Measuring of some selected herbicides in paddy surface water in the Saijo Basin, Western Japan

An analysis of the dissipation of herbicides was done in paddy water and in drainage channels of farm paddies as well as in experimental plots. A field study was conducted in three selected paddy farms of Higashi Hiroshima and an experiment was carried out in the Hiroshima Prefecture Agriculture Research Center in the paddy season of the year 2002. The herbicides were mefenacet, thiobencarb, pyributicarb and cyhalofop-butyl. The sample water was analyzed by using gas chromatography after solid phase extraction. The maximum concentration of all herbicides was detected within 1 to 3 days after herbicide application and became trace level after 21 to 28 days. The highest amount of herbicide detected was 338 mu g/L mefenacet in Farm B. No cyhalofop-butyl was detected in the experiment but was found in farm paddies until 7 days. Thiobencarb was detected until 21 and 28 days, respectively, in farm paddies and in the experiment. Pyributicarb was detected until 28 days in farm paddies and in the experiment. We found that the concentration of all herbicides in drainage channels was comparable with the paddy field at 1 day and 3 days and thiobencarb at 1 day.