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Dames & Moore, 1999 - USDA Forest Service

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Generalized Site-Wide Groundwater RechargeIDischarge<br />

Groundwater at the Site is recharged by snowmelt in the spring from the valley slopes and from the Site<br />

itself. Due to the relatively large amount of snowfall that occurred in 199611997, groundwater recharge<br />

rates were likely high compared to other years. Once snowmelt is complete, groundwater recharge results<br />

from rainfall and from water loss from surface water tributaries (Copper Creek, Portal Drainage) via<br />

infiltration through the bed. This phenomenon occurs because the water surface elevation in the tributaries<br />

is above the water surface elevation of the groundwater.<br />

Groundwater discharge occurs at the Site in response to variable recharge. Groundwater discharge via<br />

seeps was observed to peak in May and June 1997 and decrease after that (see Section 4.3.3.7). Diffuse<br />

groundwater discharge into Railroad Creek apparently follows the same trend, with the May 1997<br />

groundwater discharge observed to be larger than September 1997 discharge as quantified by flow net<br />

analysis, described below. A more detailed description of baseflow conditions in Railroad Creek is<br />

provided in Section 4.3.7.<br />

4.4.3.4 Groundwater Discharge to Railroad Creek<br />

In order to provide quantitative estimates of groundwater discharge to Railroad Creek, flow net analyses<br />

were conducted using the May and September 1997 groundwater level data. A flow net is commonly used<br />

to quantify groundwater flow through use of basic laws of groundwater movement if an appropriate cross-<br />

section can be determined in the third dimension. Referring to Figures 4.4-1 5 through 4.4-1 8, each area<br />

bounded by two flowlines is termed a "flowtube." Since no water crosses the flowlines, discharge within<br />

the flowtube is considered constant. Discharge can be calculated by measuring the dimensions of the<br />

flowtube (length, width, and saturated thickness) the hydraulic gradient (i.e., the change in groundwater<br />

elevation) over the length of the flowtube, and the hydraulic conductivity of the material within the<br />

flowtube. Summing the discharge of adjacent flowtubes provides values of total groundwater flux over'the<br />

area.of the flow net. The flow net analyses for the Site followed this procedure.<br />

The equation used to quantify groundwater flow is:<br />

where:<br />

Qr = groundwater discharge of flowtube<br />

K = hydraulic conductivity<br />

WT = width of flowtube at Railroad Creek<br />

i~ = groundwater gradient of flowtube near Railroad Creek<br />

b = saturated thickness of aquifer near Railroad Creek.<br />

Hydraulic conductivity was measured through slug tests for the alluvium, colluvium, and alluvium/reworked<br />

till unit and through infiltration tests for the tailings. Groundwater discharge is calculated based on<br />

minimum, mean, and maximum slug and infiltration test results.<br />

G:\WPDATA\W5\REPOR7SWOLDEN-2UUCI4.DOC<br />

17693-005-019Uuly 19. <strong>1999</strong>:4:51 PM:DRAFT FINAL RI REPORT

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