1988-04-11_PERMIT FILE - C1981008A ‎(15)‎

• Permit No: C1981008A
• IBM Index Class Name: PERMIT FILE
• Doc Date: 4/11/1988
• Doc Name: Probable Hydrologic Consequences
• Section_Exhibit Name: Tab 17
• Media Type: D
• Archive: Yes

Nucla East mining areas. The waters are close to equilibrium with dolomite. Dolomite
typically does not exhibit the same control that calcite does; the larger variation in
dolomite's SI (compared to that of calcite) indicates that at Nucla and Nucla East,
dolomite may affect, but probably does not "control" the water chemistry. The presence of
dolomite has not been confirmed.
An exception to the above statement about calcite and gypsum solubility controls on
chemistry is GW-N20. The well is completed in a pyrite-rich coal . This discussion
addresses the water sample with the lowest pH from either the Nucla or Nucla East areas.
No alkalinity was measured in this sample. Therefore, an SI for calcite could not be
calculated, and presumably calcite equilibrium exercised little control on the chemistry
of this sample. The low pH suggests the absence of carbonate minerals. The acid
was produced by oxidation of pyrite, producing sulfate and precipitating gypsum. The
aluminum concentration, probably resulting from dissolution of clay minerals, is high, and
appears to be controlled by precipitation of gibbsite (or perhaps amorphous Al(OH)3). The
water is undersaturated with fluorite, because of an aluminum fluoride complex that makes
the fluoride unavailable for precipitation of fluorite.
The geochemical analyses can be used to evaluate the behavior of trace metals. Where
cadmium and lead are present in detectable quantities, they appear to be in equilibrium
with their respective carbonate minerals. This suggests that precipitation of otavite and
cerrusite limit the concentrations of these metals. Because these are carbonate minerals,
their solubility is pH sensitive.
Concentrations of, for example, cadmium appears to be strongly controlled by the
solubility of otavite (CdCO3). In order to investigate the solubility of cadmium as a
function of pH, the analyses of the October, 1978 sample from GW-N15 were allowed (using
PHREEOE) to equilibrate with calcite, gypsum, cerrusite, barite, and otavite, after
setting the pH to several pH's in the range 4 to 10. The results are shown in Figure
17-15. At pH about 7.5, the equilibrium cadmium concentration is approximately constant
at 5 to 6 ug/l. As pH decreases, the equilibrium concentration increases, especially
below pH 6. In this instance, the Drinking Water Standard would be exceeded at pH's less
than approximately 6.2 or 6.3. Waters with slightly different chemistry might behave a
little differently. Lead is predicted to behave in a similar manner. Both lead and
cadmium form complexes with sulfate, so that their concentrations will be a function of
the sulfate concentration, which, in turn, is controlled by the solubility of gypsum.
17-47 Revised 04/11/88