| 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. |
| Numerous studies that have
investigated potential saltwater 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necessarily the only reports (published or unpublished) that address the issue of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 in 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, Georgia Department of Natural Resources, Environmental Protection Division, and South Carolina Water Resources Commission reports; unpublished reports, proposals, memoranda, and letters, have also been reviewed and summary discussions are provided herein. The intent of this review of the literature is to provide as broad a base of scientific input as possible to determine relevant and appropriate technical concerns relative to the proposed Savannah Harbor deepening project as well as 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 and are presented in approximate 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 report. Readers should be aware that the terms Principal Artesian Aquifer, Tertiary Limestone Aquifer and Floridan aquifer refer to the same aquifer system. The term Floridan aquifer is the currently preferred name of the aquifer system that encompasses parts of South Carolina, Georgia, Alabama, and Florida. More specifically, the Upper Floridan aquifer is the aquifer from which most potable and industrial use water is drawn from in the Savannah area. |
This report indicated a need for more information regarding ground-water in coastal Georgia due to the rapid rate of development of artesian water resources during the previous 10 years (prior to 1944). In the six coastal counties artesian water consumption for industrial use had increased by about 40,000,000 gallons per day during this period. In 1943 the total discharge of artesian water was estimated to be about 135,000,000 gallons per day. The largest pumpage was reported to occur at Savannah and Brunswick and in 1943 ground-water consumption at these localities was estimated to be 42,000,000 gallons per day and 37,000,000 gallons per day respectively.
Water levels at Savannah have been lowered by 70 to 100 feet below the original piezometric surface of 37 feet above mean sea level. The lowering of water levels at Savannah to 50 or more feet below sea level suggested the possibility of fresh artesian water being contaminated by inflow from areas where the aquifer may contain salt water. The hydraulic gradient slopes toward Savannah on all sides for a distance of 20 miles or more, although the gradient appears to be less than one foot per mile at distances greater than 12 miles east and northeast of Savannah.
This report included a study area of about 2,300 square miles with Savannah, Georgia being near the center of the study area. This report provides descriptions of the stratigraphy of the area from the Paleocene to recent time and discusses the water-bearing properties of the various stratigraphic units.
Most of the principal artesian aquifer (Upper Floridan aquifer) is reported to be comprised of middle Eocene (Claiborne age) Lisbon Formation, Gosport Sand, and Ocala Limestone (Jackson age), and undifferentiated Oligocene units and the lower portion of the Tampa Limestone (Miocene). The Hawthorn Formation (Miocene) is reported to be important as part of the upper confining unit of the principal artesian aquifer.
Concerns relating to salt-water encroachment into the principal artesian aquifer are discussed within the report and three potential scenarios under which salt-water encroachment might occur.
The first is lateral migration of salt-water through the aquifer toward Savannah from areas where salt-water recharge may be occurring (Port Royal Sound, Beaufort River and other areas where the upper confining unit may be thin or absent). At the 1963 rate of pumping (approximately 60 million gallons per day) it appeared that less than 1/3 of the total water pumped at Savannah was replaced by encroachment of salty water toward the center of the cone of depression; 2/3 or more was thought to be moving from areas where the aquifer contains freshwater. The authors considered lateral migration of salt-water through the aquifer as the most likely source of salt-water contamination of the principal artesian aquifer and should receive first attention regarding future investigations.
The second mechanism of salt-water migration discussed focused on upward vertical movement of salty water through the lower confining unit. This scenario was deemed to be unlikely in the Savannah area, inasmuch as there was reported to be at least 100 feet of clay, silt and marl between the bottom of the aquifer and the first unit containing salt-water.
Thirdly, downward leakage from surface salt-water bodies through the upper confining unit was considered. The report indicates that the Hawthorn Formation sediments that form the upper confining unit are predominately clayey silts and that the Hawthorn Formation is thin only within the northeast part of the study area (20 to 30 miles northeast of Savannah). The report indicated that a clayey silt sample recovered from the Hawthorn Formation had a coefficient of permeability of 0.001. The authors estimated that downward leakage through the upper confining unit at the Savannah pumping center was about 37,000 gallons per day per square mile (approximately 58 gallons/acre/day) and about 9,300 gallons per day per square mile (approximately 14.5 gallons/acre/day) at distances of about 8 to 10 miles from the center of the cone of depression.
This report indicates that the Savannah area obtains most of its water supply from the principal artesian aquifer. Salty connate water is present in the lower portions of the aquifer in the eastern part of the area and sea-water is entering the upper water-yielding zones in the northeastern part of the area.
Salt water encroachment into the principal artesian aquifer is the direct result of large ground-water withdrawal from a relatively small area centered at Savannah. Contamination of freshwater in the principal artesian aquifer is occurring from two sources: the upper water-yielding zones are being contaminated in the Parris Island area by downward vertical migration of salt water into the aquifer in the Port Royal Sound area and connate salt water in the lower water-yielding zones is moving toward Savannah, thus causing salt water contamination via lateral migration due to the cone of depression created by pumping at Savannah and the subsequent reversal of the hydraulic gradient.
