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About Water Models

Water Exposure Models Used by the Office of Pesticide Programs

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Water Models

When the Office of Pesticide Programs (OPP) in EPA assesses the risk of a pesticide, it considers the toxicity of the pesticide as well as exposure to the pesticide. In assessing risk to aquatic organisms and the environment, OPP routinely estimates concentrations of pesticides in natural water bodies, such as lakes or ponds, when it develops aquatic exposure assessments. The pesticide program also estimates concentrations of pesticides in water bodies when it considers the effect of pesticides on the value of water resources in its water quality assessments. More recently, since the passage of the Food Quality Protection Act (FQPA) of 1996, OPP estimates pesticide concentrations in drinking water when it establishes maximum pesticide residues on food (tolerances).

For both drinking water and aquatic exposure assessments and for water quality assessments, OPP typically relies on field monitoring data as well as mathematical models to generate exposure estimates. Monitoring and modeling are both important tools for assessing pesticide concentrations in water and can provide different types of information. Monitoring tells us what is happening under current use practices and under typical conditions. Although monitoring data can provide a direct estimate of the concentration of a pesticide in water, it does not always provide a reliable estimate of acute exposure because sampling may not occur where the highest concentrations of a pesticide are found.

OPP typically relies on mathematical models to generate the exposure estimates for drinking water and aquatic exposure assessments and water quality assessments. These models calculate estimated environmental concentrations (EECs) using laboratory data that describe how fast the pesticide breaks down to other chemicals and how it moves in the environment. The protocols for these laboratory studies (OECD 106, 111, 305, 307, and 308) can be found at the following Web site: Exit EPA Disclaimer http://www.oecd.org/document/22/0,2340,en_2649_34377_1916054_1_1_1_1,00.html. Although computer modeling provides an indirect estimate of pesticide concentrations, these concentrations can be estimated continuously over long periods of time, and for places that are of most interest for any particular pesticide. Modeling is a useful tool for characterizing vulnerable sites, and can be used to estimate peak concentrations from infrequent, large storms. These concentrations are compared to toxicity data for the pesticide to determine the risk posed in both drinking water and aquatic systems. Computer modeling can also be used to determine how different mitigation practices affect the amount of the pesticide which can run off into water.

In estimating pesticide concentrations in water resources, OPP uses a tiered screening approach involving a combination of models and monitoring data. The intent of the screening approach is to estimate pesticide concentration in water from sites that are highly vulnerable to runoff or leaching so that the program can be confident that any pesticide that passes the screening tiers poses a low possibility of harming human health, wildlife, or the environment. Failing a tier, however, does not necessarily mean the chemical is likely to cause health or environmental problems, but rather that there is a need to move to a higher tier and conduct a more refined assessment. This tiered modeling system is designed to provide a thorough analysis of each pesticide, while at the same time focus OPP's efforts on those pesticides that pose the greatest potential risk. For more information on this approach, refer to the following website about Science Policy Issues and Guidance Documents: (https://www.epa.gov/oppfead1/trac/science.)

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Model Names and Specific Uses

For drinking water assessments, the program uses FIRST (FQPA Index Reservoir Screening Tool) as a Tier 1 screening model to estimate pesticide concentrations in surface water and SCIGROW (Screening Concentration In GROund Water) as a Tier 1 screening model to estimate pesticide concentrations in ground water. For aquatic exposure assessments, OPP uses GENEEC2 (GENeric Estimated Environmental Concentration) to estimate pesticide concentrations in surface water and SCIGROW to estimate pesticide concentrations in ground water. For Tier 2 surface water screening assessments, OPP uses the linked PRZM and EXAMS models (PRZM-EXAMS) which better accommodate the specific characteristics of the chemical and which include more site-specific information regarding the application method and impact of daily weather on the treated field over a period of 36 years. As a higher-level screening tool, PRZM-EXAMS uses maximum application rates and frequencies for a vulnerable drinking water reservoir. Additional refinements in application rates may be considered if usage data indicates they are appropriate. Although the Agency does not use computer modeling to estimate pesticide concentrations in ground water above the Tier 1 screening model, it has initiated a proposal to develop a Tier II screening model.

