Environmental Fate and Ecological Risk Assessment for the Reregistration of Xylene Range Aromatic Solvents Prepared by:

Anita Pease, Biologist James Wolf, Ph. D., Environmental Scientist U. S. Environmental Protection Agency Office of Pesticide Programs Environmental Fate and Effects Division Environmental Risk Branch IV Ariel Rios Building ( Mail Code 7507C) 1200 Pennsylvania Ave., NW Washington, DC 20460 Reviewed by: Brian Anderson, Biologist Stephanie Syslo, Risk Assessment Process Lead Dan Rieder, Branch Chief October 4, 2005 i Table of Contents I. Executive Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 A. Nature of Chemical Stressor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 B. Potential Risks to Non Target Organisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 C. Conclusions  Exposure Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 D. Conclusions  Effects Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 E. Uncertainties and Data Gaps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 II. Problem Formulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 A. Stressor Source and Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 1. Source and Intensity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2. Physical/ Chemical/ Fate and Transport Properties . . . . . . . . . . . . . . . . . 11 3. Pesticide Type, Class, and Mode of Action . . . . . . . . . . . . . . . . . . . . . . 12 4. Overview of Pesticide Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12 B. Receptors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 1. Aquatic Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2. Terrestrial Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 3. Ecosystems at Risk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 C. Assessment Endpoints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 D. Conceptual Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 1. Risk Hypotheses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 2. Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 E. Analysis Plan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 1. Preliminary Identification of Data Gaps and Methods . . . . . . . . . . . . . . 22 2. Measures to Evaluate Risk Hypotheses and Conceptual Model . . . . . . . 23 a. Measures of Exposure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 b. Measures of Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 c. Measures of Ecosystem and Receptor Characteristics . . . . . . . . 24 III. Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 A. Use Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 B. Exposure Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 1. Environmental Fate and Transport Characterization . . . . . . . . . . . . . . . 27 a. Summary of Empirical Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 b. Degradation and Metabolism . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 c. Transport and Mobility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 d. Field Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 e. Bioaccumulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 ii 2. Measures of Aquatic Exposure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 a. Aquatic Exposure Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 b. Aquatic Exposure Monitoring and Field Data . . . . . . . . . . . . . . 32 3. Measures of Terrestrial Exposure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 a. Terrestrial Exposure Modeling . . . . . . . . . . . . . . . . . . . . . . . . . 32 ( 1). Estimates for Exposure Via Consumption of Contaminated Water . . . . . . . . . . . . . . . . . . . 33 ( 2). Estimates for Exposure via Inhalation of Volatilized Xylenes . . . . . . . . . . . . . . . . . . . . . . . 34 b. Residue Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 C. Ecological Effects Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 1. Aquatic Effects Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 a. Aquatic Animals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 ( 1). Acute Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 ( 2). Chronic Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 ( 3). Sublethal Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 ( 4). Field Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 b. Aquatic Plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 2. Terrestrial Effects Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 a. Terrestrial Animals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 ( 1). Acute Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 ( 2). Chronic Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 ( 3). Sublethal Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 ( 4). Field Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 b. Terrestrial Plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 IV. Risk Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 A. Risk Estimation  Integration of Exposure and Effects Data . . . . . . . . . . . . . . . . . 47 1. Non Target Aquatic Animals and Plants . . . . . . . . . . . . . . . . . . . . . . . . . . 48 2. Non Target Terrestrial Animals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 a. Acute Risk to Mammals and Birds from Ingestion of Contaminated Water . . . . . . . . . . . . . . . . . . . . . . . . . 50 b. Acute Risk to Mammals Via Inhalation Exposure . . . . . . . . . . . 50 c. Acute Risk to Mammals from Combined Exposure via Inhalation and Contaminated Water . . . . . . . . . . . . . 51 B. Risk Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 1. Risks to Aquatic Organisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 2. Risks to Terrestrial Organisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 3. Review of Incident Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 4. Endocrine Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 iii 5. Federally Threatened and Endangered ( Listed) Species Concerns . . . . . . . 58 a. Action Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 b. Taxonomic Groups Potentially at Risk . . . . . . . . . . . . . . . . . . . . 59 ( 1). Discussion of Risk Quotients . . . . . . . . . . . . . . . . . . . 59 ( 2). Probit Dose Response Relationship . . . . . . . . . . . . . . . 59 ( 3.) Data Related to Under represented Taxa . . . . . . . . . . . 60 ( 4.) Implications of Sublethal Effects . . . . . . . . . . . . . . . . . 60 c. Indirect Effects Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 d. Critical Habitat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 e. Co occurrence Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 C. Description of Assumptions, Limitations, Uncertainties, Strengths and Data Gaps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 1. Assumptions, Limitations, Uncertainties, Strengths and Data Gaps Related to Exposure For All Taxa . . . . . . . . . . . . . . 67 2. Assumptions, Limitations, Uncertainties, Strengths and Data Gaps Related to Exposure For Aquatic Species . . . . . . . . 67 3. Assumptions, Limitations, Uncertainties, Strengths and Data Gaps Related to Exposure For Terrestrial Species . . . . . . 68 4. Assumptions, Limitations, Uncertainties, Strengths and Data Gaps Related to Effects Assessment . . . . . . . . . . . . . . . . . . . . . 69 5. Assumptions, Limitations, Uncertainties, Strengths and Data Gaps Related to the Acute and Chronic LOCs . . . . . . . . . . . . . 70 V. Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 ACKNOWLEDGEMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 List of Appendices A. Environmental Fate Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A1 B. Estimation of Concentration in Receiving Water . . . . . . . . . . . . . . . . . . . . . . . B1 C. Data Requirements  Environmental Fate . . . . . . . . . . . . . . . . . . . . . . . . . . . . C1 D. Environmental Fate and Monitoring Bibliography . . . . . . . . . . . . . . . . . . . . . . D1 E. Ecological Effects Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . E1 F. The Risk Quotient Method and Levels of Concern . . . . . . . . . . . . . . . . . . . . . . F1 G. Detailed Risk Quotients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G1 H. Data Requirements  Ecological Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . H1 I. Ecotoxicology Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I1 J. Summary of Endangered/ Threatened Species . . . . . . . . . . . . . . . . . . . . . . . . . . J1 iv List of Tables Table 1. Physical and Chemical Properties of Xylene Isomers. . . . . . . . . . . . . . . . . . . . . . . . . 10 Table 2. Summary of Assessment Endpoints and Measures of Effect for Xylene Mixtures and Xylene Isomers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Table 3. Relationship Between Return Flow Volume and Receiving Water Volumes Used in the Plug Flow Model.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 Table 4. Exposure Estimates for Birds and Mammals via Consumption of Contaminated Water. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 Table 5. Maximum Estimated Exposure for Volatilized Xylenes. . . . . . . . . . . . . . . . . . . . . . . . 38 Table 6. Xylene Toxicity Reference Values ( TRVs) for Aquatic Organisms . . . . . . . . . . . . . . 40 Table 7. Comparison of the Range of Acute Toxicity Values for Mixed Xylenes and Xylene Isomers in Aquatic Animals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 Table 8. Toxicity Reference Values ( TRVs) for Mixed Xylenes in Terrestrial Organisms. . . . . . 46 Table 9. Acute Toxicity Values for Birds, Expressed in Terms of a Single Xylene Dose ( mg) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 Table 10. Acute Toxicity Values for Mammals, Expressed in Terms of a Single Xylene Dose ( mg). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 Table 11. Acute RQs for Freshwater Fish, Freshwater Invertebrates, Estuarine/ Marine Fish, Estuarine/ Marine Invertebrates, and Algae Exposed to Mixed Xylenes or Xylene Isomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 Table 12. Acute RQs for Birds and Mammals Exposed to Mixed Xylenes via Consumption of Contaminated Water. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 Table 13. Acute RQs for Mammals Exposed to Volatilized Mixed Xylenes Via Inhalation. . . . 55 Table 14. Composite Acute RQs for Mammals Exposed to Mixed Xylenes by Consumption of Contaminated Water and Inhalation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 Table 15. Calculated Xylene Concentrations at which Acute RQs Are Less Than Acute LOCs for Aquatic Organisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 Table 16. Time, Distance, and Dilution for Xylene Concentrations to Decrease from 10 ppm and 1 ppm to < 0.04 ppm Based on a Steady State Plug Flow Dilution Model . . . . . . . . . . . 61 Table 17. Tabulation by Taxonomic Group and Crop of Listed Species That May Occur in Mixed Xylene Use Areas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 Table 18. Tabulation by Taxonomic Group and State of Listed Species That May Occur in Mixed Xylene Use Areas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 v List of Figures Figure 1. Ecological Conceptual Model for Screening Level Risk Assessment of Mixed Xylenes Applied to Irrigation and Drainage Canals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Figure 2. Concentration of Xylene in Receiving Water Versus Dilution and Time. Dilutions are none, 10: 1, 20: 1, and 50: 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 1 I. Executive Summary A. Nature of Chemical Stressor Aromatic emulsifiable petroleum solvents are used to control weed growth in open channels which carry water for irrigation ( canals, ditches) or water for drainage ( ditches). There is currently only one registered product ( T Chem Aquatic Weed Killer EPA Reg. No. 9768 18), which is used in programs for the Bureau of Reclamation, the U. S. Dept. of Interior, and cooperating state water use organizations. The product may be used only in the 16 states specified in the Bureau of Reclamation Act. These states include Arizona, California, Colorado, Idaho, Kansas, Montana, Nebraska, Nevada, New Mexico, North Dakota, Oklahoma, Oregon, South Dakota, Utah, Washington, and Wyoming.

This product is used to control a variety of aquatic nuisances such as surface pond scum ( filamentous algae), and submerged weeds such as American pondweed ( Potamogeton nodosus Poir), horned pondweed ( Zannichellia palustris), leafy pondweed ( Potamogeton foliosus Raf ), Richardson's pondweed ( Potamogeton richardsonii), curly leaf pondweed ( Potamogeton crispus), coon tail water weed ( Ceratophyllum demers) and water star grass ( Heteranthera dubia). This registered product consists of 98% mixed xylene isomers, but may also contain small quantities of ethylbenzene and emulsifiers to aid in the dispersal of the xylenes throughout the canal or drainage ditch. It is applied to canals and drainage ditches when the weed growth begins to interfere with water flow or delivery. To minimize the risk of effects to human health, the label indicates that water from the treated drainage ditch should not flow into receiving waters if the concentration of xylene exceeds 10 ppm. The 10 ppm ( mg/ L) represents the concentration that the EPA has set as an enforceable standard called a Maximum Contaminant Level ( MCL) for safe drinking water ( USEPA, OW).

B. Potential Risks to Non Target Organisms The focus of this screening level assessment was to evaluate risks associated with exposure to mixed xylenes applied underwater for use in the control of aquatic weeds in two different types of habitat including: 1) canals and ditches which contain xylene treated irrigation water, and 2) receiving water bodies ( i. e., streams, rivers, and/ or lakes) which receive return flow from xylenetreated irrigation water. Return flow is the portion of irrigation water that finds its way back to a stream channel either as surface flow or subsurface flow. The treated canals and ditches of the irrigation systems contain water only during the growing season when they are delivering water to crops and can be dry from 3 to 6 months out of the year, depending on the area and the crops being irrigated. Given this use pattern, potential exposures pathways exist for aquatic and terrestrial ecosystems via direct exposure and potential ingestion of contaminated water and from inhalation exposure of xylenes that volatilize from contaminated water. Since mixed xylenes are applied directly below the surface of the water, terrestrial exposure pathways typically considered ( i. e., exposure of terrestrial animal food sources and exposure of terrestrial plants via direct 2 application or spray drift) were not addressed in this assessment. Given the rapid volatilization of xylenes from water ( half lives range from less than 2 days in a shallow flowing water body to 6 days in a pond), chronic exposure of aquatic and terrestrial ecosystems is also not expected.

In evaluating the possible risks of mixed xylenes to aquatic and terrestrial receptors, this assessment focuses on a range of water concentrations that include the recommended allowable exposure limits for xylenes ( the maximum label rate of 11 gallons product for each 1 cfs flow) in treated irrigation canals and drainage ditches, that is, an initial concentration of approximately 740 mg/ L, which will decline with time and over distance from point of application. However, since the maximum allowable concentration of 740 mg/ L exceeds the solubility limits of xylenes, acute risk was also assessed based on the maximum solubility limit for o xylene of 178 mg/ L ( the highest solubility limit reported for the three xylene isomers). According to the current label, xylene concentrations in return flows from treated irrigation ( i. e., portion of water applied as irrigation that flows into receiving rivers, streams, lakes, and ponds) can not exceed 10 mg/ L; therefore, this concentration was also evaluated as a potential exposure concentration. It is recognized that the concentration of xylene in the return flow may be less than 10 mg/ L due to dissipation that occurs while the water is used to irrigate fields, prior to being released into a receiving water body. In addition, it is expected that xylene concentrations will further decrease via dilution with the volume of water in the receiving water body.

