Nearly 60 per cent of New Zealand’s freshwater sources are tainted with nitrate concentrations above a threshold considered to pose higher health risks. Photo / AFP
Nearly 60 per cent of New Zealand’s freshwater sources are tainted with nitrate concentrations above a threshold considered to pose higher health risks. Photo / AFP
Nearly 60 per cent of New Zealand’s freshwater sources assessed in a first-of-its-kind study were shown to have nitrate concentrations above a threshold considered to pose higher health risks.
The new study, led by GNS Science, adds to mounting evidence of worrying levels of nitrate contamination in our drinking water supplies – something researchers say may be raising the danger of bowel cancer and other health problems.
While recent studies have assessed nitrate concentrations in waterways, wells and groundwater, the latest investigation was the first to focus on stable isotopes - naturally occurring atoms of chemicals often used to explore environmental change over time.
In this case, isotopes from samples gathered over 10 years contained rich information about where the nitrate contamination came from, with urine and urea fertiliser from farms unsurprisingly found to be a major source.
The study’s leader, GNS senior scientist Dr Karyne Rogers, said a key question was whether some regions had higher nitrate-nitrogen levels in their freshwater sources than others.
“We also wanted to know if nitrate contamination was more of a surface water problem or a groundwater problem,” she said.
“As our drinking water comes mostly from groundwater, we needed to survey where the nitrate problems were and what was causing it.”
The analysis showed higher nitrate levels in areas with intensive dairy farming – including Waikato, Taupō, Taranaki, Wairarapa, Canterbury and Southland - but also in rural regions centred around horticulture and cropping.
In all, they found that 58 per cent of freshwater analysed had concentrations higher than 0.9mg/l, while 33 per cent of sites had levels that exceeded half the MAV of 5.6 mg/L.
Rogers noted that overseas studies have suggested drinking water standards for nitrates be set at 1.0mg/l, given levels above that have been linked to colorectal cancers, along with impacts on biodiversity.
“Overall, in some regions in New Zealand, groundwater is already quite impacted by nitrates,” she said.
“The key drivers of this nitrate contamination are horticulture and high intensity farming operations.”
More surprising was the fact the surface water samples they assessed were less contaminated than groundwater, which carried median values of 2.9 mg/l.
“It is concerning to know that groundwater has much higher median and maximum nitrate-N values than surface water, as groundwater is harder to remediate than surface water,” Rogers said.
“Once groundwater becomes contaminated with nitrates, it can take years or decades for it to decline, depending on the age of the groundwater.”
Rogers said her team now wanted to investigate whether the same level of nitrates were present in rural drinking water wells.
Groundwater from about two-thirds of the wells they assessed had a five-year median concentrations greater than 1mg/l - something largely tied to human activities.
Around a quarter had concentrations higher than half the MAV – and six per cent exceeded it altogether.
The analysis showed higher nitrate levels in areas with intensive dairy farming – including Waikato, Taupo, Taranaki, Wairarapa, Canterbury and Southland. Photo / Christine Cornege
Again, those higher levels were generally found in areas of intensive agriculture, either grazing – as seen in results from Canterbury, Southland and Waikato – or vegetable farming, as observed in Pukekohe and Horowhenua.
At the time it was published, Otago University public health researcher Dr Tim Chambers said it was at those sites where levels were over half that MAV that likely required the most attention.
There was also particular a risk for levels to be higher on private supplies that weren’t covered by national monitoring, he said.
In 2021, Chambers led a study that suggested up to 100 cases of bowel cancer, and 41 deaths, may be caused by nitrate-contaminated drinking water each year - with around 800,000 Kiwis exposed to levels that international studies deemed a risk.
Aside from infrastructure improvements, he felt the bigger priority for those areas with worsening or high nitrate concentrations was to review land use practices.
The Ministry of Health continued to review new research about nitrates in drinking-water to understand links to adverse health, but have referred to nitrate levels lower than 50mg/l - or equivalent to a nitrate-nitrogen level of 11.3mg/l - being safe, according to current evidence.
While that value for short-term nitrate has been kept in recently-updated legislation, the sum of concentrations of nitrate and nitrite to each of their respective MAVs was not allowed to exceed one.
After taking over as our drinking water regulator, Taumata Arowai required all water supplies to monitor for nitrate and other chemicals in their first year of operation.
