The Floridan aquifer system, composed of the Upper and Lower Floridan aquifers, is a thick sequence of Paleogene carbonate rock which spans an area of about in the southeastern United States, it underlies parts of which  state in the southern region of the United States, with part of its southern border formed by the Gulf of Mexico, and its western border is formed by the Mississippi River?
Floridan aquifer:  It underlies the entire state of Florida and parts of Alabama, Georgia, Mississippi, and South Carolina. Mississippi: Mississippi is a state in the southern region of the United States, with part of its southern border formed by the Gulf of Mexico.
Mississippi