Since mixed xylenes are highly volatile, acute inhalation exposure is an anticipated exposure route for mammals near the application site, and acute inhalation toxicity studies were reviewed for this risk assessment. The 4 hour LC 50 value for rats for mixed xylenes is 6700 ppm ( Carpenter et al. 1975); this value is used to assess acute toxic risk of acute inhalation exposure of mammals to mixed xylenes. It should be noted that there are uncertainties related to the dosing method for the inhalation mammalian toxicity test because the study author does not specifically discuss the methodology, but rather refers to another paper.

( 2). Chronic Effects Due to the rapid volatilization of mixed xylenes, and low potential to accumulate, chronic exposure of terrestrial animals is not expected to occur; thus, chronic toxicity studies in terrestrial animals are not required.

( 3). Sublethal Effects Birds No information regarding sublethal effects of acute exposure to mixed xylenes or xylene isomers in birds was identified from the available literature. The acute dietary exposure study in Japanese quail reports that no adverse effects were observed at a dietary concentration of 5,000 mg/ kg diet, the lowest concentration tested ( Hill and Camardese 1986). This statement implies that adverse effects were observed at dietary concentrations greater than 5000 mg/ kg diet; however, no information regarding adverse effects of acute dietary exposure to xylenes was reported in this study. 45 Mammals Exposure of rats to mixed xylene over a period of 14 days resulted in decreased body weight. The change in mean body weight relative to controls was 23 to 29% lower for males that received 250, 500, and 1000 mg/ kg BW and 17% and 26% lower for females that received 125 and 1000 mg/ kg BW after 14 days. Shallow, labored breathing and prostration were observed immediately after dosing for male and female rats that received 2000 mg/ kg BW. No compound related effects were observed at necropsy.

Clinical signs of neurotoxicity were reported in an acute inhalation exposure study in rats ( Carpenter et al. 1975). At a concentration of 1300 ppm, rats exhibited poor coordination after two hours of exposure, but fully recovered following exposure. At concentrations of 2800 ppm, rats became prostrate, but recovered within an hour although coordination remained poor. The NOAEC for signs of toxicity was reported as 580 ppm.

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

Terrestrial Invertebrates No information on the acute toxicity of xylenes exposure to terrestrial invertebrates was identified from the available literature. The method of application should preclude exposure to honey bees, so honey bee testing is not required.

b. Terrestrial Plants Under the conditions of recommended use, chronic exposure of terrestrial plants to mixed xylenes is not anticipated. Thus, chronic exposure studies on mixed xylenes in terrestrial plants are not required.

IV. Risk Characterization Risk characterization is the integration of exposure and effects characterizations to determine the ecological risk from the use of mixed xylenes and the effects on aquatic life and wildlife based on recommended use scenarios. The risk characterization provides an estimation and a description of the risk; articulates risk assessment assumptions, limitations, and uncertainties; synthesizes an overall conclusion; and provides the risk managers with information to make regulatory decisions.

A. Risk Estimation  Integration of Exposure and Effects Data Results of the exposure and toxicity effects data are used to evaluate the likelihood of adverse ecological effects on non target species. For the assessment of mixed xylenes risks, the risk quotient ( RQ) method is used to compare exposure and measured toxicity values ( see Appendix F). Estimated environmental concentrations ( EECs) are divided by acute toxicity values. The resulting RQs are then compared to the Agency's levels of concern ( LOCs). These LOCs, summarized in 46 Appendix F, are the Agency's interpretive policy used to analyze potential risk to non target organisms and the need to consider regulatory action. Details of all RQs derived for this assessment are provided in Appendix G.

For exposures of non target aquatic species, three EECs were used to assess acute risk: 10 mg/ L ( maximum allowable concentration in receiving waters), 740 mg/ L ( maximum allowable concentration in irrigation canals), and 178 mg/ L ( solubility limit for o xylene). Mixed xylenes are applied to achieve an initial concentration of approximately 740 mg/ L, with the concentration in return flows of treated irrigation ( i. e., flows into receiving rivers and streams) not to exceed 10 mg/ L. 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). The value of 178 mg/ L was chosen to provide consistency with the chemical physical properties of xylenes applied without the addition of emulsifiers. Since mixed xylenes are highly volatile, chronic exposure of aquatic ecosystems is not anticipated; thus, only acute risk from xylenes exposure was assessed. Toxicity reference values for aquatic exposure to mixed xylenes and xylene isomers are summarized in Table 6.

Mixed xylenes are applied underwater; therefore, exposure of terrestrial organisms via direct application and spray drift are not considered as potential exposure pathways for terrestrial organisms. Risks to terrestrial plants via irrigation water and exposure of mammals and birds via ingestion of contaminated forage items was not considered for this assessment because exposures for these pathways are expected to minimal. 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 by 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 amount of water birds and mammals are expected to consume in one day ( Table 4). To assess risks due to inhalation exposure, the non dimensional Henry's law constant was used to estimate xylene air concentrations in a breathing zone above the water based upon the concentration of xylene in water ( solubility limit  178 mg/ L). The maximum exposure concentration of xylene in air ( 38.5 ppm) assumes no loss of xylene from the breathing zone due to wind or degradation ( Table 5). Since mixed xylenes are highly volatile, chronic exposure of terrestrial ecosystems is not anticipated; thus, only acute risk from xylenes exposure was assessed. Toxicity reference values for terrestrial organisms exposed to mixed xylenes are summarized in Table 7.

1. Non Target Aquatic Animals and Plants To assess risks of mixed xylenes to non target aquatic animals ( i. e., fish, invertebrates) and plants ( i. e., algae), three EECs were used to assess acute risk: 10 mg/ L ( maximum allowable concentration in receiving waters), 740 mg/ L ( maximum allowable concentration in irrigation canals), and 178 mg/ L ( solubility limit for o xylene). Toxicity data are available to assess acute risk to freshwater fish, freshwater invertebrates, estuarine/ marine fish, estuarine/ marine invertebrates, and non vascular aquatic plants. No toxicity data are available to evaluate acute risks to vascular aquatic plants. 47 Acute RQs for aquatic species are summarized in Table 11. The LOCs for acute risk ( LOC 0.5), are exceeded for the three water concentrations considered for this assessment. RQs for the lowest allowable exposure level of 10 mg/ L range from 3 for non vascular aquatic plants ( i. e., algae) to 10 for freshwater invertebrates. For the maximum exposure level of 740 mg/ L, acute RQs range from 231 for non vascular aquatic plants to 740 for freshwater invertebrates. All RQs based on the solubility limit for o xylene exceed all acute LOCs, ranging from 56 for non vascular aquatic plants to 178 for freshwater invertebrates. Although acute toxicity data is not available for estuarine/ marine fish, LOC exceedances for this taxonomic group are assumed based on xylene's known toxicity to freshwater fish. Thus, results of this analysis show that at all exposure concentrations considered in this assessment, including the maximum allowable concentration in return flows of treated irrigation, all aquatic animals and plants assessed are at acute risk from exposure to xylenes.

Table 11. Acute RQs for Freshwater Fish, Freshwater Invertebrates, Estuarine/ Marine Fish, Estuarine/ Marine Invertebrates, and Algae Exposed to Mixed Xylenes or Xylene Isomers.

Species Toxicity Value ( mg/ L) Acute RQ for Receiving Water Exposure a Acute RQ for Irrigation Canal Exposure b Acute RQ Based on the Solubility Limit for o Xylene c Freshwater Fish 2.6 mg/ L d 3.8 h 284 h 68 h Freshwater Invertebrates 1.0 mg/ L e 10 h 740 h 178 h Estuarine/ Marine Fish No data NA NA NA Estuarine/ Marine Invertebrates 4.1 mg/ L f 2.4 h 180 h 43 h Non vascular Aquatic Plants 3.2 mg/ L h 3 h 231 h 56 h a EEC/ LC50, where the EEC is the maximum allowable exposure of 10 mg/ L in receiving waters. b EEC/ LC50, where the EEC is the maximum exposure of 740 mg/ L in irrigation canals. c EEC/ LC50, where the EEC is the solubility limit for o xylene of 178 mg/ L. d 96 hour LC50 value in rainbow trout for p xylene ( Galassi et al. 1988). Study conducted under closed conditions. e 24 hour LC50 value in Daphnia magna for o xylene ( Galassi et al. 1988). Study conducted under closed conditions. f 96 hour EC50 value in sea urchin eggs for o xylene ( Falk Petersen et al. 1985). Study conducted under open conditions. g 72 hour EC50 value ( for growth inhibition) in Selenastrum capricornutum for p xylene ( Galassi et al. 1988). Study conducted under closed conditions. No NOAEC value is available to characterize risks to listed non vascular plant species. h RQ exceeds the acute LOCs for acute risk ( 0.5), acute restricted use ( LOC 0.1) and acute endangered species ( LOC 0.05). 48 2. Non Target Terrestrial Animals The most likely exposure pathways for terrestrial animals are ingestion of contaminated water and inhalation of volatilized xylenes. Based on these potential exposure pathways, the following scenarios were assessed for terrestrial animals: acute risks to mammals and birds from ingestion of contaminated water; acute risks to mammals via inhalation of volatilized xylenes; and acute risks to mammals from combined exposure via 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.

a. Acute Risk to Mammals and Birds from Ingestion of Contaminated Water To assess acute risks to terrestrial animals from ingestion of contaminated water, three EECs were used: 10 mg/ L ( maximum allowable concentration in receiving waters), 740 mg/ L ( maximum allowable concentration in irrigation canals), and 178 mg/ L ( solubility limit for o xylene). Acute RQs for birds and mammals exposed to xylenes via consumption of contaminated water were calculated using the derived daily exposure value for xylenes ( expressed in terms of mg/ kg BW) ( Table 4) and the adjusted LD 50 toxicity value for a daily dose of xylenes ( expressed in terms of mg/ kg BW) for birds ( Table 9) and mammals ( Table 10). Three weight classes of birds and mammals were evaluated. Additional details regarding these derived exposure and toxicity values are provided in Section III. B. 3. a and Section III. C. 2. a.( 1) of this report, respectively.

Acute RQs for birds and mammals exposed to xylenes via consumption of contaminated water are summarized in Table 12. All acute RQs for mammals and birds for exposure from drinking contaminated water are below acute endangered species LOCs ( LOC = 0.1). For birds, acute RQs range from < 0.01 ( for all three weight classes for water concentrations of 10 and 178 mg/ L) to < 0.031 ( 20 g birds for a water concentration of 740 mg/ L). Due to the failure of acute dietary toxicity studies in birds to establish a definitive acute LC 50 value ( LC 50 > 20,000 mg a. i./ kg diet), acute avian RQs are reported as " less than" values. For mammals, acute RQs range from 0.004 ( 15 g mammals for a water concentration of 10 mg/ L) to 0.059 ( 1000 g mammals consuming water at a concentration of 740 mg/ L). Based on this assessment, mammals and birds do not appear at acute risk from exposure via consumption of contaminated water. 49 Table 12. Acute RQs for Birds and Mammals Exposed to Mixed Xylenes via Consumption of Contaminated Water.

Species Body Weight ( g) Acute RQ Based on a Water Concentration of 10 mg/ L a, b Acute RQ Based on a Water Concentration of 740 mg/ L a, b Acute RQ Based on the Solubility Limit for o xylene of 178 mg/ L a, b Birds c 20 < 0.01 < 0.031 < 0.01 100 < 0.01 < 0.014 < 0.01 1000 < 0.01 < 0.01 < 0.01 Mammals 15 0.0004 0.032 0.008 35 0.0005 0.036 0.008 1000 0.0008 0.059 0.014 a Acute RQs were calculated using the exposure values ( mg/ kg BW) derived in Table G 2 and the adjusted LD50 values ( mg/ kg BW) shown in Table G 3, as follows: EEC/ Adjusted LD50 value. b RQs are below the LOCs for acute risk ( LOC 0.5), acute restricted use ( LOC 0.2), and acute endangered species ( LOC 0.1). c RQs for birds are based on the LC50 value > 20,000 mg/ kg diet ( adjusted LD50 values for 20 g, 100 g, and 1000 g birds are > 5224, > 6651, and > 9396 mg/ kg BW, respectively. Therefore, acute avian RQs are reported as " less than" values.

b. Acute Risk to Mammals Via Inhalation Exposure This assessment only considers the inhalation exposure of mammals to those vapors which are derived from the treated canal or ditch. The non dimensional Henry's law constant was used to estimate air concentrations in a breathing zone above the water based upon the concentration in water ( solubility limit  178 mg/ L). The maximum exposure concentration of 38.5 ppm assumes no loss of xylene from the breathing zone due to wind or degradation ( Table 5). The lowest 4 hour LC 50 value for rats exposed to mixed xylenes via inhalation ( 6700 ppm; Carpenter et al. 1975) was used as the toxicity value to calculate acute inhalation RQs.

The acute RQ for mammals exposed to xylenes via inhalation is below the acute endangered species LOC ( LOC = 0.1) ( Table 13). The acute inhalation RQ based on the maximum inhalation EEC is 0.006. Based on this analysis, mammals exposed to volatilized xylenes do not appear to be at acute risk. 50 Table 13 Acute RQs for Mammals Exposed to Volatilized Mixed Xylenes Via Inhalation.

Inhalation Toxicity Value a ( ppm) Acute Inhalation RQ Based on Maximum Estimated Exposure Maximum EEC in Air ( ppm) b Maximum Acute Inhalation RQ c ( EEC/ Toxicity Value)

6700 38.5 0.006 a 4 hour LC50 value in rats for mixed xylenes ( Carpenter et al. 1975). b EECs generated with the non dimensional Henry's law constant which estimates the concentration of a compound in the gas phase in relationship to its concentration in liquid phase Cg = Cl * Henry C. c RQ is below the LOCs for acute risk ( LOC 0.5), acute restricted use ( LOC 0.2), and acute endangered species ( LOC 0.1).

c. Acute Risk to Mammals from Combined Exposure via Inhalation and Contaminated Water To determine if a combination of exposures by consumption of contaminated water and inhalation of volatilized xylenes poses a risk to mammals, a composite acute RQ was derived. The composite acute RQ is defined as the sum of the acute drinking water RQ and the acute inhalation RQ. Composite acute RQs were calculated using the maximum acute drinking water RQs for the three mammalian weight classes based on the maximum concentration in water of 740 mg/ L ( Table 12 and the acute inhalation RQ for the maximum estimated exposure for volatilized xylenes in air ( Table 13) Since the minimum acute drinking water RQs and the minimum acute inhalation RQ for mammals are several orders of magnitude below the acute LOCs, composite RQs using the minimum acute inhalation RQs were not derived.

