A Case Study Of The Importance Of Hydrogeology To Mitigate Environmental And Technological Challenges Associated With Mine Closure
Muhlbauer, Ritva; Kotze, Johanita; Rutherford, Engel; Botha, Rudolph
Anglo American Coal, South Africa
Kromdraai is a coal operation in South Africa that is approaching closure early 2019. The most significant closure issues relate to the lack of progressive rehabilitation impacted by topsoil shortages, which in turn exacerbate the generation of acidic metal-rich mine water with the potential to discharge into adjacent streams. Significant Acid Mine Drainage (AMD) issues are complicated further by the complex interaction between surface water and groundwater associated with historic underground and adjacent open cut mining. The current strategy involves the operation of a liming plant, dosing with caustic soda, management of water levels in pits, extraction of water from dewatering wells and proposed treatment using reverse osmosis.
This paper focuses on the phased multi-disciplinary development of an integrated hydrogeological conceptual model. The first step included data collation of all historical data using ARCVIEW GIS software. Historical mine plans, geophysical data, remote sensing, water level and quality data were overlaid, to inform a gap analysis, which was used to plan a drilling program. Multi-disciplinary interpretation of geological, geophysical, hydrogeological, mine rehabilitation data, yielded a high confidence hydrogeological conceptual model, which was translated into a Spring finite element numerical flow model and was applied to simulate various post-closure water management scenarios. The numerical model provided more accurate decant volumes and locations, thereby reducing the uncertainty associated with the initial decant prediction and resulted in a reduction from 10 ML AMD/day to less than 5 ML AMD/day thereby mitigating water related closure costing significantly.
The calibrated numerical model output was used to undertake trade-off studies with cost benefit analyses which considered various improved surface rehabilitation and water treatment options to address long-term closure liabilities and to determine the most cost-effective closure option that reduced closure risks to an acceptable level. The case study discusses the project learnings and demonstrates the business case for improved surface rehabilitation, and resultant long-term liability reduction associated with active water treatment (Reverse Osmosis). Critical to closure liability reduction is ingress management in the form of optimised surface rehabilitation and vegetation cover as this results in reduced mine water generation and allows for the implementation of more cost-effective passive treatment systems.
The project demonstrated the importance of high-quality hydrogeological models and surface rehabilitation to optimise closure options, which reduce the long-term liability estimate and provide an executable mine closure plan with an acceptable post closure residual risk profile.