Development Of A Conceptual Understanding Of Hydrogeological Relationship Between The Skurweberg Aquifer And Wetlands In The Steenbras Catchment

The City of Cape Town (CoCT) commenced a study into the feasibility of the Table Mountain Group aquifers (TMGA) for augmenting the water supply to the city in 2002. It comprised drilling of exploration boreholes in several target areas and the establishment of a hydrogeological and ecological monitoring network. Due to the prolonged drought and associated water crisis, the CoCT decided to fast-track the TMGA development in 2017. The first wellfield is currently developed within the catchment area of the Steenbras Dam comprising production boreholes targeting the Skurweberg and the Peninsula aquifers of the TMGA. Since groundwater abstraction from the Peninsula and Skuweberg aquifers might have a short-term or long-term impact on aquatic ecosystems (i.e. streams and wetlands) that are linked to the TMGA. As a result, evaluation of the potential impact of groundwater abstraction from this aquifer system requires an understanding of the nature and extent of groundwater dependency of the ecosystems. A variety of data sets and parameters have been measured over the last decade at ecological monitoring sites across the study area, of which two sites are located within the Steenbras catchment that are probably connected to the Skurweberg Aquifer. Recently further boreholes and monitoring sites have been added. This paper describes the various methods used and results of the analysis towards a conceptual understanding and quantification of the groundwater dependency of the selected ecosystems. While groundwater contribution is only one factor in ecosystem functioning, sustainable and adaptive management of the groundwater use must be based on the conceptual model and ongoing monitoring of the ecosystem responses.