U. S. ENVIRONMENTAL PROTECTION AGENCY

Washington, D.C. 20460

OFFICE OF CHEMICAL SAFETY AND POLLUTION PREVENTION

PC Code: 060109

DP Barcode: 373378

376634

May 20, 2010      

                                                                        
                                                                        
                                                                        
                                                                        
                                                                        
                                                                        
                        

MEMORANDUM	

SUBJECT:	Tier II Drinking Water Exposure Assessment to Establish a
Tolerance of Thiamethoxam on Peanut and Alfalfa Seeds

TO:	Dennis McNeilly, Chemist/Risk Assessor

	Christina Swartz, Branch Chief

Julie Chao, Risk Manager Reviewer

	Venus Eagle, Risk Manager	

		

FROM:	Christopher M. Koper, Chemist

	Environmental Risk Branch I

	Environmental Fate and Effects Division

THROUGH:	Anita Pease, Acting Branch Chief

	Brian Anderson, Risk Assessment Process Leader

Faruque Khan, Senior Fate Scientist, Secondary Reviewer

Environmental Risk Branch I

	Environmental Fate and Effects Division

A Tier II screening-level drinking water assessment was conducted for
the proposed new use of thiamethoxam on peanut and alfalfa seeds
submitted by Syngenta Crop Protection, Inc.  Tolerances for leafy
vegetables, dry bulb onions and corn were also included in this
assessment after review of the submitted labels showed increased rates
that were not previously assessed for these crops. 

Thiamethoxam is a systemic neonicotinoid insecticide that can be applied
as a seed treatment for various crops.  This assessment was based on
information contained in the Cruiser( Insecticide (EPA Reg. No. 100-941)
and Cruiser( PD (EPA Reg. No. 100-RGAL) labels.  The estimated drinking
water concentrations (EDWCs) are based on the maximum proposed
application rates specified in the product labels.  The EDWCs for the
human health risk assessment are presented in Table 1.

For surface water, the EDWCs for the crops modeled in this assessment
did not exceed the previous EDWCs for rice reported in the previous
drinking water assessment (D363202) dated 07 April 2009. The reported
acute concentration of 131.77 µg/L, annual mean (chronic) concentration
of 11.31 µg/L and the highest 30 year annual average concentration of
11.31 µg/L from the rice model are higher than the reported EDWCs for
various uses in Table 1.  Therefore, the current EDWCs for surface water
do not supersede the drinking water concentrations previously estimated
for this chemical.

For groundwater, based on the highest annual use rate for dry bulb onion
(0.300 lbs. a.i./acre) and the proposed use rates for peanut seed (0.080
lbs. a.i./acre) and alfalfa seed (0.022 lbs. a.i./acre), the SCI-GROW
model concentrations of thiamethoxam were 4.66 µg/L, 1.24 µg/L and
0.34 µg/L, respectively (Table 1).  These values were compared to 4.14
µg/L reported for grapes in the previous drinking water assessment
(D329467) dated 10 January 2007.  It is important to note that the
predicted groundwater value for grapes is a corrected value described in
detail in a previous drinking water assessment (D368072) dated 02
February 2010. Since 4.66 µg/L for dry bulb onions is greater than 4.14
µg/L, the current EDWC supersedes the previously recommended EDWC for
groundwater.  

In conclusion, the current EDWCs for surface water do not supersede the
estimated drinking water concentrations previously derived for
thiamethoxam. The current EDWC for groundwater supersedes the estimated
drinking water concentration previously estimated for this chemical.
Therefore, the recommended EWDC for groundwater is 4.66 µg/L.

Table 1  Thiamethoxam Estimated Drinking Water Concentrations (EDWCs)
for Surface Water and Groundwater Based on Selected Crop Scenarios

Proposed Label Use	PRZM/EXAMS

Scenario

(first app. date)	Application Rate 

(interval between applications)	1-in-10 year acute

(µg/L)	1-in-10 year chronic 

(µg/L)	30 year average (µg/L)

Surface Water

Peanuts	NC Peanuts

(May 9)

	

1 app. X 0.080 lbs. ai/acre

	

1.51	

0.16

	

0.05

Alfalfa	CA Alfalfa

(January  9)

	

1 app. X 0.022 lbs. ai/acre

	

0.37	

0.07	

0.03

	MN Alfalfa

(May 25)

	

1 app. X 0.022 lbs. ai/acre

	