The results of this Tier I screening level ( deterministic) risk assessment show that exposure of aquatic animals and plants to mixed xylenes under the conditions of recommended use pose acute risk to listed ( i. e., endangered) and non listed freshwater and estuarine/ marine fish and invertebrates ( including amphibians), as well as aquatic non vascular plants. Acute risks to vascular aquatic macrophytes were not assessed due to lack of quantitative toxicity data; however, since mixed xylenes are used to control unwanted aquatic vegetation and risks are predicted for algae, the potential for risk to listed and non listed vascular aquatic plants is assumed. Acute risk quotients exceeded all acute LOCs for the range of water concentrations considered in this assessment ( 10 to 740 mg/ L). Thus, exposure of aquatic species to xylenes at the recommended water concentrations in the treated irrigation water ( canals and ditches) can be expected to reduce survival of aquatic organisms. In addition, available data indicates that reproductive effects may also occur in estuarine/ marine invertebrates following short term exposures. If listed fish, amphibians, and/ or aquatic invertebrates are present in the irrigation canals or ditches, the potential for direct and indirect effects to these species, through loss of food and habitat, is expected. Because acute risks to all aquatic species were observed at the allowable xylene concentration ( 10 mg/ L) in the return flows of treated irrigation into receiving rivers and streams, risk to listed and non listed aquatic receptors may not be restricted to drainage and irrigation ditch habitats, but may extend to receiving water, depending upon environmental conditions. Based on acute risk for freshwater and estuarine/ marine fish and invertebrates, indirect effects to listed species ( i. e., fish, mammals, birds, reptiles, and amphibians) that eat freshwater and estuarine/ marine fish and invertebrates may occur. The potential for adverse effects to those listed obligate and general species that rely on aquatic plants for food and/ or habitat and shelter also exists. 3 Based upon the available toxicity data, xylene concentrations in return flows should not exceed 0.04 mg/ L for freshwater environments and 0.05 mg/ L for estuarine/ marine environments to be protective of listed aquatic species. Therefore, additional dissipation ( i. e., volatilization, degradation, and dilution) is required at the 10 mg/ L level to achieve concentrations that are not harmful to listed aquatic receptors in receiving water bodies. Depending on the dilution and distance downstream where listed aquatic species may occur in receiving water bodies, exposure to xylene is expected to exceed the endangered species LOCs.

As previously mentioned, EFED acknowledges that xylene concentrations in return flows to receiving water bodies are likely to be less than 10 mg/ L, following application of treated irrigation water onto fields. As discussed in the following exposure characterization, EFED has used a simple steady state plug flow model to estimate the amount of dissipation that would be required to achieve a concentration of 0.04 mg/ L that is protective of listed aquatic species in receiving water bodies. EFED has considered a number of scenarios where the irrigation canals or ditches flow into different sized receiving bodies. Specifically, the model was used to predict the length of time and/ or dissipation distance required for xylene treated water to decrease from a concentration of 10 mg/ L ( as specified in the current label), as well an alternative concentration of 1 mg/ L, to a concentration of 0.04 mg/ L required to be protective of listed aquatic animal and plant species in receiving water bodies.

For terrestrial species, results of this assessment show that birds and mammals are not at acute risk from exposure to xylenes via ingestion of contaminated water. Additionally, mammals do not appear to be at acute risk from inhalation exposure or from combined exposure via ingestion of contaminated water and inhalation. Due to lack of inhalation toxicity data in birds, acute risks of exposure to birds via inhalation of volatilized xylenes and combined exposure via contaminated water and inhalation could not be assessed.

C. Conclusions  Exposure Characterization The environmental fate and transport properties of xylenes ( from other sources and uses without the addition of emulsifiers) are well known. Assuming that the emulsifiers have no significant effect on fate, other than transport ( solubility), xylene and its metabolites do not persist in surface water for long periods of time due to rapid volatilization and degradation. These compounds do not contain functional groups that are susceptible to hydrolysis under environmental conditions, nor do they absorb light in the environmental UV region (   > 290 nm); therefore, abiotic degradation via hydrolysis and photolysis are not important fate mechanisms. Xylenes are highly volatile compounds, and tend to evaporate readily from water surfaces. These compounds also biodegrade fairly rapidly under aerobic aquatic conditions, but appear to be more stable under anoxic aquatic conditions. Xylene metabolites include the methylated homologs of benzylsuccinic acid, benzylfumaric acid, and E phenylitaconate. Other degradation products identified are toluic acid, phthalic acid, and benzoic acid, which ultimately degrade to carbon dioxide.

Three exposure estimates were initially used to assess risks of mixed xylenes to non target aquatic 4 animals ( i. e., fish, invertebrates) and plants ( i. e., algae): 10 mg/ L ( maximum allowable xylene concentration released to receiving waters), 740 mg/ L ( estimated maximum xylene concentration in irrigation canals), and 178 mg/ L ( solubility limit for o xylene). Any water coming off the irrigated fields as return flow would be expected to have xylene concentrations less than 10 ppm. Limited monitoring data ( Walsh et al., 1977) of irrigation canals shows that initial xylene concentrations of approximately 500 to more than 800 ppm at the point of application decrease to 100 to 200 ppm five to ten miles downstream from application. A repeat application resulted in similar concentrations more than fifteen miles downstream from the point of application. The monitoring results show that xylene concentrations in the treated irrigation canals and ditches may exceed the 10 ppm release concentration. However, monitoring of xylene residues in return flow in the same study showed a decrease in concentration from 500 ppm in the water removed from lateral ditches for irrigation to less than 0.2 ppm ( 200 ppb) ( study detection limit) in return flow after flowing through irrigation fields ( length of irrigation rills 750 to 1320 feet). Similar findings were summarized in a Bulletin publish by the Bureau of Reclamation ( USDI BR, 1969). Recently, the Agency obtained some monitoring from the Washington State Department of Ecology ( WDE, 2005). These data reported xylene concentrations in irrigation waste water ( prior to release) to range between 0.004 and 10.9 ppm. There were 67 detections out of 108 samples. The mean value was 0.862 ppm. About 25 percent of the detections were less than 0.04 ppm and about 80 percent of the detections were less than 1.0 ppm. These data suggest that significant dissipation of xylene occurs within the field during the irrigation process. The amount of water coming off the field is expected to vary due to different management practices and type of irrigation. For example, sprinkler irrigation systems would likely result in increased dissipation of xylene in treated irrigation water as compared to flood irrigation.

Currently, the Agency has not adopted a specific model to estimate environmental concentrations of a pesticide applied to a transient flowing water body ( treated irrigation water canal), where dissipation processes are functioning as water flows downstream in the irrigation canal and is removed and subsequently applied to a crop as irrigation water. Based on concentration estimates and monitoring data, it is assumed that treated irrigation water in the canals and ditches exceed the endangered species LOCs for aquatic animals and plants. It is expected that xylene treated irrigation water will undergo dissipation in the field, with the excess treated irrigation water released as return flow. Allowable xylene concentrations ( 10 mg/ L) in the water released as return flow exceed the LOC; however, actual concentrations are likely to be lower due to in field dissipation ( additional volatilization, degradation, and leaching). The Agency modeled the dissipation of xylene in the return flow from the point of release to several sized water bodies, assuming the maximum release concentration ( 10 mg/ L), and a simple steady flow mixing model to estimate a distance or time for the concentration to drop below the LOC for aquatic listed species.

Using a simple steady state plug flow model ( PC 030001, D313280, Corbin, 2005), the dissipation of xylene in return flow with a flow rate of 1 cfs and a concentration of 10 mg/ L ( 10000 ppb) into a receiving water body was considered for several different dilution ratios ( 1: 1, 10: 1, 20: 1, and 50: 1) ( assuming a constant flow rate of 1 cfs, mixing during dilution, and a volatilization half life of 2 days). Release into a dry channel is approximated by the zero ( 0) dilution estimate. As 5 expected, the time and distance required for xylene to dissipate to concentrations that are protective of listed aquatic species in receiving water decreases with increasing dilution ( i. e., larger receiving water bodies). Depending on the dilution factor, which is a function of the geometry of the receiving water body, the time required for xylene to dissipate from 10 mg/ L to a concentration less than 0.04 mg/ L ( the concentration required to protect listed aquatic species) ranges from approximately 8 days to 7.5 hours, with dilutions ranging from 10: 1 to 50: 1. The time required for xylene to dissipate from 1 mg/ L to less than 0.04 ranges from approximately 5 days to 5 hours. Increasing the flow rate ( with same application rate) increases the time or distance necessary to lower the concentration to a level that is protective of listed aquatic species ( assuming that degradation remains constant).

Although terrestrial species are exposed to xylenes from the use and accidental release of petroleum based products, aromatic petroleum solvents are not used to control terrestrial weed growth. Therefore, the potential for direct exposure to terrestrial organisms is very low. The most likely exposure pathways for terrestrial animals are through ingestion of contaminated water and inhalation of volatilized xylenes. Exposure estimates for ingestion of contaminated water for birds and mammals were based on the allowable concentration range of 10 to 740 mg/ L and the solubility limit for o xylene of 178 mg/ L, and the calculated amount of water that birds and mammals are expected to consume in one day.

To assess risks associated with inhalation exposure to mammals, a xylene concentration in air was estimated using the nondimensional Henry's Law constant, which relates the concentration of a compound in the gas phase to its concentration in the solution phase. The solution concentration was assumed to be the maximum solubility of xylene ( 178 ppm), with no emulsifiers. The inhalation exposure is equal to the concentration of xylene in air, assuming the dimensions of the breathing zone for a potential receptor. The breathing zone was assumed to be several cm above the ground to reflect the area where small mammals breath. This zone was assumed to have a uniform xylene concentration, although in reality concentration probably decreases with height. The EEC for air is considered to be representative of a 4 hour time period, which corresponds to the reported exposure duration for the available mammalian inhalation toxicity study. For this risk assessment, the maximum estimated exposure concentration for volatilized xylenes in air was estimated to be 38.5 ppm, assuming that no losses from wind or degradation.

D. Conclusions  Effects Characterization No registrant submitted toxicity studies in which xylene mixtures or xylene isomers were the sole active ingredient were identified. Therefore, all ecological effects data reviewed in this risk assessment were from the published open literature. Since chronic exposure of aquatic and terrestrial ecosystems is not anticipated due to rapid volatilization, chronic exposure studies were not reviewed for this assessment.

In general, results of acute toxicity studies indicate that mixed xylenes and xylene isomers are moderately to highly toxic to aquatic species. The acute toxicity values used to estimate risks to 6 aquatic organisms are as follows:

° freshwater fish: 96 hour LC 50 value of 2.6 mg/ L for p xylene in rainbow trout ( Salmo gairdneri);

° freshwater invertebrates: 24 hour LC 50 value of 1.0 mg/ L for m xylene in water flea ( Daphnia magna);

° estuarine/ marine invertebrates: 96 hour LC 50 value of 7.4 mg/ L for mixed xylenes in grass shrimp ( Palaemonetes pugio);

° algae: 72 hour LC 50 value of 3.2 mg/ L for p xylene in green algae ( Selenastrum capricornutum).

Although limited information is available regarding sublethal effects of mixed xylenes or xylene isomers in aquatic animals, results of a study in rainbow trout show a dose dependent loss of equilibrium in fish that were exposed to xylene for approximately 1.4 hours, with NOAEC and LOAEC values of 0.65 mg/ L and 3.2 mg/ L, respectively. It is important to note that due to their high volatility, xylene isomers disappear rapidly from solution. Therefore, interpretation of aquatic toxicity data may be confounded by the rapid loss of xylenes from solution.

For terrestrial species, the types of toxicity studies pertinent to this risk assessment were acute oral exposure and acute inhalation exposure. For birds, no single oral exposure studies were available; results of an acute dietary study on Japanese quail suggest that mixed xylenes are practically nontoxic on an acute dietary basis ( LC 50 > 20,000 mg a. i./ kg diet). No acute inhalation studies in avian species were identified from the available literature. For mammals, the lowest toxicity values reported for mixed xylenes were a subacute oral LD 50 value of 1608 mg/ kg body weight in rats and an acute inhalation LC 50 value of 6700 ppm in rats.

E. Uncertainties and Data Gaps Uncertainties and data gaps pertaining to surface water modeling and inhalation exposure have been identified as follows:

° The aquatic exposure estimates contain considerable uncertainty due to limited label language and limited information concerning the use of xylene. The xylene containing product is applied to irrigation canals and ditches, or drainage ditches at a rate of 11 gallons product per 1 cubic foot per second ( 1 cfs) of water flow velocity for 20 or 30 minutes, which yields an initial level of 740 ppm. This level exceeds the solubility limits of xylenes; however, xylenes are applied with an emulsifier which can increase the apparent solubility of the product. Therefore, there is a degree of uncertainty regarding the actual maximum exposure level for aquatic organisms in irrigation canals and ditches as well as receiving waters. 7 Considerable uncertainties are also associated with 10 ppm exposure value from the return flow. The return flow may be lower due to volatilization and dilution before being leased into receiving water bodies. The actual concentration of xylene in the return flow and potential receiving water bodies will be a function of the initial concentration and dissipation processes including volatilization, dilution, and degradation. The size of the water bodies receiving treated irrigation water is unknown; therefore, there is uncertainty associated with the modeled dilution factors of 1: 1, 10: 1, 20: 1, and 50: 1 and no dilution.

° The concentration of xylene in the return flow will depend upon the concentration in the irrigation water ( applied to crop) and the dissipation ( degradation, volatilization, and sorption) that occurs from point of application in the field to the point where water draining from the field reaches the receiving body of water. The higher the concentration in the irrigation water, the greater the need for dissipation to occur before the return flow enters the receiving water. The xylene dissipation from the return flow to the receiving water body depends on the amount, rate, and length of irrigation, length of time it takes for the water to reach the receiving body, amount and rate of xylene and volume of water entering the receiving body, and rate of flow and volume of the receiving water body.

