The Use of Overland Flow Wetland Treatment Systems1 to Remove Nickel From Neutral Mine Drainage

Abstract

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.

The Use of Wetlands to Remove Nickel From Mine Drainage - Is Perpetual Treatment Really Possible?

“…If the treatment is to be sustainable and effective, not only must there be new metal removal capacity generated, but the metal must be retained within the wetland. Mass balances calculated on wetland test cells demonstrated that over 99% of the removed metals were associated with the 817 substrate and less than 1% of the total removal occurred in the vegetation (Eger et al, 1994). These results were consistent with earlier studies on metal removal in a white cedar wetland (Eger and Lapakko, 1988) and with data reported by others (Skousen et al, 1992, Wildeman et al, 1993.…”

“…The ultimate goal of a passive treatment system is to provide permanent treatment. The major mode of metal removal in this system is assumed to be the same as observed in the initial test cell study (Eger et al, 1994).…”

“…organic fraction of the peat. With these types of removal mechanisms, the wetland will have a finite life unless new removal sites can be generated at a rate greater than or equal to the incoming metal load (Eger et al, 1994).…”

“…Nickel contained within the substrate of the study cell accounted for essentially all of the total nickel removal that was calculated from the change in water quality data. Sequential extraction tests, conducted on a series of substrate samples collected from test cells constructed at the Dunka Mine, demonstrated that only 1-2% of the nickel was water soluble and could, therefore, be easily removed from the substrate (Eger et al, 1994). Figure 4.…”

“…The company's preferred option was a combination of passive alternatives which would reduce flow emanating from the stockpiles, and use wetland treatment to remove metals from the resulting drainage. In 1988, four overland flow test cells were built to investigate methods to optimize metal removal and to provide design data 800 for the ultimate implementation of wetland treatment at this facility (Eger and Lapakko, 1989;Eger et al,1991Eger et al, , 1993Eger et al, , 1994. Based on the results of this study, two full-scale wetland treatment systems were built in March of 1992 (Eger et al, 1996).…”