UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON D.C., 20460

OFFICE OF CHEMICAL SAFETY 

AND POLLUTION PREVENTION

PC Code: 118602 

DP Barcodes: 362654 

Date: May 13, 2010 

MEMORANDUM

SUBJECT: Tier I Drinking Water Assessment for the Section 3 Proposed Registration of 

Imazosulfuron on Turf, Tomato, Peppers (Bell and Non-bell) and Rice 

TO: James Tompkins, Risk Manager 

 Bethany Benbow, Risk Manager Reviewer 

 Herbicide Branch 

 Registration Division 

 

FROM: Faruque Khan, Senior Scientist 

 Environmental Risk Branch I 

 Environmental Fate and Effects Division 

THROUGH: Anita Pease, Acting Branch Chief 

 Environmental Risk Branch I 

 Environmental Fate and Effects Division 

A Tier I screening-level drinking water assessment was conducted to support the human health 

risk assessment for the Section 3 proposed new chemical registration for imazosulfuron. 

Modeled application rates represent the maximum application rates on the proposed label for use 

on turf grass, tomato, peppers (bell and non-bell) and rice (EPA Reg. No. pending). Foliar spray 

applications (ground spray) are proposed for turf grass, tomato, peppers (bell and non-bell) and 

aerial application is proposed for rice. Tier 1 surface water and groundwater modeling was 

conducted for two ground spray applications with 21 days interval for turf at a rate of 0.67 lbs. 

a.i./A. In addition, Tier 1 surface water modeling was also conducted for one ground spray 

application for food (tomato and peppers) uses and one aerial application for rice at a rate of 0.3 

lbs. a.i./A per year. 

Imazosulfuron is a selective sulfonylurea herbicide with major routes of degradation being 

aerobic and anaerobic biodegradation and aqueous photolysis. There are several major 

degradates identified in the environmental fate studies. Residues of Concern Knowledgebase 

Subcommittee (ROCKS) of the Health Effects Division (HED) reviewed the structures of these 

degradates to determine residues of concern (DP Barcode: 359190). As 2-chloro-N-[[(4-hydroxy6-methoxy-2-pyrimidinyl)amino]carbonyl]imidazo[1,2-α]pyridine-3-sulfonamide (HMS) retains 

 1
both imidazolyl (IM) and pyrimidinyl (PM) ring structures, its toxicity is likely comparable to 

the parent. Structure-activity relationship (SAR) and Deductive Estimation of Risk based on 

Existing Knowledge (DEREK) analyses revealed the same structural alerts as the parent 

compound; therefore, HMS is considered a residue of concern. DEREK analysis of [2-

chloroimidazo[1,2-alpha]pyridin-3-sulfonamide (IPSN), 2-(2-imino-1,2-dihydropyridin-1-

yl)oxalic acid (IHOA), 2-amino-4,6-dimethoxypyrimidine (ADPM), (4,6-dimethoxypyrimidine2-yl) aminocarbonylsulfamic acid (SDPM), and 4,6-dimethoxy-2-ureidopyrimidine (UDPM) 

showed that all of these cleavage products except IHOA (a low molecular weight acid) retained 

at least one of the same structural alerts as the parent. Therefore drinking water exposure of total 

toxic residue (parent plus five degradates listed in Table 1) is considered in this assessment 

based on the memorandum provided by ROCKS (DP Barcode: 359190). 

Table 1. Summary of Metabolites and Degradates to be included in the Risk Assessment.

