Risks of Paraquat Use to Federally Threatened 

California Red-legged Frog 

(Rana aurora draytonii) 

Pesticide Effects Determination 

Environmental Fate and Effects Division 

Office of Pesticide Programs 

Washington, D.C. 20460 

June 10, 2009 
Primary Authors: 

Tiffany Mason, Environmental Engineer 

Christina Wendel, Biologist 

Environmental Risk Branch II 

 Environmental Fate and Effects Division (7507P)

Secondary Review: 

William P. Eckel, PhD, Senior Physical Scientist 

Environmental Risk Branch II 

Environmental Fate and Effects Division (7507P) 

Donna Randall, Senior Biologist 

Environmental Risk Branch II 

Environmental Fate and Effects Division (7507P) 

Jean Holmes, PhD, Risk Assessment Process Leader 

Environmental Risk Branch II 

 Environmental Fate and Effects Division (7507P) 

Branch Chief, Environmental Risk Branch II: 

 Tom Bailey, PhD, Branch Chief 

 Environmental Fate and Effects Division (7507P) 
Table of Contents 

1.0 Executive Summary.......................................................................................... 5

2.0 Problem Formulation ..................................................................................... 13

2.1 Purpose.............................................................................................................. 13 

2.2 Scope................................................................................................................. 15 

2.3 Previous Assessments....................................................................................... 16 

2.4 Stressor Source and Distribution ...................................................................... 18 

2.4.1 Environmental Fate Assessment ................................................................... 18 

2.4.2 Mechanism of Action.................................................................................... 21 

2.4.3 Use Characterization..................................................................................... 21 

2.5 Assessed Species............................................................................................... 28 

2.5.1 Distribution ................................................................................................... 28 

2.5.2 Reproduction................................................................................................. 31 

2.5.3 Diet................................................................................................................ 31 

2.5.4 Habitat........................................................................................................... 32 

2.6 Designated Critical Habitat............................................................................... 33 

2.7 Action Area....................................................................................................... 35 

2.8 Assessment Endpoints and Measures of Ecological Effect .............................. 39 

2.8.1 Assessment Endpoints for the CRLF............................................................ 39 

2.8.2 Assessment Endpoints for Designated Critical Habitat ................................ 41 

2.9 Conceptual Model............................................................................................. 43 

2.9.1 Risk Hypotheses............................................................................................ 43 

2.9.2 Diagram......................................................................................................... 43 

2.10 Analysis Plan .................................................................................................... 45 

2.10.1 Measures to Evaluate the Risk Hypothesis and Conceptual Model.............. 46 

3.0 Exposure Assessment...................................................................................... 51

3.1 Label Application Rates and Intervals.............................................................. 51 

3.2 Aquatic Exposure Assessment.......................................................................... 52 

3.2.1 Modeling Approach ...................................................................................... 52 

3.2.2 Model Inputs ................................................................................................. 52 

3.2.3 Results........................................................................................................... 53 

3.2.4 Existing Monitoring Data ............................................................................. 54 

3.3 Terrestrial Animal Exposure Assessment......................................................... 55 

3.4 Terrestrial Plant Exposure Assessment............................................................. 57 

4.0 Effects Assessment .......................................................................................... 58

4.1 Evaluation of Aquatic Ecotoxicity Studies....................................................... 60 

4.1.1 Toxicity to Freshwater Fish .......................................................................... 62 

4.1.2 Toxicity to Freshwater Invertebrates ............................................................ 63 

4.1.3 Toxicity to Aquatic Plants ............................................................................ 64 

4.2 Toxicity of Paraquat to Terrestrial Organisms ................................................. 64 

4.2.1 Toxicity to Birds ........................................................................................... 66 

4.2.2 Toxicity to Mammals.................................................................................... 67 

4.2.3 Toxicity to Terrestrial Invertebrates ............................................................. 68 

4.2.4 Toxicity to Terrestrial Plants ........................................................................ 69 
4.3 Use of Probit Slope Response Relationship to Provide Information on the 

Endangered Species Levels of Concern............................................................ 70 

4.4 Incident Database Review ................................................................................ 70 

4.4.1 Terrestrial Incidents ...................................................................................... 71 

4.4.2 Plant Incidents............................................................................................... 71 

4.4.3 Aquatic Incidents .......................................................................................... 72 

5.0 Risk Characterization..................................................................................... 73

5.1 Risk Estimation................................................................................................. 73 

5.1.1 Exposures in the Aquatic Habitat ................................................................. 73 

5.1.2 Exposures in the Terrestrial Habitat ............................................................. 77 

5.1.3 Primary Constituent Elements of Designated Critical Habitat ..................... 81 

5.2 Risk Description ............................................................................................... 83 

5.2.1 Direct Effects ................................................................................................ 89 

5.2.2 Indirect Effects (via Reductions in Prey Base) ............................................. 99 

5.2.3 Indirect Effects (via Habitat Effects) .......................................................... 102 

5.2.4 Effects to Designated Critical Habitat ........................................................ 104 

5.2.5 Spatial Extent of Potential Effects .............................................................. 106 

6.0 Uncertainties.................................................................................................. 111

6.1 Exposure Assessment Uncertainties ............................................................... 111 

6.1.1 Maximum Use Scenario.............................................................................. 111 

6.1.2 Crops Not Grown in California................................................................... 111 

6.1.3 Aquatic Exposure Modeling of Paraquat.................................................... 111 

6.1.4 Action Area Uncertainties........................................................................... 114 

6.1.5 Usage Uncertainties .................................................................................... 115 

6.1.6 Terrestrial Exposure Modeling of Paraquat................................................ 115 

6.1.7 Spray Drift Modeling.................................................................................. 116 

6.2 Effects Assessment Uncertainties................................................................... 116 

6.2.1 Acute to Chronic Ratio ............................................................................... 116 

6.2.2 Age Class and Sensitivity of Effects Thresholds........................................ 117 

6.2.3 Use of Surrogate Species Effects Data ....................................................... 117 

6.2.4 Sublethal Effects ......................................................................................... 117 

6.2.5 Location of Wildlife Species....................................................................... 118 

7.0 Risk Conclusions........................................................................................... 119

8.0 References...................................................................................................... 124
List of Tables 

Table 1-1 Effects Determination Summary for Paraquat Use and the CRLF..................... 8 

Table 1-2 Effects Determination Summary for Paraquat Use and CRLF Critical Habitat 

Impact Analysis .............................................................................................. 10 

Table 1-3 Paraquat Use-specific Direct Effects Determinations1 for the CRLF .............. 11 

Table 1-4 Paraquat Use-specific Indirect Effects Determinations1 Based on Effects to 

Prey ................................................................................................................. 11 

Table 2-1 Summary of Paraquat Environmental Fate Properties ..................................... 19 

Table 2-2 Paraquat Uses Assessed for the CRLF1

............................................................ 22 

Table 2-3 Summary of California Department of Pesticide Registration (CDPR) Pesticide 

Use Reporting (PUR) Data from 1999 to 2006 for Currently Registered 

Paraquat Uses.................................................................................................. 25 

Table 2-4 Assessment Endpoints and Measures of Ecological Effects for Direct and 

Indirect Effects of Paraquat (in terms of the Paraquat Cation) on the CRLF. 40 

Table 2-5 Summary of Assessment Endpoints and Measures of Ecological Effect for 

Primary Constituent Elements of Designated Critical Habitata

...................... 42 

Table 3-1 Paraquat Uses and Application Information for the CRLF risk assessment1... 51 

Table 3-2 Summary of GENEEC2 Environmental Fate Data Used for Aquatic Exposure 

Inputs for Paraquat Endangered Species Assessment for the CRLF 1 Based on 

the Paraquat Cation......................................................................................... 53 

Table 3-3 Aquatic EECs (μg/L) for Paraquat Uses in California ..................................... 54 

Table 3-4 Input Parameters for Foliar Applications Used to Derive Terrestrial EECs for 

Paraquat with T-REX...................................................................................... 56 

Table 3-5 Upper-bound Kenega Nomogram EECs for Dietary- and Dose-based 

Exposures of the CRLF and its Prey to Paraquat............................................ 56 

Table 3-6 EECs (ppm) for Indirect Effects to the Terrestrial-Phase CRLF via Effects to 

Terrestrial Invertebrate Prey Items ................................................................. 57 

Table 4-1 Freshwater Aquatic Toxicity Profile for Paraquat Dichloride (expressed as the 

cation) ............................................................................................................. 61 

Table 4-2 Categories of Acute Toxicity for Fish and Aquatic Invertebrates.................... 61 

Table 4-3 Terrestrial Toxicity Profile for Paraquat Dichloride (expressed as the cation) 65 

Table 4-4 Categories of Acute Toxicity for Avian and Mammalian Studies ................... 66 

Table 4-5 Non-target Terrestrial Plant Seedling Emergence and Vegetative Vigor 

Toxicity (Tier II) Data .................................................................................... 70 

Table 5-1 Summary of Acute Direct Effect RQs* for the Aquatic-phase CRLF ............. 74 
Table 5-2 Summary of RQs* Used to Estimate Indirect Effects to the CRLF via Effects to 

Non-Vascular Aquatic Plants (diet of CRLF in tadpole life stage and habitat of 

aquatic-phase CRLF) ...................................................................................... 75 

Table 5-3 Summary of Acute RQs* Used to Estimate Indirect Effects to the CRLF via 

Direct Effects on Aquatic Invertebrates as Dietary Food Items (prey of CRLF 

juveniles and adults in aquatic habitats) ......................................................... 76 

Table 5-4 Summary of RQs* Used to Estimate Indirect Effects to the CRLF via Effects to 

Vascular Aquatic Plants (habitat of aquatic-phase CRLF)a............................ 77 

Table 5-5 Summary of Acute RQs* Used to Estimate Direct Effects to the Terrestrialphase CRLF (foliar application) From T-REX............................................... 78 

Table 5-6 Summary of Chronic RQs* Used to Estimate Direct Effects to the Terrestrialphase CRLF (foliar application) From T-REX............................................... 78 

Table 5-7 Summary of Acute and Chronic RQs* Used to Estimate Indirect Effects to the 

Terrestrial-phase CRLF via Direct Effects on Small Mammals as Dietary 

Food Items (foliar application) ....................................................................... 80 

Table 5-8 Risk Estimation Summary for Paraquat - Direct and Indirect Effects to CRLF

......................................................................................................................... 84 

Table 5-9 Risk Estimation Summary for Paraquat – PCEs of Designated Critical Habitat 

for the CRLF................................................................................................... 87 

Table 5-10a Upper-bound Kenega Nomogram T-HERPS EECs (mg/kg-diet) for Dietarybased Exposures of the CRLF and its Prey to Paraquat, the weights of small 

herbivore and insectivore mammals are 15 g. ................................................ 90 

Table 5-10b Upper-bound Kenega Nomogram T-HERPS EECs (mg/kg-diet) for Dietarybased Exposures of the CRLF and its Prey to Paraquat, the weights of small 

herbivore and insectivore mammals are 35 g. ................................................ 91 

Table 5-11a Upper-bound Kenega Nomogram T-HERPS EECs (mg/kg-bw) for Dosebased Exposures of the CRLF and its Prey to Paraquat, the weights of small 

herbivore and insectivore mammals are 15 g. ................................................ 91 

Table 5-11b Upper-bound Kenega Nomogram T-HERPS EECs (mg/kg-bw) for Dosebased Exposures of the CRLF and its Prey to Paraquat, the weights of small 

herbivore and insectivore mammals are 35 g. ................................................ 92 

Table 5-12a Revised Acute Dietary-based RQs* for CRLF consuming different food 

items (RQs calculated using T-HERPS), the weights of small herbivore and 

insectivore mammals are 15 g......................................................................... 94 

Table 5-12b Revised Acute Dietary-based RQs* for CRLF consuming different food 

items (RQs calculated using T-HERPS), the weights of small herbivore and 

insectivore mammals are 35 g......................................................................... 94 

Table 5-13a Refined Dose-based RQs* for CRLF consuming different food items (RQs 

calculated using T-HERPS), the weights of small herbivore and insectivore 

mammals are 15 g. .......................................................................................... 95 
Table 5-13b Refined Dose-based RQs* for CRLF consuming different food items (RQs 

calculated using T-HERPS), the weights of small herbivore and insectivore 

mammals are 35 g. .......................................................................................... 96 

Table 5-14a Revised Chronic Dietary-based RQs* for CRLF consuming different food 

items (RQs calculated using T-HERPS), the weights of small herbivore and 

insectivore mammals are 15 g......................................................................... 98 

Table 5-14b Revised Chronic Dietary-based RQs* for CRLF consuming different food 

items (RQs calculated using T-HERPS), the weights of small herbivore and 

insectivore mammals are 35 g......................................................................... 98 

Table 5-15 Summary of AgDrift Predicted Terrestrial Spray Drift Distances Using 

Maximum Application Rate.......................................................................... 108 

Table 5-16 Summary of AgDrift Predicted Aquatic Spray Drift Distances Using 

Minimum Application Rate .......................................................................... 108 

Table 7-1 Effects Determination Summary for Paraquat Use and the CRLF................. 120 

Table 7-2 Effects Determination Summary for Paraquat Use and CRLF Critical Habitat 

Impact Analysis ............................................................................................ 122 
List of Figures 

Figure 2-1 Paraquat Use in Total Pounds per County ...................................................... 24 

Figure 2-2 Recovery Unit, Core Area, Critical Habitat, and Occurrence Designations for 

CRLF............................................................................................................... 30 

Figure 2-3 CRLF Reproductive Events by Month........................................................... 31 

Figure 2-4 Initial area of concern, or “footprint” of potential use, for Paraquat ............. 37 

Figure 2-5 Conceptual Model for Pesticide Effects on Terrestrial Phase of the CRLF .. 44 

Figure 2-6 Conceptual Model for Pesticide Effects on Aquatic Phase of the CRLF ...... 45 

Figure 5-1 Overlap Map: CRLF Habitat and Paraquat Initial Area of Concern............ 110 

Appendices 

Appendix A Ecological Effects Data 

Appendix B Multi-ai Product Analysis 

Appendix C RQ Method and LOCs 

Appendix D Spatial Analysis of Paraquat 

Appendix E T-REX Example Output 

Appendix F Bibliography of Evaluated ECOTOX Open Literature Data 

Appendix G Bibliography of ECOTOX Open Literature Data Excluded or Not 

 Evaluated 

Appendix H Paraquat Incident Data 

Appendix I GENEEC2 Example Output 

Appendix J T-HERPS Example Output 

Appendix K Summary of Human Health Effects Data for Paraquat 

Attachment I Status and Life History of the California Red-legged Frog 

Attachment II Baseline Status and Cumulative Effects for the California Red-legged 

 Frog 
1.0 Executive Summary 

The purpose of this assessment is to evaluate potential direct and indirect effects on the 

California red-legged frog (Rana aurora draytonii) (CRLF) arising from Federal 

Insecticide, Fungicide, Rodenticide, Act (FIFRA) regulatory actions regarding use of 

paraquat dichloride on agricultural and non-agricultural sites. In addition, this 

assessment evaluates whether these actions can be expected to result in effects to the 

species’ designated critical habitat. This assessment was completed in accordance with 

the U.S. Fish and Wildlife Service (U.S. FWS) and National Marine Fisheries Service 

(NMFS) Endangered Species Consultation Handbook (U.S. FWS/NMFS 1998) and 

procedures outlined in the Agency’s Overview Document (U.S. EPA 2004). 

The CRLF was listed as a threatened species by U.S. FWS in 1996. The species is 

endemic to California and Baja California (Mexico) and inhabits both coastal and interior 

mountain ranges. A total of 243 streams or drainages are believed to be currently 

occupied by the species, with the greatest numbers in Monterey, San Luis Obispo, and 

Santa Barbara counties (U.S. FWS 1996) in California. 

 

Paraquat dichloride is a contact herbicide that causes desiccation to the plants. It is 

applied as a flowable solution from an emulsifiable concentrate. As a result, paraquat 

dichloride is already dissociated into its cation, paraquat, when it is applied. Therefore, 

this assessment will focus on the paraquat cation. Currently, labeled uses of paraquat 

include various agricultural uses such as apricots, loganberries, corn, peanuts, coffee, and 

lettuce, as well as multiple non-agricultural uses such as airport landing fields, and 

commercial, industrial and institutional premises. The uses considered as part of the 

federal action evaluated in this assessment are listed in Table 2-2. 

There are a total of three PC codes associated with paraquat; the PC code for paraquat 

dichloride is 061601, 061602 for paraquat bis (methyl sulfate), and 061603 for paraquat. 

Paraquat bis (methyl sulfate) is no longer a registered product of paraquat, but data from 

ECOTOX and past assessments including all three PC codes were incorporated into the 

paraquat cation. 

Paraquat is dissipated by rapid adsorption to biological materials and clay particles. Due 

to the apparent adsorption strength of paraquat, these bound residues do not appear to be 

environmentally available. Paraquat was shown to be very immobile in soil with batch 

equilibrium studies conducted on four soils in the laboratory. Adsorption Kds ranged 

from 68-50,000. There was no desorption of paraquat from any of these soils (MRID 

40762701). Due to high biological toxicity to plants and animals prior to adsorption, 

paraquat is subject to spray drift concerns. However, paraquat has such extremely high 

adsorption coefficients, it is not expected to volatilize once applied to the soil. In 

addition, since it is not expected to volatilize, is not a concern for atmospheric transport. 

Paraquat does not hydrolyze, does not photodegrade in aqueous solutions, and is resistant 

to biotic degradation. Essentially no aerobic or anaerobic microbial degradation of 

paraquat occurred during the laboratory studies. In short and long-term field dissipation 

5
studies, paraquat residues were shown to be persistent and to accumulate slightly with 

repeated applications. 

Adsorbed paraquat could potentially be found in surface water systems associated with 

soil particles carried by erosion. Paraquat is not expected to be a contaminant of 

groundwater. 

The only monitoring data available for paraquat was from the California Department of 

Pesticide Regulation (CPR) database. Out of 399 samples taken from July 2005 to 

October 2006, only one sample contained paraquat. The detection, at 0.24 ppb, was 

below the quantification limit of 1 ppb. 

Since CRLFs exist within aquatic and terrestrial habitats, exposure of the CRLF, its prey 

and its habitats to paraquat are assessed separately for the two habitats. A Tier-I aquatic 

exposure model, GENEEC2, was used to estimate high-end exposures of paraquat in 

aquatic habitats resulting from spray drift from different uses. Peak model-estimated 

environmental concentrations resulting from different paraquat uses ranged from 1.6 to 

33 μg/L. These estimates are supplemented with analysis of available California surface 

water monitoring data from U. S. Geological Survey’s National Water Quality 

Assessment (NAWQA) program and the California Department of Pesticide Regulation. 

NAWQA did not have any available monitoring data for paraquat in California surface 

waters or ground water. The maximum concentration of paraquat reported by the 

California Department of Pesticide Regulation surface water database (0.24 μg/L) is 

roughly 3,717 times lower than the highest peak model-estimated environmental 

concentration. 

To estimate paraquat exposures to the terrestrial-phase CRLF, and its potential prey 

resulting from uses involving paraquat applications, the T-REX model is used for spray 

treatment. The AgDRIFT model is also used to estimate deposition of paraquat on 

terrestrial and aquatic habitats from spray drift. The T-HERPS model is used as a 

refinement tool to explore amphibian-specific food intake on potential exposures to the 

terrestrial phase CRLF. 

The effects determination assessment endpoints for the CRLF include direct toxic effects 

on the survival, reproduction, and growth of the CRLF itself, as well as indirect effects, 

such as reduction of the prey base or effects to its habitat. Direct effects to the CRLF in 

the aquatic habitat are based on toxicity information for freshwater fish, which are 

generally used as a surrogate for aquatic-phase amphibians. In the terrestrial habitat, 

direct effects are based on toxicity information for birds, which are used as a surrogate 

for terrestrial-phase amphibians. Given that the CRLF’s prey items and designated 

critical habitat requirements in the aquatic habitat are dependant on the availability of 

freshwater aquatic invertebrates and aquatic plants, toxicity information for these 

taxonomic groups is also discussed. In the terrestrial habitat, indirect effects due to 

depletion of prey are assessed by considering effects to terrestrial insects, small terrestrial 

mammals, and frogs. Indirect effects due to effects to the terrestrial habitat are 

characterized by available data for terrestrial monocots and dicots. 

6
QINA (4-carboxyl-1-methylpyridinium) is a degradate formed by paraquat. However, in 

the photodegradation in water study the quantity of degradate present was only 6% of the 

radioactivity of paraquat after 85 weeks of natural sunlight irradiation. Due to paraquat 

degrading very slowly, the minor degradate, QINA, does not pose an important 

environmental concern and will not be evaluated in this assessment. 

Risk quotients (RQs) are derived as quantitative estimates of potential high-end risk. 

Acute and chronic RQs are compared to the Agency’s levels of concern (LOCs) to 

identify instances where paraquat use within the action area has the potential to adversely 

affect the CRLF and its designated critical habitat via direct toxicity or indirectly based 

on direct effects to its food supply (i.e., freshwater invertebrates, algae, fish, frogs, 

terrestrial invertebrates, and mammals) or habitat (i.e., aquatic plants and terrestrial 

upland and riparian vegetation). When RQs for each particular type of effect are below 

LOCs, the pesticide is determined to have “no effect” on the CRLF. Where RQs exceed 

LOCs, a potential to cause adverse effects is identified, leading to a conclusion of “may 

affect.” If a determination is made that use of paraquat within the action area “may 

affect” the CRLF or its designated critical habitat, additional information is considered to 

refine the potential for exposure and effects, and the best available information is used to 

distinguish those actions that “may affect, but are not likely to adversely affect” (NLAA) 

from those actions that are “likely to adversely affect” (LAA) the CRLF. Similarly for 

critical habitat, additional information is considered to refine the potential for exposure 

and effects to distinguish those actions that may or do not result in effects to its critical 

habitat. 

Based on the best available information, the Agency makes a May Affect, and Likely to 

Adversely Affect determination for the CRLF based on the direct effects to the 

terrestrial-phase CRLF, and indirect effects to the aquatic- and terrestrial-phase 

CRLF. Additionally, the Agency has determined that there is the potential for effects to 

CRLF designated critical habitat from the use of paraquat. A summary of the risk 

conclusions and effects determinations for the CRLF and its critical habitat is presented 

in Table 1-1 and Table 1-2. Use-specific determinations for direct and indirect effects to 

the CRLF are provided in Table 1-3 and Table 1-4. The non-agricultural and agricultural 

uses with the maximum application rates, along with the agricultural use with the median 

application rate, and the agricultural use with the three lowest application rates were used 

to bound the potential exposures from all registered uses. Further information on the 

results of the effects determination is included as part of the Risk Description in Section 

5.2. Given the LAA determination for the CRLF and potential effects to designated 

critical habitat, a description of the baseline status and cumulative effects for the CRLF is 

provided in Attachment II. 

7
Table 1-1 Effects Determination Summary for Paraquat Use and the CRLF 

Assessment 

Endpoint 

Effects 

Determination 1

Basis for Determination 

Potential for Direct Effects

Aquatic-phase (Eggs, Larvae, and Adults): 

The aquatic phase amphibian acute LOCs for listed species (0.05) are not 

exceeded for any uses of paraquat in California. The chronic EECs are all less 

than the estimated chronic value derived from the ACR. Therefore, there are no 

exceedances of chronic LOCs. 

Survival, growth, 

and/or reproduction 

of CRLF 

individuals 

NE1

LAA1

Terrestrial-phase (Juveniles and Adults): 

Refined acute dietary-based RQs for CRLFs consuming small insects exceed 

the acute listed species LOC (0.1) for all uses of paraquat except melons, RQs 

ranged from 1.28 (Airport/public health use/guava) to 0.06 (melons). The 

Refined acute dietary-based RQs for CRLFs consuming large insects and 15g 

small insectivore mammals resulted in paraquat use on airports/public health 

use/guava exceeding the listed species LOC (0.1), with an RQ of 0.14 for both. 

The refined acute dietary-based RQs for CRLFs consuming small herbivore 

mammals (15g) resulted in all uses exceeding the listed species LOC, RQs 

ranged from 2.17 (Airport/public health use/guava) to 0.10 (melons) and for 

35g small mammals all uses except melons exceed the listed species LOC, RQs 

ranged from 1.50 (Airport/public health use/guava) to 0.77 (melons). There are 

no exceedances for CRLFs consuming small terrestrial-phase amphibians. 

Refined dose-based RQs for CRLF of varying weights (1.4g, 37g and 238g) 

consuming small insects exceed the acute endangered species LOC (0.1) for 

only the Airport/public health and Guava uses of paraquat for all weights of 

CRLF). There are no exceedances for small sized (1.4g) CRLF consuming 

large insects and CRLF this size are too small to consume small mammals or 

small terrestrial-phase amphibians. The RQs for small sized (1.4g) CRLF are 

0.27 suggesting that small CRLF consuming small insects are potentially 

affected by acute exposures to paraquat. 

Refined dose-based RQs for medium sized (37g) CRLF consuming small 

herbivore mammals (either 15g or 35g) exceed the acute listed species LOC 

(0.1) for all uses of paraquat. There were also exceedances in the acute listed 

species LOC (0.1) for medium sized CRLF consuming small insectivore (15g) 

mammals for the airports/public health, guava, and ginger uses of paraquat. For 

medium sized CRLF consuming small insectivore (35g) mammals there were 

exceedances in the acute listed species LOC (0.1) for all uses of paraquat except 

carrots and melons. There are no exceedances for medium sized (37g) CRLF 

consuming large insects or small terrestrial-phase amphibians. Due to 

exceedances of LOCs for CRLF consuming small herbivore mammals (either 

15g or 35g) for all paraquat uses, and exceedances of LOCs for CRLF 

consuming small insectivore mammals (either 15g or 35g) for a majority of 

paraquat uses indicate that the medium sized CRLF could potentially be 

affected by acute exposures to paraquat. 

Refined dose-based RQs for large sized (238g) CRLF consuming small 

herbivore mammals (either 15g or 35g) exceed the acute listed species LOC 

(0.1) for all uses of paraquat except melons. There were no exceedances for 

8
Assessment 

Endpoint 

Effects Basis for Determination 

Determination 1

large sized (238g) CRLF consuming large insects, small insectivore mammals 

(15g or 35g), or small terrestrial-phase amphibians. The exceedances of LOCs 

for CRLF consuming small herbivore mammals (either 15g or 35g) for all 

paraquat uses except melons, indicates that the large sized CRLF could 

potentially be affected by acute exposures to paraquat. 

Refined chronic dietary-based RQs for CRLFs consuming small insects and 

small herbivore mammals (either15g or 35g) using T-HERPS model exceed the 

chronic species LOC (1.0) for all uses of paraquat. Refined chronic dietarybased RQs for CRLFs consuming large insects and small insectivore mammals 

(either 15g or 35g) using T-HERPS model exceed the chronic species LOC (1.0) 

for Airport/public health, guava, and ginger uses of paraquat. The refined 

chronic dietary-based RQs for CRLFs consuming small terrestrial-phase 

amphibians using T-HERPS model exceed the chronic species LOC (1.0) for 

Airport/public health and guava uses of paraquat (the maximum uses). 

Potential for Indirect Effects

Aquatic prey items, aquatic habitat, cover and/or primary productivity 

LOCs for non-vascular plants are exceeded for all uses. The non-vascular plant 

RQs range from 138 for airports, commercial/industrial areas, and public health 

areas to 4.1 for melons. 

LOCs for vascular plants are not exceeded for any uses. The vascular plant RQs 

range from 0.77 (airports, commercial/industrial areas, public health areas) to 

0.02 (melons). 

LOCs for aquatic invertebrates are not exceeded for any uses. The acute RQs 

range from 0.730 for airports, commercial/industrial areas, and public health 

areas to 0.004 for melons. When comparing chronic indirect effects, the 

estimated chronic value at 0.174 ppm is not exceeded for any use. 

For fish/frogs none of the uses exceed the LOCs for listed species. The RQs 

range from 0.004 for airports, commercial/industrial areas, and public health 

areas to < 0.001 for melons. When comparing chronic direct effects, the 

estimated chronic value at 1.89 ppm is not exceeded for any use. 

LAA1

LAA1 Terrestrial prey items, riparian habitat 

RQs could not be calculated for terrestrial invertebrates as the endpoint was not 

definitive. The calculated EECs were compared to the toxicity endpoint, and it 

was determined that terrestrial invertebrates would not likely adversely affect the 

CRLF indirectly as food. 

For small mammals the acute dose-based RQs exceed the Agency’s LOC (0.1) 

for all uses of paraquat, the RQs ranged from 7.63 (airports/public health 

use/guava) to 0.34 (melons). The chronic dose-based RQs exceed the Agency’s 

LOC (1.0) for all uses of paraquat, and range from 128.53 (airports/public health 

use/guava) to 5.78 (melons). The chronic dietary-based RQs exceed the 

Agency’s LOC (1.0) for all uses except melons, and range from 14.81 

(airports/public health use/guava) to 0.67 (melons). 

The RQs for small terrestrial-phase amphibians did not exceed the listed species 

LOC (0.1) for any use of paraquat. Reduction in amphibian prey items, 

9
Assessment 

Endpoint 

Effects Basis for Determination 

Determination 1

specifically other frogs is not affected from paraquat use. 

The RQs for non-target terrestrial monocot and dicot plants inhabiting semiaquatic and upland dry areas do not exceed the Agency’s LOC (1.0) for any uses. 

All aerial applications of paraquat results in spray drift exceedances for dicots 

(only). These exceedances range from 3.57 (Agricultural fallow/ideland 

maximum aerial application rate) to 1.07 (Melons, minimum aerial application 

rate). 1

 No effect (NE); May affect, but not likely to adversely affect (NLAA); May affect, likely to adversely 

affect (LAA) 

Table 1-2 Effects Determination Summary for Paraquat Use and CRLF Critical 

Habitat Impact Analysis 

Assessment 

Endpoint 

Effects 

Determination 

Basis for Determination 

Modification of 

aquatic-phase PCE 

May affect Due to aquatic non-vascular and terrestrial plant communities being reduced 

from a majority of use sites, there is potential for alteration of channel/pond 

morphology or geometry and/or increase in sediment deposition within the 

stream channel or pond. These plant communities provide shelter, foraging, 

predator avoidance, and aquatic dispersal for juvenile and adult CRLFs. In 

addition, there is potential for alteration in water chemistry/quality including 

temperature, turbidity, and oxygen content necessary for normal growth and 

viability of juvenile and adult CRLFs and their food. 

LOCs are exceeded for terrestrial riparian plants and for aquatic non-vascular 

plants from exposure to paraquat from spray drift. LOCs for non-vascular 

plants are exceeded for all uses of paraquat. 

Modification of 

terrestrial-phase 

PCE 

May affect The use of paraquat at all sites may create the following effects to PCE: 

elimination and/or disturbance of upland habitat; ability of habitat to support 

food source of CRLFs, elimination and/or disturbance of dispersal habitat, 

reduction and/or modification of food sources for terrestrial phase juveniles and 

adults, and alteration of chemical characteristics necessary for normal growth 

and viability of juvenile and adult CRLFs and their food source. 

The RQs for vascular plants did not exceed the Agency’s LOC (1.0) for any uses 

of paraquat in California. 

The RQs for non-target terrestrial monocot and dicot plants inhabiting semiaquatic and upland dry areas do not exceed the Agency’s LOC (1.0) for all uses. 

All aerial applications of paraquat results in spray drift exceedances for dicots 

(only). 

The use of paraquat on most use sites will exceed the revised acute dietary- and 

dose-based LOC and chronic LOC for prey food items of small mammals, and 

invertebrates. Food sources for the CRLF are reduced, and the CRLF is 

indirectly affected from this reduction. 

10
Table 1-3 Paraquat Use-specific Direct Effects Determinations1

 for the CRLF 

Aquatic Habitat Terrestrial Habitat Use(s) Acute Chronic Acute Chronic 

AIRPORTS/LANDING FIELDS, 

COMMERCIAL/INSTITUTIONAL/INDUSTRIAL 

PREMISES/EQUIPMENT (OUTDOOR), 

NONAGRICULTURAL AREAS (PUBLIC HEALTH USE), 

NONAGRICULTURAL RIGHTS-OFWAY/FENCEROWS/HEDGEROWS

NE NE LAA LAA 

GUAVA NE NE LAA LAA 

GINGER NE NE LAA LAA 

BEANS - SUCCULENT (LIMA/SNAP), CARROT 

(INCLUDING TOPS), PEAS (UNSPECIFIED), PEPPER NE NE LAA LAA 

CORN (SILAGE) NE NE LAA LAA 

MELONS NE NE NLAA LAA 

1 NE = No effect; NLAA = May affect, but not likely to adversely affect; LAA = Likely to adversely affect 

Table 1-4 Paraquat Use-specific Indirect Effects Determinations1 Based on Effects 

to Prey 

Aquatic 

Invertebrates 

Aquatic-phase 

frogs and fish 

Terrestrial-phase 

frogs Small Mammals 

Use(s) Algae 

Acute Chronic 

Terrestrial 

Invertebrates 

(Acute) Acute Chronic Acute Chronic Acute Chronic 

AIRPORTS/LANDING 

FIELDS, COMMERCIAL/ 

INSTITUTIONAL/ 

INDUSTRIAL 

PREMISES/ 

EQUIPMENT 

(OUTDOOR), 

NONAGRICULTURAL 

AREAS (PUBLIC 

HEALTH USE), 

NONAGRICULTURAL 

RIGHTS-OF-WAY/ 

FENCEROWS/ 

HEDGEROWS

LAA NE NE NLAA NE NE LAA LAA LAA LAA 

GUAVA LAA NE NE NLAA NE NE LAA LAA LAA LAA 

GINGER LAA NE NE NLAA NE NE LAA LAA LAA LAA 

BEANS - SUCCULENT 

(LIMA/SNAP), CARROT 

(INCLUDING TOPS), 

PEAS (UNSPECIFIED), 

PEPPER 

LAA NE NE NLAA NE NE LAA LAA LAA LAA 

CORN (SILAGE) LAA NE NE NLAA NE NE LAA LAA LAA LAA 

MELONS LAA NE NE NLAA NE NE NLAA LAA LAA LAA 

1

 NE = No effect; NLAA = May affect, not likely to adversely affect; LAA = Likely to adversely affect 

11
Based on the conclusions of this assessment, a formal consultation with the U. S. Fish 

and Wildlife Service under Section 7 of the Endangered Species Act should be initiated. 

When evaluating the significance of this risk assessment’s direct/indirect and adverse 

habitat modification effects determinations, it is important to note that pesticide 

exposures and predicted risks to the species and its resources (i.e., food and habitat) are 

not expected to be uniform across the action area. In fact, given the assumptions of drift 

and downstream transport (i.e., attenuation with distance), pesticide exposure and 

associated risks to the species and its resources are expected to decrease with increasing 

distance away from the treated field or site of application. Evaluation of the implication 

of this non-uniform distribution of risk to the species would require information and 

assessment techniques that are not currently available. Examples of such information and 

methodology required for this type of analysis would include the following: 

• Enhanced information on the density and distribution of CRLF life stages 

within specific recovery units and/or designated critical habitat within the 

action area. This information would allow for quantitative extrapolation 

of the present risk assessment’s predictions of individual effects to the 

proportion of the population extant within geographical areas where those 

effects are predicted. Furthermore, such population information would 

allow for a more comprehensive evaluation of the significance of potential 

resource impairment to individuals of the species. 

• Quantitative information on prey base requirements for individual aquaticand terrestrial-phase frogs. While existing information provides a 

preliminary picture of the types of food sources utilized by the frog, it 

does not establish minimal requirements to sustain healthy individuals at 

varying life stages. Such information could be used to establish 

biologically relevant thresholds of effects on the prey base, and ultimately 

establish geographical limits to those effects. This information could be 

used together with the density data discussed above to characterize the 

likelihood of adverse effects to individuals. 

• Information on population responses of prey base organisms to the 

pesticide. Currently, methodologies are limited to predicting exposures 

and likely levels of direct mortality, growth or reproductive impairment 

immediately following exposure to the pesticide. The degree to which 

repeated exposure events and the inherent demographic characteristics of 

the prey population play into the extent to which prey resources may 

recover is not predictable. An enhanced understanding of long-term prey 

responses to pesticide exposure would allow for a more refined 

determination of the magnitude and duration of resource impairment, and 

together with the information described above, a more complete prediction 

of effects to individual frogs and potential modification to critical habitat. 

12
2.0 Problem Formulation 

Problem formulation provides a strategic framework for the risk assessment. By 

identifying the important components of the problem, it focuses the assessment on the 

most relevant life history stages, habitat components, chemical properties, exposure 

routes, and endpoints. The structure of this risk assessment is based on guidance 

contained in U.S. Environmental Protection Agency's (EPA’s) Guidance for Ecological 

Risk Assessment (U.S. EPA 1998), the Services’ Endangered Species Consultation 

Handbook (U.S. FWS/NMFS 1998) and is consistent with procedures and methodology 

outlined in the Overview Document (U.S. EPA 2004) and reviewed by the U.S. Fish and 

Wildlife Service and National Marine Fisheries Service (U.S. FWS/NMFS 2004). 

2.1 Purpose 

The purpose of this endangered species assessment is to evaluate potential direct and 

indirect effects on individuals of the federally threatened California red-legged frog 

(Rana aurora draytonii) (CRLF) arising from FIFRA regulatory actions regarding use of 

paraquat on a range of uses such as commercial and industrial non-agricultural uses, 

fruits, vegetables, feed for livestock, ornamental shrubs, flowers, and trees. In addition, 

this assessment evaluates whether use on these sites is expected to result in effects to the 

species’ designated critical habitat. This ecological risk assessment has been prepared 

consistent with a settlement agreement in the case Center for Biological Diversity (CBD) 

vs. EPA et al. (Case No. 02-1580-JSW (JL) entered in Federal District Court for the 

Northern District of California on October 20, 2006. 

In this assessment, direct and indirect effects to the CRLF and potential effects to its 

designated critical habitat are evaluated in accordance with the methods described in the 

Agency’s Overview Document (U.S. EPA 2004). Screening level methods include use of 

standard models such as GENEEC2, T-REX, and AgDRIFT, all of which are described at 

length in the Overview Document. Additional refinements include the use of the THERPS model and PRZM/EXAMS when appropriate based on results of GENEEC2. 

The T-HERPS model is used as a refinement tool to explore amphibian-specific food 

intake on potential exposures to the terrestrial phase CRLF. Use of such information is 

consistent with the methodology described in the Overview Document (U.S. EPA 2004), 

which specifies that “the assessment process may, on a case-by-case basis, incorporate 

additional methods, models, and lines of evidence that EPA finds technically appropriate 

for risk management objectives” (Section V, page 31 of U.S. EPA 2004). 

In accordance with the Overview Document, provisions of the ESA, and the Services’ 

Endangered Species Consultation Handbook, the assessment of effects associated with 

registrations of paraquat is based on an action area. The action area is the area directly or 

indirectly affected by the federal action, as indicated by the exceedance of the Agency’s 

Levels of Concern (LOCs). It is acknowledged that the action area for a national-level 

FIFRA regulatory decision associated with a use of paraquat may potentially involve 

numerous areas throughout the United States and its Territories. However, for the 

13
purposes of this assessment, attention will be focused on relevant sections of the action 

area including those geographic areas associated with locations of the CRLF and its 

designated critical habitat within the state of California. As part of the “effects 

determination,” one of the following three conclusions will be reached regarding the 

potential use of paraquat in accordance with current labels: 

• “No effect”; 

• “May affect, but not likely to adversely affect”; or 

• “May affect and likely to adversely affect”. 

Designated critical habitat identifies specific areas that have the physical and biological 

features, (known as primary constituent elements or PCEs) essential to the conservation 

of the listed species. The PCEs for CRLFs are aquatic and upland areas where suitable 

breeding and non-breeding aquatic habitat is located, interspersed with upland foraging 

and dispersal habitat. 

