UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D.C. 20460

January 12 , 2012

PC Code: 128845

MEMORANDUM DP Barcode: 391089

SUBJECT: Tier I Drinking Water Assessment for New Use of Thifensulfuron

- methyl

on Chicory

FROM: Wm. J. Shaughnessy, Ph.D., Environmental Scientist Environmental Fate and Effects Division/ERB II (7507P)

THRU: Brian Anderson , Branch Chief Greg Orrick, Environmental Scientist Environmental Fate and Effects Division/ERB II (7507P)

TO: Barbara Madden, Risk Manager Andrew Ertman, Risk Reviewer Registration Division /RIMUERB (7505 P )

The Environmental Fate and Effects Division (EFED) has conducted a Tier I Drinking Water Assessment (DWA) of the herbicide thifensulfuron

- methyl , also known

as DPX

- M6316 ( 3

- (4

- methoxy

- 6

- methyl

- 1,3,5

- triazin

- 2

- ylcarbamoylsulfamoyl)thiophene

- 2

- carboxylic aci d ; CAS Reg. No. 79277

- 67

- 1 ) for an IR

- 4 petition ( 1E7885 ) for use on chicory

. This assessment contains estimated drinking water concentrations (EDWCs) for both surface and ground water

. Since the predict e d

EDWCs for use of t hifensulfuron

- methyl on chicory do not exceed those of the maximum use pattern on barley, EFED recommends using for dietary assessment the highest EDWCs for the use on barley: 4.429 μg/L (acute) and 0.9 μg/L (chronic) for surface water and 0.0972 μg/L (acute and chronic) for grou ndwater.

PREVIOUS ASSESSMENT

In September 2009, EFED completed a drinking water assessment for the proposed
new use of thifensulfuron

- methyl to control broadleaf weeds on safflower 1

. In this assessment, EFED compared EDWCs for a proposed use on safflowe r with those predicted for an existing use on barley (see attached)

.

The maximum application rate and maximum amount applied per season were the

same in the 2009 assessment , at 0.0188 lbs a.i./A for safflower. Current uses on barley

allow a maximum single application rate of 0.028 lbs a.i./A with up to two applications per season for a seasonal total of 0.056 lbs a.i./A per year. Table 1 contains summary information used for comparing EDWCs for thifensulfuron

- methyl use on safflower and barley.

1 Memorandum from Andrew Shelby and Nelson Thurman , ERB2, EFED to Tobi Colvin

- Snyder , RD, subj: Tier I Drinking Water Assessment for Proposed New Use of Thifensulfuron methyl on safflower, DP Barcode 363089, dtd: September 29, 2009. OFFICE OF CHEMICAL SAFETY

AND POLLUTION PREVENTIONTable 1 : Summary of maximum use information for thifensulfuron, based on Harmony SG Herbicide labels

USE SINGLE
APP. RATE (lbs a.i./A) NUMBER
OF APPS. SEASONAL
APP. RATE (lbs a.i./A) INTERVAL

BETWEEN APPS. (days) APP.
METHOD INCORP

. DEPTH (inches)

Safflower 0.0188 1* 0.0188 Not Specified* Aerial or

Ground Spray 0

Barley,

Fallow ** 0.028 2 0.056 14 Aerial or

Ground Spray 0

Source: 2009 DWA

* Given a single application of 0.0188 lbs a.i./A, the seasonal application rate is achieved and only one application can occur. Subsequent applications can occur if the maximum application rate is not used. The interval between these applications is not specified.

** Thifensulfuron

- methyl is currently registered for use on barley through an existing label

.

The acute E DW Cs in surface water, as predicted by the tier 1 FIRST model, ranged from 1.66 (safflower) to 4.43 (barley) μg/L . The chronic E DW Cs in surface water ranged from 0.34 (safflower) to 0.90 (barley) μg/L . The groundwater E DW C as predicted by the SCI

- GROW model ranged from 0.03 (safflower) to 0.09 (barley) μg/L

. Table 2 provides a summary of EDWCs predicted by the FIRST and SCIGROW mod els
in the 2009 assessment. The EDWCs for the proposed new use of thifensulfuron

- methyl

on safflower were less than the EDWCs for the existing use on barley.

Table 2 : Maximum Tier I Estimated Drinking Water Concentrations (EDWCs) for drinking water risk assessment based on ground application of thifensulfuron.

