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Groundwater Protection and Restoration Group, Department of Civil and Structural Engineering, University of Sheffield, Sheffield S1 3JD, UK s.a.banwart{at}sheffield.ac.uk
A general challenge to the environmental management of mine sites is the relatively high costs of site investigation and the associated development of conceptual site models and parameterization of reactive transport models. A particular problem is the inability to predict at field scale the rates of processes that give rise to long-term dissolved contamination due to active sulphide mineral weathering. Mineral weathering rates determined from laboratory and field observations generally do not agree, often exhibiting a discrepancy of two-three orders of magnitude.
Recent work on mine waste deposits has demonstrated that this discrepancy can be explained by considering a small number of bulk physical and chemical properties of mine rock at field sites. The apparent decrease in mass-normalized rates between bench-scale batch reactors and pilot-scale column reactors is predicted by accounting for differences in temperature, where lower temperatures in the column reactors reduced reaction rates due to activation energy effects. The columns also contain a significantly greater mass fraction of larger particles that have lower specific surface area and, thus, exhibit lower weathering rates.
The further apparent decrease in rates between the column reactors and field scale is predicted by additionally accounting for the spatial variability of sulphide-bearing rock at the site, which gives rise to only localized weathering. Localized zones of sulphide weathering are also associated with locally active weathering of silicate minerals due to lower pH. Hydrological factors are also important due to preferential flow within the field site, whereby a fraction of dissolved weathering products are retained within immobile water and do not reach the effluent stream, where ion mass flows resulting from weathering reactions are determined. These results suggest that application of compiled laboratory data to prediction of weathering rates at mine sites may be feasible. This is potentially valuable for application to Tier 1 risk assessment of mine sites, where reliable prediction of weathering rates from tabulated laboratory data would provide significant information to support the generally sparse datasets that are available, particularly for orphan mine sites.
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