Download GROUNDWATER ATLAS OF THE UNITED STATES

GROUND WATER ATLAS OF THE UNITED STATES
INTRODUCTION
AND
NATIONAL SUMMARY
By James A. Miller
MA
NT
NORTH DAKOTA
MONTANA
M
MINNESOTA
I
C
H
I
IDAHO
YOR
K
MAS
CON
S
N
N
SOUTH DAKOTA
NEW
A
IN
WISCONS
RI
ON
G
OREG
INE
N
VERMO
INGTO
NEW
HAMPS
HIRE
WASH
WYOMING
PEN
N
VA
SYL
NEW
JER
SEY
NIA
IOWA
A
ILLINOIS
INDIAN
WEST A
INI
VIRG
UTAH
CALIFO
COLORADO
RNIA
KANSAS
MISSOURI
KENTUC
VIRG
OKLAHOMA
ARIZON
A
ARKANSA
LA
ND
INIA
KY
NORT
TENNE
RY
A
DEL
NEVAD
MA
OHIO
NEBRASKA
RO
H CA
LINA
SSEE
H
SOUT A
LIN
CARO
S
NEW MEXICO
PI
M IS SI SS IP
ALABAM
A
GEORG
IA
LOUISIANA
F L
O
TEXAS
R
ID
A
HAWAII
A L A S K A
PUERTO RICO
CONTENTS
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A3
Unconsolidated sand and gravel aquifers . . . . . . . . .
A6
Semiconsolidated sand aquifers . . . . . . . . . . . . . . . . . A8
Sandstone aquifers . . . . . . . . . . . . . . . . . . . . . . . . . . A10
Carbonate-rock aquifers . . . . . . . . . . . . . . . . . . . . . . A11
Sandstone and carbonate-rock aquifers . . . . . . . . . . A14
Basaltic and other volcanic–rock aquifers . . . . . . . . A14
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A15
Cartographic design and production by Gary D Latzke
A1
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The Ground Water Atlas of the United States provides a
summary of the most important information available for each
principal aquifer, or rock unit that will yield usable quantities
of water to wells, throughout the 50 States, Puerto Rico, and
the U.S. Virgin Islands. The Atlas is an outgrowth of the Regional Aquifer-System Analysis (RASA) program of the U.S.
Geological Survey (USGS), a program that investigated 24 of
the most important aquifers and aquifer systems of the Nation and one in the Caribbean Islands (fig. 1). The objectives
of the RASA program were to define the geologic and hydrologic frameworks of each aquifer system, to assess the
geochemistry of the water in the system, to characterize the
ground-water flow system, and to describe the effects of development on the flow system. Although the RASA studies did
not cover the entire Nation, they compiled much of the data
needed to make the National assessments of ground-water
resources presented in the Ground Water Atlas of the United
States. The Atlas, however, describes the location, extent, and
geologic and hydrologic characteristics of all the important
aquifers in the United States, including those not studied by
the RASA program.
The Atlas is written so that it can be understood by readers who are not hydrologists. Simple language is used to explain technical terms. The principles that control the presence,
movement, and chemical quality of ground water in different
climatic, topographic, and geologic settings are clearly illustrated. The Atlas is, therefore, useful as a teaching tool for
introductory courses in hydrology or hydrogeology at the
college level and as an overview of ground-water conditions
for consultants who need information about an individual aquifer. It also serves as an introduction to regional and National
ground-water resources for lawmakers, personnel of local,
State, or Federal agencies, or anyone who needs to understand
ground-water occurrence, movement, and quality.
The purpose of the Ground Water Atlas of the United
States is to summarize, in one publication with a common
format, the most important ground-water information that has
been collected over many years by the USGS, other Federal
agencies, and State and local water management agencies.
The purpose of this introductory chapter is to describe the
content of the Atlas; to discuss the characteristics, use, and
limitations of the maps and other types of illustrations used in
the different chapters of the book; to summarize the locations
of the principal aquifers on a Nationwide map; and to give an
example of an aquifer in each principal hydrogeologic setting.
13
9
Figure 1. The Regional
Aquifer-System Analysis
program of the USGS studied 24
of the most important aquifers
and aquifer systems in the
United States, and one in the
Caribbean Islands.
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0
0
EXPLANATION
NT
MINNESOTA
8
M
C
12
H
MISSOURI
KENTUC
V IR G
NEW MEXICO
5
4
A L A S K A
ND
IN IA
KY
N O R TH
ARKANSA
LA
D EL
3
RY
11
WEST
NIA
VIRGI
10
OKLAHOMA
A
S
N
NEW
JERS
EY
A N IA
MA
IN DI AN A
TENNE
ARIZON
SYLV
OHIO
ILLINOIS
2
HAWAII
CON
N
PENN
NEBRASKA
KANSAS
MAS
K
A
9
A
YOR
NEW
G
W
I
ISCONSIN
SOUTH DAKOTA
COLORADO
INE
IRE
NORTH DAKOTA
ON
UTAH
CARO
LI N A
SSEE
SO U TH
LI N A
CA RO
S
MIS SIS SIP
PI
ALABAM
RG
A GEO
IA
6
TEXAS
F L
O
LOUISIANA
PUERTO RICO
R
ID
A
EXPLANATION
Hydrologic
Atlas Chapter
Content
Segment number
730 –A
Introductory material and nationwide summary
—
730 –B
California, Nevada
1
730 –C
Arizona, Colorado, New Mexico, Utah
2
730 –D
Kansas, Missouri, Nebraska
3
730 –E
Oklahoma, Texas
4
730 –F
Arkansas, Louisiana, Mississippi
5
730 –G
Alabama, Florida, Georgia, South Carolina
6
730 –H
Idaho, Oregon, Washington
7
730 – I
Montana, North Dakota, South Dakota, Wyoming
8
730 –J
Iowa, Michigan, Minnesota, Wisconsin
9
730 –K
Illinois, Indiana, Kentucky, Ohio, Tennessee
10
730 –L
Delaware, Maryland, New Jersey, North Carolina,
Pennsylvania, Virginia, West Virginia
11
730 – M
Connecticut, Maine, Massachusetts, New Hampshire,
New York, Rhode Island, Vermont
12
730 –N
Alaska, Hawaii, Puerto Rico, U.S. Virgin Islands
13
Figure 3. Diagrams that
illustrate principles of groundwater occurrence and movement,
such as this illustration from Atlas
Chapter G, can be readily photographed or converted into overhead transparencies for use as
teaching aids.
500 MILES
6
Introduction
16
500 KILOMETERS
Upper Colorado River Basin
Oahu, Hawaii
Caribbean Islands
Columbia Plateau
San Juan Basin
Michigan Basin
Edwards–Trinity
Midwestern basins and arches
Appalachian valleys and Piedmont
Puget–Willamette Lowland
Southern California alluvial basins
Northern Rocky Mountain Intermontane Basins
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16
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19
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21
22
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24
25
VERMO
MONTANA
RNIA
8
ATLAS DESIGN
WYOMING
CALIFO
7
Modified from Sun and Johnston, 1994
Regional aquifer system study areas
1 Northern Great Plains
2 High Plains
3 Central Valley, California
4 Northern Midwest
5 Southwest alluvial basins
6 Floridan
7 Northern Atlantic Coastal Plain
8 Southeastern Coastal Plain
9 Snake River Plain
10 Central Midwest
11 Gulf Costal Plain
12 Great Basin
13 Northeast glacial aquifers
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NEVAD
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24
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IDAHO
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IOWA
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I
Figure 2. The Ground
Water Atlas of the United States
consists of an introductory
chapter and descriptive
chapters that discuss the
principal aquifers in 13 multiState segments.
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INTRODUCTION
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·· · · · · REGOLITH ·· · ·
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Unsaturated
zone
Water table
Saturated
zone
Relative
storage
capacity
The regolith has 20 to 50
times the water storing
capacity of the bedrock.
Fractures
BEDROCK
Open fractures
are scarce below
400 feet
0
Volume
Modified from Daniel and Sharpless, 1983
NOT TO SCALE
The Atlas consists of 14 chapters; this introductory chapter and 13 descriptive chapters which show the principal aquifers in regional areas called Segments that collectively cover
the 50 States, Puerto Rico, and the U.S. Virgin Islands (fig. 2).
The regional areas were delineated using two criteria: to keep
each area at a size within which detail about each principal
aquifer could be shown at a reasonable, uniform map scale;
and to contain the most important regional aquifers or aquifer systems entirely within an area. Neither minor aquifers nor
the minor features of the geology and hydrology of major
aquifers are illustrated or described in the Atlas because of the
scale of the maps. The 13 descriptive chapters have been
published separately as Hydrologic Investigations Atlases 730–
B through 730–N (fig. 2) and may be purchased by mail from
the U. S. Geological Survey, Information Services, Box 25286,
Federal Center, Denver CO 80225.
Maps are the primary types of illustrations in each Atlas
chapter, but are supplemented by cross sections, block diagrams, charts, graphs, and photographs as needed to describe
the geology and hydrology of each principal aquifer. Largescale maps are necessary to describe some aquifers, such as
the Biscayne aquifer of southeastern Florida (Atlas Chapter G)
that extends over only a three-county area, but is the source
of water for millions of people. In contrast, small-scale maps
are needed for aquifers that extend over large areas, such as
the High Plains aquifer that covers parts of eight States and
four Atlas segments in the central part of the Nation. A set of
five uniform map scales has been chosen so that the maps can
be directly compared within a chapter and among different
chapters.
Most of the illustrations in the Atlas use color to emphasize the information and to increase the clarity and understandability of the complex data or interpretive material that is presented. To help maintain consistency, each principal aquifer
is assigned a unique color that is used to map the aquifer
wherever it appears in the Atlas. Expanded, simplified figure
captions describe the principal features of each illustration.
Extensive reference lists are included in each chapter to assist readers who may require additional information. Photographs are frequently used to show special geologic or hydrologic conditions, or the results of aquifer development.
Each of the 13 descriptive chapters begins with an overview of climatic, geologic, and hydrologic conditions throughout the regional area covered by the chapter. The overview
contains maps that show precipitation, runoff, physiography,
geology, and ground-water withdrawals from each county in
the regional area. The position and relation of each aquifer with
respect to overlying, underlying, and laterally adjacent aquifers is shown by block diagrams and cross sections. The largest, and perhaps most important, illustration in each chapter
is a map showing the shallowest principal aquifer throughout
the regional area. Some chapters describe areas in the northcentral and northeastern parts of the Nation, where productive surficial aquifers in glacial deposits of sand and gravel
overlie bedrock aquifers. In these chapters, the surficial aquifers and the bedrock aquifers are shown on separate regional
maps. Where aquifers are stacked atop other aquifers, the
order of discussion is from shallowest to deepest.
A variety of illustrations is used to describe each principal
aquifer in each regional area. Maps are used to show the location and extent of the aquifer, the thickness of the aquifer, the
potentiometric surface of the aquifer (a surface that represents
the level to which water will rise in wells completed in the aquifer), and the general chemical quality of the water that the
aquifer contains. Where data are available, maps are used to
show changes in aquifer water levels or the chemical quality of
the water in the aquifer over time. Correlation charts list the
geologic formations or parts of them that compose the aquifer,
and show subdivisions that might exist in complex, layered
aquifers or aquifer systems. Hydrogeologic sections show the
relation of the aquifer to the geology of the area. Arrows are
superimposed on the hydrogeologic sections and on the potentiometric-surface maps to show the direction of movement of
water in the aquifer. Hydrographs are used to show the rise and
fall of aquifer water levels in response to changes in the amount
of precipitation or the rate of ground-water withdrawal. Pie diagrams are used to show the amount of water that is pumped
for different uses or to illustrate the percentage of chemical constituents in the water. Special conditions caused by human activities are discussed and described; such conditions include
land subsidence over large areas or sinkhole development
caused by excessive ground-water withdrawals, waterlogging or
salt buildup in the soil caused by extensive irrigation, and large
water-level declines or saltwater intrusion caused by excessive
pumping of ground water.
No new data were collected for the Atlas because the large
amount of ground-water data available in the reports and files
of the USGS and other agencies was sufficient for its preparation. Published illustrations and interpretations were merged,
combined, modified, and simplified where necessary. The interpretive results and data bases of the RASA program were
major sources of information for compilation of the Atlas.
Where necessary, the results of some of the RASA studies were
combined, or the RASA regional syntheses were supplemented
by the results of smaller-scale studies. Maps and other types
of illustrations were prepared from existing data for some
aquifers for which no appropriate published illustrations were
available.
Regional maps of the aquifers in most Atlas chapters show
large to small areas that are designated either “minor aquifers,”
“not a principal aquifer,” or “confining unit.” Such areas are
underlain either by low-permeability deposits and rocks, unsaturated materials, or aquifers that supply little water because
they are of local extent, poorly permeable, or both. Permeability is the relative ease with which water will move through a
rock unit; aquifers are more permeable than confining units.
Within the areas mapped as principal aquifers, existing data
are not uniformly distributed in space and time, because hydrologic investigations mostly have been conducted in areas
where water supply or water quality problems existed, or where
large quantities of ground water were withdrawn. Except for
widely scattered places, long-term hydrologic records are rare
because data is usually collected only during the course of a
study or perhaps for a few years after the study has ended.
USE OF THE GROUND WATER
ATLAS
The Atlas is designed to give an overview of the most
important aspects of the geology, hydrology, ground-water flow
system, general water quality, and use of the water withdrawn
from the Nation’s principal aquifers. The Atlas does not
present a comprehensive description of all that is known about
each principal aquifer or aquifer system. Because many of the
aquifers and aquifer systems described extend over large areas, small-scale maps are required to show conditions
throughout the entire aquifer. Accordingly, illustrations in the
Atlas should not be used for local information or site-specific
interpretations. The listed references should be consulted for
detailed information about each principal aquifer or aquifer
system.
Many Atlas chapters contain diagrams that illustrate basic concepts concerning the occurrence and movement of
ground water. For example, some areas are underlain by fractured crystalline rock on which regolith, a combination of soil
and weathered rock, has developed (fig. 3). The storage capacity of the regolith, which is characterized by intergranular
porosity, or pore space between individual grains, is many
times greater than that of the underlying crystalline rock, which
is porous only where fractured. The cylinder-and-cone diagram
on the right of figure 3 summarizes the relative storage capacity of the physical situation shown on the left of the figure.
Overhead transparencies or slide photographs can easily be
made from such illustrations for classroom use in introductory
hydrogeology courses.
The Atlas summarizes ground-water investigations that
have been made over many years by a large number of hydrologists. It is, thus, a reference work that can be used by
planners, consultants, teachers, and present and future hydrologists. It can also be consulted by hydrologists in other
countries who wish to learn about the ground-water hydrology
of the United States.
The Atlas accurately shows the principal aquifers of the
United States and summarizes the significant characteristics
of their flow systems and water quality. However, the scales
of most of the maps used do not permit the descriptive chapters to be used to locate future water supplies, predict areas
where overdrafting (withdrawals in excess of natural recharge),
saltwater encroachment, or contamination are likely to occur,
and so on. Studies such as these require maps and reports of
much greater detail. Nevertheless, the Atlas shows significant
features of the regional geology, permeability, and groundwater movement of the principal aquifers. Maps in the Atlas
can also be used in conjunction with geologic and other types
of maps for regional or national planning activities.
A3
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P a c i f i c
Base modified from U.S. Geological Survey digital data,
Albers Equal-Area Conic projection. Standard parallels
55° 00' and 65° 00', central meridian -154° 00'
40°
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43
1
1
NEV
7
ADA
43
Figure 4. The principal aquifers of the United
U TA H
States and the Caribbean Islands are in six
types of rocks and deposits. The colored areas
show the extent of each principal aquifer at or
near the land surface.
