Guidance on Modeling Offsite 

Deposition of Pesticides via Spray Drift

Exposure Modeling Public Meeting (EMPM)

March 24, 2014

Environmental Fate and Effects Division

Office of Pesticide Programs

U.S. Environmental Protection Agency

Katrina White, Faruque Khan, 

Marietta Echeverria, Chuck Peck
Purpose of Guidance

 Establish default assumptions for modeling spray drift for 

agricultural uses

 Spray drift in terrestrial risk assessments

 Spray drift in aquatic risk assessments

 How to characterize effect of buffers on risk

http://www.etsprayers.com/2013/05/12/5-things-you-should-consider-to-avoid-spray-drift/
PRZM/EXAMS Inputs for Spray Drift Fraction for Applications of Liquids 

Without Any Spray Drift Limitations (Estimated using AgDRIFT Version 

2.1.1)

Type of 

Application

Spray Drift Fraction for Runoff and Drift 

EEC Without Buffer

Previous Default 

Inputs

Ecological Risk Assessment Using EPA Farm Pond

Aerial 0.125 0.05

Ground 0.062 0.01

Airblast 0.042 0.03

Drinking Water Assessments Using Index Reservoir

Aerial 0.135 0.16

Ground 0.066 0.064

Airblast 0.048 0.063

Granular applications: spray drift fraction = 0 

 application efficiency = 1

If there is a buffer distance, use AgDRIFT to determine the appropriate spray drift 

fraction.
AgDRIFT: Default Input Parameters 

(Agricultural Uses)

 Tier I Aerial

 Use ASAE Fine to Medium DSD unless the evidence indicates that use 

of Very Fine to Fine DSD is likely

 10 mph wind speed, 10 ft. release height

 60-foot swath width

 Tier I Ground

 Boom Height = High Boom

 Data Percentile = 90th Percentile

 DSD = ASAE Very fine to Fine

 Tier I Airblast

 Sparse (Young/Dormant) 

 Estimates a 50th percentile drift due to limitations in data

Except for the Tier I aerial 

assumption, the scenarios 

result in the highest exposure 

estimate

If a label specifies something 

different from the default 

assumptions, the label 

specifications are used in 

modeling.
Spray Drift and Terrestrial Exposure

Egrets, 

herons, and 

other birds 

in wild rice 

field.

http://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=9373
Terrestrial Animal Exposure off the Field

 Estimate Risk at a Distance from the Edge of the Field (Use 

AgDRIFT and TREX)

 Enter distance from the edge of the field as the buffer distance in 

AgDRIFT to get the ‘Fraction of Applied’

 Fraction of Applied x Application Rate = Application rate in TREX to 

estimate risk at the specified distance from the edge of the field

 Applications directly to water and bare ground 

 Assume exposure at the edge of the field will be identical to 

exposure on water body or field (e.g., use full application rate in 

TREX and assess exposure to terrestrial organisms)

 For bare ground application 

 EECs for seeds and insects reflect exposure on the field 

 EECs for residues on plants will mainly occur at the edge of the field
Spray Drift Distance: Distance from the edge of the field where 

exposure is at levels where LOCs could be exceeded
Terrestrial Spray Drift Distance - Animals

 Estimate the ‘Fraction of Applied’ and Enter into 

AgDRIFT

 Fraction of applied = fraction of application rate that is 

expected to result in some effect 

 Fraction of Applied = LOC/RQ ratio 

 Acute and chronic terrestrial animal RQs

 Use LOC relevant to organisms (listed or nonlisted) analyzed

 Use RQ based on maximum number of applications and 

maximum use rate
Output – Drift Distance

Input

Select
Terrestrial Spray Drift Distance - Plants

 Plant exposure models are not currently available that 

examine exposure due to multiple applications

 Include uncertainty language

Maximum Application Rate in lbs a.i./A

for listed plantsin lbs a.i/A 05 for nonlisted plants or NOAEC/EC 25 EC

Fraction of Applied for Plants =
Spray Drift and Aquatic Exposure
Estimate Spray Drift Fraction for 

PRZM/EXAMs using AgDRIFT for 

Ecological and Drinking Water 

Assessments

Do not use the default inputs (e.g., 

0.01, 0.05, 0.16, 0.064)
Ecological Risk: 

EPA Pond

Human Health Drinking Water: 

Index Reservoir

10 hectare 

agricultural field

1 ha 

pond •173 ha watershed

•5.3 ha reservoir

•1500 m x 4 m of stream with 

4 m buffer
Select

Input the Buffer Distance

Output: Spray 

Drift Fraction for 

PRZM/EXAMs 

Ecological 

Assessment

Spray Drift For 

Ecological 

Assessments
Calculating Spray Drift for Index Reservoir

Normalized Spray Drift Fraction for Streams (unitless) = 

Spray Drift Fraction for Streams x SA of Streams/SA of Reservoir

Adjusted Spray Drift Fraction for Index Reservoir 

(unitless) = 

Normalized Spray Drift Fraction for Streams + Spray Drift Fraction 

for Reservoir
PRZM/EXAMS Inputs for Spray Drift Fraction for Applications of Liquids 

Without Any Spray Drift Limitations (Estimated using AgDRIFT Version 

2.1.1)

Type of 

Application

Spray Drift Fraction for Runoff and Drift 

EEC Without Buffer

Previous Default 

Inputs

Ecological Risk Assessment Using EPA Farm Pond

Aerial 0.125 0.05

Ground 0.062 0.01

Airblast 0.042 0.03

Drinking Water Assessments Using Index Reservoir

Aerial 0.135 0.16

Ground 0.066 0.064

Airblast 0.048 0.063

Granular applications: spray drift fraction = 0 

 application efficiency = 1

If there is a buffer distance, use AgDRIFT to determine the appropriate spray drift 

fraction.
Estimated Amount of the EEC That Is Due to Runoff (The Higher the 

Percentage the Less the EEC was Influenced by Spray Drift)

