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

Applied Geochemistry 19 (2004) 181–200 www.elsevier.com/locate/apgeochem

Arsenic enrichment in groundwater of the alluvial aquifers in Bangladesh: 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 a Department of Geology, University of Dhaka, Dhaka 1000, Bangladesh Department of Land and Water Resources Engineering, Kungliga Tekniska Ho¨gskolan, SE-100 44 Stockholm, Sweden c Department of Geology, University of Dhaka, Dhaka 1000, Bangladesh d Department of Mineralogy, Petrology and Geochemistry, Faculty of Science, Masaryk University, 611 37 Brno, Czech Republic b

Abstract Arsenic in the groundwater of Bangladesh is a serious natural calamity and a public health hazard. Most groundwater from the shallow alluvial aquifers (< 150 m), particularly in the Holocene plain lands, are vulnerable to Asenrichment. Delta plains and flood plains of the Ganges–Brahmaputra river system are moderately to severely enriched and more than 60% of the tube wells are affected. Shallow aquifers in the Meghna river basin and coastal plains are extremely enriched with more than 80% of the tube wells affected. Aquifers in the Pleistocene uplands and Tertiary hills 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 association. The sediments can be grouped into 4 distinct lithofacies, viz. clay, silty clay, silty sand and sand. Thin section petrography of the As-enriched aquifer sands shows that the sands are of quartzolithic type and derived from the collision suture and fold thrust belt of the recycled orogen provenance. Groundwater is characterized by circum-neutral pH with a moderate to strong reducing nature. The waters are generally of Ca–Mg–HCO3 or Ca–Na–HCO3 type, with HCO3 as the principal anion. Low SO24 and NO3 , and high dissolved organic C (DOC) and NH+ 4 concentrations are typical chemical characteristics of groundwater. The presence of dissolved sulfides in these groundwaters indicates reduction of SO4. Total As concentration in the analyzed wells vary between 2.5 and 846 mg l 1 with a dominance of As(III) species (67–99%). Arsenic(III) concentrations were fairly consistent with the DOC and NH+ 4 contents. The
HNO3 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) ranged between 0.1–8.6 mg kg 1and 0.4–5.9 g kg 1, respectively. Arsenicox was positively correlated with Feox, Mnox and Alox in these sediments. Insignificant amounts of opaque minerals (including pyrite/arsenopyrite) and the presence of high As contents in finer sediments suggests that some As is incorporated in the authigenically precipitated sulfides in the reducing sediments. Moreover, the chemical extractions suggest the presence of siderite and vivianite as solid phases, which may control the aqueous chemistry of Fe and PO34 . Reductive dissolution of Fe oxyhydroxide present as coatings on sand grains as well as altered mica (biotite) is envisaged as the main mechanism for the release of As into groundwater in the sandy aquifer sediments. # 2003 Elsevier Ltd. All rights reserved.

* Corresponding author. E-mail address: [email protected] (K.M. Ahmed). 0883-2927/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2003.09.006

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Nomenclature BAMWSP Bangladesh Arsenic Mitigation Water Supply Project BDWS Bangladesh Drinking Water Standard BGS British Geological Survey BWDB Bangladesh Water Development Board DCH Dhaka Community Hospital DPHE Department of Public Health Engineering INFS Institute of Nutrition and Food Science MMI Mott MacDonald International SOES School of Environmental Sciences, Jadavpur University UNICEF United Nations International Children’s Emergency Fund XRF X-ray Fluorescence

above the BDWS, while 46% exceed the WHO provisional drinking water limit. Although the number of deep tubewells (> 150 m deep) is relatively small, only 1 and 5% of the analyzed groundwaters exceeded the BDWS and WHO limit, respectively. Analysis of nearly 6000 samples by UNICEF/INFS show that 23% of wells exceed BDWS and 42% samples exceed WHO limit (UNICEF and INFS, 2000). Estimates of major national surveys showed that about one third of the tested wells exceeded the BDWS. Accordingly, it is approximated that nearly 3 million tubewells, placed at depths of between 10 and 50 m, yield groundwater with As concentrations exceeding the BDWS. In the present paper, the authors present an overview of the nature and extent of As enrichment in groundwater of Bangladesh with reference to the geological and geomorphological characteristics of the Bengal Basin. Emphasis has been laid on discussion of the relationship between As-enriched groundwater with the hydrogeological setup of the basin, hydrogeochemical characteristics and the sedimentological, provenance and geochemical characteristics of the aquifer sediments.

