1 

OFFICE OF CHEMICAL SAFETY 

AND POLLUTION PREVENTION 

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON, D.C. 20460 

MEMORANDUM

DATE: July 10, 2014 

SUBJECT: Response to Public Comments on the EFED Registration Review Problem 

Formulation for Aminopyralid (PC Code No. 005100, 005209, 005219; DP Barcode 

D421326)

TO: Veronica Dutch, Chemical Review Manager 

 Jill Bloom, Team Leader 

Linda Arrington, Branch Chief 

 Risk Management and Implementation Branch V 

 Pesticide Re-evaluation Division (7508P) 

FROM: Edward Scollon, Ph.D., Toxicologist 

 Katrina White, Ph.D., Biologist 

Environmental Risk Branch IV 

Environmental Fate and Effects Division (7507P) 

THROUGH: James N. Carleton, Ph.D., Senior Fate Scientist 

 Thomas Steeger, Ph.D., Senior Advisor 

Sarah Winfield, Acting Branch Chief 

Environmental Risk Branch IV 

Environmental Fate and Effects Division (7507P) 

The Environmental Fate and Effects Division’s (EFED) reviewed comments from the technical 

registrant (Dow AgroSciences) on the “Registration Review: Preliminary Problem Formulation 

for Environmental Fate, Ecological Risk, Endangered Species, and Human Health Drinking 

Water Exposure Assessments for Aminopyralid, Potassium salt of Aminopyralid, and 

Triisopropanolamine Salt of Aminopyralid” (Document ID EPA-HQ-OPP-2013-0749-0011) in 

www.regulations.gov, referred to as Problem Formulation in this document). The purpose of this 

memorandum is to respond to these comments. EFED appreciates the comments and has 

considered them carefully; responses are provided below each set of paraphrased comments. 
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1. Aerobic Aquatic Metabolism Study (OCSPP Guideline 835.43001) 

Stated in the Problem Formulation: “Aerobic conditions were not maintained throughout the 

aerobic aquatic studies. In the absence of an acceptable study, the model half-life input for 

aerobic aquatic metabolism will be assumed to be twice the aerobic soil metabolism half-life, per 

EFED's input parameter guidance (USEPA, 2009b).” 

Dow AgroSciences Comment: “Dow AgroSciences disagrees with the Agency 

recommendation for the following reasons: 

Three sediments were evaluated in the aerobic aquatic study (MRID 46235731) which used a 

continuous flow of CO2-scrubbed, moist air. The two EU aqueous layers were measured as 

aerobic (>2 ppm oxygen) throughout the study period. The amount of oxygen in the US sediment 

samples started with lower oxygen content (<1 ppm) but the amount of oxygen steadily increased 

over the study to ca 3-4 ppm. Furthermore, for all three aqueous layers the redox potential 

suggested aerobic conditions, with generally positive or small negative values. The redox 

potential of the sediment suggests mildly anaerobic to anaerobic conditions, with values in the 

negative range. This is completely expected, especially as the sediment measurements are 

obtained from the bottom of the sediment (the probe rests on the container, for consistency in 

measurement). Furthermore, this is in full agreement with current guidance OECD 308, which 

states in paragraph 2, "The aerobic test simulates an aerobic water column over an aerobic 

sediment layer that is underlain with an aerobic gradient.'' 

EFED Response: OCSPP Guideline 835.4300 states, “The aerobic system described in this test 

guideline consists of an aerobic water layer (typical oxygen concentrations range from 7 to 10 

mg/L) and a sediment layer, aerobic at the surface and anaerobic below the surface (typical 

average redox potentials (Eh) in the anaerobic zone of the sediment range from -80 to -190 mV). 

Moistened air is passed over the surface of the water in each incubation unit to maintain 

sufficient oxygen in the head space.” Moistened air was used in the test systems. In the U.S. test 

systems, the initial and final dissolved oxygen concentrations were 0.2 and 3.3 mg/L. In the 

Italian test system, the initial and final dissolved oxygen concentrations were 3.0 and 4.6 mg/L. 

