Groundwater is water which occurs below the solid surface of the earth. The term groundwater usually refers to as subsurface water which occurs in fully saturated soil, sediments and rock formations, but may also include water occurring in partially-saturated formations.
Soil and rock formations contain void spaces, as pores between individual soil or sediment particles or fractures in rocks, which water can enter and move through. The body of rock or sediment bearing groundwater is called aquifer. Water can enter the aquifer from neighboring saturated aquifers, from infiltration from surface water flow and precipitation, and from deliberate groundwater recharge.
Naturally-occurring groundwater is about 4% of the total water on the earth, and is about two-thirds of the total fresh water in the world. As a result, it is an important resource for drinking and irrigation in many areas of the world. Depletion of groundwater at faster rates than natural recharge, and pollution of groundwater resources are significant concerns around the world.
The hydrologic cycle
Aquifers and aquitards
An aquifer is a layer or formation of soil or rock which is porous and permeable enough to contain groundwater which can be easily pumped to the surface. Aquifers are usually layers of sands and/or gravels with little silt or clay, heavily fractured or karstified rock formations.
An aquitard is a layer or formation which water flows through very slowly. Aquitards are usually layers of overconsolidated clay or formations of rock with little fracturing. There is no specific definition of the difference between an aquifer and an aquitard; a layer of dense sandy silt can be an aquifer if the surrounding layers are significnatly less permeable (for example, dense clays), or an aquitard if the surrounding layers are significantly more permeable (for example, clean sands).
Groundwater is used extensively for public and private water supply throughout the world. Most groundwater is obtained from wells drilled or dug into aquifers in the vicinity of the use. Occasionally, groundwater exists in an artesian condition, where the pressure in the aquifer is sufficient to lift the water to the surface when a well is installed, but much more frequently, groundwater must be pumped to the surface. Technology for pumping groundwater to the surface has varied from hand pumps and animal powered pumps used since prehistoric times, to windmills used through much of recorded history, to sophisticated motors and pumps available today.
Groundwater tends to be harder than water obtained from precipitation or surface runoff. Water hardness is a measure of its mineral content, with respect to +2 cations as Ca+2 (Calcium) and Mg+2 (Magnesium). In some areas, groundwater is treated after pumping to remove minerals or otherwise improve the water quality, though most groundwater is safe to drink untreated.
Some aquifers, especially those of highly populated dry regions, do not recharge as quickly as water is drawn from them; this leads to groundwater mining. Groundwater mining has become a significant issue in several arid areas, including the region of Texas over the Oglalla Aquifer. Some water agencies have developed systems to recharge their aquifers using reclaimed water to prevent depletion of their groundwater resources.
Depending on the geology of an area, groundwater depletion can lead to subsidence as the compressible aquifer layers consolidate due to groundwater extraction; this consolidation is generally irreversible, and will reduce the capacity of the aquifer as well as causing settlement of large areas.