Christchurch's pure drinking water could be contaminated due to farming

The Waimakariri River is the primary source of Christchurch's drinking water.
IAIN MCGREGOR/STUFF

The Waimakariri River is the primary source of Christchurch's drinking water.

Aquifers supplying Christchurch's pure drinking water may become contaminated with nitrates from intensive agriculture, research suggests.

The concerns have emerged from recent scientific work by Environment Canterbury (ECan), which shows deep groundwater from the Waimakariri district could be flowing towards Christchurch.

It is the first time ECan's modelling has showed that is a possibility.

It is understood the information has not been publicised due to ongoing scientific uncertainty, along with the risk of it becoming politicised due to the upcoming election, said one source familiar with the situation.

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There is no evidence the aquifers have been contaminated. Recent routine water test results were normal.

There are few health risks associated with drinking nitrate-contaminated water.

Most of Christchurch's drinking water is drawn from a series of deep aquifers beneath the city. They are primarily fed by the Waimakariri River, which loses water as it flows across the Canterbury Plains.

The water is of an exceptionally high quality and is untreated. It has long been a source of pride for the city.

However, scientific work for an upcoming Waimakariri sub regional plan found it was possible deep groundwater from Waimakariri may be flowing beneath the river and towards Christchurch's aquifers.

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There are currently low nitrate levels in Waimakariri's deep groundwater, but intensive agriculture in the area means it could leach into the deeper water over the long term.

If the groundwater was hydrologically connected to Christchurch, it could bring the nitrates with it. It is nearly impossible to remove nitrates from groundwater.

ECan confirmed its model showed a possibility of contamination, but cautioned there was no evidence the aquifers had been contaminated and the modelled result was just one result.

Significant work was under way to understand the situation and work had been delayed on the sub regional plan to fully investigate the issue.

The prospect of a contamination has been taken seriously within ECan, according to the source. 

It is understood that under the worst-case scenario, there would be a moderate, long-term increase in nitrate levels in the city's drinking water – however, the source emphasised that would be a worst-case prospect and there remained significant uncertainty.

The notion Waimakariri's groundwater could connect with Christchurch's aquifers was hypothesised as long ago as 2002, with research that year recommending investigating the possibility in detail.

ECan's modelling had never showed it was a possibility – until now.

It was determined about a month ago, but kept confidential among senior staff and stakeholders. It is understood ECan councillors were briefed recently, as was the regional water committee. 

ECan chief scientist Dr Tim Davie cautioned against jumping to conclusions. There was no evidence of any contamination and recent water tests had been normal, he said.

"None of our measurements show any contamination or any movement of nitrate under the Waimakariri [River]. The model has suggested it could possibly happen."

Planned work over the next six months included detailed testing of the chemistry of the groundwater over a greater area, drawn from nine new wells drilled at various depths near the river.

All of the results would be analysed by a panel of groundwater scientists from a variety of groups, he said. 

"That's what we'll use to say  . . . whether this is likely and what we need to do about it. We realise that this is a very important thing to get right."

Any information about the extent of any contamination, the nitrate levels expected, and what bores would be affected could not be inferred until more research was done.

Christchurch City Council three waters and waste manager John Mackie said the city council had been informed and there was no evidence of elevated nitrate levels in the city's aquifers.

Canterbury medical officer of health Dr Alistair Humphrey said the Canterbury District Health Board was aware of the matter and had no reason to be concerned, based on the information known so far.

"[We] have no indicators that this will have any adverse effect on the city's drinking water," he said.

Some rural parts of Canterbury already have issues with nitrates.

Nitrates are not dangerous to drink for most people and, unless found in high levels, do not present a health risk. 

The primary risk is "blue baby syndrome", which can be dangerous for infants.

In parts of mid-Canterbury, pregnant women are advised not to drink from private wells. 

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 - Stuff

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