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Dames & Moore, 1999 - USDA Forest Service

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The geochemistry of Site waters is generally consistent with oxidation of pyrite and<br />

pyrrhotite to release sulfate, iron and acid. The molar ratio of sulfate to iron in acidic<br />

solutions is consistent with the ratios in the minerals. Oxidation of iron sulfides is the<br />

main source of sulfate in the waters. Iron solubility is controlled by pH.<br />

Oxidation of sphalerite results in the release of zinc, cadmium ,and probably manganese.<br />

The low concentration of sphalerite in all of the tailings piles due to ore processing<br />

results in relatively low zinc concentrations in seepage water.<br />

Oxidation of chalcopyrite results in the release of copper. The abundance of chalcopyrite<br />

is an important limitation. The tailings piles contain very little chalcopyrite (due to ore<br />

processing) and therefore seeps from the tailings piles contain relatively low copper<br />

concentrations. Copper concentrations in surface water of Railroad Creek are low and<br />

are not controlled by the solubility of copper secondary minerals, but rather by sorption<br />

processes.<br />

Buffering of acidity produced by sulfide oxidation is occurring by the reaction of waters<br />

with alumino-silicates. The main silicates involved in the reaction appear to be calcic<br />

plagioclase, biotite and possibly chlorite. The contribution of these minerals is indicated<br />

by the correlation of calcium, magnesium, potassium and aluminum concentrations with<br />

sulfate and each other.<br />

Since alumino-silicates are ubiquitous and abundant, buffering occurs close to the source<br />

of acid generation.<br />

The release of aluminum by an acid reaction with alumino-silicates results in an<br />

important control on pH. Precipitation of aluminum hydroxide controls pH at 4.5. This<br />

limits the solubility of some metals (e.g., iron) but also allows pH to be low enough to<br />

allow copper, zinc and other metals to remain in solution. This in turn allows ,metals to<br />

reach Railroad Creek with little attenuation.<br />

The comparison of sulfate and aluminum supports the general conclusion of buffering by<br />

alumino-silicates; however, aluminum concentrations are lowered by aluminum<br />

hydroxide precipitation, hence the relationship is only stable at higher sulfate and lower<br />

pH.<br />

6.5 GEOGRAPHICAL DESCRGPTION OF CHEMICAL PROCESSES<br />

This section presents difference of the processes that are occurring in different parts of the Site and that<br />

reflect unique characteristics associated with specific sources. The differences in processes at these<br />

source areas are generally related to the movement of air and the movement of water. Air movement is a<br />

fundamental process controVfactor because it is regulates the degree to which oxidation occurs. If oxygen<br />

is readily available, oxidation is not limited by oxygen supply but can occur at a rate dictated by the<br />

properties of the minerals. Water movement is also an important controlling factor because it controls the<br />

dissolution and transport of weathering products. Seasonal variations in water flow also control variations<br />

in load release of drainage waters that discharge to Railroad Creek.<br />

\U)M-SWI\VOLI\COMMO~WR~~Q~U)O~I~~~~~MO.~OC 6-24<br />

17693M)5419Uuly 27.<strong>1999</strong>;4:11 PM;DRAF7 FINAL RI REPORT<br />

DAMES & MOORE

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