DRINKING WATER SOURCE (MODEL USED) USE (rate modeled) MAXIMUM ESTIMATED
DRINKING WATER CONCENTRATION (EDWC) ( μg/L ) Groundwater (SCIGROW) Safflower (0.0188 lbs a.i./A; 1 application) Acute and Chronic 0.0326

Barley, Fallow (0.028 lbs a.i./A; 2 applications) Acute and Chronic 0.0972

Surface water (FIRST) Safflower (0.0188 lbs a.i./A; 1 application) Acute 1.656

Safflower (0.0188 lbs a.i./A; 1 application) Chronic 0.336

Barley, Fallow (0.028 lbs a.i./A; 2 applications) Acute 4.429

Barley, Fallow (0.028 lbs a.i./A; 2 applications) Chronic 0.900

EFED concluded that thifensulfuron

- methyl is a somewhat persistent chemical that partitions readily to water. Acid hydrolysis and photodegradation in water are the primary routes of degradation . R apid leaching is not unlikely because of the low sorption coefficients. In soils, hydrolysis under acidic conditions and metabolism will likely be the dominant routes of degradation

. As a result, while collectively these data suggest that some attenuation in the aquatic environment may occur

, these data indicate that surface and ground drinking water supplies may be impacted by this herbicide. Further, the highest surface water EDWC (as predicted by the FIRST model) was from the barley use: 4.429 ppb (acute) and 0.9 ppb (chronic). The SCI

- GROW estimated ground water EDWC from the barley use was 0.0972 ppb (acute and chronic)

.ANALYSIS

In the current assessment, EFED is assessing the potential for exposure to drinking water supplies from the proposed new use of thifensulfuron

- methyl on chicory. According to the proposed supplemental label 2 , the maximum seasonal application rate is 0.3 oz a.i./A or 0.0187 lbs. a.i./A. This is the same application rate that was proposed for

use on safflower and assessed in the September 2009 DWA ; and is one

- third the approved rate for use on barley

. The supplemen tal label for use on chicory does not specify the method of application ; however, EFED assumes that the herbicide can be applied in the same manner as described on the label s for use on barely and safflower ; either through ground or aerial applications. Since the application rate and method proposed for chicory are the same as th ose for safflower, and there ha ve been no changes to the environmental fate model input values, EFED expects that model output EDWCs

will be the same for these proposed uses

.

Mo nitoring Data

EPA ’s STORET database reports 82 non

- targeted water analyses f or thifensulfuron

methyl conducted in Iowa from June to July, 2005 and 26 analyses of aquatic organisms in North Dakota from April to August, 1994 (as of Dec. 29, 2011)

. There we re no detections in these analyses

. Thifensulfuron

- methyl is not an analyte in the United States Geological Survey’s (USGS) National Water Quality Assessment Program (NAWQA)

program ( as of Dec. 29, 20 11).

Drinking Water Treatment

No data are available to indicate the effectiveness of drinking water treatment on the degradation of thifensulfuron.

CONCLUSIONS

Since the predict e d EDWCs for use of t hifensulfuron

- methyl on chicory are expected to be the same as those predicted in the September 2009 DWA for use on safflower and do not exceed those of the maximum use pattern on barley , EFED recommends using for dietary assessment the highest EDWCs for the use on barley: 4.429 μg/L (acute ) and 0.9 μg/L (chronic) for surface water and 0.0972 μg/L (acute and chronic) for groundwater

.

2 DuPont BASIS Herbicide, EPA Reg. No. 352

- 571; supplemental label

.UNITED STATES ENVIRONMENTAL PROTECTION AGENCY WASHINGTON D.C., 20460

MEMORANDUM DATE: 29 September 2009
PC code: 128845

DP #: 363089

SUBJECT: Tier I Drinking Water Assessment for New Use of Thifensulfuron on
Safflower

FROM: Andrew Shelby, Physical Scientist Nelson Thurman, Senior Scientist
Environmental Fate and Effects Division/ERB II (MC 7507P)

THRU: Tom Bailey, Branch Chief Environmental Fate and Effects Division/ERB II (MC 7507P)

TO: Tobi Colvin

- Snyder Registration Division (7505C)

This assessment contains estimated drinking water concentrations (E DWCs) for both surface and ground water for thifensulfuron, also known as DPX

- M6316 ( 3

- (4

- methoxy

- 6

- methyl

- 1,3,5

- triazin

- 2

- ylcarbamoylsulfamoyl)thiophene

- 2

- carboxylic acid ; CAS Reg. No. 79277

- 67

- 1 ) on
safflower. Assessed here is the parent thifensulfuron compound. Contact Andrew Shelby at

703

- 347

- 0119 if there are any questions.

