U.S. Department of the Interior 

U.S. Geological Survey

Ground-Water Flow in the Gulf Coast 

Aquifer Systems, South-Central I 

United States I

Regional Aquifer-System Analysis

Professional Paper 1416-F

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To whom it may concern:

A short time ago, you received one or more copies of Professional Paper 1416-F, 

"Ground-Water Flow in the Gulf Coast Aquifer Systems, South-Central United States," 

by Alex K. Williamson and Hayes F. Grubb. Since that time, two errors were found on 

the title page that we believe should be corrected. "Coast" is misspelled and the report 

number is incorrect. Attached is a replacement page. Please replace the original title page 

with this new one. The information on the back of the title page is correct, so you can 

simply staple this new title page onto the original one. Thank you for your help with this 

matter.

GROUND-WATER FLOW IN THE GULF COAST 

AQUIFER SYSTEMS, SOUTH-CENTRAL 

UNITED STATES

By ALEX K. WILLIAMSON and HAVES F. GRUBB

REGIONAL AQUIFER-SYSTEM ANALYSIS 

GULF COASTAL PLAIN___________________________

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1416-F

GROUND-WATER FLOW IN THE GULF COST 

AQUIFER SYSTEMS, SOUTH-CENTRAL 

UNITED STATES

By ALEX K. WILLIAMSON and HAYES R GRUBB

REGIONAL AQUIFER-SYSTEM ANALYSIS 

GULF COASTAL PLAIN___________________________

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1415-B
U.S. DEPARTMENT OF THE INTERIOR 

GALE A, NORTON, Secretary

U.S. GEOLOGICAL SURVEY 

Charles G. Groat, Director

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by 

the U.S. Government

Reston, Virginia 2001

Library of Congress Cataloging- in-Publication Data

Williamson, Alex K.

Ground-water flow in the Gulf Coast aquifer systems, south central United States / by Alex K. Williamson and Hayes F. Grubb. 

p. cm. (Regional aquifer-system analysis -Gulf coastal plain) (U.S. Geological Survey professional paper: 1416-F) 

Includes bibliographical references.

Supt.ofDoc.no.: I 19.16:1416-F 

1. Groundwater flow-Gulf Coast (U.S.) 2. Aquifers-Gulf Coast (U.S.) I. Grubb, Hayes F. H. Title. HI. Series. IV. Series:

U.S. Geological Survey professional paper: 1416-F.

GB1197.7.W57 1998 97-34922 

551.49'0976-dc21 CIP

ISBN 0-607-97120-7

For sale by the U.S. Geological Survey 

Information Services, Box 25286, 

Federal Center, Denver, CO 80225-0286
FOREWORD

THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM

The Regional Aquifer-System Analysis (RASA) Program was started in 1978 

following a congressional mandate to develop quantitative appraisals of the major groundwater systems of the United States. The RASA Program represents a systematic effort to 

study a number of the Nation's most important aquifer systems, which in aggregate 

underlie much of the country and which represent an important component of the Nation's 

total water supply. In general, the boundaries of these studies are identified by the 

hydrologic extent of each system and accordingly transcend the political subdivisions to 

which investigations have often arbitrarily been limited in the past. The broad objective 

for each study is to assemble geologic, hydrologic, and geochemical information, to 

analyze and develop an understanding of the system, and to develop predictive capabilities 

that will contribute to the effective management of the system. The use of computer 

simulation is an important element of the RASA studies, both to develop an understanding 

of the natural, undisturbed hydrologic system and the changes brought about in it by 

human activities, and to provide a means of predicting the regional effects of future 

pumping or other stresses.

The final interpretive results of the RASA Program are presented in a series of U.S. 

Geological Survey Professional Papers that describe the geology, hydrology, and 

geochemistry of each regional aquifer system. Each study within the RASA Program is 

assigned a single Professional Paper number, and where the volume of interpretive 

material warrants, separate topical chapters that consider the principal elements of the 

investigation may be published. The series of RASA interpretive reports begins with 

Professional Paper 1400 and thereafter will continue in numerical sequence as the 

interpretive products of subsequent studies become available.

Charles G. Groat 

Director

CONTENTS

Page

Foreword..................................................................................................... IV

Abstract....................................................................................................... Fl

Introduction................................................................................................. 2

Purpose and scope................................................................................ 5

Acknowledgments............................................................................... 5

Related investigations.......................................................................... 5

Adjacent Regional Aquifer-System Analysis studies................. 5

Edwards-Trinity................................................................... 5

Central Midwest................................................................... 5

Southeastern Coastal Plain................................................... 5

Floridan................................................................................ 6

Gulf Coast RASA subregional studies........................................ 6

Previous investigations................................................................ 6

Physical setting.................................................................................... 7

Topography................................................................................. 7

Climate........................................................................................ 7

Surface water............................................................................... 11

Geohydrologic framework.......................................................... 12

Approach: Digital modeling of ground-water flow............................. 15

Factors controlling regional ground-water flow.......................................... 19

Topography.......................................................................................... 19

Estimation of predevelopment water-table

altitude.................................................................................. 19

Outcrop and subcrop pattern and geometry of aquifers,

permeable zones, and confining units......................................... 20

Hydraulic properties of aquifers, permeable zones, and

confining units............................................................................ 20

Horizontal hydraulic conductivity of aquifers and

permeable zones................................................................... 20

Horizontal hydraulic conductivity

of sand beds................................................................. 24

Effective horizontal hydraulic conductivity of

regional aquifers........................................................... 24

Vertical hydraulic conductivity of

fine-grained sediments......................................................... 28

Depth dependence of hydraulic conductivity.............................. 30

Brines and density................................................................................ 34

Estimation of density.................................................................. 37

Hydraulic head and flow hi variable-density aquifers......................... 41

Effects of density......................................................................... 43

Formation of brine waters........................................................... 46

Leakage at base of flow system........................................................... 47

Flow across Midway confining unit............................................ 47

Leakage from the geopressured zone and effect

of deep fluid injection........................................................... 49

Predevelopment ground-water flow............................................................ 49

Hydraulic head and horizontal flow..................................................... 49

Recharge and discharge budget........................................................... 51

Distribution of recharge and discharge................................................ 53

Vertical flow between layers................................................................ 55

Changes due to ground-water development................................................ 58

Ground-water pumpage....................................................................... 59

Water-level changes............................................................................. 59

Hydraulic head and horizontal flow............................................................ F68

Mississippi River Valley alluvial aquifer and 

permeable zone A (Holocene-upper 

Pleistocene deposits)............................................................ 69

Permeable zone B (lower Pleistocene-upper

Pliocene deposits)................................................................. 69

Permeable zone C (lower Pliocene-upper

Miocene deposits)................................................................ 70

Permeable zone D (middle Miocene deposits)........................... 70

Permeable zone E (lower Miocene-upper

Oligocene deposits).............................................................. 70

Upper Claiborne aquifer.............................................................. 70

Middle Claiborne aquifer............................................................ 71

Lower Claiborne-upper Wilcox aquifer...................................... 71

Middle Wilcox aquifer................................................................ 72

Lower Wilcox aquifer................................................................. 72

Regional recharge and discharge................................................................ 72

Distribution of regional recharge and discharge.................................. 75

Vertical flow between layers................................................................ 75

Aquifer storage.................................................................................... 77

Land subsidence.......................................................................... 78

Ground-water velocity and potential for saline-water

intrusion...................................................................................... 78

Potential and limitations for future development........................................ 80

Ground-water management information using sensitivity to pumping

stress........................................................................................... 81

Winter Garden area.............................................................................. 84

Northeastern Texas area....................................................................... 96

Western embayment area..................................................................... 96

Mississippi Alluvial Plain area............................................................ 96

Eastern embayment area...................................................................... 97

Southern Texas area............................................................................. 97

Southeastern Texas area....................................................................... 97

Southwestern Louisiana area............................................................... 99

Eastern coastal lowlands area.............................................................. 100

Summary and conclusions.......................................................................... 100

Selected references...................................................................................... 103

Supplement: Simulation details.................................................................. Ill

Differences in simulation from preliminary report.............................. 112

Transient flow under elastic and inelastic

storage conditions................................................................. 112

Pumpage estimation............................................................. 112

Estimation of observed 1962-87

hydraulic head.............................................................. 113

Deep pressure data............................................................... 114

Areally variable hydraulic conductivity related

to sand percentage and depth............................................... 114

Approach to calibration and parameter

estimation............................................................................. 115
VI CONTENTS

Page Page

Method to estimate areal sensitivity to pumping for

development potential................................................................. Fl 15

Limitations of the simulation............................................................... 115

Recharge and discharge estimates ....................................... F121

Density............................................................................... 121

Boundaries.................................................................................. 122

Model fit...................................................................................... 115 I Lateral.................................................................................. 122

Summary of parameter-estimation

results........................................................................... 119

Scale effects................................................................................ 121

Top recharge and discharge boundary and

water-table storage....................................................... 122

Bottom no-flow boundary........................................................... 123

ILLUSTRATIONS

PLATES 1 and 7 are in pocket (back cover)

PLATE 1. Generalized sections showing geohydrologic units and simulated freshwater hydraulic-head profiles for predevelopment and 1987, and depthaveraged concentrations of dissolved solids at wells along the lines of section: A-A', B-B', C-C', D-D', E-E', and F-F' [in pocket]

2-7. Maps showing: Page

2. Simulated predevelopment freshwater hydraulic head and depth-averaged dissolved-solids concentrations:

A. Permeable zone A and Mississippi River Valley alluvial aquifer (layer 11) F124-133 

B. Permeable zone B (layer 10) 

C. Permeable zone C (layer 9) 

D. Permeable zone D (layer 8) 

E. Permeable zone E (layer 7) 

F. Upper Claiborne aquifer (layer 6) 

G. Middle Claibome aquifer (layer 5) 

H. Lower Claibome-upper Wilcox aquifer (layer 4) 

/. Middle Wilcox aquifer (layer 3) 

/. Lower Wilcox aquifer (layer 2)

3. Simulated predevelopment horizontal and vertical ground-water flow direction and magnitude, and

depth-averaged dissolved-solids concentration:

A. Permeable zone A and Mississippi River Valley alluvial aquifer (layer 11) 

B. Permeable zone B (layer 10) 

C. Permeable zone C (layer 9) 

D. Permeable zone D (layer 8) 

E. Permeable zone E (layer 7) 

F. Upper Claiborne aquifer (layer 6) 

G. Middle Claibome aquifer (layer 5) 

H. Lower Claibome-upper Wilcox aquifer (layer 4) 

/. Middle Wilcox aquifer (layer 3) 

/. Lower Wilcox aquifer (layer 2)

4. Simulated drawdown from predevelopment to 1987, and depth-averaged dissolved-solids concentrations:

A. Permeable zone A and Mississippi River Valley alluvial aquifer (layer 11) F144-153 

B. Permeable zone B (layer 10) 

C. Permeable zone C (layer 9) 

D. Permeable zone D (layer 8) 

E. Permeable zone E (layer 7) 

F. Upper Claiborne aquifer (layer 6) 

G. Middle Claibome aquifer (layer 5) 

H. Lower Claibome-upper Wilcox aquifer (layer 4) 

/. Middle Wilcox aquifer (layer 3) 

J. Lower Wilcox aquifer (layer 2)
CONTENTS VH

Page

5. Simulated freshwater hydraulic head, 1987:

A. Permeable zone A and Mississippi River Valley alluvial aquifer (layer 11) F154-163 

B. Permeable zone B (layer 10) 

C. Permeable zone C (layer 9) 

D. Permeable zone D Gayer 8) 

E. Permeable zone E Gayer 7) 

F. Upper Claiborne aquifer (layer 6) 

G. Middle Claiborne aquifer (layer 5) 

H. Lower Claiborne-upper Wilcox aquifer (layer 4) 

/. Middle Wilcox aquifer (layer 3) 

/. Lower Wilcox aquifer (layer 2)

6. Simulated 1985 ground-water flow directions and magnitudes and depth-averaged dissolved-solids

concentrations:

A. Permeable zone A and Mississippi River Valley alluvial aquifer (layer 11)

B. Permeable zone B (layer 10) Fl 64-173 

C. Permeable zone C (layer 9) 

D. Permeable zone D (layer 8) 

E. Permeable zone E (layer 7) 

F. Upper Claiborne aquifer (layer 6) 

G. Middle Claiborne aquifer (layer 5) 

H. Lower Claiborne-upper Wilcox aquifer (layer 4) 

/. Middle Wilcox aquifer (layer 3) 

/. Lower Wilcox aquifer (layer 2)

7. Estimated block-averaged and simulated 1982 freshwater hydraulic head: 

A. Permeable zone A and Mississippi River Valley alluvial aquifer (layer 11) 

B. Permeable zone B (layer 10) 

C. Permeable zone C (layer 9) 

D. Permeable zone D (layer 8)

E. Permeable zone E (layer 7) [in pocket] 

F. Upper Claiborne aquifer (layer 6) 

G. Middle Claiborne aquifer (layer 5) 

H. Lower Claiborne-upper Wilcox aquifer (layer 4) 

/. Middle Wilcox aquifer (layer 3) 

/. Lower Wilcox aquifer (layer 2) 

FIGURES 1-8. Maps showing: Page

1. Location of the Gulf Coast Regional Aquifer-System Analysis study area and adjacent Regional Aquifer-System

Analysis study areas............................................................................................................................................................................

2. Gulf coast regional aquifer systems and regional and subregional model areas.......................................................................................... 4

3. Generalized average land-surface altitude and outcrop of Vicksburg-Jackson confining unit..................................................................... 8

4. Mean annual precipitation, 1951-80, and mean monthly precipitation and pan evaporation at four locations........................................... 9

5. Mean annual pan evaporation, 1951-80.................................................................................................................................................... 10

6. Unit runoff, 1951-80.......................................................................

7. Location of structural features, salt domes, and geohydrologic sections..................................................................................................... 13

8. Altitude of the base of the aquifer systems, area of occurrence of geopressure, and geophysical well-logging locations.......................... 14

9. Diagram showing correlation of aquifer systems and geohydrologic units (model layers) with previously named units........................... 16

10. Sketches showing: A. vertical relations of the confining units, aquifers, and permeable zones;

andB. idealized representation of the geohydrologic units hi the regional flow model......................................................................... 18

11. Map showing outcrop and subcrop of aquifers, permeable zones, and confining units (model layers)...................................................... 21

12. Perspective diagrams showing extent and thickness of aquifers, permeable zones, confining units, and

saline/freshwater interface of: A. the Mississippi embayment and Texas coastal uplands aquifer systems, and

B. the coastal lowlands aquifer system.................................................................................................................................................... 22

13. Geographic areas used for comparisons of sand-bed hydraulic conductivities and flow summaries.......................................................... 25

14-16. Diagrams showing:

14. Geometric mean horizontal hydraulic conductivity of sand beds by layer and area, estimated from 

aquifer-test analyses and specific-capacity data of: A. the Mississippi embayment and Texas coastal 

uplands aquifer systems, andB. the coastal lowlands aquifer system....................................................................................................26

15. Continuity of sand beds and fine-grained (clay and silt) beds in hypothetical sections composed of

10- to 90-percent sand.............................................................................................................................................................................29

16. Model blocks with varying extents of coarse- and fine-grained beds: A. beds extend beyond model blocks; B. most

beds do not extend beyond model blocks; and C. extent of beds generally much smaller than model blocks....................................... 29

17. Graph showing effective hydraulic conductivity (Kgjj) as a function of the ratio of sand to total volume............................................... 30

18. Perspective diagrams showing sand percentage in each aquifer or permeable zone (layer) of the

gulf coast aquifer systems in: A. the Mississippi embayment and Texas coastal uplands aquifer system, and
VIE CONTENTS

B. the coastal lowlands aquifer system.................................................................................................................................................... 31

19. Graph showing decrease of sand-bed hydraulic conductivity with depth..................................................................................................33

20. Graph showing relation of horizontal and vertical effective hydraulic conductivity to sand proportion in the

gulf coast aquifer systems ....................................................................................................................................................................... 34

21. Perspective diagrams showing regional effective horizontal hydraulic conductivity in each aquifer and 

permeable zone (layer) of the gulf coast aquifer systems in: A. the Mississippi embayment and 

Texas coastal uplands aquifer system, and B. the coastal lowlands aquifer system................................................................................ 35

22. Graph showing frequency distribution of concentrations of dissolved solids in water from about 20,000 sand

beds in the gulf coast aquifers .................................................................................................................................................................37

23. Perspective diagrams showing distribution of dissolved-solids concentrations in: A. the Mississippi embayment

and Texas coastal uplands aquifer systems, and B. the coastal lowlands aquifer system........................................................................ 38

24. Section showing vertical distribution of dissolved-solids concentrations in sand beds thicker than 20 feet 

along section C-C .......................................................................................................................................^^

25. Graph showing density of water in the gulf coast aquifer systems as a function of concentration of

dissolved solids and depth.......................................................................................................................................................................42

26. Map showing simulated predevelopment hydraulic head of formation water in layer 8 (permeable zone D of the

coastal lowlands aquifer system) ............................................................................................................................................................44

27. Diagram showing perspective view of Darcy flow rates and directions in layers 8 through 11 in a

22,500-square-mile area around Baton Rouge, Louisiana...................................................................................................................... 45

28. Diagram showing idealized geometry of salt dome and caprock............................................................................................................... 47

29. Map showing location of salt domes and distribution of volume of dissolved solids from ground water

with concentrations greater than 35,000 milligrams per liter.................................................................................................................. 48

30. Diagram showing generalized local- and regional-scale ground-water flows............................................................................................52

31. Graph showing simulated predevelopment ground-water budgets, by area...............................................................................................53

32. Map showing simulated predevelopment net regional ground-water recharge and discharge, and

location of hydrographs........................................................................................................................................................................... 54

33. Diagram showing simulated vertical flow between layers and recharge and discharge for:

A. Predevelopment conditions; and B. 1987 conditions.......................................................................................................................... 56

34. Graph showing ground-water pumpage by use, 1960-85.......................................................................................................................... 59

35. Map showing ground-water pumpage, 1985.............................................................................................................................................. 60

36. Hydrographs showing simulated and observed block-averaged water levels in selected areas with substantial

drawdown, 1925-87: 

A. Memphis, Tennessee..........................................................................................................................^^

B. Grand Prairie, Arkansas.......................................................................................................................»^

C. Greenville, Mississippi ..........................................................................................................................................................................63

D. Pascagoula, Mississippi..........................................................................................................................^^

E. Gulfport, Mississippi..........................................................................................................................^^

F. Baton Rouge, I^uisiana....................................................................................................................................^

G. Alexandria, Louisiana...........................................................................................................................................................................65

H. Southeast Houston, Texas .....................................................................................................................................................................65

/.West Houston, Texas............................................................................................................................................................................... ^

J. Winter Garden area, Texas..................................................................................................................................................................... 66

37. Graph showing cumulative distributions of areas of each aquifer (layer) for predevelopment to 1987 simulated

drawdown levels......................................................................................................................................................................................67

38. Graph showing change in simulated regional ground-water budget from predevelopment to 1985.......................................................... 73

39. Map showing simulated 1985 net regional ground-water recharge and discharge ....................................................................................74

40. Graphs showing simulated 1985 ground-water budgets, by area...............................................................................................................76

41. Map showing estimated land subsidence and simulated land subsidence due to ground-water withdrawals............................................79

42-50. Graphs showing:

42. Changes in the Winter Garden area regional ground-water budget from predevelopment to 1985......................................................... F81

43. Changes in the northeastern Texas area regional ground-water budget from predevelopment to 1985..................................................... 82

44. Changes in the western embayment area regional ground-water budget from predevelopment to 1985................................................... 82

45. Changes in the Mississippi Alluvial Plain area regional ground-water budget from predevelopment to 1985......................................... 82

46. Changes in the eastern embayment area regional ground-water budget from predevelopment to 1985.................................................... 82

47. Changes in the southern Texas area regional ground-water budget from predevelopment to 1985........................................................... 83

48. Changes in the southeastern Texas area regional ground-water budget from predevelopment to 1985 .................................................... 83
CONTENTS EX

Page

49. Changes in the southwestern Louisiana area regional ground-water budget from predevelopment to 1985............................................. 83

50. Changes in the eastern coastal lowlands area regional ground-water budget from predevelopment to 1985............................................ 83

51-52. Maps showing:

51. Potential for additional development of the ground-water resource:

A. Permeable zone A and Mississippi River Valley alluvial aquifer (layer 11)......................................................................................... 85

B. Permeable zone B (layer 10)................................................................................................................................................................. 86

C. Permeable zone C (layer 9)................................................................................................................................................................... 87

D. Permeable zone D (layer 8)...................................................................................................................................................................88

E. Permeable zone E (layer 7).........................................................................»

F. Upper Claiborne aquifer (layer 6).......................................................................................................................................................... 90

G. Middle Claiborne aquifer (layer 5) .......................................................................................................................................................91

H. Lower Claiborne-upper Wilcox aquifer (layer 4).................................................................................................................................. 92

/. Middle Wilcox aquifer (layer 3)............................................................................................................................................................. 93

J. Lower Wilcox aquifer (layer 2).............................................................................................................................................................. 94

52. Areal locations of layers that are least sensitive to drawdown from increased pumpage.......................................................................... 95

53. Diagrams showing ground-water budget by model layer of the southeastern Texas area around Houston: 

A. Predevelopment, and B. 19S5..........................................................................................................^^^^^^

54-56. Graphs showing:

54. Comparison of simulated and estimated block-averaged head, 1980...................................................................................................... 118

55. Model fit as a function of simulation time, 1962-82, by layer and groups of layers............................................................................... 120

56. Results of parameter estimation by regression, comparing number of parameters with statistics of fit.................................................. 121

TABLES

TABLE 1. Summary of thickness, areal extent, and sand percentage of model layers.................................................................................................... F12

2. Comparison of hydraulic conductivity of sand beds (Ksan(j) by model calibration with mean estimated

Ksan(j from aquifer-test analyses and specific-capacity data by group of model layers and areas........................................................... 27

3. Summary of 1985 ground-water pumpage by area, type of withdrawal, and model layer............................................................................ 68

4. Summary of indicators of estimated land subsidence in the study area......................................................................................................... 80

5. Distribution of pumpage in maximum development simulation according to the estimated potential for

ground-water development......................................................................................................................................................................... 100

6. Comparison of simulated hydraulic head to estimated block-averaged head for 1980, in feet...................................................................... 116
X CONTENTS

CONVERSION FACTORS, VERTICAL DATUM, AND 

ABBREVIATED WATER-QUALITY UNITS

For readers who prefer to use metric units, conversion factors for terms used in this report are listed below: 

Multiply By To obtain

inch 25.40 millimeter

inch per year (in/yr) 25.40 millimeter per year

foot (ft) 0.3048 meter

mile (mi) 1.609 kilometer

square mile (mi2) 2.590 square kilometer

cubic mile (mi3) 4.168 cubic kilometer

million acre-feet 1,233 cubic hectometer

million gallons per day (Mgal/d) 0.04381 cubic meter per second

billion gallons per day (Ggal/d) 43.81 cubic meter per second

pounds per square inch per 6.895 kilopascal per meter 

foot [(Ib/in2)/ft]

Sea level: In this report, "sea level" refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929) a geodetic datum derived from a general 

adjustment of the first-order level nets of the United States and Canada, formerly called Sea Level Datum of 1929.

Chemical concentrations, water density, and temperatures are given in metric units. Chemical concentration is given in milligrams per liter (mg/L) or grams 

per liter (g/L). Milligrams per liter is a unit expressing the concentration of chemical constituents in solution as weight (milligrams) of solute per unit volume 

(liter) of water. For concentrations of dissolved solids less than 7,000 mg/L, the numerical value is the same as for concentrations in parts per million. Water 

density is given in grams per cubic centimeter (g/cm3). Water temperature is given in degrees Celsius ( C), which can be converted to degrees Fahrenheit ( F) by 

the following equation:

°F=1.8(°C) + 32
REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, 

SOUTH-CENTRAL UNITED STATES

BY ALEX K. WILLIAMSON AND HAYES F. GRUBB

ABSTRACT

The gulf coast regional aquifer systems comprise one of the largest, 

most complicated, and interdependent aquifer systems in the United 

States. The Gulf Coast Regional Aquifer-System Analysis (Gulf Coast 

RASA) study area encompasses approximately 230,000 square miles 

onshore in parts of Alabama, Arkansas, Florida, Illinois, Kentucky, 

Mississippi, Missouri, Tennessee, Texas, and all of Louisiana. The aquifer systems (and the study area) extend offshore beneath the Gulf of 

Mexico to include an additional 60,000 square miles and are truncated 

at the edge of the Continental Shelf. The Gulf Coast RASA study is limited to coastal plain sediments mostly of Cenozoic age except for the 

northernmost part of the area, where it includes Late Cretaceous rocks. 

The thickness of the aquifer system increases toward the Gulf of Mexico and exceeds 17,000 feet near the coastline of southeastern Louisiana. The shallower parts of the aquifers contain freshwater, but the 

deeper and offshore parts contain mostly mineralized water or brine.

Nearly 10 billion gallons per day of ground water was withdrawn 

from the aquifers in 1985; most of the water was for irrigation, but substantial quantities were used for municipal and industrial purposes. 

Three aquifer systems have been delineated in the Gulf Coast RASA 

study area: (1) the Mississippi embayment aquifer system, (2) the Texas 

coastal uplands aquifer system, and (3) the coastal lowlands aquifer 

system. Although all three aquifer systems are mixtures of fine- and 

coarse-grained clastic sediments deposited in continental and marine 

environments, the clay and silt beds in the coastal lowlands aquifer 

system are generally thinner, dispersed vertically throughout the aquifers, and not areally as extensive as in the two other aquifer systems.

The base of the gulf coast regional aquifer systems in the northern 

part of the area is at the top of the Midway confining unit and in the 

southern part is at the transition zone into geopressured sediments. 

Faults are common, but fault throws generally are not great enough to 

entirely offset the regional hydrogeologic units described in this report, 

although individual beds could be offset. Many salt domes occur in 

several basins of the gulf coast.

The gulf coast regional aquifer systems were divided into 10 aquifers and 5 regional confining units. Each of the aquifers was simulated 

as a separate layer in a three-dimensional, variable density, finite-difference ground-water flow model with 10-mile-grid spacing. The 

model accounted for inelastic compaction of fine-grained beds, resulting in more water being released from storage than would be released 

from coarse-grained sediments alone, and also resulting in land subsidence. Many of the aquifer characteristics, such as thickness, sand percentage, water density (based on concentration of dissolved solids), 

temperature, and pressure, were derived from a computerized file of 

989 geophysical logs.

The factors that controlled regional ground-water flow in the aquifer systems under predevelopment conditions (before 1925) are, in 

order of importance: (1) topography; (2) outcrop and subcrop pattern

and geometry of aquifers, permeable zones, and confining units; 

(3) variation of hydraulic properties of aquifers, permeable zones, and 

confining units; (4) distribution of density and brines; and (5) downdip 

limits of geohydrologic units and geologic structure. Topography is the 

most significant factor controlling ground-water flow because the relatively humid climate maintains the aquifer systems generally full to 

overflowing with ground water. Therefore, the amount and distribution of regional recharge and discharge is generally proportional to the 

topographic gradient and aquifer conductivity rather than being controlled by precipitation or other factors. The major variations in subsurface permeability have been accounted for by delineation of aquifers 

and confining units. Hydraulic conductivity of sand beds increases 

from the western side of the study area toward the eastern side. Effective hydraulic conductivity varies as a power function of sand percentage because as the sand percentage decreases, the degree of hydraulic 

connection among sand beds also decreases. Hydraulic conductivity 

also decreases with depth due to compaction.

About one-third of the volume of the aquifer systems studied contains freshwater; in the remaining two-thirds, saline water increases 

rapidly in concentration with distance, generally downdip, to more 

than 100,000 milligrams per liter. This increase causes large differences 

in density that have substantial effects on ground-water flow both in 

the saline and the freshwater part of the system. Many forces apparently have prevented the more saline or brine part of the system from 

reaching equilibrium, such as heat and salt sources (salt domes), sediment compaction, geopressured sediments, changes of sea level 

through geologic time, and an extensive continental shelf requiring 

long flow paths along which ground water must move to adjust to 

these forces. Thus, the primary source of brines most likely is dissolution of salt domes. Geologic structure seems to have a small effect on 

ground-water flow except for the indirect effect on the geometry of the 

geohydrologic units. The effect of the small amount of flow leaking up 

through the basal Midway confining unit, which is composed of 

marine clays several hundred feet thick, was found to be insignificant 

compared to the volume of flow in the aquifer above it. This also seems 

to be true for the small amounts of flow leaking up from the geopressured zone farther downdip toward the coastline.

Predevelopment net regional recharge was occurring in 41 percent 

of the aquifer system at the average rate of about 0.48 inch per year. 

The highest predevelopment net recharge rate was about 6 inches per 

year in southwestern Mississippi. Predevelopment net regional discharge was occurring in 59 percent of the area at the rate of about 0.35 

inch per year. The largest net regional discharge was in the Mississippi 

River Valley alluvial aquifer at about 4 inches per year. The distribution 

of freshwater (dissolved-solids concentration less than 10,000 milligrams per liter) thickness correlated well with the simulated net 

regional recharge and discharge, indicating that the regional flow has 

flushed saline water (dissolved-solids concentration greater than

Fl
F2 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

10,000 milligrams per liter) out of the sediments, creating a thicker section of freshwater. Some ground water flows offshore and is discharged 

to the ocean, but most is discharged before it reaches the coastline.

Major changes have occurred in the gulf coast regional aquifer systems in response to large-scale development of ground water throughout much of the study area. Development has led to lowering of 

hydraulic heads, changes in recharge and discharge, changes in flow 

and velocity both in direction and magnitude, land subsidence, and 

changes in water quality. Total pumpage increased by a factor of 2.5 

during 1960-80 but decreased by 7 percent from 1980 to 1985.

Every aquifer or permeable zone has more than one major cone of 

drawdown where drawdown exceeds 80 feet. The 1985 pumpage is 

more than three times the rate of predevelopment regional recharge. 

Ground-water pumpage and the resulting declines in hydraulic head 

tripled the regional recharge to more than 9 billion gallons per day. The 

increase in regional recharge was derived from both an increase in natural recharge and capture of water that was previously locally discharged. The regional discharge decreased to about 1 billion gallons 

per day, nearly one-third of its predevelopment rate. Less than 10 percent of the pumpage in 1985 was supplied from ground-water storage.

By 1985, the development caused the area of net regional recharge 

to increase from 41 percent to 66 percent of the study area. The area 

that changed from having net regional discharge to net regional 

recharge includes most of the area underlain by the Mississippi River 

Valley alluvial aquifer and most of the area underlain by the coastal 

lowlands within about 40 miles of the coastline except in southern 

Texas. Offshore, near Houston, Texas, there has been a change in the 

direction of flow from onshore areas to offshore areas during predevelopment, whereas in 1985, the simulated flow was from the offshore 

areas inland.

Downward vertical flows between layers increased substantially in 

all of the aquifers and permeable zones as a result of ground-water 

pumpage and resultant reversal of vertical gradients. The upper Claiborne aquifer continued to have the most flow interactions with the 

Mississippi River Valley alluvial aquifer, probably due to the large area 

of contact. The upper Claiborne aquifer has very little pumpage but 

has large inflows and outflows. The middle Wilcox, middle Claiborne, 

permeable zone D, and the Mississippi River Valley alluvial aquifer are 

mostly self supporting; in other words, most of their pumpage was 

supplied from increased recharge and decreased discharge within the 

layer itself. In the lower Wilcox aquifer and permeable zone E, which 

have very narrow outcrop bands, much of the pumpage had to be supplied by vertical flow from adjacent layers. The upper Claiborne aquifer, which has a very wide outcrop and subcrop band, captures 

additional recharge and discharges and transmits to adjacent aquifers.

Large-scale ground-water pumpage in the freshwater part of the 

aquifer systems has markedly changed flow patterns in the brine (dissolved-solids concentration greater than 35,000 milligrams per liter) 

part of the system and induced flow updip toward pumping areas. 

However, it will take decades or centuries for the brine water to reach 

these pumping areas because of the relatively slow ground-water 

velocities when compared to the long flow paths.

Simulation indicates that there is great potential for continued further development of the ground-water resource. By carefully designing 

the pattern of pumping additions, doubling the pumpage to 20 billion 

gallons per day could be accomplished with minimal impacts (average 

of less than 60 feet of drawdown over the study area).

INTRODUCTION

Smaller scale studies of parts of aquifer systems 

commonly are hampered by not knowing how the 

hydrology of the study area relates to the hydrology 

and development of adjacent areas and by not having 

enough data to do useful statistical analyses and comparisons. Hydrologic boundary conditions for smaller 

study areas, though important, generally are unknown. 

Commonly, data or time, or both, are insufficient to 

thoroughly study certain types of data or to consider 

different methods of analysis. The Regional AquiferSystem Analysis (RASA) Program, of which this study 

is a part, was designed to minimize these problems.

The RASA Program, which was started in 1978 by 

the U.S. Geological Survey, resulted from a congressional mandate to develop quantitative appraisals of 

the major ground-water systems of the United States 

(see Foreword). More than 20 major aquifer systems 

have been studied (Sun, 1986; Sun and Weeks, 1991). 

Major objectives of the RASA Program are to analyze 

and develop an understanding of the ground-water 

flow system on a regional scale and to develop predictive capabilities that will contribute to effective management of the system (Bennett, 1979). To reach these 

objectives, the use of computer simulation of groundwater flow in the aquifer system under natural, undisturbed conditions and conditions affected by human 

activities has proven to be an important tool.

The Gulf Coast RASA study area encompasses 

approximately 230,000 mi2 onshore in parts of Alabama, Arkansas, Florida, Illinois, Kentucky, Mississippi, 

Missouri, Tennessee, Texas, and all of Louisiana. The 

aquifer systems (and the study area) extend offshore 

beneath the Gulf of Mexico to include an additional 

60,000 mi2 and are truncated at the edge of the Continental Shelf. The study area and aquifer systems have 

been terminated at natural hydrologic boundaries 

rather than political boundaries.

The Gulf Coast RASA study is limited to coastal 

plain sediments of mostly Cenozoic age except for the 

northernmost part of the area, where it includes rocks of 

Late Cretaceous age. This report does not include the 

Upper Cretaceous rocks because they were the subject 

of detailed studies reported elsewhere (Brahana and 

Mesko, 1988). The aquifer systems thicken toward the 

Gulf of Mexico in a general wedge shape and have a 

thickness of more than 17,000 ft near the coastline of 

southeastern Louisiana. The shallower parts of the 

aquifers contain freshwater, but the deeper and offshore 

parts contain mostly highly mineralized water. The Gulf 

Coast RASA study area and its relation to adjacent 

RASA study areas is shown in figure 1. An overview of 

the aquifer systems and the plan of study for the Gulf 

Coast RASA were described by Grubb (1984,1985, and 

1987).

Ground water is an important resource in the study 

area though rainfall and surface water are relatively 

abundant. Nearly 10 Ggal/d of ground water was withdrawn from the aquifers during 1980 and 1985, mostly 

for irrigation. Public supply and industrial uses of
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F3

HAWAII

EXPLANATION

REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM Numbering system 

for identification purposes only, not intended to imply priority

Map not to scale

1 Northern Great Plains

2 High Plains

3 Central Valley, California

4 Northern Midwest

5 Southwest alluvial basins

6 Floridan aquifer system

7 Northern Atlantic Coastal Plain

8 Southeastern Coastal Plain

9 Snake River Plain

10 Central Midwest

11 Gulf Coastal Plain

12 Great Basin

13 Northeast glacial aquifers

14 Upper Colorado River Basin

15 Oahu Island, Hawaii

16 Caribbean Islands

17 Columbia Plateau Basalt

18 Michigan Basin

1 9 San Juan Basin

20 Edwards-Trinity aquifer system

21 Ohio-Indiana carbonates 

	and glacial deposits

22 Appalachian Valleys and Piedmont

23 Puget-Willamette Lowland

24 Southern California basins

25 Northern Rocky Mountains 

	Intermontane basins

St. Croix 

 16

CARIBBEAN ISLANDS

FIGURE 1. Location of the Gulf Coast Regional Aquifer-System Analysis study area 

and adjacent Regional Aquifer-System Analysis study areas.

ground water also are very important in the region. The 

effect of ground-water withdrawals has become 

regional in nature (Grubb, 1984, p. 1). Water-level 

declines with resultant increased pumping costs have 

spread across local and State political boundaries 

beyond the immediate vicinity of the irrigated areas, 

cities, and industrial areas where the water is pumped. 

Land subsidence and saltwater intrusion are significant 

side effects of ground-water-level declines in the region. 

Three aquifer systems have been delineated in the 

Gulf Coast RASA study area: (1) the Mississippi embayment aquifer system, (2) the Texas coastal uplands aquifer system, and (3) the coastal lowlands aquifer system 

(fig. 2) (Grubb, 1984). The delineation of these systems

is based on the geologic structure and sedimentation 

pattern, the separation of aquifer systems by significant 

regional confining units, and the presence of more than 

two significant regional aquifers within each aquifer 

system. The topography and outcrop pattern that control the regional ground-water flow in the Mississippi 

embayment have been substantially affected by the synclinal structure of the embayment. Although all three 

aquifer systems are mixtures of fine- and coarse-grained 

clastic sediments, the fine-grained beds in the coastal 

lowlands aquifer system are generally thinner, dispersed vertically throughout the aquifers, and not 

areally as extensive as in the other two aquifer systems.
EXPLANATION

REGIONAL 10-MILE AND SUBREGIONAL 5-MILE GRID BLOCKS

o

Texas

McNairy-Nacatoch Mississippi embayment 

Regional 10-mile grid

Mississippi River 

valley alluvium

_ _ _ . Coastal lowlands of 

Alabama, Florida, 

Louisiana, and 

Mississippi

Base from U.S. Geological Survey 

Scale 1:2,500,000

100 150 MILES

n i r

50 100 150 KILOMETERS

FIGURE 2. Gulf coast regional aquifer systems and regional and subregional model areas.
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F5

PURPOSE AND SCOPE

The purpose of this report is to present an analysis 

of the regional flow in the gulf coast regional aquifer 

systems from predevelopment through 1987, to develop 

an understanding of the controlling factors, and to evaluate the potential for further development. To accomplish this, regional flow paths were determined, and 

characteristics of the system were compared from one 

area or aquifer to another. The significant human effects 

on flow are explained. The effects of future development on ground-water flow and the related problems 

are presented.

Computer simulation of ground-water flow is the 

best available tool for this study to (1) integrate most of 

the known information about the aquifer system, (2) 

test hypotheses about the regional flow system, and (3) 

provide a common basis for comparing system characteristics from one area or aquifer to another. Therefore, 

most of the results presented in this report are based on 

digital simulations. The approach and model construction are summarized in this report. Owing to the complexity of the aquifer system and the amount of 

information, the details of the methods used in the 

modeling that have not been presented elsewhere (Williamson and others, 1990; Williamson, 1987) will be presented in the supplement "Simulation Details" at the 

end of this report.

ACKNOWLEDGMENTS

Text editing was done by Gertrude MacDonnell. 

Many of the figures were prepared or modified by April 

Wild and Matt Eli. We thank them for their willing 

assistance and accept the responsibility for the errors 

and omissions in the report.

RELATED INVESTIGATIONS

Numerous related investigations have been done 

owing to the size of the study area, the complexity of 

the aquifer systems, and the intensity of development 

of the ground-water resources in the area. There are 

four adjacent RASA studies as well as five subregional 

studies done as part of the Gulf Coast RASA study. 

Numerous investigations were made, but only those 

most closely related to this report are referenced here.

Several other reports from the Gulf Coast RASA 

study are closely related to this report. Results from a 

ground-water flow model developed by Kuiper (1994), 

which used nonlinear regression to estimate parameter 

values, were used extensively in the simulations presented in this report. The geohydrologic units of the

gulf coast regional aquifer systems are described by 

Weiss (1992) and Hosman and Weiss (1991), and the 

geology is discussed by Hosman (1993). The geochemistry of these aquifer systems is described by Pettijohn (in 

press), and the maps showing the concentration of dissolved solids and temperature are presented by Pettijohn and others (1988).

ADJACENT REGIONAL AQUIFER-SYSTEM ANALYSIS 

STUDIES

The Gulf Coast RASA study area is adjacent to four 

other regional aquifer systems with completed or ongoing studies under the RASA Program (fig. 1). They are 

the Edwards-Trinity, Central Midwest, Southeastern 

Coastal Plain, and Floridan aquifer systems.

EDWARDS-TRINITY

The Edwards-Trinity Regional Aquifer-System Analysis includes Cretaceous carbonate aquifers which dip 

underneath the gulf coast aquifers in Texas and adjacent 

States (Bush, 1986; Barker and others, in press). The vertical flow from or to the Edwards-Trinity aquifer system 

is thought to have a small effect on flow in the lower 

part of the Texas coastal uplands aquifer system owing 

to the great thickness of fine-grained marine sediments 

of the Midway Group, which separates the two aquifer 

systems.

CENTRAL MIDWEST

The Central Midwest Regional Aquifer-System 

Analysis includes the Paleozoic rocks west of the northern tip of the Gulf Coast RASA study area (Jorgensen 

and others, 1993; Imes and Emmett, 1994). These Paleozoic rocks extend beneath the Upper Cretaceous sediments in the Mississippi embayment but are not used 

for water supply because they are overlain by other 

more productive aquifers. The Paleozoic rocks are generally less permeable than the Upper Cretaceous sediments, but some water may flow upward from the 

Paleozoic rocks into the Upper Cretaceous sediments.

SOUTHEASTERN COASTAL PLAIN

The Southeastern Coastal Plain Regional AquiferSystem Analysis includes Cretaceous and Tertiary sediments east of the Gulf Coast RASA study area (Renken, 

1984; Barker and Pernik, 1994). The study areas overlap 

in western Alabama and eastern Mississippi; however, 

the Southeastern Coastal Plain RASA study is restricted 

to Cretaceous aquifers that dip beneath the clays of the 

Midway Group at the base of the gulf coast regional 

aquifer systems. The Midway Group becomes more
F6 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

permeable due to facies changes and yields water in 

Georgia and east-central Alabama; but westward 

towards the gulf coast regional aquifer systems it is predominantly composed of thick marine clays yielding 

small amounts of water.

FLORIDAN

The Floridan Regional Aquifer-System Analysis 

includes the major Tertiary carbonate aquifers of the 

coastal Southeastern United States (Bush and Johnston, 

1988; Johnston and Bush, 1988). The boundary between 

the Floridan and the Gulf Coast RASA study areas 

occurs where the more permeable carbonate facies 

changes to the less permeable marine clays of the Jackson and Vicksburg Groups, which form a regional confining unit in the Gulf Coast RASA study area.

GULF COAST RASA SUBREGIONAL STUDIES

More detailed investigations of the flow systems in 

subareas or groups of aquifers were conducted in the 

gulf coast by using subregional models (at one-half of 

the grid spacing; fig. 2). The subregional models, except 

for the gulf coast aquifer in Texas, simulate only the 

part of the aquifer system containing freshwater, 

whereas the regional model includes the entire thickness of Tertiary and younger sediments except where 

they are geopressured. The regional model includes all 

of the aquifers included in any of the subregional studies, except for the McNairy-Nacatoch aquifer (contained 

in rocks of Late Cretaceous age) in the northernmost 

part of the area. The subregional models are described 

separately:

McNairy-Nacatoch aquifer (Brahana and Mesko, 1988;

Brahana and Mesko, written commun.) 

Mississippi River Valley alluvial aquifer (Ackerman,

1989; Ackerman, 1996) 

Mississippi embayment aquifer system (Arthur and

Taylor; 1990, Arthur and Taylor, 1998) 

Coastal lowlands aquifer system of Alabama, Florida,

Louisiana, and Mississippi (Martin and Whiteman,

1989; Martin and Whiteman, 1999) 

Gulf coast aquifers in Texas (Ryder, 1988; Ryder and

Ardis, 1991)

PREVIOUS INVESTIGATIONS

A detailed discussion of many of the studies since 

1901 of ground-water flow in the gulf coast regional 

aquifer systems was given by Williamson and others 

(1990). Only the more recent and comprehensive ones 

will be summarized here. By the mid-1970's reports 

documenting intensive studies of the ground-water

resources had been published for many of the 343 counties in the study area. Also many studies of special 

ground-water problems were documented; for example, 

saltwater encroachment in the Houston-Galveston area 

(Winslow and others, 1957), effects of faults on the 

ground-water flow system (Whiteman, 1979; Gabrysch, 

1984), and subsidence due to pumping ground water 

(Winslow and Doyel, 1954; Winslow and Wood, 1959; 

Gabrysch, 1969,1977,1982; Gabrysch and Bonnet, 1975; 

Whiteman, 1980).

A few studies of a regional scale were documented 

in some early reports such as those of the Texas Coastal 

Plain by Deussen (1914,1924). Much later a study of the 

ground-water resources of the Mississippi embayment 

was described in a series of reports (Cushing and others, 1970; Boswell and others, 1965, 1968; Hosman and 

others, 1968). A multi-State study of the Claiborne 

Group from eastern Mississippi to the Rio Grande in 

Texas was conducted by Payne (1968,1970,1972,1975). 

A series of ground-water appraisals for the major river 

basins of the study area was published during the 

1970's (West and Broadhurst, 1975; Baker and Wall, 

1976; Bedinger and Sniegocki, 1976; Zurawski, 1978; 

Cederstrom and others, 1979; Terry and others, 1979).

