### UNITED STATES ENVIRONMENTAL PROTECTION AGENCY WASHINGTON, DC 20460

OFFICE OF CHEMICAL SAFETY AND POLLUTION PREVENTION

PC Code: 051505 DP Barcode: 423309 Date: October 1 0,2014

MEMORANDUM Subject:



### To:

From: Addendum to 2,4

- D Choline Salt Section 3 Risk Assessment: Refined Terrestrial Plant Exposure Est im ates and Effects Determination Emily Schmid, Risk Manager Reviewer Kathryn Montague, Product Manager Team 23 D an Kenny, Branch Chief Herbicide B ranc h Pesticide Registration Division (7505P) Office of Pesticide Programs

Edward Odenk irchen , Ph.D. , Senior

# Scien~dvisor eo__ C

. -

# ~**FaruqueK.han,Ph. D.** **,SeniorScientist ~**  .

### ~ 

# rt

1 "' //_.- Meghan Radtke, Ph.D., Biologist

### (Act**in .**  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

Th e Envi r onmenta l Fate and Effects Division (E FED ) ha s issued a scree nin g 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 Januar y, 2013 (DP 400223 , 400230, 400234, 400237, 405028 , 405812). I n that assessment, r i sks to terrestrial plants were identified from pesticide spray drift and runoff exposures, using the TerrPlant mod el (ve r sion 2.2). A spray drift mitigation measure was placed on the label to ensure that 2 , 4-D choline sp ray 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" determinat i on for Federally t hr eatened and endangered (listed) species and their critical habitat.1 

### UNITED STATES ENVIRONMENTAL PROTECTION AGENCY WASHINGTON, DC 20460

OFFICE OF CHEMI CAL 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 propos ed new uses of the 2,4-D choline salt on herbicide-tolerant corn and soyb ean in January, 2013 (DP 400223, 400230, 400234, 400237, 405028, 405812). In that assessment, risks to terres trial plants were iden tified from pesticide spray drift and runoff exposures, using the Te rrPlant 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 fo r Federally threatened and endangered (listed) species and their critical habitat.2 

### Public comments on the risk assessment and eff ects 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 is sue 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 r unoff risks and the effects of th e proposed mitigation to limit offsite runoff in listed species effects determinations.
An important component of the model used for terrestrial risk assessm ent (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 disassoci ated 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 percen t 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 tem poral aspects of runoff associated with meteorological events (i.e., runoff follows precipita tion events that exceed field capacity of the soil). According to the suppor ting 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 im portant limiting factors for 2,4-D.

A refined characterization of potential pesticide runoff is routin ely 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 consid eration of degradation, partitioning, and stochastic runoff events would lead to a more mechanistically appropriate runoff exposure estimate than is possible using Te rrPlant. Using the PRZM scenarios run for the risk assessment and assuming a single applicatio n 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 ye ar is far below the mass fraction predicted by TerrPlant for a single runoff event. Furthermor e each single runoff event (i.e., rainstorm) in PRZM yields but a fraction of the total percenta ge, further suggesting that the predictions of TerrPlant are overly conservative for this 2,4-D-choline use.
To help ensure that runoff from single precip itation 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 Regi stration Eligibility Decision pr ocess. This restriction on3 

### Enlist Duo application when rainfa ll 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 leve l by 6 to 8 hours (http://craven.ces.ncsu.edu/wpcontent/uploads/2014/06/RAINFAS T-2012.pdf). Increased time for plant absorption of 2,4-D would reduce the amount available for wash-off (inc reasingly so as applications are made to more developed crop canopies as is the case fo r later season 2,4-D cho line applications) and thereby further limits the amount available for runoff following any precipi tation event. Given that the PRZM modeling conducted for this eval uation involved applicati on to bare ground, it is logical to assume that any foliar absorption wi ll further reduce the mass available for runoff.
Any refined assessment of runoff risk to terrestri al plants must also consider that 2,4-D is primarily a foliarly-absorbed herbicide with li mited 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 runo ff sheet flow leads EFED to conclude that all available lines of evidence suppor t 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. Absorpti on and translocation of radioactive 2,4- Dichloro-5-iodo 131 

### phenoxyacetic Acid by Green Plants. Plant Physiology:603-612. http://www.ncbi.nlm.nih.gov/pmc/arti cles/PMC547963/pdf/plntphys00253-0175.pdf Shaner, D. (editor). 2002. Herbicide Ha ndbook, 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 c holine using the USEPA standard pond with the MScornSTD field scenari o. The concentrations of 2,4-D choline were generated with the Surface Water Concen tration 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 ba sed 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-da y 90-day Yearly A vg 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+005 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 ****************************************************************