Prepared in cooperation with the

CITY OF MEMPHIS,

MEMPHIS LIGHT, GAS AND WATER DIVISION and the

TENNESSEE DEPARTMENT OF ENVIRONMENT AND CONSERVATION,

DIVISION OF WATER SUPPLY

Hydrogeology and Ground-Water Flow in the Memphis and Fort 

Pillow Aquifers in the Memphis Area, Tennessee

Water-Resources Investigations Report 89-4131

U.S. Department of the Interior

U.S. Geological Survey

science for a changing world
Cover photograph: Public-supply well in Shelby County, Tennessee. Photograph taken by L.B. Thomas, 

U.S. Geological Survey.
Hydrogeology and Ground-Water Flow in the Memphis and 

Fort Pillow Aquifers in the Memphis Area, Tennessee

By J.V. Brahana and R.E. Broshears

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 89-4131

Prepared in cooperation with the

CITY OF MEMPHIS,

MEMPHIS LIGHT, GAS AND WATER DIVISION and the

TENNESSEE DEPARTMENT OF ENVIRONMENT AND CONSERVATION,

DIVISION OF WATER SUPPLY

Nashville, Tennessee

2001
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

CHARLES G. GROAT, Director

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

purposes only and does not constitute endorsement by the 

U.S. Geological Survey.

For additional information write to: Copies of this report may be purchased from:

District Chief U.S. Geological Survey

U.S. Geological Survey Branch of Information Services

640 Grassmere Park, Suite 100 Federal Center

Nashville, Tennessee 37211 Box 25286

Denver, Colorado 80225
Contents iii

CONTENTS

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

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

Approach and scope..................................................................................................................................................... 2

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

Hydrologic setting ................................................................................................................................................................. 4

Climate and precipitation............................................................................................................................................. 4

Topography and drainage ............................................................................................................................................ 4

Hydrogeologic framework........................................................................................................................................... 4

Water-table aquifers........................................................................................................................................... 6

Alluvium.................................................................................................................................................. 6

Fluvial deposits........................................................................................................................................ 13

Memphis aquifer................................................................................................................................................ 13

Fort Pillow aquifer............................................................................................................................................. 15

McNairy-Nacatoch aquifer ............................................................................................................................... 22

Conceptualization of the ground-water flow system ............................................................................................................. 23

Simulation of the ground-water flow system......................................................................................................................... 26

Finite-difference grid ................................................................................................................................................... 26

Hydrologic parameters ................................................................................................................................................ 29

Initial head distributions .................................................................................................................................... 29

Boundary conditions.......................................................................................................................................... 29

Aquifer hydraulic properties.............................................................................................................................. 32

Pumping............................................................................................................................................................. 32

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

Model testing ............................................................................................................................................................... 42

Sensitivity analysis ...................................................................................................................................................... 46

Interpretation of model results..................................................................................................................................... 46

Hydrologic budget ............................................................................................................................................. 46

Areal distribution of leakage ............................................................................................................................. 49

Model limitations......................................................................................................................................................... 49

Summary and conclusions ..................................................................................................................................................... 52

Selected references ................................................................................................................................................................ 53
iv Hydrogeology and Ground-Water Flow in the Memphis and

Fort Pillow Aquifers in the Memphis Area, Tennessee

ILLUSTRATIONS

1. Map showing location of the Memphis area and hydrogeologic sections along 

lines A-A' and B-B' in the Mississippi embayment ................................................................................................ 3

2. Hydrogeologic section showing principal aquifers and confining units, 

west to east, through the Mississippi embayment along line A-A' ......................................................................... 5

3. Hydrogeologic section showing principal aquifers and confining units, 

south to north, through the Mississippi embayment along line B-B' ...................................................................... 7

4-11. Maps showing:

4. Generalized altitude of the water table in the alluvium and fluvial deposits in the Memphis area, 1980 ...... 11

5. Generalized thickness of the Jackson-upper Claiborne confining unit in the Memphis area ......................... 12

6. Generalized thickness of the Memphis aquifer in the Memphis area ............................................................. 14

7. Altitude of the potentiometric surface of the Memphis aquifer in the Memphis area, 1980 .......................... 16

8. Location of selected aquifer tests.................................................................................................................... 18

 9. Altitude of the potentiometric surface of the Fort Pillow aquifer in the Memphis area, 1980 ....................... 19

10. Generalized thickness of the Fort Pillow aquifer in the Memphis area .......................................................... 20

11. Generalized thickness of the Flour Island confining unit in the Memphis area.............................................. 21

12. Diagram showing relation between units of the geologic framework, the natural flow system 

of the conceptual model, and the simulated flow system of the ground-water flow model.................................... 24

13-23. Maps showing:

13. Areal geology of the northern Mississippi embayment .................................................................................. 25

14. Regional digital model representation of aquifer layer 2 (Memphis aquifer) 

in the northern Mississippi embayment .......................................................................................................... 27

15. Regional digital model representation of aquifer layer 3 (Fort Pillow aquifer) 

in the northern Mississippi embayment .......................................................................................................... 28

16. Estimated potentiometric surface of the Memphis aquifer prior to development in 1886.............................. 30

17. Estimated potentiometric surface of the Fort Pillow aquifer prior to development in 1924........................... 31

18. Model-derived storage coefficient of the Memphis aquifer............................................................................ 33

19. Model-derived transmissivity of the Memphis aquifer ................................................................................... 34

20. Model-derived leakance of the Jackson-upper Claiborne confining unit ....................................................... 35

21. Model-derived transmissivity of the Fort Pillow aquifer ................................................................................ 36

22. Model-derived storage coefficient of the Fort Pillow aquifer ......................................................................... 37

23. Model-derived leakance of the Flour Island confining unit............................................................................ 38

24. Graph showing actual and modeled pumpage from the Memphis aquifer and Fort Pillow 

aquifer in the Memphis area, 1886-1985 ................................................................................................................ 39

25-27. Maps showing:

25. Finite-difference grid in the Memphis study area showing location of 

pumping nodes and selected observation wells .............................................................................................. 41

26. Comparison of observed water levels and model-computed potentiometric 

surface of the Memphis aquifer, Memphis area, 1980.................................................................................... 43

27. Comparison of observed water levels and model-computed potentiometric surface 

of the Fort Pillow aquifer, Memphis area, 1980.............................................................................................. 44

28. Selected hydrographs of observed and model-computed water levels for wells in the 

Memphis aquifer and Fort Pillow aquifers in the Memphis area ............................................................................ 45

29-30. Graphs showing:

29. Relation between changes in magnitude of calibrated input (1980) parameters and root mean 

square error between observed and simulated water levels in the Memphis aquifer ...................................... 47

30. Relation between changes in magnitude of calibrated input (1980) parameters and root mean 

square error between observed and simulated water levels in the Fort Pillow aquifer ................................... 48

31. Map showing areas of significant vertical leakage in the Memphis area as determined by model calculations .... 51
Contents v

TABLES

 1. Post-Paleozoic geologic units underlying the Memphis area and their hydrologic significance............................ 8

2. Generalized ground-water characteristics and hydraulic properties of select 

hydrogeologic units in the Memphis area ............................................................................................................... 10

3. Results of selected aquifer tests .............................................................................................................................. 17

4. Water budget calculated by the flow model, 1980, for the Memphis area .............................................................. 50

CONVERSION FACTORS, VERTICAL DATUM, AND WELL-NUMBERING SYSTEM

 Multiply By To obtain 

foot (ft) 0.3048 meter (m)

foot per second (ft/s) 0.3048 meter per second (m/s)

foot per day (ft/d) 3.528 x 10-6 meter per second (m/s)

square foot per second (ft2

/s) 0.0929 square meter per second (m2/s)

cubic foot per second (ft3/s) 2.83 x 10-2 cubic meter per second (m3/s)

mile (mi) 1.609 kilometer (km)

square mile (mi2) 2.590 square kilometer (km2)

gallon per day (gal/d) 4.384 x 10-8 cubic meter per second (m3

/s)

gallon per minute (gal/min) 6.309 x 10-5 cubic meter per second (m3/s)

million gallons per day (Mgal/d) 4.384 x 10-2 cubic meter per second (m3/s)

gallon per day per foot [(gal/d)/ft] 1.438 x 10-7 square meter per second (m2/s)

inch per year (in/yr) 0.0254 meter per year (m/a)

Sea Level: In this report "sea level" refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929)—a geodetic 

datum derived from a general adjustment of the first-order level nets of both the United States and Canada, formerly called 

Sea Level Datum of 1929.

Well-Numbering System: Wells are identified according to the numbering system used by the U.S. Geological Survey 

throughout Tennessee. The well number consists of three parts: (1) an abbreviation of the name of the county in which the 

well is located; (2) a letter designating the 7-1/2-minute topographic quadrangle on which the well is plotted; and (3) a number generally indicating the numerical order in which the well was inventoried. The symbol Sh:U-2, for example, indicates 

that the well is located in Shelby County on the "U" quadrangle and is identified as well 2 in the numerical sequence. Quadrangles are lettered from left to right, beginning in the southwest corner of the county.

Abstract 1

Hydrogeology and Ground-Water Flow in the Memphis 

and Fort Pillow Aquifers in the Memphis Area, 

Tennessee

By J.V. Brahana and R.E. Broshears

ABSTRACT

On the basis of known hydrogeology of 

the Memphis and Fort Pillow aquifers in the 

Memphis area, a three-layer, finite-difference 

numerical model was constructed and calibrated 

as the primary tool to refine understanding of 

flow in the aquifers. The model was calibrated 

and tested for accuracy in simulating measured 

heads for nine periods of transient flow from 

1886-1985. Testing and sensitivity analyses 

indicated that the model accurately simulated 

observed heads areally as well as through time.

The study indicates that the flow system 

is currently dominated by the distribution of 

pumping in relation to the distribution of areally 

variable confining units. Current withdrawal of 

about 200 million gallons per day has altered 

the prepumping flow paths, and effectively captured most of the water flowing through the 

aquifers. Ground-water flow is controlled by 

the altitude and location of sources of recharge 

and discharge, and by the hydraulic characteristics of the hydrogeologic units.

Leakage between the Fort Pillow aquifer 

and Memphis aquifer, and between the Memphis aquifer and the water-table aquifers (alluvium and fluvial deposits) is a major component 

of the hydrologic budget. The study indicates 

that more than 50 percent of the water withdrawn from the Memphis aquifer in 1980 is 

derived from vertical leakage across confining 

units, and the leakage from the shallow aquifer 

(potential source of contamination) is not uniformly distributed. Simulated leakage was concentrated along the upper reaches of the Wolf 

and Loosahatchie Rivers, along the upper 

reaches of Nonconnah Creek, and the surficial 

aquifer of the Mississippi River alluvial plain. 

These simulations are supported by the geologic 

and geophysical evidence suggesting relatively 

thin or sandy confining units in these general 

locations. Because water from surficial aquifers 

is inferior in quality and more susceptible to 

contamination than water in the deeper aquifers, 

high rates of leakage to the Memphis aquifer 

may be cause for concern.

A significant component of flow (12 percent) discharging from the Fort Pillow aquifer 

was calculated as upward leakage to the Memphis aquifer. This upward leakage was generally 

limited to areas near major pumping centers in 

the Memphis aquifer, where heads in the Memphis aquifer have been drawn significantly 

below heads in the Fort Pillow aquifer. 

Although the Fort Pillow aquifer is not capable 

of producing as much water as the Memphis 

aquifer for similar conditions, it is nonetheless a 

valuable resource throughout the area.
2 Hydrogeology and Ground-Water Flow in the Memphis and

Fort Pillow Aquifers in the Memphis Area, Tennessee

INTRODUCTION

The Memphis area has a plentiful supply of 

ground water suitable for most uses, but the resource 

may be vulnerable to pollution. Withdrawal of nearly 

200 million gallons per day (Mgal/d) ranks Memphis 

second only to San Antonio, Texas, among the nation's 

cities that depend solely on ground water for 

municipal-water supply. For the past century, most of 

the city's ground water has been pumped from the 

Memphis aquifer, a Tertiary sand unit that is confined 

in most of the Memphis area. Industrial, public supply, 

and private withdrawals also have been made from the 

Fort Pillow aquifer, but these generally have amounted 

to less than 10 percent of the total pumping in the area.

