Data Gaps

The fate database is incomplete based on the environmental fate data requirements outlined in 

the Code of Federal Regulations Title 40 Part 158 (40CFR158) Subpart N. Data are needed for 

environmental chemistry methods (with independent laboratory validations; 850.6100), and 

laboratory volatility study (835.1410) in low moisture soil and under high temperature. 

Additionally, a fish BCF study (850.1730) is necessary for the EHE formulation. The log Kow

MCPA EHE is 5.87, indicating a strong potential for bioaccumulation. While EFED believes 

that this potential will be mitigated by metabolism in the receptors, a BCF study that achieves a 

reliable metabolic rate constant (Km) is necessary to confirm this assumption.

The ecological database is also incomplete based on the ecological effects data requirements. At 

this time an avian acute toxicity study (850.2100) with a passerine species is necessary for risk 

assessment. Additionally, only one of the required avian reproduction studies has been 

submitted for MCPA. A study (850.2300) conducted with a water fowl, preferably mallard duck,

is necessary for risk assessment. 

In keeping with the Agency’s proposed new paradigm for determining potential adverse effects 

to insect pollinators from conventional pesticides (USEPA 2012), two additional studies are 

being requested. An acute oral toxicity study (following OECD 213) and a larval toxicity study 

(following OECD 237) are necessary to evaluate the potential for MCPA to adversely affect bees 

and other pollinators. 

MCPA products are not the only source of MCPA in the environment. Another source is the 

phenoxy herbicide, MCPB, which degrades to MCPA. Previously, EFED conducted analysis of 

MCPA and MCPB use patterns (2003b) from the data collected by Thelin and Gianessi (USGS 

Open-File Report 00-250). These data indicated that while MCPB is not used in the highest 

MCPA use areas, there was geographic overlap between the uses of these two groups of 

chemicals. It is likely that some percentage of the MCPA detected in surface water and 

groundwater monitoring data may result from degradation of MCPB to MCPA. Therefore EFED 

seeks relevant usage data to determine what impact the degradation of MCPB to MCPA has on 

an evaluation of monitoring data, estimated drinking water concentration and aquatic EECs, to 

reduce the uncertainty from the previous risk assessment.
1. Introduction

MCPA is an herbicide in the phenoxyaceticacid (aka phenoxy) family that is used postemergence 

for selective control of broadleaf weeds. The purpose of the Problem Formulation is to broadly 

review the registered uses of MCPA, MCPA salts and ester, and identify potentially complete 

environmental exposure pathways that will be assessed during the comprehensive Registration 

Review process. This document specifically addresses the following four forms of MCPA: 

MCPA acid, MCPA sodium salt, MCPA dimethylamine salt (MCPA DMAS), and MCPA 2-

ethylhexyl ester (MCPA 2-EHE). There is a Section 3 registration for MCPA isooctyl ester 

which will also be assessed.

a. Previous Ecological Risk Assessment

The most recent ecological risk assessment on 2-methyl-4-chlorophenoxyacetic acid (MCPA) is 

the EFED Preliminary Risk Assessment for MCPA Reregistration Eligibility Decision (RED) 

Document (US EPA, 2003, D286079). The RED assessment served as the basis for this problem 

formulation document.

At the time of the previous assessment, EPA identified ecological risk of concern (Table 1) to 

plants and terrestrial animals from MCPA uses and particularly to mammals from maximum 

application rates for rice and sorghum. 

Table 1. Summary of Risk Concerns Identified for MCPA in Previous Assessments

Birds Mammals Terr.

Plants

Terr.

Inverts Fish BCF Aquatic Inverts

Aquatic 

Plants

Groundwater 

Contamination Persistence Degradates of Concern

√ a √ b √ -- -- -- √ c √ d √ --?? --

a

 Insufficient chronic data b

 Chronic c

 Listed species from EHE d

 Vascular only except for rice use

b. Drinking Water Exposure Assessments

The latest drinking water assessment was completed on April 9, 2003 (D286079). An addendum 

to this assessment was issued on June 24, 2003 to include a use on rice (D286079). Since that 

time the MCPA task force has voluntarily cancelled the use on rice and sorghum in response to 

the RED (2004).

In the last drinking water assessment, the PRZM/EXAMS generated surface water 

concentrations were recommended for use in the human health risk assessment because

monitoring data available for MCPA were not specific to areas of use of MCPA. The maximum 

acute concentration was 45.9 μg acid equivalents (ae)/l for the Minnesota pasture scenario, while 

the highest chronic concentrations were 2.1 μg ae/l for the 1 in 10 year annual mean 

concentration and 1.3 μg ae/l for the 30 year annual mean for the Pennsylvania pasture scenario 

modeled at 2 lbs ae/acre applied twice at an interval of 30 days. These model estimates were 

approximately double the peak (acute) concentration of 18.58 μg ae/l detected in the monitoring 
data and roughly equivalent to the maximum time weighted annual mean (TWM) concentration 

of 1.49 μg ae/l. 

c. Clean Water Act Programs

MCPA does not have Primary Drinking Water Standards and does not have a set Maximum 

Contaminant Level (MCL) under the Safe Drinking Water Act (SDWA), but its non-cancer 

lifetime limit Health Advisory Levels for a 70-kg adult is set at 0.01 mg/l. The chemical is 

classified Group E, “no evidence of carcinogenicity in at least two adequate animal tests in 

different species or in adequate epidemiologic and animal studies”.

MCPA is not identified as a cause of impairment for any water bodies listed as impaired under 

section 303(d) of the Clean Water Act1

. No Total Maximum Daily Load (TMDL) criteria have 

been developed for MCPA. Aquatic benchmarks have been established for all registered forms 

of MCPA and are available at 

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. 

2. Stressor Source and Distribution

Registered agricultural use sites of MCPA include: barley, flax, grasses grown for seed, oats, 

pastureland, peas, rye, and wheat. The registered non-agricultural use sites of MCPA include: 

nonagricultural areas, uncultivated areas/soils, ornamental lawns and turf. (BEAD Chemical 

Profile, 2013). Single Application rates of up to 2.0 lbs ae/A are allowed by labels.

There are four primary active ingredients associated with MCPA (Table 2): MCPA acid, MCPA 

sodium salt, MCPA dimethylamine salt (MCPA DMAS), and MCPA 2-ethylhexyl ester (MCPA 

2-EHE). A fifth active ingredient, MCPA isooctyl ester will also be assessed. Formulation types 

for registered products include solids, soluble concentrate/solid, water dispersible granules (dry 

flowable), and wettable powder. MCPA is usually applied in combination with other phenoxy 

class chemicals, such as 2,4-D, 2,4DB, MCPP-p, and MCPB (2004). According to product

labels, MCPA can be applied anytime of year, but for best efficacy application is recommended 

in early spring and early fall.

