UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON, DC 20460 

OFFICE OF CHEMICAL SAFETY AND POLLUTION PREVENTION 

PC Code: 051505 

DP Barcode: 423309 

Date: October 10,2014 

MEMORANDUM 

Subject: 

To: 

From: 

Addendum to 2,4-D Choline Salt Section 3 Risk Assessment: Refined Terrestrial 

Plant Exposure Estimates and Effects Determination 

Emily Schmid, Risk Manager Reviewer 

Kathryn Montague, Product Manager Team 23 

Dan Kenny, Branch Chief 

Herbicide Branch 

Pesticide Registration Division (7505P) 

Office of Pesticide Programs 

Edward 

FaruqueK.han,Ph.

Odenkirchen

D.

, 

,SeniorScientist

Ph.D., Senior Scien~dvisor ~ eo__C

. 

.-

~ 

~ rt 1

"' //_.-

Meghan Radtke, Ph.D., Biologist (Actin . sk Assesrment Process Leader) IUV,Jt!l /(;//O/If 

Sujatha Sankula, Ph.D., Branch Chief "~~ J'-0-~ LO\ tc\\l 

Environmental Risk Branch I ~ :::::::::=-

Environmental Fate and Effects Division (7507P) 

Office of Pesticide Programs 

The Environmental Fate and Effects Division (EFED) has issued a screening level risk 

assessment for a Federal action involving proposed new uses of the 2,4-D choline salt on 

herbicide-tolerant corn and soybean in January, 2013 (DP 400223, 400230, 400234, 400237, 

405028, 405812). In that assessment, risks to terrestrial plants were identified from pesticide 

spray drift and runoff exposures, using the TerrPlant model (version 2.2). A spray drift 

mitigation measure was placed on the label to ensure that 2,4-D choline spray drift risks to plants 

and other taxonomic groups were limited to the treated field. This mitigation step led the 

Agency to reach a "no effect" determination for Federally threatened and endangered (listed) 

species and their critical habitat. 
1 

 

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

 WASHINGTON, DC 20460

 OFFICE OF CHEMICAL SAFETY AND POLLUTION PREVENTION 

 

 PC Code: 051505 

 DP Barcode: 423309 

 Date: October 10, 2014 

 

MEMORANDUM 

Subject: Addendum to 2,4-D Choline Salt Section 3 Risk Assessment: Refined Terrestrial 

Plant Exposure Estimates and Effects Determination 

To: Emily Schmid, Risk Manager Reviewer 

 Kathryn Montague, Product Manager Team 23

 Dan Kenny, Branch Chief 

 Herbicide Branch 

Pesticide Registration Division (7505P) 

Office of Pesticide Programs 

From: Edward Odenkirchen, Ph.D., Senior Science Advisor 

 Faruque Khan, Ph.D., Senior Scientist 

 Meghan Radtke, Ph.D., Biologist (Acting Risk Assessment Process Leader) 

Sujatha Sankula, Ph.D., Branch Chief 

Environmental Risk Branch 1 

Environmental Fate and Effects Division (7507P) 

Office of Pesticide Programs 

The Environmental Fate and Effects Division (EFED) has issued a screening level risk 

assessment for a Federal action involving proposed new uses of the 2,4-D choline salt on 

herbicide-tolerant corn and soybean in January, 2013 (DP 400223, 400230, 400234, 400237, 

405028, 405812). In that assessment, risks to terrestrial plants were identified from pesticide 

spray drift and runoff exposures, using the TerrPlant model (version 2.2). A spray drift 

mitigation measure was placed on the label to ensure that 2,4-D choline spray drift risks to plants 

and other taxonomic groups were limited to the treated field. This mitigation step led the 

Agency to reach a “no effect” determination for Federally threatened and endangered (listed) 

species and their critical habitat. 
2 

Public comments on the risk assessment and effects determination pointed out that the Agency 

did not explicitly include a consideration of the risk findings for non-target plants as a result of 

off-field runoff. 

The Agency considered the spray drift exposure to be the principal risk issue associated with the 

proposed labeled use of 2,4-D choline, owing to a variety of lines of evidence, including past 

experience with other 2,4-D formulations and associated spray drift incident reporting. 

