Water-undersaturated equilibria

Water- There are few data for water-undersaturated equilibria in the Ab-0r-QrH20 undersaturated system, particularly for the ternary minima. The best data are those of Steiner et al equilibria (1975), who investigated the system at 4 kb and presented results for the water-saturated and the dry systems see Table 3.6). Luth (1969) has estimated the position of the 10 kb dry minimum and Huang and WylUe (1975) have estimated the position of the 30 kb dry quartz-alkali feldspar field boundary. Figure 3.23 shows the positions of the minima in the dry system at 4 kb and 10 kb, which may be compared with positions of the eutectics in the hydrous system. 

In a simple situation of the type just described, water undersaturated or supersaturated with respect to a mineral invades an aquifer, where the mineral dissolves or precipitates according to a kinetic rate law, adding or removing a species to or from solution. With time, the species concentration along the aquifer approaches a steady state distribution, trending from the unreacted value at the inlet toward the equilibrium concentration. 

The fluid is undersaturated if Qi is less than A). This condition indicates that Reaction 3.34 has not proceeded to the right far enough to reach the saturation point, either because the water has not been in contact with sufficient amounts of the mineral or has not reacted with the mineral long enough. Values of Qi greater than A), on the other hand, indicate that the reaction needs to proceed to the left to reach equilibrium. In this case, the fluid is supersaturated with respect to the mineral. 

Fig. 26.1. Reaction of quartz with water at 25 °C, showing approach to equilibrium (dashed lines) with time. Top diagram shows variation in SiC>2(aq) concentration and bottom plot shows change in quartz saturation. In calculation A, the fluid is initially undersaturated with respect to quartz in B it is supersaturated.

Figure 29.2 shows the mineralogic results of the calculation. Dolomite dissolves, since it is quite undersaturated in the waste fluid. The dissolution adds calcium, magnesium, and carbonate to solution. Calcite and brucite precipitate from these components, as observations from the wells indicated. The fluid reaches equilibrium with dolomite after about 11.6 cm3 of dolomite have dissolved per kg water. About 11 cm3 of calcite and brucite form during the reaction. Since calculation... 

Precipitation. An important element of any geochemical analysis of natural waters is an evaluation of which minerals are present and the extent to which the system can be represented by equilibrium models. Typical questions that need to be answered are 1) Is the water supersaturated, undersaturated, or at equilibrium with a given mineral and 2) If more than one solid phase can form for a given element, which phase is more stable in that particular environment ... 

Sol id Sol utions. The aqueous concentrations of trace elements in natural waters are frequently much lower than would be expected on the basis of equilibrium solubility calculations or of supply to the water from various sources. It is often assumed that adsorption of the element on mineral surfaces is the cause for the depleted aqueous concentration of the trace element (97). However, Sposito (Chapter 11) shows that the methods commonly used to distinguish between solubility or adsorption controls are conceptually flawed. One of the important problems illustrated in Chapter 11 is the evaluation of the state of saturation of natural waters with respect to solid phases. Generally, the conclusion that a trace element is undersaturated is based on a comparison of ion activity products with known pure solid phases that contain the trace element. If a solid phase is pure, then its activity is equal to one by thermodynamic convention. However, when a trace cation is coprecipitated with another cation, the activity of the solid phase end member containing the trace cation in the coprecipitate wil 1 be less than one. If the aqueous phase is at equil ibrium with the coprecipitate, then the ion activity product wi 1 1 be 1 ess than the sol ubi 1 ity constant of the pure sol id phase containing the trace element. This condition could then lead to the conclusion that a natural water was undersaturated with respect to the pure solid phase and that the aqueous concentration of the trace cation was controlled by adsorption on mineral surfaces. While this might be true, Sposito points out that the ion activity product comparison with the solubility product does not provide any conclusive evidence as to whether an adsorption or coprecipitation process controls the aqueous concentration. 

Observations from deep-water sediment traps have demonstrated that PIC is present in waters that are undersaturated with respect to this mineral. Thus, thermodynamic considerations are not a perfect predictor of the presence of PIC. In other words, some PIC is present out of equilibrium with the seawater it is in. This is largely a result of kinetics in which dissolution is slow enough to enable PIC to persist for some time. Much effort has been applied to determining the factors that control the rate of PIC dissolution. Marine scientists have reached agreement that the rate law for CaC03 dissolution in undersaturated waters (H < 1) can be represented as ... 

As a water mass ages, the ratio of [CO3 ] to [HCO3] declines because the continuing generation of CO2 from the remineralization of POC pushes the equilibrium reaction in Eq. 15.18 further toward the products. Thus, as a deep water mass ages, it becomes increasingly more undersaturated with respect to biogenic calcium carbonate. 

