OFFICE OF CHEMICAL SAFETY

AND POLLUTION PREVENTION

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON, D.C. 20460

August 12, 2015

PC Code: 085601, 108209, 118601, 118602, 119009, 119011, 122001, 122009, 122010, 

122020, 122021, 128721, 128820, 128845, 128887, 128901, 128969, 128973, 129002, 129008, 

129009, 129031

MEMORANDUM DP Barcode: 427429

SUBJECT: Drinking Water Exposure Assessment for the Registration Review of 22 

Sulfonylurea Herbicides

FROM: Gregory Orrick, Environmental Scientist

ERB1/EFED (7507P)

THROUGH: Rochelle F. H. Bohaty, Ph.D., Chemist

ERB3/EFED (7507P)

Brian Anderson, ERB2 Branch Chief

ERB2/EFED (7507P)

TO: Julie Van Alstine, MPH, Environmental Health Scientist

Donna Davis, Acting Branch Chief

RAB6/HED (7509P)

Margaret Hathaway, Chemical Review Manager

Kevin Costello, Branch Chief

RMIB2/PRD (7508P)

1. Executive Summary

This Environmental Fate and Effects Division (EFED) memorandum is an abbreviated 

drinking water exposure assessment based on labeled uses of the 22 currently active 

sulfonylureas (SU) in support of registration review. Coarse and refined screening drinking 

water exposure estimates were generated with PRZM-GW (v.1.07) and FIRST (v1.1.1) for use in 

the SU dietary risk assessments in support of human health assessment. A tiered approach was 

used in this assessment beginning with coarse-screen estimates that should exceed upper-bound 

chemical-specific estimated drinking water concentrations (EDWC) for all SUs. This was 

achieved by using model inputs that represent the use pattern of highest potential exposure from 

any SU, the highest soil mobility of any SU residue of concern, and stability to any route of 

degradation over time. The resulting highest coarse-screen EDWCs from PRZM-GW were

proposed in all SU dietary risk assessments as a conservative estimate of exposure. Due to the 
2

screening nature of this assessment, EFED does not expect that the EDWCs reflect actual 

potential exposures. However, if the resulting dietary exposure was not of concern, then 

exposure modeling refinements, including chemical-specific modeling, were unnecessary. If the 

resulting dietary exposure was of concern, then simple chemical-specific refinements were made 

to generate lower refined-screen EDWCs that remained conservative for the chemical (i.e., the 

refined-screen EDWCs are still coarse, but a step more refined than the coarse-screen EDWCs). 

For both screens, the SUs were conservatively treated as stable to all routes of degradation, 

making consideration of degradates of concern irrelevant. Table 1.1 lists the coarse- and 

refined-screen EDWCs for the SUs. Due to the conservative nature of these screens, the EDWCs 

presented in this assessment are not expected to occur in drinking water.

Table 1.1. Screening-level Maximum EDWCs for Sulfonylureas

Sulfonylurea (Use) PRZM-GW 

Scenario

Screen 

Level

Max. Daily 

Conc. (μg/L)

Postbreakthrough 

Mean (μg/L)

Mean 

Breakthrough 

Time (yrs)

Chlorsulfuron (non-ag areas) NC cotton Refined 78 52 5.9

Halosulfuron-methyl (fallow) NC cotton Refined 74 53 8.1

Nicosulfuron (rights-of-way) NC cotton Refined 39 26 6.2

Prosulfuron (corn) NC cotton Refined 15 13 12

Rimsulfuron (commercial areas) NC cotton Refined 105 71 6.5

Triasulfuron (rangeland) NC cotton Refined 21 19 16

Tribenuron-methyl (corn) NC cotton Refined 35 23 6.1

Triflusulfuron-methyl (sugarbeet) FL citrus Refined 23 20 9.5

All other sulfonylureas NC cotton Coarse 751 492 5.7-8.7

2. Use Characterization

The uses and use patterns of the assessed sulfonylureas (SU) are generally described by 

the LUIS reports that were prepared and posted to the dockets during the problem formulation 

phase of registration review. The risk managers assigned to the SUs were consulted to update 

the use patterns for this assessment to reflect any label changes that occurred after the LUIS 

reports were prepared. Table 2.1 lists the resulting lowest, average, and maximum single 

application rates labeled for the SUs. Maximum agricultural single application rates are listed 

separately from maximum non-agricultural single application rates.