If the results of initial screening-level assessment methods show no direct or indirect 

effects (no LOC exceedances) upon individual CRLFs or upon the PCEs of the species’ 

designated critical habitat, a “no effect” determination is made for use of paraquat as it 

relates to this species and its designated critical habitat. If, however, potential direct or 

indirect effects to individual CRLFs are anticipated or effects may impact the PCEs of the 

CRLF’s designated critical habitat, a preliminary “may affect” determination is made for 

the FIFRA regulatory action regarding paraquat. 

If a determination is made that use of paraquat within the action area(s) associated with 

the CRLF “may affect” this species or its designated critical habitat, additional 

information is considered to refine the potential for exposure and for effects to the CRLF 

and other taxonomic groups upon which these species depend (e.g., aquatic and terrestrial 

vertebrates and invertebrates, aquatic plants, riparian vegetation, etc.). Additional 

information, including spatial analysis (to determine the geographical proximity of CRLF 

habitat and paraquat use sites) and further evaluation of the potential impact of paraquat 

on the PCEs is also used to determine whether effects to designated critical habitat may 

occur. Based on the refined information, the Agency uses the best available information 

to distinguish those actions that “may affect, but are not likely to adversely affect” from 

those actions that “may affect and are likely to adversely affect” the CRLF or affect the 

PCEs of its designated critical habitat. This information is presented as part of the Risk 

Characterization in Section 5 of this document. 

The Agency believes that the analysis of direct and indirect effects to listed species 

provides the basis for an analysis of potential effects on the designated critical habitat. 

Because paraquat is expected to directly impact living organisms within the action area 

(defined in Section 2.7), critical habitat analysis for paraquat is limited in a practical 

sense to those PCEs of critical habitat that are biological or that can be reasonably linked 

to biologically mediated processes (i.e., the biological resource requirements for the listed 

species associated with the critical habitat or important physical aspects of the habitat that 

may be reasonably influenced through biological processes). Activities that may modify 

14
critical habitat are those that alter the PCEs and appreciably diminish the value of the 

habitat. Evaluation of actions related to use of paraquat that may alter the PCEs of the 

CRLF’s critical habitat form the basis of the critical habitat impact analysis. Actions that 

may affect the CRLF’s designated critical habitat have been identified by the Services 

and are discussed further in Section 2.6. 

2.2 Scope 

Please see Table 2-2 for a range of currently registered uses of paraquat in CA. 

The end result of the EPA pesticide registration process (i.e., the FIFRA regulatory 

action) is an approved product label. The label is a legal document that stipulates how 

and where a given pesticide may be used. Product labels (also known as end-use labels) 

describe the formulation type (e.g., liquid or granular), acceptable methods of application, 

approved use sites, and any restrictions on how applications may be conducted. Thus, the 

use or potential use of paraquat in accordance with the approved product labels for 

California is “the action” relevant to this ecological risk assessment. 

Although current registrations of paraquat allow for use nationwide, this ecological risk 

assessment and effects determination addresses currently registered uses of paraquat in 

portions of the action area that are reasonably assumed to be biologically relevant to the 

CRLF and its designated critical habitat. Further discussion of the action area for the 

CRLF and its critical habitat is provided in Section 2.7. 

There are three PC codes associated with paraquat; 061601 for paraquat dichloride, 

061602 for paraquat bis (methyl sulfate), and 061603 for paraquat. Paraquat bis (methyl 

sulfate) is no longer a registered product of paraquat, and the PC code for paraquat alone 

its unclear if it is the cation or not. Since paraquat dichloride and the paraquat cation are 

used interchangeably within the literature and in past assessments. For our assessment, 

data from all three PC codes and chemicals were incorporated, and converted to the 

paraquat cation. 

QINA (4-carboxyl-1-methylpyridinium) is a minor photodegradate of paraquat. 

However, in the photodegradation in water study the quantity of degradate present was 

only 6% of the radioactivity of paraquat after 85 weeks of natural sunlight irradiation. 

Due to paraquat degrading very slowly, the minor degradate, QINA, does not pose as an 

important environmental concern and will not be evaluated in this assessment. 

The Agency does not routinely include, in its risk assessments, an 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. If effects data 

are available for a formulated product containing more than one active ingredient, they 

may be used qualitatively or quantitatively in accordance with the Agency’s Overview 

15
Document and the Services’ Evaluation Memorandum (U.S., EPA 2004; USFWS/NMFS 

2004). 

Paraquat has registered products that contain multiple active ingredients. The results of 

available toxicity data for mixtures of paraquat with other pesticides are presented in 

Appendix B; however, there are no toxicity data for paraquat Multi-AI products. There 

are available open literature data from ECOTOX and three studies which contained 

information examining toxicity effects of paraquat when mixed with other chemicals. 

According to the available open literature data, other pesticides may combine with 

paraquat to produce additive, and/or antagonistic toxic effects. Mixtures included 

paraquat with linuron, and paraquat with copper sulfate. 

2.3 Previous Assessments 

Paraquat was discovered in 1882 and was registered in England in 1962 prior to being 

registered in the United States in 1964 for use as a contact herbicide to control or 

suppress a broad spectrum of emerged weeds under the name paraquat dichloride. The 

Agency classified paraquat dichloride as a restricted use pesticide due to high acute 

toxicity to animals and people in 1978 (February 9, 1978 (43 FR 5782)). Under the 

restricted use classification, only certified applicators or applicators working under their 

direct supervision, are authorized to apply paraquat dichloride end-use products. 

Also in 1978, paraquat dichloride was accepted as a candidate for the Special Review 

board process because it was believed to exceed the risk criteria under 40 CFR 162.11: 

teratogenicity, lack of emergency treatment, chronic effects, reproductive effects, 

oncogenicity (data gap), mutagenicity (data gap), and acute effects. Other areas of 

concern included mammalian toxicity and avian reproductive effects. 

In October 1982 (43 FR 30613), the Agency issued a Final Position Document which 

concluded that the available data did not support paraquat dichloride being placed into 

the Special Review status since the risk criteria identified in 1978 had not been exceeded. 

As a result, a Data Call-In (DCI) was issued requesting additional dermal and inhalation 

data, and more precise information to assess the potential acute effects to applicators. 

In June 1987, a Registration Standard for paraquat dichloride was issued (NTIS#PB88-

217005) in which the Agency evaluated the studies submitted as a result of the previous 

DCI. In 1991, additional data were requested and evaluated for use in the 1997 

Reregistration Eligibility Decision (RED). In the 1997 RED, the Agency requested 

additional data in order to establish tolerances on taro foliage, corn and soybean aspirated 

grain fractions, wheat and hay, cotton and gin byproducts, and processed grapes, as well 

as data to confirm that the existing tolerance for field corn was adequate to cover the 

specialized use of paraquat as a harvest aid. 

To mitigate risks of concern posed by the use of paraquat, the Agency noted a number of 

label amendments to address the worker, and ecological concerns. 

• The maximum application rate for all paraquat dichloride products will be 

lowered from 1.6 lb cation/A to 1.0 lb cation/A. 

16
o Broadcast applications with backpack sprayers (non-spot) should not 

exceed the application rate 0.625 lb cation/A and the application volume 

should be no less than 20 gallons/A. 

o The maximum application rate for spot spraying on all paraquat labels will 

be no more than 0.0195 lbs cation/gallon. 

• Remove the plastic acid bottle and tree injection directions for use from the resin 

soaking sections of all paraquat dichloride labels. 

• Include the hazard statement “Paraquat dichloride is toxic to non-target crops and 

plants if off-target movement occurs. Extreme care must be taken to ensure that 

off-target drift is minimized to the greatest extent possible.” 

• For sole-active-ingredients end-use products that contain paraquat, the product 

labeling must be revised to adopt the handler personal protective 

equipment/engineering control requirements and remove any conflicting PPE 

requirements. 

o For multiple-active-ingredient end-use products that contain paraquat, the 

handler personal protective equipment/engineering control requirements 

set forth in this section must be compared to the requirements on the 

current labeling and the more protective must be retained. 

• Implement best management practices (BMPs) to reduce spray drift, including 

BMPs for controlling droplet size, boom length, application height, swath 

adjustment, wind, temperature and humidity, temperature inversions, and 

sensitive areas. 

• Implement use of PPE and engineering controls for workers. 

A drinking water assessment was performed March 15, 2000, which determined paraquat 

under most circumstances was unlikely to infiltrate past the first few centimeters of soil, 

or to move off-field dissolved in runoff. Yet, monitoring data has demonstrated that 

despite its apparent immobility, paraquat can reach ground water. Paraquat was reported 

in 11 out of 971 wells sampled between 1983 and 1990 (USEPA Pesticides in 

Groundwater Database, 1992) with 65 samples taken from private drinking water wells 

having concentrations greater than 100 μg/L. These wells were located in coarse-grained 

glacial soils that are extremely permeable, with saturated hydraulic conductivities on the 

order of 20,000 ft/day (T.Mesko, USGS, personal communication). Detections were also 

reported in household wells at concentrations ranging up to 1.52 μg/L (Mostaghimi et al.

1998). Data in Smith and Mayfield (1978) suggests that the application of fertilizer may 

enhance the mobility of paraquat. The extremely slow breakdown of sorbed paraquat also 

suggests the possibility that saturation and breakthrough may occur after years of 

repeated application on some soils. Because of its strong cation-exchange sorption to 

soils, runoff modeling is not appropriate for paraquat dichloride. In most circumstances, 

the levels of paraquat residues in surface or ground water are expected to be insignificant. 

Because it should sorb to suspended sediment, coagulation and flocculation processes in 

drinking water treatment plants are likely to remove any paraquat residues present in the 

raw water. Residues of paraquat in drinking water derived from surface supplies can 

therefore be assumed to be negligible. For residues in ground water it was recommended 

to use the value of 1.52 μg/L for human exposure assessment. 

17
On March 30, 2006, a risk assessment for proposed new uses of paraquat dichloride on 

cantaloupe, cucumber, summer squash, ginger, okra, onion, tanier, and wheat was 

performed. With the exception of wheat (where aerial and/or ground spray was to be 

allowed), the proposed new uses are for ground spray only. In most cases no more than 

2.0 lbs a.i./A per season will be allowed. The exception is for the proposed use on 

ginger, for which a maximum of 6.0 lbs a.i./A per year is directed. The 2006 risk 

assessment concluded that these new uses are expected to have acute toxicological effects 

to terrestrial aquatic wildlife, aquatic plants, and terrestrial plants, and chronic 

toxicological effects on terrestrial wildlife. Likewise, the assessment determined that 

sub-lethal effects include lung damage in rats and oxidative stress in all organisms, which 

can lead to premature aging as well as early demise. 

On July 31, 2006, uses on ginger and okra were proposed, and the uses on soybeans, 

wheat, cotton, cucurbits, onions, and tanier were amended. In this Human Health Risk 

Assessment (HED): 

• Tolerances were established or amended for the uses 

• The registrant requested amended registration for use on cotton, soybean, wheat, 

ginger, dry bulb onion, okra, and tanier (March 30, 2006 EFED assessment). In 

this assessment, the registrant proposed to cancel the addition of those uses, but 

did propose an amended use on cucurbits. 

• Registrant needs to propose maximum seasonal rates for cotton, cucurbits, ginger, 

soybeans, and wheat, minimal retreatment intervals for multiple post emergence 

applications, and the directions for tanier needs to be modified to specify its 

geological restrictions in Florida. 

• Crop field trials for certain uses were requested. 

2.4 Stressor Source and Distribution 

2.4.1 Environmental Fate Assessment 

The primary route of environmental dissipation of paraquat is adsorption to soil clay 

particles. Paraquat does not hydrolyze, does not photodegrade in aqueous solutions, and 

is resistant to microbial degradation under aerobic and anaerobic conditions. Essentially 

no microbial degradation of paraquat was seen after 180 days of aerobic incubation or 

after 60 days of anaerobic incubation following a 30 day aerobic incubation. 

Paraquat was shown to be very immobile in soil with batch equilibrium studies conducted 

on four soils in the laboratory. High rates of paraquat were added because at realistic field 

application rates, paraquat was below detection in the batch equilibrium adsorption 

solution. Adsorption Kds ranged from 68-50,000, and there was no detectable 

desorption. 

In laboratory studies with radiolabeled paraquat, no radioactivity volatilized from the soil 

surface to adsorb to glass or to collect in volatile traps. With low vapor pressure and 

extremely high adsorption coefficients, paraquat would not be expected to volatilize once 

18
applied to the soil, but spray drift could potentially be an issue since paraquat is 

extremely biologically active and toxic to plants and animals. 

In short and long term field dissipation studies, paraquat residues were extractable only 

by acid reflux and were shown to be persistent and to accumulate slightly with repeated 

applications. Paraquat is dissipated by rapid adsorption to clay particles. Due to the 

apparent adsorption strength of paraquat for soil clays, these bound residues do not 

appear to be environmentally available. Adsorbed radiolabeled paraquat did undergo 

isotopic exchange when soil samples were shaken with a highly concentrated, nonlabeled paraquat solution (7440 ppm paraquat in water), so the potential for desorption 

does exist; however, since there was no apparent exchange with calcium chloride in the 

batch equilibrium study, this exchange will probably not affect the environmental 

behavior of paraquat. 

Paraquat is very persistent and is more likely to enter surface waters systems associated 

with soil particles carried by erosion that will most likely be found sorbed onto eroded 

particles at or near the application site rather than dissolved in the water column. 

Therefore, this chemical is expected to be transported primarily along with soil particles 

and subsequently redeposited onto the beds of surface water bodies or lowland areas that 

receive eroded sediments from uplands (e.g., riparian zones, wetlands). The primary 

route for dissipation of paraquat in the environment is through sorption onto solids; once 

sorbed, paraquat is very difficult to extract. As a result, paraquat is not expected to be a 

contaminant of groundwater, except in soils with very low clay content. 

In previous reviews (reviews by Jordan, 2/14/1986), a minor photodegradate, 4-carboxyl1-methylpyridinium (QINA) which comprised 6% of applied radioactivity after 85 weeks 

of natural sunlight irradiation, was determined to be mobile. According to the field data 

reviewed in this submission which showed that paraquat degraded very slowly, QINA 

would apparently not be an important environmental concern. 

Table 2-1 lists the environmental fate properties of paraquat, along with the major and 

minor degradates detected in the submitted environmental fate and transport studies. 

Table 2-1 Summary of Paraquat Environmental Fate Properties 

Study Value (units) Major Degradates 

Minor Degradates

MRID # Study Status 

Hydrolysis Stable at pH 5, 7, 9 No degradates reported Upton et 

al., 1985 

Acceptable 

Direct Aqueous 

Photolysis 

Stable No degradates reported 

CO2 (0.15%) 

4-carboxyl-1-

methylpyridinium (QINA) 

(6%) 

MRID 

40562301 

(Jordan, 

1986) 

Acceptable 

Supplemental 

Soil Photolysis Stable No degradates reported MRID 

146807 

Acceptable 

19
Study Value (units) Major Degradates 

Minor Degradates

MRID # Study Status 

Aerobic Soil 

Metabolism 

Stable Did not degrade. No 

degradates reported. 

MRID 

41319301 

Acceptable 

Anaerobic Soil 

Metabolism 

Stable No degradates reported. 

0.29% uncharacterized 

radioactivity. 

MRID 

41319302 

Acceptable 

Anaerobic 

Aquatic 

Metabolism 

No Study No Study No Study No Study 

Aerobic 

Aquatic 

Metabolism 

<2 weeks (only represents water phase. 

Did not measure amount of paraquat 

sorbed to the soil) 

No degradates reported MRID 

00055093 

Supplemental 

Kd-ads / Kd-des 

(mL/g) 

Koc- ads / Koc-des 

(mL/g) 

68 – 50,000 (no measureable 

correlation with % OC) 

15,473 to 1,000,000 

No degradates reported MRID 

40762701 

http://www

.ermanz.go

vt.nz/consu

ltations/cei

r/t.pdf 

Acceptable 

Supplemental 

Terrestrial 

Field 

Dissipation 

Half-life not calculated; however, cited 

reference indicates a half-life of > 10 

years 

No degradates reported. MRID 

41293202 

42802101 

42738701 

42738702 

42802102 

Acceptable 

Aquatic Field 

Dissipation 

No Study No Study No Study No Study 

2.4.2 Environmental Transport Assessment 

Potential transport mechanisms include pesticide spray drift, and secondary drift of soilbound residues leading to deposition onto nearby or more distant ecosystems. Spray drift 

is expected to be the major route of exposure for paraquat. 

In general, deposition of drifting pesticides is expected to be greatest close to the site of 

application. A computer model of spray drift (AgDRIFT) is used to determine potential 

exposures to aquatic and terrestrial organisms via spray drift. The distance of potential 

impact away from the use sites (action area) is determined by the distance required to fall 

below the LOC for airports/commercial/public health areas (non-agricultural use) and 

guava (agricultural use). These uses have the greatest application rate, greatest number 

of applications per season, and the least amount of time between applications. 

Due to model limitations, it may not be possible to provide a quantitative estimate of 

exposure with known uncertainty, beyond the range of AgDRIFT. 

20
2.4.2 Mechanism of Action 

Paraquat dichloride is a quaternary nitrogen compound widely used for broadleaf weed 

control. It is a fast-acting contact herbicide used to suppress or eradicate a wide spectrum 

of post-emergent weeds. It also functions as a defoliant and desiccant and is most 

effective on growing plants with abundant green tissue. It is highly toxic (EPA toxicity 

class I) but readily immobilized. Paraquat is quickly absorbed by living (esp. healthy) 

plant tissue and produces superoxides during photosynthesis, which destroy plant cells. It 

is less effective on dry, drought-stressed, woody, or fully mature plants. 

2.4.3 Use Characterization 

Analysis of labeled use information is the critical first step in evaluating the federal 

action. The current label for paraquat represents the FIFRA regulatory action; therefore, 

labeled use and application rates specified on the label form the basis of this assessment. 

The assessment of use information is critical to the development of the action area and 

selection of appropriate modeling scenarios and inputs. 

There are a few uses that are not being assessed because they are either restricted in 

California or the crop is not grown in California. The following uses will not be assessed 

because they are not grown in California: cocoa, coffee, banana, plantain, pineapple, and 

soybeans. If the use patterns indicate that these crops are grown in CA in the future, the 

conclusions of this assessment may need to be revisited. The following uses, listed with 

their respective labels, will not be assessed because they are restricted in California: 

• Label 100-1217, Label 48273-00027: asparagus, broad beans, dried beans, moth 

beans, mung beans, rice beans, lima beans, snap beans, tepary beans, urd beans, 

catjang, cowpea/blackeyed pea, garbanzos, guar, lentils, lupine, pastures, and 

dried and southern peas. 

• Label 66222-00130 and Label 74530-00032: asparagus, broad beans, dried 

beans, moth beans, mung beans, rice beans, lima beans, tepary beans, urd beans, 

catjang, cocoa, cowpea/blackeyed pea, garbanzos, guar, lentils, lupine, pastures, 

dried and southern peas, and persimmon. 

• Label 82542-00003 and Label 82557-00001: asparagus, broad beans, dried 

beans, moth beans, mung beans, rice beans, lima beans, tepary beans, urd beans, 

catjang, cocoa, cowpea/blackeyed pea, garbanzos, guar, lentils, lupine, pastures, 

and dried and southern peas. 

The labels listed below are Section 24c labels for paraquat in other states and do not 

apply to California. As a result, they will not be assessed in this assessment. 

• Label CO06000700: alfalfa. 

• Label ID08000900 and WY08000600: clover. 

• Label MS05002100: agricultural crops/soils. 

• Label NC06000400: sage. 

• Label NJ06000100 and VA06000200: unspecified vegetables. 

• Label TX06001700, TX08001900, and TX08002000: cotton 

21
Table 2-2 presents the uses and corresponding application rates and methods of 

application considered in this assessment. 

Table 2-2 Paraquat Uses Assessed for the CRLF1

Use (Application Method) 

Max. Single 

Appl. Rate 

(lb ai/A) 

Max. Number of 

Application per 

Year 

Application 

Method 

ACEROLA (WEST INDIES CHERRY), 

APRICOT, CHERRY, NECTARINE, PEACH, 

PLUM, KIWI FRUIT 

DECIDUOUS FRUIT TREES (UNSPECIFIED), 

FOREST TREES (SOFTWOODS - CONIFERS) 1.0 3 Ground 

Aircraft 

AGRICULTURAL FALLOW/IDLELAND 1.0 5 Ground 

AIRPORTS/LANDING FIELDS, 

COMMERCIAL/INSTITUTIONAL/INDUSTRIAL 

PREMISES/EQUIPMENT (OUTDOOR), 

NONAGRICULTURAL AREAS (PUBLIC 

HEALTH USE), NONAGRICULTURAL 

RIGHTS-OF-WAY/FENCEROWS/HEDGEROWS 1.0 10 Ground 

Aircraft 

ALFALFA, BROCCOLI, ENDIVE (ESCAROLE), 

LETTUCE Ground 

RHUBARB 1.0 2 Ground 

ALMOND, BEECH NUT, BRAZIL NUT, 

BUTTERNUT, CASHEW, CHESTNUT, 

CHINQUAPIN, FILBERT (HAZELNUT), 

HICKORY NUT, MACADAMIA NUT 

(BUSHNUT), PECAN, PISTACHIO, WALNUT 

(ENGLISH/BLACK), APPLE, FIG, ORCHARDS 

(UNSPECIFIED), PEAR, PRUNE, 

ARBORVITAE/ ASH /ELM /FIR /OAK /PINE 

(FOREST/SHELTERBELT), AVOCADO, 

CALAMONDIN, CITRON (CITRUS), 

GRAPEFRUIT, KUMQUAT, LEMON, LIME, 

ORANGE, PAPAYA, PUMMELO (SHADDOCK), 

GRAPES, ORNAMENTAL AND/OR SHADE 

TREES, WOODY SHRUBS AND VINES 1.0 5 Ground 

ARTICHOKE Ground 

Aircraft 

CABBAGE, CABBAGE - CHINESE, 

COLLARDS, 1.0 3 Ground 

Aircraft 

ASPARAGUS, RICE, BARLEY, SAFFLOWER 

(UNSPECIFIED), SMALL GRAINS 

(UNSPECIFIED), SORGHUM, WHEAT, 

CAULIFLOWER, CORN (UNSPECIFIED)/ 

SWEET/ FIELD/ POP, SUNFLOWER, COTTON 

(UNSPECIFIED) 1.0 3 Ground 

22
Use (Application Method) 

Max. Single 

Appl. Rate 

(lb ai/A) 

Max. Number of Application 

Application per Method 

Year 

Ground 

BEANS - SUCCULENT (LIMA/SNAP), 

CARROT (INCLUDING TOPS), PEAS 

(UNSPECIFIED), PEPPER, CHAYOTE, 

CUCUMBER, GOURD (WAX) - CHINESE, 

GOURDS, MELONS - BITTER (BALSAM 

PEAR)/ CANTALOUPE/ CITRON /MUSK 

/WATER, PEPINO (MELON PEAR), PUMPKIN, 

SQUASH (ALL OR UNSPECIFIED), 

EGGPLANT, GROUNDCHERRY 

(STRAWBERRY TOMATO/TOMATILLO), 

TOMATILLO, TOMATO, TURNIP (GREENS) Aircraft 

GARLIC, ONION 1.0 1 Ground 

BLACKBERRY, BOYSENBERRY, 

ELDERBERRY, GOOSEBERRY, 

HUCKLEBERRY, LOGANBERRY, 

RASPBERRY (BLACK - RED), BLUEBERRY, 

CURRANT 1.0 5 Ground 

Aircraft 

BRUSSELS SPROUTS, KALE, VEGETABLES 

(UNSPECIFIED), KOHLRABI 1.0 3 Ground 

CITRUS HYBRIDS OTHER THAN TANGELO, 

TANGELO, CRABAPPLE, LOQUAT, QUINCE, 

MAYHAW (HAWTHORN) 1.0 5 Ground 

CORN (SILAGE), STRAWBERRY 

GUAR Ground 

Aircraft 

PASTURES, POTATO - WHITE/IRISH 0.5 3 Ground 

Aircraft 

GHERKIN, SUGAR BEET Ground 

GRASSES GROWN FOR SEED 1.0 3 Ground 

GINGER 1.0 6 Ground 

GUAVA 1.0 10 Ground 

Aircraft 

MELONS 0.3 1 Ground 

Aircraft 

MINT/PEPPERMINT/SPEARMINT 0.8 2 Ground 

OKRA 1.0 1 Ground 

OLIVE 1.0 4 Ground 

23
Use (Application Method) 

Max. Single 

Appl. Rate 

(lb ai/A) 

Max. Number of Application 

Application per Method 

Year 

PASSION FRUIT (GRANADILLA) 1.0 10 Ground 

PEANUTS (UNSPECIFIED) 0.3 1 Ground 

PERSIMMON 0.9 5 Ground 1

 Uses assessed based on memorandum from SRRD dated February 5, 2009 

Provided below, Figure 2-1 shows the estimated poundage of paraquat uses across the 

United States. The map was downloaded from a U.S. Geological Survey (USGS), 

National Water Quality Assessment Program (NAWQA) website. 

Figure 2-1 Paraquat Use in Total Pounds per County 

The Agency’s Biological and Economic Analysis Division (BEAD) provides an analysis 

of both national- and county-level usage information (Memo: County-Level Usage for 

Propanil, Paraquat Dichloride, Pendimehtalin, Myclobutanil, Prometryn, Dicofol, 

Alachlor, and Endosulfan in California in Support of a Red Legged Frog Endangered 

24
Species Assessment, 04 February 2009) using state-level usage data obtained from 

USDA-NASS1

, Doane (www.doane.com; the full dataset is not provided due to its 

proprietary nature) and the California’s Department of Pesticide Regulation Pesticide Use 

Reporting (CDPR PUR) database2

 . CDPR PUR is considered a more comprehensive 

source of usage data than USDA-NASS or EPA proprietary databases, and thus the usage 

data reported for paraquat by county in this California-specific assessment were 

generated using CDPR PUR data. Eight years (1999-2006) of usage data were included 

in this analysis. Data from CDPR PUR were obtained for every pesticide application 

made on every use site at the section level (approximately one square mile) of the public 

land survey system. BEAD summarized these data to the county level by site, pesticide, 

and unit treated. Calculating county-level usage involved summarizing across all 

applications made within a section and then across all sections within a county for each 

use site and for each pesticide. The county level usage data that were calculated include: 

average annual pounds applied, average annual area treated, and average and maximum 

application rate across all eight years. The units of area treated are also provided where 

available. 

 

The usage data reported by CDPR PUR summarizing paraquat’s usage for all California 

use sites is provided below in Table 2-3. The uses range from commercial and industrial 

non-agricultural uses to agricultural uses such as fruits, vegetables, feed for livestock, and 

ornamental shrubs, flowers, and trees. The uses considered in this risk assessment 

represent all currently registered uses according to a review of all current labels. No 

other uses are relevant to this assessment. Any reported use, such as may be seen in the 

CDPR PUR database, represent either historic uses that have been canceled, mis-reported 

uses, or mis-use. Historical uses, mis-reported uses, and misuse are not considered part 

of the federal action and, therefore, are not considered in this assessment. 

Table 2-3 Summary of California Department of Pesticide Registration (CDPR) 

Pesticide Use Reporting (PUR) Data from 1999 to 2006 for Currently Registered 

Paraquat Uses 

Site Name Average 

Pounds All 

Uses 

Avg 

App 

Rate All 

Uses 

Avg 

95th% 

App 

Rate 

Avg 

99th% 

App 

Rate 

Avg Max App 

Rate 

ALFALFA 3,034.6 0.7 1.0 1.3 2.4

ALMOND 8,778.1 0.9 1.4 2.2 5.5

APPLE 234.2 0.8 1.6 2.5 3.2

APRICOT 79.6 0.9 1.4 2.1 3.0

ARTICHOKE, GLOBE 432.8 0.9 1.1 1.1 2.5

ASPARAGUS 189.8 1.0 1.3 1.3 1.3

AVOCADO 31.1 0.9 2.0 2.4 2.4

 

1

 United States Depart of Agriculture (USDA), National Agricultural Statistics Service (NASS) Chemical 

Use Reports provide summary pesticide usage statistics for select agricultural use sites by chemical, crop 

and state. See http://www.usda.gov/nass/pubs/estindx1.htm#agchem. 2

 The California Department of Pesticide Regulation’s Pesticide Use Reporting database provides a census 

of pesticide applications in the state. See http://www.cdpr.ca.gov/docs/pur/purmain.htm. 

25
Site Name Average 

Pounds All 

Uses 

Avg 

App 

Rate All 

Uses 

Avg Avg 

95th% 99th% 

App 

Rate 

App Avg Max App 

Rate Rate 

BARLEY 21.2 0.7 0.8 0.8 0.8

BEAN, DRIED 43.4 0.9 1.1 1.1 1.1

BEAN, SUCCULENT 46.6 1.1 1.5 2.5 2.9

BEAN, UNSPECIFIED 11.9 1.0 1.8 1.8 1.8

BLACKBERRY 18.1 0.8 1.0 4.2 4.2

BLUEBERRY 317.0 0.9 2.2 2.6 2.6

BOYSENBERRY 8.8 0.7 1.2 1.2 1.2

BROCCOLI 59.3 0.9 1.2 1.2 1.2

BRUSSELS SPROUT 0.2 1.3 1.3 1.3 1.3

BUILDINGS/NON-AG OUTDROOR 2.9 1.3 1.3 1.3 1.3

CABBAGE 9.6 1.1 1.5 1.5 1.5

CANTALOUPE 57.6 0.8 1.1 1.1 1.1

CARROT 46.7 1.1 1.1 1.1 1.1

CAULIFLOWER 20.5 1.1 1.2 2.2 2.2

CHERRY 439.4 0.9 1.6 2.3 3.5

CHESTNUT 16.3 0.7 1.3 1.3 1.3

CHINESE CABBAGE (NAPPA) 21.5 1.0 1.1 1.1 1.1

CHRISTMAS TREE 2.2 0.9 0.9 0.9 0.9

CITRUS 24.9 0.8 1.1 1.3 1.3

COLLARD 8.5 1.0 1.0 1.0 1.0

CORN (FORAGE - FODDER) 576.1 1.0 1.3 1.4 2.4

CORN, HUMAN CONSUMPTION 26.6 1.5 1.6 1.6 1.6

COTTON 15,389.3 0.5 1.0 1.1 2.6

CUCUMBER 48.6 1.0 1.8 2.0 2.0

DITCH BANK 9.7 2.3 4.8 4.8 4.8

EGGPLANT 19.5 1.1 1.6 1.6 1.6

ENDIVE (ESCAROLE) 2.2 1.4 1.4 1.4 1.4

FIG 56.6 0.8 1.1 1.3 1.3

FORAGE HAY/SILAGE 7.4 0.7 0.9 0.9 0.9

GARLIC 27.2 1.0 1.0 1.0 1.0

GRAIN 7.4 0.6 0.6 0.6 0.6

GRAPE 4,519.4 0.9 1.3 1.5 2.9

GRAPE, WINE 2,574.0 0.7 1.2 1.6 2.6

GRAPEFRUIT 22.6 0.8 1.2 3.1 3.1

GRASS, SEED 23.1 1.0 1.0 1.0 1.0

INDUSTRIAL SITE 0.2 0.9 0.9 0.9 0.9

KALE 0.4 1.2 1.2 1.2 1.2

KIWI 95.0 1.8 3.9 5.2 5.2

26
Site Name Average 

Pounds All 

Uses 

Avg 

App 

Rate All 

Uses 

Avg Avg 

95th% 99th% 

App 

Rate 

App Avg Max App 

Rate Rate 

KOHLRABI 0.6 1.3 1.3 1.3 1.3

LANDSCAPE MAINTENANCE 6.6 1.7 2.2 2.2 2.2

LEMON 96.8 0.9 1.5 3.7 3.7

LETTUCE, HEAD 320.1 1.3 1.9 2.5 3.9

LETTUCE, LEAF 57.4 1.1 1.5 1.5 1.5

MELON 41.8 0.9 1.0 1.0 1.0

MINT 373.6 0.7 0.9 2.6 2.9

NECTARINE 894.2 0.7 1.2 1.9 2.8

N-GRNHS FLOWER 2.8 1.2 1.5 1.6 1.6

N-GRNHS PLANTS IN CONTAINERS 5.0 14.6 28.0 28.0 28.0

N-OUTDR FLOWER 10.7 0.8 1.1 2.7 2.7

N-OUTDR PLANTS IN CONTAINERS 28.5 1.4 2.2 2.8 2.8

N-OUTDR TRANSPLANTS 15.1 0.9 3.0 3.5 3.5

OLIVE 132.8 0.9 1.9 2.2 2.5

ONION, DRY 86.1 1.1 1.4 1.5 1.5

ONION, GREEN 11.9 1.1 1.4 1.4 1.4

ORANGE 575.9 1.1 1.6 3.0 4.9

ORCHARD FLOOR 2.7 0.9 1.1 1.1 1.1

PASTURELAND 15.2 0.8 1.1 1.1 1.1

PEACH 747.5 0.9 1.5 1.8 3.2

PEANUT 1.3 1.2 1.2 1.2 1.2

PEAR 139.0 0.8 1.3 2.6 3.2

PEAS 8.8 1.2 1.6 1.6 1.6

PECAN 26.1 1.1 2.8 2.8 2.8

PEPPER, FRUITING 125.1 1.0 2.1 2.3 2.4

PEPPER, SPICE 411.3 1.0 1.8 1.8 2.4

PERSIMMON 3.3 0.9 1.2 1.2 1.2

PISTACHIO 1,511.7 0.8 1.3 2.1 4.8

PLUM 727.8 0.7 1.1 1.3 2.5

POTATO 46.3 1.0 1.2 1.7 1.7

PRUNE 515.7 0.8 1.3 2.1 3.3

PUMPKIN 1.3 0.9 0.9 0.9 0.9

RANGELAND 27.3 0.6 0.6 0.6 0.6

RASPBERRY 67.7 1.1 1.5 2.3 3.7

REGULATORY PEST CONTROL 5.1 0.7 0.7 0.7 0.7

RESEARCH COMMODITY 9.7 0.8 1.5 2.4 2.4

RHUBARB 3.1 1.3 1.3 1.3 1.3

RICE 58.8 1.3 2.0 2.0 2.0

27
Site Name Average 

Pounds All 

Uses 

Avg 

App 

Rate All 

Uses 

Avg Avg 

95th% 99th% 

App 

Rate 

App Avg Max App 

Rate Rate 

RIGHTS OF WAY 407.3 0.9 1.0 1.0 1.0

SAFFLOWER 98.1 0.9 1.2 1.9 1.9

SORGHUM (FORAGE - FODDER/ MILO) 10.6 0.4 0.5 0.5 0.5

SOYBEAN 9.4 0.5 0.5 0.5 0.5

SQUASH 14.1 1.1 1.3 1.3 1.3

SQUASH, SUMMER 9.9 1.6 4.3 4.3 4.3

SQUASH, WINTER 2.6 0.8 0.9 0.9 0.9

SQUASH, ZUCCHINI 6.5 0.9 1.0 1.0 1.0

STRAWBERRY 75.1 0.9 1.6 2.5 2.9

STRUCTURAL PEST CONTROL 1.4 0.5 0.5 0.5 0.5

SUDANGRASS 10.2 0.7 0.8 0.8 0.8

SUGARBEET 61.2 1.0 1.2 1.2 1.2

SUNFLOWER 326.7 0.8 1.9 2.7 2.7

TANGELO 15.9 1.1 1.7 1.7 1.7

TOMATO 270.0 0.9 1.2 1.7 1.8

UNCULTIVATED AG 394.1 1.0 1.9 2.4 2.7

UNCULTIVATED NON-AG 60.8 1.3 1.9 4.8 4.9

VEGETABLE 4.8 0.7 0.9 0.9 0.9

VERTEBRATE CONTROL 3.2 0.2 0.5 0.5 0.5

WALNUT 796.6 0.8 1.3 1.9 4.0

WATERMELON 43.1 1.2 2.4 2.4 2.4

WHEAT 41.2 0.8 1.0 1.0 1.0

2.5 Assessed Species 

The CRLF was federally listed as a threatened species by U.S. FWS effective June 24, 

1996 (U.S. FWS 1996). It is one of two subspecies of the red-legged frog and is the 

largest native frog in the western United States (U.S. FWS 2002). A brief summary of 

information regarding CRLF distribution, reproduction, diet, and habitat requirements is 

provided in Sections 2.5.1 through 2.5.4, respectively. Further information on the status, 

distribution, and life history of and specific threats to the CRLF is provided in 

Attachment I. 

Final critical habitat for the CRLF was designated by U.S. FWS on April 13, 2006 (U.S. 

FWS 2006; 71 FR 19244-19346). Further information on designated critical habitat for 

the CRLF is provided in Section 2.6. 

2.5.1 Distribution 

The CRLF is endemic to California and Baja California (Mexico) and historically 

inhabited 46 counties in California including the Central Valley and both coastal and 

28
interior mountain ranges (U.S. FWS 1996). Its range has been reduced by about 70%, 

and the species currently resides in 22 counties in California (U.S. FWS 1996). The 

species has an elevational range of near sea level to 1,500 meters (5,200 feet) (Jennings 

and Hayes 1994); however, nearly all of the known CRLF populations have been 

documented below 1,050 meters (3,500 feet) (U.S. FWS 2002). 

Populations currently exist along the northern California coast, northern Transverse 

Ranges (U.S. FWS 2002), foothills of the Sierra Nevada (5-6 populations), and in 

southern California south of Santa Barbara (two populations) (Fellers 2005a). Relatively 

larger numbers of CRLFs are located between Marin and Santa Barbara Counties 

(Jennings and Hayes 1994). A total of 243 streams or drainages are believed to be 

currently occupied by the species, with the greatest numbers in Monterey, San Luis 

Obispo, and Santa Barbara counties (U.S. FWS 1996). Occupied drainages or watersheds 

include all bodies of water that support CRLFs (i.e., streams, creeks, tributaries, 

associated natural and artificial ponds, and adjacent drainages), and habitats through 

which CRLFs can move (i.e., riparian vegetation, uplands) (U.S. FWS 2002). 

The distribution of CRLFs within California is addressed in this assessment using four 

categories of location including recovery units, core areas, designated critical habitat, and 

known occurrences of the CRLF reported in the California Natural Diversity Database 

(CNDDB) that are not included within core areas and/or designated critical habitat (see 

Attachment I). Recovery units, core areas, and other known occurrences of the CRLF 

from the CNDDB are described in further detail in Attachment I, and designated critical 

habitat is addressed in Section 2.6. Recovery units are large areas defined at the 

watershed level that have similar conservation needs and management strategies. The 

recovery unit is primarily an administrative designation, and land area within the 

recovery unit boundary is not exclusively CRLF habitat. Core areas are smaller areas 

within the recovery units that comprise portions of the species’ historic and current range 

and have been determined by U.S. FWS to be important in the preservation of the 

species. Designated critical habitat is generally contained within the core areas, although 

a number of critical habitat units are outside the boundaries of core areas, but within the 

boundaries of the recovery units. Additional information on CRLF occurrences from the 

CNDDB is used to cover the current range of the species not included in core areas 

and/or designated critical habitat, but within the recovery units. 

Other Known Occurrences from the CNDBB 

The CNDDB provides location and natural history information on species found in 

California. The CNDDB serves as a repository for historical and current species location 

sightings. Information regarding known occurrences of CRLFs outside of the currently 

occupied core areas and designated critical habitat is considered in defining the current 

range of the CRLF. See: http://www.dfg.ca.gov/bdb/html/cnddb_info.html for additional 

information on the CNDDB. 

