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Nucla East mining areas. The waters are close to equilibrium with dolomite. Dolomite <br />• typically does not exhibit the same control that calcite does; the larger variation in <br />dolomite's SI (compared to that of calcite)~indicates that at Nucla and Nucla East, <br />dolomite may affect, but probably does not "control" the water chemistry. The presence of <br />dolomite has not been confirmed. <br />An exception to the above statement about calcite and gypsum solubiiity controls on <br />chemistry is CW-N20. The well is completed in a pyrite-rich coop. This discussion <br />addresses the water sample with the lowest pN from either the Nucla or Nucla East areas. <br />No alkalinity was measured in this sample. Therefore, an SI for calcite could not be <br />calculated, and presumably calcite equilibrium exercised little control on the chemistry <br />of this sample. The low pH suggests the absence of carbonate minerals. The acid <br />was produced by oxidation of pyrite, producing sulfate and precipitating gypsum. The <br />aluminum concentration, probably resulting from dissolution of clay minerals, is high, and <br />appears to be controlled by precipitation of gibbsite (or perhaps amorphous A1(OH)3). The <br />water is undersaturated with fluorite, because of an aluminum fluoride complex that makes <br />the fluoride unavailable for precipitation of fluorite. <br />The geochemicai analyses can be used to evaluate the behavior of trace metals. Where <br />cadmium and lead are present in detectable quantities, they appear to be in equilibrium <br />with their respective carbonate minerals. This suggests that precipitation of otavite and <br />cerrusite limit the concentrations of these metals. Because these are carbonate minerals, <br />their solubility is pH sensitive. <br />Concentrations of, for example, cadmium appears to be strongly controlled by the <br />solubility of otavite (CdC03). In order to investigate the solubility of cadmium as a <br />function of pH, the analyses of the October, 1978 sample from GW-N15 were allowed (using <br />PHREEOE) to equilibrate with calcite, gypsum, cerrusite, barite, and otavite, after <br />setting the pH to several pH's in the range 4 to 10. The results are shown in Figure <br />17-15. At pH about 7.5, the equilibrium cadmium concentration is approximately constant <br />at 5 to 6 ug/1. As pH decreases, the equilibrium concentration increases, especially <br />below pH 6. In this instance, the Drinking Water Standard would be exceeded at pH's less <br />than approximately 6.2 or 6.3. Waters with slightly different chemistry might behave a <br />little differently. Lead is predicted to behave in a similar manner. Both lead and <br />• cadmium form complexes with sulfate, so that their concentrations will be a function of <br />the sulfate concentration, which, in turn, is controlled by the solubil ity of gypsum. <br />17-47 Revised 04/11/88 <br />