2

use on alfalfa and pomegranate. It is important to note that the modeled total residues of concern 

of saflufenacil included saflufenacil parent, M01, M02, M07, M08, M15, M22, and Product 8, all 

of which are assumed to have similar toxicity to saflufenacil parent.

It is also noted that the results obtained from this assessment for crops other than rice are as 

follows:

(1) For surface water EDWCs: 

a. From use on pomegranate: 17.9 ppb acute and 10.9 ppb chronic (Tier I FIRST

modeling); and

b. From use on all other crops including the new use on alfalfa: 11.9 ppb acute and 7.4 ppb 

chronic (Tier I FIRST modeling, Attachment 1: DP Barcode: 413592 dated August 12, 

2013); and

(2) For ground water EDWCs (from use in all crops including new uses on alfalfa and 

pomegranate): 94.6 ppb acute and 63.1 ppb chronic (current 1.07 version of PRZM-GW

modeling using current NCCOASTAL_STD, this assessment) increasing from 69.2 ppb 

acute and 51.5 ppb chronic (old version of PRZM-GW modeling using old 

WISANDS_STD scenario, Attachment 1: DP Barcode: 413592 dated August 12, 2013).

This increase in EDWCs is due to the PRZM-GW model version/scenario changes in 

addition to the higher application rate suggested for pomegranate (a total of 0.175 lb a.i/A for 

pomegranate compared to the previous maximum rate of 0.134 lb a.i/A for all other crops 

including rice).

Results stated in (1) and (2) above indicates that the previously obtained surface water EDWCs 

(133.1 ppb for acute and 120.4 ppb for chronic) resulting from use on dry-seeded rice represent 

upper-bound estimates of total concentrations that might be found in surface water. Ground 

water EDWCs (94.6 ppb acute and 63.1 ppb chronic) represent upper-bound estimates of total 

concentrations that might be found in ground water due to the use of saflufenacil as a herbicide 

on the current/proposed use patterns. If more information or refinement is needed, please call 

Mohammed A. Ruhman at (703) 605-0644 or email: ruhman.mohammed@epa.gov

I. BACKGROUND

Five drinking water exposure assessments were conducted:

The 1st Assessment: A surface/ground drinking water exposure assessment for section 3 new 

chemical uses on agricultural and non-agricultural areas (DP Barcode: 349860 dated April 15, 

2009): This assessment reported the modeled EDWCs for the stressor (i.e., residues of concern) 

in drinking water resulting labeled uses at that time. This assessment was based on labeled uses 

on Christmas trees; citrus; conifer and hardwood plantations; corn (field, sweet, and pop); cotton, 

fallow and post-harvest applications; grape vines; legume vegetables; non-agricultural areas; 

pome fruits; small grains; sorghum; soybeans and sunflower. The maximum label rate consisting 

from a single application rate of 0.356 lb a.i/A for use on Christmas trees; citrus; conifer and 

hardwood plantations; and non-agricultural areas. Tier 1 FIRST modeling for surface water and
3

older version of PRZM-GW for ground water were used. In this respect it is noted that input 

parameters for the current PRZM-GW were changed following this assessment.

The 2

nd Assessment: A surface drinking water exposure assessment for section 3 new chemical 

use on rice “Pre-plant burn-down” (DP Barcode: 384975 dated March, 03, 2011). This 

assessment was based on pre-plant/pre-flood use on rice at a rate of 0.0445 lb a.i/A with specific 

use restrictions. Tier 1 Rice Model (ver. 2) was used in modeling which allows aerobic soil 

degradation of the pre-flood applied chemical in addition to partitioning and degradation in the 

rice paddy for the left over chemical residues.

The 3rd Assessment: A Tier II Drinking Water Exposure Assessment for Saflufenacil (Section 3 

New Chemical Uses as a harvest aid on dry edible beans, dry peas, soybean, oilseeds “sunflower 

subgroup 20B”, oilseeds “cotton subgroup 20C”, and oilseeds canola “subgroup 20A” (DP 

Barcodes: 380641 and 381294 dated April 1, 2011).

The 4

th Assessment (Attached): A surface drinking water exposure assessment for the proposed

new use pattern on rice (EPA Reg. No. 7969-278) and the new master label for saflufenacil 

formulations (EPA Reg. No. 7969-323 (DP Barcode: 413592 dated August 12, 2013 ). This 

assessment was based on these two labels which included the following changes:

(1) A maximum application rate for all crops, including rice, is 0.134 lb a.i/A instead of the 

previous maximum rate of 0.356 lb a.i/A ; 

(2) The aerobic soil half-life and partitioning coefficient (Koc) were entered as per guidance to 

the current version of PRZM-GW (refer to referenced document for these changes)

(3) New crop-specific use restrictions/limitations for rice (refer to referenced document for these 

changes).

The 5

th Assessment: Drinking Water Exposure Assessment for Saflufenacil: New Chemical Use 

pattern on Barley, Wheat and Triticale; Perennial forage grasses (grown in pastures and range 

land or in field grown for forage, silage and hay; and on Olive Trees (DP Barcode 414007, 

414474 & 417504 dated May 16, 2014).
4

II. CURRENT ASSESSMENT

This current assessment is for the new use patterns on alfalfa and pomegranate trees. Important 

application parameters for the new use labels are summarized in Table 1.

