Provenance of groundwater in the crystalline aquifer of Johannesburg area, South Africa

Abstract

Recharge dynamics, possible flow path and human impact within the crystalline aquifer of the Johannesburg area have been described based on the hydrogeochemical characteristics and environmental isotopes. The dominant circulation zone has also been identified in the study. Understanding of the groundwater dynamics is very important for strategic management of the resource as the country is dependent on groundwater for its different activities. It has also been observed that mixing of fresh and polluted groundwater takes place at shallow zones within the weathered crystalline rocks and dissolution cavities in dolomites where the rocks are characterized by the lithological and structural complexities. From the compositional features of the groundwater in the Johannesburg area, it follows that the solute composition of the water basically results from two basic contributions: The reaction of meteoric water with weathered zone in the near surface area and direct infiltration through dolomite dissolution cavities. The results further indicate that the main constituent movement mechanism within the crystalline aquifer is through a diffusive process, which could be due to few fractures with depth and possible sealing by calcite precipitation. Therefore, the provenance of groundwater circulation is limited to the depth of 40 m. The natural water quality is found to be highly impacted by acid mine drainage that is derived from gold mining centers distributed in the upper Witwatersrand basin. Key words: Crystalline aquifer, diffusive circulation, dissolution cavities, Johannesburg, water supply.