2

sugar beet 1.98 lbs a.i/A. Terbufos applications are limited to one application per year for each 

crop. 

This drinking water assessment was performed using a parent only approach (terbufos) as well as 

a total toxic residue approach (TTR; i.e. parent plus terbufos sulfoxide and terbufos sulfone), as 

recommended by the Residues of Concern Knowledgebase Subcommittee (ROCKS) of the

Health Effects Division (US EPA 2014; DP417464). The surface water and groundwater 

modeling were conducted according to labeled use directions, which listed a maximum annual 

application rate of 1.30 lbs. a.i./A to 1.98 lbs a.i/A for various crops. Modeling for the ground 

application used the coupled models PRZM and EXAMS for surface water and SCI-GROW as 

well as PRZM-GW for groundwater. Recommended EDWCs for terbufos and total toxic 

residues of terbufos in surface water and groundwater are summarized in Table 1. 

Table 1. Recommended EDWCs for Drinking Water Risk Assessment for Terbufos and 

its Degradates 

Source of Drinking 

Water

Estimated Drinking Water Concentration (μg/L)

1-in-10-year Peak 

Exposure

1-in-10-year Annual 

Mean Exposure 

30-year Mean 

Exposure 

Total Toxic Residues of Terbufos (Terbufos plus Terbufos Sulfoxide and Terbufos Sulfone)

Surface Water a,b 63.06 11.53 6.13

Groundwater c 33.54 -- 14.02

Terbufos (Parent only) 

Surface Water b,d 46.02 0.28 0.12

Groundwater e 0.02 0.02 0.02

a EDWCs based on PRZM/EXAMS model and residue summation method for TX Sorghum OP Scenario

b PCA adjusted modeled values

c Highest EDWCs based summation of terbufos, terbufos sulfoxide and terbufos sulfone derived from PRZM-GW 

model for Delmarva scenario

d EDWCs based on PRZM/EXAMS model for parent terbufos alone for MS corn STD Scenario

e EDWCs for parent terbufos were derived from SCI-GROW model

Drinking Water Exposure Modeling

Surface Water

A Tier II drinking water assessment was performed using PRZM (v3.12.2)/EXAMS (v. 

2.98.04.06) modeling with the index reservoir scenario. The EDWCs were generated for the 

parent only and for terbufos parent plus its two major degradates, terbufos sulfoxide and terbufos 

sulfone (TTR approach). Available environmental fate data for terbufos and individual 

degradates were used in exposure assessments. For the TTR approach, the residue summation 

method was used in estimating concentrations for individual residues of concern, which were 

then summated to represent the TTRs. Description of the residue summation method can be 

found in the document related to the methods for assessing ecological risks of pesticides with 

persistent, bioaccumulative and toxic characteristics (USEPA, 2008). Application rates for 

transformation products were adjusted to account for the parent and the normalized maximum 

percentage of transformation product formed as well as for molecular weight ratios of parent to 

the metabolite. The two scenarios with the highest EDWCs for the parent (i.e. MS corn STD and 

TX Sorghum OP scenarios) were simulated to generate EDWCs for the TTR. Post-processing of 
3

estimated EDWCs generated for terbufos and its degradates was applied using EXCEL 

spreadsheet software in estimating the 1-in-10 year exposure concentrations for TTR. 

The Pesticide Root Zone Model, (PRZM, Carsel et al. 1997) and the Exposure Analysis 

Modeling System (EXAMS, Burns 2000) were used in tandem to generate surface water 

EDWCs. PRZM (3.12.2 dated May 12, 2005) simulates fate and transport on the agricultural 

land whereas EXAMS (2.98.04.06, dated April 25, 2005) simulates the fate and resulting daily 

concentrations in the water body. Simulations were carried out with the linkage program shell, 

PE5V01.pl (dated November 15, 2006), which incorporates the standard agricultural scenarios 

developed by the Environmental Fate and Effects Division (EFED). The PRZM model simulates 

pesticide movement and transformation from crop application through soil residue processes. 

The EXAMS model simulates pesticide loading via runoff, erosion, and spray drift assuming a 

standard watershed of 172.8 ha that drains into an adjacent standard drinking water index 

reservoir of 5.26 ha with an average depth of 2.74 m. A more detailed description of the index 

reservoir watershed can be found in Jones et al., 1998. Standard percent cropped areas (PCA) 

were used for proposed uses as estimates of the extent of watershed on which crops are grown. 

The default PCA of 0.91 was used for the proposed uses because currently terbufos can be used 

in agricultural settings only (USEPA 2012b). 

PRZM/EXAMS input parameters are shown in Tables 3, 4 and 5 for the parent, terbufos 

sulfoxide and terbufos sulfone, respectively. Simulations are run for multiple (usually 30) years, 

and the EDWCs represent peak values that are expected once every ten years based on the thirty 

years of daily values generated during the simulation. Sample outputs and results of 

PRZM/EXAMS modeling are provided in Appendix B. The EDWCs for the parent and TTRs

are provided in Table 6. PRZM/EXAMS generated “time series” files of terbufos for MS corn 

STD and TTR for TX sorghum OP scenarios are also attached in Appendix B. Additional 

information on these models can be found at: http://www.epa.gov/oppefed1/models/water 

/index.htm.

Table 3. PRZM/EXAMS Input Parameters for Terbufos

Parameter Input Value Source Comment

Application Rate 

Corn: 1.30 lbs a.i/A

Sorghum: 1.68 lbs a.i./

Sugar beet: 1.98 lbs a.i/A

EPA Reg # 241-314

EPA Reg# 5481-562

These are maximum 

application rates for 

specified crops

Number of 

application/year 

1 

EPA Reg # 241-314

EPA Reg# 5481-562

Label directions

Application method Ground Current Labels Label directions

CAM 6 PRZM Manual

To simulate subsurface 

incorporation of applied 

terbufos

Depth of Incorporation 1 inch Current labels

Label direction for corn.