Matrix Residues included in Risk 

Assessment 

Residues included in 

Tolerance Expression 

Drinking Water Imazosulfuron, HMS, IPSN, 

ADPM, UDPM, and SDPM 

Not Applicable 

Imazosulfuron = 2-chloro-N-[[(4,6-dimethoxy-2-pyrimidinyl)amino]carbonyl]imidazo[1,2-α]pyridine-3-

sulfonamide 

HMS = 2-chloro-N-[[(4-hydroxy-6-methoxy-2-pyrimidinyl)amino]carbonyl]imidazo[1,2-α]pyridine-3-

sulfonamide 

IPSN = 2-chloroimidazo[1,2-alpha]pyridin-3-sulfonamide 

ADPM = 2-amino-4,6-dimethoxypyrimidine 

UDPM = 4,6-dimethoxy-2-ureidopyrimidine 

SDPM = (4,6-dimethoxypyrimidine-2-yl)aminocarbonylsulfamic acid 

The recommended estimated drinking water concentrations (EDWCs) for the human health risk 

assessment are based on highest predicted value for surface water and groundwater (Table 2). 

The recommended EDWCs in surface water are based on aerial spray application of 0.3 lbs. 

a.i./A per year for rice. The concentrations in ground water are based on ground spray on turf at 

1.34 lbs. a.i./A per year. Additional EDWCs for various scenarios can be found in Appendix A. 

If additional refinements are necessary, please contact Faruque Khan at 703-305-6127 or 

khan.faruque@epa.gov.

Table 2. Tier I EDWCs for Imazosulfuron for ground spray applications to Turf

Drinking Water Source Model Use rate (lbs 

ai/A/year) 

Acute (μg/L) Chronic (μg/L) 

Surface water RICE 0.3 278.9 278.9 

Groundwater SCI-GROW 1.34 4.8 4.8 

1.1 Drinking Water Exposure Modeling 

Tier 1 modeling was used to estimate the drinking water exposure for use in the dietary risk 

assessment of imazosulfuron. The following is a description of the models used, the selection of 

the model input parameters, and a characterization of the output from these simulations. 

 2
1.1.1 Models 

Tier 1 EDWCs for surface water were generated using FIRST, dated August 5, 2001. FIRST is a 

screening model designed by the Environmental Fate and Effects Division (EFED, 2001a) of the 

Office of Pesticide Programs (OPP) to estimate the concentrations found in drinking water from 

surface water sources for use in human health risk assessment. As such, it provides upper bound 

values on the concentrations that might be found in drinking water due to the use of a pesticide. 

FIRST is a single-event model (one runoff event), but can account for spray drift from multiple 

applications. FIRST is hardwired to represent the Index Reservoir, a standard water body used 

by OPP to assess drinking water exposure (Office of Pesticide Programs, 2002). It is based on a 

real reservoir, Shipman City Lake in Illinois, which 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 due to degradation on the field and the effects of binding 

to soil in the field. FIRST also uses a Percent Cropped Area (PCA) factor to adjust for the area 

within the watershed that is planted to the modeled crop. The national default PCA of 0.87 was 

applied in this assessment. 

The EFED Rice Model (USEPA, 2007) was used to estimate the concentration of imazosulfuron 

expected in surface water potentially used for drinking water following application to rice. The 

model calculates exposure as a function of the amount of pesticide applied and the pesticide’s 

soil-to-water partition coefficient. Concentrations are assumed to immediately and evenly 

distribute throughout the rice paddy water. Dissipation processes, such as degradation, 

volatilization, and potential dilution with uncontaminated water outside the paddy are not 

accounted for by the model. The single exposure estimate produced with the Rice Model is used 

as both an acute and a chronic screening exposure estimate that is expected to represent high-end 

exposure in released flood water (tailwater). The EDWC produced by the Rice Model was not 

adjusted by an agricultural Percent Cropped Area (PCA) value because these values may not 

apply to semi-aquatic agriculture. PCA values were designed for exposure estimates in runoff 

from terrestrial crops, not for exposure estimates in tailwater from flooded crops. 

Tier 1 EDWCs for groundwater were generated with SCIGROW 2.3, dated July 29, 2003 

(EFED, 2001b). SCIGROW is a regression model used as a screening tool for ground water 

used as drinking water. SCIGROW was developed by regressing the results of Prospective 

Ground Water studies against the Relative Index of Leaching Potential (RILP). 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. 

