Minutes of the January 11, 2011 Exposure Modeling Public Meeting

Office of Pesticide Programs (OPP) 4th Floor Conference Center

2777 South Crystal Drive – Potomac Yard Two, North

Arlington, VA 22202

Attendees:

Name

Amanda Solliday EPA

Andrew Jacques RegNet

Andrew Newcombe ARCADIS

Avery Fellow Daily Environment Report

Bayazid Sarkar EPA

Catherine Aubee EPA

Clifford Habig Exponent

Cody Howard USDA/ARS

Dana Spatz EPA

David Lieu EPA

Greg Orrick EPA

Jay Overmyer Syngenta

Jeff Evans EPA

Jenny Tao EPA

Jim Breithaupt EPA

Katherine Carr Monsanto

Katrina White EPA

Khin Oo EPA

Kristian Paul DuPont

Marrietta Echeverria EPA

Mary Clock Rust EPA

Melissa Panger EPA

Michael Barrett EPA

Michael Xiao Huang DuPont

Mike Leggett CLA

Mike Rexrode EPA

Mohammed Ruhman EPA

Natalia Peranginangin Syngenta

Nathan Mottl EPA
Nick Mastrota EPA

Patrick Havens Dow

Paul Hendley Syngenta

Pete Coody Bayer Cropscience

Phillip Villanueva EPA

Qingli Ma Exponent

R. David Jones EPA

Rick Reiss Exponent

Robert Miller EPA

Rochelle Bohaty EPA

Rueben Baris EPA

Russell Jones ECPA Kinetics Group

Sarah Winfield EPA

Scott Jackson BASF

Steve Wente EPA

Stuart Cohen

Environmental & Turf 

Services

Subijoy Dutta EPA

Tammara Estes Stone Environmental Inc

Tanja Crk EPA

Tiffany Mason EPA

William Stiteler ARCADIS US Inc.

Welcome and Introductions

The USEPA/OPP/EFED hosts biannual Exposure Modeling Public Meetings (EMPMs),

which provide a forum for exchanging information between OPP/EFED and stakeholders with 

similar technical expertise on current issues related to pesticide exposure modeling in support of 

risk assessment. The overarching theme of this EMPM was “Spatial Context to Terrestrial 

Exposure Modeling.” Rochelle Bohaty, Tanja Crk, Kris Garber and Andrew Shelby chaired the 

meeting in the capacity of co-chairs of the Terrestrial Exposure and the Water Quality Technical 

Teams. Former Water Quality Technical Team co-chair, Chuck Peck, also assisted in the 

coordination of the meeting.

The minutes and presentation slides for the January 11, 2011 EMPM are posted in the 

docket (EPA-HQ-OPP-2009-0879), which can be accessed at http://www.regulations.gov/. The 

docket ID # is reported on the EMPM webpage: 

http://www.epa.gov/oppefed1/models/water/empm_top.htm

Brief Updates:

None

Presentations:
Presentation 1:

“Estimating Confidence Intervals for Metabolite Degradation Rates”

Russell Jones, ECPA Kinetics Group and Bayer CropScience

Regulatory agencies around the world are moving towards using formation decline 

kinetics for estimating degradation rates of metabolites. Estimating confidence intervals of 

metabolite degradation rates using such kinetic analysis has become an important element of the 

registration process, especially in Europe. When confidence intervals are estimated using 

ordinary least squares regression techniques, the resulting confidence intervals are overly broad. 

Other standard techniques such as IRLS (Iteratively Reweighted Least Squares) and MCMC 

(Markov ChainMonte Carlo) provide more appropriate estimates of confidence intervals since 

these two techniques do not assume that the error variance is the same for parent and 

metabolites.

Presentation notes:

Because confidence intervals, which are estimated using ordinary least squares (OLS) 

regression techniques, may be overly broad, there is a need to look at other methods for 

determining these intervals. In his presentation, Russell Jones described his project for 

exploring other standard techniques for determining confidence intervals, namely Iteratively 

Reweighted Least Squares (IRLS) and Markov Chain Monte Carlo (MCMC). Using empirical 

data, Jones compared upper and lower confidence limits using OLS, IRLS, and MCMC. Based 

on the data from this comparison, both IRLS and MCMC techniques are better than OLS because 

they account for the error variance between the parent and the metabolite or degradate. Both 

IRLS and MCMC, though, are comparable.

