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Physical limnology of the Equity Mine pit-lakes Leung, Albert

Abstract

Two adjacent pit-lakes - Waterline and Main Zone - are located at the Equity Silver Mine site near Houston, British Columbia, Canada. The upstream Waterline is the smaller of the two pit-lakes at 43 m deep and 460 m long, while the Main Zone Pit-lake is larger at 120 m deep and 710 m long. The Waterline Pit-lake drains into the Main Zone Pit-lake through the Waterline Outflow. Treated acid rock drainage (ARD) water and sludge are being discharged into the Main Zone. Water is also withdrawn from the Main Zone Pit-lake at a depth of 20 m and discharged to nearby creeks. A study of the physical limnology of the two pit-lakes was undertaken during 2001 and 2002 in conjunction with a study of their chemistry. Marked differences between the two pit-lakes in terms of the intra-annual and inter-annual evolution were observed. The Waterline Pit-lake is meromictic, while overturn occurred at least once a year in the Main Zone Pit-lake during the observation period. Anoxia was observed in the Waterline Pit-lake below the epilimnion, likely the result of chemical oxygen demand and lack of oxygen replenishment. On the other hand, the Main Zone Pitlake was always well oxygenated due to the circulation generated by overturn and the sludge discharge. Turbidity in the Waterline Pit-lake occurred mainly at the base of the epilimnion due to the formation of oxidation precipitates, whereas in the Main Zone Pitlake, the treated ARD and sludge discharge generated turbid water near the bottom. Phytoplankton were absent in the Waterline Pit-lake, but near surface peaks in chlorophyll a (chl a) were observed in the Main Zone. Preliminary investigation indicates that upwelling was not likely in the Waterline Pit-lake, but possible in the Main Zone. Conductivity-temperature-depth (CTD) and thermistor data revealed possible groundwater intrusion into the Waterline Pit-lake and portrayed the impact of the accidental sludge discharge to the Waterline. Destratification in the Main Zone Pit-lake was observed in summer of both 2001 and 2002. It may have been due to wind-driven upwelling or alternatively due to sludge discharge. A more detailed heat and salt budget is needed to determine the exact cause(s) of destratification.

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