Table 1 New use pattern for saflufenacil herbicide on alfalfa, mixed Stands of alfalfa and coolseason forage grasses, and pomegranate trees for broadleaf weed control (MSR= Maximum single 

application rate in lbs a.i/A; MNOA= Maximum number of applications; MS= Maximum seasonal application rate in lbs a.i/A;

and MAI= Minimum application intervals in days; Note: Application procedure is not stated in the label but assumed to be 

ground and aerial)

Application Types/Purpose MSR MNOA MS MAI Notes

Alfalfa: Broad leaf weed control in season 

(crop planted in Fall or Spring) after 1st cutting: 

One application OR

Alfalfa: Broad leaf weed control in dormantseason (Fall/Winter): One OR Two sequential 

application

0.0223 One 0.0223

N/A

Sharpen®; PA Reg. No. 7969-

278 label for in season 

(between cutting)

0.0223 One 0.0223 Sharpen®; PA Reg. No. 7969-

0.0223 Two 0.0446 14 278 label for dormant season 

Mixed alfalfa and cool season grasses: inseason and dormant-season applications

0.0223 One 0.0223

Refer to 

next 

column

Sharpen®: One application in 

the spring plus two 

applications in fall or winter 

14 days apart

0.0223 One 0.0223

0.0223 One 0.0223

Pomegranate trees: Post-emergence directed 

Broad leaf weed control: 

(a) Tree bloom to harvest 

 OR

(b) Post-harvest to tree bloom (dormant 

Trees) Plus

Tree bloom to harvest 

0.0438 Three 0.1313 21

Treevix®: From one to four 

applications. Four applications 

can only be added if one or 

two of the applications were 

applied during the tree 

dormant period.

0.0438 Two

0.175

21

0.0438 Two 21

1 N/A= Not applicable (only one application)

II.1. Surface Water Modeling

Tier 1 FIRST modeling is to be used to calculate surface EDWCs because previous assessments 

were based on this type of modeling. Data for the new use patterns (Table 1) indicate that the 

maximum total application rates needed for Tier 1 FIRST modeling are as follows:

(1) for alfalfa: 0.0669 lbs a.i/A (1+2= 3 applications at 0.0223 lbs a.i/A each at 14-day 

intervals); and

(2) For pomegranate: 0.175 lbs a.i/A (4 applications at 0.0438 lbs a.i/A each: two during the 

growing period and two during the dormant period at 21-day intervals).

In case of the new use on alfalfa, the total maximum application rate is 0.066 lbs a.i/A which is 

almost half of the rate used in previous Tier 1 FIRST surface water modeling. Therefore, 

surface water EDWCs for the new alfalfa use will be much lower than that obtained in the 

attached EFED assessment which was based on an application rate of 0.132 lbs (Attachment 1, 
5

DP Barcode: 413592 dated August 12, 2013). Therefore, surface water EDWCs reported earlier 

can cover the use on alfalfa and mixed stands of alfalfa and cool-season forage grasses.

In case of the new use on pomegranate, the total maximum application rate is 0.175 lbs a.i/A

which is higher than before (0.175 lbs a.i/A). Therefore, a new Tier 1 FIRST modeling is 

needed for the use on pomegranate. A Tier 1 FIRST model run was executed using the high 

pomegranate application rate with the same parameters in the previous DW assessment (refer to

Table 5 in Attachment 1). The application rate, used in this modeling run, was 0.044 lbs a.i/A in 

four applications for a total of 0.175 lbs a.i/A/year with 21-day application intervals). The 

results, of this modeling run, suggest that use on pomegranate is expected to cause higher surface 

water exposure EDWCs compared to all other crops, except rice. These results were: 17.9 ppb 

acute EDWC and 10.9 ppb chronic EDWC. 

II.2. Ground Water Modeling

The new maximum use rate on pomegranate (a total of 0.175 lb a.i/A) is higher than the previous 

maximum rate of 0.134 lb a.i/A for all other crops. Additionally, there were changes in PRZMGW model version/scenario. Therefore, new modeling runs were executed for:

(1) The previous maximum rate 0.134 lb a.i/A to arrive at ground water EDWCs for all other 

crops (including the rice and new use on alfalfa); and

(2) The new maximum rate 0.175 lb a.i/A on pomegranate to arrive at ground water EDWCs for 

this crop; and

Two batch runs were executed for all scenarios: one for the use on all crops, except 

pomegranate and the others for pomegranate. The results obtained for the first run are 

summarized in Table 2 while those for the second run are summarized in Table 3.

Table 2 Modeled EDWCs resulting from use on all crops, except pomegranate

Scenario

Highest Daily

Value (ppb)

Post Breakthrough 

Average (ppb)

Overall Simulation 

Average (ppb)

Average Breakthrough 

Time in Days (Years)

NCCOASTAL_STD.SCN 73.1 48.5 38.6 2,175 (6.0)

DELMARVA_STD.SCN 54.4 38.4 32.1 1,358 (3.7)

WISANDS_STD.SCN 49.2 42.5 32.6 2 693 (1.9)

FLCITRUS_STD.SCN 45.2 35.2 32.4 957 (2.6)

FLPOTATO_STD.SCN 37.4 27 24.4 1,064 (2.9)

GACOASTAL_STD.SCN 20.1 15.8 13.7 1,611 (4.4)
6

Table 3 Modeled EDWCs resulting from use on pomegranate

Scenario

Highest Daily

Value (ppb)

Post Breakthrough 

Average (ppb)

