Hamilton's dirty little secret: High levels of E coli recorded downstream of Hamilton

Sunday, 7 October 2018

Niwa freshwater chief scientist John Quinn believes the high E coli readings at Horotiu probably come from Hamilton.

Choose your swimming spot carefully this summer.

It's technically unsafe to swim in the Waikato River at Horotiu, yet just upstream of Hamilton at Narrows Bridge, it's fine.

Hamilton city sewage is the biggest point source discharge into the Waikato River and there are strict conditions around this consent.

High levels of faecal bacteria make downstream of Hamilton unsafe and the city's to blame, a Niwa expert says.

A number of high E coli readings have been recorded just north of Hamilton and the regional council has no idea whether it's cows, birds or humans causing them.

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Tracking down the source is time-consuming and costly.

It takes five days to test the water and the only way to tell E coli's origin is to use faecal source tracking, Waikato Regional Council senior scientist Eloise Ryan said.

That's too expensive to undertake as part of the council's routine monitoring. The council will use it if there's a trend seen or increasing E coli contamination at a site that's regularly used for swimming.

The council sets a conservative threshold when it comes to red-flagging its 10 monitoring sites and it takes only five bad samples to deem an area unswimmable.

Five of the 100 samples tested throughout the year at Horotiu were found to have E coli levels higher than 550 parts per 100ml, which was enough for the council to red-flag swimming in that area, Ryan said.

'We're very cautious,' she said.

With good reason. E coli is a bacteria commonly found in the gut of warm-blooded animals - that includes people - and if ingested via contaminated food or water, it can cause severe stomach cramps, mild to severe diarrhoea and vomiting.

However, council's monitoring site upstream of Hamilton at Narrows Bridge is perfectly safe to swim in. Furthermore, it's safe at every other monitoring site upstream from there.

Hamilton straddles the river between Narrows and Horotiu, but Ryan insisted the Horotiu sampling site is largely unaffected by urban pollution.

'On average, Narrows and Horotiu have the same bacteria, but Horotiu has at least five out of 100 samples with high E coli.

'Occasionally at Horotiu, you might get a spike and we haven't seen that at Narrows [Bridge].'

Instead of human faeces, Ryan believes the most likely scenario for the spikes is heavy rain occurring at Horotiu just before the samples were taken, where bacteria would then travel in the rainwater off the land and into the river.

'There is also the possibility of high numbers of birds using an area above the sampling site, as well as accidental or diffuse discharges.'

Niwa chief scientist for freshwater and estuaries John Quinn said Hamilton is the most likely source of the E coli.

It is not surprising there are E coli spikes at Horotiu since all of the city's stormwater pipes empty into the river, he said.

'It's the Hamilton city stormwater discharges - there's a whole lot of stormwater pipes going into the river and there will be dog turds on the streets [and] there may be the odd leaky septic line that seeps into the stormwater system.

'The fact that out of 100 samples, five are over that 550 threshold, that's probably just due to rain events.'

However, Ryan said discharges from Hamilton make little difference to the river's overall water quality.

'It's such a big beast of a river that it only has a small impact because it's so huge,' Ryan said.

Within Hamilton, city sewage was the biggest point source discharge into the river and there were strict conditions around its consent.

'They actually almost discharge water that's cleaner than the Waikato River,' Ryan said.

Other point source consent holders, such as the Affco meat processing plant near Horotiu, have a similar negligible effect, she said.

'Again, the impact on the Waikato River is so small, we are talking a few per cent. Between Narrows and Huntly, there's Hamilton sewage, Ngāruawāhia sewage and a meatworks and they account for only 5 per cent of the total bacteria found in the river.'

This showed most of the pollution found in the river comes from non-point sources, such as rural runoff.

Once it passed by the city, the major source of contamination for the river was when the Waipā River merged into the Waikato River near Ngāruawāhia.

The Waipā carries with it loads of sediment and bacteria brought about mostly from agriculture, Ryan said.

'If it wasn't for the Waipā, the Waikato River would be in pretty good shape.

'With point sources, not so much. It really is runoff from agricultural land that affects water quality on the Waikato River.'

*An earlier version of this story incorrectly attributed quotes to the Hamilton City Council instead of the Waikato Regional Council.