Cover design considerations for mine waste rock dumps in subtropical environment

During mining activities in Australia, overburden is removed and deposited on the land in the form of waste rock dumps. When these waste rocks are exposed to atmospheric conditions and during rainfall events, they can create adverse environmental impacts through acidity and salt generation and release of metals, affecting surrounding environments, particularly final voids and water ways. The potential for impacts can be managed by rehabilitation, for example, by using soil and vegetation covers, and in some cases a barrier layer that excludes most of the rainfall from the underlying rocks. Apart from the generic objectives of rehabilitation i.e. safe, stable, non-polluting and being able to sustain a post-mining land use, cover design objectives include quantitative performance measures. Covers are intended to reduce oxygen ingress and/or net percolation into waste rock dumps. There are examples of failed covers in Queensland, Australia, due to a combination of factors such as inappropriate predictions of material performance and/or water balance. Even a very small water percolation may flush the reaction products out of waste dumps and release a significant load of acidity as seepage. Therefore, the efficiency of the cover depends not only on net percolation, but also on a range of other actors including the pre-cover conditions of the dump, the mineralogy and particle size distribution of rocks, structure of the dump and temperature and microbial activity. In Queensland, during development of Progressive Rehabilitation and Closure (PRC) plans, cover design planning needs to include identification and specification of the objectives of the cover system. If the objective is to reduce or eliminate residual environmental risks, then all relevant factors should be considered in design. In the case of coal mines of Queensland, a series of case studies have been conducted to classify, characterise and evaluate the ability of salt generation for different waste rocks. This provides an opportunity to recommend a guideline for the most effective cover design in a subtropical environment. These studies showed that firstly, the intensity and longevity of salt release are waste rock specific, and are linked to their original lithology, intrinsic salts, the presence of minerals and the magnitude of the dissolution and adsorption–desorption processes. The typical rehabilitation technique in Australia involves creating a landform using waste rocks, placement of soil on top of the waste rock pile and seeding, to create conditions for plant growth on the landform. Salt release from the waste rocks is one of the key factors that can limit plant growth, not only by creating saline seepage, but also by upward salt movement during dry periods. Therefore, attention should be given to the waste rocks sizes and their degradation degree to help determine their optimal placement in the cover design, as initial particle size and their dispersivity govern salt generation. Hydro-geochemical models, which can perform complex scenarios and mechanisms, and also include climatic scenarios, can be used to predict the salt and water movement within and from a cover design and thus predict the efficiency of the designed cover.