For sorghum and sugar beet, 

incorporation depths were 

assumed based on seeding 

depths.
4

Table 3. PRZM/EXAMS Input Parameters for Terbufos

Parameter Input Value Source Comment

Use Site

Scenario and 

Application Date 

(Month-Date)

Corn 

CA corn OP (04-01)

IA corn STD (05-25

IL corn STD (05-01)

IN corn STD (05-15)

KS corn STD (05-10)

MN corn STD (05-15)

MS corn STD (04-10)

NC CornE STD (04-15)

NC CornW OP (04-25)

ND Corn OP (05-05)

NE corn STD (05-25)

OH corn STD (05-01)

PA corn STD (04-16)

TX corn OP (03-16)

FL sweetcorn OP (10-16)

OR Sweetcorn OP (05-16)

Suger beets

CA Sugar Beet Wirrg OP (02-

01)

MN Sugar Beet STD (05-16)

Sorghum

KS Sorghum STD (05-20)

TX Sorghum OP (05-10)

Label directions

and available 

scenarios 

The following scenarios 

were used in generating 

EDWCs. 

Corn

11 standard and 5 organo 

phosphate specific scenarios

Suger beets

1 standard and 1 organo 

phosphate specific scenarios

Sorghum

1 standard and 1 organo 

phosphate specific scenarios

Spray drift fraction Not applicable EFED Guidance (US 

2013) ---

Molecular weight 288.4 g/mole MRID 4104495-02 ---

Solubility in water 

25 °C) 5.4 mg/L MRID 4104495-02 ---

Vapor pressure 6.6E-04 (mmHg @25°C) MRID 4104495-02 Additional parameters such 

as DAIR and Enthalpy

were used for semi-volatile 

characteristic of terbufos

Henry’s Law constant 

(20 °C) 2.46E-05 atm.m3

/mol Footprint

DAIR 3372 cm2/s Estimated Schwarzenbach et al, 1993

Enthalpy 12.88 http://www.chemical

book.com/ MSDS for terbufos

Hydrolysis (t1/2)

1 1.5 days @ 25°C @ pH 7 MRID 44862501 Appendix A contains DT50

calculations

Aquatic photolysis 

(t1/2)

1 1.77 days MRIDs 00161567 

and 41181101

Appendix A contains DT50

calculations

Aerobic soil 

metabolism (t1/2)

1 14.7 days MRIDs 00156853 

and 41749801 Appendix A contains DT50

calculations. The 90% of the 

upper confidence limit (UCL)

of the mean metabolism halflife2

.

Aerobic aquatic 

metabolism (t1/2)

1 36.2 days 

MRID 44672004
5

Table 3. PRZM/EXAMS Input Parameters for Terbufos

Parameter Input Value Source Comment

Anaerobic aquatic 

metabolism (t1/2)

Stable2 ---

Since terbufos is sensitive to 

hydrolytic degradation, 

anaerobic aerobic aquatic 

metabolism was assumed 

stable according to Input 

Parameter Guidance 

(USEPA 2009)2 

Partition coefficient Kd 11.11 mL/g MRID 41373604 Represent average Kd for 4 

soils

1 DT50s were recalculated using NAFTA Guidance for Evaluating and Calculating Degradation Kinetics in 

Environmental Media (USEPA 2012). Appendix A contains revised estimated half-lives.

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

Table 4. PRZM/EXAMS Input Parameters for Terbufos Sulfoxide

Parameter Input Value Source Comment

Use Site

Scenario1and 

Application (MonthDay)

Corn 

MS corn STD (05-10)

Sorghum

TX Sorghum OP (06-10)

---

Application dates were 

adjusted based on maximum 

terbufos sulfoxide formation 

in laboratory study

Application Rate

Corn: 0.72 lbs

Sorghum: 0.93 lbs

Sugar beet: 1.08 lbs

Estimated See sample calculations 

below2

Number of applications 

per year 

1 

EPA Reg # 241-

314

EPA Reg# 5481-

562

---

Application method Ground ---

Degradation product of 

terbufos

CAM 1 ---

Spray drift fraction Not applicable ---

Molecular weight 304.42 g/mole EPISUITE 4.1 ---

Solubility in water 

25 °C) 3214 mg/L MRIDS 44672001 and 44672002

Solubility of terbufos 

sulfoxide is higher terbufos

Vapor pressure 3.42E-05 (mmHg @25°C) EPISUITE 4.1

Estimated using EPISUITE 

model

Henry’s Law constant 

(25 °C) 9.13E-08 atm.m3

/mol EPISUITE 4.1

Hydrolysis (t1/2) 65.1 days @ 25°C MRID 44862501 ---

Aquatic photolysis (t1/2) Stable ---

In absence of data, assumed 

stable according to Input 

Parameter Guidance 

(USEPA 2009)3

Aerobic soil 

metabolism (t1/2)

136 x 3 (408 days) MRIDs 00156853 

Single value is available. 3X 

was used according to Input 

Parameter Guidance 

(USEPA 2009)3
6

Table 4. PRZM/EXAMS Input Parameters for Terbufos Sulfoxide

Parameter Input Value Source Comment

Aerobic aquatic 

metabolism (t1/2) Stable ---

In absence of data, assumed 

stable

Anaerobic aquatic 

metabolism (t1/2)

