REGIONAL SUMMARY
INTRODUCTION
The four States-Alabama, Florida, Georgia, and South Carolina-that comprise Segment 6 of this Atlas are located adjacent to the Atlantic Ocean or the Gulf of Mexico, or both. These States are drained by numerous rivers and streams, the largest being the Tombigbee, Alabama, Chattahoochee, Suwannee, St. Johns, Altamaha, and Savannah Rivers. These large rivers and their tributaries supply water to cities such as Columbia, S.C., Atlanta, Ga., and Birmingham, Ala. However, the majority of the population, particularly in the Coastal Plain which comprises more than one-half of the four-State area, depends on ground water as a source of water supply. The aquifers that contain the water are mostly composed of consolidated to unconsolidated sedimentary rocks, but also include hard, crystalline rocks in parts of three of the States. This chapter describes the geology and hydrology of each of the principal aquifers throughout the four-State area.
Precipitation is the source of all the water in the four States of Segment 6. Average annual precipitation (1951-80) ranges from about 48 inches per year over a large part of central South Carolina and Georgia to about 80 inches per year in mountainous areas of northeastern Georgia and western South Carolina. (fig. 1) In general, precipitation is greatest in the mountains (because of their orographic effect) and near the coast, where water vapor, which has been evaporated primarily from the ocean and the gulf, is picked up by prevailing winds and subsequently condenses and falls as precipitation when reaching the shoreline.
Much of the precipitation either flows directly into rivers and stream as overland runoff or indirectly as baseflow discharging from aquifers where the water has been stored for a short time. Accordingly, the areal distribution of average annual runoff from 1951 to 1980 (fig. 2) directly reflects that of average annual precipitation during the same period: runoff is greater in mountainous areas and near the coast. Average annual runoff in the four-State area ranges from about 8 inches per year in parts of north-central Florida to about 50 inches per year in the mountains of northeastern Georgia.
Comparison of the precipitation and runoff maps shows precipitation is greater than runoff everywhere in the four-State area. Much of the precipitation that falls on the area is returned to the atmosphere by evapotranspiration-evaporation from surface-water bodies, such as lakes and marshes, and transpiration from plants. However, a substantial part of the precipitation is available for aquifer recharge throughout the area.
MAJOR AQUIFERS
There are numerous aquifers in Segment 6, that range in composition from unconsolidated sand of the surficial aquifer system to hard, crystalline rocks of the Piedmont and Blue Ridge aquifers. These aquifers are grouped into nine major aquifers or aquifer systems on the basis of differences in their rock types and ground-water flow systems. An aquifer system consists of two or more aquifers that are hydraulically connected-that is, their flow systems function similarly, and a change in conditions in one aquifer affects the other aquifer(s).
The areas where eight major aquifers are exposed at land surface are shown in figure 3 (see opposite page). Many of these aquifers extend underground far beyond the limits of outcrop, and, accordingly, may be used for water supply in much larger areas than the size of their outcrop may indicate. In places, deeper aquifers that contain freshwater underlie the major aquifers mapped here. For example, in southeastern South Carolina, the surficial aquifer system shown on the map is underlain by the Floridan aquifer system, which in turn is underlain by the Southeastern Coastal Plain aquifer system, all of which contain mostly freshwater. In other places, such as the areas where aquifers of the Piedmont, Blue Ridge, Valley and Ridge, and Appalachian Plateaus physiographic provinces are mapped, deeper aquifers are nonexistent. In places in Alabama, Georgia, and Florida, a clayey confining unit that overlies the Floridan aquifer system is exposed at land surface, and wells need to be drilled through this clayey confining unit to penetrate the underlying aquifer.
The surficial aquifer system consists mostly of unconsolidated sand, but also contains a few beds of shell and limestone. The sand and gravel and Biscayne aquifers are separately recognized parts of the surficial aquifer system that consist of distinctive rock types. The sand and gravel aquifer consists of complexly interbedded lenses and layers of coarse sand and gravel, and the Biscayne aquifer consists predominantly of limestone. The intermediate aquifer system consists of sand and limestone and lies between the surficial aquifer system and the Floridan aquifer system. The intermediate aquifer system does not crop out, and, accordingly, is not shown on the map.
The Floridan aquifer system consists of limestone and dolomite, and is the most productive of the aquifers in the mapped area, in terms of total water yield. The Southeastern Coastal Plain aquifer system consists of four regional aquifers that are predominately sand, but these aquifers also contain some beds of gravel and limestone. All the aquifers from the surficial aquifer system down through the Southeastern Coastal Plain aquifer system are present in the Coastal Plain physiographic province (fig. 4). Water in all of the Coastal Plain aquifers is present primarily in intergranular pore spaces. However, solution openings in carbonate rocks of the Biscayne aquifer and Floridan aquifer system yield large volumes of water.
Piedmont and Blue Ridge aquifers consist of indurated metamorphic rocks, such as gneiss and schist, and igneous rocks, such as granite, that underlie the rolling hills of the Piedmont physiographic province and the mountains of the Blue Ridge physiographic province. Water is present in these rocks in fractures, but locally a large volume of water is stored in the regolith, or blanket of weathered material that overlies the rock.
Folded Paleozoic rocks underlie the Valley and Ridge physiographic province, and flatlying Paleozoic rocks underlie the combined Appalachian Plateaus and Interior Low Plateaus physiographic provinces. In these three provinces, the Paleozoic rocks consist of indurated sedimentary rocks; the major aquifers consist of limestone. However, the ground-water flow system is different where these rocks are folded and where they are not.
GEOLOGY
Two categories of sedimentary rocks comprise most of the rocks underlying the four States of Segment 6: well-indurated rocks of Paleozoic age and poorly indurated to unconsolidated rocks of Cretaceous age and younger. The Paleozoic sedimentary rocks crop out in northern Alabama and northwestern Georgia; whereas, the Cretaceous and younger rocks underlie the Coastal Plain and form a broad, arcuate, coast-parallel band. Both categories have been divided into numerous formations, as shown on correlation charts in the discussions of the major aquifers in following sections of this chapter.
The majority of the water-yielding Paleozoic rocks are limestone; however, some water also is obtained from sandstone and, locally, from chert beds and fractured shale.
Most Coastal Plain strata are clastic rocks; however, the carbonate rocks of the Floridan aquifer system also are important. Triassic, Jurassic, and Lower Cretaceous rocks are present only in the deep subsurface of the Coastal Plain and do not form aquifers except in a local area in Alabama where Lower Cretaceous rocks form a small part of the Southeastern Coastal Plain aquifer system.
The geologic map (fig. 5) shows the distribution of rocks by major age category and also shows that an extensive area is underlain by crystalline rocks. These are metamorphic and igneous rocks that crop out in a broad, northeast-trending band that widens from eastern Alabama into eastern Georgia and western South Carolina. The crystalline rocks are hard, and generally are more resistant to weathering and erosion than sedimentary rocks. The gently rolling hills of the Piedmont physiographic province and the rugged mountains of the Blue Ridge physiographic province were formed as a result of these crystalline-rock characteristics. Radiometric dating of the crystalline rocks has determined that they range in age from late Precambrian to Permian. Locally, they have been intruded by diabase dikes of Late Triassic to Early Jurassic age. Detailed mapping shows that the crystalline rocks are complex; for example, they have been separated into about 90 units on the 1976 geologic map of Georgia. Because the crystalline rocks have similar hydraulic characteristics, they are mapped and discussed as a single aquifer.
Several major faults are shown in figure 5. Some of these faults form boundaries between major rock categories; for example, a fault marks the contact between metamorphic rocks of the Blue Ridge physiographic province and tightly folded Paleozoic rocks of the Valley and Ridge physiographic province.
The area mapped in figure 5 can be divided into four broad categories of geologic structure. From northwest to southeast, these are: (1) flatlying Paleozoic sedimentary rocks that underlie the combined Appalachian Plateaus and Interior Low Plateaus physiographic provinces; (2) the same rocks folded into a series of anticlines and synclines in the Valley and Ridge physiographic province, where resistant rocks form the ridges and soft rocks underlie the valleys; (3) intensely deformed metamorphic rocks of the Piedmont and Blue Ridge physiographic provinces that have been intruded by small to large bodies of igneous rocks; and (4) gently dipping, poorly consolidated to unconsolidated sediments of the Coastal Plain physiographic province. The block diagram in figure 6 shows the general relations of the four major categories. The combination of rock type and geologic structure largely determines the hydraulic character of the rocks. These factors, plus topography and climate, determine the characteristics of the ground-water flow system throughout the mapped area.
VERTICAL SEQUENCE OF AQUIFERS
Some of the major aquifers and aquifer systems in Segment 6 lie atop others. For example, the Biscayne aquifer in southern Florida overlies the Floridan aquifer system, but the two are separated by a thick, clayey confining unit (fig. 7). Water is able to move vertically between some of these aquifers. Movement is in the direction of decreasing hydraulic head, and occurs most easily where the confining units separating the aquifers are absent, thin, or leaky.
The sequence of maps on this page shows the extent of each aquifer or aquifer system. Comparison of the maps shows the places where aquifers are stacked upon each other. The three uppermost aquifers in the Coastal Plain are shown in figure 8. These aquifers, the surficial aquifer system, sand and gravel aquifer, and Biscayne aquifer are all the same geologic age (primarily Pleistocene and younger), and all contain water mostly under unconfined (water table) conditions. However, even though these aquifers are lateral equivalents, the lithology and permeability of each are different. The surficial aquifer system is a thin, widespread layer of unconsolidated sand beds that commonly contains a few beds of shell and limestone. This aquifer system generally yields small volumes of water, and primarily is used for domestic supplies. The sand and gravel aquifer consists largely of interbedded layers of coarse sand and gravel that were deposited by streams. Thin clay beds in this aquifer locally create semiconfined conditions. The sand and gravel aquifer yields moderate volumes of water, and is an important source of supply for several counties in westernmost panhandle Florida and southwestern Alabama. Westward, in Mississippi, the sand and gravel aquifer grades into the Coastal lowlands aquifer system. The Biscayne aquifer, the source of water supply for several large cities along the southeastern coast of Florida, is a highly permeable sequence of mostly carbonate rocks that were deposited in marine waters.
The intermediate aquifer system (fig. 9) underlies the surficial aquifer system and overlies the Floridan aquifer system. The intermediate aquifer system is bounded above and below by clayey confining units. The system is not exposed at land surface and is recharged primarily by downward leakage from overlying aquifers. Sand beds and limestone lenses comprise the permeable parts of the system. The intermediate aquifer system is an important source of municipal supply in Sarasota, Charlotte, and Glades Counties, Fla.; elsewhere, it primarily is used for domestic supplies.
The Floridan aquifer system (fig. 10) consists of a thick sequence of carbonate rocks and is the most productive aquifer in Segment 6. The Floridan underlies the intermediate aquifer system where the latter is present; it also underlies the surficial aquifer system, the sand and gravel aquifer, and the Biscayne aquifer, but is separated from them practically everywhere by a thick, clayey confining unit. Where the surficial aquifer system overlies the Floridan, the clayey confining unit between the systems is thick in some places and thin or absent in other places. The Floridan supplied more than 3 billion gallons of water per day during 1985, primarily for municipal and agricultural purposes.
The Southeastern Coastal Plain aquifer system (fig. 11) underlies the Floridan aquifer system in some places (mostly in western Georgia and westward) and grades laterally into the Floridan in other places (mostly in southeastern Georgia and southwestern South Carolina). The upper part of the Southeastern Coastal Plain aquifer system grades laterally into the Mississippi embayment aquifer system in western Alabama (fig. 3). The Southeastern Coastal Plain aquifer system consists of four regional aquifers that are primarily sand beds, but which contain some gravel and limestone. The four regional aquifers generally yield large volumes of water in updip areas, where they are mostly sand, but the aquifers are less permeable in a coastward direction due to increasing clay content toward the coast. The system is an important source of water supply for all purposes throughout the inner part of the Coastal Plain.
Although rocks of the Piedmont, Blue Ridge, Valley and Ridge, and the combined Appalachian Plateaus and Interior Low Plateaus physiographic provinces (fig, 12) extend under the Southeastern Coastal Plain aquifer system, these rocks generally are not used as aquifers there because water can be more readily obtained from the shallower, unconsolidated Coastal Plain sediments. Piedmont and Blue Ridge aquifers consist of a complex sequence of metamorphic and igneous rocks, and primarily supply domestic or agricultural wells. Well yields generally are small; the water is obtained from fractures in the unweathered crystalline rock and from the mantle of regolith (weathered materials, soil, and alluvium) that overlies it. Major fault systems separate the Piedmont and Blue Ridge aquifers from the Valley and Ridge aquifers to the northwest.
The Valley and Ridge and the combined Appalachian Plateaus and Interior Low Plateaus aquifers consist of indurated sedimentary rocks of Paleozoic age. Water is obtained primarily from limestone in these provinces and secondarily from sandstone, chert beds, or fractured shale. In the Valley and Ridge province, these sedimentary rocks have been tightly folded into a sequence of northeast-trending anticlines and synclines that have been displaced by thrust faults in many places. Ground-water circulation extends to greater depths in these folded rocks than in the Appalachian Plateaus and Interior Low Plateaus provinces to the northwest where the same rocks are almost flatlying. In the Appalachian Plateaus province, the flatlying beds are topped with a resistant cap of sandstone; in the Interior Low Plateaus province, the sandstone has been dissected by erosion, and underlying limestone beds are exposed. The contact between the Valley and Ridge and Appalachian Plateaus provinces is distinct in some places where it follows faults and is gradational from nearly horizontal strata to folds in other places.
