Natural Attenuation of Septic System Nitrogen by Anammox

On-site disposal of sewage in septic systems can lead to groundwater plumes with NO3-N concentrations exceeding the common drinking water limit of 10 mg/L. Currently, denitrification is considered as the principal natural attenuation process. However, at a large seasonal-use septic system in Ontario (256 campsites), a suboxic zone exists where nitrogen removal of up to 80% occurs including removal of NH4+N. This zone has both NO3-N and NH4+N at >5 mg/L each. In the distal NH4+-rich zone, NH4+N concentrations (8.1 +/- 8.0 mg/L) are lower than in the proximal zone (48 +/- 36 mg/L) and NH4+N is isotopically enriched (concentration-weighted mean d15N of +15.7 parts per thousand) compared to the proximal zone (+7.8 parts per thousand). Furthermore, d15NNH4+ isotopic enrichment increases with depth in the distal zone, which is opposite to what would result if nitrification along the water table zone was the mechanism causing NH4+ depletion. Bacterial community composition was assessed with molecular (DNA-based) analysis and demonstrated that groundwater bacterial populations were predominantly composed of bacteria from two Candidatus genera of the Planctomycetales (Brocadia and Jettenia). Together, these data provide strong evidence that anaerobic ammonium oxidation (anammox) plays an important role in nitrogen attenuation at this site.