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Exposure Modeling Public Meeting (EMPM)

March 19, 2013

Agenda and Abstracts

Time Presentation Number and Title Presenter(s)

9:00 – 9:10 Welcome and Introductions Marietta Echeverria, Gabe 

Rothman, and Jim Carleton

9:10 - 9:20 Opening Remarks Don Brady

EPA Updates and Presentations

9:20 – 9:45 PRZM GW: Implementation and Case Studies Reuben Baris, Michael Barrett, 

Rochelle Bohaty, Marietta 

Echeverria, Dirk Young, and 

James Wolf

9:45 – 10:10 Surface Water Calculator Dirk Young

10:10 – 10:35 Rice Modeling: A Case Study Chuck Peck

10:35 – 10:45 Break

10:45 – 11:10 Status of Drinking Water Intake Watershed PCA 

Development Project

Jim Carleton

11:10 – 11:35 Spatial Aquatic Model (SAM) Pilot Project Nelson Thurman

11:35 – 12:00 Audrey III – A Tier II Plant Exposure Estimation Tool R. David Jones, Chuck Peck, 

and Frank Farruggia

12:00 – 1:00 Lunch

Public Presentations

1:00 – 1:30 Implementation of Vegetative Buffer Strip 

Effectiveness in Removing Pesticides in Regulatory 

Assessments

Bjoern Roepke, Russell L. 

Jones, and George Sabbagh

1:30 – 2:00 iSTREEM® – A Web-Based River Chemical 

Concentration Estimation Model

Xinhao Wang and Paul DeLeo

2:00 – 2:30 Monitoring and Modeling the Fate and Transport of a 

Pesticide Metabolite from Groundwater Recharge to 

Drinking Water

R. Sur, T. Oertel, M. Himml, 

and G. Görlitz

2:30 – 2:45 Break

2:45 – 3:15 Development and Validation of an Approach for 

Modeling Pyrethroid Residues in Urban Streams

Michael Winchell, Scott 

Jackson, and Gary Mitchell

3:15 – 3:45 Evaluation of a Simplified SWAT Model Approach for 

Prediction of Insecticide Concentrations in a Small 

Watershed in Northwestern Oregon

Natalia Peranginangin, Michael 

F. Winchell, and Solomon Folle

3:45 – 4:00 Meeting Wrap-Up
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Abstracts

1. PRZM GW: Implementation and Case Studies

Reuben Baris, Michael Barrett, Rochelle Bohaty, Marietta Echeverria, Dirk Young, 

James Wolf

Environmental Fate and Effects Division. United States Environmental Protection 

Agency

The Pesticide Root Zone Model for Groundwater (PRZM-GW) was developed by 

U.S. EPA as a regulatory model to estimate pesticide concentrations in drinking 

water for vulnerable groundwater supplies. PRZM-GW permits assessment of 

multiple years of pesticide use and is being used by U.S. EPA’s Office of Pesticide 

Programs to conduct standard screening (Tier I) and refined (Tier II) drinking water 

assessments. Input parameters for PRZM-GW include pesticide fate properties, 

environmental factors such as soil type, aquifer depth and weather, and agronomic 

practices, including application method, timing, and frequency of application. PRZMGW output data include peak and time-weighted drinking water concentrations. This 

presentation will review the conceptual model and model evaluation, including 

comparisons to water monitoring data. The presentation will also discuss the 

implementation of PRZM-GW for screening and refined drinking water assessments. 

Case studies will be presented to illustrate how PRZM-GW is being used in drinking 

water assessments.

2. Surface Water Calculator

Dirk Young

Environmental Fate and Effects Division. United States Environmental Protection 

Agency

EFED will soon release a new Surface Water Calculator (SWC) that will replace PE5 

and EXPRESS. The new SWC will include a new version of PRZM (PRZM5), an 

EXAMS replacement (the VVWM), and a new user interface. These programs will 

retain the most user-requested features of the older programs, implement bug fixes, 

and improve maintainability. PRZM5 is upgraded to 100% standard Fortran 2003--no 

compiler-specific extensions and no obsolete functions such as goto exist in PRZM5. 

