ARSENIC MOBILIZATION AND GROUNDWATER FLOW AT A FIELD SITE IN BANGLADESH

Groundwater, the primary source of drinking water in Bangladesh, is severely contaminated by arsenic. Although it is generally agreed that most dissolved arsenic was mobilized from metal oxides as subsurface conditions became increasingly reducing, little is known about the timing of mobilization or why arsenic concentrations vary by orders of magnitude over short distances within geologic formations. Analysis of geochemical and hydrologic conditions at our field site in the district of Munshiganj indicate that arsenic was mobilized concomitantly with an inflow of young carbon. Analysis of solid material indicates that arsenic concentrations are sensitive to geochemical perturbations because the sorption capacity of sediments is saturated where arsenic concentrations are high, and field injection experiments show that arsenic concentrations respond very quickly to geochemical perturbations. Irrigation pumping at our site draws young water into the subsurface at a rate sufficient to explain the young radiocarbon ages of both inorganic carbon and methane. I will use the results of transient three-dimensional groundwater flow modeling to discuss the relative importance to groundwater flow of (1) topography, (2) seasonal exchanges with surface water, and (3) pumping. Finally, I will consider the time for recharge to reach various depths, and how irrigated agriculture may have changed patters of flow and chemical transport.