.

Redox sensitive constituents (Fe(II) and sulfideions) were measured colorimetrically with a HachDR 2010 spectrophotometer. Dissolved Fe(II) wasmeasured using the o-phenanthroline method

(Vogel, 1989) and dissolved O2 by the Winklermethod. Alkalinity and pH were measured on-sitewith a manual field titrator and a WTW 197 pHmeter, respectively.

3.1.3. Arsenic analysis

Total As was measured by ICP-AES (PerkinElmer 3300 DV Optima) with a detection limit of20 lg L (0.26 lM L1) or by hydride generationatomic fluorescence spectrometer (HG-AFS, Mille-nium Excalibur System, PS Analytical Ltd., Kent,

UK) with a detection limit of 200 ng L1

(2 nM L1). All total As values were well abovethe detection limits.

To selectively analyze for As(III), two methodswere used. In the analytical speciation method(Kuhn and Sigg, 1993), aliquots from the filteredsolutions were diluted at ratios between 1:500 and1:50 into a 0.5 M Na2Hcitrate buffer. As(III) isselectively converted to AsH3 under these condi-tions and measured by AFS. It was verified thatAs oxidation in the citrate buffer continued onlyat a very slow rate. Total As values measured afterpreliminary KI reduction of the solution representthe sum of all toxicologically relevant species i.e.As(III), As(V), and methylated As-acids (monom-ethyl arsenic acid, MMAA; dimethyl arsenic acid,DMAA). In the second method (Kneebone andHering, 2000), species were separated in the fieldusing an anion-exchange resin (Dowex 1 8; 100200 mesh, acetate form). Arsenic is analyzed, as dis-cussed above, after separation of As(III) and As(V)with the anion-exchange resin which retains As(V)but not As(III). The resin is provided in chloride

form and must be converted to the acetate form

with an hydroxyl form as intermediate. For this pur-pose, 3 mL 1 M NaOH are first injected into the col-umn. After rinsing the resin with distilled water toeliminate the excess of NaOH, 5 mL 1 M acetic acidare injected. The resin is rinsed with water, and

50 mL of sample are filtered and acidified with0.5 mL of concentrated HCl to avoid the oxidationof As(III) to As(V). Kim et al. (2000) suggested thataqueous As(III)HCO3 negatively charged com-plexes may form, which solubilize As and affectanionic resin based field speciation studies. How-ever, this was not observed in the present study, asthe fraction of As not retained by the anionexchange resin (and expected to be dominated byAs(OH)3 species) was identical to the As(III) frac-tion measured by HG-AFS. The absence ofAs(III)carbonate complex has been recently con-

firmed by ICP-MS (Wallschlager D., pers. com.).Speciation and saturation state calculations were

carried out using PHREEQC (Parkhurst andAppelo, 1999).

3.2. Sampling and analysis of sediments

Bulk sediment was sampled from various bore-holes, including a paleomeander near the HooghlyRiver (Fig. 2). An undisturbed sediment core wasextracted at 18 m depth from the silty/clayey layer

found at this depth in Borehole B-4 (Fig. 2). The60-cm tube was sealed, put in a sealed bag filled withN2 and transported in an ice box to Grenoble,France, where the bag and the tube were openedin a glove box. Sediments were characterized forporosity using the core method (Blake and Hartge,1986). In this method a cylindrical metal sampleris pressed into the undisturbed sediment core andcarefully removed to preserve a known volume ofsample. This sample was then stored at 10 C andweighed. The material was later dried to constantweight in a freeze-dryer (Christ, Alpha Ia) in thelaboratory.

Samples for sequential extractions were taken atthe middle of the core, in order to avoid the effectof any oxidation which may have occurred duringtransport, and immediately frozen and freeze-dried.Sequential extractions were performed on the


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hydroxylamine hydrochloride; Vogel, 1989). Crys-talline phases were identified using X-ray diffraction(Siemens 5000) and mineral grain-size distribution

was determined by a Coulter Laser Granulometer.

3.3. Hydrological and geophysical methods

Groundwater table levels were measured in 44pre-existing tube wells using an electronic tape withan accuracy of 1 cm (Fig. 2). All wells were mea-sured with respect to mean sea level (msl) usingthe GTS Bench Mark. The water levels were thenused to construct a water level iso-contour mapusing the triangulation method included in SUR-

FER 7.0 software.Satellite imagery was used to obtain information

about geomorphological features and land use pat-terns, an IKONOS satellite image of the study areain Chakdaha was purchased from Space ImagingInc. to map the area. This image provides reflec-tance data in four wave bands at 4-m spatial resolu-tion and panchromatic data at 1-m resolution(www.spaceimaging.com; Fig. 1).

