UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON, D.C. 20460

OFFICE OF 

PREVENTION, PESTICIDES, AND

TOXIC SUBSTANCES 

PC Code: 005209

DP BARCODE: D361926

MEMORANDUM July 24, 2009

SUBJECT: Tier I Drinking Water Exposure Assessment for the Section 3 New Use of 

Triisopropanolamine Salt of Aminopyralid on Field Corn and Field Corn 

Grown for Ensilage.

TO: Douglas Dotson, Chemist

Health Effects Division (7509P)

Kathryn Montague, Product Manager

Registration Division (7505P)

FROM: Reuben Baris, Environmental Scientist

Environmental Fate and Effects Division (7507P)

Office of Pesticide Programs

REVIEWED BY: Marietta Echeverria, Senior Environmental Scientist

Environmental Fate and Effects Division (7507P)

Office of Pesticide Programs

THRU: Elizabeth Behl, Branch Chief

Environmental Fate and Effects Division (7507P)

Office of Pesticide Programs

This memo summarizes the Tier I estimated drinking water concentrations (EDWC) for 

Triisopropanolamine Salt of Aminopyralid (TIPA Salt) in surface water and in ground water in

support of the Section 3 New Use on field corn and field corn grown for ensilage. Aminopyralid 

is a pyridine carboxylic acid herbicide to be used for annual and perennial broadleaf weed 

control for field corn and field corn grown for ensilage. Based on available data, aminopyralid is

moderately persistent to persistent and highly mobile. The EDWCs for both surface and ground 

water recommended for use in HED’s human health risk assessment are summarized in Table 1

and are based on the registered use on rangeland grasses and wheat. The EDWCs resulting from 

the proposed application rate are lower than the EDWCs for surface and ground water in the

previous drinking water assessment for aminopyralid (D301682). Since the proposed application 
rates for field corn and field corn grown for ensilage are lower than other registered uses, it is 

expected that the EDWCs resulting from the proposed application rates for field corn and field 

corn grown for ensilage will be lower, all other variables being equal. 

EFED used the screening model FIRST (FQPA Index Reservoir Screening Tool, version 

1.1, 01/01/07) to calculate the surface water EDWCs. The FIRST-calculated acute value 

represents a 1-in-10 year peak value and the chronic value represents the 1-in-10 year annual 

mean. Estimated ground water EDWCs were calculated using the screening model SCI-GROW 

(Screening Concentrations in Ground Water, version 2.3, 07/29/03). The SCI-GROW-calculated 

value should be used to represent both acute and chronic exposure. 

Table 1. Tier I EDWCs for drinking water risk assessment based on aminopyralid use on 

rangeland grasses (application rate of 0.11 lbs a.e./A/yr) (D301682)

Surface water drinking water sources acute: 10.024 ppb

chronic: 1.937 ppb

Groundwater drinking water sources 0.630 ppb

The concentrations presented in Tables 2 and 3 are the result of an implied maximum

application rate (0.57 fluid oz/acre) on the proposed supplemental label for use on field corn and

field corn grown for ensilage. The label indicates, in the section titled “Maximum Application 

Rate”, “Use Precautions and Restrictions” section:

“An application rate of Milestone in excess of 0.57 fluid oz per 

acre can [cause] [injury] to corn and reduce yield.”

While our assessment, EFED assumes this represents the maximum application rate, in 

fact there is no maximum rate on the label. Revising the label to stipulate a maximum rate 

would eliminate this uncertainty (e.g. “do not exceed 0.57 fluid oz per acre for a single 

application.”). Although there is little difference in the peak and chronic EDWCs resulting from 

3 applications at the lower rate vs. a single application at the maximum annual rate, the revised 

language will avoid misinterpretation and potential injury to the crop.

This assessment uses data from registrant-submitted fate studies for aminopyralid. Both 

the aminopyralid TIPA salt and aminopyralid acid will dissociate in the environment to yield the 

free anion (PC code: 005100). The dissociation process is governed by an equilibrium constant 

which is so low (pKa of 2.56) that greater than 99.9% of the compound will be in the anion form 

at environmental pH ranges, regardless of the original molecular species (US EPA, 2005). 

Aminopyralid is expected to be moderately persistent to persistent and highly mobile in the 

environment. Since it is stable to hydrolysis and aerobic and anaerobic aquatic metabolism, and 

moderately degraded by aerobic metabolism in some soils, transport of aminopyralid to surface 

water and ground water is likely. 
The 2005 Aminopyralid: Tier I Estimated Drinking Water Concentrations (EDWC) for 

Use in the Human Health Risk Assessment was conducted for the parent compound. The only

degradates of aminopyralid were observed in the aqueous photolysis study (MRID 46235727), 

identified as oxamic acid (CAS#471-47-6) and malonamic acid (CAS# 2345-56-4), plus 

unidentified degradates 2-3-carbons long, with O, N, and other substituents. No environmental 

degradates other than CO2 and bound residues were observed in the remaining fate studies

(D301682). Based on the available information, HED does not believe that it is appropriate to 

include residues of either oxamic acid or malonamic acid in dietary risk assessments; therefore, 

these compounds should not be included in estimates of residues of concern in drinking water

(D301689). Both chemicals are small amino acid analogs. Following uptake, HED indicated 

they are expected to be readily metabolized and/or rapidly excreted without any significant 

biological effects. 