 1

http://iaspub.epa.gov/tmdl_waters10/attains_nation_cy.cause_detail_303d?p_cause_group_id=885
Table 2. MCPA, MCPA Salt, Amine Salt, and Ester

PC Code Names (CAS and IUPAC)1 Abbreviation

Acid

030501 2-methyl-4-chlorophenoxyacetic acid (MCPA) Acid/MCPA

Salts

030502 Sodium salt of MCPA: Na 

Amine Salts

030516 Dimethylamine salt of MCPA: DMAS

Esters

030564 2-ethylhexyl ester of MCPA:

2-ethylhexyl 2-(4-chloro-2-methylphenoxy)acetate

(RS)-2-ethylhexyl 4-chloro-o-tolyloxyacetate

2-EHE

1

Selected physico-chemical properties of MCPA forms can be obtained from Appendix A

Various phenoxy herbicides are known contain dioxin/furan impurities as a result of 

manufacturing processes (Holt, et. al. 2010). Data pertaining to dioxin/furan impurities in

MCPA formulations (up to 17 known dioxin/furan congeners) will be considered in the 

upcoming environmental risk assessment. Previously submitted dioxin/furan data for MCPB 

were below levels of quantitation (LOQ) (MRIDs 45524105 and 47374801). However, the 

Agency is aware that lower limits of quantitation of dioxins/furans by one or more orders of 

magnitude are now achievable through technological advances in analytical chemistry. 

Therefore, the Agency requests any additional information on the presence of dioxins/furans 

impurities in MCPA formulated products that may be available based on currently achievable 

LOQs. In absence of additional information, the Agency will use best available information 

(including appropriate assumptions related to reported LOQs) for determining the potential for 

exposures of concern at the time of risk assessment.

3. Environmental Fate and Transport

The agency established a strategy for bridging the environmental fate data requirements for the 

MCPA ethylhexyl ester (EHE) and MCPA dimethylamine salt (DMAS) to the MCPA acid. 

Bridging data verified that MCPA DMAS and MCPA EHE will be rapidly converted to the free 

acid in the environment. A selection of environmental fate and transport properties of MCPA is 

summarized in Table A.2 of the Appendix A.
In aqueous environments, MCPA-DMAS will completely dissociate to form MCPA (USEPA 

2004b) while MCPA EHE will hydrolyze to MCPA acid. MCPA EHE hydrolysis appears to be 

pH dependent. It hydrolyzed with half-life <117 hours at pH 9, much slower at pH 7, and was 

stable at pH 5. In a non-sterile soil:CaCl2 system at pH 5.6 and 6.8, MCPA EHE was initially 

adsorbed to the soil particles, and then available for degradation to MCPA acid with a half-life of 

less than 12 hours. A soil microbial degradation study, conducted under OECD GLP guidelines 

and called soil hydrolysis, indicated that MCPA EHE underwent microbial degradation to 

MCPA with half-lives ranging from 4.5 to 12 hours in two tested soils under 50% or 70% of the 

maximum water holding capacity (MWHC).

There may be environments, however, where this bridging strategy is not applicable, such as dry 

soils which may limit the dissociation of the MCPA DMAS and acid environments which may 

limit the abiotic hydrolysis of the MCPA EHE. There is also evidence for the phenoxy esters as 

a class to suggest that the conversion of MCPA EHE to MCPA acid may not be rapid under all 

conditions. For example, according to Smith and Hayden (1980) data, MCPA EHE which was 

surface applied to soils in Saskatchewan was rapidly converted to MCPA acid, but under dry 

conditions (15% of field capacity) the ester persisted for days with greater than 90% present after 

48 hours. In a typical setting a farmer will irrigate to add moisture to the soil or abandon the 

crop. Therefore, EFED believes the available data support the environmental fate bridging 

strategy for MCPA DMAS and MCPA EHE with the exception of environmental conditions 

previously described. 

MCPA acid is moderately stable in the natural environment and mobile. Based on the available 

data, MCPA in soil degrades mainly through aerobic soil metabolism, and in aquatic 

environments through photodegradation and aerobic aquatic metabolism. It can enter surface 

water via spray drift during application and run off after rainfall event. In the field it showed a 

low potential for leaching as it was not detected at soil depths greater than six inches. MCPA 

has been shown to be stable in the anaerobic soil and in aerobic aquatic environment.

MCPA acid did not hydrolyze in sterile, buffered solutions at pH's ranging from 5 to 9 (MRID 

42665301). In sterile buffer solution at pH 5 when irradiated with natural sunlight MCPA acid 

photodegraded with a half-life of approximately 25 days; forming one degradate, 4-chloro-ocresol (4-CC). 4-CC comprised up to 12% of the initial radioactivity. MCPA acid 

photodegraded slowly when applied to soil surfaces and irradiated with natural sunlight; the 

calculated half-life was 67 days. In the aerobic soil metabolism study MCPA acid degraded with 

a half-life of 24 days (MRID 41586001). No major degradates were present, but 14CO2 accounted 

for 64% of the initial applied radioactivity after 209 days of aerobic incubation. Under aerobic 

aquatic conditions MCPA acid degraded with an observed degradation half-life of >30 days in 

the water-sandy clay loam sediment system, and half-life values of 13 to 15 days in water-loamy 

sand sediment system. The observed degradation half-life of MCPA acid in the water-light clay 

sediment system was >100 days in the total system. In another aerobic aquatic biodegradation 

study MCPA acid degraded in a ditchwater/sediment system from the Netherlands with a whole 

system half life of 16.3 days and in a river water/sediment system from Netherlands with whole 

system half life of 16.8 days. MCPA acid did not degrade anaerobically in either an anaerobic 

soil metabolism or an anaerobic aquatic metabolism study.
Three laboratory batch equilibrium studies indicated that MCPA acid is mobile (overall 

Kf-ads = 0.021-1.2 and Kfoc = 8.9-59 mL/g). MCPA Freundlich Kads values ranged from 0.45 to 

1.20 mL/g in clay loam, silt loam, sandy loam soils, in a sandy loam aquatic sediment and in a 

beach sand in one study (MRID 42596903). In second study Freundlich Kads values ranged from 

0.0212 to 1.11 mL/g and corresponding Kf-oc values were from 9.6 to 46 in clay, silt loam, sandy 

loam and loam soils (MRID 40555801). In the last study, uniformly phenyl ring-labeled 

[

14C]MCPA acid was studied in sand (0.7% o.m.), sandy loam, loam and silty clay loam 

soil:solution slurries and sandy clay loam sediment:solution slurries (1.7% o.m.) and its 

FreundlichKads values ranged from 0.21 to 0.55. Corresponding Koc values ranged from 31 to 

59 mL/g (MRID 44239601). The study also shown that MCPA adsorption could not be 

correlated with soil organic matter, pH, or clay content. 

In various terrestrial field dissipation studies (TFD), MCPA dissipated with half-lives ranging 

from 3.3 to 23 days with a median half-life of 5.6 days. When the product was applied as MCPA 

EHE, the parent dissipated with half-lives of 4 to 9 days (MRID 43815701) while MCPA 

isooctyl ester dissipated with half-lives of 9 and 23 days (MRIDs 42134001 and 42134101). 

When the product was applied as MCPA DMAS, the parent dissipation half-lives ranged from 3 

to 10 days (MRIDs 43883001 and MRID 43697501). Total MCPA residues dissipated in the 

grass and thatch with registrant-calculated half-lives of 8.5to 15.5 days on grass and 9.6 to 11 

days on thatch following the second application of MCPA DMAS to CA, FL, and NY plots 

(MRID 43883001). Sodium salt of MCPA dissipated with a half-life of 15 days from a barley 

plot in Washington State (MRID 42133901). All concentrations in the TFD studies were reported 

as total MCPA residues. There was no obvious leaching of MCPA observed below 6 inches of 

soil depth in any of the mentioned studies with the exception of the single replicates as indicated 

by data in the Table A.2.

