Dames & Moore, 1999 - USDA Forest Service

9.2.4 Component Inflow Sources and Transport Mechanisms to Railroad Creek and Geochernistq
Processes
Component inflow sources to Railroad Creek were identified and the Site geochemistry was
characterized.
Consistent geochemical processes are occurring across the Site including iron sulfide mineral
oxidation, oxidation of sphalerite and chalcopyrite, and metal attenuation. Specific processes
include the release of metals (iron, copper, zinc, cadmium), the release of metals exerting pH
control (iron, aluminum), and differing seep chemistry for different portions of the site
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 concentrated iron sulfides and associated iron
alumino-silicates.
Host rock mineralogy is the primary factor affecting 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 wall rocks are dominated by biotite schist.
Secondary mineralization and precipitates produced by weathering processes are visibly
evident at 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 rbck piles, and
efflorescent crusts (mekil sulfates) in the mill building and where seepage emerges along the
toes of the tailings piles.
The differences in oxygen availability and water flow in the Site source areas influence the
geochemical characteristics of water quality at the Site. Portions of the underground mine are
well-oxygenated through the winter months due to airflow induced by temperature differences
between the underground mine and the ambient air. Active oxidation occurs in open stopes
above the 1500-level of the mine. Random water flow occurs in fractures and 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. The tailings piles are only
oxygenated near the surface; therefore, chemical processes leading to the release of metals
occur primarily in this zone and not at depth. Acid neutralization occurs at depth in the tailings
piles. Groundwater beneath the tailings piles contains reduced iron which rapidly oxidizes
upon emergence in seeps, forming ferricrete and flocculent.
The metal attenuation processes that occur downgradient of source areas 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.
Comparison of sulfate and aluminum supports the general conclusion of buffering by alumino-
silicates.
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17693-005-019Uuly 28.1999;10:07 AMDRAFT FINAL RI REPORT