Risks of Atrazine Use to 

Federally Listed Endangered 

Barton Springs Salamanders 

(Eurycea sosorum) 

August 22, 2006 
Risks of Atrazine Use to Federally Listed 

Endangered Barton Springs Salamanders 

(Eurycea sosorum) 

Pesticide Effects Determination 
 

Environmental Fate and Effects Division 
 

Office of Pesticide Programs 
 

Washington, D.C. 20460 
 

August 22, 2006 
 
Table of Contents 

1. Executive Summary.................................................................................................. 6 
 

2. Problem Formulation ............................................................................................... 8 
 

2.1 Purpose................................................................................................................ 9 
 

2.2 Scope................................................................................................................. 10 
 

2.3 Previous Assessments ....................................................................................... 10 
 

2.4 Stressor Source and Distribution ...................................................................... 13 
 

2.4.1 Environmental Fate and Transport Assessment........................................ 13 
 

2.4.2 Mechanism of Action................................................................................ 14 
 

2.4.3 Use Characterization................................................................................. 14 
 

2.5 Assessed Species............................................................................................... 17 
 

2.6 Action Area..............................................................................20 

2.7 Assessment Endpoints and Measures of Ecological Effect .............................. 23 
 

2.8 Conceptual Model............................................................................................. 25 
 

2.8.1 Risk Hypotheses........................................................................................ 25 
 

2.8.2 Diagram..................................................................................................... 25 
 

3. Exposure Assessment.............................................................................................. 28 
 

3.1 Label Application Rates and Intervals.............................................................. 28 
 

3.2 Aquatic Exposure Assessment.......................................................................... 29 
 

3.2.1 Background........................................................................30 

3.2.2 Geology/Hydrogeology............................................................................. 34 
 

3.2.3 Conceptual Model of Exposure ................................................................ 37 
 

3.2.4 Existing Monitoring Data ......................................................................... 39 
 

3.2.5 Modeling Approach .................................................................................. 43 
 

3.2.5.1 Model Inputs ..........................................................................................44 
 

3.2.6 Individual Scenario Results ...................................................................... 48 
 

3.2.6.1 Residential.............................................................................................49 
 

3.2.6.2 Turf .......................................................................................................54 
 

3.2.6.3 Fallow/Idle Land...................................................................................54 
 

3.2.6.4 Rights-of-Way.......................................................................................55 
 

3.2.7 Characterization ........................................................................................ 59 
 

4. Effects Assessment .................................................................................................. 64 
 

4.1 Evaluation of Aquatic Ecotoxicity Studies....................................................... 65 
 

4.1.1 Toxicity to Freshwater Fish ...................................................................... 67 
 

4.1.1.1 	 Freshwater Fish: Acute Exposure (Mortality) Studies..........................67 
 

4.1.1.2 	 Freshwater Fish: Chronic Exposure (Growth/Reproduction) Studies ..67 
 

4.1.1.3 	 Freshwater Fish: Sublethal Effects and Additional Open Literature 

 Information ............................................................................................68 
 

4.1.2 Toxicity to Aquatic-phase Amphibians .................................................... 69 
 

4.1.2.1 Amphibians: Open Literature Data on Mortality..................................69 
 

4.1.2.2 Amphibians: Open Literature Data on Sublethal Effects .....................69 
 

4.1.3 Toxicity to Freshwater Invertebrates ........................................................ 71 
 

2
 
4.1.3.1 	
Freshwater Invertebrates: Acute Exposure Studies ..............................71 
 

4.1.3.2 	
Freshwater Invertebrates: Chronic Exposure Studies...........................72 
 

4.1.3.3 	 Freshwater Invertebrates: Open Literature Data...................................73 
 

4.1.4 	 Toxicity to Aquatic Plants ........................................................................ 73 
 

4.1.4.1 	 Aquatic Plants: Laboratory Data............................................................73 
 

4.1.5 Freshwater Field Studies........................................................................... 74 

4.2 	 Community-Level Endpoints: Threshold Concentrations........................75 

4.3 	 Use of Probit Slope Response Relationship to Provide Information on the 
 

Endangered Species Levels of Concern............................................................ 78 
 

4.4 	 Incident Database Review................................................................................. 78 
 

5. Risk Characterization............................................................................................. 79 
 

5.1 Risk Estimation................................................................................................. 80 

5.1.1 Direct Effects ............................................................................................ 80 

5.1.2 Indirect Effects.......................................................................................... 81 

5.1.2.1 	 Evaluation of Potential Indirect Effects via Reduction in Food Items 

 (Freshwater Invertebrates.......................................................................82 
 

5.1.2.2 	 Evaluation of Potential Indirect Effects via Reduction in Habitat 
 

and/or Primary Productivity (Freshwater Aquatic Plants).....................84 
 

5.2 Risk Description................................................................................................ 86 

5.2.1 	 Direct Effects to the Barton Springs Salamander ..................................... 87 
 

5.2.2 	 Indirect Effects via Reduction in Food Items (Freshwater Invertebrates) 88 
 

5.2.3 	 Indirect Effects via Reduction in Habitat and/or Primary Productivity 
 

(Freshwater Aquatic Plants)...................................................................... 90 
 

6. Uncertainties............................................................................................................ 92 
 

6.1 	 Exposure Assessment Uncertainties ............................................................. 92 
 

6.1.1 Modeling Assumptions ............................................................................. 92 

6.1.2 	 Impact of Vegetative Setbacks on Runoff ................................................ 92 
 

6.1.3 	 PRZM Modeling Inputs and Predicted Aquatic Concentrations .............. 93 
 

6.2 	 Effects Assessment Uncertainties................................................................. 93 
 

6.2.1 	 Age class and sensitivity of effects thresholds ......................................... 93 
 

6.2.2 	 Use of surrogate species effects data ........................................................ 94 
 

6.2.3 	 Acute freshwater invertebrate toxicity data for the midge........................ 94 
 

6.2.4 	 Extrapolation of long-term environmental effects from short-term
 

laboratory tests.......................................................................................... 94 
 

6.2.5 	 Use of threshold concentrations for community-level endpoints ............. 95 
 

6.3 Assumptions Associated with the Acute LOCs...............................95 

7. Summary of Direct and Indirect Effects to the Barton Springs Salamander ... 96 
 

8. References................................................................................................................ 97 
 

3
 
List of Tables 

Table 1.1. Effects Determination Summary for the Barton Springs Salamander......... 8 
 

Table 2.1. Summary of Assessment Endpoints and Measures of Ecological Effect .. 25 
 

Table 3.1. Information for the Barton Springs Salamander Endangered Species 
 

Assessment................................................................................................ 29 
 

Table 3.2. Summary of USGS Monitoring Data from the Four Springs Comprising 
 

Barton Springs .......................................................................................... 40 
 

Table 3.3. Summary of PRZM/EZAMS Environmental Fate Data Used for Aquatic 
 

Exposure Inputs for Atrazine Endangered Species Assessment for the 
 

Barton Springs Salamander....................................................................... 45 
 

Table 3.4. Land Cover Adjustment Factors for the Action Area in the Barton Springs 
 

Segment of the Edwards Aquifer (BSSEA).............................................. 47 
 

Table 3.5. Summary of PRZM Output EECs for all Modeled (Edge of Field 
 

Concentrations with Base Flow Incorporated). ........................................ 58 
 

Table 3.6. Comparison of Maximum, Typical, and 90th Percentile Label Rates and 
 

Number of Applications............................................................................ 59 
 

Table 3.7. Comparison of Residential and Rights-of-Way EECs Assuming Variable 
 

Percentages of Overspray (0, 1, and 10%)................................................ 61 
 

Table 3.8. Comparison of Residential EECs (granular) with 1% Over Spray and 
 

Variable Percentages of Impervious Surface (10 and 30%) ..................... 62 
 

Table 3.9. Comparison of Residential EECs (granular) with 10% Overspray and 
 

Variable Percentages of Impervious Surface (10 and 30%) ..................... 62 
 

Table 3.10. Comparison of Residential EECs (granular) Assuming Various 
 

Percentages of Treated 1⁄4 Acre Lot (10 and 50%).................................... 63 
 

Table 4.1. Aquatic Toxicity Profile for Atrazine........................................................ 66 
 

Table 4.2. Categories of Acute Toxicity for Aquatic Organisms ............................... 67 
 

Table 5.1. Summary of Direct Effect RQs for the Barton Springs Salamander ......... 81 
 

Table 5.2. Summary of RQs Used to Estimate Indirect Effects to the Barton Springs 
 

Salamander via Direct Acute Effects on Dietary Items ............................ 83 
 

Table 5.3. Summary of RQs Used to Estimate Indirect Effects to the Barton Springs 
 

Salamander via Direct Chronic Effects on Dietary Items......................... 84 
 

Table 5.4. Summary of RQs Used to Estimate Indirect Effects to the Barton Springs 
 

Salamander via Direct Effects on Aquatic Plants.................................... 85 
 

Table 5.5. Summary of RQs Used to Assess Potential Risk to Freshwater Invertebrate 
 

Food Items of the Barton Springs Salamander ......................................... 89 
 

Table 5.6. Summary of Modeled Scenario Time-Weighted EECs with Threshold 
 

Concentrations for Potential Community-Level Effects........................... 91 
 

Table 7.1. Effects Determination Summary for the Barton Springs Salamander....... 96 
 

4
 
List of Figures 

Figure 2.1 National Extent of Atrazine Use (lbs) ............................................................ 16 
 

Figure 2.2. Barton Springs Complex ................................................................................ 19 
 

Figure 2.3. Barton Springs Salamander Action Area ....................................................... 23 
 

Figure 2.4. Conceptual Model for Barton Springs Salamander........................................ 26 
 

Figure 3.1. Barton Springs Segment of the Edwards Aquifer with HydroZones ............. 31 
 

Figure 3.2. Hydrogeologic Cross Section of the Barton Springs Segment of the Edwards 
 

Aquifer Showing Dominant Flow Pathways Within Each Hydrozone .......... 33 

Figure 3.3. Conceptual Model of Surface and Subsurface Flow Within the Barton Springs 

Segment of the Edwards Aquifer Relative to the Barton Springs Salamander34 

Figure 3.4. Flow paths within Recharge Zone of the Barton Springs Segment of the 

Edwards Aquifer ............................................................................................. 36 

Figure 3.5. Location of Surface Water Sites within the Barton Springs Segment............ 41 

Figure 3.6. Location of Groundwater Sites Within the Barton Springs Segment............. 42 

Figure 3.7. Flow Hydrograph Data for Barton Springs .................................................... 43 

Figure 3.8. Conceptual Model of Paired Residential/Impervious Scenarios.................... 50 

Figure 3.9. Percentage of Impervious Surface Coverage in Vicinity of Barton Springs.. 52 

Figure 3.10. Representative Time Series Output from Paired Residential/Impervious 

PRZM Scenario for Granular Applications .................................................... 53 

Figure 3.11. Conceptual Model of Rights-of-Way Scenario ............................................ 56 

Figure 4.1. Summary of Reported Acute LC50/EC50 Values in Freshwater Invertebrates 

for Atrazine ..................................................................................................... 72 
 

Figure 4.2. Use of Threshold Concentrations in Endangered Species Assessment.......... 77 
 

Appendices 

Appendix A Ecological Effects Data 

Appendix B Supporting Information for the Aquatic Community-Level 

Threshold Concentrations 

Appendix C Supporting Information for the Scenario Development 

Appendix D Status and Life History of the Barton Springs Salamander 

Appendix E Stepwise Approach to Modeling for the Barton Springs 

Salamander Endangered Species Assessment (Using the 

Residential Scenario as an Example) 

Appendix F Incident Database Information 

Appendix G RQ Method and LOCs 

5
 
1. Executive Summary 

The purpose of this assessment is to make an “effects determination” for the Barton 

Springs salamander (Eurycea sosorum) by evaluating the potential direct and indirect 

effects of the herbicide atrazine on the survival, growth, and reproduction of this 

Federally endangered species. This assessment was completed in accordance with the 

U.S. Fish and Wildlife Service (USFWS) and National Marine Fisheries Service (NMFS) 

Endangered Species Consultation Handbook (USFWS/NMFS, 1998), the August 5, 2004 

Joint Counterpart Endangered Species Act Section 7 Consultation Regulations specified 

in 50 CFR Part 402 (USFWS/NMFS, 2004a; FR 69 47732-47762), and procedures 

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

The range of the Barton Springs salamander is restricted to four spring outlets that 

comprise the Barton Springs complex, which is located near downtown Austin, Texas. 

Subsurface flow from the Barton Springs segment of the Edwards Aquifer and its 

contributing zone supply all of the water in the springs that make up the Barton Springs 

complex. Therefore, the action area for the Barton Springs salamander is defined by 

those areas within the hydro geologic framework of the Barton Springs segment of the 

Edwards Aquifer. 

Environmental fate and transport models were used to estimate high-end exposure values 

expected to occur in the Barton Springs action area as a result of agricultural and nonagricultural atrazine use in accordance with label directions. Modeled concentrations 

provide “edge-of-field” estimates of exposure which are intended to represent atrazine 

concentrations transported with runoff water directly to Barton Springs via subsurface 

flow through the fractured karst limestone of the Edwards Aquifer. Estimated high-end 

exposure values were compared with available monitoring data, although the monitoring 

data are unlikely to capture the upper bounds of exposure due to sampling frequency. In 

general, the modeled peak exposure estimates are two to ten times higher than 

concentrations seen in the monitoring data, while the annual average concentrations 

based on modeling are consistent with those seen in monitoring. The highest overall 

modeled exposures were predicted to occur from residential uses of atrazine within the 

action area. 

The assessment endpoints for the Barton Springs salamander include direct toxic effects 

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

effects, such as reduction of the prey base and/or modification of its habitat. Direct 

effects to the Barton Springs salamander are based on toxicity information for freshwater 

vertebrates, including fish, which are generally used as a surrogate for amphibians, as 

well as available aquatic-phase amphibian data from the open literature. Given that the 

salamander’s prey items and habitat requirements are dependant on the availability of 

freshwater aquatic invertebrates and aquatic plants, respectively, toxicity information for 

these taxonomic groups is also discussed. In addition to the registrant-submitted and 

open literature toxicity information, indirect effects to Barton Springs salamanders, via 

impacts to aquatic plant community structure and function are also evaluated based on 

6
 
time-weighted threshold concentrations that correspond to potential aquatic plant 

community-level effects. 

Degradates of atrazine include hydroxyatrazine (HA), deethylatrazine (DEA), 

deisopropylatrazine (DIA), and diaminochloroatrazine (DACT). Comparison of available 

toxicity information for the degradates of atrazine indicates lesser aquatic toxicity than 

the parent for freshwater and estuarine/marine fish, invertebrates, and aquatic plants. 

Because the degradates are not of greater toxicological concern than atrazine, 

concentrations of the atrazine degradates are not assessed further, and the focus of this 

assessment is parent atrazine. 

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 atrazine use within the action area has the potential to adversely 

affect the Barton Springs salamander via direct toxicity to the salamander or indirectly 

based on direct effects to their food supply (i.e, freshwater invertebrates) or habitat (i.e., 

aquatic plants). When RQs for a particular type of effect are below LOCs, the potential 

for adverse effects to the Barton Spring salamander is expected to be negligible, leading 

to a conclusion of “no effect”. Where RQs exceed LOCs, a potential to cause adverse 

effects is identified, leading to a preliminary conclusion of “may effect”. If a 

determination is made that use of atrazine within the action area “may affect” the Barton 

Spring salamander, additional information is considered to refine the potential for 

exposure and effects, and the best available data are used to distinguish those actions that 

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

adversely affect” the Barton Springs salamander. 

The best available data suggest that atrazine will either have no effect or is not likely to 

adversely affect the Barton Springs salamander by direct toxic effects or by indirect 

effects resulting from effects to aquatic invertebrates and plants. A summary of the risk 

conclusions and effects determination for the Barton Springs salamander is presented in 

Table 1.1. Further information on the results of the effects determination is included as 

part of the Risk Description in Section 5.2. 

7
 
Table 1.1. Effects Determination Summary for the Barton Springs Salamander 

Assessment Endpoint Effects Determination Basis for Determination 

Survival, growth, and 

reproduction of Barton 

Springs salamander 

individuals via direct 

effects 

No effect No acute and chronic LOCs are exceeded. 

Indirect effects to Barton 

Springs salamander via 

reduction of prey (i.e., 

freshwater invertebrates) 

May affect, but not likely 

to adversely affect 

Acute LOCs are exceeded based on the most sensitive 

ecotoxicity value for the midge; however RQs for other dietary 

items (amphipods, leeches, snails) are less than LOCs. Based 

on the non-selective nature of feeding behavior in the Barton 

Springs salamander and low magnitude of anticipated 

individual effects to all evaluated prey species, atrazine is not 

likely to indirectly affect the Barton Springs salamander via a 

reduction in freshwater invertebrate food items. This finding is 

based on insignificance of effects (i.e., effects to freshwater 

invertebrates are not likely to result in “take” of a single Barton 

Springs salamander) and discountability (i.e., the effect to 

freshwater invertebrates is extremely unlikely to occur given 

the estimated individual event probability of 1 in 45.5 million). 

Indirect effects to Barton 

Springs salamander via 

reduction of habitat and/or 

primary productivity (i.e., 

aquatic plants) 

May affect, but not likely 

to adversely affect 

Although atrazine use may directly affect individual vascular 

and non-vascular aquatic plants in Barton Springs, its use 

within the action area is not likely to adversely affect the 

Barton Springs salamander via indirect community-level 

effects to aquatic vegetation. Predicted 14-, 30-, 60-, and 90­

day EECs for all modeled atrazine use scenarios within the 

action area are well below the threshold concentrations 

representing community-level effects. This finding is based on 

insignificance of effects (i.e., community-level effects to 

aquatic plants are not likely to result in “take” of a single 

Barton Springs salamander). 

2. 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. This assessment was completed in accordance with the August 5, 

2004 Joint Counterpart Endangered Species Act (ESA) Section 7 Consultation 

Regulations specified in 50 CFR Part 402 (USFWS/NMFS, 2004a; FR 69 47732-47762). 

The structure of this risk assessment is based on guidance contained in EPA’s Guidance 

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

Consultation Handbook (USFWS/NMFS, 1998) and procedures outlined in the Overview 

Document (U.S. EPA, 2004). 

8
 
2.1 Purpose 

This ecological risk assessment is a component of the settlements for Center for 

Biological Diversity and Save Our Springs Alliance v. Leavitt, No. 1:04CV00126-CKK 

(filed January 26, 2004) and Natural Resources Defense Council, Civ. No: 03-CV-02444 

RDB (filed March 28, 2006). The purpose of this ecological risk assessment is to make 

an “effects determination,” as directed in Section 7(a) (2) of the ESA, for the Barton 

Springs salamander (Eurycea sosorum) by evaluating the potential direct and indirect 

effects resulting from use of the herbicide atrazine (6-chloro-N-ethyl-N-isopropyl-1, 3, 5­

triazine-2, 4-diamine) on the survival, growth, and/or reproduction of this Federally 

endangered species. The Barton Springs salamander was federally listed as an 

endangered species on May 30, 1997 (62 FR 23377-23392) by the U.S. Fish and Wildlife 

Service (USFWS or the Service). No critical habitat has been designated for this species. 

In this endangered species assessment, direct and indirect effects to the Barton Springs 

salamander are evaluated in accordance with the screening-level methodology described 

in the Agency’s Overview Document (U.S. EPA, 2004). It should be noted, however, 

that the indirect effects analysis in this assessment utilizes more refined data than is 

generally available to the Agency. Specifically, a robust set of microcosm and mesocosm 

data and aquatic ecosystem models are available for atrazine that allowed EPA to refine 

the indirect effects associated with potential aquatic community-level effects (via aquatic 

plant community structural change and subsequent habitat modification) to the Barton 

Springs salamander. Use of such information is consistent with the guidance provided in 

the Overview Document, which specifies that “the assessment process may, on a case-bycase 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). 

As part of the “effects determination”, the Agency will reach one of the following three 

conclusions regarding the potential for atrazine to affect the Barton Springs salamander: 

• “No effect”; 

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

• “Likely to adversely affect”. 

If the results of the screening-level assessment show no indirect effects and levels of 

concern (LOCs) for the Barton Springs salamander are not exceeded for direct effects, a 

“no effect” determination is made, based on atrazine’s use within the action area. If, 

however, indirect effects are anticipated and/or exposure exceeds the LOCs for direct 

effects, the Agency concludes a preliminary “may affect” determination for the Barton 

Springs salamander. 

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

Barton Springs salamander, 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 Barton Springs salamander and potential 

community-level effects to aquatic plants. Based on the refined information, the Agency 

9
 
will use the best available information to distinguish those actions that “may affect, but 

are not likely to adversely affect” from those actions that are “likely to adversely affect” 

the Barton Springs salamander. This information is presented as part of the Risk 

Characterization in Section 5. 

2.2 Scope 

Atrazine is currently registered as a herbicide in the U.S. to control annual broadleaf and 

grass weeds in corn, sorghum, sugarcane, and other crops. In addition to food crops, 

atrazine is also used on a variety of non-food crops, forests, residential/industrial uses, 

golf course turf, recreational areas, and rights-of-way. It is one of the most widely used 

herbicides in North America (U.S. EPA, 2003a). 

The end result of the EPA pesticide registration process 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, 

acceptable methods of application, approved use sites, and any restrictions on how 

applications may be conducted. Thus, the use, or potential use, of atrazine in accordance 

with the approved product labels is “the action” being assessed. 

This ecological risk assessment is for currently registered uses of atrazine in the action 

area associated with the Barton Springs salamander. Further discussion of the action area 

for the Barton Springs salamander is provided in Section 2.6. 

Degradates of atrazine include hydroxyatrazine (HA), deethylatrazine (DEA), 

deisopropylatrazine (DIA), and diaminochloroatrazine (DACT). Comparison of available 

toxicity information for the degradates of atrazine indicates lesser aquatic toxicity than 

the parent for freshwater fish, invertebrates, and aquatic plants. Specifically, the 

available degradate toxicity data for HA indicates that it is not toxic to freshwater fish 

and invertebrates at the limit of its solubility in water. In addition, available aquatic plant 

degradate toxicity data for HA, DEA, DIA, and DACT report non-definitive EC50 values 

(i.e., 50% effect was not observed at the highest test concentrations) at concentrations 

that are 700 to 10,000 times higher than the lowest reported aquatic plant EC50 value for 

parent atrazine. Given the lesser toxicity of the degradates, as compared to the parent, the 

focus of this assessment is parent atrazine. A detailed summary of the available 

ecotoxicity information for all of the atrazine degradates is presented in Appendix A. 

2.3 Previous Assessments 

The Agency completed a refined ecological risk assessment for aquatic impacts of 

atrazine use in January 2003 (U.S. EPA, 2003a). This assessment was based on 

laboratory ecotoxicological data as well as microcosm and mesocosm field studies found 

in publicly available literature, a substantial amount of monitoring data for freshwater 

streams, lakes, reservoirs, and estuarine areas, and incident reports of adverse effects on 

aquatic and terrestrial organisms associated with the use of atrazine. In the refined 

assessment, risk is described in terms of the likelihood that concentrations in water bodies 

10
 
(i.e., lakes/reservoirs, streams, and estuarine areas) equaled or exceeded concentrations 

shown to cause adverse effects to aquatic communities and populations of aquatic 

organisms. The results of the refined aquatic ecological assessment indicated that 

exposure to atrazine is likely to result in adverse community-level and population-level 

effects to aquatic communities at concentrations greater than or equal to 10-20 μg/L on a 

recurrent basis or over a prolonged period of time. 

During this time, the Agency extensively reviewed a probabilistic ecological risk 

assessment submitted by the registrant (Giddings et al., 2000). The Agency’s review of 

Syngenta’s probabilistic risk assessment is included in Appendix XVII of the 2003 

atrazine IRED. EPA’s refined risk assessment incorporates some of the data submitted 

by the registrant in its probabilistic risk assessment. 

The results of the Agency’s ecological assessments for atrazine are fully discussed in the 

January 31, 2003, Interim Reregistration Eligibility Decision (IRED)1. Because the 

Agency had determined that atrazine shares a common mechanism of toxicity with the 

structurally-related chlorinated triazines simazine and propazine, a cumulative human 

health risk assessment for the triazines was necessary before the Agency could make a 

final determination of reregistration eligibility. However, the Agency issued the interim 

decision in order to identify risk reduction measures that were necessary to support the 

continued use of atrazine. The January 2003 IRED requires extensive drinking water 

monitoring in Community Water Systems (CWSs) where atrazine levels have exceeded 

or are predicted to have the potential to exceed drinking water levels of concern. In 

addition, the need for the following information related to potential ecological risks was 

established: 1) an ecological monitoring program of potentially vulnerable water bodies 

in corn, sorghum, and sugarcane use areas; and 2) further information on potential 

amphibian gonadal developmental responses to atrazine. 

EPA issued an addendum on October 31, 2003 that updated the IRED issued on January 

31, 2003 (U.S. EPA, 2003b). This addendum describes new scientific developments 

pertaining to ecological monitoring and mitigation of watersheds and potential effects of 

atrazine on endocrine-mediated pathways of amphibian gonadal development. 

