Document ID: EPA-HQ-OPP-2014-0284-0006
Agency: epa
Document Type: Supporting & Related Material
Title: 
Posted Date: 2015-07-08T04:00Z

OFFICE OF CHEMICAL SAFETY
AND POLLUTION PREVENTION
                 UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
                            WASHINGTON, D.C.  20460
              OFFICE OF CHEMICAL SAFETY
AND POLLUTION PREVENTION
                 UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
                            WASHINGTON, D.C.  20460

                                                                PC Code: 108800
                                                             DP Barcode: 420047
                                                                               
                                                        Date: December 16, 2014
MEMORANDUM	

Subject:	Tier II Drinking Water Assessment for the Proposed S-Metolachlor Use on Lettuce (Head and Leaf), Low-Growing Berry Subgroup 13-07G (excluding Cranberry), Cucurbit Vegetable Group 9, Sunflower Subgroup 20B and Fruiting Vegetable Group 8-10.  

To:		Bethany Benbow, Risk Manager Reviewer
		Kathryn Montague, Product Manager Team 23
		Dan Kenny, Branch Chief
		Herbicide Branch
		Registration Division (7505P)

From:	Christopher M. Koper, M.S., Chemist
		Sujatha Sankula, Ph.D., Branch Chief
		Meghan Radtke, Ph.D., Risk Assessment Process Leader (RAPL), Acting
            Environmental Risk Branch 1
            Environmental Fate and Effects Division (7507P)
            
This drinking water assessment was conducted for a petition submitted by the Interregional Research Project Number 4 (IR-4) to establish S-metolachlor tolerances for lettuce (head and leaf), low-growing berry subgroup 13-07G (excluding cranberry), cucurbit vegetable group 9, sunflower subgroup 20B and fruiting vegetable group 8-10.  S-Metolachlor is a broad spectrum herbicide that acts to inhibit seedling development.  It can be applied to the proposed crops as a pre-plant, pre-emergent or post-emergent broadcast application up to a yearly maximum application rate of 3.7 lb a.i./A (single maximum application rate varies according to crop) according to the Dual Magnum (EPA Reg. No. 100-816) label.  Dual Magnum may be applied to direct seeded or transplanted crops for residual control or suppression of annual grasses, yellow nutsedge and small seeded broadleaf weeds.   

Previous drinking water assessments provided separate estimated drinking water concentrations (EDWCs) for parent metolachlor/S-metolachlor and their degradates metolachlor ESA (CGA-354743) and metolachlor OA (CGA-51202).  Following the recent recommendation of the Residues of Concern Knowledgebase Subcommittee (D424250), parent metolachlor, CGA-51202 (metolachlor-OA), CGA-40172, and CGA-50720 will be assessed together in the drinking water assessment using a total toxic residues (TTR; USEPA 2009a) approach where half-lives were recalculated (PestDF; USEPA, 2009b) to collectively account for the parent and the combined residues of concern.  These values will be utilized in surface and ground water modeling to generate aquatic exposure estimates.  

The highest annual application rate for the proposed IR-4 label (3.7 lbs a.i./A; note that sunflower maximum annual application rate is unspecified) is less than what EFED believes is the highest previously registered application rates (4 lbs a.i./A to corn or cotton). Therefore, EFED is providing new EDWCs based on the scenario/annual application rate combination that will produce the highest EDWCs because degradates of concern and the calculation method have changed.  Recommended EDWCs of S-metolachlor in surface water and ground water are reported in Table 1.

Table 1.  Recommended EDWCs for S-Metolachlor Use on Various Crops
Drinking Water Source
Crop Scenario

                              1-in-10 year acute 
                                    (ug/L)

                             1-in-10 year chronic
                                    (ug/L)
                                       
                    30- year average cancer chronic (ug/L)
Surface Water
MS Corn [1] (aerial)
                                      371
                                      --
                                      --

CA Lettuce [2] (aerial)
                                      --
                                     43.70
                                     21.80
Ground Water
NC Cotton 3 (aerial)
                                     1060
                                      NA
                                      --

WI Corn 3 (aerial)
                                      --
                                      NA
                                      978
[1] EDWC calculated from the maximum rate (2 apps x 2.0 lbs a.i./A) registered on corn and cotton generated using
  SWCC model (aerial application; TTR approach) with a PCA factor of 0.91. 
[2] Current proposed use on lettuce (2 apps x 0.95 lbs a.i./A x 2 seasons).
[3] Highest EDWCs generated using PRZM-GW model based on a 30 year simulation of 2 foliar applications of  
   2.0 lbs a.i./A for a maximum annual rate of 4.0 lbs a.i/A for corn/cotton (Metolachlor RED; D185152, 172022).