There has been increasing concern that contaminated ground water in the area's surficial aquifers may 

leak downward to the Memphis aquifer (Parks and 

others, 1982; Graham and Parks, 1986; M.W. Bradley, 

U.S. Geological Survey, written commun., 1987). To 

assess the potential for such leakage, a cooperative 

investigation was initiated in 1978 between the City of 

Memphis, Memphis Light, Gas and Water Division 

(MLGW) and the U.S. Geological Survey. This investigation is part of a series of studies pursuing a more 

complete understanding of ground-water flow and 

chemistry in the area. The main tool of this investigation is a ground-water flow model of the major aquifers in the Memphis area. This flow model integrates 

all available information on the geology, hydrology, 

and ground-water chemistry of the region. The model 

has helped to quantify the potential for leakage 

between principal aquifers, and it may be a valuable 

predictive tool to assist water managers in managing 

ground-water resources.

Approach and Scope

The necessary approaches to this investigation 

were: 

1. to describe the hydrogeologic framework of the 

Memphis area, with emphasis on the Memphis 

aquifer and Fort Pillow aquifer;

2. to develop a conceptual model of ground-water 

flow in the Memphis area;

3. to test the conceptual model through the application 

of a multilayer, finite-difference ground-water flow 

model.

As defined for this investigation, the Memphis 

area comprises a rectangular zone of roughly 

1,500 square miles (mi2), measuring about 45 miles 

from east to west by 35 miles from north to south. The 

Memphis area lies near the center of the northern part 

of the Mississippi embayment and includes all of 

Shelby County, Tennessee, and parts of Fayette and 

Tipton Counties, Tennessee, DeSoto and Marshall 

Counties, Mississippi, and Crittenden and Mississippi 

Counties, Arkansas (fig. 1).

The study area includes all of metropolitan 

Memphis, as well as undeveloped, outlying areas 

where ground water is affected by pumping from metropolitan well fields. Although the study focuses on 

the Memphis area, the aquifers and confining units are 

regional in occurrence, and extend far beyond the 

Memphis area boundaries. Descriptions and maps necessary to define the regional hydrogeology are 

included within this report only as an aid to understanding ground-water flow in the Memphis area. 

Readers interested in a full discussion of the regional 

hydrogeology of the Memphis and Fort Pillow aquifers in the northern Mississippi embayment are 

referred to Arthur and Taylor (1990).

Previous Investigations

A substantial body of literature exists on the 

hydrology and hydrogeology of aquifer systems in the 

Memphis area. The most recent, comprehensive studies include those of Graham and Parks (1986), who 

studied the potential for leakage in the Memphis area, 

and Parks and Carmichael (1989a, 1989b, 1989c), who 

described the geology and ground-water resources of 

three aquifers in West Tennessee. Extensive bibliographies of previous ground-water studies are included in 

Brahana (1982a, table 2 and p. 35-40) and in Graham 

and Parks (1986, p. 41-44). A series of potentiometric 

maps and a description of historic water-level changes 

and pumpage from the Memphis aquifer and Fort Pillow aquifer in the Memphis area are included in Criner 

and Parks (1976). Historic water levels in individual 

wells are also documented by the U.S. Geological Survey (1936-1973). The potentiometric surface in the 

Memphis aquifer for 1978 and 1980 in the Memphis 

area is shown in Graham (1979, 1982), and for 1985 

for West Tennessee is shown in Parks and Carmichael 

(1989d). The potentiometric surface of the Fort Pillow 

aquifer for 1980 for the northern Mississippi embayment is shown in Brahana and Mesko (1988, fig. 11), 

and for 1985 for West Tennessee is shown in Parks and 

Carmichael (1989e, fig. 2).
I

ntroduction3

Crowleys

Ridge

Figure 1. Location of the Memphis area and hydrogeologic sections along lines A-A′ and B-B′ in the Mississippi embayment.
4 Hydrogeology and Ground-Water Flow in the Memphis and

Fort Pillow Aquifers in the Memphis Area, Tennessee

Water quality in aquifers in the Memphis area 

has been summarized by Brahana and others (1987), 

and data describing selected water-quality parameters 

in the water-table aquifers in the Memphis area have 

been described by McMaster and Parks (1988). Parks 

(1973, 1974, 1975, 1977b, 1978, 1979a, 1979b) 

mapped the surface and shallow subsurface geology of 

the Memphis metropolitan area. A summary of some 

current and possible future environmental problems 

related to geology and hydrology in the Memphis area 

is given in a report by Parks and Lounsbury (1976). 

Parks and others (1982) described the installation and 

sampling of observation wells at selected wastedisposal sites.

Analog simulation of water-level declines in the 

Sparta aquifer (equivalent to the upper part of the 

Memphis aquifer) in the Mississippi embayment was 

summarized by Reed (1972). A two-dimensional digital flow model of the Memphis aquifer was described 

by Brahana (1982a). This model was used as a predictive tool to estimate aquifer response to various hypothetical pumpage projections (Brahana, 1982b). Arthur 

and Taylor (1990) evaluated the Memphis and Fort 

Pillow aquifers (as part of the Mississippi embayment 

aquifer system) in a regional study that encompassed 

the northern Mississippi embayment. Fitzpatrick and 

others (1989) described the geohydrologic characteristics and digital model-simulated response to pumping 

stresses in the Sparta aquifer (equivalent to upper part 

of Memphis aquifer) in east-central Arkansas.

Reports describing the general geology and 

ground-water hydrology of the Memphis area include 

Fisk (1944), Schneider and Blankenship (1950), 

Caplan (1954), Stearns and Armstrong (1955), Stearns 

(1957), Cushing and others (1964), Krinitzsky and 

Wire (1964), Moore (1965), Boswell and others (1965, 

1968), Hosman and others (1968), and Cushing and 

others (1970). 

In addition to published reports, there is a substantial body of unpublished hydrogeologic data for 

the Memphis area. These data include borehole geophysical logs, well-completion data, driller's records, 

geologic logs, summaries of pumping tests, inventories of pumpage, and individual well records and maps 

of water levels. Most of these records are located in 

the files of the U.S. Geological Survey, Water 

Resources Division; Tennessee Division of Geology; 

Tennessee Division of Water Resources; and City of 

Memphis, Memphis Light, Gas and Water Division.

HYDROLOGIC SETTING

Climate and Precipitation

The Memphis metropolitan area is characterized 

by a temperate climate, with a mean annual air temperature of about 62o F, and abundant precipitation. 

About 48 inches of precipitation per year is typical, 

although annual amounts recorded have ranged from 

31 to 77 inches. 

The distribution of rainfall is nonuniform in 

space and time. Mean annual precipitation increases 

approximately 4 inches per year from west to east 

across the Mississippi embayment (Cushing and others, 1970). The driest part of the year is late summer 

and fall, and the wettest is late winter.

Topography and Drainage

Land-surface altitudes in the Memphis area 

range from about 200 feet above sea level on the flat 

alluvial plain of the Mississippi River to about 

400 feet above sea level in the upland hills of eastern 

Shelby County. A bluff 50 to 150 feet high separates 

the alluvial plain from the upland. Other than the bluff, 

local relief seldom exceeds 40 feet.

The Mississippi River dominates surface-water 

flow in the area. From the upland in the east, it 

receives drainage from three main tributary streams—

Nonconnah Creek, Wolf River, and Loosahatchie 

River. Along most reaches, these three tributaries flow 

throughout the year. One notable exception is Nonconnah Creek upstream from the mouth of Johns Creek. 

Since the 1950's, Nonconnah Creek has been dry in its 

upstream reaches for short periods during the dry season from July to October (Criner and others, 1964). 

Hydrogeologic Framework

The Memphis area is located near the axis of the 

Mississippi embayment, a regional downwarped 

trough of Paleozoic rock that has been filled with more 

than 3,000 feet of unconsolidated sediments (Criner 

and Parks, 1976). These sediments include uncemented sand, clay, silt, chalk, gravel, and lignite. On a 

regional scale, the sediments form a sequence of 

nearly parallel, sheetlike layers of similar lithology. 

The layers reflect the trough-like shape of the Paleozoic strata (fig. 2).
Hydrologic Setting 5
6 Hydrogeology and Ground-Water Flow in the Memphis and

Fort Pillow Aquifers in the Memphis Area, Tennessee

On a local scale, however, there are complex lateral and vertical gradations in the lithology of each 

layer. Of particular interest to this study are variations 

in thickness and sand percentage of the major clay layers. These confining clay units control the groundwater interchange between the sand layers that form 

the major aquifers. Zones where the confining clays 

are thin or sandy are potential sites of high leakage, 

and the most likely pathways for pollutant migration 

(Graham and Parks, 1986). 

The structural axis of the northern Mississippi 

embayment is approximately coincident with the Mississippi River, passing south-southwest through the 

western part of the study area in eastern Crittenden 

County, Ark. (fig. 1). The sedimentary rock layers 

which comprise the embayment gently dip 10 to 

35 feet per mile from both the west and east toward the 

axis of the embayment (fig. 2). These layers thicken to 

the south-southwest (fig. 3).

The thickness, lithology, and hydrologic significance of each stratigraphic unit in the Memphis area 

are described briefly in table 1. Five of these units represent major water-bearing zones: the alluvium, the 

surficial fluvial deposits, the Memphis Sand, the Fort 

Pillow Sand, and the Ripley Formation and McNairy 

Sand. With the exception of the alluvium and fluvial 

deposits, water-bearing zones are confined by clay 

layers over much of the Memphis area. Reported 

ground-water conditions and hydraulic characteristics 

of selected units that are the focus of this report have 

been generalized in table 2.

Water-Table Aquifers

Water-table aquifers in the Memphis area consist of the alluvium and fluvial deposits which are 

mostly unconfined (Graham and Parks, 1986, p. 5). 

These aquifers outcrop throughout the study area, and 

generally occur at shallow depths (table 2).

An interpretive water-table map of the alluvium 

and fluvial deposits was constructed for "average," 

steady-state conditions, designated 1980 (fig. 4). The 

map was based on the most complete set of water-level 

data available (Graham and Parks, 1986), supplemented by historic water-levels (Wells, 1933), stream 

stages, and where no other data were available, estimates based on topographic maps, land surface elevations, and extrapolated depths to water (Brahana and 

Mesko, 1988).

Alluvium

Alluvium occurs at land surface in the stream 

valleys of the study area. The alluvium is not a major 

ground-water source in the Memphis area, even 

though it is a major water-bearing zone and can supply 

large quantities of water to wells. This lack of use is 

related to its limited area of occurrence and to the 

hardness and high iron concentration of the water. 

West, north, and south of the study area, the alluvium 

of the Mississippi River alluvial plain is one of the 

most productive regional aquifers in the Mississippi 

embayment, supplying over a billion gallons per day 

to irrigation wells in Arkansas and Mississippi 

(Boswell and others, 1968; Ackerman, 1989).

The thickness of the alluvium may vary significantly over very short distances (Krinitzsky and Wire, 

1964). In the Mississippi River alluvial plain, which 

lies west of the bluffs (fig. 4), the alluvium is commonly 100 to 175 feet thick (Boswell and others, 

1968); along valleys of upland streams tributary to the 

Mississippi River east of the bluffs (fig. 4), thickness 

generally is less than 50 feet (Graham and Parks, 

1986). Alluvium includes gravel, sand, silt, and clay; 

the latter is commonly rich in organic matter. Abrupt 

vertical and horizontal variations in lithology are 

common.

The alluvium is separated from the Memphis 

aquifer by a confining unit made up of clays and finegrained sediments of the Jackson Formation and 

underlying upper part of the Claiborne Group, which 

has variable thickness and lithology. Where this confining unit is thin or sandy, leakage of ground water 

from one aquifer to the other may be substantial. The 

generalized thickness of this confining unit is shown 

in figure 5.

Rivers dominate the hydrology of the watertable aquifers. Local streams, as shown by figure 4, are 

in direct hydraulic connection with these aquifers, 

functioning as drains during much of the year. Seasonal variations of water level in the alluvium are typically less than 10 feet, although variations of as much 

as 15 feet have been reported (Plebuch, 1961; Broom 

and Lyford, 1981; Brahana and Mesko, 1988, fig. 13). 

