UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON D.C., 20460
OFFICE OF
PREVENTION, PESTICIDES AND
TOXIC SUBSTANCES
OFFICE OF
PREVENTION, PESTICIDES AND
TOXIC SUBSTANCES
	

                                                                PC Code: 060101
                                                       Chemical: Thiabendazole 
                                                            DP Barcode: D410805


DATE:	February 24, 2014
      
MEMORANDUM

SUBJECT:	Thiabendazole: Drinking Water Assessment for Proposed New Uses of the fungicide Mertect[(R)] 340-F (EPA Reg. No. 100-889) as a seed treatment.

TO:	Tony Kish, PM 22 (7505P)
            Rosemary Kearns Reviewer (7505P)
            Registration Division, Fungicide Branch
            
            Bonnie Cropp-Kohlligian
            Elissa Reaves, Branch Chief
            Registration Action Branch 4
            Health Effects Division (7509P)
            
FROM:	James K. Wolf, Environmental Scientist
            Environmental Risk Branch 3

	James A. Hetrick, Ph.D. Senior Scientist
	Rosanna Louie-Juzwiak, RAPL
            Environmental Risk Branch 3
            Environmental Fate and Effects Division (7507C)
	
            Dana Spatz, Branch Chief
            Environmental Risk Branch 3
            Environmental Fate and Effects Division (7507C)


Syngenta Crop Protection has submitted an application to amend the Mertect[(R)] 340-F (EPA Registration No. 100-889) label with the addition of seed treatment uses on vegetables and small grains crops and amend the label to add a higher rate on soybeans.  A Tier 1 drinking water assessment was conducted for thiabendazole in surface water using the FIRST model and Tier 1 groundwater using PRZM-GW for the proposed new seed treatment uses and soybean rate increase.  

The Estimated Drinking Water Concentrations (EDWC) for the proposed new uses and the application rate increase for soybeans are based on the maximum seed treatment use rate of 0.15 lb a.i./A planted to a depth of 1 inch. For this assessment EFED also assumed that there was only 1 cropping season per year. These proposed rates are lower than the currently registered wheat seed treatment rate of 0.20 lb a.i./A, which results in the highest EDWCs. The highest EDWCs in surface water for thiabendazole were previously prepared for the 2002 Reregistration Eligibility Document (RED), using the Tier 1 GENEEC model (which has since been replaced by the FIRST model) and the SCI-GROW model (which has been replaced with PRZM-GW) for groundwater. Input parameters used by the models also followed current model input guidance. The surface water and groundwater EDWCs for the proposed new uses and reevaluated here for the wheat seed treatment rate are presented in this document (Table 1). 

Based on the proposed new uses of thiabendazole, the Tier 1 peak (acute) and annual average (chronic and cancer) EDWCs in surface source drinking water are 2.85 and 0.35 μg/L, respectively. For the wheat seed treatment, the reassessed surface water concentrations are 3.80 and 0.47 μg/L for peak (acute) and annual average (chronic and cancer), respectively. The peak concentrations are higher than previously determined for the RED primarily due to changes in the conceptual model (GENEEC vs. FIRST) (e.g., differences of field size and shape and the implementation of the Index Reservoir) and difference in fate input values for modeling. The chronic EDWCs are lower than before due the inclusion of the Index Reservoir (e.g., size and flow through the reservoir).

For groundwater as a source drinking water, the PRZM-GW model-estimated peak concentrations are 0.46 μg/L and 0.61 ug/L for 100 year simulation, respectively, for the proposed new uses and the wheat seed treatment use. Breakthrough average concentrations of thiabendazole in groundwater could not be estimated because no breakthrough of thiabendazole into groundwater was predicted. 

EFED recommends that 3.80 μg/L (peak) and 0.47 μg/L (annual average) concentrations be used in the human health dietary risk assessment.

Table 1.Summary of Tier 1 Estimated Drinking Water Concentrations in Surface and Ground Water for the New Uses and Previously Assessed Wheat Use.

New Uses and Reassessment
Thiabendazole


Peak Concentration
Annual Average
Surface Water
Crop (app rate lb a.i./A)
Model
ug/L

New Uses (0.15)
FIRST
2.85
0.35

Wheat (0.20)
FIRST
3.80
0.47

From RED
Peak Concentration
56-day Average

Wheat  (0.20) 
GENEEC
2.40[a]
1.55[a]
Groundwater


Highest Daily Concentration
Average Concentration after Breakthrough

New Uses (0.15)
PRZM-GW
0.46
-

Wheat (0.20)
PRZM-GW
0.62
-
[a] EDWCs reported in the 2002 RED.


USE CHARACTERIZATION

The seed treatment rates for the proposed new uses of thiabendazole are summarized in Table 2. Mertect 340-F is applied to seeds as a water-based slurry using standard seed treatment equipment. Treated seeds may be planted at any time of the year. While it is assumed that only one application (i.e., one planting of treated seed) may be made per year, in some regions, more than one crop cycle may occur per year. Maximum seed treatment rates as defined by the label are given in Table 2.

