Acid mine drainage (AMD) occurs when pyritic mine tailings are exposed to oxygen and water. Surface runoff and groundwater seepage through waste piles are typically very acidic and contain high concentrations of dissolved metals including iron, aluminum, manganese, copper, lead, nickel and zinc. Conventional treatment of AMD usually involves liming the runoff to neutralize the water and chemically precipitate the metals. Liming is expensive and may be required long after the mine has ceased operating. Recently, attempts have been made to develop low-maintenance, low-cost AMD treatment systems that can restore water quality. One method of passive treatment utilizes the vegetation and sediment microbial communities found in natural wetlands to reduce acidity and precipitate the metals.

This study examined natural wetlands near six base metal mines in eastern Canada for evidence of biological AMD mitigation. Potentially suitable wetlands were identified by Environment Canada in consultation with mining companies and provincial resource departments. A field program was conducted to describe and compare these sites in terms of water and sediment chemistry, hydrology, and vegetation. Two sampling periods, summer and late fall, were originally planned to enable within-site comparisons between presumed “biologically-active” and “inactive” periods. The project was started too late to conduct a summer campaign. This report presents the results of the early fall sampling period.

There were large differences among wetlands in metal loadings, groundwater inputs, water retention times, metal concentrations in wetlands sediments, and vegetation communities. None of the sites, however, appeared to significantly reduce downstream loadings of metals of acidity levels during the fall. Prolonged drought at one site, Nickel Rim, severely reduced flows and prevented calculations of influent and effluent metal loadings. The apparent lack of biologically-related improvement of AMD was most likely a consequence of (1) low temperature, ca. 4°C, (2) channelized flow patterns within the wetlands, and (3) limited contact of AMD with anoxic sediments. Inputs of other seeps and groundwaters to the wetlands were significant complicating factors, especially at the Kam-Kotia site. High metal concentrations in wetlands sediments indicate that the wetlands had removed metals in the recent path.