During floods when stream stage is temporarily higher 

than the water table, some recharge to the alluvium 

occurs. No long-term declines in water level in the 

alluvium in the Memphis area are known.

Aquifer hydraulic characteristics of the Mississippi River alluvial aquifer in Arkansas and Missouri 

have been reported by Halberg and Reed (1964), Albin 
H

ydrologicSetting7

MISSISSIPPI

TENNESSEE

Figure 3. Hydrogeologic section showing principal aquifers and confining units, south to north, through the Mississippi embayment along line B-B′.
8 Hydrogeology and Ground-Water Flow in the Memphis and

Fort Pillow Aquifers in the Memphis Area, Tennessee

Table 1. Post-Paleozoic geologic units underlying the Memphis area and their hydrologic significance[Modified frcm Criner and Parks, 1976; Moore and Brown, 1969; Plebuch, 1961; Schneider and Blankenship, 1950]

System Series Group Stratigraphic

unit

Thickness Hydrologic unit Lithology and hydrologic significance

Quaternary

Holocene and

Pleistocene Alluvium 0-175

Surficial Aquifer

Sand, gravel, silt, and clay. Underlies the Mississippi Alluvial Plain and alluvial 

plains of streams in the Gulf Coastal Plain. Thickest beneath the Alluvial Plain, where commonly between 100 and 150 feet thick; generally less than 50 feet thick elsewhere. Provides water to farm, industrial, and irrigation wells in the Mississippi Alluvial Plain.

Pleistocene Loess 0-65

Silt, silty clay, and minor sand. Principal unit at the surface in upland areas of the 

Gulf Coastal Plain. Thickest on the bluffs that border the Mississippi Alluvial Plain; thinner eastward from the bluffs. Tends to retard downward movement of water-providing recharge to the fluvial deposits.

Quaternary 

and

Tertiary(?)

Pleistocene and

Pliocene (?)

Fluvial Deposits

(terrace deposits) 0-100

Sand, gravel, minor clay and ferruginous sandstone. Generally underlies the loess 

in upland areas, but are locally absent. Thickness varies greatly because of erosional surfaces at top and base. Provides water to many domestic and farm wells in rural areas.

Tertiary

Eocene

Jackson Formation 

and upper part of 

Claiborne Group 

(“capping clay”)

0-370 Confining Unit

Clay, silt, sand, and lignite. Because of similarities in lithology, the Jackson Formation and upper part the Claiborne Group cannot be reliably subdivided based on 

available information. Most of the preserved sequence is equivalent to the Cook Mountain and overlying Cockfleld Formations, but locally the Cockfield may be overlain by the Jackson Formation. Serves as the upper confining unit for the Memphis Sand.

Claiborne

Memphis Sand

(“500-foot” sand) 500-890 Memphis aquifer

Sand, clay, and minor lignite. Thick body of sand with lenses of clay at various 

stratigraphic horizons and minor lignite. Thickest in the southwestern part of the Memphis area; thinnest in the northeastern part. Principal aquifer providing water for municipal and industrial supplies east of the Mississippi River; primary source of water for the City of Memphis.

Wilcox

Flour Island

Formation 140-310 Confining unit

Clay, silt, sand, and lignite. Consists primarily of silty clays and sandy silts with 

lenses and interbeds of fine sand and lignite. Serves as the lower confing unit for the Memphis Sand and the upper confining unit for the Fort Pillow Sand.

Paleocene

Fort Pillow Sand

(“1400-foot” sand) 92-305 Fort Pillow aquifer

Sand with minor clay and lignite. Sand is fine to medium. Thickest in the southwestern part of the Memphis area; thinnest in the northern and northeastern 

parts. Once the second principal aquifer supplying the City of Memphis; still used by an industry. Principal aquifer providing water for municipal and industrial supplies west of the Mississippi River.

Old Breastworks

Formation 180-350 Midway confining unit Clay, silt, sand, and lignite. Consists primarily of silty clays and clayey silts with lenses and interbeds of fine sand and lignite. Serves as the lower confining unit for the Fort Pillow Sand, along with the underlying Porters Creek Clay, Clayton 

Formation, and Owl Creek Formation.

?

?
Hydrologic Setting 9

System Series Group Stratigraphic

unit

Thickness Hydrologic unit Lithology and hydrologic significance

Tertiary Paleocene Midway

Porters Creek Clay 250-320

Midway confining 

unit

Clay and minor sand. Thick body of clay with local lenses of clayey, glauconitic 

sand. Principal confining unit separating the Fort Pillow Sand and the Ripley Formation and McNairy Sand.

Clayton Formation 40-120

Clay, sand, and minor limestone. Calcareous clay and glauconitic sand with local 

lenses of limestone in basal part; fossiliferous. Because of lithologic similarities, upper boundary is difficult to recognize. Confining unit.

Cretaceous Upper Cretaceous

Owl Creek Formation 40-90

Clay and sand. Calcareous clay and glauconitic sand; fossiliferous. Because of 

lithologic similarities, the Owl Creek Formation is difficult to distinguish from the overlying Clayton Formation without fossil verification. Confining unit.

Ripley Formation

and McNairy Sand 360-570 McNairy-Nacatoch aquifer

Sand and clay; minor sandstone, limestone, and lignite. Ripley changes facies 

northeast of Memphis to McNairy Sand. Ripley consists primarily of glauconitic sands and calcareous clays with minor interbeds of calcareous sandstone or sandy limestone; McNairy consists primarily of nonglauconitic sands and noncalcareous clays with local lenses of lignite. Aquifer with low potential for use in Memphis area because of lesser amounts of sand and poorer quality of water than aquifers above. Base of Ripley and McNairy is base of freshwater in the Memphis area.

Coon Creek

Formation 0-60

Confining unit

Clay and sand. Shaley clays with thin interbeds of fine sand; locally glauconitic and 

fossiliferous; locally contains some thin layers of rock. Probably present only in northeastern Shelby and northwestern Fayette Counties, Tenn. Confining unit.

Demopolis

Formation 270-390

Clay and chalk. Calcareous clays and chalks; glauconitic and fossiliferous. Some 

layers of chalk form indurated layers. Serves as the principal confining unit separating the Ripley Formation and McNary Sand and Coffee Sand.

Coffee Sand 0-120 Coffee aquifer

Sand and minor clay. Sand is fine to medium; locally glauconitic or lignitic. Clay 

occurs as local lenses, particularly at the base. Absent locally in north-central Shelby County, Tenn., where the Demopolis Formation overlies igneous intrusive rock. Contains brackish or saline water; not considered a freshwater aquifer in the Memphis area. Underlain by Paleozoic dolomitic limestones of Ordovician age.

Table 1. Post-Paleozoic geologic units underlying the Memphis area and their hydrologic significance—Continued

?

?
10 Hydrogeology and Ground-Water Flow in the Memphis and

Fort Pillow Aquifers in the Memphis Area, Tennessee

Table 2. Generalized ground-water characteristics and hydraulic properties of select hydrogeologic units in the Memphis area(a) Results from test conducted in the northern Mississippi Embayment, see table 3.(b) Results for the Memphis area from Criner and others, 1964; Moore, 1965; Hosman and others, 1968; Brahana, 1982a; Arthur and Taylor, 1990; and Parks and Carmichael, 1989a.

Hydrogeologic 

unit

Generalized 

present-day 

flow directions

Depth commonly encountered (feet) Thickness (feet) Water-bearing character Hydraulic properties of unit T (ft2/d) S (unitless) K′ (ft/d)

Alluvium Toward major 

streams—downstream.

Surface 0-175 Unconfined aquifer 

Mississippi River alluvium confined in many places.

8,500-50,000 (a) 1x10-4 to 4x10-2 (a) --

Terrace (fluvial) 

deposits.

To valleys Surface 0-100 Unconfined aquifer No measurements No measurements --

Jackson Formation and upper 

part of Claibome Group (capping clay).

-- 0-100 0-370 Confining layer -- -- No measurements

Memphis Sand Into pumping 

center

0-600

500 common

500-890 Confined aquifer in 

most of Memphis area; unconfined in southeast part of area.

2,700-45,000 (a)6,700-54,000 (b)

1x10-4 to 6x10-4 (a)

1x10-4 to 2x10-1 (b)

--

Flour Island 

Formation

-- 1,000-1,400 140-310 Confining layer -- -- .8-4.4x10-11

Fort Pillow Sand Into pumping 

center, primarily east to west.

 1,200-1,500

1,400 common

92-305 Confined aquifer 2,700-21,000 (a)

12,000-19,000 (b)

2x10-4 to 2x10-3 (a)

1.2x10-4 to 6.1 x10-4 (b)

--

Porters Creek 

Clay, Clayton and Owl Creek Formations.

-- 1,400-1,700 150-770 Confining layer -- -- No measurements

McNairy Sand Southeast to 

northwest

2,650 360-430 Confined aquifer No measurements No measurements --
Hydrologic Setting 11
12 Hydrogeology and Ground-Water Flow in the Memphis and

Fort Pillow Aquifers in the Memphis Area, Tennessee
Hydrologic Setting 13

and Hines (1967), Broom and Lyford (1981), and 

Luckey (1985). Transmissivity ranges from 8,500 to 

50,000 ft2/d, and storage coefficient for the deeper, 

more confined part of the aquifer ranges from 1 x 10-4

to 4 x 10-2 (table 2). No values of aquifer hydraulic 

characteristics of alluvium at other locations in the 

Memphis area have been reported.

Water from the alluvium is hard and has relatively high concentrations of iron, dissolved solids, 

and barium (Brahana and others, 1987, tables 2 and 3). 

Lenses of clay rich in organic matter and associated 

geomicrobial activity are thought to be the source of 

high concentrations of hydrogen sulfide, carbon dioxide, and iron in this formation (Wells, 1933).

Fluvial Deposits

Fluvial deposits occur at land surface in the 

uplands east of the bluffs (fig. 4). Although at one time 

the fluvial deposits were an important source of 

domestic water, present pumpage from this formation 

is negligible. Since about 1950, when the city of Memphis expanded its municipal supplies to serve outlying 

areas, few wells have been drilled into the fluvial 

deposits. Many of the wells that existed in 1950 have 

not remained operational and have been abandoned, 

plugged, or destroyed. Wells in the fluvial deposits are 

capable of large yields, greater than 100 gal/min, signifying a potentially large source of water in the study 

area.

Fluvial deposits range in thickness from 0 to 

100 feet (table 1). Thickness is highly variable, 

because of surfaces at both top and base (Graham and 

Parks, 1986). Locally, the fluvial deposits may be 

absent. The lithology of fluvial deposits is primarily 

sand and gravel, with minor layers of ferruginous 

sandstone.

Fluvial deposits are separated from the Memphis aquifer by sediments of the Jackson Formation 

and the upper part of the Claiborne Group (fig. 5). As 

with the alluvium, if the underlying confining unit is 

thin or sandy, leakage between water-table aquifers 

and the Memphis aquifer may be substantial.

Wells (1933), Graham (1982), and Graham and 

Parks (1986, fig. 8) reported seasonal water-level fluctuations in the fluvial deposits in the range of from 2 to 

10 feet. Long-term declines of water levels within the 

fluvial deposits have not been documented, except in 

one location in the southern part of Sheahan well field 

(fig. 4). During the period 1943 to 1955, pumpage from 

the Memphis aquifer in the south Sheahan area dewatered the fluvial deposits around the southern part of 

the well field (Graham and Parks, 1986, figs. 7 and 8). 

Before pumping began in 1933 from the Sheahan well 

field, the fluvial deposits in the southern part of the 

well field supplied small domestic wells, but these 

wells were reported to be dry in 1985 (W.S. Parks, U.S. 

Geological Survey, written commun., 1985).

No measurements of aquifer hydraulic characteristics have been reported for the fluvial deposits in 

the Memphis area. Based on lithology, saturated thickness, and mode of occurrence, transmissivity probably 

is within the range of 5,000 to 10,000 ft2/d, and storage coefficient probably is in the range of 0.1 to 0.2 

(Freeze and Cherry, 1979).

