4.3.1 Acute Toxicity Data - All Studies

In order to apply the BC Environment water quality guideline derivation procedures, data collected from all suitable acute studies were combined and are presented in Table 4.10, as follows:

Note: * - denotes value that was used in the regression analysis

Linear regression was performed on the toxicity test data denoted by an asterisk in Table 4.10, values which generally represented the lowest acute manganese concentrations for each of the hardness values. The lowest values were chosen because the objective of establishing freshwater guidelines is to protect sensitive aquatic receptors; the most sensitive test results correspond to the lowest manganese concentrations and guidelines developed from lower values should result in lower guidelines that will be more protective of sensitive species. At a water hardness of 25 mg/L CaCO₃, the coho salmon 96 Hour LC₅₀ value of 2.1 mg/L was used in the regression. As discussed in Section 4.2.4, low water hardness likely contributed to toxic effects observed in the Daphnia magna 48 Hour LC₅₀. Thus, toxicity was unlikely to be due only to concentrations of manganese and the 0.8 mg/L concentration was not included in the regression analysis. The brown trout 96 Hour LC₅₀ at a hardness of 454 mg/L CaCO₃ was also omitted from the regression analysis. This decision was based on the following:

1. The absence of other data points at high hardness values, making in unclear whether brown trout was a sensitive species at high water hardnesses as compared to rainbow trout or other organisms for which no test data were available.
2. The observed decrease in slope of the resulting regression line when the 49.9 mg/L value was excluded, thus resulting in more conservative (lower concentration) values on which to base acute guidelines.
3. Water hardness values >300 mg/L CaCO₃ are uncommon in British Columbia fresh waters.

The concentrations denoted by an asterisk in Table 4.10 were used in the regression analysis. All acute values used were 96 Hour LC₅₀ concentrations with the exception of the 72 Hour IC₅₀ value for S. capricomutum. The 72 Hour test duration was based on Environment Canada's standard procedures for this test (see Appendix A). The resultant equation and the statistical data associated with the regression line are provided in Appendix E and summarized below.

Y = 0.0441X + 1.81

where X = hardness in mg/L CaCO₃ and Y = Mn concentration in mg/L

correlation r² = 0.902 standard error = 1.46

For a water hardness of zero, the predicted manganese concentration would be 1.81 mg/L. A positive Y-intercept value makes sense because some level of tolerance of manganese would be expected even at very low water hardnesses. Manganese is a naturally occurring substance and it is expected that a threshold level would exist, below which no toxic responses would occur in aquatic organisms exposed to manganese regardless of variations in water hardness or other physical properties.

Table 4.11 summarizes predicted manganese concentrations for various hardness values:

The manganese concentrations predicted by the regression equation ranged from 2.9 mg/L for a hardness of 25 mg/L CaCO₃ to 15.0 mg/L for a hardness of 300 mg/L CaCO₃. The hardness range of 25 to 300 mg/L covers the likely range of values that occur naturally in B.C. fresh waters.
4.3.2 Chronic Toxicity Data - All Studies

Chronic toxicity test data for the B.C. Environment tests and data from literature sources screened in Section 4.3 were also combined for application of the B.C. Environment water quality guideline derivation procedures. The results are presented in Table 4.12:

Note: Bolded and italicized values were used in the regression analysis

Linear regression analysis was performed on the manganese concentrations denoted by an asterisk in Table 4.12. The chosen values were the lowest concentrations at each of the test water hardness values. The lowest values were chosen because the objective of establishing freshwater guidelines is to protect sensitive aquatic receptors; the most sensitive test results correspond to the lowest manganese concentrations and guidelines developed from lower values should result in lower guidelines that will be more protective of sensitive species. The 7 Day E-Test result at a water hardness of 25 and the brown trout 62 Day IC₂₅ result at a water hardness of 30 were not used as the values were considered to be too high (not sufficiently conservative). Other chronic data were available with similar hardnesses (36.8 and 37.5) and, in the case of brown trout, two chronic test results (hardnesses of 30 and 37.5 mg/L CaCO₃) were available and the more conservative value (2.7 mg/L for 4 month growth/survival) was considered to be the most appropriate choice.

The resultant equation and the statistical data associated with the regression line are presented in Appendix E and are summarized below.

