The Gulf Coast Regional Aquifer-System Analysis (RASA) covers an area of 230,000 square miles onshore and 60,000 square miles of the adjacent Continental Shelf (fig. 1) (Grubb, 1987). The aquifer system consists of Cenozoic sediments that were divided into aquifers, permeable zones, and confining units (Grubb, 1987, p. 104). This division was accomplished by: 1) Identifying areally extensive units of low permeability; 2) identifying large hydraulic conductivity contrasts between adjacent permeable zones not separated by a regional confining unit; and 3) identifying variations in hydraulic head with depth (Weiss and Williamson, 1985; Weiss, 1990; and Hosman and Weiss, 1988).

The layer numbers shown above will be used throughout the rest of this report for convenience. The Midway confining unit is the base of the flow system throughout much of the study area (Grubb, 1984).

Salt domes in the study area are found in five distinct regions. These regions, known as salt basins, probably reflect thick accumulations of sedimentary salt. Overburden pressures and density differences created by later sedimentary deposits then caused the salt to flow into salt structures. The salt basins (fig. 1) are: South Texas, East Texas, North Louisiana, Mississippi, and Gulf Coast. The Gulf Coast salt basin, the largest, underlies southeastern Texas, southern Louisiana, and the adjacent Continental Shelf