Fig. 14. Change in ealcile saturation state during single-step adiabatic boiling (vapour saturation pressure) of aquifer water from four wet-steam wells in three areas Krafla. Iceland Momotnmho. Nicaragua and Zunil. Guatemala. A positive Sl-value corresponds to oversaluralion and a negative value to undersaturation. An SI of zero corresponds with equilibrium. SI is on a log Scale so an Sl-value of +1 indicates tenfold oversaturation. The numbers indicate well numbers. The calculated Sl-values for the aquifer waters (dots) depart a little from equilibrium. In view of all errors involved in the calculation of the Sl-values. the departure front equilibrium is. however, not significant. Nine that variations in Sl-values are more accurately calculated than absolute values.

As mentioned earlier, sea water seems to be in equilibrium with CuO. An apparent undersaturation is seen for Zn and Cd. Nevertheless, assuming that CuO, ZnO, and CdC03 are minor constituents of solid solutions—such as occur in manganese nodules—it is possible that the ocean is slightly oversaturated with respect to these three elements. A small oversaturation would indeed agree with the residence times given here. 

Consequently, because the equilibrium value of the ion activity product is 10 8-35, the water is slightly undersaturated with respect to calcite, its Ca2+ content is fixed by C02 pressure and pH in other words, because the product of aCa2+ and aCo32 is a constant, aCa2+ and hence mca2+ can be expressed as a negative term on the right side of the electrical balance equation. After sepiolite precipitates, it can be handled similarly. The... 

Typical vertical saturation profiles for the North Atlantic, North Pacific, and Central Indian oceans are presented in Figure 4.10. The profiles in the Atlantic and Indian oceans are similar in shape, but Indian Ocean waters at these GEOSECS sites are definitely more undersaturated than the Atlantic Ocean. The saturation profile in the Pacific Ocean is complex. The water column between 1 and 4 km depth is close to equilibrium with calcite. This finding is primarily the result of a broad oxygen minimum-C02 maximum in mid-water and makes choosing the saturation depth (SD) where Oc = 1 difficult (the saturation depth is also often referred to as the saturation level SL). 

Both authors calculations also indicated that it is possible for solutions of reasonable compositions for natural waters to produce mixtures of freshwater and seawater that were undersaturated with respect to calcite but supersaturated with respect to dolomite. This observation is a cornerstone for some dolomitization models that are discussed later in this chapter. It is also important to note that the extent of undersaturation which results from mixing is strongly dependent on the initial Pco2 °f the dilute water when it is in equilibrium with calcite. Waters high in CO2 can cause more extensive dissolution. If these waters enter a vadose zone where CO2 can be degassed, they will become supersaturated and calcium carbonate can precipitate. This process provides an excellent mechanism for cementation near the water table. Because the water table can oscillate vertically, a considerable zone of cementation can result. 

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. 

How does the water quality change when assuming that the retention times in the underground are so short that only a 50 %-saturation with regard to the predominant mineral phase will occur (Note Using the key word EQUILIBRIUM PHASES, it is not only possible to specify equilibria, but to determine defined disequilibria with the help of the saturation index as well. A saturation of 80% (undersaturation) would mean e.g. IAP/KT = 80% = 0.8 log IAP/KT = SI = log 0.8 -0.1 see also Eq.35). 

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). 

Schmidt also measured the H2 content of surface waters in the North and South Atlantic Oceans. The data varied from 0.8 to 5.0 x 10 mL/L(H20), corresponding to saturation factors of E = 0.8 5.4, where F = represents equilibrium conditions between surface water and air, while F < 1 or E > 1 indicate undersaturation or supersaturation, respectively. The H2 supersaturation in ocean water may be due to the production of hydrogen by microbiological activity. 

In Figure 5 we have plotted S.I. values for melanterite indicating a trend towards saturation for the Hornet effluent (labeled B). All of the other waters (collected at downstream sites) have been diluted and oxidized and therefore appear undersaturated. The results of these calculations compare quite favorably with field observations. Unfortunately, there is a large uncertainty associated with the thermodynamic solubility constant for melanterite. Although its solubility is well-known, the thermodynamic equilibrium constant is difficult to obtain because the compound is highly soluble and therefore becomes saturated only at high ionic strenths. 

At 60°C, the equilibrium constant is 8.10(10 ), which is greater than that at 25°C. Thus, at this temperature, the water is undersaturated and will not deposit CaC03. Ans... 

Many different forms of models are utilized, usually dictated by the objectives of research. Conceptual models are the most fundamental. All of us have some kind of concept of water-rock interactions. For a groundwater interacting with the aquifer minerals during its evolution, one might conceive that most minerals would be undersaturated in the area of recharge but that some minerals (those that dissolve fastest) would become saturated at some point down gradient, having reached their equilibrium solubility... 

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