Both vertical electrical sounding (VES) and elec-trical imaging techniques were used to investigate

resistivity in both vertical and lateral directions. InVES, the Schlumberger configuration was used, inwhich the distance between the two current elec-trodes (AB) is successively expanded, while the dis-tance between the two potential electrodes (MN) iskept at a minimum (MN 61/5 AB). The apparentresistivity (qa) is measured using the relation

qa KdV=I

where K is the geometrical factor and dV and I arethe measured potential difference and current,respectively. Apparent resistivity values are plotted

against AB/2 on a loglog graph to give the vertical

electrical sounding curves, which were interpretedusing Resix-P software to determine the true resis-tivity and thickness of the geo-electric layer, defined

here as the computed layer based on the electricalresistivity values of the subsurface aquifersediments.

Electrical imaging involves measuring a series ofconstant separation traverses along a single line asthe electrode spacing is increased with each succes-sive traverse. The Wenner array was used to obtaina reasonable resolution and moderate sensitivity rel-ative to geological noise. The initial electrode sepa-ration was 10 m in the first traverse, which was thensuccessively increased to 20, 30, 40, 50 and 60 m. As

electrode separation leads to greater depth penetra-tion, the measured apparent resistivity can be usedto construct a vertical contour section usingRES2DINV code, which displays the variation ofresistivity both laterally and vertically in the section.

3.4. Reactive transport model

A 1D advective transport model was developedbased on the formulation presented by Appeloand Postma (1993). Vertical flow during the mon-soon season recharges the upper few meters of theaquifer, since 5 m annual fluctuations of groundwa-ter level are observed (Charlet and Polya, 2006, anddiscussion in Section 4.3). However, this verticalflow is neglected in the present model whichdescribes the groundwater flow at 2535 m depthas a horizontal water flow. A 3000-m 1D column,or flow tube, was defined by a series of thirty 100-m-long cells. All cells have the same physical charac-teristics (Table 2), and reactive mineral set up equalto 83 mg Fe per kg solid sediment in agreement withascorbic acid selective extraction measurements.

Four series of cells were distinguished based on their

Table 1Sequential chemical extraction treatments for As-bearing sediment (silty clay sediment sampled at 18 m depth, borehole B-4) solid phases

Fraction Extractant Target phase

F1 1 M MgCl2 pH 8, 2 h, 25 C Ionically bound As (Keon-Blute et al., 2001)F2 1 M NaH2PO4 pH 5, 16 and 24 h, 25 C As strongly adsorbed on Fe oxyhydroxides and humic acids

(Keon-Blute et al., 2001, Zheng et al., 2004)

F3 1 N HCl, 1 h, 25 C As coprecipitated with AVS, carbonates Mn oxides and veryamorphous Fe oxyhydroxides (Keon-Blute et al., 2001)

F4 0.1 M ascorbic acid 0.2 M C6H5Na3O7 tri-sodiumcitrate, 0.6 M sodium hydrogen carbonate 24 h pH 8

As coprecipitated with amorphous Fe oxyhydroxides (Kotskaand Luther, 1994)

F5 0.3 M sodium dithionite, 0.35 M sodium acetate, 0.2 Msodium citrate, 5 h pH 4.4, 60 C

As coprecipitated with crystalline Fe oxyhydroxides (Kotskaand Luther, 1994)

F6 H2O235%, pH 2, 5 h, T 85 C Orpiment and remaining recalcitrant As minerals (Tessier et al.,1979)

http://www.spaceimaging.com/http://www.spaceimaging.com/


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solution chemistry (Table 3). The water composi-tion in Zones 0, 1, 2, and 3 corresponds to that of2535 m deep wells #8, 42, 28 and 30, located alongthe transect BB 0B00 (Fig. 2).

The groundwater composition from the fourwells given in Table 3 was used and 8.95 mmol offerrihydrite/L solid phase was initially equilibratedwith this groundwater. The actual Fe oxyhydroxyde

sand coatings are mixed Fe(II)/Fe(III) solid com-pounds (Horneman et al., 2004), but since no com-plexation constant is available for these poorlydefined solid phases (Charlet and Polya, 2006), fer-rihydrite is used here as a model sorbent. TheDzombak and Morel (1990) and Appelo et al.(2002) surface complexation constants were usedfor As(III) and As(V), and the surface complexationcapacity was coupled to the amount of ferrihydritein PHREEQC-2 (Appelo et al., 2002). The PHRE-EQC numerical simulation employed an explicitfinite difference scheme, upgradient for advectivetransport and central in space for dispersion, with

the spatial and temporal discretization adjusted toensure that grid Peclet and Courant criteria weremet (Pe < 3 and Cr < 1). After the initial equilibra-tion of the solid phase with a groundwater of knowncomposition, a pore volume of groundwater withineach of the 30 cells is moved to the next respectivecell. The groundwater is then equilibrated onceagain with the solid phase and the dissolved As

composition in the flow tube is calculated. A seriesof six shifts was performed, and compositiongraphed as computed after 532 (i.e. 6 88.33) years.Residence time in one cell was obtained consideringa water flow velocity of 9.27 102 m/day and atotal flow path of 3000 m.

