Dames & Moore, 1999 - USDA Forest Service

The inflow of groundwater in the form of seepage from the alluvial aquifer was observed in the south
bank of Railroad Creek in the area of RC-4 (Figure 4.4-21). This seepage is suspected to reflect recharge
from the lagoon, portal drainage, and direct precipitation.
There were no observations that allow the development of estimates of groundwater inflow from the north
bank portion of the alluvial aquifer. However, the segment of Railroad Creek adjacent to the Site is
generally situated on the north side of the valley floor. Therefore, the alluvial deposits underlying the
north bank are suspected to be less extensive, resulting in the assumption that groundwater contribution
from this bank is smaller than for the south bank. The septic drainfield for Holden Village is located on
the north bank (Figure 4.4-19). It is estimated to contribute less than 0.05 cfs, assuming an average
Holden Village water use of 50 gallday per person, and a population of 300 persons. The resulting
discharge from the alluvial aquifer on the north bank is, therefore, estimated to be on the order of two-
thirds or less of the south bank; less than 0.6 cfs for MaylJune 1997 and 0.2 cfs for September 1997.
Qal, an assumed loss from Railroad Creek back into the aquifer, is estimated to range from 0 to 1 cfs.
The relatively low, estimated alluvial aquifer discharge rates appear to be supported by the results of flow
measurements collected within the reach of Railroad Creek between the portal drainage confluence (P-5)
and the vehicle bridge' during the 1997 and 1998 RI field efforts. An apparent small gain in baseflow
within this reach was documented from the average flow measurements made during the 1997 RI field
effort. However, the more detailed survey conducted in October 1998 suggested no significant changes in
flow rates between RC-l and RC-4 (see Section 4.3.7.3).
In summary, the Qa value for Reach 1 in MaylJune is 1.5 to 0.5 cfs and 0.5 to -0.5 cfs in September.
Reach 2
Estimates of discharge from the alluvial aquifer into Reach 2 of Railroad Creek were developed utilizing
flow net analyses completed for groundwater monitoring wells screened beneath the tailings piles as
presented in Section 4.4.3.4 (Figures 4.4-15 and 4.4-17). Based on these analyses, the flow estimates for
MayIJune 1997 are 3.2 cfs and 1.3 cfs for September 1997 (Tables 4.4-4 and 4.4-5). The resulting
alluvial aquifer discharge on the north bank is estimated to be approximately two-thirds or less of the
southbank, 2.1 cfs in MayIJune, and 0.8 cfs (Qag) in September 1997.
The September 1997 flow net analysis (Figure 4.4-17) appears to indicate a loss of flow from Railroad
Creek into the alluvial aquifer beneath the tailings just upstream of RC-2. This flow loss from Railroad
Creek (and gain to the aquifer) is estimated to be approximately 0.5'cfs (Qal) based on observed head
differences between water levels in Railroad Creek and adjacent groundwater, and would be considered a
flow loss in the water balance. The flow loss occurs because water levels in Railroad Creek are above
water levels in the alluvial aquifer in this portion of the stream reach. The apparent flow loss provides
recharge to the wetland area immediately east of tailings pile 3, replenishes groundwater storage, and is
assumed to discharge back into Railroad Creek along the reach in and near SP-21, 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 seep SP-21. The presence of the bedrock, and absence of alluvial
material, indicates that the groundwater likely becomes surface water (Railroad Creek) at this location.
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17693-005-019Uuly 19.1999;4:51 PM;DRAFT FINAL IU REPORT