Groundwater saturation states

Figure. 7.5. Saturation state of mixtures of groundwater solutions in equilibrium with calcite, at different initial Pc02 values> and seawater at 25°C. (After Plummer, 1975.)...

White (1995) found that the apparent thermodynamic supersaturation of silicate minerals in most soil pore waters resulted from excessive values for total dissolved aluminum. In reality, much of this aluminum is complexed with dissolved organics in shallow soils and does not contribute to the thermodynamic saturation state of silicate minerals. Solubility calculations involving low dissolved organic concentrations in deeper soil horizons and in groundwater appear to produce much clearer equilibrium relationships (Paces, 1972 Stefansson and Amorsson, 2000 Stefansson, 2001). 

THE CARBONATE MINERAL SATURATION STATE OF SOME REPRESENTATIVE GROUNDWATERS AND SEAWATER... 

Chemical analyses of 10 groundwaters from springs and wells in carbonate rocks are shown in Table 6.7, along with their apparent CO2 pressures and saturation indices with respect to calcite and dolomite, which have been calculated using the computer model SOLMINEQ.88 (Kharaka et al. 1988). The composition of seawater and its modeled carbonate-mineral saturation state is also shown. SOLMINEQ.88 calculates the concentrations of ion pairs, such as CaHCO and MgSO, and uses the Truesdell-Jones equation to compute ion activity coefficients. (See Chap. 4.)... 

Based on 152 laboratory and groundwater analyses between pH 1.8 and 9.2, he proposes that x and Xjp are both functions of pH, with jc = 1.24 - 0.135 pH and log = -5.89 + 1.59 pH. In his review of the saturation state of the 152 waters. Paces (1978) found that kaolinite was supersaturated by lO to l(y times, whereas allophane and halloysite were both generally close to equilibrium. 

The saturation states of these waters with respect to a range of minerals were calculated using the computer code PHREEQE. This approach indicated that the groundwaters are approximately in equilibrium with respect to quartz, calcite, kaolinite, fluorite, ferrosilite, ferric hydroxide and siderite (Table 2). The water/rock interactions deduced from the SEM-EDS analyses and the theoretical calculations using PHREEQE, are summarized in Table 3. 

The problem gives the required equilibrium concentrations, since it is stated that the groundwater sample is saturated (that is, at equilibrium) with respect to these ions. 

The vendor states that tetrachloroethane (PCE), trichloroethene (TCE), and other volatile compounds are difficult to remove from saturated soils because they are relatively insoluble. The vendor states that the technology is especially applicable to sites contaminated with dense non-aqueous-phase liquids (DNAPLs). Using the ISSZT technology creates an unsaturated zone from which these contaminants can be readily air stripped. Other contaminants such as polychlorinated biphenyls (PCBs) or metals can be isolated from groundwater and contained within barriers preventing the spread of contamination. 

It would lie far beyond the aim of this chapter to introduce the state-of-the art concepts that have been developed to quantify the influence of colloids on transport and reaction of chemicals in an aquifer. Instead, a few effects will be discussed on a purely qualitative level. In general, the presence of colloidal particles, like dissolved organic matter (DOM), enhances the transport of chemicals in groundwater. Figure 25.8 gives a conceptual view of the relevant interaction mechanisms of colloids in saturated porous media. A simple model consists of just three phases, the dissolved (aqueous) phase, the colloid (carrier) phase, and the solid matrix (stationary) phase. The distribution of a chemical between the phases can be, as first step, described by an equilibrium relation as introduced in Section 23.2 to discuss the effect of colloids on the fate of polychlorinated biphenyls (PCBs) in Lake Superior (see Table 23.5). 

In a nuclear waste repository located in basalt, solution pH is controlled by interactions between groundwater and the reactive glassy portion of the Grande Ronde basalt (10). In situ measurements and experimental data for this system indicate that equilibrium or steady-state solutions are saturated with respect to silica at ambient temperatures and above. Silica saturation and the low, total-dissolved carbonate concentration indicate the pH may be controlled by the dissolution of the basalt glass (silica-rich) with subsequent buffering by the silicic acid buffer. At higher temperatures, carbonate, sulfate, and water dissociation reactions may contribute to control the final pH values. 

Zone of groundwater stagnation. At depths below sea level all the rock systems of the continents are filled with water to their full capacity—they are saturated. Being below sea level, the water stored in these rocks is under no hydraulic potential difference, and therefore this water does not flow—it is static, or stagnant (Fig. 2.14). This situation is similar to that of water stored in a tub, it cannot flow out, and hence is stagnant. Additional water reaching the tub overflows. The same is observed in the sand-filled aquarium experiment (Fig. 2.13) after steady state is reached, all the new rainwater infiltrates down to the level of the rim and flows out, whereas the deeper water remains static. 

In groundwater a vertical flux in the water unsaturated zone as well as a horizontal flux in the water saturated zone has to be stated and, consequently, an associated transport of dissolved and particle bound contaminants can be observed. Concurrently, corresponding aerobic and anaerobic zones have to be differentiated with respect to the microbial degradation processes. 

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