Abstract-The triazine herbicide atrazine (2-chloro-4-ethylamino-6-isopropyl-amino-s-triazine) is one of the most used pesticides in North America. Atrazine is principally used for control of certain annual broadleaf and grass weeds, primarily in corn but also in sorghum, sugarcane, and, to a lesser extent, other crops and landscaping. Atrazine is found in many surface and ground waters in North America, and aquatic ecological effects are a possible concern for the regulatory and regulated communities. To address these concerns an expert panel (the Panel) was convened to conduct a comprehensive aquatic ecological risk assessment. This assessment was based on several newly suggested procedures and included exposure and hazard subcomponents as well as the overall risk assessment. The Panel determined that use of probabilistic risk assessment techniques was appropriate. Here, the results of this assessment are presented as a case study for these techniques. The environmental exposure assessment concentrated on monitoring data from Midwestern watersheds, the area of greatest atrazine use in North America. This analysis revealed that atrazine concentrations rarely exceed 20 g/L in rivers and streams that were the main focus of the aquatic ecological risk assessment. Following storm runoff, biota in lower-order streams may be exposed to pulses of atrazine greater than 20 g/L, but these exposures are short-lived. The assessment also considered exposures in lakes and reservoirs. The principal data set was developed by the U.S. Geological Survey, which monitored residues in 76 Midwestern reservoirs in 11 states in 1992-1993. Residue concentrations in some reservoirs were similar to those in streams but persisted longer. Atrazine residues were widespread in reservoirs (92% occurrence), and the 90th percentile of this exposure distribution for early June to July was about 5 g/L. Mathematical simulation models of chemical fate were used to generalize the exposure analysis to other sites and to assess the potential effects of reduction in the application rates. Models were evaluated, modified, and calibrated against available monitoring data to validate that these models could predict atrazine runoff. PRZM-2 overpredicted atrazine concentrations by about an order of magnitude, whereas GLEAMS underpredicted by a factor of 2 to 5. Thus, exposure models were not used to extrapolate to other regions of atrazine use in this assessment. The effects assessment considered both freshwater and saltwater toxicity test results. Phytoplankton were the most sensitive organisms, followed, in decreasing order of sensitivity, by macrophytes, benthic invertebrates, zooplankton, and fish. Atrazine inhibits photophosphorylation but typically does not result in lethality or permanent cell damage in the short term. This characteristic of atrazine required a different model than typically used for understanding the potential impact in aquatic systems, where lethality or nonreversible effects are usually assumed. In addition, recovery of phytoplankto...

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Exposure to agrochemicals in the aquatic environment often occurs as time-varying or repeated pulses. Time-varying exposures may occur due to runoff events and spray drift associated with precipitation and application events. Hydrologic dilution, dispersion, and degradation also produce pulsed exposures. Standard laboratory toxicity tests using constant exposure concentrations typically do not investigate the toxicity of time-varying or repeated exposures. Detoxification, elimination, and recovery may occur within organisms or populations during the periods between exposures. The difficulty of estimating effects of realistic time-varying exposures from measurements made under constant exposure conditions is often an important source of uncertainty in ecological risk assessment of pesticides. This article discusses the criteria and tools for deciding whether time-varying exposures are relevant in a particular risk assessment, approaches for laboratory toxicity testing with time-varying exposure, modeling approaches for addressing effects oftime-varying exposure, deterministic and probabilistic ecological risk characterization of time-varyingexposures and toxicity, and uncertainty analysis.

