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

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 μg/L As), but little is known about how aquifers tapped by these wells are recharged. Within a 25 km2 area of Bangladesh east of Dhaka, groundwater from 65 low-As wells in the 35-240 m depth range was sampled for tritium (3 H), oxygen and hydrogen isotopes of water (18 O/16 O and 2 H/1 H), carbon isotope ratios in dissolved inorganic carbon (DIC, 14 C/12 C and 13 C/12 C), 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 3 H of up to 6 TU, indicating the presence of groundwater recharged within 60 years. Radiocarbon (14 C) ages in DIC range from modern to 10 kyr. In the 90-240 m depth range, however, only 5 wells shallower than 150 m contain detectable 3 H (<0.3 TU) and 14 C ages of DIC cluster around 10 kyr. The radiogenic helium (4 He) content in groundwater increases linearly across the entire range of 14 C ages at a rate of 2.5×10-12 ccSTP 4 He g-1 yr-1 . Within the samples from depths >90 m, systematic relationships between 18 O/16 O, 2 H/1 H, 13 C/12 C and 14 C/12 C, 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.