HYDROGEOLOGY AND ANALYSIS OF GROUND-WATER 

WITHDRAWAL FROM THE CATAHOULA AQUIFER SYSTEM IN THE 

NATCHEZ AREA, ADAMS COUNTY, MISSISSIPPI

By Eric W. Strom, David E. Burt Jr., and William T. Oakley

U.S. GEOLOGICAL SURVEY 

Water-Resources Investigations Report 95-4171

Prepared in cooperation with 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:

District Chief U.S. Geological Survey

U.S. Geological Survey Earth Science Information Center

Suite 710, Federal Building Open-File Reports Section

100 \V. Capitol Street Box 25286, MS 517, Federal Center

Jackson, Mississippi 39269 Denver, Colorado 80225
CONTENTS

Page

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

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

General setting of the study area................................................................................ 2

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

Hydrogeology...................................................................................................................... 4

The Catahoula aquifer system..................................................................................... 4

Ground-water withdrawal.......................................................................................... 4

Analysis of ground-water withdrawal............................................................................ 8

Analysis of current conditions.................................................................................... 15

Sensitivity analyses....................................................................................................... 15

Projected effects of ground-water withdrawals....................................................... 17

Limitations of the analysis........................................................................................... 20

Summary.............................................................................................................................. 25

References............................................................................................................................. 27

ILLUSTRATIONS

Figure 1. Map showing location of study area and trace of geologic section........... 3

2. Geologic units and principal aquifers in the study area............................. 5

3. Geologic section showing the north to south dip and relation of

geologic units in the Natchez area, Adams County, Mississippi............ 6

4-6. Grid cells, well locations, and top of Glendon Limestone used in 

drawdown calculations for the:

4. upper Catahoula sand...................................................................................... 9

5. middle Catahoula sand.................................................................................... 10

6. lower Catahoula sand....................................................................................... 11

7-9. Maps showing calculated drawdown of water levels for currently 

(March 1995) pumping wells in the:

7. upper Catahoula sand...................................................................................... 12

8. middle Catahoula sand.................................................................................... 13

9. lower Catahoula sand....................................................................................... 14

10. Generalized drawdown surfaces viewed from southwest of the 

Natchez, Mississippi, area for the upper, middle, and lower 

Catahoula sands calculated from currently (March 1995) pumping 

wells................................................................................................................. 16

in
ILLUSTRATIONS-Continued

Page

Figures 11-13. Maps showing projected drawdown-surface based on current 

pumping rates 10 years beyond March 1995 in the:

11. upper Catahoula sand...................................................................................... 18

12. middle Catahoula sand.................................................................................... 19

13. lower Catahoula sand....................................................................................... 21

14-16. Maps showing projected drawdown-surface based on current pumping 

rates 20 years beyond March 1995 in the:

14. upper Catahoula sand...................................................................................... 22

15. middle Catahoula sand.................................................................................... 23

16. lower Catahoula sand....................................................................................... 24

TABLES

Table 1. Current wells in the Natchez, Mississippi, area and data used in the

investigation................................................................................................... 7

2. Estimated data used in projection analyses for planned wells in the

Natchez area................................................................................................... 17

IV
CONVERSION FACTORS AND VERTICAL DATUM

Multiply By To obtain

foot 0.3048 meter

foot per mile 0.1894 meter per kilometer

inch 25.4 millimeter

mile 1.609 kilometer

million gallons per day 0.04381 cubic meter per second

cubic foot per second 0.02832 cubic meter per second

million cubic feet per day 0.3278 cubic meter per second

square mile 2.590 square kilometer

foot squared per day 0.0929 meter squared per day

Temperature in degrees Fahrenheit (°F) can be converted to degrees Celsius (°C) as 

follows: °C = (°F-32)/1.8.

Sea level: In this report "sea level" refers to the National Geodetic Vertical Datum of 

1929 a geodetic datum derived from a general adjustment of the first-order level nets of 

both the United States and Canada, formerly called Sea Level Datum of 1929.

The standard unit for transmissivity is cubic foot per day per square foot times foot of 

aquifer thickness. In this report, the mathematically reduced form, foot squared per day, 

is used for convenience.
HYDROGEOLOGY AND ANALYSIS OF GROUND-WATER

WITHDRAWAL FROM THE CATAHOULA AQUIFER SYSTEM IN THE

NATCHEZ AREA, ADAMS COUNTY, MISSISSIPPI

By Eric W. Strom, David E. Burt Jr., and William T. Oakley

ABSTRACT

The city of Natchez, located in Adams County, Mississippi, relies on ground water 

for public supply and industrial needs. Most public supply and industrial wells are 

developed in Catahoula Formation sands of Miocene age. In 1991, an investigation 

began to describe the hydrogeology, analyze the effects of ground-water withdrawal 

from currently pumped wells, and project the possible effects of increased ground-water 

withdrawals on water levels in the Catahoula aquifer system within the Natchez area.

