Researchers from Princeton University and the University of Arizona have created a simulation that maps underground water on a continental scale. The result of three years’ work studying groundwater from coast to coast, the findings plot the unseen path that each raindrop or melted snowflake takes before reemerging in freshwater streams, following water from land surface to depths far below and back up again, emerging up to 100 miles away, after spending from 10 to 100,000 years underground.

The simulation, published January 6 in Nature Water, shows that rainfall and snowmelt flows much farther underground than previously understood and that more than half the water in streams and rivers originates from aquifers once thought to be so deep as to be walled-off from streams. These unexpected findings have major implications for tracking pollution and predicting the effects of climate change on groundwater, which supplies half of all drinking water in the United States.