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

Abstract

Pumpage from the Upper Floridan aquifer has caused a lowering of the potentiometric surface and has increased the potential for saltwater intrusion into the aquifer in coastal areas of west-central Florida, Groundwater withdrawals are likely to increase because of the expected population growth, especially in coastal areas. To gain a better understanding of the potential for and mechanisms of saltwater intrusion, two sites were selected for study. Data were collected at each site from a centrally located deep well that completely penetrates the Upper Floridan aquifer. Digital models, based on this and other information, were developed to simulate groundwater flow and solute transport. The northern site is in Hernando County near the town ofAripeka. Tfie test well in the area was drilled about 1 mile from the coast to a depth of 820 feet. Freshwater was present in the carbonate rock aquifer to a depth of about 500 feet, and saltwater occurred in a low permeability zone from 560 feet to the base of the aquifer at about 750 feet. Between the freshwater and saltwater is the zone of transition, also referred to as the freshwater-saltwater interface. At this site, the transition zone is thin and the slope is fairly steep. Aquifer tests indicated that high permeability occurs between 450 and 500 feet, which accounts for a transmissivity of 56,000 feet squared per day. The southern site is in Manatee County near the town of Rubonia. Drilling of the test well was completed at 1,260 feet, just below the base of the Upper Floridan aquifer. The transition zone in this well occurs between 875 and 975 feet. Transmissivity in the study area was determined to be about 8,500 feet squared per day and is anomalously low, especially in contrast to the transmissivity on the other side of Tampa Bay in Pinellas County. Specific-capacity tests andflowmeter logs indicate that more than 80 percent of the flow comes from the upper 124 feet of uncased well between 462 and 586 feet. Hie digital models show flow patterns that are similar to the cyclic flow of seawater and interface theory. Large groundwater withdrawals simulated in the northern study area suggest that saltwater intrusion could cause chloride concentrations to increase in freshwater supply wells in coastal areas, depending upon distance from the shoreline, well depth, and pumping rate. In the southern area, saltwater is present below Manatee County in the lower permeable zone and could migrate farther inland and to upper permeable zones as a result of pumpage for irrigation and mining. The simulations have shown that saltwater contamination of coastal wells would not be noticed as quickly as water-level declines resulting from inland pumpage. Similarly, improvements to water quality in coastal wells will not be apparent as quickly as rising water levels resulting from restoration of groundwater resources.