However, in light of the public comments, it is evident that the Agency should provide an 

explicit consideration of the runoff risks and the effects of the proposed mitigation to limit offsite runoff in listed species effects determinations. 

An important component of the model used for terrestrial risk assessment (TerrPlant) is the 

assumption that the mass of pesticide running off the treated field is directly related to the 

pesticide’s solubility in water. In the case of 2,4-D choline, the disassociated salt yields a highly 

soluble 2,4-D acid. The TerrPlant model assumes that the high solubility of the acid results in an 

assumed runoff mass of 5 percent of the field-applied mass. The model also includes an 

assumption that the non-target plant receptors in the runoff zone are exposed to a single pulse of 

the total mass. The TerrPlant model does not account for any pesticide degradation or 

partitioning, nor does it account for the temporal aspects of runoff associated with 

meteorological events (i.e., runoff follows precipitation events that exceed field capacity of the 

soil). According to the supporting ecological risk assessment, 2,4-D is not environmentally 

persistent. Therefore, TerrPlant is likely providing a highly conservative estimate of runoff 

loading at any given time because the processes of degradation and partitioning combined with 

the stochastic nature of runoff are important limiting factors for 2,4-D. 

A refined characterization of potential pesticide runoff is routinely considered in aquatic system 

exposure modeling through the use of the Pesticide Root Zone Model (PRZM). EFED has 

turned to this model to ascertain the extent to which explicit consideration of degradation, 

partitioning, and stochastic runoff events would lead to a more mechanistically appropriate 

runoff exposure estimate than is possible using TerrPlant. Using the PRZM scenarios run for the 

risk assessment and assuming a single application of 2,4-D at 1 lb ae/A (same assumption as 

TerrPlant) to bare ground, EFED has determined that the total yearly mass of 2,4-D in runoff 

from a 10 hectare (24.7 acre) treated field is <1% of the applied mass (see Appendix 1 for 

calculations). This fraction for an entire year is far below the mass fraction predicted by 

TerrPlant for a single runoff event. Furthermore each single runoff event (i.e., rainstorm) in 

PRZM yields but a fraction of the total percentage, further suggesting that the predictions of 

TerrPlant are overly conservative for this 2,4-D-choline use. 

To help ensure that runoff from single precipitation events remains well below the TerrPlant 

estimates, a proposed mitigation based on the rainfast period for 2,4-choline has been 

incorporated into the product labeling. Such a restriction is consistent with requirements for 

other 2,4-D products during the Registration Eligibility Decision process. This restriction on 
3 

Enlist Duo application when rainfall or irrigation is imminent in 24 hours would allow additional 

time for the pesticide to be absorbed by on-field vegetation. Available data for 2,4-D applied to 

corn, bean and sunflower leaves suggest that 24 hours of contact provides ample time for leaf 

absorption (Gallup and Gustafson 1952) and other sources suggest that the amine salts of 2,4-D 

reach an effective target plant absorption level by 6 to 8 hours (http://craven.ces.ncsu.edu/wpcontent/uploads/2014/06/RAINFAST-2012.pdf). Increased time for plant absorption of 2,4-D 

would reduce the amount available for wash-off (increasingly so as applications are made to 

more developed crop canopies as is the case for later season 2,4-D choline applications) and 

thereby further limits the amount available for runoff following any precipitation event. Given 

that the PRZM modeling conducted for this evaluation involved application to bare ground, it is 

logical to assume that any foliar absorption will further reduce the mass available for runoff. 

Any refined assessment of runoff risk to terrestrial plants must also consider that 2,4-D is 

primarily a foliarly-absorbed herbicide with limited root uptake (Shaner 2002). This would 

suggest that for mature off site plants, with foliage in a canopy above shallow sheet flow runoff, 

the mass of 2,4-D available for plant absorption is limited by the inefficiency of root uptake 

process. 

Combining the predictions of PRZM modeling with the requirement of a 24 hour rainfast period 

to reduce mass runoff, and the expectation that much of the off-site plant community will not 

experience foliar contact with the herbicide in runoff sheet flow leads EFED to conclude that all 

available lines of evidence support a “no effects” determination for runoff exposure for off-field 

listed plants for the proposed labeled use of 2,4-D choline. 

