A comparison of the results of freshwater aquatic toxicity testing of pulp and paper mill effluents

Freshwater short-term chronic toxicity tests were conducted quarterly on samples from pulp and paper mills under Cycle I of Canada's Environmental Effects Monitoring program. The toxicity tests included the 7-dayCeriodaphnia dubia partial life-cycle test for survival and reproduction, the 7-day rainbow trout (Oncorhynchus mykiss) embryo viability test, and the 72-hr Selenastrum capricornutum algal growth inhibition test. All three tests were performed on a routine basis with few failures. The responses of the different species were generally not correlated, suggesting that they responded to different constituents in the effluents. On a statistical basis, the algal test exhibited the greatest sensitivity, followed by the trout embryo and Ceriodaphnia tests. In general, the no-observable effect concentrations (NOECs) were lower than the corresponding IC25 estimates. Due to variability between samples from each mill, it was not possible to distinguish between different mill production processes with respect to their effect on toxicity. In general, mills that treated their effluent using aerated stabilization basins exhibited less toxicity than other treatment types. However, this observation is preliminary and also affected by variability in test results and limited sample size. Overall, the results suggest that toxicity tests have a useful role in identifying toxicity in these effluents and that Toxicity Identification Evaluations should be conducted to identify the cause of toxicity so that treatment and/or source control can be initiated as appropriate.