Geology and ground-water resources of Montgomery County, Alabama

Abstract

Montgomery County includes an area of 790 square miles in east-central Alabama. The economy of Montgomery County is related primarily to the growing and processing of agricultural products. The county is in the northern part of the Coastal Plain. It consists of parts of four divisions of the Coastal Plain: the terraces, the Black Prairie, the Chunnennuggee Hills, and the flood plains. The county drains north and northwest into the Alabama and Tallapoosa Rivers, except for a small area in the southern part of the county that is drained by tributaries of the Conecuh River. Sedimentary rocks of Late Cretaceous age underlie Montgomery County. They are divided, in ascending order, into the following: Coker and Gordo formations of the Tuscaloosa group; Eutaw formation; and Mooreville and Demopolis chalks, Ripley formation, Prairie Bluff chalk, and Providence sand of the Selma group. The Clayton formation of Tertiary age crops out in a small area in the southern part of the county. Pleistocene terrace deposits of the ancestral Alabama River overlie the older rocks in the northern part of the county. Recent alluvium underlies the flood plains of the larger streams. The Cretaceous and younger rocks consist chiefly of clay, chalk, sandstone, sand, and gravel, and a few thin beds of limestone. These deposits are underlain by a basement complex of pre-Cretaceous crystalline rocks. Large-scale withdrawals of water began in the Montgomery area about 1885. Pumpage by the city of Montgomery in 1958 averaged about 15 million gallons per day. It is estimated that an additional 10 to 15 million gallons per day was pumped in the county for industrial, irrigation, domestic, and stock use. The principal aquifer in the country is the Eutaw formation. It supplies water to the city of Montgomery municipal wells, to industrial wells in the Montgomery area, and to most domestic and stock wells in the northern two-thirds of the county. Irrigation wells also tap the Eutaw. Yields from wells range from 350 to 600 gallons per minute. The Gordo formation, the upper part of the Coker formation, and the Pleistocene terrace deposits in the Montgomery area also yield moderate to large quantities of water to municipal and industrial wells. The lower part of the Coker formation is not developed as a source of water supply, but information obtained during the investigation rthat led to this report indicates that it may be a potential source of water to wells of large capacity. Sand beds in the Ripley formation, Providence sand, and Recent alluvium in -the southern part of the county yield adequate amounts of water to domestic and stock wells. Most of the ground water used in Montgomery County occurs under artesian conditions, although water-table conditions occur in the Pleistocene terrace deposits and Recent alluvium, and in the outcrop areas of the Eutaw and Eipley formations and the Providence sand. Most of the water recharging the Ooker, Gordo, and Eutaw formations in their areas of outcrop also is discharged in these areas; only a small quantity of water moves downdip beneath the overlying chalk beds. The natural discharge, and hence the natural recharge, is estimated to be 0.2 to 0.3 million gallons per day per square mile of outcrop. All ground water in the county is of chemical quality that is satisfactory for most uses, although locally it is high in iron or chloride content and is hard. Water from the Eutaw formation a few miles southwest of Montgomery's West well field is very high in chloride content. This water moves toward the cone of depression in the piezometric surface produced by pumping in the West well field. Much additional ground water could be pumped from the Eutaw formation, especially south of Montgomery's West well field. Additional water also is available from the upper part of the Coker formation. Before large groundwater developments are planned, however, the problems of well spacing and pumping rates should be studied in order to determine the maximum development permitted by the supply. Observation wells should be installed in the Eutaw formation southwest of Montgomery's West well field to detect encroachment of water of high chloride content from adjacent Lowndes County.