° The environmental parameters for xylenes used in this assessment represent values obtained from open literature for the various xylene isomers and do not consider the influence of the emulsifiers ( excluding the use of emulsifiers to increase the " solubility"). The estimates of the volatilization of the xylenes from water is influenced by the solubility ( considered in Henry's Law), which is uncertain due to the influence of emulsifiers.

° Given the uncertainties associated with the presence of the emulsifier in the end use product and its potential effect on the solubility and volatility of xylene in irrigation ditches and canals, further information on the comparative volatility of xylene with and without the emulsifier is needed to characterize the dissipation of the herbicide under field conditions.

° The geometries and flow rate of the irrigation canals and ditches where xylene is applied are not known; therefore, there is uncertainty associated with estimated exposure concentrations. The geometry and environmental parameters of the ditch used in this assessment is intended to be representative of a small segment of a ditch or canal to which xylenes may be released. It is understood that the physical properties of any drainage ditch or canal may vary significantly from location to location; therefore, the assumed model drainage ditch used in this assessment represents a source of uncertainty in the assessment.

° Inhalation exposure was estimated using a nondimensional Henry's law constant to 8 estimate a " maximum air concentration" from a concentration in solution assuming ideal gas and steady state conditions. The transfer process of xylene from water to the atmosphere is dependant upon the chemical and physical properties of xylene and the physical properties ( e. g., flow velocity, depth, and turbulence) of the water body. Factors that control volatilization of xylene are solubility, molecular weight, and vapor pressure of xylene and the nature of the air water interface through which it must pass. The volatilization rates from water are highly variable. The air concentration would not be expected to exceed the value estimated from the Henry's constant and solubility values.

The actual flux rate of xylenes from a model drainage ditch will vary over a 4 hour exposure period; therefore, the actual duration of inhalation exposure is uncertain. The loss of xylenes out of the air compartment is highly uncertain since it is dependent upon several environmental parameters ( e. g. wind velocity) which will vary based upon location and time of year. The influence of the emulsifiers on the volatilization of xylene is not known ( except to increase solubility). Solubility is assumed to be the 178 ppm ( without emulsifiers). In practice, this flux can be expected to vary with changes in the water concentration, such that the flux peaks shortly after the initial application to water and decreases as the concentration in water decreases due to transport and degradation.

The ecological effects database for mixed xylenes is incomplete. Data gaps and uncertainties for ecological effects have been identified as follows:

° No registrant submitted toxicity studies in which xylene mixtures or xylene isomers were the sole active ingredient were identified; all ecological effects data reviewed in this risk assessment were obtained from the published open literature. Although some of the acute aquatic studies were conducted in accordance with OECD guidelines, no consistent experimental protocols were followed for estuarine/ marine invertebrates. Therefore, there is a high degree of uncertainty to the toxicity data for this taxonomic group.

° Toxicity data are not available for estuarine/ marine fish; therefore, risks for this taxonomic group are assumed based on xylene's known toxicity to freshwater fish.

° Toxicity data are not available for vascular aquatic macrophytes; therefore, risks for this taxonomic group are assumed based on xylene's mode of action as an aquatic herbicide and available information on the toxicity of xylene to non vascular aquatic plants.

° There is uncertainty associated with the acute dietary toxicity study for birds, given xylene's high volatility. Therefore, the registrant should provide information on a avian LD 50 value for xylene or submit an acute avian oral toxicity study ( LD 50 ), 9 using either a bobwhite quail or mallard duck, to satisfy the § 71 1 guideline requirements.

° Acute inhalation toxicity data are not available for birds; therefore, avian risk from inhalation exposure cannot be assessed. 10 II. Problem Formulation A. Stressor Source and Distribution 1. Source and Intensity Aromatic petroleum products that are used for aquatic weed killers are mixed xylene isomers ( m, o, and p isomers of xylene) that may contain minor amounts of other compounds such as ethylbenzene. Depending upon the manufacturing process, xylene may also initially contain benzene, toluene, trimethylbenzene, phenol, thiophene, and pyridine; however, the combined volume of these non xylene hydrocarbons is only a fraction of a percentage of the composition of mixed xylene ( ATSDR 1995). The composition of a mixed xylene depends on the manufacturing method used. Currently, most mixed xylene is produced as a catalytic reformate of petroleum and consists of approximately 44% m xylene, 20% o xylene, 20% p xylene, and 15% ethylbenzene ( ATSDR, 1995). Mixed xylene may also be manufactured from coal tar, yielding a mixture of approximately 45  70% m xylene, 23% p xylene, 10  15% o xylene, and 6  10% ethylbenzene ( ATSDR 1995). Other production processes include gasoline pyrolysis and disproportionation of toluene, both of which produce a mixture of xylenes that are virtually free of ethylbenzene ( ATSDR 1995). According to the aquatic weed killer product labels ( Sim Chem Aquatic Weed Killer, EPA Reg. No. 11682 8; T Chem Aquatic Weed Killer, EPA Reg. No. 9768 18), most formulations applied to drainage ditches to control unwanted vegetation are 98 99% mixed xylenes ( isomeric ratios unspecified) and about 1% ethylbenzene.

Formulations of mixed xylenes are used to control unwanted aquatic vegetation drainage and irrigation ditches by application below the surface of the water. To apply these products to a drainage ditch, a boom section from an ordinary sprayer is connected to the pump by a hose and then lowered to the bottom of the channel so that the nozzle flows directly into the water without hitting the bottom, weeds or other obstacles. One application method is to apply at several locations, each typically two to four miles apart. At each location, 11 gallons of the product are added for each cubic foot per second ( cfs) of canal flow, over an application period of 20 to 30 minutes. The second application method is to apply six gallons of product per cfs flow, over a 20 to 30 minute time period, at successive stations one mile apart down channel. An application is typically considered to be an effective dose if it results in a treatment concentration of 740 mg/ L. It is noted that this concentration exceeds the solubility limits of xylenes ( solubility of the xylene isomers in water is approximately 160 180 ppm); however, xylenes used to treat drainage ditches are usually applied with an emulsifier, which may result in reduced volatility and a greater apparent solubility ( and thus higher exposure levels) than the actual solubility limits in water.

Aromatic petroleum solvents are used to control submerged weeds and pond scum in drainage ditches or canals, and cannot be used in rivers, lakes, or streams. As specified on the label, any water from a treated ditch flowing into receiving waters should contain less than 10 ppm xylene and treated water should not be used for domestic or livestock purposes. 11 2. Physical/ Chemical/ Fate and Transport Properties The most important dissipation pathway for xylenes applied to a drainage ditch is volatilization. Xylenes are also susceptible to biodegradation under aerobic conditions, but the rate of volatilization ( half life of about 2 days in a shallow water body; 1.2 days in typical river and 6.0 days in a pond ( http:// www. epa. gov/ OGWDW/ dwh/ t voc/ xylenes. html) is significantly greater than the rate of degradation ( half life on the order of 20 days) ( API 1994). Abiotic degradation mechanisms, such as hydrolysis and photolysis, are not important for aromatic petroleum solvents. Xylenes have a high to moderate mobility in soils, thus when volatilization does not readily occur xylenes could leach to groundwater. For example, when applied directly to water, leaching of xylene to the soil from the canal or ditch is possible; however, groundwater below the ditch is not likely to be a source of drinking water. The chemical and physical properties of the three isomers of xylene are provided in Table 1. Other than increasing the " solubility", the influence of the emulsifiers used during the addition of xylene to the irrigation water is not known.

Table 1. Physical and Chemical Properties of Xylene Isomers.

Property m Xylene a, b o Xylene a, b p Xylene a, b Molecular Weight 106.16 106.16 106.16 Melting point ( oC)  47.8  25.2 13.2 Boiling point ( oC) 139.1 144.5 138.3 Density ( g/ cm3) 0.864 0.88 0.861 Water solubility ( mg/ L) 161 178 162 log Kow 3.2 3.12 3.15 Vapor pressure ( mm Hg) 8.29 6.61 8.84 Henry's law constant ( atmm3 mol) 0.00718 0.00518 0.00690 Aerobic soil metabolism half life 5.8 days 5 days c 7 days c Aerobic aquatic metabolism halflife 14 23 days d 9 20 days d 14 25 days d Anaerobic aquatic metabolism half life > 400 days d > 400 days d > 400 days d Koc ( adsorption) 39 365 e 39 365 e 39 365 e Table 1. Physical and Chemical Properties of Xylene Isomers.

Property m Xylene a, b o Xylene a, b p Xylene a, b 12 a ATSDR 1995 b HSDB 2005 c Tsao et al. 1998 d API 1994 e Pavlostathis and Mathavan 1992 3. Pesticide Type, Class, and Mode of Action Xylenes are considered a contact herbicide, and are phytotoxic to most vegetation. Contact herbicides act quickly by destroying plant cells; however, they do not kill the roots and reapplication several weeks or months following the initial application may be required. Aquatic weeds have limited cuticle to protect cell membranes. Xylene solubilize the hydrophobic tail of the phospholipid bilayer that makes up the cell membrane, causing the membrane to leak and allowing internal fluids to escape. When the effect is severe, weeds are killed.

The mechanisms by which xylene produces toxic effects in animals is not established. However, several theories exist, including alterations of lipid cell membrane fluidity, alteration of cell membrane protein conformation, changes in central nervous system neurotransmitters, and inhibition of microsomal enzymes ( ATSDR 1995).

4. Overview of Pesticide Usage Chem Aquatic Weed Killer ( EPA Reg. No. 9768 18), the only registered pesticide product containing aromatic emulsifiable petroleum solvents, is used to control submerged weeds in irrigation and drainage canals. Targeted weeds include a variety of aquatic nuisances such as surface pond scum ( filamentous algae), American pondweed ( Potamogeton nodosus Poir), horned pondweed ( Zannichellia palustris), leafy pondweed ( Potamogeton foliosus Raf ), Richardson's pondweed ( Potamogeton richardsonii), curly leaf pondweed ( Potamogeton crispus), coon tail water weed ( Ceratophyllum demers) and water star grass ( Heteranthera dubia). Usage data for aromatic emulsifiable petroleum solvents is non existent. No pesticide usage data are available in publicly available databases including the National Center for Agricultural and Food Policy ( NCFAP 2005), National Agricultural Statistics Service Chemical Usage Database ( NASS 2005), and the California Department of Pesticide Regulation Pesticide Use Reporting system ( CDPR 2005). The Washington State Department of Ecology reported that over 750,000 gallons of xylene were used to treat drainage ditches in Oregon, Washington and Idaho in 1967 ( WSDE 2002). Data from 1969 ( USDI BR, 1969) reports similar use levels ( 800,000 gallons per season) including more than 10,000 applications to 40,000 miles ( equivalent to 1 application every 4 miles) of canals, laterals, and drains throughout the Pacific Northwest.

Based on information on the label, Chem Aquatic Weed Killer may be used only in states specified 13 in the Bureau of Reclamation, Reclamation Act/ Newlands Act of 1902. States included in the Reclamation Act include Arizona, California, Colorado, Idaho, Kansas, Montana, Nebraska, Nevada, New Mexico, North Dakota, Oklahoma, Oregon, South Dakota, Utah, Washington, and Wyoming.

B. Receptors 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. For mixed xylenes, ecological measurement endpoints for the screening level risk assessment are based on a selection of toxicity studies performed on a limited number of organisms in broad groupings. Within each of these very broad taxonomic groups, the most sensitive acute endpoint is selected from the available test data. A complete discussion of the toxicity data selected for this risk assessment for each taxonomic group is provided in Appendix E.

The chemical of interest for this risk assessment is mixed xylenes, which is a mixture of the three isomeric xylene compounds ( ortho , meta, and para xylene). To assess risk to aquatic and terrestrial receptors, data obtained from toxicity studies using mixed xylenes and the xylene isomers were considered. The lowest toxicity value from data on mixed xylenes and the individual xylene isomers was used to estimate risk.

1. Aquatic Effects Acute toxicity data from the open literature was used to evaluate potential effects for aquatic receptors. Data sufficient for use in a quantitative risk assessment for mixed xylenes and/ or xylene isomers are available for freshwater fish, freshwater invertebrates, estuarine/ marine invertebrates, and algae. Although no studies suitable for quantitative risk assessment were identified for estuarine/ marine fish and vascular aquatic plants, risks for these taxonomic groups were qualitatively evaluated. Since mixed xylenes are highly volatile, surface water concentrations are expected to decrease rapidly; thus, chronic exposure to aquatic plants and animals is not anticipated.

2. Terrestrial Effects The anticipated exposure pathways for terrestrial animals are oral exposure via drinking of contaminated water, and inhalation exposure of volatilized xylene. Based on these exposure scenarios, open literature data sources were queried for acute and subacute oral and acute inhalation toxicity studies in birds and mammals. Acceptable subacute oral toxicity studies were identified for mammals, an acute dietary study was identified for birds, and acute inhalation toxicity studies were identified for mammals; the lack of acute inhalation data in birds was treated as a data gap in this assessment. Due to rapid volatilization, chronic exposure of terrestrial receptors is not 14 anticipated. In addition, given that mixed xylenes are applied directly below the surface of water, exposure of terrestrial plants is not anticipated. Although terrestrial plants may potentially be exposed via treated ditch or canal water used as irrigation water, exposures are expected to be minimal given xylene's high rate of volatilization.

