PRZM-GW:

Implementation and Case Stu

Presentation to the 

Environmental Modeling Public Meeting

March 19 2013 March 19, 2013

Reuben Baris

U S EPA Office of Pesticide Programs U.S. EPA, Office of Pesticide Programs, 

Environmental Fate and Effects Division
Presentation Outline Presentation Outline

• Pur

pose

• Background and Milestones• Implementation of PRZM Implementation of PRZM

-GW in OPP GW in OPP

• One year evaluation period• Case Studies
Purpose

• N-methyl Carbamate Cumulative assess

– Groundwater modeling approach was deve

refine Tier I estimates

• Provide efficient refinements of Tier I e• Provide health protective estimated 

concentrations for vulnerable private we

• Advance the science Advance the science

• Address SAP comments on current Tier 

modeling approach for groundwater modeling approach for groundwater
Background: NAFTA Proj

Background: NAFTA Proj

• Developed a harmonized conceptual mode

Developed

a harmonized conceptual mode

• Reviewed 19 models• Intensel e al ated PRZM LEACHM an

Intensely 

e

val

uated PRZM, LEACHM, an

PEARL

• C l d d th t PRZM i b t f NAFTA 

Conc

l

u

d

e

d th

a

t PRZM

is 

bes

t

for NAFTA 

• Evaluated PRZM-GW with monitoring da

• Completed final report and harmonized gu

documents
Milestones

• February 2005 SAP• A t 2005 SAP August 2005 SAP

– Proposed refined GW modeling approach

• 2006 PRZM-GW used in N-methyl carba

cumulative assessment

• 2006-2012 NAFTA Project 

– Harmonized GW modeling pp a roach with PM

• January 2013 implementation
Expectations

• Tier I screening level estimates may be 

Tier I screening level estimates may be 

than previous GW concentrations

• Fewer chemicals will pass the initial scr

Fewer chemicals will pass the initial scr

• Tier II refinements based on actual prac

may add i k ress risk concerns

• Spatial refinements focus on areas vulne

to GW contamination (soils, weather, aqdepth)
NAFTA Conceptual Mod

Potable well

Declining aerobic degradation with depth

1 m

Hydrolysis 

or other 

relevant 

Water Table

processes 

below 1m 

The conceptual model represents known drinking water sources (i.e., there is 

identifiable population that uses vulnerable groundwater wells as source drin
Standard Scenarios Locati
Implementation Strategy

• One year evaluation period

Implementation Strategy

• Use PRZM-GW as Tier 1 and Tier 2 tool• Continue to Continue to report SCI report SCI

-GROW 2 3 as part 

GROW

2.3 as part 

Tier 1 analysis during a 1 year evaluation p

• Recommend the highest DW concentration

Tier 1Tier

1
Tier 2 Refinement Strateg

Tier

2 Refinement Strateg

• Sensitivity analysis Sensitivity analysis

– Usage info from BEAD and/or FOCUS meet–

Impact of well setbacks mpact of well setbacks

– Fate inputs (e.g., half-life, Koc, subsurface 

degradation)

• Modeling additional scenario• Exposure di l di 

durat

ions re

levant to 

durat

ion o

concern (e.g., annual, 90-day, time series 

etc.

)
Evaluation Period

• Collect data on refinements and outcomes

– How often do we have to refine? How often do we have to refine?– What refinements are effective?– Risk assessment and management outcomes?

• What is being tracked?

– Ti I t ti (SG PRZM) Tier 

I concen

tration (SG or PRZM)

– Inputs– Re

ported concentration and scenario

– Maximum concentration from monitoring (wh

available)

– Refinements Tier II concentration and how th

Refinements, Tier II concentration, and how th

refinement(s) impacted the assessment
Case Studies

• Three chemicals: moderately persistent 

Case Studies

Three chemicals: moderately persistent 

persistent

• Stable to hydrolysis Stable to hydrolysis• Moderately mobile (Koc’s range from 3

1730)

• Refinements were explored
Chemical 1 Chemical 1

• Low throughputs for 30-year simulation• Ran extended weather file for 100 years

l db kh h( )