In the French test system, the initial and final dissolved oxygen concentrations were 4.2 and 7.3 

mg/L, respectively. All but one of these dissolved oxygen concentrations are below the range 

recommended in the OCSPP guideline. Additionally, electron activity (pe) plus pH values at the 

initial and final measurements in water were all less than 12, indicating redox potentials where 

aerobic microorganisms that utilize oxygen to oxidize organic matter do not function2. The 

boundary between oxic and suboxic conditions can be seen in Sposito, 1989 Figure 6.1 to be near 

a pe + pH of 12. Also, ferric iron (Fe3+) is eliminated as an electron acceptor near pe + pH 

<12. This methodology of determining anaerobic versus aerobic conditions takes into account 

that both oxidation-reduction potential (Eh) and pH must be considered when determining which 

microbial populations may be active. Additionally, the observed aerobic aquatic metabolism 

 

1

 USEPA. 2008. Fate, Transport, and Transformation Guidelines. OPPTS 835.4300 Aerobic Aquatic Metabolism; 

OPPTS 835.4400 Anaerobic Aquatic Metabolism. EPA 712-C-08-018 & E. 712-C-08-019. October 2008. Office of 

Prevention, Pesticides, and Toxic Substances. United States Environmental Protection Agency. Available at 

http://www.epa.gov/ocspp/pubs/frs/publications/Test_Guidelines/series835.htm (Accessed May 17, 2012). 2

 Sposito, G. 1989. The Chemistry of Soils. Oxford Univ. Press, New York. 
3

half-life values (462 to 990 days) are much longer than the observed aerobic soil metabolism 

half-life values (26.4 to 408 days). These results together with the data indicating reducing 

conditions and low oxygen concentrations in the aerobic aquatic test systems suggest that the 

available values underestimate aerobic aquatic degradation (e.g., result in a high half-life), and 

their use in modeling could result in very conservative estimated environmental concentrations 

(EECs). Having additional data will reduce uncertainty in the potential for risk to terrestrial 

plants from residues in irrigation water for terrestrial uses. For applications directly to water, 

this study will reduce the uncertainty for the risk assessment for aquatic and terrestrial plants. In 

the absence of additional data, the available aerobic aquatic metabolism study results will be 

used in the risk assessment or a value of 2x the aerobic soil metabolism value will be used. 

2. Dissipation study in compost (Non-Guideline)

Stated in the Problem Formulation: “Aminopyralid applied to vegetative matter used as 

animal feed, from which manure is produced that is subsequently composted, has been found to 

remain present in sufficient quantity to damage non-target plants (see Section 6c Ecological 

Incidents). A study is requested to quantify rates of aminopyralid degradation with in composted 

material.” 

Dow AgroSciences Comment: 

Dow AgroSciences has conducted a bench scale manure/straw composting study. 

Aminopyralid residues did not decline appreciably during the thermophilic or mesophilic phases 

of the composting process. These results indicate that composting process is not likely to be a 

useful technique to mitigate aminopyralid residue in compost inputs. Upon request by EPA, 

"Depuration and Composting of Aminopyralid in Ruminant Manure Systems" (study ID 081082, 

2008) would be submitted. 

EFED Response: EFED recommends the study be submitted for review and subsequent 

determination of how those data will be used. In the absence of information submitted to better 

understand the rate of degradation in compost, aminopyralid will be assumed to be stable in 

composted materials. 

3. Prospective Groundwater Monitoring Study (OCSPP 835.71003

)

Stated in the Problem Formulation: “Residues of aminopyralid have been detected in 

groundwater and other picolinic acids have incidents where residues of picolinic acids in 

groundwater used for irrigation resulted in damage to terrestrial plants. Additionally, risk 

assessments for aminopyralid have identified this pathway as a potential concern for damage to 

terrestrial plants. Additional data are recommended to better understand the potential for risk 

due to residues of aminopyralid in groundwater. Studies that would be useful include a 

prospective groundwater monitoring study or studies to refine groundwater modeling inputs for 

the Pesticide Root Zone Model for Groundwater (PRZM-GW) model. A prospective groundwater 

 

3 USEPA. 2008. Fate, Transport, and Transformation Guidelines. OPPTS 835.7100: Guidance for Prospective Ground-Water 

Monitoring Studies. EPA 712-B-10-001. August 25, 2008. Office of Prevention, Pesticides, and Toxic Substances. U.S. 