The examples presented were based on single, first-order modeling; however, Jones 

stated that other models can be used. He has performed similar analysis for 20-30 other 

individual studies (3-4 individual studies per chemical), and the results were similar (MCMC and 

IRLS provide similar results which are better than OLS). In response to questions related to the 

statistical analysis, Jones referred attendees to the following articles, which will be published in 

the near future:

Görlitz, Linus; Gao, Zhenglei; Schmitt, Walter. Statistical Analysis of Chemical 

Transformation Kinetics using Markov-Chain Monte-Carlo Methods. Submitted to 

Environmental Science & Technology and accepted for publication. 

Gao, Zhenglei; Green, John; Vanderborght, Jan; Schmitt, Walter. Improving Uncertainty 

Analysis in Kinetic Evaluations Using Iteratively Reweighted Least Squares. Submitted 

to Environmental Toxicology and Chemistry. 

Jones RL, Beigel C, Erzgraeber B, Gao Z, Green JW, Harvey B, Hayes S, Mackay N, 

Paul KW, Schmitt W, Yon D Estimating Confidence Intervals for Metabolite 

Degradation Rates. Accepted for presentation and inclusion in the proceedings at the 
XIV Symposium in Pesticide Chemistry, 30th August - 1st September 2011, Piacenza, 

Italy. 

Presentation 2:

“Determining the Fate and Transport of Pesticide Emissions in the Chesapeake Bay Region”

Cody Howard, USDA/ARS

Environmental scientists have primarily focused on the fate, transport, and toxicity of 

pesticide active ingredients, while relatively little research has been conducted on the 

environmental fate of pesticide inactive ingredients, sometimes termed formulants or inerts. 

Many formulants are volatile organic compounds that may contribute to ground-level ozone 

pollution. In California, a system of formulation testing using thermogravimetric analysis (TGA) 

is used to estimate VOC emission potential for pesticide products. In this study, several 

available methods for estimating VOC emissions and ozone formation potential (OFP) of 

pesticide products were evaluated. The methods examined included TGA alone; TGA with 

calculated OFP from confidential formulation information; TGA-FTIR of evolved gases and 

calculation of OFP; and gas-chromatography-mass spectrometry and calculation of OFP. 

Further research studies are focused on measuring the concentration of selected 

pesticides, agricultural-related emerging contaminants, and VOCs in surface agricultural soils 

collected from representative sites across the agriculture-urban interface of the Chesapeake Bay. 

The collected data will be used to develop emissions inventories for selected gaseous compounds 

using the Pesticide Emissions Model. Input files for initial model calibration will be populated 

with soil concentration, soil property, soil moisture, and meteorological data from a multi-year 

study of pesticide fate carried out at the USDA.

Presentation notes:

Dr. Howard explained the motivation behind this work, citing the Presidential Executive 

Order 13508, which mandates that the governments at the local, state, and federal levels restore 

Chesapeake Bay to an acceptable level. This project is currently being directed by a multiagency government task force. In addition, there is interest in quantifying agricultural sources of 

SVOC ozone-precursors since acceptable ozone standards are proposed to be more restrictive in 

the future. 

Throughout the U.S., urban and agricultural communities have become more spatially 

intertwined, resulting in blurred land use boundaries. Emissions from urban areas such as NOx, 

particulate matter, and VOCs can contribute to ground-level ozone pollution, causing damage to 

crops. Agricultural operations also emit pollutants to the atmosphere, including ammonia, 

odorous VOCs, and particulate matter which can create nuisance problems in residential 

communities. Agricultural emissions can also interact with urban pollutants that contribute to 

poor air quality. In addition to air pollutants, pesticides and other organic pollutants used in 

both urban and agricultural communities can have negative effects on ecosystems. In order to 

mitigate the risk from these chemicals, the project team will seek to discern the chemical nature, 