Stable ---

Since terbufos sulfoxide is 

sensitive to hydrolytic 

degradation, anaerobic 

aerobic aquatic metabolism 

was assumed stable 

according to Input 

Parameter Guidance 

(USEPA 2009)3 

Partition coefficient Kd 1.12 mL/g MRID 41373604 Represent average Kd for 4 

soils

1 The highest EDWCs for parent (i.e. MS corn STD and TX Sorghum OP scenarios) were used for generating 

TTR EDWCs

2Terbufos Sulfoxide application rate = Terbufos application rate of [1.30 lbs x (0.523, the maximum conversion 

rate from the degradation of terbufos to terbufos sulfoxide in laboratory studies) x (1.055, the molecular weight 

ratio of terbufos sulfoxide to terbufos]

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

Table 5. PRZM/EXAMS Input Parameters for Terbufos Sulfone

Parameter Input Value Source Comment

Use Site

Scenario1and Application 

(Month-Day)

Corn 

MS corn STD (06-10)

Sorghum

TX Sorghum OP (07-10)

---

Application dates were 

adjusted based on maximum 

terbufos sulfone formation 

in the laboratory study

Application Rate1 

Corn: 0.29 lbs

Sorghum: 0.38 lbs

Sugar beet: 0.44 lbs

Estimated See a sample calculations 

below2

Number of applications 

per year 

1 

EPA Reg # 241-

314

EPA Reg# 5481-

562

Application method Ground ---

Degradation product of 

CAM 1 terbufos

Spray drift fraction Not applicable ---

Molecular weight 320.42 g/mole EPISUITE 4.1 ---

Solubility in water 

25 °C) 407 mg/L

MRIDS 

44672001 and 

44672002

More soluble than terbufos

Vapor pressure 7.88E-06 (mmHg @25°C) EPISUITE 4.1

Estimated using EPISUITE 

model

Henry’s Law constant 

(25 °C) 4.10E-08 atm.m3

/mol EPISUITE 4.1

Hydrolysis (t1/2) 43.8 days @ 25°C MRID 

444862501

Aquatic photolysis (t1/2) Stable --- Assumed stable
7

Table 5. PRZM/EXAMS Input Parameters for Terbufos Sulfone

Parameter Input Value Source Comment

Aerobic soil metabolism 

(t1/2)

174 x 3 (522 days)

MRIDs 

00156853 

Single value is available. 3X 

was used according to Input 

Parameter Guidance 

(USEPA 2009)2

Aerobic aquatic 

metabolism (t1/2) Stable 

----

In absence of data, assumed 

stable according to Input 

Parameter Guidance 

(USEPA 2009)3

Anaerobic aquatic 

metabolism (t1/2)

Stable ---

Since terbufos sulfoxide is 

sensitive to hydrolytic 

degradation, anaerobic 

aerobic aquatic metabolism 

is assumed stable according 

to Input Parameter 

Guidance (USEPA 2009)3

Partition coefficient Kd 1.26 mL/g MRID 41373604 Represent average Kd for 4 

soils

1The highest EDWCs for parent (i.e. MS corn STD and TX Sorghum OP scenarios) were used for generating 

TTR EDWCs

2Terbufos Sulfone application rate = Terbufos application rate of [1.30 lbs x (0.201, the maximum conversion 

rate from the degradation of terbufos to terbufos sulfone in laboratory studies) x (1.11, the molecular weight ratio 

of terbufos sulfoxide to terbufos]

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

Table 6. Surface Water EDWCs for Drinking Water Risk Assessment for Terbufos and 

its Total Toxic Residue 

Senarios Estimated Drinking Water Concentration (μg/L)

1-in-10-year Peak

Exposure

1-in-10-year Annual 

Mean Exposure 

30-year Mean 

Exposure 

Parent only

CA corn OP 2.49 0.02 0.00

IA corn STD 15.58 0.12 0.04

IL corn STD 10.42 0.08 0.04

IN corn STD 11.50 0.08 0.03

KS corn STD 15.65 0.12 0.06

MN corn STD 4.97 0.04 0.02

MS corn STD 46.02 0.28 0.12

NC CornE STD 16.14 0.10 0.02

NC CornW OP 12.37 0.09 0.05

ND Corn OP 5.84 0.05 0.02

NE corn STD 28.95 0.22 0.10

OH corn STD 11.01 0.11 0.05

PA corn STD 4.32 0.04 0.02

TX corn OP 8.63 0.07 0.03

FL sweetcorn OP 22.99 0.17 0.09

OR Sweetcorn OP 1.18 0.01 0.00

KS Sorghum STD 14.59 0.12 0.06

TX Sourghum OP 36.71 0.23 0.09
8

Table 6. Surface Water EDWCs for Drinking Water Risk Assessment for Terbufos and 

its Total Toxic Residue 

Senarios Estimated Drinking Water Concentration (μg/L)

1-in-10-year Peak

Exposure

1-in-10-year Annual 

Mean Exposure 

30-year Mean 

Exposure 

CA Sugarbeet OP 10.52 0.08 0.02

MN Sugarbeet STD 7.31 0.05 0.03

Total Toxic Residue1

MS corn STD 60.25 6.84 3.61

TX Sorghum OP 63.06 11.58 6.13

1 Total Toxic residues are based on two scenarios with the highest EDWCs for parent (i.e. MS corn STD and TX 

Sorghum OP scenarios) 

Groundwater

Screening Concentration in Ground Water (SCI-GROW v2.3, Jul. 29, 2003) is a regression 

model used as a screening tool to estimate pesticide concentrations found in groundwater used as 

drinking water. The SCI-GROW model and user’s manual may also be downloaded from the 

EPA Water Models web-page (http://www.epa.gov/oppefed1/models/water/#scigrow). A 

summary of the model input parameter values used in the SCI-GROW model is listed in Table 7. 

SCI-GROW EDWCs are presented in Table 8. 

