From Bureau of Economic Geology, The
University of Texas at Austin (www.beg.utexas.edu).
Bureau Seminar, October 27, 2006
Deeper Groundwater Flow and Chemistry in the Arsenic-Affected Western Bengal Basin, West Bengal, India
A regional-scale hydrogeologic study was conducted in a ~21,000-km2 area of West Bengal (Murshidabad, Nadia, North and South 24 Parganas districts), India, with severe, natural arsenic contamination of shallow groundwater jeopardizing ~13.5 million inhabitants. The study evaluated the suitability of deeper water as an alternate drinking water source.
A hydrostratigraphic model (resolution: 1000 m × 1000 m × 2 m, ≤ 300 m below MSL) indicates a continuous, semi-confined sand aquifer (main aquifer) underlain by a thick clay aquitard. The aquifer deepens toward the east and south. In the south, there are several deeper, laterally connected, confined aquifers. Based on observed hydrostratigraphy, eight seasonal groundwater models (resolution: 1000 m × 1000 m × 15 m) were developed with presence of pumping (2001), absence of pumping (pre-1970s), and projected irrigational pumping (2011 and 2021). Simulations indicate topographically controlled, seasonally variable regional groundwater flow, which has been severely distorted by pumping.
Samples of deep groundwater, river water, and rainwater were collected for major and minor solutes, dissolved gases, and stable isotopes. A d18O-d2H bivariate plot of groundwater falls subparallel to the constructed local meteoric water line (d2H = 7.24 d18O + 7.73), suggesting a predominance of meteoric recharge (from monsoonal rainfall of present-day composition) with some evaporation. A trend of d18O depletion of groundwater was observed northward and westward from the Bay of Bengal, indicating a continental effect. Major solutes indicate the presence of hydrochemically distinct water bodies in the main and deeper isolated aquifers. Spatial trends of major and redox-sensitive solutes and geochemical modeling indicate that carbonate dissolution, silicate weathering, and cation exchange, along with seawater and connate-water mixing, control the major-ion chemistry. The suboxic main aquifer water exhibits partial redox equilibrium, with the possibility of oxidation in microscale environments. The redox processes are depth-dependent and hydrostratigraphically variable.
Deeper water was found unsafe with elevated (≥ 10 µg/L) As in ~60% of the samples. The presence of As is related to coupled Fe-S redox cycles. Deeper water is probably polluted by groundwater flow through interconnected aquifers with reduced sediments. Deep irrigational pumping has potentially attracted shallower, polluted water to greater depths.