Raising the bar for swimmable rivers


The government has released the ‘Clean Water’ package of proposed reforms, aimed at making more of our rivers swimmable. But how is ‘swimmable’ to be measured, and do these measures stack up? NIWA's Chief Executive - John Morgan explains...

When the proposals were released there was considerable confusion about the swimmable rivers component related to E. coli. Much of this confusion was centred around the proposed new way of grading microbiological water quality for swimming, and whether this would expose swimmers to greater health risk or not.

Given this confusion, and to help people make informed submissions on the proposals, NIWA prepared a report that independently analysed the proposal’s technical merits.

NIWA has about 200 staff with expertise on freshwater, including a well-recognised world expert in this specialist area, Graham McBride. Graham and a US colleague with complementary expertise, Jeff Soller, were the ideal people to do this analysis.

Their technical report, released in May this year (see below), backed the government proposal to grade the suitability of rivers for swimming on the basis of how often they exceed E. coli thresholds.

Clean Water proposals would clearly raise the bar

They found that the Clean Water proposals would clearly raise the bar from the current minimum requirement (for wading quality) of a median E. coli less than 1000 per 100 millilitres to one of less than 130 per 100 millilitres to meet the proposed swimmable waters requirement.

In addition to meeting this median requirement, the Clean Water proposals introduce a grading system that determines a river’s suitability for swimming, based on the proportion of time it is beneath E. coli thresholds that equate to various risks of infection. For example, the minimum threshold for swimming requires that for 80% of the time E. coli concentrations are below 540 per 100 millilitres. This means that for a random swimmer on a random day the infection risk is no more than 3.1%.

"Moving from the old wading water quality to the new swimming quality raises the bar significantly"

The report’s analysis shows that under the provisions of the current National Policy Statement for Freshwater Management, 97% of measured sites comply with the minimum standard. However, under the Clean Water swimmable proposals only 43% of these sites would comply, and significant improvements would be required to raise this to the government target for 90% of rivers and lakes to be swimmable by 2040.

Is the proposed swimming quality bar set at the right place?

So, moving from the old wading water quality to the new swimming quality raises the bar significantly, but is the proposed swimming quality bar set at the right place?

In a comparison of different approaches to defining swimmable waters, the report concludes that the Clean Water proposals fall in the middle of approaches taken overseas. The Clean Water proposals are more stringent than the European approach, but less stringent than the United States (although there are caveats to these comparisons because of the different sampling and statistical approaches taken).

The report also compared the stringency of the Clean Water swimmable threshold with the equivalent in the current National Policy Statement (while recognising that the NPS does not actually have swimmable waters as a minimum requirement). This comparison shows that the Clean Water proposal is less stringent, with 43% of sites meeting its swimmable threshold versus 30% using the NPS swimming threshold. But then the NPS swimming threshold is set very high – equivalent to ‘Excellent’ in the EU system.

The report points out that Clean Water also proposes that when a single sample of E. coli is above a trigger threshold then authorities must alert the public and continue daily sampling until E. coli returns to acceptable levels. This requirement recognises that even in waters that are of high quality most of the time there may be short periods when E. coli levels are elevated and they are unsuitable for swimming.

Graham McBride and Jeff Soller’s report was influential in shaping NIWA’s submission on Clean Water, and it has helped many others as well. From my perspective, science-informed policy is always good policy.

John Morgan is Chief Executive of NIWA

Download the report:

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

[This feature appeared in Water & Atmosphere 18]

Research subject: Water Quality