| This information is presented as a working reference
to enable the development of a Plan of Study as agreed to in the December 2000
meeting. Its purpose is to provide shared information for use of committee members
towards that end. To review the listing of concerns regarding the potential effects on the
Floridan Aquifer resulting from a deeper channel, click here. To review the existing gaps in aquifer knowledge, click here. Unless otherwise indicated, summaries of reports do not the reflect the opinion of the Aquifer Committee. Summaries 1 through 29 were drafted by John Cox. Future summaries will have their drafter identified if otherwise. This clarification was requested in the 2/2/01 Aquifer Committee meeting. |
Several studies that have investigated potential salt-encroachment within the Floridan aquifer and reports identifying lithostratigraphic units in the lower Georgia and South Carolina coastal plains have been completed. Those studies reviewed and summarized herein are not necessarily the only reports (published or unpublished) that address the issue of salt-water encroachment within the Floridan aquifer; however, they are significant, inasmuch, as they account for the majority of the current level of understanding of the local and regional hydrostratigraphy, hydrogeology and nature of ground-water flow and salt-water encroachment upon the Upper Floridan aquifer. It should be noted that an exhaustive literature search and review has not yet been conducted. Additional documents will be obtained and reviewed as the Draft Study Plan is developed.
In addition to published United States Geological Survey (USGS), Georgia Department of Natural Resources Environmental Protection Division (GAEPD), and South Carolina Water Resources Commission (now part of South Carolina Department of Health and Environmental Protection [SCDHEC]), unpublished reports, proposals, memoranda, and letters, have been reviewed and summary discussions are provided herein. The intent of completing reviews of these documents and providing summaries herein, is to provide as broad a base of scientific input as possible to determine relevant and appropriate concerns relative to the proposed Savannah Harbor deepening and consequent impact, if any, upon the ambient water quality of the Upper Floridan aquifer; and to develop a plan of study that will satisfactorily address the concerns raised by the proposed project.
Summaries of the document reviews are provided in the following paragraphs. The documents reviewed and summarized are presented in chronological order. No weight of scientific validity, integrity, importance, or relevance should be inferred from the order of discussion or the amount of discussion dedicated to each.
This report was one of the first to quantify and evaluate the ground-water resources of southeastern South Carolina and is generally considered to be the study that called attention to the issue of salt-water intrusion at the northern end of Hilton Head Island. The estimated ground-water withdrawal from the principal artesian aquifer (Floridan aquifer) in the subject counties was about 6.2 billion gallons. During that same period The City of Savannah and nearby industries pumped approximately 75 mgd (million gallons per day).
This report indicated that the principal artesian aquifer (Floridan aquifer) in the study area consists of an upper permeable zone, which provided about 75% of the water pumped from the aquifer in Hampton County and nearly all of the water pumped from the aquifer in Beaufort and Jasper Counties; a middle zone of low permeability that yields small quantities of ground-water in Hampton and Colleton Counties; and a lower permeable zone that provides most of the ground-water pumped from the aquifer in Colleton County. This report also provided estimates of transmissivity of the two identified permeable zones as follows: 10,000 ft2/day to 50,000 ft2/day for the upper permeable zone, and 500 ft2/day to about 5,000 ft2/day for the lower zone.
The report identified two sources of salt-water contamination of the aquifer in the study area. The first was sea water directly entering the aquifer through areas where the overlying confining unit (Hawthorn Formation) was locally absent or through thinned areas having relatively high permeability and secondly by means of upward migration of connate salt-water from the underlying formations. Chloride concentrations of greater than 1,500 mg/l (milligrams per liter) were reported throughout the aquifer at Parris Island, Fripp Island, Edisto Beach, and probably other small sea islands southeast of Beaufort. Saline water was indicated within the middle and lower zones of the principal artesian aquifer (Floridan aquifer) in Beaufort County, southern Colleton County, and possibly in southern Jasper County.
Chloride concentrations of about 50 mg/l were reported in the upper permeable zone of the principal artesian aquifer on Hilton Head Island. Saline ground-water was reported to be moving laterally toward Hilton Head from the northeast and east and vertically upward from the lower permeable zones. The rate of salt-water migration toward Hilton Head Island was estimated to range from 140 to 360 ft/yr (feet per year).
The report indicated a permeable zone within the Hawthorn Formation (Hawthorn Group) that consists of sandy dolomitic limestone capable of providing 50 to 200 gal/min (gallons per minute) in western Beaufort County and in Jasper County.
The authors refined and expanded a two-dimensional finite-difference model of the principal artesian aquifer (Floridan aquifer) that had been developed by Counts and Krause (1976). The steady-state model indicated that the flow system prior to development was sluggish and only about 65 mgd flowed through the modeled area. Simulation of the present day (1984) system and pumping stresses of about 85 mgd in the Savannah-Hilton Head area indicated through flow of about 250 mgd. The vertical inflow (recharge) from the overlying surficial aquifer more than doubled over that of the pre-development simulation.
A hypothetical 25-percent increase in pumpage over the model area was used to approximate future industrial and municipal pumpage over the next 20 to 30 years. The modeled increase produced water levels of 165 feet below sea level or a maximum decline of 30 feet at the center of the cone of depression and a 5 foot decline at a radius of 20 miles from the pumping center.
Other hypothetical pumping increases evaluated by the model included a doubling of pumpage on Hilton Head and the introduction of agricultural pumping north of Savannah. On Hilton Head, the increase represented about 9.7 mgd distributed throughout the island. The modeled simulation resulted in a maximum drawdown of 8 feet on the island and about 2 feet in the Savannah area. The agricultural development was represented by an increase in pumpage of 29.7 mgd. The result was the development of a cone of ground-water level decline centered in the northeast corner of Bulloch County, Georgia with a maximum decline of 29 feet.