EXPLANATION
CAL
2
Rio Grande aquifer system
3
California Coastal Basin aquifers
4
Pacific Northwest basin-fill aquifers
5
Puget–Willamette Lowland aquifer system
6
Northern Rocky Mountains Intermontane Basins
aquifer system
7
Central Valley aquifer system
8
High Plains aquifer
9
Pecos River Basin alluvial aquifer
10
Mississippi River Valley alluvial aquifer
11
Seymour aquifer
12
Surficial aquifer system
13
Unconsolidated-deposit aquifers (Alaska)
14
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A
20
n
e a
O c
Basin and Range aquifers
RNI
3
Unconsolidated sand and gravel aquifers
1
IFO
3 5°
43
3
ARIZ
Sandstone aquifers
ONA
1
1
20
Colorado Plateaus aquifers
21
Denver Basin aquifer system
22
Lower Cretaceous aquifers
23
Rush Springs aquifer
24
Central Oklahoma aquifer
25
Ada–Vamoosa aquifer
26
Early Mesozoic basin aquifers
27
New York sandstone aquifers
35
Southern Nevada volcanic-rock aquifers
28
Pennsylvanian aquifers
36
Northern California volcanic-rock aquifers
29
Mississippian aquifer of Michigan
37
Pliocene and younger basaltic-rock aquifers
Base modified from U.S. Geological Survey
digital data, Albers Equal-Area Conic
projection. Standard parallels 29° 30'
and 45° 30', central meridian -96° 00'
Basaltic and other volcanic-rock aquifers
South Coast aquifer (Puerto Rico)
Semiconsolidated sand aquifers
15
Coastal lowlands aquifer system
30
Cambrian–Ordovician aquifer system
38
Miocene basaltic-rock aquifers
16
Texas coastal uplands aquifer system
31
Jacobsville aquifer
39
Volcanic- and sedimentary-rock aquifers
17
Mississippi embayment aquifer system
32
Lower Tertiary aquifers
40
Snake River Plain aquifer system
18
Southeastern Coastal Plain aquifer system
33
Upper Cretaceous aquifers
41
Columbia Plateau aquifer system
19
Northern Atlantic Coastal Plain aquifer system
34
Upper Tertiary aquifers (Wyoming)
42
Volcanic-rock aquifers—Overlain by
sedimentary deposits where patterned (Hawaii)
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22°
Niihau
22°
Kauai
M
E
Carbonate-rock aquifers
43
Basin and Range carbonate-rock aquifers
44
Roswell Basin aquifer system
45
Ozark Plateaus aquifer system
46
Blaine aquifer
47
Arbuckle–Simpson aquifer
48
Silurian–Devonian aquifers
49
Ordovician aquifers
50
Upper carbonate aquifer
51
Floridan aquifer system
52
Biscayne aquifer
158°
Oahu
157°
21°
157°
H A W A I I
Maui
156°
Lanai
P
a
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O
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e
a
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n
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SCALE 1:2,500,000
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19°
Base modified from U.S. Geological Survey digital data,
Albers Equal-Area Conic projection. Standard parallels
20° 40' and 23° 20', central meridian -157° 58'
0
10
20 MILES
10 20 KILOMETERS
y
;
;
y
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New York and New England carbonate-rock aquifers
54
Piedmont and Blue Ridge carbonate-rock aquifers
55
Castle Hayne aquifer
56
North Coast Limestone aquifer system (Puerto Rico)
57
Kingshill aquifer (St. Croix)
Glacial deposit aquifers overlie
bedrock aquifers in many areas
Not a principal aquifer
Sandstone and carbonate-rock aquifers
58
Edwards–Trinity aquifer system
;;
;
Molokai
59
Valley and Ridge aquifers—Carbonate-rock
aquifers are patterned
60
Mississippian aquifers
61
Paleozoic aquifers
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NATIONWIDE MAP OF PRINCIPAL AQUIFERS
The distribution of the principal aquifers of the United
States, Puerto Rico, and the U.S. Virgin Islands is shown in figure 4. The aquifers shown on the map are the shallowest principal aquifers; some are underlain by other productive aquifers, whereas others are overlain by minor aquifers. For example, the Mississippi River Valley alluvial aquifer overlies
aquifers that are part of the Mississippi embayment aquifer
system from southeastern Missouri to northeastern Louisiana,
and also overlies aquifers that are part of the coastal lowlands
aquifer system in east-central Louisiana. Local stream-valley
alluvial aquifers that yield small to large amounts of water are
in the valleys of many major streams that cross principal
aquifers, but the stream-valley aquifers are not mapped on
figure 4 because of the map scale. Many of the principal aquifers are overlain by confining units and extend into the subsurface beyond the areas shown on the map.
The nationwide aquifer map was constructed by juxtaposing the regional maps of principal aquifers from the descriptive chapters. Regional maps for some chapters might show
more detail than the nationwide map because minor aquifers
that are important local sources of water were mapped in
some States. On the nationwide map, however, such local
aquifers are included in a category called “not a principal
aquifer,” along with confining units that might be mapped
separately in some descriptive chapters. However, productive
aquifers might underlie parts of the area mapped in this category: for example, the prolific Floridan aquifer system underlies the areas mapped as “not a principal aquifer” in the
Coastal Plain of Florida, Georgia, and Alabama. Also included
in this category are low-yielding aquifers that extend over large
areas, such as those in the fractured crystalline rocks of the
Appalachian and Blue Ridge region of the eastern United
States.
Large areas of the eastern, northeastern, and north-central parts of the Nation are underlain by crystalline rocks.
These igneous and metamorphic rocks are permeable only
where they are fractured and generally yield only small
amounts of water to wells. However, because these rocks extend over large areas, large volumes of ground water are withdrawn from them, and, in many places, they are the only reliable source of water supply. Accordingly, the crystalline rocks
of northern Minnesota and northeastern Wisconsin, northeastern New York and the New England States, and the Piedmont
and Blue Ridge Physiographic Provinces that extend from
eastern Alabama to southeastern New York are mapped as
aquifers in the Atlas chapters that describe those areas. Because the crystalline rocks have minimal permeability, they
are not mapped as principal aquifers on figure 4.
In the north-central and northeastern parts of the conterminous United States, numerous local productive aquifers are
in glacial deposits of sand and gravel. The map scale of the
nationwide map is too small to allow individual aquifers in these
glacial deposits to be shown. The general distribution of the
glacial deposits is indicated by the dot patterned area on figure 4, and the locations of the principal bedrock aquifers that
underlie them are mapped in the figure. These bedrock aquifers are used primarily where glacial-deposit aquifers are thin
or yield little water.
The principal aquifers mapped in figure 4 are in six types
of permeable geologic materials: unconsolidated deposits of sand
and gravel, semiconsolidated sand, sandstone, carbonate rocks,
interbedded sandstone and carbonate rocks, and basalt and other
types of volcanic rocks. Rocks and deposits with minimal permeability, that are not considered to be aquifers, consist of intrusive igneous rocks, metamorphic rocks, shale, siltstone,
evaporite deposits, silt, and clay. There is, thus, a direct relation
between permeability and type of geologic material. For this
reason, the aquifers mapped in figure 4 are categorized according to their general geologic character. Each category is described and illustrated in the following sections of this report.
C U B
A
A tlan
t i c
18°30'
67°
Oce a n
66°30'
66°
St. Thomas
65°
64°45'
56
65°15'
St. John
Isla de
Culebra
65°30'
18°
Isla de
Vieques
14
n
C a r i b b e a
VIRG
Sea
IN
AN
ISL
St. Croix 64°45'
17°45'
57
P U E RT O R I C O
SCALE 1:2,500,000
0
0
Base modified from U.S. Geological Survey digital data,
Albers Equal-Area Conic projection. Standard parallels
18° 02' and 18° 26', central meridian -66° 26'
10
20 MILES
10 20 KILOMETERS
DS
Figure 5. Basin-fill aquifers
are in unconsolidated deposits
of sand and gravel that partly
fill basins which formed by
faulting or erosion or both.
INTRODUCTION
The unconsolidated sand and gravel aquifers shown in
figure 4 can be grouped into the following three categories:
basin-fill aquifers, referred to as valley-fill aquifers in many
reports; blanket sand and gravel aquifers; and glacial-deposit
aquifers. A fourth type, called stream-valley aquifers, is located
beneath channels, floodplains, and terraces in the valleys of
major streams. The stream-valley aquifers are not shown on
figure 4 because they are too small to map accurately at the
scale of the figure. The most important stream-valley aquifers
are mapped in the descriptive Atlas chapters.
All the unconsolidated sand and gravel aquifers are characterized by intergranular porosity and all contain water primarily under unconfined, or water-table, conditions. However,
each of the four categories occupies a different hydrogeologic
setting. Examples of each category are used to illustrate these
differences.
Unconsolidated
sand and gravel
aquifers
NO
TT
OS
CA
LE
Va
ll
mi ey m
gh arg
fau t re ins
s
l
,
or ting oult fr
o
bo
th r ero m
sio
n
Mo
dif
ied
EXPLANATION
Unconsolidated deposits—Sand, gravel, silt,
and clay. Generally coarse near valley
margins and streams
Confining unit—clay
fro
m
Wh
ite
he
ad
, 19
94
Sedimentary rocks
Igneous, metamorphic, and sedimentary
rocks
Fault—Arrows show relative direction of
movement
Mountain-front
recharge
e fl o w
S u r fa c d
an
o w in
u n d e r fl
A
Mountain-front
recharge
n
ltratio
ow infi
fl
Stream
Wate
Evapotranspiration
ow
S u rf ac e fl
an d
out
u n d er fl o w
r table
B
EXPLANATION
Alluvium
Modifie
d from A
nderso
ers,
n and oth
1988
tion
anspira
Evapotr
Direction of ground-water movement
B ed ro ck
Figure 6. Basin-fill aquifers in open basins (A) are hydrologically connected by through-flowing streams and ground-water
underflow. Closed basins (B) discharge only through
evapotranspiration, mostly at playas or lakes near the basin centers.
Both types of basins receive recharge primarily from infiltration of flow
from streams that originate in the surroun-ding mountains.
Some basins are bounded or underlain by permeable bedrock such as carbonate rocks or fractured crystalline rocks.
Where such rocks surround a basin, some ground water can
enter and leave the basin through the permeable bedrock.
Some carbonate rocks are highly permeable and might be cavernous where they have been partially dissolved by circulating ground water. Several basins might be hydraulically connected, especially where they are underlain by carbonate
rocks, so that water moves through and between the basins
as a regional or subregional ground-water flow system. In
southeastern Nevada, for example, major flow systems that are
described in Atlas chapter B extend for as much as 250 miles
in basin-bounding carbonate rocks, and the flow mostly discharges to large springs.
Examples of basin-fill aquifers discussed in the Atlas include the Basin and Range aquifers of chapters B, C, and H;
the Rio Grande aquifer system of chapters C and E; the Pacific Northwest basin-fill aquifers of chapters B and H; and the
Northern Rocky Mountains Intermontane Basins aquifer system of chapters H and I. Some basin-fill aquifer systems, such
as the Central Valley aquifer system described in chapter B and
the Puget-Williamette Lowland aquifer system described in
chapter H, are in extremely large basins.
Alluvium
d from
Modifie
Anderso
n and oth
ers, 1988
B ed ro ck
SCALE
NOT TO
100°
45°
110°
105°
Figure 7.
The High
Plains aquifer is the largest
blanket sand and gravel
aquifer in the Nation and
extends over about
174,000 square miles in
parts of eight States.
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yy
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yyyyy
yy
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yy
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yy
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yy
yy
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yy
;
yyy
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yy
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yy;;
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yy
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y
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yy
;
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yy
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;
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S O U T H D A K O TA
8
or
N
th
Pla
t
WYOMING
te
Riv
er
NEBRASKA
Riv
er
er
Pla t
te
Ri
40°
95°
3
COLORADO
River
KANSAS
iv
A r ka nsa s
er
R
Widespread sheetlike aquifers that consist mostly of medium to coarse sand and gravel are collectively called blanket sand and gravel aquifers in this report. These aquifers
mostly contain water under unconfined, or water-table, conditions but locally, confined conditions exist where the aquifers contain beds of low-permeability silt, clay, or marl. Where
stream-valley alluvial aquifers, that also consist of sand and
gravel, cross the blanket sand and gravel aquifers, the two
types of aquifers are hydraulically connected and the streamvalley alluvial aquifers are not mapped separately.
The blanket sand and gravel aquifers largely consist of
alluvial deposits. However, some of these aquifers, such as the
High Plains aquifer, include large areas of windblown sand,
whereas others, such as the surficial aquifer system of the
southeastern United States, contain some alluvial deposits but
are largely comprised of beach and shallow marine sands.
Except for the Seymour aquifer in north Texas, which is
underlain by low-permeability rocks, the blanket sand and
gravel aquifers partly overlie, and are hydraulically connected
to, other aquifers. Where they are in contact with aquifers in
older rocks, the blanket sand and gravel aquifers store water
that subsequently leaks downward under natural conditions to
recharge the deeper aquifers.
The High Plains aquifer is the most widespread blanket
sand and gravel aquifer in the Nation. This aquifer extends over
about 174,000 square miles in parts of eight States and four
Atlas segments (fig. 7). The principal water-yielding geologic
unit of the aquifer is the Ogallala Formation of Miocene age,
a heterogeneous mixture of clay, silt, sand, and gravel that was
deposited by a network of braided streams which flowed eastward from the ancestral Rocky Mountains. Because it consists
largely of the Ogallala Formation, the High Plains aquifer has
also been called the Ogallala aquifer in many reports. Dune
sand is part of the High Plains aquifer in large areas of Nebraska and smaller areas in the other States (fig. 7). This permeable dune sand quickly absorbs precipitation, some of
which percolates downward to the water table of the aquifer.
R epublican
e
BLANKET SAND AND GRAVEL
AQUIFERS
v
Pla
tt
Basin-fill aquifers consist of sand and gravel deposits that
partly fill depressions which were formed by faulting or erosion or both (fig. 5). These aquifers are also commonly called
valley-fill aquifers because the basins that they occupy are
topographic valleys. Fine-grained deposits of silt and clay,
where interbedded with the porous sand and gravel, form
confining units that retard the movement of ground water. In
basins that contain thick sequences of deposits, the sediments
become increasingly more compacted and less permeable with
depth. The compacted, lithified deposits form sedimentary
rocks. The basins are generally bounded by low-permeability
igneous, metamorphic, or sedimentary rocks.
The sediments that comprise the basin-fill aquifers mostly
are alluvial deposits, but locally include windblown sand,
coarse-grained glacial outwash, and fluvial sediments deposited by streams that flow through the basins. The alluvial deposits consist of sediments eroded by streams from the rocks
in the mountains adjacent to the basins. The streams transported the sediments into the basins and deposited them primarily as alluvial fans at the base of the mountains. The
coarser sediment (boulders, gravel, and sand) was deposited
near the basin margins and finer sediment (silt and clay) was
deposited in the central parts of the basins. Some basins contain lakes or playas (dry lakes) at or near their centers. Windblown sand might be present as local beach or dune deposits
along the shores of the lakes. Deposits from mountain, or alpine, glaciers locally form permeable beds where the deposits consist of outwash transported by glacial meltwater. Sand
and gravel of fluvial origin are common in and adjacent to the
channels of through-flowing streams. Basins in arid regions
might contain deposits of salt, anhydrite, gypsum, or borate,
produced by evaporation of mineralized water, in their central
parts.
