Groundwater is the water present beneath Earth 's surface in rock and soil pore spaces and in the fractures of rock formations. About 30 percent of all readily available fresh water in the world is groundwater.
The depth at which soil pore spaces or fractures and voids in rock become completely saturated with water is called the water table. Groundwater is recharged from the surface; it may discharge from the surface naturally at springs and seeps , and can form oases or wetlands.
Groundwater is also often withdrawn for agricultural, municipal, and industrial use by constructing and operating extraction wells. The study of the distribution and movement of groundwater is hydrogeology , also called groundwater hydrology. Typically, groundwater is thought of as water flowing through shallow aquifers, but, in the technical sense, it can also contain soil moisture , permafrost frozen soil , immobile water in very low permeability bedrock , and deep geothermal or oil formation water.
Groundwater is hypothesized to provide lubrication that can possibly influence the movement of faults. It is likely that much of Earth 's subsurface contains some water, which may be mixed with other fluids in some instances.
Groundwater is often cheaper, more convenient and less vulnerable to pollution than surface water. Therefore, it is commonly used for public drinking water supplies. For example, groundwater provides the largest source of usable water storage in the United States , and California annually withdraws the largest amount of groundwater of all the states. Many municipal water supplies are derived solely from groundwater.
