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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 PHREEpE <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 />Mineral ogic Controls. To illustrate the concept of saturation indices, consider the <br />analysis for the Spoil Spring Xl. The water appears to be effectively at equilibrium with <br />calcite, dolomite, and gypsum (CaC03, CaMg(C03)2, and Ca504 2HT ). 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 />(CdC03) and cerrusite (PbC03) because the dissolved concentrations were less than the <br />detection limit. The water is effectively at equilibrium with fluorite (CaFZ), barite <br />(Ba504), magnesite (MgC03), and rhodochrosite (MnC03). 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 />