4. Results and discussion

4.1. Geomorphology and remote sensing

An IKONOS satellite image of the study area inChakdaha was purchased from Space Imaging Inc.to map the study site. This image provides reflec-tance data in four wave bands at 4-m spatial resolu-tion and panchromatic data at 1-m resolution. TheIKONOS data were processed to emphasize in darkblue or purple the presence of water in streams andponds, and in light purple the presence of irrigatedfields without vegetation (Fig. 1). Contrasting brightareas of the IKONOS image indicate dry, presum-ably uncultivated fields. Green areas show the vege-tation of the fields that are cultivated as well tree

cover that identifies villages. The largest stream

Table 2Concentration ranges of groundwater constituents sampled at Chakadaha, Nadia district, West Bengal between April 2001 and April 2002

Range at study site Zone 0 (cells 05) Zone 1 (cells 612) Zone 2 (cell 1317) Zone 3 (cells 1830)

pH 6.717.67 6.93 7.36 6.97 6.93Pe

2.27 to 0.31 2.05 1.66 1.66 1.66As(III) 0.15.83 2.79 1.52 1.82 1.25

As(V) 2.79 1.52 1.82 1.25Alk (HCO3 483010,300 8710 8150 10,320 6.686Cl 302270 390 950 1600 420P 868 27 35 27 8Ca 19404890 2600 2890 3400 2320Mg 6801750 1230 1150 1750 810Na 3303580 1030 670 1690 730K 70620 110 100 120 120Fe(II) 10177 160 140 170 90Mn(II) 1.928.2 5.6 3.1 3.1 6.2Al 0.154.19 1.04 1.22 1.30 0.15Si 151334 276 284 325 288

The measured groundwater compositions of the four zones given below were used as initial ( t = 0) concentrations in the reactive transport

model. All concentrations are total concentrations, except for Fe(II) and Mn(II) and are reported in lmol L

1.

Table 3Hydrodynamic model parameters

Parameter Units Modelvalue

Measuredrange

vp cm day1 20

E 0.4 0.40 0.42Conductivity 2.34 Am. Fe oxide

content

mg kg1 83 20014,000



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meandering in a southerly direction west from thestudy area is the so-called Hooghly River, a branchof the Ganges which flows downstream throughKolkata City. The flood plain, located on the westside of the study area, is agricultural land with high

moisture content (typical of deltaic alluvium) andno habitation. It is characterized by small fields,shown as pink or green stripes in Fig. 1, distributedperpendicular to former meanders of the HooglyRiver where small streams are now flowing. Irriga-tion is used for the newly introduced dry-season ricecalled Boro that now provides more yield thanthe traditional rice grown during the monsoon sea-son, called amman, and boro rice cultivation andirrigation increased together from 1970 to the pres-ent day. A natural levee, marked as a dark curvedline on the west edge of Fig. 1, separates this flood

plain from the inhabited area and the tree cover.The village is organized along the NS railway trackand around the railway station, where most streetsconverge. Villages and fields share the available landin roughly equal proportion in the study area, wherethe population density is nearly three times higherthan the West Bengal state average. Throughoutthe village, dark blue rectangles denote local ponds,from which clay-rich building materials were exca-

vated. These ponds, which cover in some place$15% of the land, are disconnected from the rivers,but act as engineered aquifer recharge systems dur-ing the wet season, until in the dry season their bot-tom sediments become hydraulically disconnected

from the aquifer, and their water levels remain sev-eral meters above the water table. The distributionof wells in the same area is given in Fig. 3.Responses to a questionnaire collected with the wellsamples indicate that the vast majority of these wellswere paid for by individual households and there-fore are privately owned. The survey data also indi-cate that half the existing wells in the study areawere installed between 1995 and 2000, i.e., afterthe discovery of the epidemic of arsenicosis in WestBengal. Recently however, these wells have been lessand less used for drinking-water supply, since four

150 m deep DPHE wells were drilled and rich fam-ilies connected to their water distribution system.

4.2. The arsenic plume

Total As concentrations in the groundwater aretypical of a hotspot, with a maximum of480 lg L1, surrounded by waters of lower As con-tent (e.g. 10 lg L1) (Fig. 3). The As plume present

0.0

0.5

1.0

1.5

2.02.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

N2305'

E8831' E8832'

Hoogly river Railway stationrailway School

[As]totmol/l

Zone0

Zone1

Zone2

Zone3

Fig. 3. Aqueous As (total) iso-concentration map at the end of the dry season, calculated using SURFER 7.0.



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in the Chakdaha aquifer has significantly differentphysico-chemical characteristics, compared withother analogous sites in the Bangladesh part of theBDP aquifer (Charlet and Polya, 2006). The Chak-daha aquifer has a slightly higher proportion of

As(V) than As(III) species (42% As(III) versus58% As(V)). EhpH diagrams show the oxidationpotential to be controlled by the Fe2+/Fe(OH)3(s)couple (Charlet et al., 2005), and not by the sul-fide/sulfate couple as observed elsewhere (Harveyet al., 2002). Indeed, at sites located in Bangladesh,As(III) is the dominant thermodynamically stableform, and S is present as amorphous acid volatilesulfides (AVS) and aqueous species (Harvey et al.,2002). Under these conditions, As(III), which is lessstrongly sorbed than As(V) on ferrihydrite, becomesadsorbed to some extent on FeS(s) (Wolthers et al.,

2005), as shown to be the case in Bangladesh (Wol-thers et al., 2003).

