Reuben Baris, Michael Barrett, Rochelle Bohaty, 

Marietta Echeverria, Ian Kennedy (PMRA), 

Greg Malis (PMRA), James Wolf, Dirk Young

Exposure Modeling Public Meeting

March 20, 2012

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A North American Free Trade Agreement project:

U.S. Environmental Protection Agency & Health Canada
US EPA Model Guidance

Unique Qualities of Regulatory Models

-Transparency

- Non Proprietary

http://www.epa.gov/crem/library/cred_g

uidance_0309.pdf

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US EPA Quality Assurance Guidance

Combination of G-5M 

and CREM Guidance

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General Steps to Produce a 

Regulatory Model

 Conceptual Model Development

 Implementation

 Evaluation

 Application

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Evaluation

 “The process used to generate information that will 

determine whether a model and its analytical results are 

of a sufficient quality to inform a decision.” (CREM)

INCLUDES:

 Verification- Model Code Check

 Validation

 Data Corroboration – CREM-preferred term

 Peer Review –used throughout the process

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Application

 Internal evaluation by Tech Teams

Review of acceptability of evaluation

Creation of plan for use in assessments

Management review

Communications Officer review

 Final approval by Division Director

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Groundwater Conceptual Model

Hydrolysis 

or other 

relevant 

processes 

below 1m

Depth 

to 

aquifer

Declining 

aerobic 

degradation 

with depth

1 m

Well 

Screen 

Length

Potable well 

drawing from 

surficial aquifer

Water Table

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Implementation of the 

Conceptual Model

 Reviewed 19 models

 Capabilities, ease of use, maintenance, proprietary state

 Intense evaluation of PRZM, LEACHP, and PEARL

 Performance checks, user issues

 Selected PRZM for more evaluations

 Developed User interface for PRZM-GW

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PRZM: Pesticide Root Zone Model; LEACHP: Leaching Estimation and Chemistry Model; PEARL: Pesticide Emission Assessment at 

Regional and Local Scales 
PRZM-GW User Interface

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QA Objectives for Evaluation

 Data Quality Objective

 Non-targeted Data: Groundwater simulations should 

not be below those observed in the test studies

 Targeted studies where use data is known: 

Groundwater simulations should be similar to those 

observed in the test studies

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Monitoring Data

Non-targeted data

 U.S. Geological Survery, National Water Quality 

Assessment Program (USGS NAWQA)

Semi-targeted Study:

 National Alachlor Well-Water Survey (NAWWS)

Targeted Studies

 Prospective Groundwater Monitoring (PGW) study

 U.S. Dept. of Ag., Pesticide Data Program 

(USDA PDP) data from California

 Acetochlor Registration Partnership Midwest Corn 

Production Area (ARP/MwCPA Program) 11
Non-targeted Evaluation:

NAWQA Data and Maximum Use PRZM-GW Screen Simulated Concentrations

Observed Concentrations 12
Semi-targeted:

National Alachlor Well-Water Survey (NAWWS)

 Existing private drinking water wells down gradient of 

agricultural fields 

 History of use for alachlor, atrazine, metolachlor

 PRZM-GW, when conservatively parameterized as a 

screen, produces conservative estimates

Holden et al. (1988, 1992), and Liddle et al. (1990) 13
Targeted Evaluation:

Prospective Groundwater Study with Known Use 

PGW Max = 4.55 ppb

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Targeted Evaluation : PDP Diuron

2590 ppb

Screen:

Hydrolysis is stable

Soil Degradation= 1116 days

Max Label Rate = 8.96 kg/hA

Refinement:

Hydrolysis = 1240 days

Soil Degradation = 372 days

Actual Rate = 0.31 kg/hA

Diuron Measured Value = 0.123 ppb

Reduction Factors

Hydrolysis = 70

Soil Degradation= 5.9

Label Rate = 30

0.278 ppb

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Targeted Evaluation: 

Acetochlor Registration Partnership Mid-Western 

Corn Production Area (ARP-MwCPA)

Chemical Monitoring Peak

(ppb)

PRZM–GW

(Refined)

Acetochlor 0.74 0.99

Alachlor 15.59 18.17

Atrazine 7.76 46.76

Metolachlor 5.98 2.92

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Peer Review

 SAP* February 2005

 August 2005

 EMPM (several 2005 to 2010)

 User comments

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*Scientific Advisory Panel
Summary

 With known pesticide use & good fate input 

parameters, PRZM-GW does well

 When maximum labeled rates are used, minimum 

treatment intervals, and annual application events, 

PRZM-GW provides conservative estimates of GW 

concentrations

 PRZM-GW is still in the evaluation phase

 Currently developing plan for consistent use in EFED 

assessments 18
References

Holden, L.R., J.A. Graham, R.W. Whitmore, W. J.. Alexander, R.W. Pratt, S.K. Liddle, L.L. Piper. 1992. 

Results of the National Alachlor Well Water Survey. Environmental Science & Technology 26 (5), 935-

943.

Liddle, S. K., R. W. Whitmore, R. E. Mason, W. J. Alexander, and L. R. Holden. 1990. Accounting for 

temporal variations in large-scale retrospective studies of agricultural chemicals in ground water. 

Ground Water Monitoring Review. Winter, pp. 142-145.

Holden, L.R., R.W. Whitmore, and l.A. Graham. 1988. The national alachlor well water survey: Statistical 

design and current status. In Proceedings of the Agricultural Impacts on Ground Water Conference, 

National Water Well Assoc. Des Moines, Iowa. pp. 237-254.

USEPA CREM Guidance for Developing Regulatory Models: 

http://www.epa.gov/crem/library/cred_guidance_0309.pdf

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