Wetland treatment has successfully removed nickel, copper, cobalt, and zinc from neutral mine drainage in northeastern Minnesota. Pilot and full-scale overland flow wetlands have removed up to 90% of the incoming nickel and 50% to 90% of the other metals. Nickel is the major contaminant in the mine drainage, with average concentrations from 2 to 5 mg/L. The other metals were generally present at concentrations less than 0.1 mg/L. In short-term tests, using drainage that contained around 1 mg/L zinc and 0.4 mg/L of copper and cobalt, concentrations were reduced abont 90%. Input and output flow and water quality measurements were collected, and a mass balance was calculated. To determine the fate of the metals and the removal mechanisms, samples of vegetation and peat were collected to examine the distribution, form, and depth of metal removal. Although the overall metal concentrations in the predominant vegetation types (cattail, grass, and sedge) had increased by factors ranging from 3 for above ground vegetation to around 14 for cattail roots, the mass of metal removed by the vegetation was less than 1 % of the total mass removed. The remaining metals were associated• with the peat substrate. Core samples of the peat were collected and divided into 2 to 4 cm sections. Each section underwent a sequential extraction procedure and was analyzed for total concentration and metal forms. Nickel removal occurred to a depth of 20 cm, with about 60% of the nickel being organically bound.

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In 1992, two overland flow wetland treatment systems were built in existing natural wetlands in northeastern Minnesota to remove copper, nickel, cobalt and zinc from neutral mine drainage. Typical input metal concentrations ranged from 2-5 mg/L for nickel, to less than 0.1 mg/L for copper, cobalt and zinc. Flow rates were on the order of 75 L/min for both systems. The treatment systems covered 4200 m 2 and 7000 m' and contained a series of soil berms installed across the wetland, and about a 30 cm layer of a mixture of peat and peat screenings (a waste material generated during the processing of horticultural peat). Although these systems have been successful in removing about 70-90% of the input metals, output nickel concentrations in one of the wetlands exceeded the discharge standard by as much as a factor of four. Average flow rates were greater than the design value by a factor of two, and the wetland was unable to adequately treat this flow volume. In 1993 and 1994, changes made to reduce the hydraulic gradient and minimize channeling improved performance, but nickel concentrations still exceeded permit requirements during periods of high flow and during the fall as temperature decreased. In 1995, the mining company constructed an additional 10,000 m' of wetland to provide additional treatment. After the expansion, the discharge was in compliance with permit requirements, until high flows and decreasing temperatures in the fall caused nickel concentrations to exceed standards.

Although wetland treatment systems have been shown to be effective for treating both coal and metal mine drainage, the longevity of the treatment has always been a question. Data collected from a wetland in northeastern Minnesota suggests it may be possible to build a wetland that will provide long term treatment. A 7000 square meter overland flow wetland was built in 1992 to treat a mine drainage with an av798798erage pH of 7.2 and an average nickel concentration of 5.1 mg/L. Nickel removal exceeded 90% for the first three years of operation. In 1995, the stockpile which contributed the major input to the wetland was capped, and both flow and concentrations in the drainage were reduced. An intensive study was conducted on one section of the wetland where a large percentage of the overall removal was occurring. Nickel concentrations in the substrate reached 1.5% by weight and the calculated nickel mass in the substrate was about the same as the overall mass removal calculated from the water quality and flow data. Based on a model of substrate accumulation in wetlands, the wetland generates 7 kg of nickel removal capacity each year. Since the annual input of nickel has been reduced to around 10 kg, the projected lifetime of the wetland is about 300 years.