Prepared in cooperation with the

Department of the Navy, Southern Division,

Naval Facilities Engineering Command

Hydrogeology and Ground-Water-Flow Simulation in the 

Former Airfield Area of Naval Support Activity Mid-South, 

Millington, Tennessee

Scientific Investigations Report 2004-5040

U.S. Department of the Interior

U.S. Geological Survey
Cover. See figure 6b, page 13.
Hydrogeology and Ground-Water-Flow 

Simulation in the Former Airfield Area of 

Naval Support Activity Mid-South, 

Millington, Tennessee

By Connor J. Haugh, John K. Carmichael, and David E. Ladd

Prepared in cooperation with the

Department of the Navy, Southern Division, Naval Facilities Engineering Command

Scientific Investigations Report 2004-5040

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2004

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

Box 25286, Denver Federal Center

Denver, CO 80225

For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS

World Wide Web: http://www.usgs.gov/

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted materials contained within this report.

Suggested citation: 

Haugh, C.J., Carmichael, J.K., and Ladd, D.E., 2004, Hydrogeology and ground-water-flow simulation in the former 

airfield area of Naval Support Activity Mid-South, Millington, Tennessee: U.S. Geological Survey Scientific Investigations Report 2004-5040, 31 p.
 iii

Contents

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

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

Purpose and scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Previous investigations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

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

Structure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Shallow aquifer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Simulation of ground-water flow. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Conceptual model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Model assumptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Model boundaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Model construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Model calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Sensitivity analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Model limitations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Advective flow particle tracking. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Selected references. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Figures

1-6a. Maps showing:

1. Location of Naval Support Activity Mid-South, Millington, Tennessee. . . . . . . . . . . . . . . . . . . . . . . . .2

2.. Naval Support Activity Mid-South, the Northside and Southside, and property 

transferred to the city of Millington. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3

3. Locations of study area, Area of Concern A, and interpreted plumes of 

trichloroethene in the alluvial-fluvial deposits aquifer beneath the 

former airfield area part of the Naval Support Activity 

Mid-South Northside, Millington, Tennessee . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5

4. Altitude of the base of the loess or silt and clay in the upper alluvium at Area of 

Concern A, Naval Support Activity Mid-South, Millington, Tennessee. . . . . . . . . . . . . . . . . . . . . . . . .9

5. Altitude of the base of sand and gravel in the lower alluvium or fluvial 

deposits at Area of Concern A, Naval Support Activity Mid-South, 

Millington, Tennessee . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

6a. Location of hydrogeologic section A-A′ at Area of Concern A, Naval Support 

Activity Mid-South, Millington, Tennessee . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

6b. Hydrogeologic section A-A′ showing the A1 aquifer and related features at Area

of Concern A, Naval Support Activity Mid-South, Millington, Tennessee . . . . . . . . . . . . . . . . . . . . . . . . . . 13

7. Map showing altitude of the potentiometric surface of the A1 aquifer, February and 

March 2000; location of well Sh:U-101 for which continuous water-level data 

were collected; and Area of Concern A model boundary, Naval Support 

Activity Mid-South, Millington, Tennessee. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

8. Hydrograph showing water levels recorded in well Sh:U-101 at Naval Support Activity 

Mid-South, May 1995 through March 2004. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
iv 

9-17. Maps showing:

9. Altitude of the top of the A1 aquifer at Area of Concern A, Naval Support 

Activity Mid-South, Millington, Tennessee . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

10. Altitude of the base of the A1 aquifer at Area of Concern A, Naval Support 

Activity Mid-South, Millington, Tennessee . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

11. Model grid cell types for the Area of Concern A flow model, Naval Support 

Activity Mid-South, Millington, Tennessee . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

12. Thickness of the modeled A1 aquifer, Naval Support Activity Mid-South, 

Millington, Tennessee . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

13. Hydraulic conductivity zones for the Area of Concern A flow model, 

Naval Support Activity Mid-South, Millington, Tennessee. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

