1

OFFICE OF CHEMICAL SAFETY

AND POLLUTION PREVENTION

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON, D.C. 20460

Date: November 27, 2013

PC Codes: 031402, 031403, 031465

DP Barcode: D413486

MEMORANDUM

SUBJECT: Registration Review: Preliminary Problem Formulation for Environmental 

Fate, Ecological Risk, Endangered Species, and Drinking Water Exposure 

Assessments for 2,4-DP-p Containing R Isomer Compounds (2,4-DP-p 

acid, 2,4-DP-p Amine salt, and 2,4-DP-p 2-ethylhexyl ester) 

FROM: Meghan Radtke, Ph.D., Biologist

Faruque Khan, Ph.D., Senior Fate Scientist

Environmental Risk Branch I

Environmental Fate and Effects Division (7507P)

REVIEWED

BY: Michael Lowit, Ph.D., Biologist

Stephen Wente, Ph.D., Biologist

Environmental Risk Branch I

Environmental Fate and Effects Division (7507P)

THROUGH: Sujatha Sankula, Ph.D., Branch Chief

Monica Wait, Acting RAPL

Environmental Risk Branch I

Environmental Fate and Effects Division (7507P)

TO: Kaitlin Keller, Chemical Review Manager

Tracy Perry, Team Leader

Patricia Parrott, Branch Chief

Risk Management and Implementation Branch III

Pesticide Re-Evaluation Division (7508P)
2

1. Executive summary

The Environmental Fate and Effects Division (EFED) has completed the preliminary 

problem formulation for the ecological risk, environmental fate, endangered species, and 

drinking water assessments to be conducted as part of the registration review of the 

herbicide propanoic acid, 2-(2,4-dichlorophenoxy)- (R)-; propanoic acid, 2-(2,4-

dichlorophenoxy)- (R)- compound with N-methylmethanamine; and propanoic acid, 2-

(2,4-dichlorophenoxy)- (R)- compound with 2-ethylhexyl ester. Abbreviated names for 

these chemicals are: 2,4-DP-p acid; 2,4-DP-p dimethylamine (DMAS); and 2,4-DP-p 2-

ethylhexyl ester (EHE), respectively. From here forward, these compounds are referred 

to collectively as 2,4-DP-p and individually as acid, DMAS, and EHE. The problem 

formulation describes the methods planned to be used during the completion of drinking 

water and ecological risk assessments in support of registration review and provides an 

overview of the environmental fate, ecological effects, and potential risks associated with

the use of 2,4-DP-p as well as uncertainties unique to risk assessment of 2,4-DP-p. This 

document also identifies additional studies that would be beneficial to the conduct of an 

ecological risk assessment. Major findings include:

• The fate database of 2,4-DP-p is partially complete. EFED recommends the 

following studies be performed with the acid form: aerobic soil metabolism, 

anaerobic soil metabolism, anaerobic aquatic metabolism, fish bioaccumulation,

and environmental chemistry methods (with independent laboratory validations). 

• The ecotoxicity profile is partially complete. EFED recommends the following 

studies: acute freshwater invertebrate (EHE), acute saltwater invertebrate (EHE 

and acid/amine), acute saltwater fish (EHE and acid/amine), chronic freshwater 

invertebrate (acid/amine), chronic freshwater fish (acid/amine), chronic saltwater 

fish (acid/amine), chronic saltwater invertebrate (acid/amine), acute avian oral

(any compound), avian reproduction (any compound), honeybee larval toxicity

(any compound), seedling emergence (ester – soybean only), and vegetative vigor 

(acid/amine), vegetative vigor (ester – soybean only). In the absence of this 

information, data from other phenoxy (2,4-dichlorophenoxy) herbicides may be 

used, which may be more conservative.

• The Agency has not received aquatic ecotoxicity studies for the major degradate, 

2,4-DCP. A search of the ECOTOX database indicates that some toxicity data 

may be available; these studies will be reviewed. If these studies are not 

considered valid for risk assessment purposes, conservative assumptions about 

2,4-DCP’s toxicity to aquatic organisms may be made using ECOSAR or other 

toxicity endpoint modeling tools. The Agency is interested in receiving any 

relevant, existing toxicity data for 2,4-DCP.

• Some labels are missing information about the pounds of active ingredient applied 

on a per acre basis, number of applications per year, and minimum application 

interval. In the absence of this information, EFED will utilize the use information 

that results in maximum exposure estimates for a given use pattern or make other 

conservative assumptions. 
3

• A maximum application rate (on a per acre basis) should be identified for the spot 

treatments. In the absence of this information it will be assumed that no more 

than 1000 ft2 are treated per acre.

• For the granular formulations, the Agency will need to know the size of the 

granule and the amount of active ingredient contained within a granule. In the 

absence of this information, it may be assumed that birds and mammals could 

reasonably consume enough granules to induce an acute toxic effect.

• 2,4-DP-p is formulated into a number of different products and may produce 

varied toxicological effects in plants, depending on the formulation. Existing 

terrestrial plant toxicity data with different formulations would reduce that 

uncertainty. In the absence of data, the most protective toxicity values will be 

selected for terrestrial plant endpoints. 

2. Introduction

There are active registrations for three different forms of 2,4-DP-p – the acid form (active 

ingredient) and two other forms (one amine and an ester) that degrade into the acid upon 

contact with the environment. The amine is an R isomer 2,4-DP-p amine salt (DMAS).

The ester is 2,4-DP-p 2-ethylhexyl ester (EHE) and the acid is 2,4-DP-p acid (acid). 

Collectively, the three forms will be referred to as 2,4-DP-p. The structure of 2,4-DP-p, 

as well as the chemical names and other identifiers can be found in the chemical identity 

table (Appendix A).

2,4-DP-p is a non-systemic herbicide in the phenoxy or phenoxyacetic acid family. 2,4-

DP-p causes disruption of auxin hormones in plants by affecting proteins in the plasma 

membrane, interfering with RNA production, and changing the properties and integrity of 

the plasma membrane. The plant's vascular system becomes blocked from excessive cell 

division and the resulting growth crushes the vascular transport system. Plant damage

may include growth and reproduction abnormalities, especially on new growth. 

Symptoms may appear on young growth almost immediately after application, but death 

may not occur for several weeks.

2,4-DP-p is primarily used on lawns, rights-of-ways, residential areas, and non-cultivated 

agricultural land. According to the Biological and Economics Analysis Division (BEAD), 

an average of 8,000 lb ae/year of 2,4-DP-p was used on an annual basis from 2004 to 2010

in pastures. 2,4-DP-p is used on < 1% to < 2.5% of the total pastureland in the United 

States (USEPA, 2013a). According to technical and product labels, liquid and granular 

applications are made using ground equipment; many are also made as spot treatments and 

stump treatments.

The most recent risk assessment of 2,4-DP-p was the EFED Re-registration Eligibility 

Document Chapter (RED; USEPA, 2007a). It assessed the acid, DMAS (R enriched), 

and EHE forms. In this problem formulation, EFED considered the latest Agency 

science policies and risk assessment methodologies.
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3. Use Characterization

2,4-DP-p is a non-systemic herbicide in the phenoxy or phenoxyacetic acid family that is 

applied post-emergence to a wide variety of weeds. It is registered for use on turf/lawns 

(commercial/industrial, residential, golf course, recreational, sod), airports/landing fields, 

outdoor household/domestic uses, non-agricultural rights-of-way/fencerows/hedgerows, 

non-agricultural uncultivated areas/soils, fallow land, pastures, rangeland, paths and 

patios, paved areas, grasses grown for seed, forest trees, and pine trees (USEPA, 2013b). 

