Dames & Moore, 1999 - USDA Forest Service

If water flow is available to dissolve weathering products on, a mineral surface, then the rate of flow and
flow variability become important considerations. At the Holden Mine, flow of water is highly variable
and results in significant seasonal variations in water quality associated with the source areas. These
variations are illustrated conceptually in Figure 6.3-3. The sequence is shown as beginning in summer
because this typically represents the time when accumulation of weathering products on mineral surfaces
begins after extensive leaching by melt-waters in the spring. Summer is typically a dry season when little
water infiltrates onto the surfaces of sources except during rain-storms. Under these conditions,
weathering products accumulate on the surfaces of sulfide minerals but are not leached. Seepage water
quality under these conditions typically represents groundwater baseflow, with little or no contribution
from surface water drainage.
In the fall, rainfall leads to infiltration of water on and into rock piles. The downward moving flow front
dissolves weathering products that accumulated in the summer. The flow front contacts buffering
minerals and pH increases. Since the water flow rate is relatively low and slow, conditions are optimal
for raising pH by contact with weaker buffering minerals (such as silicates).
In the winter, the formation of snow-pack ties up the moisture and causes flow to decrease again, and
weathering products again begin to accumulate on mineral surfaces.
Melting of the snowpack in spring leads to the release of relatively large volumes of water and thorough
flushing (though never complete) of weathering products From mineral surfaces. The resulting solutions
are acidic and contain high solute concentrations. Buffering minerals may be less effective in the spring
than in the fall due to the inability to fully react with solute because the water is relatively fast moving.
As the melt event proceeds, solute loads and concentrations both increase. However, as weathering
products are leached, it is possible for concentrations and loads to gradually decrease. This may occur in
thin relatively coarse deposits such as waste rock where leaching can be nearly complete and not
constrained by solubility of secondary minerals.
I
I 633 Natural Metal Removal Processes
Weathering and leaching of mine workings, waste rock or tailings results in water with variable pH
(depending on the degree of acid consumption) and elevated sulfate and metal concentrations. These
leachates are produced in oxidizing to reducing elivironrnents by relatively slow moving groundwaters.
Upon exiting the areas, the leachates are exposed to contact with an unlimited supply of oxygen,
atmospheric carbon dioxide concentrations, and mixing with dilute and frequently alkaline waters. Since
these conditions differ from those in which the leachates formed, the solution chemistry adjusts
(equilibrates) to the new condition. The following sections discuss these re-equilibration processes and
the effect they have on concentrations of heavy metals in solution.
The processes described include:
Dilution.
pH Control.
Eh control.
Efflorescence.
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