Predicting the behavior of paddy pesticides in a river basin using a simulation model (PADDY-Large): Application to a tributary of the Chikuma River under rice cultivation

Eleven paddy pesticides and 3 of their metabolites were monitored in Hiroi stream (a tributary of the Chikuma River in Japan) during rice cultivation season for 5 years (2001-2005). An improved simulation model for predicting pesticide behavior in drainage canals and rivers based on PADDY-Large was developed to include the metabolic pathways and drift process following aerial and unmanned helicopter spraying and applied to herbicides as well as nursery-box- and foliage-applied fungicides and insecticides. The nursery-box-applied insecticide carbosulfan was rarely detected in stream water during the monitoring period, whereas a clear peak for its metabolite (carbofuran) was observed. Detection of herbicides other than cyhalofop-buthyl was dependent on the timing of herbicide application. The fungicide tricyclazole, the insecticide trichlorfon, and the trichlorfon metabolite (dichlorvos) were detected at a few dozen mu g L(-1) immediately after aerial spraying and the concentrations decreased rapidly. In contrast, these peak concentrations sprayed by unmanned helicopter were one-tenth to one-half of those by aerial application. The improved PADDY-Large model successfully simulated changes in concentrations of paddy pesticides and 3 of their metabolites in stream water during the rice cultivation season, although there were some discrepancies between simulated and measured concentrations. (C) Pesticide Science Society of Japan