ABSTRACT
Hydrochemical and geochemical processes can play a critical role in deciding whether an artificial recharge scheme will meet its objectives. Case studies from Atlantis and Calvinia, two recharge sites in South Africa, are used to illustrate this principle. At Atlantis, artificial recharge provides an economical method of increasing sustainable groundwater yields. The scheme has been in operation for two decades and owes much of its success to the careful management of recharge water chemistry. Quartzitic aquifer sands add little by way of solutes to the groundwater chemistry after infiltration and act as a natural sand filter to improve the quality of treated wastewater and stormwater used for recharge. The smaller-scale recharge trials at Calvinia involve injection and storage of high quality drinking water in a complex mineralogical environment in a breccia pipe. Although hydrologically suitable, the fractured rocks contain metal sulphides and fluoride-bearing minerals which present a threat to water quality. The mineral sources of fluoride are slow to react, but dewatering of the breccia has disturbed subsurface redox equilibria causing increased salinity after injection and the release of dissolved species associated with sulphide oxidation. Treatment of the recovered water, and the associated increase in cost, will be unavoidable if this scheme is to proceed as an emergency backup for drinking water supply.