The hydrologic input data for hydraulic conductivity necessary for determining estimates of leakance (ratio of hydraulic conductivity of the confining unit to the confining unit thickness) were based on laboratory analyses. The hydraulic conductivity of the confining unit was determined to be 8.6 x 10-5 ft/day which is representative of low-permeability clays and silts. Additionally, the authors assumed the hydraulic conductivity to be uniform throughout the confining unit and consequently used the single value of 8.6 x 10-5 ft/day for the entire model area.
The Ladies and St. Helena Islands area consists of approximately 175 square miles in southeastern South Carolina. Ground-water supplies are available from several underlying stratigraphic units: the Middendorf, Black Creek, PeeDee, Black Mingo, Santee and shallow undifferentiated formations. However, the Upper Floridan aquifer is the most productive of the high water-quality bearing formations.
Recharge to the Upper Floridan aquifer occurs primarily in northern Ladies Island and the central area of St. Helena Island. Recharge to the Upper Floridan aquifer is the result of direct rainfall entering the surficial aquifer and then into the Upper Floridan aquifer in areas where the overlying confining unit is thin or absent and possibly through sinkholes. Ground-water flows radially away from the island up-land areas toward rivers, estuaries, and the Atlantic Ocean at a hydraulic gradient of as much as 15 feet per mile.
The chemical quality of ground-water in the Upper Floridan aquifer is generally good. Chloride, dissolved solids, and hardness concentrations nearest to recharge areas were reported to be significantly below U.S. Environmental Protection Agency drinking water standards. Chloride and dissolved solids concentrations are reported to increase toward areas of lower potentiometric elevation. Reported dissolved iron and sulfide concentrations are locally elevated in several areas.
This report provides data regarding the hydraulic characteristics for more than 100 multiple-well and single-well aquifer tests throughout the coastal plain of South Carolina. A well located in Jasper County, South Carolina (County well No. JAS-104; South Carolina Water Resources well No. 29II -o1) that is open to the Floridan aquifer from 145 feet below land surface to 330 feet below land surface was tested during May 1957 at a pumping rate of 1,600 gallons per minute (gpm). The specific capacity of this well was calculated to be 100 gallons per minute per foot of drawdown. The transmissivity of the Floridan aquifer at the well was calculated to be 47,000 ft2/day.
Several wells located in Beaufort County (Fripp Island, Hilton head Island, and the Waddell Mariculture Center) that are also open to the Floridan aquifer were also tested. Pumping rates during the pumping tests ranged from ranged from 280 gpm to 2,255 gpm and calculated specific capacities ranged from 6.7 to 250 gallons per minute per foot of drawdown.
As part of an investigation of soil and ground-water contamination at Marine Corps Air Station (MCAS) Beaufort, the author evaluated the potential for dissolved petroleum hydrocarbons to impact ground-water quality within the Upper Floridan aquifer. Site specific values of vertical hydraulic conductivity, porosity and vertical hydraulic gradient were determined as part of this study. Sediment samples were collected at the drill site using Shelby tubes; sub-samples were then extruded for analysis. A total of fourteen samples were subjected to permeameter testing to determine vertical hydraulic conductivity; of these, nine samples were lithologically classified as clays and the remaining five were classified as sands.
The clay samples had reported vertical hydraulic conductivity values ranging from 2.1 x 10-2 ft/day to 8.2 x 10-5 ft/day; vertical hydraulic conductivity values for sand samples ranged from 1.6 x 10-1 ft/day to 6.2 x 10-3 ft/day. Vertical hydraulic conductivities of the upper confining unit were previously determined from aquifer tests at two sites on Port Royal Island and yielded values of 5 x 10-3 ft/day and 1.5 x 10-2 ft/day respectively (Hantush-Jacob method). While the vertical hydraulic conductivity values of all of the site specific samples collected for this study ranged over four orders of magnitude, the mode and the median of the distribution from the permeameter tests were between the values calculated from the aquifer tests.
Releveling surveys completed in 1975 over parts of lines surveyed in 1955 provided an opportunity to determine further land-surface change and to test hypotheses presented by Davis, et al. (1976). Land subsidence at Savannah resulting from pumping of the Floridan aquifer was found to be insufficient to be recognized as a serious engineering concern, rather it is of interest more as a consideration in the adjustment of precise leveling and as a complicating factor in modeling of the ground-water system.
Artesian head declines, which corresponded closely to changes in rates of ground-water withdrawals, showed marked acceleration in the mid 1930s, leveling off during the mid 1940s, acceleration from 1953 to 1963, and stability thereafter. Precise leveling in 1918, 1933, 1935, and 1955 indicated that subsidence of as much as 100 mm had occurred , mostly since 1933. By 1933, an area of more than 130 km2 had subsided at least 20 mm. Davis et al. (1963) postulated that the observed subsidence was related to declines in the head of the Floridan aquifer, which by 1955 had reached 50 meters, of which 40 meters had occurred between 1933 and 1955. They concluded that the limestone matrix of the Floridan aquifer was not compacting, rather that compaction was more likely occurring in interbedded clay and marl and/or silt and clay of the overlying confining unit (Hawthorn Group).
The releveling of 1975 indicated that subsidence had continued in much the same area at Savannah despite majors changes in the amount and areal distribution of head decline in the Floridan aquifer. In the vicinity of the area of maximum head decline and maximum subsidence for the period prior to 1955, subsidence has continued at about the previous maximum rate of about 4 mm/yr despite near complete cessation of head decline there since 1963. This offers strong support to the earlier conclusion that subsidence is being caused by compaction of the fine-grained sediments of the Miocene confining unit through slow drainage rather than by compaction of the limestones of the Floridan aquifer.
