Haytons Stream, a headwater tributary of the Heathcote River/ Ōpāwaho, is consistently ranked with the worst water quality of all annually-monitored Christchurch city waterways, and has been reported to be a significant source of contaminants to the downstream Heathcote River (Margetts and Marshall, 2016; Marshall and Burrell, 2017). Previous investigations have shown that the stream is elevated in a number of contaminants (particularly trace elements, ammoniacal-nitrogen and dissolved reactive phosphorous). Haytons Stream drains a dense industrial land use catchment in Christchurch’s Hornby and Wigram suburbs, with poor instream water quality thought to be a result of this heavily urbanised land use (commonly known as the ‘urban stream syndrome’). However, previous studies have reported evidence of point-source discharges to the stream, suggesting that there are illicit discharges of contaminants from industry in the catchment. The sources of contaminants throughout Haytons Stream are still unclear, and the mechanisms for how and where contaminants are transported/discharged to the stream unknown.

This study adopted a multifaceted approach to understanding contaminants throughout Haytons Stream. Stream bed sediment was characterised for pseudo-total and labile/ bioavailable trace elements across the catchment, accompanied with three baseflow and three stormflow events of the stream water column for trace element, nutrient, suspended solid and E. coli concentrations. Potential subsurface pollution was monitored via continuous data logging of water level and conductivity trend in the catchment, to determine the occurrence of any dry-weather fluctuation in water level indicative of a discharge. Stormwater sumps on selected industrial sites were sampled, to gain a snapshot of potential industrial pollution that may contaminate Haytons Stream.

The bed sediment contained elevated concentrations of both pseudo-total and labile trace elements throughout the length of the stream, with particularly high concentrations of Zn across the catchment. These findings suggest that the bed sediment may be leaching metals back into the water column, and may be a key factor resulting in the very poor to non-existent instream ecosystem in Haytons Stream. Remediation, in the form of dredging, may be necessary to improve long-term water and ecosystem quality in the stream.

Instream sampling revealed that total and dissolved trace element concentrations were elevated during dry and wet weather, with several metals commonly exceeding ANZECC 80% species protection guidelines during baseflow. Upper catchment sites (within industrial land) generally had higher metal concentrations than lower catchment sites (re-landscaped/ pastoral land use), suggesting that the industrial land use is a major source of metals to the stream at all times. The Wigram Retention Basin in the lower catchment did not remove or decrease metal concentrations appreciably during baseflow, with metal concentration exports out of the basin common, leading to Haytons Stream polluting the main stem of the Heathcote River with elevated trace element concentrations. Ammoniacal-N and dissolved reactive phosphorous concentrations spiked at the mid catchment site of Washbournes Road during dry and wet weather, suggesting a specific source of nutrients near this site. Wet weather sampling at this site in particular resulted in nutrient concentrations far in excess of ANZECC trigger values.

Continuous logging of water level revealed the frequent inappropriate discharge of contaminants to the stormwater network at Waterloo Road and Washbournes Road, represented by fluctuations (rises) in stream water level in absence of rainfall. At Washbournes Road, discharges observed from stormwater pipes were soapy or opaque-coloured, and can be tracked back up specific pipes to potentially responsible businesses. Discharges at Waterloo Road where the stream originates were frequently associated with hydrocarbon slicks, suggesting automotive or similar-based industries may be commonly (yet not solely) responsible for part of these discharges. As the stream emerges from the stormwater network at Waterloo Road, discharges could not be tracked further upstream. Therefore the upper catchment should be intensively targeted for further industrial pollution prevention and education, to avoid further industrial discharges to Haytons Stream via the stormwater network.

Conductivity logging revealed frequent wet weather pollution of nutrients (and subsequently conductivity trend spikes) at the mid catchment site of Washbournes Road. Small to moderate-intensity rainfall events produced large spikes in conductivity at this site (as well as elevated instream nutrient concentrations) which were not seen at the upper or lower catchment sites. This was corroborated with previous studies where a specific source(s) of nutrients in the mid catchment was identified, which was responsible for generating poor quality stormwater that eventually contaminates Haytons Stream (Brown et al., 1996).

Future research for pollution prevention in Haytons Stream or the wider Heathcote River would benefit from tracking of these mid catchment nutrients up the stormwater network to a specific source, and further understanding of the bioavailability of metals in sediment. Further research is also needed to understand the processes of absorption and release of contaminants from stream bed sediments in Haytons Stream or similar waterways. Greater implementation and uptake of at-source stormwater treatment (particularly for trace elements) would greatly benefit the water and sediment quality in the catchment, and is likely needed to mitigate the continual trace element pollution from the industrial land use. Expansion out of this catchment to conduct similar research on the neighbouring Curletts Stream (which has similar contaminant concentrations with a similar impact on the Heathcote River) would benefit the wider Heathcote River environment, whilst a continued strong pollution prevention and educational presence is recommended for both catchments.