0.27	

0.03	

0.02

	NC Alfalfa

(March 25)

	

1 app. X 0.022 lbs. ai/acre

	

0.55	

0.08	

0.03

	PA Alfalfa

(March 9)

	

1 app. X 0.022 lbs. ai/acre

	

0.37	

0.05	

0.03

	TX Alfalfa

(August 25)

	

1 app. X 0.022 lbs. ai/acre

	

0.89	

0.10	

0.05

	IL Alfalfa

(May 25)

	

1 app. X 0.022 lbs. ai/acre

	

0.58	

0.06	

0.03

Leafy Vegetables

	CA Lettuce 

(Feb. 9)	

1 app. X 0.266 lbs. ai/acre

	

23.78 1

	

3.68 1	

1.29 1

Dry Bulb Onions	CA Onion

(Jan. 9)

	

1 app. x 0.300 lbs ai/acre	

1.57	

0.25	

0.06

	GA Onion

(Sept. 8)	

1 app. x 0.300 lbs ai/acre

	

20.45	

1.54	

0.44

Corn	KS Corn

(May 3)	

1 app. X 0.210 lbs. ai/acre

	

11.88

	

1.34	

0.63

	MS Corn

(April 3)	

1 app. X 0.210 lbs. ai/acre

	

17.93	

1.81	

0.75

	NE Corn

(May 18)	

1 app. X 0.210 lbs. ai/acre

	

15.43

	

1.73	

0.70

	OH Corn

(April 24)	

1 app. X 0.210 lbs. ai/acre

	

12.46	

1.50	

0.39

Groundwater

Peanuts	NA	

1 app. x 0.080 lbs ai/acre

	

1.24	

1.24	

1.24

Alfalfa	NA	

1 app. x 0.022 lbs ai/acre

	

0.34	

0.34	

0.34

Dry Bulb Onion	NA	

1 app. x 0.300 lbs ai/acre

	

4.66 1	

4.66 1	

4.66 1

Surface water values denote maximum PCA adjusted EDWC values with PCA
factor of 0.87.

Seed treatment is the method of application for all crop scenarios with
a 2.54 cm. incorporation depth, 100

% application efficiency and 0% spray drift.

NA = Not Applicable    

1 = Bolded EDWCs are the maximum concentrations predicted by PRZM/EXAMS
and SCI-GROW models



Monitoring Data

Thiamethoxam is not included as one of the analytes monitored in U.S.
surface and groundwater under the USGS’s National Water Quality
Assessment (NAWQA) program.  

Two small-scale prospective groundwater (PGW) monitoring studies (MRID
47486201) were initiated in southwest Michigan in June 2001 and
southwestern Georgia in the spring of 2002 to determine the potential
for the active ingredient thiamethoxam and its degradates (i.e:
CGA-322704, CGA-355190, CGA-353042, NOA-404617, NOA-407475) to reach
groundwater when used in a vulnerable hydrogeologic setting. Analytical
data were collected from spring 2001 through the 47-months after
treatment (MAT; May 2005) sampling event for the Michigan study and from
spring 2002 through 36 months after application (MAA; May 2005) sampling
event for the Georgia study.  The monitoring data demonstrate that
concentrations of the analytes of interest have peaked in the
groundwater and have either completely dissipated or have leveled off
for an extended period of time.  Further details concerning this study
can be found in the following memorandum (D325449) dated 12 June 2006. 

In Michigan, thiamethoxam residues found in soil-pore water samples
clearly peaked at 3.5 µg/L 14 MAT and then declined. No quantifiable
residues of thiamethoxam > LOQ (0.05 µg/L) were determined in any of
the 3, 6, and 9-ft lysimeters after the 36, 16, and 41-MAT sampling
events, respectively. By the 47-MAT sampling event, only trace level
detections of thiamethoxam were observed in three of the 15-ft.
lysimeter units (maximum concentration 0.11 µg/L).

In Georgia, thiamethoxam residues were confirmed in only four shallow
monitoring wells (MW-1S, MW-3S, MW-5S, and MW-8S) during 47 months of
sampling. Thiamethoxam residues in groundwater peaked at 43-MAT, and by
47-MAT, residues were confined to two shallow monitoring wells only
(maximum concentration 0.12 µg/L).  Thiamethoxam was not detected (LOQ
= 0.050 µg/L) in any groundwater samples collected from the deep
monitoring wells during 47 months of sampling.

