SUMMARY OF HYDROLOGY OF THE

REGIONAL AQUIFER SYSTEMS, 

I GULF COASTAL PLAIN, 

SOUTH-CENTRAL UNITED STATES

PROFESSIONAL PAPER I416-A

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Summary of Hydrology of the

Regional Aquifer Systems, Gulf Coastal Plain,

South-Central United States

By HAYES F. GRUBB

REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1416-A

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1998
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, 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

Library of Congress Cataloging-in-Publication Data

Grubb, Hayes F.

Summary of hydrology of the regional aquifer systems, Gulf coastal plain, south-central United States / 

by Hayes F. Grubb.

p. cm. (Regional aquifer-system analysis Gulf coastal plain) (U.S. Geological Survey 

professional paper ; 1416-A)

Includes bibliographical references.

1. Aquifers Gulf Coast Region (U.S.) I. Title. II. Series. III. Series: U.S. Geological Survey 

professional paper; 1416-A.

GB1199.3.G85G78 1997 97-38786 

551.49'0976 dc21 CIP

For sale by the U.S. Geological Survey Map Distribution

Box 25286, Mail Stop 306, Denver 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

AIII

CONTENTS

	Page

Foreword................................................................................................... AIII

Abstract..................................................................................................... Al

Introduction.............................................................................................. 1

Previous investigations.................................................................... 3

Regional analysis of the gulf coast aquifer systems .................... 5

Geography......................................................................................... 6

Population and economy................................................................. 6

Geology.............................................................................................. 10

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

Coastal lowlands aquifer system.................................................... 12

Texas coastal uplands aquifer system............................................ 18

Mississippi embayment aquifer system........................................ 20

Hydraulic conductivity.................................................................... 23

Ground-water chemistry........................................................................ 24

Dissolved-solids concentrations..................................................... 24

Water types........................................................................................ 25

Geochemical processes..................................................................... 25

History of ground-water development................................................ 28

Memphis............................................................................................ 29

Houston.............................................................................................. 30

New Orleans...................................................................................... 31

Baton Rouge....................................................................................... 32

Mississippi Alluvial Plain................................................................ 32

	Page

History of ground-water development Continued

Southwestern Louisiana.................................................................. A33

Winter Garden................................................................................... 35

Regional ground-water flow.................................................................. 35

Predevelopment................................................................................ 36

Recharge and discharge.......................................................... 36

Flow patterns............................................................................ 37

Permeable zone B.............................................................. 38

Middle Claiborne aquifer................................................. 43

Ground-water budget.............................................................. 43

Effects of water density........................................................... 44

Flow during the 1980's..................................................................... 45

Recharge and discharge .......................................................... 45

Flow patterns............................................................................ 46

Permeable zone B .............................................................. 46

Middle Claiborne aquifer................................................. 46

Ground-water budget.............................................................. 51

Decrease in saturated thickness............................................. 52

Land subsidence....................................................................... 53

Saltwater encroachment.......................................................... 54

Potential for ground-water development............................................. 54

References................................................................................................. 55

ILLUSTRATIONS

FIGURES 1-7. Maps showing:

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

Analysis study areas......................................................................................................................................................................... A2

2. Generalized average land-surface altitude, bathymetry, major rivers, and physiographic provinces.................................... 7

3. Mean annual precipitation, 1951-80.................................................................................................................................................... 8

4. Distribution of population in 1985 and population growth, 1800-1990........................................................................................ 9

5. Generalized geology and structural features, Gulf Coastal Plain, south-central United States................................................. 11

6. Generalized outcrop of aquifer systems, intervening regional confining units, and designated geographic areas............... 13

7. Generalized outcrop of regional geohydrologic units...................................................................................................................... 14

8. Graph showing areal extent of aquifers and permeable zones ...................................................................................................................... 16

9. Idealized section from boundary of study area in Zavala County, Texas, southeastward through Kleberg County, Texas,

to the Continental Slope, showing relation of geohydrologic units ......................................................................................................... 18

10. Map showing thickness of permeable zone B (lower Pleistocene-upper Pliocene deposits), coastal lowlands aquifer system.......... 19

11. Map showing thickness of the middle Claiborne aquifer, Texas coastal uplands and Mississippi embayment aquifer systems....... 21

12. Idealized section from near northern boundary of study area in Mississippi County, Missouri, to Iberville Parish, Louisiana,

showing relation of geohydrologic units...................................................................................................................................................... 22

13. Graph showing geometric mean horizontal hydraulic conductivity of aquifers and permeable zones by geographic area............... 24

14. Map showing dominant water types in permeable zone B (lower Pleistocene-upper Pliocene deposits), coastal lowlands

aquifer system...................................................................................... 26

15. Map showing dominant water types in the middle Claiborne aquifer, Texas coastal uplands and Mississippi embayment

aquifer systems...................................................................................................................................................................................................... 27

V
VI CONTENTS

16-24. Graphs showing:

16. Ground-water pumpage by major category of use, 1960-90.......................................................................................................... 28

17. Ground-water pumpage by aquifer system, 1960-90...................................................................................................................... 29

18. Water levels in the Memphis area, Tennessee, 1928-90.................................................................................................................. 29

19. Water levels in the Houston area, Texas, 1932-92........................................................................................................................... 30

20. Water levels in the New Orleans area, Louisiana, 1949-90............................................................................................................ 31

21. Water levels in the Baton Rouge area, Louisiana, 1940-90............................................................................................................. 32

22. Water levels in the Mississippi Alluvial Plain, 1929-87.................................................................................................................. 33

23. Water levels in southwestern Louisiana, 1938-90............................................................................................................................ 34

24. Water levels in the Winter Garden area, Texas, 1929-86................................................................................................................ 35

25-27. Maps showing predevelopment regional recharge and discharge areas, and thickness:

25. Coastal lowlands aquifer system........................................................................................................................................................ 36

26. Texas coastal uplands aquifer system................................................................................................................................................ 37

27. Mississippi embayment aquifer system ............................................................................................................................................ 38

28-31. Maps showing direction and relative magnitude of simulated predevelopment horizontal flow, area where 

vertical flow relative to base of permeable zone was downward, altitude and configuration of top, 

depth-averaged dissolved-solids concentration, and location of salt domes that penetrate base of:

28. Permeable zone A, coastal lowlands aquifer system, and the Mississippi River Valley alluvial aquifer................................ 39

29. Permeable zone B, coastal lowlands aquifer system ....................................................................................................................... 40

30. Permeable zone D, coastal lowlands aquifer system....................................................................................................................... 41

31. Middle Claiborne aquifer, Texas coastal uplands and Mississippi embayment aquifer systems............................................ 42

32. Diagram showing simulated predevelopment ground-water budget from the regional-scale model.................................................... 44

33-36. Maps showing regionalized horizontal hydraulic conductivity, areas where simulated vertical flow relative

to base of unit reversed direction from upward to downward (predevelopment to 1987), and 1987 simulated 

direction and relative magnitude of horizontal flow in permeable zones and aquifers:

33. Permeable zone A, coastal lowlands aquifer system, and the Mississippi River Valley alluvial aquifer................................ 47

34. Permeable zone B, coastal lowlands aquifer system ....................................................................................................................... 48

35. Permeable zone D, coastal lowlands aquifer system....................................................................................................................... 49

36. Middle Claiborne aquifer, Texas coastal uplands and Mississippi embayment aquifer systems ............................................ 50

37-38. Diagrams showing:

37. Simulated 1987 ground-water budget from the regional-scale model......................................................................................... 52

38. Simulated ground-water budget for the Mississippi River Valley alluvial aquifer, predevelopment through 1987............ 53

TABLES

TABLE 1. Geologic units and geohydrologic units of the gulf coast aquifer systems......................................................................................... 15

2. Summary of thickness and sand percentage of aquifers and permeable zones.................................................................................. 17
VII 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 25.4 millimeters

inch per year (inch/yr) 25.4 millimeters per year

foot (ft) 0.3048 meter

foot per day (ft/ d) 0.3048 meter per day

foot per year (ft/yr) 0.3048 meter per year

foot squared per day (ft2 / d) 0.0929 meter squared per day

mile (mi) 1.609 kilometer

square mile (mi2) 2.590 square kilometer

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

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

SUMMARY OF HYDROLOGY OF THE REGIONAL AQUIFER SYSTEMS, GULF 

COASTAL PLAIN, SOUTH-CENTRAL UNITED STATES

BY HAYES F. GRUBB

ABSTRACT

The gulf coast regional aquifer system consists of regional aquifers 

in sediments of mostly Cenozoic age in an area of about 230,000 square 

miles in the Gulf Coastal Plain of Alabama, Arkansas, Florida, Illinois, 

Kentucky, Louisiana, Mississippi, Missouri, Tennessee, and Texas and 

an additional 60,000 square miles offshore. Three aquifer systems (the 

Mississippi embayment, Texas coastal uplands, and coastal lowlands) 

have been divided into six regional aquifers and five permeable zones. 

The aquifers and permeable zones range in areal extent from about 

32,000 to 170,000 square miles and typically thicken from their outcrop 

area toward the Gulf of Mexico or toward the axes of major embayments. The average thickness of aquifers and permeable zones ranges 

from 140 to 2,010 feet. Horizontal hydraulic conductivity of aquifers 

and permeable zones ranges from about 20 to 315 feet per day.

Water in the gulf coast aquifer systems is generally fresh [less than 

1,000 milligrams per liter (mg/L) dissolved solids] in and near outcrop 

areas of the permeable zones and aquifers. Dissolved-solids concentrations in water from all aquifers and permeable zones generally increase 

toward the Gulf of Mexico and toward principal discharge areas; concentrations typically increase from 10,000 to 35,000 mg/L over distances of 10 to 40 miles. In the deeper parts of the permeable zones and 

aquifers, the dissolved-solids concentrations typically exceed that of 

seawater by as much as a factor of 2. The dominant water type in and 

near aquifer outcrop areas typically is calcium bicarbonate or a mixture 

of calcium bicarbonate and sodium bicarbonate. The water type is 

dominantly sodium bicarbonate in middip areas of the aquifers. The 

dominant water type typically is sodium chloride near the downdip 

limit of the aquifers and permeable zones. A variety of geochemical 

processes contribute to the water chemistry in the gulf coast aquifer 

systems. The major postulated processes are (1) leaching of soluble 

salts from the unsaturated zone; (2) alteration of albite; (3) cation 

exchange; (4) mixing of water by the vertical flow of water from underlying units; and (5) the dissolution of halite from salt diapirs.

Simulation of regional ground-water flow (excludes local flow of 

several miles or less) indicates that regional ground-water flow prior to 

withdrawal of ground water (pre development) generally was from 

areas of high water-level altitude toward major rivers or broad extensive areas at low land-surface altitude. Of a total 2,900 million gallons 

per day (Mgal/d) of simulated predevelopment recharge, the most 

to a single aquifer was to the Mississippi River Valley alluvial 

aquifer (830 Mgal / d) and the least was to the lower Wilcox aquifer 

(14 Mgal/d). The largest discharge was also to the Mississippi River 

Valley alluvial aquifer (about 1,200 Mgal/d) and the smallest was also 

to the lower Wilcox aquifer (12 Mgal/d). Net vertical flows ranged 

from less than 1 Mgal/d between the lower Wilcox aquifer and the 

middle Wilcox aquifer to about 300 Mgal/d between permeable zone B 

and permeable zone A.

More than 90 years of large ground-water withdrawals (about 

10,000 Mgal/d during 1980) had lowered water levels and greatly 

altered regional ground-water flow patterns by the 1980's. Many predevelopment regional discharge areas became regional recharge areas.

Regional recharge increased from about 2,900 Mgal/ d under simulated 

predevelopment conditions to about 9,600 Mgal/d during 1985, and 

discharge decreased from about 2,900 Mgal/d to about 1,100 Mgal/d 

during the same period. Horizontal flow directions have been altered; 

flow lines that extended from the shoreline toward the edge of the 

Continental Shelf prior to ground-water withdrawal currently (1985) 

converge toward pumping centers located in coastal areas of Texas and 

Louisiana. Likewise, vertical flow direction reversed at many locations 

such as at Houston, Texas, and in the vicinity of Memphis, Tennessee. 

Long-term decreases in saturated thickness have resulted from the 

withdrawal of ground water in the lower Claiborne-upper Wilcox 

aquifer in southern Texas and in the Mississippi River Valley alluvial 

aquifer in eastern and central Arkansas. Saturated thickness decreased 

at the rate of 1.5 feet per year in southern Texas during the period 

1965-85 and at the rate of about 2 feet per year at some locations in 

eastern and central Arkansas during the 1980's. Other local effects of 

ground-water withdrawal are land subsidence as much as 10 feet 

near Houston, Texas and saltwater encroachment in southwestern 

Louisiana, where chloride concentration increased at the rate of 5 to 15 

mg/L per year during the period 1963-83.

There is potential for further development of ground-water supplies in the Gulf Coastal Plain because of the abundance of water in 

streams, lakes, and swamps and the generally good hydraulic connection between the water table and underlying water-yielding units. The 

aquifer systems have the potential to support additional withdrawals 

of as much as 10,000 Mgal/d if pumping centers are carefully located. 

The most favorable conditions for further development of groundwater supplies are generally in the upper permeable zones and aquifers, and the potential for development typically increases from west to 

east.

INTRODUCTION

The Gulf Coast Regional Aquifer-System Analysis 

(Gulf Coast RASA), a study of ground water in Gulf 

Coastal Plain sediments in the south-central United 

States, is part of the Regional Aquifer-System Analysis 

Program of the U.S. Geological Survey (see Foreword). 

The program began in 1978 and was designed to provide a regional understanding and assessment of the 

Nation's ground-water resources (Bennett, 1979). A 

detailed discussion of the background leading to the 

initiation of the RASA Program is given by Sun (1986). 

Four other RASA studies have been conducted in areas 

adjacent to the Gulf Coast RASA study area (fig. 1).

Al
EXPLANATION

Regional Aquifer-System 

Analysis (RASA) study areas

|___| Central Midwest

| | Gulf Coast

|___I Southeastern Coastal Plain| | Floridan

I___I Edwards-Trinity

70

500 KILOMETERS

FIGURE 1. Location of Gulf Coast Regional Aquifer-System Analysis study area and adjacent Regional Aquifer-System Analysis study areas.
SUMMARY OF HYDROLOGY OF THE REGIONAL AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES A3

The RASA studies in areas adjacent to the Gulf Coast 

RASA study are the Edwards-Trinity, Central Midwest, 

Southeastern Coastal Plain, and Floridan (fig. 1). The 

Edwards-Trinity aquifer system lies to the northwest of 

the Gulf Coast RASA study area in Texas and adjacent 

parts of Oklahoma and Arkansas. The Edwards-Trinity 

aquifer system consists of indurated limestones and 

sandstones of Cretaceous age and is separated from 

aquifer systems that are the subject of this study by 

massive clays and limestones of minimal permeability. 

The Ozark aquifer of the Central Midwest RASA consists of limestones, dolostones, and sandstones of Late 

Cambrian through Devonian age. The Ozark aquifer is 

exposed at the land surface to the northwest of the Gulf 

Coastal Plain in northern Arkansas and southern Missouri and dips beneath Gulf Coastal Plain sediments in 

southeastern Missouri and northeastern Arkansas. The 

massive clays that separate the two aquifer systems 

become thin near the northwestern boundary of the 

Gulf Coastal Plain, and the potential exists for groundwater to flow upward from the Ozark aquifer to Gulf 

Coastal Plain sediments. During the course of this 

study, ground water was found to flow upward from 

the Ozark aquifer in substantial amounts only in a localized area near the northwestern boundary of the Gulf 

Coastal Plain. The Southeastern Coastal Plain aquifer 

system mostly consists of unconsolidated sands of Cretaceous and Tertiary age to the east of the Gulf Coastal 

Plain. The aquifers of Cretaceous age dip beneath the 

sediments that constitute the Gulf Coast RASA in 

Mississippi, southwestern Alabama, and Florida and 

are separated from the overlying aquifers by massive 

clays of minimal permeability; thus, there is little potential for ground water to flow between the two systems. 

The Floridan aquifer system, which mostly consists of 

permeable carbonate rocks of Tertiary age, lies largely 

to the east of the Gulf Coast RASA study area. However, in southwestern Alabama and western Florida, the 

rocks of the Floridan aquifer undergo a facies change 

from carbonate rock to marine clay, which forms a 

regional confining unit throughout most of the Gulf 

Coast RASA study area.

Large quantities of water are withdrawn from the 

streams and aquifers of the Gulf Coastal Plain. The 

quantity of ground water withdrawn during 1985 was 

about one-half the quantity of surface water withdrawn. Rates of ground-water withdrawal have 

increased unevenly throughout the years as the demand 

for public supply, industry, and irrigation (the three 

largest uses in the study area) generally has grown. 

Ground-water withdrawals have caused lowering of 

water levels in and near pumping centers, induced the 

movement of the fresh-saltwater interface into parts of

aquifers that previously contained freshwater, accelerated land subsidence due to the compaction of interbedded clays within the aquifers, and increased the lift for 

pumps as a result of the lower water levels. Much work 

has been done to study these effects and to evaluate the 

ground-water resources in an effort to provide a rational guide for the management and development of the 

ground-water resources of the Gulf Coastal Plain. Following is a short summary of earlier studies and those 

that were done as part of the Gulf Coast RASA.

PREVIOUS INVESTIGATIONS

The aquifers of the Gulf Coastal Plain of the southcentral United States have been the subject of study 

for the past 90 years. The importance of the groundwater resource was recognized early during this period, 

and R.T. Hill noted that the Coastal Plain of Texas, 

"***constitutes one of the most productive artesian 

regions of America, if not in the world ****" (Hill, 1901, 

p. 398). Throughout the years since 1901, many types of 

problems related to the ground-water resource have 

been studied and various types of reports have been 

published. By the middle of the second decade of the 

20th century, reports had been published describing the 

ground-water resources in Coastal Plain sediments for 

every State in the study area (Harris, 1904; Crider and 

Johnson, 1906; Glenn, 1906; Veatch, 1906; Shepard, 1907; 

Smith, 1907; Taylor, 1907; Matson and Sanford, 1913; 

and Stephenson and Crider, 1916). These reports are 

characterized by extensive tables of data on wells, surficial geologic maps, and general discussions of the 

occurrence of ground water. As the last of these studies 

was completed, attention was directed to more detailed 

studies of smaller areas and specific problems. In a 

report on La Salle and McMullen Counties in Texas 

(county and city locations shown in fig. 28), the first 

detailed county study within the Gulf Coast RASA 

study area, Deussen and Dole (T916) noted that close 

spacing of wells resulted in interference among them.

A more detailed evaluation of the ground-water 

resources was typical of many studies that began to be 

published in the 1930's for individual Texas counties 

(Lonsdale, 1935; Livingston and Bridges, 1936; Sayre, 

1937; and Lonsdale and Day, 1937). Thus began a process in which detailed studies of the ground-water 

resources of counties in Texas were released over a 

period of several decades, examples of which are 

reports by Follett (1947), Rasmussen (1947), Winslow 

(1950), Petitt (1955), Mason, (1960), Broom (1965), Shafer 

(1970), Follett (1974), and Rettman (1984). Detailed 

county studies of ground water in other States began
A4 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

later than those in Texas, but by the end of the 1970's, 

reports had been released for most counties in the study 

area. Examples of reports for Louisiana parishes are 

Maher (1941), U.S. Geological Survey (1951), Poole 

(1961), Marie (1971), Case (1979), and Tomaszewski 

(1988). Examples of reports for other States are 

Mississippi, Callahan and others (1963); Wasson and 

others (1965); Taylor and Thomson (1971); Dalsin and 

Bettandorff (1976); Spiers and Dalsin (1979); Arkansas, 

Engler and others (1945); Hewitt and others (1949); 

Counts (1957); Albin (1964); Plebuch and Hines (1967); 

Lamonds and others (1969); Ludwig (1972); Alabama, 

Toulmin and others (1951); Newton and others (1961); 

Reed and McCain (1971); Davis and others (1980); and 

Tennessee, Wilson and Criner (1969); Parks and others 

(1985). In Kentucky, detailed ground-water studies 

were released as a series of map reports of each 7.5- 

minute quadrangle, examples of which are MacCary 

(1964), Davis (1965), MacCary and Davis (1966), Hansen 

(1967), and Lambert (1968). As development of groundwater supplies increased, so did the interest in areas 

smaller than counties and in problems usually associated with large ground-water withdrawals.