Table 2.2 lists the maximum seasonal or annual application rates for agricultural uses of 

SUs that were assessed to estimate drinking water exposure using the Tier I screening drinking 

water exposure models, PRZM-GW and FIRST. Maximum single application rates were often

the same as maximum seasonal or annual application rates. Uses with single application rates 

less than seasonal or annual application rates generally did not provide a minimum reapplication 

interval, as indicated in Table 2.2. Upper-bound usage rates provided by the Biological and 

Economic Analysis Division (BEAD) were used to estimate maximum annual application rates 

for uses without maximum annual application rates on the label. Clarification of the maximum 

annual application rate and minimum reapplication interval is recommended for all labeled SU 

use sites for which these directions are not currently provided on the label.
3

Table 2.1 Sulfonylurea Single Application Rates A

Chemical

Lowest 

Max 

Application 

Rate

Highest 

Max 

Application 

Rate

Rate to Model - Ag Rate to Model -

Non-Ag

Average Rates

Across Uses 

Where Available 

(Supplied by 

BEAD)

Application 

Methods Notes

Bensulfuron-methyl 0.047 0.063 0.063 - only used on rice none 0.032; up to 2 apps Ground or Air Labeled only for rice

Chlorimuron-ethyl 0.0039 0.08 0.08 - soybean 0.0313 - several 

non-ag uses

0.004 to 0.017 Ground or Air –

Chlorsulfuron 0.0012 0.14 0.062 - rangeland/fallow 0.14 0.007 to 0.047 Ground or Air –

Flazasulfuron 0.047 0.093 0.045 0.093 0.045 Ground only –

Foramsulfuron 0.0009 0.052 0.038 - corn 0.052 - all non Ag 0.029 to 0.032

Ground only for 

tur and ground 

or air for corn

–

Halosulfuron-methyl 0.03 0.14 0.094 – corn (most uses at 

0.047) 0.136 0.016 to 0.077 Ground or air Most uses are ground only, but 

some allow aerial applications

Imazosulfuron - 0.66 0.3 - melons, vegetables, 

rice 0.66 Not available Ground or air

Uses are ground only, except 

aerial applications are allowed 

on rice

Iodosulfuron-methylNa 0.00013 0.0089 0.0089 - wheat 0.0089 - most turf 0.002 Ground only –

Mesosulfuron-methyl 0.0028 0.013 0.013 - triticale and wheat none 0.002 to 0.011 Ground or air –

Metsulfuron-methyl 0.00023 0.15

0.15 - ag uncultivated 

areas and some forestry 

uses

0.15 - several 

non-ag uses

0.002 to 0.024 Ground or air Labeled for mainly non-ag uses 

with several agricultural uses

Nicosulfuron 0.012 0.07 0.066 - corn

0.07 - several 

non-ag areas

0.011 to 0.043 Ground or air –

Orthosulfamuron 0.066 0.066 0.066 - rice none 0.061 Ground or air Labeled only for rice

Primisulfuron-methyl 0.018 0.036 0.036 - corn 0.036 - bluegrass 0.009 to 0.022 Ground or air Labeled only for bluegrass and 

corn

Prosulfuron 0.0089 0.036 0.036 – corn (most uses at 

0.018) none 0.003 to 0.018 Ground or air No non-ag uses labeled
4

Chemical

Lowest 

Max 

Application 

Rate

Highest 

Max 

Application 

Rate

Rate to Model - Ag Rate to Model -

Non-Ag

Average Rates

Across Uses 

Where Available 

(Supplied by 

BEAD)

Application 

Methods Notes

Rimsulfuron 0.0078 0.063 0.063 - most uses 0.0625 - most 

uses

0.01 to 0.062 Ground or air

Numerous ag and non-ag uses; 

some uses only allow ground 

applications

Sulfometuron-methyl 0.0281 0.0281 0.38 – fallow, 

uncultivated areas

0.281 - forestry 

and more Ground or air

Numerous ag and non-ag uses; 

some uses only allow ground 

applications, but most allow 

both

Sulfosulfuron 0.03 0.125 0.094 - pastures 0.094 - almost all 

uses

0.008 to 0.07 Ground or air

Most uses are non-cropped 

areas; most uses allow only 

ground applications

Thifensulfuronmethyl 0.0023 0.028 0.028 - almost all uses none 0.002 to 0.015 Ground or air Labeled for mainly agricultural 

uses

Triasulfuron 0.022 0.028 0.028 - most uses were 

0.0275 or 0.028 none 0.013 to 0.019 Ground or air –

Tribenuron-methyl 0.0012 0.0313 0.031 - field corn, 

blueberries none 0.002 to 0.017 Ground or air Labeled mainly for agricultural 

uses

Trifloxysulfuron-Na 0.007 0.036 0.028 – sugarcane 0.036 - turf 0.006 to 0.017 Ground or air –

Triflusulfuron-methyl 0.016 0.031 0.031 - sugar beets (all 

other uses at 0.016) none 0.008 to 0.012 Ground or air Labeled only for beets, sugar 

beets, chicory, and endive

A All rates are in units of pounds of active ingredient per acre (lbs a.i./A). “Ag” means “agricultural”; “Non-ag” means “non-agricultural”.
5

