The reservoir is known as a "perennial firn aquifer" because water persists within the firn, layers of snow and ice that don't melt for at least one season.
Rick Forster, lead author and professor of geography at the University of Utah, said that of the current sea level rise, the Greenland Ice Sheet is the largest contributor and it is melting at record levels.
So understanding the aquifer's capacity to store water from year to year is important because it fills a major gap in the overall equation of meltwater runoff and sea levels, Forster asserted.
In 2010, the Forster's team drilled core samples in three locations on the ice for analysis.
Team members returned in 2011 to approximately the same area, but at lower elevation.
Of the four core samples taken then, two came to the surface with liquid water pouring off the drill while the air temperatures were minus 4 degrees Fahrenheit.
The water was found at about 33 feet below the surface at the first hole and at 82 feet in the second hole.
Forster said that the discovery was a surprise because although water discharge from streams in winter had been previously reported, and snow temperature data implied small amounts of water, no one had yet reported observing water in the firn that had persisted through the winter.
The aquifer is extensive, covering 27,000 square miles and is similar in form to a groundwater aquifer on land that can be used for drinking water.
"Here instead of the water being stored in the air space between subsurface rock particles, the water is stored in the air space between the ice particles, like the juice in a snow cone," Forster said.
Forster added that the surprising fact is that the juice in this snow cone never freezes, even during the dark Greenland winter as large amounts of snow fall on the surface late in the summer and quickly insulates the water from the subfreezing air temperatures above, allowing the water to persist all year long.
The study is published in the journal Nature Geoscience.