Water level data were collected from July 28 to August 5, 1986 from 640 wells known to be open exclusively to the Floridan aquifer (as defined in this report). Ground-water flow within the Floridan aquifer was found to be generally toward the southeast (seaward) except where locally influenced by recharge, natural discharge, and pumping from water wells. Water levels were reported range from approximately 200 feet above sealevel in the northwest to 90 feet below sealevel near Savannah, Georgia.
In the Beaufort, South Carolina area locally significant recharge to the Floridan aquifer was reported in the northern part of Port Royal Island and on the barrier islands. The large mounding in the potentiometric surface in northern Port Royal Island is attributed to significant recharge within an area of relatively low transmissivity in the Floridan aquifer. Transmissivity was reported to increase to about 70,000 ft2/day in the southwest.
The northern portion of Hilton Head Island may be an of recharge; however, the potentiometric contours become dominated by the northeastern edge of the cone of depression in the potentiometric surface of Floridan aquifer that is centered at Savannah.
Steady-state flow in the upper Floridan aquifer was simulated for 1984 and for prior to pumping (1884) with finite-difference model. The simulations indicated that downward vertical leakage through the confining unit above the aquifer on northern Hilton Head Island were between 0 and 4 inches per year prior to pumping and between 3 and 8 inches per year in 1984. Downward leakage on the sea islands north and east of Port Royal Sound remained essentially unchanged in 1984 compared to pre-pumping conditions.
The study also reported that brackish and saltwater in the aquifer were moving toward the northeastern shore of Hilton Head Island at 50 to 80 ft/yr in 1984. The saltwater-fresh water interface beneath Port Royal Sound was reported to have moved, but was still near the theoretical steady-state interface estimated for the period prior to pumping.
Simulation of Saltwater Movement in the Floridan Aquifer System , Hilton Head Island, South Carolina. U.S. Geological Survey Water-Supply Paper 2331. Bush, Peter W., 1988.A finite-element model for fluid-density-dependent ground-water flow and solute transport (SUTRA) were used to simulate pre-pumping, recent, and future ground-water flow in the Floridan aquifer beneath the north end of Hilton Head Island and Port Royal Sound.
The simulation of pre-development conditions was typical of a coastal aquifer having a seaward gradient in the freshwater. The freshwater was shown to flow toward Port Royal Sound over an intruding wedge of saltwater. The simulated flow-field at the end of 1983 showed that ground-water in the Floridan aquifer beneath most of Hilton Head Island had reversed its pre-development direction and was moving toward Savannah. The distribution of chloride concentrations, based on simulations for the end of 1983, was about the same as the pre-development distribution of chloride concentrations obtained from the model simulations.
The results of two 50-year simulations from 1983 to 2034 suggest that there would be no significant threat of saltwater intrusion into the upper permeable zone of the Upper Floridan aquifer if hydraulic heads on Hilton Head Island remain at current levels for the next 45 to 50 years. However, if hydraulic head decline continues at the historical rate, any flow that presently occurs from the north end of the island toward Port Royal Sound would cease, allowing lateral intrusion of saltwater to proceed. Even under those conditions, chloride concentrations in the upper permeable zone of the Upper Floridan aquifer beneath Hilton Head were predicted to remain below 250 mg/l for the next 45 to 50 years.
This paper is a Regional Aquifer System Analysis (RASA model) of the Floridan aquifer throughout the four states in which it is present. The regional flow system has not been significantly altered by development; however, pumpage that reached three billion gallons per day by 1980 has resulted in long-term regional water level declines of more than 10 feet in three broad areas: coastal Georgia and adjacent South Carolina and northeast Florida; west-central Florida; and the Florida panhandle. Saltwater encroachment resulting from pumping has occurred locally in coastal areas.
The Savannah, Georgia-Beaufort, South Carolina area is classified as being within a semi-confined area of the Upper Floridan aquifer based upon the thickness of the upper confining unit (generally less than 100 feet), breaching of the confining unit, or both. The estimated transmissivity of the Upper Floridan aquifer in this area ranges from 10,000 to 50,000 ft2/day. Just to the south of Savannah the transmissivity is reported to increase to 50,000 to 100,000 ft2/day. The leakage coefficient estimated from simulations for the confining unit in the Savannah area is reported to be less than 0.01 inches per year per foot of confining material.
The pre-development potentiometric surface of the Upper Floridan aquifer in the Savannah area was estimated to range from about 30 feet to 40 feet above sea level. By 1980 the reported potentiometric surface at Savannah was about -90 feet sea level. The -10 foot level contour extended several miles off-shore of Tybee Island and the 0 foot level contour appears to extend well off-shore; however, there were no off-shore control points in the area. The net decline in the potentiometric surface of the Upper Floridan at Savannah is greater than 100 feet.
This study concluded that lateral movement of saltwater in response to reductions in fresh water hydraulic head is the dominant mechanism causing saltwater encroachment within the Floridan aquifer within the study area. Other, but less significant mechanisms contributing to saltwater contamination of the Floridan aquifer were identified as vertically downward flow through thin or absent confining beds, up-coning from underlying portions of the aquifer containing connate saline water, and poorly or improperly constructed water supply wells.
The report also provides discussions of the hydrostratigraphy of the study area and potentiometric elevations of the Floridan aquifer.
The hydrogeology and
ground-water flow system in the Floridan aquifer was modeled using a three-dimensional
finite-difference digital model. The
ground-water flow in the Floridan aquifer was simulated under pre-development conditions
and 1980 conditions in southeast Georgia and nearby parts of Florida and South Carolina.
