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Geochemistry and speciation of solid and Aqueous Phase Arsenic in the Bengal Delta Plain Aquifers

Author: 
Debashis Chatterjee, Bibhash Nath, Joydeb Jana, Aishwarya Goswami, Sudipto Chakraborty, Partha Mukherjee, Debasish Shome, Debatri Bagchi, Madhab Jyoti Sarkar, Prosun Bhattacharya, Gunnar Jacks, Kazi Matin Ahmed
Source: 
Bhujal News Quarterly Journal, April-Sept, 2009

INTRODUCTION


Bengal delta plain (BDP), an integral part of the world's largest delta, is a major natural storehouse of high As groundwater where millions of people are now suffering from serious health hazards including arsenic induced cancer (Guha Mazumder et al., 1999). The unconsolidated alluvium of Pleistocene-Recent age is the source horizon that rests upon Tertiary rock sequence (Bhattacharya et al., 1997).

The scale of the problem is grave and unprecedented both in terms of human exposure (~ 60-80 million) and geographical area coverage (173x 103 km2) (PHED, 1993; Bhattacharya et al., 1997; Smedley and Kinniburgh, 2002; Bhattacharya et al., 2003). The presence of As in groundwater, higher than the stipulated Indian standard (50 μg L-1, drinking water quality standard for most countries) and WHO guideline value (10 μg L-1) for human consumption was first reported from West Bengal, India during early 1980’s with first diagnosed case of arsenicosis (Saha, 1984). Later on, within a span of decade, human suffering from natural dissolved inorganic As was widespread in the West Bengal part of BDP and with frequent cases of arsenicosis was reported from 72 blocks spread over nine districts covering ~ 3700 km2 in West Bengal, starting from Malda in the north to the 24- Pargana (s) in the south (Bhattacharya et al., 1997; Bhattacharya, 2001). In west Bengal high As groundwater areas (barring Purbasthali, Bardhaman district and Balaghar, Hugli district) are confined to the west of Bhagirathi River. However, in the eastward extension of BDP (Bangladesh), the natural arsenic “hot spots” have much wider spatial distribution In those ’hot spots” more than 90% of the shallow wells are shown to have elevated levels of arsenic compared to Bangladesh National Standard (50 μg L-1), though the resultant health problem was first diagnosed only in 1993 (BGS and DPHE, 2001; Smedley and Kinniburgh, 2002; Ahmed et al., 2004).

In nature, arsenic (0, -3, +3, +5 oxidation states) exists in both inorganic as well as organic form. Dissolved forms of arsenic in the natural water includes arsenate (+5), arsenite (+3), mono-methyl arsonic acid (MMA) and di-methyl arsinic acid (DMA). However, the degree of toxicity solely depends on the form (inorganic/organic) and the oxidation state of the element. The inorganic forms (arsenate/arsenite) are more toxic than organic forms (MMA/DMA). Among the inorganic forms, the trivalent form (AsO33-) are likely to be more toxic than pentavalent ones (AsO43-). Therefore, the precise concentration (chemical speciation) and chemical forms of dissolved inorganic arsenic (both aqueous and solid phases) are important because more toxic and labile As (III) is now globally identified as a major public health issue.

The understanding of the behavior and distribution of redox species in natural waters are essential to explain recently found large-scale groundwater contamination reported from various part of the world. Therefore, the present paper focuses on the hydrochemistry of high As groundwater as well as interaction between water and arsenic traps to understand the release of redox sensitive species into groundwater under local reducing condition (orthodox redox traps). Moreover, understanding the distribution of redox species in natural waters is also essential to explain the large-scale groundwater contamination in the BDP. The different forms of As (in both aqueous and solid phase) will also be identified to assess their roles in As mobilization as well as level of toxicity during human exposure. Attempts have also been made to visualize the geochemistry of the deltaic environment in relation with speciation of As.