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The triazine herbicide atrazine (2‐chloro‐4‐ethylamino‐6‐isopropyl‐amino‐s‐triazine) is one of the most used pesticides in North America. Atrazine is principally used for control of certain annual broadleaf and grass weeds, primarily in corn but also in sorghum, sugarcane, and, to a lesser extent, other crops and landscaping. Atrazine is found in many surface and ground waters in North America, and aquatic ecological effects are a possible concern for the regulatory and regulated communities. To address these concerns an expert panel (the Panel) was convened to conduct a comprehensive aquatic ecological risk assessment. This assessment was based on several newly suggested procedures and included exposure and hazard subcomponents as well as the overall risk assessment. The Panel determined that use of probabilistic risk assessment techniques was appropriate. Here, the results of this assessment are presented as a case study for these techniques. The environmental exposure assessment concentrated on monitoring data from Midwestern watersheds, the area of greatest atrazine use in North America. This analysis revealed that atrazine concentrations rarely exceed 20 μg/L in rivers and streams that were the main focus of the aquatic ecological risk assessment. Following storm runoff, biota in lower‐order streams may be exposed to pulses of atrazine greater than 20 μg/L, but these exposures are short‐lived. The assessment also considered exposures in lakes and reservoirs. The principal data set was developed by the U.S. Geological Survey, which monitored residues in 76 Midwestern reservoirs in 11 states in 1992‐1993. Residue concentrations in some reservoirs were similar to those in streams but persisted longer. Atrazine residues were widespread in reservoirs (92% occurrence), and the 90th percentile of this exposure distribution for early June to July was about 5 μg/L. Mathematical simulation models of chemical fate were used to generalize the exposure analysis to other sites and to assess the potential effects of reduction in the application rates. Models were evaluated, modified, and calibrated against available monitoring data to validate that these models could predict atrazine runoff. PRZM‐2 overpredicted atrazine concentrations by about an order of magnitude, whereas GLEAMS underpredicted by a factor of 2 to 5. Thus, exposure models were not used to extrapolate to other regions of atrazine use in this assessment. The effects assessment considered both freshwater and saltwater toxicity test results. Phytoplankton were the most sensitive organisms, followed, in decreasing order of sensitivity, by macrophytes, benthic invertebrates, zooplankton, and fish. Atrazine inhibits photophosphorylation but typically does not result in lethality or permanent cell damage in the short term. This characteristic of atrazine required a different model than typically used for understanding the potential impact in aquatic systems, where lethality or nonreversible effects are usually assumed. In addition, recovery of phytoplan...

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The distribution and chemical fate of (36)Cl-ClO2 gas subsequent to fumigation of tomatoes or cantaloupe was investigated as were major factors that affect the formation of chloroxyanion byproducts. Approximately 22% of the generated (36)Cl-ClO2 was present on fumigated tomatoes after a 2 h exposure to approximately 5 mg of (36)Cl-ClO2. A water rinse removed 14% of the radiochlorine while tomato homogenate contained ∼63% of the tomato radioactivity; 24% of the radiochlorine was present in the tomato stem scar area. Radioactivity in tomato homogenate consisted of (36)Cl-chloride (≥80%), (36)Cl-chlorate (5 to 19%), and perchlorate (0.5 to 1.4%). In cantaloupe, 55% of the generated (36)Cl-ClO2 was present on melons fumigated with 100 mg of (36)Cl-ClO2 for a 2 h period. Edible cantaloupe flesh contained no detectable radioactive residue (LOQ = 0.3 to 0.4 μg/g); >99.9% of radioactivity associated with cantaloupe was on the inedible rind, with <0.1% associated with the seed bed. Rind radioactivity was present as (36)Cl-chloride (∼86%), chlorate (∼13%), and perchlorate (∼0.6%). Absent from tomatoes and cantaloupe were (36)Cl-chlorite residues. Follow-up studies have shown that chlorate and perchlorate formation can be completely eliminated by protecting fumigation chambers from light sources.

This is the first in a series of five papers that assess the risk of the cotton pyrethroids in aquatic ecosystems in a series of steps ranging from the analysis of effects data through modeling exposures in the landscape. Pyrethroid insecticides used on cotton have the potential to contaminate aquatic systems. The objectives of this study were to develop probabilistic estimates of toxicity distributions, to compare these among the pyrethroids, and to evaluate cypermethrin as a representative pyrethroid for the purposes of a class risk assessment of the pyrethroids. The distribution of cypermethrin acute toxicity data gave 10th centile values of 10 ng/L for all organisms, 6.4 ng/L for arthropods, and 380 ng/L for vertebrates. For bifenthrin, cyfluthrin, lambda-cyhalothrin, and deltamethrin, the 10th centile values for all organisms were 15, 12, 10, and 9 ng/L, respectively, indicating similar or somewhat lower toxicity than cypermethrin. For tralomethrin and fenpropathrin, the 10th centiles were <310 and 240 ng/L, respectively. The distribution of permethrin toxicity to all organisms, arthropods, and vertebrates gave 10th centiles of 180, 76, and 1600 ng/L, respectively, whereas those for fenvalerate were 37, 8, and 150 ng/L. With the exception of tralomethrin, the distributions of acute toxicity values had similar slopes, suggesting that the variation of sensitivity in a range of aquatic nontarget species is similar. The pyrethroids have different recommended field rates of application that are related to their efficacy, and the relationship between field rate and 10th centiles showed a trend. These results support the use of cypermethrin as a reasonable worst-case surrogate for the other pyrethroids for the purposes of risk assessment of pyrethroids as a class.