1. Introduction 2. Geological and sedimentological characteristics Arsenic enrichment in Bangladesh groundwater is considered to be one of the greatest current environmental disasters in the world. Since the first detection of As in Bangladesh groundwater, a number of studies have been conducted which provide substantial data and discussion about the extent and distribution of the problem. These data have been acquired by both field test kits (UNICEF/DPHE, BAMWSP, 2002, pers. comm.) and laboratory analyses (NRECA, 1997; SOES/ DCH, 2000; BGS and DPHE, 2001; UNICEF and INFS, 2000) (acronyms and other abbreviations used in the text are presented in the box Nomenclature). The largest database on the occurrence of As in groundwater is available from the DPHE/UNICEF field kit survey where 29% of the tested 50,998 wells exceeded the BDWS (50 mg l 1). Complete screening of more than one million wells in 64 upazillas using field kits by BAMWSP, UNICEF and DPHE showed that 53% of the tested wells exceeded the BDWS (BAMWSP, pers. comm.). The higher percentage of wells exceeding the BDWS 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% samples exceed the WHO provisional drinking water limit (10 mg l 1) (SOES/DCH, 2000, pers. comm.). BGS and DPHE (2001) conducted the most systematic national survey where 3534 wells from different parts of the country were analyzed. The results indicate that nearly 27% of the shallow wells (< 150 m) contain As concentrations

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 surrounded by the Indo-Burman range in the east, an uplifted block of Precambrian Shield (Shillong Plateau) in the north, and Precambrian basement complex (Indian Shield) in the west (Fig. 1). More than 16 km thick synorogenic Cenozoic sediments are deposited in the basin derived from the Himalayan and Indo-Burman range (Uddin and Lundberg, 1998). Tertiary sediments in Bangladesh are represented by mainly sandstone and shale sequences, while Pleistocene sediments are represented mostly by clay, overlain by Holocene alluvium. The Ganges, Brahmaputra and Meghna river systems transport a huge amount of sediments and converge at the lower reaches to form the great delta complex—the Ganges–Brahmaputra– Meghna (GBM) delta that is prograding to date. Various geomorphological units were mapped in the Holocene plain lands (Morgan and McIntire, 1959; Umitsu, 1987, 1993; Brammer, 1996), which include piedmont plains, flood plains, delta plains and coastal plains. The geomorphological units in the Holocene landmasses within the Bangladesh part of GBM delta include fan deltas of the Tista and Brahmaputra; fluvial flood plains of the Ganges, Brahmaputra, Tista and Meghna rivers; delta plain of the lower GBM system south of the Ganges and Meghna valleys, including the

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

moribund Ganges delta and the Chandina plain; Pleistocene Terraces (Barind and Madhapur Tracts) and the subsiding basins in the eastern Ganges tidal delta and the Sylhet basin adjacent to the Dauki Fault (BGS and DPHE, 2001). Occurrence of As-enrichment in Bangladesh groundwater shows a close relationship with the major geomorphological units. The landform units have been correlated with the aerial distribution pattern of enriched aquifers to assess the status of enrichment in the major geomorphological units (Ravenscroft, 2001). A broad classification, based on satellite imagery (LANDSAT TM Band), is presented here (Fig. 2) where 3 major units and 5 sub-units have been identified. Characteristic features of these units and the status of As occurrences are described in the Table 1. 2.2. Stratigraphy and lithofacies sequence Quaternary sediments provide good aquifers in Bangladesh and As-enrichment is mainly restricted to the Holocene 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 sedimentation in the Bengal Basin is largely controlled by huge sediment supply, active tectonics and sea level changes (Goodbred and Kuehl, 2000). Table 2 presents a simplified Quaternary stratigraphy of Bangladesh along with the status of As in the sediments. A vertical lithofacies sequence of the sediments from highly As-enriched areas of the country was prepared from exploratory borelogs available from BWDB. Considering texture and lithology, the sediments can be categorized into 4 distinct lithofacies, viz. sand, silty sand, silty clay, and clay. Two distinct lithofacies associations—a dominantly sandy channel-fill association and a fine-grained over bank association (Fig. 3) have been identified in most cases (Ahmed et al., 2001). Grain size profiles characteristically fine upward in all cases indicating a fluvial environment of deposition. It is possible to identify a number of sedimentary structures in the core samples. Sedimentary structures like trough cross lamination, horizontal lamination and ripple laminations are also indicative of a fluvial environment. The median diameters of the aquifer samples represent fine to very fine sand class, which are moderate to well sorted.

