RUNOFF PROPERTIES AND CHANGE IN CONCENTRATIONS OF AGRICULTURAL PESTICIDES IN A RIVER SYSTEM DURING A RICE CULTIVATION PERIOD

Residues of 12 pesticides in water and sediment were surveyed in a river system during a rice cultivation period. The highest concentrations of pesticides were 36 mug l-1 for fenobucarb (cabamate insecticide), 11.3 mug l-1 for simetryn (herbicide) and 17.5 mug 1-1 for iprobenfos (fungicide). The downstream flux was largest for iprobenfos, which showed a broader peak than the other pesticides. The runoff rates estimated from the ratio of the downstream flux of a pesticide to its retail amount was highest for fenobucarb and lowest for an organochlorine herbicide, chlornitrofen. These rates ranged over two orders according to the difference in the octanol-water partition coefficients or water solubility of the pesticides. Chlornitrofen was persistent in the stream sediment judging from the regression coefficient between concentrations in water and sediment.