Wadeable vs Swimmable

A useful post at The Spinoff by Jenny Webster-Brown on the clean water package. Some extracts:

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 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.

So the threshold has stayed the same, and is the standard one used internationally.

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.

So instead of a binary system, there are five grades.

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.

This part is the key. Much of the time both an A grade and C grade river will be very safe – massively under the threshold. But heavy rain (hard for Government to control) will push them over.

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.

An app listing the status for every waterway would be great.



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