Geochemical Modelling of Water Quality and Solutes Transport from Mining Environments

AMD is a well-documented issue that results from the oxidation of sulphide minerals, leading to the release of acidic and metal-rich leachates into surface and groundwater. In the Witwatersrand Basin, South Africa, the gold-containing ores also contain a variety of silicates, pyrite, and other minerals, which contribute to the leaching of sulphates and elements such as Zn, Ni, As, U, Th, and rare earths. The geochemical processes involved in AMD include the oxidation of sulphide minerals, the release of ferrous iron, and the subsequent oxidation of ferrous iron to ferric iron. This reaction is catalysed by the acidophilic Thiobacillus ferrooxidans bacteria and is dependent on pH and oxygen. The resulting acidity can lead to the precipitation of secondary minerals, such as ferric hydroxide and goethite, which can further contribute to the acid-generating process. Geochemical modelling is a useful tool for comprehensively assessing these processes and predicting the long-term evolution of geochemical systems. Several geochemical modelling codes have been developed, including the Geochemist's Workbench, Minteq, PHREEQC, and others. For this study, the PHREEQC Interactive code was used to model the geochemical processes involved in AMD. A conceptual model of a polluted abandoned mine site was developed to explain the key concepts and assumptions behind the simulations. The model includes a tailings storage facility (TSF) as a source of contaminants, which are released into surface and groundwater. The leachates then flow into a pond, where they undergo changes in elemental speciation, neutralisation, adsorption onto and desorption from hydrous iron oxides (HFOs), and evaporation of leachate solutions. The analytical data used for modelling were obtained from two samples: a leachate from oxidised tailings and a pond water composite. The data included major metals and a selection of minor metals, which were used to construct a geochemical model. The PHREEQC geochemical modelling code was used to model the geochemical processes involved in AMD, and the results were used to predict the long-term evolution of the geochemical system. The geochemical model was based on a comprehensive database that included aqueous, mineral, and gas phase constituents and reactions. The database was used to define the thermodynamic parameters, such as equilibrium constants, entropy, and enthalpy values, which are necessary for the modelling process. The results of the modelling were used to predict the changes in elemental speciation, neutralisation, adsorption onto and desorption from HFOs, and evaporation of leachate solutions. The geochemical modelling code used in this study, PHREEQC, is a widely used and versatile tool for modelling geochemical processes. It has the capability to use a number of databases, or incorporate new information into its existing databases. The code is available for free online and has been widely used in various applications, including the prediction of geochemical processes, design and optimisation of remediation strategies, and management of environmental impact. The results of the geochemical modelling were used to predict the long-term evolution of the geochemical system, including the changes in elemental speciation, neutralisation, adsorption onto and desorption from HFOs, and evaporation of leachate solutions. The modelling results were used to identify the parameters of importance in the geochemical system and to predict the potential environmental impacts of the AMD.