3. Ecosystems at Risk Aquatic ecosystems potentially at risk include the irrigation and drainage canals where xylene is applied to control aquatic weeds, as well as return flows of treated irrigation waters into receiving rivers, streams, or lakes. For use in coastal areas, aquatic habitat also includes marine ecosystems including estuaries. Given that the drainage canals are used as a source of irrigation water, it is assumed that these systems are primarily freshwater, whereas receiving water bodies may be either freshwater or estuarine/ marine, depending on proximity to coastal areas. In addition, it should be noted that irrigation canals and drainage ditches contain water only during the growing season when they are delivering water to crops. As such, they can be dry from 3 to 6 months out of the year, depending on the area and crops being irrigated. Although these canals and ditches are likely to be intermittent, these aquatic systems provide viable habitat for such organisms as larval amphibians, aquatic reptiles, benthic aquatic invertebrates ( including a number of emergent insects), and diapausal pelagic ( i. e., living in the open water) invertebrates and fish; spawning habitat for fishes; and feeding habitat for piscivorous and insectivorous wildlife.

Given the method of application, exposure of terrestrial ecosystems to xylene is expected to be minimal; however, terrestrial mammals and birds may be exposed via drinking water and inhalation pathways. In addition, terrestrial ecosystems may be potentially exposed to treated irrigation water used for furrow or flood irrigation, although exposures are expected to be minimal given xylene's high rate of volatilization.

Ecosystems potentially at risk are expressed in terms of the selected assessment measures of effect. The typical assessment measures of effect for screening level pesticide ecological risk assessments are reduced survival, and reproductive and growth impairment for both aquatic and terrestrial animal species. Aquatic species of potential concern include freshwater fish and invertebrates, estuarine/ marine fish and invertebrates, amphibians, algae, and aquatic macrophytes. Terrestrial species of potential concern include birds and mammals. For both aquatic and terrestrial animals, direct acute exposures are considered. Due to the rapid volatilization of mixed xylenes, chronic exposures are not considered in this assessment. Since mixed xylenes are applied under the surface of the water, exposures of terrestrial plants and topical exposure of beneficial insects are not considered in this assessment. The types of toxicity studies available for mixed xylenes and isomers of xylene are summarized in Table 2.

In order to protect threatened and endangered ( listed) species, all assessment endpoints are measured at the individual level. Although all endpoints are measured at the individual level, they provide insight about risks at higher levels of biological organization ( e. g. populations and communities). For example, pesticide effects on individual survival have important implications for 15 both population rates of increase and habitat carrying capacity.

The ecological relevance of selecting the above mentioned assessment measures of effect is as follows: 1) complete exposure pathways exist for these receptors; 2) the receptors may be potentially sensitive to pesticides in affected media; and 3) the receptors could potentially inhabit areas where pesticides are applied.

Table 2. Summary of Assessment Endpoints and Measures of Effect for Xylene Mixtures and Xylene Isomers.

Assessment Endpoint Measure of Effect 1. Abundance ( i. e., survival, reproduction, and growth) of individuals and populations of birds. 1a. Japanese quail 5 day oral LC50 ( non guidelinerecommended species). 1b. Acute inhalation LC50: No data.

2. Abundance ( i. e., survival, reproduction, and growth) of individuals and populations of mammals. 2a. Laboratory rat subacute oral LD50. 2b. Laboratory rat acute inhalation LC50.

3. Survival and reproduction of individuals and communities of freshwater fish and invertebrates. 3a. Rainbow trout and bluegill sunfish acute LC50 ( guideline recommended species). Acute LC50 in fathead minnow, guppy, goldfish, and white sucker fish ( non guideline recommended species). 3b. Water flea acute LC50 ( guideline recommended species). Snail acute LC50 ( non guidelinerecommended species).

4. Survival and reproduction of individuals and communities of estuarine/ marine fish and invertebrates. 4a. No data for estuarine/ marine fish. 4a. Acute LC50 in grass shrimp, brine shrimp, and sea urchin eggs ( non guideline recommended species).

6. Survival and reproduction of individuals and communities of non vascular and vascular aquatic plants. 6a. Acute EC50 in algae Selenastrum capricornutum and Skeletonema costarum (( guideline recommended species). 6b. No data for vascular aquatic plants.

LD50 = Lethal dose to 50% of the test population. LC50 ( EC50) = Lethal ( effective) concentration to 50% of the test population. NOAEC = No observed adverse effect concentration. LOAEC = Lowest observed adverse effect concentration.

C. Assessment Endpoints Assessment endpoints are defined as " explicit expressions of the actual environmental value that is to be protected." Defining an assessment endpoint involves two steps: 1) identifying the valued attributes of the environment that are considered to be at risk; and 2) operationally defining the assessment endpoint in terms of an ecological entity ( i. e., a community of fish and aquatic invertebrates) and its attributes ( i. e., survival and reproduction). Therefore, selection of the 16 assessment endpoints is based on valued entities ( i. e., ecological receptors), the ecosystems potentially at risk, the migration pathways of pesticides, and the routes by which ecological receptors are exposed to pesticide related contamination. The selection of clearly defined assessment endpoints is important because they provide direction and boundaries in the risk assessment for addressing risk management issues of concern.

A summary of the assessment endpoints and measures of effects selected to characterize potential ecological risks associated with exposure to mixed xylenes and isomers is provided in Table 2.

This ecological risk assessment considers exposures for the label recommended range of surface water concentrations for mixed xylenes applied directly below the water surface to control aquatic weeds. Mixed xylenes are applied to achieve an initial concentration of approximately 740 ppm, with the concentration in return flows of treated irrigation ( i. e., flows into receiving rivers and streams) not to exceed 10 ppm. However, since the maximum allowable concentration of 740 ppm exceeds the solubility limits of xylenes, acute risk was also assessed based on the maximum solubility limit for o xylene of 178 ppm ( the highest solubility limit reported for the three xylene isomers). This exposure estimate was chosen to provide consistency with the chemical physical properties of xylenes applied without the addition of emulsifiers. This assessment is not intended to represent a site or time specific analysis. Instead, this assessment is intended to represent highend exposures at a national level. Likewise, the most sensitive toxicity measures of effect are used from surrogate test species to estimate treatment related direct effects on acute mortality. Toxicity tests are intended to determine effects of mixed xylenes exposure on birds, mammals, fish, aquatic invertebrates, and aquatic plants. These short term acute studies are typically arranged in a hierarchical or tiered system that progresses from basic laboratory tests to applied field studies. The toxicity studies are used to evaluate the potential of mixed xylenes and/ or xylene isomers to cause adverse effects, to determine whether further testing is required, and to determine the need for precautionary label statements to minimize the potential adverse effects to non target animals and plants ( CFR 40 § 158.202, 2002).

D. Conceptual Model A conceptual model provides a written description and visual representation of the predicted relationships between the stressor, potential routes of exposure, and the predicted effects for the assessment endpoint. The conceptual model consists of two major components: the risk hypotheses and a diagram.

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 ( US EPA 2004). For this assessment, the risk is stressor initiated, where the stressor is the release of mixed xylenes to the environment. The following risk hypothesis is presumed for this screening level assessment: 17 Non target aquatic animals and plants and terrestrial animals may be exposed to mixed xylenes that are applied underwater according to the label to control aquatic weeds in drainage and irrigation ditches. Based on available information regarding volatilization, persistence, and direct and indirect toxicity, mixed xylenes have the potential to compromise survival and cause sublethal effects in non target aquatic animals and plants, and terrestrial mammals and birds.

Aquatic receptors that may be exposed to mixed xylenes include aquatic animals ( i. e., freshwater and estuarine/ marine fish and invertebrates, and aquatic phase amphibians) and plants. Terrestrial receptors that may be exposed to mixed xylenes include terrestrial and semi aquatic wildlife ( i. e., mammals, birds, reptiles, and terrestrial phase amphibians). Since mixed xylenes are applied underwater, exposure of terrestrial plants and topical exposure of insects is not anticipated. Furthermore, given the high volatility of mixed xylenes, chronic exposure of aquatic and terrestrial receptors is not anticipated. The open literature was reviewed to provide data for identifying potential endpoints, stressors, and ecological effects associated with uses of mixed xylenes and xylene isomers.

Based on the available acute toxicity data from the open literature, measures of effect suitable for a quantitative assessment of xylene mixtures have been identified. Aquatic toxicity data can be used to quantify potential adverse effects associated with reduction of aquatic invertebrates and fish, indirect effects on aquatic communities due to loss of species that are sensitive to mixed xylenes, and changes in structure and functional characteristics of the affected communities. For the aquatic ecosystem, acute toxicity data are available for freshwater fish and invertebrates, estuarine/ marine invertebrates, and aquatic algae. For the terrestrial ecosystem, acute toxicity data were identified to quantify mammalian and avian risks associated with exposure to mixed xylenes via consumption of contaminated water. In addition, inhalation toxicity data is available to characterize risks associated with inhalation of contaminated air by mammals.

2. Diagram The conceptual model used to depict the potential ecological risk associated with mixed xylenes is fairly generic and assumes that as a pesticide, mixed xylenes are capable of affecting aquatic organisms at the anticipated environmental concentrations resulting from proposed label uses. All potential routes of exposure are considered and presented in the conceptual model ( Figure 1). The conceptual model generically depicts the potential source of mixed xylenes, release mechanisms, abiotic receiving media, biological receptor types, and effects endpoints of potential concern.

In order for a chemical to pose an ecological risk, it must reach ecological receptors in biologically significant concentrations. An exposure pathway is the means by which a contaminant moves in the environment from a source to an ecological receptor. For an ecological exposure pathway to be complete, it must have a source, a release mechanism, an environmental transport medium, a 18 point of exposure for ecological receptors, and a feasible route of exposure. The assessment of ecological exposure pathways, therefore, includes an examination of the source and potential migration pathways for constituents, and the determination of potential exposure routes ( e. g., ingestion, inhalation, dermal contact).

Exposure to aquatic organisms and plants is expected from surface waters contaminated with mixed xylenes through direct application and from treated irrigation water that flows into receiving rivers, streams, or lakes. Due to the high volatility of mixed xylenes, chronic exposure of aquatic and terrestrial ecosystems is not expected to occur. Since mixed xylenes are applied underwater, exposure of terrestrial plants via direct application or spray drift or contamination of plants and insect forage sources for terrestrial animals are not anticipated exposure pathways. Based on the use pattern for xylenes, the main exposure pathways for terrestrial animals are exposure via ingestion of contaminated water and inhalation of volatilized xylenes. 19 Application of Mixed Xylenes to Irrigation and Drainage Canals Direct Subsurface Application Surface Water Ingestion of Contaminated Water Mammals Birds Reduced Survival Sublethal Effects Receptors Attribute Changes Exposure Point Source/ Exposure Media Source/ Transport Pathways Surface Water Gill/ Integument Uptake Aquatic Invertebrates Aquatic Vertebrates Amphibians Algae and Aquatic Plants Reduced Survival Sublethal Effects Surface Water Inhalation ( Xylenes Volatilized from Water)

Mammals Birds Reduced Survival Sublethal Effects Treated Irrigation Water &

Return Flow Direct Contact Water Ingestion Inhalation Aquatic Invertebrates Aquatic Vertebrates Amphibians Aquatic Plants Mammals Birds Figure 1. Ecological Conceptual Exposure Model for Screening Level Risk Assessment of Mixed Xylenes Applied to Irrigation Canals and Drainage Ditches 20 E. Analysis Plan The Agency's reregistration eligibility science chapter for mixed xylenes consists of a deterministic screening level risk quotient analysis. This document characterizes the environmental fate and effects of mixed xylenes to aquatic and terrestrial environments. Since mixed xylenes are highly volatile, chronic exposure of aquatic and terrestrial ecosystems is not anticipated; thus, this analysis plan focuses on acute exposure only.

For aquatic animals, the pathway of mixed xylenes exposure is by direct application to water. Risks to aquatic species are based on the maximum allowable concentrations in water in irrigation canals and receiving water; that is, an initial concentration of 740 ppm, with the concentration in return flows of treated irrigation ( i. e., flows into receiving rivers and streams) not to exceed 10 ppm. Since the maximum allowable concentration of 740 ppm exceeds the solubility limits of xylenes, acute risk was also assessed based on the maximum solubility limit for o xylene of 178 ppm. This exposure estimate was chosen to provide consistency with the chemical/ physical properties of xylenes applied without the addition of emulsifiers.

Since mixed xylenes are applied directly below the water surface, terrestrial exposure pathways and receptors typically considered in EFED science chapters, i. e., exposure of terrestrial animal food sources and plants via direct application or spray drift, are not considered. Any potential exposure to terrestrial ecosystems via treated irrigation water is expected to be minimal. Based on the use pattern for xylenes, the main exposure pathways for terrestrial animals are exposure via ingestion of contaminated water and inhalation of volatilized xylenes. To assess exposure of terrestrial animals via ingestion of contaminated water, the maximum allowable concentrations in irrigations canals and receiving water ( 740 and 10 ppm, respectively), and the maximum solubility limit for o xylene ( 178 ppm) were used. To determine the average amount of drinking water that birds and mammals may consume, allometric equations or empirical values given in the Wildlife Exposure Factors Handbook were used. Xylene concentrations in air were estimated using Henry's Law constant, which relates the concentration of a compound in the gas phase to its concentration in the liquid phase. Assumptions include steady state conditions and two phases ( air and water) which are partitioned by the non dimensional Henry's law constant. Insufficient toxicity data are available to assess risk of acute inhalation exposure of avian species. To assess the risk to mammals from a combination of exposure via drinking water and inhalation, a " composite" risk quotient ( RQ) was derived. The composite RQ was defined as the sum of oral ( drinking water) and inhalation RQs.

