Study uncovers secrets in water’s underground treks

When rain or snow falls, much of it seeps into the soil. From there it flows down through porous rock, gravel and sand, forming underground streams. This groundwater will sometimes travel long distances. Scientists have now attempted to calculate its path in North America. Their findings suggest water can travel below ground much farther and deeper — and for a much longer time — than had been expected.

For instance, some groundwater can travel up to 238 kilometers (148 miles), the new study reports. And it can seep to depths of more than 90 meters (300 feet).

Or consider the deep groundwater pumped up for drinking. Most was believed to be separate from polluted water that often resurfaces in streams and rivers. But through long, buried treks, groundwater can link areas thought to be separate — sharing pollution it picked up along the way. 

All that water now appears “a lot more connected,” says Reed Maxwell, an author of the new study. He works at Princeton University in New Jersey. As a hydrologist, he studies the movement of water.

His team shared its findings on how water moves in the January 6 issue of Nature Water.

Understanding these connections between water sources, they say, is important. It could help water managers work together to make better decisions about how to use water for drinking, irrigating crops and supporting ecosystems.

How they investigated water’s movement

A watershed (also known as a catchment) is the broad expanse of land that funnels snowmelt and rain downhill to a body of open water (such as a lake or ocean). Some rain and snowmelt will remain above ground. But much will seep underground as it moves.

Water managers decide who gets to use the water within a watershed and how. Watersheds are managed separately. But water can flow freely underground across watershed boundaries, says Laura Condon. She’s a hydrologist at the University of Arizona in Tucson. 

If watersheds are connected, then problems in one might affect another, says Maxwell. This got him thinking: “If you dump pollution in one watershed, does it get to another?” 

Others had mapped groundwater flows in various parts of the United States. But how distant sources of underground water might be connected hadn’t been clear. That was one motivation for this study, says Maxwell. Understanding how water moves across a continent underground can help show how, when and where impacts to that water may occur.

Maxwell and Condon’s team decided to track groundwater’s flow beneath most of continental North America. They started with where this water reemerges in streams. Then, working backward, they figured out where it must have first seeped into the Earth.

To do this, they built a computer model. It simulated North America’s underground layers of rock to depths of 392 meters (1,200 feet). Next, they divided the continent into little blocks. Each was one kilometer long by one kilometer wide.

Then they asked the model to calculate how much water would likely flow through each block. That would depend on the shape and thickness of Earth’s underground layers and the space between the rocks.

Each block is like a Lego brick, Maxwell says. Once they calculated water’s flow in each one, they stacked them together — 85 million of them.

Then they added simulated water droplets to the computer model. The model “pushes” these droplets around based on how water would flow through the layers of below ground rock. And the scientists tracked everywhere they would go.

“Think of a video game: Where would the little ball go? It’ll be pushed by the velocity this way,” says Condon.  Then, “where would it go next?”

Water often travels more than 97 kilometers (60 miles), the model showed. That wouldn’t be far if water was streaming across the surface, says Maxwell. But it was far longer than it had been expected to flow below ground.

Most water stays below the surface for five to 10 years, the model found. But buried aquifers (underground layers of water-filled rock) also exist. Water can collect in them for 100,000 to 1 million years.

Impacts to water

Knowing how far water travels and for how long helps determine impacts to water. How? “If you have pollution underground, you need to understand where it’s going to go,” Maxwell explains.

Consider, for instance, the nitrate fertilizers that farmers apply to soils. In large amounts, nitrates can harm people and aquatic life. Rain can wash these chemicals into groundwater. From there, they can potentially travel far from the farms on which they had been used. That can pollute water once thought to be free of such chemicals.

Models might also point to where water being pumped from the ground at one site (for drinking or irrigation) had been on its way to hydrate a distant wetland. Knowing this, says Claudia Faunt, managers might then make sure they leave enough water in the ground to sustain that wetland — and the ecosystem it supports. Faunt didn’t take part in the new study. But as a hydrologist with the U.S. Geological Survey in San Diego, Calif., she understands these issues. 

“Everything’s connected,” Maxwell says. By knowing how one watershed may be linked to others, you can understand how the system works as a whole, he explains. And that could be key to managing Earth’s life-giving resource.