UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON D.C., 20460

OFFICE  OF 

PREVENTION, PESTICIDES AND 

TOXIC SUBSTANCES

MEMORANDUM	July 12, 2006

SUBJECT: 		Evaluation of Risk Mitigation Proposals from the U. S. Apple
Assoc. for Use of Azinphos methyl 

DP Barcode:	D307577

PC Code:	058001

TO:			Katie Hall, Chemical Review Manager

			Reregistration Branch 2

Special Review and Reregistration Division

FROM:		R. David Jones, Ph.D., Senior Agronomist

			Environmental Risk Branch 4

Colleen Flaherty, Biologist

Environmental Risk Branch 3

THROUGH:		Elizabeth Behl, Chief 

Environmental Risk Branch 4

Dan Rieder

Environmental Risk Branch 3

Environmental Fate and Effects Division

As a follow up to the interim reregistration of azinphos methyl in 2002
(OPP, 2002), additional data was submitted regarding the exposure of
workers to azinphos methyl. A reassessment of the occupational risk from
azinphos methyl is being conducted pursuant to the submission of this
data. Revised ecological risk assessments have also been conducted
(D307567, D307568) in concert with this reassessment to ensure Agency
compliance with the Endangered Species Act.

	This document assesses potential mitigation practices to determine
their efficacy, and some alternative scenarios to further characterize
the risk to aquatic organisms. In both cases, the only crop considered
was apples. Two mitigation practices were considered: a reduction in the
seasonal maximum application rate from 4.5 lb/acre to 3 lb/acre, and
increasing the spray drift buffer from 25 feet to 60 or 100 feet. For
these simulations, changing the seasonal maximum rate to 3 lb/acre
effectively limits the number of applications to 2, since standard
practice is to use the maximum single application rate for each
application; for azinphos methyl this is 1.5 lb/acre. 

	In addition to the standard eastern and western apple scenarios in
Pennsylvania and Oregon respectively, an additional scenario in central
Washington State has been added so as to better characterize the risks
in this location and to confirm whether the Oregon apple scenario is a
suitable surrogate. For the spray drift simulations, the ‘orchard’
option was chosen rather than the ‘sparse’ orchard, as this may
better reflect drift from apple orchards with a full canopy.

  SEQ CHAPTER \h \r 1 			Models  tc "a.	Aquatic Exposure Modeling " \l 4


	For Tier 2 surface-water assessments, two models are used in tandem. 
PRZM simulates fate and transport on the agricultural field.  The
version of PRZM (Carsel et al., 1998) used was PRZM 3.12 beta, dated May
24, 2001.  The water body is simulated with EXAMS version 2.98, dated
July 18, 2002 (Burns, 1997).  Tier 2 simulations are run for multiple
(usually 30) years and the reported estimated environmental
concentrations (EECs) are the concentrations that are expected once
every ten years based on the thirty years of daily values generated by
the simulation.  PRZM and EXAMS were run using the PE4 shell, dated May
14, 2003, which also summarizes the output.  Spray drift was simulated
using the AgDrift model version 2.01 dated May 24, 2001.

  SEQ CHAPTER \h \r 1 	For air blast applications, buffer strips were
modeled with AgDrift in Tier I Orchard Airblast mode.  A sparse orchard
was modeled, and the output value for the loading into the standard pond
was tripled in order to reflect the upper 95% confidence bound on the
drift.  The only management practice that was varied was the buffer
strip width.  A buffer strip of 25 ft was evaluated, which is consistent
with label requirements for almonds, apples, Brussels sprouts, cherries,
grapes, pears, pistachios and walnuts.  The modeled spray drift from
AgDrift for the 25 ft buffer was 4.5%.  In comparison, an AgDRIFT
simulation with no buffer strip results in estimated spray drift of
12.5%. 

