Analysis of Ground-Water Flow in the Catahoula 

Aquifer System in the Vicinity of Laurel and 

Hattiesburg, Mississippi

By Keith J. Halford and Nancy L. Barber

U.S. GEOLOGICAL SURVEY 

Water-Resources Investigations Report 94-4219

Prepared in cooperation with the

PAT HARRISON WATERWAY DISTRICT and the 

MISSISSIPPI DEPARTMENT OF ENVIRONMENTAL QUALITY, 

OFFICE OF LAND AND WATER RESOURCES

Jackson, Mississippi 

1995
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director

For additional information Copies of this report can be

write to: purchased from:

	U.S. Geological Survey

District Chief Earth Science Information Center

U.S. Geological Survey Open-File Reports Section

100 W. Capitol St., Suite 710 Box 25286, MS 517, Federal Center

Jackson, MS 39269 Denver, CO 80225
CONTENTS

Abstract................................................................................. 1

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

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

Physiography and climate .......................................................... 2

Previous investigations............................................................. 2

Geohydrology of the Catahoula aquifer system near Laurel and Hattiesburg...................... 4

Geology.......................................................................... 4

Hydrogeology .................................................................... 6

Water use. ....................................................................... 17

Effects of pumping................................................................ 17

Analysis of the ground-water flow system................................................... 22

Description of model. ............................................................. 22

Boundary conditions....................................................... 27

Hydraulic properties. ...................................................... 30

Model calibration................................................................. 32

Steady-state simulation .................................................... 34

Transient simulation....................................................... 34

Sensitivity analysis................................................................ 48

Limitations of model application ................................................... 48

Simulated aquifer response to pumping scenarios............................................ 54

Summary............................................................................... 71

Selected references....................................................................... 72

ILLUSTRATIONS

1. Map showing location of study area............................................. 3

2. Map showing outcrop of the Catahoula Formation in the modeled area and the

location of selected wells. ................................................... 5

3. Sketch showing geophysical logs and aquifers in the Laurel and Hattiesburg,

Mississippi, areas........................................................... 7

4. Map showing altitude of base of the lower Catahoula aquifer in the modeled area..... 8

5. Diagrammatic sections A-A' and B-B' in the modeled area.......................... 9

6-12. Maps showing:

6. Sand thickness of the upper Catahoula aquifer in the study area................ 10

7. Sand thickness of the middle Catahoula aquifer in the study area................11

8. Sand thickness of the lower Catahoula aquifer in the study area. ............... 12

9. Thickness of confining units overlying the upper Catahoula aquifer in the study

area.................................................................. 14

10. Thickness of confining units overlying the middle Catahoula aquifer in the study

area.................................................................. 15

11. Thickness of confining units overlying the lower Catahoula aquifer in the study

area.................................................................. 16

12a. Pumping centers in the modeled area....................................... 18

12b. Pumping centers in the study area ......................................... 19

13. Graph showing pumpage from the upper, middle, and lower Catahoula aquifers in the

Laurel-Hattiesburg area, Mississippi, 1960-90. ................................ 20

14-16. Hydrographs showing:

14. Water levels for wells 67C034, 67F020, and 67G055 near Laurel, Mississippi. ..... 21

15. Water levels for wells 35B006,35D008, and 35D130 near Hattiesburg,

Mississippi............................................................ 23

16. Water levels for wells 35L079,73L063, and 111M011 generally unaffected by major

pumping centers in the modeled area..................................... 24

111
ILLUSTRATIONS-Continued

17-19. Maps showing:

17a. Model grid and extent. ................................................... 25

17b. Model grid in study area.................................................. 26

18. Altitude of water table (layer 1) in the modeled area. ......................... 28

19. Maximum potential recharge available in the modeled area.................... 29

20. Diagram showing conceptual model for the Catahoula aquifer system. ............. 31

21. Graph showing effect of parameter modifiers as applied to a hypothetical section of

lateral hydraulic conductivity .............................................. 33

22. Maps showing simulated predevelopment potentiometric surfaces of the upper,

middle, and lower Catahoula aquifers in the modeled area...................... 35

23. Diagram showing simulated predevelopment volumetric flow budget for the

Catahoula aquifer system in the modeled area................................. 36

24. Maps showing simulated potentiometric surfaces of the upper, middle, and lower

Catahoula aquifers in the modeled area in 1992................................ 38

25-27. Hydrographs showing:

25. Measured and simulated water levels for wells 67C034,67F020, and 67G055 near

Laurel, Mississippi..................................................... 40

26. Measured and simulated water levels for wells 35B006,35D008, and 35D130 near

Hattiesburg, Mississippi................................................ 41

27. Measured and simulated water levels for wells 35L079,73L063, and 111M011

generally unaffected by major pumping centers in the modeled area.......... 42

28. Diagram showing simulated volumetric flow budget for the Catahoula aquifer system

in the modeled area in 1992................................................. 43

29-35. Maps showing:

29. Simulated water-level declines from predevelopment to 1992 in the upper, middle,

and lower Catahoula aquifers in the modeled area. ........................ 44

30. Calibrated leakance of confining units above the upper Catahoula aquifer in the

modeled area.......................................................... 45

31. Calibrated leakance of confining units above the middle Catahoula aquifer in the

modeled area.......................................................... 46

32. Calibrated leakance of confining units above the lower Catahoula aquifer in the

modeled area.......................................................... 47

33. Calibrated transmissivity of upper Catahoula aquifer in the modeled area. ...... 49

34. Calibrated transmissivity of middle Catahoula aquifer in the modeled area. ..... 50

35. Calibrated transmissivity of lower Catahoula aquifer in the modeled area........ 51

36. Graph showing model sensitivity to independent changes in transmissivity, leakance,

and storage coefficient. .................................................... 52

37. Graph showing model sensitivity to simultaneous changes in transmissivity and

leakance.................................................................. 53

38. Diagram showing simulated volumetric flow budget for the Catahoula aquifer system

in 2020 at the end of the low-growth scenario. ................................ 55

39. Maps showing simulated water-level declines from 1992 to 2020 in the:

39a. Upper, middle, and lower Catahoula aquifers in the modeled area at the end of the

low-growth scenario. .................................................. 57

39b. Upper Catahoula aquifer in the study area at the end of the low-growth

scenario.............................................................. 58

39c. Middle Catahoula aquifer in the study area at the end of the low-growth

scenario. ............................................................. 59

39d. Lower Catahoula aquifer in the study area at the end of the low-growth

scenario.............................................................. 60

IV
ILLUSTRATIONS-Continued

40. Diagram showing simulated volumetric flow budget for the Catahoula aquifer system

in 2020 at the end of the moderate-growth scenario. ........................... 61

41. Maps showing simulated water-level declines from 1992 to 2020 in the:

41a. Upper, middle, and lower Catahoula aquifers in the modeled area at the end of the

moderate-growth scenario. ............................................. 62

41b. Upper Catahoula aquifer in the study area at the end of the moderate-growth

scenario. ............................................................. 63

41c. Middle Catahoula aquifer in the study area at the end of the moderate-growth

scenario. ............................................................. 64

41d. Lower Catahoula aquifer in the study area at the end of the moderate-growth

scenario. ............................................................. 65

42. Diagram showing simulated volumetric flow budget for the Catahoula aquifer system

in 2020 at the end of the high-growth scenario. ............................... 66

43. Maps showing simulated water-level declines from 1992 to 2020 in the:

43a. Upper, middle, and lower Catahoula aquifers in the modeled area at the end of the

high-growth scenario. ................................................. 67

43b. Upper Catahoula aquifer in the study area at the end of the high-growth

scenario. ............................................................. 68

43c. Middle Catahoula aquifer in the study area at the end of the high-growth

scenario. ............................................................. 69

43d. Lower Catahoula aquifer in the study area at the end of the high-growth

scenario. ............................................................. 70

TABLES

1. Geologic units underlying southeastern Mississippi........................... 6

2. Ranges of reported lateral hydraulic conductivity and storage coefficient for the

Catahoula aquifer system in southeastern Mississippi. ..................... 30

3. Simulated and measured water levels used to calibrate the transient model. ..... 39

4. Summary of alternate pumping scenarios with total pumpage in 2020 .......... 54
Analysis of Ground-Water Flow in the Catahoula 

Aquifer System in the Vicinity of Laurel and 

Hattiesburg, Mississippi

By Keith J. Halford and Nancy L. Barber

Abstract

The upper, middle, and lower Catahoula 

aquifers in the vicinity of the cities of Laurel 

and Hattiesburg in southern Mississippi are 

made up of irregular, discontinuous sand zones 

in the Catahoula Formation of Miocene age. 

Withdrawal from the Catahoula aquifer system 

increased from 28 to 41 million gallons per 

day from 1970 to 1985, and decreased to 38 

million gallons per day during 1990. Most 

withdrawal in the Laurel area is from the lower 

and middle Catahoula, and most withdrawal in 

the Hattiesburg area is from the middle and 

upper Catahoula aquifers. In the Laurel area, 

water levels in the lower Catahoula aquifer 

declined at rates ranging from about 1 to 3.6 

feet per year from 1964 until the late 1980's in 

response to the increase in pumping.

A three-dimensional model was developed to represent ground-water flow in the 

Catahoula aquifer system. Simulated water 

levels in the lower Catahoula aquifer, the layer 

most affected by pumping, were lowered from 

predevelopment levels as much as 130 feet in 

the Laurel area and 100 feet in the Hattiesburg 

area, according to the model analysis of 1992 

conditions. Three scenarios of increased 

pumpage for the period 1992-2020 were simulated. Under the low-growth scenario, waterlevel declines would be 20 feet or less below 

1992 water levels in the middle and upper Catahoula aquifer in the Hattiesburg area, and 

about 60 feet in the lower Catahoula aquifer in

the Laurel area. Under the moderate-growth 

scenario, water-level declines would be 40 feet 

or less below 1992 water levels in the middle 

Catahoula aquifer in the Hattiesburg area. 

Water-level decline would be about 110 feet in 

the lower Catahoula aquifer in the Laurel area, 

and water levels would be near the top of the 

aquifer. Under the high-growth scenario, 

water-level decline would be 40 feet or less in 

the upper Catahoula aquifer and about 80 feet 

in the middle Catahoula, with the largest 

decline occurring in the Hattiesburg area. 

Water levels would decline about 130 feet and 

would be drawn down below the top of the 

lower Catahoula aquifer in the Laurel area 

under the high-growth scenario.

INTRODUCTION

The Catahoula aquifer system, the lowermost aquifers of the Miocene-age series in 

Mississippi, is a major source of water for 

industry and public supply throughout the 

Laurel and Hattiesburg areas in southern 

Mississippi. In the Laurel area, most public 

and industrial water supplies are obtained from 

the lower and middle Catahoula aquifers. Supplies for the city of Laurel are obtained from 

the lower Catahoula exclusively. In the 

Hattiesburg area, most pumpage is from the 

middle and upper Catahoula aquifers, although 

the lower Catahoula also is used. Withdrawal 

from the aquifers in the Laurel and Hattiesburg
areas increased from about 17 Mgal/d during 

1960 to 41 Mgal/d during 1985, and decreased 

to 38 Mgal/d during 1990. Water levels near 

pumping centers in Laurel and Hattiesburg 

declined at rates ranging from about 1 to 

3.6 ft/yr between 1955 and 1985. However, the 

rates of decline have decreased from 1985 to 

1993, and in some areas, primarily in Laurel, 

water levels have recovered in response to 

decreases in ground-water pumpage and to 

changes in the pumping distribution. If 

ground-water use increases in response to population growth or industrial development, 

water levels may decline below the contemporary (1992) or historical low levels. The ability 

to estimate the potential of the Catahoula 

aquifer system would facilitate the management and plans for additional development of 

ground-water resources in the Laurel and 

Hattiesburg areas.

A study of the Catahoula aquifer system 

in the Laurel and Hattiesburg areas was performed as part of a larger study by the U.S. 

