UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON, D.C. 20460



	

	

OFFICE OF

PREVENTION, PESTICIDES AND

TOXIC SUBSTANCES



MEMORANDUM

PC Code:	128901

DP Barcode:	358804

SUBJECT:	Tier 1 Drinking Water Assessment for the DuPont CLASSIC®
Herbicide Containing the Active Ingredient Chlorimuron-ethyl for use on
OPTIMUM® GAT® Herbicide Tolerant Corn and Soybeans

TO:		James Tompkins, Risk Manager

		Registration Division (7505C)

FROM:	Wm. J. Shaughnessy, PhD, Environmental Scientist

		Environmental Risk Branch II

		Environmental Fate and Effects Division

THROUGH:	Tom Bailey, PhD, Chief

		Environmental Risk Branch II

		Environmental Fate and Effects Division

DATE:	June 15, 2009

	This memorandum presents the results of a Tier 1 drinking water
assessment of CLASSIC and its active ingredient chlorimuron-ethyl for
aerial spray application on OPTIMUM®GAT® tolerant field corn and
soybeans for pre-emergence and post-emergence control of certain annual
grass and broadleaf weeds.  The acute estimated environmental
concentrations (EECs) in surface water, as predicted by the FIRST model,
ranged from 10.67 (soybeans) to 11.98 (corn) ppb.  The chronic EECs in
surface water ranged from 4.47 (soybeans) to 5.02 (corn) ppb.  The
groundwater EEC as predicted by the SCI-GROW model was 6.99 ppb.  These
EECs are higher than EECs predicted in previous assessments for other
uses of the herbicide; therefore, EFED recommends using the current
results to predict potential health risks associated with
chlorimuron-ethyl concentrations in surface and groundwater.

BACKGROUND	

CLASSIC® is a water soluble granule which is mixed in water and applied
as either an aerial or ground spray.  The active ingredient,
chlorimuron-ethyl [ethyl
2-(4-chloro-6-methoxypyrimidin-2-ylcarbamoylsulfamoyl) benzoate], is a
sulfonylurea class herbicide with a herbicidal mode of action of
inhibiting acetolactate synthase.  Chlorimuron-ethyl is used for the
pre- and post-emergent control of certain weeds, such as buttercup and
yellow nutsedge and for the suppression of weeds such as purple aster
and silverleaf.

The current proposed label allows for up to three aerial and ground
spray applications per growing season for a not to exceed total of 0.25
lb, of CLASSIC® per acre (lb. a.i./A) per growing season.

	In July 2007 EFED completed a Tier 1 assessment of chlorimuron-ethyl
and the demethylated-parent (a major metabolite) for use on cranberries
and subgroup 13H berries.  In that assessment usage rates were limited
to 0.063 lb. a.i./A.  The FIRST and SciGrow models were used to generate
expected environmental concentrations (EECs) in surface and groundwater,
respectively.  For the proposed label use rates, the FIRST model
predicted raw surface drinking water acute and chronic exposure
concentrations of 5.7 ppb and 2.4 ppb, respectively.  The SciGrow model
predicted an acute and chronic groundwater exposure concentration of
1.76 ppb.  These EECs were comparable to those calculated in an April
2004 DWA for use on soybeans, peanuts and non-crop land.  In that
assessment, drinking water concentrations were 5.4 ppb (acute) and 2.3
ppb (chronic) for surface water.  The acute and chronic groundwater
concentration was 2.2 ppb.  In the current assessment the acute EECs
predicted by the FIRST model ranged from 10.67 (soybeans) to 11.98
(corn) ppb.  The chronic EECs ranged from 4.47 (soybeans) to 5.02 (corn)
ppb.  The groundwater EEC as predicted by the SCI-GROW model was 6.99
ppb.

ENVIRONMENTAL FATE

		The expected major route of degradation for chlorimuron-ethyl is by
metabolism in soil, with disappearance half-lives (for parent plus
demethylated parent) of 75 to 112 days measured in sandy loam
(Woodstown) and silt loam (Flanagan) soils.  Terrestrial field
dissipation studies in Delaware and North Caroline yielded soil
half-lives of 6.4 to 27 days for the disappearance of the parent. 
Abiotic hydrolysis is as fast as soil metabolism at pH 5 (half-lives 17
to 27 days) but is slow at pH 7 and 9.  Aqueous and soil photolysis were
found not to be significant processes.  Aerobic aquatic metabolism was
not tested; anaerobic aquatic metabolism yielded half-lives of 2-3 weeks
in a Florida sediment-water system, and 5-6 weeks in a Pennsylvania
sediment-water system.

