Suspended sediment and faecal contamination in a stormflow plume from the Hutt River in Wellington Harbour, New Zealand

ABSTRACT

Muddy, faecally-contaminated river flood plumes in coastal waters are a hazard to contact recreation and bivalve shellfish consumption but are difficult to study, being episodic and transient. We used a new underway flow-through sampler in a small, fast boat, to map a flood plume within Wellington Harbour, while simultaneously sampling water in the Hutt River inflow. Faecal contamination (indexed by E.coli) correlated with flow, salinity, coloured dissolved organic matter, total suspended solids (TSS) and water clarity (light beam attenuation and visual clarity). The freshwater content of the plume agreed well with time-integrated river discharge. Despite the relatively short time-scale (<12 h) of the event, a 21% loss of TSS (particle flocculation and settling), and 30% loss in E. coli (suggesting some die-off) occurred in the plume compared to river loads. E. coli relative to TSS varied up to two orders of magnitude over a year of river flood sampling. A rapid survey of plumes combined with long-term river observations is expected to augment monitoring and inform the extension of remote sensing and modelling efforts to faecal contamination of New Zealand coastal waters.

KEYWORDS:

• Water quality
• water clarity
• pollutants
• flood event
• estuary
• E. coli
• total suspended solids
• coloured dissolved organic matter
• light beam attenuation

Acknowledgments

Pete Notman (NIWA-Wellington) skippered RV Rukawai for the plume survey on 24 September 2019, with Ryan Evison and Patrick Butler (both of NIWA-Wellington) providing the crew and assisting MG in the field. Ryan and Patrick are also thanked for operating a water quality site on the Hutt River at Melling Bridge – including an auto-sampler and a sonde programmed and calibrated by Stephan Heubeck (NIWA-Hamilton) and installed by Gary DeRose (then of NIWA-Wellington). The authors also thank Andrew Swales (Catchments to Estuaries Portfolio leader, NIWA-Hamilton) for organising funding for this work and Manawa Huirama for review comments. Special thanks to Mark Heath (then of GWRC) for supporting the extension of the Hutt River monitoring, and Wendy Purdon (GWRC) for collection of river water samples between August and December 2019.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

Manuscript preparation was funded by SSIF project FWCE2201, ‘Microbial pollution of estuaries’ (led by RD-C) with a contribution from FWWQ2206, ‘Nowcasting of recreational water quality’ (led by RS). Experimental work was funded by several projects through NIWA’s Freshwater and Estuaries Centre via the Strategic Science Investment Fund (SSIF) from Ministry of Business Innovation and Employment (MBIE).