A numerical model for subsurface solute transport is developed and applied to a contaminated field site. The model is capable of depicting multiple species transport in a three-dimensional, anisotropic, heterogeneous domain as influenced by advection, dispersion, adsorption, and biodegradation. Various hydrocarbon contaminants are simulated as electron donors for microbial growth, with electron acceptors utilized in the following sequence: oxygen, nitrate, Mn(IV), Fe(III), sulfate, and CO₂. In addition, the model accounts for products of biodegradation such as Mn (II), Fe(II), H₂S, and CH₄. Biodegradation of each hydrocarbon substrate follows Monod kinetics, modified to include the effects of electron acceptor and nutrient availability. Inhibition functions permit any electron acceptor to inhibit utilization of all other electron acceptors that provide less Gibbs free energy to the microbes. The model assumes that Fe(III) and Mn(IV) occur as solid phase ions, while the other electron acceptors are dissolved in the aqueous phase. Microbial biomass is simulated as independent groups of heterotrophic bacteria that exist as scattered microcolonies attached to the porous medium. Diffusional limitations to microbial growth are assumed to be negligible. In order to verify the accuracy of the computer code, the model was applied to simple, hypothetical test cases, and the results were compared to analytical solutions. In addition, a sensitivity analysis showed that variations in model inputs caused logical changes in output. Finally, the capabilities of the model were tested by comparing model output to observed concentrations of hydrocarbons, electron acceptors, and endproducts at a leaking UST site. The model was calibrated using historical site data, and predictive capabilities of the model were tested against subsequent sets of field data. The model was used to examine the effect of porous media heterogeneities on contaminant transport and biodegradation. The turning bands method was used to produce hypothetical, yet realistic heterogeneous fields describing hydraulic conductivity, initial biomass concentration, and the maximum rate of substrate utilization. When the available electron acceptor concentrations were small compared to the hydrocarbon concentration, the overall rate of hydrocarbon mass loss increased with time, even as hydrocarbon concentrations decreased. This trend is the opposite of what would be predicted by a first order decay model. / Ph. D.