Composite acute RQs for mammals range from 0.038 ( 15 g mammals) to 0.065 ( 1000 g mammals) ( Table 14.) Based on this assessment, mammals do not appear to be at risk from the combination of exposure by ingestion of contaminated water and exposure by inhalation of volatilized xylenes. 51 Table 14 Composite Acute RQs for Mammals Exposed to Mixed Xylenes by Consumption of Contaminated Water and Inhalation.

Mammalian Body Weight ( g) Maximum Acute RQ for Consumption of Water Containing 740 mg/ L Xylenes a Maximum Acute Inhalation RQ b Maximum Acute Composite RQ c, d 15 0.032 0.006 0.038 35 0.036 0.006 0.042 1000 0.059 0.006 0.065 a Details of RQ calculation are provided in Table G 4. b Details of RQ calculation are provided in Table G 5. c RQ is calculated as the sum of the acute RQ for consumption of contaminated water and the acute inhalation RQ. d RQs are below the LOCs for acute risk ( LOC 0.5), acute restricted use ( LOC 0.2), and acute endangered species ( LOC 0.1).

B. Risk Description The focus of this screening level assessment is 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) water bodies ( i. e., streams, rivers, and/ or lakes) which receive return flow from xylene treated irrigation water. In evaluating the possible risks of mixed xylenes to aquatic and terrestrial receptors in and around treated ditches, the range of potential water concentrations include the recommended application concentration of approximately 740 mg/ L, and lower concentrations in return flows of treated irrigation ( i. e., flows into receiving water bodies). Given this use pattern, potential exposure pathways for aquatic and terrestrial ecosystems are from direct exposure to contaminated water ( aquatic organisms) and from ingesting treated water and inhalation exposure of xylenes that volatilize from contaminated water ( terrestrial animals). The focus of the assessment was on acute effects because no useful chronic data were found. Chronic testing is not recommended because chronic exposure of aquatic and terrestrial ecosystems outside of the treated ditches is not expected to significantly contribute to impact, due to the rapid volatilization of xylenes from water ( half life of approximately 1 6 days in various water bodies ( Smith and Harper 1980). Since mixed xylenes are applied directly below the surface of the water, terrestrial exposure pathways such as exposure of terrestrial animal food sources and plants via direct application or spray drift, were not considered likely.

The results of this screening level risk assessment suggest the potential for direct adverse acute effects to fish and aquatic invertebrates and aquatic plants in the irrigation canals and ditches ( where concentrations of xylene may be as high as 740 mg/ L). Although terrestrial mammals and birds may be exposed to xylene via drinking water and inhalation pathways, these routes of exposure are unlikely to result in acute adverse effects. The potential for effects in receiving waters is less certain. 52 The current label permits release of up to 10 ppm, and at this rate of release, significant potential for downstream effects exist. However, if this level is reduced to ~ 1 ppm or lower, the potential for effects to aquatic animals in the receiving waters ( where the maximum allowable concentration would not exceed 1 ppm), would drop considerably both in magnitude, duration and distance downstream.

1. Risks to Aquatic Organisms As previously mentioned, the focus of the aquatic assessment was to evaluate risks in two habitat types including the irrigation canals and ditches were xylene is applied and the receiving water bodies which receive return flow of xylene treated irrigation water. Given the high exposure levels associated with the recommended initial concentration of mixed xylenes in irrigation canals and ditches, acute risks are predicted for all freshwater aquatic animals and plants inhabiting the canals and ditches. However, it is unlikely that estuarine/ marine animals and plants would be exposed to xylene treated water in the irrigation canals because these systems are assumed to be freshwater; therefore, risks to estuarine/ marine species are not anticipated in irrigation canals.

Acute risks to freshwater and estuarine/ marine animals and plants species in receiving water bodies were assessed using two exposure scenarios. First assuming return flows of treated irrigation into receiving rivers and streams at the maximum allowable concentration of 10 mg/ L. Then, an assumption of a maximum return flow concentration of 1 mg/ L was assessed. If return flows of treated water are to large, rapidly moving water bodies, the concentration of xylene would be expected to decrease rapidly, minimizing acute risks to non target aquatic organisms. However, if return flows of treated water are to small, slow moving or stagnant water bodies, only minimal dilution of xylene may occur.

To determine the concentrations at which listed aquatic organisms in receiving waters would not be at acute risk from xylene in return flows, a concentration at which no acute LOCs are exceeded for aquatic organisms was calculated, as summarized in Table 18. The maximum water concentrations that yield acute RQs below the lowest acute LOC ( acute endangered risk LOC for aquatic animals = 0.05; aquatic endangered risk LOC for aquatic plants = 1.0) were calculated for each aquatic species category using the following relationship:

EEC/ LC 50 = acute RQ that is less than the acute endangered LOC ( 0.05 or 1.0).

Concentrations that are protective of listed aquatic species were calculated using the lowest LC 50 / EC 50 values reported for aquatic organisms and target RQs of 0.04 for aquatic animals and 0.90 for aquatic plants. In order to ensure that listed aquatic organisms in receiving waters are not at acute risk from xylene in return flows, xylene concentrations in return flows should not exceed 0.04 mg/ L for freshwater environments and 0.16 for estuarine/ marine environments. Calculated concentrations that are protective of listed aquatic species in return flows are approximately 250 and 63 fold lower than the current allowable concentration of 10 mg/ L for freshwater and estuarine/ marine environments, respectively. 53 Table 15. Calculated Xylene Concentrations at which Acute RQs Are Less Than Acute LOCs for Aquatic Organisms .

Species Toxicity Value ( mg/ L) Target Acute RQ EEC Protective of Listed Aquatic Species ( mg/ L) a Freshwater Fish b 2.6 mg/ L 0.04 0.10 Freshwater Invertebrates c 1.0 mg/ L 0.04 0.04 Estuarine/ Marine Invertebrates d 4.1 mg/ L 0.04 0.16 Algae e 3.2 mg/ L 0.90 2.88 a EEC = Target Acute RQ × Toxicity Value. b 96 hour LC50 value in rainbow trout for p xylene ( Galassi et al. 1988). c 24 hour LC50 value in Daphnia magna for o xylene ( Galassi et al. 1988). d 96 hour LC50 value in sea urchin eggs for o xylene ( Peterson Falk et al. 1985). e 72 hour EC50 value ( for growth inhibition) in Selenastrum capricornutum for p xylene ( Galassi et al. 1988).

A simple steady state plug flow dilution model ( see Appendix B) was used to estimate the amount of dissipation that would be required to achieve a xylene concentration of 0.04 mg/ L that is protective of listed aquatic species in receiving water. 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 as an alternative concentration of 1 mg/ L, to a concentration of 0.04 mg/ L required to be protective of listed aquatic species. Given the uncertainties associated with the actual size of the receiving water body, the model was run assuming a range of dilution factors. In addition, the water flow rate was assumed to be 1 cfs and the volatilization half life was assumed to be 2 days. The results of the model, which are summarized in Table 19, show that the time and distance required for xylene to dissipate to a concentration of 0.04 mg/ L in receiving water decreases with increasing dilution. The distance required to dissipate to 0.04 mg/ L is dependant on the geometries of the both the receiving water and the return flow systems. The point to stress is that the reduction in xylene concentration occurs over both time and distance. Generally, the larger the volume of the receiving water, the shorter the downstream return flow component in the irrigation canal or drainage ditch necessary to obtain the desired decrease in xylene concentrations. Note that the duration of exposure to a given organisms is not determined by the time frames reported; these only represent how long a slug of water containing xylene takes to dissipate. The duration of exposure to specific organisms or communities is a function of the duration of release of ditch water. As long as the ditch water is being released exposure will continue downstream from the release point.

In as much as the risk management goal for xylene is to reduce the concentration of the release water to 1 mg/ L, the remaining discussion will focus on that scenario.

Assuming a one to one dilution, it would take approximately 4.5 days ( or a length of 76 miles) for 54 xylene treated water to dissipate from 1 mg/ L to a protective concentration of 0.04 mg/ L. As previously mentioned, the amount of dilution in a particular body of water is dependant on the size and environmental conditions of the receiving water body. Increasing the dilution factor by 10, 20, and 50 decreases the amount of time and distance ( required for xylene treated water to dissipate from 1 mg/ L to 0.04 mg/ L) by the same factor, resulting in dissipation times/ distances of approximately 20 hours/ 13.8 miles, 10.5 hours/ 7.2 miles, and 4.8 hours/ 3 miles, respectively.

Table 16. Time, Distance, and Dilution for Xylene Concentrations to Decrease from 10ppm and 1ppm to < 0.04 ppm Based on a Steady State Plug Flow Dilution Model Condition Reduction from 1 ppm to 0.04 ppm 1: 1 dilution 10: 1 dilution 20: 1 dilution 50: 1 dilution Time ( days) 4.64 0.84 0.44 0.2 Distance ( miles) 76 13.8 7.2 3.0 The important point to consider is that at a 1.0 mg/ L, only a 1: 25 dilution would be needed to reach a concentration of 0.04 mg/ L. At a 1.0 mg/ L concentration, the amount of xylene in the receiving water body will continue to dissipate due to volatilization and mixing. Therefore, this supports reducing from 10 mg/ L to 1 mg/ L or some other lower value. In addition, it is recommended that the revised label language include a statement that treated irrigation water be used to irrigate fields. Since xylene is highly volatile, this common agricultural practice will greatly reduce the amount of xylene in the canal outflows that ultimately reach natural water bodies. If treating the fields is not feasible, the treated water should be held until measurable xylene concentrations are 1 mg/ L or less.

2. Risks to Terrestrial Organisms Results of this assessment show that LOCs for birds and mammals are not exceeded when exposed via ingestion of contaminated water ( Table 12). Additionally, mammals do not appear to be at acute risk from inhalation exposure ( Table 13) or from the combination of ingestion of contaminated water and inhalation exposure ( Table 14). However, there are several uncertainties that should be considered in interpreting the results of the assessment for terrestrial organisms.

To estimate acute risks to birds and mammals from consumption of contaminated water for the maximum exposure concentration of 740 mg/ L, it was assumed that the concentration of xylene remained constant over the exposure period of one day. Given the high volatility of xylenes, this assumption provides a very conservative exposure estimate, and results in use of an EEC that is higher than that which can be reasonably anticipated. Thus, acute RQs derived using the maximum exposure concentration of 740 mg/ L are likely to result in an over estimation of risk. However, even with this conservative approach, acute RQs for birds and mammals for consumption of contaminated 55 water are below all acute LOCs.

The acute avian toxicity values used to assess acute risks from consumption of contaminated water were reported as " greater than" values; thus, avian acute RQs are likely to be lower than those reported. However, since all acute avian RQs are below acute LOCs, uncertainties associated with the avian toxicity value do not affect the conclusions of this risk assessment: birds do not appear to be at acute risk from ingestion of contaminated water, even at the highest exposure level.

Results of this assessment indicate that mammals are not at risk from exposure via inhalation of volatilized xylenes. Acute inhalation RQs for mammals were derived using the 4 hour LC 50 value of 6700 ppm for mixed xylenes in rats ( Carpenter et al. 1975) and one exposure scenario, which represents the maximum air concentration of xylene predicted by Henry's Law, and assuming a solubility of 178 mg/ L ( Table 13). It is likely that this inhalation exposure concentration is overestimates as it does not consider degradation and dilution due to wind. Major limitations associated with the estimated inhalation exposure concentration include uncertainties associated with the potential effect of the emulsifier on volatilization and uncertainties related to the actual height at which exposures occur. Although there is uncertainty associated with the inhalation exposure estimate, use of the most conservative exposure estimate results in RQs that are below acute LOCs.

Avian exposure via inhalation of volatilized xylenes was not characterized due to a lack of inhalation toxicity data for birds. If inhalation toxicity of xylenes to birds is similar to that of mammals, risks associated with this exposure pathway would be unlikely for avian receptors. In order for there to be LOC exceedances for birds, the 4 hour inhalation toxicity LC 50 value for birds would have to be 385 ppm or roughly 175 times less than the inhalation toxicity for mammals.

The composite acute RQ derived for mammals for exposure via consumption of contaminated water and inhalation of volatilized xylenes indicates that mammals are not at acute risk from xylenes applied to irrigation ditches. However, this assessment does not consider dermal exposure of water mammals, such as beavers or otters, that may spend a substantial amount of time in contaminated water. It is anticipated that the animals' coats would act as a barrier to xylenes, thereby, substantially limiting dermal exposure. However, insufficient information is available to assess whether the dermal absorption represents a pathway of significant exposure.

3. Review of Incident Data Incident reports submitted to EPA since approximately 1994 have been tracked by assignment of EIIS ( Environmental Incident Information System) in an Incident Data System ( IDS). There are no incident reports for xylenes involving aquatic or terrestrial organisms.

4. Endocrine Effects Results of a study on sea urchin eggs shows that exposure to xylenes at 4.1 mg a. i./ L results in embryo death to 50% of the test population ( Falk Petersen et al. 1985). The results of this study indicate that reproductive effects in sea urchins may result from short term exposures to o xylene. 56 Although there is no conclusive evidence that exposure to xylenes alters developmental effects in aquatic species, this possibility cannot be ruled out. Therefore, EFED must consider the possibility that xylenes may have detrimental effects on the endocrine system.

