Ground-water withdrawals, water-level changes, land-surface subsidence, and ground-water quality in Fort Bend County, Texas, 1969-87
Fort Bend County, which has one of the fastest growing populations of all counties in the United States, is dependent entirely on ground water for public supply. Since 1969, at least 90 large-capacity wells have been drilled, of which 57 were public supply wells, 23 were irrigation wells, and 10 were industrial wells. All but seven of the new public-supply wells are located in the northeastern part of the county.
The withdrawal of ground water in Fort Bend County increased from 56 million gallons per day in 1969 to 72 million gallons per day in 1982, and then decreased to 53 million gallons per day in 1986. Withdrawals for public supply increased from 4 million gallons per day in 1969 to 28 million gallons per day in 1986. Withdrawals for irrigation averaged 38 million gallons per day during 1969-82 and in 1984, but averaged 20 million gallons per day in 1983, 1985, and 1986. Withdrawals for industrial use declined from 13 million gallons per day in 1969 to 5 million gallons per day in 1986.
Water levels in wells screened in the upper unit of the Chicot aquifer generally fluctuated less than 4 feet between 1968-69 and 1987. During the same period, water-level declines in wells screened in the lower unit of the Chicot aquifer ranged from less than 10 feet in most of the western part of the county to 100 feet in the northeastern corner. In the southwestern part of the county, the decline was less than 20 feet. Hydrographs of wells completed in the lower unit of the Chicot showed that water levels continued to decline from 1969 to the early 1980's. The hydrographs of wells located outside of the northeast area generally show a stabilization of water levels corresponding to the reduction in withdrawals after 1982 in Fort Bend County and in the neighboring Houston metropolitan area to the northeast.
Withdrawals from the Evangeline aquifer increased from 15 percent of the total in 1969 to 50 percent in 1986. Water-level declines in wells screened in the Evangeline aquifer during the same period ranged from less than 25 feet in the northwestern part of the county to 125 feet in the northeastern part. In the southwestern one-quarter of the county, the decline was less than 50 feet.
Declines in the potentiometric surfaces of the aquifers have caused compaction of clay resulting in land-surface subsidence. Between 1906 and 1978, about 65 percent of the county subsided more than 0.5 foot. Studies in the Houston area have shown that most clay compaction occurs in the Chicot aquifer and that the compressibility of the clays increases towards the coast. The total thickness of clays in the Chicot aquifer increases from less than 150 feet in the northwestern part of Fort Bend County to more than 350 feet along the eastern border. The total thickness of clay in the Evangeline aquifer increases from an average of about 700 feet in the northwest to about 1,100 feet in the east. The northeastern part of Fort Bend County is most susceptible to future subsidence because it is the area where the largest water-level declines have occurred and where the thickest, most compressible clays in the county are found.
The concentrations of dissolved solids in water from wells in Fort Bend County have not changed appreciably from 1969 to 1987. The median concentrations of dissolved solids are 475 milligrams in water from wells in Fort Bend per liter in water from the upper unit of the Chicot aquifer, 337 milligrams per liter in water from the lower unit of the Chicot aquifer, and 307 milligrams per liter in water from the Evangeline aquifer.
|USGS Numbered Series
|Ground-water withdrawals, water-level changes, land-surface subsidence, and ground-water quality in Fort Bend County, Texas, 1969-87
|Water-Resources Investigations Report
|U.S. Geological Survey
|Texas Water Science Center
|v, 155 p.
|Fort Bend County
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