For surface water, the acute, chronic, and cancer chronic EDWCs generated in this assessment (Table 2) exceeded the EDWCs in the previous drinking water assessment (D368735) dated 22 April 2010.  As mentioned, the current assessment followed the total toxic residue approach with a revised tolerance expression based on the recent ROCKS meeting.  For reference, the Health Effect Division's (HED) tolerance expression for acute dietary assessment is 219 ug/L for surface water [77.60 ug/L (S-metolachlor) + 47.61 ug/L (degradate ESA) + 93.57 ug/L (degradate OA)] and the tolerance expression for chronic dietary assessment is 126 ug/L for ground water [32.8 ug/L (S-metolachlor) + 63.7 ug/L (degradate ESA) + 29.7 ug/L (degradate OA)].  Therefore, the acute and chronic EDWC values for surface water are superseded in this assessment.

For ground water, the PRZM-GW (USEPA, 2012a) and SCI-GROW models were employed to model the highest application rate of all metolachlor/S-metolachlor uses (2 applications at 2.0 lb a.i./A for a maximum yearly application rate of 4.0 lbs a.i/A on corn and cotton) in addition to the proposed uses to determine drinking water exposure in ground water (Table 2). The recommended EDWCs for ground water include the acute concentration of 1060 ug/L and 30-year average (chronic) concentration of 978 ug/L.

Table 2. S-Metolachlor Estimated Drinking Water Concentrations (EDWCs) for Surface and Ground Water based on Selected Crop Scenarios
                             Drinking Water Source
                              Proposed Label Use
                                     Model
                                   Scenario

                               Application Rate 
                  (interval between applications; # seasons)
                                       
                                    Acute 
                                    (ug/L)
                                       
                                    Chronic
                                    (ug/L)
                                Cancer Chronic
                                    (ug/L)
Surface Water

(SWCC)
Corn[1]
KS Corn

                           2 app. x 2.0 lbs ai/acre
                                   (5 days)
                                    122.00
                                     19.80
                                     13.20

MS Corn
                           2 app. x 2.0 lbs ai/acre
                                   (5 days)
                                    371.00
                                     40.60
                                     15.20

Cotton[1]
MS Cotton
                           2 app. x 2.0 lbs ai/acre
                                   (5 days)
                                    189.00
                                     20.40
                                     10.30

Curcurbit
FL Cucumber

                          2 app. x 0.635 lbs ai/acre
                             (14 days; 3 seasons)
                                       
                                    136.00
                                       
                                     17.90
                                       
                                     10.10

MI Melon

                          2 app. x 0.635 lbs ai/acre
                             (14 days; 3 seasons)
                                       
                                     38.30
                                       
                                     10.30
                                       
                                     7.67

MO Melon

                          2 app. x 0.635 lbs ai/acre
                             (14 days; 3 seasons)
                                       
                                     52.40
                                       
                                     9.00
                                       
                                     6.05

NJ Melon

                          2 app. X 0.635 lbs ai/acre
                             (14 days; 3 seasons)
                                       
                                     87.40
                                       
                                     14.20
                                       
                                     7.58

Low-Growing Berry 13-07G
FL Strawberry

                           3 app. X 1.27 lbs ai/acre
                                   (14 days)
                                       
                                     99.30
                                       
                                     12.90
                                       
                                     6.64

Lettuce (Leaf)

CA Lettuce 

                           1 app. X 0.95 lbs ai/acre
                           1 app. X 0.32 lbs ai/acre
                             (14 days; 3 seasons)
                                       
                                    105.00
                                       
                                     39.30
                                       
                                     21.80

Lettuce (Head)

CA Lettuce 

                           2 app. X 0.95 lbs ai/acre
                             (14 days; 2 seasons)
                                       
