Rehabilitation risk assessment model

Micro-gravity, electrical resistivity and time domain electromagnetics located the boundary between old workings and virgin ground, but no ground technique tested could resolve individual bords and pillars at the depths typical of the Witbank and Highveld coalfields. Airborne thermal infrared surveys worked well as a low cost regional reconnaissance tool for shallow sinkholes, fires and possible pollution plumes.

Ion exchange was assessed as a treatment route for brine arising from mine water cleaning, from a market rather than a plant performance standpoint. World demand for ion exchange resins was expected to pass 535 million dollars by 2015, driven by urbanisation, pollution and shortages of clean water in developing regions.

Conventional rehabilitation relies on lime, fertiliser and compost to build fertility and select adaptive species, which produces short term cover but rarely a self sustaining ecosystem. The work argues for treating soil microbes as part of the system, since bacteria drive nutrient cycling, organic matter breakdown and soil aggregation, and proposes conditioning plants with adapted rhizosphere bacteria on coal waste substrates.

Stockpiling degraded every soil property measured across four Witbank area mines. Clay content of topsoil and subsoil rose by 50 to 60 per cent, surface bulk density increased from stockpiling through to rehabilitation, and effective depth on rehabilitated areas was under 17 cm in most cases. Topsoil organic carbon fell by almost half and microbial species numbers dropped in the stockpiles.