FRESH GROUND-WATER WITHDRAWALS
Ground water is the source of water supply for almost 11 million people, or about 73 percent of the population in the four-State area.
About 5,600 million gallons per day was withdrawn from all the principal aquifers during 1985; 46 percent was used in rural areas for domestic and commercial supplies and for agricultural supplies. Withdrawals for public supply were somewhat less, accounting for 35 percent of the total water withdrawn.
Total withdrawals of fresh ground water, by county, are shown in figure 13. Counties with the largest withdrawals are those that have large population centers except for south-central Florida, where combined agricultural and mining uses account for most of the withdrawals. Fresh ground-water withdrawals for most water use categories are increasing, according to a recent (1990) nationwide compilation of water-use data by the U.S. Geological Survey.
Total withdrawals of freshwater during 1985 from each of the principal aquifers in the four-State area are shown in figure 14. About 3,181 million gallons per day was withdrawn from the Floridan aquifer system, almost four times as much water as was withdrawn from the second most used aquifer, the Biscayne aquifer (786 million gallons per day), and almost twice as much water as was withdrawn from all the other principal aquifers combined. More water was withdrawn from the Biscayne aquifer, although it only extends throughout a small area in the southeastern tip of Florida, than from either the Southeastern Coastal Plain aquifer system (574 million gallons per day) or the surficial aquifer system (361 million gallons per day), even though both have a much larger areal extent. This is because the Biscayne is the source of supply for several large cities, including Miami, West Palm Beach, and Fort Lauderdale, along the southeast coast of Florida. About 298 million gallons per day was withdrawn from the intermediate aquifer system, about 150 million gallons per day from the sand and gravel aquifer, and about 149 million gallons per day from the combined Valley and Ridge, Appalachian Plateaus, and Interior Low Plateaus aquifers. Only about 100 million gallons per day, or about 2 percent of the total freshwater withdrawn, was obtained from the Piedmont and Blue Ridge aquifers because surface water is the primary source of supply in the area underlain by these aquifers.

SURFICIAL AQUIFER SYSTEM
INTRODUCTION
The surficial aquifer system (fig. 15) in the southeastern United States includes any otherwise undefined aquifers that are present at the land surface. Even though the sand and gravel aquifer of Florida and southwestern Alabama, and the Biscayne aquifer of southern Florida are present at the land surface and are the lateral equivalents of the surficial aquifer system, they are treated separately in this Atlas because of their importance as water sources. The sand and gravel, and the Biscayne aquifers supply large municipalities; the surficial aquifer system, although used by a large number of people, principally is used only for domestic, commercial, or small municipal supplies.
The thickness of the surficial aquifer system is typically less than 50 feet, but its thickness in Florida is as much as 400 feet in Indian River and St. Lucie Counties; 250 feet in Martin and Palm Beach Counties; and 150 feet in eastern St. Johns County. In southeastern Georgia, thicknesses of about 60 feet have been mapped for the system. The system generally thickens coastward.
HYDROGEOLOGIC UNITS
The surficial aquifer system consists mostly of beds of unconsolidated sand, shelly sand, and shell. Locally, in southwestern Florida, limestone beds form an important and highly permeable part of the system. In places, clay beds are sufficiently thick and continuous to divide the system into two or three aquifers; mostly, however, the system is undivided. Complex interbedding of fine- and coarse-textured rocks is typical of the system.
The rocks that comprise the surficial aquifer system range from late Miocene to Holocene in age. Although figure 16 shows that nine geologic formations are part of the system at different places in Florida, the entire sequence of formations is not present at any one location. The formations are thin and mostly lens-like, and it is unusual for more than three or four of them to comprise the aquifer system at any place. Many of the geologic formations shown interfinger with each other, and some of them, such as the Caloosahatchee Marl, are not particularly productive aquifers. In Georgia and South Carolina, unnamed, sandy, marine terrace deposits of Pleistocene age and sand of Holocene age comprise the system. These sandy beds commonly contain clay and silt. In Alabama, a thin, unnamed sand of Holocene age comprises the system.
Limestone beds of the Tamiami and Fort Thompson Formations, mostly restricted to southern and southwestern Florida, are the most productive parts of the surficial aquifer system. Yields from these formations are especially large where large-scale openings have been developed by dissolution of part of the limestone. In places where the combined Pamlico Sand and overlying sand deposits of Holocene age are 40 feet or more thick, moderate yields are obtained; elsewhere, the system generally does not yield much water.
GROUND-WATER FLOW
Ground water in the surficial aquifer system is under unconfined, or water-table, conditions practically everywhere. Locally, thin clay beds create confined or semiconfined conditions within the system. Most of the water that enters the system moves quickly along short flowpaths and discharges as baseflow to streams.
The general movement of water within the system is illustrated in figure 17, which is an idealized diagram representing hydrologic conditions in Indian River County, Fla. Water enters the system as precipitation. A large percentage of this water is returned to the atmosphere by evapotranspiration. Water that is not returned to the atmosphere by evapotranspiration, or that does not directly run off into surface-water bodies, percolates downward into the surficial aquifer system and then moves laterally through the system until it discharges to a surface-water body or to the ocean.
In places, some water leaks upward from the underlying Floridan aquifer system through the clayey confining unit separating the Floridan and surficial systems (fig. 17). In other places, where the hydraulic head of the Floridan is lower than the water table of the surficial aquifer, leakage can occur in the opposite direction.
Because the surficial aquifer system extends seaward under the Atlantic Ocean, saltwater can encroach into the aquifer in coastal areas. Encroachment is more extensive during droughts because there is less freshwater available in the surficial aquifer system to keep the saltwater from moving inland.
The configuration of the long-term, average water table of the surficial aquifer system, where it has been mapped in the eastern and southern part of the Florida peninsula, is shown in figure 18. The water-table configuration is generally a subdued reflection of the topography of land surface. Steep gradients occur between streams and ridges or hills, and gentle gradients occur in broad, flat interstream areas and under broad topographic highs.
The arrows in figure 18 show that the general direction of ground-water movement is toward the Atlantic Ocean, the Gulf of Mexico, or toward major rivers. The water-table surface is complex, reflecting the fact that water in the surficial aquifer system moves quickly toward the nearest surface-water body. Accordingly, local directions of ground-water movement change markedly within short distances.
The wide spacing of the contours in Collier County and adjacent areas reflects two conditions: (1) the Big Cypress Swamp, which is virtually flat, is present throughout much of this area; and (2) the surficial aquifer system largely consists of highly permeable limestone in this area. Steeper gradients elsewhere are more typical of a sand aquifer in an area of gentle topography.
The transmissivity of the surficial aquifer system is extremely variable. Most reported values range from 1,000 to 10,000 feet squared per day; in places, values of 25,000 to 50,000 feet squared per day have been reported. The larger values are primarily for beds of shell or limestone. Well yields range from less than 50 gallons per minute in most of Georgia and South Carolina, to 450 gallons per minute in St. Johns County, Fla., to 1,000 gallons per minute in Indian River County, Fla.
FRESH GROUND-WATER WITHDRAWALS
Water-use data are available for the surficial aquifer system only from Florida. About 361 million gallons per day of freshwater was withdrawn from the surficial aquifer system in Florida during 1985. Nearly equal volumes were withdrawn for public supply and for domestic and commercial uses (fig. 19), with withdrawals for these categories being about 154 and 157 million gallons per day, respectively. Agricultural withdrawals accounted for about 13 million gallons per day, and withdrawals for industrial, mining, and thermoelectric-power uses were about 4 million gallons per day, primarily for industrial use.

SAND AND GRAVEL AQUIFER
INTRODUCTION
The sand and gravel aquifer underlies an area of about 6,500 square miles in southwestern Alabama and the westernmost part of panhandle Florida (fig. 20). The aquifer is presently (1990) called the Miocene-Pliocene aquifer in Alabama; in the past, it has been called the Citronelle or Citronelle-Miocene aquifer in that State by some authors. In Mississippi, the sand and gravel aquifer grades laterally into part of the Coastal lowlands aquifer system that extends westward into southern Texas. The sand and gravel aquifer is the primary source of water in Baldwin, Washington, and western Escambia Counties, Ala., and in Santa Rosa and Escambia Counties, Fla. The aquifer also supplies most of the water used by small communities in the rural parts of Mobile County, Ala.; the city of Mobile in that county, however, is supplied by surface water. About 150 million gallons per day was withdrawn from the sand and gravel aquifer for all uses during 1985. About 80 percent was withdrawn in the Pensacola, Fla. area, and the majority of the remaining 20 percent was withdrawn in Mobile County, Ala.
As its name indicates, the sand and gravel aquifer consists largely of interbedded layers of sand and gravel. Clay beds and lenses are common in the aquifer and form local confining beds. Water in the aquifer is under unconfined conditions where the clay beds are thin or absent, and is under artesian conditions where such beds are thick. Movement of ground water is generally coastward.
GEOLOGY
The sand and gravel aquifer consists of rocks ranging in age from middle Miocene to Holocene that were mostly deposited in a deltaic environment. In Alabama, Miocene rocks are all included in the undifferentiated Catahoula Sandstone, a thick, predominantly nonmarine sequence of sand and clay beds. The Miocene units shown in figure 21 are overlain by the Citronelle Formation of Pliocene age. The Citronelle is mostly fine- to coarse-grained sand that is locally gravelly, and is the most important water-yielding formation in the upper part of the sand and gravel aquifer. The Citronelle locally contains layers of hardpan, or cemented iron oxide, that retard ground-water movement. The principal geologic units that comprise the aquifer in the westernmost part of the Florida panhandle are shown in figure 21. The Alum Bluff Group and the Choctawatchee Formation, which were deposited in a more marine environment, are most easily recognizable near the coast. Northward, these beds grade into undifferentiated coarse sand and gravel, which comprise the major water-yielding unit of the lower part of the sand and gravel aquifer.
THICKNESS
The sand and gravel aquifer is approximately wedge-shaped and thickens southwestward from a feather edge at its northern and eastern limit to about 1,400 feet in southwestern Alabama (fig. 22). Throughout the southern two-thirds of the area underlain by the aquifer, the confining unit forming the base of the aquifer consists of either the upper or lower clay members of the Pensacola Clay (fig. 23). Analysis of aquifer-test data, supplemented by the results of laboratory testing of cores from the Pensacola Clay, indicates that the permeability of this confining unit is so small that practically no water passes across it. To the northeast, the clay beds are absent and the sand and gravel aquifer is in direct contact with the Upper Floridan aquifer.
HYDROGEOLOGIC UNITS
In most places, the sand and gravel aquifer can be divided into two high-permeability zones, the upper surficial and lower main producing zones, separated by a less permeable sand and clay unit. The upper, or surficial, zone is mostly fine- to medium-grained sand, with gravel beds and lenses, and contains water that is mostly under unconfined conditions. This zone is recharged directly by precipitation, and ground-water flow in it is mostly lateral along short flowpaths to discharge points along small streams. Some of the water percolates downward and recharges the lower high-permeability zone. The upper zone consists mostly of the Citronelle Formation combined with stream-valley alluvium and terrace deposits. Along major streams, such as the Mobile River, alluvial deposits are as much as 150 feet thick and wells completed in them yield as much as 850 gallons per minute. The upper zone contains clay and hardpan layers that create local perched water tables or, in places, artesian conditions. The upper zone is mostly used for water supply in southern Mobile, southern Baldwin, and southwestern Escambia Counties, Ala., because the lower zone contains much clay in these counties, and, accordingly, yields less water. The hydraulic characteristics of the upper zone are extremely variable. Yields of as much as 1,000 gallons per minute are reported for wells completed in the upper zone, and a transmissivity of 11,000 feet squared per day was reported for the zone based on results of an aquifer test conducted in Escambia County, Ala.
In the westernmost part of panhandle Florida, the lower of the two high-permeability zones is called the ³main producing zone² because most of the ground water used in Santa Rosa and Escambia Counties is withdrawn from this zone. This zone also is the main source of water supply for Washington, northern Mobile, northern Baldwin, and eastern Escambia Counties, Ala. The zone consists mostly of coarse sand and gravel beds, all of Miocene age. Water in this zone is confined everywhere. Recharge to the zone is by downward leakage from the upper zone; discharge is to major streams, bays, sounds, and the Gulf of Mexico. Yields of more than 1,000 gallons per minute are commonly reported for wells completed in this zone, and results of aquifer tests have indicated that the transmissivity of the zone is as much as 20,000 feet squared per day.
GROUND-WATER FLOW
Water enters the sand and gravel aquifer as recharge from precipitation, and moves generally downward and then either discharges to streams or moves coastward in the aquifer. Discharge is primarily to streams, bays, and sounds. Small volumes of water leak upward to the Gulf of Mexico and still smaller volumes are discharged by wells. Most of the well discharge is in Mobile County, Ala., and Escambia and Santa Rosa Counties, Fla.