29
Recovery Units

1. Sierra Nevada Foothills and Central Valley

2. North Coast Range Foothills and Western 

Sacramento River Valley

3. North Coast and North San Francisco Bay

4. South and East San Francisco Bay

5. Central Coast

6. Diablo Range and Salinas Valley

7. Northern Transverse Ranges and Tehachapi

Mountains

8. Southern Transverse and Peninsular Ranges

Core Areas

1. Feather River

2. Yuba River- S. Fork Feather River

3. Traverse Creek/ Middle Fork/ American R. Rubicon

4. Cosumnes River

5. South Fork Calaveras River*

6. Tuolumne River*

7. Piney Creek*

8. Cottonwood Creek

9. Putah Creek – Cache Creek*

10. Lake Berryessa Tributaries

11. Upper Sonoma Creek

12. Petaluma Creek – Sonoma Creek

13. Pt. Reyes Peninsula

14. Belvedere Lagoon

15. Jameson Canyon – Lower Napa River

16. East San Francisco Bay

17. Santa Clara Valley

18. South San Francisco Bay

19. Watsonville Slough-Elkhorn Slough

20. Carmel River – Santa Lucia

21. Gablan Range

22. Estero Bay

23. Arroyo Grange River

24. Santa Maria River – Santa Ynez River

25. Sisquoc River

26. Ventura River – Santa Clara River

27. Santa Monica Bay – Venura Coastal Streams

28. Estrella River

29. San Gabriel Mountain*

30. Forks of the Mojave*

31. Santa Ana Mountain*

32. Santa Rosa Plateau

33. San Luis Ray*

34. Sweetwater*

35. Laguna Mountain*

* Core areas that were historically occupied by the California red-legged frog are not included in the map

Recovery Units

1. Sierra Nevada Foothills and Central Valley

2. North Coast Range Foothills and Western 

Sacramento River Valley

3. North Coast and North San Francisco Bay

4. South and East San Francisco Bay

5. Central Coast

6. Diablo Range and Salinas Valley

7. Northern Transverse Ranges and Tehachapi

Mountains

8. Southern Transverse and Peninsular Ranges

Core Areas

1. Feather River

2. Yuba River- S. Fork Feather River

3. Traverse Creek/ Middle Fork/ American R. Rubicon

4. Cosumnes River

5. South Fork Calaveras River*

6. Tuolumne River*

7. Piney Creek*

8. Cottonwood Creek

9. Putah Creek – Cache Creek*

10. Lake Berryessa Tributaries

11. Upper Sonoma Creek

12. Petaluma Creek – Sonoma Creek

13. Pt. Reyes Peninsula

14. Belvedere Lagoon

15. Jameson Canyon – Lower Napa River

16. East San Francisco Bay

17. Santa Clara Valley

18. South San Francisco Bay

19. Watsonville Slough-Elkhorn Slough

20. Carmel River – Santa Lucia

21. Gablan Range

22. Estero Bay

23. Arroyo Grange River

24. Santa Maria River – Santa Ynez River

25. Sisquoc River

26. Ventura River – Santa Clara River

27. Santa Monica Bay – Venura Coastal Streams

28. Estrella River

29. San Gabriel Mountain*

30. Forks of the Mojave*

31. Santa Ana Mountain*

32. Santa Rosa Plateau

33. San Luis Ray*

34. Sweetwater*

35. Laguna Mountain*

* Core areas that were historically occupied by the California red-legged frog are not included in the map

Figure 2-2 Recovery Unit, Core Area, Critical Habitat, and Occurrence 

Designations for CRLF 

30
2.5.2 Reproduction 

CRLFs breed primarily in ponds; however, they may also breed in quiescent streams, 

marshes, and lagoons (Fellers 2005a). According to the Recovery Plan (U.S. FWS 2002), 

CRLFs breed from November through late April. Peaks in spawning activity vary 

geographically; Fellers (2005b) reports peak spawning as early as January in parts of 

coastal central California. Eggs are fertilized as they are being laid. Egg masses are 

typically attached to emergent vegetation, such as bulrushes (Scirpus spp.) and cattails 

(Typha spp.) or roots and twigs, and float on or near the surface of the water (Hayes and 

Miyamoto 1984). Egg masses contain approximately 2000 to 6000 eggs ranging in size 

between 2 and 2.8 mm (Jennings and Hayes 1994). Embryos hatch 10 to 14 days after 

fertilization (Fellers 2005a) depending on water temperature. Egg predation is reported 

to be infrequent and most mortality is associated with the larval stage (particularly 

through predation by fish); however, predation on eggs by newts has also been reported 

(Rathburn 1998). Tadpoles require 11 to 28 weeks to metamorphose into juveniles 

(terrestrial-phase), typically between May and September (Jennings and Hayes 1994, 

U.S. FWS 2002); tadpoles have been observed to over-winter (delay metamorphosis until 

the following year) (Fellers 2005b; U.S. FWS 2002). Males reach sexual maturity at 2 

years, and females reach sexual maturity at 3 years of age; adults have been reported to 

live 8 to 10 years (U.S. FWS 2002). Figure 2-3 depicts CRLF annual reproductive 

timing. 

J F M A M J J A S O N D

Light Blue = Breeding/Egg Masses

Green = Tadpoles (except those that over-winter)

Orange = 

Adults and juveniles can be present all year

Young Juveniles

J F M A M J J A S O N D

Light Blue = Breeding/Egg Masses

Green = 

Orange = 

Adults and juveniles can be present all year

Tadpoles (except those that over-winter)

Young Juveniles

Figure 2-3 CRLF Reproductive Events by Month 

2.5.3 Diet 

Although the diet of CRLF aquatic-phase larvae (tadpoles) has not been studied 

specifically, it is assumed that their diet is similar to that of other frog species, with the 

aquatic phase feeding exclusively in water and consuming diatoms, algae, and detritus 

(U.S. FWS 2002). Tadpoles filter and entrap suspended algae (Seale and Beckvar 1980) 

via mouthparts designed for effective grazing of periphyton (Wassersug 1984; 

Kupferberg et al. 1994; Kupferberg 1997; Altig and McDiarmid 1999). 

Juvenile and adult CRLFs forage in aquatic and terrestrial habitats, and their diet differs 

greatly from that of larvae. The main food source for juvenile aquatic- and terrestrialphase CRLFs is thought to be aquatic and terrestrial invertebrates found along the 

shoreline and on the water surface. Hayes and Tennant (1985) report, based on a study 

31
examining the gut content of 35 juvenile and adult CRLFs, that the species feeds on as 

many as 42 different invertebrate taxa, including Arachnida, Amphipoda, Isopoda, 

Insecta, and Mollusca. The most commonly observed prey species were larval alderflies 

(Sialis cf. californica), pillbugs (Armadilliadrium vulgare), and water striders (Gerris sp). 

The preferred prey species, however, was the sowbug (Hayes and Tennant 1985). This 

study suggests that CRLFs forage primarily above water, although the authors note other 

data reporting that adults also feed under water, are cannibalistic, and consume fish. For 

larger CRLFs, over 50% of the prey mass may consists of vertebrates such as mice, frogs, 

and fish, although aquatic and terrestrial invertebrates were the most numerous food 

items (Hayes and Tennant 1985). For adults, feeding activity takes place primarily at 

night; for juveniles feeding occurs during the day and at night (Hayes and Tennant 1985). 

2.5.4 Habitat 

CRLFs require aquatic habitat for breeding, but also use other habitat types including 

riparian and upland areas throughout their life cycle. CRLF use of their environment 

varies; they may complete their entire life cycle in a particular habitat or they may utilize 

multiple habitat types. Overall, populations are most likely to exist where multiple 

breeding areas are embedded within varying habitats used for dispersal (U.S. FWS 2002). 

Generally, CRLFs utilize habitat with perennial or near-perennial water (Jennings et al.

1997). Dense vegetation close to water, shading, and water of moderate depth are habitat 

features that appear especially important for CRLF (Hayes and Jennings 1988). 

Breeding sites include streams, deep pools, backwaters within streams and creeks, ponds, 

marshes, sag ponds (land depressions between fault zones that have filled with water), 

dune ponds, and lagoons. Breeding adults have been found near deep (0.7 m) still or slow 

moving water surrounded by dense vegetation (U.S. FWS 2002); however, the largest 

number of tadpoles have been found in shallower pools (0.26 – 0.5 m) (Reis 1999). Data 

indicate that CRLFs do not frequently inhabit vernal pools, as conditions in these habitats 

generally are not suitable (Hayes and Jennings 1988). 

CRLFs also frequently breed in artificial impoundments such as stock ponds, although 

additional research is needed to identify habitat requirements within artificial ponds (U.S. 

FWS 2002). Adult CRLFs use dense, shrubby, or emergent vegetation closely associated 

with deep-water pools bordered with cattails and dense stands of overhanging vegetation 

(http://ecos.fws.gov/speciesProfile/SpeciesReport.do?spcode=D02D). 

In general, dispersal and habitat use depends on climatic conditions, habitat suitability, 

and life stage. Adults rely on riparian vegetation for resting, feeding, and dispersal. The 

foraging quality of the riparian habitat depends on moisture, composition of the plant 

community, and presence of pools and backwater aquatic areas for breeding. CRLFs can 

be found living within streams at distances up to 3 km (2 miles) from their breeding site 

and have been found up to 30 m (100 feet) from water in dense riparian vegetation for up 

to 77 days (U.S. FWS 2002). 

During dry periods, the CRLF is rarely found far from water, although it will sometimes 

disperse from its breeding habitat to forage and seek other suitable habitat under downed 

32
trees or logs, industrial debris, and agricultural features (U.S. FWS 2002). According to 

Jennings and Hayes (1994), CRLFs also use small mammal burrows and moist leaf litter 

as habitat. In addition, CRLFs may also use large cracks in the bottom of dried ponds as 

refugia; these cracks may provide moisture for individuals avoiding predation and solar 

exposure (Alvarez 2000). 

2.6 Designated Critical Habitat 

In a final rule published on April 13, 2006, 34 separate units of critical habitat were 

designated for the CRLF by U.S. FWS (U.S. FWS 2006; FR 51 19244-19346). A 

summary of the 34 critical habitat units relative to U.S. FWS-designated recovery units 

and core areas (previously discussed in Section 2.5.1) is provided in Attachment I. 

‘Critical habitat’ is defined in the ESA as the geographic area occupied by the species at 

the time of the listing where the physical and biological features necessary for the 

conservation of the species exist, and there is a need for special management to protect 

the listed species. It may also include areas outside the occupied area at the time of 

listing if such areas are ‘essential to the conservation of the species.’ All designated 

critical habitat for the CRLF was occupied at the time of listing. Critical habitat receives 

protection under Section 7 of the ESA (Section 7) through prohibition against destruction 

or adverse modification with regard to actions carried out, funded, or authorized by a 

federal Agency. Section 7 requires consultation on federal actions that are likely to result 

in the destruction or adverse modification of critical habitat. 

To be included in a critical habitat designation, the habitat must be ‘essential to the 

conservation of the species.’ Critical habitat designations identify, to the extent known 

using the best scientific and commercial data available, habitat areas that provide 

essential life cycle needs of the species or areas that contain certain primary constituent 

elements (PCEs) (as defined in 50 CFR 414.12(b)). PCEs include, but are not limited to, 

space for individual and population growth and for normal behavior; food, water, air, 

light, minerals, or other nutritional or physiological requirements; cover or shelter; sites 

for breeding, reproduction, rearing (or development) of offspring; and habitats that are 

protected from disturbance or are representative of the historic geographical and 

ecological distributions of a species. The designated critical habitat areas for the CRLF 

are considered to have the following PCEs that justify critical habitat designation: 

• Breeding aquatic habitat; 

• Non-breeding aquatic habitat; 

• Upland habitat; and 

• Dispersal habitat. 

Further description of these habitat types is provided in Attachment I. 

Occupied habitat may be included in the critical habitat only if essential features within 

the habitat may require special management or protection. Therefore, U.S. FWS does not 

include areas where existing management is sufficient to conserve the species. Critical 

33
habitat is designated outside the geographic area presently occupied by the species only 

when a designation limited to its present range would be inadequate to ensure the 

conservation of the species. For the CRLF, all designated critical habitat units contain all 

four of the PCEs, and were occupied by the CRLF at the time of the Final Rule (FR) 

listing notice in April 2006 (71 FR 19243, 2006). The FR notice designating critical 

habitat for the CRLF includes a special rule exempting routine ranching activities 

associated with livestock ranching from incidental take prohibitions. The purpose of this 

exemption is to promote the conservation of rangelands, which could be beneficial to the 

CRLF, and to reduce the rate of conversion to other land uses that are incompatible with 

CRLF conservation. Please see Attachment I for a full explanation on this special rule. 

U.S. FWS has established adverse modification standards for designated critical habitat 

(U.S. FWS 2006). Activities that may destroy or adversely modify critical habitat are 

those that alter the PCEs and jeopardize the continued existence of the species. 

Evaluation of actions related to use of paraquat that may alter the PCEs of the CRLF’s 

critical habitat form the basis of the critical habitat impact analysis. According to U.S. 

FWS (2006), activities that may affect critical habitat and therefore result in adverse 

effects to the CRLF include, but are not limited to the following: 

(1) Significant alteration of water chemistry or temperature to levels beyond the 

tolerances of the CRLF that result in direct or cumulative adverse effects to 

individuals and their life-cycles. 

(2) Alteration of chemical characteristics necessary for normal growth and viability 

of juvenile and adult CRLFs. 

(3) Significant increase in sediment deposition within the stream channel or pond or 

disturbance of upland foraging and dispersal habitat that could result in 

elimination or reduction of habitat necessary for the growth and reproduction of 

the CRLF by increasing the sediment deposition to levels that would adversely 

affect their ability to complete their life cycles. 

(4) Significant alteration of channel/pond morphology or geometry that may lead to 

changes to the hydrologic functioning of the stream or pond and alter the timing, 

duration, water flows, and levels that would degrade or eliminate the CRLF 

and/or its habitat. Such an effect could also lead to increased sedimentation and 

degradation in water quality to levels that are beyond the CRLF’s tolerances. 

(5) Elimination of upland foraging and/or aestivating habitat or dispersal habitat. 

(6) Introduction, spread, or augmentation of non-native aquatic species in stream 

segments or ponds used by the CRLF. 

(7) Alteration or elimination of the CRLF’s food sources or prey base (also 

evaluated as indirect effects to the CRLF). 

As previously noted in Section 2.1, the Agency believes that the analysis of direct and 

indirect effects to listed species provides the basis for an analysis of potential effects on 

the designated critical habitat. Because paraquat is expected to directly impact living 

organisms within the action area, critical habitat analysis for paraquat is limited in a 

practical sense to those PCEs of critical habitat that are biological or that can be 

reasonably linked to biologically mediated processes. 

34
2.7 Action Area 

For listed species assessment purposes, the action area is considered to be the area 

affected directly or indirectly by the federal action and not merely the immediate area 

involved in the action (50 CFR 402.02). It is recognized that the overall action area for 

the national registration of paraquat is likely to encompass considerable portions of the 

United States based on the large array of agricultural uses. However, the scope of this 

assessment limits consideration of the overall action area to those portions that may be 

applicable to the protection of the CRLF and its designated critical habitat within the state 

of California. The Agency’s approach to defining the action area under the provisions of 

the Overview Document (U.S. EPA 2004) considers the results of the risk assessment 

process to establish boundaries for that action area with the understanding that exposures 

below the Agency’s defined Levels of Concern (LOCs) constitute a no-effect threshold. 

For the purposes of this assessment, attention will be focused on the footprint of the 

action (i.e., the area where pesticide application occurs), plus all areas where offsite 

transport (i.e., spray drift, downstream dilution, etc.) may result in potential exposure 

within the state of California that exceeds the Agency’s LOCs. 

Deriving the geographical extent of this portion of the action area is based on 

consideration of the types of effects that paraquat may be expected to have on the 

environment, the exposure levels to paraquat that are associated with those effects, and 

the best available information concerning the use of paraquat and its fate and transport 

within the state of California. Specific measures of ecological effect that define the 

action area include any direct and indirect toxic effect and any potential modification of 

its critical habitat, including reduction in survival, growth, and fecundity as well as the 

full suite of sublethal effects available in the effects literature. Therefore, the action area 

extends to a point where environmental exposures are below any measured lethal or 

sublethal effect threshold for any biological entity at the whole organism, organ, tissue, 

and cellular level of organization. In situations where it is not possible to determine the 

threshold for an observed effect, the action area is not spatially limited and is assumed to 

be the entire state of California. 

The definition of action area requires a stepwise approach that begins with an 

understanding of the federal action. The federal action is defined by the currently labeled 

uses for paraquat. An analysis of labeled uses and review of available product labels was 

completed. Several of the currently labeled uses are special local needs (SLN) uses or are 

restricted to specific states other than California and are excluded from this assessment. 

In addition, a distinction has been made between food use crops and those that are nonfood/non-agricultural uses. The agricultural uses as well as the non-agricultural and nonfood uses relevant to the CRLF can be found in Table 2-2. 

Following a determination of the assessed uses, an evaluation of the potential “footprint” 

of paraquat use patterns (i.e., the area where pesticide application occurs) is determined. 

This “footprint” represents the initial area of concern, based on an analysis of available 

land cover data for the state of California. The initial area of concern is defined as all 

land cover types and the stream reaches within the land cover areas that represent the 

35
labeled uses described above. A map representing all the land cover types that make up 

the initial area of concern for paraquat is presented in Figure 2-4. 

The uses represented by paraquat are depicted by the following land cover types: 

cultivated crops (51%), developed/high intensity (0.83%), developed/low intensity 

(1.7%), developed/open space (0.83%), forest (9.9%), orchards/vineyards (33%), and 

pasture/hay (2.5%). Cultivated crops are areas used for the production of annual crops, 

such as corn, soybeans, vegetables, and cotton. This class also includes all land being 

actively tilled. Developed/high intensity areas are where people reside, or work in high 

numbers. The impervious surfaces account for 80-100% of the total cover. 

Developed/low intensity areas include a mixture of constructed materials and vegetation 

with impervious surfaces accounting for 20-40% of total cover. Developed/open space 

includes areas with a mixture of constructed materials, but mainly vegetation in the form 

of lawn grasses. Impervious surfaces account for lass than 20% of total cover. Forest 

represents deciduous, evergreen, and mixed vegetation. These areas are dominated by 

trees that are generally greater than 5 meters tall, and greater than 20% of total vegetation 

cover. Orchards/vineyards represent areas used for the cultivation of crops, such as fruits 

and nuts, which grow on vines or trees. Lastly, pasture/hay represents areas of grasses, 

legumes, or grass-legume mixtures planted for livestock grazing or the production of seed 

or hay crops. Typically, pasture/hay vegetation accounts for greater than 20% of total 

vegetation. For more information regarding which specific uses are represented by each 

land cover type, see Appendix D. 

Paraquat appears to be applied in 55 out of 58 counties in California per the CAPUR 

database. In addition, paraquat has both a wide range of agricultural and non-agricultural 

uses throughout the state. Likewise, when modeling with AgDRIFT, the buffer zone 

needed to prevent ecological effects from spray drift is greater than 1,000 feet. As a 

result, the entire state of California will be the action area. 

36
Figure 2-4 Initial area of concern, or “footprint” of potential use, for Paraquat 

37
Once the initial area of concern is defined, the next step is to define the potential 

boundaries of the action area by determining the extent of offsite transport via spray drift 

and runoff where exposure of one or more taxonomic groups to the pesticide exceeds the 

listed species LOCs. 

As previously discussed, the action area is defined by the most sensitive measure of 

direct and indirect ecological toxic effects including reduction in survival, growth, 

reproduction, and the entire suite of sublethal effects from valid, peer-reviewed studies. 

The primary route of environmental dissipation of paraquat dichloride is adsorption to 

biological material and soil clay particles. Paraquat dichloride has been shown to adsorb 

to clay crystalline lattices with no apparent correlation between organic matter content 

and paraquat adsorption. Paraquat does not hydrolyze, does not photodegrade in aqueous 

solutions, and is resistant to microbial degradation under aerobic and anaerobic 

conditions. Paraquat is also very immobile in soil and is not expected to volatilize once 

applied. However, spray drift could potentially be a problem because paraquat is very 

biologically active and toxic to plants and animals before it becomes adsorbed to soil clay 

particles. 

The AgDRIFT model (Version 2.01) is used to define how far from the initial area of 

concern an effect to a given species may be expected via spray drift. The spray drift 

analysis uses the most sensitive endpoint. The cocklebur (EC25 = 0.014 lb cation/A) was 

used to represent the non-listed plant species. The AgDRIFT results suggest that ground 

applications for non-listed plants need a spray drift buffer distance of approximately 361 

feet. However, aerial applications yielded a spray drift buffer distance that was out of 

range for the AgDRIFT model (1,000 feet). Therefore, a maximum spray drift distance 

of greater than 1,000 feet was derived. Further detail on the spray drift analysis is 

provided in Section 5.1.4.1. 

In addition to the buffered area from the spray drift analysis, the final action area also 

considers the downstream extent of paraquat that exceeds the LOC (discussed in Section 

5.1.4.2). An evaluation of usage information was conducted to determine the area where 

use of paraquat may impact the CRLF. This analysis is used to characterize where 

predicted exposures are most likely to occur, but does not preclude use in other portions 

of the action area. A more detailed review of the county-level use information was also 

completed. These data suggest that paraquat has historically been used on a wide variety 

of agricultural and non-agricultural uses in approximately 55 counties in California. As a 

result, since paraquat has both agricultural and non-agricultural uses, it is applied in 55 of 

58 counties, and has a buffer zone greater than 1,000 feet for aerial application, with such 

widespread use the action area is the entire state of California. 

38
2.8 Assessment Endpoints and Measures of Ecological Effect 

Assessment endpoints are defined as “explicit expressions of the actual environmental 

value that is to be protected.”3 Selection of the assessment endpoints is based on valued 

entities (e.g., CRLF, organisms important in the life cycle of the CRLF, and the PCEs of 

its designated critical habitat), the ecosystems potentially at risk (e.g., waterbodies, 

riparian vegetation, and upland and dispersal habitats), the migration pathways of 

paraquat (e.g., runoff, spray drift, etc.), and the routes by which ecological receptors are 

exposed to paraquat (e.g., direct contact, etc.). 

2.8.1 Assessment Endpoints for the CRLF 

Assessment endpoints for the CRLF include direct toxic effects on the survival, 

reproduction, and growth of the CRLF, as well as indirect effects, such as reduction of 

the prey base or effects to its habitat. In addition, potential effects to critical habitat is 

assessed by evaluating potential effects to PCEs, which are components of the habitat 

areas that provide essential life cycle needs of the CRLF. Each assessment endpoint 

requires one or more “measures of ecological effect,” defined as changes in the attributes 

of an assessment endpoint or changes in a surrogate entity or attribute in response to 

exposure to a pesticide. Specific measures of ecological effect are generally evaluated 

based on acute and chronic toxicity information from registrant-submitted guideline tests 

that are performed on a limited number of organisms. Additional ecological effects data 

from the open literature are also considered. It should be noted that assessment endpoints 

are limited to direct and indirect effects associated with survival, growth, and fecundity, 

and do not include the full suite of sublethal effects used to define the action area. 

According the Overview Document (U.S. EPA 2004), the Agency relies on acute and 

chronic effects endpoints that are either direct measures of impairment of survival, 

growth, or fecundity or endpoints for which there is a scientifically robust, peer reviewed 

relationship that can quantify the impact of the measured effect endpoint on the 

assessment endpoints of survival, growth, and fecundity. 

A complete discussion of all the toxicity data available for this risk assessment, including 

resulting measures of ecological effect selected for each taxonomic group of concern, is 

included in Section 4.0 of this document. A summary of the assessment endpoints and 

measures of ecological effect selected to characterize potential assessed direct and 

indirect CRLF risks associated with exposure to paraquat is provided in Table 2-4. 

 

3

 U.S. EPA (1992). Framework for Ecological Risk Assessment. EPA/630/R-92/001. 

39
Table 2-4 Assessment Endpoints and Measures of Ecological Effects for Direct and 

Indirect Effects of Paraquat (in terms of the Paraquat Cation) on the CRLF. 

Assessment Endpoint Measures of Ecological Effects4

Aquatic-Phase CRLF 

(Eggs, larvae, juveniles, and adults)a

 Direct Effects 

1. Survival, growth, and reproduction of 

CRLF 

1a. Rainbow trout (Oncorhynchus mykiss) LC50 

1b. No Chronic Fish Data 

Indirect Effects and Critical Habitat Effects 

2. Survival, growth, and reproduction of 

CRLF individuals via indirect effects on 

aquatic prey food supply (i.e., fish, 

freshwater invertebrates, non-vascular plants) 

2a. Algae (Navicula pelliculosa) EC50 

2b. No Chronic Invertebrate Data 

2c. Water flea (Daphnia magna) EC50

3. Survival, growth, and reproduction of 

CRLF individuals via indirect effects on 

habitat, cover, food supply, and/or primary 

productivity (i.e., aquatic plant community) 

3a. Duckweed (Lemna gibba) EC50

3b. Algae (Navicula pelliculosa) EC50

4. Survival, growth, and reproduction of 

CRLF individuals via effects to riparian 

vegetation 

4a. Monocots: Seedling Emergence, Vegetative Vigor EC25

4b. Dicots: Seedling Emergence, Vegetative Vigor EC25

Terrestrial-Phase CRLF 

(Juveniles and adults)b

 Direct Effects 

5. Survival, growth, and reproduction of 

CRLF individuals via direct effects on 

terrestrial phase adults and juveniles 

5a. Japanese Quail (Coturnix coturnix japonica) LC50 

5b. Mallard duck (Anas platyrhynchos) NOEAC 

Indirect Effects and Critical Habitat Effects 

6. Survival, growth, and reproduction of 

CRLF individuals via effects on terrestrial 

prey (i.e.,terrestrial invertebrates, small 

mammals , and frogs) 

6a. Laboratory rat (Rattus norvegicus) Female LD50 

6b. Wistar Derived Rats-Alderley Park Strain NOAEL 

6c. Honey bee (Apis mellifera) LD50 

6d. Northern Bobwhite Quail (Colinus virginianus) LD50

6e. Japanese Quail (Coturnix coturnix japonica) LC50 

6f. Mallard duck (Anas platyrhynchos) NOEAC 

7. Survival, growth, and reproduction of 

CRLF individuals via indirect effects on 

habitat (i.e., riparian and upland vegetation) 

7a. Monocots: Seedling Emergence, Vegetative Vigor EC25

7b. Dicots: Seedling Emergence, Vegetative Vigor EC25

a

 Adult frogs are no longer in the “aquatic phase” of the amphibian life cycle; however, submerged adult 

frogs are considered “aquatic” for the purposes of this assessment because exposure pathways in the water 

are considerably different that exposure pathways on land. 

b

 Birds are used as surrogates for terrestrial phase amphibians. 

 

4

 All registrant-submitted and open literature toxicity data reviewed for this assessment are included in 

Appendix A. 

40
2.8.2 Assessment Endpoints for Designated Critical Habitat 

As previously discussed, designated critical habitat is assessed to evaluate actions related 

to the use of paraquat that may alter the PCEs of the CRLF’s critical habitat. PCEs for 

the CRLF were previously described in Section 2.6. Actions that may affect critical 

habitat are those that alter the PCEs and jeopardize the continued existence of the CRLF. 

Therefore, these actions are identified as assessment endpoints. It should be noted that 

evaluation of PCEs as assessment endpoints is limited to those of a biological nature (i.e., 

the biological resource requirements for the listed species associated with the critical 

habitat) and those for which paraquat effects data are available. Adverse modification to 

the critical habitat of the CRLF includes, but is not limited to, those PCEs listed in 

Section 2.6. 

Measures of such possible effects by labeled use of paraquat on critical habitat of the 

CRLF are described in Table 2-5. Some components of these PCEs are associated with 

physical abiotic features (e.g., presence and/or depth of a water body, or distance between 

two sites), which are not expected to be measurably altered by use of pesticides. 

Assessment endpoints used for the analysis of designated critical habitat are based on the 

adverse modification standard established by U.S. FWS (2006). 

41
Table 2-5 Summary of Assessment Endpoints and Measures of Ecological Effect for 

Primary Constituent Elements of Designated Critical Habitata

Assessment Endpoint Measures of Ecological Effect4

Aquatic-Phase CRLF PCEs 

(Aquatic Breeding Habitat and Aquatic Non-Breeding Habitat)

Alteration of channel/pond morphology or geometry 

and/or increase in sediment deposition within the 

stream channel or pond: aquatic habitat (including 

riparian vegetation) provides for shelter, foraging, 

predator avoidance, and aquatic dispersal for juvenile 

and adult CRLFs. 

1a. Algae (Navicula pelliculosa) EC50 

1b. Duckweed (Lemna gibba) EC50 

1c. Monocots: Seedling Emergence, Vegetative Vigor EC25

1d. Dicots: Seedling Emergence, Vegetative Vigor EC25

Alteration in water chemistry/quality including 

temperature, turbidity, and oxygen content necessary 

for normal growth and viability of juvenile and adult 

CRLFs and their food source.

2a. Algae (Navicula pelliculosa) EC50 

2b. Duckweed (Lemna gibba) EC50 

2c. Monocots: Seedling Emergence, Vegetative Vigor EC25

2d. Dicots: Seedling Emergence, Vegetative Vigor EC25

Alteration of other chemical characteristics necessary 

for normal growth and viability of CRLFs and their 

food source. 

3a. Water flea (Daphnia magna) EC50 

3b. Rainbow trout (Oncorhynchus mykiss) LC50

3c. No chronic fish or aquatic-phase amphibians or aquatic 

invertebrates data

Reduction and/or modification of aquatic-based food 

sources for pre-metamorphs (e.g., algae) 4a. Algae (Navicula pelliculosa) EC50 

Terrestrial-Phase CRLF PCEs 

(Upland Habitat and Dispersal Habitat)

Elimination and/or disturbance of upland habitat; 

ability of habitat to support food source of CRLFs: 

Upland areas within 200 ft of the edge of the riparian 

vegetation or dripline surrounding aquatic and riparian 

habitat that are comprised of grasslands, woodlands, 

and/or wetland/riparian plant species that provides the 

CRLF shelter, forage, and predator avoidance 

Elimination and/or disturbance of dispersal habitat: 

Upland or riparian dispersal habitat within designated 

units and between occupied locations within 0.7 mi of 

each other that allow for movement between sites 

including both natural and altered sites which do not 

contain barriers to dispersal 

Reduction and/or modification of food sources for 

terrestrial phase juveniles and adults 

Alteration of chemical characteristics necessary for 

normal growth and viability of juvenile and adult 

CRLFs and their food source. 

5a. Monocots: Seedling Emergence, Vegetative Vigor EC25

5b. Dicots: Seedling Emergence, Vegetative Vigor EC25 

5c. Laboratory rat (Rattus norvegicus) Female LD50 

5d. Wistar Derived Rats-Alderley Park Strain NOAEL 

5e. Honey bee (Apis mellifera) LD50 

5f. Northern Bobwhite Quail (Colinus virginianus) LD50

5g. Japanese Quail (Coturnix coturnix japonica) LC50 

5h. Mallard duck (Anas platyrhynchos) NOEAC 

a Physico-chemical water quality parameters such as salinity, pH, and hardness are not evaluated because these processes are not 

biologically mediated and, therefore, are not relevant to the endpoints included in this assessment. 

 

4

 All registrant-submitted and open literature toxicity data reviewed for this assessment are included in 

Appendix A. 

42
2.9 Conceptual Model 

2.9.1 Risk Hypotheses 

Risk hypotheses are specific assumptions about potential adverse effects (i.e., changes in 

assessment endpoints) and may be based on theory and logic, empirical data, 

mathematical models, or probability models (U.S. EPA 1998). For this assessment, the 

risk is stressor-linked, where the stressor is the release of paraquat to the environment. 

The following risk hypotheses are presumed for this endangered species assessment: 

The labeled use of paraquat within the action area may: 

• directly affect the CRLF by causing mortality or by adversely affecting growth or 

fecundity; 

• indirectly affect the CRLF by reducing or changing the composition of food 

supply; 

• indirectly affect the CRLF or modify designated critical habitat by reducing or 

changing the composition of the aquatic plant community in the ponds and 

streams comprising the species’ current range and designated critical habitat, thus 

affecting primary productivity and/or cover; 

• indirectly affect the CRLF or modify designated critical habitat by reducing or 

changing the composition of the terrestrial plant community (i.e., riparian habitat) 

required to maintain acceptable water quality and habitat in the ponds and streams 

comprising the species’ current range and designated critical habitat; 

• affect the designated critical habitat of the CRLF by reducing or changing 

breeding and non-breeding aquatic habitat (via modification of water quality 

parameters, habitat morphology, and/or sedimentation); 

• affect the designated critical habitat of the CRLF by reducing the food supply 

required for normal growth and viability of juvenile and adult CRLFs; 

• affect the designated critical habitat of the CRLF by reducing or changing upland 

habitat within 200 ft of the edge of the riparian vegetation necessary for shelter, 

foraging, and predator avoidance; 

• affect the designated critical habitat of the CRLF by reducing or changing 

dispersal habitat within designated units and between occupied locations within 

0.7 mi of each other that allow for movement between sites including both natural 

and altered sites which do not contain barriers to dispersal; or 

• affect the designated critical habitat of the CRLF by altering chemical 

characteristics necessary for normal growth and viability of juvenile and adult 

CRLFs. 

2.9.2 Diagram 

The conceptual model is a graphic representation of the structure of the risk assessment. 

It specifies the paraquat release mechanisms, biological receptor types, and effects 

endpoints of potential concern. The conceptual models for terrestrial and aquatic 

exposures are shown in Figure 2-5 and Figure 2-6, respectively, which include the 

43
conceptual models for the aquatic and terrestrial PCE components of critical habitat. 

Exposure routes shown in dashed lines are not quantitatively considered because the 

contribution of those potential exposure routes to potential risks to the CRLF and effects 

to designated critical habitat is expected to be negligible. 

Stressor

Source 

Receptors 

Attribute 

Change 

Paraquat applied to use site 

Direct 

deposition Spray drift

Birds/terrestrialphase 

amphibians/ 

 reptiles/mammals 

Terrestrial 

insects 

 Individual organisms 

 Reduced survival 

 Reduced growth 

 Reduced reproduction 

Food chain

Reduction in prey 

Effects to PCEs 

related to prey 

availability 

Habitat integrity 

 Reduction in primary productivity 

 Reduced cover 

 Community change 

 Effects to PCEs related to habitat 

Terrestrial/riparian plants

grasses/forbs, fruit, seeds 

(trees, shrubs) 

Runoff 

Mammals

/ Birds 

Exposure 

Media 

Soil

Ingestion 

Ingestion 

Ingestion

Ingestion 

Dermal 

Long range 

atmospheric 

transport 

Root 

Wet/dry deposition 

Terrestrialphase 

amphibians 

Ingestion 

Figure 2-5 Conceptual Model for Pesticide Effects on Terrestrial Phase of the 

CRLF 

44
** Route of exposure includes only ingestion of aquatic fish and invertebrates 

Stressor

Source 

Receptors 

Attribute 

Change 

Paraquat applied to use site 

Spray drift 

Fish/aquaticphase amphibians 

**Piscivorous 

mammals and 

birds 

 Individual organisms 

 Reduced survival 

 Reduced growth 

 Reduced reproduction 

Food chain

Reduction in algae 

Reduction in prey 

Effects to PCEs 

related to prey 

availability 

Habitat integrity 

 Reduction in primary productivity 

 Reduced cover 

 Community change 

 Effects to PCEs related to habitat 

Surface water/ 

Sediment 

Runoff

 Aquatic Animals

 Invertebrates 

 Vertebrates 

Uptake/gills 

or integument 

Ingestion Ingestion 

Uptake/gills 

or integument 

Aquatic Plants

 Non-vascular 

 Vascular 

Uptake/cell, 

roots, leaves Riparian plant

terrestrial 

exposure 

pathways see 

Figure 2-5 

Long range 

atmospheric 

transport 

Wet/dry deposition 

Soil Groundwater

Exposure 

Media 

Figure 2-6 Conceptual Model for Pesticide Effects on Aquatic Phase of the CRLF 

2.10 Analysis Plan 

In order to address the risk hypothesis, the potential for direct and indirect effects to the 

CRLF, its prey, and its habitat is estimated. In the following sections, the use, 

environmental fate, and ecological effects of paraquat are characterized and integrated to 

assess the risks. This is accomplished using a risk quotient (ratio of exposure 

concentration to effects concentration) approach. Although risk is often defined as the 

likelihood and magnitude of adverse ecological effects, the risk quotient-based approach 

does not provide a quantitative estimate of likelihood and/or magnitude of an adverse 

effect. However, as outlined in the Overview Document (U.S. EPA 2004), the likelihood 

of effects to individual organisms from particular uses of paraquat is estimated using the 

probit dose-response slope and either the level of concern (discussed below) or actual 

calculated risk quotient value. 

45
2.10.1 Measures to Evaluate the Risk Hypothesis and Conceptual Model 

2.10.1.1 Measures of Exposure 

The environmental fate properties of paraquat along with available monitoring data 

indicate that runoff and spray drift are the principle potential transport mechanisms of 

paraquat to the aquatic and terrestrial habitats of the CRLF. In this assessment, transport 

of paraquat through runoff and spray drift is considered in deriving quantitative estimates 

of paraquat exposure to CRLF, its prey and its habitats. Paraquat is not expected to 

volatilize once applied to soil due to its extremely high adsorption coefficients. As a 

result, atmospheric transport is unlikely. See Section 3.2.4 for an explanation of existing 

monitoring data. 

Measures of exposure are based on aquatic and terrestrial models that predict estimated 

environmental concentrations (EECs) of paraquat using maximum labeled application 

rates and methods of application. The model used to predict aquatic EECs was the 

GENeric Estimated Environmental Concentration Model (GENEEC2) which mimics the 

Tier II models the Pesticide Root Zone Model coupled with the Exposure Analysis Model 

System (PRZM/EXAMS). The model used to predict terrestrial EECs on food items is 

T-REX. These models are parameterized using relevant reviewed registrant-submitted 

environmental fate data. 

GENEEC2 (v2.0, August 2001) is a Tier I computer program, that uses the soil/water 

partition coefficient and degradation kinetic data to estimate runoff from a ten hectare 

field into a one hectare by two meter deep “standard” pond. This first tier is designed as a 

coarse screen and estimates conservative pesticide concentrations in surface water from a 

few basic chemical parameters and pesticide label use and application information. Tier I 

is used to screen chemicals to determine which ones potentially pose sufficient risk to 

warrant higher level modeling. It calculates acute as well as longer-term estimated 

environmental concentration (EEC) values. It considers reduction in dissolved pesticide 

concentration due to adsorption of pesticide to soil or sediment, incorporation, 

degradation in soil before runoff to a water body, direct deposition of spray drift into the 

water body, and degradation of the pesticide within the water body. It is designed to 

mimic a PRZM-EXAMS simulation. 

PRZM (v3.12.2, May 2005) and EXAMS (v2.98.4.6, April 2005) are screening 

simulation models coupled with the input shell pe5.pl (Aug 2007) to generate daily 

exposures and 1-in-10 year EECs of paraquat that may occur in surface water bodies 

adjacent to application sites receiving paraquat through runoff and spray drift. PRZM 

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. EXAMS simulates the fate of the pesticide and resulting concentrations in the 

water body. The standard scenario used for ecological pesticide assessments assumes 

application to a 10-hectare agricultural field that drains into an adjacent 1-hectare water 

body, 2-meters deep (20,000 m3

 volume) with no outlet. PRZM/EXAMS was used to 

estimate screening-level exposure of aquatic organisms to paraquat. The measure of 

46
exposure for aquatic species is the 1-in-10 year return peak or rolling mean concentration. 

The 1-in-10 year peak is used for estimating acute exposures of direct effects to the 

CRLF, as well as indirect effects to the CRLF through effects to potential prey items, 

including: algae, aquatic invertebrates, fish and frogs. The 1-in-10-year 60-day mean is 

used for assessing chronic exposure to the CRLF and fish and frogs serving as prey 

items; the 1-in-10-year 21-day mean is used for assessing chronic exposure for aquatic 

invertebrates, which are also potential prey items. 