14. Simulated steady-state water levels for layers 1, 2, and 3 of the Area of Concern A 

flow model, Naval Support Activity Mid-South, Millington, Tennessee. . . . . . . . . . . . . . . . . . . . . . . 23

15. Calibrated transmissivities for layer 1 of the Area of Concern A flow model, 

Naval Support Activity Mid-South, Millington, Tennessee. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

16. Calibrated transmissivities for layer 2 of the Area of Concern A flow model, 

Naval Support Activity Mid-South, Millington, Tennessee. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

17. Calibrated transmissivities for layer 3 of the Area of Concern A flow model, 

Naval Support Activity Mid-South, Millington, Tennessee. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

18. Graph showing composite scaled sensitivities for Area of Concern A flow model

parameters, Naval Support Activity Mid-South, Millington, Tennessee. . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

19. Map showing advective flow path lines from the “grassy” area and “north edge 

of apron” area at Area of Concern A, Naval Support Activity 

Mid-South, Millington, Tennessee . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Tables

1. Post-Wilcox Group geologic units underlying Naval Support Activity Mid-South, 

Millington, Tennessee, and their hydrologic significance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6

2. Recharge and hydraulic conductivity parameters defined in the Area of Concern A 

flow model, Naval Support Activity Mid-South . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

3. Relation between hydraulic conductivity parameters and hydraulic conductivity by 

layers for the Area of Concern A flow model, Naval Support Activity Mid-South. . . . . . . . . . . . . . . . . . . 22

4. Calibrated hydraulic conductivities of the Area of Concern A flow model, 

Naval Support Activity Mid-South . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

5. Correlation coefficients between estimated parameters of the Area of Concern A 

flow model, Naval Support Activity Mid-South . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

6. Estimated parameter values using alternate advective travel times, Area of 

Concern A flow model, Naval Support Activity Mid-South. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
 v

Conversion Factors, Datums, Well-Numbering 

Information, and Acronyms

Temperature in degrees Fahrenheit (oF) can be converted to degrees Celsius (oC) as follows:

oC = 5/9 x (oF-32)

Transmissivity: In this report transmissivity is expressed as foot squared per day (ft2/d)—The 

standard unit for transmissivity (T) is cubic foot per day per square foot times foot of aquifer 

thickness “[(ft3

/d)/ft2]ft” or cubic meter per day per square meter times meter of aquifer thickness “[(m3

/d)/m2]m.” These mathematical expressions reduce to foot squared per day “(ft2/d)” or 

meter squared per day “(m2

/d).”

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 

(NGVD 29). Horizontal coordinate information is referenced to the North American Datum of 1927 

(NAD27).

Well-numbering system: The U.S. Geological Survey assigns each well in this report a local Tennessee well number. The local well number in Tennessee 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-98, for example, indicates that the well is located in Shelby County on the “U” quadrangle and is identified as well 98 

in the numerical sequence. Quadrangles are lettered from left to right, beginning in the southwest corner of the county.

Multiply By To obtain

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

acre 4,047 square meter (m2)

acre 0.4047 hectare (ha)

square foot (ft2) 0.0929 square meter (m2)

square mile (mi2) 259.0 hectare (ha)

square mile (mi2) 2.590 square kilometer (km2)

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

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

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

cubic foot per second per square mile 

[(ft3/s)/mi2] 0.01093

cubic meter per second per square kilometer [(m3/s)/km2]

gallon per minute (gal/min) 0.06308 liter per second (L/s)

million gallons per day (Mgal/d) 0.04381 cubic meters per second (m3/s)

inch per year (in/yr) 25.4 millimeter per year (mm/yr)

gallon per minute per foot [(gal/min)/ft] 0.2070 liter per second per meter [(L/s)/m]