1.1.2 Modeling Approach and Input Parameters 

Tables 3 and 4 summarize the model input parameter values used in FIRST and SCIGROW, 

respectively. These parameters were selected in accordance with EFED’s input parameter 

guidance (Environmental Fate and Effects Division, 2009). An aerobic soil metabolism first 

order half-life based on total toxic residue (imazosulfuron and listed degradates in Table 1) was 

used for surface water modeling with FIRST and groundwater modeling with SCIGROW. First 

order degradation half-lives for aerobic aquatic and aquatic photolysis for total toxic residue 

were also used for surface water modeling. Since the experimental phototransformation halflives of IM and PM labels are based on the continuous irradiation used in the laboratory study, 

 3
estimated half-lives were adjusted by multiplying with 0.5 based on a 12-hour light/12-hour dark 

cycle. Also, the intensity of the xenon lamp over the wavelengths 290-800 nm was 558 W/m2

, 

compared to 380 W/m2 for natural sunlight over a 12-hour (solar) day at 40°N latitude during 

June through August. Therefore, 1 hour of artificial light was equivalent to 1.47 hours of natural 

sunlight. Calculations of first-order degradation rates for total toxic residue can be found in 

Appendix B. The soil-water partitioning coefficient used in FIRST and SCIGROW was 98 

ml/goc, the lowest non-sand Koc. 

Table 3. FIRST (v 1.1) input parameter values for total residuea of imazosulfuron applied by ground spray

Parameter Value Source Comments

Application Rate (lb a.i./A) 0.67 (turf) 

0.3 (food uses) 

As per V-10142 label 

Direction --- 

Number of Applications 

2 (turf) 

1 (food uses) As per V-10142 label 

Direction --- 

Minimum Application 

Intervals (days) 21 (turf only) As per V-10142 label Direction For Ground Application 

Soil Partitioning Coefficient 

(Kfoc; ml/goc) 98 MRID 47305122 lowest non-sand valueb

Aerobic Soil Metabolism t1/2

(d) 207.15 

MRID 473051-17 

MRID 477668-01 

 

90th percentile upper confidence 

bound on the mean half-life of 

total toxic residue of 

imazosulfuron 

Wetted in? No foliar applied As per V-10142 label Direction 

Depth of Incorporation 

(inches) 0 foliar applied As per V-10142 label Direction 

Methods of Application Ground spray As per V-10142 label 

Direction As per V-10142 label Direction 

Percent Cropped Area 0.87 National default 

Input parameter guidance 

(USEPA, 2002) 

Solubility in Water (mg/L) 419 Product chemistry data 

Input parameter guidance 

(USEPA, 2009) 

Aerobic Aquatic Metabolism 

t1/2 (d) 192.03 

MRID 476397-02 

MRID 476791-05 

90th percentile upper confidence 

bound on the mean half-life of 

total toxic residue of 

imazosulfuron 

Hydrolysis t1/2 at pH 7.0 

(days) 

0 MRID 473051-19 Stable to hydrolysis @ pH 7.0 

 4
Table 3. FIRST (v 1.1) input parameter values for total residuea

 of imazosulfuron applied by ground spray

Parameter Value Source Comments

Aquatic Photolysis t1/2 (d) 61.69 MRID 473051-20 

90th percentile upper confidence 

bound on the mean of IM and PM 

rings of imazosulfuron 

a

 = Residues of concern (Parent plus degradates, i.e., HMS, IPSN, UDPM, ADPM and SDPM) recommended by 

Residues of Concern Knowledgebase Subcommittee (ROCKS) 

b

 = According to Guidance for selecting input parameters in modeling for environmental fate and transport of 

pesticides. Version II. October, 2009. 

Table 4. SCIGROW 2.3 input parameter values for total residue of Imazosulfuron applied by ground spray.