Aged column leaching studies and bioconcentration in fish studies are not available. Based on 

physico-chemical properties (log Kow = 2.83) MCPA itself has low potential for bioaccumulation 

hence fish bioconcentration studies are not requested at this time. However, a fish BCF study 

(850.1730) is necessary for the EHE formulation. The log Kowfor MCPA EHE is 5.87, 

indicating a strong potential for bioaccumulation. While EFED believes that this potential will 

be mitigated by metabolism in the receptors, a BCF study that achieves a reliable metabolic rate 

constant (Km) is necessary to confirm this assumption. 

a) Atmospheric Transport

It is known that the various phenoxy ester forms are volatile to various degrees depending on the 

ester and will potentially volatilize both during and after their application. The degree of 

volatility of the phenoxy ester varies depending on the molecular weight of the alcohol used to 

make the ester. The overall rule is the heavier molecules are less volatile than lighter molecules. 

Current open literature will be researched further to address the knowledge gap of potential 

MCPA atmospheric transport. The search emphasis will be placed on the short term transport, 

droplet drift at time of application, and long term transport, evaporation from the droplet and 

from the surface of the soil and plants after the spray has landed after application. 
A volatility study indicated that MCPA does not have potential to volatilize off soil surface. In 

the laboratory study conducted on MCPA 2-EHE applied to sandy clay loam soil (pH 6.7, 

organic carbon 2.2%) that was microbially active and adjusted to 40-60% of 0.33 bar moisture, 

no volatile organics were produced during the 15-day study (MRID 47075203). Based the study, 

the majority of radioactivity (ca. 97-100%) of MCPA 2-EHE converted to MCPA acid. Since the 

study was conducted at 25°C and the relative humidity was 95-100%, the conditions are 

favorable to abiotic and/or microbial hydrolysis of the ester form to acid. There is evidence for 

the phenoxy esters as a class to suggest that the conversion of MCPA EHE to MCPA acid may 

not be rapid under all conditions; e. g. low moisture soils, as discussed previously. Microbial 

mediated hydrolysis rates in soils may be dependent on clay mineralogy, organic carbon content, 

temperature, and moisture content (Wolfe, et al, 1989 and Wolfe, 1990). Although, the open 

literature data suggest that under normal environmental conditions MCPA EHE will be expected 

to rapidly convert to MCPA acid, analysis of terrestrial field dissipation data collected for 2,4-D 

EHE indicates that the ester remains in the field with half lives between one and 14 days. That 

increases the time when the chemical can be available for evaporation of the soil surface.

a. Transformation Products

Several degradates were detected in the laboratory fate studies. The degradates detected were 4-

chloro-o-cresol (4-CC), 5-chlorosalicylaldehyde, and 14CO2. 4-chloro-2-methylanisole (4-MCA) 

was postulated by the registrant to be a potential degradate of MCPA but was not 

detected/identified in any of the laboratory or field studies. At the time of completion of the 

RED in 2004, the Metabolite Assessment Review Committee (MARC) has determined that none 

of these degradates are of toxicological concern, therefore, no degradates were included in the 

previous risk assessments. The potential inclusion of degradates may be revisited at the time of 

risk assessment but it is not expect that our previous conclusion will change.

Note that 4-CC is a common degradate for MCPA and 2,4-D. The chemical is also used in 

synthesis of phenoxy herbicides; i.e. MCPA, MCPB, MCPP (EUR, 2002). In addition, MCPA is 

a major degradate of MCPB formed in soil. The chemical structures for 4-CC, 5-

chlorosalicylaldehyde, and 4-chloro-2-methylanisole (4-MCA) are provided in Table A.3 of 

Appendix A. In addition, various phenoxy herbicides are known contain dioxin/furan impurities 

as a result of manufacturing processes (Holt, et. al. 2010). 

4. Receptors

Table 3 and Table 4 provide a summary of the most sensitive aquatic and terrestrial acute and 

chronic endpoints that will be used in the risk assessment. In addition, the tables provide a 

preliminary overview of the potential acute toxicity of MCPA by providing the acute toxicity 

classifications. There are no submitted data on the potential toxicity of the degradates on either 

aquatic or terrestrial organisms. 
4.1. Effects to Aquatic Organisms

In the available studies for fish and invertebrates, most of the toxicity endpoints are within one 

order of magnitude when restricted to evaluating the MCPA acid, sodium salt and DMAS. The 

toxicity of MCPA EHE tends to be two to three orders of magnitude greater than the toxicity of 

the acid and salts. No EHE studies regarding chronic exposure are available; in the absence of 

these data, EFED will consider the use of acute-to-chronic ratios when possible. As expected 

with an herbicide, effects to vascular and nonvascular plants are evident in the available studies.

In the RED risk assessment, EFED hypothesized that the primary reason for the differences in 

the levels of toxicity between the ester formulation relative to the salts and acid is because of the 

fact that esters have a greater affinity for uptake through cell wall membranes. The increased 

toxicity, coupled with the fact that MCPA EHE may be more stable in acidic aquatic 

environments is explanation for the inclusion of the ester drift scenario in the ecological risk 

assessment. EFED assumes that this scenario only applies to acute exposure because biotic 

processes should result in conversion of MCPA EHE to MCPA acid after an initial acute 

exposure. 

The most sensitive of the available endpoints from the acid, salt or DMAS and the endpoint for 

the EHE (if available) are presented in Table 3. For the isooctyl ester, toxicity will be considered 

equivalent to the EHE where data are lacking.

No studies have been submitted for degradates of MCPA. However, a risk assessment conducted 

by the EU is available for the 4-CC degradate (2002), which indicates there is potential toxic 

effects to aquatic organisms for this degradate. The current thinking is that exposure to this 

degradate will be below the toxic thresholds indicated in the report, but EFED will explore this 

potential exposure in the risk assessment.

The aquatic effects dataset for MCPA is largely complete and further data are not being 

requested at this time.

Table 3. Summary of the Most Sensitive Endpoints from Aquatic Toxicity Studies for MCPA.

Taxonomic 

Group

Study 

Type

Surrogate Species Toxicity 

Value

Acute 

Toxicity 

Classification 

Source

(Study 

classification)

Freshwater fish1 Acute

Rainbow trout 

(Oncorhynchus

mykiss)

Bluegill Sunfish

(Lepomismacrochirus)

LC50= 96 mg 

ae/L (DMAS)

LC50= 0.76 mg 

ae/L (EHE)

Practically 

nontoxic

Highly toxic

40062005

(Acceptable)

42624404

(Acceptable)
Taxonomic 

Group

Study 

Type

Surrogate Species Toxicity 

Value

Acute 

Toxicity 

Classification 

Source

(Study 

classification)

Chronic 

(Early 

LifeStage)

Fathead Minnow 

(Pimephalespromelas)

NOAEC = 12

mg ai/L

LOAEC = 29

mg ai/L

Endpoint: 

reduction in 

growth

(DMAS)

-- 44407202

(Acceptable)

Freshwater 

invertebrates

Acute Water Flea 

(Daphnia magna)

EC50>82 mg 

ae/L (DMAS)

EC50> 0.18 mg 

ae/L (EHE)

Slightly toxic

Highly toxic

41800906

(Acceptable)

42613601

(Acceptable)

