Geology topics

Research about northeastern Florida

Source-linked reports with geographic coverage including northeastern Florida.

4 recordsLinked to original sources

USGS2001

Geochemistry and origins of mineralized waters in the Floridan aquifer system, northeastern Florida

Increases in chloride concentration have been observed in water from numerous wells tapping the Floridan aquifer system in northeastern Florida. Although most increases have been in the eastern part of Duval County, Florida, no spatial pattern in elevated chloride concentrations is discernible. Possible sources of the mineralized water include modern seawater intrusion; unflushed Miocene-to-Pleistocene-age seawater or connate water in aquifer sediments; or mineralized water from deeper zones of the aquifer system or from formations beneath the Floridan aquifer system. The purpose of this study was to document the chemical and isotopic characteristics of water samples from various aquifer zones, and from geochemical and hydrogeologic data, to infer the source of the increased mineralization. Water samples were collected from 53 wells in northeastern Florida during 1997-1999. Wells tapped various zones of the aquifer including: the Fernandina permeable zone (FPZ), the upper zone of the Lower Floridan aquifer (UZLF), the Upper Floridan aquifer (UFA), and both the UFA and the UZLF. Water samples were analyzed for major ions and trace constituents and for isotopes of carbon, oxygen, hydrogen, sulfur, strontium, chlorine, and boron. Samples of rock from the aquifer were analyzed for isotopes of oxygen, carbon, and strontium. In general, water from various aquifer zones cannot be differentiated based on chemistry, except for water from FPZ wells. Major-ion concentrations vary as much within the upper zone of the Lower Floridan aquifer and the Upper Floridan aquifer as between these two zones. Simple models of mixing between fresh ground water and either modern seawater or water from the FPZ as a mineralized end member show that many water samples from the UZLF aquifer and the UFA are enriched in bicarbonate, calcium, magnesium, sulfate, fluoride, and silica and are depleted in sodium and potassium (as compared to concentrations predicted by simple mixing). Chemical mass-balance models of mixing and reactions between a hypothetical initial seawater and aquifer minerals cannot account for the observed water chemistry in a few wells, implying a source other than seawater, either ancient or modern, or the occurrence of other more complex rock-water reactions. Hydrogeologic and geochemical data from water and aquifer samples indicate that the most likely source of mineralized water in some wells yielding water with increasing chloride concentrations is water from the FPZ. In other wells, the flushing of Miocene-to-Pleistocene-age seawater can account for the observed chloride concentrations. The fact that most of the water samples collected are a mixture of less than one percent of mineralized water with more than 99 percent fresh or recharge water makes identifying the source of the mineralized water difficult. Differences in carbon-14 and sulfur-34 values probably reflect areal differences in aquifer mineralogy and distribution of organic carbon related to paleokarst features. Geochemical mass-balance models of seawater-rock interaction are unable to account for the chemical and isotopic composition of mineralized water from the FPZ, which implies another source of mineralized water, such as a brine, or the occurrence of more complex water-rock reactions.

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USGS1995

The distribution of bromide in water in the Floridan aquifer system, Duval County, northeastern Florida

Although Duval County, Florida, has ample ground-water resources for public supply, the potential exists for a problem with excessive disinfectant by-products. These disinfectant by-products result from the treatment of raw water containing low concentrations of bromide and naturally occurring organic compounds. Because of this potential problem, the relation of bromide concentrations to aquifer tapped, well location and depth, and chemical characteristics of water in the Floridan aquifer system underlying Duval County were studied to determine if these relations could be applied to delineate water with low-bromide concentrations for future supplies. In 1992, water samples from 106 wells that tap the Floridan aquifer system were analyzed for bromide and major dissolved constituents. A comparison of bromide concentrations from the 1992 sampling with data from earlier studies (1979-80) indicates that higher bromide concentrations were detected during the earlier studies. The difference between the old and new data is probably because of a change in analytical methodology in the analysis of samples. Bromide concentrations exceeded the detection limit (0.10 milligrams per liter) in water from 28 of the 106 wells (26 percent) sampled in 1992. The maximum concentration was 0.56 milligrams per liter. There were no relations between bromide and major dissolved constituents, well depth, or aquifer tapped that would be useful for determining bromide concentrations. Areal patterns of bromide occurrence are not clearly defined, but areas with relatively high bromide concentrations tend to be located in a triangular area near the community of Sunbeam, Florida, and along the St. Johns River throughout Duval County.

