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Georgia Water Resources Conference
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ItemRevised hydrogeologic framework of the Floridan aquifer system in Florida, and parts of Georgia, Alabama, and South Carolina(Georgia Institute of Technology, 2013-04) Williams, Lester J.The hydrogeologic framework for the Floridan aquifer system has been revised throughout its extent in Florida and parts of Georgia, Alabama, and South Carolina. The updated framework generally conforms to the original framework established by the U.S. Geological Survey in the 1980s except for adjustments made to the internal boundaries of the Upper and Lower Floridan aquifers and the individual permeable zones that comprise these aquifers. The revised boundaries of the Floridan aquifer system were mapped by taking into account results from local studies and regional correlations of geologic and hydrogeologic units. Additional high and low permeability zones have been incorporated into the framework to allow for finer discretization of permeability variations in the two regional aquifers or within the same aquifer of a local or subregional area. These additional units can be used to progressively divide the system into discrete hydrologic units that may be important for assessing groundwater and surface water interaction, saltwater intrusion, and offshore movement of groundwater. The extent and altitude of the freshwater/saltwater interface in the aquifer system has been mapped to define the freshwater part of the flow system that will be a focus for future groundwater availiabilty assessments.
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ItemDevelopment of a revised regional hydrogeologic framework for the Floridan aquifer system using geophysical log marker horizons(Georgia Institute of Technology, 2013-04) Williams, Lester J. ; Raines, Jessica E. ; Lanning, Amanda E.During the process of updating the regional hydrogeologic framework of the Floridan aquifer system a series of subtle geophysical markers where found to be important in helping to define and correlate high and low permeable zones in the relatively thick carbonate rocks that comprise this aquifer system. Although none of the marker horizons are regionally persistent, they are critical in understanding the position and lateral continuity of the water-producing zones. The factors that control permeable variations in the Floridan aquifer system include (1) rock type, (2) structure, and (3) the proximity of the rocks to the recharge areas and the active flow system where more vigorous dissolution occurs. Among these factors, rock type is the principal control. Many of the highly permeable beds or zones in the system occur at major lithologic contacts or in thinly bedded carbonate sequences that are more susceptible to dissolution and development of secondary porosity and permeability. Once a particular sequence of rocks was identified that was important to the framework, geophysical markers were used to map these units up and down dip to better define the internal structure of the system. Distinctive geophysical log patterns are identified in a soft, poorly indurated limestone lithology, a massive dolostone lithology, thinly bedded limestonedolostone and evaporite lithology, and a fine-grained limestone lithology.
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ItemRelation of Aquifer Confinement and Long-Term Groundwater-Level Decline in the Floridan Aquifer System(Georgia Institute of Technology, 2011-04) Williams, Lester J. ; Dausman, Alyssa D. ; Bellino, Jason C.The rate and magnitude of long-term groundwater- level decline in the Floridan aquifer system in the Southeastern United States (Fig. 1) was evaluated to gain a better understanding of hydrologic responses to groundwater pumping and the effects of confinement on recharge rates in the aquifer system. Because of the large geographic area and widely varying time periods covered, a simple screening approach was used. To conduct the analysis, water-level data from monitoring wells with at least 20 years of record were analyzed by using linear regression to determine the slope of water-level change—a negative slope indicates a decreasing water-level trend and a positive slope indicates an increasing water-level trend. The slopes were then converted to a 10-year rate-of-decline coefficient and were subsequently used for mapping decline patterns in the aquifer system (Fig. 1A). The period covered by the analysis, 1970 –2010, was a period of substantial growth in irrigated agriculture and municipal and domestic water use across the region. Twenty-three percent of the wells used for the evaluation had water-level data covering the entire 40-year period; 52 percent had water-level data covering a 30-year period, 75 percent had water-level data covering a 25-year period, and 100 percent had water-level data covering a 20-year period. The rate-of-decline map, developed by using the linearinterpolation screening approach, assumes the following: (1) the long-term declining (or increasing) trends in the aquifer can be described with a simple linear-regression equation, and the slope of a best-fit line through the data points is a good measure of the actual water-level decline; (2) the varying time periods available for individual wells does not substantially influence the overall decline patterns mapped in the aquifer; and (3) the spatial distribution of the data points is adequate to describe the spatial patterns. To test these assumptions the time periods were varied from the longest of 40 years (Fig. 1A) down to the shortest of 5 years (not shown). Analysis of the shorter periods of time between 5 and 20 years provided useful information on the shorter-term trends but the 40-year period provided the best spatial distribution used to identify decline patterns and the relation of declines to aquifer confinement. The results of the study indicate that areas with the greatest 10-year rate-of-decline coefficients generally coincide with confined areas of the aquifer system (compare Fig. 1A to 1B). One exception to this is a rising trend in a confined area of the aquifer in the southeastern coastal region of Georgia where reductions in groundwater withdrawals over the past 10 years has probably resulted in increasing water levels in that area. Another exception is an area of decreasing trends located in the unconfined portion of the aquifer in Alabama; the reason for those declines is not known at this time, however, rocks that compose the upper confining unit probably vary greatly in thickness and lithology and locally influence the leakage rate in that area. Identification of the groundwater-level decline patterns provides useful insight into recharge-discharge relations on a regional scale. The presence of a fairly large contiguous area of decline in the confined parts of the aquifer system extending from south-central and southeastern Georgia into the Florida Panhandle and northeastern Florida (Fig. 1A) suggests downward leakage through the upper confining unit (or upward leakage from deeper aquifers) cannot fully satisfy pumping demand. It is theorized that as water levels have slowly declined beneath the confined part of the aquifer, broad areas of decreased water levels have expanded out into adjoining recharge (karst) areas where induced recharge is probably occurring. Numerical modeling is being conducted to test the relations of aquifer confinement and recharge on a regional scale and the potential effects on springflow and streamflow.
