Masters Degrees (Institute for Groundwater Studies (IGS))

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Browsing Masters Degrees (Institute for Groundwater Studies (IGS)) by Author "Allwright, Amy"

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    Geohydrological modelling to determine cumulative impact on groundwater at the Letšeng Diamond Mine, Lesotho
    (University of the Free State, 2023) Theko, Thato; Allwright, Amy
    Letšeng 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.
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    Investigating groundwater abstraction impacts on surface water levels and surface water – groundwater interaction with analytical and numerical models
    (University of the Free State, 2023) Maluleke, Ntsako Victor; Allwright, Amy
    Groundwater is an important source of water for many human needs, including agriculture, water supply and industry. However, the use of groundwater may have adverse consequences. Various studies have been conducted to analyse the effect of groundwater pumping on nearby streams, and many analytical models and numerical models have been developed for various aquifer and stream conditions. Numerical models are time-consuming and require a significant amount of data input but can simulate complex systems. On the other hand, analytical models are suitable to use since they are ease to use, and have minimum data requirements and exact solutions, but they are limited to less complex problems. Streamflow impacts from groundwater pumping continue to be one of the main issues with groundwater resource development. The primary objective of this dissertation is to investigate GW-SW interaction and the impact of groundwater abstraction on surface water bodies. A secondary objective is to then develop a conceptual model that highlights the natural system behaviour of groundwater abstraction in nearby streams to evaluate alternative approaches for managing and understanding streamflow depletion. The developed hypothetical model of an abstraction borehole near the stream is incorporated by realistic available data from the previous UFS/WRC test site along the Modder River, Free State, South Africa. Four analytical solutions, namely Jenkins (1968), Hantush (1965), Hunt (1999) and Hunt (2003) have been simulated using the developed hypothetical models (simple stream-aquifer conceptualisation to improved stream-aquifer conceptualisation) which aid in the quantification of streamflow depletion rates. To identify parameters that greatly influence streamflow depletion, parameters are varied to understand their sensitivity and effect on the rate of streamflow depletion. Furthermore, statistical analysis is used to visualise parameters that greatly influence streamflow depletion using ANOVA (Analysis of Variance). Groundwater abstraction is found to be the most sensitive parameter that greatly affects the streamflow depletion; therefore, groundwater abstraction rates should be monitored, and data collection should be accurately acquired to give out the best recommendation as this can greatly impact the surface water. Distance separation is also considered sensitive and greatly influences streamflow depletion. The period of pumping is less sensitive due to the fact that only time simulation is being extended with no variation of any parameter. The sensitivity analyses improve the understanding of how the analytical models simulate streamflow depletion rates and provide information to water managers on which parameters are most sensitive and require focused data collection. Numerical models are applied and designed to represent the analytical solutions to assess how the solution would be implemented in a numerical model. Two approaches have been used within this dissertation namely, 1) analytical model condition which aims at creating a numerical model to simulate analytical condition as closely as possible to understand the implication of the analytical model assumptions and 2) complex numerical model conditions considering a real-world complex numerical model and assess how analytical solutions are able to represent streamflow depletion. The approximation in the numerical model, limited aquifer extent of the numerical model domain, and differences in parameter and boundary condition applied are the most significant factors that cause differences between the analytical and numerical model results. Analytical solutions overestimate and underestimate streamflow depletion when addressing a complex setting, although they fail to address complex hydrogeological settings, these analytical solutions can act as conservative tools which aid water managers in decision-making on the quantification of streamflow depletion and its mitigation. This study's thorough investigation using different models highlights the complexities of groundwater pumping's impact on streamflow. By examining factors like borehole distance, aquifer hydraulic parameters, and model conceptualisations, it exposes limitations in simplistic analytical models while emphasising the complex nature of aquifer-stream dynamics. Understanding time delays, geological variations, and model assumptions provides crucial insights for water resource management. These findings offer valuable guidance for balancing groundwater use and preserving streamflow, urging a holistic approach integrating the strengths of numerical simulations. This research advocates sustainable strategies to address streamflow depletion, promoting responsible groundwater utilisation for the future.
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    Numerical groundwater modelling as a tool to quantify shallow aquifer water ingress through mining-related open voids
    (University of the Free State, 2023) van Dyk, George Pieter; Allwright, Amy
    The quantification of water ingress volumes and rates from different aquifer systems through open voids can aid in related studies concerning water quality, decanting potential, and stability evaluations especially in areas where historic mining has played a significant role in altering the groundwater environment. The objective of the research and approach is to evaluate if numerical groundwater modelling can be used as a tool to quantify ingress rates from shallow aquifer systems during rapid recharge events when these aquifer systems become saturated and water seeps through individual mine related open voids on a regional scale, eventually reaching historic mining infrastructure and deeper hard rock aquifers. A case study that focused on the East Rand Basin was used to evaluate if numerical modelling can be used to produce quantified ingress rates from shallow aquifer systems through mine-related open voids. The East Rand Basin has a rich history of mine related activities which still has a large influence on the regional aquifer systems. The research included gathering spatial and site-specific data required to construct and represent a numerical groundwater model in FEFLOW groundwater modelling software. The representative model was used to simulate a scenario that included monthly rainfall records that indicated elevated rainfall events and applied as time dependant recharge to the shallow aquifer systems identified along the Blesbokspruit and associated tributaries. The mapped mine-related open voids that fell within the shallow aquifers systems were assigned as discrete features to represent open voids leading to the mapped historic underground mine workings. The simulation included hydraulic head raises with each consecutive rainfall event to saturate the shallow aquifer systems and the ingress rates were recorded at each individual open void. The outcome of the numerical modelling assessment proved that ingress rates can be quantified with results indicating that a maximum of 12 000 m³/d can flow from the shallow aquifer systems during heavy rainfall events through 33 (out of 69) mapped open voids. The results were compared to a similar study (Waal, 2013), that indicated a good comparison in inflow rates. The model illustrated that roughly 16% of surface-related water (including ingress through shallow aquifer systems) could come from 46% of mine related open voids. Numerical groundwater modelling proved to be a valuable tool to quantify ingress rates from aquifer systems through open voids, however data availability and data quality add major limitations to the result accuracy and model confidence.