Y = 0.0176X + 2.42

where X = hardness in mg/L CaCO₃ and Y = Mn concentration in mg/L

correlation r² = 0.702 Standard Error = 2.03

As with the acute data, a positive Y-intercept value is predicted by the equation. As discussed in Section 4.3.1, this is logical because a threshold concentration of manganese tolerable to most or all aquatic organisms would be expected to exist, below which no toxic responses would be anticipated. The slope of the chronic regression line (0.0176) is flatter than the slope of the acute regression line (0.0441); this also makes sense because a higher level of sensitivity would be expected under chronic exposure conditions.

The higher chronic Y-intercept (2.42 vs. 1.81 mg/L at a water hardness of 0) is a product of the data used to derive the regression lines. With sufficient data, it would be expected that the chronic Y-intercept would be lower than the acute Y-intercept. Although both the acute and chronic equations were based on six data points, the correlation factor (r²) of 0.902 for the acute equation was notably higher. Substitution of the acute Y-intercept value was therefore given consideration as a conservative measure. This would result in the equation Y = 0.0176X + 1.81 and would predict chronic values that are 0.61 mg/L (2.42 - 1.81) lower than those predicted the chronic regression equation. However, application of a factor of safety (0.1 to 0.5 as outlined in Section 4 of Appendix B) would result in modified chronic manganese concentrations differing by 0.06 to 0.3 mg/L. This was not considered significant given other uncertainties associated with extrapolating toxicity test data (e.g. species differences, variable environmental conditions).

Table 4.13 presents the predicted manganese concentrations for the chronic regression equation.

The predicted manganese concentrations ranged from 2.9 mg/L at a water hardness of 25 mg/L CaCO₃ to 7.7 mg/L at a water hardness of 300 mg/L CaCO₃. The predicted acute and chronic values were the same at a water hardness of 25 mg/L CaCO₃, but were lower for all water hardnesses >25 mg/L.

4.4 DERIVATION OF FRESHWATER GUIDELINES

The document "Derivation Of Water Quality Criteria To Protect Aquatic Life In British Columbia" (BCMELP, 1995). was referenced to derive proposed manganese guidelines. The predicted acute and chronic manganese concentrations presented in Tables 4.11and 4.13 will be used to derive acute and chronic interim guidelines for the protection of fresh water aquatic life. The acute interim guidelines would apply to short term exposure only while the chronic interim results would be intended for general application.

Section 4.1.1 of the B.C. Derivation document provides guidance on the application of safety factors in the derivation of aquatic life guidelines. The appropriate safety factors are listed as typically falling between 0.1 and 0.5 and are decided on a case by case basis using scientific judgement. A factor of safety of 0.25 was chosen for both the acute and chronic data sets based on the following:

1. The bulk of the toxicity test data was in the hardness range of 25 to 250 mg/L CaCO₃, which encompasses the range of hardnesses that would occur in most BC surface fresh waters.
2. The overall number of test results (19 acute, 10 chronic) meeting primary and/or secondary data requirements.
3. The variety of organisms for which suitable data were available (sufficient to meet full acute and interim chronic guideline requirements)
4. Use of minimum or near minimum concentrations (i.e. the most sensitive receptors in the toxicity test data set) in the acute and chronic regression line derivations.

The chronic safety factor chosen was the same as the acute safety factor despite the larger data set available for acute effects. The smaller data set was offset by the longer durations of the chronic tests, which typically result in more reliable toxicity values as opposed to acute values which tend to show more variability due to the shorter duration of the tests. Some uncertainty was also associated with the acute data due to the BC Environment 96 Hour LC₅₀ test on rainbow trout, which did not fit the pattern of increasing manganese tolerance with increasing water hardness.

Four of the six chronic values used in the chronic regression equation derivation were the IC₂₅ concentrations for Daphnia magna at water hardnesses of 100 and 250 mg/L CaCO₃ and for brown trout at water hardnesses of 100 and 250 mg/L CaCO₃. The NOEC (no observed effect concentrations) values reported for these tests and the ratios of the NOEC to IC₂₅ values are presented in Table 4.14:

The NOEC values are concentrations at which no adverse impacts were observed for chronic exposure to manganese. The NOEC/IC₂₅ ratios varied between 0.67 and 1.0 for one fish species and one invertebrate species; this suggests that a factor of safety of 0.25 should be sufficiently protective for chronic exposure of aquatic life to manganese. A less conservative factors of safety (e.g. 0.4 or 0.5) was not chosen because the available toxicity data did not meet the requirements for full guideline derivation. There were not sufficient chronic tests on invertebrates and the types and numbers of species in the data set do not encompass all potentially sensitive species that exist in BC fresh waters. In addition, the 4 month chronic toxicity test value of 0.79 mg/L for rainbow trout at a hardness of 36.8 mg/L CaCO₃ (Davies and Brinkman, 1994) would be exceeded if a safety factor of 0.4 or 0.5 had been chosen. Rainbow trout is an important species in BC fresh water and the need to ensure protection of such a species was taken into account.