EXECUTIVE SUMMARY

Thifensulfuron is an herbicide of the triazinylsulfonylura family. The chemical’s mode of action is systemic and inhibits synthesis of amino acids. Current uses of thifensulfuron include wheat, barley, oats, soybeans, fallow land, field corn, flax and canola. The pesticide can be applied by air or ground. The proposed new use is for broadleaf weed control on safflower. This n ew use assessment is in response to a proposed Section 3 new use of Harmony SG Herbicide in North Dakota, South Dakota, Nebraska, Montana (East of Route 87 or East of I

- 15), and Wyoming ( East of I

- 25 or North of I

- 90).

Screening Concentration in Ground Water (SCIGROW) and FQPA Index Reservoir Screening Tool (FIRST) models were parameterized for maximum application rates and used to determine Estimated Drinking Water Concentrations (EDWCs) . The following table summarizes results for hazard maximizing modeling scenarios. OFFICE OF CHEMICAL SAFETY

AND POLLUTION PREVENTIONTable 1: Maximum Tier I Estimated Drinking Water Concentrations (EDWCs) for drinking water risk assessment based on ground application of thifensulfuron.

DRINKING WATER SOURCE (MODEL USED) USE (rate modeled) MAXIMUM ESTIMATED
DRINKING WATER CONCENTRATION (EDWC) (ppb)
Groundwater (SCIGROW) Safflower (0.0188 lbs a.i./A; 1 application) Acute and Chronic 0.0326

Barley, Fallow (0.028 lbs a.i./A; 2 applications) Acute and Chronic 0.0972

Surface water (FIRST) Safflower (0.0188 lbs a.i./A; 1 application) Acute 1.656

Safflower (0.0188 lbs a.i./A; 1 application) Chronic 0.336

Barley, Fallow (0.028 lbs a.i./A; 2 applications) Acute 4.429

Barley, Fallow (0.028 lbs a.i./A; 2 applications) Chronic 0.900

The EDWCs for the proposed new safflower use are less than the EDWCs for the highest applications for existing uses on barley (Table 1).

Thifensulfuron degrades primarily into two amines: thiophene and triazine. Other common degradation routes involve demethylation, hydroxyl substitution or formyl group substitution. This drinking water assessment does not take into account the potential toxicity of the primary degradates. The extent to which EPA could assess potential contributions of these degradates, if necessary, depends on how much information is available on the fate and transport properties of the chemicals. However, estimated conc entrations of the degradates would be lower than the EDWCs for the parent reported in Table 1.

Though thifensulfuron has many chemical characteristics that would indicate potential concerns in groundwater (i.e. low sorption coefficients and degradation h alf

- lives on the order of weeks), the low application rate results in low EDWCs in shallow groundwater. The only significant data gap for determining EDWCs of thifensulfuron is the determination of an aerobic aquatic

metabolism half

- life. In the absence of this study, EPA derived an estimated rate from the aerobic soil metabolism study. This Tier I assessment provides conservative EDWCs. A Tier II assessment can take into account specific crop scenarios, refine application methods and account

for time o f application. Better characterization of these parameters would allow for a more
accurate determination of EDWCs if needed.

PROBLEM FORMULATION

This is a Tier I drinking water assessment that uses modeling and available monitoring data to

estimate the ground water and surface water concentrations of pesticides in drinking water source water (pre

- treatment) resulting from pesticide use on sites that are highly vulnerable. This initial tier screens out chemicals with low potential risk and provides esti mated exposure concentrations for the human health dietary risk assessment.ANALYSIS

Use Characterization

The new use assessed here calls for a maximum application rate and maximum amount applied per season of 0.0188 lbs a.i./A for safflower. Appli cations can occur up to 81 days prior to harvest. This new use, like current uses, can be applied aerially or by ground spray. Current uses allow application rates up to 0.028 lbs a.i./A and 0.056 lbs a.i./A per year.

Table 2: Summary of maximum use i nformation for thifensulfuron, based on Harmony SG
Herbicide labels

USE SINGLE
APP.