Geological Survey designed to improve understanding of ground-water flow for the 

Miocene-age aquifers in selected areas of 

southern Mississippi. This report was prepared 

in cooperation with the Pat Harrison Waterway 

District and with the Mississippi Department 

of Environmental Quality, Office of Land and 

Water Resources (OLWR).

Purpose and Scope

This report describes the ground-water 

flow system in the Catahoula aquifer system 

near two areas of ground-water development: 

the cities of Laurel, in Jones County, and Hattiesburg, in Forrest County. The report is based 

on a study that included the collection of 

water-level and water-use data, and the analysis of electric-resistivity well logs to help 

define the hydrogeologic framework.

Although the cities of Laurel and Hattiesburg and adjacent areas are of specific interest

to this study, ground-water flow model investigations encompassed a much larger area that 

covered much of southeastern Mississippi and 

parts of Louisiana and Alabama (fig. 1). The 

model was used to estimate the changes in 

water levels in the upper, middle, and lower 

Catahoula aquifers under three scenarios of 

simulated pumpage.

Physiography and Climate

Most of the area included in the groundwater flow model is located in the Long-Leaf 

Pine Hills physiographic district (Stephenson 

and others, 1928), an area of rolling to moderately rugged hills ranging from 100 to 500 ft 

above sea level. The southern part of the area 

is located in the Coastal Pine Meadows, a narrow physiographic district near the gulf coast 

that is relatively flat and contains extensive 

areas of swamp and marsh.

Southern Mississippi is characterized by a 

humid, subtropical climate. Average precipitation over the study area ranges from 54 to 60 

in/yr (National Oceanic and Atmospheric 

Administration, 1985). Rainfall is distributed 

evenly throughout the year except during drier 

summer months. The average yearly temperature is 68 °F. Summer temperatures range from 

65 to 98 °F, and winter temperatures range 

from 20 to 65 °F.

The major population centers in the study 

area are Hattiesburg, with a 1990 population of 

41,882, and Laurel, with a 1990 population of 

19,730 (U.S. Bureau of the Census, 1991). 

Larger cities are located on the gulf coast, but 

these are not within the area of development of 

the Catahoula aquifer system because of the 

availability of shallower aquifers in the coastal 

area.

Previous Investigations

The water-yielding properties of the Catahoula Formation in Mississippi were first
90 C

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40 KILOMETERS

Approximate extent of modeled area

Figure 1. Location of study area.
described by Stephenson and others (1928). 

Shows and others (1966), Boswell and others 

(1970), and Brahana and Dalsin (1977) discussed the water sources, quantity, and quality 

for groups of counties in the study area in a 

series of reports that describe water availability for industrial development. The groundwater resources of Jones County are described 

by Boswell and others (1987). Withdrawal 

data for public and industrial water supplies in 

southern Mississippi were compiled for 1970 

(Callahan, 1971) and for 1974 (Callahan, 

1975). Newcome (1975) first discussed the 

hydrogeology of the Miocene aquifer system 

(which encompasses the Catahoula aquifer 

system) and Sumner and others (1989) investigated and simulated ground-water flow in the 

Miocene aquifer system in the coastal region 

of Mississippi. Martin and Whiteman (1989) 

performed a regional analysis of the coastal 

lowland ground-water flow system, including 

the Miocene aquifer system.

GEOHYDROLOGY OF THE 

CATAHOULA AQUIFER SYSTEM NEAR 

LAUREL AND HATTIESBURG

This section of the report describes the 

geohydrology of the Catahoula aquifer system 

near Laurel and Hattiesburg, Mississippi. The 

section includes descriptions of the geology, 

hydrogeology, water use, and the effects of 

pumping.

Geology

Geologic units at land surface in the study 

area are sediments of Miocene age consisting 

of a complex series of alternating and lenticular beds of sand and clay, and other sediments 

of Pliocene and younger age. The beds of 

Miocene age dip to the southwest 30 to 

100 ft/mi, and the steepest dips are nearest the

coast (Newcome, 1975). The gentle dip of the 

Miocene beds is modified in some places by 

salt domes which have uplifted the younger 

sediments. However, none of the salt domes in 

the study area penetrate the Miocene sediments, and their effect is limited to displacing 

other Miocene units to shallower depths near 

the domes (Spiers and Gandl, 1980).

Rocks of the Catahoula Formation of 

Miocene age crop out along the northeastern 

boundary of the modeled area (fig. 2) and are 

the oldest surface or near-surface rocks in the 

study area (table 1). The Catahoula Formation 

consists of beds of medium to coarse sand containing many small to large clay lenses 

(Stephenson and others, 1928, p. 55). The formation thickness ranges from 100 ft or less in 

areas of outcrop to more than 500 ft downdip. 

Miocene-age rocks of the Hattiesburg and Pascagoula Formations overlie the Catahoula and 

are also composed of interbedded, lenticular 

sands and clays. The Hattiesburg and Pascagoula Formations contain a higher percentage 

of clay than the Catahoula Formation, but the 

three formations are so similar that they cannot 

be separated in the subsurface based on lithologic or geophysical characteristics (Newcome, 1975). Identification of these formations 

in the subsurface usually is inferred from their 

relative position in the stratigraphic sequence.

In much of the modeled area, the Miocene 

sediments are unconformably overlain by the 

Citronelle Formation, a discontinuous deposit 

of sand, clay, and gravel of Pliocene and Pleistocene age, or by terrace or alluvial deposits of 

Holocene age. These overlying deposits are 

thicker and more continuous near the coast, 

but generally are less than 100 ft thick 

throughout the area (Gandl, 1982). The 

Miocene sediments are underlain by several 

hundred feet of clay and limestone of Oligocene age: the Paynes Hammock Formation, 

the Chickasawhay Limestone, and the Vicksburg Group (Sumner and others, 1989, p. 3).
32 1-

31'

pproximate extent of modeled ar

1

a urel'<igB7C 034,0055 

7F020

O 35B006

 *.&= I

Hattiesburg]*^350 ! 30 

35DOD8

.1______ -J

20 40 MILES

0 20 40 KILOMETERS

Wllllllllllllllllllh

35L079

155

o

EXPLANATION 

AREA OF CATAHOULA FORMATION OUTCROP

WELL WITH HYDROGRAPH AND NUMBER 

WELL WITH GEOPHYSICAL LOG AND NUMBER

Figure 2. Outcrop of the Catahoula Formation in the modeled area and the 

location of selected wells.
Table 1. Geologic units underlying southeastern Mississippi (Modified from Dockery, 1981)

System

Quaternary

Tertiary

Series

Holocene

Pleistocene

Pliocene

Miocene

Oligocene

Eocene

Group

Vicksburg

Claiborne

Geologic unit

Undifferentiated alluvium and 

terrace deposits

Citronelle Formation

Graham Ferry Formation

Pascagoula Formation 

Hattiesburg Formation 

Catahoula Formation

Paynes Hammock Formation 

Chickasawhay Limestone

Bucatunna Formation 

Byram Formation 

Glendon Limestone 

Marianna Limestone 

Mint Spring Formation

Cockfield Formation

Hydrogeology

The principal sources of ground water in 

the Laurel and Hattiesburg areas are sands 

within the Catahoula Formation which compose the Catahoula aquifer system. Generally 

the Catahoula Formation contains a thick 

water-bearing sand near the base of the formation and two other sands higher in the 

sequence; however the sands generally cannot 

be correlated regionally. Between and within 

the sands are units of clay that vary in thickness and areal extent. Following Boswell and 

others (1987, p. 18), the sands in this report are 

referred to as the lower, middle, and upper 

Catahoula aquifers. The base of the lower 

Catahoula aquifer system in the study area was 

identified using borehole geophysical logs and 

the Glendon Formation, a highly resistive 

limestone unit within the Vicksburg Group, as 

the primary marker bed. Typical electricresistivity log patterns for the Catahoula

Formation in the Laurel and Hattiesburg areas 

(fig. 3) illustrate the alternating sand and clay 

zones.

The configuration of the base of the lower 

Catahoula aquifer is irregular (fig. 4) but generally dips to the south and west; the dip of the 

middle and upper Catahoula aquifers is similar. Generalized hydrogeologic sections 

through the modeled area (fig. 5) illustrate the 

irregular thickness and dip of the Catahoula 

aquifer system and the intervening confining 

units. Within the Jones-Forrest County area, 

the three aquifers can be correlated in an area 

approximately 10 to 15 mi east-west and 30 to 

40 mi north-south. Outside of this area, the 

correlation of aquifers and confining units 

between borehole locations is less certain.

The sand thicknesses of the upper, middle, and lower Catahoula aquifers and related 

confining units also were measured using borehole geophysical logs (figs. 6-8). The aquifers 

are irregular in thickness, are discontinuous in
Jones County 71

General American Oil

Company of Texas

City of LaurelBotleler Unit No. 1

Forrest County 155

Department of EnergyMH7A

Sec. 12.T.8N..R.12W. 

Altitude 271ft.

Sec. 9,T.5N.,R.13W. 

Altitude 210 ft.

Spontaneous - potential 

millivolts

Resistivity 

ohms - m2/m

Spontaneous - potential 

millivolts

Resistivity 

ohms - m2/m

Middle

Catahoula

aquifer

Lower

Catahoula

aquifer

Glendon 

Limestone

Upper

Catahoula

aquifer

Middle

Catahoula

aquifer

y

Total depth 

11,352

Lower

Catahoula

aquifer

Glendon Limestone

Note: location of wells are 

shown on figure 2.

Total depth

2,844

Figure 3. Geophysical logs and aquifers in the Laurel and Hattiesburg, 

Mississippi areas.
32

pproximate extent of modeled area 1 "4

Modified from Sumner and others (1989)

20 40 MILES 

_J T ' ' ' ' I ' ' ' ' I

0 20 40 KILOMETERS

--2,000-

A A'

EXPLANATION

SUBSURFACE CONTOUR--Shows altitude of 

base of lower Catahoula aquifer. 

Dashed where approximately located. 

Contour interval variable, in feet. 

Datum is sea level

TRACE OF CROSS SECTION (See fig. 5)

Figure 4. Altitude of base of the lower Catahoula aquifer in the modeled area.
apso'

c« 

ft 

r-f oi

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03o

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ft

A-A SECTION 

FORREST

RIVER PEARL 

FORREST

DAV1S JEFFERSON 

COVINGTON

COVINGTON 

JONES

Hattiesburg

B-B' SECTION 

^MISSISSIPPI 

"ALABAMA

COoo£tr* Os

"o

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td X> o 

>^ 2 O o

COo

"os

tr-
89° 30'

31° 45'

31° 15'

10 i i i15 MILES

5 10 15 KILOMETERS

EXPLANATION

 100 LINE OF EQUAL SAND THICKNESS-Hachures 

indicate areas of lesser sand thickness. Contour 

interval 50 feet 

+ CONTROL POINT

Figure 6. Sand thickness of the upper Catahoula aquifer in the study area.

10
89° 30'

31° 45'

31° 15' 

10 15 MILES

5 10 15 KILOMETERS

EXPLANATION

 ! oo LINE OF EQUAL SAND THICKNESS-Hachures 

indicate areas of lesser sand thickness. Contour 

interval 50 feet 

+ CONTROL POINT

Figure 7. Sand thickness of the middle Catahoula aquifer in the study area.

11
89° 30'

31° 45' 

31° 15'

10 15 MILES

0 5 10 15 KILOMETERS

EXPLANATION

 100 LINE OF EQUAL SAND THICKNESS-Hachures 

indicate areas of lesser sand thickness. Contour 

interval 50 feet 

+ CONTROL POINT

Figure 8. Sand thickness of the lower Catahoula aquifer in the study area.