The hydrogeologic setting of a typical basin generally is
one of two types: open (fig. 6A) or closed (fig. 6B). Recharge
to both types of basin is primarily by infiltration of streamflow
that originates as precipitation which falls on the mountainous areas that surround the basins. This recharge, called
mountain-front recharge, is inter mittent because the
streamflow that enters the valleys is intermittent. As the
streams exit their bedrock channels and flow across the surface of the alluvial fans, the streamflow infiltrates the permeable deposits on the fans and moves downward to the water
table. In basins which are located in arid climates, much of the
infiltrating water is lost by evaporation or as transpiration by
riparian vegetation (plants on or near stream banks).
Open basins contain through-flowing streams (fig. 6A)
and commonly are hydraulically connected to upstream or
downstream basins or both. Some recharge might enter an
open basin as surface flow and underflow (ground water that
moves in the same direction as streamflow) from an adjacent
upstream basin, and recharge occurs as streamflow infiltration
from the through-flowing stream. Before development, water
discharges from an open basin largely by evapotranspiration
within the basin but also as surface flow and underflow into
downstream basins. After development, most discharge is by
withdrawals from wells.
No ground-water or surface-water flow leaves closed
basins (fig. 6B). Streamflow and ground-water movement are
from the basin boundaries toward a lake or playa usually located near the center of the basin. Predevelopment discharge
in the closed basins was by evaporation and transpiration from
the lake or playa area; in developed closed basins, discharge
is primarily through wells.
Fault—Arrows show relative direction of
movement
ble
er ta
Wat
Playa
Sou
th
BASIN-FILL AQUIFERS
A6
Direction of surface-water movement
2
Riv e r
Cana
35°
OKLAHOMA
d i an
NEW MEXICO
4
TEXAS
EXPLANATION
Base modified from U.S.
Geological Survey digital
data, 1:2,000,000, 1972
SCALE 1:7,500,000
0
25
50 MILES
0 25 50 KILOMETERS
High Plains aquifer
Dune sand
3
30°
Modified from Guntentag and others, 1984;
and Weeks and others, 1988
Atlas segment boundary and number
Nebraska
16
Texas
14
12
10
8
6
4
2
0
1954
1959
1964
1969
1974
1978
1985
Figure 9. From predevelopment conditions to 1980,
water levels in the High Plains
aquifer have remained stable in
Nebraska and in large parts of
Kansas and Colorado, but have
declined more than 100 feet in a
large area in the Texas panhandle and in small areas of
Oklahoma and Kansas. Water
levels have risen in local areas
where surface water is applied
for irrigation.
S O U T H D A K O TA
No
rth
Pla
t
NEBRASKA
te
Riv
er
WYOMING
Riv
1990
Modified from Weeks and others, 1988;
and Dugan and others, 1994
Figure 8. Ground-water withdrawals from the High Plains
aquifer for irrigation account for most of the discharge from the
aquifer. Irrigation withdrawals have been largest in Texas and
Nebraska.
Ri
v
40°
R epublican
95°
River
COLORADO
KANSAS
er
R
iv
A r ka nsa s
Riv e r
OKLAHOMA
C a n a di a n
35°
NEW MEXICO
EXPLANATION
Water-level change, in feet
Rise more than 10
Rise less than 10, decline less than 10
Decline
Base modified from U.S.
Geological Survey digital
data, 1:2,000,000, 1972
SCALE 1:7,500,000
0
25
10 to 50
TEXAS
50 to 100
50 MILES
More than 100
0 25 50 KILOMETERS
30°
Modified from Weeks and others, 1988
100°
100°
45°
110°
110°
105°
105°
Figure 11. Water in the
High Plains aquifer moves
regionally eastward, following
the eastward slope of the water
table and the land surface. The
water moves locally toward
major streams where it
discharges from the aquifer as
base flow to the streams.
S O U T H D A K O TA
No
rth
5000
WYOMING
er
;;
yy
yy
;;
y
;
;;
yy
;;y
yy
;
;;
;
;;
yy
y;
;
yy
;
yy
yyy
;;
;;
yy
yyy
;;
;;
y
;
y
yy
;;
;
y
;
y
Riv
er
Pla t
te
er
v
Ri
40°
40°
R epublican
00
16
R epublican
95°
Pla
tt
er
KANSAS
A r ka nsa s
00
28
00
y;y;
Area of little or no saturated thickness
Increase less than 10, decrease less than 10
0
25
More than 25
50 MILES
0 25 50 KILOMETERS
3200
Base modified from U.S.
Geological Survey digital
data, 1:2,000,000, 1972
10 to 25
SCALE 1:7,500,000
Modified from Weeks and others, 1988
TEXAS
SCALE 1:7,500,000
0
25
Water-table contour—Shows altitude of water
table, spring 1980. Contour interval 400
and 1000 feet. Datum is sea level
Direction of ground-water movement
50 MILES
0 25 50 KILOMETERS
30°
EXPLANATION
28
Increase more than 10
Decrease
00
00
Saturated-thickness change, in percent
TEXAS
32
36
EXPLANATION
OKLAHOMA
2400
40
00
NEW MEXICO
00
C a n a d i an
35°
NEW MEXICO
Base modified from U.S.
Geological Survey digital
data, 1:2,000,000, 1972
Riv e r
28
OKLAHOMA
C a n a di a n
35°
00
Riv e r
36
400
0
3200
24
00
R
er
2000
R
River
00
iv
40
COLORADO
KANSAS
A r ka nsa s
0
River
COLORADO
500
Sou
th
Pla
tt
e
95°
v
er
e
Ri
NEBRASKA
00
50
Riv
Pla t
te
Riv
er
0
NEBRASKA
600
Riv
er
00
Pla400
tt e 0
te
WYOMING
24
20
00
Pla
t
00
28
or
th
00
32
00
36
N
S O U T H D A K O TA
00
50
Figure 10. The saturated
thickness of the High Plains
aquifer decreased 10 percent or
more in large areas in Texas and
Kansas, and smaller areas in
Nebraska, Colorado, Oklahoma,
and New Mexico between
predevelopment conditions and
1980.
iv
45°
er
er
Pla t
te
1600
1949
Sou
th
IRRIGATION WITHDRAWLS, IN MILLIONS OF ACRE-FEET
Kansas
18
105°
Large areas of the north-central and northeastern United
States are covered with sediments that were deposited during
several advances and retreats of continental glaciers. The
massive ice sheets planed off and incorporated soil and rock
fragments during advances and redistributed these materials
as ice-contact or meltwater deposits or both during retreats.
Thick sequences of glacial materials were deposited in former
river valleys cut into bedrock, whereas thinner sequences were
deposited on the hills between the valleys. The glacial ice and
meltwater derived from the ice laid down several types of
deposits, which are collectively called glacial drift. Till, which
consists of unsorted and unstratified material that ranges in
size from boulders to clay, was deposited directly by the ice.
Outwash, which is mostly stratified sand and gravel, and glacial-lake deposits consisting mostly of clay, silt, and fine sand,
were deposited by meltwater. Ice-contact deposits consisting
of local bodies of sand and gravel were deposited at the face
of the ice sheet or in cracks in the ice. The glacial sand and
gravel deposits form numerous local but highly productive
aquifers in the area shown in figure 4. These glacial-deposit
aquifers overlie bedrock aquifers in many places. Holocene
alluvium that forms productive aquifers in many river valleys
in the glaciated areas is derived from reworked glacial deposits
and is not distinguished from the glacial deposits in the Atlas.
Likewise, sand and gravel deposited by mountain, or alpine,
glaciers in Alaska, the northern Rocky Mountains, and the
Puget Sound area form local aquifers that are mapped together
with alluvial sand and gravel with which they commonly are
connected.
Colorado, New Mexico,
Oklahoma, South Dakota,
and Wyoming
20
110°
GLACIAL-DEPOSIT AQUIFERS
24
22
45°
e
The High Plains aquifer is the most intensively pumped
aquifer in the United States. During 1990, about 15 billion
gallons per day, or about 17 million acre-feet per year, of
water was withdrawn from the aquifer. An acre-foot is the
volume of water that will cover one acre of land to a depth
of one foot, or 43,560 cubic feet of water. Most of this water
(almost 16 million acre-feet) was withdrawn for irrigation (fig.
8). Withdrawals during 1978 were much greater, with irrigation withdrawals amounting to almost 23 million acre-feet.
Average annual withdrawals of water from the aquifer are
much larger (2 to 35 times larger) than natural recharge to
the aquifer. By 1980, withdrawals had resulted in water-level
declines of more than 100 feet in parts of the aquifer in southwestern Kansas and the Texas panhandle (fig. 9). Declines
were greatest in Texas, Kansas, and Oklahoma. Water levels rose locally in the aquifer, particularly in Nebraska, in
response to increased recharge where surface water that was
applied for irrigation infiltrated the aquifer.
The saturated thickness of the High Plains aquifer is the
vertical distance between the water table and the base of the
aquifer. In 1992, the saturated thickness of the aquifer ranged
from 0 where the sediments that comprise the aquifer are
unsaturated to about 1,000 feet in parts of Nebraska and averaged about 190 feet. Ground-water development has
caused changes in the saturated thickness of the aquifer,
because this thickness changes as aquifer water levels
change. Between predevelopment conditions and 1980, the
saturated thickness of the aquifer decreased in many places
(fig. 10), but locally increased in Texas and Nebraska. The
areas of increase are the result of increased recharge to the
aquifer by one or more of the following factors: greater than
normal precipitation; decreased withdrawals; or downward
100°
Pla
tt
leakage of surface-water irrigation and water from unlined canals and reservoirs. Decreases in saturated thickness of 10
percent or more result in a decrease in well yields and an increase in pumping costs because the pumps must lift the water from greater depths.
Water in the High Plains aquifer generally is unconfined. Locally, clay beds confine the water, but regionally, water-table
conditions prevail. The configuration of the 1980 water-table
surface of the aquifer (fig. 11) generally conforms to the configuration of the land surface. Regional movement of water in
the aquifer is from west to east; locally, the water moves toward
major streams. The water-table contours bend upstream where
they cross the North Platte, the Republican, and the Canadian
Rivers (fig. 11), indicating that water moves from the aquifer
to the rivers. By contrast, the contours are either straight or bend
downstream where they cross the Arkansas River, indicating that
the Arkansas is a losing stream and water from the river recharges the aquifer.
Significant parts of the High Plains aquifer in Kansas, Colorado, and New Mexico are unsaturated, as shown in figure 11.
In these areas, the water table is discontinuous and only local
supplies of water can be obtained from filled channels that have
been eroded into bedrock.
Other blanket sand and gravel aquifers include the
Seymour aquifer of Texas (fig. 4) which, like the High Plains
aquifer, was deposited by braided, eastward flowing streams
but has been dissected into separate pods by erosion; the Mississippi River Valley alluvial aquifer, which consists of sand and
gravel deposited by the Mississippi River as it meandered over
an extremely wide floodplain; and the Pecos River Basin alluvial aquifer, which is mostly stream-deposited sand and gravel,
but locally contains dune sands.
Sou
th
BLANKET SAND AND GRAVEL
AQUIFERS— Continued
30°
Modified from Gutentag and others, 1984
A7
GLACIAL-DEPOSIT AQUIFERS—
70°33'
44°18'
29'
70°25'
Continued
A
sc
o
SCALE 1:100,000
Riv
0
1
2 KILOMETERS
360
;;;;
;;;;
;;;;
M A I N E
14'
South
Paris
la
At
nt
e
Oc
ic
an
340
O X F O R D
3
320 40
Norway
36
0
360
320
W
10'
E
M
ea
OXFORD
0
w
Lake
do
34
Little
Small stream
0
34
650
2 MILES
380
0
Feet
1,000
River
1
er
E
Water in valley-fill glacialdeposit aquifers such as the one in the Little
Androscoggin River valley in southwestern
Maine moves toward the river from the
valley sides and then moves down-valley,
parallel to the direction of streamflow.
gg i n
700
Figure 13.
fer and moves along more lengthy flow paths before discharging to larger streams. Valley-fill glacial-deposit aquifers in
bedrock valleys are much more common than glacial-deposit
aquifers in outwash plains.
Aquifers in the sand and gravel of glacial outwash plains
(fig. 12B) commonly are exposed at the land surface and
receive direct recharge from precipitation. Water in the upper
parts of these aquifers discharges to local streams, lakes, or
wetlands. Water in the deeper parts of the aquifers, however,
flows beneath the local surface-water bodies and discharges
to large rivers that are regional drains. Lenslike beds of clay
and silt are interspersed with the permeable sand and gravel,
and locally create confined conditions in the aquifers. Large
yields are common from wells completed in aquifers composed of glacial outwash.
Local movement of water in the glacial valley-fill aquifers
generally is from the valley walls toward streams (fig. 13);
regional movement of water is down the valley in the direction of stream flow. Where these aquifers occupy channels in
permeable bedrock, they generally receive much of their recharge
from the bedrock, but locally discharge some water to it.
Yields of wells completed in the glacial-deposit aquifers
formed by continental glaciers are as much as 3,000 gallons
per minute where the aquifers consist of thick sand and gravel.
Locally, yields of 5,000 gallons per minute have been obtained
from wells completed in glacial-deposit aquifers that are located near rivers and can obtain recharge from the rivers.
Aquifers that were formed by mountain glaciers yield as much
as 3,500 gallons per minute in Idaho and Montana, and wells
completed in mountain-glacier deposits in the Puget Sound
area yield as much as 10,000 gallons per minute.
o
nd r
W
Feet
750
Littl e
The distribution of the numerous sand and gravel beds
that make up the glacial-deposit aquifers and the clay and
silt confining units that are interbedded with them is extremely complex. The multiple advances of lobes of continental ice originated from different directions and different
materials were eroded, transported, and deposited by the ice,
depending on the predominant rock types in its path. When
the ice melted, coarse-grained sand and gravel outwash was
deposited near the ice front, and the meltwater streams deposited successively finer material farther and farther downstream. During the next ice advance, heterogenous deposits
of poorly permeable till might be laid down atop the sand and
gravel outwash. Small ice patches or terminal moraines
dammed some of the meltwater streams, causing large lakes
to form. Thick deposits of clay, silt, and fine sand accumulated in some of the lakes and these deposits form confining
units where they overlie sand and gravel beds. The glacialdeposit aquifers are either localized in bedrock valleys or are
in sheetlike deposits on outwash plains.
The valley-fill glacial-deposit aquifers (fig. 12A) might be
buried beneath till, glacial-lake deposits, or shallower sand and
gravel aquifers. Where the bedrock valleys are completely
filled, the locations of the ancestral channels and the coarsegrained sediments they contain may not be evident at the land
surface. In the example shown in figure 12A, two glacial-deposit aquifers are separated by a clay and silt confining unit
that restricts the flow of water between them. The shallower
aquifer receives direct recharge from precipitation, and water
moves along short flow paths in the aquifer to discharge at
local streams. Some water percolates downward through the
fine-grained confining unit to recharge the deeper, buried aqui-
900
Br
0
32
oo k
600
800
550
And
Oxford
●3
40
500
700
450
ros
cog
gin
30
River
0
Hogan
320
Pond
600
Modifed from Spieker, 1968
Modified from Bailey and others, 1985
VERTICAL SCALE GREATLY EXAGGERATED
Direction of ground-water movement
Direction of ground-water
movement
ke
Bedrock
La
Water-table contour—Shows altitude of the
water table, June 1981. Contour interval
20 feet. Datum is sea level
300
GG
on
6'
Generally clay and silt
CO
EXPLANATION
Sand and gravel aquifers
ps
Most glacial-deposit aquifers are in bedrock valleys
(A) and might be hydraulically separated from deeper aquifers by
clay and silt confining units. Sand and gravel in glacial outwash
plains (B) receive recharge directly from precipitation, and the thick
sequences of sand and gravel yield large volumes of water to wells.
om
Figure 12.
