By Richard E. Krause (1) and John S. Clarke (2)
AUTHORS: (1) Hydrologist, U.S. Geological Survey (Volunteer for Science); (2) Hydrologist, U.S. Geological Survey, 3039 Amwiler Road, Suite 130, Peachtree Business Center, Atlanta, GA 30360-2824.
REFERENCE: Proceedings of the 2001 Georgia Water Resources Conference, held March 26-27, 2001, at the University of Georgia, Kathryn J. Hatcher, editor, Institute of Ecology, The University of Georgia, Athens, Georgia, p. 756-759.

Abstract

Sustained pumping from the Upper Floridan aquifer in the coastal area of Georgia and adjacent lowcountry of South Carolina has resulted in substantial reductions in artesian pressure in the aquifer, which has resulted in saltwater intrusion at two locations in the area. At Brunswick, Georgia, brine from deeply buried paleokarst zones has intruded the aquifer by migrating upward through solution-enlarged breaks in the confining units. At the northern end of Hilton Head Island, South Carolina, lateral encroachment of seawater has occurred. Understanding the conditions under which these types of intrusion occur is of importance to managing the water resources of the coastal area.

Introduction

Ground water is the main source of water supply in coastal Georgia. The first "deep" well in the coastal area was completed in 1885 in the water-bearing zone now known as the Upper Floridan aquifer, an extremely permeable, carbonate sequence. (The zone was considered "deep" at that time, because existing wells generally tapped one of three overlying aquifers.) The Upper Floridan became the water supply of choice because of its capacity to yield extremely large quantities of fresh water and the simplicity of well construction, including not having to use well screen. By 1900, more than 200 wells had been completed in the aquifer in coastal Georgia, and pumpage was more than 10 million gallons per day (Mgal/d). This apparent plentiful water supply attracted numerous industries that required large quantities of fresh water. Pumpage increased at varying rates, and today (2000), more than 300 Mgal/d is pumped from the aquifer in the coastal area of Georgia and adjacent lowcountry of South Carolina. Pumping is widespread throughout the area, but is concentrated at several pumping centers. This pumping has resulted in substantial reduction in artesian pressure regionally, and has caused large, deep, cones of depression in the potentiometric surface at pumping centers.

As a result of the heavy pumpage and resulting reduction in pressure, saltwater intrusion of the aquifer was observed in the 1950's at Brunswick, and in the early 1970's at Hilton Head Island, South Carolina (fig.1). Saltwater contamination at these two coastal locations has constrained further development of the Upper Floridan aquifer in the coastal area and created competing demands for the limited supply of water. The Georgia Department of Natural Resources, Environ-mental Protection Division (GaEPD) has capped permitted withdrawal of water from the Upper Floridan aquifer in parts of the coastal area (including the Savannah and Brunswick areas) at 1997 rates, and also has restricted permitting of additional pumpage in all 24 coastal-area counties to 36 Mgal/d above 1997 rates. This strict management action has prompted interest in alternative management of the aquifer and in the development of alternative, supplemental sources of water supply, including supply from the shallower Miocene and surficial aquifers and the underlying Lower Floridan aquifer.

GEOLOGY AND GROUND-WATER RESOURCES

The coastal area of Georgia and adjacent lowcountry of South Carolina is blanketed and underlain by unconsolidated sedimentary strata that overlie carbo-nate rocks—limestone and dolostone—at varying depth. The sedimentary strata are thickest and most deeply buried in the Brunswick area and south into northeastern Florida, where more than 500 feet of sand and clay overlie more than 2,000 feet of carbonate rocks. The sequence is thinner and at shallower depth toward the north in the Savannah, Georgia—Hilton Head Island, South Carolina, area where the top of the carbonate rocks are 50-150 feet below land surface and the thickness is less than 500 feet.

Predevelopment Ground-Water Flow System

Prior to development of the Upper Floridan aquifer in the 1880's, recharge to the aquifer system was roughly offset by natural discharge. The Upper Floridan aquifer was replenished (recharged) by rainfall in areas where aquifer sediments are at or near land surface, generally west and northwest of the coast.Water flowed from the recharge area in the western and northwestern part of the area downgradient toward the coast. This downgradient flow with increasing depth and decreasing land-surface altitude, together with confinement exerted by low-permeability sediments overlying the aquifer, resulted in increasing artesian pressure toward the coast and offshore. The aquifer was under confined, or artesian conditions, and the water level was sufficient that wells flowed at land surface throughout most of the coastal area. The artesian water level was about 65 feet above sea level at Brunswick, and 35 feet above sea level at Savannah. Ground water discharged naturally to springs; as seepage to rivers, ponds, wetlands, and other surface-water bodies; and as diffuse upward leakage into adjacent aquifers and offshore to the Atlantic Ocean.

