Featured Reprint/
From Volume 7, No. 3, 1969
Conceptual Models for Carbonate Aquifers
by William B. Whitea
Abstract
The very diverse types of ground-water behavior in
carbonate terrains can be classified by relating the flow
type to a particular hydrogeologic environment each
exhibiting a characteristic cave morphology. The ground
water may move by diffuse flow, by retarded flow, or by
free flow. Diffuse flow occurs in less soluble rocks such
as extremely shaley limestones or crystalline dolomites.
Integrated conduits are rare. Caves tend to be small,
irregular, and often little more than solutionally widened
joints. Retarded flows occur in artesian environments and
in situations where unfavorable stratigraphy forces ground
water to be confined to relatively thin beds. Network cave
patterns are characteristic since hydrodynamic forces are
damped by the external controls. Solution occurs along
many available joints. Free flowing aquifers are those in
which solution has developed a subsurface drainage system logically regarded as an underground extension of
surface streams. These streams may have fully developed
surface tributaries as well as recharge from sinkholes and
general infiltration. Characteristic cave patterns are those
of integrated conduit systems which are often truncated
into linear, angulate, and branchwork caves. Free Flow
aquifers may be further subdivided into Open aquifers
lying beneath karst plains and Capped aquifers in which
significant parts of the drainage net lie beneath an insoluble cap rock. Other geologic factors such as structure,
detailed lithology, relief, and locations of major streams,
control the details of cave morphology and orientation of
the drainage network.
Introduction
There has been much interest during the past few years
in the hydrology of carbonate terrains. Carbonate aquifers
have long posed a headache for hydrologists because of
the localized character of the ground-water flow and the
lack of response to standard techniques for aquifer evaluation. There has also been a tendency to lump all carbonate
a
William White joined Penn State University in 1963 and is now
Professor Emeritus in The Department of Geoscientists there. He is
a leading authority on caves and karst.
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aquifers together although their flow behavior varies
widely, depending on the geology of the system. This may
be demonstrated in an over-simplified way by contrasting
the prevailing tendency in the United States to treat carbonate aquifers as diffuse flow aquifers with perturbing
peculiarities with the attitude of some French hydrologists
who, following Martel, still regard most karst aquifers as
subterranean stream systems (Gezé, 1965).
This paper suggests a series of models for carbonate
aquifers determined by the hydrogeology of the drainage
basin. The models are conceptual in nature and over-simplified in detail. The object is to provide idealized endmember aquifer types with which the more complicated
real aquifers may be compared.
Effects of Relief
There is value in making an initial distinction between
aquifer systems occurring in regions of low to moderate relief (such as most of continental United States) and
alpine areas of extreme relief (such as the Dachstein Alps
or Pyrenees carbonate aquifer systems). In regions of moderate relief hydraulic gradients are sufficiently low that the
cave systems that carry the water are in equilibrium with
the position of local base level. In alpine regions sudden
floods and snow-melt runoffs often follow underground
routes perched well above local base levels and the “water
table’’ of the massif. This has led to a prolonged European
controversy over the existence of a permanent karst water
body in mountainous areas and to controversy about the
usefulness of the water table concept. These problems,
discussed by Gezé (1965) and Lehmann (1932), have now
been partially resolved by the careful quantitative work
of Zötl (1961) and his colleagues. For simplicity’s sake,
alpine karst systems will be neglected in the discussion
that follows.
Classification of Carbonate Aquifers
The principal objective of this paper is to classify carbonate aquifers on the basis of their principal hydrologic
elements and to establish some useful criteria for determining the nature of the flow system from geological observables. An outline of the classification is given in Table 1.
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TABLE 1. TYPES OF CARBONATE AQUIFER SYSTEMS IN REGIONS OF LOW TO MODERATE
RELIEF
Flow Type
I. DIFFUSE FLOW
Hydrological Control
Associated Cave Type
GROSS LITHOLOGY
Caves rare, small, have irregular patterns.
