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Risk Assessment Tiers

The US Environmental Protection Agency (USEPA) uses multiple tiers for aquatic exposure assessment.

Tier 1

For drinking water assessments, the USEPA Office of Pesticide Program uses FIRST (FQPA Index Reservoir Screening Tool) as a screening model to estimate agrochemical concentrations in surface water, and SCI-GROW (Screening Concentration In GROund Water) as a Tier 1 screening model to estimate agrochemical concentrations in groundwater. If aquatic exposure estimates are required, GENEEC2 (GENeric Estimated Environmental Concentration) is used to estimate agrochemical concentrations in surface water and SCI-GROW to estimate agrochemical concentrations in groundwater (USEPA, 2007).

Tier 2

For Tier 2 surface water screening assessments, USEPA uses the linked PRZM and EXAMS models (PRZM-EXAMS) to accommodate the specific characteristics of the chemical and include site-specific information regarding the application method and impact of daily weather on the treated field over a period of 30 plus years. For many crops, standard scenarios have been developed (USEPA, 2007).

Tier 3

Tier 3 differs from Tiers 1 and 2 in that both use site (percent area treated, percent crop treated, and application rate), and weather parameters (use of local weather stations) are varied. The aim of Tier 3 assessments is to examine various hypothetical circumstances that are representative of the potential application area and conditions under which the agrochemical is likely to be used. Tier 3 modeling results in development of a distribution of EECs that might be expected across use markets, recognizing that both soil properties and weather patterns will vary significantly by market region and years of use. Tier 3 analysis is used by chemical companies to address environmental exposure concerns that arise during product re-registration processes (Whiteford et al., 2003).

Tier 4

Tier 4 assessments are complex analyses that assess how agrochemicals are likely to interact with a landscape composed of hundreds of thousands of acres. The landscape has diverse soils and climates, varied proximities of treated fields to receiving waters, and randomly distributed bodies of water. Geographic information systems (GIS) are commonly used at Tier 4. GIS allows graphical evaluation of concurrent risk factors (within the regions of use) that heighten concerns. In other words, GIS can help to distinguish high risk versus low risk areas of use on a regional basis. Modeling and monitoring often are combined within Tier 4 to provide a more complete understanding of the distribution of exposure occurring within treated watersheds (Whiteford et al., 2003)

References

Ecological Committee for FIFRA Risk Assessment Methods (ECOFRAM), 1999. ECOFRAM Aquatic Report. Sponsored by the US Environmental Protection Agency, Office of Pesticide Programs.

Whitford F., D. Urban, M Mayes and Jeff Wolt, 2003, Pesticides And Ecological Risk Assessment,History, Science, And Process, Editor A, Blessing, , Purdue Pesticide Programs PPP-41, Available from http://www.btny.purdue.edu/Pubs/PPP/PPP-41/PPP41.html#epa [Accessed May 22, 2008]

USEPA, 2007, About Water Models - Water Exposure Models Used by the Office of Pesticide Programs, Available from http://www.epa.gov/oppefed1/models/water/models4.htm [Accessed May 22, 2008]