Environmental Modeling Public Meeting 

September 20, 2011

Rochelle F. H. Bohaty, PhD

Office of Chemical Safety and Pollution Prevention

Office of Pesticide Programs

Environmental Fate and Effects Division

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Disclaimer: Views expressed in this presentation are those of the presenter and may not 

represent EPA’s current thinking or policy.
 Background  Conceptual Model  Model Selection  Field Studies  Evaluation  Monitoring Programs  Implementation ◦ Project Schedule

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 Vulnerable private drinking water wells in the vicinity of agricultural 

environments 

 The conceptualization of the groundwater scenario evolved from 

meetings between EPA and PMRA and from advice taken from the 2005 

N-Methyl Carbamate SAP meetings (USEPA, 2005a, 2005b). 4
 22 models were considered in primary screen 

◦ Considerations included:

 familiarity with and scope of the model 

 knowledge of programming language

 continued technical support

 public access

 runtime 

 reviewability

 Three finalist models were identified

◦ Pesticide Emission Assessment at Regional and Local Scales 

(PEARL)

◦ Pesticide Root Zone Model (PRZM)

◦ Leaching Estimation and Chemistry Model (LEACHM)

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 Acceptable studies

 Areas susceptible to leaching

 Use of tracer

 Sampling: soil cores, pore 

water, and groundwater

 Temperature probes
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0

0.2

0.4

0.6

0.8

1

1.2

0 100 200 300 400 500 600

DAT

6 ft Flow (m)

PRZM

PEARL

LEACHM

0

0.2

0.4

0.6

0.8

1

1.2

0 100 200 300 400 500 600

DAT

3 ft Flow (m)

PRZM

PEARL

LEACHM

0

0.2

0.4

0.6

0.8

1

1.2

0 100 200 300 400 500 600

DAT

9 ft Flow (m)

PRZM

PEARL

LEACHM

0

0.2

0.4

0.6

0.8

1

1.2

0 100 200 300 400 500 600

DAT

12 ft Flow (m)

PRZM

PEARL

LEACHM

Field-scale leaching studies/prospective groundwater studies
Measured Soil 

Temperature

LEACHM

PRZM

PEARL

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Day Conc

0

200

400

600

800

1000

1200

1400

0

200

400

600

800

1000

1200

1400

3 ft

9 ft

0 100 200 300 400 500

6 ft

12 ft

0 100 200 300 400 500

Model

LEACHM

PEARL

PRZM

Data

Fate

EPA

High

Low

Data
 Models performed similar for soils prone to leaching

 Additional screening required

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 Selection of Pesticide Root Zone Model (PRZM) 

◦ EPA and PMRA currently use PRZM for modeling runoff

◦ PRZM uses a capacity type algorithm for water flow rather 

than the more complex Richard’s Equation

◦ EPA maintains PRZM

◦ PRZM is freely available
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0.001

0.01

0.1

1

10

100

1000

10000

100000

0.001 0.01 0.1 1 10 100 1000 10000

PREDICTED

NAWQA OBSERVED CONCENTRATION (PPB)

Florida Citrus

Water Monitoring Data Comparison: non-targeted
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Pesticide

Peak 

NAWQA 

Concetration

(PPB)

Hydrolysis t

1/2 

(days)

Aerobic t

1/2

(days) Koc

Malathion 0.1 6.21 3 151

Carbaryl 0.781 12 12 198

Azinphos-methyl 0.18 37 95 700

Chlorpyrifos 0.07 72 76.9 6070

Propanil 0.21 Stable 0.5 487

Thiobencarb 0.036 Stable 40 909

Ethalfluralin 0.09 Stable 138 4000

Dacthal 10 Stable 20 5000

Trifluralin 0.15 Stable 219 7300

Benfluralin 0.006 Stable 65 10750

Pendimethalin 0.12 Stable 172 20000

Actual hydrolysis rate is slower than 

modeled hydrolysis rate.

• pH dependent

Modeled rate was faster 

than environmental 

metabolism,

• lab rate faster than 

environmental rate 

Mobility in soil is faster than model • lack of data

simulation

• Preferential flow

• Micro particle transport

Other factors??

Use scenarios
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Milestone/Specific Task Due Date 

Final Report (joint with PMRA) Under Peer Review 

(with PMRA)

Attachment 1: Scenario Development 

Guidance (joint with PMRA) Completed

Attachment 2: Chemical Input Parameter 

Guidance (joint with PMRA) Completed

Guidance for use of PRZM-GW in Drinking 

Water Assessments (EPA specific) In progress

User guidance for PRZM-GW/ PRZM-GW 

GUI (EPA specific) In progress

Implementation and Director’s Approval Late Fall 2011
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 EFED is working with HED on determining 

appropriate “standard” exposure durations for model 

outputs 
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 EFED is working on identifying scenarios for standard 

use in risk assessment

 EFED is incorporating mitigation measures—i.e. less 

vulnerable soils, greater depth to groundwater, well 

setbacks, etc.—into user guidance 
 EPA Project Team ◦ Reuben Baris ◦ Michael Barrett ◦ Rochelle Bohaty ◦ Dirk Young ◦ James Wolf  PMRA Project Team ◦ Greg Malis ◦ Ian Kennedy  WQTT Management Representative ◦ Marietta Echeverria  WQTT Science Advisors ◦ Jim Hetrick ◦ Nelson Thurman  WQTT Co-chairs ◦ Rochelle Bohaty ◦ Andrew Shelby

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