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Dames & Moore, 1999 - USDA Forest Service

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8.2.3.7 Aquatic Reference Reaches<br />

The three aquatic reference reach streams are relatively similar to relevant segments of Railroad Creek in<br />

terms of hydrologic conditions. However, the reference reach segment selected in Bridge Creek was noted<br />

to have deeper pools than Railroad Creek. The selected segment of the South Fork of Agnes Creek was<br />

similar in gradient and character as Railroad Creek, but with lower strearnflows. The selected segment of<br />

Company Creek was situated near the confluence of the Stehekin River.<br />

8.2.4 Groundwater<br />

8.2.4.1 Railroad Creek Watershed<br />

Groundwater within the Railroad Creek valley exists as several relatively isolated occurrences: 1) a shallow<br />

occurrence within the near-surface sand and gravel (reworked glacial till), and perched above the less<br />

permeable glacial till; 2) a deeper occurrence within the glacial till unit; and 3) within the bedrock.<br />

The permeability of the uppermost occurrence is anticipated to be relatively high; it is anticipated that the<br />

majority of near-surface groundwater occurrence is within reworked glacial till unit. Groundwater<br />

permeability within the glacial till and bedrock are anticipated to be orders of magnitude lower than the<br />

near-surface occurrence, but are difficult to measure due to the random distribution of fractures and/or joints<br />

in which water flows, and the relatively low probability of intercepting these fractures during a field testing<br />

program (i.e., drilling and monitoring well installation). Groundwater within the glacial till is likely limited<br />

to isolated zones of higher permeability sand and gravel. The bedrock groundwater is anticipated to be<br />

limited to fractures and joints. The groundwater within the watershed flows eventually into Railroad Creek.<br />

8.2.4.2 Site<br />

General<br />

The groundwater underlying the Site generally exists as the same general three occurrences discussed above<br />

for the Railroad Creek watershed. Groundwater and surface water conditions on the Site are considered to<br />

be dynamic (refer to Section 4.0 for further details). Surface water infiltrates into the ground surface, travels<br />

along surfaces of relatively low permeability soil layers such as glacial till, and either emanates from the<br />

base of slopes as springs or seeps, or eventually enters Railroad Creek as diffuse groundwater. Groundwater<br />

movement through the bedrock in the mine is anticipated to be significantly greater than the surrounding<br />

non-mined bedrock. In addition, isolated groundwater occurrences exist within the tailings piles.<br />

The groundwater levels are highest during spring snowmelt, which is reflected in the relatively frequent<br />

occurrences of springs and seeps flowing during that period of time, and by the relatively high portal<br />

drainage flow rates. The groundwater levels decline as the source of groundwater recharge, snowmelt and<br />

precipitation, decrease over the summer and early fall periods. This is reflected in the reduction in measured<br />

Site seeps from a maximum of 26 in the period of the May-June 1997, to three in September 1997, and the<br />

reduction in discharge rates from the portal drainage, from a peak of approximately 3.5 cfs to a measured<br />

low of approximately 0.1 cfs.<br />

8- 1 4<br />

\U~M~SEAI\VOLI\COMMOMWP~WPDATA\OO~\REPORTSWOLDEN-ZW\~-<br />

17693-005-0 19Uuly 28,<strong>1999</strong>; 10:24 AM;DRAFT FINAL FU REPORT

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