Hydrogeological modeling of a coastal wetland: The development of a steady-state numerical groundwater flow model for Rietvlei Wetland, Cape Town

The Rietvlei Wetland, located in the Western Cape of South Africa is well recognised for harbouring numerous bird species, and is ranked the 6th most important coastal wetland in the South-western Cape. Researchers perceive that the wetland could be threatened by the growing drought hazards, and increased water demand in Cape Town. The extent of the effects is however unquantified and unknown. This therefore calls for extensive research and novel approaches to understand and quantify wetland hydrodynamics, to shape wetland management frameworks. Conducting thorough field work to understand wetland processes, and the use of numerical models for future prediction of black swan events are well recommended. Thus, the study aims to develop a conceptual hydrogeological model for Rietvlei Wetland, and to develop a numerical model to quantify the wetland’s groundwater budget. To achieve this, historical data was gathered, and field work which included groundwater monitoring, collection of sediment profiles and water quality analysis was undertaken. Preliminary results show that the wetland is underlain by an unconsolidated aquifer, largely overlain by different types of sand, mixed with clay and silt, and precipitation is the main source of groundwater into the wetland. A distinct relationship is seen between elevation, soil type and soil structure, such that during the peak rainy season, groundwater tends to be above the ground surface in the low-lying salt pans, dominated by clayey layers on their surfaces. These surfaces tend to crack during the dry season, facilitating preferential flow pathways at onset of rain. This information, and other historical data will be used to develop a numerical model using MODFLOW-NWT and ModelMuse. The numerical model is perceived to be the basis of groundwater modeling using open-source software for Rietvlei Wetland, and may be used for predicting the impacts of drought and increased groundwater abstraction on the wetland’s hydrodynamics.