groundwater

Water present underground in porous rock strata and soils; it emerges at the surface as springs and streams. The groundwater's upper level is called the water table. Rock strata that are filled with groundwater that can be extracted are called aquifers. Aquifers must be both porous (filled with holes) and permeable (full of holes that are interconnected so that the water is able to flow).

Most groundwater near the surface moves slowly through the ground while the water table stays in the same place. The depth of the water table reflects the balance between the rate of infiltration, called recharge, and the rate of discharge at springs or rivers or pumped water wells. The force of gravity makes underground water run ‘downhill’ underground, just as it does above the surface. The greater the slope and the permeability, the greater the speed. Velocities vary from 100 cm/40 in per day to 0.5 cm/0.2 in.

Ground-water supplies vary from region to region depending on recharge rates and well use. Some areas such as western Texas and Oklahoma are in danger of depleting their groundwater supplies. In other areas, such as Long Island and New Jersey, overpumping has led to the encroachment of sea water into continental aquifers. Overall, however, the amount of usable fresh water in the ground is enormous.