Nitrous oxide as an indicator of nitrogen transformation in a septic system plume

- Groundwater nitrous oxide was studied in a septic system plume in Ontario, Canada.
- High concentrations of nitrous oxide were found in the septic system plume.
- Nitrous oxide indicated active nitrification and denitrification in the plume.
- Nitrous oxide isotope ratios exhibited a very large range in the ground water.
- The isotope ratios provided qualitative information on nitrogen cycling processes.

SummaryThis study evaluates the use of ground water N2O concentration and stable isotope composition for providing insights into nitrogen cycling processes in a large septic system plume in southern Ontario, Canada. An extremely large range of dissolved N2O concentrations were measured (0.4-1071 μg N/L) that were higher than atmospheric equilibrium values of ∼0.3 μg N/L, demonstrating substantial N2O production in the subsurface. The highest N2O concentrations occurred around the periphery of a mid-depth zone where NO3− attenuation, elevated DOC concentration, and NO3− stable isotope ratios provided evidence that denitrification was occurring. Broad ranges in δ15N-N2O (−45.8‰ to +30.6‰) and δ18O-N2O (+20.4‰ to +96.0‰) were evident. Using literature isotopic enrichment factors, which differ for N2O produced during nitrification and denitrification, and measured ranges of plume NH4+ and NO3− isotopic ratios, zones of both nitrifier-derived N2O (shallow zone) and denitrifier-N2O (mid-depth and deeper zones) could be identified. Time series sampling showed that nitrifier N2O was present early in the summer season (June) but then denitrifier N2O was more dominant later in the season. In a mid-depth NO3− depleted zone, the production of denitrifier-N2O was evident early in the season when 15N and 18O enrichment of NO3− was not sufficiently advanced to be indicative of denitrification, although δ15N and δ18O values of NO3− increased later in the season. The analysis of N2O concentrations and stable isotopic composition, in conjunction with conventional chemical analyses, provides insights into N-cycling processes in the Long Point ground water septic plume. However, large ranges in the isotopic composition of N2O produced by nitrifiers and denitrifiers meant that δ15N and δ18O analysis of ground water N2O provided qualitative, rather than quantitative, information on denitrifier versus nitrifier production of N2O at this site.