CARBONATE-ROCK AQUIFERS
Aquifers in carbonate rocks are most prominent in the central and southeastern parts of the Nation, but also occur in small areas as far west as southeastern California and as far east as northeastern Maine (fig. 4a) and in Puerto Rico (fig. 4b). The rocks that comprise these aquifers range in age from Precambrian to Miocene. Most of the carbonate-rock aquifers consist of limestone, but dolomite and marble locally are sources of water. The water-yielding properties of carbonate rocks are highly variable; some yield almost no water and are considered to be confining units, whereas others are among the most productive aquifers known.
Most carbonate rocks form from calcareous deposits that accumulate in marine environments ranging from tidal flats to reefs to deep ocean basins. The deposits are derived from calcareous algae or the skeletal remains of marine organisms that range from foraminifera to molluscs. Minor amounts of carbonate rocks are deposited in fresh to saline lakes, as spring deposits, geothermal deposits, or dripstone in caves. The original texture and porosity of carbonate deposits are highly variable because of the wide range of environments in which the deposits form. The primary porosity of the deposits can range from 1 to more than 50 percent. Compaction, cementation, and dolomitization are diagenetic processes which act on the carbonate deposits to change their porosity and permeability.
The principal post-depositional process that acts on carbonate rocks is dissolution. Carbonate rocks are readily dissolved to depths of about 300 feet below land surface where they crop out or are covered by a thin layer of material. Precipitation absorbs some carbon dioxide as it falls through the atmosphere, and even more from organic matter in the soil through which it percolates, thus forming weak carbonic acid. This acidic water partially dissolves carbonate rocks, initially by enlarging pre-existing openings such as pores between grains of limestone or joints and fractures in the rocks. These small solution openings become larger especially where a vigorous ground-water flow system moves the acidic water through the aquifer. Eventually, the openings join as networks of solution openings, some of which may be tens of feet in diameter and hundreds to thousands of feet in length. The end result of carbonate-rock dissolution is expressed at the land surface as karst topography, characterized by caves, sinkholes, and other types of solution openings, and by few surface streams. Where saturated, carbonate-rock aquifers with well-connected networks of solution openings yield large volumes of water to wells that penetrate the solution cavities, even though the undissolved rock between the large openings may be almost impermeable (fig. 27). Because water enters the carbonate-rock aquifers rapidly through large openings, any contaminants in the water can rapidly enter and spread through the aquifers.
Some of the common types of karst features that develop on the land surface where limestone is exposed in the Mammoth Cave area of central Kentucky are shown in figure 28. Recharge water enters the aquifer through sinkholes, swallow holes, and sinking streams, some of which terminate at large depressions called blind valleys. These depressions, along with karst valleys and sinkholes, form when all or part of a cavern roof collapses. Uncollapsed remnants of the cavern roof form natural bridges. Surface streams are scarce because most of the water is quickly routed underground through solution openings. In the subsurface, most of the water moves through caverns and other types of large solution openings.
Solution cavities riddle the Mississippian limestones that underlie the Mammoth Cave Plateau and the Pennyroyal Plain that borders the plateau to the south and southwest. Some of these cavities form the large, extensive passages of Mammoth Cave (fig. 29), one of the Nation's largest and best studied cave systems. As the cave's network developed, surface streams were diverted into the passages through sinkholes and flowed as underground streams through openings along bedding planes. Sand and other sediment carried by the underground streams abraded the limestone, thus further enlarging the solution openings through which the stream flowed. Vertical passages, usually developed at the intersections of joints, connect the horizontal bedding plane openings. Dissolution and erosional processes are still active at Mammoth Cave.
Aquifers in carbonate rocks of Cretaceous to Precambrian age yield water primarily from solution openings. Except for a basal sandstone aquifer, the Ozark Plateaus aquifer system consists of carbonate-rock aquifers whose hydrologic characteristics are like those of the limestones at Mammoth Cave. The Silurian-Devonian aquifers, the Ordovician aquifers, the Upper Carbonate aquifer of southern Minnesota, the Arbuckle-Simpson aquifer of Oklahoma, and the New York carbonate-rock aquifers are all in layered limestones and dolomites of Paleozoic age, in which solution openings are locally well developed. The Blaine aquifer in Texas and Oklahoma likewise yields water from solution openings, some of which are in carbonate rocks and some of which are in beds of gypsum and anhydrite interlayered with the carbonate rocks.
