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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON D.C., 20460

OFFICE OF 

CHEMICAL SAFETY AND POLLUTION PREVENTION

DP Barcode: 432215

PC Code: 288008

March 15, 2016

MEMORANDUM

SUBJECT: Aminocyclopyrachlor acid – Addendum to the Drinking Water Assessment for 

Non-Crop Uses

TO: Mindy Ondish, Risk Manager Reviewer

Reuben Baris, Product Manager Team 25

Herbicide Branch

Registration Division

FROM: William P. Eckel, Ph.D., Senior Science Advisor

Environmental Risk Branch VI

Environmental Fate and Effects Division

THRU: Mark Corbin, Branch Chief

Monica Wait, RAPL

Environmental Risk Branch VI

Environmental Fate and Effects Division

CONCLUSIONS

A revised drinking water assessment (DWA) for aminocyclopyrachlor acid (ACP) and its 

degradate IN-V0977 was completed on January 8, 2010 (DP 368238). This memorandum 

updates the previous DWA by reviewing the Tier I surface water analysis (FIRST model) and by 

updating the ground water analysis using the PRZM-GW model.

Surface Water. The previous FIRST surface water modeling was found to be appropriately 

conservative by testing in the Tier II model SWCC, including both the experimental Tier I 

scenario and the PA turf scenario. The acute and chronic estimated drinking water 

concentrations (EDWCs) for surface water were reported in the 2010 DWA to be 25.6 and 15.9

ug/L (ppb), respectively for a single ground or aerial application at 0.284 lb/A. However, a 

percent cropped area (PCA) of 0.87 was applied to these results, which is not appropriate for this 
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use pattern. The corrected results, with a PCA value of 1.0, are 29.4 ppb (acute) and 18.3 

ppb (chronic). The results for the degradate IN-V0977 are 1.7 ppb (acute) and 1.2 ppb (chronic) 

after correcting the PCA factor. These results apply to both aerial and ground application. Pease 

refer to the 2010 DWA for details of the environmental fate profile and properties of ACP.

Ground Water. For ground water, the six PRZM-GW scenarios were run, and the highest result 

was for the NC Coastal scenario. The acute EDWC is 116 ug/L (ppb), and the chronic EDWC is 

78 ppb. 

Recommended EDWCs. EFED therefore recommends the following EDWCs for the non-crop 

areas use.

Surface water (parent), acute: 29.4 ppb; chronic, 18.3 ppb.

Surface water (IN-V0977), acute, 1.7 ppb; chronic 1.2 ppb.

Ground water, acute (parent only): 116 ppb; chronic 78 ppb.

Ground Water Modeling

Due to the stability and mobility of the parent aminocyclopyrachlor acid, the compound may 

move from the treated field to ground water through leaching. Since the parent is not susceptible 

to soil photolysis (t 1/2 = 129 days; MRID 47560213), the degradate is not an issue for ground 

water contamination; therefore, no PRZM-GW modeling was performed on the degradate. 

Table 1 details the input parameters for the Tier I/II level groundwater model, PRZM-GW, for 

the parent aminocyclopyrachlor acid only. Model outputs are included in the Appendix.

Table 1: PRZM-GW Input Parameters for Aminocyclopyrachlor Acid for Non-Crop 

Use

Parameter: Input 

Value: Data Source/Rationale 

Application Rate (lbs a.i./acre)

(kg/hectare)

0.284

0.32 

EPA Reg. No. 352-TII (non-crop – parent)

Maximum Number of 

Applications/Year

1 EPA Reg. No. 352-TII (non-crop – parent)

Aerobic Soil Metabolism Half-life 

(days)

Parent 373

Value represents the upper 90% confidence

limit on the mean value 

(MRID 47560214 and 47560221).

Hydrolysis Half-life stable MRID 47835701

Koc

Parent 12 L/kg oc Average Koc value: MRID 47560219
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Appendix

Groundwater Analysis for aminocyclopyrachlor and the NC Eastern Coastal Plain -

Norfolk Loamy Sand - Cotton Scenario

Estimated groundwater concentrations and breakthrough times for aminocyclopyrachlor are 

presented in Table 1 for the NC Eastern Coastal Plain - Norfolk loamy sand - Cotton 

groundwater scenario. A graphical presentation of the daily concentrations in the aquifer is 

presented in Figure 1. These values were generated with the PRZM-GW (Version 1.07). Critical 

input values for the model are summarized in Tables 2 and 3.

Table 1. Groundwater Results for aminocyclopyrachlor and the NC Eastern Coastal Plain -

Norfolk loamy sand - Cotton Scenario.

Peak Concentration (ppb) 116

Post-Breakthrough Mean 

Concentration (ppb)

 78.1

Entire Simulation Mean 

Concentration (ppb)

 63.3

Average Breakthrough 

Time (days)

2063.52

Throughputs 5.311313

Table 2. Chemical Properties for Groundwater Modeling of aminocyclopyrachlor.

Koc (ml/g) 12

Surface Soil Half Life (days) 373

Hydrolysis Half Life (days) 0

Diffusion Coefficint Air (cm2/day) 0

Henry's Constant 0.0

Enthalpy (kcal/mol) 0.0

Table 3. Pesticide application scheme used for aminocyclopyrachlor. This application 

scheme was applied every year of the simulation.

Application Days 

Relative to 

Emergence Date 

(05/15)

Application Method Application Rate

(kg/ha)

7 Ground application with mass 

distribution increasing 

0.32
4

proportionally with depth to 4 cm

Figure 1. Aquifer Breakthrough Curve for aminocyclopyrachlor and the NC Eastern 

Coastal Plain - Norfolk Loamy Sand - Cotton Scenario