Stored in the Earth’s crust are some six quintillion gallons of water, according to a new study in Nature Geoscience, and cited in an article on latimes.com.

The research was undertaken to update a study done back in the 70’s, in which scientists at the time estimated how much water was buried beneath the ground.  The new study also looked at the age of the water in an attempt to understand if that water could be quickly replenished as we continue to pull that water out of the ground.

Called “Groundwater” this water seeps through cracks and cervices of the Earth’s surface after rains and settles on the hard rock below the surface.  Many areas rely on groundwater for drinking and watering crops by pulling it from wells.

Tom Gleeson, a hydrogeologist at the University of Victoria in Canada who led the study said their maps and estimates show where the ground water is located and identifies areas of water that is being renewed and areas of old, stagnated water.

In trying to determine how much of the water is “modern” water, defined as entering the ground less than 50 years ago, the researchers looked at how much tritium was found in groundwater across the world.  A spike in tritium was noted about 50 years ago as a result of above-ground thermonuclear testing.  Tritium is a radioactive isotope of hydrogen.

Reviewing 3,700 measurements of tritium in 55 countries, the researchers determined that only 5.6 percent of all ground water is defined as modern.  Gleeson said that finding was the most surprising fact uncovered in the study.

The team says they were interested in the amount of modern water because of several reasons, notably modern water is a more renewable resource than older “fossil” water, and also that modern water is more vulnerable to contamination from agriculture and industrial pollution.

Gleeson and his team say, if you could pump all that modern water out and spread it evenly across all the continents, it would cover the land to about the height of a stop sign, but still this new study indicates that source of that water is finite.

Saying the team now wants to find out, with current usage of the water, how long will it take before this resource is depleted.