Although exposure models make it easy to evaluate the impacts of numerous variables in the environment, the results of these models are highly dependent on the accuracy of the chemical parameters which are used as inputs to the model and the ability of the model to reproduce what occurs in the environment. In order to improve the confidence level of these models, scientists in the Environmental Fate and Effects Division(EFED) of the Office of Pesticide Programs (OPP) are working with the modeling community through the Exposure Modeling Public Meetings (EMPM) and with EPA's Office of Research and Development (ORD) to validate their performance in the field at the intended scale. In addition, EFED/OPP has drafted an input parameters selection manual which will provide guidance in choosing input values when using these drinking water and ecological models. This manual is expected to be finalized later in 2001.

The following is a summary of OPP's current Tier 1 and Tier 2 water screening models along with more detailed model descriptions and user manuals which can be downloaded on a computer. OPP is also working with the U.S. Geological Survey to develop regression-based modeling tools to supplement those listed below. After these models have been validated and documented, they will be added to this web site.

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Surface Water Models

The Agency uses a combination of monitoring data and exposure models to estimate the concentration of pesticides in surface water. In the absence of adequate monitoring data, EPA uses a surface water model called GENEEC for assessing exposure to aquatic organisms. This model is based on a standard scenario in which a ten-hectare watershed is completely cropped and drains into a one-hectare pond, two meters deep. GENEEC is derived from a more sophisticated surface water model, PRZM-EXAMS, but it requires fewer inputs and less time and effort to use. For estimating pesticide concentrations in drinking water, the Agency uses a surface water model called FIRST. Like GENEEC, FIRST is derived from PRZM-EXAMS, but it is based on an Index Reservoir (defined in the next paragraph) watershed and includes Percent Cropped Area (PCA) factors which account for the percent of a watershed that is planted with specific crops.

The Index Reservoir scenario and the PCA were intended to improve the quality and accuracy of EPA's modeling of drinking water exposure to pesticides. The Index Reservoir is based on a drinking water reservoir in Illinois called Shipman City Lake.

Index Reservoir Site
Shipman City Lake IL

photo of Shipman City Lake

This reservoir was selected because it is representative of a number of reservoirs in the central Midwest that are known to be vulnerable to pesticide contamination. It is used in a similar manner to the standard pond except that flow rates through the reservoir are set to reflect site-specific long-term average runoff. Like the standard pond, pesticide loadings to the water body are modeled using local soils and weather data to reflect crop-specific scenarios around the country. Based on best professional judgment, these scenarios are intended to represent sites at roughly the upper 90th percentile of runoff-vulnerability for pesticide transport to surface waters. For more information on the index reservoir, refer to the following Web sites:

For drinking water assessments, EPA uses a factor called the Percent Cropped Area (PCA) which accounts for land use. The results generated by the surface water model are multiplied by the maximum percent of crop area in any watershed (expressed as a decimal) for the crop or crops of interest. Currently, OPP uses PCA adjustments for four major crops - corn, soybeans, wheat, and cotton. For pesticides applied to these crops, drinking water exposure assessments utilize the appropriate index reservoir scenario and corresponding PCA(s). For pesticides applied to other crops, a default PCA adjustment is made. Application of the PCA, though, requires professional judgment, particularly when a pesticide has both agricultural and urban uses. OPP will be developing additional crop-specific PCA's as the necessary information becomes available.

