Dames & Moore, 1999 - USDA Forest Service

pathway for water to infiltrate into the pile (Figure 6.1-3a). Surface water h m the Copper Creek diversion
also flows across the westemmost portion of tailings pile 1.
The water that infiltrates tailings migrates downward and accumulates (based on groundwater monitoring)
in the lower portions of the pilesas a thin water-bearing zone (Figures 6.5-15 and 6.5-16). The water within
this zone appears to be perched on a lower permeability layer at the base of the tailings. Evidence of this
layer was observed in test pits completed during the RI. Groundwater elevations in monitoring wells
screened in the tailings indicate that groundwater generally moves to the north toward Railroad Creek
(Figures 6.5-7 and 6.5-8). This water discharges fiom the base of the tailings piles as a series of intermittent
seeps which flow into Railroad Creek.
The differences in water levels in clustered monitoring wells installed in tailings piles 2 and 3 indicate that
the groundwater within the tailings is not in direct hydraulic connection with the groundwater in the
underlying alluvium/reworked till (Figure 6.5-15). The water levels in the monitoring wells screened in the
tailings remain relatively constant throughout the year, only fluctuating approximately 3 feet during the
1997 field season. Water levels in the monitoring wells screened in the alluvium/reworked till underlying
the tailings were observed in several cases to fluctuate more than 20 feet.
In the spring, when the recharge is greatest to the alluviumlreworked till, water levels in this unit increase
relatively rapidly and the groundwater appears to be confined by the overlying low permeability tailings.
This results in an upward vertical hydraulic gradient in the southern portions of the tailings piles which
causes water fiom the alluvium/reworked till to move into the overlying tailings (Figure 6.5-15). In the
northern portions of the piles, the water levels are higher in the tailings than in the alluvium/reworked till
and water moves downward from the tailings into the underlying deposits. As water levels decline in the
summer, the vertical hydraulic gradient reverses beneath the southern portions of the tailings piles and water
within the tailings moves downward into the alluviurn/reworked till (Figure 6.5- 16).
Groundwater within the alluvium/reworked till generally moves in a northerly direction beneath the tailings
piles (Figures 6.5-7 and 6.5-8). In May, the groundwater discharges into a series of seeps that flow into
Railroad Creek. The groundwater also discharges as diffise baseflow into Railroad Creek, as indicated by
the apparent gaining conditions measured in this reach of the stream and the relationship between the water
levels in the alluviumlreworked till which are higher than those observed in the creek. Some of the
discharge to Railroad Creek is believed to occur through the abandoned Railroad Creek channel which is
approximately parallel to the northern edges of the tailings piles. Flow within the abandoned channel is
assumed to move in an easterly direction rather than to the north.
In September, the occurrence of seeps decreases significantly with only a few seeps observed (SP-1, SP-2
and SP-3) and seep flow rates decreasing significantly. In September, the groundwater flow within the
abandoned Railroad Creek channel is evident based on the water levels in the wells near the former channel.
Diffuse groundwater continues to discharge along the base of tailings piles 1 and 2, but portions of the reach
of Railroad Creek near tailings pile 3 appear to be in a losing condition.
Groundwater Chemistry
The geochemistry of tailings groundwater, based on &mples collected and analyzed fmm groundwater
monitoring wells and seeps, is primarily influenced by oxidation of the surface tailings by oxygen entering
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17693-005-019Uuly 27. 1999;4:11 PM;DRAFT FINAL RI REPORT