Engineering, hydrogeological and vadose zone hydrological aspects of Proterozoic dolomites (South Africa)

Abstract

Large parts of the urban land between Pretoria (City of Tshwane Municipality, South Africa's capital city) and Johannesburg (South Africa's largest city) are situated on Proterozoic dolomites of the Malmani Subgroup (Chuniespoort Group, Transvaal Supergroup) formed in the Transvaal Basin. As South Africa's economic hub, development is progressively becoming denser, higher, and deeper underground, increasing the risks posed by surface subsidence and sinkholes. Tshwane also sources more than 40 million liters of drinking water per day (5–8% of requirements) from the dolomite aquifers. A second basin comprising the Ghaap Group is found to the more arid western portions of the country. Even though a fair understanding exists with respect to the karst aquifer hydraulics and the surface stability issues, a distinct knowledge gap exists in the karst vadose zone. Recent research efforts focused on vadose zone hydrology has resulted in a significantly improved understanding of the behaviour of variable saturated soil and fractured rock systems, both at discreet localities and on a regional scale. However, with more than 90% of the sinkholes in this region forming due to water ingress from leaking pipelines (especially given that the latter non-revenue water is estimated in the order of 30% of the reticulated supply), the flow mechanisms, and unsaturated or vadose zone behaviour of the karst system becomes increasingly important. The ingress scenario results in downward erosion of soil (chert rubble, residual dolomite, transported soil or other residual products of younger caprock) with percolating water. The high lithological variability in South African karst adds to the uncertainty, and is related to chert abundance, the presence of residual dolomite (wad), subsurface cavities (receptacles) and so forth contributing to the heterogeneity and anisotropy. The review paper presents state-of-the-science on the karst vadose zone, including recent advances, appropriate geological models, and knowledge gaps.