Shaley limestones; crystalline dolomites; high
primary porosity.
II. FREE FLOW
THICK, MASSIVE SOLUBLE ROCKS
Integrated conduit cave systems.
Karst system underlain by impervious rocks near
or above base level.
Cave streams perched – often have free air
surface.
1. Open
Soluble rocks extend upward to level surface.
Sinkhole inputs; heavy sediment load; short
channel morphology caves.
2. Capped
Aquifer overlain by impervious rock.
Vertical shaft inputs; lateral flow under
capping beds; long integrated caves.
Karst system extends to considerable depth below
base level.
Flow is through submerged conduits.
1. Open
Soluble rocks extend to land surface.
Short tubular abandoned caves likely to be
sediment-choked.
2. Capped
Aquifer overlain by impervious rocks.
Long, integrated conduits under caprock.
Active level of system inundated.
A. PERCHED
B. DEEP
III. CONFINED FLOW
STRUCTURAL AND STRATIGRAPHIC
CONTROLS
A. ARTESIAN
Impervious beds which force flows below regional
base level.
Inclined 3-D network caves.
B. SANDWICH
Thin beds of soluble rock between impervious beds.
Horizontal 2-D network caves.
Fig. 1. Schematic diagram of a diffuse flow carbonate aquifer.
Diffuse Flow Aquifers are those in which the carbonate
rocks have suffered the least amount of solutional modification (Figure 1). They are most nearly “classical’’ in
their behavior. Darcy’s law is obeyed or nearly obeyed.
Solutional cavities are limited in size and number, often
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being mainly solutionally widened joints or bedding
planes. True caves are rare and those that occur are small
and poorly integrated. There is a high degree of interconnectivity between these small solution cavities. When the
aquifer is exposed at the ground surface, the associated
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karst landforms are subdued. The water table is likely to
be well defined and discharge is through a large number
of small springs and seeps. Evaluation of diffuse flow
aquifers by standard pumping test methods should give
results which are a reasonable estimate of the aquifer
characteristics.
Diffuse flow aquifers occur where solutional activity
of the moving ground water has been retarded by lithologic
factors. Shaley limestones and coarsely crystalline dolomites seem less susceptible to solution than are the more
normal limestones, and these form the main aquifers of
this type. Among the best-studied examples is the Silurian
dolomite aquifer of the DuPage County-Chicago Region
of Illinois (Zeizel, Walton, Sasman and Prickett, 1962). A
French example pointed out because of its deviation from
the more usual highly karsted examples is the Region of
Barle-Duc in Meuse (DeMassieux, 1966).
Free Flow Aquifers are those in which the ground-water
flow paths have been localized by solutional modification
into well integrated systems of conduits. Large flows take
place in these conduits or channels while nearby rock may
have little porosity or permeability. Flows reach velocities of tenths of feet/second and are often in the turbulent
regime. These are largely gravity flows which are governed
mainly by the hydrostatic head, the hydraulic characteristics of the conduit and the volume of recharge. Drainage
patterns are mainly integrated branchworks and the main
conduits should be more properly regarded as underground
extensions of surface streams rather than as ground water.
For example, they carry a sediment load both as suspended
load and as bedload, a feature not found in diffuse groundwater flows. Discharge is through big springs in which the
drainage of tens or hundreds of square miles is concentrated at a single outlet. The gradients of the conduits are
typically low and the water levels may be essentially flat
for miles from the discharge point on a base level stream.
Aquifer thickness may be limited by impervious beds
either above or below the main limestone sequence and the
various combinations of capping or perching beds give rise
to four subvarieties of maturely karsted aquifers.
