YDROLOGY OP^WPTEXAS GULF COAST 

I AQUIFER SYSTEMS

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PROFESSIONAL PAPER 1416-E

Hydrology of the

Texas Gulf Coast Aquifer Systems

By PAUL D. RYDER and ANN F. ARDIS

REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1416-E

2002
U.S. DEPARTMENT OF THE INTERIOR 

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

Any use of trade, product, or firm names in this publication is for

descriptive purposes only and does not imply endorsement by the

U.S. Government

Library of Congress Cataloging-in-Publication Data

Ryder, Paul D.

Hydrology of the Texas Gulf Coast aquifer systems / by Paul D. Ryder and Ann F. Ardis.

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

Includes bibliographical references.

Supt.ofDocs.no. : I 19.16 : 1416-E

1. Hydrology Texas Gulf Coast. 2. Aquifers Texas Gulf Coast.

I. Ardis, Ann F. II. Title. III. Series. IV. Series: U.S. Geological Survey professional paper ; 1416-E. 

GB1025.T4R93 1998

551.49'09764 dc21 97-29135 

ISBN 0-607-88126-7 CIP

For sale by U.S. Geological Survey, Information Services

Box 25286, Federal Center

Denver, CO 80225
FOREWORD

THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM

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 important components 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 to develop an understanding of the natural, undisturbed hydrologic system and the changes brought about 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 beginning with Professional Paper 1400.

Charles G. Groat 

Director

CONTENTS

	Page

Foreword............................................................................................................................................................ Ill

Abstract............................................................................................................................................................. El

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

Gulf Coast aquifer systems regional study ............................................................................................. 1

Texas Gulf Coast subregional study ........................................................................................................ 2

Purpose and scope ........................................................................................................................... 2

Method of investigation ................................................................................................................... 2

Description of the area .................................................................................................................... 3

Previous investigations ................................................................................................................... 5

Hydrogeology........................................... ........................................................................................................ 7

Hydrogeologic framework ....................................................................................................................... 7

Hydraulic properties ................................................................................................................................ 11

Horizontal hydraulic conductivity and transmissivity .................................................................... 11

Vertical hydraulic conductivity ....................................................................................................... 13

Storage coefficient ........................................................................................................................... 13

Dissolved-solids concentration and density of water ............................................................................. 15

Regional flow system ....................................................................................................................... ............... 16

Predevelopment steady-state flow system ............................................................................................... 17

Conceptual model of system ........................................................................................................... 17

Recharge and interaquifer leakage................................................................................................... 18

Flow system, predevelopment to 1982..................................................................................................... 18

Ground-water development.............................................................................................................. 18

Recharge, discharge, and distribution of flow, 1982 ....................................................................... 21

Subregional flow systems and hydrologic effects of development.................................................................. 24

Houston-Galveston area............................................................................................................................ 25

Coastal rice-irrigation area ....................................................................................................................... 30

Evadale-Beaumont area............................................................................................................................ 32

Kingsville area.......................................................................................................................................... 35

Winter Garden area................................................................................................................................... 40

Lufkin-Nacogdoches area......................................................................................................................... 41

Other areas................................................................................................................................................ 43

Bryan-College Station area ............................................................................................................ 43

Orange County ................................................................................................................................. 43

Potential for ground-water development.......................................................................................................... 44

Summary ........................................................................................................................................................... 51

Selected references............................................................................................................................................ 53

Model documentation........................................................................................................................................ 56

Introduction............................................................................................................................................... 56

Digital model of predevelopment steady-state flow system .................................................................... 56

Digital model of transient flow system, predevelopment to 1982........................................................... 56

Model grid and boundary conditions............................................................................................... 56

Hydrologic input data....................................................................................................................... 61

Calibration procedure and results .................................................................................................... 62

Sensitivity analysis........................................................................................................................... 69
VI CONTENTS

ILLUSTRATIONS

[Plates are in pocket]

PLATES 1-8. Maps of the Texas Gulf Coast aquifer systems showing:

1. Simulated transmissivity of permeable zones A, B, C, D, and E, and of the upper Claiborne, middle Claiborne, lower 

Claiborne-upper Wilcox and middle Wilcox aquifers.

2. Estimated dissolved-solids concentrations in water from permeable zones A, B, C, D, and E, and the upper Claiborne, middle 

Claiborne, lower Claiborne-upper Wilcox, and middle Wilcox aquifers.

3. Simulated recharge and discharge in the outcrop areas of aquifers, permeable zones, and confining units, predevelopment and 1982.

4. Estimated pumpage in 1980, by 25-square-mile areas, for permeable zones A, B, C, D, and E, and the upper Claiborne, 

middle Claiborne, lower Claiborne-upper Wilcox, and middle Wilcox aquifers.

5. Decline of potentiometric surfaces, predevelopment to 1982, and simulated 1982 horizontal flow vectors, permeable zones A, B, and 

C, Houston-Galveston and Evadale-Beaumont areas; and ground-water levels in Harris County and subsidence 

in the Houston-Galveston area, predevelopment to 1973.

6. Potential areas for development of large ground-water supplies from permeable zones A, B, C, D, E, and the upper Claiborne, 

middle Claiborne, lower Claiborne-upper Wilcox, and middle Wilcox aquifers.

7. Measured and simulated altitude of the potentiometric surface of permeable zones A, B, and C in the Houston-Galveston area, 

1982; and measured and simulated water levels in wells in the Houston-Galveston and the Evadale-Beaumont areas, 

and in Jackson and Wharton Counties.

8. Measured and simulated altitude of the potentiometric surface of the lower Claiborne-upper Wilcox aquifer, 1982; and measured 

and simulated water levels in wells in the Winter Garden area.

FIGURES 1-5. Maps showing:

1. Relation of the Texas Gulf Coast study area to the onshore part of the Gulf Coast Regional Aquifer-System Analysis study area E3

2. Relation of the Texas subregional modeled area to other subregional modeled areas and to the regional modeled area................. 4

3. Location of the Texas Gulf Coast study area.................................................................................................................... ............... 5

4. Mean annual precipitation in the Texas Gulf Coast study area......................................................................................................... 6

5. Generalized outcrops of aquifers, permeable zones, and confining units, Texas Gulf Coast study area.......................................... 10

6. Hydrogeologic section A-A' ....................................................................................................................................................................... 11

7. Map showing field and simulated hydraulic conductivity of coastal uplands aquifers and coastal lowlands permeable zones................. 12

8. Diagram showing conceptual model of Texas Gulf Coast aquifer systems and generalized representation in the digital model.............. 19

9. Diagram showing simulated predevelopment vertical flow rates across hydrogeologic units in the study area........................................ 20

10. Graph showing simulated pumpage, recharge, and depletion of storage, 1910-82 ................................................................................... 22

11. Map showing change in simulated recharge and discharge in the outcrop areas of aquifers, permeable zones, and confining

units, predevelopment to 1982......................................................................................................... ............... . .................................. 23

12. Diagram showing simulated 1982 vertical flow rates across hydrogeologic units in the study area.................... ............................ ........ 24

13-15. Maps showing:

13. Locations of observation wells and precipitation stations in selected intensively pumped areas, and model subareas......... .. ....... 26

14. Ground-water pumpage in selected counties in the central part of the study area, 1960-85............................................................. 27

15. Location of areas of intensive ground-water withdrawals within the Houston-Galveston area and average rates of

pumping for 1979......................................................................................................................................................................... 28

16. Diagram showing simulated 1982 vertical flow rates across hydrogeologic units in model subarea II..................................................... 30

17. Graphs showing ground-water levels in Jackson County and precipitation data in southwestern Wharton County .................................. 32

18. Diagram showing simulated 1982 vertical flow rates across hydrogeologic units in model subarea III............ ....................................... 33

19. Map showing ground-water pumpage in selected counties in the eastern part of the study area, 1960-85............................................... 34

20. Graph showing ground-water levels in wells in Jasper and Hardin Counties............................................................................................ 35

21. Diagram showing simulated 1982 vertical flow rates across hydrogeologic units in model subarea I....................................................... 36

22. Map showing ground-water pumpage in selected counties in the western part of the study area, 1960-85.............................................. 37

23. Map showing decline of potentiometric surface of permeable zone B, predevelopment to 1982, and simulated 1982 horizontal

flow direction and relative magnitude, Kingsville area ........................................................................................................................ 38
CONTENTS VII

Graph showing ground-water levels in well 19, Kleberg County .................................................................................................................... 39

Diagram showing simulated 1982 vertical flow rates across hydrogeologic units in model subarea IV ......................................................... 39

Map showing decline of potentiometric surface of the lower Claiborne-upper Wilcox aquifer, predevelopment to 1982, and simulated

1982 horizontal flow direction and relative magnitude, Winter Garden area............................................................................................. 41

27-30. Graphs showing:

27. Ground-water levels in wells and precipitation data in Zavala County................................................................................................. 42

28. Ground-water levels in well 37, Nacogdoches County ......................................................................................................................... 43

29. Projected fresh ground-water use for selected counties, 1985-2030..................................................................................................... 45

30. Measured and simulated water levels in wells in the Houston-Galveston area, projected to 2030....................................................... 47

31. Map showing projected land-surface subsidence in the Houston-Galveston area, 1973-2030 ....................................................................... 48

32. Graph showing simulated water levels near well 9, southeastern Harris County, projected to 2030 with 1980 pumpage.............................. 49

33. Graph showing measured and simulated water levels in wells in the Winter Garden area, projected to 2030................................................ 49

34. Diagram showing simulated 2030 vertical flow rates across hydrogeologic units in the study area............................................................... 50

35-37. Maps showing:

35. Finite-difference grid superimposed on the modeled area and study area.......................................................... .................................. 58

36. Estimated altitude of the water table for the model constant-head boundary........................................................................................ 59

37. Measured and simulated altitude of the potentiometric surface of permeable zone B in the Kingsville area in 1982.......................... 65

38. Graphs showing measured and simulated water levels in wells in the Kingsville area................................................................................... 66

39. Graphs showing measured and simulated water levels in wells in the Lufkin-Nacogdoches area.................................................................. 67

40. Map showing measured and simulated land-surface subsidence in the Houston-Galveston area, predevelopment to 1973........................... 68

41-46. Graphs showing:

41. Sensitivity of simulated 1982 heads to change in horizontal hydraulic conductivity............................................................................ 70

42. Sensitivity of simulated 1982 heads to change in vertical hydraulic conductivity................................................................................ 73

43. Sensitivity of simulated 1982 heads to changes in water density.......................................................................................................... 74

44. Sensitivity of simulated 1982 heads to changes in aquifer storage coefficient...................................................................................... 75

45. Sensitivity of simulated water levels, 1910-82, to changes in storage coefficient, Harris County ....................................................... 76

46. Sensitivity of simulated water levels, 1910-82, to changes in storage coefficient. Winter Garden area............................................... 77

TABLES

Page

TABLE 1. Stratigraphic and hydrogeologic units of the Texas coastal lowlands aquifer system................................................... ...................................E8

2. Stratigraphic and hydrogeologic units of the Texas coastal uplands aquifer system........................................................................................... 9

3. Horizontal hydraulic conductivity and transmissivity of aquifers and permeable zones based on model calibration...................................... 14

4. Vertical hydraulic conductivity of aquifers, permeable zones, and confining units based on calibrated model............................................... 15

5. Calibrated elastic storage coefficient for aquifers and permeable zones, and inelastic specific storage for compacting clays........................ 16

6. Estimated density of water in the hydrogeologic units ...................................................................................................................................... 17

7. Estimated average pumping rates for digital model 1910-82............................................................................................................................ 21

8. Simulated recharge, storage, and discharge in the Texas Gulf Coast aquifer systems, predevelopment and 1982 .......................................... 25

9. Simulated rates of water released from storage in 1982 from permeable zones A, B, and C........................................................................... 31

10. Summary of changes in simulated heads and recharge rates resulting from a hypothetical increase in pumping rate, 1983 to 2030 ............ 51

11. Summary of simulated and measured predevelopment altitudes of potentiometric surfaces ............................................................................ 57

12. Specific yield for grid blocks representing the outcrop of the lower Claiborne-upper Wilcox aquifer ........................................................... 60

13. Pumping periods and average pumping rates for digital model simulation....................................................................................................... 61

14. Summary of simulated and measured 1982 altitudes of potentiometric surfaces .............................................................................................. 63

15. Summary of simulated and measured water levels in wells shown in figure 13............................................................................................... 64

16. Changes in model hydraulic properties to test sensitivity of model for 10 model simulations ........................................................................ 69

17. Summary of calibration residual errors for 1982 heads and errors resulting from changes in model variables .............................................. 71

18. Summary of calibration residual errors for land-surface subsidence and errors resulting from changes in model variables .......................... 72

19. Comparison of measured water levels to simulated water levels resulting from changed values for model storage coefficient..................... 77
VIII CONTENTS

CONVERSION FACTORS,VERTICAL DATUM, AND ABBREVIATED 

WATER-QUALITY UNITS

Multiply By To obtain

inch (in.) 25.4 millimeter

inch per year (in./yr) 25.4 millimeter 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

cubic foot per day (ft-Vd) 0.02832 cubic meter per day

mile (mi) 1.609 kilometer (km)

square mile (mi2) 2.590 square kilometer

gallon per minute (gal/min) 0.06309 liter per second

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

billion gallons per day (Ggal/d) 3.785 million cubic meters per day

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

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

Chemical concentrations, water density, and water temperature are given in metric units. Chemical concentrations 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=(1.8x°C)+32.
REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

HYDROLOGY OF THE TEXAS GULF COAST 

AQUIFER SYSTEMS

By PAUL D. RYDER and ANN F. ARDIS

ABSTRACT

A complex, multilayered ground-water flow system exists in the Coastal 

Plain sediments of Texas. The Tertiary and Quaternary clastic deposits have 

an areal extent of 114,000 square miles onshore and in the Gulf of Mexico. 

Two distinct aquifer systems are recognized within the sediments, which 

range in thickness from a few feet to more than 12,000 feet. The older 

system, the Texas coastal uplands aquifer system, consists of four aquifers and 

two confining units in the Claiborne and Wilcox Groups. It is underlain by 

the practically impermeable Midway confining unit or by the top of the 

geopressured zone. It is overlain by the nearly impermeable Vicksburg-Jackson confining unit, which separates it from the younger coastal lowlands aquifer system. The coastal lowlands aquifer system consists of five permeable 

zones and two confining units that range in age from Oligocene to Holocene. 

The hydrogeologic units of both systems are exposed in bands that parallel the 

coastline. The units dip and thicken toward the Gulf. The quality of water in 

the aquifer systems is highly variable, with content of dissolved solids ranging 

from less than 500 to 150,000 milligrams per liter.

Substantial withdrawal from the aquifer systems began in the early 1900's 

and increased nearly continuously into the 1970's. The increase in withdrawal 

was relatively rapid from about 1940 to 1970. Adverse hydrologic effects, 

such as saltwater encroachment in coastal areas, land-surface subsidence in 

the Houston-Galveston area, and long-term dewatering in the Winter Garden 

area, were among the factors that caused pumping increases to slow or to 

cease in the 1970's and 1980's.

Ground-water withdrawals in the study area in 1980 were about 1.7 billion 

gallons per day. Nearly all of the withdrawal was from four units: Permeable 

zones A, B, and C of Miocene age and younger, and the lower Claiborne-upper Wilcox aquifer. Ground-water levels have declined hundreds of 

feet in the intensively pumped areas of Houston-Galveston, Kingsville, Winter 

Garden, and Lufkin-Nacogdoches. Water-level declines have caused inelastic 

compaction of clays which, in turn, has resulted in land-surface subsidence of 

more than 1 foot in an area of about 2,000 square miles. Maximum subsidence of nearly 10 feet occurs in the Pasadena area east of Houston.

A three-dimensional, variable-density digital model was developed to simulate predevelopment and transient flow in the aquifer systems. The modeled 

area is larger than the study area and includes adjacent parts of Louisiana and 

Mexico. The transient-model calibration period was from 1910 (predevelopment) to 1982. Model-generated head distributions, water-level hydrographs, 

and land-surface subsidence were matched to measured data in selected, 

intensively pumped areas.

For the study area, mean horizontal hydraulic conductivity in the calibrated 

model ranges from 10 feet per day for the middle Wilcox aquifer to 25 feet 

per day for permeable zone A. Mean transmissivity ranges from about 4,600 

feet squared per day for the middle Claiborne aquifer to about 10,400 feet 

squared per day for permeable zone D. Mean vertical hydraulic conductivity

ranges from l.lxlO'5 feet per day for the Vicksburg-Jackson confining unit, to 

3.8x10 ! feet per day for permeable zone A. Mean values of calibrated storage 

coefficient range from 5.2xlQ-4 for the middle Claiborne aquifer to 1.7x10'' 

for the middle Wilcox aquifer and permeable zone C. Calibrated inelastic 

specific storage values for clay beds in permeable zones A, B, and C in the 

Houston-Galveston area are 8.5xlO'\ 8.0xlO-\ and 8.0x10-" per foot, respectively.

Recharge rates were mapped for predevelopment conditions as determined 

from a steady-state model calibration. A maximum rate of 3 inches per year 

was simulated in small areas, and the average rate for the study area was 0.34 

inch per year. Total simulated recharge was 85 million cubic feet per day in 

the outcrop area. Recharge was equal to discharge in outcrop areas (79 

million cubic feet per day) plus net lateral flow out of the study area (6 million 

cubic feet per day).

Rates of inflow and outflow to the ground-water system have nearly tripled 

from predevelopment to 1982 (85 to 276 million cubic feet per day) based on 

model simulation. Withdrawal of 231 million cubic feet per day was supplied 

principally by an increase in outcrop recharge and, to a lesser extent, from a 

decrease in natural discharge and release of water from storage in aquifers and 

compacting clay beds. The average simulated 1982 recharge rate for the study 

area was 0.52 inch per year, with a maximum simulated rate of 6 inches per 

year in Jackson and Wharton Counties.

Because withdrawal has caused problems such as saltwater intrusion, 

land-surface subsidence, and aquifer dewatering, the Texas Department of 

Water Resources has projected that ground-water use will decline substantially in most of the study area by the year 2030. Some areas remain favorable 

for development of additional ground-water supplies. Pumping from older 

units that are farther inland and in areas where potential recharge is greater 

will minimize adverse hydrologic effects.

INTRODUCTION

GULF COAST AQUIFER SYSTEMS REGIONAL 

STUDY

The Gulf Coast Regional Aquifer-System Analysis (Gulf 

Coast RASA) study was begun in 1980 as part of a federally 

funded program of the U.S. Geological Survey to provide a 

regional understanding and assessment of major aquifer systems in the United States. The Gulf Coast RASA study is 

focused on the Gulf Coastal Plain sediments of Tertiary and

El
E2 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

Quaternary age. The study area consists of about 230,000 

mi2 onshore and about 60,000 mi2 offshore (about 290,000 

mi2 total) in parts of Alabama, Arkansas, Florida, Illinois, 

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

all of Louisiana (fig. 1). A complete discussion and description of the Gulf Coast RASA study is given in Grubb (1984).

Many reports and abstracts resulting from the study have 

been published or are in press. These reports generally cover 

all or parts of the aquifer systems in the Gulf Coast RASA 

study. They describe one or a combination of various 

aspects of the gulf coast aquifer systems such as geology and 

stratigraphy, geochemistry, ground-water hydraulics and flow, 

ground-water development and use, and digital-model development.

Final reports, regional and subregional in scope, that 

describe the hydrogeologic framework, hydrology, or 

geochemistry are in Professional Paper 1416, which consists 

of several chapters.

TEXAS GULF COAST SUBREGIONAL STUDY

The Texas Gulf Coast study is a part of the Gulf Coast 

RASA study (fig. 1). Parts of the Texas Gulf Coast aquifer 

systems have sustained intensive ground-water development 

that has resulted in problems associated with large decreases 

of ground-water levels, land subsidence, and saltwater 

encroachment. Ground-water withdrawals were more than 

600 Mgal/d in the Houston-Galveston area in 1980. Water 

levels in some wells declined from at or above land surface 

in the early 1900's to about 350 ft below land surface in the 

1980's. The decreased artesian pressure head caused land 

subsidence of almost 10 ft in the Pasadena area east of Houston during 1906-78 (Gabrysch, 1984b, p. 21). Extensive 

withdrawal has caused land subsidence in other areas, 

although less severe than in the Houston area.

Potential for saltwater encroachment is particularly great 

in the southwestern part of the study area. The cities of 

Alice in Jim Wells County and Brownsville in Cameron 

County have supplemented ground-water supplies with surface water because of saltwater encroachment (Texas Department of Water Resources, 1984a, p. III-22-1). Because of 

saltwater encroachment, the cities of Agua Dulce, Banquette, 

Driscoll, and Bishop in Nueces County, and Kingsville in 

Kleberg County have begun to or plan to supplement 

ground-water supplies with water from the Nueces River 

(Texas Department of Water Resources, 1984a, p. Ill-22-1).

Other areas within the Texas part of the Gulf Coast 

RASA study area have potential for significant additional 

ground-water development, but the effects of large 

increases in development are not known. Management of 

the regional ground-water resource will require quantitative 

evaluation of the geologic, hydrologic, and chemical-quality characteristics of the system in addition to definition of

the hydrogeologic boundaries that affect development 

potential.

PURPOSE AND SCOPE

The objectives of the Texas Gulf Coast study are to (1) 

define the hydrogeologic framework and hydraulic characteristics of the aquifer systems, (2) delineate the extent of fresh 

to slightly saline water in the various hydrogeologic units, 

(3) describe and quantify the ground-water flow system, (4) 

analyze the hydrologic effects of human development on the 

flow system, and (5) assess the potential of the aquifer systems for further development. A preliminary or interim 

report (Ryder, 1988) described in detail the hydrogeologic 

framework and the steady-state predevelopment flow system. 

Thus, a brief summary of these topics is given in this report, 

and the emphasis of this report is on the effects of development and the potential for development.

METHOD OF INVESTIGATION

The basic approach to meet the study objectives was to 

collect and analyze hundreds of borehole geophysical logs, 

to review previously published interpretive reports, and to 

use data files from the U.S. Geological Survey and other 

Federal and State agencies and commercially available files 

from the petroleum industry. From these analyses a regionally consistent hydrogeologic framework was defined. Additionally, the analyses provide (1) the hydrologic boundaries 

of the aquifer systems, (2) a definition of selected 

water-quality properties, (3) initial estimates of aquifer and 

confining-unit hydraulic characteristics, (4) estimates of 

withdrawal in each of the various aquifers, and (5) where 

sufficient data exist, potentiometric-surface maps for determining any long-term decline in water levels from predevelopment to modern-day conditions. All of these resulting 

data were incorporated into a multilayered, digital 

ground-water flow model. Calibration of the model provides 

an improved estimate of aquifer and confining-unit hydraulic 

characteristics, a quantitative analysis of flow within and 

between each of the various aquifers, a better understanding 

of the total flow system, and a useful tool for assessing the 

potential of the aquifer systems for further development.

The Texas Gulf Coast study is one of five subregions of 

the Gulf Coast RASA study area for which digital models 

have been developed. The five subregional models are 

nested within the Gulf Coast RASA regional model. The 

relation of the regional modeled area, with a grid having 102 

rows, 58 columns, and 10-mi grid-block spacing, to the five 

subregional modeled areas, each with 5-mi grid-block spacing, is shown in figure 2. The model grid is explained in
HYDROLOGY OF THE TEXAS GULF COAST AQUIFER SYSTEMS 

EXPLANATION

| | MIDWAY CONFINING UNIT 

I I TEXAS COASTAL UPLANDS AQUIFER SYSTEM 

| | MISSISSIPPI EMBAYMENT AQUIFER SYSTEM 

| | VICKSBURG-JACKSON CONFINING UNIT 

I I COASTAL LOWLANDS AQUIFER SYSTEM 

^H UPPER CRETACEOUS AQUIFER

E3

Study area

0 100

I

200 MILES

_I

0 100 200 KILOMETERS 

FIGURE 1. Relation of the Texas Gulf Coast study area to the onshore part of the Gulf Coast Regional Aquifer-System Analysis study area.

more detail in the section on "Model Documentation" at the 

end of this report.

DESCRIPTION OF THE AREA

The study area is within the coastal plain of Texas and 

extends from Mexico in the west to Louisiana in the east. It 

consists of about 90,000 mi 2 onshore and an additional 

24,000 mi2 in the Gulf of Mexico for a total surface area of 

about 114,000 mi 2 (fig. 3). The northern boundary is the 

updip limit of the Texas Gulf Coast aquifer systems the 

practically impermeable, predominantly marine clays of the

Midway Group. The southern boundary extends to the edge 

of the continental shelf in the Gulf of Mexico.

The study area encompasses all or parts of 109 counties. 

The principal physiographic province is the Gulf Coastal 

Plain, and the major structural features are the Rio Grande 

embayment, San Marcos arch, East Texas embayment, Sabine uplift, and the Gulf of Mexico geosyncline (fig. 1). 

Land surface is characterized by a smooth, low-lying 

coastal plain that gradually rises toward the north and 

northwest where the more dissected and rolling terrain 

reaches altitudes of as much as 600 ft in the eastern and 

central areas, and to as much as 900 ft in the west. The 

coastal uplands end at the contact with Cretaceous clay and
EXPLANATION

REGIONAL 10-MILE AND SUBREGION 5-MILE GRID BLOCKS

Mississippi River 

valley alluvium

McNairy-Nacatoch - Coastal lowlands of

Alabama, Florida, 

Louisiana, and

Mississippi embayment Mississippi

McNairy-Nacatoch

Regional 10-mile grid V^VoX Subregional 5-mile grid 5>V 93° \

\ 34 ^ Mississippi 

embayment 

aquifer 

system

Texas coastal

,'' Study area, 31 

boundary

Coastallowlands \ 

aquifer system \ jV

GULF W MEXICO

/ \

\ continental \Sfi"otf

in

0 50 100 150 MILES

I______|_____I______|

0 50 100 150 KILOMETERS

m o

o r

I

I

50 

t/3 <

FIGURE 2. Relation of the Texas subregional modeled area to other subregional modeled areas and to the Gulf Coast Regional Aquifer System modeled area.
HYDROLOGY OF THE TEXAS GULF COAST AQUIFER SYSTEMS E5

LOCATION OF STUDY AREA

5t S Y^ o i

COLORADO \ Jk3£>^T f

V-i-L^.^

ZAPATA i J1MHOGG i ' 

1 . BROOKS I

100 MILES

100 KILOMETERS

FIGURE 3. Location of the Texas Gulf Coast study area.

limestone where the land surface generally rises steeply. 

Twelve major streams drain the area, flowing generally 

south-southeastward toward the Gulf of Mexico. These are, 

from west to east, the Rio Grande, Nueces, Frio, San 

Antonio, Guadalupe, Colorado, Brazos, Navasota, Trinity, 

Neches, Angelina, and Sabine Rivers (fig. 4). Many large 

reservoirs that have been constructed on these and other 

streams supply substantial quantities of water for irrigation 

and municipal supply.

Average annual precipitation during 1951-80 varied 

greatly, ranging from about 21 in. in most of the Rio 

Grande valley in the southwest, to about 56 in. at the

Texas-Louisiana border in the east (fig. 4). Average annual 

runoff for 1951-80 ranged from about 0.2 in. at the Rio 

Grande to more than 18 in. from the Houston area eastward into Louisiana (Gebert and others, 1987).

PREVIOUS INVESTIGATIONS

Numerous reports concerning the geology and hydrogeology of all or part of the study area have been written by 

personnel of the U.S. Geological Survey and other Federal 

and State agencies, consulting firms, and others. Several 

reports are regional in nature, including a report by the Texas
E6 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

100 MILES

FIGURE 4. Mean annual precipitation (in.),Texas Gulf Coast study area. Based on National Oceanic and Atmospheric Administration records for 1951-80. Modified from Texas Department of Water Resources (1984a, fig. 5).

