Arsenic enrichment in groundwater of the alluvial aquifers inBangladesh: an overview

K. Matin Ahmeda,*, Prosun Bhattacharyab, M. Aziz Hasana,S. Humayun Akhtera, S.M. Mahbub Alamc, M.A. Hossain Bhuyiana,

M. Badrul Imama, Aftab A. Khana, Ondra Sracekd

aDepartment of Geology, University of Dhaka, Dhaka 1000, BangladeshbDepartment of Land and Water Resources Engineering, Kungliga Tekniska Hogskolan, SE-100 44 Stockholm, Sweden

cDepartment of Geology, University of Dhaka, Dhaka 1000, BangladeshdDepartment of Mineralogy, Petrology and Geochemistry, Faculty of Science, Masaryk University, 611 37 Brno, Czech Republic

Abstract

Arsenic in the groundwater of Bangladesh is a serious natural calamity and a public health hazard. Most ground-water from the shallow alluvial aquifers (<150 m), particularly in the Holocene plain lands, are vulnerable to As-enrichment. Delta plains and flood plains of the Ganges–Brahmaputra river system are moderately to severely enrichedand more than 60% of the tube wells are affected. Shallow aquifers in the Meghna river basin and coastal plains areextremely enriched with more than 80% of the tube wells affected. Aquifers in the Pleistocene uplands and Tertiaryhills are low in As. The vertical lithofacies sequence of the sediments from highly enriched areas of the country show

two distinct lithofacies associations—a dominantly sandy channel-fill association and a fine-grained over bank associ-ation. The sediments can be grouped into 4 distinct lithofacies, viz. clay, silty clay, silty sand and sand. Thin sectionpetrography of the As-enriched aquifer sands shows that the sands are of quartzolithic type and derived from the col-

lision suture and fold thrust belt of the recycled orogen provenance. Groundwater is characterized by circum-neutralpH with a moderate to strong reducing nature. The waters are generally of Ca–Mg–HCO3 or Ca–Na–HCO3 type, withHCO3

� as the principal anion. Low SO42� and NO3

�, and high dissolved organic C (DOC) and NH4+ concentrations are

typical chemical characteristics of groundwater. The presence of dissolved sulfides in these groundwaters indicatesreduction of SO4. Total As concentration in the analyzed wells vary between 2.5 and 846 mg l�1 with a dominance ofAs(III) species (67–99%). Arsenic(III) concentrations were fairly consistent with the DOC and NH4

+ contents. TheHNO3 extractable concentrations of As AsNO3

� �in the sediments (0.5–17.7 mg kg�1), indicate a significant positive

correlation with FeNO3 , MnNO3 , AlNO3 and PNO3 . The concentrations of SNO3 (816–1306 mg kg�1) peaked in the clay

sediments with high organic matter (up to 4.5 wt.%). Amounts of oxalate extractable As (Asox) and Fe (Feox) rangedbetween 0.1–8.6 mg kg�1and 0.4–5.9 g kg�1, respectively. Arsenicox was positively correlated with Feox, Mnox and Aloxin these sediments. Insignificant amounts of opaque minerals (including pyrite/arsenopyrite) and the presence of highAs contents in finer sediments suggests that some As is incorporated in the authigenically precipitated sulfides in thereducing sediments. Moreover, the chemical extractions suggest the presence of siderite and vivianite as solid phases,

which may control the aqueous chemistry of Fe and PO43�. Reductive dissolution of Fe oxyhydroxide present as coat-

ings on sand grains as well as altered mica (biotite) is envisaged as the main mechanism for the release of As intogroundwater in the sandy aquifer sediments.

# 2003 Elsevier Ltd. All rights reserved.

0883-2927/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/j.apgeochem.2003.09.006

Applied Geochemistry 19 (2004) 181–200

www.elsevier.com/locate/apgeochem

* Corresponding author.

E-mail address: kmahmed@udhaka.net (K.M. Ahmed).

1. Introduction

Arsenic enrichment in Bangladesh groundwater is

considered to be one of the greatest current environ-mental disasters in the world. Since the first detection ofAs in Bangladesh groundwater, a number of studieshave been conducted which provide substantial data

and discussion about the extent and distribution of theproblem. These data have been acquired by both fieldtest kits (UNICEF/DPHE, BAMWSP, 2002, pers.

comm.) and laboratory analyses (NRECA, 1997; SOES/DCH, 2000; BGS and DPHE, 2001; UNICEF andINFS, 2000) (acronyms and other abbreviations used in the

text are presented in the box Nomenclature). The largestdatabase on the occurrence of As in groundwater isavailable from the DPHE/UNICEF field kit surveywhere 29% of the tested 50,998 wells exceeded the

BDWS (50 mg l�1). Complete screening of more thanone million wells in 64 upazillas using field kits byBAMWSP, UNICEF and DPHE showed that 53% of

the tested wells exceeded the BDWS (BAMWSP, pers.comm.). The higher percentage of wells exceeding theBDWS in this survey can be explained by the fact that

known high risk areas were included for this study.The SOES and DCH hold the largest database of

laboratory analyses of more than 34,000 water samples

where 37% samples exceed the BDWS and 56% samplesexceed the WHO provisional drinking water limit (10 mgl�1) (SOES/DCH, 2000, pers. comm.). BGS and DPHE(2001) conducted the most systematic national survey

where 3534 wells from different parts of the countrywere analyzed. The results indicate that nearly 27% ofthe shallow wells (<150 m) contain As concentrations

above the BDWS, while 46% exceed the WHO provi-sional drinking water limit. Although the number ofdeep tubewells (>150 m deep) is relatively small, only 1and 5% of the analyzed groundwaters exceeded the

BDWS and WHO limit, respectively. Analysis of nearly6000 samples by UNICEF/INFS show that 23% ofwells exceed BDWS and 42% samples exceed WHO

limit (UNICEF and INFS, 2000). Estimates of majornational surveys showed that about one third of thetested wells exceeded the BDWS. Accordingly, it is

approximated that nearly 3 million tubewells, placed atdepths of between 10 and 50 m, yield groundwater withAs concentrations exceeding the BDWS.

In the present paper, the authors present an overviewof the nature and extent of As enrichment in ground-water of Bangladesh with reference to the geological andgeomorphological characteristics of the Bengal Basin.