Water movement in the upper zone of the aquifer is complex because this zone contains numerous discontinuous clay layers and some layers of iron oxide (hardpan). Because of the low permeability of the hardpan and the clay, and the confined conditions they produce, perched water-table conditions, artesian conditions, and true water-table conditions can all exist in one area. Such conditions prohibit drawing a representative map of the potentiometric surface of the aquifer, except for local areas. Where hardpan or clay beds are near the land surface, ponds may be perched on them or springs may issue at the top of such beds where they are exposed in small stream valleys. Some water percolates downward across all these confining beds to recharge deeper permeable zones in the aquifer. Water levels generally decrease with depth in the aquifer, a condition that allows downward leakage almost everywhere.
The saturated thickness of the aquifer is everywhere less than its total thickness because the water table ranges from a few feet to about 50 feet below land surface. The water table is just below land surface in low-lying areas and is deepest under hills and ridges.
The general coastward movement of water in the main producing zone of the sand and gravel aquifer is shown by the potentiometric contours in figure 24. The arrows show that the water is moving mostly toward Choctawatchee Bay from recharge areas where water levels are highest. The contours are smooth and evenly spaced because the water in this zone is confined. A similar map for the surficial zone of the aquifer would show the same general seaward movement of water, but the contours would be convoluted because of the effects of topography and streams.
GROUND-WATER QUALITY
Water in the sand and gravel aquifer is suitable for drinking practically everywhere. The quartz-rich sediments that comprise the aquifer are practically insoluble; accordingly, water in the aquifer has concentrations of dissolved solids that ordinarily are less than 50 milligrams per liter. Chloride concentrations also are ordinarily less than 50 milligrams per liter everywhere except in a few locations near the coast and adjacent to large bays and sounds where there is a transition zone of freshwater and saltwater; there, chloride concentrations greater than 1,000 milligrams per liter are reported in water from some wells. Water in the aquifer is usually slightly acidic, with a pH of about 6.0; locally, the water is more acidic (pH 4.5). Dissolved-iron concentrations may locally be objectionable; concentrations as large as 4,300 micrograms per liter have been reported.
The sand and gravel aquifer, like other shallow aquifers, is readily susceptible to contamination. Contamination of the upper zone has occurred at several places in the three westernmost counties of Florida. One such place is a site near Pensacola where creosote waste products from a wood-preserving plant have been detected in a large part of the upper zone of the aquifer.
FRESH GROUND-WATER WITHDRAWALS
Withdrawals of freshwater from the sand and gravel aquifer totaled 150 million gallons per day during 1985. About 44 percent, or about 66 million gallons per day, was withdrawn for public supply (fig. 25). About 9 million gallons per day was withdrawn for domestic and commercial uses, and about 18 million gallons per day was withdrawn for agricultural uses. About 57 million gallons per day was withdrawn for industrial, mining, and thermoelectric-power uses.

BISCAYNE AQUIFER
INTRODUCTION
The Biscayne aquifer underlies an area of about 4,000 square miles and is the principal source of water for all of Dade and Broward Counties and the southeastern part of Palm Beach County in southern Florida (fig. 26). During 1985, an average of about 786 million gallons per day was withdrawn from the Biscayne aquifer for all uses; pumpage at present (1990) is somewhat greater. About 70 percent of the water was withdrawn for public supply. Major population centers that depend on the Biscayne aquifer for water supply include Boca Raton, Pompano Beach, Fort Lauderdale, Hollywood, Hialeah, Miami, Miami Beach, and Homestead. The Florida Keys also are supplied primarily by water from the Biscayne aquifer that is transported from the mainland by pipeline.
Because the Biscayne aquifer is highly permeable and lies at shallow depths everywhere, it is readily susceptible to contamination. The aquifer is the only source of drinking water for about 3 million people.
Water in the Biscayne aquifer is under unconfined, or water-table, conditions and the water table fluctuates in direct and rapid response to variations in precipitation. The aquifer extends beneath Biscayne Bay, from whence it was named, and the Atlantic Ocean. The aquifer is highly permeable where it forms part of the floor of the bay and the ocean, and contains saltwater there. Some of this saltwater has migrated inland in response to the lowering of inland ground-water levels adjacent to canals constructed for drainage of low-lying areas and near large well fields.
HYDROGEOLOGIC UNITS
The Biscayne aquifer consists of highly permeable limestone and less-permeable sandstone and sand. Most of the geologic formations comprising the aquifer are of Pleistocene age but, locally, Pliocene rocks also are included in the aquifer (fig. 27).
Most of the formations are thin and lens-like, and the entire sequence shown in figure 27 is not present at any one place. Some of the units interfinger and some are lateral equivalents of each other. For example, the Anastasia Formation and Key Largo Limestone interfinger with the Fort Thompson Formation; in places, the Miami Oolite is equivalent to the Key Largo Limestone and in other places to the upper part of the Fort Thompson Formation; and so on. The thickest and most extensive geologic unit in the Biscayne aquifer is the Fort Thompson Formation, which is the surficial unit in northwestern Broward County and part of Palm Beach County (fig. 28); this unit is the major water-producing unit of the aquifer. The Anastasia Formation comprises much of the Biscayne at Fort Lauderdale and northward into Palm Beach County. However, the Pamlico Sand is the surficial unit in this area (fig. 28). The Miami Oolite, although thin, is a very porous, oolitic limestone that is present at the land surface throughout much of Dade County and parts of Broward and Monroe Counties (fig. 28). In general, the entire aquifer is more sandy in its northern and eastern parts, and contains more limestone and calcareous sandstone to the south and west.
The Fort Thompson, Anastasia, and Key Largo Formations yield the most water of any of the geologic formations of the Biscayne aquifer. The Fort Thompson is the most used of these three units. At Fort Lauderdale, the Tamiami Formation is a productive aquifer where it consists of calcareous sandstone. Most of the water is obtained from solution cavities in the sandstone. Yields of as much as 7,000 gallons per minute are reported for some wells completed in the Tamiami Formation, and drawdowns in the wells are less than 10 feet. The Biscayne aquifer is most permeable in a band near the coast in Dade and Broward Counties, but a cavity-riddled zone in the northern part of the aquifer in Palm Beach County yields as much as 1,000 gallons per minute to wells.
The Biscayne aquifer grades northward and westward into sandy deposits that are part of the surficial aquifer system. Well yields from these sandy deposits are small compared to well yields from the Biscayne. A sequence of low-permeability, largely clayey deposits about 1,000 feet thick separates the Biscayne aquifer from the underlying Floridan aquifer system. The Floridan contains saltwater in southeastern Florida, and is not hydraulically connected to the Biscayne aquifer.
BASE AND THICKNESS
The base of the Biscayne aquifer in Dade County and southern Broward County is a low-permeability sandy silt that is part of the Tamiami Formation. Farther north, the base is not as distinct; rather, it consists of a transition zone that changes from a mixture of moderately permeable calcareous sand, shell, and silt, which probably are part of the Anastasia Formation, to low-permeability silty clay which is part of either the Anastasia or Tamiami Formations.
The altitude and configuration of the base of the Biscayne aquifer are shown in figure 29. The base is somewhat irregular but generally slopes seaward from the western limit of the aquifer, where it is at the land surface, to a depth of about 240 feet below sea level near Boca Raton. Throughout much of the mapped area, the top of the aquifer is at or near the land surface. Accordingly, thickness of the aquifer can be estimated by subtracting the altitude of the base of the aquifer from the altitude of the land surface at a given point. The aquifer is wedge-shaped and ranges in thickness from a few feet near its western limit to about 240 feet near the coast.
Saltwater locally has entered the Biscayne aquifer, mostly near its base. The approximate extent of saltwater encroachment in 1982 is shown in color in figure 29.
HYDROLOGIC SYSTEM
Ground water and surface water form an integrated hydrologic system in southern Florida. Before development of these water resources, a large proportion of the abundant precipitation that fell on the flat, low-lying area drained southward to the Gulf of Mexico and Florida Bay. Most of this drainage was in the form of wide, shallow sheets of water that moved sluggishly southward during the wet season, when as much as 90 percent of areas, such as the Everglades, was inundated. This drainage was the major source of recharge to the underlying aquifers. During the dry season, water moved only through the deeper sloughs and covered probably less than 10 percent of the Everglades. Lake Okeechobee, the second largest freshwater lake wholly within the conterminous United States, was a major water-storage component in the system, functioning as a retarding basin for streams, such as the Kissimmee River, that drained southward into the lake.
Today, the shallow, southward-moving sheet of surface water still is a major source of recharge to the Biscayne aquifer in addition to the precipitation that falls directly on the aquifer. Where the Biscayne is either exposed at the land surface or is covered only by a veneer of soil, the slowly moving surface water passing over the recharge area of the aquifer is able to readily percolate downward into the aquifer.
Freshwater Controls
Canals have been used extensively in southern Florida for drainage and flood control. Levees also were constructed, first to prevent flooding from Lake Okeechobee, and subsequently to impound excess water in three large water-conservation areas for later release. These alterations to the natural hydrologic system have culminated in a regional water system; major features of this system are shown in figure 30. The South Florida Water Management District utilizes a system of canals, levees, control structures, pumping stations, and water-conservation (storage) areas (Conservation Areas 1 through 3 in fig. 30) to manage the freshwater resources of southern Florida. The system conserves freshwater, provides flood control, and minimizes saltwater encroachment.
Impoundments, such as the water-conservation areas, provide water to the extensive canal system during dry periods. Seepage from the canals into the Biscayne aquifer during such periods helps maintain the water level in the aquifer. A network of major pumping stations provides flood protection by pumping excess stormwater from canals into the conservation areas. During prolonged droughts, this water is released to maintain canal flow. Natural land-surface elevations and levee heights are such that the overall movement of impounded water is from Lake Okeechobee to Conservation Area 1, and, thence, sequentially to Conservation Areas 2 and 3. Some of the water released from Conservation Area 3 sustains the flow of freshwater into the Everglades National Park.
Canals concentrate and channel what had been natural sheetflow. The drainage of wetlands is perhaps the most important aspect of the canal network, and was the primary reason for canal construction. However, a network of control structures also allows water to be diverted through the canal system to points where it may be needed to help maintain ground-water levels, such as near municipal well fields. Rapid interchange of water from the canals to the Biscayne aquifer is possible in most places because of the high permeability of the aquifer. Control structures near the coast on the major canals are particularly important in helping to prevent encroachment of saltwater into the canals, and subsequently into the aquifer, during periods of less than normal precipitation.
Ground-Water Flow
The major features of the flow system in the Biscayne aquifer are shown by a generalized water-table map (fig. 31). The configuration of the water table is a subdued replica of the land surface; that is, the water table is at a higher altitude under hills and at a lower altitude under valleys. The water table fluctuates rapidly in response to variations in recharge (precipitation), natural discharge, and pumpage from wells. Natural discharge is by seepage into streams, canals, or the ocean; by evaporation; and by transpiration by plants.
The contours in figure 31, and the arrows superimposed on them, show that the general movement of water in the Biscayne aquifer is seaward. Water levels are generally highest near the water-conservation areas and lowest near the coast. Contours are not drawn in the conservation areas because they represent impoundments, and, accordingly, there is no slope in the water table there. The effects of natural surface drainage and uncontrolled canals on the water table are shown by the irregular patterns of the contours, particularly where they point upstream in a sharp ³V² shape, showing that the aquifer is discharging to the canals. Near the coast, the contours point downstream, showing that the aquifer is being recharged from the canals. The water level of an unconfined aquifer typically is markedly affected by surface drainage.
Some of the local variations in the water table are due to other causes. The local high area in eastern Palm Beach County (fig. 31), where the water table is higher than 16 feet, is due to a local topographic high. The closed depressions in eastern Broward and Dade Counties reflect large-scale pumpage from major well fields supplying Miami and Fort Lauderdale (compare figs. 31 and 32). Withdrawal of large volumes of ground water has locally reversed the natural flow direction (note westward-pointing arrows adjacent to depressions), thereby increasing the possibility of saltwater encroachment. The wide spacing of contours in Dade County and southeastern Broward County indicates a slight gradient (slope) in the water table, as compared to a steep gradient to the north where the contours are closely spaced. The wide spacing of contours reflects areas where the Biscayne aquifer consists mostly of highly permeable limestone; permeability is less in the steep-gradient areas where the aquifer is sandier.
Water-Table Fluctuations
Major fluctuations in the water table of the Biscayne aquifer result from variations in recharge and natural or artificial discharge, or both. Fluctuations may range from 2 to 8 feet per year, depending primarily on variations in precipitation and pumpage. Pumpage is generally greater during periods of less than normal precipitation, as farmers and homeowners apply irrigation water to maintain crop production and lawn growth. Extremely low water-table conditions, such as those shown in figure 33, result from prolonged periods of less than normal precipitation. Total precipitation for the 2 years preceding the date of the water levels shown in figure 33 barely exceeded the long-term average precipitation for a single year. As a result, water levels declined slightly below sea level throughout a large area in southern Dade County, primarily due to transpiration by plants coupled with domestic pumpage. Water levels also were below sea level in a smaller area at Miami Springs, due to pumpage from the municipal well field. Under these conditions, saltwater migrated inland for considerable distances. Most of the drainage canals also were uncontrolled at the time (1940ıs) represented by figure 33, thus the lowering of the water table; saltwater encroachment was accelerated by continuous drainage to canals.
Extensive flooding also occurs during periods of greater than average precipitation, such as that preceding the high-stage water levels of October 1947, shown in figure 34. Water overflowed the banks of many of the canals, and a large part of the inland area was inundated. West of Biscayne Bay, water levels were almost 11 feet higher than those shown in figure 33. In Hialeah, water levels that had declined to about 0.5 foot above sea level in 1945 rose to almost 9 feet above sea level in 1947. The numerous types of control structures in southern Florida were constructed largely to avoid the problems associated with such extreme water-level fluctuations as those indicated by these two figures.
