Photochemical Processes Influencing Pesticide Degradation in Rice Paddies

Abstract

Degradation by sunlight is an important route of dissipation for pesticides applied to rice paddies. The clear and shallow water filters very little of the incident light and allows both direct and indirect photochemical processes to occur within the water column and at the sediment-water interface. Of the indirect processes, degradation by hydroxyl and carbonate radical and singlet oxygen have been shown to be important for selective classes of pesticide chemistry. The ability of pesticides to undergo direct photolysis can be easily evaluated by available protocols conducted by pesticide registrants, while the reactivity of pesticides to radical oxidants are best evaluated by competition kinetic methods. Rate constants for pesticides obtained by these methods can be incorporated into the computer exposure model EXAMS2 to compare the importance of the photochemical processes to other dissipation pathways occurring simultaneously in the rice paddy ecosystem.