foot squared per day (ft2/d) 0.09290 meter squared per day (m2/d)
vi 

Acronyms

AOC Area of Concern

BRAC Base Closure and Realignment

DCE Dichloroethene

IRP Installation Restoration Program

NAS Naval Air Station

NATTC Naval Aviation Technical Training Center

NSA Naval Support Activity

PCE Tetrachloroethene

RCRA Resource Conservation and Recovery Act

RFI RCRA Facility Investigation

RMSE Root mean square error

TCE Trichloroethene

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

Former Airfield Area of Naval Support Activity Mid-South, 

Millington, Tennessee

By Connor J. Haugh, John K. Carmichael, and David E. Ladd

Abstract

Naval Support Activity Mid-South is a Department of the 

Navy base located in Millington, Tennessee. The facility was 

home to the Naval Aviation Technical Training Center from 

1943 until 1996. As part of the Base Closure and Realignment 

Act of 1990, the primary training mission of the facility was 

realigned and most of the northern part of the base, referred to 

as the Northside and consisting primarily of an airfield, was 

transferred to the city of Millington in January 2000. During 

environmental investigations at the base, plumes of dissolved 

chlorinated solvents resulting from past aircraft maintenance 

and training operations were identified in shallow ground water 

beneath the airfield area. The airfield area containing the 

plumes has been designated as Area of Concern (AOC) A. 

Chlorinated solvents, primarily trichloroethene (TCE), are the 

principal contaminants in ground water at AOC A, with TCE 

identified in concentrations as high as 4,400 micrograms per 

liter. The nature and extent of these plumes at AOC A were 

addressed during a Resource Conservation and Recovery Act 

Facility Investigation, and selected options for remediation currently are being implemented under a corrective action program. As part of these efforts, the U.S. Geological Survey 

(USGS) is working with the Navy and its consultants to study 

the hydrogeologic framework of the base and surrounding area, 

with a focus on AOC A. 

Since 1997, investigations at and near the facility have produced data prompting revisions and additions to information 

published that year in two USGS reports. The updates are presented in this report and consist primarily of (1) refinements to 

selected hydrogeologic maps presented in the 1997 reports, on 

the basis of data collected from new wells at on- and off-base 

locations, (2) additional hydraulic-conductivity data collected 

for the alluvial-fluvial deposits aquifer at AOC A, and 

(3) construction of a potentiometric-surface map of the shallow 

aquifer for the former part of the Naval Support Activity MidSouth Northside and adjacent off-base locations for February 

and March 2000 water-level conditions. Additionally, a numerical ground-water-flow model of AOC A was developed and 

calibrated to the February and March 2000 potentiometricsurface data, the results of which also are presented in this 

report. Particle-tracking simulations were used with the model 

to simulate ground-water-flow paths from two sites suspected 

of being contaminant source areas at AOC A. The flow paths 

indicated by the particle tracking simulations agree reasonably 

well with maps of the interpreted extents of TCE plumes. The 

time-of-travel plots show that advective travel times from the 

two suspected source areas to the model boundary are controlled by relative proximities of the source areas to a part of 

AOC A identified from investigations and simulated with the 

model as having the highest horizontal hydraulic conductivity.

Introduction

Naval Support Activity (NSA) Mid-South is a Department 

of the Navy (Navy) base located in Millington, Tennessee 

(fig. 1). The facility was commissioned in 1942 as the Naval 

Reserve Aviation Base. From 1943 until 1995, the base was 

known as the Naval Air Station (NAS) Memphis. During this 

period, NAS Memphis was the home of the Naval Aviation 

Technical Training Center (NATTC) and consisted of two parts 

informally referred to as the Northside, which included an airfield and supporting structures, and the Southside, which 

included housing and training facilities (fig. 2). In 1993, as part 

of the Base Closure and Realignment (BRAC) Act of 1990, 

NAS Memphis was designated for realignment of its primary 

training mission and partial closure. In 1995, NAS Memphis 

became NSA Memphis as part of the BRAC process. In 1996, 

the Navy relocated its NATTC operations from NSA Memphis 

to NAS Pensacola. In 1998, NSA Memphis became NSA MidSouth, and the Navy’s Bureau of Personnel was relocated to the 

facility. In January 2000, a large part of the Northside, including 

the airfield, was transferred to the city of Millington. Currently 

(2004), the base includes all of the Southside and the southernmost part of the Northside (fig. 2).

