From Salt Balance to Contaminant Flux: Managing Water Quality Risk Using a Systems Approach

摘要

We used a salt balance to describe contaminant transfer within a mine water system. The system included mine workings, mineral processing plants, water storages (both clean and dirty), and tailings storage facilities as components. Analysis of modelling results identified sub-systems associated with potential water quality risks. In particular, approximately 6 000 tonnes of dissolved salts associated with the tailings component could not be accounted for. Under the prevailing regulatory regime, this could be a significant water quality risk. However, the fragmented monitoring network and poorly characterised hydrogeology was not able to resolve the magnitude of the risk.The cost of developing a tailings source term was significantly less than the cost of upgrading the groundwater monitoring network. Therefore, the tailings sub-system was characterised in detail. This involved collecting tailings process water and drain samples. We also collected tailings samples from various depths in the facility. Geochemical and hydraulic analysis of the tailings allowed an indicative source term to be developed. This quantified the contaminant flux through the tailings during deposition and after closure.During the operational phase approximately 1 000 tonnes per year of dissolved salts actually leaves the tailings as seepage through the footprint. The residual salt mass leaves the tailings at the rate of approximately 250 tonnes per year once the tailings have desaturated.The improved resolution of the tailings sub-system indicated that the tailings are a sink for dissolved salts in the mine water system. The fine particle size and low permeability of the tailings under compaction, is a natural mitigation that limits the rate of salt release. Therefore, the magnitude of the water quality risk was found to be moderate