Under the Federal Food, Drug and Cosmetic Act ( FFDCA), as amended by the Food Quality Protection Act ( FQPA), EPA is required to develop a screening program to determine whether certain substances ( including all pesticide active and other ingredients) " may have an effect in humans that is similar to an effect produced by a naturally occurring estrogen, or other such endocrine effects as the Administrator may designate". Following the recommendations of its Endocrine Disruptor Screening and Testing Advisory Committee ( EDSTAC), EPA determined that there was scientific basis for including, as part of the program, the androgen and thyroid hormone systems, in addition to the estrogen hormone system. EPA also adopted EDSTAC's recommendation that the Program include evaluations of potential effects in wildlife. For pesticide chemicals, EPA will use FIFRA and, to the extent that effects in wildlife may help determine whether a substance may have an effect in humans, FFDCA authority to require the wildlife evaluations. As the science develops and resources allow, screening of additional hormone systems may be added to the Endocrine Disruptor Screening Program ( EDSP). When the appropriate screening and or testing protocols being considered under the Agency's Endocrine Disruptor Screening Program have been developed, mixed xylenes may be subjected to additional screening and or testing to better characterize effects related to endocrine disruption.

5. Federally Threatened and Endangered ( Listed) Species Concerns a. Action Area The Endangered Species Act defines the action area for a Federal action as being the footprint of possible direct and indirect effects stemming from the action, not necessarily limited to where the immediate action occurs. The screening level risk assessment conservatively assumes that listed species are co located with the pesticide treatment area. For example, terrestrial wildlife potentially occur adjacent to the irrigation canals and/ or potential receiving waters, and aquatic organisms potentially occur downstream of where xylene is initially applied. This assessment recognizes the potential that the listed species are located within an assumed area which has the relatively highest potential exposure to xylene, and that exposures are likely to decrease with distance from the treated area.

If these assumptions result in RQs that are below the listed species LOCs, a " no effect" conclusion is made with respect to listed species in that taxa, and no further refinement of the action area is necessary. Furthermore, RQs below the listed species LOCs for a given taxonomic group indicate no concern for indirect effects upon listed species that depend upon the taxonomic group covered by the RQ as a resource. However, in situations where the screening assumptions lead to RQs in excess of the listed species LOCs for a give taxonomic group, the potential for a " may affect" conclusion exists and may be associated with direct effects on listed species that depend upon that taxonomic group or may extend to indirect effects upon listed species that depend upon that taxonomic group as a resource. In such cases, additional information on the biology of listed species, the locations of 57 these species, and the locations of xylene use sites would be considered to determine the extent to which screening assumptions regarding an action area apply to a particular listed organism. These subsequent refinement steps would consider how this information would impact the action area for a particular listed organism and may potentially include exposures that are downstream of the pesticide use site.

b. Taxonomic Groups Potentially at Risk ( 1). Discussion of Risk Quotients Should estimated exposure levels occur in proximity to listed resources, the available screening level information suggests a potential concern for direct effects on listed freshwater fish and invertebrates ( including amphibians), estuarine/ marine invertebrates, and aquatic plant species associated with the use of xylene in irrigation canals. Direct effects are possible in downstream receiving waters at the estimated exposure levels. Regarding estuarine/ marine fish and aquatic plants, LOC exceedances are assumed for estuarine/ marine fish in receiving waters and vascular plants both in ditches and in receiving waters, given xylene's known toxicity to freshwater fish and its intended use to kill aquatic weeds. It should be noted, however, that although LOCs were exceeded for non vascular plants ( i. e., algae), there are no endangered species of unicellular ( non vascular) plants.

( 2). Probit Dose Response Relationship The Agency uses the probit dose response relationship as a tool for providing additional information on the listed animal species levels of concern. The acute listed species LOC of 0.05 is used for aquatic animals. As part of the risk characterization, an interpretation of acute LOCs for listed species is discussed. This interpretation is presented in terms of the chance of an individual event ( i. e, mortality or immobilization) should exposure at the estimated environmental concentration actually occur for a species with sensitivity to mixed xylenes on par with the acute toxicity endpoint selected for RQ calculation. To accomplish this interpretation, the Agency uses the slope of the dose response relationship available from the toxicity study used to establish the acute toxicity measures of effect for each taxonomic group. The individual effects probability associated with the LOCs is based on the mean estimate of the slope and an assumption of a probit dose response relationship. In addition to a single effects probability estimate based on the mean, upper and lower estimates of the effects probability are also provided, if available, to account for variance in the slope. The upper and lower bounds of the effects probability are based on available information on the 95% confidence interval of the slope. A statement regarding the confidence in the estimated event probabilities for each taxonomic group is not included because the raw data was not presented as part of the open literature toxicity data. Therefore, it was not possible to determine if the data exhibited good probit fit characteristics or to derive 95% confidence intervals for the reported slope values.

Individual effect probabilities are calculated based on an Excel spreadsheet tool IEDV1.1 ( Individual Effect Chance Model Version 1.1) developed by the U. S. EPA, OPP, Environmental Fate and Effects Division ( June 22, 2004). The model allows for such calculations by entering the mean slope estimate ( and the 95% confidence bounds of that estimate, if available) as the slope parameter for 58 that spreadsheet. In addition, the LOC ( 0.05 for aquatic animals) is entered as the desired threshold.

( 3.) Data Related to Under represented Taxa Mixed xylene data related to under represented taxa, such as amphibians, are not available from the open literature. ( 4.) Implications of Sublethal Effects Results of this risk analysis indicate that at the exposure levels considered in this assessment, direct effects on survival of aquatic animals and growth inhibition of aquatic plants are likely. However, sublethal effects have also been reported in freshwater fish. 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 exposed to xylene ( type not specified) for approximately 1.4 hours, with NOAEC and LOAEC values of 0.65 mg/ L and 3.2 mg/ L, respectively ( Walsh et al. 1977). Loss of equilibrium is likely to leave fish more vulnerable to predation pressure, which may result in reduced survival of aquatic populations. Sublethal effects, including loss of equilibrium, are reported to occur in fish at exposure levels below the highest level currently allowed to be released ( 10 mg/ L). If the maximum release level is reduced to 1 mg/ L, the concentration in receiving waterways would be expected to relatively quickly decline to below the NOAEC of 0.65 mg/ L.

There is evidence to suggest that fish also exhibit avoidance behavior in response to xylene. In rainbow trout exposed to 0.001, 0.01, and 0.1 mg 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). Avoidance behavior may result in decreased exposure to xylenes. However, depending upon the conditions of the waterways where mixed xylenes may occur, it may not be possible for fish to avoid exposure to xylene levels associated with toxicity.

c. Indirect Effects Analysis The Agency acknowledges that pesticides have the potential to exert indirect effects upon the listed organisms by, for example, perturbing forage or prey availability, altering the extent of nesting habitat, etc. In conducting a screen for indirect effects, direct effect LOCs for each taxonomic group are used to make inferences concerning the potential for indirect effects upon listed species that rely upon non endangered organisms in these taxonomic groups as resources critical to their life cycle. When a taxonomic group shows an RQ higher than the listed species LOC, there is a potential concern for indirect effects to any listed species in any taxonomic group that has a dependency on the taxa for which the RQ is in excess of the LOC and which is co located with the pesticide use site.

Because screening level acute RQs for freshwater fish and invertebrates and estuarine/ marine invertebrates exceed the listed species acute LOCs, the Agency uses the dose response relationship from the toxicity study used for calculating the RQ to estimate the probability of acute effects associated with an exposure equivalent to the EEC. It should noted, however, that data on the dose 59 response relationship was not available for estuarine/ marine invertebrates; therefore, a default slope value of 4.5 with confidence intervals of 2 and 9 was used. The uncertainty associated with the estimated even probabilities for estuarine/ marine invertebrates is increased by the large default confidence intervals associated with the slope.

In instances where information on the dose response is available, it serves as a guide to establish the need for and extent of additional analysis that may be performed using Services provided " species profiles" as well as evaluations of the geographical and temporal nature of the exposure to ascertain if a " not likely to adversely affect" determination can be made. The degree to which additional analyses are performed is commensurate with the predicted probability of adverse effects from the comparison of dose response information with the EECs. The greater the probability that exposures will produce effects on a taxa, the greater the concern for potential indirect effects for listed species dependant upon that taxa, and therefore, the more intensive the analysis on the potential listed species of concern, their locations relative to the use site, and information regarding the use scenario ( e. g., timing, frequency, and geographical extent of pesticide application).

Indirect effect analyses for freshwater and estuarine/ marine fish and invertebrates and aquatic plants are described in further detail below.

Freshwater Fish Freshwater fish acute endangered species LOCs are exceeded, based on maximum allowable concentrations of xylene in receiving waters of irrigation canals. Therefore, there are potential concerns for indirect effects on listed animals that eat fish or amphibians ( e. g., fish, mammals, birds, reptiles), or in the case of freshwater mussels, use a fish as a necessary host in the life cycle.

Because raw data was not provided as part of the acute toxicity study for the rainbow trout, information is unavailable to derive 95% confidence intervals for the reported slope value of 1.1 ( Galassi et al., 1988). An event probability was calculated for the listed species LOC ( 0.05) based on the reported slope value of 1.1. Based on an assumption of a probit dose response relationship with a slope of 1.1, the corresponding estimated chance of individual mortality associated with the listed species LOC of the acute toxic endpoint for freshwater fish is 1 in 131 (~ 0.76%). The uncertainty in estimated event probabilities for this taxonomic group is increased by the lack of established confidence intervals surrounding the mean slope value.

Freshwater Invertebrates Given that acute LOCs are exceeded for freshwater invertebrates, indirect effects to listed species ( e. g., freshwater fish, mammals, birds, amphibians) that rely on freshwater invertebrates ( i. e., daphnids) as a primary food source may be of concern.

Raw data was also not provided as part of the acute toxicity study for D. magna; therefore, it was not possible to derive 95% confidence intervals for the reported slope value of 1.4 ( Galassi et al., 1988). Based on an assumption of a probit dose response relationship with a slope of 1.4, the 60 corresponding estimated chance of individual mortality/ immobilization associated with the listed species LOC of the acute toxic endpoint for freshwater invertebrates is 1 in 292 (~ 0.34%). Similar to freshwater fish, the uncertainty in estimated event probabilities for freshwater invertebrates is increased by the lack of established confidence intervals for the mean slope value.

Estuarine/ Marine Fish and Invertebrates Although no acute toxicity data was available for estuarine/ marine fish, risks to this taxonomic group are assumed given xylene's known toxicity to freshwater fish. Acute endangered species LOCs are exceeded for estuarine/ marine invertebrates ( and assumed to be exceeded for estuarine/ marine fish); therefore, there are potential concerns for indirect effects on listed animals that eat estuarine/ marine fish or invertebrates ( e. g., fish, mammals, birds, reptiles).

The toxicity endpoint for estuarine/ marine invertebrates was obtained from a study on sea urchin eggs by Falk Peterson et al. ( 1985). Because raw data was not provided as part of this study, information was not available to estimate a slope for the dose response curve. An event probability was calculated for the listed species LOC based on a default slope assumption of 4.5 ( Urban and Cook, 1986). Based on an assumption of a probit dose response relationship with a default slope of 4.5, the corresponding estimated chance of individual mortality and/ or reproductive effect associated with the listed species LOC ( 0.05) of the acute toxic endpoint for estuarine/ marine invertebrates is 1 in 4.17E+ 08 (~ 2.4E 09%). It is recognized that extrapolation of very low probability events is associated with considerable uncertainty in the resulting estimates. In order to explore the possible bounds to such estimates, the upper and lower default values for the mean default slope estimate ( 95% C. I.: 2 to 9, Urban and Cook, 1986) were used to calculate upper and lower estimates of the effects probability associated with the listed species LOC. The respective lower and upper effects probability estimates are 1 in 216 (~ 0.46%) and 1 in 1.0E+ 16 (~ 1.0E 16%). The uncertainty in estimated event probabilities for estuarine/ marine invertebrates is increased by the large confidence intervals associated with the default slope.

Aquatic Plants Although aquatic plant data suitable for use in risk assessment is available for only non vascular aquatic algae, it is assumed that application of mixed xylenes to irrigation canals at the recommended label rate would result in exceedances of non listed and listed LOCs vascular and non vascular plants. Therefore, there is potential concern for adverse effects to those listed species that depend on plants in very narrow and broad ways for some important aspect of their life cycle. Listed species of concern would include those species with very narrow habitat, feeding, and/ or larval substrate dependencies ( termed obligates) as well as those with more general herbivory or cover type dependencies. There is concern for listed species that rely on aquatic plants for food and/ or habitat and shelter. Species specific concerns for mixed xylenes indirect effects to listed organisms requires a determination of the coincidence of pesticide use with locations of listed species and the biologically based resources upon which they depend.

d. Critical Habitat 61 In the evaluation of pesticide effects on designated critical habitat, consideration is given to the physical and biological features ( constituent elements) of a critical habitat identified by the U. S. Fish and Wildlife and National Marine Fisheries Services as essential to the conservation of a listed species and which may require special management considerations or protection. The evaluation of impacts for a screening level pesticide risk assessment focuses on the biological features that are constituent elements and is accomplished using the screening level taxonomic analysis ( RQs) and listed species LOCs that are used to evaluate direct and indirect effects to listed organisms.

The screening level risk assessment has identified potential concerns for indirect effects on listed species for those organisms dependant upon freshwater and estuarine/ marine fish and invertebrates and aquatic plants. In light of the potential for indirect effects, the next step for EPA and the Service( s) is to identify which listed species and critical habitat are potentially implicated. Analytically, the identification of such species and critical habitat can occur in either of two ways. First, the agencies could determine whether the action area overlaps critical habitat or the occupied range of any listed species. If so, EPA would examine whether the pesticide's potential impacts on non endangered species would affect the listed species indirectly or directly affect a constituent element of the critical habitat. Alternatively, the agencies could determine which listed species depend on biological resources, or have constituent elements that fall into, the taxa that may be directly or indirectly impacted by mixed xylenes. EPA would then determine whether mixed xylenes usage overlaps the critical habitat or the occupied range of those listed species. At present, the information reviewed by EPA does not permit the use of either analytical approach to make a definitive identification of species that are potentially impacted indirectly or of critical habitats that are potentially impacted directly by the use of mixed xylenes in irrigation canals. EPA and the Service( s) are working together to conduct the necessary analysis.

