| Chemical Abstract Number (CAS #) |
94746
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| Synonyms | MCPA |
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Acetic acid, (4-chloro-2-methylphenoxy)- | Metaxon | Agroxon | 2-Methyl-4-chlorophenoxyacetic acid | Mephanac | Netazol | Rhonox |
| Analytical Methods |
EPA Method 555 |
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EPA Method 615 |
EPA Method 8150B |
EPA Method 8151 |
| Molecular Formula | C9H9ClO3 |
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| Use | MCPA is used as a postemergence herbicide for control of broadleaf weeds in agricultural
applications.
HERBICIDE FOR CONTROL OF ANNUAL & PERENNIAL WEEDS
Post-emergence control of annual and perennial broad leaved weeds (including thistles and docks)
in cereals (alone or undersown), herbage seed crops, flax, rice, vines, peas, potatoes, asparagus,
grassland, turf, under fruit trees, and on roadside verges and embankments. Control of
broad-leaved and woody weeds in forestry. Control of aquatic broad leaved weeds.
Translocated herbicide in small grains, rice, peas, grassland, sugarcane, tree crops, turf and
noncrop areas for postemergent control of many annual and perennial broadleaf weeds.
Used as the sodium salt; powerful selective weed killer.
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| Consumption Patterns | HERBICIDE, OF WHICH APPROXIMATELY 69% IS USED ON WHEAT, 18% ON
OTHER GRAINS, & 13% ON RICE (1975)
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| Apparent Color | LIGHT BROWN SOLID ; PLATES FROM BENZENE OR TOLUENE ; WHITE
CRYSTALLINE SOLID (PURE COMPD) ; Colorless crystalline solid (pure)
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| Melting Point | 120 DEG C
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| Molecular Weight | 200.6
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| Density | 1.56 @ 25 DEG C/15.5 DEG C
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| Sensitivity Data | SRP: Irritation of skin, eyes, nose, and throat may also occur.
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| Environmental Impact | 2-Methyl-4-chlorophenoxyacetic acid (MCPA) is released directly to the environment
through its applications as a commercial herbicide. Field studies have shown that aerial drift from
spray applications can transport MCPA to nearby ponds and streams. Field runoff (via rainfall)
can transport MCPA to streams in the local watershed. If released to soil, microbial degradation
will be the major degradation process. Biodegradation occurs faster in media previously exposed
to MCPA. Some photodegradation can occur on surfaces exposed to sunlight. MCPA leaches
readily in soil. It may undergo some volatilization from dry plant and soil surfaces. The result and
average soil persistence of MCPA at recommended application rates is up to 1 month in moist
conditions and up to 6 months under drier climates; typical soil half-lives of 2-3 weeks have been
observed under normal growing conditions. If released to water, MCPA can degrade through bio-
and photodegradation. Bioconcentration in aquatic organisms is not an important process. If
released to the atmosphere, MCPA can exist in both the vapor and particulate phases; vapor phase
MCPA degrades readily by reaction with photochemically produced hydroxyl radicals (estimated
half-life of about 30 hours). Physical removal from the atmosphere may occur through wet and
dry deposition. Occupational exposure to MCPA occurs through dermal contact and inhalation of
dusts and sprays; dermal absorption appears to be the major route exposure.
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| Environmental Fate | Terrestrial fate: In soil, metabolism involves degradation of the side-chain to
4-chloro-2-methylphenol, ring hydroxylation, and ring opening. Duration of residual activity in
soil is approximately 3-4 months.
TERRESTRIAL FATE: The primary degradation process for MCPA (and similar
phenoxyalkanoic acid herbicides) in soil is biodegradation ; the microbial degradation rate will
depend upon soil pH, soil type, organic matter, herbicide formulation and concn, moisture content
and temperature . MCPA is susceptible to photodegradation in sunlight(1-2); however,
photodegradation at the soil surface is not considered a major degradation pathway since it is
applied post-emergence to growing crops, and residues reaching the soil will be protected from
sunlight by the crop canopy . Residues exposed to sunlight will experience some
photodegradation. MCPA is readily leached from soil as indicated by Koc values of
50-60(4,SRC). MCPA may undergo some volatilization from dry plant and soil surfaces ;
although, the volatility of MCPA salt derivatives will be low . The resultant average persistence
of MCPA at recommended application rates is up to 1 month in moist conditions and up to 6
months under drier climates .
