8 Nov 2021

Contaminated water may lead to 40 deaths a year in NZ - study

9:23 pm on 8 November 2021

New research has found nitrate contamination in our drinking water could lead to 40 deaths a year from bowel cancer.

A glass of water showing a farm

Photo: RNZ/Vinay Ranchhod

The research by the Universities of Otago, Loughborough, Auckland and Victoria found 800,000 people in New Zealand are exposed to nitrate levels in water above those found to be a risk in international studies.

University of Otago senior research fellow at the department of Public Health, Dr Tim Chambers, said it is the first study to assess the level of nitrate exposure in New Zealand and collate a national dataset of drinking water quality.

"So we found that around 800,000 people may be on water supplies that are above those thresholds seen in international studies that link nitrate with bowel cancer.

"We also estimated based on that, that potentially in New Zealand there could be around 100 cases and around 40 deaths from bowel cancer attributable to nitrate in drinking water."

Chambers said it showed a substantial number of New Zealanders were on water supplies with nitrate levels that could increase their risk of bowel cancer.

"This research highlights the potential health burden of nitrate contamination, particularly if further evidence on the link between nitrate and bowel cancer reinforces existing studies.

"The results support the need to take a precautionary approach towards nitrate contamination in New Zealand."

However the study did not directly assess the link between nitrate exposure and bowel cancer as some overseas epidemiological studies had.

It is the first of future research to assess the potential public health impact of water quality in New Zealand following the 2016 Havelock North campylobacter outbreak which caused an estimated 8320 people to get sick, 42 hospitalisations and four deaths.

Chambers said the results showed the need for water reforms, as recent Official Information Act requests showed some councils had not tested for key contaminants in drinking water for years as legislation did not require them to.

The research sat alongside a wider set of evidence linking nitrate in drinking water to pre-term births, low birthweight and congenital abnormalities which are a growing concern, he said.

Other environmental research also shows the negative impact of nitrate contamination on freshwater ecosystems and the wider environment.

"While more research is required in this area, particularly around the biological mechanism and additional epidemiological studies, the potential health and environmental impacts of nitrate contamination provide a compelling case for intervention."

Nitrate contamination in New Zealand is mainly driven by pastoral farming, specifically from intensive dairy farming. It has increased substantially since 1990 and many groundwater sites continue to degrade.

The likely biological mechanism for nitrate increasing the risk of bowel cancer is complicated and is influenced by dietary factors, especially the balance of red meat, fruit and vegetables and gut microflora.

New Zealand experiences one of the highest rates of bowel cancer in the world with an average of 3000 cases per year. It is our second biggest cancer killer, causing an average of 1200 deaths per year.

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