Dames & Moore, 1999 - USDA Forest Service

Evidence of significant surface and channel erosion in Site drainages was not observed and therefore,
there does not appear to be a direct water borne pathway for significant quantities of metal-containing
sediments to enter Railroad Creek fiom the Site. Under extreme flow events, bank erosion in Railroad
Creek and channel shifting with subsequent erosion of tailings in Copper Creek can potentially transport
tailings into Railroad Creek. Channel scour and re-suspension of flocculent and fine particulates
originating as iron oxide (and other) metal precipitates, and which settle into the Railroad Creek
streambed, is also a transport mechanism which can carry metals downstream. Although large channel
scour events in Railroad Creek which transport bed material long distances downstream are not common
(due to the size of the dominant bed material and bed armoring), transport of iron-oxides as fine
suspended material was observed several times during high flow events in 1997.
63 GEOCHEMISTRY INTERPRETATION TOOLS
Interaction of minerals with the atmosphere and the formation of new minerals control the chemistry of
waters originating from the Site. A critical component of the geochemical interpretation is therefore the
determination of which minerals in the rocks are controlling the original source of the water, and which
new minerals are forming and removing metals from solution. These two groups of minerals are referred
to as primary, indicating that they are present in the rocks, and secondary, indicating that they are formed
when metals released from the primary minerals react with other dissolved or solid constituents. The first
group does not include all minerals in the rocks because some minerals are inherently unreactive.
The following subsections describe the interpretative tools that were used to evaluate chemical processes
occurring at the Site.
6.2.1 Contribution of Primary Minerals
Subsection 6.1.1 described the types of primary minerals reported to be present in the rocks within the
Holden Mine. The processes that contribute the components of primary minerals to mine water chemistry
are introduced below and described in more detail in'subsection 6.3.
The controlling process on water chemistry at the Site is the oxidation of iron sulfide (pyrite),
summarized as:
The acid (IT) produced by this reaction, can then dissolve other minerals, for example, calcite:
or, other more abundant minerals (for example, potassium feldspar):
It can be seen from reaction (6-1) that each mole of pyrite produces two moles of sulfate ion and four
moles of hydrogen ions (representing acidity). Reactions (6-2) and (6-3) indicate that these four hydrogen
ions then react with calcite and produce two ions of calcium or four ions of potassium. Therefore, if
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