The study area covers about 80 square miles in Adams County, southwestern 

Mississippi. The study area contains several aquifers; however, the most important 

aquifers in terms of water supply are the Mississippi River alluvial aquifer and the 

Catahoula aquifer system. In the Natchez area, the Catahoula aquifer system consists of 

three main sand intervals that form the upper, middle, and lower Catahoula aquifers.

Ground-water withdrawal from the Catahoula aquifer system in the study area 

currently (March 1995) is from 24 wells screened in the three aquifers. The current daily 

rate of withdrawal is about 9.2 million gallons of water per day. Analysis of the effect of 

ground-water withdrawal from these wells was made using the Theis nonequilibrium 

equation and applying the principle of superposition. The calculated drawdown 

surfaces under current conditions indicate cones of depression surrounding the 

principal wells. In the upper Catahoula sand, most of the drawdown is concentrated 

about 1 mile east of the downtown Natchez area, where a maximum drawdown of 

95 feet was calculated. Most of the drawdown in the middle Catahoula sand occurred in 

the same general vicinity as in the upper sand, with a maximum calculated drawdown 

of about 113 feet. Drawdown in the lower Catahoula sand was concentrated about 

4 miles northeast of downtown Natchez, with a maximum calculated drawdown of 

about 31 feet.

Drawdown-surface maps were made using calculations based on current pumping 

rates for 10 years and 20 years beyond March 1995. Planned changes in the pumping 

configuration were incorporated into these analyses. The drawdown surface calculated 

for 10 years beyond March 1995 indicates an average total increase in drawdown of
about 7.3 feet for the upper Catahoula sand, with a maximum increase of about 28 feet. 

An average total increase in drawdown of only 1.2 feet was calculated for the middle 

Catahoula sand due to the planned discontinued pumping of many of the wells. An 

average total increase in drawdown of about 19 feet was calculated for the lower 

Catahoula sand, with a maximum increase of about 41 feet. The drawdown surface 

calculated for 20 years beyond March 1995 indicates an average total additional increase 

in drawdown over the 10 year drawdown surface of about 1.9,0.6, and 2.7 feet for the 

upper, middle, and lower Catahoula sands, respectively.

INTRODUCTION

The city of Natchez, located in Adams County, Mississippi, relies on ground water 

for public supply and industrial needs. Most public supply and industrial wells are 

developed in Catahoula Formation sands of Miocene age. In 1982, about 7.4 million 

gallons of water per day was withdrawn from the Catahoula aquifer system in Adams 

County (Boswell and Bednar, 1985). Currently (March 1995), more than 9 million gallons 

of water per day is withdrawn from the Catahoula aquifer system in the Natchez area 

alone. Increased ground-water withdrawals from a growing population and the 

development of new industries may increase water-level declines and affect water 

quality. In 1991, the U.S. Geological Survey, in cooperation with the Mississippi 

Department of Environmental Quality, Office of Land and Water Resources (OLWR), 

began an investigation to describe the hydrogeology, to analyze the effects of groundwater withdrawal from currently pumped wells, and to project the possible effects of 

increased ground-water withdrawals on water levels in the Catahoula aquifer system 

within the Natchez area. This report presents the results of that study.

General Setting of the Study Area

The study area covers about 80 square miles in Adams County, southwestern 

Mississippi (fig. 1). The area is located in the Mississippi Alluvial Plain and the Bluff 

Hills of the Gulf Coastal Plain physiographic province (Fenneman, 1938). Most of the 

study area is hilly due to the erosion of loess deposits; however, a broad flat alluvial 

plain formed by the Mississippi River is located in the western part of the study area. 

The mean annual temperature at Natchez is about 64.4 degrees Fahrenheit; annual 

precipitation is about 53.4 inches (National Weather Service, oral commun., 1995).

Previous Investigations

Previous investigations that include all or part of the study area have been published 

by Stephenson and others, (1928); Vestal (1942); Callahan and others (1963,1964); 

Childress and others (1976); Boswell and Bednar (1985); and Martin and Whiteman 

(1989).
91°25' 91° 17'

31°37' - 31°30' -Base modified from U.S. Geological Survey digital data, 1:100,000,1983

Lambert Conformal Conic projectionStandard parallels 33° and 45°, central meridian -90°

1 MILE

1 KILOMETER

202

EXPLANATION

A1 TRACE OF GEOLOGIC 

SECTION

LOCATION OF WELL AND 

GEOPHYSICAL LOG NUMBER

Figure 1. Location of study area and trace of geologic section.
HYDROGEOLOGY

The study area contains several aquifers; however, the most important aquifers in 

terms of water supply are the Mississippi River alluvial aquifer and the Catahoula 

aquifer system (fig. 2). The Catahoula aquifer system consists of three main sand 

intervals in the study area, although some of the uppermost sand may belong to the 

Hattiesburg Formation in parts of the study area (E.H. Boswell, OLWR, oral commun., 

1995). The Catahoula aquifer system is the focus of this investigation.