4.3. Groundwater hydrodynamics

Groundwater pumping during the dry seasondraws more water than is being recharged, as indi-cated by the cone of depression below the Chakdahacity centre (Fig. 2, wells 21, 9, 10 and 11). The centerof the depression cone coincides with the 150 m-deep public water supply well, located near tube

well #9, which supplies a quarter of the drinkingwater for the 75,000 inhabitants of Chakdaha.Groundwater pumping for domestic and industrialpurposes may therefore be responsible for recentlydrawing groundwater from the shallow aquifer intodeeper aquifer levels.

Groundwater recharge is composed of a mixtureof C-rich surface waters which enter the aquifer allyear round, and rainwater which enters the aquiferfrom April until flooding starts in June (Harveyet al., 2002; see Charlet and Polya, 2006, fo r areview). On the onset of monsoon (JuneAugust)groundwater levels rose by $2 m indicating rain-and pond-water as the major source of recharge.However, outside the monsoon period groundwateris mainly recharged from surface water bodies andthe Hooghly River, as indicated from the water leveliso-contour map of pre- and post-monsoon times(Nath et al., submitted for publication). The resultof the detailed investigation on groundwaterrecharge/discharge by the Hooghly River, rainfalland surface water bodies during the pre- and post-monsoon period in the study area are presented

elsewhere (Nath, 2006; Nath et al., submitted for

publication). Recharge by local artificial ponds usedmore than 30 a ago to supply drinking water (e.g.those located near wells #12 and #42, Fig. 2) alsocreate local maxima in the water level, e.g. in June

just before the monsoon started (Fig. 2).

4.4. Aquifer structure and sediment solid phase

Resistivity is primarily a function of porosity,pore fluid resistivity (salinity), temperature and claycontent. Of all these sediment properties, the claycontent and related porosity are the most impor-tant; resistivity generally increases as porosityincreases. The geometry and lithology of the upperaquifer system can thus be defined by soundingand resistivity profiling. The inverse model resistiv-ity section (Fig. 4a) clearly distinguishes the silt/clay

rich, low resistivity surface layer in the upper 13 m(1525 X m) from the deeper productive aquifer(1525 m depth, 2648 X m), where most wells areextracting groundwater. Iso-resistivity contoursrecorded at 15 and 25 m depth indicate a conductiveaquifer and changes in porosity as one gets awayfrom the river (Fig. 4b). The resistivity, and thusporosity, is high below the floodplain at bothdepths, and steadily decreases as one moves towardsthe SE, i.e. along a groundwater flow line.

The deeper productive aquifer sediment material

is a rather homogeneous gray, fine to medium-grained sand, made of Fe(II)/Fe(III) oxide-coatedquartz sand particles, micas, feldspars and carbon-ates, with the finer particles made of illite, magnetiteand illmenite. No peat was found, which has beenpreviously hypothesized to be associated withhigher concentrations of dissolved As, throughreductive dissolution of Fe oxyhydroxides orthrough adsorption of As(V) on detrital organic C(McArthur et al., 2001). Inverse resistivity profilesshow the occurrence of low resistivity in the upperlayer (Fig. 4a) where IKONOS satellite imaged sur-face morphology shows the presence of channelscars and remnants of former courses of the GangaRiver and its tributaries (Fig. 1). These paleomean-ders and oxbow lakes were hypothesized to be thelocation of As deposition (Nath et al., 2005), wheremangrove vegetation may have induced the precipi-tation of As-rich sulfides (As-rich-pyrite and arseno-pyrite). Various authors (Charlet and Polya, 2006,and references included therein) have suggested thatAs may have been recently released through sulfideoxidation by O2, introduced into the aquifer by the

massive increase in dry-season irrigation pumping.



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Bulk sediment samples were collected from various

boreholes (Figs. 2 and 5) and core samples (siltyclay layer at 18 m depth) from Borehole 4 (B-4)where it was assumed, based on geophysical resultsand on the IKONOS image, there were paleomean-ders, but the expected clay-, sulfide- and As-richlenses were not found.

Selective As extraction studies on B-4 sedimentcore samples are summarized in Fig. 6. The totalextracted As concentration was 3.0 0.4 ppm.Two major sinks for As are the amorphous Fe(III)oxide (34% 8% of total extracted As; fractionF4) and acid volatile sulfides and/or carbonates(19% 7% of total extracted As; fraction F3). Theheterogeneity observed in the 30-cm-long sedimentcore collected at 18 m depth is high as shown inFig. 5. Attempts to determine the bulk As speciationby X-ray adsorption near-edge spectroscopy(XANES) at the European synchrotron researchfacility (ESRF) in Grenoble failed, as on-line oxida-tion of As was observed while making the measure-ment. Oxidation of As was tentatively attributed toreaction with Fe(III) oxyhydroxides under the highenergy beam. No evidence of crystallized or amor-

phous solid sulphide was observed. In conclusion

As is sorbed, i.e. adsorbed or coprecipitated (Spos-

ito, 1986), on amorphous Fe oxyhydroxides, AVSand/or carbonates, in addition to micas (Charletet al., 2005).

