Probabilistic risk assessment of cotton pyrethroids: IV. Landscape-level exposure characterization.

Aquatic exposure arising from the use of pyrethroid insecticides on cotton in the United States was estimated as part of an extensive pyrethroid aquatic risk assessment. The exposure analysis was based on the standard U.S. Environmental Protection Agency tier II regulatory scenario for cotton, which assumes a 1-ha pond surrounded by 10 ha of treated crop, with high levels of runoff, erosion, and drift entering the pond. This regulatory scenario was modified to include simplified landscape-level information on the proximity of cotton to ponds derived from a remote sensing study. This scenario also accounts for the no-spray buffers between pyrethroid applications and surface waters mandated on all cotton pyrethroid labels (which differentiate applications made by air and by ground-based equipment), and for the percentage of cotton area that is treated with pyrethroids. Incorporation of these landscape-level factors into the analysis reduced the predicted aquatic exposure concentrations approximately 50- to 100-fold. Because many other conservative assumptions in the original tier II exposure analysis were not revised, the modified exposure predictions are still overestimates of true field exposure concentrations.