No registrant submitted studies in which mixed xylenes or xylene isomers were the sole active ingredient were identified. Thus, all ecological effects data reviewed in this risk assessment were obtained from the published open literature. Four sources were used to identify appropriate open literature publications: EPA's Ecotoxicology database ECOTOX, the Agency for Toxic Substances and Disease Registry ( ATSDR) Toxicological Profile for Xylene ( ATSDR 1995), the World Health Organization ( WHO) Environmental Health Criteria Document for Xylenes ( WHO 1997), and the National Toxicology Program ( NTP) Technical Report on Xylene ( NTP, 1986). 21 Risks were estimated based on a deterministic approach, where a single point estimate of toxicity is divided by an upper and lower bound exposure estimate to calculate a risk quotient ( RQ). The acute RQ values for each taxonomic group identified as an assessment endpoint were compared to the Agency's Levels of Concern ( LOCs). LOCs serve as criteria for categorizing potential risk to non target organisms. RQ values were calculated in the risk estimation section for each endpoint, and characterization and interpretation of risk is described in the risk description. Risks for each taxonomic group were described based on available lines of evidence from open literature data on mixed xylenes and the three xylene isomers. In addition, a preliminary assessment of listed species of concern was also completed.

1. Preliminary Identification of Data Gaps and Methods The adequacy of the submitted data was evaluated relative to Agency guidelines. The following identified data gaps for ecological fate and effects measures of effect result in a degree of uncertainty in evaluating the ecological risks of xylenes.

° There is uncertainty regarding the maximum exposure concentrations for aquatic organisms in both the irrigation canals and receiving waters. The solubility limits for xylene isomers are approximately 160 180 mg/ L; however, they are applied with an emulsifier which may result in a greater apparent solubility, possibly approaching the initial 740 ppm concentration level. There is also uncertainty in assuming the concentration in receiving water bodies and water ways. The label permits concentrations of xylene of up to 10 ppm to be released in receiving waters; however, it is unlikely that large volumes of irrigation water would be released to receiving water bodies ( given its high value in western states), and concentrations in receiving water are likely to be less than 10 ppm due to dissipation and dilution.

° No registrant submitted toxicity studies in which mixed xylenes or xylene isomers were the sole active ingredient were identified; all ecological effects data reviewed in this risk assessment were identified from published open literature. Although some of the acute aquatic studies were conducted in accordance with OECD guidelines, no consistent experimental protocols were followed for estuarine/ marine invertebrates. Therefore, there is a high degree of uncertainty to the toxicity data for this taxonomic group.

° Toxicity data on estuarine/ marine fish and vascular aquatic plants suitable for quantitative used in risk assessment was not identified from the available literature. Given xylene's acute toxicity to freshwater fish, it is assumed that similar adverse effects are likely for estuarine/ marine fish. In addition, because mixed xylenes are used to control aquatic vegetation, it is anticipated that exposure will produce adverse effects in vascular aquatic plants, including macrophytes.

° Acute inhalation data on avian species were not identified from the available 22 literature; therefore, risks associated with avian inhalation of xylene that has volatilized from xylene treated waters is not possible.

° There is uncertainty associated with the acute dietary toxicity study for birds, given xylene's high volatility. Therefore, the registrant should provide information on a avian LD 50 value or submit an acute avian oral toxicity study ( LD 50 ), using either a bobwhite quail or mallard duck, to satisfy the § 71 1 guideline requirements.

2. Measures to Evaluate Risk Hypotheses and Conceptual Model a. Measures of Exposure Water Exposure Water in canals and ditches treated with xylene is expected to have xylene concentrations ranging from 740 ppm ( the initial maximum concentration in the treated canal or ditch based on 11 gallons of product applied over a 20 30 minute period to water flowing at 1 cfs) at the point of application to 10 ppm ( current maximum allowable concentration in return flow). When concentrations fall below 100 or 200 ppm, additional xylene may be applied as a " booster application" ( Walsh et al. 1977), suggesting that xylene concentrations must be greater than 100 ppm to be effective in controlling aquatic weeds. The xylene concentrations at the release points for return flow should not exceed 10 ppm, but may be less based on limited monitoring data.

The change in concentration of a chemical being released into a flowing water channel depends upon its concentration in the water entering the channel ( return flow concentration  Co), the rate the solution water is entering the channel ( return flow rate  Q), the initial concentration in the channel ( receiving water  assumed to be 0 for xylene), and the amount of water in the channel ( volume  Vol). Concentration reductions were approximated by a simple steady state plug flow model.

Several receiving water body geometries were considered: equal volumes of return flow and receiving water, and 10, 20, and 50 times the return flow volumes. Larger volumes such as in a lake or a river could result in an even larger dilution. Since it is assumed that the flow rates are the same for the released water and the receiving water, the dilution would reflect increases in the area ( width times depth) of the water channel. A " no dilution" scenario was also presented to represent xylene treated flowing water in a dry channel and to provide an upper bound of the time necessary to hold the water before being released. 23 Inhalation Exposure The rapid volatilization of xylene from water suggests that mammalian and avian species may be exposed to levels of volatilized xylene over short time durations. Inhalation exposure is related to the flux rate ( i. e., mass per unit time transported to air), the exposure duration associated with toxicity, and the dimensions of the breathing zone for a potential receptor. For this assessment, the xylene concentration in the air following application to a drainage ditch was estimated using Henry's Law, which relates the concentration of a compound in air to its concentration in water.

Xylene concentrations are assumed to be uniform through the " breathing zone" To estimate an inhalation exposure concentration, the averaging time of 4 hours was used to correspond with the exposure duration used in the available mammalian toxicity studies.

b. Measures of Effect No registrant submitted studies in which mixed xylenes or xylene isomers were the sole active ingredient were identified. Thus, all ecological effects data reviewed in this risk assessment were obtained from the published literature. Four sources were used to identify appropriate open literature publications: EPA's Ecotoxicology database ECOTOX, the ATSDR Toxicological Profile for Xylene ( ATSDR 1995), the WHO Environmental Health Criteria Document for Xylenes ( WHO 1997), and the National Toxicology Program ( NTP) Technical Report on Xylene ( NTP, 1986). There is an extensive body of literature on the ecological effects of mixed xylenes and xylene isomers. However, a full review of all open literature is beyond the scope of this risk assessment. Therefore, the review of the available open literature data was focused on publications identified in the ECOTOX database. Additional publications including ATSDR, WHO, and NTP were also consulted in order to provide a comprehensive review of available toxicity information.

The chemical of interest for this risk assessment is mixed xylenes, which is a mixture of the three isomeric xylene compounds ( ortho , meta, and para xylene). To assess risk to aquatic and terrestrial receptors, data obtained from toxicity studies using mixed xylenes and xylene isomers were considered. The most sensitive endpoint from either the mixed xylene or individual isomer data was used to estimate risk.

A complete summary of the measures of effect based on toxicity studies for different ecological receptors and measures of effect for mixed xylenes and isomers is provided in Table 1, with study details provided in Appendix E. Given the rapid volatilization of xylenes, chronic toxicity tests are not required; however, the Agency reserves the right to request additional data if label usage changes.

c. Measures of Ecosystem and Receptor Characteristics The terrestrial and aquatic ecosystems considered in this assessment are intended to be generally representative of any aquatic or terrestrial ecosystem associated with areas where mixed xylenes are used. The receptors addressed by the aquatic and terrestrial risk assessments are summarized in 24 Table 1. For aquatic assessments, freshwater fish and invertebrates, estuarine/ marine fish and invertebrates, and algae are represented. For terrestrial assessments, birds and mammals are represented by three different weight categories. Detailed information regarding the data available for these various classes of aquatic and terrestrial receptors is provided in Appendix E.

III. Analysis A. Use Characterization Chem Aquatic Weed Killer, the only registered pesticide product ( EPA Reg. No. 9768 18) containing aromatic emulsifiable petroleum solvents, is used to control submerged weeds in irrigation and drainage canals. The product is used by the Bureau of Reclamation, the U. S. Dept. of Interior, and cooperating state water use organizations to control a variety of aquatic nuisances such as surface pond scum ( filamentous algae), American pondweed, horned pondweed , leafy pondweed, Richardson's pondweed, curly leaf pondweed, coon tail water weed, and water star grass. Applications depend on the individual water body, but in general the product is applied to drainage ditches as a liquid when weed growth begins to impede water flow. The formulated product is highly toxic to fish and other aquatic organisms, and consists of 98% mixed xylene isomers and small quantities of ethylbenzene and emulsifiers to aid in the dispersal of the xylenes throughout the channel. The influence of the emulsifiers on the fate and toxicity test values is not known; however, for the purposes of this assessment, their influence will not alter the conclusions significantly since the toxic thresholds for aquatic organisms is significantly lower than the solubility of xylene.

Current usage data for aromatic emulsifiable petroleum solvents is non existent. No pesticide usage data are available in publicly available databases including the National Center for Agricultural and Food Policy ( NCFAP 2005), National Agricultural Statistics Service Chemical Usage Database ( NASS 2005), and the California Department of Pesticide Regulation Pesticide Use Reporting system ( CDPR 2005). No current estimates of usage are available at the state level and historical estimates are sporadic. One reference from the Washington State Department of Ecology indicates that in 1967 over 750,000 gallons of xylene were used to treat drainage ditches in Oregon, Washington and Idaho ( WSDE 2002). Data from 1969 ( USDI BR, 1969) reports similar use levels ( 800,000 gallons per season) including more than 10,000 applications to 40,000 miles ( equivalent to 1 application every 4 miles) of canals, laterals, and drains throughout the Pacific Northwest.

Based on information on the label, Chem Aquatic Weed Killer may be used only in states included in the Bureau of Reclamation, Reclamation Act/ Newlands Act of 1902. These states include Arizona, California, Colorado, Idaho, Kansas, Montana, Nebraska, Nevada, New Mexico, North Dakota, Oklahoma, Oregon, South Dakota, Utah, Washington, and Wyoming. While these are the states where it is theoretically used, actual use is probably in far fewer states. 25 B. Exposure Characterization 1. Environmental Fate and Transport Characterization a. Summary of Empirical Data Following application to a drainage ditch, xylenes is expected to volatilize rapidly. Xylene has a sweet smell and an odor threshold of about 1 ppm. Photolysis and hydrolysis are not important environmental fate pathways since these compounds do not absorb photons of light with a wavelength greater than 290 nm, nor do they possess functional groups that are susceptible to hydrolysis under environmental conditions. In aquatic systems, xylenes undergo biodegradation under aerobic conditions relatively quickly, with half lives on the order of 20 days, but degradation under anaerobic conditions proceeds very slowly. Other than increasing the " solubility, the influence of the emulsifiers on the fate the xylenes are not known. The Agency assumes that the emulsifiers will also lower " volatility."

b. Degradation and Metabolism The degradation rate of xylene under aerobic and anaerobic conditions have been summarized in peer reviewed papers from the open literature and several review articles and databases ( ATSDR 1995; HSDB 2005). Although much of the published literature seeks to determine the environmental fate of xylene when released from petroleum storage tanks, spills, etc., these data may still be useful when assessing the fate of xylenes applied to control aquatic vegetation in a drainage ditch. In general, under aerobic conditions, the half lives of the isomers of xylene in soil range from several days to a few weeks, although at very high concentrations ( as might be encountered at a petroleum spill), these compounds can be toxic to microorganisms, resulting in long lag periods and a slow rate of degradation. Since aromatic petroleum solvents used as aquatic weed killers are not applied to soil surfaces or terrestrial vegetation, aerobic and anaerobic biodegradation in soils will not be an important fate process for these products.

In aquatic systems, xylenes were shown to degrade readily under aerobic conditions, but were generally not degraded under anaerobic conditions. Using PS 6 gasoline as a substrate, and microcosms constructed of 3 g of sandy soil and 20 mL of groundwater, the individual xylene isomers maintained under aerobic environments had biodegradation half lives of approximately 9 25 days depending upon the initial starting conditions ( API 1994). Under anaerobic conditions, very little degradation was observed over a 400 day incubation period.

c. Transport and Mobility The large vapor pressure ( approximately 6.6 to 9.6 mm Hg) and Henry's law constants ( approximately 0.005 to 0.008 atm m3/ mol) for the isomers of xylene suggest that volatilization from water surfaces will be an important route of dissipation for these compounds ( ATSDR 1995). An experiment which measured the rate of evaporation of xylenes from a 1: 1000 jet fuel: water mixture found that this rate averaged approximately 0.6 times the oxygen re aeration rate ( Smith 26 and Harper 1980). Combining this ratio with oxygen reaeration rates, the half life for evaporation of xylenes from a typical river or pond is estimated as 29 and 144 hours, respectively. While volatilization from water is a significant route of dissipation, volatilization from water is controlled by the water phase rather than the vapor phase.

