Saturation indices value

Spring Creek, Virginia, they observed some calcite precipitation at SI, (calcite saturation index) values between +0.1 and -t-0.7. Major precipitation ofcalcite as travertine took place at a waterfall where the local turbulence facilitated COt oulgassing. The CO2 loss caused the water to exceed calcite saturation by up to 20 times SI, = +1.3). 

Figure 2. Major and trace component saturation index values (in solid lines) for (Ca,Cd)C03 (insets A, B) and (Ca,Ni)C05 (insets C, D) solids at congruent stoichiometric saturation. Miscibility gap lines (short-dashed) and spinodal gap lines Oong-dashed) are also shown.

Finally, the saturation indexes of each precipitated species are calculated. When a precipitated species is present, the corresponding value of saturation index is appended to the matrix of the residues for the Newton-Raphson method. The saturation index values are calculated by the sum of the product of the stoichiometric coefficients corresponding to the precipitates (Ep) and the logarithm of the corresponding values of the concentrations of the master species minus the logarithm of the corresponding solubility product constant ... 

Fig. 6-4. The dissolution rate as a function of the saturation index, omega-1, where omega = co3s cas/csat is the ion activity product divided by the saturation value.

Figure 8-2 shows the depth profiles of the saturation index omegadel), the solution rate, and the respiration rate. At the shallowest depths, the saturation index changes rapidly from its supersaturated value at the sediment-water interface, corresponding to seawater values of total dissolved carbon and alkalinity, to undersaturation in the top layer of sediment. Corresponding to this change in the saturation index is a rapid and unresolved variation in the dissolution rate. Calcium carbonate is precipitating... 

Supersaturation of up to nearly 4 orders of magnitude is indicated relative to a log K= 4.9 which reflects freshly precipitated HFO. When elimination of all data points which are below the detection limits for Fe(lll) and for electrode measurements, values of Eh measured agree with Eh calculated from Fe(ll/lll) determinations and speciation calculations and the revised ferrihydrite saturation index diagram looks like fig. 3. 

Biliary cholesterol saturation index (CSI) in nonacromegalic patients with cholesterol GBS (CH-GBS), acromegalic patients with Octreotide-associated GBS (OT-GBS), and stone-free patients before Octreotide treatment (No OT-GBS). "Hie value of CSI = 1.0 indicates the limit of cholesterol solubility points above the line are supersaturated, whereas those below the line are unsaturated with cholesterol. (All graphs show mean values SEMs.) Data taken from reference 18. 

The saturation index SI indicates, if a solution is in equilibrium with a solid phase or if under-saturated and super-saturated in relation to a sohd phase respectively. A value of 1 signifies a ten-fold supersaturation, a value of -2 a hundred-fold undersaturation in relation to a certain mineral phase. In practice, equilibrium can be assumed for a range of -0.2 to 0.2. If the determined SI value is below -0.2 the solution is understood to be undersaturated in relation to the corresponding mineral, if SI exceeds +0.2 the water is assumed to be supersaturated with respect to this mineral. 

The following example shows how this can be modeled in PHREEQC. First of all, a master- ami a solution species tritium T or T1 have to be defined. Since the input of data for log k und -gamma within the key word SOLUTION SPECIES is required, but unknown, any value can be entered here as a free parameter ( dummy , e g. 0.0). This value is not used for kinetic calculations and thus, does not cause any problems. However, all results based on equilibrium calculations (e.g. the calculation of the saturation index) are nonsense for this species . The tritium values have to be entered in tritium units. However, in order not to have to define or convert them in an extra step, they are entered fictitiously with the unit umol/kgw instead of TU in PHREEQC. As no interactions of tritium with any other species are defined, the unit is eventually irrelevant. After modeling, remember that the result is displayed in mol/kgw as always in PHREEQC and has to be recalculated to the fictitious tritium unit umol/kgw. Entering mol/kgw in the input file, the solution algorithm quits due to problems with too high total ionic strengths. 

Considering calcite equilibrium a pHc of 7.076 results, that is 0.376 pH units above the measured pH value of 6.7. The permitted deviation of 0.2 is exceeded. Since pH-pHc is negative, the water is calcite aggressive, i.e., it can still dissolve calcite and present a danger for pipe corrosion. Undersaturation can also be determined without calculation of the pHc, because within initial solution calculations in the PHREEQC output, calcite already shows a saturation index of -0.63 (= 23% saturation). 

The existing input file is extended by setting up equilibrium not only with pyrite but also with calcite. 2.621 mmol of calcite dissolve. The amount of pyrite dissolved is the same as in the absence of calcite (1.347 mmol). The pH value of 7.58 is in the neutral range. Thus, to neutralize the pH approximately 2 moles of calcite must be added for every mol of pyrite. The saturation index of gypsum is still clearly undersaturated (SI = -1.09), i.e. that gypsum is not a limiting mineral phase and hence the sulfate contents stay more or less invariable. 

The calculation of montmorillonite saturation index present at the end of each 0.5-pH interval from the kinetically generated solution composition and the equilibrium constant for the Aberdeen montmorillonite was presented on Figure 6. A rapid Increase in saturation at lower values of pH slowing at higher pH values is indicated. This behavior suggests that the rate of production of soluble cations is greater than the rate at which species required for montmorillonite precipitation are removed from solution. Note that it has not been stated that montmorillonite precipitates in the classical sense that is, as a simple crystalline substance. 

Surface water temperature is approximately 32°C throughout the year. Water temperature is 34.25°C at a depth of 51 meters (R.J. Hoffman, U.S. Geological Survey, 1990, personal communication). Water in Devils Hole is slightly supersaturated with respect to calcite (saturation index averages about 0.18) with calculated Pco2 values from 0.0123 to 0.0141 atm (Plummer et al., 2000). The water has been supersaturated with calcite for at least 500,000 years. 

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... 

Note The more common rock-forming minerals are italicized (cf. Deer et ai. 1992). Single pATjp values are presumably for well-crystallized, least soluble forms. Where p/T p ranges are given, they reflect the solubility range between relatively amorphous and well-crystallized forms, values have been computed assuming specific solution speciation models (e.g., specific complexes and complex stability constants), which must also be used when these constants are employed in mineral saturation index calculations. 

One should compute about the same saturation index (SI value) for a water with respect to a carbonate mineral when the computation is done ignoring ion pairs as when considering them, as long as the respective /Qp values used in the two calculations were also determined by one ignoring and the other assuming the same ion pairs. Explain how this statement can be true. 

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