Environmental risk evaluation for anionic surfactants based on the characteristics of the appearance of a high concentration was developed together with statistical analysis of MBAS data in rivers of Hyogo Pref., western Japan. Annual average MBAS at different monitoring points generally decreased during past 10 years and their geometric mean reduced by 70%. This rate of decrease agreed well with that of estimated environmental loads of anionic surfactants in the study area. The proportions of monitoring point with high risk (risk ratio over 1.0) to fish increased in the order of the weakness of toxicity in both 1989 and 1998. Their proportions were 2% for acute toxicity, 17% for chronic toxicity and 66% for avoidance response in 1989, which surpassed the proportion evaluated using average concentration in all cases. The proportions for whole toxicities decreased in 1998, however, the proportion for avoidance response kept considerably high level (42%). These results indicated the usefulness of environmental risk evaluation for surfactants based on the characteristics of appearance of high concentration, and also suggested the need for field investigation for fish avoidance response.

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