USGS Georgia Water Science Center
This report is available online in pdf format (9 MB): USGS WRIR 02-4020 (
)
L. Elliott Jones, David C. Prowell, and Morris L. Maslia
U.S. Geological Survey Water-Resources Investigations Report 02-4020, 35 pages (Published 2002)
In 1978, the U.S. Geological Survey drilled a 2,727-foot-deep test well (TW-26) on Colonels Island in Glynn County about 3 miles west-southwest of downtown Brunswick, Georgia. The well was constructed to gain a better understanding of the hydrogeology and geochemistry of the carbonate Floridan aquifer system, which is comprised of the Upper and Lower Floridan aquifers, each of which is divided further into multiple water-bearing zones. At least since the 1940's, some of the shallower water-bearing zones of the Floridan aquifer system have been contaminated by saltwater. Analytical methods included examination of cuttings and core sections, geophysical logging, and collection of water-quality samples during construction and after completion.
Lithologic and paleontological data from TW-26 indicate strata ranging in geologic age from Late Cretaceous to Tertiary. Structural interpretations using geophysical logs suggest that these strata may be locally affected by faulting. Acoustic-televiewer and caliper logs reveal numerous layers within the carbonate Floridan aquifer system that have sizable dissolution cavities that commonly are adjacent to layers of relatively non-porous limestone or dolomite. Dissolution may increase as ground water fills openings caused by fracturing of brittle, non-porous carbonate rocks, and probably provides conduits for the circulation of large quantities of ground water. At a depth near 2,475 feet (ft), large dissolution cavities were observed near features that appear to be high-angle fault and/or fracture zones. These zones may allow upward migration of saline water to shallower water-bearing zones where hydraulic head has been reduced by pumping.
Chloride-concentration data from TW-26 indicate wide variation in salinity of ground-water in the Floridan aquifer system, including sharp increases and decreases in salinity (interfaces) with increasing depth, and long intervals of consistent salinity. During drilling, the first interface of increased salinity in the system was in the lower water-bearing zone of the Upper Floridan aquifer (955—960 ft, chloride concentration in the drilling fluid increased from 20 to 100 milligrams per liter (mg/L)). Salinity increased gradually through the lower-water bearing zone, the middle semiconfining unit, and the uppermost water-bearing zone of the Lower Floridan aquifer (chloride concentration in the drilling fluid had a local maximum of 604 mg/L at 1,245 ft). The interval 1,245—1,660 ft has low salinity—five bailer samples collected from 1,260 to 1,515 ft, during a period of free flow when the well had a temporary open interval of 598—1,528 ft, had chloride concentration ranging from 20 to 37 mg/L. Chloride concentration in drilling fluid decreased gradually from 1,260 to 1,520 ft; and during the next phase of drilling, chloride concentration in drilling-fluid samples remained generally less than 30 mg/L until 1,665 ft. The second interface of increased salinity during drilling was in the middle of the Lower Floridan aquifer (1,660—1,675 ft, from 24 to 580 mg/L), followed by an interval of almost 500 ft having moderate variation in salinity (1,675—2,145 ft, from 108 to 660 mg/L). A third interface of increased salinity was near the top of the Fernandina permeable zone in the lower part of the Lower Floridan aquifer (2,145—2,320 ft, from 282 to 16,500 mg/L). A bailer sample from the Fernandina permeable zone near the bottom of the well (2,710 ft) after completion had a chloride concentration of 33,000 mg/L, much greater than that of modern seawater (19,000 mg/L).
Because ground water in the Fernandina permeable zone is much more saline than a high-chloride plume in the Upper Floridan aquifer at Brunswick (maximum sample chloride concentration 3,040 mg/L in 1970 and 1975), the Fernandina permeable zone is thought to be the source of the high-chloride water in the Upper Floridan aquifer at Brunswick. However, one sample from a shallower depth interval (1,612—1,712 ft) had a chloride concentration of 2,400 mg/L, suggesting there may be water-bearing zones above the Fernandina permeable zone that supply high-chloride water to the Upper Floridan aquifer in Brunswick. Comparison of sample chloride concentrations and specific conductances at various depths indicate a change in water type between ground water above and below a depth of about 1,500 ft.
Graphical analysis of the relative content of major cations and anions indicates that high-chloride water from TW-26 is not a simple mixture of freshwater from the Upper Floridan aquifer and modern seawater. Dissimilarities include: the percentage of dissolved calcium in all samples was higher than seawater and lower than freshwater; all samples had a markedly lower percentage of dissolved bicarbonate than freshwater; and three samples contained percentages of sodium-plus-potassium and/or chloride that lie beyond percentages of the freshwater and seawater end points.
Abstract
Introduction
Purpose and scope
Location of TW-26
Previous investigations
Well construction
Data collection
Lithologic description and paleontologic sampling
Geophysical logs and video images
Water-quality sampling and water-level monitoring
Acknowledgments
Hydrogeology
Geologic units
Upper Cretaceous strata
Paleocene strata
Lower and middle Eocene strata
Upper Eocene strata
Oligocene strata
Miocene strata
Post-Miocene strata
Hydrogeologic units
Surficial aquifer
Upper and lower Brunswick aquifers
Upper Floridan aquifer
Lower Floridan aquifer
Ground-water quality
Chloride concentration and specific conductance
Middle drilled section (segments I—IV)—598—1,528 feet
Lower drilled section (segments V—VIII)—1,528—2,727 feet
Relation between chloride concentration and specific conductance
Ionic content
Summary
Selected references
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