Dames & Moore, 1999 - USDA Forest Service

Discharge from the alluvial aquifer on the north bank, similar to Reach I, is assumed to be less than the
south bank because bf less extensive alluvial deposits on the north side of the valley. The discharge
estimate is assumed to be less than one-half, and possibly in the range of one-third of discharge from the
south bank.
As stated previously, a baseflow gain of approximately 0.65 cfs was not accounted for from precipitation
falling on the portion of the Site between RC-I and RC-2, on the south side of Railroad Creek, based on
the climatic water budget (Section 4.3.5). The measured apparent loss of flow from Railroad Creek to the
aquifer between the vehicle bridge and RC-4 (in the range of 0.5 cfs) is of the same magnitude as the 0.65
cfs needed to correct the climatic water balance. Additionally, if loss from Copper Creek (several tenths
of a cfs) is included, the discharge from the alluvial aquifer beneath the tailings as estimated from the
flow net analysis can be accounted for entirely by direct infiltration through the tailings and recharge via
flow loss from Copper Creek and Railroad Creek. However, conclusions based on the 1998 low flow
survey should be made with caution as the standard deviations are in the same general range as the
measured flow rates. Consequently, one should be able to conclude that the apparent loss or gain within
this reach of Railroad Creek is small relative to the flow in Railroad Creek.
In summary, the Qa value for Reach 2 in MayIJune 1997 is 5.3 cfs and 1.6 cfs in September 1997.
Accuracy
The accuracy of the Reach 1 south bank inflow estimates is principally controlled by the assumed
recharge rates, and the assumption of no change in storage. Estimates of inflow from the north bank are
based on comparative judgment. Although the percentage error in these values may exist, the assumption
that the north bank inflows are less than the south bank inflows is reasonable, and the relative contribution
of north bank inflow to the site water balance is small compared to other water sources. The estimated
groundwater baseflow values within this reach of Railroad Creek are within the range observed during the
October 1998 baseflow survey which provides justification of the estimated values used.
Errors associated with the Reach 2 estimates are based on the hydraulic conductivity (K) values used.
These values were based on averages for the groundwater monitoring wells screened within the alluvial
materials beneath the tailings piles. Although there is a large range of (K) values, the use of the average
values are justified since the estimates fall within the range of baseflow values observed in Railroad
Creek.
4.4.4.9 Groundwater Contribution from the Bedrock Aquifer and Mine Stopes
Reach 1 contains the portal drainage which is the only measurable outflow from the bedrock aquifer (see
Section 4.4.4.5 above). The average flow in this drainage at Station P-1 (Qb) for MayIJune 1997 is
estimated to be 1.8 cfs, of which 0.5 cfs is estimated to be from runoff (assuming a contributing
watershed area of 60 acres). Thus, direct inflow from this source to Railroad Creek is approximately 1.3
cfs (Table 4.4-9). As previously mentioned, however, an estimated 0.35 cfs from this source during the
MayIJune 1997 period apparently exfiltrates out of the drainage into the underlying sediments, recharging
the alluvial aquifer. For the September 1997 period, flow from the portal drainage to Railroad Creek is
estimated to be 0.1 5 cfs, of which 0.06 cfs recharges the underlying alluvial aquifer.
17693-003-01 Wuly 19.1999;4:51 PM;DRAR FMAL R1 REPORT