Dames & Moore, 1999 - USDA Forest Service

snowmelt on the adjacent valley slopes, so that primary groundwater flow directions are perpendicular to
the trend of Railroad Creek. Groundwater discharge to Railroad Creek decreases over the summer.
Through the summer months as groundwater levels decrease, groundwater beneath the Site begins to flow
downstream to the east rather than directly toward Railroad Creek. The flow loss occurs because water
levels in Railroad Creek m above water levels in the alluvial aquifer near the eastern portion of the site.
The area immediately east of tailings pile 3 replenishes groundwater storage and is assumed to discharge
back to Railroad Creek along the reach in and near SP-2 I, immediately east of RC-2. This assumption is
based on the observed exposure of bedrock on the south bank of Railroad Creek, immediately
downstream of SP-21. The presence of bedrock, and absence of alluvial material, indicates that the
groundwater likely becomes surface water (Railroad Creek) at this location. Seep discharge is largest in
the spring and essentially stops by late summer, indicating snowmelt as the primary source of seep flow.
6.13.2 West Side of Site
The west side of the Site includes the Honeymoon Heights drainage area, mine area, underground mine
workings, the east and west waste rock piles, mill building area, and the maintenance yard (Figures 6.1-1
and 6.1-la). Some of the upslope run-on probably enters near-surface discontinuities in bedrock south
and upslope of the Site and flows downward through bedrock fractures, through abandoned stopes and
mineralized (unmined) portions of the underground mine and contacts residual mineralization on rock
faces of the stopes and tunnels as shown on Figures 6.1-2 and 6.1-2a. The water emerges as either surface
water overland flow (i.e., 1500-level main portal drainage) and can reinfiltrate as seeps that emerge as
overland flow andlor as diffuse groundwater discharge to Railroad Creek.
Some of the overland flow from upslope run-on also moves across the Honeymoon Heights waste rock
piles (800 and 1100 level) or the mill area, and the maintenance yard and then travels downslope to other
drainage features such as the lagoon and other miscellaneous drainage channels (Figures 6.1 - la and 6.1-
2b). Not all groundwater comes into contact with the underground mine workings. Some portion of
groundwater flow from the west side of the site is assumed to be diverted into the abandoned Railroad
Creek channel and also flows beneath the tailings piles.
6.133 East Side of Site
The east side of the Site includes tailings piles 1, 2, and 3 (Figure 6.1-la). Upslope surface water
overland flow from direct precipitation and snow melt is transported to Copper Creek and also infiltrates
across and through tailings piles 1,2, and 3. Surface water is further transported to other drainage feitures
including ditches that divert water to Copper Creek, an abandoned decant tower near the southern margin
of tailings pile 1, the Copper Creek diversion, and the sauna dipping pool. Groundwater recharge from
upslope run-on and infiltration occurs through the fractures within the bedrock found along the valley
sidewalls and in the alluvium/reworked till, where present (Figures 6.1-3 and 6.1-3a). Infiltration occurs
through the tailings piles from a combination of sources including upslope run-on as well as snow-melt
and direct precipitation on the tailings piles. Infiltration through these features contributes recharge to
groundwater in the alluvium/reworked till beneath the tailings piles which eventually discharges as seeps
and groundwater baseflow to Railroad Creek. The discharge rate decreases after the spring snow melt
period. Some portion of groundwater flow from the west portion of the site is assumed to be diverted into
the abandoned Railroad Creek channel and also flows beneath the tailings piles.
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17693-00U)19Vuly 27.1999;4:11 -RAFT FINAL RI REPORT