A report on the water supply of Memphis, Tenn. 

(Wells, 1932), was one of the earliest attempts to relate 

cause and effect (that is, pumping rates to water-level 

declines in wells). The report included a profile of the 

observed pressure surface and location of pumping 

from the lower Wilcox aquifer at four different times. In 

a study of the Houston District, Texas, White and others 

(1944) presented drawdown curves at increasing distances from a pumping well, after 1, 2, and 10 years of 

pumping. These curves were based on the results of 

aquifer tests, assumed pumping rates, and the Theis 

(1935) nonequilibrium formula. Special problems 

related to ground-water withdrawals, such as saltwater 

encroachment in the Houston-Galveston area (Winslow, and others, 1957; Jorgensen, 1977) and in the 

Baton Rouge area (Rollo, 1969), effects of faults on 

ground-water flow (Whiteman, 1979; Gabrysch 1984), 

and land subsidence (Winslow and Doyel, 1954; Winslow and Wood, 1959; Gabrysch, 1969, 1977, and 1982; 

Gabrysch and Bonnet, 1975; Gabrysch and Coplin, 1990; 

Whiteman, 1980; Ratzlaff, 1982) were documented in 

many reports. The complexity of ground-water problems increased as the number of wells and volume of 

water withdrawn increased, thus making it more difficult to clearly relate cause and effect. Consequently, 

ground-water flow simulation techniques were applied 

with increasing frequency in an attempt to evaluate the 

effect of multiple factors on the ground-water resource.

Simulation of ground-water flow in the Gulf Coastal 

Plain was introduced by Patton (as described in Wood 

and Gabrysch, 1965) with the application of an analog

model in the Houston District, Texas. Subsequently, 

analog models were used to simulate ground-water 

flow relative to specific problems at several locations, 

such as water-level declines in the Sparta Sand in the 

northern Mississippi embayment (Reed, 1972); streamaquifer interactions in Arkansas (Broom and Reed, 

1973); the effects of recharge wells on the potentiometric surface of aquifers in the vicinity of the Houston, 

Tex., ship channel (Jorgensen, 1973); and declines in the 

potentiometric surfaces of aquifers in the Houston District, Texas, in response to alternative locations and 

magnitudes of ground-water withdrawals (Jorgensen, 

1975; Jorgensen and Gabrysch, 1974).

Simulation of ground-water flow within the study 

area continued as digital computer models replaced 

analog models, and attention continued to be focused 

on specific problems at various locations. Examples 

include simulation of water levels in an alluvial aquifer 

adjacent to a reservoir in eastern Texas (Garza, 1974); 

simulation of decline in water levels due to widespread 

ground-water withdrawals in southern Texas (Klemt 

and others, 1976); simulation of stream-aquifer interactions in Arkansas (Reed and Broom, 1979); simulation 

of declines in potentiometric surfaces of aquifers and 

land subsidence in the Houston area, Texas (Meyer and 

Carr, 1979; Carr and others, 1985); and simulation of 

water-level rises due to construction of navigation facilities on the Red River in northern Louisiana (Ludwig 

and Terry, 1980; Rogers, 1983). Simulations were made 

of water-level declines due to large ground-water withdrawals for northeastern Arkansas (Broom and Lyford, 

1981); for the Memphis area, Tennessee (Brahana, 1982); 

for northwestern Mississippi (Sumner and Wasson, 

1984a, 1984b); for the Baton Rouge area, Louisiana 

(Torak and Whiteman, 1982; Huntzinger and others, 

1985; Kuniansky, 1989); and for the Gulf Coastal Plain 

of Texas (Carr and others, 1985). Simulation of groundwater flow has been used within the study area to evaluate various activities such as the use of piercementtype salt domes for the isolation of radioactive waste in 

eastern Texas (Fogg and others, 1983); solution mining 

of uranium in southern Texas (Henry and others, 1982); 

and open-pit mining of lignite in east-central Texas 

(Charbeneau and Wright, 1983). Also, simulation of 

ground-water flow has been used recently to evaluate 

the potential for the movement of saline water from 

adjacent aquifers into aquifers with large withdrawals 

(Groschen, 1985; Trudeau and Buono, 1985).

These problem-oriented studies were conducted 

with little consideration of the regional ground-water 

flow system. The regional study of the Mississippi 

embayment (Gushing and others, 1970; Boswell and 

others, 1965, 1968; Hosman and others, 1968) and the 

multi-State study of the Claiborne Group (Payne, 1968,
SUMMARY OF HYDROLOGY OF THE REGIONAL AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES A5

1970, 1972, 1975) were comprehensive and largely 

descriptive, and the summary appraisals of the groundwater resource in major river basins (West and Broadhurst, 1975; Baker and Wall, 1976; Bedinger and 

Sniegocki, 1976; Zurawski, 1978; Cederstrom and others, 1979; Terry and others, 1979) were qualitative evaluations of the availability of the resource and 

discussions of various management strategies rather 

than a quantitative cause-and-effect analysis. All of the 

studies cited above, plus many more that are not cited, 

provide a large body of data and interpretive material. 

Although these data are not uniformly distributed in 

time or space, they provided a base upon which the 

Gulf Coast RASA study could build and thus develop 

an understanding of the regional ground-water flow 

system.

REGIONAL ANALYSIS OF THE GULF COAST 

AQUIFER SYSTEMS

The major objectives of the Gulf Coast RASA study 

are to delineate the geohydrologic framework, describe 

the ground-water chemistry, and determine regional 

flow patterns. A regionally consistent geohydrologic 

framework was developed, a regional description of 

ground-water chemistry was made, and ground-water 

flow models were used to simulate regional flow patterns. The following discussion highlights the efforts 

involved in meeting the study objectives.

Many interim reports (Sun and Weeks, 1991) were 

released to describe progress or to document techniques, procedures, and data used during the study. 

Progress on the study was reported by Grubb (1985, 

1986b, 1987, 1992); Wilson and Hosman (1988) 

described a geophysical well-log data base; Weiss and 

Williamson (1985) presented a method for vertically 

subdividing thick sedimentary units; and maps, sections, and diagrams of the geohydrologic framework 

were constructed by Hosman (1982, 1988); Arthur 

and Taylor (1986); Mesko (1988, 1990); Parks and 

Carmichael (1989, 1990b, 1990c); Taylor and Arthur 

(1989); and Whiteman and Martin (1984). Potentiometric surface maps were prepared for several aquifers, 

some for the first time, by Ackerman (1987a, 1987b, 

1989b); Boswell and Arthur (1988); Garza and others 

(1987); Martin and Whiteman (1985a, 1985b, 1986); Martin and others (1988); and Parks and Carmichael (1990a, 

1990b). Williams and Williamson (1989) presented a 

technique for estimating predevelopment water-table 

altitudes for regional ground-water studies of the Gulf 

Coastal Plain.

The areal distribution of dissolved-solids concentrations, major ion concentrations, and primary water

types in ground water from the study area were shown 

by Pettijohn (1988); Pettijohn and others (1988); Pettijohn, Busby, and Beckman (1992, 1993a, 1993b, 1993c); 

Pettijohn, Busby, and Cervantes (1993a, 1993b, 1993c); 

and Pettijohn, Busby, and Layman (1993a, 1993b, 

1993c). The geochemistry was described by Pettijohn 

(1996). Procedures for processing water-chemistry data 

were described by Pettijohn (1986), and techniques 

related to water chemistry interpretations of geophysical well logs were described by MacCary (1984) and 

Weiss (1987). Brahana and others (1985) presented 

ground-water quality data for the northern Mississippi 

embayment.

Preliminary simulations of predevelopment groundwater flow were made by Ackerman (1989a), Arthur 

and Taylor (1990), Brahana and Mesko (1988), Martin 

and Whiteman (1989), Ryder (1988), Williamson (1987), 

and Williamson and others (1990). Kuiper presented 

techniques for simulating variable density groundwater flow (1983, 1985), evaluated several methods for 

the solution of the inverse problem (1986), evaluated 

several numerical techniques (1987a), provided a computer program for general application (1987b), and documented the use of a nonlinear regression model, or 

parameter estimation model for regional ground-water 

flow simulation (1994). Martin and Whiteman (1990) 

presented a calibration and sensitivity analysis of a 

ground-water flow model. Brahana and Mesko (in 

press) presented a hydrologic analysis based on simulation of ground-water flow in the McNairy-Nacatoch 

aquifer in the northern Mississippi embayment. 

Regional interpretations of ground-water flow, based 

largely on computer-model simulations, were made by 

Grubb (1986a) and Grubb and Arthur (1991).

Ground-water withdrawals, as apportioned to each 

25-square-mile area of the study area, were documented by Mesko and others (1990). Beckman and Williamson (1990) described locations of salt domes that 

penetrate each regional aquifer in the gulf coast aquifer 

systems. Prudic (1991) analyzed about 6,000 aquifer and 

specific capacity tests from the study area.

The final results of the Gulf Coast RASA are contained in chapters of U.S. Geological Survey Professional Paper 1416. Each chapter deals with a specific 

subject or with specific parts of the flow system. A 

regionally consistent geohydrologic framework is documented in chapters B (Hosman and Weiss, 1991) and C 

(Weiss, 1992), regional stratigraphy and subsurface 

geology is contained in chapter G (Hosman, 1996), and 

chapters D, E, H, and I present a hydrologic analysis 

based on simulations of ground-water flow in different 

parts of the aquifer system: the Mississippi River Valley 

alluvial aquifer (chapter D, Ackerman, 1996); the Texas
A6 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

Gulf Coastal Plain (chapter E, Ryder and Ardis, in 

press); Coastal Plain sediments of southern Louisiana, 

southern Mississippi, southwestern Alabama, and adjacent Florida (chapter H, Martin and Whiteman, in 

press); and the Mississippi embayment (chapter I, 

Arthur and Taylor, in press). Chapter F (Williamson 

and Grubb, in press) is a regional hydrologic analysis 

based on simulation of ground-water flow that provides an evaluation of the effects of historical groundwater withdrawals and an evaluation of the regional 

potential for development of the ground-water 

resource. Chapter A (this report) is a summary of the 

hydrology of the regional aquifer systems in the Gulf 

Coastal Plain of the south-central United States and, in 

large part, is taken from the other chapters of Professional Paper 1416.

GEOGRAPHY

The study area underlies about 230,000 mi2 of the 

Gulf Coastal Plain in parts of Alabama, Arkansas, Florida, Illinois, Kentucky, Louisiana, Mississippi, Missouri, 

Tennessee, and Texas, and about 60,000 mi of the Gulf 

of Mexico between the coast and the edge of the Continental Shelf (fig. 2). The topography, climate, and major 

streams of the area influence recharge, discharge, and 

ground-water flow patterns, whereas population and 

economic activity influence ground-water withdrawals 

(pumpage).

Land-surface altitude ranges from sea level along 

the coast to more than 800 ft in a small area of southern 

Texas (fig. 2). The gently sloping coastal plain is dissected by streams that are typically perpendicular to the 

coastline. Gently rolling hills and locally rugged topography are common except in a band across coastal Texas 

and Louisiana and in the broad Mississippi Alluvial 

Plain that extends from the mouth of the Mississippi 

River to the southern tip of Illinois. Land-surface altitude is typically less than 50 ft above sea level in the 

coastal band across Texas and Louisiana that is 25 to 

50 mi wide. The Mississippi Alluvial Plain is the dominant topographic feature in the study area (fig. 2). It is 

typically 80 to 90 mi wide but is as much as 120 mi wide 

near the latitude of Little Rock, Arkansas, and as little 

as 30 mi wide between Vicksburg and Natchez, Mississippi. The essentially flat alluvial plain slopes southward from about 330 ft above sea level in Illinois, 

Kentucky, and Missouri to about 25 ft above sea level at 

the latitude of Baton Rouge, Louisiana. From Baton 

Rouge southward, the Mississippi Alluvial Plain 

merges with the coastal band of low topography that 

extends across Texas and Louisiana. The hills that border the Mississippi Alluvial Plain on the east extend

southward to near the coast of Mississippi and Alabama.

Several major streams that drain the area originate 

beyond the study area boundary and have substantial 

flow at the point where they enter the study area. The 

major rivers that have large drainage areas beyond the 

Gulf Coastal Plain are, from west to east, the Rio 

Grande, Colorado, Brazos, Red, Arkansas, Mississippi, 

Ohio, and Tombigbee. Other rivers that have relatively 

small drainage areas beyond the study area are the Frio, 

Guadalupe, Trinity, Sabine, Ouachita, and Escambia. 

The entire drainage areas of several rivers, such as the 

Neches, Yazoo, Pearl, and Pascagoula, are totally within 

the study area, and they typically have less flow than 

the rivers that originate outside the study area. Much 

water is available in the stream systems; the mean 

annual flows range from 775 ft /s for the Nueces River 

near Mathis, Tex., to 452,000 ft3 /s for the Mississippi 

River at Hickman, Ky., near the northernmost part of 

the study area.

The climate of the area is mostly humid subtropical, with mean annual temperature ranging from about 

57°F in the north to about 70°F in the south. Mean 

annual precipitation ranges from less than 24 inches in 

southern Texas to more than 64 inches in southern 

Mississippi and adjacent parts of Louisiana and Alabama (fig. 3). Pan evaporation decreases from about 

100 inches/yr in southern Texas to 60-65 inches/yr 

throughout most of the coastal areas of Louisiana, Mississippi, and Alabama. Pan evaporation also decreases 

from about 65 inches/yr along the Louisiana coast to 

less than 45 inches/yr in southern Illinois.

A large part of the precipitation leaves the area as 

surface runoff. Mean annual runoff ranges from 0.2 inch 

in southern Texas to about 28 inches in southern Alabama (Gebert and others, 1987). Much of the precipitation that does not run off is removed from the area by 

evapotranspiration. Most recharge to the water table is 

discharged nearby and provides base flow of streams. 

Only a small fraction, typically less than 1 inch/yr, is 

left to recharge the regional ground-water system 

except in areas with large ground-water pumpage (Williamson and others, 1990).

POPULATION AND ECONOMY

Population within the study area grew at the rate of 

about one-quarter million per decade during the period 

1970-90 and reached a total of 20.4 million by 1990 

(fig. 4). This growth rate was about a factor of 2 greater 

than the fairly steady population growth rate during 

the previous four decades. In addition to the increase in 

population, there was a large movement of people from
SUMMARY OF HYDROLOGY OF THE REGIONAL AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES A7

EXPLANATION 

 200 Land-surface contour Shows

altitude of land surface and sea floor. 

Contour interval, in feet, is variable. 

Datum is sea level

Study area boundary

Physiography from Fenneman, 1938 

Land-surface altitude from Godson, 1981

50 100 150 MILES

j______|______1 1 i I 

0 50 100 150 KILOMETERS

Base from U.S. Geological Survey 

Scale 1:2,500,000

FIGURE 2. Generalized average land-surface altitude, bathymetry, major rivers, and physiographic provinces.
A8 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

EXPLANATION 

*% Line of equal mean annual precipitation 

Interval 4 inches

KENTUCKY 

. 37°

 36°

96

33

0 50 100 150 MILES

I I I

Study area boundary

Unpublished map compiled by D.A. Olson, National 

Oceanic and Atmospheric Administration

0 50 100 150 KILOMETERS

Base from U.S. Geological Survey 

Scale 1:2,500,000

FIGURE 3. Mean annual precipitation, 1951-80.
SUMMARY OF HYDROLOGY OF THE REGIONAL AQUIFER SYSTEMS SOUTH-CENTRAL UNITED STATES A9

KENTUCKY 

, 37=

"- ' ! 4. 'v (-FLORIDA

A

EXPLANATION

Boundary of study area

Population Each dot 

represents 1,000 people

6 50 100 150 KILOMETERS

FIGURE 4. Distribution of population in 1985 and population growth, 1800-1990.
A10 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

rural areas to urban areas, with Louisiana and Texas 

having more people living in cities than in rural areas 

by 1950. Large pumping centers have developed near 

the more densely populated areas of figure 4. Both the 

increase in population and the movement of people to 

cities have contributed to the increase in ground-water 

use in the study area.

Agricultural activities are widespread throughout 

the study area and have been important to the economy 

of the various States for many years. Some of the highvalue crops are rice, commercial vegetables, row crops 

such as corn and soybeans, and fish farming or aquaculture. Agricultural activities that typically demand large 

quantities of water are the growing of rice and commercial vegetables, and aquaculture; an increase in each 

activity has caused an increase in ground-water withdrawals over the years.

Industrial activities are economically important in 

the study area, and large quantities of water are used by 

industry. Growth of industrial activities such as the petrochemical and paper industries has contributed to 

increased ground-water withdrawals.

GEOLOGY

The aquifers of interest are in a gulfward-thickening 

wedge of sedimentary deposits of mostly Cenozoic age. 

Maximum thicknesses of normally pressured sediments 

range from about 14,000 to 18,000 ft throughout a large 

area of coastal Louisiana. The sediments crop out in a 

typical offlap relation. Older sediments are at the land 

surface at the updip limit of the study area, and progressively younger sediments crop out gulfward in 

bands parallel to the Gulf of Mexico or the axes of major 

embayments. The typically unconsolidated to semiconsolidated sediments are a complexly interbedded 

sequence of sand, silt, and clay with minor beds of lignite, gravel, and limestone. The following summary of 

the geology was adapted from much more extensive 

discussions by Hosman (1996), Weiss (1992), and Hosman and Weiss (1991).

Deposition during the Tertiary Period was affected 

by regional crustal subsidence, inflow of sediment from 

beyond the Gulf Coastal Plain, and eustatic sea-level 

change (Galloway, 1989). The principal structural features such as the Gulf Coast geosyncline and the Mississippi embayment (fig. 5) reflect the configuration and 

deformation of the underlying Paleozoic rocks. Subsidiary structural features such as uplifts, embayments, 

and fault zones began to form prior to the Tertiary 

Period and continued to develop as localized subsidence occurred at the mouths of large stream systems 

where sediment carried from the interior of the continent was deposited in vast quantities (Wilhelm and 

Ewing, 1972). Gravity flow of buried sediments near the 

margins of thick deltaic deposits resulted in growth 

faulting and thickening of the strata in a gulfward 

direction. Excess fluid pressure (also called geopressure) developed as sediments were buried more rapidly 

than fluids could be expelled from compacting sediments. The landward migration of marine environments resulted in the deposition of massive marine 

clays as sea levels rose periodically throughout the Cenozoic Era.

Marine clays of the Midway Group (Paleocene) form 

the basal confining unit of the ground-water flow system in most of the study area. All stratigraphic units 

plunge into the Gulf Coast geosyncline to the south, 

and the sand aquifers pinch out by facies change to clay 

and silt or are truncated by the geopressured zone. The 

Wilcox Group (Paleocene and Eocene) overlies the Midway Group and contains sand beds of widely varying 

thickness and texture. The Wilcox Group contains massive sand beds in places, notably in the northern part of 

the Mississippi embayment, although most of the group 

is composed of interbedded thin sands, clays, and silts. 

The overlying Claiborne Group (Eocene) is generally a 

thick sequence of massive sand beds separated by thick, 

extensive marine clay beds. The sand beds of the Wilcox 

and Claiborne Groups form regionally extensive aquifers that are typically confined by areally extensive 

marine clay beds.

The marine clays of the combined Jackson (Eocene) 

and Vicksburg (Oligocene) Groups overlie the Claiborne Group and form an extensive regional confining 

unit throughout the southern part of the study area. 

Gulfward of the outcrop area of the Vicksburg Group, 

strata of the overlying Miocene Series contains sand 

beds of varying thickness that are complexly interbedded with silt and clay. Typically, Miocene sand beds are 

not as massive, or they do not extend throughout as 

large an area, as the sand beds of the Claiborne Group. 