Chronic Water Flea 

NOAEC = 11

mg ae/L

LOAEC = 22

mg ae/L

Endpoint: 

reproduction

--

44407201

(Acceptable)

Estuarine/marine 

fish

Acute

Atlantic silverside

(Menidia menidia)

Sheepshead Minnow 

(Cyprinodon

variegatus)

LC50= 179 mg 

ae/L (Acid)

LC50> 2.7 mg 

ae/L (EHE)

Practically 

nontoxic

Moderately 

toxic

40062002

(Acceptable)

43086501

(Acceptable)

Chronic No data

Estuarine/marine 

invertebrates

Acute

Eastern Oyster 

(Cassostrea virginica; 

shell deposition)

Mysid (Americamysi

sbahia)

EC50 = 4.9 mg 

ae/L (Salt)

LC50 = 4.2 mg 

ai/L(EHE)

Moderately 

toxic

Moderately 

toxic

41939506

(Acceptable)

43086502

(Acceptable)

Chronic No data

Aquatic plants Vascular Duckweed 

(Lemna gibba)

NOAEC = 

0.016mg ae/L

IC50 =0.13mg 

ae/L (DMAS)

NOAEC = 

0.007 mg ae/L

IC50 =0.02 mg 

ae/L (EHE)

--

44903501

(Acceptable)

43083214

(Acceptable)
Taxonomic 

Group

Study 

Type

Surrogate Species Toxicity 

Value

Acute 

Toxicity 

Classification 

Source

(Study 

classification)

Nonvascular

Green algae

(Pseudokirchneriella

subspicatus)

Diatom

(Skeletonema

costatum)

NOAEC < 

0.027 mg ae/L

IC50 =0.16 mg 

ae/L (DMAS)

NOAEC < 

0.0019mg ae/L

IC50 =0.056mg 

ae/L (EHE)

--

42461301

(Supplemental)

44287310

(Supplemental)

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

4.2. Effects to Terrestrial Organisms

For terrestrial organisms, there does not appear to be differentiation in the toxicity levels for 

MCPA among the acid, salts, and ester based on the available data. Within an organism group 

(i.e., birds, mammals, honey bee, monocot plants, and dicot plants), the variation in the toxicity 

endpoints is less than two orders of magnitude, and for some groups, the variation is less than 

one order of magnitude. For those organism groups for which toxicity data were submitted to 

the Agency for MCPA EHE, there does not appear to be an increase in the toxicity relative to the 

acid and salts.

Across all forms, MCPA is classified as moderately toxic to birds, slightly toxic to mammals on 

an acute exposure basis and practically nontoxic to honey bees on an acute contact basis. As 

expected with an herbicide, MCPA adversely affects plants. Effects were seen in both the 

seedling emergence and vegetative vigor studies and in both monocots and dicots well below the 

maximum label rates. 

The most sensitive available endpoints across all studies for each taxon are reported in Table 4.

Table 4. Summary of the Most Sensitive Endpoints from Terrestrial Toxicity Studies for Propazine

Taxonomic 

Group

Study 

Type

Surrogate Species Toxicity Value Acute 

Toxicity 

Classification

Source 

(Study 

classification)

Birds

Acutea Bobwhite quail

(Colinusvirginianus)

LD50 = 221 mg 

ae/kg-bw

Moderately 

toxic

40019202

(Acceptable)

Sub-acute 

dietary

Mallard duck

(Anasplatyrhynchos)

LC50> 4608 mg 

ae/kg-diet

Practically 

non-toxic

40555802

40555803

(Acceptable)

Chronicb Bobwhite quail

NOAEC = 1000mg 

ae/kg-diet

LOAEC> 1000mg 

ae/kg-diet

-- 43505201

(Acceptable)
Taxonomic 

Group

Study 

Type

Surrogate Species Toxicity Value Acute 

Toxicity 

Classification

Source 

(Study 

classification)

Mammals

Acute Oral Laboratory rat 

(Rattusnorvegicus)

LD50 = 1383 mg 

ae/kg-bw Slightly toxic Acc. 21972

(Acceptable)

Chronic Laboratory rat 

NOAEL = 150mg 

ae/kg-diet

LOAEL = 450 mg 

ae/kg-diet

Endpoint: ovary 

weights and 

decreased pup 

weight gain

-- 40041701

(Acceptable)

Insects Acute 

contact

Honey bee 

(Apismellifera) LD50 > 17 μg ai/bee Practically non-toxic

42197801

(Acceptable)

Terrestrial 

plants

Seedling 

Emergence

Dicot (Cabbage; 

Brassica oleracea)

IC25=0.008 lb ae/A

NOAEC = 0.0027 

lbs ae/A

--

43083205

(Acceptable) Monocot (Onion; 

Allium cepa)

IC25= 0.028 lb ae/A

NOAEC = 0.012

lbs ae/A

--

Vegetative 

Vigor

Dicot (Lettuce; 

Lactuca sativa

IC25 = 0.04 lb ae/A

NOAEC = 0.0004

lb ae/A

--

47216806

(in review) Monocot (Onion; 

Allium cepa)

IC25 = 0.35 lb ae/A

NOAEC = 0.09 lbs 

ae/A

--

a

data for passerine necessary

b

no data for mallard duck; therefore endpoint remains uncertain

4.3. 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 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. Further, some incidents may be reported 

as ‘aggregate incidents’; these are contained in a separate database.

As of 01/08/2014, there are 60 reported incidents in EIIS for MCPA; seven for MCPA (under PC 

Code 030501), 20 for MCPA DMAS (under PC Code 030516) and 33 for MCPA EHE (under 

PC Code 030564). All of the reported incidents involved damage to plants and most are ascribed 

to registered uses. Incidents were reported for agricultural, residential and vegetative 
management uses. Additionally, 68 incidents (across all forms) are reported in the aggregate 

incident database, all for plant damage.

As of 01/08/2014, AIMS identified no ecological incidents involving any form of MCPA.

5. Exposure Pathways of Concern

The environmental fate and toxicity profile for MCPA suggests that direct spray, spray drift, 

leaching and runoff represent the likely exposure pathways for terrestrial and aquatic non-target 

organisms. Drinking water and inhalation exposure pathways were screened using the SIP 

(Screening Imbibition Program) and STIR (Screening Tool for Inhalation Risk) screening 

methods and were found to be exposure pathways of low concern (no LOC exceedances are 

expected). SIP and STIR are described in detail at: 

http://www.epa.gov/oppefed1/models/terrestrial/index.htm.

Risk Hypothesis

The Registration Review ecological risk assessment will test the following risk hypothesis:

Based on the application methods, mode of action, fate and transport, and the 

sensitivity of non-target aquatic and terrestrial species, MCPA has the potential to 

reduce survival, reproduction, and/or growth in non-target terrestrial and aquatic 

organisms as well as negatively affect the structure, productivity, and function of 

plant communities when used in accordance with the current labels. These nontarget organisms include Federally-listed threatened and endangered species as well 

as non-listed species.

6. Analysis Plan

This analysis plan may be revised depending upon a full review of the data available in the open 

literature and the information submitted by the public in response to the opening of the 

registration review docket.

a. Stressors of Concern 

The stressors of ecological concern for terrestrial and aquatic organisms are the four forms of 

MCPA. MCPA acid and salts are classified as practically non-toxic to moderately toxic, while 

the ester formulation of MCPA is classified as moderately toxic to highly toxic. Consequently, 

the esters will be considered separately for aquatic organisms, as has been done in the past risk 

assessment (USEPA 2003b). For terrestrial scenarios, the most sensitive endpoint, regardless of 

the MCPA form (ester, salt, acid) will be used in the risk assessment.

