Much of New Zealand's freshwater flows underground, and a team from GNS Science is in the process of mapping it.

GNS Science researchers are mapping where water flows underground across New Zealand, which will help with better management of groundwater resources.

Waikoropūpū Springs in Golden Bay are fed by groundwater springs.

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Te Whakaheke o Te Wai is a five-year project funded by MBIE's Endeavour Fund to map where groundwater comes from and where it flows to.

"Groundwater is all of the water beneath the ground," says GNS scientist Catherine Moore. "It's also the source of base flow in rivers, so when river flows get low that's basically groundwater."

New Zealand's groundwater resources include about 20 significant aquifers, which are places where the water flows through permeable rocks and can be easily extracted.

Canterbury and Hawke's Bay are examples of places where aquifers created by large gravel alluvial plains supply significant amounts of water for urban use and rural irrigation.

How old is the water?

The age of the water is a vital clue in this mapping project.

Uwe Morgenstern says that he uses radioactive isotopes of hydrogen, such as tritium, to date the water.

"We are measuring isotopic signatures of water molecules. That can tell us where the water has been recharged and how long it took the water to flow through the aquifer," says Uwe.

He compares the concentration of tritium in rain and groundwater to get a date for the age of the water.

Uwe says that most aquifers that are being used in New Zealand have water that is less than a hundred years old and it is more commonly 10-20 years old.

In Christchurch, however, the city's water supply comes from deep aquifers and is thousands of years old.

Future problems that we have already created

Uwe says the age of groundwater can have large implications for its quality in the future.

He says that Lake Rotorua is a prime example of what groundwater experts call 'the load to come.'

Since the 1970s, efforts have been underway to remediate growing levels of nutrient pollution from nitrate and phosphate runoff, but in that time water quality has continued to deteriorate.

Age measurements show that the water in streams and rivers in the catchment that feed the lake is between 50-100 years old.

"So the nitrate load we see going into the lake today is actually the load from 50 years ago," says Uwe…

Go to this episode on rnz.co.nz for more details