Parameter Value Source Comments

Maximum Application 

Rate (lb a.i./A/application) 

0.67 Ground Application As per V-10142 label Direction 

Maximum Number of 

Applications per Year 

2 Ground Application As per V-10142 label Direction 

Aerobic Soil Metabolism 

Half-life (d) 

207.15 MRID 473051-22 90th percentile upper confidence 

bound on the mean half-life of total 

toxic residue of imazosulfuron 

Organic Carbon Partition 

Coefficient (Kfoc, ml/goc) 

98 MRID 473051-17 Lowest non-sand value a

 

a = According to Guidance for selecting input parameters in modeling for environmental fate and transport of 

pesticides. Version II. October, 2009. 

Input parameters for the Rice Model are listed in Table 5; data source and justification 

descriptors accompany values for each parameter. The mean Freundlich soil-to-water partition 

coefficient (Kf) from four soils (1.19) was used to model partitioning to rice paddy sediment. 

The organic carbon partition coefficient (KOC) was not used, as it is a less precise descriptor of 

imazosulfuron’s mobility. 

Table 5. Rice Model input parameters for imazosulfuron. 

Input Parameter Value Justification Source 

Application Rate 

(kg a.i./ha) 

0.3 0.3 lbs a.i./A x 1.121 kg·A/ha·lbs 

= 0.336 kg a.i./ha 

Proposed label 

Soil-to-Water Partition 

Coefficient (Kf) (L/kg) 

1.19 Mean Kf is used when mean KOC use is 

not appropriate. 

MRID 46219074 

MRID 46578971 

1.1.3 Modeling Results 

The FIRST, RICE and SCI-GROW output files are located in Appendix A. Based on the results 

of these simulations, EFED recommends acute and chronic surface water EDWC of 278.9 μg/L

 5
for use in the human health risk assessment. These recommendations are based on the use of 

imazosulfuron on rice at an annual application rate of 0.3 lb a.i./A/year. The recommended 

EWDC for groundwater is 4.8 μg/L and is based on maximum applications at 0.67 lb a.i./A of 

two application per year for the use of imazosulfuron on turf. The estimated drinking water 

concentrations (EDWCs) for various scenarios are listed in Table 5. 

Table 5. Tier I EDWCs for Imazosulfuron for Turf, food uses and rice

Drinking Water Source Models Use rate (lbs 

ai/A/year) 

Acute (μg/L) Chronic (μg/L) 

RICE 0.3 278.9.0 278.9.0 

1.34* 101.25 33.27 

Surface water 

FRIST 

0.3** 23.47 7.71 

Groundwater SCI-GROW 1.34* 4.8 4.8 

* Application rate for turf 

** Application rate for tomato and pepper 

1.2 Monitoring Data 

There were no national-scale monitoring data available for this new chemical assessment. 

1.3 Drinking Water Treatment 

There is no available information on the drinking water treatment effects on imazosulfuron. 

2 CONCLUSIONS 

The estimated concentrations provided in this assessment are conservative estimates of 

imzosulfuron concentrations in untreated drinking water. A major source of uncertainty is the 

nature of the non-extractable residues formed during degradation in soils. Non-extractable 

residues were identified at maximums of 33-88% of applied radioactivity in aerobic soil, aerobic 

and ananerobic aquatic metabolism studies. The non-extractable residues are uncharacterized and 

it is uncertain whether they consist of degradates of risk concern. Under sterile conditions these 

non-extractables were not formed suggesting that the formation of them is microbially mediated. 

If dietary risks require refinement, higher tiered models and modeling scenarios can be utilized 

to characterize EDWCs upon request. 

 6
References 

Environmental Fate and Effects Division. 2001a. FIRST (F)IFRA (I)ndex (R)eservoir 

(S)creening (T)ool. Tier 1 Model for Drinking Water Exposure: User's Manual. 

http://www.epa.gov/oppefed1/models/water/first_users_manual.htm

Environmental Fate and Effects Division. 2001b. SCI-GROW - (S)creening (C)oncentration 

(I)n (GRO)und (W)ater: User's Manual. 

http://www.epa.gov/oppefed1/models/water/scigrow_users_manual.htm

Environmental Fate and Effects Division. 2009. Guidance for Selecting Input Parameters in 