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

2.3. Petrographic characteristics 2.3.1. Detrital composition Twenty-one samples of As-enriched Holocene aquifer sands from 14 different locations across the country were studied by thin section petrography, to describe the characteristic detrital modes and to infer their provenance (Fig. 4a–d). The sands are typically composed of quartz (47– 70%), lithic grains (4–17%), mica (muscovite and biotite, 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). However, some percentage of the matrix may have been artificially incorporated due to the breakdown of lithic grains (shale and silt) during sample preparation. Quartz is mostly monocrystalline (Qm) with minor polycrystalline (Qp) types with thick to thin and discontinuous ferruginous coatings (Fig. 4b and c). Feldspar and biotite flakes show alteration (Fig. 4d). Lithic grains are mainly sedimentary and metasedimentary (Ls) which include fragments of shale, siltstone and

Table 1 Groundwater occurrences and As status in different geomorphological unitsa of Bangladesh Groundwater occurrence

Arsenic 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.

The upper aquifer is strongly enriched ( >80% of tested wells) and the deeper aquifer 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.

Moderately 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.

Ganges Delta plains are moderately to severely enriched (60–80% of tested wells) and the Meghna 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.

Moderately to severely enriched (60-80% of tested 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.

Mostly low As ( <20% of tested wells are enriched).

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 Tila aquifer is confined to leaky confined in nature.

The Dupi Tila aquifer is low in As.

(continued on next page)

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Description

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Units

186

Numbers within parenthesis in column 1 refer to geomorphological units of Fig. 2.

chert. Some metamorphic lithic grains (Lm) such as schist and gneiss are also observed. Volcanic lithic fragments (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 the most abundant heavy mineral in all the samples. Other unstable and semi-stable heavy minerals like garnet, epidote and kyanite are common whereas stable and ultra-stable heavies like zircon, tourmaline and rutile are present in trace amounts. Significant occurrence of shale and siltstone fragments, chert, altered feldspar, chloritized biotite and chlorite in the sample indicate that the sands are derived from sedimentary and metasedimentary terrains. Dominance of hornblende (green variety) together with garnet, epidote and kyanite in the heavy mineral assemblages suggest high rank crystalline metamorphic provenance (Pettijohn, 1975; Arribas et al., 2000).

a

Limited data show the aquifers are basically As safe. Due to complex structural geometry (folding and faulting), the aquifers are not uniformly distributed as in the other geomorphic units. Dupi Tila and Tipam sands form the main aquifers. Includes the folded belt at the eastern and northeastern part of the country. Tertiary Hills (1)

Arsenic Status Groundwater occurrence Description Units

Table 1 (continued)

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2.3.2. Provenance studies Quantitative detrital modes, calculated from point counts of thin sections have been used to infer sandstone provenance (Ingersoll and Suczek, 1979; Dickinson, 1985). Quartz (Qm and Qp), feldspar (F) and lithic grains (Ls and Lv) are the primary detrital grain types used for the purpose. Compositional fields of sands characterizing the different provenances are shown as standard triangular diagrams (Fig. 5). In the QtFL plot (Fig. 5a) all the samples are in the upper part of the Qt–L leg indicating a recycled orogen source characterized by a high content of quartz and lithic grains 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 sands from a quartzose recycled orogen. However, one sample suggests 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 triangular diagrams representing absolute dominance of sedimentary and metasedimentary lithic fragments. The sands are of quartzolithic type and derived from the collision suture and fold thrust belt, and indicate the recycled orogen provenance.

3. Hydrogeology, distribution and mobilization of arsenic 3.1. Hydrogeological characteristics Groundwater occurs at very shallow depths all over the country where the major aquifers are the Holocene alluviums and fan deposits and Pliocene fluvio-deltaic (Dupi Tila) sediments. Mio-Pliocene Tipam sands form minor aquifers in the hilly areas. The aquifers are highly transmissive and generally multi-layered. The aquifer

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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.

Pleistocene

Madhupur Clay Dupi Tila

Reddish brown mottled clay and silt.

High in finer and peaty layers, low in coarser gravel layers. Most of the As is oxalate extractable Low, not oxalate extractable