It is not registered on any food/feed crops. 2,4-DP-p is formulated as a granule, 

pressurized liquid, ready-to-use, soluble concentrate, and emulsifiable concentrate. 

Treatment equipment includes spreader, sprayer, atomizer type sprayer, controlled 

droplet applicator, low volume ground sprayer, spinning disc sprayer, drencher, trigger 

spray bottle, hose-end sprayer, pump-up sprayer, sprinkler can, tank-type sprayer, high 

volume ground sprayer, product container, pressure sprayer, hand-held sprayer, backpack 

sprayer, tractor-mounted sprayer, hand pressure sprayer, boom sprayer, compression 

sprayer, hand held spray wand, and power sprayer.

The Screening Level Usage Analysis produced by BEAD (USEPA, 2013a) lists the use 

patterns of maximum exposures for the current uses of 2,4-DP-p. EFED will use 

application scenarios that result in maximum exposure scenarios for a given use for the 

risk assessment. In the absence of information on the labels (e.g., maximum single 

application rate, the application interval, the annual maximum rate allowed, rates in lbs 

ae/year, application methods), EFED will utilize the use information that results in 

maximum exposure estimates for a given use pattern or make other conservative 

assumptions. Specifically, there are several uses for which this information is missing. 

In addition, some of the spot treatments result in application rates on a per acre basis that 

are very large. Revising the label to include a maximum yearly application rate (on a per 

acre basis) or providing additional information about the pounds of material expected to 

be used in an acre would allow for more realistic modeling of 2,4-DP-p spot treatment 

uses. Please see Appendix B for more details.

4. Conclusions from Previous Risk Assessments

a. Ecological Risk Assessment 

The Agency only has one ecological risk assessment on 2,4-DP-p that serves as a basis 

for this problem formulation document – the EFED Chapter of the Reregistration 

Eligibility Decision (USEPA, 2007a). The EFED RED chapter was conducted on the

acid, DMAS (R enriched compound), and EHE. The major degradate, 2,4-DCP, was 

acknowledged in the RED, but not quantitatively incorporated into the assessment. The 

major ecological conclusions presented in the RED were as follows:
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Table 1. Summary of Risk Concerns Identified for 2,4-DP-p in the RED

Birds Mammals Terr.

Plants

Terr.

Inverts Fish BCF Aquatic Inverts

Aquatic 

Plants

Groundwater 

Contam. Persistence Degradates of Concern

Yesb Yesb Yes NAc No No No Yesa No No Presentd

a LOC only exceeded for one non-vascular plant use (6 lb ae/A; aerial application of 2,4-DP-p DMAS). b Chronic and acute risks were identified. c No risks based on toxicity (hazard) classification.

Yes = at least one LOC has been exceeded in previous assessments; No = LOCs have not been exceeded in previous

assessments; NA = RQs have not been calculated previously. d 2,4-DCP is a common degradate for many phenoxy herbicides. 

b. Drinking Water Exposure Assessments

EFED conducted an evaluation of concentrations of 2,4-DP-p to which humans may be 

exposed through the ingestion of drinking water (USEPA, 2007b). The assessment also 

included modeling and an evaluation of surface water and groundwater monitoring data. 

A number of modeling scenarios were used to provide estimated drinking water 

concentrations (EDWCs) for drinking water. Ground application on Christmas trees was 

the use producing the maximum estimated drinking water concentrations (EDWC) (US 

EPA, 2007b). EDWCs were 40.2 μg/L (1-in-10-year peak) and 2.69 μg/L (30-year 

mean) for surface water and 0.55 μg/L for ground water.

c. Clean Water Act Programs

2,4-DP-p is not identified as a cause of impairment for any water bodies listed as 

impaired under section 303(d) of the Clean Water Act. No Total Maximum Daily Load 

(TMDL) criteria have been developed for 2,4-DP-p and aquatic benchmarks have not 

been established 

(http://www.epa.gov/oppefed1/ecorisk_ders/aquatic_life_benchmark.htm). Any data 

submitted or otherwise located as part of the registration review process may be used to 

update aquatic life benchmarks, if applicable. 

5. Environmental Fate and Transport

The physicochemical properties suggest that 2,4-DP-p acid is soluble in water (729 

mg/L). The vapor pressure (1.28 × 10-6 mm Hg) and Henry's Law Constant (2.43 x 10-10

atm-m3/mol) indicate that the acid has a low volatility. The acid has low potential to

bioaccumulate in aquatic organisms given the low value of the log n-octanol/water 

partition coefficient (log Kow 3.07). It is non-persistent to moderately persistent and 

mobile in terrestrial and aquatic environments. Major routes of dissipation are 

photodegradation in water (DT50 of 4 days), microbial-mediated degradation (DT50s of

14 to 34 days), and leaching. 2,4-DP-p is stable to abiotic hydrolysis in pH 5, 7, and 9 

buffer solutions. The pKa of 2,4DP-p acid is 3.7 indicating that this compound will exist 

in an anionic form (X-COO- H+) in the environment (anions generally do not adsorb 

strongly to soils containing organic carbon and clay). Since the acid will be present 

primarily as an anion in soil at environmental pH values, there is a potential for both surface 
6

runoff and leaching to groundwater. Several field dissipation studies were performed for 

these chemicals; they are currently under review. 

2,4-DP-p formulations contain either the acid, DMAS, or EHE. Given the rapid and 

complete dissociation of DMAS and the rapid degradation of EHE in soil via microbialmediated or surface catalyzed hydrolysis processes (DT50s of 12 to 24 hours) leading to the 

formation of the acid, EFED employed an environmental fate bridging strategy of DMAS 

and EHE to the acid. The one exception is exposure to the aquatic environment from the

direct deposition from spray drift because EHE may persist in waters with an acidic to neutral 

pH. The abiotic hydrolysis ranges from a DT50 of 14 days in pH 9.0 to a DT50 of 445 

days in pH 5.0. The exposure assessment for DMAS and EHE will be represented by the 

acid equivalent. The physical, chemical and environmental fate properties (of the acid) 

are presented in Table 2.

Table 2. Physical Chemical and Environmental Fate Properties of 2,4-DP-p Acid

Parameter Value Source

Selected Physical/Chemical Parameters

Chemical Structure1

TOXNET

IUAPC Name (R)-2-(2,4-dichlorophenoxy)propionic 

acid

MRID 47163502

CAS Name (2R)-2-(2,4-dichlorophenoxy)propanoic 

acid

MRID 47163502

CAS No. 15165-67-0 MRID 47163502

Molecular Weight (molecular formula) 235.07g/mol (C9H8Cl2O3) U.S. EPA, 2005a

Smiles Code c1cc(Cl)cc(Cl)c1OC(C)C(=O)O EPISUITE 4.1

Vapor pressure (20°C) and (25°C) 6.22E-07 mbar (4.67E-07 mm Hg) @ 

(20°C)

1.70 E-06 mbar (1.28E-06 mm Hg) @ 

(25°C)

MRID 437748-05

Aqueous solubility (20°C) 729 mg/L MRID 47163502

Dissociation constants (pKa ) in water 

(20°C)

3.67 MRID 438029-02

Henry’s Law Constant (25°C) 2.43E-10 atm-m3/mol MRID 46591916

Log octanol-to-water partition coefficient

(log KOW)

3.07 MRID 438029-03
7

Parameter Value Source

Persistence

Hydrolysis half-life (25°C)

2,4-DP-p (acid)

2,4-DP-p (EHE)