Whitney
Pond
Th
EXPLANATION
OS
A
IN
B
DR
VERTICAL SCALE GREATLY EXAGGERATED
44°05'
AN
350
340
400
Base modified from U.S. Geological Survey, Lewiston, 1986
Modified from Morrissey, 1983
SEMICONSOLIDATED SAND AQUIFERS
Sediments that primarily consist of semiconsolidated
sand, silt, and clay, interbedded with some carbonate rocks,
underlie the Coastal Plains that border the Atlantic Ocean and
the Gulf of Mexico. The sediments extend from Long Island,
New York, southwestward to the Rio Grande, and generally
form a thick wedge of strata that dips and thickens seaward
from a featheredge at its updip limit. Coastal Plain sediments
are water-laid and were deposited during a series of transgressions and regressions of the sea. Depositional environments
ranged from fluvial to deltaic to shallow marine, and the exact location of each environment depends upon the relative
position of land masses, shorelines, and streams at a given
point in geologic time. Complex interbedding and variations
in lithology result from the constantly-changing depositional
environments. Some beds are thick and continuous for tens
to hundreds of miles, whereas others are traceable only for
short distances. Consequently, the position, shape, and number of the bodies of sand and gravel that form aquifers in these
sediments varies greatly from place to place.
Semiconsolidated
sand aquifers
y
;
yyyyyyy
;;;;;;;
y
;
;;;;;;;
yyyyyyy
;;;;;;;
yyyyyyy
;;
yy
;;;;;;;
yyyyyyy
;;
yy
35°
90°
85°
80°
S O U T H
CA R O L I N A
MISSISSIPPI
5
The semiconsolidated sand aquifers have been grouped
into five aquifer systems (fig. 4) which interfinger with and
grade into each other. Within each aquifer system, the numerous local aquifers have been grouped into regional aquifers
that are separated by regional confining units consisting primarily of silt and clay, but locally are beds of shale or chalk.
The rocks that comprise these aquifer systems are of Cretaceous and Tertiary age. In general, the older rocks crop out
farthest inland, and successively younger rocks are exposed
coastward.
The Southeastern Coastal Plain aquifer system in Atlas
Segments 5 and 6 (fig. 14) is a representative example of a
complex aquifer system in semiconsolidated sediments. The
predominantly clastic sediments that comprise the aquifer
system crop out or are buried at shallow depths in large parts
of Mississippi and Alabama, and in smaller areas of Georgia
and South Carolina; toward the coast, the aquifer system is
covered either by shallower aquifers or confining units. Some
of the aquifers and confining units of the Southeastern Coastal
Plain aquifer system grade laterally into adjacent clastic aquifer
systems in North Carolina, Tennessee, and Mississippi and adjacent States to the west; some also grade vertically and laterally southeastward into carbonate rocks of the Floridan
aquifer system.
Numerous geologic formations have been identified in the
complexly interbedded rocks of the Southeastern Coastal Plain
aquifer system. Likewise, many local aquifers and confining
units are present in the area underlain by the aquifer system.
Based on similarities in their hydraulic characteristics and
G E ORG IA
A L A B A M A
ce
an
Mississippi
North
30°
L
O
R
0
25
50 MILES
0 25 50 KILOMETERS
Lake
Okeechobee
Georgia
East
West
Florida
East
Southwest
Northeast
Surficial aquifer system
Alluvium and terrace deposits
Upper confining unit of Floridan aquifer system
Hawthorn Formation
Glendon, Marianna and Mint Spring Formations
Forest Hill Formation
Yazoo Formation
Ocala Limestone
Gosport Sand
Cook Mountain Formation
Sparta Sand
Lisbon Formation
Tallahatta
Formation
Meridian Sand Member
Lower part of Wilcox Group
Ripley
Formation
Floridan aquifer system
Suwannee Limestone
Nanafalia
Formation
Selma
Avon Park Formation
Tallahatta Formation
Tuscahoma Formation
Clayton Formation
Providence Sand
Ripley Formation
Cusseta Sand
Santee
Limestone
Oldsmar
Formation
Cedar Keys
Formation
Lawson
Limestone
Barnwell
Formation
Chickasawhay River aquifer
Black Mingo
Pearl River confining unit
Formation
Peedee
Formation
Pearl River aquifer
Black Creek Formation
Chattahoochee River confining unit
Blufftown Formation
Middendorf Formation
Group
Eutaw Formation (upper part)
Coffee Sand
Tuscaloosa Group
Rocks of Early Cretaceous age
Cape Fear
Formation
Eutaw Formation (lower part)
Tuscaloosa
Formation
Atkinson Formation
Unnamed
Modified from Miller and Renken, 1988
25°
Modified from Miller, 1992; and Barker and Pernik, 1994
A8
Chattachoochee River aquifer
Black Warrior River confining unit
Eutaw Formation (middle part)
Eutaw and McShan Formations
yy
;;
;;
yy
yy
;;
Southeastern Coastal Plain aquifer system
Undifferentiated rocks of Early Cretaceous, Jurassic, Triassic, and (or) Paleozoic age
Figure 14. The Southeastern Coastal Plain aquifer system
underlies the coastal plains of Mississippi, Alabama, Georgia, and
South Carolina, and is in parts of Atlas Segments 5 and 6.
EXPLANATION
Catahoula Sandstone
Porters Creek Formation
Cretaceous rocks
SCALE 1:7,500,000
West
Alluvium and terrace deposits
Citronelle Formation
A
Southeastern Coastal Plain aquifer
system—Outcrop or shallow subcrop is
patterned
Atlas segment boundary and number
D
EXPLANATION
6
I
Gulf of Mexico
Tertiary and Quaternary rocks
nt
Atla
F
Alabama
East-central
ic
O
6
Base modified from U.S.
Geological Survey digital
data, 1:2,000,000, 1972
water levels, sequences of local aquifers can be grouped into
regional aquifers; sequences of local confining units can be
grouped into regional confining units in the same manner.
The sediments of the Southeastern Coastal Plain aquifer
system have been grouped into four regional aquifers separated
by three regional confining units. The principal geologic formations that comprise these regional hydrogeologic units are shown
in figure 15. The regional aquifers chiefly consist of coarse to fine
sand, but some locally include beds of gravel and limestone. In
each aquifer, the coarser-grained material, such as gravel and
coarse sand, generally is farthest inland near sediment source
areas; the amount of silt and clay increases coastward. The regional confining units are mostly clay and mudstone with local
shale beds except for a thick sequence of chalk in Alabama and
Mississippi (the Selma Group) which forms an effective confining unit. Each of the regional aquifers is named for a major river
along which the sediments that comprise the aquifer are well exposed. From shallowest to deepest, the regional aquifers are the
Chickasawhay River aquifer, the Pearl River aquifer, the
Chattahoochee River aquifer, and the Black Warrior River aquifer (fig. 15). Each regional confining unit bears a name similar
to that of the aquifer it overlies; for example, the Pearl River
confining unit overlies the Pearl River aquifer and separates it from
the shallower Chickasawhay River aquifer (fig. 15). All the regional hydrogeologic units are not present everywhere in the
coastal plain. For example, the Chickasawhay River aquifer occurs only in southern Mississippi and southwestern Alabama; the
Chattahoochee River aquifer is absent in a large area in western
Alabama and southeastern Mississippi, and so on.
Figure 15. The large number of geologic formations that comprise the
Southeastern Coastal Plain aquifer system can be grouped into four regional aquifers,
each named for a major river that crosses the aquifer outcrop area, and three regional
confining units. The chart does not show exact correlations of the geologic units.
Black Warrior River aquifer
Base of Southeastern Coastal Plain
aquifer system
Hydrogeologic unit absent
0
40
Potentiometric contour—Shows altitude at
which water-level would have stood in
tightly cased wells before development.
Dashed where approximately located.
Contour interval 50 and100 feet. Datum is
sea level
25
50 MILES
0 25 50 KILOMETERS
Lake
Okeechobee
General direction of ground-water movement
Figure 16.
Water enters the Pearl River aquifer in topographically high
outcrop areas and mostly moves toward major streams, where it discharges.
Some water moves down the hydraulic gradient of the aquifer to become part
of a deep, regional, confined ground-water flow system.
35°
90°
25°
Modified from Miller, 1992
80°
85°
30
0
S O U T H
150
700
600
CA R O L I N A
an
R
MA
500
300 400
20
0
O
ce
NE
RG
IN O F
L PL
C O ASTA
LI N
E)
0
400 35
Base modified from U.S. 400
Geological Survey digital 300
data, 1:2,000,000, 1972
G E ORG IA
A L A B A M A
nt
ic
(F
AI N
L
AL
300
200
F
O
Base modified from U.S.
Geological Survey digital
data, 1:2,000,000, 1972
L
30°
ce
an
O
R
I
o
A
of Mexic
D
Potentiometric contour—Shows altitude at
which water-level would have stood in
tightly cased wells before development.
Dashed where approximately located.
Contour interval 50 and100 feet. Datum
is sea level
Gulf
A
o
Outcrop of Chattahoochee River aquifer
300
EXPLANATION
D
of Mexic
O
IN
ic
300
I
Gulf
O
SCALE 1:7,500,000
0
Updip limit of aquifer—Dashed where
approximately located
R
EXPLANATION
ic
15
0
300
Atla
30°
nt
Atla
100
Outcrop of Pearl River aquifer
MISSISSIPPI
400
0
25
100
15
15 0
0
of Mexic
o
Gulf
EXPLANATION
0
L
O
15
0
F
Base modified from U.S.
Geological Survey digital
data, 1:2,000,000, 1972
L
30°
200
20
Base modified from U.S.
Geological Survey digital
data, 1:2,000,000, 1972
150
00
3
F
0
0
200
0
20
100
10
0
400
300
150
0
500
25
100
400
100
300
0
10
0
an
15
40
A L A B A M A
100
CA R O L I N A
300
250
30
MISSISSIPPI
0
2
200
300
ce
nt
0
200
30
G E ORG IA
200
150
Atla
0
1
50
250
400
0
0
35
10
50
0
30
A L A B A M A
300
250
200
150
10
G E ORG IA
400
50
50
200
5000
40
450
30
200
MISSISSIPPI
0
20
10 150
0
20
0
150
U T H
S O 300
150
100
80°
A R O L I N A
C300
250
A
40
0
S O U T H
300
D
0
0
80°
85°
I
85°
35
90°
R
90°
30
35°
35°
water movement is thought to occur downdip of the 10,000
milligrams per liter dissolved-solids line. Coastal plain aquifers commonly contain unflushed saline water in their deep,
downdip parts.
The general movement of water in the southeastern
Coastal Plain is summarized in figure 19, which represents
conditions in southeastern Georgia. The figure shows the relation between the Southeastern Coastal Plain aquifer system
and the carbonate rocks of the Floridan aquifer system. Water enters the Pearl River aquifer where it crops out adjacent
to the crystalline rocks that form the base of the Southeastern Coastal Plain aquifer system. Some of the water moves
coastward in the clastic sediments of the Pearl River aquifer
and laterally into the Floridan aquifer system where sands
change to limestone; some water moves downward into the
Chattahoochee River aquifer from the Pearl River aquifer where
the two are in contact. The Black Warrior River aquifer is recharged in this area only by downward leakage across the confining unit that completely overlies it. Where the
Chattahoochee River and the Black Warrior River aquifers are
confined, water moves laterally through the aquifers. Near the
coast, flow is blocked either by an increase in the amount of
clay in the aquifer (Chattahoochee River aquifer) or by stagnant saline water (Black Warrior River aquifer). The flow becomes predominantly vertical as the water leaks upward to
shallower aquifers or to the ocean. Water leaks downward from
the Floridan aquifer system to the Southeastern Coastal Plain
aquifer system, or leaks in the opposite (upward) direction,
depending on the direction of decreasing hydraulic head between the two aquifer systems. The horizontal flow arrow
shown in the Black Warrior River aquifer near the right side of
the figure represents the coast-parallel direction of flow in this
aquifer.
Other semiconsolidated sand aquifers are grouped into
extensive aquifer systems as shown in figure 4. The Northern
Atlantic Coastal Plain aquifer system extends from North
Carolina through Long Island, New York, and locally contains
as many as 10 aquifers. The Mississippi Embayment aquifer
system consists of six aquifers, five of which are equivalent to
aquifers in the Texas coastal uplands aquifer system to the
west. The coastal lowlands aquifer system contains five thick,
extensive permeable zones.
200
Recharge to the regional aquifers is from precipitation that
falls on inland, topographically high aquifer outcrop areas. A
map of the potentiometric surface of the Pearl River aquifer
(fig. 16) shows that in outcrop areas, where unconfined conditions exist, the water moves from high altitudes toward
streams. As the water moves coastward, down the hydraulic
gradient (slope of the potentiometric surface), it becomes
confined and the potentiometric surface becomes smoother,
in contrast to its highly irregular shape in updip areas. Water
in the aquifer moves along short flow paths in and near outcrop areas and along longer, regional flow paths in
downgradient areas. The Pearl River aquifer grades into carbonate rocks of the Floridan aquifer system in southern Alabama, southern Georgia and southeastern South Carolina (fig.
15), and into clastic beds of the Mississippi embayment aquifer
system in southwestern Mississippi.
The potentiometric surface of the Chattahoochee River
aquifer (fig. 17) resembles that of the Pearl River aquifer. The
contours are irregular in outcrop areas, reflecting the influence
of incised streams between high-altitude recharge areas. In the
confined part of the aquifer, water moves along gentler hydraulic gradients and generally down the dip of the beds. The
downdip limit of this aquifer in Mississippi, Alabama, and
Georgia is the area in which the sands that comprise the aquifer change to clays.
Although movement of water in the outcrop areas of the
Black Warrior aquifer is similar to that of the shallower aquifers in the aquifer system, water in the confined parts of the
aquifer moves differently. As shown by the flow-direction arrows in figure 18, important components of flow in the deep,
confined parts of the aquifer are parallel to the outcrop belt of
the aquifer in Mississippi and are coast-parallel in South Carolina. The water also moves along extremely long flow paths
toward deeply entrenched regional drains such as the
Tombigbee, the Alabama, and the Chattahoochee Rivers once
it enters the confined parts of the aquifer. The downdip limit
of ground-water movement in the Black Warrior River aquifer
is defined as the point at which the aquifer contains water
having dissolved-solids concentrations of 10,000 milligrams
per liter. Although permeable sediments equivalent to this
aquifer extend to the Gulf and Atlantic Coasts, little ground-
350
400
SEMICONSOLIDATED SAND
AQUIFERS— Continued
Outcrop of Black Warrior River aquifer
300
SCALE 1:7,500,000
0
25
50 MILES
0 25 50 KILOMETERS
Updip limit of aquifer—Dashed where
approximately located
Potentiometric contour—Shows altitude at
which water-level would have stood in
tightly cased wells before development.
Contour interval 50 and100 feet. Datum
is sea level
Southern extent of ground water containing
less than 10,000 milligrams per liter
dissolved soilds
Approximate downdip limit of aquifer
Approximate updip limit of aquifer
General direction of ground-water movement
General direction of ground-water movement
25°
Movement of water in the Chattahoochee River
aquifer is similar to that in the overlying Pearl River aquifer. The
downdip limit of the Chattahoochee River aquifer is marked by a
facies change from permeable sand to almost impermeable clay in
a coastward direction.
25
50 MILES
0 25 50 KILOMETERS
Lake
Okeechobee
Figure 17.
SCALE 1:7,500,000
0
Figure 18.
Water enters the Black Warrior River aquifer in
outcrop areas and moves either to streams or down the dip of the
aquifer. In confined areas, however, the water moves parallel to the
outcrop bands or to the Atlantic coastline. Movement is along
extremely long flow paths and the water eventually discharges to
deeply entrenched rivers.