Yellowish brown fine to medium sand

Very low

Plio-Pleistocene

conditions vary from unconfined to leaky-confined in the shallow alluvial deposits and confined in the Dupi Tila and deeper alluvial deposits. Traditionally the aquifer system has been described as (from top to bottom): upper aquitard (silt and clay), composite aquifer (very fine to fine sand, low discharge hand tube wells tap water from this aquifer), main aquifer (medium to coarse sand with occasional gravel, high discharge irrigation wells and municipal water supply wells tap this aquifer), lower aquitard (deep clay and silt layer separating the upper water bearing sediments from deeper ones) and deep aquifer (occurring in the coastal area, mainly water supply hand and municipal wells tap water). This classification holds for the alluvial deposits. A more logical approach however, is to classify the aquifers 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 formations by a thick blanket of clay). The shallow aquifers are further subdivided into upper and lower parts. Water levels lie within a few meters of the ground surface and fluctuate with the annual dry and wet season conditions. The aquifers are recharged during the monsoon season (July–September) when the area receives more than 80% of its annual precipitation (around 2000 mm a 1). Huge amounts of annual floodwater standing on a large part of the country also contribute to the recharge process. Groundwater movements are controlled by the surface topography which is very gentle over most of the country. The regional groundwater flow direction is from the north to south, but the flow rate is very slow. The magnitude of horizontal flow is much lower as compared to the amount of vertical movement. Annual fluctuations in groundwater levels are controlled by the local hydrogeological conditions and withdrawal of groundwater for irrigation. In general, the fluctuations are more pronounced in the northwestern part of the country as compared to the coastal plains. Groundwater in general is fresh over most of the country, except for the coastal region. However, pockets of saline/brackish groundwater are reported in areas far from the coastline (Ahmed, 1994). As the Holocene

alluvial sediments are rich in organic matter and reactive minerals, water is mostly of Ca–Mg–HCO3 and Ca– Na–HCO3-type (Bhattacharya et al., 2002b). The aquifers are mostly reducing in nature and even reach the stage of CH4 formation (Ahmed et al., 1998a; Bhattacharya et al., 2002a). The amount of dissolved salts in groundwater of the Dupi Tila aquifers is less, compared to the Holocene aquifers. The Dupi Tila aquifers were thoroughly oxidized during its geological evolution, which resulted in a decrease in the quantities of organic matter 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 of BGS and DPHE data indicates that most As-rich regions are located in the central, south and southeastern parts of the country, while the northwestern and the uplifted areas of the north-central parts are least affected (Fig. 6). Isolated ’’hot spots’’ are, however, encountered in the southwestern, northwestern, northeastern and north-central regions of Bangladesh. Based on 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 are Chandpur, Comilla, Noakhali, Munshiganj, Faridpur, Madaripur, Gopalganj, Shariatpur and Satkhira. However, within the broad regional distribution pattern of occurrences of As-enriched groundwater, there are extreme local variations. The BGS and DPHE (2001) reported that maximum As concentrations occur at depths between 20 and 50 m whereas samples shallower than 10 m and deeper than 150 m are basically As free (Fig. 7). Most of the deep well samples for this study were collected from the southern coastal area of the country. Other studies from different parts of the country showed a basically similar pattern (NRECA, 1997; Burren, 1998; Mather, 1999; Broms and Fogelstro¨m, 2001). However, a recent study by van Geen et al. (2002) at Araihazar Thana of NE Bangladesh indicated low As levels in wells below 30–60 m. It is therefore apparent that the As distribution in groundwater is not only controlled by depth but rather

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

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

to a major extent by subsurface geology, i.e. age and grain size of sediments. It has been established from the different studies that As is mobilized in tube wells placed in the fine grained Holocene aquifers, while those penetrating the coarser Holocene sediments and the PlioPleistocene (Dupi Tila) aquifers are found to be safe all over the country, thereby revealing that distribution of As in the wells is largely dependent on the facies characteristics 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 Bangladesh. Since its first discovery, use of fertilizers, pesticides, insecticides, waste disposal, As compound treated wooden poles etc. were blamed as the anthropogenic sources of As-enrichment in groundwater (NRECA, 1997). Regional occurrence of As in groundwater of Bangladesh is however not attributable to any anthropogenic activities, and the current status of knowledge indicates predominantly geogenic source and its release in groundwater through natural processes (Bhattacharya et al, 1997; Nickson et al., 1998, 2000). Oxidation of pyrite (FeS2) or arsenopyrite (FeAsS) was postulated as the dominant process for As mobilization due to lowering of water table following excessive pumping of groundwater (Mandal et al., 1996; Mallik and Rajagopal, 1996) that was widely accepted at the

beginning. An alternate theory, known as the Fe oxyhydroxide reduction hypothesis, is now widely accepted as 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 be triggered by intensive extraction of groundwater for irrigation and application of phosphate fertilizer (Acharyya et al., 2000). On the other hand, Harvey et al. (2002) report that As mobilization is associated with recent inflow of C due to large scale irrigation pumping. Foster et al. (2000) and Breit et al. (2001) report that apart from Fe(III)-oxyhydroxides, other solid phases such as phyllosillicates also play an important role in As cycling and mobilization. It is therefore evident that As mobilization in Bangladesh groundwater is a complex natural geochemical processes.

