Dames & Moore, 1999 - USDA Forest Service

MTCA defines the methods and describes the criteria for determining background concentrations (WAC
173-340-708). A total of 20 simples are required to define area background and were collected from
areas that appeared not to have been influenced by releases from Howe sound Company or Site activities.
Two surface soil samples (DMSS-26 and DMSS-27) were collected in 1997 west of the Wilderness Area
boundary to preliminarily evaluate background concentrations. The sample locations are shown in Figure
5.2- 1 and the data are summarized in Table 5.2- 1. Additional historic information was discovered during
the RI that indicated that road building activities utilizing waste rock materials may have occurred at or
near these sample locations prior to the establishment of the Glacier Peak Wilderness, precluding these
data from background evaluation.
Area background metal concentrations were calculated from sample data collected in the fall of 1998.
Twenty samples were collected within the Railroad Creek watershed but outside of areas suspected to be
affected by mine activities associated with the Howe Sound Company and the Holden Mine. The
analytical data are summarized in Table 5.2-2. The sample locations are shown on Figure 5.2-2.
The results of the 20 area background samples collected. in 1998 were statistically analyzed. Samples
DMSS-26 and DMSS-27 were not included in the statistical calculations due to potential affects in terms
of the reported use of waste rock for road construction at these locations. Statistical calculation methods
to determine background concentrations are outlined in WAC 173-340-708(11) and in Ecology's
statistical guidance (1992). The statistical guidance and Ecology's MTCAStat program were used to
perform the analyses. The MTCAStat program is recommended for background and site cleanup
calculations. The background module of the MTCAStat program was used to determine the distribution
of data. Based on the data distribution, the 90th percentile, mean, standard deviation, median, minimum.
and maximum were calculated. The results of the statistical analysis are summarized with the data in
Table 5.2-2.
The 90th percentile value is a default value for the MTCAStat background evaluation. A 90th percentile
was calculated for all of the metals evaluated. Generally, distribution was lognormal or normal. There
were some data sets that were distribution free (non-parametric). These included arsenic, beryllium,
calcium, chromium, lead, molybdenum, nickel, silver, and thallium. The non-parametric distribution
generally results 'from data sets that include a wide range of detected concentrations or data sets with
concentrations that are similar and whose distribution tend to bunch rather than spread.
The calculated area background values (90th percentile) were compared to the Yakima Basin published
background values (Ecology) where available. Area background values for aluminum, beryllium,
chromium, iron, and nickel were below Yakima Basin values. Arsenic, cadmium, copper, lead,
manganese, and zinc were above Yakima Basin values. Higher concentrations of copper and zinc in
native soil'in the area are consistent with the natural mineralization in the Railroad Creek valley. Area
background values were also compared to MTCA Methods A and B cleanup levels. Background values
were below the cleanup levels with the exception of arsenic and cadmium which were above MTCA
Method B and MTCA Method A cleanup levels, respectively.
Site data were compared to the area background concentrations. For purposes of this discussion, "above
background" refers to the statistically calculated area background. Per MTCA (Chapter 173-303-740), if
background concentrations are used as cleanup levels, no single sample concentration in a data set may be
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17693-005-019~uly '28.1999;11:09 AMSRAFT FINAL RI REPORT