Table. 7. SCI-GROW Input Parameters for Terbufos, Terbufos Sulfoxide and

Terbufos Sulfone

Model Input Variable Input Values Sources Comments

The following input parameters are applicable to parent terbufos

Application Rate1

Corn: 1.30 lbs a.i/A

Sorghum: 1.68 lbs a.i./A

Sugar beet: 1.98 lbs

a.i/A

EPA Reg # 241-

314

EPA Reg# 5481-

562

Label directions

Number of Applications 1

Product Label as 

above Label directions

Aerobic Soil Metabolism 

Half-Life 8.03 days MRIDs 00156853 and 41749801 Mean value

Koc 1460 mL/g MRID 41373604 Median value

The following input parameters are applicable to terbufos sulfoxide

Application Rate1

Corn: 0.72 lbsA

Sorghum: 0.93 lbs/A

Sugar beet: 1.08 lbs/A

Estimated See a sample calculation in 

Table 4

Number of Applications 1

EPA Reg # 241-

314

EPA Reg# 5481-

---
9

Table. 7. SCI-GROW Input Parameters for Terbufos, Terbufos Sulfoxide and

Terbufos Sulfone

Model Input Variable Input Values Sources Comments

Aerobic Soil Metabolism 

Half-Life 136 x 3 (408 days) MRID 00156853 

Single value is available. 3X 

was used according to Input 

Parameter Guidance 

(USEPA 2009)2

Koc 112 mL/g MRID 41373604 Median value

The following input parameters are applicable to terbufos sulfone

Application Rates

1

Corn: 0.29 lbs/A

Sorghum: 0.38 lbs/A

Sugar beet: 0.44 lbs/A

Estimated See a sample calculation in 

Table 5

Aerobic Soil Metabolism 

Half-Life 174 x 3 (522 days) MRIDs 00156853 

Single value is available. 3X 

was used according to Input 

Parameter Guidance 

(USEPA 2009)2

Koc 141 mL/g MRID 41373604 Median value

1 SCI-GROW modeling was performed based on the highest application rates for terbufos and its degradates

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

Table 8. SCI-GROW Simulated Groundwater EDWCs for Drinking Water Risk 

Assessment for Terbufos and its Total Toxic Residues

Residue of Concern Estimated Drinking Water concentration (μg/L)1

Peak Exposure Chronic Exposure

Terbufos 0.02 0.02

Terbufos Sulfoxide 7.28 7.28

Terbufos Sulfone 2.76 2.76

Total toxic Residue2 10.06 10.06

1 EDWCs were generated for the highest application rate of 1.98 lbs a.i./ A for sugar beet

2Sum of residues of concern (i.e. terbufos, terbufos sulfoxide and terbufos sulfone)

In addition, Tier 1 groundwater EDWCs for terbufos and its total toxic residues, resulting from 

its use on sugar beet for its highest application rate of 1.98 lbs a.i./A were estimated using the 

PRZM-GW model (USEPA, 2012c), with the GW-GUI (Graphical User Interface, version 1.0, 

August 31, 2012a). PRZM-GW is a one-dimensional, finite-difference model that estimates the 

concentrations of pesticides in groundwater. It accounts for pesticide fate in the crop root zone 

by simulating pesticide transport and degradation through the soil profile after a pesticide is 

applied to an agricultural field. PRZM-GW permits the assessment of multiple years of pesticide 

application (up to 100 years) on a single site. Six standard scenarios, each representing a 

different region known to be vulnerable to groundwater contamination, are available for use with 

PRZM-GW for risk assessment purposes. Each of these standard scenarios was used for PRZMGW simulations. PRZM-GW output values represent pesticide concentrations in a vulnerable 

groundwater supply that is located directly beneath a rural agricultural field following many 

years of pesticide application. Breakthroughs were observed in all modeled scenarios. A 
10

summary of the model input parameter values used in PRZM-GW model is listed in Table 9. 

PRZM-GW values are presented in Table 10. A sample output of PRZM-GW model can be found 

in Appendix B. 

Table 9. PRZM-GW Input Parameters1for Terbufos, Terbufos Sulfoxide and Terbufos 

Sulfone 

Variable Name Data Value

Data Source

MRID(s) Comment

The following input parameters are applicable to parent Terbufos

Application Method Ground (Granular) potato Current labels See Table 4

Application Rate 

(lbs a.i./acre) [kg/ha]

1.98 [2.22] Sugar beet Current labels Used highest 

application rate 

Application Frequency 1 time/year Current labels See Table 4

Application Interval (days) Not Applicable Current labels See Table 4

Hydrolysis, DT50 (Days) 1.5 days @ 25°C MRID 46902201 ---

Aerobic Soil Metabolism 

DT50 (Days) 14.7 days MRIDs 00156853and 41749801

The 90% of the upper 

confidence limit (UCL)

of the mean metabolism 

half-life.

Partition Coefficient Kd 11.11 mL/g MRID 41373604 Represents average 

Kd for 4 soils

The following input parameters are applicable to Terbufos Sulfoxide

Application Method Ground (Granular) potato Current labels Application Method

Application Rate 

(lbs a.i./acre) [kg/ha]

1.08 [1.21] Sugar beet Current labels Application Rate 

(lbs a.i./acre) [kg/ha]

Application Frequency 1 time/year Current labels Label Directions

Hydrolysis, DT50 (Days) 65.1 MRID 46902201 ---

Aerobic Soil Metabolism 

DT50 (Days) 136 x 3 (408 days) MRID 00156853 

Single value is 

available. 3X was 

used according to 

Input Parameter 

Guidance (USEPA 

2009)1

Partition coefficient Kd 11.11 mL/g MRID 41373604 Represents average 

Kd for 4 soils

The following input parameters are applicable to Terbufos Sulfone
11

Table 9. PRZM-GW Input Parameters1for Terbufos, Terbufos Sulfoxide and Terbufos 

Sulfone 

Variable Name Data Value

Data Source

MRID(s) Comment

Application Method Ground (Granular) potato Current labels Application Method

Application Rate 

(lbs a.i./acre) [kg/ha]