The Catahoula Aquifer System

The Catahoula Formation is described by May and Marble (1976) as being fluvial to 

marginal-deltaic in depositional origin, with sediments probably representing a 

regressive, of flap sequence. The formation mainly consists of deltaic silt and clay 

deposited in a low energy environment, and intervals of sand deposited in a high energy 

environment. In the Natchez area, the Catahoula Formation contains three main sand 

intervals that are referred to as the 400-foot, 600-foot, and 1,000-foot sands (Callahan and 

others, 1963; Boswell and Bednar, 1985) and correspond to the upper, middle, and lower 

Catahoula Formation sands, respectively. These three sand intervals comprise the 

Catahoula aquifer system in the Natchez area (fig. 3). Because the sand intervals are 

southward dipping, depths are variable, and assignment of a sand bed into a particular 

sand interval is determined by the interval's altitude above the Glendon Limestone: the 

upper, middle, and lower sand intervals average 1,100 feet, 900 feet, and 650 feet above 

the limestone, respectively (Boswell and Bednar, 1985).

The Catahoula aquifers are described by Boswell and Bednar (1985) as being under 

confined conditions in the study area. The base of freshwater ranges from about 500 feet 

below sea level in the northern part of the study area, to about 800 feet below sea level in 

the southern part. Recharge probably occurs in outcrop areas northeast of Natchez. 

Locally, ground-water movement is toward pumping wells; regionally, ground-water 

movement generally is westward. Analysis of the flow system indicates regional 

discharge is upward into the central and western parts of the Mississippi River valley in 

southern Louisiana (Martin and Whiteman, 1989). The Miocene aquifers are highly 

permeable; however, aquifer thickness and transmissivity are highly variable within the 

study area. Transmissivities from aquifer tests range from 2,000 to 16,000 feet squared 

per day.

Ground-Water Withdrawal

Ground-water withdrawal from the Catahoula aquifer system in the study area 

currently (March 1995) is from 24 wells screened in the upper, middle, and lower sands 

of the Catahoula Formation. Information regarding each well is listed in table 1. Values 

of transmissivity listed are either from an aquifer test at the well site or are estimated 

using the value of transmissivity from the nearest test site. Test values (mostly singleborehole tests) come from currently pumping wells (table 1) and from wells that are no 

longer in use. Pumpage for the wells represents an average steady pumping rate, or the 

rate at which the wells would be pumped if the pumps ran continuously throughout the
Erathem

Cenozoic

System

Quaternary

I

Series

Holocene

Pleistocene 

and 

Pliocene (?)

Miocene

Oligocene

Group

Vicksburg

Geologic unit

Alluvial deposits

Loess deposits

Natchez Formation 

and terrace deposits

Hattiesburg Formation 

and

Catahoula Formation 

(undifferentiated)

Bucatunna Formation

Byram Formation

Glendon Limestone

Marianna Limestone

Forest Hill Formation

Hydrogeologic properties

Alluvial aquifer. Water-table aquifer 

yielding very large amounts of water.

Unimportant as an aquifer.

Natchez aquifer. Not commonly used by larger wells.

Catahoula aquifer system. Confined aquifers yielding large amounts of water. Most public 

and industrial wells screened in one of three 

main sand intervals of the Catahoula Formatic

Not an aquifer.

Not an aquifer.Unimportant as an aquifer.

Unimportant as an aquifer.Unimportant as an aquifer.

Figure 2. Geologic units and principal aquifers in the study area (modified from Boswell and Bednar, 1985).
A1

FEET 500-

SEA 

LEVEL

500i

1,000-

1,500-

2,000-

upper Catahoula 

sand

middle Catahoula 

sand

2 MILES

KILOMETERS

Note: trace of section shown in figure 1

VERTICAL SCALE GREATLY EXAGGERATED

Figure 3. Geologic section showing the north to south dip and relation of geologic units in the Natchez area, Adams County, Mississippi (modified from Boswell and Bednar, 1985).
Table 1. Current wells in the Natchez, Mississippi, area and data used in the investigation

Well 

number

C003

COOS

C007

C015

C016

C018

C031

C032

C034

C035

C037

C048

C063

C064

C069

C073

C102

D040

D046

D061

D073

D080

D081

P075

Latitude

313339

313336

313338

313341

313343

313337

313359

313400

313134

313140

313136

313409

313213

313337

313156

313340

313335

313359

313456

313457

313407

313518

313524

313050

Longitude

912326

912321

912316

912238

912244

912252

912311

912311

912537

912557

912543

912242

912158

912321

912158

912326

912244

911704

911713

911712

911657

911932

911958

912230

Land 

surface 

altitude 

(feet)

232

212

210

209

207

203

210

210

90

90

90

183

119

205

119

230

200

293

275

275

325

248

220

190

Screen interval 

(feet below 

land surface)