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ItemSaline Aquifers in Coastal Georgia: Assessment Using Borehole Geophysical Logs at the Fort Pulaski Core Site, Chatham County, Georgia(Georgia Institute of Technology, 2011-04) Ostrowicki, Katrina ; Williams, Lester J.Borehole geophysical logs were collected in March 2010 from a 1,020-foot deep core hole drilled at Fort Pulaski, Chatham County, Georgia. Using these logs, a relation was developed between borehole geophysical log response and known water-quality variations in the Floridan aquifer system. Formation factors determined from this analysis ranged from 4.5 to 5.6. A value of 0.75 for the tortuosity and a value of 1.6 for the cementation factor provided a fairly good fit between log-derived salinity values and water-quality values from the well samples. Formation factors determined at the Fort Pulaski core site could be applied to similar rock types in the subsurface of coastal Georgia; and this methodology could be applied to geophysical log data to expand the current knowledge about saline aquifers in coastal Georgia and throughout the southeastern United States.
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ItemAn Update on the Thickness and Extent of the Surficial Aquifer System and Its Potential Use as an Alternative Water Source in Coastal Georgia(Georgia Institute of Technology, 2011-04) Gill, Harold E. ; Williams, Lester J. ; Bellino, Jason C.The Georgia Environmental Protection Division (GaEPD) has capped withdrawals from the Upper Floridan aquifer in coastal Georgia because of saltwater contamination at Hilton Head Island, South Carolina, and Brunswick, Georgia. To offset demand, GaEPD recommends seeking alternative groundwater sources, including the surficial and Brunswick aquifer systems in the coastal area. The surficial aquifer system is a relatively untapped resource in coastal Georgia that could have potential use as an alternative water source for irrigation or other water uses in the region. The surficial aquifer system in coastal Georgia consists mostly of sands and clays of Pleistocene and Pliocene age and, in some areas, hydraulically connected sediments of Miocene age (Fig. 1). The aquifer system, formerly called the surficial aquifer (Clarke and others, 1990), consists of three zones—the shallow water-table zone and two deeper zones identified as the confined upper and confined lower water-bearing zones (Leeth, 1999). The areal extent of the confined units of the surficial aquifer system is currently unknown; however, Leeth (1999) reported two confined water-bearing zones in Camden County, and Clarke and others (1990) reported one confined water-bearing zone at Brunswick, Glynn County, and one at Skidaway Island, Chatham County (Clarke, 2003). A map showing the total composite thickness of permeable layers in the water-table and confined zones of the surficial aquifer system was compiled using data from published reports and additional data recently compiled from the files of the U.S. Geological Survey (USGS; Fig. 2). In coastal Georgia, the surficial aquifer system is thickest in the southeast Georgia embayment, with thickness exceeding 200 feet (ft) in much of Glynn County, and is greater than 100 ft in parts of Camden, Charlton, Brantley, Wayne, Liberty, Tattnall, Toombs, McIntosh, and Chatham Counties. In these areas, the surficial aquifer system is composed of both a water-table zone and one or more confined zones. Thick deposits of alluvial materials also lie in the flood plains of major rivers, such as the Altamaha River in Wayne County, where thicknesses of 120 ft in are reported for the water table zone in some areas (Watson, 1979). In the central part of the coastal plain, a coarse-to-fine gravelly layer about 30 to 50 ft below the land surface was identified by Watson (1979) as part of his “surface” aquifer (hatched area in Fig. 2). This layer is reported to vary in thickness from 5 to 15 ft and is laterally extensive within the water-table zone. Well yields vary greatly over the area depending on the thickness and permeability of zones that make up the surficial aquifer system. For the water-table zone, Clarke and others (1990) and Leeth (1999) reported well yields ranging from 2 to 140 gallons per minute (gal/min) and transmissivity ranging from 14 to 6,700 feet squared per day (ft2/d) in Glynn and Camden Counties (Clarke, 2003). For the confined surficial zones, Clarke and others (1990) reported well yields ranging from 40 to 180 gal/min and transmissivity ranging from 150 to 6,000 ft2/d. Leeth (1999) reported well yields from 15 to 100 gal/min and a transmissivity of 180 ft2/d at Camden County. In Wayne County, industrial supply wells at Jesup yielded about 250 gal/min, with a total average withdrawal of 0.86 million gallons per day in 1986 (Clarke and others, 1990). A recent 100-ft deep well constructed in a confined zone of the surficial aquifer system by the