Response to Recharge
The highly permeable rocks of the Biscayne aquifer are covered in most places only by a veneer of porous soil. Accordingly, water levels in the aquifer rise rapidly in response to rainfall. The rise in the water level in well G-86, located in Miami, following two periods of intense rainfall in April 1942, is shown in figure 35. Eleven inches of rainfall during a 4-hour period in the early morning of April 16 produced a 4.5-foot rise in the water level in the well within a few hours. Six inches of rainfall during the late morning and early afternoon of April 17 was responsible for an additional rise of 1.5 feet in the water level in the well, also within only a few hours.
Canal-Aquifer Connection
The hydraulic connection between the Biscayne aquifer and the canals that cross it is direct. Water passes freely from the canals into the aquifer and vice versa. A decline in the water level of a canal lowers the adjacent water table of the aquifer almost immediately. Similarly, a rise in the water level in a canal is rapidly followed by a rise in the water table of the aquifer adjacent to the canal. These canal-aquifer water-level relations are shown schematically in figure 36. The arrows show the direction that water moves when the water level of the canal is lower (fig. 36A) and higher (fig. 36B) than the water table in the aquifer. The degree of connection decreases as fine sediment settles out of the canal water and lines the canal bottom. Accordingly, the degree of connection may change from time to time because of either accumulation of these sediments or their removal during runoff from intense storms.
The hydraulic connection between the canals and the aquifer results in both benefits and problems. Perhaps the most obvious benefit is the ability of the canals to rapidly remove excess surface and ground water, thereby preventing flooding in low-lying interior areas. A more subtle benefit is the ability to move water from inland parts of the aquifer to coastal areas through the canals, allowing ground-water levels near the coast to remain high enough to retard saltwater encroachment during periods of less than normal precipitation. Problems also can result from the direct hydraulic connection. For example, aquifer contamination by any pollutants in the canal water can be both rapid and widespread. In addition, the canals provide channels by which saltwater can encroach into the aquifer for considerable distances inland during periods of low water. The latter problem has been greatly alleviated by the construction of large-scale canal control structures near the coastal ends of the major canals (fig. 37). These structures prevent the movement of saltwater up the canals when water levels in the canals are low.
SALTWATER ENCROACHMENT
The delicate natural balance between freshwater and saltwater in the Biscayne aquifer is tipped when canals and well fields are superimposed on it. Where a highly permeable aquifer, such as the Biscayne, is hydraulically connected to the ocean, inland movement of saltwater is offset by a slightly higher column of freshwater. Because freshwater is lighter than saltwater, a 41-foot column of freshwater is necessary to balance a 40-foot column of saltwater. This means that, for each foot of freshwater above sea level, there is approximately a 40-foot column of freshwater below sea level. Accordingly, lowering of freshwater levels by drainage canals or by intensive pumping creates an imbalance that causes the inland movement of saltwater.
How saltwater can encroach coastal areas as a result of development is shown diagrammatically in figure 38. In the natural, balanced condition shown in figure 38A, saltwater is present only near the shoreline and is balanced by a thick inland column of freshwater. Construction of a drainage canal, however (fig. 38B), lowers freshwater levels and allows landward movement of saltwater in the canal and aquifer. In addition, the canal becomes a tidal channel that conveys saltwater inland and, thence, laterally into the Biscayne aquifer. Where municipal well fields withdraw large quantities of ground water, the water level in the aquifer is lowered still farther, and saltwater can enter the well field (fig. 38C). Some coastal well fields have been abandoned for this reason. Control structures (fig. 38D) placed near the coast dam the water in the canal, thus, artificially raising water levels in both the canal and the adjacent aquifer. Thus, further saltwater encroachment is prevented and, in some instances, has even been reversed.
The saltwater body in the aquifer is approximately wedge-shaped, as shown in figure 39, being thickest near the coast and tapering inland. Therefore, the maximum inland extent of saltwater is located near the base of the aquifer. The cross section shown in figure 39 represents conditions near Biscayne Bay, where the aquifer is highly permeable and free interchange of freshwater and saltwater is possible. Farther northward, especially in Palm Beach County, the Biscayne aquifer is sandy and less permeable, and saltwater encroachment does not extend as far inland. The exact position of the saltwater front, defined by a chloride concentration of 1,000 milligrams per liter, varies in response to the height of freshwater in the aquifer, which in turn varies directly with precipitation. Movement of the saltwater front is inland and upward in response to low ground-water levels and seaward and downward in response to high ground-water levels. The arrows in figure 39 show that freshwater at the bottom of the aquifer flows upward and then discharges seaward along the saltwater front.
The sequence of maps in figure 40 shows the inland movement of saltwater in the Biscayne aquifer in response to development. The colored area on all the maps shows the inland extent of saltwater at the base of the aquifer. Under natural conditions, as shown by the 1904 map, saltwater was limited to a narrow band along the coastline and to short tidal reaches of natural water courses. Urban and agricultural development and the resulting drainage of the land had not yet begun. Before 1946, canal flow was virtually uncontrolled and groundwater levels were greatly lowered because of extensive pumpage. The threat of contamination of inland municipal well fields spurred remedial action. Salinity-control structures were constructed in coastward reaches of the major canals and halted or reversed saltwater encroachment, particularly adjacent to the canals (compare the 1953 and 1969 maps of fig. 40). By 1977, additional control structures and effective water-management practices had reduced the area of saltwater contamination considerably from its maximum extent in 1953.
SUSCEPTIBILITY TO CONTAMINATION
Because the Biscayne aquifer is highly permeable and is at or near the land surface practically everywhere, it is readily susceptible to ground-water contamination. Because of the high permeability of the aquifer, most contaminants are rapidly flushed. Major sources of contamination are saltwater encroachment and infiltration of contaminants carried in canal water. Additional sources include direct infiltration of contaminants, such as chemicals or pesticides applied to or spilled on the land, or fertilizer carried in surface runoff; landfills; septic tanks; sewage-plant treatment ponds; and wells used to dispose of storm runoff or industrial waste. Most disposal wells are completed in aquifers containing saltwater that underlie the Biscayne aquifer, but they are a potential source of contamination where they are improperly constructed. Numerous hazardous-waste sites have been identified in the area underlain by the Biscayne aquifer, and three unlined landfills are known to have contaminated the aquifer. Remedial action to prevent further contamination is underway at many of these sites.
Uncontaminated water in the Biscayne aquifer is suitable for drinking and most other uses. The water is hard, is a calcium bicarbonate type, and contains small concentrations of chloride and dissolved solids. Locally, the water contains large concentrations of iron. In places in southern Broward County and northern and central Dade County, the water is darkly colored, reflecting large concentrations of organic material.
FRESH GROUND-WATER WITHDRAWALS
Withdrawals of freshwater from the Biscayne aquifer during 1985 totaled 786 million gallons per day. Public-supply withdrawals were almost three-quarters, or about 569 million gallons per day, (fig. 41). Domestic and commercial withdrawals were about 19 million gallons per day. Agricultural withdrawals were about 180 million gallons per day. Withdrawals for industrial, mining, and thermoelectric-power uses were about 18 million gallons per day.

INTERMEDIATE AQUIFER SYSTEM
INTRODUCTION
Collectively, aquifers in southwestern Florida that lie between the surficial aquifer system and the Floridan aquifer system are called the intermediate aquifer system in this Atlas. The approximate extent of the intermediate aquifer system is shown in figure 42. This aquifer system contains water under confined, or artesian, conditions, but does not yield as much water as the underlying Floridan aquifer system. Accordingly, the intermediate aquifer system is not extensively used, and its characteristics are not well known, especially where the Floridan is near the land surface and contains freshwater. The intermediate aquifer system is the main source of water supply in Sarasota, Charlotte, and Lee Counties, Fla., where the underlying Floridan aquifer system is deeply buried and contains brackish or saltwater.
The intermediate aquifer system consists of sand beds and limestone lenses that are parts of the Tampa Limestone and Hawthorn Formation of Miocene age; and sand, limestone, and shell beds of the Tamiami Formation of Pliocene age (fig. 43). Clay confining units isolate the aquifers in the system from the Floridan and surficial aquifer systems. Where the rocks of the intermediate aquifer system grade into slightly yielding or nonyielding clayey beds, they become part of the upper confining unit of the Floridan aquifer system. Locally, in Clay, Brevard, and Indian River Counties, Fla., the Hawthorn Formation yields water, but its water-yielding beds are not continuous. In Glynn County, Ga., sand beds in the Hawthorn Formation are pumped locally for water supplies where the underlying Floridan aquifer system contains brackish water. These local aquifers in southeastern Georgia and northeastern Florida are not considered to be part of the intermediate aquifer system.
HYDROGEOLOGIC UNITS
The top of the intermediate aquifer system slopes gently southward and southwestward. Its top is highest in western Polk County, Fla., and lowest in southern Charlotte County, Fla., (fig. 44). South of the area shown in figure 44, the top of the aquifer system becomes flatter, then rises slightly.
In many places, the intermediate aquifer system can be divided into two aquifers, the Tamiami-upper Hawthorn and the lower Hawthorn-upper Tampa aquifer, separated in most places by an unnamed confining unit. The aquifer system thickens southward from Polk County into Charlotte County, Fla., (fig. 45). Farther southward, in Collier County, the aquifer system thins as the lower Hawthorn-upper Tampa aquifer becomes predominately a clay with little permeability. The Tamiami-upper Hawthorn aquifer is the principal water-yielding part of the intermediate aquifer system in Glades, Hendry, Charlotte, Lee, and Collier Counties; elsewhere, the lower Hawthorn-upper Tampa aquifer is the major source of supply.
GROUND-WATER FLOW
The water-yielding beds of the intermediate aquifer system lie between clayey confining units. Therefore, the water in the aquifer system is under confined conditions except locally, where the upper confining unit is absent and the system is in direct hydraulic contact with the overlying surficial aquifer system. In most places, water moves downward from the surficial aquifer system and through the upper confining unit of the intermediate aquifer system; most of this water then follows short flowpaths and discharges to surface drainage. Some water, however, percolates downward through the lower confining unit of the system to recharge the underlying Floridan aquifer system. Locally, in western Charlotte and Lee Counties, some water leaks upward from the Floridan to the intermediate aquifer system.
The lateral direction of water movement in part of the intermediate aquifer system is shown in figure 46. The flow arrows, which are drawn perpendicular to the potentiometric contours, show that water moves outward in all directions from two recharge areas in southwestern Polk County, where the potentiometric surface is more than 120 feet above sea level. From these points, lateral flow is toward major surface streams and the Gulf of Mexico. Two local pumping centers are shown by the depressions in the potentiometric surface in western Sarasota County.
Well yields of as much as 1,800 gallons per minute from the intermediate aquifer system have been reported. Most wells, however, yield 200 gallons per minute or less. Most transmissivity values reported for the intermediate aquifer system are 10,000 feet squared per day or less.
FRESH GROUND-WATER WITHDRAWALS
Withdrawals of freshwater from the intermediate aquifer system totaled about 298 million gallons per day during 1985. About 31 million gallons per day was withdrawn for public supply, and about 19 million gallons per day was withdrawn for domestic and commercial uses (fig. 47). About 233 million gallons per day was withdrawn for agricultural purposes, the principal water use. About 15 million gallons per day was withdrawn for industrial, mining, and thermoelectric-power uses.

FLORIDAN AQUIFER SYSTEM
INTRODUCTION
The Floridan aquifer system is one of the most productive aquifers in the world. This aquifer system underlies an area of about 100,000 square miles in southern Alabama, southeastern Georgia, southern South Carolina, and all of Florida (fig. 48). The Floridan aquifer system provides water for several large cities, including Savannah and Brunswick in Georgia; and Jacksonville, Tallahassee, Orlando, and St. Petersburg in Florida. In addition, the aquifer system provides water for hundreds of thousands of people in smaller communities and rural areas. Locally, the Floridan is intensively pumped for industrial and irrigation supplies. During 1985, an average of about 3 billion gallons per day of freshwater was withdrawn from the Floridan for all purposes. Withdrawals during 1988 were somewhat greater. Despite the huge volumes of water that are being withdrawn from the aquifer system, water levels have not declined greatly except locally where pumpage is concentrated or the yield from the system is minimal.
The Floridan is a multiple-use aquifer system. Where it contains freshwater, it is the principal source of water supply. In several places where the aquifer contains saltwater, such as along the southeastern coast of Florida, treated sewage and industrial wastes are injected into it. Near Orlando, Fla., drainage wells are used to divert surface runoff into the Floridan. South of Lake Okeechobee in Florida, the aquifer contains saltwater. Some of this saltwater is withdrawn for cooling purposes and some is withdrawn and converted to freshwater by desalinization plants. Desalinization is especially important in the Florida Keys, which have no other source of freshwater except that which is imported by pipeline.
