Hydrogeochemical processes and evolution of groundwater chemistry in a karst environment in Gauteng Province, South Africa

Groundwater chemical composition is dependent on various natural and anthropogenic factors. This study assessed factors influencing the groundwater chemical composition in an environment controlled by geological structures. The study further examined the suitability of groundwater quality for domestic and irrigation use. A hydrochemical approach was used to evaluate the main processes and activities influencing the groundwater chemistry. The main hydrochemical facies identified were mixed facies dominated by Na+ and HCO3− in the southwestern area, and MgCl / MgClSO4 / MgClHCO3, which dominated the northern area. Hydrogeochemical processes that were associated with the southwestern area groundwater were silicate weathering, ion exchange, carbonate and gypsum dissolution. The groundwater composition in the northern area, although controlled by carbonate dissolution, was influenced more by anthropogenic activities as indicated by the Cl/Br mass ratios of >88 and high Cl− concentrations. The high Cl− could not be attributed to halite dissolution due to its undersaturation in solution. Additionally, the northern area groundwater showed excess SO42−, which was linked to anthropogenic contamination. Groundwater in the southwestern area was suitable for domestic use, while that in the northern area was classified as unacceptable. The multi-tool approach used in this study provided a clear contrast between the northern and southwestern areas' groundwater, showing that these were influenced by distinct factors. This underscores the importance of developing area-based protection and management strategies that consider both natural processes and anthropogenic impacts.