Xylenes have moderate to high mobility in soils based upon K oc values in the range of 39 365 ( Lindhardt et al. 1994; Pavlostathis and Mathavan1992; Vowles and Mantoura 1987). Since aromatic petroleum solvents are used for aquatic weed control, they are not directly applied to terrestrial vegetation ( sites); however, irrigation water containing xylene flows onto terrestrial sites Therefore, irrigation of fields with xylene treated water may result in runoff and/ or return flow of xylene treated water in receiving water bodies. Since, the xylene is applied to a flowing water body ( canal or ditch), leaching out ( or leakage) of the canal is possible. Because many of these canals or ditches only contain water part of the year, it is unlikely that any groundwater present around these ditches would be used for drinking water. When water is present in the canals or ditches, water would tend to move away from the channel, when the channel is dry, water would tend to move back toward the channel.

d. Field Studies Terrestrial and groundwater contaminated field studies have been conducted to predict the environmental fate of xylene in the case of an accidental spill or leaking underground petroleum tank. These studies are not highly relevant to the environmental fate of aromatic petroleum solvents applied to a drainage ditch to control unwanted surface vegetation.

Six irrigation canals located in Wyoming, Colorado, and Washington were treated with aquatic weed killer containing emulsified xylenes ( Walsh et al. 1977). This product was released at a rate of 10 gallons per cfs of water flow within a 30 minute emission period, corresponding to an initial theoretical xylene level of 740 ppm ( Walsh et al. 1977). In one of the canals, xylene levels dissipated from ~ 555 ppm to < 10 ppm one mile downstream from its release point. In the other canals with greater water velocity, xylene levels remained at 100 ppm or greater for at least 4 miles downstream. In general, the xylene levels declined to 100 200 ppm or less after travelling 5 10 miles downstream. However, monitoring of xylene residues in return flow in the same study showed a decrease in concentration from 500 ppm in the water removed from lateral ditches used for irrigation to less than 0.2 ppm ( 200 ppb) ( study detection limit) in return flow after flowing through irrigation fields ( length of irrigation rills 750 to 1320 feet). No half life was reported nor was any attempt made to discern the most important dissipation pathway, although it was assumed volatilization was responsible for dissipation in the canals.

Recently the Agency obtained limited monitoring from the Washington State Department of Ecology ( WDE, 2005). The samples were collected, from one to several sampling stations and from one to several times, during 2002 to 2004 from four irrigation districts. These data reported xylene concentrations in irrigation waste water ( prior to release) to range between 0.004 and 10.9 ppm. There were 67 detections out of 108 samples. The mean value was 0.862 ppm. About 25 percent of the detections had concentration less than 0.04 ppm and about 80 percent of the 27 detections were less than 1.0 ppm.

Additional data on the affect emulsifiers on the properties of xylene in water would be useful to quantify the rates of dissipation associated with different irrigation methods, furrow lengths, field conditions, channel geometries, and flow rates and how these factors influence the concentration of xylene in return flows. These data have moderate value and would be useful in further refining exposure to nontarget and listed species.

e. Bioaccumulation Bioconcentration factors ( BCF) ranging from 14 to 24 have been reported for fish exposed to xylene isomers( Ogata and Miyaka 1978; Ogata et al. 1984). These BCF values suggest that bioconcentration and bioaccumulation in aquatic organisms is low.

2. Measures of Aquatic Exposure a. Aquatic Exposure Modeling Water in canals and ditches treated with xylene are expected to have xylene concentrations ranging from 740 ppm ( the initial maximum concentration in the treated canal or ditch) at the point of application to 10 ppm ( current maximum allowable concentration in return flow ). When concentrations fall below 100 ppm ( effective concentration), an additional xylene application may be required downstream from initial point of application ( Walsh et al., 1977). The xylene concentrations of the return flow at the release points to receiving water bodies should not exceed 10 ppm, but may be less.

The xylene concentration in the treated irrigation and drainage canals and channels depend upon the flow rates of the water, the geometry of the ditch or canal, and the dissipation rates of xylene in the treated canal. The concentration in the receiving water body depends upon the concentration in the return flow, the individual flow rates of the return flow and receiving water body, the geometry of the receiving water body, and the degradation rates of xylene in the receiving water body.

A simple steady state plug flow model was used to estimate the exposure concentrations in the receiving waters ( Appendix B). The change in concentration of a chemical being released into a flowing water channel depends upon its concentration in the water entering the channel ( return flow concentration  Co), the rate the solution water is entering the channel ( return flow rate  Q), the initial concentration in the channel ( receiving water  assumed to be 0 for xylene) and the amount of water in the channel ( volume  Vol). The model assumes that mixing is ideal ( constantly stirred), Q is constant, and the same amount of water enters the channel as leaves the channel ( with no leaching or evaporation). Assuming steady state flow system, the mass balance describing the time variation of the concentration in a flowing channel can be estimated by the following equation:

C( t) = C o * exp(( TVol/ Q)* R) 28 where:

C( t) = Concentration in mixed water body ( mass/ volume) at time t C o = initial concentration in return flow ( mass/ volume), assumed = 10 ppm R = first order degradation rate constant for volatilization ( 1/ time); median of literature values for flowing water.

Q = Flux density ( length3/ time)

TVol = Volume of return flow + volume of receiving water ( i. e., dilution) ( length3)

and where TVol = Cross sectional area of receiving water + cross sectional area of return flow ( length2) * flow rate ( length/ time). When there is no receiving water volume, the volume of receiving water is 0 and the Tvol is equal to the volume of return flow.

Several receiving water body geometries were considered: a dry ditch containing no water ( dilution 0), equal volumes of return flow and receiving water ( dilution 1: 1), and 10, 20, and 50 times the return flow volumes ( Tables 3). Larger volumes such as those in a lake or a river could result in an even larger dilution.

Table 3. Relationship Between Return Flow Volume and Receiving Water Volumes Used in the Plug Flow Model.

Dilution Q Discharged Return Flow Receiving water Total Flux ( ft3/ s) Velocity ( ft/ s) Area ( ft2) vol ( ft3) Velocity ( ft/ s) Area ( ft2) vol ( ft3) TVol ( ft3)

no dilution dry channel 1 1 1 1 0 0 0 1 1: 1 1 1 1 1 1 1 1 2 10: 1 1 1 1 1 1 10 10 11 20: 1 1 1 1 1 1 20 20 21 50: 1 1 1 1 1 1 50 50 51 The model was used to provide an estimate of the time or travel distance downstream needed to reduce the xylene concentration in the return flow ( assumed 10 ppm) to a concentration less than the 0.04 ppm ( mg/ L). The 0.04 mg/ L level represents a concentration of xylene in water that is 29 expected to protective of listed aquatic species ( see the Risk Description to Aquatic Organisms, Section IV. B. 1). When the receiving water flow rate is equal to return flow rates, almost 8 days are required for the concentration to drop below 0.04 ppm, 10 to 1 ratio ( volume receiving to volume return flow) about 1.5 days are required, 20: 1 ratio about 0.76 days, and 30 to 1 about 0.55 days. Additional discussion can be found in Appendix B. Table 4 also shows that almost 16 days would be required for xylene concentrations to dissipate to 0.04 mg/ L if the return water flows into a dry ditch.

Figure 2 shows the decline in xylene concentrations as calculated from the plug flow model with different dilution ratios. Estimated dilution factors are derived from the assumed geometry ( shape) of the receiving water body ( width and depth). The wider and/ or deeper the channel, the greater the volume available for dilution. Figure 2 shows the length of time it takes for xylene to decline to a concentration that is protective of listed aquatic species with no dilution( degradation volatilization only) at, 1 to 1 equal volumes, 10 to 1, 20 to 1, and 50 to 1 volume of receiving water to volume of return flows ( assume = flow rate).

Figure 2. Conce ntration of Xylene in Receiving Water Versus Dilution and Time. ( Dilutions are none, 10: 1, 20: 1, and 50: 1).

b. Aquatic Exposure Monitoring and Field Data 30 Data from a paper published by Walsh predicts xylene concentrations of < 0.2 ppm in return flows from treated fields.

Xylenes are ubiquitous in the environment due to fugitive emissions from industrial sources, automobiles, and accidental spills of petroleum solvents. Typical xylene levels usually range from about 4 to 130 : g/ m3 in outdoor air ( ATSDR 1995). According to groundwater surveys conducted in the United States, xylenes are generally detected in less than 5% of groundwater samples. However, xylene concentrations in contaminated groundwater have been reported as high as 10,000 ppb ( ATSDR 19955). Less than 6% of drinking water samples collected during U. S. drinking water surveys contained xylenes, and mean concentrations in positive samples were typically less than 2 ppb. Since xylenes volatilize rapidly from surface water, little data exists regarding background levels in streams, lakes, etc. Limited data were located in EPA's STORET database ( US EPA 2005) regarding the levels of total xylenes ( m xylene, p xylene, and o xylene) in treated drainage canals. STORET contained samples collected by Dade Environmental Resource Management ( Florida) for 25 drainage canals with concentrations of 1.2 : g/ L or less, however there is little indication as to the source of the xylene. STORET also reports that the Alaska Department of Environmental Conservation detected xylene concentrations as high as 75,000 : g/ L in a ground water monitoring well contaminated by a former gasoline service station ( site 102.26.003MW004). It is important to note that these concentrations are the result of groundwater contamination by leaking underground storage tanks, and not pesticide usage. These concentrations are not expected to result from pesticide application, but do provide an illustration of the ubiquitousness and magnitude of xylenes introduced into the environment from non pesticide sources.

The m, o, and p isomers of xylene have been detected in both surface water and ground water in the USGS NAQWA monitoring program ( drinking water). In surface water, the m xylene isomer was detected in 13.5 percent of the samples, and the o and p xylene isomers were detected in 18.9 percent ( n= 56) of the samples. In ground water, m xylene was detected in 5.3 percent and p and oxylene were detected in 10.8 percent of the samples ( n = 78). The maximum detected concentrations of o xylene in surface water and groundwater was 1.16 : g/ L and 0.93 : g/ L, respectively.

3. Measures of Terrestrial Exposure a. Terrestrial Exposure Modeling The anticipated exposure pathways for terrestrial animals are oral exposure via consumption of contaminated water and inhalation exposure of volatilized xylene. Exposure estimates for these pathways were derived as described in the following sections. 31 ( 1). Estimates for Exposure Via Consumption of Contaminated Water To determine the exposure of mammals and birds to xylenes via consumption of contaminated water, a single daily dose of xylenes was estimated using the calculated volume of water that birds and mammals are expected to consume per day and the concentration of xylenes in water as follows:

Daily Exposure ( mg/ kg BW) = Daily Water Consumption ( L) × Water Concentration ( mg/ L) Body Weight ( kg)

For this risk assessment, water concentrations were taken as the maximum allowable concentration range for mixed xylenes of 10 to 740 mg/ L and the solubility limit for o xylene of 178 mg/ L. Mixed xylenes are applied to achieve an initial concentration of approximately 740 ppm, with the concentration in return flows of treated irrigation ( i. e., flows into receiving rivers and streams) not to exceed 10 ppm. However, since the maximum allowable concentration of 740 ppm exceeds the solubility limits of xylenes, acute risk was also assessed based on the maximum solubility limit for oxylene of 178 ppm ( the highest solubility limit reported for the three xylene isomers). This exposure estimate was chosen to provide consistency with the chemical physical properties of xylenes applied without the addition of emulsifiers.

To estimate the volume of water that mammals and birds are expected to consume per day, allometric equations from the EPA Wildlife Exposure Factors Handbook ( US EPA 1993) were used. For birds, the daily water consumption ( L) was calculated using the following equation: " ( body weight in kg) $, where " = 0.059 and $ = 0.67 ( US EPA 1993, Equation 3 15, p. 3 8, for all birds). For mammals, the daily water consumption ( L) was calculated as using the following equation: " ( body weight in kg) $, where " = 0.099 and $ = 0.9 ( US EPA 1993, Equation 3 7, p. 3 10, for all mammals). As summarized in Table 4, a single daily exposure via drinking water containing 10 and 740 mg/ L xylenes was calculated for three weight classes of birds ( 20, 100, and 1000g) and mammals ( 15, 35, and 1000g). 32 Table 4. Exposure Estimates for Birds and Mammals via Consumption of Contaminated Water.

Species Body Weight ( g) Daily Water Consumption ( L) a Estimates for Exposure from Contaminated Water b Minimum Exposure ( mg/ kg BW) Based on a Water Concentration of 10 mg/ L Maximum Exposure ( mg/ kg BW) Based on a Water Concentration of 740 mg/ L Exposure ( mg/ kg BW) Based on a Water Concentration of 178 mg/ L c Birds 20 0.0043 2.15 160 38.5 100 0.013 1.30 96 23 1000 0.059 0.59 43.7 10.5 Mammals 15 0.0023 1.53 113 27.3 35 0.0048 1.37 103 24.3 1000 0.099 0.99 73.3 17.6 a For birds, the daily water consumption ( L) was calculated using the following equation: " ( body weight) $, where " = 0.059 and $ = 0.67 ( US EPA 1993, Equation 3 15, p. 3 8, for all birds). For mammals, the daily water consumption ( L) was calculated as using the following equation: " ( body weight) $, where " = 0.099 and $ = 0.9 ( US EPA 1993, Equation 3 7, p. 3 10, for all mammals). b Exposure estimates based were calculated as follows: Daily exposure ( mg/ kg BW) = ( Daily water consumption ( L) × Water concentration ( mg/ L))/ BW ( kg). b Concentration based on the solubility limit for o xylene.

