Environmental Engineering Reference
In-Depth Information
centiles from the fi tted species sensitivity distribution were selected to represent the
range of sensitivities of birds to CPY. A hypothetical dose-response curve was
derived for each of these centile species by combining the estimated LD 50 with a
probit slope. Because of the toxicity mitigation problems noted above with dietary
exposures, the acute effect metrics in this assessment were based upon the results of
acceptable acute oral gavage toxicity studies (Table 4 ).
The following sections describe how the acute and chronic effects metrics were
derived. A NOEL and LOEL were selected as the chronic effects metrics because
the available chronic toxicity studies did not have a suffi cient number of treatments
(i.e., fi ve or more) to enable derivation of dose-response curves. There were an
insuffi cient number of tested species to permit development of a SSD for chronic
toxicity data.
Acute dose-response relationships for focal species . The LD 50 , based on oral gavage,
for the red-winged blackbird ( A. phoeniceus ) was 13.1 mg ai kg −1 bwt (Schafer and
Brunton 1971 ), however, no probit slope was reported. To generate a dose-response
curve for red-winged blackbird in this assessment, a geometric mean probit slope of
3.45 was calculated from the studies listed in Table 4 . For northern bobwhite
( C. virginianus ), two LD 50 s have been reported: 32 and 119 mg ai kg −1 bwt (Hill and
Camardese 1984 ; Kaczor and Miller 2000 ). The corresponding probit slopes from
these studies were 4.6 and 3.88, respectively. The resulting geometric mean LD 50
and probit slope were 61.7 mg ai kg −1 bwt and 4.22. These values were used to gen-
erate the acute dose-response curve for northern bobwhite in this assessment.
SSD for acute toxicity of CPY to untested focal species . The data used in the deriva-
tion of the SSD for avian species are shown (Table 4 ). Multiple toxicity values were
reported for several species. Variation in toxicity for a species could be the result of
differences in experimental conditions, species strain, and/or test protocol. Using
multiple toxicity results for the same species would disproportionately infl uence the
SSD. In these situations, the geometric means were calculated (Table 4 ). Each bird
species was then ranked according to sensitivity and its centralized position on the
SSD determined using the Hazen plotting position equation ( 1 ) (Aldenberg et al.
2002 ):
i
- 0.5
PP
=
( 1 )
N
Where:
PP is the plotting position;
i is the species rank based on ascending LD 50 s; and
N is the total number of species included in the SSD derivation.
The SSD was derived using SSD Master v2.0, which includes fi ve models: nor-
mal, logistic, Weibull, extreme value (=Gompertz) and Gumbel (=Fisher-Tippett)
(CCME 2013 ). All analyses were conducted in log space, except the Weibull model,
which was conducted in arithmetic space because a log-Weibull model is the
same as the Gumbel model. The log-normal model had the best fi t of the fi ve
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