Characterizing aquatic ecological risks from pesticides using a diquat dibromide case study. I. Probabilistic exposure estimates

Diquat dibromide (6,7-dihydrodipyrido [1,2-a:2',1'-c] pyrazinediium dibromi de; diquat) is one of the few herbicides registered for direct application to water systems in the United States to control noxious aquatic weeds. The dissipation of diquat dibromide in aquatic environments is highly dependen t on the hydrodynamic and physicochemical characteristics of the receiving water body. Computer simulations (EXAMSII) were conducted to evaluate the v ariability of diquat cation dissipation in farm ponds, lakes, and canals fo llowing applications to control aquatic weeds across the United States. Wit hin each scenario, probabilities were assigned to varying values for physic al properties (velocity, dispersion, suspended sediment concentrations, bed sediment properties, and plant biomass) using a joint probability method: and the results were expressed as spatially and temporally explicit cumulat ive probability distributions by water-body type on a regional scale. The r esults showed that the dissipation of diquat cation in water was very rapid , with the highest exposure concentrations (90th percentile) predicted to b e 0.194 mg/L diquat cation 1 h after treatment and typically reaching 0.01 mg/L by 96 h.