1.1 MB pdf - Bolsa Chica Lowlands Restoration Project

SECTION 3: ANALYSIS
the subsurface sediment samples and represents the 97th percentile value. All three
cumulative percent plots for lead rise in a relatively smooth line to the break points, where
the curves become less continuous and begin to rise more sharply (Appendix E, Figures E-22
through E-24).
Mercury
Background levels for mercury in Bolsa Chica sediments were estimated at < 0.28 mg/Kg dw.
This value represents the 98th percentile for the all sample data set. When only detected
values were considered in the all sample data set, the value represents the 93rd percentile. A
value of < 0.23 mg/Kg dw (96th percentile) was estimated from surface sediment data set and
a value of < 0.15 mg/Kg dw (95th percentile) was estimated from the subsurface sediment
data set. This data set also had a large number of nondetect values with 391 samples out of the
total 581 samples that were flagged as nondetects. The resulting cumulative percent plots
(Appendix E, Figures E-25a, E-26, and E-27) show a two-stepped curve. When only the detect
values were considered, the resulting cumulative percent plot shows a smooth curve to the
break point, where the curve starts to rise more steeply (Appendix E, Figure E-25b).
Nickel
Background levels for nickel in the Bolsa Chica Lowland sediments were estimated to be
< 30 mg/Kg dw. This value represents the 95th and 93rd percentile levels for the all sample
and surface data sets, respectively. A lower value of < 19.1 mg/Kg dw was estimated from
the subsurface sediment samples and represents the 95th percentile value. All three
cumulative percent plots for nickel rise in a relatively smooth line to the break points, where
the curves become less continuous and begin to rise more sharply (Appendix E, Figures E-28
through E-30).
Selenium
The background levels for selenium in Bolsa Chica sediments were estimated at
< 0.54 mg/Kg dw. This value represents the 78th and the 96th percentile values for the all
sample and surface data sets, respectively. When the nondetect values are removed, this
value represents the 94th percentile value for the all sample group. The different cumulative
percent plots for the two data sets are presented in Figures 3-5a and 3-5b. The break point
on the cumulative percent plot without nondetects (Figure 3-5b) provides a clearer view of
the change in slope for selenium concentrations. The estimated values for the subsurface
(< 0.49 mg/Kg dw) sediment samples were similar to that estimated from the all sample
and surface sample groups. This data set also had a large number of nondetect values with
368 samples out of the total 580 samples that were flagged as nondetects. The resulting
cumulative percent plots for selenium (Appendix E, Figures E-31a and E-33) show a stepped
curve. The cumulative percent plots for the surface sediments (Figure E-32) and the all
sediment plot with only the detected values (Figure E-31b) show a smooth curve to the
break points, where the curves become discontinuous and rise more steeply.
Silver
The background level for silver in Bolsa Chica sediments is estimated to be < 0.22 mg/Kg dw.
This value represents the 80th percentile value for the all sample data set. As was done for
selenium, the all sample data set was also plotted without nondetect values in order to more
ERA REPORT 3-16 SAC/143368(003.DOC)
7/31/02