Dames & Moore, 1999 - USDA Forest Service

The acid in the water can be removed by interaction of the solutions with other types of minerals, or by
mixing of acidic solutions with waters containing dissolved alkalinity produced by contact with these
minerals. An example is the buffering of acidity by contact with calcium carbonate minerals.. These
control pH at between 6 and 8:
At the Holden Mine, carbonate minerals generally appear to be rare, but alumino-silicate minerals can
potentially buffer acidity, primarily resulting in the release of aluminum, for example, reaction of the
calcium end-member of plagioclase feldspar (anorthite):
The crystallinity of the silicates limits their reactivity. Framework silicates such as feldspars, have
limited reactivity. Layer silicates (for example, chlorite, biotite and sericite) can be very effective buffers
due to their weak crystalline structure. Bioite is a relatively abundant mineral at the Holden Mine. It
consumes acidity, releasing magnesium, iron, aluminum and silica. Diopside, a relatively abundant
mineral at the Holden Mine, also reacts with acid releasing calcium and magnesium.
The aluminum released when alumino-silicates dissolve in acid can be hydrolyzed to aluminum
hydroxide, releasing acidity:
This buffer controls pH at between pH 4 and 5 and commonly results in the formation of pale precipitates
in mine drainage (for example, the 1500-level main portal, P-1 discharge station). These precipitates are
typically amorphous (non-crystalline) and incorporate other elements.
Ferric hydroxide also buffers acidity, though at a level considered undesirable because the pH of 3 to 3.5
is below the pH at which hydroxides of heavy metals such as copper will precipitate. The fenic
hydroxide buffer reaction is:
63.2 Seasonal Leaching Effects
The above processes describe how acidity and metals are possibly released and neutralized by reaction
with nearby minerals. An important component in these interactions is the rate and flow of water. If water
flow is completely absent, the processes largely proceed in isolation. For example, sulfide minerals will
oxidize producing soluble weathering products but the resulting acidity and metal load will remain stored
on the mineral surface. Neutralizing minerals will also slowly weather but the alkalinity produced will
not be available. This is often the case for mine workings and waste rock associated with hard rock metal
mine sites since preferential flow paths (for example, along fractures in the bedrock within the
underground mine) develop. The weathering products remain stored and do not influence leachate quality
unless contacted by water from a storm event or flooding.
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17693M)5019Uuly 21. 1999:4:11 PMDRAFT FINAL RI REPORT