Geochemical model

Lindberg, R. D. and Runnells, D. D. (1984). Ground-water redox reactions An analysis of equilibrium state applied to Eh measurements and geochemical modeling. Science 225,925-927. 

Lasaga, A.C., Berner, R.A. and Garrels, R.M. (1985) An improved geochemical model of atmospheric CO2 fluctuations over the past 100 million years. In Sundquist, E.T. and Broecker, W.S. (eds.). The Carbon Cycle and Atmospheric CO2 Natural Variations Archean to Present. Washington, D.C. Am. Geophysical Union, pp. 397-411. 

AlbarMe F (1995) Introduction to Geochemical Modeling. Cambridge University Press, Cambridge... 

If osmotic effects are possible, several other effects would need to be considered in a geochemical-fate assessment, depending on whether the solute concentration is increased or decreased. If solute concentrations are increased, pressures associated with injection would increase beyond those predicted without osmotic effects. Also, the movement of ions to the injection zone from the aquifer with lower salinity (above the clay confining layer) would increase the salinity above those levels predicted by simple mixing of the reservoir fluid and the injected wastes. This action could affect the results of any geochemical modeling. 

FIGURE 20.10 Proposed geochemical model of waste after injection into the subsurface. (From U.S. EPA, Assessing the Geochemical Fate of Deep-Well-Injected Hazardous Waste A Reference Guide, EPA/625/ 6-89/025a, U.S. EPA, Cincinnati, OH, June 1990.)... 

The conceptual geochemical model of acidic waste after injection into the subsurface, proposed by Leenheer and Malcolm,102 involves a moving front of microbial activity with five zones as shown in Figure 20.10 ... 

Field studies are an important complement to geochemical modeling and to laboratory studies. The following are two ways to investigate the interactions between injected wastes and reservoir material ... 

Apps, J.A., Current Geochemical Models to Predict the Fate of Hazardous Wastes in the Injection Zones of Deep Disposal Wells, Lawrence Berkeley Laboratory, Report LBL-26007, 1988. 

Wilson, L. and A. M. Pollard (2002), Here today, gone tomorrow Integrated experimentation and geochemical modeling in studies of archaeological diagenetic change, Acc. Chem. Research 35(8), 644-651. 

Hollywood may never make a movie about geochemical modeling, but the field has its roots in top-secret efforts to formulate rocket fuels in the 1940s and 1950s. Anyone who reads cheap novels knows that these efforts involved brilliant scientists endangered by spies, counter-spies, hidden microfilm, and beautiful but treacherous women. 

Garrels and Thompson s calculation, computed by hand, is the basis for a class of geochemical models that predict species distributions, mineral saturation states, and gas fugacities from chemical analyses. This class of models stems from the distinction between a chemical analysis, which reflects a solution s bulk composition, and the actual distribution of species in a solution. Such equilibrium models have become widely applied, thanks in part to the dissemination of reliable computer programs such as SOLMNEQ (Kharaka and Barnes, 1973) and WATEQ (Truesdell and Jones, 1974). 

The first and most critical step in developing a geochemical model is conceptualizing the system or process of interest in a useful manner. By system, we simply mean the portion of the universe that we decide is relevant. The composition of a closed system is fixed, but mass can enter and leave an open system. A system has an extent, which the modeler defines when he sets the amounts of fluid and mineral considered in the calculation. A system s extent might be a droplet of rainfall, the groundwater and sediments contained in a unit volume of an aquifer, or the world s oceans. 

In the simplest class of geochemical models, the equilibrium system exists as a closed system at a known temperature. Such equilibrium models predict the distribution of mass among species and minerals, as well as the species activities, the fluid s saturation state with respect to various minerals, and the fugacities of different gases that can exist in the chemical system. In this case, the initial equilibrium system constitutes the entire geochemical model. 

Conceptualizing a geochemical model is a matter of defining (1) the nature of equilibrium to be maintained, (2) the initial composition and temperature of the equilibrium system, and (3) the mass transfer or temperature variation to occur over the course of the reaction process envisioned. 

Geochemical models can be conceptualized in terms of certain false equilibrium states (Barton et al., 1963 Helgeson, 1968). A system is in metastable equilibrium when one or more reactions proceed toward equilibrium at rates that are vanishingly small on the time scale of interest. Metastable equilibria commonly figure in geochemical models. In calculating the equilibrium state of a natural water from a reliable chemical analysis, for example, we may find that the water is supersaturated with respect to one or more minerals. The calculation predicts that the water exists in a metastable state because the reactions to precipitate these minerals have not progressed to equilibrium. 

Reaction models, despite their simple conceptual basis (Fig. 2.1), can be configured in a number of ways to represent a variety of geochemical processes. Each type of model imposes on the system some variant of equilibrium, as described in the previous section, but differs from others in the manner in which mass and heat transfer are specified. This section summarizes the configurations that are commonly applied in geochemical modeling. 

Calculating a geochemical model provides not only results, but uncertainty about the accuracy of the results. Uncertainty, in fact, is an integral part of modeling that deserves as much attention as any other aspect of a study. To evaluate the sources of error in a study, a modeler should consider a number of questions ... 

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