Hydrochemical modelling for preliminary assessment of minewater pollution

A conceptual model for preliminary assessment of minewater pollution within the risk-based corrective action (RBCA) framework for environmental manage ment is developed. The model aims to assist classification of a site regard ing the potential threat to the environment and help assess whether the ass umptions used in this classification are appropriate. The model estimates c ontamination source strength, longevity and possible future changes in disc harge quality and can be applied with sparse data sets. The model relates s olute export in the discharge to source minerals which includes sulphide ph ases that produce acidity and metals contamination and carbonate and alumin osilicate phases which provide natural attenuation to neutralise acidity an d immobilise metals. We present and apply limited data from three sites representing a mine rock waste deposit located above the water table, a flooded abandoned coal mine with deep workings and a mine tailings deposit. Results from the rock wast e deposit indicate that calcite no longer provides significant attenuation of the present acidity load and that acid generation and associated loads o f Cu2+ may persist for a period of up to two centuries. The abandoned coal mine has a discharge that is presently alkaline, with calcite depletion exp ected to occur before pyrite is consumed, possibly yielding a future drop i n pH. The lifetime for these minerals is similar at this site, and on the o rder of several centuries, thus rendering the estimate of future water qual ity evolution very uncertain. The mill tailings deposit is expected to prod uce acidic discharge on a time scale of one century. However, conclusive qu antification of calcite weathering was not possible, leaving open the possi bility that the weathering of Mg-silicate minerals provides important atten uation of the present acidity load. (C) 2001 Elsevier Science B.V. All righ ts reserved.