Stable

445 days at pH 5.0

312 days at pH 7.0

14 days at pH 9.0

Degradate

2,4-DP-p acid (76.2% of applied)

Unidentified Residue (6.1% of applied)

MRID 426830-01

MRID 429176-01

Hydrolysis half-life in Soil/Water system 

(25°C)

Silty clay loam 

Sandy loam

12 hours

24 hours

Degradates

2,4-DP-p acid (28.0% of applied)

4 chlorophenoxy-propionic acid (4-

CPP; 2.0% of applied)

Unidentified/ unextractable (2.0% of 

applied)

MRID 42937006

Aqueous photolysis half-life (25°C) 4.0 days

Degradates

2,4 DCP (not quantified) 

Unidentified degradates (>10% of 

applied)1 

MRID 43101501

Soil photolysis half-life 12.15 days

Degradates

2,4 DCP (26.5% of applied)

Unidentified degradate (4.1% of applied)

MRID 42899601

Aerobic soil metabolism half-life (days) Arkport soil – 14.0 days

Degradates 

2,4-DCP (3.9% of applied) 

2,4-Dichloroanisole (DCA) (4.2% of 

applied)

Unidentified degradate (2.8% of applied)

MRID 42935301

Aerobic aquatic metabolism half-life (total 

system)

Krempe sediment/water – 34.6 days

Ohlau sediment/water – 26.6 days

Degradates

CO2 (79-90% of applied)

MRID 42845806

Anaerobic aquatic metabolism half-life 

(total system)

No acceptable data ---
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Parameter Value Source

Mobility

Adsorption/desorption

Kd-ads / Kd-des (mL/g)

Kfoc-ads / Kfoc-des (mL/g)

Adsorption

Freundlich adsorption Kfads values 

Arkansas Clay - 0.79

Georgia Sand - 0.25

Sandy Loam 1.7

Timmerman sandy Loam 0.41

Desorption

Freundlich adsorption Kf-des values 

Arkansas Clay – 1.3

Georgia Sand - 0.17

Sandy Loam 3.1

Timmerman sandy Loam 2.5

Adsorption

Freundlich adsorption Kfoc values 

Arkansas Clay – 83.7

Georgia Sand - 32.7

Sandy Loam - 81.5

Timmerman sandy Loam - 78.2

Desorption

Freundlich adsorption Kfoc-des values 

Arkansas Clay – 143

Georgia Sand - 21.9

Sandy Loam 149

Timmerman sandy Loam 474

MRID 44028901

Field Dissipation

Terrestrial field dissipation half-life Bare ground - 4.2 days

Turf – 2.6 days

MRID 44130103

Fish Bioconcentration

Fish bioconcentration factors (steadystate); depuration rate No data. ---

1 Structures and other physicochemical properties of various chemical forms can be obtained from 

Appendix A

a. Transformation Products

The major degradation product of 2,4-DP-p is 2,4-dichlorophenol [2,4-DCP (a common 

degradate for other phenoxy herbicides), a maximum of 26.5% of applied] was observed 

in environmental fate studies (Table 3). Recent problem formulations conducted for 2,4-

D (DP barcode 402410; 4/12/2013 2), 2,4-DB, and 2,4-DB-DMAS (DP barcode 
9

410362_Draft) detail the environmental fate properties of these compounds. Similar to 

these compounds, 2,4-DP-p is also primarily degraded through microbially-mediated 

metabolism and photolysis forming 2,4-DCP. Minor degradation products were 2,4-

dichloroanisol (2,4-DCA) (a maximum of 4.2%) in an aerobic soil metabolism study and 

unidentified compounds (maximum of 4.1%) in various environmental fate studies.

Table 3 provides the transformation products of 2,4-DP-p and their maximum amounts 

formed.

Table 3. Major Degradates of 2,4-DP-p Detected in Environmental Fate Studies

Code Name/ 

Synonym

Chemical 

Name

Chemical 

Structure Study Type Ref.

(MRID)

Maximum 

%AR (day)

2,4-DCP1 2,4-

dichlorophenol Soil

photolysis 42899601 26.4% (8)

CO2 Carbon dioxide Aerobic 

aquatic 42845806 90 % (92)

1 A major degradate with toxicological significance and a common degradate for many phenoxy 

herbicides.

6. Receptors

Table 4 through 6 provide a summary of the aquatic and terrestrial taxonomic groups and 

the most sensitive surrogate species tested to characterize the potential acute and chronic 

ecological effects of 2,4-DP-p. Given that the amine and acid forms of 2,4-DP-p are 

expected to differ from the ester for aquatic exposures, they will be considered separately 

here for aquatic organisms. The ester form of 2,4-DP-p is expected to be more toxic to 

aquatic species than the amines and acid, based on available fish toxicity data for 2,4-DPp and comparable aquatic data for other phenoxy herbicides. In addition, the amine form

immediately dissociates into the acid whereas the ester does not; toxicity data from 2,4-

DP-p and other phenoxy herbicides demonstrate similar endpoints for amines and acids. 

Assessing amine/acid forms separately from ester forms for aquatic exposures is 

consistent with other phenoxy herbicides. 

For terrestrial species, all forms of 2,4-DP-p are expected to be equivalent in toxicity, 

based on data from other phenoxy herbicides; consequently, a pair of ester and 

acid/amine data are not required. The one exception is for terrestrial plants because the 

risk assessment has an aquatic component (runoff). Consequently, plants are expected to 

be exposed to the ester form for a longer period of time than the amine, which 

immediately dissociates into the acid. Ecological studies with the major degradate, 2,4-

DCP, have not been submitted to the Agency; however, a preliminary look at the open 

literature suggests it may be more toxic to some aquatic organisms than 2,4-DP-p.
10

a. Effects to Aquatic Organisms

Table 4. Summary of the Most Sensitive Endpoints from Aquatic Toxicity Studies 

for 2,4-DP-p Acid and Amines

Taxonomic 

Group

Study 

Type

Test Species and 

Chemical

Toxicity Value

(all units in 

terms of acid

equivalents)

Acute 

Toxicity 

Classification 

MRID

Freshwater fish1 Acute

Rainbow Trout

(Oncorhynchus 

mykiss) – DMAS

LC50 > 91.5 mg

ae/L

No mortality

Practically 

non-toxic 42767001

Freshwater 

invertebrates

Acute

Water Flea 

(Daphnia magna) -

Acid

EC50 = 558 mg 

ae/L* Practically 

non-toxic 43867603

Chronic

Water Flea 

(Daphnia magna) -

Acid

NOAEC = 100 

mg ae/L

LOAEC > 100 

mg ae/L

No effects

-- 46591903

Aquatic plants 

and algae

Vascular

Duckweed 

(Lemna gibba) -

DMAS

NOAEC = 1.87

mg ae/L

EC50 = 32 mg

ae/L

Based on 

reduction in 

number of 

fronds

-- 46856701

Nonvascular

Freshwater Diatom 

(Navicula

pelliculosa) -

DMAS

NOAEC = 

0.0013 mg ae/L

EC50 = 0.077

mg ae/L

Based on cell 

density

-- 42665701

1 Freshwater fish may be surrogates for aquatic-phase amphibians.

*Several tests with lower endpoints are available, but they yield “greater than” (non-definitive) values (ranged from > 

82.3 to > 100 mg ae/L), indicating that the EC50 is higher than what was tested; there was no mortality. The cited 

study, although tested with a formulation, tested a wider range of doses and there is greater confidence that it has 

identified a value closer to the true EC50 than the non-definitive endpoint tests.