The interest in quantitative analysis of ground-water 

flow, primarily related to areas of intensive groundwater pumpage, led to the early application of electric 

analog simulation techniques in the study area. The following analog model studies were made: the Houston 

District, Texas, by Wood and Gabrysch (1965); Jorgensen (1973, 1975), and Jorgensen and Gabrysch 

(1974); the Sparta Sand in the Mississippi embayment 

by Reed (1972); and the Bayou Bartholomew alluvium 

in Arkansas by Broom and Reed (1973).

By the mid-1970's, studies began to appear where 

digital models were used to simulate ground-water 

flow, such as: an alluvial aquifer and adjacent CarrizoWilcox aquifers near a reservoir on the Trinity River in 

eastern Texas by Garza (1974); the Carrizo aquifer in 

southern Texas by Klemt and others (1976); along the 

Colorado River of Texas by Thorkildsen and others 

(1989); the Bayou Bartholomew alluvium in Arkansas 

by Reed and Broom (1979); and the heavily pumped 

Houston area of Texas by Meyer and Carr (1979) and 

Jorgensen (1981).

The number of areas and the types of problems to 

which simulation techniques were applied continued to 

increase in the decade of the 1980's, and the digital 

model replaced the electric analog model. The use of 

simulation techniques within the study area is discussed herein under five categories: (1) shallow alluvial 

aquifers, (2) aquifers simulated with multilayer models, 

(3) effects of pumpage on water levels and salinity in a 

single layer, (4) effects of development on other
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F7

resources of the ground-water system, and (5) the use of 

optimization models for managing ground-water 

resources.

Parts of the shallow Mississippi River Valley alluvial 

aquifer were the subject of two separate ground-water 

simulation studies the Cache and St. Francis River 

basin in northeastern Arkansas by Broom and Lyford 

(1981) and the "Delta" area in western Mississippi by 

Sumner and Wasson (1984a, 1984b).

Aquifers simulated by multilayer models include 

the Texas Gulf Coastal Plain underlain by the Chicot 

and Evangeline aquifers (Carr and others, 1985); three 

zones in the Baton Rouge area, Louisiana (Torak and 

Whiteman, 1982; Huntzinger and others, 1985); the 

Memphis Sand of the Memphis area, Tennessee (Brahana, 1982); and the Jasper aquifer in eastern Texas 

(Baker, 1986).

Studies involving water-level declines and resultant 

movement of saline water into freshwater due to 

ground-water pumping also have been conducted. 

Groschen (1985) studied the Chicot and Evangeline 

aquifers southwest of Corpus Christi, Texas, and 

Trudeau and Buono (1985) studied the Sparta aquifer 

near West Monroe, Louisiana.

The effects of development of other resources on the 

ground-water flow system have been evaluated using 

simulation techniques. A multilayer model study of the 

Wilcox Group near the Oakwood salt dome in eastern 

Texas was made by Fogg and others (1983); a study of 

ground-water flow along a fault zone where uranium 

mining was being conducted by the solution method in 

Miocene-age sediments in southern Texas was reported 

by Henry and others (1982); and the effects of open-pit 

lignite mining and in-situ gasification of lignite on 

ground-water flow in Eocene-age sediments in east-central Texas was made by Charbeneau and Wright (1983).

Optimization models to manage pumpage such that 

a minimum saturated thickness is maintained have 

been proposed by Peralta and Peralta (1984), Peralta 

and Killian (1985), Peralta and others (1986), and Yazdanian and Peralta (1986) for the Grand Prairie area of 

Arkansas where the Mississippi River Valley alluvial 

aquifer is heavily pumped.

The primary purpose of most of the studies cited 

above has been to address a specific water-management 

problem, and only secondarily to develop an understanding of the flow in the aquifer under study and how 

it interacts with the larger regional ground-water flow 

system. The design and use of the model described 

herein, as noted earlier, is directed toward the goal of 

increased understanding of the regional ground-water 

flow system.

PHYSICAL SETTING

The study area is a gently sloping coastal plain in 

a mostly humid temperate climate and is underlain 

by thick, clastic sediments deposited in a gulfward 

offlapping sequence. Much agricultural and industrial 

development has occurred in the area, supporting a 

fast-growing population of more than 18 million as of 

1980 (Mesko and others, 1990) and requiring substantial 

ground- and surface-water resources.

TOPOGRAPHY

The land-surface altitude in the study area varies 

from sea level to over 800 ft (fig. 3). The dominant feature of the topography is the flat, low-lying Mississippi 

Alluvial Plain (Fenneman, 1938). The topography of the 

Mississippi embayment is asymmetrical in that the alluvial plain lies to the west side of the embayment and the 

topographically higher hills are mostly to the east side. 

The Mississippi River is an important feature of the 

hydrologic system because of its size; it generally 

traverses the east side of the alluvial plain. Toward the 

coast, the alluvial plain slopes toward the Gulf of Mexico and is cut through by large stream valleys that are 

approximately perpendicular to the coastline.

CLIMATE

The warm, relatively humid climate of the area is 

very favorable for agriculture. Mean annual precipitation in the area averages 48 inches and ranges from 

about 24 inches near the Mexico border to about 64 

inches along the gulf coast of Louisiana, Mississippi, 

and Alabama (fig. 4). The precipitation is evenly distributed throughout the year, though the spring is somewhat wetter and the summer and fall are somewhat 

drier, except in the western part of the area where most 

of the rain falls in the spring and summer (fig. 4). The 

mean annual temperature ranges from about 16°C in 

the north to about 21°C in the south, and the potential 

evapotranspiration is less variable than precipitation 

across the area.

In part of the area or during part of the time, or both, 

the actual evapotranspiration is limited to the amount 

of rainfall, which is less than the potential evapotranspiration. Pan evaporation, which is generally higher but 

closely related to potential evapotranspiration, ranges 

from about 45 in/yr to about 100 in/yr and averages 

about 64 in/yr (fig. 5). In the western part of the area in 

Texas, potential evapotranspiration generally exceeds 

the rainfall (figs. 4, 5), especially in the summer and 

fall, so most of the rainfall returns to the atmosphere in 

a short time. During wetter times or in wetter areas, or
F8 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

EXPLANATION 

Land-surface altitude In feet above sea level

 37°

KENTUCKY

 36°

32°

401 to 500 

501 to 600 

601 to 700 

701 to 800 

801 to 900

Vicksburg-Jackson confining unit outcrop

Base from U.S. Geological Survey 

Scale 1:2,500,000 0 50 100 150 200 KILOMETERS

FIGURE 3. Generalized average land-surface altitude and outcrop of Vicksburg-Jackson confining unit.
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F9

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EXPLANATION 

1 1 Precipitation

1 1 Evaporation

 52 Mean annual precipitation 

Contour interval 4 inches

89° 

ILLINOIS, 1 

90" S^^) \ ^\&. c .

MISSOURI/^ r /~>-^\ KENTUCKY

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FIGURE 4. Mean annual precipitation, 1951-80, and mean monthly precipitation and pan evaporation at four locations.
F10 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

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11 4

' LU

; Q 2 

o

Nor

1

theast Texc

1

js-Northwe

["[

st Louisiana

" IV; J FMAMJ JASOND

EXPLANATION

I___I Precipitation 

I I Evaporation

 45 Line of equal mean annual 

pan evaporation Contour 

interval 5 inches

\ 37°

. KENTUCKY

!£.

Z (/)

O LU

0 8

LU _-

£o

^" < ^

X cc

ZO. ,

0< *

2 Q Q

0

Near Brownsville. Texas

-

-

r

-

\ r

r

[ f r

.

r

-

r r

J FMAMJ JASOND

/ [ 29°

950 GULF OF MEXICO

Base from U.S. Geological Survey 

Scale 1:2,500.000 J FMAMJ JASOND

FIGURE 5. Mean annual pan evaporation, 1951-80.
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES Fll

both, when rainfall exceeds potential evapotranspiration, the remainder either infiltrates directly into the 

ground or runs off into streams. In the eastern part of 

the area, rainfall exceeds potential evapotranspiration 

and provides abundant surface-water runoff. The part 

of rainfall that recharges the aquifers is usually only a 

small part of the total, so it is difficult to estimate from a 

water-budget approach.

EXPLANATION 

Unit runoff, in inches

I I Less than 0.2

I I 0.2 to 6

dD 6 to 12

I I 12 to 18

dD 18 to 24

I I More than 24

 11 Line of equal mean annual 

runoff Interval, in inches, 

is variable

SURFACE WATER

Many large rivers flow through the study area. The 

unit runoff (mean annual flow divided by drainage area 

and converted to a depth over drainage area) (fig. 6) 

varies from about 0.2 in/yr in the southwestern part of 

the area to about 20 in/yr in the northern and eastern 

parts and averages about 14 in/yr. The mean annual 

flow of the larger rivers is over 467 million acre-ft/yr or

KENTUCKY

Base from U.S. Geological Survey 

Scale 1:2,500,000

50 100 150 200 MILES

0 50 100 150 200 KILOMETERS

FIGURE 6. Unit runoff, 1951-80.
F12 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

416 Ggal/d from a combined drainage area of more 

than 1.5 million mi2 (Williamson and others, 1990, table 

1). Streamflow rates are so large that the portion mov- ' 

ing into or out of the aquifer is not detectable, within 

the streamflow measurement error, because the groundwater surface-water interflow is generally less than 1 

percent of the streamflow. The combined storage (conservation pool or nonflood storage) in major reservoirs 

in or near the study area is more than 30 million acre-ft 

(Williamson and others, 1990, table 1). However, the I 

surface-water storage is very small relative to the vol- , 

ume, more than 2,000 million acre-ft, of freshwater in j 

storage in the top 200 ft of the aquifer systems (assuming an aquifer outcrop area of 180,000 mi2, a depth to 

the water table of 25 ft, and a specific yield of 0.10). 

Nevertheless, surface water provides about one-half of 

the water used in the study area (excluding nonconsumptive uses associated with hydroelectric power generation) (Mesko and others, 1990).

GEOHYDROLOGIC FRAMEWORK

The sediments of the gulf coast regional aquifer systems were deposited mostly during Cenozoic time.

Deposition was in fluvial, deltaic, or shallow marine 

environments, resulting in an interbedded sequence of 

sand, silt, and clay with some gravel, lignite, and limestone. Changes in land-surface elevation (caused by 

basin subsidence) and sea level resulted in transgressions and regressions of the sea and thus cyclical sedimentation. Depositional environments alternated from 

predominantly continental to predominantly marine. In 

general, the more sandy continental deposits have 

higher permeabilities characteristic of aquifers, and the 

more clayey marine deposits have lower permeabilities 

characteristic of confining units. Sedimentary units crop 

out in roughly parallel bands that are progressively 

younger gulfward in a typical offlap sequence.

The Gulf Coast geosyncline and the Mississippi 

embayment are the major structural features of the 

study area (fig. 7) and largely control the pattern and 

thickness of sedimentation (fig. 8). The base of the aquifer system (fig. 8) in the northern part of the area is at 

the top of the Midway confining unit and in the southern is specified as the transition zone into geopressured 

sediments. Except where affected by local uplift, the 

general pattern of sedimentation is one of increasing 

thickness in a gulfward, downdip direction. Hosman

TABLE 1. Summary of thickness, areal extent, and sand percentage of model layers 

[From Williamson and others, 1990, table 3. Some numbers may not agree due to rounding]

Layer Area

Thickness 

(feet) Sand percentage

Mean aggregate 

thickness 

(feet)

number ( 1 ,000 square

(see fig. 9)

11

10

9

17

8

16

7

15

6

14

5

13

4

3

2

12

miles)

150

130

140

28

120

45

90

92

90

92

140

110

110

170

100

170

Number 

of logs

286

334

379

52

408

115

331

361

321

390

461

420

403

631

423

575

Mean

560

1,890

1,960

470

1,760

520

1,340

580

500

360

470

230

260

1,250

300

850

Standard 

deviation

340

1,600

1,500

470

1,300

600

970

710

470

520

340

190

210

1,200

250

500

Maximum

1,220

5,650

6,330

1,960

7,850

3,990

4,450

6,580

2,490

3440

2,530

1,240

1,300

5,210

1,290

3,770

Number 

of logs

279

334

379

52

407

115

331

361

318

362

456

420

400

625

421

575

Mean

56

42

38

9

41

5

37

4

51

5

47

5

74

41

63

1

Standard 

deviation

23

17

15

5

15

3

13

4

16

6

19

7

19

17

18

4

Sand

310

790

740

42

720

26

500

23

260

18

220

12

190

510

190

9

Finegrained 

(silt and

clay)

250

1,100

1,200

430

1,000

490

840

560

240

340

250

220

68

740

110

840
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES

ILLINOIS^ 

90°

37°

MISSOURI

 v 37°

KENTUCKY

EXPLANATION

Top of Salt Dome

 Above top of Midway confining unit

 Below top of Midway confining unit

 Depth unknown

New Madrid 

Fault Zone

; 31°

\'< ( -FLORIDA

Base from U.S. Geological Survey 

Scale 1:2,500,000 50 100 150 200 KILOMETERS

FIGURE 7. Location of structural features, salt domes, and geohydrologic sections (shown in fig. 24 and plate 1).

(1988) presents a fence diagram of the study area and 

describes the lithology of the geohydrologic units.

Faults are common throughout the area, although 

their effect on regional ground-water movement is not 

well understood. In general, fault throws are not great 

enough to entirely offset the regional geohydrologic 

units described in this report, although individual beds 

could be offset. Numerous growth faults, which occur 

contemporaneously with deposition, exist farther gulfward. Whiteman (1979) describes the significance of the

Baton Rouge fault zone as a hydraulic barrier, and it is 

possible that other growth faults could act similarly 

Due to the scale of this analysis, the effects of faults 

were not considered on an individual basis.

Geopressured zones (fig. 8) have fluid pressures that 

are substantially greater than the normal hydrostatic 

pressure (Jones, 1969). Geopressured zones encroach 

younger units as the geopressured zones occur higher 

in the section in a gulfward direction. The most probable cause for the development of these abnormally high
F14 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

EXPLANATION

89° 

ILLINOIS*. I 90° ' '

 1.000 Subsurface contour Shows altitude of 

the base of the aquifer system. Contour 

interval 1,000 feet. Datum is sea level

° Geophysical well-logging location

----- Updip limit of area where top of

geopressured zone defines the base 

of the aquifer system

37°

MISSOURI,

x 37°

- KENTUCKY

33°

200 MILES

Base from U.S. Geological 

Survey Scale 1:2,500,000 0 50 100 150 200 KILOMETERS

FIGURE 8. Altitude of the base of the aquifer systems, area of occurrence of geopressure, and geophysical well-logging locations.

fluid pressures is the restriction of the escape of fluids 

during sediment compaction, thus causing pressure 

buildup and undercompaction of sediments (Fertl, 

1976, p. 16). The high pressures could not exist unless 

flow out of these zones was small, and thus the boundaries of these zones are considered flow barriers. However, these barriers are not completely impermeable, 

and given a very long time, free from other influences, 

the abnormally high pressures will eventually reduce to 

near normal hydrostatic pressures.

Salt domes occur throughout the gulf coast regional 

aquifer systems (fig. 7), particularly in bands near and 

roughly parallel to the coastline. The source of salt for 

the domes is the deeply buried Louann Salt of Jurassic 

age. The salt is extruded in diapirs that penetrate varying thicknesses of Cenozoic strata. The extent to which 

each dome penetrates the gulf coast regional aquifer 

systems is described by Beckman and Williamson 

(1990). The structural effects of the domes are relatively 

localized. However, the domes can have a substantial
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F15

effect on water quality in the deeper sediments due to 

dissolution of salt by ground water. Simulation demonstrates that the highly mineralized ground water affects 

regional flow by creating areas with high fluid densities.

The gulf coast regional aquifer systems were subdivided into geohydrologic units for analysis in the Gulf 

Coast RASA study. Weiss and Williamson (1985) 

described the methods used for subdividing the thick 

sequence of sediments. The relations of the geohydrologic units to aquifer systems and a selection of previously named units and the model layer numbering 

system used in this study are shown in figure 9. The 

geohydrologic units are referred to in the text by their j 

layer number for convenience, whereas both number 

and names generally are used in figures and tables. The 

term "layer" is used in place of "aquifer or permeable 

zone" for convenience as well. The relation of the various units are shown in the hydrologic sections on plate 

1. The thickness of each unit is a block average for 

blocks spaced 5 mi apart. Layer 11 includes both the 

Mississippi River Valley alluvial aquifer of the Mississippi embayment aquifer system and permeable zone A 

(Holocene-upper Pleistocene deposits) of the coastal 

lowlands aquifer system. The two areas of occurrence 

(fig. 12) of layer 11 are connected horizontally across a 

narrow band in central Louisiana. Due to the similarity 

in ages and deposition of the sediments of the Texas 

coastal uplands and Mississippi embayment aquifer 

systems, they were subdivided into similar geohydrologic units.

Three approaches were used in three situations to 

subdivide the aquifer systems into geohydrologic units 

(Weiss and Williamson, 1985):

1. Borehole geophysical logs were used to map regionally significant confining units (model layers 12-17), 

with aquifers in between (model layers 5 and 6 and 

parts of 7 and 8).

2. Logs were also used to identify large hydraulicconductivity contrasts between adjacent waterbearing zones not separated by regional confining 

units (model layers 2 and 4 that represent massive 

sands and layer 3 that represents thin, complexly 

interbedded coarse- and fine-grained deposits).

3. In most of the coastal lowlands aquifer system (model 

layers 7-11) where approaches 1 and 2 could not be 

used, hydraulic-head and well-opening data at 

various depths were used to subdivide the system 

so that the minimum vertical head change occurs 

within a unit. The zones were extended to areas 

where the hydraulic-head gradient is unknown by 

keeping the zone a constant proportion of the total 

system thickness, thereby avoiding abrupt discontinuities in thickness and thus preserving horizontal 

hydrologic continuity.

APPROACH: DIGITAL MODELING OF 

GROUND-WATER FLOW

A computer program (Kuiper, 1983, 1985) was used 

to simulate variable-density ground-water flow in three 

dimensions using the finite-difference method. Effective 

vertical hydraulic conductivity is specified for active 

aquifer layers as well as for inactive confining units. 

Inactive confining units have no storage nor horizontal 

flow. Kuiper's model calculates the total leakance (effective vertical conductivity divided by thickness) between 

the centers of all adjacent model layers by using the 

appropriate thicknesses and vertical hydraulic conductivities and their harmonic means. The water density, 

although variable in space, is assumed to be constant in 

time. This assumption simplifies the simulation substantially and is justified because model simulations of 

several decades are relatively short spans of time compared to the amount of time necessary for the average 

density simulated in very large model blocks to be considerably altered by the transport of dissolved solids. 

The model also accounted for inelastic compaction of 

fine-grained beds, resulting in more water being 

released from storage and in land subsidence (Kuiper, 

1994).

The areal extent of the gulf coast regional aquifer 

systems required that the regional model horizontal 

grid spacing be large (fig. 2) 10 mi on a side and vertical discretization be limited to 10 active aquifer layers. 

Even with these large blocks, there are 5,916 total blocks 

per layer (102 rows by 58 columns) times 10 active layers, which equals 59,160 blocks, although only about 

one-third of these are active due to the geometry of the 

systems. The horizontal grid is oriented approximately 

45 degrees from north-south (fig. 2). This orientation 

was chosen to minimize the size of the matrices 

required to fit the entire areal extent of the aquifer systems.

The geohydrologic unit (model layer) tops, thicknesses, and sand percentages used in these simulations 

are described in detail by Hosman and Weiss (1991) and 

Weiss (1992). The one exception is in the Mississippi 

embayment aquifer system north of the 34th parallel, 

where Hosman and Weiss describe the Memphis aquifer that is equivalent to both active layers 4 and 5 as 

modeled in this study. Layer 5 was defined as the top 

one-third of the Memphis aquifer with layer 4 composing the lower two-thirds of the aquifer for the purpose 

of preserving the horizontal hydraulic continuity of the 

model layers. South of the 34th parallel, layers 4 and 5
F16 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

Mississippi embayment and Texas coastal uplands aquifer systems

GEOLOGIC UNIT

SYSTEM

QUATERNARY

TERTIARY

SERIES

PLEISTOCENE 

AND 

HOLOCENE

EOCENE 

AND OLIGOCENE

LU

Z

LU 

0

O

LU 

LU

Z

LU

0 

0

LU 

£

GROUP

JACKSON 

AND VICKSBURG

CLAIBORNE

WILCOX

MIDWAY

GEOHYDROLOGIC UNITS 

DEFINED BY 

PREVIOUS STUDIES

Mississippi River Valley 

alluvial aquifer (Boswell 

and others, 1968)

Jackson and 

Vicksburg Groups

Cockfield aquifer system 

(Payne, 1970)

Cockfield Formation (Hosman 

and others, 1968)

Cook Mountain Formation

Sparta hydraulic system 

(Payne, 1968)

Sparta Sand (Hosman and 

others, 1968)

Memphis aquifer (Hosman 

and others, 1968) 

(layers 4 and 5)

Cane River Formation

Carrizo and Meridian Sand 

aquifer (Payne, 1975)

Carrizo Sand and Meridianupper Wilcox aquifer 

(Hosman and others, 1968)

Wilcox Group (Hosman and 

others, 1968)

Lower Wilcox aquifer 

(Hosman and others, 1968)

Midway Group

GULF COAST REGIONAL AQUIFER-SYSTEM ANALYSIS

MODEL 

LAYER 

NUMBER

11

15

6

14

5

13

4

3

2

12

GEOHYDROLOGIC UNITS

Mississippi River Valley 

alluvial aquifer*

Vicksburg-Jackson confining 

unit 1

Upper Claiborne aquifer

Middle Claiborne 

confining unit

Middle Claiborne aquifer

Lower Claiborne 

confining unit

Lower Claiborneupper Wilcox aquifer

Middle Wilcox aquifer

Lower Wilcox aquifer*

Midway confining unit 1

1 The Midway confining unit was referred to as the coastal uplands confining unit and the Vicksburg-Jackson

confining unit was referred to as the coastal lowlands confining unit by Grubb (1984, p. 11). 

* Not present in the Texas coastal uplands aquifer system.

FIGURE 9A. Correlation of aquifer systems and geohydrologic units (model layers) with previously named units.
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F17

B

Mississippi embayment and Texas coastal uplands aquifer systems

GEOLOGIC UNIT

SYSTEM

QUATERNARY

TERTIARY

SERIES

PLEISTOCENE 

AND 

HOLOCENE

PLIOCENE

MIOCENE

EOCENE 

AND OLIGOCENE

GROUP

JACKSON 

AND VICKSBURG

GEOHYDROLOGIC UNITS 

DEFINED BY 

PREVIOUS STUDIES

Upper Chicot aquifer 

(Jorgensen, 1975)

Chicot aquifer (Meyer and 

Carr, 1979)

Evangeline aquifer 

(Whitfield, 1975) 

(Meyer and Carr, 1979)

'2,000-foof sand of 

the Baton Rouge area 

(Torak and Whiteman, 

1982) 

Jasper aquifer 

(Whitfield, 1975)

GULF COAST REGIONAL AQUIFER-SYSTEM ANALYSIS

MODEL 

LAYER 

NUMBER

11

10

9

17

8

16

7

15

GEOHYDROLOGIC UNITS

Permeable (Holocene-upper 

zone A Pleistocene 

deposits)

Permeable (Lower Pleistocenezone B upper Pliocene 

deposits)

Permeable (Lower Pleistocenezone C upper Miocene 

deposits)

Zone D confining unit

Permeable (Middle Miocene 

zone D deposits)

Zone E confining unit

Permeable (Lower Miocenezone E upper Oligocene 

deposits)

Vicksburg-Jackson confining 

unit 1

1 The Midway confining unit was referred to as the coastal uplands confining unit and the Vicksburg-Jackson 

confining unit was referred to as the coastal lowlands confining unit by Grubb (1984, p. 11).

FIGURE 9B. Correlation of aquifer systems and geohydrologic units (model layers) with previously named units.

have an intervening confining unit, the lower Claiborne 

confining unit (layer 14).

Many of the aquifer characteristics such as thickness, sand percentage, and water density (based on concentration of total dissolved solids and temperature) 

were derived from a computerized file of 989 geophysical logs (fig. 8) (Wilson and Hosman, 1988). The logs 

were chosen to show regional trends of both onshore 

(895 logs) and offshore (94 logs) parts of the aquifer system. Williamson and others (1990) gave the procedures 

for preparing the data sets for the model by using computer contouring algorithms.

The comparison of the vertical relations of geohydrologic units within the aquifer systems (A) with the 

representation of the geohydrologic units in the 

regional flow model (B) is shown in figure 10. The section shown in figure 10 is diagrammatic, but it shows 

the general position of aquifers and confining units

along the axis of the Mississippi embayment. Where 

interior layers pinch out, the adjacent layers that still 

exist are directly connected with no more additional 

resistance to flow than is inherent in the conductivities 

and thicknesses of the respective units which do exist. 

The condition of resistance to flow also applies to the 

Mississippi River Valley alluvial aquifer (layer 11) that 

overlies the Mississippi embayment aquifer system 

(layers 2-6).

The base of the flow system for the regional flow 

model is assumed to be zero flow and is located at the 

top of the Midway confining unit, or the top of the zone 

of geopressure, whichever is shallowest (fig. 8). All of 

the lateral boundaries of the model were assumed to be 

no-flow. The eastern boundary was assumed to be noflow because the aquifers undergo a facies change to a 

much less permeable unit. The approach used at the 

southwestern boundary in Mexico was to use estimates
F18 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

A

North

MISSISSIPPI EMBAYMENT 

AQUIFER SYSTEM

COASTAL LOWLANDS 

AQUIFER SYSTEM

South 

Gulf of Mexico

EXPLANATION

R/D

Aquifer and model layer number 

Identified in figure 9. Shaded area 

contains saline water

Confining unit and model layer 

number Identified in table 1

No flow recharge/discharge top 

model layer

Unit truncated at edge of continental 

shelf

FEET 

5,000 .-

50 MILES

0 50 KILOMETERS

B

North

MISSISSIPPI EMBAYMENT 

AQUIFER SYSTEM

COASTAL LOWLANDS 

AQUIFER SYSTEM 

Coast -|

South

Recharge - Discharge

V V V . 7 V V V V V

14

13

Midway confining unit

10

16

EXPLANATION

3pT] Constant head 

7 | Aquifer and model layer number

16 i Confining unit and model layer 

 number

\\ Direct flow between layers

RGURE 10. A. Vertical relations of the confining units, aquifers, and permeable zones; and 

B. idealized representation of the geohydrologic units in the regional flow model.
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F19

in Mexico to extend the model to the aquifer system 

outcrop rather than try to force an arbitrary boundary at 

the United States border.

A constant head boundary was simulated in a layer 

above the top aquifer layer to simulate recharge and 

discharge. The head value specified was that of the 

water table (Williams and Williamson, 1989). The flow 

into or out of the top layer is simulated by using the difference in head between the first active aquifer block in 

the appropriate outcropping layer and the water-table 

head value above it times the conductance for vertical 

flow between the two blocks.

Most of the model calibration depended on automatic parameter estimation results described by Kuiper 

(1994). The regression methodology allowed testing of a 

wide range of parameters for simulation of the aquifer 

system. It also provided estimates of the accuracy of 

results and a mechanism to determine sources of model 

error. Kuiper used more than 40 different multipleregression models with 2 to 31 regression parameters. 

More than 3,000 values for grid-element volumeaveraged head and volume-averaged hydraulic conductivity were used as observations for the regression 

models. He estimated error bands for each model. 

Kuiper found that the root mean squared-weighted 

residual decreases little with an increase in the number 

of parameters, so that the models with a small number 

of parameters appear to be the most reliable for prediction of head.

Further discussions of the simulations are presented 

in the sections to which they relate, and the differences 

between simulations described in this report from the 

preliminary simulations of Williamson and others 

(1990) are discussed in the supplementary section "Simulation Details" at the back of this report. All of the data 

sets used in the model, along with original data sets 

used to map the geohydrologic units and ground-water 

chemistry, are archived in ASCII files on a WORM 

(Write Once, Read Many) disk that was prepared by 

Kirkpatrick (1993).

FACTORS CONTROLLING REGIONAL 

GROUND-WATER FLOW

The factors which controlled regional ground-water 

flow in the aquifer systems under predevelopment conditions are, in order of significance of effect, (1) topography; (2) outcrop and subcrop pattern and geometry of 

aquifers, permeable zones, and confining units; (3) variation of hydraulic properties of aquifers, permeable 

zones, and confining units; (4) distribution of brine 

water and water density; and (5) downdip limits of

aquifers, permeable zones, and geologic structure. They 

will be discussed in the following sections in the order 

of the significance of their effect on predevelopment 

regional ground-water flow.

TOPOGRAPHY

Topography has a major effect on the water-table 

altitude, which in turn has an effect on natural regional 

ground-water flow (Williams and Williamson, 1989; 

Williamson and others, 1990, p. 97). In terrains of humid 

climate and gentle hydraulic gradients, aquifer systems 

are generally full to overflowing with ground water. 

Therefore, the amount of recharge to the regional 

ground-water flow system is probably not related to the 

mean annual rainfall over most of the study area. Williamson (1987) reported that there was no correlation 

between the depth to the water table and the mean 

annual rainfall in this study area. It is more likely that 

the amount of recharge to the regional flow system is 

limited by the capacity of the regional flow system to 

transmit ground water away from recharge areas to discharge areas. Where the flow capacity is limited by the 

low regional hydraulic gradients or by the low conductance of the aquifer system, rainfall, which is potential 

regional recharge, is discharged to local surface-water 

bodies such as creeks and streams or to evapotranspiration. Under predevelopment conditions, areas with 

high land-surface altitude (fig. 4) were regional 

recharge areas, and areas of low land-surface altitude 

were regional discharge areas. The water-table altitude 

follows the land-surface altitude. The distribution of the 

thickness of relatively fresh water also demonstrates 

that the regional recharge areas are determined by their 

high relief, causing the deep brines to be more flushed 

out than in discharge areas. In the western part of the 

study area, where the climate is much drier (fig. 4), 

recharge to the regional ground-water flow system 

probably is limited by the smaller rainfall amount, most 

of which falls in the hot summer months when it is 

quickly evaporated or transpired.

ESTIMATION OF PREDEVELOPMENT WATER-TABLE 

ALTITUDE

Average predevelopment water-table altitudes for 

25-mi2 blocks were estimated by subtracting an estimated depth to water from the land-surface altitude calculated from very detailed digital data (Williams and 

Williamson, 1989; summarized in Williamson and others, 1990, p. 67). The traditional method of manually 

contouring measured water-table altitudes at wells was 

tried but not used owing to the large spacing of wells
F20 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

relative to small-scale variations in land-surface altitude. Most of the variation in predevelopment watertable altitude is a function of the variation in land-surface altitude rather than the variation of depth to water. 

Linear regression equations of depth to water were 

developed for five subareas on the basis of data from 

6,825 wells and average land-surface altitude (Williams 

and Williamson, 1989.) These equations also included a 

measure of local topographic variation. The resulting 

estimate of predevelopment water-table altitude over 

the entire study area is a subdued replica of the topography (fig. 3) and was presented by Williamson and 

others (1990, fig. 21).

In offshore areas, the source-sink layer was assigned 

a constant head value of 0 (sea level), a density of 1.025 

(that of seawater), and a thickness equal to the ocean 

depth, so that the model would calculate equivalent 

freshwater hydraulic-head values to use in the constant 

head blocks.

OUTCROP AND SUBCROP PATTERN AND

GEOMETRY OF AQUIFERS, PERMEABLE

ZONES, AND CONFINING UNITS

The second most significant factor controlling 

regional ground-water flow is the outcrop and subcrop 

pattern (fig. 11) and geometry of the aquifers, permeable zones, and confining beds. The major variations in 

subsurface permeability have been accounted for simply by delineation of aquifers and confining units 

(Grubb, 1986). Grubb qualitatively showed how the 

outcrop pattern, together with land-surface altitude, 

controlled the regional recharge and discharge areas. In 

many localities the water that was recharged to the 

regional flow system moved laterally to an area within 

the same outcrop area and was discharged at a lower 

altitude. Martin and Whiteman (in press) state that 59 

percent of the predevelopment recharge in the outcrop 

areas was also discharged in the outcrop areas in the 

coastal lowlands aquifer system in Louisiana, Mississippi, Alabama, and Florida. Some examples of the 

effect of the outcrop pattern will be discussed later in 

the section "Predevelopment Ground-Water Flow."

The areal variation of the thickness of the regional 

aquifers and permeable zones and confining units 

(fig. 12) is also a very important factor determining 

regional flow. For example, the middle Wilcox aquifer 

(layer 3) immediately above the Midway confining unit 

in the Texas coastal uplands aquifer system, and above 

the lower Wilcox aquifer in the Mississippi embayment 

aquifer system, is composed predominantly of interbedded coarse- and fine-grained beds. The coarse-grained 

beds have varying degrees of hydraulic connection and

therefore have a relatively low effective horizontal permeability. In many locations, the middle Wilcox is considered to be a confining unit. However, because of its 

vast thickness (fig. 12, table 1), the horizontal component of regional flow was substantial (pi. 6). Although 

recognized in this study as an aquifer, the fine-grained 

beds within the middle Wilcox aquifer are the major 

restriction to vertical flow between overlying and 

underlying aquifers.

HYDRAULIC PROPERTIES OF AQUIFERS,

PERMEABLE ZONES, AND

CONFINING UNITS

The third most significant factor controlling flow in 

the aquifer systems is hydraulic conductivity and its 

areal distribution within the aquifers, permeable zones, 

and confining units. Hydraulic conductivity (K) of 

entire aquifers, permeable zones, and confining units is 

difficult to measure and may vary widely across relatively short distances. However, maximum use was 

made of field measurements of horizontal hydraulic 

conductivity (Kh) to estimate effective horizontal 

hydraulic conductivity (Kheff) in order to avoid including model simulation errors of all kinds in modelderived values of Kheff. The Kh values used are directly 

related to aquifer properties, and they have a greater 

potential transfer value to other aquifer studies than 

model-derived Kheff values. The available data used 

included sand-bed thicknesses at locations, aquifer-test 

and specific-capacity data for several thousand wells, 

and the geometry of the units. The approach used to 

obtain estimates of Kh is discussed below and can be 

summarized as follows: (1) use aquifer-test and specificcapacity data to develop areal averages for sand-bed Kh 

for each model layer, (2) map sand percentages from the 

geophysical-log data, (3) develop a relation between 

sand percentage and Kheff, and (4) adjust the Kheff estimates for the variation in K with depth. Model-derived 

values for vertical K of confining units were used 

because the field data are sparse, highly variable, and 

difficult to extrapolate from specific locations to blockaveraged values.

HORIZONTAL HYDRAULIC CONDUCTIVITY OF 

AQUIFERS AND PERMEABLE ZONES

The horizontal hydraulic conductivity (Kh) of sand 

beds is many times larger than the conductivity of 

fine-grained beds; therefore, the thickness of the finegrained beds can be ignored in the calculation of 

transmissivity of the aquifers. The transmissivity of 

the aquifer layers can be calculated as the product of 

the total thickness of the sand beds in a layer and the
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F21

EXPLANATION

MODEL LAYERS

_____ Subcrop beneath Mississippi

River Valley alluvium 

 Downdip limit of system

Layer 11 consists of the Mississipi River Valley 

alluvial aquifer north of updip limit of subcrop of 

1 5 layer 7 (permeable zone E) and permeable 

zone A to the south.

Study area boundary

v 37°

Note: Layers 13-15 are confining units; 

confining units represented by layers 

16 and 17 are only in the subsurface

27° !

Base from U.S. Geological Survey 

Scale 1:2,500,000

50 100 150 MILES

_|______I______I

0 50 100 150 KILOMETERS

FIGURE 11. Outcrop and subcrop of aquifers, permeable zones, and confining units (model layers).
F22 REGIONAL AQUIFER-SYSTEM ANALYSIS GULP COASTAL PLAIN

EXPLANATION °

Layer thickness, 10 

in feet

2,500 50 PERMEABLE ZONE A AND MISSISSIPPI RIVER VALLEY 

r ALLUVIAL AQUIFER (LAYER 111

UPPER CLAIBORNE AQUIFER (LAYER 6)

Downdip 10 

from this 

line is 20 

more than 

35,000 30 

milligrams 

per liter 40

MIDDLE CLAIBORNE CONFINING UNIT (LAYER 14)

MIDDLE CLAIBORNE AQUIFER (LAYER 5)

LOWER CLAIBORNE CONFINING UNIT (LAYER 13)

OhH

LOWER CLAIBORNE-UPPER WILCOX AQUIFER (LAYER 4]

MIDDLE WILCOX AQUIFER (LAYER 3)

OM-H

i/ Mexico

50 1- LOWER WILCOX AQUIFER (LAYER 2) 

100 65 60 55 50 45 40 35 30 25 20 15 10 5 0

FIGURE 12. Perspective diagrams showing extent and thickness of aquifers, permeable zones, confining units, and 

saline/freshwater interface of: A. the Mississippi embayment and Texas coastal uplands aquifer systems, and

B. the coastal lowlands aquifer system.
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F23

B

100 95 90 85 80 75 70 65 60 55 50 45

EXPLANATION

Layer thickness, 

infect

More 

than 

5,000

2,500

1,500

800

300

1

Depthaveraged 

dissolved- 50 

solids concentration 

of water 

Downdip '0 

from this 

line is 20 

more than 

35,000 30 

milligrams 

per liter 40

PERMEABLE ZONE A AND MISSISSIPPI RIVER 

VALLEY ALLUVIAL AQUIFER (LAYER 11)

I I I II ' I I

PERMEABLE ZONE B (LAYER 10)

111 1111111 i n 1111111 i 11111 i 11111111111 1111

PERMEABLE ZONE C (LAYER 9)

I I I I I I I I

ZONE D CONFINING UNIT (LAYER 17)

I I I I I I I I I

PERMEABLE ZONE D (LAYER 8)

I II I I I I I i I I

ZONE E CONFINING UNIT (LAYER 16)

I I ' I ' ' II I I I I I I ' I I I I I I ' ' I I I'' I I ' ' I ' I I

PERMEABLE ZONE E (LAYER 7)

I I I I I I I I I

VICKSBURG-JACKSON CONFINING UNIT (LAYER 15)

100 95 90 85 80 75 70 65 60 25 20 15 10 5 0

FIGURE 12. Perspective diagrams showing extent and thickness of aquifers, permeable zones, confining units, and

saline/freshwater interface of: A. the Mississippi embayment and Texas coastal uplands aquifer systems, and

B. the coastal lowlands aquifer system Continued.
F24 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

effective hydraulic conductivity of the sand beds. The 

effective hydraulic conductivity (Keff) of a geohydrologic unit is the K value that would produce the same 

hydraulic gradients in a model (with one value per 

model block) as those in the aquifer, though the aquifer 

is composed of many kinds of sediments with different 

Kh values. The Kheff varies widely in the aquifer systems and is difficult to estimate. The factors that control 

Kheff are: Kh of sand beds, the sand-bed connectedness, 

and the depth. These factors will be discussed in the following sections.

HORIZONTAL HYDRAULIC CONDUCTIVITY OF SAND BEDS

Prudic (1991) summarized estimates of horizontal 

hydraulic conductivity for sand beds from about 1,500 

aquifer-test analyses and more than 5,000 specificcapacity tests in the study area. He found that there was 

a significant regional pattern of variation, although the 

extremely heterogeneous nature of variations at very 

close distances prohibited mapping the conductivities. 

The variation of geometric mean hydraulic conductivity 

of sand (Ksand) between areas, ignoring layer differences, was as great as the variation between layers, 

ignoring areal differences (Prudic, 1991, p. 23).

The study area was divided into nine geographic 

areas for subsequent discussions of areal comparisons 

(fig. 13). The regional variation showed up very clearly 

when all of the data within one geographic area (fig. 13) 

and layer were averaged and compared with averages 

for 40 or so other area-layer combinations (fig. 14). Prudic (1991) chose the geometric mean for the averaging 

statistic to compare because of the log-normal distribution of the hydraulic conductivities.

The regression modeling of Kuiper (1994) indicated 

that the models with fewer numbers of parameters produced a narrower confidence interval in predicted head; 

yet the great range between the geometric mean 

hydraulic conductivities of some area and layer combinations, when compared to others (fig. 14), indicates a 

compromise solution (discussed in the supplement 

"Simulation Details"). Grouping the area and layer combinations according to their mean conductivity into six 

groups (table 2) minimized the number of parameters 

for the regression model while preserving the differences in mean hydraulic conductivity indicated by the 

aquifer-test data. The regression model then estimated 

conductivities for each group which were near the estimated values, yet allowed the model to optimize the fit 

of hydraulic-head data. The model calibration was done 

using an initial estimate of the geometric mean Ksan(i 

(table 2) for each group from Prudic (D.E. Prudic, U.S. 

Geological Survey, written commun., 1990). Prudic 

(1991) revised his estimates using a slightly different

method of calculating K from specific-capacity data, 

resulting in a general overall increase of about 30 percent in the value of K. The model calibration was not 

redone because the change would not substantially 

affect the results except to increase, by about 30 percent, 

the K and hence flow and velocity. This amount of 

change is within the estimated error bands of the 

regression model.

EFFECTIVE HORIZONTAL HYDRAULIC CONDUCTIVITY OF 

REGIONAL AQUIFERS

The sand and fine-grained beds of the aquifer systems in many areas were deposited in geometrically 

complex shapes and highly variable sizes in three 

dimensions. The ratio of effective hydraulic conductivity (Keff) to Ksand depends on the sand-bed connectedness, which depends on the geometry of the beds and 

the scale used in the simulation of ground-water flow 

(Freeze and Cherry, 1979, p. 32-34; Bear, 1972, p. 153- 

155; Bear and Verruijt, 1987, p. 35-37). What is referred 

to here as Keff is called equivalent conductivity by 

Freeze and Cherry (1979).

In such a complex depositional environment, even a 

simple question such as, "Can the lithology be 

described as sand beds in a fine-grained unit or as finegrained beds in a sand unit?" is actually difficult to 

answer. This question does not have a yes or no answer, 

but rather the answer lies somewhere in between the 

two extremes. In nearly every locality, both extremes 

exist. In areas where deposition is predominantly sand, 

the sediments could be characterized generally as finegrained beds in a sand unit, where the reverse would be 

true in other areas (fig. 15). Figure 15 is an attempt to 

represent the observed pattern of sand and fine-grained 

beds that compose the gulf coast regional aquifer systems. It was produced by moving through an array representing a cross section of an aquifer and randomly 

locating sand, or fine-grained beds, or both, using the 

sand percentage as the probability of existence of a bed 

in any one location. The geometry of the beds (length to 

width ratio) and spatial correlation are implicit due to 

the vertical exaggeration of the hypothetical sections.

The appropriate method of calculating Keff for a 

model depends on the geometry of coarse- and finegrained beds in an aquifer. In the case where a heterogeneous aquifer is composed of alternating sand and 

fine-grained beds whose extent is larger than that of a 

model block (fig. 16A), the Keff is calculated as follows. 

If a given flow path went through a specific pattern of 

very low-conductivity clay beds and varying conductivity sand beds (analogous to series flow in electrical 

circuits), the effective conductivity would be equal to 

the harmonic mean (inverse of the mean of the inverses
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F25

Mississippi

Alluvial.

