Dames & Moore, 1999 - USDA Forest Service

interceptor ditches likely carry primarily runoff that is generated up slope of the tailings piles, with the
majority of runoff and snowmelt generated on the tailings piles infiltrating. The ditches probably
decrease the amount of infiltration from runoff generated from up slope areas that would otherwise drain
onto the surfaces of the tailings piles.
Consequently, infiltration of precipitation (rain and snowmelt) is assumed to approach nearly 100 percent
on the surfaces of the tailings piles. Based on these assumptions, available precipitation (precipitation
minus evaporation, see Table 4.3-6b) over the 1997 field season is approximately 52.6 inches, which
translates to an average discharge of 1.1 cfs (this assumes the tailings piles cover 90 acres, and the
infi~trationldischar~e occurs over the period April through September). It is assumed that this total
discharge infiltrates vertically and recharges the underlying alluvial aquifer, and reemerges as flow into
Railroad Creek, forming a part of the total Qa component in the water balance equation.
Within the Railroad Creek watershed, snowmelt on the south facing slopes (north side of the valley) and
exposed areas (such as the tailings piles) occurs earlier than on the north facing side of the valley and in
shadedlwooded areas. Because of this, the timing of snowmelt is associated with the slope aspect and
location of a given area of study within the watershed. During the 1997 field season, it was observed that
the snowmelt on the tailings piles occurred earlier than the surrounding forested areas, as well as on the
exposed south facing slopes. This phenomenon is important when evaluating the seasonal water balance
and relative contributions of water from different watershed locations.
Accuracy
The accuracy of the estimated outflow from the tailings and waste rock piles, based on infiltration, is
controlled by the accuracy of precipitation and snow depth estimates which are extrapolated from the
weather station in Holden Village, and the assumption that nearly 100 percent of the precipitation
infiltrates and discharge over a six month period. Although these assumptions cannot be directly
validated with the data available, measurements of seep flow from the base of the waste rock piles
corroborate the order of magnitude of the estimates.
4.4.4.7 Estimates of Precipitation Runoff
Direct runoff of precipitation (Qru in the water balance equation) which does not recharge underlying
aquifers or evapotranspire was estimated based on observed runoff gain in the portal drainage for a
specific contributing watershed area. The portal drainage during snowmelt runoff was observed for
MayIJune 1997 (3 1 days) to gain an average net flow of approximately 0.5 cfs from apparent snowmelt
runoff as the portal drainage crosses the slope to the west and before entering Railroad Creek (~i~ure'4.4-
20). This net gain in flow due to surface water runoff does not include estimated losses from exfiltration
or loss of water through the bottom of the portal drainage. The loss of water through the bottom of the
portal drainage is estimated to be 0.35 cfs in MayIJune 1997 and 0.06 cfs in September 1997 based on the
comparison of flow measurements at P-l (at the 1500-level main portal) and P-5 (at Railroad Creek)
(Figure 4.3-7). Therefore, accounting for the loss of flow, the total gain is estimated to be 0.85 cfs. The
gain in flow is contributed from an upslope watershed area estimated to be approximately 60 acres in size.
Based on these values, the runoff snowmelt component during AprilIMaylJune 1997 is estimated to be
approximately 10 inches. However, only about one-half to two-thirds of the runoff occurred over the
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17693-005619Uuly 19. 1999:4:51 P M:DW FINAL RI REPORT