Photochemical processes influencing pesticide degradation in rice paddies

Degradation by sunlight is an important route of dissipation for pesticides applied to rice paddies. The clear and shallow water filters very little o f the incident light and allows both direct and indirect photochemical proc esses to occur within the water column and at the sediment-water interface. Of the indirect processes, degradation by hydroxyl and carbonate radical a nd 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 regi strants, while the reactivity of pesticides to radical oxidants are best ev aluated by competition kinetic methods. Rate constants for pesticides obtai ned by these methods can be incorporated into the computer exposure model E XAMS2 to compare the importance of the photochemical processes to other dis sipation pathways occurring simultaneously in the rice paddy ecosystem.