GEOLOGY AND PHYSIOGRAPHY OF THE BENGAL DELTA PLAIN


The Bengal basin is a large asymmetrical pericratonic basin and located in the northeastern part of the Indian sub-continent and one of the largest sedimentary basins of the world. The BDP is an integral part of this basin, with an accumulation of fluviodeltaic to deltaic-estuarine sediment. The West Bengal part is characterized by a long depositional history of sedimentation (Mesozoic to Recent) that was deposited on a Precambrian basement. The zone is demarked by subsurface domal structures of varying dimension bordering east by a row of enechelon faults. The BDP has characteristic topography, geomorphic and geologic features (through space and time) within the sediments. Throughout the Holocene period, the BDP seems to have acted as fluvial-estuarine-marine platform where both marine (lower part of delta) and nonmarine (upper delta and valley margin fan) sedimentation took place. The BDP sediment constitutes dissected uplands with rolling topography and high alluvial plain above the regime of the present day river system. These sediments often disconformably overlie the older deposits, parts of which have been configured as low-lying swamp providing base for tidal inlets beyond the level of inundation of the rising sea-level at the beginning of the Holocene. Towards end of the mid-Holocene, the sea level has retreated to the south of Ranaghat-Khulna axis of the delta. The large inland swamp areas are often associated with vegetation cover, that could possibly be the source of Sedimentary Organic Carbon (SOC) that may have played as an active agent of metal deposition and isolation (Chatterjee et al., 2004).

The Bangladesh part of BDP, located at the head of the Bay of Bengal, occupies most of the Bengal basin. 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. More than 16 km thick syn-orogenic 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 mainly by sandstone and shale sequences, while Pleistocene sediments are represented mostly by clay, overlain by Holocene alluvium.

Various geomorphological units were mapped in the BDP (Morgan and McIntire, 1959; Umitsu, 1987; 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 delta include fan, flood plains, the moribund delta, the Chandina plain and upland Pleistocene Terraces (Barind and Madhapur Tracts) (BGS and DPHE, 2001).

STUDY AREA


Bay of Bengal The study area (Nadia district, ~ 65 km north of Calcutta, West Bengal, India, Latitude 23oN and Longitude 88oE) forms an integral part of the Ganga- Brahmaputra-Meghna (GBM) fluviodeltaic system (Figure 1). The vast alluvium plain spreading from Karimpur (north-east of Calcutta) down to the Bay of Bengal is characterized by succession of a fining upward sequence with occasional clay layers. The fluviatile-estuarine deposits has been influenced by grain size variation (reworked as well as distributed), mineral deposition, organic matter etc. Fluvio-marine deposits are also present in the near surface with organic matter and variable thickness of clay lenses are often observed in such environment. Overall formations are complex and interfingering in nature.

The geomorphology is characterized by a series of meander scars of varying wavelength and amplitude, abandoned channels, ox-bow lakes etc. Common land form features are natural levees, back swamps and inter-distributory swamps. The area has a natural southward slope with a relief difference of few meters (~ 2-5 m). The climate is tropical, hot and humid (temperature range 16-42oC, average relative humidity > 65%) with annual rainfall ranging between 1295-3945 mm (mostly concentrated during the monsoon, June – October).

The Bangladesh study area (Figure 1) is different from the West Bengal part in geomorphological aspects because Lower Delta Plain (LDP) was severely dissected in drainage pattern as well as relief. Climatically, Bangladesh is not much varied from West Bengal except heavy monsoon and high humidity. Groundwater development is almost similar to West Bengal. However, exploitation of shallow aquifers for irrigation are in much larger scale to sustain agriculture. Quaternary sediments provide good aquifers in West Bengal and Bangladesh but As-enrichment is mainly restricted to the Holocene alluvial aquifers at shallow and intermediate depths (Guha Mazumder et al., 1999; BGS and DPHE, 2001; Ahmed et al., 2001; Bhattacharya et al., 2002a). Quaternary sedimentation in the BDP is largely controlled by huge sediment supply, active tectonics and sea level changes (Goodbred and Kuehl, 2000).

