Third national assessment

4.0 Effluent Quality

4.1 Sublethal Toxicity Testing

Sublethal toxicity (SLT) testing is conducted on metal mining effluent from the final discharge point with potentially the most adverse environmental impact as per the MMER (Schedule 5, subsection 5(2)). Like effluent characterization and water quality monitoring,^{Footnote 9}SLT testing provides supplementary information for the biological monitoring studies, including measures of year-to-year changes in effluent quality and site-specific estimates of the potential effects of effluent on biological components in the receiving environment.

Sublethal toxicity testing involves exposing test organisms to a range of effluent concentrations under laboratory conditions. The effluent concentration that causes 25% inhibition (IC₂₅) in a test organism is determined. The test method that is used dictates the inhibition parameter, such as inhibition of growth or reproduction. A low IC₂₅ (e.g., 10%) indicates a higher level of sublethal toxicity, because the inhibition occurred at a low effluent concentration. A higher IC₂₅ indicates a lower level of sublethal toxicity. When the IC₂₅ is reported as ≥100%,^{Footnote 10} the effluent is considered to be non-toxic for the sublethal effect being considered for the organism concerned.

Mines are required to conduct SLT testing twice per calendar year for the first three years, and once per year thereafter. Tests are conducted using standardized methods referenced in the MMER (Schedule 5, section 5) and several different tests are required including a fish early-life-stage development test, an invertebrate reproduction test, and plant and algal growth inhibition tests.

SLT testing results from the first 10 years of MMER implementation were compiled to assess the trends in effluent quality presented in this report. Effluent sublethal toxicity was examined for all mines combined and for mines with different ore types: precious metals (gold, silver), base metals (e.g., copper, zinc, and nickel), uranium, iron, and other ore types (e.g., tantalum, titanium, tungsten). Trends were examined over the 10-year period from 2003 to 2012. During this period, a total of 6,761 test results were submitted by 125 mines.

Annual geometric means^{Footnote 11} of IC₂₅ values were calculated for all mines combined and for each ore type, for each year. IC₂₅ values from each year were sorted into three SLT categories: higher toxicity (IC₂₅ ≤ 20%), lower toxicity (IC₂₅ > 20% and <100%) and no toxicity (IC₂₅ ≥ 100%). The geometric mean IC₂₅ and percent of IC25 values in each SLT category is shown for each year for each of the five freshwater SLT tests in Appendix E. The annual geometric mean IC₂₅ for each ore type for freshwater SLT test is shown in Appendix F.

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4.1.1 Sublethal Toxicity of Mine Effluent

With the exception of the algal growth inhibition test, the overall sublethal toxicity of mine effluent remained stable between 2003 and 2012 (Figure 12). Algal growth inhibition decreased from 2007 to 2011, as shown by increases in IC₂₅ geometric means and decreases in the proportion of “high toxicity” results (IC₂₅ ≤ 20%) (Figure E4). Sublethal toxicity testing results for fish (Pimephales promelas) larval growth, invertebrate (Ceriodaphnia dubia)reproduction, and plant (Lemna minor) growth inhibition tests showed year-to-year variation, but no consistent trends over time (Figure 12).

Note: Ranges in number of tests conducted per year were as follows: Pimephales promelas, n = 111–140; Pseudokirchneriella subcapitata, n = 119–152; Lemna minor dry weight, n = 117–142; Ceriodaphnia dubia, n = 121–155 ; Lemna minor frond number, n = 116–145.

Trends in effluent sublethal toxicity were more variable for different ore types, likely due to the smaller data set size for ore type analyses (Figures F1-F5). The results from base and precious metal mines on algal growth showed decreases in toxicity between 2007 and 2011 and an increase in 2012. Although some trends may be apparent for uranium, iron and “other” ore types, they should be interpreted with caution given the small sample sizes.

In locations where Pimephales promelas (fathead minnow) is not an indigenous species, an analogous fish test is conducted using rainbow trout (Oncorhynchus mykiss). Eight base and two precious metal mines submitted SLT results for tests conducted on rainbow trout between 2003 and 2012. These mines were located predominantly in western Canada. For base metal mines, rainbow trout IC₂₅ values ranged from 54 to 100%, and 62% of tests indicated no sublethal toxicity (IC₂₅ ≥ 100%). For precious metal mines, in all 22 tests conducted, no sublethal toxicity (IC₂₅ ≥100%) to rainbow trout was observed.