Numerous interbedded clays, which are not traceable 

throughout large geographic areas, are a substantial 

restriction to vertical ground-water flow in the Miocene 

sands. Deposits of the Pliocene Series overlie Miocene 

deposits gulfward of the Miocene outcrop area, and 

Pliocene sands generally are hydraulically connected 

and contain no regionally extensive confining units. 

Terrace (Pleistocene) and alluvial (Holocene) deposits 

overlie Pliocene sediments in a wide band adjacent to 

the Gulf of Mexico and extend northward throughout 

most of the central and western parts of the Mississippi 

embayment where they overlie strata of Miocene 

through Paleocene age (fig. 5). These terrace and alluvial deposits are hydraulically connected to streams 

and underlying Tertiary deposits. Sediments of Oli-
SUMMARY OF HYDROLOGY OF THE REGIONAL AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES All

EXPLANATION

QJ Holocene, Pleistocene, and Pliocene 

1 Undifferentiated deposits

Tmu Miocene Undifferentiated deposits

Tvj 

Tc 

Tw 

Tm 

Kr

o

Oligocene and Eocene Vicksburg and 

Jackson Groups

Eocene Claiborne Group

Eocene and Paleocene Wilcox Group

Paleocene Midway Group

Upper Cretaceous Undifferentiated deposits

Syncline 

Anticline

Fault zone

Broad structural feature

MISSOURI

N 37

.- KENTUCKY

Jackson 

Dome

Study area boundary

100 150 MILES

I______| I I I 

0 50 100 150 KILOMETERS

Base from U.S. Geological Survey 

Scale 1:2,500,000

FIGURE 5. Generalized geology and structural features, Gulf Coastal Plain, south-central United States.
A12 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

gocene age and younger commonly lack areally extensive marine clays and therefore contain no regional 

confining units.

Major structural features (fig. 5) affected sediment 

deposition during the Tertiary Period. Sediments thin 

toward and over uplifted features such as the San Marcos arch, Sabine uplift, Monroe uplift, and Jackson 

dome. Sediments typically are thicker in depressions 

such as the Rio Grande, Houston, Terrebonne, and Mississippi embayments and the Gulf Coast geosyncline.

The regional growth-fault zones that typically parallel the coastline do not appear to be a major control on 

regional ground-water flow but may be significant controls locally. However, these faults as well as the major 

structural features are important controls on the accumulation and movement of hydrocarbons.

Salt-dome basins are present across the area from 

southern Texas to southwestern Alabama (Halbouty, 

1979). The domes can penetrate most or all Tertiary 

strata at any given location although only a few are at 

or near the land surface. Salt domes that penetrate geohydrologic units within the study area were identified 

by Beckman and Williamson (1990). The source of the 

salt is generally considered to be the deeply buried 

Louann Salt of Jurassic age (Andrews, 1960). Faulting 

associated with the salt domes is localized; the domes 

typically are only 1 to 3 mi in diameter. The effects of 

salt domes on regional ground-water flow in the freshwater part of the flow system is also localized. However, dissolution of salt in the deeply buried parts of the 

salt basins may affect regional ground-water movement 

in strata that contain highly mineralized water.

GEOHYDROLOGIC FRAMEWORK

Three aquifer systems are present in the study area: 

the coastal lowlands aquifer systems, Texas coastal 

uplands, and the Mississippi embayment (fig. 6). These 

aquifer systems, in sediments of Cenozoic age, have 

been divided into six regional aquifers and five permeable zones (fig. 7 and table 1). The geohydrology of each 

of the three aquifer systems is described by Hosman 

and Weiss (1991) and Weiss (1992); both of these reports 

present maps of the thickness, percentage of sand, and 

altitude and configuration of the top of each wateryielding unit, along with a discussion of the general 

geology. Hosman and Weiss (1991) also include maps 

for the McNairy-Nacatoch aquifer in sediments of Cretaceous age that underlie the Mississippi embayment 

aquifer system in the northern part of the study area.

The aquifers and permeable zones range in areal 

extent from about 32,000 mi2 (the Mississippi River Valley alluvial aquifer) to about 170,000 mi2 (the middle

Wilcox aquifer, fig. 8). The Mississippi River Valley 

alluvial aquifer has the smallest average thickness (140 

ft), and permeable zone C (lower Pliocene-upper 

Miocene deposits) has the largest average thickness 

(2,010 ft). Average percentage of sand ranges from 34 

percent in the middle Wilcox aquifer to 86 percent in 

the lower Claiborne-upper Wilcox aquifer (table 2). The 

permeable zones and aquifers typically thicken from 

outcrop toward the Gulf of Mexico or toward the axes 

of major embayments; however, near their downdip 

limits the water-yielding units become thinner as the 

sediments grade into marine clay facies or the top of the 

geopressured zone truncates the ground-water flow 

system.

The aquifers in the Mississippi embayment and laterally equivalent Texas coastal uplands aquifer systems 

typically consist of one or more massive sand beds and 

are separated from each other by confining units of 

regionally extensive clay beds. The sediments of the 

coastal lowlands aquifer system typically consist of 

interbedded sand and clay beds that are not regionally 

extensive. Accordingly, these sediments were divided 

vertically into permeable zones on the basis of sharp 

changes in vertical hydraulic head gradients and local 

lithology as explained by Weiss (1992) and Weiss and 

Williamson (1985).

A regional permeable zone (permeable zone B) and 

a regional aquifer (the middle Claiborne aquifer) are 

used for illustrative purposes in this summary. Thus in 

the following discussion of geohydrologic units, maps 

of thickness are shown only for permeable zone B and 

the middle Claiborne aquifer. Thickness and altitude of 

the top of each regional geohydrologic unit and percentage of sand for aquifers and permeable zones are 

presented by Hosman and Weiss (1991) and Weiss 

(1992).

COASTAL LOWLANDS AQUIFER SYSTEM

The base of the coastal lowlands aquifer system is 

the Vicksburg-Jackson confining unit except for an area 

that includes the Continental Shelf and a narrow band 

onshore along the coast of Texas and Louisiana. In this 

area, the base of the flow system is the top of the 

geopressured zone, which is above the top of the sediments that comprise the confining unit (Grubb, 1986b). 

The confining unit has a maximum thickness of about 

6,000 ft in southeastern Louisiana and an average thickness of about 500 ft.

There are five permeable zones and two confining 

units within the coastal lowlands aquifer system 

(figs. 7, 9). The two confining units are present only in 

the subsurface. The permeable zones typically consist of
SUMMARY OF HYDROLOGY OF THE REGIONAL AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES A13

EXPLANATION 

Coastal Lowlands Aquifer System

Vicksburg-Jackson Confining Unit 

Texas Coastal Uplands Aquifer System

Mississippi Embayment Aquifer System 

Midway Confining Unit 

Boundary of geographic areas

.KENTUCKY 

37°

Study area boundary

1 T 

0 50 100 150 KILOMETERS

Base from U.S. Geological Survey 

Scale 1:2,500,000

FIGURE 6. Generalized outcrop of aquifer systems, intervening regional confining units, and designated geographic areas.
A14 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

EXPLANATION

Coastal lowlands aquifer system permeable zones A-E

I . . I Zone A (Holocene-upper Pleistocene deposits. 

I -I South of Vicksburg-Jackson confining unit)

I I Zone B (lower Pleistocene-upper Pliocene deposits)

[ | Zone C (lower Pliocene-upper Miocene deposits)

[ j Zone D (middle Miocene deposits)

[ | Zone E (lower Miocene-upper Oligocene deposits)

n Vicksburg-Jackson confining unit Base of the coastal 

lowlands aquifer system

Mississippi embayment and Texas coastal uplands 

aquifer systems

Mississippi River Valley alluvial aquifer (north of 

permeable eli zone E) 

|__| Upper Claiborne aquifer 

I__I Middle Claiborne confining unit 

|__I Middle Claiborne aquifer 

^H Lower Claiborne confining unit 

|__| Lower Claiborne-upper Wilcox aquifer 

[ I Middle Wilcox aquifer 

I__I Lower Wilcox aquifer

a Midway Confining Unit Base of Missisippi embayment 

and Texas coastal uplands aquifer systems 96o

McNairy-Nacatoch aquifer

Study area boundary

 Indicates units subcrop beneath

Mississippi River Valley alluvial aquifer

* Not present in Texas coastal uplands aquifer 

system

32° ;

150 MILES

50 100 150 KILOMETERS

Base from U.S. Geological Survey 

Scale 1-2,500.000

FIGURE 7. Generalized outcrop of regional geohydrologic units.
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A16 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

180

160

80

60

20

45678 

AQUIFER OR PERMEABLE ZONE NUMBER

10

AQUIFER OR PERMEABLE 

ZONE NUMBER

1 Mississippi River Valley 

alluvial aquifer

2 Permeable zone A

3 Permeable zone B

4 Permeable zone C

5 Permeable zone D

6 Permeable zone E

7 Upper Claiborne aquifer

8 Middle Claiborne aquifer

9 Lower Claiborne-upper 

Wilcox aquifer

10 Middle Wilcox aquifer

11 Lower Wilcox aquifer

FIGURE 8. Areal extent of aquifers and permeable zones.

interbedded sand and clay rather than the areally extensive and massive sands that are typical of the other two 

aquifer systems. Also, the permeable zones in the 

coastal lowlands aquifer system generally contain a 

smaller percentage of sand than the aquifers in the 

other two aquifer systems except tor the middle Wilcox 

aquifer in the Mississippi embayment aquifer system 

(table 2). The permeable zones were delineated on the 

basis of hydraulic heads and local lithology as 

described by Weiss and Williamson (1985) because of 

the complexly interbedded nature of the sediments and 

the absence of areally extensive marker beds that are 

hydrologically significant. The method that they 

describe divides the entire thickness of the sediments 

above the Vicksburg-Jackson confining unit such that 

the hydraulic-head differences between permeable 

zones are greater than those within the permeable 

zones, especially in areas where ground-water withdrawals have created large vertical hydraulic-head gradients. The permeable zones were designated, from 

youngest to oldest, A through E, as shown in table 1.

Permeable zone A (Holocene-upper Pleistocene 

deposits) has an area of about 120,000 mi2. The zone 

directly overlies zone B (table 1), except for a narrow 

band about 50 mi wide along the Mississippi River, and 

extends northward from the main body of the zone to 

the subcrop of the Vicksburg-Jackson confining unit. In 

this small area that extends northward from the Vicksburg-Jackson confining unit and parallels the Mississippi River, permeable zone A overlies the other

1 permeable zones of the coastal lowlands aquifer system. 

Permeable zone A has an average thickness of about

| 540 ft and a maximum thickness of about 1,200 ft offshore from east-central Louisiana. The zone has an 

average sand content of about 65 percent (table 2). Sand 

content exceeds 80 percent in several small, isolated 

areas (a few hundred square miles in extent) in Texas 

and throughout a large area of several thousand square 

miles in central Louisiana. A large, nearly continuous 

area where the sand content is less than 20 percent is 

present adjacent to the downdip limit of the zone.

Permeable zone B (lower Pleistocene-upper Pliocene 

deposits) (fig. 10) has an area of about 130,000 mi2. It 

directly overlies zone C (lower Pliocene-upper Miocene 

deposits) (table 1) and the two zones are not separated 

by a confining unit. Zone B has an average thickness of 

about 1,900 ft (table 2) and a maximum thickness of 

about 5,700 ft offshore from east-central Louisiana. The 

thickest part of permeable zone B extends throughout a 

large area of offshore Louisiana. The permeable zone 

thickens gulfward from its outcrop area toward the 

edge of the Continental Shelf and from west to east 

toward the Terrebonne embayment. The small area 

where the zone is less than 3,000 ft thick near the 

edge of the Continental Shelf offshore from Louisiana (fig. 10) is due to the truncation of the permeable 

zone from below by the geopressured zone. Permeable 

zone B has an average sand content of about 48 percent. 

There are four small isolated areas (a few hundred 

square miles in extent) where the sand content exceeds
SUMMARY OF HYDROLOGY OF THE REGIONAL AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES A17

80 percent. Three of these areas extend to the updip limit 

of the zone in Texas east of the San Marcos uplift. There 

are only two small areas where the sand content is less 

than 20 percent in updip areas. Both are in or near the 

outcrop area of the permeable zone in Mississippi. A

TABLE 2. Summary of thickness and sand percentage of aquifers and 

permeable zones

[Values for thickness and sand percentage are the average of the data from 

the well-log file described by Grubb (1986b, p. 157); --, not present]

Mississippi embayment aquifer system

Aquifer

Thickness 

(feet) Sand percentage

Maximum Average Average Standard 

deviation

Mississippi River Valley 

alluvial

Upper Claiborne

Middle Claiborne

Lower Claiborne-upper 

Wilcox

Middle Wilcox aquifer

Lower Wilcox aquifer

350

1.180

1,090

940

4,060

1,290

140

310

485

270

1,020

330

70

52

57

71

34

67

23

22

26

18

24

Texas coastal uplands aquifer system

Aquifer

Thickness 

(feet) Sand percentage

Maximum Average Average Standard 

deviation

Mississippi River Valley 

alluvial

Upper Claiborne

Middle Claiborne

Lower Claiborne-upper 

Wilcox

Middle Wilcox aquifer

Lower Wilcox aquifer

(D

2,490

2,530

1,300

5,200

(D

970

570

360

1,790

-

51

60

86

55

--

15

19

14

15

--

Coastal lowlands aquifer system

Permeable zone

Thickness

(feet) Sand percentage

Maximum Average Average Standard 

deviation

Zone A (Holocene-upper 

Pleistocene deposits

Zone B (lower Pleistoceneupper Pliocene deposits)

Zone C (lower Plioceneupper Miocene deposits)

Zone D (middle Miocene 

deposits)

Zone E (lower Mioceneupper Oligocene deposits)

1,220 540

5,650 1,880

6,330 2,010

7,580 1,830

4,450 1,420

65

48

45

44

42

28

21

20

19

18

(1) Aquifer not present in the Texas coastal uplands aquifer system.

sand content of less than 20 percent is present in a large, 

nearly continuous area adjacent to the downdip limit of 

the zone.

Permeable zone C (lower Pliocene-upper Miocene 

deposits) has an area of about 140,000 mi2 and 

extends gulfward to the edge of the Continental Shelf. It 

overlies permeable zone D (middle Miocene deposits) 

(table 1) in the outcrop and adjacent downdip area 

(about 25,000 mi2) and overlies the zone D confining 

unit (which is present only in the subsurface, fig. 9) 

throughout its remaining downdip area of occurrence. 

Permeable zone C has an average thickness of about 

2,000 ft (table 2) and a maximum thickness of about 

6,300 ft on the Continental Shelf about midway between 

the east-central Louisiana shoreline and the edge of 

the shelf. The zone has an average sand content of about 

45 percent (table 2). There are three relatively small 

areas (a few hundred square miles in extent) in southern Mississippi where the sand content is more than 80 

percent. Four small isolated areas where the sand content is less than 20 percent are present two in southern 

Texas and two in southern Mississippi. A large, continuous area (tens of thousands of square miles in extent) 

where the sand content is less than 20 percent is present 

adjacent to the downdip limit of the permeable zone.

Permeable zone D (middle Miocene deposits) has an 

area of about 120,000 mi2. It overlies permeable zone E 

(lower Miocene-upper Oligocene deposits) (table 1) in 

the outcrop and adjacent updip area (about 50,000 mi2) 

and overlies the zone E confining unit (which is present 

only in the subsurface, fig. 9) throughout its remaining 

downdip area of occurrence (about 44,000 mi ). Permeable zone D has an average thickness of about 1,800 ft 

(table 2) and a maximum thickness of more than 7,500 ft 

in southeastern Louisiana. The zone has an average 

sand content of about 44 percent (table 2). There are 

only three small areas (a few hundred square miles in 

extent) in the eastern part of the outcrop area where the 

sand content exceeds 80 percent. There are several isolated areas (a few thousands of square miles in extent) 

in both updip and downdip areas where the sand content is less than 20 percent.

Permeable zone E (lower Miocene-upper Oligocene 

deposits) overlies the Vicksburg-Jackson confining unit 

and has an area of about 90,000 mi2. The zone has an 

average thickness of about 1,400 ft and a maximum 

thickness of 4,450 ft in southern Texas about midway 

between the Rio Grande and the axis of the San Marcos 

arch. The permeable zone has an average sand content 

of about 42 percent sand (table 2) and does not have any 

large area where the sand content is more than 80 percent. However, permeable zone E has a sand content of 

60-80 percent in three areas of several thousand square 

miles in extent. Two areas are in Texas, one near and
A18 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

NORTHWEST

Midway confining unit

SOUTHEAST

FEET 

2,000 -i

10,000 -

12,000

Texas coastal uplands 

aquifer system

Vicksburg-Jackson confining unit

Coastal lowlands aquifer system

Edge of

continental

shelf

20 40 MILES

I i I

0 20 40 KILOMETERS

Vertical scale greatly exaggerated 

EXPLANATION

Coastal lowlands aquifer system

IT"

I 10

[ 9

17 | Zone D confining unit 

| 8

16 | Zone E confining unit 

I 7

Permeable zone A (Holocene-upper 

Pleistocene deposits)

Permeable zone B (Lower Pleistoceneupper Pliocene deposits)

Permeable zone C (Lower Plioceneupper Miocene deposits)

Permeable zone D (middle Miocene 

deposits)

Permeable zone E (lower Mioceneupper Oligocene deposits)

Vicksburg-Jackson confining unit 

Base of the coastal lowlands aquifer system

Texas coastal uplands aquifer system 

5 Upper Claiborne aquifer

14 Middle Claiborne confining unit

5 Middle Claiborne aquifer

I 13 Lower Claiborne confining unit

4 Lower Claiborne-upper Wilcox aquifer

3 I Middle Wilcox aquifer

12 Midway confining unit Base of Texas coastal

 uplands aquifer system

 ' Contact Dashed where approximately located 

Queried where uncertain

FIGURE 9. Idealized section from boundary of study area in Zavala County, Texas, southeastward through 

Kleberg County, Texas, to the Continental Slope, showing relation of geohydrologic units.

parallel to the coast from the Rio Grande to the vicinity 

of the axis of the San Marcos arch, and the other near 

the coast in southeastern Texas. The third area extends 

from the outcrop area in southeastern Mississippi to the 

coast at the Mississippi-Louisiana State boundary. 

There are several small areas (several hundred square 

miles in extent) near the updip limits of the zone where 

the sand content is less than 20 percent. Another narrow 

band where the sand content is less than 20 percent 

occurs near the downdip limit of permeable zone E 

from western Louisiana to western Florida.

TEXAS COASTAL UPLANDS AQUIFER SYSTEM

The aquifers and confining units that compose this 

aquifer system range in age from Paleocene to late 

Eocene and are laterally equivalent to those of the Mississippi embayment aquifer system. The base of the system is the Midway confining unit except along a band 

that is as much as 30 mi wide where the top of the 

geopressured zone is the base. Near the downdip limit 

of the aquifer system, the geopressured zone is above 

the top of the sediments that compose the Midway confining unit. There are four aquifers and two confining
SUMMARY OF HYDROLOGY OF THE REGIONAL AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES A19

KENTUCKY 

37°

EXPLANATION

Outcrop or subcrop area 

Dashed where concealed

Line of equal thickness 

Interval, in feet, is variable

Depth-averaged dissolved-solids 

concentration More than 10,000 

milligrams per liter where shaded

Downdip limit of permeable zone

Study area boundary

100 150 KILOMETERS

Base from U.S. Geological Survey 

Scale 1:2,500,000

FIGURE 10. Thickness of permeable zone B (lower Pleistocene-upper Pliocene deposits), coastal lowlands aquifer system.
A20 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

units within the Texas coastal uplands aquifer system 

(table 1). The aquifers in the Texas coastal uplands aquifer system typically are thicker and contain more sand 

than equivalent aquifers in the Mississippi embayment 

aquifer system (table 2). Two aquifers, the middle Claiborne and the lower Claiborne-upper Wilcox, are not 

present throughout an area of about 6,000-9,000 mi2 

centered on the Texas-Louisiana State boundary. In this 

area, lack of deposition or erosion of younger units on 

the Sabine uplift has resulted in the middle Wilcox 

aquifer being the uppermost geohydrologic unit (fig. 7). 