Water quality in the fluvial deposits is highly 

variable. The distribution of dissolved-solids concentrations, which ranges from 76 mg/L iron to 440 mg/L, 

shows more variation in these deposits than in any 

other aquifer in the area (Brahana and others, 1987, 

tables 2 and 3). Some of the variation may be related 

to the thickness of overlying loess, which may contribute much of the dissolved solids in the aquifer (Wells, 

1933). Dissolved-solids concentrations are lowest in 

the east-central part of the Memphis area, between the 

Loosahatchie and Wolf Rivers (Brahana and others, 

1987, fig. 5).

Memphis Aquifer

The Memphis aquifer is the most productive 

aquifer in the study area, providing approximately 

98 percent of total pumpage (188 Mgal/d) to the city 

of Memphis in 1980 (Graham, 1982). Total pumpage 

since 1886 is calculated to be more than 3.2 trillion 

gallons, using published pumping values (Criner and 

Parks, 1976, fig. 2; Graham, 1982, table 2).

The Memphis aquifer is a fine- to coarsegrained sand interbedded with layers of clay and 

minor amounts of lignite. The formation occurs at 

depths ranging from 0 to 600 feet (table 2) and varies 

in thickness from 500 to 890 feet (table 1) based on 

interpretations of geophysical logs. Generalized thickness of the Memphis aquifer in the Memphis area, 

based on work by Parks and Carmichael (1989a), has 

been extrapolated to a slightly wider range from less 

than 500 to more than 900 feet (fig. 6).

The Memphis aquifer is separated from the 

underlying Fort Pillow aquifer by 140 to 310 feet of 

clay of the Flour Island Formation, and from the overlying alluvium and terrace deposits by 0 to 370 feet of 

clay and sandy clay of the Jackson Formation and 
14 Hydrogeology and Ground-Water Flow in the Memphis and

Fort Pillow Aquifers in the Memphis Area, Tennessee
Hydrologic Setting 15

upper part of the Claiborne Group. The effectiveness 

of the Jackson Formation and upper part of the Claiborne Group as a confining unit appears to vary 

because of areal differences in sand content and layer 

thickness (Graham and Parks, 1986). Due to this variability, rates of leakage from surficial aquifers are spatially heterogeneous. 

Water levels in the Memphis aquifer are 

strongly influenced by pumping (fig. 7). Water levels 

within the outcrop area, which occurs in the southeastern part of the Memphis area, range from about 280 to 

290 feet above sea level (Graham, 1982, plate 1; Parks 

and Carmichael, 1989a, fig. 7). Recharge to the Memphis aquifer occurs primarily in the outcrop area 

(fig. 7). The deepest pumping cone of depression in 

the Memphis aquifer is less than 100 feet above sea 

level; the water levels at most other pumping centers 

are in the range of 120 to 170 feet above sea level 

(Graham, 1982, plate 1; Parks and Carmichael, 1989a, 

fig. 7). The widespread and irregular distribution of 

pumping centers in the Memphis aquifer in the Memphis area causes a complex flow pattern as ground 

water flows inward from all directions to several 

pumping centers (fig. 7).

Long-term water-level declines in the Memphis 

aquifer are greater than 120 feet in the area of maximum drawdown near the Mallory well field. East of 

the pumping centers near the areas of outcrop, longterm declines have not been detected (Parks and Carmichael, 1989a, fig. 10). Seasonal variations in water 

levels are commonly less than 2 feet in areas unaffected by pumping. 

Data from 23 representative aquifer tests in the 

Memphis aquifer (table 3; fig. 8) from throughout the 

northern Mississippi embayment show transmissivity 

ranges from 2,700 to 45,000 ft2

/d, and storage coefficients range from 1 x 10-4 to 6 x 10-4. Confined conditions are typical for the Memphis aquifer, except in 

areas of outcrop.

The Memphis aquifer in the Memphis area 

(table 2) is reported to have a range of transmissivity 

from 6,700 to 54,000 ft2/d, and a range of storage 

coefficients from 1 x 10-4 to 2 x 10-1 (Criner and others, 1964; Moore, 1965; Hosman and others, 1968; 

Brahana, 1982a; Arthur and Taylor, 1990; Parks and 

Carmichael, 1989a, p. 27).

Ground water in the Memphis aquifer is a calcium-magnesium-sodium bicarbonate type (Hosman 

and others, 1968; Brahana and others, 1987, table 2). 

In the study area, water in the Memphis aquifer is 

characterized by a pH generally less than 7, and except 

for a limited area in the northwestern part of the study 

area, the dissolved-solids concentration is generally 

less than 100 mg/L. 

Fort Pillow Aquifer

The Fort Pillow aquifer is a major regional aquifer throughout much of the northern Mississippi 

embayment (Hosman and others, 1968; Arthur and 

Taylor, 1990; Parks and Carmichael, 1989b). In the 

Memphis study area, the Fort Pillow aquifer currently 

(1989) provides water to supplement supplies at Millington, Tenn., the U.S. Naval Air Station near Millington, one industrial user in Memphis, and the Shaw 

well field east of Memphis (fig. 9). The Fort Pillow 

aquifer is the sole source of water for West Memphis, 

Marion, and other small towns in eastern Arkansas, 

and for the town of Walls in Mississippi (fig. 9). In 

1984, pumpage from the Fort Pillow aquifer averaged 

about 10 Mgal/d (Graham and Parks, 1986). Although 

the Fort Pillow aquifer is much deeper in the subsurface than the Memphis aquifer, the Fort Pillow is the 

preferred aquifer in eastern Arkansas for municipal 

and domestic supplies because it provides water that 

requires less treatment than water from the Memphis 

aquifer.

The Fort Pillow aquifer is characteristically a 

fine- to medium-grained sand containing clay lenses 

and minor amounts of lignite. Thickness of the aquifer 

is commonly about 250 feet and ranges from about 

125 to 305 feet (table 1). The generalized thickness of 

the Fort Pillow aquifer in the Memphis area, based on 

work of Parks and Carmichael (1989b), is shown in 

figure 10.

The Fort Pillow aquifer is confined above by 

140 to 310 feet of clay of the Flour Island Formation, 

as defined by interpretation of geophysical logs 

(table 1). The Flour Island Formation is thought to be 

a leaky confining unit. Generalized thickness of the 

Flour Island confining unit in the Memphis area is 

based on the work of Graham and Parks (1986, fig. 5) 

and E. Mahoney, Vanderbilt University (written commun., 1989) (fig. 11). Head differences between the 

Memphis aquifer and Fort Pillow aquifer (Graham and 

Parks, 1986) occur as a result of pumping and are 

affected by the vertical hydraulic characteristics and 

thickness of the Flour Island Formation.

Water levels in the Fort Pillow aquifer (fig. 9) in 

1980 were from slightly less than 160 to more than 

240 feet above sea level. Water levels are highest in 
16 Hydrogeology and Ground-Water Flow in the Memphis and

Fort Pillow Aquifers in the Memphis Area, Tennessee
Hydrologic Setting 17

Table 3. Results of selected aquifer tests

[Data source: 1, Davis and others (1973); 2, Moore (1965); 3, Newcome (1971); 4, Hosman and others (1968); 5, Luckey (1985); 6, Broom and Lyford 

(1981); 7, Albin and Hines (1967); 8, Halberg and Reed (1964); --, not reported; ft2

/d, square feet per day; ft/d, feet per day]

Test no. Location Transmissivities (T) Hydraulic Storage Water-bearing Data

(keyed to (ft2/d) conductivity coefficient formation source

fig. 8) (K) (ft/d) (S)

1 Mayfleld, Ky. 37,000-41,000 -- 0.0001-0.0004 Memphis Sand 1

2 Union City, Tenn. 8,300 -- .0003 Memphis Sand 1

3 Tiptonville, Tenn. 18,000 -- .0003 Memphis Sand 2

4 Dresden, Tenn. 7,200 -- .0006 Memphis Sand 2

5 Kenton, Tenn. 15,000 -- -- Memphis Sand 2

6 Dyersburg, Tenn. 19,000 -- .0004 Memphis Sand 2

7 Milan, Tenn. 16,000 -- -- Memphis Sand 2

8 Ripley, Tenn. 22,000 -- -- Memphis Sand 2

9 Bells, Tenn. 5,600 -- .0005 Memphis Sand 2

10 Covington, Tenn. 29,000 -- -- Memphis Sand 2

11 Stanton, Tenn. 27,000 -- .0001 Memphis Sand 2

12 Arlington, Tenn. 21,000 -- Memphis Sand 2

13 Memphis, Tenn. 41,000 -- .0014 Memphis Sand 2

14 Somerville, Tenn. 2,700 -- -- Memphis Sand 2

15 Memphis (McCord), Tenn. 43,000 -- .0002 Memphis Sand 2

16 Memphis (Mallory), Tenn. 26,000 -- Memphis Sand 2

17 Memphis, Tenn. 45,000 -- Memphis Sand 2

18 Memphis (Sheahan), Tenn. 35,000 -- Memphis Sand 2

19 Memphis (Allen), Tenn. 31,000 -- Memphis Sand 2

20 Memphis (Lichterman), Tenn. 27,000 -- Memphis Sand 2

21 Germantown, Tenn. 23,000 -- Memphis Sand 2

22 Collierville, Tenn. 23,000 -- Memphis Sand 2

23 Clarksdale, Miss. 6,600 100 .0006 Memphis Sand 3

24 Blytheville, Ark. 21,000 -- .002 Fort Pillow Sand 4

25 Memphis (Mallory), Tenn. 17,000-19,000 -- .0002-.0006 Fort Pillow Sand 4

26 Madison Co., Tenn. 10,000 -- .0015 Fort Pillow Sand 4

27 Marks, Miss. 2,700 29 -- Fort Pillow Sand 3

28 Stoddard Co., Mo. 15,000 -- .002 Alluvium 5

29 Stoddard Co., Mo. 20,000 -- .001 Alluvium 5

30 Wayne Co., Mo. 47,000 -- .0009 Alluvium 5

31 Butler Co., Mo. 50,000 -- .001 Alluvium 5

32 Clay Co., Ark. 30,000 360 .0011 Alluvium 6

33 Jackson Co., Ark. 39,000 320 .022 Alluvium 7

34 Craighead Co., Ark. 37,000 380 .022 Alluvium 6

35 Jackson Co., Ark. 8,500 -- -- Alluvium 6

36 Jackson Co., Ark. 10,000 100 .007 Alluvium 6

37 Poinsett Co., Ark. 48,000 390 .001 Alluvium 6

38 St. Francis Co., Ark. 43,000 330 .04 Alluvium 8

39 Lee Co., Ark. 13,000-19,000 130 .00073 Alluvium 6

40 Monroe Co., Ark. 24,000 -- -- Alluvium 6

41 Monroe Co., Ark. 32,000 290 .0004 Alluvium 6

42 Phillips Co., Ark. 34,000 247 .0001 Alluvium 6
18 Hydrogeology and Ground-Water Flow in the Memphis and

Fort Pillow Aquifers in the Memphis Area, Tennessee
Hydrologic Setting 19
20 Hydrogeology and Ground-Water Flow in the Memphis and

Fort Pillow Aquifers in the Memphis Area, Tennessee
Hydrologic Setting 21
22 Hydrogeology and Ground-Water Flow in the Memphis and

Fort Pillow Aquifers in the Memphis Area, Tennessee

the eastern part of the area, nearest the outcrop, and 

lowest in the west near the centers of pumping. The 

regional movement of ground water in the Fort Pillow 

aquifer is toward the axis of the Mississippi embayment (Hosman and others, 1968). 

The hydrograph for well Fa:R-1 (location on 

fig. 9), which taps the Fort Pillow aquifer about 

27 miles east of the center of pumping at Memphis, 

shows a long-term decline of about 0.4 foot per year 

(ft/yr) (Graham, 1982). Regionally, declines of about 

1 ft/yr are not uncommon (Hosman and others, 1968; 

Brahana and Mesko, 1988, fig. 13). Graham (1982) 

noted that the hydrograph of well Sh:O-170 (location 

on fig. 9) near the center of historic pumping in Memphis showed approximately 20 feet of recovery when 

all municipal (MLGW) pumpage from the Fort Pillow 

aquifer ceased in the early 1970's. Seasonal variations 

of nonstressed water levels are commonly less than 

2 feet (Graham, 1982, fig. 4).

