Open Cut » Environment
A seminal study has provided the mining industry, regulators and the community with a true picture of what is needed to produce successful rehabilitation on Queensland mine sites. Replacing inappropriate agricultural research methods with techniques relevant to mining , highly regarded environmental scientists from the Queensland Department of Natural Resources and Mines monitored the impact of open cut mine rehabilitation on erosion and water quality under natural rainfall conditions. The six-year study gave scientists the opportunity to gather data over the range of climatic conditions that occur in central Queensland, from crippling drought to heavy wet seasons. Using mine scale plots and catchments, they were able to generate meaningful data that could be used on site to produce effective rehabilitation programs.
The outstanding final report is devoid of the usual scientific jargon and is presented in an easy-to-use format. The key points are well summarised. High-tech photographic morphing clearly illustrates changes in rehabilitation over the six-year period and why erosion occurred. The report is a compelling communication tool.
Prior to this research, there was little information on erosion rates from open cut coal mine rehabilitation in Australia, particularly under natural rainfall. Most erosion research had been conducted on agricultural soils and land use, and on lower slope gradients.
The project's objectives were to:
- Monitor the long-term impact of open cut mine rehabilitation on erosion and water quality under natural rainfall conditions
- Evaluate physical and biophysical indicators for sustainable rehabilitation
- Use the monitoring sites as an educational resource and promote outcomes from the study to the industry and the wider community.
In particular, this research assessed runoff, erosion and water quality from rehabilitated land at Curragh, Goonyella Riverside and Oaky Creek mines at two scales - plot (0.01ha) and catchment (0.4ha to 0.9ha) - and three slope gradients - 10 percent, 20 percent and 30 percent. Pasture and tree vegetation treatments were imposed on topsoil and spoil materials and a number of topsoil and spoil plots at each site were left bare to compare with the vegetative treatments.
Pasture cover is the preferred indicator of rehabilitation sustainability and stability, and topsoil is the more suitable material for vegetation establishment on rehabilitated land. The use of topsoil and pasture cover produced the least runoff, sediment and soluble salt transport. A minimum 50 percent ground cover should be attained for erosion control.
Pasture establishment on spoil was poor at all mine sites, especially where the spoil was hard setting and dispersive. Annual erosion rates from spoil remained unacceptably high throughout the study. Where spoil was high in in situ soluble salts, the electrical conductivity levels from the resulting runoff were also high.
By the final year of the study, soluble salt levels in runoff from all topsoil pastured slopes had reduced to less than 200mS/cm, which is comparable to the water quality found in local creeks.
Runoff and erosion rates from the rehabilitated contoured catchments progressively declined during the study, with a corresponding improvement in water quality.
Research outcomes were shared with mine environmental staff, government agencies, landholders and consultants during a series of Landcare and Central Queensland Mine Rehabilitation Group field days.
The key findings of this research are:
- Rainfall is the major limiting factor associated with successful rehabilitation. It is critical that pasture cover is established rapidly in order to maximise rainfall infiltration.
- A rehabilitated landscape is at greatest risk of erosion before grass cover is established. The window-of-erosion risk occurs before vegetative growth reaches 50 percent ground cover.
- Pasture establishment to 50 percent cover should be a minimum target indicator for coal mine rehabilitation. Further increases in pasture cover (greater than 80 percent) and biomass are required to reduce erosion rates on 30 percent slopes to negligible levels.
- Topsoil erosion rates declined between slopes once a dense sward of grass cover established (greater than 80 percent cover).
- Vegetative cover reduces the risk of salt movement on-site and off-site through runoff.
- The development of a hard dispersive crust on the spoil material reduced infiltration, produced very poor pasture and tree establishment and resulted in unacceptably high runoff and erosion.
- Surface ripping of slopes greater than 20 percent should be used to improve infiltration and reduce runoff and erosion losses.
- Supplementary irrigation should be used to assist rapid pasture establishment during periods of low rainfall.