UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON D.C., 20460 

OFFICE OF 

CHEMICAL SAFETY AND 

POLLUTION PREVENTION 

DP Barcode: D424724 

PC Code: 080807 

Date: April 13, 2016 

MEMORANDUM 

Subject: 

To: 

From: 

Preliminary Ecological Risk Assessment for Simazine 

Steven Snyderman, Chemical Review Manager 

Kelly Sherman, Branch Chief 

Risk Management and Implementation Branch 3 

Pesticide Re-evaluation Division (7508P) 

~ Frank T. Farruggia, :h.D., Biologist . . . 7/~~fl 

Colleen M. Rossme1sl, D.V.M., M.S., B1olog1st 8 , ,t.__-. 

James A. Hetrick, Ph.D., Senior Science Advis ..v? • ':fi · 

Melanie Biscoe, M.E.M., Physical Scientist ;l{J~t?r. :.o ./I .(} 

Rosanna 

Dana Spatz, 

Louie-Juzwiak, 

M.S., Branch 

Risk 

Ch~~~ 

Assessment Process Leader ~~ ~ 

Environmental 

Environmental Fate 

Risk Branch 

and Effects 

Ill 

Division (7507P) 

~" ~ 

Attached is the preliminary ecological risk assessment conducted by the Environmental Fate 

and Effects Division for the Registration Review of Simazine. 
DRAFT – INTERNAL DELIBERATIVE – DO NOT CITE OR DISTRIBUTE

PRELIMINARY ECOLOGICAL 

RISK ASSESSMENT FOR 

SIMAZINE 

Simazine is used as an herbicide in the U.S. to control annual broadleaf and grass

weeds. It is one of the most widely used herbicides in North America. This

preliminary risk assessment evaluates the risks of all registered simazine uses to

non‐target species of animals and plants in terrestrial and aquatic environments.

Simazine is mobile and persistent in the environment; the main routes of

dissipation are microbial degradation under aerobic conditions, runoff, and

leaching. Because of its persistence and mobility, simazine has the propensity to

move into surface and groundwater. This is confirmed by the widespread

detections of simazine in surface water and groundwater. Based on available

toxicity data for birds and mammals, the primary degradates of concern for

simazine (deisopropylatrazine (DIA), hydroxysimazine (HS), and

diadealkylatrazine (DDA)) are generally of equal toxicity or slightly more toxic

than simazine. When estimates of simazine exposure in terrestrial environments

are compared to the available ecotoxicity data, the results indicate potential risk

to birds, mammals and plants (see Section 14). Risk to birds and mammals is

primarily through chronic exposure, although some Levels of Concern (LOCs) are

exceeded for acute risk to birds under scenarios involving higher use rates.

Levels of concern for terrestrial plants are exceeded following spray drift and/or

runoff after applications according to all currently labeled maximum application

rates, as well as rates as low as 0.5 lb a.i./A (a hypothetical reduced rate). Using

the new pollinator guidance to estimate terrestrial invertebrate risk, an RQ value

for acute contact toxicity in the honey bee was calculated as 0.11; less than the

LOC of 0.4 for acute exposure. However, risk to pollinators (e.g., honey bees) is

an uncertainty due to the lack of data available in order to complete the Tier 1

risk assessment (Table 28). Based on available toxicity data for aquatic

organisms, including fish, aquatic invertebrates, aquatic phase amphibians, and

aquatic plants, the chlorotriazine degradation product, deisopropyltriazine (DIA),

is not more toxic than simazine, and some of the reported toxicity levels

exceeded the maximum solubility of the compound. For these reasons, aquatic

exposure modeling was based on simazine only. Risks to fish, aquatic phase

amphibians and aquatic invertebrates were identified from chronic exposure to

simazine, although some LOCs were also exceeded for acute risk to listed species

for freshwater invertebrates. RQs exceeded the LOCs for non‐vascular plants for

almost all predicted estimated exposure concentrations (EECs) and exceeded

the LOC for vascular plants for a number of predicted EECs.

Frank T. Farruggia, Ph.D.

Colleen M. Rossmeisl, D.V.M., M.S.

James A. Hetrick, Ph.D.

Melanie Biscoe, M.E.M.

Environmental Risk Branch III

Environmental Fate and Effects Division

Office of Pesticide Programs

U.S. Environmental Protection Agency

April 13, 2016
2

TABLE OF CONTENTS

EXECUTIVE SUMMARY............................................................................................ 11

Nature of Chemical Stressor............................................................................ 11

Environmental Exposure Assessment.............................................................. 12

Risk to Terrestrial Animals and Plants............................................................. 13

Birds, Mammals, Reptiles and Terrestrial Phase Amphibians..................... 13

Terrestrial Invertebrates.............................................................................. 15

Terrestrial Plants.......................................................................................... 15

Risk to Aquatic Animals, Plants and Plant Communities.................................. 16

Fish ............................................................................................................... 16

Aquatic Phase Amphibians .......................................................................... 16

Aquatic Invertebrates.................................................................................. 17

Aquatic Plants .............................................................................................. 17

Aquatic Plant Communities ......................................................................... 17

INTRODUCTION .................................................................................................... 18

BACKGROUND ...................................................................................................... 19

Endangered Species Assessments ............................................................... 20

HERBICIDAL MECHANISM OF ACTION................................................................... 21

Overview of Pesticide Use and Usage ................................................................... 21

Formulations................................................................................................... 28

Application Methods and Timing .................................................................... 28

Non‐Agricultural Use Sites............................................................................... 29

Non‐Agricultural Usage ................................................................................... 29

Agricultural Usage........................................................................................... 29

Screening Level Usage Analysis Data........................................................... 29

National Mapping of Use Data..................................................................... 31

ANALYSIS PLAN..................................................................................................... 33

Risk Hypothesis............................................................................................... 33

Conceptual Model........................................................................................... 33

Measures of Exposure..................................................................................... 33

Measures of Effect .......................................................................................... 35

Integration of Exposure and Effects................................................................. 36

ENVIRONMENTAL FATE AND TRANSPORT............................................................. 36

Physical and Chemical Properties of Simazine................................................. 36

Physical and Chemical Properties................................................................ 36

Environmental Fate Summary ..................................................................... 37

Degradation Products.................................................................................. 40

AQUATIC EXPOSURE ASSESSMENT........................................................................ 43

Aquatic Exposure Modeling............................................................................. 43

Tier II Exposure Assessment using Surface Water Concentration Calculator43

Aquatic Exposure Modeling for the Cranberry Use..................................... 51

Aquatic Exposure Monitoring.......................................................................... 52

Surface Water Monitoring Data Evaluated.................................................. 52
3

Monitoring Data Analysis............................................................................. 53

Characterization of Monitoring Data........................................................... 60

Uncertainties in Aquatic Exposure Modeling and Monitoring ......................... 62

Stressors of Concern ............................................................................................. 63

Evaluation of Simazine Toxicity to Specific Taxa.................................................... 64

Toxicity to Plants............................................................................................. 65

Toxicity to Terrestrial Plants........................................................................ 65

Toxicity to Aquatic Non‐Vascular Plants...................................................... 66

Toxicity to Aquatic Vascular Plants.............................................................. 67

Toxicity to Aquatic Plant Communities........................................................ 68

Effects to Animals................................................................................................. 68

Toxicity to Terrestrial Animals......................................................................... 68

Birds, Reptiles and Terrestrial Phase Amphibians....................................... 69

Toxicity to Mammals.................................................................................... 69

Toxicity to Terrestrial Invertebrates............................................................ 72

Toxicity to Aquatic Animals............................................................................. 73

Toxicity to Fish ............................................................................................. 77

Toxicity to Aquatic Invertebrates: ............................................................... 77

Toxicity to Amphibians (aquatic‐phase and terrestrial) .............................. 78

Simazine Degradates....................................................................................... 78

Incident Data ........................................................................................................ 79

Terrestrial Incidents..................................................................................... 79

Plant Incidents ............................................................................................. 79

Aquatic Incidents ......................................................................................... 79

METHODOLOGY FOR DETERMINING THE LEVELS OF CONCERN FOR Simazine....... 80

The Risk Quotient Method and Levels of Concern for Terrestrial Plants and

Terrestrial and Aquatic Animals. ..................................................................... 80

TERRESTRIAL RISK CHARACTERIZATION AND CONCLUSIONS................................. 82

Terrestrial Animals Exposure and Risk Quotients (RQ) Values......................... 82

Terrestrial Exposure to Animals................................................................... 82

Risk Quotient (RQ) Values for Terrestrial Animal Species........................... 89

Risks to Birds.............................................................................................. 101

Risks to Mammals...................................................................................... 103

Off‐Field Risk to Birds and Mammals......................................................... 105

Risk to Reptiles and Terrestrial‐phase Amphibians................................... 109

Risk to Terrestrial Invertebrates................................................................ 110

Risks to Terrestrial Plants.............................................................................. 110

Runoff and Spray Drift Exposure to Terrestrial and Semi‐Aquatic Plants. 110

Risk Quotient (RQ) Values for Terrestrial Plant Species............................ 111

Off‐Field Risk to Terrestrial Plants............................................................. 113

Risks to Terrestrial Plant Communities...................................................... 119

Aquatic Animal Risk Characterization and Conclusions....................................... 119
4

Aquatic Risk Characterization and Conclusions ............................................. 119

Risks to Fish................................................................................................ 135

Risks to Aquatic Invertebrates................................................................... 135

Risk to Aquatic‐phase amphibians............................................................. 136

Risk to Non‐Vascular and Vascular Aquatic Plants.................................... 136

Risks to Aquatic Plant Communities.......................................................... 136

Description of uncertainties, limitations and assumptions.................................. 137

Impact of simazine on chemical mixtures in the environment....................... 137

Effects uncertainties – general ...................................................................... 139

Endangered Species............................................................................................ 139

Endocrine Disruptor Screening Program: ............................................................ 140

References.......................................................................................................... 142
5

TABLE OF TABLES

Table 1.  Maximum Application Rates, Maximum Applications, Minimum Application Intervals, and

Application Methods for Section 3 Simazine Labels. ......................................................................... 22

Table 2.  Maximum Application Rates, Maximum Applications, Minimum Application Intervals, and

Application Methods for Section 24(c) Simazine Labels. ................................................................... 27

Table 3. Screening‐Level Estimates of Agricultural Uses of Simazine (2004‐2013) (USEPA, 2015)............30

Table 4. Structure and Physical and Chemical Properties of Simazine....................................................... 37

Table 5. Environmental Fate Parameters for Simazine............................................................................... 38

Table 6. Chemical Names for Simazine Degradation Products................................................................... 41

Table 7. Identification of Simazine Degradation Products in Environmental Fate Studies........................41

Table 8.  Soil Sorption Coefficients for Simazine Degradation Products.................................................... 42

Table 9. Spray Drift Fractions for Required Spray Drift Buffers on Simazine Labels ..................................43

Table 10. Criteria for Surrogate Model Scenarios in SWCC Modeling for Simazine...................................44

Table 11. Application Inputs for SWCC Modeling....................................................................................... 44

Table 12. Fate Inputs for SWCC Modeling1 ................................................................................................. 47

Table 13. SWCC EECs for Simazine in the Standard Pond........................................................................... 48

Table 14. EECs for the Simazine Cranberry Use.......................................................................................... 52

Table 15. Characteristics of Representative Monitoring Programs for Simazine and Its Degradation

Products in Surface Water ................................................................................................................. 53

Table 16. Descriptive Statistics of Simazine Concentrations in Ambient Surface Water Monitoring

Programs............................................................................................................................................54

Table 17. Distribution of Maximum Simazine Concentrations in Ambient Surface Water from Site‐Years

from Different States ......................................................................................................................... 55

Table 18. Distribution of Maximum 21‐day Simazine Concentrations in Ambient Surface Water from Site‐

Years from Different States................................................................................................................ 57

Table 19. Distribution of Maximum 60‐day Simazine Concentrations in Ambient Surface Water from Site‐

Years from Different States................................................................................................................ 58

Table 20. Distribution of Median Sampling Intervals from 27,717 Site‐Years of Ambient Surface Water

Monitoring..........................................................................................................................................60

Table 21. Linear Regression Equations for BF Estimation from a Stratified Random Sampling Design .....61

Table 22. Bias Factors for Application to Simazine Monitoring Data from Ambient Surface Water..........62

Table 23. Nontarget Terrestrial Plant Seedling Emergence and Vegetative Vigor Toxicity Endpoints (Tier

II) for Simazine....................................................................................................................................65

Table 24. Most Sensitive Data for Non‐vascular Aquatic Plant Toxicity of Simazine .................................67

Table 25. Most Sensitive Data for Vascular Aquatic Plant Toxicity of Simazine......................................... 68

Table 26. Summary of the Most Sensitive Endpoints from Terrestrial Animal Toxicity Studies for Simazine

............................................................................................................................................................68

Table 27. Summary of the Most Sensitive Reproductive and Developmental Mammalian Endpoints for

Simazine .............................................................................................................................................70

Table 28. Potential Pollinator Data Needs.................................................................................................. 73

Table 29. Summary of the Most Sensitive Endpoints from Aquatic Toxicity Studies for Simazine ............74

Table 30. Risk Presumptions and LOCs....................................................................................................... 81

Table 31.  Input Parameters for Deriving Terrestrial EECs for Simazine (T‐REX v. 1.5.2). ..........................82
6

Table 32.  Dose‐based EECs (mg/kg‐bw) as Food Residues for Birds, Reptiles, and Terrestrial‐Phase

Amphibians from Labeled Uses of Simazine (T‐REX v. 1.5.2, upper‐bound Kenaga).........................84

Table 33. Dose‐based EECs (mg/kg bw) as Food Residues for Mammals from Labeled Uses of Simazine (T‐

REX v. 1.5.2, upper‐bound Kenaga).................................................................................................... 86

Table 34.  Dietary‐based EECs (mg/kg diet) as Food Residues for Birds, Reptiles, Terrestrial‐phase

Amphibians, and Mammals from Labeled Uses of Simazine (T‐REX v. 1.5.2, upper‐bound Kenaga).

............................................................................................................................................................88

Table 35.  Acute Dose‐based RQ values for Birds, Reptiles, and Terrestrial‐Phase Amphibians from

Labeled Uses of Simazine (T‐REX v. 1.5.2, upper‐bound Kenaga). Shaded cells identify LOC

exceedances for listed species and bolded values indicate non‐listed LOC exceedances.................90

Table 36.  Acute Dietary‐based RQ values for Birds, Reptiles, and Terrestrial‐Phase Amphibians from

Labeled Uses of Simazine (T‐REX v. 1.5.2, upper‐bound Kenaga). Shaded cells identify LOC

exceedances for listed species and bolded values indicate non‐listed LOC exceedances.................92

Table 37. Chronic Dietary‐based RQ values for Birds, Reptiles, and Terrestrial‐Phase Amphibians from

Labeled Uses of Simazine (T‐REX v. 1.5.2, upper‐bound Kenaga). Shaded cells identify LOC

exceedances. ......................................................................................................................................94

Table 38. Acute Dose‐based RQ values for Mammals from Labeled Uses of Simazine (T‐REX v. 1.5.2,

upper‐bound Kenaga). Shaded cells identify LOC exceedances for listed species and bolded values

indicate non‐listed LOC exceedances................................................................................................. 96

Table 39. Chronic Dose‐based RQ values for Mammals from Labeled Uses of Simazine (T‐REX v. 1.5.2,

upper‐bound Kenaga). Shaded cells identify LOC exceedances for listed species and bolded values

indicate non‐listed LOC exceedances................................................................................................. 98

Table 40.  Chronic Dietary‐Based RQs for Mammals of Different Feeding Classes (T‐REX v. 1.5, upper‐

bound Kenega). Shaded cells identify LOC exceedances for listed species and bolded values indicate

non‐listed LOC exceedances. ........................................................................................................... 100

Table 41. Range of RQs for Birds of Different Feeding Classes (T‐REX v. 1.5.2, upper‐bound Kenaga)1

. .102

Table 42. Range of RQs for Mammals of Different Feeding Classes (T‐REX v. 1.5.2, upper‐bound Kenaga)1

.

..........................................................................................................................................................104

Table 43.  EECs for Terrestrial and Semi‐Aquatic Plants Near Simazine Use Areas (TerrPlant v. 1.2.2)...110

Table 44. RQs for Terrestrial and Semi‐Aquatic Plants Near Simazine Use Areas (TerrPlant v. 1.2.2).....112

Table 45: EECs and RQs for Simazine Uses except Corn and Cranberry.  Shaded cells identify LOC

exceedances for listed species and bolded values indicate non‐listed LOC exceedances...............121

Table 46. EECs and RQs for Simazine from Corn Uses.  Maximum and minimum estimates of water

concentrations, and RQs are provided.  Shaded cells identify LOC exceedances for listed species

and bolded values indicate non‐listed LOC exceedances.  There were a total of 17 scenarios run for

SWCC corn modeling........................................................................................................................ 126

Table 47. EECs and RQs for the Simazine Cranberry Uses........................................................................ 127

Table 48. State Summary of the number of Site‐Years that detected simazine concentrations greater

than LOCs for aquatic organisms. Data restricted to site‐years with 4 or more samples. ..............128

Table 49. Monitoring Program Summary of the number of Site‐Years that detected simazine

concentrations greater than LOCs for aquatic organisms.  Data restricted to only site‐years with 4

or more samples............................................................................................................................... 130

Table 50. State Summary of the number of Site‐Years that detected simazine concentrations greater

than LOCs for aquatic organisms.  Data restricted to only site‐years with 12 or more samples.....131
7

Table 51. Monitoring Program Summary of the number of Site‐Years that detected simazine

concentrations greater than LOCs for aquatic organisms. Data restricted to only site‐years with 12

or more samples............................................................................................................................... 134

Table 52. The most common unique mixtures of pesticides and degradates found in stream waters with

agricultural watersheds (USGS, 2006).............................................................................................. 137

   
8

TABLE OF FIGURES

Figure 1. Chemical Structures of the Chlorotriazines and their Chlorotriazine Degradation

Products. ............................................................................................................................... 12

Figure 2. Chemical Structures of the Chlorotriazines and their Chlorotriazine Degradation

Products. ............................................................................................................................... 19

Figure 3. Estimated Agricultural Use of Simazine in The United States, 2009‐2012.................... 32

Figure 4.  Structure of Major Simazine Degradation Products and Minor Chlorodegradate DDA

............................................................................................................................................... 40

Figure 5. Spray drift deposition curves for various droplet spectra following a single aerial

application of 1.6 lb a.i/A. Horizontal lines represent the most sensitive non‐listed species

mammals and birds............................................................................................................. 106

Figure 6. Spray drift deposition curves for various droplet spectra following a single ground

application of 2 lb a.i/A. Horizontal lines represent the most sensitive non‐listed species of

mammals and birds............................................................................................................. 107

Figure 7. Spray drift deposition curves for various droplet spectra following a single ground

application of 4 lb a.i/A. Horizontal lines represent the most sensitive non‐listed species of

mammals and birds............................................................................................................. 108

Figure 8. Spray drift deposition curves for various droplet spectra following a single ground

application of 0.5 lb a.i/A. Horizontal lines represent the most sensitive non‐listed species

mammals and birds............................................................................................................. 109

Figure 9. Spray drift deposition curves for various droplet spectra following a single ground

application of 2 lb a.i/A. Horizontal dotted lines represent the most sensitive non‐listed

species endpoint from tested monocot and dicot species. Horizontal dashed lines

represent the most sensitive listed species endpoint from tested monocot and dicot

species................................................................................................................................. 115

Figure 10. Spray drift deposition curves for various droplet spectra following a single ground

application of 4 lb a.i/A.  Horizontal dotted lines represent the most sensitive non‐listed

species endpoint from tested monocot and dicot species. Horizontal dashed lines

represent the most sensitive listed species endpoint from tested monocot and dicot

species................................................................................................................................. 116

Figure 11. Spray drift deposition curves for various droplet spectra following a single ground

application of 0.5 lb a.i/A.  Horizontal dotted lines represent the most sensitive non‐listed

species endpoint from tested monocot and dicot species. Horizontal dashed lines

represent the most sensitive listed species endpoint from tested monocot and dicot

species................................................................................................................................. 117
9

Figure 12. Spray drift deposition curves for various droplet spectra following a single aerial

application of 1.6 lbs a.i/A.  Horizontal dotted lines represent the most sensitive non‐listed

species endpoint from tested monocot and dicot species. Horizontal dashed lines

represent the most sensitive listed species endpoint from tested monocot and dicot

species. Horizontal dashed –dotted lines represent the least sensitive definitive IC25

endpoints from vegetative vigor and seedling emergence studies on simazine. .............. 118

TABLE OF APPENDICES

Appendix A. ECOTOX Simazine Refresh September 2014

Appendix B. Simazine Master Monitoring Data File

Appendix C:

C.1. AEEMP Watershed Properties

C.2. AEEMP Bias Factors Stats

C.3. AEEMP Bias Factor Regressions

C.4. AMP Flowing Regressions

C.5. AMP Static Regressions

C.6. AMP Bias Factor Data

C.7. Heidleberg Regressions

C.8. Heidleberg Bias Factor Summary

C.9. Crystal Ball Bias Factor Calculator

   
10

ABSTRACT

Simazine is used as an herbicide in the U.S. to control annual broadleaf and grass weeds. It is one

of the most widely used herbicides in North America. This preliminary risk assessment evaluates

the risks of all registered simazine uses to non‐target species of animals and plants in terrestrial

and aquatic environments. Simazine is mobile and persistent in the environment; the main

routes of dissipation are microbial degradation under aerobic conditions, runoff, and leaching.

Because of its persistence and mobility, simazine has the propensity to move into surface and

groundwater. This is confirmed by the widespread detections of simazine in surface water and

groundwater. Based on available toxicity data for birds and mammals, the primary degradates of

concern for simazine (deisopropylatrazine (DIA), hydroxysimazine (HS), and diadealkylatrazine

(DDA)) are generally of equal toxicity or slightly more toxic than simazine. When estimates of

simazine exposure in terrestrial environments are compared to the available ecotoxicity data,

the results indicate potential risk to birds, mammals and plants (see Section 14). Risk to birds and

mammals is primarily through chronic exposure, although some Levels of Concern (LOCs) are

exceeded for acute risk to birds under scenarios involving higher use rates. Levels of concern for

terrestrial plants are exceeded following spray drift and/or runoff after applications according to

all currently labeled maximum application rates, as well as rates as low as 0.5 lb a.i./A (a

hypothetical reduced rate). Using the new pollinator guidance to estimate terrestrial

invertebrate risk, an RQ value for acute contact toxicity in the honey bee was calculated as 0.11;

less than the LOC of 0.4 for acute exposure. However, risk to pollinators (e.g., honey bees) is an

uncertainty due to the lack of data available in order to complete the Tier 1 risk assessment

(Table 28). Based on available toxicity data for aquatic organisms, including fish, aquatic

invertebrates, aquatic phase amphibians, and aquatic plants, the chlorotriazine degradation

product, deisopropyltriazine (DIA), is not more toxic than simazine, and some of the reported

toxicity levels exceeded the maximum solubility of the compound. For these reasons, aquatic

exposure modeling was based on simazine only. Risks to fish, aquatic phase amphibians and

aquatic invertebrates were identified from chronic exposure to simazine, although some LOCs

were also exceeded for acute risk to listed species for freshwater invertebrates. RQs exceeded

the LOCs for non‐vascular plants for almost all predicted estimated exposure concentrations

(EECs) and exceeded the LOC for vascular plants for a number of predicted EECs.
11

EXECUTIVE SUMMARY   

Nature of Chemical Stressor

Simazine is used as an herbicide in the U.S. to control annual broadleaf and grass weeds on

alfalfa, almond, apple, avocado, blueberry, broccoli, Brussels sprouts, cabbage, caneberries,

cherry, citrus, corn (field, sweet), cranberry, filbert (hazelnut), grapefruit, grapes, kale, kohlrabi,

lemon, macadamia nut (bushnut), nectarine, olive, orange, peach, pear, pecan, persimmon,

plum, strawberry, and walnut (English/black).  Simazine is also registered for use on nursery

stock (ornamental plants, ornamental woody shrubs and vines, ornamental trees), forestry

(black walnut, oak, loblolly pine, slash pine, white pine), christmas tree plantations, shelterbelt

plantings, turf (golf course, ornamental, sod farms, residential lawns), ornamental ponds and

aquaria, uncultivated agricultural areas, farm buildings, and other non‐agricultural areas. It is

one of the most widely used herbicides in North America. This preliminary risk assessment

evaluates the risks of all registered simazine uses to non‐target species of animals and plants in

terrestrial and aquatic environments.  

Three chlorotriazine herbicides have current registrations in the United States: propazine (6‐

chloro‐N,N′‐bis(1‐methylethyl)‐1,3,5‐triazine‐2,4‐diamine), PC code 080808; atrazine (6‐chloro‐

N‐ethyl‐N′‐(1‐methylethyl)‐1,3,5‐triazine‐2,4‐diamine), PC code 080803; and simazine (6‐chloro‐

N,N′‐diethyl‐1,3,5‐triazine‐2,4‐diamine), PC code 080807. These three chemicals differ only in

number and/or position of methyl groups on moieties branched in identical positions off the

triazine ring, thus are structurally nearly identical (Figure 1). Based upon the available guideline

and non‐guideline toxicity studies, these chlorotriazines result in similar herbicidal effects to

terrestrial and aquatic plants. These molecules bind with a protein complex of the photosystem

II in chloroplast photosynthetic membranes (Schulz et al., 1990). The result is an inhibition in

the transfer of electrons through the light reactions of photosynthesis that in turn inhibits the

formation and release of oxygen, production of adenosine triphosphate, and the fixation of

carbon dioxide into sugars.

In terrestrial animals, the guideline studies for all three chlorotriazines have shown similar

effects at similar doses and concentrations (USEPA 2015, USEPA 2016). These effects include

food avoidance and/or reductions in frank weights or weight gain, although post‐exposure

recovery is frequently noted. The similarities of effects in mammalian studies have led OPP’s

Health Effects Division to consider atrazine, simazine, propazine, and their chlorotriazine

degradates in a cumulative human health risk assessment. Unlike atrazine, propazine and

simazine do not have the wealth of ecological effects data (e.g., mesocosms, amphibians,

academic research) that have informed the atrazine registration review ecological risk

assessment.  The ecological risk assessment presented in this document is for the active

ingredient simazine and its degradates only.   
12

Figure 1. Chemical Structures of the Chlorotriazines and their Chlorotriazine Degradation

Products.

Environmental Exposure Assessment

Simazine is mobile and persistent in the environment; the main routes of dissipation are

microbial degradation under aerobic conditions, runoff, and leaching. Because of its persistence

and mobility, simazine has the propensity to move into surface and groundwater. This is

confirmed by the widespread detections of simazine in surface water and groundwater.

The major degradates of concern for ecological risk of simazine are deisopropylatrazine (DIA),

diadealkylatrazine (DDA), and hydroxysimazine (HS). The degradate diadealkylatrazine is often

commonly referred to as DACT. Scientific open literature indicates that simazine and these

degradates have similar toxicity to terrestrial animals and that these effects are manifested at

low exposure concentrations.  However, in the aquatic environment, hazard from parent

simazine is the greater concern.  

For terrestrial exposure, this assessment considered dietary exposure to simazine and its

degradates to animals on‐field as well as off‐field as a result of spray drift. Simazine alone was

evaluated for potential risks to terrestrial plants through off‐field exposure from drift and

runoff. Maximum labeled rates according to the currently registered labels were assessed for all

uses.   

The aquatic exposure assessment (see Section 8) includes standard ecological modeling (i.e.,

Surface Water Concentration Calculator [SWCC] and modified Tier 1 Rice Model) and surface
13

water monitoring data. Although each of the exposure assessment methods have limitations in

estimating environmental exposure concentrations (EECs) of simazine, the combination of the

modeling approaches and monitoring data, taken together, provides a comprehensive

depiction of simazine occurrence in surface water at various spatial scales.  

To characterize the geospatial extent of simazine in surface water, surface water monitoring

data were evaluated (see Section 8.2). This data set includes over 20 years of ambient surface

monitoring data for simazine. Because the various programs monitoring for simazine were

designed with different objectives, it is generally difficult to directly evaluate simazine

occurrence data in the context of exact simazine use rates, application timing, and source of

simazine for a specific monitoring site. In order to characterize the uncertainty in simazine

occurrence data due to sampling frequency, bias factors were estimated based on regression

analyses developed for the ecological risk assessment in support of atrazine registration review.

The bias factors normalize simazine occurrence data to account for the impact of sampling

frequency on capturing peak or high‐end simazine concentrations. Reported monitoring data

without bias factor adjustment are used to identify aquatic environments where the ecological

levels of concern (LOC) may be exceeded. The bias factors provide additional information on

the potential extent of LOC exceedances, particularly in the Midwest (see Section 15).   

  

Risk to Terrestrial Animals and Plants

Simazine is slightly toxic to birds and mammals and is practically non‐toxic to terrestrial

invertebrates on an acute exposure basis. In most terrestrial animal species, chronic effects are

the predominant concern and are discussed further below. Based on the mechanism of action,

i.e., disruption of photosynthesis, simazine is toxic to most photoautotroph organisms including

unicellular algae and flowering plants.  

When estimates of simazine exposure in terrestrial environments are compared to the available

ecotoxicity data, the results indicate potential risk to birds, mammals and plants (see Section

14). Risk to birds and mammals is primarily through chronic exposure. Although some LOCs are

exceeded for acute risk to birds under scenarios involving higher use rates, there is greater

uncertainty in the estimated risk because the endpoints were non‐definitive. Levels of concern

for terrestrial plants are exceeded following spray drift and/or runoff after applications

according to all currently labeled maximum application rates, as well as rates as low as 0.5 lb

a.i./A.  

Birds, Mammals, Reptiles and Terrestrial Phase Amphibians

The most sensitive acute endpoint for birds (LD50) is greater than 4,640 mg a.i./kg‐bw for the

mallard duck. The most sensitive chronic endpoint for birds is reported in reproduction studies

in the northern bobwhite quail at simazine concentrations of 100 mg a.i./kg‐diet based on the
14

reduction in number of eggs laid, viable embryos, live embryos, number of hatchlings, and 14‐

day old chick survivors at 500 mg a.i./kg.  

On an acute exposure basis, simazine is practically non‐toxic to mammals (LD50 >5,000 mg/kg‐

bw). The most sensitive chronic endpoint in mammals is reported in a reproductive study in the

Norway rat. The reported NOAEL was 10 mg/kg‐diet (0.56 mg/kg‐bw) based on decreases in

body weights, and body weight gains. A number of additional mammalian toxicity studies are

available and are also considered in this assessment. Many of these studies capture the same

types of endpoints (decrease in body weight, decreased food consumption) at similar exposure

concentrations.   

Based on available toxicity data for birds and mammals, the primary degradates of concern for

simazine (DIA, DDA, and HS) are generally of equal toxicity or slightly more toxic than simazine.

In order to account for this toxicity, a default upper‐bound foliar dissipation rate of 35 days is

assumed in terrestrial exposure modeling, allowing the assumed persistence of simazine to

serve as a surrogate for degradate toxicity.  

Based on labeled uses for simazine the listed and non‐listed avian chronic LOC is exceeded for

all uses for chronic exposures. Chronic risks were identified with RQ values as high as 11.2 and

exceed LOCs for all modeled uses. Results from the model runs considering a single application

of 0.5 lb a.i./A show reductions in the RQs, however there still remain exceedances of the LOC

based on chronic exposure.   

Acute LOCs for mammals are exceeded only for the highest use rates on grapefruit, lemon, and

orange. The highest chronic RQs occur for the small mammal with short grass as the primary

food item in the citrus use scenario (RQ=869); however, many of the simazine uses allow for an

application rate of 4 lb a.i./A which results in RQs for small mammals of 744. Chronic risks are of

primary concern with RQ values are as high as 869 and LOCs exceeded for 100% of the

scenarios modeled for all uses. For the reduced rate (0.5 lb a.i./A), RQs based on acute risk to

mammals are less than the LOC for listed and non‐listed species. However, the LOC for chronic

risk is still exceeded for a single application of 0.5 lb a.i./A with RQs of 0.6‐93 and 0.8‐12 for

chronic dose and dietary based risks, respectively.  

Spray drift exposure to birds following a ground application of 2 or 4 lb a.i./A may result in near

field (within 100 feet) chronic risks for all droplet sizes for both low and high boom releases.

The reduced use rate of a single application of 0.5 lb a.i./A reduces this distance to the field

edge. Spray drift exposure to mammals results in potential chronic risks extending 200 – 650 ft,

450 – 1000 ft, and 50 – 225 ft from the field following a single application of 2, 4 or 0.5 lb a.i./A

respectively. Therefore off‐field spray drift presents a risk concern for birds and mammals for

all labeled uses.  
15

Terrestrial Invertebrates

Simazine is practically non‐toxic to honey bees (Apis mellifera L.) based on acute contact

toxicity. The reported LD50 value is >97 μg a.i./bee with 6.5% mortality reported at the highest

dose tested. Additional studies on honey bees were not available.  

The EPA has recently issued interim guidance for assessing the potential risks of pesticides to

bees and the data needed to support such assessments (Brady, 2014). The guidance document

indicates that if exposure of bees to a pesticide is expected, a Tier I risk assessment is

conducted. Using the new pollinator guidance to estimate terrestrial invertebrate risk, an RQ

value for acute contact toxicity in the honey bee was calculated as 0.11; less than the LOC of 0.4

for acute exposure. However, risk to pollinators (e.g., honey bees) is an uncertainty due to the

lack of data (i.e., oral or larval honey bee exposure with simazine) available in order to

complete the Tier 1 risk assessment (Table 28). The EPA is currently determining whether

additional pollinator data are needed for active ingredients in registration review whose

dockets opened prior to June 2014, such is the case for simazine. For those active ingredients

where the EPA determines that additional pollinator exposure and effects data are necessary to

help make a final registration review decision, the EPA intends to issue a DCI to obtain these

data. In the near future, the EPA will provide further information and guidance on this effort.

Terrestrial Plants

At the maximum single application rate for each of the modeled labeled uses, RQs for listed and

non‐listed monocots and dicots are above the LOCs for upland (dry areas) and wetland (semi‐

aquatic areas) habitats based on spray drift exposure alone as well as through the combination

of runoff and spray drift exposure. In addition, a reduced single application rate of 0.5 lb a.i./A

resulted in RQs that are above the levels of concern for runoff and spray drift to upland and

wetland habitats.  

The RQs resulting from ground spray applications result in lower drift concerns than those

resulting from aerial applications (only registered for aerial application on alfalfa); however,

these application methods contribute equally to runoff concerns. Because of the relatively high

solubility of simazine and its persistence in the environment, any species of plant that is

downhill of the application area is likely to receive runoff from use sites.  

The diversity of species that are sensitive to simazine in the vegetative vigor and seedling

emergence studies suggests that a broad diversity of plants are sensitive to simazine exposure.  

The breadth of species and families of plants potentially impacted by simazine use at current

maximum labeled rates, as well as following application at a reduced rate of 0.5 lb a.i./A

suggest that terrestrial plant biodiversity and communities are likely to be impacted from off‐

field exposures via runoff and spray drift.  
16

Risk to Aquatic Animals, Plants and Plant Communities

Simazine is moderately toxic to freshwater and estuarine/marine fish and estuarine/marine

invertebrates, and highly toxic to freshwater aquatic invertebrates on an acute exposure basis

(see Section 11.2). Risks to aquatic animals were identified from chronic exposure to simazine,

although some LOCs were also exceeded for acute risk to listed species for freshwater

invertebrates. RQs exceeded the LOCs for non‐vascular plants for almost all predicted EECs and

exceeded the LOC for vascular plants for a number of predicted EECs.  

Based on available toxicity data for aquatic organisms, including fish, aquatic invertebrates,

aquatic phase amphibians, and aquatic plants, the chlorodegradation products

(deisopropyltriazine and deethyltriazine) are not more toxic than simazine to aquatic

organisms, with some of the reported toxicity levels exceeding the maximum solubility of the

compound. For these reasons, aquatic exposure modeling was based on simazine only.

Fish

The most sensitive freshwater fish acute study is the fathead minnow with an 96‐hour LC50 of

6,400 μg a.i./L (MRID 00033309).  The most sensitive chronic exposure endpoint for fish was

reported in an early life‐stage study with common carp (Cyprinus carpio L.; Velisek et al. 2012).

The authors reported no effects on hatchability or embryo viability; however, there were

statistically significant reductions in body weights following exposures at 600 μg a.i./L (NOAEC =

60 μg a.i./L). This endpoint is used to quantify risks to freshwater fish.

On an acute exposure basis, there are no risk concerns to non‐listed or listed freshwater or

estuarine/marine fish (Table 45, Table 46, and Table 47). On a chronic exposure basis,

freshwater species RQs exceed levels of concern for corn, blueberries/caneberries,

strawberries, citrus, cranberries, apple, pear, hazelnut, pecan, nurseries, christmas trees, and

shelterbelts, at maximum labeled use rates and application methods with RQs ranging from 0.1

– 5.7.

The chronic endpoint for risk to fish in the atrazine PRA was established at 5 μg a.i./L based on a

short term reproduction test in Japanese medaka (Papoulias et al. 2014). The design of this

study allowed for the evaluation of daily and cumulative egg production as endpoints. No such

study is available for simazine. While these chronic fish data for atrazine indicate potential

sublethal effects at low exposure concentrations, it is unclear to what degree those data

represent simazine.

Aquatic Phase Amphibians

The Office of Pesticide Programs (OPP) standard in ecological risk assessments is to use the

available fish toxicity data to represent surrogate data for aquatic phase amphibians when data
17

on amphibians are not available. The modeled surface water concentrations indicate that

chronic risks to freshwater fish are expected for several of the currently labeled simazine uses

and application rates.  

While there are amphibian‐specific data for atrazine which indicate potential sublethal effects

at low exposure concentrations (low ppb), it is unclear to what degree those data represent

simazine.

Aquatic Invertebrates

Simazine is classified as highly toxic to freshwater invertebrates and moderately toxic to

saltwater invertebrates on an acute exposure basis. The most sensitive chronic endpoint for

freshwater invertebrates is 40 μg a.i./L, calculated from an ACR, using the acute endpoint of

EC50 = 1,000 μg a.i./L,  and an ACR of 25 calculated from atrazine D. magna toxicity data

(3,500/140 = 25; MRIDs 45087413 and 00024377, respectively). The estuarine/marine

invertebrate chronic endpoint is 63 μg a.i./L from a chronic mysid toxicity study (Guideline

Study 850.1300) using the Saltwater Mysid (Americamysis bahia).

The SWCC results for labeled rates of application result in acute LOC exceedances for

freshwater invertebrates for several of the uses; however, the majority of tested scenarios

(Table 45, Table 46, and Table 47) do not result in LOC exceedances. Lower application rates

result in fewer scenarios exceeding the LOCs for listed species (RQs range from 0.01 to 0.37).  

The estimated chronic exposure levels result in many of the uses with exceedances of the non‐

listed and listed invertebrate LOCs (RQs range from 0.2 – 9).   

Aquatic Plants

The Tier II results indicate that freshwater blue‐green algae (Anabaena) is the most sensitive

non‐vascular plant to simazine (IC50 = 36 μg a.i./L, IC05 = 5.4 μg a.i./L; MRID 42662401) based on

reduction in growth rates observed by day 5. Freshwater vascular plants are as sensitive to

simazine as freshwater non‐vascular plants, with the most sensitive vascular plant EC50 value of

67 μg a.i./L, based on fresh weight reduction (biomass reduction) in Vallisneria (Wilson and

Wilson, 2010).

The predicted EECs using SWCC indicate that, when based on the most sensitive taxon, the non‐

listed non‐vascular plant LOCs are exceeded for all labeled uses and rates (RQs range from 0.8

to 46).

Aquatic Plant Communities

The Concentration Equivalent Level of Concern (CELOC) was established for atrazine using a

body of aquatic plant community toxicity studies (meso and microcosm). As mentioned in
18

Section 15.1.5, simazine does not have this body of research. However, because atrazine and

simazine share a common mechanism of action and similar potency in plants, and coupled with

their propensity to move into aquatic ecosystems and their persistence in water, these

chemicals are both expected to pose a potential risk to aquatic plant communities.  

INTRODUCTION

Simazine is one of the most widely used herbicides in North America. It is a chlorotriazine

herbicide registered in the U.S. to control annual broadleaf and grass weeds in alfalfa, almond,

apple, avocado, blueberry, broccoli, Brussels sprouts, cabbage, caneberries, cherry, citrus, corn

(field, sweet), cranberry, filbert (hazelnut), grapefruit, grapes, kale, kohlrabi, lemon, macadamia

nut (bushnut), nectarine, olive, orange, peach, pear, pecan, persimmon, plum, strawberry, and

walnut (English/black). Simazine is also registered for use on nursery stock (ornamental plants,

ornamental woody shrubs and vines, ornamental trees), forestry (black walnut, oak, loblolly

pine, slash pine, white pine), christmas tree plantations, shelterbelt plantings, turf (golf course,

ornamental, sod farms, residential lawns), ornamental ponds and aquaria, uncultivated

agricultural areas, farm buildings, and other non‐agricultural areas.

