5.1 REVIEW OF THESIS OBJECTIVES

The objectives of this thesis research were as follows:

1. To review the existing freshwater aquatic life guideline for manganese;
2. To evaluate the practicality of application of the guideline;
3. To review the information available in the literature on manganese; and
4. To use new toxicity test data generated by the BC Ministry of Environment, Lands and Parks for native BC species in order to improve the existing freshwater aquatic life guideline.

Objectives 1 and 2 revealed that the existing freshwater aquatic life guideline is not toxicologically based and is not based on the protection of aquatic life. In order to fulfill Objective 4, toxicity testing was conducted on British Columbia aquatic species and the data generated were used in conjunction with supplemental data from the literature (Objective 3) to improve the existing guideline. Enhancement/modification of the existing manganese freshwater aquatic life guideline (Objective 4) resulted in a hardness dependent relationship, with manganese concentrations increasing with increased water hardness; the proposed guidelines are presented in Section 5.2.

5.2 PROPOSED ACUTE AND CHRONIC GUIDELINES

The modified acute and chronic concentrations presented in Tables 4.15 and 4.16 are proposed as surface water guidelines for manganese. For water hardnesses falling between increments of 25, it is proposed that the lower value be used as a guideline. The proposed acute and chronic guidelines are presented in Tables 5.1 and 5.2:

Note: H = hardness in mg/L CaCO₃

Note: H = hardness in mg/L CaCO₃

The modified manganese concentrations from the chronic data set (Table 5.1) are proposed as interim guidelines for protection of freshwater aquatic life to replace the existing manganese guideline of 0.1 mg/L, which applied to all water hardness values. For acute exposure ( less than 96 hour), manganese concentrations presented in Table 5.2 are proposed as interim guidelines.

A hardness dependent relationship where tolerable manganese concentrations increased with increasing water hardness was well supported by most of the BC Environment toxicity test data and the literature data. The exception was the 96 Hour LC₅₀ acute toxicity test on rainbow trout, where the manganese concentrations were lower at a hardness of 250 mg/L CaCO₃ than at a hardness of 100 mg/L CaCO₃. Although this does not support the manganese/hardness relationship, the chronic regression equation predicted manganese concentration at a hardness of 250 mg/L was 6.8 mg/L while the rainbow trout LC₅₀ concentration was 12.7 mg/L. The proposed guideline manganese concentration of 1.7 mg/L is well below the 12.7 mg/L value. In addition, if a trend exists for rainbow trout where the manganese concentration at which a toxic response occurs decreases with increasing hardness at values >250 mg/L CaCO₃, such hardness values are not commonly found in BC fresh waters.

From an aquatic life protection perspective, the modified manganese concentrations proposed are considered to be sufficiently protective of rainbow trout as well as other species. The factor of safety of 0.25 used in the derivation was considered to be suitably conservative given the quality and amount of acute and chronic toxicity tests and the range of species for which data were available. A less conservative factor of safety was not chosen because the data did not meet the requirements for full guideline derivation and uncertainties remain regarding sensitive species present in BC fresh waters for which no toxicity data is available. For a hardness range of 25 - 50 mg/L CaCO₃, a less conservative safety factor would have resulted in a guideline that exceeded the 4 month chronic toxicity value of 0.79 mg/L determined for rainbow trout (Davies and Brinkman, 1994), a species of importance in BC fresh waters.

Application of the guidelines to surface water should also reflect the presence of naturally occurring manganese. Where anthropogenic sources are to be regulated, measurement of manganese concentrations prior to discharge to surface water would help to separate non-anthropogenic manganese that may be at elevated levels due to sediment loads in surface waters. Applying the guidelines to end of the pipe effluent concentrations and to concentrations in ground water immediately adjacent to a surface water body may alleviate concerns regarding naturally occurring manganese versus anthropogenic manganese. For ground water, the presence of dissolved rather than total manganese may better reflect the mobile fraction that may discharge to surface water.

The former guideline range of 0.1 to 1 mg/L was modified to a range of 0.6 mg/L at a hardness of zero to 1.9 mg/L at a water hardness of 325 mg/L CaCO₃.

5.3 RECOMMENDATIONS FOR FURTHER STUDY

The toxicity testing program commissioned by BCMELP and conducted at Environment Canada's Aquatic Toxicity Laboratory were not comprehensive enough to permit derivation of full guidelines; consequently, interim guidelines were developed. In order to meet the requirements for full guideline development, additional aquatic toxicity testing would be required. Use of flowthrough tests or confirmation of Day 0 and final day manganese concentrations in the test water would be required. For invertebrates, an additional chronic study on a non-planktonic species would be required to meet the BCMELP full guideline requirements.

Additional studies on rainbow trout are also needed to establish whether a manganese/hardness relationship exists for this species or whether manganese tolerance in rainbow trout peaks at an intermediate manganese concentration. As discussed in Section 4, the rainbow trout 96 Hour LC₅₀ results from the BCMELP toxicity testing program were the only data that did not fit the pattern of increasing manganese concentration with increased water hardness. Possible explanations for the decrease in tolerable manganese concentrations between hardnesses of 100 mg/L and 250 mg/L CaCO₃ are not clear at this time, but may include test organism or species specific intolerance of higher water hardness. Further studies at additional water hardness values such as 50, 150, 200 and 300 would be needed to identify whether manganese tolerance in rainbow trout peaks at a water hardness of between 25 and 250 mg/L or whether the data in the BCMELP study are somewhat anomalous.

Chronic toxicity testing would also be needed to determine if the observed effect would occur under chronic exposure. The data from the Stubblefield et. al. (1997) 62 day chronic study on brown trout, a species that is physiologically similar to rainbow trout and present in BC waters, are in direct contrast to the BCMELP acute rainbow trout data with respect to the manganese/hardness relationship. The chronic toxicity values derived in the brown trout study were also lower than those determined from the rainbow trout tests.

Infilling of these data gaps may allow future enhancement of the proposed guidelines by providing additional data that may further refine the regression equations developed to define the manganese/hardness relationship.