The digital model simulations
indicated that about 900 million gallons per day of water flowed through the system prior
to development. About 65% of the modeled flow
was in the area up-gradient of the Gulf Trough. Ground-water
flow in this area consisted of recharge in areas between streams, lateral movement
down-gradient and discharge to major rivers. The
flow system in most of the area down-gradient of the Gulf Trough was characterized by
relatively slow lateral movement. Throughout
the study area , almost all ground-water circulation was found to be within the upper
Floridan. Pumpage of about 625 million
gallons per day (in 1980), concentrated mainly down-gradient of the Gulf Trough was shown
to have changed the flow system substantially. Significant
head declines within the upper Floridan aquifer were noted , especially along the
immediate coast where the greatest ground-water withdrawals were occurring.
Plate 7 of this report shows
that transmissivity values within the Floridan aquifer within the lower Savannah River
area range from 19,800 ft2/day to 53,000 ft2/day. An average laboratory-derived vertical hydraulic
conductivity of 1.3 x 10-3 ft/day for the Miocene confining unit is reported
for 52 core samples collected in Chatham County. Figure
11 of this report shows that the estimated leakance distribution of the Miocene confining
unit in the vicinity of the Savannah River to be on the order of 10-5 to 10-4
feet per day per foot. Leakance of the
Miocene confining unit is reported to vary inversely with thickness of the unit.
The Saturated-Unsaturated Transport (SUTRA) model of the U.S. Geological Survey was used for the simulation of density-dependent ground-water flow and solute transport for a vertical section of the Upper Floridan aquifer and upper confining unit beneath Hilton Head Island and Port Royal Sound.
The report indicated that saltwater encroachment in the Upper Floridan aquifer could be slowed by reducing ground-water withdrawal or by injecting freshwater into the aquifer to create a hydraulic barrier. Ground-water withdrawals at Savannah, Georgia and Hilton Head, South Carolina has lowered water levels within the Upper Floridan aquifer and has caused saltwater in the aquifer beneath Port Royal Sound to move toward Hilton Head Island.
Hydraulic conductivity values for the Miocene confining unit from aquifer tests conducted on Port Royal Island, South Carolina were reported to be 1.5 x 10-3 and 4.6 x 10-3 m/day respectively (Hantush-Jacob method). Additionally, 23 core samples collected from 8 sites beneath Port Royal Sound, South Carolina and analyzed by permeameter tests (Burt, et al., 1987) and converted to log 10 had a distribution mode of 1.7 x 10-3 m/day and median of 2.6 x 10-3 m/day. These values are between the values calculated from the aquifer tests (Hayes, 1979). The vertical hydraulic conductivity values varied over four orders of magnitude, but the largest number of samples per log 10 cycle were between 10-3 and 10-2 m/day.
Porosity values of upper confining unit materials were calculated by gravimetric method from samples cored beneath Port Royal Sound and two samples from northern Hilton Head Island. The calculated porosities ranged from 21% to 72%. The median value was 44% and the mean was 45%.
The model was less sensitive to changes in aquifer permeability than to changes in permeability of the upper confining unit. The effect of permeability changes in the silt and clay fractions of the confining unit caused by physical and chemical responses to the replacement of freshwater with saltwater was suggested for future study.
Over the past twenty years, ten separate ground-water models of the Floridan aquifer have been developed for coastal Georgia and South Carolina. Four of these models are still active:
The SUTRA Model approximates salt-water encroachment from the vicinity of Port Royal Sound toward Savannah; the remaining flow models are used to approximate water levels in the Floridan aquifer under variable pumping conditions. Each of the flow models predict different water levels at the northern end of Hilton Head Island in response to pumping changes in Savannah/Chatham County and/or elsewhere in Georgia. The respective state agencies of Georgia and South Carolina disagree as to which model most accurately predicts water levels at the northern end of Hilton Head Island.
In order to resolve the disagreement, it was proposed that the three flow models be validated by comparing their predicted results to actual measurements in newly constructed observation wells. The model that most closely predicted the measured water levels would become the model of choice for assessing the SUTRA gradient. Some of the observation wells were to be constructed at locations such that the salt-water encroachment velocity could be actually measured, thus allowing the Smith 1994 SUTRA Model to be calibrated against measured data.
As a part of the Georgia Geologic Survey's "Evaluation of the Miocene Aquifers in the Coastal Area of Georgia Project," a pumping test was conducted at the Tybee Island well cluster. A test of the Upper Brunswick Aquifer was conducted from March 19 through March 23, 1997 using a pumping well screened in the Upper Brunswick Aquifer. The data indicated a transmissivity of 21,500 ft2/day and storativity of 0.0001 for the unit tested. An observation well screened in the unconfined surficial aquifer had no response to pumping, indicating no leakage across the confining unit separating the Upper Brunswick Aquifer and the surficial aquifer. Leakage was reported to have occurred across the confining unit separating the Upper Brunswick from the underlying Upper Floridan.
Note: discussion with ACOE-Savannah indicate that the Upper Brunswick aquifer is not present within the study area and that the pumping well may have been withdrawing from the Upper Floridan aquifer.
The Georgia Environmental Protection Division's (GAEPD) stated objective is to stop the intrusion of salt water before municipal water supply wells on Hilton Head Island, South Carolina and Savannah, Georgia are contaminated, and to prevent an existing salt-water problem at Brunswick, Georgia from worsening.
During February 1996, GAEPD proposed a draft Interim Strategy to protect the Upper Floridan aquifer in 24 southeastern Georgia counties from salt-water intrusion. GAEPD subdivided the 24 county area into three subareas (northern, central, and southern) based upon recognizable geological and hydrogeologic differences between the subareas. During March and April 1996, GAEPD held nine public meetings to receive public comments. GAEPD received in excess of 400 responses to the proposed draft Interim Strategy during the public comment period.