Department of Water Resources (1984a) that describes the 

hydrology of the area and presents a comprehensive plan for 

future water development; a report by Baker and Wall (1976) 

that includes most of the study area and emphasizes the 

desirability of conjunctive use of ground water and surface 

water in any plans for future development; a report by Carr 

and others (1985) that includes much of the study area and 

applies digital modeling techniques to simulate flow in 

Miocene and younger deposits; a report by Baker (1979) that 

includes an area from Mexico to Louisiana and emphasizes 

the hydrogeologic framework of Miocene and younger units;

a report by Baker (1986) that applies digital modeling to 

simulate predevelopment flow in Miocene deposits in a 

25,000-mi2 area in the eastern part; and a report by Jones 

and others (1976) that describes the hydrogeology of the 

Wilcox Group. Some of the many reports dealing with more 

local ground-water problems are referenced throughout this 

report where the published data or conclusions are helpful 

for describing and analyzing the regional ground-water flow 

system.

Several recent reports have been generated by the Gulf 

Coast RASA study. Grubb (1984, 1987) presented an
HYDROLOGY OF THE TEXAS GULF COAST AQUIFER SYSTEMS E7

overview ;he Gulf Coast RASA. Hosman and Weiss 

(1991) described in detail the hydrogeologic framework for 

the Texas coastal uplands aquifer system. Weiss (1987) 

described methods for estimating dissolved-solids concentrations in water. Pettijohn and others (1988) presented 

maps of dissolved-solids concentrations. Ryder (1988) 

described the hydrogeology and predevelopment flow in the 

Texas Gulf Coast aquifer systems.

HYDROGEOLOGY

A detailed description of the hydrogeologic framework is 

given in Ryder (1988). A brief summary follows. Some 

model-derived values of transmissivity and horizontal and 

vertical hydraulic conductivity presented here are significantly different from those previously reported for the predevelopment model calibration. These properties are described 

in detail, as well as storage coefficient, which was estimated 

for the transient model calibration. Estimates of dissolved-solids concentrations and density of water are more 

accurate than reported previously.

HYDROGEOLOGIC FRAMEWORK

Tertiary and Quaternary deposits of clay, silt, sand, and 

gravel as much as 12,000 ft thick underlie the Gulf Coastal 

Plain in Texas. The clastic sediments dip toward the Gulf 

of Mexico and thicken in that direction. The thick 

sequence of alternating fine- and coarse-grained sediments 

are subdivided into hydrogeologic units that can be represented as layers in a digital ground-water flow model. The 

number of model layers (hydrogeologic units) chosen was 

based on practical considerations for the scale of the system, objectives of the model study, and computer-cost limitations. Weiss and Williamson (1985) discussed the 

rationale and methodology for subdividing the Coastal Plain 

sediments into discrete hydrogeologic units for the Gulf 

Coast RASA study.

Pre-Miocene sediments are distributed as relatively uniform sequences of predominantly fine- or coarse-grained 

material. Borehole geophysical data can be used to identify the intervals and to designate aquifers and confining 

units.

Miocene and younger deposits differ from pre-Miocene 

deposits because the younger deposits show more heterogeneity the interfingering of many thin beds of differing 

texture and limited areal extent. Where the aquifer system is heterogeneous, Weiss and Williamson (1985) stated 

that the subdivision into model layers should include an 

evaluation of depths of producing zones and the resultant

vertical distribution of hydraulic head. In addition, borehole geophysical data were used to suggest relative permeabilities and to delineate model layers that are more likely 

to have uniform hydraulic properties. Thus, Miocene and 

younger deposits at large pumping centers in Houston, 

Texas, and Baton Rouge, Louisiana, were subdivided into 

discrete units (Weiss and Williamson, 1985). These units 

are called "permeable zones" and confining units. The 

permeable zones typically contain discontinuous clay beds, 

in contrast to the pre-Miocene aquifers that are typically 

massive sand beds separated by regionally extensive clay 

beds.

The hydrogeologic units of the Gulf Coast RASA study 

compose three major aquifer systems (fig. 1). The Texas 

coastal uplands aquifer system occurs only in Texas; the 

coastal lowlands aquifer system occurs in Texas and several 

nearby States; the Mississippi embayment aquifer system 

occurs in several States outside Texas. The two systems 

(tables 1, 2) in Texas are separated by the poorly permeable 

Vicksburg-Jackson confining unit and are underlain by the 

practically impermeable Midway confining unit. The Texas 

coastal uplands (hereafter called simply the coastal uplands) 

and coastal lowlands aquifer systems consist of four aquifers, 

five permeable zones, and six confining units (including the 

Midway and Vicksburg-Jackson confining units). The definitions of the hydrogeologic units and the names assigned to 

them may not conform to conventional definitions and names 

as found in the published literature. The hydrogeologic units 

in the coastal uplands aquifer system are named for the 

group designation of the sediments that make up the units 

(Hosman and Weiss, 1991). Grubb (1987) applied the designations "zone A" through "zone E" for the five permeable 

zones of the coastal lowlands aquifer system, and Weiss 

(1990) presented thickness maps of both the permeable 

zones and confining units. Correlation of stratigraphic and 

hydrogeologic units is shown in table 1 for the coastal lowlands aquifer system and in table 2 for the coastal uplands 

aquifer system.

A note of explanation concerning the "Frio" Formation 

(table 1) and the Frio Clay (table 2) is due here. The "Frio" 

Formation is from Texas oil-field nomenclature. It refers to 

a thick section of sand that begins to appear about 3,000 ft 

below the land surface. The "Frio" Formation has no correlation with the Frio Clay, which is the updip nonmarine, 

time-equivalent unit of the subsurface Vicksburg Group 

(Baker, 1979, p. 36).

The hydrogeologic units are, from youngest to oldest 

(tables 1, 2), permeable zone A (Holocene-upper Pleistocene deposits), permeable zone B (lower Pleistocene-upper Pliocene deposits), permeable zone C (lower 

Pliocene-upper Miocene deposits), zone D confining unit 

(middle Miocene deposits), permeable zone D (lower to 

middle Miocene deposits), zone E confining unit (lower
E8 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

TABLE 1. Stratigraphic and hvdrogeologic units of the Texas coastal lowlands aquifer system

SYSTEM

QUATERNARY

TERTIARY

SERIES

LU

z

LU 

O

g

o

i

LU

Z

LU 

O

O

CO

LU

_l 

QLU

Z

LU

O

o

_i

0.

LU

Z

LU 

O

O

^

Modified from Baker (1979)

STRATIGRAPHIC 

UNITS

Alluvium

/

Beaumont Clay 

Montgomery Formation 

Bentley Formation 

WillisSand

Goliad Sand

Fleming Formation

Oakville

Sandstone

Catahoula 

or Tuff2

Sandstone

Anahuac 

Formation 1

"Frio" 

Formation 1

HYDROGEOLOGIC 

UNITS

cc

LU 

LL

=>

0

<

o 

o

I 

o

LU

Z(E 

-1 LU 

LU LL

£5$°<

LU

BURKEVILLE CONFINING 

SYSTEM

CATAHOULA CONFINING 

SYSTEM (RESTRICTED) JASPER AQUIFER

From Grubb (1987)

HYDROGEOLOGIC 

UNITS IN 

THIS REPORT

Permeable zone A

Permeable zone B

Permec

Permec

able zone C

zone D 

confining unit1

able zone D

zone E 

confining unit1

Permeable zone E

HYDROGEOLOGIC 

UNIT NUMBERS 

IN THIS REPORT

i

2

3

4

5

6

7

1 Present only in the subsurface

2 Catahoula Tuff west of Lavaca County

Miocene deposits), permeable zone E (lower Miocene-upper 

Oligocene deposits), Vicksburg-Jackson confining unit, 

upper Claiborne aquifer, middle Claiborne confining unit, 

middle Claiborne aquifer, lower Claiborne confining unit, 

lower Claiborne-upper Wilcox aquifer, middle Wilcox aquifer, and Midway confining unit.

All but two of the units (zone D and zone E confining 

units exist only in the subsurface) crop out in bands that 

are essentially parallel to the coastline (fig. 5). Generally, 

the units dip south-southeastward toward the Gulf of

Mexico and thicken in that direction. An exception is in 

the Sabine uplift in the northeast, where older units are 

exposed at the surface and dip away in all directions from 

the uplift center.

The arrangement and subsurface extent of the hydrogeologic units that make up the aquifer systems are shown by 

the hydrogeologic section in figure 6. The hydrogeologic 

section is useful in that it presents a simple, two-dimensional view of the interrelationship of aquifers, confining 

units, and hydrologic boundaries. The section was drawn
HYDROLOGY OF THE TEXAS GULF COAST AQUIFER SYSTEMS 

TABLE 2. Stratigmplric and hydrogeologic units of the Texas coastal uplands aquifer system

E9

SYSTEM

TERTIARY

SERIES

OLIGOCENE

EOCENE

PALEOCENE

STRATIGRAPHIC UNITS 

Modified from Hosman 

and Weiss (1991)

SOUTH

VicksFrio Clay burg 

Group1

Jackson Group

Q. 

D 

0

CJ

CD

C

0

J2

CO

(J

Wilcox Group

Midway Group

Whitsett 

Formation

Manning Clay

Wellborn 

Sandstone

Caddell 

Formation

Yegua 

Formation

La redo 

Formation

El Pico Clay

Bigford 

Formation

Carrizo Sand

Undifferentiated

Wills Point 

Formation

Kincaid 

Formation

SOUTHEAST and 

NORTHEAST

VicksFrio Clay burg 

Group1

Whitsett 

Formation

Manning Clay

Wellborn 

Sandstone

Caddell 

Formation

Yegua 

Formation

Cook Mountain 

Formation

Sparta Sand

Weches 

Formation

Queen City Sand

Reklaw 

Formation

Carrizo Sand

Undifferentiated

Wills Point 

Formation

Kincaid 

Formation

TEXAS MAJOR 

AND MINOR 

AQUIFERS 

(Muller and Price, 

1979)

Sparta2 

(Minor aquifer)

Queen City2 

(Minor aquifer)

Carrizo-Wilcox 

(Major aquifer)

HYDROGEOLOGIC 

UNITS IN 

THIS REPORT 

(Hosman and Weiss, 

1991)

Vicksburg-Jackson 

confining unit

Upper Claiborne 

aquifer

Middle Claiborne 

confining unit

Middle Claiborne 

aquifer

Lower Claiborne 

confining unit

Lower Claiborneupper Wilcox aquifer

Middle Wilcox 

aquifer

Midway confining unit

HYDROGEOLOGIC

UNIT NUMBERS 

IN THIS REPORT

8

9

10

11

12

13

14

1 Present only in the subsurface

2 Not recognized west of Frio County

through about the center of the study area and along a row 

of a finite-difference grid (fig. 2) that was used for digital 

modeling (discussed in the section on "Model Documentation"). It was constructed by using mean-layer-thickness 

values for the 25-mi2 grid blocks. The line of section is 

nearly parallel to the dips of the units. The vertical scale is 

greatly exaggerated (more than 25 times); an absence of vertical exaggeration would show the attitude of the units as 

being more nearly horizontal.

The base of the combined systems is defined here as 

either the top of the Midway confining unit or the top of the 

geopressured zone. The geopressured zone is above the top 

of the Midway confining unit in downdip areas, where there 

is a transition from a predominantly sand facies to a predominantly clay facies. Very high pressures occur in this 

zone, which indicates that hydraulic conductivities are very 

small with little or no upward migration of water. The top 

of the zone of geopressure was mapped by Wallace and
E10 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

EXPLANATION

MIDWAY CONFINING UNIT (PALEOCENE)

MIDDLE WILCOX AQUIFER (LOWER EOCENE-UPPER PALEOCENE)

LOWER CLAIBORNE-UPPER WILCOX AQUIFER (LOWER EOCENE)

LOWER CLAIBORNE CONFINING UNIT (LOWER EOCENE)

MIDDLE CLAIBORNE AQUIFER (MIDDLE EOCENE)

MIDDLE CLAIBORNE CONFINING UNIT (MIDDLE EOCENE)

UPPER CLAIBORNE AQUIFER (MIDDLE EOCENE)

VICKSBURG-JACKSON CONHNING UNIT (LOWER OLIGOCENE-UPPER EOCENE)

LOWER MIOCENE-UPPER OLIGOCENE PERMEABLE ZONE E

MIDDLE MIOCENE PERMEABLE ZONE D

LOWER PLIOCENE-UPPER MIOCENE PERMEABLE ZONE C

LOWER PLEISTOCENE-UPPER PLIOCENE PERMEABLE ZONE B

HOLOCENE-UPPER PLEISTOCENE PERMEABLE ZONE A

100 MILES

100 KILOMETERS
HYDROLOGY OF THE TEXAS GULF COAST AQUIFER SYSTEMS Ell

0 10 20 30 40 50 MILES 

I 

0 10 20 30 40 50 KILOMETERS

VERTICAL SCALE GREATLY EXAGGERATED

EXPLANATION

HYDROGEOLOGIC UNITS

Permeable zone A 

Permeable zone B 

Permeable zone C 

Zone D confining unit 

Permeable zone D 

Zone E confining unit 

Permeable zone E

Vicksburg-Jackson confining unit 

Upper Claiborne aquifer 

Middle Claiborne confining unit 

Middle Claiborne aquifer 

Lower Claiborne confining unit 

Lower Claiborne-upper Wilcox aquifer 

Middle Wilcox aquifer

FIGURE 6. Hydrogeologic section A-A, Texas Gulf Coast study area. Line of section is shown in figure 5.

others (1981). The irregular nature of the top of the 

geopressured zone and of the units themselves is evident in 

the section. In some areas, the units may be exposed but 

pinch out downdip, or they may not be exposed but exist 

only in the subsurface.

HYDRAULIC PROPERTIES

Initial estimates of aquifer properties were obtained from 

the literature or from analyses of field data. These estimates 

were tested in a digital ground-water flow model, and 

calibration of the model led to refined estimates in some 

areas and for most hydrogeologic units. The hydraulic 

properties described below were obtained from the calibrated 

model.

FIGURE 5 (facing page). Generalized outcrops of aquifers, permeable 

zones, and confining units, Texas Gulf Coast study area.

HORIZONTAL HYDRAULIC CONDUCTIVITY AND 

TRANSMISSIVITY

Hydraulic conductivity for aquifers and permeable zones 

in the Gulf Coast aquifer systems was estimated by analyzing and averaging hundreds of aquifer tests and specific-capacity tests (Prudic, 1991). Statistical analyses of 

hydraulic conductivity showed that four subareas could be 

distinguished within the study area that contain significantly 

distinct geometric-mean values (fig. 7). The mean values for 

the coastal uplands aquifers in subarea 1 are higher than for 

those in subarea 2, whereas the values for the coastal lowlands permeable zones in subarea 4 are higher than for those 

in subarea 3 and are the highest in the entire study area.

The initial estimates of hydraulic conductivity were 

refined during calibration of steady-state and transient 

ground-water flow models. A comparison of field determinations of hydraulic conductivity with those derived from 

model calibration is shown in figure 7 for the four subareas.
E12 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

TEXAS COASTAL 

UPLANDS SUBAREA 2

EXPLANATION

BOUNDARY OF SUBAREAS

MEAN HYDRAULIC CONDUCTIVITY, 

IN FEET PER DAY From Prudic (1991). 

Number of values used in estimate shown 

in parentheses

33 (340) Estimated from aquifer-test analysi

<& 

0°

,3K

(465) Estimated from specific-capacity data ^

MEAN HYDRAULIC CONDUCTIVITY -^£

IN CALIBRATED MODEL, IN FEET 

PER DAY

TEXAS COASTAL 

UPLANDS SUBAREA 1

co VV

*- L J- JD '-/- ^ *^r ^ f ^ i. --

18 (23) 

| 13 (99)

TEXAS COASTAL 

LOWLANDS SUBAREA 4

10 (43) 

14 (30)

^T-'-i '\ 

/ ' \\-~~, '

150 MILES

100 150 KILOMETERS

TEXAS COASTAL 

LOWLANDS SUBAREA 3

FIGURE 7. Field and simulated hydraulic conductivity of coastal uplands aquifers and coastal lowlands permeable zones, Texas Gulf Coast study area.
HYDROLOGY OF THE TEXAS GULF COAST AQUIFER SYSTEMS E13

Simulated >.. jes range from not greatly different from field 

values to about 1.5 times greater or lesser than field values in 

subareas 2 and 4.

Mean, minimum, and maximum grid-block values of 

hydraulic conductivity for each aquifer and permeable zone 

in the study area as derived from model calibration are 

shown in table 3. The values are designated as uniform in 

four of the units (table 3). This designation is made because 

the units are relatively undeveloped and field-test data are 

lacking, and further refinement of hydraulic conductivity 

with resulting areal variation among the grid blocks was not 

warranted. The hydraulic conductivity values range from 1 

ft/d in the lower Claiborne-upper Wilcox aquifer (downdip 

in the Winter Garden area) to 102 ft/d in permeable zone A 

(in eastern Texas). Means range from 10 ft/d in the middle 

Wilcox aquifer to 25 ft/d in permeable zone A.

Tests to analyze the sensitivity of the model to changes 

in selected model variables (discussed fully in the section 

on "Model Documentation") showed that the model is most 

sensitive to a decrease in horizontal hydraulic conductivity.

The thickness of each aquifer and permeable zone was 

described in detail by Ryder (1988). The transmissivity of 

each aquifer and permeable zone as derived through the calibration of the digital flow model is shown on plate 1. In 

general, the transmissivity is smallest in the extreme updip 

and downdip grid blocks, where thicknesses generally 

approach zero. Transmissivity within the study area reaches 

a maximum of more than 40,000 ft2/d in three units: permeable zone A, permeable zone D, and permeable zone E (pi. 

1). In permeable zone A, the largest transmissivity is at the 

Texas-Louisiana boundary where the hydraulic conductivity 

of the zone is high. The largest transmissivity in permeable 

zones D and E is in downdip areas where the units have a 

large thickness.

Mean, minimum, and maximum grid-block values of 

transmissivity for each aquifer and permeable zone in the 

study area are shown in table 3. Values range from less 

than 500 ft2/d in each layer to about 50,000 ft2/d in permeable zone A. Means range from about 4,600 ft2/d in the 

middle Claiborne aquifer to about 10,400 ft2/d in permeable 

zone D.

VERTICAL HYDRAULIC CONDUCTIVITY

The vertical hydraulic conductivity values in table 4 

were derived through the calibration of a digital 

ground-water flow model, wherein flow across a confining 

unit is the product of the vertical hydraulic-head gradient, 

the area of the grid block, and the effective leakance. The 

vertical hydraulic conductivity of a unit divided by the 

unit's thickness is defined as leakance. Effective leakance

is the harmonic mean of the leakances of the confining 

unit, the underlying aquifer, and the overlying aquifer.

The grid-block means of the vertical hydraulic conductivity of the aquifers and permeable zones in table 4 are generally a few orders of magnitude smaller than the 

horizontal hydraulic conductivity of the aquifers and permeable zones (table 3). Means of the vertical hydraulic conductivity of confining units are smaller than those of the 

aquifers and permeable zones, and the vertical conductivity 

of confining units below the Vicksburg-Jackson confining 

unit is smaller than for the confining units above. Vertical 

hydraulic conductivity ranges from l.OxlO'6 to l.OxlO" 1 

ft/d. Means range from l.lxlO"5 ft/d for the Vicksburg-Jackson confining unit to 3.8xlO~3 ft/d for permeable 

zone A.

Tests to analyze sensitivity of the model to changes in 

selected model variables (discussed in the section on 

"Model Documentation") showed that the model is moderately sensitive to change in vertical hydraulic conductivity.

STORAGE COEFFICIENT

The storage coefficient for the confined aquifers and permeable zones was estimated by applying a specific storage 

value of l.OxlO'6 per foot times the layer thickness. This 

specific storage value is an approximate value for most confined aquifers (Lohman, 1972, p. 8).

The storage coefficient of unconfined aquifers is virtually 

equal to the specific yield, which means that nearly all of the 

water is released from storage by gravity drainage (Lohman, 

1972, p. 8). The lower Claiborne-upper Wilcox aquifer has 

an exceptionally high percentage of sand relative to other 

aquifers and permeable zones in the study area (Ryder, 1988, 

table 3; Hosman and Weiss, 1991, fig. 33). The sand has 

been significantly dewatered in parts of the outcrop because 

of intense withdrawal in the agricultural Winter Garden area. 

An initial specific yield of 0.2 was assumed for this unit in 

its outcrop area; this is the average value between the general 

limits of 0.1 and 0.3 for unconfined aquifers (Lohman, 1972, 

p. 54). The value was reduced during model calibration to a 

maximum of 0.15.

Some of the shallow sands in the outcrop of units other 

than the lower Claiborne-upper Wilcox aquifer undoubtedly 

have storage-coefficient values appropriate for unconfined 

aquifers. However, for modeling purposes, it is assumed that 

there is no significant change in the shallow water table 

because (1) the sands are recharged sufficiently by precipitation and return flow from applied irrigation water, or (2) 

there is no significant ground-water withdrawal near the 

sands. Thus, in the digital model a confined storage coefficient is assigned to these units.

Values of storage coefficient in table 5 are as large as 

0.15, which is the estimated specific yield in the unconfined
E14 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

TABLE 3. Horizontal hydraulic conductivity and transmissivity of aquifers and permeable zones based on model calibration, Texas Gulf

Coast aquifer systems 

[<, less than]

Aquifer Horizontal hydraulic 

or conductivity 

permeable (feet per day)

zone Mean Minimum Maximum

Permeable 

zone A. ........ 25 12 102

Permeable 

zone B. ........ 23 2 25

Permeable 

zone C. ........ 15 7 31

Permeable 

zone D. ........ 16 8 21

Permeable 

zone E. ........ 15 15 15

Upper Claiborne 

aquifer. ....... 15 15 15

Middle Claiborne 

aquifer. ....... 15 15 15

Transmissivity 

(feet squared per dav)

Mean Minimum Maximum

6,956 <500 50,065

10,262 <500 27,534

9,298 <500 26,813

10,383 <500 49,258

7,766 <500 47,020

5,998 <500 23,101

4.602 <500 22.915

Lower Claiborneupper Wilcox 

aquifer.......

Middle Wilcox 

aquifer....

23

10 10

72

10

4,659 <500 30,922

10,071 <500 34,688

part of the lower Claiborne-upper Wilcox aquifer. Means 

range from 5.2xlO"4 for the middle Claiborne aquifer to 

1.7xlO"3 for permeable zone C and the middle Wilcox 

aquifer.

Storage in confining units is considered negligible. However, significant amounts of water may be released from storage in younger clay beds in permeable zones A, B, and C 

when sufficient declines in head cause the beds to compact 

inelastically. The release of water by inelastic compaction of 

fine-grained deposits in these permeable zones causes 

land-surface subsidence. A digital model code incorporated 

from Leake and Prudic (1989) provides for the release of 

water by inelastic compaction of fine-grained deposits by

converting from an elastic to an inelastic specific storage. 

This conversion is done for the clay part of a permeable zone 

when the head in a grid block declines below the critical 

head. The critical head is defined as the head that coincides 

with the maximum effective stress to which the deposits had 

previously been subjected.

When the critical-head decline is reached in model 

simulation, the initial specific storage value of l.OxlO"6 

per foot is increased by factors of 85, 80, and 8 for the 

clay parts of permeable zones A, B, and C, respectively, 

in the Houston-Galveston area. These factors were determined during model calibration and resulted from matching land subsidence in model simulations to measured
HYDROLOGY OF THE TEXAS GULF COAST AQUIFER SYSTEMS E15

TABLE 4. Vertical hydraulic conductivity of aquifers, permeable zones, and confining units based on calibrated model for

the Texas Gulf Coast aquifer systems

Aquifer,

permeable zone, or 

confining unit

Vertical hydraulic conductivity 

________(feet per day)_______

Mean Minimum Maximum

Permeable zone A..........

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

Permeable zone C..........

Zone D confining unit.....

Permeable zone D..........

Zone E confining unit.....

Permeable zone E..........

Vicksburg-Jackson

confining unit..........

Upper Claiborne aquifer...

Middle Claiborne

confining unit..........

Middle Claiborne aquifer..

Lower Claiborne

confining unit..........

Lower Claiborneupper Wilcox aquifer....

Middle Wilcox aquifer.....

3.8xlO~3 

1.3xlO~3 

S.OxlO"4 

l.lxlO"4 

1.3xlO~4 

l.lxlCT4 

2.1xlO~4

l.lxlO"5 

2.3xlO"3

A.lxlO"5 

4.6xlO~4

2.1xlO~5

2.0xlO~3 

l.OxlO"4

2.4xlO~6 

l.OxlO"6 

5.3xlO~6 

l.OxlO"4 

l.OxlO"5 

l.OxlO"4 

1.0x10-4

l.OxlO"5 

l.OxlO'3

4.0xlO~5 

l.OxlO'5

2.0xlO~5

2.0xlO"6 

1.2xlO"6

l.OxlO"1 

S.OxlO"2 

S.OxlO"2 

1.7xlO~4 

8.3xlO~4 

1.3xlO~4 

l.OxlO"2

l.SxlO"5 

S.OxlO"2

S.AxlO"5 

l.OxlO"2

3.4xlO"5

l.OxlO"2 

1.2xlO"3

values. The details of observed and simulated 

land-surface subsidence are discussed in the section on 

"Model Documentation."

DISSOLVED-SOLIDS CONCENTRATION AND 

DENSITY OF WATER

The concentration of dissolved solids in water is commonly used as an indication of the water's suitability for use. 

The terms used to describe water salinity in this report are as 

follows (Hem, 1985, p. 157):

Dissolved-solids concentration (mg/L)

Fresh

Slightly saline 

Moderately saline 

Very saline 

Briny

0-1,000

1,000-3,000

3,000-10,000

10,000-35,000

More than 35,000

Freshwater requires little or no treatment for most public 

and industrial use. Slightly saline water is marginal for 

many uses and often requires treatment or mixing with less
E16 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

TABLE 5. Calibrated elastic storage coefficient for aquifers and permeable zones, and inelastic specific storage for compacting clays,

Texas Gulf Coast aquifer systems 

[Leaders ( ), assumed to have no clays subject to inelastic compaction]

Aquifer or 

permeable zone

Permeable zone A. ..........

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

Permeable zone C. ..........

Permeable zone D. ..........

Permeable zone E. ..........

Upper Claiborne aquifer. . . . 

Middle Claiborne aquifer... 

Lower Claiborne-upper 

Wilcox aquifer. ..........

Middle Wilcox aquifer. .....

Storage coefficient 

(dimensionless)

Mean

6.0xlO"4 

1.3xlO"3 

1.7xlO~3 

1.5xlO"3 

1.2xlO~3

7.8xlO~4 

5.2xlO~4

5.7xlO"4 

1.7xlO"3

Maximum

9.4xlO~4 

4.4xlO~3 

4.9xlO~3 

5.1xlO"3 

4.4xlO~3

2.5xlO~3 

2.5xlO~3

5.2xlO"3

Inelastic 

specific 

storage 

(per foot)

8.5xlO"5 

S.OxlO"5 

S.OxlO'6

Specific yield assigned in outcrop area.

mineralized water. The general term "saltwater" is used to 

describe water that is not fresh.

The areal distributions of dissolved-solids concentrations 

in water for the aquifers and permeable zones are shown on 

plate 2. The maps, modified from Pettijohn and others 

(1988), were generated from two types of data: (1) Chemical analyses of water where dissolved-solids concentrations 

are less than 10,000 mg/L, and (2) borehole geophysical log 

interpretations where concentrations are equal to or greater 

than 10,000 mg/L. Because concentrations of dissolved 

solids vary with depth within a given aquifer or permeable 

zone, a depth-integrated average dissolved-solids 

concentration was calculated for each 100-mi2 grid block 

(regional grid, fig. 2) in order to construct the maps (Pettijohn and others, 1988).

Dissolved-solids concentrations generally increase in a 

downdip, gulfward direction. For most of the units, particularly the coastal lowlands permeable zones, dissolved-solids 

concentrations are higher in the west than in the east. 

Dissolved-solids concentrations range from less than 500 

mg/L for nearly all of the units to 150,000 mg/L in a downdip area of permeable zone E (pi. 2). The areal extent of 

fresh to slightly saline water (dissolved-solids concentration 

less than 2,000 mg/L) is particularly small in permeable 

zone E and in the upper Claiborne aquifer (pi. 2). This 

small area is apparently due, at least in part, to the proximity of zone E confining unit and to the intervening, poorly 

permeable Vicksburg-Jackson confining unit; thus, there is a 

restricted and sluggish flow system for permeable zone E 

and the upper Claiborne aquifer.

