Intra- and intertreatment variability in reference toxicant tests: Implications for whole effluent toxicity testing programs

Whole effluent toxicity tests are used in permitting programs across the United States to determine whether effluents are potentially toxic to aquatic biota in receiving environments. In cases where whole effluent toxicity tests indicate unacceptable toxicity, corrective measures or further testing (e.g., field tests) may be required. To be consistent and fair to permit holders, whole effluent toxicity test outcomes (e.g., pass or fail) should not be strongly influenced by intra- and interlaboratory variability. In this study, we quantified intra- and interlaboratory variability for four species-data type combinations using the results of reference toxicant tests compiled from many laboratories in recent years. For each set of test results, we conducted a regression analysis using the generalized linear models framework. The results indicated that the coefficient of variation (CV) for intralaboratory 25% effective concentration (i.e., EC25) values varied from 15.7% for number of young of Ceriodaphnia dubia in laboratory CD4 to 122% for mortality of Menidia beryllina (inland silverside) in laboratory MB3. Interlaboratory variability was small for both mortality (CV = 17.3%) and number of young (CV = 13.4%) of C. dubia. Interlaboratory variability for mortality (CV = 65.8%) and biomass (CV = 117%) of M. beryllina, however, was very high. Our study shows that permit toxicity limits can be exceeded because of factors other than effluent toxicity, particularly when the limits are based on testing of M. beryllina.