4. Groundwater chemistry and geochemical characteristics of the aquifer sediments 4.1. Groundwater chemistry and behavior of arsenic The chemistry of the BDP groundwaters has been studied extensively during recent years (Nickson, 1997; Burren, 1998; BGS/MMI, 1999; Mazumdar, 2000; BGS

190 K.M. Ahmed et al. / Applied Geochemistry 19 (2004) 181–200 Fig. 5. Provenance discrimination plots: (a) QtFL, (b) QmFLt and (c) QpLvLs showing the characteristic detrital modes and the inferred provenance for aquifer sands of Bangladesh.

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

and DPHE, 2001; Broms and Fogelstro¨m, 2001; Mukherjee and Bhattacharya, 2001; Bhattacharya et al., 2002a,b). Groundwater pH is predominantly near neutral to slightly alkaline (pH 6.5–7.6) with low dissolved O2 (DO). The Eh values (+0.594 to 0.444 V) suggest a mildly oxidizing to moderate/strongly reducing character of the aquifers. Groundwaters are generally Ca– HCO3 or Ca–Mg–HCO3 type, although Ca–Na–HCO3 type and Na–Cl type water are also found locally in several parts of the country (Ahmed, 1994; Bhattacharya et al., 2002a,b). Major ion composition is dominated by HCO3 (320– 600 mg l 1) and shows an apparent depth and lithological control (Bhattacharya et al., 2002b). Concentrations of SO24 ( < 3 mg l 1) and NO3 (40.22 mg l 1) are

generally low, except for some local variations (BGS and DPHE, 2001; Broms and Fogelstro¨m, 2001; Bhattacharya et al., 2002b). Phosphate concentrations on the other hand are high (up to 8.75 mg l 1) in the groundwaters. 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 with depth 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) in the groundwater samples both as a function of depth and 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).

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

Table 3 Calculated values of SI for selected minerals along with PCO2 values Site/value Laxmipur (n=5) Average Faridpur (n=5) Average Munshiganj (n=4) Average

SIcalcite SIdolomite SIsiderite SIvivianite LogPCO2 (atm)

0.425

0.39

0.75

1.44

1.53

0.15

0.06

0.42

0.03

1.24

0.47

0.73

0.68

0.77

0.73

Irontot indicates moderately strong positive correlation with HCO3 and PO34 (Fig. 8a and b), while it exhibits a relatively low correlation with Astot (r2=0.42; Fig. 8c). This is commonly expected in reducing groundwaters with elevated HCO3 and PO34 levels, where Fe2+ may precipitate as siderite (FeCO3) and vivianite (Fe3(PO4)2.8H2O). Saturation indices (SI) for selected minerals, calculated by the hydrogeochemical code PHREEQC (Parkhurst, 1995) using the groundwater 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 the previous findings (Bhattacharya et al., 1998; Nickson et al., 2000; Sracek et al., 2001). Thus, precipitation of

these minerals may explain the relationships between Astot 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 the carbonates 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 groundwaters indicate distinct trends of variation with both As(III) and Fetot concentrations (Mukherjee and Bhattacharya, 2001). Although DOC characteristics in Bangladesh groundwaters are not investigated in detail, organic matter in the Holocene sediments can be considered as an active source for DOC in groundwater (see Routh et al., 2001). Sulphate and NO3 levels in groundwater are generally very low, and there is no significant correlation with As. Microbial degradation of the organic matter in the aquifers results in the reduction of both SO24 and NO3 , thereby increasing the concentrations of sulfide (42 mg l 1; Broms and Fogelstro¨m, 2001) and 1 NH+ 4 (up to 13.2 mg l ) in groundwater (Bhattacharya, 2002). While Astot does not show any correlation with SO24 (Fig. 8e), fairly high correlation is observed 2 between As(III) and NH+ 4 (r =0.57; Fig. 8f) in the groundwater samples. Extremely high values of calculated log PCO2 (in some cases > 1.0) indicate generation of CO2 through the sequence of redox reactions involving organic matter in the sediments, including reduction of NO3 and SO24 as well as the reductive

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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 PO34 ; (c) Fetot vs Astot; (d) HCO3 vs DOC; (e) Astot vs SO24 ; and (f) As(III) vs NH+ 4 (n=36).

