Investigation of environmental impacts of disposing coal slurry and discard in mined out pits with the focus on groundwater pollution

Abstract

The coal processing waste is traditionally disposed of as slurry or discard in large engineered stockpiled dumps on surface. The presence of sulphide minerals, especially pyrite, in these wastes can result in potential environmental impacts that include ground water contamination. The study on the potential impacts of backfilling with coal discard and slurry was conducted. In this study, a review of available reports, analysis of sampled monitoring data, mine plans and previous studies was done. A systematic analysis of samples was implemented to evaluate the potential impact on the streams, private boreholes, and farms over time. Impact identification was performed by using an Input-Output model to assess all possible instances of ecological and socio-economic change, pollution and resource consumption that may be associated with the discard and slurry disposal in mining operations. Potential receptors assessed were defined as humans, stream or natural ecosystems that depend on the groundwater. These would be impacted negatively if the groundwater quantity and quality are affected because of mining activities. In terms of material characterisation, the slurry material had a high sulphur content and low neutralising ability, leading to an acid mine drainage potential. Samples showed a potential for acid mine drainage whilst other samples were classified as potentially acid neutralising. The slurry and discard material leaches concentrations of F, Al, Mn and Fe were above the water use licence quality objectives for the study area. These material leaches concentrations of SO4, Cd, B, Co, Fe, Mn, and Ni were above the South African National Standard guideline values and if stored on surface it will require lining. These leachate results correlate with the monitoring results observed on site. Both waste samples of discard and slurry showed a relatively large carbon component in the XRD and not in XRF results because of its high combustibility. The correlation difference between the materials lost on ignition in the XRF analysis and the coal content in the XRD results were less than 15%. The acid base accounting and net acid generating results indicated high calcite and kaolinite contents of the discard and slurry material showed a slight and moderate fizz at the start of the test indicating the higher neutralising potential. The total percentage sulphur content in all samples was well above the 0.25% margin and thus allowing all rocks to be classed as a rock type I. While the net neutralising potential of the Discard and Primary E3 material showed a potential to generate acid, the slurry and fines had higher net potential to neutralise acid. The outcome of the study showed that the NAG results of all the samples excluding the fines generated H2SO4 at test pH levels of both 7 (neutral) and 4.5. Taken all parameters and ratios into account with the neutralising potential of the slurry and fines. The material samples were leached under acidic conditions gave worst case scenario indications. The resultant fluid from the leaching procedure reported on in mg/L, indicating the leachable concentrations (LC) from the material. These concentrations were then classed against the SANS 241 drinking water standards to assess the potential risk of any metal and ion leach from the disturbed material, to potentially contaminate the surface water and groundwater in the receiving environment. The end results showed no real risk for metal leach picked up in leachable concentration results. It is thus recommended that the material be subjected to long term Kinetic tests to determine the potential for metal leach over a prolonged period. These tests should be performed under acidic conditions to simulate the potential acid formed from the oxidation of the ore and waste rock.