Stable Isotope Techniques for the Evaluation of Water Sources for Domestic Supply in Stellenbosch, South Africa

During 2017-2018, the City of Cape Town, South Africa faced an unprecedented drought crisis with the six main water storages supplying Cape Town falling to a combined capacity of just under 20%. Throughout the crisis, groundwater was considered the most important alternative urban water supply source but also the most vulnerable to contamination through accidental return flows from the municipal network, private residences and agricultural industries. This project aimed to constrain the stable isotope chemistry of the water supply network in the Stellenbosch municipality and monitor the augmentation of groundwater into the network using stable isotopes. Long-term monitoring points have been established at 35 tap water sites, 20 private wells as well as at the 3 supply reservoirs that feed the municipal network. Rainwater (4 locations) and local surface water (6 locations) were also monitored over the one-year sampling period in 2021. Preliminary data show distinct isotopic signals associated with each supply reservoir as well as in the local groundwater. Rainfall is predominantly received in the winter season (May-Aug) and typically has more negative isotope delta values. Typical residence times in storage dams and reservoirs appear to be between 2-3 weeks in the winter and 3-4 weeks in the summer, according to stable isotope hydrograph separations. Domestic water supply is consolidated at 2 water treatment facilities in Stellenbosch, where isotope values of all 3 supply reservoirs mix. The amounts of water received from each reservoir changes throughout the year according to dam levels, this change is evident in the stable isotope values at the tap water sample locations. The data also shows significant return flow into the alluvial aquifer system during warmer months when private stakeholders’ water consumption is at its highest. Groundwater is expected to supplement this urban supply network in Q1-2 of 2022 and will likely disrupt the current distribution of stable isotopes in the network, providing further insight into the potential return flow into the local groundwater system. For longer term monitoring, tap water locations that receive the same supply have been identified and single locations (8 in total) have been selected to monitor through 2022 to optimise the monitoring network. Similarly, only 4 rainfall collection sites will continue monitoring. Interested stakeholders and policy makers include municipal supply managers as well as local farmers and industry that can use this data to develop water management strategies and identify areas where leaks or overuse is likely. Hydrograph separations would be more accurate with longer term monitoring rainfall, reservoir and tap water by identifying trends in storage residence time, mixing and release schedules throughout the supply systems. Although the data indicates that there are return flows into local groundwater, these results could not distinguish the mechanisms by which water is entering the system. Further monitoring in targeted areas is needed to constrain if return flows originate from leaks, irrigation or other urban recharge processes.