ABSTRACT
Water is beyond any doubt the most widespread liquid inside the earth’s crust. Most of the permeable porous or fissured rocks – the environment in which aquifers1 are located –, and even many low-permeability rocks such as clays, contain water down to considerable depths (several kilometres, occasionally more than ten). But the volumes of water and their contents of dissolved solids are highly variable: the volume of invisible water present below one hectare of land may vary from a few thousand to several million cubic metres (m3). It is therefore difficult to calculate the volume of groundwater in a region or country – and calculating the volume of groundwater stored in the entire world is even more difficult.