The proposed use rate for seed treatments are expressed as mg (milligrams) of active ingredient (a.i.) per seed with the per seed rates ranging from 0.002 to 0.10 mg a.i. per seed or as fluid ounces (fl oz) of product per 100 pounds of seed (or 100 cwt) with rates ranging from 2.6 to 5.8 fl (fl oz) per 100 cwt (Table 2).  

Table 2.  Maximum Application Rates for Proposed New Seed Treatment Uses of Thiabendazole[a]
Crop or Crop Group
Crop Group ID
Max. Seed Treatment Rate


mg a.i./seed
fl oz product per 100 lb seed
lb a.i. per
100 lb seed
Alfalfa
NA
0.004
5.8
0.18578
Cereals, Small Grain[b]
    Barley
NA
0.03
2.6
0.08328
    Oat
NA
0.03
3.4
0.10891
    Rye
NA
0.03
3.2
0.1025
    Triticale
NA
0.03
2.6
0.08328
    Wheat
NA
0.03
3.1
0.09930
Brassica, head and stem
Subgroup 5A
0.002
-
-
Cucurbit
Group 9
0.1
-
-
Onion, bulb
Subgroup 3-07A
0.002
-
-
Root vegetables 
(except sugar beet)
Subgroup 1B
0.002
-
-
Spinach
NA
0.006
-
-
Soybeans (rate increase)
NA
0.0302
-
0.020
[a] Mertect 340-F 889 MAS 0510 AMEND 0313 HiLite-dt/df 3/15/13; 000100-00889.20130318 Mertect_340-F-Amend-0313-HiLite.PDF. 
[b] "Small grains" is not designated as an EPA crop group.

The proposed new use rates, on a per acre basis, vary depending on the crop, the size of seed, seed treatment rate (mg a.i. per seed), and the seeding rate per acre. To determine the potential thiabendazole loading within a given acre, upper bound seeding rates for the available crops were used using information from a report prepared by the Biological and Economic Analysis Division (BEAD) for some of the proposed crops (Becker and Ratnayake, 2011). Based on the proposed wheat use, the maximum seed treatment rate on a pound of active ingredient per acre (lb a.i./A) basis would be 0.15 lb a.i./A.  The seeding rates for the relevant crops are summarized in Table 3.  In correspondence from the registrant, Syngenta, it was indicated that the following would be added to the label "regardless of type of application (seed treatment, soil, or foliar), do not apply more than 0.15 lb a.i./A (68 g a.i./A) per year."

Table 3.   Proposed use rates for thiabendazole seed treatment for the new uses.
Mertect[(R)] 340-F  EPA Reg. No. 100-889 [e-submission #4085; see Table 2]
Crop Group
Representative Crops
Seed Treatment Rate
Range of Seeding Rates
(# seeds/A)[b]
Max. Appl. Rate
(lb a.i./A)[c]


fl. oz. product/100 lb seed[a]
mg a.i./seed[a]


Alfalfa
 
5.8
0.004 
3,405,000
0.030
Cereals (small grains)
Barley
2.6
0.03
1,222,000 to 1,300,000
0.081 to 0.086

Oats
3.4
0.03
1,305,000
0.0863

Rye
3.2
0.03
1,962,000
0.130

Triticale
2.6
0.03
1,744,000
0.115

Wheat
Wheat, winter
Wheat, forage
3.1

0.03

2,250,000[d]
600,000 to 1,350,000 
1,740,000
0.149
0.040 to 0.089
0.115
Brassica, head and stem
Broccoli

0.002
8,297 to 210,845
0.00004 to 0.0009

Cabbage

0.002
7,260 to 98,010
0.00003 to 0.00043
Bulb Vegetables
Onion

0.002
87,120 to 1,254,528
0.00038 to 0.0055
Cucurbit Vegetables
Cucumber
Muskmelon
Squash, summer
Squash, winter

0.1
4,840 to 139,392
 2,420 to 34,848
1,452 to 15,374
2,723 to 7,260
0.0011 to 0.031
0.00053 to 0.0077
0.00032 to 0.0034
0.00060 to 0.0016
Spinach
-

0.006
196,020 to 653,400
0.0026 to 0.0086
Root Vegetables 
(except sugarbeet)
Beet, garden
Carrot
Horseradish
Radish

Parsley
Parsnip

0.002
261,360 to 2,090,880
104,544 to 2,090,880
9,680 to 10,249
418,176 to 1,045,440 
52,272 to 1,568,160
87,120 to 435,600
0.0012 to 0.0092
0.00046 to 0.0092
0.00004 to 0.00005
0.0018 to 0.0046

0.00023 to 0.0069
0.00038 to 0.0019
Soybeans
-

0.0302
50,000 to 250,000
0.0033 to 0.017
a  Proposed label MERTEC 340-F  889 MAS 0313 HILITE  - dt/df 3/15/13 (Density = 4.1 lb a.i./gallon of product; 42.3 % active ingredient (a.i.).[e-submission #4085; MRID 4907800]
[b] Becker and Ratnayake. 2011.
[c] mg a.i./seed * seeds/A * (1 lb/453.59 g) * 1 g/1000 mg = lb a.i./A.
[d] Conservative maximum seeding rate proposed by Syngenta.
[e] Current label EPA Reg. No. 100-889 April 5, 2013. Maximum rate of soybeans is 0.16 fl oz. per 100 lb of seed = 0.00512 lb a.i./100 lb of seed.

