Genesis of arseniferous groundwater in the alluvial aquifers of Bengal Delta Plains and strategies for low-cost remediationProsun Bhattacharya, Maria Larsson, Andrea Leiss, Gunnar Jacks, Andre Sracek and Debashis Chatterjee
The occurrence, origin and mobility of arsenic in natural waters has received significant attention in recent years. Mobilization of arsenic in groundwater is governed by the geochemical processes involving leaching of continental rocks as well as sediments. Anthropogenic inputs particularly due to the application and use of arsenical wood preservatives (Bhattacharya et al. 1996, 1998) as well as pesticides could also lead to significant emission of arsenic in groundwater, especially under anoxic conditions. The risk for arsenic contamination in groundwater is therefore higher than in surface waters. Arsenic contaminated groundwater as the primary source of drinking water in several areas has particularly accentuated the problem because of the effects of arsenic exposure on human health.
Presence of arsenic in concentrations above the maximum permissible limit (> 50 mg/L) in groundwater from the alluvial aquifers has been reported from several parts of the world such as USA, Argentina, Chile, New Zealand, Taiwan, India as well as in Bangladesh (Ahmed et al. 1997; Bhattacharya et al., 1996; 1997 and references therein). The commonly existing As-species in groundwater are in the form of As(V) as arsenate and As(III) as arsenite, the later being more mobile and toxic for living organisms. Methylation of inorganic arsenic to monomethyl- and dimethylarsenic acids is coupled to the biological activity in water.
The problem of arsenic contamination in groundwater in the vast tract of alluvial aquifers in Bengal Delta Plains is a subject of global concern and known to have affected a population of about 38 million in West Bengal and another 40 million in different districts of Bangladesh (ACIC, 1998). During the last two decades, need of water for domestic as well as irrigation purposes prompted development of groundwater resources. Such overdraft of groundwater could be envisaged as one of the key factors responsible for the spreading of arsenic epidemic in this part of the world. Consumption of groundwater with high-As content over a prolonged period of time has manifested in adverse health effects among the population such as arsenical dermatosis, hyperkeratosis and several other symptoms of arsenicosis (Goriar et al., 1984; Chakraborty et al., 1987; Guha Mazumder et al., 1988; Das et al., 1996). The rural and semi-urban areas are affected to a greater extent because the groundwater is used to a major extent as a source of drinking water.
The Bengal Delta Plains is characterized by a thick succession of fluviatile sediments pertaining to Quaternary age. The arseniferous belts located in the Upper Delta Plains (UDP) are mostly characterized by complete or truncated cycles of fining upward sequences dominated by course to medium sand, fine sand, silt and clay sediments. Geochemical and hydrogeological characteristics of these alluvial sediments influence the mobility of arsenic in groundwater, but the source of arsenic in these sediments is dependent on the geology of the source terrain. Interaction of the aqueous phase with the different mineral phases of the aquifer sediments play a predominant role in controlling the retention and/or mobility of As under different redox conditions within the subsurface environment. Chemical processes as adsorption-desorption, precipitation-dissolution of unstable As minerals, organic content, biological activity are known to control the redox conditions within the aquifers (Robertson, 1986; Bhattacharya et al., 1997). The present contribution is therefore aimed to highlight the need for an integrated research in order to understand the mechanisms governing the mobilization of arsenic from the sedimentary aquifers. Characterization of sedimentary fills of the basin should be an integral part of any research engagement in order to understand the phenomenon of remobilization of As. The role of secondary surface-reactive mineral phases such as hydrous ferric oxides (HFO) and hydrous aluminum oxides (HAO) are especially important as they act as the potential adsorbents of the inorganic arsenic phases. The adsorption characteristics of HFO as well as HAO could be easily manipulated due to changes in the redox conditions of the sedimentary environment thereby facilitating mobilization of As under the modified redox conditions during groundwater development. The land use pattern is another factor which may create physico-chemical conditions favoring the mobilization of As.
The data on As-chemistry of groundwater samples from several pumped wells in Nadia District indicate arsenic concentrations in the range of 100-300 mg/L. Predominance of As(V) species is evident from the consistent ratios of As(III)/As(V) less than 1. Analytical data on the sediment samples from the bore hole sites in the region reveal significant variations in the content of total As within the aquifers at different depths. The sandy aquifers at depths of 27-63 m indicate concentration in the range of 40-55 mg/kg. The silty-clayey and clayey sediments at intermediate depths indicate high Astot content (133 mg/kg). The deeper aquifers at depths 70-122 m reveal Astot contents of 44-61 mg/kg while the clayey sediments underlying these sandy aquifers are characterized by lower Astot contents (ca. 20 mg/kg). The concentrations of Fe, Al and Mn show similar trends. Similar pattern of variation is noted for the other trace elements like Mo, V and Cr although their concentrations are lower. Sequential leaching batch experiments for the sediments using deionized water and 0.01M NaHCO3 have indicated that the amount of leachable As in these sediments range between 116-383 mg/L, similar to the As concentrations in groundwater.
Studies on selective extraction of these sediments using oxalate and pyrophosphate media has been carried out to understand the relationship of As with the secondary Fe, Al and Mn phases as well as with the organically bound As. Oxalate extraction of these sediments reveal that HFO as the predominant fraction (Feox=264-1238 mg/kg) as compared to HAO as secondary minerals in these sediments (Alox=27-294 mg/kg). Oxalate extractable Mnox (54-325 mg/kg) is however very low in these sediments. The clayey sediments at depths however indicate presence of both Feox (983 mg/kg) and Alox (294 mg/kg) fractions and complimented by high Siox (229 mg/kg) indicate presence of secondary aluminosilicates. Asox (30 mg/kg) fraction is also comparatively high in these sediments. The amount of pyrophosphate extractable Fe, Al, Mn and As are significantly low, suggesting that bulk of the secondary Fe, Al and Mn phases are inorganic in nature. The results of our investigations reveal that these surface reactive secondary Fe and Al phases play an important role in adsorbing the bulk of As in the sedimentary aquifers in the Bengal Delta Plains. These Fe- and/or Al-phases are characterized by variable surface charge, negative at higher pH and positive at lower pH. At lower pH, these surface reactive phases attain net positive charge leading to significant adsorption of As(V) species. The occurrence of As in groundwater is a process driven by the changing redox conditions where the arsenic phases are selectively desorbed as a response to the reduction of Fe3+ phases to soluble Fe2+ species. High-As occurrences concomitant with the increased Fe contents in groundwater supports this hypothesis. It can be conjectured that part of the As in the groundwater is quantitatively related to the release of As phases mainly as As(V) form adsorbed on the surface reactive Fe-oxides and hydroxides. Although the geological sources of As in the alluvial sediments of the Bengal Delta plains could be proved unequivocally, more detailed research is needed to characterize the chemistry of the aquifer materials in order to understand the water-solid-phase reactions operating in conjunction groundwater development.
Another important aspect of research was to highlight the need to develop low-cost geochemical techniques for the removal of As suitable for application in the developing countries. Laterite, a local raw material ubiquitously found in India has been tested as an adsorbent for for arsenic using the groundwater samples collected from the village Ghetugachi in Nadia district through a series of laboratory investigations (Larsson and Leiss, 1997). The laterite was found to have an efficiency to adsorb 50-90 percent of arsenic depending mainly on the natural variability of the laterite and the variations in groundwater chemistry. Long term groundwater management strategies must be adopted to prevent mobilization of arsenic to the groundwater by regulating the land use pattern in the region. Laterite can be used as a small-scale alternative for the people drinking water from arsenic affected aquifers.
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