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UBC Theses and Dissertations

Sulphate toxicity to freshwater organisms and molybdenum toxicity to rainbow trout (Oncorhynchus mykiss) Davies, Trevor Dickson

Abstract

The current "Ambient Water Quality Guidelines for Sulphate (Singleton 2000)" is largely based on studies that have serious flaws that do not accurately assess the toxicity of sulphate. Replication of the principle studies used in the current sulphate guideline rational indicates that the original studies were either flawed in their methodology or the results were misinterpreted. The authors used organisms representing a number of trophic levels and examined sulphate toxicity to the amphipod Hyalella azteca (Davies 2002a), the cladoceran Daphnia magna (Davies 2002b), the stripped bass (Morone saxatilus) (Davies 2002c) and the aquatic moss (Fontinalis antipyretica) (Davies 2002d). These replicated studies can be found in the appendices of this report. These studies, as well as others in the literature, indicate that organisms exhibit reduced toxicity from sodium sulphate exposure in waters of increasing hardness and more specifically, waters with increasing calcium content. The incorporation of these new studies into the available literature indicate that in consideration of the relatively low toxicity of sulphate, the current guideline is overly conservative and proposes unnecessarily strict discharge limits. Therefore, a new guideline, taking into consideration the water hardness of the receiving waters is proposed. In waters of hardness (as CaCO3) of less than 50 mg/L, a maximum allowable discharge that should be sufficient in the protection of ecosystem integrity is proposed at 200 mg/L sulphate. In the range of hardness of 50 to 100 mg/L and in waters of over 100 mg/L hardness, allowable discharge limits of 300 and 400 mg/L sulphate respectfully are proposed. Two experiments examining the toxicity of molybdenum to rainbow trout (Oncorhynchus mykiss) indicate that the results of an earlier study which reported a 29 day LC50 to be 0.79 mg/L molybdenum (Birge et al. 1980) highly exaggerated the toxicity of molybdenum (Davies 2002e). An experiment mimicking the experimental conditions of Birge et al. (1980) study did not cause sufficient mortality to calculate an LC50 up to molybdenum concentrations of 400 mg/L. A second study, which used standard methodology, also did not cause sufficient mortality up to concentrations of 1500 mg/L molybdenum to calculate an LC50.

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