Dames & Moore, 1999 - USDA Forest Service

As noted for Reach 1 of Railroad Creek, the general flow characteristics of the seeps (i.e.,
presencelabsence, location, etc.) were observed during the 1998 RI and found to be similar to those
observations made during the 1997 RI.
Based on the seep flow data, seep measurements would not hlly account for Qa and were not included in
the water balance.
Accuracy
Seep flow measurements at SP-6 through SP-9, SP-15 through SP-19, SP-21, SP-23 and SP-23B were*
determined utilizing streamflow measurement methods with an accuracy of 5 25 percent. Flow in
drainages that were measured using velocity meters (portal drainage, SP-17 and SP-21) were assumed to
have accuracies on the order off 20 percent.
The remainder of the seeps observed within this reach had relatively low flows (SP-I through SP-5, SP-
10E, SP-IOW through SP-13, and SP-24 through SP-25) and, therefore, did not allow the use of either
streamflow measurements and/or velocity meters. The flows were estimated and are assumed to be
accurate within 5 50 percent of the actual values. However, as discussed in Section 6, the lower flow
seeps contribute a relatively minor amount of metals loading to Railroad Creek. Therefore, the
inaccuracy of the flow measurements for the low flow seeps is not considered significant for this analysis.
4.4.4.6 Infiltration and Outflow from the Tailings and Waste Rock Piles
Estimates of outflow from the waste rock piles were developed based on estimated values of snowmelt
infiltration, and outflow over a two month period. Based on these estimates, outflow from the base of the
waste rock piles are on the order of 0.1 to 0.15 cfs. These estimates compare well with the observed
MayIJune seep flows in SP-6 and SP-7 (western waste rock pile), and SP-19 (eastern waste rock pile).
Along with infiltration, the outflow from the waste rock piles has no surface flow into Railroad Creek.
The outflow from these sources is considered to contribute to downslope seeps and ultimately to the
alluvial aquifer.
The surfaces of the three tailings piles are relatively flat and slope gently toward the south. Surface water
interceptor ditches were constructed near the southern margin of each of the tailings piles during Site
rehabilitation efforts completed by the USFS between 1989 and 1991. The ditches are connected to
diversion channels that direct runoff and snowmelt to either Copper Creek, the Copper Creek diversion,
or to SP-21 east of tailings pile 3. Based on field observations in 1997, the drainage ditches intercept
meltwater and runoff from areas upslope and runoff from along an access road which traverses the south
side of tailings piles 1,2 and 3.
A limited amount of runoff is also assumed to be intercepted from the tailings piles themselves; however,
because the tailings are essentially flat, there is little slope to encourage meltwater and rainfall to runoff.
The snowpack and the overlying gravel cover on the tailings hold water as a porous media, and since the
vertical gradient is much larger than the horizontal gradient (which is nearly flat), runoff is encouraged to
infiltrate much more readily than flow toward the drainage ditches. Infiltration rates measured within the
surface of tailings pile 2 during the RI (see Section 4.3.3.7), and infiltration rates measured within the
surface of tailings pile 3 by Hart Crowser in 1975 (Appendix E) indicate moderate permeability. The
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17693-005-0 19UuIy 19.1999:4:51 PM:DRAFT FINAL RI REPORT