UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON D.C., 20460

OFFICE  OF 

PREVENTION, PESTICIDES AND 

TOXIC SUBSTANCES

MEMORANDUM	October 24, 2006

SUBJECT: 		Evaluation of Risk Mitigation Proposals for Application of
Azinphos methyl to Almonds

DP Barcode:	D311129

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 the efficacy of buffer strips on the risks to
aquatic organisms from the application of azinphos methyl to almonds.
Buffer strips of 100 and 500 feet were assessed, as well as a scenario
with no drift, which simulates a very wide buffer (e.g., greater than
1000 feet) and assumes that the exposure is due only to runoff.

	Estimated environmental concentrations (EECs) on almonds in California
are substantially reduced as the spray drift buffer is increased from
the current 25 ft (3.1 ppb) to 500 ft (0.6 ppb).  Further increase in
buffer strip width does not significantly decrease simulated exposures
– the EEC for the no drift simulation is 0.59 ppb. However, the levels
of concern for acute risk to fish and invertebrates are still exceeded,
except when the risk quotient is calculated using the EEC from the no
drift scenario and the LC50 for brook trout. Endangered species levels
of concern are exceeded for all simulations. These simulations assumed
the application was made on July 15 for control of the navel orangeworm.
Current labels also recommend azinphos methyl use for the peach twig
borer, and use for control of this insect occurs earlier in the season,
around the end of flowering. EECs for control peach twig borer would be
higher than those in this assessment.

  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. 

Management Practices

The current maximum label practice for application of azinphos methyl to
almonds is 2 lb a.i./acre with a single application per season. Aerial
application is prohibited, and a 25 ft buffer is required around all
water bodies on the current label. This use pattern has been simulated
using an application of 2  lb/acre applied on July 15 using spray blast
equipment (Table 1). As noted above, buffers of 100 ft, 500 ft and “no
drift” were assessed. The no spray drift simulation reflects the
contribution from runoff alone and 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, according to standard practice, 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
almonds*.



Crop	

App Rate 

(lbs a.i./A)	

Max. No. Apps.	

App. Interval (days)	

Buffer Width (ft)	

App. Method

(% drift)	

First App. Date

Label	2	1	NA	25	air blast (4.5%)	July 15

100 ft buffer	2	1	NA	100	air blast (0.9%)	July 15

500 ft buffer	2	1	NA	500	Air blast (0.2%)	July 15

“No drift”	2	1	NA	NA	no drift	July 15



* 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.



Scenarios

	The California almond scenario was used for this assessment. The
California almond scenario is in San Joaquin County, California and uses
weather data from Sacramento. The soil is a Manteca fine sandy loam.
This is the standard Tier 2 scenario for assessing risks from almonds.

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 2. The input
parameters for PRZM and EXAMS are in Table 3.  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 assumption has 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 near the 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 conducted and
Fruendlich parameters were provided 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 calculating 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, 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. This usually results in substantial error.

Table 2. 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 measurement
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 3. Chemistry input parameters for Tier-II (PRZM/EXAMS) simulation
of azinphos methyl for aquatic assessment.

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, a simulation from the previous assessment was
included (Table 4) which used the current label practice: 1 application
2 lb/acre applied with spray blast equipment with a 25 ft buffer. 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. 

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

25 ft buffer	3.1	2.9	2.2	1.4	1.0

100 ft buffer	1.0	0.94	0.72	0.45	0.34

500 ft buffer	0.6	0.56	0.44	0.28	0.22

No drift	0.59	0.55	0.43	0.27	0.20



Risk quotients based on EECs from the above simulations and toxicity
thresholds for several aquatic animals are presented in Table 5. The
acute risk level of concern is 0.5 for non-endangered species and 0.05
for endangered species. The level of concern for chronic risk is 1. More
detail on the toxicity data and levels of concern can be found in the
original ecological risk assessment for the Group 3 crops (D308568).

Table 5. Risk quotients for acute risk to selected aquatic species for
azinphos methyl use on almonds with various buffer widths around water
bodies. The acute EEC is the 1 in-10-year peak concentration. All levels
of concern are exceeded except for high risk for brook trout in the no
drift scenario.

Buffer Width	Acute EEC

(μg L-1)	Brook Trout

(LC50 = 1.2 μg L-1)	Northern Pike

(LC50 = 0.6 μg L-1)	Gammarus

(EC50 = 0.36 μg L-1)

25 ft	3.1 	2.5	5.1	8.6

100 ft	1.0 	1.2	1.7	2.8

500 ft	0.60 	0.5	1.0	1.7

No drift	0.59 	0.49	0.98	1.6



.

Table 6. Risk quotients for chronic risk to selected aquatic species for
azinphos methyl use on almonds with various buffer widths around water
bodies. The exposure endpoint is the 1-in-10-year 21-day mean
concentration for Daphnia chronic and 1-in-10-year 60-day mean
concentration for the rainbow trout chronic. Risk Quotients that exceed 
levels of concern are in bold

Buffer Width	Chronic EECs (21 and 60 d)

μg L-1)	Rainbow Trout

(NOAEC = 0.44 μg L-1)	Daphnia

(NOAEC = 0.25 μg L-1)

25 ft	2.2 / 1.4	1.8	8.8

100 ft	0.72 / 0.45	1.0	2.9

500 ft	0.44 / 0.28	0.64	1.76

No drift	0.43 / 0.27	0.61	1.72



EECs are substantially reduced from the current listed label practice
with a 25 ft buffer to 500 ft buffer from a peak 1-in-10-year peak EEC
of 3.1 to 0.6 μg L-1. Even so, all acute risk quotients exceed the
endangered species level of concern, and all exceed the acute level of
concern except for brook trout in the no drift scenario (Table 5). All
scenarios exceed the chronic risk level of concern for freshwater
invertebrates (Table 6). The chronic freshwater fish LOC is exceeded for
25 and 100 ft buffer but not for 500 ft buffer or the no drift scenario.

It is important to note that these simulations were run using the
application date of July 15 which is consistent with the time periods
used for control of navel orange worm. Simulations done in response to
comments by the almond growers indicate that there is little difference
in EECs done in July and those for May and June, but they are
substantially reduced from those in March. March application is more
typical of the timing for control of peach twig borer, another pest
listed on the label for azinphos methyl use on almonds (D311127). The
simulations presented in this document represent the label practice if
peach twig borer was taken off the label and application timing was
restricted to that required to control navel orangeworm.

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.

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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.

D311127. Flaherty, Colleen and R. David Jones. 2006. Azinphos
Methyl—OPP Response to Stakeholder Comments Regarding Ecological Risks
for the Proposed Decision for “Group 3” Uses. Internal EPA
Memorandum to Katie Hall, in preparation.

  SEQ CHAPTER \h \r 1 Appendix

Aquatic Exposure Model Input File Names

Input files archived for azinphos methyl applied to almonds.



File Name	

Date	

Description



W23232.dvf	

	

weather for CA almonds scenarios (Sacramento, CA)



Caalmond0C.txt	

June 17, 2004	

PE4 scenario file for CA almonds, unirrigated



Pond298.exv	

August 29, 2002	

standard pond scenario for exams



PE4 simulation input files (.PZR extension)



058001 CA almond 05	

September 14, 2006	

CA almonds, spray blast, 25 ft buffer, 7/15 app



058001 CA almond 06	

October 24, 2006	

CA almonds, spray blast, 100 ft buffer, 7/15 app



058001 CA almond 07	

October 24, 2006	

CA almonds, 500 ft buffer, 7/15 app



058001 CA almond 08	

October 24, 2006	

CA almonds, no drift



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