Geology, hydrology, and water quality of the surficial aquifer system in Volusia County, Florida

Abstract

The surficial aquifer system in Volusia County comprises Miocene to Hotocene age sediments that overlie the Floridan aquifer system. The sediments consist of sand, sandy day, shell, and calcareous silty day, that together range in thickness from about 40 to more than 100 feet Locally, the surficial aquifer system can be subdivided into upper and lower permeable zones separated by 5 to 10 feet of arealry discontinuous day or silty sand. Of the recorded 4,500 wells that tap the surficial aquifer system, more than 3,200 are used for irrigation and about 800 are used for domestic supply. Water use from the surficial aquifer system in 1987 totaled about 4.2 million gallons per day. The water level can be 30 feet or more below land surface in ridge areas, but is less than 10 feet below land surface on terraces and in the interridge area near the St Johns River. In 1986, water levels in the upper permeable zone generally ranged 3 to 6 feet higher at the end of the wet season (fall) than in the dry season (spring). Water levels in wells tapping the lower permeable zone generally fluctuated less than 2 feet, except when influenced by pumping from the underlying Upper Floridan aquifer. Recharge to and discharge from the surficial aquifer system is strongly influenced by heads in the underlying Upper Floridan aquifer. Important recharge areas are along the De Land Ridge and the western part of the Talbot Terrace; some recharge occurs along the Atlantic Coastal Ridge. The recharge rate in the ridge areas probably ranges from 9 to 18 inches per year, whereas in nonridge areas the rate is about 0 to 8 inches per year. Reported laboratory hydraulic conductivities for surficial aquifer system core samples ranged from 7.6 x 10~5 to 3.4 x 10~1 feet per day with a median of 1.0 x 10~2 feet per day, and reported field hydraulic conductivities ranged from 3.0 x 10~2 to 12.8 feet per day with a median of 2.9 x 101 feet per day. The transmissivity of the lower permeable zone in Oak Hill (southeastern Volusia County), calculated from an aquifer test, is 1,200 feet squared per day, and the corresponding hydraulic conductivity is about 30 feet per day. In the Oak Hill area, pumpage from the lower permeable zone is constrained by the potential for upconing of saltwater, rather than by the hydraulic properties of the aquifer. Chloride concentrations of water from wells tapping the upper permeable zone ranged from 1.2 to 15,000 milligrams per liter; for the lower permeable zone, the range was from 5.7 to 340 milligrams per liter. In both zones, nutrient concentrations at some sites were higher than would be expected for natural ground water, indicating some effect from infiltrating surface water or human activity. INTRODUCTION :2 Volusia County covers an area of about 1,200 mi (square miles) in east-central Florida (fig. 1). Rapid population growth in the county has been spurred by increased tourism along the Atlantic Coast, by the growth of Orlando to the south (making some areas of south Volusia County a bedroom community of Orlando) and by a strong agricultural industry in the western and northwestern part of the county. The increase in population has also increased the demand for water for public and industrial supply, increased the need for development of areas that were once unpopulated, and increased the need for additional areas for disposal of municipal waste. These activities and demands either directly, or indirectly, affect the water resources of the county. Previous hydrologic studies in the area focused almost entirely on the Floridan aquifer system, which is the main source of potable water in the county. The surficial aquifer system, which consists of the sediments overlying the Floridan, has not been studied in detail. The surficial aquifer system is not a major source of drinking water except in the southeastern part of the county. Its most important hydrologic function is to store and transmit water to and from the underlying Floridan aquifer system. Information about the surficial aquifer system is needed to help county planners and environmental managers make informed decisions when evaluating plans for future development. To that end, the U.S. Geological Survey, in cooperation with Volusia County, conducted this study during 1985 through 1988 to provide geologic and hydrologic information on the surficial aquifer system. Purpose and Scope This report presents and interprets all the data collected during the hydrologic study of the surficial aquifer system of Volusia County in 1985-88. Because the study was of a reconnaissance nature and covered the entire county, the resulting interpretations are somewhat generalized. More detailed site-specific investigations may