Drinking Water Exposure Modeling

Estimated drinking water concentrations (EDWCs) were generated using
EFED’s standard suite of models.  Application rates for the proposed
new uses on peanut and alfalfa seeds were assessed, based on information
provided on the labels.  Since there is no standard scenario for
alfalfa, the specialty OP (Organophosphate) and NMC (N-Methyl Carbamate)
scenarios for alfalfa were utilized.  In addition, EDWCs for leafy
vegetables, dry bulb onions and corn were also included in this
assessment after review of the submitted labels showed increased rates
that were not previously assessed for these crops.

Models

Pesticide Root Zone Model (PRZM v3.12.2, May 12, 2005) and Exposure
Analysis Modeling System (EXAMS v2.98.04.06, April 25, 2005) are
simulation models coupled with the input PE5.pl shell (PE5, PE Version
5.0, November 15, 2006) to generate EDWCs of thiamethoxam residues that
may occur in surface water used as drinking water.  The PRZM model
simulates pesticide movement and transformation on and across the
agricultural field resulting from crop applications.  The EXAMS model
simulates pesticide loading via runoff, erosion, and spray drift
assuming a standard watershed of 172.8 ha that drains into an adjacent
standard drinking water index reservoir of 5.26 ha, an average depth of
2.74 m.  A more detailed description of the index reservoir watershed
can be found in Jones et al., 1998.  The coupled PRZM/EXAMS model and
user’s manuals may be downloaded from the U.S. Environmental
Protection Agency (EPA) Water Models web-page (USEPA, 2010).  Percent
Crop Areas (PCA) that account for the maximum area within a watershed
that may be planted with the modeled crop are applied to concentrations
predicted by PRZM/EXAMS.

Screening Concentration in Ground Water (SCI-GROW v2.3, Jul. 29, 2003)
is a regression model used as a screening tool to estimate pesticide
concentrations found in ground water used as drinking water.  The
SCI-GROW model was developed by fitting a linear model to ground water
concentrations with the Relative Index of Leaching Potential (RILP) as
the independent variable.  Ground water concentrations were taken from
90-day average high concentrations from Prospective Ground Water
studies.  The RILP is a function of aerobic soil metabolism and the
soil-water partition coefficient.  The output of SCI-GROW represents the
concentrations of residues that might be expected in shallow unconfined
aquifers under sandy soils, which is representative of the ground water
most vulnerable to pesticide contamination and likely to serve as a
drinking water source.  The SCI-GROW user’s manual is currently under
development; however, information concerning the model can be found on
the EPA Water Models web-page (USEPA, 2010).  

 

Input Parameters

Input parameters for the PRZM/EXAMS and SCI-GROW models are listed in
Table 2 and Table 3.  Explanations of various model input parameters are
discussed below.

Table 2.  PRZM/EXAMS Input Parameters for Thiamethoxam.  

Input Parameter:	Value:	Comment:	Source:

Scenario(s):	Peanuts: NC Peanuts

Alfalfa: CA, MN, NC, PA, TX, IL Alfalfa

Leafy Vegetables:  CA Lettuce

Dry Bulb Onions: CA, GA Onions

Corn:  KS, MS, NE, OH Corn	representative scenarios for each use	N/A 

Maximum Single 

Application Rate

lbs a.i./A (kg a.i./ha)	Peanuts:  0.080  (0.090)

Alfalfa:  0.022  (0.025)

Leafy Vegetables:  0.266 (0.299)

Dry Bulb Onions: 0.300  (0.337)

Corn:  0.210  (0.236)	proposed rates	proposed  label

Applications per Year	Seed Treatments:

Peanuts:  1

Alfalfa:  1

Leafy Vegetables:  1

Dry Bulb Onions: 1

Corn:  1	label directions.  Label specifies rates per season.  If crops
are rotated with those on which thiamethoxam is used, yearly rates may
be higher. 	proposed label

Application Interval (days)	N/A (single application only) 	intervals
were selected to reflect labeled application patterns.	proposed label

Date of Initial Application

(scenario/day/month)	Peanuts:

NC Peanuts:  09-05

Alfalfa:

CA Alfalfa:  09-01

MN Alfalfa:  25-05

NC Alfalfa:  25-03

PA Alfalfa:  09-03

TX Alfalfa:  25-08

IL Alfalfa:  25-05

Leafy Vegetables:  

CA Lettuce:  09-02

Dry Bulb Onions: 

CA Onions:  09-01

GA Onions:  08-09

Corn:

KS Corn:  03-05

MS Corn:  03-04

NE Corn:  18-05

OH Corn:  24-04	7 days before crop emergence was selected for seed
treatment scenarios.  

