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About Water Exposure Models Used in Pesticide Assessments

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Water Models

When EPA's Office of Pesticide Programs (OPP) assesses the risk of a pesticide, it considers the exposure to the pesticide as well as the toxicity of the pesticide. For both drinking water and aquatic exposure assessments, reliable field monitoring data, when available, as well as mathematical models can be used 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 the user 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 at a particular time and at a particular location, it may not provide reliable estimates for exposure assessments because sampling may not occur where and when the highest concentrations of a pesticide are found.

For drinking water and aquatic exposures assessments, OPP typically relies on mathematical models to generate exposure estimates. 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 guidelines for these laboratory studies can be found at the following website: Series 835 - Fate, Transport and Transformation Test Guidelines. Although computer modeling provides an indirect estimate of pesticide concentrations, models can estimate concentrations continuously over long periods of time and for vulnerable areas of interest for a particular pesticide. Modeling can also be used to compare estimated concentrations with toxicity data to determine the risk a pesticide poses to both drinking water and aquatic systems. Another benefit of computer modeling is in determining how various mitigation practices affect the amount of the pesticide that can run off into water.

In estimating pesticide concentrations in aquatic environments, OPP uses a tiered approach. The intent of this approach is to estimate pesticide concentrations in water from sites that are highly vulnerable to runoff or leaching. With this approach, pesticides that pass Tier I will likely pose 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 archives about Science Policy Issues and Guidance Documents related to Tolerance Reassessment Advisory Committee (TRAC). Search EPA Archive

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Model Names and Specific Uses

For estimating upper bound concentrations of pesticides in drinking water, OPP uses FIRST (FQPA Index Reservoir Screening Tool) as a Tier I model for surface water exposure assessments and PRZM-GW for groundwater exposure assessments. For estimating upper bound concentrations of pesticides in other aquatic environments, OPP uses the Tier I model GENEEC2 (GENeric Estimated Environmental Concentration) for surface water exposure assessments. View these and other models.

For Tier II surface water exposure assessments, OPP uses the Surface Water Concentration Calculator (SWCC), which accommodates the specific characteristics of the chemical and includes more site-specific information regarding the application method and impact of local daily weather on the treated field over a period of 30 years. At the Tier II level, the SWCC uses maximum application rates and frequencies for a vulnerable drinking water reservoir or vulnerable pond. Additional refinements in application rates may be considered if usage data indicate they are appropriate. Currently, scientists in the Environmental Fate and Effects Division (EFED) of the Office of Pesticide Programs (OPP) are exploring the use of the SWCC for Tier I level assessments. View the SWCC and other models.

For Tier II groundwater exposure assessments, refinement strategies for PRZM-GW can be used to estimate pesticide concentrations in groundwater. These refinement strategies include consideration of representative scenarios, additional fate parameters, annual application retreatment, well setbacks, and representative exposure durations of concern.

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 that are used as inputs and the ability of the model to represent what occurs in the environment. In order to improve transparency and confidence in these models, EFED Scientists present new model developments at the Environmental Modeling Public Meetings (EMPM), which are held on a semiannual basis. In addition, the code and documentation for all EFED/OPP water models are posted on the web page for models used in pesticide risk assessment.

The following is a more detailed summary of OPP's current Tier I and Tier II aquatic exposure models along with links to user manuals that can be downloaded.

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Surface Water Models

Pesticides can enter surface waters through runoff, spray drift, and deposition. Once pesticides have entered surface waters, they are exposed to a number of physical, chemical, and microbial processes that impact the fate of the pesticides. These processes include photodegradation, volatilization, biodegradation, absorption/adsorption, chemical degradation, leaching, and sedimentation. To better understand the fate of pesticides in surface waters, OPP has developed a number of models that capture these processes and predict the concentration of pesticides in surface waters. These models range from simple screening models that require few inputs to more complex models that reflect the dynamics of the surface water ecosystem. Below is a description of the surface water models that OPP uses in its pesticide exposure assessments.

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GENEEC2

The GENeric Estimated Environmental Concentration (GENEEC 2.1) is a screening model to predict environmental concentrations of pesticides in surface water for aquatic exposure assessments. The model, which was recompiled to operate in the Microsoft® Windows 7® environment, is a legacy model for EPA and is currently available on the Water Models - Previous Versions Web page, in the EPA Web archive. Search EPA Archive For the most part, the Surface Water Concentration Calculator (SWCC) has replaced GENEEC2 for estimating environmental concentrations of pesticides in surface water for aquatic exposure assessments. View current models.

In the past, OPP used GENEEC2 as a Tier I screening model for assessing exposure of aquatic organisms and the environment to pesticides. GENEEC2 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, which occurs on a 10-hectare field and which removes a large quantity of pesticide at one time from the field to a pond. In this case, the pond has a 20,000 cubic water volume and is 2 meters deep. The GENEEC2 program is generic in that it does not consider differences in climate, soils, topography or crop in estimating potential pesticide exposure.

GENEEC2 is expected to overestimate pesticide concentrations in surface water for most sites and may be inappropriate for some chemicals, especially those that are persistent and/or have a high sorption coefficient, as well as frequently applied pesticides. In these cases, users should go directly to a higher tiered assessment using the more sophisticated Surface Water Concentration Calculator discussed below.

