What Is Isotope Hydrology?

About three percent of the Earth's water is freshwater. This is the water we drink, we bathe in, and we irrigate our farms with.

Besides the water seen flowing in lakes and rivers or frozen in glaciers and ice caps, most freshwater goes unseen in underground aquifers.

Isotope hydrology, a form of nuclear science, comes into the picture to reveal the life story of water and help us understand the links among these resources.

Based on the isotopic fingerprint, scientists can determine the origin, age, quality and renewal rate of water resources.

Access to reliable and safe drinking water is a major factor in meeting the needs of a growing population and to maintain human health. Freshwater is also crucial for sustainable food and energy production, industry and environmental protection.

Isotope hydrology is valuable as the world's freshwater resources are being drained faster than they are being replenished.

Protecting water resources is an important goal for all countries, especially as we see the impacts of climate change becoming more and more pronounced.

"Isotope hydrology is a really useful tool for countries to help do this because it gives us a fingerprint of the water molecule," explains Jodie Miller, head of the Isotope Hydrology section at the IAEA.

"And when we have that fingerprint, we can much better understand how water is moving between different water resources. So for example, between rivers and lakes, between surface water and ground water.

"We need to know how much water is where and where it's coming from in order to be able to take steps to protect the pathways from which it's transferring from one water resource to another."

Like everything we see in the world, isotopes are a type of atom, the smallest unit of matter that retains all the chemical properties of an element. Isotopes are forms of a chemical element with specific properties.

You can see the different chemical elements on the periodic table.

Each element is distinguished by the number of protons, neutrons and electrons that it possesses. The atoms of each chemical element have a defined number of protons and electrons, but - crucially - not neutrons, whose numbers can vary.

Atoms with the same number of protons but different numbers of neutrons are called isotopes. They share almost the same chemical properties, but differ in mass and therefore in physical properties.

There are stable isotopes, which do not emit radiation, and there are unstable isotopes, which do emit radiation. The latter are called radioisotopes.

Learn more about isotopes here.

Bob Kalin, professor of environmental engineering for sustainability at Strathclyde University in Glasgow, Scotland, explains: "Stable isotopes change because of physical processes in nature. For example, the evaporation of seawater into clouds changes the stable isotopes.

"So if we're trying to find out where something has come from, like it's come from the ocean, to rainfall on the top of a mountain down to a river, then we can use the stable isotopes. But we don't know how long it's taking to do all that. And this is where the radioisotopes really come into their best application because radioisotopes, through their natural decay over time, can give us an indication of the time.

"How long did it take the water from the top of the mountain to get down to my well? That's something the radioisotopes are able to tell us. We can look to very old waters and come up with an age of the water by using the radioactive decays."

"There's some places in the world where water hasn't gone into the ground for thousands of years," explains the professor. "It's like any other raw material - it's not renewable.

"And therefore, if we start pumping lots of old groundwater out of the ground and it's not going back in, we're basically emptying the bank of water underneath the ground.

"So by understanding how fast water is going in, how fast it's moving, and what the total bank is and the age of that bank, we could determine the balance of this and then manage the water resources much more effectively."

"Each water molecule has a signature. Water moves through many different forms. It could be ice. It could be snow. It could be rain. It could be vapor.

"The water molecule starts off with one signature as ocean water, has a different signature as the vapor once it's evaporated, has a different signature when it first falls on land, and then the residual vapor ends up with a different rainfall signature on the top of mountain.

"Each one of those processes can end up changing the ratio of the heavy to the light isotope. So each one of those will have a unique sort of fingerprint for that one, but the overall signature of that movement is something we can look across continents and across the globe."

The IAEA promotes and transfers know-how on the use of isotope hydrology as an effective tool for sustainable water management. It provides assistance and training on analytical services via its Isotope Hydrology Laboratory.

The IAEA is also spearheading efforts to increase water cooperation through its Global Water Analysis Laboratory (GloWAL) Network initiative.

Another key part of the puzzle, according to the IAEA's Jodie Miller, is monitoring: "You need to monitor because if you don't monitor, you can't manage something. At the IAEA, we have a monitoring programme for precipitation, which we call the global network of isotopes in precipitation.

"It's been running since 1960 and it helps us to understand how precipitation patterns are changing and how different types of rainfall events are transferring down to the groundwater system. So understanding the time scales of groundwater flow is really essential to understanding whether we are pumping groundwater out at a rate that it's not being recharged."

The quotes in this article come from a podcast on the topic: Nuclear Explained - What is Isotope Hydrology? Listen to the full interviews, and others on the same topic, here.