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.