The width of the outcrop areas is generally narrower in 

the Texas coastal uplands aquifer system than in the 

Mississippi embayment aquifer system.

The upper Claiborne aquifer has an area of about 

45,000 mi2 and overlies the middle Claiborne confining 

unit (table 1). The aquifer has an average thickness of 

about 970 ft and is about 51 percent sand. There are 

no extensive areas where the aquifer contains more 

than 80 percent or less than 20 percent sand. The middle 

Claiborne confining unit separates the upper Claiborne 

aquifer from the underlying middle Claiborne aquifer 

and has an average thickness of about 150 ft.

The middle Claiborne aquifer (fig. 11) has an area of 

about 36,000 mi2 and overlies the lower Claiborne confining unit (table 1). The aquifer has an average thickness of about 570 ft and is about 60 percent sand 

(table 2). The thickest part of the middle Claiborne aquifer is in the Rio Grande embayment of southern Texas, 

where it is more than 2,500 ft thick. The major cause of 

thinning of the middle Claiborne aquifer near its downdip limit is the decrease in permeability due to facies 

change. The geopressured zone forms the base of the 

aquifer along a band about 20 mi wide and 100 mi long 

adjacent to the downdip limit of the aquifer, in southern 

Texas and trending northeastward from the Rio 

Grande. The aquifer is more than 80 percent sand in 

two small areas near the Sabine uplift; the total of the 

combined areas is less than 2,000 mi2. The aquifer is 

less than 40 percent sand in a narrow band of several 

thousand square miles near its downdip limit. The middle Claiborne aquifer is separated from the underlying 

lower Claiborne-upper Wilcox aquifer by the lower 

Claiborne confining unit, which has an average thickness of about 300 ft.

The lower Claiborne-upper Wilcox aquifer typically 

consists of massive sand beds; it directly overlies the 

middle Wilcox aquifer (table 1) and is present throughout an area of about 30,000 mi2. The lower Claiborneupper Wilcox aquifer has an average thickness of about 

360 ft and is about 86 percent sand (table 2). The aquifer 

is more than 80 percent sand throughout most of its 

areal extent. The three areas where the unit contains 

between 60 and 80 percent sand total about 10,000 mi2.

The largest of the three areas is in southern Texas and is 

about 7,000 mi2; the smallest area is in northeastern 

Texas, northwest of the Sabine uplift, and is less than 

1,000 mi2. The third area is just east of the San Marcos 

arch and is about 2,000 mi2.

The middle Wilcox aquifer overlies the Midway confining unit (table 1) and has an area of about 56,000 mi2. 

The aquifer typically is composed of thin alternating 

beds of sand and clay; it has an average thickness of 

about 1,800 ft and has a sand content of about 55 percent (table 2). The aquifer contains more than 80 percent 

sand in one small area of a few hundred square miles 

west of the Sabine uplift. The aquifer everywhere contains more than 40 percent sand except for an area of 

about 3,000 mi2 in southern Texas and a small area of a 

few hundred square miles near the Sabine uplift in eastern Texas.

MISSISSIPPI EMBAYMENT AQUIFER SYSTEM

The base of the Mississippi embayment aquifer system is the Midway confining unit except along a narrow 

band that is as much as 20 mi wide across southern 

Louisiana where the top of the geopressured zone is 

the base of the aquifer system. In this band the 

geopressured zone is above the top of the sediments 

that compose the Midway confining unit (Grubb, 

1986b). The confining unit has a maximum thickness of 

about 3,500 ft in Louisiana and an average thickness of 

about 750 to 850 ft. There are six aquifers and three confining units within the Mississippi embayment aquifer 

system (table 1, figs. 7, 12). The McNairy-Nacatoch 

aquifer, which underlies the Midway confining unit in 

the northern Mississippi embayment, was included in 

the Mississippi embayment aquifer system by Grubb 

(1984). However, results from simulation of groundwater flow in this aquifer indicate that only a small volume of water leaks upward from the McNairy-Nacatoch aquifer through the Midway confining unit and 

that the aquifer functions independently of the aquifers 

above the confining unit throughout most of its areal 

extent (Brahana, 1987).

Extensive and massive sand beds, especially in the 

updip and freshwater part of the system, are characteristic of the Mississippi embayment aquifer system. 

However, there is considerable variation among aquifers in the system. Some are separated by areally extensive confining units that restrict vertical movement of 

water. Other aquifers are not separated by areally 

extensive confining units but are defined on the basis of 

differences in lithology; the vertical resistance to flow 

within and between these aquifers is due to interbedded fine-grained sediments.
SUMMARY OF HYDROLOGY OF THE REGIONAL AQUIFER SYSTEMS. SOUTH-CENTRAL UNITED STATES A21

EXPLANATION

Outcrop or subcrop area 

Dashed where concealed

Line of equal thickness 

Interval, in feet, is variable

Depth-averaged dissolved-solids

concentration More than 10,000

milligrams per liter where shaded

Downdip limit of aquifer

Boundary between Mississippi 

embayment and Texas coastal 

uplands aquifer systems

KENTUCKY 

37°

Study area boundary

Geometry from Hosman and Weiss, 1991 

Dissolved solids from Pettijohn and others. 1988

50 100 150 KILOMETERS

Base from U.S. Geological Survey 

Scale 1:2,500,000

FIGURE 11 Thickness of the middle Claiborne aquifer, Texas coastal uplands and Mississippi embayment aquifer systems.
A22 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

NORTH SOUTH

FEET 

1,000 

SEA 

LEVEL

1,000 -

2,000 -

3,000 -

4,000 -

5,000 -

6,000 -

7,000 -

8,000 -

9,000 -

10,000 -

11,000 -

12,000 -

13,000 -

14,000 -

15,000

Mississippi embayment aquifer system

11

Coastal lowlands aquifer system

EXPLANATION

Mississippi embayment aquifer system 

[" 'i»j ' ' I Mississippi River Valley alluvial aquifer

I g Upper Claiborne aquifer

I 14 I Middle Claiborne confining unit

I 5 Middle Claiborne aquifer

I -J3 I Lower Claiborne confining unit

I 4 Lower Claiborne-upper Wilcox aquifer

3 Middle Wilcox aquifer

| 2 I Lower Wilcox aquifer

Midway confining unit Base of Mississippi

embayment aquifer system 

Vicksburg-Jackson confining unit Base

of the coastal lowlands aquifer system

 Contact Dashed where approximately located

50 MILES

0 50 KILOMETERS 

Vertical scale greatly exaggerated

FIGURE 12. Idealized section from near northern boundary of study area in Mississippi County, Missouri, to 

Iberville Parish, Louisiana, showing relation of geohydrologic units.

There are two aquifers in the Mississippi embayment aquifer system that are not present in the Texas 

coastal uplands aquifer system. These are the lower 

Wilcox aquifer and the Mississippi River Valley alluvial 

aquifer. The other aquifers of the Mississippi embayment aquifer system are laterally equivalent to aquifers 

present in the Texas coastal uplands aquifer system and 

they are called by the same names (table 1).

The Mississippi River Valley alluvial aquifer overlies aquifers and confining units in sediments of Tertiary age throughout an area of about 32,000 mi2 in the 

northern Mississippi embayment, mostly in Missouri, 

Arkansas, northwestern Mississippi, and northeastern 

Louisiana (fig. 7). The aquifer has an average thickness 

of about 140 ft and is about 70 percent sand (table 2). 

The aquifer is more than 80 percent sand throughout an

elongate area of about 12,000 mi2 from southern Illinois 

to west-central Mississippi.

The upper Claiborne aquifer overlies the middle 

Claiborne confining unit (table 1) and has an area of 

about 45,000 mi2. The aquifer has an average thickness 

of about 300 ft and contains about 52 percent sand 

(table 2). There are several isolated areas, extending 

from western Kentucky into central Mississippi, where 

the aquifer contains more than 80 percent sand. Except 

for one area of about 2,000 mi2, which extends from 

south-central Arkansas into north-central Louisiana, 

these areas are only several hundreds of square miles in 

areal extent. The aquifer contains less than 20 percent 

sand in only two areas of a few hundred square miles in 

extent located in south-central Mississippi. The upper 

Claiborne aquifer is separated from the middle Clai-
SUMMARY OF HYDROLOGY OF THE REGIONAL AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES A23

borne aquifer by the middle Claiborne confining unit, 

which has an average thickness of about 150 ft.

The middle Claiborne aquifer (fig. 11) typically consists of massive sand beds but has more fine-grained 

sediments than the underlying lower Claiborne-upper 

Wilcox aquifer. The middle Claiborne aquifer has an 

area of about 100,000 mi2 and overlies the lower Claiborne confining unit (table 1) except for the area north 

of about 35° latitude (figs. 7 and 12). The aquifer has an 

average thickness of about 490 ft and is about 57 percent 

sand (table 2). The aquifer is more than 80 percent sand 

in an area of several tens of thousands of square miles 

in the northern Mississippi embayment. The middle 

Claiborne aquifer is less than 20 percent sand in two 

areas, each several thousand square miles in extent, 

located in southwestern Alabama, southern Mississippi, 

and southeastern Louisiana. As defined herein, the 

thickness of the aquifer north of about latitude 35° 

north is less than that shown by Hosman and Weiss 

(1991). The difference is because about the lower onethird of this massive sand unit was included as an 

extension of the underlying lower Claiborne-upper Wilcox aquifer to facilitate the simulation of ground-water 

flow. This subdivision of the permeable units helps 

ensure that the hydraulic-head difference between aquifers is greater than that within the aquifers, especially in 

the vicinity of Memphis, Tenn., where large groundwater withdrawals have created considerable vertical 

hydraulic-head gradients (Weiss and Williamson, 1985).

The lower Claiborne-upper Wilcox aquifer typically 

consists of massive sand beds. It directly overlies the 

middle Wilcox aquifer (table 1) and has an area of about 

80,000 mi . The lower Claiborne-upper Wilcox aquifer 

has an average thickness of about 270 ft and is about 

71 percent sand (table 2). The aquifer is more than 

80 percent sand throughout an area of several tens of 

thousands of mi2 from western Kentucky to southwestern Arkansas. The aquifer is less than 20 percent sand 

only in a small area of about 1,000 mi2 in southeastern 

Mississippi. This aquifer is overlain everywhere by the 

lower Claiborne confining unit except for an area of 

about 20,000 mi2 north of about 35° north latitude in the 

northern part of the Mississippi embayment. The confining unit has an average thickness of about 300 ft 

where it is present in the Mississippi embayment aquifer system.

The middle Wilcox aquifer directly overlies the 

lower Wilcox aquifer (table 1) and has an area of about 

110,000 mi2. The aquifer typically consists of thin alternating beds of sand and clay; the aquifer has an average 

thickness of about 1,000 ft and contains about 34 percent sand (table 2). The aquifer contains more than 

80 percent sand in a small area of less than 1,000 mi2 in 

the northernmost part of the Mississippi embayment. 

The aquifer contains less than 20 percent sand throughout an area of about 15,000 mi2 in southeastern Mississippi and southwestern Alabama.

The lower Wilcox aquifer overlies the Midway 

confining unit (table 1) and has an area of about 

104,000 mi2. The aquifer has an average thickness of 

about 330 ft and is about 67 percent sand (table 2). The 

aquifer is more than 80 percent sand throughout most 

of the northern part of the Mississippi embayment, but 

the sand content gradually decreases to the southeast 

where it is less than 40 percent throughout a large part 

of southern Mississippi.

HYDRAULIC CONDUCTIVITY

Much variability characterizes the permeability of 

aquifers and permeable zones in the study area, as indicated by hydraulic-conductivity data; however, 

hydraulic conductivity generally increases from west to 

east (Prudic, 1991). The geographic areas with the largest geometric mean horizontal hydraulic conductivity 

are centers of deposition by the ancestral Mississippi 

River. The most permeable aquifer is the Mississippi 

River Valley alluvial aquifer, which has a horizontal 

hydraulic conductivity of about 315 ft/d (fig. 13), 

whereas the middle Wilcox aquifer, the least permeable, 

has a horizontal hydraulic conductivity of only 20 ft/d 

(Prudic, 1991).

The variability of horizontal hydraulic conductivity 

among geographic areas, disregarding aquifers, is as 

great as that among aquifers (fig. 13). Prudic (1991) analyzed about 6,600 estimates of horizontal hydraulic conductivity and found statistically significant variability 

of the geometric mean among aquifers, geographic 

areas, and the combination of aquifer and geographic 

area. The values for horizontal hydraulic conductivity 

shown in figure 13 are not for the full thickness of the 

aquifer but more closely represent the horizontal 

hydraulic conductivity of individual sand beds or the 

more permeable parts of the aquifer.

To obtain estimates of horizontal hydraulic conductivity throughout the study area, values of sand percentage mapped by Hosman and Weiss (1991), and 

Weiss (1992) for each permeable zone or aquifer were 

multiplied by the geometric mean hydraulic conductivity of each geographic area in which the aquifer or permeable zone exists. The resulting areal distributions of 

regionalized horizontal hydraulic conductivity are 

shown in figures 33-36 and reflect all sand beds in the 

aquifer. Because the regional aquifers are rarely 100 

percent sand throughout any extensive area, the estimates shown in figures 33-36 are smaller than the horizontal hydraulic-conductivity values shown in figure 

13. The regionalized horizontal hydraulic conductivity
A24 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

300 

250 

> 200 

150 

50 

-

AQUIFER OR PERMEABLE ZONE

D

Miss ssippi River Valley alluvial aquifer in ! M ssissippi Alluvial Plain only. Permeable

1 ff

WINTER NORTHGARDEN EASTERN

-

T

1

zone A elsewhere

| _ | Permeable zone B

Permeable zone C

Permeable zone D

| [ Permeable zone E

| | Upper Cla borne aquifer

[I M ddle Claiborne aquifer

| | Lower Cla borne-upper Wilcox aquifer

[^Pl Middle W Icox aquifer

[ [ Lower Wil

i

-

;ox aquifer

K

-i

l

-

-

I

WESTERN : MISSISSIPPI EASTERN SOUTHERN SOUTH- : SOUTH- EASTERN

EMBAYMENT i ALLUVIAL EMBAYMENT i TEXAS EASTERN : WESTERN

TEXAS PLAIN TEXAS : LOUISIANA

COASTAL

LOWLANDS

GEOGRAPHIC AREAS (See figure 6)

FIGURE 13. Geometric mean horizontal hydraulic conductivity of aquifers and permeable zones by geographic area.

(See figure 6 for location of geographic areas.)

values were the basis for transmissivities used in the 

regional ground-water flow simulations.

GROUND-WATER CHEMISTRY

The chemistry of water from permeable zones and 

aquifers of the gulf coast aquifer systems is highly 

variable as indicated by changes in dissolved-solids 

concentrations (Pettijohn and others, 1988), and water 

types (Pettijohn, Busby, and Beckman, 1992, 1993a, 

1993b, 1993c; Pettijohn, Busby, and Cervantes, 1993a, 

1993b, 1993c; Pettijohn, Busby, and Layman, 1993a, 

1993b, 1993c). Several geochemical processes contribute to the observed variability in water chemistry, 

and the variability is significant with regard to water 

use and regional ground-water flow. Relatively little 

saline ground water is used in the study area (about

210 Mgal/d during 1990), and only a relatively small 

part of each aquifer or permeable zone contains freshwater that is suitable for most uses.

DISSOLVED-SOLIDS CONCENTRATIONS

Water contained in the gulf coast aquifer systems is 

generally fresh in and near outcrop areas of the permeable zones and aquifers. Each permeable zone and aquifer in at least one area contains water with a dissolvedsolids concentration near that of precipitation or only a 

few tens of milligrams per liter. These areas of small 

concentrations of dissolved solids are typically in and 

near the aquifer outcrop area, and water samples are 

from shallow depths. In areas of several thousands of 

square miles, each permeable zone and aquifer contains 

water with dissolved-solids concentrations that range 

from a few tens of milligrams per liter to 500 mg/L.
SUMMARY OF HYDROLOGY OF THE REGIONAL AQUIFER SYSTEMS. SOUTH-CENTRAL UNITED STATES A25

Dissolved-solids concentrations in outcrop areas generally are less than 1,000 mg/L, except for a few 

areas in southern Texas where the dissolved-solids concentrations of water range from about 1,000 mg/L to 

3,000 mg/L (Pettijohn and others, 1988).

Dissolved-solids concentrations in water from all 

aquifers and permeable zones generally increase 

toward the Gulf of Mexico and toward principal discharge areas. Dissolved-solids concentrations typically 

increase from 10,000 to 35,000 mg/L over distances of 

10 to 40 mi. The deeper parts of the permeable zones 

and aquifers contain brine with dissolved-solids concentrations that typically exceed that of seawater by as 

much as a factor of 2. Each permeable zone and aquifer 

underlies at least one area of several tens of square 

miles where the water has a dissolved-solids concentration that ranges from 70,000 to 150,000 mg/L. Exceptions are the Mississippi River Valley alluvial aquifer, 

which is exposed at land surface throughout its areal 

extent, and permeable zone A, which is also at land surface throughout its onshore extent. In contrast, throughout thousands of square miles, permeable zones C, D, 

and E contain water with dissolved-solids concentrations that range from 70,000 to 150,000 mg/L. Permeable zones C, D, and E typically are buried beneath 

several thousand feet of sediments in the areas having 

large dissolved-solids concentrations.

The aquifers of the Mississippi embayment aquifer 

system contain freshwater throughout a greater geographic area than the aquifers in the Texas coastal 

uplands aquifer system. This is primarily because of a 

greater area! extent of the aquifers updip from the 

Vicksburg-Jackson confining unit in the Mississippi 

embayment aquifer system, and perhaps secondarily 

because of much less rainfall in the western part of the 

study area than in the eastern part. The permeable 

zones of the coastal lowlands aquifer system in southern Texas have only small areas of a few hundred 

square miles where they contain water with dissolvedsolids concentrations less than 1,000 mg/L. East of the 

Brazos River in Texas, most of the permeable zones contain freshwater, or water with dissolved-solids concentrations less than 1,000 mg/L, throughout thousands of 

square miles.

WATER TYPES

The water type in and near aquifer outcrop areas 

typically is calcium bicarbonate or calcium bicarbonate 

and sodium bicarbonate; locally, other water types are 

present. The water type is dominantly sodium bicarbonate in middip areas of the aquifers. The dominant 

water type typically is sodium chloride near the downdip limit of the aquifers and permeable zones. The variability of water type is demonstrated by the areal 

distribution of water types in permeable zone B of the 

coastal lowlands aquifer system (fig. 14) and the middle 

Claiborne aquifer of the Texas coastal uplands and Mississippi embayment aquifer system (fig. 15).

The dominant water type is sodium bicarbonate in 

the outcrop area of permeable zone B across Mississippi 

and Louisiana (fig. 14). The dominant water types in 

outcrop areas of the permeable zone are calcium bicarbonate in southeastern Texas and sodium chloride in 

southern Texas. Sodium bicarbonate is the dominant 

water type downdip from the outcrop area of permeable zone B throughout several thousands of square 

miles in Mississippi and Louisiana. About midway 

between the area of outcrop and the downdip limit of 

the permeable zone across southern Louisiana, sodium 

chloride is the dominant water type. Permeable zone B 

contains several water types downdip from the outcrop 

area in Texas, commonly calcium bicarbonate and 

sodium bicarbonate in southeastern Texas and sodium 

chloride in southern Texas.

Several water types are present in the outcrop area 

of the middle Claiborne aquifer (fig. 15). The aquifer 

typically contains calcium bicarbonate and sodium 

bicarbonate water types in outcrop areas of the Mississippi embayment aquifer system. Sodium sulfate and 

sodium chloride water types are common in addition to 

calcium bicarbonate and sodium bicarbonate where the 

aquifer outcrops as part of the Texas coastal uplands 

aquifer system. Sodium bicarbonate is the dominant 

water type downdip from the outcrop area of the aquifer in the Mississippi embayment aquifer system except 

in southern Mississippi, where sodium chloride is the 

dominant water type. Sodium bicarbonate is the dominant water type downdip from the aquifer outcrop area 

in southeastern Texas, but in southern Texas the aquifer 

contains sodium bicarbonate, sodium chloride, and 

sodium sulfate water types. Sodium chloride is the 

dominant water type at all locations near the downdip 

limit of the aquifer.

