Development and Use of GENEEC Version 2.0 for Pesticide
Aquatic Ecological Exposure Assessment
May 1, 2001
On this Page
- Development of a Tiered System of Modeling
- Development of GENEEC Version 1.0
- Differences Between GENEEC Version 1.0 and Version 2.0
- Relationship Between Binding and the Dissolved Concentration
- Use of the Binding Parameter, Kd in Preference to Koc
- Changes in Recommendations for Type and Depth of Incorporation
- 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
- Change in the Time Durations of the Output Values to Better Match the Durations of Relevant Toxicity Tests
- Other Processes Simulated
The USEPA Office of Pesticide Programs (OPP) is required by the Federal Insecticide Fungicide Rodenticide Act (FIFRA) to assess the risk posed by pesticides to human health and the environment. The OPP Environmental Fate and Effects Division (EFED) is charged with carrying out the environmental portion of this assessment. To assess the risk to aquatic life posed by each chemical, EFED estimates pesticide concentrations which would be expected in the environment from normal use (exposure) and compares them to concentrations known to be toxic from laboratory tests (hazard). This exposure / hazard ratio is used as an indication of potential ecological risk to non-target species in the environment.
EFED has been performing pesticide aquatic exposure assessments as a part of the ecological risk assessment process for a number of years. Exposure assessments draw on both measured pesticide quantities in the field as well as computer modeling to establish the concentration levels which might be expected in significant aquatic habitats. With the development of enhanced environmental fate and transport models such as the Pesticide Root Zone Model (PRZM) (Carsel et.al.,1984), Groundwater Loading Effects of Agricultural Management Systems (GLEAMS) (Leonard et al., 1989) and the EXposure Analysis Modeling System (EXAMS) (BURNS et.al., 1991), computer modeling of pesticide exposure began to play a larger role in EFED's risk assessments in the early 1990's.
At that time, in response to EFED's need to have a standard aquatic environment in which all chemicals could be assessed and compared on an equal footing, a "standard agricultural field-farm pond" scenario was selected for all aquatic exposure assessments. This "standard pond" scenario assumes that rainfall onto a treated, 10 hectare agricultural field causes pesticide-laden runoff into a one hectare; 20,000 cubic meter volume; 2.00 meter deep water-body. Although this "standard scenario" was designed to predict pesticide concentrations in the standard farm pond, it has been shown to be a good predictor of upper level pesticide concentrations in small but ecologically important upland streams (Effland et al., 1999).
A system was developed using the electronically linked PRZM and EXAMS models and this "standard pond" scenario to simulate pesticide applications to most US agricultural crops simulating local soil, weather and farm management in the areas in which each are grown. Output from this modeling is daily pesticide concentrations in the standard farm pond over the thirty-six year period for which rainfall data is available. This became the EFED standard method for pesticide aquatic ecological exposure assessment.
As this method was relatively labor-intensive and therefore time consuming, a "trigger" or "screening" mechanism was used to establish which chemicals were most likely to pose higher risk and should be assessed in this manner. The screening mechanism established was termed the "Back-of-the-Envelope" calculation and was based largely on the solubility of the chemical. With the advent of better models and doubts about the usefulness of a chemical's solubility as a relevant screen, work was begun in 1994 to develop a new screen that would be more consistent with other modeling approaches and better represent the pesticide parameters that are linked to pesticide transport to and persistence in surface water. The result was the GENEEC (GENeric Estimated Exposure Concentration) (Parker, et. al., 1995) model which replaced the "Back-of-the-Envelope" calculation in mid-1995 (World Wildlife Fund, 1992).
Development of a Tiered System of Modeling.
Along with development of new modeling tools and methods, EFED has developed a tiered approach to determine the appropriate level of modeling needed to perform a risk assessment for each chemical. This tiered approach is designed to minimize the amount of analysis required to evaluate any given chemical. Each of the tiers is designed to screen out pesticides by requiring higher, more complex levels of investigation only for those that have not passed the previous tier. Each tier screens out a percentage of pesticides from having to undergo a more rigorous review prior to registration or reregistration. "Passing" a given assessment tier indicates that there is a low possibility of risk to the aquatic environment. "Failing" an assessment tier, however, does not mean the chemical is likely to cause environmental problems, but that the assessment should continue on to the next higher assessment tier. The end result of this tiered modeling system will ideally be as thorough an analysis as is necessary for each pesticide and will focus greatest resources and efforts toward areas of greatest potential ecological threat. As a matter of policy, OPP does not take significant regulatory action based upon the results of tier 1 screening models.