                                    129.00
                                       
                                     43.70
                                       
                                     21.80

Fruiting Vegetables (Group 8-10)
FL Pepper
                           1 app. X 1.60 lbs ai/acre
                           1 app. X 1.60 lbs ai/acre
                           1 app. X 0.5 lbs ai/acre
                                   (14 days)
                                       
                                    111.00
                                       
                                     11.10
                                       
                                     7.74

CA Tomato
                           1 app. x 1.60 lbs ai/acre
                           1 app. x 1.60 lbs ai/acre
                           1 app. x 0.50 lbs ai/acre
                                   (14 days)
                                       
                                     40.80
                                       
                                     10.30
                                       
                                     6.67

FL Tomato
                           1 app. X 1.60 lbs ai/acre
                           1 app. X 1.60 lbs ai/acre
                           1 app. X 0.50 lbs ai/acre
                                   (14 days)
                                       
                                    123.00
                                       
                                       
                                     12.60
                                       
                                     6.67

PA Tomato
                           1 app. X 1.60 lbs ai/acre
                           1 app. X 1.60 lbs ai/acre
                           1 app. X 0.50 lbs ai/acre
                                   (14 days)
                                       
                                     56.80
                                       
                                     12.20
                                       
                                     7.35

Sunflower 
(20B)
ND Canola
                           1 app. x 1.90 lbs ai/acre
                           1 app. x 1.80 lbs ai/acre
                                   (14 days)
                                       
                                     65.20
                                       
                                     17.40
                                       
                                     11.60
Ground Water

Cotton[1]
NC Cotton 
(PRZM-GW)
                           2 app. x 2.0 lbs ai/acre
                                   (5 days)
                                     1060
                                      NA
                                      865

Corn[1]
WI Corn 
(PRZM-GW)
                           2 app. x 2.0 lbs ai/acre
                                   (5 days)
                                      --
                                      NA
                                      978

Proposed Use[2]
(Sunflower)
NC Cotton
(PRZM-GW)
                           1 app. x 1.9 lbs ai/acre
                           1 app. x 1.8 lbs ai/acre
                                   (14 days)
                                      979
                                      NA
                                      801

Proposed Use[2]
(Sunflower)
WI Corn
(PRZM-GW)
                           1 app. x 1.9 lbs ai/acre
                           1 app. x 1.8 lbs ai/acre
                                   (14 days)
                                      974
                                      NA
                                      906

Corn, Cotton[1,3]
SCI-GROW
                           2 app. x 2.0 lbs ai/acre
                                       
                                     14.3
                                     14.3
                                     14.3

Various Crops (sunflower)[2,3]
SCI-GROW
                  Various apps not exceeding 3.7 lbs ai/acre
                                     13.2
                                     13.2
                                     13.2
Bold values denote maximum EEC values.
[1] Since HED's tolerance expression has changed and the assessment is now following the total toxic residue (TTR) approach, corn and cotton were modeled in addition to the proposed application rates since they have the highest application rates.
[2] EDWCs generated for proposed uses represented by sunflower by applying 1 app. at 1.9 lbs a.i/A followed by 1 app. 1.8 lbs a.i./A (14 day interval).
[3] EDWCs generated from SCI-GROW model represent both acute and chronic values.

Environmental Fate and Transport 

Metolachlor is a racemic mixture of R- and S-enantiomers (stereo isomers that are mirror images). Metolachlor is composed of approximately equal proportions of the R and S enantiomers, whereas S-metolachlor is an enrichment comprised almost solely of the S enantiomer (88% S-isomer). Enantiomers have the same chemical and physical properties except for the direction they rotate plane polarized light.  The Agency has concluded that environmental behavior of metolachlor and S-metolachlor is expected to be similar.  Therefore, the environmental fate data were bridged from metolachlor to S-metolachlor (USEPA, 2002a).

Environmental fate data indicate that parent metolachlor/S-metolachlor appear to be moderately persistent to persistent.  Metolachlor/S-metolachlor degradation appears to be dependent of microbially-mediated (aerobic soil metabolism t1/2 = 13.9 to 67 days; anaerobic soil metabolism t1/2 =81 days) and abiotic processes (photodegradation in water t1/2 = 70 days under natural sunlight and photodegradation on soil t1/2 = 8 days under natural sunlight).