The threat of salt water contamination in the immediate Savannah area is reported to be minimized because only about 20 percent of the water flowing toward the center of the cone of depression is coming from areas where salt water encroachment is taking place. Ground-water users located within the path of salt water movement will be affected before salt water reaches the center of the cone of depression. Ground-water consumers west and south of Savannah probably will not be affected by the encroaching salt water. Periodic sampling of the lower water-yielding zones indicated only slight movement of high-chloride content water toward Savannah. Further monitoring of salt water movement was recommended.
The principal artesian aquifer is reported to be composed of the Lisbon Formation (middle Eocene), the Ocala Limestone (late Eocene) and undifferentiated sediments of Oligocene and Miocene ages. The Lisbon Formation is described as a granular, glauconitic limestone that becomes progressively finer toward the northeastern part of the study area. The Ocala Limestone resembles the Lisbon Formation within its lower portions and also becomes finer grained in the northeastern part of the study area. The upper portion of the Ocala Limestone is composed of an indurated blue-gray limestone.
The Oligocene sediments unconformably overlie the Ocala Limestone. These sediments consist of fossiliferous limestones over most of the study area. The sediments consist of fine quartz sands in a limey mud matrix in the extreme eastern and northeastern parts of the area. The Miocene sediments are reported to consist of two lithologic groups; an upper clayey sand unit that consist of two beds separated by a thin layer of sandy, dolomitic limestone. The lower unit is reported to consist of a conglomeratic limestone.
The principal artesian aquifer is reported to consist of five major water-yielding zones that are separated by relatively lower permeability units. Zone 1 (uppermost) is situated at the top of the Ocala Limestone. Zone 2 is reported to occur about 50 feet beneath Zone 1. Zone 3 is situated at the base of the Ocala Limestone. Zones 4 and 5 are reported to be present within the Lisbon Formation. In general the upper water-yielding zones are reported to be thicker than the lower zones. Chloride concentrations are reported to increase in each zone toward the east and northeast of the study area and with increasing depth.
Salt water was reported to be present in the upper permeable zones at Parris Island, Beaufort River and Port Royal Sound areas. At the present (1964) pumping rate of 62 million gallons per day and the present head decline at the pumping center of 160 feet, it was estimated that it should take about 400 years for salt water in the upper permeable zones to reach the pumping center in Savannah. The hydraulic gradient within the lower permeable zones were reported to be steeper and it was estimated that salt water might reach the pumping center in about 90 years at the present pumping rate (1964).
This study reports that the dynamic and equilibrium relations of fresh and salt water in aquifers have been intensified at several locations along the southeastern United States Atlantic coast. The areas discussed in this report are the North Carolina Outer Banks, Savannah Georgia and nearby parts of South Carolina, Brunswick Georgia, Fernandina Florida, and southeast Florida.
At Savannah Georgia artesian water is reported to occur in limestones ranging in age from middle Eocene to early Miocene. The water-bearing formations include the Lisbon Formation, the Ocala Limestone and limestones of Oligocene and Miocene age. Most of the water produced in the Savannah area is reported to be from the Ocala Limestone (upper Eocene). The Hawthorn Formation (middle Miocene) overlies the principal artesian aquifer and is the upper confining bed. It is reported to be as much as 300 feet thick at some locations, but thins toward the northeast of Savannah. The Hawthorn Formation is reported to consist of green clay, silt interbedded with marl, and slightly dolomitic limestone beds. In areas where it is thin or absent, as in the Parris Island area, the upper part of the aquifer may be exposed to sea water in deep channels between coastal islands, especially in dredged channels. Discharge of fresh ground-water probably occurred by upward vertical leakage prior to development of the aquifer in the areas where the Hawthorn Formation is thin or absent.
Five major water-yielding zones were reported by McCollum and Counts (1963) to be present in the Ocala Limestone and Lisbon Formations. Zone 1 near the top of the Ocala Limestone was reported to be 20 to 50 feet thick and to thin toward the coast east and northeast of Savannah. Zone 2 was reported to be also in the Ocala Limestone about 50 feet below Zone 1 and to 50 to 70 feet thick near Savannah and about 25 feet thick northeast of Savannah. Zone 3 was reported to be near the base of the Ocala Limestone and to be from 10 to 30 feet thick. Zone 4 was reported to occur at the top of the Lisbon Formation and to be from 10 to 30 feet thick. Zone 5 is also reported to be within the upper portion of the Lisbon Formation, about 70 feet below Zone 4, and to be from 10 to 40 feet thick.