Overall Simulation 

Average (ppb)

Average Breakthrough 

Time in Days (Years)

NCCOASTAL_STD.SCN 94.6 63.1 50 2,175 (6.0)

DELMARVA_STD.SCN 69 49.8 41.5 1,358 (3.7)

WISANDS_STD.SCN 63 53.9 41 2,693 (7.4)

FLCITRUS_STD.SCN 59.5 45.7 41.8 957 (2.6)

FLPOTATO_STD.SCN 49.5 35.2 31.6 1,064 (2.9)

GACOASTAL_STD.SCN 25.6 20.5 17.8 1,611 (4.4)

Data in Tables 2 and 3 indicate that the maximum ground water EDWCs (from use in all crops 

including new uses on alfalfa and pomegranate) are 94.6 ppb acute and 63.1 ppb chronic 

(Table 3) and it also indicate that ground water EDWCs (from use in all crops including new 

uses on alfalfa and excluding the new use on pomegranate) had changed from the previous DW 

assessment to 73.1 ppb acute and 48.5 ppb chronic (due to the use of the current 1.07 version 

of PRZM-GW/scenarios).

Hereunder, is the results for the maximum EDWCs from running a single run using pomegranate 

application rate and the Eastern Coastal Plain - Norfolk Loamy Sand-Cotton scenario:

Estimated groundwater concentrations and breakthrough times for saflufenacil are presented in 

Table 4 for the NC Eastern Coastal Plain - Norfolk loamy sand - Cotton groundwater scenario. 

A graphical presentation of the daily concentrations in the aquifer is presented in Figure 1. 

These values were generated with the PRZM-GW (Version 1.07). Critical input values for the 

model are summarized in Tables 5 and 6.

Table 4. Groundwater Results for saflufenacil and the NC Eastern Coastal Plain - Norfolk loamy 

sand - Cotton Scenario.

Peak Concentration (ppb) 94.6

Post-Breakthrough Mean 

Concentration (ppb)

 63.1

Entire Simulation Mean 

Concentration (ppb)

 50

Average Breakthrough 

Time (days)

2174.558

Throughputs 5.040105
7

Table 5. Chemical Properties for Groundwater Modeling of saflufenacil

Koc (ml/g) 24

Surface Soil Half Life (days) 6833

Hydrolysis Half Life (days) 0

Diffusion Coefficent Air 

(cm2/day) 0.0

Henry's Constant 0.0

Enthalpy (kcal/mol) 0.0

Table 6. Pesticide application scheme used for saflufenacil. This application scheme was 

applied every year of the simulation

Application Date

 (Month/Day) Application Method

Application Rate

(kg/ha)

04/15

Ground application with mass 

distribution increasing 

proportionally with depth to 4 cm 0.0491

05/06

Ground application with mass 

distribution increasing 

proportionally with depth to 4 cm 0.0491

11/15

Ground application with mass 

distribution increasing 

proportionally with depth to 4 cm 0.0491

12/06

Ground application with mass 

distribution increasing 

proportionally with depth to 4 cm 0.0491
8

Figure 1. Aquifer Breakthrough Curve for saflufenacil and the NC Eastern Coastal Plain -

Norfolk Loamy Sand - Cotton Scenario

 [-----------------------------30 Years-------------------------------]
9

Attachment 1

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON, D.C. 20460

OFFICE OF

PREVENTION, PESTICIDES, AND

TOXIC SUBSTANCES

PC Code: 118203

DP Barcode: 413592

August 12, 2013

MEMORANDUM

SUBJECT: Revised Drinking Water Exposure Assessment for Saflufenacil: New Chemical 

Use pattern on Rice and Change in Label Application Rates for Other Crops.

TO: Bethany Benbow, Risk Manager Reviewer

Kathryn Montague, M.S., Risk Manager, Rm 23

Daniel Kenny, Branch Chief

Herbicide Branch

Registration Division (RD) (7505P)

AND: George F. Kramer, Ph.D., Chemist

Charles Smith, Acting Branch Chief

Risk Assessment Branch I

Health Effects Division (7509P)

FROM: Mohammed Ruhman, Ph.D., Senior Agronomist

Environmental Risk Branch 5

Environmental Fate and Effects Division (7507P)

THROUGH: Mah Shamim, Ph.D., Branch Chief

Environmental Risk Branch 5

Environmental Fate and Effects Division (7507P)

I. EXECUTIVE SUMMARY

This is a revision for previously calculated tier I screening level EDWCs for saflufenacil total 

toxic residues. The revision is necessary due to the proposed new use pattern on rice (EPA Reg. 

No. 7969-278) and the changes in label application rates for some uses (EPA Reg. No. 7969-

323). These revised EDWCs may be used in support of human health risk assessment. 

The revised highest maximum EDWCs are 133.1 ppb for acute and 120.4 ppb for chronic

resulting from use on dry-seeded rice (Tier I Rice modeling).
10

This conclusion is based on the following EDWC results:

(4) For surface water: 133.1 ppb acute and 120.4 ppb chronic resulting from use on dryseeded rice (Tier I Rice modeling) and 11.9 ppb acute and 7.4 ppb chronic from 

application to crops other than rice (Tier I FIRST modeling); and

(5) For ground water: 69.2 ppb acute and 51.5 ppb chronic (PRZM-GW modeling using WI 

SANDS scenario). 