TERRESTRIAL FATE: A literature review of available laboratory persistence studies of MCPA
in soil found soil half-lives ranging from <7 to 41 days with an avg of about 2-3 weeks . In
laboratory tests using a sandy clay soil from Finland, initial MCPA half-lives of 2-4 weeks were
observed at initial concns ranging from 10-2000 ppm ; soil moisture was found to be an
important factor as the degradation rate in air-dried soils was less than 30% after 579 days of
incubation ; no degradation occurred at temperatures ranging from -30 to 0 deg C . The
persistence time of MCPA in soil (75-100% removal) has been reported to be about 3 months .
4-Chloro-o-cresol and 3-methyl-5-chlorocatechol are two major metabolites of MCPA by
microbial degradation in soil . In a field study, the initial MCPA half-life was approximately 29
days when using a soil that had no previous MCPA applications(6); in soil that had received 1-18
years of previous exposures, the half-life was approximately 10 days(6).
AQUATIC FATE: MCPA can degrade in water through biodegradation and photodegradation.
Various screening studies (particularly with soil microbes) have demonstrated that MCPA will
biodegrade(1-3); the rate of microbial degradation has also been shown to increase with microbial
adaptation(1-2). The direct photolysis half-life of MCPA in water (at surface conditions under
summer sunlight) is about 19-20 days ; the photolysis can be sensitized by the formation of
sunlight-produced hydroxyl radicals and may increase the photodegradation rate by up to an
order of magnitude or more(4-5); the photosensitized rate will vary among natural waters and will
depend upon the availability of photocatalytic agents. Based upon Koc values of
50-60(1,SRC), partitioning to sediment and suspended material in water will not be an important
fate process. The results of an aquatic ecosystem study have indicated that bioconcentration in
aquatic organisms is not an important process(6). Volatilization from water is also not an
important fate process(7).
ATMOSPHERIC FATE: Based upon a reported vapor pressure of 5.9X10-6 mm Hg at 25 deg
C , 2-methyl-4-chlorophenoxyacetic acid can exist in both the vapor and particulate phases in
the ambient atmosphere(2,SRC). It will degrade relatively rapidly in the vapor phase by reaction
with photochemically produced hydroxyl radicals with an estimated half-life of about 30
hr(3,SRC). Physical removal of particulates by dry deposition (settling) and wet deposition
(rainfall, washout) will also occur. Aerial drift from herbicidal spraying operations can
transport 2-methyl-4-chlorophenoxyacetic acid to nearby ponds and streams .
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| Drinking Water Impact | SURFACE WATER: In a study conducted between 1975 and 1977,
2-methyl-4-chlorophenoxyacetic acid (MCPA) was detected (detection limit of 0.1 ug/L) in 6 of
949 stream water samples collected from 11 agricultural watersheds in Ontario, Canada ;
positive samples coincided with spraying operations . Levels of <0.1 to 1.7 ug/L were detected
into 2 of 98 water samples collected from 8 agricultural watersheds in Ontario in 1974 . Water
samples were collected at the mouths of the Grand, Saugeen and Thames Rivers in Ontario
between Jan 1981 and Dec 1985 ; MCPA was detected in 1 of 100 Grand River samples (0.1
ug/L), 4 of 200 Thames River samples (1.8 ug/L mean concn), and no Saugeen samples . In a
monitoring study of the Anglican Water region in the United Kingdom (an agricultural area),
MCPA was detected in 1% of all samples at concns of <0.1 to 16.9 ug/L (detection limit not
specified) .
GROUND WATER: 2-Methyl-4-chlorophenoxyacetic acid was detected in 2 of 237 wells
monitored between 1969 and 1978 in Ontario, Canada ; concns were 1.1-10 ppb in one well
and 1001-10000 ppb in the other ; the high concns were the result of spillage . During
monitoring of 359 Ontario wells between 1979 and 1984, 2-methyl-4-chlorophenoxyacetic acid
was detected only twice ; the detections were the result of spillage . In a monitoring study of
the Anglican Water region in the United Kingdom (an agricultural area), MCPA was detected in
(detection limit of 0.1 ug/L) 0.5% of all samples at concns of 0.12 ug/L . MCPA was detected
in 0.1% of 725 wells sampled in Minnesota at a levels of 0.33 ug/L .
EFFL: 2-Methyl-4-chlorophenoxyacetic acid was detected in one of 5 leachates collected from 5
sanitary landfills in Denmark during Feb 1983 ; the concn of the positive sample was 1-10
ppm .
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