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At a press conference this afternoon the council's chief executive Sue Bidrose emphasised there was nothing wrong with the township's water supply and no contamination had been found in the bore supply which meet the current New Zealand drinking water standards.
But the untreated bore water posed a potential risk to residents which the council was a risk the council was not willing to take, Dr Bidrose said.
The town's supply will be switched over first thing on Monday morning.
"This is isn't about saying there is something wrong with that water, what it's about saying is as we look into the future at some point those bores might become contaminated and the first we would know about it was when people became sick.
Dr Bidrose said in a press release this morning the council understood it would be a change for Mosgiel residents, but it had to prioritise the health of residents before anything else.
“Mosgiel’s bore water meets the national drinking standards, but we want to give the community the safest water possible and that means moving to treated water.”
The council is responsible for delivering clean, safe drinking water to our communities and visitors.
While most of the city’s water is chlorinated, a large area of Mosgiel still receives untreated water drawn from underground bores.
International drinking water expert Dr Colin Fricker has assessed the city's drinking water supply to help the council understand any risks in the system.
His advice is that the chance of contamination is higher than was previously understood and will increase.
Dr Fricker briefed the council late last month and recommended all Dunedin’s water supplies be treated and chlorinated to prevent bacterial contamination.
Dr Bidrose said there is a robust monitoring and maintenance programme for Mosgiel’s bore water system but based on the latest information, the risks associated with untreated water could not be ruled out.
"We are fast-tracking the changeover to treated water because we want to be as sure as we can that we are supplying safe water," she said.
Mayor Dave Cull said the decision to switch the water supply is an operational decision, because it is the chief executive's responsibility to manage the safety of the water supply.
"However, I am confident the decision is the right one. councillors have been well briefed on the situation and we agree making this change is the right thing to do.”
The changeover will happen on Monday, December 11.
Operationally, it is straightforward and centres on turning off the bores and opening valves so city water can enter the Mosgiel reservoir and network.
A flyer drop to Mosgiel residents and businesses will be carried out today and tomorrow.
Council group manager Tom Dyer said the change will affect most of Mosgiel – about 10,000 residents – who currently use water drawn from a deep aquifer, via the bores.
The changeover means those residents will receive treated water from the Mount Grand treatment plant. This water, which is chlorinated and contains fluoride, is used throughout the city.
Mr Dyer said some people may notice a chlorine taste or smell initially, but it should reduce as chlorine levels settle through the network.
Following the Havelock North water supply contamination in August 2016, a second national report on drinking water standards is due to be released shortly.
Dr Fricker was involved in the enquiry as an independent expert.
Dr Bidrose said the report will highlight the risks associated with untreated water.
The council anticipates the report will change national thinking and standards for drinking water and is likely to recommend chlorination of urban drinking water.