Characterization of groundwater types and residence times in the Verlorenvlei catchment, South Africa to constrain recharge dynamics and hydrological resilience

Abstract

• The Verlorenvlei catchment is dominated by four groundwater types that belong to three aquifer systems. • 14 C and 3 H/ 3 He residence time constraints indicate distinctly different age spectrums in the three aquifer systems. • Discrepancies between the 3 H/ 3 He and 14 C results suggests mixing along the flow paths with repeated modern recharge packages entering shallower groundwater systems. • Groundwater resilience to climate change is dependent on understanding these mixing relationships. The Verlorenvlei catchment on the west coast of South Africa is a semi−arid region that is growing progressively more reliant on groundwater due to increased variability in precipitation and increasing agricultural productivity. The groundwater systems have been put under additional stress given the recent 2015–2017 El Ninõ system that led to drought conditions along the west coast of South Africa. This increased reliance puts the natural environment and human dependence on groundwater into direct conflict. Groundwater was sampled for δ 18 O, δ 2 H, δ 13 C, 14 C, 3 H and 3 H/ 3 He ratios in addition to cation and anion concentrations from seventeen production boreholes and two springs in the catchment. Residence times were estimated using tritium and radiocarbon, and in the case of the latter, the Pearson Model has been used to correct for possible carbonate dissolution. Hydrochemistry and residence times of groundwater in the Verlorenvlei catchment have successfully been used to distinguish between the primary−porosity alluvial aquifer, the secondary−porosity Malmesbury shale aquifer (MG) and the fractured rock aquifers associated with the Table Mountain Group (TMG). Groundwater mixing has been identified within the catchment and this plays an important role in the variation in groundwater chemistry and residence time between the aquifer systems. The discrepancy between the calculated radiocarbon and 3 H/ 3 He ages in the TMG and alluvial aquifers has been evaluated using a lumped parameter model that confirms that the TMG is strongly dominated by young groundwaters. Young 3 H/ 3 He residence times calculated for groundwater in the TMG (34–57 years) and alluvial aquifers (34–47 years) implies that these systems are most susceptible to reduced groundwater recharge. Given the interconnected nature of the aquifer systems, reduced recharge rates into the TMG aquifer will decrease groundwater flow to both the alluvial and MG aquifer and this will impact the long−term sustainability of the RAMSAR listed Verlorenvlei estuarine lake and wetlands at the catchment outlet. Future residence time studies should consider using a lumped parameter model (LPM) to calculate the age distribution of groundwater in these aquifers as this would provide additional insight into the sustainability of the groundwater systems.