Chemicals failing to pass the GENEEC2 program move on to the Tier II 

(PRZM/EXAMS) modeling. However, in the case of paraquat, all direct aquatic, indirect 

aquatic, and vascular aquatic plant EECs do not result in an exceedance. The nonvascular aquatic plant EECs result in an exceedance, but paraquat is an herbicide and 

exceedances for plants is expected. 

Exposure estimates for the terrestrial-phase CRLF and terrestrial invertebrates and 

mammals (serving as potential prey) assumed to be in the target area or in an area 

exposed to spray drift are derived using the T-REX model (version 1.3.1, 12/07/2006). 

This model incorporates the Kenega nomograph, as modified by Fletcher et al. (1994), 

which is based on a large set of actual field residue data. The upper limit values from the 

nomograph represented the 95th percentile of residue values from actual field 

measurements (Hoerger and Kenega 1972). For modeling purposes, direct exposures of 

the CRLF to paraquat through contaminated food are estimated using the EECs for the 

small bird (20 g) which consumes small insects. Dietary-based and dose-based exposures 

of potential prey (small mammals) are assessed using the small mammal (15 g) which 

consumes short grass. The small bird (20g) consuming small insects and the small 

mammal (15g) consuming short grass are used because these categories represent the 

largest RQs of the size and dietary categories in T-REX that are appropriate surrogates 

for the CRLF and one of its prey items. Estimated exposures of terrestrial insects to 

paraquat are bound by using the dietary based EECs for small insects and large insects. 

For an additional refinement of terrestrial-phase CLRF dose and dietary-based exposures 

the T-HERPS model is employed. The T-HERPS is used as a refinement tool to explore 

amphibian-specific food intake on potential exposures to the terrestrial phase CRLF. It 

incorporates the same inputs as T-REX with equations adjusted for poikilotherm food 

intake. 

EECs for terrestrial plants inhabiting dry and wetland areas are derived using TerrPlant 

(version 1.2.2, 12/26/2006). This model uses estimates of pesticides in runoff and in 

spray drift to calculate EECs. EECs are based upon solubility, application rate and 

minimum incorporation depth. 

The spray drift model, AgDRIFT, is used to assess exposures of terrestrial phase CRLF 

and its prey to paraquat deposited on terrestrial habitats by spray drift. In addition to the 

buffered area from the spray drift analysis, the downstream extent of paraquat that 

exceeds the LOC for the effects determination is also considered. 

47
2.10.1.2 Measures of Effect 

Data identified in Section 2.8 are used as measures of effect for direct and indirect effects 

to the CRLF. Data were obtained from registrant submitted studies or from literature 

studies identified by ECOTOX. The ECOTOXicology database (ECOTOX) was searched 

in order to provide more ecological effects data and in an attempt to bridge existing data 

gaps. ECOTOX is a source for locating single chemical toxicity data for aquatic life, 

terrestrial plants, and wildlife. ECOTOX was created and is maintained by the U.S. EPA, 

Office of Research and Development, and the National Health and Environmental Effects 

Research Laboratory's Mid-Continent Ecology Division. 

The assessment of risk for direct effects to the terrestrial-phase CRLF makes the 

assumption that toxicity of paraquat to birds is similar to or less than the toxicity to the 

terrestrial-phase CRLF. The same assumption is made for fish and aquatic-phase CRLF. 

Algae, aquatic invertebrates, fish, and amphibians represent potential prey of the CRLF 

in the aquatic habitat. Terrestrial invertebrates, small mammals, and terrestrial-phase 

amphibians represent potential prey of the CRLF in the terrestrial habitat. Aquatic, semiaquatic, and terrestrial plants represent habitat of CRLF. 

The acute measures of effect used for animals in this screening level assessment are the 

LD50, LC50 and EC50. LD stands for "Lethal Dose", and LD50 is the amount of a material, 

given all at once, that is estimated to cause the death of 50% of the test organisms. LC 

stands for “Lethal Concentration” and LC50 is the concentration of a chemical that is 

estimated to kill 50% of the test organisms. EC stands for “Effective Concentration” and 

the EC50 is the concentration of a chemical that is estimated to produce a specific effect in 

50% of the test organisms. Endpoints for chronic measures of exposure for listed and 

non-listed animals are the NOAEL/NOAEC and NOEC. NOAEL stands for “No 

Observed-Adverse-Effect-Level” and refers to the highest tested dose of a substance that 

has been reported to have no harmful (adverse) effects on test organisms. The NOAEC 

(i.e., “No-Observed-Adverse-Effect-Concentration”) is the highest test concentration at 

which none of the observed effects were statistically different from the control. The 

NOEC is the No-Observed-Effects-Concentration. For non-listed plants, only acute 

exposures are assessed (i.e., EC25 for terrestrial plants and EC50 for aquatic plants). 

It is important to note that the measures of effect for direct and indirect effects to the 

CRLF and its designated critical habitat are associated with impacts to survival, growth, 

and fecundity, and do not include the full suite of sublethal effects used to define the 

action area. According the Overview Document (USEPA 2004), the Agency relies on 

effects endpoints that are either direct measures of impairment of survival, growth, or 

fecundity or endpoints for which there is a scientifically robust, peer reviewed 

relationship that can quantify the impact of the measured effect endpoint on the 

assessment endpoints of survival, growth, and fecundity. 

48
2.10.1.3

2.10.1.4

Integration of Exposure and Effects 

Risk characterization is the integration of exposure and ecological effects characterization 

to determine the potential ecological risk from agricultural and non-agricultural uses of 

paraquat, and the likelihood of direct and indirect effects to CRLF in aquatic and 

terrestrial habitats. The exposure and toxicity effects data are integrated in order to 

evaluate the risks of adverse ecological effects on non-target species. For the assessment 

of paraquat risks, the risk quotient (RQ) method is used to compare exposure and 

measured toxicity values. EECs are divided by acute and chronic toxicity values. The 

resulting RQs are then compared to the Agency’s levels of concern (LOCs) (U.S. EPA 

2004) (see Appendix C). 

For this endangered species assessment, listed species LOCs are used for comparing RQ 

values for acute and chronic exposures of paraquat directly to the CRLF. If estimated 

exposures directly to the CRLF of paraquat resulting from a particular use are sufficient 

to exceed the listed species LOC, then the effects determination for that use is “may 

affect”. When considering indirect effects to the CRLF due to effects to animal prey 

(aquatic and terrestrial invertebrates, fish, frogs, and mice), the listed species LOCs are 

also used. If estimated exposures to CRLF prey of paraquat resulting from a particular 

use are sufficient to exceed the listed species LOC, then the effects determination for that 

use is a “may affect.” If the RQ being considered also exceeds the non-listed species 

acute risk LOC, then the effects determination is a LAA. If the acute RQ is between the 

listed species LOC and the non-listed acute risk species LOC, then further lines of 

evidence (i.e. probability of individual effects, species sensitivity distributions) are 

considered in distinguishing between a determination of NLAA and a LAA. When 

considering indirect effects to the CRLF due to effects to algae as dietary items or plants 

as habitat, the non-listed species LOC for plants is used because the CRLF does not have 

an obligate relationship with any particular aquatic and/or terrestrial plant. If the RQ 

being considered for a particular use exceeds the non-listed species LOC for plants, the 

effects determination is “may affect”. Further information on LOCs is provided in 

Appendix C. 

Data Gaps 

The long-term stability of sediment-bound paraquat under various conditions (such as 

changes in pH, Eh, etc.) is unknown. There is insufficient data to determine to what 

extent paraquat will accumulate at the same site with successive applications, and what 

the effects of accumulation might be. Likewise, paraquat is expected to adsorb to soils 

with high clay content. Therefore, a batch equilibrium study using soils with varying 

percents of clay content as well as soils with low clay content would be beneficial to 

better understand which types of soils paraquat may not adsorb to and possibly leach to 

ground water. 

The effect of consuming soil-bound paraquat on organisms is unknown. The fate of 

paraquat dichloride residues in desiccated plant material (that is, how tightly bound to 

biological material does it remain relative to clay-bound paraquat) is unknown. 

49
Therefore, conservative assumptions were made when running the models to determine 

the effects. Please see Section 3 and Section 5 for further explanation on assumptions 

made for modeling. 

Paraquat is likely to reach the aquatic environment via erosion and runoff, and persist in 

the aquatic sediment resulting in exposure to benthic dwelling freshwater invertebrates 

and fish. Currently, however no chronic toxicity studies for freshwater invertebrates or 

fish have been submitted to the Agency. Therefore, there is a data gap for chronic 

freshwater invertebrates and fish toxicity. Due to this data gap we used the chemical 

diquat, which is similar in structure to calculate an acute to chronic ratio (ACR). Only 

the freshwater invertebrate data were used as two different species of fish were studied 

and an ACR could not be calculated across species. 

There is only one multi-active ingredient product containing paraquat and Carfentrazoneethyl as Cyclone Star (EPA Reg Number 00010001316). There is no toxicity data 

relative to this compound (Carfentrazone-ethyl) in the literature or in the assessment 

provided by Human Helath and Effects Division (HED). Therefore, no difnitive 

statement can be made as to whether this product poses any toxic risk greater or less than 

paraquat product alone. The best available information suggests that the assessment 

based on paraquat alone is adequate to understand risk to non-target receptors. 

50
3.0 Exposure Assessment 

Paraquat is formulated as an emulsifiable concentrate. Application methods include 

ground, aerial, and by various sprayers (band, hooded, low-pressure, and shielded). Risks 

from ground boom and aerial applications are expected to result in the highest off-target 

levels of paraquat due to generally higher spray drift levels. Ground boom and aerial 

modes of application tend to use lower volumes of application applied in finer sprays 

than applications coincident with sprayers and spreaders and thus have a higher potential 

for off-target movement via spray drift. 

3.1 Label Application Rates and Intervals 

Paraquat labels may be categorized into two types: labels for manufacturing uses 

(including technical grade paraquat and its formulated products) and end-use products. 

While technical products, which contain paraquat of high purity, are not used directly in 

the environment, they are used to make formulated products. The formulated product 

labels legally limit paraquat’s potential use to only those sites that are specified on the 

labels. 

Currently registered agricultural and non-agricultural uses of paraquat within California 

are listed in Table 2-2. The uses being assessed are summarized in Table 3-1 below. The 

uses assessed were the non-agricultural use and agricultural use with the maximum 

application rate/largest number of applications per year/smallest application interval, the 

median agricultural use, and the agricultural use with smallest application rate/the least 

number of applications per year /largest application interval. 

Table 3-1 Paraquat Uses and Application Information for the CRLF risk 

assessment1

Uses Represented by Scenario Application 

Rate (lb 

cation/A)

Number of 

Applications 

Application 

Interval 

Application 

Method 

Aircraft 

AGRICULTURAL FALLOW/IDLELAND 1.0 5 5 Ground 

AIRPORTS/LANDING FIELDS, 

COMMERCIAL/INSTITUTIONAL/INDUSTRIAL 

PREMISES/EQUIPMENT (OUTDOOR), 

NONAGRICULTURAL AREAS (PUBLIC 

HEALTH USE), NONAGRICULTURAL 

RIGHTS-OF-WAY/FENCEROWS/HEDGEROWS 1.0 10 5 Ground 

GUAVA, PASSION FRUIT (GRANADILLA) 1.0 10 5 Ground 

Aircraft 

MELONS, PEANUTS (ground only) 0.3 1 1 Ground 1

 Uses assessed based on memorandum from SRRD dated February 5, 2009 

51
3.2 Aquatic Exposure Assessment 

3.2.1 Modeling Approach 

Aquatic exposures are quantitatively estimated for all of assessed uses using scenarios 

that represent high exposure sites for paraquat use. Each of these sites represents a 10 

hectare field that drains into a 1-hectare pond that is 2 meters deep and has no outlet. 

Exposure estimates generated using the standard pond are intended to represent a wide 

variety of vulnerable water bodies that occur at the top of watersheds including prairie 

pot holes, playa lakes, wetlands, vernal pools, man-made and natural ponds, and 

intermittent and first-order streams. As a group, there are factors that make these water 

bodies more or less vulnerable than the standard surrogate pond. Static water bodies that 

have larger ratios of drainage area to water body volume would be expected to have 

higher peak EECs than the standard pond. These water bodies will be either shallower or 

have large drainage areas (or both). Shallow water bodies tend to have limited additional 

storage capacity, and thus, tend to overflow and carry pesticide in the discharge whereas 

the standard pond has no discharge. As watershed size increases beyond 10 hectares, at 

some point, it becomes unlikely that the entire watershed is planted to a single crop, 

which is all treated with the pesticide. Headwater streams can also have peak 

concentrations higher than the standard pond, but they tend to persist for only short 

periods of time and are then carried downstream. 

Crop-specific management practices for all of the assessed uses of paraquat were used for 

modeling, including application rates, number of applications per year, application 

intervals, and the first application date for each crop. 

3.2.2 Model Inputs 

Paraquat is an herbicide used on various crops such as apricots, loganberries, corn, 

peanuts, coffee, lettuce, and non-crop areas such as airport landing fields, and 

commercial, industrial and institutional premises. Paraquat’s environmental fate data 

used for generating model parameters is listed in Table 2-2. The input parameters for the 

GENEEC2 model are in Table 3-2. 

Most of the inputs used in the GENEEC2 model followed the Input Parameter Guidance; 

however, since paraquat’s adsorption Kds ranged from 68-50,000 (mobile to highly 

immobile), the average Kd would not necessarily be an accurate depiction of paraquat’s 

mobility. Paraquat is an interesting chemical due to its high affinity to clay (negatively 

charged particles) as a result of its cationic charge in the molecule. Therefore, paraquat 

may be more mobile in soil that is less charged, such as sand, and will become 

increasingly less mobile in soils with higher percentages of clay. The Kd was determined 

by plotting the Kds along with their corresponding concentrations (lb cation/A) in order 

to determine the linear regression. Once this was established, the maximum allowable 

application rate (1 lb cation/A) was plugged into the equation given by the regression to 

determine the corresponding Kd. This was done for all four soils in the study (MRID 

40762701). Then, the average Kd was determined and used as the input. Determining 

52
Kd in this manner is believed to be very conservative because the Kd value calculated by 

this method assumes paraquat to be more mobile than expected. 

Table 3-2 Summary of GENEEC2 Environmental Fate Data Used for Aquatic 

Exposure Inputs for Paraquat Endangered Species Assessment for the CRLF 1

Based on the Paraquat Cation 

Fate Property Value (unit) MRID (or source) 

Molecular Weight 186 g/mol http://toxnet.nlm.nih.gov

Henry’s constant 1.9 x 10-9 atm m3 mol-1

http://toxnet.nlm.nih.gov

 

Vapor Pressure 0 

http://toxnet.nlm.nih.gov

 

Solubility in Water 7,000,000 mg/l (700,000* mg/l x 

10 per input guidance) 

*http://www.inchem.org/docu

ments/icsc/icsc/eics0005.htm

Per Input Parameter Guidance 

Photolysis in Water 0 (Stable) MRID 40562301 

Aerobic Soil Metabolism Half-lives 0 (Stable) MRID 41319301 

Hydrolysis 0 (Stable) MRID 41319302 

Aerobic Aquatic Metabolism (water 

column) 

0 (0.5 x Aerobic Soil Metabolism 

study) Per Input Parameter Guidance 

Anaerobic Aquatic Metabolism 

(benthic) 

0 (0.5 x Anaerobic Soil 

Metabolism study) 

Per Input Parameter Guidance 

Kd 295 MRID 40762701 

Application rate and frequency Various (see Table 3.3) Per Label Instructions 

Application intervals Various (see Table 3.3) Per Label Instructions 

1 Inputs determined in accordance with EFED “Guidance for Chemistry and Management Practice Input 

Parameters for Use in Modeling the Environmental Fate and Transport of Pesticides” dated February 28, 

2002 

3.2.3 Results 

The aquatic EECs for the various application practices using the GENEEC2 model are 

listed in Table 3-3. The maximum non-agricultural and agricultural application 

rate/interval/applications per year were calculated, along with the median use application 

rate/interval/applications per year, and minimum use application rate/interval/applications 

per year. See Appendix I for a summary of the outputs. Peak EECs ranged from 1.6 ppb 

to 55 ppb for use on melons, and airports/commercial/public health areas (impervious 

areas) and guava/passion fruit respectively. 

Since some of the application data needed for modeling were not stated on the labels, 

assumptions were made by EFED analysts regarding the maximum number of 

53
applications allowed per season, and/or the interval between applications. The 

assumptions were as follows: 

• For the application intervals that were not stated, the most conservative 

(minimum) known application interval was used (5 days). Five days was chosen 

because it was the minimum known interval that was registered. 

• If the maximum number of applications was not stated, the most conservative 

(maximum) known number of applications was used (10 applications). Ten 

applications were chosen because 10 was the maximum known number of 

applications allowed that was registered. 

• If the maximum number of applications was not stated, but the uses had an 

application timing of pre-emergence or pre-plant, and was not listed as having 

multiple crop seasons per year in California, per a table created by the Biological 

and Economic Analysis Division (BEAD), the maximum application number was 

assumed to be one. 

Table 3-3 Aquatic EECs (μg/L) for Paraquat Uses in California 

Application 

Rate (lkg 

cation/ha) Crops Represented Peak 

EEC 

21-day 

average EEC 

60-day 

average 

EEC 

33 20 10 

1.0 AGRICULTURAL FALLOW/IDLELAND 27 17 8.5 

1.0 

AIRPORTS/LANDING FIELDS, 

COMMERCIAL/INSTITUTIONAL/INDUSTRIAL 

PREMISES/EQUIPMENT (OUTDOOR), 

NONAGRICULTURAL AREAS (PUBLIC 

HEALTH USE), NONAGRICULTURAL 

RIGHTS-OF-WAY/FENCEROWS/HEDGEROWS 55 34 17 

1.0 GUAVA, PASSION FRUIT (GRANADILLA) 55 34 17 

2.0 1.2 0.62 

0.3 MELONS, PEANUTS (ground only) 1.6 1.0 0.51 

3.2.4 Existing Monitoring Data 

Whenever it is available, monitoring data is included in assessments in order to better 

characterize the EECs calculated in the modeled estimates. In this assessment, 

monitoring data was sought from the following sources: the USGS NAWQA program 

(http://water.usgs.gov/nawqa), the California Department of Pesticide Regulation 

(CDPR), and the California Air Review Board. 

3.2.4.1 USGS NAWQA Surface Water Data 

The USGS has not looked for any samples containing paraquat. Therefore, no surface 

water data are available. 

54
3.2.4.2

3.2.4.3

3.2.4.4

USGS NAWQA Groundwater Data 

The USGS has not collected any samples looking for paraquat. Therefore, no ground 

water data are available. 

California Department of Pesticide Regulation (CDPR) Data 

The California Department of Pesticide Regulation (CDPR) has been collecting surface 

water data on paraquat for many years. Out of 399 samples, 398 of the samples did not 

contain paraquat. One sample showed a detection of paraquat at 0.24 ppb; the detection 

limit was 1 ppb. 

The positive sample was taken on May 16, 2006 in the Merced River. 99.7 % of the 

samples did not detect any paraquat from July 2005 to October 2006. The samples were 

taken in the Summer, Spring, and Fall seasons. 

Atmospheric Monitoring Data 

The California Air Review Board has not conducted ambient air monitoring of paraquat. 

Therefore, no atmospheric monitoring data are available. 

3.3 Terrestrial Animal Exposure Assessment 

T-REX (Version 1.3.1) is used to calculate dietary and dose-based EECs of paraquat for 

the CRLF and its potential prey (e.g. small mammals and terrestrial insects) inhabiting 

terrestrial areas. EECs used to represent the CRLF are also used to represent exposure 

values for frogs serving as potential prey of CRLF adults. T-REX simulates a 1-year time 

period. For this assessment, spray applications of paraquat were considered, as discussed 

below. 

Instead of running all of the uses, the maximum non-agricultural use, the maximum 

agricultural use, the median agricultural use, and the minimum agricultural use were run 

using T-Rex in order to get an upper and lower bound for the terrestrial effects. 

Maximum is defined as the use with the largest application rate, the shortest intervals, 

and the most applications. 

Terrestrial EECs for foliar formulations of paraquat were derived for the uses 

summarized in Table 3-4. The non-agricultural and agricultural uses with the maximum 

application rates, along with the agricultural use with the median application rate, and the 

agricultural use with the lowest application rate were all modeled. The third and second 

lowest application rates were also modeled, to determine the lowest rates in which there 

were no exceedances. Given that no data on interception and subsequent dissipation from 

foliar surfaces was available for paraquat, a default foliar dissipation half-life of 35 days 

was used based on the work of Willis and McDowell (1987). Use specific input values, 

including number of applications, application rate and application interval are provided in 

Table 3-4. An example output from T-REX is available in Appendix E. 

55
Table 3-4 Input Parameters for Foliar Applications Used to Derive Terrestrial 

EECs for Paraquat with T-REX 

Use (Application method) 

Application 

rate 

(lbs ai/A) 

Number of 

Applications 

Intervals Between 

Applications 

(Days) 

Airports (Maximum, Non-agricultural) 1 10 5 

Guava (Maximum, Agricultural) 1 10 5 

Ginger (Median, Agricultural) 1 6 30 

Corn (Third lowest, Agricultural) 0.5 3 5 

Carrots (Second lowest, Agricultural) 1 1 1 

Melons (Lowest, Agricultural) 0.3 1 1 

T-REX is also used to calculate EECs for terrestrial insects exposed to paraquat. Dietarybased EECs calculated by T-REX for small and large insects (units of a.i./g) are used to 

bound an estimate of exposure to terrestrial insects. Available acute contact toxicity data 

for bees exposed to paraquat (in units of μg a.i./bee), are converted to μg a.i./g (of bee) 

by multiplying by 1 bee/0.128 g. The EECs are later compared to the adjusted acute 

contact toxicity data for bees in order to derive RQs. 

For modeling purposes, exposures of the CRLF to paraquat through contaminated food 

are estimated using the EECs for the small bird (20 g) which consumes small insects. 

Dietary-based and dose-based exposures of potential prey are assessed using the small 

mammal (15 g) which consumes short grass. Upper-bound Kenega nomogram values 

reported by T-REX for these two organism types are used for derivation of EECs for the 

CRLF and its potential prey (Table 3-5). Dietary-based EECs for small and large insects 

reported by T-REX as well as the resulting adjusted EECs are available in Table 3-6. An 

example output from T-REX v. 1.3.1 is available in Appendix E. 

Any RQs that exceed the LOC for listed species will be analyzed in T-HERPS. The THERPS model will therefore be employed as a refinement tool to explore amphibianspecific food intake on potential exposures to the terrestrial phase CRLF. The T-HERPS 

model incorporates the same inputs as T-REX with equations adjusted for poikilotherm 

food intake. An example output from T-HERPS is available in Appendix J. 

Table 3-5 Upper-bound Kenega Nomogram EECs for Dietary- and Dose-based 

Exposures of the CRLF and its Prey to Paraquat 

EECs for CRLF 

(Avian, 20 g) 

EECs for Prey 

(small mammals, 15 g) 

Use Dietary-based 

EEC (ppm) 

Dose-based EEC

(mg/kg-bw) 

Dietary-based 

EEC (ppm) 

Dose-based EEC

(mg/kg-bw) 

Airports 900 1025 1600 1530 

Guava 900 1025 1600 1530 

Ginger 293 333 520 500 

Corn 184 209 327 175 

Carrots 135 154 240 229 

Melons 41 46 72 69 

56
Table 3-6 EECs (ppm) for Indirect Effects to the Terrestrial-Phase CRLF via 

Effects to Terrestrial Invertebrate Prey Items 

Use Small Insect Large Insect 

Airports 900 100 

Guava 900 100 

Ginger 293 33 

Corn 184 20 

Carrots 135 15 

Melons 41 4.5 

3.4 Terrestrial Plant Exposure Assessment 

AgDRIFT (Version 2.01) is used to calculate spray drift; it was also used to estimate 

terrestrial plant exposure. The spray drift analysis uses the most sensitive endpoint. See 

section 5.2.5.1 for the spray drift analysis. 

57
4.0 Effects Assessment 

This assessment evaluates the potential for paraquat to directly or indirectly affect the 

CRLF or modify its designated critical habitat. As previously discussed in Section 2.7, 

assessment endpoints for the CRLF effects determination include direct toxic effects on 

the survival, reproduction, and growth of CRLF, as well as indirect effects, such as 

reduction of the prey base or effects to its habitat. In addition, potential effects to critical 

habitat are assessed by evaluating effects to the PCEs, which are components of the 

critical habitat areas that provide essential life cycle needs of the CRLF. Direct effects to 

the aquatic-phase of the CRLF are based on toxicity information for freshwater fish, 

while terrestrial-phase effects are based on avian toxicity data, given that birds are 

generally used as a surrogate for terrestrial-phase amphibians. Because the frog’s prey 

items and habitat requirements are dependent on the availability of freshwater fish and 

invertebrates, small mammals, terrestrial invertebrates, and aquatic and terrestrial plants, 

toxicity information for these taxa are also discussed. Acute (short-term) and chronic 

(long-term) toxicity information is characterized based on registrant-submitted studies 

and a comprehensive review of the open literature on paraquat. 

As described in the Agency’s Overview Document (U.S. EPA 2004), the most sensitive 

endpoint for each taxon is used for risk estimation. For this assessment, evaluated taxa 

include aquatic-phase amphibians, freshwater fish, freshwater invertebrates, aquatic 

plants, birds (surrogate for terrestrial-phase amphibians), mammals, terrestrial 

invertebrates, and terrestrial plants. 

Toxicity endpoints are established based on data generated from guideline studies 

submitted by the registrant, and from open literature studies that meet the criteria for 

inclusion into the ECOTOX database maintained by EPA/Office of Research and 

Development (ORD) (U.S. EPA 2004). Open literature data presented in this assessment 

were obtained from RED 1997, the EFED and HED new use chapters 2006 as well as 

ECOTOX information obtained on December 31, 2008. In order to be included in the 

ECOTOX database, papers must meet the following minimum criteria: 

(1) the toxic effects are related to single chemical exposure;5

(2) the toxic effects are on an aquatic or terrestrial plant or animal species; 

(3) there is a biological effect on live, whole organisms; 

(4) a concurrent environmental chemical concentration/dose or application 

rate is reported; and 

(5) there is an explicit duration of exposure. 

Data that pass the ECOTOX screen are evaluated along with the registrant-submitted 

data, and may be incorporated qualitatively or quantitatively into this endangered species 

assessment. In general, effects data in the open literature that are more conservative than 

 

5

 The studies that have information on mixtures are listed in the bibliography as rejected due to the 

presence of mixtures. These studies are evaluated by EFED when applicable to the assessment; however, 

the data is not used quantitatively in the assessment. 

58
the registrant-submitted data are considered. The degree to which open literature data are 

quantitatively or qualitatively characterized for the effects determination is dependent on 

whether the information is relevant to the assessment endpoints (i.e., maintenance of 

CRLF survival, reproduction, and growth) identified in Section 2.8. For example, 

endpoints such as behavior modifications are likely to be qualitatively evaluated, unless 

quantitative relationships between modifications and reduction in species survival, 

reproduction, and/or growth are available. Although the effects determination relies on 

endpoints that are relevant to the assessment endpoints of survival, growth, or 

reproduction, it is important to note that the full suite of sublethal endpoints potentially 

available in the effects literature (regardless of their significance to the assessment 

endpoints) are considered to define the action area for paraquat. 

Citations of all open literature not considered as part of this assessment because they 

were either rejected by the ECOTOX screen or accepted by ECOTOX but not used (e.g., 

the endpoint is less sensitive) are included in Appendices F and G. Both Appendix F and 

G also include a rationale for rejection of those studies that did not pass the ECOTOX 

screen and those that were not evaluated as part of this endangered species risk 

assessment. A detailed bibliography of the available ECOTOX open literature data, 

including the full suite of lethal and sublethal endpoints are presented in Appendices F 

and G. A summary of the human health effects data for paraquat is found in Appendix K. 

In addition to registrant-submitted and open literature toxicity information, other sources 

of information, including use of the acute probit dose response relationship to establish 

the probability of an individual effect and reviews of the Ecological Incident Information 

System (EIIS), are conducted to further refine the characterization of potential ecological 

effects associated with exposure to paraquat (Appendix H). A summary of the available 

aquatic and terrestrial ecotoxicity information, use of the probit dose response 

relationship, and the incident information for paraquat are provided in Sections 4.1 

through 4.4, respectively. 

Paraquat has registered products that contain multiple active ingredients. The results of 

available toxicity data for mixtures of paraquat with other pesticides are presented in 

Appendix B; however, there is no toxicity data for paraquat Multi-AI products. There is 

available open literature data from ECOTOX and three studies contained information 

examining toxicity effects of paraquat when mixed with other chemicals. The 

bibliographic information of these studies is presented in Appendix F. According to the 

available open literature data, other pesticides may combine with paraquat to produce 

additive, and/or antagonistic toxic effects. Mixtures included paraquat with linuron, and 

paraquat with copper sulfate. 

If chemicals that show additive effects with paraquat are present in the environment in 

combination with paraquat, the toxicity of paraquat may be increased, offset by other 

environmental factors, or even reduced by the presence of antagonistic contaminants if 

they are also present in the mixture. The variety of chemical interactions presented in the 

available data set suggest that the toxic effect of paraquat, in combination with other 

pesticides used in the environment, can be a function of many factors including but not 

59
necessarily limited to (1) the exposed species, (2) the co-contaminants in the mixture, (3) 

the ratio of paraquat 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 given taxa with confidence is beyond the 

current abilities of ecotoxicology. However, a qualitative discussion of implications of 

the available pesticide mixture effects data involving paraquat on the confidence of risk 

assessment conclusions for the freshwater vascular plant (duckweed, Lemna minor vs L. 

gibba), and mammals is part of the uncertainty analysis for this effects determination. 

Paraquat plus copper sulfate mixture was found to exhibit both an antagonistic and 

additive relationship in the vascular aquatic plant Lemna minor (E 102140). In the 

presence of both paraquat and copper sulfate there was a reduction in the percent 

inhibition of Fv/Fm, a measurement of the efficiency of Photosystem II (PSII), indicating 

an antagonistic relationship. An additive relationship was observed in two other 

parameters; O2 evolution and the fraction of inhibited centers (FIC). This study was 

completed in 48h, whereas guideline studies on vascular plants range from 7-14 days, 

therefore the effects observed with this mixture may potentially change over time. 

The mixture of paraquat plus linuron (formulations used for potatoes) was found to not be 

genotoxic, but was found to be cytotoxic to adult wistar rats (E104633). However, this 

mixture (paraquat plus linuron) would not be found in California as linuron is not 

registered for use on potatoes in California. The final mixture study in ECOTOX was a 

field study that examined the impacts of herbicides on species found in a field that was 

treated with herbicides (E147983). Other studies examine impacts to non-target 

organisms off the pesticide treated field as a result of runoff or spray drift. The 

treatments examined compared herbicide treatments to other cultural practices (tillage), 

which showed impacts to the “native field species” from tilling alone. This paper is only 

mentioned and will not be used quantitatively or qualitatively, it is simply an additional 

reference that demonstrates the potential impacts of cultural practices on the reduction in 

CRLF food sources. Bibliographic information is presented in Appendix F. 

4.1 Evaluation of Aquatic Ecotoxicity Studies 

Table 4-1 summarizes the most sensitive aquatic toxicity endpoints for the CRLF, based 

on an evaluation of both the submitted studies and the open literature, as previously 

discussed. A brief summary of submitted and open literature data considered relevant to 

this ecological risk assessment for the CRLF is presented below. Additional information 

is provided in Appendix A. 

60
Table 4-1 Freshwater Aquatic Toxicity Profile for Paraquat Dichloride (expressed 

as the cation) 

Assessment 

Endpoint Species 

Toxicity Value 

Used in Risk 

Assessment 

Describe effect 

(i.e. mortality, 

growth, 

reproduction) 

Citation MRID 

# (Author & 

Date) 

Study 

Classification 

Direct Toxicity to Aquatic-Phase CRLF 

Acute Bluegill 

sunfish 

96h LC50 = 13 

mg/L 

Mortality 40098001 

(Mayer & 

Ellersieck 1986) 

Acceptable 

Chronic Estimated – See Section 5 

Indirect Toxicity to Aquatic-Phase CRLF 

via Acute 

Toxicity to 

Freshwater 

Invertebrates 

(i.e. prey items) 

Daphnia 

magna 

48h EC50 = 1.2 

mg/L 

Mortality 00114473 

(Wheeler 1978) 

Acceptable 

via Chronic 

Toxicity to 

Freshwater 

Invertebrates 

(i.e. prey items) 

 Estimated – See Section 5 

via Acute 

Toxicity to 

Freshwater Fish 

(i.e. prey items) 

Bluegill 

sunfish 

96h LC50 = 13 

mg/L 

Mortality 40098001 

(Mayer & 

Ellersieck 1986) 

Acceptable 

via Chronic 

Toxicity to 

Freshwater Fish 

(i.e. prey items) 

 Estimated – See Section 5 

via Toxicity to 

Non-vascular 

Aquatic Plants 

Navicula 

pelliculosa

(freshwater 

diatom) 

4d EC50 = 0.396 

μg/L 

Growth 42601006 

(Smyth et al. 

1992) 

Acceptable 

via Toxicity to 

Vascular 

Aquatic Plants 

Lemna gibba

(Duckweed) 

14d EC50 = 71 

μg/L 

Growth and 

Reproduction 

42601003 

(Smyth et al. 

1992) 

Acceptable 

Toxicity to aquatic fish and invertebrates is categorized using the system shown in Table 

4-2 (U.S. EPA 2004). Toxicity categories for aquatic plants have not been defined. 

Table 4-2 Categories of Acute Toxicity for Fish and Aquatic Invertebrates 

LC50 (ppm) Toxicity Category 

< 0.1 Very highly toxic 

> 0.1 - 1 Highly toxic 

> 1 - 10 Moderately toxic 

> 10 – 100 Slightly toxic 

> 100 Practically nontoxic 

61
4.1.1 Toxicity to Freshwater Fish 

No scientifically valid data are available for aquatic-phase amphibians, and therefore, 

freshwater fish data were used as a surrogate to estimate direct chronic risks to the CRLF. 

Freshwater fish toxicity data were also used to assess potential indirect effects of 

Paraquat to the CRLF. Effects to freshwater fish resulting from exposure to Paraquat 

may indirectly affect the CRLF via reduction in available food. As discussed in Section 

2.5.3, over 50% of the prey mass of the CRLF may consist of vertebrates such as mice, 

frogs, and fish (Hayes and Tennant 1985). 

A summary of acute and chronic freshwater fish data, including data from the open 

literature, is provided below in Sections 4.1.1.1 through 4.1.1.3. 

4.1.1.1

4.1.1.2

4.1.1.3

Freshwater Fish: Acute Exposure (Mortality) Studies 

Freshwater fish data were used as a surrogate to estimate direct acute risks to the CRLF. 

Effects to freshwater fish from direct exposure to Paraquat could also indirectly affect the 

CRLF from reduction in available food. 

Paraquat is classified as slightly toxic to warm and cold freshwater fish on an acute 

exposure basis. The most sensitive freshwater species was the bluegill sunfish, with a 

96h LC50 of 13 mg/L (MRID 40098001 Mayer and Ellersieck 1986). Freshwater fish 

acute toxicity values range from a 96h LC50 of 13 mg/L to 156 mg/L (both endpoints are 

bluegill sunfish, MRID 40098001 Mayer and Ellersieck 1986). Other species within that 

range include the rainbow trout (15, 29, and 38.68 mg/L) (MRIDs 40098001, 00162738, 

and 00162736). These studies were completed using formulated products that contained 

21.2% to 29% of the paraquat cation. In general, the toxicity of these formulated 

products are similar in toxicity to a range of different species of freshwater fish. 

Freshwater Fish: Chronic Exposure (Early Life Stage and 

Reproduction) Studies 

There are no available freshwater fish chronic exposure toxicity studies conducted with 

Paraquat. The direct chronic toxicity values for the CRLF were calculated using an acute 

to chronic ratio (ACR) using a similar chemical compound, diquat dibromide (Section 

5.1.1). 

Freshwater Fish: Sublethal Effects and Additional Open 

Literature Information 

There were numerous studies found within the ECOTOX database, however they were 

not integrative in the measurements of growth and reproduction. Potential sublethal 

effects on fish are evaluated qualitatively and not used as part of the quantitative risk 

characterization. One study did provide some information regarding different 

biochemical factors influenced by exposures to paraquat (E104191). Further details on 

62
ECOTOX studies are provided in Appendix F, which also contains the rejection codes 

and other information as to why studies from ECOTOX were not used. 

4.1.1.4

4.1.2.1

4.1.2.2

Aquatic-phase Amphibian: Acute and Chronic Studies 

Studies were found in ECOTOX that used aquatic-phase amphibians as study organisms. 

However, there were some fatal flaws with all the frog studies that were reviewed and 

they were therefore, not used in this risk assessment. In particular there were concerns 

with the husbandry of the organisms (i.e., the amount of individuals per treatment 

replicate), as well as the lack of detailed information within the published literature. Also 

of concern was the presence of the solvent dimethyl sulfoxide (DMSO) as a co-solvent, 

which confounds the experiment as the solvent could be resulting in increased toxicity, 

and not the chemical being tested. In general there was an overall lack of information 

within the studies, especially in regards to the controls and the chemical solutions that 

were used within the experiments. There is also some concern with the testing methods 

used, Frog Embryo Teratogenesis Assay-Xenopus (FETAX). The study conditions do 

not provide the ability to deduce the chronic endpoints of the frogs, and the age of 

organisms used in this type of assay results in inherent variability. There are problems 

with the rearing of the frogs out past stage 54 as mortality levels are too high, resulting 

from the loading rate within the containers, and the feeding on the yolk until further 

development. Appendix F contains information as to why these studies and others from 

ECOTOX were not used within the assessment. 

4.1.2 Toxicity to Freshwater Invertebrates 

Freshwater aquatic invertebrate toxicity data were used to assess potential indirect effects 

of paraquat to the CRLF. Effects to freshwater invertebrates resulting from exposure to 

paraquat could indirectly affect the CRLF via reduction in available food items. As 

discussed in Section 2.5.3, the main food source for juvenile aquatic- and terrestrialphase CRLFs is thought to be aquatic invertebrates found along the shoreline and on the 

water surface, including aquatic sowbugs, larval alderflies and water striders. 

A summary of acute and chronic freshwater invertebrate data, including data published in 

the open literature, is provided below in Sections 4.1.2.1 through 4.1.2.3. 

Freshwater Invertebrates: Acute Exposure (Mortality) Studies 

Paraquat is classified as moderately toxic to freshwater invertebrates based on acceptable 

studies of the water flea (Daphnia magna). This species exhibited a 48h EC50 value of 

1.2 mg/L (MRID 0014473 Wheeler 1973). 

Freshwater Invertebrates: Chronic Exposure (Reproduction) 

Studies 

There are no available freshwater invertebrate chronic exposure toxicity studies 

conducted with Paraquat. The indirect chronic toxicity values for the CRLF were 

63
calculated using an acute to chronic ratio (ACR) using a similar chemical compound, 

diquat dibromide (Section 5.1.1). 

4.1.2.3

4.1.3.1

4.1.3.2

Freshwater Invertebrates: Sublethal Effects and Open 

Literature Data 

There were numerous studies found within the ECOTOX database, however they were 

not integrative in the measurements of growth and reproduction. The studies instead 

provided information on the sublethal effects observed. Additionally, the studies were 

not usable in this Risk Assessment. Further details on ECOTOX studies are provided in 

Appendix F, which also contains the rejection codes and other information as to why 

studies from ECOTOX were not used. 

4.1.3 Toxicity to Aquatic Plants 

Aquatic plant toxicity studies were used as one of the measures of effect to evaluate 

whether paraquat may affect primary production and the availability of aquatic plants as 

food for CRLF tadpoles. Primary productivity is essential for indirectly supporting the 

growth and abundance of the CRLF. 

Laboratory studies were used to determine whether paraquat may cause direct effects to 

aquatic plants. A summary of the laboratory data and freshwater field studies for aquatic 

plants is provided in Sections 4.1.3.2 and 4.1.3.2. 

