Open Cut » Environment
Sustainable closure of coal mines in Australia remains one of the industry's biggest challenges. This issue is amplified when spoil characteristics make rehabilitation difficult and costly. A significant proportion of mines in the Bowen Basin of Queensland have dispersive spoil. It is estimated that the current instantaneous liability for rehabilitating dispersive spoil dumps is $2 to $3 billion for the Bowen Basin alone and significantly more nationally.
Dispersive spoils present problems in post-mining rehabilitated land-forms due to their increased potential for both surface and tunnel erosion, compromising the ability to achieve the fundamental mine closure objectives of a safe, non-polluting, stable and self-sustaining post-mining land form. Weak aggregate stability and dispersion of clay particles may result in clay particles being mobilised and transported in water, even at low flow velocities. Dispersion of clay particles may lead to blocking of pores, resulting in surface sealing and crusting, the formation of hard-setting layers, and reduced permeability which, in turn, further increases runoff.
Rehabilitation of dispersive spoil is technically complex and requires detailed understanding of the key site characteristics affecting spoil behaviour and their interaction with numerous management decisions. While the potential for dispersion is a function of fundamental spoil properties, the expression of erosion as a consequence of dispersive spoil in rehabilitated mine landforms is a function of design and management. The appropriate rehabilitation approach may vary from site to site and within sites. Detailed spoil physical and chemical characterisation is critical to inform effective interventions.
The study objective was to develop a risk-based decision support framework to inform practical, cost-effective management of dispersive mine spoil, together with a set of Best Management Practices (BMPs) and costed, risk-based decision support tools.
Results and Discussion
To assist mine environmental staff to understand the key considerations of dispersive spoil rehabilitation, a framework founded on an expanded form of the Universal Soil Loss Equation (USLE) was adopted. Opportunities for intervention, the collective outcome of which represent the residual vulnerability to erosion, were grouped into five classes comprising: (i) spoil characteristics; (ii) slope characteristics; (ii) practice control factors; (iv) crop management factors; and (v) tunnelling influences. Rainfall factors, representing exposure to erosive forces, were grouped into a sixth climate class.
A process-based approach to integrating contingent interactions between site characteristics and management interventions was developed using Bayesian modelling. Bayesian models provide the capacity to conceptualise and analyse complex management system by enabling the likelihood of a particular outcome to be predicted given the condition or state of each factor in the model.
The conditional probabilities informing the Bayesian model were developed using established literature; assessment of prior dispersive spoil rehabilitation outcomes; preliminary results from field trials; and expert knowledge. The resulting model was built into a user-friendly web-tool linked to a spoil chemistry calculator and rehabilitation cost model. The model presents the outputs of surface erosion and tunnelling risk as a matrix of exposure by vulnerability, in a similar manner to the intersection of likelihood and consequence in conventional risk assessment. The model is available for operational use through an online webtool located at: http://minerehabilitation.net.au/. Login details for access to spoil nutrition, climate rehabilitation performance and rehabilitation cost tools are provided in the full report.
Results of field trials established using the principles embodied in the model support its utility. Fifteen months after rehabilitation, during which period a number of significant rainfall events occurred, a site displaying extreme exchangeable sodium percentage (ESP), severe plant nutritional deficiencies, and severe surface and tunnel erosion pre-treatment, remained well vegetated with stable slope conditions.
Spoil behaviour from field experience and modelling was captured in a set of Best Management Practices. Each individual practice is presented as a risk matrix of management intervention by the interaction of sodicity and soil salinity. The BMP's provide a guide for inputs to management decisions selected in the Bayesian model. The Bayesian model and associated BMPs provide field practitioners with the capacity to explore alternative management options and make informed decisions based on cost and risk. An iterative process, using feedback from industry will support adaptive, evidence-based best practice dispersive mine spoil management and ongoing model refinement.
Based on the expanded USLE framework, a critical finding of this study, is that all elements of the framework must be addressed to reduce the risk of erosion. Failure to address an issue in any element that contributes a significant proportion of soil loss will result in potential rehabilitation failure. This failure to address inherent spoil characteristics has been a consistent basis for poor rehabilitation outcomes in past practice.
Benefits of improved mine spoil management include enhanced capacity to meet closure criteria; improved regulator consideration of required closure criteria; improved post-closure land capability; improved community acceptance of post-closure land condition; reduced contribution to cumulative impacts; and enhanced social licence to operate.