ESTIMATED DRINKING WATER CONCENTRATIONS 

Previous Drinking Water Assessments 

As part of the Thiabendazole RED, a drinking water assessment was conducted to predict thiabendazole EDWC from seed treatment for dry peas, chickpeas, lentils, wheat, and corn. The Tier 1 EDWCs from GENEEC were 2.4 ug/L and 1.55 ug/L for the peak and average 56-day concentration, respectively, for the maximum wheat seed treatment of 0.20 lb a.i./A [or 3.6 fl oz a.i. per 100 pounds of seed and 90 lb of seed per acre].  The Tier 1 SCI-GROW model EDWC was less than 0.01 ug/L. Section 18s for Brussels sprout, cabbage, and cauliflower in California (D326593) and Washington (D327512) assumed the same drinking water exposure concentrations as the RED. 

For a new use assessment for seed treatment on dry peas @ 1 inch planting depth (D315436), the Tier 1 drinking water concentrations using FIRST, which was developed to replace GENEEC for drinking water assessment, was 2.0 ug/L for acute and 0.5 ug/L for chronic exposure. For a new use on seed treatment on corn at an application rate of 0.0055 lb ai/A and 2 inch seed planting depth (D363978), the highest Tier 2 PRZM/EXAMS for thiabendazole were 0.018 μg/L, 0.0048 ug/L, and 0.0034 μg/L, for the peak, annual average and 30 year average concentrations, respectively.


ENVIRONMENTAL FATE SUMMARY

There have been no additional environmental fate data submitted since the previous assessments.  
Pertinent physico-chemical and environmental fate and transport properties of thiabendazole are summarized in Table 4

Environmental fate studies show that thiabendazole is persistent and immobile (Table 4). Thiabendazole is stable to hydrolysis and soil photolysis and degrades slowly in aerobic (half-life ~ 700 days) and anaerobic soil environments (>200 days). Aquatic photolysis, with a half life of 1.2 days in clear water, appears to be the only significant degradation pathway. Under anaerobic aquatic conditions thiabendazole showed no evidence of abiotic or biotic degradation but moved rapidly from water phase to the sediment phase. Thiabendazole binds tightly to soil due to its high soil/water partitioning coefficients (Kocs > 1,000 mL/goc), thus limiting the amount available for leaching into ground water and for solution phase runoff into surface water. With the high sorption capacity, thiabendazole would be transported on eroded sediment. Sorption was weakly correlated to organic carbon. Terrestrial field dissipation studies showed thiabendazole to be extremely persistent with no observable leaching. Vapor pressure and Henry's Constant indicate that thiabendazole has a low potential to volatilize from soil and water. 

Table 4.  Physical/Chemical Properties, Environmental Fate Parameters and Fish Bioconcentration of Thiabendazole.
Parameter
Value
Description
Source
(MRID #)
Selected Physical/Chemical Parameters
Molecular mass 
 201.1 g/mol 
--
(Calculated)
Molecular formula
 C10H7N3S
--

Vapor pressure (25°C)
 3.45 x 10[-9] mm Hg 
 4.0 x 10[-9]  mm Hg
--
48354601
Thiabendazole RED (2002)
Henry's Law Constant (25°C)
  3.26 x10[-11] atm-m[3]/mol
--
(Calculated)
Dissociation Constants
 pKa1 = 4.5
 pKa2 = 12.01
--
48354604
40947302
Aqueous solubility 
 Buffered solution (25C) at   pH 5:  30 mg/L
 pH 7:  28 mg/L
 pH 9:  28 mg/L
 
 Water:  22.74 mg/L
 Buffer solution (20C) at:
 pH4:     79.82 mg/L
 pH7:     22.77 mg/L
 pH10:   23.75 mg/L
--
 40947301




48354601
48354604

Log octanol-to-water partition coefficient (log KOW)
 1.61
@ Room temperature = 16.6°C
48254601
48354604
Persistence
Hydrolysis half-life (22C)
 Stable @ pHs 5, 7, 9
Rates[a] of transformation were extrapolated resulting in t(1/2) (198 days @pH 7) far beyond the length of the study (30 days)
41265301
(D245780; 02/04/99)
Aqueous photolysis half-life
(22.5 to 23.2C)
  1.2 days
96 hour study @ pH5
43328305
Soil photolysis half-life (25C)
 Stable
--
41297301
41397302
Aerobic soil metabolism half-life  (25°C)
 688 days
Registration Document D245780
 One soil.
41791201 
Anaerobic soil metabolism half-life  (25°C)
 211 day
Only degradate detected was benzimidazole in a 90 day study.
(max 13.7 % AR;).
41559601 
Anaerobic aquatic metabolism half-life 
No evidence of biotic or abiotic degradation.
Rapidly moved from water phase to the sediment phase.
00122888
00070753
Aerobic aquatic metabolism half-life [dissipation half-life ] (25C)
--
Study requirement waived
(D169687; 12/30/92)
41559601 