399-444

371-421

553-613

532-592

405-455

417-467

382-442

515-575

580-674

639-679

489-560

517-578

539-599

600-650

537-597

555-595 

630-652

365-380 

395-437 

448-471

1,000-1,030

898-958

910-971

1,022-1,051

800-841 

857-928

768-788 

798-858

780-800

Sand 

interval

upper

upper

middle

upper

upper

upper

upper

middle

middle

middle

upper

middle

upper

middle

upper

middle

upper

lower

lower

lower

lower

lower

lower

middle

Months 

pumped

670

551

501

614

578

466

369

368

363

362

298

181

440

136

450

139

8

203

188

183

122

93

93

261

Pumpage 

(million 

gallons per day)

0.288

0.384

0.384

0.624

0.464

0.484

0.288

0.288

0.231

0.231

0.231

0.480

0.154

0.720

0.154

0.720

0.324

0.042

0.231

0.224

0.336

0.936

0.936

0.024

Transmissivity 

(feet squared 

per day)

8,400

a8,400

4,000

a5,000

"5,000

a5,000

"8,400

4,700

2,000

2,000

a5,500

2,100

"2,000

6,700

9,600

11,100

7,200

a!6,000

a!6,000

a !6,000

a16,000

a!4,700

13,800

"2,000

a Estimated value.
year. The current daily rate of withdrawal is about 9.2 million gallons of water per day. 

The total time the wells have been pumped (table 1) is calculated through March 1995.

ANALYSIS OF GROUND-WATER WITHDRAWAL

Analysis of the effect of ground-water withdrawal from the 24 wells listed in table 1 

was made by applying the Theis nonequilibrium equation to each of the three Catahoula 

sand intervals in the study area to calculate drawdown. This method is applicable 

because pumping tests and water-level data indicate that the aquifers are confined and 

generally non-leaky in the study area. The Theis nonequilibrium equation computes 

drawdown in a confined aquifer for a given time at a specified distance from a pumping 

well. To perform the analysis, pumpage from the well, the length of time pumping 

occurred, and the transmissivity and storage coefficient of the aquifer must be known. 

The equation given by Theis (1935) is of the form:

_ I, - ~ _ *£ f

~ ~ 4nT 4nT 1

/fs

47Y

where

h0 is the initial head (length) at some distance r (length) from the well,

h is the head (length) at some time t (time),

s is the resulting drawdown (length),

Q is the pumping rate (length cubed per time),

T is the transmissivity (length squared per time), and

S is the storage coefficient (dimensionless).

The Theis nonequilibrium equation is a solution to the radial form of the diffusion 

equation for a given set of initial and boundary conditions. Because the diffusion 

equation is linear, the principle of superposition can be applied to determine the total 

drawdown caused by multiple wells being pumped simultaneously by summing the 

drawdown determined for each individual well.

The study area was discretized into three equally spaced grids representing the 

upper, middle, and lower Catahoula Formation sand intervals. Each grid cell was 

500 feet on a side for a total of 9,000 cells in each grid (figs. 4-6). This discretization 

provided the resolution necessary to delineate the drawdown surface caused by 

pumpage from multiple wells, and to position each well in the center of a cell. The Theis 

nonequilibrium equation was applied at the center of each cell for each well in a 

particular layer using numerical approximations. Drawdown for each well in each cell 

was summed and plotted on a map (figs. 7-9). Drawdown was calculated using a well 

radius of 1 foot in cells that contained a well. The calculated drawdown does not account 

for drawdown caused by other factors, such as turbulent flow, regional water-level 

declines, or previously existing wells. A storage coefficient of 0.0002 was used for each 

well in the analysis.

8
91°25

31°3T i"

31°30' 

90

EXPLANATION

 -1,600 STRUCTURE CONTOUR-Shows approximate 

altitude of top of Glendon Limestone. Datum is sea level (Boswell and Bednar, 1985)

C069 LOCATION OF WELL AND NUMBER

© 9,600 LOCATION OF AQUIFER TEST AND VALUE 

OF TRANSMISSIVITY--in feet squared per day

+ 4 LOCATION OF PLANNED WELL AND NUMBER 

DESIGNATION

Base modified from U.S. Geological Survey digital data, 1:100,000,1983 Lambert Conformal Conic projection Siandard parallels 33° and 45°, central meridian -90° 1 MILE 1 KILOMETER

Figure 4. Grid cells, well locations for the upper Catahoula sand, and top of Glendon Limestone used in drawdown calculations.
91°25' 91° 17'

31°37' ;:

EXPLANATION

1,600- STRUCTURE CONTOUR-Shows approximate 

altitude of top of Glendon Limestone. Datum 

is sea level (Boswell and Bednar, 1985)

LOCATION OF WELL AND NUMBER

LOCATION OF AQUIFER TEST AND VALUE 

OF TRANSMISSIVITY~in feet squared per day 31°30' =:

Base modified from U.S. Geological Survey digital data, 1:100,000,1983

Lambert Conformal Conic projection Standard parallels 33° and 45°, central meridian -90° 1 MILE 1 KILOMETER

Figure 5. Grid cells, well locations for the middle Catahoula sand, and top of Glendon Limestone used in drawdown calculations.
31°37' i31°30' 

