Groundwater is water stored underground, in the pore spaces of rocks and soil. While groundwater does not make up a large percentage of the Earth's total water, it does account for approximately 30% of Earth's freshwater. Not all of earth's groundwater is fresh, but groundwater located beneath land surfaces - where freshwater (which falls as precipitation) seeps into the ground - typically is.


Where does it come from & why is it important?
Groundwater comes from precipitation seeping through the soil. An aquifer is an underground layer of permeable rock, from which groundwater can be easily extracted using a well. Groundwater is located in the zone of saturation, where all of the available spaces in the rock and soil is filled with water. Above this zone is the zone of aeration, where pore spaces are filled with both water and air; we are unable to pump water from this zone. Between the zones of aeration and saturation lies the water table, the top surface layer of groundwater.


Groundwater is an important reservoir because many people rely on water collected from aquifers. For example, the Ogallala aquifer, which underlies the Midwestern part of the United States, provides the majority of the region's drinking water and is an important source of water for irrigating crops.

Groundwater Depletion
Natural and human activities can cause groundwater depletion. Groundwater levels can change naturally over time due to changes in weather patterns, precipitation amounts, stream flow, and geologic processes such as erosion. However, the largest contributor to groundwater depletion is pumping from wells. If water is withdrawn from the ground at a faster rate than it is recharged by precipitation, the water table will drop.

There are several negative effects of groundwater depletion:

• Lowering of the Water Table: or water to be pumped from a well, the well must reach below the level of the water table. If the water table is lowered too far it would have economic consequences for well owners. Depending on the characteristics of the rock layer the groundwater is located in, impacts to the water table may be short- or long-term, and the water level may fall only a small amount or hundreds of feet. Excessive pumping can lower the water table so much that wells dry up and can no longer provide water.
• Reduces water in streams & lakes: generally, a large amount of the water flowing into a stream or river comes from groundwater seeping into the riverbed. Pumping groundwater can therefore alter the movement of water between an aquifer and surface water or wetland, influencing aquatic and wetland habitats.
• Land subsidence: as water is removed from rocks and soil, it creates instability and causes the ground to sink. In the United States more, than 17,000 square miles in 45 states have been directly affected by subsidence, most of which is attributed to excessive pumping of groundwater.
  

Sources: http://ga.water.usgs.gov/edu/watercyclegwstorage.html, http://ga.water.usgs.gov/edu/earthgwlandsubside.html and http://ga.water.usgs.gov/edu/gwdepletion.html