Dames & Moore, 1999 - USDA Forest Service

The October 1998 baseflow survey included five stations (BF-I through BF-5) between RC-6 and RC-4
(Figure 4.3-10). Over the course of the measurements (approximately 5 hours) the stage in Railroad
Creek was measured continuously with an electronic data logger situated at RC-4. The data indicated that
the stage in Railroad Creek did not change, indicating that the flow measurements are directly
comparable.
-.. Referring to Figure 4.3-10 and Table 4.3-9, the results of the survey indicate an apparent increase in flow
between the 1500;level main portal drainage (P-5) and Railroad Creek adjacent to the Holden Village
septic field (approximately midway between P-5 and RC-4), with a subsequent loss of a similar quantity
of flow between the septic field and the vehicle bridge (upstream of RC-4).
Statistical analyses were completed in order to evaluate the results of the October 1998 baseflow survey
in, terms of accuracy and precision; the results of analyses are presented in Appendix N and are
summarized herein. Referring to Figure 4.3-10 and Table 4.3-9, the mean flow measured at stations BF- 1
and BF-5 during the October 1998 flow survey appear to remain relatively constant. However, there is an
apparent increase in flow noted between stations BF-2 and BF-3 (8.5 percent), and a similar decrease in
flow between statibns between BF-3 and BF-4. The standard deviation was calculated for each station;
the values were found to range between 0.8 and 2.2 cfs. Taking the standard deviation into account, the
data appear to suggest no significant change between the stations.
Consequently, additional statistical analyses were performed in an attempt to further evaluate the
accuracy and precision of measurements. The statistical differences between the means (utilizing the one-
tailed students t-test) appears to confirm that there is no significant difference between the means of BF- I
through BF-5 at the 95 percent confidence level. Thus, there remains a question as to the significance and
magnitude of the observed flow loss and gain within this reach of Railroad Creek. Due to this question,
the water balance analysis has assumed that the loss (indicated as Qal in the water balance equation), may ,
range from 0 to 1.0 cfs. The gain is (Qag) estimated to be 1.5 cfs in the spring and 0.5 cfs in the fall for
the entire reach (see Section 4.4.4.8).
4.3.7.4 Other Related Observations
Water surface elevations observed on May 20, 1997 indicated that the water surface in the segment of
Railroad Creek adjacent to the wetland area immediately east of tailings pile 3 (see Figure 4.3-3a) was
nearly 2 feet higher than the elevation of the wetland. This appears to verify the observed flow loss within
the reach between RC-2 and RC-5. This apparent condition may be the result of stream confinement, which
has restricted the floodplain and associated sediment storage areas, with the result that higher permeability
alluvial materials have been transported and deposited downstream of the tailings piles where the channel is
no longer confined by the riprap and tailings piles.
A zone of mixing of surface water and groundwater likely exists beneath ailr road Creek. This area of
mixing is called the hyporheic zone which is defined as a zone of mixed surface water and groundwater that
may occur in the interstices of the bed sediment in direct contact with the water (Benner et al., 1995).
However, the presence of ferricrete, as described in Section 4.3.9, may limit direct mixing between
groundwater originating From the tailings piles and Railroad Creek water by armoring the streambed. The
implications of the hyporheic zone in terms of the fate and transport of constituents of potential concern are
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17693005-019Uuly 19.19994:51 PM;DRAFT FMAL RI REPORT