The January 2003 IRED required atrazine registrants to develop a watershed monitoring 

protocol. The resulting protocol identifies 40 indicator watersheds in corn and sorghum 

growing areas in which monitoring has been required for a two-year period within each 

watershed. The first 20 watersheds were monitored in 2004 and 2005. The second set of 

20 watersheds was monitored in 2005, and the second year of sampling for these 

watersheds is currently in progress. The goal of the monitoring is to ascertain the extent 

to which any of the watersheds have streams with atrazine concentrations that could 

cause significant changes in aquatic plant community structure, the most sensitive 

endpoint in the aquatic ecosystem. Streams in watersheds exceeding the Agency's levels 

of concern will be subject to mitigation consistent with watershed management principles 

1 The 2003 Interim Reregistration Eligibility Decision for atrazine is available via the 

internet at http://www.epa.gov/oppsrrd1/REDs/0001.pdf 

11
 
described by the Agency’s Office of Water program requirements 

(http://www.epa.gov/owow/tmdl/). These monitoring sites are representative of 1,172 

watersheds determined to be among the most vulnerable to atrazine surface water loading 

from use on corn and sorghum. Therefore, the results from the 40 watersheds will be 

used to determine if further monitoring or remedial efforts are needed in the larger 

population of watersheds. EPA has selected an atrazine level of concern (LOC) that is 

based on significant aquatic community effects consistent with those described in the 

Office of Pesticide Programs (OPP) 2003 ecological risk assessment (U.S. EPA, 2003a 

and b) and the Office of Water’s (OW) draft atrazine aquatic life criteria (U.S. EPA, 

2003c). Further discussion of the aquatic community-level LOC is provided in Section 

4.2 and Appendix B of this assessment. Aqueous atrazine concentrations obtained from 

monitoring studies can be interpreted with the LOC to determine if a water body is likely 

to be significantly affected. 

As discussed in the October 2003 IRED, the Agency also conducted an evaluation of the 

submitted studies regarding the potential effects of atrazine on amphibian gonadal 

development and presented its assessment in the form of a white paper for external peer 

review to a FIFRA Scientific Advisory Panel (SAP) in June 20032. In the white paper 

dated May 29, 2003, the Agency summarized seventeen studies consisting of both open 

literature and registrant-submitted laboratory and field studies involving both native and 

non-native species of frogs. The Agency concluded that none of the studies fully 

accounted for environmental and animal husbandry factors capable of influencing 

endpoints that the studies were attempting to measure. The Agency also concluded that 

the current lines-of-evidence did not show that atrazine produced consistent effects across 

a range of exposure concentrations and amphibian species tested. 

Based on this assessment, the Agency concluded and the SAP concurred that there was 

sufficient evidence to formulate a hypothesis that atrazine exposure may impact gonadal 

development in amphibians, but there were insufficient data to confirm or refute the 

hypothesis (http://www.epa.gov/oscpmont/sap/2003/June/junemeetingreport.pdf). 

Because of the inconsistency and lack of reproducibility across studies and an absence of 

a dose-response relationship in the currently available data, the Agency determined that 

the data did not alter the conclusions reached in the January 2003 IRED regarding 

uncertainties related to atrazine’s potential effects on amphibians. The SAP supported 

EPA in seeking additional data to reduce uncertainties regarding potential risk to 

amphibians. Subsequent data collection has followed the multi-tiered process outlined in 

the Agency’s white paper to the SAP (U.S. EPA, 2003d). In addition to addressing 

uncertainty regarding the potential use of atrazine to cause these effects, these studies are 

expected to characterize the nature of any potential dose-response relationship. A data 

call-in for the first tier of amphibian studies was issued in 2005 and studies are on-going; 

however, as of this writing, results are not available. 

2

 The Agency’s May 2003 White Paper on Potential Developmental Effects of Atrazine on Amphibians is 

available via the internet at http://www.epa.gov/oscpmont/sap/2003/june/finaljune2002telconfreport.pdf. 

12
 
2.4 Stressor Source and Distribution 

2.4.1 Environmental Fate and Transport Assessment 

The following fate and transport description for atrazine was summarized based on 

information contained in the 2003 IRED (U.S. EPA, 2003a). In general, atrazine is 

expected to be mobile and persistent in the environment. The main route of dissipation is 

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

atrazine is expected to reach surface and ground water. This is confirmed by the 

widespread detections of atrazine in surface water and ground water. Atrazine is 

persistent in soil, with a half-life (time until 50% of the parent atrazine remains) 

exceeding 1 year under some conditions (Armstrong et al., 1967). Atrazine can 

contaminate nearby non-target plants, soil and surface water via spray drift during 

application. Atrazine is applied directly to target plants during foliar application, but preplant and pre-emergent applications are generally far more prevalent. 

The resistance of atrazine to abiotic hydrolysis (stable at pH 5, 7, and 9) and to direct 

aqueous photolysis (stable under sunlight at pH 7), and its only moderate susceptibility to 

degradation in soil (aerobic laboratory half-lives of 3-4 months) indicates that atrazine is 

unlikely to undergo rapid degradation on foliage. Likewise, a relatively low Henry’s 

Law constant (2.6 X 10-9 atm-m3/mol) indicates that atrazine will probably not undergo 

rapid volatilization from foliage. However, its relatively low octanol/water partition 

coefficient (Log Kow = 2 .7), and its relatively low soil/water partitioning (Freundlich Kads 

values < 3 and often < 1) may somewhat offset the low Henry’s Law constant value, 

thereby possibly resulting in some volatilization from foliage. In addition, its relatively 

low adsorption characteristics indicate that atrazine may undergo substantial washoff 

from foliage. It should also be noted that foliar dissipation rates for numerous pesticides 

have generally been somewhat greater than otherwise indicated by their physical 

chemical and other fate properties. 

In terrestrial field dissipation studies performed in Georgia, California, and Minnesota, 

atrazine dissipated with half lives of 13, 58, and 261 days, respectively. The 

inconsistency in these reported half-lives could be attributed to the temperature variation 

between the studies in which atrazine was seen to be more persistent in colder climate. 

Long-term field dissipation studies also indicated that atrazine could persist over a year in 

such climatic conditions. A forestry field dissipation study in Oregon (aerial application 

of 4 lb ai/A) estimated an 87-day half-life for atrazine on exposed soil, a 13-day half-life 

in foliage, and a 66-day half-life on leaf litter. 

Atrazine is applied directly to soil during pre-planting and/or pre-emergence applications. 

Atrazine is transported indirectly to soil due to incomplete interception during foliar 

application, and due to washoff subsequent to foliar application. The available laboratory 

and field data are reported above. For aquatic environments, reported half-lives were 

much longer. In an anaerobic aquatic study, atrazine overall (total system), water, and 

sediment half-lives were given as 608, 578, and 330 days, respectively. 

13
 
A number of degradates of atrazine were detected in laboratory and field environmental 

fate studies. Deethyl-atrazine (DEA) and deisopropyl-atrazine (DIA) were detected in all 

studies, and hydroxy-atrazine (HA) and diaminochloro-atrazine (DACT) were detected in 

all but one of the listed studies. Deethylhydoxy-atrazine (DEHA) and 

deisopropylhydroxy-atrazine (DIHA) were also detected in one of the aerobic studies. 

All of the chloro-triazine and hydroxy-triazine degradates detected in the laboratory 

metabolism studies were present at less than the 10% of applied that the Agency uses to 

classify degradates as “major degradates” (U.S. EPA, 2004), however, several of these 

degradates were detected at percentages greater than 10% in soil and aqueous photolysis 

studies. Insufficient data were available to estimate half-lives for these degradates from 

the available data. The dealkylated degradates are more mobile than parent atrazine, 

while HA is less mobile than atrazine and the dealkylated degradates. 

2.4.2 Mechanism of Action 

Atrazine inhibits photosynthesis by stopping electron flow in Photosystem II. Triazine 

herbicides associate with a protein complex of the photosystem II in chloroplast 

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

of electrons that in turn inhibits the formation and release of oxygen. 

2.4.3 Use Characterization 

An analysis of available usage and land cover information, including extensive 

discussions with local experts in the fields of agriculture and soil science, was completed 

to determine which atrazine uses are likely to be present in the action area. This 

evaluation is intended to place priority on those atrazine use areas likely to be in closest 

proximity to the salamander’s habitat. The analysis indicates that of all registered uses 

for atrazine, the non-agricultural uses are likely to result in the highest exposures to the 

salamander. This is due to the preponderance of potential residential and other nonagricultural use sites (i.e., recreational and rights-of-way) in the immediate vicinity of 

Barton Springs, and the fact that very little agricultural crops other than fallow uses for 

the Conservation Reserve Program (CRP) are actually grown in the action area. Further 

details on the analysis used to make this determination are discussed below and included 

in Appendix C. 

Critical to the development of appropriate modeling scenarios and to the evaluation of the 

appropriate model inputs is an assessment of usage information. The Agency’s 

Biological and Economic Analysis Division (BEAD) provided an analysis of both 

national and local use information for atrazine (Kaul et al., 2005, Zinn and Jones, 2006, 

Kaul, et al., 2006). State level usage data were used to calculate county level usage 

because no reliable county level data are available for Texas. State usage data were 

14
 
obtained from USDA-NASS3

 and EPA proprietary data4 sources. Data from both sources 

were averaged together over the years 2000 to 2004 to calculate average annual usage 

statistics by state and crop for atrazine, including pounds of active ingredient applied, 

percent of crop treated, number of applications per acre, application rate per acre, and 

base acres treated. 

Because no reliable county level usage data are available for Texas, average annual 

pounds applied and acres treated by county were calculated by apportioning the estimated 

state level usage to counties based on the proportion of total state acres grown of each 

crop in each county. The most recently available acreage data were obtained from 

USDA’s 2002 Census of Agriculture. Estimates of the percent of each crop treated, the 

number of applications and the application rate in each county are assumed to be the 

same as the state level estimates. Apportioning the usage in this manner may 

underestimate or overestimate the actual usage in a particular county. 

In this analysis, the Agency gathered information on the agricultural uses of atrazine in 

the three counties (Hays, Travis, and Blanco) located within or adjacent to the action area 

for atrazine in the context of the Barton Springs salamander. Information was available 

on crops for which atrazine is registered, amounts of atrazine used by county, application 

rates, methods of application, application timing, and intervals between applications. 

Usage information is critical in determining which uses should be modeled, while the 

application methods, intervals, and timing are critical model inputs for estimating 

atrazine exposure. While the modeling described below relies initially on maximum 

label application rates and numbers of applications, the information on typical ranges of 

application rates and number of applications is useful for characterization of the modeling 

results. In general, for agricultural uses, atrazine is used in limited amounts relative to 

national use patterns in Hays, Travis and Blanco counties. 

Nationally atrazine has the second largest poundage of any herbicide in the U.S. and is 

widely used to control broadleaf and many other weeds, primarily in corn, sorghum and 

sugarcane. As a selective herbicide, atrazine is applied pre-emergence and postemergence. Figure 2.1 presents the national distribution of atrazine use from data 

collected between 1998 and 2004 and used in the cumulative triazine assessment (U.S. 

EPA, 2006a; Kaul et al., 2005). 

3 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. 4

 US EPA proprietary usage databases provide estimates of pesticide usage for select agricultural use sties 

by chemical, crop and state. 

15
 
Figure 2.1 National Extent of Atrazine Use (lbs) 

Locally, county level estimates of atrazine were derived using state level estimates from 

USDA-NASS and EPA proprietary data. State level data from 1998 to 2004 were 

averaged together and extrapolated down to the county level based on apportioned to 

county level crop acreage from the 2002 USDA Agriculture of Census (AgCensus) data. 

In general, this information suggests that, in the three county area, approximately 20,000 

lbs of atrazine were used on corn, sorghum, wheat, cotton, and pecans in descending 

order of total pounds applied. 

Subsequent information based on land cover data (City of Austin, 2003a and b; USGS, 

2003) and discussions with local experts (Davis, 2006; Garcia, 2006; Perez, 2006; see 

Appendix C for more detail) indicates that most of the agricultural commodities listed 

above are actually grown to the east of the action area and thus are not included in this 

assessment. The land cover analysis indicates that, of possible agricultural uses for 

atrazine, only the fallow/idle land use is likely to be present in the action area. Land 

cover data also suggest that many of the currently registered non-agricultural atrazine 

uses could not be excluded from the assessment (see Appendix C). However, the nonagricultural forestry use of atrazine on conifers was not evaluated as part of this 

16
 
assessment because forest land cover data from the U.S. Geological Survey and the U.S. 

Forest Service indicate that pine plantations are not present within the action area for the 

Barton Springs salamander (http://nationalatlas.gov/atlasftp.html). The usage analysis 

suggests that atrazine may be used on outdoor ornamental nurseries, although subsequent 

information (Shay, 2006, personal communication; DeLong-Amaya, 2006, personal 

communication; City of Austin, 2003a and b) suggests these uses are very limited in 

nature and are not assessed (see Appendix C for more detail). Based on this analysis, a 

suite of scenarios was developed, including a single agricultural scenario (fallow/idle 

land) and four non-agricultural scenarios (residential, impervious, rights-of-way, and turf) 

using local land cover, soils, and agronomic and climatic data specific to Travis and Hays 

counties in Texas. 

Application rates, number of applications, and application intervals were also estimated 

(Zinn and Jones, 2006) for the three-county area. The minimum and maximum annual 

application rates for atrazine were obtained from EPA data sources. Application rates are 

provided at the state level for only crops grown in Blanco, Hays, and Travis counties on 

which atrazine is registered. The minimum application rate was reported as the minimum 

rate range. The 90th percentile application rate was reported as the highest application 

rate at which at least 90% of the averaged total area is treated. Therefore, at least 90% of 

the area is treated at this rate or less. 

The only typical information available for a use site included in this assessment is for 

fallow land (Kaul et al., 2006). This was reported as meadow use; however for this 

analysis, it is used as a surrogate for atrazine use on fallow land under the CRP. 

Application rates are in units of pounds per acre. The minimum reported application 

rates for fallow ranged between 0.25 lbs/acre and 0.5 lbs per acre. The typical rate was 

reported as 0.8 lbs/acre for fallow, while the 90th percentile application rate for fallow 

was 2 lbs/acre. For fallow uses, the typical number of applications was 1. Information 

on typical intervals for fallow was not available. Overall, atrazine is applied as a preplant or pre-emergent herbicide to most sites in this part of Texas in late winter to mid 

spring. No information was available for other agricultural crops and no data were 

available for non-agricultural uses. 

2.5 Assessed Species 

A brief introduction to the Barton Springs salamander, including a summary of habitat, 

diet, and reproduction data relevant to this endangered species risk assessment is 

provided below. Further information on the status and life history of the Barton Springs 

salamander is provided in Appendix D. 

The Barton Springs salamander, shown in Figure D.1 of Appendix D, is aquatic 

throughout its entire life cycle. As members of the Plethodontidae Family (lungless 

salamanders), they retain their gills, and become sexually mature and eventually 

reproduce in freshwater aquatic ecosystems. The available information indicates that the 

Barton Springs salamander is restricted to the immediate vicinity of the four spring 

outlets that make up the Barton Springs complex (Figure 2.2), located in Zilker Park near 

17
 
downtown Austin, Texas. As such, this species has one of the smallest ranges of any 

vertebrate species in North America (Chippindale, 1993). The Barton Springs segment of 

the Edwards Aquifer and its contributing zone supply all of the water in the springs that 

make up the Barton Springs complex. Flows of clean spring water are essential to 

maintaining well-oxygenated water necessary for salamander respiration and survival. 

The subterranean component of the Barton Spring salamander’s habitat may provide a 

location for reproduction (USFWS, 2005); however, little is known about the 

reproductive biology of the Barton Springs salamander in the wild. It appears that 

salamanders can reproduce year-round, based on observations of gravid females, eggs, 

and larvae throughout the year in Barton Springs (USFWS, 2005). 

Based on survey results, Barton Springs salamanders appear to prefer areas near the 

spring outflows, with clean, loose substrate for cover, but may also be found in the 

aquatic plants, such as moss. In addition to providing cover, moss and other aquatic 

plants harbor a variety and abundance of the freshwater invertebrates that salamanders 

eat. 

18
 
Figure 2.2. Barton Springs Complex (from Hauwert et al., 2005) 

19
 
2.6 Action Area 

It is recognized that the overall action area for the national registration of atrazine uses is 

likely to encompass considerable portions of the United States based on the large array of 

both agricultural and non-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 Barton Springs salamander as they occur within hydrogeologic 

framework of Barton Springs. Deriving the geographical extent of this portion of the 

action area is the product of consideration of the types of effects atrazine may be 

expected to have on the environment, the exposure levels to atrazine that are associated 

with those effects, and the best available information concerning the use of atrazine and 

its fate and transport within Barton Springs. 

Unlike exposure pathways for most aquatic organisms, where stressors are transported 

via surface water to the receptor within a defined watershed, the Barton Springs 

salamander resides in a unique environment in which the source of the water, hence the 

stressor, reaches the salamander via subsurface flow. Thus, the fate and transport of 

atrazine is an important factor in defining the action area for the Barton Springs 

salamander. The fate profile (see Section 2.4.1) describes why runoff from treated fields, 

transported through the fractured limestone of the Edwards Aquifer, is considered the 

principal route of exposure for the salamander. Thus, the action area for this assessment 

is defined by those areas within the hydrogeologic “watershed” that drain to the springs. 

In this case, the area draining to the springs is defined by the subsurface geologic 

framework as opposed to surface hydrology. Figure 2.3 depicts the extent of the action 

area based on this hydrogeologic framework. More detail on the definition of the action 

area follows. 

The Barton Springs salamander is known to inhabit only 4 springs (Main Barton Springs, 

Eliza Springs, Old Mill Springs, and Upper Barton Springs; see Figure 2.2), located in the 

Barton Springs Segment of the Edwards Aquifer (BSSEA), and associated subterranean 

areas in the aquifer itself (USFWS, 2005). Barton Springs, located in Zilker Park near 

downtown Austin, Texas is an aquifer-fed system consisting of four hydrologically 

connected springs: (1) Main Springs (also known as Parthenia Springs or Barton Springs 

Pool); (2) Eliza Springs (also known as the Elks Pit); (3) Old Mill Springs (also known as 

Sunken Garden or Walsh Springs); and (4) Upper Barton Springs (Pipkin and Frech, 

1993). Collective flow from this group of springs represents the fourth largest spring 

system in Texas (Brune, 1981). The springs themselves are fed by the BSSEA, and thus 

groundwater input is the primary determinant of water quality for the salamander. Main 

Springs supply the water for Barton Springs Pool, and during high groundwater flow 

conditions, the surface water flow from Barton Creek may enter the pool if it overtops the 

dam at the upper end of the pool. Thus, any pesticide used in the land areas contributing 

to the groundwater in the Barton Springs segment of the aquifer or the surface water in 

Barton Creek could potentially be transported to these areas. 

Flow to the Barton Springs is controlled by the geology and hydrogeology of the BSSEA. 

Numerous geological and groundwater studies (Slade et al., 1986, Hauwert et al., 2004) 

20
 
have been conducted that define the extent of the area contributing to the Barton Springs. 

The BSSEA represents an approximately 150 square mile portion of the Edwards Aquifer 

system in central Texas. Within the BSSEA, both surface water and groundwater flow 

are controlled by the subsurface geology principally by the fracture nature of limestone 

within portion of the BSSEA. This is particularly relevant for Barton Springs because 

surface water flow from Barton Creek into the pool system is diverted via a bypass 

channel upstream from the main pool to limit the input of surface water from Barton 

Creek. Thus, the dominant source of water to the pool system is via subsurface flow. 

Subsurface flow in the BSSEA as it relates to Barton Springs is well defined and includes 

the Barton Creek watershed upstream of the springs accounting for potential surface 

water inputs into Barton creek. The BSSEA is characterized as a karst system, which 

permits relatively rapid transit of groundwater, with velocities along the dominant flow 

path of 1-5 miles/day, depending on groundwater flow conditions (USFWS, 2005) 

particularly within the fracture portions. Based on dye tracer studies, pesticides applied 

within the recharge and contributing zones could potentially be present in the water of the 

springs on a time scale of days to weeks (Hauwert et al., 2004). 

Four hydrogeologic zones characterize the BSSEA. These are, from west to east, the 

Contributing Zone, the Recharge Zone, the Transition Zone, and the Artesian Zone. Of 

these zones, the Contributing and Recharge Zones have the greatest and most direct 

influence on Barton Springs. There is evidence that the Transition Zone has some limited 

input into the Barton Springs, while the Artesian Zone contributes no subsurface flow to 

the springs (Slade et al., 1985, Hauwert et al., 2004). A more detailed description of the 

geology and hydrogeology of the BSSEA is provided in Section 3.2.2. 

In addition, an evaluation of usage information was completed to determine whether any 

or all of the area defined by the BSSEA should be included in the action area. Current 

labels and local use information were reviewed to determine which atrazine uses could 

possibly be present within the defined area. These data suggest that limited agricultural 

uses are present within the defined area and that non-agricultural uses cannot be 

precluded. Finally, local land cover data (City of Austin, 2003a and b; USGS, 2003) was 

analyzed and interviews with the local agricultural sector (Davis, 2006; Garcia, 2006; 

Perez, 2006; see Appendix C for more detail) were conducted to refine the 

characterization of potential atrazine use in the areas defined by Hays, Travis, and Blanco 

counties. The overall conclusion of this analysis was that while certain agricultural uses 

could be excluded, and some non-agricultural uses of atrazine were unlikely, no areas 

could be excluded from the final action area based on usage and land cover data. 

Finally, the environmental fate properties of atrazine were evaluated to determine which 

routes of transport are likely to have an impact on Barton Springs. Review of the 

environmental fate data as well as physico-chemical properties suggests that transport via 

overland and subsurface flow are likely to be dominant routes. Spray drift and/or longrange atmospheric transport of pesticides could also potentially contribute to 

concentrations in the aquatic habitat used by the salamander. Given the physicochemical profile for atrazine and the fact that atrazine has been detected in both air and 

21
 
rainfall samples, the potential for long range transport from outside the area defined by 

the BSSEA cannot be precluded, but is not expected to approach concentrations predicted 

by modeling (see Section 3.2.5). However, because areas where the atmospheric 

component of atrazine loading is considered significant are typically high use areas 

(Midwest corn belt), and the area surrounding Barton Springs is not a high use atrazine 

area, the expected loadings from atmospheric transport of atrazine are not expected to 

approach concentrations predicted by modeling (see Section 3.2.5). 

Atrazine has been documented to be transported away from the site of application by both 

spray drift and volatilization. The Agency typically addresses spray drift as a localized 

route of transport off of the application site in exposure assessments. In the case of the 

Barton Springs salamander assessment, spray drift is not considered to be a significant 

route of exposure because the source area for atrazine reaching the springs is generally 

removed from the spring system where the salamander resides, and the atrazine 

exposures that reach the springs do so via subsurface flow. Therefore, there is no direct 

pathway between the application site and receptor for drift to occur (no applications of 

atrazine are reportedly made within the immediate vicinity of the springs). The Agency 

does not currently have quantitative models to address the long range transport of 

pesticides from application sites. The environmental fate profile of atrazine, coupled 

with the available monitoring data, suggest that long range transport of volatilized 

atrazine is a possible route of exposure to non-target organisms. The full extent of the 

action area could hypothetically be influenced by this route of exposure. However, given 

the amount of direct use of atrazine within the immediate area surrounding the species 

(Kaul, et al., 2006), the magnitude of documented exposures in rain (Majewski et al., 

2000; Majewski and Capel, 1995; Capel et al., 1994) at or below available surface water 

and groundwater monitoring data (as well as modeled estimates for surface water), the 

extent of the action area is defined by the transport processes of runoff and subsequent 

overland and subsurface flow for the purposes of this assessment. 

Based on this analysis, the action area for atrazine as it relates to the Barton Springs 

salamander is defined by the contributing, recharge, and transition zones within the 

BSSEA. Figure 2.3 presents the action area graphically. 

22
 
Figure 2.3. Barton Springs Salamander Action Area 

2.7 Assessment Endpoints and Measures of Ecological Effect 

Assessment endpoints are defined as “explicit expressions of the actual environmental 

value that is to be protected.”5 Selection of the assessment endpoints is based on valued 

entities (i.e., Barton Springs salamanders), the ecosystems potentially at risk (i.e, Barton 

Springs), the migration pathways of atrazine (i.e., runoff), and the routes by which 

ecological receptors are exposed to atrazine-related contamination (i.e., direct contact). 

Assessment endpoints for the Barton Springs salamander include direct toxic effects on 

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

such as reduction of the prey base and/or modification of its habitat. Each assessment 

endpoint requires one or more “measures of ecological effect,” which are defined as 

changes in the attributes of an assessment endpoint itself or changes in a surrogate entity 

or attribute in response to exposure to a pesticide. Specific measures of ecological effect 

are evaluated based on acute and chronic toxicity information from registrant-submitted 

5

 From U.S. EPA (1992). Framework for Ecological Risk Assessment. EPA/630/R-92/001. 

23
 
guideline tests that are performed on a limited number of organisms. Given that the 

results of the required registrant-submitted amphibian toxicity tests are not available for 

this assessment, it is assumed that fish and aquatic-phase amphibian toxicities are similar. 