Aquatic Plants: Laboratory Data 

Both the vascular and non-vascular aquatic plant studies that include the most sensitive 

species are Tier II toxicity tests. The freshwater diatom (Navicula pelliculosa) was the 

most sensitive non-vascular plant with a 4d EC50 of 0.396 μg/L (MRID 42601006 Smyth 

et al. 1992). The vascular plant Lemna gibba was the most sensitive vascular plant with a 

14d EC50 of 71 μg/L (MRID 42601003 Smyth et al. 1992). 

Freshwater Field Studies 

There were no submitted field studies. 

4.2 Toxicity of Paraquat to Terrestrial Organisms 

Table 4-3 summarizes the most sensitive terrestrial toxicity endpoints for the CRLF, 

based on an evaluation of both the submitted studies and the open literature. A brief 

summary of submitted and open literature data considered relevant to this ecological risk 

assessment for the CRLF is presented below. 

64
Table 4-3 Terrestrial Toxicity Profile for Paraquat Dichloride (expressed as the 

cation) 

Assessment 

Endpoint 

Species Toxicity Value 

Used in Risk 

Assessment 

Describe 

effect (i.e. 

mortality, 

growth, 

reproduction) 

Citation 

MRID # 

(Author & 

Date) 

Study 

Classification 

Direct Toxicity to Terrestrial-phase CRLF

Acute Dosebased Colinus 

virginianus 

(Northern 

Bobwhite) 

LD50 = 128.1 

mg/kg bw 

Mortality 00029001 

(Fink et al. 

1979) 

Acceptable 

Acute 

Dietary-based 

Coturnix 

coturnix 

japonica

(Japanese 

Quail) 

5d LC50 = 703 

ppm Mortality 

00022923 

(Hill et al. 

1975) 

Supplemental 

Chronic Anas 

platyrhynchos

(Mallard 

Duck)

NOAEC = 30 

ppm 

% viable eggs, 

eggs set, 

normality of 

hatchlings and 

# of 14d old 

survival 

00110455 

(Fink et al. 

1982) 

Acceptable 

Indirect Toxicity to Terrestrial-phase CRLF 

via acute 

toxicity to 

mammalian 

prey items

Rat LD50 = 90 mg/kg 

bw (F)

Mortality 00054573 

(Rittenhouse 

1977) 

Acceptable 

via chronic 

toxicity to 

mammalian 

prey items

Wistar 

Derived RatsAlderley Park 

Strain 

NOAEL= 7.5 

mg/kg-bw 

NOAEC = 108 

ppm 

Reproductive 

Toxicity- 3 

generation 

study 

Reproduction 

00126783 

00149748 

00149749 

Acceptable 

(HED 2006) 

via acute 

toxicity to 

terrestrial 

invertebrate 

prey items

Apis mellifera

(Honey bee) 

48h LD50 > 34.8 

μg/bee

Mortality 05001991 

(Stevenson 

1978) 

Acceptable 

via acute 

toxicity to 

terrestrial 

invertebrate 

prey items

Colinus 

virginianus 

(Northern 

Bobwhite) 

LD50 = 128.1 

mg/kg bw 

Mortality 00029001 

(Fink et al. 

1979) 

Acceptable 

via acute 

toxicity to 

terrestrial 

invertebrate 

prey items

Coturnix 

coturnix 

japonica

(Japanese 

Quail) 

5d LC50 = 703 

ppm 

Mortality 00022923 

(Hill et al. 

1975) 

Supplemental 

65
Assessment 

Endpoint 

Species Toxicity Value 

Used in Risk 

Assessment 

Describe 

effect (i.e. 

mortality, 

growth, 

reproduction) 

Citation Study 

MRID # Classification 

(Author & 

Date) 

via chronic 

toxicity to 

terrestrial 

invertebrate 

prey items

Anas 

platyrhynchos

(Mallard 

Duck)

NOAEC = 30 

ppm 

% viable eggs, 

eggs set, 

normality of 

hatchlings and 

# of 14d old 

survival 

00110455 

(Fink et al. 

1982) 

Acceptable 

Seedling 

Emergence

Monocots 

EC25 = 0.84 lbs 

cation/A (using 

most sensitive 

species (dicot 

data)) 

Growth 42639601 

(Canning and 

White 1992) 

Acceptable 

Seedling 

Emergence

Dicots 

(Cocklebur)

EC25 = 0.84 lbs 

cation/A 

Growth 42639601 

(Canning and 

White 1992) 

Acceptable 

Vegetative 

Vigor

Monocots 

(Corn)

EC25 = 0.16 lbs 

cation/A

Growth 42601001 

(Canning and 

White 1992) 

Acceptable 

via toxicity to 

terrestrial 

plants 

Vegetative 

Vigor

Dicots 

(Cocklebur ) 

EC25 = 0.014lbs 

cation/A

Growth 42601001 

(Canning and 

White 1992) 

Acceptable 

Acute toxicity to terrestrial animals is categorized using the classification system shown 

in Table 4-4 (U.S. EPA 2004). Toxicity categories for terrestrial plants have not been 

defined. 

Table 4-4 Categories of Acute Toxicity for Avian and Mammalian Studies 

Toxicity Category Oral LD50 Dietary LC50

Very highly toxic < 10 mg/kg < 50 ppm 

Highly toxic 10 - 50 mg/kg 50 - 500 ppm 

Moderately toxic 51 - 500 mg/kg 501 - 1000 ppm 

Slightly toxic 501 - 2000 mg/kg 1001 - 5000 ppm 

Practically non-toxic > 2000 mg/kg > 5000 ppm 

4.2.1 Toxicity to Birds 

As specified in the Overview Document, the Agency uses birds as a surrogate for 

terrestrial-phase amphibians when amphibian toxicity data are not available (U.S. EPA 

2004). No terrestrial-phase amphibian data are available for paraquat; therefore, acute 

and chronic avian toxicity data are used to assess the potential direct effects of paraquat 

to terrestrial-phase CRLFs. 

66
4.2.1.1

4.2.1.2

4.2.1.3

Birds: Acute Exposure (Mortality) Studies 

There were two acute avian oral studies that were submitted, one classified as acceptable, 

and the other classified as supplemental (MRID 00029001 Fink et al. 1979, & 00160000 

Hudson et al. 1984). The most sensitive species was the Northern Bobwhite quail with 

an acute oral LD50 value of 128.1 mg/kg (MRID 00029001 Fink et al. 1979). The 

mallard duck acute oral toxicity classification to paraquat ranges from moderately toxic 

(144.8 mg/kg) to slightly toxic (436.8 mg/kg), both studies are supplemental (Fink et al. 

1979). Paraquat is classified as being moderately toxic to birds on an acute oral basis. 

Subacute dietary tests are required to establish toxicity of paraquat to birds. The 

preferred test species are the mallard duck and the bobwhite quail. The most sensitive 

subacute dietary 5d LC50 value was 703 ppm for the Japanese quail (a non-guideline test 

species, indicating that the study was supplemental MRID 00022923 Hill et al. 1975). 

However, the Japanese quail is believed to be an acceptable representative of avain 

subacute dietary toxicity. Acceptable studies (from guideline bird species, the mallard 

duck and bobwhite quail) resulted in 5d LC50 values of 711 ppm for the northern 

bobwhite quail, and 2914.6 ppm for the mallard duck. Paraquat is classified as being 

moderately toxic to slightly toxic to birds on a subactue dietary basis. 

Birds: Chronic Exposure (Growth, Reproduction) Studies 

Avian reproductive toxicity endpoints were submitted for two species, the northern 

bobwhite quail (MRID 00110453 Fink et al. 1981) and the mallard duck (MRID 

00110455 Fink et al. 1982). The northern bobwhite quail appeared to be the less 

sensitive of the two species with an 8d NOAEC value of 100 ppm with adult mortality as 

the endpoint. The mallard duck was the most sensitive species with an 18 week NOAEC 

of 30 ppm, and a LOAEC value of 100 ppm (the highest concentration tested). 

Concentrations of the paraquat cation for the one generation mallard duck toxicity test 

were 0, 10, 30, and 100 ppm. Adverse affect endpoints were the reduction in the % 

viable eggs, the number of eggs set, the number of 14d old survivors, and normality of 

hatchlings (MRID 00110455 Fink et al. 1982). 

Terrestrial-phase Amphibian Acute and Chronic Studies 

There were no terrestrial-phase amphibian acute or chronic studies submitted or available 

in the open literature. 

4.2.2 Toxicity to Mammals 

Mammalian toxicity data are used to assess potential indirect effects of paraquat to the 

terrestrial-phase CRLF. Effects to small mammals resulting from exposure to paraquat 

could also indirectly affect the CRLF via reduction in available food. As discussed in 

Section 2.5.3, over 50% of the prey mass of the CRLF may consist of vertebrates such as 

mice, frogs, and fish (Hayes and Tennant 1985). 

67
4.2.2.1

4.2.2.2

4.2.3.1

Mammals: Acute Exposure (Mortality) Studies 

Paraquat is classified as moderately toxic on an acute oral basis to mammals. This is 

based on an acute oral LD50 value of 90.98 mg/kg-bw (MRID 00054573 Rittenhouse 

1977). See the HED Table from the most recent HED Human Health Risk Assessment in 

Appendix K. 

Mammals: Chronic Exposure (Growth, Reproduction) Studies 

In a 3-generation reproduction and fertility study in rats, paraquat was administered to 

Wistar Derived Rats-Alderley Park Strain (MRIDs 00126783, 00149748, & 00149749). 

The offspring and reproductive NOAEL were both 7.5 mg/kg-bw, and the both NOAECs 

were 108 ppm (MRIDs 00149748 & 00149749). Reproductive NOAEC was used to 

calculate RQ values. This study is classified as acceptable/guideline and satisfies the 

guideline for a reproduction toxicity study, OPPTS 870.3800. The parental NOAEL was 

0.9 mg/kg-bw, and the parental NOAEC was 18 ppm (MRID 00126783 Lindsay et al. 

1982). The parental LOAEL was 2.7 mg/kg-bw, and the parental LOAEC was 54 ppm 

(MRID 00126783 Lindsay et al. 1982). Parental toxicity endpoints were mortality and 

lung damage (particularly increased incidences of alveolar histiocytes), indicating a 

sublethal impact lower than the acute oral endpoints. HED determined that paraquat 

dichloride was not a mutagen or a carcinogen (HED 2006). Paraquat was found to be 

weakly positive in the mouse lymphoma assay and human lymphocyte cytogenetic assay 

and was positive in the sister chromatid exchange assay (HED 2006). Paraquat was not 

mutagenic in the bacterial gene mutation assay, not genotoxic in the unscheduled DNA 

synthesis assay in vitro or in vivo, was negative for chromosomal aberration in the bone 

marrow test, and no evidence was found for suppressed fertility or dominant lethal 

mutagenicity in mice. There was also no evidence of carcinogenicity in animal studies, 

and paraquat was classified as a Group E chemical (evidence of non-carcinogenicity in 

humans) (Cancer Peer Review Committee and the Science Advisory Committee 1989, 

HED 2006). See the HED Table from the most recent HED Human Health Risk 

Assessment in Appendix K. 

4.2.3 Toxicity to Terrestrial Invertebrates 

Terrestrial invertebrate toxicity data are used to assess potential indirect effects of 

paraquat to the terrestrial-phase CRLF. Effects to terrestrial invertebrates resulting from 

exposure to paraquat could also indirectly affect the CRLF via reduction in available 

food.

Terrestrial Invertebrates: Acute Exposure (Mortality) Studies 

A honeybee acute contact study (TGAI) resulted in a 48h LD50 > 34.8 μg/bee (MRID 

05001991 Stevenson 1978). Paraquat is classified as practically non-toxic to honeybees. 

68
4.2.3.2 Terrestrial Invertebrates: Open Literature Studies 

There were no terrestrial invertebrate studies available in the open literature. 

4.2.4 Toxicity to Terrestrial Plants 

Terrestrial plant toxicity data are used to evaluate the potential for paraquat to affect 

riparian zone and upland vegetation within the action area for the CRLF. Impacts to 

riparian and upland (i.e., grassland, woodland) vegetation may result in indirect effects to 

both aquatic- and terrestrial-phase CRLFs, as well as effects to designated critical habitat 

PCEs via increased sedimentation, alteration in water quality, and reduction in upland 

and riparian habitat that provides shelter, foraging, predator avoidance and dispersal for 

juvenile and adult CRLFs. 

Plant toxicity data from both registrant-submitted studies and studies in the scientific 

literature were reviewed for this assessment. Registrant-submitted studies are conducted 

under conditions and with species defined in EPA toxicity 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. Guideline studies generally evaluate toxicity to ten crop species. A drawback to 

these tests is that they are conducted on herbaceous crop species only, and extrapolation 

of effects to other species, such as the woody shrubs and trees and wild herbaceous 

species, contributes uncertainty to risk conclusions. 

Commercial crop species have been selectively bred, and may be more or less resistant to 

particular stressors than wild herbs and forbs. The direction of this uncertainty for 

specific plants and stressors, including paraquat, is largely unknown. Homogenous test 

plant seed lots also lack the genetic variation that occurs in natural populations, so the 

range of effects seen from tests is likely to be smaller than would be expected from wild 

populations. 

The most sensitive results of the Tier II seedling emergence and vegetative vigor toxicity 

tests on non-target plants are summarized below in Table 4-5. For seedling emergence 

only one of four monocot species had scientifically sound results, and it was higher than 

dicot values. Therefore, it was assumed that monocots were as sensitive as the most 

sensitive dicot. An addendum DER to MRID 42601001 (Canning and White 1992) was 

made as the statistics for the non-target plants vegetative vigor phytotoxicity study were 

recalculated using the NUTHATCH statistical program. 

69
Table 4-5 Non-target Terrestrial Plant Seedling Emergence and Vegetative Vigor 

Toxicity (Tier II) Data 

Crop Type of 

Study 

Species 

NOAEC 

(lb cation/A) 

EC25

(lb cation/A) 

Most sensitive 

parameter 

Seedling Emergence 

Monocots Assumed as sensitive as dicots 

Dicots Cocklebur 0.423 0.85 Dry weight 

Vegetative Vigor 

Monocots Corn 0.064 0.16 

(95% CI 0.073–0.36) 

Dry weight 

Dicots Cockelbur EC05 = 0.0065 

 (95% CI 0.0040–0.01) 

0.014 

(95% CI 0.01–0.019) 

Dry weight 

4.3 Use of Probit Slope Response Relationship to Provide Information on the 

Endangered Species Levels of Concern 

The Agency uses the probit dose response relationship as a tool for providing additional 

information on the potential for acute direct effects to individual listed species and 

aquatic animals that may indirectly affect the listed species of concern (U.S. EPA 2004). 

As part of the risk characterization, an interpretation of acute RQ for listed species is 

discussed. This interpretation is presented in terms of the chance of an individual event 

(i.e., mortality or immobilization) should exposure at the EEC actually occur for a species 

with sensitivity to paraquat on par with the acute toxicity endpoint selected for RQ 

calculation. To accomplish this interpretation, the Agency uses the slope of the dose 

response relationship available from the toxicity study used to establish the acute toxicity 

measures of effect for each taxonomic group that is relevant to this assessment. The 

individual effects probability associated with the acute RQ is based on the mean estimate 

of the slope and an assumption of a probit dose response relationship. In addition to a 

single effects probability estimate based on the mean, upper and lower estimates of the 

effects probability are also provided to account for variance in the slope, if available. 

Individual effect probabilities are calculated based on an Excel spreadsheet tool IECV1.1 

(Individual Effect Chance Model Version 1.1) developed by the U.S. EPA, OPP, 

Environmental Fate and Effects Division (June 22, 2004). The model allows for such 

calculations by entering the mean slope estimate (and the 95% confidence bounds of that 

estimate) as the slope parameter for the spreadsheet. In addition, the acute RQ is entered 

as the desired threshold. 

4.4 Incident Database Review 

A review of the EIIS database for ecological incidents involving paraquat and paraquat 

dichloride, which are used interchangeably, was completed on April 17, 2009. The 

results of this review for terrestrial, plant, and aquatic incidents are discussed below in 

Sections 4.4.1 through 4.4.3, respectively. A total of 4 incidents involving paraquat (PC 

code 061603) were reported, and a total of 26 incidents involving paraquat dichloride (PC 

70
code 061601) were reported. There were no incidents reported for paraquat bis (methyl 

sulfate) (PC code 061602). A complete list of the incidents involving paraquat and 

paraquat dichloride including associated uncertainties is included as Appendix H. 

4.4.1 Terrestrial Incidents 

There were no reported terrestrial incidents for paraquat (PC code 061603). There were 

three reported terrestrial incidents for paraquat dichloride (PC code 061601). Two 

incidents resulted in the mortality of birds, and the other reported both bird mortality and 

plant damage (I008168-001, I000097-015, and I007334-001). Two of the incidents 

occurred in Virginia one was a formulation with a registered use on corn that resulted in 

the mortality of 5 Canadian geese where paraquat dichloride was the probable cause of 

the incident in June 1998 (I008168-001). The other incident was from the consumption 

of granules resulting in the death of approximately 12 birds. However, paraquat 

dichloride was unlikely the cause of the incident on May 1989 (I000097-015), as there 

are no granular forms of paraquat. Paraquat was reported since it was applied within the 

area of the observed incident. The other incident occurred in Illinois resulting in plant 

damage on 18 of 103 acres of corn, and the death of 4 unknown species of birds in June 

1998 from drift/spray exposure (I007334-001), the legal classification was that paraquat 

was of probable cause for the incident. 

4.4.2 Plant Incidents 

There were a total of four plant incidents listed for paraquat (PC code 061603). Plant 

damage was sustained to three non-target species; radishes, apples, and ornamentals 

(I014409-001, I013884-038, and I013884-014), while plant mortality was reported for 

alfalfa (I014409-024). All incidents reported were from Washington, one from a 

registered use on peas where ornamentals were affected, and paraquat was the probable 

cause of plant damage; one from a misuse (alfalfa) where paraquat was a possible cause 

of mortality; and the other two the use was of undetermined legality, and paraquat was a 

possible cause of plant damage. All exposures occurred from drift, with apples also 

being exposed from direct spray. The total magnitude of exposure was not reported. 

There were a total of 17 plant incidents listed for paraquat dichloride (PC code 061601). 

Two accidental misuse incidents occurred in an agricultural area in Wisconsin resulting 

in plant damage to alfalfa, ash, and ornamentals (I005660-005), and ash, oats and 

discoloration to alfalfa (I005880-005) from drift exposure. Paraquat was the probable 

cause of these incidents. Three accidental misuse incidents on corn resulted in plant 

damage to grass (I007371-033, I007371-034, and I007371-035) on June 17, 1997, and 

paraquat was the propable cause of these incidents. 120 acres of a corn field was 

damaged after undetermined legality of broadcast application occurred May 9, 2000, and 

paraquat was a possible cause of this incident (I012366-023). Plant damage to 181 acres 

of peppermint occurred after registered use May 1, 1996, and paraquat was a possible 

cause of this incident (I013636-029). Undetermined legality use of paraquat dichloride 

application was the possible cause of 150 (75% of 200) acres of corn damaged on May 

11, 1999 in Alabama (I009573-009). After the registered use application of paraquat 

71
dichloride 10 acres of peanuts were damaged June 19, 2001 in North Carolina, and 

paraquat was a possible cause of this incident (I011838-055). Carryover from the 

broadcast application of paraquat dichloride was the probable cause of damage to 1040 

acres of corn May 24, 2002 in Illinois (I013554-040). The registered use application of 

paraquat dichloride was a possible cause of the damage to 60 acres of pasture grass April 

23, 2003 in Georgia (I014034-009). The registered application of paraquat dichloride on 

peanuts was a possible cause of plant damage to 5 acres of peanuts in Virginia May 23, 

2001 (I012684-010). Undetermined legality use of paraquat dichloride on peanuts was a 

possible cause of plant damage to 80 acres of peanuts in Oklahoma (I011838-091) and all 

25 acres in Georgia (I011838-038). Registered use of broadcast application of paraquat 

dichloride to soybeans was a possible cause of plant damage to an unknown tree as a 

result of drift June 27, 1994 in Arkansas (I001131-001). It is highly probable that the 

accidental misuse of paraquat dichloride on soybeans resulted in plant damage to 

unknown amounts of soybeans as the result of drift May 20, 1997 in Pennsylvania 

(I007371-008). Intentional misuse of paraquat dichloride by spray application on wheat 

was the probable cause of plant damage to 120 of 184 acres from drift January 26, 2005 

in California (I016940-005). 

4.4.3 Aquatic Incidents 

There were no reported aquatic incidents listed for paraquat (PC code 061603). There 

were six reported aquatic incidents listed for paraquat dichloride (PC code 061601). 

Undetermined legality use of paraquat dichloride on an agricultural area was a possible 

cause of the mortality of 54 fish (1 largemouth bass, and 53 sunfish) due to runoff June 4, 

1981 in Virginia (B0000-502-18). Registered usage of paraquat dichloride application on 

an agricultural area in North Carolina was unlikely the cause of the mortality to unknown 

amount of unknown fish November 29, 1993 (I003654-012). Registered use of paraquat 

dichloride on corn was unlikely the cause of the mortality of numerous unknown fish 

from runoff in Kentucky April 1, 1992 (B000175-001). The application of paraquat 

dichloride on a field was a possible cause of the mortality of fish (bass, bluegill, and 

crappie) in Indiana June 2, 1997 (I009314-005). An undetermined legality application of 

paraquat dichloride was a possible cause of the mortality of an unknown amount of bass, 

bluegill, and crappie to die in Indiana January 1, 1997 (I005805-001). Paraquat 

dichloride was was a possible cause of the 200 bass and bluegills that were found dead 

after an undetermined legality application of paraquat dichloride June 3, 1999 (I008768-

007). 

72
5.0 Risk Characterization 

Risk characterization is the integration of the exposure and effects characterizations. 

Risk characterization is used to determine the potential for direct and/or indirect effects to 

the CRLF or for effects to its designated critical habitat from the use of paraquat in CA. 

The risk characterization provides an estimation (Section 5.1) and a description (Section 

5.2) of the likelihood of adverse effects; articulates risk assessment assumptions, 

limitations, and uncertainties; and synthesizes an overall conclusion regarding the 

likelihood of adverse effects to the CRLF or its designated critical habitat (i.e., “no 

effect,” “likely to adversely affect,” or “may affect, but not likely to adversely affect”). 

5.1 Risk Estimation 

Risk is estimated by calculating the ratio of exposure to toxicity. This ratio is the risk 

quotient (RQ), which is then compared to pre-established acute and chronic levels of 

concern (LOCs) for each category evaluated (Appendix C). For acute exposures to the 

CRLF and its animal prey in aquatic habitats, as well as terrestrial invertebrates, the LOC 

is 0.05. For acute exposures to the CRLF in its terrestrial environment and mammals, the 

LOC is 0.1. The LOC for chronic exposures to CRLF and its prey, as well as acute 

exposures to plants is 1.0. 

Risk to the aquatic-phase CRLF is estimated by calculating the ratio of exposure to 

toxicity using 1-in-10 year EECs based on the label-recommended paraquat usage 

scenarios summarized in Table 3-3 and the appropriate aquatic toxicity endpoint from 

Table 4-1. Risks to the terrestrial-phase CRLF and its prey (e.g. terrestrial insects, small 

mammals and terrestrial-phase frogs) are estimated based on exposures resulting from 

applications of paraquat (Table 3-5 and Table 3-6) and the appropriate toxicity endpoint 

from Table 4-3. Exposures are also derived for terrestrial plants, as discussed in Section 

3.4 and toxicity summarized in Section 4.2.4, based on the highest application rates of 

paraquat use within the action area. 

Due to Paraquat being a cation, the risk estimated may be more conservative for soils 

with higher clay contents due to paraquat’s ability to dissipate by rapid adsorption to 

biological materials and clay particles. The bound residues do not appear to be 

environmentally available because they are so strongly adsorbed. Paraquat is also very 

immobile in soil with adsorption Kds ranging from 68-50,000. Paraquat has such 

extremely high adsorption coefficients, it is not expected to desorb from the soil. 

5.1.1 Exposures in the Aquatic Habitat 

5.1.1.1 Direct Effects to Aquatic-Phase CRLF 

Direct acute effects to the aquatic-phase CRLF are based on peak EECs in the standard 

pond and the lowest acute toxicity value for freshwater fish. In order to assess direct 

chronic risks to the CRLF, 60-day EECs and the lowest chronic toxicity value for 

freshwater fish are used. However, since freshwater fish chronic toxicity values are not 

73
available at this time, the direct chronic toxicity values for the CRLF were calculated 

using an acute to chronic ratio. The acute to chronic ratio (ACR) was completed using a 

similar chemical compound, diquat dibromide. However, the ACR could not be 

calculated for freshwater fish (direct effects to the CRLF) using the same method as the 

indirect effects, as the species of fish were different in the diquat acute and chronic 

studies for the direct effects. There is greater uncertainty when using two different 

species of fish, in addition to the general uncertainty related with comparing toxicity of 

two different compounds. The ACR for invertebrates was completed and the estimated 

indirect chronic toxicity of paraquat to the CRLF was approximately 0.174 ppm (174 

ppb). For qualitative purposes, although it is not general practice, the chronic direct 

effect was calculated using the calculated chronic ratio from the invertebrates divided by 

the acute LC50 value from the freshwater fish. Because these species are different, it 

increases the uncertainty, but can provide valuable information in helping to discuss the 

possible toxicity to the aquatic phase CRLF directly. The ACR for freshwater fish and 

the estimated direct chronic toxicity of paraquat to the CRLF was found to be 

approximately 1.89 ppm (1890 ppb). As seen in Table 5-1, none of the EECs exceed 1, 

890 ppb. As a result, it appears that paraquat is determined to have no direct effect on 

the aquatic-phase of the CRLF on a chronic basis. 

Based on the data presented in Table 5-1, it appears that paraquat has no acute direct 

affects on the aquatic-phase CRLF since there are no LOC exceedences.

Table 5-1 Summary of Acute Direct Effect RQs* for the Aquatic-phase CRLF 

Use EEC 

(μg/L)b RQ 

Probability of 

Individual 

Effect at 

RQC

LOC 

Exceedance 

and Risk 

Interpretation 

33 0.003 No exceedance NO 

AGRICULTURAL FALLOW/IDLELAND 27 0.002 No exceedance NO

AIRPORTS/LANDING FIELDS, 

COMMERCIAL/INSTITUTIONAL/INDU

STRIAL PREMISES/EQUIPMENT 

(OUTDOOR), NONAGRICULTURAL 

AREAS (PUBLIC HEALTH USE), 

NONAGRICULTURAL RIGHTS-OFWAY/FENCEROWS/HEDGEROWS 

55 0.004 No exceedance NO

GUAVA, PASSION FRUIT 

(GRANADILLA) 55 0.004 No exceedance NO

2.0 <0.001 No exceedance NO 

MELONS, PEANUTS (ground only) 1.6 <0.001 No exceedance NO a

 RQs associated with acute direct toxicity to the CRLF are also used to assess potential indirect effects to the 

CRLF based on a reduction in freshwater fish and frogs as food items. No chronic data is available. b

 The highest EEC based on maximum application rate per use (see Table 3-3). c

 A probit slope value for the acute bluegill sunfish toxicity test is not available; therefore, the effect probability 

was calculated based on a default slope assumption of 4.5 with upper and lower 95% confidence intervals of 2 

and 9 (Urban and Cook, 1986). (Only calculated when there was an exceedance). *

 RQ < acute endangered species LOC of 0.05. 

^ The most sensitive species used to determine the acute direct effects (surrogate species) was the bluegill 

Sunfish (96h LC50 = 13000 ppb). 

74
5.1.1.2 Indirect Effects to Aquatic-Phase CRLF via Reduction in Prey 

(non-vascular aquatic plants, aquatic invertebrates, fish, and 

frogs) 

a) Non-vascular Aquatic Plants

Indirect effects of paraquat to the aquatic-phase CRLF (tadpoles) via reduction in nonvascular aquatic plants in its diet are based on peak EECs from the standard pond and the 

lowest toxicity value EC50 for aquatic non-vascular plants. RQ’s exceed the modeled 

EECs (Table 5-2); therefore, paraquat may affect the aquatic-phase CRLF 

indirectly via effects to non-vascular aquatic plants. 

Table 5-2 Summary of RQs* Used to Estimate Indirect Effects to the CRLF via 

Effects to Non-Vascular Aquatic Plants (diet of CRLF in tadpole life stage and 

habitat of aquatic-phase CRLF) 

Use 

Peak 

EEC 

(μg/L)

Indirect effects RQ a

(food and habitat) 

33 82 

AGRICULTURAL FALLOW/IDLELAND 27 69 

AIRPORTS/LANDING FIELDS, 

COMMERCIAL/INSTITUTIONAL/INDUSTRIAL 

PREMISES/EQUIPMENT (OUTDOOR), NONAGRICULTURAL AREAS 

(PUBLIC HEALTH USE), NONAGRICULTURAL RIGHTS-OFWAY/FENCEROWS/HEDGEROWS 

55 138 

GUAVA, PASSION FRUIT (GRANADILLA) 55 138 

2.0 4.9 

MELONS, PEANUTS (ground only) 1.6 4.1 

a

 RQs used to estimate indirect effects to the CRLF via toxicity to vascular aquatic plants are summarized in Table 5-4 

* LOC exceedances (RQ > 1) are bolded and shaded. RQ = use-specific peak EEC/ most sensitive non-vascular 

aquatic plant endpoint (Acute: Navicula pelliculosa EC50= 0.396 ppb).

b) Aquatic Invertebrates

As mention in Section 5.1.1.1, in the absence of chronic data, the acute to chronic ratio 

(ACR) between diquat and paraquat was calculated. The ACR for invertebrates and the 

estimated indirect chronic toxicity of paraquat to the CRLF was found to be 

approximately 174 ppb (0.174 ppm). As seen in Table 5-3, RQs do not exceed the LOC 

and therefore, paraquat has no indirect effects on the CRLF via reduction in 

freshwater invertebrate prey items on a chornic basis. 

A summary of the acute RQ values for exposure to aquatic invertebrates (as prey items of 

aquatic-phase CRLFs) is provided in Table 5-3. As described above for Table 5-1, probit 

slope analysis was not performed because there were no exceedances for any use. Based 

on the data presented in Table 5-3 paraquat has no indirect effect on the CRLF via 

reduction in freshwater invertebrate prey items on an acute basis. 

75
Table 5-3 Summary of Acute RQs* Used to Estimate Indirect Effects to the CRLF 

via Direct Effects on Aquatic Invertebrates as Dietary Food Items (prey of CRLF 

juveniles and adults in aquatic habitats) 

Use Peak EEC 

(μg/L)

Indirect 

Effects 

Acute 

RQ* 

Probability of 

Individual Effect at 

RQa 

33 0.025 No exceedance 

AGRICULTURAL FALLOW/IDLELAND 27 0.021 No exceedance 

AIRPORTS/LANDING FIELDS, 

COMMERCIAL/INSTITUTIONAL/INDUSTRIAL 

PREMISES/EQUIPMENT (OUTDOOR), 

NONAGRICULTURAL AREAS (PUBLIC HEALTH 

USE), NONAGRICULTURAL RIGHTS-OFWAY/FENCEROWS/HEDGEROWS 

55 0.042 No exceedance 

GUAVA, PASSION FRUIT (GRANADILLA) 55 0.042 No exceedance 

2.0 0.002 No exceedance 

MELONS, PEANUTS (ground only) 1.6 0.001 No exceedance 

* = LOC exceedances (acute RQ > 0.05) are bolded and shaded. Acute RQ = use-specific peak EEC / acute freshwater 

invertebrate endpoint (Daphnia magna EC50 =1200 ppb). No chronic data is available. a

 A probit slope value for the acute Daphnia magna toxicity test is not available; therefore, the effect probability was 

calculated based on a default slope assumption of 4.5 with upper and lower 95% confidence intervals of 2 and 9 (Urban 

and Cook, 1986). 

c) Fish and Frogs

Fish and frogs also represent potential prey items of adult aquatic-phase CRLFs. RQs 

associated with acute and chronic direct toxicity to the CRLF (Table 5-1) are used to 

assess potential indirect effects to the CRLF based on a reduction in freshwater fish and 

frogs as food items. However, since freshwater fish chronic toxicity values are not 

available at this time, the indirect chronic toxicity values for the CRLF were estimated 

using the ACR of freshwater invertebrates exposed to diquat. Based on Table 5-1 it can 

be concluded that paraquat will have no indirect effects on the aquatic-phase of the 

CRLF on an acute or chronic basis from fish or other aquatic-phase frogs as prey 

items. 

5.1.1.3 Indirect Effects to CRLF via Reduction in Habitat and/or 

Primary Productivity (Freshwater Aquatic Plants) 

Indirect effects to the CRLF via direct toxicity to aquatic plants are estimated using the 

most sensitive non-vascular and vascular plant toxicity endpoints. Because there are no 

obligate relationships between the CRLF and any aquatic plant species, the most sensitive 

EC50 values, rather than NOAEC values, were used to derive RQs. Based on the data 

presented in Tables 5-2 and 5-4, paraquat may affect the CRLF via reduction in nonvascular plants, but will have no indirect effect via reduction in vascular plants. 

76
Table 5-4 Summary of RQs* Used to Estimate Indirect Effects to the CRLF via 

Effects to Vascular Aquatic Plants (habitat of aquatic-phase CRLF)a 

Use Peak EEC 

(μg/L)

Indirect effects 

RQ a

(food and 

habitat) 

33 0.46 

AGRICULTURAL FALLOW/IDLELAND 27 0.38 

AIRPORTS/LANDING FIELDS, COMMERCIAL/INSTITUTIONAL/INDUSTRIAL 

PREMISES/EQUIPMENT (OUTDOOR), NONAGRICULTURAL AREAS (PUBLIC 

HEALTH USE), NONAGRICULTURAL RIGHTS-OFWAY/FENCEROWS/HEDGEROWS 

55 0.77 

GUAVA, PASSION FRUIT (GRANADILLA) 55 0.77 

2.0 0.03 

MELONS, PEANUTS (ground only) 1.6 0.02 

a

 RQs used to estimate indirect effects to the CRLF via toxicity to non-vascular aquatic plants are summarized in Table 5-2 

* = LOC exceedances (RQ > 1) are bolded and shaded. RQ = use-specific peak EEC / vascular aquatic plant endpoint (Lemna 

gibba EC50= 71 ppb). 

5.1.2 Exposures in the Terrestrial Habitat 

5.1.2.1 Direct Effects to Terrestrial-phase CRLF 

As previously discussed in Section 3.3, potential direct effects to terrestrial-phase CRLFs 

are based on broadcast foliar (ground and aerial) application of paraquat. Potential direct 

acute effects of paraquat to the terrestrial-phase CRLF are derived by considering doseand dietary-based EECs modeled in T-REX for a small bird (20 g) consuming small 

invertebrates (Table 3-5) and acute oral and subacute dietary toxicity endpoints for avian 

species. Acute effects are estimated using the lowest available toxicity data for birds. 

EECs are divided by toxicity values to estimate acute dietary-based RQs (Table 5-5). 

The Japanese quail was the most sensitive to paraquat on a subacute dietary basis (LC50 = 

703ppm), and the Northern bobwhite quail was the most sensitive on an acute dietary 

basis (LD50 = 128.1 mg/kg-bw) were therefore selected to serve as a surrogate for the 

CRLF. Resulting acute dietary and dose-based RQs for all but one use (melons with the 

lowest application rate) of paraquat exceed the Agency’s acute endangered species LOC 

of 0.1 for the CRLF (Table 5-5). The probability of individual effect at the endangered 

species LOC (0.1) ranges from 1 in 1.46 with a 95% CI of 1 in 1.71 to 1 in 1.20 (for 

airports) to 1 in 1.71*103

 with a 95% CI of 1 in 1.34*101 to 1 in 2.35*1010 (for carrots). 

The effect probability was calculated based on a default slope assumption of 4.5 with 

upper and lower 95% confidence intervals of 2 and 9 (Urban and Cook, 1986). 

Potential direct chronic effects of paraquat to the terrestrial-phase CRLF are derived by 

considering dietary-based exposures modeled in T-REX for a small bird (20g) consuming 

small invertebrates (Table 3-5). Chronic effects are estimated using the lowest available 

toxicity data for birds. EECs are divided by toxicity values to estimate chronic dietarybased RQs (Table 5-6). Chronic reproductive effects for the Mallard duck were observed 

77
with a NOAEC of 30 ppm. The chronic dietary-based RQs for the terrestrial phase CRLF 

exceed the Agency’s chronic LOC of 1.0 for all uses of paraquat (Table 5-6). The 

probability of individual effect probit slope analysis is not applicable for chronic 

endpoints. Based on the potential for both acute and chronic effects (Tables 5-5 and 

5-6) paraquat may directly affect the terrestrial-phase of the CRLF. 

Table 5-5 Summary of Acute RQs* Used to Estimate Direct Effects to the 

Terrestrial-phase CRLF (foliar application) From T-REX 

Use 

Application Rate (lb cation/acre) 

Dietary-based 

Acute RQ1 

Probability of 

Individual Effect at 

RQa 

AIRPORTS/LANDING FIELDS, 

COMMERCIAL/INSTITUTIONAL/INDUSTRIAL 

PREMISES/EQUIPMENT (OUTDOOR), 

NONAGRICULTURAL AREAS (PUBLIC HEALTH USE) 

(1 lb cation/acre; 10 times/yr; 5 day intervals) (maximumnon-ag) 

1.28 1 in 1.46 

(1 in 1.71 to 1 in 1.20)a

Guava (1 lb cation/acre; 10 times/yr; 5 day intervals) 

(maximum- ag) 1.28 1 in 1.46 

 (1 in 1.71 to 1 in 1.20)a

Ginger (1 lb cation/acre; 6 times/yr; 30 day intervals (ag)) 0.42 

1 in 2.22 E+01 

 (1 in 4.43 E+01 to 1 in 

2.87 E+03) a

CORN (SILAGE) (0.5 lb cation/acre; 3 times/year; 5 day 

intervals) 0.26 

1 in 2.36 E+02 

(1 in 8.27 to 1 in 1.43 

E+07)a

BEANS - SUCCULENT (LIMA/SNAP), CARROT 

(INCLUDING TOPS), PEAS (UNSPECIFIED), PEPPER 

(1 lb cation/acre; 1 time/yr; 1 day intervals) 

0.19 

1 in 1.71 E+03 

(1 in 1.34E+01 to 1 in 

2.35 E+10) a

Melons (0.3 lb cation/acre; 1 time/yr; 1 day intervals) 

(minimum-ag) 0.06 No exceedance 

* = LOC exceedances (Acute RQ > 0.1) are bolded and shaded. 1

 Based on Japanese quail LC50 = 703 ppm. a

 A probit slope value for the acute avian toxicity test is not available; therefore, the effect probability was 

 calculated based on a default slope assumption of 4.5 with upper and lower 95% confidence intervals of 2 

 and 9 (Urban and Cook, 1986).

Table 5-6 Summary of Chronic RQs* Used to Estimate Direct Effects to the 

Terrestrial-phase CRLF (foliar application) From T-REX 

Use 

(Application Rate) Dietary-based Chronic RQ1 

AIRPORTS/LANDING FIELDS, 

COMMERCIAL/INSTITUTIONAL/INDUSTRIAL 

PREMISES/EQUIPMENT (OUTDOOR), 

NONAGRICULTURAL AREAS (PUBLIC HEALTH 

USE) (1 lb cation/acre; 10 times/yr; 5 day intervals) 

(maximum- non-ag) 

30.00 

Guava (1 lb cation/acre; 10 times/yr; 5 day intervals) 

(maximum- ag) 30.00 

78
Use Dietary-based Chronic RQ1 

(Application Rate) 

Ginger (1 lb cation/acre; 6 times/yr; 30 day intervals 

(ag)) 9.76 

CORN (SILAGE) (0.5 lb cation/acre; 3 times/year; 5 day 

intervals) 6.13 

BEANS - SUCCULENT (LIMA/SNAP), CARROT 

(INCLUDING TOPS), PEAS (UNSPECIFIED), 

PEPPER (1 lb cation/acre; 1 time/yr; 1 day intervals) 

4.50 

Melons (0.3 lb cation/acre; 1 time/yr; 1 day intervals) 

(minimum-ag) 1.35 

* = LOC exceedances (Chronic RQ > 1) are bolded and shaded. 1

 Based on Mallard duck NOAEC = 30 ppm.