Completely open aquifers are those in which the soluble rocks extend from the land surface to considerable
depth below base level (Figure 2). Most recharge will be
derived from surface runoff draining into myriad sinkholes. Sinking streams from bordering clastic rocks may
contribute highly localized recharge and provide upstream
surface connections with major underground stream systems. Large sediment loads are carried into the aquifer
under these conditions and many solution openings are
likely to be choked with clastic material. Caves will occur
as short, truncated fragments because of sinkhole collapse
and other erosion from the surface. They will often be sediment-choked. The depth below base level to which large
solution cavities exist is a subject of debate. Bore hole data
indicate depths of 200 feet as about the lower limit where
artesian conditions are not present (c.f. Grant and Schmidt,
1958). The main water-carrying conduits apparently flow
at or somewhat below regional base level. Examples of this
type of aquifer may be found in the Missouri Ozarks or in
southern Indiana.
Capping beds of impervious rock have the effect of limiting recharge to the periphery of the capped area (Figure
3). Flow through the underlying limestone is by lateral
flow, often through conduits of considerable size. Cave
remnants are longer and better integrated than in uncapped
aquifers. Two types of recharge occur: a lateral flow from
sinking streams on adjacent clastic rocks (or from adjacent
uncapped karst plains) and a vertical flow through vertical shafts which carry water from the overlying ridges or
plateaus to the base level of the main aquifer unit. This
geologic setting allows explorable water-carrying conduits
at depths of up to 1000 feet below the land surface. The
water table gradient remains low in spite of irregularities
in surface topography. Many of the karst aquifers of the
Appalachian Mountains and Interior Plateaus are of this
type. Examples are the Greenbrier aquifer of eastern West
Fig. 2. Schematic diagram of a maturely karsted free flow aquifer system without confining beds.
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Fig. 3. Schematic diagram of free flow aquifer with both capping and perching beds. This section is typical of much of the Greenbrier
aquifer of West Virginia.
Virginia (White & Schmidt, 1966), the Central Kentucky
Plateau (Brown, 1966, Watson, 1966), the Highland Rim
Country of Tennessee, and the thick Mississippian aquifer
of northern Alabama.
The effect of perching beds in both capped and
uncapped aquifers is to force ground water into very shallow flow paths. Since the large conduits of a free flow
aquifer have little resistance to support much hydrostatic
head, water recharging the aquifer is quickly drained out.
Cave passages tend to be longer and better integrated. Freeair-surface streams are often found running directly on the
perching beds. The active levels of perched aquifers are
more accessible to direct exploration and survey. Storage
in such aquifers is apt to be very small. The portion of the
Mississippian Greenbrier aquifer described by White and
Schmidt (1966) is of this type.
Confined Flow Aquifers are those in which some sort
of geological boundaries rather than simple hydraulics are
the flow-rate limiting factors. One such arrangement is
the usual artesian flow situation. A second is what may be
termed a “sandwich’’ aquifer.
Artesian aquifers (Figure 4) are created by impervious
capping beds which are tilted or folded in such a way as to
force the ground water to flow at depth under hydrostatic
Fig. 4. Schematic diagram of artesian carbonate aquifer system. The portion of the Dakota aquifer near the Black Hills is of this general type.
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head. Artesian karst aquifers are not different in principle
from other artesian aquifers except that the size of the
solutional openings offers very little friction and thus very
little head loss. However, the flow velocities in artesian
karst aquifers seem to be slower than those in the free flow
aquifers, possibly because of longer paths, and possibly
because of other rate limiting factors such as discharge
into overlying rocks of lower permeability. The effect of
retarded flow is to allow many more joints to be dissolved.
The flow is not concentrated in a few master conduits. The
characteristic pattern is the network cave whose passages
slope parallel to the confining bed. Examples of artesian
karst aquifers are the Floridian aquifer (Stringfield, 1966)
and the Dakota aquifer (Swenson, 1968). The network pattern of the Black Hills Caves has been fitted by Howard
(1964) into an artesian flow model. The Black Hills Caves
are complex networks with little evidence for high flow
velocities. Similar conditions prevail in Breathing Cave,
Virginia, a much more localized example of karst artesian
flow (Deike, 1960).
