The dynamics of fluvial transportation of uranium in the Rietspruit river system in South Africa

Abstract

Mining of precious metals started many years ago, for instance, gold (Au) and the offshoot uranium (U) mining in South Africa's Witwatersrand Basin. Consequently, mining activities contaminate the environment with potentially toxic elements, which could be useful but dangerous as well. The presence of U in the atmosphere, biosphere, lithosphere, and hydrosphere has a negative impact on the exposed population. The main aim of this research is to investigate the dynamics of fluvial transportation of U in the Rietspruit river system located in the Gauteng Province of South Africa. In this study, the concentrations of U in the Rietspruit fluvial system were quantified using an inductively coupled plasma mass spectrometry (ICP-MS) machine at the Eco-Analytical Laboratory of the North-West University, August 2020. The river is an important stream because it is utilized for various purposes which include domestic, farming, and spiritual purposes. However, the main source of the water stems from the Peter Wright dam which is on the premises of a Au and U mine. Within the mining impacted source area, elevated concentration of U was logged. Uranium concentration of about 781 μg/L was logged in the furrow taking water to the Peter Wright dam. In the slimes dam and sand dump, a concentration of 72.7 mg/kg and 1043 mg/kg were recorded, respectively. The highest concentrations of U were however recorded about 6 km and 24 km downstream of the mining area in a marshland, with concentration above 1200 mg/kg recorded at both sites. This shows that dams and wetland are sinks for U leading to the immobilization of U in the dam’s and wetland’s sediments. Other toxic elements such as arsenic, cadmium, mercury and lead were also recorded in the Rietspruit water and sediment at a concentration exceeding the acceptable limits. In regions where human contact was observed, the amount of U in the collected water sample was over the acceptable limit of U in drinking water 30 μg/L. Due to this, both carcinogenic and non-carcinogenic health risk assessment was done based on the recorded concentration of U in the water. The result showed that the observed human exposure is more at risk of chemical toxicology than radiological toxicology. To study the mobility of U in the water-sediment system, a large amount of sediment was collected from a U polluted dam downstream of the Rietspruit. After drying under normal atmospheric conditions, the sediment was rewetted with rainwater. Uranium concentration of 1211 μg/L was logged in the water sample collected instantly after rewetting. This was mainly due to the dissolution of the salt crust rich in the U. This was the highest concentration of dissolved U recorded after rewetting the sediment. It was observed that the release of U into the water column was not linear due to the different phases that U was adsorbed into, for example, iron oxide, manganese oxide, sulphates and carbonates which all have different dissolving rates in water. In conclusion, the Rietspruit is polluted by the U because of the Au mining operations that took place around the Rietspruit headwater. About 24 km downstream of the Rietspruit, the water is not suitable for domestic, agricultural, and religious activities. Residents of Lawley, farmers, and worshippers, observed using the Rietspruit water, should be sensitized about the dangers involved in the continuous usage of the Rietspruit water. Further study that will quantify the concentrations of U and other toxic alloys in agricultural products – crops and animals within this environment should be done.