Mines that discharge to marine or estuarine environments are required to conduct SLT tests using marine organisms and different test methods than those used for freshwater tests. Two base metal mines conducted marine SLT tests. Sand dollar IC₂₅values ranged from 4 to 18%, white sea urchin IC₂₅ values ranged from 10 to ≥100%, red macroalgae IC₂₅ values ranged from 4 to 56%, topsmelt IC₂₅ values ranged from 67 to 73% and inland silverside IC₂₅ values ranged from 16 to ≥100%. There were no apparent trends in effluent sublethal toxicity over time at either mine.

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4.1.2 Responsiveness of Sublethal Toxicity Tests

The effluent concentration at which inhibition occurs is influenced by which standardized test is used. The test that shows inhibition at the lowest effluent concentration is considered the most responsive test. The sublethal toxicity result obtained with one type of test compared to the other tests for the same effluent sample can be used to assess relative responsiveness. It is important to monitor the responsiveness of SLT tests to ensure that the tests being used are still relevant for the effluent being evaluated (for example, species that are consistently non-responsive could be removed from the testing requirements in the future). Changes to effluent quality over time are better captured using responsive tests. The responsiveness of each test compared to other tests can help predict the dominant toxicant. For example, fish are known to be more responsive to ammonia (toxic to fish) than invertebrates, whereas invertebrates are often more responsive to metals than fish. The annual geometric means of IC₂₅ and the proportion of tests indicating no sublethal toxicity for all mines show the relative responsiveness of each test (Figures E1 to E5).

The tests that were the most responsive to effluent were the invertebrate reproduction and plant growth (frond number) inhibition tests, for which the annual geometric means of IC₂₅ were in the range from ~20 to 40%. The plant growth (frond dry weight) inhibition test was slightly less responsive, with IC₂₅ annual geometric means ranging from ~40 to 50%, followed by the algal growth inhibition test, with a wider range of annual geometric means of IC₂₅, specifically ~50 to 80%. The fish early-life-stage development inhibition test was the least responsive test, with IC₂₅ annual geometric means in the ~80 to 90% range. The relative responsiveness of these different tests was found to be the same when the proportion of tests that indicated no sublethal toxicity was compared. These results corroborate the findings of the Second National Assessment (Environment Canada 2012b), thus indicating that the relative responsiveness of SLT tests to metal mine effluent has remained constant through the first 10 years of testing.

The most responsive SLT tests among ore types with large data sets (precious and base metals) were the invertebrate reproduction and plant growth (frond number) inhibition tests (Figures E1, E2). Although the data for uranium and “other” ore types are more variable, the invertebrate reproduction and plant growth (frond number and dry weight) inhibition tests appeared to be the most responsive tests, and the fish early-life-stage development inhibition test, the least responsive (Figures E3, E4). For iron ore mines, there were no consistent differences in test responsiveness (Figure E5).

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4.1.3 Stimulation in Algal and Plant Growth Inhibition Tests

The algal and plant growth inhibition test methods require that the occurrence of growth stimulation be reported. Stimulation refers to an increase in growth of test organisms relative to controls after effluent exposure. If stimulation at low concentrations is followed by an inhibitory response at higher concentrations, this low-dose stimulation may be related to an organism’s response to low levels of a toxic substance. This effect is referred to as hormesis. If the stimulatory effect is observed across all effluent concentrations, or increases with increasing effluent concentration, the results may be indicative of an enrichment effect related to increased nutrient availability rather than hormesis.

From 2010 to 2012, stimulation was reported in 55% of algal growth tests and 19% of plant growth inhibition tests. Stimulation appears to be more frequent for base and precious metal mines than for other ore types, particularly in the case of the plant growth inhibition test (Table 1).

The overall stimulation results presented here include both types of stimulation observed--hormesis or enrichment effect--and thus likely overestimate the enrichment effect. Additional test information is needed to differentiate between these types of stimulation.

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4.2 Effluent Characterization

Effluent characterization is conducted by analyzing a sample of effluent from each final discharge point (FDP) to determine the concentrations of substances in mine effluent that are potential contaminants. The annual mean concentration of each of the nine specified substances (MMER, Schedule 5, section 4) was calculated for two different groups of FDPs. The first group contained the FDPs associated with the biological monitoring studies and the second group contained all other FDPs. These annual means are presented in Appendix G to give a general overview of effluent chemistry.

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