Birds are generally considered as surrogates for terrestrial-phase amphibians; however, 

Barton Springs salamanders are neotenic (i.e., retain gills throughout their lives) and are 

aquatic-phase amphibians. Therefore, fish are used as a surrogate for 

amphibian/salamanders, in accordance with guidance specified in the Agency’s Overview 

Document (U.S. EPA, 2004). Additional ecological effects data from the open literature, 

including effects data on salamanders and aquatic freshwater microcosm and mesocosm 

data were also considered. 

Measures of effect from microcosm and mesocosm studies provide an expanded view of 

potential indirect effects of atrazine on aquatic organisms, their populations and 

communities in the laboratory, in simulated field situations, and in actual field situations. 

With respect to the microcosm and mesocosm data, threshold concentrations on aquatic 

community effects were determined from complex time variable atrazine exposure 

profiles (chemographs) within these experimental studies. Methods were developed to 

estimate ecological community responses for any possible atrazine chemograph based on 

the relationships in the micro- and mescocosm study results. This information was used 

to determine whether a certain exposure profile within a particular use site and/or action 

area may have exceeded a level of concern. Ecological modeling with the 

Comprehensive Aquatic Systems Model (CASM) (Bartell et al. 2000, Bartell et al. 1999, 

and DeAngelis et al., 1989) was used to calibrate the measured atrazine chemographs in 

order to estimate direct and indirect effects of atrazine and to project potential changes in 

aquatic community structure and function. 

A complete discussion of all the toxicity data available for this risk assessment, including 

use of the CASM model and associated aquatic community-level threshold 

concentrations, and the resulting measures of ecological effect selected for each 

taxonomic group of concern are included in Section 4 of this document. A summary of 

the assessment endpoints and measures of ecological effect selected to characterize 

potential Barton Springs salamander risks associated with exposure to atrazine is 

provided in Table 2.1. 

24
 
Table 2.1. Summary of Assessment Endpoints and Measures of Ecological Effect 

Assessment Endpoint Measures of Ecological Effect 

1. Survival, growth, and reproduction of Barton 

Springs salamander individuals via direct effects 

1a. Rainbow trout acute LC50 

1b. Brook trout chronic NOAEC 

1c. Open literature lab and field NOAEC data for 

salamanders 

2. Survival, growth, and reproduction of Barton 

Springs salamander individuals via indirect effects 

on prey (i.e., freshwater invertebrates) 

2a. Midge acute EC50 

2b. Scud chronic NOAEC 

2c. Acute EC/LC50 data for freshwater invertebrates 

that are potential food items for the Barton Spring 

salamander 

3. Survival, growth, and reproduction of Barton 

Springs salamander individuals via indirect effects 

on habitat and/or primary productivity (i.e., aquatic 

plant community) 

3a. Vascular plant (duckweed) acute EC50 

3b. Non-vascular plant (freshwater algae) acute 

EC50 

3c. Microcosm/mesocosm threshold concentrations 

showing aquatic primary productivity communitylevel effects 

2.8 Conceptual Model 

2.8.1 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 atrazine to the environment. 

Based on the results of the 2003 atrazine IRED (U.S. EPA, 2003a), the following risk 

hypotheses are presumed for this endangered species assessment: 

• Atrazine in groundwater, surface water, and/or runoff from treated areas may 

directly affect Barton Springs salamanders by causing mortality or adversely affecting 

growth or fecundity; 

• Atrazine in groundwater, surface water, and/or runoff from treated areas may 

indirectly affect Barton Springs salamanders by reducing or changing the composition of 

prey populations; and 

• Atrazine in groundwater, surface water, and/or runoff from treated areas may 

indirectly affect Barton Springs salamanders by reducing or changing the composition of 

the plant community in the springs, thus affecting primary productivity and/or cover. 

2.8.2 Diagram 

The conceptual model is a graphic representation of the structure of the risk assessment. 

It specifies the stressor, release mechanisms, abiotic receiving media, biological receptor 

types, and effects endpoints of potential concern. The conceptual model for the potential 

effects of atrazine on the Barton Springs salamander is shown in Figure 2.4. Exposure 

routes shown in dashed lines are not quantitatively considered because these exposures 

are expected to be sufficiently low as not to cause direct or indirect effects to the Barton 

Springs salamander. 

25
 
Atrazine applied to agricultural 

fields, residential lawns, golf 

courses, and rights-of-way 

Springs 

Aquatic plants 

Aquatic invertebrates 

Aquatic vertebrates 

Individual 

salamander 

Reduced survival 

Reduced growth 

Reduced reproduction 

Food chain 

Decrease in abundance 

Shift in prey base 

Habitat 

integrity 

Reduced cover 

Stressor 

Receptor 

Attribute 

Change 

Source Runoff Spray drift 	 Groundwater/ Atmospheric 

Surface Water transport 

Figure 2.4. Conceptual Model for Barton Springs Salamander 

The conceptual model provides an overview of the expected exposure routes for Barton 

Springs salamanders within the atrazine action area previously described in Section 2.6. 

In addition to freshwater aquatic vertebrates including Barton Springs salamanders, other 

aquatic receptors that may be potentially exposed to atrazine include freshwater 

invertebrates and aquatic plants. For freshwater vertebrate and invertebrate species, the 

major routes of exposure are considered to be via the respiratory surface (gills) or the 

integument. Direct uptake and adsorption are the major routes of exposure for aquatic 

plants. Direct effects to freshwater invertebrates and aquatic plants resulting from 

exposure to atrazine may indirectly affect the Barton Springs salamander via reduction in 

food and habitat availability. The available data indicate that atrazine is not likely to 

bioconcentrate in aquatic food items, with fish bioconcentration factors (BCFs) ranging 

from 2 to 8.5 (U.S. EPA, 2003c). Therefore, bioconcentration of atrazine in salamanders 

via the diet was not considered as a significant route of exposure. 

Individual Barton Springs salamanders with the greatest potential to experience direct 

adverse effects from atrazine use are those that occur in surface water and/or groundwater 

with the highest concentrations of atrazine. Water passing into, and through Barton 

Springs comes from groundwater in the Barton Springs Segment of the Edwards Aquifer. 

26
 
When Barton Creek floods, some of the surface flow enters Barton Springs Pool; 

however, during normal flow, the water from Barton Creek enters a bypass channel 

upstream from the main pool and does not enter the pool itself. 

Based on historical records of pesticide use in Zilker Park and the area surrounding 

Barton Springs dating to 1997, atrazine has not been used in this area (personal 

communication with Elizabeth McVeety, pesticide applicator at Zilker Park, April 21, 

2006). According to the City of Austin Parks and Recreation Department (PARD) 

Integrated Pest Management Plan (IPM) (2005), the main concern within the Park is 

control of fire ants, and spot treatment of Round-up (glyphosate) is the only herbicide 

specified for control of Johnson grass and poison ivy. Although the IPM does not 

specifically address atrazine use within Zilker Park, it is currently being revised to 

specifically restrict atrazine use within the Park in the future (personal communication 

with Elizabeth McVeety, pesticide applicator at Zilker Park, July 24, 2006). Given that 

atrazine is not used within the Barton Springs area, it is unlikely that atrazine in runoff 

would indirectly affect Barton Springs salamanders by reducing or changing the 

composition of riparian zone vegetation and increasing sedimentation of the springs in 

the main pool. Increased sedimentation in the main pool is more likely to result from 

high groundwater flow conditions, when the surface water flow from Barton Creek 

overtops the dam at the upper end of the pool. Therefore, potential indirect effects to 

Barton Springs salamanders via reduction or change in the riparian zone vegetation (i.e., 

terrestrial plants) and resulting sedimentation are not considered a significant route of 

exposure and are not further addressed in this risk assessment. 

The source and mechanism of release of atrazine into surface and groundwater are 

ground and aerial application via foliar spray and coated fertilizer granules to agricultural 

(i.e., fallow/idle land) and non-agricultural sites (i.e., golf courses, residential lawns, 

rights-of-way, etc). Surface water runoff from the areas of atrazine application is 

assumed to follow topography, resulting in direct runoff to Barton Creek and/or runoff to 

the recharge area of the Barton Springs Segment of the Edwards Aquifer, where it 

becomes groundwater that discharges to the surface water of Barton Springs. Additional 

release mechanisms include spray drift and atmospheric transport via volatilization, 

which may potentially transport site-related contaminants to the surrounding air. 

However, spray drift is not considered to be a significant route of exposure because the 

source area for atrazine is generally removed from the spring system where the 

salamander resides, and the atrazine exposures that reach the springs do so via subsurface 

flow. Atmospheric transport is not considered as a significant route of exposure for this 

assessment because the magnitude of documented exposures in rainfall are at or below 

available surface water and monitoring data, as well as modeled estimates of exposure 

(Majewski et al., 2000; Majewski and Capel, 1995; Capel et al., 1994). 

27
 
3. Exposure Assessment 

3.1 Label Application Rates and Intervals 

Atrazine labels may be categorized into two types: labels for manufacturing uses 

(including technical grade atrazine and its formulated products) and end-use products. 

While technical products, which contain atrazine of high purity, are not used directly 

in the environment, they are used to make formulated products, which can be applied 

in specific areas to control weeds. The formulated product labels legally limit 

atrazine’s potential use to only those sites that are specified on the labels. 

In the January and October 2003 IREDs, EPA stipulated numerous changes to the use of 

atrazine including label restrictions and other mitigation measures designed to reduce risk 

to human health and the environment (U.S. EPA, 2003a and b). Specifically pertinent to 

this assessment, the Agency entered into a Memorandum of Agreement (MOA) with the 

atrazine registrants. In the MOA, the Agency stipulated that certain label changes must 

be implemented on all manufacturing-use product labels for atrazine and on all end-use 

product labels for atrazine prior to the 2005 growing season including cancellation of 

certain uses, reduction in application rates, and requirements for harmonization across 

labels including setbacks from waterways. Specifically, the label changes stipulate no 

use of atrazine within 50 feet of sinkholes, within 66 feet of intermittent and perennial 

streams, and within 200 feet of lakes and reservoirs. The modeling discussed below 

predicts edge of field concentrations and thus spray drift is not quantitatively included in 

the predicted exposures. It is expected that a setback distance will result in a reduction in 

loading due to runoff across the setback zone; however, current models do not address 

this reduction quantitatively. Therefore, these restrictions are not quantitatively evaluated 

in this assessment. A qualitative discussion of the potential impact of these setbacks on 

estimated environmental concentrations of atrazine for the Barton Springs salamander is 

discussed further in Section 3.2.3. Table 3.1 provides a summary of label application 

rates for atrazine uses evaluated in this assessment. 

Currently registered non-agricultural uses of atrazine within the Barton Springs action 

area include residential areas such as playgrounds and home lawns, turf (golf courses 

and recreational fields), and rights-of-way. Agricultural uses within the Barton 

Springs action area include fallow/idle land6

 including Conservation Reserve 

Program (CRP) lands in Texas. According to use data gathered by EPA, there is no 

agricultural use of atrazine on corn or sorghum within the Barton Springs action area, 

although corn and sorghum represent the greatest use nationally. 

Atrazine is formulated as liquid, wettable powder, dry flowable, and granular 

formulations. Application equipment for the agricultural uses includes ground 

6

 Fallow or ideland is defined by the Agency as arable land not under rotation that is set at rest for a period 

of time ranging from one to five years before it is cultivated again, or land usually under permanent crops, 

meadows or pastures, which is not being used for that purpose for a period of at least one year. Arable land, 

which is normally used for the cultivation of temporary crops, but which is temporarily used for grazing, is 

also included. 

28
 
Table 3.1. Label Application Information for the Barton Springs Salamander 

Endangered Species Assessment1 

Scenario 

Maximum 

Application 

Rate 

(lbs/acre) 

Maximum 

Number of 

Applications 

Date of 

First 

Application 

Formulation 

Method of 

Application 

Interval 

Between 

Applications 

Residential 2.0 2 April 1 Granular Ground 30 days 

Residential 1.0 2 April 1 Liquid Ground 30 days 

Rights-ofWay 1.0 1 June 1 Liquid Ground NA

Fallow/ 

Idle land 2.25 1 November 1 Liquid Ground and 

Aerial NA

Turf 2.0 2 April 1 Granular Ground 30 days 

Turf 1.0 2 April 1 Liquid Ground 30 days 

application (the most common application method), aerial application, band 

treatment, incorporated treatment, various sprayers (low-volume, hand held, 

directed), and spreaders for granular applications. Risks from ground boom and 

aerial applications are considered in this assessment because they are expected to 

result in the highest off-target levels of atrazine 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. 

1 – Based on 2003 IRED and Label Change Summary Table memorandum dated June 12, 2006 (U.S. EPA, 

2006b). 

3.2 Aquatic Exposure Assessment 

The exposure assessment represents an application of the standard approach outlined in 

the Overview Document (U.S. EPA, 2004) for the hydrogeologic conditions of the 

springs. The Agency’s PRZM model was used to provide edge of field estimates of 

exposure, which are assumed to be the concentrations of atrazine transported with runoff 

water directly to Barton Springs via subsurface flow through the fractured limestone of 

the Edwards Aquifer. Actual conditions are likely to result in lower atrazine 

concentrations through dilution, mixing, retention, and degradation. Available 

monitoring data from the spring systems were also evaluated and compared with model 

estimates. While of high quality and targeted to the Barton Springs system, the 

monitoring data are not considered to be robust in terms of capturing peak atrazine 

concentrations (i.e. the sample frequency is likely to miss the peak concentration). 

29
 
New regionally-specific PRZM scenarios representing both agricultural and nonagricultural use sites were developed following standard methodology (U.S. EPA, 2005) 

to capture the upper bounds of exposure. Residential uses were modeled using pervious 

(1/4 acre lot) and impervious surface scenarios and weighting the output based on local 

data on the percentage of impervious surfaces in the action area region. Durations of 

exposure were used to match available ecotoxicity thresholds. The highest overall 

exposures were predicted to occur from the residential uses of atrazine that are in closest 

proximity to the spring system. In general, the exposure assessment yields modeled peak 

exposure estimates that are two to ten times higher than those seen in monitoring data, 

while the annual average concentrations are consistent with those seen in monitoring. 

Intermediate duration exposures (14-day, 21-day, 30-day, 60-day, and 90-day averages) 

cannot be estimated from the monitoring data due to insufficient sample frequency. 

3.2.1 Background 

The Barton Springs salamander resides in a geographically limited area defined by a set 

of spring fed pools in the outskirts of the city of Austin. These pools represent the total 

aerial extent of the salamander, as defined in Sections 2.5 and D.4 of Appendix D. The 

pools are a unique system in that they are fed via two sources of water. Surface water has 

historically reached the pool system via overland flow through Barton Creek. However, 

water from Barton Creek is currently diverted near the inflow to the pool system and 

provides only limited input to the pool system during high flow (flood) events. The bulk 

of the water reaching the pool system is fed via a series of springs. The springs consist of 

the Main Spring, Upper Spring, Old Mill Spring, and Eliza Spring with approximately 

80% of the flow originating from the Main Spring. All of the springs are fed via 

subsurface flow originating in fractured limestone aquifer of the Edwards Aquifer, which 

trends south-southwest away from the pool system. Groundwater from the fractured 

limestone (karst) is derived from perennial groundwater flow and via recharge that 

originates from both surface streams and infiltration of rainfall. Therefore, the basic 

conceptual model of exposure for this assessment focuses on the subsurface pathway 

delivering groundwater to the pools via the karst system. 

The hydrogeology of the Barton Springs Segment of the Edwards Aquifer (BSSEA) 

defines the action area (see Section 2.6) of atrazine use for the Barton Springs 

salamander. Several hydrogeologic zones define the BSSEA. From west to east, these 

are the Contributing Zone, the Recharge Zone, the Transition Zone, and the Artesian 

Zone. The relevance and route of exposure relative to the Barton Springs system is 

different for each zone and is defined by the geology of the system. Given the basic 

geology and hydrogeology of these zones within the BSSEA, the Contributing Zone and 

the Recharge Zone (and to a lesser extent the Transition Zone) are likely to contribute 

directly to the Barton Springs pool systems. Therefore, land use patterns within these 

zones were considered to determine the potential for atrazine exposure to the Barton 

Springs salamander. Figure 3.1 shows the extent of the BSSEA. 

30
 
Figure 3.1. Barton Springs Segment of the Edwards Aquifer with HydroZones 

31
 
Groundwater flow within the Recharge Zone is dominated by subsurface flow via 

fractured limestone. Numerous studies have been conducted which document the nature 

of the subsurface geology and the nature and extent of groundwater flow via these 

fractures (Slade et al., 1986; Hauwert et al., 2004; Mahler, 2005a). Flow within these 

fractures has been documented to travel from the point of origin to outflow at the springs 

within hours to days of individual precipitation events, suggesting that atrazine reaching 

the Recharge Zone is likely to have the most immediate and significant impact on Barton 

Springs. 

The Contributing Zone lies due west of the Recharge Zone. In this zone, runoff from 

sites treated with atrazine is transported via overland flow to surface water streams and 

ponds. Atrazine may then be transported via surface water streams to the Recharge Zone, 

where it is available for infiltration into the network of karst fractures that ultimately feed 

the Barton Springs system. Unlike stressors originating within the Recharge Zone, some 

dilution and degradation is expected during this transport process. “Losing” streams 

(defined as a stream where flow is lost to groundwater recharge) within the Recharge 

Zone have been reported to provide as much as 85% (Slade et al., 1986) of the annual 

recharge to groundwater. Historically, surface water flow through Barton Creek has 

contributed to the loading of water, sediment, and contaminants to the Barton Springs 

pools. However, in the current configuration of Barton Creek relative to the Barton 

Springs pools, the creek has been artificially routed past the pools to ensure that the 

springs are providing the bulk of the recharge to the pools. Occasionally, large 

precipitation events may result in a bypass of this configuration overflowing of the pool 

system. In general, the pools are typically fed by groundwater flow through the karst 

fractures of the Recharge Zone that can receive stressors from both direct infiltration and 

“loss” from surface water streams. 

The Barton Springs system consists of a series of connected pools located within the city 

limits of Austin, Texas. The Barton Springs salamander has been found within the 

fractures (springs) feeding the pool system and within the pools themselves. Each 

receptor location is somewhat unique from the other in how exposures are expected to 

interact with the salamander. 

Exposures to stressors for salamanders residing within the fracture system are due to a 

combination of base flow with occasional runoff derived from pulses of increased flow. 

With the increased flow comes the potential for an increase in the magnitude of exposure 

that is of short duration depending on the climatic event. Base flow within the spring 

systems is fed by loss of volume from surface streams as they traverse the Recharge Zone 

of the BSSEA and from groundwater movement out of the Contributing Zone into the 

fractured limestone of the Recharge Zone. The short term pulsed increases in runoffderived water through the springs are the result of increased loss through surface streams 

originating in the Contributing Zone and direct infiltration of precipitation and runoff 

from surface areas of the Recharge Zone. Thus, salamanders residing within the fracture 

system of the springs are likely to be exposed to longer-term base flow concentrations of 

atrazine with occasional shorter duration pulses of higher concentrations correlated with 

precipitation derived runoff events transported through the fractures. 

32
 
Figure 3.2. Hydrogeologic Cross Section of the Barton Springs Segment of the 

Edwards Aquifer Showing Dominant Flow Pathways Within Each Hydrozone 

(Taken from Mahler, 2005a) 

Salamanders have also been found to reside within the pools themselves. In general, the 

organisms residing in the pools will be exposed to the same sources of exposure. 

However, it is expected that the magnitude and duration of exposure will be somewhat 

different given the tendency of water to move through the pools (except in the most 

extreme climatic events) more slowly. This suggests that exposures in the pools will be 

generally lower in magnitude than in the springs, but will also tend to have a longer 

duration of exposure than in the springs. 

Figures 3.2 and 3.3 present the conceptual models of both of these potential exposure 

pathways. More details on the geology and hydrogeology may be found in the following 

section. Finally, a more complete description of the Barton Springs pool system in which 

the salamander resides is provided in Section D.4 of Appendix D. 

33 
Contributing Zone 

Recharge 

Zone 

Fracture Flow 

Barton Creek flow Barton Spring 

Surface Flow 

Infiltration 

Figure 3.3. Conceptual Model of Surface and Subsurface Flow Within the Barton 
 

Springs Segment of the Edwards Aquifer Relative to the Barton Springs 
 

Salamander
 

3.2.2 Geology/Hydrogeology 

The Barton Springs pool system lies at the extreme northern end of the BSSEA, which is 

a portion of a larger fractured limestone aquifer system known as the Edwards Aquifer. 

The Edwards Aquifer and BSSEA are major sources of groundwater used for drinking 

water and represent a critical source of water necessary to replenish surface water 

resources for both recreational and ecological uses throughout the eastern half of Texas. 

The Edwards Aquifer is a karst system of limestone and dolomite of Cretaceous age 

(Slade et al., 1986). The aquifer covers roughly 6,000 square kilometers and stretches 

from north of Austin to an area southwest of San Antonio. In general, the physical trend 

of the Edwards Aquifer (and Barton Springs Segment) is south to north, and the 

carbonate rocks within the aquifer dip to the east except where broken by fractures within 

the Recharge Zone (Slade et al., 1986). The thickness of the aquifer generally increases 

from north to south and is typically 400 to 450 feet thick (Slade et al., 1986). It is a 

principal source of groundwater for drinking water in Texas, and where it discharges to 

34
 
the surface, it is critical for providing freshwater for both recreational and ecological 

needs. 

The Barton Springs Segment extends from the Colorado River south roughly 20 miles 

into Hays County and covers 391 square kilometers. The Barton Springs Segment is 

separated from the rest of the Edwards Aquifer by a hydrogeologic divide with 

groundwater north of the divide flowing north-northeast towards the Colorado River and 

south of the divide flowing south-southwest. In general, the BSSEA discharges at a 

number of springs along the Colorado River and Barton Creek. Flow through the BSSEA 

is typically around 35 cubic feet per second (cfs) during low flow periods, but can reach 

above 75 cfs during high flow conditions, while the average flow is reported to range 

between 53 cfs (Hauwert et al., 2004) and 56 cfs (Mahler, 2005a). Slade et al. (1986) 

also estimated that up to 85% of the recharge reaching the BSSEA was derived from 

infiltration from the main creeks crossing the Recharge Zone. The remaining infiltration 

was derived from water coming from minor tributaries and from upland areas in the 

Contributing Zone and from direct infiltration of precipitation. 

Hauwert et al. (2004) conducted dye trace studies of the flow systems in the BSSEA 

between 1996 and 2002. In these studies, the authors attempted to discern specific flow 

patterns within the Recharge Zone using dye tracing, mapping of the potentiometric table, 

water chemistry, local knowledge of geology, and cave mapping. Non-toxic dye 

injection into caves, sinkholes, and wells was used to define the route of groundwater 

flow, estimate flow velocities, and approximate travel times. The important finding of 

this study relative to this assessment is that travel times within the Recharge Zone range 

from hours up to one week in close proximity to the springs (defined by Travis County), 

while farther south and west, travel times can increase to approximately 4 weeks. Flow 

through fractures also may occurr within the Transitional Zone that separates the 

Recharge Zone from the eastern artesian portion of the BSSEA. Figure 3.4 presents a 

summary of the flow paths defined by this study (Hauwert et al., 2004). 

35
 
36

Figure 3.4. Flow paths within Recharge Zone of the Barton Springs Segment of the 

Edwards Aquifer (Taken from Mahler, 2005a; originally published in Hauwert et 

al., 2004)
3.2.3 Conceptual Model of Exposure 

Given the understanding of the geology/hydrogeology described above, a combination of 

modeling and monitoring data is needed to assess the potential exposures from atrazine to 

the Barton Springs salamander. Routes of exposure are dependent on the location of 

registered use sites for atrazine within the action area (defined in Section 2.6 as the 

Contributing, Recharge, and Transition Zones), the location of those uses, and locations 

within the pool system (fractures versus pools) where the salamander resides. For 

instance, uses that are predominantly within the Recharge Zone of the BSSEA are likely 

to reach the springs via direct transport through the fractures within the karst zone. 

Atrazine originating from within the Recharge Zone is analogous to “edge of field” 

concentrations because atrazine applied within this area may be transported directly from 

the site of application to the subsurface fractures. Thus, atrazine may move directly from 

the edge of the treated field (or even from within it) into the fracture system. This route 

of exposure is expected to be the most conservative (e.g. represent the highest potential 

exposure). 

The interconnected nature of the subsurface network can have a significant influence on 

mixing, dilution, storage and degradation of flow through karst (Field, 2004). The 

simplest, and for purposes of this assessment, most conservative assumption, is a straight 

conduit from the source to the springs (defined as a Type I karst network in Field, 2004). 

For the BSSEA, it is unclear how much, if any, interconnectedness exists in the Recharge 

Zone. Therefore, a conservative assumption is that the system is represented by a straight 

conduit network of fractures, and atrazine reaching the springs from source areas has 

limited potential for degradation and dilution. The conservativeness of this assumption is 

apparent when considering that source areas further removed from Barton Springs have 

the potential for some dilution and degradation both at the surface and subsurface. 

