Nitrogen Attenuation in Septic System Plumes

The persistence of inorganic nitrogen is assessed in a set of 21 septic system plumes located in Ontario, Canada, that were studied over a 31-year period from 1988 to 2019. In the plume zones underlying the drainfields, site mean NO₃⁻ values averaged 34 ± 27 mg N/L and exceeded the nitrate drinking water limit (DWL) of 10 mg N/L at 16 of 21 sites. In plume zones extending up to 30 m downgradient from the drainfields, site mean NO₃⁻ values averaged 24 ± 20 mg N/L and exceeded the DWL at 9 of 13 sites. Site mean total inorganic nitrogen (TIN; NH₄⁺ + NO₃⁻ − N) removal averaged 34 ± 26% in the drainfield zones and 36 ± 44% in the downgradient plume zones, indicating that much of the removal occurred within the drainfields. Removal was much higher at nine sites where drainfield TIN included >10% NH₄⁺ (62 ± 25% removal). TIN removal was not correlated with wastewater loading rate, system age, or sediment carbonate mineral content, but was correlated with water table depth, where shallower water table sites had generally less complete wastewater oxidation. At many of these sites, both NO₃⁻ and NH₄⁺ were present together in the plumes and were lost concomitantly, suggesting that the anammox reaction was making an important contribution to the observed TIN loss. When groundwater nitrate contamination is a concern, considering on-site treatment system designs that lead to a lesser degree of wastewater oxidation, could be a useful approach for enhancing N removal.