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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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No abstract

Runoff characteristics of particulate pesticides from paddy fields have been intensively observed in the Koise River in Japan. The 8 pesticides that are applied to paddy fields were analyzed in both particulate and dissolved forms. The concentrations and the detection frequencies of particulate pesticides were lower than those of dissolved pesticides. The particulate pesticide concentrations in the river water were evaluated based on the soil sorption coefficient, particulate organic carbon concentration, and dissolved pesticide concentrations. The particulate pesticide concentrations in the river were higher than evaluated concentrations because the paddy soil contained more pesticides than did suspended solids in the river water discharged during rain events, and because the desorption rates of pesticides were slow. In observations made during rains, the particulate pesticide concentrations increased with the increases in both the discharge rate and the concentrations of suspended solids. The particulate loading was slight compared with dissolved loading, but particulate pesticides may be influenced by enclosed areas such as a lake or estuary because under such conditions particulate matter settles vertically and the pesticide decomposition rate in sediment is slow compared with that in water.