METHODS


Sampling (both aqueous and solid phase)
Groundwaters were sampled from existing domestic tubewells in nine As affected districts of Bangladesh during January 1999 and 2001. The location of each tube well was determined using a hand held global positioning system (GPS). Groundwater pH, redox potential (Eh), temperature and electrical conductivity were measured in the field. pH was measured using a Radiometer Copenhagen PHM 80 instrument using a combination electrode (pH C2401-7). Redox potential was measured in a flow-through cell using a combined platinum electrode (MC408Pt) equipped with a calomel reference cell. Water samples collected for analyses included: i) filtered (using Sartorius 0.45 μm online filters) for anion analyses; ii) filtered and acidified with suprapure HNO3 (14 M) for the analyses of cations and other trace elements including arsenic (Bhattacharya et al., 2002b). Arsenic speciation was performed with disposable cartridges® (MetalSoft Center, PA) in the field, following the methodology described by Meng et al. (1998). The cartridges, strongly adsorbs As(V) while allowing As(III) to pass through. Similar protocols and methodology were practiced during groundwater sampling from number of affected villages of Nadia, West Bengal.

Bulk sediment was collected from various boreholes drilled during field investigation. Sediment samples were sealed in polythene bags under inert atmosphere and then stored in an airtight ice-cooled box (temp ~ 4oC). Details of sampling protocols, measurement techniques for both water and borehole sediment has been described in previous publications (Bhattacharya et al., 2003).

Analysis (both aqueous and solid phase) Anions were analyzed by using ion chromatograph (Dionex 120/Metrohm 761), simultaneously Tecator AQUATEC 5400 analyzer was used to measure NO3- (540 ηm) and PO43- (690 ηm). Cation analyses were done by ICP-MS (Varian/Jobin-Yvon/Perkin- Elmer). Arsenic [Astot and As(III)] were determined using AAS (Perkin-Elmer) and a few bulk sediment samples were also analyzed by ICP-AES (Perkin-Elmer). Redox sensitive species were also analyzed spectrophotometrically (Perkin-Elmer, Lambda-20). During the measurement, ultra trace elements [mostly As(III)] were diluted in various ratios depending on their concentration and buffered (0.5M citrate buffer) for their selective measurement under controlled condition. Measured As(III) was also verified with disposable cartridge separation method and found that there was no significant difference in As(III) concentration (~ 5%). Dissolved organic carbon (DOC) in the water samples was determined on a Shimadzu 5000 TOC analyzer (0.5 mg L-1 detection limit with a precision of ±5 -10%). Certified standards, SLRS-4 (National Research Council, Canada) and GRUMO 3A (VKI, Denmark) and synthetic chemical standards prepared in the laboratory, and duplicates were analyzed after every 10 samples during each run. Trace element concentrations in standards were within 90-110% of their true values. Relative percent difference between the original and duplicate samples were within ±5 - 10%.

Chemical partitionings, using acid (7M HNO3), oxalate (0.2M NH4-oxalate, pH~3.5) and buffer (Na-acetate, pH~5.4), were performed in finer sediments (< 65 mm fraction) following the methodology detailed in Bhattacharya et al. (2001). Sequential extraction technique thus adopted to measure the specific solid phase (oxide/oxyhydroxide/carbonate), where As was associated. Extractant were analyzed for Astot and associated elements (Fe, Mn, Al, P and S). Crystalline phases were also identified using an X-ray Diffractometer (Philips). Magnetic and non-magnetic fraction are separated by using a hand magnet. Sediments from different depths and individual mineral phases separated from each sample were then treated for As and associated elements analysis by using AAS. Organic carbon of the air dried borehole sediment samples are determined as discussed in Bhattacharrya et al. (2003).

