Rock-water systems

We assume that the 180/160 ratio of the (water+rock) system is preserved through the hydrothermal reaction (closed system)... 

Fein, J. B. (2000). Quantifying the effects of bacteria on adsorption reactions in water-rock systems. Chemical Geology, 169, 265-80. 

Isotopic investigations of the water-rock system in the deep crystalline rock of northern Switzerland. In Saline Water and Gases in Crystalline Rocks. Special Paper 33 (eds. P. Fritz and S.K. Frape). Geological Association of Canada, Memorial University, Newfoundland, pp. 175-195. 

Ra sorption coefficient for a contaminant in a water/rock system... 

Sorption overview. Both empirical and mechanistic approaches have emerged since the 1970s to describe interactions between radionuclides and geomedia. These are based on conditional constants, which are valid for specific experimental conditions, or more robust intrinsic constants, which are valid over a wider range of conditions. The empirical approach involves measurements of conditional radionuclide distribution or sorption coefficients or Rsite-specific water-rock systems using synthetic or natural ground waters and crushed rock samples. Mechanistic-based approaches produce intrinsic, thermodynamic surface-complexation constants for simple electrolyte solutions with pure mineral phases. 

Secondary hydrocarbon migration generally occurs through water-saturated sedimentary rocks, i.e. through rocks that are water-wet. As water is generally considered a perfect wetting fluid (Schowalter, 1979), the contact angle 0 in Equation 4.14 for hydrocarbon-water-rock systems can be taken to be zero. If, in addition, the hydrocarbon-water interface is assumed to be spherical, then Equation 4.14 becomes identical with Equation 4.13. 

Several key questions must be answered initially in a study of reaction chemistry. First, is the reaction sufficiently fast and reversible so that it can be regarded as chemical-equilibrium controlled Second, is the reaction homogeneous (occurring wholly within a gas or liquid phase) or heterogeneous (involving reactants or products in a gas and a liquid, or liquid and a solid phase) Slow reversible, irreversible, and heterogeneous (often slow) reactions are those most likely to require interpretation using kinetic models. Third, is there a useful volume of the water-rock system in which chemical equilibrium can be assumed to have been attained for many possible reactions This may be called the local equilibrium assumption. 

The dissolved species composition of most deep, confined groundwater and of some deep unconfined groundwater may remain constant for periods of years or even thousands of years. Such constancy suggests that reactions involving those species have come to thermodynamic equilibrium. When this is the case, the water/rock system involved can be considered a closed system with respect to those species and their controlling reactions in a thermodynamic sense (the system is open to the flow of energy, but closed to the flow of matter remember that equilibrium is the time-invariant state of a closed system). For these conditions equilibrium concepts can be used to explain concentrations of the aqueous species involved. 

In some shallow confined or unconfined water/rock systems, and in large surface-water bodies, the chemistry may appear constant in time without the attainment of equilibrium. Such systems... 

Following are some rate studies and models of geochemical interest. They have been chosen to illustrate the concepts presented in this chapter and to suggest where we are in the development and application of kinetic principles to low-temperature water/rock systems. 

If similar rates apply to natural water/rock systems, which wilt generally have much longer residence times than considered in this study, we can expect that organic matter similar to G, will have disappeared (fi = 14 days). G2 kinetics will still be important. However, rates of decomposition of more recalcitrant organics (G r), because they include perhaps most of toxic organics, will still be important. 

Empirical studies of silicate rock or mineral solution rates at low temperatures, under conditions where the water is far from equilibrium with the solid, obey zero-order kinetics (cf. Apps 1983 Paces 1983, Bodek et al. 1988), also called linear kinetics (White and Claassen 1979). The best example of such behavior is the dissolution of S1O2 polymorphs (see Rimstidt and Barnes 1980 and Section 2.7.8). Linear or zero-order kinetics is observed when the area of reacting mineral exposed to a volume of solution or volume of the water-rock system (also called the specific wetted surface, A, in cm or m /m ) may be considered constant with time. The general form of the empirical rate law is... 

Equation (2.101) corresponds to transport-controlled kinetics (cf. Stumm 1990). White and Claassen conclude that after long times in natural water/rock systems parabolic rates tend to become linear. Helgeson et al. (1984) show that feldspar dissolution rates are linear if the feldspar is pretreated to remove ultrafine reactive particles. In other words initial parabolic rates are probably an artifact of sample preparation. It seems likely that, in general, the dissolution or weathering of most silicates in natural water/rock systems obeys zero-order kinetics. 

Most reactions in gas/water/rock systems involve or are controlled by the pH of the system. 

