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Reefton town water supply will need a shot of disinfectant once a week until the end of summer to keep it safe.
But the ageing system will also need big money spent on it, sooner rather than later, Buller District Council staff and specialists hired to investigate the town's water woes concluded.
The council's three waters co-ordinator Mark Wiechern updated the Inangahua Community Board meeting on Tuesday on efforts to solve a recurring problem of high coliform levels and e.coli contamination in the Reefton reservoir.
The council took the reservoir out of commission in September and installed tanks as a temporary fix.
Mr Wiechern said that had seen a vast improvement in overall bacteria levels, but the water was still not testing 100% clear.
But it could also be coming from the reticulation system — the old network of pipes that supply water from the reservoir to Reefton homes and businesses, he said.
"The water is pure when it leaves the treatment plant but it's pumped back through part of the reticulation system to get it up to the reservoir. So somewhere along the line, it's picking up these bacteria."
Another factor could be the roof over the reservoir, installed in 2011.
"It seemed like a good idea at the time, but in reality it's created a hot, humid environment and a bug paradise."
The Canterbury District Health Board had been pushing for chlorination of the Reefton supply for public health reasons, and its water regulators were watching the Reefton situation closely, Mr Wiechern said.
"Since the Havelock North episode, where 38% of the population (5200 people) were crook because of e.coli in the water, the Government's requiring us to test for total coliform levels — not just the nasties — because they can show a potential problem."
In December, as holiday-makers arrived, water demand began to exceed supply from the tanks and the council brought the reservoir back into service.
But on January 22 it had to put Reefton on a 'boil water' notice for several days, after a sudden rise in bacteria levels — a repeat of last summer's episode.
Sampling this time found no e.coli anywhere in the reticulation network; 'shock dosing' of the reservoir and tanks with chemical hypochlorite had disinfected them, and the notice was lifted, Mr Weichern said.
But tests showed the treated water was still being re-contaminated, post-treatment, making regular dosing necessary.
Cr David Hawes asked if the cause could be leaks from the town sewerage system.
"We know there are leaks in the water pipes; they run very close to the old sewer pipes; the ground is very porous, and you could end up with cross-contamination if both pipes were damaged in the same place," Cr Hawes said.
Mr Weichern said that would be a fair assumption.
"That is the very point that was in the Ministry of Health's comments."
Council infrastructure manager Mike Duff said that since 2019 the council had tried everything in its power to avoid chlorination, investing $80,000 in the UV treatment plant, an automated switchboard and new pumps. The next step could be to replace the rising and falling mains and separate them from the reticulation network, though the council had not planned to do that for another 20 years.
"But we recognise that is not acceptable — we have to recognise that provision in the long-term plan is not timely enough to deal with the situation we are facing now."
Local Democracy Reporting is a public interest news service supported by RNZ, the Newspaper Publishers' Association and NZ on Air.