The O-, H-, and Sr-isotope composition of Cape Town groundwater; implications for recharge and sustainability

Between 2015 and 2017, the Western Cape region of South Africa experienced its worst drought in over 100 years, culminating in a threat to cut off municipal water supply to over 4 million people in the Cape Town municipality. The oxygen and hydrogen isotope composition in groundwater from boreholes, natural springs, and rivers, with the Sr-isotope composition of selected samples. The mean and median EC of all groundwater samples are 1624 and 757 μS/cm (n = 414), respectively. The δD and δ 18 O values of groundwater cluster around the local meteoric water line, which is consistent with aquifers being recharged by local, present-day rainfall. Low EC, and the lack of correlation between EC and isotope composition, is not consistent with seawater infiltration. Where EC is high, it is attributed to dissolution of salt from the sandy substrate. The range in 87 Sr/ 86 Sr ratios was 0.7094–0.7311; the lowest 87 Sr/ 86 Sr ratios appear dominated by strontium from sea spray or dissolved shells in the substrate, whereas higher 87 Sr/ 86 Sr ratios reflect strontium derived from bedrock. Seawater Sr dominates at low altitude in shallow boreholes whereas the component of bedrock Sr dominates at higher altitude and in deeper boreholes. Combined, these data are consistent with rapid recharge of groundwater by ambient rainfall, and groundwater use being sustainable at the current level of abstraction. • Over 500H- and O-isotope data reported for spring and borehole water in the Cape Town urban area. • The median EC for borehole samples is 720 µS/cm but some have EC > 50000 µS/cm due to dissolution of salt in the substrate. • The groundwater δ 2 H and δ 18 O values are similar to those of ambient rainfall; recharge is local, present-day precipitation. • Lack of correlation between EC and δ 2 H and δ 18 O values of the groundwater samples is not consistent with seawater ingress. • The Sr is sourced from sea spray or from marine shells; limited influence from bedrock Sr except where the boreholes are deep or at higher altitude.