A PRELIMINARY
BIOLOGICAL AND GEOLOGICAL ASSESSMENT OF SUBAQUEOUS TAILINGS DISPOSAL
IN BENSON LAKE, BRITISH COLUMBIA
Mine Environment Neutral Drainage at CANMET-MMSL |
MEND Project 2.11.1c-a
March 1991
EXECUTIVE
SUMMARY
Available
evidence suggests that the chemical and biological reactivity of
mine tailings is inhibited by storage underwater and thus appears
to be a promising alternative to the less environmentally benign
practice of land disposal. While impacts on the biotic community
resulting from subaqueous disposal have been documented, the duration
of these impacts appears to be transitory. To assess the above,
we examined the water quality, geochemical conditions of the sediments
and biotic communities in Benson Lake near Port Alice, British Columbia.
Benson Lake had been used as a tailings repository for approximately
eleven years prior to cessation of operations in 1973.
Benson Lake
is a small, deep, oligotrophic coastal mountain lake situated in
the coastal western hemlock biogeoclimatic zone on the northern
end of Vancouver Island. During the period August 1962 through January
1973, the Benson Lake Coast Copper Mine, operated by Cominco Ltd.
disposed of its mine tailings by depositing them, under permit,
into the deep basin of the lake. As a result, the lake was subject
to extended periods during which lake turbidity levels were elevated
beyond levels normally found in the lake. Furthermore, tailings
fines smothered the lake's profundal sediments thereby eradicating
all traces of benthic invertebrate life. Tailings fines were also
found in the Lower Benson River. Finally, tailings deposition also
resulted in elevated levels of zinc, relative to background control
levels, in the water and in the flesh of fish taken from the lake.
In 1973, Cominco
Ltd. ceased operations and halted their tailings disposal activities.
A subsequent evaluation of the lake, conducted 10 months later,
found that colloidal tailings material in the lake's waters had
settled and that Benson Lake's clarity had returned to premine conditions.
Also, the elevated levels of zinc noted in the water column during
active tailings deposition had subsided. However, it was noted that
the profundal zone of the lake was still devoid of invertebrate
life.
In September
1990, more than 17 years after cessation of tailings disposal, it
was found that Benson Lake showed little evidence of the fact that
it was the recipient of mine waste. Physical and chemical water
quality sampling conducted at three stations in the lake indicated
that lake water was similar in virtually all respects to the waters
of a nearby control lake, Keogh Lake. Some differences were noted,
however, the differences were attributable to inherent characteristics
of Benson Lake's drainage basin and to the presence of a fish farm
sighted in the control lake. Benson Lake was characterized by higher
conductivity, total dissolved solids, alkalinity, calcium and potassium
than the control lake, but the levels of each of these parameters
reflected their levels in the water flowing into the lake via the
Benson and Raging Rivers and Craft Creek. The fish farm on Keogh
Lake appeared to be eliciting a fertilization effect that resulted
in higher levels of nutrients (phosphorous and nitrogen) and plankton
growth in the control lake compared to Benson Lake.
Lake and surface
samples of tailings-rich sediments were collected and examined in
detail. Metal and petrographic analyses of lake samples indicated
that tailings are areally widespread in the lake. A surficial organic
layer is accumulating over the tailings which may be helping to
prevent benthic effluxes of metals to the overlying water column.
Sequential extractions of tailings-dominated lake and land tailings
samples revealed that underwater samples did not release any significant
quantities of metals from the water-soluble or exchangeable cation
phases. In contrast, metals were released in the water-soluble phase
of the land tailings sample. Most of the metals appeared to be associated
with primarily the unreactive residual phase in both samples. However,
a significant quantity of metals were associated with the oxidizable
phase (organs-metal complexes, metal sulphides) in the lake tailings
sample which was not apparent in the land tailings sample. These
preliminary results suggest that the chemical reactivity of the
underwater tailings is minimal and that their presence is not degrading
the biochemical environment of Benson Lake.
The biota
of Benson Lake was examined in considerable detail. It was found
that the benthic invertebrate community in the lake had re-established
itself to reflect the community structure and organism density typical
of oligotrophic lakes throughout Canada and the world. Net phytoplankton
densities and community structure were similar to densities and
assemblages found in the control lake and in other coastal mountain
lakes in British Columbia. Zooplankton species compositions in Benson
Lake were also similar to those in the control lake, but their densities
were significantly lower in both lakes than in other oligotrophic
coastal lakes.
Aquatic vegetation
was well established in the littoral zone of the lake particularly
along its southern and eastern shorelines. Compared to the control
lake, aquatic vegetation in Benson Lake was found to contain elevated
levels of arsenic and copper. arsenic accumulated in both the tops
and roots of horsetail (Equisetum sp.) and pond weed (Potamogeton
sp.), while copper only accumulated in the roots of horsetail
and in both the tops and roots of pond weed.
Fish sampling
confirmed the presence of rainbow trout in both Benson and Keogh
Lakes, a species of char in Benson Lake, and cutthroat trout in
Keogh Lake. Fish from Benson Lake were significantly larger and
had significantly higher condition factors than fish from the control
lake. In addition, the concentrations of metals in the flesh of
fish from Benson Lake were lower than body metal burden in fish
from the control lake, but the concentrations of metals in the livers
were higher. However, the concentrations of all metals in the fish
from both lakes were within the range of concentrations for the
same metals in fish tissues and livers from unpolluted Canadian
waters. The stomach contents of fish from Benson Lake suggested
that the fish in the lake were incorporating the re-established
benthic invertebrate community in their diet.
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