Zones 1 and 2 were reported to be the most productive and to yield about 70 percent of the water withdrawn from the aquifer. The various zones do not appear to be interconnected. The sediments between the zones is relatively impermeable and chloride content of water from several of the zones is reported to be different. Previous reports suggested that the lower water-bearing zones were not as permeable as the upper zones and that flushing of connate salt water would not be as complete as in the upper zones. Chloride concentrations were reported to be greater in the lower zones. The salty water in the lower zones was suggested to be unflushed connate water, whereas salty water in the aquifer at Parris Island is thought to be sea water that has entered the aquifer though thin or absent sections of the upper confining unit.
Limestones of middle Eocene to early Miocene age and clastic sediments of Paleocene to early Eocene age constitute the major water-bearing Tertiary formations along coastal South Carolina which have been intruded by sea water during Recent and past geologic time. Deposits of low regional dip, characterized by low permeabilities and hydraulic gradients and having sluggish circulation are probably discharging water of Pleistocene or earlier age.
Upper zones of Eocene limestones that have been incised by estuaries during Pleistocene and Recent time are subject to salt-water encroachment. Encroachment is also thought to occur along the sub-sea contact of Eocene and Oligocene deposits. The most extensive encroachment at the present time is reported to occur both in limestones of Tertiary age and clastic beds of Cretaceous age extending in a belt from the Beaufort Basin to areas adjacent to the Cape Fear Arch.
This report was prepared to address two main objectives: 1) to determine whether mining of phosphate in eastern Chatham County would compromise the principal artesian aquifer, and 2) determine the extent, depth, grade, volume, and value of the phosphate matrix.
The study indicated that the confining layer above the artesian aquifer is thick enough and impermeable enough to allow the phosphate matrix to be removed and still prevent significant salt water leakage into the principal artesian aquifer.
In an area of Chatham County south of the South Carolina state line, north of Ossabaw Sound, east of most residential development and within the oceanic three-mile limit, mining could be accomplished presently (1969) or in the future. Evaluation of the study area indicated that the overburden thickness varies from a minimum of 70 feet under Savannah Beach to 130 feet under Ossabaw Sound. In the same area the phosphate matrix thickness was reported to vary from 15 feet under the mouth of the Savannah River up to 50 to 60 feet under Skidaway Island. In the area where mining was considered, Wilmington, Cabbage, and Little Tybee Islands, the matrix was reported to be 20-30 feet thick.
The Hawthorn Formation within the study area was described as being comprised of mostly marl, sandy clay, and clayey sand. The lower portions of the Hawthorn Formation were reported to be more calcareous and the upper portion was reported to consist of mostly intermixed sand and clay. The thickness of the Hawthorn Formation was reported to ranged from a minimum of 20 feet under Wilmington Island to a maximum of 55 feet under the marshes to the east; however over most of the study area, the Hawthorn Formation was reported to be at least 40 feet thick. The Hawthorn Formation sediments within the study area were reported to be mostly tough sandy clays having very low permeability.
The Duplin Marl (upper Miocene) was reported to unconformably overlie the Hawthorn Formation in the Savannah area. In eastern Chatham County the Duplin Marl was reported to consist of olive-green sand, sandy clay, and clayey sand and was further reported to be often difficult to distinguish visually from the upper Hawthorn Formation sediments. However, the two formations were reported to differ significantly in phosphate content. The Hawthorn Formation was reported to contain two to three percent bone phosphate of lime (BPL) and to nowhere contain more than five percent BPL. At the base of the Duplin Marl the phosphate content increases abruptly to 13 to 30 percent BPL.
Furlow reported that the only barrier to salt water contamination of the principal artesian aquifer is the confining unit (Hawthorn Formation and Duplin Marl) that immediately overlies the Oligocene sediments. The permeability of the confining unit was reported to be extremely low, so that for all practical purposes it is nearly impermeable. If the phosphorite in eastern Chatham County were to be mined, part of the confining unit would be removed. In general the phosphate ore zone extends down to the base of the Duplin Marl. If the overburden and the ore zone were removed, only the Hawthorn sediments would remain to protect the aquifer.
In order to determine the effectiveness of the Hawthorn Formation as a confining unit, 52 core samples were submitted to the Water Resources Division of the U.S. Geological Survey for permeability testing. The average vertical permeability reported for the Hawthorn Formation samples was 0.0096 gallons per day per square foot under one foot of hydraulic head. Using this value of vertical permeability, a vertical hydraulic gradient of 0.375, and an area of one square foot, it was calculated that about 0.0036 gallons per day per square foot of salt water would pass through the Hawthorn Formation into the underlying Oligocene section. At a seepage rate of 0.0036 gallons per day per square foot, one acre of mine pit would allow about 160 gallons of salt water per day into the Oligocene section.