Three chlorotriazine herbicides have current registrations in the United States: propazine (6‐

chloro‐N,N′‐bis(1‐methylethyl)‐1,3,5‐triazine‐2,4‐diamine), PC code 080808; atrazine (6‐chloro‐

N‐ethyl‐N′‐(1‐methylethyl)‐1,3,5‐triazine‐2,4‐diamine), PC code 080803; and simazine (6‐chloro‐

N,N′‐diethyl‐1,3,5‐triazine‐2,4‐diamine), PC code 080807. These three chemicals differ only in

number and/or position of methyl groups on moieties branched in identical positions off the

triazine ring, thus are structurally nearly identical (Figure 2). Based upon the available guideline

and non‐guideline toxicity studies, these three herbicides result in similar herbicidal effects to

terrestrial and aquatic plants. These molecules bind with a protein complex of the photosystem

II in chloroplast photosynthetic membranes (Schulz et al., 1990). The result is an inhibition in

the transfer of electrons through the light reactions of photosynthesis that in turn inhibits the

formation and release of oxygen, production of adenosine triphosphate, and the fixation of

carbon dioxide into sugars.   

In terrestrial animals, the available guideline studies for all three chlorotriazines have shown

similar effects at similar doses and concentrations (USEPA 2015, USEPA 2016). These effects

include food avoidance and/or reductions in frank weights or weight gain, although post‐

exposure recovery is frequently noted. The similarities of effects in mammalian studies have led

OPP’s Health Effects Division to consider atrazine, simazine, propazine, and their chlorotriazine

degradates in a cumulative human health risk assessment. Unlike atrazine, propazine and

simazine do not have the wealth of ecological effects data (e.g., mesocosms, amphibians,

academic research) that have informed the atrazine registration review ecological risk

assessment. The ecological risk assessment presented in this document is for the active

ingredient simazine and its degradates of concern.  
19

Figure 2. Chemical Structures of the Chlorotriazines and their Chlorotriazine Degradation

Products.

BACKGROUND

Simazine is mobile and persistent in the environment. The main routes of dissipation are

microbial degradation under aerobic conditions, runoff, and leaching. Because of its persistence

and mobility, simazine moves into surface and groundwater. This is confirmed by the

widespread detections of simazine in surface water and groundwater.  

A drinking water assessment and an ecological risk assessment for registered uses was

conducted as part of EPA’s Reregistration Eligibility Decision (RED) for simazine in 2006 (DPs

324226, 325446, 307018) (USEPA, 2005). In 2006 EPA also conducted a chloro‐s‐triazine

cumulative drinking water assessment inclusive of simazine (DP 317976). Based on data needs

identified in the RED, a Data‐Call‐In (DCI) was issued on August 19, 2008 that required the

simazine registrants to conduct several environmental fate and ecological effects studies, as

well as establish a simazine drinking water monitoring program.  

The RED assessment identified risks to:  

   

Aquatic Organisms:

 Amphibians (uncertainty); the RED stated that future studies on atrazine’s potential

sublethal effects to amphibians, specifically impacts on gonadal development in frogs,

may be applicable to simazine because the two chemicals share a similar mechanism of

toxicity, a similar mechanism of herbicidal action, and similar degradates.  
20

 Aquatic non‐vascular and vascular plants, and communities.

Terrestrial Organisms:

 Chronic risk to both birds and mammals, based on sublethal effects on weight gain.

 Non‐target terrestrial plants, including both monocots and dicots.

In addition to the potential risks to non‐target organisms noted above, the endangered species

level of concern was exceeded for the following taxa and use scenarios:

 Freshwater fish – granular application for nonselective weed control on turf and other

non‐crop land.

 Freshwater invertebrates – all non‐granular application on citrus, pine trees, nuts,

peaches, apples, and corn; granular application for nonselective weed control on turf

and other non‐crop land.

 Vascular aquatic plants – all non‐ granular application on citrus, nuts, peaches, and corn;

granular application for nonselective weed control on turf and other non‐crop land as

well as apples.

 Birds – non‐granular application for all uses.

 Mammals – non‐granular and granular application for all uses.

 Monocot and dicot terrestrial plants – non‐granular and granular application for all uses.

The RED required revisions to simazine labels that included mitigation measures to address

non‐target terrestrial exposures to animals and plants. These mitigation measures were placed

on the labels in 2011 and were as follows:

 Prohibit use as “non‐selective weed control on non‐crop land” on product labels;

 Prohibit formulation of simazine into granular formulations for agricultural use; and

 Reduce the maximum application rates to typical rates, and harmonize application rates

on simazine product labels.

o Single application rates for use on corn set to 2.0 lb a.i./A, and a maximum of 2.5

lb a.i./A/year.

o Further restrict use on corn in areas with highly erodible soils (<30% plant

residue cover) to a single preemergence application of 1.6 lb a.i./A/year.

Previous Endangered Species Assessments

The EPA conducted several simazine ecological risk assessments for federally‐listed endangered

species from 2000‐2010. The EPA conducted an assessment for the Endangered and

Threatened Salmon and Steelhead in 2003 (USEPA, 2003). The EPA made a “no effect”

determination for Pacific anadromous salmonids.  

The EPA conducted an assessment for the California Red‐legged Frog (Rana aurora draytonii) in

2007 (USEPA, 2007). The EPA made a Likely to Adversely Affect (LAA) determination for the
21

aquatic‐phase California Red‐legged Frog, and also made a LAA determination for the

terrestrial‐phase California Red‐legged Frog for chronic direct effects and indirect effects.

Additionally, the EPA determined that there is the potential for modification of designated

critical habitat for the California Red‐legged Frog from the uses of simazine in California.  

In 2007, the EPA also conducted an assessment for the Barton Springs Salamander (Eurycea

sosorum) (USEPA, 2007). The EPA made a “no effect” determination for the Barton Springs

salamander.  

The EPA conducted an assessment for the Delta smelt (Hypomesus transpacificus) in 2010

(USEPA, 2010). The EPA made a LAA determination for the Delta Smelt from the use of simazine

in California. Additionally, the EPA determined that there is the potential for modification of

designated critical habitat for the Delta Smelt from the uses of simazine in California.

HERBICIDAL MECHANISM OF ACTION

Simazine is in the Weed Science Society herbicide group 5 (s‐triazine) and the Herbicide

Resistance Action Committee (HRAC) group C1, with the mechanism of action of inhibiting

photosynthesis in photosystem II (PSII). Triazine herbicides such as simazine bind with a protein

complex of the Photosystem II in chloroplast photosynthetic membranes (Schulz et al., 1990).  

The result is an inhibition in the transfer of electrons through the light reactions of

photosynthesis that in turn inhibits the formation and release of oxygen, production of

adenosine triphosphate, and the fixation of carbon dioxide into sugars.

OVERVIEW OF PESTICIDE USE AND USAGE

Information on use sites, formulations, application methods, and application timing has been

obtained from various EPA sources, including databases such as OPPIN, the Label Use

Information System (LUIS), summaries of the currently registered product labels that were

provided by the simazine registrants, and the Biological and Economic Analysis Division review

of the registrants’ use summary tables (USEPA 2015). This information was used to identify the

application parameters for assessing risk in this assessment and is provided in Table 1 and

Table 2.   
22

Table 1.  Maximum Application Rates, Maximum Applications, Minimum Application

Intervals, and Application Methods for Section 3 Simazine Labels.

Crop Geographic

Restrictions

Application

Method Application Timing

Min

single

rate     

(lb

a.i./A)

Max

single

rate     

(lb

a.i./A)

Max

no. of

apps

Min

app

interval

(days)

Max

rate

per

year (lb

a.i./A)

Almond CA

Ground: 2‐4

ft. band

each side of

tree row

Preemergence before

weed emergence in late

fall or early winter

1 2 1 NA 2

Apples ‐‐  

Ground:

Broadcast,

band, spot

Before weeds emerge

or following removal of

weed growth or

Preemergence before

weed emergence in late

fall or early winter

2 4 1 NA 4

Avocados CA, FL

Ground:

Broadcast,

band, spot

After final preparation

of grove or

Preemergence before

weed emergence in late

fall or early winter

2 4 1 NA 4

Blueberries  ‐‐ 

Ground:

Broadcast,

band, spot

Spring or split in Spring

+ Fall 2 4 2 NS 4

Caneberries  

(Black,

Boysen,

Logan, Rasp)

 ‐‐ 

Ground:

Broadcast,

band, spot

Spring or split in Spring

+ Fall 2 4 2 NS 4

Cherries,

Sour  ‐‐ 

Ground:

Broadcast,

band, spot

Before weeds emerge

or following removal of

weed growth

2 4 1 NA 4

Cherries,

Sweet

Use only in

MO and

states east

of the MS

River except

TN.

Ground:

Broadcast,

band, spot

Late Fall to early Spring

prior to weed

emergence or Before

weeds emerge or

following removal of

weed growth  

1.6 or

2 4 1 NA 4

Cherries, Tart  ‐‐ 

Ground:

Broadcast,

band, spot

Before weeds emerge

or following removal of

weed growth or

Preemergence before

weed emergence in late

fall or early winter

2 4 1 NA 4

Christmas

Tree

Plantings

 ‐‐ 

Ground:

Broadcast or

banded

Fall or Split in Spring +

Fall 2 4 2 NS 4
23

Crop Geographic

Restrictions

Application

Method Application Timing

Min

single

rate     

(lb

a.i./A)

Max

single

rate     

(lb

a.i./A)

Max

no. of

apps

Min

app

interval

(days)

Max

rate

per

year (lb

a.i./A)

Christmas

Tree

Plantings and

Shelterbelts  

(Forestry)

 ‐‐ 

Ground:

Broadcast or

banded

Fall and Early Spring, or

split applications in Fall

and Spring or  

Fall or Split Fall + Spring

2 4 2 NS 4

Christmas

Trees    ‐‐  Ground Between Fall and EarlySpring 1 or 2 2 2 NS 4

Corn  ‐‐  Ground

Spring: Preplant, At

Planting, Preemergence 0.8 1 2* NS 2.5*

Spring: Split*** 0.54 0.54 2*** NS 2.5***

Fall: Preemergence 0.54 0.54 1 NS 0.54

Spring: Split*** 0.625 0.625 2*** NS 2.5***

Fall: Preemergence   NS 0.625 1 NS 0.625

Corn (Field) ‐‐   Ground

Spring: Preemergence

to weeds and corn or

Preplant before, during

or after seedbed

preparation**

1.6** 2 2 NS 2.5

Corn (Sweet)  ‐‐  Ground

Spring: Preemergence

to weeds and corn or

Preplant before, during

or after seedbed

preparation**

1.6** 2 2 NS 2.5

Cranberries

MA

Ground,

including

chemigation

Before Spring or in Fall

after harvest

4 or

NS 4 1 NS 4

Non‐MA

Ground,

including

chemigation

Before Spring 2 or

NS 2 1 NS 2

Filbert

(Hazelnuts) ‐‐  

Ground:

Broadcast,

band, spot

Fall or Split Fall + Spring

or Spring or split in

Spring + Fall

2 4 2 NS 4

Grapefruit

CA

Ground:

Broadcast,

band, spot

Before weeds emerge

or following removal of

weed growth or Single

or split in Fall + Spring

2 4 2 NS 4

FL

Ground:

50% band

Spring: 50% band

between Jan. and April NS

8 in a

50%

band

1 NA

4 per

field

acre

Ground:

Broadcast,

band, spot

Spring and/or Fall

Prior to emergence of

weeds or after emerged

weeds have been

controlled with a

contact herbicide

NS 4 2 NS 8
24

Crop Geographic

Restrictions

Application

Method Application Timing

Min

single

rate     

(lb

a.i./A)

Max

single

rate     

(lb

a.i./A)

Max

no. of

apps

Min

app

interval

(days)

Max

rate

per

year (lb

a.i./A)

TX

Ground:

Broadcast,

band, spot

Before weeds emerge

or following removal of

weed growth or NS

NS 4 2 or

NS NS 4

Grapes  ‐‐ 

Ground:

Broadcast,

band, spot

Between harvest and

early spring 2 4 1 NS 4

Lemons

AZ

Ground:

Broadcast,

band, spot

Split Spring + Fall 1.6 1.6 2 NS 3.2

CA

Ground:

Broadcast,

band, spot

Before weeds emerge

or following removal of

weed growth or Single

or split in Fall + Spring

2 4 2 NS 4

Macadamia

Nuts  ‐‐ 

Ground:

Broadcast,

band, spot

Before weeds emerge

or following removal of

weed growth or Before

harvest and just prior to

weed emergence or NS

NS or

2 4 1 NA 4

Nectarines CA

Ground: 2‐4

ft. band

each side of

tree row

Preemergence before

weed emergence in late

fall or early winter

1 2 1 NA 2

Nurseries  ‐‐ 

Ground:

Broadcast or

banded

Spring or Fall, at least 1

year after transplanting 2 3 1 NS 3

Nurseries for

Conifers,

Deciduous

Trees and

Woody

Ornamentals

(Forestry)

 ‐‐ 

Ground:

Broadcast or

banded

Spring or Fall 2 3 1 NS 3

Olives ‐‐  

Ground:

Broadcast,

band, spot

Fall: after grove

preparation or Before

weeds emerge or

following removal of

weed growth

2 or

NS 4 1 NA 4

Oranges

AZ

Ground:

Broadcast,

band, spot

Split Spring + Fall 1.6 1.6 2 NS 3.2

CA

Ground:

Broadcast,

band, spot

Single or split in Spring

+ Fall or Before weeds

emerge or following

removal of weed

growth

2 4 2 NS 4
25

Crop Geographic

Restrictions

Application

Method Application Timing

Min

single

rate     

(lb

a.i./A)

Max

single

rate     

(lb

a.i./A)

Max

no. of

apps

Min

app

interval

(days)

Max

rate

per

year (lb

a.i./A)

FL

Ground:

50% band

Spring: 50% band

between Jan. and April NS

8 in a

50%

band

1 NA

4 per

field

acre

Ground:

Broadcast,

band, spot

Single or  Spring + Fall

or Spring and/or Fall

Prior to emergence of

weeds or after emerged

weeds have been

controlled with a

contact herbicide

NS 4 2 NS 8

TX

Ground:

Broadcast,

band, spot

Before weeds emerge

or following removal of

weed growth or NS

NS 4 2 or

NS NS 4

Ornamental

Ponds and

Aquaria

 ‐‐  Water

Treatment

Initial or subsequent

maintenance

0.00468563 lb

a.i./720 gal NS 6 NS

Peaches CA

Ground: 2‐4

ft. band

each side of

tree row

Preemergence before

weed emergence in late

fall or early winter

1 2 1 NA 2

Peaches

AR, LA, MO,

OK, TX,

states east

of the MS

River

Ground:

Broadcast,

band, spot

Late Fall to early Spring

prior to weed

emergence

1.6 or

2 4 1 NA 4

Pears  ‐‐ 

Ground:

Broadcast,

band, spot

Before weeds emerge

or following removal of

weed growth or

Preemergence before

weed emergence in late

fall or early winter

2 4 1 NA 4

Pecans

Do not use

west of the

Pecos River

in TX or in

AZ, CA, or

NM

Ground:

Broadcast,

band, spot

Before weeds emerge

in the spring 2 4 1 NA 4

Plums

Use only in

MO and

states east

of the MS

River, except

TN

Ground:

Broadcast,

band, spot

Before weeds emerge

or following removal of

weed growth or Late

Fall to early Spring prior

to weed emergence

2 or

1.6 4 1 NA 4
26

Crop Geographic

Restrictions

Application

Method Application Timing

Min

single

rate     

(lb

a.i./A)

Max

single

rate     

(lb

a.i./A)

Max

no. of

apps

Min

app

interval

(days)

Max

rate

per

year (lb

a.i./A)

Shelterbelts  

(Forestry)  ‐‐ 

Ground:

Broadcast or

banded

Spring or split in Spring

+ Fall 2 4 2 NS 4

Strawberries OR, WA only

Ground:

Broadcast,

band, spot

See Comment 1 1 1 NA 1

Tree

Plantations

for Timber  

(Forestry)

‐‐  

Ground:

Broadcast,

band, spot

Spring before weeds

emerge or in the fall

following the removal

of weed growth; before

or after seedling

transplanting

2 4 1 NS 4

Turfgrass For

Fairways,

Lawns, and

Similar Areas

(includes

farm

buildings,

non‐

agricultural

areas, and

uncultivated

agricultural

areas)

Do not use

north of NC,

(except may

be used in

VA Coastal

Plains)  

Ground:

Broadcast  

After Sept. 1 through

June 1 1 2 2 30 days 3

Turfgrass For

Sod

FL Ground:

Broadcast  

NS or Dependent on

soil texture and beds 1 4 2 NS 6

States other

than FL for

use on St.

Augustine‐

grass,

Centipede‐

grass, and

Zoysiagrass

only

Ground:

Broadcast   NS 1 orNS 2 2 NS 3

Walnuts ‐‐  

Ground:

Broadcast,

band, spot

After leveling and

furrowing operations or

Before leveling and

furrowing operations

2 4 1 NA 4
27

Crop Geographic

Restrictions

Application

Method Application Timing

Min

single

rate     

(lb

a.i./A)

Max

single

rate     

(lb

a.i./A)

Max

no. of

apps

Min

app

interval

(days)

Max

rate

per

year (lb

a.i./A)

Winter

Annual

Broadleaf

Control On

Land To be

Planted To

Corn The

Following

Spring.

   Ground:

Broadcast  

Fall after harvest of

preceding crop for land

to be planted to corn in

the spring.

1 1 NS NS 2.5

NA = Not Applicable; NS = Not Specified on the label; bolded or indicates directions from two labels that differ are included

in the same cell of the table.

* Only one application of 0.8 lb. simazine is allowed per acre per calendar year on highly erodible soils if < 30% of the soil is

covered with plant residues.

**1.6 lb a.i./A is the maximum single application rate on highly erodible soils.

*** Split application is only limited to soils not highly erodible if at least 30% of the soil is covered with plant residues.  

Table 2.  Maximum Application Rates, Maximum Applications, Minimum Application

Intervals, and Application Methods for Section 24(c) Simazine Labels.

Crop Geographic

Restrictions

Application

Method

Application

Timing

Min

single

rate     

(lb

a.i./A)

Max

single

rate     

(lb

a.i./A)

Max

no.

of

apps

Min app

interval

(days)

Max

rate

per

year

(lb

a.i./A)

Alfalfa Grown for

Seed WA Aerial orGround

Between Fall

(after harvest)

and early Spring

(before Spring

growth starts)

0.8 1.6 1 NS 1.6

Cherries, Sweet OR

Ground:

Broadcast,

banded, or

spot treatment

Late fall to early

spring 1.6 4 1 NA 4
28

Crop Geographic

Restrictions

Application

Method

Application

Timing

Min

single

rate     

(lb

a.i./A)

Max

single

rate     

(lb

a.i./A)

Max

no.

of

apps

Min app

interval

(days)

Max

rate

per

year

(lb

a.i./A)

Citrus

CA in

Fresno and

Tulare

Counties

Microsprinkler

irrigation

Single or split in

fall and spring.

Preemergence to

weeds

2 4 2 NS 4

Crops Grown For

Seed:  Brussels

sprout and

Cabbage

WA Ground Fall and Spring 0.8 0.8 2 NS 1.6

Crops Grown For

Seed: Broccoli,

Brussels sprout,

Cabbage, Chinese

cabbage, Chinese

mustard, Kale,

Kohlrabi, Radish,

Rutabaga and

Turnip  

OR Ground Late Fall 0.8 1 2 NS 2

Windbreaks &

Shelterbelts  

(Forestry)

SD

Ground:

Broadcast or

banded

Single or split in

fall and spring.

Preemergence to

weeds

2 4 2 NS 4

NA = Not Applicable; NS = Not Specified on the label.

Formulations

Simazine is formulated as a liquid, emulsifiable concentrate, dry flowable, or a flowable

concentrate. There are 32 formulated product registrations. Simazine is the single active

ingredient in 30 formulations, and is co‐formulated with atrazine in 2 formulated products.  

Application Methods and Timing

Simazine may be applied by ground broadcast, band application, chemigation, or spot

treatment, and for the Section 24(c) alfalfa use in Washington it can be applied aerially. Like

atrazine, simazine has many preemergence uses in corn, and an overall similar weed spectrum;

however, simazine tends to be a better grass herbicide (USEPA 2013).
29

Non‐Agricultural Use Sites

Simazine is also registered for use on nursery stock (ornamental plants, ornamental woody

shrubs and vines, ornamental trees), forestry (black walnut, oak, loblolly pine, slash pine, white

pine), christmas tree plantations, shelterbelt plantings, turf (golf course, ornamental, sod farms,

residential lawns), ornamental ponds and aquaria, uncultivated agricultural areas, farm

buildings, and other non‐agricultural areas (USEPA 2013).

Non‐Agricultural Usage

Simazine use on non‐agricultural sites has fluctuated based on available data reported from

2002 to 2006, with a noted increase in the institutional and lawn care operator sectors in 2004,

with reported usage of up to 329,000 pounds of simazine on institutional turf, and 220,000

pounds by lawn care operators. By 2006, while institutional turf usage significantly decreased

(to 10,000 pounds of simazine), use by lawn care operators remained high, but somewhat

decreased (186,000 pounds) (USEPA 2013).  

Agricultural Usage

Between 2006 and 2010, the annual total agricultural usage averaged approximately 5 million

pounds of simazine active ingredient (a.i.). During this time frame, the crops with the most use

in terms of total pounds of active ingredient were corn (53%), oranges (22%), and wine grapes

(7%). The crop with the most use in terms of total area treated was also corn (71%). On

average, the states with the most agricultural usage in terms of pounds a.i. applied are

California (23%), Illinois and Florida (16% each), and Indiana (13%) (USEPA 2013).   

Screening Level Usage Analysis Data

Table 3 provides the most recent Screening Level Usage Analysis (SLUA) (USEPA 2015). The

SLUA provides available estimates of pesticide usage data for simazine on agricultural crops in

the United States.

Limitations to the data include the following:

•Additional registered uses for certain crops may exist but are not included because the

available surveys do not report usage (e.g., small acreage crops).

•Lack of reported usage data for the pesticide on a crop does not imply zero usage.

•Usage data on a particular site may be noted in data sources, but not quantified. In

these instances, the site would not be reported in the SLUA.

•Non‐agricultural use sites (e.g., turf, post‐harvest, etc.) are not reported in the SLUA.

Some sites show use even though they are not on the label. This usage could be due to various

factors, including, but not limited to data collection or reporting errors, or application errors.
30

Table 3. Screening‐Level Estimates of Agricultural Uses of Simazine (2004‐2013) (USEPA,

2015).

Crop Lb. A.I.

Percent Crop Treated (PCT)

Average PCT Maximum PCT

1 Alfalfa         6,000    <1 <2.5

2 Almonds       100,000    15    25

3 Apples       100,000    15    25

4 Apricots         <500     5    5

5 Asparagus         4,000     5     5

6 Avocados        10,000     5    15

7 Blueberries        10,000    15    20

8 Cabbage         4,000    <1 <2.5

9 Caneberries        20,000    40    55

10 Cherries        20,000     5    15

11 Corn     2,600,000     5    5

12 Grapefruit       100,000    25    35

13 Grapes       500,000    30    45

14 Hazelnuts        20,000    40    55

15 Lemons       20,000    10    20

16 Nectarines         4,000    15    20

17 Olives        10,000    15    20

18 Oranges     1,100,000    35    45

19 Pastureland+        30,000    <1 <2.5

20 Peaches        50,000    20    30

21 Pears        30,000    15    25

22 Pecans        40,000     5     5

23 Plums/Prunes         3,000 <2.5     5

24 Strawberries         3,000     5    10

25 Sweet Corn         8,000 <2.5 <2.5

26 Tangelos         2,000    10    15

27 Tangerines        10,000    10    15

28 Walnuts      200,000    25    35

All numbers are rounded. <2.5: less than 2.5 percent of crop is treated; <1: less than 1 percent of crop is treated.

+:  Crops not known to be listed on active end use product registrations when this report was run.

SLUA data sources include: USDA‐NASS (United States Department of Agriculture's National Agricultural Statistics Service); Private Pesticide

Market Research
31

National Mapping of Use Data

Another measure of usage is the use intensity. For the purposes of this assessment, use

intensity is expressed as pounds a.i. applied per square mile. This differs from the application

rate, which is expressed as the pounds a.i. applied per treated acre. The USGS pesticide usage

data maps in Figure 3 illustrate the national agricultural simazine usage at the Crop Reporting

District (CRD) level and spatially represents simazine use intensity in the US. As shown, citrus

areas such as California and Florida and the corn producing states (colored in dark brown) have

the highest reported use intensity.

The usage estimates from USGS are based on private market surveys of pesticide use in

agriculture. They are intended for broad‐scale assessments such as at the national or regional

level, and are not suitable for sub‐state quantitative analyses. This is because the survey data

are limited to the states that represent the top 80‐90 percent of acreage for the individual

crops; therefore, use may be occurring in regions outside the scope of the survey. CRDs

showing no usage of pesticides may be due to either the lack of pesticide use in the region or

non‐participation in the agricultural surveys. In addition, across the years, there may be

variations in the specific crops included in the CRD survey. This may result in a lower annual

average for the CRD. Therefore, this assessment uses these data for qualitative purposes only,

in the form of the maps below, to provide a geographic footprint of a pesticide’s use.
32

Figure 3. Estimated Agricultural Use of Simazine in The United States, 2009‐2012.
33

ANALYSIS PLAN

Risk Hypothesis

Based on the application methods, mode of action, fate and transport, and the sensitivity of

non‐target aquatic and terrestrial species, simazine has the potential to reduce survival,

reproduction, and/or growth in non‐target terrestrial and aquatic animals and plants as well as

negatively affect the structure, productivity, and function of aquatic plant communities when

used in accordance with the current labels. These non‐target organisms include listed and non‐

listed species.  

Conceptual Model

The environmental fate properties of simazine indicate that runoff, leaching, spray drift and

direct spray represent potential transport mechanisms to aquatic and terrestrial habitats where

non‐target organisms may be exposed. Additional pathways are considered for the evaluation

to identify other potential routes of exposure that may be of concern.  These transport

mechanisms (i.e., sources) are depicted in the conceptual diagrams in EFED Guidance for the

Development of Conceptual Models for a Problem Formulation Developed for Registration

Review1 along with the receptors of concern and the potential attribute changes in the

receptors from exposures to simazine.  

Measures of Exposure

In order to estimate risks of simazine exposures in aquatic and terrestrial environments, all

exposure modeling and resulting risk conclusions were initially made based on maximum

application rates for the currently registered uses as discussed in Section 5, with the exception

of the aquaria and ornamental pond use. The aquaria and ornamental pond use was not

assessed due to the limited area of application at use sites and the low probability of off‐site

transport environmental exposures from this use. Measures of exposure were based on aquatic

and terrestrial models that estimate environmental concentrations of simazine using maximum

labeled application rates and application methods that have the greatest potential for off‐site

transport of the chemical. Measures of exposure were also based on simazine concentrations

measured in ambient surface water across the US.

The Surface Water Concentration Calculator (SWCC version 1.106) and the EFED Provisional

Cranberry Model (v 2.0) were used to generate aquatic EECs. Model input values were selected

consistent with the most recent version of the Input Parameter guidance (current version 2.1).  

The SWCC generates daily exposures and 1‐in‐10‐year EECs of simazine that may occur in

surface water bodies adjacent to application sites receiving atrazine through runoff and spray

1 http://www2.epa.gov/pesticide‐science‐and‐assessing‐pesticide‐risks/guidance‐development‐conceptual‐

models‐problem
34

drift. SWCC simulates pesticide application, movement and transformation on an agricultural

field and the resultant pesticide loadings to a receiving water body via runoff, erosion, and

spray drift, and then simulates the fate of the pesticide and resulting concentrations in the

water body. The standard watershed geometry used for ecological pesticide assessments

assumes application to a 10‐hectare agricultural field that drains into an adjacent 1‐hectare

water body that is 2 meters deep (20,000 m3 volume) with no outlet. The SWCC is used to

estimate screening‐level exposure of aquatic organisms to atrazine. The measure of exposure

for aquatic species is the 1‐in‐10‐year peak or rolling mean concentration. The 1‐in‐10‐year

peak is used for estimating acute exposures of direct effects to aquatic organisms. The 1‐in‐10‐

year 60‐day mean is used for assessing the effects to fish and aquatic‐phase amphibians from

chronic exposure. The 1‐in‐10‐year 21‐day mean is used for assessing the effects on aquatic

invertebrates from chronic exposure. Additional information on the SWCC can be found at:

http://www2.epa.gov/pesticide‐science‐and‐assessing‐pesticide‐risks/models‐pesticide‐risk‐

assessment

The EFED Provisional Cranberry Model (v 2.0) is a modification of the Tier 1 Rice Model. It

simulates application of pesticide to a dry field and degradation in soil (using the aerobic soil

metabolism half‐life) before introducing water to the field. The resulting pesticide mass is

transferred to the water, where the model employs the assumption that the pesticide

instantaneously partitions between water and sediment, as determined by the chemical’s

sorption coefficient, according to:

  w sedsed db

ai

w dd K

m C

 3

2

0 10

'10

   

Where,

Cw0 = initial water concentration [μg/L]

mai' = mass applied per unit area [kg/ha]

Kd = 0.01Koc

KOC = organic carbon partitioning coefficient [L/kg]

dw = water column depth = 0.305 m

dsed = sediment depth = 0.01 m

θsed = porosity of sediment = 0.509

ρb = bulk density of sediment = 1300 kg/m3

In this model, the depth of the water body was refined to reflect concentrations in the water of

a cranberry bog rather than that of a rice paddy (Cape Cod Cranberry Growers Association,

2001).  

The provisional cranberry model will overestimate exposure to pesticides in ambient water to

the extent that dilution and microbially‐mediated degradation occurs in the waterbody outside

the cranberry bog. The use of the provisional cranberry model to estimate aquatic exposure

from uses on cranberries has not been formally approved in EFED; however, models which
35

provide substantially refined estimates of pesticide residues in ambient water from use on wet‐

harvested cranberries are not available.

Surface water monitoring data are also considered in the aquatic exposure assessment.

Monitoring data for simazine occurrence in surface waters were obtained from federal, state,

registrant, and university monitoring programs. The monitoring data represent simazine

occurrence data from 1986 to 2015 in streams, rivers, lakes, and reservoirs. The monitoring

data were analyzed by site‐year to obtain a maximum daily concentration, maximum 4‐day

average concentration, maximum 21‐day average concentration, maximum 60‐day average

concentration, and maximum 90‐day average concentration. Average simazine concentrations

for each site‐year were estimated from simulated chemographs by stair‐step interpolation

between measured monitoring data. In order to characterize the downward bias in

quantification of monitoring data resulting from low sampling frequencies, peak, 21‐day

average, and 60‐day average bias factor equations developed for the atrazine Preliminary Risk

Assessment linear regression equations were used to estimate sampling bias factors (BFs) for

simazine.  

The model used to produce terrestrial EECs on food items is T‐REX, while the model used to

derive EECs relevant to terrestrial and wetland plants is TerrPlant. The AgDRIFT spray drift

model (v2.1.1; December 2011) is used to assess exposures of organisms to simazine deposited

on terrestrial or aquatic habitats by spray drift. Detailed information about the models T‐REX,

TerrPlant and AgDrift, can be found on the EPA’s website at http://www2.epa.gov/pesticide‐

science‐and‐assessing‐pesticide‐risks/models‐pesticide‐risk‐assessment.

For terrestrial vertebrates, the major routes of exposure to simazine is considered to be via

dietary ingestion of food items such as plants and/or animals that have simazine residues as a

result of direct application, spray drift, and/or runoff. Exposure to terrestrial vertebrates

through inhalation is considered unlikely given simazine’s low vapor pressure; a more

quantitative analysis of risk through inhalation exposure via EFED’s Screening Tool for

Inhalation Risk (STIR v. 1.0) is not applicable because most of the avian endpoints and the

lowest rat inhalation endpoint are non‐definitive (LC50 > 1.7 mg/L; MRID 00148899). However,

when these endpoints are considered as definitive endpoints the exposure is determined to be

not likely significant at the highest labeled application rates for mammals, and potentially

significant for birds. The avian risk through inhalation is an overestimation due to the non‐

definitive endpoints and is not expected to lead to RQs exceeding LOCs. EFED’s Screening

Imbibition Program (SIP v. 1.0) suggests no concern for risk to birds and mammals from acute

and chronic exposure through the consumption of drinking water with simazine residues.

Measures of Effect

  

Ecological effects data are used as measures of direct and indirect effects to biological

receptors. Data are obtained from registrant‐submitted studies or from literature studies

identified by ECOTOX (USEPA 2007c). The ECOTOX database provides more ecological effects

data in an attempt to bridge existing data gaps, and is a source for locating single chemical
36

toxicity data and potential chemical mixture toxicity data for aquatic life, terrestrial plants, and

wildlife. ECOTOX was created and is maintained by the USEPA, Office of Research and

Development, and the National Health and Environmental Effects Research Laboratory's Mid‐

Continent Ecology Division.

Information on the potential effects of simazine on non‐target animals is also collected from

the Ecological Incident Information System (EIIS; USEPA 2007d) and the Incident Data System

(IDS) which report on non‐target organisms where such effects have been associated with the

use of pesticides.

Incidents reported in the aggregate incident reports and the Avian Incident Monitoring System

(AIMS) was also searched. AIMS is a database administered by the American Bird Conservancy.

It contains publicly available data on reported avian incidents involving pesticides

(https://abcbirds.org/avian‐incident‐monitoring‐system/).

Integration of Exposure and Effects

Risk characterization is the integration of exposure and ecological effect characterizations to

determine the potential ecological risk from the use of simazine and the likelihood of effects to

non‐target organisms in aquatic and terrestrial habitats. Due to the similarity in structure, mode

of action and degradates, atrazine and propazine data may also be used to characterize toxicity

to target organisms, particularly when data deficiencies are identified for simazine. For the

assessment of simazine risks, the Risk Quotient (RQ) method is the first approach used to

compare estimated exposure data to the measured single‐species toxicity values. Acute and

chronic EECs are divided by acute and chronic single‐species toxicity values. The resulting RQs

are then compared to the EPA’s Levels of Concern (LOC) (USEPA, 2004). Lastly, the exposure

estimates and environmentally measured values from monitoring data as well as the breadth of

available ecotoxicity literature are used to indicate when simazine use, as directed on the

labels, has the potential to cause adverse effects to non‐target terrestrial and aquatic

organisms and communities. In addition, incident data from EIIS, IDS, and AIMS are considered

as part of the risk characterization.

ENVIRONMENTAL FATE AND TRANSPORT

Physical and Chemical Properties of Simazine    

Physical and Chemical Properties

Simazine’s structure and physicochemical properties are shown in Table 4. Simazine is

moderately soluble in water with reported aqueous solubility values ranging from 3.5 to 11

mg/L at 20‐25oC. Volatilization losses of simazine from soil and water systems are expected to

be low because of low vapor pressure (6.1 x 10‐9 torr at 20C) and low Henry's Law Constant

(3.2 x 10‐10 atmm3/mol at 25C). Bioaccumulation of simazine is expected to be low due to a
37

low octanol water partitioning coefficient (Kow = 122) and low bioconcentration factors (BCF)

(MRID 00043668).  

Table 4. Structure and Physical and Chemical Properties of Simazine

Structure

Physical/Chemical Property Units Value

CAS Reg. Number NA 122‐34‐9

Chemical Formula NA C7H12ClN5

Physical State NA Powder

Color NA White

Melting Point 0C 225

Molecular Weight g/mole 201.66

Water Solubility

mg/L

3.5 (25°C)           MRID 0023955

4.8 (25°C)           MRID 47847001

5.3 (20°C)           MRID 47733701

6.2 (20–25°C)    Gunther and Gunther, 1970*

11 (21°C)            MRID 43979901

Vapor Pressure@ 20°C torr 6.1 x 10‐9

Henrys Law Constant  

(calculated for 25C) atm‐m3 mole‐1 3.2 x 10‐10

Kow unitless 122

* Cited in LeBaron, McFarland, and Burnside. 2008. The Triazine Herbicides: 50 Years Revolutionizing

Agriculture.

Environmental Fate Summary

Simazine is mobile and persistent in the environment (KFOC of 123.25 ml/gOC; 80‐day to over

1,000‐day half‐lives in lab studies where degradation occurred). The main routes of dissipation

are microbial degradation under aerobic conditions, runoff, and leaching. Because of its

persistence and mobility, simazine can move into surface and groundwater. This is confirmed

by the detections of simazine in surface water and groundwater in monitoring programs.

Summaries of the environmental fate data are shown in Table 5.  

Although the submitted guideline studies for simazine, atrazine and propazine would indicate

that triazines do not degrade by abiotic hydrolysis at pH 5, 7, and 9 (MRID 00027856), open‐

literature studies show variable acid‐catalyzed hydrolysis half‐lives for triazines in different

matrices including soils, clay suspensions, organic matter, and groundwater (Mabey and Mill,

1978; Hance, 1967; Armstrong et al., 1967; Burkhard and Guth, 1981; Khan, 1978; Widmer et

al., 1993; Gamble et al., 1983; Plust et al., 1981; Navarro et al., 2004). With regard to simazine,

a study by Navarro, et al. in 2004 calculated a hydrolysis half‐life of 310 days in non‐sterile 20°C

groundwater with a pH of 6.66.  
38

Abiotic triazine hydrolysis can be catalyzed by acid sites on organic compounds (e.g., humic and

fulvic acid, organic acids, phenols) and mineral surfaces in soil (Laird and Koskinen, 2008;

Cessna, A., 2008). Additionally, triazine hydrolysis can occur through microbial‐mediated

processes (Mandelbaum, et al. 2008). The hydrolysis of triazines leads to formation of

hydroxytriazine compounds.            

Simazine is persistent to direct photodegradation in water (t1/2 > 386 days; MRID 42503708),

and persistent to photodegradation on soil (t1/2= 132 days; MRID 42739101).   

Simazine is persistent in aerobic loam soils (t1/2 ranges from 80 to 126 days) and aerobic aquatic

environments (t1/2= 74 days; MRID 43004501; 46561301; 43004502). Simazine is also persistent

in anaerobic aquatic environments (t1/2 > 1,766 days) and potentially persistent in anaerobic soil

environments (no degradation occurred within a 51‐day study duration) (MRIDs 40614411,

00027857).   

Soil sorption coefficients for simazine (KF) range from 0.5 to 4.3 ml/g (

1/n=0.79‐1.40) (MRID

41442903). Simazine KF values are significantly correlated with soil organic carbon content (r2 =

0.99, p‐value = 0.003), indicating that soil organic carbon content is a primary determinant of

simazine’s mobility in soil. The average KFOC of 123.25 ml/gOC (n=4; MRID 41442903) indicates a

UN Food and Agriculture Organization (FAO) mobility classification of moderately mobile in soil

(FAO, 2000).  

Field dissipation studies show that simazine dissipation is driven by microbial‐mediated

degradation and by leaching. The half‐life of simazine in one terrestrial field study was 36 days

in the top 8 inches of soil in bare ground plots in Florida (MRID 40634202). Another terrestrial

field study detected simazine 480 days after application in 18 to 24 inches of soil in Nebraska

(half‐life could not be calculated; MRID 00027863). The half‐life in four aquatic field dissipation

studies in multiple states ranged from 10 days to stable (MRIDs 00027829, 40614420,

40614421, 40614422). Microbial‐mediated degradation is an important route of dissipation in

the cited aquatic field studies. The half‐lives in the terrestrial and aquatic field dissipation

studies were often shorter than the half‐lives in the laboratory studies, which may be attributed

to the availability of multiple routes of dissipation and degradation in the field dissipation

studies.

Table 5. Environmental Fate Parameters for Simazine.

Parameter Value Source and Classification

Persistence

Hydrolysis (20C) Stable at pH 5, 7, and 9 MRID 00027856

Acceptable

Aqueous photolysis half‐life (25C) 386‐3496 days* at pH 7

MRID 42503708

Acceptable

Soil photolysis half‐life (25C) 132 days MRID 42739101
39

Parameter Value Source and Classification

Acceptable

Aerobic soil metabolism half‐life

80 days (20°C)

MRID 46561301

Supplemental

126 days (25°C) MRID 00158638

Supplemental

89 days (25°C) MRID 43004501

Supplemental

Anaerobic soil metabolism (25°C) No degradation within  

51‐day study duration

MRID 00027857

Supplemental

Aerobic aquatic metabolism half‐

life (25°C) 74.4 days MRID 43004502

Acceptable

Anaerobic aquatic metabolism half‐

life (25C)

858 days             

(sandy clay)

MRID 40614411

Acceptable

Mobility

KF (ml/g)

4.3, 1

/n = 0.84 (clay)   

0.7, 1/n = 1.08 (sand)   

1.3, 1/n = 0.79 (sandy loam)   

0.5, 1/n = 1.40 (loam) MRIDs 41442903, 41257903

KFOC (ml/gOC)

153 (clay) Acceptable

123 (sand)   

114 (sandy loam)   

103 (loam)   

Field Dissipation

Terrestrial field dissipation half‐life

and/or leaching depth

36 days in 0‐8 inches of  

bare soil (Florida)

MRID 40634202

Supplemental

Present at 480 days in 18‐24 inches

of soil (Nebraska)

MRID 00027863†

Supplemental

Aquatic field dissipation half‐life;

leaching depth

60 to 700 days‡ MRID 00027829

in lakes Supplemental

10 days (Iowa)     

man‐made pond – water MRID 40614421

146 days (Iowa)   Supplemental

man‐made pond – sediment   

120 days (Georgia)

man‐made pond – water MRID 40614422

1,551 days (Georgia) Supplemental
40

Parameter Value Source and Classification

man‐made pond – sediment

Bioaccumulation

Fish bioconcentration factor BCFsteady state = 4.92 (whole fish)

BCFkinetic = 7.30 (whole fish) MRID 00043668   

Supplemental

Depuration rate constant 0.06

* Analysis conducted for one soil using Normal Phase Thin Layer Chromatography resulted in a half‐life of 386

days, while analysis using Reverse Phase Thin Layer Chromatography resulted in a half‐life of 3,946 days.

