Dames & Moore, 1999 - USDA Forest Service

period of interest (May I5 to June 15, 1997), thus the runoff component for the water balance analysis
during the MaylJune 1997 time period is estimated to be approximately 6 inches over each contributing
watershed.
Using the 6-inch runoff estimate, runoff was then estimated for the contributing watersheds on both banks
of Railroad Creek for Reach 1 and Reach 2. Runoff (Qsr) for Reach 1 during the MaylJune 1997 time
period is estimated to be 0.8 cfs from the south bank, and 1.3 cfs from the north bank. Runoff for Reach 2
during MaylJune 1997 from the north bank is estimated to be 3.4 cfs. Runoff generated upslope of the
tailings piles was assumed to be routed around the tailings by the interceptor drainage ditches and,
therefore, is assumed not to contribute significant flow to the south bank in Reach 2 (i.e., it is already
accounted for in Copper Creek flow). Runoff estimates for the ~e~tember time period are assumed to be
zero for both Reach 1 and 2 because there was no observed runoff fiom rainfall until the end of the
month.
Accuracy
The accuracy of the runoff estimates is based on limited observations of runoff gain in the portal drainage.
This calculation is based on the absence of a continuous record of flow data for the portal drainage in
1997 (Figure 4.3-7). If a continuous record of flow data had been available for the portal drainage in
1997, as was the case in Station P-1 in 1998 (see Figure 4.3-7a), it is likely that the record would have
documented short-term events of higher flow. It should be noted that it was not possible to utilize the
1998 P-1 data due to the limited flow data collected at portal drainage station P-5 during the 1998 RI field
effort.
4.4.4.8 Groundwater Contribution from the Alluvial Aquifer
Reacb 1
Estimates of groundwater' flow into Railroad Creek (Qa in the water balance equation) from the alluvial
aquifer were developed based on estimated recharge rates from the portal drainage, inflow to the lagoon
and direct precipitation (minus evapotranspiration = 52.6 inches) over a 20 acre portion of the valley floor
which overlies the aquifer (in the vicinity of the lagoon). It was assumed that over, the course of the
summer, the change in alluvial aquifer storage was negligible, and that average recharge rates would
equal the discharge.
The components of Qa for Reach 1 include: direct precipitation into the valley bottom area, recharge
from the portal drainage, recharge from the lagoon (SP- I SE and SP-15W), north bank contribution, and
estimated loss from Railroad Creek to groundwater.
Recharge from the lagoon was assumed to equal the inflow rate of 0.19 cfs (seep SP-15). The estimated
runoff from infiltration due to precipitation was 0.24 cfs (52.6 inches times 20 acres over 183 days). The
recharge fiom the portal drainage was estimated to average 0.2 cfs (average of 0.35 plus 0.06). he total
discharge rate was, therefore, estimated to be 0.6 cfs. Based on the assumption that this discharge follows
seasonal trends, the peak outflow (Qag) in MaylJune was estimated to be 0.9 cfs, and the discharge (Qag)
in September 1997 was estimated to be 0.3 cfs.
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17693-005-019UuIy 19, 1999;4:5 1 PM;DRAFT FINAL RI REPORT