0.44 [0.49] Sugar beet Current labels Application Rate 

(lbs a.i./acre) [kg/ha]

Application Frequency 1 time/year Current labels Application 

Frequency

Hydrolysis, DT50 (Days) 43.8 MRID 46902201 ---

Aerobic Soil Metabolism 

DT50 (Days) 174 x 3 (408 days) MRID 00156853 

Single value is 

available. 3X was 

used according to 

Input Parameter 

Guidance (USEPA 

2009)2

Partition coefficient Kd 11.11 mL/g MRID 41373604 Represents average 

Kd for 4 soils

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

Table 10. PRZM-GW Simulated Groundwater EDWCs for Drinking Water Risk 

Assessment for Terbufos and its Total Toxic Residues

Crop Scenario

Highest

Daily

Value (μg/L)

Average (μg/L)

Average Simulation 

Breakthrough Time 

(days)

Terbufos (Parent only)

Sugar beet

(1.98 lbs a.i./A x 1 

application)

Delmarva sweet corn 3.16E-14 2.56E-15 475

FL citrus 0 0 1170

FL potato 1.61E-17 2.63E18 306

GA peanut 3.10E-131 2.53E-14 328

NC cotton 0 0 1142

WI corn 0 0 1043

Terbufos Sulfoxide

Sugar beet

(1.08 lbs a.i./A x 

1application)

Delmarva sweet corn 27.3 11.6 224

FL citrus 17.9 9.48 246

FL potato 10.8 6.11 266

GA peanut 0.952 0.0454 1007

NC cotton 0.563 0.233 1062

WI corn 0.658 0.00768 885

Terbufos Sulfone

Sugar beet

(0.44 lbs a.i./A x 

1application)

Delmarva sweet corn 6.24 2.42 226

FL citrus 3.36 1.61 246

FL potato 2.08 1.04 269

GA peanut 0.00324 0.00145 1010

NC cotton 0.0366 0.0146 1063
12

Table 10. PRZM-GW Simulated Groundwater EDWCs for Drinking Water Risk 

Assessment for Terbufos and its Total Toxic Residues

Crop Scenario

Highest

Daily

Value (μg/L)

Average (μg/L)

Average Simulation 

Breakthrough Time 

(days)

WI corn 0.00455 0.000502 885

Total Toxic Residue (EDWCs Summation of terbufos, terbufos sulfoxide and terbufos sulfone)

Not applicable

Delmarva sweet corn 33.54 14.02

Not applicable

FL citrus 21.26 11.09

FL potato 12.88 7.15

GA peanut 0.10 0.05

NC cotton 0.60 0.25

WI corn 0.07 0.01

1 Bolded values are the highest EDWCs for parent, terbufos sulfoxide, terbufos sulfone and total toxic residues.

Monitoring Data

A surface water and groundwater monitoring study was conducted for terbufos and its 

degradates, terbufos sulfoxide and terbufos sulfone. The study was required by USEPA's Interim 

Reregistration Eligibility Decision (IRED) to confirm exposures of terbufos and its degradates 

terbufos sulfoxide and terbufos sulfone in drinking water sources (USEPA, 2008). For the 

surface water, a total of 502 samples were collected from 33 sites between 1999 and 2005. For 

the groundwater, 73 samples were collected from 2003 to 2005. From 1999 to 2003, samples 

from numerous watersheds were provided by the NAWQA (National Water Quality Assessment) 

program. In surface water, terbufos and terbufos oxon were not detected above the reporting 

limits in any samples. Terbufos sulfoxide was detected in four samples at 0.092 to 0.205 μg/L, 

with an additional nine estimated detections of 0.045 to 0.262 μg/L. Terbufos sulfone was 

detected in six samples at 0.046 to 0.114 μg/L, with 30 additional estimated detections of 0.012 

to 0.034 μg/L. There were no detections of terbufos or any degradates in any of the groundwater 

samples.

Monitoring data from 2006 to up-to-date data from the USGS- NAWQA program were accessed 

on March 18, 2014 to evaluate the current trend of terbufos and its degradates concentrations in 

surface water and groundwater. All data of filtered surface water and groundwater was 

downloaded since the evaluation of a monitoring study of terbufos and its degradates in drinking 

water issued 2008 (USEPS, 2008). For surface water, a total of 6740 water samples were 

analyzed for terbufos. Terbufos was detected in only one sample and the concentration was 0.02 

μg/L. There were two detections of 0.07 μg/L and 0.17 μg/L terbufos sulfone in surface water 

samples out of 6198 water samples. For groundwater, a total of 3582 water samples were 

analyzed for terbufos. Terbufos was detected in one sample with a concentration of 0.01 μg/L. 

There were no detections of terbufos sulfone in any of the groundwater samples.

The National Water-Quality Assessment Program of the U.S. Geological Survey began 

monitoring the quality of source water and finished water of aquifers and major rivers used by 
13

some of the larger community water systems in the United States (USGS 2010). The 295 

anthropogenic organic compounds (AOCs) including terbufos and terbufos sulfone were

monitored for the Source Water-Quality Assessments (SWQAs) studies during 2002–2010 

(Carter et al., 2010). The SWQA studies are intended to complement drinking-water monitoring 

required by Federal, State, and local programs, which focus primarily on post-treatment 

compliance monitoring. A total of 221 surface water samples were analyzed for terbufos and its 

oxygen analog, terbufos sulfone. There were no detections of terbufos and terbufos sulfone in 

any samples.

