Ground-water geology of Grayson County, Texas

Abstract

Grayson County in north-central Texas is near the north edge of the West Gulf Coastal Plain. The county has an area of 927 square miles and had an estimated population of 79,500 in 1957. The major town is Sherman, which has an estimated population of 31,000. The northern two-thirds of the county is drained by tributaries of the Red River; the southern one-third is drained by tributaries of the Trinity River Sedimentary rocks exposed at the surface in Grayson County are of Cretaceous and Quaternary age. Sand, clay, marl, and limestone of Cretaceous age, having a maximum thickness of about 3,600 feet, underlie the county; the beds dip regionally to the southeast. Quaternary alluvium mantles part of the surface along the Red River and occurs in scattered patches elsewhere in the county. The Trinity group and Woodbine formation of Cretaceous age are the principal water-bearing formations. Other stratigraphic units that yield water to wells are, in order of importance, the Quaternary alluvium and the Pawpaw formation, Eagle Ford shale, and Austin chalk of Cretaceous age. Ground water in Grayson County generally moves eastward and southward from areas of recharge to areas of discharge. Average rates of water movement in the Trinity group and Woodbine formation are estimated to be about 1.5 and 15 feet per year, respectively. The chief source of recharge to these aquifers is precipitation on the outcrop, although Lake Texoma contributed some recharge to the Trinity where it crops out in the lake. Ground water discharges naturally by evapotranspiration, by vertical leakage, through springs, artificially through wells, and by underflow out of the county to the southeast. The withdrawal of ground water in Grayson County in 1957 was about 5 mgd. Of this amount, about 61 percent came from the Woodbine formation, about 36 percent from the Trinity group, and about 3 percent from the other water-bearing formations. About 65 percent of the ground water pumped in Grayson County is withdrawn in the Sherman area. Increased withdrawal of water since World War II has resulted in a rapid decline of the water levels in parts of Grayson County. The maximum decline in the Trinity group at Sherman from 1945 to 1958 was 113 feet, or about 8 feet per year. During the same period, water levels in the Woodbine formation at Sherman declined as much as 156 feet, an average of 12 feet per year. Total declines since the early part of the 20th century were at least 180 feet in the Trinity group and about 240 feet in the Woodbine formation. Water levels in the area of outcrop of the principal aquifers, fluctuating chiefly in response to rainfall or changes in the natural rate of recharge, showed no appreciable decline from 1957 to 1959. Coefficients of transmissibility, determined from pumping tests in Grayson County, averaged 2,800 gpd per ft for the Trinity group and 3,200 gpd per ft for the Woodbine formation. Kesults of chemical analyses of water samples indicate that the ground water in Grayson County is suitable for most purposes. The Trinity group generally yields soft water that has a high sodium bicarbonate content and is of questionable quality for irrigation. The water from the Woodbine formation ranges more widely in chemical composition than the water from the Trinity. It generally is soft but has a high iron content; it is usually suitable for irrigation in the outcrop area but unsuitable in the downdip area. Water from the other water-bearing formation, though generally hard, is suitable for most purposes, judging from the few analyses available. The ground-water resources of Grayson County have been only partly developed. The volume of fresh water in transient storage in the Trinity group and Woodbine formation is estimated to be about 60 and 25 million acre-feet, respectively. Most of this water is not practicably recoverable because of the depth at which it occurs, but relatively high artesian heads and large available drawdowns in much of the county are favorable to future development within economic limits of pumping lift. In the Sherman area, however, concentrated pumping has caused large declines in the water levels, resulting in some dewatering of the Woodbine. Because of the large margin of 'safety before dewatering of the Trinity group begins, the Trinity is the most favorable source of additional ground water for Sherman. However, the higher lifting costs should be considered. Large to moderate amounts of additional ground water can be obtained from the Trinity group and Woodbine formation in most presently undeveloped areas in the county. Water suitable for irrigation is available in moderate to large amounts from the Woodbine formation in places on its outcrop. A limiting factor to any large ground-water development, however, is the extent and thickness of saturated fresh-water sand available in the area. The thickness of saturated fresh-water sand in the Trinity decreases northward; the thickness of the sand in the Woodbine is more erratic and has little definite pattern. Moderate to large supplies of water may be available from the alluvium near the Red River, but more information is needed before definite conclusions can be reached.