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Off-Target Deposition of Pesticides from Agricultural Aerial Spray Applications

Ecosystems Res. Div., National Exposure Res. Lab., U.S. Environmental Protection Agency, 960 College Station Rd., Athens GA 30605;

David M. Esterly

E.I. duPont de Nemours & Company, Experiment Station, Bldg. 361, Wilmington DE 19880;

Steven G. Perry

Atmospheric Sciences Modeling Div,, Air Resources Lab., National Oceanic and Atmospheric Administration, Research Triangle Park, NC on assignment to National Exposure Res. Lab., U.S. Environmental Protection Agency.

^* Corresponding author (bird-sandra{at}cpamail.epa.gov).

ABSTRACT

Drift of pesticides at the times of aerial spray application is a source of concern because of potential impacts on human health, contamination of crops and livestock, and endangerment of sensitive ecological resources. A substantial body of information from field trials aimed at evaluating off-target deposition of pesticides from aerial spray applications was analyzed. Forty-five trials previously reported in the open literature were extrapolated and normalized and quantitatively compared to trials performed during the summer of 1992 in Plainview, TX, by the Spray Drift Task Force (SDTF). In general, the results observed in the SDTF studies are consistent with those reported in the open literature. Both sets of data show median values of pesticide deposition dropping from 5% of the nominal application rate at 30 m downwind to 0.5% at 150 m during low-flight applications. Based on results from these trials and others reported in the literature, droplet size is consistently the primary application variable controlling off-target drift during low-flight applications. Application of a fine spray with a volume median diameter (VMD) <200 µm in an oil-based carrier, typical of ultra-low volume (ULV) applications, can yield up to a 10-fold increase in off-target drift, whereas use of solid stream nozzles producing a relatively coarse spray (VMD > 500 µm) can reduce drift 10-fold relative to conventional application equipment producing a medium spray (VMD 300 µm). Off-target drift increases with increasing wind speeds, but the magnitude of the effect is less significant than effects of droplet size. The SDTF trials were performed under neutral to unstable atmospheric conditions and the effects of stability for this range of conditions were not distinguishable from the effects of wind speed. In previously reported studies under a wider range of conditions, stability effects are most significant 100 m downwind and beyond with an increase in deposition of more than 10-fold reported on occasion between unstable and very stable conditions. Currently, pesticide use labels restrict application during strongly stable atmospheric conditions and during high winds. In conjunction with these weather-related restrictions, clear specification of nozzles and operating conditions is the best approach for effective control of off-site drift and deposition from aerial applications.

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