1.1 MB pdf - Bolsa Chica Lowlands Restoration Project

SECTION 3: ANALYSIS
For five of the selected elements (cadmium, mercury, selenium, silver, and thallium), the
data sets contained a relatively large number of samples that were classified as nondetects
(i.e., flagged with “U” in the analytical database). Even though these nondetect reported
values were divided by 2, the reporting limits were elevated enough so that the sample
entries were still observed as elevated concentrations in the cumulative percent plots (for
examples refer to Figure 3-5a and Figure 3-6a).
In these cases, the cumulative percent plots did not clearly show a single break point
between background and elevated concentrations because the plots show two separate
locations where sample concentrations rise steeply. To clarify the break point between
background and elevated levels for these constituents, the cumulative percent plots were
regenerated with the nondetect entries removed from the data sets. Examples of the
regenerated cumulative percent plots are provided in Figure 3-5b for selenium and
Figure 3-6b for silver, which show a single break point once the nondetect entries are
removed.
Arsenic
Background levels of arsenic in the Bolsa Chica Lowland sediments are estimated to be
< 11 mg/Kg dw. This value was relatively consistent among surface, subsurface samples,
and the entire sample group. For surface samples, this background level represents the 88th
percentile (i.e., 12 percent of the samples tested had concentrations above this value). This
concentration represented the 95th and 91st percentile for subsurface samples and all
samples, respectively. All three cumulative percent plots for arsenic (surface, subsurface,
and all sample groups) rise in a smooth line to the break points, where the curves become
less smooth as they rise more sharply (Appendix E, Figures E-1 through E-3).
Barium
Background levels of barium in the Bolsa Chica Lowland sediments are estimated to be
< 110 mg/Kg dw based on the evaluation of the all sample data sets. This value represents
the 89th percentile for the all sample group. When the surface samples were considered
alone, a background level of < 92 mg/Kg dw was estimated (81st percentile level). When
the subsurface samples were considered, a background level of < 75.9 mg/Kg dw was
estimated (86th percentile level). All three cumulative percent plots for barium rise in a
smooth line to the break points, where the curves become less smooth and rise more sharply
(Appendix E, Figures E-4 through E-6).
Beryllium
Background levels of beryllium in the Bolsa Chica Lowland sediments are estimated to be
< 0.94 mg/Kg dw based on the evaluation of the all sample data set. This value represents
the 91st percentile for the all sample groups. When the surface samples were considered
alone, a background level of < 0.88 mg/Kg dw was estimated (83rd percentile level).
When the subsurface samples were considered alone, a background level of < 0.8 mg/Kg dw
was estimated (95th percentile level). The differences among the surface, subsurface, and all
sample values are probably not significant. All three cumulative percent plots for beryllium
rise in a smooth to slightly stepped line to the break points, where the curves rise more
sharply (Appendix E, Figures E-7 through E-9).
ERA REPORT 3-14 SAC/143368(003.DOC)
7/31/02