12
places, and may contain discontinuous lenses 

of clay. Between clay lenses, two aquifers coalesce to form one aquifer. Accordingly, the 

three Catahoula aquifers form a complex system (which will be referred to as the Catahoula 

aquifer system in this report), in which the 

individual aquifers may be hydraulically well 

connected because of missing or thin confining 

units. In the Hattiesburg area, where the upper 

and middle Catahoula aquifers are the primary 

water sources, the sand thickness of the upper 

Catahoula is about 200 ft, and the sand thickness of the middle Catahoula generally is less 

than 100 ft. In the Laurel area, the sand thickness of the lower Catahoula aquifer (the water 

source for the city of Laurel) generally is less 

than 100 ft. However, the irregular thickness 

and extent of the aquifers can cause variations 

in sand thickness of 50 ft between wells that 

are 100 ft apart.

The confining units overlying the upper, 

middle, and lower Catahoula aquifers are primarily clays within the Catahoula, Pascagoula, 

and Hattiesburg Formations. These clays vary 

in thickness from nearly zero to several hundred feet (figs. 9-11). The thick clays of the 

Paynes Hammock Formation, the 

Chickasawhay Limestone, and the Vicksburg 

Group underlie and are the base of the Catahoula aquifer system in various parts of the 

study area.

Beyond the boundaries of the study area, 

aquifer and confining unit maps were highly 

generalized by extending thickness values 

determined near the study area to the periphery 

of the modeled area. This generalization effectively extends the hydrogeologic framework 

identified within the study area to realistic 

hydrologic boundaries useful for groundwater-flow model calibration. Pumping is 

small within this area of generalization compared to withdrawals at Laurel and Hattiesburg, and no long-term water-level changes are 

known to have occurred.

The primary recharge area for the Catahoula aquifer system is in the northern part of

the modeled area where the aquifers crop out 

in Clarke, Covington, Greene, Jasper, Jones, 

Smith, and Wayne Counties, Mississippi and 

in Washington County, Alabama (figs. 1-2). 

The recharge areas for the individual Catahoula aquifers have not been mapped, but generally the upper Catahoula aquifer crops out 

along the southern edge of the outcrop area 

shown in figure 2, the middle Catahoula crops 

out parallel to the upper Catahoula and farther 

to the north and east, and the lower Catahoula 

aquifer crops out along the northern edge of 

the modeled area. The upper Catahoula aquifer 

(fig. 6) is not present in the Laurel area.

The principal source of water for the Catahoula aquifer system is the infiltration of rainfall in the outcrop areas. Most of the water 

available for recharge either runs off directly to 

streams or is discharged locally as base flow to 

perennial streams. The water that does infiltrate the outcropping sands of the Catahoula 

aquifer system moves within a regional 

ground-water flow system downgradient 

toward the Gulf of Mexico and toward the 

Tombigbee River (Martin and Whiteman 1989, 

fig. 33). The regional ground-water flow from 

outcrop areas toward the gulf coast is distorted 

locally in the Laurel and Hattiesburg areas by 

cones of depression caused by pumping. In 

addition to the lateral, downgradient movement of water within the aquifers, water also 

moves vertically through overlying and underlying confining units. The vertical movement 

is much slower than the lateral movement 

because the clays of the confining units do not 

transmit water as readily as the sands of the 

aquifers.

The southern limit of freshwater (water 

having less than 10,000 mg/L dissolved-solids 

concentration) in the Catahoula aquifer system 

is near the gulf coast. Water with a dissolvedsolids concentration between 1,000 and 10,000 

mg/L generally is not considered freshwater; 

however, for simulation purposes the density 

of such waters is sufficiently similar to that of

13
89° 30'

31° 45'

31° 15' 

10 15 MILES

5 10 15 KILOMETERS

EXPLANATION

 100 LINE OF EQUAL CONFINING UNIT THICKNESSHachures indicate areas of lesser confining unit 

thickness. Contour interval 100 feet 

+ CONTROL POINT

Figure 9. Thickness of confining units overlying the upper Catahoula aquifer in the 

study area.

14
89° 30'

31° 45' 

31° 15'

10 15 MILES

EXPLANATION

5 10 15 KILOMETERS 10Q LINE OF EQUAL CONFINING UNIT THICKNESSHachures indicate areas of lesser confining unit 

thickness. Contour interval 50 feet 

+ CONTROL POINT

Figure 10. Thickness of confining units overlying the middle Catahoula aquifer in 

the study area.

15
89° 30'

31° 45' 

31° 15' 

10 15 MILES

0 5 10 15 KILOMETERS

EXPLANATION

 100 LINE OF EQUAL CONFINING UNIT THICKNESSHachures indicate areas of lesser confining unit 

thickness. Contour interval 50 feet 

+ CONTROL POINT

Figure 11. Thickness of confining units overlying the lower Catahoula aquifer in 

the study area.

16
freshwater to consider these waters a continuum of freshwater.

Water Use

Water-use data for areas of heavy pumping in and near Laurel and Hattiesburg (fig. 12) 

have been collected since 1972 at 5-year intervals as part of a cooperative program with the 

Mississippi Office of Land and Water 

Resources (OLWR). Additional data for the 

period before 1972 for some water users are 

available from U.S. Geological Survey files 

and published reports. Much of the reported 

water use is based on telephone interviews, 

with supplemental information from OLWR 

permit files and Mississippi State Department 

of Health records. For this study, water-use 

data were compiled for the years 1960, 1965, 

1970, 1975,1980, 1985, and 1990.

Water-use data are of varying reliability. 

Many water users do not meter their withdrawals, and water use must be estimated from 

other information such as the population 

served or the yield of the well. Larger public 

suppliers and industries generally meter their 

withdrawals and can provide more accurate 

water-use data than the estimated amounts. 

Well locations for larger water users generally 

have been field-verified. Well locations for 

smaller users may be field-verified or may be 

based on a reported location.

Withdrawals from the Catahoula aquifer 

system in the modeled area are concentrated in 

the Laurel and Hattiesburg areas (fig. 12). In 

the Laurel area, most public-supply and industrial withdrawal is from the lower and middle 

Catahoula aquifers, and some additional 

pumpage is from the deeper Cockfield Formation of Eocene age. The city of Laurel withdraws water from the lower Catahoula 

exclusively. In the Hattiesburg area, most 

withdrawal is from the middle and upper Catahoula aquifers, although some withdrawal also 

is from the lower Catahoula aquifer. South of

Hattiesburg, water is available from sand 

zones in the overlying Hattiesburg and Pascagoula Formations, and the Catahoula aquifers 

are not widely pumped.

From 1970 to 1985 the total estimated 

pumpage from the Catahoula aquifer system in 

the modeled area increased from 28 Mgal/d to 

41 Mgal/d; during 1990 pumpage decreased to 

38 Mgal/d (fig. 13). The decline in total pumpage from 1985 to 1990 primarily resulted from 

declines in withdrawals for thermoelectric 

power generation and for industrial purposes. 

Industrial withdrawals declined steadily from 

1970 to 1990. Public-supply withdrawals 

increased more than 120 percent from 1970 to 

1990, reflecting population growth, increasing 

numbers of public water-supply systems, and 

increased water use per person.

Effects of pumping

Withdrawal from the Catahoula aquifer 

system has affected water levels in the aquifers. In the Laurel area, the water level in the 

lower Catahoula aquifer declined until the late 

1980's in response to increases in the amount 

of water pumped from the aquifer. The water 

level in well 67G055 (fig. 14), located in a 

Laurel well field west of the center of the city 

(fig. 2), declined about 83 ft between 1964 and 

1987, an average rate of 3.6 ft/yr. However, 

between 1987 and 1993 the water level rose 27 

ft. (All hydrographs in figs 14-16 are shown 

for the period 1970-92 for ease of comparison. 

Several water-level measurements were made 

prior to 1970 and were used to calculate longterm rates of water-level decline.) The water 

level in well 67F020, located in the Laurel airport well field, and well 67C034, located in the 

same well field as well 67G055, declined at 

average rates between 1 and 2 ft/yr before 

1988; water levels at these wells increased 

from 1988 to 1993. The average rate of waterlevel decline for wells 67F020 and 67C034 

prior to 1988 is less than the rate of decline

17
0 20 40 MILES

| i 'i i 1 i '| i 1 1 I i '| -1 ' ' '

0 20 40 KILOMETERS

EXPLANATION

WITHDRAWAL FROM THE 

D UPPER CATAHOULA AQUIFER 

O MIDDLE CATAHOULA AQUIFER 

V LOWER CATAHOULA AQUIFER

V MAXIMUM WITHDRAWAL GREATER THAN

500,000 GALLONS PER DAY FROM 1960 TO 

1990--Solid symbol indicates withdrawal

greater than 500,000 gallons per day

during entire period

Figure 12a. Pumping centers in the modeled area.

is
89°30' 89 C

V

1 V

V

V V V

o

V.

o

Laurel

D

V 

O

O

V

O

D

V

V

T T

I

V

V

o

o

Hattiesburg

D

V

o

V

D

0 5 10 15 20 MILES

I l ill I 111 I I I I I I I I I I I I

0 5 10 15 20 KILOMETERS

Figure 12b. Pumping centers in the study area.

19
25

20

K H (XO 15

10

O 

^spCU

' ||iill|::lil: Upper Catahoula aquifer 

Middle Catahoula aquifer 

Lower Catahoula aquifer

980 1985 1990

Figure 13. Pumpage from the upper, middle, and lower Catahoula aquifers 

in the Laurel-Hattiesburg area, Mississippi, 1960-90.
<, u u

150

100

50

H

H °

-4

< 200

H 

CO

> 150

PQ

EH 100

H

S

J 5 ° 

H

" 0

K

H 200 E-H

150

100

50

0

i

n

ji A, ,j

 " '/ vi. / >, f' >/' ; / ^-! V

-

- Well 67C034

Lower Catahoula aquifer

-

-

t*V(r ||nJ' '1 » I ! V >! ^ \ A ^ ^ ( ^^ "^^1 *' *

*J *? i ' i I / / ~\ r* f- i \ ' \ -\ 1 t ~ \

: i v v' ' | v y i' i' v | :

Well 67F020

Lower Catahoula aquifer

i , , , , i . , , , i , , , , i

 , , , , | i , i , | , i , , | i i i i | i i i i 

-

~~~~--~ A /

~~~~-^ <v\ ;\y\ '--. > ^~' \_ , '--/ ' ^ .---"" V

_

Well 67G055

Lower Catahoula aquifer

i , . , , i , , , , i i i , i i 

1970 1975 1980 1985 1990 19

Figure 14. Water levels for wells 67C034,67F020, and 67G055 near 

Laurel, Mississippi.

21
computed for well 67G055 because of a 

shorter and more recent period of record. 

Water levels in these wells were not measured 

during the 1960's and 1970's, when the water 

levels in the lower Catahoula declined more 

rapidly. The recovery of water levels in the 

lower Catahoula during the late 1980's and 

early 1990's resulted as the city of Laurel 

began actively managing pumpage, using 

water-level changes to decide the distribution 

of withdrawal between the two well fields.

Long-term water-level declines also 

occurred in the Hattiesburg area as groundwater withdrawals increased (fig. 15). The 

water level in well 35D008 in Hattiesburg, 

screened in the middle Catahoula aquifer, 

declined at an average rate of about 2 ft/yr 

from 1964 to 1992. The water level in well 

35B006, screened in the upper Catahoula aquifer, declined at a rate of less than 1 ft/yr from 

1955 to 1992. The average water-level decline 

in well 35D130 in Hattiesburg, also screened 

in the upper Catahoula aquifer, was about 

1 ft/yr from 1946 to 1981.

Water levels in areas outside of the major 

pumping centers near Laurel and Hattiesburg 

also have declined but at smaller rates (fig. 16) 

than water levels near the pumping centers. At 

well 35L079 in southern Forrest County and 

screened in the upper Catahoula aquifer, the 

water level declined at an average rate of 0.7 

ft/yr between 1970 and 1992. Water levels in 

well 73L063, located in Lamar County and 

screened in the middle Catahoula aquifer, 

declined at a rate of 1.2 ft/yr between 1975 and 

1992, although most of this decline took place 

before 1980. In well 111M011, located in 

Perry County and also screened in the middle 

Catahoula aquifer, water levels declined 0.6 

ft/yr between 1979 and 1993.