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FIRST

OPP uses the Tier I model, 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 (chronic) average concentrations of pesticides in water. Like GENEEC, it is based upon the linked PRZM and EXAMS models and is a single-event process. However, it is different from GENEEC in several aspects. As with the Tier II modeling for drinking water, FIRST uses an Index Reservoir watershed based on the Shipman City Lake in Illinois.

FIRST also uses Percent Cropped Area (PCA) factors, which consider the percentage of the watershed that is cropped rather than assuming that the whole watershed is cropped. The program automatically adjusts the output in accordance with the user-specified maximum percent of crop area in any watershed. For more information, see the FIRST User's Manual and Model Description Search EPA Archive.

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Surface Water Concentration Calculator

Currently, OPP uses the Surface Water Concentration Calculator (SWCC) for higher level, refined (Tier II) estimations of pesticide concentrations in surface waters for drinking water and aquatic exposure assessments. The SWCC is designed to simulate the environmental concentration of a pesticide in the water column and sediment and is used for regulatory purposes by the EPA’s Office of Pesticide Programs (OPP). The SWCC uses the Pesticide Root Zone Model (PRZM) version 5.0+ (PRZM5) and the Variable Volume Water Body Model (VVWM), replacing the older PE5 shell (last updated November 2006), which used PRZM3 (Carousel et al., 2005) and EXAMS (Burns, 2004). This updated model was designed to improve users' interactions with the program and facilitate maintenance and operation of the software.

For aquatic assessments, the SWCC uses the standard pond scenario, and for drinking water assessments, the SWCC uses the index reservoir/percent crop area factors.

PRZM5 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 evapotranspiration 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 (NRCS, 2003; Wischmeier and Smith, 1978).

Evapotranspiration of water is estimated from pan evaporation data. 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 curve number. The chemical transport component simulates pesticide application on the soil or on the plant foliage. Dissolved, sorbed, and vapor-phase concentrations in the soil are estimated by considering surface runoff, erosion, degradation, volatilization, foliar washoff, advection, dispersion, retardation, among others.

Each PRZM5 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 simulation is conducted using multiple years of rainfall data to cover year-to-year variability in runoff. Daily edge-of-field loadings of pesticides dissolved in runoff waters and sorbed to sediment, as predicted by PRZM5, are discharged into a standard water body (either the standard pond or the Index Reservoir) simulated by the VVWM model. Additional information about the PRZM5 model can be found on our models page.

The VVWM simulates the processes that occur in the water body by using the runoff and spray drift loading generated by PRZM5 to estimate the fate, persistence, and concentration of a pesticide in a water body on a day-to-day basis. As such, the model accounts for volatilization, sorption, hydrolysis, biodegradation, and photolysis of the pesticide. The VVWM has the ability to vary its volume on a daily scale and to include sediment burial (unlike its predecessor EXAMS) although these feature are only used for higher tiered assessments.

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 VVWM, see our information on aquatic models.

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Pesticide in Water Calculator

The Pesticide in Water Calculator (PWC) is an updated version of the tool previously known as the Surface Water Concentration Calculator (SWCC). The tool’s name was changed to better reflect that the PWC can now simulate both surface water and ground water.  In addition, the PWC has an improved volatilization routine and more batch run capabilities.  PWC version 1.52 is the latest version approved for regulatory use.  It comprises a graphical user interface, a field model (PRZM version 5.02) and a water body model: (VVWM version 1.02). View more information about PWC.

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Tier 1 Rice Model

The Tier 1 Rice Model (version 1.0) is used to estimate surface water exposure from the use of pesticides in rice paddies. This screening-level model provides short- and long-term concentrations that can be used for both aquatic ecological risk assessments and drinking water exposure assessments. Guidance for using the Tier 1 Rice Model can be found on our models page.

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Pesticide in Flooded Application Model (PFAM)

Compared to the Tier 1 Rice Model, PFAM allows for a more advanced estimate of surface water exposure from the use of pesticides in flooded fields such as rice paddies and cranberry bogs. Some of the advanced features incorporated into PFAM include specifications for water and pest management practices, degradation data for soil and aquatic environments and post-processing information of discharged paddy waters to a stream. Additional information concerning PFAM can be found on our models page.

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Aquatic Bioaccumulation Model

EPA uses the model KABAM version 1.0 (Kow (based) Aquatic Bioaccumulation Model) to estimate potential bioaccumulation of hydrophobic organic pesticides in freshwater aquatic food webs and subsequent risks to mammals and birds via consumption of contaminated aquatic prey. The model can also be used to estimate pesticide concentrations in fish tissues consumed by humans. KABAM is composed of two parts: 1) a bioaccumulation model estimating pesticide concentrations in aquatic organisms and 2) a risk component that translates exposure and toxicological effects of a pesticide into risk estimates for mammals and birds consuming contaminated aquatic prey. The users manual and executable file for KABAM can be found on our models page.