Table 2.2. Sulfonylurea Application Rate Patterns of Highest Expected Exposure A

Sulfonylurea Maximum Rate Pattern

Bensulfuron-methyl 0.0625 lbs/A/yr

Chlorimuron-ethyl 0.080 lbs/A/yr

Chlorsulfuron 0.14 lbs/A x ?/yr B

Flazasulfuron 0.15 lbs/A/yr

Foramsulfuron 0.052 lbs/A x ?/yr B

Halosulfuron-methyl 0.125 lbs/A/yr

Imazosulfuron 0.66 lbs/A x 2/yr A

Iodosulfuron-methyl-Na 0.009 lbs/A/yr

Mesosulfuron-methyl 0.013 lbs/A x ?/yr B

Metsulfuron-methyl 0.15 lbs/A x ?/yr B

Nicosulfuron 0.070 lbs/A x ?/yr B

Orthosulfamuron 0.067 lbs/A/yr

Primisulfuron-methyl 0.036 lbs/A x ?/yr B

Prosulfuron 0.036 lbs/A x ?/yr B

Rimsulfuron 0.0625 lbs/A x ?/yr B

Sulfometuron-methyl 0.375 lbs/A x 1/yr

Sulfosulfuron 0.125 lbs/A/yr

Thifensulfuron-methyl 0.031 lbs/A/yr

Triasulfuron 0.039 lbs/A/yr

Tribenuron-methyl 0.031 lbs/A x 1/yr

Trifloxysulfuron-Na 0.080 lbs/A/yr

Triflusulfuron-methyl 0.078 lbs/A/yr A The highest maximum rate pattern is bolded and in red. B Question marks indicate application rate patterns for which maximum annual application rates and maximum 

numbers of applications per year are not stated on the label.

3. Environmental Fate Characterization

Sulfonylureas tend to be anions. Therefore, the compounds tend to be hydrophilic and do 

not bioaccumulate in the environment. Water solubility limits range from 26 (trifloxysulfuron) 

to 3,180,000 mg/L (chlorsulfuron). The highest available whole fish bioconcentration factor 

(BCF) is 3.0x (tribenuron-methyl).

Based on the soil mobility data presented in Table 5.1, the SUs as a class may be 

characterized as “mobile” to “moderately mobile” on the FAO scale (USEPA, 2006). This 

means they will have some tendency for leaching into ground water and for transport away from 

the site of application dissolved in runoff.

Aerobic soil metabolism (ASM) half-lives for the SUs indicate a wide range of stability 

to metabolism by soil microbes, ranging from 3.6 days (shortest half-life for iodosulfuronmethyl) to 240 days (longest half-life for metsulfuron-methyl). Bensulfuron-methyl and 

chlorsulfuron are assumed stable to ASM in the absence of acceptable data. Iodosulfuron-methyl 
6

degrades with a half-life of 3.6-51 days to another SU, metsulfuron-methyl, that degrades slower 

with a half-life of 26-240 days. Thus, SU half-lives for ASM range from several days up to 

many months, for the same compound in some cases.

Aerobic aquatic metabolism (AAM) half-lives for the SUs indicate a wide range of 

stability to metabolism by aquatic microbes, similar to ASM half-lives, ranging from 9.1 days 

(shortest half-life for triflusulfuron-methyl) to 187 days (longest half-life for sulfometuronmethyl). Bensulfuron-methyl and chlorsulfuron are assumed stable to AAM in the absence of 

acceptable data. 

Aquatic photolysis is not a significant degradation process for the SUs, except for a few 

SUs whose photolysis half-lives were on the order of a few days (flazasulfuron, imazosulfuron, 

sulfosulfuron, and thifensulfuron-methyl).

Overall, the data for the SUs indicate that their mobility and stability to degradation are 

conducive to leaching into ground water and off-site transport via runoff, resulting in the 

contamination of water resources.

4. Residues of Concern

The drinking water residues of concern (ROC) are clearly defined for eight SUs (see 

Table 4.1). For the 14 remaining SUs, the ROCs are unclear for a variety of reasons, e.g., 

studies with new degradates may have been submitted after the ROCs were determined or the 

ROCs may not have been clearly determined in past assessments. The drinking water ROCs are 

irrelevant in this assessment, however, because the SUs are treated as completely stable to 

degradation in the screening approach used. Therefore, there was no need to clarify for this 

assessment the drinking water ROCs for any SUs.

Table 4.1. Status of SU Drinking Water Residues of Concern (ROC)

Pesticide DW ROCs Citation/Rationale

SUs with Clearly Defined ROCs

Chlorimuron-ethyl Demethylated parent Only deg with SU group (DP 

358796, 2009 HHRA)

Flazasulfuron DTPU, DTPP, HTPP, TPSA, ADMP, and 

GTF (all major degs) ROCKS decision (DP 386767)

Imazosulfuron HMS, IPSN, ADPM, UDPM, and SDPM DEREK flags (DP 377141)

Mesosulfuron-methyl AE F154851, AE F160459, AE F160460 2004 MARC decision (DP 298760+)

Nicosulfuron IN-37740, IN-HYY21 DP 414505 (DWA) (1 new study w/ 

no new degradates)