Table 5. Summary of the Most Sensitive Endpoints from Aquatic Toxicity Studies 

for 2,4-DP-p Ester

Taxonomic 

Group

Study 

Type

Test Species and 

Chemical

Toxicity Value

(all units in 

terms of acid 

equivalents)

Acute 

Toxicity 

Classification 

MRID

Freshwater fish1 Acute

Bluegill sunfish

(Lepomis 

macrochirus) –

EHE

LC50 = 5.21 mg 

ae/L

Moderately 

toxic 42767004

1 Freshwater fish may be surrogates for aquatic-phase amphibians.
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b. Effects to Terrestrial Organisms

Table 6. Summary of the Most Sensitive Endpoints from Terrestrial Toxicity 

Studies for 2,4-DP-p

Taxonomic 

Group

Study Type Test Species

and Chemical

Toxicity Value

(all units in terms 

of acid equivalents)

Acute 

Toxicity 

Classification

MRID

Birds1

Acute

Bobwhite quail

(Colinus 

virginianus) -

DMAS

LD50 = 242 mg

ae/kg-bw

Moderately

toxic 42987901

Sub-acute 

dietary

Bobwhite quail

(Colinus 

virginianus) -

EHE

LC50 > 3804 mg 

ae/kg-diet

No mortality

Practically 

non-toxic 43811401

Chronic

Japanese quail 

(Coturnix 

coturnix 

japonica) -

DMAS

NOAEC = 244 mg 

ae/kg-diet

LOAEC = 732 mg 

ae/kg-diet

Based on mortality 

and reduction in 

food 

consumption/weight 

gain

-- 46879201

Mammals

Acute Oral

Laboratory rat 

(Rattus 

norvegicus) -

DMAS

LD50 = 534 mg

ae/kg-bw Slightly toxic 42614601

Acute 

Inhalation

Laboratory rat 

(Acid) LC50 >2.7 mg ae/L Toxicity 

Category IV 42985310

Chronic

Laboratory rat 

(Rattus 

norvegicus) -

Acid

NOAEL = 40 mg

ae/kg-bw

LOAEL = 219.6 mg

ae/kg-bw

Based on reduced 

body weight and 

food consumption

-- 46721401

Insects

Acute 

contact

Honey bee 

(Apis mellifera 

L.) - EHE

LD50 >17 μg ae/bee Practically 

non-toxic 42204601

Acute oral

Honey bee 

(Apis mellifera 

L.) - Acid

LD50 > 100 μg 

ae/bee

Practically 

non-toxic 46591910

Terrestrial 

plants

Seedling 

Emergence 

(acid/amine)2

Dicot – lettuce

(Lactuca sativa) 

- DMAS

NOAEC = 0.005 lb 

ae/A

EC25 = 0.0083 lb 

ae/A

Based on dry 

weight

-- 43016702
12

Taxonomic 

Group

Study Type Test Species

and Chemical

Toxicity Value

(all units in terms 

of acid equivalents)

Acute 

Toxicity 

Classification

MRID

Monocot –

onion

(Allium cepa) -

DMAS

EC05 = 0.0034 lb/A

EC25 = 0.029 lb/A

Based on dry 

weight

-- 43016702

Vegetative 

vigor (acid/

amine)2

Dicot –

cucumber

(Cucumis 

sativa) - DMAS

EC05 = 0.00021 lb 

ae/A

EC25 = 0.0047 lb 

ae/A

Based on dry 

weight

-- 43016701

Monocot –

onion

(Allium cepa) -

DMAS

NOAEC <0.003 lb 

ae/A

EC25 = 0.014 lb 

ae/A

Based on dry weight

-- 43016701

Seedling 

emergence 

(ester)2

Dicot – oilseed 

rape (Brassica 

napus) - EHE

EC05 = 0.00541 lb 

ae/A

EC25 = 0.0358 lb 

ae/A

Based on dry weight

-- 47786303

Monocot –

onion (Allium 

cepa) - EHE

NOAEC = 0.0500 lb 

ae/A

EC25 = 0.078 lb 

ae/A

Based on dry weight

-- 47786303

Vegetative 

vigor (ester)2

Dicot – oilseed 

rape (Brassica 

napus) - EHE

NOAEC = 

0.0000686 lb ae/A

EC25 = 0.00139 lb 

ae/A

Based on dry weight

-- 47786304

Monocot –

onion (Allium 

cepa) - EHE

NOAEC = 0.00186 

lb ae/A

EC25 = 0.0162 lb 

ae/A

Based on dry weight

-- 47786304

1 Birds represent surrogates for terrestrial-phase amphibians and reptiles. 2

Ester and acid/amine values are listed for each terrestrial plant endpoint because separate assessments will be 

performed for the ester and acid/amine forms.

c. Ecological Incidents

The ecological incident information system (EIIS) is an OPP database that houses 

ecological incidents that have been reported to the Agency. When available, EIIS 
13

includes date and location of an incident, type and magnitude of effects observed in 

various species, use(s) of pesticides known or suspected of contributing to the incident, 

and results of any chemical residue analysis or other analyses conducted during incident 

investigation. EIIS incidents are categorized according to the certainty that the incident 

resulted from pesticide exposure. The Avian Monitoring System (AIMS) is a database 

administered by the American Bird Conservancy that contains publicly available data on 

reported avian incidents involving pesticides. Many of the incidents listed in this 

database are also in the EIIS. The Aggregate Summary Module of OPP’s Incident 

Database was also consulted for reports of minor incidents to non-target organisms.

EIIS returned one terrestrial plant incident in Indiana attributed to DMAS use with 

“possible” certainty (USEPA, 2007a). Incident #I024272-225 occurred in June of 2012

when DMAS was applied in a residential area. Approximately 45% of the roses and 

ornamental plants in the vicinity were damaged. 

As of August 29, 2013, AIMS identified no ecological incidents involving 2,4-DP-p. 

Registrants reported one minor wildlife incident and 867 minor plant incidents with 2,4-

DP-p between 1995 and 2012 (aggregate incident database). Unless additional 

information on these aggregated incidents becomes available, they will be assumed to be 

representative of registered uses of 2,4-DP-p in the risk assessment.

7. Exposure Pathways of Concern

The environmental fate properties and use patterns of 2,4-DP-p indicate that direct spray, 

spray drift, leaching through soil (to groundwater or in subsurface lateral flow), and 

runoff represent potential transport mechanisms of 2,4-DP-p to non-target aquatic and 

terrestrial organisms. Drinking water and inhalation exposure pathways were screened 

using the SIP (Screening Imbibition Program) and STIR (Screening Tool for Inhalation 

Risk) screening methods. Inhalation was found to be an exposure pathway of low 

concern (no RQs are expected to exceed LOCs) for mammals and birds; however, 

exposure concerns from the acid and amine forms (the ester form was not a concern) via 

drinking water are reasonably expected to occur for mammals and birds, given the results 

of the modeling. SIP and STIR are described in detail at: 

http://www.epa.gov/oppefed1/models/terrestrial/index.htm and results are in Appendix C.

