Abstract:

East Kemptville Mine is a former tin mine that has been in active care and maintenance since 1999. The active care activities include collection and lime treatment of mine impacted water at the site. Site surface and seepage collection water remains around pH 3 or 4. Three prediction models for the site were developed to estimate future lime treatment requirements and anticipate the end of the active care and maintenance phase of closure: one in 1995, one in 1996, and one in 2007. The lime consumption predictions were based on measured acidity, mined material acid base accounting, and estimated acidity release from oxidizing sulfides. To support decision making for the site, the historical models were compared against 30 years of site lime consumption records in 2023. The decrease in lime consumption over time was consistent with the most optimistic model estimates until 2012. Since then, the mine’s lime consumption (acidity levels) appears to have stabilized. The stabilization may relate to pH buffering from iron and aluminum hydrolysis and/or the dissolution of acidic oxidation products. Jarosite was identified in the tailings storage facility, but the mass of potential acidic secondary oxidation products at the site has not been quantified. Site records for other parameters of interest were also reviewed. Zinc and copper concentrations show a similar exponential decrease as acidity and sulfate. Projecting the empirical trends forward suggests that achieving discharge criteria could take on the order of 40 years longer than the best fit acidity-based prediction for lime consumption. However, the water chemistry-based extrapolations are consistent with the later range of the original acidity-based predictions. Updating the ML/ARD inventory and model is planned.

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Dr. Diana B. Loomer