Groundwater recharge deduced from He isotopes in the Botswana Kalahari

The long-term average areal recharge was evaluated for the Mmamabula aquifer in the eastern fringe of the Kalahari Basin using helium isotopes and abundances. Helium concentrations (He-4) and isotopic He-3/He-4 ratios were found to vary considerably both in the Mmamabula aquifer and along the He-4 contents range from 50 to 350 nccSTP/gH(2)O whereas He-3/He-4 ratios lie between 0.2 and 0.8R(A). The highest He-4 concentration (similar to 50 mu ccSTP/gH(2)O) and the lowest He-3/He-4 ratios (similar to 0.014R(A)) are found in the confined parts of the aquifer. The long-term average areal recharge rate is similar to 2.2 to 3.1 mmyr(-1) using the He-4 method. This range of recharge values is within the same order of magnitude as those obtained from the chloride mass balance, groundwater flow and hydrochemical-isotope modeling.