SOURCE TRANSFORMATION OF NITROGEN COMPOUNDS IN HAYTON’S STREAM, A LOW LYING URBAN DRAINAGE SYSTEM IN CHRISTCHURCH

Concentrations of nitrogen compounds in Haytons Stream, an urban waterway in Christchurch New Zealand, have been reported to be elevated. 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. Numerous environmental and physical factors such as organic matter content, hydrology/hydraulics, temperature, sediment characteristics and interactions with other contaminants can cause transformation or dilution of nitrogen compounds along the stream. The aim of this project was thus to assess the sources, types, and transformation of nitrogen compounds in Haytons stream through water quality monitoring at various locations along the stream, over time and in stormflow and baseflow conditions. Our initial results have confirmed previous reports. Levels of oxidized nitrogen were found to exceed the local guidelines in 90% of samples taken while ammoniacal nitrogen in some of the samples was up to 8 times higher than local guidelines. Our results also show that for both baseflow and stormflow conditions, there is an increase of total nitrogen downstream and that the predominant nitrogen form changes with flow conditions. During baseflow, nitrogen is mostly in its inorganic form at the upper and middle parts of the stream and in organic form at the lower part of the stream; during stormflow, the majority of the nitrogen is in its organic form. Nevertheless, the total nitrogen at each site is of about the same magnitude in both conditions, except at the lower part of the stream where the total nitrogen is considerably lower. A preliminary analysis suggests that both sediments in the stream bed and street runoff water are contributing to the nitrogen load of Haytons stream. Further research will be conducted to better understand the increase of nitrogen loads originating from stream bed sediments.