RESULTS AND DISCUSSION


Groundwater chemistry and behavior of arsenic
Most groundwater are of Ca-/Ca-Mg- HCO3 type, although Ca-Na-/Na-Ca- HCO3- type are also found in the saline tracts of Bangladesh where Cl- concentration goes up to ~ 4000 mg L-1. The characteristics chemical features of the natural high As groundwater of BDP (both West Bengal and Bangladesh) are low to very low dissolved oxygen (< 0.1 mg L-1), high redox-sensitive metal species (Mn > 0.4 mg L-1, Astot > 1 μg L-1, Fe > 0.2 mg L-1), HCO3- (> 300 mg L-1), PO43- (> 0.6 mg L-1) and DOC (> 2 mg L-1) and with low Eh (generally < 100 mV), NO3- (< 1.0 mg L-1), SO42- (< 3.0 mg L-1) and nearly neutral pH (6.5-7.5). Therefore, the characteristic features of groundwater from BDP clearly demonstrate the typical anoxic nature of the aquifers (mostly shallow/intermediate). Major ion composition is HCO3- (300-620 mg L-1) that varies with depth and lithology. Distribution of major anions (NO3-, SO42-, PO43-) indicates low variability, while mapping of groundwater PO43- indicates that the high groundwater PO43- areas are also arsenical (Figure 2a). Distribution of the major cations (Ca2+ ~ 21-184 mg L-1, Mg2+ ~ 14-96 mg L-1, Na+ ~ 7-170 mg L-1 and K+ ~ 1.5-20 mg L-1) showed significant aerial variations with depth. Results of chemical analyses also showed large variation in the concentration of metal redox species [Astot ~ 2.5-1020 μg L-1, As (III) ~ 6-970 μg L-1, Fetot ~ 0.2-15.7 mg L-1, Fe (II) ~ 0.2-15.3 mg L-1], thus indicating the presence of elevated levels of both As and Fe in groundwater. Arsenic (Astot) concentration in the groundwater varies over 3-4 orders of magnitude in some of the shallow wells, and frequently exceeding the WHO guideline value. As concentrations showed little distinct regional trend with other measured water quality parameters and exhibits a significant short-range spatial variability. Speciation data indicate that the ratio of As(III)/(V) is varying largely over a large geographical area. However, the distribution of Astot and As (III)/(V) ratio in the groundwater is largely fluctuating. There is a tendency of high Astot and As(III) concentration (> 210 μg L-1) of wells located in the low-laying areas (local sagging zones).

There is a positive correlation between Astot, PO43- and HCO3- whereas the correlation with Fe is not significant (Figure 2b). On the other hand in Bangladesh, Fetot indicates positive correlation with HCO3- (Figure 3a). This is commonly expected in BDP groundwater with reducing environment where dissolved Fe(II) concentration is largely controlled through precipitation/co-precipitation of iron carbonate/phosphate (e.g., siderite, vivianite etc.). The high alkalinity load in the groundwater is due to the breakdown of fresh organic matter during the activity of microbes that has been regulating the process. Groundwater temperature is relatively high (26-31oC) and elevated groundwater temperature further facilitating the microbial process that leads to increase in the local reducing condition. Partial pressure of CO2 is also high in contaminated groundwater (log pCO2 ~ 1-2.5) and calcite, dolomite, siderite and vivianite has been found in supersaturated phase. Hydrogeochemical model (Parkhurst, 1995), suggests that the dissolved Astot and Fetot concentrations is depending on the dissolutions/precipitations of above mineral phases, therefore, it is unlikely to have a strong positive correlation between As and Fe in groundwater. On the other hand, elevated HCO3- levels are not only controlled by the dissolution of carbonates, where HCO3-concentrations shows overall positive correlation with DOC (Figure 3b). Thus favouring the breakdown of organic matter, and is important in controlling the thermodynamically favoured microbial reactions. Both SO42- and NO3- does not show any correlation with Astot and are generally in the low to very low concentration range in groundwater. Low NO3- concentration in groundwater is mainly due to consumption during microbial process before Fe reduction (de-nitrification process). There is a moderate to strong correlation between As(III) and NH4+ (r2 = 0.57, Figure 3c) in Bangladesh groundwater where local reducing environments are more predominant and is observed because nitrogen (breakdown product of nitrification process) can be further reduced to NH4+ under strongly reducing condition. Therefore, such areas frequently show the presence of more toxic/labile As(III) in groundwater. This could be the reason of large number of patients have been identified from BDP hot spot areas (Guha Mazumder et al., 1999; Karim, 2000).