Plumes of dissolved chlorinated solvents have been identified in ground water in the alluvial-fluvial deposits aquifer, part 

of the shallow aquifer that is present from about 40 to 100 ft 

below land surface, beneath the airfield area of the former 

northern part of the NSA Mid-South Northside. The plumes, 

identified during environmental investigations conducted under 

Introduction 3
4 Hydrogeology and Ground-Water-Flow Simulation...Naval Support Activity Mid-South, Millington, Tennessee

the Navy’s Installation Restoration Program (IRP) at the base, 

are the result of past aircraft maintenance and training operations conducted in the airfield apron area when the base was 

NAS Memphis. Chlorinated solvents, primarily trichloroethene 

(TCE), are the principal contaminants in ground water beneath 

the airfield. TCE has been identified in ground-water samples 

near one of the suspected primary source areas in concentrations as high as 4,400 micrograms per liter (μg/L). Tetrachloroethene (PCE), dichloroethene (DCE), and carbon tetrachloride 

also have been detected in lower concentrations. The presence 

of low concentrations of the cis- isomer of DCE and weakly 

reducing geochemical conditions identified in ground water at 

some locations indicates that natural attenuation of PCE and 

TCE by reductive dechlorination and possibly other processes 

may be occurring within the plumes.

The identification of multiple chlorinated-solvent plumes 

in the airfield area has led to interpretations of multiple source 

areas, most of which are thought to have resulted from relatively small-volume releases of the contaminants. The two largest plumes, however, appear to be the result of significantly 

larger releases; the plume containing the highest solvent concentrations extends about 1 mi downgradient of the suspected 

source areas. Defining the nature and extent of individual 

source areas and resulting plumes has been complicated by the 

apparent coalescing of some of the plumes as they migrate 

northwestward with ground-water flow in the alluvial-fluvial 

deposits aquifer. Rather than attempt to investigate all possible 

sources and plumes within the airfield, an area has been delineated that contains all confirmed and probable locations of solvent contamination in the alluvial-fluvial deposits aquifer that 

are suspected of originating from releases in and near the airfield. This area is designated as Area of Concern (AOC) A. 

Work conducted under the IRP in compliance with the 

Resource Conservation and Recovery Act (RCRA) Correction 

Action Program at AOC A has focused primarily on the two 

largest plumes containing the highest concentrations of chlorinated solvents present at NSA Mid-South (fig. 3). The nature 

and extent of these plumes were addressed during a RCRA 

Facility Investigation (RFI), and selected options for remediation, including monitored natural attenuation and enhanced biodegradation, currently are being implemented under a corrective action program. As part of the RFI process, the U.S. 

Geological Survey (USGS) has been assisting the Navy, Southern Division, Naval Facilities Engineering Command, and its 

contractors in hydrogeologic investigations, including development and calibration of a numerical ground-water-flow model 

of AOC A.

Purpose and Scope

This report provides revisions to maps and interpretations 

of the hydrogeology of NSA Mid-South, including AOC A, that 

appear in previously published reports prepared by the USGS. 

Information from the previous reports is updated for use in the 

design and construction of a ground-water-flow model of 

AOC A and nearby areas. This report documents the AOC A 

model, including data and parameters used in model development and calibration, and presents a description and the results 

of advective-flow particle-tracking simulations that were used 

to estimate ground-water-flow direction and time-of-travel 

from selected locations suspected as being contaminant-plume 

source areas within AOC A.

