Dames & Moore, 1999 - USDA Forest Service

It is likely that much of the water enters the mine from the ground surface through fractures andlor joints in
bedrock. The slopes above the 1100-level mine portal were mapped as being devoid of glacial soil cover,
which would limit the percolation of meteoric water into the bedrock, and eventually into the mine. In
contrast, the slopes below the 1100-level mine portal are mapped as generally covered with the glacial soil:
therefore, both the infiltration of surface water into the bedrock, as well as exfiltration from the underground
mine through fractures and joints in the bedrock, is anticipated to be limited below this level of the mine.
8.2.3.5 Surface Water RunonIRunoff Features
General
Surface water run on at the Site occurs primarily in the form of near-surface and overland flow from the
slopes to the south during the spring snowmelt period. As the snowmelt proceeds and diminishes. the run
on appears to make a transition to being predominantly subsurface flow within the near-surface soils.
Surface water runoff at the Site also varied ,seasonally. All surface water runoff was noted to eventually
flow into Railroad Creek. During the spring snowmelt period, the runoff was noted to be principally in the
form of overland flow and seeps (as noted earlier in this report, the term seeps utilized in this RI includes
surface expressions of groundwater that are sources of potential metals loading principally from the mill.
mine support, Honeymoon Heights, and tailings pile areas into Railroad Creek).
During the May to June spring snowmelt period, one area of overland flow was observed emanating from
the lagoon which collects surface water runoff from the mill and waste rock piles. In addition, 26 seeps
were observed flowing and/or collecting water during the May-June event. Later in the spring and summer
seasons, the surface water runoff decreased significantly when compared to the spring snowmelt period. In
September of 1997 and 1998, no indications of overland flow were noted and only three seeps located at the
base of the tailing piles were observed flowing. .
Western portion of Site
The principal surface water run on features observed in the mill and mine support area, including the
maintenance yard, were a series of seeps (groundwater expressions) that flow overland from the mill
building and from near the base of the two waste rock piles during the May-June event. This overland flow
discharges into the lagoon which flows directly into Railroad Creek for intermittent periods during the
spring snowmelt. However, by September, the seeps were no longer present and the lagoon no longer
contained standing water. The decrease in the water level of the lagoon results from a combination of
infiltration into the underlying soils and evaporation.
The lagoon is coincident with the mapped location of a pre-existing Railroad Creek channel before the
tailings piles were constructed. Consequently, the abandoned stream bed may be acting as a preferential
pathway for the draining of the lagoon feature. The pre-existing stream bed was observed on historic Site
maps to intersect Railroad Creek near the northwest comer of tailings pile 1, near seeps SP-1 and SP-2.
Intermittent surface water flowing adjacent to two mine waste rock piles in the Honeymoon Heights area
(1 100 and 800 levels) eventually disappears into talus rock which appears to contain some waste rock.
Intermittent seepage from the 1100-level'portal also infiltrates into the waste rock pile,adjacent to the mine
opening. The intermittent drainage from this portion of the Site appears to be coincident with overland
\WM-SW\I\VOLI\COMMOMWP\WDATA\~~~WPORTSWOLDEN-2.
8- 12
17693-005-019Uuly 28, 1999;10:24 AM;DRAFT FINAL Rf REPORT