It is noted that the maximum of the previously calculated EDWCs was obtained from modeling 

using an older version of the current PRZM-GW with input parameters that are different from 

the current PRZM-GW. Table 1 contains a summary of previous results noting that reasons for 

the revision are detailed in section II (Background), below. 

Table 1 Previously calculated 1-in-10-year EDWCs for the proposed saflufenacil uses contained 

in EDWC Memo dated April, 15, 2009 (DP Barcode: 349860)

Source (Tier: Model) Peak Exposure (μg/L) Annual Mean Exposure (μg/L)

Surface water (Tier I: FIRST) 37.3 23.8

Ground water (PRZM GW) 180 173

The above stated values represent upper-bound estimates of total concentrations that might be 

found in surface water and ground water due to the use of saflufenacil as a herbicide on the 

current/proposed use patterns. If more information or refinement is needed, please call 

Mohammed A. Ruhman at (703) 605-0644 or email: ruhman.mohammed@epa.gov

II. BACKGROUND

Two drinking water exposure assessments were conducted:

(1) A surface/ground drinking water exposure assessment for section 3 new chemical uses on 

agricultural and non-agricultural areas (Appendix 1): This assessment reported the modeled 

EDWCs for the stressor (i.e., residues of concern) in drinking water resulting labeled uses at 

that time. This assessment was based on the following:

a. Labeled uses: Christmas trees, Citrus, Conifer and hardwood plantations, Corn (field, sweet, 

and pop), Cotton, Fallow and post-harvest, Grape vines, Legume vegetables, Nonagricultural areas, Pome fruits, Small grains, Sorghum, Soybeans and Sunflower;

b. Five labels: BAS 800 04H (FiRoCrop: Saflufenacil 29.74%); BAS 804 00H (LegVeg: 

Saflufenacil 17.80% + Imazethapyr 50.20%); BAS 781 02H (Saflufenacil 6.24% + 

Dimethenamid-P 55.04%); BAS 800 01H (TNV: Saflufenacil 70%); and BAS 800 02H 

(SharpenTM: Saflufenacil 12.27%);

c. A maximum label rate consisting from a single application rate of 0.356 lb a.i/A for use on 

Christmas tree, conifer, and hardwood plantations and non-agricultural areas (SharpenTM 

label);

d. The maximum rate of 0.356 lb a.i/A used in all surface/ground water modeling; and

e. The stressor included parent compound and it’s degradates M800H01, M800H02, M800H07, 

M800H08, M800H15, M800H22; and
11

f. Tier I FIRST modeling for surface water and older version of PRZM-GW for ground 

water. In this respect it is noted that input parameters for the current PRZM-GW are 

different.

Reported EDWCs for this assessment is summarized in Table 2. 

Table 2 Previously calculated 1-in-10-year EDWCs for the proposed saflufenacil uses reported 

in EDWC Memo dated April, 15, 2009 (DP Barcode: 349860, Appendix 1)

Source (Tier: Model) Peak Exposure (μg/L) Annual Mean Exposure (μg/L)

Surface water (Tier I: FIRST) 37.3 23.8

Ground water (Tier II: PRZM GW) 180 173

(2) A surface drinking water exposure assessment for section 3 new chemical use on rice “Preplant burn-down” (Appendix 2) This assessment was based on the following:

a. Labeled use: pre-plant/pre-flood use on rice only;

b. A suggested label amendment (Sharpen EPA Reg. No. 7969-278);

c. A maximum label rate consisting from single maximum application rate of 0.0445 lb a.i/A;

d. Crop-specific use restrictions/limitations that include (1) DO NOT apply after rice planting; 

(2) DO NOT irrigate (flush) between application and rice seeding; (3) DO NOT initiate 

permanent flood to rice fields until 30 days or more after planting; and (4) DO NOT apply 

within 45 days of permanent flooding in water-seeded rice paddies;

e. The stressor included parent compound and it’s degradates M800H01, M800H02, M800H07, 

M800H08, M800H15, M800H22; and

f. Tier I Rice Model (ver. 2) modeling which allows aerobic soil degradation of the pre-flood 

applied chemical in addition to partitioning and degradation in the rice paddy for the left over 

chemical residues.

Reported EDWCs for this assessment is summarized in Table 3.

Table 3 Surface water EDWCs for the proposed new use on rice as reported in EDWC Memo 

dated March, 03, 2011 (DP Barcode: 384975, Appendix 2)

Holding Time Peak (μg/L) Annual Mean (μg/L)

 1-day 45.57 41.22

 3-day 45.50 41.19

21-day 45.04 40.78

This current revised assessment is based on the proposed new use pattern on rice (EPA Reg. No. 

7969-278)1and the master label for saflufenacil formulations (EPA Reg. No. 7969-323)

2

. Based 

on these two labels, this assessment took in consideration the following:

 

1 EPA Reg. No. 7969-278 (Sharpen® Powered by KIXOR® Herbicide): A supplemental Label 

For broadleaf weed control in rice production.

2 EPA Reg. No. 7969-323 (KIXOR® Herbicide): A broad leaf herbicide for use in field and row 

agricultural crops, bearing and nonbearing tree and vine crops, and in non-crop.
12

(6) The maximum application rate for all crops, including rice, is 0.134 lb a.i/A instead of 0.356 

lb a.i/A;

(7) Changes in values for the aerobic soil half-life and partitioning coefficient (Koc) which is 

entered in the current version of PRZM-GW (refer to Table 4). 

