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Image: Michael Bradley/Getty Images
Image: Michael Bradley/Getty Images

ScienceMarch 1, 2017

Wadeable, swimmable, indecipherable: cutting through the crap in the Nick Smith water row

Image: Michael Bradley/Getty Images
Image: Michael Bradley/Getty Images

The government’s Clean Water package quickly became bogged down in claim and counter-claim. What did it really amount to? Jenny Webster-Brown of the Waterways Centre for Freshwater Management demystifies the policy.

Last Friday, Nick Smith revealed a target to make 90% of rivers and lakes swimmable by 2040, the key outcome of the government’s proposed Clean Water Package. The environment minister’s announcement attracted accusations of simply changing the definition of “swimmable” in order to achieve this goal, thereby increasing the risk of becoming ill when recreating in a so-called swimmable river, as well as counter-accusations that such claims are based on “junk science”.

The rhetoric is clouding the debate, and is ultimately unhelpful to those who want to swim with confidence in our country’s rivers and lakes, now and in the future. So let’s look at the facts …

Is the threshold for safe swimming changing?

Not really.

The current National Policy Statement for Freshwater (NPS) requires rivers and lakes to have less than 540 E. coli/100mls to be safe for swimming. This is also the threshold proposed in the Clean Water package. This threshold corresponds to a 5% (or 1 in 20) risk of infection during full immersion in a water, which is a level of risk accepted internationally for swimming.

In the NPS there is also a lower threshold (less than 260 E. coli/100mls) recommended for the best quality river and lake waters (the “A” grade waters), but it is not the limit for a water body being considered safe for swimming.

A key criticism of the NPS was that the “National Bottom Line” for recreational use of water was set at 1000 E. coli/100ml; a less stringent threshold than is considered safe for swimming. Hence the apparently low NPS aspiration of “wadeable” rather than “swimmable” for the default water quality of our rivers and lakes.

So what is changing?

The amount of time (or number of monitoring occasions) that a river or lake swimming site must be below the threshold value.

In the NPS, 95% of the monitoring data has to be below the 540 E. coli/100 ml threshold for a water to be considered swimmable. However, under the proposed Clean Water package, a river or lake swimming site can exceed this threshold more than 5% of the time, and still be considered swimmable.

Instead, there are different grades proposed: A, in which the threshold is exceeded less than 5% of the time; B, exceeded 5-10% of the time; and C, exceeded 10-20% of the time. All of these grades are considered safe for swimming, with access to further information recommended for C. There are two lower grades: D, in which the threshold is exceeded 20-30% of the time, and E, exceeded more than 30% of the time, which are considered only intermittently safe and unsafe for swimming respectively.

While it may appear that the risk of infection could be up to four times higher in a C grade water compared to an A grade water, this is not likely to be the case. In the types of rivers and lakes most amenable to swimming, E. coli concentrations are typically only high after heavy rainfall. Swimming in a flooded or swollen river is a bad idea for many reasons other than risk of infection. At the time you choose to go for a swim in, say, a C grade (or “fair”) river water, the risk of infection may very well be exactly the same as for an A (or “excellent”) river water. However, if you were to swim every day, under all river and weather conditions, in a C grade river, your risk of infection would indeed be higher than if you did the same in an A grade river.

Hence the need for regional swimming maps and access to clear information on when and where waterways are safe for swimming and, presumably, informative signage at popular swimming sites.

A dip in the Waitangi River. Photo by Michael Bradley/Getty Images

Will this help us achieve 90% swimmable rivers by 2040?

Probably … and not simply by redefining “swimmable”.

The Clean Water package is designed to make 80% of rivers and lakes swimmable by 2030 and 90% by 2040. This does require a significant improvement in water quality, as currently only around 70% of routinely monitored rivers and lakes have less than 540 E. coli/100mls, more than 80% of the time, as required for safe swimming under the proposed package.

The proposed new system for assessing swimmability, with five different grades based on how often a water exceeds the E. coli threshold, provides a “step ladder” for water quality improvement. This appears to be a more practical and motivational approach for regulators and communities seeking safer swimming in their waterways, than the system currently embodied in the NPS. The NPS has different thresholds, and methods of determining compliance with these thresholds, for swimming and wading, and offers little incentive to achieve swimmable status. The Clean Water package also proposes better mechanisms for the public to access useful information on the swimmable status of a waterway.

Obviously, simply setting thresholds, and monitoring water quality will not create the change required. There needs to be action to reduce E. coli levels, which will simultaneously target other water quality attributes affected by runoff and sewage waste discharges, such as turbidity, phosphate and various other contaminants. Other parts of the Clean Water package, specifically the Freshwater Improvement Fund and Stock Exclusion requirements, are designed to stimulate this action. Whether this will achieve the degree of change anticipated will depend on many things, but scientific evidence indicates that these are the types of actions that lead to water quality improvement.

And finally …

It is important not to lose sight of the fact that many different factors affect how safe a water body is for swimming – not just E. coli. High or low flow conditions, the presence of toxic algae, abundant algal growths (slime) or other irritating or unpleasant organisms, and high turbidity are just a few conditions that often preclude safe swimming. These may have little in common with the degree of E. coli contamination.

The Spinoff’s science content is made possible thanks to the support of The MacDiarmid Institute for Advanced Materials and Nanotechnology, a national institute devoted to scientific research.

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