Apart from the world’s oceans, groundwater is one of the largest stores of water on Earth. Although it might appear that the planet is covered in vast lakes and river systems, they make up only 0.01 per cent of the Earth’s water. In fact, there is 100 times as much groundwater on this planet as there is freshwater on its surface.
Groundwater is stored in the tiny cracks found within rock and the spaces between soil particles. It can extend deep below the surface, at least as much as 10 kilometers.
Scientists know little about the groundwater stored more than one kilometer deep but they have determined that rain and snow falling in North America can circulate to depths of one to four kilometers. Below these depths there is only ancient water with other origins, last in contact with the atmosphere more than tens of thousands of years ago, but sometimes in excess of a billion years ago.
The circulation of this deep groundwater is controlled by the forces that drive flow, such as topography and rock permeability. For example, snowmelt and rainwater circulate more deeply in the mountainous areas than flatter regions. Groundwater can flow at speeds of meters per year in sandstones and limestones, or nanometers per year in intact igneous and metamorphic rocks, due to extreme variations in rock permeability.
Although deep groundwaters may only be weakly connected to the rest of the hydrological cycle, this does not mean they are unimportant to the functioning of our planet. Microbes have been found in most subsurface environments with temperatures below 80 degrees celsius, which is typical for depths of three to four kilometers. This subsurface life likely accounts for more than 10 per cent of the Earth’s total biomass, and yet the connection between deep groundwater circulation and subsurface life are largely unexplored at present.