Simultaneous rice-transplanting and subsequent application of pesticides to the paddy fields in the catchment of a rural river on weekends from late May to mid June caused high concentrations and high loadings of pesticides in the river. Imperfections in irrigation water management and the overflow discharge from paddy fields during heavy storm events increased the risk of drinking water contamination and threatened the aquatic ecosystems. Several pesticides were detected at nearly ten ppb in the biofilms on the river bed. In order to reduce the risks due to runoff pesticides, it is necessary to improve the management of irrigation and to strengthen the biodegradation capacity of biofilms on the river bed by maintaining a long retention time in drainage channels.

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