In studying aquatic chemistry, there are many applications for chemical equilibria.  For instance, it is possible to be looking into the mechanisms that control the movement of solutes in groundwater or surface water; or one could be interested in controlling the pH, alkalinity or corrosivity of drinking water; or one may be interested in the toxicological effects of dissolved metals on the biota. Actually, there are so many possible applications of chemical equilibria to aqueous systems that it would be hard to list them all.

For a person interested in aquatic systems (or soil systems, or marine systems, for that matter) a few assumptions hold true: Dissolved ions in solution interact with each other (form complexes), interact with particulate surfaces (adsorb) and possibly form solid phases (precipitate). In a typical natural system, say a stream water, there may be 10 to 20 major chemical components dissolved in solution. These components have the potential to form hundreds of dissolved chemical complexes, solids phases or adsorbed species. Some of these chemical species may be biologically active or even toxic while others may be inert. All of this depends on factors like the total concentration of each component, the pH, pe, ionic strength and temperature.

This is where calculating chemical equilibrium is helpful. As the name implies, chemical equilibrium assumes that all reactions have gone to completion and are in equilibrium with one another. So time dependent reactions -- those reactions that have kinetic restrictions -- are not addressed in this approach. In essence, the chemical equilibrium approach provides you with a thermodynamic snapshot of your system: the pH, ionic strength, the distribution of dissolved chemical species, how much solid phase formed, etc. This is the type of information that is critical to understanding what happens chemically in water.