HYDROGEOLOGIC UNITS
A thick sequence of carbonate rocks (limestone and dolomite) of Tertiary age comprise the Floridan aquifer system. The thickest and most productive formations of the system are the Avon Park Formation and the Ocala Limestone of Eocene age (fig. 49). The Suwannee Limestone (Oligocene age) also is a principal source of water, but it is thinner and much less areally extensive than the Eocene formations. The Tampa Limestone of Miocene age is part of the Floridan in only a few places where it is sufficiently permeable to be an aquifer. Both the Suwannee and the Tampa Limestones are discontinuous. The lower part of the Avon Park Formation, the Oldsmar Formation of early Eocene age, and the upper part of the Cedar Keys Formation of Paleocene age also are included in the Floridan where they are highly permeable. Limestone beds in the lower part of the Hawthorn Formation of Miocene age are considered part of the Floridan by some, but are excluded from it in this Atlas because the permeability of these beds is thought to be minimal. The base of the aquifer system in much of Florida consists of nearly impermeable anhydrite beds in the Cedar Keys Formation. In northern peninsular Florida, the Paleocene and lowermost Eocene rocks contain sand and are much less permeable than the carbonate rocks of the Floridan. Due to the contrast in permeability, these sandy strata form the base of the Floridan aquifer system in this area. Locally, in south-central Georgia and northern peninsular Florida, evaporite minerals have filled the pore spaces in upper Eocene rocks, and these low-permeability beds comprise the base of the system.
Aquifers and Confining Units
The Floridan aquifer system has been defined on the basis of permeability. In general, the system is at least 10 times more permeable than its bounding upper and lower confining units. The aquifer system is thick and widespread, and the rocks within it generally vary in permeability. In most places, as shown by the idealized layers in figure 50, the system can be divided into the Upper and Lower Floridan aquifers, separated by a less-permeable confining unit.
The geology and hydraulic properties of the Upper Floridan aquifer have been extensively studied, and this is the part of the system described by most reports. The Upper Floridan is highly permeable in most places and includes the Suwannee and Ocala Limestones, and the upper part of the Avon Park Formation. Where the Tampa Limestone is highly permeable, it also is included in the Upper Floridan. In most places, the Upper Floridan aquifer yields sufficient water supplies for most purposes, and there is no need to drill into the deeper Lower Floridan aquifer. The confining unit separating the Upper and Lower Floridan aquifers, informally called the middle confining unit (or semiconfining unit where it allows water to leak through it more easily), is present at different altitudes and consists of different rock types from place to place. The confining unit actually consists of seven separate, discrete units that are idealized into a single layer in figure 50. At some locations, the confining unit consists of clay; at others, it is a very fine-grained (micritic) limestone; at still other places, it is a dolomite with the pore spaces filled with anhydrite. Regardless of rock type, wherever the middle confining unit is present, it restricts the movement of ground water between the Upper and Lower Floridan aquifers.
The geologic characteristics and hydraulic properties of the Lower Floridan aquifer are not as well known as those of the Upper Floridan aquifer because the Lower Floridan is at greater depths, and, therefore, fewer borehole data are available. The Lower Floridan includes the lower part of the Avon Park Formation, the Oldsmar Limestone, and the upper part of the Cedar Keys Formation. Much of the Lower Floridan aquifer contains saltwater. Two important, highly permeable zones are present within the Lower Floridan. One of these is a partly cavernous zone in northeastern Florida and southeastern coastal Georgia, called the Fernandina permeable zone, named after the Fernandina Beach area of Nassau County, Fla. This zone is the source of a considerable volume of fresh to brackish water that moves upward through the middle semiconfining unit and ultimately reaches the Upper Floridan aquifer. The second zone is an extremely permeable cavernous zone in southeastern Florida, known as the Boulder Zone. This name is applied to the zone not because it consists of boulders, but because it is difficult to drill into, having the same rough, shaking, grabbing effect on the drill stem and drilling rig as boulders would. The Boulder Zone contains saltwater and is used as the receiving zone for treated sewage and other wastes disposed through injection wells in the Miami-Fort Lauderdale area. The zone is overlain in most places by a confining unit that prevents upward movement of the injected waste. The cavernous nature of the Fernandina permeable zone and the Boulder Zone was created by the vigorous circulation of ground water through the carbonate rocks in the geologic past, and does not result from the present ground-water flow system.
Thickness
The Floridan aquifer system generally thickens seaward from a thin edge near its northern limit. The variations in thickness of the aquifer system are shown in figure 51. The contours represent the combined thicknesses of the Upper and Lower Floridan aquifers, and the middle confining unit where it is present. Some of the large-scale features on the thickness map are related to geologic structures. For example, the thick areas in Glynn County, Ga., and in Gulf and Franklin Counties, Fla., coincide with two downwarped areas, the Southeast and Southwest Georgia embayments, respectively. In north-central peninsular Florida, the limestone units that comprise the aquifer system are thin over the upwarped Peninsular arch.
A series of small faults bounds downdropped, trough-like crustal blocks (grabens) in southern Georgia and southwestern Alabama (fig. 51). Within these grabens, respectively called the Gulf Trough and Mobile graben, clayey sediments have been downdropped opposite permeable limestone of the Floridan aquifer system. This juxtaposition creates a damming effect that restricts the flow of ground water across the grabens.
VARIATIONS IN THE FLORIDAN AQUIFER SYSTEM
The variations among and complexity of various parts of the Floridan aquifer system along a southeast-trending line from south-central Georgia to southern Florida are shown in figure 52. The most obvious variation is the substantial thickening of the aquifer system to the southeast. The left side of the figure, representing conditions in south-central Georgia, shows that the Floridan is only about 250 feet thick in this area. The right side of the figure, representing southern Florida, shows that the aquifer system is about 3,000 feet thick in places. The break in this gradual thickening, shown between the faults near the left side of the figure, is the graben known as the Gulf Trough. The downward movement of this crustal block produced a depression where a greater than average thickness of the clayey upper confining unit of the Floridan accumulated, thus restricting or partially damming the southward flow of ground water. This damming is reflected on maps of the potentiometric surface of the Floridan.
Another prominent feature shown in figure 52 is the increasing complexity of the Floridan aquifer system toward the southeast. In south-central Georgia, where the system is thin, it contains only scattered, local confining units or none at all. In such areas, the system is hydraulically connected and generally functions as a single water-yielding unit, the Upper Floridan aquifer. Near the Georgia-Florida State line and southeastward, the aquifer system contains regionally-extensive middle confining units that separate it into two aquifers. In places, such as in southern Florida, two or three of these middle confining units are stacked. All of the regional and local confining units within the Floridan consist of carbonate rocks that are less permeable than the main, water-yielding parts of the aquifer system, and all of these confining units retard or partially restrict the movement of ground water in the system.
The Boulder Zone, a deeply-buried, cavernous zone filled with saltwater and used as a receiving zone for injected wastes, is shown near the right side of figure 52 along with the confining bed that overlies it. Also shown in southern Florida is the Biscayne aquifer, which is separated from the Floridan aquifer system by a clayey confining unit that is about 1,000 feet thick in this area.
Near the left side of figure 52, the Southeastern Coastal Plain aquifer system is shown directly underlying the Floridan aquifer system. Throughout much of southern Georgia, these two aquifer systems are in direct contact, and ground water passes freely between them. The permeability of the aquifers in the Southeastern Coastal Plain aquifer system, however, is generally much lower than that of the aquifers in the Floridan aquifer system. The carbonate rocks of the Floridan either had substantial intergranular porosity when they were first formed, or pores in the rocks were enlarged by the dissolving action of circulating, slightly acidic ground water, or both (fig. 53). As these carbonate rocks grade northward into the predominantly clastic rocks of the Southeastern Coastal Plain system, the porosity and permeability of the rocks decreases and they yield less water than the more productive aquifers of the Floridan aquifer system.
Southeastward from about the Georgia-Florida State line, the confining unit that forms the base of the Floridan consists of beds of anhydrite in the Cedar Keys Formation of Paleocene age. These beds consist of calcium sulfate, which, when dissolved, contributes excessive concentrations of sulfate to the ground water. When the sulfate is chemically reduced to hydrogen sulfide, an objectionable, ³rotten-egg² taste and odor are produced.
BOULDER ZONE
The deeply buried zone of cavernous permeability, called the Boulder Zone, developed in fractured dolomite in the Lower Floridan aquifer, underlies a 13-county area area in southern Florida (fig. 54). The Boulder Zone is not a single, simple, almost flatlying horizon of caves; rather, as shown by the contours in figure 54, its top is irregular and is as shallow as about 2,000 feet below sea level and as deep as about 3,400 feet below sea level. The zone is thought to represent caverns developed at several different levels and connected by vertical ³pipes² or solution tubes similar to a modern cave system. A 90-foot-high cavern reported in the subsurface in southern Florida probably is one of these vertical solution tubes rather than a large cavern.
The permeability of the Boulder Zone is extremely high because of its cavernous nature. This anomalous permeability, which prevents pressure buildup in injection wells, coupled with the fact that the Boulder Zone contains saltwater, makes it an ideal zone for receiving injected wastes. The Boulder Zone has been used for years to store vast quantities of treated sewage injected into it by Miami, Fort Lauderdale, West Palm Beach, and Stuart. Because the salinity and temperature of the water in the Boulder Zone are similar to those of modern seawater, the zone is thought to be connected to the Atlantic Ocean, possibly about 25 miles east of Miami where the sea floor is almost 2,800 feet deep along the Straits of Florida. The Boulder Zone is overlain by 500 to 1,000 feet of low-permeability limestone and dolomite, which retard the upward movement of injected fluids to shallower parts of the Floridan aquifer system that contain fresher although still brackish water.
EFFECTS OF CONFINEMENT
Effects on Dissolution
The carbonate rocks of the Floridan aquifer system are readily dissolved where they are exposed at land surface or are overlain by only a thin layer of confining material. Precipitation absorbs some carbon dioxide from the atmosphere as the precipitation falls and much more carbon dioxide from organic matter in soil as the precipitation percolates downward through the soil, thus forming weak carbonic acid. This acidic water dissolves the limestone and dolomite of the Floridan aquifer system, initially by enlarging pre-existing openings such as pores between grains of limestone or fractures (joints) in the rock. These small solution openings become larger as more of the acidic water moves through the aquifer; eventually the openings may be tens of feet in diameter. The end result of dissolution of carbonate rocks is a type of topography called karst, named for the Karst Plateau of Yugoslavia, that is characterized by caves, sinkholes, and other types of openings caused by dissolution, and by few surface streams.
Dissolution of carbonate rocks is greatest where groundwater circulation is most vigorous. Water is able to enter, move through, and discharge from the Floridan aquifer system more readily and rapidly where it is unconfined or where the upper confining unit is thin. Such areas are shown in figure 55. In these unconfined areas, the aquifer is either exposed or, more commonly, is covered by a thin layer of sand or by clayey, residual soil. In adjacent areas, the Floridan is confined, but the upper confining unit is less than 100 feet thick (fig. 55). In these areas, sinkholes that locally breach the confining unit and allow precipitation to move quickly downward into the aquifer are common. Where the confining unit is thick and unbreached (fig. 55), there has been little dissolution of the aquifer system except in deeply buried zones of paleokarst, such as the Fernandina permeable zone of southeastern Georgia and northeastern Florida, and the Boulder Zone of southern Florida. However, these deeply buried zones chiefly formed in the geologic past, when the rocks comprising the zones were at or near the land surface, and are not the result of the modern ground-water flow system.
Effects on Transmissivity
The large-scale solution porosity that develops as a result of dissolution of the carbonate rocks in the Floridan aquifer system creates large conduits in some places that store and transmit ground water. These conduits, which include caves, solution channels, and sinkholes, are like large-diameter pipes or channels in that they allow tremendous volumes of water to pass quickly through the aquifer with little resistance to flow. Transmissivity, or the capacity of an aquifer to transmit water, is one way of measuring the relative ease with which ground water moves. The greater the transmissivity, the more readily water is able to move through the aquifer.
The distribution of transmissivity values in the Upper Floridan aquifer, the best-known part of the Floridan aquifer system, is shown in figure 56. All of the area having a transmissivity greater than 1,000,000 feet squared per day and most of the area having transmissivity from 250,000 to 1,000,000 feet squared per day are where the upper confining unit of the Floridan aquifer system is less than 100 feet thick, or is absent (compare figs. 55 and 56). These areas are where large solution openings developed in the carbonate rocks allow water to be conveyed through the aquifer rapidly. Where the upper confining unit is greater than 100 feet thick, transmissivity values generally are lower and flow is not so rapid. Where the aquifer is more thickly confined and less affected by dissolution, variations in the original porosity of the aquifer chiefly are responsible for the changes in transmissivity. The lower transmissivity values (less than 50,000 feet squared per day) shown in figure 56 mark places either where the upper confining unit is thick (southern Florida) or where the aquifer system is thin or its porosity and permeability are low, or both (areas near the updip limit of the aquifer system).
Effects on Springs
Springs are places where ground water discharges through natural openings in the ground. Springs may vary greatly in the volume of water they discharge; some springs are small enough to be expressed only as seeps where water oozes slowly from the aquifer, whereas others are large enough to form the headwaters of large rivers. The water discharged by a spring may be from an aquifer that is unconfined (water-table conditions) or confined (artesian conditions). Springs issuing from an unconfined aquifer tend to have a small, extremely variable flow and are directly and quickly affected by variations in precipitation. These springs may cease flowing during periods of less than normal precipitation. In contrast, springs issuing from a confined aquifer have a more constant flow because their flow is supplied by a much greater replenishment area. Accordingly, such springs tend to be unaffected by variations in precipitation unless there is prolonged drought. Springs are common in areas of karst topography. Spring flow is controlled by the size of the replenishment area, the difference in altitude between the spring opening or openings and the water level in the aquifer, and the size of the opening or openings through which the springs issue. Factors that have lesser effects on spring flow include atmospheric-pressure changes, earth and oceanic tides, and pumping of wells located near springs.
