Use of PRZM-3 to validate a laboratory to field degradation conceptual model

The test substance, boscalid, was applied at two field sites, but, depending on how kinetic calculations were performed, the time required for 50% of the initial compound concentration to dissipate (half-life) ranged from 27 to 200 days. Laboratory aerobic soil studies indicated that the DT50 for the compound was about 108 days, and since compound dissipation rates are typically shorter in the field than those observed in the laboratory, confidence in the field half-life calculations was questioned. Researchers have asked whether exposure models might be useful for relating laboratory to field behavior, especially when results are difficult to reconcile between the two systems. In order to determine how realistic field dissipation-time kinetic calculations were, a 60-day DT50 (some masked data), 150-day DT50 and a 196-day rate constant (t1/2) kinetic result were entered into the Pesticide Root Zone Model (PRZM-3) for a California site. A 27-day DT50, and a 200 day t1/2 kinetic result were also entered into the model for a Florida site. Results indicate that using the shortest DT50 solution in PRZM-3 provided the best fit to actual field data. Results from this study suggest that PRZM-3 can be used successfully to address uncertainty between a laboratory conceptual understanding of a compound's behavior and the actual behavior observed in the field.