Hydraulic conductivity of the Fort Pillow aquifer throughout its area of occurrence in the northern 

Mississippi embayment is reported to range from 25 to 

470 ft/d. This corresponds to a range of transmissivity 

from about 670 to 85,000 ft2/d. Storage coefficient is 

reported to range from 2 x 10-4 to 1.5 x 10-2 (Hosman 

and others, 1968; Boswell, 1976; Parks and Carmichael, 1989b). Data from aquifer tests of the Fort 

Pillow aquifer (table 3, fig. 8) indicate that transmissivity ranges from 2,700 to 21,000 ft2/d, and storage 

coefficients range from 2 x 10-4 to 2.0 x 10-3.

Within the Memphis area, hydraulic characteristics have a narrower range (table 2) than described 

previously for the entire embayment. In the Memphis 

area, transmissivity of the Fort Pillow aquifer is 

reported to range from 12,000 to 19,000 ft2

/d, and 

storage coefficient is reported to range from 1.2 x 10-4

to 6.1 x 10-4 (Criner and others, 1964).

Water from the Fort Pillow aquifer is a soft, 

sodium bicarbonate type with a median dissolvedsolids concentration of 116 mg/L (Brahana and others, 

1987). Iron concentrations range from 170 to 

1,900 micrograms per liter, and pH typically is about 

7.4.

McNairy-Nacatoch Aquifer

The McNairy-Nacatoch aquifer, which encompasses sands of the Ripley Formation, McNairy Sand 

(table 1), and equivalent Upper Cretaceous Nacatoch 

Sand in Arkansas, is the basal freshwater aquifer in the 

study area. The McNairy-Nacatoch aquifer has not 

been used as a source of water supply in Memphis, but 

it has the potential for such use; north and east of the 

study area, it is a major regional aquifer (Brahana and 

Mesko, 1988).

The McNairy-Nacatoch aquifer ranges in thickness from 360 to 570 feet and is fine- to coarsegrained, glauconitic sand. The McNairy-Nacatoch 

aquifer occurs deeper than 2,500 feet below land surface at Memphis, and is confined and hydraulically 

separated from the overlying Fort Pillow Sand by 

about 750 feet of clays of the Midway and lower Wilcox Groups (table 1). These confining clays, herein 

called the Midway confining unit, are a major hydrologic boundary in the northern Mississippi embayment. Arthur and Taylor (1990) simulated the Midway 

confining unit as a lower no-flow boundary. Brahana 

and Mesko (1988) used flow modeling to evaluate 

leakage across the Midway confining unit; they found 

less than 0.5 ft3/s moved across this confining unit in 

the study area.

Hydrogeologic evaluation of the McNairyNacatoch aquifer in the Memphis area is based on 

unpublished data from a single observation well in the 

Mallory well field and on extrapolation of regional 

data (Boswell and others, 1965; Davis and others, 

1973; Luckey and Fuller, 1980; Edds, 1983; Brahana 

and Mesko, 1988). The static water level in this well is 

approximately 350 feet above sea level, which is about 

100 feet above land surface (W.S. Parks, U.S. Geological Survey, written commun., 1985). Seasonal variation in water level is about 2 feet, and no long-term 

decline is evident. Head values in the McMairyNacatoch aquifer are approximately 180 feet higher 

than heads measured in the overlying Fort Pillow aquifer (Brahana and Mesko, 1988, figs. 10 and 11). 

Water-level declines in the McNairy-Nacatoch aquifer 

due to pumping in the overlying Fort Pillow aquifer 

have not been observed.

In addition to head differences, significant differences in water quality exist between the McNairyNacatoch aquifer and the Fort Pillow aquifer. Concentrations of dissolved solids, for example, are 10 times 

greater in the McNairy-Nacatoch aquifer than in the 

Fort Pillow aquifer. 

Although the data from the McNairy-Nacatoch 

aquifer are sparse, they are consistent on both a local 

and regional scale. These differences in hydrology and 

water chemistry strongly support the contention that 

clays in the Midway confining unit (Porters Creek 

Clay, Clayton Formation, and Owl Creek Formation, 
Conceptualization of the Ground-Water Flow System 23

table 2) act as an effective confining unit (figs. 2 

and 3), and isolate the Fort Pillow aquifer from deeper 

aquifers.

CONCEPTUALIZATION OF THE 

GROUND-WATER FLOW SYSTEM

The hydrogeologic information presented in the 

previous section forms the basis for a conceptual 

model of ground-water flow in the Memphis area. This 

conceptualization accounts for the ability of each 

major unit to store and transmit water, as indicated by 

its lithology and stratigraphy, and by hydrologic data. 

Water-quality data are also used to lend credence to 

hypotheses regarding the hydrologic isolation or communication between aquifers. The conceptual model 

represents a simplification of reality but preserves and 

emphasizes the major elements controlling groundwater flow in the study area. This conceptual model 

can be tested quantitatively by depicting each of its 

elements mathematically in a digital model of groundwater flow. The relation between the hydrogeologic 

framework, the conceptual model, and the digital 

ground-water flow model is shown in figure 12.

The alluvium and fluvial deposits form the 

uppermost water-table aquifers in the conceptual 

model. Water levels respond seasonally to recharge, 

evapotranspiration, and minor pumping, but on the 

time scale of interest to this investigation, the watertable aquifers are at steady state. The one documented 

exception to steady state occurred about 1943 in the 

southern area of the Sheahan well field. Conceptually, 

the water-table aquifers serve the important function 

of providing a potentially large reservoir of vertical 

leakage to the underlying confined aquifers. Horizontal flow in the water-table aquifers are defined by the 

water-level map (fig. 4), but are of incidental interest 

in this investigation. Recharge to the aquifer is primarily from the infiltration of rainfall on the outcrop. Discharge from these aquifers is primarily to streams, as 

baseflow, and vertically to deeper aquifers as downward leakage.

The Jackson-upper Claiborne confining unit is 

conceptualized as a leaky confining unit with variable 

thickness (fig. 5) and lithology. Leakance values for 

this confining unit were poorly defined by aquifer test 

data (table 2), and much quantitative testing of alternative leakance parameters and distributions were undertaken. In general, pumping from the Memphis aquifer 

has induced flow from the shallow water-table aquifers downward to the Memphis aquifer through the 

Jackson-upper Claiborne confining unit. Leakage has 

increased with time as the head difference between the 

water-table aquifers and the Memphis aquifer has 

increased.

Flow in the Memphis aquifer has been transient 

since the onset of pumping in 1886. Recharge occurs 

in the outcrop area in the southeastern and eastern 

parts of the study area (fig. 13), and flow is predominantly into the centers of pumping from all directions 

(fig. 7). An increasing component of recharge is 

derived from leakage through time from the super and 

subjacent aquifers across nonhomogeneous confining 

units. Pumping represents the major source of discharge from the system, and the areal and temporal 

variation of pumping through time is the major reason 

this aquifer is not at steady state. Prior to pumping, 

discharge was westward to the subcrop of the Memphis aquifer beneath the alluvium, and upward beneath 

the Mississippi River alluvial plain. Up dip pinch out 

of the Memphis Sand defines the limit of occurrence 

of the Memphis aquifer, and no-flow boundaries 

around the eastern, northern, and western boundaries 

conceptually represent ground-water conditions where 

the pinch out occurs. A major effort of quantitative 

testing was focused on the Memphis aquifer and its 

related hydrogeology, including its transmissivity, 

storage, boundary configuration, and pumping.

The Flour Island confining unit is conceptualized as a confining unit that is less variable in thickness (fig. 11) and less leaky than the Jackson-upper 

Claiborne confining unit. Flow directions across the 

Flour Island confining unit are in response to dynamically changing heads in the overlying Memphis aquifer and underlying Fort Pillow aquifer. Quantitative 

testing of the vertical hydraulic conductivity of this 

unit was a specific focus of this investigation.

Flow in the Fort Pillow aquifer has been transient since about 1924, not only in response to pumping from this aquifer in the study area, but to major 

regional pumping in Arkansas. Recharge to the Fort 

Pillow aquifer occurs primarily in the outcrop areas 

east and north of the study area. Vertical leakage provides some recharge at locations where heads in the 

overlying Memphis aquifer are higher than heads in 

the Fort Pillow aquifer. Discharge from the system is 

primarily to a temporally and areally varying pumping 

distribution particularly in Arkansas (Arthur and 

Taylor, 1990). Some discharge from the Fort Pillow 

aquifer occurs as horizontal flow southward, and some 
24 Hydrogeology and Ground-Water Flow in the Memphis and

Fort Pillow Aquifers in the Memphis Area, Tennessee
Conceptualization of the Ground-Water Flow System 25
26 Hydrogeology and Ground-Water Flow in the Memphis and

Fort Pillow Aquifers in the Memphis Area, Tennessee

occurs as vertical flow upward. No-flow boundaries 

define the up-dip limits of the Fort Pillow aquifer. 

Higher leakance through the overlying Flour Island 

confining unit simulates horizontal outflow to the 

south, more than 50 miles from the study area. Quantification of hydraulic parameters of the Fort Pillow 

aquifer (transmissivity, storage coefficient, boundary 

configuration, and pumping) was the focus of quantitative testing and verification.

The Midway confining unit was conceptualized 

as being a no-flow boundary. The concept was tested 

by Brahana and Mesko (1988) and found to be a valid 

assumption. Alternative testing was not undertaken in 

this study.

SIMULATION OF THE GROUND-WATER 

FLOW SYSTEM

The validity of the conceptual model can be 

assessed in part by constructing a digital model of the 

ground-water flow system. In the digital model, differential equations depicting the physical laws governing 

ground-water flow in porous media are solved to simulate the movement of water through the system. The 

digital model code used in this study was developed 

by McDonald and Harbaugh (1988) and has the following attributes:

1. Flow is simulated in a sequence of layered aquifers 

separated by confining units;

2. Flow within the confining units is not simulated, 

but the hydraulic effect of these units on leakage 

between adjacent aquifers is taken into account;

3. A modular design facilitates hydrologic simulation 

by several alternative methods; and

4. The model code has been documented and validated 

in hydrogeologic settings similar to those which 

occur in the study area.

For this model the study area is discretized in 

space and time, and finite-difference approximations 

of differential equations depicting ground-water flow 

are solved at each node. The solution algorithm 

employs an iterative numerical technique known as 

the strongly implicit procedure—SIP (Weinstein and 

others, 1969). The theory and use of the model is documented by McDonald and Harbaugh (1988).

A three-layer model (fig. 12) was constructed to 

simulate the regional flow system in the Memphis and 

Fort Pillow aquifers. The uppermost layer represents 

the shallow aquifer. Flow within the shallow aquifer 

was not simulated; rather, the layer consisted of an 

array of constant-head nodes representing water levels 

at steady state during any given stress period. This 

layer serves as the ultimate source of recharge to the 

aquifers, either by leakage, or where the Memphis and 

Fort Pillow aquifers outcrop, as a source of simulated 

direct recharge.

The second and third layers represent the Memphis and Fort Pillow aquifers, respectively. The areal 

extent of the formations that make up the Memphis 

and Fort Pillow aquifers are shown in figure 13.

Layers of the model are separated by leaky confining units. These units are depicted by arrays of leakance terms. Leakance is calculated by dividing the 

vertical hydraulic conductivity by the thickness of the 

confining unit (McDonald and Harbaugh, 1988, 

p. 5-11). Leakance values are high in areas where confining units are thin or absent, and are low where the 

units are thick and tight.

Finite-Difference Grid

The area simulated by the digital model (fig. 14) 

is much larger than the Memphis study area. Evaluation of the larger area allows simulation of regional 

flow in the aquifer using realistic representations of 

the natural boundaries of the Memphis and Fort Pillow 

aquifers on the western, northern, and eastern margins 

of the Mississippi embayment.

