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

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 δ¹⁵N_NO3 and δ¹⁸O_NO3, 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.