Fig-

Distribution of arsenic species


Groundwater As problem is serious in terms of both geographic distribution (discrete source) and scale of exposure (exposure risk from health point of view ) even if within safe limits i.e. low As (<50ppb) groundwater. Speciation of As will provide better understanding (presence/absence) of the more toxic as well as labile As(III) which is important to protect human health. Depth distribution of groundwater As(III) concentration shows a high risk around the depth of 15 – 20 ±5 m (Figure 4a) and the As(III) concentration is in several orders of magnitude higher than the stipulated guidelines (WHO, India and Bangladesh). On the other hand, the depth distribution of groundwater As(V) does not show similar pattern (Figure 4b), while the depth distribution of Astot exhibits a depth coverage of much wider areas (Figure 4c). Therefore, in many areas it appears that the tubewells having depth greater than 150 m (deep aquifer) can only provide low As water. The increasing concentration of As (III) is alarming due to the increased recognition of the significant outbreak of As induced cancers and internal health problems in BDP resulting from the extensive use of shallow groundwater which contains largely As(III) compared to As(V) (Bhattacharya et al., 1997). Therefore, aquifer mapping is essential to decipher the high- and low-As zones to ensure the level of As(III) in groundwater.

Sediment chemistry


The distribution of As (Astot ~ 10-26.3 mg kg-1), organic matter (SOCtot ~ 4.2-9.5g kg-1) and iron (Fetot ~ 0.5-1.6 g kg-1 ) demonstrates that the fine grained sediments (silt/silty clay) were found to be associated with high groundwater As areas. A close examination of the chemical characteristic of sediment further reveals that there is a positive correlation (r2 = 0.67) between elemental As and Fe, although the elemental As is absent in the structure of arsenic traps. On the other hand, there is a weak correlations (r2 = 0.22) between elemental As and C, where average C content of the sandy sediment is often less as compared to silty/silty clay horizons.

Acid oxalate extractable fractions of Feox, Mnox, Alox, Pox and Asox also reveal considerable variability with depth and lithology. The range of Feox, Mnox, Alox, Pox and Asox varied between 0.4-5.9 g kg-1, 0.005-0.45 g kg-1, 0.1-1.32 g kg-1, 5.1-144 mg kg-1 and 0.1-8.6 mg kg-1, respectively. A positive correlation was observed amongst Asox and Feox, Mnox as well as Alox (Figure 5a-c). While the coarse-grained sediments indicated strong positive correlations between Asox and Pox (Figure 5d), their distribution in the fine-grained sediments (silty clay and clay) revealed an inverse trend (Figure 5d). 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 (0.02-0.37 mg 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. 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 the hydrogeochemical speciation modeling also suggests that the groundwater are supersaturated with respect to these minerals, which act as sinks for Fe and Man as well as PO43- and hence control their solubility in groundwater (Bhattacharya et al., 1998; Nickson et al., 2000). Oxalate extraction of the aquifer sediments demonstrate that the As and other associated elements (Fe , Mn , Al , P) were dominated the solid phase (Feox ~ 0.2 -5.9 g kg-1, Mnox ~ 0.005-0.45 g kg-1, Alox ~ 0.02-1.32 g kg-1, Pox ~ 5.1-144 mg kg-1, Asox ~ 12-101 mg kg-1), and the significant variability was observed with depth, topography, geomorphic and geologic features. It has also been observed that Asox and Pox showed positive correlation (Figure 5d) in coarser sediments rather than their distribution in the finegrained sediments.