Aquifers in carbonate rocks of Tertiary and younger age have different permeability and porosity characteristics than aquifers in Cretaceous and older carbonate rocks. The aquifers in Tertiary and younger rocks are the Castle Hayne aquifer of North Carolina (in Eocene limestone), the Biscayne aquifer of South Florida (in Pliocene and Pleistocene limestones and interbedded sands), the North Coast limestone aquifer system in Puerto Rico (in limestone of Oligocene to Pliocene age), and the Floridan aquifer system of the southeastern United States (in limestone and dolomite of Paleocene to Miocene age). The strata that comprise these aquifers were deposited in warm marine waters on shallow shelves and are mostly platform carbonate deposits in which intergranular porosity is present as well as large solution openings. The Floridan aquifer system described below is the most productive and most studied example of this type of carbonate-rock aquifer.
The Floridan aquifer system underlies an area of about 100,000 square miles in southeastern Mississippi, southern Alabama, southern Georgia, southern South Carolina, and all of Florida (fig. 30). The Floridan is one of the most productive aquifer systems in the world; an average of about 3.4 billion gallons per day of freshwater was withdrawn from it during 1990. Despite the huge withdrawals, water levels in the Floridan have not declined regionally; however, large declines have occurred locally at a few pumping centers.
The Floridan aquifer system is extremely complex because the rocks that compose the system were deposited in highly variable environments, and their texture accordingly varies from coarse coquina that is extremely permeable to micrite that is almost impermeable. Diagenesis has changed the original texture and mineralogy of the carbonate rocks in many places. The principal diagenetic processes that influence the porosity and permeability of the aquifer system are dolomitization (which increases the volume of connected pore space in fine-grained limestones) and calcite or dolomite overgrowths (which fill part or all of the connected pore space in pelletal limestone or coquina). Dissolution of the limestone has produced small to large conduits at different levels in the aquifer system. These factors produce much local variability in the lithology and the permeability of the aquifer system, but regionally the system consists of an upper and a lower aquifer, which are separated by a less-permeable confining unit (fig. 31). In parts of northeastern and northwestern Florida and southwestern Georgia, no confining unit exists and the Upper and Lower Floridan aquifers are directly connected. Two cavernous zones, the Fernandina permeable zone and the Boulder Zone, are present in the Lower Floridan aquifer. Low-permeability confining units bound the aquifer system above and below.
The thickness of the Floridan aquifer system ranges from a thin edge at its northern limit to more than 3,400 feet in parts of southern Florida (fig. 32). The map shows 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 southeastern Georgia and in the eastern panhandle of Florida coincide with downwarped areas which are called the Southeast and Southwest Georgia Embayments, respectively. In north-central peninsular Florida, the aquifer system thins over an upwarp which is called the Peninsular Arch. Faults in southern Georgia and southwestern Alabama form the boundaries of trough-like grabens. Clayey sediments within these downdropped structural blocks have been juxtaposed opposite permeable limestone of the Floridan aquifer system, and the low-permeability clay creates a damming effect that restricts the lateral movement of ground water across the grabens.
Some of the variations within the Floridan aquifer system and the complexity of the system are shown by a geohydrologic section that extends from south-central Georgia to southern Florida (fig. 33). The aquifer system thickens southeastward; it is only about 250 feet thick in south-central Georgia, but is more than 3,000 feet thick in southern Florida. A graben called the Gulf Thrugh, shown near the left side of the section, is between two faults that completely cut the aquifer system; thick clay of the upper confining unit of the Floridan accumulated in the graben. In Georgia, the aquifer system contains only scattered, local confining units or none at all. By contrast, in most of Florida, the system contains one to several thick confining units of regional extent. These confining units consist of carbonate rocks that are much less permeable than the water-yielding strata of the aquifer system, and retard the vertical movement of water within it. The Boulder Zone in southern Florida is a deeply-buried, cavernous zone that is filled with saline water and used as a receiving zone for injected wastes. In Georgia, the Floridan aquifer system directly overlies the Southeastern Coastal Plain aquifer system, which consists of interbedded sand aquifers and clayey confining units, all of which are much less permeable than the carbonate rocks of the Floridan.
