Data from a controlled fuel source emplacement field experiment were used to explore the potential for predictive modeling using an approximate model and parameters from laboratory-scale studies, and to quantify the effects of biodegradation on the natural attenuation of volatile organic compound (VOC) mixtures in the vadose zone. The application of a species grouping technique is investigated in an attempt to reduce the computational cost for predicting the fate of multicomponent mixtures with a minimum loss in accuracy. We show how several difficulties in vadose zone transport modeling may be reasonably well overcome by simplifications that are supported by field data and sensitivity calculations. The estimated case-specific field-scale biodegradation values for the majority of mixture compounds were found to fall within the range of biodegradation rate constants determined from experiments performed in columns that were filled with the field soil. A sensitivity analysis showed that using the upper or lower bounds of literature values for the biodegradation constants had a negligible effect on gas concentrations at the early stages of the contamination process, when contaminant concentrations were still high. This is a very encouraging result regarding the usefulness of model calculations in risk assessment procedures.