Age of water

Australia is an ancient country, geologically speaking, and its groundwater aquifers reflect this: water in the Murray-Darling Basin has been dated to as old as 200,000 years, while some reservoirs in the Great Artesian Basin are thought to be nearly two million years old.

That means the oldest Australian groundwater tested to date began trickling into aquifers around the same time as the first hominids were tiptoeing out of Africa.

But unlike the remains of those early humans, whose fossilised existence is trapped in static rock, water moves. Which means working out the age of water requires a slightly different way of thinking about the dating process: that glass of water you’re sipping might be 100,000 years old, two years old and 156 years old all at the same time.

“It’s not an accurate science as such, but you can say that 10% is five years old, 20% is 13 years old and so forth,” says hydrogeologist Dr Ilka Wallis, from Flinders University in Adelaide.

So how do scientists try to date water – and why do they bother?

Praise for pollution

Water age is measured by the concentration of environmental ‘tracers’, both man-made and natural.

The concentration of each tracer in the atmosphere or in soil at any given time has been measured and recorded, so a water sample will contain equivalent amounts.

The water is ‘born’, in terms of age dating, when it leaves the atmosphere carrying those tracers and hits the ground.

“There is a clock that starts when it hits the water table,” Wallis says.

Man-made contaminants are useful for dating water that is younger than 70 years old. These include such things as tritium, or hydrogen-3, which was released into the atmosphere during nuclear bomb tests from 1945 to 1963, Wallis says.

The water is ‘born’, in terms of age dating, when it leaves the atmosphere and hits the ground.

CFCs began to leak into the atmosphere from industrial use in the 1960s until they were phased out in the 1990s. Sulfur hexafluoride (SF₆) emissions began in the 1950s and continue today. And per- and poly-fluoroalkyl substances (PFAS), which are water soluble, have been aggregating since the 1950s onwards. 

This information is useful for divining water that is moving relatively quickly from recharge (when it enters an aquifer or water body) to discharge, and therefore understanding how vulnerable it is to human-made pollution.

Deep diving

But for determining the age of water bodies such as Australia’s Great Artesian Basin, science needed to lean on some new tricks and measure naturally occurring radioactive isotopes.

Water in the Great Artesian Basin moves at a rate of 1–5 metres per year through porous rock formations. Because the basin is so large – it’s one of the world’s largest sedimentary aquifer systems, covering almost a quarter of Australia’s landmass – it takes as long as 1–2 million years for water entering the system in northern Queensland to filter out in discharge springs in South Australia, according to research by basin expert Dr Rein Habermehl.

To date water this old, researchers use the well-known carbon-14 method (which has a half-life of 5,730 years – the time it takes for half of the radioisotope present to decay), as well as argon-39 and krypton-81.

“The biggest change we’ve seen over the last 20 years is that there’s more techniques to date groundwater,” says Professor Peter Cook, a hydrogeology expert at Flinders University. “But the techniques are not as widely used as they might be.”

Argon-39 and krypton-81 added two new strings to the isotopic testing regime.

Argon-39 has a half-life of 269 years, which bridges the gap between carbon-14’s lower limit of 500 years and when human-made pollutants enter the scene. On the other end of the spectrum, Krypton-81’s half-life of 229,000 years allows dating of very old water.

But it’s still a rough science.

Water’s age isn’t determined by when it came out of the tap. You might be drinking the same water as your hominid ancestors.

“Age dating groundwater is like hole punching a book,” Cook says. “You punch some holes and you get a bunch of letters on the little round bits of paper. Then you’ve got to try to find out what the story is from half a dozen tiny pieces of paper. That’s what we’re trying to do. We’re trying to understand how the whole groundwater system behaves from information that we have from a few places where we’ve punched holes in the system.”

Ice cores

If groundwater is a slow drip of information, ice cores are more like static fossils.

Ice cores, the oldest of which come from Antarctica and go back 800,000 years, tell a story of temperature, precipitation, atmospheric makeup, volcanic activity and even wind patterns.

Researchers like Professor Chris Turney, director of Earth and Sustainability Science Research Centre (ESSRC) at UNSW Sydney, are using this data to guess what higher temperatures might mean for rainfall and other weather events in the future.

The ratio of oxygen-18 to oxygen-16 isotopes in ice cores can reveal a lot about the Earth’s climate at any one point and can be cross-referenced with other data such as volcanic ashes to find age.

When the climate is cooler, ice cores contain more oxygen-16. The lighter isotope is more likely to evaporate into rain or into the ocean at cooler temperatures. But oxygen-18 is heavier and needs more energy to evaporate, so when the climate is warmer it begins to appear more in ice cores.

Who cares?

Determining the age of water may not be an accurate science, but is an important one, giving us valuable insights into such things as when climatic change happened at different times in the past, and the climatic impact of events such as volcanic eruptions.

Understanding the age of water in aquifers and above-ground water bodies such as large lakes tells us how vulnerable they might be to contamination and how much water we can use without over-extracting. 

Lake Rotorua – a New Zealand water body marked by the stench of volcanic sulphur and neighbouring hot springs – has become increasingly polluted over the past 50 years by agricultural runoff, with ongoing algal blooms and fish deaths becoming a regular blight.

A time-lag of about 50 years, for water carrying fertiliser nitrates to enter the underground system and then discharge into the lake, means it will continue to be affected by historical agricultural practices for years to come, according to research led by hydrogeologist Uwe Morgenstern, who works for New Zealand Crown research institute GNS Science.

“Younger groundwaters, with their higher nitrate load, have not yet worked their way fully through the groundwater system,” Morgenstern wrote.  

“For Hamurana Stream, the largest stream to Lake Rotorua, it takes more than a hundred years for the groundwater-dominated stream discharge to adjust to changes in land-use activities.

“With increasing arrival of this nitrate from historic land uses, a further increase of the nitrate load to the lake must be expected in the future.”

In Thailand, scientists are using groundwater age to determine the vulnerability of the country’s main river system to climate change. 

Researchers Pinit Tanachaichoksirikun and Uma Seeboonruang said in December 2020 that water age is a new indicator of sustainability and climate change stress: younger water indicates enough rainfall is feeding the system; older water coming through bores was a cause for concern.

“The new indicator is groundwater age, because it can be analysed by groundwater travel time and is not associated with aquifer type, depth and size,” they wrote.

“The fluctuation of groundwater recharge led to change in groundwater age, which was an indicator of the groundwater level, which, in turn, allowed for land subsidence, seawater intrusion and increased salinity.”

Given the likely challenges to fresh-water supply in coming decades, it’s a good thing that new tools – such as the science of age dating – are emerging to assist with water management.

So next time you’re having a long cool glass of water, remember: its age isn’t determined by when it came out of the tap. You might be drinking the same water as your hominid ancestors.