Orthosulfamuron DBS acid, DOP urea, DB amine, odesmethyl orthosulfamuron HHRA (DP 319264)

Tribenuron-methyl None SU degradates not major or not 

prominent (USEPA 2009 - DEA)

Trifloxysulfuron-Na CGA-053052, CGA-382997, CGA368732 2003 MARC decision (DP 293085)
7

Pesticide DW ROCs Citation/Rationale

SUs with Unclear ROCs

Bensulfuron-methyl

Unclear

--

Chlorsulfuron --

Foramsulfuron --

Halosulfuron-methyl --

Iodosulfuron-methyl-Na --

Metsulfuron-methyl --

Primisulfuron-methyl --

Prosulfuron --

Rimsulfuron --

Sulfometuron-methyl --

Sulfosulfuron --

Thifensulfuron-methyl 2004 MARC included IN-L9226 -

but 2 new SU degradates ID'ed since

Triasulfuron --

Triflusulfuron-methyl --

5. Exposure Modeling

5.1. Ground Water (PRZM-GW)

Screening ground water-sourced drinking water exposure estimates were generated with 

PRZM-GW (v.1.07) for use in the SU dietary risk assessments in support of human health 

assessment. Rather than produce chemical-specific EDWCs following the usual assessment 

procedures a tiered approach was used in this assessment beginning with coarse-screen

estimates that should exceed upper-bound chemical-specific EDWCs for all SUs. This was 

achieved by using model inputs that represent the use pattern of highest potential exposure from 

any SU, the highest soil mobility of any SU residue of concern, and stability to degradation over 

time. The resulting highest coarse-screen EDWCs from PRZM-GW were proposed in all SU 

dietary risk assessments as a conservative estimate of exposure. Due to the screening nature of 

this assessment, EFED does not expect that the EDWCs reflect actual potential exposures. 

However, if the resulting dietary exposure was not of concern, then exposure modeling 

refinements, including chemical-specific modeling, were unnecessary. If the resulting dietary 

exposure was of concern, then simple chemical-specific refinements were made to generate 

lower refined-screen EDWCs that remained conservative for the chemical (i.e., the refinedscreen EDWCs are still coarse, but a step more refined than the coarse-screen EDWCs). Due to 

the conservative nature of these screens, the EDWCs presented in this assessment are not 

expected to occur in drinking water.

5.1.1. Coarse Screen

For the coarse screen, the application rate pattern of highest expected exposure (i.e., 

maximum rate pattern) of any SU was modeled with PRZM-GW to produce coarse-screen 

EDWCs. The maximum application rate pattern for each SU is summarized above in Table 2.2. 

Many current labels do not provide maximum annual application rate limits, resulting in 

uncertainty in the maximum rate pattern for the individual SU in the product. Despite this 

uncertainty, the maximum rate pattern from all SUs appears to be for imazosulfuron use on turf 

(0.66 lbs/A x 2 applications/year).
8

The lowest sorption coefficients for any SU, considering the available data for residues of 

concern, were used as model inputs for PRZM-GW (representing orthosulfamuron’s degradate 

DBS acid and primisulfuron-methyl). Table 5.1 lists the SU-specific mean sorption coefficients 

for the residues of concern. Kd or KF was used when the coefficient of variation (CV) for the 

parameter was less than the CV for KOC or KFOC. Otherwise, KOC or KFOC was used. 

Freundlich parameters were used when available and when less than similar non-Freundlich 

parameters, if available. The representative compound was the residue of concern with the 

lowest mean sorption coefficient if sorption coefficient data were available. Therefore, the 

parent compound was the representative compound for one of three reasons: 1) the mean 

sorption coefficient for the parent compound was less than that for the degradates of concern, 2) 

data were not available for degradates of concern, or 3) there were no degradates of concern.

Table 5.1. Sulfonylurea Mean Sorption Coefficients for Residues of Concern A

Sulfonylurea Mean Kd or 

KF

Mean KOC

or KFOC

Representative 

Compound

Source

Bensulfuron-methyl KOC=315 Parent compound Acc# 73657

Chlorimuron-ethyl KOC=91 Parent compound MRID 143120

Chlorsulfuron KFOC=21 Parent compound MRID 42156705

Flazasulfuron KOC=29 TPSA MRID 46930001, 49030002 

(in review)

Foramsulfuron KFOC=78 Parent compound MRID 45109723

Halosulfuron-methyl KFOC=109 Parent compound MRID 42139411

Imazosulfuron KOC=242 Parent compound MRID 47305117

Iodosulfuron-methyl-Na KFOC=21 Metsulfuron MRID 49154301

Mesosulfuron-methyl KFOC=78 AE F154851 MRID 45386433

Metsulfuron-methyl KFOC=21 Metsulfuron MRID 49154301 (in review)

Nicosulfuron KOC=35 Parent compound MRID 40924222

Orthosulfamuron KF=0.11 DBS acid MRID 49308108 (in review)

