The oil sands industry in Northern Alberta produces large volumes of oil sands process water (OSPW) containing high concentrations of persistent naphthenic acids (NAs; CnH2n+ZO2). Due to the growing volumes of OSPW that need to be reclaimed, it is important to understand the fate of NAs in aquatic systems. A recent laboratory study revealed several potential markers of microbial biodegradation for NAs; thus here we examined for these signatures in field-aged OSPW on the site of Syncrude Canada Ltd. (Fort McMurray, AB). NA concentrations were lower in older OSPW; however parent NA signatures were remarkably similar among all OSPW samples examined, with no discernible enrichment of the highly cyclic fraction as was observed in the laboratory. Comparison of NA signatures in fresh oil sands ore extracts to OSPW in active settling basins, however, suggested that the least cyclic fraction (i.e. Z = 0 and Z = −2 homologues) may undergo relatively rapid biodegradation in active settling basins. Further evidence for biodegradation of NAs came from a significantly higher proportion of oxidized NAs (i.e. CnH2n+ZO3 + CnH2n+ZO4) in the oldest OSPW from experimental reclamation ponds. Taken together, there is indirect evidence for rapid biodegradation of relatively labile Z = 0 and Z = −2 NAs in active settling basins, but the remaining steady-state fraction of NAs in OSPW appear to be very recalcitrant, with half-lives on the order of 12.8–13.6 years. Alternative fate mechanisms to explain the slow disappearance of parent NAs from OSPW are discussed, including adsorption and atmospheric partitioning.