MEND Project 2.11.1c-b
June 1992

EXECUTIVE SUMMARY

Pyrite-rich mine tailings from the Coast Copper mine were discharged into Benson Lake from 1962 to 1973. The tailings in the central basin of the lake are now covered by about 30 cm of natural organic-rich sediments containing about 7 wt. % organic carbon. The relatively high C:N weight ratio (~22) indicates that the organic matter is largely of terrigenous origin. Nearby Keogh Lake, which was sampled to provide a control site, is in a different watershed and never received input of any mining-related discharges. Sediments in the centre of this basin contain ~11-22 wt. % organic carbon. The organic fraction had a C:N weight ratio ranging from 16-20 suggesting an origin which was partly terrigenous and partly planktonic.

One sediment core from each lake was collected (in August, 1991 in Benson Lake, and November, 1991 in Keogh) and processed under nitrogen to extract interstitial waters. Solid phase elemental analyses and dissolved metals and sulphate determinations were made on sediment samples and pore waters, respectively . The Benson core consisted of about ')O cm of "natural" (but still copper-enriched) sediments overlying Cu-bearing but Zn- and Pb-deplete, sulphide-rich tailings. The Keogh Lake core consisted or organic-rich natural deposits punctuated by a six-centimetre thick grey clay layer between about 24 and 30 cm depth. A thin veneer of manganese and iron oxyhydroxide-rich material mantles the sediments in both basins. In Keogh, this layer appears to be at most 5 mm thick, in Benson, it may be slightly thicker.

High dissolved iron concentrations (relative to bottom water) at 1.5 cm depth in the Keogh deposits and below 1.5 cm in the Benson sediments indicate that the natural sediments are suboxic or anoxic at very shallow sub-bottom depths in both lakes.

Dissolved metal concentrations are low in the bottom waters of both lakes, being approximately equal to or less than B.C. MOE and/or CCREM guidelines. Sulphate levels are also very low, being <10 mol/L (0.09 mg/L) in the bottom waters of both basins.

High-resolution profiles of dissolved Zn, Pb and Cd in the pore waters of both lakes show that concentrations of these metals decrease across the sediment-water interface, and are invariably lower than the levels measured in core-top (bottom) water. These data confirm that at the time of sampling there was no efflux of these metals to the overlying water in either basin. Profiles of the dissolved copper concentration in pore waters suggest that some Cu may be diagenetically cycled just below the sediment-water interface in the Keogh deposits. Concentrations are low, however, and there was no indication of a benthic efflux of the metal. The Benson Lake profile yields a similar conclusion: there is no evidence of a benthic efflux of dissolved copper from the sediments to bottom waters in the lake despite the fact that the "natural" sediments accumulating at present, and progressively burying the tailings, still contain higher concentrations of solid-phase copper than would be expected in a pristine basin.

Last Modified: 2003-11-26		Important Notices

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