As rehabilitation areas are established and develop, it is essential to monitor their success. The minerals industry and, for this particular study the coal industry in central Queensland, is facing increasing pressure to show proof of the sustainability of its rehabilitation strategies. Currently, despite much activity in this area, there are no generic recognised criteria in Australia for determining when rehabilitation is complete. Many approaches are being pursued, however, actual demonstration of achievement still provides the most powerful proof of rehabilitation sustainability.
Sustainability implies long time-frames and there are relatively few examples of rehabilitation trials in the central Queensland coal fields that have a known establishment history, are well-documented over more than a few years and remain intact. One of the few trials in central Queensland which meets these criteria is the Tertiary Spoil Trial established in 1991 at the Saraji Mine. To investigate the constructed ecosystems at the Saraji Mine trial, it was important to first recognise that long-term rehabilitation monitoring is concerned with trends and that by understanding these ecosystem trends, the rehabilitation decision-making process is assisted.
The research undertaken as part of this thesis provided documented evidence of trends through time for the ecosystem types at the Tertiary Spoil Trial at Saraji Mine. In the early stages of ecosystem development, vegetation growth was influenced by the media characteristics and the treatment species mix. Twelve years after establishment, it was evident that the initial surface media characteristics continued to influence both the potential of the system and the rate of system development. However, investigation of infiltration characteristics on the trial treatments after twelve years identified that vegetation cover had become the dominant influencing factor on infiltration, regardless of the media treatment or vegetation treatment.
Investigation of the trends within each ecosystem type showed a number of attributes had reached a steady state, such as ground cover on the topsoil treatments, giving an indication of ecosystem sustainability. However, other attributes such as the canopy foliage projective cover, continued to change through time and these require future monitoring to identify when steady state has been achieved. The comparison of the trial monitoring data with reference communities from the surrounding landscape found that although the trial treatments were more alkaline, more saline, and more sodic through the profile, the rehabilitation landscape after 12 years of development had many similarities to those in the surrounding region, indicating a level of sustainability within the trial treatments. One exception to this was identified, where all vegetation attributes on a trial treatment were below the range observed in the reference communities.
Overall, these studies highlighted the importance of media characterisation prior to rehabilitation . Given the dominance of the vegetation influence on infiltration characteristics, the rapid establishment of ground cover at these tertiary spoil rehabilitation sites is desirable. Additionally, the low nutrient concentrations identified in one of the trial media at trial establishment may have future detrimental effects. Those spoils requiring rehabilitation that have media characteristics which restrict seedling emergence and vegetation establishment should be avoided or capped with a deeper application of topsoil to restrict accumulation of salts at the soil surface.
The study also showed that those media treatments initially slower to establish a vegetative cover can develop over time to produce attributes that approach the more 'rapidly developing' media treatments. However, as these treatments initially provided lower ground cover than the topsoil treatments, landscape stability, decreased infiltration and increased runoff would all be issues requiring consideration in the early stages of rehabilitation.
There are many other facets of these rehabilitation ecosystems and reference communities that could be investigated to give an improved understanding of the underlying processes. This series of studies however, provides a valuable tool to those surrounding operations with similar climatic conditions and similar overburden (or spoil) media. In addition, it has reinforced the importance of on-going rehabilitation monitoring and the valuable nature of this long-term data in mapping ecosystem trends and providing indications of sustainability. Inclusion of reference communities within these investigations and comparison of to trial treatment information will continue to provide a valuable tool for evaluation of the rehabilitation and assist in showing sustainable ecosystem development.