Multimedia exposure assessment models are being developed for the U.S. Environmental Protection Agency. Models are comprised of an atmospheric transport, degradation, dispersion and deposition module; a surface runoff module; and a surface water quality module. The modules are linked to describe the effect of airborne, land based, and water bourne toxic organic pollutants on surface water quality. The data required for the air module include such input parameters as the settling velocity, deposition velocity, washout ratio and Henry's Law constant for the specific chemical being considered as well as the usual meteorological data for the particular area being modeled. The surface runoff module will require data on the various physical watershed characteristics, which are related to topography, soil characteristics, land surface conditions, land use, etc. Individual storm hydrographs will also be required. The surface water quality module will need to simulate equilibrium partitioning of the toxic organic between dissolved, sediment adsorbed, or biomass adsorbed phases. Volitilization, hydrolysis, microbial transformation, photolysis, and oxidation need to be simulated. Environmental characterization of the surface water body would be required as well as the appropriate transformation rate constants and partition coefficients for the chemical being simulated.