Even without a mining operation, Furlow indicated that salt water was seeping through the confining unit at rate of similar magnitude. However, when compared with the volume of fresh water in the aquifer and the volume pumped from the aquifer (63 million gallons per day [1969]), the amount of salt water that would enter the principal artesian aquifer would be negligible.
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 Low Country Capacity Use Investigation was initiated in 1973 by the South Carolina Water Resources Commission at the request of the South Carolina legislature and local officials in the four county area comprising the Low Country Capacity Use study area. Per the South Carolina Water Use Act, the South Carolina Water Resources Commission was required to report on ground-water problems within a capacity use study area. The results of a technical ground-water investigation, completed by the U.S. Geological Survey Water Resources Division in cooperation with the South Carolina Water Resources Commission are reported in "The Ground-Water Resources of Beaufort, Colleton, Hampton, and Jasper Counties South Carolina." These two reports were submitted to fulfill the requirement of the Act, and to make the findings of the investigation available to the public.
Several major ground-water problems were identified that were occurring at the time of publication as well as several other problems that were suspected of becoming major problems unless a comprehensive ground-water management plan was initiated. Major ground-water problems that were noted in the report were:
- Regional water-level declines (loss of artesian pressure) throughout large areas of the Low Country and adjacent counties of Georgia;
- Salt-water contamination of the Tertiary Limestone Aquifer in the coastal area, primarily in Beaufort County;
- Local well interference, where water levels have been lowered below some pump intakes;
- Interaquifer transfer, resulting in artesian pressure losses and/or water quality impairment;
- Inadequate requirements relating to well location, spacing, construction, and abandonment; and
- No requirements for proper water-use, well-construction, and hydraulic data reporting.
Potential problems that were identified were:
- Subsidence of the land surface (compaction subsidence) caused by excessive, concentrated ground-water withdrawals;
- Local dewatering of the Tertiary Limestone Aquifer
- Land-surface subsidence and collapse, if certain conditions are created by improperly-planned well location and spacing, or by dewatering operations, and
- Ground-water pollution of aquifers within the Hawthorn-Recent formations and in the Tertiary Limestone Aquifer Systems.
Several administrative problems having a bearing upon effective ground-water management were:
Assessment of the identified technical and administrative problems indicated that:
This report also provided an extensive history of ground-water development and the accompanying problems in the study area as well as summaries of previous ground-water investigations. Notable previous studies discussed included, but were not limited to:
This study began in 1938 and marked perhaps the first systematic ground-water investigation of the Tertiary Limestone Aquifer in the Savannah Georgia-Low Country South Carolina. Significant aspects of this report include development of three potentiometric maps of the Tertiary Limestone Aquifer in 12 Georgia counties. Potentiometric maps for pre-development conditions prior to 1880, the 1943 potentiometric surface, and a predicted potentiometric surface if pumping at Savannah were increased to 60 million gallons per day were provided within the report. The report also postulated that tidal scour channels in the Port Royal Sound area were the sources of salt water in the Tertiary Limestone Aquifer underlying Parris Island South Carolina. The study also reported that chloride concentrations increased with depth in the Tertiary Limestone Aquifer thus suggesting that low permeability in the lower portions of the aquifer prevented or slowed complete flushing of connate water.
This reconnaissance study provided the first potentiometric map of the Tertiary Limestone Aquifer in the Beaufort area. The potentiometric map indicated that the aquifer was probably in hydraulic connection with salt-water channels in the vicinity of Port Royal Sound suggesting that the aquifer was vulnerable to salt-water contamination. This observation was further substantiated after Mundorff demonstrated that Battery Creek was the likely source of chlorides that had necessitated reduction of pumping at the Jericho well field that had supplied potable water to the Marine Corps Recruit Depot at Parris Island, South Carolina. He also concluded that wells withdrawing water from the Tertiary Limestone Aquifer at Beaufort, Burton and the Marine Corps Air Station should be limited to a few feet of drawdown to avoid salt-water contamination.
This report suggested that a head decline of one to four feet in the Tertiary Limestone Aquifer in an area of southern Hampton and Jasper Counties and eastern Beaufort County South Carolina had occurred due to pumpage of the aquifer. He provided explanations for several ways that the Tertiary Limestone Aquifer could become contaminated with salt-water and he suggested that undrained depressions occurring throughout the area could be the result of breaks in the confining unit and could thus contribute to salt-water contamination of the aquifer. Siple recommended that future water supply wells should be located northwest of Burton, west of the Broad River, and north of Spring Island.
This U.S. Geological Survey report discussed the lithology and position of phosphate beds occurring in the Hawthorn Formation and other sediments overlying the Tertiary Limestone Aquifer. The findings of this report were in contrast to Furlow's conclusions (1969) and suggested that if the confining bed were to be deeply cut or breached, sea water would move downward into the Tertiary Limestone Aquifer and contaminate the freshwater supply.