The concept of a factor that accounts for land use was first proposed in a presentation to the Scientific Advisory Panel (SAP) in December, 1997 (Jones and Abel, 1997). Later in May 1999, OPP presented to the SAP a more detailed methodology for using PCA's entitled Proposed Methods For Determining Watershed-derived Percent Crop Areas And Considerations For Applying Crop Area Adjustments to Surface Water Screening Models. For more information on this methodology and comparisons of monitoring and modeling results for selected pesticide/crop/site combinations, refer to the following websites:

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The Office of Pesticide Programs (OPP) uses GENEEC (GENeric Estimated Environmental Concentration) Version 2.0 as a Tier 1 screening model for assessing exposure of pesticides to aquatic organisms and the environment. It provides a rapid screen to separate the low risk pesticides from those that need more refined assessments. The model estimates high level exposure values of pesticides in surface water from a few basic chemical characteristics and pesticide label use and application information. GENEEC2 considers adsorption of the pesticide to soil or sediment, incorporation of the pesticide at application, direct deposition of spray drift into the water body, and degradation of the pesticide in soil before runoff and within the water body. It is a single event model, meaning that it assumes one single large rainfall/runoff event occurs on a 10-hectare field and removes a large quantity of pesticide at one time from the field to a pond which has a 20,000 cubic water volume and is 2-meter deep. The GENEEC2 program is generic in that it does not consider differences in climate, soils, topography or crop in estimating potential pesticide exposure.

GENEEC is expected to overestimate pesticide concentrations in surface water for most sites because it uses maximum pesticide application rates, assumes that no buffer exists between the pond and the treated field, simulates runoff from a 6-inch rainfall over a 24-hour period, represents a water body that is smaller than a drinking water reservoir, and assumes that the entire watershed is cropped and the pesticide is applied to the entire crop. Currently, GENEEC is used for aquatic risk assessments only. The model calculates acute as well as longer-term, multiple-day average concentration values (chronic) which are based on the peak day value and values which consider how fast the pesticide degrades in the environment. GENEEC is a meta-model. This means that it uses simplified mathematics to mimic the more sophisticated PRZM-EXAMS simulation discussed below but requires fewer inputs and much less time and effort to use. See the GENEEC2 User's Manual and GENEEC2 Model Description for more information.

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OPP uses the FQPA Index Reservoir Screening Tool (FIRST) to assess exposure to pesticides in drinking water. Using a few basic chemical parameters (e.g. half-life in soil) and pesticide label application information, FIRST estimates peak values (acute) and long-term average concentrations (chronic) of pesticides in water. Like GENEEC2, it is based upon the linked PRZM and EXAMS models discussed below and is a single-event process. However it is different from GENEEC2 in several aspects. As with the Tier 2 modeling for drinking water, FIRST uses an Index Reservoir watershed based on the Shipman City Lake in Illinois. This scenario is used to represent a higher-end exposure site for pesticide residues found in drinking water derived from surface water. Pesticide concentration values estimated using this scenario should be exceeded only rarely in drinking water taken from most community water supply (CWS) systems in the United States.

In addition, FIRST uses Percent Cropped Area (PCA) factors which consider the percentage of the watershed which is cropped rather than assuming that the whole watershed is cropped. The model calculates reductions in dissolved concentration, adsorption of pesticide to field soil and to reservoir suspended or bottom sediment, incorporation of the pesticide at the time of application, degradation in soil before washoff into the reservoir, and degradation of the pesticide within the water body. Reservoir water concentrations may be increased due to deposition of spray drift into the feeding stream or directly into the reservoir itself. Like GENEEC2, FIRST is a meta-model of PRZM and EXAMS.

While estimates of pesticide concentrations based on a Midwestern index drinking water reservoir may not be representative of other parts of the country, this scenario provides an effective screening tool to screen out pesticides that do not pose a potential problem and to determine if a more refined assessment is needed. The modeling scenarios currently account for region-specific rainfall, soil, and hydrology/runoff factors. For more information, see the FIRST User's Manual and FIRST Model Description for more information.

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OPP currently uses the linked field-scale models, PRZM-3 and EXAMS II, for a higher level, refined (Tier 2) estimation of pesticide concentrations in surface waters for drinking water and aquatic exposure assessments. These values are still screens, albeit finer screens than GENEEC2 and FIRST. For aquatic assessments, PRZM-EXAMS uses the standard pond scenario, while the Index Reservoir/Percent Crop Area is used for drinking water assessments.

