Risks to aquatic organisms from use of chlorpyrifos in the United States.

The risk of chlorpyrifos (CPY) to aquatic organisms in surface water of North America was assessed using measured concentrations in surface waters and modeling of exposures to provide daily concentrations that better characterize peak exposures.Ecological effects were compared with results of standard laboratory toxicity tests with single species as well as microcosm and mesocosm studies comprised of complex aquatic communities. The upper 90th centile 96-h concentrations(annual maxima) of chlorpyrifos in small streams in agricultural watersheds in Michigan and Georgia were estimated to be :-:;0.02 llg L-1; in a reasonable worstcase California watershed, the 90th centile 96-h annual maximum concentrations ranged from 1.32 to 1.54 llg L - 1• Measured concentrations of chlorpyrifos are less than estimates from simulation models. The 95th centile for more than I 0,000 records compiled by the US Geological Survey was 0.008 llg L -1• Acute toxicity endpoints for 23 species of crustaceans ranged from 0.035 to 457 llg L -I; for 18 species of aquatic insects, from 0.05 to 27 llg L -I; and for 25 species of fish, from 0.53to >806 llg L -I. The No Observed Adverse Effect Concentration (NOAECeco) in more than a dozen microcosm and mesocosm studies conducted in a variety of climatic zones, was consistently 0.1 llg L -1• These results indicated that concentrations of CPY in surface waters are rarely great enough to cause acute toxicity to even the most sensitive aquatic species. This conclusion is consistent with the lack of fish kills reported for CPY's normal use in agriculture in the U.S.Analysis of measured exposures showed that concentrations in surface waters declined after labeled use-patterns changed in 2001, and resulted in decreased risks for crustaceans, aquatic stages of insects, and fish. Probabilistic analysis of 96-h time-weighted mean concentrations, predicted by use of model simulation for three focus-scenarios selected for regions of more intense use of CPY and vulnerability to runoff, showed that risks from individual and repeated exposures to CPY in the Georgia and Michigan watersheds were de minimis. Risks from individual exposures in the intense-use scenario from California were de minimis for fish and insects and low for crustaceans. Risks from repeated exposures in the Californiain tense-use scenario were judged not to be ecologically relevant for insects and fish,but there were some risks to crustaceans. Limited data show that chlorpyrifos oxon(CPYO), the active metabolite of CPY is of similar toxicity to the parent compound.Concentrations of CPYO in surface waters are smaller than those of CPY and less frequently detected. Risks for CPYO in aquatic organisms were judged to be deminimis.Several uncertainties common to all AChE inhibitors were identified. Insufficient data were available to allow interpretation of the relevance of effects of CPY (and other pesticides that also target AChE) on behavior to assessment endpoints such as survival, growth, development, and reproduction. Data on the recovery of AChE from inhibition by CPY in fish are limited. Such data are relevant to the characterization of risks from repeated exposures, and represent an uncertainty in the assessment of risks for CPY and other pesticides that share the same target and toxico dynamics. More intensive monitoring of areas of greater use and more comprehensive models of cumulative effects that include rates of accumulation, metabolism and recovery of AChE in the more sensitive species would be useful in reducing this uncertainty.