Emphasis has been laid on discussion of the relationshipbetween As-enriched groundwater with the hydro-geological setup of the basin, hydrogeochemical char-

acteristics and the sedimentological, provenance andgeochemical characteristics of the aquifer sediments.

2. Geological and sedimentological characteristics

2.1. Geology and geomorphology

Bangladesh, located at the head of the Bay of Bengal,occupies most of the Bengal Basin, one of the largest

sedimentary basins of the world. The basin is sur-rounded by the Indo-Burman range in the east, anuplifted block of Precambrian Shield (Shillong Plateau)

in the north, and Precambrian basement complex(Indian Shield) in the west (Fig. 1). More than 16 kmthick synorogenic Cenozoic sediments are deposited in

the basin derived from the Himalayan and Indo-Bur-man range (Uddin and Lundberg, 1998). Tertiary sedi-ments in Bangladesh are represented by mainlysandstone and shale sequences, while Pleistocene sedi-

ments are represented mostly by clay, overlain byHolocene alluvium. The Ganges, Brahmaputra andMeghna river systems transport a huge amount of sedi-

ments and converge at the lower reaches to form thegreat delta complex—the Ganges–Brahmaputra–Meghna (GBM) delta that is prograding to date.

Various geomorphological units were mapped in theHolocene plain lands (Morgan and McIntire, 1959;Umitsu, 1987, 1993; Brammer, 1996), which include

piedmont plains, flood plains, delta plains and coastalplains. The geomorphological units in the Holocenelandmasses within the Bangladesh part of GBM deltainclude fan deltas of the Tista and Brahmaputra; fluvial

flood plains of the Ganges, Brahmaputra, Tista andMeghna rivers; delta plain of the lower GBM systemsouth of the Ganges and Meghna valleys, including the

Nomenclature

BAMWSP Bangladesh Arsenic Mitigation WaterSupply Project

BDWS Bangladesh Drinking Water StandardBGS British Geological SurveyBWDB Bangladesh Water Development

Board

DCH Dhaka Community HospitalDPHE Department of Public Health

Engineering

INFS Institute of Nutrition and FoodScience

MMI Mott MacDonald International

SOES School of Environmental Sciences,Jadavpur University

UNICEF United Nations InternationalChildren’s Emergency Fund

XRF X-ray Fluorescence

182 K.M. Ahmed et al. / Applied Geochemistry 19 (2004) 181–200

moribund Ganges delta and the Chandina plain;Pleistocene Terraces (Barind and Madhapur Tracts)

and the subsiding basins in the eastern Ganges tidaldelta and the Sylhet basin adjacent to the Dauki Fault(BGS and DPHE, 2001).

Occurrence of As-enrichment in Bangladesh ground-water shows a close relationship with the major geo-morphological units. The landform units have beencorrelated with the aerial distribution pattern of enri-

ched aquifers to assess the status of enrichment in themajor geomorphological units (Ravenscroft, 2001). Abroad classification, based on satellite imagery (LAND-

SAT TM Band), is presented here (Fig. 2) where 3 majorunits and 5 sub-units have been identified. Character-istic features of these units and the status of As occur-

rences are described in the Table 1.

2.2. Stratigraphy and lithofacies sequence

Quaternary sediments provide good aquifers in Ban-gladesh and As-enrichment is mainly restricted to theHolocene alluvial aquifers at shallow and intermediate

depths (BGS and DPHE, 2001; Ahmed et al., 2001;McArthur et al., 2001; Mukherjee and Bhattacharya,2001; Bhattacharya et al., 2002a,b). Similar findings

from West Bengal in India have also been reported(Acharyya et al., 2000). The sediments are dominantly

composed of sands, silts and clays. Quaternary sedi-mentation in the Bengal Basin is largely controlled byhuge sediment supply, active tectonics and sea level

changes (Goodbred and Kuehl, 2000). Table 2 presentsa simplified Quaternary stratigraphy of Bangladeshalong with the status of As in the sediments.A vertical lithofacies sequence of the sediments from

highly As-enriched areas of the country was preparedfrom exploratory borelogs available from BWDB. Con-sidering texture and lithology, the sediments can be

categorized into 4 distinct lithofacies, viz. sand, siltysand, silty clay, and clay. Two distinct lithofacies associ-ations—a dominantly sandy channel-fill association and

a fine-grained over bank association (Fig. 3) have beenidentified in most cases (Ahmed et al., 2001). Grain sizeprofiles characteristically fine upward in all cases indi-

cating a fluvial environment of deposition. It is possibleto identify a number of sedimentary structures in the coresamples. Sedimentary structures like trough cross lami-nation, horizontal lamination and ripple laminations are

also indicative of a fluvial environment. The median dia-meters of the aquifer samples represent fine to very finesand class, which are moderate to well sorted.

Fig. 1. Regional geological setting of the Bengal Basin (redrawn after Uddin and Lundberg, 1998).

K.M. Ahmed et al. / Applied Geochemistry 19 (2004) 181–200 183

2.3. Petrographic characteristics

2.3.1. Detrital compositionTwenty-one samples of As-enriched Holocene aquifer

sands from 14 different locations across the countrywere studied by thin section petrography, to describe thecharacteristic detrital modes and to infer their prove-nance (Fig. 4a–d).

The sands are typically composed of quartz (47–70%), lithic grains (4–17%), mica (muscovite and bio-tite, 2–15%), feldspar (1–8%) and chlorite (trace–8%).

The sands are texturally immature to submature with

matrix content ranging from 4 to 14% (Fig. 4a). How-ever, some percentage of the matrix may have beenartificially incorporated due to the breakdown of lithic

grains (shale and silt) during sample preparation.Quartz is mostly monocrystalline (Qm) with minorpolycrystalline (Qp) types with thick to thin and dis-continuous ferruginous coatings (Fig. 4b and c). Feld-

spar and biotite flakes show alteration (Fig. 4d). Lithicgrains are mainly sedimentary and metasedimentary(Ls) which include fragments of shale, siltstone and

Fig. 2. Broad classification of the major geomorphological units in the Bengal Basin based on LANDSAT-TM Band mosaic.

184 K.M. Ahmed et al. / Applied Geochemistry 19 (2004) 181–200

Table 1

Groundwater occurrences and As status in different geomorphological unitsa of Bangladesh

Units Description Groundwater occurrence senic Status

Coastal Plains

(Unit 3e)

Narrow flat lands along

the coast characterised by

numerous channels with

swamps and mangrove

forests.