ANALYSIS OF THE GROUND-WATER 

FLOW SYSTEM

A three-dimensional model was used to 

quantitatively analyze the ground-water

flow in the Catahoula aquifer system in the 

study area. The McDonald and Harbaugh 

(1988) modular finite-difference model 

(MODFLOW) was used to simulate flow in the 

Catahoula aquifer system and solve the governing equation:

where

V is del, the vector differential operator

K is hydraulic conductivity,

Q is the source/sink term,

h is hydraulic head,

Ss is specific storage, and

t is time.

Description of Model

The use of a finite-difference model 

requires the discretization of the aquifer system into cells. Each cell is specified by layer, 

row, and column. The size of these cells in the 

model for this study was based on the distribution of estimated pumpage during 1960-90 and 

on the distribution of water-level and budget 

data used to calibrate the model. The active 

cells ranged in size from 0.25 to 114 mi2. Variably spaced, small cells were used in the Laurel and Hattiesburg areas (fig. 17), where 

water-level data are relatively numerous and 

pumping rates have changed greatly over time. 

The largest cells were in areas of little or no 

pumping and outside of the study area. The 

model grid covered an area greater than 13,500 

mi2 and was divided into 56 rows of 48 

columns.

The grid was oriented parallel to the outcrop area throughout most of the model 

because the outcrop area was better defined 

than the other lateral boundaries. No measurements of anisotropy were available and a lateral anisotropy ratio of 1 to 1 was used for 

simulation. Values of aquifer and confining 

unit hydraulic properties were assigned to the 

center of each cell, defined as a node, by

22
m

o 

PQ

I

200

150

100

50

0

200

150

100

50

Well 35B006

Upper Catahoula aquifer

200

150 -

100

50

Well 35D008

Middle Catahoula aquifer

Well 35D130

Upper Catahoula aquifer

_i__i i i l i i i 

1970 1975 1980 1985 1990 1995

Figure 15. Water levels for wells 35B006,35D008, and 35D130 near 

Hattiesburg, Mississippi.

23
200

150

100

50

Well 35L079

Upper Catahoula aquifer

^ 0 I , , , , I , , , ,__! , , , , I , , , , 1_

, 200 

CO

o 15 ° |-"- 

m

H 100

P-H

I IJ 50

H L Well73L063

Middle Catahoula aquifer

0 I . i . . I i . . . I , , i i L

K

H 200 i , , , , i , , , , i , , , , i , , , , r

I

150

100

50

Well 111M011

Middle Catahoula aquifer

i . . . i

1970 1975 1980 1985 1990 1995

Figure 16. Water levels for wells 35L079, 73L063, and 111M011, generally 

unaffected by major pumping centers in the modeled area.

24
90

32

pproximate extent of modeled are 1

20 40 MILES

I ' ' ' ' I ' ' ' '\

0 20 40 KILOMETERS

Figure 17a. Model grid and extent.

25
31°45'

Approximate extent 

of modeled area

) £ 

.1. ,i . i. .1. i , i i 1 i

j 10 15 2

! i i ,!,,,, 1 : i i ,

i | i i i i | i i i i |

20 MILES

Figure 17b. Model grid in study area.

26
averaging observed or estimated point values 

within the cell.

The simulation approach was based on the 

assumptions that (1) flow is primarily lateral 

within the aquifers, (2) flow through the confining units is primarily vertical, and (3) the 

amount of water released from storage from 

the confining units is negligible compared to 

the total lateral flow within the aquifers. The 

assumptions of flow directions in the aquifers 

and confining units are reasonable because the 

hydraulic conductivity of sand composing the 

aquifers is much greater than that of clay composing the confining units. The model was vertically discretized into four layers, three of 

which were used to simulate the Catahoula 

aquifer system. Layer 1 represented the surficial aquifer and was the specified-head upper 

boundary for the model (fig. 18). Layers 2, 3, 

and 4 represented the upper, middle, and lower 

Catahoula aquifers, respectively.

Vertical flow between aquifers was simulated with leakance values assigned at each 

cell between model layers. The leakance is the 

vertical hydraulic conductivity of the confining 

unit divided by its thickness in units of feet per 

day per foot. Throughout most of the model 

area the leakances used represent the resistance to flow across individual confining units. 

The zone parallel to the outcrop from Hattiesburg to the south and between layers 1 and 2 is 

an exception. The leakances in this zone represent the combined vertical resistance to flow 

across all confining units and aquifers between 

the surficial aquifer and the upper Catahoula 

aquifer.

Boundary Conditions

Proper representation of model boundary 

conditions is one of the most important aspects 

in the simulation of an aquifer system. Model 

boundaries are assigned to represent the actual 

hydrologic boundaries as accurately as possible. Where model boundaries are necessarily 

highly generalized, they are placed far enough 

away from the influence of hydrologic stresses 

in the model area to minimize their effects on 

simulation results.

The upper boundary model, layer 1, is a 

source or sink for the Catahoula aquifer system. A specified-head was used to define the 

upper boundary and represents the long-term, 

average water-table altitude throughout the 

model area. There has been no significant 

long-term decline or rise in the water table in 

the outcrop area for the Catahoula aquifer system or elsewhere in the model area (W.T. Oakley, U.S. Geological Survey, oral commun., 

1993).

Estimates of potential recharge to the Catahoula aquifer system were determined from a 

climatic budget. Assuming that the spatial 

variation in evapotranspiration corresponds 

with the spatial variation in rainfall, a contour 

map of rainfall available for recharge was 

derived by adjusting contoured values of average annual rainfall (not shown, Lamonds and 

Boswell, 1986, p. 297) by:

Fraction of = Total rainfall - Total evapotranspiration 

rainfall remaining Total rainfall

For southern Mississippi, with an average 

rainfall of 56 in/yr and evapotranspiration of 

about 35 in/yr (Callahan and Barber, 1990, p. 

321), the fraction of rainfall remaining after 

evapotranspiration is:

56 in/yr - 35 in/yr aQ 'f7r~- / ~ U.Jo,

56 m/yr

The maximum potential recharge map 

(fig. 19) was made by subtracting spatiallydistributed values of surface-water runoff 

(Lamonds and Boswell, 1986, p. 297) from the 

spatially corresponding values of available 

rainfall. Thus, the potential recharge available

27
32 L

Approximate extent of modeled area 1 "4

20

Modified from Sumner and others, 1989, fig. 6 

40 MILES

I I I I I I ' ' ' ' I

0 20 40 KILOMETERS

EXPLANATION

WATER-TABLE CONTOUR--Shows altitude of 

water table. Contour interval 100 feet. 

Datum is sea level

Figure 18. Altitude of water table (layer 1) in the modeled area.

28
32

31'

Approximate extent of modeled area

20 40 MILES

1 i ' ' ' n

20 40 KILOMETERS

EXPLANATION

LINE OF EQUAL POTENTIAL RECHARGEInterval 1 inch per year. 

Negative value indicates discharge

Figure 19. Maximum potential recharge available in the modeled area.

29
to the Catahoula aquifer system within the outcrop area ranges from 3 to 6 in/yr. The potential recharge available in the outcrop area 

(3,300 mi2) is approximately 700 Mgal/d, 

which is considerably greater than the maximum historical ground-water withdrawal of 

41 Mgal/d from the Catahoula aquifer system.

The lower model boundary was considered a no-flow boundary and corresponds to 

the top of clays previously assigned to the 

Paynes Hammock Formation, the Chickasawhay Limestone, and the Vicksburg Group. 

Results of the Gulf Coast Regional AquiferSystem Analysis (GC RASA) indicated negligible flow across the confining layer which 

underlies the lower Catahoula aquifer (Martin 

and Whiteman, 1989).

The model area is much larger than the 

study area to satisfy boundary-condition 

requirements. All the lateral model boundaries 

in each layer are no-flow boundaries (fig. 20). 

The northern edge of the model was the updip 

extent of the lower Catahoula aquifer (layer 4). 

Layer 2 extends to the northern boundary 

although the upper Catahoula aquifer pinches 

out south of the northern edge of the model. 

Extending layer 2 provides continuity between 

layer 1, the upper specified-head boundary, 

and lower layers. The southern boundary 

approximated the downdip limit of the freshwater system (Sumner and others, 1989, p. 42).

Lateral movement of water across this boundary was considered negligible. The location of 

the freshwater/saltwater interface also was 

considered to be stable over time. The eastern 

no-flow boundary is along the valley of the 

Tombigbee River, which lies above a deep graben feature. The Tombigbee River is a groundwater divide because the river is a regional 

drain for all the Catahoula aquifer system 

(Martin and Whiteman, 1989). Water moves 

upward from deeper to shallower aquifers 

along this boundary and is discharged to the 

specified head boundary (layer 1) which incorporates the river stage. The western boundary 

corresponds to a wide zone of parallel flow in 

all layers (Martin and Whiteman, 1989, 

fig. 33). This divide constitutes a no-flow 

boundary because water flows parallel to but 

not across the boundary. Previous development 

has not affected ground-water flow in the 

vicinity of this divide and the effects of further 

development are expected to be negligible.

Hydraulic Properties

Initial estimates of values for lateral 

hydraulic conductivity and storage coefficient 

were obtained from aquifer tests performed 

between 1942 and 1984 (Slack and Darden, 

1991). The largest number of tests (table 2) 

were performed in the lower Catahoula aquifer

Table 2. Ranges of reported lateral hydraulic conductivity and storage coefficient for the Catahoula aquifer system 

in southeastern Mississippi

Lateral hydraulic conductivity, In feet per day Storage coefficient

Minimum

Average

Maximum

Standard 

deviation

Upper Catahoula 

aquifer

4

94

180

54

Middle Catahoula 

aquifer

18

52

80

20

Lower Catahoula 

aquifer

12

76

190

44

Lower Catahoula 

aquifer

1 xlO'4

3 xlO'4

5 xlO'4

1 xlO'4

Number of tests 12 53 18

30
32 l

31" _

20 40 MILES

20 40 KILOMETERS

EXPLANATION

NORTHERN BOUNDARY--Pinchout of aquifers 

EASTERN BOUNDARY--A ground-water divide

SOUTHERN BOUNDARY Dissolved-solids 

concentration of 10,000 milligrams 

per liter or greater

WESTERN BOUNDARY A ground-water divide

Figure 20. Conceptual model for the Catahoula aquifer system.

31
near Laurel and Hattiesburg. Only a few tests 

were performed in the upper and middle Catahoula aquifers near Hattiesburg. The initial 

transmissivity arrays input to the model were 

calculated by multiplying the average lateral 

hydraulic conductivity for each layer by the 

corresponding sand thickness for that layer.

No aquifer tests have been performed in 

the study area that permit calculation of the 

vertical conductance or the vertical hydraulic 

conductivity of the confining units. Bear 

(1979, p. 68) reports that the hydraulic conductivity of clay ranges from 3xlO~8 to SxlO" 1 

ft/d. Vertical hydraulic conductivity of clay 

samples from similar confining units in southeastern Louisiana are about IxlO"5 ft/d 

(Whiteman, 1980, p. 18). Because the confining units in the study area contain considerable 

silt and sand, the average vertical hydraulic 

conductivity was considered to be about 

IxlO"4 ft/d. The initial leakance arrays input to 

the model were calculated dividing the average 

vertical hydraulic conductivity by the confining unit thickness between each layer.

Model Calibration

Calibration is the attempt to reduce the 

difference between model results and measured data by adjusting model input. Calibration is generally accomplished by adjusting 

input values of transmissivity and leakance 

until an acceptable calibration criterion is 

achieved. The difference between simulated 

and measured water levels was the acceptable 

match for this model. Calculated water levels 

from a calibrated, deterministic ground-water 

model usually depart from measured water 

levels by a considerable amount, even after 

diligent effort has been made to have model 

input closely approximate field observations. 

