Modelling the long-term salinization of the Cape Flats Aquifer in the Philippi Horticultural Area

The Philippi Horticultural Area (PHA) has been farmed for decades on sandy soil with an area of 34.9 km2 , producing vegetables for the semi-urban area of Cape Town, South Africa. Farmers mostly rely on groundwater pumped from the Cape Flats Aquifer, which is stored in farm ponds and used for irrigation. Water evaporates from the ponds and concentrates the remaining water. Moreover, salinization is accelerated by the clay lenses as it creates localized water tables and prevents drainage of irrigation water. Irrigation agriculture in semi-arid areas, where groundwater is being used and recycled through poor irrigation practices, is progressively deteriorating through the positive feedback cycle of salinization, which is detrimental to the potential development of this resource for municipal water supply augmentation. The realities of lower projected rainfall due to climate change may exacerbate aquifer salinization and increase water and food insecurities. This study aims to use a threedimensional steady-state model to estimate how irrigation-induced salinity affected groundwater in the past and present. Moreover, this study assessed how future climate change scenarios and increased abstraction may affect the salinity in the agricultural region of the Cape Flats Aquifer. Groundwater levels and chloride concentrations were used to calibrate the groundwater flow and transport model for the past state and validate the model for the present state using MODFLOW and MT3D codes, respectively. Then, a variable density model (SEAWAT) was used to delineate the freshwater-saltwater interface for past (1983) and present states(2023/24). Thereafter, the validated model has been used to simulate the impacts of three projected Global Climate Models under Socioeconomic Pathways 245 and 585 (i.e. an average of long-term forecasted rainfall for 2050-2060 and 2090-2100) on groundwater recharge and solute concentrations. Out of the Global Climate models (HadGEM3-GC31-LL, AWI-CM-1-1-MR, BBC-CSM2-MR) under Shared Socioeconomic Pathway 245 and 585, one climate model, HadGEM3-GC31-LL, was selected to perform sensitivity analysis of abstraction scenarios.