4.5. Aquifer geochemistry

The groundwater chemistry changes significantlyas water flows along a BB 0 flow line (Fig. 2). TheHooghly River has a pH of 8.25 0.15 and togetherwith irrigated fields it recharges the groundwaterduring post-monsoon (Nath et al., submitted forpublication). Within the first kilometer from theriver shore, pH decreases rapidly to pH 6.9, the aver-age pH value in the aquifer (Fig. 7), while at the sametime alkalinity increases from 6.1 0.5 mmolc L

1

in the river to 1522 mmolc L1 in the aquifer. In

the recharge area, Eh is high and the groundwateris nearly free of aqueous Fe(II).

Groundwater temperature is high, and remainsbetween 26 C and 28 C, except for a few wellsclose by the river, where groundwater temperaturereaches 29 C. Alkalinity and pH measurements,combined with carbonic acid equilibria, lead to a

PCO2 ranging from 102.15 to 100.85 atm, with an

Fig. 4. Geophysical data: (a) resistivity profile along cross-section AA0 (Fig. 2); horizontal and lateral distances are given in meters. Fromtop to bottom: measured apparent resistivity, calculated apparent resistivity and inverse model resistivity pseudosections. (b) Isoresistivitycontours at 15 and 25 m depth, respectively.



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average value of 101.3 atm, i.e. a value far greaterthan atmospheric PCO2 . Calcium is rather constantthroughout the aquifer, with an average concentra-tion of 2.8 0.6 mmol L1. The groundwater isroughly at equilibrium with calcite, with a satura-tion index X ranging from 0.22 to +0.44, withan average X value of +0.14. Other major cations,in order of decreasing concentration, are Mg2+,Na+, K+, Fe2+ and Mn2+ (Table 2). The groundwa-ter is hard, with an average hardness of 140 mgequivalent CaCO3 L

1 (based on the sum of Ca2+

and Mg2+ concentrations).Bicarbonate represents up to 90% of the inor-

ganic anion equivalent concentration. Other anions,in order of decreasing concentration, are Cl,SO24 ; NO

3 and PO

34 . No trend in Cl

concentra-tion could be used to trace the groundwater

recharge or the mixing of different waters. Total

PO34 ranges from 8 to 68 lmol L1. Dissolved

SiO2 is nearly constant, ranging from 0.15 to0.33 mmol L1 throughout the aquifer.

4.6. Anoxia and aqueous Fe(II)

Surface water infiltrating into the upper aquifer isheavily loaded with untreated waste from the 75,000inhabitants of Chakdaha. Consequently, thegroundwater concentration of dissolved organic Cis generally high (0.97.5 mg C L1), with an aver-age concentration of 4.4 1.9 mg C L1, similarto infiltrates from organic-rich sediments (Harveyet al., 2002).

Outside the paddy fields located in the flood plainrecharge area along the Hooghly River (see Metralet al., submitted for publication, for a detailed study

of this recharge area), Fe2+concentrations range

Resistivity contours

at 20m depth

Resistivity contoursat 24m depth

88

88

23

23

b

Fig. 4 (continued)



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between 0.010 and 0.017 mmol L1. Groundwater isat equilibrium with siderite (FeCO3), with a satura-

tion index, X, ranging from 0.80 to +0.47, andaverage X value of 0.05. Dissolved O2 is alwaysbelow 0.06 mg L1, the detection limit of the Win-kler method. Under these conditions, Fe oxidesare likely being reductively dissolved by dissolvedorganic matter, resulting in the groundwaterrecharge becoming enriched in Fe2+and HCO3(Appelo et al., 2002):

4FeOOH CH2O 7H2CO3

4Fe2 8HCO3 6H2O 1

Ferrous iron concentrations shown in Fig. 8 suggestreaction (1) does not occur at the river bed, butrather at the border between the rice field plainand the terrace where the city has been built, thatis, at the location of recent paleomeanders(Fig. 1). Hence, the boundary between oxic and an-oxic groundwaters may be due to: (i) intense re-charge of the western part of the aquifer duringthe monsoon when rice fields are flooded, (ii) thepresence of a former riverbed at the field-cityboundary exhibiting intense microbiological activ-

ity, or (iii) a combination of the two.

4.7. Competitive sorption

In addition to oxidationreduction processes,competition for adsorption sites may also act tomobilize As in groundwater. The groundwaters ofthis aquifer contain high amounts of HCO3 ; PO

34 and dissolved H4SiO4 (Table 2).

Sources of PO34 in the groundwater include oxida-tion of organic matter and fertilizer used in mod-ern agricultural practices (McArthur et al., 2001).Aqueous silica, which results from strong weather-ing of the silicate minerals in Ganges sediments,could also displace As from Fe hydroxide surfaces(Appelo et al., 2002). But silicate concentrationsare nearly equal throughout the groundwater inthe area, which means silica displacement ofadsorbed As will not vary much spatially. Thus,HCO3 (and to a small extent, PO

34 ) anions repre-

sent the most important species which competewith As species for adsorption sites at mineral sur-faces (e.g., Fe/Mn oxyhydroxides, clay mineralsand weathered mica), consequently releasing Asinto the groundwater.