Table 4 A summary of the difference in input parameters between older version of PRZM-GW

(strikethrough parameters) and the current PRZM-GW version (parameters in normal text). 

Parameter (units) Value Source and Calculation Procedure

Soil Metabolism 

Half-life 

(days) 6,833

90th percentile half-life using four soils with half-life values of 987; 

1,433; 1,822; and 9,613 (MRID 47445901) 

Koc 

(mL/g O.C) 

5.9 (the 

mean of the 

lowest 

reported 

KOC values)

24

Overall average value calculated for all of the constituents of the 

stressor (MRIDs 47127829 and 47127830) as shown in the 

following Table:

Stressor Constituents Soil 1 Soil 2 Soil 3 Soil 4 Soil 5 Soil 6

Parent 9.3 19 22 23 25 55

Metabolite M1 4.8 7 7.1 8.6 13 27

Metabolite M2 0 6.7 10 12 13 41

Metabolite M7 3.3 32 59 67 97 111

Metabolite M8 4.8 5.6 7.1 12 18 20

Metabolite M22 0 5.2 6.4 13 23 25

Overall Average 24

Overall Median 13

(8) The change in crop-specific use restrictions/limitations for rice which includes:

a. The pesticide can be applied to drilled, dry and water-seeded rice;

b. The 1st application: A maximum of 0.089 lb a.i/A (0.1 kg/ha) is applied pre-plant 

through pre-emergence “within 3 days of rice planting”;

c. The 2nd application: A maximum of 0.045 lb a.i/A (0.05 kg/ha) is applied as a 

post-emergence weed control that may be applied either before or after flooding 

only when rice has reached the 2-leaf stage;

d. The 1st and 2nd application may be applied sequentially but the two applications 

should be separated by at least 14 days;

e. The maximum seasonal rate should not exceed 0.134 lb a.i/A (0.150 kg/ha); and

f. Water may be released from treated fields after 7-days following the “last?” 

pesticide application noting that the life cycle of rice is assumed to be a minimum 

of 110 day. Thus, water is released is released after 110+7= 117 days which is 72 

days after flooding in dry-seeded rice and 117 days after flooding in water seeded 

rice. 

In rice, the length for the recognized growth stages is highly dependent on variety, type of rice 

culture (i.e., dry or water seeded), temperature and other environmental conditions3. For 

example, germination occurs within two days when temperatures are between 70° to 97°F; below 

 

3 Rice Growth and Development by Karen Moldenhauer and Nathan Slaton: URL 

http:/ / baegrisk.ddns.uark.edu/ test/ Books/ PDF/ chapter1sl3.pdf
13

or above this temperature, germination requires more time. Therefore, the following variable 

time periods were reported for rice growth stages in the same reference:

g. From planting, through germination/seedling emergence, to the 2nd leaf stage: 8-

30 days;

h. From planting, through the 2nd leaf stage to the start of internode elongation (the 

green ring stage): 44 to 112 days; and

i. Life cycle of rice cultivars in Arkansas ranges from 110 to 150 days from 

germination to maturity.

In the rice label, the 2nd application (post-emergence weed control) may be applied before or 

after flooding when rice has reached the second leaf stage up to internode elongation. This means 

that the window for the second application is from 45 to 112 days after planting. However, the 

label states that the minimum time intervals between the 1st and second application is 14 days 

and that was the interval used in modeling.

Based on rice culture, growth stages and label specific use restrictions/limitations, the following 

may be stated:

(1) In dry seeded rice: the 1st application within 3 days of planting and the 2nd application 

within 14 days of the 1st application with flooding occurring within 30 to 45 days of 

planting (normal agronomic practice in dry seeded rice).

(2) In water seeded rice: the 1st application within 3 days of planting and the 2nd application 

within 14 days of the 1st application with flooding occurring at planting (water-seeded).

Saflufenacil and its residues (the stressor) is expected to be highly persistent in soils before 

flooding (t 1⁄2 ranges 987 to 9,613days; 6,833 days is used in modeling “90th % value”) but it is expected 

to be less persistent in the rice paddy after flooding (aerobic aquatic t 1⁄2= 415.7 days; 1,247 days is 

used in modeling “3x the single value”).

Based the fate properties of the stressor and all of the label and rice agronomic practices, the 

following may be considered as the most conservative exposure scenarios:

(1) For dry seeded rice: the 1st application at planting, the second application 14 days after 

planting and flooding at 45 days after planting; and

(2) For water seeded rice: the 1st application at planting, the second application 14 days after 

planting and flooding at planting.
14

III. DRINKING WATER EXPOSURE MODLING

1. Surface Water Modeling

a. All Crops Except Rice

The FQPA Index Reservoir Screening Tool (FIRST v1.1.1, Mar. 25, 2008; USEPA, 2008) was 

used in modeling EDWCs for drinking water from surface water sources. FIRST is a Tier I 

screening model that simulates the upper-end exposure of the standard water body, the Index 

Reservoir, to pesticide residues in runoff and spray drift from an application within the standard 

watershed. Peak and annual mean EDWCs are generated to estimate acute and chronic exposure. 

The Index Reservoir covers 5.2 hectares (ha) with an average depth of 2.74 meters (m) in a 

standard watershed of 172.8 ha. A more detailed description of the index reservoir watershed 

can be found in Jones et al., 1998. The FIRST model and user’s manual are available from the 

EPA Water Models web-page (USEPA, 2009). Input parameters, for modeling the stressor, are 

summarized in Table 5 and modeling results are summarized in Table 6. 

