Dames & Moore, 1999 - USDA Forest Service

discussed in' Section 6.8.2 of this report. It is possible that during baseflow periods the interaction between
surface water and groundwater is limited for those portions of Railroad Creek adjacent to the tailings piles,
with interaction increasing immediately downstream of tailings pile 3. This is indicated by the apparent
flow loss between RC-2 and RC-5, followed by an apparent gain in Railroad Creek flow measured
approximately 100 feet downstream of RC-5, as discussed above.
Other observations with respect to surface water and groundwater interaction include the relationship
between precipitation and flow rates in Railroad Creek and the 1500-level main portal drainage. Referring
to Figures 4.3-4a and 4.3-7b, both Railroad creek and the portal drainage appear to respond relatively
rapidly (i.e., within approximately one day) to precipitation events that occur during the spring snowrnelt
period. This appears to suggest that the soil and bedrock are saturated during this period of time. However,
as the snowmelt diminishes through the summer months, the response to precipiiation becomes less
pronounced for both the portal drainage and Railroad Creek, likely due to the soil and bedrock becoming
less saturated.
4.3.7.5 Summary of Baseflow Survey
Railroad Creek upstream of the Site appears to be a gaining stream between RC-6 and RC-1 (Figure 4.3-3a).
However, adjacent to the Site, flow gains and losses are variable. It appears that, at least during parts of the
year, flow is lost between upstream stations RC-I and RC-4, and immediately downstream of RC-2. Flow
appears to be gained due to groundwater inflow (subtracting inflow from Copper Creek and the Copper
Creek diversion) between RC-4 and RC-2 during spring. Gains and losses potentially attributed to
groundwater flow are within the accuracy of the measurements during the fall and are, therefore,
inconclusive when considered independent of other site information.
4.3.8 Hydrologic Conditions During the 1998 Wand Comparison With 1997 Rate
During 1998, stream flow measurements were collected in Railroad Creek with a data logger installed at the
RC-4 and portal drainage P-l stations (Figure 4.3-3a). In general, the discharge of Railroad Creek during
the May 1997 sampling appeared higher than the discharge during the May 1998 sampling (compare
Figures 4.3-4 and 4.3-4a). The discharge in the portal drainage was also higher in 1997 than 1998 (compare
Figures 4.3-7 and 4.3-7a).
Based on these data, both the 1997 and 1998 Rl spring runoff water quality samples were apparently
collected on the rising limb or near the peak of the initial hydrograph rise (Figures 4.3-4 and 4.3-4a).
4.3.9 Interstitial Iron-Oxide Precipitate and Ferricrete
4.3.9.1 Summary of Historical Findings
The U.S. Bureau of Mines (USBM) completed an assessment of the Railroad Creek substrate in April 1994.
The assessment included the mapping of the presence or absence of interstitial iron-oxide precipitates and
ferricrete. The methods employed the probing of the substrate as part of the sampling of interstitial fluid
(see Section 5.4.3). The probing did not allow the differentiation between non-cemented and cemented
interstitial iron-oxide precipitates. However, it appeared that the cemented iron-oxide precipitates
(ferricrete) were generally limited to the banks of Railroad Creek near the northeast comer of tailings pile 1
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