This screening level risk assessment for critical habitat provides a listing of potential biological features that, if they are constituent elements of one or more critical habitats, would be of concern. These correspond to the taxa identified above as being of potential concern for indirect effects and include the following: freshwater and estuarine/ marine fish and invertebrates, aquatic plants, birds, and mammals. This list should serve as an initial step in problem formulation for further assessment of critical habitat impacts outlined above, should additional work be necessary.

e. Co occurrence Analysis Given the usage of xylene in 16 western states, a large number of listed aquatic species are likely to occur in counties where xylene is used to control aquatic weeds in irrigation canals. A summary of the number of listed species by taxonomic group for crops where irrigation water from irrigation canals is used ( assumed to be potential xylene use areas) is provided in Table 17. The list of crops likely to be irrigated with water from irrigation canals was obtained from the USDA National Agricultural Statistics Service ( NASS) website and information on irrigated crops presented in a paper by Walsh et al ( 1977). A total of 487 listed species that may potentially co occur with xylene use areas were identified. Table 18 provides a summary of the listed species by State. A comprehensive list of these listed species is provided by State in Appendix J. Given the large number of listed species, EFED recommends restricting xylene use to a smaller number of states. 62 Restriction of xylene usage to a smaller number of states would reduce the number of listed species that may be potentially affected.

The registrant must provide information on the proximity of Federally listed endangered species to mixed xylenes use sites. This requirement may be satisfied in one of three ways: 1) having membership in the FIFRA Endangered Species Task Force; 2) citing FIFRA Endangered Species Task Force data; or 3) independently producing these data, provided the information is of sufficient quality to meet FIFRA requirements. The information will be used by the OPP Endangered Species Protection Program to develop recommendations to avoid adverse effects to listed species. 63 Birds Mammals Amphibians Fish Crustaceans Reptile Insects Snails Clams Plants Table 17. Tabulation by Taxonomic Group and Crop of Listed Species That May Occur in Mixed Xylene Use Areas Crop Taxonomic Group Alfalfa ( 16 States) 26 42 9 71 9 8 23 10 3 246 Barley ( 16 States) 23 41 9 57 7 8 20 7 0 183 Carrots ( 16 States) 23 35 8 44 8 7 24 6 0 200 Citrus ( AZ and CA) 18 29 7 25 8 8 18 2 0 171 Corn for grain ( 16 States) 22 37 8 42 8 8 13 10 3 166 Corn for silage or greenchop ( 16 States) 24 38 7 58 9 8 19 8 2 197 Cotton ( AZ, CA, KA, and NM) 20 24 6 30 5 5 6 1 1 77 Dry edible beans, excluding limas ( 15 States  all except NV) 20 35 7 38 6 7 16 8 0 156 Forage  land used for all hay and haylage, grass silage, and greenchop ( 16 States) 26 42 9 71 9 9 27 10 3 259 Lima beans ( CA, KA, NM, OK, OR, and WA) 17 22 5 15 8 4 7 3 0 105 Lettuce ( 14 States  all except ND and WY) 23 37 8 48 7 9 26 3 0 219 Oats for grain ( 16 States) 23 40 8 66 8 9 21 8 3 223 Potatoes ( 16 States) 23 40 8 60 9 8 26 10 2 222 Rice ( CA and OK) 9 9 1 10 3 3 6 0 1 30 Sorghum for grain ( 13 States  all except MO, OR, and WA) 20 25 7 32 7 7 7 5 3 76 Soybeans ( CO, KA, MO, NE, NM, ND, OK, and SD) 8 7 0 7 0 0 1 0 3 4 Sugarbeets ( 11 States  all except AR, NV, NM, OK, and SD) 14 15 4 17 4 4 4 6 0 43 Tomatoes ( 16 States) 26 41 8 60 9 9 27 10 2 249 Wheat for grain ( 16 States) 25 36 8 57 9 8 22 9 3 202 Birds Mammals Amphibians Fish Crustaceans Reptile Insects Snails Clams Plants Crop Taxonomic Group 64 Birds Mammals Amphibians Fish Crustaceans Reptile Insects Snails Clams Plants Total Unique Species 26 42 9 71 9 9 27 10 3 259 Total States 16 14 4 16 3 5 8 4 2 16 Total Counties 731 289 48 362 39 39 74 9 5 203 Table 18. Tabulation by Taxonomic Group and State of Listed Species That May Occur in Mixed Xylene Use Areas State Taxonomic Group Arizona 9 1 2 17 0 2 0 1 0 18 California 16 22 6 29 8 8 22 1 0 180 Colorado 3 2 0 6 0 0 2 0 0 13 Idaho 2 4 0 7 0 0 0 6 0 3 Kansas 4 2 0 4 0 0 1 0 0 2 Montana 4 3 0 4 0 0 0 0 0 2 Nebraska 4 1 0 2 0 0 0 0 1 3 Nevada 2 0 0 23 0 1 2 0 0 9 New Mexico 7 5 1 12 2 1 0 5 0 0 North Dakota 4 0 0 1 0 0 0 0 0 1 Oklahoma 7 3 0 4 0 0 1 0 2 2 Birds Mammals Amphibians Fish Crustaceans Reptile Insects Snails Clams Plants State Taxonomic Group 65 Oregon 5 1 0 24 1 0 2 0 0 13 South Dakota 4 1 0 2 0 0 1 0 0 1 Utah 2 2 0 8 0 1 0 0 0 24 Washington 5 5 0 17 0 0 1 0 0 6 Wyoming 1 4 1 2 0 0 0 0 0 2 C. Description of Assumptions, Limitations, Uncertainties, Strengths and Data Gaps 1. Assumptions, Limitations, Uncertainties, Strengths and Data Gaps Related to Exposure For All Taxa There are a number of areas of uncertainty in the aquatic and terrestrial risk assessments. The toxicity assessment for terrestrial and aquatic animals is limited by the number of species tested in the available toxicity studies. Use of toxicity data on representative species does not provide information on the potential variability in susceptibility to acute and chronic exposures.

Inhalation exposure to mammals and birds was estimated by using the non dimensional Henry's law constant, which relates the concentration of a compound in the gas phase to its concentration in the liquid phase. This approach does not take into account the many factors which influence volatilization from water such as channel geometry, flow characteristics, and temperature. Considerable uncertainties also are related to the limits of the breathing zone ( exposure zone). This method should provide a conservative estimate of the possible xylene concentrations in air.

2. Assumptions, Limitations, Uncertainties, Strengths and Data Gaps Related to Exposure For Aquatic Species For exposures of non target aquatic species, several EECs were used to assess acute risk: 1 mg/ L ( optional reduced concentration in outflow waters) 10 mg/ L ( maximum allowable concentration in 66 receiving waters), 740 mg/ L ( maximum allowable concentration in irrigation canals), and 178 mg/ L ( solubility limit for o xylene). Mixed xylenes are applied to achieve an initial concentration of approximately 740 mg/ L, with the concentration in return flows of treated irrigation ( i. e., flows into receiving rivers and streams) not to exceed 10 mg/ L. 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). The value of 178 mg/ L was chosen to provide consistency with the chemicalphysical properties of xylenes applied without the addition of emulsifiers.

Since mixed xylenes are highly volatile, xylene concentrations are expected to rapidly decrease. However, this assessment assumes that xylene concentrations remain constant over the exposure period.

There is significant uncertainty in estimating concentrations in receiving water bodies because, while the label permits release of irrigation water with up to 10 ppm xylene, such releases are thought to be infrequent. In many of the areas were xylene treatment occurs, water is a valuable commodity which will be retained and used to the maximum extent possible. The concentration of xylene in irrigation water will rapidly decline to below 1 ppm, and in many cases to below the concentration that results in risk exceeding the listed species LOC. Therefore, 10 ppm is considered an upper bound and the expected concentrations will usually be lower.

The major limitations associated with the simple plug flow model is that it assumes steady state conditions which may not reflect environmental conditions. The geometry and flow rates and degradate rates will also not be equal or constant.

The limited monitoring was not collected for comparison with the 0.04 ppm ( assumed safe level for endangered species) but with the 10 ppm the current MCL for xylene.

3. Assumptions, Limitations, Uncertainties, Strengths and Data Gaps Related to Exposure For Terrestrial Species For exposures of non target terrestrial species via drinking contaminated water, the maximum solubility ( 178 mg/ L) was used to estimate inhalation exposure, and three EECs were used to assess acute risk from wildlife drinking treated water: 10 mg/ L ( maximum allowable concentration in receiving waters), 740 mg/ L ( maximum allowable concentration in irrigation canals), and 178 mg/ L ( solubility limit for o xylene). Since mixed xylenes are highly volatile, xylene concentrations are expected to rapidly decrease. However, this assessment assumes that xylene concentrations remain constant over the exposure period. 67 4. Assumptions, Limitations, Uncertainties, Strengths and Data Gaps Related to Effects Assessment The ecological effects database to support the aquatic and terrestrial assessments for acute exposures is not complete. Since xylenes are highly volatile, chronic exposure is not considered likely for aquatic or terrestrial ecosystems. Thus, requirements for chronic toxicity studies in aquatic and terrestrial receptors are not required.

Data gaps, uncertainties, and limitations for the acute aquatic and terrestrial assessments are summarized 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.

° Acute 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.

° The acute dietary toxicity study in birds failed to establish an acute LC 50 value ( the LC 50 was expressed as " greater than" the highest dietary concentration tested); thus, significant uncertainty was introduced into the avian RQ calculations.

° 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 ), using either a bobwhite quail or mallard duck, to satisfy the § 71 1 guideline requirements.

° There is uncertainty related to the methodology used to dose the rats in the acute inhalation mammalian study used to derive inhalation RQs ( Carpenter et al 1975) because the study authors did not specifically discuss the dosing methodology, but rather referred to an approach used in another study ( Carpenter et al 1975a).

° Acute inhalation toxicity data are not available for birds; therefore, risk from inhalation exposure cannot be assessed. 68 Age class and sensitivity of effects thresholds It is generally recognized that test organism age may have a significant impact on the observed sensitivity to a toxicant. The screening risk assessment acute toxicity data for fish are collected on juvenile fish weighing between 0.1 and 5 grams. Aquatic invertebrate acute testing is performed on recommended immature age classes ( e. g., first instar for daphids, second instar for amphipods, stoneflies and mayflies, and third instar for midges). Similarly, acute dietary testing with birds is also performed on juveniles, with mallard being 5 10 days old and quail 10 14 days old.

Testing of juveniles may overestimate toxicity at older age classes for pesticidal active ingredients, such as mixed xylenes, that act directly ( without metabolic transformation) because younger age classes may not have the enzymatic systems associated with detoxifying xenobiotics. The screening risk assessment has no current provisions for a generally applied method that accounts for this uncertainty. In so far as the available toxicity data may provide ranges of sensitivity information with respect to age class, the risk assessment uses the most sensitive life stage information as the conservative screening endpoint.

Use of the Most Sensitive Species Tested Although the screening risk assessment relies on a selected toxicity endpoint from the most sensitive species tested, it does not necessarily mean that the selected toxicity endpoints reflect sensitivity of the most sensitive species existing in a given environment. The relative position of the most sensitive species tested in the distribution of all possible species is a function of the overall variability among species to a particular chemical. In the case of listed species, there is uncertainty regarding the relationship of the listed species' sensitivity and the most sensitive species tested.

The Agency is not limited to a base set of surrogate toxicity information in establishing risk assessment conclusions. The Agency also considers toxicity data on non standard test species when available.

5. Assumptions, Limitations, Uncertainties, Strengths and Data Gaps Related to the Acute and Chronic LOCs The risk characterization section of the assessment includes an evaluation of the potential for individual effects at an exposure level equivalent to the LOC. This evaluation is based on the median lethal dose estimate and dose/ response relationship established for the effects study corresponding to each taxonomic group for which the LOCs are exceeded.

The certainty associated with the confidence in estimated event probabilities for taxonomic groups including estuarine/ marine fish and invertebrates is low because no information was available to estimate a slope for the dose response curve for these taxonomic groups. Event probabilities for listed estuarine/ marine fish and invertebrate species were calculated based on a default slope assumption of 4.5 with upper and lower confidence intervals of 2 and 9 ( Urban and Cook, 1986). Given the large uncertainty associated with the probability estimates it is not possible to predict the chance of an individual mortality event for listed estuarine/ marine fish and invertebrate species. 69 V. Literature Cited Open Literature and Government Reports Abernethy S, Bobra AM, Shui WY, Wells PG, and Mackay D. 1986. Acute lethal toxicity of hydrocarbons and chlorinated hydrocarbons to two planktonic crustaceans: the key role of organisms water partitioning. Aquatic Toxicology, 8: 163 174.

API. 1994. A study to characterize air concentrations of methyl tertiary butyl ether ( MTBE) at service stations in the Northeast. American Petroleum Institute.

ATSDR. 1995. Toxicological Profile for Xylene. U. S. Department of Health and Human Services. Agency for Toxic Substances and Disease Registry ( ATSDR). August, 1995. Available at: http:// www. atsdr. cdc. gov/ toxprofiles/ tp71. pdf.

Benville PE, and Korn, S. 1977. The acute toxicity of six monocyclic aromatic crude oil components to striped bass ( Morone saxatilis) and bay shrimp ( Crago franciscorum). Calif. Fish and Game 63( 40): 204 209.

Caldwell RS, Caldarone EM, and Mallon MH. 1977. Effects of a seawater soluble fraction of Cook Inlet crude oil and its major aromatic components on larval stages of the Dungeness crab, Cancer magister Dana. In: Wolfe DA ed. Fate and effects of petroleum hydrocarbons in marine ecosystems and organisms. Oxford, New York, Pergamon Press, pp 210 220.