When the dissolved-solids concentration of water 

approaches about 10,000 mg/L, the greater density of the 

water will begin to substantially affect its flow characteristics within the aquifer system. The technical aspects of 

variable-density ground-water flow are explained by Kuiper 

(1985), whose model was used to simulate the flow system. 

Density of water in the aquifers and permeable zones was 

calculated by using a linear relation between density and 

dissolved-solids concentration (J.S. Weiss, U.S. Geological 

Survey, written commun., 1985). Density was corrected for 

the effects of mean water temperature and hydrostatic pressure at each grid block (A.K. Williamson, U.S. Geological 

Survey, written commun., 1987) before inclusion into the 

variable-density flow model. Grid-block values of density 

estimated for aquifers and permeable zones were assigned to 

subjacent confining units for modeling purposes. A statistical summary of water density (corrected for temperature and 

pressure) is given in table 6. Mean values range from 1.001 

g/cm3 for the lower Claiborne-upper Wilcox aquifer to 

1.027 g/cm3 for zone D confining unit.

REGIONAL FLOW SYSTEM

The predevelopment flow system was described in detail 

by Ryder (1988). He presented simulated predevelopment 

potentiometric-surface maps for each aquifer and permeable 

zone discussed herein. Further model calibration has 

resulted in considerably less flow in the simulated 

predevelopment flow system than was previously reported 

but little change in the simulated potentiometric surfaces.
HYDROLOGY OF THE TEXAS GULF COAST AQUIFER SYSTEMS 

TABLE 6. Estimated density of water in the h\drogeologic unit.s of the Texas Gulf Coast aquifer systems

Water density at atmospheric pressure,

20°C 

____(grams per cubic centimeter)____

Aquifer,

permeable zone, or 

confining unit Mean Minimum Maximum

E17

Permeable zone A........... 1.004

Permeable zone B........... 1.006

Permeable zone C........... 1.019

Zone D confining unit...... 1.027

Permeable zone D........... 1.019

Zone E confining unit...... 1.024

Permeable zone E........... 1.020

Vicksburg-Jackson

confining unit........... 1.016

Upper Claiborne aquifer.... 1.011

Middle Claiborne

confining unit........... 1.010

Middle Claiborne aquifer... 1.003

Lower Claiborne

confining unit........... 1.003

Lower Claiborne-upper

Wilcox aquifer........... 1.001

Middle Wilcox aquifer...... 1.005

0.995

.995

.994

.997

.994

.995

.990

.990

.988

.988

.990

.994

.988

.989

1.029

1.048

1.069

1.068

1.087

1.087

1.072

1.072

1.047

1.047

1.046

1.046

1.020

1.047

The newer flow values are presented, including a map (pi. 3) 

of recharge and discharge in the outcrop areas and a summary of vertical flow components for each layer. Changes 

in the simulated flow system from predevelopment to 1982 

are described, and a quantitative analysis of the simulated 

regional flow system for 1982 is presented.

PREDEVELOPMENT STEADY-STATE FLOW SYSTEM

The flow system prior to development is assumed to be a 

steady-state system in which inflow from the recharge areas 

is equal to outflow in the discharge areas. There is no net 

accretion to or release of water from storage in the aquifer

systems. The assumption of steady state seems reasonable 

over long-term conditions. Climatic changes, sea-level 

changes, and tectonic movements that may have been occurring over the past hundreds or thousands of years have been 

neither rapid nor significant enough to preclude the 

establishment of steady-state conditions.

CONCEPTUAL MODEL OF SYSTEM

Water in the Texas Gulf Coast aquifer systems generally 

originates as precipitation that falls on the outcrops of the 

various hydrogeologic units. Most of the precipitation is 

returned to the atmosphere by evaporation and by plant
E18 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

transpiration. Much of the remaining precipitation runs into 

streams that drain into the Gulf of Mexico, while a small 

amount, on the order of a few inches per year, infiltrates to 

the water table in topographically high parts of the outcrops. 

After reaching the water table, most of the water moves 

downgradient for relatively short distances and is discharged 

in topographically low areas in the outcrop in the form of 

evapotranspiration, seepage, and stream baseflow. A smaller 

but significant part of the water enters a deeper confined 

part of the aquifer where vertical-head gradients cause 

upward or downward leakage into adjacent units.

A variation of the above conceptual model occurs in the 

southwestern part of the study area. Westward from a line 

through Corpus Christi and San Antonio, average annual 

precipitation ranges from about 30 to about 21 in. In this 

area of lesser precipitation, many of the streams are intermittent, and streamflow that does occur is often a source of 

recharge to the aquifers. Although springs and seeps generally are not visible, it is probable that a small, net 

ground-water discharge occurs in topographically low areas. 

Ground water is discharged principally by evapotranspiration in this part of the study area, primarily by phreatophytes, such as pecans, salt cedars, and mesquite.

The conceptual model of the aquifer systems consists of 

four aquifers, five permeable zones, and six confining units 

(fig. 8). The water table in the uppermost 150 ft of the 

outcrop areas of the units functions as a source/sink; it is 

assumed to be at a constant altitude and provides recharge to 

and discharge from the deeper parts of the flow system. The 

relation between the conceptual model of the system and the 

digital model also is shown in figure 8. The construction of 

the digital model was discussed in detail by Ryder (1988) 

and is summarized in the section on "Model Documentation."

RECHARGE AND INTERAQUIFER LEAKAGE

Simulated values of recharge and discharge in the outcrop 

areas of the hydrogeologic units under predevelopment conditions are shown on plate 3. The pattern of recharge and 

discharge is a reflection of topography; recharge occurs on 

topographically high areas, and discharge occurs in the 

stream valleys, low-lying coastal areas, and in the gulf. The 

highest values of recharge, 1 to 3 in./yr, were simulated over 

relatively small areas. Generally, values of recharge and 

discharge are between 0 and 1 in./yr. The average recharge 

in the study area is 0.34 in./yr. Because the discharge area 

is larger, the average discharge rate is only 0.18 in./yr. A 

more detailed discussion of the technical aspects of recharge 

and interaquifer leakage is in Ryder (1988, p. 102-103).

A summary of the upward, downward, and net vertical 

flow rates for the outcrop and downdip parts of each aquifer 

and permeable zone is given in figure 9. The rates are for 

the study area only. The net leakage is always upward into

the overlying unit. Net leakage rates range from nearly zero 

out of the lowermost unit to 24 million ftVd into the uppermost unit.

Simulation indicates a total recharge in the outcrop areas 

of 85 million ftVd. The middle Claiborne aquifer receives 

the largest share, 16 million ftVd, closely followed by permeable zones B, A, and C with 15, 13, and 13 million ft3/d, 

respectively (fig. 9). Because steady-state conditions are 

assumed, the total recharge rate of 85 million ftVd is offset 

by an equal rate consisting of discharge in the outcrop (79 

million ftVd) and net lateral flow out of the study area (6 

million ftVd). Of the discharge in the outcrop areas, permeable zone A was simulated as discharging the most 37 million ft3/d or nearly 50 percent of the total. Factors that 

account for the large rates of recharge and discharge in permeable zone A are its relatively large outcrop area and its 

relatively large values of vertical and horizontal hydraulic 

conductivity.

FLOW SYSTEM, PREDEVELOPMENT TO 1982

A digital ground-water flow model was calibrated for 

transient conditions, predevelopment to 1982, and is 

discussed in detail in the section on "Model Documentation." Some model input data and results are presented here to quantify changes in the flow system 

from predevelopment to 1982 and to provide a quantitative description of the regional flow system in 1982.

GROUND-WATER DEVELOPMENT

The estimated average pumping rates for selected time 

intervals for 1910-82 are shown in table 7 (for modeling 

purposes, no significant regional development prior to 1911 

is assumed). The pumping rates were relatively small and 

constant from 1911 to the late 1930's; rates nearly doubled 

during the 1940's and doubled again during the 1950's. 

Increases in pumping rates were smaller after 1957, and a 

leveling-off of rates is apparent after 1968.

Cumulative volumes of withdrawal, net recharge, and 

release of water from storage in the Texas Gulf Coast aquifer systems are shown in figure 10. These values are simulated by the model for the period 1910-82. By 1982, an 

estimated 3.2 trillion ft3 of water had been withdrawn from 

the aquifer systems. Of this, 2.4 trillion ft3 or 75 percent 

had been derived from recharge, and 0.8 trillion ft3 or 25 

percent had been derived from storage in aquifers and compacting clay beds.

The withdrawal distribution for 1980 was used for the 

simulation periods 1977-81 and 1982. Details of the withdrawal estimates are in the section on "Model Documentation" under "Hydrologic Input Data." In 1980, total 

withdrawal from the aquifer systems in the study area was
NORTHWEST

Top of geopressured zone

EXPLANATION 

PREDOMINANTLY SAND PREDOMINANTLY CLAY 

5 MODEL LAYER NUMBER

SECTION OF CONCEPTUAL MODEL Not to scale

CONSTANT-HEAD LAYER 

(WATER TABLE)

Outcrops

Horizontal and 

vertical flow

IDEALIZED SECTION 

OF DIGITAL MODEL

CORRELATION OF STRATIGRAPHIC 

AND HYDROGEOLOGIC UNITS

Fm. = Formation

FIGURE 8 Conceptual model of Texas Gulf Coast aquifer systems and generalized representation in the digital model

9 i

-No-flow boundary I 11 -J.

12

O

on

lo

rn
E20 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

CONSTANT-HEAD WATER-TABLE LAYER

72 (30) 42 (24) ' 25 

4 » ®

(14) * 16 ©

13 (11) ( 5 > 2

. = -2 ®

L = -1 ©

3 (2) (0) 0 0 L = -1

L = 0

L = -2

4 (1) 1

©

CD

EXPLANATION

Aquifer or permeable zone and identifier

(T) Permeable zone A

(2) Permeable zone B

(3) Permeable zone C

(5) Permeable zone D

(j) Permeable zone E

(9) Upper Claiborne aquifer

(TJ) Middle Claiborne aquifer

^3) Lower Claiborne-upper Wilcox aquifer

@) Middle Wilcox aquifer

Confining unit

L = -2 Net lateral flow in to (+) or out of {-) study 

area

Total recharge or discharge in outcrop 

area Includes flow through superjacent 

confining unit where the confining unit is 

exposed at the surface. Number is flow rate, 

in million cubic feet per day

Recharge Discharge

Net recharge or discharge in outcrop 

area Includes flow through superjacent 

confining unit where the confining unit is 

exposed at the surface. Number is flow rate, 

in million cubic feet per day

(3)

Recharge Discharge

(4)

Total leakage through bottom of

equifer Number is flow rate, in million 

cubic feet per day

 f Upward Downward

Net leakage through bottom of

aquifer Number is flow rate, in million 

cubic feet per day

Upward

FIGURE 9. Simulated predevelopment vertical flow rates across hydrogeologic units in the Texas Gulf Coast study area. 

Sum of flow rates may not exactly balance because of independent rounding.

231 million ftVd (1.73 billion gal/d) (fig. 10). About 90 

percent of the withdrawal was from four units, permeable 

zones A, B, and C, and the lower Claiborne-upper Wilcox 

aquifer. The remaining two permeable zones and three aquifers were relatively undeveloped. The areal distribution of 

1980 withdrawal for each aquifer and permeable zone is 

shown on plate 4. The withdrawal is shown by 25-mi2 grid 

blocks, where withdrawal for a block equals or exceeds 0.5 

Mgal/d.

The largest withdrawal from permeable zone A is centered in Jackson and Wharton Counties (pi. 4). Moderate to

large withdrawal extends southward to the coast and eastward into Harris County. In 1980, permeable zone A was 

the most intensively pumped unit in the study area.

Intense withdrawal from permeable zone B is centered in 

Harris County, with moderate withdrawal extending into 

Galveston County (pi. 4). There is moderate withdrawal in 

updip areas in Colorado, Lavaca, Jackson, and Victoria 

Counties. Permeable zone B was the second most intensively pumped unit in 1980.

Intense withdrawal from deep wells in permeable zone C 

also is centered in Harris County (pi. 4). Withdrawal from
HYDROLOGY OF THE TEXAS GULF COAST AQUIFER SYSTEMS E21

TABLE 7. Estimated average pumping rates for digital model, 1910-82, 

Texas Gulf Coast aquifer systems

Average pumping rate

Pumping 

period

1..............

2..............

3...... ........

4.... ..........

5........ ......

6..............

7........ ......

8....... ...... .

9..............

10..............

Time 

interval

1910 

1911-37 

1938-47 

1948-57 

1958-62

1963-67 

1968-72 

1973-76 

1977-81 

1982

(Million 

cubic feet 

per day)

0 

34 

62 

122 

156

189

221 

222 

231 

231

(Million 

gallons 

per day)

0 

254 

464 

913 

1,167

1,414 

1,653 

1,661 

1,728 

1,728

permeable zones D and E, and from the upper and middle 

Claiborne aquifers, is relatively small (pi. 4).

Withdrawal from the lower Claiborne-upper Wilcox 

aquifer is moderate to large in the western counties of 

Zavala, Dimmit, Frio, La Salle, Atascosa, and Wilson (pi. 

4). Withdrawal is relatively small and scattered in the central and eastern areas. The lower Claiborne-upper Wilcox 

aquifer was the third most intensively pumped unit in 

1980. Scattered and relatively small withdrawal from the 

middle Wilcox aquifer occurs in the central and eastern 

areas (pi. 4).

RECHARGE, DISCHARGE, AND DISTRIBUTION OF FLOW, 1982

The concept of a shallow water table that is replenished 

by precipitation and kept at an essentially constant altitude 

on an average annual basis is valid for undeveloped areas 

and probably for developed areas receiving a large amount 

of precipitation such as in the eastern part of the study area. 

In some areas, however, such as in the west where withdrawal is large and precipitation is relatively small, dewatering has caused the water table to decline over the years.

The head for the water table is held constant through 

time, but the flow into or out of the boundary layer is the 

product of the vertical gradient between the constant head 

and the head in the aquifer, and the conductance to flow 

between the two layers. This conductance is the harmonic 

mean conductance of the bottom one-half of the 

constant-head grid block and the top one-half of the aquifer 

grid block. The amount of flow into or out of the 

constant-head layer can be reduced (to zero, if desired) by

reducing the conductance in the desired grid blocks during 

model calibration.

For the permeable zones of the coastal lowlands system, 

the extremely large amount of vertical anisotropy, caused by 

the numerous interspersed clay layers, precludes the existence of water-table conditions except in the shallowest sands 

in the upper few feet or few tens of feet in the outcrop 

areas. These shallow sands are assumed to be recharged 

sufficiently by precipitation and by return flow from applied 

irrigation water so that there is no long-term increase or 

decrease in the shallow water table.

Some investigators suggest a long-term rise in the shallow 

water table in agricultural areas because of return flow from 

applied irrigation water. Evidence to substantiate this rise is 

lacking. Long-term observation of the water level in a shallow well in an intensively pumped area of Jackson County 

shows a long-term decline (discussed in detail in the next 

section of this report). It is not known whether the 

water-level change in this one well represents a regional 

water-level change in the shallow sands. In the absence of 

further evidence, it is assumed that there is no long-term 

change in the shallow water table that would significantly 

affect the rates of recharge, discharge, and storage changes 

as computed in the model simulations.

Simulated values of recharge and discharge in the outcrop 

areas of the hydrologic units for the final year of the model 

simulation, 1982, are shown on plate 3. Two features are 

apparent when comparing 1982 recharge to predevelopment 

recharge, the decrease in size of discharge areas compared 

to recharge areas, and the appearance of high rates of 

recharge in the central and western parts of the study area 

(pi. 3). Thus, ground-water development causes a decrease 

in natural discharge as well as an increase in recharge.

The average 1982 recharge in the study area was 0.52 

in./yr, and the average discharge rate, excluding withdrawal, 

is 0.15 in./yr. As much as 6 in./yr of recharge occurs in 

Wharton and Jackson Counties. In this and adjacent areas, 

there is concentrated withdrawal for irrigation (mainly rice) 

from updip parts of permeable zones A and B. In these 

areas, the water withdrawn is supplied by local recharge. 

Where ground water is withdrawn for irrigation in outcrop 

areas, a considerable amount of applied irrigation water may 

reenter the aquifer by downward percolation. Jorgenscn 

(1975, p. 55) estimated that as much as 30 percent of 

ground water pumped for irrigation in the Katy area 

returned to the Chicot aquifer (permeable zones A and B; 

table 1). The Katy area is west of Houston and includes 

parts of Harris, Waller, and Fort Bend Counties; recharge in 

this area is as much as 5 in./yr (pi. 3). Thus, the recharge 

rates shown on plate 3, and as discussed generally in this 

report, may also include a considerable amount of return 

flow from irrigation in certain areas.

Another location of relatively high recharge, as much 

as 4 in./yr, is in and near the outcrop of the lower
E22 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

 300

,o

J co >-

f => <

t O Q

i z DC 

?o^

1910 1930 1940 1950 

YEAR

1960

;&- » 0 

19801982

=)-^S

FIGURE 10. Simulated pumpage, recharge, and depletion of storage, Texas Gulf Coast aquifer systems, 1910-82.

Claiborne-upper Wilcox aquifer in northern Zavala and 

northern Frio Counties (pi. 3). This is also an area of 

intensive withdrawal for irrigation, and return flow from 

irrigation water probably makes up a substantial part of 

the recharge as shown on plate 3. Mackey (1987) estimated that as much as 54 percent of irrigation withdrawal from the sand-and-gravel High Plains aquifer is 

returned to the aquifer in part of the panhandle area of 

northwest Texas. The similar nature of the aquifer 

materials and hydrologic conditions suggest that this 

higher value of return flow could apply to Zavala and 

Frio Counties.

Concentrated withdrawal for municipal and industrial use 

in the Houston-Galveston area is mainly from deep wells in 

highly confined permeable zones. Excess water is generally 

discharged to surface drainage systems, with little opportunity for return to the ground-water system. Recharge in the 

immediate vicinity is restricted by intervening clay beds. 

Deep cones of depression are developed until the cones 

intercept sufficient recharge or derive additional water from 

storage mainly from inelastic compaction of clays to satisfy 

withdrawal. Recharge tends to be less in the Houston-Galveston area compared to the rice-irrigation areas in 

Wharton and Jackson Counties, as shown on plate 3.

The difference in simulated rates of recharge and discharge in the outcrop areas between predevelopment and 

1982 is shown in figure 11. The area of greatest increase in 

recharge or decrease in discharge is in the updip areas of 

permeable zones A and B extending from the Guadalupe 

River in the south to the Trinity River in the east. Although 

the area south of Houston has changed from a discharging 

to a recharging area, the increased recharge is small, considering that there is large withdrawal in this area. Gabrysch

(1977) reported that the presence of the Beaumont Clay 

overlying the permeable zones in this area restricts recharge.

A more moderate increase in recharge is in the coastal 

uplands units in western and northern Zavala County, and in 

the northern parts of Frio, Atascosa, and Wilson Counties. 

Another area of substantial increase in recharge, but of 

much less areal extent, is in the coastal uplands units along 

the Brazos River, near the junction of Milam, Robertson, 

Burleson, and Brazos Counties. Moderate to large withdrawal for irrigation and municipal supplies in this area is 

from several aquifers. Substantial but relatively smaller 

increases in recharge in other areas are scattered and of 

small areal extent.

A summary of outcrop recharge, outcrop discharge, vertical leakage, withdrawal, and change in storage in each 

hydrogeologic unit in the study area for 1982 is shown in 

figure 12. For the combined units, net recharge in the outcrop areas supplied 76 percent of the total discharge (withdrawal plus a small amount of lateral outflow), whereas 24 

percent of the discharge was derived from depletion of storage in aquifers and inelastic compaction of clay beds. Highest net recharge rates were in the outcrops of the four most 

intensively pumped units, permeable zones A, B, and C, and 

the lower Claiborne-upper Wilcox aquifer. Nearly 47 percent, or 84 million ft3/d, of the net recharge was into the 

most intensively pumped unit, permeable zone A.

Net downdip leakage tended to be from a unit with lesser 

withdrawal into an overlying or underlying unit with greater 

withdrawal. The highest rate of net vertical leakage was 6 

million ft3/d upward into permeable zone C from permeable 

zone D.

The change in the flow system between predevelopment 

and 1982 is dramatic. Flow through the system has more
HYDROLOGY OF THE TEXAS GULF COAST AQUIFER SYSTEMS E23

EXPLANATION

SIMULATED INCREASE IN RECHARGE OR 

DECREASE IN DISCHARGE, IN INCHES 

PER YEAR

] i--'-^Corpus Christ

, I KLEBE

1

50

50

I

I

100 KILOMETERS

100 MILES 

_J

FIGURE 11. Change in simulated recharge and discharge in the outcrop areas of aquifers, permeable zones, and confining units,

predevelopment to 1982, Texas Gulf Coast study area.
E24 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

CONSTANT-HEAD WATER-TABLE LAYER !

122 195) 27

nt=W

ttt

i i r ... +,; f

L = -1 S = 1 P = -86

(5l .1 11

I I A | 35 (33) A » 4 L = !

TTT Tl

= 1 S = 14 P = 51

L = -1 S = 13 P = -29

L=-2 S = 3 P = -4 (D

4(4) 10) 0 -1 S = 1 P = -1

lit'» L = -1 S = 0 P =

L=1 S=1 P = -43®

L = 0 S=18 P = -43

L = -1 S = 6 P = -10

EXPLANATION

AQUIFER OR PERMEABLE ZONE AND IDENTIFIER

(T) Permeable zone A

(2) Permeable zone B

(3) Permeable zone C

(§} Permeable zone D

(7) Permeable zone E

(?) Upper Claiborne aquifer

{Pj} Middle Claiborne aquifer

(J3) Lower Claiborne-upper Wilcox aquifer

{14} Middle Wilcox aquifer

CONFINING UNIT

TOTAL WATER BALANCE Numbers are million 

cubic feet per day. Million gallons per day in 

parentheses

Total pumpage (P) = -231 (1,728) 

Net lateral flow (L) = -5 (37) 

Total release of water

from storage (S) = 57 (426) 

Net recharge from

constant heads = 179 (1,339)

L = -2 NET LATERAL FLOW IN (+) OR OUT (-) OF STUDY 

AREA

TOTAL RECHARGE OR DISCHARGE IN OUTCROP 

AREA Includes flow through superjacent 

confining unit where the confining unit is 

exposed at the surface. Number is flow rate, 

in million cubic feet per day

Recharge Discharge

NET RECHARGE OR DISCHARGE IN OUTCROP 

AREA Includes flow through superjacent 

confining unit where the confining unit is 

exposed at the surface. Number is flow rate, 

in million cubic feet per day

1 Recharge

(7)

TOTAL LEAKAGE THROUGH BOTTOM OF 

AQUIFER Number is flow rate, in million 

cubic feet per day

Upward Downward

NET LEAKAGE THROUGH BOTTOM OF 

AQUIFER Number is flow rate, in million 

cubic feet per day

(2)

Upward Downward

FIGURE 12. Simulated 1982 vertical flow rates across hydrogeologic units in the Texas Gulf Coast study area. Sum of 

flow rates may not exactly balance because of independent founding.

than tripled. Withdrawal in 1982 was almost three times the 

predevelopment recharge rate (table 8).

SUBREGIONAL FLOW SYSTEMS AND

HYDROLOGIC EFFECTS OF

DEVELOPMENT

For a more detailed analysis of the flow system, the study 

area was divided along model rows into four model subareas 

(fig. 13). The boundaries of the subareas were chosen to be

nearly parallel to flow lines in the various hydrogeologic 

units or to approximately follow the axes of ground-water 

divides between cones of depression developed on the 1982 

potentiometric surfaces of intensively pumped hydrologic 

units (model layers).

Six intensively pumped areas were selected for further 

description and model analysis (fig. 13). The areas were 

selected on the basis of large withdrawal that has caused 

significant declines in heads over a substantial area,^ with 

present or potential adverse hydrologic effects. The six 

areas nearly coincide with six "critical areas" proposed for
HYDROLOGY OF THE TEXAS GULF COAST AQUIFER SYSTEMS 

TABLE 8. Simulated recharge, storage, and discharge in the Texas Gulf Coasi aquifer systems, predevelopmenl and 1982

E25

Recharge and storage 

(million cubic feet per day)

Discharge 

(million cubic feet per day)

Predevelopment conditions

Total recharge........ 85 Outcrop discharge...... 79

Lateral outflow........ 6

1982 conditions

Total recharge.........

Decrease in storage..

Total.............

.219

.. 57

276

Pumpage.. ..............

Outcrop discharge..... 

Lateral outflow.....

231

.. 40 

... 5

276

this part of the State by the Texas Water Commission 

(written commun., 1986). The commission defined a critical area as an area that has or that is expected to have critical ground-water problems. The six areas in figure 13 are 

referred to in this report as the:

1. Houston-Galveston area

2. Coastal rice-irrigation area

3. Evadale-Beaumont area

4. Kingsville area

5. Winter Garden area

6. Lufkin-Nacogdoches area

HOUSTON-GALVESTON AREA

The Houston-Galveston area (fig. 13) comprises all or 

parts of eight counties: Chambers, Galveston, Brazoria, Fort 

Bend, Waller, Harris, Montgomery, and Liberty. 

Ground-water withdrawal data for 1960-85 for counties 

with the most intense withdrawal within area 1 are shown in 

figure 14. Withdrawal in Waller and Fort Bend Counties in 

the west is predominantly for irrigation use, and withdrawal 

in Brazoria County in the south is about evenly divided 

between irrigation use and nonirrigation use. Withdrawal in 

Harris and Galveston Counties in the east is predominantly 

for municipal and industrial use. Withdrawal in Harris 

County is larger than the combined withdrawal from the 

other counties in area 1, with a total pumping rate of 380 

Mgal/d in 1985. The most apparent trends in withdrawal are 

the declines in pumping rates in Harris and Galveston Counties since 1970 (fig. 14).

In reports that describe this area, several local areas have 

been defined (fig. 15). These are: Katy, Houston, Pasadena,

Baytown-Laporte, Johnson Space Center, Alta Loma, and 

Texas City. Average rates of withdrawal in each local area 

for 1979 are shown in figure 15. In the Katy area, recent 

municipal withdrawal has been added to the agricultural 

withdrawals that were used primarily for rice irrigation. The 

Houston area includes the city of Houston and the surrounding metropolitan area. Ground-water withdrawals are primarily for municipal use. Flowing wells (with depths ranging 

from about 700 to 1,300 ft) were common in the area in the 

1890's; heads were 15 to 30 ft above land surface. Historically, withdrawals in the Houston area have steadily 

increased. In 1930, average private and public withdrawal 

in the Houston area was 52 Mgal/d. By 1932, water levels 

had lowered to 80 ft below land surface in downtown Houston (White and others, 1932, p. 10-11). Vertical gradients 

are now downward from the water table in most of the 

Houston area, which means that there is a potential for 

deeper units to receive recharge from the water table. 

Recharge from the water table is consistent with model simulation results.

The Pasadena area is east of Houston and includes a 

heavily industrialized zone along the Houston Ship Channel. 

In 1937, a Pasadena paper mill added 19 Mgal/d of 

ground-water withdrawals to the area, which accelerated 

water-level declines (White, 1938). This area has had large 

ground-water withdrawals since 1937, with peak usage in 

1968. Surface water has also been used throughout the history of development, but in 1977 surface water became the 

primary source of water for the area. The adjoining areas of 

Baytown-Laporte and the Johnson Space Center also had 

drastic reductions in industrial ground-water withdrawal 

when surface-water use was increased in the mid-1970's.
E26 REGIONAL AQUIFER-SYSTEM ANALYSIS- - GULF COASTAL PLAIN

24

EXPLANATION

INTENSIVELY PUMPED AREA AND 

IDENTIFIER

BOUNDARY AND IDENTIFIER OF 

MODEL SUBAREA

PRECIPITATION STATION AND 

IDENTIFIER

OBSERVATION WELL AND NUMBER

^u

EVADALE- x .. 

