CHARACTERIZATION OF SHALLOW AND DEEP AQUIFERS IN BANGLADESH AND IMPLICATIONS FOR GROUNDWATER MANAGEMENT

In Bangladesh alone, an estimated 50 million people are being exposed to Arsenic levels that exceed the WHO guideline of 10 μgL^-1 for drinking water by up to two orders of magnitude. In our study area (Araihazar upazila, 30 km east of Dhaka) the As concentrations in existing wells show a large spatial variability. As concentrations increase as a function of depth, peak at 20-30m depth and then decline again to below the drinking water standards. Hydraulic and environmental tracer data show that the shallower and deeper aquifers (less or more than 20-30 m depth) are characterized by residence times of the order of 10's of years and 100's to 1000's of years, respectively. Hydraulic head time series and conductivities show that the aquifers are more or less separated, and that at 6 monitoring sites in slightly elevated villages, flow is downward during most of the year, except for the beginning of the rainy season. The deeper, low As aquifers potentially provide an alternate supply of drinking water. Personal use of deep groundwater would correspond to an abstraction rate of only about 2 cmyr^-1. However, if the deeper aquifers would be used for irrigation as well, this rate would increase by at least an order of magnitude potentially causing pumping-induced leakage between aquifers as well as subsidence of the land surface.