In response to the public comments, GAEPD contracted with the School of Public Policy Studies of Georgia State University (GSU) to investigate non-regulatory avenues of implementing the Interim Strategy. GSU completed its analysis on October 1, 1996 and recommended that the Interim Strategy follow a policy of Rational Use. GSU's recommendation was that a policy of Rational Use would be conducive to economic development.
GAEPD released a proposed Revised Interim Strategy on December 20, 1996; followed by three public meetings held in January 1997. About 90 additional responses were received and the following consistent themes were expressed:
The Interim Strategy is intended to continue the protection of the Upper Floridan aquifer in southeastern Georgia from salt-water intrusion. Implementation of the Interim Strategy will continue until December 31, 2005. During the period of implementation will work with a stakeholder advisory committee to accept input for both the implementation of the Interim Strategy and the development of a final strategy.
GAEPD's interim strategy involves the following:
The Interim Strategy will, upon full implementation:
The Interim Strategy includes discussions of water supply planning, conservation, permitting, reallocation of water, and the Sound Science Initiative in addition to those items noted above.
The Savannah Harbor Expansion Feasibility Study is a multi-faceted study to determine the feasibility of expanding and deepening the present Savannah Harbor and entrance channel. The study is the first step in determining the engineering, environmental, and economic feasibility of the proposed project. The Potential Ground-Water Impacts study was prepared as part of a separate technical study and was prepared as an attachment to the Engineering Appendix of the Feasibility Study.
The Ground-Water Impact study included geophysical investigations (seismic reflection survey), drilling of core holes, analysis of cores to determine vertical hydraulic conductivity, grain-size distribution and other geotechnical parameters, borehole geophysical logging, installation of observation wells, and a water well survey of wells open to the surficial aquifer and the deeper Miocene sediments.
Discussions of regional and local geologic structure, stratigraphy, and hydrogeology were provided, as well as the results of field data acquisition effort. A total of 22 undisturbed core samples were collected from six boreholes. The range of vertical hydraulic conductivity for samples collected from Miocene sediments ranged from 4.3 x 10-2 to 6.0 x 10-5 ft/day; the mean vertical hydraulic conductivity for all Miocene samples analyzed was 5.7 x 10-3 ft/day. Four core samples collected from the in-fill material within buried relict stream channels was 7.7 x 10-3 ft/day.
The ACOE assessed conditions under both leakage trough the Miocene confining unit exclusively and leakage partially through through a paleochannel and partially through the Miocene confining unit. Both conditions were assessed in the vicinity of the Tybee High, where the top of the Upper Floridan aquifer is shallowest. Leakage through the Miocene confining unit was estimated to be about 900 gpd/acre and about 1160 gpd/acre through the paleochannel areas. Removal of ten feet of confining unit material through dredging was estimated to increase leakage by about 300 gpd/acre.
This document presents the ground-water management plan for the State of Georgia and compares the Georgia plan with the United States Environmental Protection Agency's (EPA) expectations of a "Core" Comprehensive State Ground-Water Protection Plan (CSGWPP). EPA approved Georgia's Ground-Water Management Plan as a "Core" CSGWPP on September 24, 1997.
Georgia's ground-water protection goal is established by the Georgia Water Quality Control Act; a succinctly phrased version of the legislative intent of the ground-water protection goal was developed in 1983 and is consistent with EPA's ground-water protection goal. Georgia implements its ground-water protection goal through a policy of non-degradation of the resource. The State's ground-water management plan implements the non-degradation policy through the following primary elements:
Protection of ground-water quality entails: (a) the prevention of pollution through proper siting, construction, operation, and monitoring of environmental facilities through EPD's permit programs, wellhead protection and prudent land-use planning by local governments; (b) detection and mitigation of existing water quality problems; (c) development of protective standards where permits are not required; and (d) education of the public regarding ground-water pollution and the need for ground-water protection.
Management of ground-water quantity involves allocation of the State's ground-water so that the resource will be available for both the present and the future. Monitoring of ground-water quality and quantity requires continual assessment of the resource so that changes can be identified and corrective action implemented as needed.
Published and unpublished seismic and geological data were reviewed, evaluated, and synthesized. Eight hundred miles of high-resolution seismic reflection data extending from Port Royal Sound to the north and Wassaw Sound to the south, collected between 1970 and 1997 were compiled and interpreted using standard methods of analysis. Borehole lithology-log and gamma-log data were used to ground truth the seismic-stratigraphic interpretations and to provide additional control in areas where seismic coverage was limited.
Five potential sites of saltwater intrusion were identified and ranked (highest to lowest) as Areas of Concern (AOC):
The Phase I report recommended that the identified AOCs and the region bounded by St. Helena Sound, Port Royal Sound, St. Helena Island, and Beaufort be investigated during Phase II.
Hydraulic characteristics (transmissivity and storage coefficient) of the Upper Floridan aquifer in the Savannah and St. Marys areas of Georgia were evaluated by analyzing the results of water-level recovery tests. Tidal corrections were applied to data from one well for the Savannah test area and to data from the four wells for the St. Marys area test. Data from one St. Marys well were also corrected for the effects of nearby pumpage. The anisotropy of the aquifer was evaluated in both the Savannah and St. Marys test sites.
The calculated transmissivity of the Upper Floridan aquifer in the Savannah area is reported to range from 32,000 to 43,000 ft2/day and the storage coefficient is reported to range from 6.3 x 10-4 to 1.3 x 10-3. Transmissivity of the aquifer has a reported anisotropy ratio of 1.2:1 and an angle of anisotropy of about 108 degrees. The larger principal transmissivity value is aligned approximately north-northwest.
