Abstract: In 1991, LTV Steel Mining decided to close an open-pit taconite mine in northeastern Minnesota using a passive-treatment approach consisting of limiting infiltration into the stockpiles and wetland treatment to remove metals. More than 50 Mt (55 million st) of sulfide-containing waste had been stockpiled adjacent to the mine during its 30 years of operation. Drainage from the stockpiles contained elevated levels of copper, nickel, cobalt and zinc. Nickel is the major trace metal in the drainages. Before the closure, the annual median concentrations ranged from 1.5 to 50 mg/L. Copper, cobalt and zinc are also present but they are generally less than 5% of the nickel values. Median pH levels range from 5 to 7.5, but most of the stockpile drainages have pH levels greater than 6.5. Based on the chemical composition of each stockpile, a cover material was selected. The higher the potential that a stockpile had to produce acid drainage, the lower the permeability of the capping material required. Covers ranged from overburden soil removed at the mine to a flexible plastic liner. Predictions of the reduction in infiltration ranged from 40% for the native soil to more than 90% for the plastic liner. Five constructed wetlands have been installed since 1992. They have removed 60% to 90% of the nickel in the drainages. Total capital costs for all the infiltration reduction and wetlands exceeded $6.5 million, but maintenance costs are less than 1% of those for an active treatment plant. Because mine-drainage problems can continue for more than 100 years, the lower annual operating costs should pay for the construction of the wetland-treatment systems within seven years.

Abstract: Much work has been undertaken on the design of treatment systems to remove iron from ochreous mine water discharges. Unlike iron, manganese removal is far more difficult and generally requires active chemical dosing rather than passive treatment. The need for manganese removal can therefore significantly change the economics, management attention and sustainability of a site. Understanding natural attenuation of manganese in river systems is therefore key to deciding whether (active) manganese treatment is needed to protect downstream receptors. Nuttall (2002, this volume) describes the effectiveness of the passive treatment system at Whittle in reducing both iron and manganese concentrations in ochreous mine waters. This paper discusses the results of in-river monitoring and provides evidence for manganese removal downstream of the discharge point. In addition to dilution, attenuation appears to be in the order of 20 to 50%, depending on relative rates of mine water discharge and river flows. Such attenuation means that active treatment may not be needed for the long-term operation of the Whittle scheme. Operation of the scheme commenced in July 2002, with monitoring to further examine evidence for manganese attenuation and any impact on the ecology of the recipient watercourses.