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.

The equilibrium state of a chemical system composed of an arbitrary number of thermodynamic components is described by a set of nonlinear equations that can be evaluated only using iterative methods. This chapter provides a detailed description of the process by which the Newton–Raphson method can be applied to solve for the equilibrium distribution of chemical mass in a multicomponent system composed of aqueous, solid, and gaseous phases.

Whereas determining the equilibrium point of a single chemical reaction is a straightforward application of thermodynamics, calculating the distribution of chemical mass among aqueous species, solids, and gases in a system composed of an arbitrary number of thermodynamic components requires that perhaps hundreds or thousands of reactions be evaluated at the same time. This chapter lays out a general set of equations by which a computer algorithm can quickly and reliably determine the equilibrium state of a multicomponent chemical system.

The sorption of aqueous species onto solid surfaces exerts an important control on the mobility and bioavailability of dissolved mass in the natural environment. This chapter shows how sorption reactions can be incorporated into multicomponent speciation models using distribution coefficients, Freundlich isotherms, Langmuir isotherms, and ion exchange theory.