System formulation, at a general, basic level as opposed to detailed application, appears to be the area in which the art of modeling of multiphase multicontaminant transport in groundwater could most benefit from experiences reported in the petroleum literature. Matters such as phase appearance and disappearance have been handled efficiently by generalized compositional models based on extensions of p-s formulations, total concentrations or analogues as primary variables, IMPES or partially implicit coupling, and Newton-Raphson iteration. The modularity of these codes allows for substitution of appropriate fluid-property calculations, inclusion of reactions, and so on. Nonequilibrium is the major phenomenon necessitating significant new concepts.