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Nitrate numbers surprise

Sections of Wairarapa rivers, particularly those surrounding Masterton, are among the worst 25 per cent of all sites in the country for nitrate pollution.

A new GNS study has uncovered some surprising results about nitrates in freshwater across New Zealand.

GNS has been monitoring, analysing, and collecting groundwater and surface water nitrate data for years as part of its National Isotope Monitoring research to understand the sources of nitrates in freshwater.

GNS Science forensic geochemist Karyne Rogers said she wanted to know if some regions had higher nitrates in their freshwater sources than others.

“Freshwater is under threat from nitrate contamination, and as leading experts in groundwater research in New Zealand, we need to ensure we are collecting data like this,” Rogers said.

The paper found that almost 60 per cent of freshwater sources analysed across the country had nitrate concentrations of more than 0.9 mg per litre.

According to Land Air and Water Aotearoa [LAWA] data, collected by Greater Wellington Regional Council, eight river sites surrounding Masterton were in this category.

High levels of nitrate in water can be a result of runoff or leakage from fertilised soil, wastewater, landfills, animal feedlots, septic systems, and urban drainage.

Generally, nitrate levels in the upper part of the region north of Greytown were in the worst 25 per cent in the country. Further south, nitrate levels ranged between the best 25 per cent in the country and the worst 50 per cent in the country.

This is not to say that overall water quality improves in southern Wairarapa, as there are other indicators of water quality, including bacteria like E. coli levels.

Nitrate levels change depending on a range of factors, including groundwater upwelling, proximity to headwaters [the source of rivers], and various polluting factors, including land use.

Rogers said the current national drinking water standard for nitrate-N is 11.3 mg/l as a maximum acceptable value.

“However, some overseas studies suggest that the national drinking water standard for nitrate-N should be 1.0 mg/L rather than 11.3 mg/l.”

She said many researchers believed that 11.3mg/l is too high.

“Ecological studies have also shown that above 1.0 mg/l, the environmental quality of stream habitats are affected, and biodiversity begins to decline.”

The study also wanted to find out if nitrate contamination was more of a surface water problem or a groundwater problem.

Rogers said she and her team of researchers were surprised to see that surface water is less contaminated than groundwater.

“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. Once groundwater becomes contaminated with nitrates, it can take years or decades for it to decline,” she said.

Dr Rogers said GNS would also like to investigate rural drinking water.


  1. Poor Farmers being picked on.
    Perhaps . National Candidate,
    Farmer can explain how it should be ok to continue as is

Comments are closed.

Grace Prior
Grace Prior
Grace Prior is a senior reporter at the Wairarapa Times-Age with a keen interest in environmental issues. Grace is the paper’s health reporter and regularly covers the rural sector, weather, Greater Wellington Regional Council, and coastal stories.

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