Environmental Protectin Agency. Available at http://www.regulations.gov/#!documentDetail;D=EPA-HQ-OPPT-2009-0152-

0044 (Accessed July 8, 2014).
4

monitoring study could be used to determine where groundwater concerns were likely to occur 

and whether what we predict based on laboratory data, is observed in the field. Additionally, it 

could be used to determine vulnerability of different areas. Data to refine inputs to PRZM-GW 

may be used to refine modeling estimates of groundwater concentrations. Studies may involve 

understanding sorption or degradation in subsurface environments. It is recommended that a 

protocol be submitted, prior to conducting any of these studies. In the absence of more suitable 

inputs or scenarios, estimated environmental concentrations (EECs) generated from PRZM-GW 

in the same fashion as estimated drinking water concentrations (EDWCs) for groundwater used 

as drinking water, will be used to characterize risks to terrestrial plants from residues in 

irrigation water obtained from groundwater sources.” 

Dow AgroSciences Comment: 

“The agency intends to use the PRZM-GW model to estimate water concentrations in 

groundwater, for use as inputs into drinking water and non-target plant risk assessments. Recent 

presentations (e.g., Negley et. al.) have shown that proper parameterization of PRZM-GW is 

critical to yield representative estimates of predicted concentrations. The characterization of soil 

sorption has been found to be especially critical and it has been found that including a time 

dependent formulation of the sorption greatly improves the predictive power of the model (as 

compared to field data). The current implementation of PRZM-GW, however, does not support 

this representation, but various groups have shown that this can be accomplished quite readily. 

In keeping with EPA's statements that “additional data are recommended to better understand 

the potential for risk due to residues of aminopyralid (2) or clopyralid (3), Dow AgroSciences 

proposes to parameterize PRZM-GW assessments using best available information and 

methodology. DAS will undertake further field studies or monitoring efforts if refined modeling 

efforts do not yield sufficient refinements of exposure.” 

EFED Response: EFED agrees in principle that a tiered study approach can be useful for 

refining risk estimates. EFED completed some sensitivity analysis on incorporation of nonlinear 

sorption and degradation in subsurface soil layers using the PRZM-GW model, to determine how 

useful conducting additional studies in refining risk estimates. The analysis is shown in 

Appendix B. EFED is currently exploring the issue of time-dependent sorption and how it may 

be used in PRZM-GW modeling; however, EFED recommends against pursuing the timedependent residue study at this time. 

EFED explored the potential for considering nonlinear sorption and a degradation rate in 

subsurface layers. When PRZM-GW was run using the highest and lowest measured sorption 

coefficient (Kd) at a single equilibrium concentration in water in all of the sorption studies, it was 

found that aminopyralid concentrations in groundwater still had the potential to result in 

exposure levels that could exceed levels of concern for terrestrial plants (see Appendix B). 

EFED also evaluated aminopyralid concentrations assuming a subsurface degradation rate of 100 

days. Currently, aminopyralid has been shown to be stable to hydrolysis (based on data 

evaluating hydrolysis over 31 days), which is typically the only dissipation pathway currently 

considered in PRZM-GW modeling below 1 meter. It was shown that assuming a subsurface 

degradation rate of 100 days could significantly reduce estimated exposure. EFED recommends 

if any additional analyses are performed, that these analyses focus on better 

understanding/quantifying the potential for degradation in subsurface layers (either via longer 

term hydrolysis studies or degradation in soils from subsurface layers). 
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Based on currently available data, aminopyralid is considered persistent and mobile. The surface 

layer degradation half-lives for parent plus unextracted residues are near 100 days, and it is 

unlikely that subsurface degradation rates will be faster than this. It is likely that whether inputs 

to PRZM-GW can be refined or not, the compound has the potential in vulnerable areas to reach 

groundwater at concentrations that could result in a risk concern if the water is used to irrigate 

sensitive terrestrial plants. A prospective groundwater (PGW) monitoring study could be used to 

determine where groundwater concerns are likely to occur, and whether modeled concentrations 

estimated based on laboratory data, are observed under actual field conditions. It is 

recommended that protocols be submitted for review and approval prior to initiating any 

additional studies. Finally, in the absence of additional data, EFED will use PRZM-GW to 

estimate the potential exposure to terrestrial plants from residues in irrigation water obtained 

from groundwater sources. 