GEOCHEMICAL PROCESSES

A variety of geochemical processes affect the 

water chemistry in the gulf coast aquifer systems. The 

major postulated processes are (1) leaching of soluble 

salts from the unsaturated zone, (2) alteration of 

albite, (3) cation exchange, (4) mixing of water by the 

vertical flow of water from underlying units, and (5) the 

dissolution of halite from salt diapirs (Pettijohn, 1996). 

The leaching of soluble salts from the unsaturated zone 

is a process that is limited to the outcrop area of permeable zones and aquifers. The alteration of albite typi-
A26 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

EXPLANATION

Water type

^ Calcium bicarbonate * Sodium bicarbonate 

 Calcium chloride Sodium chloride 

^ Magnesium bicarbonate * Sodium sulfate

| | Outcrop area of permeable zone B

 Downdip limit of permeable zone is edge of Continental Shelf

Study area boundary

50 100 150 KILOMETERS

Base from U.S. Geological Survey 

Scale 1:2,500,000

FIGURE 14. Dominant water types in permeable zone B (lower Pleistocene-upper Pliocene deposits), coastal lowlands aquifer system.
SUMMARY OF HYDROLOGY OF THE REGIONAL AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES A27

Water type

+ Calcium bicarbonate

 Calcium chloride

* Calcium sulfate

EXPLANATION

* Magnesium bicarbonate

 Magnesium chloride

v Magnesium sulfate

v 37°

KENTUCKY

* Sodium bicarbonate

* Sodium chloride

* Sodium sulfate

| | Outcrop area of the Middle Claiborne aquifer Dashed where concealed

 Downdip limit of aquifer

- - - - Boundary of Mississippi embayment and Texas Coastal Uplands 

aquifer system

nf*\ 1^ \. Vv r {J/

50 100 150 KILOMETERS

Base from U.S. Geological Survey 

Scale 1.2,500,000

FIGURE 15. Dominant water types in the middle Claiborne aquifer, Texas coastal uplands and 

Mississippi embayment aquifer systems.
A28 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

cally is an important process downdip from the outcrop 

area of a permeable zone or aquifer throughout several 

thousands of square miles; alteration of albite may also 

be important near the downdip limit of the outcrop area 

if the permeable zone or aquifer is thick. Cation 

exchange is important in permeable zones and aquifers 

when water with moderately large concentrations of 

calcium comes in contact with interbedded clays that 

have exchange sites saturated with sodium. Vertical 

flow upward from underlying permeable zones or aquifers becomes an important process when the dissolvedsolids concentration of water in the underlying unit is 

much larger than that of water in the receiving permeable zone or aquifer, and the volume of vertical flow is 

10 percent or more of the volume of horizontal flow. 

The dissolution of halite is an important process in 

areas where salt diapirs penetrate a permeable zone or 

aquifer at depths greater than 4,000 ft. At shallow 

depths (less than about 4,000 ft in the Gulf Coastal 

Plain) salt diapirs commonly are sealed by a relatively 

impervious cap rock derived from an accumulation of 

insoluble residues precipitated from ground water acting on the diapir's salt mass (Halbouty, 1979) in the geologic past; therefore; currently, dissolution of halite is 

minimal.

The major geochemical processes leave a typical 

imprint on the water, either increasing or decreasing 

certain ions or groups of ions. The leaching of soluble 

salts from the unsaturated zone results in an increase in 

the concentration of most major chemical ions. The 

alteration of albite results in an increase in sodium and 

bicarbonate ions and an increase in pH. Cation 

exchange results in the removal of the calcium ion from 

solution and the addition of the sodium ion as calcium 

replaces sodium on the clay colloids. The mixing of 

water that moves from an underlying aquifer or permeable zone with water moving horizontally in the receiving permeable zone or aquifer results primarily in an 

increase in the sodium and chloride ion because water 

in deeper sediments typically have larger concentrations of sodium and chloride. The dissolution of halite 

from salt diapirs results in an increase in the concentration of the sodium and chloride ion because the principal component of the salt diapirs is sodium chloride.

HISTORY OF GROUND-WATER 

DEVELOPMENT

Ground-water withdrawals for cities or for agricultural use began during the late 1800's at many locations 

in the Gulf Coastal Plain. By 1900 the city of Memphis, 

Tennessee, was withdrawing about 30 Mgal/d and

Houston, Texas, was withdrawing about 20 Mgal/d. 

Withdrawal of ground water for irrigation of rice began 

west of Houston during the 1890's and in southwestern 

Louisiana and central Arkansas in the early 1900's. 

Pumpage has continued to increase at uneven rates 

throughout the Coastal Plain. Increases in population, 

development and growth of industry, and expansion of 

irrigated agriculture have been the primary factors contributing to increases in ground-water withdrawals. 

The following discussion will focus briefly on trends in 

ground-water withdrawals from the perspective of the 

total study area and individual aquifer systems, then 

shift to a more detailed look at trends for four cities and 

for three areas with large-scale irrigated agriculture. 

These four cities and three irrigated areas accounted for 

about 80 percent of the ground water withdrawn in the 

study area during 1985.

Estimates of ground-water withdrawals for the 

entire study area have been compiled for the period 

1960-90, at 5-year intervals. Withdrawals during 1960 

were about 4,000 Mgal/d and increased steadily to 

almost 10,000 Mgal/d during 1980 (fig. 16). Withdrawals decreased about 7 percent from 1980 to 1985, and 

increased about 20 percent (to about 10,900 Mgal/d) 

from 1985 to 1990. The quantity of ground water withdrawn for public supply increased continually from 

1960 to 1990, whereas ground-water withdrawals for 

industrial purposes decreased continually from 1975 to 

1985 but increased slightly by 1990. Withdrawals for 

irrigation increased by a factor of about 3 during the 

period 1960-80, increasing from 2,200 to 7,200 Mgal/d.

12,000

CO 10,000

z 

o

8,000

? 6,000

=> 4,000

2,000

Irrigation 

Industrial 

Public Supply

1960 1965 1975 1980 1985

FIGURE 16. Ground-water pumpage by major 

category of use, 1960-90.
SUMMARY OF HYDROLOGY OF THE REGIONAL AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES A29

However, irrigation withdrawals decreased about 7 

percent from 1980 to 1985 but increased again from 1985 

to 1990, reaching about 8,400 Mgal/d.

1 1 ,UUU

10,000

> 9,000 

Q

DC

LU

w 8,000

z 

g

0 7,000

z 

o

j| 6,000

z

i±T 

0 5,000

D.

£ 4,000

LU

d) 3,000

z 

O 

0 2,000

1,000

n

CU 1960

CH 1965

- I I 1970

CH 1975

I I 1980

- L7H 1985 

I I 1990

-

-

-

_

_

-

-

rfTTTTfl

nl|

I

[ in

-

r

"

-

.,

, -

-

-

.

_

-

-

TEXAS MISSISSIPPI COASTAL 

COASTAL EMBAYMENT LOWLANDS 

UPLANDS

FIGURE 17. Ground-water pumpage by 

aquifer system, 1960-90.

TOTAL

60 

o 70

LlDC

=)

w 80

90

100

£ 110

120

130

140

Well Sh: P-76 at Memphis 

Land-surface altitude: 287 feet 

Well depth: 879 feet 

Middle Claiborne aquifer

1925 1931 1937 1943 1949 1955 1961 1967 1973 1979 1985 

FIGURE 18. Water levels in the Memphis area, Tennessee, 1928-90.

1991
A30 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

The distribution of ground-water withdrawals 

among aquifer systems is shown in figure 17. Of the 

three aquifer systems, the Mississippi embayment aquifer system supplied the most ground water for all uses 

during the period 1975-90. However, during the period 

1960-70 the largest ground-water withdrawals were 

from the coastal lowlands aquifer system (fig. 17). Withdrawals from the Texas coastal uplands aquifer system 

were the least for the period 1960-90, ranging from 270 

to 460 Mgal/d. Withdrawals during the period 1960-90 

increased the least from the Texas coastal uplands aquifer system and increased the most from the Mississippi 

embayment aquifer system.

MEMPHIS

The trends in ground-water withdrawals in the 

Memphis area, Tennessee, are related to growth in population and development of industry. Ground-water 

withdrawals at Memphis were about 30 Mgal/d from 

1895 to 1920 and then began to increase at a rate of 

about 2 Mgal/d per year until about 1940. Beginning in 

1946, pumpage at Memphis increased at a rate of about

40

80

UJ

o

2: 120

EC

Q 160

1

§ 200

O

UJ

 240

UJ 

UJ

LL

Z 280 

_i

LU

uj 320

360

400

440

Wells: LJ-65-23-138 and 139 

Land surface altitude: 18 feet 

Screen: 624-805 feet 

Permeable zone B Wells: LJ-65-23-220 

Land-surface altitude: 20 feet 

Screen: 329-465 feet 

Permeable zone A

Well: LJ-65-23-219 

Land surface altitude: 21 feet 

Screen: 698-1,235 feet 

Permeable zone B

1932 

B

1941 1946 1951 1966 1971 1991

2 120

EC

3 

W

Q 160

1

§ 200

O

LJJ

 240

LJJ 

LJJ

LL

Z 280 

_T

LU

uj 320

EC 

LU

< 360 

400 

440

Well: LJ-65-21-805 

Land-surface altitude: 67 feet 

Screen: 706-1,007 feet 

Permeable zone B

Well: LJ-65-21-302 

Land surface altitude: 46 feet 

Screen: 710-1,650 feet 

Permeable zones B and C

1931 1936 1941 1946 1951 1956 1961 1976 1981 1986 1991

FIGURE 19. Water levels in the Houston area, Texas, 1932-92.
SUMMARY OF HYDROLOGY OF THE REGIONAL AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES A31

4 Mgal/d per year until 1974, at which time rates stabilized at between 190 and 200 Mgal/d (Criner and Parks, 

1976) and remained in that range through 1990. Thus, 

water levels that continuously declined as much as 70 ft 

from 1934 to 1974 have stabilized near the center of 

pumping and have fluctuated about 15 ft seasonally 

since the mid-1970's (fig. 18).

HOUSTON

Trends in ground-water withdrawals in the Houston area, Texas, are related to population growth,

development and growth of industry, replacement of 

ground water by surface water in some industrial 

parts of the area, and a shift from irrigated agriculture 

to public supply as a result of urban expansion. 

Ground-water withdrawals more than doubled every 

20 years in the Houston area from 1900 to 1960 (Wood 

and Gabrysch, 1965). By 1985, the combined pumpage 

from Galveston and Harris Counties (includes Houston) was about 390 Mgal/d (Mesko, and others, 1990). 

Until 1954, ground water was the sole source of water 

for the city of Houston. At that time, surface water was 

introduced from the San Jacinto River, beginning 

with about 1 Mgal/d in 1954 and increasing to 24-

65

75

85

95

105

145

Well: OR-42 at New Orleans 

Land-surface altitude: 10 feet 

Screen: 664-775 feet 

Permeable zone A

1946 1950 1954 1958 1962 1966 1970 1974 1978 

FIGURE 20. Water levels in the New Orleans area, Louisiana, 1949-90.

1982 1986 1990
A32 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

29 Mgal/ d by 1967. Since 1968, use of surface water has 

increased periodically and ground-water withdrawals 

have decreased in the industrial district of Houston. 

Water levels stabilized in 1973 and began to recover in 

1977 (fig. 19A) as a result of decreased withdrawals. 

However, ground-water withdrawals have continued 

to increase because of urban development to the north 

and west of Houston. Thus, water levels in these areas 

continued to decline until as recently as 1989 (fig. 19B).

NEW ORLEANS

Trends in ground-water withdrawals in the New 

Orleans area, Louisiana, reflect, almost exclusively, 

changes in industrial water use. During 1990, about 

65 percent of the ground-water withdrawn was used in 

the generation of electricity. The withdrawal of ground 

water in the New Orleans area began as early as the 

mid-1800's with the drilling of the "Canal Street" well in 

1854, and many other wells were drilled in the late 

1800's and early 1900's (Harris, 1904). The water has 

been used mostly by industries because the most productive sands yield water that has a yellow color. Based 

on few data, Rollo (1966) estimated that ground-water 

withdrawals were about 5 Mgal/d by 1900 and 

increased at an annual rate of about 0.5 Mgal/d until 

1955 when pumpage was about 35 Mgal/d. Withdrawals increased at an annual rate of about 1.5 Mgal/d from 

1955 to 1970 when pumpage was about 55 Mgal/d. 

Withdrawals decreased from 1970 at an annual rate of 

slightly more than 1 Mgal/d to 1990 when pumpage 

was about 33 Mgal / d. Thus water levels near the center 

of pumping declined for a long period of time and then 

recovered during the late 1970's to levels observed during the 1950's (fig. 20).

BATON ROUGE

Trends in ground-water withdrawals in the 

Baton Rouge area, Louisiana, reflect growth in population and industry and a shift in pumpage from 

shallower to greater depths as demand for water 

increased. Large withdrawals of ground-water began

Well:EB-15

Land-surface altitude: 65 feet 

Screen: 320-679 feet 

Permeable zone A

Well: EB-78

Land-surface altitude: 65 feet 

Screen: 332-423 feet 

Permeable zone A

i i i i i i i i

Well: EB-870

Land-surface altitude: 50 feet 

Screen: 687-692 feet 

Permeable zone A

i i i i

Well: EB-90 

Land-surface altitude: 59 feet 

Screen: 2025-2120 feet 

Permeable zone C

i i i I i i i i I i i i i

1945 1955 1960 1965 1970 1975 1980 1985 1990

FIGURE 21. Water levels in the Baton Rouge area, Louisiana, 1940-90.
SUMMARY OF HYDROLOGY OF THE REGIONAL AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES A33

in the Baton Rouge area with the construction of a well 

drilled for public supply in 1892 (Harris, 1904), and 

withdrawals were estimated at 2 Mgal/d by 1900 

(Meyer and Turcan, 1955). Withdrawals increased to 

an estimated 10 Mgal/d by 1920. Three periods of rapidly increasing withdrawals, 1930-40, 1950-55, and 

1965-70, resulted in an increase in pumpage from about 

12 Mgal/d in the 1930's to 140 Mgal/d during 1970. 

Since 1970, pumpage has varied between 120 and 150 

Mgal/d. An increasingly larger percentage of pumpage 

shifted from the shallower sands to the deeper sands 

since 1950; thus, water-level declines in the shallower 

sands of permeable zone A have recovered to levels 

existing prior to the 1940's, as indicated by the water 

levels shown in figure 21.

MISSISSIPPI ALLUVIAL PLAIN

Trends in ground-water withdrawals in the Mississippi Alluvial Plain mostly reflect an increase in irrigation of rice, although about 5 percent of the volume 

withdrawn during the 1980's was for industry and public supply and an increasing volume has been withdrawn for aquaculture or fish farming. The withdrawal 

of large quantities of ground water for the irrigation of 

rice in the Mississippi Alluvial Plain north of about 

Natchez, Mississippi, followed the successful planting 

and harvesting of 80 acres of rice in Lonoke County, 

Arkansas, during 1904 (Stephenson and Crider, 1916). 

By 1910, about 75 Mgal/d was withdrawn from the 

Mississippi River Valley alluvial aquifer for the irrigation of rice in Arkansas (Engler and others, 1963). The

-3 130

110

Arkansas County, Arkansas

1929 1932 1935 1938 1941 1944 1947 1950 1953 1956 1959 1962

200

190

170

O 160

140

120

100

90

80

Bolivar County, Mississippi

^ Sr

LU >

HO 

< -i

> LU

> m

20

30

50

60

1953 1956 1959 1962 1965 1968 1971 1974 1977 1980 1983 1986

FIGURE 22. Water levels in the Mississippi Alluvial Plain, 1929-87.
A34 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

average annual withdrawal was about 180 Mgal/d 

from 1912 through 1941. In 1942, when the first of two 

periods of rapid increase in ground-water use for rice 

irrigation began, withdrawals increased annually at 

about 70 Mgal/d until 1954, then remained relatively 

constant at about 1,600 Mgal/d until the early 1970's. It 

was during this period of rapid increases in rice acreage, and consequently increasing ground-water withdrawals, that rice began to become an important crop in 

States underlain by the Mississippi River Valley alluvial 

aquifer adjacent to Arkansas. Withdrawals increased 

annually at about 340 Mgal/d from the early 1970's to 

the early 1980's during the second period of rapid 

increase in ground-water use for irrigation of rice. 

Withdrawals have continued at about 5,000 Mgal/d 

from the early 1980's through 1990. Thus, water levels 

near the center of the early withdrawals in Arkansas

declined over the years until they stabilized in the midto late 1950's and have fluctuated about 5 ft seasonally 

since about the mid-1950's (fig. 22A). In other areas, 

declines have continued as ground-water was withdrawn to support the expansion of rice acreage (fig. 

22£>, C). A decrease in saturated thickness of 30 ft or 

more has occurred in eastern and central Arkansas 

(Ackerman, 1996).

SOUTHWESTERN LOUISIANA

Trends in ground-water withdrawals in southwestern Louisiana reflect growth in population and the 

irrigation of rice and large increases in industrial 

use up to the mid-1970's followed by reductions in 

industrial use of ground water. Several hundred wells 

were used to withdraw ground water for the irrigation

I 80

100

120

Well: JD-9

Land-surface altitude: 24 feet 

Screen: 238-318 feet 

Permeable zone A

Well: CU-445

Land-surface altitude: 12 feet 

Screen: 460-540 feet 

Permeable zone A

1948 1953 1958 1963 1968 1973 1978 

FIGURE 23. Water levels in southwestern Louisiana, 1938-90.

1983
SUMMARY OF HYDROLOGY OF THE REGIONAL AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES A35

of rice in southwestern Louisiana by 1903 (Fuller, 1904), 

but no estimate was made of the quantity withdrawn. 

During the early 1900's, about 300,000 acres of rice 

were cultivated in southwestern Louisiana; by 1946 

about 600,000 acres of rice were cultivated, and 

about 240,000 acres were irrigated with ground water 

(Jones and others, 1956). The quantity of ground water 

withdrawn during 1947 for rice irrigation was about 

280Mgal/d (Jones and others, 1956); this quantity had 

increased to about 800 Mgal/d by 1980 (Mesko and 

others, 1990). Withdrawals for irrigation averaged 

about 450 Mgal/d for the period 1952-61 and 

about 670 Mgal/d for the period 1962-90. About 

100 Mgal/d of ground water was withdrawn during 

1947 for other uses, such as public supply and industry, 

and this quantity increased to about 300 Mgal/d by 

1975. Ground water withdrawn for industrial use 

decreased about 50 percent from 1975 to 1985. 

Total ground-water withdrawals decreased from 

about 1,100 Mgal/d during 1980 to about 810 Mgal/d 

during 1990. Thus, water levels near the centers of 

pumping for industrial use (well CU-445, fig. 23) show 

a long-term decline until about 1970 when they became 

relatively stable. Water levels recovered substantially

during 1982 and remained relatively stable through 

1990. Water-level changes were much less dramatic at 

locations remote from the centers of pumping for industrial use (well JD-9, fig. 23). The decrease in total 

ground-water withdrawals for 1980-90 may be the 

beginning of a reversal in the trend of increasing withdrawals that began early in the 20th century. However, 

many factors affect ground-water withdrawals, especially ground water used for irrigation. For example, 

during 1951 (a period of less than normal rainfall in 

southwestern Louisiana) about 70 Mgal/d more ground 

water was withdrawn than in 1985, a period of nearly 

normal rainfall.

