Risks to Aquatic Organisms from Use of Chlorpyrifos in the United States - Reviews of Environmental Contamination and Toxicology - page 151

recovery in cosms (Sect. 3.4 ). Only three or four events (for the 96-d and 28-d
half-lives, respectively) had durations of 4 d or greater and recovery times less than
56 d. These results suggest that ecologically signifi cant single and repeated expo-
sure events were rare, even in the high-exposure California scenario.

5 Conclusions

This ecological risk assessment of CPY and its oxon CPYO built upon a previous
assessment (Giesy et al. 1999 ) and was refi ned to address changes in the labeled
uses, different use patterns, and new toxicity data. Exposure data were taken from
Williams et al. ( 2014 ), which characterizes measured and modeled concentrations
of CPY in surface waters of the U.S.
The major pathway for exposures to CPY in surface waters is direct accumulation
from water, rather than through diet or from sediments. CPY adsorbs strongly to
sediments, and this mitigates exposures to benthic invertebrates via sediment. CPY's
sediment behavior is consistent with the fact that toxicity is less frequently observed
to occur via sediment than water under fi eld testing conditions. The focus of the
ERA was thus directed mostly to surface waters and water-column organisms.
Because exposures to CPY in fl owing surface waters are episodic with durations
usually less than 2 d (Williams et al. 2014 ), recovery of organisms between pulses
can reduce overall risks, but frequent pulses with short recovery periods could result
in cumulative damage and cumulative risks. The few studies that have characterized
recovery of the target enzyme (AChE) from CPY suggest that invertebrates recover
more rapidly than fi sh. These recovery periods were from 1 to ~7 d for invertebrates,
and periods of the order of 4-8 wk might be required for complete recovery of
AChE in fi sh. These periods were considered in the risk assessment. In situations
where there is potential for multiple pulsed exposures, a more complex model could
be developed that includes accumulation, time to effects and species-specifi c rates
of recovery of AChE. In this assessment of risk, to be conservative, it was assumed
that recovery in all organisms would be at the upper bound of observed times
(2-8 wk). This assumption likely results in an overestimate of risk.
Characterization of acute toxicity of CPY showed that crustaceans were most
sensitive to CPY (HC5 = 0.034 μ g CPY L −1 ), closely followed by insects

(HC5 = 0.087 g CPY L −1 ). The

little data available for aquatic stages of amphibians suggested that they were less
sensitive to CPY than fi sh. Thus fi sh were protective of amphibians, and amphibians
would only need to be considered in an ERA if fi sh were affected. This was not the
case for CPY.
Assessment of the results of a large number of studies of the effects of CPY in
cosms suggested that the no observed adverse effect concentration in these systems
(NOAEC eco ) was 0.1

μ g CPY L −1 ). Fish were less sensitive (HC5 = 0.812 μ

g L −1 . These data were derived from single and multiple expo-
sures to CPY and support the conclusion that the HC5s for insects and crustaceans
from acute toxicity studies are predictive and protective of toxicity under conditions

μ