Groundwater occurs at very shallow depth in

the upper shallow aquifers that is composed

of fine to very fine sand. The intermediate

aquifer (the lower shallow in other plains)

is saline all along the coast line. The deep

aquifer is composed of fine to medium sand

with occasional coarse sand. The upper

aquifers are unconfined to leaky confined

whereas the deeper aquifer is confined.

e upper aquifer is strongly enriched

80% of tested wells) and the deeper

uifer is As safe.

Depressed Land (3d) Large gentle depressions

characterised by numerous

lakes and swamps locally

known as Haors and Bills.

Due to relatively thick upper aquitard, the

upper shallow aquifer is deeper in these units.

The aquifer is composed of fine to very fine sand

and confined to leaky confined in nature.

oderately enriched (40–60% of tested wells).

Delta Plains (3c) The vast flat land south of

the country constitutes the

Delta plains of Ganges,

Brahmaputra and Meghna

river systems namely Ganges

Delta and Meghna Delta

The aquifer is mostly unconfined to leaky confined

and groundwater occurs within a few meters of the

surface. Fine to very fine sand comprises the upper

part of these shallow aquifers whereas the lower

part comprises of medium to coarse sand.

nges Delta plains are moderately to severely

riched (60–80% of tested wells) and the

eghna Delta plains are severely enriched

80% of tested wells).

Flood Plains (3b) Flat lands formed by the major

rivers include the Brahmaputra

flood plain, Ganges flood plain,

Meghna flood plain and Tista

flood plain

Groundwater occurs within a few meters of the surface,

and the aquifers are unconfined with leaky confined

nature locally. The upper part of the aquifer is composed

of fine to very fine sand whereas the lower part is composed

of medium to coarse sand with occasional gravels.

oderately to severely enriched (60-80% of

ted wells).

Piedmont Plains (3a) Gently sloping land deposited

at the foothills as alluvial fans.

The Himalayan piedmont plain

occupies the northwestern part

of the country. Narrow belt of

piedmont plains also occurs along

the margins of northeastern part

As the upper aquitard is missing, the upper shallow aquifer

continues from the ground surface where groundwater

occurs at very shallow depths. Medium to coarse sand and

gravels form unconfined aquifers with high transmissivity.

ostly low As (<20% of tested wells are

riched).

Pleistocene Uplands (2) The uplifted blocks, the Madhupur

and Barind Tracts, situated at the

central and lower northwestern part

of the country.

Due to the presence of thick Pleistocene clay, the underlying

Dupi Tila aquifer occurs at varying depth from a few meters

to a few tens of meters. The fine to medium grained Dupi Til

aquifer is confined to leaky confined in nature.

e Dupi Tila aquifer is low in As.

(continued on next page)

K.M.Ahmedetal./Applied

Geochemistry

19(2004)181–200

185

Ar

Th

(>

aq

M

Ga

en

M

(>

M

tes

M

en

a

Th

Table1(continued)

Units

Description

Groundwateroccurrence

ArsenicStatus

TertiaryHills(1)

Includesthefoldedbeltattheeastern

andnortheasternpartofthecountry.

Duetocomplexstructuralgeometry(foldingandfaulting),

theaquifersarenotuniformlydistributedasintheother

geomorphicunits.DupiTilaandTipamsandsformthe

mainaquifers.

Limiteddatashowtheaquifersare

basicallyAssafe.

aNumberswithinparenthesisincolumn1refertogeomorphologicalunitsofFig.2.

chert. Some metamorphic lithic grains (Lm) such asschist and gneiss are also observed. Volcanic lithic frag-ments (Lv) were not identified in the studied samples.Non-opaque heavy mineral ranges between 2 and 8%

and opaque heavies are very few. Hornblende is themost abundant heavy mineral in all the samples. Otherunstable and semi-stable heavy minerals like garnet,

epidote and kyanite are common whereas stable andultra-stable heavies like zircon, tourmaline and rutile arepresent in trace amounts. Significant occurrence of shale

and siltstone fragments, chert, altered feldspar, chlor-itized biotite and chlorite in the sample indicate that thesands are derived from sedimentary and metasedimen-

tary terrains. Dominance of hornblende (green variety)together with garnet, epidote and kyanite in the heavymineral assemblages suggest high rank crystalline meta-morphic provenance (Pettijohn, 1975; Arribas et al.,

2000).

2.3.2. Provenance studies

Quantitative detrital modes, calculated from pointcounts of thin sections have been used to infer sand-stone provenance (Ingersoll and Suczek, 1979; Dick-

inson, 1985). Quartz (Qm and Qp), feldspar (F) andlithic grains (Ls and Lv) are the primary detrital graintypes used for the purpose. Compositional fields of

sands characterizing the different provenances areshown as standard triangular diagrams (Fig. 5). In theQtFL plot (Fig. 5a) all the samples are in the upper partof the Qt–L leg indicating a recycled orogen source

characterized by a high content of quartz and lithicgrains and low feldspar. The QmFLt plot (Fig. 5b)shows clustering of the data in the upper part of the

Qm–Lt segment, indicating the provenance of the sandsfrom a quartzose recycled orogen. However, one samplesuggests an interior craton source. The QpLvLs ptot

(Fig. 5c) is used to infer the subtype of recycled orogen.All the samples plot on the Qp-Ls leg of the triangulardiagrams representing absolute dominance of sedimen-tary and metasedimentary lithic fragments. The sands

are of quartzolithic type and derived from the collisionsuture and fold thrust belt, and indicate the recycledorogen provenance.

3. Hydrogeology, distribution and mobilization of

arsenic

3.1. Hydrogeological characteristics

Groundwater occurs at very shallow depths all overthe country where the major aquifers are the Holocenealluviums and fan deposits and Pliocene fluvio-deltaic

(Dupi Tila) sediments. Mio-Pliocene Tipam sands formminor aquifers in the hilly areas. The aquifers are highlytransmissive and generally multi-layered. The aquifer

186 K.M. Ahmed et al. / Applied Geochemistry 19 (2004) 181–200

conditions vary from unconfined to leaky-confined in

the shallow alluvial deposits and confined in the DupiTila and deeper alluvial deposits. Traditionally theaquifer system has been described as (from top to bot-tom): upper aquitard (silt and clay), composite aquifer

(very fine to fine sand, low discharge hand tube wells tapwater from this aquifer), main aquifer (medium tocoarse sand with occasional gravel, high discharge irri-

gation wells and municipal water supply wells tap thisaquifer), lower aquitard (deep clay and silt layer separ-ating the upper water bearing sediments from deeper

ones) and deep aquifer (occurring in the coastal area,mainly water supply hand and municipal wells tapwater). This classification holds for the alluvial deposits.

