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Reducing Uncertainty in Long Term Void Water Quality Predictions for Final Void Management

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Published: May 24Project Number: C29047

Get ReportAuthor: Sue Vink | The University of Queensland

The aim of this project was to generate new data and undertake predictive water quality modelling to assist the closure of final voids as water bodies. The research reviewed available pit lake data to ascertain the range of water quality conditions currently existing in water filled voids on mine sites. Additionally, temporal changes in selected parameters were examined which showed that while the salinity in many voids has been increasing over the past decade or so this trend was not universal. Salinity and sulfate concentrations were highly correlated suggesting that sulfate is predominantly derived from salts rather than from sulfide mineral oxidation (i.e. from acid mine drainage processes). Acidic Mine Drainage (AMD) is rare based on reported pH values which were typically neutral slightly alkaline. Average long term pH also does not materially change although seasonal cycles related to biological production in the lakes were apparent.

Fieldwork was undertaken to generate new data detailing water quality changes in two pits and to provide supporting evidence for the primary processes controlling water quality. Vertical profiles of salinity, temperature, pH, dissolved oxygen and other constituents were measured. Parameter values did not vary between sampling stations in each pit.

The field data and findings informed the water balance and water quality GoldSim model to predict water quality evolution in the voids. Coupled hydrological, hydrochemical and hydrogeological processes were included in the models. Results from recent research, specifically related to estimating evaporation from the voids and inputs of salts through spoil, were also incorporated. The models were well calibrated to existing records of water level and salinity for the two voids.

The modelling showed that salinity was likely to be the biggest risk in both voids. The water level in both pits is predicted to be relatively stable and suggests overflow should not occur; due to the impact of evaporation, water volumes decreased during the model timeframe with the equilibrated void water elevations a function of the relative magnitude of evaporation and groundwater inflows. Water levels reached equilibrium elevations after about 70 years, with the salinity continuing to increase due to evaporation over the modelling period.

Simulated long term water quality at both pits indicates the significant impact of climatic conditions, such as evaporation, may pose to the final void system. The evaporation factor was identified to be the most sensitive model parameter, associated with the predictive uncertainty for void water quality, especially for the shallower pit lake (Pit 2). This also highlighted the significant roles of the laboratory to field load scaling factors, and the site specific geochemical characteristics of spoils, in predicting the long term final void water quality.

Overall, this project demonstrated the utility of field monitoring and coupled hydrological, hydrochemical and hydrogeological modelling using GoldSim to predict water quality evolution in pit lakes. Ongoing monitoring and evaluation against model outputs will be required to reduce uncertainty in water quality predictions required to evaluate the final use of water filled voids.

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