The prospects for stormwater harvesting in Cape Town: Part 2 – catchment-scale managed aquifer recharge with stormwater

Abstract

The City of Cape Town in South Africa faced the possibility of taps running dry in 2018 due to a prolonged drought that commenced in 2015. With such droughts expected to reoccur frequently in future, this study investigated the prospects for managed aquifer recharge (MAR) with stormwater. This would require temporary storage to collect and hold the stormwater during and immediately after rainfall events while it seeps into the aquifer. The 89 km² Zeekoe Catchment located in the southern part of Cape Town was selected as a case study as it had both existing surface storage (61 stormwater ponds) and was lying above a large unconfined aquifer. As the stormwater ponds were largely designed for flood control, they would need to be modified for MAR. In this desktop study, the main objective was to model temporary detention of stormwater in the ponds with the aim of predicting infiltration into, and thus augmentation of, the aquifer. The requirement that the flood control function be maintained, combined with the limited capacity in the ponds, was a key consideration. The study determined that the physical characteristics in the Zeekoe Catchment, i.e., largely flat terrain, pervious sandy soils, and a relatively deep (20–50 m) unconfined aquifer, could support managed aquifer recharge and borehole abstraction rates of 3.5–8.1 L/s per borehole from some 140 boreholes. This could provide a mean annual groundwater yield of 29–33 Mm³ (about 15% of Cape Town water demand in 2018).