Allwright, AmyTheko, Thato2025-01-032025-01-032023http://hdl.handle.net/11660/12908Dissertation (M.Sc.(Geohydrology))--University of the Free State, 2023Letšeng diamond mine (LDM) uses the split-shell open-pit mining method. The method includes excavation from the surface to access the ore at depth. Material from excavation and that separated during diamond extraction form primary mine waste. Approximately 20 million tonnes of waste is produced as fine-grained waste deposited into Tailings Storage Facilities (TSF) and coarse-grained waste deposited onto Waste Rock Dumps (WRDs). The waste can contain toxic chemical substances that can seep into groundwater or flow into surface water, contaminating it. For this reason, the study aims to determine the cumulative impacts of mining on groundwater quality. To achieve this, groundwater modelling techniques were employed to simulate groundwater flow and groundwater travel times and travel paths for contaminant transport. Geological, hydrological, hydrogeological and hydrochemical data were collected, analysed, and integrated to conceptualise the hydrogeology of LDM to provide a better understanding of the hydrogeological system. The conceptual model also indicated possible sources of contamination and their possible receptors incorporating the Source Pathway Receptor concept. This was achieved by collecting and analysing the water quality data. Mine water sampling results show that most constituents are within the acceptable limits, while nitrates and sulphates were elevated above limits at locations down gradient from the Old Slimes Dam, WRDs and Patiseng TSF suggesting these areas are potential on-site sources of contamination. The conceptual model formed the basis of the groundwater model. Based on the prevailing hydrogeological conditions in the mining area, a groundwater flow model was developed using MODFLOW 6. Due to data limitations, instead of simulating the contaminant from the source to the receptor using transport modelling, particle tracking with MODPATH was used to simulate the pathway and travel time of contaminants in groundwater. Particle tracking results show that the simulated extent of contaminant movement is generally in line with the water quality sampling results, where sampling points beyond the mine lease areas show acceptable water quality as opposed to samples closer to potential sources which show elevated levels of contamination, suggesting contamination is mostly contained within the mine lease area. Similarly, MODPATH results indicate that most particles do not exceed the lease area, also indicating that contamination from potential sources is mostly limited within the mine lease area. Over a five-year simulation period, there is very limited movement of contaminants, although some particles from the Old Slimes Dam are released into the Mothusi Dam, thus posing a risk on its quality. Based on these findings, the Dam was identified as the most sensitive receptor. Over a 10-year period, there is further movement of particles, and some start to move beyond the lease area as some boreholes start showing increased nitrate concentrations such as borehole L_WE_010 down-gradient from the WRD east, suggesting groundwater contamination from the WRD. Over 20 years, a maximum travel distance of 5 km from the eastern WRD is observed, however no sensitive receptor was reached. While the calculated extent of the cumulative migration from the Old Slimes Dam is contained on- site over the 20 years, there is potential impact on the quality of groundwater as some particles intercept the boreholes downstream of the Mothusi Dam, borehole L_WE_015 and borehole L_WE_016. More boreholes are also intercepted over the 20 years. Over time, contaminants will migrate off-site into the weathered shallow layer through the streams therefore reaching some of the receptors in the surrounding environment, either by percolation or conveyed as surface contaminants into the weathered layer. Due to low hydraulic conductivity in the types of rocks in the area, the migration of contaminates is very slow and is mostly limited within the first layer. The results show that there is some seepage of contaminants from potential sources, mostly affecting surface water compared to groundwater. However, the groundwater modelling results also indicate that over long-term evaluations, mining activities at LDM have significant impacts on the groundwater quality, where contaminant particles from different sources are observed at monitoring boreholes suggesting groundwater contamination. This study clarifies the slow but significant effects of mining operations on groundwater and surface water resources over time at Letšeng diamond mine. The results highlight the need for ongoing monitoring and mitigation measures to reduce cumulative effects on vulnerable receptors including the Khubelu, Qaqa Rivers and Mothusi Dam. The conclusion of this assessment emphasizes the critical need for ongoing environmental management within mining practices to protect the quality of nearby water resources. This is achieved by addressing the sources and pathways of contamination that have been identified, putting in place rigorous monitoring, and strengthening mitigation measures.enDissertationUniversity of the Free State