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Dames & Moore, 1999 - USDA Forest Service

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Remainder of Year - After the spring snowmelt, the amount of water flowing into the<br />

Railroad Creek from the valley sidewalls decreases significantly. The discharge from the<br />

'mine portal also decreases. For the remainder of the year, the majority of water coming<br />

into contact with the base of the tailings piles is groundwater that flows generally parallel to<br />

Railroad Creek within the glacial sands and gravels; however, base groundwater flow<br />

beneath the Site continues to discharge to Railroad Creek.<br />

A Site-specific water balance conducted for the Site accounted for the component inflow<br />

sources to Railroad Creek.<br />

Surface Water Quality in Railroad Creek.<br />

Seasonal fluctuations in the water quality were observed in Railroad Creek and a direct<br />

relationship between streamflow rates and concentrations of dissolved metals in Railroad<br />

Creek was observed, (i.e., concentrations of metals increase and decrease with<br />

increases/decreases in streamflows).<br />

Dissolved metal concentrations of copper, cadmium, and/or zinc were periodically above<br />

State water quality. criteria in Railroad Creek adjacent to the Site 'from RC-4 to RC-5<br />

between April and July- 1997. Dissolved copper and/or zinc concentrations at RC-3 were<br />

above State water quality criteria in April and May 1997. Dissolved metal concentrations<br />

above State water quality criteria in Railroad Creek decline as streamflow rates decline<br />

from spring snow melt to fall. By September, State water quality 'criteria were slightly<br />

exceeded for copper only at RC-4 (south bank) and for zinc only at RC-4 (south bank), RC-<br />

2 and RC-5.<br />

Component Inflow Sources and Transport Mechanisms to Railroad Creek and Geochemistry<br />

Processes<br />

Component inflow sources to Railroad Creek were identified and the Site geochemistry<br />

was characterized.<br />

Consistent geochemical processes are occurring across the Site including iron sulfide<br />

mineral oxidation, oxidation of sphalerite and chalcopyrite, and metal attenuation. Specific<br />

processes include the release of metals (iron, copper, zinc, cadmium), the release of metals<br />

exerting pH control (iron, aluminum), and differing seep chemistry for different portions of<br />

the site reflecting different rock types (mine vs. tailings). This dictates the difference<br />

between water chemistry in the east and west parts of the Site. The underground mine,<br />

waste rock piles and mill'building area are dominated by the effect of residual ,zinc and<br />

copper mineralization, whereas the tailings piles are dominated by concentrated iron<br />

sulfides and associated iron alumino-silicates.<br />

Host rock mineralogy is the primary factor affecting water chemistry at the Site.<br />

Weathering of these minerals, especially sulfide minerals, dominates Site water chemistry.<br />

Non-sulfide mineralogy of the tailings is expected to be dominated by minerals contained<br />

in the ore and in diabase dikes whereas the mine wall rocks are dominated by biotite schist.<br />

G:\WPDATA\OOS\REPORTS\HOLDEN-2UlNZXECUTlVE<br />

SUMMARY.doc<br />

17693-00S-019Uuly 28.<strong>1999</strong>,12:16 PM;DRAFI' FMAL RI REPORT ES-5 DAMES & MOORE

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