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Dames & Moore, 1999 - USDA Forest Service

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Water Flow<br />

The mine drainage area encompasses the underground mine, 1500-level main portal, 1100-level mine,-and<br />

the 1500-level ventilator portal. The anticipated transport pathways for these areas are shown on Figure 6.5-<br />

4. Conceptual flowpaths for spring conditions (snowmelt period, roughly May to June) from the mine<br />

workings has been observed primarily hm the 1500-level main portal, with relatively minor discharges .<br />

h m the 1 100-level portal and the 1500-level ventilator portal. Mine water discharge occurring h m any of<br />

the surface exposures of faults or shear zones which intersect the orebody have not been observed.<br />

In the spring, snowmelt enters near-surface discontinuities in the bedrock south of the Site and flows<br />

downward through the open stopes above the 1500-level of the underground mine. Intiltrating groundwater<br />

flows through mineralized but unmined portions of the mine and contacts residual mineralization on rock<br />

faces of the stopes and tunnels. The water emerges at the 1500-level main portal (portal drainage), flows ,<br />

overland, and discharges to Railroad Creek. Infiltration to groundwater in the alluviumlreworked till may<br />

occur during overland flow transport which eventually &,hes Railroad Creek as baseflow. The 1500-level<br />

main portal drainage and potential loading contribution is further discussed below under Portal Drainage.<br />

Infiltration fiom upslope run-on also seasonally perches in the 1100-level tunnel which then emerges as<br />

Seep A-I. This water infiltrates the surfaces of the 800- and 1100-level waste rock piles. An intermittent<br />

seep was observed near the base of the 800-level waste rock pile; the seep w& sampled as it entered the<br />

intennittent drainage (SP-14 lower). The intermittent drainage then eventually infiltrates colluvium and<br />

glacial till and is assumed to discharge into the Railroad Cmk as baseflow; however, it is not known for '<br />

certain whether seep SP-23 is the discharge point for the infiltrated water h m the intermittent drainage; - see<br />

Section 6.5.1.4 for further discussion.<br />

The 1500-level ventilator portal is located approximately one-half mile west of the 1500-level main portal<br />

(Figures 6.1-la). As mentioned in Section 4.1.3.2, a civil survey of the 1500-level ventilator portal<br />

indicated that the opening is approximately 20 feet higher in elevation than the 1500-level main portal. In<br />

addition, continuous flow measurements collected by a data logger installed at the 1500-level main portal<br />

(as discussed in Section 4.3.3.6) indicate relatively rapid and significant responses (within approximately<br />

one day) to precipitation events, which suggests that the pool behind the dammed portal is relatively low.<br />

Also, as discussed in Section 6.5.1.4, the chemistry of the water sampled at the 1500-level ventilator portal<br />

indicated very dilute concentrations of metals when compared with the 1500-level main portal drainage (P-<br />

1). Consequently, it is likely that the water observed flowing h m the 1500-level ventilator portal is<br />

meteoric groundwater seeping out of the glacial soil through which the portal was noted to have been<br />

timbered for the fvst 300 feet.<br />

In the event that the water was actually backed up behind the failed portion of the 1500-level main portal to<br />

a level that would allow water to flow from the 1500-level ventilator portal, the water would not likely flow<br />

out of the opening but more likely would drain down through the "300 feet of gravels" noted on the mine<br />

map. in such an event, water would most likely flow through the subsurface into' Railroad Creek<br />

downstream of the ventilator portal. A seep was observed below the 1500-level ventilator portal (SP-26).<br />

However, as discussed in Section 6.5.1.4, the chemistry of the SP-26 water indicated very dilute<br />

concentrations of metals when compared with the 1500-level main portal drainage (P-I), and SP-26 likely<br />

reflects meteoric water affected by an abandoned surface water retention area with tailings materials, and<br />

\UIM-SEAI\VOLI\COMMOMWP\~W)~~~~\~~I~~~W.~~~<br />

6-26<br />

17693-0054 19Uuly 27,<strong>1999</strong>.4: 11 -RAFT FINAL RI REPORT

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