In situ denitrification and DNRA rates in groundwater beneath an integrated constructed wetland

Abstract

Evaluation of the environmental benefits of constructed wetlands (CWs) requires an understanding of their impacts on the groundwater quality under the wetlands. Empirical mass-balance (nitrogen in/nitrogen out) approaches for estimating nitrogen (N) removal in CWs do not characterise the final fate of N; where nitrate (NO3−-N) could be reduced to either ammonium (NH4+-N) or N2 with the potential for significant production of N2O. Herein, in situ denitrification and DNRA (dissimilatory nitrate reduction to ammonium) rates were measured in groundwater beneath cells of an earthen lined integrated constructed wetland (ICW, used to remove the nutrients from municipal wastewater) using the 15N-enriched NO3−-N push-pull method. Experiments were conducted utilising replicated (n = 3) shallow (1 m depth) and deep (4 m depth) piezometers installed along two control planes. These control planes allowed for the assessment of groundwater underlying high (Cell 2, septic tank waste) and low (Cell 3) load cells of the ICW. Background piezometers were also installed off-site. Results showed that denitrification (N2O-N + N2-N) and DNRA were major NO3−-N consumption processes accounting together for 54–79% of the total biochemical consumption of the applied NO3−-N. Of which 14–16% and 40–63% were consumed by denitrification and DNRA, respectively. Both processes differed significantly across ICW cells indicating that N transformation depends on nutrient loading rates and were significantly higher in shallow compared to the deep groundwater. In such a reduced environment (low dissolved oxygen and low redox potential), higher DNRA over the denitrification rate can be attributed to the high C concentration and high TC/NO3−-N ratio. Low pH (6.5–7.1) in this system might have limited denitrification to some extent to an incomplete state, evidenced by a high N2O-N/(N2O-N+N2-N) ratio (0.35 ± 0.17, SE). A relatively higher N2O-N/(N2O-N+N2-N) ratio and higher DNRA rate over denitrification, suggest that the end products of N transformations are reactive. This N2O can be consumed to N2 and/or emitted to the atmosphere. The DNRA rate and accumulation of NH4+-N indicated that the ICW created a suitable groundwater biogeochemical environment that enhanced NO3−-N reduction to NH4+-N. This study showed that CWs significantly influence NO3−-N attenuation to reactive forms of N in the groundwater beneath them and that solely focusing on within wetland NO3−-N attenuation can underestimate the environmental benefits of wetlands.