Groundwater is both the immobile and the flowing fresh water that exists beneath the earth's surface in spaces within rock formations, gravel, sand and sediments. It constitutes about thirty percent of the world's total fresh water volume and about 99 percent of the world's fresh water in liquid form, although it is only about 0.76 percent of the world's total water volume inclusive of oceans and permanent ice. It is naturally replenished by water from precipitation, melting snow, streams, and rivers.
Groundwater has become a major source of water for consumer, agricultural and industrial use because it is often more convenient, cheaper and less vulnerable to contamination than surface water. However, there are several major environmental issues related to its use. One is that in many parts of the world it is not sustainable because it is being consumed faster than it can be replenished. Changing weather patterns due to climate change, including reduced rainfall, are also contributing to the lack of sustainability in some locations.
A second issue is that it is becoming increasingly polluted by many sources, including industrial, agricultural and household chemicals, landfills, septic systems, leaking underground oil storage tanks and pipelines, industrial waste lagoons, fracking, oil field brine pits, and sewage sludge. Another source of contamination that is expected to become increasingly severe is saltwater intrusion as sea levels continue to rise due to the melting of the polar ice caps.
A third major issue regarding groundwater is that its depletion can cause land subsidence. This is becoming an increasingly serious problem, particularly in urban areas. It is especially serious in coastal cities because both their declining elevations and rising sea levels are making them more prone to flooding.