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Scientific Investigations Report 2005—5124

Hydrogeology, Water Quality, and Water-Supply Potential of the Lower Floridan Aquifer, Coastal Georgia, 1999–2002

U.S. Geological Survey Scientific Investigations Report 2005—5124, 89 pages and one plate (Published 2005)
ONLINE ONLY

By W. Fred Falls, Larry G. Harrelson, Kevin J. Conlon, and Matthew D. Petkewich

Prepared in cooperation with the Georgia Department of Natural Resources

This report is available online in pdf format: USGS sir2005-5124 (4 MB), Plate 1

ABSTRACT

Cover of SIR 2005—5124.

The hydrogeology and water quality of the upper permeable and Fernandina permeable zones of the Lower Floridan aquifer were studied at seven sites in the 24-county study area encompassed by the Georgia Coastal Sound Science Initiative. Although substantially less than the Upper Floridan aquifer in coastal Georgia, transmissivities for the Lower Floridan aquifer are in the same range as other water-supply aquifers in Georgia and South Carolina and could meet the needs of public drinking-water supply. Water of the upper permeable zone of the Lower Floridan aquifer exceeds the Federal secondary drinking-water standards for sulfate and total dissolved solids at most coastal Georgia sites and the Federal secondary drinking-water standard for chloride at the Shellman Bluff site.

The top of the Lower Floridan aquifer correlates within 50 feet of the previously reported top, except at the St Simons Island site where the top is more than 80 feet higher. Based on the hydrogeologic characteristics, the seven sites are divided into the northern sites at Shellman Bluff, Richmond Hill, Pembroke, and Pineora; and southern sites at St Marys, Brunswick, and St Simons Island. At the northern sites, the Lower Floridan aquifer does not include the Fernandina permeable zone, is thinner than the overlying Upper Floridan aquifer, and consists of only strata of the middle Eocene Avon Park Formation. Transmissivities in the Lower Floridan aquifer are 8,300 feet squared per day at Richmond Hill and 6,000 feet squared per day at Shellman Bluff, generally one tenth the transmissivity of the Upper Floridan aquifer at these sites. At the southern sites, the upper permeable zone of the Lower Floridan aquifer is thicker than the Upper Floridan aquifer and consists of porous limestone and dolomite interbedded with nonporous strata of the middle Eocene Avon Park and early Eocene Oldsmar Formations. Transmissivities for the upper permeable zone of the Lower Floridan aquifer are 500 feet squared per day at the St Simons Island site and 13,000 feet squared per day at the St Marys site. The Lower Floridan aquifer at the Brunswick and St Marys sites includes the Fernandina permeable zone, which consists of saltwater-bearing dolomite.

Hydrographs of Coastal Sound Science Initiative wells and other nearby wells open to the Upper Floridan aquifer, and the upper permeable and Fernandina permeable zones of the Lower Floridan aquifer have similar trends. Water levels in wells open to the Upper and Lower Floridan aquifers are below land surface at the northern sites and the St Simons Island site, and above land surface at the Brunswick and St Marys sites, as of January 1, 2004.

Freshwater is present in the Lower Floridan aquifer at Pineora, Pembroke, and St Marys, and from 1,259 to 1,648 feet below land surface at Brunswick. Slightly saline water is present in the Lower Floridan aquifer at Richmond Hill, Shellman Bluff, St Simons Island, and from 1,679 to 1,970 feet below land surface in well 34H495 at Brunswick. The upper permeable zone of the Lower Floridan aquifer contains bicarbonate water at the Pembroke site, sulfate-bicarbonate water at the Brunswick site, and sulfate water at the St Simons Island, Shellman Bluff, St Marys, and Richmond Hill sites. The bicarbonate, sulfate-bicarbonate, and sulfate waters are saturated relative to calcite and dolomite, and undersaturated with gypsum and anhydrite.

The Fernandina permeable zone in well 34H495 includes moderately saline water, very saline water, and brine. The Fernandina permeable zone of the Lower Floridan aquifer beneath downtown Brunswick contains chloride water that is slightly undersaturated to saturated with gypsum and anhydrite. Concentrations of total dissolved solids, sulfate, and chloride exceeded the Federal secondary drinking-water standards. The chloride-contaminated plumes beneath downtown Brunswick would require at least a 12- to 20-percent contribution of very saline water from the Fernandina permeable zone to result in observed concentrations in the Upper Floridan aquifer.

Waters from the upper permeable zone of the Lower Floridan aquifer at the St Marys, Brunswick, Richmond Hill, and Pembroke sites had carbon-14 concentrations and stable oxygen and hydrogen isotopic compositions that were similar to waters from the Upper Floridan aquifer at these respective sites. The data indicate that waters in both aquifers at a specific well site probably entered the recharge area under similar climatic conditions and, therefore, could have similar ages. Two freshwater samples from the upper permeable zone of the Lower Floridan aquifer at Pembroke and St Marys have strontium isotope ratios that fall in the range of Oligocene or Miocene seawaters. Four ground-water samples from the upper permeable zone of the Lower Floridan aquifer at Shellman Bluff, Richmond Hill, and St Simons Island, and St Marys well 33D074 have strontium isotope ratios in the range of Eocene seawater.


CONTENTS

Abstract

Introduction

Purpose and Scope

Location of the Study Area

Previous Investigations

Well-Identification System

Acknowledgments

Methods of Data Collection and Analysis

Test Well Drilling and Completion

Hydrogeologic Data Collection

Water-Quality Sampling

Geophysical Logging

Hydrogeologic Setting

Surficial Aquifer and Upper Confining Unit

Floridan Aquifer System

Ground-Water Supply

Historical Water Levels

Ground-Water Flow, Salinity, and Potential for Saltwater Contamination

Hydrogeology of the Lower Floridan Aquifer

Upper Permeable Zone

Northern Sites

Southern Sites

Fernandina Permeable Zone

Lower Confining Unit

Water Levels in the Floridan Aquifer System

Northern Sites

Southern Sites

Water Quality

Chloride Results for Reverse-Air Rotary Discharge-Water Samples

Salinity

Major Ions, Other Constituents, and Saturation Indices

Water Type I—Bicarbonate

Water Type II—Sulfate-Bicarbonate

Water Type III—Sulfate

Water Type IV—Chloride

Isotopes

Stable Hydrogen and Oxygen Isotopes

Carbon Isotopes

Stable Carbon Isotopes and Water Types With Respect to Geochemical Processes

Carbon-14

Tritium

Strontium Isotopes of Water

Water-Supply Potential of the Lower Floridan Aquifer

Summary

Selected References

Appendix 1. Lithology, bit penetration rate, water quality, and well construction for Coastal Sound Science Initiative well 33D073 at the St Marys site, Georgia

Appendix 2. Lithology, bit penetration rate, water quality, and well construction for Coastal Sound Science Initiative wells 34H495 and 34H500 at the Brunswick site, Georgia

PLATE

1. Cross section of the geology and hydrogeology of the six Coastal Sound Science Initiative sites and the St Simons Island site from St Marys to Pineora, Georgia


REPORT AVAILABILITY

This report is available online in pdf format: USGS SIR 2005-5124 (4 MB), Plate 1
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., Harrelson, L.G., Conlon, K.J., and Petkewich, M.D., 2005, Hydrogeology, water quality, and water-supply potential of the Lower Floridan aquifer, coastal Georgia, 1999–2002: U.S. Geological Survey Scientific Investigations Report 2005–5124, 89 p. + 1 pl.

For more information, contact the South Carolina Publications Unit.

USGS South Carolina Publications South Carolina Water Science Center

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