Third national assessment of environmental effects monitoring data from metal mines: chapter 6


6.0 Key Findings

Prevalence of Effects

Most mines (62/82 or 76%) that completed studies to assess effects confirmed the presence of at least one effect, and half of the mines reporting effects (32/62 or 52%) confirmed effects on both fish and fish habitat. One mine confirmed the absence of effects on fish, fish habitat and the use of fisheries resources. Another mine observed an effect on the use of fisheries resources (mercury in fish tissue), which it attributed to historical contamination of the exposure area rather than current mine effluent.

For fish specifically, 66% (44/66) of mines that completed studies to assess effects confirmed at least one effect on fish and three mines confirmed an absence of effects on all fish indicators. Changes (increases and decreases) in the relationship between body weight and length, known as body condition, was the most prevalent fish effect observed, but changes in survival, growth, reproduction and liver condition were also observed. Survival, growth and liver condition indicators were more often larger in the area exposed to the effluent and reproduction and body condition indicators were more often smaller in the exposure area.

When considering fish habitat, 64% (52/81) of mines undertaking studies to assess effects confirmed at least one effect on fish habitat and three mines confirmed an absence of effects on all fish habitat indicators. The most prevalent confirmed fish habitat effect was a change in benthic invertebrate community structure (measured using the Bray-Curtis Index), followed by a decrease in taxon richness, changes in density (increases and decreases) and changes in the distribution of individuals among the different taxa (measured with an evenness index). The number of mines with a confirmed increase in density in the area exposed to effluent was higher than the number of mines with confirmed decreases in density in the exposure area. Confirmed effects on evenness consisted equally of increases or decreases in the area exposed to effluent. The comparative analysis, described in section 2.3, confirmed that there were no observations of false positive effects resulting from the use of the methodology specified in the Environmental Effects Monitoring technical guidance document (Environment Canada 2012a) for calculating the statistical significance of differences observed in the Bray-Curtis index.

Magnitude of Effects

Almost all mines (57/62 or 92%) with confirmed effects observed at least one effect of a magnitude that may be indicative of a higher risk to the environment.Footnote20 Sixty-four percent (28/44) of mines with confirmed fish effects and all but one of the mines with confirmed fish habitat effects, observed at least one effect of a magnitude that may be indicative of a higher risk to the environment. Confirmed effects on fish reproduction and liver condition and on all fish habitat indicators were more often of a magnitude that may be indicative of a higher risk to the environment.

Type of Effects

The types of effects identified by this third national assessment were similar to those identified by previous national assessments published by Environment and Climate Change Canada (Lowell et al. 2008, Environment Canada 2012b). Effects on growth, survival, reproduction and the condition of the body and liver were observed in fish from exposure areas. Fish habitats exposed to effluent had experienced a change in benthic invertebrate community structure and the number of individuals and species present (Table 2).

Table 2. Summary of confirmed effects observed on fish and fish habitat in aquatic environments receiving metal mine effluent

Fish
Effect Indicator Compared to Reference (number of mines) Compared to Reference (number of mines)
body condition fatter fish (10) thinner fish (16)
liver condition larger livers (9) smaller livers (6)
reproduction larger gonads (8) smaller gonads (11)
growth faster growing (12) slower growing (9)
survival older fish (10) younger fish (7)
Fish Habitat (benthic invertebrate community)
Effect Indicator Compared to Reference (number of mines) Compared to Reference (number of mines)
density more individuals per unit area (11) fewer individuals per unit area (5)
taxon richness more species (4) less species (16)
evenness index more even distribution among taxa (4) less even distribution among taxa (5)
similarity index change in community structure (45) change in community structure (45)

Effluent Quality

The overall sublethal toxicity of mine effluent remained stable during the first 10 years of MMER implementation. Algal growth inhibition decreased from 2007 to 2011, suggesting a possible trend of decreasing effluent sublethal toxicity to algal growth; however, sublethal toxicity increased again in 2012. Among individual mine ore types, the same trends of decreasing algal sublethal toxicity were noted for base and precious metal mines and sublethal toxicity increased for algal growth in 2012. For uranium, iron, and “other” ore types, meaningful trends were difficult to identify because of the small number of tests conducted each year and high year-to-year variability. The most responsive tests for metal mining effluent were the invertebrate reproduction and aquatic plant growth (frond number) inhibition tests. The least responsive test was the fish larval growth inhibition test. During the time period in which incidences of stimulation (increased growth) were reported, between 2010 and 2012, stimulation was observed in 19% and 55% of plant and algal growth tests, respectively.

Magnitude and Geographic Extent of Effects

Most mines (25/29 or 86%) assessing the magnitude and extent (M&E) of effects confirmed in the exposure area near the point of effluent discharge (near-field area) observed one or more of the same effects in the exposure area farther from the point of effluent discharge (far-field area). Half of the mines (14/25 or 56%) assessing the M&E of multiple confirmed effects (near-field area) observed the same multiple effects in far-field areas. Near-field confirmed effects on fish and fish habitat were also observed in far-field areas 31% and 55% of the time, respectively. More than 50% of the time, fish effects observed in far-field areas were smaller in magnitude than the confirmed near-field effects. The magnitude of effects on fish habitat observed in far-field areas was similar to the magnitude of near-field confirmed effects, which is indicative of a greater risk to the environment. There was no relationship between the occurrence of effects in far-field areas and the distance of those far-field areas from the mine effluent discharge points.

Causes of Effects 

Most studies on the cause of effects examined multiple confirmed effects. About half of the 35 mines conducting investigation of cause (IOC) studies investigated inhibitory effects, where the indicators measured were smaller in the exposure area compared to the reference area. The remaining mines conducting IOC studies investigated stimulatory effects, where the indicators measured were larger in the exposure area than in the reference area, or a mix of inhibitory/stimulatory effects.

Of the 26 mines that had completed their IOC studies, 77% identified current mine effluent as a primary or possible contributing cause of effects. Two mines identified effluent substances as a cause, but did not indicate if current mine effluent was the source, and four mines indicated that effects were caused by non-mine-related factors.  

The following effluent substances were identified as possible causes of observed effects:

  • major ions
  • metals
  • nitrogen compounds (ammonia and nitrate)
  • total suspended solids
  • phosphorus
  • selenium

Major ions and phosphorus tended to be associated with stimulatory effects, whereas metals, selenium and total suspended solids were more often associated with inhibitory effects. Nitrogen compounds were associated with both stimulatory and inhibitory effects. 

Potential causes identified for further investigation by the nine mines conducting ongoing IOC studies included nitrogen compounds, selenium, and major ions in mine effluent, metals in mine effluent and/or sediment, and non-mine-related factors.  

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