Approximately 10,000 mi2 of the northern Mississippi embayment is divided by a variably-spaced, 

finite-difference grid of 58 rows, 44 columns, and 

3 layers. The grid, in relation to the areas of outcrop 

and subcrop of the Memphis and Fort Pillow aquifers, 

is shown in figures 14 and 15 and is oriented to minimize the number of inactive nodes. Directional properties of transmissivity were not used to determine grid 

alignment, because on a regional scale there is no evidence of anisotropic transmissivity in the Mississippi 

embayment area (Hayes Grubb, U.S. Geological Survey, oral commun., 1986). An evaluation of an aquifer 

test of the Memphis aquifer in the Memphis area using 

tensor analysis (Randolph and others, 1985) was conducted after the grid was aligned. This evaluation indicated a slight anisotropy (2.3 to 1) with respect to 

principal axes oriented within 15o

 of the grid of this 

model (Morris Maslia, U.S. Geological Survey, written commun., 1985).
Simulation of the Ground-Water Flow System 27

Memphis area
28 Hydrogeology and Ground-Water Flow in the Memphis and

Fort Pillow Aquifers in the Memphis Area, Tennessee

Memphis area
Simulation of the Ground-Water Flow System 29

The grid spacing varies from a minimum of 

3,200 feet in the Memphis area to 100,000 feet at the 

western boundary of the model. This variable spacing 

provides computational efficiency while affording the 

highest node density within the Memphis study area. 

Grid block size within the Memphis study area varies 

from 0.45 mi2

 to slightly more than 8 mi2 (see fig. 25). 

A grid block size of about 1 mi2

 is typical for the area 

of intense pumping in metropolitan Memphis. To 

reduce the potential for numerical instability during 

model simulation, block dimensions varied by no 

more than 1.5 times the dimensions of adjacent blocks.

Hydrologic Parameters

The flow model requires arrays of input data 

that define the distribution of "average" hydrologic 

parameters and conditions affecting ground-water 

flow within each grid block. These parameters include 

initial head distributions, boundary conditions, 

hydraulic properties of the aquifers and confining 

beds, and pumping stresses.

Initial Head Distributions

The initial head distributions used in the model 

are general estimates of pre-development, steady-state 

conditions. Data are sparse, and many data points were 

extrapolated. Initial water levels for the shallow aquifer (layer 1) in the Memphis area are estimated to be 

the same as water levels in 1980 (fig. 4), except that 

the cone of depression in the area of the south Sheahan 

well field was not present under initial conditions. 

Prior to pumping, water levels in the shallow aquifers 

in the south Sheahan area are estimated to be about 

240 feet above sea level. Initial heads for the shallow 

aquifer (layer 1) in the Memphis area are based on 

data from Wells (1933), Boswell and others (1968, 

plate 1), Krinitzsky and Wire (1964), and Graham and 

Parks (1986, fig. 7).

Initial heads in the Memphis aquifer for the 

entire modeled area prior to development were derived 

from Arthur and Taylor (1990), Hosman and others 

(1968, plate 7), and Reed (1972). Within the Memphis 

area, estimated potentiometric surface of the Memphis 

aquifer prior to development in 1886 is shown in 

figure 16 (Criner and Parks, 1976, fig. 4).

Initial head data for the Fort Pillow aquifer in 

the modeled area are from Arthur and Taylor (1990), 

Criner and Parks (1976, fig. 4), Hosman and others 

(1968, plate 4), Plebuch (1961), and Schneider and 

Cushing (1948). The estimated potentiometric surface 

of the Fort Pillow aquifer within the Memphis area 

prior to development in 1924 is shown in figure 17.

Boundary Conditions

Boundary conditions include lateral no-flow 

boundaries for the Memphis and Fort Pillow aquifers, 

a no-flow condition beneath the Fort Pillow aquifer, 

and constant heads for the uppermost layer. To the 

north, east, and west for the Memphis and Fort Pillow 

aquifers, no-flow boundaries correspond with the 

updip extent of respective outcrop and subcrop areas 

(figs. 14 and 15). On the south, a no-flow boundary is 

specified that is roughly perpendicular to water-level 

contours (parallel to ground-water flow). This boundary is not truly "no flow"; however, the low aquifer 

transmissivity and distance from the area of interest 

are assumed to cause negligible effects on simulation 

in the area of interest. 

Constant heads in the uppermost layer, which 

corresponds to the water-table aquifer, represent longterm, steady-state water-table altitudes. Head declines 

have been documented in only one isolated area in the 

shallow water-table aquifer. In this area of water-level 

decline, the water levels were decreased step-wise in 

sequential stress periods to reflect estimated declines 

in the local water table. 

Simulated flow to and from the uppermost layer 

represents deep recharge and discharge from the system. Inasmuch as the focus of the study was on the 

deeper aquifers, a detailed evaluation of the hydrologic budget of the shallow aquifer was outside the 

scope of this report. However, the calculated value of 

regional recharge used in the model was hydrologically reasonable and compared favorably with values 

used in Arthur and Taylor (1990) and Brahana and 

Mesko (1988).

The Midway confining unit underlying the Fort 

Pillow aquifer is assumed to be impermeable, and its 

upper surface is specified as a "no-flow" boundary. 

This assumption is supported by lithologic, chemical, 

and hydrologic data (Brahana and Mesko, 1988, 

figs. 8, 10, and 11, and table 2).
30 Hydrogeology and Ground-Water Flow in the Memphis and

Fort Pillow Aquifers in the Memphis Area, Tennessee
Simulation of the Ground-Water Flow System 31
32 Hydrogeology and Ground-Water Flow in the Memphis and

Fort Pillow Aquifers in the Memphis Area, Tennessee

Aquifer Hydraulic Properties

Average storage coefficient and transmissivity 

for each grid block for each aquifer were required for 

model simulation. Initial estimates for these hydraulic 

properties were based on pumping tests, geologic data 

such as lithology and layer thickness, and estimates 

and calculations made by other investigators 

(Schneider and Cushing, 1948; Criner, Sun, and 

Nyman, 1964; Halberg and Reed, 1964; Bell and 

Nyman, 1968; Boswell and others, 1968; Hosman and 

others, 1968; Cushing and others, 1970; Newcome, 

1971; Reed, 1972; Parks and Carmichael, 1989a 

and b). The model-derived storage coefficient and 

transmissivity for the Memphis aquifer represent the 

values that provided the best fit between calculated 

and observed potentiometric levels (heads) (table 2 

and figs. 18 and 19).

Transmissivity values determined by calibration for the Memphis aquifer in the Memphis area 

ranged from less than 10,000 ft2/d to 50,000 ft2/d, with 

values commonly in the range from 20,000 ft2

/d to 

50,000 ft2

/d (fig. 19). These values agree with the 

average transmissivity determined by flow-net analyses (U.S. Geological Survey, unpublished data, 1985), 

and are within the range of reported values (table 2). 

Transmissivity decreases south of Shelby County, 

which reflects the change to clay facies in the middle 

part of the Memphis Sand (Hosman and others, 1968). 

The best match of heads was simulated using values of 

transmissivity that more closely matched those of the 

Sparta aqufier (Fitzpatrick and others, 1989) than 

those of the entire clay and sand unit. The storage 

coefficients for the Memphis aquifer ranged from 

2 x 10-4 to 2 x 10-1 (fig. 18). 

Leakance values were initially determined by 

dividing estimates of the vertical hydraulic conductivity of reported lithologies (U.S. Geological Survey, 

unpublished data, 1984; Freeze and Cherry, 1979) by 

the generalized thickness of the confining units (Graham and Parks, 1986, figs. 3-6). These values were 

refined during the calibration process; areal distribution of leakance by calibration is shown in figure 20.

Leakance of the upper confining layer, the Jackson Formation and upper part of the Claiborne Group, 

was characterized by a wide range of values, from 

1 x 10-8 feet per day per foot to 1 x 10-3 feet per day 

per foot. This range reflects the diverse lithology of 

the Jackson-upper Claiborne confining unit as well as 

variations in thickness of the unit (fig. 5). 

Most transmissivity values determined by calibration for the Fort Pillow aquifer in the Memphis area 

ranged from 6,000 to 24,000 ft2/d (fig. 21). The storage coefficients used in the calibrated model for the 

Fort Pillow aquifer in the Memphis area varied by less 

than a factor of 2, from 5 x 10-4 to 1 x 10-3 (fig. 22), 

sigifying uniformly confined conditions for the Fort 

Pillow aquifer. Leakance values for the lower confining unit, the Flour Island Formation, were from 

1 x 10-12 feet per day per foot to 2 x 10-12 feet per day 

per foot (fig. 23), reflecting similar lithology and little 

variation in thickness (fig. 11) of the Flour Island confining unit within the Memphis area.

Pumping

Pumping from the Memphis aquifer began in 

1886, and pumping from the Fort Pillow aquifer began 

in 1924. Withdrawals from these two major aquifers 

have occurred at varying rates and with a changing 

areal distribution. Because of variation with time, 

pumping data were introduced in the model in nine 

discrete stress periods. The total modeled pumpage 

and the corresponding total reported pumpage for the 

nine periods are shown in figure 24. The length of the 

stress periods ranged from 5 to 39 years. Seasonal 

variations in pumping were not simulated. Mean 

annual pumping was used to calculate average stress at 

each node for each of the stress periods.

Delineation of stress periods was based on 

abrupt changes in pumpage rates, variations in the 

areal distribution of pumping centers, and on availability of water-level maps. The number of well nodes 

simulating pumping in the Memphis area increased 

from 18 in stress period 1 to 88 in stress period 9. Total 

pumping from the Memphis and Fort Pillow aquifers 

increased from 0 in 1885 to about 190 Mgal/d in 1985.

Pumpage data for the Memphis and Fort Pillow 

aquifers in the Memphis area are based on the published reports of Criner and Parks (1976) and Graham 

(1982). Areal distribution was assigned based on 

extensive unpublished documents of water use 

reported to the U.S. Geological Survey in Memphis 

(W.S. Parks, U.S. Geological Survey, written 

commun., 1984).

Model Calibration

Calibration of the flow model is the process of 

adjusting the input data to produce the best match 

between simulated and observed water levels. The 
Simulation of the Ground-Water Flow System 33
34 Hydrogeology and Ground-Water Flow in the Memphis and

Fort Pillow Aquifers in the Memphis Area, Tennessee
Simulation of the Ground-Water Flow System 35
36 Hydrogeology and Ground-Water Flow in the Memphis and

Fort Pillow Aquifers in the Memphis Area, Tennessee
Simulation of the Ground-Water Flow System 37
38 Hydrogeology and Ground-Water Flow in the Memphis and

Fort Pillow Aquifers in the Memphis Area, Tennessee
Simulation of the Ground-Water Flow System 39
40 Hydrogeology and Ground-Water Flow in the Memphis and

Fort Pillow Aquifers in the Memphis Area, Tennessee

model was calibrated by simulating the stress periods 

from 1886-1980, a time interval during which flow in 

both the Memphis and Fort Pillow aquifers was 

thought to be transient. Calibration was concentrated 

on stress periods from 1961 to 1980. Ground-water 

conditions were transient in both the Fort Pillow and 

the Memphis aquifers during the period 1961 to 1980, 

whereas conditions in the shallow aquifer were 

thought to be at steady state. It should be noted that 

water-level and pumping data exist for the entire 

period of development of the Memphis aquifers; the 

early data are sparse, however, and are less well documented than data collected after 1960.

An enlarged view of part of the model grid in 

the Memphis study area, including locations simulated 

as major centers of pumping, is shown in figure 25.

The strategy for calibration was dictated by the 

availability of data, and in partcular, by availability of 

detailed water levels and pumping information for 

specified wells. In general, there is a wealth of waterlevel and pumpage data for the Memphis and Fort Pillow aquifers since 1960. There are many records that 

are adequate for general interpretation for the period 

1924 to 1960, but prior to 1924, there are few reliable 

records at all.

For example, the prepumping (1886) potentiometric surface of the Memphis aquifer is based on four 

data points (Criner and Parks, 1976), all of which were 

extrapolated (fig. 16). Data points for the Fort Pillow 

aquifer in the Memphis area likewise are lacking for 

this period. Because of this data, no formal steadystate calibration to these few prepumping data was 

attempted, although the match of prepumping conditions by removing pumping from the calibrated model 

(transient) provided a reasonable match with the estimated maps. 