This report describes the potentiometric surface of the Upper Floridan aquifer in Georgia and adjacent parts of Alabama, Florida and South Carolina for May 1998, and water-level trends in Georgia for the period May 1990 to may 1998. The Georgia Coastal Plain has been informally divided into four subareas for purposes of discussion of the potentiometric surface and water-level trends.
The configuration of the potentiometric surface of the Upper Floridan in the Georgia Coastal subarea is characterized by four significant cones of depression that are the result of pumpage. Major pumping centers located at Savannah, Jesup-Doctortown, Brunswick, and St. Marys, Georgia-Fernandina Beach, Florida dominate the configuration of the Upper Floridan aquifer potentiometric surface.
In 1998 the water-level near the center of the Savannah cone of depression was -97 feet. The cone of depression centered at Savannah covers a larger area and is deeper than in other coastal areas, although ground-water withdrawal rates are similar, because of generally lower transmissivity of the Upper Floridan aquifer in the Savannah vicinity. The overall water levels in the Upper Floridan aquifer rose during the period 1990-98 due to reductions in industrial pumping.
Evaluation of United States Geological Survey Ground-Water Flow Models of Coastal Georgia and South Carolina. Georgia Department of Natural Resources, Environmental Protection Division, Georgia Geologic Survey, Project Report 38, 1999.Several finite difference MODFLOW models were reviewed and evaluated by independent consulting firms (ARCADIS Geraghty & Miller, Camp Dresser & McKee, and Law Engineering and Environmental Services, Inc.) as part of the Sound Science Initiative; the models evaluated were:
The report was intended to review the appropriateness of USGS assumptions; appropriateness of USGS quality assurance procedures; appropriateness of the models' grid discretization and cell sizes; appropriateness of hydrogeologic boundaries; documentation of model input parameters; input parameters assigned to appropriate grid cell; appropriateness of steady-state simulations; justification of steady-state versus transient simulations; model input parameters; and data weaknesses.
The review by ARCADIS Geraghty & Miller provided the following conclusions about the various models.
The RASA Model was found to adequately represent hydrologic conditions in the Upper/Lower Floridan aquifers and is suitable to understand regional flow conditions. The model is not current state of the art strictly on the basis of grid resolution. A similar current modeling study would use substantially more grid cells to represent the modeled domain. The model is suitable to examine flow conditions on a regional scale; however, it contains limitations that reduce its usefulness for managing ground-water resources that are threatened by saltwater intrusion.
The Savannah Model was developed to evaluate the effects of additional pumping on water levels in proximity to known sites of saltwater encroachment at Hilton Head, South Carolina and Brunswick, Georgia. It is a subregional model developed from the RASA Model; it has the same limitations as the RASA Model and differs only in size and resolution.
The objective of this model was to evaluate the development potential of the Upper Floridan aquifer in coastal Georgia, such that any development would not cause ground-water levels to change in areas of known saltwater intrusion. It was also developed in response to concerns about upward migration of saline water the the potential for contamination of the Upper Floridan aquifer at Brunswick.
This model was developed in a similar fashion to the Savannah Model and it contains similar limitations. Although the model is not as well calibrated as the Savannah Model, it is generally adequate for regional flow analysis.
This model was developed to evaluate the development potential of the Upper Floridan aquifer in coastal Georgia. The model covers a larger area than the Savannah or Glynn County models and has a resolution of two miles per side for each grid block. It has greater resolution than the RASA Model but lower resolution than the Glynn County Model. However, it can simulate hydraulic effects over a larger area than either the Savannah or Glynn County Models, but does not have as high a degree of accuracy as these models. The assumptions and limitations are the same as the RASA and other USGS models for coastal Georgia.
This model departed from the USGS family of telescoping models. It is inherently different from the other USGS models and it does not rely on the RASA Model to predict boundary flows. While the boundary flow values it uses may be appropriate, it lacks the flexibility of the USGS telescoping models to simulate a wide range of system stresses. The model can not evaluate interactions between the Upper and Lower Floridan aquifers.
The Camp Dresser & McKee review provided the following conclusions and concerns:
Law Engineering and Environmental Services, Inc. provided the following comments:
The various coastal Georgia and adjacent parts of Florida and South Carolina ground-water flow models of the Floridan aquifer were revised and updated. The revised models are the RASA, Glynn County, and the Savannah Models. Changes were made to the hydraulic property arrays of the RASA and Glynn County Models to ensure consistency among all of the models.
After the revisions were completed, 32 hypothetical pumping change scenarios were simulated; the pumping changes ranged from approximately 83 million gallons per day (mgd) to about 438 mgd higher than the May 1985 pumping rate (308 mgd). The pumping scenarios were grouped by pumping locations; the groups were as follows: 1) the entire 24 county coastal area, 2) Glynn-Wayne-Camden County subarea, and 3) Savannah-Hilton Head Island subarea.
For those scenarios wherein pumping was decreased water levels at both Brunswick and Hilton Head Island increased, thus decreasing the vertical hydraulic gradient and lessening the potential for saltwater contamination. The converse was reported when increased pumpage was simulated.
The authors reported the occurrence of Late Pleistocene-Holocene, tectonic, orthogonal fracture sets in Upper Pleistocene deposits of the lower Coastal Plain. Subparallel sets occur in Middle Eocene and Lower Pliocene sediments in quarry exposures and in the walls of Dort Dorchester which was damaged by the 1886 Charleston earthquake. The fracture sets observed provide the orientation of the regional modern day stress field and can provide data on active faults that lack surface scarps.
