1.1 MB pdf - Bolsa Chica Lowlands Restoration Project

SECTION 3: ANALYSIS
dredge footprint) were grouped together. (The analytical results from the Focused Sampling
and Analyses and the Tetra Tech Phase II sampling were not included in the development
of the background values). Using samples from potentially contaminated areas (i.e., focused
sites) is reasonable because even if the samples have been contaminated with one or more
metals, they may still have background levels for a number of other metals (Cal/EPA, 1997).
The reported analytical values for all samples that were considered as non-detects (i.e., those
results accompanied by the laboratory flag “U” or "UJ") were evaluated at one-half the
reporting limit. All other laboratory results were left unchanged. This transformed data set
was then plotted in a manner similar to the method for displaying analytical data shown in
the Cal/EPA (1997) guidance for evaluating inorganic constituents. The transformed data
were sorted in order of ascending concentration for each metal and plotted against the
cumulative percent of samples. The cumulative percent plots for selected inorganic
constituents are included in Appendix E. Where available, sediment screening benchmark
values (Low and Median Effect Range [ER-L and ER-M] after Long et al., 1995 and other
sources) are also provided on the cumulative percent plots for comparative purposes.
When inorganics are measured for a relatively large number of background or site soils, the
plotted cumulative percent curves describe a distribution of the sample results. When only
a few data points are available, the distributions of inorganic levels are more difficult to
describe and often only the central tendency may be described with confidence. When
large data sets are available, the extremes of distribution are more easily characterized.
Depending on the size of the background sample data set, an upper percentile (e.g., 95th
or 99th) might be considered an appropriate criterion for the upper range of background
conditions (Cal/EPA, 1997).
To estimate the background levels for selected metals, each of the cumulative percent plots
was examined for certain characteristics as described in Appendix E. For most metals, nondetected
sample data were kept within the data sets although these data were represented
as one-half the reporting limit. There were only five metals for which the nondetects
appeared to significantly affect the cumulative distributions (see text below). The estimated
break point between the background and apparently elevated concentrations for the three
sample groups (all sample, surface, and subsurface) of each element, along with the
percentile values and sample size summary, are presented in Table 3-10. Although the
background evaluation results for all three groups are presented, the estimated background
level for each of the selected inorganics was consistently taken from the all sample group.
The percentile values corresponding to the break point values are reported in Table 3-10 but
the percentile values were not used as a selection criterion for the break point.
In addition to the short descriptions of background levels given below for the selected
inorganics, selected cumulative percent plots are also included to illustrate how the
background levels were determined. Figure 3-4 shows the cumulative percent curve for
copper, with an arrow showing the break point for the distribution of sample concentrations
for this constituent. Break points were chosen in a similar manner for the other selected
inorganic constituents where the cumulative percent distribution shows a distinct point
where a marked slope increase was noted.
SAC/143368(003.DOC) 3-13 ERA REPORT
7/31/02