Defin­ing swimmable rivers

The Tribune (NZ) - - NEWS - RACHEL KEEDWELL

What is a swimmable river? Is it a river that has a low risk of mak­ing swim­mers sick? Or is it how pleas­ant it is to swim in?

A muddy brown river is not at­trac­tive for swim­ming, nor is get­ting cov­ered in swathes of green al­gae.

The most widely used def­i­ni­tion of what is con­sid­ered swimmable is based around the risk of get­ting sick, as de­fined in the Min­istry of Health guide­lines.

These guide­lines are based on lev­els of E. coli – a fae­cal co­l­iform bac­te­ria that can be eas­ily mea­sured and is a good in­di­ca­tor of whether more serious pathogens are present.

These guide­lines are broadly de­scribed us­ing a traf­fic light sys­tem where green is con­sid­ered swimmable and red is un­safe for swim­ming. Green or ac­cept­able has less than 260 units of E. coli per 100ml; am­ber or alert, 261-550 units; and red or ac­tion has 550 units.

To put this into per­spec­tive, drink­ing wa­ter stan­dards are a whole lot tougher and re­quire the E. coli count to be less than one unit of E. coli per 100ml.

So how do these Min­istry of Health guide­lines com­pare with the re­cently in­tro­duced Na­tional Pol­icy State­ment on fresh­wa­ter man­age­ment? These stan­dards set ‘bot­tom lines’ for rivers and the re­cent de­bate has cen­tred on whether set­ting wade­able as the bot­tom line is ac­cept­able.

The stan­dards are di­vided into bands A,B, C and D.

The bot­tom line is 1000 E. coli per 100ml or the up­per limit of band C, which is de­scribed as low risk for wad­ing with an E. coli count be­tween 540 and 1000.

This means that wa­ter in B and C is classed as ac­cept­able un­der the new stan­dards and can be waded in with lit­tle risk, but it is not con­sid­ered safe to swim in with­out sig­nif­i­cant risk of ill­ness. And this is the essence of the wade­able ver­sus swimmable de­bate.

It is likely there will be a fur­ther round of con­sul­ta­tion about whether this stan­dard is ac­cept­able.

The choice is ei­ther to set a low stan­dard and ac­cept that poor qual­ity wa­ter is here to stay, or set the bar higher and make swimmable a pri­or­ity.

SUPPLIED

Hori­zons Re­gional Coun­cil signs of­fer guide­lines for safe swim­ming in lo­cal rivers.

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