A new study has mapped the continental flow of groundwater, showing that rain and snow melt much further and deeper underground than previously known. More than half of stream water comes from deep aquifers, highlighting important implications for pollution monitoring and groundwater management.
Researchers from Princeton University and the University of Arizona have developed a revolutionary simulation that maps the movement of groundwater on a continental scale. The culmination of three years of intensive research is a study that traces the hidden journey of water—from raindrops or melting snow that seep into the ground to their eventual appearance in freshwater streams. The simulation shows that water can travel to great depths, resurface up to 100 miles from the point of entry, and remain underground for periods ranging from 10 years to an astonishing 100 years.
The study, published Jan. 6 in the journal NatureWater , demonstrates that rain and snowmelt penetrate much further into the Earth than previously thought. It also shows that more than half of the water flowing in rivers and streams comes from deep aquifers that were previously thought to be isolated from surface water systems. The findings have important implications for managing groundwater resources, tracking pollution, and predicting how climate change may affect groundwater—a critical source of drinking water for half the U.S. population.
Broad scope and methodology
The simulation, covering the continental United States and parts of Canada and Mexico, tracks the flow of groundwater and measures the vast distances and depths it travels before flowing into streams over an area of more than 3 million square miles (7,85 million square kilometers). The researchers achieved this using high-resolution hydrological modeling, which allowed them to track the movement of water through underground systems.
The research team included Reed Maxwell, the William and Edna MacAleer Professor of Engineering and Applied Science at Princeton University and a professor at the Princeton High Meadows Environmental Institute; Chen Yang, a former associate research fellow at Princeton University (now Sun Yat-sen University in China); and University of Arizona professor Laura Condon.

They discovered that groundwater can travel hundreds of kilometers underground before emerging as a stream. In the Midwest, groundwater flows long distances—especially where mountains meet plains. One groundwater flow along the foothills of the Rocky Mountains covered 148 miles (238 kilometers). The study also revealed vast groundwater networks: nearly 90% of US watersheds take water from one neighbor and transfer it to another.
Consequences for water resources and pollution
The findings have dramatic implications. While they are not yet visible, groundwater accounts for 99 percent of the world’s unfrozen freshwater and provides drinking water for 145 million Americans. It is also essential to our food supply, irrigating 60 percent of the world’s agriculture. But groundwater is being depleted at an alarming rate—and has long been difficult to model. This study’s new retrospective analysis and predictive modeling provide a way to track this vital resource and understand the far-reaching consequences of leaks from oil and gas wells.
"Connectivity between watersheds is important for more than just flow," Maxwell said. “It also tells us how long the contamination will persist in the groundwater. Widespread pollutants such as nitrates and PFASs can make these long journeys to the stream, making them difficult to manage and making them last longer."
A second important new finding is that groundwater from very deep aquifers contributes significantly to the flow. Maxwell's team found that deep groundwater from aquifers 10 to 100 meters below the surface contributed more than half of the base flow in 56% of the subbasins. The greatest depths were in regions with the steepest topographic gradients, such as the Rockies and Appalachians.