1

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 (CGA354743) and metolachlor OA (CGA-51202). Following the recent recommendation of the 

Residues of Concern Knowledgebase Subcommittee (D424250), parent metolachlor, CGA51202 (metolachlor-OA), CGA-40172, and CGA-50720 will be assessed together in the drinking 
2

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 

(μg/L)

1-in-10 year 

chronic

(μg/L)

30- year average 

cancer chronic 

(μg/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 μg/L for 

surface water [77.60 μg/L (S-metolachlor) + 47.61 μg/L (degradate ESA) + 93.57 μg/L

(degradate OA)] and the tolerance expression for chronic dietary assessment is 126 μg/L for 

ground water [32.8 μg/L (S-metolachlor) + 63.7 μg/L (degradate ESA) + 29.7 μg/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 μg/L and 30-

year average (chronic) concentration of 978 μg/L.
3

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

(μg/L)

Chronic

(μg/L)

Cancer 

Chronic

(μg/L)

Surface Water

(SWCC)

Corn1 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

Cotton1 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
4

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

(μg/L)

Chronic

(μg/L)

Cancer 

Chronic

(μg/L)

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 Cotton1 NC Cotton 

(PRZM-GW)

2 app. x 2.0 lbs ai/acre

(5 days)

1060 NA 865

Corn1 WI Corn 

(PRZM-GW)

2 app. x 2.0 lbs ai/acre

(5 days)

-- NA 978

Proposed Use2

(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 Use2

(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, Cotton1,3 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).
5

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 halflives 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. 
6

Drinking Water Exposure Modeling

Surface Water

The Surface Water Concentration Calculator (SWCC v 1.106) model1 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) 2nd 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) 2nd 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

 

1

http://www.epa.gov/oppefed1/models/water /index.htm
7

Table 3. SWCC Input Parameters for S-Metolachlor. 

Input Parameter: Value: Comment: Source:

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 10-5product chemistry data US EPA, 2002b TRED

Henry’s Law Constant

(atm-m3/mol)

2.2 x 10-8calculated 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) values1

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 study1

MRID 41185701

Anaerobic Aquatic 

Metabolism Half-life (days)

217 TTR value1 MRID 41185701

Hydrolysis Half-lives (days) 0 stable US EPA, 2002b TRED

MRID 00015662

MRID 40430201

Aqueous Photolysis

Half-life (days)

115.8 TTR value1 US EPA, 2002b TRED

MRID 40430202
8

Table 3. SWCC Input Parameters for S-Metolachlor. 

Input Parameter: Value: Comment: Source:

-N/A = Not Applicable 

1Where 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 nonpaired adsorption dataMRID 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
9

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 nonpaired 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
10

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 μg/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 μg/L. 

For the degradate ESA, 1,245 detects were obtained within the monitoring data with a maximum 

concentration of 8.42 μg/L in surface water. For ground water, 548 detects were obtained within 

the monitoring data with a maximum concentration of 9.81 μg/L. For the degradate OA, 1,122 

detects were obtained within the monitoring data with a maximum concentration of 4.95 μg/L in 

surface water. For ground water, 298 detects were obtained within the monitoring data with a 

maximum concentration of 7.92 μg/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 

Table 6. PRZM-GW Estimated Drinking Water Concentrations in Ground Water Resulting from 

the Use of S-Metolachlor

Application Rate and 

Frequency Scenario1 Simulation 

Duration 

(years)

Highest Daily

Value (μg/L)

Post 

Breakthrough 

Average 

(μg/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. 
11

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 μg/L (corn; 

registered), the annual mean (chronic) concentration of 43.70 μg/L (lettuce; proposed) and the 30 

year annual average concentration of 21.80 μg/L (lettuce; proposed).

For ground water, the recommended EDWCs include the acute concentration of 1060 μg/L and 

30-year average (chronic) concentration of 978 μg/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.
12

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; 90th percentile = 447 days)

MRID 41309801

MRID 45499606
13

Aqueous Photolysis (115.7 days)

MRID 40430202

Aerobic Aquatic Metabolism (38.9 days)

MRID 41185701
14

Anaerobic Aquatic Metabolism (217 days)

MRID 41185701
15

APPENDIX B

SWCC Output: Mississippi Corn – Aerial Application
16

APPENDIX C

SCI-GROW Model Output: Corn/Cotton and Sunflower
17

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)
18