Dames & Moore, 1999 - USDA Forest Service

The pH to copper relationship (Figure 6.4-12) shows some indication of pH controls. Copper is higher in
concentration in the west area of the site than the east area. The abandoned mill building, portal drainage
and waste rock piles indicate a negative correlation between copper and pH implying a constraint on
copper concentrations for these data. In comparison, much lower copper concentrations are seen in the
tailings pile seeps. The data do not have sufficient pH range to demonstrate a relationship between
copper and pH, though tailings pile 1 has lower copper concentrations at higher pH. The tailings pile data
probably indicate a limit to the availability of copper in the tailings but also possibly indicate the effects
of co-precipitation of ferric hydroxides. In the case of limited copper availability in tailings, the
correlation of pH and copper reflects only the increased leaching of copper by stronger acidic solutions.
Copper concentrations in surface waters of Railroad Creek are very low and do not appear to be
controlled by the solubility of copper secondary minerals. Coprecipitation and adsorption on iron
coatings and other materials on stream sediments is expected to attenuate copper concentrations in stream
waters.
The plot of pH and zinc (Figure 6.4-13) is similar to copper except that the mill building, portal drainage
and waste rock piles appear to have relatively stable zinc concentrations (between 0.1 and 1 m a). This
may imply a mineral solubility constraint, although zinc minerals are far more soluble than indicated by
these data. The tailings pile data appear to indicate a limit to the availability of zinc, as zinc would be
expected to occur at much higher concentrations in low pH water (by extrapolation of the pH to zinc trend
for the mill building, portal drainage, and waste rock piles). The correlation of zinc and pH may, like
copper, be caused by co-precipitation of zinc with ferric hydroxides, with the effect decreasing as pH
decreases. omp par is on of sulfate and aluminum also supports the general conclusion of buffering by
alumino-silicates (Figure 6.4-9) shown by examination of magnesium and potassium concentrations.
sulfate is correlated with aluminum at high aluminum and sulfate concentrations indicating that
generation of acid, represented by sulfate results in attack on alumino-silicates releasing aluminum in
proportion to acidity. However, aluminum concentrations are lowered by aluminum hydroxides
precipitation, hence the relationship is only stable at higher sulfate and lower pH. Aluminum is removed
rather than sulfate as pH increases introducing scatter in the relationship between the parameters in Figure
6.4-9.
The cadmium to pH relationship is similar to Zn-pH indicating that zinc and cadmium show similar
geochemical behavior (Figure 6.4- 14).
Water ch'emistry indicates that pH is controlled by the formation of amorphous hydroxide precipitates of
iron and aluminum. The formation of these precipitates constrains pH, which allows copper to remain in
solution, but precipitation also allows attenuation by co-precipitation and provides a substrate for
adsorption.
6.4.6 Conclusions
Evaluation of water chemistry .indicates that geochemical behavior consistent with the well-known
processes discussed in Section 6.3 is occurring throughout the Holden Mine Site. Conclusions fiom this
section are as follows:
Consistent geochemical processes are occurring throughout the Site, specifically iron
sulfide oxidation, sphalerite and chalcopyrite oxidation, buffering and metal attenuation.
\\DM-SEA I\VOLI\COMMOMWP\~W)~~2\n160.dos 6-23
17693-005-019Vuly 27.1999.4:ll PMDRAFf FINAL RI REPORT