Christchurch City Council to vote on chlorinating water

Christchurch residents are split on whether to chlorinate the water supply, some of which comes from shallow bores, but the council's drinking water manager backs the idea.

Christchurch City Council is considering chlorinating drinking water for up to 80,000 residents due to a high risk of contamination.

Havelock North is still counting the cost after more than 5000 people fell sick when its wells were contaminated, the same sort of shallow wells that are now causing alarm in Christchurch.

Christchurch has the largest supply of untreated drinking water in the country.

While most of its water is sourced from wells deep underground, for residents in the north west of the city it comes from a series of shallow bores, some as close as 16 metres to the surface.

Until recently the council decided it was sufficient just to bring forward by a year the drilling of deeper, more secure wells.

Then it received a letter from Canterbury District Health Board's medical officer of health Alistair Humphrey asking if it was enough to ensure there would not be a contamination on the scale of the one in Havelock North.

At the time, the council's manager for drinking water, John Mackie, said that until work on the northwest bores was completed, the assurance the DHB was seeking would be very difficult to provide.

This was because there was an unavoidable lag between when the untreated water was tested for contaminants and when the results were available.

A spokesperson said the council now believed the risk was too great and was considering chlorinating the water in the northwest until deeper wells had been drilled.

Dr Humphrey said the move was prudent.

"I had assumed that unconfined shallow aquifers would be treated but despite the fact that there was regular testing and provisions put in place by the council, I discovered that the water itself was not treated."

The 22 wells in question had a "D" rating, meaning their level of risk was deemed unsatisfactory.

Dr Humphrey said one was just 16m deep.

"In an unconfined shallow bore that's only 16 metres deep you can get all sorts of contamination," he said.

"You can get bacterial contamination, as we have seen in Havelock North, certainly you can get viruses, you tend to have higher levels of nitrate in the shallow bores."

Today, Mr Mackie said he supported chlorination.

"But I understand and fully appreciate that Christchurch people love their untreated drinking water."

"And I support that because what the chlorination does, it gives you a residual defence against any contamination event that might occur, not just in the bore but in the network itself."

"As a water engineer I'd always be recommending full treatment drilling work on deeper, more secure bores started in 2012 but the earthquakes had caused delays," he said.

"Even as late as the Valentines Day earthquake in February, some of the new bores we had drilled after that shake, they started to produce sand again, so you've got to re-clean all of those bores and start again."

A resident of affected suburb Bishopdale, Greg McKenzie said water contamination did not bear thinking about.

"I'm a pre-school teacher, so I would hate for that to happen because that would devastate all of the kids," he said.

"When something goes around [the children] are the first ones to feel it, so I don't want that to happen here."

Katherine Broughton was less keen to see her water chlorinated.

"Have the long-term health effects been researched on putting chlorine in the water, because isn't chlorine a carcinogen?" she said.

"I know in Invercargill where I grew up there was chlorine in the water but Invercargill also has one of the highest rates of colon cancer in the world.

"I think there should be further research before they decide to chlorinate the water."

A full meeting of the council is set to vote on Thursday whether to chlorinate the water.

The water could start to be treated within two weeks if the decision is made to chlorinate.