†

This is the only soil compartment analyzed; simazine concentrations were not measured at day 0 and

consequently the half‐life was not calculated.  Plant bioassay was used to determine simazine or simazine‐

equivalents in other soil compartments.   

‡

Sediment samples were not analyzed.

Degradation Products

Environmental fate data indicate that deisopropylatrazine (DIA) and hydroxysimazine (HS) are

the major degradates of simazine (>10% applied a.i.) and that diadealkylatrazine (DDA) is a

minor chlorotriazine degradation product (<10% applied a.i.). The degradate diadealkylatrazine

is often commonly referred to as DACT. This assessment considers all three degradates

stressors of concern for terrestrial species. Figure 4 shows the chemical structures for DIA, HS,

and DDA. DIA and DDA are formed through dealkylation of the amino groups on the simazine

molecule, while hydroxysimazine is formed through substitution of a chlorine by a hydroxy

group. Dealkylation is a microbial‐mediated process, while hydroxylation is both abiotic and

microbial‐mediated process. DIA and DDA are also degradation products of atrazine and

propazine.

DIA HS DDA

Figure 4.  Structure of Major Simazine Degradation Products and Minor Chlorodegradate DDA

Apart from DDA, a number of other minor degradates were identified in the environmental fate

studies, including s‐Triazin‐2‐ol, 4‐amino‐6‐(ethylamino)‐ (8CI) (DIHA), Ammeline (OAAT), 2‐

Amino‐4‐ethylamino‐6‐methoxy‐s‐triazine (MAET), o‐methyl hydroxysimazine, and 2‐chloro‐4‐

ethylamino‐6‐hydroxyethylamino‐s‐triazine. Table 6 shows the chemical names and formulas

for all simazine degradation products found in laboratory studies.
41

Table 6. Chemical Names for Simazine Degradation Products   

Common Name Chemical Name Chemical

Formula

CAS

Reg No. Synonyms

Deisopropylatrazine 2‐chloro‐6‐ethylamino‐4‐

amino‐s‐triazine

C5H8ClN5 1007‐

28‐9

CEAT/DIA/G‐28279

Hydroxysimazine bis(ethylamino)‐1,3,5‐

triazin‐2‐ol

C7H13N5O 2599‐

11‐3

OEET/HS/G‐30414/

GS‐30414

Diadealkylatrazine chlordiamino‐s‐triazine C3H4ClN5 3397‐

62‐4

CAAT/DACT/DDA/

GS‐28273

Deisopropylhydroxyatrazine 2‐hydroxy‐6‐ethylamino‐4‐

amino‐s‐triazine

C5H9N5O 7313‐

54‐4

OIAT/DIHA/GS‐

17792

2‐chloro‐4‐ethylamino‐6‐

hydroxyethylamino‐s‐triazine

2‐chloro‐4‐ethylamino‐6‐

hydroxyethylamino‐s‐

triazine

C3H9ClN5O None

Found

G‐28516

Ammeline 4,6‐diamino‐2,5‐dihydro‐

1,3,5‐triazin‐2‐one

C3H5N5O 645‐92‐

1

OAAT/GS‐17791

MAET 2‐Amino‐4‐ethylamino‐6‐

methoxy‐s‐triazine

C6H11N5O 30360‐

56‐6

G‐31709/ GS‐31709

o‐methyl hydroxysimazine N2,N4‐diethyl‐6‐methoxy‐

1,3,5‐triazine‐2,4‐diamine

C8H15N5O 673‐04‐

1

G‐30044

Simazine degrades to DIA, HS, and DDA in both soil and water. In laboratory studies, DIA, HS,

and DDA formed up to 18%, 62%, and 9% of applied simazine, respectively (Table 7). When

formed in soil, DIA and DDA are expected to readily dissipate through runoff and leaching, as

their organic carbon:water and soil:water partition coefficients are lower than simazine and

would be classified as mobile under the FAO mobility classification (Table 8; FAO 2000). HS is

classified as slightly mobile in soil with an average KFOC of 4,328 (FAO 2000). As such, HS is less

susceptible to leaching than DIA or DDA. Like other compounds with high organic carbon:water

and soil:water partition coefficients; however, HS formed in soil may move to surface water

primarily via eroded sediment.

DIA and HS were detected in terrestrial and aquatic field dissipation studies (Table 7). DIA and

DDA have been widely detected in ambient surface water monitoring conducted in federal,

state, and registrant‐conducted monitoring programs. It should be noted that these detections

may not be solely attributable to simazine use, as atrazine or propazine may have been used

concurrently in the same watershed. Although HS is expected to be present in surface water, it

is rarely reported in ambient surface water monitoring and has not been analyzed in registrant‐

conducted surface water monitoring programs.   

Table 7. Identification of Simazine Degradation Products in Environmental Fate Studies

Study Degradation

Products

Maximum Percentage of

Applied Simazine (day) References

Hydrolysis   None None

MRID 00027856

Acceptable

Photodegradation in

Water

No known

degradates

identified

No known degradates

quantified

MRID 42503708

Acceptable
42

Study Degradation

Products

Maximum Percentage of

Applied Simazine (day) References

Photodegradation

on Soil 

DDA 1.0 (30 d) MRID 42739101

Acceptable DIA 16.1 (30 d)

HS 2.2 (30 d)

Aerobic Soil

Metabolism

DDA 9.2 (270 d) MRIDs 46561301,

43004501,

00158638

Supplemental

DIA 17.5 (90 d)

HS 62.08 (371 d)

Anaerobic Soil

Metabolism

DDA 2.2 (0 d) MRID 00027857

Supplemental DIA 7.7 (0 d)

HS 5.6 (70 d)

Anaerobic Aquatic

Metabolism

DDA 0.62 (30 d) MRID 40614411

Acceptable DIA 6.87 (365 d)

HS 6.98 (365 d)

Aerobic Aquatic

Metabolism

DIA 1.6 (0 d and 7 d) MRID 43004502

Acceptable HS 11.7 (30 d)

Terrestrial Field

Dissipation

DIA 1.8 (15 d) (0‐8 inches) MRIDs 40634202,

00027863

Supplemental HS 4.9 (31 d) (0‐8 inches)

Aquatic Field

Dissipation

DIA 36.8* (6 d)

MRIDs 00027829,

40614420,

40614421,

40614422

Supplemental

HS 26.4 (7 d)** water

* Percent applied based on an initial application of 2.04 mg/L, as the 0‐day simazine concentration

measurement was below the LOD. An additional application of simazine was made on day 7 of the

study.  Maximum degradate concentrations in water during the study period were 1.02 mg/L on

day 9 for DIA and 0.81 mg/L on day 8 for HS.   

** Percent applied based on a maximum simazine detection of 5.30 mg/L on day 7 of the study.

Maximum degradate concentrations during the study period were 0.74 mg/L on day 120 for DIA

and 1.40 mg/L on day 7 for HS.  Maximum degradate concentrations in sediment during the study

period were 0.28 ppm on day 120 for DIA and 0.07 ppm on day 14 for HS.

Table 8.  Soil Sorption Coefficients for Simazine Degradation Products

Degradation

Product KF 1/n Average KFOC References

DDA 0.16‐1.56 0.858‐1.09 55 MRID 41257904

DIA 0.16‐2.7 0.874‐1.025 51 MRID 41257905

HS 8.48‐468.02 0.45‐0.66 4,328 MRID 41257903
43

AQUATIC EXPOSURE ASSESSMENT

The aquatic exposure assessment is designed to consider standard ecological modeling (i.e.,

Surface Water Concentration Calculator [SWCC] and EFED Provisional Cranberry Model) and

surface water monitoring data. Although each of the exposure assessment methods have

limitations in estimating environmental concentrations (EEC) of simazine, the combination of

the modeling approaches and monitoring data, taken together, provide a comprehensive

depiction of simazine occurrence in surface water at various spatial scales.   

Aquatic Exposure Modeling

Tier II Exposure Assessment using Surface Water Concentration Calculator

The Surface Water Concentration Calculator (version 1.106) using the standard pond was used

for Tier II surface water modeling. The ecological exposure modeling was conducted for parent

simazine because DIA and DDA have equal or less toxicity (USEPA 2015) and given the half‐life

of simazine parent, the aquatic exposures are expected to be primarily simazine parent.

All simazine products are required to have a spray drift and runoff buffer restriction whereby

the product cannot be applied “within 66 feet of the points where field surface water runoff

enters perennial or intermittent streams and rivers or within 200 feet of natural or impounded

lakes or reservoirs” (USEPA 2006). Spray drift fractions were calculated according to standard

policy using Tier 1 default droplet size spectrums and release heights (Table 9; Brady 2013). For

this assessment, the ecological modeling was conducted using a 66‐foot buffer. This buffer was

selected to provide a conservative assessment of the estimated environmental concentrations

(EECs).

Table 9. Spray Drift Fractions for Required Spray Drift Buffers on Simazine Labels

Buffer Size (feet) Application Method Drift Fraction

661 Ground 0.017

Aerial 0.064

2002 Ground 0.008

Aerial 0.031

1‐Drift buffer for perennial or intermittent streams and rivers

2‐Drift buffer for natural or impounded lakes or reservoirs

Surface water modeling using SWCC was conducted for uses on the Section 3 and 24(c)

simazine labels with the exception of wet‐harvested cranberry and aquaria and ornamental

ponds (Table 1 and Table 2). Standard crop scenarios or appropriate custom crop scenarios

from cumulative drinking water assessments (e.g., Organophosphate [OP], N‐Methyl

Carbamate [NMC]) or endangered species assessments (e.g., Red‐Legged Frog [RLF], Barton

Springs Salamander [BSS]) are used in this assessment. Several use patterns such as shelter

belts, macadamia nuts, and Christmas trees do not have standard or customized scenarios.  

Surrogate model scenarios, therefore, are used to represent these specialized crop use
44

patterns. Table 10 shows the selection criteria used for adoption of certain surrogate model

scenarios for some labeled sanctioned use patterns.        

Table 10. Criteria for Surrogate Model Scenarios in SWCC Modeling for Simazine

Crop Scenario Selection Criteria for Surrogate Scenario

Shelter Belts GA pecan The GA pecan and OR filbert scenarios were used to represent the

OR filbert shelter belt.   

Macadamia Nuts CA avocado Although Hawaii has the highest macadamia nut production in the

United States, California has production areas for macadamia nuts.  

The CA avocado scenario is selected to be representative of a

California use site for a nut crop.   

Christmas Trees GA pecans Because christmas tree plantations can exist throughout the United

States, the selected scenarios represent different geographic regions

for tree crops.

MI cherries

OR christmas trees

SWCC modeling used the recommended simazine application conditions as prescribed on the

Section 3 and 24(c) labels (Table 1 and Table 2). Maximum application rates, maximum number

of applications, and minimum application intervals were used in the modeling. Application

dates were selected based label directions and recommendations from BEAD. One exploratory

model run evaluated the concentrations following a reduced application rate to corn (Table 11),

and was based on use information for corn provided by BEAD. The environmental fate inputs

used in the modeling were derived from registrant‐submitted studies to support registration.  

Table 11 and Table 12 outline the application and fate inputs, respectively, for SWCC modeling.   

Table 11. Application Inputs for SWCC Modeling

Crop Scenario No.

Apps

Application

Rate per

Application1  

(lb a.i./A)

Application

Timing2

Application

Method3

Spray

Drift

Fraction

Corn, Field

Corn, and Sweet

Corn  

(Pre‐Emergent)

KS Corn 2 2/0.5 ‐21E, ‐7E Ground 0.017

MS Corn 2 2/0.5 ‐21E, ‐7E Ground 0.017

OH Corn 2 2/0.5 ‐21E, ‐7E Ground 0.017

PA Corn 2 2/0.5 ‐21E, ‐7E Ground 0.017

IL Corn 2 2/0.5 ‐21E, ‐7E Ground 0.017

IN Corn 2 2/0.5 ‐21E, ‐7E Ground 0.017

MN Corn 2 2/0.5 ‐21E, ‐7E Ground 0.017

NC Corn 2 2/0.5 ‐21E, ‐7E Ground 0.017

NE Corn 2 2/0.5 ‐21E, ‐7E Ground 0.017

CA Corn‐OP 2 2/0.5 ‐21E, ‐7E Ground 0.017

TX Corn‐OP 2 2/0.5 ‐21E, ‐7E Ground 0.017

FL‐Sweet Corn‐OP 2 2/0.5 ‐21E, ‐7E Ground 0.017

NCcornOP 2 2/0.5 ‐21E, ‐7E Ground 0.017

ORSWcornOP 2 2/0.5 ‐21E, ‐7E Ground 0.017

STXcornNMC 2 2/0.5 ‐21E, ‐7E Ground 0.017

KS Corn 1 1.6 +14E Ground 0.017

MS Corn 1 1.6 +14E Ground 0.017
45

Corn, Field

Corn, and Sweet

Corn  

(Highly Erodible

Land, Post‐

Emergent)  

OH Corn1 1.6 +14E Ground 0.017

PA Corn1 1.6 +14E Ground 0.017

IL Corn1 1.6 +14E Ground 0.017

IN Corn1 1.6 +14E Ground 0.017

MN Corn1 1.6 +14E Ground 0.017

NC Corn1 1.6 +14E Ground 0.017

NE Corn1 1.6 +14E Ground 0.017

CA Corn‐OP1 1.6 +14E Ground 0.017

TX Corn‐OP1 1.6 +14E Ground 0.017

FL‐Sweet Corn‐OP1 1.6 +14E Ground 0.017

NCcornOP1 1.6 +14E Ground 0.017

ORSWcornOP1 1.6 +14E Ground 0.017

STXcornNMC1 1.6 +14E Ground 0.017

Corn‐Fallow

KS Corn11 +196E Ground 0.017

MS Corn11 +196E Ground 0.017

OH Corn11 +196E Ground 0.017

PA Corn11 +196E Ground 0.017

IL Corn11 +196E Ground 0.017

IN Corn11 +196E Ground 0.017

MN Corn11 +196E Ground 0.017

NC Corn11 +196E Ground 0.017

NE Corn11 +196E Ground 0.017

CA Corn‐OP11 +196E Ground 0.017

TX Corn‐OP11 +196E Ground 0.017

FL‐Sweet Corn‐OP11 +196E Ground 0.017

NCcornOP11 +196E Ground 0.017

ORSWcornOP11 +196E Ground 0.017

STXcornNMC11 +196E Ground 0.017

Corn‐Reduced

Rate

KS Corn1 0.5 +196E Ground 0.017

MS Corn1 0.5 +196E Ground 0.017

OH Corn1 0.5 +196E Ground 0.017

PA Corn1 0.5 +196E Ground 0.017

IL Corn1 0.5 +196E Ground 0.017

IN Corn1 0.5 +196E Ground 0.017

MN Corn1 0.5 +196E Ground 0.017

NC Corn1 0.5 +196E Ground 0.017

NE Corn1 0.5 +196E Ground 0.017

CA Corn‐OP1 0.5 +196E Ground 0.017

TX Corn‐OP1 0.5 +196E Ground 0.017

FL‐Sweet Corn‐OP1 0.5 +196E Ground 0.017

NCcornOP1 0.5 +196E Ground 0.017

ORSWcornOP1 0.5 +196E Ground 0.017

STXcornNMC1 0.5 +196E Ground 0.017

Alfalfa CAafalfaOP1 1.6 1/16 Ground 0.017
46

CAalfalfaOP1 1.6 1/16 Aerial 0.064

Blueberries/

Caneberries

ORberries14 6/1 Ground 0.017

ORberries24 6/1,8/30 Ground 0.017

Strawberry

CAstrawberries14 8/1 Ground 0.017

ORberries14 8/1 Ground 0.017

CAstrawberries11 11/l Ground 0.017

ORstrawberries11 11/1 Ground 0.017

Grapes NYgrapes

14 3/1 Ground 0.017

CAgrapes14 1/1 Ground 0.017

Nectarines CAfruit12 11/1 Ground 0.017

Grapefruit/

Orange

CAcitrus14 3/1 Ground 0.017

STXgrapefruitNMC14 3/1 Ground 0.017

FLcitrus18 5/1 Ground 0.017

FLcitrus22 3/1,5/30 Ground 0.017

Lemon

CAcitrus14 3/1 Ground 0.017

STXgrapefruitNMC2 1.6 6/1,8/30 Ground 0.017

Apple

ORapple14 11/1 Ground 0.017

NCapple14 11/1 Ground 0.017

PAapple14 11/1 Ground 0.017

WAorchards14 11/1 Ground 0.017

Avocado

FLavocado14 11/1 Ground 0.017

CAavocado14 11/1 Ground 0.017

Peach GApeach

14 2/1 Ground 0.017

CAfruit12 11/1 Ground 0.017

Olive CAolive14 1/11 Ground 0.017

Cherries MIcherries14 11/1 Ground 0.017

Pear/Plums

WAorchards14 11/1 Ground 0.017

ORapples14 11/1 Ground 0.017

NCapples14 11/1 Ground 0.017

PAapples14 11/1 Ground 0.017

Turf/

Residential

FLturf2 2/1 3/1, 3/30 Ground 0.017

CAturf2 2/1 3/1,3/30 Ground 0.017

Sod Farms

FLturf2 4/2 3/1, 3/30 Ground 0.017

CAturf2 2/1 3/1, 3/30 Ground 0.017

Almond CAalmond12 11/1 Ground 0.017

Hazelnut

(Filbert)

ORFilbert14 2/1 Ground 0.017

ORFilbert22 6/1,8/30 Ground 0.017

Macadamia Nut CAavocado14 4/1 Ground 0.017

Pecan GApecan14 4/1 Ground 0.017

Walnut

ORFilbert14 4/1 Ground 0.017

GAPecan14 4/1 Ground 0.017

Nursery

CAnursery13 9/1 Ground 0.017

MInursery13 9/1 Ground 0.017

NJnursery13 9/1 Ground 0.017
47

ORnursery 1 3 9/1 Ground 0.017

TNnursery 1 3 9/1 Ground 0.017

Nursery

NurseryBSS 1 3 9/1 Ground 0.017

CAnursery 1 3 3/1 Ground 0.017

MInursery 1 3 3/1 Ground 0.017

NJnursery 1 3 3/1 Ground 0.017

ORnursery 1 3 3/1 Ground 0.017

TNnursery 1 3 3/1 Ground 0.017

NurseryBSS 1 3 3/1 Ground 0.017

Christmas Trees

OrXmastrees 2 2 6/1, 8/30 Ground 0.017

MIcherries 2 2 6/1, 8/30 Ground 0.017

GApecan 2 2 6/1, 8/30 Ground 0.017

FLTurf 1 4 3/1 Ground 0.017

Shelterbelts

GApecan 1 4 3/1 Ground 0.017

ORfilbert 1 4 3/1 Ground 0.017

GApecan 2 2 6/1, 8/30 Ground 0.017

ORfilbert 2 2 6/1,8/30 Ground 0.017

1

To model uses where there may be two applications to the field with differing application rates, the forward slash

demarcates the application rate for the first application and the rate for the second application. 2

Application timing was either a specific date listed as month/day or a day selected in reference to the crop

emergence date (E) specified in the SWCC scenario. For example, ‐21E would indicate the application occurred 21

days prior to the crop emergence date specified in the SWCC scenario. The dates of application where two

applications occur are separated by a comma. 3

Application efficiency inputs for ground and aerial applications are 0.99 and 0.95, respectively, per EFED input

parameter guidance.

Table 12. Fate Inputs for SWCC Modeling1

PARAMETER ESTIMATED

VALUE (EV)

EFED MODELING

VALUE2 SOURCE

Aerobic Soil Metabolism Half‐life

(days) 126, 993

, 88.9

119 days (90th CBM)

Average=104.6

SD=13.6

t90,n‐1=1.886

n=3

MRID 00158638

MRID 43004501

MRID 46561301

Organic Carbon Partition

Coefficient (Koc) (mL/goc) 123 123 MRID 41257905

Aerobic Aquatic  

Half‐Life (days)   

74.4

(SFO)

223

(3*EV) MRID 43004502

Anaerobic Aquatic  

half‐life (days)

666

(SFO)

1999

(3*EV) MRID 40614411

Aqueous Photolysis  

half‐life (days)4 Stable Stable MRID 42503708

Hydrolysis  

half‐life (days) Stable Stable MRID 00027856
48

PARAMETER ESTIMATED

VALUE (EV)

EFED MODELING

VALUE2 SOURCE

Hydrolysis  

half‐life (days) Stable Stable MRID 00027856

Molecular Weight (g/mole) 201.7 201.7 Simazine RED

Water Solubility at 25°C

(mg/L) 3.5 3.5 MRID 00023955

Vapor Pressure (torr) 6.1E‐9 6.1E‐9 MRID 00023955

CBM = Confidence Bound on the Mean; EV = Estimated Value; SD = Standard Deviation; SFO = Single First Order 1 All SWCC modeling assumes a 0‐day foliar decay rate. 2 All SWCC Model Inputs for temperature are 25oC.    3 Temperature corrected half‐life from 20oC to 25oC. Temperature correction was done using Q10 equation.    4 Estimated Value is for a latitude of 40oN.

The EECs for simazine in surface water are not expected to exceed 205 μg/L for 1‐in‐10 year

daily peak, 198 μg/L for the 1‐in‐10 year 21‐day average, and 187 μg/L for the 1‐in‐10‐year 60‐

day average (Table 13). These maximum concentrations for simazine are associated with the

Florida Citrus scenario. For this particular scenario, the maximum label rate is 4 lb a.i./A.  

Because a minimum interval is not specified on all labels; however, the single application rate

was assumed to be equivalent to the annual application rate of 8 lb a.i./A/year. The peak EECs

for corn range from 32 to 162 μg/L for an application of 2.5 lb a.i./A/year.

The simazine EECs are intended to represent exposure that recurs with a frequency of once

every ten years at a site which is more vulnerable to pesticide movement in surface water than

90% of the sites that can be used to grow the treated crop. All available model scenarios were

used and may represent sites more or less vulnerable than the target vulnerability. The EECs

are expected to represent first‐order streams and small static water bodies adjacent to

simazine use areas.    

Table 13. SWCC EECs for Simazine in the Standard Pond

Crop Scenario Application

Method

Application

Rate  

(lb a.i./A)  

per app

1‐in‐10‐Year

Peak 21‐Day

Average

60‐Day

Average

μg/L

Corn, Field Corn,

and Sweet Corn  

(Pre‐Emergent)

ILCornSTD Ground 2/0.5 98.4 96.3 93.6

MScornSTD Ground 2/0.5 113 110 106

NCcornESTD Ground 2/0.5 34 33.3 32.4

OHCornSTD Ground 2/0.5 62.1 60.7 57.6

PAcornSTD Ground 2/0.5 57 55.9 54.2

CAcornOP Ground 2/0.5 32.3 32 31.8

FLsweetcornOP Ground 2/0.5 123 119 115

NCcornWOP Ground 2/0.5 69.8 68.5 66.5

NDcornOP Ground 2/0.5 57.3 56.3 54.8

ORswcornOP Ground 2/0.5 36.6 35.7 35.6

TXcornOP Ground 2/0.5 56.2 54.3 51.4

STXcornNMC Ground 2/0.5 70.7 68 63.6

IAcornstd Ground 2/0.5 45.9 44.2 41.5
49

INCornStd Ground 2/0.5 92.3 90.1 85.8

KSCornStd Ground 2/0.5 111 108 103

MNCornStd Ground 2/0.5 74.2 72.6 69.3

NECornStd Ground 2/0.5 162 158 151

Corn, Field Corn,

and Sweet Corn  

(Highly Erodible

Land, Post‐

Emergent)

ILCornSTD Ground 1.6 32.8 32.3 32.3

MScornSTD Ground 1.6 49 47.5 44.9

NCcornESTD Ground 1.6 18.4 17.9 16.9

OHCornSTD Ground 1.6 38.9 37.8 36.1

PAcornSTD Ground 1.6 23.3 22.8 21.8

CAcornOP Ground 1.6 8.94 8.65 8.24

FLsweetcornOP Ground 1.6 93.1 90.5 87.8

NCcornWOP Ground 1.6 41.6 40.5 38.7

NDcornOP Ground 1.6 33.4 32.5 30.9

ORswcornOP Ground 1.6 14.5 14.2 14.2

TXcornOP Ground 1.6 31.5 30.6 28.6

STXcornNMC Ground 1.6 54.6 51.7 47.7

IAcornstd Ground 1.6 24.8 24.3 23.6

INCornStd Ground 1.6 21.4 21.4 21.5

KSCornStd Ground 1.6 52.7 50.9 48.3

MNCornStd Ground 1.6 29.1 28.3 27.1

NECornStd Ground 1.6 67.6 67.6 64.4

Corn‐Fallow ILCornSTD Ground1 39.3 39.7 34.4

MScornSTD Ground1 41.9 41.4 40.4

NCcornESTD Ground1 28.9 28.2 26.9

OHCornSTD Ground1 37.7 38.2 36.1

PAcornSTD Ground1 29.5 29.4 29

CAcornOP Ground1 16 15.6 15.3

FLsweetcornOP Ground1 56.7 54.7 52.8

NCcornWOP Ground1 27.7 27.6 26.3

NDcornOP Ground1 10.6 10.4 10.1

ORswcornOP Ground1 55.7 56.2 52.9

TXcornOP Ground1 39.4 38.1 36

STXcornNMC Ground1 59.2 56.3 57.8

IAcornstd Ground1 13.8 13.6 13

INCornStd Ground1 44.7 44 42.9

KSCornStd Ground1 48.3 47.4 44.1

MNCornStd Ground1 18.1 17.9 17.4

NECornStd Ground1 33.8 33.6 26.5

Corn‐Reduced Rate ILCornSTD Ground 0.5 19.6 19.9 17.2

MScornSTD Ground 0.5 20.9 20.7 20.2

NCcornESTD Ground 0.5 14.5 14.1 13.4

OHCornSTD Ground 0.5 18.9 19.1 18

PAcornSTD Ground 0.5 14.8 14.7 14.5

CAcornOP Ground 0.5 7.99 7.81 7.63

FLsweetcornOP Ground 0.5 28.3 27.4 26.4

NCcornWOP Ground 0.5 13.9 13.8 13.2

NDcornOP Ground 0.5 5.29 5.22 5.07

ORswcornOP Ground 0.5 27.9 28.1 26.4

TXcornOP Ground 0.5 19.7 19.1 18

STXcornNMC Ground 0.5 29.6 28.2 28.9

IAcornstd Ground 0.5 6.91 6.81 6.51

INCornStd Ground 0.5 22.3 22 21.5

KSCornStd Ground 0.5 24.1 23.7 22.1

MNCornStd Ground 0.5 9.05 8.96 8.7

NECornStd Ground 0.5 16.9 16.8 13.3

CAafalfaOP Ground 1.6 24.1 23.4 22.5
50

Alfalfa Grown for

Seed

CAalfalfaOP Aerial 1.6 31.2 30.4 29.3

Blueberries/

Caneberries

ORberries Ground4 22.0 21.7 21

ORberries Ground8 60.8 61.3 60.2

Almond CAalmond Ground2 23.5 23.9 23

Nectarine CAfruit Ground2 8.18 7.95 7.75

Strawberry CAstrawberries Ground4 64.8 64.2 63.7

ORstrawberries Ground1 22.9 22.4 22.5

Grape NYgrapes Ground4 57.6 57.2 55.9

CAgrapes Ground4 37.3 36.5 35.1

Citrus CAcitrus Ground4 8.34 8.10 7.71

STXgrapefruitNMC Ground4 33.7 32.6 30.6

CAcitrus Ground 2/2 7.40 7.13 6.64

STXgrapefruitNMC Ground 2/2 30.4 28.8 26.5

FLcitrus Ground8 205 198 187

FLcitrus Ground4 102 98.9 93.6

FLcitrus Ground 2/2 87.3 83.4 78.3

STXgrapefruitNMC Ground 1.6/1.6 24.3 23.1 21.2

Apple NCappleSTD Ground4 83 81.7 80.8

ORappleSTD Ground4 105 105 106

PAappleSTD_V2 Ground4 59.6 58.3 57.6

WAorchardsNMC Ground4 16.1 16 15.5

Avocado FLavocado Ground4 13.1 13.5 13.2

CAavocado Ground4 12.5 12.1 11.6

Peach CApeach Ground2 8.18 7.95 7.75

GApeach Ground4 42.3 40.9 38.8

Olive CAolive Ground4 20.7 20.1 19.8

Cherries WAorchardsNMC Ground4 16.1 16 15.5

MICherriesSTD Ground4 55.2 54.3 52.8

Pears NCappleSTD Ground4 83 81.7 80.8

ORappleSTD Ground4 105 105 106

PAappleSTD_V2 Ground4 59.6 58.3 57.6

WAorchardsNMC Ground4 16.1 16 15.5

Plums NCappleSTD Ground4 83 81.7 80.8

ORappleSTD Ground4 105 105 106

PAappleSTD_V2 Ground4 59.6 58.3 57.6

WAorchardsNMC Ground4 16.1 16 15.5

Turf/Residential FLturf Ground 2/1 9.09 8.93 8.65

Sod Farms FLturf Ground 4/2 30.1 28.7 27.8

Hazelnut(Filbert) ORFilbert Ground 2/2 25 24.6 24.1

ORFilbert Ground4 71.2 70.4 68.5

Macademia Nut CAavocado Ground4 11.2 10.8 10.3

Pecan GApecan Ground4 114 111 105

Walnut CAalmond Ground4 20.2 19.8 19.3

Nursery TNnurserySTD_V2       Ground 3(fall) 60.3 57.8 54.5

CAnurserySTD_V2       Ground 3(fall) 6.38 6.15 5.83

FLnurserySTD_V2       Ground 3(fall) 60.9 57.7 53

MInurserySTD_V2       Ground 3(fall) 91.3 90.4 91.3

NJnurserySTD_V2       Ground 3(fall) 59.4 57.1 57.2

ORnurserySTD_V2       Ground 3(fall) 29.4 28.5 27.4

NurseryBSS_V2   Ground 3(fall) 59 57.4 54.2

TNnurserySTD_V2       Ground 3(spring) 61.1 59.5 57

CAnurserySTD_V2       Ground 3(spring) 31.9 31.3 30.5

FLnurserySTD_V2       Ground 3(spring) 32 31.2 30.1

MInurserySTD_V2       Ground 3(spring) 58.7 58.2 56.9

NJnurserySTD_V2       Ground 3(spring) 58.7 58.2 57.1
51

ORnurserySTD_V2       Ground 3(spring) 21.5 21.2 20.6

NurseryBSS_V2         Ground 3(spring) 31.1 30.5 29.2

Christmas Trees ORXmasTreeSTD         Ground 2/2 15 14.7 14.3

MICherriesSTD         Ground 2/2 58.9 57.9 55.9

GAPecansSTD           Ground 2/2 86.5 82.6 83.6

ORXmasTreeSTD         Ground 4 15.6 15.3 14.9

MICherriesSTD         Ground 4 44 43.1 41.9

GAPecansSTD           Ground 4 134 129 123

Shelter Belts GAPecansSTD           Ground 2/2 86.5 82.6 83.6

CAalmond_WirrigSTD    Ground 2/2 29.1 28.3 27

MICherriesSTD         Ground 2/2 58.9 57.9 55.9

GAPecansSTD           Ground 4 134 129 123

CAalmond_WirrigSTD    Ground 4 21.8 21.5 21

MICherriesSTD         Ground 4 44 43.1 41.9

Aquatic Exposure Modeling for the Cranberry Use

Simazine may be used on cranberry fields before spring or, in the case of Massachusetts, after

harvest. The maximum simazine application rate for cranberries is 4 lb a.i./A in Massachusetts

and 2 lb a.i./A in other states, with a maximum of one application per year.

To determine EECs for cranberry use, the SWCC (ORberries scenario) and the EFED Provisional

Cranberry Model (v 2.0) were used (https://www.epa.gov/pesticide‐science‐and‐assessing‐

pesticide‐risks/models‐pesticide‐risk‐assessment#aquatic). Results for the SWCC Oregon berries

scenario are representative of dry‐harvested cranberry fields, which are typically found in the

Pacific Northwest. Provisional Cranberry Model results are representative of wet‐harvested

cranberry fields, which are predominant in the Northeast United States and also found in the

Pacific Northwest. Wet‐harvested cranberry fields are flooded to protect plants from the cold in

the winter, to harvest, and to remove fallen leaves and control pests (Cape Cod Cranberry

Growers Association 2001). The most common flooding times when the crop is active is when

cranberry bogs are flooded a few days prior harvest followed with the release of water a few

days later. After flooding of the cranberry fields, water may be held in a holding system,

recirculated to other cranberry growing areas, or released to adjacent water bodies (rivers,

streams, lakes, or bays).  

The Provisional Cranberry Model simulates application of pesticide to a dry field and

degradation in soil (using the aerobic soil metabolism half‐life) prior to flooding of the cranberry

bog. In this case, it was assumed that simazine was applied 225 days prior to a 3‐day flood

event for harvesting.

Results of the cranberry modeling for simazine are in Table 14. The maximum concentrations

are for wet‐harvested cranberry, with a peak EEC of 371 μg/L, a 21‐day average EEC of 360

μg/L, and a 60‐day average EEC of 339 μg/L, all for an application rate of 4 lb a.i./A.   
52

Table 14. EECs for the Simazine Cranberry Use

Use Areas  

Application

Rate  

(lb a.i./A)

Dry‐Harvested Cranberry/SWCC

Results* (μg/L)

Wet‐Harvested Cranberry/Provisional

Model Results (μg/L)

1‐in‐10‐yr

Peak

1‐in‐10‐yr

21‐day

Average

1‐in‐10‐yr  

60‐day

Average

Peak 21‐day

Average

60‐day

Average

Massachusetts 4 22.0 21.7 21.0 371 360 339

Other States 2 11.0 10.8 10.5 185 180 169

*assumes an application date of March 20 and 100% crop treated.

Aquatic Exposure Monitoring

Surface Water Monitoring Data Evaluated

Second to atrazine, the surface water monitoring data available for simazine is the most robust

pesticide monitoring data available in the United States and is of particular importance in

assessing aquatic exposures. Monitoring data for simazine occurrence in surface water were

obtained from USGS National Water Information System (NWIS), EPA Storage and Retrieval

Warehouse (STORET), and USGS National Water Quality Assessment (NAWQA). Additionally,

monitoring data were obtained from California Department of Pesticide Regulation (CalDPR),

Syngenta’s Atrazine Monitoring Program (AMP), Syngenta’s Simazine Monitoring Program

(SMP), Syngenta’s Atrazine Ecological Exposure Monitoring Program (AEEMP), Louisiana

Department of Agriculture and Forestry (LDAF), Oregon ELEM (OR ELEM), Oregon LASAR (OR

LASAR), Washington Department of Agriculture (WDA), Wisconsin Department of Natural

Resources (WI DNR), USDA Pesticide Data Program (PDP) and USGS‐EPA reservoir monitoring

program (USGS‐EPA RES). There are 29,717 site‐years of simazine surface water monitoring

data considered in this monitoring data analysis (Appendix B).

Characteristics of monitoring programs for atrazine from 1986 to 2015 are shown in Table 15.  

The monitoring programs, as expected, vary regarding their objective and monitoring strategy.  

Several of the programs such as the USGS National Water Quality Assessment, California

Surface Water Monitoring Program (CSW), and USGS‐EPA Pilot Monitoring Program were

developed to assess general pesticide occurrence in ambient surface water. In contrast, other

monitoring programs such as Syngenta’s Simazine Monitoring Program (SMP) was targeted to

simazine use areas and Syngenta’s Atrazine Monitoring Program (AMP) and Atrazine Ecological

Exposure Monitoring Program (AEEMP) monitoring programs were targeted to atrazine use

areas. Additionally, of all the monitoring data considered in this assessment, only the AEEMP

collects simazine occurrence data according to a statistical sampling plan.
53

Table 15. Characteristics of Representative Monitoring Programs for Simazine and Its

Degradation Products in Surface Water

Study

Number of

States and

Territories

Number of

Sampling

Stations

Years Targeted   

Monitoring

Reported

LOD

(μg a.i./L)

California Department

of Pesticide

Regulation

1 591 1991‐2012 No 0.0049‐25

Wisconsin DNR 1 12 2009‐2010 Yes 01

USGS‐EPA Reservoir 12 20 1999‐2000 No <0.009

AEEMP 5 10 2014 Yes3 <0.05

AMP and SMP 13 1812 2003‐20135 Yes4 0.05

PDP 29 70 2004‐2009 No 0.00071‐0.05

STORET 26 3,928 1986‐2014 No 0.00024‐27

NWIS 52 5,344 1989‐2015 No 0.001‐0.8

NAWQA 49 1,813 1991‐2014 No 0.0003‐9.99

Washington

Department of

Agriculture

1 25 2003‐2013 Unknown 0.012‐0.048

Oregon LASAR 1 253 1990‐2012 Unknown 0.0005‐0.0232

Oregon ELEM 1 233 2012‐2015 Unknown 0.002‐0.443

Louisiana Department

of Agriculture and

Forestry

1 1 2010‐2014 Unknown not reported

1 LOD was reported as zero in data. 2 This is the number of Community Water Systems monitored. Each CWS may have one or more source‐water

sampling locations across site‐years. 3 Targeted to atrazine use areas. 4 SMP targeted to simazine use areas; AMP targeted to atrazine use areas. 5 EPA received a late submission of SMP data for Fairfield IL (PWSID IL1910100) for monitoring conducted in 2015.  

Results are consistent with the data described throughout the SMP and AMP program data (MRID 49829101).

Monitoring Data Analysis  

The monitoring data for simazine is analyzed by site‐year. This strategy was employed because

pesticide occurrence is dependent on spatially‐dependent site conditions including pesticide

use, agronomic practices, soil properties, meteorology, etc., as well as temporally‐dependent

conditions, including pesticide application timing and rainfall occurrence.   

The monitoring data are analyzed using a custom Python script. Each site‐year of monitoring

data with 4 or more samples in a year was analyzed by generating a stair‐step chemograph

from the first sampling date to last sampling date. The concentrations for non‐detections in the

chemograph were expressed as 1/2 the minimum reporting concentration (i.e., LOD or LOQ).  

Chemographs were not generated for site‐years with less than 4 samples in the year because

four samples per year is the minimum number of samples required under SDWA to assess

human health Maximum Contaminant Levels in ambient drinking water source water. Simazine
54

chemographs are generated by stair‐step imputation between measured values. The stair‐step

chemograph, therefore, provides a daily chemograph from the first sampling date to the last

sampling date in the year. From this chemograph, maximum daily concentration, maximum 4‐

day average concentration, maximum 21‐day average concentrations, maximum 60‐day

average concentrations, and maximum 90‐day average concentrations were derived.

Additionally, the Python script provides a count on the number of samples, number of non‐

detects, number of samples per quarter, and the average and median sampling intervals. For

site years with less than 4 samples per year, the maximum simazine concentration is reported.     

The simazine monitoring data indicate that simazine is widely used and detected in ambient

surface water. Detection frequencies are exceptionally high; 10 of the 13 data sets analyzed

have detection frequencies above 20% and 5 have detection frequencies above 50% (Table 16).

For the purpose of the analysis, ambient surface water is defined as surface water from flowing

water (rivers, streams, and springs), reservoirs, ponds, lakes, wetlands, canals, ditches,

estuaries, and oceans, as well as raw surface source water from community water systems

(CWS). Although several of the monitoring programs considered in this assessment have

undergone extensive data quality checks by the data developers, a quality assurance check was

conducted on reported simazine concentrations above 500 μg/L. The maximum daily

concentration reported is in the NWIS data 1,080 μg/L which is associated with a site in New

York State where simazine was applied to ornamental ponds (personal communication with

Bob Gilliom of USGS). Because discharges of treated water to ambient water bodies are

prohibited on ornamental pond and aquaria use product labels, it is presumed that this

concentration is a result of product misuse. Reported non‐NWIS maximum daily concentrations

range from non‐detects to 127 μg/L. The magnitude of the maximum daily concentrations in

the monitoring data collected after RED mitigation in 2006 are comparable to those collected

prior to 2006.