There are few toxicity data on degradates of MCPA. Previous assessments have considered the 

likelihood of exposure to MCPA degradates in significant quantities to be minimal. However, 
any information regarding the toxicity of MCPA degradates will be considered for the risk 

assessment. Information in this regard will reduce uncertainty in risk conclusions. EFED will 

consider the toxicity of the 4-CC degradate, as indicated the European risk assessment cited 

earlier, relative to the potential exposure to aquatic organisms in the risk assessment.

b. Measure 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.

c. Measure 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 ECOTOXicologydatabase (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.

d. Drinking Water Assessment

The drinking water assessments conducted to support future human health risk assessments of 

MCPA will incorporate modeling of MCPA acid form in surface and ground water. Since the last 

drinking water assessment was conducted, EFED has updated its models (PRZM-EXAMS), the 

drinking water modeling input parameters, and the percent crop area (PCA) guidelines. For all 

modeling, the input parameters will be selected according to the current input guidelines, and 

where in question a conservative approach will be taken in the selection of half-lives. In addition, 

the newly available PRZM-GW model will be utilized to obtain estimated drinking water 

concentrations (EDWCs) from ground water source. Descriptions of all executable versions of 

current models for drinking water exposure can be obtained from the following URL link: 

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

EFED will consider all ground and surface water monitoring data available to us, and will 

conduct a comprehensive evaluation of the data for the risk assessment. The data that may be 

considered include the United States Geological Survey (USGS) National Water-Quality 

Assessment (NAWQA) Program monitoring data.

It is likely that some unknown percentage of the MCPA detected in surface water and 

groundwater monitoring data may result from degradation of MCPB to MCPA. Therefore, in the 

event of uncertainties regarding MCPA and MCPB use pattern overlap, EFED will assess an 

impact the degradation of MCPB to MCPA on monitoring data, EDWCs, and aquatic EECs, in a 

conservative manner.
7. Endangered Species Assessment

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

Agency will evaluate risks to federally 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.

8. Preliminary Identification of Data Gaps

a. Environmental Fate

Table 5 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 to 

support the risk assessment.

Table 5. Submitted Environmental Fate Data for MCPA, MCPA sodium, MCPADMAS, and MCPAEHE.

OCSPP 

Guideline

Data 

Requirement

Submitted 

Studies 

(MRID)

Study 

Classifications

Sufficient 

for 

EFED?

Rationale for Additional Data Need

Not 

available Dissociation 42457101 Acceptable Not Applicable

835.2120 Hydrolysis

42693901A

42671501

42665301A

Acceptable

Supplemental

Acceptable

Yes Not Applicable

835.2240 Aqueous 

photolysis 42928101 Acceptable Yes Not Applicable

835.2410 Soil photolysis 43225801 Acceptable Yes Not Applicable

835.4100 Aerobic soil 

metabolism

41586001

47744901

Acceptable

Supplemental Yes Not Applicable

835.4200 Anaerobic soil 

metabolism 41586001 Acceptable Yes Not Applicable

835.4300 Aerobic aquatic 

metabolism

44732401A/

44192701

44732401B

45889303

Acceptable

Acceptable

Acceptable

Yes Not Applicable

835.4400

Anaerobic 

aquatic 

metabolism

40461901 Acceptable Yes Not Applicable
Table 5. Submitted Environmental Fate Data for MCPA, MCPA sodium, MCPADMAS, and MCPAEHE.

OCSPP 

Guideline

Data 

Requirement

Submitted 

Studies 

(MRID)

Study 

Classifications

Sufficient 

for 

EFED?

Rationale for Additional Data Need

835.1230

835.1240

Adsorption/ 

desorption 

42596903

40555801

44239601

47075204

Acceptable

Supplemental

Supplemental

SupplementalA

Yes No Applicable

Leaching 

835.1410 Volatilitylaboratory 47075203 SupplementalA No

In the study only moist soil at 25 ± 2°Cwas 

tested, dry soil, with sufficient moisture to 

still maintain microbial activity, and most 

soil in a higher temperature are 

recommended to be tested.

835.8100 Volatility Field Reserved

835.6100 Terrestrial field 

dissipation

42133901

42134001

42134101

42134201

43815701

43883001

43697501

Supplemental

Supplemental

Supplemental

Unacceptable

Acceptable 

Acceptable

Acceptable

Yes Not Applicable

835.6200 Aquatic field 

dissipation 46875802 Waived Yes Not Applicable

850.6100

Analytical 

method in soil 42134001 06/05/1990 No

The environmental chemistry method 

(EMC) for soil is from 1990(GC/Hall 

Detector) and at that time an independent 

laboratory validation method (ILV) was 

not submitted. A current ECM and ILV for 

MCPA residues in water and sediment are 

required to support current monitoring for 

MCPA residues and to evaluatethe newly 

submittedenvironmental fate and 

ecological effects data.

Analytical 

method in water 

and sediment.

44192701 12/06/1996 No

External Validation of a Method for the 

Determination of 4-Chloro-2-

methylphenoxyacetic Acid Dimethylamine 

Salt(MCPA DMAS) as its Acid 

Equivalent, 4-Chloro-2-

methylphenoxyacetic Acid (MCPA), and 

4-chloro-2-methylphenoxyacetic Acid 2-

Ethylhexyl Ester(MCPA 2-EHE) in Water 

Samples by Gas Chromatography with 

Mass Selective Detection (GC/MCD).

The EMC and ILV for water are 

from1996. The updated ECM and ILV for 

MCPA residues in water and sediment are 

required to support current monitoring for 

MCPA residues and to evaluatenewly 

submitted environmental fate and 

ecological effects data.
Table 5. Submitted Environmental Fate Data for MCPA, MCPA sodium, MCPADMAS, and MCPAEHE.

OCSPP 

Guideline

Data 

Requirement

Submitted 

Studies 

(MRID)

Study 

Classifications

Sufficient 

for 

EFED?

Rationale for Additional Data Need

165-4 

(850.1730)

Fish 

bioconcentration --- ---- No

A fish BCF study is necessary for the EHE 

formulation. The log Kow MCPA EHE is 

5.87, indicating a strong potential for 

bioaccumulation. While EFED believes 

that this potential will be mitigated by 

metabolism in the receptors, a BCF study 

that achieves a reliable metabolic rate 

constant (Km) is necessary to confirm this 

assumption. A After further review the studies were downgraded from acceptable to supplemental.

Volatility-Laboratory:

Guideline 835.1410

In the available study (MRID 47075203) only moist soil at 25±2oC was tested. Dry soil, with 

sufficient moisture to still maintain microbial activity, and moist soil at a higher temperature are 

recommended to be tested. The processes by which pesticides may be transported away from the 

target site include spray drift at the time of application and volatilization. Transport resulting 

from volatilization is highly dependent on the properties of the active ingredient (e.g. vapor 

pressure) and as well as a number of environmental parameters. Two of those parameters are 

temperature and soil moisture. The magnitude and impact of potential transport of MCPA esters 

via volatilization or long range drift away from the target site was an uncertainty in the previous 

assessment (US EPA, 2004). Reported evidence in the public literature suggests concern for 

impact to non-target organisms due to drift and volatilization of the ester formulations of the 

phenoxy herbicides (2003c). In the submitted study, the majority of radioactivity (ca. 97-100%) 

of MCPA 2-EHE converted to MCPA acid. Open literature data (US EPA, 2003c) suggests that 

thephenoxy herbicide esters may not convert rapidly under dry soil conditions to the acid form. 