The completeness and documentation of the 

data base for conditions after 1960 justified using this 

data as the major tool of calibration. The transient simulation from 1961 to 1980 was completed using four 

5-year pumping periods (fig. 24) of 10 time-steps 

each. Seasonal fluctuations in water levels were averaged to give a single annual value. The model was calibrated by minimizing the difference between model 

simulated heads and measured heads (Criner and 

Parks, 1976; Graham, 1982). In addition, differences 

between hydrographs of observed and simulated water 

levels at long-term observation wells were minimized.

Calibration was continued by adjusting the global multiplier of transmissivity, vertical conductance, 

and storage coefficients of the Memphis and Fort Pillow aquifers and their confining units until the sum of 

the squared differences between observed and calculated heads was minimized. Individual hydraulic data 

for nodes was adjusted only if geologic or hydrologic 

justification warranted such a change. Calibrated values for hydraulic properties were within the range 

determined by aquifer tests (table 2) and those estimated from published values of similar geologic materials (Schneider and Cushing, 1948; Criner, Sun, and 

Nyman, 1964; Halberg and Reed, 1964; Bell and 

Nyman, 1968; Boswell and others, 1968; Hosman and 

others, 1968; Cushing and others, 1970; Newcome, 

1971; Reed, 1972; Parks and Carmichael, 1989a and b).

Data collected from the period 1886 to 1960 

were used to make minor adjustments to parameters 

during calibration (fig. 24). These data were less well 

defined than post-1960 data, and in some instances, 

were essentially undocumented. As an example, major 

uncertainty exists about water levels and discharge 

from the Auction Avenue “tunnel,” a major source of 

municipal supply that was used from about 1906 to 

about 1924. The Auction Avenue “tunnel” was a collector tunnel for some early wells screened in the 

Memphis aquifer (Criner and Parks, 1976, p. 13). 

According to Criner and Parks (1976): “...little is 

known about the tunnel (Auction Avenue “tunnel”), 

but it is reported to have been constructed in a clay 

layer, about 85 feet below land surface and below the 

potentiometric surface of the Memphis aquifer. The 

tunnel was reported to be brick-lined, about 5 feet in 

diameter, and about one-quarter mile in length. Several wells were completed along the tunnel and constructed so that water would flow into the tunnel 

through underground outlets. Water was pumped into 

the city supply system from a large well, 40 feet in 

diameter, at the end of the tunnel at Auction Avenue 

Station.” Inasmuch as this and other dominant withdrawals during the period 1886-1924 were not well 

defined, little emphasis was given to calibrating the 

model using older data.

An important model calibration and testing criterion was an error analysis of simulated and observed 

water levels at the nodes representing the control 

points. The root mean square error (RMSE) was used 

to judge how closely the simulation matched “reality,” 

which was defined by a network of observation wells 

(Criner and Parks, 1976, fig. 1). The root mean square 

error was calculated as a measure of the difference 

between model-calculated heads and observed heads. 
Simulation of the Ground-Water Flow System 41
42 Hydrogeology and Ground-Water Flow in the Memphis and

Fort Pillow Aquifers in the Memphis Area, Tennessee

The root mean square error is described by the equation:

where

RMSE is the root mean square error;

HC is calculated head, in feet, at a model node;

HO is observed head, in feet;

n is the number of comparison points;

i is a subscript that defines any specific comparison 

point, varying between 1 and n.

Another criterion was the comparison made 

between observed and simulated hydrographs. 

Records from four wells from the Memphis aquifer 

and two wells from the Fort Pillow aquifer were of 

sufficient duration to provide reasonable comparisons 

(fig. 28). Locations of the wells from which the comparisons were made are shown on figure 25. For the 

most part, the observed and simulated hydrographs 

agree closely.

The results of the calibration are shown in figures 26, 27, and 28. A comparison of observed data 

points and simulated potentiometric surface of the 

Memphis aquifer is shown in figure 26; a similar map 

for the Fort Pillow aquifer is shown in figure 27. 

Hydrographs of observed and simulated water levels 

for selected wells are compared in figure 28.

The simulated potentiometric surfaces match 

the observed data points reasonably well for both aquifers at the end of the calibration period, stress period 8 

(figs. 26 and 27). Likewise, interpretive maps contoured from the observed data (figs. 7 and 9) are similar to simulated potentiometric surfaces. Stress periods 

4 through 7 simulated observed water levels as well or 

better than stress period 8, but because of their similarities to one another, have not been included as figures.

In addition to the areal match of water-level 

data, simulated and observed water levels agree closely 

through time for selected hydrographs (fig. 28). Variations are thought to be due to errors in the amount and 

distribution of pumping, particularly prior to 1960, 

when pumping was not accurately monitored.

Although the overall simulation of heads in the 

Memphis aquifer is considered to be good, heads 

matched poorly in one subarea lying near Nonconnah 

Creek and the Tennessee-Mississippi border in south 

Memphis (figs. 26 and 7). Many alternative representations of transmissivity, leakage, and recharge were 

attempted, but their effect on heads outside the 

problem area created more problems with overall simulation than they solved with improved subarea simulation. Hydrogeologic data from this area suggest that 

the model does not contain all relevant hydraulic or 

boundary conditions; any model application to this 

subarea should be undertaken with extreme caution. 

There is no doubt that this subarea is a source of significant recharge to the Memphis aquifer. The quantity 

and location of the concentrated recharge in this area 

as indicated by the model may be subject to error and 

the descriptions of these factors in this report should 

be considered tentative at best.

It is common in reports documenting groundwater flow models to evaluate average ground-water 

discharge to streams with calculated flux from the 

model. Inasmuch as the Mississippi River and its tributaries dominated the ground-water flow, and inasmuch as simulation of the shallow aquifer was outside 

the scope of this report, no attempt was made to 

include this comparison. Discharge to streams was not 

undertaken in this study because:

1. Flow in the Mississippi River was four to five 

orders of magnitude greater than ground-water 

inflow rates to streams, thereby masking the 

inflow component;

2. Grid dimensions for the outcrop areas of the Memphis aquifer and Fort Pillow aquifer were large. 

Simulation of streams in these large blocks 

required estimations that were poorly quantified;

3. No aquifer hydraulic tests were reported for the 

fluvial deposits; and 

4. Direct simulation of flow in the water-table aquifer 

was outside the scope of the investigation.

 Model Testing

After calibration, the model was tested to determine its ability to simulate observed water levels for 

the period 1981-85 (fig. 24). For this testing phase, no 

modification of boundary conditions or calibrated data 

was made. In this testing phase, the flow model simulated heads in the Fort Pillow aquifer and Memphis 

aquifer within 5 feet of observed water levels for at 

least 75 percent of the observation wells (this comparison used interpolated values rather than root mean 

square error values). These results increase confidence 

that the model accurately simulates ground-water flow 

in the study area. The additional criteria used to evaluate the calibration phase also were used to judge the 

accuracy of the simulated results for this testing phase.

RMSE

Hi

C HiO 〈 〉 –

n -----------------------------

2

i = 1

n

= ∑
Simulation of the Ground-Water Flow System 43
44 Hydrogeology and Ground-Water Flow in the Memphis and

Fort Pillow Aquifers in the Memphis Area, Tennessee
Simulation of the Ground-Water Flow System 45
46 Hydrogeology and Ground-Water Flow in the Memphis and

Fort Pillow Aquifers in the Memphis Area, Tennessee

Sensitivity Analysis

The response of the calibrated model to variations in model parameters, pumping, and boundary 

conditions was evaluated by sensitivity analysis. 

Transmissivity and storage of the Memphis and Fort 

Pillow aquifers, and leakance for the Jackson-upper 

Claiborne and Flour Island confining units were each 

varied uniformly in the model while the other parameters were kept constant. The subsequent effects of 

these variations on calculated water levels in the 

Memphis and Fort Pillow aquifers were evaluated by 

root mean square error (RMSE) comparison of 

observed and simulated water levels for 1980. Results 

of the sensitivity analyses are illustrated in figures 29 

and 30 for the Memphis aquifer and the Fort Pillow 

aquifer, respectively. 

The RMSE was 14 feet for the Memphis aquifer 

and about 10 feet for the Fort Pillow aquifer. These 

values, on initial evaluation, appear to define very 

poor simulation of a system. The data set that was used 

to generate the RMSE value, however, was treated in a 

nontraditional manner, and the values generated 

should be considered relative rankings rather than 

absolute measures of goodness-of-fit.

The data set for RMSE comparisons included all 

known observed water levels for the period of interest. 

Typically, for pumping periods 4 through 9 (fig. 24) 

occurring after 1955, the data set included more than 

100 points. For pumping period 8, on which figures 29 

and 30 are based, 129 comparison points were used. 

Many of the observation wells did not occur at the 

center of a model node, but fell near boundaries of 

adjacent nodes. Rather than interpolate an observed 

value to the nearest nodal center, the actual measurement was compared to the simulated head at the surrounding nodes typically either the two nearest if on a 

boundary, or the four nearest if on a corner. Because of 

the steep gradients associated with pumping, a large 

difference in head frequently occurred for such comparisons (one typically higher, one typically lower), 

giving rise to a large RMSE when in fact an interpolation of simulated conditions matched observed conditions closely.

Results of the sensitivity analysis showed that 

calculated heads in the Memphis aquifer were most 

sensitive to variations in aquifer transmissivity and 

leakance of confining unit A, and least sensitive to 

storativity (fig. 29). Calculated heads in the Memphis 

aquifer were not responsive to changes in the aquifer 

characteristics of the Fort Pillow aquifer. Calculated 

heads in the Fort Pillow aquifer were most sensitive to 

transmissivity, and least sensitive to leakance of the 

Flour Island confining unit and storativity (fig. 30). As 

a general rule, calculated heads in the Fort Pillow 

aquifer were insensitive to general changes in aquifer 

characteristics of the Memphis aquifer. Because of the 

dominating effect of the pumping stress in the Memphis aquifer, calculated heads in the Fort Pillow aquifer were sensitive to factors affecting recharge and 

leakage to the Memphis aquifer. Although not shown 

in the figures, variations in simulated pumping caused 

large variations in calculated heads in the aquifers. 

Changes in simulating the southern boundary of the 

model 20 miles closer and 20 miles farther from Memphis caused only very slight changes in calculated 

heads from calibrated values.

These results suggest that the values used in the 

calibrated model are reasonable approximations of 

actual conditions within the aquifer, particularly in 

light of the constraints made by the well-defined 

pumping data and the well-defined potentiometric surfaces. The high sensitivity of leakance of the Jacksonupper Claiborne confining unit with respect to simulated heads in the Memphis aquifer gives confidence 

that an otherwise poorly defined parameter is well 

approximated in the model.

Interpretation of Model Results

The underlying objective of ground-water flow 

modeling was to develop a tool to quantitatively assess 

the hydrogeology of the Memphis area, and thereby 

improve understanding of the factors affecting groundwater flow. Digital simulation of ground-water flow 

permitted a quantitative evaluation of flux across 

hydrogeologic boundaries and calculation of a hydrologic budget. Interpretation of these results promotes a 

more complete understanding of the flow system and 

often has direct implications for resource management.

Hydrologic Budget

One of the principal products of the digital 

model is a hydrologic budget for each layer in which 

ground-water flow is simulated. For a given stress 

period, the model calculates the simulated volume of 

water that was added to or removed from the layer. 

Flow rates are also calculated. Because pumpage was 

variable in space and time throughout the simulation, 

components of the hydrologic budget were not 
Simulation of the Ground-Water Flow System 47
48 Hydrogeology and Ground-Water Flow in the Memphis and

Fort Pillow Aquifers in the Memphis Area, Tennessee
Simulation of the Ground-Water Flow System 49

constant. The budget figures for 1980 are presented in 

table 4.

Pumpage accounted for almost all of the total 

discharge from the Memphis aquifer (table 4). Model 

simulations indicated pumped water was replaced 

from three sources: recharge and lateral inflow 

(42 percent), leakage from the shallow aquifer (54 percent), leakage from the deep aquifer (1 percent), and 

storage (3 percent). Lateral inflow refers to the essentially horizontal movement of water within the aquifer; the ultimate source of this water is recharge in the 

outcrop area.

