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AQUIFERS

The term "aquifer" refers to rocks and sediments that are saturated with good- to fair-quality water and that are sufficiently permeable to yield significant volumes of water, generally more than twenty-five gallons per minute, from wells. Well water is pumped from aquifers by individuals, municipalities, and companies for household, industrial, and agricultural use. Oklahoma's aquifers include both bedrock aquifers and Quaternary-age alluvium and terrace deposits.

Six of the major bedrock aquifers are the Central Oklahoma (Garber-Wellington), Vamoosa-Ada, Rush Springs, High Plains (Ogallala), Antlers, and Elk City. Bedrock aquifers in Oklahoma consist of sandstone, sand, limestone, dolomite, gypsum, or fractured novaculite and chert. The aquifers generally are one hundred feet to several thousand feet thick. The depth to fresh water ranges from a few feet to more than one thousand feet, and most wells are one hundred to four hundred feet deep. Wells drilled into bedrock aquifers generally yield twenty-five to three hundred gallons per minute, although some wells yield as much as six hundred to twenty-five hundred gallons per minute. Water in most bedrock aquifers has a low to moderate mineral content, about three hundred to fifteen hundred milligrams per liter dissolved solids.

Alluvium and terrace aquifers include the Tillman, Cimarron River, and Enid, and three North Canadian River (east, west, and central) aquifers. Alluvium and terrace deposits of Quaternary age (the last 1.65 million years) consist mainly of unconsolidated sand, silt, clay, and gravel laid down by streams that flow generally to the east and southeast across the state. The thickness of Quaternary deposits generally is ten to fifty feet, and locally up to one hundred feet. Wells in alluvium and terrace deposits generally yield ten to five hundred gallons per minute of water (locally several thousand gallons per minute), and most water has a low mineral content (less than one thousand milligrams per liter dissolved solids).

Fresh water stored in Oklahoma's aquifers results from downward movement of precipitation and surface waters that enter each aquifer at its recharge area. The system is dynamic; aquifers are recharged continually by percolation down to the water table. The rate of ground-water movement in the state's aquifers is highly variable, probably three to one hundred feet per year in most aquifers, and may reach one hundred to one thousand feet (or more) per year, where the rock is highly porous, cavernous, or fractured.

Large areas of the state are underlain mostly by shale or other low-permeability rocks that typically yield only enough water for household use (about one to five gallons per minute). Highly mineralized (saline) water, unfit for most uses, is present beneath fresh-water zones in these rocks, as well as beneath the fresh-water aquifers in all other parts of Oklahoma. The depth to the top of this saline water ranges from less than one hundred feet, in many areas, to as much as three thousand feet in the Arbuckle Mountains.

Oklahoma's principal aquifers contain an estimated 320 million acre-feet of fresh water, perhaps half of which is recoverable for beneficial use. Wells and springs that yield water from these aquifers currently supply more than 60 percent of the water used in the state, chiefly in the west where lakes and other surface-water supplies are less abundant.

Because of the great importance of Oklahoma's ground-water resources, special care must be taken to prevent pollution or contamination of aquifers. State rules and regulations prohibit storage or disposal of hazardous waste materials above or within principal aquifers or their recharge areas.

SEE ALSO: ENVIRONMENT AND CULTURAL ECOLOGY, RIVERS AND CREEKS, WATER QUALITY ACTS.

BIBLIOGRAPHY: Kenneth S. Johnson, Maps Showing Principal Ground-Water Resources and Recharge Areas in Oklahoma (Norman: Oklahoma State Department of Health and Oklahoma Geological Survey, 1983). Kenneth S. Johnson, "Principal Ground-Water Resources in Oklahoma," Earth Sciences and Mineral Resources of Oklahoma, Educational Publication 9 (Norman: Oklahoma Geological Survey, 2004). Oklahoma's Water Atlas (Norman: Oklahoma Water Resources Board, 1984).

Kenneth S. Johnson

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