1.1 MB pdf - Bolsa Chica Lowlands Restoration Project

SECTION 4: RISK CHARACTERIZATION
no-effect levels, low-effect levels, and acute effect levels are used to aid in the weighting
of potential risks. In addition, results of toxicity bioassays are also used to increase the
confidence in the estimation of risk to a given receptor.
Given the differences in species, the estimated risks for one species may be over-or underrepresentative
of another species.
4.3.4 Overall Uncertainty
The uncertainties that have the greatest impact on the results of the ERA and their potential
impact are listed below:
• Use of ½ non-detect limits for estimating risk – overestimation of HQs. The HQs would
be lower if the actual value in that sample was much less than 1/2 the reporting limit
and the maximum detected value had been used. In some cases the HQ would have
been within the same order of magnitude, but in others, it would have been less and
may possibly have resulted in an HQ less than 1.
• Use of “0” for reporting non-detected chemicals – results in an underestimation of the
exposure point concentrations using the 95 th UCL. These calculations typically use ½ the
non-detect value. Use of “0”s lowers the 95 th UCL and as such lowers the resulting HQs.
• Lack of any RTV for a given chemical and receptor – level of impact varies based on
whether there is an RTV for the given chemical for another receptor group (e.g., RTVs
for 4,4’-DDD were not available for terrestrial plants or invertebrates, but were available
for birds and mammals). If the chemical can be evaluated at some trophic level, then
there is less uncertainty than if the chemical could not be evaluated at all.
• Lack of site-specific RTVs – level of impact varies depending on the chemical and
receptors potentially involved. Site-specific RTVs add power to the quantification of risk
and may provide RTVs where none were available in the literature, but if they
complement those available in the literature, then the impact is minimal.
Use of RTVs with inherent uncertainty – level of impact can be minimal or large. RTVs with
some uncertainty include those taken from older references (e.g., Long and Morgan, 1990) that
are not used in more recent references as well as those that are based on toxicity bioassays in
which there were no toxic effects. The use of older references had a minimal impact as they
allowed quantification of risks to some chemicals that would otherwise have no other
low-confidence RTVs. Use of NOECs from toxicity bioassays that had no toxic response
could have a larger impact and result in overestimated risks for those chemicals. This was
most apparent in the estimates for aquatic receptor exposure to sediment (Table 4-3).
ERA REPORT 4-28 SAC/143368(004.DOC)
7/31/02