It should be noted that anion exchange is notthe only cause of high As concentrations in West

Bengal groundwater. Most of the As present in

Fig. 5. Vertical lithological distribution pattern along a flow path (cross-section BB 0B00, Fig. 2), indicating various granulometric classes.The approximate depth of the water level is shown. For example at B11, the patterns represent the following classes with increasing depth:silty clay, sandy clay, coarse sand, silty clay, sandy clay, coarse sand; fine sand is indicated in white.



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the area is sorbed, i.e. adsorbed to, or co-precipi-tated with (Sposito, 2004), sediment particles. Theselective extractions performed on the B-4 sedi-ment core give insight into the various sorptionprocesses at work in this heterogeneous medium.Arsenic sorbed to carbonate and acid volatile sul-fides amounts to 0.8 ppm, As sorbed to ferrihy-drite and other amorphous Fe(III) oxides to1.1 ppm and As sorbed to crystalline Fe(III) oxideto 0.4 ppm (Fig. 6). Although the amount of Assorbed to ferrihydrite in the river suspended parti-cles is high (Appelo et al., 2002), it may contrib-ute little to the As hotspot since reductivedissolution does not occur in the present riverbed, but rather, within the sediment at the levelof a paleomeander. Therefore, much of the Aspresent in this groundwater originates from theaquifer sediment, where it may be found as outer-and inner-sphere surface complexes (fractions F1and F2, respectively). However, most of the As

seems to be coprecipitated within amorphous

(fraction F4) and crystalline (fraction F5) Fehydroxides (Waychunas et al., 1993), and withincarbonate or AVS (fraction F3) minerals(Roman-Ross et al., 2003). Therefore, in orderto accurately assess As behavior in these ground-waters, various sorbent phases should be ideallytaken into consideration, and aqueous carbonateand Fe(II) surface complexation must also betaken into account.

4.8. Role of micas

Arsenic also adsorbs strongly to clay particles,which are transported through the river system assuspended load (Ansari, 1997). At pH 67, Frostand Griffin (1977), and Charlet et al. (2001), Char-let et al. (2005) observed 90100% of As(V) and40% of As(III) adsorbs to kaolinite and montmo-rillonite from a US landfill barrier and from aNorth Mortagne industrial site in France, respec-

tively. Saifullah (1988) highlighted the complex

35

30

25

20

15

10

5

0

0 1 2 3 4 5

As (mg/Kg)

Depth(cm)

F1

F2

F3

F4

F5

F6

Fig. 6. Dissolved As concentrations obtained by a sequence of extractions (F1F6) which target different solid phases (Keon-Blute et al.,2001, see Table 1). F1 Ionically bound As; F2 Strongly adsorbed As; F3 As coprecipitated with AVS, carbonates, Mn oxides and very

amorphous Fe oxyhydroxides; F4 As coprecipitated with amorphous Fe oxyhydroxides; F5 As coprecipitated with crystalline Feoxyhydroxides; and F6 Orpiment and remaining recalcitrant As minerals. Results are reported as a function of depth.



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15/20

role that organic matter and clay minerals play,along with Fe and Mn oxyhydroxides, in regulat-ing As concentrations. The Charlet et al. (2001)study also shows about 100% of both As(III) andAs(V) adsorb to Black slag (organic matter) from

North Mortagne, over a pH range of 38. In addi-tion, oxidation of organic matter itself may releaseAs during the reductive dissolution of As-richhydroxides.

About 30% of the total mineral mass in the Gan-ges sediments is present as micas (Ansari, 1997).Micas are transported by the Ganges River as sus-pended load for about 2500 km, during which timethey are chemically and physically weathered(Ansari, 1997). Once buried in anoxic conditions,micas (e.g., phlogopite) can adsorb As in the pres-ence of Fe(II) and transform As and other pollu-

tants into less toxic forms (Charlet et al., 2002). Inparticular As(V) may react with surface Fe(II) toform surface nanoparticles, with a mixed AsFecomposition and mixed oxidation states. A recentX-ray photoelectron spectroscopy (XPS) study ofAs sorbed on micas indicates that the oxidationstate of As, sorbed as As(V) on weathered musco-vite mica grains, may have an oxidation state of(III) or lower (Charlet et al., 2005). A more detailedstudy on adsorptiondesorption of As on mica fromthe Ganges Delta has been carried out with the oxi-

dation status of arsenic systematically studied byXPS (Chakraborty et al., in press, submitted forpublication).

The silt-sized fractions of muscovite and biotitemicas are effective for removal of arsenate and arse-nite from solution in the groundwater pH region(6.57.5; Chakraborty et al., in press). Moreover,an X-ray photoelectron spectroscopic study ofreduction of As by biotite also provides an impor-tant mechanism in the immobilization/mobilizationof As in Bengal and Bangladesh groundwaters (Cha-kraborty et al., submitted for publication). So, micasand other phyllosilicates represent an importantcomponent of the As cycle in the aquifer sediments,although lack of surface (de)protonation data, andlack of surface complexation data with species otherthan As (phosphate, carbonate, silicate) prohibit theinclusion of micas in the following model.

