Comparing the Residence Time of Deep vs Shallow Groundwater in the Karoo Basin, South Africa using 3H, 14C, 36Cl and 4He Isotopes

Abstract The Karoo Basin in South Africa is a water-stressed region but little is known about the deep groundwater in the region. Sub-thermal groundwaters, defined for the Karoo as >25 °C, emanate from springs or boreholes but there is no direct access to deep (>1500m) Karoo groundwater. Sub-thermal groundwaters are therefore taken to represent deeper groundwater since there is no heat source in the Karoo other than that generated by burial. Analysis of sub-thermal groundwaters, as a precursor to possible shale-gas development in the Karoo, has produced a complete set of 14 C, 3 H, 36 Cl, 4 He isotope data for a range of different groundwater types and allows comparison of their effectiveness for evaluating the residence time of Karoo groundwater. 14 C activities range from > 94 pmC to as low as 20 pmC, implying a range of apparent ages from modern to >20,000 years. 3 He/ 4 He ratios indicate a similar range of ages and in both cases the shallow groundwater is younger whilst the sub-thermal spring waters are generally older. 36 Cl/Cl ratios showed comparable differences between shallow and deep groundwater. Variations in 3 H activities are less clear but shallow groundwaters generally have higher 3 H activities. Deviations from this pattern suggest a range of mixing processes for the different groundwater types. In general, the sub-thermal groundwater showed much longer residence times than shallow groundwater, implying that sub-thermal groundwaters are a reasonable proxy for deep groundwater in the Karoo.