Natural attenuation of septic system nitrogen. A case study in Killarney Provincial Park, Ontario, Canada.

Nutrient concentrations in groundwater have been monitored down gradient from an onsite sewage treatment and disposal system on a campsite located at the Killarney Provincial Park (Canada, On). During July and August, the study site serves both local and visitors in 142 campsites, receiving significant wastewater loading up to 25,000 L/day. The objective of this study is to assess the processes involved in nitrogen attenuation in a well-characterized septic system plume. Isotopic and microbiological tools have been used to evaluate the role of nitrification, denitrification and anammox on the attenuation of nitrate and ammonium along the groundwater flow system. Results showed chloride (~32 mgL-1) remarkably uniform across the entire transect, suggesting that the septic system plume is composed essentially of straight sewage, excluding the effect of dilution derived by precipitation. Ammonium content of the septic tank effluent is relatively high but it is then attenuated at the bottom of the plume, in the most distal monitoring location. Conversely, nitrate at high concentration is most present on the upper fringes of the plume, where dissolved oxygen diffusion from the water table zone promotes nitrification. Therefore, nitrogen compounds are variably distributed along the transect with high (57.5 mgL-1 N-NO3-, 40.5 mgL-1 N-NH4+) near the source and low (<1 mgL-1 for N-NO3- and N-NH4+) concentrations in the distal part of the plume. Nitrate isotope signatures (15N-N-NO3-, 18O-N-NO3-) suggest that multiple transformation processes take place at the study site: (i) nitrification in the unsaturated zone and shallow part of the plume, where high nitrate concentration is accompanied by depleted δ15N-NO3- (57.5 mgL-1 and -3.2‰, respectively) and (ii) denitrification along the plume where nitrate concentration decreases (<1 mgL-1 N-NO3-) correlated with an enrichment trend for δ15N-NO3- and δ18O-NO3- (up to +44.8‰ and +26.8‰, respectively). There is a possibility that anammox is involved in the attenuation of nitrate and ammonium. The role of this process is being evaluated using microbial analysis, specifically bacterial community composition (DNA-based analysis), and δ15N data in ammonium. This study has shown that ammonium associated to high sewage loads can be present in septic system plume. Nitrification, denitrification and anammox can be all involved in nitrogen transformation and attenuation in these complex septic plumes.