Nitrate Contamination of Freshwater is not just about the risk of “blue baby” deaths

by Dr Peter Trolove, President NZFFA

New Zealanders deserve to be better informed about the risks to human and environmental health from increasing levels of nitrate pollution from New Zealand’s intensive farming systems


New Zealand has been poorly served by the Government’s and Regional Councils’ focus on short term economic growth with scant regard to the long term consequences of contamination of groundwater with high levels of nitrate – contamination that will take decades or possibly centuries to reverse.

Central Government’s limits for nitrate

For the first 20 years or more after the Resource Management Act (1991) was enacted the Government failed to issue national standards for nitrate contamination of freshwater.

The initial National Policy Statement for Freshwater Management NPS FM standards were established in 2014 and amended in 2017. The “bottom line” for nitrate (NO3-N) in freshwater was set at 6.9 mg/L. This level was not protective but politically inspired – a sop attempting to normalize the unacceptably high levels of nitrate that were occurring as a result of the then current Government’s promotion of intensive farming.

The latest NPS FM standards were set in August 2020. Still a political compromise delayed until 31 December 2024, the NPS FM set the bottom line for nitrate at 2.4 mg/L despite advice from the Ministry for the Environment’s own Scientific and Advisory Group recommending a scientifically defensible bottom line to protect aquatic environments of 1.0 mg/L NO3-N

National Policy Statement for Freshwater Management 2020 | Ministry for the Environment

Environment Canterbury’s limits for nitrate

The Canterbury Regional Council has adopted the ironic marketing title of “Environment Canterbury” or Ecan.

Ecan was appropriated by the Key Government in 2010 by replacing elected councillors with commissioners appointed to expand intensive irrigated farming throughout the Canterbury region.

The Environment Canterbury (Temporary Commissioners) and Improved Water Management Act 2010 removed protections afforded to the region’s braided rivers through Water Conservation Orders, and gave legal primacy to the non-statutory Canterbury Water Management Strategy CWMS. Canterbury was divided into 10 water zones with committees comprising largely of water users established to set standards for water quality and quantity within each zone.

This resulted in Ecan having a generic “bottom line” for nitrate contamination of 6.9 mg/L NO3-N with zone committees such as the Selwyn Water Zone setting a farmer friendly aspirational limit of 8.6 mg/L NO3-N within this zone.

The Canterbury District Health Board’s limit for nitrate

The Canterbury DHB set its bottom line at New Zealand’s Maximum Allowable Value MAV for drinking water of 11.3 mg/L NO3-N

Health risks of nitrates in drinking water | Community & Public Health (

The 11.3 mg/L limit was historically adopted by the WHO to prevent acute nitrate poisoning of human babies. This figure came from a US study investigating 400 “blue baby” deaths in America during the 1940s.

The New Zealand MAV of 11.3 mg/L has been set at the threshold of acute toxicity for babies under 6 months.

The 11.3 mg/L NO3 MAV is solely focused on the blue baby syndrome and has no relevance for limiting environmental harm or for providing any assurance that emerging public health concerns associated with much lower levels of nitrate contamination are not a threat to human health.

It takes many years for standards such as this to become established and there is a growing awareness that nitrate contamination of freshwater may have environmental and public health consequences due to chronic exposure of much lower levels of nitrate.

Summer Science: There’s something in the water | RNZ

What is an appropriate limit for nitrate in freshwater to protect the environment?

As the podcast above records, the 2000 ANZEEC limit to protect the environment used to be 0.44 mg/L NO3-N

A consensus of New Zealand scientists is that the limit should be no greater than 1.0 mg/L NO3-N.

The issue is to keep nutrient levels below the tipping point where eutrophication occurs.

Eutrophication occurs when excess nutrients allow the growth of aquatic plans and algae to strip waters of dissolved oxygen resulting in fish kills, the death of other aquatic species, and allows algal blooms to occur.

, Nitrate Contamination of Freshwater is not just about the risk of “blue baby” deaths

The 6.9 mg/L limit set in the 2014-2017 NPS FM standards was developed by NIWA scientist/toxicologist Dr Chris Hickey. Hickey simply conducted a literature search of NZ and Overseas studies of acute nitrate toxicity (96 hour LD 50s) to set his limit.

No consideration was made of chronic exposure to much lower levels of nitrate or the consequences of eutrophication as would have been the case if a freshwater ecologist was involved.

