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 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.