( 2). Estimates for Exposure via Inhalation of Volatilized Xylenes The rapid volatilization of xylene from water suggests that mammalian and avian species may be exposed to high levels of volatilized xylene over short time durations. Although volatilization from water is rapid, it is more rapid in air. Thus, the water phase controls the volatilization of xylene from water. The non dimensional Henry's law constant was used to calculate a xylene exposure concentration in air; the constant relates the concentration of a compound in the gas phase ( air) to its concentration in water. This relationship is presented in the following equation:

H nl = c g / c l where: H nl = Henrys' constant ( nondimesional)

c g = concentration in the gas phase ( mass/ volume, mg/ m3) 33 c l = concentration in the water phase ( mass/ volume, mg/ m3)

This relationship provides an upper limit of the xylene concentration in air resulting from water vaporization of xylene. Thomas ( 1990) suggests that for compounds with Henry's Law constants > than 10 3 atm mol 3/ mol, the liquid phase will control the transfer from the liquid phase to gas phase.

A conservative inhalation exposure value can be estimated from the maximum air concentration, the exposure duration associated with toxicity, and the dimensions of the breathing zone for a potential receptor. The volume of air within the breathing zone of a receptor is based on the effective surface area of the receiving water, and a height above the water body that is considered relevant to ecological receptors. The surface area of the channel is assumed to be the same as the area of the breathing zone; therefore, the area can be simply represented by a unit area ( m2), and the height of the breathing zone for terrestrial receptors ( i. e., small mammals and waterfowl) is assumed to be 0.1 m. A breathing zone height of 0.1 m was chosen to represent a uniform concentration of xylene. The average breathing zone concentration of 4 hours is used to correspond to the 4 hour exposure duration used in the available mammalian inhalation toxicity studies. The xylene concentration in the air above the stream, for this assessment is assumed to remain constant ( a conservative estimate).

The maximum estimated exposure concentration for volatilized xylenes is summarized in Table 5. To convert estimated air concentrations of xylene expressed in terms of mg/ m3 to ppm, the following relationship was used: 1 mg/ m3 = 0.23 ppm ( ATSDR 1995). The maximum exposure value does not take into account the potential loss of xylenes in the breathing zone due to wind and degradation. The EEC for air was calculated for a 4 hour time period ( assuming constant exposure during this period) to correspond to the exposure period reported for the inhalation mammalian toxicity study used to derive acute inhalation risk quotients ( 4 hour LC 50 value of 6700 ppm in rats for mixed xylenes; Carpenter 1975).

Table 5. Maximum Estimated Exposure for Volatilized Xylenes.

Exposure Scenario EEC ( mg/ m3) EEC a ( ppm)

Maximum Exposure 167.4 38.5 a To convert the EEC in expressed terms of ppm to mg/ m3, the following relationship was used: 1 mg/ m3 = 0.23 ppm ( ATSDR 1995). The EEC expressed in terms of ppm was used to derive acute inhalation risk quotients for mammals. 34 b. Residue Studies Mixed xylenes are applied under water for use in the control of aquatic weeds in drainage and irrigation ditches. Thus, exposure of terrestrial ecosystems is not associated with the use of this chemical, and residue studies for terrestrial systems were not considered.

C. Ecological Effects Characterization In screening level ecological risk assessments, effects characterization describes the types of effects a pesticide can have on aquatic or terrestrial organisms. No registrant submitted studies in which mixed xylenes or xylene isomers were the sole active ingredient were identified. Thus, all ecological effects data reviewed for this risk assessment were obtained from the published open literature. Four sources were used to identify appropriate open literature publications: EPA's Ecotoxicology database ECOTOX, the ATSDR Toxicological Profile for Xylene ( ATSDR 1995), and the WHO Environmental Health Criteria Document for Xylenes ( WHO 1997), and the National Toxicology Program ( NTP) Technical Report on Xylene ( NTP, 1986). There is an extensive body of literature on the ecological effects of mixed xylenes and xylene isomers. However, a full review of all open literature is beyond the scope of this risk assessment. Therefore, the review of the available open literature data was focused on publications identified in the ECOTOX database. Additional publications including ATSDR, WHO, and NTP were also consulted in order to provide a comprehensive review of available toxicity information.

Appendix E summarizes the results of the open literature studies used to characterize effects for this risk assessment. Toxicity testing reported in this section does not represent all species of birds, mammals, or aquatic organisms. Only a few surrogate species for both freshwater fish and birds are used to represent all freshwater fish ( 2000+) and bird ( 680+) species in the United States. For mammals, acute studies are usually limited to the Norway rat or the house mouse. Estuarine/ marine testing is usually limited to a crustacean, a mollusk, and a fish. Also, testing for reptiles and amphibians are not required. The risk assessment assumes that reptiles are not more sensitive than birds. The same assumption is used for fish and amphibians ( i. e., amphibians are not more sensitive than fish).

1. Aquatic Effects Characterization The acute toxicity reference values used to assess risks to aquatic organisms are summarized in Table 6. A more detailed summary of the aquatic toxicity data available to characterize risks associated with mixed xylenes and xylene isomers is given in Appendix E, Tables E 1 through E 5. In general, results of acute toxicity studies indicate that xylenes are moderately to highly toxic to aquatic animals. It is important to note that due to their high volatility, xylene isomers disappear rapidly from solution. In a study evaluating the acute toxicity effects of xylenes in several aquatic species using an open system, significant losses of xylenes were observed over a 96 hour monitoring period ( 19 to 99% loss from 24 to 96 hours) ( Benville and Korn 1977). Given these levels of loss, exposure concentrations can be difficult to determine. Therefore, unless toxicity tests were 35 conducted under closed conditions, interpretation of toxicity data in aquatic animals may be confounded by the rapid loss of xylenes from solution.

The chemical of interest for this risk assessment is mixed xylenes, a combination of the three isomeric xylene compounds ( ortho , meta, and para xylene). To assess acute risk to aquatic organisms, toxicity data was obtained from studies using mixed xylenes and the three isomeric xylene compounds. Comparison of available acute toxicity values from the open literature shows that the three xylene isomers and mixed xylene have similar potencies in aquatic animals ( Table 7). It should be noted, however, that aquatic toxicity data on the end use product ( mixed xylene plus the emulsifier) is not available; therefore, it is not possible to determine the potential effect of the emulsifier on the toxicity of aquatic animals and plants. 36 Table 6. Xylene Toxicity Reference Values ( TRVs) for Aquatic Organisms.

Exposure Scenario Species Exposure Duration Toxicity Reference Value ( Test Substance) Reference ( Classification)

Freshwater Fish Acute rainbow trout 96 hours LC50 = 2.6 mg/ L d ( p xylene) Galassi et al., 1988 ( Supplemental)

Chronic No Data ( not required)

Freshwater Invertebrates Acute Daphnia magna 24 hours IC50 = 1.0 mg/ L a, d ( o xylene) Galassi et al., 1988 ( Supplemental)

Chronic No Data ( not required)

Estuarine/ Marine Fish Acute No Data Chronic No Data ( not required)

Estuarine/ Marine Invertebrates Acute sea urchin 96 hours EC50 = 4.1 mg/ L d ( o xylene) Falk Petersen et al. 1985 ( Supplemental)

Chronic No Data ( not required)

Aquatic Plants Non vascular plants Selenastrum capricornutum 72 hours EC50 = 3.2 mg/ L b, c, d ( p xylene) Galassi et al 1988 ( Supplemental)

Vascular plants No Data a IC50 is the concentration resulting in immobility in 50% of the test organisms. Immobility is used as a surrogate value for lethality. b EC50 is the concentration resulting in growth inhibition in 50% of test organisms. c Purity of test material was not specified; therefore, data are not expressed in terms of the concentration of a. i. d Test conducted under closed conditions to minimize loss due to volatilization. e Test conducted under open conditions; loss due to volatilization was not controlled. 37 Table 7. Comparison of the Range of Acute Toxicity Values for Mixed Xylenes and Xylene Isomers in Aquatic Animals.

Species Category LC50 Value ( Exposure Time in Hours)

Mixed Xylenes o Xylene m Xylene p Xylene Freshwater Fish Lowest Value no data 7.6 mg/ L ( 96) b 8.4 mg/ L ( 96) b 2.6 mg/ L ( 96) b Highest Value no data 16.1mg/ L ( 48) c 12.9 mg/ L ( 96) d 10 mg/ L ( 96) a Freshwater Invertebrates Lowest Value no data 1.0 mg/ L ( 24) e 4.7 mg/ L ( 24) e 3.6 mg/ L ( 24) e Highest Value no data > 22.4 mg/ L ( 48) f 9.6 mg/ L ( 48) g 8.5 mg/ L ( 48) g Estuarine/ Marine Fish: no data Estuarine/ Marine Invertebrates Lowest Value 7.4 mg/ L ( 96) h 4.1 mg/ L ( 96) i 19.3 mg/ L ( 48) j 24.5 mg/ L ( 48) j Highest Value 14.0 mg/ L ( 24) h 24.7 mg/ l ( 48) j 19.3 mg/ L ( 48) j 24.5 mg/ L ( 48) j a Species: rainbow trout ( Folmar et al. 1976) b Species: rainbow trout ( Galassi et al. 1988) c Species: fathead minnow, bluegill sunfish, goldfish, white sucker fish ( Holcombe et al. 1987) d Species: guppy ( Galassi et al. 1988) e Species: Daphnia magna ( Galassi et al. 1988) f Species: snail ( Holcombe et al. 1987) g Species: Daphnia magna ( Abernethy et al. 1986) h Species: grass shrimp ( Tatem et al. 1978) i Species: sea urchin ( Falk Peterson et al. 1985) j Species: brine shrimp ( Abernethy et al. 1986)

a. Aquatic Animals ( 1). Acute Effects Freshwater Fish No acute toxicity studies on mixed xylenes in freshwater fish were identified from the available literature. The acute toxicity of xylene isomers has been evaluated in freshwater fish; study details 38 are provided in Appendix E, Table E 1. The acute freshwater fish toxicity data show similar levels of toxicity for each of the three isomers with the lowest LC 50 values ranging from 2.6 to 7.6 mg a. i./ L.

For o xylene, acute toxicity values range from the 24 hour LC 50 value of 7.6 mg a. i./ L in rainbow trout ( Galassi et al, 1988) to the 96 hour LC 50 value of 16.1 mg/ L in bluegill sunfish, fathead minnow, goldfish, and white sucker fish ( Holcombe et al. 1978). Results of these studies indicate that o xylene is slightly to moderately toxic to freshwater fish on an acute basis. For m xylene, acute toxicity values range from the 96 hour LC 50 value of 8.4 mg a. i./ L in rainbow trout ( Galassi et al. 1988) to the 96 hour LC 50 value of 12.9 mg a. i./ L in guppy ( Galassi et al. 1988), suggesting that mxylene is moderately toxic to freshwater fish. Acute toxicity values for p xylene range from the 96 hour LC 50 value of 2.6 mg a. i./ L in rainbow trout ( Galassi et al. 1988) to the 96 hour LC 50 value of 10 mg a. i./ L in rainbow trout ( Folmar 1976), indicating that p xylene is moderately toxic to freshwater fish on an acute basis. The lowest acute toxicity value obtained for the p xylene isomer ( 96 hour LC 50 value of 2.6 mg a. i./ L in rainbow trout ( Galassi et al. 1988) is used to assess acute risk of mixed xylenes to freshwater fish. The study by Galassi et al. ( 1988) was conducted in accordance with OECD standardized method No. 203, modified for testing volatile compounds. Closed conditions were used to minimize loss of xylenes from the test media, and toxicity values are expressed in terms of measured concentrations.

The acute toxicity of xylene ( type not specified) and xylene plus emulsifier ( 2% AD 410) were compared in a single artificial stream study using rainbow trout ( Walsh et al. 1977). Results of this study show that the addition of emulsifier does not appear to alter the acute toxicity of xylene. The 24 hour LC 50 values for xylene alone and xylene plus emulsifier were 13 mg/ L and 17.3 mg/ L, respectively. It should be noted that these acute toxicity values are not true 24 hour LC 50 values; fish were exposed for a 2 hour time period and lethality was recorded 24 hours after the start of exposure.

Freshwater Invertebrates No acute toxicity studies on mixed xylenes in freshwater invertebrates were identified from the available literature. The acute toxicity of xylene isomers has been evaluated in freshwater invertebrates; study details are provided in Appendix E, Table E 2. Results indicate that the three xylene isomers have similar toxicities to freshwater invertebrates. For o xylene, acute toxicity values range from the 24 hour LC 50 value of 1.0 mg a. i./ L in Daphnia magna ( Galassi et al. 1988) to the 48 hour LC 50 value of > 22.4 mg/ L in the snail ( Holcombe et al. 1987), indicating that the toxicity of xylene isomers ranges from slightly to highly toxic in freshwater invertebrates. For m xylene, acute toxicity values range from the 24 hour LC 50 value of 4.7 mg a. i./ L in Daphnia magna ( Galassi et al. 1988) to the 48 hour LC 50 value of 9.6 mg a. i./ L in Daphnia magna ( Abernethy et al. 1986), indicating that m xylene is moderately toxic to freshwater invertebrates. Acute toxicity values in Daphnia magna for p xylene range from the 24 hour LC 50 value of 3.6 mg a. i./ L ( Galassi et al. 1988) to the 48 hour LC 50 value of 8.5 mg a. i./ L ( Abernethy et al. 1986), indicating that p xylene is moderately toxic to freshwater invertebrates on an acute basis. The lowest acute toxicity value obtained for the o xylene isomer, the 24 hour LC 50 value of 1.0 mg a. i./ L in Daphnia magna ( Galassi et al. 1988) is used to assess acute risk of mixed xylenes to freshwater invertebrates. As 39 previously noted, the study by Galassi et al. ( 1988) was conducted under closed conditions to minimize loss of xylenes from the test media, and toxicity values are expressed in terms of measured concentrations. The study was conducted in accordance with OECD standardized method No. 202, with modifications for testing volatile compounds. The 24 hour toxicity value of 1.0 mg a. i./ L was chosen as the representative acute endpoint for freshwater invertebrates over 48 hour studies, given the rapid volatilization of xylene, and because the study was conducted in accordance with OECD guidelines.

