Dames & Moore, 1999 - USDA Forest Service

The uppermost stopes within the mine above the 1500-level are mapped to be within approximately 50 feet
of the ground surface. No historical information was identified which addressed the potential for subsidence
at the Site. Consequently, as part of the RI, a review of available underground mine maps, and a field
reconnaissance of the areas identified as having potential for mine subsidence was conducted. The methods
utilized to assess the potential for mine subsidence, and the results of the assessment, are discussed in
Section 4.2.5.
The mine was closed in 1957 and eventually became flooded to the 1500-level. Water began discharging
from the 1500-level main portal during the mid- to late 1960s (personal communication with Wes Prieb,
Holden Village, 1997, and Warner Jansen, former Holden Village Operations Manager, 1998).
4.1.3.2 Mine Discharge
Referring to Figures 4.1-5c and 4.1-5d (which show two cross-sections of the underground mine), the portal
drainage is the lowest mine opening for exit of groundwater from the mine and likely reflects the pool level
of the mine groundwater. This is supported by portal drainage flow data collected by an automated data
logger installed at the 1500-level portal. The data indicate a significant increase in flow (more than 10-fold)
in response to precipitation within approximately 18 to 24 hours of a spring snowmelt event. Such a rapid
and significant response would not be possible if flow through the failed portion of the tunnel was restricted
(these data are.discussed in Section 4.3.3.6).
Water was also observed flowing from the 1100-level portal during the May - June site visit at relatively
low flow (estimated to be less than 5 gallons per minute). However, a review of underground mine maps
strongly suggested that the water is not reflective of the actual water level in the mine; the water discharge
instead appears to represent meteoric water which infiltrates into the mine above this level and ponds behind
the rock and soil "dike-like" feature present at the portal opening due to collapse. The water stopped
flowing from the 1100-level portal after the end of June. The chemistry of the water sampled and analyzed
from the 1 100-level portal was also determined to be meteoric in nature (see Sections 5 and 6 of this report).
A discharge of water was also noted emanating from the 1500-level ventilator portal. The flow was
estimated to be less than 5 gallons per minute. A civil survey of the 1500-level portal indicated that the
opening is approximately 25 feet higher in elevation than the ,1500-level main portal. Consequently, the
water in the 1500-level main portal would need to be backed up more than approximately 25 feet before
water would start flowing from the 1500-level ventilator portal. The backup of this much water behind the
obstructed 1500-level main portal appears unlikely but possible. It is also possible that the water observed
flowing from the 1500-level ventilator portal is a combination of mine water and meteoric water seeping out
of the glacial soil through which the portal was noted to have been timbered for the first 300 feet. This is
supported by the chemistry which is discussed in more depth in Sections 5 and 6.5.1. It is possible that
water may be exiting the mine through fracture flow in the bedrock. However, based on the results of the
site-specific water balance (discussed in Section 4.4), as well as the results of the loading analysis presented
in Section 6.6.1, it does not appear likely that fracture flow is occurring.
The chemistry of the portal drainage assists us in understanding the chemical processes in the mine. The
portal drainage has been sampled and analyzed by others historically and Dames & Moore during the RI.
Data collected intermittently between 1982 and 1998, within the May to September period, are available to
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17693-005-0 19Uuly 19. 1999;4:5 1 PM;DRAFT FINAL RI REPORT