WINTER GARDEN

Trends in ground-water withdrawals in the Winter 

Garden area, Texas, are mostly related to two periods of 

growth in irrigation, largely of vegetable crops, and 

recent fluctuations related to weather, and perhaps to 

economic conditions. Ground-water withdrawals for 

irrigation in the Winter Garden area began with the 

completion of the first flowing well at Carrizo Springs, 

Texas, in 1884 (Turner and others, 1960). The quantity

560

540

520

O

5 500

LU 460

I

440

420

Well: HZ-77-42-801

Land-surface altitude: 613 feet

Screen: 698-1,235 feet

Upper Claiborne-lower Wilcox aquifer

1929 1934 1939 1949 1954 1959 1964 1969 1974 1984 1989

FIGURE 24. Water levels in the Winter Garden area. Texas, 1929-86.
A36 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

of water withdrawn throughout the first three 

decades of the 20th century probably increased, based 

on the reported increase in the number of wells (Taylor, 

1907). However, no estimates of total use were available until about 1930, when about 36 Mgal/d was withdrawn for irrigation, public supply, and industrial 

use (Sundstrom and Follett, 1950; Turner and others, 

1960). Withdrawals increased to about 80 Mgal/d by 

1950 and to about 230 Mgal/d by 1965. Since 1965, 

withdrawals have varied between 220 and 280 Mgal / d. 

Thus, water levels near the center of pumping show 

a long-term decline until the late 1960's and a relatively stable period for 1965-90 (fig. 24). Water levels in 

and near the aquifer outcrop area are still declining and 

may continue to do so for many years.

REGIONAL GROUND-WATER FLOW

Regional ground-water flow patterns have been 

changed greatly by the withdrawal of large quantities 

of ground water. These changes are best demonstrated 

by comparison of the results from several simulations of 

ground-water flow. Thus, most of the following discussion is summarized from several ground-water flow 

simulations that were done as part of the Gulf Coast 

RASA study and reported by Ackerman (1989a, 1996), 

Arthur and Taylor (1990; in press), Brahana and Mesko 

(1988; in press), Martin and Whiteman (1989; in press), 

Ryder (1988), Ryder and Ardis (in press), Williamson 

and others (1990), and Williamson and Grubb (in press). 

Few data are available to define the areal distribution of 

the potentiometric surfaces of the aquifers and permeable zones prior to development (that is prior to withdrawal of substantial quantities of ground water, also 

referred to herein as predevelopment). Therefore, predevelopment potentiometric surfaces were simulated

EXPLANATION

I I Recharge area 

| j Discharge area

| | Outcrop of Vicksburg-Jackson 

confining unit

 2,000 Line of equal thickness Shows 

thickness of coastal lowlands 

aquifer system, in feet. Interval is 

variable

- - - - Northern limit of area where 

geopressured zone is above 

top of Vicksburg-Jackson 

confining unit

I /AIABAMA

Boundary of study area

Base from U.S. Geological Survey 

Scale 1:2,500,000

Edge of

Continental

Shelf

100 200 KILOMETERS

FIGURE 25. Predevelopment regional recharge and discharge areas, and thickness: coastal lowlands aquifer system.
SUMMARY OF HYDROLOGY OF THE REGIONAL AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES A37

and used as a basis for comparison with potentiometric 

surfaces of the 1980's.

The digital computer models used to simulate 

ground-water flow are of both regional and subregional 

scale. The regional-scale model consists of a rectangular 

grid divided into squares 10 mi on each side, and the 

subregional-scale models have rectangular grids 

divided into squares 5 mi on each side. Four of the 5-mi 

squares therefore cover the same area as one square 

in the regional model. The regional-scale model 

simulated flow of variable-density ground water in 

10 regional aquifers, whereas the subregional-scale 

models generally simulated only the freshwater part of 

one of the aquifer systems in fewer regional aquifers. 

These models simulated the regional flow but did not 

simulate local flow, water that enters and leaves the 

ground-water system within the area of a single model 

grid. A detailed discussion of model construction and 

underlying assumptions are included in the references 

cited in the previous paragraph.

PREDEVELOPMENT

Predevelopment ground-water flow was controlled 

by several factors in the Gulf Coastal Plain. Chief 

among these factors are topography, permeability, and 

the geometry of the aquifers and confining units. The 

predevelopment ground-water flow, based upon results 

from simulation, will be discussed first in a general

EXPLANATION

| | Recharge area 

I | Discharge area

| | Outcrop of Vicksburg-Jackson 

confining unit

......... Downdip limit of aquifer system

- 7,000 - Line of equal thickness Shows 

thickness of Texas coastal uplands 

aquifer system, in feet. Interval is 

variable

Boundary of study area

Boundary between aquifer systems 

From Hosman and Weiss, 1991

way, highlighting instances where the major controlling 

factors are best illustrated from an aquifer-system perspective.

RECHARGE AND DISCHARGE

Simulated regional predevelopment ground-water 

flow in the coastal lowlands aquifer system generally 

was from areas of high water-level altitudes between 

the major rivers and adjacent to the outcrop band of the 

Vicksburg-Jackson confining unit, to the major rivers 

and to regional discharge areas parallel to the shoreline 

and centered about midway between the recharge areas 

and the shoreline (fig. 25). Major predevelopment 

regional discharge of freshwater to the Gulf of Mexico 

was probably only along the coast of Mississippi, Alabama, and Florida, where areas of high water-level altitudes were within a relatively short distance of the 

shoreline. The extensive flat topography with little 

relief throughout many of the coastal counties of Louisiana and Texas afforded little opportunity for development of other than local ground-water flow systems. 

The freshwater from the distant regional recharge areas 

was largely discharged landward from these coastal 

counties. Consequently, the uppermost sands throughout most of the coastal counties of Louisiana and Texas 

contain slightly saline water.

Simulated predevelopment ground-water flow in 

the Texas coastal uplands aquifer system generally was

28° f,

100 200 MILES

100 200 KILOMETERS

Base from U.S. Geological Survey 26° "I ~ 

Scale 1:2,500,000 98c

FIGURE 26. Predevelopment regional recharge and discharge areas, and thickness: 

Texas coastal uplands aquifer system.
A38 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

EXPLANATION

| | Recharge area 

| | Discharge area

| | Outcrop of Vicksburg-Jackson 

confining unit

...... Downdip limit of aquifer system

- 7,000 - Line of equal thickness Shows 

thickness of Mississippi embayment 

aquifer system, in feet. Interval is 

variable

Updip limit of the Mississippi 

embayment aquifer system

Boundary of study area

Boundary between aquifer systems 

From Hosman and Weiss, 1991

100 200 MILES

100 200 KILOMETERS

Base from U.S. Geological Survey 

Scale 1:2,500,000

FIGURE 27. Predevelopment regional recharge and discharge areas, and thickness: 

Mississippi embayment aquifer system.

from areas of high water-level altitudes between the 

major rivers to narrow discharge areas in the vicinity of 

the rivers (fig. 26). Flow paths in this system typically 

were tens of miles in length. The minimal permeability 

of the Vicksburg-Jackson confining unit, which restricts 

the vertical movement of water from underlying aquifers, and the limited downdip extent of aquifers 

beneath the Vicksburg-Jackson confining unit limit the 

development of flow in aquifers underlying the confining unit.

Generally, regional predevelopment flow in the Mississippi embayment aquifer system was from areas of 

high water-level altitudes along the eastern side of the 

study area to the Mississippi Alluvial Plain in Missouri, 

Arkansas, northwestern Mississippi, and northeastern 

Louisiana (fig. 27). Some flow paths may have been as 

long as 200 to 300 mi. For example, water moved from 

aquifer outcrop areas in east-central Mississippi southward beneath the Vicksburg-Jackson confining unit and 

then northwestward toward a large regional discharge 

area in northeastern Louisiana. The minimal permeability of the Vicksburg-Jackson confining unit, in combination with low land-surface altitudes in northeastern 

Louisiana (on the western flank of a northward extension of the confining unit), accounts for these long flow 

paths.

FLOW PATTERNS

Predevelopment horizontal flow patterns were very 

complex and, as noted above, extended for tens of miles 

to hundreds of miles from recharge areas to discharge 

areas. Predevelopment horizontal flow ranged widely, 

as indicated by the results of ground-water flow simulation (figs. 28-31). Vertical flow generally was downward in aquifer outcrop areas and nearby areas that 

have high land-surface altitudes. Ground-water flow 

simulation indicates that most of the area underlain by 

an aquifer or permeable zone had upward vertical flow. 

Very large quantities of water flowed vertically from 

one permeable zone or aquifer to other permeable zones 

or aquifers. Following is a detailed look at the predevelopment flow patterns in permeable zone B of the coastal 

lowlands aquifer system and the middle Claiborne 

aquifer of the combined Texas coastal uplands and Mississippi embayment aquifer systems.

PERMEABLE ZONE B

The largest predevelopment horizontal flows in permeable zone B (lower Pleistocene-upper Pliocene 

deposits) were in southern Texas, southeastern Texas, 

southwestern Mississippi, and west-central Louisiana 

(fig. 29). Flow direction in southern Texas was from the 

outcrop area eastward toward the coast. East of the out-
SUMMARY OF HYDROLOGY OF THE REGIONAL AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES A39

- -700 -

EXPLANATION

Generalized land-surface altitude of outcrop, in feet

Less than 25 

25 to 50 

50 to 100 

100 to 200

200 to 300 

300 to 400 

400 to 500

Area where simulated predevelopment vertical flow relative to base 

of permeable zone was downward

Horizontal flow, in million gallons per day Size of arrowhead indicates 

range of flow. Length of shaft indicates flow within stated range

1.0 to 10

 37°

KENTUCKY

Less than 0.1 0.1 to 1.0 More than 10

Structure contour Shows altitude of top of aquifer. Contour interval, in 

feet, is variable. Vertical datum is sea level

Depth-averaged dissolved-solids concentration greater than 

35,000 milligrams per liter downdip from line

Downdip limit of permeable zone

Location of salt domes that penetrate base of permeable zone

... " -3, -r KINDS ("WKK1N, ( f.\ O .

Jft .<^ MISSISSIPPI s*H JASPER %>5J 

»"/ ?V,- "Tun *4 ' >, 'f'

FLORIDA

Geometry from Weiss, 1992 

Dissolved solids from Pettijohn

and others, 1988 

Land-surface altitude from

Godson, 1981 

Ground-water from Williamson

and Grubb, in press 

Salt dome locations from

Beckman and Williamson, 1990

200 MILES

99 200 KILOMETERS

Base from U.S. Geological Survey 

Scale 1-2,500,000

FIGURE 28. Direction and relative magnitude of simulated predevelopment horizontal flow, area where vertical flow relative to

base of permeable zone was downward, altitude and configuration of top, depth-averaged dissolved-solids concentration,

and location of salt domes that penetrate base of permeable zone A, coastal lowlands aquifer system, and the

Mississippi River Valley alluvial aquifer.
A40 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

- -500 -

EXPLANATION

Generalized land-surface altitude of outcrop, in feet

I I Less than 25 I^Ji 200 to 300

I I 25 to 50 MM 300 to 400

I ...I 50 to 100 ^^V 400 to 500

I 3 100 to 200

Area where simulated predevelopment vertical flow relative to base 

of permeable zone was downward

Horizontal flow, in million gallons per day Size of arrowhead indicates 

range of flow. Length of shaft indicates flow within stated range

1.0 to 10

, 37°

 KENTUCKY

- Less than 0.1 0.1 to 1.0 More than 10

Structure contour Shows altitude of top of aquifer. Contour interval, in 

feet, is variable. Vertical datum is sea level

Depth-averaged dissolved-solids concentration greater than 

35,000 milligrams per liter downdip from line

Downdip limit of permeable zone

Location of salt domes that penetrate base of permeable zone

Study area boundary

 31°

-R.ORIDA

Geometry from Weiss, 1992 

Dissolved solids from Pettijohn

and others, 1988 

Land-surface altitude from

Godson, 1981 

Ground-water from Williamson

and Grubb, in press 

Salt dome locations from

Beckman and Williamson, 1990

200 MILES

100 150 200 KILOMETERS

Base from U.S. Geological Survey 

Scale 1 -2.500.000

FIGURE 29. Direction and relative magnitude of simulated predevelopment horizontal flow, area where vertical flow relative to

base of permeable zone was downward, altitude and configuration of top, depth-averaged dissolved-solids concentration,

and location of salt domes that penetrate base of permeable zone B, coastal lowlands aquifer system.
SUMMARY OF HYDROLOGY OF THE REGIONAL AQUIFER SYSTEMS. SOUTH-CENTRAL UNITED STATES A41

EXPLANATION

Generalized land-surface altitude of outcrop, in feet

25 to 50 

50 to 100 

100 to 200 

200 to 300

113 300 to 400

 400 to 500

 500 to 600

600 to 700 

700 to 800 

More than 800

| | Area where simulated predevelopment vertical flow relative to base 

of permeable zone was downward

Horizontal flow, in million gallons per day Size of arrowhead indicates 

range of flow. Length of shaft indicates flow within stated range

Less than 0.1 0.1 to 1.0 1.0 to 10 More than 10

 -1,000 Structure contour Shows altitude of top of aquifer. Contour interval, in 

feet, is variable. Vertical datum is sea level

 36°

Depth-averaged dissolved-solids concentration greater than 

35,000 milligrams per liter downdip from line

Downdip limit of permeable zone

Location of salt domes that penetrate base of permeabl

Study area boundary

Beckman and Williamson, 1990

100 150 200 KILOMETERS

Base from U.S. Geological Survey 

Scale 1:2,500,000

FIGURE 30. Direction and relative magnitude of simulated predevelopment horizontal flow, area where vertical flow relative to

base of permeable zone was downward, altitude and configuration of top, depth-averaged dissolved-solids concentration,

and location of salt domes that penetrate base of permeable zone D, coastal lowlands aquifer system.
A42 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

 700-

EXPLANATION

Generalized land-surface altitude of outcrop, in feet

50 to 100 

100 to 200 

200 to 300

300 to 400 

400 to 500 

500 to 600

600 to 700 

700 to 800

Area where simulated predevelopment vertical flow relative to base 

of permeable zone was downward

Horizontal flow, in million gallons per day Size of arrowhead indicates 

range of flow. Length of shaft indicates flow within stated range

Less than 0.1 0.1 to 1.0 1.0 to 10 More than 10

Structure contour Shows altitude of top of aquifer. Contour interval, in 

feet, is variable. Vertical datum is sea level

 Depth-averaged dissolved-solids concentration greater than 

35,000 milligrams per liter downdip from line

 Downdip limit of permeable zone

Boundary of Mississippi embayment and Texas Coastal Uplands 

aquifer system

Location of salt domes that penetrate 

base of permeable zone

Study area boundary

Geometry from Weiss, 1992 

Dissolved solids from Pettijohn

and others, 1988 

Land-surface altitude from

Godson, 1981 

Ground-water from Williamson

and Grubb, in press 

Salt dome locations from

Beckman and Williamson, 1990

100 150 200 KILOMETERS

Base from U.S. Geological Survey 

Scale 1:2,500,000

FIGURE 31. Direction and relative magnitude of simulated predevelopment horizontal flow, area where vertical flow relative to

base of permeable zone was downward, altitude and configuration of top, depth-averaged dissolved-solids concentration,

and location of salt domes that penetrate base of middle Claiborne aquifer,

Texas coastal uplands and Mississippi embayment aquifer systems.
SUMMARY OF HYDROLOGY OF THE REGIONAL AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES A43

crop areas, horizontal flow decreased substantially 

throughout most of the coastal counties where land-surface altitudes are generally less than 50 ft above sea 

level. The small flow vectors at the shoreline (fig. 29) 

indicate that most of the freshwater had been discharged from the aquifer. Water moved upward all 

along the flow paths from a short distance downdip 

from the outcrop area to the shoreline. At the shoreline, 

eastward-flowing freshwater met westward-moving 

saline water. The horizontal flow of saline water offshore was less than the flow of freshwater onshore, as 

indicated by the much smaller flow vectors offshore on 

the Continental Shelf than the flow vectors onshore.

Horizontal flow outward from the outcrop area of 

permeable zone B in southeastern Texas was southeastward toward the coast. Horizontal flow decreased substantially where land-surface altitudes are generally 

less than 50 ft above sea level, about 30 to 40 mi inland 

from the shoreline. Vertical flow was upward to the 

overlying permeable zone along most of the flow 

paths from an area just gulfward from the outcrop area 

to the shoreline. Small horizontal flow radiated outward from two bodies of dense water offshore from 

Texas. The dense water is approximately outlined by 

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

shown in figure 29.

The large horizontal flows in southwestern Mississippi radiated outward from the outcrop to the west, 

southwest, and south. Flow decreased substantially at 

the Mississippi River to the west and southwest where 

the river flows along the eastern side of the Mississippi 

Alluvial Plain. All flow to the west discharged upward 

to the overlying permeable zone within about 10 mi of 

the Mississippi River, as indicated by the meeting of 

eastward- and westward-pointing flow vectors (fig. 29). 

Flow decreased substantially to the south about 70 to 

100 mi from the shoreline where land-surface altitudes 

are typically less than 50 ft above sea level. Northward 

flow from the Continental Shelf met the southward flow 

as much as 50 mi inland from the coast. The northward 

flow was associated with a large body of dense water in 

this permeable zone that extended throughout most of 

offshore Louisiana and most coastal parishes of southeastern Louisiana. This body of dense water is approximately outlined by the line marking a dissolved-solids 

concentration of 35,000 mg/L in figure 29.

Large horizontal flow in west-central Louisiana was 

generally to the south and extended throughout an area 

of several hundred square miles from an area of high 

land-surface altitude near the Louisiana-Texas boundary. This area of large horizontal flow was much 

smaller than the other three discussed above, and flow 

decreased substantially within 40 mi of the outcrop

area. Most of the flow had discharged upward to the 

overlying permeable zone inland from the coast where 

flow vectors are about parallel to shoreline (fig. 29).

MIDDLE CLAIBORNE AQUIFER

Large predevelopment horizontal flows in the middle Claiborne aquifer were in southern and northeastern Texas, southern Arkansas and adjacent parts of 

Louisiana, central Mississippi, and western Tennessee 

and adjacent parts of Mississippi (fig. 31). Flow in 

southern Texas was to the east, south, and southwest 

from the outcrop area of the aquifer and decreased substantially near the downdip extent of the aquifer. The 

flow to the south and southwest discharged to the Rio 

Grande, and the flow to the east discharged in the vicinity of the Guadalupe River. Upward flow to the overlying upper Claiborne aquifer occurred all along the flow 

paths; thus, flows were reduced substantially to the east 

and southwest near the downdip limit of the aquifer.

The horizontal flow vectors in northeastern Texas 

(fig. 31) appear to be disordered compared to those in 

southern Texas. Most discharge was to the streams that 

flow across the outcrop area. Long regional flow paths 

were present only where substantially smaller flows 

were parallel to the outcrop area in the confined part of 

the aquifer. Semicircular flow was associated with a 

large body of dense water that extended from Texas 

across adjacent parts of Louisiana into southern Mississippi. The semicircular feature is centered in central 

Louisiana. The body of dense water is approximately 

delineated by the dissolved-solids concentration line of 

35,000 mg/L in figure 31.

Horizontal flow lines from southern Arkansas and 

adjacent parts of Louisiana converged along the Ouachita River and at a large regional discharge area in northeastern Louisiana. Some of the northernmost flow lines 

radiating from the outcrop area of southern Arkansas 

extended southeastward past the Mississippi River and 

converged along a discharge line northwest of the area 

of large horizontal flow in central Mississippi. The discharge line extended into the southernmost part of the 

subcrop of the middle Claiborne aquifer beneath the 

Mississippi River Valley alluvial aquifer. The relatively 

low-lying Mississippi Alluvial Plain extends several 

miles to the east of the Mississippi River in the vicinity 

of the line of discharge. Some of the southernmost flow 

lines radiating outward from the outcrop area of central 

Mississippi crossed the Mississippi River and converged to the regional discharge area in northeastern 

Louisiana.