Depending on the soil (i.e. organic matter content), metolachlor/S-metolachlor has the potential to be mobile to moderately mobile with Koc values ranging from 21.6 to 369 mL/g oc and have been detected extensively in surface water and ground water.  The Agency concluded that there is no difference in soil sorption affinity between metolachlor and S-metolachlor as there were similar Koc coefficients in the batch equilibrium studies conducted on both active ingredients independently [mean Koc=135 (racemic) and 219 (S-enriched)] (USEPA 2002a).

Field dissipation studies indicate that metolachlor/S-metolachlor is persistent in surface soil with half-lives ranging from 3 to 292 days in the upper six inch soil layer depending on geographic location.  Metolachlor/S-metolachlor was reportedly detected as deep as 36 to 48 inches in some of the studies.  

The Metolachlor Task Force (MTF) has conducted a small scale prospective groundwater study (MRIDs 47017401-04) for metolachlor to comply with the request of groundwater monitoring data for the conditional registration of technical metolachlor.  The residue sample collection was initiated in June 2003 continuing through 939 days after application.  The objective was to determine the potential for the active ingredient metolachlor and its degradates (CGA-37735, CGA-51202, CGA-67125, CGA-41638, and CGA-354743) to reach groundwater when used in a vulnerable hydrogeologic setting.  The study results show that metolachlor and two of its degradation products CGA-51202 and CGA-354743 can leach through the vadose zone into groundwater when applied in a vulnerable hydrogeologic setting.  The detected peak concentrations of the degradation products CGA-51202 and CGA-354743 are higher than the detected peak metalochlor concentrations.

Metolachlor appears to have a low potential to bioaccumulate in fish with a reported whole body bioconcentration factor of 69X and a whole body elimination of 93% after 14 days depuration.

Total Toxic Residues (TTR) Approach
                        
Following the recent recommendation of the Residues of Concern Knowledgebase Subcommittee (D424250), parent metolachlor, CGA-51202 (metolachlor-OA), CGA-40172, and CGA-50720 will be assessed together in the drinking water assessment using a total toxic residues (TTR; USEPA 2009a) approach where half-lives were recalculated (Appendix A; PestDF; USEPA, 2009b) to collectively account for the parent and the combined residues of concern. To follow the total toxic residue (TTR) approach, half-lives were recalculated to collectively account for the parent and the combined residues of concern.  These values will be utilized in surface and ground water modeling to generate aquatic exposure estimates.  TTR half-lives for aerobic soil metabolism, aerobic aquatic metabolism, anaerobic aquatic metabolism, and aqueous photolysis, correspond to 447 days, 38.9 days, 217 days and 115.8 days, respectively.  The hydrolysis half-life was considered stable since a half-life could not be calculated due to the lack of degradation needed to fit a degradation curve.  In addition, the average Koc value of 181 mL/g o.c., was selected among parent batch equilibrium studies.  

Drinking Water Exposure Modeling

Surface Water

The Surface Water Concentration Calculator (SWCC v 1.106) model was used to generate EDWCs for the Tier II aquatic exposure assessment.  The SWCC is a graphical user interface that runs the Pesticide Root Zone Model (PRZM, v 5, November 15, 2006) and the Variable Volume Water Body Model (VVWM, 3/6/2014).  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.  The national default PCA of 0.91 was used for the proposed agricultural uses (USEPA 2012a).

The SWCC input parameters are shown in Table 3 below.  EDWCs are provided in Table 2 above, and a representative SWCC output is provided in Appendix B.  

