Transformation of nitrogen compounds in a regenerated urban drainage stream in New Zealand

Abstract

ABSTRACT Haytons Stream, a low laying stormwater and groundwater-fed stream located in Christchurch, New Zealand, contains some of the highest levels of ammoniacal nitrogen and oxidised nitrogen in the country. To improve its water quality, the watercourse was reconditioned by adding retention ponds and riverbank riparian plants. The objective of this study was to assess the spatial and temporal changes in nitrogen species and concentration along the restored Haytons Stream to gain insight into the drivers of nitrogen compound transformation in urban streams. Water samples were collected from eight sampling stations along the stream in baseflow and stormflow conditions during dry and wet seasons. Results show that under baseflow, inorganic nitrogen was mostly present while under stormflow it was mainly in organic form. Overall, nitrogen concentrations increased from the upper to middle stream sections, decreasing downstream where the watercourse had been restored. The restoration of the stream was found to reduce all forms of nitrogen, except particulate organic nitrogen (PON, i.e. algae). The pond at the outlet of Haytons Stream contribute to converting dissolved inorganic nitrogen to predominantly PON, which should be removed through separation mechanisms in order to prevent PON from entering the Heathcote/Opawaho River at the Hayton’s Stream discharge.