Potential for saltwater intrusion into the Upper Floridan aquifer, Hernando and Manatee counties, Florida

Pumpage from the Upper Floridan aquifer has caused a lowering of the potentiometric surface and has increased potential for saltwater intrusion into the aquifer in coastal areas of west-central Florida. Groundwater withdrawals are likely to increase because of expected population growth, especially in coastal areas. To increase the understanding of the potential and mechanics of saltwater intrusion, two sites were selected for study. Data were collected at each site from a centrally located deep well, and digital models were developed to simulate groundwater flow and solute transport. The northern site is in Hernando County near the town of Aripeka. The test well in the area was drilled about 1 mile from the coast to a depth of 820 ft. Freshwater was present in the carbonate rock aquifer to a depth of about 500 ft and saltwater occurred from 560 ft to the base of the aquifer at about 750 ft. Between the freshwater and saltwater is the zone of transition, also referred to as the freshwater-saltwater interface. The southern site is in Manatee County near the town of Rubonia. Drilling of the test well was completed at 1,260 ft, just below the base of the Upper Floridan aquifer. The transition zone in this well occurs between 875 and 975 ft within a highly permeable zone. Digital simulations show flow patterns similar to the cyclic flow of seawater and interface theory. Simulations have shown that saltwater contamination of coastal wells would not be noticed as quickly as water-level declines resulting from inland pumpage. (USGS)