Water stable isotopes reveal surface water–groundwater relations, risks and opportunities for a small lake: Paardevlei, Cape Town

Wetlands are valuable ecosystems providing multiple benefits to humans, animals and the environment, but are endangered due to destruction or damage caused by human activity. Paardevlei near Somerset West, Cape Town, was investigated by means of water chemistry and stable isotope analysis, including surrounding rain, surface water and groundwater. The dissolved ions of all these waters were within acceptable drinking water limits, except for autumn in Paardevlei, due to evaporative enrichment. The d2H and d18O values of water in the vlei showed large increases over the summer (September 2023 to February 2024: d2H from −4 to +60‰ and d18O from −1.7 to +11.0‰), in contrast to groundwater and the Lourens River, which remained more stable (average: −15 and −4‰), suggesting the Lourens River receives baseflow all year, whilst Paardevlei receives less or no groundwater input. Management of Paardevlei therefore needs to consider that rainfall is the primary replenisher of water and with climatic changes forecast for the future, plans should be made to ensure the long-term viability of this water resource.