Florida has 27 first-magnitude springs (springs with a flow of 100 cubic feet per second or more) out of a total of 78 in the entire Nation. The location of these springs is shown in figure 57. All of them issue from the Upper Floridan aquifer, and practically all of them are located in areas where the upper confining unit of the Floridan aquifer system either is less than 100 feet thick or is absent. The distribution of large springs discharging from the Floridan aquifer system, like the areas of greatest transmissivity within the aquifer system, is the direct result of dissolution of carbonate rocks, which results in the development of large conduits. Many of these caverns channel the ground water to the point where they are exposed at land surface and become the orifices of major springs (fig. 58).
GROUND-WATER FLOW
The existence of a regional ground-water flow system in the Floridan aquifer system was first recognized in peninsular Florida in the 1930ıs and, by the 1940ıs, this system was known to extend into Georgia and South Carolina. The major features of this ground-water flow system can be illustrated by a map of the potentiometric surface of the Upper Floridan aquifer. The contours shown in figure 59 represent the altitude and configuration of the potentiometric surface of the Upper Floridan aquifer before development (that is, the condition before substantial withdrawals from the aquifer began). The altitude and configuration of the potentiometric surface in 1980, when withdrawals had changed the configuration of the surface considerably, is shown in figure 60. In both figures, the contours represent lines of equal altitude of the potentiometric surface or water level. The arrows superimposed on the maps show the direction of ground-water movement, which generally is perpendicular to the contours.
Water in the Upper Floridan aquifer moves from high to low areas on the potentiometric surface. The highest areas on the Upper Floridanıs potentiometric surface are located: (1) in a band where the aquifer is exposed at the land surface near its landward, updip limit and (2) in an area in central peninsular Florida (figs. 59 and 60). Water moves coastward from the outcrop area of the aquifer and outward in all directions from the potentiometric high in central Florida. Although recharge to the aquifer takes place throughout more than one-half of its area, recharge tends to be concentrated in outcrop areas and at potentiometric highs. Rates of recharge vary from less than 1 inch to more than 20 inches per year, depending on local geologic and hydrologic conditions. For example, in Lowndes County in south-central Georgia, the aquifer is hydraulically connected to the Withlacoochee River through swallow holes (sinkholes that develop in a bed of a stream) in the streambed and captures much or all of the streamflow during dry seasons. Recharge here is estimated to be between 10 and 20 inches per year. In contrast, little recharge (an estimated 1 to 5 inches per year) takes place at the potentiometric high in central peninsular Florida.
Before development, nearly 90 percent of the discharge from the Floridan aquifer system was to springs and streams. Upward leakage across confining units, especially in coastal areas, accounted for slightly more than 10 percent of the discharge. Discharge to offshore springs was common on both the gulf and ocean sides of the northern part of peninsular Florida where onshore hydraulic heads were 10 feet or less. Contours that extend offshore from coastal Georgia and adjacent northeastern Florida are based on freshwater heads measured during recent test drilling.
The degree of confinement of the Floridan aquifer system is the characteristic that most greatly affects the distribution of recharge and discharge, and is reflected in the character of the potentiometric surface. Where the system is unconfined or the upper confining unit is thin, there is substantial hydraulic connection between the aquifer and surface drainage. In such areas, the potentiometric surface is irregular, complex, and has many closed highs and lows. Contours are commonly distorted where they cross surface streams or where there are groups of springs. In areas of thick confinement, the aquifer is not affected by surface streams because of the intervening confining unit. Smooth contours are, accordingly, associated with confined conditions. Examples of such places are southeastern Georgia and South Carolina, western panhandle Florida, and southern peninsular Florida.
The band of closely spaced contours trending northeast across south-central Georgia (fig. 59) is located just upgradient from the Gulf Trough, a graben filled with a greater than average thickness of the clayey upper confining unit. Faults bounding this graben extend through the Floridan aquifer system and have allowed confining-unit material to be downdropped opposite the aquifer, thus impeding ground-water flow. This damming effect is represented by the closely spaced potentiometric contours.
The effect of ground-water withdrawals on the potentiometric surface of the Upper Floridan aquifer is illustrated by a map of that surface as it existed in 1980 (fig. 60). The major features of the potentiometric surface are the same as those of the predevelopment surface. That is, the direction of flow in South Carolina and Georgia was still east or southeast from outcrop areas to the Atlantic Ocean and Florida. In peninsular Florida, the general flow direction was still toward the gulf and ocean. However, the effect of withdrawals is shown by deep cones of depression at Savannah, Jesup, and Brunswick, Ga., and at Fernandina Beach and Fort Walton Beach, Fla. Also, hydraulic heads have been lowered 30 feet or more throughout a five-county area southeast of Tampa Bay on the west coast of Florida as a result of withdrawals for irrigation and industrial needs. Regional declines of more than 10 feet have occurred in three broad areas surrounding pumping centers (fig. 61): (1) southeastern Georgia and adjacent parts of northeastern Florida and southern South Carolina; (2) west-central peninsular Florida; and (3) western panhandle Florida. Predevelopment potentiometric gradients have been locally reversed in some coastal areas, creating the potential for encroachment of saltwater from the gulf or ocean or from deep parts of the aquifer that contain saltwater. However, saltwater encroachment was limited to a few localized areas as of 1986.
The major characteristics of the predevelopment flow system have not been greatly altered by ground-water development. The dominant form of discharge remains springflow and discharge to streams. The withdrawal of more than 3 billion gallons per day of freshwater during the early 1980ıs accounts for less than 20 percent of the total discharge of the Floridan aquifer system.
SINKHOLES
Sinkholes are closed depressions in the land surface formed by dissolution of near-surface rocks or by the collapse of the roofs of underground channels and caverns. Sinkholes are a natural, common geologic feature in places underlain by soluble rocks such as the limestone and dolomite that form the Floridan aquifer system. Under natural conditions, sinkholes form slowly and expand gradually. However, activities, such as dredging, constructing reservoirs, diverting surface water, and pumping ground water can accelerate the rate of sinkhole expansion, resulting in the abrupt formation of collapse-type sinkholes, some of which are spectacular (fig. 62).
The dissolution of carbonate rocks by acidic water is the cause of the subsidence that creates all sinkholes. The water enlarges pre-existing openings ranging from pore spaces between limestone particles to fractures in the rock. The enlarged spaces eventually form a network of caves, pipes, and other types of conduits, all of which collect and channel large volumes of ground water. In the Floridan aquifer system, the greatest dissolution occurs where the upper confining unit of the system is thin or absent.
Sinkholes have important effects on both surface and ground water. Lakes commonly occupy the depressions created by sinkhole collapse. Streams, such as the Withlacoochee River near Valdosta, Ga., lose their entire flow at low-flow stages to the Upper Floridan aquifer through swallow holes in the streambed. Where they are not plugged, sinkholes form a direct connection from the land surface to the Upper Floridan aquifer, allowing surface runoff to move directly and quickly into the aquifer.
Some sinkholes form by slow, gradual subsidence of the land surface in response to dissolution of limestone (fig. 63). These gradually subsiding sinkholes are usually shallow and bowl-shaped, and form in places where the limestone is either: (1) exposed at the land surface or thinly covered or (2) covered by a layer of unconsolidated sand that slumps downward to fill the sinkhole as it forms. In the latter case, infilling of the sinkhole keeps pace with dissolution of the limestone. Gradually subsiding sinkholes commonly form where slow dissolution takes place, mostly along joints in the limestone. These sinkholes tend to form naturally and are not greatly affected by human activities.
Collapse sinkholes, such as the one shown in figure 64, form suddenly by the collapse of the roof of a large solution cavity Such large cavities commonly form where ground water circulation is vigorous, thus accelerating the dissolution of the limestone. As the cavity expands laterally, its roof gradually flakes off under the effect of gravity. Continued dissolution and spalling of the cavity roof proceed until the roof suddenly collapses under the weight of the overlying material, and a steep-sided, circular sinkhole forms. Collapse sinkholes form either in places where the cavity roof consists entirely of limestone, or where clay forms a bridge over the cavity (fig. 64); they are the type of sinkhole that usually forms in response to human activities. Additional loading of the land surface by construction of surface-water impoundments or buildings, abrupt decline of the potentiometric surface of the Upper Floridan aquifer in areas of substantial pumpage or harmonic loads produced by the vibratory action of passing trains or heavy construction equipment have all been known to trigger sinkhole collapse.
GROUND-WATER DEVELOPMENT
Ground-water development of the Floridan aquifer system began in the 1880ıs when Savannah, Ga., and Jacksonville, Fla., first constructed wells for municipal supply. By the early 1900ıs, several other cities, including Brunswick, Ga., and Daytona Beach, Fernandina Beach, Fort Meyers, Orlando, St. Petersburg, Tallahassee, and Tampa, Fla., were obtaining water supplies from the Floridan. Many of these early wells flowed because hydraulic heads initially were high. However, increased use soon caused the hydraulic head in the Upper Floridan aquifer to decline until installation of pumps and deepening of supply wells became necessary. Starting in the 1930ıs, industrial withdrawals from the Floridan became an important factor as pulp- and paper-processing plants in southeastern Georgia and northeastern Florida, and phosphate mining and citrus-processing operations in west-central Florida, began to withdraw large volumes of ground water.
By 1950, withdrawals of freshwater from the Floridan for all purposes totaled about 630 million gallons per day (fig. 65); by 1980, nearly five times this volume, or about 3 billion gallons per day, was being pumped. The dominant factors causing this increase were the expansion of agriculture, industry, and mining, and the increased demand for public water supplies, especially in Florida where the population served by the Floridan aquifer system nearly tripled during this 30-year period. Significant changes in the use of water from the Floridan occurred between 1950 and 1980. The major changes have been in the percentage of withdrawal used for agricultural purposes, primarily irrigation, which more than tripled, and the percentage withdrawn for self-supplied industrial, mining, and thermoelectric power uses, which decreased by more than one-half. The majority of the withdrawals in the latter category are for industrial use; withdrawals for mining and thermoelectric power uses account for only about 3 percent of the combined pumpage. The percentage withdrawn for domestic and commercial uses remained practically the same, but that for public supply increased by one-third, directly reflecting trends in population increases.
The regional distribution of estimated withdrawals of freshwater from the Floridan aquifer system, by county, for all uses except domestic and commercial, is shown in figure 66. More water is withdrawn in central Florida than elsewhere, primarily for citrus irrigation and processing, and phosphate-mining activities. Almost 30 percent of the total pumpage is withdrawn in a five-county area-Hillsborough, Orange, Pasco, Pinellas, and Polk Counties. Polk County uses the most ground water; about 310 million gallons per day was withdrawn during 1980. Orange County was second in ground-water use, with an estimated 200 million gallons per day being withdrawn. Pumpage is primarily for irrigation in most of those counties in central Florida where 20 to 100 million gallons per day are withdrawn (fig. 66). The large withdrawals (50 to 100 million gallons per day) in Taylor County in the easternmost part of panhandle Florida are primarily for industrial use (pulp and paper production).
Substantial volumes of fresh ground water are withdrawn in several coastal counties in northeastern Florida and coastal Georgia (fig. 66). During 1980, public supply and industrial pumpage accounted for about 160 million gallons per day in the Duval-Nassau County, Fla., area (Jacksonville and Fernandina Beach); about 100 million gallons per day in Glynn County, Ga., ( Brunswick); and about 75 million gallons per day in both Wayne County (Jesup) and Chatham County (Savannah), Ga. In Georgia, the greatest withdrawals for agricultural purposes, primarily irrigation, during 1980 were in a 15-county area known as the Dougherty Plain in southwestern Georgia, where a total of about 210 million gallons per day was withdrawn.
Uncontrolled flowing wells in many counties in central and southern Florida are a commonly overlooked but important source of discharge from the Floridan aquifer system. Where hydraulic heads are high, some of these wells discharge at land surface; others discharge into shallow zones in the Floridan or overlying aquifers through deteriorated well casings or open boreholes. In 1978, there were as many as 15,000 uncontrolled flowing wells in Florida that discharged an estimated 790 million gallons per day-almost one-third as much as was pumped from the aquifer system during 1980. Many of these wells were subsequently located and plugged under an ongoing program conducted by State and local governmental agencies.
INJECTION WELLS
Parts of the Lower Floridan aquifer that contain saltwater are locally used as receiving zones for industrial and municipal wastes disposed of through injection wells in Florida. The location of injection-well sites in Florida that were operating as of January 1988 is shown in figure 67. About 208 million gallons per day of wastes are injected into these wells; about 97 percent of this volume is municipal waste. Some of the wells, such as those in Polk County, Fla., are used to inject wastes into permeable rocks below the Floridan aquifer system because the entire Floridan contains freshwater in Polk County. The majority of injection wells, however, are completed in the deeper parts of the Floridan that contain saltwater.
In central Florida, particularly in the Orlando area (Orange County), drainage wells have been used since the early 1900ıs to dispose of storm runoff into the Upper Floridan aquifer (fig. 67). Public water-supply wells in the Orlando area, accordingly, are drilled into the Lower Floridan aquifer, which is separated from the Upper Floridan aquifer by a confining unit. There is no evidence to date that the drainage wells have contaminated the Lower Floridan aquifer to any great extent, even though they provide an estimated 30 to 50 million gallons per day of recharge to the system.