Previous Investigations

Several reports describe studies of the hydrogeology of 

NSA Mid-South and the surrounding area. Kingsbury and Carmichael (1995) present the hydrogeology of the post-Wilcox 

Group stratigraphic units in the area of NSA Mid-South as interpreted from available geologic data for the area, including the 

results of a test-drilling program conducted on the NSA MidSouth Northside by the USGS in 1994 prior to the onset of sitespecific subsurface investigations on this part of the base. Carmichael and others (1997) update information on the hydrogeology at NSA Mid-South following a second series of test holes 

drilled by the USGS on the NSA Mid-South Southside in 1995 

prior to the onset of site-specific subsurface investigations on 

this part of the base, and include a description of water quality 

in the principal water-bearing units beneath the facility. Robinson and others (1997) present the results of a study conducted 

by the USGS from 1995 through 1997 to simulate ground-water 

flow in the shallow aquifer system using a numerical model calibrated for an approximately 30 square-mile (mi2) area that 

included all of the NSA Mid-South Southside and most of the 

Northside. A series of reports prepared by EnSafe, Inc., Memphis, Tenn., and other consultants to the Navy also describe the 

results of site-specific environmental investigations conducted 

under the IRP at NSA Mid-South.

Hydrogeology

Descriptions of the physical setting of NSA Mid-South 

and the hydrogeologic framework of the base and surrounding 

area are presented in previous reports by Kingsbury and Carmichael (1995), Carmichael and others (1997), and Robinson 

and others (1997). Carmichael and others (1997) describe the 

post-Wilcox Group geologic units underlying the NSA MidSouth area (table 1). Primary geologic units studied under the 

RFI at AOC A include the alluvium, loess, fluvial deposits, and 

Cockfield Formation. Brief descriptions of these units and their 

hydrogeologic significance provide a framework for the 

ground-water-flow model of AOC A.

Alluvium of Pleistocene through Holocene age is present 

in the valleys of streams in the Memphis area, including Big 

Creek Drainage Canal and its tributaries that drain NSA MidSouth (fig. 2). In the valley of Big Creek Drainage Canal and 

along the lower reaches of its larger tributaries such as North 

Fork Creek, the alluvium consists of silt and clay with minor 

amounts of sand in the upper part, and sand and gravel in the 
Hydrogeology 5
6 Hydrogeology and Ground-Water-Flow Simulation...Naval Support Activity Mid-South, Millington, Tennessee

lower part (Carmichael and others, 1997). Thickness of the alluvium ranges from 0 to 70 ft at NSA Mid-South (table 1). The 

alluvium thins as the altitude of its base rises and the coarsegrained lower part becomes absent in the middle and upper 

reaches of the valleys of the main tributaries to Big Creek 

Drainage Canal. The upper part of the alluvium is interpreted to 

grade laterally and to be hydraulically similar to the loess, and

the lower part of the alluvium is interpreted to grade laterally 

and to be hydraulically similar to the fluvial deposits. Saturated 

sand and gravel in the lower part of the alluvium constitute the 

alluvial aquifer in the Memphis area. At NSA Mid-South, 

ground water in the alluvial aquifer is confined locally by the 

overlying finer-grained upper alluvium.

Loess of Pleistocene age, which overlies the fluvial deposits in upland parts of the Memphis area outside the alluvial 

plains of streams, consists of silt with silty clay and silty, fine 

sand. Thickness of the loess ranges from less than 15 to 45 ft at 

NSA Mid-South (table 1). In general, the loess is thinnest on the 

hilltops and thickest on the valley slopes. Locally, silt and clay 

on the valley slopes may include loess reworked or redeposited 

Table 1. Post-Wilcox Group geologic units underlying Naval Support Activity Mid-South, Millington, Tennessee, and their hydrologic 

significance 

[Modified from Parks and Carmichael, 1989, 1990; Kingsbury and Parks, 1993; Kingsbury and Carmichael, 1995; Carmichael and others, 1997] 

System Series Group Stratigraphic unit

(and local name)

Thickness

(in feet) Lithology and hydrologic significance

Holocene 

and Pleistocene Alluvium 

(alluvial deposits)

0-70

Silt, clay, sand, and gravel. Underlies the alluvial plains of 

Big Creek and tributary streams. A sand and gravel horizon 

in the lower part of the unit is connected to the fluvial 

deposits and constitutes part of the alluvial-fluvial deposits 

aquifer.

