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Dames & Moore, 1999 - USDA Forest Service

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9.2.2 Site Surface WaterIGroundwater Interaction and Movement<br />

The data collected during the RI bracketed both high and low flow conditions in Railroad Creek and<br />

component inflows:<br />

All surface water and groundwater at the Site ultimately discharge to Railroad Creek.<br />

Spring Conditions - The primary component of surface water and groundwater at the Site and<br />

in the vicinity during the spring period (approximately April through July) is snowrnelt. The<br />

source areas for surface water and groundwater originate upslope of the Site and in the<br />

upstream portion of Railroad Creek. Water sources flow into Railroad Creek as overland flow.<br />

base groundwater flow through the near-surface glacial sands and gravels, overland flows that<br />

infiltrates to groundwater from source areas and as groundwater surface or subsurface<br />

expressions that represent springs, seeps, or subsurface flow into the bottom of the streambed.<br />

Water enters the mine through fractures and joints. Water discharging from the 1500-level<br />

main mine portal represents the bedrock groundwater component observed at the Site. As<br />

overland flow discharges from the 1500-level main portal to the confluence of Railroad Creek,<br />

water also infiltrates to groundwater which eventually flows to Railroad Creek. The tailings<br />

pile materials have relatively low permeability; however, some water infiltrates through the<br />

surface of the tailings piles during snowmelt, precipitation events, and ponding on the surface<br />

of the piles.<br />

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

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

also decreases. For the remainder of the year, the majority of water coming into contact with<br />

the base of the tailings piles is groundwater that flows generally parallel to Railroad Creek<br />

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

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 />

9.2.3 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 />

increaseddecreases in streamflows).<br />

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

water quality criteria in Railroad Creek adjacent to the Site from RC-4 to RC-5 between April<br />

and July 1997. Dissolved copper and/or zinc concentrations at RC-3 were above State water<br />

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

quality criteria in Railroad Creek decline as streamflow rates decline from spring snow melt to<br />

fall. By September, State water quality criteria were slightly exceeded for copper only at RC-4<br />

(south bank) and for zinc only at RC-4 (south bank), RC-2 and RC-5.<br />

G:\WPDATA\OOS\REPORTSWO~DEN-ZUU\~-O.~~E 9-2<br />

17693-005-019Uuly 28,<strong>1999</strong>;10:07 AM,DRAFT FINAL RJ REPORT

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