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The biogeochemistry of the Equity Silver Mine pit lakes Whittle, Phillip

Abstract

Between June 2001 and June 2003 a comprehensive two year study of the biological, geochemical and physical controls on trace metal behaviour in two mine pit lakes was undertaken at the Equity Silver Mine, British Columbia, Canada. Three major open pit developments occurred at this site generating the Main Zone, Waterline and Southern Tail pits. Mining operations at the Equity Silver Mine finished in 1994 and the Main Zone and Waterline pits have both since filled with water while the Southern Tail pit has been back filled with waste rock and is not considered in this study. Though located only metres apart, the two study lakes (Main Zone and Waterline) display considerably different physical and biogeochemical processes. The Main Zone pit lake (MZP) is polymictic, mesotrophic, well oxygenated year round and, for most of the year, contains relatively low concentrations of dissolved (< 0.45 µm) metals. The Waterline pit lake (WLP) on the other hand is meromictic, oligotrophic, anoxic at depths below 10 m (maximum lake depth is 40 m) and contains elevated concentrations of dissolved arsenic (2 ppm), iron (45 ppm), zinc (0.8 ppm) and manganese (4 ppm) in the hypolimnion. The geochemical differences between the two pit lakes are driven largely by the deposition of acid rock drainage (ARD) treatment (lime neutralization) sludge into the MZP by PDC-Equity, the mine operators. The waste from an ARD treatment plant is deposited into the MZP, typically during the ice-free periods from May through to November. Due to a high suspended solids content (5 to 8 % by volume), the sludge creates a density current as it sinks through the water column. This imparts physical mixing energy to the pit lake as well as dehvering dissolved oxygen to the bottom waters. Both of these factors promote a well oxygenated water column in the MZP with most metals either precipitating as, or coprecipitating with, oxyhydroxide solid phases. Unlike the majority of metals, dissolved Mn is introduced to the MZP through sludge deposition. This subsequently precipitates from the water column as a solid phase, likely MnO[sub x] , during the sludge free periods. As opposed to the MZP, the geochemistry of the Waterline pit lake is dominated by a permanent pycnocline at 4 to 10 m depth (varying seasonally) with significant mixing restricted to the epdirnnion (mixolirnnion). Sufficient biological / chemical oxygen demand coupled to the restricted transport of dissolved oxygen across the pycnocline, promotes reducing conditions below the epilimnion and elevated dissolved metal concentrations (particularly Fe) through the dissolution of oxyhydroxide solids. A combination of low rates of sulphate reduction and Fe-oxyhydroxide precipitation from the pycnocline operates to produce what appears to be a mixed pe system in the bottom waters / sediments of the WLP. Concentration profdes suggest that arsenic is predominantly controlled by adsorption to and dissolution from Fe-oxyhydroxide solid phases. Free dissolved sulphide was not detected in the water column but removal of Cu, Cd and Sb in the hypolimnion suggests their precipitation as sulphide minerals. A series of limnocorral (in situ mesocosm) experiments were conducted in both the MZP and WLP to test biologically induced metal removal strategies. Only the WLP experiments are presented as part of this thesis. Algal growth and dissolved metal removal (through algal uptake / and / or adsorption) from epilimnetic waters of the WLP was induced with nutrient additions (N and P) to the corrals. Biological sulphate reduction and precipitation of insoluble sulphide mineral-forming metals (Cd, Zn, Sb, Co, Ni) was induced through ethanol additions to the hypolimnia of these corrals. Mn removal from the epilimnion was also induced though only after the algal growth period, possibly though bacterially catalyzed Mn oxidation. The results have significant implications for the long-term management of pit lake water quality. In situ stimulation of phytoplankton growth has the potential to remove on a nearquantitative scale concentrations of zinc and other metals. The stimulation of sulphate reduction at depth in meromictic pit lakes offers a mechanism to sequester dissolved metals into underlying sediments, on a permanent basis if stable stratification can be guaranteed.

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