Atrazine residues derived from application sites that are located predominantly within the 

Contributing Zone are expected to travel via overland flow to surface streams to the 

Recharge Zone where infiltration from “losing” streams is likely to occur. Thus, 

exposures from these sources are expected to be lower relative to the “edge of field” 

exposures that originate in the Recharge Zone. Therefore, the “edge of field” approach is 

likely to over-estimate exposure originating in these areas (i.e., the Contributing Zone). 

Given the limited nature of the available monitoring data both within the spring network 

and in the surrounding groundwater and surface water, an analysis of potential use sites 

within the action area is needed. Available agricultural statistics, land cover data, usage 

information, and soils data were evaluated relative to the hydrogeologic framework 

described above. This information was used to determine whether both agricultural and 

non-agricultural uses sites are present in the Recharge Zone, the Contributing Zone, or 

both. 

37
 
Based on the hydrogeologic configuration, pesticide use is modeled in the Contributing, 

Recharge, and Transition Zones. PRZM is used to model edge-of-field runoff 

concentrations, assuming that the edge of field runoff concentration is transported 

directly to the Barton Springs system via flow through the fracture system. This 

exposure pathway represents a simplification of how the species may be exposed to 

pesticides used in the BSSEA; however, it provides an upper bound of potential 

exposures and is reasonable given that there are no available models for predicting 

pesticide fate and transport in karst systems. This approach is considered to be 

conservative because the conceptual model of transport does not include degradation or 

dilution for use sites in close proximity to the springs. 

Analysis of land cover data and usage information suggests that limited agriculture is 

present in the Contributing and Recharge Zones of the BSSEA. In order to address the 

potential for atrazine exposure from use on these sites, a suite of PRZM modeling 

scenarios was developed for the specific agronomic, soil, and climatic data available for 

the BSSEA. As noted above, the action area for the development of the Barton Springs 

scenarios is comprised of three hydrologic zones of the BSSEA (in order of importance): 

1) the Recharge Zone, which consists of a fractured karstic geology; 2) the Contributing 

Zone, where surface runoff may flow to the Recharge Zone; and 3) the Transition Zone, 

which has a remote potential to contribute to the Recharge Zone 

(http://www.edwardsaquifer.net/intro.html). Although the Transition Zone is considered 

in this assessment, primary emphasis is given to the Recharge Zone with secondary 

emphasis on the Contributing Zone. No scenarios were parameterized based solely on 

the Transition Zone. Spatial data containing the hydrozone boundaries were obtained 

from the Barton Springs/Edwards Aquifer Conservation district 

(ftp://www.bseacd.org/from/HCP Shape Files/). The areas to the east of the Recharge 

Zone are not considered relevant to the assessment because groundwater flow to the 

Barton Springs system comes either directly from transport through the Recharge Zone, 

which occurs generally south to north, or indirectly via the Contributing Zone/Recharge 

Zone interaction, where flow is dominantly west to east. 

As previously discussed in Section 3.1, label changes, including the establishment of 

setback restrictions on application of atrazine around wells and sinkholes, perennial and 

intermittent streams, lakes, and reservoirs, were implemented as part of the IRED/MOA. 

Specifically, the label changes restrict atrazine use within 50 feet of sinkholes, 66 feet of 

intermittent and perennial streams, and 200 feet of lakes and reservoirs. These 

restrictions are not quantitatively evaluated in this assessment. 

As stated previously, this assessment assumes that the estimated environmental 

concentration (EEC) is derived from an edge-of-field exposure; thus, spray drift is not a 

factor in the predicted assessment. However, the assessment also conservatively assumes 

that the edge of field exposure could occur where surface runoff enters the subsurface 

fractures via sinkholes and/or other conduits to the karst fractures adjacent to the site of 

application. The net effect of the 50 foot setback would reduce loadings to the subsurface 

where the setback zone consists of a healthy vegetated zone. Alternatively, where the 

setback zone consists of bare soil, or a poorly maintained vegetative zone, little reduction 

38
 
in runoff would be likely to occur. Current models do not estimate the effect of setbacks 

on load reduction for runoff, although it is documented in the literature that well 

vegetated setbacks can result in a substantial reduction in pesticide load to surface water 

(USDA, NRCS, 2000). Specifically for atrazine, data reported in the USDA study 

indicate that well vegetated setbacks reduce atrazine loading to surface water by as little 

as 11% and as much as 100% of total runoff without a buffer. It is expected that the 

presence of a well-vegetated 50 foot setback between the site of application of atrazine 

and sinkholes could result in loading reductions to the subsurface system. Therefore, the 

aquatic EECs presented in this assessment are likely to over-estimate exposure in areas 

with well-vegetated setbacks. However, given the lack of quantifiable estimates of load 

reduction and available data on the effectiveness of vegetated zones surrounding 

sinkholes, ranges of potential exposures cannot be estimated. The label changes also 

specify setback distances of 66 and 200 feet for atrazine applications surrounding 

intermittent/perennial streams and lakes/reservoirs, respectively. Typically, the influence 

of setback distances on spray drift loading would be evaluated using AgDrift to estimate 

the impact of the setback on the fraction of drift reaching a surface water body (U.S. 

EPA, 2004). However, as previously discussed, spray drift was not considered as a 

significant route of exposure in this assessment. 

Overall, it is expected that well vegetated and maintained setbacks will reduce overall 

loading of atrazine from runoff estimates presented in this assessment. However, these 

reductions cannot be quantified and are unlikely to be uniform across the action area. 

3.2.4 Existing Monitoring Data 

USGS provided monitoring data for surface streams, groundwater wells, and the four 

springs making up the Barton Springs system (Mahler, 2005a). Specifically, the data 

provided long-term trends within all three source types. In addition, recent data from the 

USGS targeted single runoff events within the spring systems that included high 

frequency sampling to match the hydrograph correlated with the several specific runoff 

events. 

Four springs were included in the USGS analysis, including Main Spring, Eliza Spring, 

Upper Spring, and the Old Mill Spring. All four springs represent the main source of 

inflow into the Barton Springs pool system with the Main Spring providing roughly 80% 

of overall flow. Sampling and analysis of these springs indicates that the highest 

detection of atrazine was 3 μg/L in the Upper Spring, however, most detections of 

atrazine were below 1 μg/L. Given the nature of the flow regime within the springs, it is 

unlikely that these sampling events have captured the peak exposures. 

Evaluation of long-term trends in the monitoring data suggests that atrazine 

concentrations have increased since the 1980s; however, given that recent detections 

could be related to improved sampling techniques and analytical methods with lower 

detection limits, these observations may reflect an increase in monitoring intensity rather 

than a trend in atrazine exposures. Long-term trends in the analysis of spring water data 

suggest that atrazine occurrence has been sporadic. Atrazine was not detected in spring 

39
 
Spring 

Range 

of 

Sample 

Dates 

# of 

Samples 

# of 

Detects 

Frequency 

of Detection 

Maximum 

Concentration 

(μg/L) 

Minimum 

Concentration 

(μg/L) 

Average 

Concentration 

(μg/L) 

1978­ Main 78 60 77% 0.555 0.042 0.070 2005 

2001­ Upper 2005 44 44 100% 3.190 0.018 0.164 

Old 2001 12 12 100% 0.063 0.007 0.015 Mill 2005 

2000­ Eliza 15 15 100% 0.112 0.007 0.036 2005 

Several degradates of atrazine were detected in the samples collected from the springs 

including deethyl-atrazine, 2-hydroxyatrazine, chlorodiamino-s-triazine, and 

deisopropylatrazine. All four degradates were detected in spring water at concentrations 

below those found for atrazine. In general, the concentrations of the degradates were 2 to 

3 times less than atrazine during low flow (base flow) periods, while atrazine 

concentrations were 10 to 15 times higher than deethyl-atrazine during high flow (storm 

derived runoff events). 

Analysis of the stream data suggests that trends similar to those seen in the spring data 

occurr in surface streams. Similar to the spring detections, most of the atrazine detections 

in surface streams have occurred since 2000. The highest detection of atrazine in the 

streams is 4.39 μg/L in Slaughter Creek in 2005. Figure 3.5 shows the location of stream 

samples within the BSSEA. 

samples prior to 2000; however, this may be due to changes in detection limits and an 

increased attention in recent years on more frequent sampling and selected sampling tied 

to specific runoff events (as noted above). More recent sampling suggests that longerterm concentrations in the four springs tend to be less than 1 μg/L, with most of the 

longer-term detections closer to 0.1 μg/L. Consideration of only detections yields an 

overall average concentration of 0.45 μg/L (excluding non-detections). Table 3.2 

presents a summary of the site-specific monitoring data. 

Table 3.2. Summary of USGS Monitoring Data from the Four Springs Comprising 

Barton Springs 

­

40
 
Figure 3.5. Location of Surface Water Sites within the Barton Springs Segment 

Statewide surface water data for atrazine collected by the USGS NAWQA Program 

between 1993 and 2003 shows that atrazine was detected in 792 out of 866 samples 

(92% detection frequency). The highest detected concentration of atrazine in all of the 

NAWQA samples was 20 μg/L in 1994 and 1995 from two sites in the Trinity River 

Study Unit in Ellis and Navarro counties (station ID numbers 321313096415201 and 

321017096420099) that are outside of the action area for this assessment. 

Analysis of the well data from the USGS collected between 2000 and 2005 suggests that 

atrazine was detected in groundwater from five locations within the BSSEA. In general, 

with the exception of a single sample analyzed from well # YD-58-34-617 in 2002 at 

0.192 μg/L, all atrazine detections were below 0.1 μg/L. More commonly detected were 

the principal degradates of atrazine including deethyl-atrazine, 2-hydroxyatrazine, 

chlorodiamino-s-triazine, and deisopropylatrazine, none of which are included in this 

assessment (rationale for this exclusion is provided in the atrazine 2003 IRED and 

Section 2.2 of this assessment). Overall, the low frequency and magnitude of detection 

of atrazine suggests that its occurrence in baseflow may be a minimal source of exposure 

at Barton Springs. Figure 3.6 presents the location of the groundwater wells within the 

BSSEA. 

41
 
Figure 3.6. Location of Groundwater Sites Within the Barton Springs Segment 

Overall, the monitoring data provided by the USGS indicate relatively consistent lowlevel concentrations of atrazine over time with periodic spikes related to storm-derived 

runoff events. Because of the limited nature of the runoff-related sampling, it is not 

possible to determine whether these data are representative of overall peak exposures 

(Mahler, personal communication, 2005b). Therefore, these data represent a lower bound 

on exposures and are considered to be representative of long-term baseflow exposure in 

the spring system. 

42
 
03/01/1978 

03/20/1980 

04/09/1982 

04/28/1984 

05/18/1986 

06/06/1988 

06/26/1990 

07/15/1992 

08/04/1994 

08/23/1996 

09/12/1998 

200 

180 

160 

140 

120 

100 

80 

60 

40 

20 

0 

Date 

Mean Flow (cfs)

Figure 3.7. Flow Hydrograph Data for Barton Springs 

Mean Daily Flow (cfs) 

A key component of the total load reaching Barton Springs is represented by base flow. 

For this assessment, a reasonably conservative estimate of atrazine load arriving via base 

flow is represented by the average concentration in all springs of 0.45 μg/L. This 

estimation is conservative because it excludes all non-detections of atrazine and includes 

concentrations associated with storm events. The USGS has estimated base flow 

concentrations of atrazine to be less than 0.1 μg/L (Mahler, 2005a). Information on the 

average flow rate through the spring system was evaluated. As noted above, Hauwert et 

al. (2004) estimated an average flow rate through the entire system of 53 cfs, while 

Mahler (2005a) reported an average of 56 cfs. Hydrograph data for Barton Springs from 

the USGS (Figure 3.7) yields an average flow of 62 cfs. 

Mean Flow (cfs) for Barton Springs 

3.2.5 Modeling Approach 

The analysis of available monitoring data and usage information indicates that the 

exposure assessment cannot rely exclusively on monitoring data. Although of high 

quality and in selected instances targeted to pesticide use and single runoff events, the 

unique nature of flow through the BSSEA and the relationship of the flow regime to the 

Barton Springs salamander indicates that the exposure assessment should rely on 

modeling to augment the available monitoring data. 

43
 
Typically, the Agency conducts modeling using scenarios intended to represent use sites 

in areas that are highly vulnerable to either runoff, erosion, or spray drift. Runoff 

estimates predicted by the PRZM model are linked to the Exposure Analysis Modeling 

System (EXAMS). For ecological risk assessment, the Agency relies on a standard water 

body to receive the edge of field runoff estimates. The standard water body is of fixed 

geometry and includes processes of degradation and sorption expected to occur in ponds, 

canals, and low order streams (e.g. first and second order streams), but with no flow 

through the system. 

The unique geology/hydrogeology of the BSSEA suggests that, for the use sites being 

evaluated, an estimate of exposure in surface water may not be suitable by itself. If the 

use site resides exclusively within the Recharge Zone, the principal route of exposure is 

expected to be either via “edge of field” runoff into an adjacent fracture or via overland 

flow to a stream which subsequently “loses” some of that flow to the fracture system. In 

order to provide the most conservative estimate for each scenario modeled, edge of field 

concentrations are used to represent sources originating with the Recharge Zone (instead 

of EXAMS concentrations) to mimic the pulsed nature of exposures moving through 

karst fractures. 

Loading of atrazine to the system via base flow is approximated using available 

monitoring data. The average concentration from the available USGS monitoring data 

from the four springs of 0.45 μg/L is added to the daily edge of field EEC. The volume 

associated with the base flow is accounted for by using the long-term average flow rate 

from the USGS hydrograph data for Barton Springs of 63 cfs. PRZM reports a daily flow 

at the edge of field in units of centimeters representing the depth of water leaving the 10 

hectare field. An equivalent volume is calculated by multiplying the edge of field runoff 

depth by the total area of the 10 hectare field in centimeters. In order to use the average 

flow rate from the hydrograph data, the allocation of 63 cfs was back-calculated to the 

entire BSSEA drainage area of 900 square kilometers. This estimation yields an 

approximate additional flow through the Recharge Zone of 1,000,000,000 cm3/day 

(PRZM reports this as the depth of water running off the entire 10 hectare watershed, or 1 

cm) which is added to the daily edge of field estimate. 

Peak atrazine concentrations, as well as rolling time-weighted averages of 14 days, 21 

days, 30 days, 60 days, and 90 days are calculated for comparison with various 

ecotoxicity endpoints (including aquatic community-level threshold concentrations) for 

atrazine. 

3.2.5.1 Model Inputs 

EECs from surface water sources were calculated using the Agency’s Tier II PRZM 

model. PRZM is used to simulate pesticide transport as a result of runoff and erosion 

from a standardized watershed. The linkage program shell, PE4v01.pl, which 

incorporates the site-specific scenarios developed by the Agency, was used to run PRZM 

(U.S. EPA, 2005). However, new, site-specific scenarios were developed for use in this 

assessment. Linked site-specific use scenarios and meteorological data are used to 

44
 
Fate Property Value MRID (or source) 

Molecular Weight 215.7 MRID 41379803 

Henry’s constant 2.58 x10-9 MRID 41379803

Vapor Pressure 3 x 10-7 atm-m3/mol MRID 41379803

Solubility in Water 33 mg/l MRID 41379803 

Photolysis in Water 335 days MRID 42089904 

MRID 40431301 

Aerobic Soil Metabolism Half-lives 152 days MRID 40629303 

MRID 42089906 

Hydrolysis Stable MRID 40431319

Aerobic Aquatic Metabolism (water 

column) 304 days 

2x aerobic soil metabolism 

rate constant 

estimate exposure for each modeling scenario. Weather and agricultural practices are 

simulated over 30 years to estimate the 1 in 10 year exceedence probability at the site. 

Further information on these models may be found at: 

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

The appropriate PRZM input parameters were selected from environmental fate data 

submitted by the registrant and in accordance with US EPA-OPP EFED water model 

parameter selection guidelines, Guidance for Selecting Input Parameters in Modeling the 

Environmental Fate and Transport of Pesticides, Version 2.3, February 28, 2002. These 

parameters are consistent with those used in both the 2003 atrazine IRED (U.S. EPA, 

2003a) and the cumulative triazine risk assessment (U.S. EPA, 2006a); no new 

environmental fate data were incorporated into this assessment. The date of first 

application was identified based on several sources of information including data 

provided by BEAD, crop profiles maintained by the USDA, and conversations with local 

experts. More detail on the crop profiles and the previous assessments may be found at: 

http://pestdata.ncsu.edu/cropprofiles/cropprofiles.cfm 

http://www.epa.gov/oppsrrd1/REDs/atrazine_ired.pdf 

http://www.epa.gov/pesticides/cumulative/common_mech_groups.htm#chloro 

A summary of the model inputs used in this assessment are provided in Table 3.3. 

Table 3.3. Summary of PRZM/EZAMS Environmental Fate Data Used for Aquatic 
 

Exposure Inputs for Atrazine Endangered Species Assessment for the Barton 
 

Springs Salamander 
 

45
 
Fate Property Value MRID (or source) 

Anaerobic Aquatic Metabolism 

(benthic) 

Koc 

Application Efficiency 

1 Spray Drift Fraction

608 days 

88.78 ml/g 

95 % for aerial 

99 % for ground 

5 % for aerial 

MRID 40431323 

MRID 40431324 

MRID 41257901 

MRID 41257902 

MRID 41257904 

MRID 41257905 

MRID 41257906 

2 default value

2 default value 1 % for ground 

1 – Spray drift not included in final EEC due to edge-of-field estimation approach 

2 – 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 

Unlike the Agency’s standard ecological risk assessment methodology that relies on 

EECs derived using an EXAMS standard water body, edge of field concentrations were 

predicted for those use sites expected to reside within the action area. It is expected that 

infiltration directly into the fractured limestone represents the most direct route of 

exposure and is likely to yield the highest EECs. In this instance, PRZM alone was used 

to estimate the edge of field concentration. Unlike the typical approach used to estimate 

exposure using the EXAMS water body described above, the PRZM output from the *.zts 

file (daily time series data) associated with each scenario modeled was used. In this 

instance, the *.zts file provides daily estimates of atrazine exposures. The PE4v01.pl 

script was modified to provide time series output (TSER in PRZM) as opposed to the 

standard cumulative output (TCUM in PRZM) of runoff volume, mass transported with 

runoff, infiltration volume, mass transported with infiltration, eroded sediment, and mass 

of pesticide transported with sediment. The runoff volume and runoff mass were 

extracted and converted to a runoff concentration. 

The standard approach for conducting ecological risk assessment assumes that 100% of 

the 10-hectare watershed is covered by the relevant use. This approach also assumes that 

the standard water body is adjacent to the edge of the field. In this assessment, the 

majority of the use sites, with the exception of residential uses, are either sporadically 

present within the action area or are predominant further south and west of the spring 

systems. Therefore, it is unlikely that edge of field concentrations for these use sites are 

equivalent to the standard assumption that the water body is receiving runoff from a small 

watershed that is adjacent to a field that is 100% cropped and treated. Although travel 

through the fractures is likely to be direct, there is also likely to be non-impacted fracture 

flow arriving simultaneously at the springs from the same runoff event. In order to 

account for this, an adjustment factor is applied to each of the use sites located outside 

the immediate area of the springs. These factors are based on an assessment of recent 

land cover data with specific cover types that correlate with the use sites. A summary of 

the land cover-based adjustment factors is presented in Table 3.4. 

46
 
Scenario Modeled Land Cover Adjustment Factor 

Residential 

Impervious 

Turf 

Rights-of-Way 

Fallow/Idle land 

70% of action area in vicinity of Barton Springs (% 

decreases with distance from spring system) 

30% of action area in vicinity of Barton Springs (% 

decreases with distance from spring system) 

100% of action area in vicinity of Barton Springs 

and 28% of treatable golf course (based on limited 

occurrence of golf courses within action area) 

10% of action area in vicinity of Barton Springs 

(based on density near springs; % decreases with 

distance from spring system) 

5% of action area in vicinity of Barton Springs 

(estimated from land cover of entire action area; 

majority of fallow/idle land is located at the 

southern and western edge of action area) 

Table 3.4. Land Cover Adjustment Factors for the Action Area in the Barton 
 

Springs Segment of the Edwards Aquifer (BSSEA) 
 

The edge of field concentrations are post-processed (see Appendix E for details) in order 

to provide durations of exposure. First, daily concentrations were calculated using the 

time series data described above for all 30 years of model output. Then, peak, 14-day, 

21-day, 30-day, 60-day, and 90-day average concentrations were calculated across the 

entire 30 years of data. In order to match the standard PRZM/EXAMS output, the 30 

years of daily and rolling averages were separated into individual years and the maximum 

value for each of the 30 years was calculated for both peak and rolling averages. Finally, 

the 30 years of maximum values were ranked and the 90th percentile from the rankings 

was selected as the final EEC for use in risk estimation. 

The time series output generated by PRZM provides individual runoff events and does 

not capture the influence of base flow in the fracture zone. In order to account for the 

influence of base flow, the average of all monitoring data detections (excluded nondetections) was added to the time series output. For atrazine, the estimated base flow 

value of 0.45 μg/L is considered to be conservative because it includes recently analyzed 

storm runoff data and excludes all non-detections. In addition, the flow associated with 

the base flow exposures was estimated using USGS hydrograph data for Barton Springs 

and was also added into the exposure estimate. Addition of base flow exposure to the 

estimated exposure was intended to provide an upper bound on base flow and account for 

the limitations in the sample frequency that may not completely represent base flow. 

47
 
The 0.45 μg/L estimate of base flow concentration is considered to be a conservative 

estimate for several reasons. First, the estimate is based on all detections from the sitespecific USGS monitoring data and includes samples collected during high flow events. 

Second, the value excludes all non-detections. These first two points are important 

because the USGS estimates that the actual base flow concentration of atrazine from the 

non-runoff driven sampling is less than 0.1 μg/L (Mahler, 2005a). 

Calculating an annual average concentration from the PRZM generated edge of field 

concentrations without the added 0.45 μg/L value yields an annual average concentration 

of roughly 0.15 μg/L that is consistent with the USGS estimate. Therefore, addition of 

the 0.45 μg/L atrazine concentration is assumed to be conservative and protective. The 

analysis indicates that long term averaging of the PRZM edge of field output provides 

estimated exposures consistent (within a factor of two to three times) with the estimate of 

base flow predicted by the USGS from the site specific monitoring data. Therefore, the 

approach taken in this assessment is considered to be reasonable and protective. The 

approach is reasonable given that the monitoring data, though of high quality and targeted 

to Barton Springs, is not considered to be of sufficient robustness to provide an upper 

bound on both peak and longer-term exposures due to the limited sampling frequency in 

the study design. This approach is protective because use of the 0.45 μg/L concentration 

as a base flow value is greater than any estimate of base flow available based on a limited 

monitoring data set. 

The calculations described above were completed by a straight comparison of runoff 

mass divided by runoff volume to get a daily estimated EEC. However, a second step 

was included to evaluate the influence of infiltrating water on the overall exposure. This 

additional step was included to mimic the influence of infiltrating water into the fractured 

limestone of the Recharge Zone and to account for base flow concentrations associated 

with some amount of infiltrating water not accounted for in the runoff edge of field EEC. 

The additional volume of water predicted by PRZM to leach out of the bottom of the soil 

profile was added back into the edge of field concentration. This additional step accounts 

for recharge to the fractured limestone that is likely to occur from infiltration of runoff 

water through fractures/sinkholes and direct infiltration throughout the soil profile. 

3.2.6 Individual Scenario Results 

A total of four scenarios were developed for this assessment, including residential, turf, 

rights-of-way, and fallow/idle land. Of these, two scenarios are used in tandem with the 

impervious scenario (residential and rights-of-way). Discussions with local experts 

(Markwardt, 2006; Ward, 2006; Mason, 2006) suggest that atrazine is never used in the 

rights-of-way scenario within the action area. However, the rights-of-way scenario was 

modeled, given its potential as a use site in the action area. It should be noted, however, 

that the predicted EECs associated with rights-of-way are less relevant to actual 

exposures than the remaining scenarios and are presented for a qualitative comparison. 

Model inputs were selected for atrazine using the most recent data available from the 

atrazine IRED (U.S. EPA, 2003a) and the triazine cumulative assessment (U.S. EPA, 

48
 
2006a). A discussion of each assessed exposure scenario and a summary of the results 

for each is provided below. Copies of the model input files along with the stepwise 

approach for processing model output are provided in Appendix E. 

3.2.6.1 Residential 

The residential exposure scenario represents two scenarios modeled in tandem. The first 

scenario is intended to reflect runoff and erosion from a typical 1⁄4 acre lot and reflects a 

typical urban/suburban use site with homeowner and professional applications. The 

residential lot scenario was developed using local soil information and a USDA runoff 

curve number developed specifically for 1⁄4 acre lots (USDA, 1986). In order to justify 

the assumption of 1⁄4 acre lot as a typical exposure scenario, publicly available data was 

reviewed from the United States Census (Census). Specifically, data from 2003 from the 

American Housing Survey (AHS) available at the following website was reviewed. 

http://www.census.gov/hhes/www/housing/ahs 

Initially, the data for all suburban homes available nationally was reviewed. It is 

assumed that most pesticide applications, particularly for herbicides, occur in suburban 

settings. In order to test the assumption of the 1⁄4 acre lot as the best representation, AHS 

data for suburban homes that list total number of houses by lot size and by square footage 

of house (see Table 1C-3 at the AHS website above) was reviewed. With a total of 

45,552,000 total units reported nationally for all suburban areas, 12,368,000 units (the 

largest class at 27%) were located on lots between 1/8 acre and 1⁄4 acre, while 9,339,000 

units (the second largest class at 21%) were located on lots between 1⁄4 acre and 1⁄2 acre. 