dissolution of Fe(III) oxyhydroxides in the solid phase. 4.2. Sediment geochemistry and implications on aquifer characteristics Geochemical analyses of the sediments have been carried out on the Holocene alluvial sediments from different parts of Bangladesh in recent years (viz. Yamazaki et al., 2000; Mukherjee and Bhattacharya, 2001; BGS and DPHE, 2001; Anawar et al., 2003; Akai et al., 2003). Salient geochemical characteristics of a set of 16 borehole sediment samples representing the Holocene alluvium are discussed in the following section. These sediment samples were collected by BWDB during 1999 and 2000, from two representative boreholes at Chapai Nawabganj (DW1 and DW2) at depths between 3 and 132 m, and at one borehole at Tala representing the aquifer sediments between 6 and 46 m (Table 4) A series of selective and partial extractions were carried

out at the laboratories of the Royal Institute of Technology, Stockholm and analyzed at the laboratories of the Universities at Linko¨ping and Stockholm. Acid extraction (7M HNO3) was employed to determine the budget of As and the major elements Fe, Mn, Al, P, and S associated with the non-silicate minerals following the methodology described in Bhattacharya et al. (2001). The HNO3 extracted concentrations of AsNO3, Fe NO3, MnNO3, AlNO3 and PNO3 in the shallow aquifer sediments varied considerably with depth and ranged between 0.5 and 17.7 mg kg 1, 5.8 and 26.5 g kg 1, 0.05 and 0.5 g kg 1, 3.8 and 19.1 g kg 1, and 0.16 and 0.51 g kg 1, respectively. These results were comparable to the results for the whole rock analyses carried out by XRF (BGS and DPHE, 2001). ArsenicNO3 indicated significant positive correlation with FeNO3 , MnNO3 , AlNO3 and PNO3 (Fig. 9). Sulfur concentrations are generally low. Concentrations of SNO3 ranged between 7.5 and 1306 mg kg 1, and had peaks in the clay sediments with high organic matter (Table 4). The content of

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Fig. 9. Variation in HNO3 extractable As AsNO3 with (a) FeNO3 ; (b) MnNO3 ; (c) AlNO3 ; and (d) PNO3 in the aquifer sediments.

organic matter varied between 0.1 and 4.5%. Authigenic framboidal pyrite has been documented in the clayey sediments in several regions of Bangladesh. While a moderately strong correlation was observed between AsNO3 FeNO3 in these sediments (r2=0.67), correlations between FeNO3 SNO3 , and AsNO3 SNO3 were relatively poor. Oxalate extraction (pH 3.5) releases elements that are present as oxides and to some extent even clays (Bhattacharya et al., 2001). Moreover, it also dissolves elements bound as carbonates. It is therefore important to quantify the carbonate bound fractions of the elements following extraction of the sediments with Na-acetate buffered to pH 5.4. Acetate extraction characterizes the carbonate minerals such as calcite (and solid solutions of Fe and Mn therein), siderite, rhodochrosite (MnCO3), as well as unstable phosphate mineral like vivianite (Dodd et al., 2000). Acid oxalate extractable fractions of Feox, Mnox, Alox, Pox and Asox also reveal considerable variability with depth and lithological character 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 144 mg kg 1, and 0.1 and 8.6 mg kg 1, respectively. Arsenicox was positively correlated with Feox, Mnox and Alox (Fig. 10a–c). While the coarse grained sediments indicated strong positive correlations between Asox and Pox (Fig. 10d), their distribution in the fine grained sediments (silty clay and clay) revealed an inverse trend (Fig. 10d). Distinct negative correlation between Asox and Pox fractions suggests dissolution of secondary

vivianite, rather than the release of PO4 ions adsorbed onto the Fe-oxide surfaces in these fine grained sediments. 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 extractable fractions (Table 4). Dissolution of minerals like siderite and rhodochrosite may account for the acetate extractable fractions of Feacet and Mnacet in these sediments. However, high acetate extractable P fractions (Pacet) concomitant with high Fe (Feacet) indicate the possible dissolution of vivianite in these reducing aquifer sediments (Dodd et al., 2000). It is important to note that hydrogeochemical speciation modeling also suggests that the groundwaters are supersaturated with respect to these minerals, which act as sinks for Fe and Mn as well as PO34 and hence control their solubility in groundwater (Bhattacharya et al., 1998; Nickson et al., 2000; Sracek et al., 2000, 2001). Fairly high correlation between Feox and Pox (r2=0.74; Fig. 11a), indicates reductive dissolution of the amorphous Fe-oxides together with surface bound PO34 , particularly in the coarse sediments. However, Pacet fractions do not exhibit a significant correlation with Feacet (Fig. 11b). Further, Pacet fractions are an order of magnitude lower than the Pox fractions, which suggests that the pool of PO4 related 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 FeoxFeacet as independent variables (Fig. 12) reveals two

Table 4 Salient geochemical characteristics of the aquifer sediments from Bangladesh S. No. Sample No. Lithology

