The effect of stormwater infiltration shallow urban groundwater quality: a case study in the Cape Flats, South Africa

Cape Town, South Africa, is diversifying its water supply in line with its commitment to becoming a water sensitive city (WSC). Blue-green infrastructure (BGI) is key to the implementation of a WSC and can assist in restoring the pre-development hydrological cycle and improving water quality. Approximately 200 stormwater detention ponds overlie the Cape Flats Aquifer and serve as flood control measures, but are very often vacant open spaces, at risk of becoming dumping grounds or informal settlements. One of these detention ponds has been transformed into a multifunctional BGI with stormwater harvesting via infiltration and this case study evaluated the impact of this on groundwater quality over a period of two years. After accounting for dilution effects, the mean contaminant removal during infiltration for ammonium, total organic carbon, total dissolved phosphate, aluminium and zinc were 92%, 84%, 92%, 83%, and 91% respectively. Results also indicated that denitrification of the groundwater is occurring.