Dames & Moore, 1999 - USDA Forest Service

Set 1B: RI stations located in Holden Creek, Big Creek, Copper Creek, Ten Mile Creek
and Stehekin were pooled.
The number of data points for Sets 1A and 1B are shown below. The specific data points used for the
analyses are shown in Tables 5.3-2 through 5.3-14.
Aluminum
Arsenic
Beryllium
Cadmium
Chromium
Copper
Iron
Lead
Magnesium
Manganese
Selenium
Silver
Zinc
Metal
Set lA
Railroad Creek
# Data Points
30
11
21
28
22
2 1
27
22
27
27
10
28
3 1
Set lB
Other Background
# Data Points
13
8
13
13
12
12
13
13
13
9
6
13
13
The Ecology guidance notes that MTCA has no provisions for excluding statistical outliers that have not
resulted from apparent errors. For this reason, all results for the selected sample points that were reported
above the detection limit were included in the statistical calculations. However, due to the wide.range of
detection limits reported historically and during the RI, some data points (reported as not detected) were
omitted if the detection limits reported were due to apparent methodology differences that resulted in high
detection limits that were well above the other reported data. Historically and during the RI, method
changes were made in an effort to reduce detection limits so that the data were more meaningful when
compared to water quality criteria (WQC). Tables 5.3-1 through 5.3-14 show the data points actually
used for each analysis as well as those considered for analysis but omitted.
During RI data collection in 1997, lead results in upstream stations were generally at or just above the
detection limit of the method used for analysis. Due to continuous method blank contamination in the
laboratory, detected surface water sample lead results were artificially influenced. This, in combination
with the need to clearly delineate lead concentrations above chronic water quality criteria, resulted in a
revision to the lead method in 1998 to reduce the detection limit by an order of magnitude. Detection
limits ranged from 0.01 1 to 1 pg/L in sample data collected during the RI. Due to the wide range of
concentrations representing the detection limit, the 90th percentile calculated for lead in all three data sets
did not include detection limits greater than or equal to 1.0 pg/L. Inclusion of these detection limits
skewed the 90th percentile lead concentration. Table 5.3-9 provides a list'of sample data points that were
considered but were omitted from the statistical calculations.
An additional item considered during the. statistical analysis for zinc was the impact of field filtration
during April, May, June, and July 1997. During this timeframe, it was determined that field filtration had
potentially artificially enhanced the zinc results. For statistical purposes, the total results were used where
the data for dissolved zinc clearly indicated artificially introduced zinc. These instances include only the
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