A more logical approach however, is to classify theaquifers into two major classes: the shallow aquifers(alluvial and Dupi Tila sands within a depth of 200 m)and deeper aquifers (occurring at depths greater than

200 m and separated from the upper water bearing for-mations by a thick blanket of clay). The shallow aqui-fers are further subdivided into upper and lower parts.

Water levels lie within a few meters of the groundsurface and fluctuate with the annual dry and wet sea-son conditions. The aquifers are recharged during the

monsoon season (July–September) when the areareceives more than 80% of its annual precipitation(around 2000 mm a�1). Huge amounts of annual flood-water standing on a large part of the country also con-

tribute to the recharge process. Groundwatermovements are controlled by the surface topographywhich is very gentle over most of the country. The

regional groundwater flow direction is from the north tosouth, but the flow rate is very slow. The magnitude ofhorizontal flow is much lower as compared to the

amount of vertical movement. Annual fluctuations ingroundwater levels are controlled by the local hydro-geological conditions and withdrawal of groundwater

for irrigation. In general, the fluctuations are more pro-nounced in the northwestern part of the country ascompared to the coastal plains.Groundwater in general is fresh over most of the

country, except for the coastal region. However, pocketsof saline/brackish groundwater are reported in areas farfrom the coastline (Ahmed, 1994). As the Holocene

alluvial sediments are rich in organic matter and reac-

tive minerals, water is mostly of Ca–Mg–HCO3 and Ca–Na–HCO3-type (Bhattacharya et al., 2002b). The aqui-fers are mostly reducing in nature and even reach thestage of CH4 formation (Ahmed et al., 1998a; Bhatta-

charya et al., 2002a). The amount of dissolved salts ingroundwater of the Dupi Tila aquifers is less, comparedto the Holocene aquifers. The Dupi Tila aquifers were

thoroughly oxidized during its geological evolution,which resulted in a decrease in the quantities of organicmatter and unstable mineral phases, i.e. amorphous

oxyhydroxide and carbonates (BGS and DPHE, 2001).

3.2. Distribution of arsenic in groundwater

An As distribution map prepared with the results ofBGS and DPHE data indicates that most As-richregions are located in the central, south and south-

eastern parts of the country, while the northwestern andthe uplifted areas of the north-central parts are leastaffected (Fig. 6). Isolated ’’hot spots’’ are, however,

encountered in the southwestern, northwestern, north-eastern and north-central regions of Bangladesh. Basedon the little data available from the Hill Districts, it

seems that the wells in that region are mostly unaffected.The map shows that the extremely As-rich districts areChandpur, Comilla, Noakhali, Munshiganj, Faridpur,Madaripur, Gopalganj, Shariatpur and Satkhira. How-

ever, within the broad regional distribution pattern ofoccurrences of As-enriched groundwater, there areextreme local variations.

The BGS and DPHE (2001) reported that maximumAs concentrations occur at depths between 20 and 50 mwhereas samples shallower than 10 m and deeper than

150 m are basically As free (Fig. 7). Most of the deepwell samples for this study were collected from thesouthern coastal area of the country. Other studies from

different parts of the country showed a basically similarpattern (NRECA, 1997; Burren, 1998; Mather, 1999;Broms and Fogelstrom, 2001). However, a recent studyby van Geen et al. (2002) at Araihazar Thana of NE

Bangladesh indicated low As levels in wells below 30–60m. It is therefore apparent that the As distribution ingroundwater is not only controlled by depth but rather

Table 2

Generalized quaternary stratigraphy of Bangladesh and As status

Age

Formation Lithology Arsenic status

Holocene

Alluvium Grey clay, silt, fine sand

with occasional peat and gravels.

High in finer and peaty layers,

low in coarser gravel layers. Most of

the As is oxalate extractable

Pleistocene

Madhupur

Clay

Reddish brown mottled clay and silt.

Low, not oxalate extractable

Plio-Pleistocene

Dupi Tila Yellowish brown fine to medium sand Very low

K.M. Ahmed et al. / Applied Geochemistry 19 (2004) 181–200 187

188

Fig. 3. Vertical lithofacies sequences of alluvial sediments from different regions of Ba sh.

K.M.Ahmedetal./Applied

Geochemistry

19(2004)181–200

nglade

to a major extent by subsurface geology, i.e. age andgrain size of sediments. It has been established from thedifferent studies that As is mobilized in tube wells placed

in the fine grained Holocene aquifers, while those pene-trating the coarser Holocene sediments and the Plio-Pleistocene (Dupi Tila) aquifers are found to be safe all

over the country, thereby revealing that distribution ofAs in the wells is largely dependent on the facies char-acteristics of the alluvial deposits.

3.3. Mobilization of arsenic

Several hypotheses have been put forward to explain

the problem of As-enrichment in groundwaters of Ban-gladesh. Since its first discovery, use of fertilizers, pesti-cides, insecticides, waste disposal, As compound treated

wooden poles etc. were blamed as the anthropogenicsources of As-enrichment in groundwater (NRECA,1997). Regional occurrence of As in groundwater of

Bangladesh is however not attributable to any anthro-pogenic activities, and the current status of knowledgeindicates predominantly geogenic source and its release

in groundwater through natural processes (Bhatta-charya et al, 1997; Nickson et al., 1998, 2000). Oxida-tion of pyrite (FeS2) or arsenopyrite (FeAsS) waspostulated as the dominant process for As mobilization

due to lowering of water table following excessivepumping of groundwater (Mandal et al., 1996; Mallikand Rajagopal, 1996) that was widely accepted at the

beginning. An alternate theory, known as the Fe oxy-hydroxide reduction hypothesis, is now widely acceptedas the principal mechanism of As mobilization in the

groundwaters of the alluvial aquifers of the GBM Delta(Bhattacharya et al., 1997, 2001; Ahmed et al., 1998b;Nickson et al., 2000; Routh et al., 2000; McArthur et

al., 2001; Dowling et al, 2002; Anawar et al., 2003).Arsenic mobilization in groundwater also appears to betriggered by intensive extraction of groundwater for irri-

gation and application of phosphate fertilizer (Acharyyaet al., 2000). On the other hand, Harvey et al. (2002)report that As mobilization is associated with recentinflow of C due to large scale irrigation pumping. Foster

et al. (2000) and Breit et al. (2001) report that apart fromFe(III)-oxyhydroxides, other solid phases such as phyllo-sillicates also play an important role in As cycling and

mobilization. It is therefore evident that As mobilizationin Bangladesh groundwater is a complex natural geo-chemical processes.

