2018 年 16 巻 5 号 p. 185-198
The environmental effects of combined sewer overflows (CSOs) in Toyama, Japan were evaluated using toxicity tests and chemical analyses. Chronic and sub-chronic tests with three freshwater organisms (Danio rerio, Ceriodaphnia dubia, and Raphidocelis subcapitata) were applied to seven samples collected during three rainfall events from October to December 2016. Five samples from CSOs including the first flush and the following overflows of CSOs, and two samples from river water (upstream and downstream) were collected. The chronic toxicity unit (TUc), total organic carbon (TOC), and coliform count were higher in the first flush than in other samples. In the three samples collected at the third sampling event, toxic effects were only observed on daphnia. Thus, toxic influence was the strongest on crustaceans and toxicity identification evaluation (TIE) procedures using daphnia were applied to the three samples. To further examine the effect of heavy metals on samples’ toxicity to daphnia, this study also clarified the change of toxic effect during the overflow event, and compared the Hazard Quotients (HQ) of heavy metals with TUc in samples collected at the third sampling event. The possible causes of toxicity and the contribution of heavy metals are discussed.