A summary of the model input parameter values used in FIRST is presented in Table 2. 

A summary of the model input parameter values used in SCI-GROW is presented in Table 3. 

Input parameters were selected in accordance with EFED’s “Guidance for Selecting Input 

Parameters in Modeling the Environmental Fate and Transport of Pesticides,” Version II 

(February 28, 2002). 

For FIRST surface water modeling, the soil sorption coefficient (Kd) was used instead of 

the organic carbon partition coefficient (KOC) because correlation of Kd with organic carbon was 

low (R2= 0.3656) for the eight soils tested, suggesting that the adsorption of aminopyralid is not 

closely related to organic carbon content (MRID 46235732). Therefore, Kd, the sorption

coefficient uncorrected for organic carbon, was assumed to better represent sorption in soil. The 

aerobic soil metabolism half-life was calculated by nonlinear two-parameter exponential decay

analysis, and then that value was multiplied by 3 to account for the uncertainty associated with 

using a single value. The proposed label allows for ground spray applications and tank mixes; a 

broadcast ground spray was modeled as the method of application in order to be protective of all

application scenarios. The aerobic aquatic metabolism half-lives were calculated using firstorder linear regression on log-transformed data, then the 90th percentile of the upper confidence 

bound on the mean of the three half-lives was calculated. The aqueous photolysis study was 

continuously illuminated and the aqueous photolysis half-life input value was adjusted to reflect 

photolysis in summer sunlight at 40o N latitude. There was no degradation observed in the dark 

controls and no dark-control correction necessary. 

The FIRST output file is located in Appendix 1. The SCI-GROW output file is located 

in Appendix 2. 
Table 2. FIRST (v1.1.0) input parameter values and results for TIPA Salt of aminopyralid 

applied to field corn and field corn grown for ensilage.

Parameter Value Source

Application Rate (lb a.e./A)1

0.57 fl oz/A

(0.0089 lbs a.e./A)

Annual maximum: 1.7 fl oz/A/yr

(0.0266 lbs a.e./A/yr)

Proposed label.

Number of Applications 3 Proposed label.

Interval between Applications (days) 5

Assumed. Not specified on proposed 

label.

Soil Sorption Coefficient (Kd; mL/g) 0.03

Represents the lowest non-sand value 

among eight values ranging from 0.03 to 

0.72 mL/g; 

MRID: 46235732.

Aerobic Soil Metabolism Half-life 

(days) 116.1

Determined by multiplying the 

calculated half-life (38.7) by 3 to 

account for the uncertainty associated 

with using a single value; MRID: 

46235729.

Wetted in? No Proposed label.

Depth of Incorporation (inches) 0 Proposed label.

Method of Application ground spray Proposed label.

Percent Cropped Area 0.46 PRZM input guidance (US EPA, 2002)

Solubility in Water @ 20 OC, 

unbuffered (mg/L or ppm) 2480 MRID: 46235701.

Aerobic Aquatic Metabolism Halflife (days) 1074

Represents the 90th percentile of the 

upper confidence bound on the mean of 

three half-life values (772.9). MRID: 

46235731.

Hydrolysis Half-life @ pH 7 (days) stable MRID: 46235726.

Aquatic Photolysis Half-life @ pH 

5 (days) 0.6 MRID: 46235727.

FIRST Results (EDWCs for surface water drinking water sources)

3 applications per year at 0.57 fl 

oz/A (0.0089 lbs a.e./A)

Acute Concentration: 1.25 μg/L

Chronic Concentration: 0.24 μg/L

Maximum annual application rate 

1.7 fl oz/A/yr (0.0266 lbs a.e./A/yr)

Acute Concentration: 1.28 μg/L

Chronic Concentration: 0.25 μg/L

1 Application rates are given in lbs acid equivalents (a.e.)/A. The acid equivalent for aminopyralid is 21.1% - 2lb/gal. Therefore the conversion 

from fl oz/A to lbs a.e./A is completed using the following formula:

X fl oz/A * 1 gal/128 fl oz * 2 lb a.i/gal = X lbs a.e./A
Table 3. SCI-GROW (v2.3) input parameter values and results for aminopyralid applied 

to field corn and field corn grown for ensilage.

Parameter Value Source and/or Comments

Maximum Application Rate 

(lb a.e./A)1

1.7 fl oz/A/yr

(0.0266 lb a.e./A)

Proposed label.

Number of Applications per Year 1

Represents the most conservative labelallowed scenario in which the total 

maximum rate per year is applied in one 

application.

Aerobic Soil Metabolism Half-life 

(days) 38.7

Represents one value for the aerobic soil 

metabolism of the parent compound;

MRID: 46235729.

