Dames & Moore, 1999 - USDA Forest Service

through diffusion. This is indicated by the presence of a thin oxidized zone in the near surface and
unoxidized tailings at depth. Water moving down through the tailings piles cames sulfate, acidity and
reduced iron. Some of the iron is precipitated in the immediate subsurface as cementation but most of the
iron moves downward in a reduced state. This water moves through the tailings, probably being neutralized
by contact with alumino-silicates as is apparent for the whole Site. This process probably adds additional
iron due to dissolution of hornblende. The extent of this neutralization is controlled by the length of the
flow path. Waters entering the piles along the slopes adjacent to Railroad Creek, Copper Creek, and the
Copper Creek diversion will'have relatively short flow path lengths with little oppomity for neutralization.
For this reason, the chemistry of seeps (SPl, SP2, SP3, SP4, SP5) observed along the toes of the tailings
piles represents mixing of groundwater impacted by water moving downward through the pile over different
flow path lengths.
The observed chemistry of the seeps generally reflects this hypothesis. The seep chemistry of all three piles
is very similar. The differences are in pH and changes in ratios of elements, which imply drift toward
Railroad Creek water chemistry. Seepage water probably mixes with unimpacted groundwater to varying
degrees resulting in dilution of tailings groundwater. The pH changes also cause changes in iron
concentrations, and co-precipitation of heavy metals such as copper, zinc and cadmium. The low pHs
indicate precipitation of ferric hydroxide (see equation 6-6 presented earlier in this section). Internal
groundwater pHs are generally greater than 4 and indicate silicate buffering as observed elsewhere at the
Site.
Specific comments for individual piles are provided below.
Tailings Pile 1
Referring to Section 5 of this report, water chemistry data for monitoring wells (screened in the underlying
surficial materials and within the tailings) and seeps in tailings pile 1 suggest that both affected and natural
groundwaters flow under the tailings and discharge to Railroad Creek. Well TPl-6A reported the highest.
concentrations of copper (1 100 pg/L), cadmium (100 pg/L) and zinc (1 1,400 pg/L); exceeding the levels
recorded for seeps SP-1 and SP-2. Water from well TPI-4A represents groundwater that has not been
impacted by tailings, as it contained low levels of metals and sulfate (Cu: 4 pg/L; Zn: 28 Clgn; Cd: 4.2
pg/L; Fe: 30 pg/L; Sod: 300 mg/L) and a near-neutral pH (6.7). Zinc concentrations showed a degree of
zonality within the groundwaters. Low zinc concentrations were detected near the upslope edge of the
tailings (TP1-1 and TPl-4; 466 and 28 pg/L, respectively) whereas concentrations one to two orders of
magnitude higher were observed in wells further downslope near Railroad Creek (TP1-2,3, 5 Ad 6; 2,270
to 1 1,400 pg/L). These wells were comparable in zinc concentrations to discharge from seeps SP-1 and SP-
2 (3490 and 4570 p@). The increase in zinc concentrations occurs approximately along the axis of the
former Railroad Creek channel under the tailings.
Compared to the monitoring wells that are screened in the underlying surficial materials, wells screened in
the tailings groundwater contained low concentrations of zinc. Iron concentrations were approximately
three to five times higher. This suggests that tailings are contributing dissolved reduced iron and sulfate to
the underlying groundwaters.
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176934OU)lWuIy 27.1999,4:11 PM;DItAFT m AL Ri REPORT