Résumé
The Future Water Institute (FWI) has collaborated with the University of Copenhagen on a DANIDA funded project entitled “Pathways to water resilient South African cities” (PaWS), as well as with the French Geological Survey (BRGM) and the City of Cape Town (CoCT) on an AFD funded project, both of which examined aspects of urban water quality in the Cape Flats. These overlapping projects drew together researchers from different disciplines, organisations, and continents to research water quality challenges facing the CoCT in utilising the Cape Flats Aquifer (CFA) for water storage and reuse.The CFA underlies a large portion of the city and is a shallow, sandy, primary aquifer which is vulnerable to contamination from activities taking place on the land surface. The Cape Flats is the region overlying the CFA which is relatively flat and bounded by mountains to the east and west, and the False Bay coastline to the south. A large portion of the land is built up, with a mix of uses including business, industry, agriculture, formal housing, and informal settlement. There are about 200 stormwater detention ponds that overlie the CFA, as well as eight wastewater treatment works (WWTW), sand mines, landfill sites, and the Philippi Horticultural Area (PHA). The CoCT is currently initiating the use of the CFA as part of its potable water supply system, with managed aquifer recharge (MAR) by direct injection of treated wastewater planned for the near future. As part of the PaWS project, researchers investigated the retrofitting of stormwater detention ponds to turn them into multifunctional blue-green infrastructure (BGI) including stormwater harvesting by infiltration into the CFA. A pilot study site in Mitchell’s Plain has been a node for transdisciplinary research since 2020, including stormwater and groundwater quality monitoring. As part of the CoCT-BRGM project the Mitchell’s Plain study site and three other groundwater monitoring locations were selected for non-target screening (NTS) of contaminants of emerging concern (CECs) and subsequent water quality monitoring. The four monitoring locations included wells at the PaWS site and a well at a sports ground, both in (different) formal residential areas, a deep well in the PHA, and a deep well near a WWTW. These monitoring sites showed different geochemical characteristics. The NTS and targeted analyses showed the highest number of CECs and highest total concentrations in groundwater near the WWTW. In addition, NTS identified daughter compounds of both detected and not detected parent compounds. Stormwater had a high number of CECs with lower concentrations of prescription pharmaceuticals and higher concentrations of illicit drugs compared with the groundwater near the WWTW. Results from these studies contribute to the understanding of water quality considerations for the use of the CFA as a water storage and reuse system, whether recharge is achieved through stormwater infiltration or directly with treated wastewater. Further research is required to understand the water treatment capacity of BGI linked to stormwater harvesting as well as the wider significance of this type of multifunctional infrastructure within a water resilient city such as Cape Town.