BEAUMONT' > 

AREA

, fHOUSTONGALVESTON

AREA

KINGSVILLE 

AREA

i

100 KILOMETERS

FIGURE 13. Locations of observation wells and precipitation stations in selected intensively pumped areas, and model subareas, Texas Gulf

Coast study area.

In the Alia Loma area, ground-water withdrawals are used 

for public supply for the town of Alta Loma and the city of 

Galveston. At the old Alta Loma well field in Galveston

County, heads in wells ranging from 726 to 868 ft deep 

were 28 ft above land surface in 1893; by 1939 water levels 

were 45 to 50 ft below land surface (Petitt and Winslow,
HYDROLOGY OF THE TEXAS GULF COAST AQUIFER SYSTEMS E27

BURLESON COUNTY

- 9

 

.

, r.._

15

w-,

id

 

 ^rp

8 _ R

/ -

*fU

30

20

10

n

ROBERTSON COUNTY

20

_OT

30

te^!

19

Ifi 16

 

F-s

 

PE

 

=r^

IB

^n;

*fu

30

20

BRAZOS COUNTY

31

21 ^ N

14

,.nL

n ffi

5 "i

19 19 1 1 f ,LI

1

U1

1 1 ^~~W i T

Ln,. ,1 j 5 * ' fe

 j ; ' : ;

*fU

30

20

10

i ;

^ *- ~ ' .-

WALLER COUNTY

34

26

24

n

30

^

30

~\

27

 

.

.

H 1960 1965 1970 1975 1980 1985 1960 1965 1970 1975 1980 1985

1960 1965 1970 1975 1980 1985

EXPLANATION

PUMPAGE SCALE. IN MILLION GALLONS 

PER DAY Scale may vary according to county

MEAN PUMPING RATE, IN MILLION 

GALLONS PER DAY

IRRIGATION PUMPAGE

NONIRRIGATION PUMPAGE

YEAR

BRAZORIA COUNTY

39 &

32

 

20

,_}

u

1

m

i 

n

45

 

36

. PI

i

1960 1965 1970 1975 1980 1985 1960 1965 1970 1975 1980 1985

100 KILOMETERS

FIGURE 14. Ground-water pumpage in selected counties in the central part of the Texas Gulf Coast study area, 1960-85.
E28 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

MrWTON ,'

J PASADENA 

AREA

BAYTOWNLAPORTE

AREA

T~ /^ HOUSTON 

83 ' AREA

JOHNSON SPACE 

CENTER AREA

10 20 30 MILES

_J______I______i

EXPLANATION

AREA OF INTENSIVE GROUND-WATER 

WITHDRAWALS From Gabrysch 

(1984a, p. 4-6)

OBSERVATION WELL AND NUMBER

AVERAGE PUMPAGE, IN MILLION 

GALLONS PER DAY Upper 

number is public and industrial 

pumpage. Lower number is 

irrigation pumpage

i l I I

0 10 20 30 KILOMETERS

FIGURE 15. Location of areas of intensive ground-water withdrawals within the Houston-Galveston area (area I, fig. 13) and average rates of

pumping for 1979.

1955, p. 24). Ground-water withdrawals gradually increased 

in the county, mostly as a result of industrial expansion in 

the Texas City area. Since the early 1970's, ground-water 

use in the Alta Loma and Texas City areas has decreased 

substantially because of increased reliance on surface water. 

Withdrawal in the Houston-Galveston area is from permeable zones A, B, and C. The 1980 withdrawal distribution 

by layer and by model grid block is shown on plate 4. 

Withdrawal has created large cones of depression in the 

potentiometric surfaces of the three permeable zones, with 

the centers in the general area of Houston. Declines in the 

potentiometric surface for permeable zones A, B, and C

are shown on plate 5. The maps were constructed by subtracting 1982 measured heads (mean heads in 25-mi2 grid 

blocks) from measured predevelopment heads for each permeable zone. Maximum head declines range from nearly 

200 ft for permeable zone A to more than 400 ft for permeable zone C.

Hydrographs of wells 9, 7, and 3 (pi. 5) were selected 

for analysis because of their long-term record and close 

proximity of the wells to the centers of the cones of 

depression developed in permeable zones A, B, and C, 

respectively. The hydrograph for well 9, in southeastern 

Harris County in the Baytown-Laporte area (fig. 15), shows
HYDROLOGY OF THE TEXAS GULF COAST AQUIFER SYSTEMS E29

a continuous water-level decline until about 1976, when a 

substantial recovery of water levels began and continued 

into the 1980's. The hydrographs for wells 7 and 3 in the 

Houston area (fig. 15) show a more-or-less continuous 

decline in water levels from the early 1940's to the middle 

1980's.

Well 9 is in an area of southeastern Harris County that 

has had severe land-surface subsidence. The large 

ground-water withdrawals have reduced the artesian pressure 

sufficiently to cause water from included and adjacent clay 

beds to flow out. Water flows out of the compressible clay 

beds due to inelastic compaction that is largely irreversible. 

The loss of water from the clays results in a permanent loss 

of pore space. Land subsidence is approximately equivalent 

in volume to the reduction of pore space in the compacted 

clays. As shown on plate 5, an area of about 2,000 mi2 

around Galveston and Houston has subsided more than 1 ft. 

From the beginning of human development until 1973, maximum land subsidence of nearly 9 ft (pi. 5) has been 

recorded. Gabrysch (1984b) noted that from 1906 to 1978 

the Pasadena area east of Houston may have subsided as 

much as 10 ft.

Subsidence in low-lying areas of Harris and Galveston 

Counties, combined with proximity to the Gulf of Mexico, 

has increased the risk of flood damage to residential and 

commercial properties. Subsidence has also caused activation of faults, leading to structural damage (Harris-Galveslon Coastal Subsidence District, written commun., 

1983). With the creation of the Harris-Galveston Coastal 

Subsidence District in 1975, there has been a growing 

emphasis on reduction of ground-water withdrawal coupled 

with an increased reliance on surface-water supplies. Pumping rates in the late 1970's and early and middle 1980's 

were substantially reduced in much of southeastern Harris 

County and in Galveston County; this reduction has caused 

a recovery of water levels and an end or sharp decrease in 

the rate of land-surface subsidence in that area.

In addition to the head declines in permeable zones A, B, 

and C, plate 5 shows horizontal flow for these zones as simulated by the model for 1982. The ground-water flow directions and relative magnitudes were computed at the 

intersection of four adjacent grid blocks. A flow vector 

(arrows, pi. 5) is the vector addition of the average of the 

two adjacent X-direction flows and the average of the two 

adjacent Y-dircction flows. The size of the arrowhead indicates the order of magnitude (integer component of the logarithm, base 10 (log,,,)) of the flow, and the length of the 

shaft is proportional to the mantissa of the log, 0 flow (Williamson and others, 1990, p. 76). The flow patterns are similar for the three zones, with the flow generally converging 

on areas that have the greatest amount of measured drawdown. The large withdrawals of ground water in the Houston-Galveslon area have reversed the coastward hydraulic

gradients, causing saltwater to move toward the centers of 

pumping.

An interesting circulating pattern of ground-water flow is 

shown in offshore areas of permeable zone C (pi. 5). The 

circulation cells occur because of density forces resulting 

from heat and solute gradients as the denser, heavier water 

overturns to get under the fresher, lighter water (Williamson 

and others, 1990, p. 76).

The simulated average interstitial velocity of saltwater 

moving toward the pumping centers in 1982, assuming an 

effective porosity of 0.2 for the sands, is 22 ft/yr for permeable zone A, 41 ft/yr for permeable zone B, and 111 ft/yr 

for permeable zone C. These arc average simulated velocities for water moving through the sands in each permeable 

zone (zones A, B, and C); water in individual sand beds 

may actually have greater or smaller velocity, depending 

upon permeability.

Lateral migration of saltwater was evident in 1954, when 

water in observation wells near the freshwater-saltwater 

interface in Harris County had an increase in chloride concentration of 122 mg/L since 1951 (Winslow and others, 

1957). Saltwater contamination was also occurring in the 

shallower Chicot aquifer wells near the Houston Ship Channel (Baker and Wall, 1976). Changes in chloride concentrations with depth have been analyzed along the ship channel. 

The interpretations of the chemical data, along with head 

differences, indicate that saltwater is moving both vertically 

and laterally toward the Chicot aquifer (Jorgensen, 1977). 

The city of Galveston had to relocate the Alta Loma well 

field northward because the old well field was contaminated 

by movement of saltwater from below and from downdip 

(Petitt and Winslow, 1955).

A summary of 1982 withdrawal, change in storage, and 

vertical flow rates (recharge and leakage) for each unit in 

model subarea II (fig. 13) is shown in figure 16. The area 

encompasses the major cones of depression developed in 

permeable zones A, B, and C in the Houston-Galveston 

area. About 90 percent of the total withdrawal in subarea II 

is from permeable zones A, B, and C. Small amounts of net 

lateral flow between model subareas are quantified by "L" in 

figure 16. On the basis of simulation, net recharge in the 

outcrop areas accounted for 68 percent of the total pumping 

rate of 102 million ft-Vd (763 Mgal/d) for all units in subarea 

II. Net recharge includes a decrease of natural discharge 

and an increase in induced recharge. Part of the induced 

recharge may include a return flow from applied irrigation 

water, as discussed earlier. Twenty-eight percent of the 

pumping rate was derived from depletion of ground water in 

storage, and the remaining 4 percent was net lateral inflow 

from adjacent model subareas (I and III, fig. 13). Nearly 61 

percent of the net recharge was in the outcrop of permeable 

zone A. The highest rate of net vertical leakage was 9 million ft-Vd downward into permeable zone B from permeable 

zone A.
E30 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COAS FAL PLAIN

+++ 30 (27) 3

CONSTANT-HEAD WATER-TABLE LAYER ! -^^^

nf (T) L - 1S - 1P -35

L=1 S = 12

(1) (0) 0 J

(1) HI r^f^ L= 1 S =

© ^

L = 0 S = 2 P = -1 ©

= -0 S=1 P = -1

i i A 2 <°> , ;; ° ; 

ill' . .' I i n i

L = 0 S = 0 P

(0) =H=^ L= 1 S

^^ L = OS = OP = -1

2 (2) 0 (0) L=0 S = 2 P = -4 © w

EXPLANATION

L= 1

AQUIFER OR PERMEABLE ZONE AND IDENTIFIER

(T) Permeable zone A

(5) Permeable zone B

(3) Permeable zone C

@ Permeable zone D

(7) Permeable zone E

(5) Upper Claiborne aquifer

@ Middle Claiborne aquifer

Q3) Lower Claiborne-upper Wilcox aquifer

(O) Middle Wilcox aquifer

CONFINING UNIT

TOTAL WATER BALANCE Numbers are million 

cubic feet per day. Million gallons per day in 

parentheses

Total pumpage (P) = -102 (763) 

Net lateral flow (L) = 4 (30) 

Total release of water

from storage (S) = 29 (217) 

Net recharge from

constant heads = 69 (516)

NET LATERAL FLOW IN (+) OR OUT (-) OF STUDY 

AREA

TOTAL RECHARGE OR DISCHARGE IN OUTCROP 

AREA Includes flow through superjacent 

confining unit where the confining unit is 

exposed at the surface. Number is flow rate, 

in million cubic feet per day

1

Recharge t Discharge

NET RECHARGE OR DISCHARGE IN OUTCROP 

AREA Includes flow through superjacent 

confining unit where the confining unit is 

exposed at the surface. Number is flow rate, 

in million cubic feet per day

Recharge

(5)

TOTAL LEAKAGE THROUGH BOTTOM OF 

AQUIFER Number is flow rate, in million 

cubic feet per day

Upward Downward

NET LEAKAGE THROUGH BOTTOM OF 

AQUIFER Number is flow rate, in million 

cubic feet per day

(2)

Upward Downward

FIGURE 16. Simulated 1982 venical flow rates across hydrogeologic units in model subarea II, Texas Gulf Coast study 

area. Sum of flow rates may not exactly balance because of independent rounding.

In the Houston-Galveston area, model results indicate that 

about 21.9 million ftVd or 76 percent of the 29 million ftVd 

of water released from storage from all model layers (fig. 

16) during the 1982 simulation period was from inelastic 

compaction of clays in permeable zones A, B, and C. The 

simulated rates of water released from storage from these 

permeable zones for 1982 are summarized in table 9. For 

the combined zones A, B, and C, about 26 percent of the 

pumping rate of 92 million ft-Vd is supplied from storage 

releases; about 24 percent of the pumping rate is supplied

from the component of storage that results from inelastic 

compaction of clays.

COASTAL RICE-IRRIGATION AREA

The coastal rice-irrigation area (area 2, fig. 13) consists of 

most of Jackson and Wharton Counties and parts of Colorado, Lavaca, Victoria, and Matagorda Counties. Water 

withdrawal is mainly for irrigation, as shown in figure 14 for 

Wharton, Jackson, and Matagorda Counties. Rice is the
E31

TABLE 9. Simulated rates of water released from storage in 1982 from permeable zones A, B, and C, Texas Gulf Coast

aquifer systems

Hydrogeo logic 

unit

Permeable

r7nn o A

Permeable 

zone B. . . . .

Permeable 

zone C. . . . .

Pumping 

rate 

in 1982 

(million 

cubic feet 

per day)

35 

35

22

Total rate of 

release of water 

from storage

Amount 

(million 

cubic feet 

per day)

0.9 

12.3 

10.8

Percent 

of 

pumpage

2.6 

35 

49

Rate of release 

of water resulting 

from inelastic 

compaction of clays 

Amount Percent 

(million of 

cubic feet pumpage 

per day)

0.8 

11.9 

9.2

2.3 

34 

42

major crop in the area, and the two-crop rice season requires 

large amounts of water for irrigation.

Withdrawal in area 2 is mostly from permeable zone A. 

A substantial amount of water is withdrawn from permeable 

zone B, mainly in the outcrop area, but other hydrologic 

units are essentially undeveloped. The 1980 withdrawal distribution by grid blocks is shown on plate 4 for permeable 

zone A and permeable zone B. As mentioned previously, 

model results indicate that the water withdrawn in this area 

is supplied by local recharge. Water derived from artesian 

storage in the units has been relatively small, and steep 

cones of depression, such as those in the Houston area, have 

not developed.

Ground-water withdrawal began in the early 1900's. 

Originally, artesian pressures were large enough to produce 

flowing wells from the Chicot and Evangeline aquifers 

(table 1). As pumping increased, most wells ceased to flow 

by the mid-1940's (Loskot and others, 1982, p. 33). In the 

early 1950's, there was a sharp increase in withdrawal 

because of a period of below-normal precipitation combined 

with the introduction of the two-crop rice season. The largest water-level declines from 1944 to 1964 were in the western section of Wharton County and in the southeastern 

section of Jackson County. In western Matagorda County 

from 1944 to 1967, withdrawal for irrigation resulted in 

water-level declines of as much as 52 ft (Hammond, 1969, 

p. 32).

Hydrographs of wells 14a and 15 in Jackson County are 

shown in figure 17, as is a graph of mean annual precipitation in southwestern Wharton County. Well 14a is in northwestern Jackson County near an area of intensive 

withdrawal in permeable zone A. However, the well is only

76 ft deep and is open to the Holocene alluvium above the 

intensively pumped zone. The water table in well 14a has 

declined at a rate of about 0.6 ft/yr since the late 1930's, 

with periods of partial recovery in 1958-61 and 1969-74 

when precipitation was at or above normal (fig. 17). Baker 

and Follett (1973, p. 18) used this well and other data to 

conclude that shallow sands in the area have been dewatered 

on a long-term basis (nearly 40 years) and that recharge 

from rainfall has been unable to stabilize the water table. 

They further concluded that the reduction in the gradient of 

the water table toward the Lavaca River has probably 

reduced stream baseflow. However, it is not known whether 

the water-level change in this one well represents a regional 

water-level change in the shallow sands. In the absence of 

further evidence, no long-term change in the shallow water 

table is assumed that would significantly affect the rates of 

recharge, discharge, and storage changes as computed in the 

model simulations. Well 15 is open to permeable zone A 

and is near areas of moderate to large withdrawal from that 

zone. The water level in well 15 has been declining at an 

average rate of about 1.4 ft/yr since the middle 1950's (the 

beginning of record).

The large withdrawal in the coastal rice-irrigation area has 

resulted in compaction of clays present within the permeable 

zones. From 1900 to 1975, the land surface subsided less 

than 1 ft over most of the area, but Carr and others (1985, 

fig. 37) showed as much as 1.5 ft of subsidence in eastern 

Jackson and western Matagorda Counties. Because the subsidence is fairly evenly distributed over large, mostly rural 

areas, the undesirable effects sometimes associated with 

subsidence are minimized.
E32 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

WELLUa

10

20

60

State well number: PP-66-58-601 

 Land-surface datum: 90 feet 

Well depth: 76 feet 

Aquifer: Permeable zone A (alluvium)

i i i i

WELL 15

0 

50

State well number: PP-80-06-703 

Land-surface datum: 38 feet 

Well depth: 590 feet 

Aquifer: Permeable zone A

! I I ! I I I I I I I I I I I I I

70

60

50

40

50

50

80

g w 60

^

£^40

a?

£ 20

0 

19

PRECIPITATION STATION B

/Long-term average (1931-84) _

- /

- '

i;

/

/

^LL '\hT

 

rj1

?

:

 - -j

n^ n

« -

i

---

T

n

-i-- - - 

i~^

,-; *1 H -

rn r-!

-> - -- R"r

fr M * *

-!

^n B

--

30 1935 1940 1945 1950 1955 1960 1965 1970 1975

YEAR

1::

pr-j

-'- ft

|-i ^.-

 ;:\

1980 19

FIGURE 17. Ground-water levels in Jackson County and precipitation data in southwestern Wharton County, Texas. Well locations are shown in

figure 13.

Saltwater encroachment is a potential threat in this area, 

either by lateral migration updip or by vertical migration 

where freshwater sands are overlain by saltwater-bearing 

deposits. Saltwater encroachment is not currently a serious 

threat to the quality of ground water used in the coastal 

rice-irrigation area.

Simulated horizontal flow vectors for permeable zone A 

show a general zone of convergence near the intersection of 

Jackson, Wharton, and Matagorda Counties (pi. 5) resulting 

from intense withdrawal. A summary of 1982 flow components for units within model subarea III (fig. 13) is shown in 

figure 18. The simulated net recharge in the outcrop areas 

was 98 percent of the total pumping rate of 58 million ftVd 

for all units. Nearly 67 percent of the net recharge was in 

the outcrop of permeable zone A. About 4 million ftVd of 

water was released from storage in the simulations. A net 

lateral flow out of the model subarea amounted to about 3

million ft3/d. The highest rate of net vertical leakage was 5 

million ft3/d upward into permeable zone A from permeable 

zone B.

EVADALE-BEAUMONT AREA

The Evadale-Beaumont area (area 3, fig. 13) has large 

ground-water withdrawals at Evadale in southwest Jasper 

County and moderate withdrawals at Beaumont's well field 

in southeast Hardin County (fig. 19). Withdrawal in Jasper 

County has been fairly constant since 1970, but withdrawal 

in Hardin County, mainly for nonirrigation use, has risen 

since 1975.

Withdrawal at Evadale is about evenly divided between 

permeable zone B and permeable zone C. Withdrawal in 

the Beaumont well field is from permeable zone B. The 

1980 withdrawal distribution by grid blocks is shown on
HYDROLOGY OF THE TEXAS GULF COAST AQUIFER SYSTEMS E33

CONSTANT-HEAD WATER-TABLE LAYER

m

o^« \ooj . ,

LIT , it t

L = 0 S = 0 P = -43^

T-M ^^4 ©

©

1 (1) < 1 >

L = 0 S = 1 P = -1

L = 0 S = O P = O

©

ttH =^ : -1 S = O P = O ®

= O S = O P = O

lit1 i i t L °° S ° 1P "T

1 (1) 0 (0) L = -1 S = 1 P = -1

EXPLANATION

AQUIFER OR PERMEABLE ZONE AND IDENTIFIER

(T) Permeable zone A

(5) Permeable zone B

(3) Permeable zone C

(5) Permeable zone D

(?) Permeable zone E

(9) Upper Claiborne aquifer

@ Middle Claiborne aquifer

(13) Lower Claiborne-upper Wilcox aquifer

@ Middle Wilcox aquifer

CONFINING UNIT

TOTAL WATER BALANCE Numbers are million 

cubic feet per day. Million gallons per day in 

parentheses

Total pumpage (P) = -58 (434) 

Net lateral flow (L) = -3 (22) 

Total release of water

from storage (S) = 4 (30) 

Net recharge from

constant heads = 57 (426)

L = -1 NET LATERAL FLOW IN (+) OR OUT (-) OF STUDY 

AREA

TOTAL RECHARGE OR DISCHARGE IN OUTCROP 

AREA Includes flow through superjacent 

confining unit where the confining unit is 

exposed at the surface. Number is flow rate, 

in million cubic feet per day

Recharge Discharge

NET RECHARGE OR DISCHARGE IN OUTCROP 

AREA Includes flow through superjacent 

confining unit where the confining unit is 

exposed at the surface. Number is flow rate, 

in million cubic feet per day

I <0>

1 Recharge T Discharge

(4)

TOTAL LEAKAGE THROUGH BOTTOM OF 

AQUIFER Number is flow rate, in million 

cubic feet per day

Upward I Di'ownward

NET LEAKAGE THROUGH BOTTOM OF 

AQUIFER Number is flow rate, in million 

cubic feet per day

(5)

T Upward I Downward

FIGURE 18. Simulated 1982 vertical flow rates across hydrogeolof 

Coast study area. Sum of flow rates may not exactly bal

:ic units in model subarea III (fig. 13), Texas Gulf 

ance because of independent rounding.

plate 4 for permeable /one B and permeable zone C. Prior 

to 1955, ground-water resources in the Evadale area were 

relatively undeveloped. Predevelopment water levels in the 

lowest sands of permeable zone C that contained freshwater 

(locally called the Evangeline aquifer, table 1) were 20 to 

30 ft above land surface (Wesselman, 1967). In 1955, a 

paper mill began withdrawals of nearly 18 Mgal/d. Production increased to about 42 Mgal/d by 1970 and 

remained at approximately that rate through 1980. In Jefferson County, fresh ground-water supplies are limited. 

Prior to 1958, Beaumont relied entirely on surface water.

By 1965, Beaumont was pumping 6 Mgal/d from the well 

Held in Hardin County. It was the approximate rate 

through 1980.

Sufficient data were available to map the head declines for 

permeable zone C at Evadale (pi. 5). The map was constructed by subtracting 1982 measured heads from measured 

predevelopment heads. The map shows between 150 and 

200 ft of drawdown centered at Evadale, with the cone 

extending to surrounding counties.

Hydrographs of well 16 in Jasper County and well 17 in 

Hardin County are shown in figure 20. Well 16 is near the
E34 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

NACOGDOCHES COUNTY

EXPLANATION

BEAUMONT

1960 1965 1970 1975 1980 1985

JASPER 

COUNTY

21

35

45 45

f

46 46

-

1960 1965 1970 1975 1980 1985

ORANGE COUNTY

1960 1965 1970 1975 1980 1985

PUMPAGE SCALE, IN MILLION GALLONS 

PER DAY Scale may vary according to county

MEAN PUMPING RATE, IN MILLION 

GALLONS PER DAY

IRRIGATION PUMPAGE 

NONIRRIGATION PUMPAGE 

YEAR 

n WELL FIELD AND NAME

FIGURI-: 19. Ground-water pumpage in selected counties in the eastern part of the Texas Gulf Coast study area, 1960-85.

center of the drawdown cone at Evadale and is open to permeable /one C. Between 1955 and 1967, the water level in 

well 16 declined about 190 ft. From 1968 to 1985, the water 

level declined only about 20 ft.

Well 17 is near Beaumont's well field and is open to permeable zone B. From about 1960 to 1968, the water level in

well 17 declined 50 ft (about 5.5 ft/yr). During the next 6 

years, the water level declined about 15 ft, and from 1974 to 

82 it was essentially stable (fig. 20).

In 1954, a network of bench marks was installed in and 

around the paper mill at Evadale to measure land-surface 

subsidence. The maximum differential subsidence from
HYDROLOGY OF THE TEXAS GULF COAST AQUIFER SYSTEMS E35

1 1 1 au 

(J

L? ®

CC

Z> 50

Q 100

< 150

g 20°

LU 250 

CQ

o 30°

LU

o

CQ 50

1- 0

LU

ffi 50

- 100

_l

^ 150

LU""" 200

cc

LU

1- 250

5 300,

1 1 1 1 | 1 1 1 1

- WELL 16

-

 County:

State well number:

 Land-surface datum:

Well depth:

~ Aquifer:

1 1 1 1 1 1 1 1 1

III III

- WELL 17

_

 County:

State well number

 Land-surface datum

Well depth

Aquifer

1 1 1 1 1 1 1 1

)30 1935 19

I I I I | I I I I I I I I I I I I | I I I I | l l l l | I I I l | I I I I | I I I I

\ 

\

\ _____ 

"~~~~ \

Jasper N. 

PR-614-8-209 ^\ --^^ ______ ^

46 feet ""~ ^^N^^-^^-

1,295 feet

Permeable zone C

I I I I I I I I I III I I I I I I I I I I I i I I I I I I I I I I

III I | III III I I I | I I | l l l i

____ __ ~~~~~~~~~ _____

Hardin 

LH-61 -55-204

25 feet 

800 feet

Permeable zone B

I I I I I l II I I I I I I I I I I I I I I I I I I I I I I I I I I I I I

40 1945 1950 1955 1960 1965 1970 1975 1980 19

YEAR

-50

-0

-50 - 

O

- 100 CO

-150 H

LU

 200 LL

z

LU"

Q

^

P

-50 <

_i

 0 >

CC 

LU

- 100 H

-150 ^

 200

 250

85

FIGURE 20. Ground-water levels in wells in Jasper and Hardin Counties, Texas. Well locations are shown in figure 13.

1955 to 1963 was 0.228 ft (Wesselman, 1967, p. 58). This 

subsidence was at a point where the water-level difference 

between the original reference point and the point of maximum subsidence was about 25 ft. Recent measurements of 

subsidence are not available. However, if the ratio 0.228/25 

is valid, the estimated maximum land-surface subsidence is 

nearly 2 ft at the center of the cone where the maximum 

drawdown is 210 ft.

Simulated horizontal flow vectors (pi. 5) show an area of 

convergence in permeable zone C at Evadale. A summary of 

1982 flow components for model subarea I is shown in figure 

21. Model simulations show a net recharge in the outcrop 

areas that was 84 percent of the total discharge rate (pumping plus a small lateral outflow) for all units in subarea I. 

Sixteen percent of the discharge was derived from 

ground-water storage. The withdrawal was fairly evenly distributed among permeable zones A and B, the lower Claiborne-upper Wilcox aquifer, and the middle Wilcox aquifer 

(fig. 21).

KINGSVILLE AREA

The Kingsville area (area 4, fig. 13) consists of parts of 

Kleberg, Nueces, and Jim Wells Counties. Ground-water 

withdrawals are largest in Kleberg County (fig. 22), and 

withdrawal is concentrated in Kingsville's well field near the 

Kleberg-Nueces County line. Withdrawal in area 4 is mainly 

from permeable zone B (see pi. 4 for the 1980 grid-block 

distribution). Flowing wells existed in the area in the early 

1900's, one of which was at the railroad station in Kingsville. This well, open at a depth of about 600 to 700 ft, was 

reported to have had an artesian head of 20 ft above land 

surface (Livingston and Bridges, 1936). From 1901 to 1935,

ground-water withdrawal in the Kingsville area was about 2 

Mgal/d, which included supplies to small municipalities and 

small-scale irrigation (Groschen, 1985, p. 15). Since then, a 

steady increase of municipal and industrial withdrawal has 

lowered water levels. The aggregate withdrawal for Kleberg, 

Nueces, and Jim Wells Counties has been relatively constant 

since 1970 at about 16 Mgal/d (fig. 22). Concentrated withdrawal in and near Kingsville, about 6.5 Mgal/d in 1980, has 

created a steep cone of depression in the potentiometric surface of permeable zone B, with the center of the cone near 

Kingsville.

