Understanding the speciation and migration of metal pollutants in the mining-impacted Klip River wetland, Johannesburg

Abstract

The Witwatersrand Basin in Gauteng, South Africa, is the world’s largest gold resource, having yielded more than one third of all the gold ever produced. The majority of these mines are located in Johannesburg and the surrounding areas. Waste from gold mines is considered to constitute the largest single source of waste pollution in South Africa and contributes significantly to acid mine drainage (AMD), which critically affects water resources. One of these major resources, severely affected by these activities is the Klip River wetland, which is the major catchment in the region. The sequestration of metals within the wetland is considered vitally important for water purification as water flowing through the wetland discharges into the Vaal Dam, which supplies potable water to major municipalities. To establish the sources, transport and sequestration processes of pollutant metals and how wetland attenuation is influenced by AMD contaminants, we identified specific toxic pollutants present in the Klip River wetland and mapped their movement within the wetland. The main sources of contaminants to the wetland were identified using Pb isotope ratios and metal enrichment. Finally, the speciation of As, a major by-product of gold mining, was investigated to assess the attenuation processes. Analyses reveal that the majority of pollutants are associated with contaminant plumes that emanate from mine dumps and enter the wetland via groundwater recharge. This water carries highly enriched concentrations of Co, Ni, Zn, U and rare earth elements, naturally sequestered within the wetland through precipitation and adsorption. While surface runoff from mine dumps severely contaminate watercourses within the upper catchment, surface inputs are considered relatively minor contributors to the overall pollutant load entering the Klip River wetland, although aerosol fallout is the primary source of Pb. Lead concentrations and isotope compositions were measured (208Pb/207Pb, 206Pb/207Pb and 208Pb/206Pb) to identify major sources contributing to Pb contamination in the wetland. Isotopic ratios of 206Pb/207Pb and 208Pb/207Pb in the wetland ranged between 1.13 –1.32 and2.34 –2.43, respectively.Source identification using Pb isotopes indicated mixing within the system of alkyl lead, coal and Witwatersrand pyrite. Isolating the wetland samples from the rest of the system, reveals predominant coal and alkyl lead (petroleum) signatures, with minor contributions of the Witwatersrand geology. To understand the attenuation processes of As in the wetland, its concentration, bioavailability and speciation in samples from several tailing storage facilities (TSFs), sediments from two river tributaries (the Klip River and Klipspruit) feeding into the wetland ,and surface and core samples in the wetland were measured. Total As concentrations in TSFs ranged between 77 -106 mg kg-1, 10 –90 mg kg-1in the tributaries and an average of 34 ± 25 mg kg-1 in the wetland. Arsenic enters the wetland through groundwater inflow. Speciation analysis identified As(III), As(v) and monomethylarsonic acid (MMA) present with concentrations ranging between 0.028 –1.18 mg kg-1, 0.076 –7.36 mg kg-1, and 0.012 –0.22 mg kg-1, respectively. There also appears to be an As-sulfur species present closely linked to the Stotal concentrations. The predominant As species in the wetland is As(V), and speciation appears dependant on both redox potential and microbial activity (inferred by S concentrations). These processes limit the availability of bioavailable As in the wetland and play an important role in the sequestration process. The extensive accumulation of metals and specific sources within the Klip River wetland reflects the contaminant legacy primarily associated with gold mining on the Witwatersrand, less to alkyl lead artefacts and highlights the vital role this natural system has played in trapping vast quantities of toxic pollutants and remediating downstream waters. Contaminant plumes associated with mine dumps will likely persist for decades, and preventing further deterioration of the Klip River wetlands is, thus critical for safeguarding water sources in the region