4. Groundwater chemistry and geochemical character-

istics of the aquifer sediments

4.1. Groundwater chemistry and behavior of arsenic

The chemistry of the BDP groundwaters has beenstudied extensively during recent years (Nickson, 1997;Burren, 1998; BGS/MMI, 1999; Mazumdar, 2000; BGS

Fig. 4. Photomicrographs of As-enriched aquifer sands seen in thin sections. (a) High abundance of matrix; (b) thick and dis-

continuous coating on quartz; (c) thin and partial coating on silicate grains; (d) abundance of altered biotite and few non-opaque

heavy minerals. Abbreviations: ct: coating; q: quartz; mtx: matrix; c: calcite; b: biotite; g: garnet; t: tourmaline; h: hornblende.

K.M. Ahmed et al. / Applied Geochemistry 19 (2004) 181–200 189

190

Fig. 5. Provenance discrimination plots: (a) QtFL, (b) QmFLt and (c) QpLvLs showing the characteristic detrital modes and the in provenance for aquifer sands of Bangladesh.

K.M.Ahmedetal./Applied

Geochemistry

19(2004)181–200

ferred

and DPHE, 2001; Broms and Fogelstrom, 2001;Mukherjee and Bhattacharya, 2001; Bhattacharya et al.,

2002a,b). Groundwater pH is predominantly near neu-tral to slightly alkaline (pH 6.5–7.6) with low dissolvedO2 (DO). The Eh values (+0.594 to �0.444 V) suggest a

mildly oxidizing to moderate/strongly reducing char-acter of the aquifers. Groundwaters are generally Ca–HCO3 or Ca–Mg–HCO3 type, although Ca–Na–HCO3type and Na–Cl type water are also found locally inseveral parts of the country (Ahmed, 1994; Bhatta-charya et al., 2002a,b).Major ion composition is dominated by HCO3

� (320–

600 mg l�1) and shows an apparent depth and litholo-gical control (Bhattacharya et al., 2002b). Concentra-tions of SO4

2� (<3 mg l�1) and NO3� (40.22 mg l�1) are

generally low, except for some local variations (BGSand DPHE, 2001; Broms and Fogelstrom, 2001; Bhat-

tacharya et al., 2002b). Phosphate concentrations on theother hand are high (up to 8.75 mg l�1) in the ground-waters. Distribution of the major cations such as Ca

(21–122 mg l�1), Mg (14–41 mg l�1), Na (7–150 mg l�1),and K (1.5–13.5 mg l�1) show significant variations withdepth as well as region. Considerable variability is noted

in the levels of total As (Astot=2.5–846 mg l�1), total Fe(Fetot=0.4–15.7 mg l

�1) and Mn (0.02–1.86 mg l�1) inthe groundwater samples both as a function of depthand region. Although As(V) is prevalent in some wells,

As(III) is the dominant species representing about 67–99% of Astot in these groundwaters (Bhattacharya et al.,2002b).

Fig. 6. Aerial distribution of As in groundwater from shallow and deep aquifers of Bangladesh (after BGS and DPHE, 2001).

K.M. Ahmed et al. / Applied Geochemistry 19 (2004) 181–200 191

Irontot indicates moderately strong positive corre-

lation with HCO3� and PO4

3� (Fig. 8a and b), while itexhibits a relatively low correlation with Astot (r

2=0.42;Fig. 8c). This is commonly expected in reducing

groundwaters with elevated HCO3� and PO4

3� levels,where Fe2+ may precipitate as siderite (FeCO3) andvivianite (Fe3(PO4)2.8H2O). Saturation indices (SI) for

selected minerals, calculated by the hydrogeochemicalcode PHREEQC (Parkhurst, 1995) using the ground-water chemical data from selected areas of Bangladesh(Table 3), reveal positive SI values for siderite (0.42–

0.75) as well as vivianite (0.03–1.44), consistent with theprevious findings (Bhattacharya et al., 1998; Nickson etal., 2000; Sracek et al., 2001). Thus, precipitation of

these minerals may explain the relationships betweenAstot and Fetot.SI values for calcite and dolomite are, however, close

to equilibrium, which suggests that the elevated HCO3�

levels are not controlled only by the dissolution of thecarbonates in these aquifers. Moreover, high HCO3

�

concentrations correlate with the levels of dissolved

organic C (DOC=1.15–14.2 mg l�1) in groundwater(Fig. 8d). DOC levels in the analyzed groundwatersindicate distinct trends of variation with both As(III)

and Fetot concentrations (Mukherjee and Bhattacharya,2001). Although DOC characteristics in Bangladeshgroundwaters are not investigated in detail, organicmatter in the Holocene sediments can be considered as

an active source for DOC in groundwater (see Routh etal., 2001). Sulphate and NO3

� levels in groundwater aregenerally very low, and there is no significant correl-

ation with As. Microbial degradation of the organicmatter in the aquifers results in the reduction of bothSO4

2� and NO3�, thereby increasing the concentrations of

sulfide (42 mg l�1; Broms and Fogelstrom, 2001) andNH4

+ (up to 13.2 mg l�1) in groundwater (Bhattacharya,2002). While Astot does not show any correlation with

SO42� (Fig. 8e), fairly high correlation is observed

between As(III) and NH4+ (r2=0.57; Fig. 8f) in the

groundwater samples. Extremely high values of calcu-lated log PCO2 (in some cases >�1.0) indicate genera-

tion of CO2 through the sequence of redox reactionsinvolving organic matter in the sediments, includingreduction of NO3

� and SO42� as well as the reductive

Table 3

Calculated values of SI for selected minerals along with PCO2

values

Site/value

SIcalcite SIdolomite SIsiderite SIvivianite LogPCO2

(atm)

Laxmipur

(n=5)

Average

�0.425 �0.39 0.75 1.44 �1.53

Faridpur

(n=5)

Average

0.15 0.06 0.42 0.03 �1.24

Munshiganj

(n=4)

Average

�0.47 �0.73 0.68 0.77 �0.73

Fig. 7. Distribution of As in groundwater with the depth of the aquifers (modified from Ravenscroft, 2001).