RATE (lbs a.i./A) NUMBER
OF APPS. SEASONAL
APP. RATE (lbs a.i./A) INTERVAL

BETWEEN
APPS. (days) APP.
METHOD INCORPORATION DEPTH (inches)

Safflower 0.0188 1* 0.0188 Not Specified* Aerial or

Ground Spray 0

Barley,

Fallow 0.028 2 0.056 14 Aerial or

Ground Spray 0

*

- Give n a single application of 0.0188 lbs a.i./A, the seasonal application rate is achieved and only one application can occur. Subsequent applications can occur if maximum application rate is not used. The interval between these applications is not specified.

Fate and Transport Characteristics

The primary routes of dissipation for thifensulfuron methyl are acid hydrolysis, photodegradation

in water and a erobic soil metabolism, with half

- lives of 5 to 36 days. The aerobic soil

metabolism half

- lives were 27 days and 36 days. The field dissipation half

- lives (based on lysimeters) range from 3 to 21 days, and are consistent with the laboratory half

- lives. Longer
half

- lives in the field may be associated with cooler temperatures used in the studies. Thifensulfuron methyl is also very mobile, and thus may move laterally to surface water or downward to ground water.

The rate of degradation due to chemical processes varies widely. The rate of degradation in pH 5, 7 and 9 aqueous buffered solutions were 5.5, 184, and 145 days, respectively. This indicates acid hydrolysis readily occurs. The major degradate was the thiophene portion of the molecule,

2

- ester

- 3

- sulfonamide, which increased until the end of the study. However, the pH did not affect the rate of degradation in aqueous photolysis studies, based on dark

- control adjusted half

lives of 4.9

- 5.3 days at pH values of 5,7 and 9. The half

- lives due to s oil photolysis were 41

- 54 days, with the major degradates being the intact thiophene (2

- ester

- 3

- sulfonamide) and triazine (triazine amine) rings of the molecule.

Microbial degradation also appears to degrade parent thifensulfuron methyl. The major

metabo lites were parent compound with hydroxyl and formyl group ring substitutions, the thiophene portion of the molecule (cyclized and non

- cyclized), and aminotriazine (the triazine portion of the molecule). The linear half

- lives of the parent compound in aero bic soil were 27 and 36 days. These aerobic soil half

- life estimates were determined from a slightly acidic soil and a slightly alkaline soil. A more alkaline soil would provide a more conservative estimate of aerobic soil metabolism by assuring that an a cid hydrolysis effect is not occurring. There is some indication that an acid hydrolysis effect is occurring in the referenced soils as empirical DT50 and DT90 values are substantially smaller for the slightly acidic soil when compared to theslightly alk aline soil. Further, an alkaline soil would be more representative of upper great plains soils relevant to this assessment. Thus, it is possible persistence in some areas relevant to this assessment may be greater than that predicted by the referenced st udies.

In anaerobic aquatic conditions, the half

- lives ranged from 19 to 29 days, forming 2

- ester

- 3

- sulfonamide. Triazine amine was also formed and increased until the end of the study (365 days). In the field, the degradates O

- demethyl DPX

- M6316 and 2

- ester

- 3

- sulfonamide were present in all four test sites, and remained in the top 5 inches of soil primarily. In stainless steel cylinders of silt loam soil, triazine amine (from applied chlorsulfuron) reached 23.5

- 24.9% of applied by 4

- 8 weeks after trea tment, and mostly occurred in the top 10 cm.

Sorption of thifensulfuron methyl was studied in four soils using the batch equilibrium method. The chemical was very mobile in soils with K d values of 0.08 to 1.38 mL/g. Desorption values were similar, rangin g from 0.27 to 1.69 mL/g. Adsorption was related to soil organic carbon content (r 2 = 0.71). K OC values ranged from 7.4 to 32 mL/g o.c. In soil columns, the mobility of thifensulfuron was studied in five soils. In sandy loam, silt, loamy sand, and silt loam soils, 72

- 96% of applied thiophene

- labeled thifensulfuron was present in the leachate of soil columns, while only 3

- 28% was present in soils. In silt loam soil, 69% of triazine

- labeled thifensulfuron was present in the leachate and 31% was presen t in the soils columns. Virtually all radioactivity in leachate was present as parent compound, while 29% was parent

- acid in the silt loam soil. No other metabolites reached significant levels.

