Deposition of Aerially Released Bt over a 2 km Sampling Grid

Abstract

A data set was collected to determine the downwind distance at which currently used near-field mechanistic pesticide deposition models (as formulated in AGDISP 8.26) should hand off to long-range pesticide drift models. This article presents the collected data as well as some comparison to existing, near-field deposition models. A 2 km 2 km grid of deposition samplers was deployed in the basin and range country of central Utah. Twenty aerial Bt spraying trials were undertaken. The trials showed very little deposition at 2 km downwind, but measurable material was detected in some of the tests at that distance. Mass balance was attempted but ranged from below 0.1 to over 2 due to high uncertainty in both the method and the collection efficiency of the samplers. Nevertheless, mass balance showed a strong trend with atmospheric stability in these trials, and results indicate that much material lofted out of the sampling domain during trials undertaken in unstable atmospheric conditions. This study confirms the inability of the Lagrangian model as formulated in AGDISP to adequately model fine droplet drift at distances beyond approximately 250 m depending on ambient conditions. A second article will describe the evaluation and configuration of a long-range pesticide drift model interfaced with AGDISP.