Approximately ¼ of the population of the U.S. relies on onsite wastewater treatment systems (OWTS) for disposal of domestic wastewater. Failing or improperly functioning OWTS are public health and environmental hazards because of their potential to degrade ground, surface, and coastal water quality. It has been estimated that up to 50% of the N inputs into Greenwich Bay, Rhode Island comes from septic system effluent. Comparison of an innovative OWTS using a patented leachfield aeration process (AIR) compared to a traditional leachfield (LEACH) has shown that aeration improves effluent quality, with a greater percent removal of N. Although N removal has been documented, the processes responsible have not been characterized. The main goal of this project was to characterize the fate of nitrogen (as NH₄⁺) from septic tank effluent in AIR and LEACH soil by amending it with ¹⁵NH₄Cl, and Br^-, to trace transformations of N over a 24 h period in the gas, drainage water, and soil pools. Our hypothesis was that the bulk of the ¹⁵N label would show up in the water pool as ¹⁵NO₃^-, and in the gas pool as ¹⁵N₂ and ¹⁵N₂O in the AIR treatment as a result of nitrification and subsequent denitrification. Preliminary analysis of the drainage water showed that the bulk of the inorganic N pool was composed of ¹⁵NO₃^- in the AIR treatment.  Gas isotope data indicates at least a 1000-fold ¹⁵N enrichment in the AIR treatment relative to the LEACH treatment, indicating the occurrence of denitrification in the AIR treatment.