Dames & Moore, 1999 - USDA Forest Service

covers limited portions of the Site where ground disturbance occurred in conjunction with development, and
includes portions of the mill site, Holden Village, Winston Homesites, and the roads. Hydraulic
conductivity has not been measured in the soil/fill unit. Due to the heterogeneous nature of the material, the
soil/fill material likely exhibits a wide range of hydraulic conductivity. Hydraulic conductivity is small in
areas of finer-grained or more compacted material and could be quite large in less compacted fill or that
containing large rock fragments. Differences in horizontal and vertical hydraulic conductivity may occur
due to layering oi compaction within the fill material. The soil and fill unit may be saturated to a thickness
of up to 5 feet.
Colluvium I
Colluvium consists of soil deposited in conjunction with past mass movement of soil or rock, and is
commonly found on andtor near the base of slopes. The colluvium is suspected to have been encountered
during the installation of several of the groundwater monitoring wells and is anticipated to be limited in
extent. Hydraulic conductivity within the colluvium ranges from 4.6 x 10" cm/sec to 1.2 x 1c2 cm/sec,
based on slug test results for well PZ-4A (Table 4.4-3). Groundwater appears to occur in the colluvium on
both sides of ailr road Creek. At well H-3 (HV-3) in Holden Village north of Railroad Creek, the saturated
thickness of the alluvium ranged from at least 45 feet in May to at least 10 feet in September. The
colluvium on the south side of the creek appears to be completely saturated (approximately 25 feet thick) in
May; by September, approximately 15 feet of the unit is saturated.
Alluvium
Alluvium consists of granular materials deposited by the various creeks in the Railroad Creek watershed,
and is generally limited to those areas near the creeks. The alluvium on the Site appears to range in
thickness from 5 to 25 feet, based on interpretation of geophysical data (Appendix A) and boring logs
(Appendix B). Hydraulic conductivity in'the alluvium varies frorn 7.2 x lc3 to 1.8 x lo-' cdsec, based on
slug tests at wells HBKG-1 and DS-2. The alluvium contains groundwater throughout the Site and the
saturated thickness appears to range from 5 to 25 feet.
Tailings Materials
The tailings materials were generated during the processing of the ore rock, and the 'tailings cover
approximately 90 acres of the Site and range up to approximately 150 feet in thickness. Logs of borings
(Appendix B) completed within each of the three tailings piles give indications that alternating wet and dry
layers occur within the tailings. While fine-grained layers are found throughout each of the tailings piles,
and in some instances can be correlated over parts of two of the piles, no evidence is found to suggest
laterally extensive water perching zones in any of the piles. Hydraulic conductivity of the tailings is
calculated to range from 2.0 x lo4 to 4.4 x 10J cmlsec based on infiltration tests conducted on tailings pile 2
during the RI (Appendix G) and frorn data collected by others (Hart Crowser, 1975, Appendix E). As the
infiltration tests only influence small portions near the surface of the tailings piles, the hydraulic
conductivity values calculated from the infiltration tests may not be representative of conductivity
throughout the pile. Vertical hydraulic conductivity within the tailings piles may be lower than horizontal
hydraulic conductivity due to the presence of layers of finer rnaterials within the piles, and the potential for
limited cementation.
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17693405-019Uuly 19. 1999:4:51 PM;DRAFT FINAL R1 REPORT