1.1 MB pdf - Bolsa Chica Lowlands Restoration Project

SECTION 3: ANALYSIS
easily identify the curve break point. In the latter case, the < 0.22 mg/Kg dw value represents
the 86th percentile level. Both cumulative percent plots are provided for comparison (see
Figures 3-6a and 3-6b). A slightly lower background level (< 0.19 mg/Kg dw) was estimated
from the surface sediment data set (97th percentile value). Because there were only 2 samples
out of 264 samples that were actually detects in the subsurface sample group, the background
level was not assessed for this limited data set. The all sample data set had a large number of
nondetect values with 539 samples out of the total 581 samples that were flagged as nondetects.
The cumulative percent plot for silver (Appendix E, Figures E-34a) shows a stepped curve. The
cumulative percent plots for the all sediment group with only the detected values (Figure E-34b)
shows a relatively smooth curve to the break point, where the curve becomes discontinuous and
rises more steeply. The break point on the cumulative percent plot for the surface sediments
(Figure E-35) was found to occur at the end of the first continuous string of actual detected
values, where the curve starts to rise more steeply. Due to the large number of nondetect values
in the subsurface sediments, a break point was not assessed (Figure E-36).
Thallium
The background level for thallium in Bolsa Chica sediments is estimated to be
< 0.61 mg/Kg dw. This value represents the 81st percentile value for the all sample group and
the 99th percentile when the nondetect values are removed from the data set. Slightly lower
values were estimated from the surface (< 0.52 mg/Kg dw) and subsurface (< 0.44 mg/Kg dw)
sediment data sets. This data set had an elevated number of nondetect values with 159 samples
out of the total 581 samples that were flagged as nondetects. All three cumulative percent
plots for thallium rise in a relatively smooth line to the break points, where the curves begin
to rise more sharply (Appendix E, Figures E-37 through E-39).
Vanadium
The background levels for vanadium in Bolsa Chica sediments are estimated to be
< 75 mg/Kg dw. This value represents the 91st percentile value for the all sample data set.
A slightly lower value (< 72 mg/Kg dw) was estimated from the surface sediment sample
group that represents the 84th percentile value. A lower level (< 60 mg/Kg dw) was
estimated from the subsurface sample group that represents the 93rd percentile value. All
three cumulative percent plots for vanadium rise in a relatively smooth line to the break
points, where the curves begin to rise more steeply (Appendix E, Figures E-40 through
E-42).
Zinc
The background level for zinc in Bolsa Chica Lowland sediments is estimated to be
< 103 mg/Kg dw. This value represents the 91st percentile value for the all sample data set.
Lower levels were estimated from the surface (< 92 mg/Kg dw) and the subsurface sample
group (< 89.7 mg/Kg dw) that represent the 81st and 97th percentile values, respectively.
All three cumulative percent plots for zinc rise in a relatively smooth line to the break
points, where the curves become less continuous and begin to rise more steeply
(Appendix E, Figures E-43 through E-45).
SAC/143368(003.DOC) 3-17 ERA REPORT
7/31/02