Plain

36° ,'

Northeastern 

Texas

Western 

Embayment 93°

Eastern 

Embayment

MISSISSIPPI 

EMBAYMEN 

AQU1.FER SYSTEM

Study area 

bounda

Winter 

Garden

Eastern

Coastal

Lowlands

Southwestern 

Louisiana

96 Southeastern 280 Texas

Edge of Continental Shelf

0 50 100 150 200 KILOMETERS

99°

98°

Base from U.S. Geological Survey 

Scale 1:2,500,000

Southern 

Texas

FIGURE 13. Geographic areas used for comparisons of sand-bed hydraulic conductivities and flow summaries.
F26 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

WEST A. Areas 1-5 EAST WEST B Areas 6-9 EAST

AREAS

EXPLANATION

Geometric mean hydraulic 

conductivity, in feet per day

75 to 149 

Greater than 150

Southern Southeastern Southwestern Eastern Coastal 

Texas Texas Louisiana Lowlands

AREAS

No data, or layer 

does not exist in 

the area

FIGURE 14. Geometric mean horizontal hydraulic conductivity of sand beds by layer and area, estimated from aquifer-test analyses and specific-capacity

data of: A. the Mississippi embayment and Texas coastal uplands aquifer systems, and 

B. the coastal lowlands aquifer system. Areas represent east-west variations.

of the values) of the individual bed conductivities. If the 

flow could go through any of a group of beds (analogous to parallel flows in electrical circuits), the effective 

conductivity would be equal to the arithmetic mean of 

the individual bed conductivities. For the case where 

the extent of the beds was much less than the size of the 

model blocks (fig. 16C) and the beds are randomly distributed and randomly oriented, the Keg in any direction is equal to the geometric mean of all of the K 

values. However, for the case where the extent of individual beds varies from smaller to larger than the extent

of the model blocks (fig. 16B), the horizontal Keff is 

between the geometric and arithmetic means, and for 

the vertical, Keff is between the geometric and harmonic 

means (Fogg, 1989, p. 46). This is the case in this study 

area, as well as many others, because there is so much 

variability in the extent of the beds, as well as the large 

variation in K values. As early as 1945, Cardwell and 

Parsons (1945) recognized that Keff could not be calculated either by the harmonic or the arithmetic means 

but was somewhere in between.
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F27

TABLE 2. Comparison of hydraulic conductivity of sand beds (Ksan({) by model calibration with mean estimated Ksandfrom aquifer-test

analyses and specific-capacity data by group of model layers and areas

[Estimates from Prudic, 1991, table 6. All values are in feet per day. 1 Summaries are not included for layers in an area with

less than five estimates of hydraulic conductivity]

2 Group Area

1 4

Group composite:

2 4

2 5

2 8

2 9

Group composite:

3 4

3 4

3 5

3 5

3 6

3 8

3 8

3 9

3 9

3 9

Group composite: 

4 1

4 2

4 3

4 3

4 4

4 5

4 5

4 7

4 7

4 8

4 8

4 9

4 9 

Group composite: 

5 6

5 7

5 7

5 8

5 8

5 9

5 3

5 3

5 3

Layer

11

2

5

11

11

5

6

2

4

11

9

10

8

9

10

4

4

5

6

4

3

6

8

11

7

8

6

7 

8

9

10

6

6

5

2

3

4

Number of 

values

374

374 

18

71

601

515

1,205 

147

174

107

78

16

152

293

326

334

219

1,846

44

38

492

73

31

70

32

25

231

242

196

16

186 

1,724 

6

213

330

11

11

29

10

458

18

Final estimates3

Arithmetic 

mean

376

376 

187

170

270

194

231 

119

116

145

129

201

141

147

123

129

126

131 

63

40

94

64

76

92

90

50

79

103

97

49

95 

87 

19

24

35

28

28

65

42

56

47

Standard 

deviation

203

203 

193

142

207

175

194 

123

113

162

146

235

168

154

140

121

122

139

56

31

112

75

73

137

73

43

96

121

116

38

110 

106 

9.1

18

39

13

13

70

54

85

29

Harmonic 

mean

227

227 

95

42

103

84

87 

41

51

5.4

25

15

41

48

45

45

39

30

25

14

30

24

24

9.8

19

13

30

36

26

14

15 

23 

15

15

20

19

19

1.9

17

8.3

5.3

Geometric 

mean

316

316 

128

106

189

432

156

75

78

79

71

69

82

88

78

85

82

81

42

29

57

41

48

40

60

30

49

62

59

33

56 

52 

17

19

26

24

24

20

25

26

31

Initial 

estimated Model 

geometric calibrated 

mean

240

240 247 

111

89

144

104

122 123

52

58

65

57

62

56

60

60

67

63

36

24

39

27

35

29

47

30

42

42

41

25

42

"38 37 |

' .... ... .,.,*..-< 

17

16

21

16

16

13

15

16

19
F28 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

TABLE 2. Comparison of hydraulic conductivity of sand beds (Ksantj) by model calibration with mean estimated Ksan^from aquifer-test 

analyses and specific-capacity data by group of model layers and areas Continued

[Estimates from Prudic, 1991, table 6. All values are in feet per day. 1 Summaries are not included for layers in an area with

less than five estimates of hydraulic conductivity]

9

Group Area

Group composite: 

6

6

6

6

6

6

6

Group composite:

6

6

9

1

2

2

2

Layer

9

10

3

3

3

5

6

Number of 

values

1,088 

23

27

10

10

17

224

32

22

Final estimates3

Arithmetic 

mean

42 

8.6

17

13

17

18

19

16

17

Standard 

deviation

62

7.2

18

14

23

24

17

15

21

Harmonic 

mean

10 

3.9

8.1

.9

3.2

5.5

10

5.3

5

Geometric 

mean

24 

5.8

12

4.9

8.3

10

14

11

10

Initial 

estimated 

geometric 

mean

17

5.2

9.5

2.3

4.7

7.9

12

8.5

7

Model 

calibrated

17

7

1Hydraulic conductivities exceeding 1,000 feet per day were not included in the analyses. 

2Areas shown in figure 13.

3Initial and final estimates refer to a change in the method of calculation of Ksan(j from aquifer-test analyses and specific-capacity data. Model was 

calibrated to match initial estimates before final estimates were available. See text, p.24.

The effect of different methods of calculating an 

effective hydraulic conductivity for model blocks where 

the Ksand/ Kfine, and sand percentage are either known 

or estimated is shown in figure 17, where Kgne denotes 

the hydraulic conductivity of fine-grained beds. Sand 

percentage is the only readily usable and consistent 

measurement of the degree of connectedness between 

sand beds in the study area. Desbarats (1987) has 

presented a statistically based procedure for calculating 

grid-element effective hydraulic conductivity values, 

which in effect is similar to the method presented here, 

except that the method presented here is based on 

data (sand percentage) that are easily calculated. 

Effective horizontal and vertical hydraulic conductivity 

values, Kh and Kv, are calculated as functions (eqs. 1, 2) 

of sand percentage (S%), expressed as a ratio, and 

shown in figure 18, for the case where (Kfine/Ksand)= 

0.0001. Kheff and Kveff can be calculated from:

and = tKfi

eff/ Ksand - tKfir

0.25

(1)

(2)

The exponents 4 and 0.25 were derived by Kuiper's 

(1994) regression model.

Sand percentage (S%) also varies widely among 

aquifers and across the study area (fig. 18). The S% 

implies the statistical frequency with which coarsegrained beds are interconnected, which largely determines the effective K and hence why it is a power function in equations 1 and 2.

VERTICAL HYDRAULIC CONDUCTIVITY OF 

FINE-GRAINED SEDIMENTS

Resistance to vertical flow is of two types: across 

regionally identifiable confining units, and within aquifers and permeable zones due to many beds of finegrained materials that may have considerable areal 

extent yet are not traceable over multicounty areas. The 

deep cones of drawdown resulting from large groundwater pumpage at Houston and Baton Rouge demonstrate that large vertical resistance can occur even when 

only the latter type of resistance occurs. Several hundred feet of vertical-head difference has developed 

between the water table and the deep, intensively 

pumped sands, although there are no regionally identifiable confining units.

Vertical hydraulic conductivity (Kv) is nearly impossible to estimate because (1) laboratory values are seldom representative of undisturbed field values, (2) 

values range over many orders of magnitude (Freeze 

and Cherry, 1979, p. 29; Wolff, 1982, p. 36-90), and (3) 

the large variations generally can occur over short distances, making estimation of the effective value even 

more difficult. Kuiper's (1994) regression model estimated 0.11 x 10~3 ft/d as the best estimate of vertical 

hydraulic conductivity for fine-grained materials 

within confining units and 0.22 x 10~3 ft/d for finegrained materials contained within aquifers. Note that 

the two values are quite close to each other considering 

Kv can vary several orders of magnitude. Less than 22 

percent (Texas coastal uplands aquifer system and Mississippi embayment aquifer system) or 6 percent
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F29

Sand 

Percentage

10%

20%

50%

Each of nine hypothetical sections through parts of 

the aquifer system with differing sand percentages 

represents hundreds of feet vertically and tens of miles 

horizontally. White denotes sand. Black denotes finegrained beds

FIGURE 15. Continuity of sand beds and fine-grained (clay and silt) beds in

hypothetical sections composed

of 10- to 90-percent sand.

(coastal lowlands aquifer system) of the fine-grained 

sediments is contained within the regional confining 

units. Most of the fine-grained beds occur within the 

regional aquifers and permeable zones. Vertical resistance to flow can be proportional to the net thickness of 

fine-grained beds whether they have regional lateral 

extent or not. Regionally mappable, fine-grained units 

may cause most of the overall resistance to vertical flow

where they exist, especially if they comprise the bulk of 

the thickness of fine-grained material at those locations. 

The concept of large vertical resistances to flow due 

to fine-grained beds within the aquifers can be illustrated using the results of previous studies such as that

Areally extensive layers of coarse- and fine-grained 

deposits.

Lenses of coarse-grained deposits interbedded with 

fine-grained deposits.

Numerous lenses of coarse- and fine-grained deposits. 

EXPLANATION

Coarse-grained deposits 

I I Fine-grained deposits

FIGURE 16. Model blocks with varying extents of coarse- and finegrained beds: A. beds extend beyond model 

blocks, B. most beds do not extend beyond model 

blocks, and C. extent of beds generally much 

smaller than model blocks.
F30 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

J

> o.oi

0.0001

0.4 0.6 

SAND RATIO

Graph construction assumption is that Ksancj/Kfjne equals 

10,000, although vertical axis could be adjusted to other ratios.

FIGURE 17. Effective hydraulic conductivity (K^f) as a 

function of the ratio of sand to total volume.

by Wesselman (1967). He plotted hydraulic head relative to depth (Wesselman, 1967, p. 57) for an interval 

containing the Chicot, Evangeline, and Jasper aquifers 

and the Burkeville confining unit at Evadale, Texas, 

near the type locality of the Burkeville confining unit. 

The plot shows a hydraulic head change of 67.2 ft over a 

depth interval of approximately 430 ft across the 

Burkeville, which is a vertical hydraulic gradient of 

0.16 ft/ft. However, on the same plot, a hydraulic-head 

change of 18.9 ft occurs across 130 ft of an unnamed 

fine-grained bed in the middle part of the Evangeline 

aquifer, indicating a hydraulic gradient of 0.15 ft/ft. 

This similarity in hydraulic gradients suggests that the 

numerous smaller fine-grained beds within the aquifer 

system may cumulatively provide nearly the same 

resistance to vertical flows as larger, more regionally 

mappable confining units. This is only one example, but 

numerous other examples exist. I

I DEPTH DEPENDENCE OF HYDRAULIC CONDUCTIVITY

The hydraulic conductivity of the sediments tends to 

decrease with depth due to increasing overburden pressure causing compaction and a corresponding decrease 

in porosity. However, this decrease is moderated 

because hydraulic conductivity also tends to increase 

with depth due to higher natural geothermal temperature and the resulting decrease in fluid viscosity. This 

temperature effect can change the viscosity by a factor 

of nearly five (Rouse, 1950, p. 1008; Weiss, 1982) from 

near land surface where the water temperature is about 

21°C to a depth of over 10,000 ft where the temperature 

can be as high as 149°C (Wilson and Hosman, 1988). The 

decrease in viscosity with depth is partly compensated 

by an increase in density of water downdip, due to 

increased salinity. The increase in salinity causes a 

slight increase in viscosity and a corresponding 

decrease in hydraulic conductivity. The net effect of 

these factors, however, is toward decreasing hydraulic 

conductivity with depth.

Kuiper (1994) estimated that Ksancj varies by depth 

in proportion to io(-°-000243D), where D is depth in feet. 

Kuiper used data from Loucks and others (1986) and 

Lake and Carroll (1986). Data on the sand hydraulic 

conductivity of deposits similar to those of this study 

are shown in figure 19, combining data from oilwell tests (Loucks and others, 1986) and from waterwell aquifer tests (Prudic, 1991). Kuiper's curve, 

(30)10(~"-0002493D), which is an approximation assuming 

the constant for Ksancj is 30 ft/d, is also shown in figure 

19. The exponent of this curve was assigned a regression parameter by Kuiper (1994), but because of the 

shallow depth (D) of most head observations, the 

parameter had such a large confidence interval that it 

was decided that the model would be more accurate if 

the curve was assumed fixed. Prudic (1991) found the 

variation of Ksand with depth to be significant at the 95- 

percent confidence interval but only in an analysis of 

variance where each area-layer combination was 

treated separately. However, about 9 of the more than 

40 area and layer combinations had signs for the 

coefficients in the equations, indicating the wrong 

direction of the relation. These tended to be groups of 

data where there was lower confidence in the relation as 

well. A simple log linear regression was performed on 

both data sets shown in figure 19, yielding the equation, K = (54.5)10(~°-000309ti), with an adjusted r2 of 

0.736. This equation is similar to Kuiper's (also shown 

in fig. 19), so Kuiper's equation was used.

The vertical hydraulic conductivity of the finegrained beds, like that of the sands, also tends to 

decrease with depth. The primary cause for this is 

compaction. Neglia (1979) presented data for the
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F31

100 95 90 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5

UPPER CALIBORNE AQUIFER (LAYER 6)

MIDDLE CALIBORNE AQUIFER (LAYER 5)

LOWER CLAIBORNE-UPPER WILCOX AQUIFER (LAYER 4)

I I I I I I I I I > I I I I I I II I I II I I I I I I I I II I I I I I I I I I I I I I I

MIDDLE WILCOX AQUIFER (LAYER 3)

I I I I I I I I I U I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I II I I

Gulf of Mexico 

LOWER WILCOX AQUIFER (LAYER 2)

100 95 90 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0

ROW

EXPLANATION 

Sand percentage

81tolOO 

61 to 80 

41 to 60 

21 to 40 

Oto20

FIGURE 18. Perspective diagrams showing sand percentage in each aquifer or permeable zone (layer) of the

gulf coast aquifer systems in: A. the Mississippi embayment and Texas coastal uplands

aquifer systems, and B. the coastal lowlands aquifer system.
F32 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0

PERMEABLE ZONE A AND MISSISSIPPI RIVER VALLEY ALLUVIAL AQUIFER (LAYER 11)

PERMEABLE ZONE B (LAYER 10)

I I I I I I I I ! I II I I I I I I I II I II I I I II II I I I I I I I I I I I u I u-1 i i | M i i i i i i i i i i i i i i i i

Gulf of Mexico 

PERMEABLE ZONE D (LAYER 8)

PERMEABLE ZONE E (LAYER 7)

100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0

ROW

EXPLANATION 

Sand percentage

81tolOO 

61 to 80 

41 to 60 

21 to 40 

Oto20

FIGURE 18. Perspective diagrams showing sand percentage in each aquifer or permeable zone (layer) of the

gulf coast aquifer systems in: A. the Mississippi embayment and Texas coastal uplands

aquifer systems, and B. the coastal lowlands aquifer system Continued.
1,000

100

10

o (J (J

3 0.01<EC QI

0.001

0.0001

0.00001

Regression with both 

data sets 

K = 54.5x10(-°-000309D>

Kuiper (1994) K = 30 x 1 o(-°-000293D)

Water-well test

Oil-well test

i i

 » <* 

t- r-"--. -.-

tI

i

n

O

2,000 4,000 6,000 8,000 10,000 

DEPTH, IN FEET

12,000 14,000 16,000 18,000

FIGURE 19. Decrease of sand-bed hydraulic conductivity with depth.
F34 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

decrease with depth of the hydraulic conductivity of 

various clay samples. Kuiper (1994, p. 48 and fig. 30) 

used Neglia's data (1979) and formulated a regression 

equation to approximate the measured values, (constant)10(-°-000356 D + 0.0000254 D'2), where D is in feet/

which was used in this study. As with the decrease of 

sand hydraulic conductivity with depth, a regression 

parameter that Kuiper (1994) placed into the exponent 

had a very large confidence interval and was removed 

so that the depth relation was assumed fixed.

After all of these factors are accounted for in estimating regional effective hydraulic conductivity in the 

horizontal and vertical directions for each model block 

in every layer, the resulting Keff values span a wide 

range (fig. 20). Both Kheff and Kveff span 6 to 7 orders 

of magnitude, although most of the blocks are in a more 

limited spread. The approach used maximizes the use 

of measurable data to estimate Keff across a broad range 

of values (fig. 20).

The areal distribution of effective horizontal hydraulic conductivity shows some interesting patterns, more 

obvious than any shown by component distribution 

(fig. 21). The west-to-east trend of increasing sand-bed

hydraulic conductivity and also sand percentage combine to show an even more dominant east-west trend of 

increasing effective conductivity. The pattern in layer 11 

also shows an increased conductivity in the Rio Grande 

embayment (fig. 7), probably due to deposits from 

higher energy stream deposits, similar to what appears 

in the Mississippi embayment, but to a lesser extent. A 

lower conductivity north-south band occurs in layers 2 

and 4 in north Louisiana and south-central Arkansas.

BRINES AND DENSITY

Variations in concentration of dissolved solids, and 

hence density, have a substantial effect on flow in the 

aquifer system because about two-thirds of the volume 

of the aquifer systems studied contain saline 

water that is, water having a concentration of dissolved solids greater than 10,000 mg/L (Kuiper, 1994, 

p. 12 and fig. 8). About one-half of the volume of the 

aquifer systems contains water saltier than seawater or 

brine (fig. 22). Generally, the transition from a relatively 

fresh water (dissolved-solids concentration less than

1,000 E

100

10

0.1

o

<-> 0.01

0

0.001

[5 o.oooi

0.00001

0.000001

Kfo Effective horizontal 

hydraulic conductivity

Kv Effective vertical 

hydraulic conductivity

0.0 0.2 0.4 0.6 0.8 

RATIO OF SAND THICKNESS TO TOTAL THICKNESS

FIGURE 20. Relation of horizontal and vertical effective hydraulic conductivity to 

sand proportion in the gulf coast aquifers systems.

1.0
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F35

100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0.

Gulf of Mexico 

UPPER CLAIBORNE AQUIFER (LAYER 6)

I ' ' ' I I ' ' ' I I II I I I I I ' I I I I I I I ' I ' I I I ' I ' I I

Gulf of Mexico 

MIDDLE CLAIBORNE AQUIFER (LAYER 5)

Gulf of Mexico 

LOWER CLAIBORNE-UPPER WILCOX AQUIFER (LAYER 4)

Gulf of Mexico 

MIDDLE WILCOX AQUIFER (LAYER 3)

Gulf of Mexico 

LOWER WILCOX AQUIFER (LAYER 2)

100 95 90 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0

EXPLANATION 

Effective horizontal 

hydraulic conductivity, 

in feet per day

greater than 200

FIGURE 21. Perspective diagrams showing regional effective horizontal hydraulic conductivity in each

aquifer and permeable zone (layer) of the gulf coast aquifer systems in: A. the Mississippi embayment and

Texas coastal uplands aquifer systems, and B. the coastal lowlands aquifer system.
F36 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

B

100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0,

PERMEABLE ZONE A AND MISSISSIPPI RIVER VALLEY ALLUVIAL AQUIFER (LAYER 11)

PERMEABLE ZONE B (LAYER 10)

I I II I I I I H I I I I ' I I I I I I I I ' I I I I I I I I I I I

PERMEABLE ZONE C (LAYER 9)

1 ' I I ' I ' ' I H I ' I ' I I I I I I I I I I I I I I ' I ' ' I I I ' I i I I I i I

PERMEABLE ZONE D (LAYER 8)

I I I ' ' I I I I I ' H

PERMEABLE ZONE E (LAYER 7)

100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0

EXPLANATION 

Effective horizontal 

hydraulic conductivity, 

in feet per day

greater than 200

200

100

50

20

10

5

2

1

0.1

0.01

m 

m

FIGURE 21. Perspective diagrams showing regional effective horizontal hydraulic conductivity in each

aquifer and permeable zone (layer) of the gulf coast aquifer systems in: A. the Mississippi embayment and

Texas coastal uplands aquifer systems, and B. the coastal lowlands aquifer system Continued.
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES 

CUMULATIVE FREQUENCY IN PERCENT

F37

100

50,000 100,000 150,000 200,000 

CONCENTRATION OF DISSOLVED SOLIDS, IN MILLIGRAMS PER LITER

FIGURE 22. Frequency distribution of concentrations of dissolved solids in water from 

about 20,000 sand beds in the gulf coast aquifers.

250,000

10,000 mg/L) to a brine (dissolved-solids concentration 

greater than 35,000 mg/L) occurs within a distance of 

about one to three model-block widths (10-30 mi).

Dissolved solids and temperature were mapped by 

Pettijohn and others (1988). The complicated distribution of dissolved solids in the aquifers and permeable 

zones is shown in figure 23. The approximate location 

of the 35,000-mg/L dissolved-solids concentration line 

(saline/freshwater interface) is also shown in figure 12 

for each layer. The distribution of brine concentrations 

also is variable when looking at a vertical slice of the 

aquifer system (fig. 24). For example, the well log at 

mile 40 of section G-G' shows substantial variability in 

dissolved-solids concentrations between adjacent sand 

beds, especially in layer 3. The vertically weighted average concentration of dissolved solids for each permeable zone at well locations is plotted on the sections 

shown on plate 1.

ESTIMATION OF DENSITY

Water density is affected by the concentration of dissolved solids, temperature, and hydrostatic pressure. 

Dissolved-solids concentrations were estimated by Williamson and others (1990, p. 63-65) from water resistivities obtained from the spontaneous potential curve of 

the 989 electric logs as described by Weiss (1987). A 

computer program requiring an iterative solution,

developed by Weiss (1982) and modified by Kontis and 

Mandle (1988), was used to estimate water density from 

concentration, temperature, and formation pressure 

based on equations and coefficients of Potter and Brown 

(1977). To simplify this calculation, Williamson and others (1990) presented a linear multiple regression using 

these data and estimates of density for 15,200 model 

blocks that have horizontal dimensions of 5 mi on each 

side. The regression equation estimated density within 

a standard error of estimate of 0.0020 g/cm3, about the 

value estimated by the iterative program. The regression equation is:

DENGW= 0.000648 TDS -0.000368 TEMPC +

0.0000015 D + 1.00472 (3)

where

DENG W=density of the ground water at the pressure 

and temperature in the aquifer, 

in grams per cubic centimeter, 

TDS = dissolved solids, in grams per liter, 

TEMPC= temperature, in degrees Celsius, and 

D = depth, in feet.

The equation in Williamson and others (1990, p. 65) 

is given in terms of pressure in bars rather than depth in 

feet. Equation (3) was converted to the more convenient 

value of depth by assuming a hydrostatic gradient of 

0.444 pound per square inch per foot [(Ib/in2)/ft] that
F38 REGIONAL AQUIFER-SYSTEM ANALYSIS--GULF COASTAL PLAIN

100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0 ,

100 95 90 85 55 50 45 40 35 30 25 20 15 10 5 0

EXPLANATION

Dissolved-solids concentration in 

grams per liter.

150

100

70

35

10

1

FIGURE 23. Perspective diagrams showing distribution of dissolved-solids concentrations in: A. the Mississippi embayment and 

Texas coastal uplands aquifer systems, and B. the coastal lowlands aquifer system.
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F39

B

100 95 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0

of Mexico 

PERMEABLE ZONE A AND MISSISSIPPI RIVER VALLEY ALLUVIAL AQUI

PERMEABLE ZONE B (LAYER 10)

I M I I I I I II M I I M I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I

PERMEABLE ZONE C (LAYER 9)

I I I I I I I M I I

PERMEABLE ZONE D (LAYER 8)

PERMEABLE ZONE E (LAYER 7)

100 95 90 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0

EXPLANATION

Dissolved-solids concentration in 

grams per liter.

FIGURE 23. Perspective diagrams showing distribution of concentration of dissolved solids in: A. the Mississippi embayment 

and Texas coastal uplands aquifer systems, and B. the coastal lowlands aquifer system Continued.
C1

Sea level - -2,000

-4,000 - -6,000 - p -8,000:

-10,000:-12,000 - -14,000-

-16,000

- Sea level- -2,000

- -4,000: -6,000 EXPLANATION

69 Concentration of dissolved solids 

for a sand bed, in grams per liter

63 Weighted average concentration for layer, in grams per liter

0 Weighted average concentration for layer with concentration less 

than 10 grams per liter

2 Model layer number

Line of section (also shown in fig. 7)

- -8,000

- -10,000

--12,000- -14,000

-16,000

o

240 280 

MILES

FIGURE 24. Vertical distribution of dissolved-solids concentrations in sand beds thicker than 20 feet along section C-C'.
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F41

corresponds to a density of 1.025 g/cm3. This approximation is adequate in many cases because pressure is 

the least sensitive factor. The variation of densities of 

water from the gulf coast regional aquifer systems is 

shown in figure 25. A substantial volume of the water 

has density less than 1.0 g/cm3 because temperature is 

elevated at depth and dissolved-solids concentrations 

are minimal at some locations. This is especially true in 

some of the deeper parts of the Mississippi embayment 

aquifer system, which contains mostly freshwater.

HYDRAULIC HEAD AND FLOW IN 

VARIABLE-DENSITY AQUIFERS

To understand the regional flow patterns in the gulf 

coast regional aquifer systems, one must understand 

how hydraulic head and flow are related in aquifers 

containing variable-density water. Unlike aquifers containing constant-density water, where flow is controlled 

only by the hydraulic-head gradient and the hydraulic 

conductivity, variable-density flow is also affected by 

the density and change in altitude of the aquifer top and 

bottom, which will be referred to as "gravitational 

effects." This second force affecting flow can be understood by taking the simple case where a bucketful of 

sand is saturated with freshwater except for one end, 

which is saturated with a heavier fluid such as a brine. 

Without any other force except for the difference in density, the brine will flow along the bottom of the bucket, 

displacing water vertically, until it fills the entire lower 

section of the bucket, with the freshwater "floating" on 

top of it. If the bucket is tilted, the brine will flow down 

until it fills the newly defined lowest section of the 

bucket. This is an exaggerated example of the force 

exerted in the aquifer system due to varying altitude 

and density. The actual flow in the aquifer will result 

from the vector sum of the two forces hydraulic gradient and gravitational effects due to differences in water 

density and altitude of the aquifer.

Therefore, in aquifers with variable-density water, 

hydraulic gradients, expressed either in terms of equivalent freshwater head or formation-water head, do not 

necessarily indicate flow directions or magnitudes. Neither do bottom hole pressures, commonly measured 

during drill-stem tests in the oil industry, indicate flow 

directions or magnitudes (Hanor and Bailey, 1983). 

Bond (1972,1973) combined the forces due to hydraulichead gradients and gravitational effects, defining the 

"head available to cause flow," O, between points a and b 

in the aquifer, as:

u

= (Ha- Hb) - J(r - 1 )dz (4)

where

Ha,Hb = equivalent freshwater head 

= (Z, altitude, plus P/rg, 

the pressure head), at 

points a and b,

b

\ - integral along flow path

a from point b to a,

r = relative density of the

ground water under the 

temperature and pressure 

of its sampling, compared 

with the density of pure 

water at 24°C and 1 

atmosphere pressure, and

dz = the change in altitude in the 

aquifer between points a 

andfc.

The difference in the first two terms, Hb - Ha, is the 

familiar difference in hydraulic head, with the minor 

adjustment of observed water level in a well or pressure 

data to equivalent freshwater head. The integral term 

defines the force exerted due to the variable density and 

altitude in the aquifer. Bond (1973) gives exceptions to 

this equation for two conditions. The first exception is 

where the structure of the top and bottom of the aquifer 

involves troughs, saddles, anticlines, and synclines that 

can cause gravitational effects. In this case, the head differences that are caused by the structure usually reduce 

the head that is available to cause flow. The second 

exception is where the rock contains permeability barriers that restrict horizontal flow within the aquifer or 

divert flow from horizontal paths. In this case, valid 

flow patterns cannot be deduced. These exceptions may 

be common because heterogeneity in aquifers is common (Bond, 1973).

The significance of the effect of density on hydraulic 

head and flow can be demonstrated using layer 8 (permeable zone D). Lusczynski (1961) defined "point-water 

head" [referred to as "hydraulic head" by Kuiper (1985)] 

as the water level, referred to sea-level datum, in a well 

filled sufficiently with water from the formation at the 

well screen to balance the existing pressure. This will be 

called formation-water head in this report. The variation of formation-water head for layer 8 based on simulation under predevelopment conditions is shown in 

figure 26. Note that the formation-water head map 

(fig. 26) has depressions in the brine area that appear
F42 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

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GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F43

like cones of drawdown, even though the map is from a 

steady-state, predevelopment simulation. The closed 

depressions are due to the variation in water density 

and sloping of the layers.

The freshwater head declines from more than 300 ft 

in the higher parts of the outcrop area to nearly zero 

and increases to more than 700ft near the deepest 

downdip extent of the layer (pi. 2D). This would be 

impossible in a freshwater aquifer, but it is possible in a 

system with variable density flow. The flow vectors 

(pi. 3D) are not necessarily perpendicular either to the 

freshwater head contours or the formation-water head 

contours in the brine part of the aquifer system.

EFFECTS OF DENSITY

The density of the water in the deeper parts of the 

aquifer systems has a substantial effect on the groundwater flow in the systems especially in the saline section, but also in the freshwater section. The simulations 

presented in this report do not assume that the brine is 

stagnant. Although often assumed, it is unknown 

whether the brine part of the system is in equilibrium 

with other large and long-term changes such as sealevel change and natural sediment compaction. Equilibrium conditions probably have not yet been achieved 

due to the relatively low flow velocities and the long 

flow paths to a discharge point.

The effect of spatially variable density is usually 

ignored in ground-water flow models by imposing a 

no-flow boundary at the saline/freshwater interface. In 

certain cases this sharp interface assumption is justified. 

The sharp interface assumption requires that the brine 

part of the system be in equilibrium with the freshwater 

part of the system so that freshwater flow is the only 

dynamic driving force. This assumption is adequate in 

cases like small-scale seawater intrusion where there 

are no other heat or salt sources so that the saline 

ground water has had enough time to reach equilibrium. In the study area, there are many forces which 

apparently have prevented the brine part of the system 

from reaching equilibrium, such as heat and salt 

sources, sediment compaction, geopressured sediments, 

changing sea level, and a very wide continental shelf 

that makes for longer flow paths to discharge water.

Although density is assumed to vary spatially, it is 

assumed to be constant in time so that the effect of solute transport on ground-water flow can be ignored. 

This assumption is valid over the relatively short (a few 

decades) time scales used in the transient simulations 

because the volume of water that can move into an adjacent block at maximum flow rates will not substantially

affect the average density of the water in these very 

large blocks in the regional model.

Due to the complexity of the aquifer systems and the 

effect of density on ground-water flow, the flow paths 

in and near the area containing brine are very complex 

in all three dimensions (fig. 27). The complicated threedimensional flow pattern is difficult to display and fully 

visualize; however, some patterns can be seen by looking at Darcy velocity in three dimensions as shown in 

figure 27. Recharge in outcrop areas of each layer near 

the Mississippi-Louisiana State line moves downdip 

and south and then to the west, discharging in east-central Louisiana.

Several types of simulations confirmed the large 

effect of density on flow. One test compared two simulations that were identical except that one simulation 

specified the density for all the model blocks to 1 g/cm 

(the density of freshwater) instead of the actual estimated density. Even at Memphis, Tennessee, which is 

about 200 mi updip from the limit of freshwater in layer 

5 (fig. 12), the difference in simulated head was nearly 

1 ft (Williamson and others, 1990, p. 103). Assuming the 

entire aquifer system is composed of freshwater, there 

would be less drawdown caused by the pumping an 

effect similar to moving a limited flow boundary farther 

away.

In addition, all simulations indicate that substantial 

volumes of flow occur in the saline part of the aquifer 

system (pis. 3, 6). The distribution of salinity has a high 

degree of local variability (pi. 1, figs. 23, 24), consequently, the relatively sparse spatial-data density (fig. 8) 

available for this analysis contains variability in addition to the regional trend that is still discernible. Williamson and others (1990, p. 103-106) tested the effect 

that local variation in density had on the flow simulation by smoothing the density data sets for each layer 

using an areal moving average. The smoothed and 

unsmoothed density data sets were used to make otherwise identical simulations. Some small-scale variations 

were eliminated by the smoothed density data sets; 

however, the regional pattern of flow was very similar 

to the flow using the unsmoothed density data sets 

(Williamson and others, 1990, figs. 26G, 30). All simulations presented in this report were made using the 

smoothed density data sets to show the effects of the 

regional trends in density.

Flow in the brine part of the aquifer systems appears 

not to have reached equilibrium. The time required for 

the flow system to adjust to processes such as sea-level 

change, additional sedimentation and compaction, and 

salt dissolution is long, perhaps thousands of years. 

Such time spans are owing to the very long flow paths 

necessary for water to move to a discharge point and
F44 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

EXPLANATION

 200 Line of equal formation-water head Hachures indicate depression 

Interval is 50 feet. Datum is sea level. Formation-water head 

is the altitude of water level in a well when the well is full 

of formation water

 10,000 milligrams per liter Solids

 35,000 milligrams per liter Solids

KENTUCKY

98°

Base from U.S. Geological Survey 

Scale 1:2,500,000 0 50 100 150 200 KILOMETERS

FIGURE 26. Simulated predevelopment hydraulic head of formation water in layer 8 (permeable zone D of the

coastal lowlands aquifer system).
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F45

K

25

Vertical scale greatly exaggerated

EXPLANATION

 Line of 10,000 milligrams 

^^^ per liter dissolved solids 

<CJ Red arrows indicate a net upward

component of flow 

<C1 Green arrows indicate a net downward

component of flow

Each successively larger arrowhead represents 10 

times the rate of flow. Vertical flow components are 

calculated by averaging the flows to adjacent blocks 

before vector addition of the vertical and horizontal flow

FIGURE 27. Perspective view of Darcy flow rates and directions in layers 8 through 11 in a 22,500-square-mile area

around Baton Rouge, Louisiana.
F46 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

the relatively slow ground-water velocity. Predeveloprnent simulations indicate that there is a complex, threedimensional flow pattern of brine (pi. 3) and that average velocities of about a meter per year occur in some 

blocks containing brine. The flow pattern in the brine 

part of the system is largely controlled by the distribution of ground-water density, as evidenced by looking 

at the results of a large number of simulations with 

varying parameters. Although density estimates vary 

substantially from one sand bed to the next and from 

one well log to the next, there is confidence in the 

regional trend (fig. 23) in density due to the large number of logs used (fig. 8).

Assumptions about how hydraulic conductivity varies with depth of burial, as a result of compaction, affect 

the model's sensitivity to density because most dense 

water occurs at depth. Therefore if it is assumed that 

conductivity decreases rapidly with depth, all of the 

flows at depth will appear to be much smaller and 

hence have less effect on flow in the freshwater part of 

the aquifer systems. The amount of change of the 

hydraulic conductivity with depth is uncertain. Williamson and others (1990, p. 107), for simplicity, 

assumed that hydraulic conductivity did not vary with 

depth, so the results presented in the preliminary report 

would indicate a larger value of average regional flow 

in the brine parts of the aquifer system. The simulations 

presented in this report assume that the K decreases 

substantially with depth (see section "Depth Dependence of Hydraulic Conductivity"). Therefore, much 

smaller flows are shown in the deeper parts of the aquifer system on the vector maps (pis. 3, 6).

Large differences in water chemistry on opposite 

sides of confining units or fine-grained beds can introduce osmotic gradients (Jones, 1969, p. 63, 81-86; Glasstone, 1946, p. 651; Young and Low, 1965). These osmotic 

gradients induce flow from the fresher side of the confining unit toward the more saline side in order to 

equalize the concentrations. These gradients may 

induce substantial flow. However, the estimation of the 

rate of this flow is wholly dependent on estimating 

osmotic conductivities. The values of osmotic conductivity can vary many orders of magnitude, and the characteristics of the confining units and water chemistry, 

which control the values, are largely unknown. Therefore, osmotic effects have been ignored in this study.

FORMATION OF BRINE WATERS

There has been much speculation about the source of 

salt and mechanisms causing widespread occurrence of 

the brine waters found downdip in all waters of the gulf 

coast regional aquifer systems. Jones (1969, p. 59) 

accepted the theory of widespread diagenesis (De Sitter,

1947, p. 2040) of formation waters whose original 

source was connate seawater (Clayton and others, 1966, 

p. 3873). Bredehoeft and others (1963) discussed the 

possibility of clay beds acting as membrane filters to 

concentrate brines on the side that flow is coming from. 

Hanor (1987) presented geochemical and physical evidence for the existence of density inversions in Gulf 

Coastal Plain sediments sufficient to drive large-scale 

convective fluid flow. The inversions are caused in part 

by the dissolution of salt domes and the formation of 

dense brines at shallow depths. The circulation is thus 

considered thermohaline in nature. Hanor and others 

(1986) and Bennett and Hanor (1987) suggested that 

these effects have regional significance in Gulf Coastal 

Plain sediments. Simulations done for the Gulf Coast 

RASA indicate that this is a possible mechanism to disi tribute brines throughout the gulf coast regional aquifer 

systems, even though salt domes occur only in parts of 

the systems.

The possibility of dissolution of salt domes as the 

primary source of brine in the gulf coast regional aquifer systems was explored using location (fig. 7) and 

other general information on all recognized salt domes 

in the Gulf Coastal Plain and adjacent Continental 

Shelf. The caprock is composed chiefly of anhydrite 

(Halbouty, 1979, p. 45), whereas less than 3 percent of 

the salt dome content is anhydrite (Kupfer, 1963).

I Assuming that the existing caprock is whatever remains 

from previous dissolution of salt (Goldman, 1952), 

which is the most common view (Halbouty, 1979, p. 45), 

an estimate of the volume of salt that has dissolved to 

date can be made. Beckman and Williamson (1990) compiled the information from eight published sources, recognizing 624 salt domes. Data on the depth to the top of 

salt were available for 349 of these domes (fig. 28). Data 

on the depth to the top of the caprock were available for 

185 of these domes. Estimated average diameter was 

available for most of the domes whose depth was 

known. For others, the average diameter of 1.25 mi was 

used. The total volume of caprock in the 185 domes 

with sufficient information is 15.5 mi3. Dividing the 

total volume of anhydrite by 3 percent yields about 

500 mi3 of dissolved salt. If that volume were prorated 

to estimate the total, including domes whose caprock 

depth is unknown (624/185), the volume would be 

about 1,700 mi3.

Assuming most of the sediments containing brines 

today were originally deposited in marine environI ments and thus contained seawater, it is possible to calculate the volume of salt needed to get the distribution 

of salt concentration that exists today. Using the aquifer 

geometry and estimated concentration for each model 

block to calculate that volume of salt, when summed for 

all of the blocks, yields about 600 mi3. Admittedly, both
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F47

CAPROCK 

(-100% ANHYDRITE)

SALT 

(3% ANHYDRITE)

CAPROCK 

THICKNESS

CAPROCK VOLUME=

(CAPROCK

THICKNESS/5,280)

(DIAMETER/2 )27C

FIGURE 28. Idealized geometry of salt dome and caprock.

of these calculations are only rough approximations; 

however, it is very interesting to note that the two volumes are in general agreement and demonstrate the 

possibility that the major source of salt for the brine 

ground waters could have been dissolution from salt 

domes.

More evidence that the dissolution of salt domes is 

the primary source of salt in the brines is the chemical 

composition and areal distribution. Pettijohn (in press) 

has shown, based on discriminant and factor analysis of 

tens of thousands of water analyses, that the major 

geochemical process controlling chemistry of the 

ground water in the brine area is dissolution of halite 

(salt). With a few exceptions, the distribution of salt 

domes is closely correlated with the areal distribution of 

brines with concentrations in excess of 35,000 mg/L dissolved solids (fig. 29, calculated as described in the previous paragraph). The exceptions are in the east Texas 

and north Louisiana salt basins (Beckman and Williamson, 1990) (fig. 29). In both of these basins, there is very 

little ground water with dissolved-solids concentrations 

greater than 35,000 mg/L; however, the sediments are 

predominantly much older and shallower than in the 

other salt basins. This anomaly could be explained, at 

least in part, because much of the brine in these areas 

could have been diluted and flowed out to the surface 

or could have moved downdip over the very long time 

since deposition and subsequent salt-dome penetration 

and dissolution. Another anomalous condition might 

be explained by this hypothesis. Many salt domes 

whose tops are near land surface appear to be entirely 

surrounded by freshwater. It seems possible that the 

caprock that develops on top also develops on the sides

as a thin veneer. Anhydrite is 30 times less soluble than 

salt, therefore forming a casing and preventing more 

salt from dissolving. The diverse nature of salt-dome 

uplift timing and resultant structure provides for much 

variation in the thickness of the caprock as well as the 

side casing of anhydrite.

An additional source of salts that has been given 

very little consideration is the massive salt formations 

that occur throughout the entire Continental Slope 

(Martin, 1980), downdip from this study area. These 

probably have been given little consideration previously because they occur so far offshore, away from the 

high dissolved-solids concentrations much farther 

updip and the common, though possibly mistaken, 

assumption that ground water always flows downdip. 

The Continental Slope contains a very large source of 

dissolvable salt and probably is a source of abnormally 

high temperatures because of the salt. Therefore the 

Continental Slope is a possible source of salts that have 

moved long distances coastward and updip, driven by 

density differences.

LEAKAGE AT BASE OF FLOW SYSTEM

FLOW ACROSS MIDWAY CONFINING UNIT

The base of the flow system for the regional flow 

model is assumed to be the top of the Midway confining 

unit or the top of the zone of geopressure, whichever is 

shallower (fig. 8). The effect of the small amount of flow 

leaking up through the Midway confining unit, which is 

composed of marine clays several hundred feet thick, is 

thought to be insignificant compared to the volume of 

flow in the aquifer above it. A detailed investigation 

into this possibility was done as part of the analysis of 

McNairy-Nacatoch aquifer, which directly underlies the 

Midway confining unit in the northern Mississippi 

embayment (Brahana, 1987; Brahana and Mesko, 1988). 

The McNairy-Nacatoch aquifer in this area is composed 

predominantly of sand and is distinctly different from 

the equivalent Upper Cretaceous limestone in the 

southern Mississippi embayment (approximately south 

of the 35th parallel) and below the Texas coastal 

uplands aquifer system.

Under predevelopment conditions, there was less 

than 12 Mgal/d (18.3 ftVs) (Brahana and Mesko, written commun., fig. 43) flowing up through the Midway 

confining unit, which is small relative to horizontal flow 

in the lower Wilcox aquifer (122 Mgal/d, fig. 33, pi. 3) 

(Arthur and Taylor, 1990; Arthur and Taylor, 1998). 

Under 1985 conditions, there was still less than 17 

Mgal/d (26 ft3 /s) (Brahana and Mesko, 1988, fig. 64; 

Brahana and Mesko, 1998, fig. 52) flowing up through
tinental Shelf

Base from U.S. Geological Survey 

Scale 1:2,500,000

EXPLANATION

Top of salt dome

Above top of Midway confining unit 

^ Below top of Midway confining unit 

O Depth unknown

Volume of excess 

salt Expressed as depth, in feet

1.5 1.0 

0.5 

0.0

FIGURE 29. Location of salt domes and distribution of volume of dissolved solids from ground water with concentrations greater than 35,000 milligrams per liter.
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F49

the Midway confining unit and any fracture zones penetrating it, which is small relative to the horizontal flow 

in the lower Wilcox aquifer (173 Mgal/d, fig. 33, pi. 6) 

(Arthur and Taylor, 1990; Arthur and Taylor, in press). 

Therefore, the Midway confining unit has been treated 

as a no-flow boundary in the simulations presented in 

this report. If larger volumes of pumpage were simulated in the lower Wilcox aquifer, this no-flow assumption would need modification.

An exception to the basal Midway confining unit 

being the base of the flow system exists in the extreme 

northwestern part of the Mississippi embayment. The 

Mississippi River Valley alluvial aquifer extends 

beyond the subcrop of the Midway confining unit in an 

area of a few hundred square miles and directly overlies 

the McNairy-Nacatoch aquifer and Paleozoic rocks of 

the Central Midwest aquifer system. According to Brahana and Mesko, pinching out of the Midway and occurrence of the McNairy-Nacatoch aquifer directly above 

the Paleozoic rocks allow substantial flow between the 

Paleozoic rocks and the Mississippi embayment aquifer 

system (Brahana and Mesko, written commun.). The 

flow is predominantly from aquifers in Paleozoic rocks 

discharging to the Mississippi River Valley alluvial 

aquifer, which in turn discharges as rather large flows 

to streams. Nearly all of these flows occur within the 

same regional model block and therefore are not considered regional flow.

LEAKAGE FROM THE GEOPRESSURED ZONE AND 

EFFECT OF DEEP FLUID INJECTION

Downdip and gulfward in the gulf coast regional 

aquifer systems, the top of the zone of geopressure is 

above the top of the Midway confining unit (fig. 8). 

Geopressured zones are defined as areas where the 

pressure substantially exceeds the hydrostatic pressures 

caused by the regional ground-water flow system. The 

high pressures were created by other forces. Geopressured zones exist in the transition from a predominantly 

coarse-grained facies to a predominantly fine-grained 

facies. Substantial fluid pressures in the geopressured 

zone force some flow upward into the aquifer system. 

The rate of flow is thought to be small because of the 

minimal vertical hydraulic conductivity of the predominantly shaly sediments, which is confirmed by the fact 

that the high pressures still exist. Hypotheses about the 

values of conductivities and flows were tested by 

Kuiper (1994) using various simulations. He concluded 

that the sparsity of reliable deep-head data precluded 

the possibility of determining the volume of flow from 

the geopressured zone on the basis of simulations. Most

of his model simulations assumed zero flow from the 

geopressured zone as does this report.

Oil-field brines and other hazardous wastes are 

injected into the brine section of the aquifer system, and 

oil and brine are pumped out of the same section. This 

study generally assumed that the volumes are not large 

enough to substantially affect regional ground-water 

flow or quality of water in the freshwater part of the 

system. Using data on oil production by the Louisiana 

Department of Natural Resources (1986), a simple comparison of pumpage totals for Louisiana indicates that 

this pumpage is less than 10 percent of the volume of 

the freshwater pumpage for the same period of time. 

Crude oil production for the State of Louisiana in 1985 

was about 185 million barrels (28 Mgal/d) (Louisiana 

Department of Natural Resources, 1986, p. 146), including oil and condensate but not including gas volumes. 

Ground-water pumpage for the same year for all of 

Louisiana was about 1,800 Mgal/d (Mesko and others, 

1990). Assuming a 3:1 ratio of brine to oil withdrawn, 

which varies widely but is thought to be an average, the 

total withdrawal of fluids would have been about 

110 Mgal/d, or about 6 percent of the volume of the 

freshwater pumpage for Louisiana for 1985. This would 

still be a small percentage of the total withdrawals, even 

after increasing this volume somewhat to account for 

gas production.