Table 5 FIRST input parameters for the total residues of saflufenacil based on the maximum 

seasonal application among all use patterns

Input Parameter Value Comments Source

Application rate (lbs a.i./A) 0.134

Maximum proposed single application 

rate. Proposed label.

Number of applications per 

year 1

Maximum proposed number of 

applications per year at the maximum 

proposed single application rate. Proposed label.

Percent cropped area 100% Default for non-agricultural uses. Effland et al., 1999

Organic Carbon Partition 

Coefficient (KOC) (L/kgOC) 24

Refer to Table 4, above for calculation 

procedure

MRID 47127829

MRID 47127830

Aerobic soil metabolism 

half-life (days) 6,833

Represents the upper 90% confidence 

bound on the mean of four half-lives 

for the residues of concern. MRID 47445901

Wetted in? No Not required. Proposed label.

Method of application Aerial

Proposed application method of 

highest potential spray drift. Proposed label.

Solubility in water (ppm) 2100 Solubility in pH 7 water. MRID 47127819

Aerobic aquatic metabolism

half-life (days) 1,247

Represents 3x the single available 

half-life for the residues of concern. MRID 47127827

Aqueous photolysis half-life 

(days) 90.9

Represents the maximum 

environmental photo transformation 

half-life for the residues of concern. MRID 47699901

Table 6 FIRST modeling output (ppb).

Peak EDWC (Acute) Annual Average (Chronic

11.873 7.346
15

(b) Rice

Tier I v2.0 rice model was used to generate estimated drinking water concentrations (EDWC) of 

saflufenacil residues of concern that may occur in surface water used as drinking water. The tier

I v2.0 rice model is a modification of the same model (v1.0). The two versions of the model rely 

on an equilibrium partitioning concept to provide EDWCs resulting from application of 

pesticides to rice paddies. When a pesticide is applied to a rice paddy, the model assumes that it 

will instantaneously partition between a water phase and a sediment phase. Although v 1.0 of 

the model does not account for pesticide degradation, v2.0 accounts for degradation in the rice 

paddy using the aerobic aquatic half-life. It is noted however, that both versions of the model 

produces the same peak concentration in water on the day of treatment and may produce similar 

concentrations thereafter, depending on the aerobic aquatic degradation rate of the compound. 

Peak concentrations of rapidly degraded compounds in released tail-water following a holding 

period will be lower than those produced by v1.0 of the model, as will any chronic 

concentrations averaged over time.

Both versions of the model, do not account for mass transfer between the aqueous phase and the 

sediment, volatilization, dilution, drift or other dissipation processes. It should be stated that v01 

of the model was calibrated to generate estimates that are consistent with or conservative for 

dissolved concentrations measured within rice paddies and in discharged paddy water. The 

model was not evaluated or calibrated for concentrations measured in sediment and does not 

account for residues partitions to suspended sediment. Currently, v02 of the model is considered 

to be a provisional model. 

Due to a range of environmental and other factors, v01 of the rice model is expected to generate 

conservative EDWCs that exceed peak measured concentrations of pesticides in water bodies 

well downstream of rice paddies by less than one order of magnitude to multiple orders of 

magnitude (USEPA, 2007b). Rice paddies are likely to be planted and harvested on or near the 

same date suggesting that when paddy waters are released, an influx of saflufenacil residues 

from multiple locations to the waterways may be expected. Also, EDWCs are expected to be 

more conservative downstream of rice paddies in California and inland areas of the Southeastern 

U.S. where dilution is expected to occur. Tidal influences in surface waters of the coastal areas 

of the Southeastern United States may effectively reduce dilution of saflufenacil residues from 

discharged paddy water, lengthening the period of chronic saflufenacil exposure in waterways 

influenced by rice paddies. In contrast, v02 of the rice model is expected to generate after peak 

concentrations that are lower than those generated by v01 for pesticides that are expected to 

degrade quickly in aquatic systems (short aerobic aquatic half-lives). However, these 

concentrations are expected to be less conservative than those produced by v01 model but 

possibly more representative to actual conditions in the field where degradation is expected to 

take place. Tier I v1.0 rice model and associated users’ manuals may be downloaded from the 

U.S. Environmental Protection Agency (EPA) water models web-page (USEPA, 2009). 

Exposure was specifically assessed for the proposed use of saflufenacil on rice based on the 

following two conservative exposure scenarios (refer to II (2) BACKGROUND section above 

for details used in identifying these two scenarios):
16

(1) The first scenario for dry seeded rice: the 1st application at planting, the second 

application 14 days after planting and flooding at 45 days after planting; and

(2) The second scenario for water seeded rice: the 1st application at planting, the second 

application 14 days after planting and flooding at planting.

For dry seeded rice, Modeling was done in two stages: 

(1) Stage 1: Field before flooding: to estimate the amount of pesticide left in the soil after the 

45 day in which the rice paddy is not flooded. The estimate was based on the aerobic soil 

degradation half-life of 6,833days for saflufenacil residues. Stage 1 estimate that 99.7% of 

the applied pesticide was left, in the un-flooded soil, after 45 days and two applications: one 

at 14 days from planting and the other after 45 days from planting; and

(2) Stage 2: Field after flooding (Rice paddy): to estimate daily concentrations using the tier I 

v2.0 rice model which takes into account degradation in the rice paddy. This degradation was 

based on 3 times the one aerobic aquatic half-life of saflufenacil residues which was 

determined previously to equal 1,247 days (from 3 x 415.7). Other parameters used in 

modeling included: maximum application rates (1st = 0.089 lb a.i/A and 2nd = 0.045 lb a.i/A, 

the mean for partitioning coefficient (Koc)= 24 L/kg, water column depth of 0.1 meter (m), 

sediment depth of 0.01 m, sediment organic fraction of 0.01, sediment bulk density of 1300 

kg/m3, and sediment porosity of 0.509 (50.9%).

