Dames & Moore, 1999 - USDA Forest Service

uffering 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
(