Dames & Moore, 1999 - USDA Forest Service

Mill. Mine Sup~ort and Waste Rock Piles
Sources of groundwater recharge within the glaciaYalluvial deposits beneath the western portion of the Site
are melt water from the watershed and.south of the tailings piles. This component of groundwater recharge
appears to decrease afier the spring snowmelt period. Infiltration of precipitation and melt water through the
surfaces of the waste rock piles adjacent to the mill building likely contributes relatively minor amounts of
seasonal water to the near-surface groundwater occurrence beneath the western portion of the Site. Other
contributing components to the groundwater occurrence within the glaciaYalluvial soil beneath the western
portion of the Site include groundwater infiltration from overland flow between the 1500-level main portal
and the confluence at RC-4, as well as from the Copper Creek diversion (which is piped to the hydroelectric
plant and flows out above ground in a ditch to Railroad Creek) and Copper Creek.
Eastern Portion of Site
Tailings Piles
Groundwater monitoring wells installed by others were completed both within the tailings piles and in
native soil underlying the tailings materials. Groundwater levels within the tailings materials were measured
in the wells from May-June through September, 1997. The water levels in the wells completed in the
tailings materials generally consistent throughout the period, fluctuating a maximum of approximately 2
feet. A total of seven permeability (percolation) tests were completed on the surface of tailings piles 2 and 3
by Dames & Moore and others. These data indicate that the permeability of the tailings is relatively low,
with permeability values ranging from 1 x to 1 x lo-' centimeters per second (cmlsec).
In contrast, the groundwater levels in the wells installed in the native soil underlying the tailings piles tend
to fluctuate markedly throughout the May to September sampling period. The water levels in the wells
installed near the southern margin of tailings piles 2 and 3 were observed to fluctuate in excess of 30 feet.
This compares to a fluctuation of approximately 3 feet in wells installed beneath the tailings materials
nearest Railroad Creek, near the northern margin of the tailings piles.
These data strongly suggest that the groundwater within the native reworked deposits during the spring
snowmelt period occurs as two primary components: one flowing generally parallel to Railroad Creek near
the valley floor within the glacial reworked deposit; and a second component flowing downslope towards
Railroad Creek. Using vectors to illustrate the two components, the spring snowrnelt period is believed to
consist of two relatively equal vectors which are generally perpendicular to each other until the groundwater
flow pathways intersect near the valley floor to then flow eastward parallel to Railroad Creek.
Based on the results.of both two-dimensional groundwater modeling and surface water flow measurement,
the groundwater flow into Railroad Creek during MaytJune was calculated to be on the order of 5 cfs along
tailings piles 1 and 2, and 2.1 cfs along tailings pile 3. In contrast, as the snowmelt and precipitation
diminish through the summer months, the vector parallel to Railroad Creek becomes more predominant.
The groundwater flow into Railroad Creek during September was calculated to be on the order of 1.0 cfs
along tailings piles 1 and 2, and 0.4 cfs along tailings pile 3.
These changes in predominant groundwater components throughout the spring and summer months are also
reflected in groundwater levels within the near-surface soils. The spring melt water and precipitation flows
from the slopes south and above the tailings piles through the native soil beneath the tailings piles. Since the
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17693-00S419Uuly 28. 1999;10:24 AM;DRAFT FINAL RI REPORT