³H/³HE DATING IN LOW HYDRAULIC GRADIENT ENVIRONMENTS IN THE BENGAL BASIN

In the context of our studies of factors and processes controlling arsenic concentrations in groundwater of the Bengal Basin, we measured ³H, ¹³C, ¹⁴C, noble gas, CFC, and SF₆ (see abstracts by Horneman et al.) concentrations as well as hydraulic parameters of 6 multilevel well nests and private tube wells in a 25 km2 area in Araihazar Upazilla (23.7^oE, 90.6^oN), located about 20 km east of Dhaka, Bangladesh.

Hydraulic heads in the surveyed wells show generally small horizontal hydraulic gradients (~10^-4) across the entire area and seasonal fluctuations of about 4 m, closely tracking the local rivers. Vertical hydraulic head measurements in the shallow aquifer show significant downward groundwater movement, even in areas not affected by irrigation.

Noble gas measurements indicate that some samples have been affected by degassing during sampling or during the groundwater recharge process. ³H (tritium), primarily produced during the atmospheric nuclear weapons tests in the early 1960s, was found at depths of up to 40m indicating that the age of the deeper groundwater is greater than 40 years. DIC ¹⁴C ages of groundwater in the deeper aquifer reach several thousand years and are consistent with measured radiogenic ⁴He concentrations.

Recharge rates of the shallow aquifer range from 6 to 63 cm year^-1. A comparison of the groundwater [³H]+[³He] data with the reconstructed 3H concentrations in precipitation indicate significant mixing between post 1960s and older groundwater near the bottom of the shallow aquifer, potentially caused by the large seasonal fluctuations of the water table, or in some areas by irrigation. Recharge rates are consistent with vertical hydraulic head and conductivity data.