NIWA Technical Background report for MfE “Clean Water” Swimmability Proposals for Rivers

NIWA provides technical background information on "Clean Water" swimmability proposals.

When the Government proposed amendments to the National Policy Statement for Freshwater Management (“the Clean Water proposals”) there was considerable confusion and public debate about what the swimmable rivers component related to E. coli actually meant. That confusion also occurred within the freshwater science community. We therefore decided to prepare a technical background paper to inform ourselves and our colleagues in other organisations so that we might all be able to make fact-based submissions on the proposals. 

Who did the report?

The report was written by NIWA Principal Scientist – Water Quality, Aquatic Pollution Graham McBride and Jeff Soller, Principal Scientist of US firm Soller Environmental, who are internationally regarded experts in the field of microbiological water quality risk analysis. It was peer-reviewed internally by five senior NIWA scientists and externally by Jim Graham, Principal Environmental Scientist, OPUS International.

Does the report support the proposed approach?

The report’s authors found the overall concept proposed in Clean Water of using an approach that considers how often a water body exceeds certain E. coli thresholds in determining a site’s relative suitability for swimming is technically sound.

Will the proposed approach improve swimmability?

The proposed approach provides an incentive to improve microbiological water quality in rivers. This is because while 97% of the 792 sites we have data for comply with the current National Bottom Line for boating/wading (secondary contact), only 43% of those sites would comply with the proposed swimming (primary contact) standard. 

How safe will the new standards make it?

All the proposed grades for swimmability require that the median E. coli concentration be less than 130 per 100 ml and thus the health risk is practically zero for at least half the time. The average infection risk faced by a random swimmer on a random day in waters that just meet the swimming threshold [bottom of ‘C’ /’Fair’/yellow grade] is no more than 3.1%, and it is lower than that for blue (‘A’/’Excellent’) and green (‘B’/’Good’) grades.

Are the proposed new standards tougher?

Yes and no. The Clean Water swimming threshold is more restrictive than the current secondary (boating/wading) National Bottom Line in the 2014 National Policy Statement (NPS) with an E. coli median of 130/100ml vs 1000/100ml. But, it is not as restrictive as what was deemed the ‘minimum acceptable state’ for swimming in the NPS. That said, the NPS ‘minimum acceptable state’ for swimming set the bar very high (equivalent to ‘Excellent’ in the EU system).

What’s the difference between grading and surveillance?

Grading assesses the general suitability of a site for swimming on a long-term basis, while surveillance assesses the suitability of a site for swimming in the short-term (i.e., is it ok to swim today?). A recreational site may receive an ‘A’ grade (excellent long term quality) but may not be suitable for swimming during specific circumstances, such as unusual contamination.

How does the proposed thresholds compare with overseas?

They are about middle of the road. The proposed swimming threshold is more restrictive than the EU threshold for swimming and appears to be more permissive than the US threshold – but care needs to be taken when making the comparisons because of the differences in sampling methods.

Download the report:

Technical Background for 2017 MfE 'Clean Water' Swimmability Proposals for Rivers [NIWA Report] [PDF 1.3 MB]