A dairy farm on the banks of the Waimakariri River. (file photo)

Christchurch drinking water will be able to contain more nitrates from pollution for the next 50 to 100 years, Environment Canterbury (ECan) has decided.

The elevated level of 3.8 milligrams of nitrates per litre of water was proposed by the Waimakariri Water Zone Committee, due to polluted water flowing into aquifers from North Canterbury dairy farms.

ECan councillors Lan Pham​ and Iaean Cranwell voted against the proposal at Thursday afternoon's meeting.

EMMA DANGERFIELD/STUFF A denitrification wall pilot trial is under way at the Silverstream Reserve, Clarkville. ESR groundwater scientist Lee Burbery is heading the project which aims to filter nitrates out of groundwater using a permeable barrier made from woodchip and gravel.

Earlier in the day, the Christchurch City Council dropped its previous support for what were still then proposed new limits.

READ MORE:

* Cantabrians may have higher cancer risk from poor drinking water

* Modelling shows Christchurch drinking water nitrate levels safe, but increasing

* Poisoning the wells: a history of infected drinking water in Canterbury

* Works starts on new trial to reduce nitrates at Kaiapoi's Silverstream

City council chief executive Karleen Edwards said the council wanted to withdraw a point in the council's submission that mentioned a limit of 3.8mg/l because it could affect Christchurch's aquifers.

"We've considered it's not appropriate to put another value in there."

The council would get another opportunity in mid-2019 to submit on the issue and it would provide a detailed submission on nitrate levels in Christchurch.

Under the 2008 government standards, the maximum allowable value of nitrates in drinking water is 11.3mg/l.

According to the 2017 National Policy Statement for Freshwater Management, the new level of 3.8mg/l protects 90 per cent of aquatic species from the chronic effects of nitrate toxicity.

The Waimakariri committee's proposal called the 3.8mg/l level "a precautionary limit" for "guiding nitrate reductions for the source area potentially associated with the Christchurch aquifer".

Pham said she could not support any such increased nitrate level.

"What we would be accepting here is not a precautionary limit, as it has been inappropriately and I feel somewhat misleadingly described.

"What we would be accepting is it would somehow be appropriate for the activities of a few private individuals and enterprises to put at risk the drinking water of a large metropolitan city.

"These implications prove a step too far for me to support."

Ngāi Tūāhuriri had also made it clear it did not support aspects of the proposal, she said.

Appointed councillor David Caygill said as individual councillors they had a responsibility to make their own judgements about such matters.

"But there is something I think unsatisfactory about setting up a committee, inviting them to work away on a very complex set of problems for three years or more. And then when they finally reach a set of conclusions, saying, 'well thanks, but I'm actually going to do my own thing'.

"There's something that sits very awkwardly for me with that."

Caygill voted for the new nitrate limit.

At the city council meeting, Cr Pauline Cotter said having pollutants entering the city's aquifers was unacceptable.

Cr Vicki Buck said nitrates were coming under the Waimakariri River and into the city's water.

It was a long slow process and she feared the nitrates in the water at the moment were from pre-industrial farming levels.

She was worried the situation was going to get worse.

"We need to make sure this will not impact the city's drinking water. "

She said ECan needed to act quickly and change the land use that was affecting Christchurch's drinking water.