UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON, D.C. 20460



	

	

OFFICE OF

PREVENTION, PESTICIDES AND

TOXIC SUBSTANCES



MEMORANDUM

PC Code:	128887

DP Barcode:	358877, 358880, 358882, 358885

SUBJECT:	Tier 1 Drinking Water Assessment for the DuPont EXPRESS®
Herbicide Containing the Active Ingredient Tribenuron-Methyl

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 EXPRESS and its active ingredient tribenuron-methyl for
broadcast spray on OPTIMUM®GAT field corn and OPTIMUM®GAT soybeans for
pre-plant, post-emergence, and post-harvest control of certain broadleaf
weeds.  The acute estimated environmental concentrations (EECs) in
surface water as predicted by the FIRST model ranged from 2.57 – 2.88
ppb.  The chronic EECs ranged from 0.76 – 0.86 ppb.  The SCI-GROW
model was used by EFED to estimate ground water concentrations from the
use of this herbicide.  The groundwater EEC as predicted by the SCI-GROW
model was 4.95E-03 ppb.  These EECs are lower than the EECs derived in a
2006 drinking water assessment (DWA) completed by EFED.  In that
assessment the acute and chronic surface water EECs predicted by the
FIRST model were 4.1 ppb and 2.7 ppb, respectively.  The groundwater
acute and chronic EEC as predicted by the SCI-GROW model was 6.8 ppb. 
Therefore, EFED recommends using the 2006 results to predict potential
health risks associated with tribenuron-methyl concentrations in surface
and groundwater.

BACKGROUND	

EXPRESS® is a water soluble granule which is mixed in water and applied
aerially and as a uniform broadcast spray.  The label allows for up to
three applications per growing season for a total of 0.063 lb, of
EXPRESS® per acre (lb. a.i./A) per growing season.  Its mode of action
is an ALS-inhibitor herbicide.   These herbicides slowly starve affected
plants of amino acids eventually leading to inhibition of DNA synthesis.

	In July 2004 a Tier 1 drinking water assessment was completed for
tribenuron-methyl use on wheat, barley and fallow, oats, cotton, and
flax and canola).  Application rates ranged from 0.005 to 0.016 lb
a.i./A.  The number of applications was limited to once per year.  EFED
used the Food Quality Protection Act (FQPA) Index Reservoir Screening
Tool (FIRST) model to estimate drinking water concentrations in surface
waters.  The Screening Concentration in Ground Water (SCI-GROW) model
was used by EFED to estimate ground water concentrations from use of the
herbicide.  The estimated acute environmental concentrations (EECs) as
predicted by the FIRST model ranged from 0.179 – 1.386 ppb.  The
chronic estimated environmental concentrations (EECs) ranged from 0.053
– 0.413 ppb.  Groundwater EECs as predicted by the SCI-GROW model
ranged from 0.0005 – 0.0013 ppb.

	EFED also completed a drinking water assessment of tribenuron-methyl
for use on sunflower, cotton, field corn, sorghum, soybean, canola,
flax, and rice in October 2006.  In this assessment the FIRST and
SCI-GROW modeling was based on a use rate of three (3) aerial
applications of 0.015 lb a.i./A, 14 days apart.  The acute and chronic
surface water EECs predicted by the FIRST model were 4.1 ppb and 2.7
ppb, respectively.  The groundwater acute and chronic EEC as predicted
by the SCI-GROW model was 6.8 ppb.

ENVIRONMENTAL FATE

	Tribenuron-methyl is non-persistent but very mobile in soils. The
primary routes of dissipation for tribenuron-methyl are acid and neutral
hydrolysis and microbial degradation with half-lives of 1-6 days in acid
environments (shallow wells) and 4-30 days in more basic environments. 
The field dissipation half-lives (based on lysimeters) range from <5
days in acid soils to 19 days in soils with a basic pH. 
Tribenuron-methyl is also very mobile with Kads values of 0.1-1.7 ml/g,
and may move laterally to surface water or downward to ground water. 
However, very little mobility was observed in lysimeters used in field
studies.

		The rate of degradation due to chemical processes varies widely.  The
half-lives in pH 5 and 7 aqueous buffered solutions were <1 and 3-6
days, respectively.  Degradation was negligible in pH 9 solutions.  The
major degradates were the phenyl portion of the molecule (saccharin) and
the triazine portion of the molecule (triazine amine).  In the aqueous
photolysis study, the dark control adjusted half-lives for pH 7 and 9
triazine-labeled solutions were 103 days and 358 days, respectively.  In
pH 5 solutions, the rate of degradation was identical for both dark
control and irradiated solutions.   Soil photolysis also degraded
triazine and phenyl-labeled tribenuron-methyl, with dark-control
adjusted half-lives in Gardena silt loam of 23 and 5 days, respectively.
 The major degradates were saccharin and triazine amine.  