Table 3.  SWCC Input Parameters for S-Metolachlor.  
Input Parameter:
Value:
Comment:
Source:
Scenario(s):
Corn:  KS, MS Corn
Cotton:  MS Cotton  
Cucurbit:  FL Cucumber; MI, MO, NJ Melon
Low-Growing Berry:  FL Strawberry
Lettuce (leaf):  CA Lettuce
Lettuce (head):  CA Lettuce
Fruiting Vegetables:  FL Pepper; CA, FL, PA Tomato
Sunflower:  ND Canola
representative scenarios for each use
N/A 
Maximum Single 
Application Rate
lbs a.i./A (kg a.i./ha)
Corn: 2.0 (2.25)
Cotton: 2.0 (2.25)
Cucurbit: 0.635 (0.713)
Low-Growing Berry: 1.27 (1.43)
Lettuce (leaf): 1[st] 0.95 (1.07) 2[nd] 0.32 (0.36)
Lettuce (head): 0.95 (1.07)
Fruiting Vegs: 2 app. at 1.60 (1.80) 3[rd] app at 0.5 (0.562)
Sunflower: 1[st] 1.90 (2.13) 2[nd] 1.80 (2.02)
proposed rates
proposed  label
Applications per Year
Corn: 2
Cotton: 2
Cucurbit: 2 (3 seasons)
Low-Growing Berry: 3
Lettuce (leaf):  2 (rate varies)
Lettuce (head): 2
Fruiting Vegs: 3 (rate varies)
Sunflower:  2 (rate varies)
label directions.  Label specifies rates per season.  If crops are rotated with those on which s-metolachlor is used, yearly rates may be higher.
proposed label
Application Interval (days)
Corn: 5
Cotton: 5
Cucurbit: 14
Low-Growing Berry: 14
Lettuce (leaf):  14
Lettuce (head): 14
Fruiting Vegs: 14
Sunflower: 14
intervals selected to reflect labeled application patterns
proposed label
Date of Initial Application
(relative application date)
Corn

-3
+2
Cotton

-3
+2
Cucurbit

-7
+7
+21
+35
+49
+63

Berry

-7
+7
+21

Leaf L.

-7
+7
+21
+35
+49
+63

Head L

-7
+7
+21
+35

.
Fruit.Veg.
-7
+7
+21

Sunflower

-7
+7

for corn and cotton, 3 days before crop emergence was selected for pre-emergent applications and 2 days after crop emergence was selected for post-emergent applications.  

for proposed uses, 7 days before crop emergence was selected for pre-emergent applications and 7 days after crop emergence was selected for post-emergent applications.  
Crop Scenarios
Application Method
Foliar: for pre- and post-emergent applications
label directions
proposed label
CAM Input
2
foliar  -  aerial
proposed label
IPSCND Input
3

remaining pesticide on foliage is retained as surface residue and continues to undergo decay.
US EPA, 2005
Spray Drift Fraction
0.135

foliar - aerial
Input parameter guidance (USEPA, 2013b) 
Application Efficiency
0.95

foliar - aerial

Input parameter guidance 
(USEPA, 2013b)
Molecular Mass (g/mol)
283.8
product chemistry data
US EPA, 2002b TRED
Vapor Pressure at 25°C (torr)
2.8 x 10[-5]
product chemistry data
US EPA, 2002b TRED
Henry's Law Constant
(atm-m3/mol)
2.2 x 10[-8]
calculated from volatility and solubility data
registrant submitted data
Solubility in Water at 25oC (mg/L)
480
product chemistry data
US EPA, 2002b TRED
Organic Carbon Partition Coefficient (Koc) (L/kg oc)
181
represents the average KOC
represents the average value from all non-paired adsorption data
Aerobic Soil Metabolism Half-life (days)
447
represents the upper 90th percentile of TTR (332 days, 39 days) values[1] 
US EPA, 2002b TRED
MRID 41309801
MRID 45499606
Aerobic Aquatic Metabolism Half-life (days)
38.9
TTR based on the aerobic (first 30 days) of the anaerobic aquatic metabolism study[1]
MRID 41185701

Anaerobic Aquatic Metabolism Half-life (days)
217
TTR value[1]
MRID 41185701

Hydrolysis Half-lives (days)
0  
stable
US EPA, 2002b TRED
MRID 00015662
MRID 40430201
Aqueous Photolysis
Half-life (days)
115.8
TTR value[1]
US EPA, 2002b TRED
MRID 40430202

-N/A = Not Applicable    
[1]Where the TTR approach was identified, half lives were recalculated to account for the parent including the identified residues of concern.  Draft Guidance for Modeling Pesticides Total Toxic Residues (TTR; USEPA, 2009a) in addition to the NAFTA Guidance for Evaluating and Calculating Degradation Kinetics in Environmental Media (USEPA 2009b) were utilized.  Recalculated half-lives (PestDF outputs) are listed in Appendix A.

