EVALUATION OF WATER SUPPLY SUSTAINABILITY IN THE BENGAL BASIN THROUGH REGIONAL MODELING OF FLOW PATTERNS AND ARSENIC SORPTION

Millions of people in the Bengal Basin of Bangladesh and India are drinking water containing levels of arsenic above health standards. Currently, high concentrations of arsenic occur primarily in shallow depths; deeper groundwater (>150m) is generally low in As (<50 mg/L) and a potential source of safe water for much of the region. The resource may be threatened, however, by downward migration of arsenic from shallower depths, and so assessment of sustainability is essential for management.

The As-safe water supply may be considered sustainable if the recharge source and subsurface flowpath to a well are low in arsenic, or for practical purposes, if the time to breakthrough of arsenic in the well is very long. Delayed breakthrough may occur if travel times from high-As to low-As aquifer zones are long, or if chemical processes, such as sorption, immobilize arsenic and delay arrival. These factors were investigated with a regional-scale groundwater flow and solute transport model of the Bengal Basin. Current and future pumping schemes were simulated, resulting in different flowpaths and advective travel times. Effects of varying degrees of sediment sorption, consistent with laboratory and in-situ measurements (Radloff et al., Goldschmidt 2010), were evaluated for each pumping scheme. Results indicate that high deep pumping rates consistent with current irrigation use will induce downward As migration over much of the Basin, even when sorption is considered. However, simulations indicate that a management scheme that limits deep pumping to domestic supply (while confining irrigation pumping to shallow zones only, or discontinuing groundwater irrigation) may result in an infinitely sustainable source of low-As water for more than 42% of the As-affected area, and in more than 84% of the area, advective travel times from high-As zones to deep wells would be greater than 1000y. Consideration of sorption increases the time to arsenic breakthrough at pumping wells. This indicates that sediment chemistry can provide important protection of water supply where sorbing sediments exist. However, proper management of the groundwater resource will substantially reduce the area vulnerable to As contamination in deep wells under all sorption conditions.