PRZM5 eliminates non-working, conceptually problematic, unused, and redundant 

routines such as Monte Carlo, MOC, VADOFT, Nitrogen Simulation, biphasic 

degradation, and lateral flow. In a subtle but major revision, the time loop has been 

streamlined to a single daily loop from the old PRZM convoluted looping system that 

involved an inner day loop, a middle year loop, and an outer month loop. The VVWM 

is a close replica of the two-compartment subset of EXAMS, with the exception that 

VVWM has an option for changing the water body volume as a result of water inputs 

and outputs. Finally, the user interface was designed by developers with first-hand 

knowledge of the way that EPA performs pesticide risk assessments and will be 

intuitive in its operation and installation.
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3. Rice Modeling: A Case Study

Chuck Peck

Environmental Fate and Effects Division. United States Environmental Protection 

Agency

EFED uses the Tier I rice model, which employs a pesticide’s application rate and 

adsorption coefficient, to develop estimated environmental concentrations (EECs) 

and estimated drinking water concentrations (EDWCs). While this model is useful 

for screening pesticides that are not harmful to human health or ecological 

receptors, it is not effective in refining estimates when risks of concern are triggered 

or for estimating chronic concentrations. Over the past few years, EFED has 

developed the Pesticide Flooded Application Model (PFAM) to refine aquatic 

exposure assessment for applications to rice or other flooded agricultural crops. 

PFAM adds the capability to account for water and pest management practices, 

degradation in soil and aquatic environments, as well as post processing of 

discharged paddy waters to a stream, index reservoir, farm pond, or other userdefined receiving water. We will report on a case study of a drinking water exposure 

assessment for an herbicide applied to rice, which explores the use of PFAM and 

other approaches in an effort to refine chronic drinking water exposure estimates. 

This presentation will highlight the case study, the methods used to develop 

EDWCs, and the updates made to PFAM.

4. Status of Drinking Water Intake Watershed PCA Development Project

Jim Carleton

Environmental Fate and Effects Division. United States Environmental Protection 

Agency

EFED uses percent cropped area (PCA) adjustment factors in its drinking water 

assessments to account for the fact that watersheds supporting surface drinking 

water sources may not be devoted entirely to crop agriculture. The first PCAs were 

based on cropped acreages initially within 8-digit HUCs (USGS Hydrologic Unit 

Code “cataloguing units”). Acting in collaboration with USGS, EFED has recently 

developed a dataset of 4,840 quality-assured watersheds for the locations of 

drinking water intakes that supply community water systems in the United States 

with raw water. EFED has developed and validated PCAs for these watersheds, 

compared them against existing HUC-based PCAs, and evaluated their use in 

modeling through comparisons of PCA-modified PRZM/EXAMS results against 

available pesticide monitoring data. This talk will provide an update on, and 

overview of the results of this project.
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5. Spatial Aquatic Model (SAM) Pilot Project

Nelson Thurman

Environmental Fate and Effects Division. United States Environmental Protection 

Agency

A key question pesticide aquatic exposure assessments need to address is whether 

a pesticide, when used according to its label, will occur in water at concentrations 

that may adversely impact human health or aquatic species and, if so, how often and 

where will these impacts likely to overlap with the populations at risk. 

In order to improve current aquatic exposure assessments without increasing the 

workload for the risk assessor, EFED is developing a Spatial Aquatic Model (SAM). 

SAM will use mappable data and model routines to automate the scenario-building 

and modeling steps, allowing EFED risk assessors to more effectively and efficiently 

characterize the spatial extent of any areas that may exceed levels of concern for a 

pesticide use. Such areas could be matched to drinking water intakes (for human 

health) or habitat (for aquatic species) to inform risk management decisions. 

The current pilot project will incorporate spatial variability in hydrology, soil, land use, 

crop, agricultural practices, and climate-related parameters. Results of this regional 

pilot will be used to inform the development of a national spatial aquatic model. The 

framework for the expanded model will allow incremental updates to EFED’s current 

aquatic modeling framework in order to incorporate better data and modeling 

routines as they become available. 

