CHLORPYRIFOS RELEASE RATE FROM CLAY GRANULES - EXPERIMENTAL-OBSERVATIONS AND SIMPLE ALGORITHM DEVELOPMENT FOR USE IN COMPUTER-BASED EXPOSURE ASSESSMENTS

Environmental models can approximate pesticide mass leaving a treated field in runoff water and sediment but do not typically account for co ntrolled-release formulations such as granules. Mathematical algorithm s are developed from experimental observations to predict the environm ental release rate of chlorpyrifos from a commercial formulation [Lors ban (trademark of Dow AgroSciences) 15G insecticide] into the surround ing soil environment. Chlorpyrifos release rates into flowing water we re measured by HPLC analysis of water pumped through a column containi ng 15G granules. Analysis of these observations provides a basis for p redicting water-induced release rates during natural precipitation eve nts. Chlorpyrifos diffusion and volatilization rates from the granule in the absence of precipitation were determined from experimental obse rvations using first-order kinetic modeling. Multiyear simulations and distribution analysis were performed using the GLEAMS model for the M idwestern corn belt region to determine release mechanism sensitivity and edge-of-field runoff potential for chlorpyrifos in 15G granules wh en the release mechanisms of advection, volatilization, and diffusion are accounted for. It is found that advection release was the dominate mechanism for chlorpyrifos release from clay granules, followed by di ffusion and volatilization for Midwestern U.S. scenarios, Predicted lo sses of chlorpyrifos in runoff from soil incorporated applications are <1% of applied.