Managing the mixing : assessing mine soils with enhanced small-scale variation as part of mine rehabilitation activities

Abstract

Small-scale variation (SSV) of mine soils is enhanced by stripping, mixing and stockpiling of materials. Soils with enhanced variation are difficult to assess, causing misleading results that indirectly affects mine rehabilitation outcomes. This thesis aims to determine if standard soil chemical and biological methods are applicable in the assessment of enhanced SSV mine soils from Barrick (Cowal) Gold mine. Mine soils are firstly discussed in context of rehabilitation. Rehabilitating mine sites involves ecosystem engineering, in which landscape, function, structure and composition are essential ecosystem features. Using these features, a framework for planning, implementing and monitoring for ecosystem engineering was developed. The issue of SSV in mine soils was reviewed in the literature. It was determined that SSV directly and indirectly influences mine rehabilitation outcomes. The development of sampling strategies, alternative analytical and statistical methods, and standard guidelines are required for reliable assessment of mine soils. Nineteen topsoil stockpiles at Cowal were sampled for characterisation, with sample numbers proportional to stockpile size. Electrical conductivity (EC), exchangeable cations, cation exchange capacity (CEC), total carbon (TC), nitrogen (TN), sulphur (TS), nitrate-N, ammonium-N and Colwell phosphorus (P) were analysed. The soils require stabilisation and amendments for successful revegetation. Variation in exchangeable cations, TC and TN indicate soil management may cause variation in the stockpiles. Lack of matching pre- and post-mine surveys resulted in inconclusive causes of variation. Standard organic (wood mulch and compost) and inorganic (gypsum) amendments were used in the rehabilitation of a heterogeneous mine soil. This was undertaken as a 32-week laboratory (biological) and 72-week glasshouse (chemical characteristics and plants) study. The wood mulch increased C:N, C:P, C:S, respiration and moisture, which positively influenced revegetation. Gypsum had no significant impact on revegetation. Compost, however, increased all nutrients measured and decreased C:N, C:P and C:S. The low C:N:P:S ratios resulted in poorer revegetation. Coefficients of variation (CVs) were greater than 30% for microbiological biomass carbon (MBC), EC and nitrate-N (NO3-). The high variation in EC was related to gypsum application. The variation for MBC and NO3- was probably due to underlying enhanced SSV and inconsistent interferences in methods. TC, TN, TP, TS, respiration, water-P, bicarbonate-P, EC, pH, and moisture were demonstrated to be precise and accurate indicators for the assessment of mine soils. Large CVs in the standard salicylate Berthelot reaction for ammonium-N were investigated. Gypsum and enhanced variation of the soil cause inconsistent interferences. Pre-treatment by steam distillation is recommended. The anion exchange membrane method for measuring P (AEM-P) was assessed along with a range of low ionic solutions (LIS) to limit the variability associated with mine soils. Deionised water and NH4F with AEMs are precise for a range of mine soils. This thesis identifies DI AEM-P, WP, TC, TN, TP, TS, pH, EC and respiration as accurate and precise approaches for the assessment of mine soils. Nitrate-N, ammonium-N and MBC methods are not accurate, nor precise. There is a need to standardise analytical and statistical methods, and survey techniques for mine soils. Show more