In order to investigate the mechanism of arsenic release to anoxic ground water in alluvial aquifers, we sampled ground waters from three piezometer nests, 79 shallow (< 45m) wells, and 6 deep (> 80m) wells, in an area 750 m by 450 m, just north of Barasat, near Kolkata (Calcutta), in southern West Bengal. High concentrations of arsenic (200 -1180 µg l -1 ) are accompanied by high concentrations of iron (3 -13.7 mg l -1 ) and phosphate (1 -6.5 mg l -1 ). Ground water that is rich in manganese (1 -5.3 mg l -1 ) contains < 50 µg l -1 of arsenic. The composition of shallow ground water varies at the 100m-scale laterally and the metre-scale vertically, with vertical gradients in arsenic concentration reaching 200 µg l -1 m -1 . The arsenic is supplied by reductive dissolution of FeOOH and release of the sorbed arsenic to solution. The process is driven by natural organic matter in peaty strata both within the aquifer sands and in the overlying confining unit. In well waters, thermo-tolerant coliforms, a proxy for faecal contamination, are not present in high numbers (< 10 cfu/100 ml in 85% of wells) showing that faecally-derived organic matter does not enter the aquifer, does not drive reduction of FeOOH, and so does not release arsenic to ground water.Arsenic concentrations are high (>> 50 µg l -1 ) where reduction of FeOOH oxide is complete and its entire load of sorbed arsenic is released to solution, at which point the aquifer sediments become grey in colour as FeOOH vanishes. Where reduction incomplete, the sediments are brown in colour and resorption of arsenic to residual FeOOH keeps arsenic concentrations below 10 µg l -1 in the presence of dissolved iron. Sorbed arsenic released by reduction of manganese oxides does not release arsenic to ground water because the arsenic resorbs to FeOOH. Arsenic pollution of ground water is common in the alluvial aquifers of the Bengal Basin because Himalayan erosion supplies immature sediments, with low surface-loadings of FeOOH on mineral grains, to a depositional environment that is rich in organic mater so that complete reduction of FeOOH is common.

[1] In the Bengal Basin, the land surface exposed during the last lowstand of sea level around 20 ka, and now buried by Holocene sediment, is capped by an effectively impermeable clay paleosol that we term the Last Glacial Maximum paleosol (LGMP). The paleosol strongly affects groundwater flow and controls the location of arsenic pollution in the shallow aquifers of our study site in southern West Bengal and, by implication, in shallow aquifers across the Bengal Basin and As-polluted deltaic aquifers worldwide. The presence of the LGMP defines paleointerfluvial areas; it is absent from paleochannel areas. A paleosol model of pollution proposed here predicts that groundwater in paleochannels is polluted by arsenic, while that beneath paleointerfluvial areas is not: paleointerfluvial aquifers are unpolluted because they are protected by the LGMP from downward migration of arsenic and from downward migration of organic matter that drives As-pollution via reductive dissolution of As-bearing iron oxyhydroxides. Horizontal groundwater flow carries arsenic from paleochannels toward paleointerfluvial aquifers, in which sorption of arsenic minimizes the risk of pollution.

No abstract

The consumption of groundwater polluted by arsenic (As) has a severe and adverse effect on human health, particularly where, as happens in parts of SE Asia, groundwater is supplied largely from fluvial/deltaic aquifers. The lateral distribution of the As-pollution in such aquifers is heterogeneous. The cause of the heterogeneity is obscure. The location and severity of the As-pollution is therefore difficult to predict, despite the importance of such predictions to the protection of consumer health, aquifer remediation, and aquifer development. To explain the heterogeneity, we mapped As-pollution in groundwater using 659 wells across 102 km(2) of West Bengal, and logged 43 boreholes, to reveal that the distribution of As-pollution is governed by subsurface sedimentology. Across 47 km(2) of contiguous palaeo-interfluve, we found that the shallow aquifer (<70 mbgl) is unpolluted by As (<10 μg/L) because it is capped by an impermeable palaeosol of red clay (the last glacial maximum palaeosol, or LGMP, of ref 1 ) at depths between 16 and 24 mbgl. The LGMP protects the aquifer from vertical recharge that might carry As-rich water or dissolved organic matter that might drive reduction of sedimentary iron oxides and so release As to groundwater. In 55 km(2) of flanking palaeo-channels, the palaeosol is absent, so invasion of the aquifer by As and dissolved organic matter can occur, so palaeo-channel groundwater is mostly polluted by As (>50 μg/L). The role of palaeosols and, in particular, the LGMP, has been overlooked as a control on groundwater flow and pollutant movement in deltaic and coastal aquifers worldwide. Models of pollutant infiltration in such environments must include the appreciation that, where the LGMP (or other palaeosols) are present, recharge moves downward in palaeo-channel regions that are separated by palaeo-interfluvial regions where vertical recharge to underlying aquifers cannot occur and where horizontal flow occurs above the LGMP and any aquifer it caps.

No abstract