Fig-6 Furthermore relatively high acetate fraction (Feacet ~ 0.02 – 0.37 mg kg-1 in Feox ~ 2.1 – 80%, Mnacet ~ 0.002 – 0.27 mg kg-1 in Mnox ~ 15.8 –97%, Pacet ~ 0.7 – 73.7 mg kg-1 in Pox ~ 70 %) confirms the possible dissolution of carbonate (siderire and rhodochrosite) as well as unstable phosphate mineral (vivianite) that has already been demonstrated by our speciation model during aqueous phase discussion. The study (hydrogeochemical and chemical partitioning modeling) further suggests that the groundwater were supersaturated with respect to carbonate and phosphate minerals which acted as sinks for Fe/Mn/PO43-, and thereby, control their solubility in groundwater. Carbonate minerals were important components of mineralogy of the BDP sediments and dissolution/precipitation of As from such mineral phases controls groundwater chemistry. Therefore, reaction kinetics are important in understanding the local short range spatial variations. Moreover, correlation between Feox and Pox (r2 = 0.74, Figure 6a) indicates reductive dissolution of the amorphous Fe–oxides together with surface bound PO43-, whereas Feacet and PO43-acet fraction do not exhibit such relationship. These suggest that the Fe-oxides – PO43- pool is more significant than vivianite – PO43- pool in BDP aquifers.

fig-7 Fairly high correlation between Feox and Pox (r2 = 0.74; Figure 6a), indicates reductive dissolution of the amorphous Feoxides together with surface bound PO43- , particularly in the coarse sediments. However, Pacet fractions do not exhibit a significant correlation with Feacet (Figure 6b). Further, Pacet fractions are in the order of magnitude lower than the Pox fractions, which suggest that the pool of PO4 related to Fe-oxides is more significant than the pool of PO4 related to Pminerals like vivianite in the aquifers. Oxalate extractable Asox plotted against Feacet and Feox-Feacet as independent variables (Figure 7) reveals two 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 are characterized by high Feacet fractions plot separately. These results clearly demonstrate that siderite and vivianite are also present together with amorphous Fe-oxides with adsorbed As in the Holocene sedimentary aquifers.

Geochemistry and Arsenic affinity


The redox processes are important to understand the reduction reactions that occurs when aquifers behave anoxic (Langmuir, 1997). Among the processes, iron oxides/hydroxides (FeIII/II system) redox chemistry is important since the system has direct impact on the mobility of As under anaerobic environment. The major pathway of As release in aquous phase is the reductive dissolution of the “arsenic traps” (mostly sedimentary Fe-oxides/hydroxides) under local reducing condition (redox traps, SOC). Arsenic is released to groundwater during reduction (partially and/or completely) of these arsenic traps (Fe/Mn/Al oxides/hydroxides) and the process is regulated by microbes (dissimilatory iron reducing bacteria, DIRB) where the oxidation of organic matter supplies necessary energy to drive thermodynamically favored redox processes via electron transfer reactions (Lovely and Chapelle, 1995). The redox ladder includes multiple steps and begins with consumption of dissolved oxygen available in the subsurface water and an increase in dissolved bicarbonate ions due to the decomposition of organic matter. Next is the de-nitrification process and aqueous nitrate decreases rapidly with increase in dissolved bicarbonate ions. Sedimentary metal oxides/hydroxides are then reduced from insoluble phase to soluble phase [Mn (IV) (s) to Mn (II) (aq) and/or Fe (III) (s) to Fe (II) (aq)]. As a result, dissolved concentration of redox sensitive species along with bicarbonate ions are increased in the system and sulphate reduction, methanogenesis and finally ammonia production are the sequential steps that are also important to understand redox processes. The speciation of redox species (both solid and aqueous phase) and partitioning between sediment-water interaction are now important to understand the exact sequence of geochemical processes that leads to the elevated redox species concentration in the groundwater (Sracek et al., 2004).

