Dames & Moore, 1999 - USDA Forest Service

that began at the Site in mid-September 1997, and continued off and on through October resulted in
significant increases in streamflow oflen within approximately 12 to 24 hours of the rainfall event.
However, seepage flow increases were not observed during the summer rainfall events, but were noted
during a larger rainfall event in October 1997, and then only in selected seeps along the tailings piles (SP-
2 and SP-3) and selected seeps upstream of the tailings (SP-7, SP-14, SP-15, SP-23 and SP-23B). This
indicates that a relatively large influx of rainfall infiltration is necessary to saturate the soils underlying
the Site and cause seep flow increases after a prolonged dry period.
Baseflow in Railroad Creek is sustained by discharge from lakes and subsurface flow from glaciers, and
frorn groundwater inflow along its length. Baseflow is defined as a steady low flow condition which
reflects sustained inflow from groundwater and surface water stored in the basin, and is not a direct response
to meteorological events such as rainfall and snowlice melt. In the vicinity of the Holden Mine Site,
groundwater inflow to Railroad Creek occurs from seeps and from groundwater discharge directly through
the streambed.
Baseflow conditions were observed in Railroad Creek during the mid-April 1997 (Table 4.3-7) stream
sampling event, and during the baseflow survey completed in mid-September 1997 (Table 4.3-8). An
additional baseflow survey was performed in October 1998. At thc time of these observations, the stage
(water surface elevation) in Railroad Creek remained nearly constant, indicating a baseflow condition with
runoff frorn rainfall and snowlice melt not a significant contributor to flow.
4.3.7.1 April 1997 Survey
During the April baseflow observations, stage at RC-4 increased slightly after the first flow measurements
were made (April 16, 1997), but then remained constant during the remainder of time that flow
rneasurements were collected (April 17, 1997, to April 18, 1997). Table 4.3-7 shows the magnitude of flow
measured at each station, displayed on the table from upstream to downstream (Figures 4.3-3 and 4.3-3a).
Flow measurements made on April 16, 1997, between RC-6, RC-I and RC-4 indicate that there is a gain in
flow between RC-6 and RC-1, and a loss of flow between RC-I and RC-4. Considering the assumed
accuracy of the wading measurements used to measure flow (+I-5 to 7 percent), the gain in flow between
RC-6 and RC-I may range between 0 to 16 cfs. Similarly, the loss of flow between RC-1 and RC-4 may
range between approximately - 1 1 cfs and a gain of 6 cfs. A gain in flow between RC-6 and RC-1 would be
the expected condition for Railroad Creek, as the creek generally gains flow in a downstream direction over
its length. However, the apparent loss of flow between RC-I and RC-4, although not expected, is
corroborated by the flow relationships identified in Table 4.3-4. As discussed previously (Section 4.3.3.3),
the loss of flow between RC-1 and RC-4 may reflect downvalley flow through an old Railroad Creek
channel buried beneath the tailings piles.
It is difficult to assess the groundwater inflow between RC-4 and RC-7 during the April 1997 sampling
because of the contribution fiom Copper Creek, which could not be measured due to snow cover, and a
slight increase in stage between the April 16 and 17 measurements which may have increased flow between
flow measurements at RC-4 (morning of April 16) and RC-7 (afternoon of April 16). However, assuming
that Copper Creek accounted for 4 to 10 cfs of the gain between the stations, and the increased stage
(observed between the time that flow was measured at RC-4 and RC-7) accounted for 6 cfs (based on the
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17693-005-019Uuly 19. 1999;4:51 PM;DRAR FINAL RI REPORT