Pesticide hydroxyl radical rate constants: Measurements and estimates of their importance in aquatic environments

Pesticide degradation by hydroxyl radicals produced in natural water is often cited as an important dissipation mechanism, yet its significance is rarely assessed with other dissipation processes occurring simultaneously in the aquatic environment. The computer model Exposure Analysis Modeling System (EXAMS) is useful for this purpose because it incorporates other dissipation processes and allows the assessment of their dynamics under different environmental conditions. Rate constants of reaction with photochemically generated hydroxyl radicals were measured for the herbicides sulfometuron, bensulfuron methyl, chlorsulfuron, triclopyr, dicamba, molinate, thiobencarb, 2,4-D, mecoprop, and oxadiazon, for the insecticides chlorpyrifos and imidacloprid, and for the fungicide chlorothalonil by competition kinetic methods and were used as the input value for second-order rate oxidation rate constant (KOX) in EXAMS. Hydroxyl radical degradation was estimated to be most significant in shallow water, where sunlight is less attenuated; however, in relatively clear water, it still could contribute to degradation at greater depths for compounds slowly degraded by other mechanisms. For carbaryl at shallow depths, degradation by hydroxyl radicals was predicted to contribute to less than 1% of the degradation profile at pH 8 to as much as 15% of the profile at pH 6. As unique degradation products may be generated by indirect photolysis mechanisms, it should be possible to use EXAMS to estimate when such products may occur for a particular chemical and environmental situation.