4. Missing citations 

Dow indicated that citations to aerobic soil metabolism studies MRID numbers 47925801 and 

47911201 were omitted. 

MRID 47925801 is listed under 835.4100 on page 36. MRID 47911201 was omitted and is 

listed below. EFED will be sure to include this citation in future documents, as appropriate. 

Yoder, R.; Smith, K. (2009) Aerobic Soil Degradation of XDE-750 in Four European Soils: 

Amended Report. Project Number: 020054. Unpublished study prepared by Dow AgroSciences, 

LLC. 72 p. MRID 47911201. 
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Appendix A. Summary of pe plus pH results for the aerobic aquatic metabolism study 

MRID 46235731 

French System – Aqueous Phase 

Day 

Measured 

Eh Measured pH pE pE+pH 

pE+pH < 

12? 

0 180 5.37 3.0 8.4 Yes 

0 178 6.00 3.0 9.0 Yes 

3 62 6.63 1.0 7.7 Yes 

3 142 7.90 2.4 10.3 Yes 

7 34 7.67 0.6 8.2 Yes 

7 20 7.47 0.3 7.8 Yes 

14 263 6.66 4.4 11.1 Yes 

14 111 6.26 1.9 8.1 Yes 

21 76 5.27 1.3 6.6 Yes 

21 64 6.53 1.1 7.6 Yes 

30 196 6.78 3.3 10.1 Yes 

30 168 6.56 2.8 9.4 Yes 

62 -56 6.51 -0.9 5.6 Yes 

62 212 5.94 3.6 9.5 Yes 

101 203 5.76 3.4 9.2 Yes 

101 205 5.91 3.5 9.4 Yes 

Data from Table 8 page 50 of study report. 

Italian System – Aqueous phase 

Day 

Measured 

Eh Measured pH pE pE+pH 

pE+pH < 

12? 

0 122 7.88 2.1 9.9 Yes 

0 78 7.98 1.3 9.3 Yes 

3 7 7.91 0.1 8.0 Yes 

3 -12 8.33 -0.2 8.1 Yes 

7 65 8.22 1.1 9.3 Yes 

7 94 8.10 1.6 9.7 Yes 

14 82 7.76 1.4 9.1 Yes 

14 41 7.98 0.7 8.7 Yes 

21 -97 7.39 -1.6 5.7 Yes 

21 -168 7.49 -2.8 4.7 Yes 

30 98 7.99 1.7 9.6 Yes 

30 99 8.19 1.7 9.9 Yes 

62 91 7.80 1.5 9.3 Yes 

62 -65 7.67 -1.1 6.6 Yes 

101 127 7.06 2.2 9.2 Yes 

101 110 7.56 1.9 9.4 Yes 

Data from Table 9 page 51 of study report. 
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U.S. System – Aqueous Phase 

Day Measured Eh Measured pH pE pE+pH 

pE+pH < 

12? 

0 -39 7.35 -0.7 6.7 Yes 

0 -90 8.00 -1.5 6.5 Yes 

3 -67 8.73 -1.1 7.6 Yes 

3 -155 8.49 -2.6 5.9 Yes 

7 -32 8.59 -0.5 8.1 Yes 

7 -31 8.53 -0.5 8.0 Yes 

14 -35 8.42 -0.6 7.8 Yes 

14 -188 8.32 -3.2 5.1 Yes 

21 -308 7.44 -5.2 2.2 Yes 

21 -310 7.92 -5.2 2.7 Yes 

30 35 8.67 0.6 9.3 Yes 

30 -151 8.54 -2.5 6.0 Yes 

62 16 8.54 0.3 8.8 Yes 

62 -229 7.70 -3.9 3.8 Yes 

101 37 8.59 0.6 9.2 Yes 

101 2 7.71 0.0 7.7 Yes 

Data from Table 10 page 52 of study report. 
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Appendix B. Summary of PRZM-GW Modeling and Sensitivity Analysis 

Table B1. PRZM-GW Input Parameters 

Parameter (units) Input Value Data Source 

Application Rate (kg 

a.e./hectare) 0.247 Maximum application rate of all 

the uses. 