Modified from Miller, 1992
Lake
Okeechobee
25°
Modified from Miller, 1992
EXPLANATION
Surficial aquifer system
Recharge
Floridan aquifer system
Atl
a
c
nti
Oc
ean
Aquifers in Southeastern Coastal
Plain aquifer system
Pearl Riv
er
aquifer
Confining units in Southeastern
Coastal Plain aquifer system
Freshwater-saltwater interface
;;;;;;
;;;;;;
;;;;;;
Chattaho
o
River aquchee
ifer
General direction of ground-water
movement
Base of S
outh
Plain aqu eastern Coastal
ifer system
NOT TO
SCALE
Figure 19. In southeast Georgia, the general movement of
water in the Southeastern Coastal Plain aquifer system is from
outcrop recharge areas down the hydraulic gradient of the aquifers
until the water discharges upward to the overlying Floridan aquifer
system. The downdip extent of flow is limited either by a marked
decrease in permeability in the Chattahoochee River aquifer or by
stagnant saline water in the Black Warrior River aquifer.
Black War
River aqu rior
ifer
Saltwater
Modife
d from
Miller, 1
990
A9
SANDSTONE AQUIFERS
Aquifers in sandstone are more widespread than those in
all other kinds of consolidated rocks (fig. 4). Although the
porosity of well-sorted, unconsolidated sand may be as high
as 50 percent, the porosity of most sandstones is considerably
less. During the process of conversion of sand into sandstone
(lithification), compaction by the weight of overlying material
reduces not only the volume of pore space as the sand grains
become rearranged and more tightly packed, but also the interconnection between pores (permeability). The deposition of
cementing materials such as calcite or silica between the sand
grains further decreases porosity and permeability. Sandstones
retain some primary porosity unless cementation has filled all
the pores, but most of the porosity in these consolidated rocks
consists of secondary openings such as joints, fractures, and
bedding planes. Ground-water movement in sandstone aquifers primarily is along bedding planes, but the joints and fractures cut across bedding and provide avenues for the vertical
movement of water between bedding planes.
Sandstone aquifers commonly grade laterally into finegrained, low-permeability rocks such as shale or siltstone.
Many sandstone aquifers are parts of complexly interbedded
sequences of various types of sedimentary rocks. Folding and
faulting of sandstones following lithification can greatly complicate the movement of water through these rocks. Despite
all the above limitations, however, sandstone aquifers are
highly productive in many places and provide large volumes
of water for all uses.
Sandstone
aquifers
The Cambrian–Ordovician aquifer system in the northcentral United States (fig. 20) is composed of large-scale,
predominantly sandstone aquifers that extend over parts of
seven States and three segments of the Atlas. The aquifer
system consists of layered rocks that are deeply buried where
they dip into large structural basins. It is a classic confined, or
artesian, system and contains three aquifers (fig. 21). In
descending order, these are the St. Peter–Prairie du Chien–
Jordan aquifer (sandstone with some dolomite), the Ironton–
Galesville aquifer (sandstone), and the Mount Simon aquifer
(sandstone). The aquifers are named from the principal geologic formations that comprise them. Confining units of poorly
permeable sandstone and dolomite separate the aquifers. Lowpermeability shale and dolomite compose the Maquoketa
confining unit that overlies the uppermost aquifer and is
considered to be part of the aquifer system. Wells that penetrate
the Cambrian–Ordovician aquifer system commonly are open
to all three aquifers, which are collectively called the
sandstone aquifer in many reports.
The rocks of the aquifer system are exposed in large areas of northern Wisconsin and eastern Minnesota, adjacent to
the Wisconsin Dome, a topographic high on crystalline Precambrian rocks. From this high area, the rocks slope southward into the Forest City Basin in southwestern Iowa and
northwestern Missouri, southeastward into the Illinois Basin in
southern Illinois, and eastward toward the Michigan Basin, a
circular low area centered on the Lower Peninsula of Michigan.
The configuration of the top of the Mount Simon sandstone
(that forms the Mount Simon aquifer) is shown in figure 22.
The map shows that this aquifer, which represents the lower
part of the Cambrian–Ordovician aquifer system, is buried to
depths of 2,000 to 3,500 feet below sea level in these structural basins. The configuration of the tops of the overlying
95°
Figure 20.
The Cambrian–
Ordovician aquifer system,
which consists of predominantly sandstone aquifers
separated by poorly permeable
confining units, extends over a
large part of the north-central
United States.
90°
Duluth
M I N N E S O TA
45°
Ironton–Galesville and St. Peter–Prairie du Chien–Jordan aquifers are similar to that of the Mount Simon aquifer. The deeply
buried parts of the aquifer system contain saline water.
Regionally, water in the Cambrian–Ordovician aquifer
system moves from topographically high recharge areas,
where the aquifers crop out or are buried to shallow depths,
eastward and southeastward toward the Michigan and Illinois
Basins. A map of the 1980 potentiometric surface of the St.
Peter–Prairie du Chien–Jordan aquifer (fig. 23) shows this
general direction of movement. The map also shows that water
moves subregionally toward major streams, such as the Mississippi and the Wisconsin Rivers, and toward major withdrawal
centers, such as those at Chicago, Illinois, and Green Bay and
Milwaukee, Wisconsin. In and near aquifer outcrop areas, water
moves along short flow paths toward small streams. Movement
of water in the underlying Ironton–Galesville and Mount Simon
aquifers is similar to that in the St. Peter–Prairie du Chien–
Jordan aquifer. Before development, all the water moved either toward surface streams where it discharged as base flow,
or downgradient, toward the structural basins, into deeply
buried parts of the aquifer where it discharged by upward leakage into shallower aquifers.
One of the most dramatic effects of ground-water withdrawals known in the United States is shown in figure 24.
Withdrawals from the Cambrian–Ordovician aquifer system,
primarily for industrial use in Milwaukee, Wisconsin, and Chicago, Illinois, caused declines in water levels of more than 375
feet in Milwaukee and more than 800 feet in Chicago from
1864 to 1980. Many of the wells in the Chicago–Milwaukee
area obtain water from all three aquifers of the aquifer system,
and the water-level decline map, accordingly, is a composite
map that shows the effects of withdrawals on the entire system. The declines extended outward for more than 70 miles
from the pumping centers in 1980. Movement of water in the
aquifers was changed from the natural flow direction (eastward
toward the Michigan Basin) to radial flow toward the pumping centers. Beginning in the early 1980’s, withdrawals from
the Cambrian–Ordovician aquifer system decreased as some
users switched to Lake Michigan as a source of supply. Water
levels in the aquifer system had begun to rise by 1985 as a
result of the decreased withdrawals.
85°
●
Minneapolis
★
Green Bay
Figure 21. The three
aquifers of the Cambrian–
Ordovician aquifer system are
separated by confining units.
The Maquoketa confining unit
at the top of the system creates
artesian conditions wherever it
is present.
●
WISCONSIN
9
Madison
★
MICHIGAN
Milwaukee
●
Geologic
nomenclature
Principal
lithology
Shale and
dolomite
Sioux City
Detroit
Shaly
dolomite
●
Ordovician
Rockford
●
I O WA
Chicago
●
Des Moines
★
Davenport
Sandstone
40°
Dolomite
and
sandstone
INDIANA
Indianapolis★
Springfield★
10
●
Cambrian-Ordovician aquifer system
9
Cambrian
Kansas City
Jefferson City
●
★
Saint Louis
MISSOURI
Atlas segment boundary and number
St. Peter–
Prarie du Chien–
Jordan aquifer
●
ILLINOIS
EXPLANATION
Maquoketa
confining unit
Dolomite
★
Lansing
●
Hydrogeologic
nomenclature
Dolomite and
fine-grained
sandstone
St. LawrenceFranconia confining unit
Sandstone
Ironton–Galesville
aquifer
Shaly
sandstone
Eau Claire
confining unit
Sandstone
Mount Simon
aquifer
Cambrian–Ordovician aquifer system
●
Saint Paul
Precambrian
3
Modified from Olcott, 1992
SCALE 1:7,500,000
0
Base modified from U.S.
Geological Survey digital
data, 1:2,000,000, 1972
25
50 MILES
0 25 50 KILOMETERS
Modified from Young, 1992b
95°
Figure 22.
Figure 23. Water in the
St. Peter–Prairie du Chien–
Jordan aquifer moves from
aquifer outcrop areas regionally
toward the Michigan and
Illinois Basins, subregionally
toward large rivers and major
pumping centers, and locally
toward smaller streams.
90°
85°
0
0
-50 0
0
0
1
00
-15 0
0
-20
800 900
45°
Minneapolis
★
9
0
800 Saint
8 0Paul
●
00
●
700
10
00
WISCONSIN
750
75
●
Rockford
0
100
Sioux City
900
40°
-2
●
0
0
00
-3
Springfield★ -4
-1250
0
00
40°
Indianapolis★
Illinois
Basin
INDIANA
ILLINOIS
Indianapolis★
Springfield★
i 50
0
EXPLANATION
Kansas City -1000
Jefferson City
100
300
INDIANA
ILLINOIS
●
★
●
800
Saint Louis
MISSOURI
SCALE 1:7,500,000
0
25
50 MILES
0 25 50 KILOMETERS
Base modified from U.S.
Geological Survey digital
data, 1:2,000,000, 1972
A10
Davenport
pp
Top-of aquifer contour—Shows altitude
of top of Mount Simon aquifer.
Contour interval, in feet, is variable.
Datum is sea level
★
s si
-250
Chicago
Miss i
Mount Simon aquifer
●
★
Lansing
500
50
00
-1500
●
700
6
0
00
-2
-20
-1750
EXPLANATION
00
MICHIGAN
450
●
Des Moines
●
-2
0
25
0
Rockford
500
-2
50
700
I O WA
Chicago
0
y
Cit
est in
r
o
F Bas
0
80
●
●
Milwaukee
Madison
★
0
Davenport
00
11
●
●
800
0
-1
Des Moines
★
★
Detroit
sconsin
Wi
900
700
80
I O WA
Milwaukee
Lansing
0
-50 -250
0
-10 750
00
-12
50
-15
00
Sioux City
●
900 800 0
70
★
900
500
Madison
250
0
90
0
●
60
WISCONSIN
Michigan
Basin
MICHIGAN
800
Green Bay
iv
800
Green Bay
85°
●
gan
Saint Paul
r
Michi
★
io
Lake
Wisconsin
Dome
00
Minneapolis
Super
e
Ri v e r
10
●
Duluth
M I N N E S O TA
100500
7
500
750
●
k
La
pp
i
●
1000
45°
Mississi
700
Duluth
M I N N E S O TA
90°
R
The top of the
Mount Simon aquifer slopes
from high areas near the
Wisconsin Dome downward
into the Michigan, Illinois, and
Forest City Basins. The tops of
the overlying Ironton–Galesville
and St. Peter–Prairie du Chien–
Jordan aquifers have the same
general shape.
er
95°
Modified from Olcott, 1992; and Young, 1992b
Potentiometric contour—Shows altitude at
which water-level would have stood in
tightly cased wells in 1980. Dashed where
approximately located. Hachures indicate
depression. Contour interval, in feet, is
variable. Datum is sea level
Kansas City
500
Jefferson City
★
MISSOURI
Limit of St. Peter-Prairie du Chien-Jordan
aquifer
●
Saint Louis
Ri
ve
r
SCALE 1:7,500,000
0
25
50 MILES
0 25 50 KILOMETERS
Direction of ground-water movement
Base modified from U.S.
Geological Survey digital
data, 1:2,000,000, 1972
Modified from Olcott, 1992; and Young, 1992b
Detroit
●
SANDSTONE AQUIFERS— Continued
89˚
88˚
100
OZAUKEE
50
DODGE
50
WASHINGTON
100
0
10
SCALE 1:2,500,000
20
0
0
Milwaukee
WAUKESHA
JEFFERSON
0
WISCONSIN
50
RACINE
ROCK
WALWORTH
0 KENOSHA
WISCONSIN
30
MC HENRY
LAKE
500
OGLE
ILLINOIS
42˚
EXPLANATION
Water-level decline in Cambrian–
Ordovician aquifer system,
in feet
Chicago
DU PAGE
100
DE KALB
50 MILES
50 KILOMETERS
Less than 100
800
KANE
600
50
BOONE
0
WINNEBAGO
40
0
ILLINOIS
25
an
ichig
e M
Lak
375
350
300
43˚
25
100 to 300
LEE
00
300 to 500
7
KENDALL
500 to 700
800
200
50
0
10
Greater than 700
WILL
LA SALLE
Line of equal water-level decline,
1864-1980—Dashed where
approximately located. Contour
interval, in feet, is variable
300
GRUNDY
KANKAKEE
41˚
Ground-water divide
Base modified from U.S.
Geological Survey digital
data, 1:2,000,000, 1972
Modified from Young, 1992a
Figure 24. Water levels in the Cambrian–
Ordovician aquifer system declined more than 800
feet at Chicago as a result of large withdrawals
from 1864 to 1980.
The ground-water flow system of the Cambrian–Ordovician aquifer system is summarized in figure 25. Water from
precipitation moves downward through surficial deposits of
glacial drift, shallower aquifers, and the Maquoketa confining
unit into the St. Peter–Prairie du Chien–Jordan aquifer. Some
of the water moves horizontally along short flow paths to local streams, such as the Little Cannon River, where it moves
upward to discharge to the streams as base flow. Some water
moves in a similar fashion along flow paths of intermediate
length and discharges to larger streams, such as the Cannon
River. Some of the water continues to percolate downward
through successively deeper confining units into successively
deeper aquifers. Water in these deeper aquifers moves laterally over long distances and eventually discharges to major
rivers, such as the Mississippi River. A small part of the water
moves down the regional hydraulic gradient, perpendicular to
the plane of the section shown in figure 25 and toward deeply
buried parts of the aquifer system. This deep, regional flow discharges by upward leakage to shallower aquifers or is captured
by pumping wells.
The chemical quality of the water in large parts of the
aquifer system is suitable for most uses. The water is not highly
mineralized in areas where the aquifers crop out or are buried
to shallow depths, but mineralization generally increases as the
water moves downgradient toward the structural basins. The
distribution of dissolved-solids concentrations in the St. Peter–
Prairie du Chien–Jordan aquifer (fig. 26) shows this increase.
Where the aquifer is at or near the land surface in southeastern Minnesota, northeastern Iowa, southern Wisconsin, the
Upper Peninsula of Michigan, and central Missouri, it contains
water with dissolved-solids concentrations of less than 500 milligrams per liter (the limit recommended for drinking water by
the U.S. Environmental Protection Agency) or less. Concentrations increase to more than 1,000 milligrams per liter in
western and southern Iowa, north-central Illinois, and along the
northwestern shore of Lake Michigan. Where the aquifer is
deeply buried and ground-water movement is almost stagnant
in parts of Missouri, concentrations are greater than 10,000
milligrams per liter. The lobe of water with low dissolved-solids concentrations that extends southward in central Iowa is
thought to represent unflushed subglacial meltwater that
moved into the aquifer during the Pleistocene Epoch from an
area of extremely high hydraulic head created by the weight
of the glacial ice. Dissolved solids in water from the Mount
Simon aquifer show the same trends as those in water from
the St. Peter–Prairie du Chien–Jordan aquifer.
Other layered sandstone aquifers that are exposed adjacent to domes and uplifts or that extend into large structural
basins or both are the Colorado Plateaus aquifers, the Denver
Basin aquifer system, the lower Tertiary aquifers, the Upper
Cretaceous aquifers, the Lower Cretaceous aquifers, the Wyoming Tertiary aquifers, the Mississippian aquifer of Michigan,
and the New York sandstone aquifers (fig. 4). The Rush
Springs, Central Oklahoma, and Ada–Vamoosa aquifers of
Oklahoma are small aquifers that contain water largely under
unconfined conditions and generally yield small amounts of
water. The Jacobsville aquifer on the Upper Peninsula of Michigan is also small, mostly unconfined, and yields little water;
however, the glacial deposits that cover this aquifer store precipitation and release it slowly into the underlying sandstone.
