GIS-based groundwater vulnerability modelling: A case study of the Witbank, Ermelo and Highveld Coalfields in South Africa

Abstract

In the last 20 years, the popular mineral systems approach has been used successfully for the exploration of various mineral commodities at various scales owing to its scientific soundness, cost effectiveness and simplicity in mapping the critical processes required for the formation of deposits. In the present study this approach was modified for the assessment of groundwater vulnerability. In terms of the modified approach, water drives the pollution migration processes, with various analogies having been derived from the mineral systems approach. The modified approach is illustrated here by the discussion of a case study of acid mine drainage (AMD) pollution in the Witbank, Ermelo and Highveld coalfields of the Mpumalanga and KwaZulu-Natal Provinces in South Africa.Many AMD cases have been reported in these provinces in recent years and are a cause of concern for local municipalities, mining and environmental agencies. In the Witbank, Ermelo and Highveld coalfields, several areas have been mined out while mining has not yet started in others, hence the need to identify groundwater regions prone to AMD pollution in order to avoid further impacts on the groundwater resources. A knowledge-based fuzzy expert system was built using vulnerability factors (energy sources, ligands sources, pollutant sources, transportation pathways and traps) to generate a groundwater vulnerability model of the coalfields. Highly vulnerable areas were identified in Witbank coalfield and the eastern part of the Ermelo coalfield which are characterised by the presence of AMD sources, good subsurface transport coupled with poor AMD pollution trapping properties. The results from the analysis indicate significant correlations between model values and both groundwater sulphate concentrations as well as pH. This shows that the proposed approach can indeed be used as an alternative to traditional methods of groundwater vulnerability assessment. The methodology only considers the AMD pollution attenuation and migration at a regional scale and does not account for local-scale sources of pollution and attenuation. Further research to refine the approach may include the incorporation of groundwater flow direction, rock-pollution reaction time, and temporal datasets for the future prediction of groundwater vulnerability. The approach may be applied to other coalfields to assess its robustness to changing hydrogeological conditions.