PREDEVELOPMENT GROUND-WATER 

FLOW

The dating of "predevelopment" conditions is complicated by the fact that ground-water development in a 

few areas, such as at Memphis, Tennessee, dates back to 

decades before 1900. In most of the study area, however, 

ground-water development did not occur until decades 

after 1900, with large-scale developments not active 

until the 1930's or 1940's. Predevelopment conditions in 

this report generally refer to conditions in 1925 or 

before.

HYDRAULIC HEAD AND HORIZONTAL FLOW

Simulated predevelopment freshwater hydraulic 

head for each aquifer is shown on plate 2, and the 

regional ground-water flow directions and relative 

magnitudes are shown on plate 3. The plotted vector 

equals the vector addition of the average of the two 

adjacent x-direction flows and the average of the two 

adjacent y-direction flows. The arrows show the direction of horizontal flow and its magnitude both qualitatively (by the arrowhead size) and quantitatively (by
F50 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

the arrowshaft length as well). The arrowhead size signifies the order of magnitude [(integer component of 

the logarithm, base 10 (Iog10)], of the flow, and the shaft 

length is proportional to the mantissa (decimal component) of the Iog10 flow. The type and color of the round 

symbols on the flow maps show the direction of vertical 

flow out of or into the bottom of each layer. The size of 

the symbol is proportional to the natural logarithm of 

the magnitude of the vertical flow.

Hydraulic-head data were sparse outside the outcrop of the individual aquifers for predevelopment conditions. The model was primarily calibrated using 

1960-85 hydraulic-head data. Predevelopment conditions were simulated by removing ground-water withdrawal. Flow patterns presented in this report are more 

complex than those presented in the preliminary report 

(Williamson and others, 1990) because of the addition to 

the simulations of variations of hydraulic conductivity 

(K) by area sand percentage and depth. In the earlier 

report, for simplicity, K was assumed to be constant for 

each aquifer. The simulation results should be used and 

interpreted with caution, especially on the small scale of 

individual flow values. More confidence can be placed 

in the large, regional flow trends composed of several to 

many arrows, which are noticeable on plate 3. The 

regional flow trends were similar in most of the different calibration attempts. Only such features that are 

more likely to remain constant through a broad range of 

simulation parameters will be described in the text.

In the Mississippi embayment aquifer system, the 

predominant regional flow pattern, especially in layers 

2, 4, and 5, is from major recharge areas on the eastern 

side of the embayment to discharge areas in layer 11, 

which is mostly in the central and western parts of the 

embayment (pi. 3). This flow pattern primarily is 

caused by the topography (fig. 3), which is asymmetrical, in that the valley lies on the western side of the 

embayment and the topographically higher hills are 

mostly on the eastern side. The dominant feature of the 

topography is the flat, low-lying Mississippi Alluvial 

Plain. The river, because of its size, is an important feature of the hydrologic system. It generally traverses the 

eastern side of the plain. Some recharge is simulated on 

the western side of the embayment in the uplands south 

of the Arkansas River and on Crowleys Ridge, a 1- to 3- 

mi-wide ridge extending north to south about 100 mi in 

the north-central part of the Mississippi Alluvial Plain. 

The horizontal flow rates in layer 3 are substantially 

larger in the Texas coastal uplands aquifer system than 

in the Mississippi embayment aquifer system because 

of the larger hydraulic conductivity (fig. 21).

One of the more noticeable flow paths in the aquifers of the Mississippi embayment aquifer system is 

from a major recharge area in south-central Mississippi

that radiates outward to the south and to the west 

(pi. 3). A large part of this flow moves southwestward 

and then curves northwestward to the major discharge 

area in northeastern Louisiana. This curvature in flow 

paths is partly because of the restriction of vertical flow 

downdip. More importantly, what might have been a 

discharge area for the Mississippi embayment aquifers 

in southern and western Mississippi is really a regional 

recharge area for the coastal lowlands aquifers due to 

high land-surface (and water-table) altitude, reversing 

the flow direction to downward. This gradient forces 

the flow in the lower units to turn westward to discharge in the lowland area of northeastern Louisiana. In 

layer 6, the fresh ground water flows downdip under 

the Vicksburg-Jackson confining unit and turns to the 

northwest to flow updip from under the confining unit 

to discharge to the Mississippi River Valley alluvial 

aquifer in an area of low topography in northeastern 

Louisiana. Throughout most of the areal extent of layer 

6 there is upward flow from underlying aquifers.

Another major predevelopment flow pattern 

involves recharge in the outcrop of each aquifer in 

south-central Arkansas, downward flow into layers 6,4, 

and especially 5, and then southeastward flow across 

Arkansas, towards the regional discharge area in northeastern Louisiana (pi. 3). In the northern part of the 

Mississippi embayment aquifer system, the dominant 

flow pattern is from east to west toward the flow axis of 

the embayment, which is west of both the geological 

axis and the Mississippi River (fig. 7). In the Mississippi 

River Valley alluvial aquifer, the flow directions are 

quite complex because the water moves in the direction 

of the shortest circuit for flow to one of the major surface rivers or drains. The lowest freshwater head in 

nearly every Mississippi embayment aquifer occurs in 

northeastern Louisiana (pi. 2).

One of the interesting smaller features of predevelopment flow is in southern Arkansas and northern Louisiana in layers 3 and 2 (pis. 31, 3J). Notice that the flow 

lines converge approximately toward the axis of the 

Ouachita River and that the line of equal concentration 

of dissolved solids takes a downdip bend at the same 

location, even though layers 3 or 2 do not outcrop in 

this area. Apparently, the water-table altitude decreases 

due to the Ouachita River Valley increasing leakage 

from layer 2 to the surface and drawing fresher water 

farther downdip in the section at this location. A similar 

flow pattern is seen along the Red River (pis. 31, 3J).

In the Texas coastal uplands aquifer system, the predominant regional flow pattern is from recharge areas 

between major river valleys laterally to discharge areas 

along the valleys (pi. 3). In southern and south-central 

Texas, there is a pronounced regional flow pattern. Flow 

is lateral (parallel to the strike of the outcrops of the
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F51

units) from the southwest toward the northeast, as indicated on both the head (pi. 2) and flow (pi. 3) maps, 

especially in layers 3, 4, and 5. The Vicksburg-Jackson 

confining unit covers a larger percentage of the Texas 

coastal uplands aquifer system than it does of the Mississippi embayment aquifer system. The VicksburgJackson confining unit has a substantial effect on the 

flow pattern in addition to the effect of topography. It 

impedes upward flow out of the downdip parts of the 

Texas coastal uplands aquifer system, effectively 

restricting longer flow paths in the downdip direction. 

This is demonstrated by noting that freshwater in layer 

6 becomes saline close to the outcrop except for the area 

in southwestern Mississippi, as described previously 

(pi. 3F). In the Texas coastal uplands aquifer system, 

several major flow paths are toward the Nueces, Brazos, 

Neches, and Sabine Rivers and the Rio Grande.

In the coastal lowlands aquifer system, the predominant flow pattern is from recharge areas in the updip, 

topographically higher areas along the inland edge of 

the system to discharge areas onshore near the coast. 

The horizontal flows radiate outward from the higher 

topography recharge areas in southwestern Mississippi, 

eastern Texas, west-central Louisiana, and southern 

Texas (pi. 3). In layers 7 through 10, the widest band of 

freshwater occurs where the radial flow pattern in 

southwestern Mississippi shows the effects of the largest area of regional recharge in the hills. Generally, the 

downdip flow pattern continues across the interface of 

saltier water, though the flow is diminished because at 

this point the aquifer is quite deep and the hydraulic 

conductivity has diminished.

The minimum freshwater hydraulic head in layer 11 

in the coastal lowlands aquifer system never reaches sea 

level (minimum about 3 ft) before it starts increasing 

offshore due to density differences (pi. 2). The flow map 

for layer 11 (pi. 3A) shows flow perpendicular to the 

head contours (pi. 2A) and toward the coast (lower altitude) onshore as would be expected in a freshwater system. Near the coastline in several places, the flow 

direction reverses. Offshore, the flow direction is quite 

varied, with several areas of converging flows where 

the water moves downward into the lower units.

Flow patterns in the deeper coastal lowlands aquifer 

system layers are more difficult to interpret because of 

the large variability due to the complex three-dimensional distribution of ground-water density. Layer 10 

shows a pattern of hydraulic head and flow similar to 

layer 11. There is a striking uniformity of directions of 

flow in two freshwater parts of layer 10, toward the 

southeast in the area west and south of Houston, Texas, 

and toward the east in southern Texas. Unlike layer 11, 

but more like the lower layers, the freshwater head in

layer 10 rises substantially offshore due to the effects of 

density. Some circulation cells appear in that part of the 

layer containing brine water. It is interesting to note that 

off the coast of south-central Louisiana, where layer 11 

shows convergent horizontal flow and downward flow 

out the bottom, layer 10 shows divergent flow paths 

and downward flow out the bottom.

Regional ground-water flow in layer 9 generally 

moves downdip from the outcrop except in the discharge area adjacent to the Mississippi River. The variability of flow far downdip from the outcrop area 

becomes more pronounced in layer 9 because it is 

deeper and more saline than overlying layers and therefore, more affected by the density driving force. Circulation cells that appear on plate 3C can occur solely due to 

the density forces resulting from heat and solute gradients (L.K. Kuiper, oral commun., 1985) as the denser, 

heavier water overturns to flow under the fresher, 

lighter water. These circulation cells can continue indefinitely, somewhat analogous to convection currents in 

the atmosphere if a continuous driving force is present 

in the ground-water flow system such as a heat source 

or salt dissolution. The pattern of hydraulic head in layers 7, 8, and 9 approximately follows what has already 

been described for layers 10 and 11. The flow maps are 

more complex due to the high density of the water and 

the substantial variability in the density of the water 

spatially. The part of the aquifers containing freshwater 

is fairly narrow compared to the part containing brine.

RECHARGE AND DISCHARGE BUDGET

Ground-water flow can be considered at various 

scales, from small valleys with recharge on a hill and 

discharge to a nearby creek in the valley, to regional 

flow patterns across counties or States (fig. 30). This 

investigation is concerned with regional flow that generally involves flow paths of tens of miles or more. The 

small-scale (or local) flow paths, which generally have 

higher rates because the flow paths are shorter (a few 

miles or less) and the gradients are steeper, will not be 

shown or discussed in this report. They were not simulated because the short flow paths do not cross a model 

block face (fig. 30). Only the net flow across a model 

block face is considered in these simulations. In addition, there is some canceling of effects of recharge and 

discharge within a model block. Local recharge would 

cancel out an equal amount of local discharge (fig. 30) 

and therefore would not be simulated in these regional 

models. These factors should be considered when comparing water budgets and rates of regional recharge and 

discharge from this report to detailed studies of smaller
F52 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN 

MODEL BLOCKS

-10 miles- 10 miles- 10 milesMississippi River alluvial aquifer

10 miles- 10 milesEXPLANATION

Regional-scale ground-water flow 

Simulated in regional model

Local-scale ground-water flow Not

simulated in regional model

Confining unit 

Permeable zone 

Surficial aquifer

Figure 30. Generalized local- and regional-scale ground-water flows.

areas that probably show higher rates of recharge and 

discharge.

Simulation indicates that the total predevelopment 

regional recharge is about 2.9 Ggal/d. This equals 

regional net discharge assuming steady-state conditions. Although some effects of long-term changes like 

sea-level change may exist, the effect of these transient 

conditions on the regional budget is probably minimal 

because of the size of the study area. Regional recharge 

equals only 0.6 percent of the total precipitation (about 

500 Ggal/d, fig. 4) in the study area. Most of the precipitation returns to the atmosphere in a short time as 

evapotranspiration (ET) because much of the precipitation occurs during the hot months when ET rates are 

highest (fig. 5). Most of the remaining precipitation 

becomes runoff (about 140 Ggal/d) (fig. 6) either 

directly or by recharging small, local ground-water flow 

systems that discharge to creeks and streams nearby.

The net regional recharge amounts to only about 0.5 

percent of the total surface-water flow (523 Ggal/d) 

from the many large streams that cross the study area 

(Williamson and others, 1990, table 1, p. 97). Regional 

recharge cannot reliably be estimated by stream budgets because of the contrast in sizes of the flows.

Predevelopment regional ground-water budgets 

(fig. 31) by geographic area (fig. 13) show that the Mississippi Alluvial Plain has the largest total flow. In the 

Mississippi Alluvial Plain, southwestern Louisiana, and 

to some extent the Winter Garden area, inflow from 

adjacent geographic areas is a substantial source of 

water. In the other areas, net regional recharge is the 

dominant source of water. In several geographic areas 

including the eastern embayment, western embayment, 

eastern coastal lowlands, and the Winter Garden area 

(listed in order of proportions), outflow is dominantly 

to adjacent areas rather than to net regional discharge.
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES

AREA GROUND-WATER BUDGETS, 1987

F53

NORTHEASTERN 

TEXAS

WINTER 

GARDEN

MISSISSIPPI 

ALLUVIAL PLAIN

WESTERN 

EMBAYMENT

SOUTHEASTERN 

TEXAS

SOUTHERN 

TEXAS

EASTERN 

EMBAYMENT

SOUTHWESTERN 

LOUISIANA

3

EASTERN COASTAL 

LOWLANDS

EXPLANATION

INFLOW FROM OTHER

AREAS 

NET REGIONAL

RECHARGE

NET REGIONAL DISCHARGE

OUTFLOW TO OTHER AREAS

Area of each circle indicates relative quantity of flow into the aquifer system 

in the designated area. Water-table storage from Ackerman, 1996.

FIGURE 31. Simulated predevelopment ground-water budgets, by area.

DISTRIBUTION OF RECHARGE AND DISCHARGE

Predevelopment net regional recharge occurred in 

41 percent of the modeled area at the average rate of 

about 0.48 in/yr (fig. 32). The largest rate was in southwestern Mississippi at about 6 in/yr. Predevelopment 

net regional discharge was occurring on 59 percent of 

the area at the average rate of about 0.35 in/yr (fig. 32). 

The largest net regional discharge rate was in the Mississippi Alluvial Plain at about 4 in/yr.

The distribution of regional net recharge and discharge (fig. 32) shows a high degree of correlation with 

the altitude of the predevelopment water table and the 

land-surface altitude (fig. 3). Areas with the largest

regional recharge are in the northern part of the "boot" 

in eastern Louisiana, an area in south-central Louisiana, 

and one in southern Texas. The areas with substantial 

regional recharge rates are relatively large, topographically high areas adjacent to low areas where the 

regional flow paths are generally in one layer. The flow 

paths are short, the gradients steep, and the resistance 

to flow is relatively small, partly because the vertical 

component of flow occurs all in one layer. The largest 

regional recharge area in the study occurs in southwestern Mississippi and adjacent Louisiana where the relatively high topography is a major driving force, 

inducing regional recharge that flows downward to
F54 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

\~37

\ KENTUCKY

96

33

Base from U.S. Geological Survey 

Scale 1:2.500,000

EXPLANATION

Location of model blocks and identification of 

hydrographs shown in Figure 36.

-4.0 -2.0 -1.0 -0.5 -0.2 -0.1 0.0 0.1 0.2 0.5 1.0 2.0 4.0 E

DISCHARGE RECHARGE

FIGURE 32. Simulated predevelopment net regional ground-water recharge and discharge, and location of hydrographs.
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F55

underlying layers and radiates horizontally in nearly all 

directions (pi. 3). The large area of regional recharge in 

southwestern Mississippi and adjacent Louisiana generally is coincident with the greatest thickness of freshwater mapped in Mississippi by Gandl (1982) and in 

Louisiana by Winslow and others (1968).

The location of the Vicksburg-Jackson confining unit 

(layer 15 in fig. 3) has a substantial effect on the distribution of recharge and discharge. Areas updip from the 

outcrop of the unit are regional discharge zones, probably due to two factors. As described previously, upward 

vertical discharge from the underlying aquifers is 

impeded by the Vicksburg-Jackson confining unit, forcing more discharge immediately updip from its outcrop. Also, in some places, particularly eastern and 

southern Texas, the Vicksburg-Jackson confining unit 

outcrop coincides with a band of higher topography; 

therefore, these areas are regional recharge areas.

There are many large structural features in the study 

area (fig. 7), such as the Jackson dome, the Desha Basin, 

and the San Marcos Arch, which affect the regional 

geology. The effect of these structural features on the 

predevelopment regional ground-water flow is not as 

significant as the configuration of the water table and 

the permeability of the geohydrologic units (Grubb, 

1986, p. 18). This is also indicated by comparing figure 

32 with figure 7.

The amount of recharge to the regional groundwater flow system is also probably not related to the 

mean annual precipitation (fig. 4) over most of the 

study area because of the relatively humid climate. If 

regional ground-water recharge is controlled by the 

amount of precipitation, the depth to the water table 

should be related to precipitation. Williamson and others (1990, p. 97-98) tested this possibility by several 

types of regression analyses that indicated no relationship between the measured depth to water and mean 

annual precipitation based on measurements of water 

levels made before 1960 in about 7,000 shallow wells.

It is more likely that the amount of precipitation that 

recharges the regional flow system is limited by the 

capacity of the flow system to transmit water from 

recharge areas to discharge areas. Where the regional 

ground-water flow capacity is limited by flat regional 

hydraulic gradients or by the resistance to flow in the 

aquifer system, most of the water that infiltrates from 

precipitation as potential regional recharge is discharged to local surface-water bodies such as creeks 

and streams, or to evapotranspiration. In the extreme 

southwestern part of the area, where the climate is 

much drier, recharge to the regional ground-water flow 

system may be limited by the lesser precipitation, much 

of which falls in the hot summer months and is quickly 

evaporated or transpired.

Some ground water flows offshore before being discharged to the ocean, but most is discharged before it 

reaches the coastline (fig. 32). Maximum discharge 

occurs in an inland band parallel to the coastline. In 

southern Mississippi, higher topography exists closer to 

the coast than in Texas. The higher topography produces steeper horizontal gradients in the ground-water 

system, steeper upward vertical gradients offshore 

(pi. ID), and more fresh ground-water discharge from 

offshore Mississippi than from offshore Texas (pi. I A). 

Fresh ground water extends beneath some islands offshore from southern Mississippi (Sumner and others, 

1987, p. 7, figs. 27-28); but to the west, along the Louisiana and Texas coastline, the only freshwater offshore is 

a relatively thin lens that is recharged and discharged 

locally (see layer 10 in fig. 12B). Deep, fresh ground 

water does not extend offshore to the east, but probably 

for a different reason. The aquifers that are onshore 

decrease in hydraulic conductivity to the east and offshore. On Dauphin Island, offshore from Mobile, Alabama, the only fresh ground water found was a 

relatively thin lens of freshwater in a water-table aquifer recharged on the island (Kidd, 1987).

VERTICAL FLOW BETWEEN LAYERS

Although vertical hydraulic conductivity is commonly several orders of magnitude smaller than horizontal hydraulic conductivity, the geometry of the 

aquifer system where the width greatly exceeds the 

thickness of each unit causes a large area for vertical 

flow and a large head gradient because vertical flow 

paths are relatively short. These factors cause the vertical flow to be about the same magnitude as the horizontal flow despite the large difference in conductivity. The 

upward flows between layers were generally larger 

than the downward flows (fig. 33) under predevelopment conditions. The individual flows across block 

faces in the model represent net flows across that particular face of the block. The net flow is the difference 

between positive and negative flow across the individual block face. The flows in figure 33 were obtained by 

adding separately the negative and positive net flows 

across all of the model block faces between adjacent layers. The net flows between layers (not shown in fig. 33 

due to lack of room, but easily calculated) equal the difference between the upward and the downward flows.

In the coastal lowlands aquifer system, all of the net 

flows between layers are upward, the largest between 

layers 10 and 11. The flows become progressively 

smaller down through the layers, 9-10, 8-9, 7-8 (fig. 33). 

Recharge in the topographically higher outcrop areas 

flows downdip and discharges upward through the
1221

830

RECHARGE DISCHARGE 2904

11 SUBCROP

445ftI2904 ., 0.5 - 7.8 78 7.8-

61.5

130 10.6-

228

0.5

342

t I

35

295

161 

t , 22.3 

249

'l30

245

__t_

33.5

13.2<

II 

237

-f 0.2

187

88.6

t;

113

_t_f 4.2

u

28.4 77.7

27.0

t

264 259«

I I I I 

16.7

30.1 245

63.8

83.5 _ 

t ' 30.9 «-

12.4 , 

0

92.4'

-» 4.0

I 252

* 17.9

I 

365

-+ 8.4

57.4

317

6

1.5

6.5

744

-U

495 11

10

EXPLANATION 

4 Permeable Layer Number See table 1 for names

144

148 -4rr

153

106

11.4 1.0

107

1.3

2.5

2.2

0.9

14.7

Flow to layer 11 at subcrop 30.9

3 Discharge

subcrop 

p 30.9 *-

63.8

Leakage to overlying layer

317 Leakage from 

t , overlying layer

Lt- Outcrop Recharge | j

-. 17.9 U4 

148t 

Leakage to overlying layer li153

Leakage from overlying layer

4

0.9 - 3

14.7

Unwarri leakaae whe

layer 4 is not present

layer 4 is not present

All flows are in million gallons per day

Sum of flows may not balance due to independent rounding and small mass balance error in model

o

FIGURE 33. Simulated vertical flow between layers and recharge and discharge for: A. predevelopment conditions; and B. 1987 conditions.
RECHARGE DISCHARGE 1068

t;

245

t

5204 11 SUBCROP

240

P = 5034

I 956

276

_» 0.7 

- 7.9

II 

172

89.5

24.9

65.6

t 2.5'

las

31.4t; **«-

55.2 

_ t' r c

tt

6.1

1758

0.7 197 P= 1327 -K376 

» 0

347

173

-rP=406

330 269 P = 330

213

__t_

u

44.1 

--» 0.2 25.8

H______

i

311

- 

f.

23.3 85-3

= ft

i

285 P = 322

116

II 

264 

-» 78.9 106t '

215 64.4

I I 

683

25.5 262

P = 513

430 

* 37.1

74

346 P = 408

163

4

148

i I 

236 P= 115

4.9 87.1

12.6 

2.2

, I 

150 P = 75.4

0.8

11

10

9.1

EXPLANATION 

4 Permeable Layer Number See table 1 for names

~F 0.9

0.3

147

Flow to layer 11 at subcrop 21.3

D Discharge

subcrop 

P 21.3 «-

31.4

r-ti 

Leakage to overlying layer

_-_ Leakage from £ overlying layer

' » Outcrop Recharge i i

430 *kfi 346

-» 37.1 

163

*

Leakage to overlying layer II236

Leakage from overlying layer

P = 408 4

1.4

J.

3

14.3

Uoward leakaae whe

Downward leakage where layer 4 is not present

layer 4 is not present

= Decrease in confined storage. Positive sign indicates 

water added to storage

P = Pumpage of ground weter

= Percentage of pumpage supplied by storage

All flows are in million gallons per day

Sum of flows may not balance due to independent rounding and small mass balance error in model

FIGURE 33. Simulated vertical flow between layers and recharge and discharge for: A. predevelopment conditions; and B. 1987 conditions Continued.
F58 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

overlying layers at lower altitudes. Layers 8, 9, and 10 

all have similar amounts of recharge in the outcrop 

areas. Layer 7 has about one-tenth as much recharge as 

the other three layers, probably because of its much 

smaller outcrop area and the increased resistance to vertical flow caused by the overlying layers. Potential 

regional recharge is discharged locally owing to the limited capacity of layer 7 for regional flow. Layer 11 has 

about twice as much recharge because of its wide outcrop and easy discharge path.

In the Texas coastal uplands aquifer system, the general flow pattern from updip highlands to downdip 

lowlands is affected by many large stream valleys. 

Fresh ground water occurs only in narrow bands near 

the outcrop of each permeable zone. Several examples 

of this occur in layer 2, the middle Claiborne aquifer, in ' 

eastern Texas (pi. 3G). Where the outcropping layer 

occurs in highland areas, fresh ground water moves 

downdip and then flows parallel to the outcrop though 

slightly downdip from the outcrop, and finally moves 

updip to discharge in an area of the outcrop where the 

topography is low. An example of this occurs in the 

Mississippi embayment aquifer system in layer 6 in 

southwestern Mississippi and adjacent parts of Louisiana (pi. 3F).

Less than 30 Mgal/d of leakage occurs upward 

through the Vicksburg-Jackson confining unit (layer 15) 

from the Mississippi embayment aquifer system and 

Texas coastal uplands aquifer system into the coastal 

lowlands system (between layers 5 and 7) under predevelopment conditions (fig. 33A). The small rate of 

flow is due mostly to the large thickness and small vertical permeability of the confining unit. Some vertical 

flow does occur between aquifer systems across the 

Vicksburg-Jackson confining unit; thus, the unit is not 

an impermeable barrier to flow as might be suggested 

by considering only its thickness and lithology. The 

head gradient in layer 6 trends downdip from the outcrop, underneath the overlying confining unit (pi. 3), 

indicating flow away from the outcrop that discharges 

up through the confining unit. A similar situation also 

exists below the Midway confining unit at the base of 

the aquifer system. However, the effect of this upward 

leakage on the flow system above the Midway confining unit is much less than on the flow system below. If 

not for this upward leakage, there would be very little 

or no circulation of ground water below the confining 

unit. Without upward leakage, the flow patterns in the 

upper system would change little because of other 

sources of flow that usually are larger. The VicksburgJackson confining unit operates similarly to the regional 

confining units above the Dakota Sandstone discussed 

by Bredehoeft and others (1983) in that they do conduct

significant quantities of water vertically even though 

their hydraulic conductivities are low.

The Mississippi River Valley alluvial aquifer (part of 

layer 11) receives a substantial amount of flow (more 

than 400 Mgal/d) from the older units of the Mississippi embayment aquifer system (fig. 33A). In the Texas 

coastal uplands and Mississippi embayment aquifer 

systems, only layers 4 and 5 have substantial net flow 

upward to the layer above. The net flow upward out of 

layer 6, the upper Claiborne aquifer, occurs mostly 

where it subcrops beneath and discharges to the Mississippi River Valley alluvial aquifer (layer 11). The flow 

discharging upward from layer 6 to the subcrop is 

approximately equal to the flow upward into layer 6 

from layer 5 below. Layer 5 has the largest amount of 

recharge and the largest amount of discharge in its outcrop area; much of the discharge is in outcrop areas of 

eastern and southern Texas. Layer 4 has the largest net 

recharge in its outcrop area, probably because it has a 

large area of outcrop at a higher altitude than the other 

layers and has a relatively large horizontal hydraulic 

conductivity. Upward flow is nearly balanced by the 

downward flow between layers 2,3, and 4. Layers 2 and 

3 have very little net recharge in their respective outcrops.

The vertical flow between layers can be analyzed by 

looking at hydraulic-head profiles of all aquifers shown 

on one graph (pi. 1). Where the head decreases with 

depth (decreasing layer number) and the density is 

nearly constant, downward vertical flow and regional 

recharge are indicated, as on the topographic high 

shown on plate IA in southern Texas. Where the head 

increases with depth (increasing layer number) and the 

density is nearly constant, upward vertical flow and 

regional discharge are indicated, as between the Neches 

River and Kingsville in southern Texas (pi. IA). At this 

location, the heads in layers 3 and 4 are more than 200 ft 

above the land surface. The head in layer 3 nearly 

equals the head in layer 4 because of a fairly high sand 

percentage, which indicates a high vertical leakance.

In the coastal lowlands aquifer system, where there 

is a much greater range of water density, the vertical 

relations are more difficult to interpret. At the point 

where the dissolved-solids concentration and hence 

density increase in a layer, the freshwater head rises 

substantially (pis. 1A, B, C, D).

CHANGES DUE TO GROUND-WATER 

DEVELOPMENT

Massive changes occurred in the gulf coast regional 

aquifer systems in response to large-scale development 

of the ground-water resources throughout much of the
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F59

study area. Development of ground water has led to 

lowering of hydraulic heads and changes in aquifer 

storage, changes in rates and locations of recharge and 

discharge, changes in flow direction and magnitude, 

changes in flow velocity, changes in water quality, and 

land subsidence. Digital simulations were made using 

estimates of ground-water pumpage available at 5-yr 

intervals. It was assumed that 1985 pumpage at a steady 

rate would simulate changes in ground-water levels 

that occurred between 1982 and 1987; therefore, flow 

quantities are attributed to 5-yr intervals (for example, 

1980, 1985), whereas water-level comparisons are 

lagged two years (for example, 1982,1987).

GROUND-WATER PUMPAGE

About 9 Ggal/d of ground water was pumped from 

the three aquifer systems in 1985 for use by about 19 

million people inhabiting the area (Mesko and others, 

1990). Total pumpage increased by 250 percent during 

1960-80 but decreased by 7 percent by 1985 (fig. 34). 

This was consistent with a nationwide decline of about 

12 percent from 1980 to 1985 (Solley and others, 1988). 

Irrigated agriculture uses the largest volume (about 

three-fourths) of ground water in the study area 

(Grubb, 1984, p. 17) and also represents the fastest 

growth in ground-water pumpage (fig. 34) during the 

past three decades. Much of the increase in agricultural 

pumpage was between 1970 and 1980, especially in the 

Mississippi River Valley alluvial aquifer. Other areas 

with large use of ground water for irrigation are southwestern Louisiana and the Winter Garden area of southern Texas. Figure 35 shows the distribution of groundwater pumpage in 1985 and the predominantly pumped 

layer at each location.

Over 60 percent of the municipal and industrial 

pumpage is withdrawn from the coastal lowlands aquifer system (Mesko and others, 1990). Ground-water 

usage for public supply has continued to increase 

(fig. 34) as the population has grown. Industrial pumpage at many locations has declined since 1970 (fig. 34), 

mainly in response to large drawdowns, and in some 

areas because of land subsidence and other adverse 

effects, as well as socioeconomic factors. About threefourths of the total pumpage is withdrawn from layer 

11 (Mississippi River Valley alluvial aquifer and permeable zone A). The remainder of the pumpage is generally withdrawn from whatever shallower zones exist in 

an area. The upward trend of pumpage has been 

unequal across the study area. These trends in pumpage 

in different areas are discussed in a later section "Potential and Limitations for Future Development" and 

shown in the area budget-trend bar graphs (figs. 42-50).

TY PUMPED, 

ALLONS PERDAY

Eo) co

h-d 

? 2 4

*§

ad

CD 

^ 2

n

I | Agricultural 

1 1 Industrial 

1 ___ 1 Public-supply

-

2.2

1

0.6

2.7

1

0.7

3.3

1.3

0.9

4.9

1.3

1.1

7.2

1

1.3

6.5

0.8 ~

1.4

1960 1965 1970 1975 1980 1985

FIGURE 34. Ground-water pumpage by use, 1960-85.

Although most areas showed a general increase in 

pumpage throughout the period 1960-85, there were 

some decreases in pumpage after 1965 in southern 

Texas (fig. 47).

The pumpage data used in this study were compiled 

by Mesko and others (1990) for 1960-85 at 5-yr 

intervals by using a detailed 1980 distribution of pumpage by block and the published total pumpage from 

each county for the respective year, assuming that the 

distribution of pumpage within each county has been 

constant through time, with a few exceptions. Two categories of ground-water pumpage data were chosen:

1. Point pumpage, usually for public supply and 

industry, where little water can return to the aquifer 

systems.

2. Areally dispersed pumpage for irrigation, fish farm, 

stock, and rural domestic uses, where there is a 

potential for a significant amount of water to seep 

back to the aquifer systems.

More information about the method of estimating 

pumpage is discussed in the supplement "Simulation 

Details" and more detailed pumpage information is 

available in Williamson and others (1990, p. 65-66) and 

Mesko and others (1990, p. 25-29).

WATER-LEVEL CHANGES

Ground-water pumpage has caused regional-scale 

water-level declines in most of the aquifers (fig. 36). 

Every aquifer or permeable zone has more than one 

major cone of drawdown and has maximum drawdowns (block-averaged) exceeding 80 ft. Note that
F60 REGIONAL AQUIFER SYSTEM ANALYSIS GULF COASTAL PLAIN

EXPLANATION

Pumpage, in million gallons per day 

Less 

than 

0.2 125 10 25 50

I

200 MILES

Base from U.S. Geological Survey 

Scale 1:2,500,000 0 50 100 150 200 KILOMETERS

FIGURE 35. Ground-water pumpage, 1985.
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F61

plate 4 shows simulated drawdown from predevelopment to 1987. Total drawdown has not been observed in 

most places because little hydraulic-head data exist for 

predevelopment conditions. The standard error of estimate [01 root mean-squared error (RMSE)] of the simulated 1987 hydraulic head is about 50 ft, based on the 

calibration of the model against 1972 and 1980 head 

estimates (discussed in the supplement "Simulation 

Details").

Differences in the patterns of drawdown exist 

between the coastal lowlands aquifer system and the 

Mississippi embayment aquifer system and the Texas 

coastal uplands aquifer system (fig. 36). In most of layers 2 to 6, the 1987 drawdown near the updip limit of 

the aquifer exceeds 5 ft, whereas in layers 7 through 11, 

the drawdown around the updip limit of the aquifer is 

less than 5 ft. In southern Texas and adjacent offshore 

Texas, there are large areas where drawdown is less 

than 5 ft, and the drawdown more gradually increases 

away from the updip limit of layers 7 through 11 than in 

layers 2 through 6. This can be demonstrated more 

quantitatively by using a cumulative frequency distribution of drawdown relative to area of the layer at that 

level of drawdown (fig. 37). Layers 3 and 4 have the 

largest areas of drawdown throughout the whole range 

of plotted drawdowns. Layer 3 has the largest cumulative volume of drawdown, and it is exceeded in pumpage by all of the layers except 2, 6, and 7 (table 3). This 

implies that throughout the large areal extent of layer 3 

(the largest area 170,000 mi2, table 1), it is affected by 

pumping in adjacent aquifers. Layer 5 has the next largest cumulative volume of drawdown and has the second largest volume of pumpage (table 3). Layer 9 is 

fourth largest in both pumping volume (table 3) and 

cumulative volume of drawdown (fig. 37). Layers 6 and 

7 have the least drawdown, but also have small pumpage. Layer 11 has one of the least cumulative volumes of 

drawdown (fig. 37) although it has the largest volume 

of pumpage (table 3). The relatively small cumulative 

volume of drawdown is due to the shallow depth of the 

aquifer and hence close proximity to sources of 

recharge, as well as its large horizontal hydraulic conductivity and the change in water-table storage.

Changes in simulated and observed hydraulic head 

over time for selected 10-mi blocks having substantial 

drawdown are demonstrated by the hydrographs in figure 36. The observed block-averaged heads were estimated by averaging for each well, the measurements 

made during each 2-yr period centered at the 5-yr intervals of plate 5 and then averaging at least two wells in a 

block in the same layer (discussed in the supplement 

"Simulation Details"). At most locations, the rate of 

drawdown increased around 1960 and again during the

1970's because of large increases in pumpage. At most 

locations, there is drawdown in nearly every permeable 

zone, due to pumpage, that may be several hundred or 

thousand feet vertically and several layers removed 

from the layer pumped (fig. 36).

In 1985, ground-water pumpage and resulting drawdown decreased, leveled off, or the rate of increase 

declined at most locations with considerable pumpage 

(fig. 35-36). Water levels recovered substantially to the 

east of Houston, Texas (fig. 36H) after 1977. Several feet 

of land subsidence had occurred in this area during the 

1960's and 1970's due to ground-water pumping. The 

Harris-Galveston Coastal Subsidence District was 

formed in the mid-1970's to reduce pumping and subsidence. Water levels also recovered in the Grand Prairie region of central Arkansas (fig. 36B), at Greenville, 

Mississippi (fig. 36C), and at Pascagoula, Mississippi 

(fig. 36D). Pumping and resulting drawdown at Baton 

Rouge, Louisiana, decreased during the 1970's (fig. 

36F).

Another interesting feature of the hydrographs is the 

time at which the vertical-head gradient reverses. In 

every block shown in figure 36 there is at least one layer 

whose head crosses the head for another layer or layers 

at some point in time, indicating a reversal of the vertical-head gradient. Commonly the direction of these 

reversals is from upward flow to downward flow after 

pumping begins. In many places the reversal changes 

differently above and below the major pumping zone so 

that all flows move toward that zone, such as at Memphis, Tennessee (fig. 36A), where the vertical gradient is 

downward above layer 5 and upward below it.

Hydraulic-head changes in one layer caused by 

pumpage in an adjacent layer are also shown by the 

head profiles on plate 1. Most of the pumpage in the 

Winter Garden area of southern Texas occurs in layer 4, 

but the head in layer 3 follows the head in layer 4 

closely (pi. 1A) as the leakance is substantial due to the 

high sand percentage. The heads in all the layers of the 

Texas coastal uplands aquifer system decline in this 

area, largely due to pumpage in layer 4. At Kingsville, 

also in southern Texas but nearer to the coast, the 

pumping is in layer 10, but drawdown occurs in layers 

9 and 11 as well (pi. 1A). Here, the model did not simulate enough drawdown because the area of intense 

pumping is much smaller than a model block. In addition, the pumping is allocated to more than one block 

because it does not coincide with the center of a block, 

further minimizing the simulated drawdown.

The multicounty cone of drawdown at Houston, 

most extreme in layer 9, causes drawdown in all of the 

permeable zones of the coastal lowlands aquifer system, 

and the effects extend as far down in the section as layer
F62 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

300

250

200

150

100

< 50

LLJ 

I 

O 200

DC 

D

X

150

A. Memphis, Tennessee 

Row 16, Column 12

Water table

100

50

i i i r

B. Grand Prairie, Arkansas 

Row 25, Column 11

Water table

-::-. «\ IT---

1925 1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990

YEAR

EXPLANATION

Water levels Numbers indicate 

layer number. Various colors.

 5 Measured for layer 5

 - 5 Simulated for layer 5

FIGURE 36. Hydrographs showing simulated and observed block-averaged water levels in selected areas with substantial drawdown,

1925-87: A. Memphis, Tennessee, B. Grand Prairie, Arkansas; C. Greenville, Mississippi; D. Pascagoula, Mississippi;

E. Gulfport, Mississippi; F. Baton Rouge, Louisiana; G. Alexandria, Louisiana; H. Southeast Houston, Texas;

/. West Houston, Texas; J. Winter Garden area, Texas.
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F63

200

C. Greenville, Mississippi 

Row 27, Column 18

150

Water table

100

11

50

I______I______I______I

150

100

50

D. Pascagoula, Mississippi 

Row 30, Column 44

'--5

 4-^,1

Water table

-50

-100

1925 1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990

YEAR

EXPLANATION

Water levels Numbers indicate 

layer number. Various colors.

 5 Measured for layer 5

 - 5 Simulated for layer 5

FIGURE 36. Hydrographs showing simulated and observed block-averaged water levels in selected areas with substantial drawdown,

1925-87: A. Memphis, Tennessee, B. Grand Prairie, Arkansas; C. Greenville, Mississippi; D. Pascagoula, Mississippi;

E. Gulfport, Mississippi; F. Baton Rouge, Louisiana; G. Alexandria, Louisiana; H. Southeast Houston, Texas;

/. West Houston, Texas; /. Winter Garden area, Texas Continued.
F64 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

200

150

100

50

Q

cc. 

a

-50

400

\\ I I I I I I I I I l

E. Gulf port, Mississippi 

Row 32, Column 41

-5

-7

Water table

200

-200

-400

\ I I I I I

F. Baton Rouge, Louisiana 

Row 41, Column 33

4

Water table

11

1925 1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990

YEAR

EXPLANATION

Water levels Numbers indicate 

layer number. Various colors.

 5 Measured for layer 5

 - 5 Simulated for layer 5

FIGURE 36. Hydrographs showing simulated and observed block-averaged water levels in selected areas with substantial drawdown,

1925-87: A. Memphis, Tennessee, B. Grand Prairie, Arkansas; C. Greenville, Mississippi; D. Pascagoula, Mississippi;

E. Gulfport, Mississippi; F. Baton Rouge, Louisiana; G. Alexandria, Louisiana; H. Southeast Houston, Texas;

/. West Houston, Texas; /. Winter Garden area, Texas Continued.
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F65

400

200

-200

Q

< -400

X

O 400

200

G. Alexandria, Louisiana 

Row 44, Column 24

j___ i ____i_____i_____i_____i_____I_____I

-200

-400

\ \ I I I I I I

H. Southeast Houston, Texas 

Row 63, Column 22

-t

1 

Water table

1925 1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990

YEAR

EXPLANATION

Water levels Numbers indicate 

layer number. Various colors.

 5 Measured for layer 5

- - - - 5 Simulated for layer 5

FIGURE 36. Hydrographs showing simulated and observed block-averaged water levels in selected areas with substantial drawdown,

1925-87: A. Memphis, Tennessee, B. Grand Prairie, Arkansas; C. Greenville, Mississippi; D. Pascagoula, Mississippi;

E. Gulfport, Mississippi; F. Baton Rouge, Louisiana; G. Alexandria, Louisiana; H. Southeast Houston, Texas;

/. West Houston, Texas; J. Winter Garden area, Texas Continued.
F66 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

200 

0

HYDRAULIC HEAD, IN FEET

si S§£3gO 3OO C

1

/. West Houston, Texas 

Row 64, Column 20

- -^

- -J

 

-, "" '

1 1

. I

""-^ 

:lj^-

9 -^

Jl ^

Water table

~ >>

V 

V

::>^^

^

 _ _lh -6

-7

^----j-8

- 10

X

--10

-9

J. Winter Garden area, Texas 

Row 88, Column 8

******

 ' ^ "-^.^

"1k ""-V"

~---.

"~~~~

\

\ \ 

\ \

\ N

\/\x

N 

\ 

\

Water

 '

Sv ^ X / 

\/

table

___

^ 

s . / x

^ >

>

i

__,J 5

* 

I / 

\S i'

v3^- 

\7

^4

--3

1925 1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990

YEAR

EXPLANATION

Water levels Numbers indicate 

layer number. Various colors.

 5 Measured for layer 5

- - - - 5 Simulated for layer 5

FIGURE 36. Hydrographs showing simulated and observed block-averaged water levels in selected areas with substantial drawdown,

1925-87: A. Memphis, Tennessee, B. Grand Prairie, Arkansas; C. Greenville, Mississippi; D. Pascagoula, Mississippi;

E. Gulfport, Mississippi; F. Baton Rouge, Louisiana; G. Alexandria, Louisiana; H. Southeast Houston, Texas;

/. West Houston, Texas; J. Winter Garden area, Texas Continued.
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F67

180

Layer number 

and curve symbol

10 20 30 40 50 60 70 

DRAWDOWN, IN FEET

80 90 100 110

FIGURE 37. Cumulative distributions of areas of each aquifer (layer) for predevelopment to 1987 simulated drawdown levels.
F68 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

TABLE 3. Summary of 1985 ground-water pumpage by area, type of withdrawal, and model layer, in million gallons per day

[P, point withdrawals; pumpage is mostly for public supply and industrial use. A, areal withdrawals; pumpage is mostly for irrigation use; 

quantities from data in Mesko and others (1990). The symbol x denotes absence of layer in the indicated area. Leaders ( ) indicate no 

pumpage in the area. Zero indicates pumpage less than 0.5 million gallons per day. Totals may not agree due to independent rounding]

Area1

1. Winter Garden

2. Northeastern Texas

3. Western embayment

4. Mississippi alluvial 

plain

5. Eastern embayment

6. Southern Texas

7. Southeastern Texas

8. Southwestern

Louisiana

9. Eastern coastal

lowlands

Total point

Total areal 

Grand Total

Type

P

A

P

A

P

A

P

A

P

A

P

A

P

A

P

A

P

A

Model layer

2

X

X

X

X

0

1

32 

11

29

3

X

X

X

X

61

15

76

3

1

4

61

24

6

13

1 

2

4

1

0

73

44

117

4

26

186

68

12

0

7

20

12

73

6

0

0

0

._.

0

187

223

410

5

1

1

9

20

65

34

173 

57

143

8

0

0

0

0

5

1

396

121

517

6

1

0

7

8

1

2

23 

3

6

3

0

2

2

0

0

6

1

46

19

65

7

X

X

X

X

X

X

X 

X

X

X

3

0

4

3

8

2

76

5

91

10

101

8

X

X

X

X

X

X

X 

X

X

X

3

3

14

6

61

2

213

20

291

31

322

9

X

X

X

X

X

X

X 

X

X

X

5

4

193

7

6

0

105

20

309

31

340

10

X

X

X

X

X

X

X 

X

X

X

13

5

267

54

8

5

45

10

333

74

407

11

X

X

X

X

X

X

2 68

4,910

X

X

5

20

89

386

144

596

123

20

429

5,930

6,360

All layers

29

191

145

64

74

56

317 

5,000

254

21

29

32

568

458

226

605

572

77

2,210

6,500

8,710

See figure 13 for area boundaries.

2 Small difference in number here and in figure 33B is due to southern boundary of area placed at updip limit of subcrop of Vicksburg-Jackson 

confining unit for modeling purposes rather than at downdip limit as used for this compilation.

6 in the Texas coastal uplands aquifer system, where the 

water is saline (pi. IB). Pumping near Pascagoula, on 

the coast of Mississippi, in layer 9 causes drawdown in 

adjacent layers 8 and 10 (pi. ID).

Layers 4 and 5 at Memphis, Tennessee, comprise the 

Memphis aquifer, and simulated heads in both layers 

are very similar throughout time (pi. IE). The pumping 

in layers 4 and 5 in Fulton County, Kentucky, appears to 

cause drawdown in layers 3 and 2 as shown by the narrowing drawdown contours near where there is no 

pumpage in layer 2 or 3 (pi. 4). Farther south in the 

embayment, the head profiles in all layers are lowered 

all the way across the embayment (pi. IF).

