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Model Report

Generic Estimated Environmental Concentration

Last Revision Date: 11/16/2009 View as PDF
General Information Back to Top
Model Abbreviated Name:

Model Extended Name:

Generic Estimated Environmental Concentration
Model Overview/Abstract:
The USEPA Office of Pesticide Programs (OPP) is required by the Federal Insecticide Fungicide Rodenticide Act (FIFRA) to carry out a risk/benefit assessment for pesticides which are to be registered and used in the United States. The OPP Environmental Fate and Effects Division (EFED) are charged with carrying out both the aquatic and terrestrial ecological portions of this assessment. To assess the potential exposure to non-target species living in water bodies in proximity to sites where a pesticide is applied, EFED estimates pesticide aquatic exposure concentrations to which species may be exposure through permitted use of the chemical. These concentrations are then compared to concentrations known to be toxic from laboratory tests. This exposure / hazard (effects) ratio is used as an indication of potential risk to organisms.

Concentrations of pesticide in surface water for an exposure/risk assessment may be estimated through a program of monitoring (field sampling) or through computer simulation or both. As either of these options may involve expenditure of substantial time and money, EFED has developed GENEEC2 as a simple screening model by which the need for monitoring or more advanced computer simulation can be determined.

EFED had several criteria for development of a screening model of this type. First, the model should be fast and easy to use. Second, it should require only a few, readily available input parameters. Third, the input parameters should be those most significant to represent pesticide amount and type of application as well as transport to and persistence in surface water. Fourth, the predicted concentration values should be (1) higher than most of the highest of the concentration values predicted in the next higher tier of modeling and (2) higher than most of the upper level concentration values that are measured in the field at vulnerable sites. The last requirement is design to preclude the possibility that a potentially hazardous chemical pass the screen early in the assessment process and escape sufficient review. A vulnerable site is defined as one at which high concentration levels are expected due to the occurrence of those conditions of pesticide application, weather, and soils known to favor transport to and persistence in surface water.

The GENEEC2 model was developed to meet these criteria. GENEEC2 uses a vulnerable standard field-rural water body watershed (referred to as “standard pond”) scenario which was selected for all screening level drinking water exposure assessments. This scenario assumes that sufficient rain falls on a ten-hectare, treated field to cause up to ten percent of the applied pesticide to run off into a one hectare, two-meter deep pond. Using the method, all pesticides can be compared under the same conditions.

“Passing” a GENEEC2 screening assessment (estimated concentration is lower than laboratory effects data) is an indicator or acceptable risk. “Failing” a GENEEC2 screening assessment (estimated concentration is higher than laboratory effects data) is not an indicator of unacceptable risk, but that additional assessment work must be carried out.

Keywords: Water Quality Modeling, Pesticide Environmental Fate and, Transport Modeling, Aquatic Exposure Assessment, Pesticide Exposure Assessment, Screening Model, Estimated Environmental Concentration, EEC, PEC
Model Technical Contact Information:
Ronald Parker, Ph.D.
U.S. Environmental Protection Agency
Office of Pesticide Programs
Phone: (703) 305-5505
Fax: (703) 305-6019
Model Homepage: http://www.epa.gov/oppefed1/models/water/#geneec2
Substantive Changes from Prior Version: The main differences between versions 1.0 and 2.0 include:
  • an entirely new binding curve to represent dissolved concentration as a function of Kd
  • the use of the binding parameter, Kd in preference to Koc to represent pesticide attachment to soil, to organic matter or to water-body bottom sediments
  • a change in the recommendation for depth of incorporation
  • a change in the timing of the single event rainstorm for chemicals which receive multiple applications
  • addition of a subroutine from the SDTF to estimate spray drift
  • a change in the time durations of the output values to better match the durations of relevant toxicity tests.

User Information Back to Top
Technical Requirements
Computer Hardware
Any PC
Compatible Operating Systems
Download Information
The GENEEC2 model is available to be downloaded.
Using the Model
Basic Model Inputs
Application rate, number, interval and method; incorporation depth; spray drift data; buffer data; soil/water partition coefficient (Kd or Koc); degradation inputs to the model (aerobic aquatic metabolism, abiotic hydrolysis, direct aquatic photolysis)
Basic Model Outputs
Outputs from this model are the peak daily, average 4 day, average 21 day, average 60 day, average 90 day and average 365 day pesticide concentrations in the standard pond.
User Support
User's Guide Available?
The GENEEC2 User's Guide is available to be downloaded.
User Qualifications
Understanding of pesticide fate and transport.

Model Science Back to Top
Problem Identification
GENEEC2 is a FORTRAN based meta-model of the USEPA PRZM and EXAMS environmental fate and transport models. The user answers consecutive questions presented on the screen and output is both displayed on the screen and to a printable file.
Summary of Model Structure and Methods
GENEEC2 simulates pesticide runoff, spray-drift, adsorption and degradation/dissipation from a treated, ten-hectare field to an adjacent, static, one-hectare water body. Pesticide in runoff from the field ranges from 10 percent to 0.01 percent of the applied pesticide depending on the fraction of the pesticide that is adsorbed to field soil and to water body bottom sediments. Model output concentrations are designed to exceed field-monitored values at most sites most of the time based on comparisons with measured data.

GENEEC2 equations are curve-matching functions developed to mimic output of the linked PRZM/ EXAMS models. Biological degradation in the treated field and photolytic, hydrolytic and biological degradation in the water body follow a first order processes. Concentrations from runoff and from spray-drift are calculated separately and summed to give the final output concentration. Multiple-day, mean pesticide concentrations are arithmetic averages of the appropriate number of daily values beginning with the first day.

Model Evaluation
QA/QC, peer review and code verification through Environmental Fate and Effects Division, Office of Pesticide Programs

Sensitivity and uncertainty analyses unavailable

Parker, R.D., R.D.Jones, and H.P. Nelson , 1995. GENEEC: A Screening Model for Pesticide Environmental Exposure Assessment., in Proceedings of the International Exposure Symposium on Water Quality Modeling; American Society of Agricultural Engineers, pp. 485-490; Orlando, Florida.

Key Limitations to Model Scope
GENEEC2 predictions of pesticide concentrations in surface water are designed to be higher than most concentrations from field sampling as well as most concentrations predicted by other, more complex models most of the time.
Case Studies
Many GENEEC2 applications can be seen at: http://cfpub.epa.gov/oppref/rereg/status.cfm?show=rereg

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