Dames & Moore, 1999 - USDA Forest Service

One sample (W-1, May 2, 1998) of the water discharging from the 1500 Level ventilation portal was
very dilute (pH 6.4, sulfate 3.5 mg/L, Cu 0.7 pgiL, Zn 4 pg/L), which is consistent with a local meteoric
source for the water.
Portal Drainage
Mine drainage from the 1500-level main portal (P-1) enters Railroad Creek at P-5 via a ditch. White
precipitates are observed coating the ditch. The chemistry of the water changes little between the two
points. In the spring, metal concentrations tend to be higher at P-1 than at P-5, but in the summer .
concentrations are comparable between the two points. The decrease in concentrations in the spring is
interpreted to be due to dilute meltwater water entering the ditch h m the valley sides i d diluting the
flow.
MMTEQA2 calculations indicate that the drainage is significantly over-saturated with respect to
aluminum hydroxides and sulfates, alunite, barite and various iron hydroxides and cupiicferrite.
Saturation with respect to zinc minerals is not indicated. The precipitates observed indicate that
precipitation is occumng, although some of the precipitate may not have formed in the ditch but formed
in the mine and settled in the drainage. The active formation of flocculent as very fine particles, probably
results in the over-estimation of dissolved metal concentrations in the water, hence the degree of over-
saturation is probably less than predicted by MINTEQA2. It is likely that as the portal drainage emerges
at P-1, it is in disequilibrium with the atmosphere due to the slow conversion oxidation of Fe(I1) to
Fe(lI1). This causes precipitation of iron minerals in the ditch and co-precipitation of other heavy metals
(mainly copper). However, most of the iron precipitates due to oxidation, dilution, and neutralization.
Analysis of the precipitate indicate that it contains mainly aluminum and iron with some copper and zinc.
The principal attenuating mechanisms in the ditch are assumed to be:
Dilution by surface runoff
Oxidation and precipitation of iron resulting in removal of iron and co-precipitation of
copper oxide
Precipitation of aluminum hydroxide resulting in removal of aluminum
As the 1500-level main portal drainage water enters Railroad Creek, the weakly acidic water mixes with
the weakly alkaline stream water. The rapid pH shift causes any residual aluminum and iron to
precipitate as amorphous hydroxides in the stream bed. Copper is also expected to form a fine flocculent
as basic copper carbonate. This effect was shulated using MINTEQA2 by changing the pH and
alkalinity to that of Railroad Creek. This indicated that the malachite might precipitate. Zinc would not
be expected to precipitate. The rate at which copper carbonate might form determines whether copper
would be removed from solution, or whether copper concentrations would be diluted below the solubility
limits imposed by copper carbonates before the precipitates actually formed. The loading analysis
presented subsequently in this section confirms that copper carbonates might fonn.
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1769MOS-OI9Wuly 27. 1999;4:11 PM;DRAFT FINAL R1 REPORT