Predicting the dissipation of bensulfuron methyl and azimsulfuron in rice paddies using the computer model EXAMS2

A computer model scenario using EXAMS2 was developed to estimate concentrat ions of sulfonylurea herbicides, bensulfuron methyl and azimsulfuron in pad dy water. The rapid disappearance of the two chemicals from water could be described only when:indirect photolysis (represented by hydroxyl radical re actions) was included as a dissipation pathway in the modeling scenario wit h partitioning to soil, hydrolysis, direct photolysis and metabolism. The m odel scenarios were calibrated against paddy water dissipation data from tw o 1 m(2) simulated paddies to obtain values for the dispersion coefficient and oxidative radical concentration in each plot. The EXAMS2 scenario succe ssfully predicted the partitioning and degradation that led to half-lives i n paddy water of 3-4 days observed in field studies conducted in Japan. Ver ification of the EXAMS2 model by comparison with field results was necessar y for establishing realistic environmental concentrations of bensulfuron me thyl and azimsulfuron in paddy water.