I Didn’t Know That!: Tracing Water Underground

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Imagine standing by a clear mountain spring. The water looks pure and inviting, but have you ever wondered where it starts its journey? That cold, clear water started its journey long before it reached you. It fell as rain or snow, soaked through the soil, and slowly recharged underground aquifers.

But how can scientists figure out when and where that water began its journey? We can help answer that question using tiny chemical clues that act like water’s fingerprint. These clues are known as “stable isotopes.”

Did you know?

Stable isotopes are used in many fields of science. They help paleontologists study fossils, climate scientists look at past temperatures, nutritionists track diets, and even forensic scientists solve mysteries!  

Scientists collect stable isotope data in Cedar Breaks National Monument.

NPS Photo

What are stable isotopes?

Okay, we’re about to get “science-y,” but stick with us, we’ll break it down.

Think back to chemistry class. Remember the poster hanging on the wall with the periodic elements? Elements, like hydrogen and oxygen, are atoms that have a distinct number of protons in their nucleus.

Some atoms have the same number of protons but carry extra neutrons in their nucleus. These are called isotopes.

Isotopes of the same element have similar chemical and physical properties, but their nuclear properties vary. Some isotopes are radioactive, which means they break down over time. Others are stable, which means they stay the same over time. Stable isotopes are useful tools for scientists to track the origin and movement of materials because they do not change.

Environmental scientists often track the stable isotopes of oxygen, hydrogen, carbon, and nitrogen, depending on what questions they are trying to answer. For tracing water, we will focus on the two elements that make up water: hydrogen and oxygen.

Remember, water has two hydrogen atoms and one oxygen atom. The chemical formula of a standard water molecule is H₂¹⁶O. Its weight is 18 atomic mass units.

In water, the most common isotopes are:

• Hydrogen: light (¹H) and heavy (²H, also called Deuterium, D)

• Oxygen: light (¹⁶O) and heavy (¹⁸O)

When these isotopes combine, they form slightly heavier or lighter water molecules.

These specific molecules are:

• H₂¹⁸O, weight of 20 atomic mass units

• HD¹⁶O, weight of 19 atomic mass units

Those differences may seem small, but they are powerful tools for scientists who want to understand how water moves through an environment.

How do isotopes tell a story?

Stable isotopes act like clues to help scientists understand the journey of water. They can show where the water came from, when it fell, how much fell, and how long it took to reach the groundwater.

Where it came from:

When rain forms, heavier water molecules with ¹⁸O fall first. Lighter ones with D (²H) travel farther inland and higher up mountains. By measuring the balance between heavy and light isotopes, called isotopic signature, scientists can tell where the precipitation fell.

Heavier water molecules = closer to the coast or lower elevation

Lighter water molecules = farther from the coast or higher elevation

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Text description of infographic

General Description: An infographic showing the relationship between lighter and heavier isotopes and precipitation at higher and lower elevations.

Detailed Description:

• The background image is a coastal scene of a grassy mountainside next to the ocean.
• The graphic begins with a text box over the ocean that says "heavier isotopes are more likely to be left behind in the ocean."
• There is an arrow that goes from that text box toward a cloud over the ocean that says "lighter isotopes are more likely to evaporate."
• There is an arrow that travels from that cloud toward another cloud over the land that says "heavier isotopes are more likely to condense and precipitate closer to the ocean and at lower elevations."
• There is an arrow that travels from that cloud toward another cloud that says "lighter isotopes are more likely to condense and precipitate farther from the coast and at higher elevations."
• There is a note at the bottom of the image that says "as you move farther from the coast, the proportion of heavier isotopes decreases and the proportion of lighter isotopes increases."

When it fell:

Isotopic signatures can also change with the seasons. Summer rain usually carries more heavy molecules. Winter precipitation usually carries light molecules. Measuring the isotopic signature can help scientists figure out what time of year the water entered the groundwater.

Heavier water molecules = summer

Lighter water molecules = winter

How much fell:

Since heavier molecules are rained out first, smaller storms typically have higher concentrations of heavier molecules. Higher concentrations of lighter molecules can indicate less frequent, larger storms. This is due to the sheer volume of water involved. Scientists call this the “amount effect.”

Heavier water molecules = smaller storms

Lighter water molecules = larger storms

How long it took:

Water in open lakes or rivers loses lighter water molecules through evaporation, leaving behind heavier molecules. If groundwater is recharged slowly from surface water, its isotopic signature will show a higher concentration of heavier molecules. This helps scientists understand whether water traveled quickly from precipitation seeping through the soil or if it was held in surface water before reaching the groundwater.

Heavier water molecules = slowly, through surface water

Lighter water molecules = quickly from precipitation

Scientists collect stable isotope data in Zion National Park.

NPS Photo

Using these four clues together, scientists can map the hidden paths of water through a park landscape.

How do NPS scientists use stable isotope monitoring?

NPS scientists compile long-term precipitation data into graphs called local meteoric water lines (LMWLs). By comparing them to the isotopic signatures from rivers, lakes, groundwater, and springs, they can map how water cycles through a park.

For parks, this information is important! Understanding how groundwater recharges helps protect clean water and supports wildlife.

Here are a few examples of how parks use stable isotope monitoring:

• Yellowstone National Park: Specialists collected stable isotope data at Tower Junction in Yellowstone National Park. Their analysis found that winter precipitation has a strong influence on groundwater recharge. This alerts the park to track changes in winter precipitation. Any changes could affect how available water is in the area.

• Zion National Park: Specialists collected stable isotope data for Upper Smith Creek and Hop Valley in Zion National Park. The data analysis showed that the water for these two areas came mostly from groundwater. They recommended creating a resource protection zone for these areas.

• Hovenweep National Monument: Specialists collected stable isotope data for groundwater springs at Hovenweep National Monument. This data would guide the park to choose the best restoration technique. The specialists found that surface waters influenced the recharge. So, they recommended potholes, or small pools of water, to help recharge the groundwater springs.

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Pass it on!

Did you learn something new? Pass it on! There is so much to learn about the natural world, but there's no way for everyone to be an expert in everything. That’s why sharing knowledge is so important!

Stable isotopes may be tiny, but they tell big stories about water. So next time you drink (filtered) water from a mountain spring, remember that you are drinking rain from seasons or even years past, tracked all the way through the water cycle.

Check out other I Didn’t Know That! Topics.

Text description of infographic

I Didn't Know That! Tracing Water Underground

How can we understand the journey of water? Scientists are like detectives! They use tiny chemical clues that act like water’s fingerprint. These clues are known as “stable isotopes.”

Stable isotopes can show:

• where the water came from
• when it fell
• how much fell
• how long it took to reach the groundwater

go.nps.gov/idkt