Abstract
The whole effluent toxicity（WET）test has been conducted to estimate the toxicity of complex mixtures such as industrial effluents using various bioassays in the regulatory framework of many countries. The Japanese Ministry of Environment recently started conducting an investigation to introduce the WET methods for water quality management. However, the methods used in the Japanese WET test method have not yet been established. In this study, statistical methods to calculate chronic toxicity endpoints were evaluated for the WET approach. Ceriodaphnia dubia reproduction tests were conducted for 252 effluent samples. The no observed effect concentration（NOEC）and the 25% inhibition concentration（IC25）were statistically calculated for each effluent sample. Comparison of the NOEC and IC25 showed that the IC25 values were approximately equal to the NOECs in 65.9% of the effluent samples, indicating that the IC25 values could be considered as an analogue of NOECs. Test precision and the choice of statistical methods greatly influenced the estimation of the NOEC. In some cases, the NOEC values for effluents were less than 3.125% with IC25 values greater than 100%, suggesting that many false-positives may be generated by using the NOEC values alone. Overall, the results of this study strongly suggest that the IC25 is the preferred statistical method for estimating the chronic effects of effluents.