Table 16. Descriptive Statistics of Simazine Concentrations in Ambient Surface Water

Monitoring Programs   

Monitoring

Program

Number of

samples

Number of

non‐detects

Detection

Frequency

Across All

Site‐Years

Maximum

Daily Peak

Maximum

21‐Day

Average

Maximum

60‐Day

Average

μg/L

California

Department of

Pesticide

Regulation

8,010 5,567 30% 127 104.9 63.8

Wisconsin DNR 105 100 5% 0.834 0.7 0.5

USGS‐EPA

Reservoir 310 30 90% 1.58 1.4 1.3

AEEMP 965 611 37% 3.41 2.5 1.2

AMP and SMP1 28,648 24,773 14% 44.36 19.2 13.3

PDP 2,791 436 84% 1.6 1.0 0.5

STORET 36,127 34,051 6% 25 17.1 11.7

NWIS 42,813 20,007 53% 1,080 707.7 305.5
55

Monitoring

Program

Number of

samples

Number of

non‐detects

Detection

Frequency

Across All

Site‐Years

Maximum

Daily Peak

Maximum

21‐Day

Average

Maximum

60‐Day

Average

μg/L

NAWQA 29,404 10,750 63% 64.5 29.0 18.2

Washington

Department of

Agriculture

3,584 1,982 45% 0.69 0.3 0.1

Oregon LASAR 3,474 2,725 22% 13 9.4 7.6

Oregon ELEM 1,868 1,377 26% 10.5 9.9 5.3

Louisiana

Department of

Agriculture and

Forestry

2 0 100% 0.34 not

calculated

not

calculated

1

EPA received a late submission of SMP data for Fairfield IL (PWSID IL1910100) for monitoring conducted in 2015.  

Results are consistent with the data described throughout the SMP and AMP program data (peak simazine

concentration = 9.04 ug/L; MRID 49829101).

Simazine detections are most prominent in areas with high simazine use such as the corn belt

of the United States and California. The spatial distribution of simazine occurrence in ambient

surface waters is shown in Table 17, Table 18, and Table 19.  

Table 17. Distribution of Maximum Simazine Concentrations in Ambient Surface Water from

Site‐Years from Different States

State

Percentile No. of

Site‐

years

Min 5% 25% 50% 75% 95% Max

μg/L

AK 0.00 0.00 0.00 0.00 0.00 0.01 0.01 25

AL 0.00 0.00 0.01 0.03 0.08 1.11 34.10 348

AR 0.00 0.00 0.00 0.01 0.02 0.09 1.48 737

AS 0.00 0.00 0.00 0.00 0.00 0.00 0.00 4

AZ 0.00 0.00 0.00 0.00 0.01 0.04 0.67 167

CA 0.00 0.00 0.02 0.03 0.25 1.50 127.00 2340

CO 0.00 0.00 0.00 0.01 0.01 0.06 4.56 366

CT 0.00 0.00 0.00 0.01 0.01 0.04 0.69 122

DC 0.00 0.00 0.01 0.04 0.05 0.29 1.00 17

DE 0.00 0.00 0.02 0.03 0.03 0.10 0.12 43

FL 0.00 0.00 0.00 0.01 0.03 0.26 11.00 761

GA 0.00 0.00 0.00 0.02 0.14 1.32 10.60 768

HI 0.00 0.00 0.00 0.01 0.02 0.03 0.38 61

IA 0.00 0.01 0.03 0.03 0.03 0.08 20.00 2881

ID 0.00 0.00 0.00 0.00 0.00 0.01 0.50 297

IL 0.00 0.00 0.03 0.12 0.48 5.90 22.64 659

IN 0.00 0.00 0.00 0.25 0.90 2.86 25.00 850
56

State

Percentile No. of

Site‐

years

Min 5% 25% 50% 75% 95% Max

μg/L

KS 0.00 0.00 0.00 0.00 0.15 0.15 22.00 5471

KY 0.00 0.00 0.03 0.07 0.37 1.91 9.27 196

LA 0.00 0.00 0.02 0.06 0.17 0.34 1.57 576

MA 0.00 0.00 0.00 0.00 0.01 0.01 0.01 110

MD 0.00 0.00 0.01 0.03 0.13 2.18 8.58 293

ME 0.00 0.00 0.00 0.00 0.01 0.01 0.01 9

MI 0.00 0.00 0.01 0.03 0.10 1.34 3.64 86

MN 0.00 0.00 0.00 0.04 0.05 0.25 1.30 1352

MO 0.00 0.00 0.00 0.02 0.05 0.63 20.30 810

MS 0.00 0.01 0.03 0.09 0.22 0.60 1.67 182

MT 0.00 0.00 0.00 0.00 0.01 0.05 0.20 114

NC 0.00 0.00 0.00 0.01 0.10 0.66 6.70 634

ND 0.00 0.00 0.00 0.00 0.23 0.50 2.25 424

NE 0.00 0.00 0.02 0.03 0.06 0.25 3.91 735

NH 0.00 0.00 0.00 0.00 0.00 0.00 0.01 42

NJ 0.00 0.00 0.00 0.01 0.01 0.05 3.99 793

NM 0.00 0.00 0.00 0.01 0.06 0.34 7.05 383

NV 0.00 0.00 0.00 0.01 0.02 1.00 10.00 163

NY 0.00 0.00 0.00 0.01 0.01 0.23 1080.00 887

OH 0.00 0.02 0.11 0.37 1.08 4.35 28.50 430

OK 0.00 0.00 0.00 0.01 0.06 0.50 1.58 91

OR 0.00 0.00 0.00 0.01 0.04 0.88 13.00 1339

PA 0.00 0.00 0.01 0.03 0.10 0.95 44.36 521

RI 0.00 0.00 0.00 0.00 0.00 0.00 0.00 2

SC 0.00 0.00 0.01 0.03 0.11 1.71 14.80 157

SD 0.00 0.00 0.00 0.02 0.04 0.10 0.20 189

TN 0.00 0.00 0.00 0.01 0.03 10.17 20.64 252

TX 0.00 0.00 0.00 0.01 0.11 1.17 9.08 1019

UT 0.00 0.00 0.00 0.01 0.30 0.50 2.60 368

VA 0.00 0.00 0.00 0.01 0.08 1.97 32.50 303

VT 0.00 0.00 0.00 0.00 0.00 0.01 0.01 11

WA 0.00 0.00 0.00 0.01 0.03 0.14 4.99 721

WI 0.00 0.00 0.00 0.01 0.07 0.52 10.00 415

WV 0.00 0.00 0.00 0.01 0.03 0.17 0.60 50

WY 0.00 0.00 0.00 0.00 0.01 0.01 0.08 142
57

Table 18. Distribution of Maximum 21‐day Simazine Concentrations in Ambient Surface

Water from Site‐Years from Different States

State

Percentile No. of

Site‐

years > 4

samples

No. of

Site‐

years < 4

samples

Min 5% 25% 50% 75% 95% Max

μg/L

AK NA NA NA NA NA NA NA 0 25

AL 0.00 0.01 0.03 0.07 0.20 1.49 28.70 149 199

AR 0.00 0.01 0.01 0.06 0.10 0.20 0.29 50 687

AS NA NA NA NA NA NA NA 0 4

AZ 0.00 0.00 0.00 0.01 0.02 0.04 0.05 32 135

CA 0.00 0.01 0.03 0.10 0.38 4.37 104.92 618 1722

CO 0.00 0.00 0.01 0.01 0.02 0.10 1.63 87 279

CT 0.00 0.00 0.00 0.01 0.01 0.04 0.07 50 72

DC 0.02 0.02 0.02 0.06 0.08 0.08 0.08 5 12

DE 0.07 0.07 0.07 0.07 0.07 0.07 0.07 1 42

FL 0.00 0.00 0.01 0.04 0.13 0.35 2.71 112 649

GA 0.01 0.01 0.06 0.18 0.76 3.38 9.09 224 544

HI 0.00 0.00 0.00 0.12 0.24 0.24 0.24 4 57

IA 0.00 0.01 0.01 0.03 0.05 0.18 17.09 339 2542

ID 0.00 0.00 0.00 0.00 0.01 0.03 0.04 27 270

IL 0.00 0.01 0.07 0.17 1.11 5.78 19.16 416 243

IN 0.01 0.06 0.22 0.50 0.97 2.56 14.17 427 423

KS 0.00 0.01 0.02 0.04 0.08 0.47 3.95 110 5361

KY 0.01 0.02 0.04 0.08 0.42 2.07 5.82 97 99

LA 0.00 0.01 0.05 0.12 0.19 0.29 1.23 321 255

MA 0.00 0.00 0.00 0.01 0.01 0.01 0.01 29 81

MD 0.01 0.02 0.03 0.08 1.03 3.05 5.16 61 232

ME NA NA NA NA NA NA NA 0 9

MI 0.00 0.01 0.03 0.06 0.11 1.19 3.15 41 45

MN 0.00 0.00 0.01 0.01 0.01 0.04 0.33 81 1271

MO 0.00 0.01 0.02 0.07 0.23 1.02 16.75 190 620

MS 0.03 0.05 0.11 0.18 0.27 0.70 1.26 88 94

MT 0.00 0.00 0.00 0.00 0.01 0.01 0.01 15 99

NC 0.01 0.01 0.04 0.13 0.27 1.07 2.35 141 493

ND 0.00 0.00 0.00 0.01 0.01 0.04 0.23 28 396

NE 0.00 0.01 0.02 0.03 0.07 0.25 2.80 186 549

NH NA NA NA NA NA NA NA 0 42

NJ 0.00 0.01 0.01 0.02 0.02 0.09 0.25 106 687

NM 0.00 0.00 0.00 0.02 0.02 0.05 0.05 16 367

NV 0.00 0.00 0.01 0.01 0.07 1.62 2.42 76 87

NY 0.00 0.00 0.01 0.01 0.03 0.55 707.69 147 740
58

State

Percentile No. of

Site‐

years > 4

samples

No. of

Site‐

years < 4

samples

Min 5% 25% 50% 75% 95% Max

μg/L

OH 0.01 0.06 0.26 0.51 1.06 3.42 15.80 223 207

OK 0.01 0.25 1.22 1.40 1.40 1.42 1.42 5 86

OR 0.00 0.01 0.02 0.06 0.30 1.65 9.89 370 969

PA 0.00 0.01 0.02 0.05 0.19 0.86 16.43 134 387

RI NA NA NA NA NA NA NA 0 2

SC 0.00 0.01 0.03 0.08 0.48 3.10 10.47 76 81

SD 0.01 0.01 0.01 0.01 0.02 0.02 0.02 5 184

TN 0.00 0.01 0.01 0.02 0.13 14.22 18.12 93 159

TX 0.00 0.00 0.01 0.07 0.56 2.25 7.14 280 739

UT 0.00 0.00 0.01 0.01 0.02 0.20 0.26 26 342

VA 0.00 0.00 0.05 0.12 0.52 1.82 21.58 99 204

VT 0.01 0.01 0.01 0.01 0.01 0.01 0.01 2 9

WA 0.00 0.00 0.01 0.02 0.03 0.11 0.71 303 418

WI 0.00 0.01 0.02 0.06 0.21 0.54 1.83 117 298

WV 0.00 0.01 0.02 0.02 0.18 0.37 0.42 5 45

WY 0.01 0.01 0.01 0.01 0.01 0.01 0.01 2 140

Table 19. Distribution of Maximum 60‐day Simazine Concentrations in Ambient Surface

Water from Site‐Years from Different States

State

Percentile No. of

Site‐

years > 4

samples

No. of

Site‐

years < 4

samples

Min 5% 25% 50% 75% 95% Max

μg/L

AK NA NA NA NA NA NA NA 0 25

AL 0.00 0.01 0.02 0.06 0.14 0.99 18.16 149 199

AR 0.00 0.00 0.01 0.05 0.08 0.16 0.20 50 687

AS NA NA NA NA NA NA NA 0 4

AZ 0.00 0.00 0.00 0.01 0.01 0.02 0.03 32 135

CA 0.00 0.01 0.02 0.06 0.25 2.49 63.83 618 1722

CO 0.00 0.00 0.00 0.01 0.02 0.07 0.57 87 279

CT 0.00 0.00 0.00 0.01 0.01 0.02 0.03 50 72

DC 0.01 0.01 0.02 0.04 0.06 0.06 0.06 5 12

DE 0.04 0.04 0.04 0.04 0.04 0.04 0.04 1 42

FL 0.00 0.00 0.01 0.03 0.10 0.27 1.53 112 649

GA 0.00 0.01 0.04 0.13 0.49 1.86 6.16 224 544

HI 0.00 0.00 0.00 0.05 0.09 0.09 0.09 4 57

IA 0.00 0.01 0.01 0.02 0.04 0.11 11.68 339 2542

ID 0.00 0.00 0.00 0.00 0.01 0.02 0.02 27 270
59

State

Percentile No. of

Site‐

years > 4

samples

No. of

Site‐

years < 4

samples

Min 5% 25% 50% 75% 95% Max

μg/L

IL 0.00 0.01 0.05 0.12 0.78 3.99 13.29 416 243

IN 0.01 0.04 0.14 0.32 0.62 1.66 5.60 427 423

KS 0.00 0.00 0.02 0.03 0.05 0.27 2.67 110 5361

KY 0.01 0.02 0.04 0.07 0.30 1.28 2.20 97 99

LA 0.00 0.01 0.04 0.09 0.14 0.21 0.64 321 255

MA 0.00 0.00 0.00 0.00 0.01 0.01 0.01 29 81

MD 0.01 0.01 0.02 0.07 0.72 1.86 4.22 61 232

ME NA NA NA NA NA NA NA 0 9

MI 0.00 0.01 0.02 0.04 0.08 0.62 2.24 41 45

MN 0.00 0.00 0.00 0.01 0.01 0.02 0.29 81 1271

MO 0.00 0.01 0.02 0.05 0.14 0.86 9.83 190 620

MS 0.03 0.04 0.09 0.13 0.18 0.40 0.85 88 94

MT 0.00 0.00 0.00 0.00 0.00 0.01 0.01 15 99

NC 0.01 0.01 0.03 0.10 0.19 0.83 1.86 141 493

ND 0.00 0.00 0.00 0.00 0.01 0.04 0.23 28 396

NE 0.00 0.01 0.01 0.03 0.05 0.16 1.44 186 549

NH NA NA NA NA NA NA NA 0 42

NJ 0.00 0.01 0.01 0.01 0.02 0.06 0.17 106 687

NM 0.00 0.00 0.00 0.01 0.02 0.04 0.04 16 367

NV 0.00 0.00 0.01 0.01 0.05 0.93 1.40 76 87

NY 0.00 0.00 0.00 0.01 0.02 0.34 305.45 147 740

OH 0.01 0.04 0.18 0.34 0.68 1.99 7.03 223 207

OK 0.01 0.22 1.10 1.25 1.25 1.28 1.28 5 86

OR 0.00 0.00 0.02 0.04 0.19 1.15 7.62 370 969

PA 0.00 0.01 0.01 0.04 0.14 0.56 5.87 134 387

RI NA NA NA NA NA NA NA 0 2

SC 0.00 0.01 0.02 0.06 0.29 1.81 4.55 76 81

SD 0.01 0.01 0.01 0.01 0.01 0.02 0.02 5 184

TN 0.00 0.00 0.01 0.02 0.09 10.89 14.98 93 159

TX 0.00 0.00 0.01 0.06 0.42 1.59 3.75 280 739

UT 0.00 0.00 0.00 0.01 0.01 0.08 0.10 26 342

VA 0.00 0.00 0.04 0.08 0.35 1.43 10.02 99 204

VT 0.00 0.00 0.00 0.00 0.00 0.00 0.00 2 9

WA 0.00 0.00 0.01 0.02 0.03 0.06 0.31 303 418

WI 0.00 0.01 0.01 0.04 0.13 0.41 0.65 117 298

WV 0.00 0.01 0.02 0.02 0.10 0.18 0.20 5 45

WY 0.00 0.00 0.00 0.00 0.01 0.01 0.01 2 140
60

Characterization of Monitoring Data

Evaluation of simazine surface water monitoring data collected across the United States is an

important component of the ecological exposure assessment. Evaluation of the monitoring data

requires an understanding of the objective of the monitoring program for site selection and

sampling strategy. It is important that monitoring sites are located in watersheds with simazine

use. Additionally, it is important to understand the impact of sample frequency and timing for

capturing peak simazine concentrations. The peak simazine concentration is important in

quantifying average concentrations from monitoring data. Because low sample frequencies are

a common attribute in pesticide monitoring data, and because monitoring is rarely conducted

according to a statistical sampling plan, there is a high probability that peak simazine

concentrations are not captured. This situation leads to monitoring data underestimating true

peak and average simazine concentrations.   

Table 20 illustrates the sampling frequency with which simazine sites are sampled across the

29,717 site‐years of monitoring data used in this assessment. The typical site‐year sampling

frequency is 37 days. Sample frequencies ranged from daily to 1 sample per year.     

Table 20. Distribution of Median Sampling Intervals from 27,717 Site‐Years of Ambient

Surface Water Monitoring

Percentile

Site‐Year  

Sampling Interval  

(days)

Description

Minimum 1 Daily Sampling

10th 8 Weekly Sampling

20th 14.5 Bi‐Weekly Sampling

30th 25 ‐‐ 

40th 29 Monthly Sampling

50th 37 ‐‐ 

60th 59.5 Bi‐Monthly Sampling

70th 90 Quarterly Sampling

80th 119 Two Samples Per

Year

90th 126 ‐‐ 

Maximum 336 One Sample Per Year

In order to characterize the underestimation inherent in the simazine occurrence data due to

sampling frequency, simazine bias factors were estimated from bias factor equations developed

for the atrazine ecological risk assessment to support registration review (Appendix C). These

equations were used due to simazine and atrazine’s similarities in structure, fate and transport

characteristics, and labeled use on corn. The equations, which are listed in Table 21, were

developed using atrazine concentrations from the 2009‐2014 Atrazine Ecological Exposure
61

Monitoring Program (AEEMP), Atrazine Monitoring Program (AMP), and National Center for

Water Quality Research (NCWQR). These monitoring programs were selected to develop the

atrazine bias factor equations because they have high sampling frequencies from daily to 7‐day

sampling intervals and the sites are associated with atrazine use areas. Additionally, these

monitoring programs represent a variety of hydrologic conditions from small‐flowing streams,

large rivers, and drinking water reservoirs.

Table 21. Linear Regression Equations for BF Estimation from a Stratified Random Sampling Design

Monitoring

Program

Regression Models

Exposure

Estimate Regression Equation2 R2

F‐test

Probability

Regression

Limitations1

AEEMP

(Flowing)

Peak 0.354485 +0.03741X 0.47 <0.05

21‐day 0.112909+0.027481X 0.51 <0.05 See table footnote

60‐day 0.093328+0.022318X 0.54 <0.05

AMP1

(Flowing)

Peak  ‐0.0478+0.023998X 0.28 <0.05

≥ 7 day sampling interval ; See

table footnote 21‐day  ‐0.03154+0.017807X 0.28 <0.05

60‐day  ‐0.0196+0.014011X 0.29 <0.05

AMP2

(Static)

Peak  ‐0.0345+0.015095X 0.18 <0.05

≥ 7 day sampling interval; See

table footnote 21‐day  ‐0.01638+0.007425X 0.15 <0.05

60‐day  ‐0.01084+0.005603X 0.17 <0.05

NCWQR

(Flowing)

Peak 0.039677+0.02764X 0.40 <0.05

21‐day 0.004501+0.01996X 0.46 <0.05 See table footnote

60‐day 0.006318+0.01642X 0.50 <0.05

1‐Regression models should be selected according the factors in Table 22.  State location, watershed size,

hydrology should correspond within the range of the data used to develop the regression model.

2‐Regression equations represent slopes of the relationship of log BF vs sampling interval (days); the X is the

independent variable (sampling interval in days) in the regression equation where log BF= slope*sampling

interval+intercept.

Using the atrazine bias factor equations, the bias factors estimated for the typical (i.e., 37‐day)

sampling interval range from 3.34 to 54.78 for the daily maximum, 1.81 to 13.48 for the 21‐day

average, and 1.57 to 8.30 for the 60‐day average. The selection of the proper bias factor is

dependent on the waterbody type (flowing or static), watershed area, and location.  

The statistical interpretation of the bias factors in Table 22 is that 95% of the time the bias

factor multiplied by the observed concentration (daily peak, 21‐day average, or 60‐day average)

will provide an estimated concentration equal to or higher than the true concentration.

Although non‐bias‐factor‐adjusted simazine monitoring data are used to calculate risk quotients

for aquatic taxa in the assessment, the bias factors provide additional characterization on the

magnitude of potential underestimation in assessing aquatic exposures from monitoring data.  
62

Table 22. Bias Factors for Application to Simazine Monitoring Data from Ambient Surface Water  

Applicable Sites

(equation source; see Table 21)

Exposure

Estimate

37‐day Interval

Bias Factor

(50th Percentile)

Flowing < 3rd order stream with a watershed area of 23‐253 km2 in IA, IL, IN,

KS, KY, LA, MN, MO, NE, OH, TN, or TX

(AEEMP flowing)

Peak 54.78

21‐day 13.48

60‐day 8.30

Flowing 3rd to 7th order stream with a watershed area of 0.82‐2,955,814 km2

in IA, IL, IN, KS, KY, LA, MO, OH, or TX

(AMP1 flowing)

Peak 6.92

21‐day 4.24

60‐day 3.15

Static water body with a watershed area of 1‐1,242,000 km2 in IA, IL, IN, KS,

KY, LA, MN, MO, NE, OH, TN, or TX

(AMP2 static)

Peak 3.34

21‐day 1.81

60‐day 1.57

Flowing water body with a watershed area of 1,777‐16,395 km2 in OH

(NCWQR flowing)

Peak 11.54

21‐day 5.53

60‐day 4.11

Uncertainties in Aquatic Exposure Modeling and Monitoring

Uncertainties in the aquatic exposure assessment are associated with the application rates of

simazine products co‐formulated with atrazine, environmental fate properties of simazine,

unknown variability of quality assurance in the monitoring programs, and interpolation of

monitoring data for calculation of average concentrations for the exposure assessment.   

The SWCC modeling was conducted using a single simazine aerobic aquatic metabolism half‐life

(74.4 days) and a single simazine anaerobic aquatic metabolism half‐life (666 days), each

multiplied by 3 as per model input parameter guidance (Brady, 2009). This multiplier was used

to approximate the upper 90th confidence bound on the mean half‐lives. Although the

multiplier increased the aerobic aquatic half‐life from 74.4 days to 223 days and the anaerobic

aquatic half‐life from 666 days to 1,998 days, this modification in half‐life has minimal impact

on the predicted EECs from SWCC. Additionally, for applications where the crop is present (e.g.,

turf) the 0‐day foliar decay rate used in the SWCC modeling is an uncertainty.

The simazine monitoring data were derived from available databases including NWIS, STORET,

state, registrant, etc. An extensive quality assurance analysis or evaluation of circumstances

leading to simazine occurrence in the monitoring data was not conducted. A quality assurance

check was conducted on reported simazine concentrations above 500 μg/L. A similar quality

assurance check was conducted on reported atrazine concentrations above 500 μg/L, with

results indicating that the STORET monitoring data contain reporting errors. The extent of these

errors is difficult to quantify. Additionally, simazine occurrence at some sampling sites may be

caused by unique circumstances which are not identified in the reported monitoring data.   

Average concentrations from monitoring data were calculated using a stair‐step interpolation

between measured values. The stair‐step interpolation was used to avoid assuming linear
63

interpolation between measured values. To address the uncertainty with interpolation of

monitoring data, sampling site‐years with 12 or more samples per year were used for

estimating the 21‐day and 60‐day average atrazine concentrations. Twelve samples would

represent, in the worst case, a single sample each month for a year. Because calculation of

average concentrations require at least two measured values, the selection of 12 or more

samples are expected to provide reasonable number of measured concentrations for

calculation of average concentrations.     

Bias in the monitoring data associated with sampling frequency was characterized in the

previous section through the calculation of bias factors. There is some uncertainty associated

with the extrapolation of bias factors calculated for simazine using atrazine bias factor

regression equations. Additionally, bias factors could not be calculated for use areas outside the

Midwest.

STRESSORS OF CONCERN

  

Previous ecological risk assessments for simazine have evaluated potential degradate(s) for

inclusion in the risk assessment (USEPA 2009b). Based on those analyses, the parent simazine is

considered the stressor of concern for aquatic organisms. This conclusion is based on either

available degradate toxicity data indicating that it is less toxic than parent simazine, or on the

proportion of the degradates expected to be in the environment and available for exposure

relative to simazine. In this case, focusing on parent simazine is expected to be protective for

non‐target aquatic organisms that may be exposed to the parent and any of its degradates

(USEPA, 2009b). Based on available toxicity data for birds and mammals, the primary

degradates of concern for simazine (DIA, DDA, and HS) are generally of equal toxicity or slightly

more toxic than simazine. In order to account for this toxicity, a default upper‐bound foliar

dissipation rate of 35 days is assumed in terrestrial exposure modeling, allowing the assumed

persistence of simazine to serve as a surrogate for degradate toxicity.

In its ecological risk assessments, the EPA does not routinely include a quantitative evaluation

of mixtures of active ingredients, either those mixtures of multiple active ingredients in product

formulations or those in the applicator’s tank. In the case of the product formulations of active

ingredients (that is, a registered product containing more than one active ingredient), each

active ingredient is subject to an individual risk assessment for regulatory decision regarding

the active ingredient on a particular use site. Effects data are not available for simazine

formulated products that contain more than one active ingredient.  

It is expected that the toxic effect of simazine, in combination with other pesticides used in the

environment, is likely to be a function of many factors, including but not necessarily limited to:

(1) the exposed species, (2) the co‐contaminants in the mixture, (3) the ratio of simazine and

co‐contaminant concentrations, (4) differences in the pattern and duration of exposure among

contaminants, and (5) the differential effects of other physical/chemical characteristics of the

receiving waters (e.g., organic matter present in sediment and suspended water).

Quantitatively predicting the combined effects of all these variables on mixture toxicity to any
64

given taxa with confidence is beyond the capabilities of the available data and methodologies.

However, a qualitative discussion of implications of the available pesticide mixture effects data

on the confidence of risk assessment conclusions are addressed as part of the characterization

of uncertainties.

  EVALUATION OF SIMAZINE TOXICITY TO SPECIFIC TAXA

The risk assessment for simazine relies on a surrogate species approach. Toxicological data

generated from surrogate test species, which are intended to be representative of broad

taxonomic groups, are used to extrapolate the potential effects on a variety of species included

under these taxonomic groupings.  

Acute and chronic simazine toxicity data from single‐species studies submitted by pesticide

registrants along with the available open literature were used to evaluate the potential effects

of simazine to aquatic and terrestrial species, including sublethal effects that can be directly

linked to survival, growth, or fecundity. These data include toxicity on the technical grade active

ingredient, degradates, and when available, formulated products (e.g., “Six‐Pack” studies).  

The open literature studies are identified through the EPA’s ECOTOXicology (ECOTOX 2007c)

database, which employs a literature search engine for locating chemical toxicity data for

aquatic life, terrestrial plants, and wildlife. Several ECOTOX runs have been conducted over the

years prior to this risk assessment (2003, 2004, 2006, 2007, 2008, 2011, and 2014; Appendix A).

Studies on simazine are published in the open literature every year.  This assessment attempted

to include all of the available monitoring data and relevant primary scientific literature

identified through EPA’s ECOTOX program up to September 2014.  This assessment also

includes all registrant‐submitted studies received through December 2015. EPA continues to

monitor the scientific literature and will include additional studies, as appropriate, in any future

updates to this risk assessment.

The assessment endpoints for pesticide risk assessments are growth, reproduction, and survival

of species. For this assessment, evaluated taxa include aquatic‐phase amphibians, freshwater

and saltwater fish, freshwater and saltwater invertebrates, aquatic plants, birds (surrogate for

terrestrial‐phase amphibians and reptiles), mammals, terrestrial invertebrates, and terrestrial

plants. Acute (short‐term exposure) and chronic (long‐term exposure) toxicity information is

characterized based on registrant‐submitted studies and a comprehensive review of the open

literature on simazine and its degradates.  

Simazine shares a common mechanism of toxicity with atrazine and 4 other chlorinated triazine

compounds.  Atrazine, simazine, propazine, and the 3 chlorinated degradates common to these

compounds all exhibit neuroendocrine effects seen across mammals and can alter hormone

levels in rats that may result in developmental and reproductive consequences. In addition to

this primary effect in mammals, acute and chronic exposure of animals to each of these

chlorinated triazine compounds has shown significant reduction in body weight and organ

weights across multiple mammal and bird species.   
65

A summary of the simazine data to be used for quantitative and qualitative risk assessment for

non‐target species and communities exposed to simazine in aquatic and terrestrial habitats is

provided in this section.

Toxicity to Plants

Toxicity to Terrestrial Plants   

Plant toxicity data from both registrant‐submitted studies and studies in the scientific literature

have been reviewed for this assessment. Registrant‐submitted studies are conducted under

conditions and with species defined in OCSPP test guidelines. Sub‐lethal endpoints such as plant

growth, dry weight, and biomass are evaluated for both monocots and dicots, and effects are

evaluated at both seedling emergence and vegetative life stages.  

Based on the results of the submitted and available open literature terrestrial plant toxicity

tests, it appears that the seedling emergence stage of plant development is more sensitive to

simazine than the vegetative vigor stage of development. However, all tested plants, with the

exception of corn in the seedling emergence and vegetative vigor tests, exhibited adverse

effects following exposure to simazine.  

For Tier II seedling emergence, the most sensitive dicot is lettuce and the most sensitive

monocot is onion. IC25 values, on an equivalent application rate basis, for lettuce and onion,

which are based on a reduction in dry weight, are 0.009 and 0.02 lb a.i./A, respectively; NOAEC

value for lettuce is 0.0018 lb a.i./A. For onion, the statistical NOAEC was estimated to be greater

than the IC25; thus, the IC05 is used in place of the NOAEC (IC05 = 0.0017 lb a.i./A). Table 23

summarizes the terrestrial plant seedling emergence toxicity data used to derive risk quotients

in this assessment.

For Tier II vegetative vigor studies, the most sensitive dicot is lettuce with an IC25 and NOAEC of

0.033 and 0.016 lb a.i./A respectively. The available monocot results did not define an IC25 for

onion or ryegrass (both >0.016 lb a.i./A), and NOAEC values were reported as 0.016. The most

sensitive monocot with a definitive IC25 was oat (IC25 = 0.033 lb a.i./A) and with a NOAEC of

0.016 lb a.i./A. Table 23 summarizes the terrestrial plant vegetative vigor toxicity data used to

derive risk quotients in this assessment.

Table 23. Nontarget Terrestrial Plant Seedling Emergence and Vegetative Vigor Toxicity

Endpoints (Tier II) for Simazine.

Seedling Emergence

MRID 42634603

Vegetative Vigor

MRID 42634604

Species Endpoint NOAEC EC25 Endpoint NOAEC EC25

Monocots

Corn (Zea mays) No Effect 4.0 >4.0 No Effect 4.0 >4.0

Oat (Avena sativa) Biomass 0.016b 0.031b Biomass 0.016 0.033

Onion (Allium cepa) Biomass 0.0017a 0.02 Biomass 0.016b >0.016b
66

Seedling Emergence

MRID 42634603

Vegetative Vigor

MRID 42634604

Species Endpoint NOAEC EC25 Endpoint NOAEC EC25

Ryegrass (Lolium perenne) Biomass 0.0022a 0.045 Biomass 0.016b >0.016b

Dicots

Radish (Raphanus sativus) Biomass 0.049 >0.049 Biomass 0.049b 0.063b

Soybean (Glycine max)    Biomass 0.018a 0.057 Biomass 0.049 0.085

Lettuce (Lactuca sativa)     Biomass 0.0018 0.009 Biomass 0.016 0.033

Cabbage (Brassica oleracea

alba)    Biomass 0.016b >0.016b Biomass 0.016 0.041

Tomato (Solanum

lycopersicum) Biomass 0.016b 0.038b Biomass 0.049b 0.054b

Cucumber (Cucumis

sativus)   Biomass 0.016b 0.046b Biomass 0.016 0.036

a The IC05 was included in brackets when the NOAEC was not determined or was greater than the IC25. b results based on Chetram 1993 reported values, statistics could not be verified because of illegible raw data in the

available report

Toxicity to Aquatic Non‐Vascular Plants

Guideline Tier II toxicity data for technical grade simazine are available for the following non‐

vascular plants: blue‐green algae (Anabaena flos‐aquae), marine diatom (Skeletonema

costatum and Phaeodactylum tricornutum), freshwater alga (Pseudokirchneriella subcapitata),

freshwater diatom (Navicula pelliculosa), marine algae (Isochrysis galbana), and marine green

algae (Chlorococcum sp. and Dunaliella tertiolecta). The Tier II results indicate that blue‐green

algae (Anabaena) is the most sensitive non‐vascular plant to simazine (IC50 = 36 μg a.i./L, IC05 =

5.4 μg a.i./L; MRID 42662401) based on reduction in growth rates observed by day 5. In

addition, a 28% reduction in cell density was observed at the lowest test concentration of 20 μg

a.i./L.  

Studies available in the open literature identify additional toxicity studies which have reported

more sensitive endpoints than the submitted data (Table 24). The most sensitive species

reported in the open literature is Pseudokirchneriella subcapitata with a reported 25%

reduction in biomass at 0.614 μg a.i./L and a 25% reduction in photosynthesis (oxygen

evolution) at 2.44 μg a.i./L (Turbak et al. 1986). Turbak et al (1986) is classified as supplemental

for qualitative use only in the assessment due to reporting deficiencies. Another published

study, Bednarz 1981, evaluated the effects of pesticides on several species of non‐vascular

plants, concluding that the IC50 = 8.0 μg a.i./L for the green alga, Chlorococcum sp., was the

most sensitive for simazine. This endpoint represents the most sensitive non‐vascular aquatic

plant endpoint in the available simazine toxicity data.
67

Table 24. Most Sensitive Data for Non‐vascular Aquatic Plant Toxicity of Simazine

Species

TGAI/TEP % a.i. Toxicity Value (μg a.i./L) Citation  orMRID Number1 Comment

Pseudokirchneriella

subcapitata

TGAI

Oxygen Evolution Test

IC50 = 2.24 μg/L  

Bottle Test (Biomass)

IC50 =  0.614 μg/L  

Turbak et al.

1986 ECOTOX#

11780

Supplemental/ Qualitative:  

Nominal concentrations;

no raw data provided; no

information provided to

determine NOAEC or EC05

Spirulina platensis

Chlorococcum sp

Oscillatoria sp

TGAI

IC50 = 6.0 μg/L, est IC05 = 1μg/L

IC50 = 8.0 μg/L, est IC05 = 2μg/L  

IC50 = 8.0 μg/L, est IC05 = 1.5μg/L

Bednarz 1981

ECOTOX# 17259

Supplemental/Quantitative

no raw data provided; no

information provided to

determine NOAEC or EC05.

IC05 values estimated from

figures. Results similar to

those for atrazine tested.

Nephroselmis pyriformis

TGAI

IC50 = 24.4 μg a.i./L

IC10 = 3.65 μg a.i./L

LOAEC = 4.0 μg a.i./L

Reduction in Effective Quantum

Yield. 4 hr exposure.

Magnusson et

al. 2010.  

ECOTOX#

153836

Supplemental/ Qualitative:  

no raw data provided; no

information provided to

determine NOAEC or EC05.

Results similar to those for

atrazine tested.

Anabaena flos‐aquae    

TGAI  

IC50 = 36 μg a.i./L

IC05 = 5.4 μg a.i./L

MRID 42662401 Supplemental: A NOAEC

could not be determined

based on cell density.

Existing cell density data

was used to calculate an

EC05 for use as an NOAEC.   

All definitive endpoints are used quantitatively; bold endpoints identify the most sensitive endpoint used

quantitatively in this assessment. 1 ECOTOX references are designated with an E followed by the ECOTOX reference number.  

Toxicity to Aquatic Vascular Plants

Single‐species aquatic plant toxicity studies are used as one of the measures of effect to

evaluate whether simazine may affect primary production and diversity in aquatic ecosystems.

Numerous aquatic vascular plant toxicity studies have been submitted to the EPA and/or

published in the open literature.

Freshwater vascular plants are as sensitive to simazine as freshwater non‐vascular plants, with

the most sensitive vascular plant EC50 having a value of 67 μg a.i./L, based on fresh weight

reduction (biomass reduction) in Vallisneria americana (Wilson and Wilson 2010) (Table 25). In

comparison, the registrant submitted 850.4550 guideline study using Lemna gibba (MRID

42503704) reported an IC50 of 140 μg a.i./L based on frond number reduction.  

The most sensitive single species data for aquatic vascular plants from either supplemental or

acceptable studies (Table 25) are used for risk characterization and risk quotient calculation.
68

Table 25. Most Sensitive Data for Vascular Aquatic Plant Toxicity of Simazine

Species

TGA.I./TEP % a.i. Toxicity Value Citation  orMRID Number1 Comment

Water Celery,  

Vallisneria americana

TGAI

13‐day exposure results

Fresh Weight Gain

IC50 = 67 μg a.i./L  

NOAEC = < 58 μg a.i./L

Length

IC50 = 81 μg a.i./L  

NOAEC = 58 μg a.i./L

Wilson and

Wilson 2010,

ECOTOX #

164763

Supplemental:   

LOAEC of 58 μg a.i./L based

on 44% reduction in Fresh

weight gain.

LOAEC of 116 μg/L based on

10% reduction in Length

Duckweed,  

Lemna gibba

TGAI

14‐day IC50 = 140 μg

a.i./L  

NOAEC = 54 μg a.i./L  

MRID 42503704 Acceptable:  LOAEC of 110 μg

a.i./L based on reduction in

frond number

1 ECOTOX references are designated with an E followed by the ECOTOX reference number.  

Toxicity to Aquatic Plant Communities  

A number of freshwater microcosm, mesocosm, and field studies are available for simazine,

although the lowest concentration of simazine tested in these studies was 1,000 μg a.i./L, well

above environmentally relevant concentrations (summarized in USEPA 2007). Therefore, the

available field study data are inadequate to determine whether simazine applications to aquatic

habitats or runoff from terrestrial uses resulting in water concentrations below 1,000 μg a.i./L

result in adverse effects to the aquatic community structure, productivity or function.  

EFFECTS TO ANIMALS

Toxicity to Terrestrial Animals

A summary of the most sensitive endpoints for terrestrial animals is presented in Table 26.

Table 26. Summary of the Most Sensitive Endpoints from Terrestrial Animal Toxicity Studies

for Simazine

Species Toxicity Value Used in Risk

Assessment

Citation

MRID# Comment

Mallard Duck

Anas

platyrhynchos

LD50 = >4,640 mg a.i./kg‐bw 00072798

(Fink, 1976)

Supplemental: No mortality at the highest

test concentration; however, reduced

reaction to external stimuli, wing droop, and

depression were observed at concentrations

as low as 1,000 mg a.i./kg‐bw one hour after

dosing.  

Mallard Duck

Anas

platyrhynchos

LC50 = >5,000 mg a.i./kg‐

diet

00022923

(Hill et al.,

1975)

Acceptable: No mortality at the highest test

concentration

Northern

Bobwhite Quail

NOAEC = 100 mg a.i./kg‐

diet

LOAEC = 500 mg a.i./kg‐diet

00163134

(Beavers,

1986)

Acceptable: Reduction in number of eggs

laid, viable embryos, live embryos,
69

Species Toxicity Value Used in Risk

Assessment

Citation

MRID# Comment

(Colinus

virginianus)

hatchlings, and 14‐day old chick survivors at

500 mg a.i./kg

Laboratory rat

Rattus

norvegicus

Simazine LD50 = >5,000 mg

a.i./kg‐bw

00148897

(Rosenfeld,

1985)

Acceptable: At 5,000 mg a.i./kg‐bw, 3/10

animals died

Laboratory rat

Rattus

norvegicus

Parental NOAEL = 10

mg/kg‐diet; 0.56

mg/kg/day (M); 0.7 (F)

41803601

(Epstein et

al., 1991)

Acceptable: LOAEL = 100 mg/kg‐diet; 5.61

mg/kg/day (M); 7.04 mg/kg/day (F), based

on decreased body weight/body weight gain

Honey bee acute

contact

LD50 = >96.7 μg a.i./bee

00036935

(Atkins et al.,

1975)

Acceptable: 6.5% mortality in the 96.7 μg

a.i./bee treatment group

Birds, Reptiles and Terrestrial Phase Amphibians  

Simazine is classified as practically non‐toxic to birds on an acute exposure basis (LD50 = >4,640

mg a.i./kg‐bw). Subacute avian dietary toxicity values for the technical grade and an 80% a.i.

formulation indicate that simazine is practically non‐toxic to birds.  

The available avian reproductive studies determined NOAECs 100 mg a.i./kg (MRID 00163134)

and 150 mg a.i./kg (MRID 43576901), based on the reduction in the number of eggs laid, viable

embryos, live embryos, hatchlings, and 14‐day old chick survivors.  

The ECOTOX database for simazine was reviewed for any bird toxicity studies not previously

considered in the 2013 Problem Formulation (USEPA 2013). No additional studies were

identified with more sensitive apical endpoints than those previously discussed.

Toxicity to Mammals

Rats exposed to technical grade simazine showed no mortality at the highest doses tested. The

corresponding LD50 value for the TGAI is >5,000 mg/kg‐bw, suggesting technical grade simazine

is practically non‐toxic (MRID 00148897) to mammals on an acute basis. Although a definitive

LC50 was not established in the study, mortality was noted.  

