Aquifer Rebound Investigation at a Defunct Coal Colliery in South Africa
Lourens, Paul Joël Havemann; Vermeulen, Petrus Daniël; Allwright, Amy
Institute for Groundwater Studies, University of the Free State, South Africa
The Institute for Groundwater Studies at the University of the Free State has been appointed to investigate the aquifer/aquifers response at a defunct coal colliery in South Africa. The colliery includes three subsections, the Main Underground Operation, the Strip Mine Operation and the Secondary Underground Operation. All mining operations ceased 2005. The investigation was required to determine possible risks to the natural aquifer systems to enable to the mining company to obtain a mine-closing certificate.
To evaluate and create a conceptual understanding of the study area, all available historical groundwater monitoring data was converted into a format that is compatible with the WISH software package. After evaluation of the historical data, down-the-hole chemical profiling was conducted on existing monitoring boreholes, using an YSI-Multi-parameter probe, to determine if there is any physical or chemical stratification in the aquifers. A numerical groundwater model was developed using the FEFLOW software package. The model was developed in a staged manner, initially a steady state pre-mining model was created to analyse the groundwater flow before mining took place. Building on this, a steady state model for mining conditions to analyse the groundwater flow in response to mining influences. The model for mining conditions simulates the current groundwater equilibrium and was calibrated with the latest water levels. After steady state model calibration, various scenarios was considered to assess groundwater conditions at the colliery.
All the underground workings are completely flooded and mixing of water between the mine and upper aquifers is taking place. According to the water level data, it is plausible that the natural groundwater level has recovered to its natural state before mining commenced. The pre-mining conditions suggests that the seepage areas along the tributary draining the area was naturally susceptible for seepage conditions and mine water decant will occur if a pathway exits. Therefore, regardless of the ash-backfilling (coal power station ash-slurry) of the underground mine voids, seepage would have still occurred at these areas. The risk of suspending active pumping from the southern final void for a monitoring period is low, as long as the level in the northern final void remains stable.
The authorities granted permission to the mining company to suspend active pumping for a trial period of two years to monitor the aquifer response. Criteria to reinstate pumping was drawn-up based on the conceptual understanding of the area and should be adhered to at all times.