REGIONAL TO LOCAL-SCALE EXTENT AND CONTROLS ON EXISTENCE OF DEEPER GROUNDWATER ARSENIC IN WESTERN PARTS OF BENGAL BASIN

In the western parts of the Bengal basin [West Bengal (WB) state in India], >40 million residents use groundwater from shallow, fluvial aquifers in which elevated dissolved arsenic (As) has been detected. The generalization that As contamination is largely restricted to <100 m depth and deeper aquifers are safe is not universally accurate in the Bengal basin. As concentrations >10 ppb have been observed up to 250 m depth in WB, with ~60% of deep wells (~80% north of 22.75 ^°N) having elevated As. The reason is probably a combined result of hydrostratigraphy and sediment and water chemistry, reflecting differences in geologic evolution at the sub-basinal scale (cratonic stable shelf [WB] versus foreland foredeep [Bangladesh]), along with different land use in terms of divergent starting dates and types of irrigation in India and Bangladesh.

North of ~24 ^°N in WB, potable groundwater is only available to a depth of ~80 m; below this is a regional-scale, thick clay layer. From 24.5 ^°N–22.75 ^°N, a semi-confined sand aquifer provides vertical hydraulic continuity from near land surface to a depth of 300 m (thickening southward). This is inconsistent with inferences elsewhere of an effective, basin-wide, vertically anisotropic barrier (formed by intermediate-depth, thick clay layers) between As-rich, shallow aquifers and safe, deep aquifers. No regionally extensive, oxidized stratum could be mapped to a depth of 300 m in WB. Water chemistry (major and trace solutes and redox parameters from >120 wells) and sediment chemistry (sequential extraction of solid-phase As from multiple boreholes, to 232 m depth) indicate that reducing conditions conducive to As mobilization can occur at >200 m depth.

A regional, heterogeneous, vertically and horizontally anisotropic groundwater flow model, which incorporates observed shallow and deep pumping, shows that As-laden groundwater could have moved to >150 m depth in the semi-confined aquifer without an effective hydraulic barrier since the early 1970s. This is substantiated by depth profiles of environmental tracers (¹⁸O, Cl, and ³H) at a modeled, local-scale study site in WB. Deep pumping for irrigation and public supply at >100 m depth is pervasive in WB, with >2000 large diameter wells installed since the early 1970s, in contrast to infrequent deep groundwater use in Bangladesh.