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Washita Alluvium & Terrace (Reach 1)

Reach 1 of the Washita Alluvium and Terrace aquifer extends from the western Oklahoma-Texas border to Clinton, Oklahoma and is comprised of Quaternary-age alluvium deposited by the Washita River. The alluvium is comprised of fine to medium grained sand with some gravel and interbedded clays derived predominantly from the Ogallala Formation as indicated by the presence of quartzitic material. The composition of the aquifer varies vertically and horizontally as a result of the lenticular nature of these alluvial deposits. The Quaternary age sediments from the Washita River overlie the Tertiary-age Ogallala Formation on the western end of Roger Mills County. The Washita River alluvium overlies the Cloud Chief Formation through most of its length. The Ogallala Formation consists of calcium carbonate cemented sand with gravel, silt, and clay. The Cloud Chief Formation is composed of interbedded sandstones, siltstones, mudstones, gypsum-anhydrite, and dolomite. The lithology of the Cloud Chief Formation limits yields to only a few gallons a day and acts as a confining unit that inhibits the downward movement of groundwater. The 93-mile stretch of Quaternary alluvium and terrace averages one mile wide with 90 to 200 or more feet of saturated thickness.

The predominant use of groundwater from this aquifer is agriculture. Nearly half the land in the study area is used for crops such as wheat, cotton, and sorghum, while the other half serves as pasture. Wells in this aquifer typically yield between 200 to 1400 gallons per minute, with an average of 600 gallons per minute. The variation of well yields can be attributed to sharp changes in permeability and saturated thickness due to the varying composition of the alluvium.

Water from the Washita Alluvium and Terrace aquifer Reach 1 is relatively high in dissolved solids, averaging around 3,000 parts per million, with the main constituents being bicarbonate hardness and sulfate. The water quality is affected by the underlying Cloud Chief Formation, in which intermittent beds of gypsum and dolomite are present, contributing appreciable water to the alluvium.

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