8. Analysis Plan

a. Stressors of Concern

i. Ecological Risk Assessment

The stressors of ecological concern for terrestrial and aquatic organisms include the three

forms of 2,4-DP-p. Submitted ecological studies indicate that the ester form for 2,4-DP-p 

is more acutely toxic to freshwater fish than DMAS, as holds true with other phenoxy 
14

herbicides. Consequently, the ester will be considered separately for aquatic organisms, 

as has been done in recent risk assessments (USEPA 2007a). For terrestrial scenarios, the 

most sensitive endpoint, regardless of the 2,4-DP-p form (ester, amine, acid) will be used 

in the risk assessment. 2,4-DCP is a major degradate in the terrestrial environment for all 

forms of 2,4-DP-p and there are some toxicity data for 2-DCP available in ECOTOX 

(http://cfpub.epa.gov/ecotox/) that suggest it is more toxic than 2,4-DP-p for some 

aquatic organisms. Pending results of reviewing the aquatic toxicity studies on the 

aquatic degradate 2,4-DCP, this degradate will be included with 2,4-DP-p in the residues 

of concern modeled for aquatic exposure. Since, 2,4-DCP may be more toxic than 2,4-

DP-p for some aquatic species, aquatic exposures may be conducted for 2,4-DP-p and 

2,4-DCP separately or based on total equivalency factor (TEF) depending on additional 

2,4-DCP data availability. In the absence of data, ECOSAR values may be used to 

predict the toxicity of 2,4-DCP. 

ii. Drinking Water 

An EFED drinking water assessment was conducted to support the RED human health 

risk assessment for 2,4-DP-p (USEPA 2007b). If a revised drinking water assessment is 

warranted, EFED will conduct a drinking water assessment to support future human 

health dietary risk assessments of 2,4-DP-p. The drinking water assessment will be 

performed based on EFED’s current guidance using appropriate surface and groundwater 

models. Descriptions of all executable versions of current models for drinking water 

exposure can be obtained from URL link below. 

http://www.epa.gov/oppefed1/models/water/

b. Measures of Exposure 

EFED will use standard available models to evaluate potential exposures to aquatic and 

terrestrial organisms as described at 

http://www.epa.gov/pesticides/science/models_db.htm. In addition, other available 

exposure models may be used to assess use in drains, drain lines, irrigation supply 

systems, waste water systems, sewage systems, sewer lines, and storm sewers, as 

applicable. 

i. Available Monitoring Data

The Agency is aware of monitoring conducted by federal and state agencies and this 

information will be considered in the assessment to the extent that data on 2,4-DP-p and 

its degradates are available.
15

c. Measures of Effect 

Toxicity data presented in Section 3 of this problem formulation will be used to calculate 

risk quotients. Any additional information submitted by the registrant or found in the 

open literature prior to conduct of the risk assessment will also be considered. The open 

literature studies are identified using EPA’s ECOTOXicology database (ECOTOX) 

(USEPA, 2007c), which employs a literature search engine for locating chemical toxicity 

data for aquatic life, terrestrial plants, and wildlife. The evaluation of both sources of 

data can also provide insight into the direct and indirect effects of pesticides on biotic 

communities from loss of species that are sensitive to the chemicals and from changes in 

structure and functional characteristics of the affected communities.

9. Endangered Species Assessments

Consistent with the Agency’s responsibility under the Endangered Species Act (ESA), 

the Agency will evaluate risks to federally threatened and/or endangered (listed) species 

from registered uses of pesticides in registration review. The process for evaluating 

potential risks to listed species is further described at 

http://www.epa.gov/oppfead1/endanger/litstatus/riskasses.htm.

10. Endocrine Disruptor Screening Program

2,4-DP-p was not included in the first or second group of chemicals issued/to be issued 

orders to conduct Tier 1 EDSP testing. For additional information the EDSP program, 

visit http://www.epa.gov/endo/. 

11. Preliminary Identification of Data Gaps 

a. Environmental Fate

Table 7 identifies environmental fate studies by MRID that offer data for each guideline 

requirement, as well as study classifications and whether or not further data are needed in 

order to support risk assessment. Additional environmental fate data for 2,4-DP-p, as 

noted in Table 7, will reduce uncertainty related to the exposure assessment.
16

Table 7. Submitted Environmental Fate Data and Study Classifications for 2,4-DPp.

OCSPP 

Guideline

Data 

Requirement

Submitted 

Studies 

(MRID)

Study 

Classifications

Are data 

needed to 

conduct risk 

assessment?

Justification and Assumptions 

EPA will Make in Absence of 

Data

835.2120 Hydrolysis 426830-01 Acceptable No --

835.2240 Aqueous 

photolysis 43101501 Acceptable No --

835.2410 Soil photolysis 42899601 Acceptable No --

835.4100 Aerobic soil 

metabolism 42935301 Acceptable Yes. Partially fulfilled.

Single aerobic soil metabolism 

study is acceptable for risk 

assessment. Additional data for 

three soils are required under 40 

CFR Part 158 for terrestrial 

outdoor uses. OCSPP guidance 

recommends that these tests be 

performed with four soils. 

Given that OCSPP uses an upper 

90th percentile estimate of soil 

metabolism half-lives based on 

available study data for 

modeling purposes, any 

additional data can reduce 

uncertainty related to the 

exposure assessment as well as 

the half-life value used in 

modeling. Additional data are 

considered to have a high 

potential to add value to the 

ecological risk and drinking 

water assessments.

835.4200 Anaerobic soil 

metabolism No data

--

Yes

Data are required under 40 CFR 

Part 158 for terrestrial outdoor 

uses. OCSPP guidance 

recommends that these tests be 

performed with four soils. 

Additional data are considered 

to have a low potential to add 

value to exposure assessments 

for the ecological risk and 

drinking water in presence of 

anaerobic aquatic metabolism 

(835.4400).
17

OCSPP 

Guideline

Data 

Requirement

Submitted 

Studies 

(MRID)

Study 

Classifications

Are data 

needed to 

conduct risk 

assessment?

Justification and Assumptions 

EPA will Make in Absence of 

Data

835.4300 Aerobic aquatic 

metabolism 42845806 Supplemental No --

835.4400

Anaerobic 

aquatic 

metabolism

No data. -- Yes

The assumption of compound 

stability will be made in the 

absence of acceptable data.

835.1230

835.1240

Adsorption/ 

desorption and 

leaching

44028901 Acceptable No --

835.1410 Volatility –

laboratory No data. -- No

Not expected to be volatile 

under normal use conditions 

based on low vapor pressure and 

Henry’s Law Constant.

835.6100 Terrestrial field 

dissipation

44071401

44072402

44071403

44085501

44110801

44130101

44130102

44130103

44153701

44153702

44150703

44153704

TBD1 TBD

Registrants submitted 12

terrestrial field dissipation 

studies. A preliminary review 

suggests that these studies are 

scientifically sound. Currently 

in review. 

850.6100

Analytical 

method in soil No data. TBD TBD Currently in review

Analytical 

method in water No data. -- Yes

Environmental chemistry 

method (ECM) and independent 

laboratory validations (ILV) are 

required for 2,4DP-p acid and its 

degradates.

850.1730 Fish 

bioconcentration No data. -- No

log Kow 3.07 suggests low 

potential to bioaccumulate in 

aquatic organisms 

1

TBD - To be determined after final review of these studies

Environmental chemistry methods (ECM) for 2,4-DP-p residues of concern in water as 

well as independent laboratory validation (ILV) of the methods are required to support 

monitoring for 2,4-DP-p residues and to evaluate newly submitted environmental fate and 

ecological effects data. ILV methods submitted by the registrant are used to evaluate 

analyses described in submitted environmental fate and ecological effects studies. 

