Geohydrology and susceptibility of major aquifers to surface contamination in Alabama; area 10

Abstract

The U.S. Geological Survey, in cooperation with the Alabama Department of Environmental Management, is conducting a series of geohydrologic studies to delineate the major aquifers and their susceptibility to contamination in Alabama. This report delineates and describes the geohydrology and susceptibility to contamination of the major aquifers in Area 10 which includes Choctaw, Clarke, and Washington Counties. The major aquifers in Area 10 are the Pliocene-Miocene aquifer which includes the Jackson Group and the Oligocene, Miocene, and Pliocene Series; the Lisbon aquifer which includes the upper part of the Tuscahoma Formation, the Hatchetigbee and Tallahatta Formations, the Lisbon Formation, and the Gosport Sand; and the Nanafalia-Clayton aquifer which includes the Naheola and Nanafalia Formations and the lower part of the Tuscahoma Formation (includes Clayton Formation east of the study area). Water in these aquifers generally is confined. The Nanafalia-Clayton aquifer is widely used for public water supplies in the northern part of Area 10. The Lisbon aquifer is used in the central part of the study area, and the Pliocene-Miocene aquifer is used in the southern part. The potentiometric surfaces of the major aquifers have not been lowered significantly by long-term withdrawals in Area 10. The recharge areas for all the major aquifers in the study area are susceptible to surface contamination. In most of the area, however, the major wells are some distance from the recharge area of the aquifer they tap, and are protected from surface contamination by the depth of the aquifer below the surface. Most of the recharge areas are in rural areas used for timberlands, farms, and pastures. Permeable terrace and alluvial deposits overlie the outcrops of the major aquifers along the Tombigbee and Alabama Rivers. Presently, water discharges from these areas to the rivers, but if in the future pumpage increases causing a depression in the potentiometric surface of one of the underlying aquifers, the terrace and alluvial deposits will become a source of recharge and, potentially, a source of contamination.