Dames & Moore, 1999 - USDA Forest Service

6.0 TRANSPORT AND FATE OF COMPOUNDS OF POTENTIAL CONCERN
Most hard rock metal mines, like the Holden Mine, involve excavation and processing of ore rock to
extract useful metals (e-g., copper and zinc) contained in minerals. The minerals may be smelted onsite to
produce the metals, or, as in the case of the Holden Mine, the minerals are smelted off-site. As it is not
practical to recover all the minerals (particularly those containing iron), non-recovered minerals remain
onsite in: (1) the walls of the excavations (mine openings); (2) in rock removed to access the ore (waste
rock); and (3) in residues remaining from the processing of ore (tailings). .
The metal-contaking minerals are formed under conditions in the earth's crust that are different than at
the surface. Some differences include, pressure and temperature (both greater in the crust), and water and
oxygen abundance (both greater at the surface due to exposure to the atmosphere). Because of these
differences, the minerals remaining on a site after mining are lisually unstable in the atmosphere and
begin to breakdown almost immediately, releasing the metals they contain to flowing water, if present.
An analogue for the process is the transformation of iron to rust, which happens because iron is unstable
in the atmosphere; water flowing ove; the rust will contain iron.
Therefore, water flowing through mine sites very ofien contains metals reflecting the chemical instability
of the rock. These waters are ofien dissimilar from natural waters because contact with the minerals as
described above modifies the water chemistry. The mine waters may be acidic and contain relatively high
concentrations of dissolved sulfur. As the waters flow away from a mine, waste rock, or tailings, the
metals contained in the water can be removed by processes such as aeration, contact with acid
neutralizing rock, and mixing with natural acid-neutralizing waters.
Understanding these processes as they apply specifically to the Holden Mine Site is critical to evaluating
the sources of water quality impacts observed at the site and in Railroad Creek, and the options and
benefits of remediation. The purpose of Section 6, therefore, is to: (1) describe the processes controlling
release of metals from Site sources; (2) how metals are immobilized before they enter Railroad Creek;
and (3) the processes occurring when waters from the Site mix with Railroad Creek (Figure 6.1-1).
This section has been structured to develop the current understanding of the processes occurring at the
Holden Mine Site and how these processes impact the quality of surface water and groundwater at the site
and in Railroad Creek.
Subsection 6.1 summarizes findings on Site conditions presented in previous sections of the report and
provides some new information to give context to the subsequent discussion of processes.
Subsequent sections describe the chemical interpretation of processes at the Site.
Subsection 6.2 describes specific methods used to interpret chemical processes at the Site.
Subsection 6.3 is a general introduction to chemical processes occurring when reactive minerals are
exposed to weathering by the atmosphere, and processes by which metals are removed.
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17693-OOS-019UuIy 27.1999;4:11 -RAFT FINAL RI REPORT