Dames & Moore, 1999 - USDA Forest Service

Bedrock
Bedrock underlies the entire watershed and is exposed at the ground surface along the valley sidewalls
where glacial materials and/or weathered bedrock is not present, and in isolated circumstances within and
near Railroad Creek. Hydraulic conductivity has not been measured for bedrock at the Site. Published
values range from 4.7 x 10" to 9.3 x 1 cmlsec for fractured metamorphic and igneous rocks and 1.3 x 10'
to 2.3 x 10'12 cmlsec for unfractured metamorphic and igneous rocks (Daly, 1982). Water transmitted by
bedrock at the Site is presumably moved through fractures.
4.4.3.2 Groundwater Monitoring Well Levels
Most groundwater monitoring wells at the Site include a series designation as either "A," "B," or "C" series
wells. "A" series wells monitor groundwater within the colluvium, alluvium, or alluviumlreworked till units
(i.e., native materials). "B" and "C" series wells monitor groundwater within the tailings. "B" series wells
are completed near the base of the tailings and "C" series wells are screened just above potential perching
layers. Groundwater monitoring wells without a designation monitor the same units as "A" series wells.
Well screen intervals and water level data are summarized in Table 4.4-2. The locations of the monitoring
wells are noted on Figure 4.4-1. Table 4.0- 1 presents the coordinates for all of the wells. During 1997 RI
field activities, water-level measurements of site wells were conducted on four occasions. Dates of the
measurements were May 17, June 17-20, July 13, and September 17-19. All groundwater level data
collected manually are presented in Table 4.4-2. Water-level elevation maps for each round based on
measurements in wells screened in native materials are provided as Figures 4.4-6 through 4.4-9. Surface
water elevations have been plotted in some instances to provide additional control. Groundwater elevations
in May and September 1997, for wells screened in tailings on tailings piles 2 and 3, are. shown in
- Figures 4.4- 10 and 4.4- 1 1, respectively.
Geologic units do not overlie the tailings and water within the tailings pile is in indirect contact with the
atmosphere through the pores between the grains of tailings. Water within the tailings is termed unconfined
.and "B" series wells monitor a free water surface. Although not shown on Figures 4.4-2 and 4.4-3, "C"
series wells were also found to monitor a free water surface. Water within the alluviaVreworked till unit
becomes confined between the dense till below and the tailings above, near the southern margin of the
tailings piles. The tailings act as a confining unit either because of their lower overall hydraulic
conductivity andfor because of the apparent presence of a lower permeability unit at the base of the tailings.
"A" series wells monitor either a potentiometric surface within the alluviaVreworked till unit where this unit
is confined, or monitor a Free water surface where unconfined conditions occur. In areas where groundwater
occurs under confined conditions, water levels measured in wells rise higher than the top of the contact
between the tailings and the underlying hydrostratigraphic unit since the water is under pressure transferred
from waters originating at a higher elevation. The potentiometric surface shown on Figures 4.4-2 and 4.4-3
does not imply that the tailings are saturated to the pressure head level shown for the "A" series wells, but
only that water within the "A" series wells rises to the levels shown.
Water leveldpressure heads measured during May 1997 in monitoring wells completed in both the tailings
and the alluvium/reworked till unit are shown on Figure 4.4-2 (tailings pile 2), and Figure 4.4-3 (tailings pile
3). Water leveldpressure heads in wells completed in both units are shown for September 1997 on
Figure 4.4-4 (tailings pile 2) and Figure 4.4-5 (tailings pile 3). Note that although monitoring well PZ-6A is
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17693-005-019Uuly 19. 1999;4:5l PM;DRAFT FFNAL RI REPORT
DAMES & MOORE