4.9. Modeling As migration

The displacing effect of HCO3 may offer anexplanation for high As concentrations in Chak-

daha groundwater, given that the pH values and

alkalinities of river water and groundwater are verydifferent. The Hooghly river water has a low CO2partial pressure of 103.0 atm and a high pH. Onthe other hand, the hotspot and downstreamgroundwater has a very high alkalinity of 4.8

10.3 mmol of HCO

3 L

1

, as a result of relativelyhigh temperatures, high CO2 partial pressures inthe aquifer, and presumably high microbiologicalactivity in the paleomeander.

The contribution of the various processes werecalculated by first equilibrating the groundwater ineach of the different zones (Table 2, Fig. 3) withthe model solid phase. Organic C, which reducesdissolved O2 and ferrihydrite to give aqueous Fe(II)according to Eq. (1) at the paleomeander, wasassumed to be constant downstream, thus not todissolve any more Fe(III) hydroxides and not

release As upon oxidation (Starr and Gillham,1993). Indeed, the Fe(II) concentration remainsfairly constant downstream in Zones 1, 2 and 3,ranging only from 0.14 to 0.17 mM. The log Kvalueof the reaction:

H3AsO4 2H 2e H3AsO3 H2O 2

was set equal to 16.0 instead of 18.89 (Parkhurstand Appelo, 1999) in order to qualitatively repro-duce the As(III)/As(V) balance observed through-out the study site. Model calculations were then

first performed taking into account only the 8 firstchemical reactions given in Table 4, i.e. only Asadsorption reactions and reactions of proton-ation/deprotonation of reactive surface groups.As the groundwater moves from one 100 m-longcell to the next (residence time in one cell:

Table 4Surface reactions taken into account in the reactive transportmodel (Appelo et al., 2002)

Surface complexation reactions log Kint

(1) >Few

OH + H+

= >Few

OH

2 7.29(2) >FewOH = >FewO + H+ 8.93(3) >FesOH + H+ = >FesOH2 7.29(4) >FesOH=>FesO + H+ 8.93(5) >FewOH + Fe2+ = >FewOFe+ + H+ 2.98(6) >FewOH + AsO34 + 3H

+ = >FewH2AsO4 + H2O 29.31

(7) >FewOH + AsO34 + 2H+ = >FewHAsO4 + H2O 23.51

(8) >FewOH + AsO34 = >FewH2AsO

34 + H2O 10.58

(9) >FewOH + CO23 + H+ = >FewCO23 + H2O 12.56

(10) >FewOH + CO23 + H+ = >FewHCO3 + H2O 20.61

(11) > FewOH PO34 3H > FewH2PO4 H2O 31.29

(12) > FewOH PO34 2H > FewHPO4 H2O 25.39

(13) > Few

OH PO3

4 H

> Few

HPO2

4 H2O 17.72



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88.66 a) and is again equilibrated with the modelmineral reactive surface, one may observe eithera net removal of As from the groundwater, asfor As(III) when water from Zone 1 i.e. the hot-spot enters Zone 2 (Fig. 9a), or a release of pre-

viously sorbed As as observed in Zone 2 As(V)front (Fig. 9b).Model calculations were afterwards repeated,

taking into consideration competition for adsorp-

tion sites by either HCO3 ; PO34 or Fe

2+ or bythe three ions together. The effect of sorption sitecompetition on the As concentration in West Ben-gal groundwater is graphed in Fig. 9a and b.Whereas Fe2+ competition does not affect signifi-

cantly the concentration profile (Fig. 9a), competi-tion by anionic species for reactive surface sitesaffects the propagation of the contaminant plumein different ways. Phosphate ions unexpectedly

0

50

100

150

200

250

300

350

400

0 500 1000 1500 2000 2500 3000

Distance (m)

As(III)concentration(ppb

) As(III) init

without competition

effect of carbonates

effect of Fe(II)

effect of Phosphate

Effect of all components

phosphate

0

50

100

150

200

250

300

350

400

0 500 1000 1500 2000 2500 3000

Distance (m)

As(V)con

centration(ppb) As(V) init

without competition

effect of carbonates

effect of Fe(II)

effect of Phosphate

Effect of all components

phosphate

Zone 1Zone 0 Zone 2 Zone 3

Zone 1Zone 0 Zone 2 Zone 3

a

b

Fig. 9. Results of the 1D reactive transport model predicting plume propagation: (a) As(III) and (b) As(V). The various scenarios differ

depending on whether Eqs. (1)(13) (Table 4) are taken into account or not.



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have little affect on the propagation of arsenateions but have a strong impact on the As(III)fronts: the Front in Zone 3 is accelerated, whereasit is slowed down in Zone 2. However, HCO3remains by far the most important competing spe-

cies (Fig. 9), producing an important desorption ofAs in Zone 2 and thus peak As concentrations insolution which may threaten the Chakdaha CityCentre.