		Chlorimuron-ethyl has 6 major degradates, and no minor degradates. 
The major degradates include demethylated parent, a “sulfonamide,”
and “pyrimidine-amine,” saccharin, dechlorinated pyrimidine-amine,
and demethylated pyrimidine-amine.  The demethylated parent, saccharin,
sulfonamide and pyrimidine-amine each remained at greater than 10% of
applied radioactivity at the end of some of the aerobic soil metabolism
studies (one year), and were major degradates in the field dissipation
studies.

In the environment, parent chlorimuron-ethyl is very mobile in soil,
with Kd values of <0.03 (sandy loam), 0.28 (silt loam), and >1.6 (silt
loam).  The parent is not expected to be volatile, with a reported vapor
pressure of 4E-12 atm.  In soil column leaching studies using
phenyl-ring labeled parent, saccharin and the sulfonamide were observed
at up to 28% and 4.3%, respectively, of the applied radiation in the
leachate.  Saccharin has Koc values of 4.6 to 15.5, indicating that it
is mobile (MRID 45012638).  Overall, chlorimuron-ethyl is expected to
dissipate by metabolism in soil and transport in water by run-off, or
leaching.

FINDINGS AND CONCLUSIONS

	In this assessment, the FIRST (Version 1.1.0) and SCI-GROW (Version
2.3) models were used to predict the estimated environmental
concentrations (EECs) of chlorimuron-ethyl in surface and groundwater,
respectively, for aerial applications on OPTIMUM®GAT® tolerant corn
and soybean.

While the product label allows for up to three applications per growing
season with the total amount of active ingredient applied not to exceed
0.25 lb. a.i./A, for modeling purposes the number of applications was
limited to once per year.  EFED assumes that the more conservative model
EECs would be achieved by using a single application at the maximum
application rate.    Input parameters for the FIRST model runs for
applications on corn and soybean are presented in Table 1.  Model output
is presented in Table 2 and 3.    The acute EECs as predicted by the
FIRST model ranged from 10.67 (soybeans) – 11.98 (corn) ppb.  The
chronic EECs ranged from 4.47 (soybeans) – 5.02 (corn) ppb.  The
SCI-GROW model was used by EFED to estimate ground water concentrations
from use of the herbicide.  The groundwater EEC as predicted by the
SCI-GROW model was 6.99 ppb.  The model input parameters and output
results for the SCI-GROW model are presented in Table 4.

Because of higher use rates the EECs predicted in the current assessment
are proportionately higher than those predicted in the 2007 and 2004
assessments.

Table 1.  FIRST Model Input Parameters for Chlorimuron-ethyl

Model Input	Value	Comments

Max. application rate (lb. a.i./A)	0.25	DuPont CLASSIC Herbicide Label

No. of applications per year	1	The label allows for up to three
applications per growing season at a rate not to exceed 0.25 lb. a.i./A
per growing season; 1 is used to yield more conservative model results

Percent cropped area:

    Corn

    Soybean	

46

41	From model input guidance for specific crops

Koc	2.5 ml/g for Cecil sandy loam	MRID 145778, 143120, 154436, 154440

Aerobic soil metabolism half-life (days)	106 days 90th percentile upper
confidence limit on 4 values) 

91 days (average of 4 values)	MRID 131580, 145402

Is this pesticide to be wetted-in?	No	Label

Method of application	Aerial spray	Label

Water solubility (mg/L) @ 20°C, pH 7	1200 (parent) 	  HYPERLINK
"http://sitem.herts.ac.uk/aeru/footprint/en/" 
http://sitem.herts.ac.uk/aeru/footprint/en/ 

solubility is a strong function of pH above pKa of 4.2

Aerobic aquatic metabolism half-life (days)	212 	2*106 days (aerobic
soil metabolism half-life) per input guidance.

Aqueous photolysis half-life (days) at pH 7	Stable	MRID 145779, 154438



Table 2.  FIRST Model Output - Corn

RUN No. 1 FOR Chlorimuron-ethyl ON OPTIMUM GAT Corn  *INPUT VALUES*

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

RATE (#/AC)	  No.APPS &	SOIL	SOLUBIL	APPL TYPE	%CROPPED	INCORP

ONE (MULT)	   INTERVAL	 Koc	(PPM)		(%DRIFT)	AREA		(IN)

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

0.25		     1	1	2.5	1200		AERIAL (16.0)	46.0	   0.0

FIELD AND RESERVOIR HALFLIFE VALUES (DAYS)

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

METABOLIC	   DAYS UNTIL   HYDROLYSIS  PHOTOLYSIS  METABOLIC  COMBINED

(FIELD)	   RAIN/RUNOFF	(RESERV.)    (RES.-EFF)	(RESERV.)	  (RESERV.)