Modern-Day Ground-Water Flow System

Ground-water pumping has caused the water level in the Upper Floridan aquifer to decline throughout the entire coastal area, and has resulted in the development of cones of depression in areas of heavy, concentrated pumpage, such as the Savannah, Brunswick, Jesup, and St Marys, Georgia–Fernandina Beach, Florida areas (fig.1). Wells have ceased to flow at land surface throughout much of the coastal area. Many freshwater springs and seeps have ceased to discharge; freshwater wetlands and ponds that prior to development were fed by flow from the Upper Floridan are no longer sustained by that flow. Although the cones of depression are deep, they do not intercept the top of the Upper Floridan aquifer; thus, dewatering or "mining" of the aquifer is not taking place. The aquifer is still fully satu-rated, but because of the large, sustained withdrawal of water, the artesian pressure in the aquifers has been reduced. This pressure reduction has allowed saltwater under higher pressure to intrude the freshwater part of the aquifer in at least two locations—Brunswick, Georgia, and Hilton Head Island, South Carolina.

Freshwater-Saltwater Interface

Freshwater in the Upper and Lower Floridan aquifers flows seaward until it comes in contact with seawater along the freshwater-saltwater interface. Freshwater is less dense than saltwater and tends to flow on top of it. The interface is not sharp and distinct, but is a diffuse zone in which freshwater and saltwater mix through the processes of chemical diffusion and mechanical dispersion. Data from the offshore area, hydrologic conceptual models, and results of simulation indicate that the freshwater-saltwater interface in the upper part of the aquifer system is at the coastline just north of Hilton Head Island; arcs eastward under the Atlantic Ocean, reaching a maximum distance of about 55 miles offshore; then arcs back to the coast south of Jacksonville, Florida.

The freshwater-saltwater interface is relatively flat-lying and at considerable depth in and under the Floridan aquifer system along the central part of the Georgia coast and northeastern Florida. In the Brunswick area, saltwater (hypersaline connate water) occurs naturally below freshwater in the lower part of the Fernandina permeable zone. Saltwater in the Fernandina permeable zone is at a depth of about 2,400 feet below sea level.

Saltwater Contamination at Brunswick, Georgia

Saltwater contamination at Brunswick is the result of upward intrusion of saltwater (chloride concentration greater than 30,000 milligrams per liter) from the lower part of the Fernandina permeable zone into freshwater zones of the Lower Floridan, then Upper Floridan aquifers (fig. 2). Saltwater from the Fernandina permeable zone migrates upward through solution-enlarged fractures and conduits in the limestone and dolostone confining units in response to reduced artesian pressure caused by pumping from the Upper Floridan aquifer. Upon reaching the aquifers in the southern part of the Brunswick peninsula, the diluted saltwater moves northward toward cones of depression caused by pumping in the northern part of Brunswick. Most of this saltwater intrusion and lateral downgradient transport occurs in the Upper Floridan aquifer because of greater pumpage and reduction in pressure than in the Lower Floridan aquifer.

Saltwater intrusion occurs at locations that are laterally away from the centers of pumping and the induced cones of depression. Also, intrusion moves upward through isolated conduits, not symmetrically in the form of an inverted cone as it would through porous media.Therefore, upconing is not occurring. Contami-nation of the Upper Floridan aquifer in the Brunswick area is not due to lateral encroachment nor downward intrusion of seawater, because the aquifer is deeply buried at Brunswick (greater than 500 feet deep); the freshwater interface with seawater is far from the coastline (more than 50 miles offshore); and most significantly, pressure in the aquifer was greater than sea level when saltwater contamination at Brunswick was first detected in the late 1950's.

Saltwater Contamination at Hilton Head Island, South Carolina

Saltwater contamination along the northern end of Hilton Head Island probably is the result of lateral encroachment of seawater, combined with some down-ward vertical leakage of saltwater or brackish water from sounds, estuaries, tidal creeks, and saltwater marshes where the Upper Floridan aquifer is exposed or thinly confined. In the vicinity of Port Royal Sound, and possibly other estuaries, downcutting by ancient river systems during periods of lower ocean levels that existed during the most recent ice age (about 18,000 years BP) has exposed the aquifer to seawater, resulting in a direct connection between salty ocean water and fresh ground water (fig. 3).

Regional pumping, but most significantly, pumping on Hilton Head Island, has locally lowered the water level in the Upper Floridan aquifer and reversed the natural hydraulic gradient, allowing encroachment of saltwater. What had been submarine springs prior to development, now are conduits or sinks where saltwater encroaches from the ocean, sounds, and estuaries.

SELECTED REFERENCES

Bush, P.W., and Johnston, R.H., 1988, Ground-water hydraulics, regional flow, and ground-water development of the Floridan aquifer system in Florida and in parts of Georgia, South Carolina, and Alabama: U.S. Geological Survey Professional Paper 1403-C, 80 p.

Garza, Reggina, and Krause, R.E., 1996, Water-supply potential of major streams and the Upper Floridan aquifer in the vicinity of Savannah, Georgia: U.S. Geological Survey Water-Supply Paper 2411, 36 p.

Krause, R.E, and Randolph, R.B., 1989, Hydrology of the Floridan aquifer system in southeast Georgia and adjacent parts of Florida and South Carolina: U.S. Geological Survey Professional Paper 1403-D, 65 p.

Peck, M.F., Clarke, J.S., Ransom III, Camille, and Richards, C.J., 1999, Potentiometric surface of the Upper Floridan aquifer in Georgia and adjacent parts of Alabama, Florida, and South Carolina, May 1998, and water-level trends in Georgia, 1990-98: Georgia Geologic Survey Hydrologic Atlas 22, 1 plate.