Although the thickness and sand percentage of the 

aquifers may be less significant than other factors affecting regional flow, they may have a large effect on the

shape and size of a cone of drawdown, as shown by 

comparing the cones in layer 8 in Louisiana at Alexandria and DeRidder (pi. 4D). Pumpage at these two locations is about equal (30 Mgal/d), yet the cone of 

drawdown at Alexandria is about four times deeper. An 

explanation is that the sand percentage is about three 

times larger and the aquifer thickness is about twice as 

large at DeRidder, resulting in a much larger transmissivity at DeRidder as compared to Alexandria.

HYDRAULIC HEAD AND HORIZONTAL FLOW

Ground-water pumping has largely affected the 

potentiometric surface (pi. 5) and the pattern of groundwater flow (pi. 6) of the 10 layers and permeable zones. 

The horizontal flow vectors (pis. 3, 5, 6) are resolved at
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F69

the intersection of four adjacent blocks to facilitate display of pumping conditions where pumping out of one 

block will be shown as the four surrounding vectors 

point toward the pumping block. In many locations the 

hydraulic gradient and horizontal flow directions have 

reversed from predevelopment conditions. In other 

cases the location of lowest head and consequently the 

regional discharge point have moved substantially

Plates 5 and 6 also show lines of equal dissolved-solids concentration of 10,000 and 35,000 mg/L. These 

lines represent the average concentration within 100-

f\

miz model blocks. In many areas, where it appears that 

the brine interface intrudes into areas of current pumping, this may not be true. Actually, most of the pumping 

is from the top part of the permeable zone where the 

water is fresher than what is shown as the average for 

the block, or the pumping is concentrated near the corner of the block.

Although random and unknown bias errors are 

present in the simulations, relative comparisons 

between one simulation and another or between one 

location and another have greater certainty because 

they are not absolute; therefore, similar errors in both 

parts of a comparison would offset each other. Simulated hydraulic-head maps also contain uncertainty 

because of errors in the model design and assumptions 

as well as errors in estimating relevant hydraulic-head 

values for 100-mi2 blocks using observed well data for 

appropriate comparison with simulated heads from the 

regional flow model. Estimating observed head values 

for a regional ground-water model is a complex problem owing to a variety of reasons. Models assume discrete blocks of aquifer over discrete time units, 

requiring discretization of the point well-measurement 

data over four dimensions three in space and one in 

time. Horizontal discretization is complicated due to 

the relatively flat regional hydraulic gradient, which is 

affected by large local topographic variations. It also is 

complicated by the steep horizontal-head gradients 

occurring around the steep cones of drawdown. Vertical 

discretization is important but difficult because in some 

places layers are quite thick, and considerable head difference occurs vertically within the layer. In addition, 

wells are not measured on the same days of the year or 

even every year because of changes in the monitoring 

network design and operation. The details of estimating 

block-averaged observed hydraulic head are given in 

the supplement "Simulation Details."

MISSISSIPPI RIVER VALLEY ALLUVIAL AQUIFER

AND PERMEABLE ZONE A (HOLOCENEUPPER PLEISTOCENE DEPOSITS)

In layer 11, the Mississippi River Valley alluvial 

aquifer in the northern part of the study area and permeable zone A in the southern part, the general pattern 

of hydraulic head (pi. 5A) and flow (pi. 6A) has been 

altered in several areas with large pumpage. Areas with 

some of the largest ground-water withdrawals from 

layer 11 are Houston, Texas, and a farming area about 

80 mi southwest of Houston, southwestern Louisiana, 

and the Grand Prairie area of central Arkansas. There 

would have been much more drawdown from this huge 

volume of pumping except that the layer is close to surface sources of recharge, has a high sand percentage, 

and has substantial hydraulic conductivity. Water-table 

storage may be a factor, but it is thought to be a minor 

contributor because the water levels have begun to 

reach steady state. Throughout most of coastal eastern 

Texas, coastal Mississippi, and some parts of southern 

Louisiana, upward flow into layer 11 before development became downward flow out of the layer (pi. 6A) 

as ground-water withdrawals increased. Vertical flow 

direction reversed throughout most of the area underlain by the Mississippi River Valley alluvial aquifer as 

well. The offshore horizontal flows in layer 11 (permeable zone A) are about the same as existed prior to 

development.

PERMEABLE ZONE B (LOWER PLEISTOCENEUPPER PLIOCENE DEPOSITS)

The 1985 offshore flow in layer 10 (permeable zone 

B) is substantially altered from the predevelopment 

condition (pis. 3B, 6B). Many of the cones of drawdown 

have lowered the hydraulic head to well below sea level 

(pi. 5B). In an area that extends up to 70 mi offshore 

from the Houston area, the horizontal flow toward the 

coast is larger than predevelopment flow, and in that 

part of the area extending up to 50 mi offshore, former 

upward flow from layer 9 becomes downward flow out 

of layer 10 and into layer 9. Horizontal flow in an inland 

direction increased along the coast of Louisiana. Horizontal flow toward the coastline also increased offshore 

from southern Mississippi.

Layer 10 contains more than 10 separate cones of 

drawdown, of which 3 have simulated drawdown 

exceeding 100 ft since predevelopment (pi. 4B) near 

Houston, Texas, at Baton Rouge, Louisiana, and Gulfport, Mississippi. Hydraulic head had dropped below 

sea level by 1987 at these locations and also north of 

Beaumont, Texas. The model does not simulate enough 

drawdown at Kingsville, Texas: the observed draw-
F70 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

down is more than 100 ft and the 1987 head is also 

below sea level. Despite all of the drawdown in layer 

10, throughout most of coastal eastern Texas, coastal 

Mississippi, and several parts of southern Louisiana, 

former upward flow into the bottom of layer 10 (pi. 3B) 

during predevelopment had reversed to downward 

flow out of the layer (pi. 6B) by 1987. The flow out of 

layer 10 is due to drawdown in the underlying layer 9.

PERMEABLE ZONE C (LOWER PLIOCENEUPPER MIOCENE DEPOSITS)

Layer 9, or permeable zone C, has the maximum 

simulated drawdown of any layer, with more than 

400 ft near Houston, Texas, and with five cones having 

more than 100 ft of simulated drawdown (pi. 4C). The 

four other areas with large drawdowns are north of 

Beaumont in Texas, Baton Rouge in Louisiana, and 

Gulfport and Pascagoula in Mississippi. At all five locations the simulated 1987 hydraulic head is below sea 

level (pi. 5C).

In a few places along the updip extent of layer 9, 

especially in central Louisiana, there is more downward 

vertical flow out of layer 9 in 1987 than before development. There were very few other vertical flow reversals 

at the base of the layer, except that many of the former 

upward vertical flows under predevelopment conditions (pi. 3C) became much larger (pi. 6C) after groundwater withdrawals began. Pumping onshore in and 

around Houston has changed flow directions more than 

45 degrees or flow rates more than 50 percent in the 

brine part of layer 10. The area where flow directions 

have changed is about 6,000 to 8,000 mi2 and extends as 

far as about 60 mi offshore. Changes in flow directions 

and volumes of similar magnitude were simulated 

throughout a large area in the brine part of layer 9 in 

south-central Louisiana.

PERMEABLE ZONE D (MIDDLE MIOCENE DEPOSITS)

Although there is little pumpage from layer 8 (permeable zone D) in the Houston area in Texas, very large 

drawdown is simulated in this area in response to 

pumping in shallower layers (pi. 4D). A similar situation also exists to some extent in layer 8 near the Baton 

Rouge area in Louisiana. Pumping in layer 8 has created 

cones of drawdown with drawdowns of over 200 ft at 

Pascagoula, Mississippi, and at Alexandria, Louisiana, 

and the simulated 1987 head is below sea level (pi. 5D). 

Several other areas have lesser but still substantial 

drawdown (pi. 4D). Drawdown in layer 8 (pis. 4D, 5D) 

in Claiborne County, Mississippi, is due to withdrawal 

of ground water from the Mississippi River Valley alluvial aquifer for a powerplant, beginning in the early 

1980's.

The distribution of vertical flow relative to the base 

of layer 8 did not change much from predevelopment 

conditions (pi. 3D) to 1987 conditions (pi. 6D), except 

for an area in south-central Louisiana where small 

downward flows reversed to upward flows. In this area, 

the horizontal flow magnitude also increased and flow 

direction shifted to the east, toward the cone of drawdown created by pumping in shallower layers at Baton 

Rouge. Another change in horizontal flow was in the 

cone of drawdown surrounding Bogalusa, in Washington Parish, Louisiana, on the Louisiana-Mississippi border.

PERMEABLE ZONE E (LOWER MIOCENEUPPER OLIGOCENE DEPOSITS)

Layer 7 (permeable zone E) has numerous cones of 

simulated drawdown (pi. 4£), none of which exceed 

100 ft. The simulated cone at Laurel, Mississippi, underestimates the observed drawdown, probably because 

the pumpage occurs from a small area that is not accurately represented in the coarse-mesh model. The largest and most extreme simulated drawdown occurs in 

the Alexandria area of Louisiana. A rather large cone 

exists to the west of Houston, Texas, but it is not as deep 

as the one at Alexandria.

The distribution of vertical flow across the Vicksburg-Jackson confining unit (upward into layer 7) is not 

greatly changed from predevelopment conditions. In a 

few parts of southern Louisiana, Mississippi, and Alabama, and in the western tip of Florida, small downward flows (pi. 3£) have reversed direction to upward 

flows (pi. 6£). Horizontal flows in layer 7 have not 

changed greatly either, although there are many occurrences of flow being diverted towards one of the many 

small pumping cones.

UPPER CLAIBORNE AQUIFER

By 1987, layer 6 (upper Claiborne aquifer) had at 

least seven areas with regional drawdown even though 

most of the cones of drawdown are related to pumping 

in other layers. Very little ground water is withdrawn 

from layer 6 (fig. 35, pi. 4F). For example, at Alexandria, 

Louisiana, there is drawdown in layer 6 because of 

pumping in the layers above. The drawdown at Alexandria is interesting because the ground-water withdrawals are from layer 8 above the thick Vicksburg-Jackson 

confining unit. At Bryan, Texas, and in central Arkansas, drawdown in layer 6 is mostly in response to 

pumping in layer 5 below. At Jackson and Greenville in 

Mississippi, the drawdown is due to pumping from 

layer 6. The shape of the regional hydraulic-head surface of layer 6 has not greatly changed due to groundwater pumping (pis. 2F, 5F).
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F71

Layer 6 subcrops beneath the Mississippi River Valley alluvial aquifer throughout a broad area and 

becomes a conduit from recharge areas to discharge 

areas associated with other aquifers. The vertical flow 

across the base of layer 6 was different in 1985 (pi. 6F) 

than it was under predevelopment conditions (pi. 3F), 

especially in the Mississippi embayment. In and around 

Memphis, Tennessee, and across western Arkansas, the 

upward flow of ground water to the regional discharge 

area was reversed to downward flow into layer 5, which 

is heavily pumped. In the Texas coastal uplands aquifer 

system, the pattern of flow is not greatly changed. The 

horizontal flows are not changed as much as the vertical 

flows in the Mississippi embayment aquifer system. 

The largest regional discharge area still occurs in northern Louisiana.

MIDDLE CLAIBORNE AQUIFER

Layer 5, or the middle Claiborne aquifer, had more 

than 15 different cones of drawdown in 1987 (pi. 4G). 

This includes several large cones where large quantities 

of water are pumped out of layer 5, such as at Memphis, 

Tennessee, central Arkansas, and northern Louisiana. 

The simulated drawdown in layer 5 in central Arkansas 

and northern Louisiana substantially underestimates 

the observed drawdown. This error will be discussed in 

the supplement "Simulation Details." Layer 5 also has 

some large cones of drawdown where much of the 

pumping is from adjacent aquifers, such as layer 4 in 

the Winter Garden area of southern Texas and also to 

some extent in layer 6 in central Mississippi near Jackson. Despite all of the drawdown in layer 5, the 1987 

potentiometric surface (pi. 5G) still generally resembles 

the predevelopment potentiometric surface (pi. 2G) 

with the large regional flow area in southern Arkansas 

and northern Louisiana.

Vertical flow between layer 5 and the underlying 

layer changed considerably in certain areas from predevelopment (pi. 3G) to 1985 (pi. 6G). Most notable was 

the large-scale reversal of flow in the Winter Garden 

area where generally upward vertical flow changed to 

large downward flow throughout an extensive area. 

Other areas where vertical flow direction changed from 

upward to downward are in northwestern Arkansas, 

western Tennessee, eastern Texas, and central Mississippi. In the Memphis area of Tennessee, small upward 

vertical flows became very large, maintaining the 

upward direction. Large patterns of concentric horizontal flows that did not exist prior to development (pi. 3G) 

can be seen on the 1985 simulated flow map (pi. 6G).

LOWER CLAIBORNE-UPPER WILCOX AQUIFER

Layer 4, the lower Claiborne-upper Wilcox aquifer, 

contains the largest area of any aquifers with more than 

40 ft of simulated drawdown from predevelopment to 

1987 (fig. 37). The extensive area having 40 ft or more 

drawdown is due to the large area of public-supply 

pumping around Memphis, Tennessee, in the Mississippi embayment aquifer system, and in the Winter 

Garden area of southern Texas, where intensive pumping for irrigation causes widespread drawdown 

(pi. 4H). Also, more than 80 percent of the area underlain by layer 4 had more than 10 ft of simulated drawdown from predevelopment to 1987, a much higher 

percentage than any other layer (fig. 37). This is due in 

part to the model design in which the massive sand unit 

at the top of the Wilcox Group is defined as layer 4 and 

has a more limited extent, especially downdip.

Throughout a large part of the Mississippi embayment aquifer system, the drawdown in layer 4 is caused 

by pumping in layer 5 because the two layers either are 

not separated by a regional confining unit north of 35 

latitude (Hosman and Weiss, 1991) or are separated by a 

leaky confining unit. Note, however, that where the two 

layers are not separated, the heads are not exactly equal 

(pis. 5G, 5H), but the difference may be too small to be 

observed onsite when combined with other factors 

causing differences (horizontal location of wells and 

timing of measurements). Even though there is no intervening regional confining unit and the units are generally sandy the interbedded fine-grained sediments do 

tend to restrict vertical flow.

The simulated potentiometric surface for layer 4 in 

1987 (pi. 5H) is largely changed from the predevelopment surface (pi. 2H) in the Winter Garden area, the 

Memphis area, and in a couple of cones of drawdown in 

eastern Texas. In other areas, the configuration of the 

surface is largely similar to what it was in predevelopment except that most of the heads are lowered.

Over the study area, downward flow between layer 

4 and the layer below it increased from 153 Mgal/d in 

predevelopment to 236 Mgal/d in 1985 while upward 

flow stayed about the same (fig. 33). The direction of 

vertical flow also changed in many locations. In the 

Winter Garden area, changes in vertical flow volume 

and direction developed (pis. 3H, 6H). The horizontal 

flow in the Winter Garden area also changed considerably from a southward and westward direction in predevelopment to generally horizontal radial flow in 1985 

toward a point about 30 mi downdip from the landward extent of the aquifer. Horizontal flow direction 

changed all the way to the downdip limit of the aquifer. 

In the general vicinity of the Brazos River, the vertical 

flow also reversed to become downward. Around Tyler
F72 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

in northeastern Texas, the vertical flow reversed from 

downward to upward due to pumping in layer 4 where 

there was also horizontal radial flow toward the center 

of pumping.

In the Mississippi embayment aquifer system, in 

layer 4 south of about 33.5° latitude, there are no noticeable differences in flow in the downdip part of the layer 

except for an increase in downward flow in south-central Mississippi. Farther north in the embayment, 

pumping causes several sizable changes in groundwater flow. West of the Mississippi River across from 

Tennessee, the vertical flow relative to the base of layer 

4 reversed from upward to downward in 1985. The flow 

reversal affected four adjacent aquifers, with water 

flowing downward through layers 4 and 3 to satisfy a 

small volume of pumping from layer 2. Apparently the 

distances to layer 2 outcrop areas with direct recharge 

are so great that most of the flow to the pumping centers has to be drawn vertically from above where the 

flow paths are short and the hydraulic gradients relatively steep even though the vertical permeability is 

low. The reversal of vertical flow directions is also 

shown in the head profiles shown on section N-N' 

(pi. IN). However, just east of the Mississippi River at 

Memphis, the vertical flow changed from downward in 

predevelopment to upward in 1985 to supply pumping 

in layer 5. This flow reversal also affected the vertical 

flows between layers 2 through 5.

MIDDLE WILCOX AQUIFER

Layer 3, or the middle Wilcox aquifer, has a pattern 

of drawdown that is very similar to the pattern of drawdown at the same locations in layer 4 (pi. 41). In general, 

the drawdown in layer 3 is slightly less than the drawdown at the same location in layer 4. The total pumpage 

in layer 3 is about one-fourth of the amount of pumpage 

in layer 4, and much of the pumpage occurs in the top 

part of layer 3 adjacent to pumping in layer 4. One 

exception is in Brazos County in east-central Texas at 

Bryan and College Station. Here, the most productive 

part of layer 3 is a massive sand locally known as the 

Simsboro Formation (G.E. Fogg, The University of 

Texas at Austin Bureau of Economic Geology, oral commun., 1988). The drawdown in layer 3 is greater than in 

layer 4 because almost all of the ground water withdrawn at this location is from layer 3. Except as noted, 

the changes in the hydraulic gradient in layer 3 are similar to the changes in layer 4.

In 1985, just as in predevelopment times (pi. 31), the 

horizontal flow in the middle Wilcox aquifer, layer 3, is 

much larger in the Texas coastal uplands aquifer system 

than it is in the Mississippi embayment aquifer system 

(pi. 61). The layer flows are mainly due to larger

hydraulic conductivity (fig. 14) and thickness of layer 3 

in Texas (fig. 12A) than to the east. In the Mississippi 

embayment aquifer system, some hydrologists consider 

the middle Wilcox aquifer (layer 3) to be a confining 

unit. It does act like a confining unit in many locations 

when considered on a local scale. The horizontal flow in 

layer 3 in Texas did not change much during development except for flow to some pumping centers, especially the Winter Garden area, where flows from the 

outcrop on the northern side of the Winter Garden area 

increased considerably (pis. 31, 61). The lateral flows in 

the Mississippi embayment aquifer system were generally small during predevelopment except for a few locations (pi. 31), and the changes that occurred were also 

small (pi. 61). Vertical flow relative to the base of layer 3 

was not simulated in the Texas coastal uplands aquifer 

system because layer 2 does not exist in Texas. There 

were no substantial changes in vertical flow in layer 3 in 

the Mississippi embayment aquifer system other than 

the ones noted the discussion of layer 4.

LOWER WILCOX AQUIFER

There are only a few cones of drawdown in the 

lower Wilcox aquifer (layer 2) (pi. 4/), near Memphis, 

Tennessee, central Arkansas, northern Louisiana, and 

east-central Mississippi. The simulated drawdown at 

Memphis exceeded 100 ft, whereas the other areas had 

drawdowns of only 30-50 ft. At the two locations west 

of the Mississippi River, the main pumping is much 

shallower, yet the drawdown extends far downdip 

where the concentration of dissolved solids in ground 

water exceeds 10,000 mg/L (pi. 4/). The shape of the 

horizontal hydraulic-head gradient in 1987 (pi. 5/) is 

very similar to what it was in predevelopment times 

(pi. 2J) except for the cone of drawdown at Memphis.

Some differences in horizontal flow due to groundwater pumpage in layer 2 (pi. 6/) were simulated. 

Although the general pattern in 1985 is not greatly different from predevelopment, the flow that originally 

moved across the northern embayment to discharge to 

the subcrop of layer 2 is captured by the pumpage at 

west Memphis and other well fields in layer 2 located in 

Arkansas. The change is indicated by the flow vectors 

pointing in the opposite direction from the predevelopment flow direction in the area west of Memphis (pi. 6/).

REGIONAL RECHARGE AND DISCHARGE

The rates (fig. 38) and distribution (fig. 39) of 

regional recharge and discharge were substantially 

changed by development of the ground-water resource. 

The 1985 pumpage was about three times greater than 

the rate of predevelopment regional recharge. Pumpage
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F73

and the resulting declines in hydraulic head increased 

the regional recharge by more than three times to more 

than 9 Ggal/d. Part of the increase in recharge is the 

capture of water that was previously locally discharged. 

Regional discharge decreased to about 1 Ggal/d, nearly 

one-third of its predevelopment rate. Thus, more than 

65 percent of the simulated 1985 pumpage was supplied 

from increased regional recharge. About 20 percent of 

the pumpage was captured regional discharge, and less 

than 15 percent was from decrease of ground-water 

storage (fig. 38). A large part of the increased regional 

recharge is captured local discharge. Much of the

increased recharge in the irrigated areas comes from 

seepage of irrigation water past the root zone. Jorgensen (1975, p. 55) estimated that as much as 30 percent of the ground water pumped for irrigation in the 

Katy area, west of Houston, Texas, returned to the shallow aquifer. The volumes of water from aquifer storage 

shown in figures 38 and 40 include confined storage 

summed from the simulation reported here as well as 

loss of unconfined storage in the Mississippi River Valley alluvial aquifer simulated by Ackerman (in press). 

The water-budget values shown in figures 38 and 40 

were calculated by taking the regional model-simula12,000

10,000

8,000

6,000

4,000

2,000

Water-table storage From 

the Mississippi River Valley 

alluvial aquifer, simulated by 

Ackerman, 1996

-2,000

-4,000

-6,000

-8,000

-10,000

-12,000

Predevelopment1930 1960 1965 

YEAR

1970 1975 1985

FIGURE 38. Change in simulated regional ground-water budget from predevelopment to 1985.
F74 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

-37°

KENTUCKY

96'

.'J 32°

950 ~~~ GULF OE MEXICO

50 100 150 200 MILES i i i i

Base from U.S. Geological Survey 

Scale 1:2,500,000

0 50 100 150 200 KILOMETERS

~ EXPLANATION ~

o o

E -2.0 -1.0 -0.5 -0.2 -0.1 -0.0 0.1 0.2 0.5 1.0 2.0 4.0 8.0 E

 J

DISCHARGE RECHARGE

FIGURE 39. Simulated 1985 net regional ground-water recharge and discharge.
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F75

tion results and adding Ackerman's water-table storage 

values to simulated storage and subtracting the same 

values for simulated recharge.

Although there were large changes in the groundwater budget caused by ground-water withdrawals, the 

changes are proportionately smaller when put in perspective of the overall hydrologic budget of the study 

area. The volumes of precipitation (500 Ggal/d 

summed from fig. 4) and runoff (140 Ggal/d summed 

from fig. 6) are much greater than the 6.7 Ggal/d 

change in simulated ground-water recharge. The major 

stream inflow to the study area (415 Ggal/d) as estimated by Williamson and others (1990, table 1) is 

almost as large as the volume of precipitation. Natural 

evapotranspiration is difficult to estimate but probably 

is 300 to 400 Ggal/d.

As of 1985, the Mississippi Alluvial Plain still had 

the largest volume of ground-water flow (fig. 40), as it 

did before development (fig. 31). However, the southeastern Texas area had the second largest total flow in 

1985. Areas in southwestern Louisiana, Winter Garden, 

and northeastern Texas also had large increases in total 

flow. The eastern coastal lowlands area had the largest 

proportion of flow coming from confined storage. Most 

of the change in storage in the Mississippi Alluvial 

Plain is from water-table changes that are unevaluated 

for the other areas. Many of the interarea flows stayed 

relatively constant despite substantial changes in other 

components of the ground-water budget.

DISTRIBUTION OF REGIONAL RECHARGE AND 

DISCHARGE

In 1985, the proportion of the study area that contributed net regional recharge increased from 41 percent 

of the study area to 66 percent of the study area (figs. 32, 

39). The area that changed from having net regional discharge to net regional recharge includes most of the 

area underlain by the Mississippi River Valley alluvial 

aquifer and most of the area underlain by the coastal 

lowlands aquifer system within about 40 mi of the 

coastline, except in southern Texas. In southern Texas 

along the coast, there is still net regional discharge 

because of little ground-water pumpage. Even offshore, 

there has been a change from discharge to recharge. Off 

the coast near Houston during predevelopment, flow 

was discharging from the aquifers to the Gulf of Mexico, whereas in 1985 the simulated flow was from the 

Gulf of Mexico into the aquifer. On the north and west 

edges of the Winter Garden area there have been 

changes from net discharge to net regional recharge. In 

the areas of intensive irrigation such as the Mississippi 

Alluvial Plain, southwestern Louisiana, and west and

south of Houston, Texas, a substantial portion of the 

simulated net regional recharge actually may be irrigation return flow.

VERTICAL FLOW BETWEEN LAYERS

By 1985, most of the simulated net vertical flows 

between layers changed direction or changed considerably in magnitude (fig. 33). The amount and direction 

of the net vertical flows are sensitive to the aquifer 

parameters chosen for the simulation because net flows 

are a difference between two commonly larger numbers. The absolute rates of upward and downward flow 

considered separately are less sensitive to simulation 

parameters and hence more certain than the net vertical 

flows, especially if considered as relative numbers to 

compare to other layers or prepumping conditions, or 

both. The magnitudes of vertically upward flow 

between layers remained about the same as before 

development in most layers. Exceptions to this were 

upward flow into layer 6, which decreased to less than 

one-half, and upward flow into and out of layer 10, 

which decreased to slightly more than one-half. This is 

probably because in many places with little or moderate 

pumpage, the vertical flow remained similar. However, 

in layers 6 and 10, much water that had moved upward 

into the layers during predevelopment conditions was 

moving to the large pumping areas in layers 5 and 9 by 

1985.

Downward vertical flows between layers increased 

substantially in all of the aquifers and permeable zones 

(fig. 33) as a result of ground-water pumpage and 

resultant reversal of vertical gradients in many places 

(pi. 1). Downward vertical flow increased so much that 

all the net flows between 1 ayers, which were generally upward before development, became downward 

except between layers 6 and 7. Flow across the 

Vicksburg-Jackson confining unit between layers 6 and 

7 increased slightly in both directions due to development. The smallest changes in vertical flow were across 

this unit because there is minimal pumpage in the two 

adjacent permeable layers.

In every layer, the regional recharge increased and 

some of the regional discharge in the outcrop areas was 

captured owing to ground-water pumpage (fig. 33). The 

Mississippi River Valley alluvial aquifer and middle 

Claiborne aquifer had the largest amounts of pumpage 

and the largest increases in regional recharge. The size 

ranking of all of the layers shown in figure 33 would be 

very similar when ranking amount of pumpage, or 1985 

recharge, or change in recharge from predevelopment. 

This indicates that increased pumpage is correlated 

with increased regional recharge.
F76 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

AREA GROUND-WATER BUDGETS, 1985

NORTHEASTERN 

TEXAS

MISSISSIPPI ALLUVIAL PLAIN

WINTER GARDEN

WESTERN 

EMBAYMENT

e

SOUTHEASTERN 

TEXAS

SOUTHERN 

TEXAS

EXPLANATION EASTERN COASTAL 

LOWLANDS

| | WATER TABLE

___ /CONFINED STORAGE

HI INFLOW FROM OTHER 

^^ AREAS 

^H NET REGIONAL 

RECHARGE

NET REGIONAL DISCHARGE

OUTFLOW TO OTHER AREAS

PUMPAGE

Area of each circle indicates relative quantity of flow into the aquifer system 

in the designated area. Water-table storage from Ackerman, 1996.

FIGURE 40. Simulated 1985 ground-water budgets, by area.
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F77

Upward vertical discharge from the layers subcropping the Mississippi River Valley alluvial aquifer 

decreased by nearly one-half, and downward flow into 

the subcropping layers increased to more than double 

its predevelopment value (fig. 33). Layer 6 continued to 

have the most flow interactions with the alluvium, 

probably due to the large area of contact. Layer 6 has 

minimal pumpage yet has large inflows and outflows. It 

is the only layer in which the change in recharge was 

much larger than the 1985 pumpage. Layer 6 acts as a 

large regional conduit, carrying water from recharge 

areas along regional flow paths (pi. 6) to cones of drawdown caused by pumping in deeper layers.

Layers 3 through 5 and 8 through 11 have ratios of 

net recharge (total recharge minus total discharge) in 

the outcrop (and subcrop of layer 11) to pumpage of 

between 0.68 and 1.01. These ratios indicate that on the 

average, most of the water withdrawn from the layer 

was supplied from increased recharge and decreased 

discharge within the layer itself rather than by inflow 

from adjacent layers. The ratios for layers 2 and 7, 

which have very narrow outcrop and subcrop bands 

(fig. 11), were only 0.04 and 0.17, indicating that most of 

the pumpage was supplied by vertical flow from adjacent layers. The ratio for layer 6, which has a very wide 

outcrop and subcrop band, was 2.16, indicating that it 

was capturing additional recharge and discharge. Layer 

6 supplied the excess flow as leakage to underlying layers to satisfy pumpage from other layers, primarily 

layer 5.

The direction of vertical flow and an indication of its 

magnitude during both predevelopment and 1985 conditions are shown on the sections that also show both 

hydraulic-head profiles (pi. 1). The head profiles do not 

always indicate flow directions in the brine part of the 

aquifer system. The location of brine water can be 

approximated by the concentrations of dissolved solids 

that exceed 35,000 mg/L as posted on the sections.

AQUIFER STORAGE

The simulated proportion of pumpage being supplied from confined aquifer storage in 1985 was less 

than 3 percent (fig. 38). This is a small percentage, as 

was the figure of 7 percent supplied by storage in a simulation of 1985 conditions by Kuiper (1994). The total 

simulated change in confined storage for the period 

1925-87 was about 3.5 x 1012 gal, or about 4 percent of 

the total pumpage of 88 x 1012 gal. The change in watertable storage was not simulated because a constanthead layer was used to simulate regional recharge and 

discharge and because the observed change in watertable storage volume was not estimated due to the lack

of water-table measurements over time. This limitation 

will be discussed more fully in the supplement "Simulation Details." In addition to the simulated change in 

confined storage, figures 38 and 40 also show changes 

in water-table storage simulated by a subregional 

model for the Mississippi River Valley alluvial aquifer 

(Ackerman, 1996).

Although only a small proportion of 1985 pumping 

was supplied from aquifer storage, on the average the 

lateral extents of cones of drawdown are far from where 

they would eventually stabilize if 1985 pumping conditions were continued until equilibrium was reached. As 

a cone of drawdown expands, its area increases greatly 

(in proportion to the square of the radius), so a very 

large volume of water from aquifer storage is released 

with a small change in hydraulic head. Therefore, when 

the cone becomes large, its rate of lateral expansion 

slows substantially. If no boundaries or sources are 

intersected in a large flow system, the cone will keep 

extending very slowly over a very long period of time. 

This effect was demonstrated by Williamson and others 

(1990, p. 110-112) by comparing two simulations. One 

simulated the aquifer system under 1980 conditions 

with some water being released from aquifer storage, 

and the other simulated no aquifer storage available so 

that the maximum steady-state drawdown was 

reached. The 50-ft contour of the cone of drawdown in 

layer 10 moved from 5 mi offshore in 1980 to 30 mi offshore when steady-state conditions were reached. The 

25-ft contour of drawdown in layer 10 moved from 

30 mi offshore in 1980 to near the edge of the Continental Shelf about 100 mi offshore when steady-state conditions were reached. The drawdown near the center of 

the cones is nearly equal in both simulations (Williamson and others, 1990).

The area with the largest proportion of flow coming 

from confined storage in 1987 is the eastern coastal lowlands area (fig. 40). This is partly due to the fact that 

withdrawals in some other areas that are more extensively developed actually declined in the 1980's. Most of 

the change in storage in the Mississippi Alluvial Plain 

area is from water-table changes (Ackerman, 1996) that 

are unevaluated for the other areas, as was mentioned 

at the beginning of this section. Many of the interarea 

flows stayed relatively constant (figs. 31, 40) despite 

large changes in other components of the ground-water 

budget.

Long-term changes in depth to water in wells indicate trends in ground-water storage. A summary of 

about 200,000 measurements from about 2,500 shallow 

wells shows a decline from the mid-1940's until the 

early 1970's, when the water levels began to stabilize 

(Williamson and others, 1990, fig. 22). This stabilization
F78 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

probably was due to the decrease in industrial pumpage in the areas that had previously had the largest 

declines. The areas with the largest increases in pumpage are mostly irrigated areas underlain by the Mississippi River Valley alluvial aquifer, which has large 

hydraulic conductivity, short distances to large surfacewater bodies, and, in places, a large storage coefficient, 

due to water-table conditions. All of these factors 

contribute to a relatively small drawdown. This summary of changes in ground-water storage was made 

using a procedure outlined by Williamson and others 

(1990, p. 69).

LAND SUBSIDENCE

Decreases in hydraulic head due to pumping cause 

increased loading on the skeleton of the aquifers and 

can cause compaction and land subsidence if the finegrained deposits are deformable. This has a great effect 

on the aquifers because the effective storage coefficient 

during compaction could be two or more orders of magnitude larger than the elastic storage coefficient of a 

confined aquifer, thus slowing the head decline by providing a source of water to the pumping wells. Prudic 

and Williamson (1986) used a technique to simulate a 

compacting regional aquifer in the Central Valley of 

California similar to the method Meyer and Carr (1979) 

used in the Houston area of Texas. More recently, Leake 

and Prudic (1987, 1988) and Leake (1990) have 

enhanced the method to an implicit formulation that is 

more accurate. Kuiper (1994) independently developed 

a similar technique that is used in this study.

The subsidence algorithm in the model operates so 

that if the hydraulic head in the aquifer drops more 

than a set amount, called the critical head (80 ft was 

used), the elastic storage coefficient is switched to an 

inelastic value, which is commonly two or more orders 

of magnitude larger than the confined storage value. 

The critical head is then reset to the new lowest head 

value in that model block. Thus the process inside finegrained beds of the aquifer system is simulated where 

there is compaction. If the head rises above the previous 

lowest value, called the critical head, then the storage 

switches back to the confined value. The simulated land 

subsidence is estimated by summing the volumes of 

change in confined storage in a grid cell and dividing 

by the area to get a depth of water removed from storage. The amount of water removed from confined storage approximately equals the volume of land 

subsidence over a large area. The part of this land-surface change that is caused by change in elastic confined 

storage would rebound if the water levels rose again, as 

is the case in the San Joaquin Valley in California and 

elsewhere.

Simulated land subsidence caused by withdrawal of 

ground water generally approximates the major features of estimated land subsidence in the study area 

(fig. 41). Subsidence has been observed for the period 

1906-83 in the Houston area (Gabrysch, 1969, 1977, 

1982, 1984), and although it was simulated for the 

period 1925-87, both periods should show similar subsidence. A maximum of 1.2 ft of land subsidence has 

also been observed in the Baton Rouge area of Louisiana from 1934 to 1976 (Whiteman, 1980, fig. 1). 

Although subsidence has only been documented in 

some detail in those two areas, it may have occurred 

elsewhere. Subsidence over large areas commonly goes 

unnoticed until it reaches several feet in magnitude, 

especially in an inland area where it will not contribute 

to flooding. At inland locations, subsidence may occur 

without any other effects if it proceeds slowly enough 

and over a large enough area, as is commonly the case 

when it is caused by widespread ground-water withdrawals.

Two national studies (Chi and Reilinger, 1984; Holdahl and Morrison, 1974) of geodetic data for evidence of 

land subsidence at benchmarks have isolated vertical 

movements that might be related to land subsidence in 

the study area (fig. 41, table 4). At Robstown, Texas, the 

estimated subsidence probably resulted from oil production (R.K. Gabrysch, U.S. Geological Survey, oral 

commun., 1991). Near New Orleans, Louisiana, the estimated land subsidence is probably mostly due to compaction of peat soils (Snowden, Simmons, and others, 

1977; Snowden, Ward and Studlick, 1980). No inelastic 

compaction was simulated in the Eocene-age sediments 

because it was presumed that they had already undergone previous consolidation stresses. That assumption 

may not hold for all locations, which could account for 

some of the discrepancies shown in figure 41.

Little of the water pumped comes from confined 

aquifer storage even when the water from inelastic storage is included in the totals. In the Houston area, only 8 

percent of the pumpage was from confined and compaction storage by 1987 (fig. 53). Nearly all of the water 

derived from storage in the Houston area was from 

layer 9. About 31 percent of the water pumped from 

layer 9 in this area was from confined and inelastic storage combined.

GROUND-WATER VELOCITY AND POTENTIAL FOR 

SALINE-WATER INTRUSION

Large-scale ground-water pumpage in the freshwater part of the aquifer systems has markedly changed 

flow patterns in the brine part of the systems and 

induced flow updip toward pumping areas (compare 

pis. 3, 6). However, it will take decades or centuries for
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F79

Rena Lara

3.42

1939-57

EXPLANATION

Estimated land subsidence, 1906-83, in feetInterval is variable

Simulated land subsidence, 1925-87, in feetInterval is variable

Estimated subsidence for period and location 

indicated, in feet

ILLINOIS x

37°

MISSOURI,

\ 37°

 KENTUCKY

96'

Rena Lara 

v 3.42 

1939-57

I > ARK

___ _ _ ___ Monjcoe 

I Shreveport 0.69 .

1 1.06 1890-1966 ^Vf^'0.39

i 1920-69 -&Cf«.*cQ ,1934-69

Alexandria

0.74 Study are 1934-66 <"*"

boundary

Baton Rouge 

- 2- " 7 

1934-76

Iowa LOUISIANA 

0.2

Venice

0.25

1938-64

GULF OF MEXICO

 

» \ 97°

Aransas Pass 

0.25 / 

1951-59/

Edge of Continental Shelf

Robstown\ 

: 2.20 (oil related) 

i1951-59

200 MILES

99°

98°

Base from U.S. Geological Survey 

Scale 1:2,500,000

FIGURE 41. Estimated land subsidence and simulated land subsidence due to ground-water withdrawals.
F80 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

TABLE 4. Summary of indicators of estimated land subsidence in the study area 

[Negative numbers indicate land-surface rise; , no data]

Location

Alexandria, La.

Aransas Pass, Tex.

Baton Rouge, La.2

Beaumont, Tex.

Biloxi, Miss.

Galveston, Tex.3

Houston, Tex.3

Iowa, La.

Jackson, Miss.

Kenner, La.

Meridian, Miss.

Mobile, Ala.

Monroe, La.

New Orleans, La.

Pensacola, Fla.

Port Isabel, Tex.

Robstown, Tex.4

Shreveport, La.

Venice, La.

Period

1934-66

1951-59

1934-76

_

1906-78

1906-78

1934-70

1934-69

1938-68

1890-1966

1934-69

...

~-

1951-59

1920-69

1938-64

Source l a

Subsidence 

(feet)

0.74

.25

1.20

_

1.40

9.00

.20

.39

1.23

_ 

1.19

...

2.20

1.06

.25

Source 2b

Extent 

(square mile) *

13

12

20

_

10

80

12

20

20

31

12

17

2

Subsidence 

rate

Standard 

deviation

(foot per decade)

0.11

.16

.08

.02

.16

.53

-.03

-.06

.25

.04

.00

_

.03

.32

0.04

.04

.03

.04

.02

.02

.04

.04

.04

.02

.04

_

.05

.06

aFrom Chi and Reilinger, 1984. 

bFrom Holdahl and Morrison, 1974.

Approximate diameter of the subsiding area. 

2From Whiteman, 1980, figure 1. 

3From Gabrysch, 1982, figure 13.

Related to oil withdrawals.

the brine water to reach these pumping areas because of 

the relatively slow ground-water velocities and the long 

flow paths. For example, freshwater pumping onshore 

in and around Houston had changed flow directions 

more than 45 degrees or flow rates more than 50 percent 

in the brine part of layer 10 or permeable zone B (lower 

Pleistocene-upper Pliocene deposits) throughout an 

area of 6,000-8,000 mi2 that extends as far as about 

60 mi offshore. Block-averaged Darcian velocities (specific discharge) in that part of layer 10 containing brine 

water approach 10 ft/yr at a maximum. Assuming an 

effective porosity of 20 percent, this would mean average particle velocities of about 50 ft/yr or about 100 yr 

to move 1 mi. The actual ground-water velocities could 

differ from these estimates by an order of magnitude or 

more because these estimates are averages across a 10- 

mi model block face. The large variability resulting

from heterogeneity of the deposits allows for large differences in velocities between different pathways that 

water may actually take. Therefore, the velocities will 

generally give traveltimes slower than what actually 

occurs in the aquifer. However, they do indicate the 

possibility of long-term problems that may not be 

noticed for years or even decades, yet which need consideration for prudent management of the groundwater resource.

POTENTIAL AND LIMITATIONS FOR 

FUTURE DEVELOPMENT

Historically when ground-water development in a 

particular area expands enough to cause detrimental 

effects, water managers usually recommend that pump-
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F81

ing be restricted and that alternative supply sources, 

generally surface water, be sought (Harris-Galveston 

Coastal Subsidence District, written commun., 1987; 

Mississippi Water Resources Research Institute, 1987, p. 

3). Many ground-water studies describe only the effects 

of existing and proposed water development. Osborne 

and others (1986) have shown that water-use forecasts, 

regardless of the method used, are likely to be highly 

inaccurate owing to their dependence on unpredictable 

economics and politics. Typically, ground-water flow 

models are used to simulate a few hypothetical future 

pumpage scenarios, which are difficult to estimate, may 

never occur and still do not show where there is capacity for further development with minimal effects. 

Instead, studies need to contain the information water 

managers could use to determine what areas and permeable zones have potential to supply more water for 

expanding ground-water development with minimal 

adverse effects.

A method was developed to estimate relative potential for ground-water development using the groundwater flow model that will be described in the following 

section. The potential for development will then be 

described for each of the nine geographic areas delineated in figure 13. As each geographic area is discussed, 

the historical trends of the ground-water budget of the 

respective area will be discussed to put the potential for 

ground-water development in the perspective of the 

effects of the current development. The changes from 

predevelopment to 1985 in water budgets prepared for 

parts of the aquifer systems (fig. 13) are shown in 

figures 42-50. Those changes will be discussed as the 

potential of each geographic area is discussed in the following sections. The pumpage trends by geographic 

area are also described by Mesko and others (1990, 

p. 15-19, fig. 12, table 6). There are some general similarities among all of the trends shown in the water budgets for the different areas. The budget trend in all 

geographic areas shows a common pattern of generally 

increasing pumpage causing decreased natural regional 

discharge and increased net regional recharge. Confined storage, the storage shown in all budgets except 

the one for the Mississippi Alluvial Plain, is a small 

fraction of the total aquifer-system water budget. 

Changes in water-table storage are considered with 

regional recharge or discharge due to the way the flow 

system was conceptualized (see supplement "Simulation Details"). The only alternative is to measure hundreds or thousands of wells and study the observed 

changes in water levels in very shallow wells over a 

long period of time. The exact proportions of the simulated net recharge that actually represents change in 

water-table storage is probably small because in most

areas, the water-table altitude has probably changed 

very little since predevelopment time. There are some 

notable exceptions to this, such as in the Mississippi 

River Valley alluvial aquifer, in a large area in southwestern Louisiana where rice is irrigated, and in the 

Winter Garden area of southern Texas.

GROUND-WATER MANAGEMENT INFORMATION 

USING SENSITIVITY TO PUMPING STRESS

Simulated drawdown from a uniform increase in 

pumpage is an integrated result of most of the hydrologic properties known to affect the hydraulic response, 

such as distance to recharge sources, aquifer thickness,

WINTER GARDEN GROUND-WATER BUDGET

h-'DC

LULU

QW 

DZ

OQO

8R

400

200

-400

m H

H

i i 1 1 1 1

Predevel-1930 1940 1950 1960 1965 1970 1975 1980 1985 

opment

YEAR

EXPLANATION 

Inflow from other areas 

Storage 

Recharge

Discharge 

Pumpage

Outflow to 

other areas

FIGURE 42. Changes in the Winter Garden area regional ground-water 

budget from predevelopment to 1985.
F82 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

NORTHEASTERN TEXAS GROUND-WATER BUDGET WESTERN EMBAYMENT GROUND-WATER BUDGET

HUU

?Q 200

t-"cc

LULU

dc/J

coo

EJ o

1°

is

§ -i -200

2

400

1 1 1 1 1 1 1 1 1 1

_

-

^_

_

i 

_

i

r i

__

3

_

T

=

_

 

 

_

 

=

__

 

_

 

=

_

m

=

__

.

_

-

1 1 1 1 1 1 1 1 1 1

4UU

Q 200

LULU

Bz coo

DC^

Is§zo°

cc^

z

-Ann

1 1 1 1 1 1 1 1 1 1

_

. P

Q p

= =

 

. ^.

_

-

1 1 1 1 1 1 1 1 1 1 Predevel-1930 1940 1950 1960 1965 1970 1975 1980 1985 

opment

YEAR

EXPLANATION 

Inflow from other areas 

Storage 

Recharge

Discharge 

Pumpage

Outflow to 

other areas

FIGURE 43. Changes in the northeastern Texas area regional 

ground-water budget from predevelopment to 1985.

Predevel-1930 1940 1950 1960 1965 1970 1975 1980 1985 

opment

YEAR

EXPLANATION 

Inflow from other areas 

Storage 

Recharge

Discharge 

Pumpage

Outflow to 

other areas

FIGURE 44. Changes in the western embayment area regional 

ground-water budget from predevelopment to 1985.

MISSISSIPPI ALLUVIAL PLAIN GROUND-WATER BUDGET

-400

Predevel-1930 1940 1950 1960 1965 1970 1975 1980 1985 

opment

YEAR

EXPLANATION 

Inflow from other areas 

Storage 

Recharge

Discharge 

Pumpage

Outflow to 

other areas

FIGURE 45. Changes in the Mississippi Alluvial Plain area regional 

ground-water budget from predevelopment to 1985.