Management Practices

The current maximum label practice for application of azinphos methyl to
apples is 1.5 lb a.i./acre per single application with a seasonal
maximum of 4.5 lb a.i./acre. A minimum interval of 7 d is required
between applications. Aerial application cannot be used on apples, and a
25 ft buffer is required around all water bodies. This use pattern has
been simulated using three applications of 1.5 lb/acre, 7 d apart
applied with spray blast equipment with the 25 ft buffer. In practice,
there are often more applications made at a lower single application
rate, but the simulated pattern results in the greatest EECs. The U. S.
Apple Association has recommended reducing the seasonal application rate
to 3 lb/acre and increasing the spray drift buffer to 60 ft. In
addition, simulations have been run for a 100 foot buffer and for when
there is no spray drift. These last simulations (no spray drift) reflect
the contribution from spray drift alone and reflect the EECs that might
occur when application is a long distance from the water body, e.g.
several thousand feet.

  SEQ CHAPTER \h \r 1 	For air blast applications, buffer strips were
modeled with AgDrift in Tier I Orchard Airblast mode.  The generic
“orchard” was modeled, and the output value for the loading into the
standard pond was tripled in order to reflect the upper 95% confidence
bound on the drift.  The only management practice that was varied was
the buffer strip width. In comparison, an AgDRIFT simulation with no
buffer strip results in estimated spray drift of 12.5%. 

Table 1. Management practices simulated for azinphos methyl use on
apples*.



Crop	

App Rate 

(lbs a.i./A)	

Max. No. Apps.	

App. Interval (days)	

Buffer Width (ft)	

App. Method

(% drift)	

First App. Date

Label	1.5	3	7	25	air blast (4.5%)	May 1

U. S Apple Assoc. 1	1.5	2	7	60	air blast (1.4%)	May 1

U. S Apple Assoc. 2	1.5	2	7	100	air blast (1.4%)	May 1

No drift	1.5	2	7	NA	no drift	May 1



* For all simulations, IPSCND, the disposition of foliar pesticide
residues on foliage at harvest was set to 1, so that the residues are
applied to the soil.



	The percent spray drift used to represent each buffer width is in Table
2. The value for no buffer (‘0 buffer width’) is provided for
reference and was not used in a simulation. 

Table 2. Fraction of applied spray drift as a fraction of the
application rate loaded to the standard pond as a function of buffer
width . Values were estimated using AgDrift 2.01.

Buffer Width (ft)	Fraction of Application Rate

0	0.0654

25	0.0279

60	0.0144

100	0.0093



Scenarios

	Three scenarios were used in this assessment (Table 3). Two scenarios,
apples in Lancaster County, Pennsylvania, and Marion County Oregon were
the same scenarios as were used in the assessment of apples in the prior
assessment (D307568). This provides a standard of comparison against
which the new use patterns can be compared. A scenario for apples in
central Washington was added in order to demonstrate differences in
aquatic exposure between apples grown in Oregon and Washington. This
scenario is not a standard scenario but was developed for assessment of
the carbamate insecticide in the carbamate cumulative.  As such, it has
been through EFED QA procedures for scenarios .

Table 3.  Scenarios used to represent crops for PRZM/EXAMS modeling of
azinphos methyl uses.

Crop	Location	Soil	Weather

Apples, Eastern	Lancaster Co, PA	Elioak silt loam	Allentown, PA

Apples, Western, OR	Marion Co, OR	Cornelius silt loam	Portland, OR

Apples, Western, WA	Grant Co, OR	Taunton silt loam	Yakima, WA



Chemistry Input Parameters tc \l4 "i.         Chemistry Input Parameters


Azinphos methyl is an organophosphate insecticide used on a wide variety
of food and non-food crops.  Azinphos methyl environmental fate data
used for generating model parameters is listed in Table 4. The input
parameters for PRZM and EXAMS are in Table 5.  Descriptions of special
considerations used to select environmental fate parameters or to
generate modeling input values are described below.

Hydrolysis.  As noted above, measurements of the hydrolysis rates were
made at 30( C and 40( C rather than the standard 25(C. The Arrhenius
Rate Law was used to calculate the degradation rate by hydrolysis at
25(C for use in EXAMS.

