Document ID: EPA-HQ-OPP-2013-0676-0004
Agency: epa
Document Type: Supporting & Related Material
Title: 
Posted Date: 2014-01-30T05:00Z

Use of AgDRIFT and AGDISP in OPP Risk Assessments
      
      Scientific Basis for the Models
      EPA uses two peer-reviewed spray drift models (AgDRIFT and AGDISP) to estimate the contribution of spray drift to ecological and human health risk assessments. Both models estimate drift fractions, as applicable to spray of liquid materials. In addition, AGDISP has limited capability to estimate drift fractions from dry materials application. 
      The Spray Drift Task Force (SDTF), a coalition of pesticide registrants, was formed in 1990, with the goal of developing a generic database of off-target drift information in support of spray drift data requirements for the registration of pesticides. In developing test protocols for laboratory and field studies, the SDTF consulted with technical experts from research institutions around the world and compiled a list of over 2,000 drift-related studies from the scientific literature. Between 1992 and 1995, the SDTF conducted a series of field and laboratory studies that provided the scientific basis for spray deposition and downwind drift predictions. These SDTF studies were designed and conducted in consultation with scientists at universities and research institutions, and were intended to provide a quantitative measure of the potential environmental exposure of non-target organisms to a chemical of concern as well as for use in predicting human exposure from spray drift.  More than 300 applications were made in 10 field studies to quantify spray drift from aerial, ground, and orchard airblast applications. After the field studies were completed, the results were peer reviewed at two public meetings of the Federal Insecticide Fungicide and Rodenticide Act (FIFRA) Scientific Advisory Panel (SAP).  The report from the December 1997 meeting is available at http://www.epa.gov/scipoly/sap/meetings/1997/121097_mtg.htm  and the report from the July 1999 meeting is available at http://www.epa.gov/scipoly/sap/meetings/1999/072099_mtg.htm.  
      Using the data from the SDTF field trials, the AgDRIFT model was developed to assess a variety of spray drift conditions.  AgDRIFT, a modified version of the AGricultural DISPersal (AGDISP) model developed by the US Forest Service, was created under a Cooperative Research and Development Agreement between the EPA, the US Department of Agriculture's Forest Service, and the SDTF for estimating spray drift from aerial, ground, and orchard airblast applications.  AGDISP is a "first-principles," science-based aerial model which has been well established and can be used beyond the STDF test conditions. AGDISP/AgDRIFT algorithms have been formally validated using 161 SDTF field trial data for a wide range of aerial application and meteorological conditions  .   
      Based on the SDTF data, AgDRIFT can predict Tier 1 (screening-level) drift fractions for aerial, ground, and orchard air blast applications.  AgDRIFT can also predict drift fractions for higher-tiered modeling for aerial applications.   Input features of the AGDISP and AgDRIFT models provide the capability to alter over 30 parameters related to the aerial application method including application technique, meteorology, spray material and atmospheric stability.  AgDRIFT can also provide alternate, empirically based, predictions for ground and airblast sprayers which represent applications under different conditions. The AGDISP model also has application modules for ground spray and limited capability to model the spray of dry materials. 
      In general, OPP 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. Both of these spray drift models are widely accepted in the scientific community for determining off-site spray drift and have been used by many US federal agencies and by pesticide regulatory authorities worldwide.  
      Several updates have occurred in recent years to improve the performance and reliability of AgDRIFT and AGDISP.  When updates occur, OPP evaluates the latest versions of these models, and determines if changes are needed to OPP's methodologies for estimating exposure and risk.  For more information regarding these models, refer to:  http://www.epa.gov/pesticides/science/models_db.htm.
      
      Harmonized Model-Generated Spray Drift Fractions Usage in OPP Risk Assessments
      The Agency has determined to use the AgDRIFT model generated spray drift fractions to assess spray drift exposure for pesticide applications. A harmonized understanding of the process for estimating spray drift fractions by both ecological and human health risk assessors will ensure consistency and provide more realistic risk estimates. Both of the draft guidances use harmonized AgDRIFT model scenarios for risk assessment purposes.  In the absence of application parameters such as droplet size spectrum, release height, wind speed, and percent of swath displacement, as well as relevant meteorological conditions, on the product label, the following model input parameters for various application scenarios will be used to estimate spray drift fractions:
          *              Ground Applications: high boom height, ASAE very fine to fine Drop Size Distribution (DSD), and a 90[th] percentile data without extended settings  
          * Orchard/Airblast:  Sparse (Young/Dormant) AgDRIFT
          # Aerial Application: Tier I module. Additional parameters for aerial application include the following:
                o ASAE Fine to Medium DSD, 
                o a wind speed of 10 mph
                o 60 foot swath width,  
                o release height of 10 feet,
                o Boom Length of 76.3%, and
                o Swath Displacement of 37%. 
      
      Most of these parameters are recommended in the AgDRIFT model by the SDTF.  The recommended droplet size spectra were also suggested in university extension material for various pesticide types or uses and were consistent with SDTF recommendations.
      Currently, this harmonized approach is being implemented for both ecological and human health risk assessments.