Dames & Moore, 1999 - USDA Forest Service

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 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 processes leading
to the release of heavy metals occur primarily in this zone and not at depth. Acid
neutralization occurs at depth. Groundwaters contain 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 aluminum, iron and copper flocculent probably
occurs when seeps mix with slightly alkaline Railroad .Creek water and groundwater
adjacent to Railroad Creek.
The magnesium (conservative parameter) balance indicated that all major sources were
identified and that the required flow balances were consistent with measured magnesium
concentrations. Therefore, the magnesium balance corroborates the site-specific water
balance in Section 4.4.
Mass balance calculations for Railroad Creek at the Site indicate that the mass of zinc,
cadmium, copper and iron originating from the underground mine, waste rock piles, and
tailings piles has been accounted for. Seasonal and yWly variations are apparent
reflecting changing variations in flow characteristics and timing of sampling with respect
to the spring snowmelt.
Copper and zinc loads to Railroad Creek from measured point sources and other
groundwater (baseflow) sources are highest during the spring snowmelt and groundwater
discharge period when groundwater levels are highest in the deep wells beneath the
tailings, and high flow occurs at the 1500-level portal drainage. During the May round
when flows are the highest, the portal drainage is the .primary source of loading of
cadmium, copper and zinc to Railroad Creek.
Seeps SP-23 and SP-23B are the two next highest point sources that are estimated to
contribute copper, cadmium and zinc during May; however, this load drops to zero later in
the year as seep SP-23 dries up.
Iron enters Railroad Creek primarily by groundwater and iron loads are greater in
September than May. Iron loads enter Railroad Creek downstream of the load sources
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17693405419Uuly 27. 1999;4:11 PM;Dm FINAL RI REPORT