Modelling the Long-term Salinization of the Cape Flats Aquifer in the Philippi Horticultural Area

Abstract

For decades, the City of Cape Town relied heavily on groundwater as its primary source of supply for agricultural activity in the Phillipi Horticultural Area (PHA). Groundwater is extracted from boreholes, held in holding ponds, and then used for crop irrigation. The IPCC expects that semi-arid regions in the Southern Hemisphere will be the most affected by rising surface temperatures, decreased rainfall, and increased evaporation. This increase will put tremendous pressure on the quality and quantity of groundwater resources. This study hypothesizes that if groundwater and irrigation holding ponds are subjected to climate variability forces, groundwater and irrigation water salinity will increase. This study aims to develop a three-dimensional steady state model to estimate the extent to which irrigation-induced salinity affected groundwater in the past and present. Moreover, another aim is to assess how future climate change scenarios and increased groundwater abstraction may affect the salinity of Cape Flats Aquifer (CFA). 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 and present states. 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 rainfall, predicted rainfall 2050-2060 and 2090 -2100) on groundwater recharge and groundwater solute concentrations. From the existing 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. The results of this study found that depth to water decreased from 18 to 16m from winter to summer in regions of the aquifer with clay lenses. Moreover, the chloride concentration is greater in the summer season compared to the winter season (e.g. Borehole EM-08) for most boreholes for both past and present state. As a result of summer irrigation more saline water has a higher chance of being recharged compared to winter where rainfall may act to dilute the aquifer. The HadGEM3-GC31-LL was selected as it is widely used in climate studies for weather generation for the future state. The results showed that the SSP245 (2050-2060) scenario had a slightly lower salinity (less than 10%) when compared with the SSP585 (2050-2060). Lower rainfall during SSP585 has led to a slight increase in salinity when compared to SSP245, probing the need to abstract more water and overall increasing irrigation return flow. Moreover, the seawater freshwater interface has encroached much further inland in the SSP585 scenario in comparison with the SSP245 scenario. This study is important to inform farmers to improve water management, especially if abstraction rates should be significantly higher in the future, on which crops to plant currently as most of the plants are sensitive to moderately sensitive to salt. The farmers are advised to improve irrigation techniques and plant salt-resistant crops in future, especially under increased abstraction scenarios. The municipality is advised to monitor the water quality of the Cape Flats Aquifer as the municipality plans to augment the domestic water supply during time of low rainfall.