Irrigation provided by Amuri Irrigation Company in North Canterbury has intensified in recent years.

Nitrate levels soared across farmland in North Canterbury last year, leaving some drinking water bores so polluted they posed a danger to pregnant women and babies.

Concentrations of the chemical rose by more than 50 per cent in a single year in some bores around Culverden, Rotherham and Waiau in the Amuri Basin.

The area is supplied with irrigation water by Amuri Irrigation Company (AIC), which has strict conditions over nitrate levels under its Environment Canterbury (ECan) resource consent.

Stacy Squires/Stuff Canterbury Medical Officer of Health Dr Alistair Humphrey (Canterbury) says high nitrate levels are regarded as unsuitable for everyone, not just pregnant women and bottle-fed babies.

Monitoring of sample groundwater bores in 2018 found nitrate-nitrogen levels had risen across the board on the previous year, in one case more than doubling.

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Levels in some were so high they broke drinking-water standards.

AIC was forced to test drinking water bores, alert property owners and check if vulnerable pregnant women or babies were among them.

Two such bores on private properties in Rotherham and Culverden exceeded maximum nitrate level when tested in June and August, though AIC said no pregnant women or babies used them.

But AIC's 2018 consent compliance report, released to conservation group Forest & Bird, revealed a delay between the initial discovery of high nitrates in test bores in October and November 2017 and when the firm examined drinking water bores the following year – meaning residents could have unknowingly drunk potentially unsafe water for eight months.

Despite breaking national regulations, AIC did not breach its resource consent.

ENVIRONMENT CANTERBURY/SUPPLIED A denitrification wall at Silverstream Reserve, North Canterbury, has already significantly reduced nitrate levels in less than two months. (Video first published in January 2019)

​ECan northern zone manager Andrew Arps said the company had been "compliant" with the consent conditions and that ECan takes nitrate levels "very seriously".

"AIC has taken appropriate action, as per the consent conditions, to notify all landowners where nitrate exceeded allowable levels and confirm that there were no pregnant women or infants under six months at each address."

But Canterbury Medical Officer of Health Dr Alistair Humphrey branded the conditions "bizarre", questioning whether it was right for an irrigation firm to determine whether a woman is pregnant.

He told Stuff homes with nitrate levels above acceptable levels were not suitable for anyone to live in, not just pregnant women.

SUPPLIED ECan's North Canterbury zone manager Andrew Arps insists AIC has not breached its resource consents.

"The consent conditions are not consistent with the law – not with the Health Act, not with New Zealand drinking water standards ...

"The water supplies to the buildings supplied by this are not potable [drinkable], and therefore under law those buildings are insanitary and not considered habitable. The council and the regional council have to deal with it."

AIC chief executive Andrew Barton accepted increased irrigation was affecting groundwater and said he was "definitely not happy" with rising nitrate levels, saying the company and farmers were working hard to reduce their impact and improve irrigation efficiency.

"Whilst we've improved practises on farms and reduced the amount of nitrogen we're losing each year we've also reduced the amount of recharge that the aquifer is receiving, probably to a greater extent than we've reduced the amount of nitrogen coming through into the aquifer, so that results in an increased concentration of nitrogen in the groundwater."

SUPPLIED Forest & Bird's freshwater advocate Annabeth Cohen branded the consent conditions imposed on AIC as "lax" and questioned what ECan was doing to ensure pollution is reduced.

He also said the delay in testing drinking water bores was not deliberate and that it would not happen if such a situation arose again.

But Forest & Bird freshwater conservation advocate Annabeth Cohen said the testing had put people at risk because it was too slow and infrequent.

"There is too much pollution going into the system, more than the environment can naturally process, so much that we're putting babies and pregnant women's lives at risk.

"We know the environment is munted, but this is taking it to an unacceptable level."

TONY BENNY/STUFF Amuri Basin in North Canterbury is a haven for farmers in the dairying industry, but there are questions over the impact of irrigation on the environment.

Criticising ECan's consent conditions for AIC as "lax", she said: "It's almost as if they recognise there's a risk to human health, but they haven't included any sort of deterrent for more pollution going into the system or any requirements to reduce the pollution or stop polluting in the first place."

AIC's 131 shareholders, predominantly dairy farmers, irrigate more than 28,600 hectares in the Amuri Basin, an increase of 1,600ha since 2015.