90

-1,700

D040 

10,000

+ 1

EXPLANATION

STRUCTURE CONTOUR-Shows approximate 

altitude of top of Glendon Limestone. Datum is sea level (Boswell and Bednar, 1985)

LOCATION OF WELL AND NUMBER

LOCATION OF AQUIFER TEST AND VALUE 

OF TRANSMISSIVITY--in feet squared per day

LOCATION OF PLANNED WELL AND NUMBER 

DESIGNATION

iffi

Base modified from U.S. Geological Survey digital data, 1:100,000,1983

Lambert Con formal Conic projection Standard parallels 33° and 45°. central meridian -90° 1 MILE 1 KILOMETER

Figure 6. Grid cells, well locations for the lower Catahoula sand, and top of Glendon Limestone used in 

drawdown calculations.
91°25' 91° IT

N>

31°37' - 31°30'

Base modified from U.S. Geological Survey digital data, 1:100,000,1983

Lambert Conformal Conic projection

Standard parallels 33° and 45°, central meridian -90°

EXPLANATION

-45- LINE OF EQUAL DRAWDOWNInterval 5 feet

i MILE

1 KILOMETER

Figure 7. Calculated drawdown of water levels in the upper Catahoula sand interval for currently 

(March 1995) pumping wells.
91°25' 91° 17'

31°37

31°30' -Base modified from U.S. Geological Survey digital data, 1:100,000,1983

Lambert Conformal Conic projection

Standard parallels 33° and 45°, centra) meridian -90°

EXPLANATION 1 MILE

 55- LINE OF EQUAL DRAWDOWNInterval 5 feet 1 KILOMETER

Figure 8. Calculated drawdown of water levels in the middle Catahoula sand interval for currently 

(March 1995) pumping wells.
91° 25' 91° 17'

31°37' - 31°30' -Base modifled from U.S. Geological Survey digital data, 1:100,000,1983

Lambert Conformal Conic projectionStandard parallels 33° and 45°, central meridian -90°

EXPLANATION 1 MILE

 12- LINE OF EQUAL DRAWDOWNInterval 2 feet 1 KILOMETER

Figure 9. Calculated drawdown of water levels in the lower Catahoula sand interval for currently 

(March 1995) pumping wells.
Analysis of Current Conditions

Drawdown analysis for current conditions was made using the Theis nonequilibrium 

equation with pumpage through March 1995 and information listed in table 1. The 

calculated drawdown surfaces indicate cones of depression surrounding the principal 

wells. In the upper Catahoula sand (fig. 7) most of the drawdown is concentrated about 

1 mile east of the downtown Natchez area, where a maximum drawdown of 95 feet was 

calculated. Most of the drawdown in the middle Catahoula sand (fig. 8) occurred in the 

same general vicinity as the upper sand, with a maximum calculated drawdown of 

about 113 feet. Significant drawdown also occurred in the middle sand about 3 miles 

southwest of downtown Natchez due to relatively low transmissivity values in the area. 

Drawdown in the lower Catahoula sand (fig. 9) was concentrated about 4 miles 

northeast of downtown Natchez, with a maximum calculated drawdown of about 

31 feet. Water-level measurements in the Natchez area indicate that the potentiometric 

surfaces of the upper, middle, and lower Catahoula aquifers are greater than 100,200, 

and 600 feet, repectively, above the top of the water-bearing sands. A generalized view of 

drawdown in all three sands is shown in figure 10.

Sensitivity Analyses

The Theis nonequilibrium equation was applied using the value of transmissivity 

determined from an aquifer test or estimated for each individual well. An alternative 

method would be to assume a single average value of transmissivity for all of the wells; 

however, error likely is reduced by using site-specific values of transmissivity for each 

individual well (Strom and Oakley, 1995) because calculated drawdown is greatest at the 

center of a well and decreases exponentially with distance from the well. However, 

because the Theis nonequilibrium equation is based on the assumption of a 

homogeneous aquifer, and because site-specific values of transmissivity were used for 

each individual well, sensitivity analyses were performed using the geometric mean of 

the test-determined values of transmissivity for each sand interval.

A geometric mean transmissivity of 6,000 feet squared per day for the upper 

Catahoula sand resulted in an average calculated drawdown about 3 feet less than the 

drawdown values calculated using site-specific values of transmissivity. A maximum 

difference of about 13 feet less than the drawdown values calculated using site-specific 

values of transmissivity occurred at well C063 (fig. 4) due to the relatively large 

difference between the test-determined and geometric mean value of transmissivity. 