Depth (m) FeNO3 Feox Feoxa Feacet Feacetb MnNO3 Mnox Mnoxa Mnacet Mnacetb AlNO3 Alox Aloxa AsNO3 Asox Asoxa PNO3 Pox g kg

a b

3–3.6

%

g kg

1

%

g kg

1

%

g kg

1

%

g kg

34.3

18.8

1

mg kg

1

%

0.39 2.1

4.7

1.5

30.8 586

144.0 24.6 73.7

51.2 107 66.5 498

%

mg kg

1

Pacetb SNO3

%

mg kg

1

%

mg kg

23.6

1.3

5.5 0.04

3.0

0.39

0.13 32.8

0.04

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

0.5

0.2

30.1 160

5.1

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

3.7

0.5

12.9 450

135.0 30.0 4.9

3.6

291

60–60.6 22.8 132–132.6 24.5

1.7 5.8

7.7 0.37 23.8 0.33

2.1 5.7

0.45 0.50

0.45 10.0 0.25 50.3

0.27 0.18

60.2 71.7

15.3 16.4

0.44 2.9 0.78 4.7

6.1 7.4

0.1 2.0

2.1 509 27.3 491

84.7 16.6 7.3 126.2 25.7 6.4

8.6 5.1

816 1203

3–3.6 18–18.6

19.6 8.3

1.3 1.5

6.8 0.04 18.6 0.10

3.0 6.4

0.39 0.13

0.13 33.6 0.09 65.9

0.08 0.03

58.1 33.5

15.2 5.6

0.37 2.5 0.23 4.1

4.2 1.9

1.3 1.0

30.5 378 53.8 214

50.5 13.4 4.4 20.3 9.5 8.1

8.7 7.5 39.7 207

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

1.1

0.3

30.1 148

7.3

69.9 180

57–57.6 25.6 108–108.6 17.1

5.9

22.9 0.37 – 0.32

6.3 –

0.50 0.27

0.28 55.5 –

0.14 0.12

52.0 –

19.1 10.0

1.32 6.9 – –

9.9 4.3

2.8 –

28.2 439 – 459

107.9 24.6 4.8 – – 8.0

4.5 –

876 1306

6.1–6.71 12.2–12.8 18.3–18.9 27.4–28

17.6 18.1 19.6 20.1

3.5 3.4 3.6 3.9

20.2 18.6 18.3 19.6

0.23 0.25 0.29 0.13

6.4 7.5 8.0 3.4

0.22 0.38 0.43 0.24

0.09 0.15 0.16 0.16

42.2 38.5 37.3 67.3

0.08 0.14 0.16 0.10

84.1 94.3 96.8 59.7

12.2 9.6 11.2 8.8

0.61 0.34 0.37 0.41

5.0 3.6 3.3 4.7

5.3 4.0 6.2 17.7

1.9 1.6 2.5 8.6

36.2 40.5 40.0 48.8

82.3 68.1 105.5 132.6

0.7 4.1 11.3 1.9

147 72 128 871

36.6–37.2 26.5 45.7–46.3 7.4

2.2 0.4

8.5 0.17 5.7 0.34

7.4 80.1

0.48 0.10

0.37 76.6 0.06 57.2

0.15 0.01

39.7 15.8

16.8 3.7

0.46 2.7 0.06 1.7

14.1 0.5

3.5 0.2

24.5 342 31.7 278

235 380 433 326

3.2 3.4

4.9 5.1

35.1 17.9 24.4 40.7

0.6 2.8 12.0 2.6

69.4 20.3 1.5 10.9 3.9 1.7

2.2 72 15.8 135

1

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Chapai Nawabganj (DW1) 1 CN-1-1 Fine sand to silt 2 CN-1-2 Fine sand with mica 3 CN-1-3 Silty clay with kankar 4 CN-1-4 Clay 5 CN-1-5 Bluish sticky clay Chapai Nawabganj (DW2) 6 CN-2-1 Fine sand to silt 7 CN-2-2 Medium to fine sand 8 CN-2-3 Fine sand with mica 9 CN-2-4 Sticky clay 10 CN-2-5 Sticky clay Tala, Satkhira 11 TALA-1 Very fine sand 12 TALA-2 Silt clay intercalation 13 TALA-3 Silt-sand intercalation 14 TALA-4 Silty clay with org. matter 15 TALA-5 Very fine sand/silt 16 TALA-6 Fine-sand with org. matter

1

Poxa Pacet

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

195

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

siderite and vivianite are also present together with amorphous Fe-oxides with adsorbed As in the Holocene sedimentary aquifers of Bangladesh.