Primisulfuron-methyl KFOC=12 Parent compound MRID 41869304

Prosulfuron KF=0.22 Parent compound MRID 42685257

Rimsulfuron KFOC=47 Parent compound MRID 41356336

Sulfometuron-methyl KFOC=63 Parent compound MRID 42789301

Sulfosulfuron KF=0.29 Parent compound MRID 44295728

Thifensulfuron-methyl KFOC=28 Parent compound MRID 161290

Triasulfuron KF=0.34 CGA-195660 MRID 42782002

Tribenuron-methyl KFOC=31 Parent compound MRID 46101501

Trifloxysulfuron-Na KF=0.26 CGA-382997 MRID 45371821

Triflusulfuron-methyl KF=0.64 Parent compound MRID 42496861

A The lowest mean sorption coefficient inputs are bolded and in red.

The PRZM-GW inputs for the coarse-screen EDWCs are listed in Table 5.2. Application

inputs represent the maximum rate pattern for imazosulfuron, which is the highest of any SU 
9

(two applications per year at 0.66 lbs/acre, with a 21-day reapplication interval). The initial 

application date (7 days since emergence) is a post-emergence date within the application season. 

Alternative initial application dates of 37 and 67 days post-emergence were also explored with 

the North Carolina scenario.

Chemical inputs (i.e., sorption coefficients) for the sulfonylurea residues of concern

follow the PRZM-GW Input Parameter Guidance (USEPA, 2012). The chemicals were modeled 

as stable to degradation (hydrolysis and aerobic soil metabolism) as part of the coarse-screen 

approach. Sorption coefficients reflect the highest soil mobility of any SU residue of concern, 

consistent with the coarse-screen approach.

Table 5.2. PRZM-GW Input Parameters for Sulfonylurea Coarse-Screen EDWCs

Input Parameter Value Justification Source

Application Rate (kg a.i./ha) 0.74 Maximum labeled single application 

rate for use of imazosulfuron on turf 

(see Table 2.2)

EPA Reg. No.

59639-155

(Mar. 29, 2011)

Applications per Year 2; every year

Reapplication Interval (days) 21 Minimum labeled interval

Initial Application Date (days 

since emergence) 7 Post-emergence application date (calculated)

Chemical Application 

Method 2 Foliar application EPA Reg. No.59639-155

Hydrolysis Half-life (days) 0 Stability was modeled as a screen Screening assumption

Surface Soil Half-life (days) 0 Stability was modeled as a screen Screening assumption

Sorption Coefficient (ml/g) Kd=0.11 or

KOC=12

Represent the lowest mean Kd and 

KFOC values for sulfonylurea 

residues of concern in Table 5.1

MRID 49308108 (in 

review); MRID 

41869304

All six standard scenarios were modeled as surrogates for the use site (turf) with 

maximum application rates. PRZM-GW outputs are listed in Table 5.3. The KOC of 12 

produced shorter mean breakthrough times than the Kd of 0.11. Coarse-screen EDWCs were 

higher with the KOC input as well, with two exceptions. The North Carolina scenario produced 

the highest daily and post-breakthrough mean EDWCs. The highest daily concentration was 

produced with the KOC input and the 37-days post-emergence initial application date. The 

highest post-breakthrough mean concentration was produced with the Kd input and the 7-days

post-emergence initial application date. The initial application date input was not as sensitive as 

the sorption coefficient, which is expected due to the assumptions that the residues are stable and 

are not allowed (by the PRZM-GW conceptual model) to run-off, leaving them available for 

leaching whenever precipitation or irrigation events occur.
10

Table 5.3. PRZM-GW Coarse-Screen EDWCs for Sulfonylureas A

Modeled 

Scenario Kd or KOC

Initial Application 

Dates (days postemergence)

Max. Daily 

Conc. (μg/L)

Mean 

Breakthrough 

Time (yrs)

Postbreakthrough 

Mean (μg/L)

Wisconsin (WI)

corn

Kd 7 449 11.5 415

KOC 7 504 6.9 425

Delmarva (DMV)

corn

Kd 7 489 5.3 369

KOC 7 560 3.3 378

Florida (FL) citrus

Kd 7 429 3.2 351

KOC 7 467 2.3 349

Florida (FL)

potato

Kd 7 375 2.6 272

KOC 7 407 2.2 275

Georgia (GA)

peanut

Kd 7 197 5.2 156

KOC 7 206 3.9 158

North Carolina 

(NC) cotton

Kd 7 640 8.7 492

Kd 37 641 8.7 491

Kd 67 637 8.7 490

KOC 7 742 5.7 484

KOC 37 751 5.7 483

KOC 67 744 5.7 482 A Maximum values are bolded.

Figure 5.1 displays ground water concentrations over time for the North Carolina 

scenario with the Kd input and the 7-days post-emergence initial application date.