fate, and transport of critical agricultural pollutants emitted to the atmosphere and consider the 

potential risks posed by reactivity and/or deposition of these chemicals to sensitive ecosystems. 
Dr. Howard is currently working to develop a pesticide emission inventory using land use 

data and sales data in tandem with the Pesticide Emission Model (PEM). The emissions 

inventory will be processed in the SMOKE (Sparse Matrix Operator Kernel Emission Model) 

pre-processor program in order to utilize the CMAQ (Community Multi-scale Air Quality) 

model to ultimately determine the loading in the Bay from deposition processes associated with 

pesticide concentrations in the air. The PEM model for soil emissions will be calibrated with a 

robust USDA 10-year field volatility study on atrazine and metolachlor. Additional field 

volatility studies will be needed for crop canopy and orchard type of applications in order to 

calibrate PEM.

Presentation 3:

“Developments in Terrestrial Exposure Modeling”

Ed Odenkirchen, USEPA/OPP/EFED

The Environmental Fate and Effects Division, Office of Pesticide Programs, United States 

Environmental Protection Agency (EFED) has been working to enhance its methods for problem 

formulation and exposure assessment phases of the ecological risk assessment process for 

terrestrial wildlife to be more mindful of the potential for toxicologically significant exposures 

from a variety of pathways beyond the traditionally assessed dietary route. These enhanced 

methods are intended to provide transparent, reasonable, concise, and consistent evaluations of 

the potential pesticide risks posed by the drinking water, inhalation, and dermal exposure routes. 

This presentation discusses past assumptions, present problem formulation screening tools for 

prioritizing data collection and risk assessment efforts, and development efforts for first tier 

screening risk assessment tools.

Presentation notes:

Historically, EPA’s ecological risk assessments for pesticides have focused solely on 

dietary routes of exposure to birds and mammals. Although the LD50/ft2 tool in the T-REX model 

addresses potential exposure to non-target organisms from all possible routes, this analysis does 

not distinguish which pathways (diet, drinking water, inhalation and/or dermal) are significant 

sources of chemical exposure. Current modeling efforts within the Office of Pesticide Programs 

are addressing this need to examine other potential exposure pathways. New risk assessment 

tools include the drinking water Screening Imbibition Program (SIP), the Screening Tool for 

Inhalation Risk (STIR) and a soon-to-be-developed Dermal Uptake Screening Tool (DUST). The 

purpose of these tools is to utilize both chemical fate and toxicity information to identify when a 

route of exposure can be considered negligible during the problem formulation stage of 

Registration Review. If an exposure pathway is determined to be a potential source of risk based 

on the conservative estimates provided by these screening tools, then a clear rationale exists to 

request additional data to address these concerns. T-REX, SIP, and STIR can be found online at 

http://www.epa.gov/oppefed1/models/terrestrial/

Question and Answer Session:

Q: Is your ultimate goal to include these pathways in T-REX?
A: Yes

Q: For the dermal model, why do you assume that 100% of the pesticide is absorbed by the skin 

of birds and mammals?

A: If the potential for risk is identified with this conservative assumption, then this highlights 

the need for additional information. If sufficient additional data is submitted, then the percentage 

of pesticide absorbed could be adjusted. 

Q: Do you anticipate a peer review, such as a Scientific Advisory Panel (SAP) before employing 

these tools?

A: No, a Scientific Advisory Panel is not necessary as the underlying basis for these tools has 

previously been reviewed by former SAPs.

Q: Can STIR handle inhalation of granulars?

A: The tool can evaluate a chemical if the vapor pressure is known.

Presentation 4:

“Dermal Contact, Movement, and Amphibian Pesticide Exposure”

Tom Purucker, EPA/ORD/NERL

The more terrestrial an amphibian’s life cycle is, the more likely its skin will be used to regulate 

water content in order to maintain hydration. Therefore, amphibian dermal contact from 

enhanced skin permeability can be a key exposure pathway compared to non-amphibian 

receptors. Hydrophilic and lipophilic molecules have separate pathways for dermal exposure, 

with neutral, lipophilic molecules traditionally receiving the most attention in humans and other 

mammals. However, preferential water uptake by the seat patch in amphibians increases the 

effective surface area for osmosis and may enhance the potential for low Kow dermal 

contaminant exposures. Although amphibian epidermis has some capability to limit exposures to 

contaminants, positive linear relationships between Kow and contaminant flux across amphibian 

skin are significantly higher in amphibians versus mammals. In addition, sustained direct 

contact with soil from daily movement patterns, overnight burrowing, and 

aestivation/hibernation periods further complicates chronic exposure estimation for amphibians. 

