EXECUTIVE SUMMARY
The Myra Falls (British Columbia) mine site study is one of four field evaluations carried out in 1997 under the Aquatic Effects Technology Evaluation (AETE) Program, a joint government-industry program to evaluate the cost-effectiveness of technologies for the assessment of mining-related impacts in the aquatic environment. The other three mines studied were Dome (Ontario), Mattabi (Ontario) and Heath Steele (New Brunswick). Results
of all four studies are summarized and evaluated in a separate summary report.
The Myra Falls operations of Boliden (Westmin) are located in central Vancouver Island, and produce base metal concentrates (zinc, copper; lead) as well as gold and silver. The operations discharge treated effluent to Myra Creek and seepages from other sources at the mine reaches Myra Creek, which flows into Buttle Lake, a large, deep impoundment in the Campbell River watershed. The mine historically discharged tailings into the south end of Buttle Lake (until the mid-1980s).
The objectives of the 1997 field program were to test 13 hypotheses formulated under four guiding questions:
- are contaminants getting into the system (and to what degree and in which compartments)?
- are contaminants bioavailable?
- is there a measurable (biological) response? and
- are contaminants causing the responses?
The hypotheses are more specific questions about the ability or relative ability of different monitoring tools to answer these four general questions about mine effect. The evaluation of tools included: sediment monitoring (sediment toxicity tests); fish monitoring (tissue metallothionein and metal analyses, and population/community indicators), and; integration of tools (relationships between exposure and biological responses and use of effluent sublethal toxicity).
Of the 13 hypotheses, 6 were tested at Myra Falls as outlined in Table 1.1. The remaining seven hypotheses not tested at Myra Falls related to responses in fish. Fish sampling was not included at Myra Falls because it was concluded that the site conditions were less optimal to test fish hypotheses than at the other three mine sites tested in 1997.
The sediment quality triad was used as an additional means of evaluating the linkages between sediment toxicity, sediment chemistry and benthic community response (H10 and H11) in Buttle Lake. The triad provides a more holistic means of evaluating the tools.
Study Design
A reconnaissance survey was carried out in Myra Creek and Buttle Lake to assess the feasibility of collecting fish and benthos in Myra Creek, to identify metal concentration gradients in Buttle Lake sediments, and to assess the abundance of benthic invertebrates in the profundal zone of the lake. The final study design was formulated based on the results of this reconnaissance.
The study design at Myra Falls was based on lake sampling for benthos, sediment chemistry and sediment toxicity using a nearfield-farfield-reference design. The nearfield area was in southern Buttle Lake, the farfield in northern Buttle Lake, and nearby Brewster Lake served as a reference. Seven stations were sampled within each of the three areas.
Sampling in Myra Creek followed a reference-exposure (Control-Impact) design, and allowed for qualitative testing of benthos-water quality and effluent toxicity-benthos hypotheses. Ten stations were sampled for benthos in ruffle areas within each of the two sampling areas.
Sampling Program
The field survey at Myra Falls was completed in mid-September 1997, and included:
- water sampling in Myra Creek and Buttle Lake in each sampling area for determination of dissolved (0.45 micron filtered) and total metal concentrations. Only the Myra Creek water quality data were used in hypothesis testing;
- surficial sediment sampling at 21 profundal lake stations (3 areas) using a petite Ponar, for determination of “total” metal concentrations, partial metal concentrations (i.e., the Fe and Mn oxide-bound fraction) and concentrations of acid volatile sulphide (AVS) and simultaneously extracted metals (SEM);
- surficial sediment sampling at the above 21 stations for benthic macroinvertebrate community analysis and for sediment toxicity testing Hyalella azteca survival and growth, Chironomus riparius survival and growth, Tubifex tubifex survival and growth);
- benthic macroinvertebrate sampling in stream riffles at ten effluent-exposed stations and ten reference stations in Myra Creek using a T-sampler; and
- testing of chronic effluent toxicity, based on four samples of final effluent from the mine. Tests included Ceriodaphnia dubia survival and reproduction, fathead minnow survival and growth, Selenastrum capricornutum growth and Lemna minor
Data Overview
Water Quality
Zinc and copper concentrations in Myra Creek were greater than Canadian Water Quality Guidelines (CWQGs) downstream of the mine, and indicated a metal source from Myra Falls. Much of the metal loading appeared to originate from sources other than the treated effluent.
Concentrations of zinc were also elevated in the nearfield area of Buttle Lake, with maximum values approximating the CWQG value. Metal concentrations were lower in the farfield and lowest in the reference area.
Dissolved and total metal concentrations showed similar spatial patterns and generally similar values for key metals (Zn, Cu, Cd). There was some evidence of minor sample contamination in dissolved metal samples.
Sediment Chemistry
Sediment total metal concentrations were highest in the nearfield, lower in the farfield and lowest in the reference area (Zn, Cu, Cd, Pb, As). Concentrations of all of these metals exceeded the Canadian Interim Sediment Quality Assessment Values; especially in the nearfield area where concentrations were greater than the probable effect level (PEL) values. Partial metal concentrations followed a similar spatial pattern, although the partial metal fraction generally accounted for a relatively small portion of the total concentrations.
The SEM/AVS molar ratios in sediments were highly variable within areas, and were generally greatest at nearfield stations, lower at farfield stations, and lowest at reference stations. The results implied that nearfield sediments, and to a lesser extent farfield sediments, are potentially toxic to sediment-dwelling organisms.
Sediment Toxicity
Nearfield, farfield and reference lake sediments showed different degrees of toxicity to Chironomus, Hyalella and Tubifex. Nearfield sediments were toxic to the former two species in terms of survival and growth. Hyalella also showed a survival and growth response in farfield sediments relative to the reference site. No response was seen in Tubifex survival, and reproductive responses to nearfield and farfield sediments were minor.
Benthic Macroinvertebrates
Benthic macroinvertebrates did not respond to exposure to metal-enriched sediments in terms of densities of organisms, numbers of taxa present or the abundance of chironomids. Harpactacoids and Pisidium, however, were nearly absent in the exposure area (Buttle Lake) but were common in the reference area (Brewster Lake).
Reference-exposure differences in Myra Creek benthos were relatively small, and included slightly reduced organism densities, numbers of taxa and numbers of sensitive EPT taxa (Ephemeroptera-Plecoptera-Trichoptera) at the exposed stations.
Effluent Toxicity
Myra Falls effluent was non-toxic to fathead minnow. Chronic IC25 values were similar for Ceriodaphnia, Selenastrum and Lemna. That is, reproduction (Ceriodaphnia) or growth (the other species) was inhibited by 25% when exposed to 35% to 45% effluent on average.
Hypothesis Testing
Hypothesis testing results are summarized in Table 5.2. Results of testing indicate that measurable biological responses occur at Myra Falls and that contaminants (metals) appear to cause these biological responses.
Technology Evaluation
Overall, most of the monitoring tools evaluated at Myra Falls were effective in demonstrating a mine effect, with the exception of the fathead minnow chronic toxicity test and the SEM/AVS analysis. Of the tools that were effective, some were slightly more effective than others as predictors of biological response. A summary of the effectiveness of various monitoring tools tested at Myra Falls is presented in Table 6.2. Table 6.3 provides a comparison of the effectiveness of similar tools in measuring aquatic effects at Myra Falls.
Conclusions on the cost-effectiveness of the tools based on results from all four mine sites studied in 1997 are found in a separate document “Summary and Cost-Effectiveness of Aquatic Effects Monitoring Technologies Applied in the 1997 AETE Field Evaluation Program.”