In 1970 The U.S. Geological Survey, in cooperation with the City of Savannah, Chatham County, and the State of Georgia, began work on a digital model of the Tertiary Limestone Aquifer in the Savannah area. Some of the results of that effort were publish in the above referenced report.
The model was used to predict water-level changes in the aquifer that might occur with changes in redistribution of pumpage, increase or decrease in pumpage, or combinations of these conditions. The model was calibrated using Warren's 1880 potentiometric map of the pre-development potentiometric surface of the Tertiary Limestone Aquifer and the model was verified by comparing computed water levels with water level measurements made in 1956, 1960, and 1970. Model verification indicated a reasonably good match between computed and measured water- levels.
Three computed maps were included that showed the projected water-level change predicted to occur from 1970 to 2000 by adding to the 1970 pumpage (1) 10 mgd at theoretical sites on Hilton head Island, (2) 10 mgd at Hutchinson Island Georgia, and (3) 6 mgd at Montieth Georgia. Two other maps showed the predicted water-level changes expected by (1) transferring 10 mgd of the 1970 pumpage from Savannah to Bloomingdale, 10 miles to the west, and (2) decreasing the 1970 pumpage 20 mgd at the center of the cone of depression in Savannah.
The report indicates that the water-level and water-quality monitoring networks in the Savannah area are distributed in a manner to give emphasis to portions of the aquifer where possible impending problems may occur. The network will detect changes that could affect the availability and quality of water in the principal artesian aquifer. The decline in water-levels is most significant in the Savannah, Georgia-Hilton Head Island South Carolina area where ground-water withdrawals of approximately 85 mgd has caused a decline in water-level in excess of 150 feet since pumping began in the late 1800s.
The subsequent development of an areally extensive cone of depression within the potentiometric surface of the aquifer may be causing seawater encroachment from Port Royal Sound and adjacent areas. Although brackish-water zones underlying the principal artesian aquifer are reported to have chloride concentration of up to 13,000 mg/l, the water-level decline has not caused a significant increase in chloride concentrations in the monitored wells during the past 20 years (prior to 1984).
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.
The pre-development potentiometric surface of the Upper Floridan aquifer as documented by Warren (1944) indicated that regional ground-water flow of freshwater within the aquifer was seaward toward Port Royal Sound and ground-water discharged into the sound under artesian pressure. Because of the steady-state condition within the flow system (seaward hydraulic gradient), freshwater head in the area surrounding western Port Royal Sound and further to the west in Georgia, were higher than sea level, thereby preventing appreciable landward movement of the freshwater-saltwater interface. Since ground-water withdrawals in the Upper Floridan aquifer centered at Savannah began, the zero-foot contour has migrated northward into Beaufort and Jasper Counties South Carolina.
Problems associated with water quality in the upper permeable zone were first documented at Parris Island, South Carolina. Typically, wells would initially yield good quality water, but would eventually become too salty for human consumption. Two of three wells drilled in 1899 were reported to have been abandoned by 1903 because of high salt content.
To quantify the extent of seawater encroachment into the aquifer, the concentration of chloride ions and indirectly specific conductance was used as an indicator of seawater contamination. Chloride concentration and specific conductance in ground-water samples collected from the study area showed a direct correlation, hence specific conductance was used to indirectly estimate chloride concentration in field samples. Water supply wells in Beaufort began to have chloride concentration increases in the early 1940s. By 1944, the main supply well in downtown Beaufort was being pumped at a rate of 250,000 gpm day and the chloride concentration had reached 190 mg/l in November 1943. Previously, the chloride concentration in this well had been reported to be 30-40 mg/l. The well was taken out of service in 1946. During aquifer tests conducted using this well in 1955, the chloride concentration was reported to have decreased to 44 mg/l, presumably due to the cessation of pumping post 1946.
Several possible mechanisms were suggested to account for the presence of saltwater in the upper permeable zone of the Floridan aquifer:
This report identified two areas of the Upper Floridan aquifer as having elevated chloride concentrations. One area encompasses and extends slightly south of the theoretical steady-state freshwater-saltwater interface. The second area is beneath the town of Port Royal, South Carolina where the upper confining unit is thin or absent.
An observation well (BFT-1810) that is completed within the upper permeable zone of the Upper Floridan aquifer and instrumented with specific conductance probes at 170, 190 and 200 depths indicated a distinct specific conductance profile with time. From October 1987 through September 1993, monitoring of BFT-1810 indicated little change in specific conductance at 170 foot depth; however, the probes at 190 and 200 foot depths indicated specific conductance increases to 4,000 microsiemens/centimeter ( FS/cm) and 10,000 FS/cm, respectively. Well BFT-1810 is located close to one of the two saltwater plumes delineated in the Upper Floridan aquifer and long-term chloride concentration increases are the result of movement of saltwater within the aquifer toward Hilton Head Island resulting from regional ground-water withdrawals.
