The Saturation State of Natural Waters with Respect to Calcite and Dolomite

THE SATURATION STATE OF NATURAL WATERS WITH RESPECT TO CALCITE AND DOLOMITE 

Laboratory and field studies indicate that calcite dissolves more rapidly than dolomite and that approaching equilibrium with respect to these minerals by dissolution, starting with fresh, undersaturated waters (the only way one can approach dolomite saturation), takes from weeks to months (cf. Plummer et al 1979 Herman and White 1985). A rapid increase in temperature, mixing of waters. 

In Chap. 1 we introduced the concept of the saturation index (SI), which is defined as SI = logio (IAP/A sp). In this expression f AP is the ion activity product of the mineral computed from the water analysis and K p its solubility product for the same temperature and pressure. For a water at saturation, lAP = K p and SI = 0. lAP K p and SI 0 indicates the water is supersaturated with the mineral, with lAP K p and 5/ 0 denoting undersaturated conditions. 

In order to make the saturation indices of carbonates roughly comparable, one can normalize their SI values to the same mole number of carbonate groups or of cations. Thus, if 5/ is the saturation index of dolomite, and AT,/ and lAP y its solubility and ion activity products, we write 

Taking into account probable uncertainties in the field-measured pH (usually about 0.05 pH units) lab-analyzed Ca , Mg , and HCOj values, ionic strength and equilibrium constants involved, SI values for the carbonates so defined are probably known to 0.1 units at best (Langmuir 1971c). 

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