Number of Applications 1 Label 

Application Dates 

(month/day) 14 day since emergence 

Application Method 2 Above canopy application 

Annual Application Retreatment Once every year Label 

Hydrolysis Half-life (days) See Table B2 MRID: 46235726 

Soil Metabolism Half-life (days) 98.8 

Represents the 90th percentile 

upper confidence bound on the 

mean (109) of eight half-life 

values (aminopyralid + 

unextracted residues). Values 

were adjusted to 25o

C. 

average of Koc (mL/goc) See Table B2 See text. 

Table B2. PRZM-GW version 1.07: Modeling Results for Aminopyralid and Sensitivity 

Analysis of the Koc/Kd input and Hydrolysis input. Results are shown for the Wisconsin 

sand scenario. The simulation was run for 30 years for the mean and lowest single Kd and 

100 years for the highest single Kd. 

Kd/Koc input, hydrolysis input Highest Daily 

value (μg/L) 

Post 

Breakthrough 

Average (μg/L) 

Average 

Simulation 

Breakthrough 

Time (days) 

Mean Koc of 8.96, stable 55.5 49 2491 

Lowest single Kd (0.0042 mL/g)1, stable 56.8 50.3 2440 

Highest single Kd (0.93 mL/g)2, stable 11 9.84 17674 

Highest single Kd (0.93 mL/g)2, DT50 of 100 days 0.103 0.0861 17674 

Mean Koc of 8.96, DT50 of 100 days 1.31 0.509 2490 

Bolded value represents the highest EDWC for aminopyralid acid. 

1

 Measured in M549 soil at aminopyralid equilibrium concentration in solution of 0.949 μg/mL. 2

 Measured in M616 soil at aminopyralid equilibrium concentration in solution of 0.028 μg/mL. 

To estimate exposure to plants when groundwater contaminated by aminopyralid is applied to 

crops and to determine whether changing input parameters would influence risk conclusions, the 

following method was used. 

Assume a field is irrigated with one inch of water containing an estimated aminopyralid 

concentration in water in μg/L using the following equation. 

EEC lb a.i./Acre = (EEC in water in mg a.i./L) × (102,790 L water/ Acre-in) × (1 inch) × (1 lb/ 

453,592 mg) = 9.84 μg a.e./L ×1000 μg/mg × 0.226613 = 0.00223 lbs a.e./A 

 
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For evaluating the potential for nonlinear sorption to influence risk conclusions, an aminopyralid 

concentration in water of 9.84 μg/L was assumed and resulted in an estimated application rate of 

0.00223 lbs a.i./A. The lowest no observed adverse effect concentration (NOAEC) available for 

terrestrial plants (i.e., 0.0004 lb a.e./A, MRID 462358-25) is well below this estimated level of 

exposure. Therefore, considering the impact of nonlinear sorption on the estimated exposure is 

unlikely to reduce the exposure estimate to below levels of concern. 

Assuming a 100-day half-life for hydrolysis and the highest Kd, results in an estimated exposure 

level (EEC in groundwater = 0.0861 μg/L; EEC for 1 inch of irrigation water =1.95 ×10-5 lbs 

a.e./A) that reduce risk estimates below levels of concern. However, the highest Kd is not expected 

to be conservative for all soil types and more than an inch of water may be used to irrigate. 

Assuming a 100-day half-life for hydrolysis and the mean Kd, results in an estimated exposure 

level (EEC in groundwater = 1.31μg/L; EEC for 1 inch of irrigation water =0.0003 lbs a.e./A) that 

would in turn reduce risk estimates to just below levels of concern; however, it is expected that 

many areas will receive more than an inch of irrigation water.