Leakage to the Memphis aquifer occurred both 

from the surficial aquifers and the Fort Pillow aquifer. 

As water-levels in the Memphis aquifer declined in 

response to pumpage, hydraulic gradients favored the 

flow of water across the overlying and underlying confining units. Approximately 98 percent of the simulated leakage to the Memphis aquifer was attributable 

to flow across the Jackson-upper Claiborne confining 

unit. In 1980, this leakage from water-table aquifers 

contributed more than 50 percent of the water pumped 

from the Memphis aquifer. Because water in the 

water-table aquifers is inferior in quality and more susceptible to contamination than water in the Memphis 

aquifer, this substantial contribution may be cause for 

concern. The third source of water pumped from the 

Memphis aquifer was storage, which refers to water 

made available by compression of the aquifer and 

expansion of the water column. Storage contributes a 

minor part (3 percent) of the budget of the Memphis 

aquifer, based on simulation of 1980 conditions.

The hydrologic budget for the Fort Pillow aquifer in 1980 also is defined in table 4. Water was 

removed from this aquifer both by pumpage 

(88 percent) and leakage to the Memphis aquifer 

(12 percent). Most of the water removed from this 

aquifer was derived from recharge and lateral inflow 

(87 percent). About 13 percent of the water was 

derived from storage.

Areal Distribution of Leakage

Downward leakage from the water-table aquifer 

through the Jackson-upper Claiborne confining unit to 

the Memphis aquifer poses a potential threat to the 

quality of water used for public supply in the Memphis 

area. To facilitate management and protection of this 

resource, it is important to identify those areas where 

leakage is most significant.

In the flow simulation, a small amount of downward leakage to the Memphis aquifer occurred 

throughout the study area. In certain zones, however, 

leakage was more pronounced (fig. 31). In most places 

leakage did not exceed 0.01 cubic feet per second per 

square mile, which is equivalent to an infiltration 

velocity of 0.14 inch per year (in/yr). Near the outcrop 

area and around Lichterman well field in southeastern 

Memphis, there was a zone in which leakage was 

greater than other areas. Near the outcrop area, leakage rates varied from 0.01 to 0.1 cubic feet per second 

per square mile, which is equivalent to an infiltration 

velocity of 0.14 to 1.4 in/yr. In this zone the confining 

unit is known to be relatively thin (fig. 5).

Simulated leakage rates were substantially 

higher in several other locations, as well. These locations included: (1) Johns Creek, Nonconnah Creek, 

and the South Sheahan area (fig. 31, area 1); (2) the 

Wolf River between Sheahan and McCord well fields 

(fig. 31, area 2); (3) along the Mississippi River near 

Mallory well field (fig. 31, area 3); and (4) a zone east 

of Lichterman well field (fig. 31, area 4). The large 

leakage rates indicated by the simulation agree with 

other evidence supporting substantial flow between 

the surficial aquifers and the Memphis aquifer at these 

locations. Other evidence includes isotopic data, 

water-level measurements, and thermal anomalies 

(Graham and Parks, 1986).

Model Limitations

Models by their very nature are only approximations, and are not exact replicas of natural systems. 

The success of a model in approximating the natural 

system is limited by such factors as scale, inaccuracies 

in estimating hydraulic characteristics and stresses, 

inaccurate or poorly defined boundary or initial conditions, and the degree of violation of flow-modeling 

assumptions (P. Tucci, U.S. Geological Survey, written 

commun., 1988).

For example, the minimum grid block size for 

this model is about 0.45 mi2, an area much too large to 

simulate ground-water levels in individual wells. The 

model was neither designed for nor should it be used 

for site-specific applications. It was designed for intermediate to regional evaluation of "average" transient 

ground-water conditions within the Memphis area, and 

within this application, the model has been shown to 

simulate observed conditions to a reasonable degree of 

accuracy.
50 Hydrogeology and Ground-Water Flow in the Memphis and

Fort Pillow Aquifers in the Memphis Area, Tennessee

Table 4. Water budget calculated by the flow model, 1980, for the Memphis area

Sources and discharges Flow, in cubIc feet per second Percentage of total

Memphis Aquifer

Sources:

Recharge 106 36

Boundary flux 17 6

Leakage from shallow aquifer 157 54

Leakage from deep aquifer 2 1

Storage 10 3

Total 292 100

Discharge:

Boundary flux out 3 1

Pumping 289 99

Leakage (net in) 0 0

Total 292 100

Fort Pillow Aquifer

Sources:

Recharge 5 31

Boundary flux in 9 56

Leakage from Memphis aquifer 0 0

Storage 2 13

Total 16 100

Discharge:

Boundary flux out 0 0

Pumping 14 88

Leakage to Memphis aquifer 2 12

Total 16 100
Simulation of the Ground-Water Flow System 51
52 Hydrogeology and Ground-Water Flow in the Memphis and

Fort Pillow Aquifers in the Memphis Area, Tennessee

Selection of model boundary conditions can 

greatly influence model results. Model boundaries 

should closely correspond to natural hydrologic 

boundaries whenever possible (E. Weeks, U.S. Geological Survey, written commun., 1975), and, with the 

exception of the southern boundary, this concept was a 

guiding approach that was followed in this (figs. 14 

and 15) and previous models of the area (Brahana, 

1982a, fig. 5). The variable spacing of the grid, however, has the potential of introducing “average” 

approximations within the larger grid cells (the largest 

are about 8 mi2) that are significantly different than 

actual conditions. For example, representation of 

hydrologic features such as divides or drains is difficult in large grid cells, because the feature represents 

only a small percentage of the total area of the cell. For 

this reason, any but regional interpretations regarding 

head and flow in grid cells larger than several square 

miles should be avoided, and, as with the actual development of the model, emphasis should be limited to 

the Memphis study area.

Continuing reassessment will be very important 

in the evolution of the model. As ongoing studies fill 

the gaps in the data base and improve understanding of 

this complex flow system, the model can be modified 

and recalibrated to include those changes. Newly 

developed techniques of aquifer parameter estimation 

would be particularly useful as an aid to understanding 

the system, as would an optimization model (Larson 

and others, 1977; Lefkoff and Gorelick, 1987). 

Though the USGS does not develop them, an optimization model might be useful to resource managers in 

evaluating placement of future well fields and pumping configurations.

Despite the limitations discussed in this section, 

the model provided useful insights into the workings 

of the hydrologic system of the study area. Model 

results support the conceptual model of the groundwater flow system that the Memphis aquifer and Fort 

Pillow aquifer are partially isolated by the Flour Island 

confining unit. Leakage between aquifer layers represents a large component of the hydrologic budget 

(table 4), and if the model is to be used for predictive 

purposes using pumping configurations with locations 

significantly different than those tested for the calibration and validation phases, simulated results may vary 

from measured results. Extreme caution is recommended in interpreting results in such simulations.

SUMMARY AND CONCLUSIONS

The Memphis area has a plentiful supply of 

ground water suitable for most uses, but the resource 

may be vulnerable to contamination. Current withdrawals totalling about 200 million gallons per day 

have caused water-level declines in the major aquifers, 

increasing the potential for contaminated ground water 

in the surficial aquifer downward into the major aquifers. This study describes the hydrologic framework, 

simplifies and conceptualizes the hydrogeologic system to preserve and emphasize the major elements 

controlling ground-water flow, and quantitatively tests 

each of the major elements. The main tool for the 

investigation is a digital ground-water flow model; the 

ultimate objective of the study is an improved understanding of the factors affecting ground-water flow in 

the Memphis area.

The hydrogeologic framework of the area consists of approximately 3,000 feet of unconsolidated 

sediments that fill a regional downwarped trough, the 

Mississippi embayment. For the most part, the sediments are interbedded clays and sands, with varying 

amounts of silt, gravel, chalk, and lignite present. On a 

regional scale, the sediments form a sequence of 

nearly parallel, sheetlike layers of similar lithology. 

On a local scale, complex lateral and vertical gradations in lithology are common.

Clays of the Owl Creek Formation, Clayton Formation, Porters Creek Clay, and Old Breastworks Formation effectively define the base of freshwater 

aquifers. Overlying this base, the hydrogeologic 

framework includes the Fort Pillow Sand, the Flour 

Island Formation, the Memphis Sand, the Jackson Formation and upper part of the Claiborne Group, and 

alluvial and fluvial deposits.

Ground-water flow in this framework of aquifers (sands and gravels) and confining units (clays) is 

controlled by the altitude and location of sources of 

recharge and discharge, and by the hydraulic characteristics of the hydrogeologic units. Leakage between 

the Fort Pillow aquifer (Fort Pillow Sand) and Memphis aquifer (Memphis Sand), and between the Memphis aquifer and the shallow aquifer (alluvium and 

fluvial deposits) is a major component of the hydrologic budget. Pumping from the Fort Pillow and Memphis aquifers has significantly affected flow in these 

aquifers in the study area. Net discharge to the Mississippi River alluvial plain from the subcropping Fort 

Pillow and Memphis aquifers has decreased or ceased 

since predevelopment time; pumpage has captured 
Selected References 53

most of present-day flow by lowering potentiometric 

surfaces. The shallow surficial aquifer has not been 

pumped intensively (<1 Mgal/d), and with the exception of one limited area, is thought to have remained at 

steady state throughout the period of evaluation.

A three-layer finite-difference flow model was 

constructed to simulate the regional flow system in the 

Memphis area. The model area was much larger than 

the area of immediate concern, so that natural boundaries of the aquifers could be incorporated. Initial conditions, boundary conditions, hydraulic characteristics, 

and stresses were input values into 58 row by 44 column matrices. The model calculated heads and hydrologic budgets. In the model, the uppermost aquifer 

layer represents the shallow aquifer. Flow within the 

shallow aquifer was not simulated; rather, the layer 

consisted of an array of constant-head nodes representing water levels at steady state during any given 

stress period. The second and third layers represent the 

Memphis aquifer and Fort Pillow aquifer, respectively, 

where horizontal flow was simulated. Layers of the 

model are separated by leaky confining units. These 

units are depicted by arrays of leakance terms. Leakance values are high in areas where confining units 

are thin or absent, and are low in areas where the confining units are thick and hydraulically tight. The 

model was calibrated and tested using standard 

accepted practices of the U.S. Geological Survey.

This study has provided an improved understanding of the hydrogeology and ground-water flow 

in the Memphis and the Fort Pillow aquifers in the 

Memphis area. Calibration and validation of a multilayer finite-difference flow model indicated that leakage through the upper confining layer was a 

significant part of the hydrologic budget of the Memphis aquifer. The model attributes more than 50 percent of water withdrawn from this aquifer in 1980 to 

leakage. Although a significant portion of this leakage 

occurs near the outcrop area where the confining unit 

is thin, the implications for the Memphis aquifer 

remain the same. The potential exists for contamination of the Memphis aquifer in areas where surficial 

aquifers are contaminated and head gradients favor 

downward leakage.

Leakage was not uniformly distributed. The 

assumption of zones of high leakage along the upper 

reaches of the Wolf and Loosahatchie Rivers, the 

upper reaches of Nonconnah Creek, and in the area of 

the surficial aquifer in the Mississippi River alluvial 

plain was essential in simulating observed water levels 

in the Memphis aquifer. Geologic and geophysical 

data from these suspected zones of leakage suggest 

relatively thin or sandy confining units. On a regional 

basis, simulated vertical leakage through the upper 

confining unit was almost an order of magnitude 

greater than leakage through the lower confining unit. 

A significant component of flow (12 percent) 

from the Fort Pillow aquifer was calculated to occur in 

the form of upward leakage to the Memphis aquifer. 

This upward leakage generally was limited to areas 

near major pumping centers in the Memphis aquifer, 

where heads in the Memphis aquifer have been drawn 

significantly below heads in the Fort Pillow aquifer. 

Although the Fort Pillow aquifer is not capable of producing as much water as the Memphis aquifer for similar conditions, it is nonetheless a valuable resource 

throughout the area.

The multilayer finite-difference flow model is a 

valuable tool for hydrogeological research and 

resource management in the Memphis area. The model 

integrates boundary conditions as suggested by available information on the geology, hydrology, and water 

chemistry of the area; it can be updated as new data 

are collected.

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