Mobility
Freundlich soil-water partition coefficients (KF); organic carbon-normalized Freundlich coefficients (KFOC)
KF (mL/g)/ KFoc(mL/goc):
(soil textural class)

2.49/1,104: (sand)
15.97/3,992: (sandy loam)
21.75/1,812: (silt loam)
269.6/22,467: (clay)
 
 Mean = 7,343 mL/goc
 Median = 2,902 mL/goc

41170102
Field Dissipation
Terrestrial field dissipation half-life in upper 6 inch soil layer; (max. depth of observed leaching)
 t(1/2) = 1100 days; (6 inches). 
 t(1/2) = 990 and 1444 days; (12 inches)
 t(1/2)= 833 and 1093 days: (6 inches).
Winter wheat/loamy sand/WA
Soybean and bare ground/sandy loam/IL
Soybean and bare ground/sandy loam/GA
43187201
43187202
43187201
Octanol/Water Partition Coefficient (logPow)
 1.61
--
48354604
Fish Bioconcentration
Fish bioconcentration factors (steady-state); depuration rate
 86.5 (whole); 19.7 (edible), 747 (viscera), 3-day depuration: 96%  in viscera and 50% in edible tissue.
Low bioconcentration potential in edible tissue, some potential to bioaccumulate in viscera but with rapid depuration
41631801
42022701
[a] Transformation rates of thiabendazole: pH 5 (-0.0019 days[-1]); pH 7 (-0.0035 days[-1]); pH 9 (-0.0026 days[-1])

Transformation Products

Three degradates were identified in the aqueous photolysis study (benzimidazole-2-carboxamide as a major degradate; benzimidazole and benzimidazole-2-carboxylic acid as minor degradates), two minor degradates were identified in the aerobic soil metabolism study (benzimidazole and 5-hydroxythiabendazole (maximum observed 0.33%)), and one major degradate was identified in the anaerobic soil metabolism study (benzimidazole) (Appendix 1 Table 1). The metabolite 5-hydroxythiabendazle and benzimidazole are included in the tolerance dietary exposure for food (40 CFR 180.242), but not included in drinking water. The common name, chemical names, CAS No., molecular formula and mass, chemical structure, and maximum and final percent of applied radioactivity (AR) of the major degradates are summarized in Appendix 2 Table 1. The only degradate that was analyzed for in the terrestrial field dissipation studies (TFD) is benzimidazole, since it is a major metabolite in a soil study (anaerobic soil metabolism). Results TFD studies showed that benzimidazole was not detected in any soil layers. No data were available for the other three degradates to verify their presence in the field. However, since they comprised a relatively small fraction (less than 10%) of the total applied radioactivity in the laboratory studies, EFED believes that any transformation to these degradates in the fields will also be negligible. 

While photodegradation is a route of degradation, the dissipation of thiabendazole is expected to be dependent on water conditions (e.g., sediment loading, depth, etc.) affecting sunlight penetration. For example, thiabendazole should not persist in clear shallow water bodies, but will be more persistent in turbid and/or deeper waters, particularly those with long hydrological residence times.

Due the low vapor pressure and burial of treated seeds when planted, volatilization is not expected to be an important route of dissipation. Spray drift, foliar dissipation, and wash-off are also not expected to be important for seed treatments. It is also assumed that the intent is to bury all seed, which would reduce the presence of a.i. at the surface and keep losses due to runoff at minimum. 

Monitoring Data

There are no detections of thiabendazole reported in the California Department of Pesticide Regulation (CDPR) Surface Water Monitoring Database available and it is not included in CDPR Groundwater Protection List.  Thiabendazole is not included in the surface water database collected in the USGS National Water Quality Assessment (NAWQA) Program. There were no detections of thiabendazole reported in groundwater (49 samples from CA, DE, FL,MD, NJ) in USGS NAWQA data warehouse with limits of detection <0.06 ug/L.  

Simulation Modeling

Surface Water 

This Tier I drinking water assessment utilizes the Tier I FQPA Index Reservoir Screening Tool (FIRST, Version 1.1.1, released 03/26/08) to assess surface water.  Exposure was estimated for a seed treatment application at 0.15 lb a.i./A, which is to be the maximum annual application rate allowed on the Mertect[(R)] 340-F label, and 0.20 lb a.i./A for the wheat seed-treatment use. Description of the FIRST model can be found on the EPA's website at: http://www.epa.gov/pesticides/science/models_pg.htm.

Model Inputs 

The fate data used in modeling have been selected (Table 5) according to current modeling guidance. Using current guidance for preparing the environmental fate data for input into the EFED water models results in slightly different values from the previous assessments. Previous drinking water assessments used the single aerobic soil metabolism half-life value of 688 days and the median Koc of 2,903 mL/goc which does not agree with current model input guidance (see Table 4). While inputs (average vs. median Koc and aerobic soil metabolism half-life) may have changed due to updated guidelines, the conclusions concerning the fate of thiabendazole does not change. For PRZM-GW the application date (i.e., planting date) is assumed to occur 7 days before the emergence date as specified in each scenario.

The assessment assumes that 100 percent of the a.i. comes off the seed and is available for runoff or leaching (EFED, 1999).  Thus, the runoff prediction of thiabendazole is expected to be conservative.

