Origin and quantity of groundwater on the earth

Origin

Most groundwater originates as meteoric water from precipitation in the form of rain or snow. If it is not lost by evaporation, transpiration or to stream runoff, water from these sources may infiltrate into the ground. Initial amounts of water from precipitation onto dry soil are held very tightly as a film on the surfaces and in the micro pores of soil particles in a belt of soil mixture. At intermediate levels, films of water cover the solid particles, but air is still present in the voids of the soil. This region is called unsaturated zone or zone of aeration, and the water present is vadose water. At lower depths and in presence of adequate amounts of water, all voids are filled to produce a zone of saturation, the upper level of which is the water table. Water present in a zone of saturation is called groundwater [3].

The porosity and structure of the ground determine the type of aquifer and underground circulation. groundwater may circulate and be stored in the entire geological stratum: this is the case in porous soils such as sand, sandstone and alluvium. It may circulate and be stored in fissures or faults in compact rocks that are not themselves permeable, like most of volcanic and metamorphic rocks. Water trickles through the rocks and circulates because of localized and dispersed fissures. Compact rocks of large fissures or caverns are typical of limestone.

Quantity in the earth

On the earth, approximately 3% of the total water is fresh water. Of this groundwater comprises 95%, surface water 3.5% and soil moisture 1.5%. Out of all the fresh water on the earth, only 0.36% is readily available to use (Leopold, 1974).
Groundwater is an important source of water supply. 53% of the population of US receives its water supply from groundwater sources. Groundwater is also a major source of industrial and agricultural uses.

We are withdrawing water from underground aquifers at a faster rate that it can be replenished. Although immense, world's aquifers are not bottomless and in many areas water levels are sinking fast. The water in some aquifers is millennia old and lies beneath what are now some of the driest regions on Earth. Although people have drown water from from springs and wells since the earliest civilizations, in the past 50 years multiplying populations have needed more food and water and the rate of withdrawal has increased drammatically.
In some coastal areas so much fresh water has been withdrawn from aquifers that saltwater has started to intrude, turning well water brackish and unusable. For more information check our seawater intrusions web page.
In some areas the emptying of aquifers has caused serious subsidence or high decreasing of the water table level. Some striking cases are listed here [13]:

USA:
San Joachim Valley, CA - 10 meters
Phoenix, AZ - more than 1 metre
Houston - Galveston, TX - 1 metre
Milwaukee - water table dropped 114 meters by 1976
Chicago, IL - water table dropped 274 meters by 1979. Partly recovered since then because reduced extraction
Ogallala Aquifer - some wells have run out in Oklahoma, Kansas and in Texas, where the water table has dropped by 30 meters

Mexico:
Mexico City - the city centre has subsided by 7.5 meters since 1950
Ciudad Juarez/El Paso (US border) - the aquifer that supports 1.5 million people is expected to be depleted within 30 years

Libya:
1 billion cubic meters of water per year is being mined by Libya from beneath the Sahara and piped to its farms and cities in the north

Lebanon:
mining the aquifer beneath Tripoli is leading to an annual deficit of 3.8 million cubic meters

Yemen:
water table dropping by about 2 meters per year. Wells have been dug 2 km deep without success

Baluchistan, Pakistan:
water table is dropping by 3.5 meters per year

Punjab, India and Pakistan:
water table is dropping by 1 meter per year

North China Plain:
water table dropping by 3 meters per year

Groundwater is also affected by water engineering: for decades and centuries, through improper disposal of wastes to the environment and subsurface areas many groundwater have become contaminated. Efforts to protect the quality and quantity of groundwater have been made by cooperation between all government agencies, industrial parties and researchers. [4]

Here you find a map of groundwater distribution on the earth [11].

Click here for definitions concerning groundwater, to learn more about its properties, its sources in Europe or the problem of its contamination.
For more information about global water distribution on the earth check our Water Quantity FAQ.