Overall, the median lot size was 0.37 acre. This analysis suggests that the 1⁄4 acre lot is a 

reasonable approximation of suburban pesticide use. The selection of the 1⁄4 acre lot was 

an assumed to provide an estimate of potential exposure in an urban/suburban scenario 

where it is expected that most herbicide use will occur. It is believed that this 

representation provides a reasonable estimate of typical uses in an urban/suburban 

watershed particularly as it relates to the City of Austin and its rapidly developing 

outskirts. 

The second scenario was developed to represent general impervious surfaces expected to 

be present in an urban/suburban watershed. Examples of representative impervious 

surfaces include roads, parking lots, and buildings. These surfaces are distinct from the 

impervious surfaces inherent in the 1⁄4 acre lot (driveways, sidewalk, and house). The 

impervious surface uses a high-end curve number (98 out of a maximum of 100) to 

mimic the runoff expected from these surfaces. Using these in tandem allows for a 

weighting of the runoff potential of both surface types within a residential watershed that 

is different from the standard agricultural watershed that assumes uniform land cover. 

Figure 3.8 presents the conceptual model of the paired impervious/pervious scenario 

approach. 

49
 
10 Hectare Field – 50% Impervious/50% 

Pervious (1/4 acre lot) Draining to 1 

Hectare Water body 

Figure 3.8. Conceptual Model of Paired Residential/Impervious Scenarios (green 

square represents 1⁄4 acre lot while black square represents impervious surface 

scenario. Ratio of pervious to impervious surface based on best available land cover 

data) 

For edge of field EECs, the output is weighted based on the percentage of impervious 

surface present in the action area and by the percentage of the 1⁄4 acre lot treated. For this 

assessment, it is assumed that 30% of the action area in the vicinity of the spring system 

is impervious (see Figure 3.9). This assumption is reasonable given the density of 

residential development surrounding the springs. Outlying areas are likely to have lower 

percentages of impervious surfaces; however, residential areas in close proximity to the 

springs are likely to be most representative of the expected edge of field concentrations. 

Additional analysis of the impact of alternative assumptions for percent impervious 

50
 
surface, overspray, and percentage of lot treated is included in Section 3.2.7. Because of 

the unique nature of the karst system, it is assumed that no direct spray drift will reach the 

spring system (personal communication with Elizabeth McVeety, pesticide applicator at 

Zilker Park, April 21, 2006). However, it is likely that some overspray may reach 

impervious surfaces in the residential setting. In order to account for overspray, the 

impervious surface was modeled using three separate assumptions. For the purposes of 

risk assessment, it is assumed that 1% of the application rate could reach the impervious 

surfaces surrounding each residential lot. This amount of overspray is not based on any 

empirical data (studies of this type were not identified); however, the assumption seems 

reasonable given the principal drift assumption for ground spray in ecological risk 

assessments is 1%. In order to test the assumption of 1% overspray and address the 

uncertainty associated with the lack of data for overspray, two alternate scenarios were 

modeled. The impervious surface was modeled with 0% overspray and 10% overspray to 

provide a lower bound (0% overspray) and an upper bound (10% overspray) on the 1% 

assumption. The results of these alternate modeling exercises are discussed more fully in 

Section 3.2.7 of this assessment. 

In this exercise, it is also assumed that that 50% of the 1⁄4 acre lot is treated with atrazine. 

This assumption was based on data from the AHS website and from professional 

judgment about typical features and the percentage of a typical lot those features might 

require. For example, the AHS survey data reports that of a total of 43,328,000 single 

detached homes in suburban areas, 10,124,000 (the largest group at 23%) were between 

1,500 and 2,000 square feet, while 7,255,000 (the third largest group at 17%) were 

between 2,000 and 2,500 square feet, and 9,513,000 (the second largest group at 22%) 

were between 1,000 and 1,500 square feet. From this data, it is assumed that a typical 

home is 2,000 square feet with a 1,000 square foot footprint. Lower sized houses less 

than 1,500 square feet are more likely to represent single floor structures; thus, the 1,000 

square foot estimate for a house footprint is considered to be reasonable. 

In addition to the footprint of the typical house, it is also assumed that a typical house has 

a driveway of approximately 25 by 30 feet or 750 square feet and roughly 250 square feet 

of sidewalk. A typical suburban home was also assumed to have roughly 300 square feet 

of deck space and 900 square feet of garage. Finally, it was assumed that a substantial 

portion of the typical home would be landscaped with an estimate of 2,000 square feet. 

All of the previous estimates are based on professional judgment and are not derived 

from the AHS data. All of these areas are assumed to not be treated with a turf herbicide, 

resulting in a total area not treated with atrazine of 5,200 square feet. Taking a total 1⁄4 

acre lot size of 10,890 square feet and subtracting the untreated square footage yields a 

total remaining area of 5,690, or roughly 50% of the total lot that could be potentially 

treated. 

The first assumption may result in an underestimation of exposure, given that more 

overspray of impervious surfaces is possible. The impact of this assumption is tested and 

characterized in Section 3.2.7. Note that this scenario represents general impervious 

surfaces within a watershed not part of the 1⁄4 acre lot and includes roads, parking lots, 

and buildings among others where overspray from residential lots is expected to be 

51
 
minimal. The 1⁄4 acre lot, by comparison, was developed with a curve number reflective 

of the fact that the lot is covered with both pervious surfaces (grass and landscaped 

gardens) and impervious surfaces (driveways, sidewalks, and buildings). In this case, the 

assumption that 50% of the lot is treated likely overestimates the amount of landscaped 

area treated, but underestimates unintentional overspray of driveways and sidewalks. 

The impact of this assumption is also evaluated in Section 3.2.7. Overall, these are 

simplifying assumptions that are likely to provide a reasonable high-end estimate of 

exposure, given the limitations of the modeling approach. 

Figure 3.9. Percentage of Impervious Surface Coverage in Vicinity of Barton 
 

Springs 
 

The combined edge of field concentrations are estimated using the *.zts output from 

PRZM as described above. In this paired scenario approach, the *.zts output from both 

the impervious and residential scenarios are weighted and added together to provide an 

52
 
1

23456789123456789

80 

70 

60 

Concentration (ppb) 50 

40 

30 

20 

10 

0 

1/1/1961/1/1961/1/1961/1/1961/1/1961/1/1961/1/1961/1/1961/1/1961/1/19701/1/1971/1/1971/1/1971/1/1971/1/1971/1/1971/1/1971/1/1971/1/1971/1/19801/1/19811/1/19821/1/19831/1/19841/1/19851/1/19861/1/19871/1/19881/1/19891/1/1990 

Date 

Edge of Field EEC 

Figure 3.10. Representative Time Series Output from Paired Residential/Impervious 

PRZM Scenario for Granular Applications 

overall estimate of exposure. Non-detects in the weighted output were converted to 0.45 

μg/L in order to capture the potential influence of base flow (described in more detail 

above). 

Two categories of formulations are currently registered for atrazine use on residential 

sites, including granular and liquid formulations (wettable powder dry flowables). Both 

formulations were modeled separately because application rates are different (2 lbs/acre 

for granular and 1 lb/acre for liquid) and the standard assumption for modeling granular 

formulations is different from liquid formulations. Granular formulations are typically 

modeled as soil applied (CAM is set to 8 with a minimized incorporation depth of 1 cm) 

and 0% spray drift, as compared with a foliar application (CAM isset to 2 with a 4-cm 

depth of incorporation), which assumes the standard spray drift of 1% for ground 

applications. However, because spray drift is not assumed to contribute to the loadings 

in the spring and some overspray is expected to impervious surfaces, both residential 

scenarios (liquid and granular) were modeled assuming that 1% of the application rate is 

applied to the impervious surface. 

Figure 3.10 graphically presents the runoff only time series output for the edge of field 

concentrations predicted for the granular application of atrazine to the paired 

residential/impervious scenario, assuming an overspray of 1% of the application rate to 

the impervious surface. 

Residential (Granular) Edge of Field Concentration (ppb) 

53 
3.2.6.2 Turf 

The turf scenario was developed consistent with the current PRZM scenarios for turf in 

Pennsylvania and Florida (no pre-existing turf scenario for Texas is available). For the 

Barton Springs assessment, the turf scenario is intended to represent golf course turf and 

recreational fields because residential uses are captured in the residential scenario 

(standard turf scenarios are typically used to represent both golf course, recreational, and 

residential uses). The standard approach for conducting ecological risk assessments 

assumes 100% of the 10-hectare watershed is covered by the relevant use. This approach 

also assumes that the receptor (EXAMS standard water body) is adjacent to the edge of 

the field. In this assessment, with the exception of the residential use sites described 

above, most of the other potential atrazine use sites are not adjacent to the receptor. 

Although travel through the fractures is likely to be direct, there is also likely to be nonimpacted fracture flow arriving simultaneously at the springs. In order to account for 

this, an adjustment factor was applied to each of the non-residential use sites. For turf, it 

was assumed that 100% of the watershed feeding the fractures is represented by golf 

course turf, but that the percentage of the golf course expected to be treated in this case is 

represented by the golf course adjustment factor of 28%. This seems reasonable given 

that the land cover analysis indicates that only a few golf courses are present within the 

action area. As with the residential scenario, a base flow concentration of 0.45 μg/L was 

added to the overall exposure. 

Similar to the residential scenario, two categories of formulations are registered for 

atrazine use on turf sites. These are granular and liquid formulations (wettable powder 

dry flowables). Both formulations were modeled separately because application rates are 

different (2 lbs/acre for granular and 1 lbs/acre for liquid) and the standard assumption 

for modeling granular formulations is different from liquid formulations. Granular 

formulations are typically modeled as soil applied (CAM is set to 8 with a minimized 

incorporation depth of 1 cm) and 0% spray drift, as compared with a foliar application 

(CAM is set to 2 with a 4-cm depth of incorporation), which assumes the standard 1% 

spray drift for ground applications. 

3.2.6.3 Fallow/Idle Land 

The fallow/idle land scenario represents the only agricultural use present within in the 

action area. Generally, this scenario conceptually represents the potential application of 

atrazine to fallow lands under the Conservation Reserve Program (CRP) as most of the 

available information suggests that no agricultural use of atrazine occurs in the action 

area. It is also expected, given the available usage information, that fallow/idle land 

represents a minor use in the action area, although there are limitations associated with 

this analysis. Regardless, the scenario was included in the assessment to address all 

potential atrazine uses. 

54
 
This scenario was developed similar to the standard PRZM scenarios. The bulk of this 

use pattern is located south and west of the receptor site; therefore, a land cover 

adjustment factor was applied to the modeled output. Although travel through the 

fractures is likely to be direct, non-impacted fracture flow is likely to arrive 

simultaneously at the springs, especially given that most of this use site is located within 

the Contributing Zone. To account for this, an adjustment factor was applied for each of 

the non-residential use sites. For the fallow/idle land scenario, this factor represents the 

percentage of the land cover within the action area that is in fallow/idle land. As shown 

in Table 3.4, the adjustment factor for fallow/idle land is 5%. As with the residential 

scenario, a base flow concentration of 0.45 μg/L was added to the overall exposure. 

3.2.6.4 Rights-of-Way 

The rights-of-way scenario represents a vegetated buffer strip where atrazine could be 

applied adjacent to a water body. The vegetative strip as modeled is intended to represent 

treated buffers along roadways, railroad lines, and utility rights-of-way. Each of these 

sites is conceptualized as a naturally vegetated strip that runs linearly adjacent to a 

sensitive water body. Figure 3.11 presents the conceptual model of this scenario relative 

to a roadway, although a similar layout would be expected for rail and power rights-ofway. It is expected that the density of roads, railroads, and utility rights-of-way define 

the density of rights-of-way use within the watershed where pesticides may be applied. 

Land cover data was used to define this percentage. For this assessment, it is assumed 

that the maximum density of treated rights-of-way within the action area is 10%, which is 

expected to represent a slight over-estimation. 

55
 
Figure 3.11. Conceptual Model of Rights-of-Way Scenario 

56
 
In addition, the potential impact of impervious surfaces within the treated area was 

addressed to estimate its influence on overall runoff and exposure. For the roadway 

rights-of-way use, it is assumed that there is an equal amount of impervious and pervious 

surface within the treatment area, but that the atrazine treatment is likely to be restricted 

to the pervious portion of the rights-of-way. As with the residential scenario, this 

assumption accounts for a potential overspray to the impervious surface of 1%. 

Conversations with local experts (Markwardt, 2006; Ward, 2006; Mason, 2006) suggest 

that the rights-of-way scenario is likely to be conservative, given that atrazine is not 

typically used (glyphosate is reportedly the herbicide of choice in Hays and Travis 

counties) and treatment zones are typically one to four feet wide. As with the residential 

scenario, the impact of 0% and 10% over spray of the impervious surface on rights-ofway is characterized in Section 3.2.7 of this assessment. 

As with the residential scenario, the edge of field EECs for both the runoff only and 

runoff plus infiltration scenarios were estimated. Similar to the residential scenario, a 

base flow concentration of 0.45 μg/L was added to the overall exposure. 

Table 3.5 presents the summary of all relevant time-weighted concentrations for each 

scenario modeled at the 90th % of exposure for the edge of field exposure. The EECs 

presented in Table 3.5 are used to derive risk quotients, which are presented as part of the 

Risk Characterization in Section 5 

57
 
Table 3.5. Summary of PRZM Output EECs for all Modeled Scenarios (Edge of Field Concentration

Incorporated). 

90th Percentile of 30 Years of Output 

Use Site Application Rate (lbs/acre) Number of Applications (interval) First Application Date Peak (one-day) EEC (μg/L) 14-day EEC (μg/L) 21-day EEC (μg/L) 30-day EEC (μg/L) 60-day EEC (μg/L) 

Residential Granular1 2 2 April 1 41.2 3.5 2.5 1.9 1.2 Residential Liquid1 1 2 April 1 26.6 2.5 1.8 1.5 1.0 Right-ofWay1 1 1 June 1 6.2 1.1 0.9 0.8 0.6 Fallow/Idle land 2.25 1 November 1 7.5 1.0 0.8 0.7 0.6 Turf – Granular 2 2 April 1 22.4 2.0 1.5 1.2 0.8 Turf - Liquid 1 2 April 1 16.2 1.7 1.3 1.0 0.7 

58
In general, the exposure assessment yields modeled peak exposure estimates that are two 

to ten times higher than those seen in monitoring data, while the annual average 

concentrations are consistent with those seen in monitoring. The intermediate duration 

exposures (14-day, 21-day, 30-day, 60-day, and 90-day averages) cannot be estimated 

from the monitoring data due to insufficient sample frequency. 

3.2.7 Characterization 

Reported use information provides a sense of the actual use on sites similar to those 

assessed including fallow/idle land. In this instance, the data for fallow/idle land 

suggests that the 90th percentile application rate is similar to the maximum labeled use 

rate while the typical use application rate (equivalent to the average of all reported 

applications) is roughly half the labeled maximum rate used in this assessment. Table 3.6 

summarizes the typical and 90th percentile rates and number of applications relative to 

those used in this assessment. If it were assumed that this pattern holds true for all uses 

(agricultural and non-agricultural), then modeling with the typical application rates would 

yield predicted exposures that are roughly half of those presented in Table 3.5. 

Table 3.6. Comparison of Maximum, Typical, and 90th Percentile Label Rates and 

Number of Applications 

Scenario 

Maximum 

Application 

Rate 

(lbs/acre) 

Maximum 

Number of 

Applications 

90th 

Percentile 

Application 

Rate 

(lbs/acre) 

90th Percentile 

Number of 

Applications 

Typical 

Application 

Rate 

(lbs/acre) 

Typical 

Number of 

Applications 

Fallow/Idle 

land 2.25 1 2 1 0.9 1 

In order to account for the variability in overspray, the residential scenario was modeled 

assuming two alternate scenarios of 0% and 10% overspray to impervious surfaces. The 

alternate assumptions are intended to provide a bound on the 1% assumption. Because 

both the residential and rights-of-way scenarios were modeled using the paired 

pervious/impervious approach, the alternate scenarios were modeled for both scenarios 

(residential was modeled for both granular and liquid formulations). The 

conservativeness of these assumptions is unknown, given a lack of data on this 

phenomenon. However, given that the impervious scenario is intended to represent nontarget surfaces such as roads, parking lots and buildings, it is seems reasonable to assume 

that 10% overspray is an over-estimation of what would likely occur to these off-site 

areas, while 0% may be an under-estimation. 

In order to model the overspray, the binding coefficient was set to zero and the aerobic 

soil metabolism half-life was set to stable in lieu of actual data for the impervious 

scenario. It is assumed that non-binding would occur on these surfaces and that limited 

degradation would occur. The percentage overspray was then multiplied by the total 

application rate to yield an effective application rate for the overspray to impervious 

59
 
surfaces. This analysis yielded an application rate on the impervious surface of 0.2 

lbs/acre (0.23 kg/ha) for 10% overspray and 0.02 lbs/acre (0.023 kg/ha) for 1% 

overspray. 

Comparison of the resulting EECs indicates that with 10% overspray, the overall increase 

in peak EECs is roughly 30%, while the longer-term EECs are increased by nearly 50%. 

For the 1% overspray assumption, there is very little increase in overall EECs for both 

peak and average EECs, as compared to the 0% overspray assumption. This is not 

unexpected, given the increased runoff, lack of binding, and lack of degradation being 

modeled. Without actual data on these processes, it is impossible to determine whether 

these exposures reflect reality, especially given that none of the monitoring data indicate 

concentrations approaching any of these EECs with or without overspray. The overspray 

comparison is presented in Table 3.7. 

Other assumptions that can have a significant impact on the overall predicted EECs 

include the percentage of impervious surface and the percentage of 1⁄4 acre lot that is 

treated. In both instances, the relationship between the assumption and the predicted 

EEC is linear. The assumed action area impervious surface percentage of 30% in the 

vicinity of the Barton Springs decreases dramatically further south and west from the 

salamander’s habitat. It is apparent from the available data that this value decreases to 

less than 10% the further south and west from the springs. The impact of this assumption 

was evaluated by readjusting the output to reflect the impact of a 10% impervious cover 

assumption on predicted exposures. In general, peak and longer-term average 

concentrations are generally doubled as the percentage of impervious decreases. The 

comparison of this analysis is presented in Table 3.8. This is likely due to the increase in 

treated area contributing more pesticide mass and an increase in the impervious surface, 

which yields greater amounts of non-contaminated runoff. 

This impact of a decrease in impervious surface will hold only with the assumption of 

limited overspray. This assumption was explored by comparing the impact of the change 

in percentage of impervious surface on the 10% overspray scenario discussed above. In 

this case, peak EECs increase by roughly 50% while the averages are only slightly 

increased. The comparison of this analysis is presented in Table 3.9. 

Finally, in this assessment it is assumed that 50% of the 1⁄4 acre lot is treated. In order to 

test the significance of this assumption, the exposure scenario was re-run using a different 

assumption of 10% treatment of the 1⁄4 acre lot. As expected, peak EECs are reduced by 

roughly a factor of five, while the longer term exposures are reduced by a factor of two to 

three times. The results of this comparison are presented in Table 3.10. 

60
 
Table 3.7. Comparison of Residential and Rights-of-Way EECs Assuming Variable Percentages of Oversp

90th Percentile of 30 Years of Output 

Use Site 

Application 

Rate 

(lbs/acre) 

Number of 

Applications 

(interval) 

First 

Application 

Date 

Peak 

EEC 

(μg/L) 

14-day 

EEC 

(μg/L) 

21-day 

EEC 

(μg/L) 

30-day 

EEC 

(μg/L) 

60-day 

EEC 

(μg/L) 

Residential - 

1% 

Overspray1 

Residential – 

No 

Overspray1 

Residential - 

10% 

Overspray1 

Rights-ofWay - 1% 

Overspray 

Rights-ofWay – No 

Overspray 

Rights-of 

Way - 10% 

Overspray 

2.0 2.0 2.0 

1.0 1.0 

1.0 

2 

(30 days) 

2 

(30 days) 

2 

(30 days) 

1 1 

1 

April 1 41.2 3.5 2.5 April 1 40.0 3.3 2.4 April 1 51.7 5.6 4.1 

June 1 6.2 1.1 0.9 June 1 5.9 1.1 0.9 

June 1 9.5 1.9 1.7 

1.9 1.8 3.4 

0.8 0.8 

1.5 

1.2 1.1 2.3 0.6 0.6 

1.1 

1 – Only the granular application was tested for characterization 

61
Table 3.8. Comparison of Residential EECs (granular) with 1% Over Spray and Variable Percentages of 

(10 and 30%) 

90th Percentile of 30 Years of Output 

Use Site 

Applicati 

on Rate 

(lbs/acre) 

Number of 

Applications 

(interval) 

First 

Application 

Date 

Peak EEC 

(μg/L) 

14-day 

EEC 

(μg/L) 

21-day 

EEC 

(μg/L) 

30-day 

EEC 

(μg/L) 

60-day 

EEC 

(μg/L) 

Residential - 30% Impervious 1.0 2 (30 days) April 1 41.2 3.5 2.5 1.9 1.2 Residential – 10% Impervious 1.0 2 (30 days) April 1 62.8 5.0 3.5 2.6 1.6 

Table 3.9. Comparison of Residential EECs (granular) with 10% Overspray and Variable Percentages of 

(10 and 30%) 

90th Percentile of 30 Years of Output 

Use Site 

Applicati 

on Rate 

(lbs/acre) 

Number of 

Applications 

(interval) 

First 

Application 

Date 

Peak 

EEC 

(μg/L) 

14-day 

EEC 

(μg/L) 

21-day 

EEC 

(μg/L) 

30-day 

EEC 

(μg/L) 

60-day 

EEC 

(μg/L) 

Residential - 30% Impervious 1.0 2 (30 days) April 1 51.7 5.6 4.1 3.4 2.3 Residential – 10% Impervious 1.0 2 (30 days) April 1 67.1 6.0 4.2 3.2 2.0 

62
Table 3.10. Comparison of Residential EECs (granular) Assuming Various Percentages of Treated 1⁄4 Acr

90th Percentile of 30 Years of Output 

Use Site 

Application 

Rate 

(lbs/acre) 

Number of 

Applications 

(interval) 

First 

Application 

Date 

Peak 

EEC 

(μg/L) 

14-day 

EEC 

(μg/L) 

21-day 

EEC 

(μg/L) 

30-day 

EEC 

(μg/L) 

60-day 

EEC 

(μg/L) 

Residential – 50% Treated Residential – 10% Treated 

1.0 1.0 

2 

(30 days) 

2 

(30 days) 

April 1 41.2 3.5 2.5 April 1 9.5 1.3 1.0 

1.9 

0.9 

1.2 

0.7 

63
4. Effects Assessment 

This assessment evaluates the potential for atrazine to adversely affect the Barton Springs 

salamander. As previously discussed in Section 2.7, assessment endpoints for the Barton 

Springs salamander include direct toxic effects on the survival, reproduction, and growth 

of the salamander itself, as well as indirect effects, such as reduction of the prey base 

and/or modification of its habitat. Direct effects to the Barton Springs salamander are 

based on toxicity information for freshwater vertebrates, including fish, which are 

generally used as a surrogate for amphibians, as well as available salamander toxicity 

data from the open literature. Given that the salamander’s prey items and habitat 

requirements are dependent on the availability of freshwater aquatic invertebrates and 

aquatic plants, toxicity information for various freshwater aquatic invertebrates and 

plants is 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 atrazine. In addition to registrant-submitted and open literature 

toxicity information, indirect effects to Barton Springs salamanders, via impacts to 

aquatic plant community structure and function are also evaluated based on communitylevel threshold concentrations. 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 

atrazine. A summary of the available freshwater ecotoxicity information, the 

community-level endpoints, use of the probit dose response relationship, and the incident 

information for atrazine are provided in Sections 4.1 through 4.4, respectively. 

With respect to atrazine degradates, including hydroxyatrazine (HA), deethylatrazine 

(DEA), deisopropylatrazine (DIA), and diaminochloroatrazine (DACT), it is assumed 

that they are of lesser toxicity as compared to the parent compound. Comparison of 

available toxicity information for the degradates of atrazine indicates lesser aquatic 

toxicity than the parent for freshwater fish, invertebrates, and aquatic plants. 

Specifically, the available degradate toxicity data for HA indicate that it is not toxic to 

freshwater fish and invertebrates at the limit of its solubility in water. In addition, 

available aquatic plant degradate toxicity data for HA, DEA, DIA, and DACT report nondefinitive EC50 values (i.e., 50% effect was not observed at the highest test 

concentrations) at concentrations that are 700 to 10,000 times higher than the lowest 

reported aquatic plant EC50 value for parent atrazine. Therefore, given the lesser toxicity 

of the degradates, as compared to the parent, concentrations of the atrazine degradates are 

not assessed, and the focus of this assessment is limited to parent atrazine. The available 

information also indicates that aquatic organisms are more sensitive to the technical grade 

(TGAI) than the formulated products of atrazine; therefore, the focus of this assessment is 

on the TGAI. A detailed summary of the available ecotoxicity information for all of the 

atrazine degradates and formulated products is presented in Appendix A. 