HydroVision Inc reviewed the ACOE report under contract with the City of Savannah and concluded that sufficient study has not been done regarding proposed harbor dredging and that substantial additional work would be required to answer questions regarding the impact of proposed harbor dredging on ground-water in the Savannah area.
Specific criticisms of significance were as follows:
HydroVision also provided recommendations to the City of Savannah to rectify their perceived shortcomings of the ACOE report:
Law Engineering and Environmental Services, Inc. (Law) conducted a field examination and evaluation of the potential hydrologic characteristics of fractures in the Cenozoic sediments of Georgia and South Carolina coastal plain. As part of this evaluation, nine outcrop areas were visited, examined, photographed, and described.
The report concluded that there are numerous fractures present in Coastal Plain sediments of all ages. However, the fractures rarely cross contacts between various stratigraphic units. Most of the fractures that were observed and described are tight and many are infilled with clay. The limited vertical extent of the fractures, together with the clay infilling, minimizes any potential ground-water flow between the surficial aquifer and underlying aquifers (i.e. The Upper Floridan aquifer).
The fractures in the Coastal Plain sediments probably have some effect on the hydraulic properties of the material; however, the fractures and the material matrix combine to control the hydrologic properties of a given hydrostratigraphic unit. In general, it was reported that fractures would have a limited role in the hydrogeology of the area.
The report also indicated that it would be difficult to assess how much flow occurs through the matrix versus flow within the fractures. Any large scale hydraulic testing would be affected by both fracture and matrix permeability. Rather than relying on such testing to assess local hydraulic properties, an evaluation of the regional ground-water flow can be assessed to evaluate the potential hydrologic significance of the combination of fracture and matrix hydraulic conductivity. As an example, significant drawdown of the potentiometric surface of the Upper Floridan aquifer is well documented; however, there has not been a corresponding decline in the hydraulic head of the surficial aquifer. The total vertical leakage can therefore be estimated by calibration of ground-water flow models appropriate to the local area.
This letter addressed three concerns that had been communicated regarding the effect of Savannah Harbor expansion on the Floridan aquifer in the Savannah, Georgia area.
In addressing the first question, it was noted that if the confining was jointed or fractured, springs and other discharge points should have been noted in the Savannah area prior to development of the Floridan aquifer, inasmuch as there was a net vertically upward hydraulic gradient from the Floridan aquifer toward land surface; and the potentiometric surface of the Floridan aquifer during pre-development conditions was above land surface.
At the time of the Charleston earthquake, there was still high upward hydraulic pressure in the Floridan aquifer as evidenced by the presence of artesian (flowing) wells. Again, there is no reported record of springs and/or seeps occurring at the time of the Charleston event.
It was also noted that the large decline in the piezometric surface notwithstanding, there has been no indication of salt water leakage into the Floridan aquifer. It was therefore concluded that the confining unit is structural intact and is efficient in preventing vertical flow of water from the surface into the aquifer.
Several old Savannah River channels (paleochannels) were identified and cored as part of the ACOE 1998 study. It was noted that the lithologic logs from these cores indicate (a) that the strata underlying the river contain significant quantities of impermeable sediments and should act as effective parts of the overall confining unit and (b) there were two indications of non-horizontal fractures. It was concluded that the presence of the old river channels has had no effect on the integrity of the confining unit as a whole.
It was also concluded that it is unlikely that the removal of an additional 10 feet (about 15 % of the thickness of the confining unit below the channel where it is relatively thin near Ft. Pulaski) would have any negative effect.
The letter concluded that all lines of evidence indicate that the confining unit overlying the Floridan aquifer at Savannah, does not transmit any significant quantity of saltwater to the aquifer; and that it is unlikely that 10 feet of additional dredging would have significant impact on the integrity of the confining unit or cause significant decline in water quality in the Floridan aquifer.
Geology and Ground-Water Resources of the Coastal Area of Georgia. Georgia Geologic Survey Bulletin 113. Clarke, John S., Hacke, Charles M., and Peck, Michael F., 1990.
This report provides detailed discussions of the hydrostratigraphy of the Georgia coastal plain. The upper and lower Brunswick aquifers were delineated and named as part of this study. The report also discussed the hydraulic properties of the various water-bearing geologic units and of the various confining units. The report cites evidence of downward movement of ground-water from shallow units into deeper units based on both hydraulic head and geothermal gradient data. Seven wells located within the Savannah area cone of depression were found to have geothermal gradients that were less than normal (0.8 degrees Celsius per 100 feet) as determined by Wait and Gregg (1973) suggesting that ground-water was moving from shallow to deeper water-bearing units.
The report also discusses the Eocene and later stratigraphy of the study area. It was noted that the total thickness of the Miocene unit (Miocene A, B, and C units) was reported to be about 53 feet at Fort Pulaski; the report also indicated that the confining unit above the Upper Floridan is about 14 feet at Fort Pulaski.
Vertical hydraulic conductivity values reported by Furlow (1969) ranged from 1.3 x 10-5 ft/day to 5.3 x 10-2 ft/day. The reported ratio of vertical hydraulic conductivity to horizontal hydraulic conductivity was reported to range from 0.14 to 1.5; suggesting highly variable, anisotropic hydraulic properties of the confining unit
Water-Supply Potential of Major Streams and the Upper Floridan Aquifer in the Vicinity of Savannah, Georgia. USGS Water-Supply Paper 2411, by Garza, Reggina, and Krause, Richard E., 1996
This report has three major components: 1) an evaluation of flow and water quality characteristics at selected gauging stations on the Savannah River (Clyo Station) and the Ogeechee River (Eden Station); 2) a description of the ground-water flow model developed and calibrated for 1985 conditions using finite-difference techniques described by McDonald and Harbaugh (1988); and 3) an assessment of the water-supply potential of the Upper Floridan aquifer and an evaluation of theoretical ground-water pumping alternatives.