References 

A.H. Gallup and F.G. Gustafson. 1952. Absorption and translocation of radioactive 2,4-

Dichloro-5-iodo131 phenoxyacetic Acid by Green Plants. Plant Physiology:603-612. 

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC547963/pdf/plntphys00253-0175.pdf

Shaner, D. (editor). 2002. Herbicide Handbook, 10th Edition. Weed Science Society of 

America, Lawrence, KS. pp. 16-19. 

 
4 

Appendix 1 

Contribution of 2,4-D Choline from Runoff and Erosion to a Waterbody 

Estimated Environmental Concentrations were calculated for 2,4-D choline using the USEPA 

standard pond with the MScornSTD field scenario. The concentrations of 2,4-D choline were 

generated with the Surface Water Concentration Calculator (SWCC Version 1.106)1

. The 

transport from the field to the water body is less than 1.0 % of the annual applied mass via runoff 

and erosion. Estimated values in Table 1 are based on one application rate of 1.12 ai kg/ha/yr (1 

lb ae/A) of 2,4-D choline. The SWCC output is attached to the document. These values are 

comparable to PRZM3 model predictions. 

Table 1. Fractional and total Contribution of 2,4-D Choline Transport Processes to 

Waterbody 

Sources Contribution 

Fraction 

Total Mass 

(Kg) 

Based on 30 

years 

Total Mass 

Applied 

Over 30 

year 

Period 

Percentage of 

2,4-D choline 

over 30 year 

period 

Average 

Annual 

Loss 

(%) 

Runoff 0.91 6.64 

336 

1.9 0.06 

Erosion 0.02 0.11 0.03 0.001 

Total 0.93 6.75 2.0 0.07 

1 http://www.epa.gov/oppefed1/models/water /index.htm

Variable Volume Water Model, SWCC Compatible Version 

 

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

Performed on: 10/10/2014 at 13: 0 

 

Peak 1-in-10 = 33.1 ppb 

Chronic 1-in-10 = 7.35 ppb 

Simulation Avg = 2.76 ppb 

4-day avg 1-in-10 = 32.4 ppb 

21-day avg 1-in-10 = 29.9 ppb 

60-day avg 1-in-10 = 24.6 ppb 

90-day avg 1-in-10 = 21.3 ppb 

 

Benthic Pore Water Peak 1-in-10 = 15.1 ppb 

Benthic Pore Water 21-day avg 1-in-10 = 15.1 ppb 

Benthic Conversion Factor = 2.84 -Pore water (ug/L) to (total mass, ug)/(dry sed mass,kg) 

Benthic Mass Fraction in Pore Water = 0.130 

 

