Numerical prediction of the long-term evolution of acid mine drainage at a waste rock pile site remediated with an HDPE-lined cover system

Remediation at former mining sites containing waste rock piles (WRPs) commonly involves the installation of a cover system over the waste rock to limit water and oxygen ingress and attenuate the impacts of acid mine drainage (AMD) to the environment. Cover systems containing high-density polyethylene (HDPE) liners have the attributes to be highly effective; however, their performance over the long-term is unknown. The objective of this study was to assess the long-term effectiveness of an 'in-service' HDPE-lined cover system for reducing AMD contamination at WRP sites. A numerical investigation of a former mining site containing a large WRP reclaimed with an HDPE cover is presented. A 3-D groundwater flow and contaminant transport model of the site was developed in FEFLOW to predict the spatial and temporal evolution of AMD over 100 years. Field parameters observed at 46 monitoring wells over a 5-year monitoring period (including hydraulic head, recharge, hydraulic conductivity and water quality) were used as key input and calibration parameters. The HDPE cover significantly reduced both water recharge to the waste rock (i.e., 512 to 50 mm/year) and AMD seepage to groundwater. Both the groundwater flow and contaminant transport (sulfate was used as an AMD tracer) components of the model were calibrated and verified to the observed field data, with strong correlations evident between observed and simulated hydraulic heads and sulfate concentrations, respectively. Long-term model predictions of AMD evolution indicated significant and continual reductions in sulfate concentrations over time at all well locations. Background concentration levels (25 mg/L) are expected to be reached within 40 years. This study has demonstrated that HDPE-lined cover systems can be highly effective in reducing AMD loading from WRPs and its impacts on the receiving environment.