1.1 MB pdf - Bolsa Chica Lowlands Restoration Project

SECTION 4: RISK CHARACTERIZATION
4.3.1 Problem Formulation
The Scoping Assessment (CH2M HILL, 1998b) was the basis for the problem formulation for
the EEC Report (CH2M HILL, 1999) and this ERA report, and uncertainties are primarily
associated with the limited availability of chemical stressor information at the time that the
document was produced. Other uncertainties are associated with the selection of
representative species and the identification of exposure pathways.
The stressor data available for the identification of COPECs was limited to hard-copy
reports from the Phase II sampling conducted by Tetra Tech (1996). The data were not
available in electronic format at that time, so evaluations were limited to screening the
maximum detected concentrations in each medium against screening-level benchmarks. In
addition, the nature and extent of contamination across the Lowlands could not be
evaluated because the Tetra Tech results were based on a focused sampling plan, whereby
only those locations with suspected toxicity were evaluated, and the ERA Sampling and
Analyses was just beginning. The electronic version of the Tetra Tech data was obtained, but
was found to be incomplete and not in a structure conducive to incorporation into the
database format necessary to calculate exposure point concentrations, estimate exposures, or
conduct Geographic Information System (GIS) mapping. This uncertainty was rectified in
this ERA with the acquisition and incorporation of the electronic database from the Tetra
Tech sampling (1996).
Representative species are selected to reduce uncertainty and to focus on species that are
both maximally exposed and representative of the wildlife using the site. However,
differences between species, including physiology, reproductive biology, or foraging habits,
can result in different exposures and sensitivities to different chemicals.
4.3.2 Analysis
The analysis consists of the exposure characterization and the ecological effects
characterization. Uncertainties related to these tasks are presented below.
4.3.2.1 Exposure Characterization
The uncertainties associated with the exposure characterization include limitations in the
background evaluation, assumptions made in calculating exposure point concentrations,
selection of exposure routes to quantify, and identification of species-specific exposure
parameters.
The evaluation of background inorganic levels in sediments included all samples collected
in the ERA Sampling and Analysis, including those samples collected from the dredge
footprint area. Some of these samples have been impacted by contaminants from onsite
activities or drainage to the Lowlands, which would have increased the levels of some of the
inorganic constituents. Dredge area sediment samples could not be readily separated by
depth because of different sampling approaches used there. For example, the surface
interval in the dredge area included at least the top 2 feet bgs, and may have included the
entire core (8 feet) if the core material was uniform rather than just the top 6 inches of
material (sampling was conducted to characterize each distinct layer of sediment that was at
least 2 feet thick). Therefore, the “surface sediment” data set includes samples that were
actually sampled to depths greater than 6 inches bgs. For this reason, a statistical analysis to
ERA REPORT 4-24 SAC/143368(004.DOC)
7/31/02