		Microbial degradation also appears to metabolize parent
tribenuron-methyl.  In aerobic silt loam soils, the calculated
half-lives were 1.4-2.2 days in an acid soil and 18.3-30.7 days in a
basic soil.   The major metabolite was triazine amine.  Under anaerobic
aquatic conditions the calculated total system half-lives for phenyl and
triazine labels ranged from 2-2.6 days for the Landenberg system (low
pH) and 4.9-11.3 days for the Bradenton system (high pH).  Triazine
amine and saccharin were the major degradates, which were primarily
associated with the water phase.  In field dissipation studies using
lysimeters, tribenuron-methyl dissipated with calculated half-lives of
5.3 and 2.1 days in Idaho and Illinois, respectively.  These half-lives
are consistent with the aerobic soil metabolism half-lives (1.4-18.3
days) and less persistence in acidic conditions.  The primary
metabolites were triazine amine and saccharin.  In field studies in
Delaware, the half-lives were <2 days for parent compound in an acid
soil.  In Madera, CA, the half-life of tribenuron-methyl was 19 days in
a high pH soil.  Again, triazine amine and saccharin were the major
metabolites found in the studies.  In all the field studies, minimal
leaching of residues was observed.

Tribenuron-methyl was very mobile in tested soils with Kf values of
0.1-1.7 ml/g and Kfoc values of 10-74 ml/g.  Adsorption was inversely
related to soil pH (r2=0.90).  Desorption Kf values ranged from 0.3-2.2
ml/g, with Kfoc values of 29-69 ml/g.  Triazine amine was very mobile to
mobile in soil.  Adsorption Kf values ranged from 0.93 to 3.18 ml/g,
with Kfoc values of 53-138 ml/g.  Desorption Kf values ranged from 1.52
to 3.88 ml/g, with Kfoc values from 69-169 ml/g.   Sorption was related
to clay content (r2=0.99), soil organic carbon (r2=0.23), and soil pH
(r2=0.99).  The correlation with soil pH is associated with the effect
of the soil pH on the clay in the tested soils.   N-demethyl triazine
amine, a metabolite of tribenuron-methyl, was also very mobile in soil. 
Adsorption Kf values ranged from 0.1 to 0.6 ml/g, with Kfoc values of
12-26 ml/g.  Desorption Kf values were only possible for the sandy loam
soil (1.1 ml/g) because of limited sorption.  The Kfoc value was 46
ml/g.  Sorption was related to clay content (r2=0.94), soil organic
carbon (r2=0.44), and soil pH (r2=0.95).  The correlation with soil pH
is associated with the effect of the soil pH on the clay in the tested
soils.	

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) in surface and groundwater, respectively, for
aerial applications on OPTIMUM GAT 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 0.063 lb.
a.i./A, for modeling purposes the number of applications was limited to
once per year.  EFED assumed 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 2.57 – 2.88 ppb.  The chronic EECs ranged from 0.76 – 0.86 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 4.95E-03 ppb.  The model input
parameters and output results for the SCI-GROW model are presented in
Table 4.

The EECs predicted by the models in the 2004, 2006 and the current
assessment are different and are proportionate to their varying use
rates and application methods.    

Table 1.  FIRST Model Input Parameters for Tribenuron-methyl

Model Input	Value	Comments

Max. application rate (lb. a.i./A)	0.063	DuPont Express 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 0.063 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

Kd	0.3	MRID 46101501

Aerobic soil metabolism half-life (days)	63	MRID 40245522

Upper 90th confidence bound on mean of non-linear half-lives

Is this pesticide to be wetted-in?	No	Label

Method of application	Aerial spray	Label

Water solubility (mg/L) @ 20°C, pH 7	2040
http://sitem.herts.ac.uk/aeru/footprint/en/

Aerobic aquatic metabolism half-life (days)	126	2*63 days (aerobic soil
metabolism half-life) per input guidance.

Aqueous photolysis half-life (days) at pH 7	103	MRID 40245520



Table 2.  FIRST Model Output - Corn

RUN No. 1 FOR Tribenuron-methyl ON OPTIMUM GAT Corn  *INPUT VALUES*

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

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

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

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

0.063		     1	1	0.30	2040		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.)

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

63.0		     	2		N/A	          103.0-12772.0	      126.0	      124.77

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

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

     PEAK DAY (ACUTE)	ANNUAL AVERAGE (CHRONIC)

       CONCENTRATION		CONCENTRATION

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

0.86

Table 3.  FIRST Model Output - Soybean

RUN No. 2 FOR Tribenuron-methyl ON OPTIMUM GAT Soybean  *INPUT VALUES*

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

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

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

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

0.063		     1	1	0.30	2040		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.)

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

63.0		     	2		N/A	          103.0-12772.0	      126.0	      124.77

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

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

     PEAK DAY (ACUTE)	ANNUAL AVERAGE (CHRONIC)

       CONCENTRATION		CONCENTRATION

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

0.76

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		      0.063	             1		  15		   9.9	     	  4.95E-03

Soybean	      0.063	             1		  15		   9.9	     	  4.95E-03

 Product label

2 MRID 46101501

3 MRID 40245522

Monitoring Data

EFED consulted the   HYPERLINK
"http://infotrek.er.usgs.gov/traverse/f?p=NAWQA:HOME:0"  USGS National
Water Quality Assessment Data Warehouse  for available groundwater and
surface water quality data for tribenuron-methyl.  Approximately 1000
monitoring station records nationwide were reviewed.  All reported
values were below detection limits.