• Delayed breakthrough (~88 years)• Scenario: Wisconsin
Chemical 1 Continued Chemical

1 Continued

• 15 years of consecutive applications

Period of application
Chemical 2 Chemical 2

• Reduce number of # of applications to 1

Reduce number of # of applications to 1

• Increasing organic carbon content in the 

layer(s) of the soil increased hold up in layer(s) of the soil increased hold up in 

meter of the soil profile
Chemical 3
Questions?
Chem 1 Chem 2 

Application rate (lbs a.i./A;

# off applications)

0.022

(2)

0.08

(1)

Hyd l ro ysis Stable Stable 

Aerobic Soil Metabolism 1730 days 1298 days† 3

Sorption coefficient Koc = 1760 L/g oc Kfoc = 481 L/g oc Peak Tier I concentration concentration 8.82 μg/L 22.4 μg/L 

(PRZM‐GW)

22.4 μg/L

(PRZM‐GW)

Post‐breakthrough Average

concentration

7.2 μg/L

(approx. 80 years)

19.3 μg/L

(approx. 26 years)

Simulation Average 2.11 μg/L

(100 years)

5.5 μg/L

(30 years)

Drinking water concern? No Yes Yes, 

Refinements Refinements NA 1) Reduced Reduced number 1) R

of applications on

label;

2) refinements

related to soil

1) R

d

2) 

u

related to soil 

type restrictions

Tier II peak concentration NA 19.4 μg/L 1) 65.

2) 32
Implementation:

Tier I Process Diagram

Run the six standard Run the six standard

scenarios using PRZMGW 30 year simulation 

If a standard scenario 

doesn’t represent use 

move to Tier II

Run SCI-GROW 

2 3

Rep

move 

to Tier II

Report EDWCs for 

scenarios where 

breakthrough was Was breakthrough observed for all

2.3. 

reco

ED

H

(i

Yes

observed. 

observed for all

scenarios? PRZ

and 

If E

l

Run 100 year 

simulation for 

scenarios that 

breakthrough was 

tb d

No

l

not o

bserve

d. 

Report EDWCs for 

100 year simulation.
Implementation:

Tier II Process Diagram

Consider refinement options including: 

Incorporate usage info from BEAD; Report EDWC(s) and 

t ti fi 

Consider impact of well setbacks; 

Develop representative scenario;

Consider additional fate inputs (e.g., subsurface 

degradation); and

most conservative refi

use in HED as

Yes

Determine exposure durations relevant to duration of 

concern (e.g., annual, 90-day, time series data, etc.)

Run PRZM-GW representative scenarios with refined 

input parameters. Run 100 year simu

scenarios where bre

tb 

No

Was breakthrough observed for all scenarios?

was not o

bserv

Report EDWCs for 

simulations
Tier 1 PRZM-GW Results Comp

Human Health BenchmarkHuman Health Benchmark

• 19% exceed the 19% exceed the acute HHBP (of those HHBP (of those

chemicals with a HHBP

†

)

• 23% exceed the chronic 23% exceed the chronic†† HHBP (of tho

†† HHBP (of tho

chemicals with a HHBP

‡

)

• 71% d excee

d MCL (5 f th 7 h i l 

(5 o

f the 

7 c

hem

ica

ls 

have an MCL established)

htt

p p pg p p p ://ias

pub.e

pa.

gov/a

pex/pesticides/f?

p=HHBP:home

†41 of 66 NAWQA chemicals list an Acute HHBP †† Assumes 100% Relative Source Contribution‡51 of 66 NAWQA chemicals list a Chronic HHBP 
1 Year IP Summary Compared

SCI

‐GROW 2 3 GROW

2.3

10000

100000

100

1000

ppb

0.1

1

10

[PRZM-GW] p

0.001

0.01 All Chemicals Ev

one to one lineSCI-GROW

0.0001

0.0001 0.001 0.01 0.1 1 10 100 1000 

[NAWQA] ppb

Note: The “IP” (Implementation Plan) plotted value (black diamonds) is the laPRZM‐GW hybrid and SCI‐GROW 2.3 values. Comparisons are made to the all‐

maximum of all samples from all wells in NAWQA regardless of well type and

characteristics.