Ground Water
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 ground water 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 SCI-GROW is listed in Table 4, below. SCI-GROW EDWCs are presented in Table 2 (above) and a representative output is provided in Appendix C.  

Table 4.  SCI-GROW Input Parameters for S-Metolachlor
Input Parameter
Value
Comment
Source
Maximum Application Rate per growing season (lbs a.i./A)
2.0 (corn, cotton)
3.7 (sunflower)
maximum rate from all registered uses
maximum rate from proposed label

EPA Reg. No. 100-816

Applications per Year
2  (corn, cotton)
1  (sunflower)
based on maximum rate and yearly total applied
EPA Reg. No. 100-816

Organic Carbon Partition Coefficient (KOC) (L/kgOC)
181

represents the average KOC value from all non-paired adsorption data
MRID 43928937
MRID 40494604

Aerobic Soil Metabolism
Half-life (days)
447
represents the upper 90th percentile of TTR (332 days, 39 days) values 
US EPA, 2002b TRED
MRID 41309801
MRID 45499606

Since the new ground water model (PRZM-GW) has been implemented (USEPA, 2012a), the highest application rate of all metolachlor/S-metolachlor uses (2 apps. at 2.0 lbs a.i/A for a yearly maximum application rate of 4.0 lb a.i./A on corn and cotton) in addition to the proposed uses for S-metolachlor were modeled to determine drinking water exposure in ground water.  PRZM-GW is a one-dimensional, finite-difference model that estimates the concentrations of pesticides in ground water.  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 ground water contamination, are available for use with PRZM-GW for risk assessment purposes.  In the PRZM-GW simulations, each of the standard scenarios was used.  PRZM-GW output values represent pesticide concentrations in a vulnerable ground water supply that is located directly beneath a rural agricultural field following many years of pesticide application.  A summary of the model input parameter values used in the PRZM-GW model is listed in Table 5.  PRZM-GW results are presented in Table 6.  A representative output of the PRZM-GW model can be found in Appendix D.

Table 5. PRZM-GW (v1.0) Input Parameter1 Values for S-Metolachlor
                                  Parameters
                                     Value
                                  Data Source
                                   Comments
Application Method
Aerial    (Corn, Cotton)
Aerial    (Sunflower)

EPA Reg. No. 100-816

                                      ---
Application Rate 
(lbs. a.i./acre) [kg/ha]
2.0 [2.25]  (Corn, Cotton)
1[st]  app.1.90 [2.13]  (Sunflower)
2[nd] app.1.80 [2.02]  (Sunflower)

EPA Reg. No. 100-816

                                      ---
Application Number
2  (Corn, Cotton)
1  (Sunflower)

EPA Reg. No. 100-816

                                      ---
Application Interval (days)
5    (Corn, Cotton)
14  (Sunflower)

EPA Reg. No. 100-816

                                      ---

Days since Emergence

Corn/Cotton:
-3
+2
Sunflower:
-7
+7
Crop Scenarios and proposed label info
For corn and cotton, 3 days before crop emergence was selected for pre-emergent applications and 2 days after crop emergence was selected for post-emergent applications.  

For proposed uses, 7 days before crop emergence was selected for pre-emergent applications and 7 days after crop emergence was selected for post-emergent applications.  
Hydrolysis Half-Life (days)
0
stable
US EPA, 2002b TRED
MRID 00015662
MRID 40430201
Aerobic Soil Metabolism Half-life (days)
447
represents the upper 90th percentile of TTR (332 days, 39 days) values 
US EPA, 2002b TRED
MRID 41309801
MRID 45499606
Koc (mL/goc)
181

represents the average KOC value from all non-paired adsorption data
MRID 43928937
MRID 40494604

1 [PRZM-GW Input Guidance]:  [http://www.epa.gov/oppefed1/models/water/przm_gw/wqtt_przm_gw_input_guidance.pdf]

Table 6. PRZM-GW Estimated Drinking Water Concentrations in Ground Water Resulting from the Use of S-Metolachlor
                        Application Rate and Frequency
                                  Scenario[1]
                                       