Modeling results are summarized in Table 7, below.

Table 7 Tier I v2.0 Rice model output (EDWCs in ppb) for tail 

water following a 7 days holding period

Scenario Peak Annual Mean

The first scenario: dry seeded rice 133.1 120.4

The second scenario for water seeded rice 129.8 117.5

Data in Table 7 show that the maximum EDWC is 138.0 ppb for peak and 125.4 ppb for the 

annual mean.
17

Figure 1 Change in EECs with time following two applications in dry and water-seeded rice (in 

dry seeded rice flooding is after 45 days of planting while it is at planting in water seeded rice) Aqueous Concentration (μg/L)

Days After Planting 

EECs vs Time following Two Applications (Dry-Seeded Rice)

Aqueous Concentration (μg/L)

Days After Planting 

EECs vs Time following Two Applications (Water-seeded rice)
18

2. Ground Water Modeling

As per current EFED policy, ground water exposure was modeled using Tier I PRAZM-GW

and SCI-GROW models. Parameters used in modeling runs were as follows:

(1) Scenarios and application rates and associated parameters:

PRZM-GW calls for using the six standard scenarios representing vulnerable ground waters, 

namely: (1) DELMARVA_STD GW, (2) FLCITRUS_STD GW, (3) FLPOTATO_STD, (4) 

GACOASTAL_STD GW, (5) NCCOASTAL_STD, and (6) WISANDS_STD GW. Table 8

contains a summary of the input parameters used in modeling.

Table 8 PRZM-GW and SCI-GROW input parameters used for modeling saflufenacil

Parameter (units)

Input Values

PRZM-GW SCI-GROW Data Source/Comments

Application Rate 0.150 Kg a.i/Ha 0.134 lb a.i/A

Suggested amendment to label: maximum 

rate is changed from 0.356 to 0.134 lb a.i/A 

Hydrolysis Half-life Stable N/A MRID 47127823

Soil Metabolism 

Half-life (days) 6,833 1,628

90th percentile half-life for PRZM-GW and 

median half-live for SCI-GROW using four 

soils with half-life values of 987; 1,433; 

1,822; and 9,613 (MRID 47445901) 

CAM (chemical 

Application Model) 2 N/A As per guidance1

Koc (mL/g O.C) 24 13

Average for PRZM-GW and Median for 

SCI-GROW (MRIDs 47127829 and 

47127830 refer to Table 4, above)

1 URL: http://www.epa.gov/oppefed1/models/water/przm_gw/wqtt_przm_gw_guidance.htm

Modeling results are summarized in Figure 2 for PRZM-GW and Table 9 for SCI-GROW and

PRZM-GW.
19

Figure 2 PRZM-GW estimated drinking water concentrations (EDWCs μg/L) in ground water 

resulting from the stressor (total toxic residues of saflufenacil)

1 0

1 5

2 0

2 5

3 0

3 5

4 0

4 5

5 0

5 5

6 0

6 5

7 0

7 5

7-Dec 6-Jan 5-Feb 7-Mar 6-Apr 6-May 5-Jun 5-Jul 4-Aug 3-Sep 3-Oct 2-Nov 2-Dec

EDWC (ppb)

Application Date

Highest Daily EDWCs Values

WI SANDS NC COASTAL DELMARVA

FL CITRUS FL POTATO GA COASTAL

1 0

1 5

2 0

2 5

3 0

3 5

4 0

4 5

5 0

5 5

7-Dec 6-Jan 5-Feb 7-Mar 6-Apr 6-May 5-Jun 5-Jul 4-Aug 3-Sep 3-Oct 2-Nov 2-Dec

EDWC (ppb)

Application Date

Post Breakthrough EDWCs Average

WI SANDS NC COASTAL DELMARVA

FL CITRUS FL POTATO GA COASTAL
20

Table 9 PRZM-GW and SCI-GROW estimated drinking water concentrations (EDWCs μg/L) 

in ground water resulting from the stressor (total toxic residues of saflufenacil)

PRZM-GW SCI-GROW

Application 

Dates Scenario*

Highest 

Daily

Value

Post 

Breakthrough 

Average

Average Simulation 

Breakthrough Time 

(Days/Years)

Peak and 

Average

Mar 15 DELMARVA_STD 42.4 27.5 413.5849 (1.1)

160 ppb

SCI-GROW 

was 

developed 

using KOC

values 

ranging from 

32-180 mL 

gOC

-1

and 

half-lives 

from 13-1000 

days. 

