A methodology to quantify the groundwater impacts of mega-tailings dams for the gold mining industry, South Africa

Abstract

The recovery of base and precious metals (gold, silver, copper, platinum etc.) often entails the removal of a large quantity of rock strata by means of deep shaft and opencast mining practices. The crushing and milling of ore results in a waste product that ranges from sand to clay sized particles, referred to as tailings. Tailings are deposited as slurry and stored in purpose built impoundments known as Tailings Storage Facilities (TSFs), where it will remain for the duration of the mine and well after mining has ceased. One of the most significant impact of gold mine tailings is the seepage of contaminated water from these impoundments into the surrounding groundwater and surface water bodies. As a result of diminishing ore reserves and increasing pressure on mining houses to rehabilitate old mining sites, the focus has shifted to the reclamation of dormant TSFs. This development has given rise to new deposition strategies, with increased stability, resulting in larger TSFs. These “new” TSFs have also given designers the opportunity to correct previous design errors that have resulted in serious environmental impacts in the past. The aims of this dissertation will attempt to combine the findings from previous studies that focused on the impacts of decommissioned TSFs so as to develop a workable methodology for contamination prediction, utilising an effective water balance as a basis and ultimately evaluating the extent of groundwater contamination by means of numerical models. The established methodology is tested in order to evaluate the effectiveness thereof. The case study was also used to determine the extent of groundwater contamination through seepage from the mega tailings facility. Focus was placed on the geotechnical properties of specifically gold tailings material in order to establish the hydrological character of water movement through the TSF. The chemical properties of gold tailings were also investigated by means of a literature study to establish major contaminants associated with the ore. Natural aquifer parameters where obtained during field investigations. The seep/W modelling package was used to determine seepages from the base of the mega tailings using an operational and post operational water balance applied as a flux on top of the TSF. Seepage fluxes from seep/W were applied to a numerical groundwater model PMWIN so as to evaluate the extent of contaminant migration from the base of the TSF through the natural aquifer system. During the operational phase of the TSF, increased seepage fluxes result in an elevated phreatic surface which causes a localised groundwater mound situation. Once deposition ceases, there is a rapid decrease in the phreatic surface and subsequent decrease in seepage fluxes as natural rainfall becomes the only source of water to the TSF. The increased contaminant loads from the gold TSFs raise the overall contaminant loads found in the natural groundwater system surrounding these facilities. Predictive modelling revealed that there is a large contaminant plume moving in a downstream direction eventually contributing to a major South African river