Dames & Moore, 1999 - USDA Forest Service

uff~ng by other magnesium-based minerals lacking potassium such as chlorite. The lower potassium
to magnesium ratio for tailings piles 1 and 2 indicates that bioite is less significant. This is consistent
with the presence of diabase, which has little biotite or sericite.
comparison of sulfate and aluminum also supports the general conclusion of buffering by alumino-
silicates (Figure 6.4-9); however, aluminum concentrations are lowered by aluminum hydroxides
precipitation, hence the relationship is only stable at higher sulfate and lower pH.
In summary, abundant alumino-silicates (calcic plagioclase, biotite and possible chlorite) are involved in
buffering acid produced by oxidation of iron sulfide minerals.
6.4.5 Evidence of Metal Attenuation
Secondary mineral precipitation controls can be examined principally by comparing metal concentrations
with pH. Further discussion of mineral solubility controls using MINTEQA2 (Allison et al. 1991) is
provided in Section 6.5 for each of the water quality monitoring locations.
The relationship between iron and pH indicates a strong negative correlation (Figure 6.4-10). Lower pHs
(0.5) are associated with high iron concentrations in the tailings pile seeps. Scatter at higher pHs can be
caused by colloidal flocculent indicating high "dissolved" iron concentrations (for example, in surface
waters and emergent groundwater) and ferrous iron (ground waters), which remains in solution at neutral
pH. Scatter can also be the result of concentrations near laboratory detection limits. The general
relationship between iron and pH indicates pH control by precipitation/dissolutian of iron oxyhydroxides.
Difference in iron concentrations at various locations at' the Site are therefore strongly controlled by
solubility of secondary iron minerals produced when ferrous iron oxidizes to ferric iron which then
precipitates as femc hydroxide:
The pH to aluminum relationship (Figure 6.4-1 1) is similar to iron except that the transition to low
aluminum concentrations occurs between pH of 4 and 5. Above pH 5, aluminum concentrations are very
low or are not detected above detection limits. The strong correlation implies aluminum solubility control
by precipitation of aluminum hydroxide in mine waters.
Surface water in Railroad Creek has fairly constant aluminum concentrations (0.001 mmoVL) which are
probably not high enough to be controlled by 'aluminum hydroxide (AI(OW3) solubility.
The strong relationships between pH to iron and pH to aluminum, and the abundance of both iron and
aluminum in the rocks and tailings indicate that precipitates form when these minerals are released and
serve as important pH buffers for the Holden Mine Site. It is therefore likely that the solubility of other
minerals will also be limited due to the buffering capacity of these minerals.
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17693-005-019Wuly 27.1999;4:11 PM;DRAFT RI REPORT