Figure 5.1 Sulfonylurea Coarse-screen Ground Water EDWCs (μg/L) per Time (years) for 

the North Carolina Scenario, Kd of 0.11, and 7-days post-emergence initial application date
11

5.1.2. Refined Screen

Refined-screen exposure estimates were generated for 8 sulfonylureas (chlorsulfuron, 

halosulfuron-methyl, nicosulfuron, prosulfuron, rimsulfuron, triasulfuron, tribenuron-methyl, and 

triflusulfuron-methyl) for which the coarse-screen EDWCs (i.e., acute EDWC of 751 μg/L and 

chronic EDWC of 492 μg/L) were problematic. The PRZM-GW inputs for refined-screen 

EDWCs are listed in Table 5.4. Application inputs represent the maximum application rate 

pattern for the chemical, considering the maximum single application rate first, then the 

maximum seasonal application rate, using national-scale upper-bound usage rates provided by 

BEAD if the labels do not provide a maximum seasonal rate. The modeled sorption coefficient 

is also chemical-specific. All other inputs, including the screening assumptions of stability to 

degradation and application every year, remained the same as for the coarse screen.

Table 5.4. PRZM-GW Input Parameters for Sulfonylurea Refined-Screen EDWCs

Sulfonylurea (Use) App. Rate

(kg a.i./ha)

App.

per 

Year

Reapp.

Interval 

(days)

Initial App.

Date (days 

since 

emergence)

App.

Method

Hydrolysis 

and Surface 

Soil Half-life 

(days)

Sorption 

Coefficient 

(ml/g)

Chlorsulfuron (non-ag areas) 0.157 1 N/A

7 2 0

KOC=21

Halosulfuron-methyl (fallow) 0.157 1 A N/A KOC=109

Nicosulfuron (rights-of-way) 0.078 1 N/A KOC=35

Prosulfuron (corn) 0.040 1 N/A Kd=0.22

Rimsulfuron (commercial areas) 0.071 3 B 7 C KOC=47

Triasulfuron (rangeland) 0.031 2 D 7 C Kd=0.34

Tribenuron-methyl (soybeans) 0.035 2 E 7 C KOC=31

Triflusulfuron-methyl (sugarbeet) 0.035 3 F 7 C Kd=0.64

A One application per year was assumed in the absence of label directions based on national-scale usage data 

collected by BEAD indicating halosulfuron-methyl was not applied more than once to 27,303 acres of fallow land.

B Three applications per year were assumed in the absence of label directions, based on national-scale usage data

collected by BEAD indicating that rimsulfuron was not applied more than three times per year to any crop.

C A 7-day reapplication interval was assumed in the absence of label directions, based on the assumption that 

herbicides are not applied more often, giving time to evaluate the impact of the previous application before the next.

D The 2nd application is at 0.013 kg a.i./ha to reach the seasonal maximum rate of 0.039 lbs a.i./A.

E Two applications per year were assumed in the absence of label directions based on usage data provided by BEAD 

that indicates tribenuron-methyl was never applied more than twice to 4,831,149 acres of soybeans.

F The 2nd and 3rd applications are at 0.026 kg a.i./ha each to reach the seasonal maximum rate of 0.078 lbs a.i./A.

All six standard scenarios were modeled as surrogates for the use sites listed in Table 

5.4. PRZM-GW outputs for the most vulnerable scenario are listed in Table 5.5. The North 

Carolina scenario produced the highest daily and post-breakthrough mean EDWCs for each 

modeled use pattern except that for triflusulfuron-methyl on sugarbeets, for which the Florida 

citrus scenario provided the highest EDWCs. Additional EDWC refinements were not pursued 

because the refined-screen EDWCs were found by the Health Effects Division not to be a risk 

concern.
12

Table 5.5. PRZM-GW Refined-Screen EDWCs for Sulfonylureas

Sulfonylurea (Use) PRZM-GW 

Scenario

Max. Daily Conc. 

(μg/L)

Post-breakthrough 

Mean (μg/L)

Mean Breakthrough 

Time (yrs)

Chlorsulfuron (non-ag areas) NC cotton 78 52 5.9

Halosulfuron-methyl (fallow) NC cotton 74 53 8.1

Nicosulfuron (rights-of-way) NC cotton 39 26 6.2

Prosulfuron (corn) NC cotton 15 13 12

Rimsulfuron (commercial areas) NC cotton 105 71 6.5

Triasulfuron (rangeland) NC cotton 21 19 16

Tribenuron-methyl (corn) NC cotton 35 23 6.1

Triflusulfuron-methyl (sugarbeet) FL citrus 23 20 9.5

5.2. Surface Water (FIRST)

5.2.1. Coarse Screen

Screening surface water-sourced drinking water exposure estimates were generated with 

FIRST (v.1.1.1) to confirm that estimated exposure to SUs via surface water sources is less than 

that via ground water sources. Rather than produce chemical-specific EDWCs, a coarse-screen 

estimation approach was used that should exceed upper-bound chemical-specific EDWCs for all 

SUs, similar to the coarse-screen assessment approach used for ground water exposure. This was 

achieved by using model inputs that represent the use pattern of highest exposure from any SU, 

the highest soil mobility of any SU residue of concern, and stability to any route of degradation 

over time.