                          Simulation Duration (years)
                                 Highest Daily
                                 Value (ug/L)
                       Post Breakthrough Average (ug/L)
                  Average Simulation Breakthrough Time (days)
                                       
                                 Corn/Cotton:
                          2 app. X 2.0 lbs a.i./acre
Florida Citrus
                                      30
                                      843
                                      742
                                     2537

Florida Potato
                                      30
                                      182
                                      173
                                     4514

Georgia Peanut
                                      30
                                      315
                                      289
                                     4214

North Carolina Cotton
                                      30
                                     1060
                                      865
                                     3627

Delmarva Sweet Corn
                                      30
                                      835
                                      734
                                     3206

Wisconsin Corn
                                      30
                                     1050
                                      978
                                     4795
                                  Sunflower:
1[st]  app.1.90 lbs a.i./acre
2[nd] app.1.80 lbs a.i./acre
                                       
Florida Citrus
                                      30
                                      782
                                      684
                                     2537

Florida Potato
                                      30
                                      169
                                      161
                                     4514

Georgia Peanut
                                      30
                                      290
                                      267
                                     4214

North Carolina Cotton
                                      30
                                      979
                                      801
                                     3627

Delmarva Sweet Corn
                                      30
                                      769
                                      678
                                     3206

Wisconsin Corn
                                      30
                                      974
                                      906
                                     4795
1 These scenarios are regional vulnerable sites for groundwater evaluation.  

Monitoring Data

Metolachlor/S-metolachlor and its degradates ESA and OA have been detected extensively in surface water and groundwater.  A search of the available NAWQA monitoring data was performed on December 10, 2014 to ensure that the most current monitoring data were considered in this assessment.  For parent metolachlor, 23,089 detects (16.6% were classified as estimated values) were obtained within the monitoring data with a maximum concentration of 9.84 ug/L in surface water.  For ground water, 1,634 detects (33.5% were classified as estimated values) were obtained within the monitoring data with a maximum concentration of 9.54 ug/L.  

For the degradate ESA, 1,245 detects were obtained within the monitoring data with a maximum concentration of 8.42 ug/L in surface water.  For ground water, 548 detects were obtained within the monitoring data with a maximum concentration of 9.81 ug/L.  For the degradate OA, 1,122 detects were obtained within the monitoring data with a maximum concentration of 4.95 ug/L in surface water.  For ground water, 298 detects were obtained within the monitoring data with a maximum concentration of 7.92 ug/L.  In addition, the maximum metolachlor concentration from the 2012 Atrazine Ecological Monitoring Program (AEMP) was 59.09 ppb.  

Uncertainties 

For PRZM-GW modeling, the values generated are for vulnerable ground water supplies such as a rural drinking water well that exists below or is directly influenced by an agricultural field treated yearly at the maximum proposed s-metolachlor labeled rates.  The EDWCs generated were from a 30 year simulation with a 13 year breakthrough period.

Conclusions

Following the recent recommendation of the Residues of Concern Knowledgebase Subcommittee (D424250), parent metolachlor, CGA-51202 (metolachlor-OA), CGA-40172, and CGA-50720 were assessed together using a total toxic residues (TTR) approach.  Since the approach and the tolerance expression was revised since the previous drinking water assessment, the highest application rate of all metolachlor/S-metolachlor uses (2 applications at 2.0 lb a.i./A for a maximum yearly application rate of 4.0 lbs a.i/A on corn and cotton) was modeled in addition to the proposed uses to determine drinking water exposure in surface and ground water.
The recommended EDWCs for surface water include the acute concentration of 371 ug/L (corn; registered), the annual mean (chronic) concentration of 43.70 ug/L (lettuce; proposed) and the 30 year annual average concentration of 21.80 ug/L (lettuce; proposed). 

For ground water, the recommended EDWCs include the acute concentration of 1060 ug/L and 30-year average (chronic) concentration of 978 ug/L, based on the highest application rate (yearly maximum rate of 4.0 lbs a.i/A on corn, cotton) of all metolachlor/s-metolachlor uses.  Extensive monitoring data was available for metolachlor and its degradation products ESA and OA.

References

USEPA. 1995. Reregistration Eligibility Decision (RED). Data Package Barcode: 185152, 
	172022 Metolachlor. April 1995.