The discrepancy between simulation results 

and measured water levels (model error) usually is caused by grid scale and the difficulty of

obtaining sufficient measurements to account 

for all of the spatial variation in hydraulic 

properties throughout the model area.

The initial arrays of transmissivity and 

leakance input to the model were adjusted to 

calibrate the model. Six parameters were used 

as global multipliers, three for each confining 

unit leakance array between each aquifer and 

three for each transmissivity array of the Catahoula aquifers (layers 2,3, and 4). Global multipliers change the value of either leakance or 

transmissivity by a fixed amount throughout 

the model. Two additional parameters used 

during calibration were weight-matrix multipliers, which change an array value by varying 

amounts throughout the model. The weight 

matrices were based on the lateral hydraulic 

conductivity distributions from aquifer tests 

within the upper and lower Catahoula aquifers 

(layers 2 and 4) respectively. No weight matrix 

was generated for layer 3 because the available 

data for the middle Catahoula aquifer were 

from wells located exclusively in Hattiesburg. 

The weight matrices define hydraulic conductivity variations over the model area within 

individual aquifers. Weight-matrix multipliers 

are used to modify the matrices because the 

best estimates of the difference in hydraulic 

conductivity from one location to another are 

not the same as the initial estimate of the 

hydraulic conductivity distribution. The combined effect of the global and weight matrix 

multipliers is shown in figure 21. Additional 

information on the use of weight-matrix multipliers may be found in Halford (1992). A uniform storage coefficient of 3xlO~4 (the average 

value from available aquifer tests of the lower 

Catahoula) was assigned to each layer and not 

adjusted to calibrate the model. The use of a 

uniform storage coefficient was based on the 

insensitivity of models with source/sink upper 

boundaries to changes in storage coefficient 

and not on the certainty with which this value 

was known.

32
~~l ' Effect of "~~ 

weight matrix multiplier Effect of

Global

Multiplier

Initial parameter estimate 

i i i i i i

DISTANCE

Figure 21. Effect of parameter modifiers as applied to a hypothetical section of 

lateral hydraulic conductivity.

Calibration improvement was determined 

by decreases in root-mean-square error 

(RMSE) which is defined by:

RMSE=»

N

i = I

N

where

*m

is the simulated water level in feet,

is the measured water level in feet,

and 

N is the total number of water-level

comparisons.

As measured water levels rarely coincide 

with node locations, simulated water levels 

were interpolated laterally to points of measurement from surrounding nodes. Simulated 

water levels were interpolated because they 

were assumed to be part of a continuous distribution representing the aquifer. Vertical interpolation usually is inappropriate because 

model layers correspond to confined aquifers 

and the water levels are considered discontinuous across layers.

Model calibration also was facilitated by a 

parameter estimation program (Halford, 1992). 

The parameter estimation process began by 

using the model to establish the initial differences between simulated and measured water 

levels. These differences, or residuals, were 

minimized by the parameter estimation program. The sensitivity coefficients, the derivatives of simulated water-level change with 

respect to parameter change, were calculated 

by the influence coefficient method (Yeh, 

1986) using the initial model results. This 

method required changing each parameter a 

small amount and using MODFLOW to compute new water levels. A quasi-Newton procedure (Gill and others, 1981, p. 137) was used 

to compute new values of the parameters that 

should improve the model. The model was 

updated to reflect the latest parameter estimates and a new set of residuals was calculated. The entire process of changing a 

parameter in the model, calculating new residuals, and computing a new value for the 

parameter, was continued iteratively until 

model error or model-error change was 

reduced to a specified level or until a specified 

number of iterations were made.

33
Steady-State Simulation

A steady-state version of the model was 

used for preliminary parameter estimation. 

Because only one set of water levels 

described the steady-state flow system and 

the model solved for them, the initial water 

levels in layers 2, 3, and 4 did not need to be 

representative of the water levels being simulated. The period from 1980 to 1981 was 

used for steady-state calibration because a 

comprehensive water-level survey was completed in 1981 and the system was close to 

equilibrium conditions. Hydrographs indicate that during this period water levels, on 

average, were neither rising or declining 

(figs. 14-16). Pumpage data for the steadystate simulation were based on water-use 

data collected for 1980.

After calibration, pumpage was 

removed from the steady-state model to simulate predevelopment conditions. The resulting potentiometric surfaces for the upper, 

middle, and lower Catahoula aquifers 

(fig. 22) indicate ground water flows from 

the outcrop areas toward the gulf coast. In 

the Laurel and Hattiesburg areas, the simulated predevelopment surfaces agree with the 

ground-water flow pattern from the gulf 

coast regional aquifer-system analysis by 

Martin and Whiteman (1989, fig. 49). The 

simulated potentiometric surfaces provided 

the initial water levels for the transient-state 

simulations.

The volumetric flow budget for the Catahoula aquifer system prior to development 

is shown schematically in figure 23. The left 

side of the diagram represents the part of the 

aquifer system in the outcrop area of the Catahoula aquifer system, and the right side represents the downdip area of the aquifer 

system. The arrows drawn between the various layers show the flow across the boundary 

between those layers, and the arrows drawn 

between the outcrop area and the downdip 

area show the amount of water moving 

through each layer. The amounts of water

that move in and out of a layer within the outcrop or downdip area represent the shallow 

flow system, whereas the water in the three 

deeper flow systems can be inferred from the 

net flow amounts.

The predevelopment flow rate through the 

Catahoula aquifer system was about 29 

Mgal/d, of which about 12 Mgal/d entered the 

middle Catahoula and nearly 8 Mgal/d entered 

the upper Catahoula aquifer in the outcrop area 

(fig. 23). The remaining 9 Mgal/d entered the 

aquifer system in the downdip area. The inflow 

of 20 Mgal/d translates into an effective 

recharge rate of 0.13 in/yr over a 3,300 mi2 

area, compared to the potential recharge of 3 to 

6 in/yr in the outcrop area. The large differences are the result of a substantial and relatively immediate discharge of infiltrated 

rainfall to nearby streams in the outcrop area 

that is not simulated by this model. The net 

flow rate downgradient from the outcrop area 

was about 10 Mgal/d.

Transient Simulation

The response of the Catahoula aquifer system to ground-water pumping was simulated 

from 1941 to 1992 using eight stress periods: 

1941-57,1958-62,1963-67,1967-72, 

1973-77, 1978-82, 1983-87, and 1988-92. 

Five-year stress periods were used from 1958 

to 1992 because the most complete pumpage 

data were collected for the years 1960,1965, 

1970, 1975, 1980, 1985, and 1990. Pumpage 

during the first stress period, 1941-57, was 

approximated as half the 1960 rates. Pumpage 

was distributed spatially by assigning the locations shown in figure 11 to whatever model 

cell surrounded it.

Simulated water levels also were interpolated to the time a water level was measured. 

To avoid undue bias toward wells with a large 

number of water-level measurements, only one 

average measured water level from each of the 

simulation periods of 1941-57, 1958-62, 

1963-67, 1968-72, 1973-77, 1978-82,

34
ISI

32 L

90 32"

Upper Catahoula aquifer (Layer 2)

9(T

89" 88 C

I C, 1 1 /

ximate extent of mode

Middle Catahoula aquifer (Layer 3)

0 20 40 60 

" I " " I " ' 1111111 

0 20 40 60 80 KILOM

EXPLANA

POTENTIOMETRIC C

at which water l

in tightly cased 

50 feet. Datum is 

Lower Catahoula aquifer (Layer 4)

Figure 22. Simulated predevelopment potentiometric surfaces of the upper, 

middle, and lower Catahoula aquifers in the modeled area.
EXPLANATI

 - DOWNGRADIENT LIMIT 

 > FLOW ACROSS A BOUND

 > NET FLOW ACROSS A BO

9.75 FLOW RATE, IN MILLION 

NORTH

4.31

WATER TABLE

5.97

7.85 1.88

I

 2.43

 1.67

0.76"

2.39

S

.75 18.95

9.20

UPPER CATAHOULA AQU

3.08 13.28

13.18 7.87 2.60 0.21

1.12

 9.51

 7.46

2.05- MIDDLE CATAHOULA AQU

0.62 1.67

1.74 0.62

 0.79

 0.62

0.17-

1.05

LOWER CATAHOULA AQU

OUTCROP DOWNDIP

Figure 23. Simulated predevelopment volumetric flow budget for the 

Catahoula aquifer system in the modeled area.
1983-87, and 1988-92 was used for comparison. For example, for a well having 200 waterlevel measurements made uniformly across 

four periods, only four comparisons between 

simulated and measured water levels were 

made. Each measured water level used for 

comparison was an average of 50 measurements from each period.

The error statistics for the calibrated 

model over all stress periods are:

RMSE

Average

Maximum

Minimum

16.92 ft

1.84ft

38.57 ft

-35.43 ft

Standard deviation 16.92 ft.

Measured water levels were obtained from historical records and from water-level surveys 

conducted during 1981 and 1991. The greater 

number of water-level measurements available 

after 1980 biased model calibration toward the 

last three stress periods (1978-92). The simulated potentiometric surfaces of the Catahoula 

aquifer system for 1992 agreed reasonably 

well with measured water levels (fig. 24). The 

cones of depression in the upper and middle 

Catahoula aquifers near Hattiesburg, and the 

cones in the lower Catahoula near Laurel and 

near Hattiesburg, are reflected in the simulated 

potentiometric surfaces. Residuals from the 

last stress period (1988-92) generally show the 

smallest differences between simulated and 

measured values in the Laurel and Hattiesburg 

areas. Simulated and measured water levels 

from the calibrated model are listed in table 3. 

The period of record for the hydrographs 

used to compare measured and simulated 

water levels was less than the total simulation 

period. Simulated water levels agreed well 

with hydrograph records (figs. 25-27) but generally tended to be higher. The average model 

error was 2 ft based on 85 observations. 

Agreement between simulated and measured 

water levels was best for wells 67F020 

(fig. 25) and 35D130 (fig. 26). The worst

match between simulated and measured water 

levels occurred at well 35L079 (fig. 27), where 

the simulated water levels were consistently 

higher than the measured water levels. However, even for this well the trend in simulated 

water levels was in close agreement with the 

trend in measured water levels.

Simulated budget results at the end of 

stress period 8 (1988-92) are shown in 

figure 28. Approximately 55 Mgal/d of water 

entered the Catahoula aquifer system during 

this period, and about 1.9 Mgal/d was released 

from storage. This rate of inflow was about 25 

Mgal/d more than during predevelopment and 

some predevelopment-flow directions were 

altered. About 37 Mgal/d, or 67 percent of all 

inflow to the aquifers, occurred as recharge in 

the outcrop area under 1992 conditions. Most 

of the flow leaving the Catahoula aquifer was 

by pumpage (38 Mgal/d or 69 percent). The 

remaining discharge (17 Mgal/d or 31 percent) 

occurred as leakage to overlying aquifers. The 

rate of leakage to overlying aquifers during 

1992 was 42 percent less than the corresponding predevelopment rate. Net flow in the 

downdip area, which had been upward to overlying model layers, was reversed under 1992 

conditions.

Lateral flow directions also were altered 

from predevelopment to 1992 conditions. 

Flow in the Catahoula aquifer system converged from all directions toward pumping 

centers in Laurel and Hattiesburg (fig. 24) during 1992. Water levels in the lower Catahoula 

aquifer, the most heavily pumped aquifer, were 

as much as 130 ft lower in Laurel and 100 ft 

lower in Hattiesburg (fig. 29) than predevelopment levels. Water-level declines were less 

than 30 ft in all aquifers south of Forrest 

County and east of Jones County.

Calibrated leakance values for all confining unit arrays between and above the Catahoula aquifer system were highest toward the 

outcrop of the modeled area and generally 

decreased from north to south (figs. 30-32). 

