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Scientific Investigations Report 2005—5134

Hydrogeology, Water Quality, and Saltwater Intrusion in the Upper Floridan Aquifer in the Offshore Area near Hilton Head Island, South Carolina, and Tybee Island, Georgia, 19992002

U.S. Geological Survey Scientific Investigations Report 2005—5134, 48 pages (Published 2006)
ONLINE ONLY

By W. Fred Falls, Camille Ransom, James E. Landmeyer, Eric J. Reuber, and Lucy E. Edwards

This report is available online in pdf format: USGS SIR 2005-5134 (4 Mb) and a high-resolution format: USGS SIR 2005-5134 (42 Mb)

ABSTRACT

Cover of SIR 2005—5134.

To assess the hydrogeology, water quality, and the potential for saltwater intrusion in the offshore Upper Floridan aquifer, a scientific investigation was conducted near Tybee Island, Georgia, and Hilton Head Island, South Carolina. Four temporary wells were drilled at 7, 8, 10, and 15 miles to the northeast of Tybee Island, and one temporary well was drilled in Calibogue Sound west of Hilton Head Island.

The Upper Floridan aquifer at the offshore and Calibogue sites includes the unconsolidated calcareous quartz sand, calcareous quartz sandstone, and sandy limestone of the Oligocene Lazaretto Creek and Tiger Leap Formations, and the limestone of the late Eocene Ocala Limestone and middle Avon Park Formation. At the 7-, 10-, and 15-mile sites, the upper confining unit between the Upper Floridan and surficial aquifers correlates to the Miocene Marks Head Formation. Paleochannel incisions have completely removed the upper confining unit at the Calibogue site and all but a 0.8-foot-thick interval of the confining unit at the 8-mile site, raising concern about the potential for saltwater intrusion through the paleochannel-fill sediments at these two sites. The paleochannel incisions at the Calibogue and 8-mile sites are filled with fine- and coarse-grained sediments, respectively.

The hydrogeologic setting and the vertical hydraulic gradients at the 7- and 10-mile sites favored the absence of saltwater intrusion during predevelopment. After decades of onshore water use in Georgia and South Carolina, the 0-foot contour in the regional cone of depression of the Upper Floridan aquifer is estimated to have been at the general location of the 7- and 10-mile sites by the mid-1950s and at or past the 15-mile site by the 1980s. The upward vertical hydraulic gradient reversed, but the presence of more than 17 feet of upper confining unit impeded the downward movement of saltwater from the surficial aquifer to the Upper Floridan aquifer at the 7- and 10-mile sites.

At the 10-mile site, the chloride concentration in the Upper Floridan borehole-water sample and the pore-water samples from the Oligocene and Eocene strata support the conclusion of no noticeable modern saltwater intrusion in the Upper Floridan aquifer. The chloride concentration of 370 milligrams per liter in the borehole-water sample at the 7-mile site from the Upper Floridan aquifer at 78 to 135 feet below North American Vertical Datum of 1988 is considerably higher than the chloride concentration of 25 milligrams per liter measured at the 10-mile site. The higher concentration probably is the result of downward leakage of saltwater through the confining unit at the 7-mile site or could reflect downward leakage of saltwater through an even thinner layer of the upper confining unit beneath the paleochannel to the northeast and lateral movement (encroachment) from the paleochannel to the 7-mile site. Carbon-14 concentrations at both sites, however, are low and indicate that most of the water is relict fresh ground water.

The hydrogeology at the 15-mile site includes 17 feet of the upper confining unit. The chloride concentration in the Upper Floridan aquifer is 6,800 milligrams per liter. The setting for the Upper Floridan aquifer beneath the 15-mile site is interpreted as a transitional mixing zone between relict freshwater and relict saltwater.

At the Calibogue site, 35 feet of fine-grained paleochannel-fill sediments overlies the Oligocene strata of the Upper Floridan aquifer. The vertical hydraulic conductivity of the paleochannel fill at this site is similar to the upper confining unit and effectively replaces the missing upper confining unit. Chloride concentrations and low carbon-14 and tritium concentrations in borehole water from the Upper Floridan aquifer, and low chloride concentrations in pore water from the upper confining unit indicate relict freshwater confined in the Upper Floridan aquifer at the Calibogue site.

The coarse-grained paleochannel-fill sediments at the 8-mile site along with the downward hydraulic gradient that began in the 1950s potentially created a pathway for downward leakage of saline water from the surficial aquifer to the Upper Floridan aquifer. The presence of very saline water in the Oligocene strata of the Upper Floridan aquifer at the 8-mile site supports the occurrence of saltwater intrusion or encroachment. Beneath the Oligocene strata, pore-water chloride concentration decreases to less than 500 milligrams per liter in the Eocene part of the Upper Floridan aquifer. The water in the Oligocene strata at the 8-mile site has a carbon-14 concentration of 2.3 percent modern carbon; however, this site has the highest tritium concentration (4 tritium units) of all the temporary wells. The calculated freshwater/modern saltwater mixing of the water at the 8-mile site is 55/45 percent. Therefore, the carbon-14 and tritium concentrations do not support the concept of relatively modern saline water mixing with relict freshwater in the Upper Floridan aquifer at the 8-mile site. Instead, the carbon-14 and tritium results indicate that the intruding water predominantly is relict saline water from the surficial aquifer with only a minor component of modern saltwater.


CONTENTS

Abstract

Introduction

Purpose and Scope

Location of the Study Area

Previous Investigations

Well-Identification System

Acknowledgments

Offshore Drilling and Methods

Hydrogeology

Hydrogeologic Setting

Ground-Water Supply and Saltwater Contamination

Ground-Water Flow and Potential for Saltwater Intrusion

Geologic and Hydraulic Characteristics of Hydrogeologic Units

Surficial Aquifer

Paleochannel Fill

Upper Confining Unit

Upper Floridan Aquifer

Offshore Water Levels in the Upper Floridan Aquifer

Water Quality

Borehole-Water Samples

Stable Oxygen and Hydrogen Isotopes

Carbon Isotopes

Tritium

Pore-Water Samples

Saltwater Intrusion

The 7- and 10-Mile Sites

The 15-Mile Site

The 8-Mile and Calibogue Sites

Summary

References

Appendix 1. Identification, location, construction, and water-level data for one well site in Calibogue Sound and four offshore well sites southeast of Hilton Head Island, South Carolina

Appendix 2. Dinoflagellate taxa identified in core samples collected at the four offshore and Calibogue sites, and interpretation of age and geologic unit

Appendix 3. Field and laboratory analyses for water samples collected from the Calibogue and four offshore wells; well 39Q025, Tybee Island, Georgia; well HAM-122, Hampton County, South Carolina; and the Atlantic Ocean


REPORT AVAILABILITY

This report is available online in pdf format: USGS SIR 2005-5134 (4 Mb) and a high-resolution format: USGS SIR 2005-5134 (42 Mb)
To view the PDF document, you need the Adobe Acrobat® Reader installed on your computer. (A free copy of the Acrobat® Reader may be downloaded from Adobe Systems Incorporated.)

Suggested citation:

Falls, W.F., Ransom, Camille, Landmeyer, J.E., Reuber, E.J., and Edwards, L.E., 2005, Hydrogeology, water quality, and saltwater intrusion in the Upper Florida aquifer in the offshore area near Hilton Head Island, South Carolina, and Tybee Island, Georgia, 19992002: U.S. Geological Survey Scientific Investigations Report 20055134, 48 p.

For more information, contact the South Carolina Publications Unit.

USGS South Carolina Publications South Carolina Water Science Center

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