5.1.2.2 Indirect Effects to Terrestrial-Phase CRLF via Reduction in 

Prey (terrestrial invertebrates, mammals, and frogs) 

a) Terrestrial Invertebrates 

In order to assess the risks of paraquat to terrestrial invertebrates, which are considered 

prey of CRLF in terrestrial habitats, the honey bee is used as a surrogate for terrestrial 

invertebrates. The toxicity value for terrestrial invertebrates is calculated by multiplying 

the lowest available acute contact LD50 > 34.6μg cation/bee by 1 bee/0.128g, which is 

based on the weight of an adult honey bee. EECs (μg a.i./g of bee) calculated by T-REX 

for small and large insects are divided by the calculated toxicity value for terrestrial 

invertebrates, which is > 270.3 μg cation/g of bee, to calculate RQs. Indirect effects 

cannot be precluded from available data because the LD50 exceeds the levels tested even 

though pesticide related mortality was observed Therefore, it is determined that 

paraquat may indirectly affect the CRLF via reduction in terrestrial invertebrate 

prey items. 

b) Mammals 

Risks associated with ingestion of small mammals by large terrestrial-phase CRLFs are 

derived for dietary-based and dose-based exposures modeled in T-REX for a small 

mammal (15g) consuming short grass. Acute and chronic effects are estimated using the 

most sensitive mammalian toxicity data (LD50 = 90.98 mg/kg-bw). EECs are divided by 

the toxicity value to estimate acute and chronic dose-based RQs as well as chronic 

dietary-based RQs (Table 5-7). RQs representing acute and chronic exposures of small 

mammals consuming short grass on the treated field contaminated with paraquat exceed 

endangered species LOC of 0.1 (acute) and 1 (chronic) for all uses except melons (Table 

5-7). Melons chronic dietary based RQ does not exceed the endangered species LOC of 

0.1. The probability of individual effect at the endangered species LOC (0.1) is 1 in 

2.94*105 (95%CI: 1 in 4.40*101 to 8.86*1018). The effect probability was calculated 

based on a default slope assumption of 4.5 with upper and lower 95% confidence 

intervals of 2 and 9 (Urban and Cook, 1986). 

79
The probability of individual effect probit slope analysis is not applicable for chronic 

endpoints. HED determined that paraquat dichloride was not a mutagen or a carcinogen 

(HED 2006). Paraquat was found to be weakly positive in the mouse lymphoma assay 

and human lymphocyte cytogenetic assay and was positive in the sister chromatid 

exchange assay (HED 2006). Paraquat was not mutagenic in the bacterial gene mutation 

assay, not genotoxic in the unscheduled DNA synthesis assay in vitro or in vivo, was 

negative for chromosomal aberration in the bone marrow test, and not evidence was 

found for suppressed fertility or dominant lethal mutagenicity in mice. There was also no 

evidence of carcinogenicity in animal studies, and paraquat was classified as a Group E 

chemical (evidence of non-carcinogenicity in humans) (Cancer Peer Review Committee 

and the Science Advisory Committee 1989, HED 2006). Based on the acute and 

chronic LOC exceedances of paraquat on small mammal prey (Table 5-7), paraquat 

may indirectly affect the CRLF via reduction in small mammal prey items.

Table 5-7 Summary of Acute and Chronic RQs* Used to Estimate Indirect Effects 

to the Terrestrial-phase CRLF via Direct Effects on Small Mammals as Dietary 

Food Items (foliar application) 

Use Chronic RQ Acute RQ 

(Application Rate) Dose-based 

Chronic RQ1 

Dietary-based 

Chronic RQ2 

Dose-based 

Acute RQ3 

Probability of 

% Effect at 

Acute RQa

AIRPORTS/LANDING FIELDS, 

COMMERCIAL/INSTITUTIONAL/INDU

STRIAL PREMISES/EQUIPMENT 

(OUTDOOR), NONAGRICULTURAL 

AREAS (PUBLIC HEALTH USE) (1 lb 

cation/acre; 10 times/yr; 5 day intervals) 

(maximum- non-ag) 

128.53 14.81 7.63 

100% 

1 in 1 (1 in 

1.04 to 1 in 1)a 

Guava (1 lb cation/acre; 10 times/yr; 5 day 

intervals) (maximum- ag) 128.53 14.81 7.63 

100% 

1 in 1 (1 in 

1.04 to 1 in 1) a

Ginger (1 lb cation/acre; 6 times/yr; 30 

day intervals (ag)) 41.82 4.82 2.48 

96% 

1 in 1.04 ( 1 in 

1.27 to 1 in 1) a

CORN (SILAGE) (0.5 lb cation/acre; 3 

times/year; 5 day intervals) 26.28 3.03 1.56 

81% 

1 in 1.24 (1 in 

1.54 to 1 in 

1.04) a

BEANS - SUCCULENT (LIMA/SNAP), 

CARROT (INCLUDING TOPS), PEAS 

(UNSPECIFIED), PEPPER (1 lb 

cation/acre; 1 time/yr; 1 day intervals) 

19.28 2.22 1.14 

60% 

1 in 1.66 (1 in 

1.83 to 1 in 

1.44) a

80
Melons (0.3 lb cation/acre; 1 time/yr; 1 day 

intervals) (minimum-ag) 5.78 0.67 0.34 

1.75% 

1 in 57.1 (1 in 

5.73 to 1 in 

8.07E+04) a

* = LOC exceedances (acute RQ > 0.1 and chronic RQ > 1) are bolded and shaded. 

1

 Based on dose-based EEC and paraquat rat NOAEL = 5.4 mg/kg-bw. 2 Based on dietary-based EEC and paraquat rat NOAEC = 108 mg/kg-diet. 3

 Based on dose-based EEC and paraquat rat acute oral LD50 = 90.98 mg/kg-bw. a

 A probit slope value for the acute avian toxicity test is not available; therefore, the effect probability was calculated 

 based on a default slope assumption of 4.5 with upper and lower 95% confidence intervals of 2 and 9 (Urban and 

 Cook, 1986).

c) Frogs

An additional prey item of the adult terrestrial-phase CRLF is other species of frogs. In 

order to assess risks to these organisms, dietary-based and dose-based exposures modeled 

in T-REX for a small bird (20g) consuming small invertebrates are used. See Section 

5.1.2.1 and associated table (Table 5-5 and 5-6) for results. The acute LOC is exceeded 

for all uses of paraquat except Melons, and the chronic LOC is exceeded for all uses of 

paraquat. Based on the acute and chronic LOC exceedances, paraquat may indirectly 

affect the CRLF via reduction in frogs as prey items. 

5.1.2.3

5.1.3.1

Indirect Effects to CRLF via Reduction in Terrestrial Plant 

Community (Riparian and Upland Habitat) 

Potential indirect effects to the CRLF resulting from direct effects on riparian and upland 

vegetation are assessed using the most sensitive terrestrial plant data, in this case 

vegetative vigor EC25 data, as a screen. The vegetative vigor endpoint is EC25 = 0.014 lb 

cation/acre. To determine the reduction in terrestrial plants, a spray drift analysis using 

AgDRIFT was performed. Please see Section 5.2.5.1 for an explanation of the effects of 

spray drift. 

Paraquat may indirectly affect the CRLF via reduction in terrestrial plants from 

both aerial and ground application for vegetation that is located between the site of 

application to the border of where plant exposure is expected. The distance of effect 

is expected to be 1,000 feet or possibly greater.

5.1.3 Primary Constituent Elements of Designated Critical Habitat 

Aquatic-Phase (Aquatic Breeding Habitat and Aquatic NonBreeding Habitat) 

Three of the four assessment endpoints for the aquatic-phase primary constituent 

elements (PCEs) of designated critical habitat for the CRLF are related to potential 

effects to aquatic and/or terrestrial plants: 

81
• Alteration of channel/pond morphology or geometry and/or increase in sediment 

deposition within the stream channel or pond: aquatic habitat (including riparian 

vegetation) provides for shelter, foraging, predator avoidance, and aquatic 

dispersal for juvenile and adult CRLFs. 

• Alteration in water chemistry/quality including temperature, turbidity, and 

oxygen content necessary for normal growth and viability of juvenile and adult 

CRLFs and their food source. 

• Reduction and/or modification of aquatic-based food sources for pre-metamorphs 

(e.g., algae). 

Based on the risk estimation for potential effects to aquatic and/or terrestrial plants 

provided in Sections 5.1.1.2, 5.1.1.3, and 5.1.2.3, paraquat is likely to affect aquaticphase PCEs of designated habitat related to effects on aquatic and terrestrial plants. 

Reduction of aquatic based food sources may occur from most use sites. 

Because reduction of aquatic based food sources may occur from most use sites, paraquat 

may be likely to indirectly affect the CRLF. Likewise, due to paraquat’s ability to reduce 

aquatic non-vascular plants used as food source and habitat for CRLF, paraquat may be 

likely to indirectly affect the CRLF. Since there are LOC exceedances on non-target 

terrestrial dicot plants from spray drift at the minimum application rate, paraquat may 

indirectly affect the CRLF via reduction in terrestrial plants from aerial application as 

well. As a result, due to aquatic vascular and terrestrial plant communities being reduced 

from most use sites, there is potential for alteration of channel/pond morphology or 

geometry and/or increase in sediment deposition within the stream channel or pond and 

for alteration in water chemistry/quality including temperature, turbidity, and oxygen 

content necessary for normal growth and viability of juvenile and adult CRLFs and their 

food. 

The remaining aquatic-phase PCE is “alteration of other chemical characteristics 

necessary for normal growth and viability of CRLFs and their food source.” To assess 

the impact of paraquat on this PCE (i.e., alteration of food sources), acute and chronic 

freshwater fish and invertebrate toxicity endpoints, as well endpoints for aquatic nonvascular plants are used as measures of effects. RQs for these endpoints were calculated 

in Sections 5.1.1.1 and 5.1.1.2. Based on acute LOC exceedances for aquatic plants, 

paraquat may result in effects to aquatic-phase PCEs. 

5.1.3.2 Terrestrial-Phase (Upland Habitat and Dispersal Habitat) 

The first two assessment endpoints for the terrestrial-phase PCEs of designated critical 

habitat for the CRLF are related to potential effects to terrestrial plants: 

• Elimination and/or disturbance of upland habitat; ability of habitat to support food 

source of CRLFs: Upland areas within 200 ft of the edge of the riparian 

vegetation or dripline surrounding aquatic and riparian habitat that are comprised 

of grasslands, woodlands, and/or wetland/riparian plant species that provides the 

CRLF shelter, forage, and predator avoidance 

82
• Elimination and/or disturbance of dispersal habitat: Upland or riparian dispersal 

habitat within designated units and between occupied locations within 0.7 mi of 

each other that allow for movement between sites including both natural and 

altered sites which do not contain barriers to dispersal 

The risk estimation for terrestrial-phase PCEs of designated habitat related to potential 

effects on terrestrial plants is provided in Section 5.1.2.3. These results will inform the 

effects determination for effects to designated critical habitat for the CRLF. There were 

no LOC exceedances for non-target monocot plants (inhabiting upland dry, semi-dry 

areas) or resulting from spray drift. Although there were no LOC exceedances for nontarget dicot plants inhabititing upland dry and semi-dry area RQ’s, there was an 

exceedance for spray drift. The buffer determined from AgDRIFT (Section 5.2.5.1) 

yielded a buffer of at least 1,000 feet. Therefore, any plants with in a 1,000 foot radious 

from the application site may potentially be affected. Based on the results paraquat 

may affect the first and second terrestrial-phase PCEs. 

The third terrestrial-phase PCE is “reduction and/or modification of food sources for 

terrestrial phase juveniles and adults.” To assess the impact of paraquat on this PCE, 

acute and chronic toxicity endpoints for birds, mammals, and terrestrial invertebrates are 

used as measures of effects. RQs for these endpoints were calculated in Section 5.1.2.2. 

Based on acute and chronic LOC exceedances for CRLF prey items of small 

mammals, terrestrial invertebrates and other frogs, paraquat may result in effects 

to the third terrestrial-phase PCE. 

The fourth terrestrial-phase PCE is based on alteration of chemical characteristics 

necessary for normal growth and viability of juvenile and adult CRLFs and their food 

source. Direct acute and chronic RQs for terrestrial-phase CRLFs are presented in 

Section 5.2.1.2. Due to acute and chronic LOC exceedances at all but one use sites 

(acute only) to terrestrial-phase CRLFs, paraquat may result in effects to the fourth 

terrestrial-phase PCE. 

5.2 Risk Description 

The risk description synthesizes an overall conclusion regarding the likelihood of adverse 

impacts leading to an effects determination (i.e., “no effect,” “may affect, but not likely 

to adversely affect,” or “likely to adversely affect”) for the CRLF and its designated 

critical habitat. 

Based on the RQs presented in the Risk Estimation (Section 5.1) a preliminary 

effects determination is “May Affect” for the CRLF and critical habitat. 

83
A summary of the risk estimation results are provided in Table 5-8 for direct and indirect 

effects to the CRLF and in Table 5-9 for the PCEs of designated critical habitat for the 

CRLF. 

Table 5-8 Risk Estimation Summary for Paraquat - Direct and Indirect Effects to 

CRLF 

Assessment Endpoint 

LOC 

Exceedances 

(Y/N) 

Description of Results of Risk Estimation 

Aquatic Phase 

(eggs, larvae, tadpoles, juveniles, and adults)

Direct Effects 

Survival, growth, and reproduction 

of CRLF individuals via direct 

effects on aquatic phases No 

The aquatic phase amphibian acute LOCs for listed species 

(0.05) are not exceeded for any uses of paraquat in California. 

The RQs range from 0.004 for airports, commercial/industrial 

areas, and public health areas to < 0.001 for melons. 

When comparing chronic direct effects, the estimated chronic 

value at 1.89 ppm is not exceeded for any use. 

Indirect Effects 

Survival, growth, and reproduction 

of CRLF individuals via effects to 

food supply (i.e., freshwater 

invertebrates, non-vascular plants) 

Yes 

LOCs for aquatic invertebrates are not exceeded for any uses. 

The acute RQs range from 0.042 for airports, 

commercial/industrial areas, and public health areas to < 0.01 for 

melons. 

When comparing chronic indirect effects, the estimated chronic 

value at 0.174 ppm is not exceeded for any use. 

Indirect Effects 

Survival, growth, and reproduction 

of CRLF individuals via effects on 

habitat, cover, and/or primary 

productivity (i.e., aquatic plant 

community) 

Yes 

RQs for vascular aquatic plants do not exceed the Agency’s 

LOC (1.0) for any uses. These range from 0.77 (airports, 

commercial/industrial areas, public health areas) to 0.02 

(melons). 

LOCs for non-vascular plants are exceeded for all uses. The 

RQs range from 138 (airports, commercial/industrial areas, and 

public health areas) to 4.1 (melons). 

Indirect Effects 

Survival, growth, and reproduction 

of CRLF individuals via effects to 

riparian vegetation, required to 

maintain acceptable water quality 

and habitat in ponds and streams 

comprising the species’ current 

range. 

Yes 

RQs for non-target terrestrial monocot and dicot plants 

inhabitating semi-aquatic and upland dry areas do not exceed 

the Agency’s LOC for any uses except for dicot plants exposed 

to agricultural follow/ideland, and melon applications for spray 

drift. Spray drift RQs for these applications range from 3.57 to 

1.07 respectively. 

Terrestrial Phase 

(Juveniles and adults)

Direct Effects 

Survival, growth, and reproduction 

of CRLF individuals via direct 

effects on terrestrial phase adults and 

juveniles 

yes 

Refined acute dietary-based RQs for CRLFs consuming small 

insects exceed the acute listed species LOC (0.1) for all uses of 

paraquat except melons, RQs ranged from 1.28 (Airport/public 

health use/guava) to 0.06 (melons). The Refined acute dietarybased RQs for CRLFs consuming large insects and 15g small 

insectivore mammals resulted in paraquat use on airports/public 

health use/guava exceeding the listed species LOC (0.1), with 

an RQ of 0.14 for both. The refined acute dietary-based RQs 

for CRLFs consuming small herbivore mammals (15g) resulted 

84
LOC 

Assessment Endpoint Exceedances Description of Results of Risk Estimation 

(Y/N) 

in all uses exceeding the listed species LOC, RQs ranged from 

2.17 (Airport/public health use/guava) to 0.10 (melons) and for 

35g small mammals all uses except melons exceed the listed 

species LOC, RQs ranged from 1.50 (Airport/public health 

use/guava) to 0.77 (melons). There are no exceedances for 

CRLFs consuming small terrestrial-phase amphibians. 

Refined dose-based RQs for CRLF of varying weights (1.4g, 

37g and 238g) consuming small insects exceed the acute 

endangered species LOC (0.1) for only the Airport/public 

health and Guava uses of paraquat for all weights of CRLF). 

There are no exceedances for small sized (1.4g) CRLF 

consuming large insects and CRLF this size are too small to 

consume small mammals or small terrestrial-phase amphibians. 

The RQs for small sized (1.4g) CRLF are 0.27 suggesting that 

small CRLF consuming small insects are potentially affected by 

acute exposures to paraquat. 

Refined dose-based RQs for medium sized (37g) CRLF 

consuming small herbivore mammals (either 15g or 35g) 

exceed the acute listed species LOC (0.1) for all uses of 

paraquat. There were also exceedances in the acute listed 

species LOC (0.1) for medium sized CRLF consuming small 

insectivore (15g) mammals for the airports/public health, 

guava, and ginger uses of paraquat. For medium sized CRLF 

consuming small insectivore (35g) mammals there were 

exceedances in the acute listed species LOC (0.1) for all uses of 

paraquat except carrots and melons. There are no exceedances 

for medium sized (37g) CRLF consuming large insects or small 

terrestrial-phase amphibians. Due to exceedances of LOCs for 

CRLF consuming small herbivore mammals (either 15g or 35g) 

for all paraquat uses, and exceedances of LOCs for CRLF 

consuming small insectivore mammals (either 15g or 35g) for a 

majority of paraquat uses indicate that the medium sized CRLF 

could potentially be affected by acute exposures to paraquat. 

Refined dose-based RQs for large sized (238g) CRLF 

consuming small herbivore mammals (either 15g or 35g) 

exceed the acute listed species LOC (0.1) for all uses of 

paraquat except melons. There were no exceedances for large 

sized (238g) CRLF consuming large insects, small insectivore 

mammals (15g or 35g), or small terrestrial-phase amphibians. 

The exceedances of LOCs for CRLF consuming small 

herbivore mammals (either 15g or 35g) for all paraquat uses 

except melons, indicates that the large sized CRLF could 

potentially be affected by acute exposures to paraquat. 

Refined chronic dietary-based RQs for CRLFs consuming small 

insects and small herbivore mammals (either15g or 35g) using 

85
LOC 

Assessment Endpoint Exceedances Description of Results of Risk Estimation 

(Y/N) 

T-HERPS model exceed the chronic species LOC (1.0) for all 

uses of paraquat. Refined chronic dietary-based RQs for 

CRLFs consuming large insects and small insectivore mammals 

(either 15g or 35g) using T-HERPS model exceed the chronic 

species LOC (1.0) for Airport/public health, guava, and ginger 

uses of paraquat. The refined chronic dietary-based RQs for 

CRLFs consuming small terrestrial- phase amphibians using THERPS model exceed the chronic species LOC (1.0) for 

Airport/public health and guava uses of paraquat (the maximum 

uses). 

Indirect Effects 

Survival, growth, and reproduction 

of CRLF individuals via effects on 

prey (i.e., terrestrial invertebrates, 

small terrestrial mammals and 

terrestrial phase amphibians) 

yes 

The acute-dose based RQs for small mammals exceed the 

endangered species LOC for all uses except Melons, and ranged 

from 7.63 (Airports/public health use, maximum application 

rate) to 0.34 (Melons). These results are summarized in Table 

5-7. 

The chronic dietary-based RQs for small mammals exceed the 

endangered species LOC for all uses except Melons, and ranged 

from 14.81 (Airports/public health use, maximum application 

rate) to 0.67 (Melons). The chronic dose-based RQs for small 

mammals exceed the endangered species LOC for all uses, and 

ranged from 128.53 (Airports/public health use) to 5.78 

(Melons). These results are summarized in Table 5-7. 

Indirect Effects 

Survival, growth, and reproduction 

of CRLF individuals via effects on 

habitat (i.e., riparian vegetation) 

yes 

The RQs for non-target terrestrial monocot and dicot plants 

inhabiting semi-aquatic and upland dry areas do not exceed the 

Agency’s LOC (1.0) for all uses. All aerial applications of 

paraquat results in spray drift exceedances for dicots (only). 

These exceedances range from 3.57 (Agricultural 

fallow/ideland maximum aerial application rate) to 1.07 

(Melons, minimum aerial application rate). 

86
Table 5-9 Risk Estimation Summary for Paraquat – PCEs of Designated Critical 

Habitat for the CRLF 

Assessment Endpoint Habitat Effects 

(Y/N) Description of Results of Risk Estimation 

Aquatic Phase PCEs 

(Aquatic Breeding Habitat and Aquatic Non-Breeding Habitat)

Alteration of channel/pond morphology or 

geometry and/or increase in sediment 

deposition within the stream channel or 

pond: aquatic habitat (including riparian 

vegetation) provides for shelter, foraging, 

predator avoidance, and aquatic dispersal 

for juvenile and adult CRLFs. 

Yes LOCs are exceeded for terrestrial riparian plants 

and for aquatic non-vascular plants from exposure 

to paraquat from spray drift. 

Alteration in water chemistry/quality 

including temperature, turbidity, and oxygen 

content necessary for normal growth and 

viability of juvenile and adult CRLFs and 

their food source. 

Yes LOCs are exceeded for terrestrial riparian plants 

and for aquatic plants from exposure to paraquat 

from spray drift. Alteration of riparian and nonvascular plants may result in alteration of 

temperature, turbidity, and oxygen content. 

Alteration of other chemical characteristics 

necessary for normal growth and viability of 

CRLFs and their food source. 

Yes LOC is exceeded for indirect effects on terrestrial 

phase CRLF from most paraquat applications. 

Reduction and/or modification of aquaticbased food sources for pre-metamorphs 

(e.g., algae) 

Yes LOCs for non-vascular plants are exceeded for all 

uses. 

Terrestrial Phase PCEs 

(Upland Habitat and Dispersal Habitat)

Elimination and/or disturbance of upland 

habitat; ability of habitat to support food 

source of CRLFs: Upland areas within 200 

feet of the edge of the riparian vegetation or 

dripline surrounding aquatic and riparian 

habitat that are comprised of grasslands, 

woodlands, and/or wetland/riparian plant 

species that provides the CRLF shelter, 

forage, and predator avoidance 

yes 

The AgDRIFT model was used to evaluate 

potential distances beyond which exposures would 

be expected to be below the LOC. The Tier I 

ground application output for the maximum 

application rate of paraquat indicated a buffer zone 

of >1,000 feet for non-listed plants to be below the 

LOC. For the minimum application rate, the output 

indicated a buffer zone of of >1,000 feet for nonlisted species. 

Both the Tier I and Tier II aerial application output 

for the maximum and minimum application rates of 

paraquat indicated that the buffer zone required 

would be greater than 1,000 feet for non-listed 

plants. Since the model is restricted to accurately 

discerning a buffer within 1,000 feet of the 

application, the exact distance needed for a buffer 

to protect listed and non-listed plants is unknown. 

Elimination and/or disturbance of dispersal 

habitat: Upland or riparian dispersal habitat 

within designated units and between 

occupied locations within 0.7 mi of each 

other that allow for movement between sites 

including both natural and altered sites 

yes 

Effects are expected to non-target plants over 1,000 

feet from the use site from aerial application of 

paraquat at the maximum application use rate. 

87
Habitat Effects Assessment Endpoint Description of Results of Risk Estimation (Y/N) 

which do not contain barriers to dispersal 

Reduction and/or modification of food 

sources for terrestrial phase juveniles and 

adults 

yes 

Acute RQs for small mammals and birds exceed the 

endangered species LOC for all uses of paraquat 

except Melons. 

Chronic dietary RQs for birds exceed the endangered 

species LOC for all uses of paraquat. For small 

mammals the chronic dietary-based RQs exceed the 

endangered species LOC for all uses of paraquat 

except Melons. The chronic dose-based RQs for 

small mammals exceed the endangered species LOC 

for all uses of paraquat. 

Alteration of chemical characteristics 

necessary for normal growth and viability of 

juvenile and adult CRLFs and their food 

source. 

yes 

All aerial applications of paraquat results in spray 

drift exceedances for non-target terrestrial plants 

inhabiting semi-aquatic and upland dry areas. 

Following a “may affect” determination, additional information is considered to refine 

the potential for exposure at the predicted levels based on the life history characteristics 

(i.e., habitat range, feeding preferences, etc.) of the CRLF. Based on the best available 

information, the Agency uses the refined evaluation to distinguish those actions that 

“may affect, but are not likely to adversely affect” from those actions that are “likely to 

adversely affect” the CRLF. 

The criteria used to make determinations that the effects of an action are “not likely to 

adversely affect” the CRLF include the following: 

• Significance of Effect: Insignificant effects are those that cannot be meaningfully 

measured, detected, or evaluated in the context of a level of effect where “take” 

occurs for even a single individual. “Take” in this context means to harass or 

harm, defined as the following: 

 Harm includes significant habitat modification or degradation that 

results in death or injury to listed species by significantly impairing 

behavioral patterns such as breeding, feeding, or sheltering. 

 Harass is defined as actions that create the likelihood of injury to listed 

species to such an extent as to significantly disrupt normal behavior 

patterns which include, but are not limited to, breeding, feeding, or 

sheltering. 

• Likelihood of the Effect Occurring: Discountable effects are those that are 

extremely unlikely to occur. 

• Adverse Nature of Effect: Effects that are wholly beneficial without any adverse 

effects are not considered adverse. 

 

A description of the risk and effects determination for each of the established assessment 

endpoints for the CRLF is provided in Sections 5.2.1 through 5.2.3. 

88
5.2.1 Direct Effects 

5.2.1.1

5.2.1.2

Aquatic-Phase CRLF 

The aquatic-phase considers life stages of the frog that are obligatory aquatic organisms, 

including eggs and larvae. It also considers submerged terrestrial-phase juveniles and 

adults, which spend a portion of their time in water bodies that may receive spray drift 

containing paraquat. 

Paraquat is considered “moderately toxic” to the freshwater fish, which are surrogates for 

the aquatic phase CRLF. The aquatic animal acute LOCs for listed species (0.05) was 

not exceeded for any of the uses. The RQ’s ranged from 0.004 for airports to <0.001 for 

melons. 

As discussed in Section 5.1.1.1, the acute to chronic ratio (ACR) was used to estimate 

chronic exposure to aquatic animals. With this method, the chronic exposure values are 

estimated to be at levels far less than 0.05 LOC for listed species. It is important to note 

that since there isn’t any actual data from paraquat to determine the actual chronic 

exposure to aquatic animals, the estimated chronic exposure determined using the ACR 

contains uncertainty. 

Of the aquatic animal incidents that were reported for paraquat, and in all of the reported 

incidents the certainty of paraquat being responsible was either unlikely or possible. 

Three were from registered uses of paraquat, and three were from the undetermined 

legality use of paraquat dichloride. Two of the incidents related to the registered use of 

paraquat dichloride believed that paraquat dichloride was the unlikely cause of mortality 

seen; the total mortality of the unknown fish was also unknown in 1993 and 1992 

(I003654-012 and B000175-001 respectively). The other incident related to the 

registered use of paraquat was possibly responsible for the deaths of unknown amounts of 

bass, bluegill sunfish, and crappie in 1997 (I009314-005). The other three paraquat 

incidents were the possible result of undertermined legality usage of paraquat. A total of 

54 dead fish (1 largemouth bass, and 53 sunfish) were observed in 1981 (B0000-502-18), 

an unknown amount of dead bass, bluegill, and crappie were observed in 1997 (I005805-

001), and a total of 400 dead fish (200 each of bass and bluegill) were observed in 1999 

(I008768-007). 

Because there are no LOC exceedances from registered uses of paraquat to the 

CRLF surrogate species (freshwater fish), because monitored concentrations in 

surface water are far below modeled estimates, and because the non-target incidents 

resulting from paraquat use are judged to be unlikely to possible, the Agency 

concludes that paraquat will have no direct effects on aquatic-phase CRLF. 

Terrestrial-Phase CRLF 

The RQs representing acute dietary-based exposures exceed the Agency’s LOC (0.1) for 

all uses of paraquat except melon (Section 5.1.2.1.). The RQs ranged from 1.28 

89
(Airport/public health use) to 0.06 (Melons) (Table 5-5). The chronic dietary-based RQs 

all exceed the Agency’s LOC (1.0) for all uses of paraquat (Section 5.1.2.1.). The RQs 

ranged from 30.0 (Airports/public health use) to 1.35 (Melons) (Table 5-6). These RQs 

were derived using the T-REX model, which estimates exposures that are specific to food 

intake equations for birds. RQs generated for birds are used as surrogates to represent 

RQs for the terrestrial-phase CRLF. Based on these exceedances to the terrestrial-phase 

CRLF, a “May Affect” determination was made. 

The T-HERPS model was therefore employed as a refinement tool to explore amphibianspecific food intake on potential exposures to the terrestrial phase CRLF. The T-HERPS 

model incorporates the same inputs as T-REX with equations adjusted for poikilotherm 

food intake. The dietary-based and dose-based EECs generated by T-HERPS are found 

in Tables 5-10a & b and 5-11a & b, respectively. An example output from T-HERPS is 

available in Appendix J. 

Table 5-10a Upper-bound Kenega Nomogram T-HERPS EECs (mg/kg-diet) for 

Dietary-based Exposures of the CRLF and its Prey to Paraquat, the weights of small 

herbivore and insectivore mammals are 15 g. 

Scenario Small 

Insects 

Large 

Insects 

Small 

Herbivore 

Mammals

Small 

Insectivore 

Mammals

Small 

Terrestrial 

Phase 

Amphibians 

AIRPORTS/LANDING FIELDS, 

COMMERCIAL/INSTITUTIONAL/IN

DUSTRIAL PREMISES/EQUIPMENT 

(OUTDOOR), NONAGRICULTURAL 

AREAS (PUBLIC HEALTH USE) (1 

lb cation/acre; 10 times/yr; 5 day 

intervals) (maximum- non-ag) 

899.99 100.00 1525.46 95.34 31.24 

Guava (1 lb cation/acre; 10 times/yr; 5 

day intervals) (maximum- ag) 899.99 100.00 1525.46 95.34 31.24 

Ginger (1 lb cation/acre; 6 times/yr; 30 

day intervals (ag)) 292.84 32.54 496.36 31.02 10.16 

CORN (SILAGE) (0.5 lb cation/acre; 3 

times/year; 5 day intervals) 184.01 20.45 311.89 19.49 6.39 

BEANS - SUCCULENT 

(LIMA/SNAP), CARROT 

(INCLUDING TOPS), PEAS 

(UNSPECIFIED), PEPPER (1 lb 

cation/acre; 1 time/yr; 1 day intervals) 

135.00 15.00 228.82 14.30 4.69 

Melons (0.3 lb cation/acre; 1 time/yr; 1 

day intervals) (minimum-ag) 40.50 4.50 68.65 4.29 1.41 

90
Table 5-11b Upper-bound Kenega Nomogram T-HERPS EECs (mg/kg-diet) for 

Dietary-based Exposures of the CRLF and its Prey to Paraquat, the weights of small 

herbivore and insectivore mammals are 35 g. 

Scenario Small 

Insects 

Large 

Insects 

Small 

Herbivore 

Mammals

Small 

Insectivore 

Mammals

Small 

Terrestrial 

Phase 

Amphibians 

AIRPORTS/LANDING FIELDS, 

COMMERCIAL/INSTITUTIONAL/IN

DUSTRIAL PREMISES/EQUIPMENT 

(OUTDOOR), NONAGRICULTURAL 

AREAS (PUBLIC HEALTH USE) (1 

lb cation/acre; 10 times/yr; 5 day 

intervals) (maximum- non-ag) 

899.99 100.00 1054.30 65.89 31.24 

Guava (1 lb cation/acre; 10 times/yr; 5 

day intervals) (maximum- ag) 899.99 100.00 1054.30 65.89 31.24 

Ginger (1 lb cation/acre; 6 times/yr; 30 

day intervals (ag)) 292.84 32.54 343.05 21.44 10.16 

CORN (SILAGE) (0.5 lb cation/acre; 3 

times/year; 5 day intervals) 184.01 20.45 215.56 13.47 6.39 

BEANS - SUCCULENT 

(LIMA/SNAP), CARROT 

(INCLUDING TOPS), PEAS 

(UNSPECIFIED), PEPPER (1 lb 

cation/acre; 1 time/yr; 1 day intervals) 

135.00 15.00 158.15 9.88 4.69 

Melons (0.3 lb cation/acre; 1 time/yr; 1 

day intervals) (minimum-ag) 40.50 4.50 47.44 2.97 1.41 

Table 5-12a Upper-bound Kenega Nomogram T-HERPS EECs (mg/kg-bw) for 

Dose-based Exposures of the CRLF and its Prey to Paraquat, the weights of small 

herbivore and insectivore mammals are 15 g. 

Scenario CRLF 

Size (g) 

Small 

Insects 

Large 

Insects 

Small 

Herbivore 

Mammals

Small 

Insectivore 

Mammals

Small 

Terrestrial 

Phase 

Amphibians 

1.4 34.97 3.89 N/A N/A N/A 

37 34.36 3.82 618.43 38.65 1.19 

AIRPORTS/LANDING FIELDS, 

COMMERCIAL/INSTITUTIONAL/IN

DUSTRIAL PREMISES/EQUIPMENT 

(OUTDOOR), NONAGRICULTURAL 

AREAS (PUBLIC HEALTH USE) (1 

lb cation/acre; 10 times/yr; 5 day 

intervals) (maximum- non-ag) 

238 22.52 2.50 96.14 6.01 0.78 

1.4 34.97 3.89 N/A N/A N/A 

37 34.36 3.82 618.43 38.65 1.19 

Guava (1 lb cation/acre; 10 times/yr; 5 

day intervals) (maximum- ag) 

238 22.52 2.50 96.14 6.01 0.78 

1.4 11.38 1.26 N/A N/A N/A 

37 11.18 1.24 201.23 12.58 0.39 

Ginger (1 lb cation/acre; 6 times/yr; 30 

day intervals (ag)) 

238 7.33 0.81 31.28 1.96 0.25 

91
1.4 7.15 0.79 N/A N/A N/A 

37 7.03 0.78 126.44 7.90 0.24 

CORN (SILAGE) (0.5 lb cation/acre; 3 

times/year; 5 day intervals) 

238 4.60 0.51 19.66 1.23 0.16 

1.4 5.24 0.58 N/A N/A N/A 

37 5.15 0.57 92.77 5.80 0.18 

BEANS - SUCCULENT 

(LIMA/SNAP), CARROT 

(INCLUDING TOPS), PEAS 

(UNSPECIFIED), PEPPER (1 lb 

cation/acre; 1 time/yr; 1 day intervals) 238 3.38 0.38 14.42 0.90 0.12 

1.4 1.57 0.17 N/A N/A N/A 

37 1.55 0.17 27.83 1.74 0.05 

Melons (0.3 lb cation/acre; 1 time/yr; 1 

day intervals) (minimum-ag) 

238 1.01 0.11 4.33 0.27 0.04 

Table 5-131b Upper-bound Kenega Nomogram T-HERPS EECs (mg/kg-bw) for 

Dose-based Exposures of the CRLF and its Prey to Paraquat, the weights of small 

herbivore and insectivore mammals are 35 g. 

Scenario CRLF 

Size (g) 

Small 

Insects 

Large 

Insects 

Small 

Herbivore 

Mammals

Small 

Insectivore 

Mammals

Small 

Terrestrial 

Phase 

Amphibians 

1.4 34.97 3.89 N/A N/A N/A 

37 34.36 3.82 997.31 62.33 1.19 

AIRPORTS/LANDING FIELDS, 

COMMERCIAL/INSTITUTIONAL/IN

DUSTRIAL PREMISES/EQUIPMENT 

(OUTDOOR), NONAGRICULTURAL 

AREAS (PUBLIC HEALTH USE) (1 

lb cation/acre; 10 times/yr; 5 day 

intervals) (maximum- non-ag) 

238 22.52 2.50 155.04 9.69 0.78 

1.4 34.97 3.89 N/A N/A N/A 

37 34.36 3.82 997.31 62.33 1.19 

Guava (1 lb cation/acre; 10 times/yr; 5 

day intervals) (maximum- ag) 

238 22.52 2.50 155.04 9.69 0.78 

1.4 11.38 1.26 N/A N/A N/A

37 11.18 1.24 324.50 20.28 0.39 

Ginger (1 lb cation/acre; 6 times/yr; 30 

day intervals (ag)) 

238 7.33 0.81 50.45 3.15 0.25 

1.4 7.15 0.79 N/A N/A N/A

37 7.03 0.78 203.91 12.74 0.24 

CORN (SILAGE) (0.5 lb cation/acre; 3 

times/year; 5 day intervals) 

238 4.60 0.51 31.70 1.98 0.16 

1.4 5.24 0.58 N/A N/A N/A

37 5.15 0.57 149.60 9.35 0.18 

BEANS - SUCCULENT 

(LIMA/SNAP), CARROT 

(INCLUDING TOPS), PEAS 

(UNSPECIFIED), PEPPER (1 lb 

cation/acre; 1 time/yr; 1 day intervals) 238 3.38 0.38 23.26 1.45 0.12 

1.4 1.57 0.17 N/A N/A N/A

37 1.55 0.17 44.88 2.80 0.05 

Melons (0.3 lb cation/acre; 1 time/yr; 1 

day intervals) (minimum-ag) 

238 1.01 0.11 6.98 0.44 0.04 

92
Acute Exposures 

Refined acute dietary-based RQs for CRLFs consuming small insects and small herbivore 

mammals exceed the acute listed species LOC (0.1) for all uses of paraquat except 

Melons (only small insects when the small mammals weigh 15g). The acute dietarybased RQs for CRLFs consuming large insects exceed the acute listed species LOC for 

Airports/public health uses and Guava for both the 15g and 35g small mammals. The 

acute dietary-based RQs for CRLFs consuming small insectivore mammals exceed the 

acute listed species LOC for Airports/public health uses and Guava for the 15g small 

herbivore mammal only. No acute dietary based LOCs were exceeded for CLRF 

consuming small terrestrial phase amphibians for any paraquat use. Results are presented 

in Tables 5-12a and 5-12b 

Refined dose-based RQs for CRLF of varying weights (1.4g, 37g and 238g) consuming 

small insects exceed the acute endangered species LOC (0.1) for only the Airport/public 

health and Guava uses of paraquat for all weights of CRLF (Table 5-13a and Table 5-

13b). There are no exceedances for small sized (1.4g) CRLF consuming large insects 

(Table 5-13a), and CRLF this size are too small to consume small mammals or small 

terrestrial-phase amphibians. The RQs for small sized (1.4g) CRLF are 0.27 suggesting 

that small CRLF consuming small insects are potentially affected by acute exposures to 

paraquat. 