Scenario

Peak Run-off only EECs 

Peak Run-off + Drift EECs (Percent)

CAnurserySTD 98%

CAForestryRLF 95%

CAcornOP At Plant 94%

CAColeCropRLF 81%

CAcotton_WirrigSTD 66%

CAsugarbeet_WirrigOP 

At Plant 43%

CAtomato_WirrigSTD 30%

CAfruit_WirrigSTD 12%

CAMelonsRLF 6%

Log Kow =6
Aquatic Spray Drift Distance/Buffer Distance

 Method 1: Considering Exposure to Spray Drift Only (use 

AgDRIFT and acute aquatic endpoints)

 This is needed for all assessments with all LOC exceedances 

because the drift distance may be greater than the 

terrestrial drift distance. If it is, it would establish the 

baseline potential area of effect

 Method 2: Considers Exposure to Both Spray Drift and Runoff 

 Estimate spray drift fraction that will eliminate LOC 

exceedance by adjusting spray drift fraction input in 

PRZM/EXAMS using trial and error 

 Find corresponding drift distance in AgDRIFT using Tables in 

Guidance or AgDRIFT

 Considers both acute and chronic risk
Determine 

PRZM/EXAMs 

EECs without drift

a. Do not calculate a 

spray drift distance 

because there is no risk

c. Calculate a spray drift distance that 

considers risk due to exposure to 

both runoff and spray. Estimate 

spray drift distances using Method 1 

(acute endpoint and AgDRIFT) and 

Method 2 (AgDRIFT and 

PRZM/EXAMS).

Runoff only EEC does not result in 

LOC exceedance but the EEC that 

considers both drift and runoff 

does result in an LOC exceedance

No LOC 

Exceedance

Runoff only EEC 

results in LOC 

exceedance

b. Risk cannot be 

mitigated using a 

spray drift buffer; 

however, any buffer 

used will help to 

reduce risk. Estimate 

a spray drift distance 

using Method 1 (acute 

endpoint, AgDRIFT)

Determine PRZM/EXAMS EECS with Drift 

Calculate RQs and 

Compare to LOCs

LOC 

Exceedance
Calculating the Aquatic Spray Drift 

Distance: Method 1

Input in AgDRIFT is the Initial Average Concentration

 Represents the average concentration within a defined water 

body, in ng/L

 Average concentration in a defined water body in ng/L that 

will not result in LOC exceedance

Initial Average Concentration

 Aquatic Organisms:

 = LC50 × LOC 

 Aquatic plants:

= EC50 × 1 (for non-listed plants)

= EC05 or NOAEC × 1 (for listed plants) 
Calculating the Aquatic Spray 

Drift :Example of Method 2

Example 1: Esfenvalerate, CACole crop, 2 applications, 0.06 kg/ha, 7 day interval

Acute nonlisted LOC = 0.5, Acute listed LOC = 0.05, Chronic LOC = 1

Scenario Buffer

Spray 

Drift 

Fraction peak EEC LC50 RQ

Chronic 

EEC NOAEC RQ

PRZM/EXAMS EEC 

with drift and runoff 0 0.24 0.50 3.00 0.17 0.07 0.04 2.00

PRZM/EXAMS EECs 

without drift 0 0.00 0.03 3.00 0.01 0.01 0.04 0.26

PRZM/EXAMS EECs 

with drift and runoff 

with a buffer

800-997 0.04 0.09 3.00 0.03 0.02 0.04 0.53

600-750 0.05 0.11 3.00 0.04 0.02 0.04 0.61

450-550 0.06 0.13 3.00 0.04 0.02 0.04 0.69

350-400 0.07 0.15 3.00 0.05 0.03 0.04 0.76
Spray Drift Fraction: EPA pond

Distance from edge of 

field to water body 

(feet)

Fraction of Applied or Spray Drift Fraction

Ground Application Aerial Application

ASAE very 

fine to fine

ASAE fine to 

medium/coarse

ASAE very 

fine to fine

ASAE fine to 

medium

ASAE 

medium to 

coarse

0 0.06 0.02 0.24 0.13 0.09

1 0.06 0.01 0.24 0.12 0.09

5 0.05 0.01 0.23 0.12 0.08

10 0.04 0.01 0.23 0.11 0.07

25 0.03 0.01 0.21 0.09 0.06

50 0.02 0.01 0.18 0.07 0.04

100 0.01 0.00 0.15 0.05 0.03

150 0.01 0.00 0.12 0.04 0.02

200 0.01 0.00 0.10 0.03 0.02

250 0.01 0.00 0.09 0.03 0.01

300 0.01 0.00 0.08 0.02 0.01

350 0.00 0.00 0.07 0.02 0.01

400 0.00 0.00 0.07 0.02 0.01

450 0.00 0.00 0.06 0.02 0.01

500 0.00 0.00 0.06 0.02 0.01

550 0.00 0.00 0.06 0.02 0.01

600 0.00 0.00 0.05 0.01 0.01

650 0.00 0.00 0.05 0.01 0.01

700 0.00 0.00 0.05 0.01 0.01

750 0 00 0 00 0.05 0 01 0 01
Questions?

Katrina White

703-308-4536

White.Katrina@epa.gov

Published as Federal Registry Notice: 

http://www.regulations.gov/#!docketDetail;D=EPA-HQ-OPP-2013-0676

Comments accepted through end of April