Dames & Moore, 1999 - USDA Forest Service

Upstream of the tailings piles, significant fdcrete deposits have not been observed. However, it generally
apm that most contribution in the spring is by surface flow and in the fall the load contributed by
groundwater is small. A hyporheic zone, if present, would be significant only in the fall.
6.9 SEDIMENT
As noted in Section 5, a number of stream sediment samples were collected historically from Railroad
Creek by others (reported in Kilburn, et al., 1994; U.S. Bureau of Mines or Lambeth, R.E., 1995; and
Ecology, or Johnson A., et al., 1997) and in Lake Chelan by Dames & Moore as part of the RI in 1998.
Sediments were collected historically upstream, within and downstream of the Site from 11 sampling
stations; no duplicate samples to measure variation within a particular sampling station were collected.
Analyses for total metals were performed on the medium to fine sand, silt and clay fraction of the
sediment. Metal (aluminum, arsenic, cadmium, copper, iron, manganese, lead, and zinc) concentrations
for samples collected during 1994 show a slight increase in concentration in the vicinity of the Site in
relation to the upstream concentrations. Data from tributary streams along Railroad Creek had similar but
slightly lower concentrations of metals in sediments. Lake sediments were collected offshore of the
mouth of Railroad Creek in 1998. Compared to the Railroad Creek sediment data, metals in lake
sediments from the Lucerne bar have a similar but slightly higher concentration range. In general, the
concentrations remain relatively constant fiom the Site to Lake Chelan.
Assuming that the sediment samples are representative, the uniformity of the metal concentrations .
downstream of the Site suggests that the stream sediment is not significantly diluted during downstream
transport between the Site and Lake Chelan, andor the tributaries contribute sediment with metals.
Railroad Creek is characterized by a coarse (70 to 90 percent cobble-boulder matrix) grain size that is
related to channel morphology and gradient. Fine sediment sources include limited areas of Railroad
Creek, tributaries and the streambanks upstream and downstream of the Site. Sediment derived from the
watershed area are transported downstream and deposited eventually into Lake Chelan.
Although downstream sediment transport in Railroad Creek is a potential compound of concern migration
pathway, several physical mechanisms reduce the potential for this pathway to be significant. The majority
of metals that have been deposited in the sireambank sediments become progressively attenuated in a
downstream direction as they migrate. Downstream sediment becomes interspersed with sediments from
tributary and Railroad Creek streambed sources. Copper and zinc remain slightly elevated (approximately
two-fold higher than upstream of the Site) from the Site to Lake Chelan. However, the metals are presumed
to be present in the sediment as iron oxides andor manganese oxides which are relatively inert and not
readily available due to the neutral pH of both Railtoad Creek and Lake Chelan.
\\oM-S~l\VOLI\COMMOMWRwpQn~\baldm-2\n7M).dos 6-50
17693405419Uuly 27.1999;4:11 PMDRAFT FINAL RI REPORT
DAMES & MOORE