Soil and Aquatic Metabolism.  Only one anaerobic and one aerobic soil
metabolism value were available for azinphos methyl.  No aquatic
metabolism data are currently available.  Current policy for generating
input parameters for PRZM 3 when only one value is available is to
multiply the half-life by three resulting in a PRZM input parameter for
aerobic soil degradation of 95.3 d.  In previous modeling for azinphos
methyl, the anaerobic soil metabolism value was used as input to PRZM
representing the degradation rate in the sub-surface horizons.  For this
set of simulations, the aerobic soil metabolism half-life was used for
all depths.  This will have no effect on the EECs. 

Since no aquatic metabolism data were available, current policy is to
use the value of the corresponding half-life for aerobic soil metabolism
and multiply that value by 2 to represent aerobic aquatic metabolism. 
This is done as there is usually some correspondence between soil and
aquatic metabolism rates and in the absence of aquatic data this is
judged to be a reasonable conservative surrogate. The aerobic aquatic
metabolism input parameters were multiplied by 2 again to estimate the
degradation rate in the pond sediment. The resulting half lives for
aerobic and anaerobic aquatic metabolism are 190.8 and 381.6 d
respectively. In practice these values are of little importance as the
degradation in the water column will be dominated by hydrolysis.

Soil Water Partition Coefficients.  In previous modeling, a Koc value
based on Kf's was used in the simulations.  The method for generating
soil-water partition coefficient input values has changed substantially
from this in the new simulations.  In selecting a value for the
soil-water partition coefficient to use in the simulations, four issues
needed to be considered.  First, adsorption and desorption isotherms are
not equal, so it must be decided whether to use the adsorption or
desorption isotherm.  Current policy is to use the desorption values in
PRZM because the dominant process during a runoff event is desorption
and to use the adsorption isotherm in EXAMS as that it is the dominant
process in the pond.  Secondly, the data for each of the three soils
(both adsorption and desorption processes) were fitted to a Fruendlich
isotherm and the 1/n or "curvature" term in the equation was
significantly different than 1, indicating that concentration adsorbed
to soil was curvilinearly related to concentration in solution. 
Unfortunately, PRZM and EXAMS only have a linear (Kd) partition model
for handling soil-water partitioning of pesticides.  For the desorption
isotherm, this was handled by calculating the partitioning between soil
and water at the maximum concentration it would be expected to occur in
each media.  While this method does not give the most accurate
soil-water partitioning of the pesticide over the range of the isotherm,
it should be most accurate at near application rate, where the greatest
portion of the runoff occurs.  For the calculated desorption Kd's for
PRZM 2, the soil concentration of 17.2 µg kg-soil-1 was used, which
corresponds to the concentration resulting from the application rate
being mixed into the top 1 cm of soil.  The soil water was content was
assumed to be 0.35 cm3-H2O ( cm-3-soil and the bulk density of the soil
was assumed to be 1.3 kg L-1.  The partitioning under these conditions
was used to calculate a Kd appropriate for this soil content.  Note that
for each soil, four different desorption experiments were done and
Fruendlich parameters were given for each separate experiment. The
average of the four sets of parameters was used to calculate a single Kd
for the soil at the application rate rather than four different Kds
being calculated and then averaged.

Finally, a Pearson's Correlation Analysis of the of the calculated Kd
with organic carbon content was used to calculate a Koc. for neither
adsorption nor desorption was there a significant correlation between
the calculated Kd's and organic carbon content, so the Kd value for the
silty clay soil, 8.414 L (kg-1 for desorption and 7.55 L (kg-1 for
adsorption was used.  Finally it should be noted that the concentrations
in the soil-water partitioning study are only about 1 tenth the
concentration of pesticide that could be found in the soil at the
application rate.  Hence, we are extrapolating considerably beyond the
range of the experimental data for calculating the EEC and this usually
results in substantial error.

Table 4. Environmental fate parameters for azinphos methyl.