Ultimately, SAM will be a desktop or web application capable of estimating pesticide 

concentrations in water for use in regulatory purposes, as well as exploring risk 

mitigation options. The model will be amenable to methods for automating data input 

and for providing outputs in terms of summary tables and maps as needed. The 

automated process will allow risk assessors to devote more time to analyzing and 

characterizing results and working with risk managers to evaluate mitigation options 

as needed. 

This presentation will provide an overview of the goals, conceptual approach, inputs, 

and expected outputs for a regional pilot underway to develop a working version of 

SAM.

6. Audrey III – A Tier II Plant Exposure Estimation Tool

R. David Jones, Chuck Peck, and Frank Farruggia
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Environmental Fate and Effects Division. United States Environmental Protection 

Agency

This presentation will discuss the current progress toward a replacement for the 

Agency’s existing plant exposure model TerrPlant. Audrey III will be built into the 

Agency’s new PRZM-EXAMS modeling shell and will rely on the same scenarios 

used for aquatic exposure assessment. Like TerrPlant, it will generate exposure 

estimates for plants in both terrestrial and semi-aquatic habitats. Conceptual models 

for both environments will be presented. The terrestrial conceptual model is based 

on a Plant Exposure Zone (PEZ), whose width is determined by distance from the 

edge of field traveled by overland flow and whose depth is determined by the plant 

root zone. Exposure will be estimated for loading from runoff, eroded sediment, and 

spray drift; exposure outside the PEZ will be estimated for spray drift only. For semiaquatic environments, concentrations will be estimated using VVWM (Variable 

Volume Water Model) using a shallow wetland.

7. Implementation of Vegetative Buffer Strip Effectiveness in Removing 

Pesticides in Regulatory Assessments

Bjoern Roepke, Russell L. Jones, and George Sabbagh 

Bayer CropScience

During the past decade significant advances have been made in the modeling of 

pesticide removal by vegetative buffer strips. Several models have been developed 

and tested. Comparisons of model predictions with measured data have also been 

performed. In the U.S. interest in such modeling effectiveness of buffer strips for 

pesticide removal has been shown by registrants and regulators, but such modeling 

has not yet been incorporated into regulatory risk assessments. In Europe modeling 

of buffer strip effectiveness has been included into mitigation incorporated into 

surface water risk assessments. This paper will review the developments in 

modeling of pesticide removal by vegetative buffer strip and the development of 

scenarios and software tools for implementing vegetative buffer strip modeling in 

European assessments.

8. iSTREEM® – A Web-Based River Chemical Concentration Estimation Model

Xinhao Wang1 and Paul DeLeo2

1 University of Cincinnati

2 American Cleaning Institute

iSTREEM® is a web-based computer model that predicts the concentration of 

chemicals used in ‘down-the-drain’ products, such as consumer use pesticides, in 

the effluent of more than 10,000 wastewater treatment plants throughout the 

continental United States, their resultant mixing zones, riverine concentrations and 

drinking water intake concentrations downstream of wastewater discharges. The 

model has been scaled at both the national level (e.g., approximately 30,000 river 

reaches covering over 200,000 river miles) and at the regional/watershed scales 

(e.g., Trinity River, TX; Willamette River, OR). The model permits exposure 
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estimates to humans or ecological receptors from ingredients used in consumer 

products.

 

iSTREEM® is a valuable new tool being made available to the public at no cost by 

the American Cleaning Institute to promote product stewardship and regulatory 

compliance for chemical suppliers and manufacturers of formulated products of all 

sizes. For more information about iSTREEM®, go to: 

http://www.aciscience.org/iSTREEM.aspx.