Monitoring data for surface water, groundwater, and sediment from the California Department of 

Pesticide Regulation (CDPR) were searched on March 18, 2014. Terbufos was detected in only 

one sample and the concentration was 0.04 μg/L out of 2538 surface water samples. There were 

no detections of terbufos or its degradates in any of the groundwater samples

Uncertainties

Current labels of terbufos require certain setback distances or vegetative buffers between treated 

areas and the bodies of surface water. A well maintained vegetative buffer could potentially

intercept sediment laden pesticides via runoff from terbufos applied to the field. However, the 

current surface water model does not have the capability to account for prescribed setbacks or 

vegetative buffer distances, thus the PRZM/EXAMS model generated EDWCs are considered 

upper bound exposures. In addition, a lack of the full suite of environmental fate data for major 

degradates of terbufos is an uncertainty in this assessment. Selected persistence and mobility of 

terbufos sulfoxide and terbufos sulfone were estimated from registrant submitted environmental 

fate data. However, several fate parameters were assumed stable in absence of data. Selected 

physicochemical parameters were also estimated using the EPISUITE, which is a Windows®-

based suite of physical/chemical properties and environmental fate estimation programs 

developed by the EPA’s Office of Pollution Prevention Toxics and Syracuse Research 

Corporation (SRC). Additional environmental fate and physicochemical properties of major 

degradates can reduce the uncertainties in the drinking water assessment.

Conclusions

This assessment provides estimated drinking water concentrations (EDWCs) of terbufos and its 

major metabolites, terbufos sulfoxide and terbufos sulfone, in surface water and groundwater in 

support of human health risk assessment for use of terbufos in various crops. The surface water 

and groundwater modeling were conducted according to a label-recommended maximum annual 

application rate of 1.30 to 1.98 lbs a.i./A for granular applications using the coupled models 

PRZM and EXAMS for surface water and SCI-GROW for ground water. In addition, the PRZMGW model was used in determining groundwater EDWCs for terbufos and its total toxic 

residues. 

Recommended EDWCs for terbufos and total toxic residue of terbufos in surface water and 

groundwater are summarized in Table 1. The maximum acute concentration of 63.06 μg/L and 

chronic concentration of 6.13 μg/L for surface water were associated with application to 

sorghum. The maximum acute and chronic estimated concentrations of TTR in shallow 
14

groundwater using PRZM-GW model are 33.54 and 14.02 μg/L, respectively. For parent 

terbufos, the maximum acute concentration of 46.02 μg/L and chronic concentration of 0.12 

μg/L for surface water were associated with application to corn. The maximum acute and 

chronic estimated concentrations of terbufos in shallow groundwater is 0.02 μg/L, derived from 

SCIGROW model. 

Since the review of terbufos and its degradates terbufos sulfoxide and terbufos sulfone in 

drinking water sources (USEPA, 2008), limited numbers of terbufos and terbufos sulfone have 

been detected in surface water and groundwater samples collected for NAWQA and CDPR.

However, NAWQA monitoring data were not targeted specifically to terbufos use areas or 

during times of known terbufos use. Terbufos was not detected in sediment monitoring of the 

NAWQA program. For SQWA, a total of 221 surface water samples were analyzed for terbufos 

and its oxygen analog, terbufos sulfone. There were no detections of terbufos and terbufos 

sulfone in any samples.
15

References

Carter, J.M., Kingsbury, J.A., Hopple, J.A., and G.C. Delzer. 2010. Concentration data for 

anthropogenic organic compounds in groundwater, surface water, and finished water of 

selected community water systems in the United States, 2002–10: U.S. Geological 

Survey Data Series 544.

Jones, R. D., S. Abel, W. R. Effland, R. Matzner, R. Parker. 1998. An Index Reservoir for Use 

in Assessing Drinking Water Exposure. Proposed Methods for Basin-scale Estimation of 

Pesticide Concentrations in Flowing Water and Reservoirs for Tolerance Reassessment. 

Presentation to FIFRA Science Advisory Panel, June 29-30, 1998. Online at: 

http://www.epa.gov/scipoly/sap/1998/index.htm

Schwarzenbach R.P., Gschwend P.M. and D.M. Imboden. 1993. Environmental Organic 

Chemistry, Wiley-Interscience, NJ

USEPA, 2008. White Paper on Methods for Assessing Ecological Risks of Pesticides with 

Persistent, Bioaccumulative and Toxic Characteristics. Office of Chemical Safety and 

Pollution Prevention. U. S. Environmental Protection Agency.

http://www.epa.gov/scipoly/sap/meetings/2008/october/sap_pbt_whitepaper_final_Oct_7

_08d.pdf

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 Prevention, Pesticides and Toxic Substances, Office of Pesticide Programs, 

Environmental Fate and Effects Division, October 22, 2009. 

USEPA. 2010. Water Models. U.S. Environmental Protection Agency, Pesticides: Science and 

Policy. Online at: http://www.epa.gov/oppefed1/models/water/

US EPA. 2012a. Development and Use of Percent Cropped Area and Percent Turf Area 

Adjustment Factors in Drinking Water Exposure Assessments: 2012 Update 

http://www.epa.gov/oppefed1/models/water/pca_adjustment_dwa.html

US EPA. 2012b. Memorandum: Approval of PRZM-GW for use of Drinking Water Exposure 

Assessment. Environmental Fate and Effects Division, Office of Chemical Safety and 

Pollution Prevention. U. S. Environmental Protection Agency.