The Sandwich Aquifer is the extreme limit of a carbonate unit which is both capped and perched, and is thin
compared with the total thickness of beds above base
level (Figure 5). Limestones with the characteristic sandwich feature are typically less than 40 feet thick. Flow
is retarded by lack of concentrated recharge from overlying beds and thus channelization does not take place.
Solution takes place along many available joints generating a very dense network pattern. An extreme example is
Anvil Cave, Alabama in which 13 miles of openings have
been surveyed on one plane in an area of .25 square mile
(Figure 6). Caves with this pattern have been discovered
most frequently near major streams and base level backflooding, a form of bank storage, may contribute to their
development.
Geologic Boundary Conditions
The main characteristics of carbonate aquifers are
determined by the gross geological environment just
described. Other “controls’’ serve to modify the shape of
the drainage net and the orientation and geometry of particular conduits.
Structural controls work in two ways. Folding and
faulting in a drainage basin determines the position of
aquifer beds relative to recharge and discharge areas and
thus determines the main orientation of the drainage net.
In the folded Appalachians, limestone basins tend to be
localized because the karsted limestones outcrop in long
narrow bands. More on the hydrology of folded carbonate
rocks has been discussed in a review paper by Parizek (in
preparation). Joints, fractures and fracture zones are the
routes through which initial ground-water flow prior to
solutional modification must be channeled (Kiersch and
Hughes, 1952). The solutionally modified conduits thus,
in local detail, usually follow the dominant joint systems.
This relationship has been applied by Lattman and Parizek
(1964) to the prediction of optimum water well location by
drilling on intersections of fracture traces determined from
air photographs.
Stratigraphic and lithologic controls are more poorly
delineated than structural controls. The existence of
Fig. 5. Schematic diagram of a sandwich type carbonate aquifer. Some recharge is by diffuse flow from overlying clastics. More direct
recharge is provided by back-flooding from flood rises in the surface stream.
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Fig. 6. Map of Anvil Cave, Alabama. This pattern is typical of retarded flow situations. Map reproduced courtesy of the Alabama
Cave Survey.
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insoluble beds (typically shale, dolomite or continuous
chertbeds) in the carbonate sequence will cause local
perching of ground water and off-sets in vertical shafts.
Underground streams flowing above the main cave base
level are often perched on these insoluble horizons. White
and Schmidt (1966) described some West Virginia examples. Variations in solubility between one carbonate bed
and another control the selection of the initial route to be
followed by ground water. In folded rocks, particularly,
where the water has a choice of beds from a steeply dipping sequence, certain formations are selected in preference to others (Rauch and White, 1968).
There is now fairly impressive evidence that the principal ground-water flow conduits form close to or just below
regional base levels. Davies (1958) pointed out this control in the Appalachians. Confirming evidence has been
provided by White (1960) and Ek (1961) among others.
Exceptions to base level controls occur when the aquifers
are perched on impervious beds or when artesian conditions are present.
Conclusions
Carbonate aquifers have been subdivided into three
major types with a number of subtypes. The classification
of a particular aquifer into one of these types can be made
on the basis of easily observed hydrogeological conditions.
Each type has associated with it a particular flow pattern
and a characteristic pattern for the fragments of cave that
are left behind as the water table is lowered. The usual
controls of structure, lithology, and position of base level
are shown to act mainly to perturb the gross pattern and to
determine some of the detailed morphology of the resulting drainage network.
Acknowledgments
Many of the ideas expressed in this paper were derived
from field work carried out with the assistance of the Cave
Research Foundation. I am indebted to T. Aley, H. W.
Rauch and E. T. Shuster for a number of suggestions, and
to the Alabama Cave Survey for permission to publish the
Anvil Cave map.