Differences up to about 9 feet less than the drawdown values calculated using sitespecific values of transmissivity also occurred in the vicinity of wells C015 and C016. A 

geometric mean transmissivity of 4,300 feet squared per day for the middle Catahoula 

sand resulted in an average calculated drawdown about 4 feet less than the drawdown 

values calculated using site-specific values of transmissivity. Maximum differences from 

about 18 to 23 feet less than the drawdown values calculated using site-specific values of 

transmissivity occurred at wells C034, C35, and C48, and about 18 feet more than the 

drawdown values calculated using site-specific values of transmissivity occurred at well 

C073 (fig. 5) due to the relatively large differences between the test-determined and 

geometric mean value of transmissivity. A geometric mean transmissivity of 12,500 feet

15
Figure 10. Generalized drawdown surfaces viewed from southwest of the Natchez, Mississippi area 

tor the upper, middle, and lower Catahoula sand intervals calculated from currently (March 1995^1 

pumping wells. '

16
squared per day for the lower Catahoula sand resulted in an average calculated 

drawdown about 2 feet greater than the drawdown values calculated using site-specific 

values of transmissivity. The maximum difference of about 5 feet greater than the 

drawdown values calculated using site-specific values of transmissivity occurred at all 

of the pumping wells except D040, which had a maximum difference of about 3 feet 

(fig. 6).

Because storage coefficient data were limited, a sensitivity analysis of the change in 

the storage coefficient was made. Decreasing the storage coefficient from 0.0002 to 0.0001 

resulted in an increase in drawdown of about 4.5,5.9, and 1.4 feet for current conditions 

for the upper, middle, and lower Catahoula sands, respectively. Increasing the storage 

coefficient from 0.0002 to 0.0003 resulted in a decrease in drawdown of about 2.6,3.4, 

and 0.8 feet for current conditions for the upper, middle, and lower Catahoula sands, 

respectively.

Projected Effects of Ground-Water Withdrawals

Projected drawdown-surface maps based on current pumping rates were made for 

10 years and 20 years beyond March 1995. Several major changes in the pumping 

configuration are planned by the city of Natchez in the near future; table 2 lists relevant 

information regarding the planned addition of several new wells. Planned locations for 

the new wells are shown on figure 4 and figure 6. In addition, wells C3, C5, C7, C31, C32, 

C48, C73, and C64 are expected to discontinue production in May 1996. These changes in 

the pumping configuration were incorporated into the projection analyses. The 

projection analyses determined the total drawdown caused by pumping the existing and 

planned wells 10 and 20 years beyond March 1995; however, water-level recoveries are 

to be expected for the upper and middle Catahoula sands where wells are planned to be 

taken out of production.

Table 2. Estimated data used in projection analyses for planned wells in the Natchez area

Well 

number

1

2

3

4

Latitude

313506

313520

313516

313133

Longitude

911819

911942

911910

912535

Sand 

interval

lower

lower

lower

upper

Date pumping 

to begin

May 1996

May 1996

May 1996

April 1995

Pumpage 

(million gallons 

per day)

1.184

1.184

1.184

0.528

Transmissivity 

(feet squared 

per day)

16,000

14,700

16,000

5,500

The drawdown surface calculated for 10 years beyond March 1995 indicates an 

average total increase in drawdown of about 7.3 feet for the upper Catahoula sand 

(fig. 11), with a maximum increase of about 28 feet at the planned location of well 4 (fig. 4 

and table 2). An average total increase in drawdown of only 1.2 feet was calculated for 

the middle Catahoula sand (fig. 12) due to the planned discontinued pumping of many

17
91°25' 91° IT

00

31°37' - 31°30' -Base modified from U.S. Geological Survey digital data, 1:100,000,1983

Lambert Conformal Conic projectionStandard parallel! 33° and 45°, central meridian -90°

EXPLANATION 1 MILE

 55- LINE OF EQUAL DRAWDOWNInterval 5 feet 1 KILOMETER

Figure 11. Projected drawdown map based on current pumping rates in the upper Catahoula sand 10 years 

beyond March 1995.
91°25' 91° 17'

31°37' - 31°30' -Base modified from U.S. Geological Survey digital data, 1:100,000,1983

Lambert Conformal Conic projectionStandard parallels 33° and 45°, central meridian -90°

EXPLANATION 1 MILE

-55- LINE OF EQUAL DRAWDOWNInterval 5 feet 1 KILOMETER

Figure 12. Projected drawdown map based on current pumping rates in the middle Catahoula sand 10 years 

beyond March 1995.
of the wells. An average total increase in drawdown of about 19 feet was calculated for 

the lower Catahoula sand (fig. 13), with a maximum increase of about 41 feet at the 

planned locations of wells 1,2, and 3 (fig. 6 and table 2). The drawdown surface 

calculated for 20 years beyond March 1995 (figs. 14-16) indicates an average total 

additional increase in drawdown over the 10-year estimates of about 1.9,0.6, and 2.7 feet 

for the upper, middle, and lower Catahoula sands, respectively.

Projection analyses are based on the assumption that the planned new wells will 

have properties similar to those listed in table 2. However, if the completion date of the 

planned wells is as much as 1 year later than anticipated, the drawdown surfaces 

calculated for 10 years beyond current conditions would be less than 0.3 foot different 

than the values calculated above.

