Rehabilitation of Open Cut Mines Using Native Pasture Species

It is a requirement of law that coal mining companies in the Hunter Valley rehabilitate mined land to a standard at least equal to that existing prior to mining activities. In the Hunter Valley most areas prior to mining consist of Class IV, V and VI agricultural land, as defined by the Soil Conservation Service's land capability assessment system. This land largely consists of cleared land dominated by native grass communities and scattered trees. Current recommendations for open cut coal mine rehabilitation in the Hunter Valley are based on a set of introduced pasture species, and in some areas where there is a topsoil deficit, native trees are planted or seeded. At present no native grasses or other understorey species are included.

While the use of introduced pasture grasses and legumes has been successful in satisfying the requirements for surface stabilisation, these species have high input requirements in terms of seeding rate, fertiliser and topsoil. In addition, current ideas suggest that they may not be ecologically sound in the long term. Species such as Rhodes Grass, although sown in a mixture, tend towards a monoculture over time, and unless grazed repeatedly is of fairly low nutritive value to stock.

Many native grasses are tolerant of low soil fertility, can persist in diverse communities under erratic climatic conditions, can flourish under grazing and provide useful stock feed.

A project was undertaken to research the potential for using native grasses in open cut coal mine rehabilitation in the Hunter Valley. The project was proposed by the Hunter Coal Environment Group (HCEG), which provides a forum for those professionally concerned with environmental issues relating to coal mines.

The HCEG consists of representatives from the coal mines, government agencies, consultancies and others. Funding for the 3 year project was supplied by the Australian Coal Association Research Program ACARP), which directs a proportion of the Australian 5% coal levy towards research. The project was administered by the Department of Land and Water Conservation (DLWC, formerly Conservation and Land Management, and the NSW Soil Conservation Service), who provided the infrastructure to employ staff, provide resources and administer ACARP funds. The project was conducted by a full - time research officer based at DLWC's Scone Research Service Centre, with assistance from departmental and mines staff, and under the direction of an industry monitor and steering committee of HCEG members.

The goal of the proposed project was to produce a set of practical and cost effective methods and materials for incorporating native pasture species in the rehabilitation of open cut coal mine spoil in NSW.

Conclusions & Recommendations

Based on the results of this project the following summary and recommendations can be made:

• Recommended cool season native grasses for mine rehabilitation include species of Wallaby Grass (Danthonia spp.) and Plains Grass (Stipa aristiglumis). 
• Recommended warm season native grasses include Queensland Bluegrass (Dichanthium sericeum), Couch (Cynodon dactylon and Windmill Grass (Chloris truncata). 
• Autumn sowings for both cool and warm season species are recommended in preference to spring sowings. Because many native grasses are slow to establish compared to introduced species, they may not be sufficiently established before summer. 
• Topsoil is still preferable to raw overburden as a growth substrate. Because of the potential economic benefits of eliminating topsoil, however, further research into making the overburden more conducive to plant growth is justified. 
• High sowing rates, equal to those used for introduced is preferable. Obviously seed supply and cost will affect the feasibility of this approach. 
• Weed control is of utmost importance during the first year at least after sowing to allow natives to establish. Native grasses tested in this trial were resistant to Grazon Ò and are possibly resistant to others. The alternative is to use as high sowing rate which will help the native species to compete. 
• Seed germination and quality testing of native grass is essential to calculate seed viability, dormancy and correct sowing rates. 
• Characterisation of overburden and topsoil is very important. Obviously, the differences among overburden and topsoil material in the Hunter are considerable, and will affect species selection for, and success rates of, rehabilitation areas. As such, each rehabilitation site should be assessed for its unique characteristics.

Hopefully this project has provided valuable information on the main issues concerning the use of native grasses for coal mine rehabilitation. Some promising species have been found, and some of the constraints and advantages of their use identified.

Project Extension

ACARP have funded a 2 - year extension to this project which will build on the results already achieved. The aim of the extension is to carry out longer term monitoring and evaluation of promising species from the original project, and to develop the extension of practical establishment and management techniques into the field. More specifically the extension will include:

• Develop a set of decommissioning criteria, following a review of information available at the end of the extension period. 
• Collaboration of this project with the understorey species project currently being funded by ACARP, and run by the Centre for Mined Land Rehabilitation. 
• Assess management techniques for incorporating native grasses into existing pastures. 
• Optimise spoil amelioration techniques so as to maximise plant growth. 

Develop methods of handling and sowing local seed by:.

• Further developing local seed sources, ie. locate additional existing stands and secure them for future seed harvest or if necessary establish seed areas within rehabilitation areas.
• Improve seed harvesting methods, using a brush harvester purchased during the current project, and located in the Hunter Valley. 

• Develop cost effective methods of handling the seed of selected species and investigate appropriate mechanical seeding methods. 
• Develop and assess mixed species sowings. 
• Expand the range of spoil lithologies assessed for species suitability. Because of the wide variation in lithology types, each species must be assessed for its suitability to each type, so that confident recommendations can be made. The species used in these trials will have shown good potential in the original project, or be considered as having potential. 

Monitor existing field trials for a further two years to asses:

• ability to form, maintain or increase ground cover over time
• nutrient value relative to:
• soil fertility, seasonal conditions, and ability to compete with weeds
• potential to proliferate
• continued drought tolerance
• herbicide tolerance/resistance
• response to follow-up fertiliser application

It is to be hoped that this project and its extension will lead to the incorporation of native grasses into open cut coal mine rehabilitation in the Hunter Valley. This will provide options for more environmentally friendly, low cost rehabilitation which is sustainable in the long term and in harmony with the local environment.