Leakance values ranged from greater than

37
32"

90

89"

31"

Upper Catahoula aquifer (Layer 2)

32 C

90

89"

31'

Lower Catahoula aquifer (Layer 4)

32"

90

31"

Middle Catahoula aquifer (Layer 3)

0 20 40 60 8

0 20 40 60 80 KILOM

EXPLANA

POTENTIOMETRIC C

at which water le

in tightly cased 

indicate depress

25 feet. Datum is 

RESIDUAL--Differ

simulated and m

levels, in feet

Figure 24. Simulated potentiometric surfaces of the upper, middle, 

and lower Catahoula aquifers in the modeled area in 1992.
Table 3. Simulated and measured water levels used to calibrate the transient model

Layer

2

2

2

4

4

2

4

4

4

2

4

4

2

2

2

3

3

3

4

4

4

4

2

2

3

3

3

3

3

3

4

4

4

4

4

4

4

4

4

4

Well Simulated Measured

Stress period 1

35D130 163.69

35D130 160.87

35D130 160.70

67C034 166.79

67G104 160.89

Stress period 2

35D130 142.04

67G104 107.33

Stress period 3

153N001 175.09

67G104 105.72

Stress period 4

35L079 138.30

67G004 89.30

67G004 87.17

Stress period 5

91G006 159.30

67J037 161.22

35L079 135.38

41D001 138.33

73L063 132.44

111M011 136.32

67D072 183.13

67G004 68.81

67F020 69.59

67G055 113.11

Stress period 6

35L079 131.99

35B115 142.55

41A021 154.84

41U025 120.68

73L063 128.58

35B002 77.04

111M011 131.51

111Q024 121.64

35D008 86.63

67J061 157.85

67N012 156.43

111H009 136.46

67F020 63.86

35B119 142.11

35D008 80.53

67G055 110.27

67L051 148.72

111C040 139.08

(1941-57)

156.60

157.32

152.76

149.00

149.00

(1958-62)

146.23

128.01

(1963-67)

157.85

122.18

(1968-72)

122.29

92.32

76.72

(1973-77)

155.00

167.70

119.24

159.54

127.00

125.90

187.04

88.35

80.65

99.16

(1978-82)

114.49

138.58

165.50

96.00

118.37

84.40

119.95

98.33

67.85

166.30

169.50

115.20

74.26

160.88

62.03

99.25

135.40

142.08

Residual

7.09

3.55

7.94

17.79

11.89

-4.19

-20.68

17.24

-16.46

16.01

-3.03

10.45

4.30

-6.48

16.14

-21.21

5.44

10.42

-3.91

-19.54

-11.06

13.95

17.50

3.97

-10.66

24.68

10.21

-7.37

11.56

23.31

18.78

-8.45

-13.08

21.26

-10.40

-18.77

18.50

11.02

13.32

-3.00

Layer

2

2

3

3

3

3

3

3

4

4

4

4

4

4

4

4

4

4

4

4

4

4

4

4

4

4

4

4

4

4

4

2

2

2

3

3

3

3

3

4

4

4

4

4

4

Well

35B006

35B006

35E214

41L019

111R002

73L063

41P039

111M011

35B071

35B102

35C064

67G001

67B079

67A003

67H035

67J050

35D008

67C031

67C034

67C067

67C096

67C153

67F020

67F044

67F059

67F060

67F061

67F062

67F063

35D107

67G055

35L079

35B006

35L079

35D108

35D108

41P039

73L063

111M011

73E214

35D008

67C034

67F020

67G055

73E214

Simulated

Stress period 7

115.29

111.37

119.63

133.61

118.58

107.63

124.13

125.95

128.90

136.33

123.00

113.89

163.14

194.37

171.02

158.56

86.00

90.55

90.11

127.04

99.74

71.05

58.72

39.81

68.03

81.16

48.49

70.24

79.69

108.59

86.18

Stress period 8

124.88

115.25

124.70

86.41

86.64

123.26

111.84

125.75

111.51

87.55

93.61

81.28

86.09

112.12

Measured

(1983-87)

126.36

127.23

116.72

117.00

81.30

111.00

94.15

117.67

154.75

155.65

145.81

119.00

156.94

218.10

197.43

172.00

63.17

81.85

82.95

88.47

86.02

83.75

64.16

74.38

76.76

61.25

57.99

73.99

60.10

121.00

77.52

(1988-92)

109.22

127.00

109.64

121.84

120.42

88.60

110.18

113.50

119.25

57.71

78.85

71.54

92.69

118.25

Residual

-11.08

-15.86

2.91

16.61

37.28

-3.37

29.98

8.28

-25.85

-19.32

-22.81

-5.12

6.20

-23.73

-26.41

-13.44

22.83

8.70

7.16

38.57

13.72

-12.70

-5.44

-34.57

-8.73

19.91

-9.50

-3.75

19.59

-12.41

8.66

15.66

-11.75

15.06

-35.43

-33.78

34.66

1.66

12.25

-7.74

29.84

14.76

9.74

-6.60

-6.13

39
250

200 -

150

100

_J 50

W SEA 

t-J LEVEL

CO

-50

25O

Well 67C034

Lower Catahoula aquifer

200 -

PQ

O i 5 o

o 100

50

SEA 

LEVEL Well 67F020

Lower Catahoula aquifer

SEA 

LEVEL

-50

MEASURED

CALIBRATED

LOW GROWTH SCENARIO

MODERATE GROWTH SCENARIO

HIGH GROWTH SCENARIO

Well 67G055

Lower Catahoula aquifer

i

1940 1960 1980 2000 2020

Figure 25. Measured and simulated water levels for wells 67C034,67F020, and 

67G055 near Laurel, Mississippi.

40
250

200

150

100

50

H SEA 

t-J LEVEL

H 

CO

K

o

-50 

250

200

150

100

50

SEA 

LEVEL

-50 

350

200

150

100

50

SEA

LEVEL

-50

Well 35B006

Upper Catahoula aquifer

"Well 35DOOB

Middle Catahoula aquifer

Well 35D130

Upper Catahoula aquifer

i

 MEASURED

 CALIBRATED

 LOW GROWTH SCENARIO

 MODERATE GROWTH SCENARIO 

- HIGH GROWTH SCENARIO

1940 1960 1980 2000 2020

Figure 26. Measured and simulated water levels for wells 35B006, 

35D008, and 35D130 near Hattiesburg, Mississippi.

41
250

200

150

100

50

SEA 

LEVEL

CO

o

PQ

K 

O

O 

PQ

<

E-

-50

250

200 -

150

100

50

SEA 

LEVEL

-50 

250

K 200 -

150

100

50

SEA 

LEVEL

-50

MEASURED 

CALIBRATED

- LOW GROWTH SCENARIO

- - - MODERATE GROWTH SCENARIO"

- - - - HIGH GROWTH SCENARIO

Well 35L079

Upper Catahoula aquifer

Well 73L063

Middle Catahoula aquifer

Well 111M011

Middle Catahoula aquifer

i

1940 1960 1980 2000 2020

Figure 27. Measured and simulated water levels for wells 35L079,73L063, and 

111MO11 generally unaffected by major pumping in the modeled area.

42
NORTH

1.48

21.90 20.42 

P=2.53

S=0.04

p

s

EXPLANATIO

DOWNGRADIENT LIMIT O

FLOW ACROSS A BOUNDA

NET FLOW ACROSS A BOU

PUMPAGE RATE FROM AQ

FLOW RATE FROM STORA

WATER TABLE

1.82

13.65 FLOW RATE, IN MILLION 

SO

13.65

15.10 13.38 

P=0.12

s=o.oo

I P=7.15 16.44 3.79

S=0.54

0.57 '

0.65

UPPER CATAHOULA AQUIF

9.49

7.38 6.73

 14.82

 13.40

P=15.67

12.10 3.61

S=0.60

1.43 MIDDLE CATAHOULA AQUIF

0.23 1.83

P=10.49

S=0.04

11.48 11.26

 1.03

 0.81

P=2.39

2.77 0.94

S=0.64

LOWER CATAHOULA AQUIF

OUTCROP DOWNDIP

Figure 28. Simulated volumetric flow budget for the Catahoula aquifer 

system in the modeled area in 1992.
33 C

90 C 89^ 88 1-

31 C

32

90

Upper Catahoula aquifer (Layer 2)

32

90 C 89 C 88 l

31 C

C, 1

pbroximate extent of mode 

NL *v I ^

Lower Catahoula aquifer (Layer 4)

Middle Catahoula aquifer (Layer 3)

0 20 40 60 

' ' L -!.._

0 20 40 60 80 KILOM

EXPLANATION

LINE OF EQUAL DR

Interval 25 feet

Figure 29. Simulated water-level declines from predevelopment to 1992 in the upper, 

middle, and lower Catahoulamaquifersmin the modeled area.
pproximale extent of modeled are

\

20 40 MILES

20 40 KILOMETERS

EXPLANATION

LINE OF EQUAL LEAKANCE-- 

Vertical hydraulic conductivity/confining 

unit thickness. Hachures indicate lesser 

leakance. Interval, in feet per day per 

foot xlO~ 6 . is variable

Figure 30. Calibrated leakance of confining units above the 

upper Catahoula aquifer in the modeled area.

45
90 C 89" 88

32*-

31*

Approximate extent o^modeled area

0

20 40 MILES

1 I ' ' ' ' I

20 40 KILOMETERS

EXPLANATION

LINE OF EQUAL LEAKANCE 

Vertical hydraulic conductivity/confining 

unit thickness. Hachures indicate lesser 

leakance Interval, in feet per day per 

foot xlO , is variable

Figure 31. Calibrated leakance of confining uints above the 

middle Catahoula aquifer in the modeled area.

46
32 C

31 C

Approximate extent of modeled area

0 20 40 MILES

[-r1 ! i 1 i '| ,', I i '| ' ' ' '

0 20 40 KILOMETERS

EXPLANATION

LINE OF EQUAL LEAKANCE-- 

Vertical hydraulic conductivity/confining 

unit thickness. Hachures indicate lesser 

leakance. Interval, in feet per day per 

foot xlO , is variable

Figure 32. Calibrated leakance of confining units above the 

lower Catahoula aquifer in the modeled area.

47
IxlO"5 1/d in Jones County to less than 5xlO~7 

1/d in southeast Forrest County.

Transmissivity estimates in the Catahoula 

aquifer system generally were lower than 

transmissivities determined from aquifer tests 

(figs. 33-35). The effective transmissivity represented by the calibrated model arrays generally is less than the average determined from 

aquifer tests because individual wells in the 

study area are characteristically screened in the 

most permeable sands. These sands generally 

are lensoidal and discontinuous and probably 

are not representative of the entire volume of 

clastic sediments within the area corresponding to the model cell, which characteristically 

include sands as well as clayey-sands and 

clays. Spatial variation in model-estimated and 

measured transmissivity values was similar. 

Transmissivity estimates from the calibrated 

model ranged from 300 ft2/d in the lower Catahoula aquifer (fig. 35) to 20,000 ft2/d in the 

middle Catahoula aquifer (fig. 34) in Jones 

County.

Sensitivity Analysis

To determine how transmissivity, leakance, and storage coefficient affected simulation results, each parameter was varied, 

independently, over a range of four orders of 

magnitude. This range was greater than the 

uncertainties associated with these parameters 

but gave a more complete perspective on 

model sensitivity. Model sensitivity was 

described in terms of RMSE. The model was 

determined to be more sensitive to changes in 

transmissivity than to changes in leakance 

(fig. 36), and more sensitive to decreases in 

transmissivity and leakance than to increases. 

Figure 36 shows the sensitivity of the model to 

changing one parameter while all others are 

held at their calibrated values. Like other models simulating confined aquifer systems with a 

specified-head source/sink boundary, this 

model was quite insensitive to changes in storage coefficient. Boundary conditions and 

pumpage were not adjusted during calibration 

and sensitivity analysis was not performed for 

these parameters.

Two parameters, transmissivity and leakance, were varied simultaneously, and the sensitivity of the model to these changes is shown 

as a RMSE map in figure 37. The sensitivity of 

the model to changes in transmissivity and leakance shown in figure 36 are cross sections 

through the RMSE map. The model was more 

sensitive to decreases in transmissivity than to 

increases in transmissivity even when leakance 

values were something other than the calibrated value. This same sensitivity pattern 

existed for leakance even when transmissivity 

values were something other than the calibrated values (fig. 37). In all cases the lowest 

RMSE was associated with the calibrated 

model.