Acute mammalian oral toxicity data are also available for one degradate of simazine, DIA (MRID

43012301). In this study both the female and male LD50 values for the degradate showed that

the degradate appears to be more toxic to laboratory rats than technical grade values for the

parent simazine with respective values of 810 mg/kg for the female and 2,290 mg/kg for the

male. The combined LD50 value for males and females is 1,240 mg/kg.

Reproductive and developmental mammalian toxicity studies provide adequate toxicity data on

chronic developmental and reproductive effects of simazine (Table 27; USEPA 2007). Chronic

studies using laboratory rats show consistent reductions in adult body weight gain and adult

body weight at simazine concentrations of 100 mg/kg‐diet. The corresponding NOAEL value for
70

these studies is 10 mg/kg‐diet (0.56 and 0.7 mg/kg/day for males and females respectively;

MRIDs 41803601 and 40614405) and are occurring at concentrations below the NOAEL for LH

surge (5.0 mg/kg/day; MRID 45471002). In addition, reproductive effects including increased

abortions, reduced fetal weight, and increased skeletal variations were observed in New

Zealand white rabbits at a concentration of 200 mg/kg/day, with a corresponding NOAEL value

of 75 mg/kg/day (MRID 00161407). The chronic endpoint from the rat reproduction study

(MRID 41803601; NOAEL = 0.56 mg/kg/day) will be used for quantitative risk assessment.

Table 27. Summary of the Most Sensitive Reproductive and Developmental Mammalian

Endpoints for Simazine    

Guideline No./

Study Type

MRID No. (year)/

Classification /Doses

Results

870.3100

90‐Day oral toxicity

(rat)

00143265 (1985)

0, 14.25, 142, or 276

mg/kg/day

NOAEL = not identified.

LOAEL = 14.25 mg/kg/day, based on decreased body

weight gain, decreased food consumption and

hematological changes.

870.3150

13‐Week dietary  

toxicity (dog)

00146655

M: 6.9, 65.2, 133.6

F: 8.2, 64.3, 136.7

NOAEL = 6.9 mg/kg/day (M); 8.2 mg/kg/day (F)

LOAEL = 65.2 mg/kg/day (M); 64.3 mg/kg/day (F) based

on decreased body weight/body weight gain, decreased

food consumption, organ weight changes, decreased

serum glutamate oxaloacetate (SGOT) and reduced

alkaline phosphatase activities (females).

870.3700a

Prenatal

developmental in

Rat

40614403 (1986)

0, 30, 300 or 600 mg/kg/day

Maternal NOAEL = 30  mg/kg/day

LOAEL = 300 mg/kg/day based on decreased body

weight/body weight gain, and decreased food

utilization.

Developmental NOAEL = 30 mg/kg/day

LOAEL = 300 mg/kg/day based on skeletal variations.

870.3700b

Prenatal

developmental in

Rabbit

00161407 (1984)

0, 5, 75 or 200 mg/kg/day

Maternal NOAEL = 5  mg/kg/day

LOAEL = 75  mg/kg/day based on decreased body

weight gain, decreased food consumption, increased

tremors, and stool alterations.

Developmental NOAEL = 75  mg/kg/day

LOAEL = 200 mg/kg/day based decreased fetal weight

and increased skeletal variations.   

870.3800

Reproduction and

fertility effects

(Rat)

41803601 (1991)

0, 10, 100, or 500 ppm

M: 0, 0.56, 5.61, 28.9

mg/kg/day

F: 0, 0.7, 7.04, 34.96

mg/kg/day

Parental/Systemic NOAEL = 0.56 mg/kg/day (M); 0.7 (F)

LOAEL = 5.61 mg/kg/day (M); 7.04 mg/kg/day (F), based

on decreased body weight/body weight gain.

Offspring NOAEL = 31.93 mg/kg/day

LOAEL = not identified

870.4100b

Chronic toxicity

(dog)

40614402

Acceptable‐guideline

NOAEL  = 3.41 mg/kg/day (M);   0.76 mg/kg/day (F)
71

Guideline No./

Study Type

MRID No. (year)/

Classification /Doses

Results

M: 0, 0.68, 3.41, 42.9

mg/kg/day

F: 0, 0.76, 3.64, 44.9

mg/kg/day  

LOAEL = 42.9  mg/kg/day (M) based on decreased body

weight gains, increased platelet counts, and increased

adrenal/brain weight ratio;   3.64 mg/kg/day (F), based

on decreased body weight gain, hematological effects

(decreased levels of red blood cell counts, hemoglobin

and hematocrit) and increased adrenal weight,

adrenal/brain weight ratio, and adrenal/body weight

ratio.   

Special Study ‐ in

vivo endocrine

effects in rats.

Acceptable‐Non‐

guideline

43598614 In a special study  (MRID 43598614) on in vivo

endocrine effects, atrazine and simazine (>96 % a.i.)

were administered to 11 female rats/dose/strain (both

Sprague‐Dawley and Fischer 344 rats were used) by oral

gavage at dose levels of  0, 100, and 300 mg/kg/day for

14 to 23 days  depending on time to achieve proestrus.

The LOAEL for systemic toxicity is 100 mg/kg/day for

both atrazine and simazine, based on body weight

effects and reproductive organ weight effects for

atrazine.  The NOAEL for toxicity cannot be determined.

The LOAEL for endocrine effects of atrazine is 100

mg/kg/day based on organ weight effects, plasma

hormone changes (estradiol), estrus cycle lengthening,

and vaginal cytology.  The NOAEL for endocrine effects

of atrazine cannot be determined.

The LOAEL for endocrine effects of simazine is 300

mg/kg/day based on organ weight effects and vaginal

cytology.  The NOAEL for endocrine effects of simazine

is 100 mg/kg/day.

Special Study ‐ LH

surge in rats

Acceptable‐

Nonguideline

45471002 In a special study (MRID 45471002) on the effects of

chlorotriazines on luteinizing hormone (LH) surge,

simazine (100%, batch no. SG202028GB10),

diaminochlorotriazine (DACT) (96.8%, batch no.

GP720301) and atrazine (97.1% , batch no.

SG8029BA10) were administered to 20 Sprague‐Dawley

Crl:CD BR female rats/dose/group by oral gavage at

dose levels of 0, 2.5, 5, 40, 200 mg/kg bw/day

(equivalent to 12.4, 24.8, 198.3, and 991.6

μmol//kg/day for simazine; for 17.2, 34.4, 274.9, 1374.6

μmol//kg/day for DACT; and 11.6, 23.2, 185.4, 927.2

μmol//kg/day for atrazine) once daily for at least 4

weeks.  

The LOAEL for systemic toxicity is 40 mg/kg/day for

simazine, DACT, and atrazine, based on body weight

effects.  The NOAEL for all three compounds is 5

mg/kg/day.
72

Guideline No./

Study Type

MRID No. (year)/

Classification /Doses

Results

The LOAEL for endocrine effects for simazine, atrazine,

and DACT  is 40 mg/kg/day, based on analyses of  

pre‐peak, peak, and post‐peak LH concentrations,

adjusted peak LH response, and comparison of

responses between compounds (at the same dose

levels).  The NOAEL for endocrine effects for simazine

atrazine, and DACT is 5.0 mg/kg/day.

Toxicity to Terrestrial Invertebrates  

The use of simazine may result in exposure to non‐target terrestrial invertebrates, such as the

honey bee. The results of acute contact toxicity testing of simazine on the adult honey bee (Apis

mellifera) show that by 48 hours in the contact test (MRID 00036935), 6.5% mortality was

observed in the 96.7 μg a.i./bee treatment group; therefore, the LD50 value for the contact test

is >96.7 μg a.i./bee. As a result, simazine is categorized as practically non‐toxic to honeybees on

an acute contact basis.   

Three open literature studies on simazine effects to non‐target insects including earthworms

and beetles were located and are summarized in the 2007 California Red‐legged Frog

assessment (USEPA 2007). The results of two earthworm studies (Martin, 1982: ECOTOX

#58170; Lydy and Linck, 2003; ECOTOX #71459) showed no adverse effects of mortality and

growth, following 96‐hours of exposure at 10 μg/cm2 and 7 days of exposure at 100 ppm, the

highest simazine concentrations tested. Samsoe‐Peterson (1987; ECOTOX # 70278) evaluated

the effects of Gesatop 50‐WP (50% simazine) on the rove beetle, Aleochara bileneata.  

Following 5‐days of exposure, no mortality or reduction in egg production were observed in the

simazine‐treated adult female beetles at an application rate of 600 L/ha. According to the

standard used by the International Organization of Biological Control (IOBC) working group

“Pesticides and Beneficial Organisms,” simazine was classified as “harmless” to the rove beetle.   

When initially scoping the registration review for simazine, the EPA did not identify the need for

any additional studies to evaluate potential effects on pollinators. However, since the issuance

of the June 2014 Guidance for Assessing Pesticide Risks to Bees2]

, the EPA has begun to require

these data where applicable. The EPA is currently determining whether additional pollinator

data are needed for active ingredients in registration review whose dockets opened prior to

June 2014, such is the case for simazine. For those active ingredients where the EPA determines

that additional pollinator exposure and effects data are necessary to help make a final

registration review decision, the EPA intends to issue a DCI to obtain these data. In the near

future, the EPA will provide further information and guidance on this effort. As discussed

above, an acceptable acute contact toxicity test (OCSPP Guideline 850.3020) has been

submitted to the EPA (MRID 00036935); the EPA may require the additional data it believes are

                                                       

2 http://www2.epa.gov/sites/production/files/2014‐

06/documents/pollinator_risk_assessment_guidance_06_19_14.pdf
73

necessary to help inform the pollinator risk assessment. The pollinator studies that could be

required are included in Table 28.

Table 28. Potential Pollinator Data Needs.

*Tier 1 (Laboratory‐based studies)

**Tier 2 and 3 (Semi‐field and full field colony‐level studies) The need for a higher tier test for pollinators will be determined

based upon lower‐tiered tests and/or other lines of data and the need for a refined pollinator risk assessment.  

Toxicity to Aquatic Animals

A considerable number of aquatic toxicity and field studies are available for simazine. While

simazine concentrations in water would appear to be stable to hydrolysis and photolysis for the

duration of the acute static studies, the actual exposure levels are uncertain because mean‐

measured concentrations are not available, and precipitation is frequently reported in the

acute studies. Test concentrations were rarely measured to verify exposure levels; therefore, a

high degree of uncertainty exists for the freshwater toxicity data for simazine.

Table 29 summarizes the most sensitive ecological toxicity endpoints for aquatic organisms.

Below the table, an additional summary discussion of simazine toxicity to the listed taxonomic

groups is provided.

Guideline # Study

850.3030* Honey bee toxicity of residues on foliage

850.3040** Field testing for pollinators

Non‐Guideline*

(OECD 213)

Honey bee adult acute oral toxicity

Non‐Guideline*

(OECD 237)

Honey bee larvae acute oral toxicity

Non‐Guideline* Honey bee adult chronic oral toxicity

Non‐Guideline* Honey bee larvae chronic oral toxicity

Non‐

Guideline** Residues in pollen and nectar/field residue analysis

Non‐

Guideline**

(OECD 75)

Semi‐field testing for pollinators (tunnel or colony feeding

studies)
74

Table 29. Summary of the Most Sensitive Endpoints from Aquatic Toxicity Studies for Simazine

AssessmentEndpoint Acute/Chronic Species TGA.I./TEP % a.i. Toxicity Value Classification Citation or MRID Number1 Freshwater fish Acute Fathead Minnow,

Pimephales promelas

(TGAI)

96‐hour LC50 =6,400 μg a.i./LNOAEL = 2500μg a.i./L

ModeratelyToxic

MRID00033309

Acute Common Carp

Cyprinus carpio(TGAI)

Calculated96‐hour LC50 =

400 μg a.i./L.

NA 

Chronic Common Carp

Cyprinus carpio(TGAI)

NOAEC = 60 μga.i./L

NA Velisek et

al. 2012.Ecotox#167125

Freshwaterinvertebrates

Acute Daphnid,

Daphnia magna(TGAI)

48‐hour EC502 =

1,000 μg a.i./L

Highly Toxic MRID

45088221
75

AssessmentEndpoint Acute/Chronic Species TGA.I./TEP % a.i. Toxicity Value Classification Citation or MRID Number1 

Chronic Daphnid,

Daphnia magna

(80% formulatedproduct)

CalculatedNOAEC = 40 μga.i./L

NA  ‐  

Estuarine/marine fish

Acute Seabream

(Sparus aurata) yolk‐sac larvae

(90% simazineformulation Gesatop®GW)

72‐ hour LC50 =

4,190 μg a.i./L

ModeratelyToxic

E76270Arufe et al.,2004

Chronic Seabream

(Sparus aurata)Calculated usingFathead minnow ACR4

(6.7)

NOAEC = 628 μga.i./L

NA  ‐‐  

Estuarine/marineinvertebrates

Acute Eastern oyster,

Crassostrea virginica(TGAI)

96‐hr EC50 =>3,700 μg a.i./L

ModeratelyToxic

MRID42503703
76

AssessmentEndpoint Acute/Chronic Species TGA.I./TEP % a.i. Toxicity Value Classification Citation or MRID Number1 

Acute Saltwater Mysid

Americamysis bahia(TGAI)

Calculated96‐hour LC50 =

995 μg a.i./L.

NA 

Chronic Saltwater Mysid

Americamysis bahia(TGAI)

NOAEC = 63 μga.i./L

NA MRID

48680006

1 ECOTOX references are designated with an E followed by the ECOTOX reference number.2 Threshold Limit, term used by authors to identify the threshold concentration in which immobility was observed at a certain level, in 

immobilization which is for our purpose, the same as an EC50 and used in calculations in the same way.

3ACR calculations from atrazine data: data from MRIDs 45087413 and 00024377 showed atrazine acute and chronic toxicity endpoint

and 140 μg a.i./L, respectively; therefore, 3500/140 = 25.

4ACR calculations for fathead minnow simazine toxicity data above: 6400/960 = 6.67.5ACR calculations for saltwater mysid based on propazine toxicity data MRIDs 44184501 and 48125801; 4200/266 = 15.79.
77

Toxicity to Fish

It appears that simazine is acutely toxic to some freshwater fish and aquatic invertebrates at

concentrations greater than 1,000 μg a.i./L. While there are no submitted freshwater fish early

life‐stage tests using the TGAI, two fish life‐cycle tests with fathead minnow were submitted for

simazine formulated products; however, these have been reclassified as supplemental. The

decision to reclassify these studies from acceptable to supplemental was based on the lack of

raw data for the study, mature fish were used at the start of the study (i.e., not less than 24‐hr

old embryos), and the total number of spawns and eggs, as well as survival, is not clear from

the study report. A NOAEC value of 960 μg a.i./L was based on 12% reduction in growth (length)

to 30‐day old fry at a continuous exposure treatment level of 2,000 μg a.i./L (MRID 00043676).  

In addition to registrant submitted studies, data available from the open literature show

sublethal effects to freshwater fish at lower concentrations than those previously used in risk

assessments (e.g., MRID 00043676). Although these studies report potentially sensitive

endpoints (olfactory detection of female priming pheromone and priming response of male),

effects on survival, growth, or reproduction were not observed in the available full life‐cycle

studies. Consequentially, these studies will be used qualitatively for risk characterization, but

not in RQ calculations.  

In the simazine Problem Formulation the chronic endpoint for estuarine/marine fish was

calculated using an acute to chronic ratio (ACR) (USEPA 2013). This relied upon the available

acute toxicity data for Seabream (Sparus aurata) and the acute and chronic endpoints for

fathead minnow. The calculated ACR (6.7) provided and estimated NOAEC of 625 μg a.i./L for

Seabream.

The most sensitive chronic exposure endpoint for fish was reported in an early life‐stage study

with common carp (Cyprinus carpio L.; Velisek et al. 2012). The authors reported no effects on

hatchability or embryo viability; however, there were statistically significant reductions in body

weights following exposures at 600 μg/L (NOAEC = 60 μg/L). This endpoint will be used to

evaluate the quantitative risks to freshwater fish from chronic exposure.  

Based on the Velisek et al 2012 study, the common carp is a more sensitive species than the

fathead minnow (MRID 00043676) on a chronic exposure basis. There is not a submitted acute

study for the common carp, therefore the acute to chronic ratio from the fathead minnow

studies (described above) was used to obtain an acute endpoint for carp. This estimated LC50 of

400 μg a.i./L is used to evaluate the quantitative risks to freshwater fish from acute exposure.  

Toxicity to Aquatic Invertebrates:  

Acute toxicity data for simazine are available for daphnids (Daphnia magna), seed shrimp

(Cypridopsis vidua), scuds (Gammarus lacustris and G. fasciatus), stoneflies (Pteronarcys

californica), sowbugs (Asellus brevicaudus), glass shrimp (Palaemonetes kadiakensis) and

crayfish (Orconectes nais), and the estuarine/marine species Eastern oyster (Crassostrea
78

virginica). Results of acute toxicity tests are found in the 2007 California Red‐legged Frog

assessment (USEPA 2007). Based on these data, simazine is classified as highly toxic to

freshwater invertebrates and moderately toxic to saltwater invertebrates on an acute exposure

basis.  

A freshwater invertebrate life‐cycle test using the TGAI has not been submitted for simazine;

however, a comparable study was submitted that used the simazine formulated product

Aquazine (80% formulation; MRID 00043676). No treatment‐related adverse effects to D.

magna, including parental mortality and offspring production, occurred during the 21‐day study

at the highest concentration tested, 2,000 μg a.i./L. This chronic endpoint, however, is higher

than the acute endpoint (Table 29). Concentrations were confirmed in the chronic test but not

the acute test. In the acute test, technical grade simazine was used along with ethanol as a

carrier solvent. In the chronic test, an 80% formulated product was used and results were

adjusted for concentration of a.i. Therefore, to estimate a chronic value for D. magna, the most

sensitive acute endpoint of EC50 = 1,000 μg a.i./L was adjusted with an ACR of 25 calculated

from atrazine D. magna toxicity data (3500/140 = 25; MRIDs 45087413 and 00024377

respectively), resulting in a chronic endpoint of 40 μg a.i./L. These atrazine data are similar to

propazine acute and chronic toxicity endpoints for D. magna of 5320 (MRID 44287305) and 47

μg a.i./L (MRID 44332760), respectively.   

A registrant‐submitted chronic mysid toxicity study (850.1300) using the Saltwater Mysid

(Americamysis bahia) was submitted after the completion of the problem formulation. The

chronic exposure study resulted in statistically significant reductions in the number of offspring

produced per day, and total adult male and female length at 608 μg a.i./L. Additionally,

biologically significant (15%) reduction in F0 survival was reported at 151 μg a.i./L (NOAEC = 63

μg a.i./L). Therefore, the estuarine/marine invertebrate endpoint for quantitative evaluation of

risk is 63 μg a.i./L. No acute toxicity studies on mysids have been submitted to the EPA.

Toxicity to Amphibians (aquatic‐phase and terrestrial)

There are no direct toxicity data for simazine effects on aquatic phase amphibians at

environmentally relevant concentrations.   

Simazine Degradates

The degradates of simazine have been identified based on environmental fate studies. The two

major degradates are HS and DIA. As described above, there is evidence that DIA is more

acutely toxic than simazine to the laboratory rat. The endpoint for DIA will be used to evaluate

the risk to terrestrial mammals from the exposure to DIA on an acute basis. The remaining

registrant‐submitted data available for simazine terrestrial or aquatic degradates suggests that

the degradates are either as toxic or much less toxic than the parent.  
79

INCIDENT DATA  

A review of the Ecological Incident Information System (EIIS) database for ecological incidents

involving simazine was completed on January 25, 2015. The results of this review for terrestrial

animal, plant, and aquatic animal incidents are discussed below. The Aggregate Incident

Summary report in IDS shows six simazine related incidents, including two involving plant

damage; and the others were single reports involving moderate property damage, minor to

moderate effects on domestic animals, fatal domestic animal event, and one unspecified

human event. The Avian Monitoring System (AIMS) is a database administered by the American

Bird Conservancy that contains publicly available data on reported avian incidents involving

pesticides. The simazine incidents listed in this database are also in the EIIS.   

Terrestrial Incidents

Only three simazine incidents have been reported involving terrestrial animals, two with an

“unlikely” certainty index and one “probable.”  In the incident with a certainty index of

“probable,” which occurred on June 26, 1998, five Canada geese were found dead in a corn

field in Rockingham County, Virginia, following spray application of Princep 4L (#I008168‐001).  

Soil and vegetative samples were collected along the bank near the creek in which the dead

geese were found. Substantial concentrations of simazine and atrazine were found in the

samples. Simazine detections ranged from 0.16 to 2.3 ppm in soil and 8.5 to 20.5 ppm in

foliage.   

Plant Incidents

Four simazine incidents have been reported for terrestrial plants, with two rated as being

“unlikely” a result of simazine. In one incident, water from a simazine‐treated swimming pool

affected a section of lawn grass. The certainty index for the lawn incident (# I003567‐001) is

“highly probable.” Another incident report from New York, was a Syngenta 6(a)2 submission

regarding an allegation that the spraying of three herbicide formulations (a.i. included were

atrazine, simazine and primisulfuron‐methyl) caused loss of milk production and loss of corn

and hay production. The treatment site nor the magnitude of the incident were given, neither

was the magnitude or any analysis (IO16790‐007, September 20, 2005). The certainty index for

the incident involving simazine is listed as “probable.”

Aquatic Incidents

Ten freshwater aquatic incidents involving fish kills have been reported for simazine between

the years of 1976 and 2004. Seven incidents have a certainty index of “highly probable” or

“probable,” and the other three have certainty indices of “possible” and “unlikely.” One

incident resulted from runoff into a lake due to a valve failure in a chemical application hose

that was dispensing glyphosate and simazine to golf course turf. The incident resulted in the

death of 6 unknown species of fish. Six incidents resulted from treatment of a lake, pond, or

lagoon; two incidents were associated with simazine use on corn and from simazine use along
80

railroad tracks; one was due to a spill from golf course use, and the treatment site for the other

incident was not reported. In a number of the incidents involving direct application of simazine

to lakes, ponds, and lagoons, the legality of use was listed as “misuse” or “undetermined.” For

those incidents where the legality of use is reported as “registered use,” the volume of the

water bodies is not provided; therefore, it is unclear whether simazine was applied in

accordance with its intended use. The six incidents involving direct application of simazine to

water all occurred prior to 1996, when label language was clarified to restrict direct applications

to ornamental ponds and aquaria. It is important to note that in a number of the incidents

involving direct application of simazine to water, low dissolved oxygen, caused by decaying

aquatic vegetation, is attributed as an indirect effect related to the fish kills. The certainty index

associated with the remaining three incidents (those resulting from use on corn, railroad tracks,

and an unspecified treatment site) was reported as “unlikely.”  

Of the ten reported incidents, three were reported in California, two were reported in

Nebraska, two were reported in South Carolina, and one was reported in Michigan, Oklahoma,

and in Tennessee. Fish species listed in these kills include smelt, bullheads, stickleback, striped

bass, bluegills, channel catfish, croaker, menhaden, mullet, northern pike, pinfish, yellow perch,

sea trout, black bullhead, and fathead minnows.   

The total number of actual incidents associated with the use of simazine may be higher than

what is reported to the EPA.  Incidents may go unreported since effects may not be

immediately apparent and/or readily attributed to the use of a chemical.

METHODOLOGY FOR DETERMINING THE LEVELS OF CONCERN FOR SIMAZINE.

The Risk Quotient Method and Levels of Concern for Terrestrial Plants and Terrestrial

and Aquatic Animals.

The Risk Quotient Method is used to integrate the results of exposure and ecotoxicity data. For

this method, Risk Quotients (RQs) are calculated by dividing exposure estimates by the acute

and chronic ecotoxicity values (i.e., RQ = EXPOSURE/TOXICITY). These RQs are then compared

to OPP's levels of concern (LOCs). These LOCs are criteria used by OPP to indicate potential risk

to non‐target organisms and the need to consider regulatory action. The EPA has defined LOCs

for acute risk, acute restricted use classification, acute and chronic risk to endangered species.

Risk presumptions, along with the corresponding RQs and LOCs are summarized in Table 30.

   
81

Table 30. Risk Presumptions and LOCs

Risk Presumption RQ LOC

Birds1

Acute Risk EEC/LC50 or LD50/sqft or LD50/day 0.5

Acute Endangered Species EEC/LC50 or LD50/sqft or LD50/day   0.1

Chronic Risk EEC/NOEC 1

Wild Mammals1

Acute Risk EEC/LC50 or LD50/sqft or LD50/day 0.5

Acute Endangered Species EEC/LC50 or LD50/sqft or LD50/day   0.1

Chronic Risk   EEC/NOEC 1

Aquatic Animals2    

Acute Risk EEC/LC50 or EC50 0.5

Acute Endangered Species EEC/LC50 or EC50 0.05

Chronic Risk EEC/NOEC 1

Terrestrial and Semi‐Aquatic Plants  

Acute Risk EEC/EC25 or IC25 1

Acute Endangered Species EEC/EC05 or IC05 or NOEC 1

Terrestrial Invertebrates: Honeybees

Acute Risk EEC/LC50 0.4

1

  LD50/sqft = (mg/sqft) / (LD50 * wt. of animal)   

   LD50/day = (mg of toxicant consumed/day) / (LD50 * wt. of animal)

2  EEC = (ppm or ppb) in water

   
82

TERRESTRIAL RISK CHARACTERIZATION AND CONCLUSIONS

Terrestrial Animals Exposure and Risk Quotients (RQ) Values

Terrestrial Exposure to Animals

Terrestrial wildlife exposure estimates are typically calculated for birds and mammals by

emphasizing the dietary exposure route of uptake of pesticide active ingredients. These

exposures are considered to be surrogates for exposures to terrestrial‐phase amphibians and

reptiles. For exposures to terrestrial organisms, such as birds and mammals, pesticide residues

on food items are estimated based on the assumption that organisms are exposed to pesticide

residues as a function of the pesticide use pattern. For simazine, application methods for the

registered uses include aerial applications for alfalfa grown for seed in Washington and ground

applications for all other uses.   

T‐REX (v. 1.5.2) is used to calculate dietary and dose‐based EECs of simazine residues on food

items for mammals and birds generated by spray applications for the labeled uses. Input values

for deriving EECs using T‐REX are located in Table 31. The default 35‐day foliar half‐life was

used for modeling. Upper‐bound Kenaga nomogram values are used to derive EECs for simazine

exposures to terrestrial mammals and birds (Table 32, Table 33 and Table 34), based on a 1‐

year time period. Consideration is given to different types of feeding strategies for mammals,

including herbivores, insectivores and granivores. Dose‐based exposures are estimated for

three weight classes of birds (20 g, 100 g, and 1000 g) and three weight classes of mammals (15

g, 35 g, and 1000 g).

Table 31.  Input Parameters for Deriving Terrestrial EECs for Simazine (T‐REX v. 1.5.2).

Crop (lb a.i./A) Max App Rate

(lb a.i./A) Max Apps

Min App

Interval

(days)

Max Annual

Rate

(lbs/A)

Alfalfa (1.6) 1.6 1 NA 1.6

Alfalfa (1.4) 1.4 1 NA 1.4

Corn (2/0.5) 2/0.5 2 14 2.5

Corn Fallow (1) 1 1 NA 1

Corn Postemergent (1.6) 1.6 1 NA 1.6

Blueberry/Caneberry (4) 4 1 NA 4

Blueberry/Caneberry (2/2) 2 2 90 4

Cranberry (4) 4 1 NA 4

Strawberry (1) 1 1 NA 1

Strawberry (4) 4 1 NA 4

Grapes (4) 4 1 NA 4

Grapefruit/Orange/Lemon (4) 4 1 NA 4

Grapefruit/Orange/Lemon (2/2) 2 2 90 4

Grapefruit/Orange/Lemon (4/4) 4 2 90 8

Lemon (1.6/1.6, 141‐days) 1.6 2 141 3.2

Lemon (1.6/1.6, 90‐days) 1.6 2 90 3.2

Apple (4) 4 1 NA 4

Avocado (4) 4 1 NA 4
83

Crop (lb a.i./A) Max App Rate

(lb a.i./A) Max Apps

Min App

Interval

(days)

Max Annual

Rate

(lbs/A)

Peach (4) 4 1 NA 4

Peach (2) 2 1 NA 2

Olive (4) 4 1 NA 4

Cherry (4) 4 1 NA 4

Pear (4) 4 1 NA 4

Plum (4) 4 1 NA 4

Nectarine (2) 2 1 NA 2

Turf and Residential Lawns (2/1; 140‐days) 2/1 2 141 3

Turf and Residential Lawns (2/1; 30‐days) 2/1 2 30 3

Sod Farms (4/2; 141‐days) 4/2 2 141 6

Sod Farms (4/2; 40‐days) 4/2 2 40 6

Almond (2) 2 1 NA 2

Hazelnut (4) 4 1 NA 4

Hazelnut (2/2) 2 2 90 4

Macadamia (4) 4 1 NA 4

Pecan (4) 4 1 NA 4

Walnut (4) 4 1 NA 4

Nursery (3) 3 1 NA 3

Christmas Trees (4) 4 1 NA 4

Shelter Belts (4) 4 1 NA 4

Shelter Belts (2/2) 2 2 90 4

Cherries (4) 4 1 NA 4

Reduced Rate (0.5) 0.5 1 NA 0.5

Screening level EECs (upper‐bound Kenaga) for the T‐REX analysis are contained in Table 32,

Table 33 and Table 34. Differences in exposures between ground and aerial applications cannot

be assessed with the current T‐REX model; therefore exposure and risk estimates from the T‐

REX model are considered relevant to both application scenarios.
84

Table 32. Dose‐based EECs (mg/kg‐bw) as Food Residues for Birds, Reptiles, and Terrestrial‐Phase Amphibians from (T‐REX v. 1.5.2, upper‐bound Kenaga).

Primary Feeding Strategy  Herbivores and Omnivores Insectivores Animal Size  Small Med Large Small MeDietary Items 

Short Grass

Tall Grass

Broad‐leaf

Plants

Fruits, pods,

seeds, etc.

Short Grass

Tall Grass

Broadleaf

Plants

Fruits, pods,

seeds, etc.

Short Grass

Tall Grass

Broadleaf

Plants

Fruits, pods,

seeds, etc.

Arthro

Use(s) (lb a.i./A)Alfalfa (1.6) 437.3 200.4 246.0 27.3 249.4 114.3 140.3 15.6 111.7 51.2 62.8 7.0 171.3 97.Alfalfa (1.4) 382.7 175.4 215.3 23.9 218.2 100.0 122.7 13.6 97.7 44.8 55.0 6.1 149.9 85.Corn (2/0.5) 551.0 252.5 309.9 34.4 314.2 144.0 176.7 19.6 140.7 64.5 79.1 8.8 215.8 123Corn Fallow (1) 273.3 125.3 153.8 17.1 155.9 71.4 87.7 9.7 69.8 32.0 39.3 4.4 107.1 61.Corn Postemergent (1.6) 437.3 200.4 246.0 27.3 249.4 114.3 140.3 15.6 111.7 51.2 62.8 7.0 171.3 97.Blueberry/Caneberry (4) 1093.3 501.1 615.0 68.3 623.5 285.8 350.7 39.0 279.1 127.9 157.0 17.4 428.2 244Blueberry/Caneberry (2/2) 638.6 292.7 359.2 39.9 364.2 166.9 204.9 22.8 163.0 74.7 91.7 10.2 250.1 142Cranberry (4) 1093.3 501.1 615.0 68.3 623.5 285.8 350.7 39.0 279.1 127.9 157.0 17.4 428.2 244Strawberry (1) 273.3 125.3 153.8 17.1 155.9 71.4 87.7 9.7 69.8 32.0 39.3 4.4 107.1 61.Strawberry (4) 1093.3 501.1 615.0 68.3 623.5 285.8 350.7 39.0 279.1 127.9 157.0 17.4 428.2 244Grapes (4) 1093.3 501.1 615.0 68.3 623.5 285.8 350.7 39.0 279.1 127.9 157.0 17.4 428.2 244Grapefruit/Orange/Lemon(4) 1093.3 501.1 615.0 68.3 623.5 285.8 350.7 39.0 279.1 127.9 157.0 17.4 428.2 244Grapefruit/Orange/Lemon(2/2) 638.6 292.7 359.2 39.9 364.2 166.9 204.9 22.8 163.0 74.7 91.7 10.2 250.1 142Grapefruit/Orange/Lemon(4/4) 1277.3 585.4 718.5 79.8 728.4 333.8 409.7 45.5 326.1 149.5 183.4 20.4 500.3 285Lemon (1.6/1.6, 141‐days) 464.1 212.7 261.1 29.0 264.7 121.3 148.9 16.5 118.5 54.3 66.7 7.4 181.8 103Lemon (1.6/1.6, 90‐days) 510.9 234.2 287.4 31.9 291.3 133.5 163.9 18.2 130.4 59.8 73.4 8.2 200.1 114Apple (4) 1093.3 501.1 615.0 68.3 623.5 285.8 350.7 39.0 279.1 127.9 157.0 17.4 428.2 244Avocado (4) 1093.3 501.1 615.0 68.3 623.5 285.8 350.7 39.0 279.1 127.9 157.0 17.4 428.2 244Peach (4) 1093.3 501.1 615.0 68.3 623.5 285.8 350.7 39.0 279.1 127.9 157.0 17.4 428.2 244Peach (2) 546.7 250.6 307.5 34.2 311.7 142.9 175.4 19.5 139.6 64.0 78.5 8.7 214.1 122Olive (4) 1093.3 501.1 615.0 68.3 623.5 285.8 350.7 39.0 279.1 127.9 157.0 17.4 428.2 244Cherry (4) 1093.3 501.1 615.0 68.3 623.5 285.8 350.7 39.0 279.1 127.9 157.0 17.4 428.2 244Pear (4) 1093.3 501.1 615.0 68.3 623.5 285.8 350.7 39.0 279.1 127.9 157.0 17.4 428.2 244
85

Primary Feeding Strategy  Herbivores and Omnivores Insectivores Animal Size  Small Med Large Small MeDietary Items 

Short Grass

Tall Grass

Broad‐leaf

Plants

Fruits, pods,

seeds, etc.

Short Grass

Tall Grass

Broadleaf

Plants

Fruits, pods,

seeds, etc.

Short Grass

Tall Grass

Broadleaf

Plants

Fruits, pods,

seeds, etc.

Arthro

Use(s) (lb a.i./A)Plum (4) 1093.3 501.1 615.0 68.3 623.5 285.8 350.7 39.0 279.1 127.9 157.0 17.4 428.2 244Nectarine (2) 546.7 250.6 307.5 34.2 311.7 142.9 175.4 19.5 139.6 64.0 78.5 8.7 214.1 122Turf and Residential Lawns

(2/1; 140‐days) 546.7 250.6 307.5 34.2 311.7 142.9 175.4 19.5 139.6 64.0 78.5 8.7 214.1 122

Turf and Residential Lawns

(2/1; 30‐days) 575.1 263.6 323.5 35.9 328.0 150.3 184.5 20.5 146.8 67.3 82.6 9.2 225.3 128

Sod Farms (4/2; 141‐days) 1093.3 501.1 615.0 68.3 623.5 285.8 350.7 39.0 279.1 127.9 157.0 17.4 428.2 244Sod Farms (4/2; 40‐days) 1093.3 501.1 615.0 68.3 623.5 285.8 350.7 39.0 279.1 127.9 157.0 17.4 428.2 244Almond (2) 546.7 250.6 307.5 34.2 311.7 142.9 175.4 19.5 139.6 64.0 78.5 8.7 214.1 122Hazelnut (4) 1093.3 501.1 615.0 68.3 623.5 285.8 350.7 39.0 279.1 127.9 157.0 17.4 428.2 244Hazelnut (2/2) 638.6 292.7 359.2 39.9 364.2 166.9 204.9 22.8 163.0 74.7 91.7 10.2 250.1 142Macadamia (4) 1093.3 501.1 615.0 68.3 623.5 285.8 350.7 39.0 279.1 127.9 157.0 17.4 428.2 244Pecan (4) 1093.3 501.1 615.0 68.3 623.5 285.8 350.7 39.0 279.1 127.9 157.0 17.4 428.2 244Walnut (4) 1093.3 501.1 615.0 68.3 623.5 285.8 350.7 39.0 279.1 127.9 157.0 17.4 428.2 244Nursery (3) 820.0 375.8 461.3 51.3 467.6 214.3 263.0 29.2 209.4 96.0 117.8 13.1 321.2 183Christmas Trees (4) 1093.3 501.1 615.0 68.3 623.5 285.8 350.7 39.0 279.1 127.9 157.0 17.4 428.2 244Shelter Belts (4) 1093.3 501.1 615.0 68.3 623.5 285.8 350.7 39.0 279.1 127.9 157.0 17.4 428.2 244Shelter Belts (2/2) 638.6 292.7 359.2 39.9 364.2 166.9 204.9 22.8 163.0 74.7 91.7 10.2 250.1 142Cherries (4) 1093.3 501.1 615.0 68.3 623.5 285.8 350.7 39.0 279.1 127.9 157.0 17.4 428.2 244Reduced Rate (0.5) 136.7 62.6 76.9 8.5 77.9 35.7 43.8 4.9 34.9 16.0 19.6 2.2 53.5 30.
86

Table 33. Dose‐based EECs (mg/kg bw) as Food Residues for Mammals from Labeled Uses of Simazine (T‐REX v. 1.5.2

Primary Feeding Strategy  Herbivores and Omnivores InsectiAnimal Size  Small Med Large Small MeDietary Items 

Short Grass

Tall Grass

Broad‐leaf

Plants

Fruits, pods,

seeds, etc.

Short Grass

Tall Grass

Broadleaf

Plants

Fruits, pods,

seeds, etc.

Short Grass

Tall Grass

Broadleaf

Plants

Fruits, pods,

seeds, etc.

Arthro

Use(s) (lb a.i./A)Alfalfa (1.6) 366.1 167.8 205.9 22.9 253.0 116.0 142.3 15.8 58.7 26.9 33.0 3.7 143.4 99Alfalfa (1.4) 320.4 146.8 180.2 20.0 221.4 101.5 124.5 13.8 51.3 23.5 28.9 3.2 125.5 86Corn (2/0.5) 461.2 211.4 259.4 28.8 318.8 146.1 179.3 19.9 73.9 33.9 41.6 4.6 180.7 124Corn Fallow (1) 228.8 104.9 128.7 14.3 158.1 72.5 89.0 9.9 36.7 16.8 20.6 2.3 89.6 61Corn Postemergent (1.6) 366.1 167.8 205.9 22.9 253.0 116.0 142.3 15.8 58.7 26.9 33.0 3.7 143.4 99Blueberry/Caneberry (4) 915.3 419.5 514.8 57.2 632.6 289.9 355.8 39.5 146.7 67.2 82.5 9.2 358.5 247Blueberry/Caneberry (2/2) 534.6 245.0 300.7 33.4 369.5 169.4 207.8 23.1 85.7 39.3 48.2 5.4 209.4 144Cranberry (4) 915.3 419.5 514.8 57.2 632.6 289.9 355.8 39.5 146.7 67.2 82.5 9.2 358.5 247Strawberry (1) 228.8 104.9 128.7 14.3 158.1 72.5 89.0 9.9 36.7 16.8 20.6 2.3 89.6 61Strawberry (4) 915.3 419.5 514.8 57.2 632.6 289.9 355.8 39.5 146.7 67.2 82.5 9.2 358.5 247Grapes (4) 915.3 419.5 514.8 57.2 632.6 289.9 355.8 39.5 146.7 67.2 82.5 9.2 358.5 247Grapefruit/Orange/Lemon (4) 915.3 419.5 514.8 57.2 632.6 289.9 355.8 39.5 146.7 67.2 82.5 9.2 358.5 247Grapefruit/Orange/Lemon(2/2) 534.6 245.0 300.7 33.4 369.5 169.4 207.8 23.1 85.7 39.3 48.2 5.4 209.4 144Grapefruit/Orange/Lemon(4/4) 1069.3 490.1 601.5 66.8 739.0 338.7 415.7 46.2 171.3 78.5 96.4 10.7 418.8 289Lemon (1.6/1.6, 141‐days) 388.5 178.1 218.6 24.3 268.5 123.1 151.1 16.8 62.3 28.5 35.0 3.9 152.2 105Lemon (1.6/1.6, 90‐days) 427.7 196.0 240.6 26.7 295.6 135.5 166.3 18.5 68.5 31.4 38.6 4.3 167.5 115Apple (4) 915.3 419.5 514.8 57.2 632.6 289.9 355.8 39.5 146.7 67.2 82.5 9.2 358.5 247Avocado (4) 915.3 419.5 514.8 57.2 632.6 289.9 355.8 39.5 146.7 67.2 82.5 9.2 358.5 247Peach (4) 915.3 419.5 514.8 57.2 632.6 289.9 355.8 39.5 146.7 67.2 82.5 9.2 358.5 247Peach (2) 457.6 209.8 257.4 28.6 316.3 145.0 177.9 19.8 73.3 33.6 41.3 4.6 179.2 123Olive (4) 915.3 419.5 514.8 57.2 632.6 289.9 355.8 39.5 146.7 67.2 82.5 9.2 358.5 247Cherry (4) 915.3 419.5 514.8 57.2 632.6 289.9 355.8 39.5 146.7 67.2 82.5 9.2 358.5 247Pear (4) 915.3 419.5 514.8 57.2 632.6 289.9 355.8 39.5 146.7 67.2 82.5 9.2 358.5 247Plum (4) 915.3 419.5 514.8 57.2 632.6 289.9 355.8 39.5 146.7 67.2 82.5 9.2 358.5 247
87

Primary Feeding Strategy  Herbivores and Omnivores InsectiAnimal Size  Small Med Large Small MeDietary Items 

Short Grass

Tall Grass

Broad‐leaf

Plants

Fruits, pods,

seeds, etc.