64
 
4.1 	 Evaluation of Aquatic Ecotoxicity Studies 

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 the 2003 atrazine IRED (U.S. EPA, 2003a) as well as information 

obtained on February 16, 2006. The February 2006 ECOTOX search included all open 

literature data for atrazine (i.e., pre- and post-IRED). 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; 

(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 

the registrant-submitted data are considered. Based on the results of the 2003 IRED for 

atrazine, potential adverse effects on sensitive aquatic plants and non-target aquatic 

organisms including their populations and communities, are likely to be greatest when 

atrazine concentrations in water equal or exceed approximately 10 to 20 μg/L on a 

recurrent basis or over a prolonged period of time (U.S. EPA, 2003a). Given the large 

amount of microcosm/mesocosm and field study data for atrazine, only effects data that 

are more conservative than the 10 μg/L aquatic-community effect level identified in the 

2003 atrazine IRED were considered. In addition, data for taxa that are directly relevant 

to the Barton Springs salamander (i.e., aquatic-phase amphibians) were also considered. 

The degree to which open literature data are quantitatively or qualitatively characterized 

is dependent on whether the information is relevant to the assessment endpoints (i.e., 

maintenance of Barton Springs salamander survival, reproduction, and growth) identified 

in Section 2.7. For example, endpoints such as behavior modifications are likely to be 

qualitatively evaluated, because quantitative relationships between modifications and 

reduction in species survival, reproduction, and/or growth are not available. 

As described in Agency’s Overview Document (U.S. EPA, 2004), the most sensitive 

endpoint for each taxa is evaluated. For this assessment, evaluated taxa include 

freshwater fish, freshwater aquatic invertebrates, and freshwater aquatic plants. 

Currently, no guideline tests exist for salamanders. Therefore, surrogate species were 

used as described in the Overview Document (U.S. EPA, 2004). In addition, aquaticphase amphibian ecotoxicity data from the open literature are qualitatively discussed. 

Table 4.1 summarizes the most sensitive ecological toxicity endpoints for the Barton 

Springs salamander, based on an evaluation of both the submitted studies and the open 

literature, as previously discussed. A brief summary of submitted and open literature 

65
 
data considered relevant to this ecological risk assessment for the Barton Springs 

salamander is presented below. Additional information is provided in Appendix A. It 

should be noted that Appendix A also includes ecotoxicity data for taxonomic groups that 

are not relevant to this assessment (i.e., birds, estuarine/marine fish, invertebrates, and 

plants) because the Agency is completing endangered species assessments for other 

species concurrently with this assessment. 

Table 4.1. Aquatic Toxicity Profile for Atrazine 

Assessment Endpoint Species Toxicity Value Used in 

Risk Assessment 

Citation 

MRID # 

(Author & 

Date) 

Comment 

Acute Direct Toxicity to 

Salamander 

Rainbow 

trout

1 

96-hour LC50 = 5,300 

μg/L 

Probit slope = 2.72 

000247-16 

(Beliles and 

Scott, 1965) 

Acceptable 

Chronic Direct Toxicity 

to Salamander 

Brook 

trout

1 

NOAEC = 65 μg/L 

LOAEC = 120 μg/L 

000243-77 

(Macek et al., 

1976) 

Acceptable: 7.2% 

reduction in 

length; 16% 

reduction in 

weight 

Indirect Toxicity to 

Salamander via Acute 

Toxicity to Freshwater 

Invertebrates (i.e. prey 

items) 

Midge 48-hour LC50 = 720 

μg/L 

Probit slope unavailable 

000243-77 

(Macek et al., 

1976) 

Supplemental: 

raw data 

unavailable 

Indirect Toxicity to 

Salamander via Chronic 

Toxicity to Freshwater 

Invertebrates (i.e. prey 

items) 

Scud NOAEC = 60 μg/L 

LOAEC = 120 μg/L 

000243-77 

(Macek et al., 

1976) 

Acceptable: 25 % 

reduction in 

development of F1 

to seventh instar 

Indirect Toxicity to 

Salamander via Acute 

Toxicity to Non-vascular 

aquatic plants 

4 species 

of 

freshwater 

algae 

1-week EC50 = 1 μg/L 000235-44 

(Torres & 

O’Flaherty, 

1976) 

Supplemental: 41 

to 98% reduction 

in chlorophyll 

production; raw 

data unavailable 

Indirect Toxicity to 

Salamander via Acute 

Toxicity to Vascular 

aquatic plants 

Duckweed 14-day EC50 = 37 μg/L 430748-04 

(Hoberg, 1993) 

Supplemental: 

50% reduction in 

biomass; NOAEC 

not determined 1

 Used as a surrogate for the Barton Springs salamander. Open literature data for the salamander are 

presented in Section 4.1.2. 

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. 

66
 
Table 4.2. Categories of Acute Toxicity for Aquatic Organisms 

LC50 (ppm) Toxicity Category 

< 0.1 

> 0.1 - 1 

> 1 - 10 

> 10 - 100 

> 100 

Very highly toxic 

Highly toxic 

Moderately toxic 

Slightly toxic 

Practically nontoxic 

4.1.1 Toxicity to Freshwater Fish 

As previously discussed, no guideline tests exist for salamanders; therefore, freshwater 

fish are used as surrogate species for amphibians including salamanders (U.S. EPA, 

2004). The available open literature information on atrazine toxicity to aquatic-phase 

amphibians, which is provided in Section 4.1.2, shows that acute and chronic ecotoxicity 

endpoints for amphibians are generally less sensitive than fish. Therefore, endpoints 

based on freshwater fish ecotoxicity data are assumed to be protective of potential direct 

effects to aquatic-phase salamanders including the Barton Springs salamander. A 

summary of acute and chronic freshwater fish data, including sublethal effects, is 

provided below. 

4.1.1.1 Freshwater Fish: Acute Exposure (Mortality) Studies 

Freshwater fish acute toxicity studies were used to assess potential direct effects to the 

Barton Springs salamander because direct acute toxicity guideline data on salamanders 

are unavailable. Atrazine toxicity has been evaluated in numerous freshwater fish 

species, including rainbow trout, brook trout, bluegill sunfish, fathead minnow, tilapia, 

zebrafish, goldfish, and carp, and the results of these studies demonstrate a wide range of 

sensitivity. The range of acute freshwater fish LC50 values for atrazine spans one order of 

magnitude, from 5,300 to 60,000 μg/L; therefore, atrazine is categorized as moderately 

(>1,000 to 10,000 μg/L) to slightly (>10,000 to 100,000 μg/L) toxic to freshwater fish on 

an acute basis. The freshwater fish acute LC50 value of 5,300 μg/L is based on a static 

96-hour toxicity test using rainbow trout (Oncorhynchus mykiss) (MRID # 000247-16). 

No sublethal effects were reported as part of this study. A complete list of all the acute 

freshwater fish toxicity data for atrazine is provided in Table A-8 of Appendix A. 

4.1.1.2 Freshwater Fish: Chronic Exposure (Growth/Reproduction) Studies 

Similar to the acute data, chronic freshwater fish toxicity studies were used to assess 

potential direct effects to the Barton Springs salamander because direct chronic toxicity 

guideline data for salamanders do not exist. Freshwater fish full life-cycle studies for 

atrazine are available and summarized in Table A-12 of Appendix A. Following 44 

weeks of exposure to atrazine in a flow-through system, statistically significant 

reductions in brook trout mean length (7.2%) and body weight (16%) were observed at a 

concentration of 120 μg/L, as compared to the control (MRID # 000243-77). The 

67
 
corresponding NOAEC for this study is 65 μg/L. Although the acute toxicity data for 

atrazine show that rainbow trout are the most sensitive freshwater fish, available chronic 

rainbow trout toxicity data indicates that it is less sensitive to atrazine, on a chronic 

exposure basis, than the brook trout, with respective LOAEC and NOAEC values of 

1,100 μg/L and 410 μg/L. Further information on chronic freshwater fish toxicity data 

for atrazine is provided in Section A.2.2 of Appendix A. 

4.1.1.3 Freshwater Fish: Sublethal Effects and Additional Open Literature 

Information 

In addition to submitted studies, data were located in the open literature that report 

sublethal effect levels to freshwater fish that are less than the selected measures of effect 

summarized in Table 4.1. 

Reported sublethal effects in rainbow trout show increased plasma vitellogenin levels in 

both female and male fish and decreased plasma testosterone levels in male fish at 

atrazine concentrations of approximately 50 μg/L (Wieser and Gross, 2002 [MRID 

456223-04]). Vitellogenin (Vtg) is an egg yolk precursor protein expressed normally in 

female fish and dormant in male fish. The presence of Vtg in male fish is used as a 

molecular marker of exposure to estrogenic chemicals. It should be noted, however, that 

there is a high degree of variability with the Vtg effects in these studies, which confounds 

the ability to resolve the effects of atrazine on plasma steroids and vitellogenesis. 

In salmon, endocrine-mediated olfactory functions were affected at 0.5 μg/L atrazine 

(Moore and Lower, 2001). The reproductive priming effect of the female pheromone 

prostaglandin F2α on the levels of expressible milt in males was reduced after exposure to 

atrazine at 0.5 μg/L. Overall, the relationship between reduced olfactory response of 

males to the female priming hormone in the laboratory and reduction in salmon 

reproduction (i.e., the ability of male salmon to detect, respond to, and mate with 

ovulating females) in the wild is not established. In addition, EPA did not use these data 

in development of the aquatic life water quality criteria for atrazine because the test 

material was not adequately described or translated. Furthermore, the study did not 

determine whether the decreased response of olfactory epithelium to specific chemical 

stimuli would likely impair similar responses in intact fish. 

Although these studies raise concern about the effects of atrazine on endocrine-mediated 

functions in freshwater and anadromous fish, these effects are difficult to quantify 

because they are not clearly tied to the assessment endpoints for the Barton Springs 

salamander (i.e., survival, growth, and reproduction of individuals). In addition, 

differences in habitat and behavior of the tested fish species compared with the Barton 

Springs salamander suggest that the results are not readily extrapolated to salamanders. 

Furthermore, there is uncertainty associated with extrapolating effects observed in the 

laboratory to more variable exposures and conditions in the field. Therefore, potential 

sublethal effects on fish are evaluated qualitatively and not used as part of the 

quantitative risk characterization. Further detail on sublethal effects to fish is provided in 

Sections A.2.4a and A.2.4b of Appendix A. 

68
 
4.1.2 Toxicity to Aquatic-phase Amphibians 

Available toxicity information on potential atrazine-related mortality and sublethal 

effects to aquatic-phase amphibians (including salamanders) from the open literature is 

summarized below in Sections 4.1.2.1 and 4.1.2.2, respectively. Guideline ecotoxicity 

studies for amphibians are not available. 

4.1.2.1 Amphibians: Open Literature Data on Mortality 

Available acute data for amphibians, including the leopard frog (Rana pipiens), wood 

frog (R. sylvaticas), and American toad (Bufo americanus), indicate that they are 

relatively insensitive to atrazine with acute LC50 values > 20,000 μg/L (Allran and 

Karasov, 2001). Acute toxicity data are not available for the salamander. 

Chronic mortality data for aquatic-phase amphibians confirm that exposure to atrazine 

does not cause direct mortality to frogs and salamanders at concentrations ranging from 

approximately 200 to 2,000 μg/L; these concentrations represent the highest tested 

atrazine treatment levels within each of the studies. Salamander-specific chronic 

mortality data are available for the spotted salamander (Ambystoma maculatum), smallmouthed salamander (A. texanum), streamside salamander (A. barbouri), and the longtoed salamander (A. macrodactylum). The available salamander data show no effect to 

mortality at the highest treatment concentrations of atrazine in each of the respective 

studies, ranging from approximately 200 to 400 μg/L (Boone and James, 2003; Rohr et 

al., 2003, Forson and Storfer, 2006). Rohr et al. (2004) reported decreased embryo 

survival through Day 16 in streamside salamanders following exposure to 400 μg/L 

atrazine (NOAEC = 40 μg/L). However, most embryo mortality was associated with a 

white film covering the embryo, suggesting the presence of a fungal pathogen, which 

may have decreased survival. According to the study authors, it is unknown whether the 

fungi caused or simply followed mortality. In addition, reduced survival was reported in 

only one of the two years tested; therefore, there is a high degree of uncertainty 

associated with the reported results. 

4.1.2.2 Amphibians: Open Literature Data on Sublethal Effects 

An evaluation and review of the results of submitted studies regarding potential atrazine 

effects data on amphibian gonadal development are presented in the Agency’s White 

Paper (U.S. EPA, 2003d) and discussed in Section A.2.4c of Appendix A. As previously 

discussed in Section 2.3, the Agency has concluded that there is currently insufficient 

evidence to confirm or refute the hypothesis that atrazine exposure may impact gonadal 

development in amphibians. Therefore, the Agency has requested additional data from 

the registrant to reduce uncertainties regarding potential risk to amphibians. In addition 

to addressing uncertainty regarding the potential of atrazine to cause these effects, these 

studies are expected to characterize the nature of any potential dose-response 

69
 
relationship. The initial results of these amphibian studies are expected to become 

available in late 2006 to early 2007; therefore, as of this writing, they are not available for 

inclusion in this endangered species risk assessment for the Barton Springs salamander. 

Open literature data on sublethal effects of atrazine to aquatic-phase amphibians, 

including frogs and salamanders, are summarized below and discussed in greater detail in 

Section A.2.4d of Appendix A. The following information includes studies identified as 

part of the 2006 open literature search that were not reviewed as part of the Agency’s 

2003 White Paper (U.S. EPA, 2003d). 

 Frogs (Anurans) 

The reviewed studies were classified as qualitative because they address issues of 

concern to the risk assessment, but are not appropriate for quantitative use due to 

uncertainties related to a lack of raw data and limitations in the study design. Further 

information on the study design and uncertainties associated with each of the reviewed 

studies are provided in Section A.2.4d and Table A-16 of Appendix A. In summary, the 

microcosm/mesocosm and chronic lab data for frogs indicate that sublethal effects to 

amphibians, such as reduced mass and length at metamorphosis may occur at atrazine 

exposure concentrations of approximately 200 μg/L and higher under the conditions 

tested (Diana et al., 2000; Boone and James, 2003; and Gucciardo, 1999). Decreased 

frog weight (and length) at metamorphosis at > 200 μg/L atrazine is hypothesized to 

result from atrazine’s effect on algal populations, which are a primary source of food for 

developing anurans (Diana et al., 2000). Other factors, such as decreasing DO, pH, and 

macrophyte biomass following atrazine exposure may also contribute to observed 

sublethal effects. 

In the lab, plasma testosterone was reduced in male frogs at atrazine concentrations of 

259 μg/L; however, an increase in aromatase activity (aromatase increases synthesis of 

17β-estradiol resulting in depletion of testosterone levels) was not observed (Hecker et 

al., 2005). Therefore, the mechanism associated with decreased testosterone levels in 

adult males is unclear. 

The observed effect levels of ~200 μg/L are greater than the aquatic community-level 

effects of 10-20 μg/L documented in the January 2003 atrazine IRED (U.S. EPA, 2003a). 

In addition, uncertainties and associated limitations in the design of the reviewed studies 

are similar to the conclusions previously reported (U.S. EPA, 2003d). 

 Salamanders (Caudates) 

The reviewed sublethal studies contain variable results with respect to atrazine exposures 

and sublethal effects to aquatic-phase salamanders. Two chronic studies on the 

streamside salamander (A. barbouri) and long-toed salamander (A. macrodactylum) show 

reduced mass and snout-vent length (SVL) at metamorphosis, in addition to accelerated 

metamorphosis, relative to controls, at atrazine concentrations ranging from 184 to 400 

μg/L (Rohr et al., 2004; Forson and Storfer, 2006). The NOAEC values for these studies 

70
 
range between 18.4 and 40 μg/L. In another study, the time to metamorphosis was 

increased in small-mouthed salamanders (A. texanum) at the only concentration of 

atrazine tested (197 μg/L); however, no effect on the time to metamorphosis was 

observed in spotted salamanders at the same concentration of atrazine (Boone and James, 

2003). 

The interaction of atrazine and one of the iridoviruses (the Ambystoma tigrinum virus 

[ATV]) was studied in long-toed salamanders by Forson and Storfer (2006). Larvae 

exposed to both atrazine and ATV had lower levels of mortality and ATV infectivity 

compared to larvae exposed to virus alone, suggesting that atrazine may compromise 

virus efficacy or improve salamander immune competency. Behavioral changes in 

locomotion (i.e., increased activity following tapping on tanks) were observed in 

streamside salamanders exposed to 400 μg/L; however, this endpoint is not relevant to 

the assessment endpoints chosen for this risk assessment. It is unclear how increased 

larval salamander activity due to tank tapping in the lab would translate into reduced 

fitness in the wild. 

All of the reviewed salamander studies from the open literature were classified as 

qualitative because they address issues of concern to the risk assessment, but are not 

appropriate for quantitative use due to uncertainties related to a lack of raw data and 

limitations in the study design. Further information on the study design and uncertainties 

associated with each of the reviewed studies is provided in Section A.2.4d and Table A17 of Appendix A. 

4.1.3 Toxicity to Freshwater Invertebrates 

Freshwater aquatic invertebrate toxicity data were used to assess potential indirect effects 

of atrazine to the Barton Springs salamander. Direct effects to freshwater invertebrates 

resulting from exposure to atrazine may indirectly affect the Barton Springs salamander 

via reduction in available food. As discussed in Section D.5.1 of Appendix D, Barton 

Springs salamanders feed on a wide range of freshwater aquatic invertebrates including 

ostracods, copepods, chironomids, snails, amphipods, mayfly larvae, leeches, and adult 

riffle beetles. Based on analysis of the stomach and fecal samples from a limited number 

of adult and juvenile Barton Springs salamanders, the most prevalent organisms found 

were ostracods, amphipods, and chironomids (USFWS, 2005). However, data on the 

relative percentage of each type of aquatic invertebrate in the salamander’s diet are not 

available. 

A summary of acute and chronic freshwater invertebrate data, including published data in 

the open literature since completion of the IRED (U.S. EPA, 2003a), is provided below in 

Sections 4.1.3.1 through 4.1.3.3. 

4.1.3.1 Freshwater Invertebrates: Acute Exposure Studies 

Atrazine is classified as highly toxic to slightly toxic to aquatic invertebrates. There is a 

wide range of EC50/LC50 values for freshwater invertebrates with values ranging from 

71
 
720 to >33,000 μg/L. The freshwater LC50 value of 720 μg/L is based on an acute 48­

hour static toxicity test for the midge, Chironomus tentans (MRID # 000243-77). Further 

evaluation of the available acute toxicity data for the midge shows high variability with 

the LC50 values, ranging from 720 to >33,000 μg/L. With the exception of the midge, 

reported acute toxicity values for the other five freshwater invertebrates (including the 

water flea, scud, stonefly, leech, and snail) are 3,500 μg/L and higher. All of the 

available acute toxicity data for freshwater invertebrates are provided in Section A.2.5 

and Table A-18 of Appendix A. The LC50/EC50 distribution for freshwater invertebrates 

is graphically represented in Figure 4.1. The columns represent the lowest reported value 

for each species, and the positive y error bar represents the maximum reported value. 

Values in parentheses represent the number of studies included in the analyses. 

Summary of Reported Acute LC50/EC50 Values in Freshwater Invertebrates 

for Atrazine 

0 

5000 

10000 

15000 

20000 

25000 

30000 

35000 

Midge (3) Waterflea (5) Scud (3) Stonefly (1) Leech (1) Snail (1) 

Species 

LC50/E

C50

(pp

b) Figure 4.1. Summary of Reported Acute LC50/EC50 Values in Freshwater 
 

Invertebrates for Atrazine 
 

4.1.3.2 Freshwater Invertebrates: Chronic Exposure Studies 

The most sensitive chronic endpoint for freshwater invertebrates is based on a 30-day 

flow-through study on the scud (Gammarus fasciatus), which showed a 25% reduction in 

the development of F1 to the seventh instar at atrazine concentrations of 140 μg/L; the 

corresponding NOAEC is 60 μg/L (MRID # 000243-77). Although the acute toxicity 

data for atrazine show that the midge (Chironomus tentans) is the most sensitive 

freshwater invertebrate, available chronic midge toxicity data indicate that it is less 

sensitive to atrazine, on a chronic exposure basis, than the scud, with respective LOAEC 

and NOAEC values of 230 μg/L and 110 μg/L. Additional information on the chronic 

toxicity of atrazine to freshwater invertebrates is provided in Section A.2.6 and Table A20 of Appendix A. 

72
 
4.1.3.3 Freshwater Invertebrates: Open Literature Data 

One additional acute study for an underrepresented taxa of freshwater mussels was 

located in the open literature. The results of the study by Johnson et al. (1993) suggest 

that 48-hour exposures at atrazine concentrations up to 60 mg/L do not affect the survival 

of juvenile and mature freshwater mussels, Anodonta imbecilis; therefore, A. imbecilis is 

less acutely sensitive to atrazine than other freshwater invertebrates. 

4.1.4 Toxicity to Aquatic Plants 

Aquatic plant toxicity studies were used as one of the measures of effect to evaluate 

whether atrazine may affect primary production. In Barton Springs, primary productivity 

is essential for indirectly supporting the growth and abundance of the Barton Springs 

salamander. In addition to providing cover, moss and other aquatic plants harbor a 

variety of aquatic invertebrates that salamanders eat. 

Two types of studies were used to evaluate the potential of atrazine to affect primary 

productivity. Laboratory studies were used to determine whether atrazine may cause 

direct effects to aquatic plants. In addition, the threshold concentrations, described in 

Section 4.2, were used to further characterize potential community level effects to Barton 

Springs salamanders resulting from potential effects to aquatic plants. A summary of the 

laboratory data for aquatic plants is provided in Section 4.1.4.1. A description of the 

threshold concentrations used to evaluate community-level effects is included in Section 

4.2. 

4.1.4.1 Aquatic Plants: Laboratory Data 

Numerous aquatic plant toxicity studies have been submitted to the Agency. A summary 

of the data for freshwater vascular and non-vascular plants is provided below. Section 

A.4.2 and Tables A-40 and A-41 of Appendix A include a more comprehensive 

description of these data. 

The Tier II results for freshwater aquatic plants indicate that atrazine causes a 41 to 98% 

reduction in chlorophyll production of freshwater algae; the corresponding EC50 value for 

four different species of freshwater algae is 1 μg/L, based on data from a 7-day acute 

study (MRID # 000235-44). Vascular plants are less sensitive to atrazine than 

freshwater non-vascular plants with an EC50 value of 37 μg/L, based on reduction in 

duckweed growth (MRID # 430748-04). 

Comparison of atrazine toxicity levels for three different algae endpoints suggests that the 

endpoints in decreasing order of sensitivity are cell count, growth rate and oxygen 

production (Stratton, 1984). Walsh (1983) exposed Skeletonema costatum to atrazine and 

concluded that atrazine is only slightly algicidal at relatively high concentrations (i.e., 

500 & 1,000 μg/L). Caux et al. (1996) compared the cell count IC50 and fluorescence 

LC50 and concluded that atrazine is algicidal at concentrations affecting cell counts. 

73
 
Abou-Waly et al. (1991) measured growth rates on days 3, 5, and 7 for two algal species. 

The pattern of atrazine effects on growth rates differs sharply between the two species. 

Atrazine had a strong early effect on Anabaena flos-aquae followed by rapid recovery in 

clean water (i.e., EC50 values for days 3, 5, and 7 are 58, 469, and 766 μg/L, 

respectively). The EC50 values for Selenastrum capricornutum continued to decline from 

day 3 through 7 (i.e., 283, 218, and 214 μg/L, respectively). Based on theses results, it 

appears that the timing of peak effects for atrazine may differ depending on the test 

species. 

It should be noted that rrecovery from the effects of atrazine and the development of 

resistance to the effects of atrazine in some vascular and non-vascular aquatic plants has 

been reported and may add uncertainty to these findings. However, reports of recovery 

are often based on differing interpretations of recovery. Thus, before recovery can be 

considered as an uncertainty, an agreed upon interpretation is needed. The Agency 

believes that recovery has a simple and straight forward interpretation: a return to preexposure levels for the affected population, not for a replacement population of more 

tolerant species. Further research is needed to quantify the impact that recovery and 

resistance would have on aquatic plants. 

4.1.5 Freshwater Field Studies 

Microcosm and mesocosm studies with atrazine provide measurements of primary 

productivity that incorporate the aggregate responses of multiple species in aquatic plant 

communities. Because plant species vary widely in their sensitivity to atrazine, the 

overall response of the plant community may be different from the responses of the 

individual species measured in laboratory toxicity tests. Mesocosm and microcosm 

studies allow observation of population and community recovery from atrazine effects 

and of indirect effects on higher trophic levels. In addition, mesocosm and microcosm 

studies, especially those conducted in outdoor systems, incorporate partitioning, 

degradation, and dissipation, factors that are not usually accounted for in laboratory 

toxicity studies, but that may influence the magnitude of ecological effects. 