Development of GENEEC Version 1.0.
EFED had several criteria for development of a first tier screening model.
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
- higher than most of the highest of the values predicted in the next higher tier of modeling and
- 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 potentially hazardous chemicals 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.
GENEEC Version 1.0 was developed with these conditions in mind. It was designed to mimic a much more sophisticated PRZM/EXAMS simulation but requires far fewer inputs and much less time and effort to use. The model uses a chemical's label application information, its soil/water partition data and its degradation kinetics to estimate high level exposure values in the same EFED "standard" agricultural field / farm pond scenario as used with PRZM/EXAMS simulations. The program is generic in that it does not consider differences in climate, soils, topography or crop in estimating potential pesticide exposure.
GENEEC is also simpler than the PRZM and EXAMS models in its treatment of hydrology. The linked PRZM and EXAMS models simulate the impact of daily weather on the treated agricultural field over a period of thirty-six years. During this time, pesticide is washed-off of the field into the water-body by twenty to forty rainfall/runoff events per year. Each new addition of pesticide to the water-body adds to the pesticide which has arrived earlier either through previous runoff events or through spray-drift and begins degrading on the day it reaches the water. GENEEC, on the other hand, is a single event model. It assumes one single large rainfall/runoff event occurs that removes a large quantity of pesticide from the field to the water all at one time. Longer-term, multiple-day average concentration values are calculated based on the peak day value and subsequent values considering degradation processes.
Differences Between GENEEC Version 1.0 and Version 2.0
GENEEC Version 2.0 was developed in response to suggestions for improvements by model users, by the desire to stay current with the newer versions of the PRZM (Carsel, 1997) and EXAMS (Burns, 1997) programs upon which GENEEC is based and by availability of much improved data on spray drift and quantitative methods of estimation of offsite drift developed by the Spray Drift Task Force (SDTF). 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 and suspended 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; and
a change in the time durations of the output values to better match the durations of relevant toxicity tests.
These changes were made as follows:
Relationship Between Binding and the Dissolved Concentration.
The main operator in GENEEC Version 1.0 is a pesticide's organic carbon normalized equilibrium partition coefficient (Koc). The Koc is defined as the equilibrium adsorption coefficient (Kd) normalized to the soil's organic carbon (OC) content. It is calculated by dividing the Kd value by the organic carbon fraction. Initial development of Version 1.0 of the program began by exploring the exposure impact of full range of chemical Koc values on the dissolved pesticide concentration in the standard field/pond system using the linked PRZM1 and EXAMS2.94 models. This was accomplished by repetitively increasing the Koc value in both programs by an order of magnitude, running each new simulation and then recording the resulting instantaneous dissolved concentration. This gave a series of dissolved pesticide concentrations in the pond as a function of Koc. The S-shape of the resulting curve suggested that a four-parameter Morgan-Mercer-Flodin (1975) type model might replicate the function most closely. This function was fit and the resulting curve was programmed into the model. The EXAMS parameter PRBEN remained at its default value of 0.5 to equally divide influent adsorbed pesticide between the water column and the bottom sediments.
The same process was followed in development of Version 2.0 except that the Kd value was used in place of the Koc and the PRZM3.12 and EXAMS2.97.7 models were used in place of the earlier versions of those programs. The curve described by these successive points was not easy to match with a standard fitting routine, so a process of linear interpolation between the points was used instead. The resulting relationship is used in both GENEEC Version 2.0 and FIRST Version 1.0.