Table 5. Summary of environmental fate parameters for thiabendazole and other values used for modeling FIRST, PRZM-GW, and SCI-GROW. 
Parameter
Value or Range 
Value Used
Source
Solubility (water)[a]
28 mg/L 
28 mg/L
MRID 40947301
Hydrolysis t1/2
stable at pH 7
0 day[a] - surface water
198 days[b] -  groundwater
MRID 41265301
Aquatic Photolysis[a] t1/2
29 hours
1.2 days[b]   
MRID 43328305
Aerobic Soil Metabolism t1/2
688 days (single value)
2,064 (3 x 688)
2064 days[a,b]
MRID 41791201
 (DER 2)
Aerobic Aquatic Metabolism[a] t1/2
Stable
(0 day)[b]
No data available
Koc
See Table 3                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
Mean = 7,343 mL/goc[a,][b]
Median = 2,902 mL/goc[c]
MRID 41170102
Application Method
Granular 
 D
Model option 
Application Rate
New uses
Wheat rate (RED)
0.15 lb a.i./A
0.20 lb a.i./A
Max rate
Number of Applications per Year
 -
1
-
Watered In
-
No
-
DEPI (depth of soil incorporation) 
1 to 2 inches
1 and 2 inch[b]
Beale, 2007 
Percent Crop Area (PCA)[d]
All agriculture (0.91)
0.91[b]
EFED, 2011[d]
[a] Input for FIRST
[b] Input for PRZM-GW
[c] Median Koc value used for SCI-GROW (earlier assessments used median Koc of 2903 mL/goc).
[d] http://www.epa.gov/oppefed1/models/water/pca_adjustment_dwa.html

      
The Tier 1 surface water EDWCs for thiabendazole are summarized in Table 6. Maximum values are shown in bold in Table 6. Output from FIRST for the two planting depths are located in Appendix 3. A percent crop area (PCA) of 0.91 was used. 


New Uses

For the proposed new seed treatment rate of 0.15 lb a.i./A, the EDWCs for thiabendazole in surface water source of drinking water are 2.85 and 0.35 μg/L, for the peak and annual average concentrations, respectively.  

Updated Drinking Water Concentrations for Wheat Use Rate

For the wheat seed treatment rate of 0.20 lb a.i./A, the EDWCs for thiabendazole in surface water source of drinking water are 3.80 and 0.47 μg/L, for the peak and annual average concentrations, respectively.  

Table 6. FIRST Model Estimated drinking water concentrations (EDWCs) of thiabendazole for proposed maximum annual application rate of 0.15 lb a.i./A and two seeding depths and for the Maximum Wheat Rate Assessed in the RED (0.20 lb a.i./A). 
                                     Crop
                                   Scenario
                             EDWCs (ug a.i./L)[a]
                                       
                                       
                                     Peak
                                Annual Average
All New Uses 
0.15 lb a.i./A
    Generic granular/seed treatment scenario with 1-inch soil incorporation
                                     2.85
                                     0.35

    Generic granular/seed treatment scenario with 2-inch soil incorporation
                                     1.42
                                     0.18 
Wheat Rate
0.20 lb a.i./A
    Generic granular/seed treatment scenario with 1-inch soil incorporation
                                     3.80
                                     0.47

    Generic granular/seed treatment scenario with 2-inch soil incorporation
                                     1.90
                                     0.23
[a] All agriculture PCA = 0.91


Ground Water

The Tier 1 drinking water assessment used the PRZM-GW and SCI-GROW models for predicting thiabendazole concentrations in groundwater source drinking water.

PRZM-GW 

The six standard PRZM-GW scenarios used in the groundwater simulations were developed to be representative of known vulnerable sites -- shallow, unconfined aquifers under an agricultural field with leaching prone soil and adequate precipitation and/or irrigation.  Output values represent pesticide concentrations that may be observed in private drinking water wells resulting from the use of groundwater vulnerable to pesticide contamination. The results are expected to represent the upper bound values on the concentrations of thiabendazole that might be found in drinking water supplied by vulnerable groundwater sources. The selection and procedures for selecting model input parameters followed EFED model input guidance.  


Groundwater Concentrations

PRZM-GW is a graphical user interface (GUI) for running the Pesticide Root Zone Model (PRZM) to estimate exposure in a shallow, unconfined, aquifer following pesticide application to an agricultural field. PRZM was developed by the U.S. EPA to simulate the vertical movement of pesticides in the unsaturated soil, within and below the plant root zone, and extending to the water table using generally available input data (Carsel et al., 1984). PRZM is also currently used by EFED in combination with EXAMS to estimate pesticide concentrations in surface water. Groundwater EDWCs with PRZM-GW are simulated for 30 (and 100) years as done with PRZM/EXAMS for surface water. Unlike SCI-GROW, estimated groundwater concentrations using PRZM-GW are influenced by both the soil and hydrology of the specific scenario.  