Atrazine has been the subject of many mesocosm and microcosm studies in ponds, 

streams, lakes, and wetlands. The durations of these studies have ranged from a few 

weeks to several years at exposure concentrations ranging from 0.1 μg/L to 10,000 μg/L. 

Most of the studies have focused on atrazine effects on phytoplankton, periphyton, and 

macrophytes; however, some have also included measurements on animals. 

As described in the 2003 IRED for atrazine (U.S. EPA, 2003a), potential adverse effects 

on sensitive aquatic plants and non-target aquatic organisms including their populations 

and communities are likely to be greatest when atrazine concentrations in water equal or 

exceed approximately 10 to 20 μg/L on a recurrent basis or over a prolonged period of 

time. A summary of all the freshwater aquatic microcosm, mesocosm, and field studies 

that were reviewed as part of the 2003 IRED is included in Section A.2.8a and Tables A22 through A-24 of Appendix A. Given the large amount of microcosm and mesocosm 

and field study data for atrazine, only effects data less than or more conservative than the 

74
 
10 μg/L aquatic community effect level identified in the 2003 IRED were summarized 

from the open literature search that was completed in February 2006. Field study data for 

aquatic-phase amphibians, including frogs and salamanders, are summarized in Section 

4.1.2 and discussed in greater detail in Sections A.2.3 and A.2.4 of Appendix A. Based 

on the selection criteria for review of new open literature, all of the available studies 

show effects levels to freshwater fish, invertebrates, and aquatic plants at concentrations 

greater than 10 μg/L. 

Community-level effects to aquatic plants that are likely to result in indirect effects to the 

rest of the aquatic community, including the Barton Springs salamanders, are evaluated 

based on threshold concentrations. These screening threshold concentrations, which are 

discussed in greater detail in Section 4.2 and Appendix B, incorporate the available 

micro- and mesocosm data included in the 2003 IRED, as well as additional information 

gathered following completion of the 2003 atrazine IRED (U.S. EPA, 2003b and 2003e). 

4.2 Community-Level Endpoints: Threshold Concentrations 

In this endangered species assessment, direct and indirect effects to the Barton Springs 

salamander are evaluated in accordance with the screening-level methodology described 

in the Agency’s Overview Document (U.S. EPA, 2004). If aquatic plant RQs exceed the 

Agency’s non-listed species LOC (because the salamander does not have an obligate 

relationship with any one particular plant species, but rather relies on multiple plant 

species), based on available EC50 data for vascular and non-vascular plants, risks to 

individual aquatic plants are assumed. 

It should be noted, however, that the indirect effects analysis in this assessment is unique, 

in that the best available information for atrazine-related effects on aquatic communities 

is significantly more extensive than for other pesticides. Hence, atrazine effects 

determinations can utilize more refined data than is generally available to the Agency. 

Specifically, a robust set of microcosm and mesocosm data and aquatic ecosystem 

models are available for atrazine that allowed EPA to refine the indirect effects 

associated with potential aquatic community-level effects (via aquatic plant community 

structural change and subsequent habitat modification) to the Barton Springs salamander. 

Use of such information is consistent with the guidance provided 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). This information, which represents the best scientific data available, 

is described in further detail below and in Appendix B. 

As previously mentioned in Section 2.3, the Agency has selected an atrazine level of 

concern (LOC) in the 2003 IRED (U.S. EPA, 2003a and b) that is consistent with the 

approach described in the Office of Water’s (OW) draft atrazine aquatic life criteria (U.S. 

EPA, 2003c). Through these previous analyses (U.S. EPA, 2003a, b, and c), which 

reflect the current best available information, predicted or monitored aqueous atrazine 

concentrations can be interpreted to determine if a water body is likely to be significantly 

75
 
affected via indirect effects to the aquatic community. Potential impacts of atrazine to 

plant community structure and function that are likely to result in indirect effects to the 

rest of the aquatic community, including the Barton Springs salamander, are evaluated as 

described below. 

As described further in Appendix B, responses in microcosms and mesocosms exposed to 

atrazine were evaluated to differentiate no or slight, recoverable effects from significant, 

generally non-recoverable effects (U.S. EPA, 2003e). Because effects varied with 

exposure duration and magnitude, there was a need for methods to predict relative 

differences in effects for different types of exposures. The Comprehensive Aquatic 

Systems Model (CASM) (Bartell et al., 2000; Bartell et al., 1999; DeAngelis et al., 1989) 

was selected as an appropriate tool to predict these relative effects, and was configured to 

provide a simulation for the entire growing season of a 2nd and 3rd order Midwestern 

stream as a function of atrazine exposure. CASM simulations conducted for the 

concentration/duration exposure profiles of the micro- and mesocosm data showed that 

CASM seasonal output, represented as an aquatic plant community similarity index, 

correlated with the micro- and mesocosm effect scores, and that a 5% change in this 

index reasonably discriminated micro- and mesocosm responses with slight versus 

significant effects. The CASM-based index was assumed to be applicable to more 

diverse exposure conditions beyond those present in the micro- and mesocosm studies. 

To avoid having to routinely run the CASM model, simulations were conducted for a 

variety of actual and synthetic atrazine chemographs to determine 14-, 30-, 60-, and 90­

day average concentrations that discriminated among exposures that were unlikely to 

exceed the CASM-based index (i.e., 5% change in the index). It should be noted that the 

average 14-, 30-, 60-, and 90-day concentrations were originally intended to be used as 

screening values to trigger a CASM run (which is used as a tool to identify the 5% index 

change LOC), rather than actual thresholds to be used as an LOC (U.S. EPA, 2003e). 

The following threshold concentrations for atrazine were identified (U.S. EPA, 2003e): 

• 14-day average = 38 μg/L 

• 30-day average = 27 μg/L 

• 60-day average = 18 μg/L 

• 90-day average = 12 μg/L 

Effects of atrazine on aquatic plant communities that have the potential to subsequently 

pose indirect effects to the Barton Springs salamander are best addressed using the robust 

set of micro- and mesocosm studies available for atrazine and the associated risk 

estimation techniques (U.S. EPA, 2003a, b, c, and e). The 14-, 30-, 60-, and 90-day 

threshold concentrations developed by EPA (U.S. EPA 2003e) are used to evaluate 

potential indirect effects to aquatic communities for the purposes of this endangered 

species assessment. Use of these threshold concentrations is considered appropriate 

because: (1) the CASM-based index meets the goals of the defined assessment endpoints 

for this assessment; (2) the threshold concentrations provide a reasonable surrogate for 

the CASM index; and (3) the additional conservatism built into the threshold 

concentration, relative to the CASM-based index, is appropriate for an endangered 

76
 
species risk assessment (i.e., the threshold concentrations were set to be conservative, 

producing a low level (1%) of false negatives relative to false positives). Therefore, these 

threshold concentrations are used to identify potential indirect effects (via aquatic plant 

community structural change) to the Barton Springs salamander. If modeled atrazine 

EECs exceed the 14-, 30-, 60- and 90-day threshold concentrations following refinements 

of potential atrazine concentrations with available monitoring data, the CASM model 

could be employed to further characterize the potential for indirect effects. A step-wise 

data evaluation scheme incorporating the use of the screening threshold concentrations is 

provided in Figure 4.2. Further information on threshold concentrations is provided in 

Appendix B. 

Figure 4.2. Use of Threshold Concentrations in Endangered Species Assessment 

77
 
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 atrazine 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 (i.e., 

freshwater fish used as a surrogate for aquatic-phase amphibians and freshwater 

invertebrates). 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. The upper and lower bounds of the effects probability are based 

on available information on the 95% confidence interval of the slope. A statement 

regarding the confidence in the estimated event probabilities is also included. Studies 

with good probit fit characteristics (i.e., statistically appropriate for the data set) are 

associated with a high degree of confidence. Conversely, a low degree of confidence is 

associated with data from studies that do not statistically support a probit dose response 

relationship. In addition, confidence in the data set may be reduced by high variance in 

the slope (i.e., large 95% confidence intervals), despite good probit fit characteristics. 

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 number of incidents have been reported in which atrazine has been associated with 

some type of environmental effect, with variable levels of certainty that atrazine caused 

the effects, ranging from unlikely to highly probable. As of the writing of the 2003 IRED 

(U.S. EPA, 2003a), 109 incidents were listed in the Ecological Incident Information 

System (EIIS) files under atrazine: 4 cases were listed as highly probable, 40 as probable, 

50 as possible, 13 as unlikely, and 2 as unrelated. Atrazine alone is not very toxic to the 

birds, mammals, and aquatic animals cited in most of these incidents. In none of these 

cases has evidence been provided that firmly demonstrates that atrazine has produced the 

reported effects. Atrazine residues in fish tissue were measured in only one incident 

78
 
reported as a fish kill (# I004021-004); however, many chemicals were identified and 

high profenofos levels were found. Therefore, the organophosphate was determined to be 

responsible for the large fish kill. In many cases, the inference of these reported incidents 

to atrazine is likely due to the widespread use of atrazine and the proximity of the 

atrazine application and timing to the occurrence of the incident. 

Between October 26, 2000 and June 9, 2006, 8 incidents were listed in the EIIS involving 

the use of atrazine: 6 cases are listed as possible and 2 are listed as unlikely. The effects 

of these incidents ranged from major fish kills to minor burning of garden plants adjacent 

to a field treated with atrazine. Of these incidents, 5 were caused by drift, 1 by runoff 

and 2 because of misuse. 

Of the 6 cases that were listed as “Possible,” all were terrestrial and, therefore, not 

relevant to this assessment. In the two cases listed as “Unlikely,” one resulted in the 

death of 50-60 bass, 2,000 crappie and 300-400 bluegills (IN: #I013987-001). Three 

chemicals, including terbufos, atrazine and acetochlor, were used in a product suspected 

to be present in the runoff. Tests were conducted in the two affected ponds and terbufos 

was the only chemical listed as being detected in both. It is not clear if atrazine and 

acetochlor were measured in the pond water analysis. However, it is likely that terbufos 

was responsible for the fish kill because it has a greater lethality to fish than atrazine and 

acetochlor. 

One of the two reported “misuse” incidents caused substantial damage to aquatic animals 

(TN: # I016990-001) resulting in the death of 2,000 bluegill sunfish, 400 catfish, and a 

snake. This incident was credited to the dumping of 4 to 5 gallons of a product suspected 

of containing atrazine into a one half acre pond. 

Based on the available incident information, supporting data is not available to clearly 

demonstrate that atrazine is the cause of the observed aquatic effects (i.e., death to fish). 

In addition, the best available toxicity information shows that atrazine is not directly toxic 

to freshwater fish (and aquatic-phase amphibians) at environmentally relevant 

concentrations (see Sections 4.1.1 and 4.1.2). Further information on the atrazine 

incidents reported in the 2003 IRED (U.S. EPA, 2003a) and a summary of uncertainties 

associated with all reported incidents are provided in Appendix F. 

5. Risk Characterization 

Risk characterization is the integration of the exposure and effects characterizations to 

determine the potential ecological risk from varying atrazine use scenarios within the 

action area and likelihood of direct and indirect effects on the Barton Springs salamander. 

The risk characterization provides an estimation and a description of the likelihood of 

adverse effects; articulates risk assessment assumptions, limitations, and uncertainties; 

and synthesizes an overall conclusion regarding the effects determination (i.e., “no 

effect,” “likely to adversely affect,” or “may affect, but not likely to adversely affect”) for 

the Barton Springs salamander. 

79
 
5.1 Risk Estimation 

Risk was estimated by calculating the ratio of the estimated environmental concentrations 

(EECs; see Table 3.5) and the appropriate toxicity endpoint (see Table 4.1). 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 G). Screening-level 

RQs are based on the most sensitive endpoints and modeled surface water concentrations 

from the following scenarios for atrazine: 

•	 residential granular use @ 2 lb ai/A; 2 applications with 30 days between 

applications (assumes 1% over-application of atrazine granules to impervious 

surfaces) 

•	 residential liquid use @ 1 lb ai/A; 2 applications with 30 days between 
 

applications (assumes 1% over-spray of atrazine to impervious surfaces) 
 

•	 turf granular use @ 2 lb ai/A; 2 applications with 30 days between applications 

•	 turf liquid use @ 1 lb ai/A; 2 applications with 30 days between applications 

•	 rights-of-way liquid use @ 1 lb ai/A; 1 application (assumes 1% over-spray of 

atrazine to impervious surfaces) 

•	 fallow/idle land use @ 2.25 lb ai/A; 1 application 

In cases where the screening-level RQ exceeds one or more LOCs, additional factors, 

including Barton Springs salamander life history characteristics, refinement of the EECs 

using available monitoring data, and consideration of community-level threshold 

concentrations, are considered and used to characterize the potential for atrazine to affect 

the Barton Springs salamander. Risk estimations of direct and indirect effects of atrazine 

to the Barton Springs salamander are provided in Sections 5.1.1 and 5.1.2, respectively. 

As previously discussed in the effects assessment, the toxicity of the atrazine degradates, 

including HA, DEA, DIA, and DACT, is assumed to be less than the parent compound; 

therefore, RQ values were not derived for the degradates. 

5.1.1 Direct Effects 

Direct effects associated with acute and chronic exposure to atrazine in Barton Springs 

are not expected to occur for the Barton Springs salamander. Risk quotients used to 

estimate direct effects to the Barton Springs salamander are provided in Table 5.1 below. 

Risk quotients were calculated only for the use that resulted in the highest EEC (granular 

residential use) because none of the acute or chronic LOCs were exceeded. These risk 

quotients are further characterized in Section 5.2.1. 

80
 
Table 5.1. Summary of Direct Effect RQs for the Barton Springs Salamander 

Effect to 

Barton 

Springs 

Salamander 

Surrogate 

Species 

Toxicity 

Value (μg/L) 

EEC (μg/L) RQ Probability of 

Individual 

Effect 

LOC 

Exceedance 

and Risk 

Interpretation 

Acute Direct 

Toxicity 

Rainbow 

trout 

LC50 = 5,300 Peak: 41.2 0.008 1 in 1.7E+08 

(1 in 1,870 to 1 

in 5.82E+15)

a 

Nob 

Chronic Direct 

Toxicity 

Brook trout NOAEC = 65 60-day: 1.2 0.018 Not calculated 

for chronic 

endpoints 

Nob 

a

 Based on a probit slope of 2.72 with 95% confidence intervals of 1.56 and 3.89 (MRID# 000247-16). b

 RQ < acute endangered species LOC of 0.05. 

5.1.2 Indirect Effects 

Pesticides have the potential to exert indirect effects upon listed species by inducing 

changes in structural or functional characteristics of affected communities. Perturbation 

of forage or prey availability and alteration of the extent and nature of habitat are 

examples of indirect effects. 

In conducting a screen for indirect effects, direct effects LOCs for each taxonomic group 

(i.e., freshwater fish, invertebrates, and aquatic plants) are employed to make inferences 

concerning the potential for indirect effects upon listed species that rely upon non-listed 

organisms in these taxonomic groups as resources critical to their life cycle (U.S. EPA, 

2004). This approach used to evaluate indirect effects to listed species is endorsed by the 

Services (USFWS/NMFS, 2004b). If no direct effect listed species LOCs are exceeded 

for non-endangered organisms that are critical to the Barton Springs salamander’s life 

cycle, the concern for indirect effects to the Barton Springs salamander is expected to be 

minimal. 

If LOCs are exceeded for freshwater invertebrates that are prey items of the Barton 

Springs salamander, there is a potential for atrazine to indirectly affect the salamander by 

reducing available food supply. In such cases, the dose response relationship from the 

toxicity study used for calculating the RQ of the surrogate prey item is analyzed to 

estimate the probability of acute effects associated with an exposure equivalent to the 

EEC. The greater the probability that exposures will produce effects on a taxa, the 

greater the concern for potential indirect effects for listed species dependant upon that 

taxa (U.S. EPA, 2004). 

As an herbicide, indirect effects to the Barton Springs salamander from potential effects 

on primary productivity of aquatic plants are a principle concern. If plant RQs fall 

81
 
between the listed species and non-listed species LOCs, a no effect determination for 

listed species that rely on multiple plant species to successfully complete their life cycle 

(termed plant-dependent species) is concluded. If plant RQs are above non-listed species 

LOCs, this could be indicative of a potential for adverse effects to those listed species 

that rely either on a specific plant species (plant species obligate) or multiple plant 

species (plant-dependent) for some important aspect of their life cycle (U.S. EPA, 2004). 

Based on the information provided in Appendix D, the Barton Springs salamander relies 

on multiple plant species, including aquatic moss, pondweed, arrowhead, water primrose, 

cabomba, and other aquatic plants for cover and as a source of habitat and food for the 

variety and abundance of aquatic invertebrates that salamanders eat. 

In summary, the potential for indirect effects to the Barton Springs salamander was 

evaluated using methods outlined in U.S. EPA (2004) and described below in Sections 

5.1.2.1 and 5.1.2.2, respectively. 

5.1.2.1 Evaluation of Potential Indirect Effects via Reduction in Food Items 

(Freshwater Invertebrates 

Potential indirect effects from direct effects on animal food items (i.e., freshwater 

invertebrates) were evaluated by considering the diet of the Barton Springs salamander 

and the distribution of the sensitivities of the prey organisms to atrazine. Barton Springs 

salamanders feed on a wide range of freshwater aquatic invertebrates including ostracods, 

copepods, chironomids, snails, amphipods, mayfly larvae, leeches, and adult riffle 

beetles. The most prevalent invertebrates found in stomach and fecal samples from a 

limited number of adult and juvenile Barton Springs salamanders were ostracods, 

amphipods, and chironomids (USFWS, 2005). However, data on the relative percentage 

of each type of aquatic invertebrate in the salamander’s diet are not available. The RQs 

used to characterize potential indirect effects to the Barton Springs salamander from 

direct acute and chronic effects on freshwater invertebrate food sources are provided in 

Tables 5.2 and 5.3, respectively. Acute and chronic RQs are based on the most sensitive 

toxicity endpoint for the midge (EC50 = 720 μg/L) and the scud (NOAEC = 60 μg/L), 

respectively. 

82
 
Table 5.2. Summary of RQs Used to Estimate Indirect Effects to the Barton Springs 
 

Salamander via Direct Acute Effects on Dietary Items 
 

Indirect Effect 

to Barton 

Springs 

Salamander 

Surrogate 

Food Item 

/ Toxicity 

Value 

(μg/L) 

Use (appl. 

method; rate; # 

appl.; interval 

between appl.) 

Peak 

EECs 

(μg/L) 

RQ Probability of 

Individual 

Effecta 

LOC 

Exceedance 

and Risk 

Interpretation 

Reduced Food 

Supply via 

Acute Direct 

Toxicity to 

Midge 

EC50 = 720 

Residential 

(granular; 2 lb 

ai/A; 2 appl.; 30 d 

interval) 

41.2 0.057 1 in 4.55E+07 Yesb 

Invertebrates Residential 

(ground liquid; 1 

lb ai/A; 2 appl.; 

30 d interval) 

26.6 0.037 1 in 6.72E+09 Noc 

Turf (granular; 2 

lb ai/A; 2 appl.; 

30 d interval) 

22.4 0.031 1 in 6.29E+10 Noc 

Turf (ground 

liquid; 1 lb ai/A; 

2 appl.; 30 d 

interval) 

16.2 0.023 1 in 3.53E+12 Noc 

Fallow/Idle land 

(aerial liquid; 

2.25 lb ai/A; 1 

appl.) 

7.5 0.010 1 in 1.46E+18 Noc 

Rights-of-Way 

(liquid; 1 lb ai/A; 

1 appl.) 

6.2 0.009 1 in 8.97E+18 Noc 

a

 Slope information on the toxicity study that was used to derive the RQ for freshwater invertebrates is not 

available. Therefore, the probability of an individual effect was calculated using a probit slope of 4.4, 

which is the only technical grade atrazine value, reported in the available freshwater invertebrates studies 

that may serve as food items for the salamander; 95% confidence intervals could not be calculated based on 

the available data (Table A-18; Taylor et al., 1991; MRID# 452029-17). 

b

 RQ > acute listed species LOC of 0.05. Further evaluation of the range of freshwater invertebrate species 

sensitivity to atrazine and dietary requirements of the Barton Springs salamander is completed in Section 

5.2.2. 
 c

 RQ < acute listed species LOC of 0.05. 
 

For freshwater invertebrates, acute RQs exceed the acute risk to the listed species LOC of 

0.05 for the residential granular use (2 lb ai/A) only. Acute RQs for the other modeled 

atrazine uses are less than the listed species LOC. Because the listed species LOC is 

exceeded for the residential granular use, atrazine use related to residential granular 

applications has the potential to indirectly affect the Barton Springs salamander via 

reduction in the availability of sensitive aquatic invertebrate food items. However, this 

analysis was based on the most sensitive aquatic invertebrate endpoint of freshwater 

species tested in laboratory studies and did not consider the range of aquatic invertebrate 

species sensitivity to atrazine or the specific dietary requirements of the Barton Springs 

83
 
salamander. Therefore, additional characterization of the potential for atrazine to affect 

freshwater invertebrate food items of the Barton Springs salamander is presented as part 

of the Risk Description in Section 5.2.2. 

Table 5.3. Summary of RQ and LOC Used to Estimate Indirect Effects to the 
 

Barton Springs Salamander via Direct Chronic Effects on Dietary Items 
 

Indirect Effect 

to Barton 

Springs 

Salamander 

Surrogate 

Food Item 

/ Toxicity 

Value 

(μg/L) 

Use (appl. 

method; rate; # 

appl.; interval 

between appl.) 

21-day 

EECs 

(μg/L) 

RQ LOC Exceedance and Risk 

Interpretation 

Reduced Food 

Supply via 

Chronic Direct 

Toxicity to 

Invertebrates 

Scud 

NOAEC = 

60 

Residential 

(granular; 2 lb 

ai/A; 2 appl.; 30 d 

interval) 

2.5 0.04 Noa 

a

 RQ < chronic risk LOC of 1.0. 

As shown in Table 5.3, the chronic LOC is not exceeded for freshwater invertebrates, 

based on the use that results in the highest EECs (granular residential use). Therefore, 

indirect effects to the Barton Springs salamander based on direct chronic effects to 

dietary items are not expected to occur. 

5.1.2.2 Evaluation of Potential Indirect Effects via Reduction in Habitat 

and/or Primary Productivity (Freshwater Aquatic Plants) 

Potential indirect effects from effects on habitat and/or primary productivity were 

assessed using RQs from freshwater aquatic vascular and non-vascular plant data as a 

screen. If aquatic plant RQs exceed the Agency’s non-listed species LOC (because the 

salamander relies on multiple plant species), potential community level effects are 

evaluated using the threshold concentrations, as described in Section 4.2. Risk quotients 

used to estimate potential indirect effects to the Barton Springs salamander from effects 

on aquatic plants primary productivity are summarized in Table 5.4. 

84
 
Table 5.4. Summary of RQs Used to Estimate Indirect Effects to the Barton Springs 
 

Salamander via Direct Effects on Aquatic Plants 
 

Indirect Effect 

to Barton 

Springs 

Salamander 

Use (appl. method; 

rate; # appl.; interval 

between appl.) 

Peak EECs 

(μg/L) 

Non-vascular 

plant RQ 

(EC50 = 1 

μg/La) 

Vascular 

plant RQ 

(EC50 = 37 

μg/Lb) 

LOC Exceedance 

and Risk 

Interpretation 

Reduced 

Habitat and/or 

Primary 

Residential (granular; 2 

lb ai/A; 2 appl.; 30 d 

interval) 

41.2 41.2 1.11 Yesc 

Productivity via 

Direct Toxicity 

to Aquatic 

Plants 

Residential (ground 

liquid; 1 lb ai/A; 2 

appl.; 30 d interval) 

26.6 26.6 0.72 Yesd 

Turf (granular; 2 lb 

ai/A; 2 appl.; 30 d 

interval) 

22.4 22.4 0.61 Yesd 

Turf (ground liquid; 1 

lb ai/A; 2 appl.; 30 d 

interval) 

16.2 16.2 0.44 Yesd 

Fallow/Idle land (aerial 

liquid; 2.25 lb ai/A; 1 

appl.) 

7.5 7.5 0.20 Yesd 

Rights-of-Way 

(liquid; 1 lb ai/A; 1 

appl.) 

6.2 6.2 0.17 Yesd 

a Based on 1-week EC50 value of 1 μg/L for four species of freshwater algae (MRID # 000235-44). 

b Based on 14-day EC50 value of 37 μg/L for duckweed (MRID # 430748-08). c RQ > non-listed species LOC of 1.0 for both non-vascular and vascular plants. Direct effects to nonvascular and vascular aquatic plants are possible. Further evaluation of the EECs relative to the threshold 

concentrations (for community-level effects) is necessary. 

d RQ > non-listed aquatic plant species LOC of 1.0 for non-vascular plants; RQ < non-listed plant species 

LOC of 1.0 for vascular plant. Direct effects to non-vascular aquatic plants are possible. Further 

evaluation of the EECs relative to the threshold concentrations (for community-level effects) is necessary. 

