MEND Project 2.11.1a-b
March 1990

EXECUTIVE SUMMARY

The Benson Lake Coast Copper Mine operated from 1962-1973, during which 3.6 million tonnes of high grade copper ore was mined. A copper concentrate mill and, a magnetite recovery plant which operated from 1963-1970, discharged tailings under permit into Benson Lake. Over the period of operations, the tailings were discharged into progressively deeper waters to combat a continuing problem with lake turbidity caused by colloidal suspension of tailing fines. A flocculating agent was also added at the tailings pond to assist settlement.

Benson Lake is a reasonably small, deep, soft watered, and oligotrophic coastal lake. It is characterized by isothermal conditions during the winter, establishment of a thermocline over the summer, and a fall turnover. Field studies by the Environmental Protection Service in 1967 and 1973 found the entire lake bottom was covered by tailings and devoid of benthic life. In 1967, even the outlet river had a layer of fines on the bottom, and was populated by turbid-water benthic invertebrates. A bathymetry map prepared in 1970 showed the depth profile had not been significantly changed by tailings deposition.

The lake supports rainbow (Oncorhynchus mykiss) and cutthroat (0. clarki) trout, Dolly Varden char (Salvelinus alpinus) and sculpin (Cottus asper). Even after the mine was shut down, the fish concentrated their feeding on drift organisms carried into the lake by inlet streams. The resident fish were found in 1967 and 1973 to have high levels of zinc in their tissues, but equally high concentrations were found in a nearby control lake (Maynard Lake).

This study constitutes the initial stage of a field assessment of the lake's recovery from tailings disposal. The 1989-90 work included mapping the lake's bathymetry, and establishing staff and crest gauges and discharge sites on inlet and outlet streams. Inlet streams include Raging River, Benson River and Craft Creek, while the outlet is through the lower Benson River. Bathymetry mapping provided a series of Iimnological morphometric parameters. Stream profiles were prepared for discharge site cross-sections.

Benson Lake is 2.1 km long, with a maximum depth of 54 m (mean depth = 25.5 m), and a surface area of 7.73 x 105 M2. The relative depth (5.4%) and shoreline development factor (1.7) describe a small and deep elliptical lake with relatively high stability and only moderate potential for littoral zone development. Comparisons between present bathymetry and the depth profile from 1970 suggest the deeper portions of the lake (>50 m) have been partly filled.

Insufficient data are available to define hydrographic rating curves for each of the inlet and outlet streams. A comparison of the net inflow (9.5 m3/s) and outflow (8.5 M3/s) for November 3, 1989 suggested gauge and discharge sites with more laminar flows should be selected. Sites were checked and re-established as required on March 4, 1990.

Last Modified: 2003-11-26		Important Notices

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