Hawea water supply report

The residents of Hawea should have received prior notice before the chlorination of the supply last week, Queenstown Lakes District Council acting water services manager John Porter said.

“We are dealing with a new contractor who considered it was acting in the best interest of the community, having discovered e-coli. The contractor was not aware of an existing protocol to give the Hawea community warning of any chlorination. It had also given the supply a higher dose than previously experienced by the community,” Mr Porter said.

The Hawea supply is UV treated and is not normally chlorinated at all, which meant people could be highly sensitive to a one-off dose.

“We now have a report from the contractor United Water that confirms two very important things,” Mr Porter said.

One is that the contractor did not detect any evidence - in a series of testing undertaken throughout the chlorination period - to indicate the level of chlorine in the supply was outside acceptable drinking water standards. The highest reading found indicated 1.69mg per litre.

The New Zealand Ministry of Health ‘maximum allowable value’ for drinking water is 5 mg per litre, with the majority of swimming pools at around the 3-5mg per litre level.

“The second issue being that the effect of the chlorination was significantly worse for residents due to the presence of a bio-film build up on the pipes (coating of microbes on the pipe interior, built up since the water was last chlorinated two years ago) which has reacted with the chlorine,” Mr Porter said.

This had resulted in a strong odour and an unpleasant taste but did not present any health risk.

“We certainly apologise to the community for the failure to issue a chlorination warning and it is unfortunate that the bio-film has had an added effect on the supply and the community,” Mr Porter said.

The council had discussed the dosage with the contractor and it had agreed that given the low water temperature and low organic content of the Hawea water a lower dose of chlorine would have been sufficient.

“The higher dose has however highlighted the issue with bio-film build-up,” Mr Porter said.

Given the build up, the contractor, in its report, recommended implementing a six month air scouring regime, given the irregular or lack of chlorination.

“Bio-film is not desirable but it is a by-product of the treatment system in place at Hawea. Air scouring can cause discoloration so it’s an issue that we want to discuss further with the community,” Mr Porter said.

Due to the requirement for mandatory compliance with Drinking Water Standards 2005 the council was looking at an upgraded UV treatment system for the Hawea supply and giving consideration to a filtration system, which should control the bio-film and ultimately should result in limiting the need to chlorinate the supply at all, Mr Porter said.

As for the root cause of the original e-coli detection, Mr Porter said he was keen to investigate that matter further.

“There are various theories that I would like to explore with the community,” he said.

The council accepted that treating the supply unannounced and at a higher dose than the community had been accustomed to, was a controversial decision for the Hawea community.

“At the end of the day the contractor considered it was working in the community’s best interests. I guess we would have also had a very unhappy community if people became unwell as a result of a contaminated water supply,” Mr Porter said.

Ironically the council was currently reviewing its procedures for minimising water supply contamination risks.

“This will be done on a scheme by scheme basis and will be documented in Public Health Risk Management Plans, which must be agreed with the Ministry of Health,” Mr Porter said.

In the meantime United Waters had agreed to adhere to the protocol with the Hawea community in future.