Saltwater intrusion is a potential threat to ground- water quality in the Floridan aquifer in Duval, Nassau, and St. Johns Counties. Five possible mechanisms of saltwater movement that could explain the increases in chloride concentrations in water in the freshwater zones of the Floridan aquifer system are: (1) unflushed pockets of relic seawater, (2) upward leakage of saltwater through failed, uncased, or improperly plugged or constructed wells, (3) lateral movement of saltwater from the north- eastern Florida coast, (4) upconing of saltwater from below pumped wells, and (5) saltwater movement into freshwater zones due to thinning, or breaching by joints, fractures, collapse features, or possibly by faults, of the overlying semiconfining beds. Total ground-water use in the study area increased more than 40 percent between 1965 and 1988, with approximately 90 percent of the water being with- drawn from the Floridan aquifer system. Pumping to satisfy this increase in water use resulted in a decline of the potentiometric surface, thus in- creasing the potential for saltwater movement upward into freshwater zones. The potential for saltwater intrusion is expected to increase as population growth continues and greater demands are placed on ground-water resources. Possible water-management strategies that could reduce this potential threat include: minimizing well depths, installation of new well fields in areas where the thickness of the freshwater lens is greatest, reducing drawdowns in wells fields and other areas where saltwater in- trusion could be occurring, and plugging the lower parts of some of the deeper wells.