192 K.M. Ahmed et al. / Applied Geochemistry 19 (2004) 181–200

dissolution of Fe(III) oxyhydroxides in the solidphase.

4.2. Sediment geochemistry and implications on aquifer

characteristics

Geochemical analyses of the sediments have been

carried out on the Holocene alluvial sediments fromdifferent parts of Bangladesh in recent years (viz.Yamazaki et al., 2000; Mukherjee and Bhattacharya,

2001; BGS and DPHE, 2001; Anawar et al., 2003; Akaiet al., 2003). Salient geochemical characteristics of a setof 16 borehole sediment samples representing the Holo-

cene alluvium are discussed in the following section.These sediment samples were collected by BWDB dur-ing 1999 and 2000, from two representative boreholes atChapai Nawabganj (DW1 and DW2) at depths between

3 and 132 m, and at one borehole at Tala representingthe aquifer sediments between 6 and 46 m (Table 4) Aseries of selective and partial extractions were carried

out at the laboratories of the Royal Institute of Tech-nology, Stockholm and analyzed at the laboratories ofthe Universities at Linkoping and Stockholm.Acid extraction (7M HNO3) was employed to deter-

mine the budget of As and the major elements Fe, Mn,Al, P, and S associated with the non-silicate mineralsfollowing the methodology described in Bhattacharya et

al. (2001). The HNO3 extracted concentrations ofAsNO3, Fe NO3, MnNO3, AlNO3 and PNO3 in the shallowaquifer sediments varied considerably with depth and

ranged between 0.5 and 17.7 mg kg�1, 5.8 and 26.5 gkg�1, 0.05 and 0.5 g kg�1, 3.8 and 19.1 g kg�1, and 0.16and 0.51 g kg�1, respectively. These results were com-

parable to the results for the whole rock analyses carriedout by XRF (BGS and DPHE, 2001). ArsenicNO3 indi-cated significant positive correlation with FeNO3 ,MnNO3 ,AlNO3 and PNO3 (Fig. 9). Sulfur concentrations are gen-

erally low. Concentrations of SNO3 ranged between 7.5and 1306 mg kg�1, and had peaks in the clay sedimentswith high organic matter (Table 4). The content of

Fig. 8. Trends for variations of the salient chemical parameters in the groundwaters of Holocene aquifers in Bangladesh and their

relationship with As. (a) HCO3� vs Fetot; (b) Fetot vs PO4

3�; (c) Fetot vs Astot; (d) HCO3� vs DOC; (e) Astot vs SO4

2�; and (f) As(III) vs

NH4+ (n=36).

K.M. Ahmed et al. / Applied Geochemistry 19 (2004) 181–200 193

organic matter varied between 0.1 and 4.5%. Authigenic

framboidal pyrite has been documented in the clayeysediments in several regions of Bangladesh. While amoderately strong correlation was observed betweenAsNO3�FeNO3 in these sediments (r2=0.67), corre-

lations between FeNO3�SNO3 , and AsNO3�SNO3 wererelatively poor.Oxalate extraction (pH 3.5) releases elements that are

present as oxides and to some extent even clays (Bhat-tacharya et al., 2001). Moreover, it also dissolves ele-ments bound as carbonates. It is therefore important to

quantify the carbonate bound fractions of the elementsfollowing extraction of the sediments with Na-acetatebuffered to pH 5.4. Acetate extraction characterizes thecarbonate minerals such as calcite (and solid solutions

of Fe and Mn therein), siderite, rhodochrosite(MnCO3), as well as unstable phosphate mineral likevivianite (Dodd et al., 2000). Acid oxalate extractable

fractions of Feox, Mnox, Alox, Pox and Asox also revealconsiderable variability with depth and lithologicalcharacter of the sediments. Amounts of Feox, Mnox,

Alox, Pox and Asox ranged between 0.4 and 5.9 g kg�1,

0.005 and 0.45 g kg�1, 0.1 and 1.32 g kg�1, 5.1 and 144mg kg�1, and 0.1 and 8.6 mg kg�1, respectively.

Arsenicox was positively correlated with Feox, Mnox andAlox (Fig. 10a–c). While the coarse grained sedimentsindicated strong positive correlations between Asox andPox (Fig. 10d), their distribution in the fine grained

sediments (silty clay and clay) revealed an inverse trend(Fig. 10d). Distinct negative correlation between Asoxand Pox fractions suggests dissolution of secondary

vivianite, rather than the release of PO4 ions adsorbed

onto the Fe-oxide surfaces in these fine grained sedi-ments.Acetate extractable fractions Feacet (0.02–0.37mg

kg�1), Mnacet, (0.002–0.27 mg kg�1) and Pacet (0.7–73.7

mg kg�1), account for a range of 2.1–80% Feox, 15.8–97% Mnox, and up to 70% Pox in the oxalate extrac-table fractions (Table 4). Dissolution of minerals like

siderite and rhodochrosite may account for the acetateextractable fractions of Feacet and Mnacet in these sedi-ments. However, high acetate extractable P fractions

(Pacet) concomitant with high Fe (Feacet) indicate thepossible dissolution of vivianite in these reducing aqui-fer sediments (Dodd et al., 2000). It is important to notethat hydrogeochemical speciation modeling also sug-

gests that the groundwaters are supersaturated withrespect to these minerals, which act as sinks for Fe andMn as well as PO4

3- and hence control their solubility in

groundwater (Bhattacharya et al., 1998; Nickson et al.,2000; Sracek et al., 2000, 2001). Fairly high correlationbetween Feox and Pox (r

2=0.74; Fig. 11a), indicates

reductive dissolution of the amorphous Fe-oxides toge-ther with surface bound PO4

3�, particularly in the coarsesediments. However, Pacet fractions do not exhibit a

significant correlation with Feacet (Fig. 11b). Further,Pacet fractions are an order of magnitude lower than thePox fractions, which suggests that the pool of PO4related to Fe-oxides is more significant than the pool of

PO4 related to P-minerals like vivianite in the aquifers.Oxalate extractable Asox plotted against Feacet and Feox-Feacet as independent variables (Fig. 12) reveals two

Fig. 9. Variation in HNO3 extractable As AsNO3

� �with (a) FeNO3 ; (b) MnNO3 ; (c) AlNO3 ; and (d) PNO3 in the aquifer sediments.

