From algal blooms to infant health: new nitrate sensors for detecting water pollution

What if you could check the health of a river as easily as checking the weather?

At a discreet lab in Albany, University of Auckland Associate Professor Ashton Partridge and his team are working towards exactly that – building low-cost sensors that can continuously monitor nitrate levels in freshwater streams and rivers.

The technology has been almost a decade in the making, with research engineers Alan Zuppicich and Callum McGaw refining the chemistry and electronics through multiple prototypes.

The project grew out of conversations between Partridge and his Ngāpuhi colleague Millan Ruka, who has spent the past 20 years monitoring nitrate levels in rivers around New Zealand.

Ruka knew how difficult it was to build up a detailed picture of what was happening in waterways when measurements relied on people physically travelling to rivers to collect samples.

The engineering challenge was to find a better way.  

The result is a deceptively simple device: a buoy with tiny rubber elements mounted on a circuit board, calibrated against known high- and low-nitrate solutions.

River water is pumped through the sensor, where it is compared against those standards to produce a nitrate reading.

The sensor can take measurements as often as required – from minute-by-minute monitoring to once a quarter – while running for up to six months on battery power.

Each reading is sent over the cellular network to a live dashboard, turning what was once a handful of isolated measurements into a continuous picture of a river’s health.

The team has already trialled the sensors in Paerata and near Helensville, with a larger trial involving 10–15 units planned for the end of the year.

"There’s a lot of moving parts there," says Partridge.

"The sensors, getting all the guts of this working properly, and how this will be deployed – it will start off small and get bigger and bigger and bigger."

Why nitrates matter

Nitrate is essential for plant growth, but excessive levels can damage freshwater ecosystems and pose risks to human health.

For Partridge, the problem begins with something that can be difficult to see until the damage is already being done.

"High nitrate levels promote algal growth, leading to algal blooms. When the algae die, their decomposition consumes oxygen in the water, creating oxygen-depleted areas," he says.

"This results in fish and other aquatic organisms dying, and their subsequent decomposition further reduces oxygen levels, ultimately contributing to the formation of large hypoxic, or "dead" zones.

"It’s happening in the Hauraki Gulf. It’s happening in the Kaipara Harbour. So what do we do about it? The first thing we do about it is we monitor it. That’s the beginning of the solution."

That monitoring is currently done by collecting water samples at particular sites and monthly intervals.

Zuppicich says a network of sensors could provide a much more detailed picture.

"Councils would be able to put multiple sensors in a river, rather than just having one spot in one river monitored by someone going out in a truck and taking a scoop once a month."

Iwi are also obvious potential users of the technology, the team says.

"There are certainly Iwi up and down the country who would love to have access to it and monitor the rivers that they hold dear," says Zuppicich.

The same sensing technology could eventually be adapted to measure other pollutants, including phosphates and chlorides, although nitrates are currently the focus.

An outdated legal limit?

The environmental consequences of excess nitrate are only part of the concern.

New Zealand’s legal limit for nitrate in drinking water is 11.3 parts per million – a threshold established in the 1950s to prevent "blue baby syndrome".

But emerging research suggests health effects may occur at much lower concentrations, with studies linking nitrate levels around 5ppm to premature births and levels around 1ppm to an increased risk of colon cancer.

Southland is among the regions where nitrate levels are a particular concern, with three of the eight sites required to be monitored monthly recording levels around 3ppm.

And those official monitoring sites represent only a fraction of the places where people get their water.

"That doesn’t count all the private bores – we’re talking thousands of locations that will be over the legal limit, including households that are drinking water, cooking in it, bathing in it, using it for everything frequently," says Zuppicich.

For the researchers, making nitrate monitoring cheaper, more frequent and more accessible could ultimately help communities make better decisions about the water around them.

And McGaw says the ambition goes beyond simply collecting more data.

"We want to bring our freshwater systems back to a point where anyone can go out into the bush, put a bottle of water into a stream and drink it.

"It just means we’re protecting our really important ecosystems for future generations, which I think everyone agrees is one of the most important things we can do."