5. Discussions and conclusions

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

distinct populations. The data sets plotted on this diagram, follow a linear trend for most of the analyzed sediments showing a clear association of Asox with the Fe-oxide phases (Feox-Feacet) in the coarse grained sediments. However, the reducing fine grained aquifer sediments characterized by high Feacet fractions plot separately. These results clearly demonstrate that

Most of the shallow aquifers of the Holocene plain lands of Bangladesh are affected by As-enrichment whereas the aquifers in the Pleistocene and Tertiary sediments are low As. In the Ganges and Meghna delta plains, the status of As-enrichment is severe, while flood plains of the Ganges, Brahmaputra and Meghna rivers are 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 aquifers of the coastal plains, while the deep aquifers and the aquifers in the piedmont plains are not enriched. Vertical sequences of sediments show two distinct lithofacies assemblages; over bank facies with predominant fine sediments and the coarser channel fill sediments. The sedimentary structures are characteristic of deposition under fluvial and estuarine/tidal environments. High-As groundwaters are produced from the Holocene alluvial aquifers comprising fine to very fine grained sand, relatively higher percentage matrix (silt and 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 evidenced by the occurrence of well-developed red clays

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

197

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

and sand and characterized by groundwaters with low As levels. Thin section petrography of aquifer sands indicates immature detritus, comprising quartz, feldspar, lithic grains, mica and chlorite. Non-opaque heavy minerals are present in most of the samples, while the opaque heavies are rare. Ferruginous grain coatings are mostly common on the framework detrital grains. Plots of detrital framework characteristics on provenance diagrams reveal sedimentary, metasedimentary and metamorphic terrains of the adjoining highlands and the Himalayan orogenic 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 with the immature aquifer sediments rich in organic matter. 3 Low SO24 and NO3 coupled with high NH+ 4 and PO4 levels typically characterizes these reducing groundwaters. Conversion of NO3 to NH+ 4 is a common process in the sequences of redox reactions, and a positive signifies its correlation between As(III) and NH+ 4 importance in mobilizing As in the aquifers. Dissolved sulfides in appreciable levels suggest SO24 reduction, in spite of the low overall status of S in the Holocene aquifers of Bangladesh. Moreover, low SO24 levels do not show any definite correlation with Astot, which implies that pyrite oxidation is not the primary mechanism for As mobilization in these aquifers. Increased levels of DOC in groundwater result through the processes of degradation of organic matter. While the oxidative degradation produces high HCO3 , microbial fermentation of organic matter leads to the generation of biogenic CH4 as reported in the aquifers from one of the most As-enriched regions in SE Bangladesh (Ahmed et al., 1998a). Influence of microbial processes in these sediments rich in organic matter create a favorable

reducing environment facilitating transformations of Fe(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 aquifer sediments also results in the release of PO4 along with As into groundwater (McArthur et al., 2001; Mukherjee and Bhattacharya, 2001; Bhattacharya et al., 2002b). Sediment extraction studies reveal AsNO3 concentrations of (0.5–17.7 mg kg 1), sufficiently high to mobilize As above the acceptable drinking water standards. SulphurNO3 and AsNO3 peaks in the clay sediments, were consistent with the earlier findings of authigenic pyrite framboids (Nickson et al., 2000; BGS and DPHE, 2001). Oxalate extractions of the aquifer sands revealed positive correlation of Asox with Feox, Mnox and Alox, and a strong positive correlation between Asox and Pox in the coarse grained sediments. Negative correlation between Asox and Pox in the fine sediments indicated dissolution of secondary vivianite, rather than the release of adsorbed PO34 onto the Fe-oxide surfaces through dissolution of amorphous Fe- and Mn-oxides. Acetate extraction of the sediments indicates the presence of carbonate minerals like siderite and rhodochrosite in these reducing aquifer sediments, while Pacet fractions concomitant with high Feacet indicates possible dissolution of vivianite. Hydrogeochemical modeling also indicates supersaturation of siderite and vivianite, both acting as sinks for Fe(II) and PO34 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. Reduction of Fe oxyhydroxides is coupled to the degradation of organic matter in the sediments. However, the extent of these processes varies considerably, and is influenced by several biogeochemical processes during sediment-water interactions in the Holocene aquifers in the country.

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Acknowledgements The authors would like to thank the Sida-SAREC for providing research funds (SWE-1998-193) for the studies on high As groundwaters in Bangladesh. KMA thanks Sida (Development Corporate Office, Dhaka) for providing travel grants for participation and presentation of the paper during the As Workshop held at Rio de Janeiro in Brazil during August 2000. We appreciate the critical comments of Gunnar Jacks and an anonymous reviewer, which helped to improve the earlier version of this manuscript.

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