A map of the head decline in permeable zone B (fig. 23) 

shows the difference between measured predevelopment 

heads and measured 1982 heads. The 1982 head measurements are from Groschen (1985, fig. 9). who related the 

heads to the confined part of the Evangeline aquifer (table 1). 

The heads also are applicable to the pumped interval of permeable zone B in this area, the upper part of the Goliad 

Sand (table 1). Drawdowns exceed 200 ft at the Kingsville 

well field, and the cone of depression extends to surrounding 

counties (fig. 23).

A hydrograph of well 19 in Kleberg County is shown in 

figure 24. Well 19 is open to permeable zone B and is a 

short distance west of the center of the drawdown cone. 

From 1933 to 1972, the water level in well 19 declined 155 

ft or at an average rate of about 4 ft/yr. From 1972 to 1985, 

the water level was essentially stable.

The large water-level declines and proximity to saltwater 

in overlying and underlying deposits and downdip have created the potential for movement of saltwater into the freshwater. In the Kingsville area, Groschen (1985, p. 17) 

considered the upper limit of dissolved-solids concentration 

of freshwater to be 2,000 mg/L. The major source of potential contamination is saltwater in the overlying deposits, as
REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

-Hr+

CONSTANT-HEAD WATER-TABLE LAYER ^4

uf , i ,i t ^= -1 S = 0 P = -5

i t t Li OS = OP = -5 ©^

CD

= OS = OP =

n . t

L = 0 S = 0 P = -1 ^

° *

 I-;-"-- 4 L = OS = OP = -1 ©

ttf^H t L

= 0 S = 2 P = -8

L = 1 S = 2 P = -5

$4) w

EXPLANATION

AQUIFER OR PERMEABLE ZONE AND IDENTIFIER

(T) Permeable zone A

(5) Permeable zone B

(3) Permeable zone C

(D Permeable zone D

Q) Permeable zone E

(§) Upper Claiborne aquifer

(Vl) Middle Claiborne aquifer

Q3) Lower Claiborne-upper Wilcox aquifer

^4) Middle Wilcox aquifer

CONFINING UNIT

TOTAL WATER BALANCE Numbers are million 

cubic feet per day. Million gallons per day in 

parentheses

Total pumpage (P) = -29 (217) 

Net lateral flow (L) = -3 (22) 

Total release of water

from storage (S) = 5 (37) 

Net recharge from

constant heads = 27 (202)

L = -2 NET LATERAL FLOW IN (+) OR OUT (-) OF STUDY 

AREA

TOTAL RECHARGE OR DISCHARGE IN OUTCROP 

AREA Includes flow through superjacent 

confining unit where the confining unit is 

exposed at the surface. Number is flow rate, 

in million cubic feet per day

10

Recharge Discharge

14

NET RECHARGE OR DISCHARGE IN OUTCROP 

AREA Includes flow through superjacent 

confining unit where the confining unit is 

exposed at the surface. Number is flow rate, 

in million cubic feet per day

1 Recharge

(4)

TOTAL LEAKAGE THROUGH BOTTOM OF 

AQUIFER Number is flow rate, in million 

cubic feet per day

 f Upward I 

' 1

Downward

NET LEAKAGE THROUGH BOTTOM OF 

AQUIFER Number is flow rate, in million 

cubic feet per day

(D . 

 f Upward I Downward

FIGURE 21. Simulated 1982 vertical flow rates across hydrogeologic units in model subarea I (fig. 13), Texas Gulf Coast 

study area. Sum of flow rates may not exactly balance because of independent rounding.

opposed to saltwater encroachment from the Gulf of Mexico 

(Groschen, 1985, p. 59). Originally, the hydrostatic pressure 

in the producing zone B was higher than in the overlying 

deposits that contain saltwater. As well discharges increased 

and exceeded recharge, the hydraulic gradient was reversed, 

creating the potential for saltwater to flow into the pumped 

zone. The natural quality of water in the producing zone is 

marginal for drinking, and any deterioration due to saltwater 

encroachment is of serious concern (Groschen, 1985, p. 5).

Some municipalities reportedly have had water-quality 

problems and have taken, or plan to take, corrective measures. The city of Alice in Jim Wells County has supplemented its ground-water supplies with surface water 

because of saltwater encroachment. Also because of saltwater encroachment, Agua Dulce, Banquette, Driscoll, and 

Bishop, in Nueces County (fig. 22), and Kingsville in Kleberg County have begun to or plan to supplement 

ground-water supplies with water from the Nueces River
HYDROLOGY OF THE TEXAS GULF COAST AQUIFER SYSTEMS E37

ZAPA I J.MHOGG

| I /BROOKS !

PUMPAGE SCALE. IN MILLION GALLONS 

PER DAY Scale may vary according to county

MEAN PUMPING RATE, IN MILLION 

GALLONS PER DAY

IRRIGATION PUMPAGE 0 1966 1965 1970 1975 19~80 1985

NOMRRIGATION PUMPAGE

YEAR

I960 1965 1970 1975 1980 1985

KINGSVILLE

n WELL FIELD AND NAME

FIGURE 22. Ground-water pumpage in selected counties in the western part of the Texas Gulf Coast study area. 1960-85. For Zavala County, 1965 and

1970 pumpage modified from Klemt and others (1976, fig. 7).
E38 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

EXPLANATION

DECLINE IN POTENTIOMETRIC SURFACE, 

PREDEVELOPMENT TO 1982, IN FEET

Less than 50 (or insufficient data)

50 - 100

100-150

150 - 200

More than 200

\_ , , I OUTCROP AREA

HORIZONTAL FLOW , IN CUBIC FEET PER 

DAY Size of arrowhead indicates range of 

flow; length of shaft indicates flow within 

stated range

5 6

0.0 to 

IxlO5

1 x 10 to 

1 x 106

1 x 10° to 

IxlO7

\ \ I 

10 20 30 KILOMETERS

FIGURE 23. Decline of potentiometric surface of permeable zone B, predevelopment to 1982, and simulated 1982 horizontal flow direction and

relative magnitude, Kingsville area, Texas.

(Texas Department of Water Resources, 1984a, p. III-22-1). 

Groschen (1985, p. 58) concluded that the pumping from 

zone B in the Kingsville area has most likely not created 

water-quality degradation on a regional scale.

Although there have been large water-level declines in 

deposits that are similar in age to those in the Houston area,

subsidence in the Kingsville area for 1917 through 1976 has 

been estimated at less than 0.3 ft (Muller and Price, 1979, p. 

42).

Simulated horizontal flow vectors (fig. 23) are shown converging on the center of the drawdown cone near Kingsville. 

The large flows to the southwest in Jim Hogg, Brooks, Starr,
HYDROLOGY OF THE TEXAS GULF COAST AQUIFER SYSTEMS E39

State well number:

Land-surface datum:

Well depth:

Aquifer:

1930 1935 1940 1945 1950 1955 1960 1965 

YEAR

1970 1975 1980 1985

FIGURE 24. Ground-water levels in well 19, Kleherg County, Texas. Well location is shown in figure 13.

CONSTANT-HEAD WATER-TABLE LAYER

*+4 ^^7 ^^

iffttr

OD L= 1 S = 0 P = -3 w

i t

= OS = OP = -2

L = -2S = OP = -1 ©

2 (2) (0)

Ut' ; f t L; = (5) -1 v-y

ttHH = 0 P = 0

Tt

L = 0 S=1 P = -1

L = 0 S = 16 P = -33

L = -1 S = 1 P = 0

EXPLANATION

AQUIFER OR PERMEABLE ZONE AND IDENTIFIER

(T) Permeable zone A

(2) Permeable zone B

(5) Permeable zone C

(5) Permeable zone D

(j) Permeable zone E

(?) Upper Claiborne aquifer

{Tj} Middle Claiborne aquifer

(fs) Lower Claiborne-upper Wilcox aquifer

(Q) Middle Wilcox aquifer

CONFINING UNIT

TOTAL WATER BALANCE Numbers are million 

cubic feet per day. Million gallons per day in 

parentheses

Total pumpage (P) =-41 (307) 

Net lateral flow (L) = -3 (22) 

Total release of water

from storage (S) = 18 (135) 

Net recharge from

constant heads = 26 (194)

L = -2 NET LATERAL FLOW IN (+) OR OUT (-) OF STUDY 

AREA

TOTAL RECHARGE OR DISCHARGE IN OUTCROP 

AREA Includes flow through superjacent 

confining unit where the confining unit is 

exposed at the surface. Number is flow rate, 

in million cubic feet per day

1

Recharge Discharge

NET RECHARGE OR DISCHARGE IN OUTCROP 

AREA Includes flow through superjacent 

confining unit where the confining unit is 

exposed at the surface. Number is flow rate, 

in million cubic feet per day

I (5> 

1 Recharge T Discharge

(5)

TOTAL LEAKAGE THROUGH BOTTOM OF 

AQUIFER Number is flow rate, in million 

cubic feet per day

Upward I Down'i ward

NET LEAKAGE THROUGH BOTTOM OF 

AQUIFER Number is flow rate, in million 

cubic feet per day

(3)

f Upward I Downward

FIGURE 25. Simulated 1982 vertical flow rates across hydrogeologic units in model subarea IV (fig. 13), Texas Gulf 

Coast study area. Sum of flow rates may not exactly balance because of independent rounding.
E40 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

and Hidalgo Counties are not the result of ground-water 

v. i irawal. Land-surface gradients, and thus the estimated 

waie:-table gradients, are steeper here than elsewhere in the 

study area, and ground water is flowing naturally in greater 

volume downdip toward the gulf.

A summary of 1982 flow components for model subarea 

IV (fig. 13) is shown in figure 25. Most of the withdrawal in 

model subarea IV is from the lower Claiborne-upper Wilcox 

aquifer in the Winter Garden area.

WINTER GARDEN AREA

The Winter Garden area is intensively pumped (area 5, 

fig. 13) and is defined as all or major parts of Zavala, 

Dimmit, Frio, La Salle, and Atascosa Counties, and minor 

parts of Bexar, Wilson, and McMullen Counties. The combination of infrequent killing frosts and fertile soils make 

the Winter Garden area ideal for growing vegetables and 

other food crops. Relatively little precipitation (about 21 to 

29 in./yr from west to east) and a general lack of surface-water resources necessitate the withdrawal of large 

amounts of ground water for irrigation. Withdrawal is 

almost entirely from the lower Claiborne-upper Wilcox 

aquifer.

During the early development of irrigation, flowing wells 

were common in the Winter Garden area. The artesian head 

southeast of Carrizo Springs was estimated to be 18 to 40 ft 

above land surface (Turner and others, 1948). A few springs 

flowed into Carrizo Creek in Dimmit County, but by 1929, 

with continued withdrawal, springs on the creek no longer 

flowed. In Atascosa County, the first flowing well was 

drilled in 1904 in the city of Poteet. In 1929-30, intensive 

withdrawal for irrigation was predominantly within 5 mi of 

Poteet. A drastic increase in withdrawal in the Winter Garden area beginning about 1950 was caused by widespread 

drought. Increases in population, industry, and irrigated 

acreage have caused higher rates of ground-water withdrawal.

Largest withdrawals for most years are in Zavala, Frio, 

Atascosa, Dimmit, and La Salle Counties, in that order (fig. 

22). Withdrawals for the three most intensively pumped 

counties in 1980 Zavala, Frio, and Atascosa were spread 

fairly evenly (fig. 22, pi. 4). The intensive withdrawal, 

which has exceeded recharge in this relatively dry area, has 

been mostly from the deep, confined parts of the lower Claiborne-upper Wilcox aquifer, and has caused a very large 

cone of depression. The drawdown map (fig. 26) shows the 

difference between measured predevelopment heads and 

measured 1982 heads in the lower Claiborne-upper Wilcox 

aquifer. The largest drawdowns, in excess of 250 ft, are in 

south-central Zavala County and at the boundary of southeastern Zavala and northeastern Dimmit Counties. Drawdowns exceed 100 ft in at least six counties (fig. 26).

Simulated horizontal flow vectors (fig. 26) converge in the 

vicinity of Zavala, Dimmit, Frio, and La Salle Counties. 

Water is flowing toward the pumping center from a very 

large area, with the largest flows coming from the outcrop 

area.

Hydrographs of wells 23 and 24 and precipitation data in 

Zavala County are shown in figure 27. Well 23 is more than 

1,000 ft deep in the confined part of the aquifer; it is near the 

center of the cone of depression in south-central Zavala 

County. From 1947 to 1956, the water level in well 23 

declined 270 ft or about 27 ft/yr. The rapid decline resulted 

in large part from widespread drought conditions in the early 

and middle 1950's, when pumping demands were great and 

recharge rates minimal. Water levels recovered significantly 

in the late 1950's when precipitation returned to normal or 

above normal, but the trend continued downward in the 

1960's until a decline of 355 ft below the 1947 level 

occurred in 1967 (fig. 27). A significant recovery occurred 

again beginning about 1968. and water levels appear to have 

been relatively stable in the 1970's and 1980's.

Well 24 (fig. 27) is in the outcrop of the lower Claiborne-upper Wilcox aquifer in the northeast corner of Zavala 

County. The well is relatively shallow (260 ft deep) and is 

near areas of moderate to large withdrawal in the downdip, 

confined parts of the aquifer. The water table in well 24 

declined about 70 ft from 1953 to 1967, or more than 4 ft/yr. 

From 1968 to 1985, the water table declined about 25 ft, or 

about 1.5 ft/yr. Total decline for 1953-85 is about 95 ft. 

which means that a minimum of 95 ft of the aquifer has been 

dewatered in this area where the aquifer is unconfined.

Water-level declines in the Winter Garden area have produced undesirable effects, one of which is greater pumping 

lifts. In some places, pump intakes have been lowered, wells 

deepened, or larger pumps installed. All of these factors 

result in increased pumping costs. In addition, saltwater is 

reported to be leaking downward from overlying deposits 

through old well bores and contaminating the aquifer. 

Muller and Price (1979, p. 18) noted that this problem is 

especially evident in Dimmit County, where saltwater from 

the overlying Bigford Formation is leaking downward 

through old well bores. The hydraulic gradient between the 

lower Caliborne confining unit and the lower Claiborne-upper Wilcox aquifer has been reversed as a result of 

the intensive withdrawal. The old wells may have been 

poorly constructed initially or have been improperly plugged, 

and saltwater moves downward to mix with freshwater in the 

aquifer. Klemt and others (1976, p. 23) cautioned that continued increase in development of the aquifer could result in 

widespread contamination as a result of interformational 

leakage.

In model subarea IV (fig. 25), about 33 million ft3/d were 

pumped from the lower Claiborne-upper Wilcox aquifer 

during 1982 (nearly all in the Winter Garden area). Modeling results suggest that about 48.5 percent of the withdrawal
HYDROLOGY OF THE TEXAS GULF COAST AQUIFER SYSTEMS E41

v | \IMcMULLEN \ 

1 K ' \ \ '

EXPLANATION V\\

DECLINE IN POTENTIOMETRIC 

SURFACE, PREDEVELOPMENT 

TO 1982, IN FEET

Less than 50 (or insufficient data)

50 - 100

100-150

150 - 200

200 - 250

More than 250

^

Ik

 ^ c± -v & 3 $ 4

^ -c* -c> X7 H <1 $ d $

-"3 -.& &$

^ $ t>

^ 4 4 4 

^ 4 __4_ J 4

f <3~^l 

^ ^ ' ^

K " ^ ^ ^ 

^ fe *

* ^ I fc x» H S~ T^v^er 

^ DUVAL / -^c°

OUTCROP AREA

HORIZONTAL FLOW. IN CUBIC FEET PER 

DAY Size of head indicates range of flow; 

length of shaft indicates flow within stated range

0.0 to 1 x 105 to 1 x 106 to

IxlO5 IxlO6 1x10'

0 10 20 30 MILES

I . ' , .' '

I 1 H T

0 10 20 30 KILOMETERS >\

FIGURE 26. Decline of potentiometric surface of the lower Claiborne-upper Wilcox aquifer, predevelopment to 1982, and simulated 1982 

horizontal flow direction and relative magnitude, Winter Garden area, Texas.

was derived from aquifer storage (including dewatcring of 

the aquifer), and about 39.4 percent was from net recharge 

in the outcrop area. The remaining 12 percent was from 

net upward leakage from the middle Wilcox aquifer (6 percent) and net downward leakage from the middle Claiborne 

aquifer (6 percent).

LUFKIN-NACOGDOCHES AREA

The Lufkin-Nacogdoches area (area 6, fig. 13) consists 

of most of Nacogdoches and Angelina Counties. There is 

concentrated withdrawal from industrial well fields 

between Lufkin and Nacogdoches and straddling the
E42 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

0

50

100

150

200

250

300

350

400

450

500

I I I I | I I I I 

WELL 23

I I I I I I I I I I I I I I I | I I I I I I I

State well number:

Land-surface datum:

Well depth:

Aquifer:

I I I I I I I I I

ZX-77-18-401

568 feet

1,054 feet

Lower Claiborne-upper Wilcox

I I I I I I I I I I I I I I I I I I I I I I I

550 

500 

450 _i

LU

400 Lu 

350 <

LU

300 "

250 O 

m

200 ^

LU

150 LU

LL

100 ?

LU

Q

ID

u_ u

-> 50

LU 100

^J 150

oc

LU 200

^ 250

300

3Rfl

1 1 1 1 1 1 l 1 1 1 1 1 1 1 1 1 1 ill l l l l 1 l l l l i i i | l l l l l 1 1 l | l l i l

- WELL 24

-

_

State well number:

Land-su

-

-

1 1 1 1

rface datum:

Well depth: 

Aquifer:

1 1 1 1

_____^^^

 ~ -~^^_ 

ZX-69-61-502 ~N ______ ^

750 feet 

260 feet 

Lower Claiborne-upper Wilcox

I I I I III I I I I l I l I I I I I I I I I I I I III

_i

- 700 _i

LU

- 650 >

_i

- 600 <r

LU

- 550 ^

- 500

- 450

 Ann

80

y t/5 eo

- 20

"1 I I I | I I I I | I I I I | I I T

PRECIPITATION STATION A

LONG-TERM AVERAGE (1949-84)

i I

< I . I » h .I'll i 1 i i i 1 11 i I i IJ I H ..I-M-. I i.l.ii.i.-l I I i-.l i l..t:l i-M'l . 1.1;. I... I i I I h I t 11 11

1930 1935 1940 1945 1950 1960 1965 1970 1975 1980 1985

YEAR

FIGURE 27. Ground-water levels in wells and precipitation data in Zavala County, Texas. Well locations are shown in figure 13.

Nacogdoches-Angelina County line (fig. 19). The two cities also maintain well fields for municipal supplies, and 

the combined withdrawal has created a steep cone of 

depression on the potentiometric surface of the lower Claiborne-upper Wilcox aquifer. Drawdowns of water levels 

near the pumping centers were as much as 500 ft from 

the late 1930's to the late 1960's (William F. Guyton and 

Associates, 1970, p. 23). The major withdrawal is in 

Angelina County, and withdrawal from both counties has 

remained nearly constant during the 1970's and 1980's 

(fig- 19).

In 1907, the hydraulic head in wells open to the Carrizo 

Sand (upper part of the lower Claiborne-upper Wilcox aquifer, table 2) in the city of Nacogdoches was as much as 40 ft 

above land surface (Deussen, 1914). The city of Nacogdoches began pumping from the Carrizo Sand in the early 

1900's. After 30 years of pumping, water levels had 

declined about 39 ft. A large paper mill and Lufkin began 

pumping from the Carrizo Sand in the late 1930's. Since 

then, water levels in the Carrizo have declined as much as

500 ft near the center of pumping. The water-level declines 

become less severe as distance increases northward away 

from the paper mill and towards the outcrop of the Carrizo 

Sand.

According to William F. Guyton and Associates (1970), 

the cone of depression was causing some brackish water to 

move toward the Lufkin and paper-mill well fields. The 

movement was slow, and periodic observations may allow 

sufficient time to relocate wells when necessary.

Head data are insufficient to construct an accurate map of 

the potentiometric surface for development conditions, but a 

few observation wells with long-term records exist near the 

pumping centers. One such well is well 37 in Nacogdoches 

County a short distance east of the major pumping center at 

the large paper mill. A hydrograph of well 37 is shown in 

figure 28. The well is 900 ft deep and opens to the lower 

Claiborne-upper Wilcox aquifer. The water level in well 37 

has declined almost continuously since the beginning of 

record in 1947, with a total decline of 300 ft. The rate of 

decline mainly reflects pumping rates; it was rapid in the
HYDROLOGY OF THE TEXAS GULF COAST AQUIFER SYSTEMS E43

State well number:

Land-surface datum:

Well depth:

Aquifer:

TX-37-36-102

200 feet

900 feet

Lower Claiborne-upper Wilcox

500

1930 1935 1940 1945 1950 1955 1960 

YEAR

1965 1970 1975 1980 1985

FIGURE 28. Ground-water levels in well 37, Nacogdoches County, Texas. Well location is shown in figure 13.

earlier years and became more gradual until the 1980's, 

when there appears to have been no further decline (fig. 28). 

The pumping rate from the lower Claiborne-upper Wilcox 

aquifer in model subarea I (fig. 13) was estimated at 8 million ft-Vd for 1982 (fig. 21). More than one-half of this withdrawal was in the Lufkin-Nacogdoches area. Of the 8 

million ftVd of withdrawal, model results indicate that 25 

percent is derived from decrease in aquifer storage, and the 

remainder is derived equally from recharge in outcrop areas 

and net downward leakage from the middle Claiborne 

aquifer.

OTHER AREAS

Intensive ground-water development has occurred in some 

areas without causing large, extensive cones of depression 

and without the threat of adverse effects, such as significant 

land-surface subsidence or aquifer dewatering. Two such 

areas are the Bryan-College Station area and most of Orange 

County.

BRYAN-COLLEGE STATION AREA

The Bryan-College Station area is along the Brazos River 

and includes parts of Brazos, Burleson, and Robertson Counties (fig. 13). Considerable amounts of ground water are 

pumped in this area for irrigation and municipal supplies 

(fig. 14). The water is being withdrawn from several aquifers, which has minimized drawdowns in any single unit. 

The Yegua Formation (upper Claiborne aquifer, table 2) 

crops out in a belt that runs through the Bryan-College Station area, and the Sparta Sand (table 2) crops out just north 

of the Brazos County line. Texas Agricultural and Mechanical University (Texas A&M) at College Station has pumped 

water from the Yegua Formation since the institution's establishment in 1886. By the 1970's, Texas A&M, College Station, and Bryan were all using water from wells in the Sparta

Sand and Wilcox Group (table 2). In 1970, Bryan was the 

largest user of ground water for public supply in Brazos and 

Burleson Counties (Follett, 1974).

In 1948, irrigation with water from the Brazos River alluvium began in Robertson County (Hughes and Magee, 1962, 

p. 1). Little water was pumped from the alluvium prior to 

this date. But the drought of the early 1950's led to the 

search for additional quantities of water for irrigation on the 

flood plain (Cronin and Wilson, 1967, p. 1). From 1950 to 

1964, the number of irrigation wells pumping from the alluvium increased at a rapid rate. Withdrawal for irrigation 

decreased from 1964 to 1969, as Brazos County increased its 

surface-water use (Follett, 1974, p. 26). Approximately 98 

percent of the ground water used for irrigation is pumped 

from the flood-plain alluvium, but no ground water is 

pumped from the alluvium for municipal uses.

By 1961, the water levels in the flood-plain alluvium had 

probably recovered from the intense pumping during 1950 to 

1957 (Follett, 1974). Then, from 1961 to 1971, the water levels again declined in the flood-plain alluvium northwest of 

Bryan. According to Cronin and Wilson (1967, p. 32), the 

decline may have been the result of withdrawals from the 

underlying Sparta Sand (table 2), which is in direct hydraulic 

connection with the alluvium. Withdrawal from the Sparta 

Sand reduced the artesian pressure, which may have caused 

water from the alluvium to move downward in response to 

the change in head.

ORANGE COUNTY

The development of ground water in Orange County, in 

southeast Texas (fig. 13), began relatively slowly. From 

1910 to 1940, ground water was pumped for public supply, 

sawmills, railroads, and oil and gas production. By 1941, 

total ground-water withdrawal was only 2.6 Mgal/d (Wesselman, 1965, p. 27). The principal source of water in this area 

is from the lower part of the Chicot aquifer (permeable zone
E44 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

B, table 1). In the early 1900's, prior to large-scale withdr.-v' .J, the deep wells had large artesian flows. In the city 

of Orange, the heads in the lower part of the Chicot aquifer 

were estimated to be about 25 to 30 feet above sea level 

(Wesselman, 1965, p. 28). The artesian pressure decreased 

as more wells were drilled and withdrawals increased. The 

demand for water increased during World War II, and by 

1950 the declining heads caused all the deep wells to stop 

flowing (Wesselman, 1965, p. 28). Because of increased 

withdrawal and below-normal rainfall, the potentiometric 

surface in 1962 was below sea level throughout most of the 

county. The largest declines have been in the southeastern 

part of the county, an area of moderate to large industrial 

withdrawal. Water levels rose throughout most of the county 

between 1980 and 1985 because of a decrease in withdrawals. Ground-water levels in the city of Orange recovered as 

much as 14 ft (Bonnett and Williams, 1987, p. 24).

In the coastal area of Orange County, there is the potential 

for vertical (upconing) and lateral encroachment of saltwater. 

Upconing is likely to occur when sand beds containing saltwater are not separated by clay beds from the freshwater 

sands. In some areas of Orange County, thin layers of clay 

separate the freshwater sands from sands containing 

saltwater. Increased withdrawal from 1963 to 1980 led to an 

increase in chloride concentrations in wells in an industrial 

area near the city of Orange. The upconing is directly 

related to pumping and is evident when wells with high chloride concentrations are surrounded by wells with lower chloride concentrations (Bonnet and Williams, 1987, p. 16-22). 

Movement of saltwater updip, toward the center of pumping, 

has also been observed in the lower part of the Chicot aquifer (table 1) in southeastern Orange County (Gabrysch and 

McAdoo, 1972, p. 10). From 1980 to 1984, the decreases in 

withdrawals have caused chloride concentrations to stabilize 

throughout the county.

Land-surface subsidence, resulting from decreased artesian 

pressures, has been measured in the area. Some parts of 

eastern and western Orange County have experienced more 

than 0.5 ft of land-surface subsidence in the years 1900 to 

1975 (Carr and others, 1985, fig. 25). This subsidence is 

much less than the amount measured in the Houston-Galveston area.

POTENTIAL FOR GROUND-WATER 

DEVELOPMENT

In the previous section, the history of development was 

described for the more intensively pumped areas along with 

some of the problems associated with that development. 

There has been little development in a large part of the study 

area, and large quantities of ground water are available for 

various uses.

The availability of ground-water supplies will depend on 

the amount of withdrawal that can be sustained without 

exceeding acceptable limits of (1) water-level declines, (2) 

land-surface subsidence, (3) saltwater encroachment at the 

coast, (4) contamination from saltwater in adjacent beds or 

from downdip, and (5) reduced streamflow.

Reports that include a regional description of 

ground-water use and availability, projected ground-water 

requirements, and potential problems associated with future 

development include those by the Texas Department of Water 

Resources (1984a, 1984b, 1985), and Baker and Wall (1976). 

A common characteristic of problem areas is that large 

ground-water withdrawal is concentrated in small areas. 

Some general ways to reduce withdrawal and the potential or 

existing adverse effects of withdrawal are (1) water-conservation practices, (2) construction of reservoirs and more 

dependence on surface-water supplies, (3) relocation of well 

fields and spreading the withdrawal over a larger area, (4) 

importation of water from nonproblem areas, (5) artificial 

recharge, and (6) desalination of salty ground water.

Pumping from older units that are farther inland can minimize land-surface subsidence and saltwater encroachment. If 

the withdrawal is in areas with more abundant precipitation, 

the potential for recharge is greater, and aquifer dewatering 

is less likely to be a problem.

Fresh ground-water use for 1985 and projected use for 

2000 and 2030 are shown for selected counties in the study 

area (fig. 29). The projections, made by the Texas Water 

Development Board, are tentative and undergo constant revision and updating depending on the many changing technical 

and socioeconomic factors. Ground-water use in eight of the 

nine most intensively pumped counties in 1985, namely Harris, Wharton, Zavala, Jackson, Frio, Jasper, Atascosa, and 

Colorado, is projected to decrease by 2030 (fig. 29). Total 

decrease for the eight counties is expected to be from 872 

Mgal/d in 1985 to 392 Mgal/d in 2030, an average decrease 

of about 55 percent.

