Geochemical and hydrogeological contrasts between shallow and deeper aquifers in two villages of Araihazar, Bangladesh: Implications for deeper aquifers as drinking water sources

Sediment and groundwater profiles were compared in two villages of Bangladesh to understand the geochemical and hydrogeological factors that regulate dissolved As concentrations in groundwater. In both villages, fine-grained sediment layers separate shallow aquifers (< 28 m) high in As from deeper aquifers (40–90 m) containing < 10 μg/L As. In one village (Dari), radiocarbon dating indicates deposition of the deeper aquifer sediments > 50 ka ago and a groundwater age of thousands of years. In the other village (Bay), the sediment is < 20 ka old down to 90 m and the deeper aquifer groundwater is younger, on the order of hundreds of years. The shallow aquifers in both villages that are high in As contain bomb-3H and bomb-14C, indicating recent recharge. The major and minor ion compositions of the shallow and deeper aquifers also differ significantly. Deeper aquifer water is of the Na+-HCO3– type, with relatively little dissolved NH4+ (76 ± 192 μmol/L), Fe (27 ± 43 μmol/L) and Mn (3 ± 2 μmol/L). In contrast, shallow aquifer water is of the Ca2+-Mg2+-HCO3– type, with elevated concentrations of dissolved NH4+ (306 ± 355 μmol/L), Fe (191 ± 73 μmol/L), and Mn (27 ± 43 μmol/L). In both villages, the quantity of As extractable from deeper aquifer sands with a 1 mol/L phosphate solution (0.2 ± 0.3 mg/kg, n = 12; 0.1 ± 0.1 mg/kg, n = 5) is 1 order of magnitude lower than P-extractable As from shallow deposits (1.7 ± 1.2 mg/kg, n = 9; 1.4 ± 2.0 mg/kg, n = 11). The differences suggest that the concentration of P-extractable As in the sediment is a factor controlling the concentration of As in groundwater. Low P-extractable As levels are observed in both deeper aquifers that are low in As, even though there is a large difference in the time of deposition of these aquifers in the two villages. The geochemical data and hydrographs presented in this study suggest that both Holocene and Pleistocene deeper aquifers that are low in As should be a viable source of drinking water as long as withdrawals do not exceed recharge rates of ∼1 cm/yr.