The importance of preserving groundwater resources is underscored by the number of places that depend on groundwater for drinking water, industrial uses and other needs. "Groundwater is an important ecological resource to critters as well as us, and we've already had serious problems in many areas where big sources of water for wildlife - waterfowl in particular - have dried up," Phillips said. Lowering the water table also decreases the water flow into lakes, rivers and streams. When that happens, the well "runs dry" and no water can be removed until the groundwater is recharged - which, in some cases, can take hundreds or thousands of years. "It's gone from being a lake in the southern San Joaquin Valley to a place where the water table is 500 feet below the surface." Groundwater lossesĮxcessive pumping of groundwater can result in a lowering of the water table in severe cases, where pumping demand is high and aquifer recharge is slow, the water table can drop so low that it's below the depth of a well. "We're seeing substantial loss of storage in the Central Valley aquifer in California," Phillips said. This scenario has repeated during subsequent droughts. In California's southern San Joaquin Valley, groundwater levels declined more than 150 feet (46 meters) during the state's severe 1976-1977 drought. Groundwater is a finite resource, and even large aquifers can be drained of much of their water, especially during droughts, when aquifers aren't recharged by precipitation. In addition to concerns about groundwater quality, the overall volume of groundwater in aquifers is also a serious environmental issue. And once groundwater is contaminated, it's notoriously difficult to remediate. This does not mean, however, that groundwater can't be contaminated: There are many cases worldwide where important groundwater resources (and drinking water supplies) have been ruined by saltwater intrusion (a particular problem in coastal areas), biological contaminants such as manure or septic tank discharge, and industrial chemicals such as pesticides or petroleum products. These factors make groundwater an important drinking water resource. And because it usually moves slowly through rock, groundwater is filtered of many contaminants, including some bacteria and viruses. While surface water quality varies due to erosion, runoff, pollution, plant matter, animal waste and other possible contaminants, groundwater quality is generally more stable. Tests on ancient groundwater dripping from fractures in a deep mine revealed the liquid to be between 1.1 to 2.7 billion years old. Groundwater can be trapped beneath the Earth's surface for millions of years. When an aquifer is confined beneath an aquitard layer, the pressure on the groundwater can be enough to force the water out of any well that's drilled into that aquifer. Through water might move through such material, it will do so very slowly (if at all). Dense, impermeable material like clay or shale can act as an "aquitard," i.e., a layer of rock or other material that is almost impenetrable to water, Phillips said. Groundwater can also be found in regions where the bedrock is made of denser material - such as granite or basalt - if that bedrock has been cracked or shattered. Some of the world's largest aquifers - vast underground reservoirs of groundwater - are found in regions with these porous bedrocks. More porous rock, such as limestone, sandstone and gravel, has innumerable small spaces that can hold water. In other areas, where the rock and soil are looser and more permeable, groundwater can move several feet in a day. Depending on the density of the rock and soil through which groundwater moves, it can creep along as slowly as a few centimeters in a century, according to Environment Canada. Groundwater does move, however, and it usually flows downward through the force of gravity because most natural replenishment takes place in the mountains, Phillips said.
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