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

106		     	2		N/A	          0.0-0.0		      212	      212

UNTREATED WATER CONC  (MICROGRAMS/LITER (PPB)) Ver 1.1.0  Jan. 1, 2007

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

     PEAK DAY (ACUTE)	ANNUAL AVERAGE (CHRONIC)

       CONCENTRATION		CONCENTRATION

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

11.98					5.02

Table 3.  FIRST Model Output - Soybean

RUN No. 2 FOR Chlorimuron-ethyl ON OPTIMUM GAT Soybean  *INPUT VALUES*

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

RATE (#/AC)	  No.APPS &	SOIL	SOLUBIL	APPL TYPE	%CROPPED	INCORP

ONE (MULT)	   INTERVAL	 Koc	(PPM)		(%DRIFT)	AREA		(IN)

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

0.25		     1	1	2.5	1200		AERIAL (16.0)	41.0	   0.0

FIELD AND RESERVOIR HALFLIFE VALUES (DAYS)

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

METABOLIC	   DAYS UNTIL   HYDROLYSIS  PHOTOLYSIS  METABOLIC  COMBINED

(FIELD)	   RAIN/RUNOFF	(RESERV.)    (RES.-EFF)	(RESERV.)	  (RESERV.)

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

106		     	2		N/A	          0.0-0.0	      		212	      212

UNTREATED WATER CONC  (MICROGRAMS/LITER (PPB)) Ver 1.1.0  Jan. 1, 2007

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

     PEAK DAY (ACUTE)	ANNUAL AVERAGE (CHRONIC)

       CONCENTRATION		CONCENTRATION

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

10.67					4.47

Table 4.  SCI-GROW Model Input and Output

Crop		Application	      No. of	       Median Koc    Aerobic Soil	 EEC
(ppb)

Rate (lb. a.i./A)1   Applications    (mL/g)2	        Metabolism

						        Half-life (days)

        (Average) 3   

Corn		      0.25	             1		  2.5		   91	     	  6.99

Soybean	      0.25	             1		  2.5		   91	     	  6.99

 Product label

2 MRID 145778, 143120, 154436, 154440

3 MRID 131580, 145402

Monitoring Data 

The US Geological Survey (USGS) reports that chlorimuron-ethyl has been
detected in the drinking water facilities presented in Table 5.  The
data show the percent of the total number of samples in which the
herbicide was detected and the maximum concentration observed at that
location.  The reported concentrations are less than those predicted by
the FIRST model in this assessment.

Table 5.  Detection Frequency and Maximum Concentration at Four Drinking
Water Facilities 

Reservoir Location	Concentration at Water-supply Intake	Concentration at
Reservoir Outflow Site	Concentration of Treated Effluent

	Detection %	Max Conc. (ppb)	Detection %	Max Conc. (ppb)	Detection %	Max
Conc. (ppb)

Indianapolis Water Co., IN	-	-	-	-	5	0.04

Higginsville Reservoir, MO	11	0.018	10	0.026	-	-

East Fork Lake, OH	47	0.05	36	0.021	-	-

Lake Mitchell, SD	5	0.021	11	0.023	9	0.026

Source: USGS Open file report 01-456 (Pesticides in Selected
Water-Supply Reservoirs and Finished Drinking Water, 1999-2000: Summary
of results from a Pilot Monitoring Program)

	The National Water-Quality Assessment (NAWQA) Program database was
searched for the latest surface and groundwater monitoring results for
chlorimuron-ethyl.  Chlorimuron-ethyl has been monitored for in 42
states where there have been 82 incidences where the herbicide has been
found to be above detection limits.  In these incidences the
concentrations have ranged from 0.010 to 0.870 ppb.  These reported
concentrations are also less than those predicted by the FIRST and
SCI-GROW models in this assessment.

 EFED Memo to Daniel Rosenblatt, Registration Division. July 2007. Subj:
Tier 1 Drinking Water Assessment for Chlorimuron-ethyl Use on Cranberry
and Low-growing Berry Subgroup 13H, Except Strawberry

