ASSESSING METAL REMOVAL MECHANISMS IN OXIC PIT LAKES USING NUTRIENT ADDITIONS: MAINZONE PIT LAKE, HOUSTON BC

Pit lakes are increasingly common on the landscape of mine sites, appearing as open-pit mines fill with water upon cessation of mining. The MainZone pit lake near Houston BC is at the site of the former Equity Silver mine. This lake is several hundred meters across, 120 m deep and is a seasonally stratified, well oxygenated water body with near-neutral pH (stratification controls discussed by Pieters et al, this session). Surface waters of the pit are characterized by concentrations of Zn, Cd, Ni and Cu that far exceed levels expected for a pristine lake, especially during the period of spring melt, due to oxidation of metal sulfides during subaerial exposure. In order to both test possible controls on seasonal variability in metal concentrations and to examine possible avenues for remediation, nutrient addition experiments were conducted during the summer of 2002 within 2-m diameter experimental enclosures, or limnocorrals. Despite elevated initial surface-water metal concentrations (e.g. [Zn] ~1000 ug/L; [Cd]~10 ug/L)) eutrophic conditions were induced in just over a week by addition of ~10^-7 mol P/m^2/week. The chl a concentrations achieved in the surface waters of the nutrient-amended enclosures (~10 ug/L) are more than an order of magnitude higher than observed in the controls yet lower than those observed following comparable P additions to pristine lakes (Schindler et al., 1973). Following 3 months of weekly nutrient additions,roughly 90%, 50% and 20% removal was observed for Zn, Cd, and Ni, respectively. Sediment trap samples are being analyzed to confirm that metal removal occurred via some combination of active metal uptake by the algae and/or adsorption to settling particulate matter. These results suggest that addition of phosphorus to P-limited, neutral-pH pit lakes constitutes a promising approach, in many cases, towards reducing metal concentrations.