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

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 produc-tivity. The groundwater systems have been put under additional stress given the recent 2015-2017 El Nino similar to 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 delta 18O, delta 2H, delta 13C, 14C, 3H and 3H/3He 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). Ground-water 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 3H/3He 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 3H/3He 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.

Automated summary

The Verlorenvlei catchment in South Africa's west coast is increasingly relying on groundwater due to changing precipitation patterns and increased agriculture. Groundwater chemistry and residence times have been used to differentiate between different aquifers, and the mixing of groundwater plays a role in variation of chemistry and residence time. Young groundwater is more susceptible to reduced recharge, which impacts the sustainability of the Verlorenvlei estuarine lake and wetlands. Future studies should use a lumped parameter model to further understand groundwater system sustainability.