GROUND-WATER QUALITY
Dissolved-solids concentrations (the sum of all cations and anions in solution) of water in the Floridan aquifer system are related to: (1) the ground-water flow system, and (2) the proximity to saltwater. In places where the aquifer system is unconfined or thinly confined, ground-water flow is vigorous. Large volumes of water move quickly in and out of the aquifer system, and dissolved-solids concentrations are minimal. Water that travels down longer flowpaths, and, thus, dissolves more limestone and possibly sulfate minerals, such as gypsum, has greater dissolved-solids concentrations. Dissolved-solids concentrations in the Upper Floridan aquifer are shown in figure 68. Near the east and west coasts of Florida, and locally in eastern South Carolina and adjacent areas of coastal Georgia, large dissolved-solids concentrations are due to the mixing of fresh ground water with deeper saltwater that migrates into the aquifer from the ocean. In western panhandle Florida and in the southern one-third of that State, large concentrations of dissolved solids result from the ground water mixing with residual saltwater that a sluggish flow system has left unflushed from the aquifer. The band of large dissolved-solids concentrations along the St. Johns River in east-central Florida likewise reflects unflushed, residual saltwater.
The most common cations in water from the Upper Floridan aquifer are calcium, magnesium, and sodium; the most common anions are bicarbonate, chloride, and sulfate. All of these ions are present either in the minerals of the aquifer or in unflushed saltwater within the aquifer. In general, water in the Lower Floridan aquifer is chemically similar to that of the Upper Floridan aquifer, except for dissolved-solids concentrations. There are more dissolved solids in the water in the Lower Floridan aquifer because this water has followed longer flowpaths and, accordingly, has had more time to dissolve aquifer minerals.


SOUTHEASTERN COASTAL PLAIN AQUIFER SYSTEM
INTRODUCTION
The Southeastern Coastal Plain aquifer system consists of four regional aquifers that are composed predominately of clastic rocks ranging in age from Cretaceous to late Tertiary. The aquifer system underlies an area of about 90,000 square miles in the Coastal Plain of Alabama, Georgia, and South Carolina and extends for a short distance into northern Florida (fig. 69). The system also extends westward throughout much of Mississippi, where it underlies an area of about 32,000 square miles. The upper part of the Southeastern Coastal Plain aquifer system grades into the Mississippi embayment aquifer system in western Alabama. The Mississippi part of the aquifer system is within Segment 5 of this Atlas.
The southern and southeastern limits of the aquifer system extend past the coastline in most places. That is, the rocks that comprise some of the water-yielding units of the aquifer system are permeable enough to maintain their character as aquifers for some distance offshore. However, the aquifers contain saltwater with dissolved-solids concentrations of 10,000 milligrams per liter or more near the coast in most areas. The limit of the aquifer system in peninsular Florida is the place at which the rocks of the lowermost regional aquifer change from a sand and clay sequence to low-permeability, calcareous clay and sand, and carbonate rocks. This change is approximately in the area shown by the dashed line in figure 69. West of Jacksonville, Fla., rocks of this lowermost aquifer are locally absent. Shallower regional aquifers in the system change to low-permeability rocks in southern Georgia. The northern limit of the aquifer system is its contact with crystalline rocks or consolidated sedimentary rocks of Paleozoic age at the Fall Line, which marks the updip extent of Coastal Plain sediments.
Rocks of the Southeastern Coastal Plain aquifer system were deposited in fluvial, deltaic and shallow-marine environments during a series of transgressions and regressions of the sea. Coarser grained, fluvial to deltaic sediments are located primarily near the updip extent of the aquifer system and consist primarily of coarse sand and gravel that form productive aquifers (fig.70). Most of the aquifers in the system, however, consist chiefly of fine to coarse sand.
Confining units within the system are mostly silt and clay, except for a thick sequence of chalk in Alabama and Mississippi. All these fine-grained materials form effective confining units that retard the vertical movement of ground water, especially where they are thick. The proportion of clay in the aquifer system generally increases in the direction of the coastline; thus, even though the system thickens in this direction, its overall transmissivity is much less toward the coastline.
The interbedding of coarse- and fine-grained sediments is complex because of fluctuating sea level and resulting changes in energy conditions and in depositional environments. Rock types and textures may change greatly within short vertical or horizontal distances.
RELATION TO ADJACENT REGIONAL AQUIFER SYSTEMS
The Southeastern Coastal Plain aquifer system is adjacent to four regional aquifer systems. It grades laterally in to the Northern Atlantic Coastal Plain aquifer system to the northeast and is partly overlain by, and partly grades laterally into, the Floridan aquifer system to the south and the Mississippi embayment and Coastal lowlands aquifer systems to the west (fig. 71). The Northern Atlantic Coastal Plain aquifer system is located in Segments 11 and 12 of this Atlas, the Mississippi embayment aquifer system is located in Segments 5 and 10, and the Coastal lowlands aquifer system is located in Segments 4 and 5.
The predominately clastic nature of the aquifers and confining units of the Southeastern Coastal Plain aquifer system persists as the system grades into the Northern Atlantic Coastal Plain aquifer system near the North Carolina-South Carolina State line. The Mississippi embayment and Coastal lowlands aquifer systems to the west likewise are composed of clastic rocks. Parts of these systems overlie the Southeastern Coastal Plain aquifer system and are separated from it by thick, low-permeability clay confining units; other parts are equivalent to, but are much thicker and more complex than, aquifers and confining units of the Southeastern Coastal Plain aquifer system.
The Floridan aquifer system to the southeast consists of a thick sequence of carbonate rocks that are partly equivalent to the clastic beds of the Southeastern Coastal Plain aquifer system. In places, thick sequences of carbonate strata of the Floridan aquifer system overlie the clastic rocks. There is no confining unit separating the two aquifer systems, and the major difference between them is that the Floridan aquifer system tends to be more permeable.
HYDROGEOLOGIC UNITS
There are many geologic formations in the complexly interbedded rocks that comprise the Southeastern Coastal Plain aquifer system. Likewise, there are many aquifers and confining units of local extent. Sequences of local aquifers may be grouped together and treated as a single, regionally extensive aquifer. One way to establish that local aquifers function as a single aquifer is to show that their hydraulic heads fluctuate in the same manner and at about the same time. Likewise, sequences of local confining units can be grouped as a single regional confining unit that impedes the vertical ground-water flow between regional aquifers.
The sediments of the Southeastern Coastal Plain aquifer system have been grouped into seven regional hydrogeologic units-four regional aquifers separated by three regional confining units. The geologic formations comprising the four regional aquifers and the three regional confining units that separate them are shown in figure 72. Also shown in figure 72 are the Floridan aquifer system and the surficial aquifer system, which is generally thin and is not present everywhere. The Floridan aquifer system is hydraulically connected in different places to three of the regional aquifers of the Southeastern Coastal Plain aquifer system (fig. 72). In most places, there is no confining unit between the two aquifer systems and ground water can pass freely between them.
Several geologic formations are included in each of the geohydrologic units shown in figure 72. In some places, the boundaries of the formations coincide with those of the hydrogeologic units; in other places, they do not. For example, the top of the Gosport Sand in Alabama is the same as the top of a regional aquifer. In contrast, the Cape Fear Formation in South Carolina is partly an aquifer and partly a confining unit. The regional hydrogeologic units differentiated are primarily units of similar permeability that hydraulically function in the same way. The regional aquifers are mostly sand with minor gravel and limestone beds, but they locally may contain clay beds. The regional confining units are primarily clay, silt, or chalk, but locally may contain sand beds.
Each of the regional aquifers of the Southeastern Coastal Plain aquifer system has been named for a major river that crosses the outcrop belt of the aquifer and, thus, exposes the aquifer. From youngest to oldest, the four regional aquifers differentiated in the system are the Chickasawhay River aquifer, the Pearl River aquifer, the Chattahoochee River aquifer, and the Black Warrior River aquifer. The regional confining units separating these aquifers bear the same name of the aquifer they overlie. For example, the Pearl River regional confining unit lies above the Pearl River regional aquifer and beneath the Chickasawhay River regional aquifer; and so on (fig. 72).
TOP OF THE AQUIFER SYSTEM
The rocks grouped into the four regional aquifers and the intervening three regional confining units of the Southeastern Coastal Plain aquifer system are exposed at the land surface, approximately as a series of arcuate bands (fig. 73). In Alabama, the rocks comprising the oldest geohydrologic unit, the Black Warrior River aquifer, are exposed farthest north, with successively younger rocks cropping out toward the Gulf Coast. In eastern Georgia and western South Carolina, the younger rocks that comprise the Pearl River aquifer are exposed at the land surface. These rocks completely cover the older rocks that comprise the Chattahoochee River and Black Warrior River aquifers and their intervening confining units.
In south-central Alabama, the clastic rocks that comprise the Chickasawhay River aquifer grade into carbonate rocks that comprise the upper part of the Floridan aquifer system (fig. 73); the Chickasawhay River aquifer does not extend east of this area. The sandy beds of the Chattahoochee River aquifer grade westward into clayey confining units in western Alabama, and the aquifer does not exist beyond that area. Carbonate rocks of the Floridan aquifer system overlap the clastic rocks of the Pearl River aquifer. No clayey confining unit separates these rocks except locally in western Alabama.
Rocks comprising the regional confining units of the Southeastern Coastal Plain aquifer system are not exposed at the land surface except in Alabama. Accordingly, permeable beds of some of the regional aquifers lie directly on top of each other in their outcrop areas in Georgia and South Carolina. However, the low-permeability rocks comprising the regional confining units are present in the subsurface in these two States.
VARIATIONS WITHIN THE AQUIFER SYSTEM
The different regional aquifers and confining units of the Southeastern Coastal Plain aquifer system thicken and thin in the subsurface, and in places are absent, as their permeability characteristics grade from those of an aquifer to those of a confining unit or vice versa. These gradational differences are shown on two hydrogeologic sections; one perpendicular and one parallel to the outcrop area of the aquifer system.
Hydrogeologic section A-Aı in southeastern Georgia (fig. 74) shows that there are three regional aquifers that comprise the aquifer system there. The different geologic formations that comprise each aquifer are identified on the section. All the aquifers dip toward the southeast from the outcrop area. The thickness of each individual aquifer, and the aquifer system as a whole, is greatest near the middle of the section. The Black Warrior River aquifer extends farther to the southeast than do the two shallower aquifers. The Pearl River and Chattahoochee River aquifers, and the Chattahoochee River confining unit between them, all end near the coast. In the coastal area, the sandy aquifer material and the clayey confining-unit material of the Southeastern Coastal Plain aquifer system grade into carbonate rocks of the Floridan aquifer system. Near the northwest side of the section, the Pearl River and Chattahoochee River aquifers are in direct hydraulic contact.
Hydrogeologic section B-Bı (fig. 75) in southern Alabama shows the Pearl River aquifer is at the land surface. The Chattahoochee River aquifer and its overlying Chattahoochee River confining unit are present only in the eastern one-third of the section. They both merge to the west into the Pearl River aquifer and are included as part of it. The Black Warrior River confining unit is everywhere thicker than 1,000 feet along the line of the section. None of the data used to construct this section are from wells deep enough to completely penetrate the Black Warrior River aquifer, which contains mostly saltwater in the area of the section; the two shallower aquifers contain freshwater.
BASE OF THE AQUIFER SYSTEM
The base of the Southeastern Coastal Plain aquifer system consists of rocks of several types, all of which have extremely low permeability. The base of the system slopes gently from its inner margin toward the Atlantic Ocean and more steeply toward the Gulf of Mexico (fig. 76). Near the Georgia-Florida State line, the rocks that comprise the base of the aquifer system are warped into two deep basins separated by a high area that is a northwestward extension of the Peninsular arch in Florida. The base of the system is about 4,500 feet below sea level in the Southeast Georgia embayment and is about 10,000 feet below sea level in south-central Alabama. Westward, in Mississippi, the base of the aquifer system trends more northward due to the effect of downwarping in the Mississippi embayment.
The low-permeability rocks comprising the base of the aquifer system can be grouped into four categories (fig. 76). From oldest to youngest, these are: predominantly Precambrian crystalline rocks, including igneous and metamorphic rocks that are buried extensions of the Piedmont; Paleozoic sedimentary rocks that are mostly sandstone and shale in western Alabama, and black shale and white quartzite elsewhere; early Mesozoic rocks that are predominately coarse-grained redbeds intruded by diabase; and Jurassic sedimentary rocks that are mostly varicolored sandstone interbedded with shale.
REGIONAL AQUIFERS
Each of the four regional aquifers that comprise the Southeastern Coastal Plain aquifer system includes several smaller-scale aquifers. Even though the regional aquifers contain clayey confining units and can be locally subdivided, their overall water-yielding characteristics are similar throughout their extent. The rocks included in the Black Warrior River aquifer, for example, are everywhere more permeable than the rocks of the overlying and underlying confining units.
Chickasawhay River Aquifer
The Chickasawhay River aquifer is present in only a small area in southwestern Alabama, and, accordingly, is not mapped. This aquifer, the uppermost regional aquifer in the Southeastern Coastal Plain aquifer system, consists of sand and limestone beds of Oligocene and Miocene age. The Chickasawhay River aquifer lies above the Pearl River aquifer and is separated from it by the Pearl River confining unit. The Chickasawhay River aquifer is much thicker and more areally extensive in Mississippi; in Louisiana it becomes part of the Coastal lowlands aquifer system. The Chickasawhay River aquifer is located mostly in Segment 5 of this Atlas.