	Crop Scenarios

Application Method	ground (seed treatment)	label directions	proposed
label

CAM Input	4	soil applied, user defined incorporation depth, uniform with
depth	proposed label

Incorporation Depth (cm.)	2.54 	label directions  (> 1 in.)	proposed
label

IPSCND Input	1	remaining pesticide on foliage is converted to surface
application to the top soil layer 	US EPA, 2005

Spray Drift Fraction	0	incorporated seed treatment	Input parameter
guidance (USEPA, 2009) 

Application Efficiency	1.00	seed treatment	Input parameter guidance 

(USEPA, 2009)

Molecular Mass (g/mol)	291.7	product chemistry data	US EPA, 2007

Vapor Pressure at 25°C (torr)	4.95 x 10-11	product chemistry data	US
EPA, 2007

Henry’s Law Constant

(atm-m3/mol)	4.65 x 10-15	calculated from volatility and solubility data
registrant submitted data

Solubility in Water at 25oC (mg/L)	4100	product chemistry data	US EPA,
2007

Organic Carbon Partition Coefficient (KOC) (L/kgOC)	70.23	represents the
average KOC	MRID 44703502

Aerobic Soil Metabolism Half-life (days)	480.92	represents the 90th
%-ile confidence bound on the mean half-life	MRID 44703418

MRID 44703419

MRID 44703501

Aerobic Aquatic Metabolism Half-life (days)	19.92	represents the 90th
%-ile confidence bound on the mean half-life	MRID 44715032



Anaerobic Aquatic Metabolism Half-life (days)	43.98	represents the 90th
%-ile confidence bound on the mean half-life	MRID 44715029

MRID 44715030

MRID 44715031

Hydrolysis Half-lives (days)	Stable	pH 7	MRID 44703417 

Aqueous Photolysis

Half-life (days)	4.46	represents the 90th %-ile confidence bound on the
mean half-life	MRID 44715032

 N/A = Not Applicable



Table 3.  SCI-GROW Input Parameters for Thiamethoxam.  

Input Parameter	Value	Comment	Source

Maximum Application Rate per growing season (lbs a.i./A)	0.300 (dry bulb
onion)

0.080 (peanuts)

0.022 (alfalfa)	maximum application rate	proposed label

Applications per Year	1	label directions	proposed label

Organic Carbon Partition Coefficient (KOC) (L/kgOC)	48	represents the
median KOC value	MRID 44703502

Aerobic Soil Metabolism

Half-life (days)	302	represents the mean half-life value	MRID 44703418

MRID 44703419

MRID 44703501



Application methods and rates were obtained from the proposed labels for
peanut and alfalfa seeds and from existing labels for leafy vegetables,
dry bulb onions and corn.  Considering the seed treatment, the date of
initial application of thiamethoxam was 7 days before the crop emergence
date from the respective crop scenarios.  In addition, among the 10
possible scenarios available for corn, the Kansas, Mississippi,
Nebraska, and Ohio corn scenarios were selected to be representative of
upper bound estimations in this assessment.  

 

Standard percent cropped areas (PCA) were used for all uses as estimates
of the extent of watershed on which crops is grown (Effland et al.,
1999).  The default agricultural PCA of 0.87 was used for the proposed
uses because they are not confined to specific regions of the U.S. 
Actual fractions of cropped areas could be less in some areas of the
country. The model input and output data for these estimates are
attached in Appendix A and B.

REFERENCES

Effland, W. R., N. C. Thurman, I. Kennedy.  1999.  Proposed Methods for
Determining Watershed-derived Percent Crop Areas and Considerations for
Applying Crop Area Adjustments to Surface Water Screening Models. 
Presentation to the FIFRA Science Advisory Panel, May 27, 1999.  Online
at:   HYPERLINK "http://www.epa.gov/scipoly/sap/1999/index.htm" 
http://www.epa.gov/scipoly/sap/1999/index.htm 

Jones, R. D., S. Abel, W. R. Effland, R. Matzner, R. Parker.  1998.  An
Index Reservoir for Use in Assessing Drinking Water Exposure.  Proposed
Methods for Basin-scale Estimation of Pesticide Concentrations in
Flowing Water and Reservoirs for Tolerance Reassessment.  Presentation
to FIFRA Science Advisory Panel, June 29-30, 1998.  Online at:  
HYPERLINK "http://www.epa.gov/scipoly/sap/1998/index.htm" 
http://www.epa.gov/scipoly/sap/1998/index.htm 

USEPA.  2005.  PRZM-3, A Model for Predicting Pesticide and Nitrogen
Fate in the Crop Root and Unsaturated Soil Zones:  Users Manual for
Release 3.0.  May 2005.

