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Exposure Modeling Public Meeting (EMPM)

March 20, 2012

Agenda and Abstracts

Time Presentation Number and Title Presenter

9:00 – 9:10 Welcome and Introductions Marietta Echeveria, Dirk

Young, and Katrina White

9:10 – 9:40 1‐ NAFTA Guidance for Calculating Degradation

Kinetics in Environmental Media

Dirk Young and Ian Kennedy

9:40 – 10:10 2‐Update: Implementation of PRZM‐GW Reuben Baris, Michael Barrett,

Rochelle F. H. Bohaty, Marietta

Echeverria, Ian Kennedy, Greg

Malis, Dirk Young, and James

Wolf

10:10 – 10:30 Break   

10:30 – 11:00 3‐Measuring and Estimating Concentrations in

Drinking Water:  A Historical Perspective

Russell L. Jones

11:00 – 11:30 4‐Improved Characterization of the Temporal and

Spatial Variability of Potential Surface Water Drinking

Water Exposure by Using Environmental and Historic

Monitoring Databases

Paul Hendley, Wenlin Chen,

Robert Joseph,  Ron Williams,

Chris Harbourt & Paul Miller

11:30 – 12:00 5‐Estimating Upper Centile Pesticide Concentrations

and Sample Size Requirement

Wenlin Chen, Paul Mosquin

12:00 – 1:00 Lunch   

1:00 – 1:30 6‐Estimation of Upper Percentiles of Chlorpyrifos

Surface Water Concentration from Yearly Monitoring

Program Data

Paul L. Mosquin, G. Gordon

Brown, Roy W. Whitmore, Nick

Poletika

1:30 – 2:00 7‐Sampling Plans for Water Quality Assessment John W. Green

2:00 – 2:30 8‐Update on Development of Drinking Water Intake

Watershed PCAs

Jim Carleton, Rochelle Bohaty,

Marietta Echeverria, Nelson

Thurman, Michelle Thawley,

Katrina White, Mohammed

Ruhman

2:30 – 2:45 Break   

2:45 – 3:15 9‐Exposure Assessment for Pronamide Drinking

Water Residues in California Central Coast Lettuce

Production Areas

Nick Poletika, Brian Bret, Mike

Winchell

3:15 – 3:45 10‐ Hybrid PRZM:"Filling in the gaps" in Field

Sampling Data Using Realistic Simulation Modeling

Paul Miller, Chris Harbourt,  

William J. Northcott,  

Paul Hendley,  and  

Jessie J. Prenger

3:45 – 4:00 Meeting Wrap‐Up
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Abstracts

1. NAFTA Guidance for Calculating Degradation Kinetics in Environmental Media 

Dirk Young1

, Ian Kennedy2 

1

Environmental Fate and Effects Division. United States Environmental Protection 

Agency 

2

Health Canada 

The USEPA and Health Canada (NAFTA partners) have developed guidance for 

calculating degradation rates for transport model inputs (i.e., PRZM and EXAMS). The 

NAFTA guidance specifies the conditions under which a first-order model can be used 

to represent pesticide degradation data in NAFTA assessments and provides a 

procedure to determine a representative rate when the data deviate from a first-order 

concept. The NAFTA guidance is based on the premise that there is no "correct" 

transformation of non-first-order data into first-order model parameters; therefore, any 

parameter transformation will require a subjective decision based on an agreed-upon 

level of protectiveness. The emphasis of the NAFTA procedure is therefore on 

standardization of a process rather than on statistics of determining "best fit" models. In 

this regard the NAFTA group has arrived at a comparatively simple method that has 

preserved some standard assumptions used by FOCUS such as use of a DT90 as a 

reference point. 

2. Update: Implementation of PRZM-GW 

Reuben Baris, Michael Barrett, Rochelle F. H. Bohaty, Marietta Echeverria, Ian 

Kennedy, Greg Malis, Dirk Young, and James Wolf 

Environmental Fate and Effects Division. United States Environmental Protection 

Agency, Arlington, VA 

Estimation of pesticide concentrations in groundwater is one of the considerations as 

part of the exposure assessment in the pesticide registration process. For this reason, 

Canada and the United States combined efforts as part of the North American Free 

Trade Agreement (NAFTA) to develop a harmonized groundwater modeling protocol. 