USEPA. 2002a. Review of Documents Related to the Equivalency of Racemic Metolachlor 
	(Metolachlor) and S-Metolachlor for Environmental Fate and Ecotoxicity. Environmental
	 Fate and Effects Division, Office of Pesticide Programs. February 28, 2002.

US EPA.  2002b.  Revised Metolachlor/S-Metolachlor Tolerance Reassessment Document 
	(TRED), D248805, D228814, D258817, D258824, D258822, D258812, D259119, 
	D259037, D259034).

USEPA.  2005.  PRZM-3, A Model for Predicting Pesticide and Nitrogen Fate in the Crop Root and Unsaturated Soil Zones:  User's Manual for Release 3.0.  May 2005.

USEPA.  2009a.  DRAFT Guidance for Modeling Pesticides Total Toxic Residues (TTR) May 
      20, 2009.   U.S. Environmental Protection Agency, Office of Pesticide Programs, Environmental Fate and Effects Division.  Arlington, VA.
      
USEPA, 2009b.  DRAFT NAFTA Guidance for Evaluating and Calculating Degradation 
      Kinetics in Environmental Media, U.S. Environmental Protection Agency, Office of Pesticide Programs, Environmental Fate and Effects Division.  Arlington, VA.

USEPA.  2010. Guidance for Selecting Input Parameters in Modeling the Environmental Fate and Transport of Pesticides http://www.epa.gov/oppefed1/models/water/input_parameter_guidance.htm
 
 US EPA. 2012a. 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.
 
USEPA. 2012b. Guidance on Development and Use of Percent Cropped Area Adjustment Factors in Drinking Water Exposure Assessments.  Environmental Fate and Effects Division, Office of Pesticide Programs, United States Environmental Protection Agency. Available at http://www.epa.gov/oppefed1/models/water/pca_adjustment_dwa.pdf.

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. 

                          APPENDIX A:  PestDF Outputs

Aerobic Soil Metabolism (322 days and 39 days; 90[th] percentile = 447 days)

MRID 41309801

MRID 45499606

Aqueous Photolysis (115.7 days)

MRID 40430202

Aerobic Aquatic Metabolism (38.9 days)

MRID 41185701

                                       
                                       
Anaerobic Aquatic Metabolism (217 days)

MRID 41185701

                                       
                                       
                                       
                                       
                                       
                                       
                                       
                                       
                                       
                                       
                                       
                                       
                                       
                                       
                                       
                                       
                                       
                                       
                                       
                                       
                                       
                                       
                                  APPENDIX B

SWCC Output:  Mississippi Corn  -  Aerial Application

                                       
                                       
                                       
                                       
                                       
                                       
                                  APPENDIX C

SCI-GROW Model Output:  Corn/Cotton and Sunflower

   1.) Corn/Cotton (maximum rate)

 SciGrow version 2.3
 chemical:S-Metolachlor use on Corn and Cotton
 time is 12/11/2014  15:27:32
 ------------------------------------------------------------------------
  Application      Number of       Total Use    Koc      Soil Aerobic
  rate (lb/acre)  applications   (lb/acre/yr)  (ml/g)   metabolism (days)
 ------------------------------------------------------------------------
      2.000           2.0           4.000      1.81E+02      447.4
 ------------------------------------------------------------------------
 groundwater screening cond (ppb) =   1.43E+01 or 14.3 ppb
 ************************************************************************
                                       
                                        

   2.) Sunflower (representing various crops)

SciGrow version 2.3
 chemical:S-Metolachlor use on Sunflower
 time is 12/ 5/2014  14:13: 7
 ------------------------------------------------------------------------
  Application      Number of       Total Use    Koc      Soil Aerobic
  rate (lb/acre)  applications   (lb/acre/yr)  (ml/g)   metabolism (days)
 ------------------------------------------------------------------------
      3.700           1.0           3.700      1.81E+02      447.4
 ------------------------------------------------------------------------
 groundwater screening cond (ppb) =   1.32E+01 or 13.2 ppb
 ************************************************************************

                                       
                                       
                                       
                                       
                                       
                                       
                                       
                                       
                                       

                                  APPENDIX D

PRZM-GW Output:  North Carolina Cotton Scenario (Corn/Cotton)