Flow lines radiating outward from the area of large 

horizontal flow in western Tennessee and adjacent Mississippi extended a relatively short distance to the west
A44 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

WATER TABLE, STREAMS, SWAMPS, AND 

OTHER NEAR-SURFACE FEATURES 2,900

MISSISSIPPI RIVER VALLEY 

ALLUVIAL AQUIFER 7. 8

\£4JouU-

-H240 

- 0.2

370 

-> 8.4

PERMEABLE 

ZONE A

PERMEABLE 

ZONEB

PERMEABLE 

ZONEC

PERMEABLE 

ZONED

PERMEABLE 

ZONEE

UPPER CLAIBORNE AQUIFER

MIDDLE CLAIBORNE AQUIFER

EXPLANATION

Flow between aquifers and 

permeeble zones In million 

gallons per day

LOWER CLAIBORNEUPPER WILCOX AQUIFER

To subcrop 

From subcrop

Flow between the area of subcrop

Discharge R echarge of aquifers and permeable zones

MIDDLE WILCOX AQUIFER

LOWER WILCOX AQUIFER

and the Mississippi River Valley 

alluvial aquifer, and recherge and 

discharge to the outcrop of 

aquifers and permeable zones 

In million gallons per day

Sum of flow rates may not balance 

due to independent rounding

FIGURE 32. Simulated predevelopment ground-water budget from the regional-scale model.

before flow was reduced substantially near the edge of 

the Mississippi Alluvial Plain. The flow from the northernmost part of the area was to the northwest toward 

Missouri. The flow lines extended past the Mississippi 

River into Arkansas before they turned to the west and 

then to the south, where they converged with flow lines 

from the west.

GROUND-WATER BUDGET

The regional ground-water budget for predevelopment (fig. 32) accounts for inflows to and outflows 

from each aquifer or permeable zone. Outflow from 

an aquifer or permeable zone is either (1) discharge to 

the water table or surface-water body within the outcrop area of the same aquifer or permeable zone, or 

(2) flow upward to overlying aquifers or permeable 

zones. Simulated predevelopment recharge was about 

1,300 Mgal/d to the combined Mississippi River Valley alluvial aquifer and permeable zone A and about 

1,600 Mgal/d to all other aquifers and permeable 

zones (fig. 32). The net recharge of about 700 Mgal/d to 

all aquifers and permeable zones except the Mississippi River Valley alluvial aquifer and permeable zone 

A was discharged to permeable zone A and the Mississippi River Valley alluvial aquifer in nearly equal volumes. Net recharge ranged from less than 1 Mgal/d to 

the lower Wilcox aquifer to about 190 Mgal/d to the 

lower Claiborne-upper Wilcox aquifer. Vertical flows 

ranged from less than 1 Mgal/d between the lower 

Wilcox aquifer and the middle Wilcox aquifer to 

about 300 Mgal/d between permeable zone B and permeable zone A (fig. 32).

Net simulated recharge among the permeable zones 

of the coastal lowlands aquifer system was least to permeable zone E and greatest to permeable zone B (Martin and Whiteman, in press; Ryder and Ardis, in press). 

The largest vertical flow between permeable zones was 

upward from permeable zone B to permeable zone A. 

Permeable zone A was a net predevelopment discharge 

area because it underlies a large area where the landsurface altitude commonly is less than 50 ft above sea 

level, and the vertical gradients in the underlying 

permeable zones are predominantly upward.

Net simulated recharge among the aquifers of the 

Texas coastal uplands aquifer system was least to the
SUMMARY OF HYDROLOGY OF THE REGIONAL AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES A45

middle Wilcox aquifer and greatest to the lower Claiborne-upper Wilcox aquifer (Ryder and Ardis, in press). 

The largest vertical flow between aquifers was upward 

from the middle Claiborne aquifer to the upper 

Claiborne aquifer. The outcrop area of the upper Claiborne aquifer was a net discharge area because the predominant vertical flow from all the underlying aquifers 

was upward, and the upper Claiborne aquifer is the 

uppermost aquifer in the aquifer system. Vertical flow 

from the upper Claiborne aquifer to the coastal lowlands aquifer system was restricted to a small quantity 

by the minimal permeability of the Vicksburg-Jackson 

confining unit.

Net simulated recharge among the aquifers of the 

Mississippi embayment aquifer system was least to the 

lower Wilcox aquifer and greatest to the lower Claiborne-upper Wilcox aquifer (Arthur and Taylor, in 

press). The largest vertical flow between aquifers was 

upward from the middle Claiborne aquifer to the upper 

Claiborne aquifer. Most of the net recharge was discharged to the Mississippi River Valley alluvial aquifer 

where the aquifers subcrop; about equal quantities were 

discharged from the upper Claiborne aquifer and all 

other aquifers combined. The large volume of vertical 

flow upward from the upper Claiborne aquifer was 

because of the predominant vertical upward gradient in 

all the aquifers underlying the area where the upper 

Claiborne aquifer subcrops the Mississippi River Valley 

alluvial aquifer. Vertical flow from the upper Claiborne 

aquifer to the coastal lowlands aquifer system was 

restricted to a small quantity by the minimal permeability of the Vicksburg-Jackson confining unit.

EFFECTS OF WATER DENSITY

The simulated flow directions in aquifers and permeable zones are substantially affected by water density that is greater than about 1.025 g/cm. The effects 

such as vertical flows that are downward, increased 

volume of horizontal flow, and semicircular horizontal 

flow patterns are illustrated by comparison of simulated results for permeable zones A, B, and D. The 

effects of water density are more evident for permeable 

zones D, C, E, and B in the coastal lowlands aquifer system and for the middle Wilcox aquifer of the Mississippi embayment aquifer system.

Vertical flow relative to the base of permeable zone 

A was downward through an area of about 12,000 mi2 

offshore from Louisiana and about 8,000 mi2 offshore 

from Texas (fig. 28). Throughout most of the area where 

the flow was downward, a column of seawater (density 

of 1.025 g/cm ) overlies permeable zone A and 

increases in height (up to 300 ft) from the shoreline 

toward the edge of the Continental Shelf. Where the

vertical flow in the offshore area was downward from 

permeable zone A, water density generally was greater 

than 1.000 g/cm3 in permeable zone A and greater than 

1.020 g/cm in permeable zone B. The less dense water 

was discharged from permeable zone A to the Gulf of 

Mexico throughout most of the Continental Shelf where 

vertical flow was not downward relative to the base of 

permeable zone A. The two points offshore where horizontal flow arrows meet in permeable zone A are the 

centers of bodies of dense water in permeable zone B 

(figs. 28 and 29).

Vertical flow relative to the base of permeable zone 

B (fig. 29) was downward through some of the same offshore area as for permeable zone A (fig. 28). However, 

the total area was less, and the volume of vertical flows 

generally were smaller for permeable zone B than for 

permeable zone A. Water density in permeable zone B 

was greater than 1.04 g/cm3 in those areas of offshore 

Louisiana where the downward vertical flows across 

the base of permeable zone B were largest. Horizontal 

flows along flow paths that originated 10 to 20 mi offshore from Galveston Bay in permeable zone B and 

extended to the east and southeast became larger offshore from southwestern Louisiana where they turned 

to the south. A substantial volume of downward vertical flow across the base of permeable zone A was 

present (fig. 28) in the area where horizontal flows 

became larger in permeable zone B (fig. 29).

Vertical flow relative to the base of permeable zone 

D (fig. 30) was downward only onshore because the 

underlying permeable zone E did not extend offshore. 

A body of dense water (generally ranging in density 

from 1.04 to 1.06 g/cm3) extended across southern Louisiana; however, downward flow relative to the base of 

permeable zone D was present throughout an area of 

about 4,500 mi2. The semicircular horizontal flow patterns in this permeable zone (fig. 30) across southern 

Louisiana are coincident with the northern margin of 

the dense water body described above.

FLOW DURING THE 1980's

Several significant changes in ground-water flow 

due to ground-water withdrawals were evident by the 

1980's throughout much of the Gulf Coastal Plain. Many 

of the changes had been in progress for many years but 

reached maximum magnitude by the 1980's. Some 

changes have been documented, such as lowering of 

water levels, land subsidence, loss of saturated thickness, and saltwater encroachment. Other changes, such 

as changes in direction of vertical ground-water flow, 

increases in regional recharge, decreases in regional discharge, and large-scale changes in direction of horizon-
A46 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

tal flow, are not as readily observable by direct 

measurement, but they are nonetheless real and significant to the long-term use and management of the 

ground-water resource. These less obvious changes in 

ground-water flow are best demonstrated by comparing and contrasting predevelopment conditions to conditions during the 1980's. Results from ground-water 

flow models that use regionally consistent data make 

possible the comparisons and contrasts that highlight 

the less obvious changes in the ground-water flow system.

RECHARGE AND DISCHARGE

Most of the onshore predevelopment regional discharge areas had become net regional recharge areas by 

1985. Three notable exceptions are: the valley of the 

Tombigbee River in Alabama, the valley of the Trinity 

River where it crosses the Texas coastal uplands aquifer 

system, and the valley of the Nueces River, which is 

connected to a discharge area along the coast of southern Texas (Williamson and others, 1990).

Many regional discharge areas along the valleys of 

major streams in the coastal lowlands aquifer system 

had become net recharge areas by the 1980's. The 

regional recharge areas across southeastern Texas 

(fig. 25) merged to form one large area of net recharge 

extending from the Guadalupe River eastward to the 

Trinity River. Discharge areas along the Texas streams 

were separated from the coastal discharge area where it 

still existed in 1982 except for the area along the Rio 

Grande (Ryder and Ardis, in press). By 1985, the coastal 

area of southwestern Louisiana that was a discharge 

area under predevelopment conditions (fig. 25) was 

greatly reduced, as the area of recharge expanded to 

encompass most of southwestern Louisiana except for 

short segments of the major stream valleys gulfward 

from the Vicksburg-Jackson confining unit (Martin and 

Whiteman, in press). Most of the Mississippi Alluvial 

Plain from Vicksburg, Mississippi, to the Gulf of Mexico 

had changed from a net regional discharge area under 

predevelopment conditions to a net regional recharge 

area by 1985 (Martin and Whiteman, in press). Discharge to the Pearl, Pascagoula, and Tombigbee Rivers 

continued during 1985.

Discharge to the valleys of the major rivers in the 

Texas coastal uplands aquifer system was the least 

changed by ground-water withdrawal. Although the 

size of the areas adjacent to the streams receiving 

regional predevelopment discharge had, in most 

instances, decreased by the 1980's, they were still net 

discharge areas. An example is a large predevelopment 

discharge area in southern Texas (fig. 26) that decreased 

in size by several tens of square miles due to withdrawal of ground water for irrigation (Ryder and Ardis, 

in press).

Most of the Mississippi Alluvial Plain was a net 

regional recharge area to the Mississippi embayment 

aquifer system by the 1980's. However, discharge continued in an area of Kentucky, Missouri, and Tennessee 

adjacent to the Mississippi River in the northern part of 

the embayment (fig. 27). Also, simulation shows discharge along a short segment of the Ouachita River in 

southern Arkansas and adjacent Louisiana, and along 

the Yazoo River and tributaries of the Pascagoula River 

in Mississippi (Arthur and Taylor, in press).

FLOW PATTERNS

Changes in regional flow patterns from predevelopment have resulted from ground-water withdrawals. 

The most notable are changes in direction of horizontal 

flow paths that extend for hundreds of miles from 

pumping centers, and a reversal in vertical flow direction from vertically upward to vertically downward 

throughout thousands of square miles. These changes 

are shown by comparing figures 28-31 with figures 33- 

36. The flow patterns in permeable zone B and the middle Claiborne aquifer will be used as examples to illustrate some of the changes.

PERMEABLE ZONE B

The largest changes in flow from predevelopment to 

the 1980's in permeable zone B (lower Pleistoceneupper Pliocene deposits) were in southeastern Texas 

and southeastern Louisiana and the adjacent offshore 

area (compare figs. 29 and 34). Ground-water withdrawals in southern Texas and west-central Louisiana 

had caused little change in either direction or quantity 

of flow by the 1980's.

By 1985 horizontal flow in southeastern Texas was 

toward an area with large pumpage centered near 

Houston in Harris County. The predevelopment flow 

direction from the outcrop area toward the gulf was still 

maintained except that near the center of pumpage 

many of the flow lines became parallel to the shoreline 

(fig. 34). Also, the convergence of predevelopment flow 

lines near the shoreline was changed by ground-water 

development. Currently (1985), flow from several tens 

of miles offshore moves toward the center of pumpage 

located several tens of miles inland from the gulf near 

Houston. The quantity of flow just inland from the gulf 

is substantially larger than during predevelopment and 

moves in the opposite direction (that is from the gulf 

toward the center of pumpage). The direction of vertical 

flow relative to the base of permeable zone B has been 

reversed throughout an area of several hundred square 

miles that is coincident with much of the area of
SUMMARY OF HYDROLOGY OF THE REGIONAL AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES A47

EXPLANATION

Horizontal flow, in million gallons per day Size of arrowhead indicates 

range of flow. Length of shaft indicates flow within stated range

Less than 0.1 0.1 to 1.0 1.0 to 10 More than 10

Area where simulated vertical flow relative to base of permeable zone A and 

Mississippi River Valley alluvial aquifer was downward during 1987, a 

reversal in direction from predevelopment conditions

Line of equal horizontal hydraulic conductivity Interval, in feet per day, 

is variable. Hachures indicate closed area of lesser hydraulic conductivity

 Updip boundary of permeable zone

- ~ ~ - Aquifer-system boundary Mississippi River Valley alluvial aquifer north 

of line and permeable zone A south of line

Downdip limit of permeable zone is edge of Continental Shelf

, 37°

KENTUCKY

 200 

Study area boundary

Ground-water flow from 

Williamson and Grubb, 

in press

200 MILES

100 150 200 KILOMETERS

Base from U S. Geological Survey 

Scale 1:2.500,000

FIGURE 33. Regionalized horizontal hydraulic conductivity, areas where simulated vertical flow relative to base of

unit reversed direction from upward to downward (predevelopment to 1987), and 1987 simulated direction

and relative magnitude of horizontal flow in permeable zone A, coastal lowlands aquifer system,

and the Mississippi River Valley alluvial aquifer.
A48 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

EXPLANATION

Horizontal flow, in million gallons per day Size of arrowhead indicates 

range of flow. Length of shaft indicates flow within stated range

Less than 0.1 0.1 to 1.0 1.0 to 10 More than 10

| I Area where simulated vertical flow relative to base of permeable zone B

was downward during 1987, a reversal in direction from predevelopment 

conditions

 70 Line of equal horizontal hydraulic conductivity Interval, in feet per day, 

is variable. Hachures indicate closed area of lesser hydraulic conductivity

 - - Updip boundary of permeable zone Dashed at subcrop or where concealed 92°

35° I1 

Downdip limit of permeable zone is edge of Continental Shelf

 37°

KENTUCKY

96»

Ground-water flow from 

Wilhamson and Grubb, 

in press

200 MILES

150 200 KILOMETERS

Base from U.S. Geological Survey 

Scale 1 -2,500.000

FIGURE 34. Regionalized horizontal hydraulic conductivity, areas where simulated vertical flow relative to base of

unit reversed direction from upward to downward (predevelopment to 1987), and 1987 simulated direction

and relative magnitude of horizontal flow in permeable zone B, coastal lowlands aquifer system.
SUMMARY OF HYDROLOGY OF THE REGIONAL AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES A49

EXPLANATION

Horizontal flow, in million gallons per day Size of arrowhead indicates 

range of flow. Length of shaft indicates flow within stated range

Less than 0.1 0.1 to 1.0 1.0 to 10 More than 10

[ | Area where simulated vertical flow relative to base of permeable zone D

was downward during 1987, a reversal in direction from predevelopment 

conditions

 7 Line of equal horizontal hydraulic conductivity Interval, in feet per day, 

is variable. Hachures indicate closed area of lesser hydraulic conductivity

 - - Updip boundary of permeable zone Dashed at subcrop or where concealed

 Downdip limit of permeable zone

Ground-water flow from 

Williamson and Grubb 

in press

50 100 150 200 KILOMETERS

Base from U.S. Geological Survey 

Scale 1:2,500,000

FIGURE 35. Regionalized horizontal hydraulic conductivity, areas where simulated vertical flow relative to base of

unit reversed direction from upward to downward (predevelopment to 1987), and 1987 simulated direction

and relative magnitude of horizontal flow in permeable zone D, coastal lowlands aquifer system.
A50 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

EXPLANATION

Horizontal flow, in million gallons per day Size of arrowhead indicates 

range of flow. Length of shaft indicates flow within stated range

KENTUCKY

Less than 0.1 0.1 to 1.0 1.0 to 10 More than 10

Area where simulated vertical flow relative to base of middle Claiborne ac 

downward during 1987. A reversal in direction from predevelopment conditions

Line of equal horizontal hydraulic conductivity Interval, in feet per day, is variable. 

Updip boundary of aquifer Dashed at subcrop or where concealed 

Downdip limit of aquifer

Boundary between Mississippi embayment and 

Texas coastal uplands aquifer systems

Ground-water flow from 

Williamson and Grubb, 

in press

200 MILES

100 150 200 KILOMETERS

Base from U.S. Geological Survey 

Scale 1:2,500,000

FIGURE 36. Regionalized horizontal hydraulic conductivity, areas where simulated vertical flow relative to base of

unit reversed direction from upward to downward (predevelopment to 1987), and 1987 simulated direction

and relative magnitude of horizontal flow in middle Claiborne aquifer,

Texas coastal uplands and Mississippi embayment aquifer systems.
SUMMARY OF HYDROLOGY OF THE REGIONAL AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES A51

ground-water withdrawal in southeastern Texas (compare figs. 29 and 34). Under predevelopment conditions, flow was upward from underlying units through 

permeable zone B to permeable zone A and eventually 

discharged to the water table, or to streams, lakes, 

swamps, and other surface-water features. As a result 

of large-scale ground-water pumpage water was flowing downward from permeable zone B by the 1980's in 

order to supply the pumpage from permeable zone C.

Flow toward a center of pumpage at Baton Rouge, 

La., resulted in a change in direction of both horizontal 

and vertical ground-water flow by 1985 (fig. 34). Flow 

from the regional recharge area in southern Mississippi 

and adjacent Louisiana continued outward in the same 

directions as during predevelopment, but pumpage has 

caused flow lines to turn parallel to the coast and 

toward the center of pumpage. Flow lines between the 

gulf and the center of pumpage changed direction due 

to the pumping, and flow from offshore now moves 

toward the center of pumpage at Baton Rouge. Horizontal flow volumes onshore between the pumping 

center and the gulf were substantially larger in 1985 

than in predevelopment. By 1985, the vertical flow of 

water relative to the base of permeable zone B was 

reversed from its predevelopment direction. Pumping 

from the underlying permeable zone C resulted in flow 

from permeable zone B to permeable zone C.

The predevelopment direction of vertical flow 

between permeable zone B and the overlying permeable 

zone A (fig. 33) was also reversed in these two pumping 

centers by ground-water development. Substantial 

quantities of water moved from the water table downward through permeable zone A into permeable zone B 

to supply pumpage from permeable zone B and underlying permeable zones in 1985.

MIDDLE CLAIBORNE AQUIFER

Horizontal flow direction in the middle Claiborne 

aquifer changed very little in the Texas coastal uplands 

aquifer system but changed substantially in the Mississippi embayment aquifer system. Vertical flow relative 

to the base of the aquifer changed direction in southern 

Texas, northeastern Arkansas, southwestern Arkansas 

and adjacent parts of Louisiana, and in local areas in 

Mississippi (compare figs. 31 and 36).