Fate Parameter	Value	Source

Molecular Mass	317.32 g mol-1	OPP, 1986

Aerobic Soil Metabolism Rate Constant	2.17 x 10-2 d-1	MRID 29900

Anaerobic Soil Metabolism Rate Constant	1.04x10-2 d-1	MRID 29900

Kd	7.6 L kg-soil-1 (sandy loam)	MRID 42959702

Solubility	25.10 mg L-1	OPP, 1986

Vapor Pressure	 2.2x10-7 torr 	OPP, 1986

Acidic Hydrolysis Rate Constant	4.78 L (mol-H+)-1 d-1	MRID 29899

Neutral Hydrolysis Constant	7.83 x 10-4 d-1	MRID 29899

Alkaline Hydrolysis Constant	82 L (mol-OH+)-1 d-1	MRID 29899

Aqueous Photolysis Constant	3.19 d	MRID 40297001

Washoff Fraction	0.937	Gunther et al., 1977

Foliar Degradation Rate Constant	7.2 d	see text



Foliar Washoff and Degradation.   Foliar dissipation is an important
process for estimating the EEC of azinphos methyl.  Data for foliar
washoff of azinphos methyl (Gunther et al., 1977) is not presented in a
manner that is most amenable to direct use in PRZM 2.  The value
available for foliar washoff is 60% of the amount applied washed off in
the first 0.33 cm of rainfall.  The PRZM foliar washoff parameter,
FEXTRC, is the amount of pesticide washed off in 1 cm of rainfall,
expressed as a fraction.  There is some indication (McDowell et al.,
1984) that a hyperbolic model (1/[a+bt]) best predicts the concentration
profile with washing volume of methyl parathion, a similar compound, in
washoff, but integration of the regression equations failed to provide
meaningful estimates of the percent washed off in 1 cm of rainfall
(Values calculated exceeded the initial concentration).  To obtain a
meaningful, if not particularly accurate or precise estimate of foliar
washoff, the following assumptions were made: first, that washoff rate
was proportional to the amount on the leaf (i.e. ∂[AM]/∂V = -k[AM],
where [AM] is the azinphos methyl concentration on the leaf, V is the
volume of runoff expressed as cm of precipitation and k is the washoff
rate constant).  The exponential removal model which was selected for
the first assumption was chosen over a linear model as there is some
indications that an exponential model better described the structure of
the data.  Based on the first assumption, the equation describing the
washoff fraction as a function of the precipitation amount, V, in 1 cm
is:

where W is the fraction remaining on the foliage.  The 40% remained
after 0.33 cm of precipitation allows calculation of a point estimate of
k as 2.78.  Using this value for k, the fraction washed off (1-W) with 1
cm of rainfall is 0.937.

For foliar degradation, 7 foliar half-lives measurements are available
(Lindquist and Krueger, 1975; Hoskins, 1962; Pree et al., 1976;
Winterlin et al., 1974, McDowell et al, 1984).   Assuming these values
are distributed normally, the value which represents the one tail upper
90% confidence limit of the mean is 9.8 d.

Table 5. Chemistry input parameters for Tier-II (PRZM/EXAMS) simulation
of azinphos methyl for aquatic assessment of the peach and potato uses.
Source data is in Table 10.

Input Parameter	Value	Justification	Quality

Molecular weight	317.32 g mol-1	calculated	excellent

Solubility	25.10 mg L-1	measured	very good

Hydrolysis	39.4 (pH 5)

37.5 (pH 7)

 6.6 (pH 9)	adjusted for temperature	excellent

Photolysis	3.19 d	measured

	Aerobic Soil Metabolism	95.4 d	single value x 3	fair

Water Column Metabolism	190.8 d	aerobic soil x 2	poor

Sediment Metabolism	381.6 d	water column x 2	poor

Foliar Degradation 	9.8 d	UCB90 on 7 values	good

Foliar Washoff Coefficient	0.937 cm-1	point estimate from 1 study	fair

Henry’s Law Constant	3.66 x 10-6 L atm mol-1	estimated from solubility
and vapor pressure	poor