California Department of Pesticide Regulation ( CDPR) Pesticide Use Reporting ( PUR). 2005. Pesticide Usage Database. Available on line at : http:// www. cdpr. ca. gov/ docs/ pur/ purmain. htm. Accessed 6 July 2005.

Carpenter CP, Kinkead ER, Geary DL Jr, Sullivan LJ, & King JM. 1975. Petroleum hydrocarbon toxicity studies. V. Animal and human response to vapors of mixed xylenes. Toxicol Appl Pharmacol, 33: 543 558.

Carpenter CP, Kinkead ER, Geary DL, Sullivan LF, & King JM. 1975a. Petroleum hydrocarbon toxicity studies. I. Methodology. Toxicol Appl Pharmacol, 32: 246 262.

EXAMS. 2002. Surface Water Model, Version 2.98.04. Environmental Fate and Effects Division, Office of Pesticide Programs, U. S. Environmental Protection Agency, Washington, D. C.

Falk Petersen, I B, Kjorsvik E, Lonning S, Naley AM, and Sydnes LK. 1985. Toxic effects of hydroxylated aromatic hydrocarbons on marine embryos. Sarsia 70: 11 16.

Folmar LC. 1976. Overt avoidance reaction of rainbow trout fry to nine herbicides. Bull Environ Contam Toxicol, 15: 509 514. 70 Frank PA, Otto NE, and Bartley TR. 1961. Techniques for evaluating aquatic weed hebicides. J Weed Soc. Am. 9( 4): 515 521.

Galassi S, Mingazzini M, Vigano L, Cesareo D, and Tosato ML. 1988. Approaches to modelling toxic responses of aquatic organisms to aromatic hydrocarbons. Ecotoxicol Environ Saf, 16: 158 169.

Herman DC, Inniss WE, Mayfield CI. 1990. Impact of volatile aromatic hydrocarbons, alone and in combination, on growth of the freshwater alga Selenastrum capricornutum. Aquatic Toxicology 18: 87 100.

Hill EF and Camardese MB. 1986. Lethal dietary toxicities of environmental contaminants and pesticides to Coturnix. Washington, DC, US Department of the Interior, Fish and Wildlife Service, 138 pp ( Fish and Wildlife Technical Report No. 2).

Hine CH and Zuidema HH. 1970. The toxicological properties of hydrocarbon solvents. Ind Med, 38: 215 220.

Holcombe GW, Phipps GL, Sulaiman AH, and Hoffmann AD. 1987. Simultaneous Multiple species testing: acute toxicity of 13 chemicals to 12 diverse freshwater amphibian, fish, and invertebrates families. Arch. Environ. Contam. Toxicol. 16: 697 710.

HSDB 2005  National Library of Medicine Hazardous Substance Databank. Avaialable at : http:// toxnet. nlm. nih. gov/ cgi bin/ sis/ htmlgen.

Lindhardt B, Christensen TH, Brun A. 1994. Volatilization of o xylene from sandy soil. Chemosphere 29: 2965 2937.

National Agricultural Statistics Service ( NASS). 2005. Agricultural Chemical Usage Database. Available on line at http:// www. pestmanagement. info/ nass/ app_ usage. cfm. Accessed 6 July 2005.

National Center for Food and Agricultural Policy ( NCFAP). 2005. Pesticide Usage Database. Available on line at http:// pestdata. ncsu. edu/ ncfap/ search. cfm. Accessed 6 July 2005.

Neff JM, Anderson JW, Cox BA, Laughlin RB, Rossi SS, and Tatem HE. 1976. Effects of petroleum on survival, respiration and growth of marine animals. In: Sources, effects and sinks of hydrocarbons. Washington, DC, American Institute of Biological Science, pp 515 523.

NTP. 1986. National Toxicology Program technical report on the toxicology and carcinogenesis studies of xylenes ( mixed) ( 60% m xylene, 14% p xylene, 9% o xylene, and 17% ethylbenzene) ( CAS No. 1330 20 7) in F344/ N rats and B6C3F1 mice ( gavage studies). Research Triangle Park, NC: U. S. Department of Health and Human Services, Public Health Service, National Institutes of Health, National Toxicology Program. NTP TR 327. NIH Publication No. 87 2583. Available at: http:// ntp. niehs. nih. gov/ ntp/ htdocs/ LT_ rpts/ tr327. pdf. 71 Ogata M, Miyaka Y. 1978. Disappearance of aromatic hydrocarbons and sulfur compounds from fish reared in crude oil suspensions. Water Res 12: 1041 1044.

Ogata M, Fujisawa K, Ogino Y, Mano E. 1984. Partition coefficients as a measure of bioconcentration potential of crude oil components in fish and shellfish. Bull. Environ. Contam. Toxicol. 33: 561 567.

Pavlostathis SG, Mathavan GN. 1992. Desorption kinetics of selected volatilie organic compounds from field contaminated soils. Environ Sci Technol 26: 532 538 ( 1992).

Smith JH, Harper JC 1980. 12th Conf on Environ Toxicol: Behavior of Hydrocarbon Fuels in Aquatic Environment; pp 336 53.

Tatem HE, Cox BA, and Anderson JW. 1978. The toxicity of oils and petroleum hydrocarbons to estuarine crustaceans. Estuar Coast Mar Sci, 6: 365 373.

Tsao CW, Song HG, Bartha AR. 1998. Metabolism of benzene, toluene, and xylene hydrocarbons in soil. Appl Environ Microbiol 64: 4924 4929.

U. S. EPA. 1993. U. S. Environmental Protection Agency. Wildlife Exposure Factors Handbook. Volume I of II. EPA/ 600/ R 93/ 187a. Office of Research and Development, Washington, D. C. 20460.

U. S. EPA. 2001. U. S. Environmental Protection Agency. Ecological Risk Assessor Orientation Package. U. S. Environmental Protection Agency, Ecological Fate and Effects Division. Draft Version, August 2001.

U. S. EPA. 2004. U. S. Environmental Protection Agency. Overview of the Ecological Risk Assessment Process in the Office of Pesticide Programs, U. S. Environmental Protection Agency: Endangered and Threatened Species Effects Determinations. Office of Prevention, Pesticide, and Toxic Substances. January 23.

U. S. EPA. 2005. USEPA Storage and Retrieval System ( STORET). Available on line at http:// www. epa. gov/ storet/ dbtop. html. Accessed 6 July 2005.

Vowles PD and Mantoura RFC 1987. Sediment water partition coefficients and hplc retention factors of aromatic hydrocarbons. Chemosphere 16: 109 16.

Walsh DF, Armstrong JB, Bartley TR, Salman HA, and Frank PA. 1977. Residue of emulsified xylene in aquatic weed control and their impact on rainbow trout salmo giardneri. REC ERC 76 11. NTIS PB 267270. Bureau Of Reclamation Denver, CO.

Washington State Department of Ecology. 2005. Xylene data for EPA ( 09 16 05). Water Quality 72 Program. Department of Ecology, Washington State.

WHO. 1997. Environmental Health Criteria for Xylenes. International Programme on Chemical Safty. World Health Organization. Available at: http:// www. inchem. org/ documents/ ehc/ ehc/ ehc190. htm.

Woodard AE, Abplanalp H, Wilson WO, and Vohra P. 1973. Japanese quail husbandry in the laboratory ( Coturinx coturnix japonica). Department of Avian Sciences, University of California, Davis CA. Available at: http:// animalscience. ucdavis. edu/ Avian/ Coturnix. pdf WSDE ( Washington State Department of Ecology) 2002. Fact sheet for aquatic weed control in irrigation system. General NPDES Permit. April 17, 2002. 73 ACKNOWLEDGEMENTS The Environmental Fate and Effects Division would like to thank Syracuse Environmental Research Associates Inc. ( SERA) ( Julie Klotzbach and Mario Citra) for their assistance in developing the draft screening level risk assessment for Xylene Range Aromatic Solvents.  A 1 APPENDIX A: Environmental Fate Studies Hydrolysis 161 1 Xylenes lack functional groups that are susceptible to hydrolysis under environmental conditions; therefore, hydrolysis is not expected to be an important environmental fate property for the aromatic solvents.

Photodegradation in water 161 2; Photodegradation on soil 161 3 Xylenes do not possess functional groups that absorb photons of light above 290 nm; therefore, photolysis in soil and water systems is not expected to be an important environmental fate process for aromatic solvents.

Aerobic soil metabolism 162 1 p Xylene, at an initial concentration of 45 ppm, was reported to be completely degraded in natural potting soil microcosms in less than 27 days at 20 /

C, but was not degraded at 6 /

C ( ATSDR 1995). Using a Nixon sandy loam from NJ, 14C labeled o and p xylene applied at a concentration of 3 µ L/ g soil, degraded with half lives of approximately 5 and 7 days respectively ( Tsao et al. 1998). A 3 4 day lag period was observed for the degradation of the p isomer, while no lag period or acclimation time was necessary for o xylene. The natural pH of the soil ( 5.5 6.0) was raised to 7.0 for the experiments by addition of CaCO 3 . The authors noted that doubling or tripling the concentration of the xylenes resulted in inhibitory effects ( toxicity to the microbial population) and a much slower rate of degradation. The main degradation products observed were 14CO 2 , but some minor degradation products such as 3,6 dimethylcatechol were also observed prior to complete mineralization.

Under aerobic conditions, xylenes are degraded in standard biodegradability tests using various inocula including sewage and activated sludge. Employing a standard biological oxygen demand ( BOD) dilution technique and a sewage inoculum, a theoretical BOD of 52, 80 and 44% was achieved over a 5 day incubation period for o, m, and p xylene respectively ( Bridie et al. 1979). m Xylene present at 100 mg/ l, reached 100% of its theoretical BOD in 4 weeks using an activated sludge inoculum at 30 mg/ l and the Japanese MITI test ( CITI 1992). These data suggest that under aerobic conditions, xylenes will degrade readily in the environment.

Anaerobic soil metabolism 162 2 Anaerobic biodegradation of aromatic compounds in soils appear to occur more readily under nitrate reducing or sulfate reducing conditions as compared to methanogenic conditions ( HSDB 2005). Xylenes were shown to degrade in an aquifer microflora laboratory test under sulfate reducing conditions, but not under methanogenic conditions ( ATSDR 1995). m Xylene was completely degraded in less than 25 days under sulfate reducing conditions and successive metabolites involved in the anaerobic degradation of xylenes were reported to be methylbenzylsuccinic acid, toluic acid,  A 2 phthalic acid, and benzoic acid ( finally degraded to carbon dioxide) ( ATSDR 1995).

Anaerobic aquatic metabolism 162 3 Using PS 6 gasoline as substrate, anaerobic microcosms constructed of 3 g of sandy soil and 20 mL of groundwater from the Borden Canada Forces Base in Ontario, were employed to study the fate of BTEX in anaerobic environments. The three isomers of xylene ( initial concentrations of 330 800 µ g/ L), were very slowly degraded with half lives of several hundred days ( API 1994). The degradation rates in these anaerobic microcosms did not appear to follow first order kinetics; however, using the initial concentration and final concentrations at day 420, estimated half lives were 432 days ( p xylene), 235 days ( m xylene), and 676 days ( o xylene). Similar results were obtained using PS 6 gasoline containing the oxygenates MTBE and methanol. At lower initial starting concentrations the rate of degradation appears to increase. Using a starting concentration of 250 µ g/ L, the half lives of the o and m xylene isomers were approximately 25 days in groundwater from Seal Beach, CA ( Beller et al. 1995). After a 60 day incubation period, over 95% of the initially applied amount of xylene isomers had been degraded. 2 Methylbenzyl succinic acid and 2 methylbenzyl fumaric acid were identified as metabolites of o xylene, while 3 methylbenzyl succinic acid and 3 methylbenzyl fumaric acid were identified as metabolites of m xylene.

Aerobic aquatic metabolism 162 4 In a study funded by the American Petroleum Industry ( API) in conjunction with Ontario University, benzene, toluene, ethylbenzene and xylenes, ( BTEX) degradation rates in soil/ water microcosms were studied ( API 1994). Using PS 6 gasoline and fully aerobic microcosms constructed of 3 g of sandy soil and 20 mL of groundwater from the Borden Canada Forces Base in Ontario, the 3 isomers of xylene were shown to degrade relatively rapidly. At starting concentrations of 168, 216 and 85 µ g/ L, the first order degradation rate constants for o, m, and p xylene were 0.030, 0.038, and 0.028 days 1, respectively; corresponding to half lives of 23, 20, and 25 days. Using the same microcosms and PS 6 gasoline with 15% MTBE ( methyl tertiary butyl ether) the rate constants were 0.047, 0.058 and 0.044 for o, m, and p xylene, respectively; corresponding to half lives of 15, 12, and 15 days. For microcosms using PS 6 gasoline with 15% methanol added, the rate of degradation increased with half lives of 14, 9 and 14 days for o, m, and p xylene, respectively.

Mobility/ Adsorption/ Desorption 163 1 Adsorption experiments conducted with 2 silty clay soils and a course sand yielded K oc values for xylenes ( mixed isomers) ranging from 39 365 ( Pavlostathis and Mathavan 1992). The properties of the soils are provided in table A 1.

Table A 1. Adsorption properties of xylenes in soils Property Soil 1 Soil 2 Soil 3 % Sand 5 ND 40  A 3 % Silt 70 ND 39 % Clay 25 ND 21 % Organic carbon 0.17 1.4 0.09 pH 8.05 7.05 6.8 K oc 365 39 311 K d 0.62 0.54 0.28 These K oc values suggest that xylenes will have high to moderate mobility in soil, depending upon the characteristics of the soil. Sediment/ water partition coefficients of 8.9 and 10.5 were calculated for o xylene and p xylene, respectively ( Vowles and Mantoura 1987). The sediment was reported to contain 4% organic carbon; therefore, K oc values of 222 and 262 were derived for o xylene and pxylene respectively.