Stream flow analyses were conducted by using flow duration values to estimate the percentage of time that the 7Q10 discharge has been equaled or exceeded in each of the river basins. Flow duration tables were constructed using classes that represent ranges of stream discharge. The report concluded that the Savannah and Ogeechee Rivers can be considered as potential water-supply sources based on the historic stream flows and water quality.
The Savannah Area Model was developed to resolve out-dated hydrologic data, limitations on grid resolution, vertical discretization issues of the previous finite-difference models that have been developed for coastal Georgia and nearby coastal South Carolina, to estimate the potential for additional Upper Floridan aquifer development in the Savannah-Hilton Head area, and to evaluate resource management alternatives at greater resolution than was previously possible. The Savannah model is based upon the larger areal extent and coarser grid, numeric model that was developed as part of the Regional Aquifer System Analysis (RASA) study. All of the subregional models use the regional model (RASA) to define boundary conditions, and each regional-subregional model pairs function as a telescoped model. The telescoping models can be used to evaluate the ground-water flow system at a greater resolution than that of the RASA model without having to extend the subregional model boundaries out to a point where they would encounter natural hydrologic boundaries.
The RASA model simulates lateral ground-water flow and ground-water levels in the Upper and Lower Floridan aquifers. The upper confining unit overlying the Upper Floridan aquifer is simulated as being vertically leaky. The surficial aquifer overlying the upper confining unit functions as a source-sink (depending upon local vertical hydraulic gradients) to the Upper Floridan and is simulated as a specified head boundary.
The Savannah Area model simulates an area of 6,680 square miles. The uniform finite-difference grid has 76 rows and 88 columns. Each cell is one mile to a side. Transmissivity values were assigned to specific cells based on results obtained during multi-well aquifer tests and specific-capacity data. The transmissivity values in the northeastern part of the model area in South Carolina were adjusted to more closely agree with those used by Smith (1988). Leakance values for the upper confining unit were taken from the RASA model. Estimates of leakance were originally derived from estimates of vertical hydraulic conductivity and the thickness of the confining unit (Krause and Randolph, 1989). Plate 6 of the report indicates that most of the Savannah River Navigation Channel is underlain by Hawthorn Group sediments having leakance values of 10-6 to 10-5. A small area near Fort Pulaski and the area of the river between Garden City and Port Wentworth have leakance values less than 10-6.
Sensitivity analyses were conducted on transmissivity values of the Upper and Lower Floridan aquifer, independently, and on vertical leakance between the surficial aquifer and the Upper Floridan aquifer, and between the Upper Floridan aquifer and the Lower Floridan aquifers. Simulated ground-water levels in the Upper Floridan aquifer were found to be most sensitive to 1) changes in transmissivity within the Upper Floridan aquifer and 2) changes in leakance between the surficial aquifer and the Upper Floridan aquifers.
The various ground-water pumping scenarios are not included herein as they have little bearing upon focus of this study.
Stratigraphy and Economic Geology of the Eastern Chatham County Phosphate Deposit. Georgia Geologic Survey Bulletin 82. Furlow, James, W., 1969.
This report provides evaluations regarding the potential impacts that phosphate mining may have on the principal artesian aquifer (Upper Floridan aquifer) and the extent, depth, grade, volume, and value of the phosphate matrix. The study indicates that the upper confining unit is sufficiently thick and impermeable to allow phosphate ore to be removed and still prevent significant saltwater infiltration into the Upper Floridan aquifer. The report further concluded that, under proper supervision, phosphate mining in eastern Chatham County would not adversely affect the water-quality within the Upper Floridan aquifer.
The Miocene unit overlying the Upper Floridan aquifer is described as being comprised of the Tampa Limestone Equivalent, Hawthorn Formation and the Duplin Marl. The Tampa Limestone Equivalent lithology is described as sand, sandy clay or marl. Furlow described the Tampa Limestone Equivalent as being from 5 to 15 feet thick within the study area.
The Hawthorn Formation is described as tough clay to tough sandy clay of very low permeability. Furlow indicated that the Hawthorn Formation ranges from about 20 feet in thickness under Wilmington Island to about 55 feet thickness under the marshes to the east. However, the Hawthorn Formation is at least 40 feet thick thoughout most of the study area. The Hawthorn Formation is capped by a dense dolomitic limestone stringer, about one foot thick, in some portions of the study area.
The Duplin Marl is described as an olive-green sand, sandy clay, and clayey sand that is difficult to distinguish visually from the upper Hawthorn Formation sediments. However, the phosphate content is markedly greater within the basal portion of the Duplin Marl compared with the Hawthorn Formation. The phosphate ore zone of the Duplin Marl is overlain by up to 50 feet of additional sediments assigned to the Duplin Marl. These sediments are lithologically similar to those of the phosphate ore zone, but contain significantly less phosphate. In the vicinity of the Savannah River entrance, the Duplin Marl appears to have been scoured and thinned by erosion.
As part of this study, fifty-two core samples were collected from the upper confining unit and were submitted to the Water Resources Division of the U.S. Geological Survey for determination of estimates of vertical permeability. The average vertical permeability of the core samples was determined to be 0.0096 gallons per day per square foot under one foot of hydraulic head. Assuming a hydraulic gradient of .375 and a coefficient of permeability of .0096 gpd/ft2, it was calculated that about 0.0036 gallons per day per square foot would pass through the upper confining unit.