In absence of the data it is assumed that MCPA has potential to volatilize of soil surface.

Environmental Chemistry Methods and Associated Independent Laboratory Validation: 

Guideline 850.6100

The environmental chemistry method for MCPA residues of concern in soil, water, and sediment 

as well as independent laboratory validation of the methods are required to support monitoring 

for MCPA residues and to evaluate newly submitted environmental fate and ecological effects 

data. Independently validated analytical methods for residues in soil, water, and sediment, and 

plant material (environmental chemistry 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 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. Presently, the available 

methods from 1990 and 1996 do not fulfill the above requirements.
An independent validation of environmental chemistry method for MCPA residues in soil has 

not been submitted by the registrant. The original methods for water and soil were submitted in 

1994 and 1990. An independent validation performed for water method by Wildlife 

International Ltd. in 1996 used only 2 reps for the MCPA 2-EHE. This is lower than the four 

typically requested for an ILV.

Fish BCF: Guideline 850.1730

A fish BCF study is necessary for the EHE active ingredient. The log Kow for MCPA EHE is 

5.87, indicating a strong potential for bioaccumulation. While EFED believes that this potential 

will be mitigated by metabolism in the receptors, a BCF study that achieves a reliable metabolic 

rate constant (Km) is necessary to confirm this assumption.

b. Effects

The effects database for MCPA formulations is not complete based on the ecological effects data 

requirements. At this time an avian acute toxicity study (850.2100) with a passerine species is 

necessary for risk assessment. Additionally, only one of the required avian reproduction studies 

has been submitted for MCPA. A study (850.2300) conducted with a waterfowl species (e.g.

mallard duck) is necessary for risk assessment. The endpoints presented in Tables 3 and 4

represent the most sensitive currently available, not necessarily those that will be used in the risk 

assessment. 

In keeping with the Agency’s proposed new paradigm for determining potential adverse effects 

to insect pollinators from conventional pesticides (USEPA 2012), two additional studies are 

being requested. An acute oral toxicity study (following OECD 213) and a larval toxicity study 

(following OECD 237) are necessary to evaluate the potential for MCPA to adversely affect bees 

and other pollinators. Although the mode of action and acute contact toxicity would not suggest 

substantial toxicity to adult and larval bees via oral routes, it may not be the only mode of action 

relevant to non-target species. Also, several the target crops are considered attractive to honey 

bees and this is a systemic chemical that is transported throughout the plant. Thus, MCPA 

residues may be available to pollinators via pollen and/or nectar.

Due to the large number of studies regarding effects of MCPA formulations to various taxa, 

please see Appendix B for a complete summary of available studies and their classifications.

Avian Acute Toxicity: Guideline 850.2100

Data regarding the acute toxicity of MCPA to a passerine species is necessary for risk 

assessment. A proposed protocol should be submitted for review prior to conducting the study.

Avian Reproduction: Guideline 850.2300

Data from an avian reproduction study with a waterfowl species (e.g. mallard duck) is necessary 

for risk assessment. Although data from bobwhite is available, it remains unknown whether 

mallard, a required test species, is more sensitive to MCPA than bobwhite quail.
9. References

EUR (2002).4-CHLORO-O-CRESOL European Union Risk Assessment Report, Valium 1. 

Office for Official Publications of the European Communities, ISBN 92-894-0809-X.

Holt. E., Weber, R., Stevenson, G., and Gaur, C. 2010. Polychlorinated Dibenzo-p-Dioxins and 

Dibenzofurans (PCDD/Fs) Impurities in Pesticides: A Neglected Source of Contemporary 

Relevance. Environ. Sci. Technol. 44, 5409-5415.

Smith, A.E. and Hayden B.J. (1980). Hydrolysis of MCPA Esters and the Persistence of MCPA

in Saskatchewan Soils. Bull. Environm. Contam. Toxicol., 25, 369-373. 

US EPA (2003a). MCPA – Drinking Water Assessment for the Health Effects Division (HED) 

Reregistration Eligibility Decision Document.April 9, 2003.D286079

US EPA (2003b). Addendum to the Drinking Water Assessment for MCPA Transmitted

to the Health Effects Division (HED) to Account for Use on Rice. June 24, 2003.D286079

US EPA (2003c). Environmental Fate and Effects Division Preliminary Risk Assessment for

the 2-methyl-4-chlorophenoxyacetic acid (MCPA) Reregistration Eligibility Decision Document. 

September 5, 2003. D286079

US EPA (2004). Reregistration Eligibility Decision (RED) for MCPA (2-methyl-4-

chlorophenoxyacetic acid) List A Case 0017. September 30, 2004.

USEPA. 2012. White Paper in Support of the Proposed Risk Assessment Process for Bees.

Submitted to the FIFRA Scientific Advisory Panel for Review and Comment September 11 – 14, 

2012. Office of Chemical Safety and Pollution Prevention Office of Pesticide Programs 

Environmental Fate and Effects Division, Environmental Protection Agency, Washington DC; 

Environmental Assessment Directorate, Pest Management Regulatory Agency, Health Canada, 

Ottawa, CN; California Department of Pesticide Regulation 

http://www.regulations.gov/#!documentDetail;D=EPA-HQ-OPP-2012-0543-0004

USEPA (2013). BEAD Chemical Profile (BCP) for Registration Review: MCPA Case (030501,

030511, 030502,030516,030563 and 030564). December 12, 2013
Appendix A. Physical Parameters of MCPA and its Forms, and MCPA Degradates

Table A.1. Selected physical chemical properties of various forms of MCPA1

PC Code 

Active 

Ingredient 

Molecular 

Formula and 

weight (g/mol)

Solubility Density

(g/cm3) Log Kow

Vapor Pressure 

(mm Hg@ ≈25°C)

030501

MCPA acid

C9H9ClO3

200.6 0.03g/100g 1.18-1.21 2.828 7.7 x 10-6

mbar at 20°C

3.12E -4 torr2

030502

MCPA Na 

Salt

C9H8ClNaO3

222.6 Water soluble NA Na salt dissociate to acid

030516

MCPA DMAS

C11H16ClNO3

245.7 NA 1.181 1.415 (25oC) NA

030564

MCPA 2-EHE C17H25ClO3

312.8

69 x 10-6 g/L (pH 5)4

57 x 10-6 g/L (pH 7)4

110 x 10-6 g/L(pH 9)4

1.06 5.37

1.77 x10-5 mbar 

8.43 E-6 mm Hg3

2E-06 mm Hg at 18°C5

1E-04 mm Hg at 45°C5

1 Data assembled for RED (USEPA, 2004)

2 Product Chemistry

3

Estimated by EpiSuite Software

4

Values at 20°C; MRID 47075204 5 MRID 47075203

NA Not available

Table A.2. Physical Chemical and Environmental Fate Properties of MCPA.