194 K.M. Ahmed et al. / Applied Geochemistry 19 (2004) 181–200

Table 4

Salient geochemical characteristics of the aquifer sediments from Bangladesh

S. No. Sample No. Lithology Depth (m) FeNO3 Feox Feoxa Feacet Feacet

b MnNO3 Mnox Mnoxa Mnacet Mnacet

b AlNO3 Alox Aloxa sox Asox

a PNO3 Pox Poxa Pacet Pacet

b SNO3

g kg�1 % g kg�1 % g kg�1 % g kg�1 % g kg�1 % 1 % mg kg�1 % mg kg�1 % mg kg�1

Chapai Nawabganj (DW1)

1 CN-1-1 Fine sand

to silt

3–3.6 23.6 1.3 5.5 0.04 3.0 0.39 0.13 32.8 0.04 34.3 18.8 0.39 2.1 .5 30.8 586 144.0 24.6 73.7 51.2 107

2 CN-1-2 Fine sand

with mica

33–33.6 5.8 0.5 9.4 0.02 4.4 0.05 0.005 10.7 0.002 44.2 3.8 0.12 3.2 .2 30.1 160 5.1 3.2 3.4 66.5 498

3 CN-1-3 Silty clay with

kankar

51–51.6 23.9 3.1 13.1 0.24 7.6 0.35 0.17 47.8 0.07 39.4 22.3 0.73 3.3 .5 12.9 450 135.0 30.0 4.9 3.6 291

4 CN-1-4 Clay 60–60.6 22.8 1.7 7.7 0.37 2.1 0.45 0.45 10.0 0.27 60.2 15.3 0.44 2.9 .1 2.1 509 84.7 16.6 7.3 8.6 816

5 CN-1-5 Bluish sticky

clay

132–132.6 24.5 5.8 23.8 0.33 5.7 0.50 0.25 50.3 0.18 71.7 16.4 0.78 4.7 .0 27.3 491 126.2 25.7 6.4 5.1 1203

Chapai Nawabganj (DW2)

6 CN-2-1 Fine sand to silt 3–3.6 19.6 1.3 6.8 0.04 3.0 0.39 0.13 33.6 0.08 58.1 15.2 0.37 2.5 .3 30.5 378 50.5 13.4 4.4 8.7 7.5

7 CN-2-2 Medium to

fine sand

18–18.6 8.3 1.5 18.6 0.10 6.4 0.13 0.09 65.9 0.03 33.5 5.6 0.23 4.1 .0 53.8 214 20.3 9.5 8.1 39.7 207

8 CN-2-3 Fine sand with

mica

36–36.6 7.0 0.6 8.0 0.08 14.7 0.08 0.02 21.7 0.01 67.5 4.0 0.13 3.4 .3 30.1 148 7.3 4.9 5.1 69.9 180

9 CN-2-4 Sticky clay 57–57.6 25.6 5.9 22.9 0.37 6.3 0.50 0.28 55.5 0.14 52.0 19.1 1.32 6.9 .8 28.2 439 107.9 24.6 4.8 4.5 876

10 CN-2-5 Sticky clay 108–108.6 17.1 – 0.32 – 0.27 – 0.12 – 10.0 – – – 459 – – 8.0 – 1306

Tala, Satkhira

11 TALA-1 Very fine sand 6.1–6.71 17.6 3.5 20.2 0.23 6.4 0.22 0.09 42.2 0.08 84.1 12.2 0.61 5.0 .9 36.2 235 82.3 35.1 0.6 0.7 147

12 TALA-2 Silt clay intercalation 12.2–12.8 18.1 3.4 18.6 0.25 7.5 0.38 0.15 38.5 0.14 94.3 9.6 0.34 3.6 .6 40.5 380 68.1 17.9 2.8 4.1 72

13 TALA-3 Silt-sand intercalation 18.3–18.9 19.6 3.6 18.3 0.29 8.0 0.43 0.16 37.3 0.16 96.8 11.2 0.37 3.3 .5 40.0 433 105.5 24.4 12.0 11.3 128

14 TALA-4 Silty clay with org.

matter

27.4–28 20.1 3.9 19.6 0.13 3.4 0.24 0.16 67.3 0.10 59.7 8.8 0.41 4.7 .6 48.8 326 132.6 40.7 2.6 1.9 871

15 TALA-5 Very fine sand/silt 36.6–37.2 26.5 2.2 8.5 0.17 7.4 0.48 0.37 76.6 0.15 39.7 16.8 0.46 2.7 .5 24.5 342 69.4 20.3 1.5 2.2 72

16 TALA-6 Fine-sand with org.

matter

45.7–46.3 7.4 0.4 5.7 0.34 80.1 0.10 0.06 57.2 0.01 15.8 3.7 0.06 1.7 .2 31.7 278 10.9 3.9 1.7 15.8 135

a Percentage of Fe, Mn, Al, As, and P in oxalate extracts compared to HNO3 extractions.b Percentage of Fe, Mn, and P in acetate extractions compared to oxalate extractions.

K.M.Ahmedetal./Applied

Geochemistry

19(2004)181–200

195

AsNO3 A

mg kg�

4.7 1

0.5 0

3.7 0

6.1 0

7.4 2

4.2 1

1.9 1

1.1 0

9.9 2

4.3 –

5.3 1

4.0 1

6.2 2

17.7 8

14.1 3

0.5 0

distinct populations. The data sets plotted on this dia-gram, follow a linear trend for most of the analyzedsediments showing a clear association of Asox with theFe-oxide phases (Feox-Feacet) in the coarse grained sedi-

ments. However, the reducing fine grained aquifersediments characterized by high Feacet fractions plotseparately. These results clearly demonstrate that

siderite and vivianite are also present together with

amorphous Fe-oxides with adsorbed As in the Holocenesedimentary aquifers of Bangladesh.

