Abstract
Vale Canada Limited retained BluMetric Environmental Inc. to assess the feasibility of using Permeable Reactive Barriers (PRBs) for in situ passive treatment of groundwater impacted by acid rock drainage. The objective of a PRB in this environment is to enhance bacterial suphate-reducing conditions to produce H2S and promote precipitation of dissolved metals as metal sulphides.
Following successful bench-scale testing, a pilot-scale PRB was constructed at a Vale facility in Sudbury, Ontario. Existing soils were excavated to bedrock within a slide-rail system to minimize excavation volume and maximize worker safety. The PRB is 20 m long perpendicular to the mean groundwater flow direction. It is 5 m long in the groundwater flow direction. The average depth to bedrock is 2.6 m. The PRB was filled with a mixture of pea gravel, chipped wood, composted leaf mulch, spent brewer’s grain, and limestone.
Forty monitoring wells were installed upgradient, within and downgradient of the PRB and were sampled quarterly for two years following installation of the pilot PRB. The performance monitoring wells showed effective treatment of nickel, copper, cobalt, and cadmium from concentrations as high as 35 mg/L, 1.3 mg/L, 1.0 mg/L and 0.0072 mg/L, respectively, entering the PRB, to average concentrations of 0.03 mg/L, 0.002 mg/L, 0.004 mg/L and <0.0001 mg/L, respectively, within the PRB. A significant reduction in metal concentrations was also observed in monitoring wells downgradient of the PRB. The concentration of sulphate also decreases through the PRB due to sulphate reduction and the subsequent precipitation of metal sulphides.