Last year, a Danish study reported a link between nitrate in drinking water and the risk of developing colorectal (bowel) cancer. This finding could have important implications for Kiwis.

New Zealand has one of the highest bowel cancer rates in the world. Recent data showed also that drinking water supplies in some parts of New Zealand had nitrate levels more than three times higher than the threshold level for colorectal cancer risk identified in the Danish study.

This study and other research raise a question about the contribution nitrate exposure through drinking water may be making to our high rates of bowel cancer.

The health implications

Nitrate fertiliser is added to pasture and crops. Much of it enters waterways with rain and irrigation or via animal urine.

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The Danish study, published in the International Journal of Cancer, included 2.7 million people over 23 years and monitored their nitrate exposure levels and colorectal cancer rates. The findings confirmed suspicions that long-term exposure to nitrate may be linked to cancer risk. The researchers propose the risk results from nitrate converting into a carcinogenic compound (N-nitroso) after ingestion.

The research found a statistically significant rise in colorectal cancer risk at 0.87ppm (parts per million) of nitrate-nitrogen in drinking water. There was a 15 per cent increase in risk at levels over 2.1ppm, compared with those who had the least exposure.

One implication is that the current nitrate standard for drinking water used in most countries, including New Zealand, is probably too high.

The Canterbury example

The cancer risk level (0.87ppm) identified in the study is less than a tenth of the current maximum allowable value (MAV) of nitrate-nitrogen of 11.3ppm (equivalent to 50ppm of nitrate). This level has been in use in many nations for decades and comes from the World Health Organisation's limit. It is based on the risk of "blue baby syndrome" (infantile methaemoglobinaemia, which reduces the ability of red blood cells to release oxygen to tissues).

Rates of bowel cancer vary in New Zealand, with the highest incidence in South Canterbury, with an age-standardised rate of 86.5 cases per 100,000 people. Bowel cancer is the second-highest cause of cancer death in New Zealand and each year about 3000 people are diagnosed and 1200 die of it.

A recent epidemiological review estimated the contribution of a range of modifiable "lifestyle" risk factors to colorectal cancer in New Zealand. In order of importance, these factors are obesity, alcohol, physical inactivity, smoking and consumption of red meat and processed meat. It would be useful to conduct more research to see if nitrate exposure in drinking water should be added to this list.

A recent Fish and Game New Zealand investigation of drinking water supplies in the Canterbury region found nitrate levels in drinking water sourced from groundwater in areas of intensive farming and horticulture were high and rising. That was consistent with Environment Canterbury data. The latest groundwater report shows that half the wells it monitors have values above 3ppm nitrate-nitrogen, more than three times the Danish study's trigger level for colorectal cancer risk.

Christchurch City Council data shows that of 420 samples collected from 2011 to 2016, 40 per cent exceeded 0.87ppm.

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Impact on ecosystems

When nitrate enters waterways, it accelerates algae growth. Freshwater scientists have long been pushing for nitrate limits to curtail algal proliferation, but restrictions have been slow and in some regions non-existent. An important coincidence is that the Australian and New Zealand guideline for healthy aquatic ecosystems for nitrate is at 0.7mg/l nitrate-nitrogen, close to the level required to stay under the colorectal cancer risk value found in the Danish study.

The Canterbury region exemplifies the problems due to failure of central and local government to protect ground and surface water. These failures cannot be blamed on a lack of awareness as these outcomes were predicted decades ago. For example, in 1986 the Ministry of Works predicted the nitrate contamination we now see as a consequence of regional irrigation schemes. It made it clear that alternative drinking water supplies would have to be found in Canterbury.

While many Kiwis face significant and increasing costs for water treatment, water bottlers pay virtually nothing for our purest water. The Government needs to put tougher limits on nitrate loss and face up to dealing with water ownership issues.

In conclusion, surface water in many parts of the land is highly contaminated with nitrates due to intensified farming. This is damaging freshwater ecosystems and biodiversity, and may harm human health.

At the very least, public health authorities need to conduct a systematic survey to assess current nitrate levels in our drinking waters. This information could then be used to provide a quantitative estimate of the colorectal cancer burden in New Zealand that can be attributed to this hazard.

Mike Joy is a senior researcher at the Institute for Governance and Policy Studies, Victoria University of Wellington. Michael Baker is a professor of public health at the University of Otago.

- The Conversation