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USGS1994

Saltwater intrusion and quality of water in the Floridan aquifer system, northeastern Florida

Saltwater intrusion into freshwater aquifers has resulted in increased chloride concentrations in water from some wells in northeastern Florida. The principal areas of saltwater intrusion in the Floridan aquifer system in the study area are east-central Duval County, the southern two-thirds of St. Johns County, and along the coast. At least five possible mechanisms of saltwater movement, some more plausible than others, could explain the observed increases in chloride concentration in the upper freshwater zones of the Floridan aquifer. They are (1) the presence of unflushed pockets of relict seawater; (2) lateral movement of the freshwater-saltwater interface off the northeastern Florida coast; (3) upconing of saltwater from deeper, salty zones below pumped wells; (4) upward leakage from deeper, salty water-bearing zones through failed, uncased, or improperly plugged or constructed wells; and (5) upward leakage from salty water-bearing zones through semiconfining units that are thin, or are breached by joints, faults, or collapse features. Ground-water withdrawals in Duval, Nassau, and St. Johns Counties increased from about 183 to 254 million gallons per day from 1965 to 1988. Approximately 90 percent of the total withdrawal is from the Floridan aquifer system, resulting in long-term declines in the potentiometric surface of the Upper Floridan aquifer of about one-third to three-fourths foot per year. Hydraulic heads in the lower part of the aquifer system are naturally higher than in the upper parts. Declines in head in the upper part have further increased the vertical head difference between zones, increasing the potential for vertical ground-water flow from lower zones of higher head upward through structural deformities, leaky confining beds, and wells, to higher zones of lower head. Lower zones typically have higher chloride concentrations than do upper zones. Concentrations of chemical constituents in water from the Floridan aquifer system vary both areally and with depth. Chloride concentrations in water in the Upper Floridan aquifer in the study area range from about 4.6 to 3,600 milligrams per liter. Data indicate that in much of the study area, water in the upper zone of the Lower Floridan aquifer generally is slightly more mineralized than water from the overlying Upper Floridan aquifer. Water from the Fernandina permeable zone varies in quality from fresh to saline. Chemical analyses of water from five monitoring wells tapping this zone indicate maximum chloride concentrations of 16,800 milligrams per liter. The potential for saltwater contamination of the freshwater-bearing zones probably will continue to increase in northeastern Florida as artesian pressure in the upper freshwater zones continues to decline. Implementation of wise water-management strategies could, however, reduce the potential for saltwater intrusion.

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USGS1983

Relation of concealed faults to water quality and the formation of solution features in the Floridan aquifer, northeastern Florida, U.S.A.

Geological and hydrological information on the Floridan aquifer in northeastern Florida indicates that isolated occurrences of water having relatively high chloride concentration in the upper part of the aquifer may be associated with buried faults. Water having chloride concentrations of more than 700 mg l −1 occurs in the deeper zone of the aquifer at depths below ∼ 600 m below sea level in the coastal and east-central part of the study area. This deep salty water is under higher artesian pressure than water in the shallower, generally freshwater zones, but it is restricted from moving upward by relatively impermeable dolomite beds. Two buried faults with vertical displacements of 30–45 m are in areas where relatively high concentrations of chloride have been detected in water in the upper part of the aquifer. Geochemical, artesian pressure, and water temperature data show that the source of the relatively high concentrations of chloride in water in the upper part of the aquifer is from the deeper zone. This indicates that the faults may have breached the dolomite confining beds and allowed the upward movement of salty water from the deeper zone. The upward movement of mineralized water along the faults may also have formed some of the solution features found in the aquifer near the faults. In this area, freshwater in the upper part of the aquifer is normally saturated with respect to calcite and dolomite. However, water from wells tapping the upper part of the aquifer near the faults is not fully saturated suggesting that the mixing of deep mineralized water with shallower freshwater produces a mixture that is not saturated with respect to these minerals and allows for the dissolution of limestone in the aquifer near the faults. Dissolution of limestone may also be occurring at the freshwater-saltwater interface in the deeper zones of the aquifer.

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