Dames & Moore, 1999 - USDA Forest Service

evaluated on the basis of judgment and comparative evidence, assuming the tield season averages are
representative of the actual conditions.
4.4.4.4 Tributary Inflow
Inflow Measurements and Characteristics
Tributary inflow from Copper Creek (including flow at CC-1 and CC-D) averaged 13 percent of the flow
at RC-4 over the course of the 1997 field season. Flow in Copper Creek (Qt) is estimated to be
represented by a consistent flow, that is 13 percent of the flow at RC-4 (Table 4.3-4, Figure 4.3-6). Based
on this average, the Copper Creek inflow (Reach 2) for the MaylJune and September periods was
estimated to be 60 cfs and 16 cfs, respectively.
Water levels in both Copper Creek and the Copper Creek diversion were observed to be above
groundwater levels in the tailings piles adjacent to these tributary channels. The channel bed in both
tributaries consist of reworked tailings and alluvial materials. Thus, water is free to infiltrate through the
beds of Copper Creek and the'copper Creek diversion into the surrounding soils. The rate of loss was not
measured in the field (and is likely well below measurement accuracy); however, estimated losses have
been developed from the observed head relationships and assumed hydraulic conductivities of the bed
material (see Appendix N). The results of the estimates indicate that combined losses to groundwater
from the tributaries may be on the order of several tenths of a cfs.
Accuracy
The accuracy of the Copper Creek tributary inflow estimates is a hnction of the flow measurement
technique (estimated to range between 7 and 10 percent) and the tield season averaging. The accuracy of
the estimates of exfiltration into the tailings piles is based on the assumed values of hydraulic
conductivity of the streambed, and the limitations of the analysis method used. The hydraulic
conductivity values used represent an average of the range of values measured in the tailings, both during
the RI (see Section 4.3.3.7) and by Hart Crowser in 1975 (see Appendix E).
4.4.4.5 Average Portal How and Seeps
Portal Flow
The hydrograph of flow in the portal drainage ( ~i~ure 4.3-7) indicates an average flow at P-5 of
approximately 1.8 cfs for MayIJune 1997, and 0.15 cfs for September 1997. The hydrograph shows
differences between flow directly downstream of the 1500-level main portal (P-I), and the confluence
with Railroad Creek (P-5). The portal drainage at P-l represents the only measurable flow from the
bedrock aquifer. During MayIJune 1997, the portal drainage .gains flow between P-l and P-5. The gain
in flow in MayIJune averages approximately 0.5 cfs (ranging from 1 cfs in mid-May to 0 cfs in mid-June).
The gain in flow is due to snowmelt and surface runoff entering the portal drainage ditch between P-1 and
P-5. For the water balance, P-l flow measurements were used to represent flow from the bedrock aquifer
(Qb).
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17693-005-019Uuly 19. 1999;4:51 PM;DRAFT FINAL RI REPORT