Have you ever wondered about that clear cool liquid that comes from our faucets? Yes, we're talking about groundwater! Not everyone realizes that groundwater use in coastal Georgia is of major significance not only to our state, but also to our neighbors in South Carolina and northern Florida. Because Georgia's 24 coastal counties are expected to experience a continued increase in population over the next several decades, our groundwater is a fundamental issue for our legislators to consider when planning to balance sustainable economic development with protection of environmental resources in southeastern Georgia. In a nutshell, the availability of sustainable long-term supplies of groundwater will be of critical importance for our future.

In coastal Georgia, studies by the U.S. Geological Survey show that we use most of our groundwater for public water supply and industrial needs. Most of that water is supplied from a single aquifer (a layer of rock or sediment that can supply economically significant quantities of water ) known as the Floridan Aquifer System. This aquifer extends from South Carolina, through Georgia, and into Florida and Alabama; it is present beneath both the coastal plain upon which we live and the continental shelf above which we fish and sail. Because of its geographic extent, the management of water in the aquifer is, and will continue to be, an inter-state issue as usage practices in one state can affect the availability and quality of water in an adjacent state. About 90% of the groundwater withdrawn from the Floridan Aquifer System in coastal Georgia is actually taken from its uppermost part which is known as the upper Floridan aquifer (UFA). The UFA is a pale-colored limestone that originally accumulated in warm subtropical to tropical seas more than 25 million years ago (for reference, the dinosaurs became extinct about 65 million years ago). It has since been periodically eroded and weathered during times of lowered sea level and buried by younger sediments during times of higher sea level. In our area, the UFA lies anywhere from 19 to 200 ft below sea level and ranges from 50 to 200 ft in thickness. If we were to somehow drop sea level by a couple of hundred feet (an event that would need the help of an ice age!) and peel off the overlying sediments that we now live upon, the top surface of the aquifer would resemble karst limestone landscapes we see in western Europe today.

Massively bedded limestones and karst topography in the Burren, western Ireland

Every day in coastal Georgia the pores, cracks, crevices, and subterranean caverns in the UFA provide approximately 350 million gallons of crystal-clear potable water for our use, a volume that has been steadily increasing ever since water was first pumped from the aquifer at Savannah about 115 years ago. To put 350 million gallons per day in perspective, that's equivalent to a column of water about 800 feet high with a base (footprint) the size of a football field. Recent data from the U.S. Geological Survey show that Chatham County alone consumes about 76 million gallons of Floridan water daily (equivalent to a similar column of water about 175 feet high).

Throughout the latter half of the 20th century, large volumes of groundwater have been pumped out of the UFA in the Savannah - Hilton Head region; as a result of this, the aquifer's potentiometric surface (which you can think of as an imaginary water table within the aquifer) is now over 120 feet lower in elevation at Savannah than it was in the late 19th century. What this means is that wells which would have flowed freely at the land surface (i.e., artesian wells) in Liberty, Bryan, and Chatham counties (Georgia); and Jasper and Beaufort counties (South Carolina) in the late 1800s now have to be pumped as the water pressure in the aquifer is no longer sufficient to move water up and out of the well bore. Prior to the 1800s, early Spanish explorers were even able to obtain fresh water from seafloor seeps offshore. When viewed on a map today, the depressed potentiometric surface forms a cone of depression (an upside-down cone) that has a radius of up to 30 miles, is centered on Savannah, and underlies eight coastal counties. The cone of depression developed, and continues to persist, because the pumped water cannot be replaced quickly enough through natural inflow from other parts of the aquifer; the inverted apex of the cone now lies about 100 feet below sea level.

Recent potentiometric map for the upper Floridan aquifer showing the cone of depression centered on Savannah.

As you might expect, when fresh groundwater is pumped from our coastal aquifer, there is the potential for sea water, and not just fresh water, to move into the aquifer and towards the pumping sites to replace the water being withdrawn. Certain conditions allow this problematic "seawater intrusion" effect to occur. The two primary conditions are (1) the local absence of a "cap rock" or aquiclude, which is a layer of rock or sediment that would restrict the flow of sea water downward through the seabed and into the underlying aquifer, and (2) a negative pressure gradient (as we would find within the Savannah - Hilton Head cone of depression) between the ocean and the aquifer which would induce sea water to move downward into the aquifer. Both of these conditions occur, unfortunately for us, in the Georgia - South Carolina coastal area.

To address this environmental concern, the Georgia Environmental Protection Division (EPD) is currently funding a two-year project at Georgia Southern University's Applied Coastal Research Lab (Jim Henry and Tony Foyle) and the Skidaway Institute (Clark Alexander) to identify areas in coastal Georgia and South Carolina where seawater has the potential to leak into the UFA. Several other academic, municipal, state, and federal entities are also being funded to tackle different aspects of EPD's Interim Strategy (1997-2005) for managing saltwater intrusion in the UFA of southeastern Georgia. Our collective results will form the scientific basis upon which EPD can formulate and adopt a comprehensive groundwater-management strategy.


Shift change on the US Army Corps of Engineers Drilling Barge "Explorer"

We use marine geophysical survey and drilling data to identify coastal areas where (1) the Miocene aquiclude overlying the UFA may be breached, thin, or missing, and (2) where the overlying water column is saline. In areas where these two conditions are met, particularly within the Savannah - Hilton Head cone of depression, there will be a strong likelihood that seawater is getting into the aquifer and is heading towards our pumping wells.

General stratigraphic scenarios for seawater intrusion into the upper Floridan aquifer

In coastal Georgia and South Carolina, the Miocene aquiclude consists mostly of sands, silts, and clays that were deposited about 5 to 25 million years ago. The aquiclude can be as much as 160 ft thick, but in localized areas it can be thin or absent as a result of two natural processes. Firstly, in many coastal creeks today, tidal currents are of sufficient strength to erode the channel bottoms and cut into or through the aquiclude and expose the UFA to seawater. Some of these tidal-scour holes are as much as 70 feet deep, a depth you wouldn't ordinarily find on the Georgia coast unless you were at least 25 miles offshore. These tidal-scour holes are potential trouble spots, especially in Beaufort County where the aquifer is shallow. Secondly, several times over the past 2 million years, sea level was as much as 300 ft lower than it is today. During these times of lowered sea level, the most recent of which occurred about 18,000 years ago, the Savannah River flowed across the exposed continental shelf to its paleo-mouth located 60 to 80 miles seaward of where it is today. At certain points along its route, the river channel cut down into, and locally through, the aquiclude. While the paleochannels have since been filled with sands and gravels, these younger (and generally coarser) sediments are not as efficient an aquiclude as the Miocene strata. These paleochannels are also potential trouble spots, especially seaward of Hilton Head Island where the aquifer is relatively shallow.


Dip-oriented digitized seismic section offshore of Hilton Head Island showing incision of the aquiclude by a Quaternary paleochannel of the Savannah River system.

Our initial data has allowed us to identify several areas of concern where the UFA is shallow enough, the Miocene aquiclude thin enough, and the UFA's potentiometric surface depressed enough for there to be a significant risk of seawater intrusion into the UFA. The next step for our group of Interim Strategy participants is for Georgia, South Carolina, and the U.S. Geological Survey to use our information to determine the locations of planned monitoring/detection wells in coastal Georgia and South Carolina. That data, in turn, will be incorporated into modeling efforts and scenario development. Approximately five years from now, our increased understanding of this coastal aquifer will form the sound scientific basis upon which coastal Georgia's future groundwater management plans can be constructed.