Submitted analytical methods are also used by various federal, state, tribal, and local 
18

agencies to detect and monitor residues that are or are suspected to be in environmental 

compartments from outdoor uses and accidental releases. Independent laboratory 

validations for submitted analytical methods are necessary to confirm that the methods 

are functional and to confirm the levels of detection and quantitation reported in 

registrant-prepared validations. 

b. Effects

Tables 8 and 9 identify ecological effects studies by MRID that offer data for each 

guideline requirement, as well as study classifications and whether or not further data are 

needed to support risk assessment. Although the ester form of 2,4-DP-p is more acutely 

toxic to freshwater fish, aquatic chronic studies are not requested given that it degrades

into the acid form; the amines also rapidly dissociate into the acid form. Consequently, 

chronic exposure is only expected to occur with the acid form.
19

Table 8. Submitted Aquatic Ecological Effects Data for 2,4-DP-p

OCSPP

Guideline

Data 

Requirement

MRID

(Compound)

Study 

Classifications

Are data 

needed to 

conduct 

risk 

assessment?

Justification and 

Assumptions EPA will 

Make in Absence of Data

850.1010 Freshwater 

invertebrate 

acute toxicity

43867603 

(Acid)

Acceptable Yes Data are needed for EHE 

because the ester form is 

generally more toxic to 

aquatic organisms than the 

acid/amine forms (based on 

other phenoxy herbicides 

and 2,4-DP-p freshwater 

fish data). Without the 

submission of an acceptable 

study, data from 2,4-DP-p 

isooctyl ester may be used

or a conservative estimate of 

the endpoint will be made 

based on data from other 

phenoxy herbicides.

850.1025

850.1035

850.1045

850.1055

(Choice of 

four 

guidelines, 

but only one 

is needed to 

fulfill the 

requirement)

Saltwater 

invertebrate 

acute toxicity 

(None) (N/A) Yes Data are needed for 1) acid 

or amine; 2) EHE. Data are 

required for both the 

acid/amine and the ester 

because the ester form is 

more toxic to aquatic

organisms (based on other 

phenoxy herbicides and 2,4-

DP-p freshwater fish data). 

Without the submission of 

acceptable studies, 

conservative estimates of 

the endpoints will be made 

using data from other 

phenoxy herbicides (see the 

2,4-DP-p RED for 

examples).

850.1075 Freshwater fish 

acute toxicity 

42767001 

(DMAS)

Acceptable No --

40098001 

(EHE)

Acceptable
20

OCSPP

Guideline

Data 

Requirement

MRID

(Compound)

Study 

Classifications

Are data 

needed to 

conduct 

risk 

assessment?

Justification and 

Assumptions EPA will 

Make in Absence of Data

850.1075 Saltwater fish 

acute toxicity 

(None) (N/A) Yes Data are needed for 1) acid 

or amine; 2) EHE. Data are 

required for both the 

acid/amine and the ester 

because the ester form is 

more toxic to aquatic

organisms (based on other 

phenoxy herbicides and 2,4-

DP-p freshwater fish data). 

Without the submission of 

acceptable studies, 

conservative estimates of 

the endpoints will be made 

using data from other 

phenoxy herbicides (see the 

2,4-DP-p RED for 

examples).

850.1300 Freshwater 

invertebrate 

life cycle 

46591903 

(Acid)

Supplemental Yes The existing study did not 

measure growth endpoints, 

which are usually more 

sensitive than mortality 

endpoints. Thus, the study 

needs to be repeated with an 

acid or amine form of 2,4-

DP-p. Without the 

submission of acceptable 

studies, conservative 

estimates of the endpoints 

will be made using data 

from other phenoxy

herbicides (see the 2,4-DP-p 

RED for examples).
21

OCSPP

Guideline

Data 

Requirement

MRID

(Compound)

Study 

Classifications

Are data 

needed to 

conduct 

risk 

assessment?

Justification and 

Assumptions EPA will 

Make in Absence of Data

850.1350 Saltwater 

invertebrates 

life cycle 

(None) (N/A) Yes Data are needed for 2,4-DPp acid or amine. Without 

the submission of an 

acceptable study, 

conservative estimates of 

the endpoints will be made 

using data from other 

phenoxy herbicides (see the 

2,4-DP-p RED for 

examples).

850.1400 Freshwater fish 

early-life stage

(None) (N/A) Yes Data are needed for 2,4-DPp acid or amine; the study 

should be performed with 

the rainbow trout because 

this was the most sensitive 

species in the acute testing. 

Without the submission of 

an acceptable study, 

conservative estimates of 

the endpoints will be made 

using data from other 

phenoxy herbicides (see the 

2,4-DP-p RED for 

examples).

850.1400 Saltwater fish 

early-life stage 

(None) (N/A) Yes Data are needed for 2,4-DPp acid or amine. Without 

the submission of an 

acceptable study, 

conservative estimates of 

the endpoints will be made 

using data from other 

phenoxy herbicides (see the 

2,4-DP-p RED for 

examples).

850.1500 Fish life cycle (None) (N/A) No --

850.4400 Aquatic plant 

Toxicity Test 

using Lemna 

spp.

46856701

(DMAS)

Acceptable No Based on 2,4-D data (a 

related phenoxy herbicide), 

the ester form of 2,4-DP-p is 

not expected to be more 

toxic to aquatic vascular 

plants than the acid/amine 

forms. Consequently data 

for the ester are not 

expected to yield a lower 

endpoint.

850.4500 Algal toxicity 42665701 

(DMAS)

Acceptable 

(freshwater 

diatom)

Yes Data are needed for 2,4-DPp EHE for the freshwater 

diatom because the ester 
22

OCSPP

Guideline

Data 

Requirement

MRID

(Compound)

Study 

Classifications

Are data 

needed to 

conduct 

risk 

assessment?

Justification and 

Assumptions EPA will 

Make in Absence of Data

42595902 

(DMAS)

Acceptable 

(green algae)

Yes form is generally more toxic 

to aquatic organisms than 

the acid/amine forms (based 

on other phenoxy herbicides 

and 2,4-DP-p freshwater 

fish data). Without the 

submission of acceptable 

studies, conservative 

estimates of the endpoints 

will be made using data 

from other phenoxy 

herbicides (see the 2,4-DP-p 

RED for examples).

42595901 

(DMAS)

Acceptable 

(marine 

diatom)

Yes

850.4550 Cyanobacteria 42681001 

(DMAS)

Acceptable No A freshwater diatom study

for EHE is being requested 

in lieu of cyanobacteria

because freshwater diatoms 

were the most sensitive nonvascular aquatic plant to 

acid/amine forms for 2,4-

DP-p.

Table 9. Submitted Terrestrial Ecological Effects Data for 2,4-DP-p

OCSPP 

Guideline

Data 

Requirement

MRID

(Compound)

Study 

Classifications

Are data 

needed for 

risk 

assessment?

Current Additional Data 

Need

850.2100 Avian oral 

toxicity

42987901 

(DMAS)

Acceptable Yes Data are required for one 

passerine species in addition 

to the northern bobwhite 

quail study that is already 

available (acid, amine, or 

ester). Without the 

submission of acceptable 

studies, conservative 

assumption will be made 

from the bobwhite quail 

study. 

850.2200 Avian dietary 

toxicity 

43811401 

(EHE)

Acceptable No --

43220202 

(EHE)

Acceptable
23

OCSPP 

Guideline

Data 

Requirement

MRID

(Compound)

Study 

Classifications

Are data 

needed for 

risk 

assessment?

Current Additional Data 

Need

850.2300 Avian 

reproduction

46879201 

(DMAS)

Supplemental Yes Data are required for one 

upland game species and one 

waterfowl species. The 

existing study is for the 

Japanese quail, which lays 

eggs continuously rather than 

cyclically, as seen in North 

American species. 

Consequently the data 

requirement for avian 

reproduction has not been 

met. Without the submission 

of acceptable studies, 

conservative estimates of the 

endpoints will be made using 

data from other phenoxy 

herbicides (see the 2,4-DP-p 

RED for examples).