By 1985, the direction of horizontal flow in the middle Claiborne aquifer of the Mississippi embayment 

aquifer system was toward major centers of pumpage in 

extreme southwestern Tennessee, southern Arkansas, 

and northern Louisiana (fig. 36). Flow relative to the 

base of the aquifer in northeastern Arkansas west of the 

pumping center in Tennessee was reversed from predevelopment; by 1985, flow was downward from the middle Claiborne aquifer to the underlying lower 

Claiborne-upper Wilcox aquifer. Some horizontal flow 

paths in the lower Claiborne-upper Wilcox aquifer had 

an eastward component into Tennessee where flow was 

upward to the center of pumpage in the middle Claiborne aquifer. However, some of the areas where flow 

was downward from the middle Claiborne aquifer in 

northeastern Arkansas coincides with an area of downward flow across the base of the lower Claiborne-upper 

Wilcox aquifer to supply pumpage from underlying 

aquifers. The most widespread change in vertical flow 

direction related to pumping in the middle Claiborne 

aquifer occurred in the overlying upper Claiborne aquifer. By 1985, downward flow from the upper Claiborne 

aquifer to the middle Claiborne aquifer extended 

throughout most of the area where the upper Claiborne 

aquifer is present in Arkansas and Tennessee. Under 

predevelopment conditions, the flow was in the opposite direction, from the middle Claiborne aquifer 

upward to the upper Claiborne aquifer.

Flow lines converged toward pumping centers in 

northeastern Louisiana by 1985. The point of convergence had moved to the south and slightly westward 

from the point of convergence under predevelopment 

conditions. Downward flow under predevelopment 

conditions in northwestern Louisiana had been 

reversed to upward flow relative to the base of the 

middle Claiborne aquifer throughout an area of about 

70 mi2 by 1985.

Horizontal flow patterns changed little in the middle Claiborne aquifer of the Texas coastal uplands aquifer system from predevelopment to 1985 because there 

has been minimal pumping from the aquifer. However, 

by 1985 the vertical flow direction throughout an area 

of several hundred square miles in southern Texas had 

been reversed from predevelopment. By 1985, water 

flowed downward from the middle Claiborne aquifer to 

the lower Claiborne-upper Wilcox aquifer because of 

the large withdrawals from the lower Claiborne-upper 

Wilcox aquifer.

GROUND-WATER BUDGET

During 1985, regional recharge increased from 

about 2,900 Mgal/d under simulated predevelopment 

conditions to about 9,600 Mgal/d, and natural discharge decreased from about 2,900 Mgal/d to about 

1,100 Mgal/d during the same time period (compare 

figs. 32 and 37). These changes are due to ground-water 

withdrawals which generally increased from the late 

1800's to about 8,900 Mgal/d during 1985. About 76 

percent of the ground water withdrawn during 1985 

was derived from an increase in recharge, about 21 percent from a decrease in discharge, and about 3 percent
A52 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

WATER TABLE, STREAMS, SWAMPS, AND

OTHER NEAR-SURFACE FEATURES

250

1,100

 H 

280

9,600

MISSISSIPPI RIVER VALLEY 

240 ALLUVIAL AQUIFER

S=+0.5 7.9 

P=5,000

J I

If

4U

If

S=2.0 

P=1,300

=7.1 

=410

S=90 

P=340

S=61 

P=320

S=41 

P=100

S=2.0 

P=510

S=+9.9 

P=410

S=24 

P=120

S=1.5 

P=75

PERMEABLE 

ZONE A

PERMEABLE 

ZONES

PERMEABLE 

ZONEC

PERMEABLE 

ZONED

PERMEABLE 

ZONEE

UPPER CLAIBORNE AQUIFER

MIDDLE CLAIBORNE AQUIFER

LOWER CLAIBORNEUPPERWILCOX AQUIFER

MIDDLE WILCOX AQUIFER From subcrop 

To subcrop

LOWER WILCOX AQUIFER

EXPLANATION

Flow between aquifers and 

permeable zones in million 

gallons per day

Flow between the area of subcrop

Discharge Recharge of aquifers and permeable zones 

and the Mississippi River Valley 

alluvial aquifer, and recharge and 

discharge to the outcrop of 

aquifers and permeable zones 

In million gallons per day

s=+9.9 Reduction in storage In million

gallons per day. Plus sign indicates 

addition to storage

P=410 Pumpage of ground water In

million gallons per day

Sum of flow rates may not balance 

due to independent rounding

FIGURE 37. Simulated 1987 ground-water budget from the regional-scale model.

from loss of aquifer system storage. Net simulated 

recharge to all aquifers and permeable zones except the 

Mississippi River Valley alluvial aquifer and permeable 

zone A was about 2,050 Mgal/d, or an increase from 

predevelopment by a factor of about 5. Net simulated 

recharge to permeable zone A was about 1,480 Mgal/d 

compared to a net predevelopment discharge of about 

300 Mgal/d. The largest change in the ground-water 

budget was for the Mississippi River Valley alluvial 

aquifer, which had a net simulated recharge of about 

5,000 Mgal/d during 1987 compared to a net predevelopment discharge of about 390 Mgal/d. The change 

from net discharge to net recharge was because the Mississippi River Valley alluvial aquifer had more than 

one-half (about 4,900 Mgal/d) of the 1985 pumpage. 

Net recharge to other aquifers and permeable zones 

ranged from about 2 Mgal/d to the lower Wilcox aquifer to as much as 550 Mgal/d to the middle Claiborne 

aquifer. Part of the increase in regional recharge was

derived from capture of discharge from the local flow 

system that was not simulated by the large-scale models. Net vertical flows were downward in all aquifers 

and permeable zones except for the upward flow from 

the upper Claiborne aquifer to permeable zone E of the 

coastal lowlands aquifer system.

With the exception of permeable zone A, net simulated recharge to the permeable zones of the coastal 

lowlands aquifer system during the 1980's was least to 

permeable zone E and most to permeable zone B (Martin and Whiteman, in press; Ryder and Ardis, in press). 

The largest net vertical flow between permeable zones 

was from permeable zone A downward to permeable 

zone B.

Net simulated recharge to the aquifers of the Texas 

coastal uplands aquifer system during the 1980's was 

least to the upper Claiborne aquifer and most to the 

lower Claiborne-upper Wilcox aquifer (Ryder and 

Ardis, in press). The largest net vertical flow between
SUMMARY OF HYDROLOGY OF THE REGIONAL AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES A53

2,000

1,000

IJ -1,000 

-2,000

-3,000

-4,000

-5,000

1-1 

i

! 

! 

! 

!

I I iv

en u 

en R

I I s

en v

Some of 

and othe 

budget.

^

lississip 

nderlyir 

iver leal 

torage 

/ells

values 

the pun 

rs, 1990

T

L-1

pi River Vail 

ig aquifers 

cage

are flow out 

ipage report 

was not incl

p.

n : n

-

BY confining unit

ed by Mesko 

uded n this

1

Prede- 1933 1942 1952 1957 

velopment

1962

If

-

1967

-

¥

1972

Jfl

n 4 "

- 

1977 1982 1987

FIGURE 38. Simulated ground-water budget for the Mississippi River Valley alluvial aquifer, 

predevelopment through 1987.

aquifers was from the middle Claiborne aquifer downward to the lower Claiborne-upper Wilcox aquifer.

Net simulated recharge to the aquifers of the Mississippi embayment aquifer system during the 1980's was 

least to the lower Wilcox aquifer and most to the middle 

Claiborne aquifer (Arthur and Taylor, in press). The 

largest net vertical flow between aquifers was from the 

upper Claiborne aquifer downward to the middle Claiborne aquifer. Net discharge to the Mississippi River 

Valley alluvial aquifer from all aquifers was reduced to 

about 14 percent of the predevelopment discharge, and 

net discharge from the upper Claiborne aquifer was 

reduced to about 3 percent of the predevelopment discharge.

The ground-water budget for the Mississippi River 

Valley alluvial aquifer is of interest because more than 

one-half of the 1985 withdrawals were from this aquifer, and the change in sources of water supplying the 

withdrawals gives a more detailed picture of the 

hydrology than the combined budgets for regional 

aquifer systems. Prior to development of the groundwater supply, the recharge through the surficial Mississippi River Valley confining unit (a locally recognized 

geohydrologic unit) and the flow upward from underlying units was discharged principally to the stream

network of the Mississippi Alluvial Plain (fig. 38). Simulations indicate that discharge to the streams 

decreased as withdrawals increased, and by 1952 the 

volume pumped was about equal to the discharge to 

streams (Ackerman, 1996). As withdrawals continued 

to increase after 1952, the quantity of water discharged 

to streams continued to decrease until sometime 

between 1972 and 1977 when the inflow from the 

streams to the aquifer exceeded aquifer discharge to the 

streams. Recharge increased substantially after 1952 

and was still the largest source of water to the aquifer, 

although the inflow from the streams was almost as 

large (fig. 38).

DECREASE IN SATURATED THICKNESS

Long-term decreases in saturated thickness have 

resulted from the withdrawal of ground water in the 

lower Claiborne-upper Wilcox aquifer in the Winter 

Garden area of southern Texas and in the Mississippi 

River Valley alluvial aquifer in eastern and central 

Arkansas. A rate of decrease in saturated thickness of 

about 1.5 ft/yr in the lower Claiborne-upper Wilcox 

aquifer was reported by Ryder and Ardis (in press) at a 

well in northeastern Zavala County, Texas, during the 

period 1965-85. The total decrease in saturated thick-
A54 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

ness was about 95 ft since withdrawals began. By 

1985, an area of about 240 mi2 is estimated to have 

experienced a substantial decrease in saturated thickness. At this rate of decrease in saturated thickness, a 

total of 125 ft of the original 200 ft of saturated thickness 

would be dewatered within the next 20 years.

A rate of decrease in saturated thickness of 

about 2 ft/yr in the Mississippi River Valley alluvial 

aquifer at some locations in east-central Arkansas during the 1980's was noted by Ackerman (1996). By 1982, 

an area of about 300 mi had simulated decreases in saturated thickness of from 40 to 60 ft since withdrawals 

began. At current (1985) withdrawal rates, the original 

saturated thickness of about 100 ft may be dewatered to 

the point that withdrawal rates cannot be sustained 

within the next 15 to 20 years.

Both areas noted above as having substantial 

decreases in saturated thickness are only a small part of 

the area underlain by the aquifers being pumped. However, they suggest that serious declines in pumpage 

could be caused by decreased saturated thickness 

locally and that perhaps increased effort is needed to 

quantify inflow and outflow at a local scale.

LAND SUBSIDENCE

Land subsidence probably has resulted from withdrawal of ground water throughout thousands of 

square miles of the Gulf Coastal Plain. Locally, up to 

10 ft of subsidence has been measured east of Houston, 

Texas, near Galveston Bay (Gabrysch and Coplin, 1990). 

Subsidence is difficult and expensive to measure; thus, 

it has been measured at only a few locations in the Gulf 

Coastal Plain. However, simulation of ground-water 

flow suggested that as much as one-half foot of land 

subsidence has resulted from ground-water withdrawals throughout tens of thousands of square miles of the 

Gulf Coastal Plain (Carr and others, 1985). The quantity 

of water released from clay beds as they are compacted 

due to the withdrawals from underlying aquifers may 

be a significant proportion of the ground water withdrawn during relatively short time periods of several 

decades. The rate of subsidence has decreased substantially in the area of maximum subsidence near Houston 

due to large decreases in pumpage. To the north and 

west of Houston, however, subsidence continued 

unabated during the 1980's as ground-water pumpage 

continued to increase.

The magnitude of subsidence due to ground-water 

withdrawals through the 1980's across the Gulf Coastal 

Plain was insignificant on a regional scale. Locally, in 

urban areas such as north and west of Houston, or in 

low-lying areas near the Gulf of Mexico, subsidence 

may continue to be a significant problem until withdrawal rates are reduced or centers of withdrawals are 

relocated, perhaps to areas that are farther inland and at

higher altitudes or at shallower depths. Subsidence 

might become a significant regional problem if groundwater withdrawals of similar magnitude to those at 

Houston were to become widespread along the coast of 

Texas and Louisiana.

SALTWATER ENCROACHMENT

The effects of saltwater encroachment range from 

the necessity to relocate well fields to the slowly 

increasing chloride content in water from wells near the 

fresh-saltwater interface. The results of ground-water 

flow simulation suggest that location of pumping centers inland, away from the interface, does not stop the 

landward movement of saltwater and may result only 

in a longer span of time before increasing concentrations of chlorides are observed at the well fields.

The relocation of well fields is illustrated by the 

example of the city of Galveston, Texas. In the 1890's, 

the city's well field was moved inland to a location 

about 20 mi northwest of the city due to the lack of sufficient freshwater on Galveston Island. By the early 

1940's another well field was installed about 4 mi farther inland because of an increase in the chloride concentration in the water withdrawn (Petitt and Winslow, 

1955). These well fields, which obtain water from permeable zone A, supplied a substantial part of the water 

supply for the city of Galveston for about 40 to 45 years 

before surface water became the principal source of 

supply during the 1980's (Barbie and Locke, 1993). 

Chloride concentrations in water from both well fields 

decreased as withdrawals slowly declined beginning in 

the early 1970's.

Simulated flows in permeable zone A for 1987 are 

consistent with the observed chloride concentrations at 

the Galveston well fields. Horizontal flow lines near the 

shoreline of Galveston County are parallel to the shore 

(fig. 33), whereas the general direction of flow inland is 

toward the center of much larger ground-water withdrawals in the Houston area in Harris County. Vertical 

flow throughout Galveston County is downward relative to the base of permeable zone A (fig. 33), thus precluding upconing of saltwater from permeable zone B. 

However, horizontal flow in permeable zone B is from 

offshore (where the water has a larger concentration of 

dissolved solids than water in permeable zone A) across 

Galveston County toward the center of pumping in 

Harris County (fig. 34). Thus, chloride concentrations 

have probably increased with depth at the location of 

the Galveston well fields.

Nyman (1984) showed gradually increasing chloride 

concentrations in water from permeable zone A at several locations in southwestern Louisiana. Extensive 

pumping centers are located several miles inland from
SUMMARY OF HYDROLOGY OF THE REGIONAL AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES A55

the shore (Mesko and others, 1990). Chloride concentrations increased at the rate of 5 to 15 [(mg/L)/yr] 

during!963-83 at locations between the shore and the 

center of maximum pumpage (Nyman, 1984). Simulated flow vectors in permeable zone A for 1987 (fig. 33) 

indicate large horizontal flow from near the shoreline 

inland toward the center of pumpage, and vertical 

flows were upward relative to the base of permeable 

zone A; both conditions indicate the potential for the 

movement of saltwater from offshore and from below.

POTENTIAL FOR GROUND-WATER DEVELOPMENT

The aquifer systems of the Gulf Coastal Plain 

have a potential for further development of water supplies. The potential to increase regional recharge is substantial owing to the abundance of water in streams, 

lakes, swamps, and other surface-water bodies and 

to the hydraulic connection between the water table 

in outcrop areas and the confined part of aquifers 

and permeable zones. An analysis by Williamson and 

Grubb (in press) indicates that the aquifer systems 

could support withdrawals of as much as an additional 

10,000 Mgal/d. The local flow system as well as the 

regional flow system needs to be considered in locating 

pumping centers to minimize adverse impacts of additional withdrawals. Areas where the Mississippi River 

Valley alluvial aquifer has potential for further development were delineated by Ackerman (1996), and they 

are principally in northeastern Arkansas and southeastern Missouri, east-central Mississippi, and northeastern 

Louisiana. Areas of the Texas coastal uplands aquifer 

system with potential for further development were 

delineated by Ryder and Ardis (in press) and are principally in and near aquifer outcrop areas. Ryder and 

Ardis (in press) reported that the potential for development generally increases from west to east. The most 

favorable conditions for further development of 

ground-water supplies in the coastal lowlands aquifer 

system are generally in the upper permeable zones. 

Areas with potential for further development in the 

coastal lowlands aquifer system have been delineated 

by Martin and Whiteman (in press), and Ryder and 

Ardis (in press).

Many factors work in concert to determine the 

potential for the development of large quantities of 

ground water at any specific location. An example from 

Arthur and Taylor (in press) illustrates the effect of a 

regional confining bed. At Marianna, Arkansas, the 

lower Claiborne confining unit is present between 

the middle Claiborne aquifer and the lower Claiborne-upper Wilcox aquifer, whereas at Wynne, 

Arkansas, which is about 30 mi to the north of Marianna, the confining unit is not present. Simulation of 

hypothetical ground-water withdrawals of 40 Mgal/d 

from a 100-mi area underlain by the middle Claiborne aquifer resulted in about 60 ft more drawdown 

after 13 years pumping at Marianna than at Wynne.

REFERENCES

Ackerman, D.J., 1987a, Generalized potentiometric surface of the 

aquifers in the Cockfield Formation, southeastern Arkansas, 

spring 1980: U.S. Geological Survey Water-Resources Investigations Report 87-4212, map, scale 1:500,000,1 sheet.

 1987b, Generalized potentiometric surface of the Sparta-Memphis aquifer, eastern Arkansas, spring 1980: U.S. Geological Survey Water-Resources Investigations Report 87-4282, map, scale 

1:500,000,1 sheet.

 1989a, Hydrology of the Mississippi River Valley alluvial aquifer, south-central United States A preliminary assessment cff the 

regional flow system: U.S. Geological Survey Water-Resources 

Investigations Report 88-4028, 74 p.

 1989b, Potentiometric surfaces of the Mississippi River Valley 

alluvial aquifer in eastern Arkansas, spring 1972 and 1980: U.S. 

Geological Survey Water-Resources Investigations Report 88- 

4075, map, scale 1:500,000,1 sheet. 

-1996, Hydrology of the Mississippi River Valley alluvial aquifer,

south-central United States: U.S. Geological Survey Professional 

Paper 1416-D, 56 p.

Albin, D.R., 1964, Geology and ground-water resources of Bradley, 

Calhoun, and Ouachita Counties, Arkansas: U.S. Geological Survey Water-Supply Paper 1779-G, 32 p.

Anders, R.B., and Naftel, W.L., 1962, Pumpage of ground water and 

fluctuation of water levels in the Houston district, and the Baytown-La Porte area, Texas, 1957-61: Texas Water Commission 

Bulletin 6211, 52 p.

Andrews, D.I., 1960, The Louann Salt and its relationship to gulf coast 

salt domes: Gulf Coast Association of Geological Societies, 

Transactions, v. 10, p. 215-240.

Arthur, J.K., and Taylor, R.E., 1986, Mississippi embayment aquifer 

system in Mississippi Geohydrologic data compilation for 

flow model simulation: American Water Resources Association, 

Water Resources Bulletin, v. 22, no. 6, p. 1021-1029.

 1990, Definition of geohydrologic framework and preliminary 

simulation of ground-water flow in the Mississippi embayment 

aquifer system, Gulf Coastal Plain, United States: U.S. Geological Survey Water-Resources Investigations Report 86-4364, 97

P-

-in press, Ground-water flow analysis of the Mississippi embayment aquifer system, south-central United States: U.S. Geological 

Survey Professional Paper 1416-1.

Baker, E.T., Jr., and Wall, J.R., 1976, Summary appraisals of the 

Nation's ground-water resources Texas-Gulf region: U.S. Geological Survey Professional Paper 813-F, 29 p.

Barbie, D.L., and Locke, G.L., 1993, Ground-water withdrawals, water 

levels, and ground-water quality in the Houston District, Texas, 

with emphasis on 1985-89: U.S. Geological Survey WaterResources Investigations Report 92-4180, 28 p.

Beckman, J.D., and Williamson, A.K., 1990, Salt-dome locations in the 

Gulf Coastal Plain, south-central United States: U.S. Geological 

Survey Water-Resources Investigations Report 90-4060, 44 p.

Bedinger, M.S., and Sniegocki, R.T., 1976, Summary appraisals of the 

Nation's ground-water resources Arkansas-White-Red region: 

U.S. Geological Survey Professional Paper 813-H, 31 p.

Bennett, G.D., 1979, Regional ground-water systems analysis: U.S. 

Army Corps of Engineers, Water Support Center, Fort Belvoir, 

Virginia, Water Spectrum, v. 11, no. 4, p. 36-42.
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SUMMARY OF HYDROLOGY OF THE REGIONAL AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES A57

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 1993b, Properties and chemical constituents in ground water 

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SUMMARY OF HYDROLOGY OF THE REGIONAL AQUIFER SYSTEMS, SOUTH-CENTRAL UNITED STATES A61

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* U.S. GOVERNMENT PRINTING OFFICE: 1998 773-274 / 24000 Region No. 8



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