Short Grass

Tall Grass

Broadleaf

Plants

Fruits, pods,

seeds, etc.

Short Grass

Tall Grass

Broadleaf

Plants

Fruits, pods,

seeds, etc.

Arthro

Use(s) (lb a.i./A)Nectarine (2) 457.6 209.8 257.4 28.6 316.3 145.0 177.9 19.8 73.3 33.6 41.3 4.6 179.2 123Turf and Residential Lawns

(2/1; 140‐days) 457.6 209.8 257.4 28.6 316.3 145.0 177.9 19.8 73.3 33.6 41.3 4.6 179.2 123

Turf and Residential Lawns

(2/1; 30‐days) 481.5 220.7 270.8 30.1 332.8 152.5 187.2 20.8 77.2 35.4 43.4 4.8 188.6 130

Sod Farms (4/2; 141‐days) 915.3 419.5 514.8 57.2 632.6 289.9 355.8 39.5 146.7 67.2 82.5 9.2 358.5 247Sod Farms (4/2; 40‐days) 915.3 419.5 514.8 57.2 632.6 289.9 355.8 39.5 146.7 67.2 82.5 9.2 358.5 247Almond (2) 457.6 209.8 257.4 28.6 316.3 145.0 177.9 19.8 73.3 33.6 41.3 4.6 179.2 123Hazelnut (4) 915.3 419.5 514.8 57.2 632.6 289.9 355.8 39.5 146.7 67.2 82.5 9.2 358.5 247Hazelnut (2/2) 534.6 245.0 300.7 33.4 369.5 169.4 207.8 23.1 85.7 39.3 48.2 5.4 209.4 144Macadamia (4) 915.3 419.5 514.8 57.2 632.6 289.9 355.8 39.5 146.7 67.2 82.5 9.2 358.5 247Pecan (4) 915.3 419.5 514.8 57.2 632.6 289.9 355.8 39.5 146.7 67.2 82.5 9.2 358.5 247Walnut (4) 915.3 419.5 514.8 57.2 632.6 289.9 355.8 39.5 146.7 67.2 82.5 9.2 358.5 247Nursery (3) 686.5 314.6 386.1 42.9 474.4 217.5 266.9 29.7 110.0 50.4 61.9 6.9 268.9 185Christmas Trees (4) 915.3 419.5 514.8 57.2 632.6 289.9 355.8 39.5 146.7 67.2 82.5 9.2 358.5 247Shelter Belts (4) 915.3 419.5 514.8 57.2 632.6 289.9 355.8 39.5 146.7 67.2 82.5 9.2 358.5 247Shelter Belts (2/2) 534.6 245.0 300.7 33.4 369.5 169.4 207.8 23.1 85.7 39.3 48.2 5.4 209.4 144Cherries (4) 915.3 419.5 514.8 57.2 632.6 289.9 355.8 39.5 146.7 67.2 82.5 9.2 358.5 247Reduced Rate (0.5) 114.4 52.4 64.4 7.2 79.1 36.2 44.5 4.9 18.3 8.4 10.3 1.1 44.8 31
88

Table 34.  Dietary‐based EECs (mg/kg diet) as Food Residues for Birds, Reptiles, Terrestrial‐

phase Amphibians, and Mammals from Labeled Uses of Simazine (T‐REX v. 1.5.2, upper‐

bound Kenaga).

Primary Feeding Strategy  Herbivores, Omnivores, and Granivores Insectivores

Dietary Items 

Short GrassTall GrassBroad‐leafPlantsFruits, pods,seeds, etc.Arthropods

Use(s) (lb a.i./A)

Alfalfa (1.6) 384.0 176.0 216.0 24.0 150.4

Alfalfa (1.4) 336.0 154.0 189.0 21.0 131.6

Corn (2/0.5) 483.8 221.7 272.1 30.2 189.5

Corn Fallow (1) 240.0 110.0 135.0 15.0 94.0

Corn Postemergent (1.6) 384.0 176.0 216.0 24.0 150.4

Blueberry/Caneberry (4) 960.0 440.0 540.0 60.0 376.0

Blueberry/Caneberry (2/2) 560.8 257.0 315.4 35.0 219.6

Cranberry (4) 960.0 440.0 540.0 60.0 376.0

Strawberry (1) 240.0 110.0 135.0 15.0 94.0

Strawberry (4) 960.0 440.0 540.0 60.0 376.0

Grapes (4) 960.0 440.0 540.0 60.0 376.0

Grapefruit/Orange/Lemon (4) 960.0 440.0 540.0 60.0 376.0

Grapefruit/Orange/Lemon (2/2) 560.8 257.0 315.4 35.0 219.6

Grapefruit/Orange/Lemon (4/4) 1121.5 514.0 630.8 70.1 439.3

Lemon (1.6/1.6, 141‐days) 407.5 186.8 229.2 25.5 159.6

Lemon (1.6/1.6, 90‐days) 448.6 205.6 252.3 28.0 175.7

Apple (4) 960.0 440.0 540.0 60.0 376.0

Avocado (4) 960.0 440.0 540.0 60.0 376.0

Peach (4) 960.0 440.0 540.0 60.0 376.0

Peach (2) 480.0 220.0 270.0 30.0 188.0

Olive (4) 960.0 440.0 540.0 60.0 376.0

Cherry (4) 960.0 440.0 540.0 60.0 376.0

Pear (4) 960.0 440.0 540.0 60.0 376.0

Plum (4) 960.0 440.0 540.0 60.0 376.0

Nectarine (2) 480.0 220.0 270.0 30.0 188.0

Turf and Residential Lawns (2/1; 140‐days) 480.0 220.0 270.0 30.0 188.0

Turf and Residential Lawns (2/1; 30‐days) 505.0 231.4 284.1 31.6 197.8

Sod Farms (4/2; 141‐days) 960.0 440.0 540.0 60.0 376.0

Sod Farms (4/2; 40‐days) 960.0 440.0 540.0 60.0 376.0

Almond (2) 480.0 220.0 270.0 30.0 188.0

Hazelnut (4) 960.0 440.0 540.0 60.0 376.0

Hazelnut (2/2) 560.8 257.0 315.4 35.0 219.6

Macadamia (4) 960.0 440.0 540.0 60.0 376.0

Pecan (4) 960.0 440.0 540.0 60.0 376.0

Walnut (4) 960.0 440.0 540.0 60.0 376.0

Nursery (3) 720.0 330.0 405.0 45.0 282.0

Christmas Trees (4) 960.0 440.0 540.0 60.0 376.0
89

Primary Feeding Strategy  Herbivores, Omnivores, and Granivores Insectivores

Dietary Items 

Short GrassTall GrassBroad‐leafPlantsFruits, pods,seeds, etc.Arthropods

Use(s) (lb a.i./A)

Shelter Belts (4) 960.0 440.0 540.0 60.0 376.0

Shelter Belts (2/2) 560.8 257.0 315.4 35.0 219.6

Cherries (4) 960.0 440.0 540.0 60.0 376.0

Reduced Rate (0.5) 120.0 55.0 67.5 7.5 47.0

Risk Quotient (RQ) Values for Terrestrial Animal Species

   

This assessment of the labeled uses of simazine relies on the deterministic RQ method to

provide a metric of potential risks. The RQ provides a comparison of exposure estimates to

toxicity endpoints (i.e., the estimated exposure concentrations are divided by acute and chronic

toxicity values, respectively). The resulting unitless RQ values are compared to the EPA’s LOCs,

as shown in Table 30. The LOCs are used by the EPA to indicate when the use of a pesticide, as

directed by the label, has the potential to cause adverse effects to non‐target organisms. For

endangered species, LOC exceedances require an additional in‐depth listed species evaluation

of the potential co‐occurrence of listed species and areas in which use crops are grown to

characterize risks. Although such a co‐occurrence evaluation has not been done in this

assessment, RQs that exceed the non‐listed species LOC are presumed to exceed the listed

species LOC.

Because the acute and sub‐acute dietary‐based toxicity endpoints are non‐definitive for birds

(LD50 > 4640 mg/kg‐bw, LC50 > 5,000 mg/kg diet), RQ values are calculated for acute risk to birds

through dietary exposure as characterization. Conclusions for acute risk based on the dose‐

based RQs are considered to be protective of acute dietary risks for birds.

EECs for upper‐bound Kenaga values are listed in Table 32 through Table 34 above, and the

associated RQs are listed in Table 35 through Table 40. At the maximum single application rate

for each of the modeled Section 3 and Section 24c labeled uses, RQs for listed and non‐listed

terrestrial animals are above the LOCs for multiple uses. In addition, the model was run

assuming potential reduced single application rate of 0.5 lb a.i./A. These resulting RQs are also

above the levels of concern for certain sizes and dietary items, particularly for chronic risks.  

More detailed discussion of these results is contained in the individual sections below on birds

and mammals.    
90

Table 35. Acute Dose‐based RQ values for Birds, Reptiles, and Terrestrial‐Phase Amphibians from Labeled Uses of Supper‐bound Kenaga). Shaded cells identify LOC exceedances for listed species and bolded values indicate non‐listePrimary Feeding Strategy  Herbivores and Omnivores Insectivores Animal Size  Small Med Large Smal l Med Dietary Items 

Short Grass

Tall Grass

Broad‐leaf

Plants

Fruits, pods,

seeds, etc.

Short Grass

Tall Grass

Broadleaf

Plants

Fruits, pods,

seeds, etc.

Short Grass

Tall Grass

Broadleaf

Plants

Fruits, pods,

seeds, etc.

Arthropods 

Use(s) (lb a.i./A) Alfalfa (1.6) 0.18 0.08 0.10 0.01 0.08 0.04 0.05 0.01 0.03 0.01 0.01 <0.01 0.07 0.03 Alfalfa (1.4) 0.16 0.07 0.09 0.01 0.07 0.03 0.04 <0.01 0.02 0.01 0.01 <0.01 0.06 0.03 Corn (2/0.5) 0.23 0.10 0.13 0.01 0.10 0.05 0.06 0.01 0.03 0.01 0.02 <0.01 0.09 0.04 Corn Fallow (1) 0.11 0.05 0.06 0.01 0.05 0.02 0.03 <0.01 0.02 0.01 0.01 <0.01 0.04 0.02 Corn Postemergent (1.6) 0.18 0.08 0.10 0.01 0.08 0.04 0.05 0.01 0.03 0.01 0.01 <0.01 0.07 0.03 Blueberry/Caneberry (4) 0.45 0.21 0.26 0.03 0.20 0.09 0.11 0.01 0.06 0.03 0.04 <0.01 0.18 0.08 Blueberry/Caneberry (2/2) 0.27 0.12 0.15 0.02 0.12 0.05 0.07 0.01 0.04 0.02 0.02 <0.01 0.10 0.05 Cranberry (4) 0.45 0.21 0.26 0.03 0.20 0.09 0.11 0.01 0.06 0.03 0.04 <0.01 0.18 0.08 Strawberry (1) 0.11 0.05 0.06 0.01 0.05 0.02 0.03 <0.01 0.02 0.01 0.01 <0.01 0.04 0.02 Strawberry (4) 0.45 0.21 0.26 0.03 0.20 0.09 0.11 0.01 0.06 0.03 0.04 <0.01 0.18 0.08 Grapes (4) 0.45 0.21 0.26 0.03 0.20 0.09 0.11 0.01 0.06 0.03 0.04 <0.01 0.18 0.08 Grapefruit/Orange/Lemon (4) 0.45 0.21 0.26 0.03 0.20 0.09 0.11 0.01 0.06 0.03 0.04 <0.01 0.18 0.08 Grapefruit/Orange/Lemon(2/2) 0.27 0.12 0.15 0.02 0.12 0.05 0.07 0.01 0.04 0.02 0.02 <0.01 0.10 0.05 Grapefruit/Orange/Lemon(4/4) 0.53 0.24 0.30 0.03 0.24 0.11 0.13 0.01 0.08 0.03 0.04 <0.01 0.21 0.09 Lemon (1.6/1.6, 141‐days) 0.19 0.09 0.11 0.01 0.09 0.04 0.05 0.01 0.03 0.01 0.02 <0.01 0.08 0.03 Lemon (1.6/1.6, 90‐days) 0.21 0.10 0.12 0.01 0.09 0.04 0.05 0.01 0.03 0.01 0.02 <0.01 0.08 0.04 Apple (4) 0.45 0.21 0.26 0.03 0.20 0.09 0.11 0.01 0.06 0.03 0.04 <0.01 0.18 0.08 Avocado (4) 0.45 0.21 0.26 0.03 0.20 0.09 0.11 0.01 0.06 0.03 0.04 <0.01 0.18 0.08 Peach (4) 0.45 0.21 0.26 0.03 0.20 0.09 0.11 0.01 0.06 0.03 0.04 <0.01 0.18 0.08 Peach (2) 0.23 0.10 0.13 0.01 0.10 0.05 0.06 0.01 0.03 0.01 0.02 <0.01 0.09 0.04 Olive (4) 0.45 0.21 0.26 0.03 0.20 0.09 0.11 0.01 0.06 0.03 0.04 <0.01 0.18 0.08 Cherry (4) 0.45 0.21 0.26 0.03 0.20 0.09 0.11 0.01 0.06 0.03 0.04 <0.01 0.18 0.08 Pear (4) 0.45 0.21 0.26 0.03 0.20 0.09 0.11 0.01 0.06 0.03 0.04 <0.01 0.18 0.08 Plum (4) 0.45 0.21 0.26 0.03 0.20 0.09 0.11 0.01 0.06 0.03 0.04 <0.01 0.18 0.08 Nectarine (2) 0.23 0.10 0.13 0.01 0.10 0.05 0.06 0.01 0.03 0.01 0.02 <0.01 0.09 0.04 
91

Turf and Residential Lawns

(2/1; 140‐days) 0.23 0.10 0.13 0.01 0.10 0.05 0.06 0.01 0.03 0.01 0.02 <0.01 0.09 0.04 

Turf and Residential Lawns

(2/1; 30‐days) 0.24 0.11 0.13 0.01 0.11 0.05 0.06 0.01 0.03 0.02 0.02 <0.01 0.09 0.04 

Sod Farms (4/2; 141‐days) 0.45 0.21 0.26 0.03 0.20 0.09 0.11 0.01 0.06 0.03 0.04 <0.01 0.18 0.08 Sod Farms (4/2; 40‐days) 0.45 0.21 0.26 0.03 0.20 0.09 0.11 0.01 0.06 0.03 0.04 <0.01 0.18 0.08 Almond (2) 0.23 0.10 0.13 0.01 0.10 0.05 0.06 0.01 0.03 0.01 0.02 <0.01 0.09 0.04 Hazelnut (4) 0.45 0.21 0.26 0.03 0.20 0.09 0.11 0.01 0.06 0.03 0.04 <0.01 0.18 0.08 Hazelnut (2/2) 0.27 0.12 0.15 0.02 0.12 0.05 0.07 0.01 0.04 0.02 0.02 <0.01 0.10 0.05 Macadamia (4) 0.45 0.21 0.26 0.03 0.20 0.09 0.11 0.01 0.06 0.03 0.04 <0.01 0.18 0.08 Pecan (4) 0.45 0.21 0.26 0.03 0.20 0.09 0.11 0.01 0.06 0.03 0.04 <0.01 0.18 0.08 Walnut (4) 0.45 0.21 0.26 0.03 0.20 0.09 0.11 0.01 0.06 0.03 0.04 <0.01 0.18 0.08 Nursery (3) 0.34 0.16 0.19 0.02 0.15 0.07 0.09 0.01 0.05 0.02 0.03 <0.01 0.13 0.06 Christmas Trees (4) 0.45 0.21 0.26 0.03 0.20 0.09 0.11 0.01 0.06 0.03 0.04 <0.01 0.18 0.08 Shelter Belts (4) 0.45 0.21 0.26 0.03 0.20 0.09 0.11 0.01 0.06 0.03 0.04 <0.01 0.18 0.08 Shelter Belts (2/2) 0.27 0.12 0.15 0.02 0.12 0.05 0.07 0.01 0.04 0.02 0.02 <0.01 0.10 0.05 Cherries (4) 0.45 0.21 0.26 0.03 0.20 0.09 0.11 0.01 0.06 0.03 0.04 <0.01 0.18 0.08 Reduced Rate (0.5) 0.06 0.03 0.03 <0.01 0.03 0.01 0.01 <0.01 0.01 <0.01 <0.01 <0.01 0.02 0.01 
92

Table 36.  Acute Dietary‐based RQ values for Birds, Reptiles, and Terrestrial‐Phase

Amphibians from Labeled Uses of Simazine (T‐REX v. 1.5.2, upper‐bound Kenaga).

Shaded cells identify LOC exceedances for listed species and bolded values indicate

non‐listed LOC exceedances.

Primary Feeding Strategy  Herbivores, Omnivores, and Granivores Insectivores

Dietary Items 

Short GrassTall GrassBroad‐leafPlantsFruits, pods,seeds, etc.Arthropods

Use(s) (lb a.i./A)

Alfalfa (1.6) 0.08 0.04 0.04 0.00 0.03

Alfalfa (1.4) 0.07 0.03 0.04 0.00 0.03

Corn (2/0.5) 0.10 0.04 0.05 0.01 0.04

Corn Fallow (1) 0.05 0.02 0.03 0.00 0.02

Corn Postemergent (1.6) 0.08 0.04 0.04 0.00 0.03

Blueberry/Caneberry (4) 0.19 0.09 0.11 0.01 0.08

Blueberry/Caneberry (2/2) 0.11 0.05 0.06 0.01 0.04

Cranberry (4) 0.19 0.09 0.11 0.01 0.08

Strawberry (1) 0.05 0.02 0.03 0.00 0.02

Strawberry (4) 0.19 0.09 0.11 0.01 0.08

Grapes (4) 0.19 0.09 0.11 0.01 0.08

Grapefruit/Orange/Lemon

(4) 0.19 0.09 0.11 0.01 0.08

Grapefruit/Orange/Lemon

(2/2) 0.11 0.05 0.06 0.01 0.04

Grapefruit/Orange/Lemon

(4/4) 0.22 0.10 0.13 0.01 0.09

Lemon (1.6/1.6, 141‐days) 0.08 0.04 0.05 0.01 0.03

Lemon (1.6/1.6, 90‐days) 0.09 0.04 0.05 0.01 0.04

Apple (4) 0.19 0.09 0.11 0.01 0.08

Avocado (4) 0.19 0.09 0.11 0.01 0.08

Peach (4) 0.19 0.09 0.11 0.01 0.08

Peach (2) 0.10 0.04 0.05 0.01 0.04

Olive (4) 0.19 0.09 0.11 0.01 0.08

Cherry (4) 0.19 0.09 0.11 0.01 0.08

Pear (4) 0.19 0.09 0.11 0.01 0.08

Plum (4) 0.19 0.09 0.11 0.01 0.08

Nectarine (2) 0.10 0.04 0.05 0.01 0.04
93

Turf and Residential Lawns

(2/1; 140‐days) 0.10 0.04 0.05 0.01 0.04

Turf and Residential Lawns

(2/1; 30‐days) 0.10 0.05 0.06 0.01 0.04

Sod Farms (4/2; 141‐days) 0.19 0.09 0.11 0.01 0.08

Sod Farms (4/2; 40‐days) 0.19 0.09 0.11 0.01 0.08

Almond (2) 0.10 0.04 0.05 0.01 0.04

Hazelnut (4) 0.19 0.09 0.11 0.01 0.08

Hazelnut (2/2) 0.11 0.05 0.06 0.01 0.04

Macadamia (4) 0.19 0.09 0.11 0.01 0.08

Pecan (4) 0.19 0.09 0.11 0.01 0.08

Walnut (4) 0.19 0.09 0.11 0.01 0.08

Nursery (3) 0.14 0.07 0.08 0.01 0.06

Christmas Trees (4) 0.19 0.09 0.11 0.01 0.08

Shelter Belts (4) 0.19 0.09 0.11 0.01 0.08

Shelter Belts (2/2) 0.11 0.05 0.06 0.01 0.04

Cherries (4) 0.19 0.09 0.11 0.01 0.08

Reduced Rate (0.5) 0.02 0.01 0.01 <0.01 0.01

   
94

Table 37. Chronic Dietary‐based RQ values for Birds, Reptiles, and Terrestrial‐Phase

Amphibians from Labeled Uses of Simazine (T‐REX v. 1.5.2, upper‐bound Kenaga).

Shaded cells identify LOC exceedances.

Primary Feeding Strategy

 Herbivores, Omnivores, and Granivores Insectivores

Dietary Items 

ShortGrassTall GrassBroad‐leafPlantsFruits,pods,seeds,etc.Arthropods

Use(s) 

Alfalfa (1.6) 3.8 1.8 2.2 0.2 1.5

Alfalfa (1.4) 3.4 1.5 1.9 0.2 1.3

Corn (2/0.5) 4.8 2.2 2.7 0.3 1.9

Corn Fallow (1) 2.4 1.1 1.4 0.2 0.9

Corn Postemergent (1.6) 3.8 1.8 2.2 0.2 1.5

Blueberry/Caneberry (4) 9.6 4.4 5.4 0.6 3.8

Blueberry/Caneberry

(2/2) 5.6 2.6 3.2 0.4 2.2

Cranberry (4) 9.6 4.4 5.4 0.6 3.8

Strawberry (1) 2.4 1.1 1.4 0.2 0.9

Strawberry (4) 9.6 4.4 5.4 0.6 3.8

Grapes (4) 9.6 4.4 5.4 0.6 3.8

Grapefruit/Orange/Lemon

(4) 9.6 4.4 5.4 0.6 3.8

Grapefruit/Orange/Lemon

(2/2) 5.6 2.6 3.2 0.4 2.2

Grapefruit/Orange/Lemon

(4/4) 11.2 5.1 6.3 0.7 4.4

Lemon (1.6/1.6, 141‐days) 4.1 1.9 2.3 0.3 1.6

Lemon (1.6/1.6, 90‐days) 4.5 2.1 2.5 0.3 1.8

Apple (4) 9.6 4.4 5.4 0.6 3.8

Avocado (4) 9.6 4.4 5.4 0.6 3.8

Peach (4) 9.6 4.4 5.4 0.6 3.8

Peach (2) 4.8 2.2 2.7 0.3 1.9

Olive (4) 9.6 4.4 5.4 0.6 3.8

Cherry (4) 9.6 4.4 5.4 0.6 3.8

Pear (4) 9.6 4.4 5.4 0.6 3.8

Plum (4) 9.6 4.4 5.4 0.6 3.8

Nectarine (2) 4.8 2.2 2.7 0.3 1.9
95

Turf and Residential

Lawns (2/1; 140‐days) 4.8 2.2 2.7 0.3 1.9

Turf and Residential

Lawns (2/1; 30‐days) 5.0 2.3 2.8 0.3 2.0

Sod Farms (4/2; 141‐days) 9.6 4.4 5.4 0.6 3.8

Sod Farms (4/2; 40‐days) 9.6 4.4 5.4 0.6 3.8

Almond (2) 4.8 2.2 2.7 0.3 1.9

Hazelnut (4) 9.6 4.4 5.4 0.6 3.8

Hazelnut (2/2) 5.6 2.6 3.2 0.4 2.2

Macadamia (4) 9.6 4.4 5.4 0.6 3.8

Pecan (4) 9.6 4.4 5.4 0.6 3.8

Walnut (4) 9.6 4.4 5.4 0.6 3.8

Nursery (3) 7.2 3.3 4.1 0.5 2.8

Christmas Trees (4) 9.6 4.4 5.4 0.6 3.8

Shelter Belts (4) 9.6 4.4 5.4 0.6 3.8

Shelter Belts (2/2) 5.6 2.6 3.2 0.4 2.2

Cherries (4) 9.6 4.4 5.4 0.6 3.8

Reduced Rate (0.5) 1.20 0.55 0.68 0.08 0.47
96

Table 38. Acute Dose‐based RQ values for Mammals from Labeled Uses of Simazine (T‐REX v. 1.5.2, upper‐bound Keidentify LOC exceedances for listed species and bolded values indicate non‐listed LOC exceedances.Primary Feeding Strategy  Herbivores and Omnivores Insectivores Animal Size  Small Med Large Small Med Dietary Items 

Short Grass

Tall Grass

Broad‐leaf

Plants

Fruits, pods,

seeds, etc.

Short Grass

Tall Grass

Broadleaf

Plants

Fruits, pods,

seeds, etc.

Short Grass

Tall Grass

Broadleaf

Plants

Fruits, pods,

seeds, etc.

Arthropo

Use(s) (lb a.i./A) Alfalfa (1.6) 0.03 0.02 0.02 <0.01 0.03 0.01 0.02 <0.01 0.02 0.01 0.01 <0.01 0.01 0.01 Alfalfa (1.4) 0.03 0.01 0.02 <0.01 0.02 0.01 0.01 <0.01 0.01 0.01 0.01 <0.01 0.01 0.01 Corn (2/0.5) 0.04 0.02 0.02 <0.01 0.04 0.02 0.02 <0.01 0.02 0.01 0.01 <0.01 0.02 0.01 Corn Fallow (1) 0.02 0.01 0.01 <0.01 0.02 0.01 0.01 <0.01 0.01 <0.01 0.01 <0.01 0.01 0.01 Corn Postemergent (1.6) 0.03 0.02 0.02 <0.01 0.03 0.01 0.02 <0.01 0.02 0.01 0.01 <0.01 0.01 0.01 Blueberry/Caneberry (4) 0.08 0.04 0.05 0.01 0.07 0.03 0.04 <0.01 0.04 0.02 0.02 <0.01 0.03 0.03 Blueberry/Caneberry (2/2) 0.05 0.02 0.03 <0.01 0.04 0.02 0.02 <0.01 0.02 0.01 0.01 <0.01 0.02 0.02 Cranberry (4) 0.08 0.04 0.05 0.01 0.07 0.03 0.04 <0.01 0.04 0.02 0.02 <0.01 0.03 0.03 Strawberry (1) 0.02 0.01 0.01 <0.01 0.02 0.01 0.01 <0.01 0.01 <0.01 0.01 <0.01 0.01 0.01 Strawberry (4) 0.08 0.04 0.05 0.01 0.07 0.03 0.04 <0.01 0.04 0.02 0.02 <0.01 0.03 0.03 Grapes (4) 0.08 0.04 0.05 0.01 0.07 0.03 0.04 <0.01 0.04 0.02 0.02 <0.01 0.03 0.03 Grapefruit/Orange/Lemon (4) 0.08 0.04 0.05 0.01 0.07 0.03 0.04 <0.01 0.04 0.02 0.02 <0.01 0.03 0.03 Grapefruit/Orange/Lemon(2/2) 0.05 0.02 0.03 <0.01 0.04 0.02 0.02 <0.01 0.02 0.01 0.01 <0.01 0.02 0.02 Grapefruit/Orange/Lemon(4/4) 0.10 0.04 0.05 0.01 0.08 0.04 0.05 0.01 0.04 0.02 0.03 <0.01 0.04 0.03 Lemon (1.6/1.6, 141‐days) 0.04 0.02 0.02 <0.01 0.03 0.01 0.02 <0.01 0.02 0.01 0.01 <0.01 0.01 0.01 Lemon (1.6/1.6, 90‐days) 0.04 0.02 0.02 <0.01 0.03 0.02 0.02 <0.01 0.02 0.01 0.01 <0.01 0.02 0.01 Apple (4) 0.08 0.04 0.05 0.01 0.07 0.03 0.04 <0.01 0.04 0.02 0.02 <0.01 0.03 0.03 Avocado (4) 0.08 0.04 0.05 0.01 0.07 0.03 0.04 <0.01 0.04 0.02 0.02 <0.01 0.03 0.03 Peach (4) 0.08 0.04 0.05 0.01 0.07 0.03 0.04 <0.01 0.04 0.02 0.02 <0.01 0.03 0.03 Peach (2) 0.04 0.02 0.02 <0.01 0.04 0.02 0.02 <0.01 0.02 0.01 0.01 <0.01 0.02 0.01 Olive (4) 0.08 0.04 0.05 0.01 0.07 0.03 0.04 <0.01 0.04 0.02 0.02 <0.01 0.03 0.03 Cherry (4) 0.08 0.04 0.05 0.01 0.07 0.03 0.04 <0.01 0.04 0.02 0.02 <0.01 0.03 0.03 Pear (4) 0.08 0.04 0.05 0.01 0.07 0.03 0.04 <0.01 0.04 0.02 0.02 <0.01 0.03 0.03 Plum (4) 0.08 0.04 0.05 0.01 0.07 0.03 0.04 <0.01 0.04 0.02 0.02 <0.01 0.03 0.03 
97

Nectarine (2) 0.04 0.02 0.02 <0.01 0.04 0.02 0.02 <0.01 0.02 0.01 0.01 <0.01 0.02 0.01 Turf and Residential Lawns

(2/1; 140‐days) 0.04 0.02 0.02 <0.01 0.04 0.02 0.02 <0.01 0.02 0.01 0.01 <0.01 0.02 0.01 

Turf and Residential Lawns

(2/1; 30‐days) 0.04 0.02 0.02 <0.01 0.04 0.02 0.02 <0.01 0.02 0.01 0.01 <0.01 0.02 0.01 

Sod Farms (4/2; 141‐days) 0.08 0.04 0.05 0.01 0.07 0.03 0.04 <0.01 0.04 0.02 0.02 <0.01 0.03 0.03 Sod Farms (4/2; 40‐days) 0.08 0.04 0.05 0.01 0.07 0.03 0.04 <0.01 0.04 0.02 0.02 <0.01 0.03 0.03 Almond (2) 0.04 0.02 0.02 <0.01 0.04 0.02 0.02 <0.01 0.02 0.01 0.01 <0.01 0.02 0.01 Hazelnut (4) 0.08 0.04 0.05 0.01 0.07 0.03 0.04 <0.01 0.04 0.02 0.02 <0.01 0.03 0.03 Hazelnut (2/2) 0.05 0.02 0.03 <0.01 0.04 0.02 0.02 <0.01 0.02 0.01 0.01 <0.01 0.02 0.02 Macadamia (4) 0.08 0.04 0.05 0.01 0.07 0.03 0.04 <0.01 0.04 0.02 0.02 <0.01 0.03 0.03 Pecan (4) 0.08 0.04 0.05 0.01 0.07 0.03 0.04 <0.01 0.04 0.02 0.02 <0.01 0.03 0.03 Walnut (4) 0.08 0.04 0.05 0.01 0.07 0.03 0.04 <0.01 0.04 0.02 0.02 <0.01 0.03 0.03 Nursery (3) 0.06 0.03 0.04 <0.01 0.05 0.02 0.03 <0.01 0.03 0.01 0.02 <0.01 0.02 0.02 Christmas Trees (4) 0.08 0.04 0.05 0.01 0.07 0.03 0.04 <0.01 0.04 0.02 0.02 <0.01 0.03 0.03 Shelter Belts (4) 0.08 0.04 0.05 0.01 0.07 0.03 0.04 <0.01 0.04 0.02 0.02 <0.01 0.03 0.03 Shelter Belts (2/2) 0.05 0.02 0.03 <0.01 0.04 0.02 0.02 <0.01 0.02 0.01 0.01 <0.01 0.02 0.02 Cherries (4) 0.08 0.04 0.05 0.01 0.07 0.03 0.04 <0.01 0.04 0.02 0.02 <0.01 0.03 0.03 Reduced Rate (0.5) 0.01 <0.01 0.01 <0.01 0.01 <0.01 0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 
98

Table 39. Chronic Dose‐based RQ values for Mammals from Labeled Uses of Simazine (T‐REX v. 1.5.2, upper‐bound K

identify LOC exceedances for listed species and bolded values indicate non‐listed LOC exceedances.

Primary Feeding Strategy  Herbivores and Omnivores InsectivAnimal Size  Small Med Large Small MeDietary Items 

Short Grass

Tall Grass

Broad‐leaf

Plants

Fruits, pods,

seeds, etc.

Short Grass

Tall Grass

Broadleaf

Plants

Fruits, pods,

seeds, etc.

Short Grass

Tall Grass

Broadleaf

Plants

Fruits, pods,

seeds, etc.

Arthro

Use(s) (lb a.i./A)Alfalfa (1.6) 297.5 136.3 167.3 18.6 254.1 116.5 142.9 15.9 136.2 62.4 76.6 8.5 116.5 99.Alfalfa (1.4) 260.3 119.3 146.4 16.3 222.3 101.9 125.1 13.9 119.2 54.6 67.0 7.4 101.9 87.Corn (2/0.5) 374.8 171.8 210.8 23.4 320.1 146.7 180.1 20.0 171.6 78.6 96.5 10.7 146.8 125Corn Fallow (1) 185.9 85.2 104.6 11.6 158.8 72.8 89.3 9.9 85.1 39.0 47.9 5.3 72.8 62.Corn Postemergent (1.6) 297.5 136.3 167.3 18.6 254.1 116.5 142.9 15.9 136.2 62.4 76.6 8.5 116.5 99.Blueberry/Caneberry (4) 743.7 340.8 418.3 46.5 635.2 291.1 357.3 39.7 340.5 156.1 191.5 21.3 291.3 248Blueberry/Caneberry (2/2) 434.4 199.1 244.3 27.1 371.0 170.1 208.7 23.2 198.9 91.2 111.9 12.4 170.1 145Cranberry (4) 743.7 340.8 418.3 46.5 635.2 291.1 357.3 39.7 340.5 156.1 191.5 21.3 291.3 248Strawberry (1) 185.9 85.2 104.6 11.6 158.8 72.8 89.3 9.9 85.1 39.0 47.9 5.3 72.8 62.Strawberry (4) 743.7 340.8 418.3 46.5 635.2 291.1 357.3 39.7 340.5 156.1 191.5 21.3 291.3 248Grapes (4) 743.7 340.8 418.3 46.5 635.2 291.1 357.3 39.7 340.5 156.1 191.5 21.3 291.3 248Grapefruit/Orange/Lemon (4) 743.7 340.8 418.3 46.5 635.2 291.1 357.3 39.7 340.5 156.1 191.5 21.3 291.3 248Grapefruit/Orange/Lemon(2/2) 434.4 199.1 244.3 27.1 371.0 170.1 208.7 23.2 198.9 91.2 111.9 12.4 170.1 145Grapefruit/Orange/Lemon(4/4) 868.8 398.2 488.7 54.3 742.1 340.1 417.4 46.4 397.8 182.3 223.8 24.9 340.3 290Lemon (1.6/1.6, 141‐days) 315.7 144.7 177.6 19.7 269.7 123.6 151.7 16.9 144.5 66.3 81.3 9.0 123.7 105Lemon (1.6/1.6, 90‐days) 347.5 159.3 195.5 21.7 296.8 136.1 167.0 18.6 159.1 72.9 89.5 9.9 136.1 116Apple (4) 743.7 340.8 418.3 46.5 635.2 291.1 357.3 39.7 340.5 156.1 191.5 21.3 291.3 248Avocado (4) 743.7 340.8 418.3 46.5 635.2 291.1 357.3 39.7 340.5 156.1 191.5 21.3 291.3 248Peach (4) 743.7 340.8 418.3 46.5 635.2 291.1 357.3 39.7 340.5 156.1 191.5 21.3 291.3 248Peach (2) 371.8 170.4 209.2 23.2 317.6 145.6 178.7 19.9 170.3 78.0 95.8 10.6 145.6 124Olive (4) 743.7 340.8 418.3 46.5 635.2 291.1 357.3 39.7 340.5 156.1 191.5 21.3 291.3 248Cherry (4) 743.7 340.8 418.3 46.5 635.2 291.1 357.3 39.7 340.5 156.1 191.5 21.3 291.3 248Pear (4) 743.7 340.8 418.3 46.5 635.2 291.1 357.3 39.7 340.5 156.1 191.5 21.3 291.3 248Plum (4) 743.7 340.8 418.3 46.5 635.2 291.1 357.3 39.7 340.5 156.1 191.5 21.3 291.3 248
99

Primary Feeding Strategy  Herbivores and Omnivores InsectivAnimal Size  Small Med Large Small MeDietary Items 

Short Grass

Tall Grass

Broad‐leaf

Plants

Fruits, pods,

seeds, etc.

Short Grass

Tall Grass

Broadleaf

Plants

Fruits, pods,

seeds, etc.

Short Grass

Tall Grass

Broadleaf

Plants

Fruits, pods,

seeds, etc.

Arthro

Use(s) (lb a.i./A)Nectarine (2) 371.8 170.4 209.2 23.2 317.6 145.6 178.7 19.9 170.3 78.0 95.8 10.6 145.6 124Turf and Residential Lawns

(2/1; 140‐days) 371.8 170.4 209.2 23.2 317.6 145.6 178.7 19.9 170.3 78.0 95.8 10.6 145.6 124

Turf and Residential Lawns

(2/1; 30‐days) 391.2 179.3 220.0 24.4 334.1 153.1 188.0 20.9 179.1 82.1 100.8 11.2 153.2 130

Sod Farms (4/2; 141‐days) 743.7 340.8 418.3 46.5 635.2 291.1 357.3 39.7 340.5 156.1 191.5 21.3 291.3 248Sod Farms (4/2; 40‐days) 743.7 340.8 418.3 46.5 635.2 291.1 357.3 39.7 340.5 156.1 191.5 21.3 291.3 248Almond (2) 371.8 170.4 209.2 23.2 317.6 145.6 178.7 19.9 170.3 78.0 95.8 10.6 145.6 124Hazelnut (4) 743.7 340.8 418.3 46.5 635.2 291.1 357.3 39.7 340.5 156.1 191.5 21.3 291.3 248Hazelnut (2/2) 434.4 199.1 244.3 27.1 371.0 170.1 208.7 23.2 198.9 91.2 111.9 12.4 170.1 145Macadamia (4) 743.7 340.8 418.3 46.5 635.2 291.1 357.3 39.7 340.5 156.1 191.5 21.3 291.3 248Pecan (4) 743.7 340.8 418.3 46.5 635.2 291.1 357.3 39.7 340.5 156.1 191.5 21.3 291.3 248Walnut (4) 743.7 340.8 418.3 46.5 635.2 291.1 357.3 39.7 340.5 156.1 191.5 21.3 291.3 248Nursery (3) 557.7 255.6 313.7 34.9 476.4 218.4 268.0 29.8 255.4 117.0 143.7 16.0 218.5 186Christmas Trees (4) 743.7 340.8 418.3 46.5 635.2 291.1 357.3 39.7 340.5 156.1 191.5 21.3 291.3 248Shelter Belts (4) 743.7 340.8 418.3 46.5 635.2 291.1 357.3 39.7 340.5 156.1 191.5 21.3 291.3 248Shelter Belts (2/2) 434.4 199.1 244.3 27.1 371.0 170.1 208.7 23.2 198.9 91.2 111.9 12.4 170.1 145Cherries (4) 743.7 340.8 418.3 46.5 635.2 291.1 357.3 39.7 340.5 156.1 191.5 21.3 291.3 248Reduced Rate (0.5) 93.0 42.6 52.3 5.8 79.4 36.4 44.7 5.0 42.6 19.5 23.9 2.7 36.4 31.
100

Table 40.  Chronic Dietary‐Based RQs for Mammals of Different Feeding Classes (T‐REX v. 1.5,

upper‐bound Kenega). Shaded cells identify LOC exceedances for listed species and bolded

values indicate non‐listed LOC exceedances.