The inputs for PRZM-GW are summarized in Table 5. In the previous drinking water exposure assessment, a single aerobic soil metabolism half-life value of 688 days had been used.  EFED model input guidance recommends multiplying a single half-life value times 3 (2064 days) to account for variability of half-lives among different soils. Also following current model input guidance, the mean Koc (7,343 mL/goc) is used with the PRZM-GW.  In contrast, the median Koc is used in SCI-GROW. The highest daily concentration of thiabendazole in predicted by PRZM-GW as groundwater source drinking water is 0.46 and 0.62 ug/L for the proposed new uses and the existing wheat use rate, respectively (Table 7). Complete breakthrough was not achieved for any of the scenarios with both 30 and 100 years of simulation. Therefore, the average concentration after breakthrough was not determined. This modeling shows that the leaching of thiabendazole to groundwater is not expected to be a significant dissipation pathway as concentrations estimates ranged from essentially 0 to less than 1 ug/L, with a 100 year of simulation. Representative output from PRZM-GW is presented in Appendix 3.


Table 7. Thiabendazole EDWC PRZM-GW (version 1.03) and updated scenarios.
Scenario
Length of Simulation
(years)
High Daily Value[a]
(ug/L)
Avg BT
(ug/L)[a]
Avg BT time (days)
Application Rate 0.15 lb a.i./A
Delmarva
30
<10[-8]
-[b]
-[b]
"
100
0.0038
-
-
FL citrus
30
0.0029
-
-
"
100
0.46
-
-
FL potato
30
<10[-10]
-
-
"
100
2.1x10[-5]
-
-
GA Coast Pl
30
<10[-16]
-
-
"
100
<9.0x10[-7]
-
-
NC Coast Pl
30
<6 x 10[-7]
-
-
"
100
0.028
-
-
WI Central Sands
30
3.2 x 10[-14]
-
-
"
100
0.00015
-
-
Application Rate 0.20 lb a.i./A 
FL citrus
100
0.62
-
-
    [a] Hydrolysis half-life @ pH7 = 198 days for all runs.
    [b] Breakthrough not achieved during simulations (30 and 100 years).

SCI-GROW

A SCI-GROW estimate of the thiabendazole concentration in groundwater is also provided as required by the implementation of the PRZM-GW model. SCI-GROW (SG23.exe, version 2.3, dated 09/10/08) is a Tier I, regression-based model that provides a groundwater screening exposure value to be used in determining the potential risk to human health from drinking water contaminated with the pesticide. SCI-GROW does not consider multiple years of use.
 
The SCI-GROW estimate considers the highest rate as a single application, 0.15 and 0.20 lb a.i. per acre for thiabendazole.  The environmental fate values used are listed in Table 5. The thiabendazole concentrations were estimated to be 9.43 x 10[-][3] ug/L and 1.26 x 10-2 ug/L, the proposed maximum new use rate and the current wheat seed treatment rate, respectively,  (Table 8). SCI-GROW output is given in Appendix 3.
      

Table 8. SCI-GROW estimated ground water concentration for thiabendazole at the maximum proposed rate new use rate and the maximum wheat use rate.
Concentration (app rate 0.15 lb a.i./A)
9.43 x 10[-]3  ug/L 
Concentration (app. rate 0.20 lb a.i./A)
1.26 x 10[-2] ug/L

 
Appendix 1.
Appendix 1. Table 1. Thiabendazole degradates identified in Environmental Fate Studies.

Degradates

Hydrolysis

Photolysis (water)

Photolysis (soil)

Aerobic Soil

Anaerobic Soil

Terrestrial Field
Benzimidazole

X

X 
X[a]

5-hydroxythiabendazole



X 


Benzimidazole-2-carboxamide


X[a]




Benzimidazole-2-carboxylic acid


X




Unidentified [[14]C]

X

X


Unknown 1



X


[14]CO2



X


[a] - major degradates (>10%)



Appendix 2. 
Appendix 2. Table 1.  Thiabendazole and Its Environmental Degradates
Name
Chemical Name CAS No.
Formula
Molecular Mass
Chemical Structure
Study Type
Maximum %AR (day)
Final %AR (study length)
PARENT
Thiabendazole
2-(4-thiazolyl)-1H-benzimidazole

CAS No.: 148-79-8

Formula: C10H7N3S
MW: 201.25 g/mol







Aerobic Soil Metabolism
Anaerobic Soil Metabolism




56.8%
78% (@ day 90)
TRANSFORMATION PRODUCTS
Thiabendazole-5-hydroxy
5-hydroxythiabendazole;
2-(4-Thiazolyl)-5-benzimidazolol

CAS No.: 948-71-0


Formula: C10H7N3S


MW:  217.25g/mol

Aerobic Soil Metabolism

 0.33% max observed through study
Benzimidazole
1H-Benzimidazole
CAS No.: 962-28-7 
Formula: C7H6N2

MW: 118.05 g/mol
            


Aq. Photolysis (minor)
Aerobic  Soil Metabolism
Anaerobic Soil Metabolism
Terrestrial Field Dissipation
-
2.2%
13.7%
-
6.49% (avg.)
0.17%
5.5% (@ day 90
-
Benzimidazole carboxylic acid

1H-Benzimidazole-2-carboxylic acid 
CAS No.:
Formula: C8H6N2O2

MW: 162.15 g/mol
   


Aq. Photolysis (minor)

