Under the Food Sanitation Act of Japan, some residual pesticides in food are regulated as total level including their metabolites. However, in the area of drinking water quality regulation, some of these metabolites are not included in the list of target substances set by Ministry of Health, Labour and Welfares. In this study, we developed and validated the methods for analysis of four pesticides (orysastrobin, prothiofos, amitraz and iprodione) and their corresponding metabolites in water samples. Using the established methods, a survey was conducted at 21 water treatment plants in Osaka and the concentration of each substance was determined before and after water treatment process. As a result, orysastrobin and (5Z)-orysastrobin, a metabolite of orysastrobin, were detected in tap water at several plants. Our result suggests that orysastrobin in drinking water should be regulated along with its metabolite.