Organic Carbon Partition 

Coefficient (Koc; mL/g)2 1.05

Represents the lowest value of eight 

values ranging from 1.05 to 24.30 mL/g 

for the parent compound; MRID: 

46235732.

Results (EDWC for groundwater 

drinking water sources) Concentration: 0.15 μg/L

1

See footnote 1 in Table 2.

2 Koc was used for ground water modeling as per US EPA, 2002 Guidance for Selecting Input Parameters in Modeling the Environmental Fate 

and Transport of Pesticides. 
References:

DP 301682. US EPA. 2005. Aminopyralid: Tier I Estimated Drinking Water Concentrations 

(EDWC) for Use in the Human Health Risk Assessment. US EPA, Office of Pesticide 

Programs, Environmental fate and Effects Division, 1200 Pennsylvania Ave., NW 

Washington, DC.

DP 301689. US EPA. 2005. Aminopyralid: Aggregate Human Health Risk Assessment for the 

Proposed Uses on Wheat, Grasses, Non-cropland Areas, and Natural Areas. US EPA, 

Office of Pesticide Programs, Environmental fate and Effects Division, 1200 

Pennsylvania Ave., NW Washington, DC.

US EPA. 2005. Environmental Fate and Ecological Risk Assessment for the Registration of 

Aminopyralid (PC Codes: 005100, 005209). US EPA, Office of Pesticide Programs, 

Environmental fate and Effects Division, 1200 Pennsylvania Ave., NW Washington, DC.
Appendix 1. FIRST Modeling Output file

Recommended Application Rate

 RUN No. 1 FOR aminopyralid ON fieldcorn * INPUT VALUES * 

 --------------------------------------------------------------------

 RATE (#/AC) No.APPS & SOIL SOLUBIL APPL TYPE %CROPPED INCORP

 ONE(MULT) INTERVAL Kd (PPM ) (%DRIFT) AREA (IN)

 --------------------------------------------------------------------

 .009( .026) 3 5 .0 2480.0 GROUND( 6.4) 46.0 .0

 FIELD AND RESERVOIR HALFLIFE VALUES (DAYS) 

 --------------------------------------------------------------------

 METABOLIC DAYS UNTIL HYDROLYSIS PHOTOLYSIS METABOLIC COMBINED

 (FIELD) RAIN/RUNOFF (RESERVOIR) (RES.-EFF) (RESER.) (RESER.) 

 --------------------------------------------------------------------

 116.10 2 N/A .60- 74.40 ****** 69.58

 UNTREATED WATER CONC (MICROGRAMS/LITER (PPB)) Ver 1.1.0 JAN 1, 2007

 --------------------------------------------------------------------

 PEAK DAY (ACUTE) ANNUAL AVERAGE (CHRONIC) 

 CONCENTRATION CONCENTRATION 

 --------------------------------------------------------------------

 1.246 .241

Annual Maximum Application Rate

 RUN No. 2 FOR aminopyralid ON fieldcorn_ * INPUT VALUES * 

 --------------------------------------------------------------------

 RATE (#/AC) No.APPS & SOIL SOLUBIL APPL TYPE %CROPPED INCORP

 ONE(MULT) INTERVAL Kd (PPM ) (%DRIFT) AREA (IN)

 --------------------------------------------------------------------

 .027( .027) 1 1 .0 2480.0 GROUND( 6.4) 46.0 .0

 FIELD AND RESERVOIR HALFLIFE VALUES (DAYS) 

 --------------------------------------------------------------------

 METABOLIC DAYS UNTIL HYDROLYSIS PHOTOLYSIS METABOLIC COMBINED

 (FIELD) RAIN/RUNOFF (RESERVOIR) (RES.-EFF) (RESER.) (RESER.) 

 --------------------------------------------------------------------

 116.10 2 N/A .60- 74.40 ****** 69.58

 UNTREATED WATER CONC (MICROGRAMS/LITER (PPB)) Ver 1.1.0 JAN 1, 2007

 --------------------------------------------------------------------

 PEAK DAY (ACUTE) ANNUAL AVERAGE (CHRONIC) 

 CONCENTRATION CONCENTRATION 

 --------------------------------------------------------------------

1.279 .247
Appendix 2. SCIGROW Modeling Output File

 SCIGROW

 VERSION 2.3

 ENVIRONMENTAL FATE AND EFFECTS DIVISION

 OFFICE OF PESTICIDE PROGRAMS

 U.S. ENVIRONMENTAL PROTECTION AGENCY

 SCREENING MODEL

 FOR AQUATIC PESTICIDE EXPOSURE

SciGrow version 2.3

chemical:aminopyralid

time is 6/10/2009 15:10:45

------------------------------------------------------------------------

 Application Number of Total Use Koc Soil Aerobic

 rate (lb/acre) applications (lb/acre/yr) (ml/g) metabolism (days)

------------------------------------------------------------------------

 0.027 1.0 0.027 1.05E+00 38.7

------------------------------------------------------------------------

groundwater screening cond (ppb) = 1.55E-01 

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