9.98% @ 96 hrs
Benzimidazole-2-carboxamide

1H-Benzimidazole-2-carboxamide

CAS NO. 5805-52-7

Formula: C8H7N3O


MW: 161.16 g/mol

Aq.  Photolysis (major)

10.2% @ 96 hrs

Appendix 3. Representative Model Output

FIRST OUTPUT

Proposed New Use Maximum Rate of 0.15 lb a.i./A

   RUN No.   1 FOR thiabendazole    ON   seed treat    * INPUT VALUES * 
   --------------------------------------------------------------------
    RATE (#/AC)   No.APPS &   SOIL  SOLUBIL  APPL TYPE  %CROPPED INCORP
     ONE(MULT)    INTERVAL    Koc   (PPM )   (%DRIFT)     AREA    (IN)
   --------------------------------------------------------------------
   .150(   .150)   1   1    7343.0   28.0   GRANUL(  .0)  91.0   1.0


   FIELD AND RESERVOIR HALFLIFE VALUES (DAYS) 
   --------------------------------------------------------------------
   METABOLIC  DAYS UNTIL  HYDROLYSIS   PHOTOLYSIS   METABOLIC  COMBINED
    (FIELD)  RAIN/RUNOFF  (RESERVOIR)  (RES.-EFF)   (RESER.)   (RESER.) 
   --------------------------------------------------------------------
   2064.00        2          N/A      1.20-  148.80      .00    148.80


   UNTREATED WATER CONC (MICROGRAMS/LITER (PPB)) Ver 1.1.0  JAN 1, 2007
   --------------------------------------------------------------------
        PEAK DAY  (ACUTE)      ANNUAL AVERAGE (CHRONIC)      
          CONCENTRATION             CONCENTRATION            
   --------------------------------------------------------------------
              2.846                       .349



   RUN No.   2 FOR thiabendazole    ON   seed treat    * INPUT VALUES * 
   --------------------------------------------------------------------
    RATE (#/AC)   No.APPS &   SOIL  SOLUBIL  APPL TYPE  %CROPPED INCORP
     ONE(MULT)    INTERVAL    Koc   (PPM )   (%DRIFT)     AREA    (IN)
   --------------------------------------------------------------------
   .150(   .150)   1   1    7343.0   28.0   GRANUL(  .0)  91.0   2.0


   FIELD AND RESERVOIR HALFLIFE VALUES (DAYS) 
   --------------------------------------------------------------------
   METABOLIC  DAYS UNTIL  HYDROLYSIS   PHOTOLYSIS   METABOLIC  COMBINED
    (FIELD)  RAIN/RUNOFF  (RESERVOIR)  (RES.-EFF)   (RESER.)   (RESER.) 
   --------------------------------------------------------------------
   2064.00        2          N/A      1.20-  148.80      .00    148.80


   UNTREATED WATER CONC (MICROGRAMS/LITER (PPB)) Ver 1.1.0  JAN 1, 2007
   --------------------------------------------------------------------
        PEAK DAY  (ACUTE)      ANNUAL AVERAGE (CHRONIC)      
          CONCENTRATION             CONCENTRATION            
   --------------------------------------------------------------------
              1.423                       .175





ReRun EDWCs for Wheat Seed Treatment at 0.20 lb a.i./A

   RUN No.   1 FOR Thiabendazole    ON   Seed Treat    * INPUT VALUES * 
   --------------------------------------------------------------------
    RATE (#/AC)   No.APPS &   SOIL  SOLUBIL  APPL TYPE  %CROPPED INCORP
     ONE(MULT)    INTERVAL    Koc   (PPM )   (%DRIFT)     AREA    (IN)
   --------------------------------------------------------------------
   .200(   .200)   1   1    7343.0   28.0   GRANUL(  .0)  91.0   1.0


   FIELD AND RESERVOIR HALFLIFE VALUES (DAYS) 
   --------------------------------------------------------------------
   METABOLIC  DAYS UNTIL  HYDROLYSIS   PHOTOLYSIS   METABOLIC  COMBINED
    (FIELD)  RAIN/RUNOFF  (RESERVOIR)  (RES.-EFF)   (RESER.)   (RESER.) 
   --------------------------------------------------------------------
   2064.00        2          N/A      1.20-  148.80      .00    148.80


   UNTREATED WATER CONC (MICROGRAMS/LITER (PPB)) Ver 1.1.0  JAN 1, 2007
   --------------------------------------------------------------------
        PEAK DAY  (ACUTE)      ANNUAL AVERAGE (CHRONIC)      
          CONCENTRATION             CONCENTRATION            
   --------------------------------------------------------------------
              3.795                       .466



   RUN No.   2 FOR Thiabendazole    ON   seed treat    * INPUT VALUES * 
   --------------------------------------------------------------------
    RATE (#/AC)   No.APPS &   SOIL  SOLUBIL  APPL TYPE  %CROPPED INCORP
     ONE(MULT)    INTERVAL    Koc   (PPM )   (%DRIFT)     AREA    (IN)
   --------------------------------------------------------------------
   .200(   .200)   1   1    7343.0   28.0   GRANUL(  .0)  91.0   2.0


