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Contaminated ground water in New Zealand: The causes of and solutions for nitrate leaching

Tests are showing a high percentage of New Zealand’s ground water is contaminated at levels that pose a serious health risk. Rebecca Reider outlines the cause and potential solutions.
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Understanding water, and how it moves through soil and landscapes, is crucial to understanding Aotearoa’s current freshwater crisis.

Soil microbes can convert various forms of nitrogen into nitrate. Having nitrate available in soils is good for farming, as it’s the easiest form of nitrogen – essential for growth – for plants to take up. However, nitrate is not easily held in the soil. The nitrate ion has a negative charge – so it doesn’t get electrically held in place by clays or organic matter. Nitrate is also water-soluble. So nitrate easily gets flushed into groundwater and surface water. This is the crisis that is now percolating through much of New Zealand’s water supply.

The cause

The cause of the crisis is well known: synthetic nitrogen fertiliser. New Zealand’s dairy herd nearly doubled in three decades, from 1990 to 2019, according to Stats NZ. That growth was built on a huge uptick in the use of synthetic nitrogen fertiliser, which increased more than 600 percent during the same period. Nitrogen can spark quick grass growth, allowing farmers to pack more cows into their paddocks. However, any excess nitrogen gets washed out of the soil, and into the water system.

“If you’re whacking a whole load of nitrogen mineral fertiliser on, you’re just loading the whole soil up with nitrogen,” agroecologist, Dr. Charles ‘Merf’ Merfield, says. “It will transform into nitrate and then, when you have a punch of rain, you get drainage and you get leaching.” And while rising fertiliser prices have made some farmers more cautious with their fertiliser applications, in the past “guys used to just lash it on as an insurance policy.”

Synthetic fertiliser is not the only cause of nitrate leaching. On New Zealand farms, cow urine is the other big culprit. Anyone who has watched the liquid fountain from a urinating cow can visualise the problem. Cows urinate in large, concentrated amounts. Each urine deposit creates an intense pocket of nitrate in the soil, which pasture plants cannot use all at once. Cows are more damaging than sheep in this regard. Sheep spread smaller blips of urine over a greater area.

The problem

The connection between nitrate leaching and freshwater pollution is not always a straight line. Water takes a different path through each landscape. Nitrate’s journey from farm soil into local waterways can take “anywhere from hours to centuries”, Merf says. It depends on soil drainage, aquifer permeability and the farm’s distance from surface waters. For example, on a farm with sandy, well-drained soil bordering a braided river, “anything you do will end up in that river almost instantaneously”, Merf says. This is the situation on many Canterbury farms.

On other farms, nitrate may take a decade to get from farm soil to groundwater. This makes it harder to monitor the problem, as there’s a lag time in the system. “There’s a pulse of nitrogen moving through Canterbury,” Merf says.

Health impacts

In drinking water supplies nationwide, the nitrate levels are reaching critical levels for human health.

Reporting of nitrate levels in New Zealand drinking water has been patchy, depending on regional councils. Activists and researchers are now working to fill the data gap to alert the public to local drinking water dangers. Over the past two years, Greenpeace and GNS Science (Geological and Nuclear Sciences Limited – the Crown Research Institute for Earth and material sciences) have been testing private water supplies for nitrates; anyone can send in a water sample to be tested.

These water tests have told a harrowing story. Unsurprisingly, the worst nitrate levels in drinking water are in dairying areas, with Otago especially high.1

Many public health scientists now argue that nitrates in drinking water should be kept below 1mg/L. “There are hundreds of thousands of rural New Zealanders who are exposed to levels higher than that, in all the areas you’d expect – areas of agricultural intensification,” says Russel Norman, Executive Director of Greenpeace.

The numbers

There are three numbers worth knowing when it comes to nitrates and human health.

The first number of concern is 11.3mg/L. This is the ‘safe’ level for nitrate-N in drinking water according to the World Health Organisation.2 This is also the drinking water standard set by the New Zealand Ministry of Health. At levels above 11.3mg/L, formula-fed babies are at risk of developing ‘blue baby syndrome’, as nitrate interferes with their blood’s ability to carry oxygen.

However, science is now showing that the government-endorsed 11.3mg/L legal ‘safe’ limit is not so safe.

Medical researchers now believe that levels of just 5mg/L of nitrate-N in drinking water can cause premature births. A recent Stanford University study of 1.4 million births found that when nitrate-N in drinking water goes above 5mg/L, the risk of premature birth increases.3 In New Zealand, “There’s a lot of people exposed over 5mg/L, and midwives are starting to warn people to get their water tested,” Norman says.

