Ellerbroek, D.A., Jones, D.R., and Townley, L.R. (1996), Managing the hydrology and water quality of final voids after mining, Proc. ACMRR Workshop on Post-mining Landform Stability and Design (Bell, L.C., and Maclean, R.W., Eds.), 17-18 September, 111-121.

Open pit mining of relatively low grade ore has expanded dramatically in Australia during the last decade. A consequence of large-scale open pit mining is the creation of pits that may contain lakes after mining and groundwater interception have ceased. Development of postmining land use plans for these features depends on the ability to predict water levels and water quality after they have filled with water and become final voids. Lakes formed in final voids have a number of important differences from natural lakes that make it difficult to utilise existing models to predict long term water quality. These differences include a relatively lower surface area to volume ratio, closed drainage basins, and the potential for inflows from reactive materials that have been disturbed during the mining process. Lakes forming in final voids may be influenced by the hydraulic properties and weathering products of waste rock and tailings that form aquifers discharging into the system. These properties may change over time, and the hydrology and water quality of the final void may evolve for many years before approaching a steady state.

This paper investigates a number of issues relevant to predicting the long term hydrology and water quality of final voids. The major hydrochemical processes affecting water quality in a final void include weathering reactions and solute transport in surrounding geologic materials, sulfide oxidation, evapo-concentration, stratification and mixing, biologically mediated redox reactions, complexation with inorganic and organic ligands and particulate matter, and precipitation of secondary minerals. Failure of pit walls or spoil adjacent to the void may also affect water quality and may be influenced by changes in the piezometric surface or water level in the void. The complexity and interdependency of these factors makes determination of water quality highly site specific. Very little will be gained by developing models without a general understanding of how these physical, chemical, and biological processes interact to influence water quality.

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Last revised: 17 January 2022