This included the development of a common conceptual groundwater modeling scenario 

for regulatory purposes designed to be protective of even the most vulnerable drinking 

water supplies. The Pesticide Root Zone Model (PRZM) was selected as the NAFTA 

regulatory tool. Evaluation of PRZM was completed by comparing simulated pesticide 

concentrations to available targeted and non-targeted monitoring data. For the majority 

of chemicals tested, PRZM-predicted pesticide concentrations represent upper bound 

estimates of exposure in groundwater when conservative input parameters are 

used (e.g., maximum application rates, annually repeated applications, conservative 

degradation assumptions). There are some pesticide detections in monitoring data that 

are not captured by PRZM model estimates. This may be a result of physical 

processes that are not accounted for in PRZM (e.g., preferential flow or macroparticle 
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transport). With site specific adjustments to the PRZM input values estimated pesticide 

concentrations compare very well to monitoring data (the same order or within an order 

of magnitude). The evaluation demonstrates that PRZM is a versatile assessment tool 

that can be used both as a screening tool and a refined site-specific tool in risk 

assessment. 

3. Measuring and Estimating Concentrations in Drinking Water: A Historical 

Perspective 

Russell L. Jones 

BayerCrop Science

After the adoption of the Food Quality Protection Act in 1996, risk assessments for 

registration had to include the contribution of residues in drinking water to dietary 

assessments. This triggered work modeling and monitoring work by both EPA and 

industry to determine satisfactory methods for estimating and measuring concentrations 

in drinking water from both ground and surface water. This presentation reviews the 

developments in modeling and monitoring of drinking water from surface water over the 

past 15 years, including identification of the most sensitive drinking water intakes, 

SWMI, WARP, and the Index Reservoir as well as the evolution in surface water 

monitoring studies. The results of the past research will be used to suggest potential 

approaches to obtaining appropriate estimates of drinking water based on either 

modeling or monitoring. 

4. Improved Characterization of the Temporal and Spatial Variability of Potential 

Surface Water Drinking Water Exposure by Using Environmental and Historic 

Monitoring Databases 

Paul Hendley1

, Wenlin Chen1, Robert Joseph1, Ron Williams1, Chris Harbourt2 & 

Paul Miller2

1

Syngenta Crop Protection LLC, Greensboro, NC 

2

Waterborne Environmental Inc., Champaign, IL 

The extensive atrazine surface water monitoring database totals over 300,000 samples 

including ~120,000 drinking water samples from high frequency seasonal monitoring 

programs covering more than 15 years and >210 Community Water Systems (CWS). 

This wealth of frequent monitoring data sampled across a wide range of years and sites 

allows for robust conclusions to be drawn with very high statistical certainty regarding 

the highest centiles of potential atrazine exposures. To complement the monitoring 

data, site-specific details have also been accumulated on CWS watersheds, source 

waters and water handling procedures; this information has been used to better 

understand the expected temporal concentration patterns at individual CWS. We will 

outline how these and related data (e.g. daily flow and chemical monitoring data from 

Heidelberg University) have been used to examine approaches for optimizing sampling 

strategies for assessing potential drinking water concentrations in surface water. 

Scenarios with different sampling frequencies have been simulated and the results have 

been compared to measured data for each year and site. This talk will demonstrate how 
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lower frequency monitoring data can be used to estimate potential maximum shorter 

duration concentrations. 

5. Estimating Upper Centile Pesticide Concentrations and Sample Size 

Requirement 

Wenlin Chen1

, Paul Mosquin2

1

Syngenta Crop Protection, LLC 

2

RTI International 

A survey sampling approach is presented for estimating upper centiles of aggregate 

distributions of surface water pesticide concentrations obtained from monitoring with 

large sample sizes but variable sampling frequency. It is applied to three atrazine 

monitoring programs of Community Water Systems (CWS) that used surface water as 

their drinking water source: 1. the nationwide Safe Drinking Water Act (SDWA) data, 

2.the Syngenta Voluntary Monitoring Program (VMP), and 3. the Atrazine Monitoring 

Program (AMP). Requisite sample sizes are determined using statistical tolerance 

limits, relative standard error, and the Woodruff interval sample size criterion. These 

analyses provide 99.9% confidence that the existing data include the 99.9th centile 

atrazine concentration for CWS raw and finished water in the Midwest atrazine high-use 

areas and in the nationwide SDWA dataset, assuming no bias due to nonrandom 

selection of CWS and monitored days during these years. The general validity of these 

approaches is established by a simulation that shows estimates to be close to target 

quantities for weights based on sampling probabilities or time intervals between 

samples. 