The FIRST inputs for the coarse-screen EDWCs are listed in Table 5.6. Application

inputs represent the maximum rate pattern for imazosulfuron, which is the highest of any SU 

(two applications per year at 0.66 lbs/acre, with a 21-day reapplication interval). A conservative 

Percent Cropped Area (PCA) of 100% was used. Chemical inputs (i.e., sorption coefficients) for 

the SU ROCs were prepared following the Water Models Input Parameter Guidance (USEPA, 

2009). The chemicals were modeled as stable to degradation (hydrolysis, photolysis, and 

metabolism processes) as part of the coarse-screen approach. Sorption coefficients reflect the 

highest soil mobility of any SU residue of concern, consistent with the coarse-screen approach.

Table 5.6. FIRST Input Parameters for Sulfonylurea Coarse-Screen EDWCs

Input Parameter Value Justification Source

Application Rate (lbs a.i./A) 0.66 Maximum labeled single application 

rate for use of imazosulfuron on turf 

(see Table 2.2)

EPA Reg. No.

59639-155

(Mar. 29, 2011)

Applications per Year 2

Reapplication Interval (days) 21 Minimum labeled interval

PCA Decimal 1.0 Conservative value for screen Screening assumption

Kd or KOC value (L/kg) Kd=0.11 or

KOC=12

Represent the lowest mean Kd and 

KFOC values for sulfonylurea 

residues of concern in Table 5.1

MRID 49308108 (in 

review); MRID 

41869304

Method of Application Aerial Conservative value for screen Screening assumption
13

Input Parameter Value Justification Source

Solubility (ppm) 26 Lowest SU value (input is well 

above and does not impact EDWCs)

MRID 45371702

Aerobic Soil Metabolism 

Half-life (days) 0 Stability was modeled as a screen Screening assumption

Aerobic Aquatic Metabolism

Half-life (days) 0 Stability was modeled as a screen Screening assumption

Hydrolysis Half-life (days) 0 Stability was modeled as a screen Screening assumption

Photolysis Half-life (days) 0 Stability was modeled as a screen Screening assumption

The resulting coarse-screen EDWCs from FIRST using a Kd of 0.11 were 138 μg/L (1-

in-10-year peak used for acute exposure) and 98 μg/L (1-in-10-year annual mean used for 

chronic exposure). Respective EDWCs using a KOC of 12 were slightly less, at 137 μg/L and 97 

μg/L. These values (using either Kd or KOC) are 18-20% of the coarse-screen ground water 

EDWCs, indicating that the coarse-screen ground water exposure assessment is protective of 

surface water exposure for SUs with dietary exposure not of concern at the coarse screen.

5.2.2. Refined Screen

Screening surface water EDWCs were generated with FIRST (v.1.1.1) for the 8 SUs for 

which refined-screen ground water EDWCs were generated, in order to further confirm that 

estimated exposure to SUs via surface water sources is less than that via ground water sources. 

The same simple chemical-specific refinements as were made for ground water assessment were 

made for surface water assessment to generate lower refined-screen EDWCs that remained 

conservative for the chemical (see Table 5.7). Application inputs represent the maximum 

application rate pattern for the chemical, considering the maximum single application rate first, 

then the maximum seasonal application rate, using national-scale upper-bound usage data 

provided by BEAD if the labels do not provide a maximum seasonal rate. The modeled sorption 

coefficient is also chemical-specific. All other inputs, including the screening assumption of 

stability to degradation, remained the same as for the coarse screen.

Table 5.7. FIRST Input Parameters for Sulfonylurea Refined-Screen EDWCs

Sulfonylurea (Use) App. Rate

(lbs a.i./A)

App.

per 

Year

Reapp.

Interval 

(days)

Solubility 

(ppm)

PCA 

Decimal 

App.

Method

Degradation 

Half-lives

(days)

Sorption 

Coefficient 

(ml/g)

Chlorsulfuron (non-ag areas) 0.14 1 N/A

26 1.0 Aerial 0

KOC=21

Halosulfuron-methyl (fallow) 0.14 1 A N/A KOC=109

Nicosulfuron (rights-of-way) 0.070 1 N/A KOC=35

Prosulfuron (corn) 0.036 1 N/A Kd=0.22

Rimsulfuron (commercial areas) 0.063 3 B 7 C KOC=47

Triasulfuron (rangeland) 0.028 2 D 7 C Kd=0.34

Tribenuron-methyl (corn) 0.031 2 E 7 C KOC=31

Triflusulfuron-methyl (sugarbeet) 0.031 3 F 7 C Kd=0.64

A One application per year was assumed in the absence of label directions based on national-scale usage data 

provided by BEAD indicating halosulfuron-methyl was not applied more than once to 27,303 acres of fallow land.