NonguidelineAvian 

inhalation

(None) (N/A) No --

850.3020 Honey bee 

acute contact 

toxicity

42204601 

(EHE)

Acceptable No --

OECD 

213

Honey bee 

acute oral 

toxicity

46591910 

(Acid)

Supplemental No --

OECD

237

Honey bee 

larval toxicity 

(None) (N/A) Yes Toxicity data for larval 

honeybees are necessary 

under the Pollinator White 

Paper (USEPA 2012). One 

study should be conducted 

with any 2,4-DP-p form.

850.3030 Honey bee 

residue on 

foliage

(None) (N/A) No --

850.3040 Field testing 

for 

pollinators

(None) (N/A) No --

850.4100 Seedling 

Emergence 

43016702 

(DMAS)

Supplemental No --
24

OCSPP 

Guideline

Data 

Requirement

MRID

(Compound)

Study 

Classifications

Are data 

needed for 

risk 

assessment?

Current Additional Data 

Need

and Seedling 

Growth

47786303

(EHE)

Supplemental Yes Toxicity data for soybean 

with an ester is needed for 

the spray drift portion of the 

risk analysis. Without the 

submission of acceptable 

studies, conservative 

estimates of the endpoints 

may be made using data 

from other phenoxy 

herbicides.

850.4150 Vegetative 

Vigor

43016701 

(DMAS)

Supplemental Yes The existing NOAEC 

endpoint for the most 

sensitive plant (onion) in the 

DMAS study yielded a nondefinitive (less than) 

endpoint. A vegetative vigor 

study should be performed 

with onion using DMAS 

(racemic). Without the 

submission of acceptable 

data, conservative estimates 

of the endpoints will be 

made using data from other 

phenoxy herbicides (see the 

2,4-DP-p RED for examples)

or an IC05 may be calculated.

47786304

(EHE)

Supplemental Yes Toxicity data for soybean 

with an ester is needed for 

the spray drift portion of the 

risk analysis. Without the 

submission of acceptable 

studies, conservative 

estimates of the endpoints 

may be made using data 

from other phenoxy 

herbicides.

There are some toxicity data for 2,4-DCP available in ECOTOX 

(http://cfpub.epa.gov/ecotox/), which suggest it is more toxic than 2,4-DP-p for selected 

aquatic organisms. There have been no registrant submitted environmental effects studies 

for this degradate, but the Agency notes that there are multiple freshwater invertebrate 

and fish studies for 2,4-DCP in the ECOTOX database. The Agency plans to review 

these studies for the registration review risk assessment and incorporate toxicity 

information from this degradate, as appropriate. Some registrant-submitted information 

is available for the environmental fate of 2,4-DCP. The registrant and/or public is 

welcome to submit additional information that would validate 2,4-DCP's environmental 

fate and effects properties. In the case that the ECOTOX studies are not found to meet 

the Agency's criteria, conservative assumptions will be made about the fate properties 
25

and toxicological effects of 2,4-DCP, including the use of predictive models to generate 

appropriate data. The Agency would be interested in any public perspective regarding 

the availability of data and its consistency with the Agency’s data evaluation criteria 

outlined in 

http://www.epa.gov/pesticides/science/efed/policy_guidance/team_authors/endangered_s

pecies_reregistration_workgroup/esa_evaluation_open_literature.htm

Honeybee Larval Toxicity Test: Non-Guideline

Honeybee larval toxicity data are needed to complete a screening level risk assessment 

for risks to honeybees. In combination with acute contact toxicity and acute oral toxicity 

data, the larval toxicity component ensures that exposure routes to representative life 

stages of honeybees have been considered. The assessment process is further described 

in the Pollinator White Paper (USEPA 2012).

12. References 

USEPA. 2007a. Re-registration eligibility document environmental fate and effects 

science chapter for 2,4-DP-p acid, 2,4-DP-p DMAS, and 2,4-DP-p EHE. 

Environmental Fate and Effects Division, Office of Pesticide Programs, U.S. 

Environmental Protection Agency. August 23, 2007 (D341297).

USEPA. 2007b. FQPA Drinking Water Assessment for Dichloroprop (2,4-DP-p).

Environmental Fate and Effects Division, Office of Pesticide Programs, U.S. 

Environmental Protection Agency. April 12, 2007 (D322697).

USEPA. 2012. White paper in support of the proposed risk assessment process for bees. 

Office of Chemical Safety and Pollution Prevention, United States Environmental 

Protection Agency, Washington, DC. September 11, 2012.

USEPA. 2013b. 2,4-DP (031416, 031419, 031463) screening level usage analysis. 

Biological and Economic Analysis Division, Office of Pesticide Programs, U.S. 

Environmental Protection Agency. January 30, 2013.

USEPA. 2013b. 2,4-DP-p Chemical Profile. Biological and Economic Analysis 

Division, Office of Pesticide Programs, U.S. Environmental Protection Agency. 

August 8, 13
26

Appendix A. Selected physical and chemical properties of various forms of 2,4-DP-p

Table A. Selected Physical Chemical Properties of Various Forms of 2,4-DP-p

PC Code 

Active 

Ingredient 

Molecular 

Formula and 

weight (g/mol)

Structure, CAS # and CAS Name

Solubility 

(mg/L)

(Source)

Dissociation 

constants (pKa ) 

in water

Vapor Pressure 

(mm Hg@) and 

Source

031402

2,4-DP-p

C9H8Cl2O3

(235.06)

CAS # 15165-67-0 

(2R)-2-(2,4-dichlorophenoxy)propanoic 

acid

729

(Table 2) 3.67 1.28E-06 mm Hg

(Table 2)

031403

2,4-DP-p 

DMAS (R 

isomer 

compound)

C12H17Cl2NO5

(280.2) 

CAS # 104786-87-0

(2R)-2-(2,4-dichlorophenoxy)propanoic 

acid - N-methylmethanamine

>1000000

MRID 

427215-01

Mean pKa = 

2.625 at 20.5°C

Rapidly 

dissociation at 

approximately 2 

minutes

MRID 427215-01

≤ 2.55E-07 mm 

Hg (≤3.4 x 10-5

Pa)

MRID 427215-03
27

PC Code 

Active 

Ingredient 

Molecular 

Formula and 

weight (g/mol)

Structure, CAS # and CAS Name

Solubility 

(mg/L)

(Source)

Dissociation 

constants (pKa ) 

in water

Vapor Pressure 

(mm Hg@) and 

Source

031465

2,4-DP-p EHE

C16H22Cl2O3

(347.3)

CAS # 865363-39-9 

2-ethylhexyl (2R)-2-(2,4-

dichlorophenoxy)propanoate

170

MRID 

440422-01

Not determined 

due to the very 

low solubility of 

the ester in water

(MRID 440422-

01)2

4.05 E-06 mm Hg 

(5.4 x 10-4 Pa at 

20°C)

MRID 440422-08

Physical and chemical properties of this compound assumed to be same as R enriched DMAS salt 

2

2,4 –DP-p 2-EHE rapidly hydrolyze in soil slurries. The DT50 of 2,4-DP-p 2-EHE was < 12 hours and < 24 hours in slurries of acidic 

silty clay loam soil and slightly-acidic sandy loam soil.
28

Appendix B. Table of Uses and Application Rates (from USEPA, 

2013b)

Use Method of Application

Maximum Application Rate

(in terms of ai*) (minimum 

application interval)

2,4-DP-p acid

Commercial/industrial lawns, golf 

course turf, ornamental lawns and 

turf, recreation area lawns, 

residential lawns

Granular broadcast 2 apps per year at 0.364 lb ai/A (30 

days) 

