Dames & Moore, 1999 - USDA Forest Service

Site has been observed by Dames & Moore personnel to be composed of dense to very dense material
ranging from clay- to boulder-size, with all intervening size fractions present.
Bedrock
Bedrock at the Site is composed primarily of quartz diorite, granodiorite, schist, and gneiss, and completely
, underlies the Site both north and south of Railroad Creek (Cater and Crowder, 1967). Based on
interpretation of geophysical data (Appendix A) the shallowest identified depth to bedrock is approximately
12 feet, both adjacent to Railroad Creek along seismic line F-F', and south of the maintenance yard along
seismic line A-A'. Greatest depth to bedrock is about 140 feet, beneath the central and northern portions of
tailings pile 3, again based on interpretation of geophysical data.
Descriptions of the geology for each Site area, as shown on Figures 4.1-3 and 4.1-3% are presented
hereafter.
Mine Sup~ort and Waste Rock Piles Area
Referring to geologic cross-section and seismic-refraction line A-A' on Figure 4.2-7, the geology for the
portion of the Site from the western waste rock pile to north of Railroad Creek in the area of the lagoon
feature consists of alluvium/reworked till over bedrock. The thickness of the near-surface soil appears to be
on the order of 10 to 15 feet thick. The alluvium/reworked till unit is underlain by approximately 15 to
more than 70 feet of moderately dense to dense material, which is most likely glacial till. The till material is
underlain by apparent bedrock materials which appear to dip to the north.
The southern segment of the geologic cross-section and seismic refraction line B-B' (Figure 4.2-8) crosses
the eastern waste rock pile. Based on review of historical information, the waste rock is comprised of .
bedrock, removed during the completion of the 1500-level mine and ventilator portals, that was determined
not to contain economic minerals in high enough concentration to warrant processing in the mill.
Combined, these two tunnels comprise over a mile of underground mine workings through non-mineralized
bedrock. The waste rock pile was placed on dense glacial till. The maximum height of the waste rock pile
is on the order of 140 feet, based on topographic maps. However, the actual thickness of the waste rock
when measured perpendicular to the slope appears to be less than 50 feet. The bedrock underlying the waste
rock pile also generally follows the topography.
A reconnaissance of the Railroad Creek streambed between RC-1 and RC-6 disclosed the presence of an
exposure of glacial till in the south bankof the creek. The soil was observed to consist of interbedded
sequences of blue-gray clayey silt (glacial lake deposits), silty sand, and silty sandy gravel. The minimum
thickness of the unit is on the order of 10 feet. Water was noted emanating from above the contact with the
clayey silt layer (discussed in subsequent sections of this report as seep SP-26).
Tailings Pile 1
Referring to Figures 4.2-6b and 4.2-8, the majority of the tailings pile, other than steep slopes facing
Railroad Creek and an isolated area near the southwest margin, is covered with approximately 4 to 6 inches
of rounded gravel placed by the USFS during site rehabilitation efforts between 1989 and 1991. The
northern portion of the B-B cross-section across tailings pile 1 indicates that the tailings beneath the surface
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17693-005-019Uuly 19, 1999;4:5 1 PM;DRAFT FINAL RI REPORT