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.
At the request of the City of Savannah, HydroVision Inc. prepared and submitted the above referenced proposal. The City of Savannah in turn distributed the proposal to the Aquifer Committee via e-mail on September 7, 2000. Subsequently, the Aquifer Committee discussed the relative merits and weaknesses during the third Aquifer Committee meeting and via postings on the Aquifer Experts Information Exchange.
The HydroVision Inc. proposal was intended to provide answers to the following questions:
HydroVision Inc proposed to determine several things in order to resolve these questions:
HydroVision Inc proposed completing several technical tasks to address the questions posed above.
Task I as proposed involved the collection, compilation, assimilation, and interpretation of previous studies, and data. Additionally, existing numerical ground-water flow, solute transport and other pertinent models would be evaluated to determine the applicability of both in-put data sets and results.
Task II was to focus upon determinations of the salinity of interstitial pore water from core samples collected from the Miocene confining unit. The primary purpose of collecting and determining the salinity concentration of interstitial pore water was to determine the salinity distribution (salinity profile) within the confining unit and to answer the questions relating to whether leakage of saline water is occurring, at what, if any, rate, and whether it has reached the Upper Floridan aquifer.
The determination of salinity profiles was proposed at a minimum of three locations: 1) where the confining unit is thinnest (Tybee High); 2) where the vertical hydraulic conductivity is greatest (paleochannel); and 3) where the hydraulic gradient between the Savannah River and the Upper Floridan aquifer is greatest (near the center of pumping at International Paper Company).
Task III was to consist of developing and using ground-water flow and solute transport model of stream aquifer flow (leakage). Leakage and salinity were to be computed for both present day and historic conditions, taking into account changes in the potentiometric surface of the Upper Floridan, ie. vertical hydraulic gradient reversal and reductions in the thickness of the confining unit attributable to previous dredging of the Savannah River navigation channel.
After calibrating the model, leakage and salinity would be computed for conditions that might arise as a result of future dredging scenarios. Comparisons between current existing conditions and future conditions within the river thalweg would be used to predict estimates of leakage increases into the Upper Floridan that might be expected as a result of the proposed dredging.
A second model would be developed to simulate the movement of saline or brackish water as it enters and migrates downgradient within the Upper Floridan aquifer. This second model would account for relative density differences between saline/brackish water and the freshwater of the Upper Floridan aquifer. This and the ground-water flow and solute transport (leakage model would be coupled. Calibration of the solute transport model was to be made by comparing existing field data regarding the salinity concentrations of water in the Upper Floridan aquifer at locations in the vicinity of the Savannah River and the pumping center at Savannah. A sensitivity analysis of various model input parameters was also to be conducted.
The need for hydrogeologic field testing for the determination of in-situ estimates hydraulic characteristics would be determined based upon the results of the numeric modeling/sensitivity analyses to be conducted during the completion of Task III. Should the numeric modeling runs and sensitivity analyses indicate that in-situ estimates of the hydraulic properties of the confining unit and the Upper Floridan aquifer would be necessary, then test wells and observation wells would be installed as Task IV Element A. Aquifer tests of the entire hydrogeologic setting (Upper Floridan aquifer and Miocene confining unit) would be completed as Task IV Element B. The aquifer test data would be interpreted and analyzed to determine estimates of the hydraulic properties (aquifer transmissivity and storage coefficient, and vertical hydraulic conductivity and specific storage of the confining unit).
Task V would involve the preparation of a report of findings and Task VI consisted of long-term monitoring of primarily salinity concentrations at five locations to be determined resultant to the conceptual hydrogeologic model and the numeric model simulations. Sampling and analyses would be conducted before, during, and after dredging was completed.
This memorandum states that the Aquifer Committee was created by the Stakeholders Evaluation Group (SEG) as the result of new concerns about the structural integrity of the Miocene confining unit and that of particular concern is the high potential for the confining unit to contain fractures, causing the hydraulic conductivity of the confining unit to be significantly higher than previously determined from drilling cores and thereby greatly increasing the estimated flow rate (leakage) of seawater into the Floridan aquifer.
The memorandum goes on to state that the established mission (as of June 26, 2000) is fourfold:
Because concerns remained, the ACOE 1998 report notwithstanding, the City of Savannah contracted HydroVision Inc to critique the ACOE report. HydroVision Inc determined that the level of confidence of minimal impact to the Upper Floridan due to dredging concluded by the ACOE 1998 report was not justified. Two specific points of concern were noted: (1) that the leakage calculations were based on averaged geotechnical properties rather than using the worst-case scenario; and (2) that core-sample hydraulic conductivities calculated by laboratory analysis are much less definitive of actual conditions than in-situ (pumping) tests.
