The Earth is truly a blue planet; 70% of its surface is covered with water. Unfortunately 97.5% of that is salt water, unusable for humans. Fresh water accounts for the other 2.5%, however, about two thirds of that is locked up in glaciers and in the icy poles. That leaves humans (and every other living creature on land) only about 1% of all the water on Earth to use.
If we break this down even further, we see more limitations. Of the 1% usable water, only 1% is actually on the surface and can be easily accessed. This includes lakes, rivers, and swamps. The rest is underground. The US Geological Survey has estimated the actual quantities in cubic miles for water distribution on Earth and is as follows:
Oceans, Seas, and Bays - 321 million
Ice caps, glaciers, and permanent snow - 5.8 million
Groundwater - 5.6 million
Lakes - 42,320
Atmosphere - 3,095
Swamps - 2,752
Rivers - 509
Biological Water - 269
When looking at these numbers, what really pops out is the enormous stores of groundwater available. 5.6 million cubic miles is a staggering sum, even when compared to the immensity of the oceans.
Groundwater is contained in aquifers, underground layers of water-bearing permeable rock or sediment such as silt, sand, or clay. Under, and sometimes over the aquifer is an aquiclude, a solid, impermeable layer such as clay that the water cannot penetrate. The top surface of the water saturated material is called the water table. For millennia, humans have dug wells to access this bountiful source of fresh water
Due to unsound environmental practices and the sheer increase in human numbers, aquifers around the world have been coming under heavy duress. There is even an Aquifer Vulnerability Index that is used by geologists and hydrogeologists to determine which areas are most being contaminated or depleted.
Aquifers with limited natural recharge capacity can be over-exploited by human habitation and agriculture. This is especially true in deep underground aquifers in arid regions such as the enormous Ogallala aquifer in the Great Plains. This aquifer contains what is known as fossil water, water deposited in the ground from the melting of ice during the last great Ice Age. Some areas are able to keep the water table leveled off with sufficient rain and recharge. However, the annual recharge in the more arid parts of the aquifer is estimated to be only 10% of annual withdrawals. Efforts have been made to conserve water by the adoption of new agricultural practices such as terracing, crop rotation, efficient irrigation methods such as center pivot and drip, and simply reducing area under irrigation.
The Ogallala and other aquifers across the United States are easily contaminated. Contamination can come from many sources, including under- and above-ground storage tanks, septic systems, hazardous waste sites, landfills, road salts, fertilizers, pesticides, and various chemicals.
Many storage tanks (particularly underground ones) contain fossil fuels such as gasoline, oil, or propane. Over time, the tanks can corrode, develop leaks, and spread contamination into the substrate to find the water table.
Septic systems are not connected to the municipal sewer system, and are meant to slowly drain human waste harmlessly into the ground where it can be biologically degraded. If the septic system is improperly designed or maintained, it can cause groundwater contamination.
Other major contaminants are the chemicals we put on our lawns to kill weeds and insects, and to fertilize the plants. These are hazardous chemicals that the rain eventually washes down into the ground towards the water table or into streams, lakes and the ocean.
It is important to remember that groundwater, just like lakes, rivers, and streams, are part of the freshwater hydrological cycle. They are all connected, so that what happens to one will affect the other. Since water is so vital to human existence, it is right to protect that source which is most abundant. Maintaining good water quality is everybody's responsibility.
For more information: http://www.groundwater.org/gi/sourcesofgwcontam.html