Pearl River Aquifer
The Pearl River aquifer is a thick sequence of sand with minor sandstone and gravel, and a few limestone beds. The sediments comprising the aquifer range in age from Paleocene to late Eocene and were deposited mostly in marine environments in the area mapped in figure 77. The aquifer is equivalent to the Mississippi embayment aquifer system to the west and southwest and to part of the Floridan aquifer system in southern Georgia and adjacent areas. The Pearl River aquifer and the Floridan aquifer system are hydraulically connected.
The top of the Pearl River aquifer slopes gently toward the Atlantic Ocean and the Gulf of Mexico (fig. 77). The seaward limit of the aquifer in Florida and Georgia is the area where it grades completely into carbonate rocks of the Floridan aquifer system. In southwestern Alabama, its limit is the area where it grades completely from sandy strata into low-permeability clay. The aquifer contains freshwater everywhere except for a small area in southwestern Alabama and panhandle Florida (fig. 77).
Chattahoochee River Aquifer
The Chattahoochee River aquifer lies above the Black Warrior River aquifer and the two are separated by the Black Warrior River confining unit. The Chattahoochee River aquifer is separated from the overlying Pearl River aquifer by the Chattahoochee River confining unit. The Chattahoochee River aquifer extends from southeastern North Carolina westward into central Alabama (fig. 78), where the lower part of the aquifer changes from sand to clay and chalk, and the clay confining unit over it pinches out (fig. 75). West of this area, equivalent permeable rocks are grouped with those of the Pearl River aquifer, and the fine-grained rocks in the lower parts of the aquifer are included in the Black Warrior River confining unit. The southern limit of the Chattahoochee River aquifer is where it grades into carbonate rocks of the lower part of the Floridan aquifer system.
Like the overlying Pearl River aquifer, the Chattahoochee River aquifer slopes gently seaward from its outcrop belt. Geologic formations included in the Chattahoochee River aquifer range in age from Late Cretaceous to Late Paleocene. The rocks are mostly sand beds with thin, lignitic clay lenses and locally include glauconitic sand and limestone. These rocks were deposited in marine environments except locally in South Carolina, where they were deposited in a fluvial environment.
Black Warrior River Aquifer
The basal aquifer of the Southeastern Coastal Plain aquifer system is the Black Warrior River aquifer. Although this regional aquifer crops out only in Alabama, Mississippi, and a small part of westernmost Georgia, it is extensive in the subsurface (fig. 79) and is the most widespread of the regional aquifers in the system. In North Carolina, the aquifer grades laterally into the basal aquifers of the Northern Atlantic Coastal Plain aquifer system. The top of the aquifer, as shown by the contours in figure 79, ranges from a few hundred feet above sea level in its outcrop area to about 7,000 feet below sea level in southwestern Alabama. The aquifer is absent in a wide band adjacent to the inner Coastal Plain margin in South Carolina and eastern Georgia. There, the Chattahoochee River aquifer lies directly on the low-permeability rocks of the base of the aquifer system.
The Black Warrior River aquifer consists mostly of Upper Cretaceous sand and clay that were deposited in fluvial, deltaic, and marine environments. Locally, sands of Early Cretaceous age are included in the aquifer in Alabama. The aquifer is displaced by faults in southwestern Alabama that have offset its top by as much as 500 feet. These faults do not affect the ground-water flow system, however, because they occur where the aquifer contains stagnant saltwater. About one-third of the aquifer contains water with dissolved-solids concentrations greater than 10,000 milligrams per liter, as shown in figure 79 .
GROUND-WATER FLOW
Recharge enters the Southeastern Coastal Plain aquifer system from precipitation on the outcrop areas of the aquifers. When reaching the water table, most of this water moves laterally to discharge at small streams in the outcrop area, evaporates, or is transpired by plants. Only a small part of the water percolates downward into the deeper parts of the aquifer system. In the outcrop areas, movement of the water is downward along generally short flowpaths until it reaches the area where the aquifers are confined. From this area, most of the movement is horizontal, along generally long flowpaths, until the water approaches discharge points, where its movement becomes again predominately vertical-but here, it moves upward, either toward a surface-water body or a shallower aquifer, either of which is a discharge area.
A map of the potentiometric surface (fig. 80) of the Black Warrior River aquifer shows that in outcrop areas, where water in the aquifer is under unconfined (water-table) conditions, it stands at high altitudes. These areas are primarily in Alabama, and the potentiometric surface is irregular because the aquifer is eroded by streams of various sizes that locally divert groundwater flow. As the aquifer becomes confined, its potentiometric surface is smoother and the contours representing that surface are more evenly spaced. Most of the water in the aquifer flows coastward and down the dip of the beds. However, an important component of flow is parallel to the Atlantic Ocean (perpendicular to dip), particularly in South Carolina where the water moves slowly northeastward along long flowpaths. The Chattahoochee River and Pearl River aquifers do not have this coast-parallel direction of flow. Most of the water in these two shallower aquifers moves down the dip of the aquifers.
In southeastern Georgia, water moves readily between the Southeastern Coastal Plain and Floridan aquifer systems. Idealized conditions and the general direction of ground-water movement from the inner margin of the Coastal Plain to the Atlantic Ocean are shown in figure 81. In southeastern Georgia, only the Pearl River aquifer (of the three Southeastern Coastal Plain regional aquifers shown) extends to the land surface and receives direct recharge from precipitation. However, the Chattahoochee River confining unit that underlies the Pearl River aquifer pinches out updip, allowing water to leak freely downward from the Pearl River aquifer to the underlying Chattahoochee River aquifer. The Black Warrior River aquifer that lies still deeper is recharged in this area only by downward leakage across the confining unit that completely covers the aquifer.
Where the aquifers become confined, the flow arrows show that the predominant movement of the water is lateral and down the dip of the aquifers. Near the coast, the arrows turn upward as flow is blocked either by saltwater or by the aquifer becoming so fine-grained that its permeability is greatly decreased. Flow is then primarily vertical to shallower aquifers or surface-water bodies, such as the ocean. Some water leaks either downward from or upward to the Floridan aquifer system from the Southeastern Coastal Plain aquifer system, depending on which aquifer system has the higher hydraulic head at a given place. The saltwater shown in the coastal area of the Black Warrior River aquifer is stagnant. The horizontal flow arrow, shown near the right side of the figure, in this aquifer represents the coastal-parallel component of ground-water flow that is unique to the aquifer.
GROUND-WATER QUALITY
Water in each of the regional aquifers of the Southeastern Coastal Plain aquifer system undergoes similar chemical changes as it moves from outcrop areas down the hydraulic gradient into deeper, confined parts of the aquifers. Initial changes are gradual and result from chemical interactions between the water and the minerals comprising the rocks. In deep parts of the aquifers, changes result from the mixing of freshwater with saltwater.
The principal water-quality constituents that are used to characterize the ground water in the Southeastern Coastal Plain aquifer system are dissolved solids, dissolved iron, and dissolved chloride. Dissolved-solids concentrations are generally less than 50 milligrams per liter in outcrop recharge areas but increase as the water moves downgradient and dissolves some of the aquifer minerals. Concentrations greater than 500 milligrams per liter generally occur only where there is mixing of freshwater and saltwater. This mixing has produced dissolved-solids concentrations as large as 10,000 milligrams per liter in places.
Dissolved-iron concentrations generally are, less than 100 micrograms per liter in recharge areas but increase markedly downgradient where concentrations ranging from 1,000 to 10,000 micrograms per liter are present in bands parallel to outcrop areas. Farther downgradient, dissolved-iron concentrations decrease as the iron precipitates. Still farther downgradient, iron concentrations again increase in the freshwater-saltwater mixing zone.
Dissolved-chloride concentrations have the same pattern as dissolved-solids concentrations; both relate in large part to the degree to which freshwater has flushed saltwater, entrapped during deposition of the sediments, from the aquifers. The distribution of dissolved-chloride concentrations in water from the middle part of the Black Warrior River aquifer is shown in figure 82 and represents the general distribution of chloride concentrations for all the aquifers. Concentrations increase downgradient, locally exceeding 3,000 milligrams per liter in west-central Alabama and 20,000 milligrams per liter in southern Georgia. The downgradient increase in concentrations is gradual to about 1,000 milligrams per liter, where there is a marked increase as the freshwater mixes with saltwater.
Classification of waters based on their dominant cations and anions are called hydrochemical facies. The distribution of these hydrochemical facies (fig. 83) shows the progressive change in the chemical character of the water as it moves through the aquifer. To demonstrate the classification used, a calcium bicarbonate water is one in which calcium ions account for more than 50 percent of the total cations in the water and bicarbonate ions account for more than 50 percent of the total anions. Where no ion exceeds 50 percent, the water is said to have ³no dominant² hydrochemical facies. In outcrop areas, the ground water is a calcium bicarbonate type. As water moves deeper into the aquifer, it becomes a sodium bicarbonate type. Finally, in the deeper parts of the aquifer, the water becomes a sodium chloride type. The water mapped in the figure as a sodium chloride type is not everywhere saltwater; some of the water is usable or even potable in many places, but its dominant ions in this area are sodium and chloride. The distribution of hydrochemical facies shown in figure 83 is representative of water in all the aquifers of the Southeastern Coastal Plain aquifer system.
FRESH GROUND-WATER WITHDRAWALS
An estimated 574 million gallons per day of freshwater was withdrawn from the Southeastern Coastal Plain aquifer system during 1985. About 210 million gallons per day was withdrawn for public supply, the principal use (fig. 84). About 89 million gallons per day was withdrawn for domestic and commercial uses. About 150 million gallons per day was withdrawn for agricultural purposes, and about 125 million gallons per day was withdrawn for industrial, mining, and thermoelectric-power uses.

PIEDMONT AND BLUE RIDGE AQUIFERS
INTRODUCTION
The crystalline-rock aquifers that underlie the Piedmont and Blue Ridge physiographic provinces in east-central Alabama, northwestern Georgia, and western South Carolina (fig. 85) are collectively called Piedmont and Blue Ridge aquifers in this Atlas. Similar aquifers extend northward throughout a large area from North Carolina into New Jersey, in a wide band near the center of Segment 11 of this Atlas. The Piedmont and Blue Ridge aquifers consist of bedrock overlain by unconsolidated material called regolith. Included in the regolith are: saprolite, which is a layer of earthy, decomposed rock developed by weathering of the bedrock; soil that develops on the upper part of the saprolite; and alluvium, which is mainly confined to stream valleys and may overlie soil, saprolite, and bedrock. The saprolite is by far the largest component of the regolith, and has a thickness of 150 feet in places. Saprolite thickness, however, is extremely variable.
Because the crystalline rocks formed under intense heat and pressure, they have few primary pore spaces, and the porosity and permeability of the unweathered and unfractured bedrock are extremely low. This does not mean, however, that these rocks will yield no water. Ground water can be obtained from two sources: (1) the regolith, and (2) fractures in the rock (fig. 86). Locally, where the crystalline rocks consist of marble, the dissolving action of slightly acidic ground water has created solution openings that yield large volumes of water.
Although there are considerable differences in the mineralogy and texture of the rocks comprising the Piedmont and Blue Ridge aquifers, the overall hydraulic characteristics of the aquifers are similar. Locally, however, the occurrence and availability of ground water varies greatly because of the complex variability in rock type. Such variability makes it impractical to describe ground-water flow regionally. Accordingly, specific examples taken from local studies are used to illustrate different aspects of the hydrology of the crystalline rocks.
GEOLOGY
Bedrock underlying the Piedmont and Blue Ridge physiographic provinces consists of many different types of metamorphic and igneous rocks that are complexly related. Rock type varies markedly from place to place. For example, Blue Ridge and Piedmont rocks are divided into more than 90 units on the 1976 geologic map of Georgia. The main rock types are gneiss and schist of various compositions; however, extremely fine-grained rocks, such as phyllite and metamorphosed volcanic tuff, ash, and flows are common in places. Locally, quartzite and marble are present. Most of these metamorphic rocks were originally sediments, but some of them were originally igneous or volcanic materials. The degree of heat and pressure to which the original rocks were subjected, coupled with the degree of structural deformation (principally folding and shearing) that they have undergone, has determined the final texture and mineralogy of the rocks. Most of the rocks have undergone several periods of metamorphism. Locally, they contain mineralized zones, some of which are ore bearing.
All the metamorphic rocks have been intruded by large to small bodies of igneous rock that varies in composition from felsic (light-colored rocks that contain large quantities of silica) to mafic (dark-colored rocks that contain large quantities of ferromagnesian minerals). Large igneous intrusions consist of granite, quartz monzonite, and gabbro; these rocks are present in plutons that cover many tens of square miles. Smaller intrusions, such as dikes and sills, consist of both felsic and mafic rocks, including syenite, andesite, diabase, and pegmatite. The rocks are displaced by several major fault zones, some of which extend for hundreds of miles. Locally, shearing along large fracture zones has produced siliceous, intensely fractured rocks, such as mylonite or phyllonite.
RELATION OF ROCK TYPE AND WELL YIELD
Although some wells completed in the Piedmont and Blue Ridge aquifers yield almost 500 gallons per minute, the average reported well yield is much less and generally is in the range of about 15 to