Dames & Moore, 1999 - USDA Forest Service

Western Portion of Site
Undermound Mine and Honemoon Heights
The bedrock underlying the Site is likely saturated (the fractures and joints are assumed to be filled with
water) during the spring snowmelt period through early summer. Water originating as snowmelt and rainfall
likely infiltrates into the upper workings of the mine through the fractures and joints in the bedrock in areas
where the bedrock is not covered with glacial till. The 1100-level is generally considered the upper limit of
the glacial till unit in the Honeymoon Heights area. A mining engineer who worked at the Site during the
period of mine operation reported that water flow into the mine was diffuse, and not focused in the areas of
mapped faults. Water was removed from the mine to the surface during operation by utilizing several
pumps and trenches, in the tunnels. Flows were reportedly higher during the spring snowmelt period.
A small volume of water seepage was observed flowing from 1100-level portal in the Honeymoon Heights
area, above the 1500-level portals. Based on the evaluation of the geochemical characteristics described in
Section 6, this seepage is assumed to be meteoric water draining through the bedrock near the tunnel
opening. The water exits the 1100-level portal and infiltrates into the 1100-level waste rock pile. The
groundwater is assumed to intercept the glacial till contact which generally prevents the water from re-
entering the bedrock groundwater system and, therefore, the underground mine. The water is assumed to
eventually enter Railroad Creek through the reworked glacial materials which are present on the valley
floor.
Surface water flow from the intermittent drainage within the avalanche chute immediately east of the 1100-
and 800-level mine portals travels downslope on the surface of the exposed bedrock before infiltrating
through a mixture of mostly talus debris and minor amounts of waste rock near the base of the slope below
the 1 100-level mine portal. Groundwater infiltrates the mixture of talus and waste rock and is assumed to
travel downslope at the contact with the relatively impermeable glacial till unit within the glacial reworked
material. The groundwater is assumed to eventually flow into Railroad Creek; a dye test was conducted
during the RI to test this hypothesis; however, the results were inconclusive.
After mining operations ceased in 1957, the underground mine eventually filled with groundwater to the
level of the lowermost mine opening at the 1500-level portal (portal drainage). The 1500-level main and
ventilator portals were surveyed as part of the RI. The 1500-level ventilator portal was determined to be
approximately 20 feet higher in elevation than the 1550-level main portal. The tunnels connect
approximately one-half mile back in the mine. The groundwater in the mine is, therefore, assumed to flow
out of the 1500-level main portal. The 1500-level ventilator tunnel has a relatively small volume of water
flowing from the portal during the spring snowmelt period (less than an estimated 5 gallons per minute).
The water observed flowing from the ventilator is assumed to be meteoric water draining through the glacial
soil near the tunnel opening.
The 1500-level main portal is partially caved at the entrance, but the caving does not currently impede the '
flow of groundwater from the mine. It is reported that during the late 1960s, a blockage prevented
groundwater discharge from the mine for a period of time. The blockage eventually broke free, resulting in
a sudden release of groundwater from the mine.
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17693-005-019Uuly 28. 1999:10:24 AM;DRAFT FINAL RI REPORT