Decrease of 4,5,6,7-Tetrachlorophthalide in Paddy Field Soil After Aerial Application

Variation in the fungicide, 4,5,6,7-tetrachlorophthalide (phthalide), in paddy field soil was investigated in order to evaluate its decrease after aerial application by a radio-controlled helicopter. The maximum concentrations of phthalide were 309–320 μg/kg dry, which were 83%–96% of the applied phthalide. The organic carbon normalized soil sorption coefficient (Koc) was calculated to be 94–96 mL/g at 1 h after the application. The calculated Koc values increased to 620–1,300 mL/g from 1 through 9 days after the application and then 4,700–7,200 mL/g 14 days after the application. The half-life of the phthalide was calculated to be 20–31 days.