Saturation index calculation

Hydrochemical analyses should be as complete and correct as possible because they are the basic prerequisite of a reliable hydrogeochemical model. They represent the essential information and errors propagate from them to the final result. Fig. 32 to Fig. 34 show an example of the saturation index calculation for calcite and dolomite, of the C02 equilibrium partial pressure, and of the consequences an incomplete analysis may have. The following analysis is given (pH = 7.4, temp. = 8.1°C, conductivity = 418 pS/crn, concentrations in mg/L) ... 

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. 

Saturation index calculations made as part of a species distribution problem allow an assessment to be made of the effect of organic acids on the likely state of heterogeneous equilibria in an aqueous system (see Drever 1988, for discussion and definitions). By comparing saturation indices for minerals in systematically different waters we can predict the likely behavior of these minerals in the presence of organic acids. The predictions about mineral stability vary with the precise constraints that are placed on the calculations, in particular whether the cations are constrained to be in equilibrium with a mineral phase or set as a total concentration, the temperature, the partial pressure of CO2, and the anionic composition of the water. Conclusions that differ from those presented here may be possible, nevertheless, some consistent trends emerge that are related to observations made in the preceding section about speciation. 

Calcium—In general, calcium (as CaCOs) below 800 ppm should not result in calcium sulfate scale. In arid climates, however, the critical level may be much lower. For calcium carbonate scaling tendencies, calculate the Langelier Saturation Index or the Ryznar Stability Index. 

Fig. 8-7. Steady-state profiles of the saturation index, omegadel = omega-1, the dissolution rate, and the respiration rate to a depth of 40 centimeters. This calculation uses a finer depth resolution. 

Fig. 16.2. Results of reacting albite at 70 °C with an NaCl solution maintained at pH 1.5, calculated as a kinetic reaction path. Top diagram shows how the saturation index of albite varies with time bottom plot shows change in amount (mmol) of albite.

In fact, the choice of CO2 fugacity has little effect on the mineralogical results of the mixing calculation. In the model, the critical property of the Fountain fluid is that it is undersaturated with respect to calcite, so that calcite dissolves when the fluid mixes into the Lyons. Because we assume equilibrium with dolomite and magnesite, the saturation index (log Q/K) of calcite is fixed by the reaction... 

Figure 3 shows a schematic diagram of the C02-E0R site, together with the calculated calcite saturation index for each produced water sample as a function of time and for each well. This figure also shows that there is a clear difference between those wells on the fracture trend and those off-trend. For most of the on-trend wells, the calcite saturation drops shortly after the onset of C02 injection, becoming negative. This is contrasted by the off-trend well behavior, for which the SI remains nearly constant, and positive through most of the period represented. The rapid decrease seen in a few wells... 

From the chemical compositions of water samples, the saturation index of calcite is calculated as follows ... 

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. 

The Saturation Index is the difference between the actual measured pH and the calculated pH s at saturation with calcium carbonate ... 

Are the calculations for Langelier Saturation Index (LSI) or Ryznar Stability Index (SI) regularly undertaken and interpreted ... 

This kind of software information is now also available to the wider marketplace through, for example, electronic books that provide an infinitely customizable, all-in-one package, combining technical manual calculations for all common water treatment areas, displays for system material balance, saturation index tables, report generator, etc. An example of this type of software is the Water Treatment Handbook On-Disk from LXF Inc. 

The product Meh+ a La h in a solution is known as the ion activity product (IAP). The IAP is a useful concept in chemical modelling and can be used to test whether certain solutions are supersaturated with respect to a particular solid phase. For implementation into computer models a saturation index is calculated using the expression ... 

Langelier Saturation Index is calculated using equation 3.10 ... 

Langelier Saturation Index (LSI) is used to determine the scaling potential of calcium carbonate. (Note that LSI is used up to about 4,000 ppm TDS higher concentrations rely on the Stiff-Davis Saturation Index.) The LSI is calculated using the following formulas... 

The program calculates total dissolved solids (TDS), conductivity, and the Langelier and Stiff-Davis Saturation Indexes (see Chapter 3.9). The screen allows the designer to use sodium, calcium, magnesium, chloride, sulfate, or bicarbonate to balance the water analysis. Water quality for up to five feed streams can be entered and blended together to make the total, combined feed water to the system. 

The logarithm of the quotient of the ion activity product (IAP) and solubility product constant (KSP) is called the saturation index (SI). The IAP is calculated from activities that are calculated from analytically determined concentrations by considering the ionic strength, the temperature, and complex formation. The solubility product is derived in a similar manner as the IAP but using equilibrium solubility data corrected to the appropriate water temperature. 

For these carbonates, the calculation of the saturation index gets more difficult. If, for instance, one considers the calcite/strontianite system, the solubility of both mineral phases is estimated by ... 

Fig. 33 Calcite saturation index of complete and incomplete water analyses (calculated with PHREEQC after data by Merkel 1992)...

In comparison to this calculation, the dissolution of gypsum in distilled water shall now be modeled by means of PHREEQC The input is very simple as it concerns distilled water and thus, the SOLUTION block contains only pH = 7 and temperature = 20 °C. To force equilibrium with gypsum, the keyword EQUILIBRIUMPHASES and the saturation index of 0 are used. 

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 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. 

After speciation and activities have been calculated for all the free ions, ion pairs, triplets, etc., a mineral saturation index can be computed. The saturation index, SI, is defined as the logarithm of the ratio of the ion-activity product, lAP, to the solubility product constant,... 

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