The effects of mine tailings on local groundwater have been recognized for decades. The oxidation of newly exposed mine tailings, particularly sulphide minerals such as pyrite, releases acids and heavy metals into the environment. The rate of acid production is far higher than that caused by natural leaching and erosion of solid rock, and is beyond the environment capacity to absorb. As a result, acid mine drainage has been identified as the largest single environmental problem facing the Canadian mining industry (Filion et al., 1990).
Recent mining practices have reduced the amount of waste rock in tailings ponds through activities such as backfill and reprocessing, and improved site engineering has reduced the flow of contaminants beyond the confinement area. Federal restrictions on groundwater acidity and heavy-metal concentration levels regulate conditions in the immediate vicinity of the mine, and Ontario mines have a respectable track record for compliance (Environment Canada, 1988). Abandoned mines, however, generally date to an era before environmental regulation, and thus may pose a considerable risk.
The high acid and heavy-metal concentration in the leachate make the groundwater extremely conductive and an excellent electromagnetic geophysical target. The example shown has been extracted from a larger mineral exploration survey conducted for the Ontario Geological Survey. The survey was flown with helicopter electromagnetics (HEM)/magnetometer/ VLF-EM sensors at a nominal terrain clearance of 30 m (HEM) and 45 m (magnetometer and VLF-EM) and a 200-m line spacing. Observations were positioned by radar ranging techniques, and results were presented originally as EM profiles at 1:20,000. The actual location and orientation of the data have been disguised.