Predicting the Dissipation of Bensulfuron Methyl and Azimsulfuron in Rice Paddies Using the Computer Model EXAMS2

Abstract

A computer model scenario using EXAMS2 was developed to estimate concentrations of sulfonylurea herbicides, bensulfuron methyl and azimsulfuron in paddy water. The rapid disappearance of the two chemicals from water could be described only when indirect photolysis (represented by hydroxyl radical reactions) was included as a dissipation pathway in the modeling scenario with partitioning to soil, hydrolysis, direct photolysis and metabolism. The model scenarios were calibrated against paddy water dissipation data from two 1m² simulated paddies to obtain values for the dispersion coefficient and oxidative radical concentration in each plot. The EXAMS2 scenario successfully predicted the partitioning and degradation that led to half-lives in paddy water of 3-4 days observed in field studies conducted in Japan. Verification of the EXAMS2 model by comparison with field results was necessary for establishing realistic environmental concentrations of bensulfuron methyl and azimsulfuron in paddy water.