Submitted to: American Chemical Society Symposium Series
Publication Type: Book / Chapter
Publication Acceptance Date: 6/1/2001
Publication Date: 1/20/2003
Citation: Wauchope R.D., Potter, T.L., Culbreath, A.K. 2003. Relating field dissipation and laboratory studies through modeling: chlorothalonil dissipation after multiple applications in peanut. In: Arthur, E.L., Barefoot, A.C., Clay, V.E., editors.Terrestrial field dissipation studies: purpose, design and interpretation. American Chemical Society Symp. Series, #842, ACS, Washington, DC. p 287-303. Interpretive Summary: One of the most important and expensive tests that manufacturers of pesticides are required to do to test the safety of a new pesticide is the ?Terrestrial Field Dissipation Test?. In this test the manufacturer applies the experimental chemical to the soil in a field and attempts to determine its fate: how it dissipates and degrades and how long the material persists. This test must be done exactly as specified by EPA and both the manufacturers and EPA are engaged in examining how the test might be modified to get more information for the same money. As part of a symposium on this subject, this paper was presented to show how running a sophisticated pesticide environmental fate computer simulation model alongside the field test can help to interpret the results. We demonstrate this for soil behavior of the important peanut fungicide chlorothalonil and we also extend the experiment to include multiple applications of chemicals and applications to crop foliage. These are realistic conditions for many pesticides but the standard experiment does not cover those conditions. The model provided excellent insights into the experimental results that would have been hidden without this analysis.
Technical Abstract: A computer simulation model can provide useful environmental risk information for pesticides based on rather limited field and laboratory data, if that data provides calibration of the model for the critical processes controlling dissipation. We used the USDA-Agricultural Research Service Root Zone Water Quality Model (RZWQM) to estimate leachate and runoff of chlorothalonil fungicide and its soil degradates during a growing season in which a total of seven applications were made at intervals of 14 days to peanut plants. RZWQM provides detailed algorithms to describe broadcast pesticide application to a combined foliage/soil target, dissipation within both targets, and movement in runoff and leachate water as a function of weather and soil moisture. The model provides an integrated analysis showing how initial partitioning of chlorothalonil between foliage and soil, washoff from foliage to soil, and degradation on the foliage and soil surface (actually the top cm of soil) limit the potential of the parent chlorothalonil and its degradates to leach. However, the model indicates that under severe rainfall conditions during the application period, significant quantities of the parent compound may be transported by runoff.