Recharge of low-arsenic aquifers tapped by community wells in Araihazar, Bangladesh, inferred from environmental isotopes

摘要

More than 100,000 community wells have been installed in the 150-300 m depth range throughout Bangladesh over the past decade to provide low-arsenic drinking water (<10 mu g/L As), but little is known about how aquifers tapped by these wells are recharged. Within a 25 km(2) area of Bangladesh east of Dhaka, groundwater from 65 low-As wells in the 35-240 m depth range was sampled for tritium (H-3), oxygen and hydrogen isotopes of water (O-18/O-16 and H-2/H-1), carbon isotope ratios in dissolved inorganic carbon (DIC, C-14/C-12 and C-13/C-12), noble gases, and a suite of dissolved constituents, including major cations, anions, and trace elements. At shallow depths (<90 m), 24 out of 42 wells contain detectable H-3 of up to 6 TU, indicating the presence of groundwater recharged within 60 years. Radiocarbon (C-14) ages in DIC range from modern to 10 kyr. In the 90-240 m depth range, however, only five wells shallower than 150 m contain detectable H-3 (<0.3 TU) and C-14 ages of DIC cluster around 10 kyr. The radiogenic helium (He-4) content in groundwater increases linearly across the entire range of C-14 ages at a rate of 2.5 x 10(-12) ccSTP He-4 g(-1) yr(-1). Within the samples from depths >90 m, systematic relationships between O-18/O-16, H-2/H-1, C-13/C-12, and C-14/C-12, and variations in noble gas temperatures, suggest that changes in monsoon intensity and vegetation cover occurred at the onset of the Holocene, when the sampled water was recharged. Thus, the deeper low-As aquifers remain relatively isolated from the shallow, high-As aquifer.