US EPA. 2014. Terbufos. Report of the Residues of Concern Knowledge base Subcommittee

(ROCKS) Office of Chemical Safety and Pollution Prevention. U. S. Environmental 

Protection Agency. (DP 417464)

US EPA. 2013b. Guidance on Modeling offsite deposition of pesticides via spray drift for 

Ecological and Drinking Water Assessments. Environmental Fate and Effects Division, 

Office of Chemical Safety and Pollution Prevention. U. S. Environmental Protection 

Agency. December 20, 2013. 
16

APPENDIX A

Terbufos

Hydrolysis (MRID 44862501

Temperature adjusted DT50 @ 25o C 1.5 days

Following guidance was used in calculating temperature adjusted DT50s.

http://www.epa.gov/oppefed/models/water/input_parameters_guidance.html
17

Aquatic Photolysis (MRID 41181101

Estimated DT50 is 1.77 days @ 25o C
18

Aerobic Soil Metabolism (MRID 00156853)

Aerobic Soil Metabolism (MRID 41181101)

90th %tile DT50 14.7 days
19

Aerobic Aquatic Metabolism (MRID 44672204)

Temperature adjusted 90th %tile DT50 @ 25o C 36.2 days
20

Terbufos Sulfoxide

Hydrolysis (MRID 44862501

Temperature adjusted DT50 @ 25o C 65.1 days

Aerobic Soil Metabolism (MRID 00156853)
21

Terbufos Sulfone

Hydrolysis (MRID 44862501

Temperature adjusted DT50 @ 25o C 43.8 days

Aerobic Soil Metabolism (MRID 00156853)
22

APPENDIX B

Table 6. Surface Water EDWCs for Drinking Water for Terbufos and its Total Toxic 

Residue

Scenario Peak 96 hr 21 Day 60 Day 90 Day Yearly

Average 

of 

yearly 

averages

Estimated Drinking Water concentration (μg/L)

Terbufos 

CA corn OP 2.49 1.08 0.29 0.10 0.07 0.02 0.00

IA corn STD 15.58 6.88 2.08 0.73 0.49 0.12 0.04

IL corn STD 10.42 4.48 1.23 0.47 0.32 0.08 0.04

IN corn STD 11.50 4.89 1.34 0.50 0.34 0.08 0.03

KS corn STD 15.65 7.58 1.93 0.72 0.49 0.12 0.06

MN corn STD 4.97 2.09 0.73 0.27 0.18 0.04 0.02

MS corn STD 46.02 19.49 4.25 1.71 1.14 0.28 0.12

NC CornE STD 16.14 6.91 1.59 0.59 0.40 0.10 0.02

NC CornW OP 12.37 5.60 1.42 0.53 0.35 0.09 0.05

ND Corn OP 5.84 3.52 0.91 0.33 0.22 0.05 0.02

NE corn STD 28.95 13.89 3.52 1.34 0.90 0.22 0.10

OH corn STD 11.01 5.57 1.78 0.66 0.45 0.11 0.05

PA corn STD 4.32 1.84 0.67 0.24 0.16 0.04 0.02

TX corn OP 8.63 4.02 1.14 0.42 0.29 0.07 0.03

FL sweetcorn OP 22.99 10.25 2.71 0.99 0.67 0.17 0.09

OR Sweetcorn OP 1.18 0.66 0.20 0.07 0.05 0.01 0.00

KS Sorghum STD 14.59 6.19 1.52 0.74 0.49 0.12 0.06

TX Sourghum OP 36.71 15.63 4.03 1.42 0.95 0.23 0.09

CA Sugarbeet OP 10.52 5.01 1.28 0.48 0.32 0.08 0.02

MN Sugarbeet STD 7.31 3.79 0.91 0.32 0.22 0.05 0.03

Total Toxic Residue1

MS corn STD 60.25 56.74 33.15 23.69 18.58 6.84 3.61

TX Sorghum OP 63.06 60.59 53.58 38.54 30.85 11.58 6.13

1 Total Toxic residues are based on two scenarios with the highest EDWCs for parent (i.e. MS corn STD and 

TX Sorghum OP scenarios) 
23

An Example of Non-Adjusted PCA PRZM/EXAMS modeling Output

stored as MScornDW.out

Chemical: Terbusfos

PRZM environment: MScornSTD.txt, modified Tueday, 29 May 2007 at 2:57:40

EXAMS environment: ir298.exv, modified Tueday, 26 August 2008 at 05:14:08

Metfile: w03940.dvf, modified Tueday, 26 August 2008 at 05:14:14

Water segment concentrations (ppb)

