Gleeson, Tom, et al. November 2015. The global volume and distribution of modern groundwater. Nature Geoscience.

The water in aquifers and wells billions of people depend upon is mostly a non-renewable resource that could run out.

Underground water is renewed very slowly. Only 5.8% is replenished within a human lifespan of 50 years, and is further reduced by climate change when this causes less rainfall.

The actual number may be closer to 1.5%, because 5.8% is likely to be an overestimate due to the types of rock in the areas where most of the measurements were taken.

In California and the Midwest (Ogallala aquifer), people are already using “non-renewable” water thousands of years old, water that produces about a third of America’s food.

Egypt is tapping into water last renewed a million years ago. Such old water isn’t just non-renewable — it’s usually saltier and more contaminated than younger groundwater.

In addition, overusing groundwater, either old or young, can lower subsurface water levels and dry up streams, which could have a huge effect on ecosystems on the surface

This water is near enough to the surface to be contaminated by pollution or evaporated by high temperatures

While many people may think groundwater is replenished by rain and melting snow the way lakes and rivers are, underground water is actually renewed much more slowly.

Over a third of the world’s population depends on groundwater for drinking, agriculture, and commercially.

Also see Emily Chung, Nov 16, 2015, Groundwater is mostly non-renewable, CBC News

Konikow, L.F., 2013, Groundwater depletion in the United States (1900-2008): U.S. Geological Survey Scientific Investigations Report 2013-5079