Dames & Moore, 1999 - USDA Forest Service

Mixing of Seeps with Railroad Creek Water
The seeps flowing into Railroad Creek between station RC-1 and the Copper Creek diversion confluence
(SP23, SP12, SP9, SPll, SP25, SP24, SPIOW, SPIOE) are all acidic to some degree. Therefore,
neutralization will occur upon mixing with the Railroad Creek water. The increase in pH will cause any
remaining aluminum in solution to be precipitated as aluminum hydroxide flocculent. Copper in solution
may precipitate as basic copper carbonate.
6.52 Tailings Piles
6.52.1 Processes in the Tailings Piles
Air Movement
Unlike the underground mine and waste rock piles, oxygen access to the tailings occurs principally by
diffusion. Consequently, seasonal variations in ambient air temperature do not result in transitions
between diffusion and convective air movement (Figure 6.5-11). In tailings deposits, only the
immediately exposed surface of the deposit is usually oxidized because the fine texture limits movement
of air into the mass. However, the fine-grained nature of the tailings results in higher surface areas than
for coarser deposits such as waste rock. During relatively dry periods, oxidation occurs resulting in the
buildup of oxidation products. During wetter periods (typically during snowmelt), saturation of the near
.. surface pores reduces oxygen diffusion but leaches accumulated weathering products.
Water Flow
This section describes the surface water and groundwater transport pathways associated with tailings piles 1,
2, and 3. The conceptual transport pathways are summarized on flow charts presented as Figures 6.5-12
, through 6.5-14. Figure 6.1-3a shows the surface water flow paths over and around the tailings piles.
Figures 6.5-15 and 6.5-16 present conceptual hydrogeologic cross-sections showing anticipated water flow
through and beneath the tailings piles in May and September, respectively.
Surface water infiltrates through the tailings piles as a result of direct precipitation and surface water run-on
from the slopes south of the tailings piles. The potential for infiltration is greatest in the spring when the
snow on the piles and the slopes south of the piles melt, and immediately following significant precipitation
events. The fine-grained nature of the tailings results in relatively low permeability (documented by Hart
Crowser in 1975 and RI data; see Section 4.4-3), which limits the infiltration of water into the piles. The
low permeability of the tailings results in isolated ponding of water on the surfaces of the piles, especially on
tailings piles 2 and 3.
Tailings piles 2 and 3 surface water run-on is collected in ditches on the piles and diverted around tailings
piles 2 and 3 (Figure 6.1-3a). A ditch located adjacent to the upslope access road of tailings pile 3 also
collects and diverts surface water run-on around tailings pile 3. Surface water run-on for tailings pile 1 is
directed into ditches on the pile which convey the run-on to the edges of the piles. The ditches are not lined
and some surface water likely infiltrates into the tailings piles. On tailings pile 1, some of this surface water
is also directed to a decant tower that was observed during the RI to not be completely sealed and provides a
17693005-019Uuly 27.1999.4:Il PMDRAFT FINAL RI REPORT