Primary Feeding Strategy  Herbivores, Omnivores, and Granivores Insectivores

Dietary Items 

Short GrassTall GrassBroad‐leafPlantsFruits, pods,seeds, etc.Arthropods

Use(s) (lb a.i./A) 

Alfalfa (1.6) 38.4 17.6 21.6 2.4 15.0

Alfalfa (1.4) 33.6 15.4 18.9 2.1 13.2

Corn (2/0.5) 48.4 22.2 27.2 3.0 18.9

Corn Fallow (1) 38.4 11.0 13.5 1.5 9.4

Corn Postemergent (1.6) 38.4 17.6 21.6 2.4 15.0

Blueberry/Caneberry (4) 96.0 44.0 54.0 6.0 37.6

Blueberry/Caneberry (2/2) 56.1 25.7 31.5 3.5 22.0

Cranberry (4) 96.0 44.0 54.0 6.0 37.6

Strawberry (1) 38.4 11.0 13.5 1.5 9.4

Strawberry (4) 96.0 44.0 54.0 6.0 37.6

Grapes (4) 96.0 44.0 54.0 6.0 37.6

Grapefruit/Orange/Lemon (4) 96.0 44.0 54.0 6.0 37.6

Grapefruit/Orange/Lemon (2/2) 56.1 25.7 31.5 3.5 22.0

Grapefruit/Orange/Lemon (4/4) 112.2 51.4 63.1 7.0 43.9

Lemon (1.6/1.6, 141‐days) 40.8 18.7 22.9 2.5 16.0

Lemon (1.6/1.6, 90‐days) 44.9 20.6 25.2 2.8 17.6

Apple (4) 96.0 44.0 54.0 6.0 37.6

Avocado (4) 96.0 44.0 54.0 6.0 37.6

Peach (4) 96.0 44.0 54.0 6.0 37.6

Peach (2) 48.0 22.0 27.0 3.0 18.8

Olive (4) 96.0 44.0 54.0 6.0 37.6

Cherry (4) 96.0 44.0 54.0 6.0 37.6

Pear (4) 96.0 44.0 54.0 6.0 37.6

Plum (4) 96.0 44.0 54.0 6.0 37.6

Nectarine (2) 48.0 22.0 27.0 3.0 18.8

Turf and Residential Lawns (2/1;

140‐days) 48.0 22.0 27.0 3.0 18.8

Turf and Residential Lawns (2/1;

30‐days) 50.5 23.1 28.4 3.2 19.8

Sod Farms (4/2; 141‐days) 96.0 44.0 54.0 6.0 37.6

Sod Farms (4/2; 40‐days) 96.0 44.0 54.0 6.0 37.6

Almond (2) 48.0 22.0 27.0 3.0 18.8

Hazelnut (4) 96.0 44.0 54.0 6.0 37.6
101

Primary Feeding Strategy  Herbivores, Omnivores, and Granivores Insectivores

Dietary Items 

Short GrassTall GrassBroad‐leafPlantsFruits, pods,seeds, etc.Arthropods

Use(s) (lb a.i./A) 

Hazelnut (2/2) 56.1 25.7 31.5 3.5 22.0

Macadamia (4) 96.0 44.0 54.0 6.0 37.6

Pecan (4) 96.0 44.0 54.0 6.0 37.6

Walnut (4) 96.0 44.0 54.0 6.0 37.6

Nursery (3) 72.0 33.0 40.5 4.5 28.2

Christmas Trees (4) 96.0 44.0 54.0 6.0 37.6

Shelter Belts (4) 96.0 44.0 54.0 6.0 37.6

Shelter Belts (2/2) 56.1 25.7 31.5 3.5 22.0

Cherries (4) 96.0 44.0 54.0 6.0 37.6

Reduced Rate (0.5) 12.0 5.5 6.8 0.8 4.7

Risks to Birds

Based on upper‐bound Kenaga values from T‐REX, and the non‐definitive acute dose and

dietary toxicity endpoints, RQs slightly exceed the LOC for multiple labeled uses for simazine in

birds for acute exposures.  Because the acute toxicity data are non‐definitive, and there were

no mortalities reported in the acute toxicity studies, there is minimal risk of mortality on an

acute exposure basis.   

However, on a chronic exposure basis, the chronic LOC is exceeded for all uses, with RQ values

as high as 11.2. Chronic risk to birds is consistent with the available chlorotriazine toxicity data

in birds. Although results from the model runs considering a single application of 0.5 lb a.i./A

show reductions in the RQs, the chronic LOC is still exceeded at this application rate. The

summary table below (Table 41) includes minimum and maximum RQ ranges for each use.

   
102

Table 41. Range of RQs for Birds of Different Feeding Classes (T‐REX v. 1.5.2, upper‐bound

Kenaga)1.

Use (lb a.i./A)

Acute Dose‐based

avian RQ range  

Acute Dietary‐

based avian RQ

range  

Chronic Dietary‐

based avian RQ

range  

Max Min Max Min Max Min

Alfalfa (1.6) 0.18 0.01 0.08 0.00 3.8 0.2

Alfalfa (1.4) 0.16 0.01 0.07 0.00 3.4 0.2

Corn (2/0.5) 0.23 0.01 0.10 0.01 4.8 0.3

Corn Fallow (1) 0.11 0.01 0.05 0.00 2.4 0.2

Corn Postemergent (1.6) 0.18 0.01 0.08 0.00 3.8 0.2

Blueberry/Caneberry (4) 0.45 0.01 0.19 0.01 9.6 0.6

Blueberry/Caneberry (2/2) 0.27 0.01 0.11 0.01 5.6 0.4

Cranberry (4) 0.45 0.01 0.19 0.01 9.6 0.6

Strawberry (1) 0.11 0.01 0.05 0.00 2.4 0.2

Strawberry (4) 0.45 0.01 0.19 0.01 9.6 0.6

Grapes (4) 0.45 0.01 0.19 0.01 9.6 0.6

Grapefruit/Orange/Lemon (4) 0.45 0.01 0.19 0.01 9.6 0.6

Grapefruit/Orange/Lemon

(2/2) 0.27 0.01 0.11 0.01 5.6 0.4

Grapefruit/Orange/Lemon

(4/4) 0.53 0.01 0.22 0.01 11.2 0.7

Lemon (1.6/1.6, 141‐days) 0.19 0.01 0.08 0.01 4.1 0.3

Lemon (1.6/1.6, 90‐days) 0.21 0.01 0.09 0.01 4.5 0.3

Apple (4) 0.45 0.01 0.19 0.01 9.6 0.6

Avocado (4) 0.45 0.01 0.19 0.01 9.6 0.6

Peach (4) 0.45 0.01 0.19 0.01 9.6 0.6

Peach (2) 0.23 0.01 0.10 0.01 4.8 0.3

Olive (4) 0.45 0.01 0.19 0.01 9.6 0.6

Cherry (4) 0.45 0.01 0.19 0.01 9.6 0.6

Pear (4) 0.45 0.01 0.19 0.01 9.6 0.6

Plum (4) 0.45 0.01 0.19 0.01 9.6 0.6

Nectarine (2) 0.23 0.01 0.10 0.01 4.8 0.3

Turf and Residential Lawns

(2/1; 140‐days) 0.23 0.01 0.10 0.01 4.8 0.3

Turf and Residential Lawns

(2/1; 30‐days) 0.24 0.01 0.10 0.01 5.0 0.3

Sod Farms (4/2; 141‐days) 0.45 0.01 0.19 0.01 9.6 0.6

Sod Farms (4/2; 40‐days) 0.45 0.01 0.19 0.01 9.6 0.6

Almond (2) 0.23 0.01 0.10 0.01 4.8 0.3

Hazelnut (4) 0.45 0.01 0.19 0.01 9.6 0.6

Hazelnut (2/2) 0.27 0.01 0.11 0.01 5.6 0.4

Macadamia (4) 0.45 0.01 0.19 0.01 9.6 0.6

Pecan (4) 0.45 0.01 0.19 0.01 9.6 0.6

Walnut (4) 0.45 0.01 0.19 0.01 9.6 0.6

Nursery (3) 0.34 0.01 0.14 0.01 7.2 0.5
103

Use (lb a.i./A)

Acute Dose‐based

avian RQ range  

Acute Dietary‐

based avian RQ

range  

Chronic Dietary‐

based avian RQ

range  

Max Min Max Min Max Min

Christmas Trees (4) 0.45 0.01 0.19 0.01 9.6 0.6

Shelter Belts (4) 0.45 0.01 0.19 0.01 9.6 0.6

Shelter Belts (2/2) 0.27 0.01 0.11 0.01 5.6 0.4

Cherries (4) 0.45 0.01 0.19 0.01 9.6 0.6

Reduced Rate (0.5) 0.06 0.01 0.02 0.01 1.2 0.1

Risks to Mammals

The LD50 value for simazine TGAI is >5,000 mg/kg‐bw, suggesting technical grade simazine is

practically non‐toxic to mammals on an acute basis (MRID 00148897). Although a definitive LC50

was not established in the study, mortality was noted. In order to gain a better understanding

of how the EECs for the maximum proposed simazine application rate relate to the toxicity data

currently available for mammals, the T‐REX model is used to calculate the ratio of the EEC and

toxicity endpoint. In this exercise the acute ratios are < 0.1, thus acute LOCs are not exceeded,

except only for the highest use rates on grapefruit, lemon, and orange. Although the highest

dose tested is above the maximum expected exposure level, the study indicates that there is a

risk of mortality to non‐target mammals from the use of this pesticide. Therefore, based on the

available data the risk to listed species following an acute exposure from the use of simazine is

an uncertainty.

Chronic risks are of primary concern with chronic RQ values as high as 869 and chronic LOCs

exceeding 100% of the use scenarios modeled. Primary concern with chronic risk is consistent

with the available chlorotriazine toxicity risks to mammals. The chronic RQs are based on

upper‐bound Kenaga values from T‐REX, with the highest maximum RQs occurring for small

mammals with short grass as the primary food item and the citrus use scenarios. Many of the

simazine uses allow for an application rate of 4 lb a.i./A which results in RQs for small mammals

of 744. The summary table below (Table 42) includes minimum and maximum RQ ranges for

each use.  

For the reduced rate modeled, RQs based on acute risk to mammals are less than the LOC for

listed and non‐listed species. However, the chronic LOC for is still exceeded for a single

application of 0.5 lb a.i./A with RQs of 0.6‐93 and 0.8‐12 for chronic dose‐ and dietary‐based

risks, respectively.  
104

Table 42. Range of RQs for Mammals of Different Feeding Classes (T‐REX v. 1.5.2, upper‐

bound Kenaga)1.

Use (lb a.i./A)

Dose‐based RQ

range mammalian

acute risk

Dose‐based RQ

range mammalian

chronic risk

Dietary‐based RQ

range mammalian

chronic risk

Max Min Max Min Max Min

Alfalfa (1.6) 0.03 0.01 297.5 1.9 38.4 2.4

Alfalfa (1.4) 0.03 0.01 260.3 1.7 33.6 2.1

Corn (2/0.5) 0.04 0.01 374.8 2.4 48.4 3.0

Corn Fallow (1) 0.02 0.01 185.9 1.2 38.4 1.5

Corn Postemergent (1.6) 0.03 0.01 297.5 1.9 38.4 2.4

Blueberry/Caneberry (4) 0.08 0.01 743.7 4.7 96.0 6.0

Blueberry/Caneberry (2/2) 0.05 0.01 434.4 2.8 56.1 3.5

Cranberry (4) 0.08 0.01 743.7 4.7 96.0 6.0

Strawberry (1) 0.02 0.01 185.9 1.2 38.4 1.5

Strawberry (4) 0.08 0.01 743.7 4.7 96.0 6.0

Grapes (4) 0.08 0.01 743.7 4.7 96.0 6.0

Grapefruit/Orange/Lemon (4) 0.08 0.01 743.7 4.7 96.0 6.0

Grapefruit/Orange/Lemon

(2/2) 0.05 0.01 434.4 2.8 56.1 3.5

Grapefruit/Orange/Lemon

(4/4) 0.10 0.01 868.8 5.5 112.2 7.0

Lemon (1.6/1.6, 141‐days) 0.04 0.01 315.7 2.0 40.8 2.5

Lemon (1.6/1.6, 90‐days) 0.04 0.01 347.5 2.2 44.9 2.8

Apple (4) 0.08 0.01 743.7 4.7 96.0 6.0

Avocado (4) 0.08 0.01 743.7 4.7 96.0 6.0

Peach (4) 0.08 0.01 743.7 4.7 96.0 6.0

Peach (2) 0.04 0.01 371.8 2.4 48.0 3.0

Olive (4) 0.08 0.01 743.7 4.7 96.0 6.0

Cherry (4) 0.08 0.01 743.7 4.7 96.0 6.0

Pear (4) 0.08 0.01 743.7 4.7 96.0 6.0

Plum (4) 0.08 0.01 743.7 4.7 96.0 6.0

Nectarine (2) 0.04 0.01 371.8 2.4 48.0 3.0

Turf and Residential Lawns

(2/1; 140‐days) 0.04 0.01 371.8 2.4 48.0 3.0

Turf and Residential Lawns

(2/1; 30‐days) 0.04 0.01 391.2 2.5 50.5 3.2

Sod Farms (4/2; 141‐days) 0.08 0.01 743.7 4.7 96.0 6.0

Sod Farms (4/2; 40‐days) 0.08 0.01 743.7 4.7 96.0 6.0

Almond (2) 0.04 0.01 371.8 2.4 48.0 3.0

Hazelnut (4) 0.08 0.01 743.7 4.7 96.0 6.0

Hazelnut (2/2) 0.05 0.01 434.4 2.8 56.1 3.5
105

Use (lb a.i./A)

Dose‐based RQ

range mammalian

acute risk

Dose‐based RQ

range mammalian

chronic risk

Dietary‐based RQ

range mammalian

chronic risk

Max Min Max Min Max Min

Macadamia (4) 0.08 0.01 743.7 4.7 96.0 6.0

Pecan (4) 0.08 0.01 743.7 4.7 96.0 6.0

Walnut (4) 0.08 0.01 743.7 4.7 96.0 6.0

Nursery (3) 0.06 0.01 557.7 3.6 72.0 4.5

Christmas Trees (4) 0.08 0.01 743.7 4.7 96.0 6.0

Shelter Belts (4) 0.08 0.01 743.7 4.7 96.0 6.0

Shelter Belts (2/2) 0.05 0.01 434.4 2.8 56.1 3.5

Cherries (4) 0.08 0.01 743.7 4.7 96.0 6.0

Reduced Rate (0.5) 0.01 0.01 93.0 0.6 12.0 0.8

Off‐Field Risk to Birds and Mammals  

Spray deposition curves calculated using AgDRIFT (version 2.1.1) (Tier I) were used to estimate

the distance from the edge of the field to the location where effects to non‐target organisms

are no longer of concern following a single application of 0.5, 2 or 4 lb a.i./A by ground

applications.

The distance estimated for birds and mammals is based on one application and does not reflect

possible cumulative exposure from multiple applications. It is recognized that a species could

receive exposure from multiple applications, in which case, this methodology may

underestimate risk. The distance estimated for terrestrial animals for multiple applications may

occur when wind is blowing consistently in one direction for all applications or when wind is

blowing in different directions during different applications as long as the organism is

downwind in each case and regardless of whether it is mobile or stationary. This may result in

an overestimation of exposure for terrestrial organisms whose spray drift distances are based

on exposure to the maximum number of applications and who are not downwind of every

application. Exposure to multiple applications is more likely to occur when agricultural

fields/use areas are on multiple sides of a terrestrial area of interest and when local wind

direction is not variable.

The following figures describe the risk associated with aerial and ground spray applications

assuming different droplet sizes and release/boom heights. Drift exposure to birds and

mammals following an aerial application of 1.6 lb/A3 may result in near‐field chronic risks to

birds; however, the exposure distance of concern for mammals extends out to 1000 feet for

most droplet spectra (Figure 5).   

                                                       

3 Aerial applications are only registered on alfalfa grown for seed in Washington.
106

Drift exposure to birds following a ground application of 2 or 4 lb a.i./A may result in near field

(within 100 feet) chronic risks for all droplet sizes; a reduction in the application rate to 0.5 lb

a.i./A reduces this distance to the field edge (Figure 6, Figure 7 and Figure 8). Following a single

ground spray application, drift exposure results in potential chronic risks for mammals

extending from 200 – 650 ft, 450 – 1000 ft, and 50 – 225 ft following a single application of 2, 4

or 0.5 lb a.i./A respectively (Figure 6, Figure 7 and Figure 8). These off‐field estimates are

considered lower‐bound but may actually underestimate risk for animals that spend more time

foraging on and adjacent to the field. Therefore, for all labeled ground application rates, off‐

field spray drift presents a chronic risk concern for mammals.  

Figure 5. Spray drift deposition curves for various droplet spectra following a single aerial

application of 1.6 lb a.i/A. Horizontal lines represent the most sensitive non‐listed species

mammals and birds.

0.001

0.01

0.1

1

0 100 200 300 400 500 600 700 800 900 1000

lb ai/A

Distance Downwind from field (ft)

1.6 lb a.i./A application rate; aerial app

Fine to Medium Medium

Medium‐Coarse Coarse

Coarse‐Very Coarse Very Coarse

Very Coarse‐Extremely Coarse Mammalian Chronic Risk, Upper Kenaga, LOC <1)

Avian Chronic Risk, Upper Kenaga, LOC <1)
107

Figure 6. Spray drift deposition curves for various droplet spectra following a single ground

application of 2 lb a.i/A. Horizontal lines represent the most sensitive non‐listed species of

mammals and birds.

0.001

0.01

0.1

1

10

0 100 200 300 400 500 600 700 800 900 1000

lb ai/A

Distance Downwind from field (ft)

2 lb/A application rate; ground app

Low‐Boom Very‐Fine to Fine Low‐Boom Fine to Medium/Coarse

High‐Boom Very‐Fine to Fine High‐Boom Fine to Medium/Coarse

Mammalian Chronic Risk, Upper Kenaga, LOC <1) Avian Chronic Risk, Upper Kenaga, LOC <1)
108

Figure 7. Spray drift deposition curves for various droplet spectra following a single ground

application of 4 lb a.i/A. Horizontal lines represent the most sensitive non‐listed species of

mammals and birds.

0.001

0.01

0.1

1

10

0 100 200 300 400 500 600 700 800 900 1000

lb ai/A

Distance Downwind from field (ft)

4 lb/A application rate; ground app

Low‐Boom Very‐Fine to Fine Low‐Boom Fine to Medium/Coarse

High‐Boom Very‐Fine to Fine High‐Boom Fine to Medium/Coarse

Mammalian Chronic Risk, Upper Kenaga, LOC <1) Avian Chronic Risk, Upper Kenaga, LOC <1)
109

Figure 8. Spray drift deposition curves for various droplet spectra following a single ground

application of 0.5 lb a.i/A. Horizontal lines represent the most sensitive non‐listed species

mammals and birds.

Risk to Reptiles and Terrestrial‐phase Amphibians

Due to the lack of reptile and terrestrial‐phase amphibian toxicity data for simazine, data for

birds are used as a surrogate. Therefore, discussion on EECs, RQs and drift analysis in Sections

14.1.3 and 14.1.5 above are applicable to risks for reptiles and terrestrial‐phase amphibians.

   

0.0001

0.001

0.01

0.1

1

0 100 200 300 400 500 600 700 800 900 1000

lb ai/A

Distance Downwind from field (ft)

0.5 lb/A application rate; ground app

Low‐Boom Very‐Fine to Fine Low‐Boom Fine to Medium/Coarse

High‐Boom Very‐Fine to Fine High‐Boom Fine to Medium/Coarse

Mammalian Chronic Risk, Upper Kenaga, LOC <1) Avian Chronic Risk, Upper Kenaga, LOC <1)
110

Risk to Terrestrial Invertebrates

The acute contact LD50 in honey bees was >97 ug a.i./bee (6.5% mortality occurred at the

highest dose level) (MRID 00036935) which is classified as practically non‐toxic. A dose of 97 μg

a.i./bee corresponds to a simazine concentration on the bee of approximately 757 mg/kg‐bw,

assuming an adult honey bee weighs 128 mg (Mayer and Johansen, 1990). The corresponding

exposure value to honey bees at an application rate of 4 lb a.i./A is approximately 60 mg/kg‐bw.  

Exposure assessments for honey bees were also calculated using the new pollinator guidance.

Based on Tier I exposure estimates for contact exposure and a maximum single application rate

of 4 lb a.i./A, the exposure estimate was 10.8 μg a.i./bee. The RQ based on the Tier I exposure

estimate and non‐definitive LD50 was 0.11. This is below the LOC of 0.4. No additional data were

available for honey bee toxicity; therefore RQs based on adult oral exposure or larval exposure

were not calculated.

Risks to Terrestrial Plants

Runoff and Spray Drift Exposure to Terrestrial and Semi‐Aquatic Plants

Exposure of non‐target terrestrial and semi‐aquatic (wetland) plant species is estimated using

OPP’s TerrPlant (v. 1.2.2) model. Loading via spray drift to dry, non‐target, adjacent areas is

assumed to occur from one acre of treated land to one acre of the non‐target area. Runoff is

also expected to be a source of pesticide loading to non‐target areas. TerrPlant calculates EECs

as a function of application rate, solubility, and default assumptions regarding spray drift. The

default spray drift assumptions are 1% of the application rate for ground spray applications and

5% for aerial spray applications (USEPA 2006b). The EECs for terrestrial and semi‐aquatic plants

for a single application of simazine at the maximum labeled rates for representative uses are

presented in Table 43.

Table 43.  EECs for Terrestrial and Semi‐Aquatic Plants Near Simazine Use Areas (TerrPlant v.

1.2.2)

Cropa (lb a.i./A)  

Single Max.

Application

Rate

(lb a.i./A)

EECs (lb a.i./A)

Spray Drift

Only

Runoff and

Spray Drift

(Dry Areas)

Runoff and

Spray Drift

(Semi‐Aquatic Areas)

Alfalfa (1.6) Ground 1.6 0.016 0.032 0.176

Alfalfa (1.6) Aerial 1.6 0.08 0.096 0.24

Corn (2/0.5) 2 0.02 0.04 0.22

Corn (0.5) 0.5 0.005 0.01 0.055

Corn Fallow (1) 1 0.01 0.02 0.11

Corn Post‐emergent (1.6) 1.6 0.016 0.032 0.176

Blueberry/Caneberry (4) 4 0.04 0.08 0.44

Blueberry/Caneberry (2/2) 2 0.02 0.04 0.22
111

Cropa (lb a.i./A)  

Single Max.

Application

Rate

(lb a.i./A)

EECs (lb a.i./A)

Spray Drift

Only

Runoff and

Spray Drift

(Dry Areas)

Runoff and

Spray Drift

(Semi‐Aquatic Areas)

Cranberry (4) 4 0.04 0.08 0.44

Strawberry (1) 1 0.01 0.02 0.11

Strawberry (4) 4 0.04 0.08 0.44

Grapes (4) 4 0.04 0.08 0.44

Grapefruit/Orange/Lemon (4) 4 0.04 0.08 0.44

Grapefruit/Orange/Lemon (2/2) 2 0.02 0.04 0.22

Grapefruit/Orange/Lemon (4/4) 4 0.04 0.08 0.44

Lemon (1.6/1.6, 141‐days) 1.6 0.016 0.032 0.176

Lemon (1.6/1.6, 90‐days) 1.6 0.016 0.032 0.176

Apple (4) 4 0.04 0.08 0.44

Avocado (4) 4 0.04 0.08 0.44

Peach (4) 4 0.04 0.08 0.44

Peach (2) 2 0.02 0.04 0.22

Olive (4) 4 0.04 0.08 0.44

Cherry (4) 4 0.04 0.08 0.44

Pear (4) 4 0.04 0.08 0.44

Plum (4) 4 0.04 0.08 0.44

Nectarine (2) 2 0.02 0.04 0.22

Turf and Residential Lawns (2/1;

140‐days) 2 0.02 0.04 0.22

Turf and Residential Lawns (2/1;

30‐days) 2 0.02 0.04 0.22

Sod Farms (4/2; 141‐days) 4 0.04 0.08 0.44

Sod Farms (4/2; 40‐days) 4 0.04 0.08 0.44

Almond (2) 2 0.02 0.04 0.22

Hazelnut (4) 4 0.04 0.08 0.44

Hazelnut (2/2) 2 0.02 0.04 0.22

Macadamia (4) 4 0.04 0.08 0.44

Pecan (4) 4 0.04 0.08 0.44

Walnut (4) 4 0.04 0.08 0.44

Nursery (3) 3 0.03 0.06 0.33

Christmas Trees (4) 4 0.04 0.08 0.44

Shelter Belts (4) 4 0.04 0.08 0.44

Shelter Belts (2/2) 2 0.02 0.04 0.22

Cherries (4) 4 0.04 0.08 0.44

a All uses are labeled only for ground application except alfalfa.

Risk Quotient (RQ) Values for Terrestrial Plant Species

At the maximum single application rate for each of the modeled labeled uses, RQs for listed and

non‐listed monocots and dicots are above the LOCs for upland (dry areas) and wetland (semi‐

aquatic areas) habitats based on spray drift exposure alone as well as through the combination
112

of runoff and spray drift exposure (Table 44). In addition, the model was run assuming potential

reduced single application rates of 0.5 lb a.i./A. The resulting RQs are above the levels of

concern for runoff and spray drift to upland and wetland habitats.  

The RQs resulting from ground spray applications result in lower drift concerns than those

resulting from aerial applications4

; however, these application methods contribute equally to

runoff concerns. Because of the relatively high solubility and mobility of simazine and its

persistence in the environment, any species of plant that is downhill of the application area is

likely to receive runoff from use sites.  

Table 44. RQs for Terrestrial and Semi‐Aquatic Plants Near Simazine Use Areas (TerrPlant v.

1.2.2).

                                                      

4 Aerial applications are only registered on alfalfa grown for seed in Washington.

Exposure Scenario 

Listed and Non‐Listed Species RQs

Runoff and

Spray Drift

(Dry Areas)

Runoff and

Spray Drift

(Semi‐Aquatic Areas)

Spray Drift Only

Plant Group 

Monocot Non‐ListedMonocotListedDicot Non‐ListedDicot ListedMonocot Non‐ListedMonocotListedDicot Non‐ListedDicot ListedMonocot Non‐ListedMonocotListedDicot Non‐ListedDicot Listed

Use(s) 

Alfalfa (1.6) Ground 1.6 18.8 3.6 17.8 8.8 103.5 19.6 97.8 0.8 9.4 1.8 8.9

Alfalfa (1.6) Aerial 4.8 56.5 10.7 53.3 12.0 141.2 26.7 133.3 5.0 2.4 8.9 44.4

Corn (2) 2.0 23.5 4.4 22.2 11.0 129.4 24.4 122.2 1.0 11.8 2.2 11.1

Corn (0.5) 0.5 5.9 1.1 5.6 2.8 32.4 6.1 30.6 0.3 2.9 0.6 2.8

Corn Fallow (1) 1.0 11.8 2.2 11.1 5.5 64.7 12.2 61.1 0.5 5.9 1.1 5.6

Corn Postemergent (1.6) 1.6 18.8 3.6 17.8 8.8 103.5 19.6 97.8 0.8 9.4 1.8 8.9

Blueberry/Caneberry (4) 4.0 47.1 8.9 44.4 22.0 258.8 48.9 244.4 2.0 23.5 4.4 22.2

Blueberry/Caneberry

(2/2) 2.0 23.5 4.4 22.2 11.0 129.4 24.4 122.2 1.0 11.8 2.2 11.1

Cranberry (4) 4.0 47.1 8.9 44.4 22.0 258.8 48.9 244.4 2.0 23.5 4.4 22.2

Strawberry (1) 1.0 11.8 2.2 11.1 5.5 64.7 12.2 61.1 0.5 5.9 1.1 5.6

Strawberry (4) 4.0 47.1 8.9 44.4 22.0 258.8 48.9 244.4 2.0 23.5 4.4 22.2

Grapes (4) 4.0 47.1 8.9 44.4 22.0 258.8 48.9 244.4 2.0 23.5 4.4 22.2

Grapefruit/Orange/Lemon

(4) 4.0 47.1 8.9 44.4 22.0 258.8 48.9 244.4 2.0 23.5 4.4 22.2

Grapefruit/Orange/Lemon

(2/2) 2.0 23.5 4.4 22.2 11.0 129.4 24.4 122.2 1.0 11.8 2.2 11.1
113

Off‐Field Risk to Terrestrial Plants

Because TerrPlant assumes a single fraction of the spray deposition, the model cannot describe

the distance to the point when an LOC is not exceeded.  Spray deposition curves calculated

using AgDRIFT (version 2.1.1) (Tier I) were used to estimate the distance from the edge of the

field to where effects to non‐target organisms are no longer of concern following a single

application of  0.5, 2, or 4 lb a.i./A by ground applications.  The deposition curves were then

compared against the most sensitive monocot and dicot IC25 values.

The distance estimated for plants is based on one application and does not reflect possible

cumulative exposure from multiple applications. It is recognized that a species could receive

exposure from multiple applications; in this case, the estimated distance may underestimate

risk. The distance estimated for terrestrial plants for multiple applications may occur when

wind is blowing consistently in one direction for all applications or when wind is blowing in

different directions during different applications as long as the organism is downwind in each

Lemon (1.6/1.6) 1.6 18.8 3.6 17.8 8.8 103.5 19.6 97.8 0.8 9.4 1.8 8.9

Apple (4) 4.0 47.1 8.9 44.4 22.0 258.8 48.9 244.4 2.0 23.5 4.4 22.2

Avocado (4) 4.0 47.1 8.9 44.4 22.0 258.8 48.9 244.4 2.0 23.5 4.4 22.2

Peach (4) 4.0 47.1 8.9 44.4 22.0 258.8 48.9 244.4 2.0 23.5 4.4 22.2

Peach (2) 2.0 23.5 4.4 22.2 11.0 129.4 24.4 122.2 1.0 11.8 2.2 11.1

Olive (4) 4.0 47.1 8.9 44.4 22.0 258.8 48.9 244.4 2.0 23.5 4.4 22.2

Cherry (4) 4.0 47.1 8.9 44.4 22.0 258.8 48.9 244.4 2.0 23.5 4.4 22.2

Pear (4) 4.0 47.1 8.9 44.4 22.0 258.8 48.9 244.4 2.0 23.5 4.4 22.2

Plum (4) 4.0 47.1 8.9 44.4 22.0 258.8 48.9 244.4 2.0 23.5 4.4 22.2

Nectarine (2) 2.0 23.5 4.4 22.2 11.0 129.4 24.4 122.2 1.0 11.8 2.2 11.1

Turf and Residential

Lawns (2/1) 2.0 23.5 4.4 22.2 11.0 129.4 24.4 122.2 1.0 11.8 2.2 11.1

Sod Farms (4/2) 4.0 47.1 8.9 44.4 22.0 258.8 48.9 244.4 2.0 23.5 4.4 22.2

Almond (2) 2.0 23.5 4.4 22.2 11.0 129.4 24.4 122.2 1.0 11.8 2.2 11.1

Hazelnut (4) 4.0 47.1 8.9 44.4 22.0 258.8 48.9 244.4 2.0 23.5 4.4 22.2

Hazelnut (2/2) 2.0 23.5 4.4 22.2 11.0 129.4 24.4 122.2 1.0 11.8 2.2 11.1

Macadamia (4) 4.0 47.1 8.9 44.4 22.0 258.8 48.9 244.4 2.0 23.5 4.4 22.2

Pecan (4) 4.0 47.1 8.9 44.4 22.0 258.8 48.9 244.4 2.0 23.5 4.4 22.2

Walnut (4) 4.0 47.1 8.9 44.4 22.0 258.8 48.9 244.4 2.0 23.5 4.4 22.2

Nursery (3) 3.0 35.3 6.7 33.3 16.5 194.1 36.7 183.3 1.5 17.6 3.3 16.7

Christmas Trees (4) 4.0 47.1 8.9 44.4 22.0 258.8 48.9 244.4 2.0 23.5 4.4 22.2

Shelter Belts (4) 4.0 47.1 8.9 44.4 22.0 258.8 48.9 244.4 2.0 23.5 4.4 22.2

Shelter Belts (2/2) 2.0 23.5 4.4 22.2 11.0 129.4 24.4 122.2 1.0 11.8 2.2 11.1

Cherries (4) 4.0 47.1 8.9 44.4 22.0 258.8 48.9 244.4 2.0 23.5 4.4 22.2
114

case. This may result in an overestimation of exposure for terrestrial plants whose spray drift

distances are based on exposure to the maximum number of applications and who are not

downwind of every application. Exposure to multiple applications is more likely to occur when

agricultural fields/use areas are on multiple sides of a terrestrial area of interest and when local

wind direction is not variable.

The single maximum label rate for corn is 2 lb a.i/A. Figure 9 describes the risk associated with

ground spray application assuming different droplet sizes and boom heights. Depending on the

boom height and droplet size, drift to terrestrial plant species following an ground application

of 2 lb a.i./A results in concentrations that exceed the non‐listed monocots and dicots out to

50‐250 ft and 100 – 450 ft respectively. Drift deposition from both high and low boom

applications result in concentrations that exceed listed species of monocots and dicots from

700 to beyond 1000 ft.  
115

Figure 9. Spray drift deposition curves for various droplet spectra following a single ground

application of 2 lb a.i/A. Horizontal dotted lines represent the most sensitive non‐listed

species endpoint from tested monocot and dicot species. Horizontal dashed lines represent

the most sensitive listed species endpoint from tested monocot and dicot species.

The single maximum label rate for most uses of simazine is 4 lb a.i/A. Figure 10 describes the

risk associated with ground spray application assuming different droplet sizes and boom

heights. Depending on the boom height and droplet size, drift to terrestrial plant species

following an ground application of 4 lb a.i./A results in concentrations that exceed the non‐

listed monocots and dicots out to 100 ‐ 400 ft and 250 – 750 ft respectively. Drift deposition

from both high and low boom applications result in concentrations that exceed listed species of

monocots and dicots to beyond 1,000 ft.

0.001

0.01

0.1

1

10

0 100 200 300 400 500 600 700 800 900 1000

lb ai/A

Distance Downwind from field (ft)

2 lb/A application rate; ground app

Low‐Boom Very‐Fine to Fine Low‐Boom Fine to Medium/Coarse

High‐Boom Very‐Fine to Fine High‐Boom Fine to Medium/Coarse

Most Sensitive Listed Moncot (IC05) Most Sensitive Non‐Listed Monocot (IC25)

Most Sensitive Listed Dicot (NOAEC) Most Sensitive Non‐Listed Dicot (IC25)
116

Figure 10. Spray drift deposition curves for various droplet spectra following a single ground

application of 4 lb a.i/A.  Horizontal dotted lines represent the most sensitive non‐listed

species endpoint from tested monocot and dicot species. Horizontal dashed lines represent

the most sensitive listed species endpoint from tested monocot and dicot species.

The reduced rate scenario modeled in this assessment is 0.5 lb a.i/A. Figure 11 describes the

risk associated with ground spray application assuming different droplet sizes and boom

heights. Depending on the boom height and droplet size, drift to terrestrial plant species

following an ground application of 0.5 lb a.i./A results in concentrations that exceed the non‐

listed monocots and dicots out to 0 ‐ 75 ft and 10 ‐ 150 ft respectively. Drift deposition from

both high and low boom applications result in concentrations that exceed listed species of

monocots and dicots from 150 ‐ 550 ft.
117

Figure 11. Spray drift deposition curves for various droplet spectra following a single ground

application of 0.5 lb a.i/A.  Horizontal dotted lines represent the most sensitive non‐listed

species endpoint from tested monocot and dicot species. Horizontal dashed lines represent

the most sensitive listed species endpoint from tested monocot and dicot species.

The only labeled aerial application use is on alfalfa in Washington. Figure 12 describes the risk

associated with the maximum‐allowed aerial spray application of 1.6 lb a.i./A assuming

different droplet sizes. Depending on the droplet size, drift to terrestrial plant species results in

concentrations that exceed the non‐listed monocots and dicots out to 0 ‐ 75 ft and 10 ‐ 150 ft

respectively. Drift deposition from both high and low boom applications result in

concentrations that exceed listed species of monocots and dicots from 150 ‐ 550 ft.  

0.0001

0.001

0.01

0.1

1

0 100 200 300 400 500 600 700 800 900 1000

lb ai/A

Distance Downwind from field (ft)

0.5 lb/A application rate; ground app

Low‐Boom Very‐Fine to Fine Low‐Boom Fine to Medium/Coarse

High‐Boom Very‐Fine to Fine High‐Boom Fine to Medium/Coarse

Most Sensitive Listed Moncot (IC05) Most Sensitive Non‐Listed Monocot (IC25)

Most Sensitive Listed Dicot (NOAEC) Most Sensitive Non‐Listed Dicot (IC25)
118

Figure 12. Spray drift deposition curves for various droplet spectra following a single aerial

application of 1.6 lbs a.i/A.  Horizontal dotted lines represent the most sensitive non‐listed

species endpoint from tested monocot and dicot species. Horizontal dashed lines represent

the most sensitive listed species endpoint from tested monocot and dicot species. Horizontal

dashed –dotted lines represent the least sensitive definitive IC25 endpoints from vegetative

vigor and seedling emergence studies on simazine.

   

0.001

0.01

0.1

1

0 100 200 300 400 500 600 700 800 900 1000

lb ai/A

Distance Downwind from field (ft)

1.6 lb/A application rate; aerial app

Fine to Medium Medium

Medium‐Coarse Coarse

Coarse‐Very Coarse Very Coarse

Very Coarse‐Extremely Coarse Most Sensitive Listed Moncot (IC05)

Most Sensitive Non‐Listed Monocot (IC25) Most Sensitive Listed Dicot (NOAEC)

Most Sensitive Non‐Listed Dicot (IC25) Least senstive SE

Least sensitve VV
119

Risks to Terrestrial Plant Communities  

No terrestrial plant community studies have been found in the open literature for simazine.

Given that the available single species toxicity tests (see Section 10.1.1) show that the taxa with

definitive IC25 values are within an order of magnitude for most species (0.033 and 0.085 lb

a.i./A for vegetative vigor and between 0.009 and 0.057 lb a.i./A for seedling emergence) and

that all of these species would result in LOC exceedances following expected exposures due to

runoff and spray drift, broad impacts to terrestrial plant populations and communities are

expected. The simazine exposure impacts from runoff would be most significant initially on the

seedling emergence phase of the plant life cycle; however, non‐lethal exposures impacting

growth are expected annually and through multiple pulses, which may lead to reduced

fecundity and furthermore may lead to impacts on community structure and composition.   

AQUATIC ANIMAL RISK CHARACTERIZATION AND CONCLUSIONS

Because the risk of runoff from the site of application differs based on the use site (crop,

geography, soil, climate) application timing and method, risks to aquatic taxa and communities

are discussed below on a crop by crop basis. This discussion relies upon EECs from the Surface

Water Concentration Calculator (SWCC, see Section 8.1.1 for details), and the Provisional

Cranberry Model (see Section 8.1.2 for details) using the maximum labeled rates and

application scenarios for each crop. The discussion also relies upon monitored concentrations

from federal, state, and registrant‐conducted monitoring programs, as described in Section 8.2.

Aquatic Risk Characterization and Conclusions

A summary of the risks for aquatic animals, plants and the aquatic plant community are

provided in Table 45 for Section 3 and 24(c) labeled uses (excluding the corn and cranberry

uses), in Table 46 for corn uses, and Table 47 for cranberry uses.

A summary of risks based on available simazine monitoring data are provided in Table 48

through Table 51. The monitoring data used to calculate RQs in these tables are inherently

underestimated because of the infrequency of sampling. As a result, the risks illustrated in the

tables underestimate the true risks. This is illustrated by a comparison of Table 48 and

Table 50. Table 48 provides a risk summary for individual states based on the data available

from monitoring sites in each state that have 4 to 11 samples taken within each site‐year.

Table 50 provides a comparable risk summary for individual states, but includes data only for

monitoring stations that have 12 or more samples taken within each site‐year. Sites with a

higher sampling frequency (i.e., 12 or more samples per year) tend to have a higher percentage

of acute LOC exceedances than sites with lower sampling frequencies. Bias factors calculated in

Section 8.2.3 illustrate the extent to which the monitoring data may underestimate aquatic
120

risks. It should be noted that chronic risks were only calculated using site‐years with 12 or more

samples.

A discussion of risk from each of the crop scenarios modeled as well as the risk from the

monitoring data is provided below the summary tables.
121

Table 45: EECs and RQs for Simazine Uses except Corn and Cranberry. Shaded cells identify LOC exceedances for list

values indicate non‐listed LOC exceedances.

