Dames & Moore, 1999 - USDA Forest Service

Background
The results of the climatic water budget (Section 4.3.5) indicate that the baseflow gain observed in
Railroad Creek from the 1997 flow measurements at RC-4 and RC-2 is not solely attributable to direct
precipitation and snowmelt in the Site area. The climatic water budget analysis estimated an approximate
23.8 inch (Table 4.3-6b) influx of water in excess of direct precipitation and snowmelt over a 120 acre
mine area from April to September (I 83 days). This quantity, averaged over the analysis period, indicates
a baseflow increase of 0.65 cfs over what would be expected solely from direct precipitation on the mine
site.
4.4.4.2 Site Water Balance Model
The general hydrogeologic model for the Railroad Creek valley at the Site includes a thin reworked
alluvial aquifer which underlies the creek in most places and forms much of the valley sediments. This
unit is believed to be underlain by a transitional unit composed of alluvium and reworked glacial till,
which in turn is underlain by compacted glacial till and then bedrock. The bedrock forms the upper
valley sidewalls. The tailings piles have been placed directly over the alluvial aquifer.
Groundwater inflow to Railroad Creek occurs from the alluvial aquifer either as seep flow andlor u p
welling through the bed and banks. Where the alluvial aquifer is not present, groundwater flows from the
underlying glacial till or bedrock. a round water contained in the bedrock may also flow directly into the
creek from seep water that flows From the valley sidewalls and the 1500-level main portal drainage.
Other contributions to Railroad Creek flow include tributaries and surface water runoff.
The general water balance 'equation for Railroad Creek is expressed as follows:
where:
Qr = flow at the Railroad Creek station of interest
Qru = flow at the Railroad Creek station upstream of the station of interest
Qa = the baseflow contribution to Railroad Creek between Qr and Qru from the alluvial
aquifer beneath the creek
Qb = the baseflow contribution to Railroad Creek between Qr and Qru from the bedrock
aquifer either as seep flow or as groundwater flow through the basal till
Qsr = surface runoff contributed to Railroad Creek between Qr and Qru
Qt = tributary inflow to Railroad Creek contributed between Qr and Qru
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17693-00S-019Uuly 19.1999:4:51 PM;DRAFT FINAL R1 REPORT