ECOLOGICAL RISK ASSESSMENT OF ATRAZINE IN NORTH-AMERICAN SURFACE WATERS

The triazine herbicide atrazine -chloro-4-ethylamino-6-isopropyl-amino -s-triazine) is one of the most used pesticides in North America. Atra zine is principally used for control of certain annual broadleaf and g rass weeds, primarily in corn but also in sorghum, sugarcane, and, to a lesser extent, other crops and landscaping. Atrazine is found in man y surface and ground waters in North America, and aquatic ecological e ffects are a possible concern for the regulatory and regulated communi ties. To address these concerns an expert panel (the Panel) was conven ed to conduct a comprehensive aquatic ecological risk assessment. This assessment was based on several newly suggested procedures and includ ed exposure and hazard subcomponents as well as the overall risk asses sment. The Panel determined that use of probabilistic risk assessment techniques was appropriate. Here, the results of this assessment are p resented as a case study for these techniques. The environmental expos ure assessment concentrated on monitoring data from Midwestern watersh eds, the area of greatest atrazine use in North America. This analysis revealed that atrazine concentrations rarely exceed 20 mu g/L in rive rs 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 mu g/L, but these exp osures are short-lived. The assessment also considered exposures in la kes and reservoirs. The principal data set was developed by the U.S. G eological 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 residu es were widespread in reservoirs (92% occurrence), and the 90th percen tile of this exposure distribution for early June to July was about 5 mu g/L. Mathematical simulation models of chemical fate were used to g eneralize the exposure analysis to other sites and to assess the poten tial effects of reduction in the application rates. Models were evalua ted, modified, and calibrated against available monitoring data to val idate that these models could predict atrazine runoff. PRZM-2 overpred icted atrazine concentrations by about an order of magnitude, whereas GLEAMS underpredicted by a factor of 2 to 5. Thus, exposure models wer e not used to extrapolate to other regions of atrazine use in this ass essment. The effects assessment considered both freshwater and saltwat er toxicity test results. Phytoplankton were the most sensitive organi sms, followed, in decreasing order of sensitivity, by macrophytes, ben thic invertebrates, zooplankton, and fish. Atrazine inhibits photophos phorylation but typically does not result in lethality or permanent ce ll 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 ar e usually assumed. In addition, recovery of phytoplankton from exposur e to 5 to 20 mu g/L atrazine was demonstrated. In some mesocosm field experiments, phytoplankton and macrophytes were reduced after atrazine exposures greater than 20 mu g/L. However, populations were quickly r eestablished, even while atrazine residues persisted in the water. Eff ects in field studies were judged to be ecologically important only at exposures of 50 mu g/L or greater. Mesocosm experiments did not revea l disruption of either ecosystem structure or function at atrazine con centrations typically encountered in the environment (generally 5 mu g /L or less). Based on an integration of laboratory bioassay data, fiel d effects studies, and environmental monitoring data from watersheds i n high-use areas in the Midwestern United States, the Panel concluded that atrazine does not pose a significant risk to the aquatic environm ent. Although some inhibitory effects on algae, phytoplankton, or macr ophyte production may occur in small streams vulnerable to agricultura l runoff, these effects are likely to be transient, and quick recovery of the ecological system is expected. A subset of surface waters, pri ncipally small reservoirs in areas with intensive use of atrazine, may be at greater risk of exposure to atrazine. Therefore, it is recommen ded that site-specific risk assessments be conducted at these sites to assess possible ecological effects in the context of the uses to whic h these ecosystems are put and the effectiveness and cost-benefit aspe ct of any risk mitigation measures that may be applied.