Because drawdown and pumpage have a linear relation in the Theis nonequilibrium 

equation, the projection maps may be used for extrapolation or interpolation of 

drawdown for other pumping rates. For example, the calculated drawdown for current 

conditions at well C063 (fig. 4) is about 71 feet (fig. 7). After 10 years of pumpage beyond 

March 1995, the calculated drawdown is about 79 feet (fig. 11); therefore, total additional 

drawdown is 8 feet. If pumpage were increased by 50 percent for all of the wells for the 

10-year period, the total additional drawdown at well C063 would be 1.5 times 8, or 

12 feet, resulting in a total drawdown of about 91 feet.

Limitations of the Analysis

To correctly apply the Theis nonequilibrium equation in the drawdown analyses, 

several assumptions were made: (1) the upper, middle, and lower sand intervals of the 

Catahoula Formation are confined and infinite in extent, (2) wells are of infinitesimal 

diameter and fully penetrate the aquifer, (3) water is released instantaneously from the 

aquifer with decline in head, (4) the aquifer is homogeneous and isotropic, and (5) there 

are no contributions of water from overlying or underlying aquifers. The first 

assumption is valid if the extent of the aquifer is large compared to the extent of the 

drawdown cones of the pumping wells. If impermeable geologic boundaries are reached 

by the drawdown cones, however, computed drawdown will be less than actual 

drawdown. Nearby sources of recharge would cause computed drawdown to be greater 

than actual drawdown. The second assumption ignores any storage in the well, which is 

a reasonable assumption for long-term analyses in which the total water pumped is large 

relative to wellbore storage. Wells in the study area generally were screened the entire 

thickness of the sand interval. The third assumption is valid for the confined case, but 

would be invalid if water-table conditions were encountered. The fourth assumption 

was made for the analysis of each individual well; however, aquifer tests indicated 

variations in transmissivity. Because site-specific aquifer properties were used at each 

individual well, the drawdown calculated near each well probably is a better 

approximation than if a constant value of transmissivity were applied to all wells. 

According to the fifth assumption, the clay and silt confining layers are impermeable. 

The calculated drawdown will be greater than the actual drawdown if water moves 

through the confining layers into the aquifers.

The drawdowns calculated herein do not necessarily represent precise drawdowns 

observed in the field. Calculated drawdown is for the currently pumping wells and does

20
91°25' 91° IT

N>

31°3T - 31°30' - x - - r «* T > *' "' - .... \

Base modified from U.S. Geological Survey digital data, 1:100,000,1983

Lambert Confonral Conic projection

Standard parallels 33° and 45°, central mridian -90°

EXPLANATION

 30 LINE OF EQUAL DRAWDOWNInterval 5 feet

1 MILE

1 KILOMETER

Figure 13. Projected drawdown map based on current pumping rates in the lower Catahoula sand 10 years 

beyond March 1995.
91°25' 91° 17'

31°37' - 31°30'

Base modified from U.S. Geological Survey digital data, 1:100,000,1983

Lambert Confonral Conic projection

Standard parallels 33° and 45°, central meridian -90°

EXPLANATION

 60- LINE OF EQUAL DRAWDOWN-- 

Interval 5 feet

o i MILE I r J0 1 KILOMETER

Figure 14. Projected drawdown map based on current pumping rates in the upper Catahoula sand 20 years beyond March 1995.
91°25' 91° 17'

S3

31°37' - 31°30' -Base modified from U.S. Geological Survey digital data, 1:100,000,1983

Laniben Conformal Conic projectionStandard parallels 33° and 45°, central meridian -90°

EXPLANATION 1 MILE

 55- LINE OF EQUAL DRAWDOWNInterval 5 feet 1 KILOMETER

Figure 15. Projected drawdown map based on current pumping rates in the middle Catahoula sand 20 years beyond March 1995.
91°25' 91° IT

31°37

31° 30' -Base modified from U.S. Geological Survey digital data, 1:100.000,1983

Lambert Conformal Conic projectionStandard parallels 33° and 45°, central meridian -90°

EXPLANATION 1 MILE

 30- LINE OF EQUAL DRAWDOWNInterval 5 feet 1 KILOMETER

Figure 16. Projected drawdown map based on current pumping rates in the lower Catahoula sand 20 years 

beyond March 1995.
not account for drawdown caused by previously existing wells or regional water-level 

declines; however, the calculated drawdowns provide a guide for estimating drawdown 

trends that may occur in the future. Water-level recoveries are to be expected for the 

upper and middle Catahoula sand intervals if numerous wells discontinue pumping. 

Projection analyses are based on the assumption that the planned new wells will have 

properties similar to those listed in table 2.

SUMMARY

The city of Natchez, located in Adams County, Mississippi, relies on ground water 

for public supply and industrial needs. Most public-supply and industrial wells are 

developed in Catahoula Formation sands of Miocene age. The U.S. Geological Survey, in 

cooperation with the Mississippi Department of Environmental Quality, Office of Land 

and Water Resources, began an investigation to describe the hydrogeology, analyze the 

effects of ground-water withdrawal from currently pumped wells, and project the 

possible effects of increased ground-water withdrawals on water levels in the Catahoula 

aquifer system within the Natchez area.

