Evaluating the vulnerability and contamination of groundwater from mining dump in Welkom, Free State, South Africa

Abstract

The quality of groundwater in and around Welkom, a city in the Free State Province of South Africa, has been steadily declining over the years. This deterioration has led public and private institutions, including schools, churches, hospitals, and universities, to cease using groundwater for drinking due to health concerns. Previous studies have suggested that contaminants from mine dumps in the Welkom area are the primary cause of groundwater pollution. This study aimed to assess the vulnerability of groundwater to contamination from a specific mine dump in Welkom. A key objective of the research was to develop a point source vulnerability assessment method tailored to identify groundwater vulnerability to contamination from mine dumps. The study introduced a new parametric method called DWAPH, which considers five parameters: Depth to groundwater, Water quality, Aquifer type, Precipitation, and Horizontal distance to the contaminant source. The DWAPH method uses an index with four vulnerability scales: low (5-10), moderate (11-15), high (16-20), and very high (20-25). The study data, informed by the parameters was mainly gathered from five boreholes at the Central University of Technology campus in Welkom and a nearby mine dump. The boreholes were used as proxies to investigate the effects a nearby mine dump on groundwater quality and vulnerability to contamination. Each of the five boreholes was characterised through electrical conductivity and dilution tests to detect the fracture points and type of aquifers. The characterisation also involved the assessment of the depth to groundwater, and the depth of each of the five boreholes using a groundwater level meter. Each of the five boreholes, together with a nearby effluent from a mine stream, were seasonally sampled in the summer and autumn seasons between 2021 and 2023 and the water was tested for physical, chemical, and microbiological elements such as Electrical Conductivity (EC), Total Dissolved Solids (TDS), Dissolved Oxygen (DO), ( Potential of Hydrogen) pH, Sodium( Na), Calcium (Ca), Magnesium (Mg) , Chloride (Cl), Fluoride (F), Sulphate (SO4) and Nitrate (NO3). The Weighted Arithmetic Index method was used to investigate the Water Quality Index using the same set of physical and chemical parameters. Further geological and hydrological characterisation of the study site was conducted through a literature review of previous studies. Results from the characterisation process indicated that all five boreholes were part of an unconfined aquifer with fracture points detected at depths of 10 m for Boreholes 1, 2 and 5; 20 m for Borehole 3; and 24 m for Borehole 4. The results indicated that for Borehole 3, four parameters, EC, TDS, Na, and Cl, exceeded the South African National Standards (SANS 241), while all the other boreholes, except Borehole 1, had parameters that were within the recommended SANS 241 standard limits. Samples from the mine effluent stream had abnormally high levels of EC, TDS, DO, Na, Ca, Mg, Cl, F, SO4 across all the months of the summer season. All boreholes, except Borehole 1 and Borehole 4, had total coliforms and faecal coliform counts that exceeded the SANS 241 drinking water guideline limits, while for the autumn season, the total coliform counts and the E. coli counts in all boreholes, except Borehole 3, were higher than the recommended SANS 241 drinking water guidelines. The results from the Water Quality Index (WQI) calculated through the Weighted Arithmetic Index (WAI) method using the SANS 241 water quality guidelines indicated that three of the five boreholes (B1, B3 and B5) had poor water quality, while only two boreholes (B2 and B4) had good water quality. The overall mean WQI of 68.16 that was calculated from the physical and chemical determinants of all five boreholes indicated that the water quality was poor and posed great health risks when used for drinking purposes. The WQI for the mine dump effluent was 544.31, which was five times beyond the minimum guideline requirements, hence supporting the possibility that it was heavily contaminated and can possibly contaminate nearby groundwater sources. The Welkom area was classified under the Witwatersrand supergroup with shale and sandstone. Using the background knowledge and data from the characterisation process, the DWAPH vulnerability assessment method was applied to assess the extent of groundwater vulnerability of the boreholes to contaminants from mine dumps. Results from the DWAPH index indicated an overall vulnerability score of 13 on a scale ranging from 5 to 25, which indicated moderate vulnerability. The outcomes from the DWAPH vulnerability method were compared against outcomes from the AVI, GOD, and RTt methods. Despite all three methods utilising the same datasets, their outcomes were not the same due to a variety of factors, which were mainly based on their design. The GOD and the DWAPH methods identified the extent of vulnerability of the Welkom area as moderate, while the AVI and RTt methods identified the extent of groundwater vulnerability as very high and low, respectively. The DWAPH method was further validated with NO3- and bacteriological counts of total coliforms and E. coli. The nitrate validation indicated a weak correlation between nitrate concentrations and the DWAPH index of R2 = 0.0894. The total coliform counts showed a weak correlation with the DWAPH index while the E. coli counts indicated an almost perfect relationship with the outcomes from the DWAPH index. The preliminary findings of this study indicate that the groundwater in Welkom exhibits a moderate vulnerability to contamination from nearby mine dumps, with water quality tests revealing varying degrees of contamination. Given these results, it is imperative to exercise extreme caution when using groundwater in the area. Implementing restoration and treatment techniques, such as pump and treat, air stripping, filtration with granulated activated carbon, and air sparging, is strongly recommended before any groundwater is utilized. Future research in the Welkom area should prioritize using alternative parameters to further refine, consolidate, or validate the DWAPH method. Expanding the scope of groundwater assessments will provide a more comprehensive understanding of the contamination risks and help improve the reliability of vulnerability assessments. Additionally, it is crucial for local authorities and mining companies to collaborate in developing sustainable solutions to address the ongoing groundwater contamination issues. By working together, they can mitigate the environmental impact of mining activities and protect the health and well-being of the community.