U.S. Army at Fort Stewart, Liberty County, obtained about 550 gal/min during a 24-hour pumping test with an estimated transmissivity of 2,500 ft2/d (Gerard Gonthier, U.S. Geological Survey, written commun., June 2010). Because of the relatively smaller well yields, the surficial aquifer system in coastal Georgia is probably a viable alternative water source for some small commercial, industrial, and irrigation water uses. Although the thickness of this aquifer is highly variable, local drilling data could be used to identify places where it is thickest and most productive. Some waterquality problems, such as iron and manganese, have been noted and could limit the use in some areas (Clarke and Others, 1990). To date, the development potential of the surficial aquifer is mostly confirmed in the southeastern Georgia embayment; however, substantial thicknesses have been mapped elsewhere, thus indicating the potential for develop–ment in other coastal areas. The effects of heavy withdrawals on streams and wetlands will be one of the considerations and limiting factors for development—groundwater level and quality monitoring would help to assess the effects of increased development.
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ItemSaline Aquifer Mapping Project in the Southeastern United States(Georgia Institute of Technology, 2011-04) Spechler, Rick M. ; Williams, Lester J.In 2009, the U.S. Geological Survey initiated a study of saline aquifers in the southeastern United States to evaluate the potential use of brackish or saline water from the deeper portions of the Floridan aquifer system and the underlying Coastal Plain aquifer system (Fig. 1). The objective of this study is to improve the overall understanding of the available saline water resources for potential future development. Specific tasks are to (1) develop a digital georeferenced database of borehole geophysical data to enable analysis and characterization of saline aquifers (see locations in Fig. 1), (2) identify and map the regional extent of saline aquifer systems and describe the thickness and character of hydrologic units that compose these systems, and (3) delineate salinity variations at key well sites and along section lines to provide a regional depiction of the freshwater-saltwater interfaces. Electrical resistivity and induction logs, coupled with a variety of different porosity logs (sonic, density, and neutron),are the primary types of borehole geophysical logs being used to estimate the water quality in brackish and saline formations. The results from the geophysical log calculations are being compared to available water-quality data obtained from water wells and from drill-stem water samples collected in test wells. Overall, the saline aquifer mapping project is helping to improve the understanding of saline water resources in the area. These aquifers may be sources of large quantities of water that could be treated by using reverse osmosis or similar technologies, or they could be used for aquifer storage and recovery systems.
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ItemRevised Hydrogeologic Framework for the Floridan Aquifer System in the Northern Coastal Areas of Georgia and Parts of South Carolina(Georgia Institute of Technology, 2011-04) Gill, Harold E. ; Williams, Lester J.The hydrogeologic framework for the Floridan aquifer system was revised for eight northern coastal counties in Georgia and five coastal counties in South Carolina (Fig. 1) as part of a regional assessment of water resources by the U.S. Geological Survey (USGS) Groundwater Resources Program. In this study, selected well logs were compiled and analyzed to determine the vertical and horizontal continuity of permeable zones that make up the aquifer system, and define more precisely the thickness of confining beds that separate individual aquifer zones. The results of the analysis indicate that permeable zones in the Floridan aquifer system can be divided into (1) an upper group of extremely transmissive zones that correlate to the Ocala Limestone in Georgia and the Parkers Ferry Formation in South Carolina, and (2) a lower group of zones of relatively lower transmissivity that correlates to the middle part of the Avon Park formation in Georgia and updip clastic equivalent units of South Carolina (Fig. 2). This new subdivision simplifies the hydrogeologic framework originally developed by the USGS in the 1980s and helps to improve the understanding of the physical geometry of the system for future modeling efforts. Revisions to the framework in the Savannah–Hilton Head area are particularly important where permeable beds control the movement of saltwater contamination. The revised framework will enable waterresource managers in Georgia and South Carolina to assess groundwater resources in a more uniform manner and help with the implementation of sound decisions when managing water resources in the aquifer system.