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upper part of the table. Saturation Indices (SI's) are presented in the lower part of the <br /> table. <br /> Ion Activity Product <br /> SI = log <br /> Solubility Constant <br /> The Ion Activity Product (IAP) is, as the name suggests, the product of thermodynamic <br /> activities of the ions directly involved in the precipitation or dissolution of the <br /> mineral . When equilibrium exists, the ratio IAP to Solubility Constant (SC) equals 1 , and <br /> the SI equals zero. When SI is greater than zero, the water is probably supersaturated <br /> with the mineral . A negative SI indicates undersaturation. Because of analytical errors <br /> and uncertainty in the thermodynamic constants, an SI of Ot 0.5 is commonly interpreted as <br /> representing equilibrium, although this range is approximate. A blank in the table <br /> indicates that an SI could not be calculated, either because an analysis or a constituent <br /> element was not performed or the element was not present at concentrations above the <br /> detection limit. The table includes SI 's for only a few minerals. The model PHREEQE <br /> actually calculates SI 's for a much larger suite of minerals; those selected to be <br /> included in this table were those that, for several different sampling sites, appeared to <br /> be approximately at equilibrium with the waters analyzed. <br /> Mineralogic Controls. To illustrate the concept of saturation indices, consider the <br /> analysis for the Spoil Spring #1. The water appears to be effectively at equilibrium with <br /> calcite, dolomite, and gypsum (CaCO3, CaMg(CO3)23 and CaSO4 ' 2H20). These minerals may <br /> be dissolving or precipitating. Typically, calcite dissolves, placing calcium ions in <br /> solution, which then combines with sulfate to form gypsum. However, both may be <br /> dissolving; the analysis by itself does not provide sufficient information to distinguish <br /> which is occurring. Continuing down the list, SI 's were not calculable for otavite <br /> (CdCO3) and cerrusite (PbCO3) because the dissolved concentrations were less than the <br /> detection limit. The water is effectively at equilibrium with fluorite (CaF2), barite <br /> (BaSO4), magnesite (MgCO3), and rhodochrosite (MnCO3). Saturation index calculations <br /> could not be made for gibbsite (A1(OH)3), because aluminum was not detected. Note the <br /> dominance of sulfate and carbonate minerals. <br /> In general, the waters are at equilibrium with calcite and gypsum, indicating that these <br /> minerals have an important role in determining the water chemistry in both the Nucla and <br /> 17-46 Revised 04/11/88 <br />