Parameter Value Source

Selected Physical/Chemical Parameters

Chemical Structure1

TOXNET

IUAPC Name 4-chloro-o-tolyloxyacetic acid U.S. EPA, 2003b

CAS Name (4-chloro-2-methylphenoxy)acetic acid U.S. EPA, 2003b

CAS No. 94-74-6 U.S. EPA, 2003b

Molecular Weight (molecular 

formula)

200.6 g/mol (C9H9ClO3) U.S. EPA, 2003b

Smiles Code Clc1cc(c(OCC(=O)O)cc1)C IUPAC web site

Physical state colorless crystal

Vapor pressure (25°C) 0.000312 mm Hg U.S. EPA, 2003b

Aqueous solubility (20°C) 1500 ppm U.S. EPA, 2003b

Dissociation constants (pKa ) 3.73 (weak acid) in water (25°C) IUPAC Web Site

Henry’s Law Constant (25°C) 0.000549 atm-m3/mol U.S. EPA, 2003b

Log octanol-to-water partition 

coefficient(log KOW)

2.828 U.S. EPA, 2003b

Persistence
Table A.2. Physical Chemical and Environmental Fate Properties of MCPA.

Parameter Value Source

Dissociation MCPA-DMA completely dissociated MRID 42457101

Hydrolysis half-life MCPA-EHE in buffer

pH 9: 75-117 hrs 

pH 7: 76 days 

pH 5: Stable

MCPA-EHE in soil:0.01 M CaCl2 (1:5) systems

After 12 hrs of incubation, MCPA-acid accounted for an avg. 

of 83.9% of in the silt loam soil system and 71.5% of the 

applied in the sandy loam soil system.

MCPA

pH 9,7, 5: Stable

MRID 42693901

MRID 42671501

MRID 42665301

Aqueous photolysis half-life 25.4 days MRID 42928101

Soil photolysis half-life irradiated with natural sunlight 

67.3 days on sandy loam soil 

incubated in the dark 

121.6 days on sandy loam soil

MRID 43225801

Aerobic soil metabolism half-life 

(days)

Sandy loam – 24 days

MCPA 2-EHE(OECD GLP:viable soil hydrolysis study)

Sandy loam soil (pH 5.6-6.3)

4.5 hrs (50% MWHC)

7.5 hrs (70% MWHC)

Clay loam soil (pH 5.5-6.3)

0.4 days (DT50 ca 8 hrs, DT90 48 hrs) (50% MWHC)

0.7 days (DT50 12 hrs, DT90 96 hrs) (70% MWHC)

Degradate: 4-CC (2 %)

MRID 41586001

MRID 47744901

Anaerobic soil metabolism half-life Stable (>62 days) MRID 41586001

Anaerobic aquatic metabolism halflife (total system)Stable MRID 40461901

Aerobic aquatic metabolism half-life 

(total system)

MCPA DMAS in water-sandy clay loam sediment

236 days (extrapolated value) 

Degradates

4-CC(1.9% at 30 d in sediment)

15 days total, 13 days water, 7 days sediment (water-loamy 

sand sediment system) 

>100 days in total system, water and sediment (water-light 

clay sediment system)

Degradates

4-CC: 44 days in total system

The Netherlands 

16.3 days (TNO ditch water-sediment) 

16.8 days (Kromme Rijn river water-sediment)

MRID 44732401A

/44192701

MRID 44732401B

MRIDs 45889303

Mobility
Table A.2. Physical Chemical and Environmental Fate Properties of MCPA.

Parameter Value Source

Adsorption/desorption

Kfoc-ads / (mL/g)

MCPA only

Adsorption

Freundlich adsorption Kf-ads values

ranged from 0.45 to 1.20 mL/g for

clay loam, silt loam, sandy loam soils, in a sandy loam 

aquatic sediment

Adsorption

Freundlich adsorption Kf-ads values:

from 0.0212 to 1.11 mL/g

Kfoc values: 9.6-46 mL/g for

clay, silt loam, sandy loam and loam soils

MRID 42596903

MRID 44239601

Adsorption/desorption

Kfoc-ads / (mL/g)

MCPA and Degradate

Adsorption: Freundlich adsorption Kfads values

0.21 sand soil (0.7% o.m.), 

0.26 sandy loam soil, 

0.55 loam soil, 

0.50 silty clay loam soil, and 

0.36 sandy clay loam sediment (1.7% o.m.); 

Corresponding Kf-oc values ranged from 31 to 59 mL/g

Desorption: Kf-des = 0.45 -1.5

corresponding Koc-des = 71-305 mL/g. 

4-chloro-o-cresol (4-CC) 

Adsorption: FreundlichKads values

0.81 sand soil (0.7% o.m.), 

1.6 sandy loam soil, 

3.1 loam soil, 

2.7 silty clay loam soil, and 

2.1 sandy clay loam sediment (1.7% o.m.); 

corresponding Koc: 198, 199, 330, 266, and 238 mL/g. 

Desorption: Freundlich Kdes values 0.87 to 4.2

Corresponding Kf-oc: 212, 311, 543, 418, and 302 mL/g.

4-MCA

Adsorption: Freundlich Kadsvalues

2.4 for the sand soil (0.7% o.m.), 

4.8 for the sandy loam soil, 

7.1 for the loam soil, 

6.2 for the silty clay loam soil, and 

5.5 for the sandy clay loam sediment (1.7% o.m.); 

Corresponding Koc: 588, 580, 755, 623, and 628 mL/g. 

Desorption: Freundlich Kdes = 1.4 to 8.5

Corresponding Koc = 346- 904 mL/g.

MRID 40555801

Adsorption/desorption

Kd / (mL/g) 

MCPA 2-EHE

Adsorption 

Single Kdvaluesat 0, 24, and 48 hours:

9.22-178 (clay loam; pH=6.6)

3.52-103 (sandy loam; pH=6.9)

3.66-106 (loam; pH=7.9)

5.39-57.9 (loamy sand; pH=7.4, oc=0.3%)

Respective single Kocvalues at 0, 24, and 48 hours:

396-7661 (clay loam; pH=6.6)

155-4514(sandy loam; pH=6.9)

87.4-2531(loam; pH=7.9)

1546-16585(loamy sand; pH=7.4, oc=0.3%)

MRID 47075204

Field Dissipation

Terrestrial field dissipation half-life The first order half-lives ranged from 3 to 23 days with a 

median half-life of 5.6 days. 

MCPA-Na salt dissipated with a half-life of 14.6 days from 

the top 6 inches of a sandy loam soil in Othello, 

Washingtonwhen applied to barley.

MRID 42133901
Table A.2. Physical Chemical and Environmental Fate Properties of MCPA.

Parameter Value Source

Total MCPA residues (reported as MCPA-isooctyl ester) 

dissipated with a half-life of 9.08 days from the top 6 inches 

of a loam soil in Californiawhen applied to barley.

MCPA isooctyl-ester (reported as total MCPA residues) 

dissipated with a half-life of 23 days from the top 6 inches of 

a sandy loam soil (pH 7.8) when applied to bareground in 

Montana.

MCPA EHE dissipated (based on total MCPA residues) from 

loamy sand soil in Georgia with half-lives of

6.2 days on bareground and 4.1 days on the wheat plot, and 

from loam soil in Kansas 8.9 days on bareground and 4.1 

days on the wheat plot.