Table 3: Summary of physical/chemical and environmental fat e and transport properties of
thifensulfuron

. PARAMETER VALUE(S) (units) SOURCE COMMENT

Chemical Name 3

- (4

- methoxy

- 6

- methyl

- 1,3,5

- triazin

- 2

- ylcarbamoylsulfamoyl)thiophen

e

- 2

- carboxylic acid Molecular Weight 373.36 Solubility (25 0 C) 2.24 g/L at pH 7 Vapor Pressure (25 0 C) 5.6 x 10 11 mm Hg MRID

47141802 pKa 4.0 MRID

47141802

Octanol

- Water
Partition Coefficient (K OW ) 0.018 MRID
47141802

Hydrolysis Half

life pH 5 5.5 days pH 7 184 days pH 9 145 days MRID
00145151

MRID
00161284 Aqueous Photolysis Half

life 5.3 days 5.7 days MRID
00161285 pH 7

Soil Photolysis

Half

- life 41 days 55 days MRID
00161286 Thiopene label

Triazine label

Aerobic Soil

Metabolism Half

life t 1/2 : 27.3 days; 36.1 days

DT50: 7

- 14 days; <3.5 days
DT90: 28

- 42 days; 3.5

- 7 days

t input = 41.3 MRID
00145154 Flanagan silt loam (pH 7.5); Keyport silt loam (pH 6.4)



### n s t t t 1

- n 90,

1/2 input 

### + = per

2002 Input Parameter
GuidanceAnaerobic Aquatic

Metabolism Half

life 29 days 21 days 19 days MRID

00161288 Loamy sand (pH 4.9) Sandy loam (pH 7.5) Silt loam (pH 6.5)

Aerobic Aquatic

Metabolism Half

life 2 x 41.3 = 82.6 days Data gap 2 x aerobic soil metabolism

half

- life parameter per 2002 Input Parameter Guidance Organic Carbon

Partition Coefficient (K OC ) 7.4 ml/g o.c. 9.1 ml/g o.c. 32 ml/g o.c. 17 ml/g o.c. MRID
00161290 Sandy loam (pH 6.6) Sandy loam (pH 6.5) Silt loam (pH 5.4) Silt loam (pH 5.2)

Terrestrial Field Dissipation Half

life linear <1 day 2.8 days 2.0 days 0.8 day MRID
00161291 Loam (pH 6.5) Silt loam (pH 6.4) Silty clay loam (pH 7.3) Silty clay loam (pH 7.9)

Drinking Water Exposure Modeling

SCIGROW (Screening Concentration in Ground Water) is a regression model used as a screening tool to estimate pesticide concentrations found in ground water used as drinking water. SCIGROW was developed by fitting a linear model to groundwater concentrations with the Relative Index of Leaching Potential (RILP) as the independent variable. Groundwater
concentrations were taken from 90

- day average high concentrations f rom Prospective Ground Water studies; the RILP is a function of aerobic soil metabolism and the soil

- water partition coefficient. The output of SCIGROW represents the concentrations that might be expected in shallow unconfined aquifers under sandy soils, which is representative of the ground water most vulnerable to pesticide contamination likely to serve as a drinking water source.

FIRST (FQPA Index Reservoir Screening Tool) is a metamodel of PRZM and EXAMS used as a screening tool to estimate pesticide concentrations found in surface water used as drinking water. FIRST was developed by making multiple runs of PRZM using varying sorption coefficients and determining the concentration in the EXAMS index reservoir scenario after a two

- inch single

storm eve nt. (The Index Reservoir is a standard water body used by the Office of Pesticide Programs to assess drinking water exposure (Office of Pesticide Programs, 2002). It is based on a real reservoir (albeit not currently in active use as a drinking water sup ply), Shipman City Lake in Illinois, that is known to be vulnerable to pesticide contamination.) The single runoff event moves a maximum of 8% of the applied pesticide into the reservoir. This amount can be reduced by degradation or effects of binding to soil in the field. Additionally, FIRST can account for spray drift and adjusts for the area within a watershed that is planted with the modeled crop (Percent Crop Area). Spray drift (modeled as direct deposition of the pesticide into the reservoir)

is a ssumed to be 16% of the applied active ingredient for aerial application, 6.3% for orchard air blast application, and 6.4% for other group spray application. Despite being a single event model, FIRST can account for spray drift from multiple applications. The default agricultural Percent Cropped Area (PCA) is 87%. The PRZM scenario used for FIRST development was among the most vulnerable, and thus resulting surface water concentrations represent the upper bound values on the concentrations that might be found in drinking water from the use of a pesticide.