9. Monitoring and Modeling the Fate and Transport of a Pesticide Metabolite from 

Groundwater Recharge to Drinking Water

R. Sur1

, T. Oertel2, M. Himml3, G. Görlitz4

1

Bayer CropScience LP, Environmental Safety 

2 Water Works 

3 GEOBIT 

4 Bayer CropScience AG, Environmental Safety

The duration and extent of future drinking water exposure was investigated for a 

pesticide metabolite caused by long-time application of a plant protection product in 

an orchard within the catchment area of a water works from 1993 to 2006.

The leaching of the metabolite into groundwater under the orchard was calculated 

with the 1-dimensional leaching model PEARL based on the actual pesticide use 

history and using local soil and weather data for model parameterization. The further 

dilution during transport in the aquifer to the drinking water abstraction point was 

simulated using a 2-dimensional modeling approach applying HYDRUS-2D. 

Horizontal hydraulic conductivities in the aquifer for model input were experimentally 

determined across the catchment. 

Monitoring results from several wells within the catchment confirm the results of the 

simulations. In total, a transfer factor of ca. 20 between the concentration of the 

metabolite in groundwater recharge and in raw water for drinking water production 

was calculated. The drinking water exposure is predicted to remain constant until 

2015 and to terminate by 2025.

10.Development and Validation of an Approach for Modeling Pyrethroid Residues 

in Urban Streams

Michael Winchell, Scott Jackson, and Gary Mitchell

Stone Environmental Inc.

Pyrethroid insecticides have been detected in urban waterways throughout 

California and other regions. Modeling pyrethroid environmental fate in a residential 

setting can help us to better understand the sources, transport pathways, and 

mitigation strategies for reducing environmental concentrations of pyrethroids. 

Currently, there is not a standard modeling approach for estimating environmental 
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concentrations of pesticides in an urban environment. One modeling approach is to 

adapt an urban water quality model, the Storm Water Management Model (SWMM) 

to serve this purpose. To test the suitability of SWMM in predicting urban pesticide 

exposure, the model was applied to a heavily developed residential neighborhood in 

southern California. This neighborhood was modeled because of the data available 

from monitoring of flow and pyrethroid concentrations conducted there over a 

several year period. One of the primary inputs to the model, the wash-off 

characteristics of the pyrethroid, were calibrated to observed plot-scale data for both 

impervious and pervious turf surfaces. A statewide survey regarding the spatial and 

temporal patterns of pyrethroid applications was used in structuring the SWMM 

model to allow realistic application variability throughout the neighborhood. Because 

of their direct effects on “dry weather” wash-off and storm drain residues, residential 

outdoor water use practices represented another important component to the model 

structure and parameterization. The hydrologic component of the model was

calibrated to 1 years’ worth of hourly flow data measured from a stormwater outfall 

draining the neighborhood. Pyrethroid samples from both storm and non-storm 

periods were compared with the SWMM model predictions to assess the model 

performance. This presentation will report on the model development, calibration 

results, and the potential for utilizing this modeling approach as part of a 

standardized risk assessment process and as a tool for evaluating the potential 

benefits of best management practices.

11.Evaluation of a Simplified SWAT Model Approach for Prediction of Insecticide 

Concentrations in a Small Watershed in Northwestern Oregon

Natalia Peranginangin1

, Michael F. Winchell2, and Solomon Folle3

1

Syngenta Crop Protection, LLC 

2

Stone Environmental, Inc.

3

University of Minnesota

The need for a modeling approach to estimate pesticide concentrations in flowing 

water bodies has become increasingly important due to the endangered species 

assessment process. To address this need, a modeling approach using the Soil and 

Water Assessment Tool (SWAT) was developed and tested for a widely used corn 

herbicide on 30 watersheds across the corn belt and southern US. The assessment 

evaluated the ability of SWAT to predict pesticide concentration distributions using a 

simplified parameterization approach. The encouraging results from this study led to 

the application of the methodology to an insecticide in a different geographic region, 

Northwest Oregon. In stark contrast to the original watersheds studied, the Oregon

watershed modeled included heterogeneous cropping patterns with a more variable 

pesticide use pattern. Simulation results from the Northwest Oregon watershed will 

be presented.