B Three applications per year were assumed in the absence of label directions, based on national-scale usage data 

collected by BEAD indicating that rimsulfuron was not applied more than three times per year to any crop.
14

C A 7-day reapplication interval was assumed in the absence of label directions, based on the assumption that 

herbicides are not applied more often, giving time to evaluate the impact of the previous application before the next.

D The two applications (0.056 lbs a.i./A total) exceed the seasonal maximum rate of 0.039 lbs a.i./A because FIRST 

cannot accept multiple application rate values.

E Two applications per year were assumed in the absence of label directions based on usage data provided by BEAD 

that indicates tribenuron-methyl was never applied more than twice to 4,831,149 acres of soybeans.

F The three applications (0.093 lbs a.i./A total) exceed the seasonal maximum rate of 0.078 lbs a.i./A because 

FIRST cannot accept multiple application rate values.

FIRST outputs are listed in Table 5.8. As expected, acute and chronic surface water 

EDWCs are less than respective ground water EDWCs for each SU. Therefore, screening 

ground water exposure estimates rather than surface water exposure estimates are used to screen 

for exposure concerns in this assessment.

Table 5.8. FIRST Refined-Screen EDWCs for Sulfonylureas

Sulfonylurea (Use) 1-in-10-year Peak

(μg/L)

1-in-10-year Annual 

Mean (μg/L)

Chlorsulfuron (non-ag areas) 14 10

Halosulfuron-methyl (fallow) 13 7.2

Nicosulfuron (rights-of-way) 7.0 4.7

Prosulfuron (corn) 3.7 2.6

Rimsulfuron (commercial areas) 19 12

Triasulfuron (rangeland) 5.7 3.9

Tribenuron-methyl (corn) 6.2 4.2

Triflusulfuron-methyl (sugarbeet) 9.0 5.8

6. Exposure Summary

Table 6.1 summarizes the maximum coarse-screen and refined-screen exposure model 

results for the sulfonylureas, which are not expected to occur in drinking water due to the 

conservative nature of these screens. Monitoring data were not collected for this assessment 

because of its screening approach that is not meant to estimate actual exposures at drinking water 

intake facilities.

Table 6.1. Coarse- and Refined-Screen Maximum EDWCs for Sulfonylureas

Sulfonylurea (Use) PRZM-GW 

Scenario

Screen 

Level

Max. Daily 

Conc. (μg/L)

Postbreakthrough 

Mean (μg/L)

Mean 

Breakthrough 

Time (yrs)

Chlorsulfuron (non-ag areas) NC cotton Refined 78 52 5.9

Halosulfuron-methyl (fallow) NC cotton Refined 74 53 8.1

Nicosulfuron (rights-of-way) NC cotton Refined 39 26 6.2

Prosulfuron (corn) NC cotton Refined 15 13 12

Rimsulfuron (commercial areas) NC cotton Refined 105 71 6.5

Triasulfuron (rangeland) NC cotton Refined 21 19 16

Tribenuron-methyl (corn) NC cotton Refined 35 23 6.1

Triflusulfuron-methyl (sugarbeet) FL citrus Refined 23 20 9.5

All other sulfonylureas NC cotton Coarse 751 492 5.7-8.7
15

7. References

USEPA. 2006. Standardized Soil Mobility Classification Guidance. U.S. Environmental 

Protection Agency, Office of Pesticide Programs, Environmental Fate and Effects 

Division. Internal memorandum. Apr. 21, 2006.

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 Pesticide Programs, Environmental Fate and Effects Division, Oct. 22, 

2009. http://www.epa.gov/oppefed1/models/water/input_parameter_guidance.htm

USEPA. 2012. Guidance for Selecting Input Parameters for Modeling Pesticide Concentrations 

in Groundwater Using the Pesticide Root Zone Model. Version 1.0. U.S. Environmental 

Protection Agency, Office of Pesticide Programs, Environmental Fate and Effects 

Division; Health Canada, Pesticide Management Regulatory Agency, Environmental 

Assessment Directorate. Oct. 15, 2012.

Appendix A. Aquatic Model Calculations

Aquatic model input and output files are contained in the attached WinZip file.

Appendix B. Environmental Fate Data

In support of this assessment, environmental fate data regarding the sulfonylureas and 

their degradates were collected from the problem formulations for registration review and, in 

some cases, directly from studies submitted to EPA. The data are tabulated in the attached 

Microsoft Excel workbook.

SU PRA Fate Data 

FINAL.xls

Appendix C. Chemical Structures

In support of this assessment, the chemical structures of the sulfonylureas and their 

degradates were collected from the problem formulations for registration review and from 

studies submitted to EPA after the problem formulations were prepared. The data are tabulated 

in the attached Microsoft Word document.

SU PRA Chemical 

Structures.doc