Following the 2nd application of MCPA DMAS,total MCPA 

residues (as MCPA equivalent) dissipated

in California (sandy loam soil) with half-life of

3.9 days on the bareground plot

5.6 days on the grass plot.

in Florida with half-life of

3.5 days on bareground

5.3 days on grass

in New York with half-life of

10.4 days on bareground

3.3 days on grass

Degradates: In CA and FL, 4-CC and 4-MCA were not 

detected above the LOQ. In New York, 4-CC was detected 

twice at 0.010-0.012 ppm (0- to 6-inch depth; 1 of 3 

replicates each) from 0 to 1 day posttreatment, and was not 

detected below the 0- to 6-inch depth. The degradate 4-MCA 

was not detected above the LOQ.

MCPA DMAS dissipated in

CA with t1/2 3.8 days on baregroundand 3.2 days on wheat,

In Kansas with t1/2 5.6 days on baregroundand 6.6 days on the 

wheat plot,

Degradates: 4-CC was detected once at 0.010 ppm (1 of 3 

replicates at 0- to 6-inch depth, in Kansas) at 28 days 

posttreatment; 

Wheat plants were not analyzed for MCPA residues.

MRID 42134001

MRID 42134101

MRID 43815701

MRID 43883001

MRID 43697501

Table A.3. Environmental Degradates of MCPA and its forms.

Degradate/

Molecular formula

Max Percent Formation in 

%AR (Study)

Source/Study 

MRID

Chemical Structure

4-chloro-o-cresol

(4-CC)

4-chloro-2-methylphenol

(PCOC)

C7H7ClO

11.6% (aq. photolysis)

2.1% (at 3 hrs, aerobic soil 

metabolism)A

1.7% (at 48 hours, aerobic soil 

metabolism)B

5.6% (soil photolysis)

16.7% (aerobic aquatic)

42928101

47744901

47744901

MRID

MRID
5-chlorosalicylaldehyde 1.2% (aq. photolysis) 42928101

4-chloro-2-methylanisole

(4-MCA)

analyzed for and 

not detected

4-Chloro-2-

hydroxymethylphenoxyacetic 

acid

(HMCPA)C

analyzed for and not detected 47744901

A Detected once in single replicate of clay loam soil under aerobic conditions at 50% MWHC, 

B Detected once in single replicate of sandy loam soil under aerobic conditions at 50% MWHC,

C Another abbreviation seen is CHPA
279

Appendix B. Summary Tables of Ecological Effects Studies

Table G-2: Ecological Effects Data Requirements for MCPA

Guideline # Data

030501 030502 030516 030564

Requirement MRID Study

Classification MRID StudyClassification MRID StudyClassification MRID StudyClassification

71-1 850.21 Avian Oral LD50 bobwhite

400192-01

core bobwhite

400192-01

core

71-2 850.22 Avian Dietary

LC50

bobwhite

405558-03 

mallard

405558-02

core 

core

71-4 850.23 Avian

Reproduction

bobwhite

435052-01

core

72-1 850.1075 Freshwater Fish

LC50

bluegill

418009-01 

rainbow

418009-02

core TEP

core TEP

bluegill

400620-04 

bluegill

426244-02 

bluegill

418009-04 

rainbow

400620-05 

rainbow

418009-05

core 

core

core TEP

core

core TEP

bluegill

426244-03 

rainbow

426244-04

core 

core

72-2 850.101 Freshwater 

Invertebrate 

Acute LC50

daphnid

418009-03

core TEP daphnid

424122-01 

daphnid

418009-06

core 

core TEP

daphnid

426136-01

core

72-3(a) 850.1075 Estuarine/Marine

Fish LC50

Atlantic silverside

400620-02

core sheepshead 419395-

04

core TEP sheepshead

430832-10 

sheepshead

419395-01

core 

core TEP

sheepshead

430865-01

core
280

Table G-2: Ecological Effects Data Requirements for MCPA

Guideline # Data

030501 030502 030516 030564

Requirement MRID Study

Classification MRID StudyClassification MRID StudyClassification MRID StudyClassification

72-3(b) 850.1025 Estuarine/Marine

Mollusk EC50

Eastern oyster 

larvae

400620-03

core Eastern oyster shell

419395-06

core TEP Eastern oyster 

shell 430932-09

Eastern oyster 

shell 432521-01

Eastern oyster 

shell 419395-03

core

core TEP

supp TEP (no 

definitive 

NOAEC)

72-3(c) 850.1035

850.1045

Estuarine/Marine

Shrimp EC50

pink shrimp

400620-01

core mysid

419395-05

core TEP mysid

419395-02 

pink shrimp

400620-06

core TEP

core

mysid

430965-02

core

72-4(a) 850.14 Freshwater Fish

Early Life-Stage

fathead minnow

444072-02

core

72-4(b) 850.1300

850.1350

Aquatic 

Invertebrate LifeCycle

daphnid

444072-01

core

72-5 850.15 Freshwater Fish

Full Life-Cycle
281

Table G-2: Ecological Effects Data Requirements for MCPA

Guideline # Data

030501 030502 030516 030564

Requirement MRID Study

Classification MRID StudyClassification MRID StudyClassification MRID StudyClassification

123-1(a) 850.4100 Seedling 

Emergence

430832-05 

(ten species)

core 426987-08 

(eight species)

432579-01 

(two species)

Together, 

these two 

studies 

contain core 

data for ten 

species.

426693-01 

(ten species)

core

123-1(b) 850.4150 Vegetative Vigor 430832-05 

(ten species)

core 47216808 Under review 426693-04 

(five species)

437882-01 

(five species)

Together, 

these two 

studies 

contain core 

data for ten 

species.

426693-02 

(ten species)

core
282

Table G-2: Ecological Effects Data Requirements for MCPA

Guideline # Data

030501 030502 030516 030564

Requirement MRID Study

Classification MRID StudyClassification MRID StudyClassification MRID StudyClassification

123-2 850.44 Aquatic Plant

Growth

Lemna gibba

431265-01

Selan. 

capricornitum

430632-03

Navic. pelliculosa

430932-02

Ana. flos-aquae

430832-04

Skel. costatum

430832-01

supp (no 

definitive 

NOAEC) 

core

core 

core 

core

Lemna gibba

431265-02

Lemna gibba

449035-01

Selan. 

capricornitum

424613-01

Selan. 

capricornitum

449035-02

Selan. 

capricornitum

453131-01

Selan. 

capricornitum

455544-03

Navic. pelliculosa

430832-07

Navic. pelliculosa

449035-04

Ana. flos-aquae

430832-08

Ana. flos-aquae

449035-03

Skel. costatum

430832-06

Skel. costatum

449035-05

supp (no 

definitive 

NOAEC) 

core

core 

core

supp 

(insufficient 

replicates) 

supp (no 

definitive 

NOAEC) 

core

supp (no 

definitive 

NOAEC) 

core

core 

core 

core

Lemna gibba

430832-14

Selan. 

capricornitum

424612-01

Navic. 

pelliculosa

430832-11

Ana. flos-aquae

430832-13

Skel. costatum

430832-12

core 

core

core

core 

core

141-1 850.302 Honey Bee Acute

Contact LD50

honey bee 421503-

01

core honeybee

421978-01

core
Table G-2: Ecological Effects Data Requirements for MCPA

Guideline # Data

030501 030502 030516 030564

Requirement MRID Study

Classification MRID StudyClassification MRID StudyClassification MRID StudyClassification

141-2 850.303 Honey Bee 

Residue on 

Foliage