The adsorption properties of o xylene were measured in 2 course sandy soils from Denmark under saturated and unsaturated moisture conditions ( Lindhardt et al. 1994). In general, o xylene was expected to have moderate to high mobility in soils, with slightly greater adsorption in the higher organic soil. It was also observed that the mean adsorption coefficients were approximately 25 35% greater under unsaturated conditions as compared to fully saturated moisture conditions. The properties of the two soils are provided in table A 2.  A 4 Table A 2. Adsorption properties of o xylene in two sandy soils Property Soil 1 Soil 2 % Course Sand ( 0.2 2 mm) 71.9 95.5 % Fine Sand ( 0.02 0.2 mm) 17.3 1.8 % Silt 3.8 0.5 % Clay 4.8 2.1 % Organic carbon 1.1 0.11 pH 5.2 6.3 K oc ( saturated) 169 64 K oc ( saturated) 139 77 K oc ( 2.5% moisture content) 207 103 K oc ( 5% moisture content) 211 80 K oc ( average value) 181 81 Field volatility 163 3 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 and Harper 1980). Combining this ratio with oxygen reaeration rates for typical bodies of water, one estimates that the half life for evaporation of xylenes from a typical river or pond is 29 and 144 hours, respectively.

A field monitoring study of an oil spill from the Trans Alaskan Pipeline which leaked into the Atigun River ( June 10, 1979) indicated xylenes were absent from the 40 km long river in the contaminated area 18 days after the spill indicating that the majority of xylenes volatilized or were biodegraded.

Accumulation in Laboratory Fish 165 4 Eels reared in seawater containing crude oil suspension ( 50 ppm for 10 days), had BCF values of 23.6 and 21.4 for m/ p xylene and o xylene, respectively ( Ogata and Miyaka 1978). The isomers of xylene were depurated relatively quickly following transfer to non contaminated water. after 10 days in clean sea water, BCF values were approximately 5 for m/ p xylene and 3 for o xylene.

Goldfish reared in water containing 1 mg/ L o, m, and p xylene had BCF values of 15 ( m, and pxylene and 14 ( o xylene)( Ogata et al. 1984).  A 5 Terrestrial field dissipation 164 1 In order to simulate the dynamics and environmental fate of xylenes following an accidental spill, xylene was sub surface applied ( depths ranging from 1.8 to 7.2 cm) to Westwood silt loam soil cores at levels of 4.6 19.0% of the pore volume, and continuously flushed with water to simulate rainfall ( Aurelius and Brown 1987). Following application, leachate was continuously removed at 33 kPa tension. The majority of the applied xylenes were degraded over the course of the 67 day experiment, with smaller amounts volatilized and leached. The results of these experiments are summarized in table A 3.

Table A 3. Summation of the fate of xylene applied to soil surfaces.

Soil Core Moisture content ( mg/ kg) Application depth ( m) Amount volatilized Amount remaining in soila Amount leached Amount degraded 1 0.15 0.036 4.1% 11.9% 0.5% 83.5%

2 0.20 0.036 1.21% 11.3% 8.6% 78.9%

3 0.20 0.072 2.65% 6.7% 32.3% 57.8%

4 0.26 0.036 1.98% 12.8% 33.6% 50.9%

5 0.26 0.018 2.5% 11.4% 6.4% 76.6%

a. 67 days post application.

Aquatic field dissipation 164 2 A study was conducted that reviewed the impact of emulsified xylenes used in aquatic weed control on rainbow trout ( Walsh et al. 1977). This study also summarized results from a previous investigation that reviewed the fate of xylenes released to 6 irrigation canals located in the states of Wyoming, Colorado, and Washington. The application rate was 10 gallons of xylene per ft3/ s of water flow within a 30 minute emission period, corresponding to a level of 740 ppm ( Walsh et al. 1977). The rate of flow in these 6 irrigation canals ranged from 11 89 ft3/ s. although no half lives were reported, dissipation curves depicting the xylene concentration as a function of distance from the release point were provided. The concentration of xylenes tended to vary widely near the release point, but became more uniform downstream. 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 traveling 5 10 miles downstream. 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  A 6 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). 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.  A 7 References ATSDR 1995. Toxicological profile for xylenes. Prepared for the Agency for Toxic Substances and Disease Registry. ATSDR Contract No. 205 1999 00024.

API. 1994. A study to characterize air concentrations of methyl tertiary butyl ether ( MTBE) at service stations in the Northeast. American Petroleum Institute.

Aurelius MW, Brown KW 1987. Fate of spilled xylene influenced by moisture content. Wat. Air Soil Pollut. 36: 23 31.

Beller HR, Ding WH, Reinhard M. 1995. Byproducts of anaerobic alkylbenzene metabolism useful as indicators of in situ bioremediation. Environ Sci Technol 29: 2864 2870.

Bridie AL, Wolf CJM, Winter M. 1979. BOD and COD of petroleum chemicals. Water Res. 13: 627 630.

CITI. 1992. Chemicals Inspection and Testing Institute. Japan Chemical Industry Ecology  Toxicology and Information Center. ISBN 4 89074 101 1.

Lahvis MA, Baehr AL, Baker RJ. 1999. Quantification of aerobic biodegradation and volatilization rates of gasoline hydrocarbons near the water table under natural attenuation conditions. Water Resources Research 35: 753 765.

Lindhardt B, Christensen TH, Brun A. 1994. Volatilization of o xylene from sandy soil. Chemosphere 29: 2965 2937.

Lysyj, I, Perkins G, Farlow JS 1980. Trace analysis of aromatic hydrocarbons in natural waters. Environ Int 4: 407 16.

Ogata M, Miyaka Y. 1978. Disappearance of aromatic hydrocarbons and sulfur compounds from fish reared in crude oil suspensions. Water Res 12: 1041 1044.

Ogata M, Fujisawa K, Ogino Y, Mano E. 1984. Partition coefficients as a measure of bioconcentration potential of crude oil components in fish and shellfish. Bull. Environ. Contam. Toxicol. 33: 561 567.

Oregon Department of Environmental Quality ( DEQ) 2002. Summary of comments and response to comments received for the proposed 2002 Mutual Agreement and Order ( MAO) for the application of acrolein, xylene and copper in irrigation systems.

Pavlostathis SG, Mathavan GN. 1992. Desorption kinetics of selected volatilie organic compounds from field contaminated soils. Environ Sci Technol 26: 532 538 ( 1992).  A 8 Smith JH, Harper JC 1980. 12th Conference on Environ Toxicol: Behavior of Hydrocarbon Fuels in Aquatic Environment; pp 336 53.

Thomas, R. G. 1990. Chapter 15. Volatilization from Water. In: Handbook of Chemical Property Estimation Methods. Ed: W. J. Lyman, W. F. Reehl, and D. H. Rosenblatt. American Chemical Society. Washington, D. C.

Tsao CW, Song HG, Bartha AR. 1998. Metabolism of benzene, toluene, and xylene hydrocarbons in soil. Appl Environ Microbiol 64: 4924 4929.

United States Department of the Interior, Bureau of Reclamation. 1969. Irrigation Operation and Maintenance, Bulletin No. 69. Division of Irrigation Operations. Office of Chief Engineer, Denver, CO.

Walsh DF, Armstrong JB, Bartley TR, Salman HA, and Frank PA. 1977. Residue of emulsified xylene in aquatic weed control and their impact on rainbow trout salmo giardneri. REC ERC 76 11. NTIS PB 267270. Bureau Of Reclamation Denver, CO.  B 1 APPENDIX B:

Estimation of Concentration in Receiving water using simple plug flow model The change in concentration of a chemical being released into a flowing water channel depends upon the concentration of 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 of the channel ( receiving water  assumed to be 0 for xylene) and the amount of water in the channel ( Vol). Concentration reductions were approximated by a simple steady state plug flow model.

Assumptions and simplifications are the mixing is ideal ( constantly stirred), Q is constant, same amount of water enters the channel as leaves the channel.

Assuming a steady state flow system, the mass balance describing the time variation of an average concentration in a flowing channel can be estimated.

C( t) = Co * exp(( TVol/ Q)* R)

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), R is median of volatilization half lives report in literature.

Q = Flux density ( length3/ time)

TVol = Volume of return flow ( 1 length3) + volume ( dilution x 1 length3) 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).

The following table shows the relationship between flow properties in the outflow water ( velocity, area, and volume) compared to the receiving water and receiving water ( velocity return flow, receiving area, and volume). The application of xylene into the irrigation canal is based upon flow ( 11 gallons per 1 ft3/ sec flow) rate. Thus, if the flow rate is double, the application rate also doubles with the concentrations remaining the same.  B 2 The dissipation of xylene released as return flow into a receiving body of water will correlated with the flow rate and cross sectional area and degradation rate ( volatilization and biotic metabolism). If the flow rates are equal, the cross section area ( essentially a dilution factor) has the greatest influence on the dissipation.

Table B1. Relationship between return flow and receiving water in plug flow model.

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

no dilution 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 Table B2 shows the influence of return flow concentration ( 10 or 1 ppm) with different dilutions.

Table B2. Time, Distance, and Dilution for Xylene Concentration to Decrease from 10ppm and 1ppm to < 0.04 ppm Based on a Steady State Plug Flow Dilution Model Condition Reduction from 10 ppm to 0.04 ppm Reduction from 1 ppm to 0.04 ppm 1: 1 dilution 10: 1 dilution 20: 1 dilution 50: 1 dilution 1: 1 dilution 10: 1 dilution 20: 1 dilution 50: 1 dilution Time ( days) 7.96 1.45 0.76 0.31 4.64 0.84 0.44 0.2 Distance ( miles) 130 23.7 12.4 5.1 76 13.8 7.2 3.0 Estimate of Xylene Concentration in Air The concentration of xylene in air was estimated by the following equation:

C g = C l * H'

C g = Concentration in gas phase ( mg/ L) ( 38.4925 mg/ L, ppm)  B 3 C l = Concentration in liquid phase ( 178 mg/ L, ppm)

H' = Nondimensional Henry's Law constant ( 0.21625) at 20 "

C This assume ideal conditions, no losses of xylene through degradation or wind. The uncertainty of these conservative assumptions are considered acceptable given the resulting EECs do not indicate risk.  C 1  C 1 APPENDIX C: Data Requirements  Environmental Fate Table A1. Status of environmental fate data adequacy/ needs for xylenes.

Guideline # Data Requirement Adequate for Risk Assessment Citation Study Classification 161 1 835.212 Hydrolysis yes HSDB 2005; ATSDR 1995 open lit.

161 2 835.224 Photodegradatio n in Water yes HSDB 2005; ATSDR 1995 open lit.

161 3 835.241 Photodegradatio n on Soil yes HSDB 2005; ATSDR 1995 open lit.

161 4 835.237 Photodegradatio n in Air yes HSDB 2005; ATSDR 1995 open lit.

162 1 835.41 Aerobic Soil Metabolism yes Tsao et al. 1998 open lit.

162 2 835.42 Anaerobic Soil Metabolism yes HSDB 2005; ATSDR 1995 open lit.

162 3 835.44 Anaerobic Aquatic Metabolism yes API 1994 open lit.

162 4 835.43 Aerobic Aquatic Metabolism yes API 1994 open lit.

163 1 835.1240 835.1230 Leaching Adsorption/ Deso rption yes Pavlostathis and Mathavan 1992 Vowles and Mantoura 1987 Lindhardt et al. 1994 open lit.

163 2 835.141 Laboratory Volatility no data open lit.

163 3 835.81 Field Volatility yes Smith and Harper 1980 open lit. Table A1. Status of environmental fate data adequacy/ needs for xylenes.

Guideline # Data Requirement Adequate for Risk Assessment Citation Study Classification  C 2 164 1 835.61 Terrestrial Field Dissipation yes Aurelius and Brown 1987 open lit.

164 2 835.62 Aquatic Field Dissipation yes Walsh et al. 1977 open lit.

165 4 850.173 Accumulation in Fish yes Ogata and Miyaka 1978 Ogata et al. 1984 open lit.  D 1 APPENDIX D: Environmental Fate and Monitoring Bibliography ATSDR 1995. Toxicological profile for xylenes. Prepared for the Agency for Toxic Substances and Disease Registry. ATSDR Contract No. 205 1999 00024.

API. 1994. A study to characterize air concentrations of methyl tertiary butyl ether ( MTBE) at service stations in the Northeast. American Petroleum Institute.

Aurelius MW, Brown KW 1987. Fate of spilled xylene influenced by moisture content. Wat. Air Soil Pollut. 36: 23 31.

Beller HR, Ding WH, Reinhard M. 1995. Byproducts of anaerobic alkylbenzene metabolism useful as indicators of in situ bioremediation. Environ Sci Technol 29: 2864 2870.

Bridie AL, Wolf CJM, Winter M. 1979. BOD and COD of petroleum chemicals. Water Res. 13: 627 630.

California Department of Pesticide Regulation ( CDPR) Pesticide Use Reporting ( PUR). 2005. Pesticide Usage Database. Available on line at : http:// www. cdpr. ca. gov/ docs/ pur/ purmain. htm. Accessed 6 July 2005.

CITI. 1992. Chemicals Inspection and Testing Institute. Japan Chemical Industry Ecology  Toxicology and Information Center. ISBN 4 89074 101 1.

EXAMS. 2002. Surface Water Model, Version 2.98.04. Environmental Fate and Effects Division, Office of Pesticide Programs, U. S. Environmental Protection Agency, Washington, D. C.

HSDB 2005  National Library of Medicine Hazardous Substance Databank. Avaialable at : http:// toxnet. nlm. nih. gov/ cgi bin/ sis/ htmlgen.

Lahvis MA, Baehr AL, Baker RJ. 1999. Quantification of aerobic biodegradation and volatilization rates of gasoline hydrocarbons near the water table under natural attenuation conditions. Water Resources Research 35: 753 765.

Lindhardt B, Christensen TH, Brun A. 1994. Volatilization of o xylene from sandy soil. Chemosphere 29: 2965 2937.

Lysyj, I, Perkins G, Farlow JS 1980. Trace analysis of aromatic hydrocarbons in natural waters. Environ Int 4: 407 16.