Vapor Pressure	2.2 x 10-7 torr	good	good

Soil Water Partition Coefficient (Kd)	7.6 L kg-soil-1	lowest non-sand Kd
good



Results

For comparison purposes, two simulations from the previous assessment
were included (Table 6), one each for Oregon and Pennsylvania. These
reflect 3 applications spaced 7 days apart, a 25 ft spray drift buffer,
a spray drift generated with AgDrift using the ‘sparse orchard’. The
sparse orchard is appropriate to reflect applications to applications in
winter to orchards of deciduous trees (‘dormant applications’),
applications to immature trees that have not achieved a closed canopy,
applications early in the season prior full leaf emergence, and sprays
made over the top of the canopy. Dormant applications of azinphos methyl
and applications made over the top of the canopy are specifically
prohibited on the label. Information is not currently available on the
prevalence of applications made to immature orchards. Applications of
azinphos methyl may be made prior to full canopy development as it is
applied to control plum curculio at petal fall (the end of bloom),
particularly in the eastern United States. However, azinphos methyl is
dominantly used to control codling moth. Applications to control this
pest are made in mid-season to mature, bearing orchards and using the
sparse orchard in AgDrift will result in overestimates of drift for
these applications.

To simulate the drift in this assessment, the “orchard” option in
AgDrift was chosen. This option represents the mean drift curve over all
the spray blast drift trials used to develop the drift curves for Spray
Blast simulation in AgDrift. They show a considerably reduced drift
loading compared to the sparse orchard. For example, for the 25 ft
buffer for orchards on the azinphos methyl label, the sparse orchard
results in 4.5% of the application rate into the standard pond while
drift while the drift using the “orchard” option is 2.8%, a 37%
reduction.

The mitigation practices suggested by the U. S. Apple Assoc. (2
applications, 60 foot buffer) along with consideration of different
spray drift scenario reduce the EECs from the 15.1 to 8.8 µg L-1 in
Pennsylvania, and from 9.9 to 3.6 µg L-1 in Oregon for the 1-in 10 year
peak concentration, reductions of 42 and 67% respectively. However, in
neither case does the reduction lower the risk below the level of
concern for acute risk of 0.3 µg L-1 for fish and 0.15 µg L-1 for
freshwater aquatic invertebrates. In addition to the 60 ft buffers
suggested by U. S. Apple Association, a 100 ft buffer was also
simulated, with 1 in10 year peak EECs of 6.5 and 3.1 µg L-1 for
Pennsylvania and Oregon respectively. In fact, assuming no drift in the
lower exposure Oregon scenario still results in a 1 in 10 year peak EEC
of 2.1 µg L-1. So, while these suggested changes do substantially
reduce the estimated exposure for the use of azinphos methyl, they do
not mitigate the risk below the level of concern.

The U. S. Apple Association had concerns that the Oregon apple scenario
was not representative of apple culture in the Pacific Northwest. A
scenario for apples in central Washington has been developed for use in
the carbamate cumulative risk assessment, one of the most prominent
apple growing areas in that region, and a simulation was made for
azinphos methyl using that scenario. Relative to the Oregon apple
orchard, the Washington orchard produced somewhat higher EECs, 4.8 µg
L-1 for 1 in 10 year peak EEC in Washington versus 3.6 µg L-1 for
Oregon with 2 applications and a 60 foot spray drift buffer. The
differences are likely due to the use of irrigation in the Washington
scenario and not in the Oregon scenario. Irrigation is, in fact, use
most of the time for apple culture in the Pacific Northwest.

Table 6. Tier 2 EECs for azinphos methyl application to apples with U.
S. Apple Assoc. mitigation practices. 

