The Seymour aquifer consists mainly of the scattered erosional remnants of the Seymour Formation of Pleistocene age. The aquifer has been referred to in the literature as the "north-central Texas alluvial aquifers" because it is in 22 separate areas of alluvium in parts of 20 Texas counties in the upper Red and upper Brazos River Basins (fig. 32). The areas are predominantly in the Central Lowland Physiographic Province; only parts of the five westernmost areas are in the Great Plains Province. Average annual precipitation in the area ranges from 19 to 26 inches, and average annual runoff ranges from 0.2 to 1 inch. The aquifer generally has less than 100 feet of saturated thickness, but it is an important source of water for domestic, municipal, and irrigation needs.
During Pleistocene time, the eroded bedrock surface, which was developed mostly on poorly permeable red beds of Permian age, was covered by the Seymour Formation. The Seymour Formation consists of clay, silt, sand, and gravel that were deposited by eastward-flowing streams. Subsequent erosion left scattered remnants of the Seymour Formation mostly in interstream areas, and some of the eroded material was redeposited, thus forming the younger alluvium and alluvial terraces in stream valleys. The younger deposits are similar in composition to the Seymour Formation and compose part of the Seymour aquifer (fig. 33).
Areal extents of the individual alluvial areas range from about 20 square miles for an area in Baylor County to about 430 square miles for an area that spans Haskell and Knox Counties. Saturated thickness locally is as much as 100 feet but usually ranges between 20 and 60 feet. Water in the aquifer generally is unconfined; however, it may be confined locally by beds of clay. The alluvium is recharged mainly by direct infiltration of precipitation that falls on the land surface. Ground water moves toward points of discharge along streams or toward pumping wells. Yields of wells completed in the alluvium range from less than 100 to as much as 1,300 gallons per minute and average about 300 gallons per minute. The chemical quality of water in the alluvial aquifer ranges from fresh to slightly saline. In some areas, the water is hard and contains dissolved-solids concentrations in excess of 2,500 milligrams per liter; consequently, its suitability for some uses is restricted.
About 4.5 million acre-feet of fresh to slightly saline water was estimated to be in storage in the Seymour aquifer in 1974. About 75 percent of this water, or about 3.4 million acre-feet, was estimated to be recoverable.
An estimated 120 million gallons per day was withdrawn from the Seymour aquifer during 1959. About 94 percent was used for irrigation, and the remainder, for public and industrial supplies. More than 50 percent of the total withdrawal was for irrigation in the area that spans Haskell and Knox Counties.
SEYMOUR AQUIFER IN HASKELL AND KNOX COUNTIES
The part of the Seymour aquifer that is most intensively developed is in Haskell and Knox Counties, and is the largest continuous part of the aquifer. The Seymour aquifer is the only available source of water for moderate to large irrigation supplies in the local area. The aquifer furnished water to more than 2,000 irrigation wells during 1976; it also is a widely used source for domestic and livestock watering supplies. The areal extent of this part of the aquifer is about 430 square miles. Saturated thickness of the aquifer is generally 20 to 40 feet but is as much as 60 feet in northern Haskell County (fig. 34). Buried channels and valleys on the surface of the Permian red beds are areas where the Seymour Formation is thick and consists of coarse grained material. Sand and gravel that form productive aquifers are generally in the lower part of the Seymour Formation.
The hydrogeologic section in figure 35 is located along the maximum length of the aquifer from western Haskell County to eastern Knox County, and shows that the Seymour Formation overlies the Clear Fork Group of Permian age. The younger alluvium and alluvial terraces along the Brazos River are at lower altitudes and are not in hydraulic connection with the Seymour Formation. The slope of the potentiometric surface of the Seymour aquifer generally conforms to the slope of the land surface and to the surface of the underlying Permian rocks. The altitude of the potentiometric surface in January 1977 (fig. 36) indicates that ground water moved generally northward toward the Brazos River from a high area on the potentiometric surface in central Haskell County.
Wells completed in the Seymour aquifer are typically 40 to 60 feet deep. Well yields average about 270 gallons per minute and are as great as 1,300 gallons per minute. Transmissivity of the aquifer ranges from 2,700 to more than 40,000 feet squared per day and averages 13,400 feet squared per day. The chemical quality of the ground water is extremely variable. Concentrations of dissolved solids range from 300 to 3,000 milligrams per liter; most values are between 400 and 1,000 milligrams per liter.
Ground-water contamination is a problem in some areas and is related mainly to pesticides and fertilizers used in agriculture and to human and animal wastes (septic tanks, barnyards, feedlots, and sewage-treatment plants). Contamination from brine disposal and leakage from wells that are or were a part of oilfield activities is expected to remain a localized problem.
FRESH GROUND-WATER WITHDRAWALS
Withdrawals of freshwater from the Seymour aquifer totaled about 121 million gallons per day during 1985 (fig. 37). Approximately 110 million gallons per day was withdrawn for agricultural purposes, the principal water use. About 9 million gallons per day was withdrawn for public supply, and about 1 million gallons per day was pumped for domestic and commercial uses. About 1 million gallons per day was withdrawn for industrial, mining, and thermoelectric-power uses.