‘Swimmability’ of New Zealand rivers

Swimming is a popular activity in Aotearoa-New Zealand. Two attributes of waters that strongly affect aesthetic quality and safety for swimming are visual clarity and faecal contamination as indicated by Escherichia coli bacteria.

It turns out that these two attributes are fairly well-correlated (inversely) in New Zealand rivers, such that (easily seen) visual clarity provides a rough-but-useful guide to (unseen) microbial quality (Davies-Colley et al. 2018). At most of the 77 sites in the National Rivers Water Quality Network (NRWQN) run by NIWA, ten years of monthly E. coli measurements were moderately correlated with visual clarity measurements by the black disc method made on water sampling visits. 

We found that, overall, NZ river waters are seldom microbially contaminated when clear (obvious exceptions include some urban waters). As a consequence, people could avoid exposing themselves to microbial hazards if they avoided river waters less than 1.6 m visibility—a long-standing NZ Ministry for the Environment (1994) guideline for swimming. We recommend that visual clarity is routinely measured as part of bathing water surveillance in NZ. Furthermore, regional councils and other agencies could protect the swimming public in ‘real’ time by continuously monitoring turbidity in river waters. The turbidity sensor would be locally calibrated to both visual clarity (to which turbidity is strongly, inversely, correlated) and E. coli (to which it is more weakly correlated). 

In on-going research we are trialling (1) use of continuous turbidity as an indicator of both visual clarity and E. coli in rivers, and (2) assessment of bathing suitability by community volunteers using SHMAK (Stream Health Monitoring and Assessment Kit) with new and upgraded capabilities.

Volunteers measuring clarity with the black disc method. [Photo: Dave Allen, NIWA]

Further information

Davies-Colley, R. J.; Valois, A.; Milne, J. M. (2018) Faecal contamination and visual clarity of New Zealand rivers. Correlation of key variables affecting swimming water quality. Journal of Water and Health 16(3). DOI: 10.2166/wh.2018.214

Contact

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Principal Scientist - Water Quality
Regional Manager - Wellington