Year Peak 96 hr 21 Day 60 Day 90 Day Yearly

1961 3.44 1.48 0.33 0.16 0.12 0.03

1962 41.63 19.19 4.43 1.59 1.06 0.26

1963 4.71 1.99 0.54 0.19 0.15 0.04

1964 20.89 8.69 2.89 1.03 0.69 0.17

1965 0.94 0.40 0.09 0.03 0.02 0.01

1966 8.42 4.20 1.29 0.59 0.39 0.10

1967 7.61 3.25 1.10 0.54 0.36 0.09

1968 4.19 1.94 0.76 0.30 0.20 0.05

1969 30.06 12.66 3.95 1.40 0.93 0.23

1970 3.10 1.30 0.40 0.24 0.16 0.04

1971 16.51 8.07 2.35 0.93 0.62 0.15

1972 4.00 1.70 0.50 0.20 0.13 0.03

1973 40.85 18.26 4.09 1.51 1.01 0.25

1974 41.50 17.40 4.08 1.47 0.98 0.24

1975 6.43 2.96 1.09 0.47 0.32 0.08

1976 3.38 1.44 0.62 0.29 0.20 0.05

1977 6.59 3.89 1.08 0.39 0.26 0.07

1978 4.29 1.82 0.73 0.50 0.34 0.08

1979 88.51 48.86 11.28 4.20 2.81 0.70

1980 76.11 45.88 11.28 4.09 2.73 0.67

1981 3.30 1.59 0.44 0.18 0.12 0.03

1982 17.91 8.44 2.09 0.75 0.51 0.13

1983 51.56 21.67 4.69 1.91 1.28 0.32

1984 4.19 1.76 0.60 0.23 0.16 0.04

1985 1.20 0.59 0.15 0.06 0.04 0.01

1986 1.99 0.97 0.53 0.22 0.15 0.04

1987 1.81 0.81 0.34 0.15 0.10 0.03

1988 8.81 3.74 0.91 0.32 0.22 0.05

1989 9.57 4.17 1.17 0.45 0.31 0.08

1990 6.12 2.60 0.92 0.38 0.25 0.06

Sorted results

Prob. Peak 96 hr 21 Day 60 Day 90 Day Yearly

0.03 88.51 48.86 11.28 4.20 2.81 0.70
24

0.06 76.11 45.88 11.28 4.09 2.73 0.67

0.10 51.56 21.67 4.69 1.91 1.28 0.32

0.13 41.63 19.19 4.43 1.59 1.06 0.26

0.16 41.50 18.26 4.09 1.51 1.01 0.25

0.19 40.85 17.40 4.08 1.47 0.98 0.24

0.23 30.06 12.66 3.95 1.40 0.93 0.23

0.26 20.89 8.69 2.89 1.03 0.69 0.17

0.29 17.91 8.44 2.35 0.93 0.62 0.15

0.32 16.51 8.07 2.09 0.75 0.51 0.13

0.35 9.57 4.20 1.29 0.59 0.39 0.10

0.39 8.81 4.17 1.17 0.54 0.36 0.09

0.42 8.42 3.89 1.10 0.50 0.34 0.08

0.45 7.61 3.74 1.09 0.47 0.32 0.08

0.48 6.59 3.25 1.08 0.45 0.31 0.08

0.52 6.43 2.96 0.92 0.39 0.26 0.07

0.55 6.12 2.60 0.91 0.38 0.25 0.06

0.58 4.71 1.99 0.76 0.32 0.22 0.05

0.61 4.29 1.94 0.73 0.30 0.20 0.05

0.65 4.19 1.82 0.62 0.29 0.20 0.05

0.68 4.19 1.76 0.60 0.24 0.16 0.04

0.71 4.00 1.70 0.54 0.23 0.16 0.04

0.74 3.44 1.59 0.53 0.22 0.15 0.04

0.77 3.38 1.48 0.50 0.20 0.15 0.04

0.81 3.30 1.44 0.44 0.19 0.13 0.03

0.84 3.10 1.30 0.40 0.18 0.12 0.03

0.87 1.99 0.97 0.34 0.16 0.12 0.03

0.90 1.81 0.81 0.33 0.15 0.10 0.03

0.94 1.20 0.59 0.15 0.06 0.04 0.01

0.97 0.94 0.40 0.09 0.03 0.02 0.01

0.10 50.57 21.42 4.67 1.88 1.26 0.31

Average 

of yearly 

averages:

0.14

Inputs generated by pe5.pl - Novemeber 2006

Data used for this run:

Output File: MScornDW

Metfile: w03940.dvf

PRZM 

scenario:

MScornSTD.txt

EXAMS 

environm

ent file:

ir298.exv
25

Chemical 

Name:

Terbusfos

Descripti

on

Variable 

Name

Value Units Comments

Molecula

r weight

mwt 288.4 g/mol

Henry's 

Law 

Const.

henry 2.46E-05 atm-m^3/mol

Vapor 

Pressure

vapr 6.60E-04 torr

Solubility sol 5.4 mg/L

Kd Kd 11.11 mg/L

Koc Koc mg/L

Photolysi

s half-life

kdp 1.77 days Half-life

Aerobic 

Aquatic 

Metabolis

m

kbacw 36.2 days Halfife

Anaerobi

c Aquatic 

Metabolis

m

kbacs 0 days Halfife

Aerobic 

Soil 

Metabolis

m

asm 14.7 days Halfife

Hydrolysi

s:

pH 7 1.5 days Half-life

Method: CAM 6 integer See PRZM manual

Incorpora

tion 

Depth:

DEPI 2.56 cm

Applicati

on Rate:

TAPP 1.46 kg/ha

Applicati

on 

Efficienc

y:

APPEFF 1 fraction

Spray 

Drift

DRFT fraction of application rate applied to pond

Applicati

on Date

Date 4-Oct dd/mm or dd/mmm or dd-mm or dd-mmm

Record 

17:

FILTRA

IPSCND 1

UPTKF

Record 

18:

PLVKRT

PLDKRT

FEXTRC 0.5
26

Flag for 

Index 

Res. Run

IR Reservoir

Flag for 

runoff 

calc.

RUNOFF total none, monthly or total(average of entire run)

Time Series Files 

MS corn_Parent.csv Sorghum_TTR.csv
27

Groundwater

SciGrow version 2.3

chemical:Terbufos

time is 2/28/2014 9:53:25

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

 Application Number of Total Use Koc Soil Aerobic

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

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

 1.980 1.0 1.980 1.46E+03 8.0

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

groundwater screening cond (ppb) = 1.95E-02 

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

SciGrow version 2.3

chemical:Terbufos SO

time is 2/28/2014 9:55:31

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

 Application Number of Total Use Koc Soil Aerobic

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

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

 1.080 1.0 1.080 1.12E+02 408.0

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

groundwater screening cond (ppb) = 7.28E+00 

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

SciGrow version 2.3

chemical:Terbufos SO

time is 2/28/2014 9:56: 3

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

 Application Number of Total Use Koc Soil Aerobic

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

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

 0.440 1.0 0.440 1.41E+02 522.0

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

groundwater screening cond (ppb) = 2.76E+00 

************************************************************************
28

Example Output of PRZM-GW modeling for Terbufos Sulfoxide for Delmarva Scenario