We reviewed recent data and methods concerning amphibian exposures to identify a set of 

algorithms and parameters for screening and higher-tier model selection purposes that accounts 

for the increased dermal contact exposure potential and movement behavior.

Presentation notes:

The purpose of this project is to review recent literature on amphibian dermal exposure, 

develop a model to estimate dermal contact exposure in amphibians, and compare the relative 

importance of dermal exposure across animal classes. This presentation explores factors related 

to dermal exposure in amphibians although knowledge is lacking on several key factors. 
Mammals have a strong hydrophobic barrier in the epidermis that limits absorption of all 

but non-ionic, lipophilic molecules. On the other hand, amphibians lack this hydrophobic layer. 

Estimates of dermal exposure doses in children vary greatly, by three orders of magnitude, 

depending on the methodology used. In mammals, decreasing relative dermal absorption is 

correlated with increasing dermal loading. This correlation has important implications for 

extrapolating from lab studies (high loading) to field conditions (low loading). Amphibians 

differ from mammals in the following ways: 1) they have lower metabolic demand than 

mammals; 2) they have greater surface area relative to body weight (due in part to the seat patch)

than mammals; 3) they have thin skin without a hydrophobic barrier to chemicals, resulting in 

greater absorption; 4) they absorb water through the skin; and 5) gas exchange is mainly through 

the skin. 

Studies have shown that dermal exposure is an important pathway for amphibians. The 

seat patch on the ventral side of amphibians’ skin is an adaptation that is responsible for fluid 

intake and osmotic release of chemicals. With their high moisture levels, irrigation fields are 

especially attractive to amphibians and are sources of chemical exposure. Amphibians may also 

receive high exposure to pesticides in agricultural soils when they burrow into soil at night and 

over the winter. When amphibians burrow over winter, they eat their shed skin, resulting in a 

dietary route of exposure in addition to dermal exposure. The more terrestrial an amphibian is, 

the more likely it will be exposed to pesticides. Chemicals with high Kow’s are likely to be more 

of a concern than those with low Kow’s. The dermal exposure model includes coefficients for 

dermal penetrability (Kp), surface area (SA), skin thickness (H), time of exposure (T), 

bioavailability (BF), and body weight (BW). Walker et al. (2003) showed that there is a close 

linear relationship between Kp and Kow in humans, while Quaranta et al. (2009) looked at this 

relationship in frogs and pigs. Both researchers found a similar relationship (i.e., similar slopes), 

but the log(Kp)was much greater for frogs than for pigs (i.e., the intercept was greater). 

Unfortunately the results from the human study (Walker et al., 2003) are not comparable to those 

from the pig and frog study (Quaranta et al., 2009) because of differences in experimental 

design. The ratio of frog permeability to pig permeability varied greatly, ranging from about 25x 

to about 300x. The dermal exposure model was used to estimate dermal and dietary doses for 

mammals, birds, reptiles, and amphibians and to compare the relative contribution of the 

exposure routes in different classes of animals. Metabolic rates are lower for cold-blooded 

reptiles and amphibians than for warm-blooded birds and mammals. The relationship between 

metabolic rate and body weight is more uncertain for reptiles and amphibians than for birds and 

mammals. 

For all classes, doses were higher for chemicals with higher Kow values. The order of 

dermal dose by class is as follows: amphibians > reptiles > birds and mammals. Kow does not 

affect dietary dose for any class. The order of dietary dose by class is as follows: birds and 

mammals > reptiles and amphibians. The relative contribution of dermal exposure is much 

greater for amphibians than for other classes. For reptiles, the relative contribution is strongly 

dependent on the Kow, and is high for high-Kow chemicals. The relative contribution of dermal 

exposure is small for birds and mammals. Dermal exposure to pesticides can be estimated by 

using: (1) exposure associated with moving animals, which is based on Kp, or (2) exposure 

based on a “finite” source in the vicinity of an aestivating animal. 

