At many sites contaminated with petroleum hydrocarbons, methanogenesis is a significant degradation pathway. Techniques to estimate CH₄ production, consumption, and transport processes are needed to understand the geochemical system, provide a complete carbon mass balance, and quantify the hydrocarbon degradation rate. Dissolved and vapor-phase gas data collected at a petroleum hydrocarbon contaminated site near Bemidji, Minnesota, demonstrate that naturally occurring nonreactive or relatively inert gases such as Ar and N₂ can be effectively used to better understand and quantify physical and chemical processes related to methanogenic activity in the subsurface. In the vadose zone, regions of Ar and N₂ depletion and enrichment are indicative of methanogenic and methanotrophic zones, and concentration gradients between the regions suggest that reaction-induced advection can be an important gas transport process. In the saturated zone, dissolved Ar and N₂ concentrations are used to quantify degassing driven by methanogenesis and also suggest that attenuation of methane along the flow path, into the downgradient aquifer, is largely controlled by physical processes. Slight but discernable preferential depletion of N₂ over Ar, in both the saturated and unsaturated zones near the free-phase oil, suggests reactivity of N₂ and is consistent with other evidence indicating that nitrogen fixation by microbial activity is taking place at this site.