Dames & Moore, 1999 - USDA Forest Service

5.3.1.3 Summary
Based on the statistical analysis performed, sufficient area background surface water data points exist for
determining background surface water quality in Railroad Creek. Limited variation exists between the
90th percentile metal concentrations in the total data set as compared to the spring and fall seasonal data
sets for dissolved aluminum, arsenic, cadmium, copper, iron, lead, magnesium, manganese, and zinc.
Additionally, limited variation exists between area background Sets 1A (Railroad Creek stations) and 1B
(other background stations). This provides justification for using the upstream Railroad Creek station(s),
RC- 1, RC-6, and/or RC- 1 1, to monitor area background surface water quality in Railroad Creek.
The 90th percentile values calculated for the total data set are provided for total and dissolved metals
(aluminum, arsenic, barium, beryllium, cadmium, calcium, chromium, copper, iron, lead, magnesium,
manganese, mercury, molybdenum, nickel, potassium, selenium, silver, sodium, thallium, uranium, and
zinc) in Table 5.3-1. For the purposes of the following discussion, the area background is defined as the
90th percentile values derived for the total data set. Surface water data are compared to WQC where
available and area background and MTCA Method B cleanup levels for those metals without WQC.
5.3.2 Stehekin River Watershed
The Stehekin River Watershed was considered a part of the reference area evaluation for comparative fish
population studies to Railroad Creek due to similarities in climate, topography, geology, hydrology, and
natural mineralization; however, the prospecting and mining activities in the Stehekin River watershed were,
not as extensive as in the Railroad Creek watershed. Water quality and benthic macroinvertebrate RI
sample collection was performed concurrent with the ecological surveys conducted during the RI in Bridge
Creek, South Fork Agnes Creek, and Company Creek. The data collected were compared to data collected
in Railroad Creek to assess similarities and differences between the Railroad Creek and Stehekin River
drainages. The locations are shown on Figure 5.3-3. Available water quality data are discussed below and
include historical data collected from the Stehekin River.
5.3.2.1 Stehekin River
Harper Owes (Patmont, 1989) collected samples from the Stehekin River on a monthly basis from
December 1986 to November 1987 and analyzed the samples for total recoverable arsenic, iron, and zinc.
The mean concentrations for the metals were 0.41 _+ 0.18 p@, 98.7 _+ 89.8 p a, and 1.0 _+ 0.4 pgL,
respectively. The data are assumed to be of acceptable quality, however, only the mean concentration for
each metal was provided in the 1989 report. The mean concentration is the result of averaging all of the
data collected and does not account for the significant seasonal flow differences. Thus, the data collected
from the Stehekin River watershed during the RI were not compared to the Harper Owes data.
Data were not collected from the Stehekin River during the RI, but were collected from Bridge Creek, South
Fork Agnes Creek, and Company creek in the Stehekin Riverwatershed.
5.3.2.2 Bridge, South Fork Agnes, and Company Creeks
Samples were collected during the RI at one station each on Bridge Creek, South Fork Agnes Creek, and
Company Creek in September/October 1997. A field duplicate was collected at Bridge Creek. Laboratory
results and field data are summarized in Table 5.3-18. Sample locations are shown in Figure 5.3-3.
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17693-005619Vuly 28. 1999;11:09 AM;DRAFT FINAL RI REPORT