The study area covers about 80 square miles in Adams County, southwestern 

Mississippi. The study area contains several aquifers; however, the most important 

aquifers in terms of water supply are the Mississippi River alluvial aquifer and the 

Catahoula aquifer system. In the Natchez area, the Catahoula Formation contains three 

principal sand intervals that are referred to as the 400-foot, 600-foot, and 1,000-foot sands 

and correspond to the upper, middle, arid lower Catahoula Formation, respectively.

Ground-water withdrawal from the Catahoula aquifer system in the study area 

currently (March 1995) is from 24 wells screened in the upper, middle, and lower sands 

of the Catahoula Formation. The current daily rate of withdrawal is about 9.2 million 

gallons of water per day. Analysis of the effect of ground-water withdrawal from the 

wells was made using the Theis nonequilibrium equation and applying the principle of 

superposition. The study area was discretized into three equally spaced grids, and 

drawdown was calculated at the center of each cell. Analysis of current conditions was 

based on the pumpage through March 1995 and the aquifer properties determined for 

each well. The calculated drawdown surfaces indicate cones of depression surrounding 

the principal wells. In the upper Catahoula sand most of the drawdown is concentrated 

about 1 mile east of the downtown Natchez area, where a maximum drawdown of 

95 feet was calculated. Most of the drawdown in the middle Catahoula sand occurred in 

the same general vicinity as in the upper sand, with a maximum calculated drawdown 

of about 113 feet. Drawdown in the lower Catahoula sand was centered about 4 miles 

northeast of downtown Natchez, with a maximum calculated drawdown of about 

31 feet.

Drawdown-surface maps were made using calculations based on current pumping 

rates for 10 years and 20 years beyond March 1995. Planned changes in the pumping 

configuration were incorporated into these analyses. The drawdown surface calculated 

for 10 years beyond March 1995 indicates an average total increase in drawdown of 

about 7.3 feet for the upper Catahoula sand, with a maximum increase of about 28 feet. 

An average total increase in drawdown of only 1.2 feet was calculated for the middle

25
Catahoula sand due to the planned discontinued pumping of many of the wells. An 

average total increase in drawdown of about 19 feet was calculated for the lower 

Catahoula sand, with a maximum increase of about 41 feet. The drawdown surface 

calculated for 20 years beyond March 1995 indicates an average total additional increase 

in drawdown over the 10-year calculations of about 1.9,0.6, and 2.7 feet for the upper, 

middle, and lower Catahoula sands, respectively.

26
REFERENCES

Boswell, E.H., and Bednar, G.A., 1985, Ground-water resources of the Natchez area, 

Mississippi: U.S. Geological Survey Water-Resources Investigations Report 84-4341, 

53 p.

Callahan, J.A., Skelton, John, Everett, D.E., and Harvey, E.J., 1963, Water resources of 

Adams, Claiborne, Jefferson, and Warren Counties, Mississippi: Mississippi 

Industrial and Technological Research Commission Bulletin 63-1,48 p.

___1964, Available water for industry-Adams, Claiborne, Jefferson, and Warren 

Counties, Mississippi: Mississippi Industrial and Technological Research 

Commission Bulletin 64-1, 45 p.

Childress, S.C., Bograd, Michael, and Marble, J.C., 1976, Geology and man in Adams 

County, Mississippi: Mississippi Geological, Economic, and Topographical Survey 

Environmental Geology Series No. 4,188 p.

Fenneman, N.M., 1938, Physiography of the Eastern United States: New York, McGrawHiU Book Co., p. 67-83.

May, J.H., and Marble, J.C., 1976, General geology and mineral resources of the 

Mendenhall west quadrangle, Mississippi: Mississippi Geological, Economic and 

Topographical Survey, Map GQ 82-NW, 1 sheet.

Martin, Angel, Jr., and Whiteman, C.D., Jr., 1989, Geohydrology and regional groundwater flow of the coastal lowlands aquifer system in parts of Louisiana, Mississippi, 

Alabama, and Florida a preliminary analysis: U.S. Geological Survey WaterResources Investigations Report 88-4100,88 p.

Stephenson, L.W., Logan, W.N., and Waring, G.A., 1928, The ground-water resources of 

Mississippi, with a discussion o/The chemical character of the waters, by C.S. Howard: 

U.S. Geological Survey Water-Supply Paper 576,515 p.

Strom, E.W., and Oakley, W.T., 1995, Hydrogeology and analysis of ground-water 

withdrawal in the Mendenhall-D'Lo area, Simpson County, Mississippi: U.S. 

Geological Survey Water-Resources Investigations Report 95-4013,18 p.

Theis, C.V., 1935, The relation between the lowering of the piezometric surface and rate 

and duration of discharge of a well using groundwater storage: Transactions of the 

American Geophysical Union, v. 16, p. 519-524.

Vestal, F.E., 1942, Adams County mineral resources: Mississippi State Geological Survey 

Bulletin 47,200 p.

U.S. GOVERNMENT PRINTING OFFICE: 1995 655-177/00033 27