4.4.1 Acute Guidelines

The acute regression equation concentration data from Table 4.11 and the concentrations resulting from application of a factor of safety of 0.25 are presented in Table 4.15:

Note: Modified Mn Concentration is the predicted Mn concentration multiplies by a factor of safety of 0.25

The modified acute manganese concentrations ranged for 0.7 mg/L to 3.8 mg/L within the range of water hardnesses from 25 to 300 mg/L CaCO₃.

4.4.2 Chronic Guidelines

The chronic regression equation concentration data from Table 4.13 and the concentrations resulting from application of a factor of safety of 0.25 are presented in Table 4.16:

Note: Modified Mn Concentration is the predicted Mn concentration multiplied by a factor of safety of 0.25

The modified chronic manganese concentrations ranged for 0.8 mg/L to 3.9 mg/L within the range of water hardnesses from 25 to 300 mg/L CaCO₃. The modified chronic values were lower than the modified acute values for all water hardnesses.

4.4.3 Application of Guidelines

The acute and chronic guidelines derived in Sections 4.4.1 and 4.4.2 can be applied to fresh water as maximum acceptable concentrations at the corresponding hardness ranges. For water hardness values of 350 mg/L CaCO₃ or greater, the equations provided in Tables 5.1 and 5.2 could be applied. The acute guidelines would only apply for exposure durations of 96 hours or less. Exposures of longer duration would be considered chronic and the chronic guidelines would apply.

The guidelines reflect total manganese concentrations in fresh water. Natural variability exists for total manganese concentrations in surface water due to environmental factors such as the range of manganese concentrations that are present in different rock and soil types, the solubility of naturally occurring manganese compounds, the weathering rate of the soil/rock, and the amount of sediment suspended in the water. Section 2.1 of this document indicates that total manganese concentrations observed in BC surface waters range from <0.001 mg/L to 1.70 mg/L (CCME, 1987; BCMELP 1998), with concentrations in excess of 1.0 mg/L rarely observed. Higher concentrations were typically associated with higher seasonal flows. Application of chronic water quality guidelines for manganese should reflect the natural occurrence of peak events and the presence of non-anthropogenic sources of manganese in surface waters.

The modified manganese concentrations, if used as guidelines, may be exceeded by naturally occurring manganese in stream water at water hardnesses below 100 mg/L CaCO₃ (acute) and 250 mg/L CaCO₃ (chronic). Surface fresh water data (BCMELP, 1998) suggest that higher concentrations occur during periods of higher stream flow (e.g. during spring runoff) and lower concentrations occur downstream of lakes (which act as settling areas for sediment). This natural variability should be taken into account when applying the proposed guidelines because the intent is to protect aquatic life from anthropogenic sources of manganese rather than naturally occurring manganese. Sampling of surface water upstream and downstream of discharge areas can provide a means of comparison. Sampling of ground water adjacent to surface waters where manganese may be of concern could also be undertaken to determine the likelihood that manganese concentrations observed in surface water are a result of human activities. End of pipe points of discharge could also be sampled to evaluate manganese concentrations prior to mixing with surface water, particularly during periods of high sediment loads.

The Contaminated Sites Regulation (Province of BC, 1997) provides standards for substance concentrations in ground water. For aquatic life water use, the current ground water standard for manganese is 1 mg/L. A dilution factor of 10 for discharge of ground water to surface water is assumed (i.e. the surface water value of 0.1 mg/L was modified by a factor of 10 to develop the 1 mg/L standard). The proposed chronic guidelines range from 0.6 mg/L to >1.9 mg/L, depending on hardness. Applying a dilution factor of 10 would result in ground water values of 6 to >19 mg/L, considerably higher than the current 1 mg/L standard. The proposed guidelines are based on toxicity test results for a number of BC species and are considered to have a more solid scientific basis. If a ground water standard for manganese for protection of aquatic life is retained, the proposed guidelines could be used to develop new ground water standards. The current ground water standard of 1 mg/L has frequently been exceeded throughout the province. A range of 6 mg/L to >19 mg/L would be founded on a more scientifically sound basis. In practical terms, it would remove many sites from "contaminated status" based on the proximity of a site to nearby surface water.