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 first-order 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 Half-life (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
label-allowed  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            

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

.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 

 ***********************************************************************
*