6. Estimation of Upper Percentiles of Chlorpyrifos Surface Water Concentration 

from Yearly Monitoring Program Data 

Paul L. Mosquin1

, G. Gordon Brown1, Roy W. Whitmore1, Nick Poletika2

1

RTI International, Department of Statistics & Epidemiology, Research Triangle Park, 

NC, 27709, United States 

2

Dow AgroSciences LLC, Department of Field Exposure and Effects, Indianapolis, 

IN, 46268, United States 

This talk presents a finite population method to estimate annual 95th, 99th and 99.9th

centiles of surface water chlorpyrifos concentrations and associated confidence 

intervals using historical surface water pesticide monitoring programs with large number 

of measurements. The method provides reliable estimates, accounting for infrequent 

sample collection, measurements below reporting limits, and the strong seasonality 

present in these datasets. Data from three monitoring programs are considered: the 

USGS National Water-Quality Assessment Program (NAWQA), the Heidelberg 

University National Center for Water Quality Research (NCWQR) monitoring data, and 

the Washington State Department of Agriculture/ Department of Ecology surface water 

monitoring program. Results for temporal trends will be provided, including a 

comparison of estimates before and after chlorpyrifos was withdrawn from residential 

use in 2001. 
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7. Sampling Plans for Water Quality Assessment 

John W. Green, Ph.D. 

DuPont Applied Statistics Group 

Sampling plans for monitoring chemical concentrations in rivers were investigated for 

two California streams of differing order to assess the relative merits of daily sampling 

versus less frequent sampling. The assessments were based on the ability to estimate 

time-weighted total and peak concentrations on a monthly or annual basis and to detect 

the presence of spikes in these concentrations. A secondary purpose was to determine 

the feasibility of predicting spikes in chemical concentrations from the river discharge 

rate. In these streams, some less frequent sampling plans were shown to provide 

almost the same utility as daily sampling for the first purpose and discharge rate was 

shown to have little correlation with chemical concentration. 

8. Update on Development of Drinking Water Intake Watershed PCAs 

Jim Carleton, Rochelle Bohaty, Marietta Echeverria, Nelson Thurman, Michelle 

Thawley, Katrina White, Mohammed Ruhman 

Environmental Fate and Effects Division. United States Environmental Protection 

Agency, Arlington, VA 

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

assessments to account for the fact that watersheds supplying surface drinking water 

sources may not be devoted entirely to agriculture. The first PCAs were developed in 

the late 1990s and mid 2000s, based on cropped acreage within 8-digit HUCs. More 

recently, EFED has developed delineations for watersheds associated specifically with 

drinking water intakes that supply community water systems. EFED is planning to use 

this new dataset to develop drinking water intake watershed-specific PCAs. This talk 

will discuss this intake watershed dataset and PCA development methodologies in 

general terms, along with unique issues and challenges associated with the use of 

these new data for PCA development. 

9. Exposure Assessment for Pronamide Drinking Water Residues in California Central 

Coast Lettuce Production Areas 

Nick Poletika1, Brian Bret2, Mike Winchell3 

1

Dow AgroSciences LLC 

2

Dow AgroSciences LLC, Regulatory and Government Affairs NAFTA 

3

Stone Environmental, Inc. Applied Information Management 

Regulatory pesticide human exposure models are intentionally designed to be 

protective of individuals in populations. Consequently, they tend to be quite 

conservative in early assessment tiers, and predictions are generally refined with data in 

subsequent assessments. For drinking water obtained from surface water, pesticide 

exposure model concentration estimates for agricultural use generally are in the μg L-1

to tens of μg L-1 range, which can lead to concern if human health endpoints for a 

specific pesticide are low values. Currently US EPA OPP accepts only targeted 

monitoring studies for refinement of model estimated concentrations. We present an 
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alternative assessment method to targeted monitoring for a pesticide with a limited 

range of label crops and geographic regions of use that relies on proximity analysis of 

use sites and drinking water intakes. We also address national security issues related 

to sensitive intake location data and consider the general utility of the approach.

10. Hybrid PRZM:”Filling in the Gaps” in Field Sampling Data using Realistic 

Simulation Modeling 

Paul Miller, PhD1

, Chris Harbourt, PhD1, William J. Northcott, PhD1, Paul Hendley, 

PhD2, and Jessie J. Prenger1

1

Waterborne Environmental, Inc. 

2

Syngenta 

The Hybrid-PRZM approach can be used to combine observations from field monitoring 

with results from simulation modeling based on local rainfall for the same period to “fill 

in” gaps between sampling occasions. The approach uses Pesticide Root Zone Model 

(PRZM) simulations based on local rainfall data together with site-specific watershed 

soil and agronomic characteristics. The model is used to estimate edge-of-field atrazine 

concentrations occurring on days with rainfall; the simulation results are scaled to 

watershed conditions without calibration. The PRZM model results supplement field 

monitoring data with estimates of potential runoff events on days when runoff inducing 

rain falls in a watershed between sampling occasions. This technique captures the 

strengths of field and simulation approaches to develop conservative daily time series 

using quality sampling results and appropriate simulation assumptions. Significant 

developments are presented here that result in improved exposure simulation results; 

these include the use of NEXRAD local precipitation data combined with a land 

workability algorithm to develop appropriate distributions of application dates across a 

watershed.