EASTERN EMBAYMENT GROUND-WATER BUDGET

400

Zg 200

COQ 

ccH

gd -200

-400

Predevel-1930 1940 1950 1960 1965 1970 1975 1980 1985 

opment

YEAR

EXPLANATION 

Inflow from other areas 

Storage 

Recharge

Discharge 

Pumpage

Outflow to 

other areas

FIGURE 46. Changes in the eastern embayment area regional 

ground-water budget from predevelopment to 1985.
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F83

SOUTHERN TEXAS GROUND-WATER BUDGET SOUTHEASTERN TEXAS GROUND-WATER BUDGET

h-'CC 

LLJLLJ

mo 

£<

^O

3°:

400

200

-200

400

-

-

I

m 

I

I

|

I

|

I

|

I

|

I

|

I

|

I

|

I

|

I

|

-

-

Predevel-1930 1940 1950 1960 1965 1970 1975 1980 1985 

opment

YEAR

EXPLANATION 

Inflow from other areas 

Storage 

Recharge

FIGURE 47. Changes in the southern Texas area regional groundwater budget from predevelopment to 1985.

1,500

> 1,000 

Q

LLjLLJ

mo

-500

 -1,000

-1,500

\ I I I I I I I I I

I I I I I I I I I I

Predevel-1930 1940 1950 1960 1965 1970 1975 1980 1985 

opment

YEAR

Discharge 

Pumpage 

Outflow to

»

EXPLANATION 

Inflow from other areas 

Storage 

Recharge

Discharge 

Pumpage

Outflow to 

other areas

FIGURE 48. Changes in the southeastern Texas area regional groundwater budget from predevelopment to 1985.

SOUTHWESTERN LOUISIANA GROUND-WATER BUDGET

1,500

> 1,000 

Q

LLJLU

mo

-500

- -1,000

-1,500

-

-

1

1

1

1

1

1

1

1

1

as 

|

1

 sz

\

\

\

\

\

\

\

\

-

-

EASTERN COASTAL LOWLANDS GROUND-WATER BUDGET 

1,200

Predevel-1930 1940 1950 1960 1965 1970 1975 1980 1985 

opment

YEAR

EXPLANATION 

Inflow from other areas 

Storage 

Recharge

FIGURE 49. Changes in the southwestern Louisiana area regional 

ground-water budget from predevelopment to 1985.

Q 

LLJLLJ

mo

800

400

0°. ~400

 -800

-1,200

-

-

\

\

\

\

\

I 

|

1

|

1

I

1

m 

\

\

\

\

m

\

\

m

\

\

I 

i

-

Predevel-1930 1940 1950 1960 1965 1970 1975 1980 1985 

opment

YEAR

Discharge 

Pumpage 

Outflow to

EXPLANATION 

Inflow from other areas 

Storage 

Recharge

Discharge 

Pumpage

Outflow to 

other areas

FIGURE 50. Changes in the eastern coastal lowlands area regional 

ground-water budget from predevelopment to 1985.
F84 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

sand percentage, hydraulic conductivity, vertical leakage, and aquifer storage. Therefore, the model was used 

to predict the relative potential for increased groundwater pumpage by doing an areal analysis of sensitivity 

to pumpage by layer. The aquifer system's response to a 

unit increase in pumpage was simulated for each model 

block containing relatively fresh water. The simulations 

were done with two slightly different approaches, both 

described in the supplement "Simulation Details." The 

product is a map for each layer (fig. 51) showing 

the potential for future ground-water development. 

The five levels of potential, as shown in the following table represent the volume of drawdown in each 

block that would result from an increase in pumpage 

of 1 Mgal/d per 100-mi2 block.

Level

5

4

3

2

1

Potential for development

Excellent

Good

Fair

Poor

Virtually none

Drawdown 

(feet)

Oto2

2 to 5

50 to 10

10 to 25

More than 25

These simulations, which ignore aquifer storage and 

compute maximum steady-state drawdown, will indicate what the maximum future effects might be. These 

maps will therefore allow a water manager to identify 

the layers and(or) geographic areas in which additional 

pumpage may have relatively greater effect and geographic areas or layers where additional ground-water 

development may have relatively less effect.

Every 10-mi model block with estimated concentration of dissolved solids less than 10,000 mg/L and a 

thickness greater than 25 ft was stressed by simulation 

of an additional 1 Mgal/d. There are 7,324 model 

blocks that contain ground water with a concentration 

of dissolved solids less than 10,000 mg/L. This dissolved-solids concentration was used because it was 

convenient and generally was identified within a few 

miles of dissolved-solids concentrations of 3,000 mg/L, 

a concentration value commonly used to delineate 

"usable water" in the western part of the study area. At 

each model grid location, equal additional pumping in 

each block of each layer with relatively fresh water is 

simulated independently (discussed in the supplement 

"Simulation Details").

A composite map (fig. 52) was made showing the 

layer at each model grid location that had the least sensitivity to pumpage (least drawdown) from the data 

shown in figure 51. Theoretically, this would be the 

layer for water managers to consider first when additional ground-water supplies are needed because drawdown would be minimized for a given amount of 

pumpage if the pumpage came from this layer. Several 

practical aspects limit the application of this information. In some cases it may be more expensive to develop 

wells in the layer shown rather than in a shallower 

layer. However, developing a deeper well field might 

still be much less costly than developing an alternative 

source of water. The reason well installation costs could 

be greater is related to the scale of this analysis. This 

analysis considers only the regional perspective of 

large-scale development of aquifer systems. A permeable zone that appears the best from this analysis, such 

as layer 6 in parts of the Mississippi embayment aquifer 

system, may have only thin sand beds in which to place 

well screens, which might limit the well's yield. However, because of some other characteristic of that permeable zone at that location, such as proximity to a 

recharge source, the layer might still have better potential for ground-water development if the water could be 

withdrawn using a larger number of smaller capacity 

wells than might be needed in some other layer with 

thicker sand beds.

Water quality and land subsidence are factors to 

consider along with potential for ground-water 

development. Attempting to minimize drawdown for a 

given level of pumpage by locating well fields in a new 

area or layer may increase the total contributing area 

(and hence spread the effects of drawdown). This has a 

disadvantage when trying to protect ground water in 

the areas contributing to well fields from becoming contaminated. There is a corresponding advantage; with a 

larger contributing area, there is a greater chance that 

any water-quality degradation will be more diluted by 

freshwater coming from the remaining contributing 

area. In areas where land subsidence has occurred, it 

may recur in the future if drawdown is greater than it 

was previously. In other parts of the coastal lowlands 

aquifer system, land subsidence may occur if drawdown from predevelopment exceeds a critical value. In 

calibrating the model, the average critical value was 

found to be about 80 ft, but that value is approximate 

and could vary spatially.

WINTER GARDEN AREA

In the Winter Garden area, the peak ground-water 

pumpage occurred in 1975 (which was only slightly 

more than in 1965) and then decreased slightly thereafter (fig. 42). Since pumpage began, there has been 

slightly more inflow from other areas than outflow to 

other areas. Natural regional discharge decreased to 

less than one-half of its predevelopment value. 

Regional recharge increased by about a factor of four. In
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F85

EXPLANATION

Potential for future ground-water development

Excellent potential for development 

Good potential for development 

Fair potential for development 

Poor potential for development 

Virtually no potential for development 

Not estimated

Area of marginal to poor water quality Dissolvedsolids concentration exceeds 1,000 milligrams per 

liter (from Pettijohn and others, 1988)

1985 saturated thickness less than 50 feet

 37°

KENTUCKY

 Downdip limit of aquifer or permeable zone I 96°

33

Mississippi River Valley 

Alluvial Aquifer

200 MILES

Base from U.S. Geological Survey 

Scale 1:2.500,000 100 150 200 KILOMETERS

FIGURE 51. Potential for additional development of the ground-water resource: A. permeable zone A and Mississippi River Valley alluvial aquifer (layer 11);

B. permeable zone B (layer 10); C. permeable zone C (layer 9); D. permeable zone D (layer 8); E. permeable zone E (layer 7); F. upper Claibome

aquifer; (layer 6); G. middle Claibome aquifer (layer 5); H. lower Claibome-upper Wilcox aquifer (layer 4);

/. middle Wilcox aquifer (layer 3); /. lower Wilcox aquifer (layer 2).
F86 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

B

EXPLANATION

Potential for future ground-water development

Excellent potential for development 

Good potential for development 

Fair potential for development 

Poor potential for development 

Virtually no potential for development 

Not estimated

Area of marginal to poor water quality Dissolvedsolids concentration exceeds 1,000 milligrams per 

liter (from Pettijohn and others, 1988)

1985 saturated thickness less than 50 feet

Downdip limit of aquifer or permeable zone I 96°

KENTUCKY

Base from U.S. Geological Survey 

Scale 1:2.500,000 100 150 200 KILOMETERS

FIGURE 51. Potential for additional development of the ground-water resource: A. permeable zone A and Mississippi River Valley alluvial aquifer (layer 11);

B. permeable zone B (layer 10); C. permeable zone C (layer 9); D. permeable zone D (layer 8); E. permeable zone E (layer 7); F. upper Claiborne

aquifer; (layer 6); G. middle Claiborne aquifer (layer 5); H. lower Claiborne-upper Wilcox aquifer (layer 4);

/. middle Wilcox aquifer (layer 3); /. lower Wilcox aquifer (layer 2) Continued.
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F87

EXPLANATION

Potential for future ground-water development

Excellent potential for development 

Good potential for development 

Fair potential for development 

Poor potential for development 

Virtually no potential for development 

Not estimated

Area of marginal to poor water quality Dissolvedsolids concentration exceeds 1,000 milligrams per 

liter (from Pettijohn and others, 1988)

1985 saturated thickness less than 50 feet

 37°

KENTUCKY

 Downdip limit of aquifer or permeable zone

96

33

200 MILES

Base from U.S. Geological Survey 

Scale 1:2,500,000 100 150 200 KILOMETERS

FIGURE 51. Potential for additional development of the ground-water resource: A. permeable zone A and Mississippi River Valley alluvial aquifer (layer 11);

B. permeable zone B (layer 10); C. permeable zone C (layer 9); D. permeable zone D (layer 8); E. permeable zone E (layer 7); F. upper Claiborne

aquifer; (layer 6); G. middle Claiborne aquifer (layer 5); H. lower Claiborne-upper Wilcox aquifer (layer 4);

/. middle Wilcox aquifer (layer 3); /. lower Wilcox aquifer (layer 2) Continued.
F88 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

D

EXPLANATION 

Potential for future ground-water development

Excellent potential for development 

Good potential for development 

Fair potential for development 

Poor potential for development 

Virtually no potential for development 

Not estimated

Area of marginal to poor water quality Dissolvedsolids concentration exceeds 1,000 milligrams per 

liter (from Pettijohn and others, 1988)

89° 

ILLINOIS I 90° -

37°

MISSOURI,

\ 37°

KENTUCKY

IvIvM-Id 1985 saturated thickness less than 50 feet

 Downdip limit of aquifer or permeable zone

96

33'

200 MILES

Base from U.S. Geological Survey 

Scale 1:2,500,000 50 100 150 200 KILOMETERS

FIGURE 51. Potential for additional development of the ground-water resource: A. permeable zone A and Mississippi River Valley alluvial aquifer (layer 11);

B. permeable zone B (layer 10); C. permeable zone C (layer 9); D. permeable zone D (layer 8); E. permeable zone E (layer 7); F. upper Claiborne

aquifer; (layer 6); G. middle Claiborne aquifer (layer 5); H. lower Claiborne-upper Wilcox aquifer (layer 4);

/. middle Wilcox aquifer (layer 3); /. lower Wilcox aquifer (layer 2) Continued.
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F89

EXPLANATION

Potential for future ground-water development

Excellent potential for development 

Good potential for development 

Fair potential for development 

Poor potential for development 

Virtually no potential for development 

Not estimated

Area of marginal to poor water quality Dissolvedsolids concentration exceeds 1,000 milligrams per 

liter (from Pettijohn and others, 1988)

1985 saturated thickness less than 50 feet

ILUNOISv I 90° " L

37°

MISSOURI,

S 37°

L- KENTUCKY

 Downdip limit of aquifer or permeable zone

33°

200 MILES

Base from U.S. Geological Survey 

Scale 1:2,500,000

50 100 150 200 KILOMETERS

FIGURE 51. Potential for additional development of the ground-water resource: A. permeable zone A and Mississippi River Valley alluvial aquifer (layer 11);

B. permeable zone B (layer 10); C. permeable zone C (layer 9); D. permeable zone D (layer 8); E. permeable zone E (layer 7); F. upper Claiborne

aquifer; (layer 6); G. middle Claiborne aquifer (layer 5); H. lower Claiborne-upper Wilcox aquifer (layer 4);

/. middle Wilcox aquifer (layer 3); /. lower Wilcox aquifer (layer 2) Continued.
F90 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

EXPLANATION

Potential for future ground-water development

Excellent potential for development 

Good potential for development 

Fair potential for development 

Poor potential for development 

Virtually no potential for development 

Not estimated

\//yy/\Area of marginal to poor water quality Dissolvedsolids concentration exceeds 1,000 milligrams per 

liter (from Pettijohn and others, 1988)

F-~-~-~-~-~-~H 1985 saturated thickness less than 50 feet 

 Downdip limit of aquifer or permeable zone

96'

KENTUCKY

Base from U.S. Geological Survey 

Scale 1:2,500,000 0 50 100 150 200 KILOMETERS

FIGURE 51. Potential for additional development of the ground-water resource: A. permeable zone A and Mississippi River Valley alluvial aquifer (layer 11);

B. permeable zone B (layer 10); C. permeable zone C (layer 9); D. permeable zone D (layer 8); E. permeable zone E (layer 7); F. upper Claiborne

aquifer; (layer 6); G. middle Claiborne aquifer (layer 5); H. lower Claiborne-upper Wilcox aquifer (layer 4);

/. middle Wilcox aquifer (layer 3); /. lower Wilcox aquifer (layer 2) Continued.
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F91

EXPLANATION

Potential for future ground-water development

Excellent potential for development 

Good potential for development 

Fair potential for development 

Poor potential for development 

Virtually no potential for development 

Not estimated

Area of marginal to poor water quality Dissolvedsolids concentration exceeds 1,000 milligrams per 

liter (from Pettijohn and others, 1988)

1985 saturated thickness less than 50 feet

MISSOURI,

X 37

.KENTUCKY

 Downdip limit of aquifer or permeable zone

96

200 MILES

Base from U.S Geological Survey 

Scale 1:2,500,000 50 100 150 200 KILOMETERS

FIGURE 51. Potential for additional development of the ground-water resource: A. permeable zone A and Mississippi River Valley alluvial aquifer (layer 11);

B. permeable zone B (layer 10); C. permeable zone C (layer 9); D. permeable zone D (layer 8); E. permeable zone E (layer 7); F. upper Claiborne

aquifer; (layer 6); G. middle Claiborne aquifer (layer 5); H. lower Claiborne-upper Wilcox aquifer (layer 4);

/. middle Wilcox aquifer (layer 3); J. lower Wilcox aquifer (layer 2) Continued.
F92 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

H EXPLANATION

Potential for future ground-water development

Excellent potential for development 

Good potential for development 

Fair potential for development 

Poor potential for development 

Virtually no potential for development 

Not estimated

Area of marginal to poor water quality Dissolvedsolids concentration exceeds 1,000 milligrams per 

liter (from Pettijohn and others, 1988)

37

KENTUCKY

C-1-I-I-I-IvI] 1985 saturated thickness less than 50 feet 

 Downdip limit of aquifer or permeable zone

200 MILES

Base from U.S. Geological Survey 

Scale 1:2,500,000 50 100 150 200 KILOMETERS

FIGURE 51. Potential for additional development of the ground-water resource: A. permeable zone A and Mississippi River Valley alluvial aquifer (layer 11);

B. permeable zone B (layer 10); C. permeable zone C (layer 9); D. permeable zone D (layer 8); E. permeable zone E (layer 7); F. upper Claiborne

aquifer; (layer 6); G. middle Claiborne aquifer (layer 5); H. lower Claiborne-upper Wilcox aquifer (layer 4);

/. middle Wilcox aquifer (layer 3); J. lower Wilcox aquifer (layer 2) Continued.
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F93

EXPLANATION

Potential for future ground-water development

Excellent potential for development 

Good potential for development 

Fair potential for development 

Poor potential for development 

Virtually no potential for development 

Not estimated

Area of marginal to poor water quality Dissolvedsolids concentration exceeds 1,000 milligrams per 

liter (from Pettijohn and others, 1988)

1985 saturated thickness less than 50 feet 

Downdip limit of aquifer or permeable zone

96

ILLINOIS x 8|9 90 -

M1SSOURI

\ 37

V - KENTUCKY

200 MILES

Base from U.S. Geological Survey 

Scale 1:2,500,000 50 100 150 200 KILOMETERS

FIGURE 51. Potential for additional development of the ground-water resource: A. permeable zone A and Mississippi River Valley alluvial aquifer (layer 11);

B. permeable zone B (layer 10); C. permeable zone C (layer 9); D. permeable zone D (layer 8); E. permeable zone E (layer 7); F. upper Claiborne

aquifer; (layer 6); G. middle Claiborne aquifer (layer 5); H. lower Claiborne-upper Wilcox aquifer (layer 4);

/. middle Wilcox aquifer (layer 3); J. lower Wilcox aquifer (layer 2) Continued.
F94 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

EXPLANATION

Potential for future ground-water development

Excellent potential for development 

Good potential for development 

Fair potential for development 

Poor potential for development 

Virtually no potential for development 

Not estimated

Y/^///^ Area of marginal to poor water quality Dissolvedsolids concentration exceeds 1,000 milligrams per 

liter (from Pertijohn and others, 1988)

 37

KENTUCKY

1985 saturated thickness less than 50 feet

 Downdip limit of aquifer or permeable zone 95

200 MILES

Base from U.S. Geological Survey 

Scale 1:2,500,000 0 50 100 150 200 KILOMETERS

FIGURE 51. Potential for additional development of the ground-water resource: A. permeable zone A and Mississippi River Valley alluvial aquifer (layer 11);

B. permeable zone B (layer 10); C. permeable zone C (layer 9); D. permeable zone D (layer 8); E. permeable zone E (layer 7); F. upper Claiborne

aquifer; (layer 6); G. middle Claiborne aquifer (layer 5); H. lower Claiborne-upper Wilcox aquifer (layer 4);

/. middle Wilcox aquifer (layer 3); J. lower Wilcox aquifer (layer 2) Continued.
EXPLANATION

2 -9 LAYER NUMBER

0 LAYER 10

1 LAYER 11

LAYER NUMBERS CORRESPOND TO THOSE IN TABLE 3

[ | AREA WHERE LAYER 11 IS NOT SHOWN 

Next least sensitive layer shown in this area 

because layer 11 is the least sensitive layer nearly everywhere it occurs; layer 11 may not 

always be the most favorable for other reasons

O

Base from U.S. Geological Survey 

Scale 1:2,500,000

0 50 100 150 MILES

I I I T

0 50 100 150 KILOMETERS

FIGURE 52. Areal locations of layers that are least sensitive to drawdown from increased pumpage.
F96 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

fact, the maximum pumpage exceeded predevelopment 

regional recharge by more than a factor of four. Contribution from confined storage reached a peak in 1965 

and was 6 percent of the pumpage. This geographic 

area has had large water-table declines (Klemt and others, 1976) and, therefore, changes in water-table storage, 

which were not simulated separately in the regional 

model. Ryder and Ardis (in press) reported results from 

their subregional simulation with a 5-mi grid spacing, 

indicating that 49 percent of the 1980 pumpage in the 

lower Claiborne-upper Wilcox aquifer (layer 4) was 

supplied from water-table and confined storage. That 

amount, 121 Mgal/d, equals about 46 percent of the 

total pumpage in the Winter Garden area during 1980. 

In 1985, confined storage was being replenished as 

water levels recovered.

In the Winter Garden area of southern Texas, the 

lower Claiborne-upper Wilcox aquifer has the most 

potential for ground-water development (fig. 52). Not 

coincidentally, this also is the aquifer that has been most 

heavily developed to date, with large water-level drawdowns. The middle Claiborne (layer 5) and upper Claiborne (layer 6) aquifers, in areas closer to the coast and 

toward the Mexico border, have little potential for 

development because of their proximity to saline water 

(fig. 52, figs. 51F, 51G). The middle Wilcox aquifer (layer 

3), at locations farther inland, has good and fair potential for development (fig. 51C).

NORTHEASTERN TEXAS AREA

In the northeastern Texas area, ground-water pumpage has not had as large an effect on the ground-water 

budget as in the Winter Garden area. Regional groundwater recharge has increased by two-thirds from predevelopment, while natural regional discharge has 

decreased by almost one-half (fig. 43). Pumpage peaked 

in 1985, whereas the maximum change in confined storage was in 1980 and was about 9 percent of the pumpage. Both inflow and outflow to other geographic areas 

were steady with outflow exceeding inflow by about a 

factor of two. The 1985 ground-water pumpage was 

larger than the value of predevelopment regional 

recharge in this area.

Throughout a large part of the northeastern Texas 

area, the middle Claiborne aquifer (layer 5) has the best 

relative potential for ground-water development 

(fig. 52). The relative potential for development in this 

aquifer is good to excellent (fig. 51G). Large areas 

underlain by the lower Claiborne-upper Wilcox aquifers and the middle Wilcox aquifer (layers 4, 3) have a 

relative potential for ground-water development that 

ranges from fair to good (fig. 51H, 511). The good potential in the middle Wilcox aquifer (layer 3) occurs where 

the aquifer is thickest (fig. 12A).

WESTERN EMBAYMENT AREA

Ground-water pumpage in the western embayment 

area has caused the regional recharge to increase by 

more than one-half (fig. 44). Pumpage has captured 

about one-half of the natural regional ground-water discharge. Ground-water pumpage in 1985 was more than 

three-fourths of the volume of predevelopment regional 

recharge in the area. Although peak ground-water 

pumpage in the western embayment occurred in 1985, 

the peak change in confined storage came in 1980 and 

was about 8 percent of the pumpage. Outflow from the 

area (mainly to the Mississippi Alluvial Plain area) 

exceeds inflow by a factor of four or five, and both outflow and inflow have changed slowly over time.

Simulations indicate that the upper Claiborne aquifer (layer 6, fig. 52) has the most potential for groundwater development in the western embayment area. 

Towards the eastern side of this area, it has excellent to 

good relative potential for development (fig. 51F). In 

south-central Arkansas, the potential for development 

in this aquifer is diminished substantially by the presence of an isolated part of the Vicksburg-Jackson confining unit (fig. 11), which decreases the vertical 

connection to recharge sources. The middle Claiborne 

aquifer (layer 5) has good relative potential for groundwater development in much of the western embayment 

area (fig. 51G). The middle Wilcox aquifer (layer 3) has 

good relative potential for development in the southwestern part of this area (fig. 511).

MISSISSIPPI ALLUVIAL PLAIN AREA

Much ground-water pumpage in the Mississippi 

Alluvial Plain area has caused the regional recharge to 

increase by more than a factor of five (fig. 45). Pumpage 

also has captured about 80 percent of the natural 

regional ground-water discharge. The maximum 

ground-water pumpage is more than seven times larger 

than the predevelopment regional recharge rate. The 

peak ground-water pumpage and change in groundwater storage was in 1980, when the decrease in confined and water-table storage was about 25 percent of 

the pumpage (Ackerman, 1996, fig. 24). Inflow from 

adjacent areas (mainly from the eastern embayment 

area) is much larger than outflow to adjacent areas and 

has changed slowly over time. The water-budget values 

shown in figure 45 were calculated by taking the 

regional model-simulation results and adding watertable storage values from the subregional model (Ack-
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F97

erman, 1996) to simulated storage while subtracting the 

same values from simulated recharge.

Simulations indicate that the aquifer with the most 

potential for ground-water development in the Mississippi Alluvial Plain area is the Mississippi River Valley 

alluvial aquifer (layer 11), followed by the middle and 

upper Claiborne aquifers (layers 5, 6) (fig. 52). The Mississippi River Valley alluvial aquifer has excellent 

potential for development throughout much of the area, 

probably because of its proximity to surface-water 

sources of recharge and also because of its position as 

the natural regional discharge of most of the underlying 

aquifers. The major limitations to further development 

of this aquifer are its current level of drawdown, which 

is large in several places (pi. 7A), and marginally poor 

quality of water at some locations (fig. 51A) (Pettijohn 

and others, 1988). The upper Claiborne aquifer (layer 6) 

has several areas with excellent and good potential for 

development (fig. 51F). The middle Claiborne aquifer 

(layer 5) has good potential for ground-water development throughout most of the Mississippi Alluvial Plain 

area (fig. 51G). This includes the northern part of the 

embayment where the Memphis aquifer includes layer 

4 and layer 5. The middle Wilcox aquifer (layer 3) has 

some areas of poor and fair potential with smaller areas 

of good and even excellent potential for ground-water 

development (fig.517). The lower Wilcox aquifer (layer 

2) has large areas with fair and some areas with good 

potential for ground-water development (fig. 51/).

EASTERN EMBAYMENT AREA

Ground-water pumpage in the eastern embayment 

area has caused the regional recharge to increase by 

about one-half the predevelopment recharge rate (fig. 

46) while capturing nearly one-half of the natural 

regional ground-water discharge. The 1985 groundwater pumpage is about two-thirds of the volume of 

predevelopment regional recharge. The peak groundwater pumpage and change in confined storage (to 

date) came in 1985 and was about 7 percent of the 

pumpage. Outflow from the area (mainly to the Mississippi Alluvial Plain area) exceeds inflow by more than 

10 times, and both outflow and inflow have changed 

slowly with time.

Simulations indicate that each of the aquifers in the 

eastern embayment area have good to excellent potential for development at some locations (fig. 52). The 

middle Claiborne aquifer (layer 5) has good to excellent 

potential for ground-water development throughout 

much of the eastern embayment area (fig. 51G). This 

includes the northern part of the embayment where the 

Memphis aquifer includes layers 4 and 5. The middle

Wilcox aquifer (layer 3) has some areas of poor and fair 

potential with smaller areas of good and excellent 

potential for ground-water development (fig. 511). The 

lower Wilcox aquifer (layer 2) has large areas with fair 

and some areas with good and excellent potential for 

ground-water development (fig. 51/).

SOUTHERN TEXAS AREA

Some pumping developed in the early 1960's along 

the Rio Grande but began decreasing by the late 1960's 

(Mesko and others, 1990, fig. 6). Of all the areas in the 

study area, ground-water pumpage has least affected 

the hydrologic budget of the southern Texas area. The 

maximum ground-water pumpage is only about threefourths of the rate of predevelopment regional recharge. 

Pumpage has caused regional recharge to increase (fig. 

47) while natural regional ground-water discharge has 

decreased. The peak of ground-water pumpage and 

change in confined storage came in 1965 and was about 

5 percent of the pumpage from permeable zone C (layer 

9). Outflow and inflow to the area are about equal, and 

both have changed slowly over time.

Compared to other areas in the Gulf Coastal Plain, 

the southern Texas area has less potential for groundwater development (fig. 51) because this area has lower 

sand percentage and lower hydraulic conductivity of 

sand than other areas as well as less rainfall (figs. 4, 14, 

18, 21). In the few areas where the aquifer has some 

potential for ground-water development, the potential 

is limited by dissolved-solids concentrations that generally exceed 3,000 mg/L (fig. 51) (Pettijohn and others, 

1988).

SOUTHEASTERN TEXAS AREA

In the southeastern Texas area, peak ground-water 

pumpage occurred in 1980 (which was only slightly 

more than in 1970) and then decreased in 1985 (fig. 48). 

Natural regional discharge decreased to less than onefourth of its predevelopment value. Regional recharge 

increased by more than a factor of four. The maximum 

pumpage is more than five times the predevelopment 

regional recharge rate. Contribution from confined storage, both elastic and inelastic, reached a peak in 1970 

and was about 11 percent of the pumpage. Water withdrawn from inelastic storage has caused land subsidence, as described previously. Since pumpage began, 

there has been slightly more outflow to other areas than 

inflow from other areas.

Focusing on the ground-water budget of Houston 

and the surrounding area (fig. 53) shows some interest-
F98

A

REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

Recharge/Discharge

26.1

11.5

B

Recharge/Discharge

449.3

16.6 EXPLANATION

\v-jS

*/ s Fresh/brine interface at

) 10,000 milligrams per liter

8 Layer number 

D Permeable zone

i Boundary between outcrop 

i and downdip

2.3

I Recharge or downward flow, 

I in million gallons per day

T Discharge, or upward flow, in 

49 million gallons per day

P = 3 Pumpage, in million gallons 

per day

:S Change in storage

AAdj Net flow to adjacent areas. Plus 

means flow out of this area.

Numbers do not exactly match due 

to independent founding

FIGURE 53. Ground-water budget by model layer of the southeastern Texas area around Houston: 

A. predevelopment, and B. 1985.
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F99

ing effects of development that are more striking than 

the budget for the entire southeastern Texas area 

(fig. 48). The Houston area is defined for this purpose as 

the part of the coastal lowlands aquifer system between 

model rows 56 and 70 (fig. 2). The outcropping part of 

each layer is defined separately from the downdip part 

of the respective model layers for the summaries of 

flows presented in figure 53. Most of the change in confined storage in 1982-87 in the southeastern Texas area 

(81 Mgal/d, fig. 48) was in the downdip portion of 

permeable zone C (layer 9) in the Houston area (49 

Mgal/d, fig. 53). Change in confined storage accounts 

for about 31 percent of the water pumped out of the 

nonoutcropping part of permeable zone C (layer 9) in 

the Houston area.

Another striking change in permeable zone C (layer 

9) was the flow from the brine part of the layer toward 

the part that has relatively fresh water. The flow 

increased by a factor of 7 to nearly 42 Mgal/d in 1985 

(fig. 53). This brine flow accounted for about 27 percent 

of the volume of flow toward the pumping wells in the 

nonoutcropping part of permeable zone C (layer 9). 

Owing to the large distances (tens of miles) from the 

locations with brine water to pumping centers and the 

relatively slow movement of ground water, wells will 

not necessarily be withdrawing any part of the brine 

water now or possibly even in the next few decades. 

Another large component of flow toward the cone of 

drawdown in permeable zone C (layer 9) was net 

downward flow from permeable zone B (layer 10), 

which amounted to about 29 percent of the pumpage in 

permeable zone C (fig. 53). Only about 8 percent of the 

pumpage from permeable zone C (layer 9) was supplied 

by recharge in the outcrop and flow downdip in the 

zone (fig. 53).

There were also changes in the flows in permeable 

zone B (layer 10) in the Houston area (fig. 53). As above, 

the net vertical flow was from the underlying permeable zone C (layer 9) in predevelopment conditions. By 

1987 the flow direction had largely reversed, with flow 

moving down from permeable zone B (layer 10) to permeable zone C (layer 9). The net recharge to outcrop 

areas of model permeable zone B (layer 10) increased by 

a factor of four, to about 82 Mgal/d. Net vertical flow 

upward to permeable zone A (layer 11), which had been 

about 47 Mgal/d, reversed to about 210 Mgal/d downward. Interestingly the flow into the brine area of permeable zone A (layer 11) increased due to development, 

probably to replenish water that flowed out of the brine 

area of permeable zone A (layer 11) and down into the 

fresher part of permeable zone B (layer 10). There were 

only small changes in the flows within permeable zones

D and E (layers 7, 8) in the Houston area because there 

was little pumpage from those zones.

Simulations indicate that no single permeable zone 

has the most potential for ground-water development 

in the southeastern Texas area (fig. 52). With a few 

exceptions the permeable zone with the most potential 

is generally the shallowest one that exists at that location. One exception is west of Houston, where permeable zone A (layer 11) is at the surface, but permeable 

zone C (layer 9) has the most potential for groundwater development in three blocks and permeable zone 

B (layer 10) in another block (fig. 52). Permeable zone A 

(layer 11) has the largest outcrop area and hence the 

most potential for ground-water development, generally good or excellent except west and south of Houston 

and in some areas along the coast (fig. 5 LA). Permeable 

zone B (layer 10) has a considerable area with good 

potential for ground-water development (fig. 51B). Permeable zone C (layer 9) has a narrow strip with good 

potential for ground-water development (fig. 51C). Permeable zone D (layer 8) has some area with good and 

some area with excellent potential for ground-water 

development, especially toward the east (fig. 51 D). Permeable zone E (layer 7) has very little potential for 

ground-water development in the southeastern Texas 

area (fig. 51E).

Land subsidence is a possible effect of future development of ground water, as it has been in the past in the 

southeastern Texas area. Subsidence is less likely at 

locations with good potential for ground-water development shown on the maps (fig. 51A-E) because those 

areas are in close proximity to sources of recharge.

SOUTHWESTERN LOUISIANA AREA

In the southwestern Louisiana area, ground-water 

pumpage peaked in 1980 (which was only slightly more 

than in 1970) and then decreased slightly during 1985 

(fig. 49). Natural regional discharge decreased to about 

one-fifth of its predevelopment value. Regional 

recharge increased by more than a factor of four. The 

maximum pumpage is nearly six times the value of predevelopment regional recharge. Contributions from 

confined storage reached a peak in 1970 and were about 

3 percent of the pumpage. Inflow from other areas was 

more than twice the outflow to other areas and was 

increasing somewhat throughout the whole period.

The greater relative potential for ground-water 

development is toward the western Louisiana border 

for most permeable zones. Permeable zone A (layer 11) 

has the largest outcrop area and hence the greatest 

potential for ground-water development, generally 

excellent or good (fig. 5LA). Permeable zone B (layer 10)
FIDO REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

has some area with good and some area (toward the 

west) with excellent potential for ground-water development (fig. 51B). One area in the central part of layer 

10 in southwestern Louisiana has only fair potential. 

Permeable zone C (layer 9) has two strips with good 

potential for ground-water development, one toward 

the west and one toward the east (fig. 51C). Permeable 

zone D (layer 8) exhibits a pattern similar to layer 9 (fig. 

51D). Permeable zone E (layer 7) has very little potential 

for ground-water development in the southwestern 

Louisiana area (fig. 51 E).

EASTERN COASTAL LOWLANDS AREA

In the eastern coastal lowlands area, ground-water 

pumpage peaked in 1980 and decreased slightly by 1985 

(fig. 50). Natural regional discharge decreased to about 

one-half of its predevelopment value. Regional recharge 

increased by almost 50 percent. The maximum pumpage is slightly larger than the value of predevelopment 

regional recharge. Contribution from confined storage 

reached a peak in 1970 and was about 22 percent of the 

pumpage. This is the largest proportion of pumpage 

coming from confined storage of any area, possibly 

because ground-water development has increased more 

recently in this area. Inflow from other areas was somewhat less than the outflow to other areas. Inflow nearly 

doubled, and outflow decreased slightly during the 

period.

Nearly all of the permeable zones exhibit a northsouth trend of greater potential for ground-water development along or slightly to the west of the Pearl River. 

Simulations show that no single permeable zone has the 

most potential for ground-water development in the 

eastern coastal lowlands area (fig. 52). With a few 

exceptions, the permeable zone with the most potential 

is generally the shallowest one that exists at that location. Permeable zone A (layer 11) has the largest outcrop area and hence the most potential for groundwater development, generally excellent or good except 

in the birdfoot delta area of the Mississippi River and in 

coastal Hancock County in Mississippi (fig. 5L4). Permeable zone B (layer 10) has an area with excellent 

potential for ground-water development north of Lake 

Pontchartrain and several narrow bands with good 

potential (fig. 51B). Permeable zone C (layer 9) has several bands with good potential for ground-water development and some narrower areas with excellent 

potential in southern Mississippi and Alabama (fig. 

51 C). Permeable zone D (layer 8) has a considerable 

area with excellent potential for ground-water development in a wide strip along the Pearl River and in southern Alabama (fig. 51D). Adjoining those areas are strips

with good potential for ground-water development in 

layer 8. Permeable zone E (layer 7) has a strip along the 

Pearl River with good potential for ground-water development (fig. 51E).

There is a large potential for further development of 

the entire ground-water resource. Doubling the pumpage to 20 Ggal/d might be accomplished without 

unreasonable consequences if the pattern of additional 

pumping was carefully designed. Doubling the pumpage was tested by using the pumping additions proportional to the potential for development (fig. 51). 

Therefore, model blocks were assigned pumpage 

according to table 5. The resulting pumpage distribution was simulated in the model, resulting in an average 

drawdown of 60 ft. Although this distribution of pumpage is impractical, it demonstrates the large potential 

for development.

TABLE 5. Distribution of pumpage in maximum development

simulation according to the estimated potential for

ground-water development

Potential for future 

ground-water development

Pumpage range

(million gallons per day per

100-square-mile block)

Excellent

Good

Fair

Poor

Virtually none

15 to 38

1.8 to 15

0.35 to 1.8

0.04 to 0.35

0 to 0.04

Potential ranges taken from figure 51. Pumpage range calculated 

from log-log curve relating pumpage to the drawdown shown in figure 51 

from constant incremental pumping. Pumpage limited to 38 million 

gallons per day. These pumpages were for 100-square-mile block values 

for each block where relatively fresh water (less than 10,000 milligrams 

per liter) is present.

SUMMARY AND CONCLUSIONS

The Gulf Coast Regional Aquifer-System Analysis 

(Gulf Coast RASA) study area encompasses approximately 230,000 mi2 onshore in parts of Alabama, Arkansas, Florida, Illinois, Kentucky, Mississippi, Missouri, 

Tennessee, Texas, and all of Louisiana. The aquifer systems (and the study area) extend offshore beneath the 

Gulf of Mexico to include an additional 60,000 mi2 and 

are truncated at the edge of the Continental Shelf. The 

Gulf Coast RASA study is limited to coastal plain sediments of mostly Cenozoic age except for the northernmost part of the area where it includes sediments of 

Late Cretaceous age. The geohydrologic units generally 

thicken toward the Gulf of Mexico and reach a combined thickness of more than 17,000 ft near the coastline 

of southeastern Louisiana. The shallower parts of the
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F101

aquifers contain freshwater, but the deeper and offshore 

parts contain mostly highly mineralized water. Nearly 

10 Ggal/d of ground water was withdrawn from the 

aquifers in 1985, mostly for irrigation, but public supply 

and industrial uses are also important.

The study area is a gently sloping coastal plain in a 

mostly humid, temperate climate, underlain by thick, 

mostly clastic sediments deposited in a gulfward offlapping sequence. Historical sea-level oscillations caused 

cyclical sedimentation. Depositional environments 

alternating from predominantly continental to predominantly marine resulted in alternating fine-grained and 

more coarse-grained deposits. The base of the aquifer 

system in the north is at the top of the Midway confining unit and in the southern part of the area is at the 

transition zone into geopressured sediments. Though 

faults are common, fault throws are generally not great 

enough to entirely offset the full thickness of the 

regional hydrologic units described in this report, 

although individual beds could be offset. Many salt 

domes are present in several basins of the gulf coast 

regional aquifer systems. The highly mineralized 

ground water, which probably has resulted from saltdome dissolution, largely affects regional flow by creating areas with high fluid densities.

Three aquifer systems have been delineated in the 

Gulf Coast RASA study area: (1) the Mississippi embayment aquifer system, (2) the Texas coastal uplands aquifer system, and (3) the coastal lowlands aquifer system. 

Although all three aquifer systems are mixtures of fineand coarse-grained sediments, the fine-grained beds in 

the coastal lowlands aquifer system are generally thinner, dispersed vertically throughout the aquifers, and 

not areally as extensive. The aquifer systems were 

divided into 10 aquifers and 5 regional confining units, 

each of which was defined as a separate layer in a threedimensional, variable density, finite-difference groundwater flow model. The model grid consists of 102 by 58 

cells that are each 10 mi on a side. The water density, 

although variable in space, is assumed to be approximately constant in time, a valid assumption given the 

large model blocks and simulation time that is short relative to transport times. The model accounted for irreversible compaction of fine-grained beds, resulting in 

water being released from storage in fine-grained beds 

and in land subsidence. Many of the aquifer characteristics such as thickness, sand percentage, and water density (based on concentration of dissolved solids, 

temperature, and pressure) were derived from a computerized file of 989 geophysical logs.

The factors that controlled regional ground-water 

flow in the aquifer systems under predevelopment conditions are, in order of importance of effect (1) topography; (2) outcrop and subcrop pattern and geometry of 

aquifers, permeable zones, and confining units; (3) variation of hydraulic properties of aquifers, permeable 

zones, and confining units; (4) distribution of density 

and brines; and (5) downdip limits of geohydrologic 

units and geologic structure.

Topography is the most significant factor controlling 

predevelopment ground-water flow because the relatively humid climate maintains the aquifer systems 

generally full to overflowing with ground water. Therefore, the amount and distribution of regional recharge 

and discharge were proportional to the topographic 

gradient and the conductance of the aquifer. The major 

variations in subsurface permeability have been 

accounted for simply by delineation of aquifers and 

confining units. Sand-bed hydraulic conductivity 

increases from the western side of the study area 

toward the eastern side. Regional effective hydraulic 

conductivity varies as a power function of sand percentage because as the sand percentage decreases, the 

degree of hydraulic connection of the sand beds also 

decreases. Conductivity also decreases with depth due 

to compaction.

About one-third of the aquifer system studied contains freshwater, whereas the dissolved-solids concentrations of the saline water increases over relatively 

short distances to more than 100,000 mg/L. The resulting large differences in water density has substantial 

effects on ground-water flow both in the saline and the 

freshwater part of the system. Many forces apparently 

have prevented the more concentrated saline part of the 

system from reaching equilibrium. These forces are heat 

and salt sources (salt domes), sediment compaction, 

geopressured sediments, changing sea level, and a very 

large continental shelf requiring long flow paths to 

move brine to make adjustments. It is possible that most 

of the salt contained in high-concentration brines came 

from dissolution of salt domes. Geologic structure 

seems to have a small effect on ground-water flow 

except indirectly through the geometry of geohydrologic units. The effect of the small amount of flow leaking up through the Midway confining unit, which is 

composed of marine clays several hundred feet thick, 

was found to be insignificant compared to the volume 

of flow in the aquifer above it. This may also be true for 

the small amounts of flow leaking up out of the 

geopressured zone farther downdip.

One of the most prominent predevelopment flow 

paths in the Mississippi embayment aquifer system is 

from a major recharge area in south-central Mississippi, 

radiating outward with a major part curving to the 

south and then west. A large part of this flow moves 

southwestward and then curves northwestward to the
F102 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

regional discharge area in northeastern Louisiana. This 

curvature in flow paths is due partly to the resistance to 

vertical flow downdip. In the Texas coastal uplands 

aquifer system, the predominant regional flow pattern 

is from recharge areas between major river valleys laterally to discharge areas along the valleys. In the coastal 

lowlands aquifer system, the predominant flow pattern 

is from recharge areas in the updip and topographically 

higher areas along the inland edge of the system to discharge areas onshore towards the coast. Flow patterns 

in the deeper coastal lowlands aquifer system are more 

variable and difficult to interpret because of the large 

variability due to the complex three-dimensional distribution of ground-water density.

Predevelopment net regional recharge occurred in 

41 percent of the modeled area at the average rate of 

about 0.48 in/yr. The model block with the highest rate 

was in Mississippi at about 6 in/yr. Predevelopment net 

regional discharge occurred on 59 percent of the area at 

the rate of about 0.35 in/yr. The model block with the 

largest regional discharge was in the Mississippi River 

Valley alluvial aquifer at about 4 in/yr. The distribution 

of freshwater thickness correlates well with the simulated net regional flow and discharge, indicating that 

the regional recharge has flushed saline water and created a thick section of freshwater. Some ground water 

flows offshore before it is discharged to the ocean, but 

most of it discharges before it reaches the coastline.

Massive changes have occurred in the gulf coast 

regional aquifer systems in response to large-scale 

development of ground water throughout much of the 

study area. Development has led to lowering of hydraulic heads, changes in recharge and discharge, changes in 

flow velocity, changes in flow direction and magnitude, 

changes in water quality, and land subsidence. Total 

pumpage increased by a factor of 2.5 during 1960-80 

but decreased by 7 percent to 1985. Irrigated agriculture 

uses the largest volume (about three-fourths) of ground 

water in the study area, and also represents the fastest 

growth in ground-water pumpage for the period 1960 

to 1985. Much of the increase in agricultural pumpage 

was between 1970 and 1980, especially in the Mississippi Alluvial Plain. Other areas with large use of 

ground water for irrigation are southwestern Louisiana 

and southern Texas. More than 60 percent of the municipal and industrial pumpage is withdrawn from the 

coastal lowlands aquifer system. Ground-water usage 

for public supply has continued to increase with the 

population, whereas industrial pumpage at many locations has declined since 1970, largely spurred by the 

adverse effects of pumping and economic factors.

Every aquifer or permeable zone has more than one 

major cone of drawdown and has maximum blockaveraged drawdowns exceeding 80 ft. The 1985 pumpage is more than three times the rate of predevelopment 

regional recharge. Pumpage and the resulting declines 

in hydraulic head caused the regional recharge to 

increase by more than a factor of three to more than 9 

Ggal/d by increasing natural recharge and capturing 

water that was previously locally discharged. Meanwhile, the regional discharge decreased to about 1 

Ggal/d, nearly one-third of its predevelopment value. 

Thus, most of the simulated 1985 pumpage was supplied from increased regional recharge. Less than 20 

percent of the pumpage was captured natural regional 

discharge, and less than 10 percent was from groundwater storage.