The memorandum presented four alternative actions recommended for consideration to assess the risk of seawater intrusion associated with harbor deepening:
The authors stated that in considering these alternatives that the highest level of confidence that no harm will be done to the aquifer, because if seawater intrusion results from deepening, remediation is unlikely both technically and economically. It is also worth considering that except for perhaps the most seaward portion, all of the channel is within the cone of depression of the Upper Floridan aquifer.
The authors opined that additional studies are required and recommended that either alternative 3 or alternative 4 be employed to develop a statement of work; and that a highly qualified independent firm be selected to complete the work and that the work be peer reviewed by appropriate entities.
Camille Ransom (South Carolina Department of Health and Environmental Control) provided an outline to Chris Schuberth to be considered by the Aquifer Committee. The points were as follows:
GAEPD performed a second technical review of the ACOE 1998 report Potential Ground-Water Impacts, Savannah Harbor Expansion Feasibility Study after considering written and oral stakeholder critiques of the report that were performed as part of the SEG process.
The ACOE report was first peer reviewed in September 1997, at the request of ACOE. The peer review was intended to identify errors and/or omissions as well as the appropriateness of the investigative methods used during the course of the study. A review meeting was held in Savannah, attended by William McLemore (GAEPD), Napoleon Caldwell (GAEPD), Richard Krause (USGS), Chris Leeth (USGS), and Vernon Henry (Georgia Southern University) and representatives of GPA and Lockwood Greene. According to the author of this letter (William McLemore, GAEPD), no specific errors or omissions were identified during the meeting nor were particular concerns raised. There was apparently agreement that ACOE's technical approach was sound and that the preliminary conclusion that channel deepening would not significantly adversely impact the Upper Floridan aquifer was reasonable.
GAEPD's 1998 technical review and recent re-review indicated that the data reported in the ACOE 1998 report are reasonable and were collected in a manner consist with the professional standard of practice.
The letter indicated that some previously suggested additional work as unnecessary:
The letter summarized that while there are considerable data that indicate that the proposed deepening of the Savannah River Navigation Channel will not adversely impact water quality in the Upper Floridan aquifer and there are no data to suggest otherwise, GPA may consider gathering additional information. Should additional information, data and interpretations be consistent with the previously gathered data and interpretations, GPA would then be able to adequately demonstrate that the proposed deepening would not significantly adversely impact the Upper Floridan aquifer. However, should new data demonstrate that the Upper Floridan aquifer would be adversely affected by harbor deepening, then such deepening would not be appropriate.
The letter went on to say that both GAEPD and SCDHEC are currently performing hydrogeologic studies in the Savannah area. Some of these studies may provide information useful for assessing the impact of harbor deepening; and that both state agencies would be willing to share technical information with ACOE and GPA.
Heavy pumpage (>300 mgd throughout coastal Georgia) and the resultant reduction in artesian pressure, saltwater intrusion of the Upper Floridan aquifer was observed at Brunswick Georgia in the 1950s and at Hilton Head Island South Carolina in the early 1970s. Saltwater contamination at these two coastal locations has constrained further development of the aquifer in coastal Georgia and has created competing demands for the available water supply The Georgia Department of Natural Resources, Environmental Protection Division has capped permitted withdrawals in parts of coastal Georgia including Savannah and Brunswick at 1997 rates and imposed a restriction in additional pumpage in all 24 coastal-area counties to 36 mgd above 1997 rates.
Saltwater contamination along the northern end of Hilton Head Island is reported to the probable result of lateral encroachment of seawater combined with some downward vertical leakage of seawater and/or brackish water from sounds, estuaries, tidal creeks, and saltwater marshes in areas where the Upper Floridan aquifer is exposed or thinly confined.
In the vicinity of Port Royal Sound and possibly other estuaries, downcutting by river systems during periods of lowered ocean levels has exposed the aquifer to seawater resulting in a direct connection between seawater having high chloride concentration and fresh ground-water within the aquifer.
Preliminary saltwater transport models in the Savannah-Hilton Head Island and Brunswick areas are part of the first phase of a modeling program that will include development of a regional flow model and refined, predictive models of selected focus areas.
Saltwater transport models are being developed using the SUTRA 3D simulator. A concurrently developed regional-scale flow model will provide boundary conditions for and encompass the smaller scale models. SUTRA 3D is a three-dimensional, variable-density finite element model that simulates density-dependent saturated-unsaturated ground-water flow and transport of a solute in ground-water.
Chloride concentrations in ground-water within the Upper Floridan aquifer beneath Port Royal Sound indicates that saltwater has intruded the aquifer. Pumpage centered at Savannah, Georgia combined with pumpage on Hilton Head Island has reversed the original seaward hydraulic gradient. Offshore of the Savannah-Hilton Head Island area, erosion has partially or completely removed the confining unit overlying the Upper Floridan aquifer at some locations.