This result confirms previous studies wherehigh As concentrations in groundwater were cor-related with high HCO3 concentrations (Welchand Lico, 1998; Nickson et al., 2000). The displac-ing effect of HCO3 on As sorbed to Fe oxyhy-droxides in the reactive transport model hasbeen confirmed using FTIR spectroscopy. In theirexperiments Voegelin and Hug (2003) oxidized

surface As(III) by H2O2, and they quantitativelyobserved the disappearance of the HCO3 peakas the As(V) peak developed, demonstrating thatoxidative desorption of As leads to increasedsorption of HCO3 .

Thus when sediments deposited from low-carbonate river water containing large amounts ofsorbed As become exposed to the high-carbonategroundwater, As may become mobilized bydisplacement from the sediment surface due to com-petitive effects of carbonate sorption. Paleohydro-

logical studies are being performed to furtherquantify this contrast between deposition and pres-ent day physico-chemical conditions (Guillot andCharlet, submitted for publication). The modelresults suggest that this effect could lead in thefuture to even higher As concentrations than thoseobserved today, and that arsenate and Fe(III)-oxidereduction is not even necessary to achieve the highAs groundwater loads. This confirms previous stud-ies which pointed out the co-occurrence of highalkalinity and high As in groundwaters in SWUSA and in India (Welch and Lico, 1998; Welchet al., 2000; Nickson et al., 2000). The mobilizationof As by HCO3 has also been used by Anawar et al.(2003, 2004) to explain the high As concentrationsin Bangladesh groundwaters, as well as by Garcia-Sanchez et al. (2005) to explain the high As concen-trations in groundwater in central Spain.

5. Conclusions

High levels of As in well water are causing wide-spread poisoning, posing the greatest threat to

human health in West Bengal. To assess the origin

and mode of As mobilization, an extensive geophys-ical and geochemical data set was collected at an Ashotspot in a typical aquifer located along theHooghly River, 60 km north of Kolkata City, nearthe city of Chakdaha.

Arsenic is present not only in amorphous andcrystalline Fe(II)/Fe(III) hydroxides, but also inother solid phases present in this sediment such ascarbonates and mica particles. Once water passesthrough bacterially active paleomeanders locatedat the border between the flood plain and the inhab-ited terraces, Fe hydroxides undergo reductive dis-solution and release As and Fe reduced species.Carbonate may dissolve under the resulting highPCO2 , also leading to the formation of the observedhotspot. Therefore, release of redox species dependson geomorphological characteristics and land-use

patterns.Total As concentrations range from 0.5 to more

than 60 lmol L1. The As plume present in theWest Bengal aquifer near Chakdaha has signifi-cantly different physicochemical characteristics,compared with other analogous sites in the Bangla-desh part of the BDP (Harvey et al., 2002). TheWest Bengal aquifer has a slightly higher proportionof As present as As(V) species with no observed sul-fide phases (As-rich pyrite or arsenopyrite) present,and redox is mostly controlled by the Fe2+/

Fe(OH)3(s) couple. Arsenic release appears to becontrolled by dissolution of Fe(II/III) hydroxidesand carbonates.

Spatial distribution maps of As and water levelsindicate the presence of a contaminant plume mov-ing during the dry season towards a major drinkingwater supply well in the most densely populatedarea of the city, near the railway station. Resultsfrom a 1D horizontal reaction transport modelshow that, due to the slow horizontal groundwatervelocity ($1 m a1) and to the efficient retardationcreated by Fe oxide sorption sites, the plume isapparently moving very slowly in the direction ofChakdaha (Fig. 9). However, this optimistic conclu-sion may not hold once the spatial variability (Figs.3 and 4) of parameters such as porosity and amor-phous Fe hydroxide content have been measuredand fully integrated into the model. For instance,model calculations which take into account thecompetition between other aqueous major ions,such as HCO3 ; PO

34 , Fe

2+ and As indicate theplume is actually moving much faster. A few dec-ades are still necessary, however, for the plume to

reach the major population center. Rather, the real



18/20

threat for the local population may come from avertical transfer of As from the shallow, contami-nated aquifer to the 150-m-deep As-free wellsrecently used to distribute large quantities of highquality drinking water.

Acknowledgements

This research was partly funded by the IndoFrench Centre for the Promotion of Advanced Re-search (IFCPAR), Project #W1-2200, and by grantsfrom the French Ministry of Research (ACI-Eau),CNRS (EC2CO) and the Embassy of France inNew Delhi and We thank Anthony Le Beux andAnne Audrey Latscha for their field work andMichel Dietrich for his help in geophysical datatreatment. A.A. is thankful to the French Ministry

of Science and Technology for a visiting scientistfellowship at LGIT. The authors acknowledge theeditorial work of Victoria L. Knowles and technicalassistance from Delphine Tisserand for runningcomparative speciation tests with HPLC-AFS.Thanks are due to Richard B. Wanty for EditorialHandling.

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