Refined dose-based RQs for medium sized (37g) CRLF consuming small herbivore 

mammals (either 15g or 35g) exceed the acute listed species LOC (0.1) for all uses of 

paraquat (Table 5-13a and Table 5-13b). There were also exceedances the acute listed 

species LOC (0.1) for medium sized CRLF consuming small insectivore (15g) mammals 

for the airports/public health, guava, and ginger uses of paraquat (Table 15-13a). For 

medium sized CRLF consuming small insectivore (35g) mammals there were 

exceedances in the acute listed species LOC (0.1) for all uses of paraquat except carrots 

and melons (Table 5-13b). There are no exceedances for medium sized (37g) CRLF 

consuming large insects or small terrestrial-phase amphibians (Table 15-13a and Table 

15-13b). Due to exceedances of LOCs for CRLF consuming small herbivore mammals 

(either 15g or 35g) for all paraquat uses, and exceedances of LOCs for CRLF consuming 

small insectivore mammals (either 15g or 35g) for a majority of paraquat uses indicate 

that the medium sized CRLF could potentially be affected by acute exposures to 

paraquat. 

Refined dose-based RQs for large sized (238g) CRLF consuming small herbivore 

mammals (either 15g or 35g) exceed the acute listed species LOC (0.1) for all uses of 

paraquat except melons (Table 5-13a and Table 5-13b). There were no exceedances for 

large sized (238g) CRLF consuming large insects, small insectivore mammals (15g or 

35g), or small terrestrial-phase amphibians (Table 15-13a and Table 15-13b). Due to 

exceedances of of LOCs for CRLF consuming small herbivore mammals (either 15g or 

35g) for all paraquat uses except melons, indicate that the large sized CRLF could 

potentially be affected by acute exposures to paraquat. 

93
Table 5-14a Revised Acute Dietary-based RQs* for CRLF consuming different food 

items (RQs calculated using T-HERPS), the weights of small herbivore and 

insectivore mammals are 15 g. 

Scenario Small 

Insects 

Large 

Insects 

Small 

Herbivore 

Mammals

Small 

Insectivore 

Mammals

Small 

Terrestrial 

Phase 

Amphibians

AIRPORTS/LANDING FIELDS, 

COMMERCIAL/INSTITUTIONAL/INDUSTRIAL 

PREMISES/EQUIPMENT (OUTDOOR), 

NONAGRICULTURAL AREAS (PUBLIC 

HEALTH USE) (1 lb cation/acre; 10 times/yr; 5 

day intervals) (maximum- non-ag) 

1.28 0.14 2.17 0.14 0.04 

Guava (1 lb cation/acre; 10 times/yr; 5 day 

intervals) (maximum- ag) 1.28 0.14 2.17 0.14 0.04 

Ginger (1 lb cation/acre; 6 times/yr; 30 day 

intervals (ag)) 0.42 0.05 0.71 0.04 0.01 

CORN (SILAGE) (0.5 lb cation/acre; 3 times/year; 

5 day intervals) 0.26 0.03 0.44 0.03 0.01 

BEANS - SUCCULENT (LIMA/SNAP), 

CARROT (INCLUDING TOPS), PEAS 

(UNSPECIFIED), PEPPER (1 lb cation/acre; 1 

time/yr; 1 day intervals) 

0.19 0.02 0.33 0.02 0.01 

Melons (0.3 lb cation/acre; 1 time/yr; 1 day 

intervals) (minimum-ag) 0.06 0.01 0.10 0.01 <0.01 

*RQs exceeding the Listed LOC (0.10) are bolded and shaded 

Table 5-15b Revised Acute Dietary-based RQs* for CRLF consuming different food 

items (RQs calculated using T-HERPS), the weights of small herbivore and 

insectivore mammals are 35 g. 

Scenario Small 

Insects 

Large 

Insects 

Small 

Herbivore 

Mammals

Small 

Insectivore 

Mammals

Small 

Terrestrial 

Phase 

Amphibians

AIRPORTS/LANDING FIELDS, 

COMMERCIAL/INSTITUTIONAL/INDUSTRI

AL PREMISES/EQUIPMENT (OUTDOOR), 

NONAGRICULTURAL AREAS (PUBLIC 

HEALTH USE) (1 lb cation/acre; 10 times/yr; 5 

day intervals) (maximum- non-ag) 

1.28 0.14 1.50 0.09 0.04 

Guava (1 lb cation/acre; 10 times/yr; 5 day 

intervals) (maximum- ag) 1.28 0.14 1.50 0.09 0.04 

Ginger (1 lb cation/acre; 6 times/yr; 30 day 

intervals (ag)) 0.42 0.05 0.49 0.03 0.01 

CORN (SILAGE) (0.5 lb cation/acre; 3 

times/year; 5 day intervals) 0.26 0.03 0.31 0.02 0.01 

BEANS - SUCCULENT (LIMA/SNAP), 

CARROT (INCLUDING TOPS), PEAS 

(UNSPECIFIED), PEPPER (1 lb cation/acre; 1 

time/yr; 1 day intervals) 

0.19 0.02 0.22 0.01 0.01 

94
Melons (0.3 lb cation/acre; 1 time/yr; 1 day 

intervals) (minimum-ag) 0.06 0.01 0.07 <0.01 <0.01 

*RQs exceeding the Listed LOC (0.10) are bolded and shaded 

Table 5-16a Refined Dose-based RQs* for CRLF consuming different food items 

(RQs calculated using T-HERPS), the weights of small herbivore and insectivore 

mammals are 15 g. 

Scenario CRLF 

Size (g) 

Small 

Insects 

Large 

Insects 

Small 

Herbivore 

Mammals

Small 

Insectivore 

Mammals

Small 

Terrestrial 

Phase 

Amphibians 

1.4 0.27 0.03 N/A N/A N/A 

37 0.27 0.03 4.83 0.30 0.01 

AIRPORTS/LANDING FIELDS, 

COMMERCIAL/INSTITUTIONAL

/INDUSTRIAL 

PREMISES/EQUIPMENT 

(OUTDOOR), 

NONAGRICULTURAL AREAS 

(PUBLIC HEALTH USE) (1 lb 

cation/acre; 10 times/yr; 5 day 

intervals) (maximum- non-ag) 

238 0.18 0.02 0.75 0.05 0.01 

1.4 0.27 0.03 N/A N/A N/A 

37 0.27 0.03 4.83 0.30 0.01 

Guava (1 lb cation/acre; 10 times/yr; 

5 day intervals) (maximum- ag) 

238 0.18 0.02 0.75 0.05 0.01 

1.4 0.09 0.01 N/A N/A N/A 

37 0.09 0.01 1.57 0.10 < 0.01 

Ginger (1 lb cation/acre; 6 times/yr; 

30 day intervals (ag)) 

238 0.06 0.01 0.24 0.02 < 0.01 

1.4 0.06 0.01 N/A N/A N/A 

37 0.05 0.01 0.99 0.06 < 0.01 

CORN (SILAGE) (0.5 lb 

cation/acre; 3 times/year; 5 day 

intervals) 238 0.04 < 0.01 0.15 0.01 <0.01 

1.4 0.04 < 0.01 N/A N/A N/A 

37 0.04 < 0.01 0.72 0.05 < 0.01 

BEANS - SUCCULENT 

(LIMA/SNAP), CARROT 

(INCLUDING TOPS), PEAS 

(UNSPECIFIED), PEPPER (1 lb 

cation/acre; 1 time/yr; 1 day 

intervals) 238 0.03 < 0.01 0.11 0.01 < 0.01 

1.4 0.01 < 0.01 N/A N/A N/A 

37 0.01 < 0.01 0.22 0.01 < 0.01 

Melons (0.3 lb cation/acre; 1 

time/yr; 1 day intervals) (minimumag) 238 0.01 < 0.01 0.03 < 0.01 < 0.01 

*RQs exceeding the Listed LOC (0.10) are bolded and shaded 

95
Table 5-17b Refined Dose-based RQs* for CRLF consuming different food items 

(RQs calculated using T-HERPS), the weights of small herbivore and insectivore 

mammals are 35 g. 

Scenario CRLF 

Size (g) 

Small 

Insects 

Large 

Insects 

Small 

Herbivore 

Mammals

Small 

Insectivore 

Mammals

Small 

Terrestrial 

Phase 

Amphibians 

1.4 0.27 0.03 N/A N/A N/A 

37 0.27 0.03 7.79 0.49 0.01 

AIRPORTS/LANDING FIELDS, 

COMMERCIAL/INSTITUTIONA

L/INDUSTRIAL 

PREMISES/EQUIPMENT 

(OUTDOOR), 

NONAGRICULTURAL AREAS 

(PUBLIC HEALTH USE) (1 lb 

cation/acre; 10 times/yr; 5 day 

intervals) (maximum- non-ag) 

238 0.18 0.02 1.21 0.08 0.01 

1.4 0.27 0.03 N/A N/A N/A 

37 0.27 0.03 7.79 0.49 0.01 

Guava (1 lb cation/acre; 10 

times/yr; 5 day intervals) 

(maximum- ag) 

238 0.18 0.02 1.21 0.08 0.01 

1.4 0.09 0.01 N/A N/A N/A 

37 0.09 0.01 2.53 0.16 < 0.01 

Ginger (1 lb cation/acre; 6 

times/yr; 30 day intervals (ag)) 

238 0.06 0.01 0.39 0.02 < 0.01 

1.4 0.06 0.01 N/A N/A N/A 

37 0.05 0.01 1.59 0.10 < 0.01 

CORN (SILAGE) (0.5 lb 

cation/acre; 3 times/year; 5 day 

intervals) 

238 0.04 < 0.01 0.25 0.02 < 0.01 

1.4 0.04 < 0.01 N/A N/A N/A 

37 0.04 < 0.01 1.17 0.07 < 0.01 

BEANS - SUCCULENT 

(LIMA/SNAP), CARROT 

(INCLUDING TOPS), PEAS 

(UNSPECIFIED), PEPPER (1 lb 

cation/acre; 1 time/yr; 1 day 

intervals) 

238 0.03 < 0.01 0.18 0.01 < 0.01 

1.4 0.01 < 0.01 N/A N/A N/A 

37 0.01 < 0.01 0.35 0.02 < 0.01 

Melons (0.3 lb cation/acre; 1 

time/yr; 1 day intervals) 

(minimum-ag) 

238 0.01 < 0.01 0.05 < 0.01 < 0.01 

*RQs exceeding the Listed LOC (0.10) are bolded and shaded 

A probit slope value for the acute avian toxicity test was not available; therefore, the 

effect probability was calculated based on a default slope assumption of 4.5 with upper 

and lower 95% confidence intervals of 2 and 9 (Urban and Cook, 1986). For all uses 

with RQs that exceed the endangered species LOCs the probability of individual effects 

were conducted to determine the probability that one individual could be impacted by 

exposure to paraquat. Using the revised acute dietary-based RQs for CRLF consuming 

different food items the chance of individual mortality for which the RQs exceed the 

LOC (0.1) range from approximately 1 in 2.94*105

 (95 % CI: 1 in 4.4*101 to 1 in 

8.86*1018) (<1%) at an RQ 0.10 (Melons, small herbivore mammals weighing 15g) to 

approximately 1 in 1 (95 % CI: 1 in 1.33 to 1 in 1) (100%) at an RQ of 2.17 

(Airports/public health use/Guava, small herbivore mammals weighing 15g) (Table 5-

96
12a). This range of RQs is relevant to CRLF consuming small and large insects, and 

small herbivore and insectivore mammals. This range is not relevant to CRLF consuming 

small terrestrial-phase amphibians modeled for any use scenario since there was no LOC 

exceedance. 

Since the CRLF also consumes larger sizes of mammals RQs were also calculated for 

small herbivore and insectivore mammals weighing 35g. Using the revised acute dietarybased RQs for CRLF consuming different food items the chance of individual mortality 

for which the RQs exceed the LOC (0.1) range from approximately 1 in 1.64*104

 (95 % 

CI: 1 in 2.28*101

 to 1 in 1.31*1014) (<1 %) at an RQ 0.14 (Airports/public health use/ 

Guava, large insects) to approximately 1 in 1.27 (95 % CI: 1 in 1.57 to 1 in 1.06) (86%) 

at an RQ of 1.50 (Airports/public health use/Guava, small herbivore mammals weighing 

35g) (Table 5-12b). This range of RQs is relevant to CRLF consuming small and large 

insects, and small herbivore mammals. This range is not relevant to CRLF consuming 

small insectivore mammals or small terrestrial-phase amphibians modeled for any use 

scenario since there was no LOC exceedance. 

Using the revised acute dose-based RQs for CRLF consuming different food items the 

chance of individual mortality for which the RQs exceed the LOC (0.1) range from 

approximately 1 in 2.94*105 (95 % CI: 1 in 4.4*101 to 1 in 8.86*1018) (<1%) at an RQ 

0.10 (Ginger, small herbivore mammals weighing 15g, 37g CRLF) to approximately 1 in 

1 (95 % CI: 1 in 1.09 to 1 in 1) (100%) at an RQ of 4.83 (Airports/public health 

use/Guava, small herbivore mammals weighing 15g, 37g CRLF) (Table 5-13a). This 

range of RQs is relevant to all sizes of CRLF consuming small insects, and small 

herbivore and insectivore mammals. This range is not relevant to CRLF (of any size) 

consuming large insects or small terrestrial-phase amphibians modeled for any use 

scenario since there were no LOC exceedances.

Using the revised acute dose-based RQs for CRLF consuming different food items the 

chance of individual mortality for which the RQs exceed the LOC (0.1) range from 

approximately 1 in 2.94*105

 (95 % CI: 1 in 4.4*101 to 1 in 8.86*1018) (<1%) at an RQ 

0.10 (Corn, small insectivore mammals weighing 35g, 37g CRLF) to approximately 1 in 

1 (95 % CI: 1 in 1.04 to 1 in 1) (100%) at an RQ of 7.79 (Airports/public health 

use/Guava, small herbivore mammals weighing 35g, 37g CRLF) (Table 5-13b). This 

range of RQs is relevant to all sizes of CRLF consuming small insects, and small 

herbivore and insectivore mammals. This range is not relevant to CRLF (of any size) 

consuming large insects or small terrestrial-phase amphibians modeled for any use 

scenario since there were no LOC exceedances. 

Chronic Exposures

Refined chronic dietary-based RQs for CRLFs consuming small insects and small 

herbivore mammals (either 15g or 35g) using T-HERPS model exceed the chronic 

species LOC (1.0) for all uses of paraquat (Table 5-14a and 5-14b). Refined chronic 

dietary-based RQs for CRLFs consuming large insects and small insectivore mammals 

(either 15g or 35g) using T-HERPS model exceed the chronic species LOC (1.0) for 

Airport/public health, guava, and ginger uses of paraquat (Table 5-14b). The refined 

97
chronic dietary-based RQs for CRLFs consuming small terrestrial- phase amphibians 

using T-HERPS model exceed the chronic species LOC (1.0) for Airport/public health 

and guava uses of paraquat (the maximum uses) (Table 5-14a and 5-14b). 

Table 5-18a Revised Chronic Dietary-based RQs* for CRLF consuming different 

food items (RQs calculated using T-HERPS), the weights of small herbivore and 

insectivore mammals are 15 g. 

Scenario Small 

Insects 

Large 

Insects 

Small 

Herbivore 

Mammals

Small 

Insectivore 

Mammals

Small 

Terrestrial 

Phase 

Amphibians 

AIRPORTS/LANDING FIELDS, 

COMMERCIAL/INSTITUTIONAL/INDUSTRIAL 

PREMISES/EQUIPMENT (OUTDOOR), 

NONAGRICULTURAL AREAS (PUBLIC 

HEALTH USE) (1 lb cation/acre; 10 times/yr; 5 

day intervals) (maximum- non-ag) 

30.0 3.33 50.85 3.18 1.04 

Guava (1 lb cation/acre; 10 times/yr; 5 day 

intervals) (maximum- ag) 30.0 3.33 50.85 3.18 1.04 

Ginger (1 lb cation/acre; 6 times/yr; 30 day 

intervals (ag)) 9.76 1.08 16.55 1.03 0.34 

CORN (SILAGE) (0.5 lb cation/acre; 3 times/year; 

5 day intervals) 6.13 0.68 10.40 0.65 0.21 

BEANS - SUCCULENT (LIMA/SNAP), 

CARROT (INCLUDING TOPS), PEAS 

(UNSPECIFIED), PEPPER (1 lb cation/acre; 1 

time/yr; 1 day intervals) 

4.50 0.50 7.63 0.48 0.16 

Melons (0.3 lb cation/acre; 1 time/yr; 1 day 

intervals) (minimum-ag) 1.35 0.15 2.29 0.14 0.05 

*RQs exceeding the Listed LOC (1.0) are bolded and shaded 

Table 5-194b Revised Chronic Dietary-based RQs* for CRLF consuming different 

food items (RQs calculated using T-HERPS), the weights of small herbivore and 

insectivore mammals are 35 g. 

Scenario Small 

Insects 

Large 

Insects 

Small 

Herbivore 

Mammals

Small 

Insectivore 

Mammals

Small 

Terrestrial 

Phase 

Amphibians

AIRPORTS/LANDING FIELDS, 

COMMERCIAL/INSTITUTIONAL/INDUSTRIAL 

PREMISES/EQUIPMENT (OUTDOOR), 

NONAGRICULTURAL AREAS (PUBLIC 

HEALTH USE) (1 lb cation/acre; 10 times/yr; 5 

day intervals) (maximum- non-ag) 

30.0 3.33 35.14 2.20 1.04 

Guava (1 lb cation/acre; 10 times/yr; 5 day 

intervals) (maximum- ag) 30.0 3.33 35.14 2.20 1.04 

Ginger (1 lb cation/acre; 6 times/yr; 30 day 

intervals (ag)) 9.76 1.08 11.43 0.71 0.34 

CORN (SILAGE) (0.5 lb cation/acre; 3 times/year; 

5 day intervals) 6.13 0.68 7.19 0.45 0.21 

98
BEANS - SUCCULENT (LIMA/SNAP), 

CARROT (INCLUDING TOPS), PEAS 

(UNSPECIFIED), PEPPER (1 lb cation/acre; 1 

time/yr; 1 day intervals) 

4.50 0.50 5.27 0.33 0.16 

Melons (0.3 lb cation/acre; 1 time/yr; 1 day 

intervals) (minimum-ag) 1.35 0.15 1.58 0.10 0.05 

*RQs exceeding the Listed LOC (1.0) are bolded and shaded 

In the available chronic study where Mallard duck were exposed to paraquat, the NOAEC 

was 30 ppm and the LOAEC was 100 ppm, based on effects to % viable egs, eggs set, 

normality of hatchlings, and number of 14-day old survivors. In comparing the LOAEC 

to the chronic dietary-based EECs for CRLF small insects and small herbivore mammals 

indicate that the EECs for all uses except melons exceed the concentration where 

reproductive effects were observed within the laboratory. For CRLF consuming large 

insects (except airport/public health/guava use), small insectivore mammals (15g or 35g), 

and small terrestrial-phase amphibians all uses except those previously listed have EECs 

which do not exceed the LOAEC. Therefore, some CRLF feeding categories, paraquat 

EECs are at levels were reproductive effects were observed in birds, which serve as 

surrogates for the CRLF. 

Of the terrestrial animal incidents that were reported paraquat was not the only chemical 

that was present in the field. Only one incident was from the registered use of paraquat, 

and paraquat was considered as being probably responsible, for the death of 

approximately 5 Canadian geese, as it had the highest acute toxicity to birds of the 

pesticides applied in the mix tank (the others were Atrazine, Simazine, Cyanazine, and 

Esfenvalerate) (I008168-001). With the other two incidents, paraquat was considered to 

be unlikely or possibly responsible for the incident (I000097-015, I007334-001). 

Avian dietary and dose-based toxicity is very different, based upon the chemical 

properties of paraquat. This is a conservative estimate as paraquat’s toxicity may vary 

due to it binding to the food residue and maybe less biologically available to the 

organism. 

Based on the line of evidence, and on these refined acute and chronic risk quotients 

(RQs) and their exceedances of the Agency’s LOC a May Affect and is Likely to 

Adversely Affect (LAA) determination is made for paraquat use in California. 

5.2.2 Indirect Effects (via Reductions in Prey Base) 

5.2.2.1 Algae (non-vascular plants) 

 

As discussed in Section 2.5.3, the diet of CRLF tadpoles is composed primarily of 

unicellular aquatic plants (i.e., algae and diatoms) and detritus. Indirect effects of 

paraquat to the aquatic-phase CRLF (tadpoles) via reduction in non-vascular aquatic 

plants in its diet are based on peak EECs from the standard pond and the lowest acute 

toxicity value for aquatic non-vascular plants. The Agency’s LOC (1.0) is exceeded for 

all uses of paraquat in California. The RQs range from 2213 for airports, 

commercial/industrial areas, and public health areas to 11 for melons (Section 5.1.1.2). 

99
The fate characteristics indicate that paraquat is expected to be persistent in aquatic 

environments. As a result, the primary food source for the aquatic-phase CRLF (nonvascular aquatic plants) is expected to be adversely affected. 

Because of non-vascular LOC exceedance from registered uses of paraquat the 

Agency concludes that there is a potential of indirect impact to the aquatic-phase of 

the CRLF from reduction of food items (algae). Therefore, paraquat May Affect 

and is Likely to Adversely Affect (LAA) the CRLF. 

5.2.2.2

5.2.2.3

Aquatic Invertebrates 

The potential for paraquat to elicit indirect effects to the CRLF via effects on freshwater 

invertebrate food items is dependent on several factors including: (1) the potential 

magnitude of effect on freshwater invertebrate individuals and populations; and (2) the 

number of prey species potentially affected relative to the expected number of species 

needed to maintain the dietary needs of the CRLF. Together, these data provide a basis 

to evaluate whether the number of individuals within a prey species is likely to be 

reduced such that it may indirectly affect the CRLF. 

The main food source for juvenile aquatic and terrestrial-phase CRLFs is thought to be 

aquatic and terrestrial invertebrates found along the shoreline and on the water surface. 

The fate properties of paraquat suggest that paraquat is expected to be transported 

primarily along with soil particles and subsequently redeposited onto the beds of surface 

water bodies or lowland areas that receive eroded sediments from uplands, such as 

riparian zones. 

Indirect acute effects to the aquatic-phase CRLF via effects to prey (invertebrates) in 

aquatic habitats are based on peak EECs in the standard pond and the lowest acute 

toxicity value for freshwater invertebrates. The acute RQs range from 0.042 for airports, 

commercial/industrial areas, and public health areas to 0.001 for melons. 

Since there are not any aquatic chronic data values for paraquat, the chronic risk to 

aquatic animals is estimated using the ACR as described in Section 5.1.1.1. There is 

believed to be no risk to invertebrates using this method due to the ACR being much 

higher than the EECs calculated using GENEEC2 for the invertebrates. 

Because there are no LOC exceedances from registered uses of paraquat to the 

CRLF surrogate species (freshwater invertebrates), because monitored 

concentrations in surface water are far below modeled estimates, and because the 

non-target incidents resulting from paraquat use are judged to be unlikely to 

possible, the Agency concludes that paraquat will not indirectly affect the CRLF. 

Fish and Aquatic-phase Frogs 

As discussed in Section 2.5.3, the diet of CRLF also includes small fish and other 

aquatic-phase frogs. Direct effects to the aquatic-phase CRLF are based on peak EECs in 

100
the standard pond and the lowest acute toxicity value for freshwater fish. In order to 

assess direct chronic risks to the CRLF, 60 day EECs and the lowest chronic toxicity 

value for freshwater fish would be used. However, since there are not any aquatic 

chronic data values, the chronic RQs could not be calculated. Therefore, the chronic 

exposure to CRLF is uncertain. 

Because there are no LOC exceedances from registered uses of paraquat to the 

CRLF surrogate species (freshwater fish), the Agency concludes that paraquat will 

not indirectly affect the aquatic-phase of the CRLF on an acute or chronic basis 

from fish or other aquatic-phase frogs as prey items. 

5.2.2.4

5.2.2.5

Terrestrial Invertebrates 

When the terrestrial-phase CRLF reaches juvenile and adult stages, its diet is mainly 

composed of terrestrial invertebrates. Terrestrial invertebrate toxicity data is used to 

assess potential indirect effects of paraquat to the terrestrial-phase CRLF. Effects to 

terrestrial invertebrates resulting from exposure to paraquat may also indirectly affect the 

CRLF via reduction in available food. 

Because the LD50 was not definitive, and there was little incidence of mortality, RQs 

were not calculated. However, EECs were compared to the highest concentration tested. 

All of estimated EEC’s the level tested for all uses for small insects and all of the uses 

greater than 6 applications per year with 30 day intervals at 1 lb cation/acre per 

application for large insects; therefore, a preliminary “May Affect” determination was 

made. However, the calculated EEC’s for Airports/public health use/Guava (899.99 ppm 

small insects and 100.00 ppm large insects) were 26 and 2.9 times the level of paraquat 

tested (>34.6 μg cation/bee), at which there was a low incidence of mortality. Therefore, 

it is reasonable to assume that the effects to terrestrial invertebrate populations will be 

negligible for the uses of paraquat in California. Therefore, based on the weight-ofevidence, the Agency concludes that there is a negligible potential impact to 

terrestrial invertebrates that the CRLF consumes, and therefore paraquat May 

Affect but is Not Likely to Adversely Affect (NLAA) the CRLF. 

Mammals 

Life history data for terrestrial-phase CRLFs indicate that large adult frogs consume 

terrestrial vertebrates, including mice. Acute dose based RQs exceed the Agency’s 

endangered species LOC (0.1) for all uses of paraquat. The dose based RQs that exceed 

the LOC range from 0.34 (Melons) to 7.63 (Airports/public health uses/Guava) (Table 5-

7). Chronic dose based RQs exceed the Agency’s LOC (1.0) for all uses and range from 

5.78 (Melons) to 128.53 (Airports/public health use/Guava) (Table 5-7). The dietary 

based chronic RQs exceed the Agency’s LOC for all uses except melons and range from 

3.03 (Corn) to 14.81 (Airports/public health use/Guava) (Table 5-7). 

Based on the mammalian dose-based acute RQs and a probit dose-response default slope 

of 4.5 the estimated mammalian prey item population reduction for which RQs exceed 

101
the Listed (endangered species) LOCs are approximately 1 in 1 for all uses except melon 

(no exceedance) (Table 5-7). The acute dose-based EECs are well above the levels 

mortality was documented at. The chronic dose-based and dietary-based EECs are also 

well above the levels that were tested, and showed mortality and sublethal effects (lung 

damage). Because environmental exposure levels are estimated to be much higher than 

the level which may cause acute effects to mammals, the CRLF may be indirectly 

affected by acute exposure to paraquat. 

Reproductive and sublethal effects (lung damage) were observed in chronic mammalian 

studies, and resulted in RQ values that exceeded the LOC (1.0) for all uses (chronic dosebased) and all uses except melons (chronic dietary-based). The RQs ranged from 128.53 

to 5.78 for the dose-based, and 14.81 to 0.67 for the dietary-based (Table 5-7). Paraquats 

toxicity, when combined in the diet is lower than the gavage (dose) based treatment, 

indicating that the toxicity may be reduced in combination with the diet. Chronic 

exposure to paraquat is likely. 

Based on effects to small mammals, there is a potential indirect impact to the CRLF 

via reduction in small mammal prey items, and therefore paraquat May Affect and 

is Likely to Adversely Affect (LAA) the CRLF. 

5.2.2.6

5.2.3.1

Terrestrial-phase Amphibians 

Terrestrial-phase adult CRLFs also consume frogs. RQ values representing direct 

exposures of paraquat to terrestrial-phase CRLFs are used to represent exposures of 

paraquat to frogs in terrestrial habitats. The T-HERPS model was therefore employed as a 

refinement tool to explore amphibian-specific food intake on potential exposure to 

terrestrial-phase amphibian food items for the CRLF. The T-HERPS model incorporates 

the same inputs as T-REX with equations adjusted for poikilotherm food intake. As 

described in Section 5.2.1.2, the RQs for small terrestrial-phase amphibians did not 

exceed the listed species LOC (0.1) for any use of paraquat. Reduction in amphibian 

prey items, specifically other frogs is not affected from paraquat use. Other items in the 

prey base all had RQs that exceeded the listed species LOC for numerous uses of 

paraquat. 

Based on this evidence, a May Affect and is Likely to Adversely Affect (LAA) 

determination is made for indirect effects via reductions in prey base to the 

terrestrial-phase CRLF. 

5.2.3 Indirect Effects (via Habitat Effects) 

Aquatic Plants (Vascular and Non-vascular) 

Aquatic plants serve several important functions in aquatic ecosystems. Non-vascular 

aquatic plants are primary producers and provide the autochthonous energy base for 

aquatic ecosystems. Vascular plants provide structure as attachment sites and refugia for 

many aquatic invertebrates, fish, and juvenile organisms, such as fish and frogs. In 

102
addition, vascular plants also provide primary productivity and oxygen to the aquatic 

ecosystem. Rooted plants help reduce sediment loading and provide stability to 

nearshore areas and lower streambanks. In addition, vascular aquatic plants are important 

as attachment sites for egg masses of CRLFs. 

Potential indirect effects to the CRLF based on impacts to habitat and/or primary 

production were assessed using RQs from freshwater aquatic vascular and non-vascular 

plant data. Indirect effects of paraquat to the aquatic-phase CRLF (tadpoles) are present 

in the reduction of non-vascular aquatic plants in the aquatic-phase CRLFs diet. The 

Agency’s LOC (1.0) for non-vascular plants is exceeded for all uses of paraquat in 

California. The non-vascular aquatic plant RQs range from 138 for airports, 

commercial/industrial/public health uses to 4.1 for use on melon (Section 5.1.1.2). 

Indirect effects of paraquat to the aquatic-phase CRLF (tadpoles) are also found via the 

reduction in vascular aquatic plants in the aquatic-phase CRLFs diet. The Agency’s LOC 

(1.0) for vascular plants is exceeded for all uses of paraquat in California. The acute RQs 

range from 0.77 for airports, commercial/industrial areas, and public health areas to 0.02 

for melons (Section 5.1.1.2). 

An analysis of the fate characteristics of paraquat indicates that paraquat is expected to be 

persistent in aquatic environments. As a result, the primary food source for the aquaticphase CRLF (both vascular and non-vascular aquatic plants) is expected to be adversely 

affected. 

Because of the non-vascular aquatic plant LOC exceedances for registered uses of 

paraquat, and verified non-target incidents resulting from paraquat use, the Agency 

concludes that there is a potential of indirect impact to the aquatic-phase of the 

CRLF from reduction of food items (algae). Therefore, paraquat May Affect and is 

Likely to Adversely Affect (LAA) the CRLF. 

5.2.3.2 Terrestrial Plants 

Terrestrial plants serve several important habitat-related functions for the CRLF. In 

addition to providing habitat and cover for invertebrate and vertebrate prey items of the 

CRLF, terrestrial vegetation also provides shelter for the CRLF and cover from predators 

while foraging. Terrestrial plants also provide energy to the terrestrial ecosystem through 

primary production. Upland vegetation including grassland and woodlands provides 

cover during dispersal. Riparian vegetation helps to maintain the integrity of aquatic 

systems by providing bank and thermal stability, serving as a buffer to filter out sediment, 

nutrients, and contaminants before they reach the watershed, and serving as an energy 

source. 

Potential indirect effects to the CRLF based on impacts to habitat and/or primary 

production were assessed using RQs from monocot and dicot plant data. The RQs for 

monocots were below the Agency’s LOC, whereas the dicot spray drift RQ exceeded the 

Agency’s RQ (1.0), for all aerial applications of paraquat (Tables 5-15 and 5-16). 

103
Twenty-one incidents have been reported for paraquat, listed under paraquat and paraquat 

dichloride, but they are one in the same. There were nine incidents from the misuse of 

paraquat that resulted in plant damage, with one incident of plant mortality (I014409-

024), and one incident of plant discoloration (I005880-005). Of the reported misuses of 

paraquat; two from general misuse, six from accidental misuse, and one from the 

intentional misuse of paraquat (I014409-024, I013554-040, I005660-005, I005880-005, 

I007371-033, I007371-034, I007371-035, I007371-008, and I016940-005 respectively). 

There were a total of six incidents from the registered use of paraquat and all resulted in 

plant damage (I013884-038, I013636-029, I011838-055, I014034-099, I012684-010, 

I001131-001). The remaining six incidents were from undetermined legality of paraquat 

use and resulted in plant damage (I014409-001, I013884-014, I012366-023, I009573-

009, I011838-091, and I011838-038). 

Based on LOC exceedances in spray drift RQs for dicots, and the twenty-one 

reported incidents that resulted in plant damage a variety of monocots, paraquat 

May Affect and is Likely to Adversely Affect (LAA) the CRLF indirectly via habitat 

degradation through reduction in terrestrial plants. Of particular concern is the 

risk from spray drift during/after aerial application. 

5.2.4 Effects to Designated Critical Habitat 

5.2.4.1 Aquatic-Phase PCEs 

Three of the four assessment endpoints for the aquatic-phase primary constituent 

elements (PCEs) of designated critical habitat for the CRLF are related to potential 

effects to aquatic and/or terrestrial plants: 

• Alteration of channel/pond morphology or geometry and/or increase in sediment 

deposition within the stream channel or pond: aquatic habitat (including riparian 

vegetation) provides for shelter, foraging, predator avoidance, and aquatic 

dispersal for juvenile and adult CRLFs. 

• Alteration in water chemistry/quality including temperature, turbidity, and 

oxygen content necessary for normal growth and viability of juvenile and adult 

CRLFs and their food source. 

• Reduction and/or modification of aquatic-based food sources for pre-metamorphs 

(e.g., algae). 

Conclusions for potential indirect effects to the CRLF via direct effects to aquatic and 

terrestrial plants are used to determine whether modification to critical habitat may occur. 

LOCs are exceeded for terrestrial riparian plants and for aquatic plants from exposure to 

paraquat from spray drift. Alteration of riparian and vascular plants may result in 

alteration of temperature, turbidity, and oxygen content. 

Aquatic non-vascular plants used as a food source and habitat for CRLF may be 

potentially affected from all paraquat uses. A reduction in these aquatic based food 

104
sources may occur from most use sites. Likewise, due to aquatic vascular and terrestrial 

plant communities being reduced from most use sites, there is potential for alteration of 

channel/pond morphology or geometry and/or increase in sediment deposition within the 

stream channel or pond. 

The remaining aquatic-phase PCE is “alteration of other chemical characteristics 

necessary for normal growth and viability of CRLFs and their food source.” Other than 

impacts to algae as food items for tadpoles (discussed above), this PCE is assessed by 

considering direct and indirect effects to the aquatic-phase CRLF via acute and chronic 

freshwater fish and invertebrate toxicity endpoints as measures of effects. 

Based on acute LOC exceedances for aquatic plants paraquat may result in effects 

to aquatic-phase PCEs of designated critical habitat related to effects of alteration of 

other chemical characteristics necessary for normal growth and viability of CRLFs 

and their food source. 

5.2.4.2 Terrestrial-Phase PCEs 

Two of the four assessment endpoints for the terrestrial-phase PCEs of designated critical 

habitat for the CRLF are related to potential effects to terrestrial plants: 

• Elimination and/or disturbance of upland habitat; ability of habitat to support food 

source of CRLFs: Upland areas within 200 ft of the edge of the riparian 

vegetation or drip line surrounding aquatic and riparian habitat that are comprised 

of grasslands, woodlands, and/or wetland/riparian plant species that provides the 

CRLF shelter, forage, and predator avoidance. 

• Elimination and/or disturbance of dispersal habitat: Upland or riparian dispersal 

habitat within designated units and between occupied locations within 0.7 mi of 

each other that allow for movement between sites including both natural and 

altered sites which do not contain barriers to dispersal. 

There is a potential for habitat effects via impacts to terrestrial plants (Section 

5.2.3.2) from paraquat use (aerial applications only). 

The risk estimation for terrestrial-phase PCEs of designated habitat related to potential 

effects on terrestrial plants is provided in Section 5.1.2.3. These results will inform the 

effects determination for effects to designated critical habitat for the CRLF. 

The third terrestrial-phase PCE is “reduction and/or modification of food sources for 

terrestrial phase juveniles and adults.” To assess the impact of paraquat on this PCE, 

acute and chronic toxicity endpoints for terrestrial invertebrates, mammals, and 

terrestrial-phase frogs are used as measures of effects. RQs for these endpoints were 

calculated in Section 5.1.2.2. There is potential for habitat effects via indirect effects 

to terrestrial-phase CRLFs via reduction in prey base (Section 5.2.2.4 for terrestrial 

105
invertebrates, Section 5.2.2.5 for mammals, and 5.2.2.6 for frogs) from paraquat 

use. 

The fourth terrestrial-phase PCE is based on alteration of chemical characteristics 

necessary for normal growth and viability of juvenile and adult CRLFs and their food 

source. Direct acute and chronic RQs for terrestrial-phase CRLFs are presented in 

Section 5.2.1.2. There is potential for habitat effects via direct (Section 5.2.1.2) and 

indirect effects (Sections 5.2.2.4, 5.2.2.5, and 5.2.2.6) to terrestrial-phase CRLFs 

from paraquat use. Paraquat use may result in habitat effects based on effects to 

the terrestrial PCE related to alteration of chemical characteristics necessary for 

normal growth and viability. 

5.2.5 Spatial Extent of Potential Effects 

A May Affect determination applies to those areas where it is expected that the 

pesticide’s use will directly or indirectly affect the CRLF or its designated critical habitat. 

To determine this area, the footprint of paraquat’s use pattern is identified, using land 

cover data that correspond to paraquat’s use pattern. The spatial extent of the effects 

determination also includes areas beyond the initial area of concern that may be impacted 

by spray drift. The identified direct and indirect effects are anticipated to occur only for 

those currently occupied core habitat areas, CNDDB occurrence sections, and designated 

critical habitat for the CRLF that overlap with the initial area of concern plus at least 

1,000 feet from its boundary. It is assumed that non-flowing waterbodies (or potential 

CRLF habitat) are included within this area. 

In addition to the spray drift buffer, the downstream dilution extent analysis results in a 

distance of 300 kilometers for forest land cover, 285 kilometers for cultivated crop, 

orchard/vineyard, and pasture/hay land cover and 88.7 kilometers for developed land 

cover. If any of these streams reaches flow into CRLF habitat, there is potential to affect 

either the CRLF or modify its habitat. These lotic aquatic habitats within the CRLF core 

areas and critical habitats potentially contain concentrations of paraquat sufficient to 

result in a May Affect determination or effects to critical habitat. 

The determination of the buffer distance and downstream dilution for spatial extent of the 

effects determination is described below. 

5.2.5.1 Spray Drift 

In order to determine terrestrial and aquatic habitats of concern due to Paraquat exposures 

through spray drift, it is necessary to estimate the distance that spray applications can 

drift from the treated area and still be present at concentrations that exceed levels of 

concern. An analysis of spray drift distances was completed using AgDrift with the most 

sensitive endpoints for terrestrial plants. 

For paraquat use with the maximum application rate relative to the terrestrial-phase 

CRLF, the results of the screening-level risk assessment indicate that spray drift for aerial 

106
applications, using both the Tier I and Tier II mode, exceeds the 1,000 foot range of the 

AgDrift model . For the Tier I ground mode (no higher tier modeling for ground 

applications is available in AgDrift), the spray drift buffer distance is found to be >1,000 

feet for non-listed plants as well. A summary of the modeled distances by application 

method is presented in Table 5-15. 

Since the maximum application rate resulted in distances exceeding the range allowed by 

the AgDRIFT model, the lowest application rate was also run for comparison purposes. 

For the Tier I and Tier II mode for the aerial application, the spray drift buffer distance 

was found to be >1,000 feet for non-listed plants. For the Tier I ground application 

mode, the spray drift buffer distance was found to be >1,000 feet for the non-listed plants 

(no higher tier modeling for ground applications is available in AgDrift). See Table 5-16 

for a summary of the modeled distances by application method. 

 

In order to characterize the spatial extent of the effects determination that is relevant to 

the CRLF (i.e. NLAA versus LAA), an analysis was conducted using the most sensitive 

non-listed terrestrial plant EC25 of 0.014 lbs cation/acre. Typically the NOAEC is used 

when there is an obligate relationship between the species being assessed and endangered 

plants (or other taxa). However, there is no obligate relationship between the CRLF and 

any endangered plant; therefore the LAA/NLAA determination is based on the area 

defined by the non-listed species LOC (i.e., EEC/EC50). 