YEAR Peak 4-day 21-day 60-day 90-day Yearly Avg Benthic Pk Benthic 21-day 

 1 1.78E+01 1.75E+01 1.62E+01 1.36E+01 1.17E+01 4.06E+00 8.46E+00 8.42E+00 
5 

 2 1.04E+01 1.02E+01 9.79E+00 8.20E+00 7.03E+00 2.48E+00 5.23E+00 5.20E+00 

 3 1.36E+00 1.33E+00 1.21E+00 1.09E+00 9.47E-01 3.93E-01 7.77E-01 7.73E-01 

 4 2.03E+01 1.99E+01 1.86E+01 1.51E+01 1.27E+01 4.18E+00 9.43E+00 9.38E+00 

 5 3.98E+01 3.93E+01 3.69E+01 3.03E+01 2.59E+01 8.89E+00 1.99E+01 1.98E+01 

 6 2.04E+00 2.01E+00 1.88E+00 1.86E+00 2.09E+00 8.56E-01 2.76E+00 2.99E+00 

 7 1.99E+00 1.94E+00 1.78E+00 1.49E+00 1.31E+00 5.02E-01 1.01E+00 1.00E+00 

 8 1.36E+01 1.34E+01 1.24E+01 1.03E+01 8.82E+00 2.99E+00 6.34E+00 6.30E+00 

 9 2.55E+00 2.49E+00 2.32E+00 1.94E+00 1.71E+00 7.05E-01 1.39E+00 1.38E+00 

 10 1.36E+00 1.33E+00 1.24E+00 1.06E+00 9.17E-01 3.32E-01 6.75E-01 6.72E-01 

 11 8.57E+00 8.43E+00 7.89E+00 6.95E+00 6.08E+00 2.09E+00 4.44E+00 4.42E+00 

 12 1.39E+00 1.36E+00 1.30E+00 1.14E+00 9.93E-01 4.02E-01 8.00E-01 7.96E-01 

 13 3.74E+00 3.67E+00 3.51E+00 2.97E+00 2.57E+00 8.80E-01 1.85E+00 1.84E+00 

 14 5.19E+00 5.08E+00 4.67E+00 3.80E+00 3.20E+00 1.16E+00 2.37E+00 2.35E+00 

 15 4.33E+00 4.25E+00 3.96E+00 3.48E+00 3.00E+00 1.07E+00 2.23E+00 2.22E+00 

 16 3.37E+01 3.31E+01 3.05E+01 2.51E+01 2.17E+01 7.51E+00 1.54E+01 1.53E+01 

 17 1.69E+01 1.65E+01 1.51E+01 1.25E+01 1.05E+01 3.77E+00 7.98E+00 7.94E+00 

 18 2.34E+00 2.29E+00 2.19E+00 1.82E+00 1.61E+00 6.65E-01 1.33E+00 1.32E+00 

 19 1.62E+01 1.59E+01 1.54E+01 1.28E+01 1.11E+01 3.84E+00 8.01E+00 7.98E+00 

 20 4.82E+01 4.74E+01 4.49E+01 3.84E+01 3.31E+01 1.10E+01 2.56E+01 2.55E+01 

 21 1.22E+01 1.21E+01 1.14E+01 9.37E+00 8.01E+00 2.99E+00 6.34E+00 6.31E+00 

 22 2.34E+00 2.30E+00 2.12E+00 1.93E+00 1.68E+00 6.60E-01 1.32E+00 1.32E+00 

 23 2.24E+01 2.21E+01 2.07E+01 1.75E+01 1.50E+01 5.18E+00 1.10E+01 1.09E+01 

 24 1.04E+01 1.02E+01 9.85E+00 8.29E+00 7.11E+00 2.62E+00 5.60E+00 5.58E+00 

 25 7.34E+00 7.19E+00 6.79E+00 5.69E+00 4.86E+00 1.73E+00 3.67E+00 3.65E+00 

 26 1.28E+00 1.26E+00 1.16E+00 1.04E+00 9.02E-01 3.60E-01 7.19E-01 7.15E-01 

 27 2.55E+00 2.51E+00 2.34E+00 1.99E+00 1.70E+00 5.78E-01 1.23E+00 1.22E+00 

 28 2.70E+01 2.65E+01 2.47E+01 2.05E+01 1.76E+01 5.89E+00 1.25E+01 1.24E+01 

 29 1.87E+01 1.83E+01 1.74E+01 1.48E+01 1.27E+01 4.52E+00 9.80E+00 9.76E+00 

 30 1.45E+00 1.43E+00 1.35E+00 1.18E+00 1.06E+00 5.03E-01 1.40E+00 1.51E+00 

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

 Effective compartment halflives averaged over simulation duration: 

 

 zero washout 0 

 water col metab halflife (days) = 66.1932967308725 

 zero hydrolysis 0 

 photolysis halflife (days) = 1176.57708312456 

 volatile halflife (days) = 3208642.19834153 

 total water col halflife (days) = 62.6664395822102 

 

 zero burial 0 

 benthic metab halflife (days) = 472.178850013557 

 zero benthic hydrolysis 0 

 total benthic halflife (days) = 472.178850013557 

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

 Fractional Contribution of Transport Processes to Waterbody & Total Mass (kg): 

 
6 

 Due to Runoff = 0.9064 6.642 

 Due to Erosion = 0.0157 0.1147 

 Due to Drift = 0.0779 0.5712 

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

 Flow in/out Characteristics of Waterbody: 

 Average Daily Runoff Into Waterbody (m3/s) = 1.549977735488095E-003 

 Baseflow Into Waterbody (m3/s) = 0.000000000000000E+000 

 Average Daily Flow Out of Waterbody (m3/s) = 1.549977735488099E-003 

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