| Summary: | 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. === Applied Science, Faculty of === Civil Engineering, Department of === Graduate
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