Shallow groundwater is typically from zero to a couple of hundred years old. Some thousand years are not uncommon and for deep aquifer (that have little hydrostatic pressure reason to flow towards an ocean). It can be millions or even billion years old.

Where is the oldest water found?

Canada The Worlds Oldest Water Lies Deep Below Canada And Is 2 Billion Years Old. The worlds oldest known water was found in an ancient pool below Canada in 2016, and is at least 2 billion years old.

What is the biggest aquifer in the world?

High Plains Aquifer The High Plains Aquifer is the largest underground freshwater source in the world. It underlies 174,000 square miles of the Central Plains and has the same amount of water as Lake Huron.