Tale of two pit lakes: Initial results of a three-year study of the Main Zone and Waterline pit lakes near Houston, British Columbia, Canada

摘要

The Main Zone and Waterline pit lakes at the Equity Silver Mine near Houston, BC, Canada, are the sites of a three-year research program. Its primary goals are to understand the controls on metal concentrations in the lakes. The program used mesocosms to test a variety of remediation strategies and verify and improve an existing coupled physical-geochemical model to improve predictions of pit lake water quality. Initial data from two summer surveys in the Main Zone pit lake reveal elevated dissolved metal concentrations (Zn as high as 800 mu g/L) in a 2-m- (6.6-ft-) deep mixed layer and reveal elevated particulate metal concentrations towards the bottom of the water column. Hydroxide sludge, a byproduct of acid rock drainage (ARD) treatment, is being discharged to the surface waters of the Main Zone pit lake. The sludge sinks rapidly to the lake bottom, which leads to deep waters with elevated levels of dissolved O_2 and total metals. By contrast, the deep waters of the proximal Waterline pit lake are mildly reducing, manifested by depletion of O_2 and elevated levels of dissolved Fe. These reducing conditions have led to elevated levels of As (up to 1,5000 mu g/L) in Waterline pit deep waters. The contrasting behavior of these two pit lakes, as well as the processes controlling metal concentrations in the lakes, is discussed.