              UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

                                     WASHINGTON, D.C.  20460												
						                                                                  
                                                                        
                                                   	

DATE:  August 09, 2007 

MEMORANDUM

(Revised from memo dated 07/31/07)

SUBJECT:	MCPB: Response to waivers for the following studies (Revised
from memo dated 07/31/07): terrestrial field dissipation (164-1), fish
early life stage (850.1400) (72-4a), Oyster acute toxicity test (shell
deposition) (850.1025) and acute fish toxicity test freshwater and
marine (data requirement 850.1075) studies for the Re-registration
Eligibility Decision (RED) for MCPB and Salts.  PC Code: 019201 (MCPB
Acid) and 019202 (MCPB Sodium Salt).  DP Barcode: D 340501

FROM: 	Marie Janson, Environmental Scientist

                        James Hetrick, Ph.D., Senior Science Advisor	  

	Nancy Andrews, Ph.D., Chief ERB 1

                        Thuy Nguyen, RAPL ERB 1              

                        Environmental Fate and Effects Division (7507 P)

TO:	James Parker

                       Chemical Review Manager

                       Re-Registration Branch I SRRD (7508 P)

Registrant waiver request for fish early-life state toxicity test (data
requirement 850.1400) MCPB technical acid and MCPB sodium salt: 

MCPB is a minor use product used mainly on peas. MCPB is applied only
once per season.  There are no direct aquatic uses so that the presence
in water could arise only from overspray/runoff/drainage, none of which
is expected to give high concentration in water.  MCPB is not persistent
in water and does not accumulate in fish (  HYPERLINK
"mailto:Kow=2.8@PH5"  Kow=2.8@PH5 ).  The molecule dissociates at
physiological PH (pKa=4.5), further reducing the possibility of
bioaccumulation.  Neither MCPB nor other herbicides in the phenoxy acid
series is associated with mammalian developmental toxicity, other than
foetotoxic effects at maternally toxic doses.  Acute toxicity to fish is
only moderate (LC50>4mg/L).

Thus, continued repeated or continued exposure of early life stages is
not anticipated and there is no reason on experimental or structure
function grounds to suspect differential toxicity to the early life
phase of fish. 

EFED response:

 

EFED requests a fish early-life state toxicity test for MCPB (data
requirement 850.1400) because chronic freshwater or chronic estuarine
marine fish or invertebrate studies were not submitted to the Agency. 
Also, MCPB is highly soluble and its acidic/anionic nature,
physicochemical properties, relatively low sorption to soil (average
soil sorption coefficient of 0.85 mL/g) and low octanol-to-water
partitioning ratio (1.33 at pH 7), indicate that MCPB is prone to leach
and runoff to freshwater and estuarine marine aquatic systems.  Although
MCPB photolyzed in water with half-lives of 2 to 3 days, the overall
aerobic soil metabolism half-life of 18 days and the hydrolysis
extrapolated half-life of greater than 500 days suggest that aquatic
organisms will be exposed to MCPB, resulting in possible chronic risks
to these species. No information is available on aquatic biodegradation
of MCPB.

The value of the requested study may effect the outcome of risk
conclusions for chronic toxicity to fish.  

In the current EPA RED chapter for MCPB, due to insufficient data, the
Agency estimated chronic risks for MCPB on freshwater fish based on
acute to chronic ratios (ACR) with the degradate MCPA which has similar
fate characteristics as MCPB.  However, a comparison of MCPB toxicity
data with the available toxicity data for MCPA published in the EPA RED
chapter indicates that some differences in toxic potency may exist
between the two compounds.  Using common aquatic species tested for each
compound (acid equivalent) the following comparisons can be made:

Species                       Test Type                    MCPB Results	
          MCPA Results

Rainbow trout            96-h LC50                   3.9 mg ae/L		96 mg
ae/L

The aquatic organism comparisons would suggest that MCPA (acid
equivalent) is less toxic than MCPB.  

  

Although there were no LOC exceedences for chronic risks to freshwater
fish the ACR method described above, there is low confidence in this
risk conclusion based on toxicity differences between MCPB and MCPA for
freshwater fish.  These endpoint differences are as follows: MCPB is
more toxic to acute freshwater fish (3.9 mg ae/L) than MCPA (96 mg ae/L)
by one order of magnitude.

Therefore, the fish early-life state toxicity test (data requirement
850.1400) will be of value to assure the public that chronic toxicity to
fish will not be at risk with the application of MCPB when sprayed near
freshwater and estuarine marine environments.

Registrant waiver request:  for Oyster acute toxicity test (shell
deposition) data requirement 850.1025) and acute fish toxicity test
freshwater and marine (data requirement 850.1075) MCPB technical acid.
MCPB is a weak acid that is fully dissociated at environmentally
relavant PH.  Thus, the studies submitted/proposed for the sodium salt
preparation are equally applicable to the acid and a wavier is sought on
this basis.

EFED response: 

No estuarine marine fish or invertebrate data were submitted for MCPB
technical acid or MCPB sodium.  However, studies submitted/proposed for
the sodium salt preparation are equally applicable to the acid.

 

Registrant waiver request for terrestrial field dissipation (data
requirement 835.6100) MCPB technical acid and MCPB sodium salt;

MCPB [4-(2-methyl-4- chlorophenoxy) butyric acid] is a selective phenoxy
herbicide used post-emergence as a broadcast foliar application to
control certain broad-leaved annual and perennial weeds.  MCPB is
registered for use on peas where typically one application is
recommended per growing season.   MCPB is rapidly converted to MCPA
under aerobic soil conditions, for which and acceptable field
dissipation study has been submitted to EPA (1988: MRID 40961301). 
Additional confirmation of the dissipation pattern of MCPA and
metabolites is provided in bareground trials conducted as a part of
wheat/bareground (MRID 43697501) and turf/bareground (MRID 43883001)
dissipation studies.  Due to rapid conversion of MCPB to MCPA under
aerobic soil conditions it is the contention of the MCPB Task Force that
sufficient data exist from available MCPA dissipation studies to allow
EPA to evaluate the dissipation characteristics of MCPB. A waiver from
the 835.6100 data requirement for MCPB is therefore requested 

 

EFED response: 

After reviewing the request and the submitted data, EFED believes that
MCPA terrestrial field dissipation studies are not sufficient to address
the terrestrial dissipation characteristic of MCPB. 

Although the degradation of MCPB could be described as bi-phasic (hockey
stick) with an initial rapid degradation half-life (t1/2=4 days) in
soil, an overall calculated aerobic soil metabolism half-life of 18 days
indicated a potential for MCPB to leach through soil under typical use
conditions.  Additionally, MCPA and MCPB have different crop use
profiles and dissipation studies of MCPA cannot be used to fulfill the
data requirement for MCPB.  MCPB is used extensively on peas, whereas
MCPA can be used on small grains, peas, beans, etc.  The agronomic and
geographic differences in pea and small grain production areas are
expected to control the MCPA and MCPB dissipation rates.  More
importantly, the submitted MCPA field dissipation studies for MCPA-DMA
were performed on turf and wheat, which is not typical of MCPB
agricultural uses.

Therefore, EFED requests submission of terrestrial field dissipation
studies conducted with MCPB to fulfill the 835.6100 data requirement for
MCPB.

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