Use Pattern	Maximum	4 Day	21 Day	60  Day	90 Day

	----------------------------------------- μg L-1
-------------------------------------------------

PA sparse orchard;

 3 apps, 25 ft buffer	15.1	14.1	11.6	8.5	6.7

OR sparse orchard;

3 apps, 25 ft buffer	9.9	9.4	8.1	5.7	4.4

PA, “orchard”;

2 apps 25 ft buffer	8.8	8.4	6.8	4.6	3.7

PA, “orchard” , 

2 apps, 60 ft buffer	7.1	6.8	5.6	3.7	2.9

PA, “orchard”;

2 apps, 100 ft buffer	6.5	6.2	5.2	3.4	2.7

OR, “orchard”;

2 apps, 25 ft buffer	5.4	5.1	4.1	2.9	2.2

OR, “orchard”, 

2 apps, 60 ft buffer	3.6	3.4	2.7	1.9	1.5

OR, “orchard”;

2 apps, 100 ft buffer	3.1	2.9	2.3	1.6	1.2

OR, no drift,

2 apps	2.2	2.0	1.6	1.0	0.74

WA, “orchard”, 

2 apps, 60 ft buffer	4.8	4.5	3.8	2.5	1.9



Literature Citations

Office of Pesticide Programs. 1986. Guidance for the Reregistration of
Pesticide Products Containing Azinphos-methyl as the Active Ingredient.
United States Environmental Protection Agency. Washington, DC. Issued
September, 1986.

Office of Pesticide Programs. 2002. Interim Reregistation Eligibility
Decision for Azinphos methyl: Case No. 0235   HYPERLINK
"http://www.epa.gov/oppsrrd1/REDs/azinphosmethyl_ired.pdf" 
http://www.epa.gov/oppsrrd1/REDs/azinphosmethyl_ired.pdf .

MRID 00029899. Wilkes, L.C., J. P. Wargo, and R. R. Gronberg. 1979.
Dissipation of Guthion in Buffered Aqueous Solution.  Analytical
Development Crop., Monument, Colorado. ADC Project 378-F, notebook
reference 79-R-126,127, Acc. No. 099216, Tab No. 67983.

MRID 00029887. M.F. Lenz. 1979. Soil Adsorption and Desorption of
Guthion.  Mobay Chemical Corp. April 11, 1979.  Accession No. 099216. 
Tab No. 66848.

MRID 00029900. Gronberg, R. R., R .J. Polluck and J.P. Wargo. 1979. The
Metabolism of Guthion in sandy loam soil. Mobay Chemical, August 27,
1979, Accession No. 099216, Tab No 68030.

MRID 40297001. J. G. Morgan. The Aqueous Photolysis of
GUTHION-Phenyl-UL-14C. Report No. 94709. 14 July 1987. Accession No.
4029701.

D307567. Flaherty, Colleen and R. David Jones. 2005.   SEQ CHAPTER \h \r
1 Azinphos Methyl Insecticide: Ecological Risk Assessment for the Use of
Azinphos Methyl on Caneberries, Cranberries, Peaches, Potatoes, and
Southern Pine Seeds (Group 2 Uses). Internal EPA Memorandum to Diane
Isbell dated June 12, 2005.

D307568. Flaherty, Colleen and R. David Jones. 2005.   SEQ CHAPTER \h \r
1 Azinphos-methyl Insecticide: Ecological Risk Assessment for the Use of
Azinphos-methyl on Almonds, Apples, Blueberries (Low- and Highbush),
Brussels Sprouts, Cherries (Sweet and Tart), Grapes, Nursery Stock,
Parsley, Pears, Pistachios, and Walnuts. Internal EPA Memorandum to
Diane Isbell dated September 29, 2005.

  SEQ CHAPTER \h \r 1 Appendix

Aquatic Exposure Model Input File Names

 Input files archived for azinphos methyl applied to apples

File Name	Date	Description

W14737.dvf	July 3, 2002	weather for Pennsylvania apple scenario
(Allentown, PA)

W24229.dvf	July 3, 2002	weather for Oregon apple scenarios (Portland,
OR)

W24243.dvf	July 3, 2002	weather for Washington apple scenario (Yakima,
WA)

ORappleC.txt	October 12, 2002	PE4 scenario file for Oregon apples,
unirrigated



PAappleC.txt	October 12, 2002	PE4 scenario file for Pennsylvania apples

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058001 PA apples 06	July 5, 2006	Pennsylvania apples, 2 applications,
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058001 WA apples 01	July 5, 2006	Washington apples, 2 applications, 60
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