Limitations of model application

The model was developed using regional 

characteristics of the aquifers to determine the 

effects of increased pumpage on the groundwater conditions of the Laurel and Hattiesburg 

areas. The water level calculated by the model 

is an approximation of the average water level 

for the area included in the model grid cell, but 

may not be a reliable approximation of the 

water level in a well or a wellfield.

Most pumping from the Catahoula aquifer 

system is from the confined aquifer system, 

therefore the model was developed primarily 

to simulate ground-water flow and evaluate the 

effects of withdrawals on water levels within 

the confined Catahoula aquifer system. The 

model does not provide an analysis of flow in 

the parts of the Catahoula aquifers which are 

under unconfined conditions. Although the 

model can be used to identify conditions and 

areas where water levels might be lowered to 

the top of one of the aquifers, the model was 

not designed to simulate the change from
20 40 MILES

1 I ' ' ' ' I 

20 40 KILOMETERS

EXPLANATION

LINE OF EQUAL TRANSMISSIVITY--Hachures 

indicate lesser transmissivity. Interval 

variable, in 1,000 feet squared per day

TRANSMISSIVITY VALUE DETERMINED FROM 

AQUIFER TEST--In 1,000 feet squared 

per day

Figure 33. Calibrated transmissivity of upper Catahoula aquifer in the modeled area.

49
Approximate extent of modeled area

i ~\ r'") 

I -\J\-V20 40 MILES

i I 1 ' ' I ' ' ' ' ! 

0 20 40 KILOMETERS

EXPLANATION

LINE OF EQUAL TRANSMISSIVITY--Hachures 

indicate lesser transmissivity. Interval 

variable, in 1,000 feet squared per day

TRANSMISSIVITY VALUE DETERMINED FROM 

AQUIFER TEST--In 1,000 feet squared 

per day

Figure 34. Calibrated transmissivity of middle Catahoula aquifer in the modeled area.

50
33 C

31 C

20 40 MILES

20 40 KILOMETERS

EXPLANATION

LINE OF EQUAL TRANSMISSIVITY--Hachures 

indicate lesser transmissivity. Interval 

variable, in 1,000 feet squared per day

TRANSMISSIVITY VALUE DETERMINED FROM 

AQUIFER TEST--In 1,000 feet squared 

per day

Figure 35. Calibrated transmissivity of lower Catahoula aquifer in the modeled area.

51
1,500

1,000 -

03

o

03

03

03

oCO

500 -

O 

O

03

- - - - LE

_ _ __ ST

o.oi o.i

MULTIPLIER OF CALIBRATED VALUE

Figure 36. Model sensitivity to independent changes in transmissivity, leakance, and 
100

0.01

0.1 -

0.01 0.1 1 10

MULTIPLIER OF CALIBRATED TRANSMISSIVITY VALUE

too

LINE OF EQUAL ERROR Shows 

root mean-squared error of model. 

Interval variable, in feet

Figure 37. Model sensitivity to simultaneous changes in transmissivity and leakance.

53
confined to unconfined conditions. If future 

pumping increased to the extent that the water 

level did drop below the top of an aquifer, the 

model would require modification to simulate 

the new conditions.

The western and eastern boundaries of the 

model are ground-water divides and were 

assumed to be no-flow boundaries. However, 

extensive pumping close to one of these 

boundaries could shift the location of the 

divide and would require modification of the 

model to simulate the new boundary. No such 

development of the Catahoula aquifers or their 

lithostratigraphic equivalents is anticipated.

SIMULATED AQUIFER RESPONSE TO 

PUMPING SCENARIOS

Three scenarios were simulated to estimate the response of the Catahoula aquifer 

system to pumping changes near Laurel and 

Hattiesburg (table 4). The scenarios were:

1. The 1992 pumpage in Laurel was 

increased 2 percent per year and 

1992 pumpage in Hattiesburg was 

increased 1.2 percent per year from

1992 to 2020, for a net increase of 

6.40 Mgal/d (low-growth 

scenario).

2. The 1992 pumpage in Laurel was 

increased 3.5 percent per year and 

1992 pumpage in Hattiesburg was 

increased 2 percent per year from 

1992 to 2020, for a net increase of 

10.95 Mgal/d (moderate-growth 

scenario).

3. The 1992 pumpage in Laurel was 

increased 4 percent per year and 

1992 pumpage in Hattiesburg was 

increased 4 percent per year from 

1992 to 2020, for a net increase of 

17.07 Mgal/d (high-growth 

scenario).

Under the low-growth scenario, approximately 5 Mgal/d of additional water would 

enter the Catahoula aquifer system compared 

to 1992 conditions, with about 1.0 Mgal/d of 

this total released from storage (fig. 38). This 

quantity represents a 9 percent increase in volumetric flow over 1992 conditions. Net flow 

directions would be little changed compared to 

1992. About 41 Mgal/d, or 69 percent of all 

inflow to the system, would enter in the outTable 4. Summary of alternate pumping scenarios with total pumpage in 2020 

[Mgal/d, million gallons per day; --, not applicable]

Laurel area

Base conditions 

(1992)

Low-growth 

scenario

Moderategrowth scenario

High-growth 

scenario

Percent

growth

2.0

3.5

4.0

Pumpage

(Mgal/d)

5.74

8.95

11.37

12.17

Hattiesburg area

Percent

growth

1.2

2.0

4.0

Pumpage

(Mgal/d)

9.50

12.69

14.82

20.14

Study area total

Percent

growth

0.7

1.1

1.8

Pumpage

(Mgal/d)

34.56

40.97

45.51

51.63

54
NORTH

1.34

A

84.64 83.30 

P=2.53

3=0.04

WATER TABLE

1.52

EXPLANATIO

 - DOWNGRADIENT LIMIT O

 > FLOW ACROSS A BOUNDA

 ^ NET FLOW ACROSS A BO

P PUMPAGE RATE FROM AQ

S FLOW RATE FROM STORA

12.54 FLOW RATE, IN MILLION 

SO

12.54

16.65 15.13 

P=0.12

S=0.02

I

 7.64

 7.08

p_7 57 17.86 5.32

3 = 0.22

0.56- UPPER CATAHOULA AQUI

0.58 8.67

.53 7.95

 15.73

 14.32

P=18.44

13.77 5.10

S=0.30

MIDDLE CATAHOULA AQUIF

1.65

P=13.70

3=0.04

14.63 14.44

 1.04

 0.78

P=2.39

S=0.33

8.93 1.88

0.86- LOWER CATAHOULA AQUIF

OUTCROP DOWNDIP

Figure 38. Simulated volumetric flow budget for the Catahoula aquifer system in 

2020 at the end of the low-growth scenario.
crop area under this scenario. Most of the flow 

leaving the Catahoula aquifer system would be 

pumpage (45 Mgal/d or 74 percent). Vertical 

leakage to overlying aquifers would be 

15 Mgal/d, which represents a small decrease 

compared to 1992 conditions.

Simulated changes in water levels from 

1992 to 2020 for the low-growth scenario are 

presented in figure 39. Water-level declines 

would be 20 ft or less in the Hattiesburg area 

for the middle and upper Catahoula aquifers. 

Water-level declines in the lower Catahoula 

aquifer in the Laurel area would be about 60 ft 

under this scenario. Water-level declines in all 

other parts of the study area would be less than 

10ft.

About 64 Mgal/d of inflow to the Catahoula aquifer system would occur under the 

moderate-growth scenario, in which withdrawal would increase 3.5 percent each year in 

Laurel and 2 percent each year in Hattiesburg. 

About 1.6 Mgal/d of this total would be 

released from storage (fig. 40). This quantity 

represents a 16 percent increase compared to 

1992 conditions. Net flow directions would be 

little changed compared to 1992 conditions 

and the low-growth scenario. About 44 

Mgal/d, or 69 percent of all inflow, would 

enter the aquifer system in the outcrop area 

under this scenario. Most of the flow leaving 

the Catahoula aquifer system would be pumpage (49 Mgal/d or 77 percent). The remaining 

15 Mgal/d of outflow would be leakage to 

overlying aquifers.

The simulated average water-level decline 

in the upper Catahoula aquifer would be 20 ft 

or less (fig. 41) under the moderate-growth 

scenario, with the largest decline occurring in 

the Hattiesburg area. Simulated declines in the 

middle Catahoula would be 40 ft or less, also 

with the largest declines occurring in the Hattiesburg area.The water-level decline in the 

lower Catahoula would be about 110 ft, which 

places the water level at or near the top of the 

aquifer. A decline in the water level below the 

top of an aquifer results in a change from confined to unconfined conditions within the aquifer. This change could cause dewatering of the 

aquifer and possible compaction of the aquifer 

material. The model was not developed to simulate such conditions.

Under the high-growth scenario, in which 

withdrawal would increase 4 percent each year 

in both the Laurel and Hattiesburg areas, 

approximately 70 Mgal/d of water would enter 

the Catahoula aquifer system. This quantity 

represents a 26 percent increase over 1992 

conditions: about 2.8 Mgal/d of this total was 

released from storage (fig. 42). Net flow directions were little changed from 1992 conditions. About 47 Mgal/d, or 68 percent of all 

inflow to the system, would enter in the outcrop area under this scenario. Most of the discharge from the Catahoula aquifer system 

would be pumpage (55 Mgal/d or 80 percent). 

The remaining 14 Mgal/d of outflow would be 

leakage to overlying aquifers.

Under the high-growth scenario, simulated water-level declines in the upper Catahoula aquifer would be 40 ft or less, with the 

largest declines occurring in the Hattiesburg 

area (fig. 43). Water-level declines in the middle Catahoula would be about 80 ft, also with 

the largest declines occurring in the Hattiesburg area. The water-level decline would reach 

a maximum of 38 ft below the top of the lower 

Catahoula aquifer in the Laurel area under the 

high-growth scenario, about 130 ft below 1992 

levels. However, with the drop in water level 

below the top of the aquifer, the aquifer would 

have changed from confined to unconfined 

conditions and the model was not designed to 

simulate this change. The magnitude of the 

decline in water level below the top of the 

aquifer would probably be less than 38 ft, as 

the effective storage coefficient of an unconfined aquifer is higher than a confined aquifer, 

yielding more water to wells and slowing the 

decline in water levels. Further analysis of 

ground-water conditions under the highgrowth scenario would require modification of 

the model.

56
<J\

32 C

31 C

32 C

90 C 89 C 88 C

31 C

32 C

Upper Catahoula aquifer (Layer 2)

90

31 C

I I

.-! 1

Middle Catahoula aquifer (Layer 3)

0 20 40 60 80 MILES

. . i I . i . i I , , i i I i . i . I

0 20 40 60 80 KILOMETERS

EXPLANATION

-10 LINE OF EQUAL DRAWDOWNInterval 10 feet

Lower Catahoula aquifer (Layer 4)

Figure 39a. Simulated water-level declines from 1992 to 2020 in the upper, middle, and lower 

Catahoula aquifers in the modeled area at the end of the low-growth scenario.
Approximate extent of modeled area

10

I I I

15

I 

10 15 20 KILOMETERS

20 MILES

i I

Figure 39b. Simulated water-level declines from 1992 to 2020 in the upper Catahoula 

aquifer in the study area at the end of the low-growth scenario.

58
31°45' _

Approximate extent of modeled area

10 15 20 MILES

10 15 20 KILOMETERS

Figure 39c. Simulated water-level declines from 1992 to 2020 in the middle Catahoula 

aquifer in the study area at the end of the low-growth scenario.

59
89°30'

Approximate extent of modeled area

T

T

I

Hattiesburg I

o 10 15 20 MILES

0 5 10 15 20 KILOMETERS

Figure 39d. Simulated water-level declines from 1992 to 2020 in the lower Catahoula 

aquifer in the study area at the end of the low-growth scenario.