Crop Scenario Application Method

Application

Rate (lb

a.i./A) per

App

Peak

μg/L

21‐day

avg

μg/L

60‐

day

Avg

μg/L

RQs

Acute

FW Fish

Chronic

FW Fish

Acute

EM Fish

Chronic

EM Fish

Acute

FW

Inverts

Chronic

FW

Inverts

Acute

EM

Inverts

Chronic

EM

Inverts

Alfalfa

CAafalfaOP Ground 1.6 24.1 23.4 22.5 0.06 0.4 0.01 0.0 0.02 0.6 0.03 0.4 

CAalfalfaOP Aerial 1.6 31.2 30.4 29.3 0.08 0.5 0.01 0.0 0.03 0.8 0.03 0.5 

Blueberries/ ORberries Ground 4 22 21.7 21 0.06 0.4 0.01 0.0 0.02 0.5 0.02 0.3 

Caneberries ORberries Ground 8 60.8 61.3 60.2 0.15 1.0 0.01 0.1 0.06 1.5 0.06 1.0 

Almond CAalmond Ground 2 23.5 23.9 23 0.06 0.4 0.01 0.0 0.02 0.6 0.02 0.4 

Nectarine CAfruit Ground 2 8.18 7.95 7.75 0.02 0.1 0.00 0.0 0.01 0.2 0.01 0.1 

Strawberry

CAstrawberri

es

Ground 4 64.8 64.2 63.7 0.16 1.1 0.02 0.1 0.06 1.6 0.07 1.0 

ORstrawberri

es

Ground 1 22.9 22.4 22.5 0.06 0.4 0.01 0.0 0.02 0.6 0.02 0.4 

Grape

NYgrapes Ground 4 57.6 57.2 55.9 0.14 0.9 0.01 0.1 0.06 1.4 0.06 0.9 

CAgrapes Ground 4 37.3 36.5 35.1 0.09 0.6 0.01 0.1 0.04 0.9 0.04 0.6 

Citrus

CAcitrus Ground 4 8.34 8.1 7.71 0.02 0.1 0.00 0.0 0.01 0.2 0.01 0.1 

STXgrapefruit

NMC Ground 4 33.7 32.6 30.6 0.08 0.5 0.01 0.0 0.03 0.8 0.04 0.5 

CAcitrus Ground 2/2 7.4 7.13 6.64 0.02 0.1 0.00 0.0 0.01 0.2 0.01 0.1 

STXgrapefruit

NMC Ground 2/2 30.4 28.8 26.5 0.08 0.4 0.01 0.0 0.03 0.7 0.03 0.5 

FLcitrus Ground 8 205 198 187 0.51 3.1 0.05 0.3 0.21 5.0 0.21 3.1 
122

Crop Scenario Application Method

Application

Rate (lb

a.i./A) per

App

Peak

μg/L

21‐day

avg

μg/L

60‐

day

Avg

μg/L

RQs

Acute

FW Fish

Chronic

FW Fish

Acute

EM Fish

Chronic

EM Fish

Acute

FW

Inverts

Chronic

FW

Inverts

Acute

EM

Inverts

Chronic

EM

Inverts

FLcitrus Ground 4 102 98.9 93.6 0.26 1.6 0.02 0.1 0.10 2.5 0.11 1.6 FLcitrus Ground 2/2 87.3 83.4 78.3 0.22 1.3 0.02 0.1 0.09 2.1 0.09 1.3 

STXgrapefruit

NMC Ground 1.6/1.6 24.3 23.1 21.2 0.06 0.4 0.01 0.0 0.02 0.6 0.03 0.4 

Apple

NCappleSTD Ground 4 83 81.7 80.8 0.21 1.3 0.02 0.1 0.08 2.0 0.09 1.3 

ORappleSTD Ground 4 105 105 106 0.26 1.8 0.03 0.2 0.11 2.6 0.11 1.7 

PAappleSTD_

V2 Ground 4 59.6 58.3 57.6 0.15 1.0 0.01 0.1 0.06 1.5 0.06 0.9 

WAorchards

NMC Ground 4 16.1 16 15.5 0.04 0.3 0.00 0.0 0.02 0.4 0.02 0.3 

Avocado

FLavocado Ground 4 13.1 13.5 13.2 0.03 0.2 0.00 0.0 0.01 0.3 0.01 0.2 

CAavocado Ground 4 12.5 12.1 11.6 0.03 0.2 0.00 0.0 0.01 0.3 0.01 0.2 

Peach

CApeach Ground 2 8.18 7.95 7.75 0.02 0.1 0.00 0.0 0.01 0.2 0.01 0.1 

GApeach Ground 4 42.3 40.9 38.8 0.11 0.6 0.01 0.1 0.04 1.0 0.04 0.6 

Olive CAolive Ground 4 20.7 20.1 19.8 0.05 0.3 0.00 0.0 0.02 0.5 0.02 0.3 

Cherries

WAorchards

NMC Ground 4 16.1 16 15.5 0.04 0.3 0.00 0.0 0.02 0.4 0.02 0.3 

MICherriesST

D Ground 4 55.2 54.3 52.8 0.14 0.9 0.01 0.1 0.06 1.4 0.06 0.9 

Pears

NCappleSTD Ground 4 83 81.7 80.8 0.21 1.3 0.02 0.1 0.08 2.0 0.09 1.3 

ORappleSTD Ground 4 105 105 106 0.26 1.8 0.03 0.2 0.11 2.6 0.11 1.7 
123

Crop Scenario Application Method

Application

Rate (lb

a.i./A) per

App

Peak

μg/L

21‐day

avg

μg/L

60‐

day

Avg

μg/L

RQs

Acute

FW Fish

Chronic

FW Fish

Acute

EM Fish

Chronic

EM Fish

Acute

FW

Inverts

Chronic

FW

Inverts

Acute

EM

Inverts

Chronic

EM

Inverts

PAappleSTD_

V2 Ground 4 59.6 58.3 57.6 0.15 1.0 0.01 0.1 0.06 1.5 0.06 0.9 

WAorchards

NMC Ground 4 16.1 16 15.5 0.04 0.3 0.00 0.0 0.02 0.4 0.02 0.3 

Plums

NCappleSTD Ground 4 83 81.7 80.8 0.21 1.3 0.02 0.1 0.08 2.0 0.09 1.3 

ORappleSTD Ground 4 105 105 106 0.26 1.8 0.03 0.2 0.11 2.6 0.11 1.7 

PAappleSTD_

V2 Ground 4 59.6 58.3 57.6 0.15 1.0 0.01 0.1 0.06 1.5 0.06 0.9 

WAorchards

NMC Ground 4 16.1 16 15.5 0.04 0.3 0.00 0.0 0.02 0.4 0.02 0.3 

Turf/Residenti

al FLturf Ground 1/2 9.09 8.93 8.65 0.02 0.1 0.00 0.0 0.01 0.2 0.01 0.1 

Sod Farms FLturf Ground 2/4 30.1 28.7 27.8 0.08 0.5 0.01 0.0 0.03 0.7 0.03 0.5 

Hazelnut(Filbe

rt)

ORFilbert Ground 2/2 25 24.6 24.1 0.06 0.4 0.01 0.0 0.03 0.6 0.03 0.4 

ORFilbert Ground 4 71.2 70.4 68.5 0.18 1.1 0.02 0.1 0.07 1.8 0.07 1.1 

Macademia

Nut CAavocado Ground 4 11.2 10.8 10.3 0.03 0.2 0.00 0.0 0.01 0.3 0.01 0.2 

Pecan GApecan Ground 4 114 111 105 0.29 1.8 0.03 0.2 0.11 2.8 0.12 1.8 

Walnut CAalmond Ground 4 20.2 19.8 19.3 0.05 0.3 0.00 0.0 0.02 0.5 0.02 0.3 

Nursery

TNnurserySTD_V2 Ground 3(fall) 60.3 57.8 54.5 0.15 0.9 0.01 0.1 0.06 1.4 0.06 0.9 

CAnurserySTD_V2 Ground 3(fall) 6.38 6.15 5.83 0.02 0.1 0.00 0.0 0.01 0.2 0.01 0.1 
124

Crop Scenario Application Method

Application

Rate (lb

a.i./A) per

App

Peak

μg/L

21‐day

avg

μg/L

60‐

day

Avg

μg/L

RQs

Acute

FW Fish

Chronic

FW Fish

Acute

EM Fish

Chronic

EM Fish

Acute

FW

Inverts

Chronic

FW

Inverts

Acute

EM

Inverts

Chronic

EM

Inverts

FLnurserySTD_V2 Ground 3(fall) 60.9 57.7 53 0.15 0.9 0.01 0.1 0.06 1.4 0.06 0.9 MInurserySTD_V2 Ground 3(fall) 91.3 90.4 91.3 0.23 1.5 0.02 0.1 0.09 2.3 0.10 1.4 NJnurserySTD_V2 Ground 3(fall) 59.4 57.1 57.2 0.15 1.0 0.01 0.1 0.06 1.4 0.06 0.9 ORnurserySTD_V2 Ground 3(fall) 29.4 28.5 27.4 0.07 0.5 0.01 0.0 0.03 0.7 0.03 0.5 NurseryBSS_V2 Ground 3(fall) 59 57.4 54.2 0.15 0.9 0.01 0.1 0.06 1.4 0.06 0.9 TNnurserySTD_V2 Ground 3(spring) 61.1 59.5 57 0.15 1.0 0.01 0.1 0.06 1.5 0.06 0.9 CAnurserySTD_V2 Ground 3(spring) 31.9 31.3 30.5 0.08 0.5 0.01 0.0 0.03 0.8 0.03 0.5 FLnurserySTD_V2 Ground 3(spring) 32 31.2 30.1 0.08 0.5 0.01 0.0 0.03 0.8 0.03 0.5 MInurserySTD_V2 Ground 3(spring) 58.7 58.2 56.9 0.15 0.9 0.01 0.1 0.06 1.5 0.06 0.9 NJnurserySTD_V2 Ground 3(spring) 58.7 58.2 57.1 0.15 1.0 0.01 0.1 0.06 1.5 0.06 0.9 ORnurserySTD_V2 Ground 3(spring) 21.5 21.2 20.6 0.05 0.3 0.01 0.0 0.02 0.5 0.02 0.3 NurseryBSS_V2 Ground 3(spring) 31.1 30.5 29.2 0.08 0.5 0.01 0.0 0.03 0.8 0.03 0.5 

Christmas

Trees

ORXmasTreeSTD Ground 2/2 15 14.7 14.3 0.04 0.2 0.00 0.0 0.02 0.4 0.02 0.2 
125

Crop Scenario Application Method

Application

Rate (lb

a.i./A) per

App

Peak

μg/L

21‐day

avg

μg/L

60‐

day

Avg

μg/L

RQs

Acute

FW Fish

Chronic

FW Fish

Acute

EM Fish

Chronic

EM Fish

Acute

FW

Inverts

Chronic

FW

Inverts

Acute

EM

Inverts

Chronic

EM

Inverts

MICherriesSTD Ground 2/2 58.9 57.9 55.9 0.15 0.9 0.01 0.1 0.06 1.4 0.06 0.9 GAPecansSTD Ground 2/2 86.5 82.6 83.6 0.22 1.4 0.02 0.1 0.09 2.1 0.09 1.3 ORXmasTreeSTD Ground 4 15.6 15.3 14.9 0.04 0.2 0.00 0.0 0.02 0.4 0.02 0.2 MICherriesSTD Ground 4 44 43.1 41.9 0.11 0.7 0.01 0.1 0.04 1.1 0.05 0.7 GAPecansSTD Ground 4 134 129 123 0.34 2.1 0.03 0.2 0.13 3.2 0.14 2.0 

ShelterBelts

GAPecansSTD Ground 2/2 86.5 82.6 83.6 0.22 1.4 0.02 0.1 0.09 2.1 0.09 1.3 

CAalmond_WirrigSTD Ground 2/2 29.1 28.3 27 0.07 0.5 0.01 0.0 0.03 0.7 0.03 0.4 MICherriesSTD Ground 2/2 58.9 57.9 55.9 0.15 0.9 0.01 0.1 0.06 1.4 0.06 0.9 GAPecansSTD Ground 4 134 129 123 0.34 2.1 0.03 0.2 0.13 3.2 0.14 2.0 CAalmond_WirrigSTD Ground 4 21.8 21.5 21 0.05 0.4 0.01 0.0 0.02 0.5 0.02 0.3 MICherriesSTD Ground 4 44 43.1 41.9 0.11 0.7 0.01 0.1 0.04 1.1 0.05 0.7 
126

Table 46. EECs and RQs for Simazine from Corn Uses. Maximum and minimum estimates of water concentrations, a

Shaded cells identify LOC exceedances for listed species and bolded values indicate non‐listed LOC exceedances. The

scenarios run for SWCC corn modeling.

Crop Estimation

Application

Rate (lb

a.i./A) per

App

Peak 21‐day

avg

60‐day

Avg RQs

μg/L

Acute

FW

Fish

Chronic

FW Fish

Acute

EM

Fish

Chronic

EM Fish

Acute

FW

Inverts

Chronic

FW

Inverts

Acute

EM

Inverts

Chronic

EM

Inverts

Non‐

Listed

Non‐

Vascul

Plants

Corn‐ Pre‐

Emergent

Maximum 2/0.5 162 158 151 0.41 2.52 0.04 0.24 0.16 3.95 0.17 2.51 20.25 

Minimum 2/0.5 32.3 32 31.8 0.08 0.53 0.01 0.05 0.03 0.80 0.03 0.51 4.04 

Number of scenarios exceeding LOC 0(14) 11(11) 0(0) 0(0) 0(5) 14(14) 0(6) 10(10) 17 

Corn‐Fallow

Maximum 1 59.2 56.3 57.8 0.15 0.96 0.01 0.09 0.06 1.41 0.06 0.89 7.40 

Minimum 1 10.6 10.4 10.1 0.03 0.17 0.00 0.02 0.01 0.26 0.01 0.17 1.33 

Number of scenarios exceeding LOC 0(9) 1(1) 0(0) 0(0) 0(0) 9(9) 0(0) 0(0) 17 

Corn‐ Highly

Erodible Land,

Post‐

Emergent

Maximum 1 93.1 90.5 87.8 0.2 1.5 0.0 0.1 0.1 2.3 0.1 1.4 11.6 

Minimum 1 8.94 8.65 8.24 0.0 0.1 0.0 0.0 0.0 0.2 0.0 0.1 1.1 

Number of scenarios exceeding LOC 0(7) 2(2) 0(0) 0(0) 0(0) 7(7) 0(1) 2(2) 17 

Corn‐Reduced

Rate

Maximum 0.5 29.6 28.2 28.9 0.07 0.48 0.01 0.05 0.03 0.71 0.03 0.45 3.70 

Minimum 0.5 5.29 5.22 5.07 0.01 0.08 0.00 0.01 0.01 0.13 0.01 0.08 0.66 

Number of scenarios exceeding LOC 0(9) 0(0) 0(0) 0(0) 0(0) 0(0) 0(0) 0(0) 15 
127

Table 47. EECs and RQs for the Simazine Cranberry Uses.

EECs for the Simazine Cranberry Use.

RQs

Use Areas

Application

Rate

Dry‐Harvested

Cranberry/SWCC Results*

(μg/L)

(lb a.i./A)

1‐in‐10‐

yr Peak

1‐in‐10‐

yr

1‐in‐10‐yr

21‐day

Average

60‐day

Average

Acute

FW Fish

Chronic

FW Fish

Acute

EM Fish

Chronic

EM Fish

Acute

FW

Inverts

Chronic

FW

Inverts

Acute

EM

Inverts

Chronic EM

Inverts

Non‐

Listed

Non‐

Vascula

Plants

Massachusetts(Dry Harvested) 4 22 21.7 21 0.06 0.4 0.01 0.0 0.02 0.5 0.02 0.3 2.8 Other States

(Dry Harvested) 2 11 10.8 10.5 0.03 0.2 0.00 0.0 0.01 0.3 0.01 0.2 1.4 

Massachusetts(Wet Harvested) 4 371 360 339 0.93 5.7 0.09 0.5 0.37 9.0 0.39 5.7 46.4 Other States

(Wet Harvested) 2 185 180 169 0.46 2.8 0.04 0.3 0.19 4.5 0.19 2.9 23.1 
128

Table 48. State Summary of the number of Site‐Years that detected simazine concentrations greater than LOCs for a

restricted to site‐years with 4 or more samples.

State Number of Site‐Years with Detections

Number of Times Non‐Listed and Listed Species LOCs were 

Acute

FW Fish

Acute

EM Fish

Acute FW

Inverts

Acute EM

Inverts

Non‐Listed Non‐

Vascular Plants

Listed Non‐

Vascular Plants

AK 0 0 0 0 0 0 0 AL 270 0 0 0 0 3 20 AR 106 0 0 0 0 0 1 AS 0 0 0 0 0 0 0 AZ 50 0 0 0 0 0 0 CA 938 0 0 0 0 55 163 CO 151 0 0 0 0 0 2 CT 75 0 0 0 0 0 0 DC 14 0 0 0 0 0 1 DE 20 0 0 0 0 0 0 FL 256 0 0 0 0 2 11 GA 499 0 0 0 0 8 62 HI 15 0 0 0 0 0 0 IA 490 0 0 0 0 2 9 ID 41 0 0 0 0 0 0 IL 517 0 0 0 0 23 133 IN 594 0 0 0 0 4 192 KS 331 0 0 0 0 11 57 KY 154 0 0 0 0 1 19 LA 451 0 0 0 0 0 4 MA 43 0 0 0 0 0 0 MD 205 0 0 0 0 1 26 
129

State Number of Site‐Years with Detections

Number of Times Non‐Listed and Listed Species LOCs were 

Acute

FW Fish

Acute

EM Fish

Acute FW

Inverts

Acute EM

Inverts

Non‐Listed Non‐

Vascular Plants

Listed Non‐

Vascular Plants

ME 0 0 0 0 0 0 0 MI 65 0 0 0 0 0 5 MN 122 0 0 0 0 0 1 MO 262 0 0 0 0 1 16 MS 126 0 0 0 0 0 5 MT 30 0 0 0 0 0 0 NA 0 0 0 0 0 0 0 NC 368 0 0 0 0 0 18 ND 153 0 0 0 0 0 11 NE 401 0 0 0 0 0 8 NH 2 0 0 0 0 0 0 NJ 439 0 0 0 0 0 1 NM 98 0 0 0 0 0 4 NV 104 0 0 0 0 2 10 NY 393 4 0 3 3 12 25 OH 361 0 0 0 0 7 118 OK 27 0 0 0 0 0 4 OR 536 0 0 0 0 5 65 PA 453 0 0 0 0 6 28 RI 0 0 0 0 0 0 0 SC 125 0 0 0 0 2 16 SD 66 0 0 0 0 0 0 TN 142 0 0 0 0 15 20 TX 599 0 0 0 0 2 68 
130

State Number of Site‐Years with Detections

Number of Times Non‐Listed and Listed Species LOCs were 

Acute

FW Fish

Acute

EM Fish

Acute FW

Inverts

Acute EM

Inverts

Non‐Listed Non‐

Vascular Plants

Listed Non‐

Vascular Plants

UT 35 0 0 0 0 0 4 VA 211 0 0 0 0 4 24 VT 4 0 0 0 0 0 0 WA 487 0 0 0 0 0 4 WI 239 0 0 0 0 4 11 WV 26 0 0 0 0 0 0 WY 5 0 0 0 0 0 0 

Table 49. Monitoring Program Summary of the number of Site‐Years that detected simazine concentrations greater t

organisms. Data restricted to only site‐years with 4 or more samples.

Source Number of Site‐Years with Detections

Number of Times Non‐Listed and Listed Species 

Acute

FW

Fish

Acute

EM

Fish

Acute

FW

Inverts

Acute

EM

Inverts

Non‐Listed

Non‐

Vascular

Plants

List

No

Vasc

Pla

AEEMP 9 0 0 0 0 0 3 AMP‐SMP_AMBIENT 409 0 0 0 0 30 21CDPR 410 0 0 0 0 32 9LDAF 2 0 0 0 0 0 0 MULTIPLE 2 0 0 0 0 0 0 NAWQA 2964 0 0 0 0 28 24NWIS 5649 4 0 3 3 54 36OR_ELEM 66 0 0 0 0 1 8 
131

Source Number of Site‐Years with Detections

Number of Times Non‐Listed and Listed Species 

Acute

FW

Fish

Acute

EM

Fish

Acute

FW

Inverts

Acute

EM

Inverts

Non‐Listed

Non‐

Vascular

Plants

List

No

Vasc

Pla

OR_LASAR 176 0 0 0 0 4 2PDP 120 0 0 0 0 0 2 STORET 1161 0 0 0 0 21 20USGS‐EPA_RES_AMBIENT 21 0 0 0 0 0 3 WDA 107 0 0 0 0 0 0 WI_DNR 3 0 0 0 0 0 0 

Table 50. State Summary of the number of Site‐Years that detected simazine concentrations greater than LOCs for a

restricted to only site‐years with 12 or more samples.

State

Number of Site‐Years withDetections

Number of Times Non‐Listed and Listed Species LOCs were Exceede

Acute

FW Fish

Chronic

FW Fish

Acute

EM Fish

Chronic

EM Fish

Acute

FW

Inverts

Chronic

FW

Inverts

Acute

EM

Inverts

Chronic

EM

Inverts

Non‐

Listed

Non‐

Vascular

Plants

AK 0 0 NA 0 NA 0 NA 0 NA 0 AL 44 0 0 0 0 0 0 0 0 0 AR 10 0 0 0 0 0 0 0 0 0 AZ 14 0 0 0 0 0 0 0 0 0 CA 248 1 0 0 0 0 0 0 0 4 CO 38 0 0 0 0 0 0 0 0 0 
132

State

Number of Site‐Years withDetections

Number of Times Non‐Listed and Listed Species LOCs were Exceede

Acute

FW Fish

Chronic

FW Fish

Acute

EM Fish

Chronic

EM Fish

Acute

FW

Inverts

Chronic

FW

Inverts

Acute

EM

Inverts

Chronic

EM

Inverts

Non‐

Listed

Non‐

Vascular

Plants

CT 32 0 0 0 0 0 0 0 0 0 DC 2 0 0 0 0 0 0 0 0 0 FL 35 0 0 0 0 0 0 0 0 0 GA 88 0 0 0 0 0 0 0 0 4 IA 141 0 0 0 0 0 0 0 0 1 ID 20 0 0 0 0 0 0 0 0 0 IL 225 0 0 0 0 0 0 0 0 20 IN 250 0 0 0 0 0 0 0 0 3 KS 41 0 0 0 0 0 0 0 0 8 KY 31 0 0 0 0 0 0 0 0 1 LA 158 0 0 0 0 0 0 0 0 0 MA 10 0 0 0 0 0 0 0 0 0 MD 22 0 0 0 0 0 0 0 0 0 MI 16 0 0 0 0 0 0 0 0 0 MN 35 0 0 0 0 0 0 0 0 0 MO 70 0 0 0 0 0 0 0 0 0 MS 54 0 0 0 0 0 0 0 0 0 MT 11 0 0 0 0 0 0 0 0 0 NC 79 0 0 0 0 0 0 0 0 0 ND 8 0 0 0 0 0 0 0 0 0 NE 89 0 0 0 0 0 0 0 0 0 NJ 47 0 0 0 0 0 0 0 0 0 
133

State

Number of Site‐Years withDetections

Number of Times Non‐Listed and Listed Species LOCs were Exceede

Acute

FW Fish

Chronic

FW Fish

Acute

EM Fish

Chronic

EM Fish

Acute

FW

Inverts

Chronic

FW

Inverts

Acute

EM

Inverts

Chronic

EM

Inverts

Non‐

Listed

Non‐

Vascular

Plants

NV 31 0 0 0 0 0 0 0 0 0 NY 75 0 0 0 0 0 0 0 0 1 OH 174 0 0 0 0 0 0 0 0 4 OK 2 0 0 0 0 0 0 0 0 0 OR 152 0 0 0 0 0 0 0 0 4 PA 55 1 0 0 0 0 0 0 0 2 SC 36 0 0 0 0 0 0 0 0 2 TN 27 0 0 0 0 0 0 0 0 4 TX 99 0 0 0 0 0 0 0 0 2 UT 9 0 0 0 0 0 0 0 0 0 VA 59 0 0 0 0 0 0 0 0 3 WA 175 0 0 0 0 0 0 0 0 0 WI 52 0 0 0 0 0 0 0 0 0 WV 3 0 0 0 0 0 0 0 0 0 WY 0 0 NA 0 NA 0 NA 0 NA 0 
134

Table 51. Monitoring Program Summary of the number of Site‐Years that detected simazine concentrations greater t

organisms. Data restricted to only site‐years with 12 or more samples.

Source

Number ofSite‐YearswithDetections

Number of Times Non‐Listed and Listed Species LOCs were Excee

Acute

FW Fish

Chronic

FW Fish

Acute

EM Fish

Chronic

EM Fish

Acute

FW

Inverts

Chronic

FW

Inverts

Acute

EM

Inverts

Chronic

EM

Inverts

Non‐

Listed

Non‐

Vascular

Plants

AEEMP 9 0 0 0 0 0 0 0 0 0 AMP‐SMP_AMBIENT 387 1 0 0 0 0 0 0 0 29 CDPR 93 0 0 0 0 0 0 0 0 1 NAWQA 957 1 0 0 0 0 0 0 0 10 NWIS 906 0 0 0 0 0 0 0 0 9 OR_ELEM 44 0 0 0 0 0 0 0 0 1 OR_LASAR 52 0 0 0 0 0 0 0 0 3 PDP 106 0 0 0 0 0 0 0 0 0 STORET 99 0 0 0 0 0 0 0 0 10 USGS‐EPA_RES_AMBIENT 9 0 0 0 0 0 0 0 0 0 WDA 105 0 0 0 0 0 0 0 0 0 WI_DNR 0 0 0 0 0 Grand Total 2766 2 0 0 0 0 0 0 0 62 
135

Risks to Fish  

On an acute exposure basis, there are few risk concerns to non‐listed and listed freshwater fish

(RQs 0.2 – 0.93) with only citrus and cranberries exceeding the non‐listed LOC. The acute and

chronic LOCs were not exceeded for estuarine/marine fish (Table 45 and Table 46). On a

chronic exposure basis, freshwater species RQs exceed levels of concern for corn,

blueberries/caneberries, strawberries, citrus, cranberries, apple, pear, hazelnut, pecan,

nurseries, christmas trees, and shelterbelts, for labeled uses and application methods with RQs

ranging from 0.1 – 5.7. The evaluation of a hypothetical reduced rate for corn (0.5 lb a.i./A)

resulted in RQs that exceeded the acute LOC for listed fish for 9 of the 17 modeled corn

scenarios, while acute non‐listed fish and chronic exposure LOCs were not exceeded for listed

or non‐listed fish.

The available monitoring data resulted in few site years with acute or chronic LOC exceedances

(Table 48 through Table 51). The only acute LOC exceedances for freshwater fish occurred in

California, Pennsylvania, and New York. There are no acute LOC exceedances for estuarine/

marine fish; nor are there chronic LOC exceedances for freshwater and estuarine/marine fish.

As previously discussed, the chronic endpoint for risk to fish in the atrazine PRA was established

at 5 μg a.i./L based on a short term reproduction test in Japanese medaka. The design of this

study allowed for the evaluation of daily and cumulative egg production as endpoints. No such

study is available for simazine. While these chronic fish data for atrazine indicate potential

sublethal effects at low exposure concentrations, it is unclear to what degree those data

represent simazine.

Risks to Aquatic Invertebrates

The SWCC results for labeled application rates result in acute LOC exceedances for freshwater

and estuarine/marine invertebrates for corn, citrus, apples, pears, plums, pecans, nursery,

christmas trees and shelterbelts uses (Table 45, Table 46, and Table 47). Lower application

rates result in fewer scenarios exceeding the LOC for listed species (RQs 0.01‐3.9).  The

estimated chronic exposure levels result in many of the uses with exceedances of the

invertebrate LOC (RQs 0.1‐9.0). The evaluation of a hypothetical reduced rate for corn (0.5 lb

a.i./A) resulted in RQs that did not exceed the acute or chronic LOCs for listed or non‐listed

invertebrates.

The available monitoring data resulted in few site years with acute or chronic LOC exceedances

(Table 48 through Table 51). The only acute LOC exceedances for freshwater and estuarine/

marine invertebrates occurred in New York. There are no chronic LOC exceedances for aquatic

invertebrates.  
136

Risk to Aquatic‐phase amphibians

The OPP standard in ecological risk assessments is to use the available fish toxicity data to

represent surrogate data for aquatic phase amphibians when data on amphibians are not

available. The modeled surface water concentrations indicate that acute and chronic risks to

freshwater fish are expected for several of the currently labeled simazine uses and application

rates.  

While there are amphibian‐specific data for atrazine which indicate potential sublethal effects

at low exposure concentrations (low ppb), it is unclear to what degree those data represent

simazine.

Risk to Non‐Vascular and Vascular Aquatic Plants

The predicted EECs using SWCC indicate when compared against the most sensitive taxon the

non‐listed non‐vascular plant LOCs are exceeded for all labeled uses and rates (RQs range from

0.8 to 46, Table 45, Table 46, and Table 47). The evaluation of a hypothetical reduced rate for

corn (0.5 lb a.i./A) resulted in RQs that exceeded the LOCs for non‐listed and listed non‐vascular

aquatic plants for 15 and 16 of the 17 modeled corn scenarios respectively. For vascular plants

no exceedances of the LOCs were predicted for non‐listed or listed taxa.  

Based on the available monitoring data, LOC exceedances for non‐listed and listed non‐vascular

aquatic plants are ubiquitous across the United States. Few LOC exceedances were observed for

non‐listed and listed vascular aquatic plants. The only LOC exceedances observed for vascular

aquatic plants occurred in California and New York.

Risks to Aquatic Plant Communities

As mentioned in Section 10.1.4, simazine does not have an extensive body of research on micro

and mesocosms (cosms) like atrazine. However, because atrazine and simazine share a

common mechanism of action and similar potency in plants, and coupled with their propensity

to move into aquatic ecosystems and their persistence in water, these chemicals both pose a

potential risk to aquatic plant communities. Based on the individual species toxicity data

presented above, there are risks to non‐vascular plants for all simazine’s uses and risks to

vascular plants for many uses. Risk to aquatic plant communities is presumed for the current

uses.  

   
137

DESCRIPTION OF UNCERTAINTIES, LIMITATIONS AND ASSUMPTIONS

Impact of simazine on chemical mixtures in the environment

As discussed in greater detail in the Atrazine risk assessment, atrazine and simazine are often

co‐formulated with other herbicides (22 different active ingredients in 52 formulated products

for atrazine, 2 active ingredients in 32 formulated products for simazine), indicating that it is co‐

applied with other active ingredients when used in standard formulations. In addition to these

multi a.i. products, it is well documented that tank mixtures and environmental mixtures occur,

resulting in the presence of chemical mixtures in both terrestrial and aquatic environments.

USGS summarized the composition of pesticide mixtures observed in surface water samples

collected throughout the US during the 1990s. The analysis determined that herbicides were

the most commonly detected pesticides within agricultural areas, with atrazine and its

degradates being the most frequently detected (found in 2/3 of all samples taken from streams

with agricultural landcovers representing their watersheds). More than 50% of the stream

samples had ≥5 different active ingredients. Atrazine and metolachlor were the most commonly

detected mixture in agricultural watersheds, followed by atrazine/prometon/ metolachlor

(USGS, 1999). A review of NAWQA data collected between 1992 and 2001 showed that

atrazine, metolachlor, and cyanazine were the most frequently detected herbicides in

agricultural watersheds (USGS, 2006). Mixture composition varied over time, with different

compositions of chemicals and relative amounts measured. Table 52 includes the most

frequently detected mixtures of pesticide active ingredients in streams with agricultural

watersheds.

Table 52. The most common unique mixtures of pesticides and degradates found in stream

waters with agricultural watersheds (USGS, 2006).

Number of

chemicals

in mixture

Chemicals present

Frequency of detection

in agricultural streams

(percentage of time )

2 Atrazine Metolachlor 77

Atrazine Deethylatrazine 77

Atrazine Simazine 64

Atrazine Prometon 50

Prometon Simazine 41

3 Deethylatrazine Metolachlor 69

Deethylatrazine Simazine 57

Atrazine Deethylatrazine Prometon 48

Atrazine Prometon Simazine 41

Atrazine Diazinon Simazine 16

Atrazine Diazinon Prometon 10

Diazinon Prometon Simazine 9

4 Atrazine Deethylatrazine Metolachlor 69
138

Number of

chemicals

in mixture

Chemicals present

Frequency of detection

in agricultural streams

(percentage of time )

Atrazine Deethylatrazine Simazine 57

Atrazine Metolachlor Simazine 57

Atrazine Deethylatrazine Metolachlor Simazine 52

Atrazine Deethylatrazine Metolachlor Prometon 45

Alachlor Atrazine Deethylatrazine Metolachlor 42

Atrazine Deethylatrazine Prometon Simazine 39

Atrazine Metolachlor Prometon Simazine 38

Atrazine Diazinon Prometon Simazine 9

5 Atrazine Deethylatrazine Metolachlor Prometon

Simazine

37

Alachlor Atrazine Deethylatrazine Metolachlor

Prometon

33

Alachlor Atrazine Deethylatrazine Metolachlor

Simazine

33

Atrazine Cyanazine Deethylatrazine Metolachlor

Simazine

33

Alachlor Atrazine Deethylatrazine Prometon Simazine 26

Atrazine Deethylatrazine Metolachlor Simazine

Tebuthiuron

19

Atrazine Deethylatrazine Prometon Simazine

Tebuthiuron

16

Atrazine Diazinon Metolachlor Prometon Simazine 8

Atrazine Deethylatrazine Diazinon Prometon Simazine 8

Atrazine Carbaryl Diazinon Prometon Simazine 2

The presence of chemical mixtures in terrestrial and aquatic environments is a concern due to

the potential for one chemical to increase the toxicity of another chemical. Although literature

reviews were not conducted for chemical mixtures involving simazine specifically, atrazine has

been reported to synergistically increase the toxicity of organophosphates in aquatic

invertebrates (Anderson and Lydy, 2002; Perez et al., 2013) and terrestrial invertebrates (Chen

et al., 2015). There are also examples of atrazine having an antagonistic effect on toxicity or a

relationship that can change from being antagonistic at low doses to becoming synergistic at

higher doses (Yang et al., 2015). As they share common modes of actions, similar effects could

be potentially found with simazine chemical interactions as with atrazine.  Due to the

complexity of these interactions, it is difficult to quantitatively predict the impact of all chemical

combinations that can exist in the environment, and this remains an uncertainty in the

assessment. However, due to the widespread detection of simazine in the aquatic environment,

the potential for simazine to increase the toxicity of other chemicals must be considered in the

overall risk picture.  
139

Effects uncertainties – general  

The toxicity assessment for terrestrial and aquatic plants and animals is limited by the number

of species tested in the available toxicity studies. Use of toxicity data on representative species

does not provide information on the potential variability in susceptibility to acute and chronic

exposures.  

Although the risk assessment relies on a selected toxicity endpoint from the most sensitive

species tested, it does not necessarily mean that the selected toxicity endpoints reflect

sensitivity of the most sensitive species existing in a given environment. The relative position of

the most sensitive species tested in the distribution of all possible species is a function of the

overall variability among species to a particular chemical. The relationship between the

sensitivity of the most sensitive tested species versus wild species (including listed species) is

unknown and a source of significant uncertainty. In addition, in the case of listed species, there

is uncertainty regarding the relationship of the listed species' sensitivity and the most sensitive

species tested.

ENDANGERED SPECIES

Consistent with the EPA’s responsibility under the Endangered Species Act (ESA), the EPA will

evaluate risks to federally listed threatened and endangered (listed) species from registered

uses of pesticides in accordance with the Joint Interim Approaches developed to implement the

recommendations of the April 2013 National Academy of Sciences (NAS) report, Assessing Risks

to Endangered and Threatened Species from Pesticides. The NAS report outlines

recommendations on specific scientific and technical issues related to the development of

pesticide risk assessments that the EPA and the Services must conduct in connection with their

obligations under the ESA and FIFRA. The EPA will address concerns specific to simazine in

connection with the development of its final registration review decision for simazine.

In November 2013, the EPA, the U.S. Fish and Wildlife Service, National Marine Fisheries (the

Services), and USDA released a white paper containing a summary of their joint Interim

Approaches for assessing risks to listed species from pesticides. These Interim Approaches were

developed jointly by the agencies in response to the NAS recommendations, and reflect a

common approach to risk assessment shared by the agencies as a way of addressing scientific

differences between the EPA and the Services. Details of the joint Interim Approaches are

contained in the November 1, 2013 white paper, Interim Approaches for National‐Level

Pesticide Endangered Species Act Assessments Based on the Recommendations of the National

Academy of Sciences April 2013 Report.
140

Given that the agencies are continuing to develop and work toward implementation of the

Interim Approaches to assess the potential risks of pesticides to listed species and their

designated critical habitat, this ecological problem formulation supporting the Preliminary

Work Plan for atrazine does not describe the specific ESA analysis, including effects

determinations for specific listed species or designated critical habitat, to be conducted during

registration review. While the agencies continue to develop a common method for ESA

analysis, the planned risk assessment for the registration review of simazine will describe the

level of ESA analysis completed for this particular registration review case. This assessment will

allow the EPA to focus its future evaluations on the types of species where the potential for

effects exists, once the scientific methods being developed by the agencies have been fully

vetted. Once the agencies have fully developed and implemented the scientific methods

necessary to complete risk assessments for listed species and their designated critical habitats,

these methods will be applied to subsequent analyses of simazine as part of completing this

registration review. The EPA will complete its effects determination and initiate consultation for

simazine by 2020.

ENDOCRINE DISRUPTOR SCREENING PROGRAM:

As required by FIFRA and FFDCA, the EPA reviews numerous studies to assess potential adverse

outcomes from exposure to chemicals. Collectively, these studies include acute, subchronic and

chronic toxicity, including assessments of carcinogenicity, neurotoxicity, developmental,

reproductive, and general or systemic toxicity. These studies include endpoints which may be

susceptible to endocrine influence, including effects on endocrine target organ histopathology,

organ weights, estrus cyclicity, sexual maturation, fertility, pregnancy rates, reproductive loss,

and sex ratios in offspring.  For ecological hazard assessments, the EPA evaluates acute tests

and chronic studies that assess growth, developmental and reproductive effects in different

taxonomic groups. As part of this risk assessment, the EPA reviewed these data and selected

the most sensitive endpoints for relevant risk assessment scenarios from the existing hazard

database. However, as required by FFDCA section 408(p), simazine is subject to the endocrine

screening part of the Endocrine Disruptor Screening Program (EDSP).  

The EPA has developed the EDSP to determine whether certain substances (including pesticide

active and other ingredients) may have an effect in humans or wildlife similar to an effect

produced by a “naturally occurring estrogen, or other such endocrine effects as the

Administrator may designate.” The EDSP employs a two‐tiered approach to making the

statutorily required determinations. Tier 1 consists of a battery of 11 screening assays to

identify the potential of a chemical substance to interact with the estrogen, androgen, or

thyroid (E, A, or T) hormonal systems. Chemicals that go through Tier 1 screening and are found

to have the potential to interact with E, A, or T hormonal systems will proceed to the next stage

of the EDSP where the EPA will determine which, if any, of the Tier 2 tests are necessary based
141

on the available data. Tier 2 testing is designed to identify any adverse endocrine‐related

effects caused by the substance, and establish a dose‐response relationship between the dose

and the E, A, or T effect.  

Under FFDCA section 408(p), the EPA must screen all pesticide chemicals. Between October

2009 and February 2010, the EPA issued test orders/data call‐ins for the first group of 67

chemicals, which contains 58 pesticide active ingredients and 9 inert ingredients. A second list

of chemicals identified for EDSP screening was published on June 14, 20135 and includes some

pesticides scheduled for registration review and chemicals found in water. Neither of these lists

should be construed as a list of known or likely endocrine disruptors. Simazine is on List 1 for

which the EPA has received all of the required Tier 1 assay data. The EPA has reviewed all of the

assay data received for the appropriate List 1 chemicals and the conclusions of those reviews

are available in the chemical‐specific public dockets (see EPA‐HQ‐OPP‐2013‐0251 for simazine

which can be accessed at http://www.regulations.gov). For further information on the status of

the EDSP, the policies and procedures, the lists of chemicals, future lists, the test guidelines and

Tier 1 screening battery, please visit our website[2].

On June 29, 2015, the EPA released the findings of the simazine EDSP weight‐of‐evidence

analysis (WoE) (http://www2.epa.gov/sites/production/files/2015‐06/documents/simazine‐

080807_2015‐06‐29_txr0057136.pdf). The EPA concluded that based on the weight‐of‐

evidence analysis, simazine has the potential to interact with the estrogen and androgen

pathways in mammals and other wildlife, and that there was not convincing evidence of

potential interaction with the thyroid pathway. The potential for interaction with the estrogen

and androgen pathway is supported by the SAP conclusion that the chlorotriazines (including

simazine) function through a neuroendocrine MOA that suppresses the hypothalamic release of

Gonadotrophin‐releasing hormone (GnRH) and therefore Luteinizing Hormone (LH), which may

result in downstream effects on estrogen and androgen signaling pathways.

   

                                                      

5 See http://www.regulations.gov/#!documentDetail;D=EPA‐HQ‐OPPT‐2009‐0477‐0074 for the final second list of

chemicals.

[2] Available: http://www.epa.gov/endo/
142

REFERENCES

Brady, D., 2014. Guidance for Assessing Pesticide Risks to Bees. Environmental Fate and Effects

Division, Office of Pesticide Programs, USEPA.

Cape Cod Cranberry Growers Association. 2001. Conservation District Grant Program of the

Commonwealth of Massachusetts Executive Office of Environmental Affairs. Cranberry Water

Use.  An Information Fact Sheet.  June 2001. Available at http://www.cranberries.org/pdf/

wateruse.pdf (Accessed October 25, 2012).

Drexel. January 15, 2015.  Simazine Use Summary Table submitted to EPA.

Drexel. May 12, 2015. Letter and email communication to US EPA responding to April 16, 2015

US EPA Review of Simazine Use Summary Table memorandum.

Food and Agriculture Organization of the United Nations (FAO). 2000. Appendix 2. Parameters

of pesticides that influence processes in the soil. In FAO Information Division Editorial Group

(Ed.), Pesticide Disposal Series 8. Assessing Soil Contamination. A Reference Manual. Rome:

Food & Agriculture Organization of the United Nations (FAO).

LeBaron, McFarland, and Burnside. 2008. The Triazine Herbicides: 50 Years Revolutionizing

Agriculture.

Oxon‐Italia. January 30, 2015.  Simazine Use Summary Table submitted to US EPA.

Oxon‐Italia. May 19, 2015. Letter and email communication to US EPA responding to April 16,

2015 US EPA Review of Simazine Use Summary Table memorandum.

Papoulias, D. M., Tillitt, D. E., Talykina, M. G., Whyte, J. J., and Richter, C. A. (2014). Atrazine

reduces reproduction in Japanese medaka (Oryzias latipes). Aquat Toxicol. 154:230‐239.

Syngenta Crop Protection. December 30, 2014. Simazine Use Summary Table submitted to US

EPA.

Syngenta Crop Protection. May 19, 2015. Email communication to US EPA responding to April

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