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Most of the materials beneath the Earth's surface are porous. That means that they contain tiny open spaces as well as solids, just like a sponge. The porosity of a material is the percentage of open pore space it contains. Loosely packed sand and gravel can have porosities as high as 25 percent. Solid rock is much less porous. Many rocks have a porosity of only a small fraction of a percent.
Another important property of Earth materials is their permeability. The permeability describes how easy it is to force a fluid to flow through the pore spaces of the material. Loose sand and gravel have high permeability. Solid rock usually has low permeability. The best sources of ground water, called aquifers, have high porosity and also high permeability. Sand, gravel, and fractured rock make the best aquifers.
Ground water flow is much slower than flow in streams and rivers. That is because the passageways through the pore spaces are very small, so there is a lot of friction with the solid walls of the pores. Speeds of flow in streams and rivers are often greater than a meter per second. Ground water flow is often as slow as meters per day.
For a large town or city to obtain its water from ground water, there needs to be a large aquifer. Several widely spaced wells are used to pump water from the aquifer, all at the same time. If the ground water is replaced as fast as it is pumped, then it is a renewable resource. If the ground water is pumped faster than it is replaced, however, then the level of the ground water falls. It becomes more and more difficult to obtain the needed water. Then the ground water is not really a renewable resource, because the replacement might take far longer than a human lifetime!
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