60
NORTH

p

s

12.28

EXPLANATION

DOWNGRADIENT LIMIT OF OUTCROP AREAFLOW ACROSS A BOUNDARYNET FLOW ACROSS A BOUNDARYPUMPAGE RATE FROM AQUIFER

FLOW RATE FROM STORAGEFLOW RATE, IN MILLION GALLONS PER DAY

 - _

WATER TABLE

1 41 12

1 1 t 1 1 1

SOUTH 

.. _ 28

* 1 i \ , It 17.65 16.24 ' P=7.85 18.51 6.23

H^a.oo s=0.42 

P=0.12 _^,.44

S=0.02 0.56^-7 UPPER CATAHOULA AQUIFER

1.28 0 56 1 8 47

1 1 1 1 if' 1 1 f

1 f 1 1 f 1 | 1 f 1 

26.51 25.23 9.26 8.70 P~20 30 14 ' 71 6 ' 24

^ 1B ' U 3=0.49

p_p rr, [>M4.69

3=0 07 i. 4g c MIDDLE CATAHOULA AQUIFER

0.18 1 1.83

lit ' 1 1 t

i 1 1 | 1 T

P=16.11 16 ' 96 16 ' 78 P=2.39 2 ' 95 1AZ 

S=0.08 h*L03 s=o.52

j^-0.75

0 . 2a c LOWER CATAHOULA

1

1

AQUIFER

OUTCROP DOWNDIP

Figure 40. Simulated volumetric flow budget for the Catahoula aquifer system 

in 2020 at the end of the moderate-growth scenario.
32 C 90

31 C

32 C 9(T 89 1-

ed area

Upper Catahoula aquifer (Layer 2)

32 C

90 C 89 C 88^

31 C

e nt of mode " ed area

IT

nt of mode

Middle Catahoula aquifer (Layer 3)

0 20 40 60 80 MILES

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

T 

0 20 40 60 80 KILOMETERS

EXPLANATION

->o LINE OF EQUAL DRAWDOWNInterval 10 feet

Lower Catahoula aquifer (Layer 4)

Figure 41a. Simulated water-level declines from 1992 to 2020 in the upper, middle, and lower Catahoula aquifers in the modeled area at the end of the moderate-growth scenario.
89°30

Approximate extent of modeled area

10 15

i I

20 MILES

10 15 20 KILOMETERS

Figure 41b. Simulated water-level declines from 1992 to 2020 in the upper Catahoula 

aquifer in the study area at the end of the moderate-growth scenario.

63
31°45'

Approximate extent of modeled area

10 15

1 i ' ' ' n ' ^ T 'i ' ' ' ' i

5 10 15 20 KILOMETERS

20 MILES

_J

Figure 41c. Simulated water-level declines from 1992 to 2020 in the middle Catahoula 

aquifer in the study area at the end of the moderate-growth scenario.

64
Approximate extent of modeled ana

T

L_

Hattiesburg I

10 15

10 15 20 KILOMETERS

20 MILES

_J

Figure 41d. Simulated water-level declines from 1992 to 2020 in the lower Catahoula 

aquifer in the study area at the end of the moderate-growth scenario.

65
NORTH

I

1.21

27.93 26.72 

p_o co

3=0 n

WATER TABLE

i

1.26

EXPLANATION

- DOWNGRADIENT LIMIT OF OUTCROP AREA

 > FLOW ACROSS A BOUNDARY

* NET FLOW ACROSS A BOUNDARY 

P PUMPAGE RATE FROM AQUIFER 

S FLOW RATE FROM STORAGE 

11.77 FLOW RATE, IN MILLION GALLONS PER DAY

SOUTH

11 77

19.10 17.84 

P = 0.12

S-0.01

P=8.54 19 ' 84 8 ' 07

S=0.78

6.16

o.57 UPPER CATAHOULA AQUIFER

+

0.52

I

10.09 9.57

0.18

I

8.08

18.34

i.4i

P~24 92 16 ' 55 8 ' 47

S=0.85

MIDDLE CATAHOULA AQUIFER

P=16.92 

3=0.13

17 ' 82 17 ' 64

o.86

P=2.39

s=0.90

f°' 64

0 , 37 LOWER CATAHOULA AQUIFER

OUTCROP DOWNDIP

Figure 42. Simulated volumetric flow budget for the Catahoula aquifer system in 2020 

at the end of the high-growth sceanrio.
32 C 90

31 C

32 C

32 C

Upper Catahoula aquifer (Layer 2)

90

31 C

31 C

ed area

Middle Catahoula aquifer (Layer 3)

0 20 40 60 80 MILES

i i i i I i i i i I

"H|MM|MII|IMI|

0 20 40 60 80 KILOMETERS

EXPLANATION

->o LINE OF EQUAL DRAWDOWNInterval 10 feet

Lower Catahoula aquifer (Layer 4)

Figure 43a. Simulated water-level declines from 1992 to 2020 in the upper, middle, 

and lower Catahoula aquifers in the modeled area at the end of the high-growth scenario.
10 15 20 MILES

10 15 20 KILOMETERS

Figure 43b. Simulated water-level declines from 1992 to 2020 in the upper Catahoula 

aquifer in the study area at the end of the high-growth scenario.

68
Approximate extent of modeled area

10 15 20 MILES

I ' ' i ' I i i i i I ' > i i I ' Pl1 I

0 5 10 15 20 KILOMETERS

Figure 43c. Simulated water-level declines from 1992 to 2020 in the middle Catahoula 

aquifer in the study area at the end of the high-growth scenario.

69
Approximate extent of modeled area

10 15 20 MILES

1 1 1 1 i i i

3 5 10

, 1 ,

i | i , 

15

1 1 1 1 

20

, 1 , , , , 1

KILOMETERS

Figure 43d. Simulated water-level declines from 1992 to 2020 in the lower Catahoula 

aquifer in the study area at the end of the high-growth scenario.

70
SUMMARY

The flow of ground-water in the Catahoula 

aquifer system in the vicinity of the cities of 

Laurel and Hattiesburg, in southern Mississippi, is described in this report. The Catahoula 

aquifer system is made up of irregular, discontinuous sand zones in the Catahoula Formation 

of Miocene age. The Catahoula Formation 

generally contains a sand zone near the base of 

the formation and two other sand zones higher 

in the sequence. These three sand zones are 

referred to as the lower, middle, and upper Catahoula aquifers. The three aquifers form a 

complex system in which specific aquifers 

may be hydraulically connected because of a 

missing or incomplete confining unit between 

them. Primary recharge for the Catahoula aquifer system is rainfall in the outcrop area in the 

northern part of the modeled area. Rain that 

infiltrates the aquifers moves downgradient 

toward the gulf coast and toward the Tombigbee River, on the eastern edge of the study 

area. The regional flow pattern is distorted by 

cones of depression, caused by pumping, in the 

Laurel and Hattiesburg areas.

Pumpage from the Catahoula aquifer system increased from 28 to 41 Mgal/d from 1970 

to 1985, and decreased to 38 Mgal/d for 1990. 

In the Laurel area, water levels in the lower 

Catahoula aquifer declined at rates ranging 

from about 1 to 3.6 ft/yr from 1964 until the 

late 1980's in response to the increase in 

pumping. Since the late 1980's, redistribution 

of withdrawals from the aquifer by the city of 

Laurel has resulted in increases in the water 

levels in the city well fields. In the Hattiesburg 

area, water levels have declined at rates from 

less than 1 to 2 ft/yr in both the lower and 

upper Catahoula aquifers, and in undeveloped 

areas water levels in the middle and upper Catahoula aquifers generally have declined less 

than 1 ft/yr.

A digital model was developed for the 

Catahoula aquifer system in the Laurel and 

Hattiesburg areas to further analyze the flow

system and to examine the effects of three scenarios of increased pumping on water levels in 

the aquifers. A specified-head upper boundary 

was used because the water table in the area 

has shown no significant decline or rise, and 

because the potential recharge for the system is 

much greater than the amount of withdrawal 

from the aquifers. The lower, no-flow boundary of the digital model represented the Vicksburg Group and underlying units. All lateral 

boundaries were also no flow, representing the 

outcrop area to the north, the regional drain for 

all the Catahoula aquifers of the Tombigbee 

River to the east, the freshwater/saltwater 

interface to the south, and a stable groundwater divide to the west. The properties of lateral hydraulic conductivity and leakance were 

adjusted to calibrate the model using a 

parameter-estimation technique. A steady-state 

model was calibrated using 1980 pumpage 

information and 1981 measured water levels. 

Pumpage was then removed from the calibrated model to simulate predevelopment conditions and to provide initial water levels for 

the transient-state model. Total predevelopment flow through the Catahoula aquifer system was about 29 Mgal/d, with about 20 

Mgal/d of this amount entering the aquifers in 

the outcrop area. The inflow from the outcrop 

area translates into a recharge rate of 0.13 

in/yr.

The transient-state model was used to simulate the Catahoula aquifer system for the 

period from 1941 to 1992 using eight stress 

periods. Simulated water levels agreed well 

with available hydrograph records, and simulated potentiometric surfaces reflected the 

cones of depression in the upper and middle 

Catahoula near Hattiesburg, and the cones in 

the lower Catahoula near Laurel and near Hattiesburg. Residuals from the last stress period 

(1988-92) generally show the smallest differences between simulated and calibrated values 

in the Laurel and Hattiesburg areas, the main 

areas of interest for the study. The RMSE of 

the calibrated model was 17 ft. About

71
55 Mgal/d of water flowed through the Catahoula aquifer system under 1992 conditions, 

with about 1.9 Mgal/d of this total released 

from storage. Under 1992 conditions, flow, 

which had been upward to overlying layers 

under predevelopment conditions, was downward, and lateral flow was primarily toward 

the pumping centers in Laurel and Hattiesburg. 

Water levels in the lower Catahoula aquifer, 

the layer most affected by pumping, were lowered from predevelopment levels as much as 

130 ft in the Laurel area and 100 ft in the Hattiesburg area. Sensitivity analysis showed that 

the model was most sensitive to changes in 

transmissivity but was also sensitive to leakance changes. The simulated aquifer system 

was insensitive to changes in storage coefficient. The model was developed primarily to 

simulate ground-water flow within confined 

aquifers, and would require modification in 

order to simulate conditions where the aquifer 

system changed from confined to unconfined 

conditions as the water level dropped below 

the top of an aquifer.

Three scenarios of increased pumpage 

were simulated with the calibrated model. 

Pumpage was increased 2, 3.5, and 4 percent 

per year from 1992 rates to the year 2020 for 

the Laurel area, and 1.2,2, and 4 percent for 

the Hattiesburg area for the low-, moderate-, 

and high-growth scenarios respectively. Under 

the low-growth scenario, the additional waterlevel decline over the 1992 levels would be 20 

ft or less in the upper and middle Catahoula 

aquifers in the Hattiesburg area, and about 60 

ft in the lower Catahoula aquifer in the Laurel 

area. Water-level declines in all other parts of 

the study area would be less than 10 ft.

Under the moderate-growth scenario, the 

water-level decline would be 20 ft or less in 

the upper Catahoula aquifer, but 40 ft or less in 

the middle Catahoula aquifer in the Hattiesburg area. The water-level decline in the lower 

Catahoula would be about 110 ft in the Laurel 

area, and the water level would be near the top 

of the aquifer.

Under the high-growth scenario, the 

water-level decline would be 40 ft or less in 

the upper Catahoula, and the water-level 

decline would be about 80 ft or less in the middle Catahoula, with the largest decline in the 

Hattiesburg area. The water level would 

decline below the top of the lower Catahoula 

aquifer in the Laurel area under the highgrowth scenario. With the decline in water 

level below the top of the aquifer, the aquifer 

would have changed from confined to unconfined conditions. The model was not designed 

to simulate this change.

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 d U.S. GOVERNMENT PRINTING OFFICE: 1995 655-177/00019 73