5. Discussions and conclusions

Most of the shallow aquifers of the Holocene plain

lands of Bangladesh are affected by As-enrichmentwhereas the aquifers in the Pleistocene and Tertiarysediments are low As. In the Ganges and Meghna delta

plains, the status of As-enrichment is severe, while floodplains of the Ganges, Brahmaputra and Meghna riversare moderately affected. The deeper aquifers (>150 m)in these regions are either less enriched or low in As.

Arsenic enrichment also persists in the shallow aquifersof the coastal plains, while the deep aquifers and theaquifers in the piedmont plains are not enriched.

Vertical sequences of sediments show two distinctlithofacies assemblages; over bank facies with pre-dominant fine sediments and the coarser channel fill

sediments. The sedimentary structures are characteristicof deposition under fluvial and estuarine/tidal environ-ments. High-As groundwaters are produced from the

Holocene alluvial aquifers comprising fine to very finegrained sand, relatively higher percentage matrix (siltand clay) and organic matter. On the other hand,aquifers in older (Pleistocene) sediments are fine to

medium grained with low organic matter content(Ravenscroft, 2001). The sediments are oxidized as evi-denced by the occurrence of well-developed red clays

Fig. 10. Variation in oxalate extractable As (Asox) with (a) Feox; (b) Mnox; (c) Alox; and (d) Pox in the aquifer sediments.

Fig. 11. Relationship between (a) Feox–Pox; and (b) Feacet–Pacetin the aquifer sediments of Bangladesh.

196 K.M. Ahmed et al. / Applied Geochemistry 19 (2004) 181–200

and sand and characterized by groundwaters with lowAs levels.

Thin section petrography of aquifer sands indicatesimmature detritus, comprising quartz, feldspar, lithicgrains, mica and chlorite. Non-opaque heavy minerals

are present in most of the samples, while the opaqueheavies are rare. Ferruginous grain coatings are mostlycommon on the framework detrital grains. Plots of det-

rital framework characteristics on provenance diagramsreveal sedimentary, metasedimentary and metamorphicterrains of the adjoining highlands and the Himalayanorogenic belt as the major source terrains for Holocene

alluvial aquifer sands.Groundwater is mostly Ca–Mg–HCO3 and Ca–Na–

HCO3 types in the shallow Holocene aquifers, with

HCO3� as the dominant anion. Elevated Ca+Mg over

Na is caused due to the interaction of groundwater withthe immature aquifer sediments rich in organic matter.

Low SO42� and NO3

� coupled with high NH4+ and PO4

3�

levels typically characterizes these reducing ground-waters. Conversion of NO3

� to NH4+ is a common pro-

cess in the sequences of redox reactions, and a positive

correlation between As(III) and NH4+ signifies its

importance in mobilizing As in the aquifers. Dissolvedsulfides in appreciable levels suggest SO4

2� reduction, in

spite of the low overall status of S in the Holoceneaquifers of Bangladesh. Moreover, low SO4

2� levels donot show any definite correlation with Astot, which

implies that pyrite oxidation is not the primarymechanism for As mobilization in these aquifers.Increased levels of DOC in groundwater result through

the processes of degradation of organic matter. While theoxidative degradation produces high HCO3

�, microbialfermentation of organic matter leads to the generation ofbiogenic CH4 as reported in the aquifers from one of the

most As-enriched regions in SE Bangladesh (Ahmed etal., 1998a). Influence of microbial processes in thesesediments rich in organic matter create a favorable

reducing environment facilitating transformations ofFe(III) to Fe(II) as well as As(V) to As(III), thereby

mobilizing As in groundwater (Routh et al., 2000).Reductive dissolution of Fe(III) from the coarse aquifersediments also results in the release of PO4 along with

As into groundwater (McArthur et al., 2001; Mukherjeeand Bhattacharya, 2001; Bhattacharya et al., 2002b).Sediment extraction studies reveal AsNO3 concentra-

tions of (0.5–17.7 mg kg�1), sufficiently high to mobilizeAs above the acceptable drinking water standards. Sul-phurNO3 and AsNO3 peaks in the clay sediments, wereconsistent with the earlier findings of authigenic pyrite

framboids (Nickson et al., 2000; BGS and DPHE,2001). Oxalate extractions of the aquifer sands revealedpositive correlation of Asox with Feox, Mnox and Alox,

and a strong positive correlation between Asox and Poxin the coarse grained sediments. Negative correlationbetween Asox and Pox in the fine sediments indicated

dissolution of secondary vivianite, rather than therelease of adsorbed PO4

3� onto the Fe-oxide surfacesthrough dissolution of amorphous Fe- and Mn-oxides.Acetate extraction of the sediments indicates the pre-

sence of carbonate minerals like siderite and rhodo-chrosite in these reducing aquifer sediments, while Pacetfractions concomitant with high Feacet indicates possible

dissolution of vivianite. Hydrogeochemical modelingalso indicates supersaturation of siderite and vivianite,both acting as sinks for Fe(II) and PO4

3� in reducing

alkaline groundwaters.These studies demonstrate that As is mainly released

in groundwaters of GBM Delta of Bangladesh due to

reductive dissolution of Fe oxyhydroxides. Reductionof Fe oxyhydroxides is coupled to the degradation oforganic matter in the sediments. However, the extentof these processes varies considerably, and is influ-

enced by several biogeochemical processes duringsediment-water interactions in the Holocene aquifersin the country.

Fig. 12. Relationship between Asox in the sediments as functions of oxide bound Fe (Feox–Feacet) and carbonate-phosphate bound Fe

(Feacet).

K.M. Ahmed et al. / Applied Geochemistry 19 (2004) 181–200 197

Acknowledgements

The authors would like to thank the Sida-SAREC forproviding research funds (SWE-1998-193) for the stud-

ies on high As groundwaters in Bangladesh. KMAthanks Sida (Development Corporate Office, Dhaka) forproviding travel grants for participation and presenta-

tion of the paper during the As Workshop held at Riode Janeiro in Brazil during August 2000. We appreciatethe critical comments of Gunnar Jacks and an anon-

ymous reviewer, which helped to improve the earlierversion of this manuscript.

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