Ecological Risk Assessment of the Uses of the Organophosphorus Insecticide Chlorpyrifos, in the United States - Reviews of Environmental Contamination and Toxicology - page 6

the greatest exposure estimates. Model simulations for Orestimba Creek in
California used actual reported applications of CPY, but fi eld-specifi c management
practices were not represented in the simulations. Two half-lives for aerobic soil
metabolism of CPY in soil, 28 and 96 d, were selected for the purposes of modeling.
These half-lives conservatively represent the fi rst and second phases of bi-phasic
degradation in aerobic soil metabolism studies.
Estimated concentrations of CPY in water were in general agreement with ambi-
ent monitoring data from 2002 to 2010. Maximum daily concentrations predicted
for the watersheds a in California, Georgia, and Michigan were 3.2, 0.041, and
0.073

g L −1 , respectively, with the 28-d aerobic soil metabolism half-life and 4.5,
0.042, and 0.122

μ

g L −1 , respectively, with the 96-d soil half-life. These estimated
values compared favorably with maximum concentrations measured in surface
water, which ranged from 0.33 to 3.96

μ

g L −1 . For sediments, the maximum daily
concentrations predicted for the watersheds in California, Georgia, and Michigan
were 11.2, 0.077, and 0.058

μ

μ g kg −1 dwt, respectively, with the 28-d half-life, and

22.8, 0.080, and 0.087 g kg −1 , respectively, with the 96-d soil half-life. Twelve

detections out of 123 analyses (10%) were contained in the USGS, CDPR, and
WDOE databases with concentrations reported from <2.0 to 19

μ

μ g kg −1 , with the

exception of one value reported at 58.6 μ g kg −1 . Again, the modeled values com-

pared favorably with measured values.
Duration and recovery intervals between peak concentrations of CPY infl uence
the potential for recovery from sublethal exposures in aquatic organisms. Recovery
intervals were characterized by using threshold values derived from toxicity data.
Based on modeling with the 28-d half-life value, no toxicologically signifi cant
exposure-recovery events were identifi ed in the focal watersheds in Georgia and
Michigan. Using the 96-d half-life value, three exposure-recovery events of 1 d
duration only were identifi ed in the Michigan focal watershed. Frequency of signifi -
cant events was greater in the focus watershed from California and the probability
of shorter recovery events was greater. However, even in the worst-case focus-
watershed in California the median duration was 1 d.

5 Risks of Chlorpyrifos to Aquatic Organisms

The fi fth paper in this series addressed the risks of CPY to aquatic organisms. In
contrast to the previous lower-tier assessments that indicated potential adverse
effects in aquatic organisms (Giesy et al. 1999 ), this paper relied on higher and more
refi ned tiers of risk assessment. Effects of CPY on aquatic organisms were evalu-
ated by comparing measured or modeled concentrations of CPY in aquatic environ-
ments to species sensitivity distributions (SSDs), cosm no observed ecologically
adverse effect concentrations (NOAEC eco ), or individual toxicity values where suf-
fi cient data to derive a SSD were not available (Giddings et al. 2014 ). Toxicity data
included in the SSDs were all of high quality. The ranges for acute toxicity end-
points for 23 species of crustaceans ranged from 0.04 to 457

μ g L −1 ; for 18 species