107
Table 5-20 Summary of AgDrift Predicted Terrestrial Spray Drift Distances Using 

Maximum Application Rate 

Risk Class Risk Description

Application

Rate (lb

cation/acre)

Toxicity

Value

Used

Fraction of 

applied

Nonvolatile

Rate (lb/a)

Minimum

Spray Volume

Rate (gal/a)

Active Rate

(lb ai/a) Distance

Non-Listed Plants

Potential for effects to nontarget, non-listed plants from

exposures

1

EC25 =

0.014 lb

cation/A 0.000004

Does not

apply

Does not

apply

Does not

apply >1,000 feet

Risk Class Risk Description

Application

Rate (lb

cation/acre)

Toxicity

Value

Used

Fraction of 

applied

Nonvolatile

Rate (lb/a)

Minimum

Spray Volume

Rate (gal/a)

Active Rate

(lb ai/a) Distance

Non-Listed Plants

Potential for effects to nontarget, non-listed plants from

exposures

1

EC25 =

0.014 lb

cation/A 0.000004

Does not

apply

Does not

apply

Does not

apply >1,000 feet

Risk Class Risk Description

Application

Rate (lb

cation/acre)

Toxicity

Value

Used

Fraction of 

applied

Nonvolatile

Rate (lb/a)

Minimum

Spray Volume

Rate (gal/a)

Active Rate

(lb ai/a) Distance

Non-Listed Plants

Potential for effects to nontarget, non-listed plants from

exposures

1

EC25 =

0.014 lb

cation/A 0.000004 0.683 0.317 1 >1,000 feet

Tier II Aerial Application

Tier I Aerial Application

Tier I Ground Application

 

Table 5-21 Summary of AgDrift Predicted Aquatic Spray Drift Distances Using 

Minimum Application Rate 

Risk Class Risk Description

Application 

Rate (lb 

cation/acre)

Toxicity 

Value 

Used

Fraction of 

applied

Nonvolatile 

Rate (lb/a)

Minimum 

Spray Volume

Rate (gal/a)

Active Rate 

(lb ai/a) Distance

Non-Listed Plants

Potential for effects to nontarget, non-listed plants from 

exposures

0.3 EC25 = 

0.014 lb 

cation/A 0.000012

Does not 

apply

Does not 

apply

Does not 

apply >1,000 feet

Risk Class Risk Description

Application 

Rate (lb 

cation/acre)

Toxicity 

Value 

Used

Fraction of 

applied

Nonvolatile 

Rate (lb/a)

Minimum 

Spray Volume

Rate (gal/a)

Active Rate 

(lb ai/a) Distance

Non-Listed Plants

Potential for effects to nontarget, non-listed plants from 

exposures

0.3 EC25 = 

0.014 lb 

cation/A 0.000012

Does not 

apply

Does not 

apply

Does not 

apply >1,000 feet

Risk Class Risk Description

Application 

Rate (lb 

cation/acre)

Toxicity 

Value 

Used

Fraction of 

applied

Nonvolatile 

Rate (lb/a)

Minimum 

Spray Volume

Rate (gal/a)

Active Rate 

(lb ai/a) Distance

Non-Listed Plants

Potential for effects to nontarget, non-listed plants from 

exposures

0.3 EC25 = 

0.014 lb 

cation/A 0.000012 0.683 0.317 0.3 >1,000 feet

Tier II Aerial Application

Tier I Ground Application

Tier I Aerial Application

5.2.5.2 Downstream Dilution Analysis 

To complete this assessment, the greatest ratio of aquatic RQ to LOC was estimated. 

Using an assumption of uniform runoff across the landscape, it is assumed that streams 

flowing through treated areas (i.e. the initial area of concern) are represented by the 

108
modeled EECs; as those waters move downstream, it is assumed that the influx of nonimpacted water will dilute the concentrations of paraquat present. 

Using a NOAEC value of 0.158 ug/L for vascular aquatic plants (the most sensitive 

species) and a maximum peak EEC for applications to airports/commercial/public health 

areas (non-agricultural use) of 892 ug/L yields an RQ/LOC ratio of 5424 (5424/1), and 

guava (agricultural use) of 26 ug/L yields an RQ/LOC ratio of 158 (158/1). Using the 

downstream dilution approach (described in more detail in Appendix J) results in a 

distance of 300 kilometers for forest land cover (which represents the maximum 

continuous distance of downstream dilution from the edge of the initial area of concern), 

285 kilometers for cultivated crop, orchard/vineyard, and pasture/hay land cover and 

88.7 kilometers for developed land cover. Similar to the spray drift buffer described 

above, the LAA/NLAA determination is based on the area defined by the point where 

concentrations exceed the EC50 value. 

5.2.5.3 Overlap between CRLF habitat and Spatial Extent of Potential 

Effects 

An LAA effects determination is made to those areas where it is expected that the 

pesticide’s use will directly or indirectly affect the CRLF or its designated critical habitat 

and the area overlaps with the core areas, critical habitat and available occurrence data 

for CRLF. 

For paraquat, the use pattern in the following land cover classes (forest land cover, 

cultivated crop, orchard/vineyard, pasture/hay land cover, and developed land cover) also 

include areas beyond the initial area of concern that may be impacted by spray drift 

overlaps with CRLF habitat. Appendix D provides maps of the initial area of concern, 

along with CRLF habitat areas, including currently occupied core areas, CNDDB 

occurrence sections, and designated critical habitat. It is expected that any additional 

areas of CRLF habitat that are located at least 1,000 ft (to account for offsite migration 

via spray drift) and 300 kilometers for forest land cover (to account for the maximum 

continuous distance of downstream dilution from the edge of the initial area of concern) 

outside the initial area of concern may also be impacted and are part of the full spatial 

extent of the LAA/effects to critical habitat effects determination. 

109
Figure 5-1. Overlap Map: CRLF Habitat and Paraquat Initial Area of Concern 

110
6.0 Uncertainties 

6.1 Exposure Assessment Uncertainties 

6.1.1 Maximum Use Scenario 

The screening-level risk assessment focuses on characterizing potential ecological risks 

resulting from a maximum use scenario, which is determined from labeled statements of 

maximum application rate and number of applications with the shortest time interval 

between applications. The frequency at which actual uses approach this maximum use 

scenario may be dependant on pest resistance, timing of applications, cultural practices, 

and market forces. 

6.1.2 Crops Not Grown in California 

Crops that are not currently grown in California according to CDPR PUR data, as well as 

the USDA NASS database, are listed on current labels. As these crops are not currently 

grown in California and are not expected to be grown in California in the future, use of 

paraquat was not assessed for these crops. The following uses will not be assessed 

because they are not grown in California: cocoa, coffee, banana, plantain, pineapple, and 

soybeans. If the use patterns indicate that these crops are grown in CA in the future, the 

conclusions of this assessment may need to be revisited. 

6.1.3 Aquatic Exposure Modeling of Paraquat 

GENEEC2 is a Tier I computer program, that uses the soil/water partition coefficient and 

degradation kinetic data to estimate runoff from a ten hectare field into a one hectare by 

two meter deep “standard” pond. This first tier is designed as a coarse screen and 

estimates conservative pesticide concentrations in surface water from a few basic 

chemical parameters and pesticide label use and application information. Tier I is used to 

screen chemicals to determine which ones potentially pose sufficient risk to warrant 

higher level modeling. It calculates acute as well as longer-term estimated environmental 

concentration (EEC) values. It considers reduction in dissolved pesticide concentration 

due to adsorption of pesticide to soil or sediment, incorporation, degradation in soil 

before washoff to a water body, direct deposition of spray drift into the water body, and 

degradation of the pesticide within the water body. It is designed to mimic a 

PRZM-EXAMS simulation. 

The standard ecological water body scenario (pond) used to calculate potential aquatic 

exposure to pesticides is intended to represent conservative estimates, and to avoid 

underestimations of the actual exposure. The standard scenario consists of application to 

a 10-hectare field bordering a 1-hectare, 2-meter deep (20,000 m3

) pond with no outlet. 

Exposure estimates generated using the pond are intended to represent a wide variety of 

vulnerable water bodies that occur at the top of watersheds including prairie pot holes, 

playa lakes, wetlands, vernal pools, man-made and natural ponds, and intermittent and 

lower order streams. As a group, there are factors that make these water bodies more or 

111
less vulnerable than the pond. Static water bodies that have larger ratios of pesticidetreated drainage area to water body volume would be expected to have higher peak EECs 

than the pond. These water bodies will be either smaller in size or have larger drainage 

areas. Smaller water bodies have limited storage capacity and thus may overflow and 

carry pesticide in the discharge, whereas the pond has no discharge. As watershed size 

increases beyond 10-hectares, it becomes increasingly unlikely that the entire watershed 

is planted with a single crop that is all treated simultaneously with the pesticide. 

Headwater streams can also have peak concentrations higher than the pond, but they 

likely persist for only short periods of time and are then carried and dissipated 

downstream. 

The Agency acknowledges that there are some unique aquatic habitats that are not 

accurately captured by this modeling scenario and modeling results may, therefore, 

under- or over-estimate exposure, depending on a number of variables. For example, 

aquatic-phase CRLFs may inhabit water bodies of different size and depth and/or are 

located adjacent to larger or smaller drainage areas than the EXAMS pond. The Agency 

does not currently have sufficient information regarding the hydrology of these aquatic 

habitats to develop a specific alternate scenario for the CRLF. CRLFs prefer habitat with 

perennial (present year-round) or near-perennial water and do not frequently inhabit 

vernal (temporary) pools because conditions in these habitats are generally not suitable 

(Hayes and Jennings 1988). Therefore, the EXAMS pond is assumed to be representative 

of exposure to aquatic-phase CRLFs. In addition, the Services agree that the existing 

EXAMS pond represents the best currently available approach for estimating aquatic 

exposure to pesticides (U.S. FWS/NMFS 2004). 

Uncertainties associated with each of these individual components add to the overall 

uncertainty of the modeled concentrations. Additionally, model inputs from the 

environmental fate degradation studies are chosen to represent the upper confidence 

bound on the mean values that are not expected to be exceeded in the environment 

approximately 90 percent of the time. Mobility input values are chosen to be 

representative of conditions in the environment. The natural variation in soils adds to the 

uncertainty of modeled values. Factors such as application date, crop emergence date, 

and canopy cover can also affect estimated concentrations, adding to the uncertainty of 

modeled values. Factors within the ambient environment such as soil temperatures, 

sunlight intensity, antecedent soil moisture, and surface water temperatures can cause 

actual aquatic concentrations to differ for the modeled values. 

Unlike spray drift, tools are currently not available to evaluate the effectiveness of a 

vegetative setback on runoff and loadings. The effectiveness of vegetative setbacks is 

highly dependent on the condition of the vegetative strip. For example, a wellestablished, healthy vegetative setback can be a very effective means of reducing runoff 

and erosion from agricultural fields. Alternatively, a setback of poor vegetative quality 

or a setback that is channelized can be ineffective at reducing loadings. Until such time 

as a quantitative method to estimate the effect of vegetative setbacks on various 

conditions on pesticide loadings becomes available, the aquatic exposure predictions are 

112
likely to overestimate exposure where healthy vegetative setbacks exist and 

underestimate exposure where poorly developed, channelized, or bare setbacks exist. 

In order to account for uncertainties associated with modeling, it is preferred to compare 

available monitoring data to GENEEC2 estimates of peak EECs for the different uses. 

However, as discussed above, only CDPR had looked for paraquat, and only detected one 

positive concentration of approximately 1 ppb. It is believed the positive detection was a 

result of runoff from agricultural use areas. Yet, the specific use patterns (e.g. application 

rates and timing, crops) associated with the agricultural areas are unknown, but they are 

assumed to be representative of potential paraquat use areas. 

Due to the lack of detections from the CDPR data, 399 samples and only one detect, it is 

believed that the GENEEC2 EECs provide a conservative measure of exposure. 

Moreover, due to the chemical properties of paraquat, it is believed that the larger the 

clay content in the soil, the greater the likelihood that paraquat will adsorb to the clay 

surface and will remain. Therefore, paraquat would become decreasingly mobile in areas 

with increased clay content, and this would decrease the probability of paraquat finding 

its way to water bodies due to run-off. If paraquat is found in aquatic environments, 

paraquat will bind quickly to clay found within the water body due to its cationic 

properties; it is a double charged cation. 

Although the potential impact of discharging ground water on CRLF populations is not 

explicitly delineated, it should be noted that, in some areas of the country, ground water 

could provide a source of pesticide to surface water bodies – especially low-order 

streams, headwaters, and ground water-fed pools. This is particularly likely if the 

chemical is persistent and mobile, the pesticide is applied to highly permeable soils 

overlying shallow unconfined ground water, and rainfall is sufficient to drive the 

chemical through the soil to ground water. Soluble chemicals that are primarily subject 

to photolytic degradation will be very likely to persist in ground water, and can be 

transportable over long distances. Similarly, many chemicals degrade slowly under 

anaerobic conditions (common in aquifers) and are thus more persistent in ground water. 

Under the right hydrologic conditions, this ground water may eventually be discharged to 

the surface – often supporting stream flow in the absence of rainfall. Continuously 

flowing low-order streams in particular are sustained by ground water discharge, which 

can constitute 100% of stream flow during baseflow (no runoff) conditions. Thus, it is 

important to keep in mind that pesticides in ground water may impact surface water 

quality during base flow conditions with subsequent impact on CRLF habitats. However, 

many smaller streams in CA are net dischargers of water to ground water that go dry 

during portions of the year and are not supplied by baseflow from ground water. 

Although concentrations in a receiving water body resulting from ground water discharge 

cannot be explicitly quantified, it should be assumed that significant attenuation and 

retardation of the chemical will have occurred prior to discharge. Nevertheless, where 

paraquat is applied to highly permeable soils over shallow ground water where there is a 

net recharge to adjacent streams, ground water could still be a consistent source of 

chronic background concentrations in surface water, and may also add to surface runoff 

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during storm events (as a result of enhanced ground water discharge typically 

characterized by the ‘tailing limb’ of a storm hydrograph). 

6.1.4 Action Area Uncertainties 

An example of an important simplifying assumption that may require future refinement is 

the assumption of uniform runoff characteristics throughout a landscape. It is well 

documented that runoff characteristics are highly non-uniform and anisotropic, and 

become increasingly so as the area under consideration becomes larger. The assumption 

made for estimating the aquatic action area (based on predicted in-stream dilution) was 

that the entire landscape exhibited runoff properties identical to those commonly found in 

agricultural lands in this region. However, considering the vastly different runoff 

characteristics of: a) undeveloped (especially forested) areas, which exhibit the least 

amount of surface runoff but the greatest amount of groundwater recharge; b) 

suburban/residential areas, which are dominated by the relationship between 

impermeable surfaces (roads, lots) and grassed/other areas (lawns) plus local drainage 

management; c) urban areas, that are dominated by managed storm drainage and 

impermeable surfaces; and d) agricultural areas dominated by Hortonian and focused 

runoff (especially with row crops), a refined assessment should incorporate these 

differences for modeled stream flow generation. As the zone around the immediate 

(application) target area expands, there will be greater variability in the landscape; in the 

context of a risk assessment, the runoff potential that is assumed for the expanding area 

will be a crucial variable (since dilution at the outflow point is determined by the size of 

the expanding area). Thus, it important to know at least some approximate estimate of 

types of land use within that region. Runoff from forested areas ranges from 45 – 

2,700% less than from agricultural areas; in most studies, runoff was 2.5 to 7 times higher 

in agricultural areas (Okisaka et al. 1997; Karvonen et al. 1999; McDonald et al. 2002; 

Phuong and van Dam 2002). Differences in runoff potential between urban/suburban 

areas and agricultural areas are generally less than between agricultural and forested 

areas. In terms of likely runoff potential (other variables – such as topography and 

rainfall – being equal), the relationship is generally as follows (going from lowest to 

highest runoff potential): 

Three-tiered forest < agroforestry < suburban < row-crop agriculture < urban. 

There are, however, other uncertainties that should serve to counteract the effects of the 

aforementioned issue. For example, the dilution model considers that 100% of the 

agricultural area has the chemical applied, which is almost certainly a gross overestimation. Thus, there will be assumed chemical contributions from agricultural areas 

that will actually be contributing only runoff water (dilutant); so some contributions to 

total contaminant load will really serve to lessen rather than increase aquatic 

concentrations. In light of these (and other) confounding factors, Agency believes that 

this model gives us the best available estimates under current circumstances.] 

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6.1.5 Usage Uncertainties 

County-level usage data were obtained from California’s Department of Pesticide 

Regulation Pesticide Use Reporting (CDPR PUR) database. Eight years of data (1999 – 

2006) were included in this analysis because statistical methodology for identifying 

outliers, in terms of area treated and pounds applied, was provided by CDPR for these 

years only. No methodology for removing outliers was provided by CDPR for 2001 and 

earlier pesticide data; therefore, this information was not included in the analysis because 

it may misrepresent actual usage patterns. CDPR PUR documentation indicates that 

errors in the data may include the following: a misplaced decimal; incorrect measures, 

area treated, or units; and reports of diluted pesticide concentrations. In addition, it is 

possible that the data may contain reports for pesticide uses that have been cancelled. 

The CPDR PUR data does not include home owner applied pesticides; therefore, 

residential uses are not likely to be reported. As with all pesticide usage data, there may 

be instances of misuse and misreporting. The Agency made use of the most current, 

verifiable information; in cases where there were discrepancies, the most conservative 

information was used. 

6.1.6 Terrestrial Exposure Modeling of Paraquat 

The Agency relies on the work of Fletcher et al. (1994) for setting the assumed pesticide 

residues in wildlife dietary items. These residue assumptions are believed to reflect a 

realistic upper-bound residue estimate, although the degree to which this assumption 

reflects a specific percentile estimate is difficult to quantify. It is important to note that 

the field measurement efforts used to develop the Fletcher estimates of exposure involve 

highly varied sampling techniques. It is entirely possible that much of these data reflect 

residues averaged over entire above ground plants in the case of grass and forage 

sampling. 

It was assumed that ingestion of food items in the field occurs at rates commensurate 

with those in the laboratory. Although the screening assessment process adjusts dryweight estimates of food intake to reflect the increased mass in fresh-weight wildlife food 

intake estimates, it does not allow for gross energy differences. Direct comparison of a 

laboratory dietary concentration- based effects threshold to a fresh-weight pesticide 

residue estimate would result in an underestimation of field exposure by food 

consumption by a factor of 1.25 – 2.5 for most food items. 

Differences in assimilative efficiency between laboratory and wild diets suggest that 

current screening assessment methods do not account for a potentially important aspect of 

food requirements. Depending upon species and dietary matrix, bird assimilation of wild 

diet energy ranges from 23 – 80%, and mammal’s assimilation ranges from 41 – 85% 

(U.S. EPA 1993). If it is assumed that laboratory chow is formulated to maximize 

assimilative efficiency (e.g., a value of 85%), a potential for underestimation of exposure 

may exist by assuming that consumption of food in the wild is comparable with 

consumption during laboratory testing. In the screening process, exposure may be 

underestimated because metabolic rates are not related to food consumption. 

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For the terrestrial exposure analysis of this risk assessment, a generic bird or mammal 

was assumed to occupy either the treated field or adjacent areas receiving a treatment rate 

on the field. Actual habitat requirements of any particular terrestrial species were not 

considered, and it was assumed that species occupy, exclusively and permanently, the 

modeled treatment area. Spray drift model predictions suggest that this assumption leads 

to an overestimation of exposure to species that do not occupy the treated field 

exclusively and permanently. 

6.1.7 Spray Drift Modeling 

Although there may be multiple paraquat applications at a single site, it is unlikely that 

the same organism would be exposed to the maximum amount of spray drift from every 

application made. In order for an organism to receive the maximum concentration of 

paraquat from multiple applications, each application of paraquat would have to occur 

under identical atmospheric conditions (e.g., same wind speed and – for plants – same 

wind direction) and (if it is an animal) the animal being exposed would have to be present 

directly downwind at the same distance after each application. Although there may be 

sites where the dominant wind direction is fairly consistent (at least during the relatively 

quiescent conditions that are most favorable for aerial spray applications), it is 

nevertheless highly unlikely that plants in any specific area would receive the maximum 

amount of spray drift repeatedly. It appears that in most areas (based upon available 

meteorological data) wind direction is temporally very changeable, even within the same 

day. Additionally, other factors, including variations in topography, cover, and 

meteorological conditions over the transport distance are not accounted for by the 

AgDRIFT model (i.e., it models spray drift from aerial and ground applications in a flat 

area with little to no ground cover and a steady, constant wind speed and direction). 

Therefore, in most cases, the drift estimates from AgDRIFT may overestimate exposure 

even from single applications, especially as the distance increases from the site of 

application, since the model does not account for potential obstructions (e.g., large hills, 

berms, buildings, trees, etc.). Furthermore, conservative assumptions are often made 

regarding the droplet size distributions being modeled (‘ASAE Very Fine’ for 

agricultural uses), the application method (e.g., aerial), release heights and wind speeds. 

Alterations in any of these inputs would change the area of potential effect. However, 

since these input values were not provided in the labels, the default values for the droplet 

size, release heights, and wind speeds were used for this assessment. 

6.2 Effects Assessment Uncertainties 

6.2.1 Acute to Chronic Ratio 

Indirect acute effects to the aquatic-phase CRLF via effects to prey (invertebrates) in 

aquatic habitats are based on peak EECs in the standard pond and the lowest acute 

toxicity value for freshwater invertebrates. For chronic risks, 21-day EECs and the lowest 

chronic toxicity value for invertebrates are used to derive RQs. However, since 

freshwater invertebrate chronic toxicity values are not available at this time, the indirect 

116
chronic toxicity values for the CRLF were unable to be calculated. As a result, there is 

uncertainty as to what degree paraquat may have chronic indirect effects on the CRLF. 

6.2.2 Age Class and Sensitivity of Effects Thresholds 

It is generally recognized that test organism age may have a significant impact on the 

observed sensitivity to a toxicant. The acute toxicity data for fish are collected on 

juvenile fish between 0.1 and 5 grams. Aquatic invertebrate acute testing is performed on 

recommended immature age classes (e.g., first instar for daphnids, second instar for 

amphipods, stoneflies, mayflies, and third instar for midges). 

Testing of juveniles may overestimate toxicity at older age classes for pesticide active 

ingredients that act directly without metabolic transformation because younger age 

classes may not have the enzymatic systems associated with detoxifying xenobiotics. In 

so far as the available toxicity data may provide ranges of sensitivity information with 

respect to age class, this assessment uses the most sensitive life-stage information as 

measures of effect for surrogate aquatic animals, and is therefore, considered as 

protective of the CRLF. 

6.2.3 Use of Surrogate Species Effects Data 

Guideline toxicity tests and open literature data on paraquat are not available for frogs or 

any other aquatic-phase amphibian; therefore, freshwater fish are used as surrogate 

species for aquatic-phase amphibians. The open literature that was available is not 

applicable as the quality of the experiments is not sound to include in the analysis of 

toxicity. Therefore, endpoints based on freshwater fish ecotoxicity data are assumed to 

be protective of potential direct effects to aquatic-phase amphibians including the CRLF, 

and extrapolation of the risk conclusions from the most sensitive tested species to the 

aquatic-phase CRLF is likely to overestimate the potential risks to those species. Efforts 

are made to select the organisms most likely to be affected by the type of compound and 

usage pattern; however, there is an inherent uncertainty in extrapolating across phyla. In 

addition, the Agency’s LOCs are intentionally set very low, and conservative estimates 

are made in the screening level risk assessment to account for these uncertainties. 

6.2.4 Sublethal Effects 

 

When assessing acute risk, the screening risk assessment relies on the acute mortality 

endpoint as well as a suite of sublethal responses to the pesticide, as determined by the 

testing of species response to chronic exposure conditions and subsequent chronic risk 

assessment. Consideration of additional sublethal data in the effects determination is 

exercised on a case-by-case basis and only after careful consideration of the nature of the 

sublethal effect measured and the extent and quality of available data to support 

establishing a plausible relationship between the measure of effect (sublethal endpoint) 

and the assessment endpoints. However, the full suite of sublethal effects from valid 

open literature studies is considered for the purposes of defining the action area. 

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There were numerous studies found within the ECOTOX database that reported on the 

sublethal effects observed after paraquat exposure. Potential sublethal effects on fish are 

evaluated qualitatively and not used as part of the quantitative risk characterization. 

Although one study did provide some information regarding different biochemical factors 

influenced by exposures to paraquat (E104191). Further details on ECOTOX studies are 

provided in Appendix G, and contain the rejection codes and other information as to why 

studies from ECOTOX were not used. 

To the extent to which sublethal effects are not considered in this assessment, the 

potential direct and indirect effects of paraquat on CRLF may be underestimated. 

6.2.5 Location of Wildlife Species 

For the terrestrial exposure analysis of this risk assessment, a generic bird or mammal 

was assumed to occupy either the treated field or adjacent areas receiving a treatment rate 

on the field. Actual habitat requirements of any particular terrestrial species were not 

considered, and it was assumed that species occupy, exclusively and permanently, the 

modeled treatment area. Spray drift model predictions suggest that this assumption leads 

to an overestimation of exposure to species that do not occupy the treated field 

exclusively and permanently. 

118
7.0 Risk Conclusions 

In fulfilling its obligations under Section 7(a)(2) of the Endangered Species Act, the 

information presented in this endangered species risk assessment represents the best data 

currently available to assess the potential risks of paraquat to the CRLF and its designated 

critical habitat. 

Based on the best available information, the Agency makes a May Affect and Likely to 

Adversely Affect (LAA) determination for the CRLF from the use of paraquat. The 

Agency has determined that there is the potential for effects to CRLF designated critical 

habitat from the use of the chemical. The direct effects and habitat modification 

determinations are summarized in Table 7.1 and Table 7.2 respectively. Given the LAA 

determination for the CRLF and potential effects to designated critical habitat, a 

description of the baseline status and cumulative effects for the CRLF is provided in 

Attachment II. 

There are three exceptions to this determination. First, for the direct affects to the CRLF, 

the aquatic-phase CRLF (from surrogate fish species), a No Effect (NE) determination 

was made, as none of the EECs for any of the uses exceeded the listed species LOC 

(0.05). Second, for the indirect effects to the CRLF, the aquatic invertebrates, a No 

Effect (NE) determination was also made, as none of the EECs for any of the uses 

exceeded the listed species LOC (0.05). Third, also for the indirect effects to the CRLF, 

the terrestrial invertebrates were determined to May Affect, But Not Likely to 

Adversely Affect (NLAA) the CRLF, as there is a negligible potential impact to 

terrestrial invertebrates that the CRLF consumes. Likewise, none of the EECs for any of 

the uses exceeded the listed species LOC (0.05). All of the other direct and indirect 

effects to the CRLF were determined to Likely to Adversely Affect (LAA) the CRLF. 

The LAA effects determination applies to those areas where it is expected that the 

pesticide’s use will directly or indirectly affect the CRLF or its designated critical habitat. 

To determine this area, the footprint of paraquat’s use pattern is identified, using land 

cover data that correspond to paraquat’s use pattern. The spatial extent of the LAA 

effects determination also includes areas beyond the initial area of concern that may be 

impacted by spray drift. The identified direct and indirect effects are anticipated to occur 

only for those currently occupied core habitat areas, CNDDB occurrence sections, and 

designated critical habitat for the CRLF that overlap with the initial area of concern plus 

at least 1,000 feet from its boundary, see Section 5.2.5 for further information. It is 

assumed that non-flowing waterbodies (or potential CRLF habitat) are included within 

this area. 

In addition to the spray drift buffer, the results of the downstream dilution extent analysis 

result in a distance of 300 kilometers for forest land cover (which represents the 

maximum continuous distance of downstream dilution from the edge of the initial area of 

concern), 285 kilometers for cultivated crop, orchard/vineyard, and pasture/hay land 

cover and 88.7 kilometers for developed land cover. Please see Section 5.2.5 for a more 

detailed description. If any of these streams reaches flow into CRLF habitat, there is 

119
potential to affect either the CRLF or modify its habitat. These lotic aquatic habitats 

within the CRLF core areas and critical habitats potentially contain concentrations of 

paraquat sufficient to result in LAA determination or effects to critical habitat. 

 

Appendix D provides maps of the initial area of concern, along with CRLF habitat areas, 

including currently occupied core areas, CNDDB occurrence sections, and designated 

critical habitat. It is expected that any additional areas of CRLF habitat that are located at 

least 1,000 ft (to account for offsite migration via spray drift) and 300 kilometers for 

forest land cover (to account for the maximum continuous distance of downstream 

dilution from the edge of the initial area of concern) outside the initial area of concern 

may also be impacted and are part of the full spatial extent of the LAA/effects to critical 

habitat effects determination. 

A summary of the risk conclusions and effects determinations for the CRLF and its 

critical habitat, given the uncertainties discussed in Section 6, is presented in Table 

7-1and Table 7-2. 

Table 7-1 Effects Determination Summary for Paraquat Use and the CRLF 

Assessment 

Endpoint 

Effects 

Determination 1

Basis for Determination 

Potential for Direct Effects

Aquatic-phase (Eggs, Larvae, and Adults): 

The aquatic phase amphibian acute LOCs for listed species (0.05) are not 

exceeded for any uses of paraquat in California. The chronic EECs are all less 

than the estimated chronic value derived from the ACR. Therefore, there are no 

exceedances of chronic LOCs. 

Survival, growth, 

and/or reproduction 

of CRLF 

individuals 

NE1

LAA1

Terrestrial-phase (Juveniles and Adults): 

Refined acute dietary-based RQs for CRLFs consuming small insects exceed 

the acute listed species LOC (0.1) for all uses of paraquat except melons, RQs 

ranged from 1.28 (Airport/public health use/guava) to 0.06 (melons). The 

Refined acute dietary-based RQs for CRLFs consuming large insects and 15g 

small insectivore mammals resulted in paraquat use on airports/public health 

use/guava exceeding the listed species LOC (0.1), with an RQ of 0.14 for both. 

The refined acute dietary-based RQs for CRLFs consuming small herbivore 

mammals (15g) resulted in all uses exceeding the listed species LOC, RQs 

ranged from 2.17 (Airport/public health use/guava) to 0.10 (melons) and for 

35g small mammals all uses except melons exceed the listed species LOC, RQs 

ranged from 1.50 (Airport/public health use/guava) to 0.77 (melons). There are 

no exceedances for CRLFs consuming small terrestrial-phase amphibians. 

Refined dose-based RQs for CRLF of varying weights (1.4g, 37g and 238g) 

consuming small insects exceed the acute endangered species LOC (0.1) for 

only the Airport/public health and Guava uses of paraquat for all weights of 

CRLF). There are no exceedances for small sized (1.4g) CRLF consuming 

large insects and CRLF this size are too small to consume small mammals or 

small terrestrial-phase amphibians. The RQs for small sized (1.4g) CRLF are 

0.27 suggesting that small CRLF consuming small insects are potentially 

affected by acute exposures to paraquat. 

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Refined dose-based RQs for medium sized (37g) CRLF consuming small 

herbivore mammals (either 15g or 35g) exceed the acute listed species LOC 

(0.1) for all uses of paraquat. There were also exceedances in the acute listed 

species LOC (0.1) for medium sized CRLF consuming small insectivore (15g) 

mammals for the airports/public health, guava, and ginger uses of paraquat. For 

medium sized CRLF consuming small insectivore (35g) mammals there were 

exceedances in the acute listed species LOC (0.1) for all uses of paraquat except 

carrots and melons. There are no exceedances for medium sized (37g) CRLF 

consuming large insects or small terrestrial-phase amphibians. Due to 

exceedances of LOCs for CRLF consuming small herbivore mammals (either 

15g or 35g) for all paraquat uses, and exceedances of LOCs for CRLF 

consuming small insectivore mammals (either 15g or 35g) for a majority of 

paraquat uses indicate that the medium sized CRLF could potentially be 

affected by acute exposures to paraquat. 

Refined dose-based RQs for large sized (238g) CRLF consuming small 

herbivore mammals (either 15g or 35g) exceed the acute listed species LOC 

(0.1) for all uses of paraquat except melons. There were no exceedances for 

large sized (238g) CRLF consuming large insects, small insectivore mammals 

(15g or 35g), or small terrestrial-phase amphibians. The exceedances of LOCs 

for CRLF consuming small herbivore mammals (either 15g or 35g) for all 

paraquat uses except melons, indicates that the large sized CRLF could 

potentially be affected by acute exposures to paraquat. 

Refined chronic dietary-based RQs for CRLFs consuming small insects and 

small herbivore mammals (either15g or 35g) using T-HERPS model exceed the 

chronic species LOC (1.0) for all uses of paraquat. Refined chronic dietarybased RQs for CRLFs consuming large insects and small insectivore mammals 

(either 15g or 35g) using T-HERPS model exceed the chronic species LOC 

(1.0) for Airport/public health, guava, and ginger uses of paraquat. The refined 

chronic dietary-based RQs for CRLFs consuming small terrestrial-phase 

amphibians using T-HERPS model exceed the chronic species LOC (1.0) for 

Airport/public health and guava uses of paraquat (the maximum uses). 

Potential for Indirect Effects

LAA1

Aquatic prey items, aquatic habitat, cover and/or primary productivity 

LOCs for non-vascular plants are exceeded for all uses. The non-vascular plant 

RQs range from 138 for airports, commercial/industrial areas, and public health 

areas to 4.1 for melons. 

LOCs for vascular plants are not exceeded for any uses. The vascular plant RQs 

range from 0.77 (airports, commercial/industrial areas, public health areas) to 

0.02 (melons). 

LOCs for aquatic invertebrates are not exceeded for any uses. The acute RQs 

range from 0.730 for airports, commercial/industrial areas, and public health 

areas to 0.004 for melons. When comparing chronic indirect effects, the 

estimated chronic value at 0.174 ppm is not exceeded for any use. 

For fish/frogs none of the uses exceed the LOCs for listed species. The RQs 

range from 0.004 for airports, commercial/industrial areas, and public health 

areas to < 0.001 for melons. When comparing chronic direct effects, the 

estimated chronic value at 1.89 ppm is not exceeded for any use. 

121
LAA1

Terrestrial prey items, riparian habitat

RQs could not be calculated for terrestrial invertebrates as the endpoint was not 

definitive. The calculated EECs were compared to the toxicity endpoint, and it 

was determined that terrestrial invertebrates would not likely adversely affect the 

CRLF indirectly as food. 

For small mammals the acute dose-based RQs exceed the Agency’s LOC (0.1) 

for all uses of paraquat, the RQs ranged from 7.63 (airports/public health 

use/guava) to 0.34 (melons). The chronic dose-based RQs exceed the Agency’s 

LOC (1.0) for all uses of paraquat, and range from 128.53 (airports/public health 

use/guava) to 5.78 (melons). The chronic dietary-based RQs exceed the 

Agency’s LOC (1.0) for all uses except melons, and range from 14.81 

(airports/public health use/guava) to 0.67 (melons). 

The RQs for small terrestrial-phase amphibians did not exceed the listed species 

LOC (0.1) for any use of paraquat. Reduction in amphibian prey items, 

specifically other frogs is not affected from paraquat use. 

The RQs for non-target terrestrial monocot and dicot plants inhabiting semiaquatic and upland dry areas do not exceed the Agency’s LOC (1.0) for any uses. 

All aerial applications of paraquat results in spray drift exceedances for dicots 

(only). These exceedances range from 3.57 (Agricultural fallow/ideland 

maximum aerial application rate) to 1.07 (Melons, minimum aerial application 

rate). 1

 No effect (NE); May affect, but not likely to adversely affect (NLAA); May affect, likely to adversely 

affect (LAA)

Table 7-2 Effects Determination Summary for Paraquat Use and CRLF Critical 

Habitat Impact Analysis 

Assessment 

Endpoint 

Effects 

Determination 

Basis for Determination 

Modification of 

aquatic-phase PCE 

May affect Due to aquatic non-vascular and terrestrial plant communities being reduced 

from a majority of use sites, there is potential for alteration of channel/pond 

morphology or geometry and/or increase in sediment deposition within the 

stream channel or pond. These plant communities provide shelter, foraging, 

predator avoidance, and aquatic dispersal for juvenile and adult CRLFs. In 

addition, there is potential for alteration in water chemistry/quality including 

temperature, turbidity, and oxygen content necessary for normal growth and 

viability of juvenile and adult CRLFs and their food. 

LOCs are exceeded for terrestrial riparian plants and for aquatic non-vascular 

plants from exposure to paraquat from spray drift. LOCs for non-vascular 

plants are exceeded for all uses of paraquat. 

Modification of 

terrestrial-phase 

PCE 

May affect The use of paraquat at all sites may create the following effects to PCE: 

elimination and/or disturbance of upland habitat; ability of habitat to support 

food source of CRLFs, elimination and/or disturbance of dispersal habitat, 

reduction and/or modification of food sources for terrestrial phase juveniles and 

adults, and alteration of chemical characteristics necessary for normal growth 

and viability of juvenile and adult CRLFs and their food source. 

The RQs for vascular plants did not exceed the Agency’s LOC (1.0) for any uses 

of paraquat in California. 

122
The RQs for non-target terrestrial monocot and dicot plants inhabiting semiaquatic and upland dry areas do not exceed the Agency’s LOC (1.0) for all uses. 

All aerial applications of paraquat results in spray drift exceedances for dicots 

(only). 

The use of paraquat on most use sites will exceed the revised acute dietary- and 

dose-based LOC and chronic LOC for prey food items of small mammals, and 

invertebrates. Food sources for the CRLF are reduced, and the CRLF is 

indirectly affected from this reduction. 

Based on the conclusions of this assessment, a formal consultation with the U. S. Fish 

and Wildlife Service under Section 7 of the Endangered Species Act should be initiated. 

When evaluating the significance of this risk assessment’s direct/indirect and adverse 

habitat modification effects determinations, it is important to note that pesticide 

exposures and predicted risks to the species and its resources (i.e., food and habitat) are 

not expected to be uniform across the action area. In fact, given the assumptions of drift 

and downstream transport (i.e., attenuation with distance), pesticide exposure and 

associated risks to the species and its resources are expected to decrease with increasing 

distance away from the treated field or site of application. Evaluation of the implication 

of this non-uniform distribution of risk to the species would require information and 

assessment techniques that are not currently available. Examples of such information and 

methodology required for this type of analysis would include the following: 

• Enhanced information on the density and distribution of CRLF life stages within 

specific recovery units and/or designated critical habitat within the action area. 

This information would allow for quantitative extrapolation of the present risk 

assessment’s predictions of individual effects to the proportion of the population 

extant within geographical areas where those effects are predicted. Furthermore, 

such population information would allow for a more comprehensive evaluation of 

the significance of potential resource impairment to individuals of the species. 

• Quantitative information on prey base requirements for individual aquatic- and 

terrestrial-phase frogs. While existing information provides a preliminary picture 

of the types of food sources utilized by the frog, it does not establish minimal 

requirements to sustain healthy individuals at varying life stages. Such 

information could be used to establish biologically relevant thresholds of effects 

on the prey base, and ultimately establish geographical limits to those effects. 

This information could be used together with the density data discussed above to 

characterize the likelihood of adverse effects to individuals. 

• Information on population responses of prey base organisms to the pesticide. 

Currently, methodologies are limited to predicting exposures and likely levels of 

direct mortality, growth or reproductive impairment immediately following 

exposure to the pesticide. The degree to which repeated exposure events and the 

inherent demographic characteristics of the prey population play into the extent to 

which prey resources may recover is not predictable. An enhanced understanding 

of long-term prey responses to pesticide exposure would allow for a more refined 

determination of the magnitude and duration of resource impairment, and together 

with the information described above, a more complete prediction of effects to 

individual frogs and potential modification to critical habitat. 

123
8.0 References 

Alvarez, J. 2000. Letter to the U.S. Fish and Wildlife Service providing comments on 

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