WATER TESTING IN THE ESTUARY

As most people are probably aware, Auckland Council regularly tests water quality at many Auckland beaches. Why then, is Point Wells not included?

Well, in fact it is: I chanced across a couple of people taking a water sample at the boat ramp as I was emerging from a swim in late March, and they were from the Council. In discussion with them and subsequently with Council’s Healthy Waters department.

I discovered that they have been testing at Point Wells since early 2019. The programme was suspended in 2020 and 21, largely due to Covid. It seems they are back on track now, having tested ten times in 2022 and three times this year to the end of March.

They have completed 39 tests in total since their 2019 start, all at the boat ramp and none of these tests has resulted in what they term an “exceedance”, jargon for a level of enterococci above which it is considered unsafe to swim. This is an excellent result and is not inconsistent with the E coli measurements recorded in our programme of boat ramp sampling.

I should explain that enterococci and E coli are different bacteria which both reside in the gut of mammals and birds. While not themselves pathogens, they are indicative of the presence of faecal pathogens in the water. Enterococci is generally the preferred bacterium to measure in sea water samples and E coli in fresh water.

My reading of the literature and our own sampling experience seems to confirm that the presence and quantity of both bacteria tends to be closely correlated. For this reason, it seems that measurement of either bacterium will give accurate readings in the slightly less salty water of an estuary.

Does the fact that the Council is sampling at Point Wells mean we can cease our programme? Unfortunately, no. The information the Council provides for Auckland beaches on their Safeswim website is based upon numerous measurements which go back many years.

The daily recommendations they provide are derived from modelled data rather than actual data: as you can imagine, they don’t have the resources to turn up at every beach every day to take and analyse water samples, so they create a mathematical model for each beach based upon this large backlog of actual rainfall and actual enterococci measurements. When the database for each beach is large enough this enables accurate prediction of what level of enterococci will be present on any particular day.

The other condition which is necessary to enable predictions is that there must be “exceedances” in some of the samples for each beach, because without instances of high levels of enterococci they don’t have a basis on which to construct their model. As an aside, we can say that at this point, the boat ramp at Point Wells is a victim of its own relatively clean condition!

So long as the PWCRA is prepared to fund our activities we shall continue sampling at the boat ramp and the other locations further upriver which we have traditionally sampled. We are also happy to consider suggestions for other places to sample if there are swimming spots not currently tested or where you think there may be sources of contamination coming into the river.

Because our sampling programme is “point-in-time” rather than ongoing, we reiterate our advice about not swimming for two to three days after heavy rain, and swimming on an oncoming or full tide rather than when it is strongly ebbing.

And we would note that the Council data confirms our own findings, that the boat ramp is consistently the safest spot for swimming.

Robin Ransom May 2023