The geohydrology and related stability of the dolomite aquifer underlying Ikageng : Potchefstroom

Abstract

Following large scale sinkhole formation on the Far West Rand as a direct result of mining related dolomite dewatering, groundwater is now known as an important factor affecting the stability of cavernous dolomite. Ikageng was developed partly on dolomitic land before the direct relationship between dolomite, dewatering and sinkhole formation was clearly understood. The Tlokwe Local Municipality (TLM) inherited the legal responsibility to ensure the safety of residents in the greater Ikageng who are at risk of subsidence and sinkhole formation. The TLM therefore initiated a dolomite risk assessment with the aim of having a dolomite risk management strategy (DRMS). The wealth of geotechnical and geophysical data in the area were interpreted to compile a sinkhole hazard zone map of dolomitic terrain in Ikageng. This map formed the basis of the risk assessment. Geohydrological factors that might be conducive to sinkhole formation were then identified as flags, and overlain on the hazard zone map. The single biggest threat identified in the area was the Kynoch Gypsum Tailings Dump. The Kynoch Fertilizer Factory in Potch-Industria was commissioned in 1967 and the resultant tailings facility was developed two kilometres to the west on dolomitic land. Gypsum precipitated out of a waste slurry for 35 years, leaving a 25 ha reservoir of highly toxic brine that is remobilised by rainwater. Seepage from the sides was measured to have a pH as low as 1.8, which is expected to dissolve the underlying dolomite. Sinkholes already developed on similar gypsum tailings facilities on carbonate rocks in Florida State in the United States of America