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Dames & Moore, 1999 - USDA Forest Service

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Infiltration of water from the portal drainage to the underlying alluvial aquifer may<br />

account for metals concentrations in seeps located downgradient from the portal drainage<br />

that flow into Railroad Creek or to groundwater baseflow.<br />

Iron enters Railroad Creek primarily by groundwater and iron loads are greater in<br />

September than May. Iron loads enter Railroad Creek downstream of the load sources<br />

(i.e., portal drainage) for cadmium, zinc and copper, which enter the creek as surface<br />

flows or seeps.<br />

Similar results were obtained in spring 1997 and 1998. Differences in absolute loadings<br />

were observed and can be attributed to differences in timing of monitoring. The 1998<br />

monitoring was.conducted during the initial part of snowmelt, likely resulting in the<br />

measurement of greater loads of copper, zinc and cadmium entering Railroad Creek.<br />

Additional source areas located at the west and east waste rock piles and the mill area are<br />

not significant loading sources to Railroad Creek. Metals loading at SP-21 may account for<br />

a component of unaccounted loads noted in September.<br />

6.7 COMPARISON OF THE HOLDEN MDW WITH OTHER MINE SITES<br />

This section provides case example comparisons of two other mines with characteristics similar to the<br />

Holden Mine. The examples are relatively typical of hard rock metal mines in northwestern North<br />

America and were selected on the basis of similar mine configuration, comparable waste types and<br />

leaching of similar elements. These cases illustrate that the chemical processes operating at the Holden<br />

Mine Site have been documented elsewhere, are not unique to the Holden Mine, and demonstrate that the<br />

Site does not represent extreme mine drainage conditions.<br />

6.7.1 Baker Mine<br />

The Baker Mine is a small unherground gold mine located in a sub-alpine area in north central British<br />

Columbia. Similar to the Holden Mine Site, the Baker Mine experiences winter conditions during which<br />

a snow pack forms. In June, the snow pack melts releasing large quantities of water in a few weeks.<br />

Summers are relatively warm and dry with occasional thunderstorms. The fall is wetter before snow<br />

starts to accumulate in October. The summer is shorter than at the Holden Mine Site, but the overall<br />

climatic conditions are comparable.<br />

Although the Baker Mine is much smaller than the Holden Mine, the configuration of the workings is<br />

comparable (Figure 6.7-1). Mine drainage exits from a single portal. A second portal is located 100 feet<br />

above this portal, and a small open pit is located on the slopes immediately above the mine; mine<br />

drainage does not flow from these two workings. The mine exploited an epithermal gold vein for a few<br />

years in the early 1980s and is now being reclaimed. The immediate mine sequence contains little<br />

carbonate. Two bulkheads were installed in the portals in 1993 with the intention of flooding the mine.<br />

However, as a result of leakage around the bulkheads, holes were drilled in the bulkheads to allow water<br />

to drain freely.<br />

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17693-0OS-019Uuly 27.<strong>1999</strong>;4:11 PMDRAFT FINAL RI REPORT<br />

6-46

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