Estimating contaminant concentrations in a re-flooded underground mine

Investigations were undertaken to characterize the potential for various underground components of the proposed Crandon Mine to release contaminants to groundwater that will re-flood the mine after operations cease. Methods used to characterize the sources included literature and data review, laboratory tests and simple calculations. The potential for backfilled tailings to release contaminants to the groundwater was shown to be limited by the rate at which oxygen can be transported into the backfilled stopes. In assessing the potential for the wall rock and crown pillar to release contaminants, key parameters were the proportion of wall that will be sulfidic, and the actual surface area of the walls. Conservative estimates of contaminant emissions from diesel equipment were used to show that underground emissions are not likely to contribute a significant amount of contaminants to the groundwater. The investigations also included an analysis of groundwater flow patterns in the re-flooded mine, which led to the conclusion that the groundwater quality will initially be dominated by contributions from wall rock in the access workings, but subsequently will be dominated by the backfilled tailings.