Writing in the journal Nature, study co-author Tom Gleeson of McGill University in Montreal and his colleagues identify India, Pakistan, Saudi Arabia, Iran, Mexico and the United States as countries straining their groundwater supplies.
The researchers developed a "groundwater footprint" measure, comparing water coming in with water going out, to gauge how large an aquifer would need to be to sustain current withdrawal levels.
A groundwater "footprint" larger than the actual aquifer means people are using water faster that it can be replenished, treating it as a nonrenewable resource, Gleeson said.
Eighty percent of the world's aquifers are operating in a sustainable footprint, he said, but countries overtapping other aquifers are draining the world's water supply.
"It's not sustainable," Gleeson said. "We don't know how long the aquifers will last."