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Aquifer Basics
Aquifers are geological formations containing saturated rock through which water can easily move. Water percolates down through the soil until it reaches a non-porous layer, called an aquitard. At this point, the water can descend no deeper and saturation begins to occur. As more groundwater accumulates, the saturation level, known as the water table, rises. The saturated layer represents an aquifer. Vast amounts of water can be stored in aquifers, making them important natural resources.
Vital Water Resource
Aquifers are an important water resource because they represent a vital source for watering livestock, irrigating crops and providing potable water to municipalities. This is especially true of deep aquifers in arid regions. The Ogallala Aquifer, located in the central United States, is used to irrigate 20 percent of all the cropland in the country. Furthermore, the crops nourished by this water provide 40 percent of the nation's feedlot beef output.
Usage and Recharging
One of the greatest threats to aquifers is excessive extraction of water. Aquifers are refilled, or recharged, by rainfall. This recharge rate is very slow because the rainwater must infiltrate through layers of soil and rock. Many aquifers were formed over long geological periods lasting centuries, such as by glacier melt at the end of the ice age. A portion of these are not even being actively recharged and their water is known as fossil water. High rates of extraction can threaten these precious water resources, even if they are being slowly recharged.
Aquifer Contamination
Contamination poses another serious threat to aquifers. The quality of water collected from aquifers is generally very good. This is because the layers of rock and sand act as a natural filter. Materials with smaller pore spaces, such as sand, provide better filtering, but recharge more slowly. On the other hand, materials with large pores and fractures provide better water flow, but limit nature's ability to eliminate contamination. Agricultural chemicals represent one of the primary types of contaminants threatening water quality in aquifers.
Noteworthy Aquifers
The Great Artesian Basin in Australia is one of the world's largest aquifers, covering more than 650,000 square miles. This represents one-fifth of the entire Australian continent. It is an important source of water for Queensland and southern Australia. The aquifer contains around 52 billion acre-feet of water. The Ogallala Aquifer is another of the world's largest aquifers. This water resource covers 174,000 square miles, including parts of South Dakota, Wyoming, Nebraska, Colorado, Kansas, Oklahoma, New Mexico and Texas. In 1990, it was estimated to hold 3.3 billion acre-feet of water. Unfortunately, its water is being extracted 10 times faster than the natural rate of recharge.