Modeling Approach and Input Parameters

Input parameters were determined based on “Guidance for Selecting Input Parameters in Modeling the Environmental Fate and Transport of Pesticides” (Version II, February 28, 2002). The following tables display input parameters for SCIGROW and FIRST models respectivelyand provide explanation for the determination of the parameter.

Table 4: SCIGROW (v2.3) input parameters for thifensulfuron u se on safflower PARAMETER (units) VALUE(s) SOURCE/RATIONALE Maximum Application Rate (lbs a.i./A) 0.0188 Proposed label

Number of Applications per

Year 1 Maximum application rate matches maximum application per season Organic Carbon Partition Coefficient (K OC ;mL/g o.c.) 9.1 MRID 00161290 ; Median non

- sand value

Aerobic Soil Metabolism Half

life (days) 31.7 MRID 00145154 ; 1 st Order Degradation

Table 5 : FIRST (v1.1.0) input parameters for thifensulfuron u se on safflower PARAMETER (units) VALUE(s) SOURCE/RATIONALE Application rate (pounds

a.i. acre - 1 ) 0.0188 Proposed label

Number of applications (per year) 1 Maximum application rate matches maximum application per season Application interval (days) N/A

Percent Crop Area 0.87 Since no PCA has been determined, 0.87 is the

recommended value from Guidance

K OC 7.4 MRID 00161290 ; Minimum non

- sand value

, 2002 Input Parameter Guidance

Aerobic soil t 1/2 (days) 41.3 MRID 00145154 ; 1 st Order Degradation



### n s t t t 1

- n 90,

1/2 input 

### + =

per 2002 Input Parameter Guidance Is the pesticide wetted

in? No

Method of application A Aerial application Depth of incorporation (inches) 0 Section 3 label ?

Solubility (mg/L) 2,240 MRID 47141802 Aerobic aquatic t 1/2 (days) 82.6 2 x (aerobic soil metabolism parameter) per 2002 Input Parameter Guidance Photolysis half

- life (days) 5.7 pH 7, MRID 00161285

Modeling Results

SCIGROW concentration (ppb) represents the groundwater concentration that might be expected in shallow unconfined aquifers under sandy soils. Output is used for both acute and chronic endpoints.

FIRST concentrations (ppb) represent untreated surface water concen trations. The one

- in

- 10

- year peak day concentration is used for acute endpoints and the one

- in

- 10

- year average concentration is used for chronic endpoints.The estimated concentrations provided in this assessment are conservative estimates of

concentrati ons in drinking water. If dietary risks require refinement, higher tiered crop

- specific models and modeling scenarios can be used.

Table 6: Maximum Tier I Estimated Drinking Water Concentrations (EDWCs) for drinking water risk assessment based on ground application of thifensulfuron.

DRINKING WATER SOURCE (MODEL USED) USE (rate modeled) MAXIMUM ESTIMATED
DRINKING WATER CONCENTRATION (EDWC) (ppb)
Groundwater (SCIGROW) Safflower (0.0188 lbs a.i./A; 1 application) Acute and Chronic 0.0326

Barley, Fallow (0.028 lbs a.i./A; 2 applications) Acute and Chronic 0.0972

Surface water (FIRST) Safflower (0.0188 lbs a.i./A; 1 application) Acute 1.656

Safflower (0.0188 lbs a.i./A; 1 application) Chronic 0.336

Barley, Fallow (0.028 lbs a.i./A; 2 applications) Acute 4.429

Barley, Fallow (0.028 lbs a.i./A; 2 applications) Chronic 0.900

Monitoring Data

For most compounds, modeled concentrations in surface water and ground water are generally higher than monitored concentrations because the frequency of sampling is inadequate to catch high concentrations and samples may not be taken in high use areas. Comparison of monitoring and modeling is not possible for thifensulfuron because EFED is not aware of any monitoring

data.

Drinking Water Treatment

No data are available to indicate the effectiveness of drinking water treatment on the degradation

of thifensulfuron.

CONCLUSIONS

Thifensulfuron is a somewhat persistent chemical that partitions readily to water. Though it degrades through many proces ses, acid hydrolysis and photodegradation in water appear to be the primary routes of degradation. Photolysis can be short

- circuited if leaching to groundwater is rapid or where the surface water bodies limit light penetration. Rapid leaching is not unli kely given the indicated low sorption coefficients. These concerns, however, are allayed given the low application rate considered in this assessment. In soils, hydrolysis in acidic conditions and metabolism will likely be dominant routes of degradation.