By 1987, the proportion of the study area that contributed net regional recharge had increased by more 

than 50 percent, from 41 percent of the study area to 66 

percent of the study area. The area that changed from 

having net regional discharge to net regional recharge 

includes most of the area underlain by the Mississippi 

River Valley alluvial aquifer and most of the lowlands 

area within about 40 mi of the coastline except in southern Texas. Even offshore, there has been a reversal in the 

direction of simulated flow from onshore to the Gulf of 

Mexico off the coast near Houston during predevelopment to flow from the Gulf of Mexico toward the coastline in 1985.

By 1985, most of the net vertical flows between aquifers changed direction, or at least the magnitude 

changed considerably. Downward vertical flows 

between aquifers increased substantially in all of the 

aquifers and permeable zones as a result of groundwater pumpage and resultant reversal of vertical-head 

gradients. The upper Claiborne aquifer continued to 

have the most flow interchange with the Mississippi 

River Valley alluvial aquifer, probably due to the large 

area of contact. The upper Claiborne has very little 

pumpage yet has large inflows and outflows, capturing 

additional recharge and discharge and transmitting 

them to adjacent layers. The other aquifers are mostly 

self-supporting, meaning that most of their pumpage 

was supplied from increased recharge and decreased 

discharge within the aquifer. In the lower Wilcox aquifer and permeable zone E, which have very narrow outcrop and subcrop bands, much of the pumpage had to 

be supplied by vertical flow from adjacent layers.

In the Houston, Texas area, in the nonoutcropping 

part of permeable zone C, change in storage accounted 

for 31 percent of the flow toward the pumping wells. 

The flow from the brine area accounted for 27 percent of 

the volume of pumpage. However, wells will not necessarily be withdrawing any part of the brine water 

immediately or possibly even in the near future owing 

to the distances from the brine water and the relatively 

slow movement of ground water. Another large compo-
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F103

nent of flow toward the cone of drawdown in permeable zone C was downward flow from layer 10, which 

amounted to 29 percent of the pumpage. Only 8 percent 

of the pumpage was supplied by recharge in the outcrop flowing downdip to the pumping center in permeable zone C.

Large-scale ground-water pumpage in the freshwater part of the aquifer systems has markedly changed 

flow patterns in the brine part and induced flow updip 

toward pumping centers. However, it will take decades 

or centuries for the brine to reach these pumping areas 

because of the relatively slow ground-water velocities 

and long flow paths. For example, freshwater pumping 

onshore in and around Houston had changed flow 

directions more than 45 degrees or flow rates more than 

50 percent in the brine part of the lower Pleistocene and 

upper Pliocene deposits (permeable zone B; layer 10) 

throughout an area of 6,000 to 8,000 mi2 that extends as 

far as about 60 mi offshore. Block-averaged Darcian 

velocities in the brine approach 10 ft/yr. Assuming an 

effective porosity of 20 percent, this would mean average particle velocities of about 50 ft/yr, or about 100 yr 

to move 1 mi. These estimates could be in error by an 

order of magnitude or more. However, they indicate a 

possibility of long-term problems that may not be 

noticed for decades, yet which need consideration for 

prudent management of the resource.

The model was used to predict the relative potential 

for increased ground-water pumpage by making an 

areal analysis by aquifer of sensitivity to pumpage by 

simulating a unit increase in pumpage in each model 

block containing relatively fresh water. In addition to 

making maps by aquifer of relative potential for 

ground-water development, a composite map was 

made showing the layer in each grid block which had 

the least sensitivity to pumpage (least drawdown) and 

therefore the most potential for development.

Simulation indicates there is great potential for continued additional development of the ground-water 

resource in the study area. Doubling the pumpage to 

20 Ggal/d produced minimal consequences (average 

60 ft of drawdown over the study area) if the pattern of 

added pumping was carefully designed. While this distribution of pumping is not very practical, it does demonstrate the potential for development.

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Nation's ground-water resources Rio Grande region: U.S. Geological Survey Professional Paper 813-D, 39 p.

White, W.N., Rose, N.A., and Guyton, W.F., 1944, Ground-water 

resources of the Houston District, Texas: U.S. Geological Survey 

Water-Supply Paper 889-C, p. 141-288.

Whiteman, C.D., Jr., 1979, Saltwater encroachment in the "600-foot" 

and "1,500-foot" sands of the Baton Rouge area, Louisiana, 1966- 

78 including a discussion of saltwater in other sands: Louisiana 

Department of Transportation and Development, Office of Public Works Water-Resources Technical Report 19,49 p.

 1980, Measuring local subsidence with extensometers in the 

Baton Rouge area, Louisiana, 1975-79: Louisiana Department of 

Transportation and Development, Office of Public Works WaterResources Technical Report 20,18 p.

Whitfield, M.S., Jr., 1975, Geohydrology of the Evangeline and Jasper 

aquifers of southwestern Louisiana: Louisiana Department of 

Conservation, Louisiana Geological Survey and Louisiana 

Department of Public Works Water-Resources Bulletin 20, 72 p.
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F109

Williams, T.A., and Williamson, A.K., 1989, Estimating water-table 

altitudes for regional ground-water flow modeling, U.S. Gulf 

Coast: Ground Water, v. 27, no. 3, p. 333-340.

Williamson, A.K., 1987, Preliminary simulation of ground-water flow 

in the gulf coast aquifer systems, south-central U.S., in Vecchioli, 

John, and Johnson, A.I., eds., Regional aquifer systems of the 

United States, Aquifers of the Atlantic and Gulf Coastal Plain: 

American Water Resources Association Monograph 9, p. 119- 

137.

Williamson, A.K., 1989, Possible cause and substantial effects of brine 

distribution in the gulf coast aquifer systems, south-central 

United States [abs.]: International Geological Congress, 28th, 

Washington, D.C., July 9-19,1989, v. 3, p. 364-365.

Williamson, A.K., Grubb, H.F., and Weiss, J.S., 1990, Ground-water 

flow in the gulf coast aquifer systems, south-central United 

States A preliminary analysis: U.S. Geological Survey WaterResources Investigations Report 89^1071,125 p.

Wilson, T.A., and Hosman, R.L., 1988, Geophysical well-log database 

for the gulf coast aquifer systems, south-central United States: 

U.S. Geological Survey Open-File Report 87-0677,213 p.

Winslow, A.G., and Doyel, W.W., 1954, Land-surface subsidence and 

its relation to the withdrawal of ground water in the HoustonGalveston region, Texas: Economic Geology, v. 49, no. 4, p. 413- 

422.

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Winslow, A.G., Hillier, D.E., and Turcan, A.N., Jr., 1968, Saline ground 

water in Louisiana: U.S. Geological Survey Hydrologic Investigations Atlas HA-310, 4 sheets, scale 1:750,000.

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distribution coefficients, and dispersivity: U.S. Geological Survey Open-File Report 82-16,123 p.

Wood, L.A., and Gabrysch, R.K., 1965, Analog model study of ground 

water in the Houston District, Texas, with a section on Design, 

construction, and use of electric analog models, by E.P. Patten, 

Jr.: Texas Water Commission Bulletin S6508,103 p.

Yazdanian, Amin, and Peralta, R.C., 1986, Sustained-yield groundwater planning by goal programming: Ground Water, v. 24, 

no. 2, p. 157-165.

Yeh, WW-G., and Yoon, Y.S., 1981, Aquifer parameter identification 

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Professional Paper 813-L, 35 p.

SUPPLEMENT: SIMULATION DETAILS
F112 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

SIMULATION DETAILS

Details of the methods and data used to prepare the 

simulations are included in this section. The information provided here is helpful for making an evaluation 

of the simulation techniques used in the study and provides a basis for continued research.

DIFFERENCES IN SIMULATION FROM 

PRELIMINARY REPORT

Most of the details of data sources and compilation 

and other simulation items are contained in a preliminary report by Williamson and others (1990). All of the 

model input and output files as well as original data 

files were archived in ASCII files on a WORM (Write 

Once, Ready Many) disk by Kirkpatrick (1993). This section will only document differences between simulations presented in this report and those from the 

preliminary report. The differences include transient 

simulations with both elastic and inelastic (compaction) 

storage, areally variable hydraulic conductivity related 

to sand percentage and depth, and the use of automatic 

parameter-estimation methods.

TRANSIENT FLOW UNDER ELASTIC AND INELASTIC 

STORAGE CONDITIONS

The preliminary report (Williamson and others, 

1990) described several simulations, most of which 

assumed steady-state conditions. Most of the simulations described in this report, except those for predevelopment conditions and estimating potential for 

development, assume transient conditions where the 

pumpage and possibly other factors are changing 

between each pumping period. This necessitates estimates of the storage properties of the aquifers. Three 

types of aquifer storage are possible: water-table 

storage (elastic) where the change in storage represents 

the upper part of the aquifer being dewatered and 

sometimes wetted again; elastic confined storage, which 

occurs at any point where the aquifer is confined; and 

inelastic storage with irreversible compaction of finegrained beds, which provides a source of water to wells. 

Only the latter two types were directly simulated in this 

study. In most areas, little is known about the changes 

in water-table altitudes over time because few studies 

have attempted to measure them. It was beyond the 

scope of this study to collect data on water-table altitudes through time because of the large area and complexity of the aquifer system. Instead, change in watertable storage was ignored in the model, implying that 

some part (probably small) of the water volume simulated as regional recharge actually comes from watertable storage.

Kuiper (1994, table 4) found in his parameter-estimation study that the model calibration was most representative of observed data when the specific storage 

value of 0.51 x 10~6 ft"1 was used. Specific storage multiplied by the thickness of the aquifer or permeable 

zone equals the storage coefficient. Kuiper found that 

this was a relatively sensitive parameter in his simulations and that over a broad range of parameterization 

approaches (Kuiper, 1994, table 4), the regression 

always indicated specific storage values at the smallest 

limit without violating physical principles. The only 

parameters more sensitive were hydraulic conductivity 

of sand and hydraulic conductivity of fine-grained 

beds. Kuiper's parameter-estimation method will be 

discussed in the "Summary of Parameter-Estimation 

Results." Elastic storage in the confining units is 

assumed to be zero in the simulations, an assumption 

that should have little effect on the simulation results 

because the total volume of regional confining units is 

only about one-fifth of the total volume of sediments in 

the aquifer systems studied. However, inelastic storage in confining beds, which contributes to land 

subsidence, is simulated.

PUMPAGE ESTIMATION

Pumpage was estimated for 1960-85 at 5-yr intervals 

using a distribution to model blocks based on detailed 

pumpage estimates for 1980 and the total county pumpage for the respective year (Mesko and others, 1990, 

p. 25-29). Some uncertainty is introduced by the 

assumption that the percentage distribution of pumpage within a county has been equal to the 1980 distribution over the 1960-85 period. Exceptions to this 

assumption were made in the central Arkansas and 

Houston, Texas, areas because the distribution was 

known to change over time (Mesko and others, 1990, 

p. 28-29). Two categories of ground-water pumpage 

data were distinguished in the proration of county data 

to model blocks:

1. Point pumpage, primarily water pumped for public 

supply and industrial uses, where little water 

returns to the aquifer system; and

2. areally dispersed pumpage, primarily for irrigation, 

fish farms, stock, and rural domestic uses, where 

there is a potential for a substantial amount of 

return flow to the aquifer system. 

Point-pumpage data for 1980 were tabulated for 

each of about 5,000 municipal and industrial wells, 

along with the well's 5-mi block location, depth, and 

model layer, if known. These data included annual 

(1976-85) withdrawals for Harris and Galveston Counties in Texas, where the law requires each of about 3,000 

wells to be metered and reported (obtained on computer disk from Ronald J. Neighbors, Harris-Galveston 

Coastal Subsidence District, written commun., 1985).
F113

GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES

The wells in the Mississippi embayment aquifer system and Texas coastal uplands aquifer system were 

manually assigned to appropriate model layers. The 

wells in the coastal lowlands aquifer system were 

assigned using a FORTRAN program that used all of 

the 5-mi grid data on layer thicknesses and elevations of 

the land surface. The altitude of the top of each layer at 

the well location was interpolated from the four adjacent 5-mi block centers, weighted by the distance from 

the well to each block center. The correct layer was 

found by checking the layer-top altitudes, starting from 

the bottom, until one was found that was higher than 

the altitude of the bottom of the well. If the well bottom 

was calculated to be in a confining unit, it was assigned 

to the closest adjacent aquifer layer.

Estimation of areally dispersed pumpage is generally subject to greater errors than the point pumpage 

because there are many more wells, most of which are 

not metered, and well locations are usually not as accurately known as industrial or municipal well locations. 

Published data were used where available, generally as 

totals by county (references given by Mesko and others, 

1990). Estimates for agricultural use were generally 

based on irrigated areas, types of crops, and typical 

crop water-use estimates. Rural domestic-use estimates 

generally were based on the population not served by 

public systems and per capita use estimates. County 

total pumpage was apportioned to 5-mi blocks by using 

estimates of the distribution to each block and layer in 

the county based on well records and local knowledge 

of water-use patterns.

The transient simulations were made using the time 

periods and pumpage outlined below, with predevelopment simulated hydraulic heads as initial conditions. 

The pumpage factors before 1960 were extrapolated by 

simple linear regression using 1960, 1970, and 1980 

pumpage totals and 1930-80 population data.

Transient simulation time periods

Period 

ended

1925

1935

1945

1955

1962

1967

1972

1977

1980

1982

1987

Length

(in years)

10

10

10

7

5

5

5

3

2

5

None

Pumpage used

1960 pumpage multiplied by factor 0.149

1960 pumpage multiplied by 0.310

1960 pumpage multiplied by 0.569

1960

1965

1970

1975

1980

1980

1985

ESTIMATION OF OBSERVED 1962-87 HYDRAULIC HEAD

Estimates of hydraulic head for model blocks at 5-yr 

intervals were needed for comparison with simulated 

heads. Over one-half million water-level measurements 

taken from data on nearly 20,000 wells stored in the U.S. 

Geological Survey's WATSTORE data base and including over 100,000 measurements from the Texas Natural 

Resources Information System files (Thomas A. Williams, U.S. Geological Survey, written commun., 1987) 

were combined into one data base. Each well in the data 

base was assigned a model layer by a computer program using the location, altitude, and depth of the well. 

A considerable amount of error screening was done by 

several automatic methods in order to delete data that 

were obviously in error. In addition to internal checks 

such as missing or impossible data values, well locations were checked by computer program to test if they 

were in or near the county specified in the data base. 

About 1 percent of the wells, which contained about 0.3 

percent of the measurements, were not used because of 

failure to pass this location check. Land-surface altitudes were checked to test if they were within a reasonable range of the 5-mi land-surface altitude interpolated 

to the well location. About 1.6 percent of the wells, 

which contained about 1 percent of the measurements, 

were ignored because of this land-surface altitude 

check. The data were then plotted as hydrographs, with 

many wells plotted on a page, to check for extreme outliers.

The desired intervals for head data were offset from 

the pumpage data by 2.5 yr. For example, pumpage 

data for 1980 were assumed to approximate the average 

for the period 1977-82, and simulation with 1980 pumpage should produce water-level changes from 1977 to 

1982. All of the measurements for each well during the 

2-yr time period on either side of the end of the pumping period (1981-83) were used to calculate an average 

head for the well at the end of the pumping period. The 

number of wells measured during each period varied 

from about 4,000 in 1957 to a maximum of about 9,500 

in 1972, decreasing to about 7,000 in 1982. The 2-yr well 

averages were used to interpolate heads for each block 

that had sufficient head data in and near it.

Using 1982 as an example, the combined data base 

contained over 37,000 head measurements taken in over 

7,000 wells between July 1981 and June 1983. Obviously, 

some wells were measured several times in that time 

period, and if so, the measurements for those wells 

were averaged.

As is generally the case, the numerical flow model 

used in this study produces an approximation to blockaveraged head values that are not equal to values for 

head at a single point in the aquifer system (Kuiper,
F114 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

1994, p. 55). Observed hydraulic-head data were averaged for each block to assure comparison of values of 

head expected to be representative of what the model 

computes. Ten-mile block-averaged head values were 

estimated using an inverse distance-squared weightedaveraging scheme of the SURFACE III computer program (Sampson, 1978, 1988). Several different neighbor 

selection criteria (to select which data points will be 

used to estimate a grid cell value) and gridding algorithms were attempted. Two different gridding algorithms were compared, one using a simple distanceweighted average of nearby sample values (Sampson, 

1988, fig. 13) and the other using distance-weighted 

averages of the projection of dips of the surface at the 

nearby values (Sampson, 1988, figs. 14,15; Sampson, 

1978, fig. 10). The projection of dip averages would be 

the best estimate of the observed head surface at the 

center of the block, whereas the simple averages would 

approximate the average head for the block. The root 

mean squared error (RMSE) between these two sets of 

block-averaged observed heads was 65 ft. The RMSE 

between simulated 1982 heads and dip projection-averaged observed heads was 85.7ft, and for the simple 

averages, 57.4 ft. This indicates that the simple-average 

head, approximating the block-averaged head, estimates values that compare better with the model-computed heads than those the method designed to 

estimate the head value at the center of the model block. 

The RMSE between different sets of neighbor selection 

criteria for simple block-averaged observed heads was 

about 25 ft. These RMSE values for different estimates 

of observed heads are large because the horizontal and 

vertical hydraulic-head gradients are commonly very 

steep in the vicinity of pumping centers and there are 

many abrupt changes in land-surface altitude, causing 

great difficulty in estimating an average head for a 

100-mi2 block for a layer. All model blocks that had at 

least one head measurement within 5 mi of the center of 

the block and at least three other measurements within 

a 12-mi radius were used in the comparison. The chosen 

set of block-averaged observed head data for 1982 

included 1,110 blocks among the 10 permeable layers. 

Kuiper (1994, p. 55-58) used the same raw data with 

different culling criteria and averaging method to come 

up with water-level estimates for 1,432 blocks for 1972 

and 1,675 blocks for 1982.

DEEP PRESSURE DATA

Nearly all of the hydraulic-head measurements 

mentioned above are taken from the upper parts of the 

aquifer systems, which contain relatively fresh water. 

Calibration of aquifer characteristics in the deep, brine 

parts of the aquifer systems (which comprise about

two-thirds of the aquifer-system volume) is dependent 

on the availability of pressure or head estimates. More 

than 40,000 final shut-in pressure measurements from 

drill-stem tests were available in a file subset from the 

Petroleum Information file (1990). Several problems 

complicate the use of these data. Many of the pressure 

measurements are affected by oil and gas withdrawals. 

Some measurements have low pressures because not 

enough time was allowed after the well was shut in for 

the pressure to build up to static condition or because of 

equipment problems, leaking packers, and so forth. 

Deeper measurements, near the bottom of the systems 

studied, are from near the geopressured zone where the 

fluid pressures are in transition from hydrostatic to 

geopressure and are much greater than immediately 

above. Pressures in the geopressured zones are commonly high enough to make equivalent freshwater 

heads several thousand feet above sea level. A few pressure measurements from the transition zone can greatly 

affect any areal averages or interpolations.

Culling the pressure data by retaining those having 

equivalent freshwater heads between 300 below to 

1,500 ft above sea level reduced the number of measurements to about 19,500. The lower bound was chosen to 

retain data affected by head declines resulting from 

freshwater pumpage and to eliminate most of the data 

strongly affected by oil pumping or test deficiencies. 

The upper bound was set to a few hundred feet above 

the maximum head calculated from the hydrostatic gradient in the modeled area to eliminate pressures 

affected by the transition to the geopressured zone. All 

but 8,933 of these pressure data were from below the 

aquifer systems modeled. Using a similar procedure as 

mentioned above for freshwater well data, equivalent 

freshwater heads were interpolated for 288 blocks that 

had at least seven pressure measurements each. There 

was generally very large variability among the seven or 

more pressure measurements for a single block.

Kuiper (1994, p. 55) used the same raw data with different culling criteria and averaging method to estimate 

deep heads for 586 blocks. Including the deep head estimates did not materially affect the parameter estimation results (Kuiper, 1994, p. 94). This is probably 

because even with the culling and averaging, the data 

are still quite noisy and cannot be simulated well by the 

model regardless of the parameters used.

AREALLY VARIABLE HYDRAULIC CONDUCTIVITY 

RELATED TO SAND PERCENTAGE AND DEPTH

Most of the details about the methods and results of 

estimating effective hydraulic conductivity, Keff are in 

the body of this report. However, Kuiper (written commun., 1987) did test other areal groupings for potential
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES

F115

improvement in the model calibration. Kuiper found 

that the area boundaries (fig. 13) used as presented in 

this report produced a model with better fit and narrower confidence limits on parameters than with any 

other of several combinations.

Kuiper also tested the exponent used in the power 

function relating sand percentage to effective hydraulic 

conductivity, Keff. He found that the model was quite 

sensitive to this parameter. Under at least 10 different 

schemes of parameter choices, the model consistently 

estimated a value within the range of 3.52 to 4.36 for the 

exponent of horizontal K and the reciprocal of those 

values for the vertical K (Kuiper, 1994, table 4).

The function relating Keff to depth was not a very 

sensitive parameter in the model according to Kuiper 

(1994, p. 73). This is probably because most of the reliable head data was quite shallow in the aquifer system 

compared to the total depth of the system modeled.

APPROACH TO CALIBRATION AND 

PARAMETER ESTIMATION

The purpose of the modeling in this study was to 

understand regional-scale flow. Therefore, no attempt 

was made to alter the parameters for an individual 

block or even groups of blocks unless data errors were 

found. Small improvements in the model fit to historical 

observed data would not substantially improve the 

understanding of the system. Instead, more emphasis 

was given to the design of the approach to estimating 

model parameters such as hydraulic conductivity to 

maximize the information content of the available data. 

An additional reason for not attempting alteration of 

individual model block parameters during calibration 

was the size and complexity of the system modeled.

Manual model calibration is an iterative process 

similar to the one used by parameter-estimation models 

to find the combination of parameter values that gives 

the best fit to available data. Automatic approaches by 

computer programs have the advantage of being able to 

test many more values and being more likely to find the 

"best fit" parameters. They have the disadvantage of not 

being able to recognize unrealistic parameters or combinations of parameters. This disadvantage can be minimized by careful application and by constraining the 

parameter selection to within reasonable ranges.

METHOD TO ESTIMATE AREAL SENSITIVITY TO 

PUMPING FOR DEVELOPMENT POTENTIAL

A starting head equal to zero was used for these simulations for easy comparison. Therefore, drawdown is 

equal to the opposite sign of hydraulic head. Running

the simulation to steady state gives quicker runs and 

the full and final effects of the pumpage. If subsidence 

was simulated in the sensitivity to pumpage runs 

(described previously in the text), areas of potential 

subsidence would look more favorable for development 

than would be true for a steady-state condition, due to 

less drawdown.

To analyze effects of pumping individual blocks, 

interference of nearby pumped blocks must be minimized. In the first attempt at the sensitivity to pumpage 

runs, every fourth block in every other layer was 

pumped. The pumped blocks in every other layer were 

offset two blocks, so the nearest interference from a 

neighboring pumping block was at least four blocks 

away, horizontally and vertically. Enough interference 

was noted that a new design was developed that 

pumped only one layer at a time and only every ninth 

block in each direction. This took 810 simulations, but 

because the simulations were all steady state, computer 

run time was small and the whole group of simulations 

executed in one night. Every model block with an average concentration of dissolved solids less than 10,000 

mg/L was included in the analysis, a total of 7,324 

blocks.

LIMITATIONS OF THE SIMULATION

Uncertainty is present in every scientific finding. 

This section will discuss some of the known sources of 

uncertainties in the findings of this study.

MODEL FIT

Hydraulic head is the only model-output quantity 

that is observable because of the previously mentioned 

accuracy problems in estimating stream gains and 

losses or other flows. Comparison of simulated with 

observed hydraulic head is a very complicated process 

when so much observed data are available and the system simulated is so complex. Data could be compared 

at six time periods for each of 10 layers in tens or hundreds of blocks in each layer, totaling many thousands 

of comparisons. A summary of comparisons for 1980 is 

shown in table 6 because the most observed-head data 

were available for that year. The model fit is adequate 

considering the method and approach to calibration. No 

attempt was made to alter the parameters for individual 

blocks or even groups of blocks unless data errors were 

found, as described in the previous section on calibration.

The overall model-fit root mean squared error 

(RMSE) is only slightly better than it was in the preliminary model (comparing table 6 with Williamson and
TABLE 6. Comparison of simulated hydraulic head to estimated block-averaged head for 1980, in feet 

[MEAS, Mississippi embayment aquifer system; TCUAS, Texas coastal uplands aquifer system; CLAS, coastal lowlands aquifer system; GC RASA, Gulf Coast 

Number of blocks

Layer system

2

3

4

5

6

MEAS and TCUAS

7

8

9

10

11

CLAS

GCRASA

Estimated 

heads

24

105

81

170

55

435

37

69

96

96

345

643

1,078

Model 

high

13

79

56

125

28

301

30

45

62

72

164

373

674

Model 

low

11

26

25

45

27

134

7

24

34

24

181

270

404

Mean

Estimated 

head

209.9

283.6

272.1

139.6

138.0

202.7

165.0

148.8

51.2

9.2

43.7

57.9

116.3

Model 

head

225.5

306.2

287.8

179.2

149.4

228.9

191.6

159.4

69.6

44.2

48.7

71.3

134.9

Mean head difference

All 

blocks

15.6

22.6

15.7

39.6

11.4

26.2

26.6

10.6

18.4

34.9

5.1

13.3

18.5

Model 

high

39.7

39.2

32.8

61.2

43.4

47.6

38.3

37.3

47.2

54.8

27.5

38.1

42.3

Model 

low

12.9

27.9

22.7

20.2

21.9

21.9

23.5

39.6

34.1

24.7

15.3

20.9

21.2

Head difference

Standard 

deviation

35.3

47.0

43.8

58.0

39.3

50.8

35.3

52.0

52.6

52.6

31.3

42.7

46.5

Minimum

-25.9

-88.4

-209.7

-75.5

-55.1

-209.7

-52.5

-248.8

130.8

-58.3

-61.9

-248.8

-248.8

M
F117

GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES

others, 1990, table 5), with a few exceptions. Confirming 

what Konikow (1978) reported that is, most improvement in model fit is obtained in the early stages of 

model calibration. There is improvement in the overall 

standard deviation of the errors from 56.5 ft to 46.5 ft. 

However, there is an increase in the mean error, from 7.4 

to 18.5 ft, which causes the RMSE to be only slightly 

less, 56.9 to 50.1. The distribution of errors shows that 

there is some bias to the errors (fig. 54). The leakance 

(hydraulic conductivity multiplied by the flow area 

divided by the flow-path length) between the source 

sink layer and the top model layer was adjusted to 

reduce the mean error, but as mean error decreased, the 

RMSE increased. Kuiper adjusted for this by decreasing 

the estimated water-table altitude used in the source 

and(or) sink layer by a factor ranging from 0.834 to 

0.932 in the various model runs reported (Kuiper, 1994, 

table 4). This adjustment was not made in the simulations presented in this report because no physical explanation for this kind of adjustment is known and because 

it would only decrease the measurement of error without producing other significant improvements. The 

model fit was improved over the preliminary model for 

layers 2, 9, and 11, was worse for layers 5, 4, and 3, and 

was similar for layers 6, 7, 8, and 10.

The details of the model fit are discussed, layer by 

layer, by comparing contours of simulated hydraulic 

head with posted values of block-averaged estimated 

head, as shown on plate 7A-J. The block-averaged estimated heads are not contoured because that is not possible without including further averaging and 

extrapolation. The block-averaged heads were estimated using the Surface III gridding algorithm 

described previously. Generally, the discussion is 

focused at locations where the model fit is less than 

ideal. At some locations, it appears that the simulated 

drawdown is not coincident with the estimated drawdown. This is usually due to the independent discretization of pumpage and estimated head data. Both data 

sets have some additional error introduced that is inherent to the discretization process. The values in one data 

set may be shifted in one direction to the nearest block, 

whereas the values in the other data set may be shifted 

in the other direction, magnifying the apparent error. 

An example is shown on plate 7G where pumping at 

Nacogdoches, in eastern Texas, is shifted to the nearest 

10-mi block to the south (estimated head = -178). A 

large simulated drawdown is shown in a slightly different location than the maximum block-averaged drawdown, which was shifted to the nearest block center 

north of the locations of estimated well data.

The simulated 1980 hydraulic head for layer 11 

matches the estimated head in permeable zone A somewhat better than the RMSE for the whole model (table 

6) because the layer is relatively shallow, and the estimated drawdown is not very large. The model underestimates the depth of the drawdown in several areas (pi. 

7A). A steep drawdown identified at New Orleans is 

not simulated very well because the cone is small and 

because the existing wells pump only from the top part 

of the layer where the transmissivity is effectively 

smaller (C.D. Whiteman, U.S. Geological Survey, oral 

commun., 1989). The model also underestimates the 

drawdown in southwestern Louisiana and the vicinity 

of Houston, Texas.

In Louisiana and Mississippi, the simulated 1980 

hydraulic head in layer 10 generally matches the estimated head in permeable zone B (pi. 7B). The simulated 

drawdown around Houston is large and quite deep, but 

not as deep as the observed cone. The simulated drawdown near Kingsville (the first area north of Mexico 

with abundant estimated heads) is also not as deep as 

estimated data indicate (pi. 7B). The situation is similar 

to that at New Orleans, where the cone is also quite 

small and the pumping is from part of the layer, making 

simulation difficult.

The simulated 1980 hydraulic head for layer 9 generally follows the estimated head estimates in permeable 

zone C (pi. 7C). Simulation of the cones of depression 

along the coast at Gulfport and Pascagoula, Mississippi, 

and Baton Rouge, Louisiana, generally matches the 

observed cones except for some differences in shape. 

The very large cone at Houston has a few more differences in shape. There are some minor differences in the 

simulated heads farther south in Texas.

Less pumpage and therefore fewer and smaller 

cones of depression are present in the 1982 heads for 

layers 6, 7, and 8 (pis. 7D, E, F). In and near the outcrop 

areas of the layers, the simulated water levels closely 

approximate observed water levels except where local 

variations in the land-surface altitude are strongly 

reflected in observed water levels. The maximum drawdown at Alexandria in central Louisiana is simulated by 

the model in layer 8, whereas the maximum observed 

block-averaged drawdown is in layer 7 (fig. 36G). The 

failure to match drawdowns is because layer 8 is very 

thin at this location, resulting in a discretization error in 

allocating excessive pumpage to layer 8.

The model overestimates the 1980 head in layer 8 

below the very large pumping at Baton Rouge. The cone 

along the Mississippi Gulf Coast is offset somewhat 

from the estimated drawdown but is similar in magnitude. The model underestimates drawdown at some 

areas in layer 7 in southern Mississippi. Layer 6 heads 

generally are simulated well by the model except for 

underestimating the drawdown in central Mississippi.
F118 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

200

Simulated heads are lower than observed heads

150

O 100

50

Simulated heads are higher than observed heads

0 

-160 -120 -80 -40 0 40 80 

SIMULATED MODEL HEAD MINUS OBSERVED HEAD, IN FEET

EXPLANATION

11 Permeable Zone A and Mississippi 

River Valley alluvial aquifer

|____110 Permeable Zone B

I | 9 Permeable Zone C

8 Permeable Zone D

7 Permeable Zone E

6 Upper Claiborne aquifer

I____ 5 Middle Claiborne aquifer

[B^* ' I 4 Lower Claiborne-Upper Wilcox aquifer

|____| 3 Middle Wilcox aquifer

|____| 2 Lower Wilcox aquifer 

FIGURE 54. Comparison of simulated and estimated block-averaged head, 1980.

120 160
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES

F119

The model fit is poorest in layer 5 (table 6), especially in southern Arkansas and northern Louisiana 

where the estimated hydraulic heads vary greatly from 

one block to the next and the simulated heads are generally too high (pi. 7G). In this area, layer 5 contains a 

substantial confining unit, effectively dividing it into 

two permeable zones with nearly all of the pumpage 

from the upper zone. The upper zone is much thinner 

than the whole unit, causing more actual drawdown 

(due to the reduced transmissivity) than is simulated in 

the model, which uses the transmissivity of the entire 

thickness of layer 5. The simulated heads near Memphis, Tennessee, match the estimated heads well.

In layer 4, the simulated hydraulic heads generally 

match the estimated heads in the Mississippi embayment aquifer system but underestimate the drawdown 

in the Winter Garden area of southern Texas (pi. 7H). 

The fit (RMSE) is poorest in the western part of La Salle 

County and adjacent Dimmit County, an area with 

some of the lowest estimated heads. The trough of lowest simulated head is much farther to the east, possibly 

due to errors in the distribution of observed pumpage.

In layer 3, most of the estimated hydraulic-head data 

are in eastern Texas, where the estimated head varies 

considerably from one block to the next (pi. 71). The 

simulation for eastern Texas is neither consistently high 

nor low. However, due to the varied nature of the estimated heads, the model fit is poor at some of the locations.

The estimated hydraulic heads are matched best 

(lowest RMSE) by the simulated heads in layer 2 (table 

6). Most of the estimated block-averaged heads are 

within 15 ft of the model-simulated head for the same 

location for 1980 (pi. 7/).

Another check of the accuracy of the simulation is to 

examine model fit over time for any trends that would 

indicate the model did not simulate some time-sensitive 

feature of the aquifer system. Usually, this is done by 

comparing a few available well hydrographs with simulated hydrographs for the nearest block. The large 

model blocks of this regional model preclude comparing individual well hydrographs because the model 

simulates many hydrologic complexities within each 

model block. An alternative approach was chosen, comparing block-averaged estimated heads, wherever and 

whenever they existed, with the equivalent model block 

head and then plotting different statistics of the fit over 

time. Simulated trends exhibit little bias over time 

(fig. 55). The RMSE is plotted for each layer and groups 

of layers in figure 55. All of the statistical measures 

included in table 6 were plotted similarly to figure 55. 

The mean differences were steady over time. The statistics that measure variability in errors, like mean absolute error and standard deviation, showed trends in 

time very similar to the ones shown in figure 55. In 

some layers the model fits the observed data consistently better, and in other layers the fits are consistently 

poorer. The estimated hydraulic-head data used to construct figure 55 include more than 5,000 individual 

block-year comparisons for the six time periods shown, 

with about 500 blocks compared in 1962, increasing to a 

maximum of 1,078 blocks in 1980, and decreasing 

slightly in 1982. Additionally, the block-averaged estimated and simulated heads for all layers in 10 selected 

blocks having large drawdowns are shown in figure 36.

SUMMARY OF PARAMETER-ESTIMATION RESULTS

Kuiper's (1994) parameter-estimation results are 

summarized here. Parameter values from Kuiper's 11- 

parameter model (model number 9) (Kuiper, 1994, table 

4) were chosen for simulations presented in this report. 

Kuiper (1994, p. 75) states that the best model to predict 

future hydraulic heads and estimates of effective conductivity was his model using four parameters. Those 

four parameters were horizontal hydrologic conductivity of sand, vertical hydraulic conductivity of finegrained deposits, specific storage, and a multiplier for 

the water-table head used for the constant-head source 

and(or) sink layer. Additional parameters chosen for 

this report were used to divide up the Kh for sand into 

six groups divided by combinations of area and layer, 

ordered by estimated Kh from aquifer tests (Prudic, 

1991, table 5, fig. 15). The layer-area combinations having the largest estimated Kh were combined in group 1, 

and so on, through the six groups with successively 

smaller values of Kh. Kuiper's model 9, chosen for use 

in this study, also included two groups of Kv, one for 

fine-grained deposits in the regional confining units 

and one for the fine-grained deposits contained within 

the regional aquifers and permeable zones. The only 

considerable change from Kuiper's model 9 is that the 

simulations presented in this report do not use a multiplier for the water-table head used in the constant- head 

source and(or) sink layer. Use of this parameter would 

have improved the fit of the simulated to estimated 

hydraulic head by lowering the mean difference from 

18.5 ft (table 6) to 3.7 ft (Kuiper, 1994, table 4). However, 

little else would have changed and there is no physical 

explanation for use of this multiplier. For comparison 

purposes, note that Kuiper's weighted RMSE (36.4 ft for 

model 9) is equivalent to an RMSE using nonweighted 

head data alone of 51.6 ft.

Part of the difference in model fit in this study compared to Kuiper's is due to Kuiper's culling of estimated-head data that appear to be outliers because of 

steep cones of drawdown. His culling criteria only
F120 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

90

85

80

75

LLJ -in _ LLJ /U 

LL

z

a: 65 h

O

cc

60 h

55 h

LLJ

cn 50 -

45 h

O 40 - 

cc.

35 - 

30 - 

25 -

20

1960 1965 1970 1975 1980

Layer

 6 

 5 

 4 

 3 

 2 

 U 

 11 

 10 

 9 

 8 

 7 

 L 

YEAR

EXPLANATION

Geohydrologic unit

Upper Claiborne aquifer

Middle Claiborne aquifer

Lower Claiborne-Upper Wilcox aquifer

Middle Wilcox aquifer

Lower Wilcox aquifer

All layers in Mississippi Embayment and 

Texas Coastal Uplands aquifer system

Permeable Zone A and Mississippi River 

Valley alluvial aquifer

Permeable Zone B

Permeable Zone C

Permeable Zone D

Permeable Zone E

All layers in Coastal Lowlands aquifer system

All layers

1985

FIGURE 55. Model fit as a function of simulation time, 1962-82, by layer and groups of layers.
F121

GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES

allowed data showing head gradients of 35 ft/mi or less 

(Kuiper, 1994).

The confidence limits (WjJ of predicted head of 

model 9 are 18 percent worse than the confidence limits 

for model 4 (fig. 56), which is not good. However the 

Chi2 value is lower and closer to a value that would 

prove that the errors were normally distributed and 

therefore random. In both cases, the Chi2 value is too 

high to accept the hypothesis of normally distributed 

errors, introducing some uncertainty about which 

model is the best to choose (Kuiper, 1994, p. 75-76). 

Kuiper's 11-parameter model was chosen for use in this 

report because the RMSE and Chi2 were less than for 

the four-parameter model, and the Wj, value was not 

much greater (fig. 56).

Approximately 95 percent of the prediction interval 

half-widths for volume-averaged freshwater heads 

exceed 108 ft. This means that about 95 percent of the 

predicted freshwater-head values would be within

60-

CC

Q 

LU

I 50- 

[u

LU 

CC.

I 45 '

O

g 40-

35-

650

550

450

350

250

150

250

240

230

220

210

200

190

180

170

160

150

140

\. 8 12 16 20 24 28 32 

NUMBER OF PARAMETERS

EXPLANATION

Wk = confidence limits 

CH|2 =

RMSE = root mean squared error

FIGURE 56. Results of parameter estimation by regression, comparing 

number of parameters with statistics of fit.

108 ft of the actual head value. For volume-averaged 

base 10 logarithm hydraulic conductivity, the prediction 

interval half-widths exceed 0.89 (Kuiper, 1994, table 4). 

All of the models are unreliable for the prediction of 

head and ground-water flow in the deeper parts of the 

aquifer system, including the amount of flow coming 

from the geopressured zone beneath the aquifer system. 

A simulation was done to test the effect of assuming 

freshwater flow with a no-flow boundary at the brine 

interface. This was done by truncating the domain of 

solution to exclude that part of the system having a 

ground-water density greater than 1.005 g/cm3 and setting the density to that of freshwater in the remaining 

shallow part. This change does not appreciably change 

the results for head and ground-water flow from the 

model, except for locations close to the truncation surface (Kuiper, 1994).

SCALE EFFECTS

The scale chosen to simulate ground-water flow systems has a significant effect on several aspects of the 

study, including recharge and discharge estimates, 

effective hydraulic conductivity as described in the 

body of this report, and water density. The following 

discussions are provided to familiarize the reader with 

the basis for these differences in order to properly interpret the conclusions presented in this report.

RECHARGE AND DISCHARGE ESTIMATES

As was stated in the body of the report, out of necessity, the only flows simulated were those that crossed 

block faces. Flows within a single model block were not 

considered (fig. 30). This has a sizable effect on estimates of recharge and discharge because the small-scale 

local flow systems commonly have large recharge and 

discharge. The head gradients in these small-scale systems are steep due to short flow paths. These factors are 

important considerations when comparing flows 

described in this report with those of smaller scale studies.

DENSITY

As mentioned previously, the distribution of concentration of dissolved solids has a very large degree of 

local as well as regional variability, both in the vertical 

and horizontal directions. To limit the effect of local 

variations in the point data used to estimate the 

regional trends of density, the density data were 

smoothed using an areal moving average as described 

by Williamson and others (1990, p. 103). Kuiper (1994) 

found that smoothing the density data had almost no 

effect on the parameter-estimation fit or prediction. The
F122 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

smoothed density-data sets were used in all simulations 

presented in this report in order to make the best 

approximation at a regional scale.

BOUNDARIES

Boundary conditions are commonly a source of 

uncertainty in ground-water simulations (Franke and 

others, 1987). Simulating a very large portion of the 

entire regional aquifer system minimizes the uncertainty due to boundaries. The following description is 

provided to alert the reader to limitations in applying 

the results of this study near the boundaries where true 

boundary conditions in the aquifer system were simulated as nearly as was practical.

LATERAL

On the eastern edge of the study area near the Gulf 

of Mexico, the lithology of most of the aquifers undergoes a facies change. Permeable zones in the coastal 

lowlands aquifer system pinch out. The fine-grained 

clastic deposits of the Vicksburg-Jackson confining unit 

gradually change into the limestone beds of the Floridan aquifer system to the east. Much of the Mississippi 

embayment aquifer system becomes calcareous with 

greatly reduced hydraulic conductivities. Most of the 

coastal lowlands aquifer system outcrop curves to the 

south and offshore. All of these factors tend to restrict 

flow across the eastern boundary of the study area. This 

boundary was treated as a no-flow boundary for the 

preliminary simulations (Williamson, 1990, p. 36).

Transient simulations highlighted the need to 

change the eastern boundary from a no-flow boundary 

to some other type; otherwise, the model would incorrectly simulate drawdowns of hundreds of feet near 

Mobile, Alabama. The eastern boundary was changed 

to a constant head boundary in layers 3, 5, 7, and 8, a 

poor approximation of conditions near this boundary 

but an improvement compared to a no-flow boundary.

The only data available on the gulf coast regional 

aquifer systems in Mexico was topography and a surficial geology map (Direccion General de Geografica del 

Territorio Nacional, 1984). The approach was to use 

speculative data at distances from any area of interest 

rather than try to force an arbitrary condition of some 

kind at the international boundary. Data from several 

geophysical logs on the United States side of the border 

were duplicated and transposed to a location in Mexico 

in about the equivalent stratigraphic position as shown 

by the surficial geology map.

TOP RECHARGE AND DISCHARGE BOUNDARY AND 

WATER-TABLE STORAGE

The constant-head layer simulating regional 

recharge and discharge was used in this study because 

estimates of the value of the net recharge and(or) discharge and its components are not available over most 

of the study area. Most of the recharge and discharge 

occurs where the ground-water system interacts with 

sources or sinks of water at or near the land surface. The 

flow into or out of the source and(or) sink layer was 

simulated by the model using the difference in head 

between the first active aquifer block in the appropriate 

outcrop layer and the constant-head value above it multiplied by the conductance of vertical flow between the 

two blocks. This conductance is based on the harmonic 

mean conductance of the top one-half of the aquifer 

block and the bottom one-half of the constant-head 

block. For simplicity, the vertical conductance for the 

constant-head layer is uniform all over the study area 

(Kv = 3 x 1CT5 ft/d and thickness = 1 ft) and simulates 

resistance to water entering or leaving the aquifer such 

as streambeds, the soil zone, lake bottoms, and so forth. 

The constant-head layer accounts for ground-water and 

surface-water interaction from all surface sources and 

sinks such as rainfall that has infiltrated into the soil, 

seepage into or out of lakes and streams, and evapotranspiration from ground water.

The method works relatively well for estimating net 

recharge and discharge for steady-state conditions, 

especially because there are no alternative methods that 

will work at the scale of the model. Other methods frequently estimate net recharge and discharge values 

with the wrong signs that is, recharge where there 

should be discharge, or the reverse. This is because, in a 

relatively humid climate with large streams crossing the 

aquifer system, the magnitude of all surface volumes of 

water is so large compared to the regional groundwater component that the regional ground-water component is usually of the same order of magnitude or 

much smaller than the error component of surfacewater measurements. Under transient conditions, the 

source-sink layer method used can overestimate the 

volume of regional recharge unless it is limited to a 

maximum reasonable amount. Under the range of 

observed stressed conditions, the simulation required 

amounts of regional recharge that were possible, given 

the rainfall and location of perennial streams.

An area of research that has not been pursued in the 

Gulf Coastal Plain is the subject of water-table change 

over time. The method used for simulating regional 

recharge and discharge in this report has a limitation 

because the constant-head source-sink layer tends to 

restrict head changes in the top aquifer layer and 

because some water is simulated as increased recharge
GROUND-WATER FLOW IN THE GULF COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES

F123

or decreasing discharge, which really is withdrawal 

from water-table storage. Therefore, those two waterbudget components cannot be calculated separately. 

Few investigations in the study area have addressed the 

changes in water levels in shallow wells, which would 

indicate how much change there has been in water-table 

storage. A much better understanding of the way the 

ground-water system responds to stress would be possible if locations with a declining or rising water table 

were identified. In the Houston area, there appear to be 

very large areal differences in the depth to the water 

table as of 1991 (Dana Barbie, U.S. Geological Survey, 

written commun., 1994), probably resulting from 

increased dow 

cal resistance.