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Ground Water Models

SCI-GROW

After the passage of the Food Quality Protection Act (FQPA) of 1996, the EPA developed SCI-GROW (Screening Concentration in Groundwater) as a screening-level tool to estimate drinking water exposure concentrations in groundwater resulting from pesticide use (Barrett, 1997). As a screening tool, SCI-GROW provides conservative estimates of pesticides in groundwater, but it does not have the capability to consider variability in leaching potential of different soils, weather (including rainfall), cumulative yearly applications or depth to aquifer. If SCI-GROW-based assessment results indicate that pesticide concentrations in drinking water exceed levels of concern, the ability to refine the assessment is limited. At the present time, SCI-GROW is considered a legacy model for EPA and has been largely replaced by PRZM-GW.

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PRZM-GW

In 2004, the EPA and the Pest Management Regulatory Agency (PMRA)-Canada initiated a project to evaluate advanced methods for estimating pesticide concentrations in groundwater. The goals of this project were to identify a common computer model for estimating pesticide concentrations in groundwater and to develop common procedures for determining model input parameters from soil survey data, pesticide environmental fate studies, and pesticide use information. After evaluating 19 modeling programs, EPA and PMRA selected a modified version of PRZM as the North American Free Trade Agreement (NAFTA) regulatory tool for estimating concentrations of pesticides in ground water. Concurrently, EPA consulted with the FIFRA Scientific Advisory Panel (SAP) twice in 2005 on the development of a groundwater conceptual model and the use of PRZM-GW to implement the conceptual model.

Figure 2 depicts the general groundwater scenario concept for estimating pesticide concentrations in drinking water as implemented in PRZM-GW. This conceptual model is based on a rural drinking water well beneath an agricultural field (a high pesticide use area), which draws water from an unconfined, high water-table aquifer.

Diagram of house with potable well and defined underground areas including: declining aerobic degradation with depth of 1m; hydrolysis or other relevant processes below 1m;  depth to aquifer; well screen length; water table.Figure 2: General Groundwater Scenario Concept for Estimating Pesticide Concentrations in Drinking Water As Implemented in PRZM-GW

The depth of the well is site-specific (i.e., scenario specific). The well extends into a shallow unconfined aquifer and has a well-screen that starts at the top and continues down into the aquifer. The length of the well-screen represents the region of the aquifer where drinking water is collected. The well-screen length is well-specific and can be adjusted. Processes included in the conceptual model that influence pesticide transport through the soil profile include water flow, chemical specific dissipation and transportation parameters (i.e., degradation and sorption), and crop specific factors, including transpiration, pesticide interception and management practices.

After developing the conceptual model for PRZM-GW, EPA compared its performance in estimating drinking water concentrations of pesticides with targeted and non-targeted groundwater monitoring data. Data from prospective ground water monitoring studies (detailed site investigations of pesticide leaching into vulnerable aquifers) were important in the development and evaluation of the PRZM-GW model. After an extensive evaluation, EPA determined that PRZM-GW was an effective tool for establishing upper bound pesticide concentrations in groundwater for national and site-specific assessments.

Initially, EPA implemented PRZM-GW using a Tier I procedure that involves simulation of 30 to 100 years of pesticide applications at labeled maximum application rates in defined scenarios that represent the most vulnerable types of aquifers utilized as drinking water sources. These studies showed that the primary pesticide-specific inputs affecting PRZM-GW exposure estimates are the application rate and timing, the aerobic soil degradation rate, the linear adsorption coefficient, and the hydrolysis rate. For volatile pesticides such as fumigants, a volatilization routine can also be incorporated in the model run.

After evaluating PRZM-GW as an effective tool for establishing Tier I screening assessments, EPA developed refinement strategies for using PRZM-GW for Tier II groundwater assessments. These refinement strategies can include consideration of representative scenarios, additional fate parameters, annual application retreatment, well setbacks, and representative exposure durations of concern. In the future, OPP may consider additional strategies to facilitate such refinements. For more information, refer to EPA’s Guidance for Using PRZM-GW in Drinking Water Exposure Assessments Search EPA Archive.

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References

Barrett, M. 1997. Initial Tier Screening of Pesticides for Groundwater Concentration Using the SCI-GROW Model. U.S. Environmental Protection Agency. Washington, D.C.

Burns, L. 2004. Exposure Analysis Modeling System (EXAMS). User’s manual and system documentation. Ecosystems Research Division. U.S. Environmental Protection Agency. Athens, GA. EPA/600/R-081. September 2000. Revision G.

Carousel, R.F., J.C. Imhoff, P.R. Hummel, J.M. Cheplick, A.S. Donigian, and L.A. Suarez. 2005. U.S. Environmental Protection Agency. Athens, GA. Pesticide Root Zone Model (PRZM)-3. PRZM: a model for predicting pesticide and nitrogen fate in the crop root and unsaturated soil zones. 3.12.2 ed.

NRCS, 2003. National Engineering Handbook Section 4: Hydrology. Natural Resources Conservation Service, U.S. Department of Agriculture, Washington DC.

Wischmeier, W. H., and D. D. Smith. 1978. Predicting rainfall erosion losses - a guide to conservation planning. Agriculture Handbook 537, U.S. Department of Agriculture, Washington, DC, USA.

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