USEPA.  2006.  Termination of Sampling Request for the Michigan and
Georgia Small-Scale Prospective Groundwater Monitoring Studies for
Thiamethoxam.  D325449.  June 12, 2006.

USEPA.  2007.  Revised Tier II Drinking Water Assessment for
Thiamethoxam. U.S. Environmental Protection Agency, Office of
Prevention, Pesticides and Toxic Substances.  DP DP329467. January 10,
2007.

USEPA.  2009.  Guidance for Selecting Input Parameters in Modeling the
Environmental Fate and Transport of Pesticides, Version 2.1.  U.S.
Environmental Protection Agency, Office of Prevention, Pesticides and
Toxic Substances, Office of Pesticide Programs, Environmental Fate and
Effects Division, October 22, 2009. 

USEPA.  2010.  Water Models.  U.S. Environmental Protection Agency,
Pesticides: Science and Policy.  Online at:   HYPERLINK
"http://www.epa.gov/oppefed1/models/water/" 
http://www.epa.gov/oppefed1/models/water/ 

Appendix A. PRZM/EXAMS Output Files

Appendix B. SCI-GROW Output Files:

Dry Bulb Onion:

SCIGROW

VERSION 2.3

ENVIRONMENTAL FATE AND EFFECTS DIVISION

OFFICE OF PESTICIDE PROGRAMS

U.S. ENVIRONMENTAL PROTECTION AGENCY

SCREENING MODEL

FOR AQUATIC PESTICIDE EXPOSURE

 

 SciGrow version 2.3

 chemical:Thiamethoxam on Dry Bulb Onion

 time is  4/16/2010  14: 7:45

 -----------------------------------------------------------------------
-

  Application      Number of       Total Use    Koc      Soil Aerobic

  rate (lb/acre)  applications   (lb/acre/yr)  (ml/g)   metabolism
(days)

 -----------------------------------------------------------------------
-

      0.300           1.0           0.300      4.80E+01      302.0

 -----------------------------------------------------------------------
-

 groundwater screening cond (ppb) =   4.66E+00 

 ***********************************************************************
*

Peanuts:

SCIGROW

VERSION 2.3

ENVIRONMENTAL FATE AND EFFECTS DIVISION

OFFICE OF PESTICIDE PROGRAMS

U.S. ENVIRONMENTAL PROTECTION AGENCY

SCREENING MODEL

FOR AQUATIC PESTICIDE EXPOSURE

 

 SciGrow version 2.3

 chemical:Thiamethoxam

 time is  5/11/2010  16:39:56

 -----------------------------------------------------------------------
-

  Application      Number of       Total Use    Koc      Soil Aerobic

  rate (lb/acre)  applications   (lb/acre/yr)  (ml/g)   metabolism
(days)

 -----------------------------------------------------------------------
-

      0.080           1.0           0.080      4.80E+01      302.0

 -----------------------------------------------------------------------
-

 groundwater screening cond (ppb) =   1.24E+00 

 ***********************************************************************
*

	

Alfalfa:	

SCIGROW

VERSION 2.3

ENVIRONMENTAL FATE AND EFFECTS DIVISION

OFFICE OF PESTICIDE PROGRAMS

U.S. ENVIRONMENTAL PROTECTION AGENCY

SCREENING MODEL

FOR AQUATIC PESTICIDE EXPOSURE

SciGrow version 2.3

chemical:Thiamethoxam

time is  5/11/2010  16:41:54

------------------------------------------------------------------------

Application      Number of       Total Use    Koc      Soil Aerobic

rate (lb/acre)  applications   (lb/acre/yr)  (ml/g)   metabolism (days)

------------------------------------------------------------------------

0.022           1.0           0.022      4.80E+01      302.0

------------------------------------------------------------------------

groundwater screening cond (ppb) =   3.42E-01

************************************************************************

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