Groundwater

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.

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, or heavily fractured 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).