1.1 MB pdf - Bolsa Chica Lowlands Restoration Project

SECTION 4: RISK CHARACTERIZATION
determine whether the mean surface and subsurface inorganic levels differed throughout
the Lowlands was not done. The estimate of background conditions was based on the “all
sample” data set.
Because there were no comparable offsite reference areas for Bolsa Chica, the estimate of
inorganic background levels was based on a qualitative evaluation of the cumulative percent
distribution curves for each constituent derived from onsite samples to indicate the
background or ambient levels. The determination of the curve break points required
professional judgment based on review of the data. Where the data sets contained a large
number of elevated non-detect (“U”-flagged) values, the curves were regenerated to
determine the distribution of detected values. This was done only for the “all sample” groups
of cadmium, mercury, selenium, silver, and thallium. There were a few elevated non-detect
values on some of the other “all sample,” surface, and subsurface cumulative percent plots.
However, the non-detects were not screened out of those data sets unless they directly
interfered with the interpretation of the break points for the cumulative percent curves.
The calculation of exposure point concentrations included assumptions that chemical
concentrations would remain constant over time, chemicals not detected or analyzed were
not present, and detected concentrations had the same bioavailability as those used in the
literature-reported toxicity tests or other toxicological studies. These assumptions may not
be realistic for all chemicals in all media, but they are generally conservative and represent
standard practice for conducting ERAs. The calculation of exposure point concentrations
was also limited by the lack of sample-specific reporting limits for non-detected chemicals in
the Tetra Tech data. Specifically, the electronic (and hardcopy version) of the Tetra Tech
data reported a “0” for non-detected chemicals rather than the detection limit. When
calculating summary statistics, non-detected chemicals are typically evaluated at one-half of
the reported detection limit. Because this information was not available, the non-detect
values were statistically evaluated at one-half of “0”, which equaled “0”. This results in a
downward or underestimation of the mean and 95th UCL. The 95th UCL was used to
estimate risks to mobile receptors (birds and mammals) and so these risks may be underestimated.
In addition, the summary statistical program used (SAS, 1990) did not
distinguish between detected chemicals and non-detected chemicals when selecting the
maximum value. If a ½ non-detect value was still greater than the maximum detected value,
the ½ non-detect value was selected as the maximum and used to estimate risks. This
resulted in an overestimation of many hazard quotients. Some were within the same order
of magnitude, but others were greater. The hazard quotients calculated using ½ non-detect
values are noted with an “*” in Tables 4-1 through 4-4.
Several exposure routes were considered minor and were not included in the exposure
analysis. Nonetheless, exposure via these other routes still contributes to the total risk to
each receptor; therefore, potential risks could have been underestimated because these
routes were not quantified. The routes of exposure that were not retained for quantitative
exposure evaluations include the following:
• Dermal contact with sediment/soil and surface water by birds or mammals
• Inhalation of volatiles from sediment/soil or surface water by birds or mammals
SAC/143368(004.DOC) 4-25 ERA REPORT
7/31/02