Some of nitrate’s most lethal health effects are now known to occur at even lower levels of pollution. Bowel cancer risk goes up in the presence of very low levels of nitrate in drinking water. A landmark Danish study in 2018, looked at millions of people’s health records and determined that the risk of bowel cancer increased at just 0.87mg/L of nitrate-N in drinking water.

A recent New Zealand-based study has determined that as many as 17 percent of New Zealanders are drinking water with enough nitrate in it to cause an increased cancer risk. The incidence of bowel cancer in New Zealand is one of the highest in the world.

At the sink tap, only a few types of water filters, such as ion exchange and reverse osmosis filters, can remove nitrate from drinking water.

Ecological limits

Humans are part of nature – so it’s no surprise that natural ecosystems also show stress from raised nitrate levels.

Around 1mg/L is often cited as a safe limit for nitrate-N in waterways. In more sensitive ecosystems, however, science is showing the need to keep nitrate even lower. In August 2023, the Environment Court issued a decision to protect Golden Bay’s Te Waikoropupu Springs, the largest freshwater springs in New Zealand, which contain some of the clearest waters ever measured. Nitrate-N levels at the springs have been measured on the rise, reaching over 0.5mg/L. The court instructed the Tasman District Council to manage the aquifer in order to keep nitrate levels in the springs below 0.41mg/L.

In waterways, nature will try to make use of excess nitrate, but the results can be disastrous for native species. Algae and cyanobacteria feed on excess nitrate and phosphorus in water and can come to dominate the aquatic ecosystem, a process known as eutrophication. As the algae and cyanobacteria die off, their decomposition sucks up available oxygen in the water and makes the ecosystem inhospitable for other living creatures.

Holding the line

From a policy perspective, there are two key metrics that the government can use to limit nitrate pollution: rates of synthetic fertiliser use, and stocking rates (how many animals are allowed on each hectare of farmland). However, while the government has instituted a modest cap on nitrogen fertiliser use, the rule is poorly enforced.

Holding the line on fertiliser use and cow numbers is crucial to getting the nitrate problem under control – because once nitrate is present in waterways, it’s not easy to get it out.

On the farm

Proactive on-farm solutions do exist. The key is to use farming practices that keep nitrogen cycling through plants rather than allowing it to leach through the soil.

Riparian planting – the practice of lining stream banks with native plants – is not a total solution. Such plantings don’t prevent nitrate leaching, Merf cautions. That’s because nitrate can leach down into groundwater before it reaches a surface stream. “Nitrogen is sliding underground and coming up under the rivers,” Merf says. (That said, riparian plantings do soak up excess phosphorus before it gets into waterways.)

There is really only one rule to prevent nitrate leaching, Merf says: Make sure there isn’t more nitrogen in the soil than the crop can take up, because nitrogen can’t be stored.

Organic solution

Switching to organic farming is a way to soften the leaching crisis, as organic farmers cannot use synthetic nitrogen fertilisers. Still, Merf cautions that organic farmers are not fully exempt from the dangers of nitrate leaching. Many organic farmers grow nitrogen-fixing cover crops, such as legumes, in order to draw nitrogen from the atmosphere into the soil. If the farmer ploughs a nitrogen-rich cover crop into the soil in a time of warm, moist conditions, the crop will decompose quickly and a flush of nitrogen will be released into the soil all at once. A major rainfall event can then wash nitrate into groundwater.

Organic farmers can reduce their leaching risk by taking care to terminate nitrogen-rich cover crops at the right moment, avoiding times of high temperatures and soil moisture. Another way to lower the risk is to chop or crush the cover crop and leave crop residues on the soil surface to break down more slowly. Having crops in the ground ready to take up the released nitrogen is also important; seeds of the next crop can be sown directly into the mulch of a terminated cover crop. Soils should never be left fallow, without living cover.

Diversity

Agroforestry plantings offer another way to combat nitrate leaching in sensitive areas. Planting deep-rooting trees which can capture nitrogen through their roots is “the closest thing to a silver bullet” to counteract leaching, Merf says.

Agroecology-inspired cropping strategies can also minimise nitrate loss. One nitrogen-smart approach is intercropping – for example, growing a nitrogen-rich ‘service crop’ of legumes directly alongside a cash crop, such as grain. The legumes secrete nitrogen into the soil as they grow, and that nitrogen can directly feed the cash crop. Living mulches – plants grown to cover the soil surface – can also supply nitrogen. Successful intercropping depends on finding specific cultivars that work well together, and is best approached with gradual experimentation.


Rebecca Reider is a facilitator, writer and researcher working at the crosswords of agriculture, ecology and health.

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