For example adult trout may survive in very high levels of nitrate contaminated water yet salmonid eggs and fry show toxic effects from around 3.0 mg/L NO3-N depending on species and other water quality parameters such as temperature and hardness.

What is the safe limit for nitrate contamination to protect public health?

The simple answer is that we do not know.

We do know there is a growing awareness that nitrate contamination is probably associated with a number of public health conditions at levels much lower than the NZ MAV of 11.3 mg/L NO3-N. Conditions such as;

  • Colorectal cancer [from 0.87 mg/L NO3-N]
  • Abortion [from 5.5 mg/L
  • Premature babies&/or short term gestation [from 5.5 mg/L
  • Thyroid cancers [from 2.5 mg/L
  • Bladder cancer
  • Spinal bifida, brain tumours, and spinal cord defects of babies

The New Zealand Government is failing to put public health before GDP. This is underscored by a “risk analysis” conducted by a couple of ESR toxicologists commissioned and funded by Fonterra and MBIE who concluded high levels of nitrate contamination in drinking water does not pose a risk to human health so long as you eat your greens. (Anti-oxidants in the diet should provide adequate protection).

The New Zealand Government actually withdrew funding for a prosed Otago University project assessing the risks of colorectal cancer and nitrate contaminated drinking water using COVID 19 as its excuse.

A current Otago study is struggling to get Government funding to study the shorter term consequences of exposure of pregnant women to > 5.5 mg/L NO3-N in drinking water and premature births. [This is a simpler project as women only need to be exposed to this lower level of nitrate from one month before conception rather than the 20 years of chronic exposure linked to bowel cancer.]

In the absence of New Zealand studies it is useful to look to first world countries prepared to spend money on much needed research.

Nitrate in Drinking Water: A Public Health Perspective | Water for Food Global Forum – Bing video

Dr Mary H. Ward provides a conservative and considered account of where scientific understanding of nitrate contamination of drinking water was at in 2021.

This hour long video is worth viewing both for providing background on an environmental problem created by our modern farming practices and the emerging evidence of the effects on human health at much lower levels of nitrate contamination than New Zealand Inc. would like the public of New Zealand to believe.

Nebraska has the 7th highest levels of nitrate contamination of groundwater of the US states.

The state of Iowa has been conducting similar studies for more than 20 years.

Drinking Water Nitrate and Human Health: An Updated Review | CHEEC | Center for Health Effects of Environmental Contamination (

The same patterns of nitrate related cancers are repeated Iowa.

It will take more time to fully understand the consequences of nitrate contaminated freshwater for human health.

A sensible approach would be to take a precautionary approach to nitrate contamination of fresh water as it will take decades if not centuries to reverse.

It appears from the limited number of recent peer reviewed studies available that the WHO MAV of 11.3 mg/L NO3-N adopted by New Zealand is out of date.

It appears it will remain unchanged until our leaders finally understand that our current farming practices are unsustainable.

My own perspective

Since August 2019 I have been testing the surface streams and drains of the lower Selwyn Water Zone.

I have conducted tests of private wells across Central Canterbury in conjunction with Greenpeace.

I have tested water for a group of Waikato lifestyle block owners whose wells have been polluted by factory waste from a Fonterra milk plant resulting in a cluster of bowel cancer cases.

It is clear that the groundwater in the Selwyn Water Zone is becoming increasingly contaminated with nitrate as evidenced by wells exceeding the 11.3 mg/L MAV and the lower Selwyn River testing as high as 9.85 mg/L in 2021.

This is not a legacy issue but a worsening situation since the Central Plains Water enhancement irrigation scheme commenced operating.

The problem is real and it is not being taken seriously by Ecan or the Selwyn District Council who still allow dairy conversions and issue consents to apply meat plant and dairy factory waste to land along with treated sewage wastewater.

, Nitrate Contamination of Freshwater is not just about the risk of “blue baby” deaths

The Selwyn District is one of the fastest growing districts on New Zealand. Residents who must rely on private wells and are left to fend for themselves by local and central government will have to pay dearly to treat their water and may face devalued property values if this shambles is left to develop further.

, Nitrate Contamination of Freshwater is not just about the risk of “blue baby” deaths

Dr Peter Trolove

President NZFFA

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1 Response to Nitrate Contamination of Freshwater is not just about the risk of “blue baby” deaths

  1. "Grand-ma" says:

    Where does it end? Our children and grandchildren will suffer the consequences.
    Have these politicians and their associated bureaucrats no conscience?

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