Calculated models of the chemistry of natural waters show how mass is distributed among aqueous species, minerals, and gases, whether individual minerals are undersaturated or supersaturated, and gas partial pressures within the waters. This chapter explores how to construct and interpret computed models of water chemistry, using seawater, river water, and deep-sea brines as examples.

In efforts to increase and extend production from oil and gas fields, as well as to keep wells operational, petroleum engineers pump a wide variety of fluids into the subsurface. In this chapter we consider how multicomponent chemical reaction analysis can be used to foresee unfavorable consequences of reservoir flooding. We take up two examples, sulfate scaling in North Sea oil fields and the alkali flooding of a hypothetical clastic reservoir.

Reaction processes can be driven by the transfer of mass into or out of a multicomponent chemical system held at equilibrium. Complex reactions, for example, can occur when a solid phase dissolves into or precipitates from a fluid held in equilibrium. This chapter discusses how such processes might be simulated and interpreted using computer modeling techniques.

Chemists commonly find need to trace the rates at which aqueous species form complexes with other species in solution, or with reactive sites on mineral surfaces, and conversely how rapidly such complexes break apart. This chapter shows how kinetic rate laws applied to association and dissociation reactions can be incorporated into multicomponent chemical reaction models and provides a fully worked calculation, using aluminum fluoride complexing as an example.

Many redox reactions in the natural environment proceed too slowly to approach their equilibrium state, absent the action of catalysts or enzymes. In this chapter, we consider how to trace multicomponent reaction paths in which the progress of redox reactions is described by kinetic rate laws. We frame our discussion in terms of three example calculations. In the first example, ferrous iron both catalyzes and provides reducing power to convert uranyl to insoluble uranium hydroxide. The autocatalysis of manganese, in which the reaction product operates as the catalyst that promotes the reaction, serves as the second example, and the microbial degradation of phenol makes up the final illustration of the method.

In this chapter we develop a numerical model of the transport of chemical mass within flowing groundwater. Starting with a derivation of the advection–dispersion equation, we consider how the equation may be cast in discrete form as a finite difference equation. We further discuss the numerical stability of the calculation procedure and the numerical dispersion inherent in the discrete solution, concluding with a worked example.

Surface complexation is the process by which aqueous species react chemically with specific sites at the aqueous–solid interface to form surface complexes. This chapter describes in detail electrostatic triple-layer theory and its cousin the CD-MUSIC model, and how the methods can be integrated into multicomponent equilibrium models. A worked example demonstrates how the theory might be applied in the study of natural waters.

The objective of groundwater geothermometry is to use a groundwater’s chemistry to estimate the temperature at which it was last in equilibrium with its environment, to better understand the water’s origins. In this chapter, we explore the use of multicomponent chemical reaction modeling as a tool in geothermometry. We first construct a synthetic example to illustrate the calculation process. We then apply the modeling to better understand the chemistry of a hot spring in Iceland as well as geothermal water from that country.

Many minerals in the geochemical environment do not react rapidly enough with coexisting fluids to maintain thermodynamic equilibrium. Kinetic laws for this reason are required to predict the rates at which such minerals dissolve and precipitate. This chapter shows how rate laws of this class can be incorporated in multicomponent chemical reaction models, illustrates how such models behave, and provides advice on how the models might be best applied to advantage by scientists and engineers.

Transformation matrices provide a quick, easy, and reliable method for balancing and manipulating chemical reactions in an arbitrary fashion. This chapter shows how the matrices can be applied to balance reactions, calculate equilibrium constants and free-energy changes of reaction, compute equilibrium temperatures, and determine the form of equilibrium equations.

An activity coefficient relates the concentration of an aqueous species to its chemical potential. This chapter describes in detail the estimation of aqueous activity coefficients using Debye–Hückel methods, the Pitzer virial equations, and Specific ion Interaction Theory (the SIT method). A series of example calculations demonstrates application of the methods and the key differences among them.

Stable isotopes serve as naturally occurring tracers that can provide much information about how chemical reactions proceed in nature, such as which reactants are consumed and at what temperature reactions occur. This chapter shows how multicomponent chemical reaction models can be adapted to account for the stable isotope fractionation of hydrogen, carbon, oxygen, and sulfur. In the modeling approach, solid mineral phases can be held in isotopic equilibrium with the aqueous fluid or be segregated from isotope exchange. In the latter case, the isotopic composition of minerals varies only in response to precipitation and dissolution reactions. A fully worked example calculation traces the dolomitization reaction of a limestone, computed assuming the minerals are segregated from isotope exchange.