The Gulf Coast Regional Aquifer-System Analysis is a study of regional aquifers in sediments of mostly Cenozoic age in an area of about 230,000 sq mi in the Central Plain of Alabama, Arkansas , Florida, Illinois, Kentucky, Louisiana, Mississippi, Missouri, Tennessee, and Texas, and an additional 60,000 sq mi offshore. Three aquifer systems have been identified: the Mississippi embayment aquifer system, the Texas coastal uplands aquifer system, and the coastal lowlands aquifer system. These aquifer systems thicken from < 100 ft near their updip limit to thousands of ft gulfward toward their downdip limits. The Mississippi embayment aquifer system exceeds 5,000 ft in thickness in central Louisiana and in southwestern Mississippi. The thickest area in southwestern Mississippi underlies most of the six Mississippi counties, centered around Jefferson County. The greatest thickness of the coastal lowlands aquifer system in Mississippi occurs in southern Hancock County where the system is composed of several individual aquifers and confining units. There are seven aquifers and three confining units in the Mississippi embayment aquifer system, five aquifers and two confining units in the Texas coastal uplands aquifer system, and five aquifers and two confining units in the coastal lowlands aquifer system. Most of the thicker parts of each aquifer system contain moderately saline to very saline water. Water in the Mississippi embayment aquifer system is moderately saline to very saline in most of a seven county area in southwestern Mississippi. About 9,600 million gal/day (gpd) of ground water was pumped from the aquifers in the study area during 1980. About 15% of that pumpage (or about 1,400 million gpd was in Mississippi, mostly from the Mississippi River Valley alluvial aquifer of the Mississippi embayment aquifer system. About 10% of the Mississippi pumpage, or 140 million gpd, was from the coastal lowlands aquifer system. Preliminary results from simulation of groundwater flow indicates that parts of Mississippi are major regional recharge areas for both the Mississippi embayment aquifer system and the coastal lowlands aquifer system.