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UBC Theses and Dissertations

Limnology of the Meromictic Island Copper Mine pit lake Fisher, Timothy Simon Richmond


The BHP Billiton owned Island Copper Mine pit near Port Hardy, Vancouver Island, BC, Canada, was flooded in 1996 with seawater and capped with fresh water to form a meromictic pit lake (maximum depth 350 m and surface area 1.71 km ). The pit lake is being utilized as a passive treatment system for acid rock drainage (ARD). Conductivitytemperature- depth profiles (July 1997 to March 2001) document the evolving structure of the pit lake. Intensive measurements with two thermistor "chains" and a meteorological station (December 1999 to November 2000) enabled physical processes to be investigated. The pit lake has developed into three distinct layers: a brackish and well mixed upper layer; an intermediate layer circulated by ARD plumes; and a thermally convecting lower layer. The upper halocline has risen due to the injection of buoyant ARD into the base of the intermediate layer and entrainment of the halocline by the impinging ARD plume. The lower halocline fluctuated seasonally due to the balance between ARD plume erosion and lower layer thermal convection. The true salinity was determined from the Practical Salinity Scale 1978 plus a time and layer dependent correction for non-seawater ions sourced from the ARD and submerged pit walls. The plume stirred intermediate layer was analogous to the filling box model (Baines and Turner, 1969), which explains the observed salinity and counter-stable temperature gradients. Lake length and width internal waves were identified at the upper halocline and their periods agreed with predictions. These internal waves appeared to degenerate by damping, not by non-linear steepening as predicted by theory. Conceptual models used to model the long-term evolution of the pit lake identified upwelling and/or erosion of the intermediate layer as potential failure modes. Clear from the conceptual models was the importance of maintaining the density stratification for the longterm stability of the meromictic structure. Detailed modeling with a one-dimensional process model DYRESM replicated some details of the pit lake, but did not adequately model wind mixing, nor did it allow for plume entrainment of the upper halocline.

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