The largest decrease, 143 Mgal/d, is projected for Harris 

County (fig. 29), closely followed by a projected decrease of 

131 Mgal/d for Wharton County. Harris, Wharton, Jackson, 

Jasper, and Colorado Counties are part of a region designated 

by the State as the Southeast Texas and Upper Gulf Coast 

Region (Texas Department of Water Resources, 1984a, fig. 

1). Although total water use is projected to increase in this 

region by about 20 percent from 1980 to 2030, ground-water 

use is projected to decrease by about 29 percent. According 

to the Texas Department of Water Resources (1984a, p. 53), 

an increasing dependence on surface-water supplies will be 

necessary to prevent further land subsidence and saltwater 

encroachment.

Zavala, Frio, and Atascosa Counties are within the Winter 

Garden area. In an area designated as the South Central 

Texas Region, the Texas Department of Water Resources 

(1984a, p. 51) projected shortages of water for irrigation,
HYDROLOGY OF THE TEXAS GULF COAST AQUIFER SYSTEMS E45

EXPLANATION

 1985

 2000 (projected)

 2030 (projected)

. 239

237

ZAVALA

50

___L 

100 150 200 250 

MILLION GALLONS PER DAY

300 350 400

FIGURE 29. Projected fresh ground-water use for selected counties, 1985-2030, Texas Gulf Coast study area. Data from

Texas Water Development Board, 1988.
E46 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

primarily in the Winter Garden area, to be about 151 Mgal/d 

by : - -0.

It is apparent that the future development of ground water 

in the study area is a complex matter, with projected large 

decreases in withdrawal for some areas and projected 

increases for other areas. Such withdrawal projections could 

be simulated in the ground-water flow model, but such complex simulations are beyond the scope of this study. However, the model was used to simulate two simplified 

projected withdrawal hypotheses. In the first hypothesis, the 

calibration period from predevelopment to 1982 was 

extended to the year 2030, with the 1980 pumping rate 

remaining in effect for 1983-2030. In the second hypothesis, the calibration period from predevelopment to 1982 was 

again extended to the year 2030 but with an additional 

pumping rate of 0.5 Mgal/d assigned to each grid block 

(25-mi 2 area) in all aquifers and permeable zones where the 

block contained mean dissolved-solids concentrations of 

2,000 mg/L or less and had a thickness greater than 25 ft. 

An analysis of the two simulations provides a qualitative 

evaluation of the response of aquifer systems to additional 

large development.

Representative water-level hydrographs for the 1910-2030 

simulations are shown for permeable zones A, B, and C in 

Harris County (fig. 30). In this section of the report, simulated water levels are discussed along with measured water 

levels in specific wells. Note that a simulated water level is 

the average water level for a large volume of aquifer sediments having the areal dimension of a model grid block (25 

mi2 ) and an average aquifer thickness for that block.

Well 9 (fig. 30) is open to permeable /one A in an area of 

southeastern Harris County where withdrawal from most 

zones was substantially reduced starting in 1976 and continuing into the 1980's. Simulation of continual pumping at the 

1980 rate resulted in little additional drawdown in well 9 

after 1982.

Wells 7 and 3 (fig. 30) are open to permeable zones B and 

C, respectively, in an area of southwest Harris County where 

pumping rates have generally continued to increase in the 

1970's and through 1980. Wells 7 and 3 had an additional 

simulated drawdown of 100 and 130 ft, respectively, between 

1982 and 2030. Both drawdown curves approach horizontal 

near the end of the simulation period, signifying little further 

change in storage.

For the second simulation, involving increased withdrawal, 

about 50 ft of additional drawdown was simulated in well 9 

after 1982. Simulated drawdowns in wells 7 and 3 after 

1982 were 160 and 220 ft, respectively. The rates of decline 

of the water levels for wells 9, 7, and 3 from 1983 to 2030 

averaged 1.0, 3.3, and 4.6 ft/yr, respectively, compared to 

rates of 0.1, 2.1, and 2.7 ft/yr for the first simulation with 

1980 withdrawal.

Overall declines in heads in the Houston-Galveston area 

during the simulations caused additional land-surface

subsidence (fig. 31). As part of the model calibration, 

land-surface subsidence in the Houston-Galveston area 

from predevelopment to 1973 was simulated by the model. 

A comparison of measured and simulated land subsidence 

is shown in figure 40 and is discussed in detail in the 

section on "Model Documentation." For the model simulation using 1980 withdrawal during 1983-2030, land subsidence in eastern Harris County and Galveston County 

increased by about 1-3 ft from 1973 to 2030 (fig. 31). 

For the same period, the land surface subsided by more 

than 4 ft in southwest Harris and northeast Fort Bend 

Counties.

Additional land-surface subsidence in southeastern Harris County beyond the 1980's may seem contradictory. 

Model results (fig. 30) indicate that a maximum water-level 

drawdown in the block containing well 9 occurred in 1976. 

The drawdown is from a block in permeable zone A. Projected drawdowns in this block to the year 2030 were never 

as large as in 1976. Thus, if water levels in the block containing well 9 are representative of ground-water levels in 

southeastern Harris County, additional land subsidence to 

the year 2030 would not be expected. Because the model 

is projecting additional subsidence in southeastern Harris 

County, and specifically in the grid block containing well 

no. 9, the model output was further analyzed. Drawdowns 

for permeable zone B in this grid block (fig. 32) follow a 

pattern similar to that for permeable zone A (fig. 30). 

However, water levels in permeable zone C (fig. 32) show 

a continuous decline from 1910 to 2030, thus accounting 

for additional land-surface subsidence in the model 

simulations.

The additional land-surface subsidence in southeastern 

Harris County and adjacent areas, as projected from 1983 to 

2030, results from 1980 pumping rates (the most recent withdrawal data available for the model simulations). Pumping 

rates in the southeastern Harris County area continued to 

decline substantially through 1986. Land-surface subsidence 

data provided by the Harris-Galveston Coastal Subsidence 

District (written commun., 1987) show a continuing marked 

decrease or total cessation in the rate of subsidence at monitored sites in southeastern Harris County and in Galveston 

County through 1986.

For the simulation with increased withdrawal superimposed on the 1980 pumping rate, land-surface subsidence 

from 1973 to 2030 is more than 5 ft in eastern Harris 

County, and more than 7 ft in northeastern Fort Bend County 

(fig. 31).

Well 23 (fig. 33) is downdip in the lower Claiborne-upper 

Wilcox aquifer in southern Zavala County. With the 1980 

pumping rate continuing for 1983-2030, simulated drawdown for this period was 45 ft. Well 24 (fig. 33) is in the 

outcrop of the lower Claiborne-upper Wilcox aquifer in 

northeastern Zavala County. The depth of the well is 

reported as 260 ft. With the 1980 pumping rate continuing,
HYDROLOGY OF THE TEXAS GULF COAST AQUIFER SYSTEMS E47

A Permeable zone A, well 9

DU 

0<

50 

100 

150

200 

250 

300 

350 

400 

At;n

I I I I I I I I I I I I I I I I I I I I I I I

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

B Permeable zone B, well 7

^o r i i i i r~ 

o (L - _....

I I I I I I I I I I I I I I I I I I

50

100

150

200

250

300

350

400

450

500

-O- ... 

i I I I I I I I I I I i i i i i i i i i i i i ^

C Permeable zone C, well 3

I I I I I I I I I I I I I I I I I I I I I I

1920 1930 1940 1950 1960 1970 1980 1990 2000 2010 2020

YEAR

2030

EXPLANATION

 MEASURED WATER LEVEL

O- O SIMULATED WATER LEVEL (WITH INCREASED PUMPAGE FOR 1983-2030)

 - - SIMULATED WATER LEVEL (WITH 1980 PUMPAGE FOR 1983-2030)

FlGURF 30. Measured and simulated water levels in wells in the Houston-Galveston area, Harris County, Texas, projected

to 2030. Well locations are shown in figure 13.

simulated drawdown between 1982 and 2030 was 60 ft. 

Because the aquifer is under water-table conditions, an additional 60 ft of the aquifer was dewatered at this location. By 

2030, about 73 percent of the aquifer would be dewatered at 

this location.

For the simulation with additional withdrawal, drawdowns 

for 1983-2030 at wells 23 and 24 were 205 and 130 ft, 

respectively (fig. 33). Average rates of decline of the water 

levels in wells 23 and 24 for 1983-2030 were 4.3 and 2.7 

ft/yr, respectively, compared to rates of 0.9 and 1.2 ft/yr for 

the first simulation. The aquifer at well 24 would be almost 

totally dewatered by the year 2030.

Major components of the ground-water budget are presented for the simulation with increased withdrawal

superimposed on the 1980 pumping rate. A summary of 

withdrawal, change in storage, and vertical flow rates in the 

exposed and downdip parts of each hydrogeologic unit in the 

modeled area for the year 2030 are shown in figure 34. Net 

recharge in the outcrop areas of all units accounted for about 

82 percent of the total pumping rate of 548 million ftVd, 

with about 16 percent of the pumping rate accounted for by 

water released from storage in aquifers and compacting clay 

beds, and about 2 percent contributed by net lateral flow into 

the study area. Highest recharge rates would occur in the 

outcrops of the most intensively pumped units, with 140 million ft-Vd or 31 percent of total net recharge entering the 

uppermost and most intensively pumped unit permeable 

zone A (fig. 34).
E48 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

96' 95°

EXPLANATION

LINE OF EQUAL SIMULATED LAND-SURFACE 

SUBSIDENCE Interval 1 foot. With 1980 

pumpage for 1983-2030 'V r\ '? >}

LINE OF EQUAL SIMULATED LAND-SURFACE 

SUBSIDENCE Interval 1 foot. With 

increased pumpage for 1983-2030

10 20 30 MILES

FIGURE 31. Projected land-surface subsidence in the Houston-Gal veston area, Texas, 1973-2030.

The highest net leakage rate was 26 million ftVd downward into the lower Claiborne-upper Wilcox aquifer, followed by 23 million ftVd downward into permeable zone B 

(fig. 34).

A summary of changes in pumping rates, heads, and 

recharge rates for each aquifer and permeable zone in the 

Texas Gulf Coast aquifer systems is given in table 10 for 

the simulation with increased withdrawal. Increases in 

pumping rates range from 10 million ft3/d for permeable 

zone E to 61 million ftVd for the lower Claiborne-upper 

Wilcox aquifer. Mean head decreases between 1982 and 

2030 range from 13 ft for permeable zone A to 276 ft for

the lower Claiborne-upper Wilcox aquifer. Head declines 

for the latter aquifer are especially large downdip in the 

Winter Garden area, where freshwater is found at unusually 

great depths but aquifer transmissivity is small. For the 

combined units, the mean increase in the recharge rate in 

the outcrops over the 1982 rate is 0.35 in./yr. The mean 

increases for the individual units range from 0.2 in./yr for 

permeable zone A to 1.1 in./yr for the lower Claiborne-upper Wilcox aquifer (table 10).

A final analysis of aquifer potential for development utilizes the difference between (1) 2030 water levels simulated 

with additional withdrawal, and (2) the 2030 water levels
HYDROLOGY OF THE TEXAS GULF COAST AQUIFER SYSTEMS E49

100

50

=! 100

Q 150

< 200

§ 250

2 300

LU

m 350

O 400

A Permeable zone B

- OI I I I I I I I I_ I I I I I I I I I I I I I I

B Permeable zone C

EETABC

u_

_J

UJ 

UJ_J

cc

UJ1-

5

1 UU

0

50

100

150 

200

250

300

350 

400

L I I I I I I I I I I I I I I I I I I I I I I I _ 

1 "O----_

- ~~Q-^ 

1 ^'"O --o _I

-_ -o- __

 """"""" T(

- 

-

~ 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010 2020 2030

YEAR

FIGURE 32. Simulated water levels near well 9, southeastern Harris County, Texas, projected to 2030 with 1980 pumpage. Well

locations are shown in figure 13.

50,

0

50

100

150

200

250

300

350

400

450

500

550

A Lower Claiborne-upper Wilcox aquifer, well 23

i i i i i i i i i i i i i i i i

B Lower Claiborne-upper Wilcox aquifer, well 24

50

100

150

200

250

300

350

 O ..

I I I i i i i i i i

1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010 2020 2030

YEAR

EXPLANATION

MEASURED WATER LEVEL

O O SIMULATED WATER LEVEL (WITH INCREASED PUMPAGE FOR 1983-2030) 

 - - SIMULATED WATER LEVEL (WITH 1980 PUMPAGE FOR 1983-2030)

FIGURE 33. Measured and simulated water levels in wells in the Winter Garden area, Zavala County, Texas, projected to 2030.

Well locations are shown in figure 13.
E50 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

CONSTANT-HEAD WATER-TABLE LAYER 1

 I | t 

314 (309) 5

2 * ' 

1 1 4 22 (1 "

° 71 (69) ,

, , A 55 (55) 27 (26) i 1 1 1 + ; 

37 (37) 12 (5)

0

.trf- iiit""i

0 T T 1 T 

I 1 A 36 (36) 23

° 20 (20)

10

150 , (140) | | L = 2S = 2P = -121®

1 R 1 /^l 

33 ?(23) L = 2 g _ 12 p__84v^

(it 4 © ' a » L = 5 S = 1 1 P = -63W

L = -2 S = 7 P = -34^

X W X

3 ' ..;. i T (7)

A " (11) . 4 L = -1 S = 5 P = -11^^

1 ° « L = 0 S=1

V * /r 

3 L = 4S=1P = -51^

' (rb 

A L =-1 S = 29 P = -104VL5/

© L = 4S= 18 P=-64^-y

P=-16®

EXPLANATION

AQUIFER OR PERMEABLE ZONE AND IDENTIFIER

(T) Permeable zone A

(5) Permeable zone B

(5) Permeable zone C

(If) Permeable zone D

(T) Permeable zone E

(§) Upper Claiborne aquifer

^j) Middle Claiborne aquifer

@ Lower Claiborne-upper Wilcox aquifer

@ Middle Wilcox aquifer

CONFINING UNIT

TOTAL WATER BALANCE Numbers are million 

cubic feet per day. Million gallons per day in 

parentheses

Total pumpage (P) = -548 (4,099) 

Net lateral flow (L) = 13 (97) 

Total release of water

from storage (S) =86(643) 

Net recharge from

constant heads = 449 (3,359)

L = -2 NET LATERAL FLOW IN (+) OR OUT (-) OF STUDY 

AREA

TOTAL RECHARGE OR DISCHARGE IN OUTCROP 

AREA Includes flow through superjacent 

confining unit where the confining unit is 

exposed at the surface. Number is flow rate, 

in million cubic feet per day

Recharge Discharge

NET RECHARGE IN OUTCROP AREA 

Includes flow through superjacent 

confining unit where the confining unit is 

exposed at the surface. Number is flow rate, 

in million cubic feet per day

X Recharge

(19)

TOTAL LEAKAGE THROUGH BOTTOM OF 

AQUIFER Number is flow rate, in million 

cubic feet per day

A Upward I Downward

NET LEAKAGE THROUGH BOTTOM OF 

AQUIFER Number is flow rate, in million 

cubic feet per day

(D

T Upward I Downward

FIGURE 34. Simulated 2030 vertical flow rates across hydrogeologic units in the Texas Gulf Coast study area. Sum of 

flow rates may not exactly balance because of independent rounding.

simulated with 1980 withdrawal continuing from 1982 to 

2030. Because water levels in the simulations are the integrated result of all hydrologic properties and conditions, such 

as aquifer transmissivity, nearness to outcrop or recharge 

source, and vertical leakage, the head difference between the 

two simulations provides some measure of the capacity of 

the aquifer systems to accept additional large development. 

This water-level difference is shown for each aquifer and 

permeable zone on plate 6. Also shown on these maps are 

areas within each aquifer and permeable zone that are designated as having more or less potential for future 

ground-water development. In addition to the difference in

head as just described, other subjective criteria for rating the 

areas include:

1. Aquifer thickness. An area having a mean grid-block 

thickness of less than 25 ft is designated as having only 

some or limited potential for additional development,

2. Dissolved-solids concentration of water. An area having a mean dissolved-solids concentration greater than 2,000 

mg/L is designated as having limited potential for additional 

development,

3. Recharge potential. Precipitation, and thus recharge 

potential, is generally progressively smaller from east to 

west.
HYDROLOGY OF THE TEXAS GULF COAST AQUIFER SYSTEMS E51

TABLE 10. Summary of changes in simulated heads and recharge rates resulting from a hypothetical increase in pumping rate, 1983 to 2030,

Texas Gulf Coast aquifer systems

Aquifer

or

permeable

zone

Permeable zone A.

Permeable zone B.

Permeable zone C.

Permeable zone D.

Permeable zone E.

Upper Claiborne

aquifer. .......

Middle Claiborne

aauifer. .......

1980

pumping

rate

(million

cubic feet

per day)

86

51

29

4

1

2

5

1983-2030

pumping

rate

(million

cubic feet

per day)

121

84

63

34

11

16

51

Increase

in

pumping

rate

(million

cubic feet

per day)

35

33

34

30

10

14

46

Mean

decrease

in head

from 1982

head

(feet)

13

28

45

98

76

48

120

Mean increase

in recharge

rate in outcrop

area from

1982 rate

(inches

per year)

0.2

.6

.5

.5

.4

.5

.8

Lower Claiborneupper Wilcox 

aquifer.......

Middle Wilcox 

aquifer....

43

10

104

64

61

54

276

160

1.1

.5

The rating of areas in each aquifer and permeable zone is 

subjective and is meant only to show areas within a given 

aquifer that have relatively more or less potential for additional large development. The ratings do not necessarily 

indicate that a particular aquifer has more potential for development than another. For the development potential, it is 

generally assumed that saltwater intrusion at the coast and 

elsewhere is minimal and within acceptable limits. It is also 

assumed that additional land-surface subsidence, as discussed 

previously, is within acceptable limits. Examination of plate 

6 shows that the potential for future development in each 

aquifer and permeable zone generally becomes higher from 

west to east across the study area.

For the middle Wilcox aquifer, the simulation with the 

superimposed withdrawal is significantly affected in the 

extreme northeast corner of the study area by nearness to the 

no-flow boundary (pi. 6). Testing of the effects of the 

no-flow lateral boundaries on the simulations (discussed in 

the section on "Model Documentation") shows that this is 

the only area and the only aquifer so affected.

SUMMARY

Gulf Coastal Plain sediments as much as several thousand feet thick were represented as discrete aquifers, permeable zones, and confining units in a multilayered digital 

model. The bases for selecting the number of hydrogeologic units (model layers) include practical considerations 

of size of the area, objectives of the model study, and computer cost limitations. Electrical-log interpretations of 

lithology and vertical-head differences at major pumping 

centers in Texas and Louisiana were used to delineate 

hydrogeologic units.

Two aquifer systems are recognized for the Texas Gulf 

Coast the Texas coastal uplands aquifer system and the 

coastal lowlands aquifer system. The coastal uplands aquifer 

system consists of four aquifers and two confining units in 

the Wilcox and Claiborne Groups. The system is bounded 

from below by the practically impermeable clays of the Midway Group or by the top of the geopressured zone. The 

overlying Vicksburg-Jackson confining unit separates the
E52 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

coa-t uplands and the younger coastal lowlands aquifer 

sysic: .s. The coastal lowlands aquifer system consists of 

five permeable zones and two confining units that range in 

age from Holocene to Oligocene. Water quality in the aquifer systems varies from freshwater to brines, with dissolved-solids concentrations ranging from a few hundred to 

as much as 150,000 mg/L.

In 1980, ground-water withdrawals in the study area, an 

area of about 114,000 mi2 , were 1.7 billion gal/d. Withdrawal has caused large cones of depression in the potentiometric surfaces, particularly in the Houston-Galveston and 

Winter Garden areas, where water levels have declined hundreds of feet over large areas since the beginning of development. Inelastic compaction of clays has resulted in 

land-surface subsidence of more than 1 ft in an area of about 

2,000 mi 2 . As much as 10 ft of subsidence has occurred in 

the Pasadena area east of Houston. Movement of saltwater 

toward pumping centers by vertical migration or from downdip has caused water-quality problems. The potential for 

contamination is greatest along the coastal margins. Saltwater contamination and land-surface subsidence have 

prompted an increasing reliance on surface water for municipal and industrial supplies.

A three-dimensional, variable-density digital flow model 

was developed for simulation of predevelopment and transient flow conditions. The area simulated extends beyond 

the study area to include parts of Louisiana and Mexico. 

No-flow boundaries were used at the updip and downdip 

extent of the hydrologic units and at the eastern and western 

limits. A constant-head water-table boundary provides 

recharge to the units in their outcrop areas.

The model calibration period was from 1910 (predevelopment) to 1982. Simulated 1982 heads, long-term hydrographs, and land-surface subsidence were matched to 

measured data in selected, intensively pumped areas.

Mean horizontal hydraulic conductivity of hydrogeologic 

units in the calibrated model ranges from 10 ft/d for the middle Wilcox aquifer to 25 ft/d for permeable zone A. Mean 

transmissivity ranges from about 4,600 ft2/d for the middle 

Claiborne aquifer to about 10,400 ft2/d for permeable zone 

D. Mean vertical hydraulic conductivity ranges from 

l.lxlO"5 ft/d for the Vicksburg-Jackson confining unit to 

3.8xlO"3 ft/d for permeable zone A. Mean values of calibrated storage coefficient range from 5.2xlO~4 for the middle 

Claiborne aquifer to 1.7xlO~3 for the middle Wilcox aquifer 

and permeable zone C. Calibrated inelastic specific storage 

values for clay beds in permeable zones A, B, and C in the 

Houston-Galveston area are 8.5xlQ-5. S.OxlQ-5 , and S.OxlQ-6 

per foot, respectively.

Model results indicate that recharge was the source of 

about 76 percent of the discharge in the study area (withdrawal plus a small net lateral outflow) for 1982; release of 

water from aquifer storage and compacting clay beds provided the remaining 24 percent. The average 1982 recharge

rate for the calibrated simulation was 0.52 in./yr; the average 

discharge, not including ground-water withdrawal, was 0.15 

in./yr. As much as 6 in./yr of recharge was simulated in the 

coastal rice-irrigation area; perhaps as much as 30 percent of 

this rate is return flow of ground water that has been pumped 

for irrigation.

A combination of intensive withdrawal and low precipitation has caused long-term dewatering of the lower Claiborne-upper Wilcox aquifer in the Winter Garden area. 

Long-term dewatering of shallow sands was observed in a 

well in Jackson County, a part of the coastal rice-irrigation 

area where precipitation is more abundant than in the Winter 

Garden area. A regional assessment of dewatering in the 

coastal rice-irrigation area is precluded because of a lack of 

data.

Ground-water use is projected to decline substantially in 

most of the study area by the year 2030 as problems such as 

saltwater intrusion, land-surface subsidence, and aquifer 

dewatering become worse. The digital flow model developed 

for this study was used to simulate the flow system to 2030 

assuming two pumping scenarios: (1) The 1980 pumping 

rate was applied to 1983-2030, and (2) an additional pumping stress of 0.5 Mgal/d per 25 mi 2 was superimposed on the 

1980 pumping rate in those areas where water contains less 

than 2,000 mg/L dissolved-solids concentration and the aquifer thickness is at least 25 ft; this rate was used for the 

period 1983-2030.

Additional water-level declines, several feet of additional 

land subsidence in the Houston-Galveston area, and 

increased recharge rates were projected for the year 2030 in 

both simulations. The difference between the 2030 heads 

generated by the two model simulations, along with other 

factors such as aquifer thickness, dissolved-solids concentration of water, and recharge potential provided the criteria for 

rating areas within the aquifers and permeable zones as having relatively more or less potential for future development. 

The potential for future development in each aquifer and permeable zone becomes higher from west to east across the 

study area.

Permeable zone A has the highest potential for development of additional large ground-water supplies. Because it 

crops out over its entire landward extent, recharge is generally always nearby and water-level declines are not large. 

The potential for development is also high for large areas of 

permeable zones B and C. Areas for potential development 

are small for permeable zones D and E and for the upper 

Claiborne aquifer, largely because these units contain relatively little freshwater. The middle Claiborne, lower Claiborne-upper Wilcox, and middle Wilcox aquifers have some 

potential for development, mostly in the eastern areas.

Some general ways to reduce ground-water withdrawal or 

the adverse effects of withdrawal are (1) water conservation 

practices, (2) construction of reservoirs and more dependence 

on surface-water supplies, (3) relocation of well fields and
HYDROLOGY OF THE TEXAS GULF COAST AQUIFER SYSTEMS E53

spreading the withdrawal over a larger area, (4) importation 

of water from nonproblem areas, (5) artificial recharge, and 

(6) use of desalinated ground water.

SELECTED REFERENCES

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E54 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

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HYDROLOGY OF THE TEXAS GULF COAST AQUIFER SYSTEMS E55

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E56 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

MODEL DOCUMENTATION

INTRODUCTION

The calibration of the digital model that was used to 

describe and quantify the regional and subregional flow systems in previous sections of the report is detailed in this section. Because the details of the predevelopment digital flow 

model were presented in a previous report (Ryder, 1988), 

only a brief summary of the updated predevelopment model 

calibration is presented. The transient-model calibration 

includes additional data, such as pumping rates, development 

heads, and storage coefficients; thus a detailed discussion 

follows.

A numerical model developed by Kuiper (1985) was used 

to simulate the ground-water flow system. The model simulates variable-density ground-water flow in three dimensions. 

The ground-water density is variable in space but is assumed 

to be constant in time. Horizontal and vertical flow is computed for aquifers and permeable zones, and vertical flow is 

computed for confining units. The model utilizes an integrated finite-difference grid, wherein the six-sided grid 

blocks are rectangular when viewed from the vertical direction. The sides of the blocks are vertical planes, but their top 

and bottom surfaces follow the curvature of the hydrogeologic units (Kuiper, 1985). The strongly implicit procedure 

(SIP) was selected for solving the approximating flow equations. Kuiper (1985) presented a complete documentation of 

the model, including detailed instructions for the use of the 

computer program.

A numerical technique used to simulate land-surface subsidence and release of water from storage resulting from 

compacting fine-grained sediments was obtained from Leake 

and Prudic (1989). The technique for the simulation of compaction and land subsidence was incorporated into the model 

by L.K. Kuiper (U.S. Geological Survey, written commun., 

1987).

DIGITAL MODEL OF PREDEVELOPMENT 

STEADY-STATE FLOW SYSTEM

As stated previously, further model calibration resulted in 

substantial differences in simulated aquifer and confining-unit properties and consequently in simulated flow compared to the preliminary model results reported in Ryder 

(1988). However, statistical analyses of the residual errors 

(simulated aquifer heads minus measured aquifer heads) as 

shown in table 8 are similar to those previously reported in 

Ryder (1988, table 6). This similarity in simulated heads 

and difference in simulated flow rates results because the 

steady-state calibration does not provide a unique solution. When the distribution and amount of both recharge

and discharge and the distribution of vertical and horizontal hydraulic conductivities are unknown, the calibration of 

a steady-state model to a set of water levels provides a 

solution, which may or may not be correct. Proportionately changing all values of hydraulic conductivities will 

result in the same set of simulated heads, but the flow 

through the system will be different because it is proportional to the assigned hydraulic conductivity values.

Residuals (table 11) range from a minimum value of-187 

ft for permeable zone D to a maximum value of 184 ft for 

the lower Claiborne-upper Wilcox aquifer. Residuals are 

largest for the lower Claiborne-upper Wilcox aquifer, with a 

standard deviation of 47 ft and an absolute mean of 53 ft. 

Permeable zone A has the smallest residuals, with a standard 

deviation of 12 ft and an absolute mean of 9 ft. For the 

combined aquifers and permeable zones, the standard deviation is 40 ft and the absolute mean is 30 ft.

The statistical analyses show that when the changed vertical and horizontal hydraulic conductivities, necessitated by 

the transient-model calibration, are incorporated into the predevelopment, steady-state model, the predevelopment-model 

calibration is statistically about the same as the original 

calibration.