The Pennsylvanian aquifers that cover large parts of the eastcentral United States cap hills and plateaus, are poorly permeable, and yield water mostly from shallow fracture systems
and interbedded, cleated coals. The early Mesozoic basin
aquifers of the eastern part of the Nation occupy titled grabens
or half-grabens, are commonly interbedded with fine-grained
sediments and intruded by traprock, and generally yield only
small amounts of water.
95°
Figure 26. Dissolved-solids
concentrations in water from
the St. Peter–Prairie du Chien–
Jordan aquifer generally are
less than 500 milligrams per liter
where the aquifer crops out, but
increase greatly downdip where
the aquifer is deeply buried.
Figure 25. Water in the stacked
sandstone aquifers moves vertically
downward, then horizontally toward major
rivers, where it moves upward to discharge
as base flow. Regional flow, not shown here,
is perpendicular to the section and into deep,
confined parts of the aquifers.
90°
Duluth
M I N N E S O TA
85°
●
M I N N E S O TA
45°
A'
Minneapolis
●
★
Saint Paul
A
N. Branch Middle
Fork River
1,400
1100
1000
900
85
80
200
0
0
75
700
700
0
850
●
MICHIGAN
Milwaukee
★
Lansing
●
Sioux City
Detroit
●
Rockford
0
70
900
950
★
800
750
600
400
Madison
Mississippi River
750
1000
950
Little Cannon River
Cannon River
950
800
●
N. Fork Zumbro River
1200
1,000
Green Bay
WISCONSIN
650
1,200
A'
A
850
Feet
●
I O WA
Chicago
●
Des Moines
★
Davenport
800
●
Sea level
200
VERTICAL SCALE GREATLY EXAGGERATED
0
0
5
5
10
10
15 MILES
15 KILOMETERS
EXPLANATION
Surficial aquifer system
Upper carbonate aquifer
Cambrian–Ordivician aquifer system
Maquoketa confining unit
St. Peter–Prairie du Chien–Jordan aquifer
yy
;;
;;
yy
Modified from Delin and Woodward, 1984
INDIANA
ILLINOIS
40°
Indianapolis★
Springfield★
●
Kansas City
Eau Claire confining unit
Jefferson City
Mount Simon aquifer
★
●
Saint Louis
MISSOURI
Crystalline-rock aquifer
Water table
900
SCALE 1:7,500,000
Line of equal hydraulic potential—Interval
50 and 100 feet
0
Base modified from U.S.
Geological Survey digital
data, 1:2,000,000, 1972
St. Lawrence–Franconia confining unit
Direction of ground-water movement
25
50 MILES
0 25 50 KILOMETERS
Ironton–Galesville aquifer
A
A' Line of hydrogeologic section
Modified from Young, 1992b
EXPLANATION
Dissolved-solids concentration in water from
the St. Peter–Prairie du Chien–Jordan
aquifer, in milligrams per liter. Dashed
where appoximately located
500
1000
5000
10,000
Aquifer absent
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 and in Puerto Rico (fig. 4). 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 wellconnected 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.
Carbonaterock aquifers
Figure 27.
Dissolution along joints and bedding planes in
carbonate rocks forms extremely permeable conduits that conduct
large volumes of water through the almost impermeable
undissolved rock.
A11
Continued
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 carbonaterock 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.
39°
85°
83°
ey
all
tv
s
r
Ka
87°
K E N T U C K Y
89°
Mammoth Cave Area
37°
y
;
;
y
y;;;yy;;yyy;y;y;y;y;;yy;;;yy;;yyy;y;y;y;y;
;
y
y
;
;
y
;
y
;
y
;
y
;
y
;
y
y
;
y;;;yyy;y;;yy;;yy;y;y;y;;yy;;yy;y;y;
y;y; y;y; y;y;y;
;
y
;
y
;
y
;
y
;
y
;
y
y;y;y;y;y;y;y;y;
y; y; y;
CARBONATE-ROCK AQUIFERS—
;;;
;;
yy
;;yyy
yy
;;;
yyy
;;
yy
Spring
Natural bridge
Sinkholes
Sinking
stream
Blind valley
Spring
Shale
Limes
Swallow
hole
Sinkhole
Cavern
tone
NOT
Blind valley
TO S
CALE
Shal
Figure 28.
e
The dissolution of exposed limestone
forms characteristic land-surface features, such as sinkholes,
karst valleys, and sinking streams. Solution features are rare where the
limestone is covered with noncarbonate rocks. Few surface streams occur because
most of the precipitation is quickly routed into and through solution openings in the limestone.
Modified from Lloyd and Lyke, 1994
35°
85°
S O U T H
★Columbia
Atlanta
★
Birmingham
●
GE O R G I A
A L A B A M A
●
Tallahessee
★
Base modified from U.S.
Geological Survey digital
data, 1:2,000,000, 1972
F
Jacksonville●
L
O
30°
Gulf of Mexico
D
A
Joe Meiman, U.S. National Park Service,
Mammoth Cave, Ky.
Figure 29. Where solution openings in limestone are large and
well connected, they may form networks of cave passages such as
this one at Mammoth Cave. The openings can be enlarged at the
bottom as the limestone is eroded by sediment-laden streams
flowing through them.
●
Orlando
EXPLANATION
Tampa
●
Floridan aquifer system
Southwest South Carolina
Georgia
and
Northeast
and
Southeast
Florida
Northwest
Georgia
Florida
East-Central
Florida
West-Central
Florida
Miami ●
SCALE 1:7,500,000
0
50
0
50
100 MILES
100 KILOMETERS
Modified from
Miller, 1990
Upper confining unit
UPPER FLORIDAN AQUIFER
35°
Figure 32.
Boulder
Zone
80°
S O U T H
CA R O L I N A
A L A B A M A
GE O R G I A
F
L
n
ea
Oc
c
nti
18
28
00
24
00
00
1400
O
140
0
0
160
I
D
Gulf of Mexico
A
20
00
00
22
240
0
26
00
30
0
0
EXPLANATION
30 28
00 00
34
Area where Floridan aquifer system is thin
due to intergranular gypsum
00
340
0
3200
30
Thickness of Floridan aquifer system, in feet
340
600
3000
3200
0
1200
3000
1800
SCALE 1:7,500,000
2400
3000
0
0
No data
2000
Line of equal thickness of Floridan aquifer
system—Interval 200 feet
Fault—Vertical or nearly so. Bar and ball on
downthrown side
A
A' Line of hydrogeologic section—Section
shown in figure 33
50
50
00
00
1
24 800
00
40 0
600
0
160
R
yy
;;
30°
00
6 00
1000
30
600
0
60
8 0 000
10
32
400
400
600
1200
1200
1400
1600
0
180
0
60
0
0
6 0 00
80 10 0
1200 140
00
200
20
4
00
Base modified from U.S.
Geological Survey digital
data, 1:2,000,000, 1972
meable 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 dir ectly connected. Two caver nous 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.
1000
0
60
200
Modified from Miller, 1986
400
600
800
Atla
Lower confining unit
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-per-
yy
;;
;;
yy
0
20
A
00
LOWER FLORIDAN AQUIFER
Middle
confining
unit
00
Middle
semiconfining
unit
18
Middle
semiconfining
unit
85°
28
Middle
confining
unit
The thickness
of the Floridan aquifer system
varies considerably. Some of
the variations reflect major
warping of the Earth’s crust
during deposition of the
carbonate rocks that compose
the system and some reflect
faulting after the rocks were
deposited.
1600
FLORIDAN AQUIFER SYSTEM
25°
South
Florida
Fernandina
permeable
zone
A12
Atla
nti
c
Oc
Savannah ●
I
The Floridan
aquifer system can generally
be divided into an Upper Floridan
aquifer and a Lower Floridan
aquifer, separated in most places
by a less-permeable confining
unit. The Lower Floridan aquifer
locally contains cavernous zones
that are extremely permeable.
ea
n
★Montgomery
R
Figure 31.
CA R O L I N A
Augusta ●
Figure 30. The Floridan aquifer
system underlies all of Florida, large parts
of Georgia, and smaller areas in South
Carolina, Alabama, and Mississippi.
Panhandle
Florida
80°
Greenville
●
00
34
2800
2600
2800
100 MILES
100 KILOMETERS
3200
3400
25°
A'
0
300
Modified from
Miller, 1986
Charleston
CARBONATE-ROCK AQUIFERS—
Feet
500
Surficial aquifer system
Sea level
Biscayne aquifer
Upper confining unit of Floridan aquifer system
Local confing unit
2000
2500
Southeastern Coastal Plain aquifer system
4000
Fault—Arrows indicate relative vertical movement
4500
BR
AD
FO
RD
W
N
MA
TO
GE
OR
GE
0
HIGHLANDS
40
O
ST LUCIE
E
SB
OU
OR
DE SOTO
MARTIN
GLADES
20
40
CHARLOTTE
Lake
Okeechobee
60
LEE
HENDRY
PALM BEACH
40
BROWARD
COLLIER
Figure 34. Water in the Upper Floridan aquifer moves
regionally coastward from high areas on the potentiometric
surface. Withdrawals at major pumping centers had developed
deep cones of depression in several places by 1980.
Miami ●
MONROE
GH
Modified from Rosenau
and others, 1977
Florida has 27 first-magnitude springs that issue
from large solution openings in the Upper Floridan aquifer. These
openings are features of the karst topography that has developed
on the carbonate rocks of the aquifer where its upper confining unit
is thin or absent.
RIO
N
DO
EN
AL
LEN
DA
LE
HARDEE
INDIAN
RIVER
Figure 35.
WN
LA
NC
AS
TE
R
CL
W
EL
L
40
RD
FO
SUMTER
MBIA
COLU
FER
ION
AR
MBIA
BR
20
BE
SUMTER
MANATEE
O
CH
LLAS
INDIAN
RIVER
EE
OK
LL
PINE
BREVARD
POLK
Direction of ground-water movement
BREVARD
40
OSCEOLA
120
100
0
Potentiometric contour—Shows altitude at
which the water level in the Upper Floridan
aquifer would have stood in tightly cased
wells in 1980. Dashed where approximately
located. Contour interval, in feet, is variable.
Datum is sea level
TA
SO
HI
OSCEOLA
POLK
Tampa
●
RA
ORANGE
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SEMINOLE
●
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80
PASCO
100 KILOMETERS
0
Orlando
HERNANDO
SA
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60
LAKE
80
GH
LAKE
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28°
100 MILES
0
OU
CITRUS
Spring discharging 100 cubic feet
per second or more
CITRUS
OR
EXPLANATION
MARION
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120
PUTNAM
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VOLUSIA
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29°
20
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100 KILOMETERS
tic
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25 50 KILOMETERS
Modified from Miller, 1986
NBU
FRANKLIN
JACKSON
N
MO
LIBERTY
F
50 MILES
VERTICAL SCALE GREATLY EXAGGERATED
RTA
COLBERT
S
OHN
ST J
Base modified from U.S.
Geological Survey digital
data, 1:2,000,000, 1972
s
25
0
85°
LAUDERDALE
HAMILTON
MADISON
ck
Paleocene rocks
SPA
35°
SA
RO
LEON
ro
The Floridan aquifer system is thin in south-central
Georgia, where it consists of a single aquifer that contains local confining
units. The system thickens greatly in Florida, where it consists of
complexly interfingering, lens-shaped bodies of permeable and lesspermeable carbonate rocks. The line of the hydrogeologic section is
shown in figure 32.
NE
TO
BAY
83°
GADSDEN
ne
Figure 33.
ES
A
OS
AR
BIA
WALTON
ce
0
DIXIE
84°
Eo
3500
Contact of geologic formation—Dashed where
approximately located
82°
JACKSON
N
TO
NG
HI
AS
W
er
3000
Lower confining unit of Floridan aquifer system
85°
HOLMES
Middle Eocene rocks
w
Boulder Zone within Floridan aquifer
LIM
NT
AM
AL
Lo
Lower Floridan aquifer
BIA
SA
ESC
OK
Oligocene rocks
ks
oc
ks
er
roc
en
us
oc
eo
tac
Cre
1500
Miocene rocks
le
Pa
Middle confining unit
Post-Miocene rocks
Oligocene rocks
Upper Eocene rocks
s
ck
ro
ne
ce
leo
Pa
1000
AM
86°
500
Upper Floridan aquifer
Base modified from U.S.
Geological Survey digital
data, 1:2,000,000, 1972
87°
Miocene rocks
Floridan aquifer system
A
NT
SA
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 dissolvedsolids 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 dissolvedsolids 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.
ESC
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 southcentral 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
A'
Georgia
Florida
A
EXPLANATION
LA
Continued
DADE
25°
Modified from Bush
and Johnston, 1988
35°
85°
35°
80°
85°
80°
S O U T H
S O U T H
CA R O L I N A
A L A B A M A
GE O R G I A
Fernandina Beach
●
Public supply
(15 million gallons
per day)
L
●
O
n
Oc
c
nti
F
Base modified from U.S.
Geological Survey digital
data, 1:2,000,000, 1972
O
R
I
Gulf of Mexico
I
D
D
Gulf of Mexico
A
A
EXPLANATION
EXPLANATION
Net water-level decline in Upper
Floridan aquifer, in feet,
predevelopment to 1980
Dissolved-solids concentration in
water from Upper Floridan
aquifer, in milligrams per liter
Less than 10
Agricultural,
mining, and
public supply
(1 billion gallons
per day)
Less than 250
10 to 30
250 to 500
30 to 50
500 to 1,000
50 to 80
Greater than 1,000
Greater than 80
No reliable data
SCALE 1:7,500,000
0
Figure 36.
L
30°
R
Base modified from U.S.
Geological Survey digital
data, 1:2,000,000, 1972
F
30°
●
Atlantic
Fort Walton
Beach
c
Industrial and
public supply
(500 million gallons
per day)
●
Jesup
Brunswick
O
●
GE O R G I A
ea
Savannah
n
ea
Atla
A L A B A M A
CA R O L I N A
Regional declines of water levels in the Upper
Floridan aquifer have occurred in three areas as a result of large
withdrawals from the aquifer. Declines represent the difference
between predevelopment and 1980 water levels.
0
50
50
SCALE 1:7,500,000
100 MILES
100 KILOMETERS
Figure 37.
25°
Modified from Bush
and Johnston, 1988
Concentrations of dissolved solids in the Upper Floridan
aquifer increase coastward, primarily as a result of mixing of freshwater
with seawater. In aquifer outcrop areas and where the aquifer is thinly
confined, flow is vigorous and dissolved-solids concentrations are
generally less than 250 milligrams per liter.
0
0
50
50
100 MILES
100 KILOMETERS
25°
Modified from Johnston
and Bush, 1988
A13
SE
Mammoth Cave Plateau
Well B
Big Clifty Sandstone
Member of Golconda
Formation
Spring
Green
River
0
Figure 39.
Water levels in
wells completed in sandstone
and carbonate-rock aquifers
respond differently to recharge.
Changes in the water level in
well A, completed in sandstone,
show that the sandstone is
recharged quickly but drains
slowly. In contrast, changes in
the water level in well B, open
to underlying limestone
formations, show that recharge
to and discharge from the
carbonate-rock aquifer are rapid
due to large solution openings
in the limestone.