The major features of the regional ground-water flow system of the Floridan aquifer system are shown by a map of the potentiometric surface of the Upper Floridan aquifer (fig. 34). The water moves regionally southeastward and southward from recharge areas in central Georgia and southern Alabama where the aquifer is exposed at the land surface or is covered by a thin layer of younger sediments. Water also moves outward in all directions from local potentiometric highs in south-central Georgia and in the northern and central parts of the Florida peninsula. Depressions on the potentiometric surface mark major withdrawal centers at Savannah, Georgia, and at Fernandina Beach, Fort Walton Beach, and the Hillsborough-Pinellas County area, Florida. The band of closely spaced contours that extends northeastward from Grady County to Jeff Davis County, Georgia, is located just up the hydraulic gradient from the Gulf Trough graben that is filled with a thick sequence of clay. This clay, which is part of the upper confining unit of the Floridan aquifer system, has been downdropped opposite the permeable limestone of the Floridan, thus impeding the coastward flow of water in the aquifer. This impedance is represented by the closely spaced contours.
Florida has 27 first-magnitude springs (fig. 35), or springs which discharge 100 cubic feet per second or more, out of 78 in the Nation. All these springs issue from the Upper Floridan aquifer, and practically all of them are located in places where the aquifer is exposed at the land surface or is covered by less than 100 feet of clayey upper confining unit. Dissolution of the carbonate rocks of the aquifer in these places has resulted in the development of large caverns, many of which channel the ground water to major spring orifices. Some of the springs are large enough to form the headwaters of surface streams.
Large withdrawals from the Upper Floridan aquifer at several major pumping centers lowered hydraulic heads in the aquifer more than 80 feet from predevelopment levels in several places by 1980 (fig. 36). Regional declines of 10 to 30 feet have developed in three multicounty areas, one of which extends over almost half the Georgia Coastal Plain. The withdrawals have locally reversed predevelopment hydraulic gradients in some coastal areas, creating the potential for the encroachment of saline water from the Gulf of Mexico, the Atlantic Ocean, or from deep parts of the Floridan aquifer system that contain saline water. However, saline water encroachment is limited to a few localized areas at present (1998 ). Although withdrawals are large, they have not greatly altered the major characteristics of the predevelopment ground-water flow system. The dominant forms of discharge from the aquifer system are springflow and baseflow to streams, just as before development began. Water-budget calculations indicate that withdrawal of about 3.4 billion gallons per day of freshwater during 1990 accounts for only about 20 percent of the total discharge from the aquifer system.
The chemical quality of water in the Floridan aquifer system
is suitable for most uses over an area of about two-thirds of
the aquifer system. Water with dissolved-solids concentrations
of 1,000 milligrams per liter or greater is not considered by
the U.S. Environmental Protection Agency to be suitable for drinking.
A map of dissolved-solids concentrations of water in the Upper
Floridan aquifer (fig. 37) shows that
mineralization of the water is greater near the coast than inland.
The distribution of dissolved solids is related to the ground-water
flow system and proximity to seawater. Where the aquifer is unconfined
or overlain by a thin confining unit, ground-water flow is vigorous.
Large volumes of water move quickly in and out of the aquifer,
and dissolved-solids concentrations are minimal. By contrast,
water that travels coastward down long, regional flow paths is
in contact with aquifer materials, such as limestone or local
gypsum beds, for a much longer time and dissolves more mineral
material. Thus, the water has larger dissolved-solids concentrations.
Near the coasts, large dissolved-solids concentrations are due
to the mixing of fresh ground water with seawater that migrates
into the aquifer from the ocean or the Gulf of Mexico. In southern
Florida and along the St. Johns River in east-central Florida,
areas of large dissolved-solids concentrations represent unflushed
seawater that was either trapped in the limestone of the aquifer
system as it was deposited or entered the aquifer system later,
during high stands of sea level. Dissolved-solids concentrations
in water from the Lower Floridan aquifer are larger than those
in the Upper Floridan aquifer because the water in the Lower Floridan
has followed longer flowpaths and, accordingly, has had more time
to dissolve aquifer minerals.
