Constructed wetlands for treatment of mine drainage

Precious and Base-metal mines

The first "passive treatment system" reported in the literature doesn't even involve wetlands! Gale and Wixson reported in 1979 that AMAX constructed a series of meanders which removed metals from the effluents of the Buick Mine-Mill Complex. The treatment process relied on algae and aquatic vegetation to remove lead and zinc from the effluents. This treatment process operated successfully for at least 7 years, protecting Strother Creek from metal contamination.

It is only recently that wetlands constructed to treat drainage from base metal mines or gold mill effluents have been considered viable solutions. For example, pilot-scale wetland cells were constructed in the late 80's at an abandoned mine in Idaho Springs, Colorado to treat a highly acidic discharge containing copper, iron, manganese and zinc. (Howard et al., 1989). Information on the Big Five Tunnel site is available on the net. The latter system is more a hybrid wetland/bioreactor, in which the flow of water is directed vertically through an organic substrate. Another example of this type of system is being tested at the Leviathan Mine near Markleeville, California.

The ERA Ranger uranium mine in Australia has constructed pilot and full-scale wetlands to remove manganese and uranium from its discharge (see diagram). Performance data for uranium removal from the pilot wetland are shown below (Jones et al., 1996). Uranium was removed by 50% in the full-scale system in its first year of operation. However, performance will improve as the system matures.

Uranium and manganese removal from pilot wetland at ERA Ranger mine.

The most ambitious effort to date is the development of a passive treatment system to treat millions of Litres of metal-contaminated water pouring daily from a disused Cornish tin mine. A pilot-scale system was developed in 1993 on 11 acres of land at the defunct Wheal Jane mine, near Truro. It consists of anoxic limestone drains, ponds, and aerobic and anaerobic wetland cells. Preliminary results indicate that it is effective in neutralizing the acidic discharge and removing metals (Dodds-Smith et al., 1995).

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