Both underground and open-cut mining have been undertaken at Collinsville in North Queensland since the 1920's, with little attention being given to the rehabilitation of mined land prior to the acquisition of the Collinsville Coal Company by MIM Holdings Ltd in 1975. Major environmental problems associated with the older mining areas included acid runoff and drainage, erosion and the lack of vegetative cover. In the early 1980's, Collinsville Coal Company recognised the need to assess the potential for similar problems to occur in association with the development of a new strip mine in the western part of its lease.
The research described in this thesis was undertaken to obtain a detailed knowledge of the geochemical properties of the overburden materials associated with the development of the western lease areas at Collinsville, such that a practical management system could be developed to enable successful progressive rehabilitation. The specific objectives of this research project were to (1) characterise toxic and potentially toxic overburden and interburden strata, (2) evaluate relationships between various overburden properties with respect to toxicity prediction and monitoring (3) assess the significance of overburden variability, and (4) establish overburden toxicity potentials for the western lease mining areas.
The reproducibility and validity of the sequential nitric acid oxidation method of Davey and Colbum (1979) for determiningpyrite (FeS2) content was evaluated on selected Collinsville overburden samples and standard materials. The procedure gave reproducible results which appeared to accurately reflect the pyrite content of the overburden material The method advocates the use of material ground to <0.2mm; however, the results obtained in this study suggested that further assessment of the effect of grinding size on FeS2 result was warranted. It was concluded that the procedure was too complicated for routine overburden characterisation, and a modified procedure was developed to assess sulphur forms in overburden material.
A linear relationship was found to exist between pyritic S and total S for 13 fresh overburden samples which could be expressed as follows:
Pyritic S (%) = Total S (%) x 0.693 - 0.015 (r2=0.997)
Following establishment of this relationship, the overburden characterisation programme developed for the assessment of the western lease areas involved the routine determination of total S using the Leco high temperature procedure with the pyritic S being calculated from total S.
A total of 441 overburden samples were evaluated in the characterisation programme which indicated that 28% of these samples could be classified as acid-toxic (pH<4), 21 % as potentially acid-toxic and 51% as non acid-toxic. This classification was based on acid-base accounting which balances the potential acidity (calculated from pyritic S) against the inherent acid neutralising capacity (ANC) for each sample. The results are expressed in CaCO3 equivalents with positive values indicating an excess of neutralising potential, while negative values highlight a net potential deficiency suggesting acid-toxicity could occur.
Multi-dimensional scaling was employed to assess the relationship between 10 parameters included in the characterisation programme. The data is presented as a two dimensional configuration with each point corresponding to one of the parameters, permitting direct interpretation. The configuration based on all of the overburden data, indicated that acid-base account, ANC and pH were well correlated while fizz rating and electrical conductivity (EC) were poorly con-elated. Fizz rating and Munsell colour value were con-elated, as were total S and EC, whereas sample depth was not highly correlated with any parameter but was con-elated most closely with colour chroma, reflecting a relationship with the degree of weathering.
With respect to prediction and monitoring implications, fizz rating was the most definitive predictor of acid-toxicity out of the four rapid semi-quantitative parameters (Munsell chroma and value, pyrite spot test and fizz rating). Any material with a fizz rating of 3 was considered non acid-toxic, whereas overburden with a pyrite spot test of 0 may be acid-toxic, depending on its fizz rating. Overburden materials with a Ph less than 7 and EC greater than 1 ds/m are likely to be acid-toxic as are samples with a total S content greater than 0.5% and an ANC less than 20t CaCO3eq./l,000t. Overall, it appeared that the ANC was the most useful individual parameter for predicting the acid-base account and hence assessing the acid-toxicity of Collinsville overburden.
The overburden strata overlying the various coal seams in the western lease mining areas at Collinsville are dominated by highly acid-toxic and potentially toxic materials. In general, the toxicity status of each area can be ranked in the following order: Blake West>Garrick West>Scott-Denison West>Scott-Denison North. The overburden strata occurring in the Blake West area are extremely toxic and have the potential to create serious environmental problems. The Garrick West area also has serious potential toxicity problems with siltstone-dominance strata having very high toxicity levels. The overburden profiles occurring in the Scott-Denison areas can be classified in relation to the depth of overburden to the coal seams. Where the coal seam is shallow, the overburden strata are likely to be dominated by toxic materials and vice-versa.
Both truck and shovel and dragline methods of overburden removal are being used to mine the various areas at Collinsville, and thus the potential exists to practice selective placement of pyritic material to isolate it from oxygen and water. Recommendations are given for the handling of soil and spoil to achieve a non-toxic root zone for plants following recontouring of the spoil piles.