Based on the results shown in Table 5.4, LOCs (RQ > 1.0) for direct effects to aquatic 

non-vascular plants are exceeded for all modeled atrazine use scenarios; LOCs for direct 

effects to aquatic vascular plants are exceeded only for the residential granular use 

scenario. Therefore, atrazine has the potential to indirectly affect the Barton Springs 

salamander via direct effects on both non-vascular and vascular aquatic plants for the 

residential granular scenario, and via direct effects on non-vascular aquatic plants for all 

modeled use scenarios. However, this screening-level analysis was based on the most 

sensitive EC50 value from all of the available freshwater aquatic plant toxicity 

information. No known obligate relationship exists between the Barton Springs 

salamander and any single freshwater plant species; therefore, listed species RQs using 

the NOAEC/EC05 values for aquatic plants were not derived. Further analysis of the 

time-weighted EECs relative to their respective threshold concentrations is necessary to 

determine whether effects to individual plant species would likely result in communitylevel effects. This analysis is presented as part of the Risk Description in Section 5.2.3. 

85
 
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 Barton Springs salamander. 

If the RQs presented in the Risk Estimation (Section 5.1) show no indirect effects and 

LOCs for the Barton Springs salamander are not exceeded for direct effects, a “no 

effect” determination is made, based on atrazine’s use within the action area. If, 

however, indirect effects are anticipated and/or exposure exceeds the LOCs for direct 

effects, the Agency concludes a preliminary “may affect” determination for the Barton 

Springs salamander. 

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 Barton Spring salamander and 

potential community-level effects to aquatic plants. 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 Barton Springs salamander. 

The criteria used to make determinations that the effects of an action are “not likely to 

adversely affect” the Barton Springs salamander 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. For example, use of dose-response 

information to estimate the likelihood of effects can inform the evaluation 

of some discountable effects. 

•	 Adverse Nature of Effect: Effects that are wholly beneficial without any 

adverse effects are not considered adverse. 

86
 
A description of the risk and effects determination for each of the established assessment 

endpoints for the Barton Springs salamander is provided in Sections 5.2.1 through 5.2.3. 

5.2.1 Direct Effects to the Barton Springs Salamander 

Respective acute and chronic RQs of 0.008 and 0.018 (based on the modeled EECs from 

the residential granular scenario assuming 1% overspray and 30% impervious surfaces) 

are well below the Agency’s acute and chronic risk LOCs for all modeled uses of atrazine 

within the action area. Using an upper bound assumption of residential granular use 

EECs, based on 10% overspray and 10% impervious surfaces (peak EEC = 67.1 μg/L and 

60-day EEC = 2.0 μg/L; see Table 3.9), also results in respective acute and chronic RQs 

of 0.013 and 0.03 that are less than the Agency’s LOCs. As previously discussed, direct 

effects to the Barton Springs salamander were based on freshwater fish data, which are 

used as a surrogate for aquatic-phase amphibians. 

The probability of an individual event to the Barton Springs salamander was calculated 

for the acute RQ of 0.008, based on the dose response curve slope from the acute toxicity 

study for the rainbow trout of 2.72 (MRID # 000247-16). The corresponding estimated 

chance of an individual acute mortality to the Barton Springs salamander at an RQ level 

of 0.008 (based on the acute toxic endpoint for surrogate freshwater fish) is 1 in 170 

million. It is recognized that extrapolation of very low probability events is associated 

with considerable uncertainty in the resulting estimates. In order to explore the possible 

bounds to such estimates, the upper and lower default values for the rainbow trout dose 

response curve slope estimate (95% C.I.: 1.56 to 3.89) were used to calculate upper and 

lower estimates of the effects probability associated with the acute RQ. The respective 

lower and upper effects probability estimates are 1 in 1,870 (0.05%) and 1 in 5.82E+15 

(~1.7E-14%). Given the low probability of an individual mortality occurrence and acute 

and chronic RQs that are well below LOCs, atrazine is not likely to cause direct adverse 

effects to the Barton Springs salamander. 

Further lines-of-evidence that atrazine is unlikely to cause direct adverse effects to the 

Barton Springs salamander are provided by the information in the open literature. As 

previously discussed, the Agency has concluded that there is currently insufficient 

evidence to confirm or refute the hypothesis that atrazine exposure may impact gonadal 

development in amphibians (U.S. EPA, 2003d). Further examination of the available 

open literature data for aquatic-phase amphibians (discussed in Section 4.1.2) shows that 

exposure to atrazine does not cause direct acute and/or chronic mortality at 

environmentally relevant concentrations similar to the upper bounds of the modeled 

EECs. Reported sublethal effects to aquatic-phase amphibians show reduced weight and 

length at metamorphosis for frogs and salamanders at atrazine exposure concentrations of 

approximately 200 and accelerated metamorphosis in salamanders at concentrations of 

approximately 184 μg/L; however, no effects to growth or time to metamorphosis have 

been reported at concentrations of < 68 μg/L, similar to the upper bound of modeled 

EECs for atrazine uses within the action area. Therefore, direct effects to the survival, 

growth, and reproduction of Barton Springs salamanders are unlikely to occur. 

87
 
As discussed in Section 4.1.1.3, several open literature studies raise concern about 

sublethal effects of atrazine on endocrine-mediated functions in freshwater fish, which 

are used as a surrogate for aquatic-phase amphibians. However, the significance of these 

effects is difficult to quantify because they are not quantitatively linked to changes in 

survival, growth, and reproduction of individuals (i.e., the assessment endpoints for the 

Barton Springs salamander). In addition, differences in habitat and behavior of the tested 

species compared with the Barton Springs salamander suggest that the results may not be 

relevant to this assessment. Furthermore, there is uncertainty associated with 

extrapolating effects observed in the laboratory to more variable exposures and 

conditions in the field. Further details on potential atrazine-related sublethal effects to 

fish are provided in Appendix A. 

A review of the available aquatic incidents shows that only two incidents involving fish 

kills have been reported from 2000 through 2006. One of the two incidents was reported 

as “unlikely” (#I0139876-001) and the other was reported as a “misuse” (#I013550-003). 

Based on all reported aquatic incidents for atrazine, none were reported in Texas and 

none of the incidents reported effects to aquatic-phase amphibians. Further information 

on all of the reported aquatic incidents for atrazine is provided in Section 4.4 and 

Appendix F. Uncertainties related to the use of incident information from the Ecological 

Incident Information System (EIIS) are also discussed in Appendix F. 

In summary, the Agency concludes a “no effect” determination for direct effects to the 

Barton Springs salamander, via mortality, growth, or fecundity, based on all available 

lines of evidence. 

5.2.2 Indirect Effects via Reduction in Food Items (Freshwater Invertebrates) 

The results of the screening-level risk assessment for the Barton Springs salamander 

suggest the potential for direct acute adverse effects to freshwater invertebrates, based on 

the residential granular use of atrazine at 2 lb ai/A (assuming 1% overspray and 30% 

impervious surfaces). The acute RQ of 0.057 exceeds the listed species of 0.05; 

therefore, atrazine use related to residential granular applications has the potential to 

indirectly affect the Barton Springs salamander via reduction in the availability of 

sensitive aquatic invertebrate food items. 

However, this analysis was based on the lowest LC50 value of 720 μg/L for the midge 

(Chironomus spp.). Consideration of all acute toxicity data for the midge shows a wide 

range of sensitivity within and between species of the same genus (2 orders of 

magnitude) with values ranging from 720 to >33,000 μg/L. Although the midge is a 

component of the Barton Spring salamander’s diet, this species reportedly consumes a 

wide range of freshwater invertebrates that also include ostracods, copepods, snails, 

amphipods, mayfly larvae, leeches, and adult riffle beetles. Available acute toxicity 

values for other freshwater invertebrates that are included in the Barton Spring 

salamander’s diet (i.e., amphipods, leeches, and snails) are 5,700 μg/L and higher. 

88
 
The potential for atrazine to elicit indirect effects to Barton Springs salamanders via 

effects on 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 Barton Springs salamander. 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 Barton Springs salamander. 

Table 5.5 presents acute RQs and the probability of individual effects for dietary items of 

the Barton Springs salamander including midges, amphipods, leeches, and snails. The 

species sensitivity distribution of all acute toxicity data for freshwater aquatic 

invertebrates tested is represented in Figure 4.1. This analysis considers only acute risk 

to aquatic invertebrate food items as chronic risk quotients for invertebrates were less 

than the Agency’s LOC. Even at the upper bound of EECs (21-day EEC of 4.2 μg/L 

from Table 3.9) based on assumptions of 10% overspray and 10% impervious surfaces 

for the residential granular use scenario), the chronic RQ of 0.07 is well below the LOC. 

Table 5.5. Summary of RQs Used to Assess Potential Risk to Freshwater 
 

Invertebrate Food Items of the Barton Springs Salamander 
 

Barton Springs 

Salamander Food 

Item Species 

Acute 

Toxicity 

Value Range 

(μg/L) (No. of 

Studies) 

RQ Range 

(based on 

an EEC of 

40 μg/L) 

Probability of 

Individual 

Effect* 

Risk Interpretation 

Midge 720 - >33,000 

(5) 

<0.01 ­

0.057 

Up to 1 in 

4.55E+07 

Atrazine may affect sensitive food 

items, such as the midge; however 

the low probability of an individual 

effect to the midge is not likely to 

indirectly affect the Barton Springs 

salamander via reduction in midge 

prey items. 

Amphipod 5,700 – 14,900 

(3) 

<0.01 <1 in 

1.46E+18 

Based on low probability of 

individual effects and RQs that are 

well below acute LOCs, atrazine is 

not likely to indirectly affect the 

Barton Springs salamander via 

reduction in amphipod, leech, or 

snail prey items. 

Leech >16,000 (1) 

Snail >16,000 (1) 

*The probability of an individual effect was calculated using a probit slope of 4.4, which is the only 

technical grade atrazine value, reported in the available freshwater invertebrates studies that may serve as 

food items for the salamander; 95% confidence intervals could not be calculated based on the available data 

(Table A-18). 

As shown in Table 5.5, the listed species LOC is exceeded for the midge (RQ = 0.057), 

based on the LC50 value of 720 μg/L. However, acute RQs based on the other acute 

toxicity data for the midge are <0.04, less than the acute risk to endangered species LOC. 

Sufficient dose-response information was not available to allow for an estimation of the 

probability of an individual effect on the midge. Therefore, the probability of an 

individual effect was calculated using a probit dose response curve slope of 4.4; this is 

89
 
the only slope for technical grade atrazine reported in available ecotoxicity data for 

freshwater invertebrates that are a component of the Barton Springs salamander’s diet 

(amphipod; MRID # 452029-17). Based on a probit slope of 4.4, the probability of an 

individual mortality to the midge at an RQ of 0.057 is approximately 1 in 45.5 million 

(2.2E-08%). 

Acute LOCs are not exceeded for the other dietary items of the Barton Springs 

salamander including the amphipod, leech or snail, based on the residential granular use 

EEC (assuming 1% overspray and 30% impervious surfaces). In addition, acute RQs 

based on the upper bound residential granular peak EEC of 67.1 μg/L (assuming 10% 

overspray and 10% impervious surfaces) are also less than acute LOCs for these food 

items in the salamander’s diet. 

Based on the non-selective nature of feeding behavior in the Barton Springs salamander 

and low magnitude of anticipated individual effects to all evaluated prey species, atrazine 

is not likely to indirectly affect the Barton Springs salamander via a reduction in 

freshwater invertebrate food items. This finding is based on insignificance of effects 

(i.e., effects to freshwater invertebrates are not likely to result in “take” of a single Barton 

Springs salamander) and discountability (i.e., the effect to freshwater invertebrates is 

extremely unlikely to occur given the estimated individual event probability of 1 in 45.5 

million). Therefore, the effects determination for the assessment endpoint of indirect 

effects on the Barton Springs salamander via direct effects on prey (i.e., freshwater 

invertebrates) is “may affect, but not likely to adversely affect.” 

5.2.3 Indirect Effects via Reduction in Habitat and/or Primary Productivity 

(Freshwater Aquatic Plants) 

Direct adverse effects to non-vascular aquatic plants are possible, based on all modeled 

atrazine uses within the action area. In addition, direct effects to vascular plants are 

possible, based on the residential granular use of atrazine. Based on these direct effects, 

atrazine may indirectly affect the Barton Springs salamander via direct effects on aquatic 

plants. Therefore, the time-weighted EECs (for 14-day, 30-day, 60-day, and 90-day 

averages) were compared to their respective time-weighted threshold concentrations to 

determine whether potential effects to individual plant species would likely result in 

community level effects. As discussed in Section 4.2, concentrations of atrazine from the 

exposure profile at a particular use site and/or action area that exceed any of the 

following time-weighted threshold concentrations indicate that changes in the aquatic 

plant community structure could be affected: 

• 14-day average = 38 μg/L 

• 30-day average = 27 μg/L 

• 60-day average = 18 μg/L 

• 90-day average = 12 μg/L 

90
 
A comparison of the 14-, 30-, 60-, and 90-day EECs for the Barton Springs salamander 

with the atrazine threshold concentrations representing potential aquatic community-level 

effects is provided in Table 5.6. 

Table 5.6. Summary of Modeled Scenario Time-Weighted EECs with Threshold 
 

Concentrations for Potential Community-Level Effects 
 

Use Scenario 

14-day 30-day 60-day 90-day 

EEC 

(μg/L) 

Threshold 

Conc. 

(μg/L) 

EEC 

(μg/L) 

Threshold 

Conc. 

(μg/L) 

EEC 

(μg/L) 

Threshold 

Conc. 

(μg/L) 

EEC 

(μg/L ) 

Threshold 

Conc. 

(μg/L) 

Res. 

(granular) 

(1% OS; 30% 

IS) / (10% 

OS; 10% IS) 

3.5 / 

6.0 

1.9 / 

3.2 

1.2 / 

2.0 

1.0 / 

1.5 

12 

Res. 

(liquid) 2.5 1.5 1.0 0.8 

Turf 

(granular) 2.0 

38 

1.2 

27 

0.8 

18 

0.7 

Turf 

(liquid) 1.7 1.0 0.7 0.6 

Rights-ofway 

1.1 0.8 0.6 0.6 

Fallow/ 

Idle land 1.0 0.7 0.6 0.6 

OS = overspray 

IS = impervious surfaces 

Based on the results of this comparison, predicted 14-, 30-, 60-, and 90-day EECs for all 

modeled atrazine use scenarios (including the upper bound residential granular use EECs 

assuming 10% overspray and 10% impervious surfaces) are well below the threshold 

concentrations representing community-level effects. Although atrazine use may directly 

affect individual aquatic plants in Barton Springs, its use within the action area is not 

likely to adversely affect the Barton Springs salamander via indirect community-level 

effects to aquatic vegetation. This finding is based on insignificance of effects (i.e., 

community-level effects to aquatic plants are not likely to result in “take” of a single 

Barton Springs salamander) Therefore, the effects determination for the assessment 

endpoint of indirect effects on the Barton Springs salamander via direct effects on habitat 

and/or primary productivity of aquatic plants is “may affect, but not likely to adversely 

affect.” 

91
 
6. Uncertainties 

6.1 Exposure Assessment Uncertainties 

Overall, the uncertainties inherent in the exposure assessment tend to result in overestimation of exposures. This is apparent when comparing modeling results with 

monitoring data. In particular, peak exposures are generally an order of magnitude above 

the highest detection found in any of the four springs. In general, the monitoring data 

should be considered a lower bound on exposure, while modeling represents an upper 

bound. Factors influencing the over-estimation of exposure include the assumption of no 

degradation, dilution, or mixing in the subsurface transport from edge of field to springs. 

The modeling exercise conservatively assumes that the spring and atrazine application 

site are adjacent. In reality, there are likely to be processes at work which cannot be 

accounted for in the modeling that will reduce the predicted exposures. In addition, the 

impact of setbacks on runoff estimates has not been quantified, although these buffers, 

especially those that are well-vegetated, are likely to result in significant reduction in 

runoff loading of atrazine. 

6.1.1 Modeling Assumptions 

Overall, the uncertainties addressed in this assessment cannot be quantitatively 

characterized. However, given the available data and the tendency to rely on 

conservative modeling assumptions, it is expected that the modeling results in an overprediction in exposure. In general, the simplifying assumptions used in this assessment 

appear from the characterization in Section 3.2.7 to be reasonable especially in light of 

the analysis completed and the available monitoring data. There are also a number of 

assumptions that tend to result in exposure over-estimation that cannot be quantified, but 

can be qualitatively described. For instance, modeling for each use site assumes (with the 

exception of the rights-of-way scenario) that the entire 10-hectare watershed is taken up 

by the respective use pattern. The assessment assumes that all applications have occurred 

concurrently on the same day at the exact same application rate. This is unlikely to occur 

in reality, but is a reasonable conservative assumption in lieu of actual data. 

6.1.2 Impact of Vegetative Setbacks on Runoff 

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 of various 

conditions on pesticide loadings becomes available, the aquatic exposure predictions are 

likely to overestimate exposure where healthy vegetative setbacks exist and 

underestimate exposure where poorly developed, channelized, or bare setbacks exist. 

92
 
6.1.3 PRZM Modeling Inputs and Predicted Aquatic Concentrations 

In general, the linked PRZM/EXAMS model produces estimated aquatic concentrations 

that are expected to be exceeded once within a ten-year period. The Pesticide Root Zone 

Model (PRZM) is a process or "simulation" model that calculates what happens to a 

pesticide in a farmer's field on a day-to-day basis. It considers factors such as rainfall and 

plant transpiration of water, as well as how and when the pesticide is applied. It has two 

major components: hydrology and chemical transport. Water movement is simulated by 

the use of generalized soil parameters, including field capacity, wilting point, and 

saturation water content. The chemical transport component simulates pesticide 

application on the soil or on the plant foliage. Dissolved, adsorbed, and vapor-phase 

concentrations in the soil are estimated by simultaneously considering the processes of 

pesticide uptake by plants, surface runoff, erosion, decay, volatilization, foliar wash-off, 

advection, dispersion, and retardation. 

Uncertainty associated with each of these individual components adds 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 open environment 

90 percent of the time. Mobility input values are chosen to be representative of 

conditions in the open 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. 

Additionally, the rate at which atrazine is applied, the percent of a watershed that is 

cropped, and the percent of crops in that watershed that are actually treated with atrazine 

may be lower than the Agency’s default assumptions including use of the maximum 

allowable application rate, treatment of the entire crop, and the estimated area within a 

watershed planted with agricultural crops. The geometry of a watershed and limited 

meteorological data sets also add to the uncertainty of estimated aquatic concentrations. 

6.2 Effects Assessment Uncertainties 

6.2.1 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 pesticidal active 

ingredients, such as atrazine, that act directly (without metabolic transformation) because 

93
 
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 Barton Springs salamander. 

6.2.2 Use of surrogate species effects data 

Guideline toxicity tests are not available for salamanders; therefore, freshwater fish are 

used as surrogate species for aquatic-phase amphibians including salamanders. The 

available open literature information on atrazine toxicity to aquatic-phase amphibians 

shows that acute and chronic ecotoxicity endpoints for aquatic-phase amphibians are 

generally about 3 to 4 times less sensitive than freshwater fish. Therefore, endpoints 

based on freshwater fish ecotoxicity data are assumed to be protective of potential direct 

effects to aquatic-phase salamanders including the Barton Springs salamander, and 

extrapolation of the risk conclusions from the most sensitive tested species to the Barton 

Springs salamander 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. 

6.2.3 Acute freshwater invertebrate toxicity data for the midge 

The initial acute risk estimate for freshwater invertebrates was based on the lowest 

toxicity value from Chironomus studies, which showed a wide range of sensitivity within 

and between species of the same genus (2 orders of magnitude). Therefore, acute RQs 

based on the most sensitive toxicity endpoint for freshwater invertebrates may represent 

an overestimation of potential direct risks to freshwater invertebrates and indirect effects 

to the Barton Springs salamander via a reduction in available food. 

6.2.4 Extrapolation of long-term environmental effects from short-term 

laboratory tests 

The influence of length of exposure and concurrent environmental stressors to the Barton 

Springs salamander (i.e., urban expansion, habitat modification, decreased quantity and 

quality of water in Barton Springs, predators, etc.) will likely affect the species response 

to atrazine. Additional environmental stressors may decrease the Barton Spring 

salamander’s sensitivity to the herbicide, although there is the possibility of 

additive/synergistic reactions. Timing, peak concentration, and duration of exposure are 

critical in terms of evaluating effects, and these factors will vary both temporally and 

spatially within the action area. Overall, the effect of this variability may result in either 

an overestimation or underestimation of risk. However, as previously discussed, 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. 

94
 
6.2.5 Use of threshold concentrations for community-level endpoints 

For the purposes of this endangered species assessment, threshold concentrations are used 

to predict potential indirect effects (via aquatic plant community structural change) to the 

Barton Springs salamander. The conceptual aquatic ecosystem model used to develop 

the threshold concentrations is intended to simulate the ecological production dynamics 

in a 2nd or 3rd order Midwestern stream; however, the model has been correlated to microand mesocosm studies, which were derived from a wide range of experimental studies 

(i.e., jar studies to large enclosures in lentic and lotic systems), that represent the best 

available information for atrazine-related community-level endpoints. 

The threshold concentrations are predictive of potential atrazine-related community-level 

effects in aquatic ecosystems, such as Barton Springs, where the species composition 

may differ from those included in the micro- and mesocosm studies. Although it is not 

possible to determine how well the responses observed in the micro- and mesocosm 

studies reflect the Barton Springs aquatic community, estimated high-end atrazine 

exposure concentrations in the action area (from modeled EECs) are predicted to be 

between 10 to 30 times lower than the community-level threshold concentrations, 

depending on the modeled atrazine use and averaging period. Given that threshold 

concentrations were derived based on the best available information from available 

community-level data for atrazine, these values are intended to be protective of the 

aquatic community, including the Barton Springs salamander. Additional uncertainties 

associated with use of the screening thresholds to estimate community-level effects are 

discussed in Section B.8 of Appendix B. 

6.3 Assumptions Associated with the Acute LOCs 

The risk characterization section of this endangered species assessment includes an 

evaluation of the potential for individual effects. 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 for the effects study corresponding to 

the taxonomic group for which the LOCs are exceeded. 

Sufficient dose-response information was not available to estimate the probability of an 

individual effect on the midge (one of the dietary food items of the Barton Springs 

salamander). Acute ecotoxicity data from the midge was used to derive RQs for 

freshwater invertebrates. Based on a lack of dose-response information for the midge, 

the probability of an individual effect was calculated using the only probit dose response 

curve slope value reported in available freshwater invertebrate ecotoxicity data for 

technical grade atrazine. Therefore, a probit slope value of 4.4 for the amphipod, which 

is also a component of the Barton Springs salamander’s diet, was used to estimate the 

probability of an individual effect on the freshwater invertebrates. It is unclear whether 

the probability of an individual effect for freshwater invertebrates other than amphipods 

would be higher or lower, given a lack of dose-response information for other freshwater 

invertebrate species. However, the assumed probit dose response slope for freshwater 

95
 
invertebrates of 4.4 would have to decrease to approximately 1 to 2 to cause an effect 

probability ranging between 1 in 10 and 1 in 100, respectively, for freshwater 

invertebrates. 

7. Summary of Direct and Indirect Effects to the Barton Springs Salamander 

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 atrazine to the Barton Springs 

salamander. A summary of the risk conclusions and effects determination for the Barton 

Springs salamander, given the uncertainties discussed in Section 6, is presented in Table 

7.1. 

Table 7.1. Effects Determination Summary for the Barton Springs Salamander 

Assessment Endpoint Effects determination Basis for Determination 

Survival, growth, and 

reproduction of Barton 

Springs salamander 

individuals via direct 

effects 

No effect No acute and chronic LOCs are exceeded. 

Indirect effects to Barton 

Springs salamander via 

reduction of prey (i.e., 

freshwater invertebrates) 

May affect, but not likely 

to adversely affect 

Acute LOCs are exceeded based on the most sensitive 

ecotoxicity value for the midge; however RQs for other dietary 

items (amphipods, leeches, snails) are less than LOCs. Based 

on the non-selective nature of feeding behavior in the Barton 

Springs salamander and low magnitude of anticipated 

individual effects to all evaluated prey species, atrazine is not 

likely to indirectly affect the Barton Springs salamander via a 

reduction in freshwater invertebrate food items. This finding is 

based on insignificance of effects (i.e., effects to freshwater 

invertebrates are not likely to result in “take” of a single Barton 

Springs salamander) and discountability (i.e., the effect to 

freshwater invertebrates is extremely unlikely to occur given 

the estimated individual event probability of 1 in 45.5 million). 

Indirect effects to Barton 

Springs salamander via 

reduction of habitat and/or 

primary productivity (i.e., 

aquatic plants) 

May affect, but not likely 

to adversely affect 

Although atrazine use may directly affect individual vascular 

and non-vascular aquatic plants in Barton Springs, its use 

within the action area is not likely to adversely affect the 

Barton Springs salamander via indirect community-level 

effects to aquatic vegetation. Predicted 14-, 30-, 60-, and 90­

day EECs for all modeled atrazine use scenarios within the 

action area are well below the threshold concentrations 

representing community-level effects. This finding is based on 

insignificance of effects (i.e., community-level effects to 

aquatic plants are not likely to result in “take” of a single 

Barton Springs salamander). 

96
 
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