Pennyroyal Plain
Well A
Perched water
table
Sinkhole
Water table
Spring
Sinkhole
Swallow hole
Sinkhole
stone tone
Lime
es
Girkin vieve Lim
e
n
e
G
Ste
St Louis Limestone
and
stone
Lime mation
w
a
s
For
War
ayne
Fort P
NOT TO SCALE
Sandstone
and carbonaterock aquifers
Chattanooga Shale
37°
8
10
12
14
16
18
20
406
408
Well
B
410
412
414
416
418
EXPLANATION
Direction of ground-water movement
87°
Mammoth Cave
Plateau
6
420
83°
Pennyroyal
Plain
89°
A
4
Modified from Brown, 1966;
and Quinlan and others, 1983
39°
85°
Well
2
WATER LEVEL, IN FEET BELOW LAND SURFACE
Q
;;
QQ
;;
Q
;;
QQ
;
QQ
;;
QQ
;;
;;
;
Q
QQ
;
NW
PRECIPITATION,
IN INCHES
Figure 38. Large volumes
of water move rapidly from
sinkholes and swallow holes
through a well-developed network of solution cavities in the
St. Louis and Ste. Genevieve
Limestones to discharge at
springs or to the Green River.
The openings were formed by
dissolution of the limestones as
water moved along bedding
planes and fractures.
K E N T U C K Y
422
20
10
0J
DJ
1953
DJ
DJ
1955
1954
DJ
1956
D
1957
Modified from Brown, 1966
Mammoth Cave Area
SANDSTONE AND CARBONATEROCK AQUIFERS
Aquifers in sandstone and carbonate rocks are most widespread in the eastern half of the Nation, but also extend over
large areas of Texas and smaller areas in Oklahoma, Arkansas, Montana, Wyoming, and South Dakota (fig. 4). These
aquifers consist of interbedded sandstone and carbonate rocks;
the carbonate rocks are the most productive aquifers, whereas
the interbedded sandstones yield less water. The aquifers in
the Mammoth Cave area of Kentucky are examples of sandstone and carbonate-rock aquifers. The development of solution openings and karst topography in the limestones of the
Mammoth Cave area are discussed in the preceding section
of this report; the following section discusses the relations of
the sandstone and carbonate-rock aquifers in that area.
Movement of water through the unconfined and confined
parts of the limestone aquifers that underlie the Pennyroyal
Plain and Mammoth Cave Plateau is summarized in figure 38.
Where the St. Louis Limestone is exposed at the land surface
in the Pennyroyal Plain, water from surface streams enters underground solution cavities through swallow holes and sinkholes. Water also enters solution cavities in the Ste. Genevieve
and Girkin Limestones through sinkholes that have developed
in the Big Clifty Sandstone Member of the Golconda Formation that caps the Mammoth Cave Plateau. The sinkholes on
the plateau are collapse sinkholes that developed when the
sandstone cap collapsed into caves which formed in the underlying limestone. Many of the solution openings in the Girkin
and Ste. Genevieve Limestones are dry because they formed
when the erosional base level in the area was at a higher altitude. Water from the saturated solution openings in the Ste.
Genevieve and St. Louis Limestones discharges to the Green
River from springs in the river channel and valley walls. Large
quantities of water move rapidly to the river through the solution openings.
Water moves slowly through intergranular pore spaces
and small fractures in the Big Clifty Sandstone Member of the
Golconda Formation. Discontinuous layers of shale in the underlying Girkin Limestone (fig. 38) impede the downward
movement of water and create a perched water table from
which small springs discharge at the escarpments bounding
the Mammoth Cave Plateau.
The presence or absence of solution openings affects
aquifer recharge and discharge and is reflected by the water
levels in wells completed in different rock types. The water
level in well A (fig. 39A), completed in the Big Clifty Sandstone Member of the Golconda Formation (fig. 38), rises
quickly in response to seasonal increases in precipitation; after
the sudden rise, the water slowly drains from the aquifer and
the water level declines slowly. In contrast, the water level in
well B (fig. 39B), which is open to the St. Louis and Ste.
Genevieve Limestones, rises sharply only in response to heavy
rains. Following the abrupt rise, the water level in this well
declines quickly as the solution cavities penetrated by the well
are drained. The large openings allow rapid recharge and
equally rapid discharge during and immediately following
periods of intense precipitation. The transmissivity (rate at
which water moves through an aquifer) of the part of the rock
that contains solution openings is extremely high, whereas that
of the undissolved rock between the solution conduits generally is very low.
zones, and the thickness of the flow. The cooling rate is most
rapid when a basaltic lava flow enters water. The rapid cooling results in pillow basalt (fig. 40), in which ball-shaped
masses of basalt form, with numerous interconnected open
spaces at the tops and bottoms of the balls. Large springs that
discharge thousands of gallons per minute issue from pillow
basalt in the wall of the Snake River Canyon at Thousand
Springs, Idaho. Interflow zones are permeable zones that develop at the tops and bottoms of basalt flows (fig. 41). Fractures and joints develop in the upper and lower parts of each
flow, as the top and bottom of the flow cool while the center
of the flow remains fluid and continues to move, and some
vesicles that result from escaping gases develop at the top of
the flow. Few open spaces develop in the center of the flow
because it cools slowly. Thus, the flow center forms a dense,
low-permeability zone between two more permeable zones.
Thin flows cool more quickly than thick flows, and accordingly
the centers of thin flows commonly are broken and vesicular
like the tops and bottoms of the flows.
The Snake River Plain regional aquifer system in southern Idaho and southeastern Oregon (fig. 42) is an example
of an aquifer system in basaltic rocks. The Snake River Plain
is a large graben-like structure that is filled with basalt of Miocene and younger age (fig. 43). The basalt consists of a large
number of flows, the youngest of which was extruded about
2,000 years ago. The maximum thickness of the basalt, as estimated by using electrical resistivity surveys, is about 5,500
feet. The basalt is bounded at the margins of the plain by silicic volcanic and intrusive rocks that are downwarped toward
the plain.
BASALTIC- AND OTHER
VOLCANIC-ROCK AQUIFERS
Basaltic
and other
Volcanic–rock
aquifers
R.L. Whitehead 1980
Figure 40.
Pillow basalt forms when basaltic lava enters water
and cools quickly. Extensive interconnected pore spaces develop in
the flow as the ball-shaped pillows cool.
Aquifers in basaltic and other volcanic rocks are widespread in Washington, Oregon, Idaho, and Hawaii, and extend
over smaller areas in California, Nevada, and Wyoming (fig.
4). Volcanic rocks have a wide range of chemical, mineralogic,
structural, and hydraulic properties. The variability of these
properties is due largely to rock type and the way the rock was
ejected and deposited. Pyroclastic rocks, such as tuff and ash
deposits, might be emplaced by flowage of a turbulent mixture of gas and pyroclastic material, or might form as windblown deposits of fine-grained ash. Where they are unaltered,
pyroclastic deposits have porosity and permeability characteristics like those of poorly sorted sediments; where the rock
fragments are very hot as they settle, however, the pyroclastic material might become welded and almost impermeable.
Silicic lavas, such as rhyolite or dacite, tend to be extruded
as thick, dense flows and have low permeability except where
they are fractured. Basaltic lavas tend to be fluid and form thin
flows that have a considerable amount of primary pore space
at the tops and bottoms of the flows. Numerous basalt flows
commonly overlap and the flows commonly are separated by
soil zones or alluvial material that form permeable zones.
Basalts are the most productive aquifers of all volcanic rock
types.
The permeability of basaltic rocks is highly variable and
depends largely on the following factors: the cooling rate of
the basaltic lava flow, the number and character of interflow
120°
48°
C o l umbia
Snake River Plain regional
aquifer system
Ri
Flow top–
vesicular,
scoriaceous
and broken;
substantial
permeability
v er
EXPLANATION
Seattle
●
●
W A S H IN G T O N
Spokane
★Oly
mpia
SCALE 1:7,500,000
115°
0
0 25 50 KILOMETERS
Ri ve r
C o lumbi a
r
●
Portland
Yellowstone
National Park
Salem
★
fic
Paci
50 MILES
ve
Flow center–
dense, few
vesicles, most
fractures are
vertical; minimal permeability
25
Ri
Ocean
ake
Sn
Eugene●
I D A H O
O R E G O N
Boise
★
43°
iv e
r
S n ake
■
Flow bottom–
vesicular and
broken; substantial permeability
Modified from Whitehead, 1992
Thousand
Springs
Base modified from U.S. Geological Survey
digital data, 1:2,000,000, 1972
Figure 41.
A typical basalt flow contains zones of
varying permeability. Vesicular, broken zones at the top and
bottom of the flow are highly permeable, whereas the dense
center of the flow has minimal permeability.
A14
Figure 42. The Snake River Plain, which is located in
southern Idaho and southeastern Oregon, is underlain
by basalt aquifers.
●
Pocatello
R
Modified from Whitehead, 1992
SE
Mammoth Cave Plateau
Well B
Big Clifty Sandstone
Member of Golconda
Formation
Spring
Green
River
0
Figure 39.
Water levels in
wells completed in sandstone
and carbonate-rock aquifers
respond differently to recharge.
Changes in the water level in
well A, completed in sandstone,
show that the sandstone is
recharged quickly but drains
slowly. In contrast, changes in
the water level in well B, open
to underlying limestone
formations, show that recharge
to and discharge from the
carbonate-rock aquifer are rapid
due to large solution openings
in the limestone.
Pennyroyal Plain
Well A
Perched water
table
Sinkhole
Water table
Spring
Sinkhole
Swallow hole
Sinkhole
stone tone
Lime
es
Girkin vieve Lim
e
n
e
G
Ste
St Louis Limestone
and
stone
Lime mation
w
a
s
For
War
ayne
Fort P
NOT TO SCALE
Sandstone
and carbonaterock aquifers
Chattanooga Shale
37°
8
10
12
14
16
18
20
406
408
Well
B
410
412
414
416
418
EXPLANATION
Direction of ground-water movement
87°
Mammoth Cave
Plateau
6
420
83°
Pennyroyal
Plain
89°
A
4
Modified from Brown, 1966;
and Quinlan and others, 1983
39°
85°
Well
2
WATER LEVEL, IN FEET BELOW LAND SURFACE
Q
;;
QQ
;;
Q
;;
QQ
;
QQ
;;
QQ
;;
;;
;
Q
QQ
;
NW
PRECIPITATION,
IN INCHES
Figure 38. Large volumes
of water move rapidly from
sinkholes and swallow holes
through a well-developed network of solution cavities in the
St. Louis and Ste. Genevieve
Limestones to discharge at
springs or to the Green River.
The openings were formed by
dissolution of the limestones as
water moved along bedding
planes and fractures.
K E N T U C K Y
422
20
10
0J
DJ
1953
DJ
DJ
1955
1954
DJ
1956
D
1957
Modified from Brown, 1966
Mammoth Cave Area
SANDSTONE AND CARBONATEROCK AQUIFERS
Aquifers in sandstone and carbonate rocks are most widespread in the eastern half of the Nation, but also extend over
large areas of Texas and smaller areas in Oklahoma, Arkansas, Montana, Wyoming, and South Dakota (fig. 4). These
aquifers consist of interbedded sandstone and carbonate rocks;
the carbonate rocks are the most productive aquifers, whereas
the interbedded sandstones yield less water. The aquifers in
the Mammoth Cave area of Kentucky are examples of sandstone and carbonate-rock aquifers. The development of solution openings and karst topography in the limestones of the
Mammoth Cave area are discussed in the preceding section
of this report; the following section discusses the relations of
the sandstone and carbonate-rock aquifers in that area.
Movement of water through the unconfined and confined
parts of the limestone aquifers that underlie the Pennyroyal
Plain and Mammoth Cave Plateau is summarized in figure 38.
Where the St. Louis Limestone is exposed at the land surface
in the Pennyroyal Plain, water from surface streams enters underground solution cavities through swallow holes and sinkholes. Water also enters solution cavities in the Ste. Genevieve
and Girkin Limestones through sinkholes that have developed
in the Big Clifty Sandstone Member of the Golconda Formation that caps the Mammoth Cave Plateau. The sinkholes on
the plateau are collapse sinkholes that developed when the
sandstone cap collapsed into caves which formed in the underlying limestone. Many of the solution openings in the Girkin
and Ste. Genevieve Limestones are dry because they formed
when the erosional base level in the area was at a higher altitude. Water from the saturated solution openings in the Ste.
Genevieve and St. Louis Limestones discharges to the Green
River from springs in the river channel and valley walls. Large
quantities of water move rapidly to the river through the solution openings.
Water moves slowly through intergranular pore spaces
and small fractures in the Big Clifty Sandstone Member of the
Golconda Formation. Discontinuous layers of shale in the underlying Girkin Limestone (fig. 38) impede the downward
movement of water and create a perched water table from
which small springs discharge at the escarpments bounding
the Mammoth Cave Plateau.
The presence or absence of solution openings affects
aquifer recharge and discharge and is reflected by the water
levels in wells completed in different rock types. The water
level in well A (fig. 39A), completed in the Big Clifty Sandstone Member of the Golconda Formation (fig. 38), rises
quickly in response to seasonal increases in precipitation; after
the sudden rise, the water slowly drains from the aquifer and
the water level declines slowly. In contrast, the water level in
well B (fig. 39B), which is open to the St. Louis and Ste.
Genevieve Limestones, rises sharply only in response to heavy
rains. Following the abrupt rise, the water level in this well
declines quickly as the solution cavities penetrated by the well
are drained. The large openings allow rapid recharge and
equally rapid discharge during and immediately following
periods of intense precipitation. The transmissivity (rate at
which water moves through an aquifer) of the part of the rock
that contains solution openings is extremely high, whereas that
of the undissolved rock between the solution conduits generally is very low.
zones, and the thickness of the flow. The cooling rate is most
rapid when a basaltic lava flow enters water. The rapid cooling results in pillow basalt (fig. 40), in which ball-shaped
masses of basalt form, with numerous interconnected open
spaces at the tops and bottoms of the balls. Large springs that
discharge thousands of gallons per minute issue from pillow
basalt in the wall of the Snake River Canyon at Thousand
Springs, Idaho. Interflow zones are permeable zones that develop at the tops and bottoms of basalt flows (fig. 41). Fractures and joints develop in the upper and lower parts of each
flow, as the top and bottom of the flow cool while the center
of the flow remains fluid and continues to move, and some
vesicles that result from escaping gases develop at the top of
the flow. Few open spaces develop in the center of the flow
because it cools slowly. Thus, the flow center forms a dense,
low-permeability zone between two more permeable zones.
Thin flows cool more quickly than thick flows, and accordingly
the centers of thin flows commonly are broken and vesicular
like the tops and bottoms of the flows.
The Snake River Plain regional aquifer system in southern Idaho and southeastern Oregon (fig. 42) is an example
of an aquifer system in basaltic rocks. The Snake River Plain
is a large graben-like structure that is filled with basalt of Miocene and younger age (fig. 43). The basalt consists of a large
number of flows, the youngest of which was extruded about
2,000 years ago. The maximum thickness of the basalt, as estimated by using electrical resistivity surveys, is about 5,500
feet. The basalt is bounded at the margins of the plain by silicic volcanic and intrusive rocks that are downwarped toward
the plain.
BASALTIC- AND OTHER
VOLCANIC-ROCK AQUIFERS
Basaltic
and other
Volcanic–rock
aquifers
R.L. Whitehead 1980
Figure 40.
Pillow basalt forms when basaltic lava enters water
and cools quickly. Extensive interconnected pore spaces develop in
the flow as the ball-shaped pillows cool.
Aquifers in basaltic and other volcanic rocks are widespread in Washington, Oregon, Idaho, and Hawaii, and extend
over smaller areas in California, Nevada, and Wyoming (fig.
