Sources and transformation of nitrogen compounds in Haytons Stream, a low lying urban drainage stream in Christchurch, New Zealand
Thesis DisciplineEnvironmental Engineering
Degree GrantorUniversity of Canterbury
Degree NameMaster of Engineering
Haytons Stream is an urban watercourse that drains Christchurch’s Wigram suburb and discharges into the Heathcote/Opawaho River. Previous studies have shown levels of ammoniacal nitrogen (NH4-N) and oxidized nitrogen (NOX-N) in Haytons Stream to be one of the highest among Christchurch City waterways, specifically exceeding local water guidelines for toxic concentration of NH4-N (ECan, 2007; Moores et al., 2009). High levels of ammonia and/or related nitrogen compounds can be toxic to aquatic organisms and can have a significant effect on the stream’s ecological health. The contamination in Haytons stream is symptomatic of other urban streams around the country and there is thus a need to understand the principal sources and composition of urban nitrogen contamination, the fate and transport of nitrogen compounds and their interaction with the stream.
Nitrogen compounds in Haytons Stream were thus characterized through spatial and temporal field sampling in order to better understand their sources, their relationship with the mainly industrial land use and to identify stormwater management practices to deal with these types of pollutants. Eight sites along the stream were sampled under storm and baseflow conditions during the dry and wet season.
Results from sampling showed that nitrogen compounds varied in location and time along the stream. Point discharges of NH4-N and dissolved organic nitrogen (DON) occurred at the upper and middle part of the stream. NH4-N was found to reach toxic levels mainly in the middle part of Haytons Stream.
Five of the highest total nitrogen concentrations found at Haytons Stream had most nitrogen compounds in dissolved form (NH4-N, DON and NOX-N). Levels of NOX-N were found to exceed the local guidelines in 90% of samples while ammoniacal nitrogen in some of the samples was found to be up to 8 times higher than local guidelines.
The predominant nitrogen form changes with flow conditions. During baseflow, nitrogen is mostly in its inorganic form at the upper and middle parts of Haytons and in organic form at the lower part; during stormflow, the majority of the nitrogen is in its organic form in all sections of the stream.
Nitrogen dynamics are affected by season. An increase in temperatures and sun incidence during the dry season (i.e. summer) increases biological processes rates, incrementing ammonification, nitrification and other processes that modify the nitrogen dynamics along Haytons Stream. During the wet season (i.e. winter), rainfall dilutes the stream water, decreasing nitrogen compounds concentrations.
Overall, results showed that nitrogen concentrations increased from the upper part to the middle part of Haytons Stream and decreased from the middle to the lower part. Retention ponds along the stream together with the recently planted riparian zone and a wetland at the lower part of the catchment were found to help in the reduction of all forms of nitrogen, except particulate organic nitrogen (PON).
It can be concluded that stormwater management along Haytons stream should be focused on the dissolved forms of nitrogen. The wetland/ponds at the outlet of Haytons Stream does a good job of converting nitrogen to a predominantly particulate form of nitrogen (PON, i.e. algae), which could be removed through filtering or other physical treatment means. Further research is necessary to identify and stop point discharges at Haytons Stream and evaluate methods to mitigate PON entering Heathcote/Opawaho River.