British Columbia Mine Reclamation Symposium

Closure of legacy waste rock piles : can we achieve passive treatment to manage residual seepage in the short term? Meiers, G.; O'Kane, Mike; Mayich, D.; Weber, P.; Bradley, C.; Shea, Joseph Michael


Closure and reclamation of waste rock piles using engineered cover systems and water treatment of seepage is a common technique for management of acid rock drainage/metal leaching (ARD/ML) to mitigate adverse impacts to the receiving environment. Linking cover system performance (i.e. net percolation and/or oxygen ingress) to impacts to the receiving environment provides a rational basis for cover system design criteria. Two general models are used in the mining industry to predict long-term seepage and closure costs. One model assumes that loading of contaminants to the environment will remain unchanged under reduced flux rates (net percolation) where contaminant concentrations increase proportionally as a function of decreasing flow; the other assumes that a reduction in the flux rate will result in decreased contaminant loads for constant contaminant concentrations in flow. Both models will have a transition point at which reduced loading to the receiving environment will occur as the flux decreases. Enterprise Cape Breton Corporation implemented a program for the closure of historic coal mines located near Sydney, Nova Scotia, Canada, with Public Works and Government Services Canada providing project management. The Victoria Junction waste rock pile was reclaimed with an engineered cover system comprising a 60 mil HDPE geomembrane, a granular drainage layer and an overlying growth medium. It was estimated following closure that active treatment of seepage waters impacted by ARD/ML would be required for ~20 years before passive treatment systems could be established; however, this has occurred within seven years with significant cost savings realised. Reclamation of the site reduced the risk of potential loading under various failure scenarios. An acidity mass balance was used to provide an understanding of past (uncovered waste rock pile and active treatment), current (covered waste rock pile and passive treatment) and long-term (100 years) loading under progressive changes to water collection and treatment activities. The acidity mass balance will serve to inform management decisions in ongoing closure planning. The case study presented here demonstrates that it is possible to eliminate the need for active treatment of seepage for legacy sites.

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