Estuarine/ Marine Fish No acceptable acute toxicity studies on mixed xylenes or xylene isomers in estuarine/ marine fish were identified from the available literature.

Estuarine/ Marine Invertebrates The acute toxicity of mixed xylenes has been evaluated in grass shrimp ( Tatem et al. 1978); study details are provided in Appendix E, Table E 4. Results of the study reports an acute 96 hour LC 50 value of 7.4 mg/ L in grass shrimp, indicating that mixed xylenes are moderately toxic to estuarine/ marine invertebrates.

Additional information on the acute toxicity of xylene in estuarine/ marine invertebrates was obtained in studies using xylene isomers. Study details are summarized in Appendix E, Table E 4. Results indicate that the xylene isomers have a similar degree of toxicity as mixed xylenes to estuarine/ marine invertebrates. For o xylene, acute toxicity values range from the 96 hour embryo lethality EC 50 value of 4.1 mg/ L in sea urchin eggs ( Falk Petersen et al. 1985) to the 48 hour LC 50 value of 24.7 mg/ L in brine shrimp ( Abernathy et al. 1986). Results of these studies indicate that oxylene is slightly to moderately toxic to estuarine/ marine invertebrates on an acute basis. For mxylene and p xylene, the respective 48 hour LC 50 values are 19.3 and 24.5 mg/ L in brine shrimp ( Abernathy et al. 1986), suggesting that the m xylene and p xylene isomers are slightly toxic to estuarine/ marine invertebrates on an acute basis.

The lowest acute toxicity value obtained for o xylene ( the 96 hour EC 50 of 4.1 mg/ L in sea urchin eggs ( Falk Petersen et al. 1985) is used to assess acute risk of mixed xylenes to estuarine/ marine invertebrates. It should be noted that there is uncertainty associated with this study because it was conducted under open conditions, and the authors report a significant decrease in xylene concentration over the course of the exposure period. However, it appears that the results were expressed in terms of measured concentrations. Despite the uncertainties associated with this study, the results suggest that reproductive effects in sea urchins may result from short term exposures to o xylene.

( 2). Chronic Effects Due to the rapid volatilization of mixed xylenes in receiving waters, chronic exposure of aquatic organisms is not expected to occur; thus, chronic toxicity studies in aquatic animals are not required. In irrigation ditches, relatively high concentrations are maintained for long periods of time which will result in mortality to aquatic animals occurring there; therefore chronic effects would not have time 40 to occur, so chronic data are not required for this exposure either.

( 3). Sublethal Effects Limited information is available regarding sublethal effects of mixed xylenes or xylene isomers in aquatic animals. A study in rainbow trout reports that fish exposed to xylene ( type not specified) concentrations of 3.2 mg/ L and higher for two hours resulted in the development of sublethal effects ( loss of equilibrium), with an NOAEC value of 0.65 mg/ L ( Walsh et al. 1977). The severity of the effect was dose dependent and the onset of effects was approximately 1.4 hours after the initiation of exposure. Effects in survivors were reversible, with fish recovering after a " short time" in untreated water. In rainbow trout exposed to 0.001, 0.01, and 0.1 mg/ L p xylene for 1 hour, fish exhibited attractant behavior at a concentration of 0.01 mg/ L, but avoidance behavior at a concentration of 0.1 mg/ L ( Folmar 1976).

( 4). Field Studies Field studies on the effects of mixed xylenes or xylene isomers in aquatic animals are not available.

b. Aquatic Plants No studies on the acute effects of mixed xylenes in algae were identified from the available literature. The acute toxicity of xylene isomers was evaluated in two studies using Selenastrum capricornutum, a green algae ( Galassi et al. 1988; Herman et al. 1990). Galassi et al. ( 1988) report 72 hour EC 50 values for growth inhibition ranging from 3.2 mg a. i./ L for p xylene to 4.9 mg a. i/ L for m xylene. Herman et al. ( 1990) report 8 day EC 50 values for growth inhibition ranging from 3.9 mg/ L for mxylene to 4.4 mg/ L for p xylene. Study details are provided in Appendix E, Table E 5. The lowest acute toxicity value obtained for the p xylene isomer, the 72 hour EC 50 value of 3.2 mg a. i./ L in Selenastrum capricornutum ( Galassi et al. 1988), is used to assess acute risk of mixed xylenes to algae. It should be noted that NOEAC data for non vascular plants is not available. The study by Galassi et al. ( 1988), which was performed in accordance with OECD standardized method No. 201, was conducted under closed conditions to minimize loss of xylenes from the test media, and toxicity values are expressed in terms of measured concentrations.

Limited information is available regarding the toxicity of mixed xylenes or xylene isomers to aquatic macrophytes and other vascular plants. Results of a single laboratory study on the efficacy of xylene ( xylene type not reported) on target aquatic macrophytes shows that xylene damaged target plants under standing and moving water conditions ( Frank et al. 1961); study details are provided in Appendix E, Table E 5. The plant species used in this study were water weed ( Elodea canadensis), American pondweed ( Potamogeton nodosus), sago pondweed ( P. pectinatus). Under static conditions, extensive damage was observed to all three plant species tested at a concentration of 100 ppm, but no damage was observed at a concentration of 5 ppm. Under moving water conditions, extensive damage was observed to all three species at test concentrations of 300 and 600 ppm. Since EC/ LC 50 values or NOAEC values were not determined, data are not suitable for quantitative 41 use. Given that mixed xylenes are used to control aquatic vegetation, toxicity of xylenes to aquatic plants is presumed.

2. Terrestrial Effects Characterization The toxicity endpoints used to characterize risks of acute mixed xylenes exposure to birds and mammals are summarized in Table 8. Results of all studies in terrestrial organisms are summarized in Appendix E, Tables E 6 and E 7. As discussed in Section II, the anticipated exposure pathways for terrestrial animals are by ingestion of contaminated water and inhalation of volatilized xylenes in air. Therefore, the types of toxicity studies pertinent to this risk assessment are acute oral exposure, acute dietary exposure, and acute inhalation exposure.

Table 8. Toxicity Reference Values ( TRVs) for Mixed Xylenes in Terrestrial Organisms.

Exposure Scenario Species Exposure Duration Toxicity Reference Value Reference ( Classification)

Mammals Subacute Dose based rat 14 days LD50 = 1608 mg/ kg BW NTP, 1986 ( Supplemental)

Acute Inhalation rat 4 hours LC50 = 6700 ppm Carpenter et al. 1975 ( Supplemental)

Chronic No Data Available   not required Birds Acute Dose based No Data Available Acute Dietary based Japanese quail 5 day dietary LC50 > 20,000 mg a. i./ kg diet Hill and Carmardese 1986 ( Supplemental)

Acute Inhalation No Data Available Chronic No Data Available   not required Plants No Data Available   not required a. Terrestrial Animals ( 1). Acute Effects Birds No acute oral or acute inhalation toxicity studies in birds with mixed xylenes or xylene isomers were identified from the available literature. A single acute dietary study in Japanese quail was identified from the available literature ( Hill and Camardese 1986); study details are provided in Appendix E, 42 Table E 6. This study failed to establish an acute lethality value in birds, with the acute dietary LC 50 value reported as > 20,000 mg a. i./ kg diet. The LC 50 value of > 20,000 mg a. i./ kg diet in Japanese quail is used to characterize acute risk to birds from ingestion of mixed xylenes in contaminated water.

Exposure estimates for birds via drinking contaminated water were expressed in terms of a single daily dose ( mg/ kg BW) for three different body weight classes of birds ( 20, 100, and 1000 g) ( Table 4). Therefore, to calculate acute RQs for birds, the acute dietary LC 50 was converted to a daily dose for exposure via drinking water ( expressed in terms of mg/ kg BW xylenes) for each of the three body weight classes as follows: the avian acute dietary LC 50 value expressed in terms of mg/ kg diet was first converted to an equivalent LD 50 value expressed in terms of mg/ kg body weight, using the following equation:

LD50 ( mg/ kg body weight) = LC50 in food ( mg/ kg diet) × daily food consumption ( kg diet) body weight ( kg)

where the average daily food consumption per bird reported in the Hill and Camardese ( 1986) study was 0.0126 kg. The body weights of Japanese quail was not reported in the Hill and Camaradese ( 1986) study; therefore, for this calculation, the average body weight for a 14 day Japanese quail was taken as approximately 0.043 kg ( Woodard et al. 1973).

Thus, the acute oral LD 50 for Japanese quail was derived as > 5,860 mg/ kg body weight [> 20,000 mg/ kg diet × 0.0126 kg diet/ 0.043 kg body weight = > 5,860 mg/ kg body weight]. To obtain the adjusted LD 50 value, the acute oral LD 50 value for the Japanese quail (> 5,860 mg/ kg body weight) was adjusted for the size of the animal tested compared with the size of the animal being assessed ( e. g., 20 g bird). Exposure estimates from contaminated water and toxicity values are relative to the animal's body weight ( mg residue/ kg BW) because consumption of the same mass of pesticide residue results in a higher body burden in smaller animals as compared with larger animals. The following equation was used for the LD 50 adjustment for birds:

Adjusted LD 50 = LD 50 ( AW/ TW)( x 1)

where:

Adjusted LD 50 = Adjusted LD 50 ( mg/ kg BW) calculated by the equation; LD 50 = > 5,860 mg/ kg BW ( acute oral LD 50 value for the Japanese quail); AW = Body weight of assessed bird ( 20 g, 100 g, and 1000 g); TW = Body weight of tested animal ( 43 g for 14 day Japanese quail); and x = Mineau scaling factor for birds; EFED default is 1.15.

The calculated toxicity value for a daily dose for each weight class of birds are summarized in Table 9. The derived daily dose values were used to calculate acute RQs for birds for exposure via consumption of contaminated water. 43 Table 9. Adjusted Acute Toxicity Values for Birds, Expressed in Terms of a Single Xylene Dose ( mg/ kg BW).

Species Body Weight ( g) Adjusted LD50 a ( mg/ kg body weight)

Birds 20 > 5224 100 > 6651 1000 > 9396 a Adjusted LD50 = LD50 ( AW/ TW)( x 1) where LD50 = > 5860 mg/ kg BW (> 20,000 mg/ kg diet for the Japanese quail; Hill and Camardese); AW = 0.02, 0.1, and 1.0 kg; TW = 0.043 kg for the 14 day Japanese quail, and x = Mineau scaling factor for birds = EFED default value of 1.15.

Mammals Results of the acute oral toxicity studies on mixed xylenes are summarized in Table E 7. Acceptable subacute oral toxicity data for mammals was located in the NTP ( 1986) study, where male and female rats were exposed to doses of mixed xylene ranging from 125 to 2000 mg/ kg BW over a duration of 14 days. Three of five male and five of five female rats that received 2000 mg/ kg BW died before the end of the study. Two other deaths at lower doses ( 1 male death at 125 mg/ kg BW and 1 female death at 250 mg/ kg BW) were attributed to gavage trauma. It should be noted that all of the mortalities were observed within the first 2 to 4 days of the study. Based on the results of this study, the subacute oral LD 50 value is 1608 mg/ kg BW. The lowest LD 50 value of 1608 mg/ kg body weight ( NTP 1986) is used to assess acute toxic risk for oral exposure of mammals to mixed xylenes.

Exposure estimates for mammals via drinking contaminated water were expressed in terms of a single daily dose ( mg/ kg BW) for three different body weight classes ( 15, 25, and 1000 g) ( Table 4). Therefore, to obtain the adjusted mammalian LD 50 value, the subacute mammalian LD 50 value for the rat ( 1608 mg/ kg body weight) is adjusted for the size of the animal tested compared with the size of the animal being assessed ( e. g., 15 g mammal). The following equation is used for the LD 50 adjustment for mammals:

Adjusted LD 50 = LD 50 ( TW/ AW)( 0.25)

where:

Adjusted LD 50 = Adjusted LD 50 ( mg/ kg BW) calculated by the equation; LD 50 = 1608 mg/ kg BW ( subacute oral LD 50 value for the rat); TW = Body weight of tested animal ( 350 g rat); and AW = Body weight of assessed mammal ( 15 g, 35 g, and 1000 g);

The calculated toxicity values for a daily dose for each weight class of mammals are summarized in 44 Table 10. The derived daily dose values were used to calculate acute RQs for mammals for exposure via consumption of contaminated water.

Table 10. Adjusted Acute Toxicity Values for Mammals, Expressed in Terms of a Single Xylene Dose ( mg/ kg BW).

Species Body Weight ( g) Adjusted LD50 a ( mg/ kg body weight)

Mammals 15 3534 35 2859 1000 1237 a Adjusted LD50 = LD50 ( TW/ AW)( 0.25) where: LD50 = 1608 mg/ kg BW ( Carpenter et al. 1975); AW = 0.015, 0.035, and 1.0 kg; and TW = 0.35 kg for rat.