Naturally occurring biodegradation of hydrocarbon compounds may offer a sustainable management option at contaminated sites. However, a sound understanding of contaminant mass loss rates is required to enable estimation of source zone longevity, serving to alleviate public concerns and inform decision makers. Under some conditions, surficial CO2 efflux measurements can be useful to delineate petroleum hydrocarbon containing source zones, and to provide estimates of depth-integrated vadose zone hydrocarbon degradation rates. However, the accuracy of degradation rate estimates is limited by our ability to separate CO2 effluxes associated with contaminant decomposition from those attributable to naturally occurring soil respiration. To understand CO2 sources and transport processes within the vadose zone, this work combines measurement of surficial CO2 effluxes with detailed analysis of soil gas composition– including the radiocarbon and stable isotopic composition of CO2. Quantitative reactive transport modeling allows further evaluation of controls on CO2 generation and fate, and CH4 generation and oxidation. Results confirm that, in the source zone at the Bemidji site, the majority of CO2 originates from degradation of the oil body. In addition, radiocarbon in CO2 proves particularly useful in determining the contribution of contaminant degradation to the measured CO2 efflux.