Acidity and alkalinity titrations determine the total capacity of natural waters to consume strong bases or acids as measured to specified pH values defined by the endpoints of titrations. Of more interest for many purposes is the ability of a water or water-rock system to resist pH change when mixed with a more acid or alkaline water or rock. This system property is called its buffer capacity. Buffer capacity is important in aqueous/environmental studies for reasons that include ... 

There are other possible explanations when a model calculation indicates a water is supersaturated with respect to one or more carbonate minerals. They include (1) the use of inaccurate, inconsistent, or incomplete thermodynamic data for carbonate minerals and aqueous complexes (2) nonstoichiometry (i.e.. solid solution) and/or small (submicron) particle sizes of the carbonates, making them more soluble than the well-crystallized pure phases assumed in the calculation (cf. Busenberg and Plummer 1989) (3) different solution models used to define the mineral and in the calculation of saturation state in a natural water (4) inhibition of carbonate nucieation by adsorbed substances (cf. Inskeep and Bloom 1986) and (5) slow nucieation and precipitation rates that require times exceeding residence times of the water in the water-rock system (cf. Herman and Lorah 1987). The same possible explanations apply to model-computed supersaturations obtained for noncarbonate minerals. 

TABLE 13.8 Important ( ) and usually unimportant ( ) oxidation states of some actinides (An) in natural water/rock systems... 

The perfect high-level waste repository (or toxic-waste site) is one in which individual waste components are at thermodynamic equilibrium with the host water-rock system. For such conditions there is no tendency for the waste components to dissolve and be transported from the site to the accessible environment. It was shown earlier that low-Eh crystalline-rock groundwaters are often near saturation with respect to UO2 (Section 13.2.3). Spent fuel UO2 in such a system should have little or no tendency to dissolve and release other radionuclides to the groundwater. An appropriate first task in characterizing a potential repository site should, therefore, be to obtain accurate groundwater analyses to determine if the groundwater is saturated with respect to UO2. 

A primary goal of adsorption measurements and models is to predict adsorption in a variety of possible water/rock systems for a range of conditions. For the actinides, such prediction is necessary to develop confidence that radionuclide releases from a nuclear waste repository will not exceed health standards at some time and distance from the repository. Most repository programs use the K t approach to describe and model radionuclide adsorption (cf. Meijer 1992 OECD 1993). In part, this surely reflects the simplicity of incorporating values in transport codes (Freeze and Cherry 1979 ... 

Discuss factors governing the attainment of radioactive and chemical equilibrium in a water-rock system. What chemical and physical processes aid or hinder the attainment of radioactive (secular) equilibrium in a groundwater ... 

Glacier-permafrost coupling creates a distinctive hydraulic regime, dominated by transient processes, which must be understood for long term safety assessments. In principle, the problem should be solved by a thermo-mechanically coupled model of the whole ice-water-rock system, driven by an external climate function. As an interim step, we use two separate models, one for... 

This equation shows that the values andwhich describe total mass transfer in a water-rock system, are linked together by sum of the individual mass transfer in water-mineral systems. However, whereas in particular reactions stoichiometric coefficients y do not change, in the summary equation of mass transfer with rock stoichiometric coefficients turn out variable. 

Geological evolution and self-organizing of the water-rock system. In 5 vol. Vol. 1 System water-rock in Earth crust interaction, kinetics, equilibrium, modeling. VA. Alezeyevet al., Resp. Editor S.L. Shvartsev. OIGGM SO RAN [and others]. Novosibirsk, Publishers SO RAN, 2005, 244 p. (in Russian). 

At first, each mechanism (reaction, diffusion, and advection) will be explained and then coupling mechanisms will be applied to water-rock system (ground water system, hydrothermal system, seawater system). 

Diffusion in pore and crack in rocks and minerals is generally more important as micro scale mass transfer mechanism in water-rock system rather than diffusion in solid phases and in free water. 

Validation of analytical results is necessary. Experimental studies on the migration of radioactive elements from an underground storage facility need to be conducted and findings need to be compared with equivalent scenarios in natural water environments and computer simulations. However, relatively few such studies have been conducted to date. While simple modeling analyses have been conducted on natural water-rock systems, numerous unresolved problems related to migration of radioactive elements in more complicated natural systems remain. 

An important role of the EBS is the control of chemical conditions to enhance barrier preservation and radionuclide immobilization, and maximize its capacity to withstand external perturbations to these conditions. In the geosphere, chemical characteristics cannot be engineered and must be assessed via site characterization efforts. In this regard, it is important to distinguish between simply characterizing ambient chemical conditions in a particular water-rock system and defining the control or buffer of... 

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