Recharge of low-arsenic aquifers tapped by community wells in AraihazarBangladesh 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 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 (³H), oxygen and hydrogen isotopes of water (¹⁸O/¹⁶O and ²H/¹H), carbon isotope ratios in dissolved inorganic carbon (DIC, ¹⁴C/¹²C and ¹³C/¹²C), 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 ³H of up to 6 TU, indicating the presence of groundwater recharged within 60 years. Radiocarbon (¹⁴C) 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 ³H (<0.3 TU) and ¹⁴C ages of DIC cluster around 10 kyr. The radiogenic helium (⁴He) content in groundwater increases linearly across the entire range of ¹⁴C ages at a rate of 2.5×10⁻¹² ccSTP ⁴He g⁻¹yr⁻¹. Within the samples from depths >90 m, systematicrelationships between ¹⁸O/¹⁶O, ²H/¹H,¹³C/¹²C and ¹⁴C/¹²C, and variations in noblegas temperatures, suggest that changes in monsoon intensity and vegetation cover occurredat the onset of the Holocene, when the sampled water was recharged. Thus, the deeperlow-As aquifers remain relatively isolated from the shallow, high-As aquifer.