Dames & Moore, 1999 - USDA Forest Service

Water Flow
. The mine drainage area encompasses the underground mine, 1500-level main portal, 1100-level mine, and
the 1500-level ventilator portal. The anticipated transport pathways for these areas are shown on Figure 6.5-
4. Conceptual flowpaths for spring conditions (snowmelt period, roughly May to June) from the mine
workings has been observed primarily from the 1500-level main portal, with relatively minor discharges
from the 1 100-level portal and the 1500-level ventilator portal. Mine water discharge occurring fiom any of
the surface exposures of faults or shear zones which intersect the orebody have not been observed.
In the spring, snowmelt enters near-surface discontinuities in the bedrock south of the Site and flows
downward through the open stopes above the 1500-level of the underground mine. Infiltrating groundwater
flows through mineralized but unmined portions of the mine and contacts residual mineralization on rock
faces of the stopes and tunnels. The water emerges at the 1500-level main portal (portal drainage), flows
overland, and discharges to Railroad Creek. Infiltration to groundwater in the alluvium/reworked till may
occur during overland flow transport which eventually reaches Railroad Creek as baseflow. The 1500-level
main portal drainage and potential loading contribution is fiuther discussed below under Portal Drainage.
Infiltration from upslope run-on also seasonally perches in the 1100-level tunnel which then emerges as
Seep A-1. This water infiltrates the surfaces of the 800- and 1 100-level waste rock piles. An intermittent
seep was observed near the base of the 800-level waste rock pile; the seep was sampled as it entered the
intermittent drainage (SP-14 lower). The intermittent drainage then eventually infiltrates colluvium and
glacial till and is assumed to discharge into the Railroad Creek as baseflow; however, it is not known for
certain whether seep SP-23 is the discharge point for the infiltrated water hm the intermittent drainage; see
Section 6.5.1.4 for further discussion.
The 1500-level ventilator portal is located approximately one-half mile west of the 1500-level main portal
(Figures 6.1-la). As mentioned in Section 4.1.3.2, a civil survey of the 1500-level ventilator portal
indicated that the opening is approximately 20 feet higher in elevation than the 1500-level main portal. In
addition, continuous flow measurements collected by a data logger installed at the 1500-level main portal
(as discussed in Section 4.3.3.6) indicate relatively rapid and significant responses (within approximately
one day) to precipitation events, which suggests that the pool behind the dammed portal is relatively low.
Also, as discussed in Section 6.5.1.4, the chemistry of the water sampled at the 1500-level ventilator portal
indicated very dilute concentrations of metals when compared with the 1500-level main portal drainage (P-
I). Consequently, it is likely that the water observed flowing from the 1500-level ventilator portal is
meteoric groundwater seeping out of the glacial soil through which the portal was noted to have been
timbered for the first 300 feet.
In the event that the water was actually backed up behind the failed portion of the 1500-level main portal to
a level that would allow water to flow from the 1500-level ventilator portal, the water would not likely flow
out of the opening but more likely would drain down through the "300 feet of gravels" noted on the mine
map. In such an event, water would most likely flow through the subsurface into Railroad Creek
downstream of the ventilator portal. A seep was observed below the 1500-level ventilator portal (SP-26).
However, as discussed in Section 6.5.1.4, the chemistry of the SP-26 water indicated very dilute
concentrations of metals when compared with the 1500-level main portal drainage (P-1), and SP-26 likely
reflects meteoric water affected by an abandoned surface water retention area with tailings materials, and
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17693005019Uuly 27.1999,4:11 WRAFT FINAL RI REPORT