1.1 MB pdf - Bolsa Chica Lowlands Restoration Project

SECTION 4: RISK CHARACTERIZATION
dose-response effect could be observed if it were present. The data had to be transformed
back to the original state after the regression analyses so that the EC 50 could be estimated.
The transformation and then back-transformation can result in some uncertainty. The
uncertainty in the estimated EC 50 values is inversely proportional to both the sample size
and r 2 for the regression model on which they are based. As both sample size and
r 2 increase, confidence in the EC 50 increases. The best EC 50 measurements (e.g., those with
the least uncertainty) are based on models with the highest r 2 (i.e., > 0.5) and the largest
sample sizes, followed by EC 50 based on models with high r 2 and small sample sizes.
Moderate uncertainty is associated with EC 50 based on models where 0.2 < r 2 < 0.5. Given
the small amount of variation they describe, the EC 50 based on models with r 2 < 0.2,
regardless of sample size, are not recommended for use in remedial decisionmaking.
For Nereis (in sediment), topsmelt (in surface water), and Mysidopsis (in surface water), it
was not possible to determine the maximum concentrations of any chemicals that would not
cause significant effects (the NOECs) or the lowest concentrations that cause effects
(LOECs). This occurred because no effects were observed at the highest exposure
concentrations that were tested. Therefore, the NOECs and LOECs for those species
represent uncertainties, may result in an overestimation of the HQ.
Uncertainties associated with the selection of RTVs for use in the ERA include the effects
data available and extrapolations made. An attempt was made to identify RTVs for each
chemical for each receptor group, but toxicological information that can be correlated to
media concentrations is generally limited for terrestrial plants, invertebrates, and birds. In
general, RTVs for terrestrial receptors were limited to chronic no-effect and low-effect levels.
RTVs for aquatic receptors included both chronic and acute effect levels. As such, the
highest level of risk that could potentially quantified was Category B for terrestrial receptors
and Category A for aquatic receptors.
Lack of RTVs for several chemicals results in uncertainty of the risk posed by these
chemicals. Receptors that had the least number of RTVs available were terrestrial plants,
terrestrial invertebrates, and birds. RTVs were available for most chemicals for mammals
and aquatic receptors.
The other main source of uncertainty in RTVs is for those chemicals for which the only RTV
available was a NOEC based on a toxicity bioassay which did not show any toxicity. Since
there were no other RTVs with which to compare the HQs, it is unknown whether the HQ
represents an accurate estimation or is over-estimated. The other site-specific RTVs
(e.g., NOECs, LOECs, LC 20 s, and LC 50 s) were generally within the same magnitude as
established benchmarks, but not in all cases. Specifically, some LC 20 values were far more
conservative than ER-Ls.
4.3.3 Risk Characterization
Uncertainties related to the risk characterization include the use of hazard quotients to
quantify potential risks and the assumption that estimated risks for the representative
species will be protective of all similar receptors.
Hazard quotients are an estimate of potential risk and, while it can be conservatively
determined that if an HQ exceeds one there is a potential for risk, the magnitude of the HQ
cannot be used as a definitive measure of the risk. Different types of effect levels such as
SAC/143368(004.DOC) 4-27 ERA REPORT
7/31/02