Researchers have developed a simulation that reveals underground water patterns across an entire continent. After three years of investigations examining groundwater from one coast to another, the results illustrate the hidden journey of each raindrop or melted snowflake before it returns to freshwater streams. This process traces the path of water from the surface down to deep underground levels and back up again, sometimes surfacing up to 100 miles away after being submerged for anywhere from 10 to 100,000 years.
Researchers from Princeton University and the University of Arizona have developed a simulation that reveals patterns of underground water on a continental scale. This outcome stems from three years of thorough examination of groundwater from coast to coast, mapping the concealed journey that every raindrop or melted snowflake undertakes before reappearing in freshwater streams. This research follows water from the land surface down to significant depths underground, then back up again, sometimes emerging as far as 100 miles away after spending anywhere from 10 to an astonishing 100,000 years underground.
The simulation, unveiled on January 6 in the journal Nature Water, indicates that rain and melted snow travel much deeper underground than previously recognized. More than half of the water found in rivers and streams is sourced from aquifers that were once believed to be too deep to connect with surface waters. These surprising findings have significant ramifications for monitoring pollution levels and forecasting how climate change may impact groundwater resources, which provide drinking water for half of all residents in the United States.
Covering the continental United States along with portions of Canada and Mexico, the simulation tracks groundwater movement and quantifies the incredible distances and depths it traverses before emptying into streams across an area of over 3 million square miles (7.85 million square kilometers). The research team achieved this using an advanced hydrological simulation that enabled them to follow the water as it traveled through underground systems.
The research group comprised Reed Maxwell, a professor at Princeton’s High Meadows Environmental Institute and holder of the William and Edna Macaleer Chair in Engineering and Applied Science at Princeton; Chen Yang, a former associate research scholar at Princeton (currently at Sun Yat-sen University in China); and Laura Condon, a professor at the University of Arizona.
They discovered that groundwater can journey underground for hundreds of kilometers before emerging as streamflow. Particularly in the Midwest, groundwater can traverse long distances — especially where mountains meet plains. One significant groundwater route along the base of the Rocky Mountains was measured at 148 miles (238 kilometers). The study also highlighted the extensive connection networks associated with groundwater: nearly 90% of U.S. watersheds receive water from neighboring areas and then transfer it to another.
These findings carry enormous significance. Although hidden from view, groundwater accounts for 99% of the world’s unfrozen fresh water and serves as a drinking source for 145 million Americans. Furthermore, it plays a vital role in our food production, providing irrigation for 60% of global agriculture. However, groundwater is being depleted at an alarming rate, and modeling it has long posed challenges. This study introduces new retrospective analyses and predictive simulations that offer a means to monitor this critical resource and comprehend the wide-reaching effects of leaks from sources such as oil and gas companies.
“The connections between watersheds are crucial not just for streamflow,” noted Maxwell. “They also indicate how long contaminants linger in groundwater. Widespread pollutants like nitrate and PFAS can undertake these extensive journeys to the streams, complicating their management and prolonging their presence.”
Another significant new finding is that groundwater from extremely deep aquifers has a notable impact on streamflow. Maxwell’s research team found that groundwater sourced from aquifers located 10 to 100 meters beneath the surface contributed over half of the baseflow in 56% of the studied subbasins. The greatest depths were found in areas with the steepest topographical gradients, like the Rocky and Appalachian mountain ranges.
