Polycyclic aromatic hydrocarbons (PAH) were used as probes to identify the processes controlling the transport and fate of aqueous semivolatile hydrocarbons (SVHCs) in a petroleum-contaminated aquifer near Bemidji, Minnesota. PAH and other SVHCs were isolated from ground water by field solid-phase extraction and analyzed using gas chromatography/mass spectrometry. Close to the oil body, aqueous aliphatic hydrocarbon compositions are substantially different from the parent oil, suggesting microbial alteration prior to or during dissolution. Aqueous PAH concentrations are elevated above oil-water equilibrium concentrations directly beneath the oil and decrease dramatically at distances ranging from the 25 to 65 m downgradient from the leading edge of the oil body. Variations in downgradient distributions of naphthalene, fluorene and phenanthrene, coupled with their biodegradation, partitioning and volatility characteristics, suggest that the PAH are useful probes for distinguishing between the biogeochemical processes affecting SVHC transport and persistence in ground water.