Completion Criteria for Pasture Based Rehabilitation in the Bowen Basin

Fundamental parameters for surface stability and vegetation performance have been developed for ungrazed rehabilitation pastures on central queensland coal mines.  It is widely recognised within the mining industry that criteria used to evaluate rehabilitation for lease surrender and to discharge the liability for mined areas are inadequate. There are no formal criteria on which to assess the success of rehabilitated areas after mining. Guidance is provided in the Technical Guidelines for Environmental Management of Exploration and Mining in Queensland (DME, 1995), however there is a need to develop completion criteria that are practical, defensible and appropriate for local conditions.

The aim of this project was to review pasture-based rehabilitation efforts in central Queensland, and to make progress towards the development of completion criteria suitable for relinquishment purposes.  The review aimed to:

• Identify factors critical in the establishment and development of pasture-based rehabilitation after open cut coal mining in central Queensland
• Assess the performance of pasture-based rehabilitation in its existing unmanaged state, with particular reference to surface stability
• Provide background information relevant to the drafting of preliminary ecologically-based completion criteria for pasture-based rehabilitation in central Queensland.

Data was obtained from Callide, Moura, Blackwater, Gregory, Norwich Park, Peak Downs, Saraji, Goonyella Riverside, South Walker Creek, and Collinsville.  Data obtained was compiled to generate a quantitative database for further analysis and interpretation. The dataset of around 500 records, which forms the core of the review, covered about 130 sites or trial areas across nine mines (Callide not included in dataset) and encompassed sites of varying age, gradient, growth media type and associated vegetation parameters.

Findings

Researchers developed fundamental parameters for surface stability and vegetation performance:

• Achieve and maintain vegetation cover of at least 70 percent - Vegetation cover forms the single most important control on erosion. Erosion rates are significantly reduced for cover levels of 50 percent but higher cover levels are considered necessary for adequate control. Soil movement can still occur at high cover, dependent on other factors.

• Regrade slopes to less than 12 percent - High levels of cover are in themselves not necessarily adequate. Significant movement can still occur for cover more than 70 percent on slopes exceeding 12 to 15 percent. Maintaining slope gradients at less than 12 percent appears to be associated with low erosion rates.
• Media properties reducing infiltration affect the influence of vegetation and slope - Surface materials which have poor particle size distribution (similar proportions of fine sand, silt and non-active or only slightly active clays) combined with elevated ESP or R1 dispersion index tend to have lower infiltration and transmission rates. Levels of runoff are higher than on rehabilitated areas with relatively favourable characteristics and erosion rates are higher through an increased volume of runoff and through dispersion. This is further compounded by lowered plant growth (as less water is available for growth). Rock content is considered to reduce erosion through surface protection in a manner similar to vegetation, and in disruption of rill development.
• Reduce root zone salinities to less than 0.6dS/m (on 1:5 basis) - Salinity levels of less than 0.6dS/m in the surface 40cm support pastures with dry matter levels capable of providing the minimum cover necessary for erosion control. However, the impacts of dry matter removal through grazing may warrant a lower "threshold" value. Low initial site salinity will assist in rapid vegetation establishment, thereby reducing the risk of erosion.
• Media properties influence salinity reduction over time - The same factors which affect infiltration, runoff (and subsequent erosion) also impact on root zone salinity through the development of leaching profiles. Surface materials which have poor particle size distribution combined with elevated ESP or R1 dispersion index tend to have lower infiltration and transmission rates. Slope and surface roughness (ripping) have an effect on infiltration in the short term but in the longer term their effects are minimised.
• A minimum CEC of 8-10 is required for adequate nutrient retention - Soil fertility (available P, N and S, plus cations (mainly K) impacts on pasture dry matter. However, past fertiliser application and spoil mixing mask longer term equilibrium levels of these nutrients. CEC provides a more usable indicator of long-term nutrient status. Low CEC is mainly associated with materials containing a high proportion of coarse particles, subject to higher leaching and nutrient rundown. As a guide, materials with a sand content of 75 percent or more are likely to have inadequate nutrient retention capacity. Clay mineralogy will be important in determining CEC.