Extrapolation 

beyond these 

values will 

increase the 

uncertainty 

of the ground 

water 

concentration

. Therefore, 

the results for 

this modeling 

exercise are 

not used in 

this risk 

assessment

Mar 15 FLCITRUS_STD 38.8 27.1 432.3471 (1.2)

Mar 15 FLPOTATO_STD 28.4 19.2 749.658 (2.1)

Mar 15 GACOASTAL_STD 19.7 15.2 1380.526 (3.8)

Mar 15 NCCOASTAL_STD 39.6 30.9 1247.013 (3.4)

Mar 15 WISANDS_STD 67.3 48.7 1248.434 (3.4)

Apr 15 DELMARVA_STD 43 27.6 413.5849 (1.1)

Apr 15 FLCITRUS_STD 39.3 27.2 432.3471 (1.2)

Apr 15 FLPOTATO_STD 28.1 19.2 749.658 (2.1)

Apr 15 GACOASTAL_STD 19.6 15.3 1380.526 (3.8)

Apr 15 NCCOASTAL_STD 39.6 30.8 1247.013 (3.4)

Apr 15 WISANDS_STD 69.2 51.5 1248.434 (3.4)

Jan 15 DELMARVA_STD 44.2 27.4 413.5849 (1.1)

Jan 15 FLCITRUS_STD 40.9 27 432.3471 (1.2)

Jan 15 FLPOTATO_STD 27.8 19.2 749.658 (2.1)

Jan 15 GACOASTAL_STD 19.8 15.3 1380.526 (3.8)

Jan 15 NCCOASTAL_STD 38.7 31.2 1247.013 (3.4)

Jan 15 WISANDS_STD 62.2 46.2 1248.434 (3.4)

Jul 15 DELMARVA_STD 43.6 27.1 413.5849 (1.1)

Jul 15 FLCITRUS_STD 43.8 26.9 432.3471 (1.2)

Jul 15 FLPOTATO_STD 26.4 19.2 749.658 (2.1)

Jul 15 GACOASTAL_STD 20.2 15.2 1380.526 (3.8)

Jul 15 NCCOASTAL_STD 40.3 30.8 1247.013 (3.4)

Jul 15 WISANDS_STD 68 51 1248.434 (3.4)

Jun 01 DELMARVA_STD 42.1 27.5 413.5849 (1.1)

Jun 01 FLCITRUS_STD 40.5 26.1 432.3471 (1.2)

Jun 01 FLPOTATO_STD 27.4 19.2 749.658 (2.1)

Jun 01 GACOASTAL_STD 19.8 15.3 1380.526 (3.8)

Jun 01 NCCOASTAL_STD 39.7 30.7 1247.013 (3.4)

Jun 01 WISANDS_STD 68.2 51.2 1248.434 (3.4)

May 15 DELMARVA_STD 43 27.5 413.5849 (1.1)

May 15 FLCITRUS_STD 40.1 27.4 432.3471 (1.2)

May 15 FLPOTATO_STD 28.2 19.2 749.658 (2.1)

May 15 GACOASTAL_STD 19.8 15.3 1380.526 (3.8)

May 15 NCCOASTAL_STD 39.9 30.7 1247.013 (3.4)

May 15 WISANDS_STD 69 51.2 1248.434 (3.4)

Oct 15 DELMARVA_STD 40.4 26.6 413.5849 (1.1)

Oct 15 FLCITRUS_STD 42 26.2 432.3471 (1.2)

Oct 15 FLPOTATO_STD 29 19 749.658 (2.1)
21

PRZM-GW SCI-GROW

Application 

Dates Scenario*

Highest 

Daily

Value

Post 

Breakthrough 

Average

Average Simulation 

Breakthrough Time 

(Days/Years)

Peak and 

Average

Oct 15 GACOASTAL_STD 19.8 15.2 1380.526 (3.8)

Oct 15 NCCOASTAL_STD 39.9 30.6 1247.013 (3.4)

Oct 15 WISANDS_STD 64.4 50.3 1248.434 (3.4)

Sep 15 DELMARVA_STD 40.3 26.8 413.5849 (1.1)

Sep 15 FLCITRUS_STD 42.9 26.2 432.3471 (1.2)

Sep 15 FLPOTATO_STD 27.5 19.1 749.658 (2.1)

Sep 15 GACOASTAL_STD 19.7 15.2 1380.526 (3.8)

Sep 15 NCCOASTAL_STD 39.9 30.6 1247.013 (3.4)

Sep 15 WISANDS_STD 65 50.8 1248.434 (3.4)

Nov 15 DELMARVA_STD 41.3 26.5 413.5849 (1.1)

Nov 15 FLCITRUS_STD 42.2 26.3 432.3471 (1.2)

Nov 15 FLPOTATO_STD 28.2 19 749.658 (2.1)

Nov 15 GACOASTAL_STD 20.1 15.2 1380.526 (3.8)

Nov 15 NCCOASTAL_STD 39.1 30.6 1247.013 (3.4)

Nov 15 WISANDS_STD 65.2 50 1248.434 (3.4)

Data in Table 9 show that the maximum EDWC is 69.2 ppb for peak and 51.5 ppb for the post 

breakthrough average with an average breakthrough time of 3.4 years.

LITERATURE CITATIONS

USEPA. 2009. Water Models. U.S. Environmental Protection Agency, Pesticides: Science and 

Policy. Last updated: Apr 23, 2008. Online at: 

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

USEPA. 2007b. Bradbury, S. Guidance for Tier I Estimation of Aqueous Pesticide 

Concentrations in Rice Paddies. U.S. Environmental Protection Agency, Office of 

Prevention, Pesticides and Toxic Substances, Environmental Fate and Effects Division. 

Internal Memorandum. May 8, 2007.