Dames & Moore, 1999 - USDA Forest Service

Dye tracer testing in an intermittent surface water drainage near historic mine features to
assess potential surface water and groundwater pathways
A baseline risk assessment to evaluate risk to human and ecological receptors
Data compilation and report preparation
The nomenclature for the surface water sampling locations in Railroad Creek was developed utilizing a
system initially developed by the USFS. The sampling station upstream of the Site was established as RC-I
(Railroad Creek station 1). The sampling station immediately downstream of the Site was designated RC-2,
and the station at the mouth of Railroad Creek was noted as RC-3. However, subsequent stations were
added during the RI as the need arose for additional data, resulting in a total of eleven Railroad Creek
stations (RC-1 through RC-1 1) which are, therefore, not in sequential order fiom upstream to downstream.
KEY FINDINGS
A summary of the key findings include the following:
Host Rock Mineralogy
The Holden Mine ore deposit is hosted by the Buckskin Schist, which is a quartz amphibole
schist sequence with at least two horizons of intermittent marble beds and calcareous
schists. The dominant silicates are plagioclase and biotite (aluminum-based). The primary
sulfide minerals in the Holden Mine ore deposit include pyrite, pyrrhotite, sphalerite and
chalcopyrite.
Site Surface Water/Groundwater Interaction and Movement
The data collected during the RI bracketed both high and low flow conditions in Railroad Creek and
component inflows:
'
All surface water and groundwater at the Site ultimately discharge to Railroad Creek.
Spring Conditions - The primary component of surface water and groundwater at the Site
and in the vicinity during the spring period (approximately April through July) is snowmelt.
The source areas for surface water and groundwater originate upslope of the Site and in the
upstream portion of Railroad Creek. Water sources flow into Railroad Creek as overland
flow, base groundwater flow through the near-surface glacial sands and gravels, overland
flows that infiltrates to groundwater fiom source areas and as groundwater surface or
subsurface expressions that represent springs, seeps, or subsurface flow into the bottom of
the.streambed. Water enters the mbe through fractures and joints. Water discharging from
the 1500-level main mine portal represents the bedrock groundwater component observed
at the Site. As overland flow discharges from the 1500-level main portal to the confluence
of Railroad Creek, water also infiltrates to groundwater which eventually flows to Railroad
Creek. The tailings pile materials have relatively low peimeability; however, some water
infiltrates through the surface of the:tailings piles during snowmelt, precipitation events,
and ponding on the surface of the piles.
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SUMMARY.doc
17693-005-019Uuly 28.1999;12:16 PM;DRAFT FMAL'RI REPORT ES-4