Garrels and Thompson (1962) first applied chemical modeling to geochemistry in 25 °C and one atmosphere total pressure. Their calculation, computed by hand, is now known as an
equilibrium model, which predicts species distributions, mineral saturation states, and gas fugacities from measurements of bulk solution composition. By removing small aliquots of
solvent water from an equilibrated spring water and repeatedly recalculating the species distribution, Garrels and Mackenzie (1967) simulated the reactions that occur as spring water evaporated. This coupling of mass transfer with an equilibrium model, known as a
reaction path model, enabled geochemists to simulate reaction processes.
Helgeson (1968) introduced the first computer program to solve equilibrium and reaction path models, which he and coworkers used to model geological processes like
weathering, sediment
diagenesis,
evaporation,
hydrothermal alteration, and
ore deposition. Later developments in geochemical modeling included reformulating the governing equations, first as
ordinary differential equations, then later as
algebraic equations. Additionally,
chemical components came to be represented in models by aqueous species, minerals, and gases, rather than by the elements and electrons which make up the species, simplifying the governing equations and their numerical solution. Geochemists are now able to construct on their laptops complex reaction path or
reactive transport models which previously would have required a supercomputer. == Setting up a geochemical model ==