The objective of this paper is to define key chemical processes and water quality constituents that control the partitioning and speciation of metals in stormwater runoff. To achieve this objective, the equilibrium model MINTEQA2 was applied to historical NURP data and to synthesized stormwater data sets as a qualitative and semi-quantitative framework. Adsorption and organic complexation were found to be key geochemical processes, and pH, adsorption solids, total metals, and DOC were found to be key water quality constituents. To refine the model for more quantitative predictions, additional site specific information is needed on adsorption and organic complexation binding constants and site densities.