The saltwater transport model for the Savannah-Hilton Head Island area represents five aquifers and three confining units and includes Chatham and adjacent counties in Georgia and South Carolina and the offshore area. The horizontal coverage is about 11,00 square miles. Vertically, the model represents aquifer units with five cell layers and confining units with three cell layers. Intrinsic permeability values assigned to the layers range from 10-16 feet squared for the confining units to 10-9 feet squared for the aquifer units. The following hydraulic boundary conditions were used in the model: the bottom boundary is no flux; the top offshore boundary is hydrostatic seawater; the top onshore boundary is atmospheric or zero pressure; the northwest vertical boundary is hydrostatic freshwater; the southeast vertical boundary is hydrostatic seawater; and the southwest and northeast vertical boundaries are no flux. A stress is applied to three vertical cells representing a pumping site that withdraws 80 mgd from the Upper Floridan aquifer at Savannah. This preliminary model is designed to test the effect of concentrated pumping on the steady-state offshore saltwater-freshwater interface and is not intended to simulate actual conditions.
The model was run to simulate steady-state conditions prior to development and transient conditions after 100,000 years of pumping. The steady-state simulation was used as the initial condition for transient simulations. Preliminary modeling results indicate that this time scale is required to move the saltwater-freshwater interface to the pumping site. Over the course of the simulation the saltwater-freshwater interface migrates toward the Savannah pumping center as saltwater moves laterally from the offshore steady-state position and downward through the confining unit. The preliminary model was designed with a grid resolution that does not capture local areas where the confining unit overlying the Upper Floridan aquifer is thin or absent. This notwithstanding, the simulated saltwater intrusion is partly the result of downward vertical transport of seawater through the upper confining unit. As the model is refined, these areas of thin upper confining unit will be more precisely and accurately simulated , and it is expected that these will act as direct conduits for seawater entry into the Upper Floridan aquifer, thus decreasing the time scale of saltwater intrusion.
Saltwater contamination along the northern part of Hilton Head Island is reported to probably be the result of lateral encroachment of seawater combined with some downward vertical leakage of seawater where the Upper Floridan aquifer is exposed or nearly exposed. In the vicinity of Port Royal Sound, and possibly other estuaries, downcutting by ancient river systems that occurred during periods of lower eustatic sea level exposed the rocks that comprise the Upper Floridan aquifer forming a direct connection between seawater and fresh ground-water. This combined with the lowered potentiometric surface of and reversed hydraulic gradient of the Upper Floridan aquifer has allowed seawater to enter the aquifer. Regional ground-water pumping, but more importantly local pumping on Hilton Head Island has caused the encroachment of seawater into the aquifer.
An assessment of the feasibility of conducting aquifer testing to determine estimates of vertical hydraulic conductivity of the Miocene confining unit was conducted using the techniques described in the U.S. Geological Survey publication "Techniques of Water-Resources Investigations of the United States Geological Survey Chapter C1 Finite-Difference Model for Aquifer Simulation in Two Dimensions with Results of Numerical Experiments" Book 7 by Trescott, Pinder, and Larsen.
The time required for a uniform gradient to be established across a confining bed and to induce leakage from an overlying water-bearing stratum or stream bed was evaluated using the following equation: tD = Kl t/ Ssb2 where: tD = dimensionless time Kl = vertical hydraulic conductivity of confining bed t = time since pumping began Ss = specific storage of the confining unit, and b = thickness of confining unit Transient effects of downward leakage from storage in the confining unit dominate when the dimensionless time is less than 0.5. After transient effects have dissipated, a uniform hydraulic gradient across the confining unit will be established; when a uniform gradient is established, a flow through component of steady leakage will be also be established. Any additional changes in the uniform gradient will cause new transient effects to occur.
Information from the Tybee Island test site and nearby vicinity were used in assessing the feasibility of conducting an aquifer test to determine an estimate of the vertical hydraulic conductivity of the Miocene confining unit at Tybee Island, Georgia. A previous aquifer test was completed at the Tybee Island test site from March 10 through March 23, 1997 using Tybee 2 as the pumping well and Tybee 3 and 4 wells as observation wells. Tybee 3 is located 14 feet from the pumping well and is screened in the Marks Head Formation (Miocene Unit B). Tybee 4 is located 48.3 feet from the pumping well and is screened over the same interval as the pumping well. The screened intervals of observation wells Tybee 3 and 4 are separated by approximately 25 feet of low permeability confining unit sediments.
Five scenarios were evaluated for the Tybee Island test site using different values for vertical hydraulic conductivity derived from laboratory and ground-water modeling leakance data and the thickness of the confining unit. Each scenario was evaluated twice using different values of specific storage for the confining unit.
The evaluation concluded that an aquifer test of 30 days duration or possibly longer could yield useful data from which an estimate of the vertical hydraulic conductivity of the confining unit could be made based on the dimensionless time evaluations from 5 of the 10 scenarios calculated.
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.