The Pesticide Root Zone Model (PRZM) is a process or "simulation" model that calculates what happens to a pesticide in a farmer's field on a day-to-day basis. It considers factors, such as rainfall and plant transpiration of water, as well as how and when the pesticide is applied. It has two major components: hydrology and chemical transport. The hydrologic component for calculating runoff and erosion of soil is based on the Soil Conservation Service curve number technique and the Universal Soil Loss Equation. Evapotranspiration of water is estimated either directly from pan evaporation data or is based on an empirical formula. Total evapotranspiration of water includes evaporation from crop interception, evaporation from soil, and transpiration by the crop. Water movement is simulated by the use of generalized soil parameters, including field capacity, wilting point, and saturation water content. The chemical transport component can simulate pesticide application on the soil or on the plant foliage. Dissolved, adsorbed, and vapor-phase concentrations in the soil are estimated by simultaneously considering the processes of pesticide uptake by plants, surface runoff, erosion, decay, volatilization, foliar washoff, advection, dispersion, and retardation.

Each PRZM modeling scenario represents a unique combination of climatic conditions, crop specific management practices, soil specific properties, site specific hydrology, and pesticide specific application and dissipation processes. Each PRZM simulation is conducted using multiple years of rainfall data to cover year-to-year variability in runoff. PRZM-3 allows the user to consider pulse loads and predict peak events. Daily edge-of-field loadings of pesticides dissolved in runoff waters and sorbed to sediment, as predicted by PRZM, are discharged into a standard water body (either the "standard pond" or the Index Reservoir) simulated by the EXAMS model described below. The PRZM-3 model system and all the support files and programs are available on diskette from the U.S. EPA National Exposure Research Laboratory in Athens, Georgia 30605. Information about PRZM is also available at the following websites:

The other Tier 2 surface water simulation model which is used in conjunction with PRZM-3 is EXAMS II (the Exposure Analysis Modeling System). It is also a process model, but it simulates the processes that occur in the water body rather than on the agricultural field. EXAMS II takes the runoff and spray drift loading generated by PRZM and estimates the concentration in the pond on a day-to-day basis.

EXAMS II can be used to assess the fate, exposure, and persistence of synthetic organic chemicals in aquatic ecosystems. It accounts for volatilization, sorption, hydrolysis, biodegradation, and photolysis of the pesticide. Since EXAMS is a steady-state model, the water bodies are modeled as having constant volume. Multiple-year pesticide concentrations in the water column are calculated from the simulations as the annual daily peak, maximum annual 96-hour average, maximum annual 21-day average, maximum annual 60-day average, and annual average. The upper 10th percentile concentrations (except annual average) are compared against ecotoxicological and human health levels of concern (LOC). For a more detailed description of the parameters, validations and assessments for EXAMS II, see the following web site:

Center for Exposure Assessment Modeling
Software, Descriptions, Abstracts

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Ground Water Models


A screening model which OPP uses most frequently to estimate concentrations of pesticides in ground water is SCI-GROW. This model was developed using data from prospective ground water monitoring studies that provided screening estimates of pesticide concentrations in shallow, vulnerable ground water. This regression model provides an estimate of the most likely ground water concentrations if the pesticide is used at the maximum allowable rate in areas with ground water which is exceptionally vulnerable to contamination. The SCI-GROW estimate is based on environmental fate properties of the pesticide (aerobic soil degradation half-life and linear adsorption coefficient normalized for soil organic carbon content), the maximum application rate, and any existing data from small-scale prospective groundwater monitoring studies.

Pesticide concentrations estimated by SCI-GROW are expected to represent conservative or high-end values because the model is based on ten ground-water monitoring studies which were conducted by applying the pesticide at maximum allowed rates and frequency to hydrogeologically vulnerable sites (i.e., shallow aquifers, sandy, permeable soils, and substantial rainfall and/or irrigation to maximize leaching). If adequate groundwater monitoring data are available for a specific pesticide, then this data will be used instead of the SCI-GROW estimate. For more information, refer to the SCI-GROW Description and the SCI-GROW Users Manual.

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