Modeling the Environmental Fate and Transport of Pesticides, Version II. U. S. Environmental 

Protection Agency. Washington, D.C. http://www.epa.gov/oppefed1/models/water/

USEPA. 2007. Policy for Estimating Aqueous Concentrations from Pesticides Labeled for Use 

on Rice. EFED Policy Memorandum. U.S. Environmental Protection Agency, Office of 

Pesticide Programs, Washington, DC. 

http://www.epa.gov/oppefed1/models/water/rice_tier_i.htm#model

Office of Pesticide Programs. 2000. Part A. Guidance for Use of the Index Reservoir in 

Drinking Water Assessments. http://www.epa.gov/oppfead1/trac/science/reservoir.pdf

 

 7
Appendix A 

Surface water (FRIST)

Ground Application 

 RUN No. 1 FOR Imazosulfuron ON Turf * INPUT VALUES * 

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

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

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

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

 .670( 1.295) 2 21 98.0 419.0 GROUND( 6.4) 87.0 .0 

 FIELD AND RESERVOIR HALFLIFE VALUES (DAYS) 

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

 METABOLIC DAYS UNTIL HYDROLYSIS PHOTOLYSIS METABOLIC COMBINED 

 (FIELD) RAIN/RUNOFF (RESERVOIR) (RES.-EFF) (RESER.) (RESER.) 

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

 207.15 2 N/A 61.69- 7649.56 192.03 187.33 

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

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

 PEAK DAY (ACUTE) ANNUAL AVERAGE (CHRONIC) 

 CONCENTRATION CONCENTRATION 

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

 101.248 33.268 

 RUN No. 2 FOR Imazosulfuron ON Food Uses * INPUT VALUES * 

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

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

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

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

 .300( .300) 1 1 98.0 419.0 GROUND( 6.4) 87.0 .0 

 FIELD AND RESERVOIR HALFLIFE VALUES (DAYS) 

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

 METABOLIC DAYS UNTIL HYDROLYSIS PHOTOLYSIS METABOLIC COMBINED 

 (FIELD) RAIN/RUNOFF (RESERVOIR) (RES.-EFF) (RESER.) (RESER.) 

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

 207.15 2 N/A 61.69- 7649.56 192.03 187.33 

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

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

 PEAK DAY (ACUTE) ANNUAL AVERAGE (CHRONIC) 

 CONCENTRATION CONCENTRATION 

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

 23.466 7.710 

 8
 

 Rice Model (USEPA, 2007). 

 The formula of the Tier I Rice Model v1.0 is as follows: 

d

ai

w K

m C

00013.0 00105.0

'

+ =

and, if appropriate: 

d = 01.0 KK oc

where: 

 Cw = water concentration [μg/L] 

mai' = mass applied per unit area [kg/ha] 

Kd = water-sediment partitioning coefficient [L/kg] 

Koc = organic carbon partitioning coefficient [L/kg] 

Example calculation: 

EEC = (0.336 kg a.i./ha) / [0.00105 + (0.00013 x 1.19 L/kg)] 

= 278.9 μg/L (ppb)

Ground water

 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:Imazosulfuron 

 time is 5/ 5/2010 15: 3:43 

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

 Application Number of Total Use Koc Soil Aerobic 

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

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

 0.670 2.0 1.340 9.80E+01 207.1 

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

 groundwater screening cond (ppb) = 4.80E+00 

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

 9
Appendix B 

Calculations of Half-life for total toxic residue 

Aerobic Soil Metabolism (MRID 473051-17 

 10
Aerobic Soil Metabolism (MRID 477668-01 

- 

 11
Aerobic Aquatic Metabolism (MRID 476397-02) 

1

 12
Aerobic aquatic Metabolism (MRID 476791-05 

 13
Aquatic Photolysis (IM Ring) 

 14
Aquatic Photolysis (PM Ring) 

Half life calculation for total system: 

PM ring 32.66 Days 

IM Ring 4.72 Days 

90th %ile 61.69 Days 

 15