Abstract
Simetryn is one of the popular herbicides and detected often in river water in Japan. Therefore, runoff behavior of simetryn from paddy field was estimated under several conditions of precipitation and irrigation by a new fate model. It uses the rates of chemical degradation, aerobic and anaerobic biodegradation in water and soil, diffusion rate in void water of soil, Freundlich type adsorption equilibrium and the rates of two step adsorption onto soil.
It was estimated that the highest runoff concentrations from Niigata and Kumamoto paddy fields were as high as several times of the water quality guide value by the Environment Agency of Japan, and that the total runoff amounts in 60 days were ca. 10-60% of the applied amounts. Besides, the concentration in soil of Niigata paddy field was estimated to be as high as several times of that of Kumamoto paddy field. Thus it was shown that the runoff load could be decreased by scheduling the weed to avoid rainy day, controlling irrigation, and applying the optimum amounts for each area.