BOTTOM NO-FLOW BOUNDARY

The effects and likelihood of the bottom boundary 

acting as a no-flow boundary or as an upward leakage 

boundary from the abnormally high geopressured zone , below the aquifer systems in the downdip areas were 

tested by Kuiper (1994). He found that, based on known 

data and the simulation results, the upward flow could

be as high as 108 ft3 /d, or about 7 percent of the total 

regional recharge in 1982 (Kuiper, 1994, table 5) without 

materially affecting the fit for the rest of the aquifer system, mostly as observed in the shallow part. A flow of 

this magnitude is possible with an upward verticalhead gradient of several thousand feet of head and 

could probably be maintained for about 100 million 

years, assuming a large value of specific storage in the 

geopressured zone (Kuiper, 1994). Flow from the 

geopressured zone could be ignored without materially 

affecting the model fit of the shallow head data, and the 

overall model fit was not very sensitive to two different 

types of model truncation (Kuiper, 1994). The truncations were to test the effects of the brine part of the system on the freshwater part of the system without 

modeling the density effects. One truncation was at the 

shallowest depth where water density was 1.005 g/cm3. 

The other truncation was at a fixed depth of 3,000 ft. 

Although the overall model fit was not very different 

when treating the saline/freshwater interface as a noflow boundary, the flows near the boundary were substantially different. Obviously, treating the saline/freshwater interface as a no-flow boundary would grossly 

underestimate the potential for saltwater intrusion into 

aquifers that previously held freshwater.
F124 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

EXPLANATION

-300 LINE OF EQUAL SIMULATED FRESHWATER 

HYDRAULIC HEAD Interval, in feet, is 

variable. Datum is sea level

- DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 10,000 

milligrams per liter

- - - DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 35,000 

milligrams per liter

96°

Model bounda

Study area bounda

Downdip limit

< 'SA

u ^/ i

""V '[ \ I V. I

2ff> f^.\ i A-26- I

98°

50 100 150 MILES

Base from U.S. Geological Survey 

Scale 1:2.500,000 0 50 100 150 KILOMETERS

Plate 2A. Simulated predevelopment freshwater hydraulic head and depth-averaged dissolved-solids concentrations: Layer 11, 

permeable zone A, and Mississippi River Valley alluvial aquifer.
GROUND-WATER FLOW IN THE GULF-COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F125

EXPLANATION

-300 LINE OF EQUAL SIMULATED FRESHWATER 

HYDRAULIC HEAD Interval, in feet, is 

variable. Datum is sea level

----- DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 10,000 

milligrams per liter

- - - DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 35.000 

milligrams per liter

 36°

96°

 32°

Model bounda

Study area boundar

31°

98°

150 MILES

Base from U.S. Geological Survey 

Scale 1:2,500,000 0 50 100 150 KILOMETERS

Plate 2B. Simulated predevelopment freshwater hydraulic head and depth-averaged dissolved-solids concentrations: Layer 10, 

permeable zone B.
F126 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

EXPLANATION

- 300 LINE OF EQUAL SIMULATED FRESHWATER 

HYDRAULIC HEAD Interval, in feet, is 

variable. Datum is sea level

----- DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 10,000 

milligrams per liter

- - - DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 35 000 

milligrams per liter

 36°

Model bounda

 32°

Study area bounda

31°

Downdip limit

50 100 150 MILES

Base from U.S. Geological Survey 

Scale 1:2.500,000

0

I >-. ' 

0 50 100 150 KILOMETERS

Plate 2C. Simulated predevelopment freshwater hydraulic head and depth-averaged dissolved-solids concentrations: Layer 9, 

permeable zone C.
GROUND-WATER FLOW IN THE GULF-COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F127

EXPLANATION

-300 LINE OF EQUAL SIMULATED FRESHWATER 

HYDRAULIC HEAD Interval, in feet, is 

variable. Datum is sea level

- - DEPTH-AVERAGED DISSOLVED-SOLIDS 

CpNCENTRATION Equal to 10,000 

milligrams per liter

- - - DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 35 000 

milligrams per liter

 36°

Model bounda

 . 32°

Study area bounda

99

Base from U.S. Geological Survey 

Scale 1:2,500,000

50 100 150 MILES

50 100 150 KILOMETERS

Plate 2D. Simulated predevelopment freshwater hydraulic head and depth-averaged dissolved-solids concentrations: Layer 8, 

permeable zone D.
F128 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

EXPLANATION

-300 LINE OF EQUAL SIMULATED FRESHWATER 

HYDRAULIC HEAD Interval, in feet, is 

variable. Datum is sea level

----- DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 10.000 

milligrams per liter

- - - DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 35.000 

milligrams per liter

 36°

29

33°

Model bounda

95° GULF OF MEXICO

Study area bounda

31 c

Downdip limit

Base from U.S. Geological Survey 

Scale 1:2,500,000

0 50 100 150 MILES I ' H i 

0 50 100 150 KILOMETERS

Plate 2E. Simulated predevelopment freshwater hydraulic head and depth-averaged dissolved-solids concentrations: Layer 7, 

permeable zone E.
GROUND-WATER FLOW IN THE GULF-COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F129

EXPLANATION

-300 LINE OF EQUAL SIMULATED FRESHWATER 

HYDRAULIC HEAD Interval, in feet, is 

variable. Datum is sea level

----- DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 10,000 

milligrams per liter

- - - DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 35.000 

milligrams per liter

96

95° GULF OF MEXICO

Model bounda

Study area boundar

 . 32°

inn° »\ v »' , 1CO ^. n ;^-/ v

 V ^/ °^V/ Edge of Continental Shelf

 27°

Lr^

99°

26C 26°

98°

100 150 MILES

Base from U.S. Geological Survey 

Scale 1:2,500,000 0 50 100 150 KILOMETERS

Plate 2F. Simulated predevelopment freshwater hydraulic head and depth-averaged dissolved-solids concentrations: Layer 6, 

upper Claiborne aquifer.
F130 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

EXPLANATION

-300- LINE OF EQUAL SIMULATED FRESHWATER 

HYDRAULIC HEAD Interval, in feet, is 

variable. Datum is sea level

----- DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 10,000 

milligrams per liter

- - - DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 35 000 

milligrams per liter

, 37°

Model bounds

 32°

/ \ i xitx/v^ y J \,- s LOUISIANA x / 

Downdip limitv

Study area bounda

29

50 100 150 MILES

Base from U.S. Geological Survey 

Scale 1:2,500,000 0 50 100 150 KILOMETERS

Plate 2G. Simulated predevelopment freshwater hydraulic head and depth-averaged dissolved-solids concentrations: Layer 5, 

middle Claiborne aquifer.
GROUND-WATER FLOW IN THE GULF-COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F131

EXPLANATION

-sac LINE OF EQUAL SIMULATED FRESHWATER 

HYDRAULIC HEAD Interval, in feet, is 

variable. Datum is sea level

- - DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 10,000 

milligrams per liter

- - - DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 35 000 

milligrams per liter

Model bounda

.-32°

Study area bounda

Base from U.S. Geological Survey 

Scale 1:2,500,000

150 MILES

50 100 150 KILOMETERS

Plate 2H. Simulated predevelopment freshwater hydraulic head and depth-averaged dissolved-solids concentrations: Layer 4, 

lower Claiborne-upper Wilcox aquifer.
F132 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

EXPLANATION

-3oo LINE OF EQUAL SIMULATED FRESHWATER 

HYDRAULIC HEAD Interval, in feet, is 

variable. Datum is sea level

----- DEPTH-AVERAGED DISSOLVED-SOLDS 

CONCENTRATION Equal to 10,000 

milligrams per liter

- - - DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 35,000 

milligrams per liter

Model bounda

29°

50 100 150 MILES

Base from U.S. Geological Survey 

Scale 1:2,500.000 0 50 100 150 KILOMETERS

Plate 21. Simulated predevelopment freshwater hydraulic head and depth-averaged dissolved-solids concentrations: Layer 3, 

middle Wilcox aquifer.
GROUND-WATER FLOW IN THE GULF-COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F133

EXPLANATION

-300 LINE OF EQUAL SIMULATED FRESHWATER 

HYDRAULIC HEAD Interval, in feet, is 

variable. Datum is sea level

- - DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 10,000 

milligrams per liter

- - - DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 35,000 

milligrams per liter

. 37°

Model bounda

32°

99

Base from U.S. Geological Survey 

Scale 1:2,500,000 0 50 100 150 KILOMETERS

Plate 2J. Simulated predevelopment freshwater hydraulic head and depth-averaged dissolved-solids concentrations: Layer 2, 

lower Wilcox aquifer.
F134 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

EXPLANATION

HORIZONTAL FLOW, IN MILLIONS OF GALLONS

PER DAY Size of arrowhead indicates range of flow. 

Length of shaft indicates flow within stated range

0.00- 0.10 - 

0.10- 1.00 i» H>-*> 

1.00- 9.99 

9.99 - 99.90

DEPTH-AVERAGED DISSOLVED SOLIDS, IN 

MILLIGRAMS PER LITER

 10,000

 - 35,000

VERTICAL FLOW ACROSS BOTTOM OF LAYER, IN 

MILLIONS OF GALLONS PER DAY Size of

symbol is proportional to natural logarithm 

of flow. Symbol indicates flow direction 96° 

r 0.1 - 10 Downward

0 O 01-10 Upward

Model bounda

Study area bounda

Base from U.S. Geological Survey 

Scale 1:2,500,000

0 50 100 150 MILES 

I ' I 

0 50 100 150 KILOMETERS

Plate 3A. Simulated predevelopment horizontal and vertical ground-water flow direction and magnitude, and depth-averaged 

dissolved-solids concentration: Layer 11, permeable zone A, and Mississippi River Valley alluvial aquifer.
GROUND-WATER FLOW IN THE GULF-COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F135

EXPLANATION

HORIZONTAL FLOW, IN MILLIONS OF GALLONS

PER DAY Size of arrowhead indicates range of flow 

Length of shaft indicates flow within stated range

. 37°

E>-0

0.00- 0.10 

0.10- 1.00 

1.00- 9.99 

9.99-99.90

DEPTH-AVERAGED DISSOLVED SOLIDS, IN 

MILLIGRAMS PER LITER

---- 10000

-- 35,000

VERTICAL FLOW ACROSS BOTTOM OF LAYER, IN 

MILLIONS OF GALLONS PER DAY Size of

symbol is proportional to natural logarithm 

of flow. Symbol indicates flow direction

- ^ 0.1-10 Downward 

0 O 0.1-10 Upward

Model bounda

96'

 32°

Study area bounda

 31°

99°

Base from U.S. Geological Survey 

Scale 1:2,500,000

50 100 150 MILES

50 100 150 KILOMETERS

Plate 3B. Simulated predevelopment horizontal and vertical ground-water flow direction and magnitude, and depth-averaged 

dissolved-solids concentration: Layer 10, permeable zone B.
F136 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

EXPLANATION

HORIZONTAL FLOW, IN MILLIONS OF GALLONS

PER DAY Size of arrowhead indicates range of flow 

Length of shaft indicates flow within stated range

0.00- 0.10 - 

0.10- 1.00 t> -t>-*> 

1.00- 9.99 

9.99-99.90

 36°

DEPTH-AVERAGED DISSOLVED SOLIDS, IN 

MILLIGRAMS PER LITER

---- 10,000

--- 35,000

VERTICAL FLOW ACROSS BOTTOM OF LAYER, IN 

MILLIONS OF GALLONS PER DAY Size of 

symbol is proportional to natural logarithm 

of flow. Symbol indicates flow direction

n O 0.1-10 Downward 

0 O 0.1-10 Upward

Model bounda

MISSOURI JKENTUCRY '

^ ^W^x^'X.v.v.v '<'///,','' 'V.v '^^?^^^^v\v ' f «'*'''>>>"**l l *''':-- -^^

Edge of Continental Shelf 

Downdip limit

Study area bounda

31°

Downdip limit

Downdip limit

50 100 150 MILES

Base from U.S. Geological Survey 

Scale 1:2,500,000 50 100 150 KILOMETERS

Plate 3C. Simulated predevelopment horizontal and vertical ground-water flow direction and magnitude, and depth-averaged 

dissolved-solids concentration: Layer 9, permeable zone C.
GROUND-WATER FLOW IN THE GULF-COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F137

EXPLANATION

HORIZONTAL FLOW, IN MILLIONS OF GALLONS

PER DAY Size of arrowhead indicates range of flow 

Length of shaft indicates flow within stated range

37°

E>-0

0.00- 0.10 

0.10- 1.00 

1.00- 9.99 

9.99-99.90

DEPTH-AVERAGED DISSOLVED SOLIDS, IN 

MILLIGRAMS PER LITER

 10,000

 35,000

VERTICAL FLOW ACROSS BOTTOM OF LAYER, IN 

MILLIONS OF GALLONS PER DAY Size of

symbol is proportional to natural logarithm 

of flow. Symbol indicates flow direction

0.1-10 Downward 

0.1-10 Upward

96°

33'

Model bounda . 32°

u<^-^-

MISSOURI ^KENTUCKY

TENNESSEE:" i 36

GULF OF MEXICO

Downdip limit

Study area bounda

27° 

99°

Base from U.S. Geological Survey 

Scale 1:2,500.000

50 100 150 MILES

50 100 150 KILOMETERS

Plate 3D. Simulated predevelopment horizontal and vertical ground-water flow direction and magnitude, and depth-averaged 

dissolved-solids concentration: Layer 8, permeable zone D.
F138 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

EXPLANATION

HORIZONTAL FLOW, IN MILLIONS OF GALLONS 

PER DAY Size of arrowhead indicates range of flow. 

Length of shaft indicates flow within stated range

89°

ILLINOISx I 90°

E>-t>

0.00- 0.10 

0.10- 1.00 

1.00- 9.99 

9.99 - 99.90

DEPTH-AVERAGED DISSOLVED SOLIDS, IN 

MILLIGRAMS PER LITER

---- 10,000

-- 35,000

VERTICAL FLOW ACROSS BOTTOM OF LAYER, IN 

MILLIONS OF GALLONS PER DAY Size of 

symbol is proportional to natural logarithm 

of flow. Symbol indicates flow direction

'- O 0.1-10 Downward 

0 O 0.1-10 Upward

Model bounda

36°

96'

 32°

Study area bounda

Y^:-S>

riX-Ak » *y. 99°

26

Base from U.S. Geological Survey 

Scale 1:2,500,000

50 100 150 MILES

50 100 150 KILOMETERS

Plate 3E. Simulated predevelopment horizontal and vertical ground-water flow direction and magnitude, and depth-averaged 

dissolved-solids concentration: Layer 7, permeable zone E.
GROUND-WATER FLOW IN THE GULF-COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F139

EXPLANATION

HORIZONTAL FLOW, IN MILLIONS OF GALLONS

PER DAY Size of arrowhead indicates range of flow 

Length of shaft indicates flow within stated range

0.00- 0.10 - - 

0.10- 1.00 o-o-o 

1.00- 9.99 

9.99-99.90

. 37°

 36°

DEPTH-AVERAGED DISSOLVED SOLIDS, IN 

MILLIGRAMS PER LITER

 - 10000

-- 35,000

VERTICAL FLOW ACROSS BOTTOM OF LAYER, IN 

MILLIONS OF GALLONS PER DAY Size of 

symbol is proportional to natural logarithm 

of flow. Symbol indicates flow direction

0.1-10 Downward 

0 o o.l - 10 Upward

Model bounda

 . 32°

Study area boundar

31°

99

Base from U.S. Geological Survey 

Scale 1:2,500,000

50 150 MILES

50 100 150 KILOMETERS

Plate 3F. Simulated predevelopment horizontal and vertical ground-water flow direction and magnitude, and depth-averaged 

dissolved-solids concentration: Layer 6, upper Claiborne aquifer.
F140 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

EXPLANATION

HORIZONTAL FLOW, IN MILLIONS OF GALLONS

PER DAY Size of arrowhead indicates range of flow 

Length of shaft indicates flow within stated range

0.00- 0.10 ~ - o.io- i.oo * *-"-

1.00- 9.99 

9.99 - 99.90

DEPTH-AVERAGED DISSOLVED SOLIDS, IN 

MILLIGRAMS PER LITER

---- 10,000

-- 35,000

VERTICAL FLOW ACROSS BOTTOM OF LAYER, IN 

MILLIONS OF GALLONS PER DAY Size of 

symbol is proportional to natural logarithm 

of flow. Symbol indicates flow direction

" 0.1-10 Downward 

0 O 0.1-10 Upward

Model bounds

 32°

Study area boundar

31°

29

50 100 150 MILES

Base from U.S. Geological Survey 

Scale 1:2,500,000 50 100 150 KILOMETERS

Plate 3G. Simulated predevelopment horizontal and vertical ground-water flow direction and magnitude, and depth-averaged 

dissolved-solids concentration: Layer 5, middle Claiborne aquifer.
GROUND-WATER FLOW IN THE GULF-COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F141

EXPLANATION

HORIZONTAL FLOW, IN MILLIONS OF GALLONS

PER DAY Size of arrowhead indicates range of flow. 

Length of shaft indicates flow within stated range

0.00- 0.10 - - 

0.10- 1.00 > H>-> 

1.00- 9.99 E> E>H> 

9.99 - 99.90

. 37°

DEPTH-AVERAGED DISSOLVED SOLIDS, IN 

MILLIGRAMS PER LITER

 10,000

 35,000

VERTICAL FLOW ACROSS BOTTOM OF LAYER, IN 

MILLIONS OF GALLONS PER DAY Size of 

symbol is proportional to natural logarithm 

of flow. Symbol indicates flow direction

0.1-10 Downward 

0 O 0.1-10 Upward

96

Model bounds

Study area bounda

. 32°

V ~*.-» -» ~a. ce 

28° -C^~*> -» -oo e 

]^ ~ "^ ioo<r~x

27° 

^iSi, -o. c-0

3 xC HS. &0 _,

xa"^ fo -ff 

S^Tt£> t>0 --5^^, 

-C> f^o D*-»r^^ 

S. CC -a,-^/^

*> o-^ 

^>C^X

-^/ :/

CO

^

fv 

r*iS ^rr 28°

^""^ 97°

>?srJV ^7

,.- 27°

Edge of Continental Shelf

T99°

26C

98°

Base from U.S. Geological Survey 

Scale 1-2.500,000

1Q 2&

50

50

100 150 MILES

100 150 KILOMETERS

Plate 3H. Simulated predevelopment horizontal and vertical ground-water flow direction and magnitude, and depth-averaged 

dissolved-solids concentration: Layer 4, lower Claiborne-upper Wilcox aquifer.
F142 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

EXPLANATION

HORIZONTAL FLOW, IN MILLIONS OF GALLONS 

PER DAY Size of arrowhead indicates range of flow. 

Length of shaft indicates flow within stated range

0.00- 0.10 - - 

0.10- 1.00 E> H>-O 

1.00- 9.99 E> £>J> 

9.99 - 99.90

DEPTH-AVERAGED DISSOLVED SOLIDS, IN 

MILLIGRAMS PER LITER

 10,000

 - 35,000

VERTICAL FLOW ACROSS BOTTOM OF LAYER, IN 

MILLIONS OF GALLONS PER DAY Size of

symbol is proportional to natural logarithm 

of flow. Symbol indicates flow direction

: 0.1-10 Downward 

0 O o.l - 10 Upward

Model bounda 32°

Study area bounda

150 MILES

99°

Base from U.S. Geological Survey 

Scale 1:2,500,000

50 100 150 KILOMETERS

Plate 31. Simulated predevelopment horizontal and vertical ground-water flow direction and magnitude, and depth-averaged 

dissolved-solids concentration: Layer 3, middle Wilcox aquifer.
GROUND-WATER FLOW IN THE GULF-COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F143

EXPLANATION

HORIZONTAL FLOW, IN MILLIONS OF GALLONS

PER DAY Size of arrowhead indicates range of flow. 

Length of shaft indicates flow within stated range

ILLINOIS, 90°

37°- -37°

E>-t>

0.00- 0.10 

0.10- 1.00 

1.00- 9.99 

9.99-99.90

DEPTH-AVERAGED DISSOLVED SOLIDS, IN 

MILLIGRAMS PER LITER

 10,000

 35,000

VERTICAL FLOW ACROSS BOTTOM OF LAYER IN 

MILLIONS OF GALLONS PER DAY Size of 

symbol is proportional to natural logarithm 

of flow. Symbol indicates flow direction

-36°

96'

0.1-10 Downward 

0.1-10 Upward

Model boundary 

32

-32°

>/ \ « *> _^:\ '* » * ^t*£->3%3I32 

yl' 'Vf^H^Cfe^S ^>,'. r>^vT^^^^% _ \ - -%y*^^^ <AV._-_^

Study area bounda

Base from U.S. Geological Survey 

Scale 1:2,500,000

50 100 150 MILES

50 100 150 KILOMETERS

Plate 3J. Simulated predevelopment horizontal and vertical ground-water flow direction and magnitude, and depth-averaged 

dissolved-solids concentration: Layer 2, lower Wilcox aquifer.
F144 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

EXPLANATION

-300 LINE OF EQUAL SIMULATED FRESHWATER 

HYDRAULIC HEAD Interval, in feet, is 

variable. Datum is sea level

- - DEPTH-AVERAGED DISSOLVED-SOLIDS 

CpNCENTRATION Equal to 10,000 

milligrams per liter

- - - DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 35,000 

milligrams per liter

Model bounda

 32°

Study area bounda

31°

50 100 150 MILES

Base from U.S. Geological Survey 

Scale 1:2,500,000 0 50 100 150 KILOMETERS

Plate 4A. Simulated drawdown from predevelopment to 1987 and depth-averaged dissolved-solids concentrations: Layer 11, 

permeable zone A, and Mississippi River Valley alluvial aquifer.
GROUND-WATER FLOW IN THE GULF-COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F145

EXPLANATION

-300 LINE OF EQUAL SIMULATED FRESHWATER 

HYDRAULIC HEAD Interval, in feet, is 

variable. Datum is sea level

----- DEPTH-AVERAGED DISSOLVED-SOLJDS 

CONCENTRATION Equal to 10,000 

milligrams per liter

- - - DEPTH-AVERAGED DISSOLVED-SOLJDS 

CONCENTRATION Equal to 35 000 

milligrams per liter

Model bounda

 f. 32°

Study area bounda

31°

50 100 150 MILES

Base from U.S. Geological Survey 

Scale 1:2.500,000 0 50 100 150 KILOMETERS

Plate 4B. Simulated drawdown from predevelopment to 1987 and depth-averaged dissolved solids concentrations: Layer 10, 

permeable zone B.
F146 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

EXPLANATION

-300 LINE OF EQUAL SIMULATED FRESHWATER 

HYDRAULIC HEAD Interval, in feet, is 

variable. Datum is sea level

- DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 10,000 

milligrams per liter

- - - DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 35,000 

milligrams per liter

 36°

Model bounds

 32°

Study area bounda

31°

Downdip 

limit

50 100 150 MILES

Base from U.S. Geological Survey 

Scale 1:2,500,000 0 50 100 150 KILOMETERS

Plate 4C. Simulated drawdown from predevelopment to 1987 and depth-averaged dissolved-solids concentrations: Layer 9, 

permeable zone C.
GROUND-WATER FLOW IN THE GULF-COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F147

EXPLANATION

-100 LINE OF EQUAL SIMULATED FRESHWATER 

HYDRAULIC HEAD Interval, in feet, is 

variable. Datum is sea level

- DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 10,000 

milligrams per liter

- - - DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 35,000 

milligrams per liter

89°

ILLINOIS- I 90° '

37°

 36°

96

Model bounda

. 32°

Study area bounda

31°

Base from U.S. Geological Survey 

Scale 1:2,500,000

50 100 150 MILES

50 100 150 KILOMETERS

Plate 4D. Simulated drawdown from predevelopment to 1987 and depth-averaged dissolved-solids concentrations: Layer 8, 

permeable zone D.
F148 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

EXPLANATION

- -300 LINE OF EQUAL SIMULATED FRESHWATER 

HYDRAULIC HEAD Interval, in feet, is 

variable. Datum is sea level

----- DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 10,000 

milligrams per liter

- - - DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 35,000 

milligrams per liter

 36°

96

Model bounda

 32°

Study area bounda

, 31°

50 150 MILES

Base from U.S. Geological Survey 

Scale 1:2.500,000 50 100 150 KILOMETERS

Plate 4E. Simulated drawdown from predevelopment to 1987 and depth-averaged dissolved-solids concentrations: Layer 7, 

permeable zone E.
GROUND-WATER FLOW IN THE GULF-COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F149

EXPLANATION

-300 LINE OF EQUAL SIMULATED FRESHWATER 

HYDRAULIC HEAD Interval, in feet, is 

variable. Datum is sea level

- DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 10,000 

milligrams per liter

- - - DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 35 000 

milligrams per liter

< 37°

 36°

Model bounda

 32°

Study area bounda

Base from U.S. Geological Survey 

Scale 1:2,500,000

0 50 100 150 MILES 

I ' 

0 50 100 150 KILOMETERS

Plate 4F. Simulated drawdown from predevelopment to 1987 and depth-averaged dissolved-solids concentrations: Layer 6, upper 

Claiborne aquifer.
F150 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

EXPLANATION

-300 LINE OF EQUAL SIMULATED FRESHWATER 

HYDRAULIC HEAD Interval, in feet, is 

variable. Datum is sea level

- - DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 10,000 

milligrams per liter

- - - DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 35,000 

milligrams per liter

37°

 36°

Model bounda

 32°

950 GULF OF MEXICO

Study area bounda

99°

Base from U.S. Geological Survey 

Scale 1:2,500,000

50 100 150 MILES

0 50 100 150 KILOMETERS

Plate 4G. Simulated drawdown from predevelopment to 1987 and depth-averaged dissolved-solids concentrations: Layer 5, 

middle Claiborne aquifer.
GROUND-WATER FLOW IN THE GULF-COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F151

EXPLANATION

-300 LINE OF EQUAL SIMULATED FRESHWATER 

HYDRAULIC HEAD Interval, in feet, is 

variable. Datum is sea level

ILLINOIS\ 90°

- DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 10,000 

milligrams per liter

- - - DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 35.000 

milligrams per liter

96°

Model bounda

 . 32°

950 GULF OF MEXICO

Study area bounda

Base from U.S. Geological Survey 

Scale 1:2,500,000 0 50 100 150 KILOMETERS

Plate 4H. Simulated drawdown from predevelopment to 1987 and depth-averaged dissolved-solids concentrations: Layer 4, lower 

Claiborne-upper Wilcox aquifer.
F152 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

EXPLANATION

-SOD LINE OF EQUAL SIMULATED FRESHWATER 

HYDRAULIC HEAD Interval, in feet, is 

variable. Datum is sea level

ILLINOISx 90°

v 37°

----- DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 10,000 

milligrams per liter

- - - DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 35,000 

milligrams per liter

 36°

96°

Model bounda

. 32°

Study area bounda

29:

50 100 150 MILES

Base from U.S. Geological Survey 

Scale 1:2,500,000 0 50 100 150 KILOMETERS

Plate 41. Simulated drawdown from predevelopment to 1987 and depth-averaged dissolved-solids concentrations: Layer 3, 

middle Wilcox aquifer.
GROUND-WATER FLOW IN THE GULF-COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F153

EXPLANATION

-see LINE OF EQUAL SIMULATED FRESHWATER 

HYDRAULIC HEAD Interval, in feet, is 

variable. Datum is sea level

- DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 10,000 

milligrams per liter

- - - DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 35,000 

milligrams per liter

89°

ILLINOIS- 1 90°

91°

 36°

96

Model boundary

 32°

Study area boundary

50 100 150 MILES

Base from U.S. Geological Survey 

Scale 1:2,500,000 50 100 150 KILOMETERS

Plate 4J. Simulated drawdown from predevelopment to 1987 and depth-averaged dissolved-solids concentrations: Layer 2, middle 

Wilcox aquifer.
F154 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

EXPLANATION

-300- LINE OF EQUAL SIMULATED FRESHWATER 

HYDRAULIC HEAD Interval, in feet, is 

variable. Datum is sea level

- - DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 10,000 

milligrams per liter

- - - DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 35 000 

milligrams per liter

ILUNOISX 90°

 36°

Model bounda

 32°

Study area bounda

99°

98° 50 100 150 MILES

Base from U.S. Geological Survey 

Scale 1:2,500,000 0 50 100 150 KILOMETERS

Plate 5A. Simulated freshwater hydraulic head, 1987, Layer 11: permeable zone A and Mississippi River Valley alluvial aquifer.
GROUND-WATER FLOW IN THE GULF-COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F155

EXPLANATION

-.wo LINE OF EQUAL SIMULATED FRESHWATER 

HYDRAULIC HEAD Interval, in feet, is 

variable. Datum is sea level

----- DEPTH-AVERAGED DISSOLVED-SOLJDS 

CONCENTRATION Equal to 10,000 

milligrams per liter

- - - DEPTH-AVERAGED DISSOLVED-SOLJDS 

CONCENTRATION Equal to 35,000 

milligrams per liter

 36°

Model bounda

'. 32°

Study area bounda

99°

Base from U.S. Geological Survey 

Scale 1:2,500,000

100 150 MILES

0 50 100 150 KILOMETERS

Plate 5B. Simulated freshwater hydraulic head, 1987: Layer 10, permeable zone B.
F156 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

EXPLANATION

-300 LINE OF EQUAL SIMULATED FRESHWATER 

HYDRAULIC HEAD Interval, in feet, is 

variable. Datum is sea level

- - DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 10,000 

milligrams per liter

- - - DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 35,000 

milligrams per liter

 36°

96°

Model bounda

 32°

Study area bounda

Downdip limit

99

Base from U.S. Geological Survey 

Scale 1:2,500,000

100 150 MILES

0 50 100 150 KILOMETERS

Plate 5C. Simulated freshwater hydraulic head: 1987, Layer 9, permeable zone C.
GROUND-WATER FLOW IN THE GULF-COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F157

EXPLANATION

-3oo LINE OF EQUAL SIMULATED FRESHWATER 

HYDRAULIC HEAD Interval, in feet, is 

variable. Datum is sea level

- DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 10,000 

milligrams per liter

- - - DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 35,000 

milligrams per liter

 36°

29:

Model bounda

Study area bounda

FLORIDA

100 150 MILES

Base from U.S. Geological Survey 

Scale 1:2,500,000 50 100 150 KILOMETERS

Plate 5D. Simulated freshwater hydraulic-head, 1987: Layer 8, permeable zone D.
F158 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

EXPLANATION

- -SOD LINE OF EQUAL SIMULATED FRESHWATER 

HYDRAULIC HEAD Interval, in feet, is 

variable. Datum is sea level

----- DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 10,000 

milligrams per liter

- - - DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 35,000 

milligrams per liter

96°

33

Model bounda

 32°

Study area bounda

50 100 150 MILES

Base from U.S. Geological Survey 

Scale 1:2,500,000 0 50 100 150 KILOMETERS

Plate 5E. Simulated freshwater hydraulic head, 1987: Layer 7, permeable zone E.
GROUND-WATER FLOW IN THE GULF-COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F159

EXPLANATION

-300- LINE OF EQUAL SIMULATED FRESHWATER 

HYDRAULIC HEAD Interval, in feet, is 

variable. Datum is sea level

- DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 10,000 

milligrams per liter

- - - DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 35,000 

milligrams per liter

96'

Model bounda

 32°

Study area bounda

99°

Base from U.S. Geological Survey 

Scale 1:2,500,000

50 100 150 MILES

0 50 100 150 KILOMETERS

Plate 5F. Simulated freshwater hydraulic head, 1987: Layer 6, upper Claiborne aquifer.
F160 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

EXPLANATION

-3oo LINE OF EQUAL SIMULATED FRESHWATER 

HYDRAULIC HEAD Interval, in feet, is 

variable. Datum is sea level

----- DEPTH-AVERAGED DISSOLVED-SOLIDS 

CpNCENTRATION Equal to 10,000 

milligrams per liter

- - - DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 35,000 

milligrams per liter

96°

Model bounda

 32°

95° GULF OF MEXICO

Study area bounda

29

50 100 150 MILES

Base from U.S. Geological Survey 

Scale 1:2,500.000 0 50 100 150 KILOMETERS

Plate 5G. Simulated freshwater hydraulic head, 1987: Layer 5, middle Claiborne aquifer.
GROUND-WATER FLOW IN THE GULF-COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F161

EXPLANATION

-300 LINE OF EQUAL SIMULATED FRESHWATER 

HYDRAULIC HEAD Interval, in feet, is 

variable. Datum is sea level

----- DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 10,000 

milligrams per liter

- - - DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 35,000 

milligrams per liter

96°

Model bounda

. 32°

FLORIDA 

 30° \ 88° 

rf Downdip limit

Study area boundar

99'

Base from U.S. Geological Survey 

Scale 1:2,500,000

50 100 150 MILES

0 50 100 150 KILOMETERS

Plate 5H. Simulated freshwater hydraulic head, 1987: Layer 4, lower Claiborne-upper Wilcox aquifer.
F162 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

EXPLANATION

-300 LINE OF EQUAL SIMULATED FRESHWATER 

HYDRAULIC HEAD Interval, in feet, is 

variable. Datum is sea level

- - DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 10,000 

milligrams per liter

- - - DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 35,000 

milligrams per liter

Model bounda

 32°

95° GULF OF MEXICO

Study area bounda

Base from U.S. Geological Survey 

Scale 1:2,500,000

50 100 150 MILES

50 100 150 KILOMETERS

Plate 51. Simulated freshwater hydraulic head, 1987: Layer 3, middle Wilcox aquifer.
GROUND-WATER FLOW IN THE GULF-COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F163

EXPLANATION

-300- LINE OF EQUAL SIMULATED FRESHWATER 

HYDRAULIC HEAD Interval, in feet, is 

variable. Datum is sea level

----- DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 10,000 

milligrams per liter

- - - DEPTH-AVERAGED DISSOLVED-SOLIDS 

CONCENTRATION Equal to 35,000 

milligrams per liter

37°

 36°

96°

Model bounda

: 32°

Study area bounda

99'

Base from U.S. Geological Survey 

Scale 1:2,500,000

50 100 150 MILES

50 100 150 KILOMETERS

Plate 5J. Simulated freshwater hydraulic head, 1987: Layer 2, lower Wilcox aquifer.
F164 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

EXPLANATION

HORIZONTAL FLOW, IN MILLIONS OF GALLONS

PER DAY Size of arrowhead indicates range of flow. 

Length of shaft indicates flow within stated range

E>-t> 36°

 32°

less than 0.10 

0.10- 1.00 

1.00- 10 

more than 10

DEPTH-AVERAGED DISSOLVED SOLIDS, IN 

MILLIGRAMS PER LITER

---- 10,000

--- 35,000

VERTICAL FLOW ACROSS BOTTOM OF LAYER, IN 

MILLIONS OF GALLONS PER DAY Size of 

symbol is proportional to natural logarithm 

of flow. Symbol indicates flow direction

« * 0.1-10 Downward

- ® 0.1-10 Upward

Model boundar

32° / 

Study area bounda

150 MILES

Base from U.S. Geological Survey 

Scale 1:2,500,000 50 100 150 KILOMETERS

Plate 6A. Simulated 1985 ground-water flow directions and magnitudes and depth-averaged dissolved-solids concentrations: 

Layer 11, permeable zone A, and Mississippi River Valley alluvial aquifer.
GROUND-WATER FLOW IN THE GULF-COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F165

EXPLANATION

HORIZONTAL FLOW, IN MILLIONS OF GALLONS 

PER DAY Size of arrowhead indicates range of flow. 

Length of shaft indicates flow within stated range

E> -O

less than 0.10 

0.10- 1.00 

1.00- 10 

more than 10

DEPTH-AVERAGED DISSOLVED SOLIDS, IN 

MILLIGRAMS PER LITER

---- 10,000

--- 35,000

VERTICAL FLOW ACROSS BOTTOM OF LAYER, IN 

MILLIONS OF GALLONS PER DAY Size of 

symbol is proportional to natural logarithm 

of flow. Symbol indicates flow direction 96°

<& 0.1-10 Downward 

« 0 1 - 10 Upward

Model boundary

33'

s^w- »/ i "«3? *-H*ik s- «A" f

Study area bounda

98

50 100 150 MILES

Base from U.S. Geological Survey 

Scale 1:2,500,000 0 50 100 150 KILOMETERS

Plate 6B. Simulated 1987 ground-water flow directions and magnitudes and depth-averaged dissolved-solids concentrations: 

Layer 10, permeable zone B.
F166 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

EXPLANATION

HORIZONTAL FLOW, IN MILLIONS OF GALLONS

PER DAY Size of arrowhead indicates range of flow. 

Length of shaft indicates flow within stated range

E>-O

less than 0.10 

0.10- 1.00 

1.00- 10 

more than 10

DEPTH-AVERAGED DISSOLVED SOLIDS, IN 

MILLIGRAMS PER LITER

---- 10,000

--- 35,000

VERTICAL FLOW ACROSS BOTTOM OF LAYER, IN 

MILLIONS OF GALLONS PER DAY Size of 

symbol is proportional to natural logarithm 

of flow. Symbol indicates flow direction

« * 0.1 - 10 Downward

- « 0.1-10 Upward

 36°

96'

Model bounda

32° /. 

Study area bounda

 32°

27° /&,/

~^-<8 ~ *"" -*"s f * 

« ;%. 

3»<V "~

Downdip limit

99 2B*3ps^jj£.5^ |

Base from U.S. Geological Survey 

Scale 1:2,500,000

50 150 MILES

50 100 150 KILOMETERS

Plate 6C. Simulated 1987 ground-water flow directions and magnitudes and depth-averaged dissolved-solids concentrations: 

Layer 9, permeable zone C.
GROUND-WATER FLOW IN THE GULF-COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F167

EXPLANATION

HORIZONTAL FLOW, IN MILLIONS OF GALLONS

PER DAY Size of arrowhead indicates range of flow. 

Length of shaft indicates flow within stated range

89°

E>-0

less than 0.10 

0.10- 1.00 

1.00- 10 

more than 10

DEPTH-AVERAGED DISSOLVED SOLIDS, IN 

MILLIGRAMS PER LITER

- 10,000

--- 35,000

VERTICAL FLOW ACROSS BOTTOM OF LAYER, IN 

MILLIONS OF GALLONS PER DAY Size of

symbol is proportional to natural logarithm 

of flow. Symbol indicates flow direction 96°

 $0.1-10 Downward

 » 0.1-10 Upward

33

Model bounds

;. 32°

I "*

GUZ.F OF MEXICO \~ '

Study area bounda

99°

Base from U.S. Geological Survey 

Scale 1:2,500,000

50 100 150 MILES

50 100 150 KILOMETERS

Plate 6D. Simulated 1987 ground-water flow directions and magnitudes and depth-averaged dissolved-solids concentrations: 

Layer 8, permeable zone D.
F168 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

EXPLANATION

HORIZONTAL FLOW, IN MILLIONS OF GALLONS

PER DAY Size of arrowhead indicates range of flow. 

Length of shaft indicates flow within stated range

£>-£>

less than 0.10

o.io- i.oo

1.00- 10 

more than 10

DEPTH-AVERAGED DISSOLVED SOLIDS, IN 

MILLIGRAMS PER LITER

 10,000

--- 35,000

VERTICAL FLOW ACROSS BOTTOM OF LAYER, IN 

MILLIONS OF GALLONS PER DAY Size of 

symbol is proportional to natural logarithm 

of flow. Symbol indicates flow direction ,

* 0.1-10 Downward 

» 0.1-10 Upward

33° i

If. 32°

^p^i^;^-'"^^-^^ ® ®- « »r__<-au \i JL** S " r » ' -* X * » 1 '» » ^-« °*\^^^*O*a -*KtLT" *

V\. <vs\< A?. XV9"- -^-^. > t? ' _ s~> _'.... .. /

Model bounda

32° , 

Study area bounda

99°

Base from U.S. Geological Survey 

Scale 1:2,500,000

50 100 150 MILES

50 100 150 KILOMETERS

Plate 6E. Simulated 1987 ground-water flow directions and magnitudes and depth-averaged dissolved-solids concentrations: 

Layer 7, permeable zone E.
GROUND-WATER FLOW IN THE GULF-COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F169

EXPLANATION

HORIZONTAL FLOW, IN MILLIONS OF GALLONS 

PER DAY Size of arrowhead indicates range of flow. 

Length of shaft indicates flow within stated range

, 37°

E>-O

less than 0.10 

0.10- 1.00 

1.00- 10 

more than 10

DEPTH-AVERAGED DISSOLVED SOLIDS, IN 

MILLIGRAMS PER LITER

---- 10,000

--- 35,000

VERTICAL FLOW ACROSS BOTTOM OF LAYER, IN 

MILLIONS OF GALLONS PER DAY Size of 

symbol is proportional to natural logarithm 

of flow. Symbol indicates flow direction _ ,

 $ 0.1 - 10 Downward

 « 0.1-10 Upward

Model bounda

 36°

 <*. 32°

Study area bounda

29

99

Base from U.S. Geological Survey 

Scale 1:2,500,000

100 150 MILES

50 100 150 KILOMETERS

Plate 6F. Simulated 1987 ground-water flow directions and magnitudes and depth-averaged dissolved-solids concentrations: 

Layer 6, upper Claiborne aquifer.
F170 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

EXPLANATION

HORIZONTAL FLOW, IN MILLIONS OF GALLONS

PER DAY Size of arrowhead indicates range of flow. 

Length of shaft indicates flow within stated range

E>-t>

 32°

less than 0.10 

0.10- 1.00 

1.00- 10 

more than 10

DEPTH-AVERAGED DISSOLVED SOLIDS, IN 

MILLIGRAMS PER LITER

 10,000

--- 35,000

VERTICAL FLOW ACROSS BOTTOM OF LAYER, IN 

MILLIONS OF GALLONS PER DAY Size of 

symbol is proportional to natural logarithm 

of flow. Symbol indicates flow direction

 « 0.1 - 10 Downward

 « 0.1-10 Upward

Model bounda

32° /. 

Study area bounda

50 100 150 MILES

Base from U.S. Geological Survey 

Scale 1:2,500.000 50 100 150 KILOMETERS

Plate 6G. Simulated 1987 ground-water flow directions and magnitudes and depth-averaged dissolved-solids concentrations: 

Layer 5, middle Claiborne aquifer.
GROUND-WATER FLOW IN THE GULF-COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F171

EXPLANATION

HORIZONTAL FLOW, IN MILLIONS OF GALLONS 

PER DAY Size of arrowhead indicates range of flow. 

Length of shaft indicates flow within stated range

less than 0.10 

0.10- 1.00 

1.00- 10 

more than 10

£> E>-O

E>E>E>

DEPTH-AVERAGED DISSOLVED SOLIDS, IN 

MILLIGRAMS PER LITER

 10,000

--- 35,000

VERTICAL FLOW ACROSS BOTTOM OF LAYER, IN 

MILLIONS OF GALLONS PER DAY Size of

symbol is proportional to natural logarithm 

of flow. Symbol indicates flow direction

 3> 0.1-10 Downward

 * 0.1-10 Upward

Model bounda

 32°

Study area bounda

29

Base from U.S. Geological Survey 

Scale 1:2,500,000

50 150 MILES

50 100 150 KILOMETERS

Plate 6H. Simulated 1987 ground-water flow directions and magnitudes and depth-averaged dissolved-solids concentrations: 

Layer 4, lower Claiborne-upper Wilcox aquifer.
F172 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

EXPLANATION

HORIZONTAL FLOW, IN MILLIONS OF GALLONS

PER DAY Size of arrowhead indicates range of flow. 

Length of shaft indicates flow within stated range

less than 0.10 - ^ 

0.10- 1.00 «> H>-B1.00- 10 

more than 10

DEPTH-AVERAGED DISSOLVED SOLIDS, IN 

MILLIGRAMS PER LITER

 10,000

--- 35,000

VERTICAL FLOW ACROSS BOTTOM OF LAYER, IN 

MILLIONS OF GALLONS PER DAY Size of

symbol is proportional to natural logarithm 

of flow. Symbol indicates flow direction

 $> 0.1-10 Downward

 « 0.1-10 Upward

Model bounda

96

26 32°

Study area bounda

5431°

50 100 150 MILES

Base from U.S. Geological Survey 

Scale 1:2.500,000 0 50 100 150 KILOMETERS

Plate 61. Simulated 1987 ground-water flow directions and magnitudes and depth-averaged dissolved-solids concentrations: 

Layer 3, middle Wilcox aquifer.
GROUND-WATER FLOW IN THE GULF-COAST AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES F173

EXPLANATION

HORIZONTAL FLOW, IN MILLIONS OF GALLONS

PER DAY Size of arrowhead indicates range of flow. 

Length of shaft indicates flow within stated range

E>-0

less than 0.10 

0.10- 1.00 

1.00- 10 

more than 10

DEPTH-AVERAGED DISSOLVED SOLIDS, IN 

MILLIGRAMS PER LITER

---- 10,000

--- 35,000

s 37°

 36°

96°

Model bounda

rrr M. \ *- * -~^J"~nr'/* I V n * *^<\ r& * \i rf\\*.fc.^*>C\'*'* K\'S\V^^? I X_\ _ i. _i»J£»»-w ^f W. >Vf s~ ' ft r^\ -«-*» . !^'7i*-G i \/i \ 'j-^\vV -. / , ' . I v . - -JvV *

..

- "" ' '' ' y.: : :^^

Study area bounda

Base from U.S. Geological Survey 

Scale 1:2,500,000

0 50 100 150 MILES

I , i ' ' 

0 50 100 150 KILOMETERS

Plate 6J. Simulated 1987 ground-water flow directions and magnitudes and depth-averaged dissolved-solids concentrations: 

Layer 2, lower Wilcox aquifer.

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