Abstract Volusia County is a 1,200-square-mile area in east-central Florida which includes the communities of Daytona Beach, Ormond Beach, New Smyrna Beach, Deland, and Deltona. The area is oriented toward tourism, especially along the coast. The two aquifers underlying the area are the unconfined surficial aquifer and the confined Floridan aquifer system. The surficial aquifer consists of sand, clay, and shell beds of Pleistocene and Holocene age, and the Floridan aquifer system consists of limestone and dolomite of Eocene age. The two aquifers are separated by confining clay beds of Miocene or Pliocene age. Both aquifers contain good quality water in most of Volusia County, but brackish water is present in the eastern and western fringes of the county in the surficial aquifer and in the upper zones of the Floridan aquifer system presently tapped by wells. Saltwater is present throughout the county in the relatively untapped lower zones of the Floridan aquifer system. Development in the fringes has caused movement of the brackish water. There is a need for an understanding of this movement so that future problems can be avoided. Average ground-water withdrawal in 1980 in Volusia County was 66 million gallons per day, of which 30 was for public supply, 24 was for agricultural irrigation, and 12 was for other uses. Ground-water use quadrupled between 1955 and 1980. More than 95 percent of the ground water pumped is from the Floridan aquifer system. Water levels in Floridan aquifer system wells declined more than 10 feet over an area of 70 square miles from 1955 to 1982. This drawdown, which occurred in the area of Ormond Beach, Daytona Beach, and Port Orange, was caused by pumping. The annual water budget of the surficial layer consists of 53 inches rainfall, 1 inch upward leakage from the Floridan aquifer system, 0.5 inch recharge from Floridan aquifer system withdrawals, and 0.5 inch stream inflow, counterbalanced with 39 inches evapotranspiration, 11 inches stream outflow, and 5 inches downward leakage to the Floridan aquifer system.
The annual water budget of the Floridan aquifer system consists of 5 inches downward leakage inflow, 1 inch upward leakage outflow, 1 inch horizontal inflow, 1.5 inches horizontal outflow, 2.5 inches discharge from springs and flowing wells, and 1 inch pumpage. Brackish water movement, or intrusion, apparently is occurring in the surficial aquifer on the barrier island in the northeast part of the study area. Intrusion is probably occurring in other sections of the barrier island, but there is no evidence that it is occurring in inland areas. Future intrusion is likely in the surficial aquifer on the barrier island as groundwater withdrawals increase. There is little evidence that intrusion is occurring uniformly over large areas in the Floridan aquifer system. Most nonpublic supply wells have not exhibited long-term changes in salinity, although significant increases in chloride concentration in water from five nonpublic supply wells in the coastal area of Ormond Beach and Daytona Beach may be associated with ground-water withdrawals in northeast Volusia County. Vertical intrusion is occurring at sites of public supply pumping because these wells are usually deeper, are pumped a greater amount of the time, and are pumped at higher rates. Many of these wells in the eastern and western fringes of Volusia County have been abandoned and replaced by new wells closer to the central part of the county. Water-management practices that may minimize future movement of brackish water in the Floridan aquifer system include minimizing well depths, minimizing drawdown, installing wells where the freshwater zone is thickest, increasing head in the freshwater zone by using injection wells, and reducing head in the saltwater zone. Drawdown can be reduced by increasing the number of supply wells per desired water-use rate. Minimizing well depth may be the single most effective step against intrusion.