Our studies revealed that significant amounts of amorphous Fe-oxides/hydroxides, carbonates as well as SOC are present in the aquifer sands/silts. Arsenic is released to groundwater during reduction of these mineral phases (arsenic traps) under local reducing conditions (~1% SOC) and the process is regulated by microbes (mostly by Dissimilator Iron Reducing Bacteria, DIRB) where oxidation of SOC supplies necessary energy to drive thermodynamically favoured redox processes. The sediment texture as well as the presence and distribution of SOC vary in space. The high concentration of redox sensitive species (As, Fe, Mn) along with high alkalinity and the absence of dissolved oxygen and low to very low NO3- in the groundwater indicates the geochemical processes (dentrification → iron reduction) that controls the groundwater chemistry of the high As aquifers (hotspot areas). However, the vertical and lateral variation in redox species concentration (geographical distribution of As and public health issues) and their heterogeneity (at least in the hot spot areas) are difficult to explain by the process of iron reduction by subsurface organic rich sediment.

In BDP, the subsurface geochemical process is activated through the presence of high redox sensitive species. It is suggested that the respiration of organic carbon via colloidal/pre-colloidal iron route plays an important role in As mobilization at best in shallow aquifers. However, mobilization may also be driven by reduction with organic carbon via carbonate/silicates dissolution during the paucity of colloidal/pre-colloidal iron oxides. This can be a possible explanation to understand the heterogeneity of the hotspot areas where iron concentration is very low or insignificant and As concentration is very high. Similar situation may also occur in deltaic/tidal/upland-inland basin areas where dissolved NH4+ and Ca2+ profiles follow the dissolved As concentration peak in natural groundwater and such observations are recently reported from various parts of BDP (Bhattacharya et al., 1997; Harvey et al., 2002; Akai et al., 2004; Sracek et al., 2004).

CONCLUSION


The present work suggests that the BDP groundwater is anoxic in nature and mostly calcium bicarbonate type. Sedimentary iron [Fe(III)/(II)] is the dominant mineral constituent that carry As might have deposited by the meandering river. Sediment mineralogy and texture along with organic matter play crucial role in release of As in groundwater. The possible geochemical pathways is Fe reduction at least in shallow aquifers. High redox sensitive species (As, Fe and Mn), high alkalinity and absence of dissolved oxygen and nitrates further demonstrate the microbial mediated and thermodynamically favoured redox processes (denitrification iron-reduction). Aqueous speciation of As reveals that both As(III) and As(V) concentrations are varying largely and As(III) is significantly increasing in low lying areas. Stratigraphic distribution of As(III) and As(V) reveals that As(III) is more dominant in near-surface aquifers rich in organic matter. Chemical partitioning further supports the presence of amorphous Feoxide together with surface bound phosphates in hotspot areas.

ACKNOWLEDGEMENTS


The principal author (BN) would like to acknowledge DAAD (Germany) for the fellowship and opportunity to carry research work in Germany. One of the author (DC) acknowledges the funding agencies (RGNDWM/IFCPAR) to carry out the research work and acknowledges the Swedish and French partners for their active supports in analysis and training of research scholars. We appreciate the critical comments of Gunnar Jacks to improve the manuscript. Another co-author (DS) likes to thank Department of Geological Sciences, J.U. for necessary infrastructural facilities to carry out the work.

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Debashis Chatterjee, Bibhash Nath, Joydeb Jana, Aishwarya Goswami, Sudipto Chakraborty, Partha Mukherjee -Department of Chemistry, University of Kalyani, Kalyani, Nadia – 741235, West Bengal, India

Debasish Shome, Debatri Bagchi, Madhab Jyoti Sarkar, - Department of Geological Sciences, Jadavpur University, Calcutta – 700 032, India

Prosun Bhattacharya, Gunnar Jacks -Department of Land and Water Resources Engineering, Royal Institute of Technology, SE-100 44 STOCKHOLM, Sweden

Kazi Matin Ahmed - Department of Geology, University of Dhaka, Dhaka 1000, Bangladesh

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