Environmental geochemistry and acid mine drainage modelling of the Sabie River water system in the vicinity of abandoned gold mines

Abstract

The purpose of this research was to learn more about the release, transport, dispersal, and fate of metal species emitted by mine tailings storage facilities (MTSFs) on the Sabie River system. Analytical techniques and geochemical modelling were used to evaluate the chemical reactions that occur after metals are released from gold mine tailings in the Sabie Goldfield. MSTFs, a rainwater runoff pond and the Sabie River system were all considered as potential metal contaminants flow paths. Seasonal variation was taken into account to learn more about the effects on metal dispersion during the wet (summer) as well as dry (winter) seasons. Very high concentrations of metal species were found in the tailings from the Nestor MTSF when compared to concentrations found in stream sediments. There was no substantial difference in metal (loid) distribution, specifically, cobalt (Co), nickel (Ni), lead (Pb), chromium (Cr), zinc (Zn), copper (Cu) and arsenic (As) between the summer and winter seasons. In addition, no indication of metal species dispersal was observed from the Nestor MTSF to the nearby water bodies, namely, the Klein Sabie and the Sabie River respectively. This is because the majority of the metal pollution is detached from the system over a short distance. When compared to concentrations found in stream sediments, the tailings from the Nestor MTSF had elevated concentrations of metal species. Acid producing minerals such as pyrite and secondary iron minerals, which are commonly found in mine wastes, were discovered to be possible polluters to the Sabie River system. Furthermore, the mineral capable of neutralizing acid dolomite present in the sediments may function as a critical buffer. Using traditional acid base accounting (ABA) criteria, gold mine tailings from the Nestor MTSF are classified as potential acid forming (PAF), whereas gold tailings from the Glynns Lydenburg MTSF are classified as non-acid forming (NAF) but may still produce alkaline drainage. Furthermore, tailings from these two sites were mixed, and ABA was performed to determine whether the alkaline tailings could neutralize the acid from the Nestor (MIX25 and MIX75). The results indicated that materials from the Glynns Lydenburg are capable of neutralizing the acid from the Nestor mine tailings. Therefore, materials from the Glynns Lydenburg MTSF can be an economically viable option to treat potential acid-generation from the Nestor MTSF by forming a base cover system in which vegetation will grow. To characterize the solid phase speciation of metal species in the vicinity of the Sabie River system, a four-stage sequential extraction procedure was performed on mine waste, soil and

stream sediment samples. Organics and sulphides-bound fractions, as well as Fe and Mn-bound fractions were considered bioavailable, with metals in these phases being possibly remobilised under varying environmental settings. Aluminium and iron were discovered to achieve maximum bioavailability in mine waste, followed in decreasing order by manganese (Mn), As, Zn, Cr, Cu, Co, and Pb. Adsorption into Mn, Al, and Fe-hydroxide precipitates observable in the drainage path flowing towards the Klein Sabie River is the crucial process for this geochemical scavenging. The PHREEQC modelling code was used in conjunction with sequential extraction to characterize the chemical reactions that transform metals. According to the results of PHREEQC modelling, cation exchange played a substantial part in regulating the chemistry of surface water in the Sabie River system. The obtained results can be used to guide environmental management of gold mine tailings.