In conclusion, amphibians and reptiles receive a significant dose of pesticides from 

dermal exposure, whereas birds and mammals receive very little. These findings have important 

implications for the surrogate use of birds to protect amphibians and reptiles from pesticides. 
Laboratory and field data sets for amphibians need to be examined to better define the 

parameters used in dermal exposure models.

Question and Answer Session:

Q: In the plots of Kp versus Kow, most of the data were for chemicals with relatively low 

log(Kow) values of 2 to 3. What would the plot show for chemicals which have much 

greater Kow values? Would the line level off?

A (Tom Purucker): My guess is that the linear relationship probably would not hold for 

chemicals with high Kow values. The slope would probably change.

Q: Will you consider the behavior and ecology of amphibian species when you predict 

their dermal exposure? For example, some amphibians come out only at night, and seek 

out areas where insect densities are higher.

A (Tom Purucker): You could account for such ecological factors to some degree. For 

example, you could adjust the time spent in field based on ecological characteristics.

A (Edward Odenkirchen): With all of these models, there needs to be a balance between 

being general and being realistic for specific species. To be efficient, the EPA needs to 

keep the model fairly generic for species so that we can apply the results to protect a wide 

range of nontarget species.

Presentation 5:

“Habitat Classification for Ecological Risk Assessment Using Aerial Photography and GIS Data 

in a Two-stage Expert System”

William Stiteler, PhD, ARCADIS US Inc.

There are many challenges in performing a land cover classification that can be used for habitat 

suitability analysis of multiple species in ecological risk assessments. The classification scheme 

must be detailed enough to accomodate several different habitat requirements, as well as 

flexible enough to be applicable to different species. Because habitat suitability often has 

minimum area requirements, but does not require regular shapes of contiguous cover, high 

spatial resolution is useful, but imagery of high spatial resolution often lacks the spectral 

resolution useful for habitat classification. We present the results of a land cover classification 

designed to allow for the modeling of habitat suitability for a variety of receptors, such as 

shrews, robins, and deer. A hybrid remote sensing / GIS approach was developed that uses both 

aerial photography and ancillary GIS data. A two-stage expert system is used to bring together 

different data sources. This expert system was designed to work with a particular set of data 

sources, but it can be easily adapted to other sources. The result is a land cover classification 

with fourteen classes, a minimum mapping unit of 1/20 acre, and distinct polygons that can be as 

narrow as four feet wide. Ground reference data from field visits showed an overall accuracy of 

approximately 80% for qualitatively different classes and 60% for the full classification scheme 
containing quantitative class divisions, such as those between closed, intermediate, and open 

canopy cover classes.

Presentation notes:

A method to classify habitat based on aerial photography conditioned on GIS coverages 

of soils data and tax parcel data was presented. Complications in developing accurate coverages 

using this method are effects of shadows, compression of digital photographs, and effects due to 

time of the year (i.e., you can’t see through a forest canopy except in winter). In some cases, 

boundary data are defined differently in different jurisdictions, and this situation requires special 

handling. In the prepared example, a dam along the boundary of two counties was described 

differently in the boundary data, and consequently the software had trouble classifying the river 

and land around the dam. The analysis was conducted in two stages: first a draft set of polygons 

from the photos and GIS coverages was developed and secondly the resulting polygons into 

habitat classes were aggregated. The resulting coverage had accuracy (~73%) similar to the 

NLCD (National Land Cover Database).

Question and Answer Session:

Q: Were standard USDA boundaries used for the assessment as it can be difficult to 

combine local coverages such as that presented in the paper with broader regional or 

national assessments?

A: USDA boundaries were not used for the assessment.

Wrap up:

The next EMPM will be held on September 13, 2011. The meeting theme will be announced 

through the “empm listserve” forum on the LYRIS list serve at:

https://lists.epa.gov/read/?forum=empmlist