A mine water management strategy for the extension of an Opencast Colliery in Mpumalanga
Water is a critical resource and can pose a huge risk to mining operations. Therefore, understanding the hydrogeological conditions at mining sites is essential in minimizing the impact on groundwater, and to develop practical and cost-effective management and mitigative solutions. Mining and mining processes are often associated with Acid Mine Drainage (AMD). These impacts are generally only identified and addressed in the post-mining operational phase. The costing associated with post-mining rehabilitation is often not adequate to address the impact of acid-mine drainage. This study focused on how an opencast colliery called Mine X in Mpumalanga will behave hydraulically and geochemically during mining, and hydrochemically post-mining if potential decant will occur. Additionally, the study presents a methodology that may be used to predict future mine water decant chemistry and the applicable cost of pH pre-treatment as a condition set by the current RO (Reverse Osmosis) plant. To address the focus of the study, numerical flow modelling, numerical transport modelling, geochemical modelling, statistical analysis and analytical modelling was performed. The results of the above showed that calculated inflows expected during mining will be 653 m3/day after which, rebound of groundwater levels upon cessation of mining will be approximately 11 years. The post-mining decant volume was calculated at 6 l/s with a calculated starting concentration of 1900 mg/l of SO4. This was determined using non-parametric multivariate statistical analysis of 48 samples between three similar mining sites which are currently decanting. Using principal component analysis as well as clustered analysis an estimated concentration was assigned to the source term in a transport model with an annual decay rate of 5% p/a based on the work of (Mack & Skousen, 2008). Based on the result of the transport model, a relationship between SO4 and pH was calculated using 1790 samples from the same sites. Geochemical modelling was subsequently performed to determine pre-treatment product volume requirements for the dynamic pH values associated with the dynamic SO4 concentrations. pH and SO4 are dependent variables in the opencast pit but are both influenced by the amount of sulphide materials present in the backfilled opencast mine. Other influencing parameters could include carbonate mineral phases. However, due to the absence of alkalinity in the mine water samples, it was assumed that carbonates are not present or depleted. Therefore, pH and SO4 in decant water remain independent due to the absence of sulphides and the fact that sulphate is not a pH dependant species. The results of the statistical model indicated a positive relationship between pH and SO4 for the study area, and identified that possible statistical relationships between other constituents, are also likely. Therefore metals concentrations can also be calculated using these relationships, and effectively targeted and treated by passive and/or active methods. From this a mathematical expression was developed to determine a relationship between the required product volume and the required pH change which is flexible enough to accommodate an evolving source term. This approach can be implemented in various post mining environments taking the listed parameters into account. This is likely to improve the dynamic apportionment of capital and operational expenditure in the management of post mining hydrochemistry.