Within the EXAMS program, it is the binding parameter (Koc or Kd) that controls not only the final equilibrium partitioning of the chemical between the dissolved (in water) and adsorbed (to soil or to bottom sediments) phases, but also the rate at which binding takes place to reach this equilibrium. For high Koc or Kd values, the binding takes place largely within the first day while for lower values of Koc or Kd, the process may not be complete for almost a year. In order to accurately mimic this EXAMS process in the GENEEC model, an empirical procedure was carried out to simulate a pseudo-binding rate as a function of Koc. This rate was determined by "turning off" all degradation processes within the PRZM and EXAMS models and calculating an apparent rate constant that would account for the continuous decline in concentration values. Another series of PRZM-EXAMS simulations were run for Koc values ranging from 10-1 to 10+8 and a daily pseudo-binding rate was determined. A four-parameter Morgan-Mercer-Flodin (1975) type function was then fit to calculate these apparent binding rate constants as a function of Koc. This ongoing adsorption within the pond is then programmed to occur simultaneously with chemical and biological degradation processes. For these reasons, pesticide concentration in Version 2.0 are likely to exceed concentrations in Version 1.2 by a small amount
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
Adsorption (binding) tests are performed on soils of different textural classes, pH's and organic matter contents. The soil/water partition coefficient (Kd) is defined as the ratio between the concentration in soil and the concentration in water. It can therefore be used to estimate the dissolved or the adsorbed fraction in a soil-water system for all chemicals. For this reason it is in general the preferred parameter for this purpose and is recommended for use in GENEEC Version 2.0.
The organic carbon normalized soil/water equilibrium partition coefficient (Koc) may be preferred for pesticides for which there is a strong positive correlation between the Kd value and the organic carbon content of the soils on which the adsorption tests were performed. If there is correlation, the multiple Koc values will be less variable than the multiple Kd values and the Koc is likely to be a more accurate estimator. If there is no correlation, use of the Kd is preferable. The Kd / Koc conversion is based on an organic matter content of 2 percent and an organic carbon content of 1.16 percent. If neither the Kd nor the Koc is available, it is recommended to use 0.35 times the Kow value (Burns, 2001, personal communication).
Changes in Recommendations for Type and Depth of Incorporation
Version 1.0 of GENEEC recommended using a depth of 1.0 inches to represent in-furrow application A literature review, however, shows that a value of 2.0 would more accurately represent actual placement of the material. For a banded-incorporated application, version 1.0 recommended a depth of 2.0 inches. A depth of 1.2 inches would be a more accurate representation. Version 1.0 suggested that no incorporation was appropriate for both aerial and ground applications. Version 2.0 suggests no incorporation for aerial and airblast applications. Incorporation of ground applications would be variable depending on the type of application being made (eg. in-furrow ground spray incorporated to 2.0 inches).
Change in the Timing of the Single Event Rainstorm for Chemicals Which Receive Multiple Applications
Some pesticides are designed to degrade very quickly in the field so that in a short time, little residue remains. In order to give "credit" to such pesticides, GENEEC was designed to allow a two day degradation period for the pesticide in the treated agricultural field prior to the single rainfall event which washes the pesticide into the farm pond. The aerobic soil metabolism rate is used to simulate the decline during this two day period as well as during the period between multiple applications prior to the rainfall/runoff event. (If the pesticide label requires that the pesticide be "wetted-in" at the time of application either through irrigation or rainfall, the two day period is not used). In GENEEC Version 1.0, the two day period between application and the rainfall event was provided only following a single application. For multiple applications, the rainstorm was programed to occur immediately on the premise that "credit" for rapid degradation in the field had already been provided by rapid degradation between successive applications. This, however, caused confusion when comparing the impact of single with multiple applications. Version 2.0 is therefore programed to provide for the two day degradation period in the field following either single or multiple applications.
Addition of a Subroutine from the SDTF to Estimate Spray Drift
In GENEEC Version 1.0, spray drift percentages and application efficiency factors were set to fixed values based on literature values and personal communication with experts in the field. Aerial and airblast spray drift directly to the pond was assumed to average 5% of the application rate across the 208 foot width of a one acre square pond. (When the scenario was metricized from acres to hectares, the decision was made to keep the original width of the pond). Ground spray drift directly to the pond was assumed to average 1% of the application rate across the width of the pond. Application efficiency was assumed to be 95% for aerial and airblast spray and 99% for ground spray.