DIGITAL MODEL OF TRANSIENT FLOW SYSTEM, 

PREDEVELOPMENT TO 1982

Transient-model simulations allow for ground-water levels 

and flow to change with time in response to changes in the 

amount of recharge and discharge caused by man-induced 

stresses such as pumpage. Transient ground-water flow was 

simulated for the period 1910-82 to determine the effects 

caused by pumpage on water levels and flow in the aquifer 

systems. Initial water levels for 1910 were assumed to be 

equal to the predevelopment heads generated from the calibrated steady-state model. Ground-water pumpage was 

added as a discharge and averaged over periods ranging from 

1 to 27 yr. Estimates of storage coefficients were added to 

the model and were adjusted during model calibration along 

with the values of vertical and horizontal hydraulic conductivity. Calibration was achieved when model-computed 

water levels reasonably matched selected hydrographs of 

wells and areally distributed water levels as measured in 

1982. In areas of known land subsidence, the inelastic specific storage values of clays were adjusted until the amount 

and distribution of simulated land subsidence reasonably 

matched measured land subsidence.

MODEL GRID AND BOUNDARY CONDITIONS

The model finite-difference grid superimposed on the 

modeled area and study area is shown in figure 35. The grid
HYDROLOGY OF THE TEXAS GULF COAST AQUIFER SYSTEMS E57

TABLE 11. Summary of simulated and measured predevelopment altitudes of potentiometric surfaces, Texas Gulf Coast aquifer systems

[Residual error, simulated aquifer head minus measured aquifer head]

Total grid 

Aquifer or blocks with 

permeable measured 

zone heads

Al 1

Permeable zone A. . .

Permeable zone B. . .

Permeable zone C...

Permeable zone D. . .

Permeable zone E...

Upper Claiborne 

aquifer. .........

Middle Claiborne 

aauif er . .........

4,588 

621

906

495

259

68

136 

544

Mean of 

residuals 

(feet)

12 

2

9

6

-29

-18

29

13

Minimum Maximum Standard 

residual residual deviation 

(feet) (feet) (feet)

-187 184 40 

-59 37 12

-69 54 20

-148 115 34

-187 103 46

-104 77 31

-33 149 39 

-59 129 41

Absolute 

mean of 

residuals 

(feet)

30 

9

17

25

43

27

34 

34

Lower Claiborneupper Wilcox 

aquifer.......

Middle Wilcox 

aquifer.....

596

963

47

16

-84

-97

184

172

47

44

53

37

consists of 106 rows and 77 columns uniformly spaced 5 mi 

apart. The modeled area extends beyond the study area to 

include parts of Louisiana and Mexico and has an areal 

extent of 149,000 mi 2 .

The general direction of ground-water flow prior to development was nearly parallel to the eastern and western boundaries of the study area; the flow paths followed the dips of 

the aquifers toward the Gulf of Mexico. Little flow could be 

expected to enter or leave the study area under predevelopment conditions or where there has been little development, 

and the lateral boundaries of the model are designated as 

no-flow boundaries.

However, intensive development in the Lake Charles area 

of southwestern Louisiana has caused a substantial amount 

of flow across the Texas-Louisiana border in the uppermost 

coastal lowlands permeable zones. Primarily for this reason, 

the eastern no-flow boundary was allowed to follow row 1 of 

the grid into southeastern Louisiana so that it increases to a 

maximum distance of about 195 mi from the Texas-Louisiana border (fig. 35).

The validity of retaining the lateral no-flow boundaries for 

the transient-model calibration was tested by placing constant heads in the grid blocks just inside the no-flow boundaries. The constant heads were placed along the lateral 

edges in all layers, and the altitudes assigned to the constant 

heads were the starting heads (predevelopment) for the transient calibration. The transient predevelopment-to-1982 simulation was made with the new boundaries, and the resulting 

simulated 1982 heads were compared to the 1982 heads simulated by the transient calibration. For all layers, the head 

changes at the boundaries of the study area were small in 

comparison to the residual errors (differences between simulated and measured heads) present in the model calibration. 

Thus, designation of the lateral boundaries as no-flow boundaries had no significant effect on the transient-model 

calibration.

Another test of the no-flow lateral boundaries was made 

for the simulation involving the superposition of additional, 

hypothetical pumpage on 1980 pumping rates, and continuing the simulation to the year 2030. For this simulation,
E58 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

EXPLANATION

A' ROW USED FOR GEOLOGIC SECTIONSection shown shown in figure 6

- BOUNDARY OF MODELED AREA

50 100 150 MILES

50 100 150 KILOMETERS

FIGURE 35. Finite-difference grid superimposed on the modeled area and the Texas Gulf Coast study area.

the constant-head lateral boundaries as described above 

were in effect for the entire simulation period predevelopment to 2030. Differences in head caused by the changed 

boundary conditions were analyzed for all model layers. 

The heads in the middle Wilcox aquifer were significantly 

affected in the extreme northeastern corner of the study 

area (pi. 6). Head changes elsewhere for all other layers 

were small and of little consequence.

The base of the flow system is a no-flow boundary, which 

represents the top of the Midway confining unit, a thick 

deposit of nearly impermeable marine clays, or the top of the 

geopressured zone where it occurs above the top of the Midway confining unit. Abnormally high pressures in the 

geopressured zone apparently are the result of greatly 

reduced permeability, which, in turn, precludes the possibility of significant flow out of this zone.
HYDROLOGY OF THE TEXAS GULF COAST AQUIFER SYSTEMS E59

EXPLANATION

WATER-TABLE CONTOUR Shows altitude of water 

table in outcrop area of the Texas Gulf Coast aquifer 

system. Contour intervals 25 feet (near coast) and 50 

feet. Datum is sea level

UPDIP LIMIT OF AQUIFER SYSTEM 

DOWNDIP LIMIT OF AQUIFER SYSTEM

50

FIGURE 36. Estimated altitude of the water table for the model constant-head boundary. Texas Gulf Coast study area.

The updip limit of" the flow system, where the aquifer system pinches out, is a no-flow boundary. The downdip limit 

of the flow system is defined as the edge of the Continental 

Shelf. It is at a great distance from the coastline and is 

treated as a no-flow boundary.

The top of the simulated flow system is bounded by a 

layer of constant-head grid blocks thai represent the water 

table (fig, 36) in the uppermost 150 ft of the exposed parts of 

the units. Details of construction of the water table are given 

in Ryder (1988) and in Williams and Williamson (1989).
E60 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

TABLE 12 Specific yield for grid blocks representing the outcrop of the lower Claiborne-upper Wilcox 

aquifer, Texas Gulf Coast aquifer systems

Specific 

Row Column yield

Specific 

Row Column yield

Specific 

Row Column yield

2...

2...

3...

3...

4...

6...

7...

8...

9...

11...

12...

13...

14...

14...

15...

15...

16...

16...

17...

17...

18...

18...

18...

19...

19...

19...

21...

22

31-35

20

38

20

38

38

38

38

10

20

20

8

20

8

20

20

21

20-21

26-28

9

20-23

25-27

10

20

25

11

0.15

.15

.15

.15

.15

.15

.15

.15

.15

.15

.15

.15

.15

.15

.15

.15

.15

.15

.15

.15

.15

.15

.15

.15

.15

.15

.15

23...

25...

31...

33...

34...

35...

39...

40...

47...

48...

52...

55...

58...

61...

62...

64...

65...

68...

69...

70...

71...

72...

73...

74...

75...

76...

77...

11

12

16

18

18

18

19

19

20

20

21

22

23

24

24

25

25

24

24

24

24

24

23

23

23

22

21-22

0.15

.15

.15

.15

.15

.15

.15

.15

.15

.15

.15

.15

.15

.15

.15

.15

.15

.15

.15

.15

.15

.15

.15

.15

.15

.15

.15

78...

79...

80...

80...

81...

82...

83...

84...

85...

86...

87...

88...

89...

90...

91...

92...

93...

94...

94...

95...

96...

97...

98...

99...

100...

101...

21

19-21

19

20

18-19

18

17

16

15-16

14-15

13-14

13

12-13

12

12

11

11

11

17-18

13-18

16-18

17-18

17-19

18-20

19-20

20

0.15

.15

.15

.15

.15

.15

.15

.15

.15

.15

.15

.15

.15

.15

.15

.15

.10

.10

.10

.10

.10

.10

.10

.10

.10

.10

The altitude of the water table ranges from essentially sea 

level at the coast and in the gulf to more than 800 ft above 

sea level in the west.

The concept of a shallow water table that is replenished by precipitation and kept at an essentially constant 

altitude on an average annual basis is valid for 

undeveloped areas and probably for developed areas 

receiving a large amount of precipitation such as in the 

east. In some areas, however, such as the west where 

withdrawal is large and precipitation is relatively small, 

dewatering has caused the water table to decline over the 

years. The model grid blocks representing the outcrop of 

the lower Claiborne upper Wilcox aquifer were assigned a

specific yield rather than an artesian storage coefficient 

(table 12).

The head for the water table is held constant through 

time, but the flow into or out of the boundary layer is 

the product of the vertical gradient between the constant 

head and the head in the aquifer, and the conductance to 

flow between the two layers. This conductance is the 

harmonic mean conductance of the bottom one-half of 

the constant-head grid block and the top one-half of the 

aquifer grid block. The amount of flow into or out of 

the constant-head layer can be reduced (to zero, if 

desired) by reducing the conductance in the desired grid 

blocks during the model calibration.
HYDROLOGY OF THE TEXAS GULF COAST AQUIFER SYSTEMS E61

TABLE 13. Pumping periods and average pumping rates fur digital model 

simulation, Texas Gulf Coast aquifer systems

Pumping Time 

period interval

1........

2........

3........

4........

5........

6........

7........

8........

9........

10.......

1910 

1911-37 

1938-47 

1948-57 

1958-62

1963-67 

1968-72 

1973-76 

1977-81 

1982

Average pumping rate 

(million (million 

cubic feet gallons 

per day) per day)

0

34 

62 

122 

156

189

221 

222 

231 

231

0 

254 

464 

913 

1,167

1,414 

1,653 

1,661 

1,728 

1,728

HYDROLOGIC INPUT DATA

Starting heads for the transient model are the calibrated 

predevelopment heads. Initial estimates of hydraulic 

conductivity were determined from the steady-state model. 

Water density was estimated as discussed earlier; it was not 

adjusted during model calibration. Thicknesses of the units 

were discussed in detail in Ryder (1988).

In general, pumping periods for the model were chosen on 

5-yr intervals to coincide with irrigation pumpage surveys 

done by the State approximately every 5 years beginning in 

1958 (Texas Water Development Board, 1986). Public and 

industrial pumpage data are more accurate than irrigation 

pumpage data and are generally available on an annual or 

more frequent basis from State and local files and from published reports. Average public and industrial pumping rates 

were calculated for the appropriate pumping periods.

Pumping rates prior to 1958 were estimated from historical pumpage data found in various published reports. These 

data were less comprehensive and sometimes less accurate 

than post-1958 rates, particularly in regard to irrigation 

pumpage. Longer pumping periods were selected for the 

pre-1958 simulations. For the Houston-Galveston area, 

detailed records of public and industrial pumpage showed a 

significant shift in pumpage distributions after 1976. Thus a 

4-yr pumping period was selected for 1973-76. A

single-year pumping period was chosen for 1982 to quantify 

the flow system for that year.

The locations and depths of the pumping wells were used 

to assign pumpages to 5-mi grid blocks for the appropriate 

model layers. As an example, the 1980 grid-block distributions of pumping rates that are applicable for 1977-82 are 

shown on plate 4. The pumping periods for the model and 

average pumping rates for the study area are summarized in 

table 13. A complete description and listing of pumpage 

data by State, county, model layer, and 5-mi grid block are 

given in Mesko and others (1990) for the Gulf Coast RASA 

study.

During the calibration process, simulated heads are 

matched to measured heads. Two forms of measured-head 

data were compiled for calibration purposes: (1) 

Potentiometric-surface maps in four intensively pumped 

areas (areas 1, 3, 4, and 5, fig. 13) for 1982, the final year of 

the calibration period; and (2) hydrographs from 39 wells in 

six intensively pumped areas (fig. 13) that show water-level 

changes with time. Water-level data were obtained from the 

files of the Texas Water Development Board and Texas Water 

Commission (formerly the Texas Department of Water 

Resources) and the U.S. Geological Survey, and from published reports and maps.

The water levels that were selected to construct the 1982 

potentiometric-surface maps generally are averages of the 

measurements in the wells for the time period beginning in 

June 1981 and ending in June 1983. Only single 

measurements that were made in the spring of 1982 were 

used for many wells; these wells provided supplementary 

data in areas where more detailed measurements were sparse. 

The water-level data were plotted, and potentiometric contours were drawn only through areas where a sufficient number of measurements existed. From the potentiometric 

contour maps, average head values for each 25-mi2 grid 

block were determined for statistical comparison with simulated heads.

The wells for which hydrographs were constructed were 

selected on the following basis: (1) Even coverage of the 

major producing zones in the intensively pumped areas, (2) 

even distribution within the cones of depression, and (3) 

long-term record.

Initial estimates of storage-coefficient values for aquifers 

and permeable zones were discussed in the earlier section, 

"Storage Coefficient." The release of water by inelastic compaction of fine-grained deposits in permeable zones A, B, 

and C makes up a large percentage of total water released 

from storage during the transient simulations. This release 

of water also causes land-surface subsidence that was briefly 

discussed earlier. A digital model code incorporated from 

Leake and Prudic (1989) provides for the release of water by 

inelastic compaction of fine-grained deposits by converting 

from an elastic to an inelastic specific storage. This conversion is done for the clay part of a permeable zone when the
E62 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

head "-> a grid block declines below the critical head. The 

critic, head is defined as the head that coincides with the 

maximum effective stress to which the deposits had previously been subjected.

If the computed head in a grid block recovers, the storage 

coefficient again becomes the elastic value, and the previous 

minimum head becomes the new critical head. If the head 

should decline below the new critical head, the storage coefficient again becomes the inelastic value, and so on. Simulated land-surface subsidence is the product of head decline 

below the critical head in each time step multiplied by the 

inelastic storage coefficient. Leake and Prudic (1989) gave a 

complete discussion and documentation of the land-subsidence simulation code, with qualifying assumptions and 

limitations.

Carr and others (1985, p. 45) concluded from a model calibration in the Houston area that a preconsolidation stress of 

70 ft approximates the maximum effective stress to which 

deposits have been subjected prior to ground-water development. Thus, 70 ft of head decline (from predevelopment 

starting heads) must occur at a model grid block before the 

first model conversion to an inelastic storage coefficient 

occurs. A critical head of 70 ft was adopted for the Texas 

Gulf Coast RASA model.

Carr and others (1985, p. 35) related subsidence of the 

land surface, clay thickness, and decrease in artesian pressure 

for 1943-73 to derive inelastic storage-coefficient values of 

the clay beds in the Houston area. In their computations, 

specific unit-compaction values were derived. These values 

are an approximation of specific storage if the resulting compaction approximates the ultimate compaction from an 

applied stress. Mean specific unit-compaction values were 

l.OxlQ-4 per foot for the Chicot aquifer and l.SxlO'5 per foot 

for the Evangeline aquifer. In an earlier study in the Houston area, Meyer and Carr (1979, p. 13) calculated a mean 

specific unit-compaction value of 8.7xlO~5 per foot for the 

Chicot aquifer and 1.5xlO~5 per foot for the Evangeline aquifer. These values were used as guidelines for initial estimates of inelastic storage coefficient for the clay intervals of 

permeable zones A, B, and C. (See table 1 for the relationship between the Chicot and Evangeline aquifers and permeable zones A, B, and C.)

CALIBRATION PROCEDURE AND RESULTS

The transient model was calibrated by adjusting horizontal 

and vertical hydraulic conductivity and elastic and inelastic 

storage coefficients until (1) measured potentiometric surfaces and hydrographs in intensively pumped areas were reasonably matched by simulated heads, and (2) the cumulative 

measured land-surface subsidence in the Houston-Galveston 

area from predevelopment to 1973 was reasonably matched 

by simulated land subsidence. Calibration was aided by 

applying the results of a parameter-estimation model (L.K.

Kuiper, U.S. Geological Survey, written commun., 1987), 

which automatically adjusts values of model variables so that 

the residual errors are minimized.

The Houston-Galveston area is shown as area 1 in figure 

13. Simulated and measured 1982 potentiometric surfaces of 

permeable zones A, B, and C in the Houston-Galveston area 

are shown on plate 7. The best fit of the simulated to the 

measured surfaces is for permeable zone B. The simulated 

potentiometric surface of permeable zone A is generally 

higher than the measured surface, while the simulated surface of permeable zone C is lower than the measured surface 

in the center of the cone of depression. Statistically, the 

absolute means of the residuals are 34 ft for permeable zone 

A, 25 ft for permeable zone B (includes residuals in the 

Kingsville area), and 35 ft for permeable zone C (includes 

residuals in the Evadale area) (table 14).

Hydrographs of water levels in wells 1, 2, 9, and 11 (fig. 

13), which are open to permeable zone A in the Houston-Galveston area, are shown on plate 7. Wells 1 and 2 are 

on the northwestern rim of the 1982 cone of depression, well 

11 is on the southern rim, and well 9 is near the center of the 

cone. Residual errors for the water levels are computed as 

the difference between simulated and measured where both 

exist at the end of a pumping period. The absolute means of 

the water-level residuals for wells 1, 2, 9, and 11 are 7, 9, 

36, and 40 ft, respectively (table 15).

Hydrographs of water levels in wells 4, 7, 8, 12, and 10 

(fig. 13) open to permeable zone B in the Houston-Galveston 

area are shown on plate 7. Wells 4, 7, and 8 are near the 

center of the 1982 cone of depression, well 10 is on the eastern rim, and well 12 is on the southeastern rim. The absolute means of the water-level residuals for wells 4, 7, 8, 12, 

and 10 are 22, 23, 55, 52, and 73 ft, respectively (table 15).

Hydrographs of water levels in wells 3, 5, and 6 (fig. 13) 

open to permeable zone C in the Houston-Galveston area are 

shown on plate 7. The wells are in the steep part of the 

1982 cone of depression and are fairly well distributed about 

its center. The absolute means of the water-level residuals 

for wells 3, 5, and 6 are 49, 36, and 63 ft, respectively (table 

15).

Hydrographs of water levels in wells 13, 14, and 15 open 

to permeable zone A in Jackson and Wharton Counties are 

shown on plate 7. All three wells are near intensive pumpage from permeable zone A for mainly rice irrigation; however, drawdowns are small in comparison to those in the 

Houston area. The absolute means of the water-level residuals for wells 13, 14, and 15 are 6, 9, and 6 ft, respectively 

(table 15).

The Evadale-Beaumont area is shown as area 3 in figure 13. Pumpage in Evadale's well field (at the Jasper-Hardin County line) is from zones B and C. The 

simulated and measured 1982 potentiometric surfaces of 

permeable zone C in the well-field area are shown on 

plate 7. The two surfaces are reasonably well matched.
HYDROLOGY OF THE TEXAS GULF COAST AQUIFER SYSTEMS E63

TABLE 14. Summary of simulated and measured 1982 altitudes of potentiometric surfaces, Texas Gulf Coast aquifer systems 

[Residual error, simulated aquifer head minus measured aquifer head]

Aquifer or 

permeable 

zone

Permeable 

zone A. ......

Permeable 

zone B. ......

Permeable 

zone C .......

Total grid 

blocks with 

measured 

heads

121

192

89

Mean of 

residuals 

(feet)

27

9

1

Minimum 

residual 

(feet)

-26

-96

-117

Maximum 

residual 

(feet)

108

72

91

Standard 

deviation 

(feet)

35

29

42

Absolute 

mean of 

residuals 

(feet)

34

25

35

Lower Claiborneupper Wilcox 

aquifer...... 216 17 -75 82 30 30

A water-level hydrograph of well 16 (fig. 13) open to permeable zone C and near the center of the 1982 cone of 

depression is shown on plate 7. The absolute mean of the 

water-level residuals for well 16 is 34 ft (table 15).

A water-level hydrograph of well 17 (fig. 13), which is 

open to permeable zone B and is just north of Beaumont's 

well field at the Jefferson-Hardin County line, is shown on 

plate 7. Pumpage in the well field is from permeable zone 

B. The absolute mean of the water-level residuals for well 

17 is 13 ft (table 15).

The Kingsville area is shown as area 4 in figure 13. 

Pumpage in Kingsville's well field in Kleberg County is 

from permeable zone B. A distinctive feature of the simulated and measured 1982 potentiometric surfaces shown 

in figure 37 is the shift of the simulated cone of depression toward the east. This shift was the result of having 

to shift pumpage, which was between grid blocks, to the 

bordering eastern grid block. Water-level hydrographs of 

wells 18, 19, 20, and 21 open to permeable zone B are 

shown in figure 38. Wells 18 and 19 are on the western 

rim of the 1982 cone of depression and nearer to the center than wells 20 and 21, which are on the eastern rim. 

The absolute means of the water-level residuals for wells 

18, 19, 20, and 21 are 55, 42, 17, and 22 ft, respectively 

(table 15).

The Winter Garden area is shown as area 5 in figure 13. 

Pumpage is mostly from the lower Claiborne-upper Wilcox 

aquifer. The 1982 simulated and measured potentiometric 

surfaces of the lower Claiborne-upper Wilcox aquifer are 

shown on plate 8. The absolute mean of the water-level 

residuals for the aquifer in the Winter Garden area is 30 ft 

(table 14).

Water-level hydrographs of 15 wells open to the lower 

Claiborne-upper Wilcox aquifer in the Winter Garden area 

are shown on plate 8. Wells 23, 25, 26, 29, and 32 (pi. 8) 

are nearest the center of the huge cone of depression. The 

remaining wells are on the flanks of the cone in all directions 

from the center. Absolute means of the water-level residuals 

for the wells range from 8 ft for well 24 to 83 ft for well 34 

(table 15).

The Lufkin-Nacogdoches area is shown as area 6 in figure 

13. A steep cone of depression has developed on the potentiometric surface of the lower Claiborne-upper Wilcox aquifer as a result of intense pumpage centered between Angelina 

and Nacogdoches Counties. Data do not exist to construct 

an accurate postdevelopment potentiometric surface. Hydrographs of water levels in wells 37, 38, and 39 open to the 

lower Claiborne-upper Wilcox aquifer in the Lufkin-Nacogdoches area are shown in figure 39. Wells 37 and 38 are east 

of and nearest the pumping center; well 39 is southeast of 

the concentrated pumpage. The absolute means of the 

water-level residuals for wells 37, 38, and 39 are 53, 91, and 

87 ft, respectively (table 15). Simulated heads are much 

higher than measured heads in the three wells near the center 

of withdrawal.

Simulated and measured cumulative land-surface subsidence in the Houston-Galveston area from predevelopment to 

1973 is shown in figure 40. The simulated subsidence tends 

to be somewhat less than measured values in the center of 

greatest subsidence and somewhat greater in far western 

Harris and northeastern Fort Bend Counties. The absolute 

mean of the subsidence residuals for the 196 grid blocks is 

0.6 ft.
E64 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

Well 

No.

1....

2....

3....

4....

5....

6....

7....

8....

9....

10....

11....

12....

13....

14....

15....

16....

17....

18....

19....

20....

Aquifer or 

permeable 

zone

Permeable

zone A.

Permeable

zone A. 

Permeable

zone C. 

Permeable

zone B.

Permeable

zone C. 

Permeable

zone C. 

Permeable

zone B. 

Permeable

zone B.

Permeable

zone A. 

Permeable

zone B. 

Permeable

zone A. 

Permeable

zone B.

Permeable

zone A. 

Permeable

zone A. 

Permeable

zone A. 

Permeable

zone C.

Permeable

zone B. 

Permeable

zone B. 

Permeable

zone B. 

Permeable

zone B.

Absolute 

mean of 

residuals 

(feet)

7

9

49

22

36

63

23

55

36

73

40

52

6

9

6

34

13

55

42

17

Well 

No.

21....

22....

23....

24....

25....

26....

27....

28....

29....

30....

31....

32....

33....

34....

35....

36....

37....

38....

39....

Aquifer or 

permeable 

zone

Permeable

zone B.

Lower Claiborneupper Wilcox aquifer. 

Lower Claiborneupper Wilcox aquifer. 

Lower Claiborneupper Wilcox aquifer.

Lower Claiborneupper Wilcox aquifer. 

Lower Claiborneupper Wilcox aquifer. 

Lower Claiborneupper Wilcox aquifer. 

Lower Claiborneupper Wilcox aquifer.

Lower Claiborneupper Wilcox aquifer. 

Lower Claiborneupper Wilcox aquifer. 

Lower Claiborneupper Wilcox aquifer. 

Lower Claiborneupper Wilcox aquifer.

Lower Claiborneupper Wilcox aquifer. 

Lower Claiborneupper Wilcox aquifer. 

Lower Claiborneupper Wilcox aquifer. 

Lower Claiborneupper Wilcox aquifer.

Lower Claiborneupper Wilcox aquifer. 

Lower Claiborneupper Wilcox aquifer. 

Lower Claiborneupper Wilcox aquifer.

Absolute 

mean of 

residuals 

(feet)

22

57

53

8

61

54

10

26

65

30

9

79

36

83

37

37

53

91

87
HYDROLOGY OF THE TEXAS GULF COAST AQUIFER SYSTEMS E65

98° 97°

28°

27C

26C

DIMMIT

McMLLLEN

VVKBM DtAW.

 IIMIIfX-C

STARR

--50-

 -50-

EXPLANATION

OUTCROP AREA

POTENTIOMETRIC CONTOUR Shows altitude at 

which water levels would have stood in tightly cased 

wells. Hachures indicate depression. Contour interval 

50 feet. Datum is sea level. Modified from Groschen 

(1985, fig. 9)

SIMULATED POTENTIOMETRIC CONTOUR Shows 

altitude at which water levels would have stood in tightly 

cased wells. Hachures indicate depression. Contour 

interval 50 feet. Datum is sea level

________I_________________________

0 20

i

30 MILES

I I 

0 10 20 30 KILOMETERS

__________________I

FIGURE 37. Measured and simulated altitude of the potentiometric surface of permeable zone B in the Kingsville area, Texas, in 1982.

Calibrated values of horizontal and vertical hydraulic conductivity and transmissivity are on plate 1 and in tables 3 

and 4. A summary of calibrated elastic storage coefficient 

values is in table 5. Calibrated inelastic specific storage

TABLE 15 (facing page). Summary of simulated and measured water 

levels in wells shown in figure 13, Texas Gulf Coast aquifer systems

values for fine-grained deposits in permeable zones A, B, and 

C in the Houston-Galveston area are 8.5xlO' 5 per foot, 

S.OxlO"5 per foot, and S.OxlO"6 per foot, respectively.

The model calibration is sufficiently accurate for its 

intended purpose to provide estimates of aquifer properties 

and to identify in a general way the sources and quantities of 

flow in the aquifer systems. Practical considerations of
E66 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

Well 18

IUU

<

0 

100

200 

300 

400 

Kfin

1 1 1 1 1 1 1

: State well number: RR-83-25-701

Land-surface datum: 114 feet 

j_ Well depth: 498 feet 

: Aquifer: Permeable zone B

I I I I I I I

i i I I i i i .

^"""""^^^^J

I I I I I i I

Well 19

LU

cc

Q

<

0

100

200

300

400 

Rnn

i i i i i i i i i i i i

~- o -_a.

'-_ ~~~^~~---------___^a'--°-^

- State well number: RR-83-25-501 ^~ 1::^-a

Land-surface datum: 106 feet

-_ Well depth: 485 feet 

: Aquifer: Permeable zone B

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

i i

-

2

-

 -o- <3 "r:

-_

i

Well 20

m 

cr.

U

LU 

>

0 

m

<

LU

LU 

U-

_l 

LU

>

_l

<

0

100

200

300

400

Rnn

i I I I I I I i I I I

_ o a ___

.

State well number: RR-83-27-802

Land-surface datum: 39 feet

-_ Well depth: 948 feet

: Aquifer: Permeable zone B

-

.

