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Dames & Moore, 1999 - USDA Forest Service

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6.4.1 Evidence of Iron Sulfide Mineral Oxidation<br />

Sulfate is the best indicator of oxidation processes because oxidation of iron sulfides (pyrite and<br />

pyrrhotite) (Figure 6.3-1) and oxidative dissolution of heavy metal sulfides (Figure 6.3-2) produce<br />

secondary iron sulfate salts. Iron is also released when pyrite and pyrrhotite are oxidized; it is only useful<br />

as an indicator of iron sulfide oxidation in low pH (SO,) represented by<br />

tailings pile 1 with Railroad Creek waters originating upstream (RC-1, SOpFe). This implies that the<br />

seep.waters from tailings piles 2 and 3 are comparable to tailings pile 1, but more dilute due to mixing<br />

wid~ailroad Creek. It would be expected that the downstream waters in Railmad Creek (for example,<br />

RC-2 would lie exactly on this trend (i.e., Fe increasing with respect to SO4 while Fe/S04

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