Household/domestic dwellings 

outdoor treatment, nonagricultural 

rights-ofway/fencerows/hedgerows, paths 

and patios, paved areas (private 

roads/sidewalks)

Stump treatment 0.0568 lb /gal (as needed)1,2

Household/domestic dwellings 

outdoor treatment, nonagricultural 

rights-ofway/fencerows/hedgerows, paths 

and patios, paved areas (private 

roads/sidewalks)

Foliar spray 0.0142 lb /gal (as needed)1,2

2,4-DP-p DMAS

Commercial/industrial lawns, Granular broadcast 2 apps per year at 0.153 lb ai/A (30 

days)

Commercial/industrial lawns Granular broadcast 1 app per year at 1.79 lb ai/A

Golf course turf Ground spray 1 app per year at 0.343 lb ai/A

Grasses grown for seed Ground spray 2 apps per year at 0.286 lb ai/A (21 

days)

Nonagricultural rights-ofway/fencerows/hedgerowsGround spray (spot 

treatment) 17.62 lb ai/A (30 days)3

Nonagricultural uncultivated 

areas/soils Ground spray 1 app per year at 6.04 lb ai/A

Ornamental lawns and turf, 

ornamental sod farm/turf

Ground spray (spot 

treatment)

2 apps per year at 59.9 lb ai/A (30 

days)

Recreational areas Ground spray 2 apps per year at 0.335 lb ai/A (30 

days)

Residential lawns Granular broadcast 2 apps per year at 0.465 lb ai/A (30 

days)

Residential lawns Ground spray (spot 

treatment)

2 apps per year at 1.37 lb ai/A (30 

days)

2,4-DP-p EHE

Agricultural uncultivated areas Stump treatment 1 app per year at 1.52 lb/23 gal1

Agricultural uncultivated areas Ground spray 1 app per year at 2.91 lb ai/A
29

Use Method of Application

Maximum Application Rate

(in terms of ai*) (minimum 

application interval)

Airports/landing fields, 

commercial/industrial lawns, 

commercial/institutional/industrial 

premises/equipment, golf course 

turf, household/domestic dwellings 

(outdoor premises), 

nonagricultural rights-ofway/fencerow/hedgerow, pave 

areas (private roads/sidewalks), 

recreation area lawns, residential 

lawns

Ground spray 2 apps per year at 0.531 lb ai/A(30 

days)

Forest trees, ornamental lawns and 

turf, ornamental sod farm (turf), 

pine (forest/shelterbelt)

Spot treatment 2 apps per year at 2.94 lb ai/A (30 

days)

Nonagricultural uncultivated 

areas/soils Basal bark treatment 1 app per year at 5.08 lb/1.3 gal1

Nonagricultural uncultivated 

areas/soils Foliar treatment 3.252 lb ai/A

*

Rates on the label report from BEAD were reported in active ingredient (ai) rather than acid 

equivalents (ae) (USEPA, 2013b)

1

Rate needs to be reported in lbs ai/A rather than in lbs ai/gal

2

Number of applications and application interval is missing

3

Number of applications is missing
30

Appendix C. Screening Level Models to Evaluate Potential Importance 

of Drinking Water and Inhalation Exposure Pathways

SIP (Screening Imbibition Program, v1.0) was used to calculate an upper bound estimate 

of 2,4-DP-p exposure via drinking water using solubility of 2,4-DP-p acid (729 mg/L)

and EHE (170 mg/L), the most sensitive acute and chronic avian toxicity endpoints 

(bobwhite quail, LD50 = 242 mg ae/kg-bw; Japanese quail NOAEC > 244 mg ae/kg-diet) 

and the most sensitive acute and chronic mammalian toxicity endpoints (rat LD50 = 534

mg ae/kg-bw and NOAEL = 40 mg ae/kg-bw). The model indicated that there are acute 

and chronic concerns to birds and mammals through the drinking water exposure 

pathway for the acid and amine forms (the amine form is represented by the acid). There 

are no concerns to birds and mammals from the ester form. Results for 2,4-DP-p acid are 

presented in Tables B.1 and B.2. 

Table B.1. SIP Mammalian Results for 2,4-DP-p

Parameter Acute Chronic

Upper bound exposure (mg/kg-bw) 125.3880 125.3880

Adjusted toxicity value (mg/kg-bw) 410.7317 30.7664

Ratio of exposure to acute toxicity 0.3053 4.0755

Conclusion*

Exposure through drinking 

water alone is a potential 

concern for mammals

Exposure through drinking 

water alone is a potential 

concern for mammals

Table B.2. SIP Avian Results for 2,4-DP-p

Parameter Acute Chronic

Upper bound exposure (mg/kg-bw) 590.4900 590.4900

Adjusted toxicity value (mg/kg-bw) 174.3439 31.8300

Ratio of exposure to toxicity 3.3869 18.5514

Conclusion*

Exposure through drinking 

water alone is a potential 

concern for birds

Exposure through drinking 

water alone is a potential 

concern for birds

*Conclusion is for drinking water exposure alone. This does not combine all routes of exposure. 

Therefore, when aggregated with other routes (i.e., diet, inhalation, dermal), pesticide exposure through 

drinking water may contribute to a total exposure that has the potential for additional effects to non-target 

animals.

STIR (Screening Tool for Inhalation Risk, v1.0) was used to calculate an upper bound 

estimate of 2,4-DP-p exposure via inhalation using 2,4-DP-p’s (acid) molecular weight 

(235.07 g/mole), vapor pressure (1.28 x 10-6 mmHg), high end application rate (5.08 lb 

ae/A), most sensitive acute avian toxicity endpoint (bobwhite quail, LD50 = 242 mg

ae/kg-bw), and most sensitive acute oral and inhalation mammalian toxicity endpoints 

(rat, LD50 = 534 mg ae/kg-bw and 4 hr LC50 > 2.7 mg ae/L, respectively). Inhalation 

alone is not a potential pathway of concern for birds (droplet inhalation) for 2,4-DP-p

acid (Table B.3); exposure is not likely significant for birds. The results for mammals 

also indicated no adverse effects from the inhalation exposure route (Table B.4). 

Exposure is not likely significant for mammals. 
31

Table B.3. STIR Avian (0.020 kg) Results for 2,4-DP-p Acid

Maximum Vapor Concentration in Air at Saturation 

(mg/m3) 1.62E-02

Maximum 1-hour Vapor Inhalation Dose (mg/kg) 2.04E-03

Adjusted Inhalation LD50

6.82E+0

0

Ratio of Vapor Dose to Adjusted Inhalation LD50 2.98E-04

Exposure not Likely 

Significant

Maximum Post-treatment Spray Inhalation Dose (mg/kg) 5.37E-01

Ratio of Droplet Inhalation Dose to Adjusted Inhalation 

LD50 7.87E-02

Exposure not Likely 

Significant

Table B.4. STIR Mammalian (0.15 kg) Results for 2,4-DP-p Acid

Maximum Vapor Concentration in Air at Saturation 

(mg/m3) 1.62E-02

Maximum 1-hour Vapor Inhalation Dose (mg/kg) 2.56E-03

Adjusted Inhalation LD50 1.61E+02

Ratio of Vapor Dose to Adjusted Inhalation LD50 1.59E-05

Exposure not Likely 

Significant

Maximum Post-treatment Spray Inhalation Dose 

(mg/kg) 6.75E-01

Ratio of Droplet Inhalation Dose to Adjusted Inhalation 

LD50 4.20E-03

Exposure not Likely 

Significant