Partial nitrification enhances natural attenuation of nitrogen in a septic system plume

Abstract

Natural attenuation of nitrogen (N) was investigated in a well characterized septic system plume at a campground in Ontario, Canada. Total inorganic N (TIN) concentrations in deeper portions of the plume were about one third of the septic tank value of 40.7mgL−1. NH4+ and NO3− isotopic characterization were used to provide insight into potential attenuation processes. Concentrations of NH4+ and NO3− were highly variable in the plume, but approached the septic tank TIN value in some shallow zones and exhibited δ15N values like the tank value of +6‰. However, isotopic enrichment (up to +24‰ for NH4+ and +45‰ for NO3−) and declining TIN concentrations in the deeper zones indicated that anaerobic ammonium oxidation contributed to the TIN attenuation. The degree of isotopic enrichment increased at lower NH4+ concentrations and was consistent with Rayleigh-type distillation with an enrichment factor (Ɛ) of −5.1‰. Additionally, decreasing DOC values with depth and the concomitant enrichment of δ15NNO3 and δ18ONO3, suggested that denitrification was also active.The N attenuation observed in the Killarney plume was partly due to incomplete nitrification that occurred because of the shallow water table, which varied from only 0.2–0.7m below the tile bed infiltration pipes. Moreover, some of the monitoring locations with the shallowest water table distances from the infiltration pipes, had the highest degree of TIN attenuation (70–90%) in the plume. This behavior suggests that controlling water table distance from the infiltration pipes could be a useful mechanism for enhancing N attenuation in septic system plumes.