   FIELD AND RESERVOIR HALFLIFE VALUES (DAYS) 
   --------------------------------------------------------------------
   METABOLIC  DAYS UNTIL  HYDROLYSIS   PHOTOLYSIS   METABOLIC  COMBINED
    (FIELD)  RAIN/RUNOFF  (RESERVOIR)  (RES.-EFF)   (RESER.)   (RESER.) 
   --------------------------------------------------------------------
   2064.00        2          N/A      1.20-  148.80      .00    148.80


   UNTREATED WATER CONC (MICROGRAMS/LITER (PPB)) Ver 1.1.0  JAN 1, 2007
   --------------------------------------------------------------------
        PEAK DAY  (ACUTE)      ANNUAL AVERAGE (CHRONIC)      
          CONCENTRATION             CONCENTRATION            
   --------------------------------------------------------------------
              1.897                       .233



PRZM-GW

1/28/2014 7:52:43 AM; thiabendazole hyd up b HL ave koc; Florida Citrus - FL Central Ridge, Polk County - (Tampa) Met File (12842.dvf) - Astatula sand, hrgb A
User Interface Version: PRZM-GW (Version 1.03)
***Record 3 
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***Record 6
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***Record 10
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***Record 12
Place Holder
***Record 13
      99       1       0       0
***Record 15
Place Holder
***Record 16
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***Record 17
       0       3       0
***Record 19
Place Holder
***Record 20
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***Record 26
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***Record 27
       4     0.1    0.90     7.0
***Record 31
  0.2  0.2  0.2  0.2  0.2  0.2  0.2  0.2  0.2  0.2  0.2  0.2 0.97 10.0
***Record 32
   21   21   21   21   21   21   21   21   21   21   21   21
***Record 32A
    2.00    25.0
***Record 33
       8
*** Record 34,36,37 Horizon 1
       1      10    1.35     0.3       0       0       0
          0.00350.000336       0
               1     0.3   0.014   0.14410.57392
              21      96     2.4       0       0
*** Record 34,36,37 Horizon 2
       2      10    1.58    0.25       0       0       0
          0.00350.000317       0
               5    0.25   0.016   0.14410.57392
              21      95     2.8       0       0
*** Record 34,36,37 Horizon 3
       3      20    1.58    0.25       0       0       0
          0.00350.000261       0
              20    0.25   0.016    1.44105.7392
              21      95     2.8       0       0
*** Record 34,36,37 Horizon 4
       4      20    1.45    0.33       0       0       0
          0.00350.000187       0
              20    0.33   0.015   0.14410.57392
              21      95     2.8       0       0
*** Record 34,36,37 Horizon 5
       5      20     1.5    0.27       0       0       0
          0.00350.000112       0
              20    0.27   0.015   0.14410.57392
              21      95     2.8       0       0
*** Record 34,36,37 Horizon 6
       6      20     1.5    0.34       0       0       0
          0.00353.73E-05       0
              20    0.34   0.015   0.14410.57392
              21      95     2.8       0       0
*** Record 34,36,37 Horizon 7
       7     200    1.59    0.25       0       0       0
          0.0035       0       0
              50    0.25   0.014   0.14410.57392
              21      96     2.8       0       0
*** Record 34,36,37 Horizon 8
       8     100    1.58  0.3997       0       0       0
          0.0035       0       0
              50  0.3997   0.001   0.14410.57392
              21      87      11       0       0
*** Record 40
       0
*** Record 42
            YEAR      10            YEAR      10            YEAR      10   0
*** Record 45
       5    YEAR
*** Record 46
    DCON1   TAVE  21  22  1.0E3
    INFL    TCUM  21  210.000121
    IRRG    TSER
    VFLX    TSER
    FPVL    TSER















Highest Daily Value(ppb) = 0.461 ug/L 
Average Breakthrough Time(days)  -  Incomplete
Post-Breakthrough Average (ppb)  -  Incomplete
Throughputs                     - 0.487
Simulation Period 100 years
Application rate 0.15 lb a.i./A
Application Date (-7 days) since emergence
Hydrolysis half-life  -  198 days
Surface Soil Half-life  -  2064 days
Sorption Coefficient  -  Koc 7343 (average Koc)




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:thiabendazole
 time is  1/23/2014  13:26: 4
 ------------------------------------------------------------------------
  Application      Number of       Total Use    Koc      Soil Aerobic
  rate (lb/acre)  applications   (lb/acre/yr)  (ml/g)   metabolism (days)
 ------------------------------------------------------------------------
      0.150           1.0           0.150      2.90E+03     2064.0
 ------------------------------------------------------------------------
 groundwater screening cond (ppb) =   9.43E-03 
 ************************************************************************


 SciGrow version 2.3
 chemical:thiabendazole
 time is  2/24/2014   8: 0:23
 ------------------------------------------------------------------------
  Application      Number of       Total Use    Koc      Soil Aerobic
  rate (lb/acre)  applications   (lb/acre/yr)  (ml/g)   metabolism (days)
 ------------------------------------------------------------------------
      0.200           1.0           0.200      2.90E+03     2064.0
 ------------------------------------------------------------------------
 groundwater screening cond (ppb) =   1.26E-02 
 ************************************************************************
 