National Agricultural Statistics Service ( NASS). 2005. Agricultural Chemical Usage Database. Available on line at http:// www. pestmanagement. info/ nass/ app_ usage. cfm. Accessed 6 July 2005.

National Center for Food and Agricultural Policy ( NCFAP). 2005. Pesticide Usage Database.  D 2 Available on line at http:// pestdata. ncsu. edu/ ncfap/ search. cfm. Accessed 6 July 2005.

Ogata M, Miyaka Y. 1978. Disappearance of aromatic hydrocarbons and sulfur compounds from fish reared in crude oil suspensions. Water Res 12: 1041 1044.

Ogata M, Fujisawa K, Ogino Y, Mano E. 1984. Partition coefficients as a measure of bioconcentration potential of crude oil components in fish and shellfish. Bull. Environ. Contam. Toxicol. 33: 561 567.

Oregon Department of Environmental Quality ( DEQ) 2002. Summary of comments and response to comments received for the proposed 2002 Mutual Agreement and Order ( MAO) for the application of acrolein, xylene and copper in irrigation systems.

Pavlostathis SG, Mathavan GN. 1992. Desorption kinetics of selected volatilie organic compounds from field contaminated soils. Environ Sci Technol 26: 532 538 ( 1992).

Smith JH, Harper JC 1980. 12th Conf on Environ Toxicol: Behavior of Hydrocarbon Fuels in Aquatic Environment; pp 336 53.

Tsao CW, Song HG, Bartha AR. 1998. Metabolism of benzene, toluene, and xylene hydrocarbons in soil. Appl Environ Microbiol 64: 4924 4929.

USEPA. 2005. USEPA Storage and Retrieval System ( STORET). Available on line at http:// www. epa. gov/ storet/ dbtop. html. Accessed 6 July 2005.

Vowles PD and Mantoura RFC 1987. Sediment water partition coefficients and hplc retention factors of aromatic hydrocarbons. Chemosphere 16: 109 16.

Walsh DF, Armstrong JB, Bartley TR, Salman HA, and Frank PA. 1977. Residue of emulsified xylene in aquatic weed control and their impact on rainbow trout salmo giardneri. REC ERC 76 11. NTIS PB 267270. Bureau Of Reclamation Denver, CO.

WSDE ( Washington State Department of Ecology) 2002. Fact sheet for aquatic weed control in irrigation system. General NPDES Permit. April 17, 2002.  E 1 APPENDIX E: Ecological Effects The primary focus of this risk assessment is to evaluate the potential adverse effects of mixed xylenes to aquatic and terrestrial ecosystems. As such, toxicity data for mixed xylenes and the three xylene isomers ( m, o, and p xylene) were considered in the derivation of risk quotients.

No registrant submitted toxicity studies in which xylene ( mixed xylenes or isomers) is the sole active ingredient were identified; thus, all toxicity data summarized in this appendix are from open literatures studies. Three sources were used to identify open literature publications on xylenes: EPA's Ecotoxicology database ECOTOX, the ATSDR Toxicological Profile for Xylene ( 1995), and the World Health Organization Environmental Health Criteria Document for Xylenes ( 1997).

I. Categories of Acute Toxicity In general, categories of acute toxicity ranging from " practically nontoxic" to " very highly toxic" have been established for aquatic organisms ( based on LC 50 values), terrestrial organisms ( based on LD 50 values), and avian species ( based on LC 50 values) ( US EPA 2001).

Categories of acute toxicity for aquatic organisms can be classified according to the toxicity reference value ( LC 50 ) given by a study:

LC50 ( ppm) Toxicity Category < 0.1 Very highly toxic 0.1  1 Highly toxic > 1  10 Moderately toxic > 10  100 Slightly toxic > 100 Practically non toxic  E 2 Categories of acute toxicity for mammalian species can be classified according to the toxicity reference value ( LD 50 ) given by a study:

LD50 ( mg a. i./ kg) Toxicity Category < 10 Very highly toxic 10  50 Highly toxic 51  500 Moderately toxic 501  2000 Slightly toxic > 2000 Practically non toxic For avian species, categories of acute toxicity for can be classified according to the toxicity reference value ( LC 50 ) given by a study:

LC50 ( ppm) Toxicity Category < 50 Very highly toxic 50  500 Highly toxic 501  1000 Moderately toxic 1001  5000 Slightly toxic > 5000 Practically nontoxic II. Toxicity to Freshwater Aquatic Animals a. Freshwater Fish, Acute Two freshwater fish toxicity studies are required to establish the acute toxicity of mixed xylenes to fish. The preferred test species are bluegill sunfish ( a warmwater fish) and rainbow trout ( a coldwater 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 several species of freshwater fish; study details are summarized in Table E 1. 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  E 3 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 m xylene 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 a acute basis. The lowest acute toxicity value obtained for the p xylene isomer ( 24 hour LC50 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 guideline ( § 72 1) is fulfilled because 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.

Information on sublethal effects was reported in a single study in rainbow trout ( Walsh et al. 1977). Dose dependent signs of toxicity ( loss of equilibrium and " anesthetic" effects) were observed in fish exposed to xylene concentrations of 3.2 and greater, yielding an NOAEC value for sublethal effects of 0.64 ppm. In addition, Folmer ( 1976) reported that rainbow trout actively avoid xylene at concentrations as low as 0.1 ppm.

Table E 1. Acute Toxicity of Xylenes to Freshwater Fish.

Species Test Substance LC50 Toxicity Category Reference Study Classification rainbow trout ( Salmo gairdneri) p xylene ( technical grade) Concentrations tested: 0.001, 0.01. and 0.1 mg/ L ( nominal concentrations)

96 hour LC50: 10 mg a. i./ L ( nominal concentration)

Trout exhibited attractant behavior at a concentration of 0.01 mg/ L, but avoidance behavior at a concentration of 0.1 mg/ L after 1 hour of exposure. moderately toxic Folmar 1976 Open literature ( Supplemental) Table E 1. Acute Toxicity of Xylenes to Freshwater Fish.

Species Test Substance LC50 Toxicity Category Reference Study Classification  E 4 rainbow trout ( Salmo gairdneri) xylene ( not specified if mixed or isomer) ( industrial grade material)

xylene plus 2% emulsifier ( AD 410) Fish exposed to averaged ( measured ) 0.64, 3.6, 7.1, 16.1 and 25.6 ppm for 2 hours. Test vessel was an artificial stream approximately 750 feet long ( 5 sections of 150 feet).

At the 16.1 and 25.6 ppm concentrations, all fish were killed within 24 hours of exposure. No fish were killed at lower concentrations. At concentrations of 3.6 and 7.2 ppm, fish exhibited " symptoms similar to anesthesia", including loss of equilibrium, after 1.4 hours of exposure. Severity of effect was dosedependent Fish recovered after a " short time" in untreated water.

Values for xylene only ( NOTE: values based on deaths at 24 and 96 hours. Exposure period was only 2 hours). 24 hour LC50 = 13.5 ppm ( 95% CI: 9.5 19.2) 96 hour LC50 = 13.5 ppm ( 95% CI: 9.5 19.2) NOAEC for lethality: 7.1 ppm NOAEC for sublethal effects: 0.64 ppm Acute LC50 values for xylene plus emulsifier : 24 hour LC50 = 17.3 ppm ( 95% CI: 11.9 19.2) 96 hour LC50 = 17.3 ppm ( 95% CI: 11.9 25.1) NOAEC values not reported. slightly toxic Walsh et al. 1977 Open literature ( Supplemental) Table E 1. Acute Toxicity of Xylenes to Freshwater Fish.

Species Test Substance LC50 Toxicity Category Reference Study Classification  E 5 rainbow trout ( Salmo gairdneri) o xylene m xylene p xylene ( highest purity) Static renewal conditions. Measured concentrations. Note: all tests conducted using a closed system to minimize loss due to volatility.

96 hour LC50 values: o xylene: 7.6 mg a. i./ L m xylene: 8.4 mg a. i./ L p xylene: 2.6 mg a. i./ L Slope functions: o xylene: 1.11 m xylene: 1.11 p xylene: 1.10 moderately toxic Galassi et al. 1988 Open literature ( Supplemental  study was performed in accordance with OECD Standard Method No. 203)

guppy ( Peocilia reticulata) o xylene m xylene p xylene ( highest purity) Static renewal conditions. Measured concentrations. Note: all tests conducted using a closed system to minimize loss due to volatility.

96 hour LC50 values: o xylene: 12.0 mg a. i./ L m xylene: 12.9 mg a. i./ L p xylene: 8.8 mg a. i./ L Slope functions: o xylene: 1.05 m xylene: 1.03 p xylene: 1.13 slightly to moderately toxic Galassi et al. 1988 Open literature ( Supplemental  study was performed in accordance with OECD Standard Method No. 203)

fathead minnow ( Pimephales promelas) o xylene ( purity not reported) Measured concentrations. Test appears to be conducted under static renewal conditions, although this is not specifically stated.

48 hour LC50: 16.1 mg/ L 95% CI: 11.6 22.4 slightly toxic Holcombe et al. 1987 Open literature ( Supplemental) Table E 1. Acute Toxicity of Xylenes to Freshwater Fish.

Species Test Substance LC50 Toxicity Category Reference Study Classification  E 6 rainbow trout ( Salmo gairdneri) o xylene ( purity not reported) Measured concentrations. Test appears to be conducted under static renewal conditions, although this is not specifically stated.

48 hour LC50: 8.05 mg/ L 95% CI: 5.59 22.6 moderately toxic Holcombe et al. 1987 Open literature ( Supplemental)

bluegill sunfish ( Lepomis machrochirus) o xylene ( purity not reported) Measured concentrations. Test appears to be conducted under static renewal conditions, although this is not specifically stated.

48 hour LC50: 16.1 mg/ L 95% CI: 11.6 22.4 moderately toxic Holcombe et al. 1987 Open literature ( Supplemental)

goldfish ( Carassius auratus) o xylene ( purity not reported) Measured concentrations. Test appears to be conducted under static renewal conditions, although this is not specifically stated.

48 hour LC50: 16.1 mg/ L 95% CI: 11.6 22.4 moderately toxic Holcombe et al. 1987 Open literature ( Supplemental)

white sucker ( Catostomus commersoni) o xylene ( purity not reported) Measured concentrations. Test appears to be conducted under static renewal conditions, although this is not specifically stated.

48 hour LC50: 16.1 mg/ L 95% CI: 11.6 22.4 moderately toxic Holcombe et al. 1987 Open literature ( Supplemental)

b. Freshwater Fish, Chronic Under the conditions of recommended use, chronic exposure of freshwater fish to mixed xylenes is not anticipated. Thus, chronic exposure studies on mixed xylenes in freshwater fish are not required. It should be noted that no chronic freshwater fish toxicity studies for mixed xylenes or xylene isomers  E 7 were located in the open literature.

c. Freshwater Invertebrates, Acute A freshwater aquatic invertebrate toxicity study is required to establish the toxicity of mixed xylenes to freshwater invertebrates. The preferred test species is the water flea ( Daphnia magna). 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 summarized in 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 xylene isomers are slightly to highly toxic to 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 ( 24 hour LC50 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. The guideline ( § 72 2) is fulfilled because the Galassi et al. ( 1988) study was conducted in accordance with OECD standardized method No. 202, with modifications for testing volatile compounds. As previously noted, the study was conducted under closed conditions to minimize loss of xylenes from the test media, and toxicity values are expressed in terms of measured concentrations.  E 8 Table E 2. Acute Toxicity of Xylenes to Freshwater Invertebrates Species Test Substance LC50 Toxicity Category Reference Study Classification Daphnia magna p xylene m xylene o xylene all test substances > 97% a. i. Nominal concentrations. Static conditions.

48 hour LC50 values : p xylene: 80 mmol/ m3 m xylene: 90 mmol/ m3 o xylene: 30 mmol/ m3 Based on Molecular Weight of 106.16 for xylene, the above LC50 values are calculated in terms of mg/ L, where 1m3 = 1000 L [ For example: 20 mmole/ m3 × 1 m3/ 1000 L × 106.16 mg/ mmol = 2.1 mg/ L]

p xylene: 8.5 mg a. i./ L m xylene: 9.6 mg a. i./ L o xylene: 3.2 mg a. i./ L Note: Airspace exposure was eliminated to minimize volatilization loss. all values: moderately toxic Abernethy et al. 1986 same data reported in Bobra et al. 1983 Open literature ( Supplemental)

Daphnia magna o xylene m xylene p xylene ( highest purity) Static renewal conditions. Measured concentrations. Note: all tests conducted using a closed system to minimize loss due to volatility.

24 hour IC50 values ( for immobility) o xylene: 1.0 mg a. i./ L m xylene: 4.7 mg a. i./ L p xylene: 3.6 mg a. i./ L Slope functions: o xylene: 1.40 m xylene: 1.97 p xylene: 1.56 moderately to highly toxic Galassi et al. 1988 Open literature ( Supplemental  study was performed in accordance with OECD Standard Method No. 202) Table E 2. Acute Toxicity of Xylenes to Freshwater Invertebrates Species Test Substance LC50 Toxicity Category Reference Study Classification  E 9 Daphnia magna o xylene ( purity not reported) Measured concentrations. Test appears to be conducted under static renewal conditions, although this is not specifically stated.

48 hour LC50: 3.82 mg/ L 95% CI: 2.61 5.59 moderately toxic Holcombe et al. 1987 Open literature ( Supplemental)

snail ( Aplexa hypnorum) o xylene ( purity not reported) Measured concentrations. Test appears to be conducted under static renewal conditions, although this is not specifically stated.

48 hour LC50: > 22.4 mg/ L slightly toxic Holcombe et al. 1987 Open literature ( Supplemental)

d. Freshwater Invertebrate, Chronic Under the conditions of recommended use, chronic exposure of freshwater invertebrates to mixed xylenes is not anticipated. Thus, chronic exposure studies on mixed xylenes in freshwater invertebrates are not required. No chronic toxicity data for freshwater invertebrates were located in the open literature.