References
Brown, R. F. 1966. Hydrology of the cavernous limestones of the
Mammoth Cave Area, Kentucky. U. S. Geol. Survey WaterSupply Paper 1837. 64 pp.
Davies, W. E. 1960. Origin of caves in folded limestone. Bull.
Nat. Speleol. Soc. v. 22, pp. 5–18.
Deike, G. H. III. 1960. Origin and geologic relations of Breathing
Cave, Virginia. Bull. Nat. Speleol. Soc. v. 22, pp. 30–42.
DeMassieux, L. 1966. Le Comportement de la nappe aquifere des
Calcaires du Barrois (Portlandien) dans la region de Bar-leDuc (Meuse). Sciences de la Terre. v. 11, pp. 163– 199.
Ek, C. 1961. Conduits souterrains en relation avec les terasses
fluviales. Ann. Soc. Geol. Belgium. v. 84, pp. 313–340.
Gezé, B. 1965. Les conditions hydrogeologiques des roches calcaires. Chronique d’Hydrogeologie, No. 7, pp. 9–39.
Grant, L. F. and L. A. Schmidt. 1958. Grouting deep solution
channels under an earth fill dam. Jour. Soil Mech. Found. Div.,
Proc. ASCE. v. 84, pp. 1813-1–1813-13.
Howard, A. D. 1964. A model for cavern development under artesian ground-water flow, with special reference to the Black
Hills. Bull. Nat. Speleol. Soc. v. 26, pp. 7–16.
Kiersch, G. A. and P. W. Hughes. 1952. Structural localization
of ground water in limestones —“Big Bend District,’’ TexasMexico. Econ. Geol. v. 47, pp. 794–806.
Lattman, L. H. and R. R. Parizek. 1964. Relationship between
fracture traces and the occurrence of ground water in carbonate rocks. Journ. Hydrology. v. 2, pp. 73–91.
Rauch, H. and W. B. White. 1968. Lithologic controls on the
development of solutional porosity in carbonate aquifers.
Trans. Amer. Geophys. Union. v. 49, pp. 173.
Stringfield, V. T. 1966. Artesian water in Tertiary limestone in the
southeastern States. U. S. Geol. Survey Prof. Paper 517, 226 pp.
Swenson, F. A. 1968. New theory of recharge to the artesian basin
of the Dakotas. Geol. Soc. Amer. Bull. v. 79, pp. 163–182.
Watson, R. A. 1966. Central Kentucky karst hydrology. Bull. Nat.
Speleol. Soc. v. 28, pp. 159–166.
White, W. B. 1960. Terminations of passages in Appalachian
caves as evidence for a shallow phreatic origin. Bull. Nat.
Speleol. Soc. v. 22, pp. 43–53.
White, W. B. and V. A. Schmidt. 1966. Hydrology of a karst area
in east-central West Virginia. Water Resources Res, v. 2, pp.
549–560.
Zeizel, A. J., W. C. Walton, R. T. Sasman and T. A. Prickett. 1962.
Ground-water resources of Dupage County, Illinois. III. State
Water Survey Coop. Ground-Water Rpt. No. 2, 103 pp.
Zötl, J. 1961. Die Hydrographie des nordostalpinen karstes.
Steirische Beiträge Hydrogeol. Jahr. 60/61, No. 2.
Editor’s Note: This reprinted paper from 1969 helps to celebrate the legacy of research in Ground Water that informed generations of
hydrogeologists.
Dr. White added this note. “Back in the 1960s hydrogeologists were just beginning to think about carbonate systems as aquifers. To
many professionals in seemed that karstic aquifers were not intrinsically different from other aquifers and that water carrying conduits
were at most a small perturbation on the flow field. I wrote that paper to illustrate the many different aquifer responses that might be
expected depending on bedrock lithology, structure, and the placement of recharge sources and conduits within the drainage basin. It
is very gratifying that the ideas seemed to have been useful.” (December 27, 2011)
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