4). Volcanic rocks have a wide range of chemical, mineralogic,
structural, and hydraulic properties. The variability of these
properties is due largely to rock type and the way the rock was
ejected and deposited. Pyroclastic rocks, such as tuff and ash
deposits, might be emplaced by flowage of a turbulent mixture of gas and pyroclastic material, or might form as windblown deposits of fine-grained ash. Where they are unaltered,
pyroclastic deposits have porosity and permeability characteristics like those of poorly sorted sediments; where the rock
fragments are very hot as they settle, however, the pyroclastic material might become welded and almost impermeable.
Silicic lavas, such as rhyolite or dacite, tend to be extruded
as thick, dense flows and have low permeability except where
they are fractured. Basaltic lavas tend to be fluid and form thin
flows that have a considerable amount of primary pore space
at the tops and bottoms of the flows. Numerous basalt flows
commonly overlap and the flows commonly are separated by
soil zones or alluvial material that form permeable zones.
Basalts are the most productive aquifers of all volcanic rock
types.
The permeability of basaltic rocks is highly variable and
depends largely on the following factors: the cooling rate of
the basaltic lava flow, the number and character of interflow
120°
48°
C o l umbia
Snake River Plain regional
aquifer system
Ri
Flow top–
vesicular,
scoriaceous
and broken;
substantial
permeability
v er
EXPLANATION
Seattle
●
●
W A S H IN G T O N
Spokane
★Oly
mpia
SCALE 1:7,500,000
115°
0
0 25 50 KILOMETERS
Ri ve r
C o lumbi a
r
●
Portland
Yellowstone
National Park
Salem
★
fic
Paci
50 MILES
ve
Flow center–
dense, few
vesicles, most
fractures are
vertical; minimal permeability
25
Ri
Ocean
ake
Sn
Eugene●
I D A H O
O R E G O N
Boise
★
43°
iv e
r
S n ake
■
Flow bottom–
vesicular and
broken; substantial permeability
Modified from Whitehead, 1992
Thousand
Springs
Base modified from U.S. Geological Survey
digital data, 1:2,000,000, 1972
Figure 41.
A typical basalt flow contains zones of
varying permeability. Vesicular, broken zones at the top and
bottom of the flow are highly permeable, whereas the dense
center of the flow has minimal permeability.
A14
Figure 42. The Snake River Plain, which is located in
southern Idaho and southeastern Oregon, is underlain
by basalt aquifers.
●
Pocatello
R
Modified from Whitehead, 1992
BASALTIC- AND OTHER VOLCANIC-ROCK AQUIFERS—Continued
Pliocene and younger basaltic-rock aquifers are the most
productive aquifers in the Snake River Plain. The saturated
thickness of the Pliocene and younger basaltic rocks is locally
greater than 2,500 feet in parts of the eastern Snake River Plain
but is much less in the western plain (fig. 44). Aquifers in
Miocene basaltic rocks underlie the Pliocene and younger
basaltic-rock aquifers (fig. 43), but the Miocene basaltic-rock
aquifers are used as a source of water only near the margins
of the plain. Unconsolidated-deposit aquifers are interbedded
with the basaltic-rock aquifers, especially near the boundaries
of the plain. The unconsolidated deposits consist of alluvial
material or soil that developed on basaltic rock, or both, and
were subsequently covered by another basalt flow.
The Pliocene and younger basaltic-rock aquifers consist
primarily of thin basalt flows with minor beds of basaltic ash,
cinders, and sand. The basalts were extruded as lava flows
from numerous vents and fissures which are concentrated
along faults or rift zones in the Snake River Plain. Some flows
spread outward for as much as 50 miles from the vent or fissure from which the flow issued. Shield volcanoes formed
around some of the larger vents and fissures (fig. 45). Flows
that were extruded from the volcanoes formed a thick complex of interbedded basalt.
Water in the Snake River Plain aquifer system occurs
mostly under unconfined (water-table) conditions. The configuration of the regional water table of the aquifer system (fig.
46) generally parallels the configuration of the land surface of
the plain. The altitude of the water table is greatest in Fremont County, Idaho, near the eastern border of the plain and
least in the Hells Canyon area along the Idaho–Oregon border. Where the water-table contours bend upstream as they
cross the Snake River (for example, near Twin Falls, Idaho),
the aquifer system is discharging to the river. In a general way,
the spacing between the contours reflects changes in the geologic and hydrologic character of the aquifer system. Widely
spaced contours in the Eastern Plain indicate more permeable or thicker parts of the aquifer system, whereas closely
spaced contours in the Western Plain indicate less permeable
or thinner parts. Water levels in the areas where shallow
aquifers or perched water bodies overlie the regional aquifer
system (fig. 46) are higher than those in the aquifer system.
These areas are underlain by rocks that have extremely low
permeability.
Other basalt aquifers are the Hawaii volcanic-rock aquifers, the Columbia Plateau aquifer system, the Pliocene and
younger basaltic-rock aquifers, and the Miocene basaltic-rock
aquifers. Volcanic rocks of silicic composition, volcaniclastic
rocks, and indurated sedimentary rocks compose the volcanic- and sedimentary-rock aquifers of Washington, Oregon,
Idaho, and Wyoming. The Northern California volcanic-rock
aquifers consist of basalt, silicic volcanic rocks, and
volcaniclastic rocks. The Southern Nevada volcanic-rock aquifers consist of ash-flow tuffs, welded tuffs, and minor flows of
basalt and rhyolite.
EXPLANATION
Unconsolidated-deposit aquifers
Pliocene and younger basaltic-rock aquifers
N
Miocene basaltic-rock aquifers
ort
Litt
h
le W
o
o
Rive d
r
Big
W
Riveood
r
Des
ert u
plan
d
Silicic volcanic rocks
Fault—Arrows show relative direction of
movement
Sna
Riv ke
er
Ag
ricu
ltur
Spr
al l
and
ing
Ag
s
ricu
lanltural
d
Salm
Cre on Fal
ek
ls
NO
TT
OS
Sp
rin
gs
CA
LE
So
uth
Figure 43. Basalt of Miocene
and younger age fills the graben-like trough
on which the Snake River Plain has formed. Lowpermeability, silica-rich volcanic rocks bound the basalt,
which is locally interbedded with unconsolidated deposits.
114°
112°
118°
WASHINGTON
116°
CLARK
difi
ed
fro
mW
hite
hea
FREMONT
d, 1
994
E
CUSTER
Mo
TT
JEFFERSON MADISON
CA
N
TO
TE
PA
YE
BOISE
GEM
EXPLANATION
CAMAS
I D A H O
ELMORE
JEROME
r
ve
K
OC
NN
BA
ORE GON
500
POWER
CARIBOU
CASSIA
Rift zone
2,500
BEAR
LAKE
Low shield
FR
A
ONEIDA
42°
Base modified from U.S.
Geological Survey digital
data, 1:2,000,000, 1972
1,500
2,000
LIN
OWYHEE
TWIN
FALLS
Low shield with
pit crater
1,000
Ri
ke
BINGHAM
LINCOLN
IDOK
A
S na
ING
OD
GO
MALHEUR
Saturated thickness of
Pliocene and younger
basaltic rocks, in feet
BONNEVILLE
BLAINE
MIN
ON
NY
BUTTE
ADA
NK
44°
Absent
Ma
jor
Modified from Whitehead, 1992
lava
tub
SCALE 1:4,000,000
Figure 44.
0
The saturated thickness of Pliocene and younger
basaltic rocks is locally greater than 2,500 feet in the eastern
Snake River Plain but is much less in the western plain.
25
0
25
Fissu
r
flow e
e fl
ow
50 MILES
50 KILOMETERS
Bu
low ried
NO
TT
shi
OS
eld
CA
LE
Fee
d
tub er
e
114°
Figure 45.
Basaltic lava
that was extruded from numerous
overlapping shield volcanoes in southern
Mo
difi
ed
Idaho has formed a thick complex of overlapping
fro
mW
hith
flows. Most flows issued from a central vent or fissure,
ead
, 19
and some are associated with large rift zones in the Earth’s crust.
94
112°
118°
WASHINGTON
2100
116°
580
500 0
0
CLARK
CUSTER
E
50
TT
YE
26
00
IN
K
KL
CASSIA
ONEIDA
42°
Base modified from U.S.
Geological Survey digital
data, 1:2,000,000, 1972
Figure 46.
BEAR
LAKE
AN
0
270
TWIN
FALLS
4000
OC
POWER
3800
Area where local aquifers or perched water
bodies overlie regional aquifer system
CARIBOU
NN
Ri
ve
BA
00
43
Twin Falls
r
0
0
JEROME
OWYHEE
EXPLANATION
BINGHAM
440
410
00
39
ORE GON
00
2800
30
00
2600
LINCOLN
00 0
37 360
00
34
ke
0
420
00
40
S na
34
ING
OD 3200
GO
00
00
32
28
I D A H O
ELMORE
Multiple basalt flows
BONNEVILLE
MIN
IDOK
A
N
24
00
Most recent basalt flow—
Contains some lava tubes
0
BLAINE
CAMAS
0
45 0
Water-table contour—Shows altitude of
regional water table during spring 1980.
Contour interval, in feet, is variable.
Datum is sea level
Direction of ground-water movement
FR
PA
YO
BUTTE
ADA
EXPLANATION
N
TO
22020200
4
4 700
6 00
ON MADISON
JEFFERS
00
l
TE
N
00CA
23
24
4800
BOISE
GEM
250
MALHEUR
00
44°
FREMONT
a
on
si res
n
Te ctu
fra
Modified from Whitehead, 1992
SCALE 1:4,000,000
The regional movement of water in the Snake River
Plain aquifer system is from east to west. Much of the discharge
from the aquifer system is to the Snake River. Low-permeability
rocks underlie shallow local aquifers or perched water bodies.
0
0
25
25
50 MILES
50 KILOMETERS
Anderson, T.W., Welder, G.E., Lesser, Gustavo, and Trujillo, A., 1988, Region
7, Central alluvial basins, in Back, William, Rosenshein, J.S., and Seaber,
P.R., eds, Hydrology: Geological Society of America, The Geology of
North America, v. 0–2, p. 81–86.
Bailey, Z.C., Greeman, T.K., and Crompton, E.J., 1985, Hydrologic effects of
ground- and surface-water withdrawals in the Howe area, LaGrange
County, Indiana: U.S. Geological Survey Water-Resources Investigations
Report 85–4163, 130 p.
Barker, R.A., and Pernik, Maribeth, 1994, Regional hydrology and simulation
of deep ground-water flow in the Southeastern Coastal Plain aquifer
system in Mississippi, Alabama, Georgia, and South Carolina: U.S.
Geological Survey Professional Paper 1410–C, 87 p.
Brown, R.F., 1966, Hydrology of the cavernous limestones of the Mammoth
Cave area, Kentucky: U.S. Geological Survey Water-Supply Paper 1837,
64 p.
Bush, P.W., and Johnston, R.H., 1988, Ground-water hydraulics, regional flow,
and ground-water development of the Floridan aquifer system in Florida
and in parts of Georgia, South Carolina, and Alabama: U.S. Geological
Survey Professional Paper 1403–C, 80 p.
Daniel, C.C., III, and Sharpless, N.B., 1983, Ground-water supply potential and
procedures for well-site selection, upper Cape Fear River basin: North
Carolina Department of Natural Resources and Community Development, 73 p.
Delin, G.N., and Woodward, D.G., 1984, Hydrogeologic setting and the
potentiometric surfaces of regional aquifers in the Hollandale embayment,
southeastern Minnesota, 1970–80: U.S. Geological Survey WaterSupply Paper 22–19, 56 p.
Dugan, J.T., McGrath, Timothy, and Zelt, R.B., 1994, Water-level changes in
the High Plains aquifer—Predevelopment to 1992: U.S. Geological
Survey Water-Resources Investigations Report 94–4027, 56 p.
Gutentag, E.D., Heimes, F.J., Kroethe, N.C., Luckey, R.R., and Weeks, J.B.,
1984, Geohydrology of the High Plains aquifer in parts of Colorado,
Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas, and
Wyoming: U.S. Geological Survey Professional Paper 1400–B, 63 p.
Johnston, R.H., and Bush, P.W., 1988, Summary of the hydrology of the
Floridan aquifer system in Florida and in parts of Georgia, South Carolina,
and Alabama: U.S. Geological Survey Professional Paper 1403–A, 24 p.
Lloyd, O.B., Jr., and Lyke, W.L., 1994, Ground Water Atlas of the United
States—Segment 10: Illinois, Indiana, Kentucky, Ohio, Tennessee: U.S.
Geological Survey Hydrologic Investigations Atlas HA–730–K, 30 p.
Miller, J.A., 1986, Hydrogeologic framework of the Floridan aquifer system in
Florida and in parts of Georgia, Alabama, and South Carolina: U.S.
Geological Survey Professional Paper 1403–B, 91 p.
—1990, Ground Water Atlas of the United States—Segment 6: Alabama,
Florida, Georgia, and South Carolina: U.S. Geological Survey Hydrologic
Investigations Atlas HA–730–G, 28 p.
—1992, Summary of the hydrology of the Southeastern Coastal Plain aquifer
system in Mississippi, Alabama, Georgia, and South Carolina: U.S.
Geological Survey Professional Paper 1410–A, 38 p.
Miller, J.A., and Renken, R.A., 1988, Nomenclature of regional hydrogeologic
units of the Southeastern Coastal Plain aquifer system: U.S. Geological
Survey Water-Resources Investigations Report 87–4202, 21 p.
Morrissey, D.J., 1983, Hydrology of the Little Androscoggin River Valley
aquifer, Oxford County, Maine: U.S. Geological Survey Water-Resources
Investigations Report 83–4018, 79 p.
Olcott, P.G., 1992, Ground Water Atlas of the United States—Segment 9:
Iowa, Michigan, Minnesota, Wisconsin: U.S. Geological Survey
Hydrologic Investigations Atlas HA–730–J, 31 p.
Quinlan, J.F., Ewers, J.O., Ray, J.A., Powell, R.L., and Krothe, N.C., 1983,
Groundwater hydrology and geomorphology of the Mammoth Cave
region, Kentucky, and of the Mitchell Plain, Indiana: Indiana Geology
Survey, Field Trips in Midwestern Geology, v. 2, p. 1–85.
Rosenau, J.C., Faulkner, G.L., Hendry, C.W., Jr., and Hull, R.W., 1977, Springs
of Florida: Florida Department of Natural Resources, Bureau of Geology
Bulletin 31 (revised), 461 p.
Spieker, A.M., 1968, Ground-water hydrology and geology of the lower Great
Miami River valley, Ohio: U.S. Geological Survey Professional Paper
605–A, 37 p.
Sun, R.J., and Johnston, R.H., 1994, Regional Aquifer-System Analysis
program of the U.S. Geological Survey, 1978–1992: U.S. Geological
Survey Circular 1099, 126 p.
Weeks, J.B., Gutentag, E.D., Heimes, F.J., and Luckey, R.R., 1988, Summary
of the High Plains regional aquifer-system analysis in parts of Colorado,
Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas, and
Wyoming: U.S. Geological Survey Professional Paper 1400–A, 30 p.
Whitehead, R.L., 1992, Geohydrologic framework of the Snake River Plain
regional aquifer system, Idaho and eastern Oregon: U.S. Geological
Survey Professional Paper 1408–B, 32 p.
—1994, Ground Water Atlas of the United States—Segment 7: Idaho, Oregon,
Washington: U.S. Geological Survey Hydrologic Investigations Atlas HA–
730–H, 31 p.
Young, H.L., 1992a, Summary of ground-water hydrology of the CambrianOrdovician aquifer system in the northern midwest, United States: U.S.
Geological Survey Professional Paper 1405–A, 55 p.
—1992b, Hydrogeology of the Cambrian-Ordovician aquifer system in the
northern midwest, United States, with a section on Ground-water quality
by D.I. Siegel: U.S. Geological Survey Professional Paper 1405–B, 99 p.
References
A15