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February 13, 2014
Modeling pesticide spray drift

Accounting for spray drift of an outdoor pesticide product is an important element in applications to the EPA to register that product.  Accordingly, the EPA has released two draft documents the Agency plans to use to review estimates of the contributions of spray drift.

One document addresses the role of pesticide spray drift in drinking water and ecological risk assessments; the second addresses residential exposure to pesticide spray drift.  The Agency will accept comments on the two drafts until March 31, 2014.

“Once final, these guidance documents will be posted on EPA’s Web site to ensure consistent risk practices and provide transparency for pesticide registrants and other interested stakeholders,” says the Agency.

Drift models

The EPA uses two peer-reviewed spray drift models (AgDRIFT and AGDISP) to assess the contributions of spray drift to ecological and human health risk assessments. Both models estimate drift fractions applicable to the spray of liquid materials.  In general, the Agency uses the AgDRIFT model to assess spray drift from agricultural applications, whereas AGDISP is used for other types of pesticide applications, such as aerial application of mosquito adulticides.  AGDISP has limited capability to estimate drift fractions from dry materials application, the EPA notes.

Spray drift in ecological risk assessments

This draft mainly revises how the EPA uses default drift values in these ecological risk assessments.  Beginning in the 1990s, EPA’s approach was to use default drift values—developed using best professional judgment—of 5 percent (aerial application), 3 percent (airblast application), and 1 percent (ground application) in terrestrial and aquatic assessments. 

In 2004, EPA staff performed a comparison study of these percentages and found they potentially underestimate off-site deposition of spray drift under certain scenarios when compared to model predicted values.  In the draft, the EPA uses more precise model input parameters.  Use of these inputs in the AgDRIFT model should result in more-realistic estimates of exposure from spray drift deposition for all terrestrial and aquatic environments, says the Agency.

Spray drift in human health risk assessments

This document is an addendum to EPA’s SOPs for Residential Exposure Assessment,which introduced procedures for assessing potential risks to human health associated with spray drift.  The primary concern with spray drift in the human health risk assessment process is indirect contact children have with pesticides on contaminated surfaces such as lawns.  The draft guidance describes scenarios for which quantitative risk assessments for spray drift would generally be appropriate, and provides the information needed to complete a residential turf assessment using spray drift fractions predicted by AgDRIFT.  Values are calculated using lawns at different distances from a treatment area— adjoining it to 300 feet away.

The Agency says it is particularly interested in receiving comments on the model-generated spray drift values described in either or both the draft documents.

Click here for the draft documents.

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