Assessing various rehabilitation strategies for coarse coal washery reject dumps in the Hunter Valley, Australia. I. Chemical characteristics

Abstract

At several coal mines in the Hunter Valley (New South Wales, Australia), the use of coarse coal washery reject (chitter) as a top-dressing material has resulted in successful establishment rates of tree and shrub species by direct seeding. However, concerns have arisen about the potential to downgrade the rural capability of the land for grazing, poor understanding of the material's long-term benefits, and spontaneous combustion issues. To ascertain the suitability of chitter as a final landform surface, a revegetation research trial was initiated at United Collieries coal mine in the Hunter Valley of New South Wales. Experimental plots were established in autumn (May 1998) and spring (November 1998) on a freshly shaped stockpile of chitter. The revegetation trials incorporated four substrate treatments, namely a 1 m and 20 cm capping of overburden, a 20 cm topsoil dressing and a treatment of only chitter. Each of the four substrate treatments incorporated five revegetation treatments, namely pasture species and mulch (straw), pasture species and no mulch, native seed (shrub and tree) and mulch, native seed and no mulch, and planted tubestock trees. Chemical characteristics (pH, EC, total N, nitrate-N, available-P, total P, total S, Ca, Na, Mg, K, organic C and soil moisture) across the substrate and vegetation treatments were assessed at 0, 3, 6 and 12 months after establishment. Laboratory analyses of soil samples from the autumn and spring substrate sowings concluded that the chitter treatment contained a significantly higher concentration of organic carbon (23 – 41%), Na (2.8 – 13 me/100g), total N (2600 – 5300 mg/kg) and S (900 – 4000 mg/kg), and pH was elevated (7.5 – 9.8), relative to the other substrates. The topsoil substrate had significantly greater moisture content and more nitrate-N in comparison to the overburden and chitter materials, and exhibited a significantly lower pH over time. The two overburden treatments showed similar chemical properties and contained significantly higher levels of available P relative to topsoil and chitter. In comparison to recommended levels, all substrates recorded adequate levels of Ca, Mg and K across the autumn and spring assessments and were generally deficient in nitrate-N. In the overburden materials, levels of organic carbon, electrical conductivity, exchangeable bases, total P and available P were generally adequate. However, the pH (8.1 – 8.9) and S (200 – 700 mg/kg) content of overburden was consistently high, and deficiencies in total and available nitrogen were commonly detected. Assessment of substrate samples across vegetation treatments showed few differences with only K, available P and nitrate-N varying significantly between treatments. In both the autumn and spring trials, the concentration of available P in pasture plots was significantly higher in comparison to the other vegetation treatments, while K levels were significantly higher in the pasture and mulch treatment relative to tubestock plots. This probably resulted from greater uptake of these nutrients by trees and shrubs compared to pasture species. In the autumn trials, seeded native tree plots contained significantly more nitrate-N than the other treatments, probably due to N-fixation by Acacia spp. Across both establishment seasons, topsoil provided the best chemical substrate for the establishment of vegetation and chitter was deemed the least hospitable substrate, while the different vegetation treatments appeared to have little effect on the chemical characteristics of the substrates. Vegetation characteristics across the treatments are presented in an accompanying paper (Charnock and Grant, Assessing various rehabilitation strategies for coarse coal washery reject dumps in the Hunter Valley, Australia. II. Vegetation characteristics. International Journal of Surface Mining, Reclamation and Environment, 2005 (in review)).