III. Toxicity to Estuarine and Marine Animals a. Estuarine and Marine Fish, Acute An estuarine/ marine fish toxicity study is required to establish the toxicity of mixed xylenes to estuarine/ marine fish. The preferred test species is the sheepshead minnow. No acute toxicity studies on mixed xylenes in estuarine/ marine fish were identified from the available open literature.

Only one acute xylene isomer toxicity study for estuarine/ marine fish ( striped bass) was located in the open literature ( Benville and Korn 1977); however, the study is classified as invalid because it does not specifically report that treatments were compared to an acceptable control ( study details are summarized in Table E 3). Therefore, the guideline ( § 72 3) is not fulfilled.  E 10 Table E 3. Acute Toxicity of Xylenes to Estuarine/ Marine Fish Species Test Substance LC50 Toxicity Category Reference Study Classification striped bass ( Morone saxatilis) m xylene o xylene p xylene (> 99% pure) Conducted under static, open conditions. Significant loss of all test substances: p xylene: 21 99% loss from 24 to 96 hrs; m xylene: 32 99% loss from 24 to 96 hrs; o xylene: 19 99% loss from 24 to 96 hours. Toxicity values expressed in terms of nominal concentrations. Water concentrations were measured at 24 hour intervals.

24 hour LC50: m xylene: 9.2 ppm ( 95% CL: 8.3 10) o xylene: 11 ppm ( 95% CL: 9.4 12) p xylene: 2.0 ( 95% CL not reported)

96 hourLC50: m xylene: 9.2 ppm ( 95% CL: 8.3 10) o xylene: 11 ppm ( 95% CL: 9.4 12) p xylene: 2.0 ppm ( 95% CL not reported)

Nearly all mortalities occurred during the first 6 hours of exposure. based on 96 hour values:

m xylene ( moderately toxic); oxylene ( slightly toxic); pxylene ( moderately toxic) Benville and Korn 1977 Open literature ( Invalid: No control data)

b. Estuarine and Marine Fish, Chronic Under the conditions of recommended use, chronic exposure of estuarine/ marine fish to mixed xylenes is not anticipated. Thus, chronic exposure studies on mixed xylenes in estuarine/ marine fish are not required. No chronic toxicity data for estuarine/ marine fish were located in the open literature.

c. Estuarine and Marine Aquatic Invertebrates, Acute  E 11 An estuarine/ marine invertebrate toxicity study is required to establish the toxicity of mixed xylenes to estuarine/ marine invertebrates. The preferred test species is the mysid shrimp. The acute toxicity of mixed xylenes has been evaluated in bay shrimp ( Neff et al. 1976) and grass shrimp ( Tatem et al. 1978); however, the results of the Neff et al. study is classified as invalid because it does not specifically report that treatments were compared to an acceptable control ( details of both studies are provided in Table E 4). Results of the Tatum et al. ( 1978) study reports an acute 96 hour LC 50 value of 7.4 mg/ L in bay shrimp, indicating that mixed xylenes are moderately toxic to estuarine/ marine invertebrates.

The acute toxicity of xylene isomers has also been evaluated in several species of estuarine/ marine invertebrates including bay shrimp ( Benville and Korn 1977), brine shrimp ( Abernathy et al., 1986), larval stage Dungeness crab ( Caldwell et al. 1977), and sea urchin eggs ( Falk Peterson et al. 1985); study details are summarized in Table E 4. Although data is available for four different estuarine/ marine invertebrate species, the studies using bay shrimp ( Benville and Korn 1977) and Dungeness crab ( Caldwell et al. 1977) are classified as invalid because no mention of control data was included as part of the study methodology or results. Available data for the brine shrimp ( Abernathy et al. 1986) shows that xylene isomers are slightly toxic to estuarine/ marine invertebrates with 48 hour LC 50 values of 19.3, 24.5, and 24.7 mg/ L for m xylene, p xylene, and o xylene, respectively.

Results of a sub acute study investigating the effect of o xylene exposure on sea urchin eggs show that embryo lethality was observed after 4 days of exposure, with a 96 hour EC50 value of 4.1 mg/ L ( Falk Peterson et al. 1985). There is uncertainty associated with this study because it was conducted under open conditions, and the authors report a significant decrease in xylene concentrations over the course of the exposure period. However, although not specifically stated, it appears that the results are expressed in terms of measured concentrations The lowest acute toxicity value obtained for o xylene ( the 96 hour EC50 of 4.1 mg/ L in sea urchin eggs ( Falk Peterson et al. 1985) is used to assess acute risk of mixed xylenes to estuarine/ marine invertebrates. The guideline ( § 72 3) is fulfilled based on the available information obtained from the open literature.

Table E 4. Acute Toxicity of Xylenes to Estuarine/ Marine Invertebrates Species Test Substance LC50 Toxicity Category Reference Study Classification bay shrimp ( Palaemonetes pugio) mixed xylenes ( purity and composition of test material not reported) 96 hour LC50: 7.4 ppm Not reported if data are expressed in terms of nominal or measured concentrations. Conditions of exposure ( static or flow through/ open or closed) not reported. moderately toxic Neff et al. 1976 Open literature ( Invalid: No control data) Table E 4. Acute Toxicity of Xylenes to Estuarine/ Marine Invertebrates Species Test Substance LC50 Toxicity Category Reference Study Classification  E 12 grass shrimp ( Palaemonetes pugio) xylenes ( purity and composition of test material not reported) Static conditions conducted under " covered" conditions to minimize volatilization loss. Report does not specify if results are expressed in terms of nominal or measured concentrations.

24 hour LC50: 14.0 ppm 48 hour LC50: 8.5 ppm 96 hour LC50: 7.4 ppm slightly to moderately toxic Tatem et al. 1978 Open literature ( Supplemental)

bay shrimp ( Crago francisocrum) m xylene o xylene p xylene (> 99% pure) Conducted under static, open conditions. Significant loss of all test substances: p xylene: 21 99% loss from 24 to 96 hrs; m xylene: 32 99% loss from 24 to 96 hrs; o xylene: 19 99% loss from 24 to 96 hours. Toxicity values expressed in terms of nominal concentrations. Water concentrations were measured at 24 hour intervals.

24 hour LC50: m xylene: 4.8 ppm ( 95% CL: 3.6 6.3) o xylene: 5.3 ppm ( 95% CL: 4.4 6.5) p xylene: 2.0 ppm ( 95% LC not reported)

96 hour LC50: m xylene: 3.7 ppm 95% CL: 2.9 4.7) o xylene: 1.3 ppm ( 95% CL: 1.1 1.6) p xylene: 2.0 ppm ( 95% LC not reported) moderately toxic to highly toxic Benville and Korn 1977 Open literature ( Invalid: No control data) Table E 4. Acute Toxicity of Xylenes to Estuarine/ Marine Invertebrates Species Test Substance LC50 Toxicity Category Reference Study Classification  E 13 brine shrimp ( Artemia sp.) p xylene m xylene o xylene all test substances < 97% a. i. Nominal concentrations. Static conditions.

48 hour LC50 values p xylene: 232 mmol/ m3 m xylene: 182 mmol/ m3 o xylene: 223 mmol/ m3 Based on Molecular Weight of 106.16 for xylene, the above LC50 values are calculated in terms of mg/ L, where 1m3 = 1000 L [ For example: 145 mmole/ m3 × 1 m3/ 1000 L × 106.16 mg/ mmol = mg/ L]

p xylene: 24.5 mg a. i./ L m xylene: 19.3 mg a. i./ L o xylene: 24.7 mg a. i./ L Note: Airspace exposure was eliminated to minimize volatilization loss. slightly toxic Abernethy et al. 1986 Open literature ( Supplemental)

Dungeness crab ( Canoer magister Dana)  larval stage m xylene o xylene ( purity not reported) Static conditions. Report does not indicate if toxicity endpoints are expressed in terms of nominal or measured values.

48 hour LC50 values: m xylene: 33 mg/ L o xylene: 38 mg/ L 96 hour LC50 values: m xylene: 12 mg/ L o xylene: 6.0 mg/ L slightly to moderately toxic Caldwell et al. 1977 Open literature ( Invalid: No control data) Table E 4. Acute Toxicity of Xylenes to Estuarine/ Marine Invertebrates Species Test Substance LC50 Toxicity Category Reference Study Classification  E 14 sea urchin eggs ( Strongylocentrotus droebachiensis) o xylene (> 98% a. i.) 4 day exposure period under static, open conditions. Test vessels were covered. Significant decrease in xylene concentration in water due to evaporative loss. Although not specifically stated, it appears that results are expressed in terms of measured concentrations.

Effect measured: embryo death 96 hour EC50: 4.1 mg a. i./ L moderately toxic Falk Petersen et al. 1985 Open literature ( Supplemental)

iv. Estuarine and Marine Invertebrate, Chronic Under the conditions of recommended use, chronic exposure of estuarine/ marine invertebrates to mixed xylenes is not anticipated. Thus, chronic exposure studies on mixed xylenes in estuarine/ marine invertebrates are not required. No chronic toxicity data for estuarine/ marine invertebrates were located in the open literature.

IV. Toxicity to Aquatic Plants Acute toxicity studies are required to establish the toxicity of mixed xylenes to aquatic plants ( both algae and aquatic macrophytes). Details of all studies are provided in Table E 5. No studies on the acute effects of mixed xylenes were identified from the available literature.

The acute toxicity of xylene isomers has been evaluated in two studies in 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. Results of these studies show that the xylene isomers have very similar toxicities to algae. The lowest acute toxicity value obtained for the p xylene isomer ( 72 hour EC50 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.  E 15 No studies suitable for use in a quantitative risk assessment on the effects of xylene on non target vascular aquatic macrophytes were identified from the available literature. Results of a single laboratory study on the efficacy of xylene ( xylene type not reported) on target aquatic macrophytes shows that exposure to xylene damaged target plants under both standing and moving water conditions ( Frank et al., 1961). Under static conditions, extensive damage was observed to the three plant species tested at a concentration of 100 ppm, but no damage was observed for the 5 ppm concentration. 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 use. The guideline ( § 122 2 and 123 22) requirements for aquatic plant toxicity testing are not fulfilled.

Table E 5. Toxicity of Xylenes to Aquatic Plants Species Test Substance LC50 Reference Study Classification Selenastrum capricornutum ( green algae) p xylene m xylene o xylene ( purity not reported) Containers were air tight and sealed to minimize volatilization loss. Measured concentrations.

Effects on growth 8 day EC50 ( mg/ L) p xylene: 4.4 m xylene: 3.9 o xylene: 4.2 Herman et al. 1990 Open literature ( Supplemental)

Selenastrum capricornutum ( green algae) o xylene m xylene p xylene ( highest purity) Static renewal conditions. Measured concentrations. Note: all tests conducted using a closed system to minimize loss due to volatility.

72 hour EC50 values ( for growth inhibition) o xylene: 4.7 mg a. i/ L m xylene: 4.9 mg a. i/ L p xylene: 3.2 mg a. i/ L Galassi et al. 1988 Open literature ( Supplemental  study was performed in accordance with OECD Standard Method No. 201) Table E 5. Toxicity of Xylenes to Aquatic Plants Species Test Substance LC50 Reference Study Classification  E 16 water weed ( Elodea canadensis), American pondweed ( Potamogeton nodosus), sago pondweed ( P. pectinatus) xylene ( not specified if mixture or isomer)

( purity not reported) Two test conditions   static ( standing water) conditions ( 4 week exposure to 5 or 100 ppm) and limited contact ( 30 minute exposure to moving water at concentrations of 300 and 600 ppm). Report does not specify nominal or measured concentrations.

Results of static test show that a concentation of 5 ppm did not damage plants. At a concentration of 100 ppm, significant damage was observed to all three plant species.

Results of limited contact test show significant damage to all three plant species at both exposure concentrations. Frank et al. 1961 Open literature ( Supplemental)

V. Toxicity to Terrestrial Animals a. Birds, Acute and Subacute An acute oral toxicity study is required to establish the toxicity of mixed xylenes to birds. The preferred test species is either mallard duck ( a waterfowl) or bobwhite quail ( an upland gamebird). No acute oral toxicity studies in birds using mixed xylenes or xylene isomers were identified from the available literature. The guideline ( § 71 1) is not fulfilled for acute oral toxicity to birds.

Two subacute dietary studies are required to establish the toxicity of mixed xylenes to birds. The preferred test species are mallard duck and bobwhite quail. A single acute dietary study in Japanese quail was identified from the available open literature ( Hill and Camardese 1986); study details are provided in Table E 6. Results of this study yield an acute dietary LC 50 value of > 20,000 mg a. i./ kg diet. Although no information describing the types of sublethal effects or signs of toxicity were provided in this study report, the authors report that no adverse effects were observed at a concentration of 5,000 ppm a. i., the lowest concentration tested. The LC50 value of > 20,000 mg a. i./ kg diet in Japanese quail is used to characterize the acute risk of birds to mixed xylenes. The guideline ( § 71 2) is fulfilled for acute dietary toxicity to birds.  E 17 Table E 6. Acute Dietary Toxicity of Xylenes to Birds Species Test Substance LC50 Toxicity Category Reference Study Classification Japanese quail mixed xylenes ( reagent grade, 100 % a. i.) 5 day dietary exposure study. Birds were 14 days old at the start of the 5 day exposure period. Weight of animals was not provided in the study report.

LC50 > 20,000 ppm a. i.

Study does not report if any deaths occurred at the highest concentration tested ( 20,000 ppm). No overt signs of toxicity at 5,000 ppm a. i. Specific signs of toxicity at higher dietary concentrations were not reported.

Average food consumption per day: Day 1: 12.2 g food/ bird/ day Day 2: 12.8 g food/ bird/ day Day 3: 12.9 g food/ bird/ day Day 4: 12.7 g food/ bird/ day Day 5: 12.2 g food/ bird/ day 5 day average food consumption: 12.6 g food/ bird/ day ( equivalent to 0.0126 kg) practically non toxic Hill and Camardese 1986 Open literature ( Supplemental)