Dames & Moore, 1999 - USDA Forest Service

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mine, Honeymoon Heights, the waste rock piles, mill building, maintenance yard, and lagoon feature. The
eastern portion of the Site principally includes the three tailings piles, the Copper Creek diversion. and
Copper Creek. The remaining pathway analyzed was Railroad Creek from the Site to Lake Chelan. The
detailed results of the transport and fate analyses are presented in Section 6.0. The following discussion is a
summary of the findings.
8.4.2 Overview of Site Geochemistry
Consistent geochemical processes are occurring across the Site including iron sulfide mineral oxidation,
oxidation of sphalerite and chalcopyrite, and metal attenuation. Specific controls include the release of
heavy metals (iron, copper, zinc, cadmium), the release of metals exerting pH control (iron, aluminum). and
seep chemistry for different facilities reflecting different rock types (mine vs. tailings). This dictates the
difference between water chemistry in the east and west parts of the Site. The underground mine. waste
rock piles and mill building area are dominated by the effect of residual zinc and copper mineralization.
whereas the tailings piles are dominated by iron sulfides and associated iron alumino-silicates.
Host rock mineralogy is the primary control in water chemistry at the Site. Weathering of these minerals,
especially sulfide minerals, dominates Site water chemistry. Non-sulfide mineralogy of the tailings is
expected to be dominated by minerals contained in the ore and in diabase dikes whereas the mine wail rocks
are dominated by biotite schist.
Secondary mineralization and precipitates produced by weathering processes are visibly evident throughout
the Site, including orange brown iron stains (iron oxyhydroxides) on waste rock and tailings, white
precipitates (amorphous aluminum hydroxide) in the 1500-level main portal drainage, green stain (copper
carbonate) on marble waste rock in the waste rock piles, and efflorescent crusts (metal sulfates) in the mill
building and where seepage emerges along the toes of the tailings piles.
The oxidation of sulfide minerals is releasing iron and acid to surface water drainages. Buffering of acidity
is occurring by the reaction of waters with alumino-silicates. This limits the solubility of some metals (e.g.,
iron) but also allows pH to be low enough to solubilize copper. However, since alumino-silicates are
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abundant, buffering occurs close to the source of acid generation.
Source controls reflect the differences in oxygen availability and water flow. Portions of the underground
mine are likely well-oxygenated through the winter months due to airflow induced by temperature
differences between the underground mine and the ambient air, and may therefore be actively oxidizing in
open stopes above the 1500-level of the mine. Random water flow in fractures dissolves weathering
products, some of which are discharged in the 1500-level main portal drainage, and some of which are
stored as salts formed by evapo-concentration. Discharge water reflects precipitation of iron in the workings
and precipitation of aluminum within the mine and in the portal drainage and Railroad Creek. The tailings
piles are only oxygenated near the surface. Chemical processe; leading to the release of heavy metals occur
primarily in this zone and not at depth. Acid neutralization occurs at depth. Groundwater contains reduced
iron which rapidly oxidizes upon emergence in seeps, forming ferricrete and flocculent.
The metal attenuation processes that occur downgradient of sources prior to entering Railroad Creek include
precipitation due to pH increase and aeration, efflorescence (causing seasonal formation of salts), co-
precipitation of heavy metals (primarily with iron), and adsorption. Precipitation of iron, aluminum, and
copper .flocculent probably occurs when seeps mix with slightly alkaline Railroad Creek water and
groundwater adjacent to Railroad Creek.
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17693-005-019Uuly 28,1999;10:24 AM:DRAFT FINAL RI REPORT