GENEEC Version 2.0 incorporates much more sophisticated spray drift treatment based on availability of much improved data on spray drift and quantitative methods of estimation of offsite drift developed by the Spray Drift Task Force (SDTF). In Version 2.0, the user may simulate application by aerial spray, by airblast spray, by ground spray or by broadcast application of granular material. Spray drift is calculated using a subroutine developed by the Spray Drift Task Force (SDTF) for this purpose. It estimates the ninetieth percentile total down-wind deposition onto the two hundred and eight foot wide water body. The program allows the user to specify either aerial, airblast or ground application, select the spray quality (droplet size distribution) and simulate a no-spray zone between the treated field and the water if one is required by the pesticide label. Note: Spray quality is the droplet size distribution as defined by American Society of Agricultural Engineers (ASAE) Standard 572 for all types of applications.
EFED default spray quality is specified for cases in which none is given on the pesticide label. The default is no buffer unless one is specified on the label. For airblast application only, a safety factor of 3.0 is applied to the SDTF drift estimate. For broadcast application, 100 percent application efficiency is assumed with no pesticide drifting directly to the pond. Biological and chemical degradation of the spray drift in the pond is assumed to begin at the time the chemical enters the pond. Degradation of the pesticide reaching the pond via runoff begins on whichever day it reaches the pond. GENEEC calculates the contributions from spray drift and from runoff independently and then combines the results to give the final concentration.
Change in the Time Durations of the Output Values to Better Match the Durations of Relevant Toxicity Tests
GENEEC Version 1.0 estimated the peak value which occurs on the day of the single large rainstorm as well as multiple day averages over periods of 4, 21, and 56 days. These averages are compared to the results toxicity tests carried out for the same durations in a screening level risk assessment.
GENEEC Version 2.0 estimates the peak value which occurs on the day of the single large rainstorm as well as multiple day averages over periods of 4, 21, 60 and 90 days. These averages are appropriate to comparison with toxicity tests currently performed.
Other Processes Simulated
Overall Degradation Rate
Calculating degradation in the pond is for the purpose of estimating annual average concentration values for chronic exposure assessment. Degradation in the pond considers aerobic aquatic metabolism, abiotic hydrolysis and direct aquatic photolysis. The EXAMS program upon which the aquatic degradation is based considers the amount of light penetration in the simulated water body to calculate an effective photolysis rate based on the rate which is input into the model. Based on EXAMS simulation, the photolysis rate constant in the relatively murky pond is 124 times slower than that in clear water. The overall degradation rate in the pond is calculated by summing the aerobic aquatic metabolism rate constant, the abiotic hydrolysis rate constant and 1/124th of the direct aquatic photolysis rate constant.
Pesticide Soil Incorporation
The program also accounts for pesticide incorporation at the time of application. Incorporation reduces the mass of pesticide available to runoff by a factor equal to the depth of incorporation in inches up to a maximum of six inches.
GENEEC considers the solubility of the chemical only as an upper limit on the dissolved concentration estimate. If the estimated concentration values exceed the user input solubility of the chemical, the concentration values are reduced to the solubility level.
Burns, L.A. 1991. Exposure Analysis Modeling System: Users Guide for EXAMS II version 2.94, Environmental Research Laboratory, Office of Research and Development, U.S. Environmental Protection Agency, Athens, GA.
Burns, L.A. March 1997. Exposure Analysis Modeling System (EXAMSII) Users Guide for Version 2.97.5, Environmental Research Laboratory, Office of Research and Development, U.S. Environmental Protection Agency, Athens, GA.
Carsel, R.F., C.N. Smith, L.A. Mulkey, J.D. Dean and P. Jowise. 1984. Users manual for pesticide root zone model (PRZM): Release 1, Rep. EPA-600/3-84-109, 219 pp. Environmental Research Laboratory, Office of Research and Development, U.S. Environmental Protection Agency, Athens, GA.
R.F. Carsel, J.C.Imhoff, P.R.Hummel, J.M.Cheplick and J.S.Donigian, Jr. 1997. PRZM-3, A Model for Predicting Pesticide and Nitrogen Fate in Crop Root and Unsaturated Soil Zones: Users Manual for Release 3.0; Environmental Research Laboratory, Office of Research and Development, U.S. Environmental Protection Agency, Athens, GA.
Effland, W.R., Thurman, N.C., Kennedy, I. Proposed Methods For Determining Watershed-Derived Percent Cropped Areas and Considerations for Applying Crop Area Adjustments To Surface Water Screening Models; USEPA Office of Pesticide Programs; Presentation To FIFRA Science Advisory Panel, May 27, 1999.
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.