I i 1 I 1 1 I I I I i

' ' ' I

:

u=r^:ro- o -

_~

-

I

-

_

III"

Well 21

IUU

<

0

100

200

300

400

Knn

I I I I I I I I I I I I » ____ i o ~_.__a__

~ S^ ^~ QT--.Q-_ _-.

- State well number: RR-83-35-201

Land-surface datum: 41 feet

-_ Well depth: 964 feet

; Aquifer: Permeable zone B

-

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

i i

-

-

~~o --5 "".

i

~

2

-

-

\ \

1910 1915 1920 1925 1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985

YEAR

EXPLANATION

 MEASURED WATER LEVEL

O- O SIMULATED WATER LEVEL Circles indicate model time steps 

FIGURE 38. Measured and simulated water levels in wells in the Kingsville area, Kleberg County, Texas.

objectives of the model study, scale of the system, and computer-cost limitations have determined important factors such 

as the number of model layers and grid-block size. Some of 

the differences between simulated and measured heads that 

were just described may seem large, on the order of a few 

tens of feet. A listing of some important sources of error in 

model calibration and resulting simulated heads and flow 

rates is as follows:

1. There is inherent difficulty in estimating values for a 

set of model variables that minimize residual errors in head 

and land subsidence in a complex, interrelated system of 

numerous aquifers, permeable zones, and confining units.

2. The physical system is overgeneralized by the 

restricted number of model layers, particularly for the coastal 

lowlands aquifer system where there is a large amount of 

heterogeneity. Additional layers would improve model
HYDROLOGY OF THE TEXAS GULF COAST AQUIFER SYSTEMS E67

200

100

0

100

200

300

400 

< 200

Well 37

I I I I I I I I I I I \ I I

County: Nacogdoches 

State well number: TX-37-36-102 

Land-surface datum: 200 feet 

Well depth: 900 feet

Aquifer: Lower Claiborne-upper Wilcox

I I I I I I I I I I I I I I

-O -

Well 38

o 100 <>--

100

200

300

400

1 I I I I I I I I I I I I I

County: Angelina 

State well number: AD-37-36-403 

- Land-surface datum: 204 feet 

Well depth: 847 feet

Aquifer: Lower Claiborne-upper Wilcox

I I I I I I I I

O _O 2

Well 39

100 ( 

0 

100 

200

300 

/nn

>--

I I 1 I I I I I I I I

County: Angelina /s__x__^ ~~~-C 

State well number: AD-37-36-801 ^^"""^^^^ 

Land-surface datum: 228 feet s"~--- 

Well depth: 1,056 feet 

Aquifer: Lower Claiborne-upper Wilcox

1 1 1 1 1 1 I I 1 I 1

ill.

^o-o--~oi

III"

1910 1915 1920 1925 1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985

YEAR

EXPLANATION

 MEASURED WATER LEVEL 

O O SIMULATED WATER LEVEL Circles indicate model time steps

FIGURE 39. Measured and simulated water levels in wells in the Lufkin-Nacotidoches area, Texas.

accuracy in areas such as Houston, where the vertical 

distributions of numerous pumpage and head data are reasonably well known. A related source of error is that the model 

simulations assume that pumping and observation wells are 

open to the full length of the aquifer or model layer generally, this is not true. Thus, simulated heads can only be 

approximations of measured heads.

3. In general, the accuracy of simulation models 

decreases with increasing grid-block size. The large 

grid-block size, 25 mi 2 , may have led to overgeneralization 

of the input data and poor resolution of model results in 

some areas. For example, in the Houston area pumpage and 

head data are of sufficient density to warrant smaller grid 

spacing. Another example is Kingsville, where significant 

error was introduced because the city's well field lies almost 

exactly between the centers of the large grid blocks.

4. The accuracy of simulated development heads and 

drawdowns is dependent upon correct predcvelopment 

starting heads. Simulated starting heads are subject to considerable error in some areas.

5. The model must simulate average conditions over long 

time periods (generally of 5-yr duration after 1958). This 

length of time is necessitated by long periods of unknown or 

poorly understood irrigation pumpage data, as well as variations in recharge.

6. A considerable part of modeled recharge rates in some 

areas may consist of return flow from irrigation pumpage to 

the ground-water system. Such rates of return flow generally 

are unknown or poorly understood.

7. It is uncertain whether or not there is a long-term net 

dewatering of shallow sands in the eastern part of the study 

area, where precipitation is more abundant. Such knowledge
E68 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

EXPLANATION

 3 LINE OF EQUAL LAND-SURFACE SUBSIDENCE Dashed where approximately 

located. Interval 0.5 and 1 foot. From 

Carr and others (1985, fig. 31)

3 LINE OF EQUAL SIMULATED LAND-SURFACE 

SUBSIDENCE Interval 0.5 and 1 foot

FIGURE 40. Measured and simulated land-surface subsidence in the Houston-Galveston area, Texas, predevelopment to 1973.

may be important in evaluating modeled estimates of the 

amount of ground-water pumpage derived from recharge and 

from releases from storage.

8. Simulation of land-surface subsidence is subject to the 

following additional sources of error (Prudic and Williamson, 

1984): (a) The critical head in the clay part of a permeable 

zone is assumed to be equal to the head in the sand deposits. 

However, time is needed for a head change to propagate 

through the clay deposits. Ignoring the time lag affects the 

distribution and amount of the simulated subsidence, (b) 

Water is assumed to be released from the inelastic

compaction of clays instantaneously with a head decline 

below the critical head. There may be a lag time of several 

years between observed subsidence and a head change in 

coarse-grained deposits as the head change gradually propagates through the fine-grained deposits, (c) The inelastic 

specific storage is assumed to be constant. However, laboratory tests indicate that the amount of water released from 

inelastic compaction is a function of applied stress. In addition, the vertical hydraulic conductivity of the compacting 

deposits, treated as a constant in the model, should 

theoretically decrease with time. Most of the errors
HYDROLOGY OF THE TEXAS GULF COAST AQUIFER SYSTEMS E69 

TABLE 16. Changes in model hydraulic properties to test sensitivitv of model for 10 model simulations, Texas Gulf Coast aquifer systems

Model 

simulation Property change

1.......... Horizontal hydraulic conductivity (KH) of all aquifers and permeable zones equals calibrated KH

multiplied by 1.5.

2.......... Horizontal hydraulic conductivity (KH) of all aquifers and permeable zones equals calibrated KH

multiplied by 0.5.

3.......... Vertical hydraulic conductivity (KV) of all model layers equals calibrated KV multiplied by 1.5.

4.......... Vertical hydraulic conductivity (KV) of all model layers equals calibrated KV multiplied by 0.5.

5.......... Water density (DEN) equals 1.000 g/cm3 in all model layers.

6.......... Water density (DEN) equals calibrated DEN+0.010 g/cm3 for grid blocks in all model layers

where DEN is greater than 1.002 g/cm3 .

7.......... Storage coefficient (S) of all aquifers and permeable zones equals calibrated S multiplied by 1.5.

8......... Storage coefficient (S) of all aquifers and permeable zones equals calibrated S multiplied by 0.5.

9......... Initial critical head (D) for conversion to inelastic storage coefficient equals calibrated D

multiplied by 1.5.

10......... Initial critical head (D) for conversion to inelastic storage coefficient equals calibrated D

multiplied by 0.5.

associated with item 8 tend to be minimized because of the 

large time periods for averaging pumpages in the model 

simulations.

Insight into the relative amount of error in simulated heads 

and land-surface subsidence resulting from incorrect estimates of aquifer and confining-unit properties can be gained 

by analyzing the sensitivity of the model calibration to 

changes in selected model variables.

SENSITIVITY ANALYSIS

Ten transient-model simulations were made in order to 

test the sensitivity of the model to changes in the calibrated 

values of horizontal and vertical hydraulic conductivity, 

water density, storage coefficient, and initial critical head 

(for land-surface subsidence). The changes for the 10

model simulations are summarized in table 16. Each 

change was uniform over the modeled area with the exceptions noted in table 16. When the value of each property 

was changed for a model simulation, the values of all other 

properties remained unchanged from their calibrated values. 

Because recharge and discharge are simulated by a 

head-dependent flux boundary, these values are allowed to 

change with any change in head in the sensitivity 

simulations.

The sensitivity simulations are evaluated by (1) showing 

graphically the change from 1982 calibrated heads in permeable zones A, B, and C along row 43 of the model grid (fig. 

35), and in the lower Claiborne-upper Wilcox aquifer along 

row 91; (2) statistically comparing the new residual errors 

for 1982 simulated and measured potentiometric surfaces 

with calibration residuals; (3) statistically comparing the new 

residual errors for simulated and measured predevelopment
E70 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

25

-25

A Permeable zone A

-50

100

50

0

-50

-100

-150

150

100

50

0

-50

-100

-150

-200 

O -250

LLl -300

9£ -350

1.5 X CALIBRATED (KH) 

0.5 X CALIBRATED (KH)

MODEL ROW 43

6. Permeable zone B

~ MODEL ROW 43

C. Permeable zone C

- MODEL ROW 43

100

50

0

-50

-100

-150

-200

-250

D. Lower Claiborne-upper Wilcox aquifer

_ MODEL ROW 91

10 20 30 40 50 

MODEL COLUMN NUMBER

60 70 80

FIGURE 41. Sensitivity of simulated 1982 heads to change in horizontal hydraulic conductivity (KH), Texas Gulf Coast study area.

to 1973 land-surface subsidence with calibration residuals; 

and (4) comparing graphically and statistically simulated and 

measured hydrographs for selected wells in the Houston and 

Winter Garden areas.

Row 43 was selected because it extends approximately 

through the centers of the cones of depression in permeable 

zones A, B, and C in the Houston-Galveston area. Row 91 

extends approximately through the center of the cone of 

depression in the lower Claiborne-upper Wilcox aquifer in 

the Winter Garden area.

The horizontal hydraulic conductivity of the aquifers and 

permeable zones was increased by 50 percent for the first 

sensitivity simulation. The changes in 1982 heads in 

intensively pumped units along the two model rows .are 

shown in figure 41. Heads are greater than calibrated 

heads within the cones of depression developed in permeable zones A, B, and C in the Houston-Galveston area, and 

in the lower Claiborne-upper Wilcox aquifer in the Winter

Garden area. For each unit, the peak departure from calibrated heads occurs where the cone of depression is steepest. Peak departures range from 20 ft for permeable zone 

A to 150 ft for permeable zone C. For permeable zones A, 

B, and C, the departure of heads from calibrated heads is 

almost negligible beyond the coastline (beyond about column 50).

Statistical analyses of the residual errors for 1982 simulated and measured potentiometric surfaces resulting frqm 

the increased horizontal hydraulic conductivity are shown in 

table 17. Absolute means of the residuals increased from 34 

ft (calibrated value) to 41 ft for permeable zone A, from 25 

to 42 ft for permeable zone B, from 35 to 54 ft for permeable zone C, and from 30 to 56 ft for the lower Claiborne-upper Wilcox aquifer.

The effect of increased conductivity on land-surface subsidence in the Houston-Galveston area is shown statistically 

in table 18. The absolute mean of the residual errors for
HYDROLOGY OF THE TEXAS GULF COAST AQUIFER SYSTEMS E71

TABLE 17. Summary of calibration residual errors for 1982 heads and errors resulting from changes in model variables, Texas Gulf Coast aquifer systems 

[KH, horizontal hydraulic conductivity; KV, vertical hydraulic conductivity; S, storage coefficient or specific yield; >, greater than; residual error, simulated

aquifer head minus measured aquifer head]

Aquifer

or 

permeable

zone

Grid 

blocks 

with 

measured 

heads

Absolute 

Change in Mean of Minimum Maximum Standard mean of

model residuals residual residual deviation residuals 

variable (feet) (feet) (feet) (feet) (feet)

Permeable 121 CALIBRATION...... 27

zone A. 1.5KH............ 34

0.5KH............ 7

1.5KV............ 34

0.5KV............ 5

Densities=1.0.... 24

Densities>1.002

=+0.01......... 24

1.5S............. 29

0.5S............. 14

Permeable 192 CALIBRATION...... 9

zone B. 1.5KH............ 36

0.5KH............ -48

1.5KV............ 12

0.5KV............ -4

Densities=l.0.... 6

Densities>1.002

=+0.01......... 5

1.5S............. 24

0.5S............. -24

Permeable 89 CALIBRATION...... 1

zone C. 1.5KH............ 40

0.5KH............ -84

1.5KV............ 7

0.5KV............ -14

Densities=l.0.... -4

Densities>l.002

=+0.01......... -1

1.5S............. 20

0.5S............. -36

Lower 216 CALIBRATION...... 17

Claiborne- 1.5KH............ 52

upper 0.5KH............ -66

Wilcox 1.5KV............ 36

aquifer. 0.5KV............ -14

Densities=l.0.... 14

Densities>1.002

=+0.01......... 19

1.5S............. 38

0.5S............. -20

26

32

69

25

69

39

38

35

 46

108

102

87

104

77

95

95

101

84

35

39

35

35

39

36

35

37

34

34

41

28

37

35

34

34

37

31

-96

-63

-405

-94

-125

-94

-96

-82

-147

72

105

38

78

62

73

72

82

69

29

33

64

29

38

32

31

32

39

25

42

54

25

30

26

25

34

36

 117

 68

 454

 109

 125

 124

 115

 68

 190

91

137

28

90

96

116

77

117

44

42

54

108

41

46

44

41

45

51

35

54

87

35

39

35

34

38

45

-75

-89 

235

-50 

116

-75

-74

-55

-114

82

114

90

85

78

81

83

128

46

30

38

61

29

35

28

31

33

30

30

56

74

39

30

26

30

42

27
E72 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

1 ' 3LE 18. Summary of calibration residual errors for land-surface subsidence and errors resulting from changes in model variables, Texas Gulf

Coast aquifer svstems

 ,\H, horizontal hydraulic conductivity; KV, vertical hydraulic conductivity; S, storage coefficient; D, initial head decline that must occur before 

model converts to inelastic storage value; >, greater than; residual errors, simulated aquifer head minus measured aquifer head]

Grid blocks

with

measured

subsidence

196... ....

Change in model

variable

CALIBRATION. ....

1.5KH. ..........

0.5KH. ..........

1.5KV.. .........

0.5KV. ..........

Density=l .0. ....

Density>1.002=

density+0.01. .

1.5S.. ..........

0.5S. ...........

1.5D=105 feet.. .

0.5D=35 feet. . . .

Mean of

residuals

(feet)

-0.2

-.6

.4

-.4

.2

-.2

-.2

-.02

-.5

-.5

.3

Minimum

residual

(feet)

-2.5

-3.7

-1.2

-2.8

-2.0

-2.5

-2.5

-1.9

-3.9

-3.2

-1.9

Maximum

residual

(feet)

2.1

1.6

7.4

2.0

3.2

2.4

2.4

4.5

1.3

1.8

3.1

Standard

deviation

(feet)

0.7

.8

1.3

.7

.9

.8

.8

1.0

.8

.7

.9

Absolute

mean of

residuals

(feet)

0.6

.8

.9

.7

.7

.6

.6

.7

.7

.7

.7

simulated and measured predevelopment to 1973 land-surface subsidence increased from a calibrated value of 0.6 ft to 

0.8 ft.

A 50-percent decrease in horizontal hydraulic conductivity 

results in head departures that are nearly mirror images of 

those shown for the 50-percent increase (fig. 41). However, 

the departures are larger, with peak departures ranging from 

-37 ft for permeable zone A to -350 ft for permeable zone 

C. Absolute means of the 1982 head residuals are 28 ft for 

permeable zone A, 54 ft for permeable zone B, 87 ft for permeable zone C, and 74 ft for the lower Claiborne-upper Wilcox aquifer (0.5KH, table 17). The residuals for permeable 

zone A are an improvement over the calibration residuals. 

The reduced horizontal hydraulic conductivity causes additional drawdown that is generally needed in the Houston-Galveston area for an improved calibration. However, 

statistical data for the other units shown in table 17 indicate 

that the reduced hydraulic conductivity resulted in much 

poorer matches of simulated and measured heads. The absolute mean of the land-surface subsidence residuals is 0.9 ft 

(0.5KH, table 18).

For the third model sensitivity simulation, the vertical 

hydraulic conductivity of all model layers was increased 

by 50 percent. Generally, the increase had the same 

effect as the increase in horizontal hydraulic conductivity to raise the heads above the calibrated heads (fig. 

42). Peak departures from calibrated heads are much 

smaller, ranging from 11 ft for permeable zone A to 30 ft 

for the lower Claiborne-upper Wilcox aquifer. Absolute

means of the 1982 head residuals are 37 ft for permeable 

zone A, 25 ft for permeable zone B, 35 ft for permeable 

zone C, and 39 ft for the lower Claiborne-upper Wilcox 

aquifer (1.5KV, table 17). The absolute mean of the 

land-surface subsidence residuals is 0.7 ft (1.5KV, table 

18).

A 50-percent decrease in vertical hydraulic conductivity 

results in head departures that are nearly mirror images of 

those shown for the 50-percent increase (fig. 42). As for 

the case with horizontal conductivity, the departures are 

larger, with peak departures ranging from -25 ft for permeable zone A to -50 ft for the lower Claiborne-upper Wilcox 

aquifer. For permeable zone C, there is a rise in heads of 

about 10 to 15 ft extending from column 43 to the downdip 

limit.

Absolute means of the potentiometric-surface residuals are 

35 ft for permeable zone A, 30 ft for permeable zone B, 39 ft 

for permeable zone C, and 30 ft for the lower Claiborne-upper Wilcox aquifer (0.5KV, table 17). The absolute 

mean of the land-surface subsidence residuals is 0.7 ft 

(0.5KV, table 18).

For the fifth model sensitivity simulation, the density of 

the water in all model layers was given a uniform value 

equal to 1.000 g/cm3 . For permeable zones A, B, and C, 

the changed density produced essentially no change from 

the calibrated heads where the units originally contained 

fresh to slightly saline water (dissolved-solids concentration 

less than about 3,000 mg/L) (fig. 43). Farther downdip in 

the three zones, the heads departed from the calibrated
HYDROLOGY OF THE TEXAS GULF COAST AQUIFER SYSTEMS E73

25

-25

A. Permeable zone A

-50

1.5 X CALIBRATED (KV) 

0.5 X CALIBRATED (KV)

MODEL ROW 43

25

B. Permeable zone B

-25

-50

MODEL ROW 43

25

-25

-50

50

25

-25

-50

C. Permeable zone C

MODEL ROW 43

D. Lower Claiborne-upper Wilcox aquifer

MODEL ROW 91

10 20 30 40 50 

MODEL COLUMN NUMBER

60 70 80

FIGURE 42. Sensitivity of simulated 1982 heads to change in vertical hydraulic conductivity (KV), Texas Gulf Coast study

area.

heads in an upward direction, with peak departures ranging 

from 15 ft for permeable zone A to 240 ft for permeable 

zone C. For the lower Claiborne-upper Wilcox aquifer 

along model row 91, heads departed from calibrated heads 

in a downward direction, with a peak departure of -43 ft 

(fig. 43).

Absolute means of the 1982 potentiometric-surface residuals were nearly identical to the calibrated values for permeable zones A, B, and C (density=1.0, table 17). The absolute 

mean for the lower Claiborne-upper Wilcox residuals is 26 

ft, an improvement over the calibrated value of 30 ft because 

of the additional drawdown downdip in the Winter Garden 

area. The statistical data for the land-surface subsidence 

residuals are nearly the same as for the calibrated values 

(density^ 1.0, table 18).

For the sixth model simulation, the water density in all 

model layers was increased by 0.010 g/cm3 for those grid 

blocks where the density was greater than 1.002 g/cm3 . The 

changed density produced essentially no change from the 

calibrated heads where the units originally contained fresh to

slightly saline water (fig. 43). For downdip blocks in 

permeable zones A, B, and C, the heads were a few feet 

lower than calibrated heads. For the extreme downdip 

blocks in the lower Claiborne-upper Wilcox aquifer, the 

heads were a few feet higher. Statistical data for the potentiometric-surface and land-surface subsidence residuals were 

nearly identical to the. calibrated values (density>1.002=+0.01, tables 17, 18).

The seventh model sensitivity simulation consisted of 

increasing the storage coefficient of all aquifers and 

permeable zones by 50 percent. The increase tended to raise 

the heads above the 1982 calibrated heads for the inland 

parts of model rows 43 and 91 (1.5S, fig. 44). Peak departures from calibrated heads range from 7 ft for permeable 

zone A to 42 ft for permeable zone C.

Absolute means of the 1982 potentiometric-surface residuals are 37, 34, 38, and 42 ft for permeable zones A, B, C, 

and the lower Claiborne-upper Wilcox aquifer, respectively 

(1.5S, table 17). The absolute mean of the land-surface subsidence residuals is 0.7 ft (table 18).
E74 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

50

25

-25

A. Permeable zone A

 ADDED 0.01 TO DENSITIES > 1.002

 ALL DENSITIES = 1.000

MODEL ROW 43

-50

50

25

6. Permeable zone B

-25

-50

250

200

150

100

E 50

LI.

O 0

LLJ

§ -50

cc

< 50

LLJ

° 25

-25

-50

MODEL ROW 43

C. Permeable zone C

MODEL ROW 43

D Lower Claiborne-upper Wilcox aquifer

MODEL ROW 91

10 20 30 40 50 

MODEL COLUMN NUMBER

60 70 80

FIGURE 43. Sensitivity of simulated 1982 heads to changes in water density, Texas Gulf Coast study area.

A 50-percenl decrease in the storage coefficient had the 

opposite effect of the 50-percent increase. The heads are 

lower than the 1982 calibrated heads (fig. 44), with peak 

departures ranging from -17 ft for permeable zone A to -75 

ft for permeable zone C. The departures are larger for the 

decrease in storage coefficient compared to the increase.

Th'e effect of the decreased storage coefficient on 1982 

potentiometric-surface residuals was mixed. For permeable 

zone A and the lower Claiborne-upper Wilcox aquifer, the 

statistical fits of simulated to measured heads are slightly 

better than the calibration fits, whereas the fits for zones B 

and C are considerably worse than calibrated fits (0.5S, table 

17). The absolute mean of the land-surface subsidence residuals is 0.7 ft (0.5S, table 18).

An additional test of the sensitivity of the model to the 

change in storage coefficient consists of comparing the new 

drawdown distributions over time to calibrated values and to 

measured values. This comparison was done at five observation wells near the centers of large cones of depression. 

Three wells in the Houston area are open to permeable zones

A, B, and C (fig. 45). Two wells in the Winter Garden area 

are open to the lower Claiborne-upper Wilcox aquifer (fig. 

46).

Simulated water levels for the three model simulations (calibration, 50-percent increase, and 50-percent 

decrease in storage coefficient) were compared to measured water levels in the five wells at the end of each 

time step (where measured water levels exist). The 

results, in terms of absolute means of the residuals, are 

in table 19.

For the five wells, the increase in storage coefficient produces residuals that are substantially larger than the calibration residuals. However, the decrease in storage coefficient 

produces residuals that are substantially lower for four of the 

five wells. The calibration residuals are smaller only for 

well 7, open to permeable zone B in the Houston area. 

Although a smaller storage coefficient may seem indicated, 

at least for some areas in some layers, the following factors 

may suggest otherwise: (1) The specific storage estimate of 

l.OxlO"6 per foot approaches a minimum value for confined
HYDROLOGY OF THE TEXAS GULF COAST AQUIFER SYSTEMS E75

50

25

0

-25

-50

50

25

0

-25

-50

-100

50

25

0

-25

-50

-75

-100

50

25

0

-25

-50

-75

-100

A. Permeable zone A

1.5 X CALIBRATED (S) 

0.5 X CALIBRATED (S)

MODEL ROW 43

O

B. Permeable zone B

 MODEL ROW 43

C. Permeable zone C

_ MODEL ROW 43

 .

D. Lower Claiborne-upper Wilcox aquifer

MODEL ROW 91

0 10 20 30 40 50 

MODEL COLUMN NUMBER

60 70 80

FIGURE 44. Sensitivity of simulated 1982 heads to changes in aquifer storage coefficient (S), Texas Gulf Coast study area.

clastic sediments; (2) the number of hydrographs (five) is a 

small sample considering the large areal extent of the major 

cones of depression in the study area; (3) the head as measured in a well may be an average over a smaller vertical 

thickness of aquifer than the thickness assigned to the model 

block. Thus, to obtain a match of simulated and measured 

water levels, the storage coefficient may have to be adjusted 

downward to account for the lesser thickness open to the 

well; and (4) other factors that may contain considerable 

errors, such as estimates of starting heads and pumpage, 

exert a large influence on the configuration of the simulated 

hydrographs.

Final model sensitivity simulations were made to test the 

effect of raising and lowering the initial critical head on 

land-surface subsidence residuals. A 50-percent increase in 

initial critical head, from 70 to 105 ft, produces a rise in the 

absolute mean of the residuals from a calibrated value of 0.6 

to 0.7 ft (1.5D=105 ft. table 18). A 50-percent reduction of 

the initial critical head, from 70 to 35 ft, produces the same

absolute mean of the residuals as did the increase 0.7 ft. 

Thus, the critical head does not seem sensitive to the 

simulation of land subsidence, but it may affect the simulation of drawdowns in the model.

The following statements summarize the sensitivity 

analysis:

1. The sensitivity of the model calibration is tested by 

analyzing the impact of a changed model variable on 1982 

potentiometric-surface residuals, hydrograph residuals, and 

land-surface subsidence residuals.

2. The model is most sensitive and has the highest 

residuals when horizontal hydraulic conductivity is decreased 

by 50 percent. An exception is for permeable zone A, where 

a 50-percent decrease causes smaller 1982 head residuals.

3. The model is moderately sensitive to 50-percent 

changes in vertical hydraulic conductivity and storage coefficient.

4. The model is least sensitive to changes in water 

density.
E76 REGIONAL AQUIFER-SYSTEM ANALYSIS GULF COASTAL PLAIN

50

50

100

150

200

250

300

350

400

450

50

A Permeable zone A, well 9

I I I I I I I I I I I I I I

J____I____I____I____I____I____I____l____I____I____I____I

B Permeable zone B, well 7

50

100

150

200

250

300

350

400

450

C

50

0

50

100

150

200

250

300

350

400

450

1910

" o^--.

I I I

Permeable zone C, well 3

J____|____|____|____|____|____|____\_____|____|____|____|____L 

1915 1920 1925 1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985

YEAR

EXPLANATION

 MEASURED WATER LEVEL

O O SIMULATED WATER LEVEL, CALIBRATED

»- « SIMULATED WATER LEVEL, 1.5x CALIBRATED (S)

 - - SIMULATED WATER LEVEL, 0.5 x CALIBRATED (S)

FIGURE 45. Sensitivity of simulated water levels, 1910-82, to changes in storage coefficient (S), Harris County,

Texas.
HYDROLOGY OF THE TEXAS GULF COAST AQUIFER SYSTEMS E77

50

0

50

100

150

200

250

300

350

400

450

50

0

50

100

150

200

250

300

350

400

A Lower Claiborne-upper Wilcox aquifer, well 23, Zavala County

B Lower Claiborne-upper Wilcox aquifer, well 29, Frio County

i i i i

1910 1915 1920 1925 1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985

YEAR

EXPLANATION

 MEASURED WATER LEVEL

O- -O SIMULATED WATER LEVEL, CALIBRATED

e-----« SIMULATED WATER LEVEL, 1.5 X CALIBRATED (S)

 - - SIMULATED WATER LEVEL, 0.5 X CALIBRATED (S)

FIGURE 46. Sensitivity of simulated water levels, 1910-82, to changes in storage coefficient (S), Winter Garden

area, Texas.

TABLE 19. Comparison of measured water levels to simulated water levels resulting from changed values for model storage coefficient, Texas

Gulf Coast aquifer systems

Well No. 

(figs. Aquifer or 

13, 45, permeable 

and 46) zone

9...... A.................

7...... B.... ............

3...... C.......... ......

23...... Lower ClaiborneAbsolute mean of residuals (feet)

Period of 

comparison

1957-82 

1947-82 

1947-82

50-percent 

Calibration increase 

in storage 

coefficient

36 52 

23 45 

49 73

50-percent 

decrease 

in storage 

coefficient

19 

28

23

upper Wilcox. 1947-82

Lower Claiborneupper Wilcox. 1957-82

53

65

60

84

40

33