Oklahoma Water Resources Board the Water Agency

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Gerty Sand

The Gerty Sand aquifer, located in southeast Oklahoma, is comprised of an unconsolidated Quaternary-age sedimentary deposit unconformably overlying Pennsylvanian and Permian-age formations. The Gerty Sand is primarily fine-grained sand with interbedded silt and clay and a base composed of medium to coarse-grained sand containing larger grains of quartzite, chert, limestone, and granite. The underlying upper Pennsylvanian Ada and Vanoss Formations and the lower Permian Stillwater, Stratford, and Wellington Formations are primarily shale and mudstone with well-indurated arkosic sandstones and limestone conglomerates. These formations largely inhibit the downward flow of groundwater, though limited quantities have been produced locally. The average thickness of the terrace deposit is slightly less than 50 feet and ranges from 10 to 100 feet with the greatest thickness near the central portion of the study area.

Total reported use has declined since the 1960s from a high of more than 2,500 acre-feet in 1969 to little more than 500 acre-feet in 2015. The aquifer's primary use was irrigation through the 1990s, which subsequently declined as public water supply use grew. In the past few years reported use between the two has been near equal.

Well yield ranges from 5 to 850 gallons per minute. The highest-yielding wells are those completed for irrigation and public water supply. These wells are centrally located where the deposits and saturated thicknesses are greatest. The water is chemically similar across the study area and of good quality, hard, and moderately alkaline due to calcium and sodium-bicarbonate. Total dissolved solids were less than 400 milligrams per liter. Nitrate and Phosphorus were 2.81 and 0.15 milligrams per liter, respectively, and the only metals detected were Barium, Zinc, and Copper with concentrations well below the EPA standard. Other metals, including Arsenic, were not detected.

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