Evolution of the Island Copper Mine pit lake
Creators: name: Wilton, Mike
Publisher: name: University of British Columbia
Year: 2009

description: The Island Copper Mine is located on the north shore of Rupert Inlet on Vancouver Island, British Columbia. During the twenty-five years of the mine's operating life, an approximately 400 metre deep open pit was excavated. In the summer of 1996 shortly after the mine ceased operations, the pit was flooded with seawater by digging a channel to Rupert Inlet. The innovative Island Copper Mine closure plan proposes that the flooded pit be used as a passive treatment system for acid rock drainage (ARD) that is generated from surrounding waste rock piles. The ARD is injected into the pit lake where it rises as a buoyant plume through the more dense salt water. In addition to dilution and neutralization, the plan is to treat the ARD by metal-sulphide precipitation. This expected precipitation of metal-sulphides will be driven by anaerobic sulphate-reducing bacteria (SRB). SRB, which oxidize simple organic compounds, respire sulphate and produce hydrogen sulphide. As the success of this passive treatment system is dependent on a stable stratification within the lake, various models were developed before the pit was flooded in an attempt to predict how this system would evolve. Monitoring of the water quality in the pit lake has been carried out since the pit was flooded. Samples collected by BCL Biotechnologies Ltd. have been analyzed for salinity, dissolved oxygen, dissolved metals, pH, and alkalinity. Temperatures have been measured in-situ using reversing thermometers. In addition, the Environmental Fluid Mechanics Group has visited the site five times with a Conductivity-Temperature-Depth meter (CTD) and dissolved oxygen probe. The CTD yielded continuous profiles of temperature, salinity, and dissolved oxygen through the water column. The pit lake has evolved into three distinct layers as predicted by the models: a brackish upper layer extending down to a depth of approximately 5-10 m; a well mixed intermediate layer extending down to the depth of the ARD discharge (~ 224 m); and a quiescent lower layer extending from approximately 224 m to the bottom of the pit. Examination of the average temperature, salinity, dissolved oxygen, and dissolved metals in the upper, intermediate, and lower layers has revealed how the pit lake has evolved to date. As of March, 1998, the dissolved oxygen of the lower layer has decreased to below 1 mg/L . The dissolved oxygen in the intermediate layer has decreased at a rate approximately half that of the lower layer to 4.4 mg/L. A number of factors may have contributed to the higher concentration in the intermediate layer including: a lower consumption than in the lower layer; the injection of DO with the ARD; and the transport of oxygen from the upper layer to the intermediate layer. To predict the future evolution of the lake water properties, a simple two layer box model was developed. The upper and intermediate layers were modeled as "continuously stirred reactors". The model output shows that without the precipitation of copper by the sulphate reducing bacteria or uptake by organisms, copper concentrations in the surface water of the lake may exceed the discharge requirements due to mixing from the intermediate layer into the upper layer. Results to date have been compared with the predictions of the model developed by Dunbar (1995). Recommended refinements to future models include the use of the dynamically relevant potential density rather than the absolute density and the incorporation of mixing across the interface between the upper and intermediate layers due to the re-direction of the buoyant plume. Further study is required before the long term behaviour of the pit lake can be predicted with certainty. In particular, the transfer of dissolved oxygen across the interface between the upper and intermediate layer should be investigated as this may hinder the development of anoxia in the intermediate layer.