Quaternary Pleistocene Loess 15-45

Silt, clay, and sand. Predominantly silt with silty clay and 

silty, fine sand at various horizons. Principal unit at the surface in upland areas. Thinnest on the tops of hills and 

ridges; thickest on the valley slopes. Generally serves as the 

upper confining unit for the alluvial-fluvial deposits aquifer. Locally contains perched water tables in the upper part.

Quaternary

and

Tertiary(?)

Pleistocene 

and 

Pliocene(?)

 Fluvial deposits 

 (terrace deposits)

5-80

Sand and gravel; minor clay and ferruginous sandstone. 

Underlies the loess in upland areas. Thickness varies 

greatly because of erosional surfaces at top and base. Constitutes part of the alluvial-fluvial deposits aquifer. Provides 

water to some domestic and farm wells in the NSA MidSouth area.

Cockfield 

Formation

0-185

Sand, silt, clay, and lignite. Interbedded and interlensed. 

Thickness of formation is variable because of erosional surfaces at top and base. Locally contains sand lenses (which 

compose the Cockfield aquifer) in which domestic and farm 

wells are screened. Sand lenses are more prevalent in northern and eastern NSA Mid-South. Commonly consists predominantly of fine sediments and serves as part of the upper 

confining unit for the Memphis aquifer.

Tertiary Eocene Claiborne Cook Mountain 

Formation

10-60

Clay, silt, and sand. Generally consists of clay and silt, but 

locally contains some very fine sand. Locally serves as part 

of the lower confining unit for the Cockfield aquifer and is 

the principal part of the upper confining unit for the Memphis aquifer.

Memphis Sand 865-880

Sand, silt, clay, and minor lignite. Consists of a thick body 

of sand with clay lenses at various horizons. Sand is fine to 

medium or medium to coarse. Upper part contains lenses of 

fine sand and clay. Constitutes the Memphis aquifer—the 

principal aquifer providing water for most domestic, commercial, industrial, and municipal supplies in the Memphis 

area. Provides water to three wells at NSA Mid-South and 

four wells at Millington.
Hydrogeology 7

as colluvial or alluvial deposits. At NSA Mid-South, the lithologic similarity and subtle transition laterally from undisturbed 

loess to reworked loess or alluvium makes differentiation 

between these deposits difficult based solely on general lithologic descriptions (Carmichael and others, 1997). A transitional 

zone ranging from about 3 to 7 ft thick and grading from sandy 

silt at the top to silty sand at the base is present between silt in 

the loess and sand and gravel in the fluvial deposits at NSA 

Mid-South. Because of the low permeability of the silt, the loess 

generally serves both to retard downward movement of 

recharge to and as the upper semi-confining to confining unit 

for the fluvial deposits. Locally, however, the base of the loess 

lies above the potentiometric surface of the fluvial deposits 

beneath the higher land-surface altitude parts of the NSA MidSouth area. The loess commonly contains a perched saturated

zone with water levels that fluctuate seasonally from about 1 to 

15 ft below land surface.

The fluvial (terrace) deposits of Pliocene(?) and Pleistocene age underlie the loess in upland parts of the Memphis 

area and consist of sand and gravel with minor amounts of clay. 

Thickness of the fluvial deposits ranges from 5 to 80 feet at 

NSA Mid-South (table 1). Locally, the sand in the upper part of 

the formation is very fine to fine and silty or clayey, and is similar in character to the transitional zone between the loess and 

fluvial deposits described by Carmichael and others (1997). 

Gravel is present in the fluvial deposits as lenses at various horizons but is more common in the lower part. Therefore, the fluvial deposits may be described as a coarsening downward 

sequence that generally is composed of silty sand or sandy silt 

in the upper part, and sand or sand and gravel through the 

remaining part, but with gravel more common in the lower part. 

Two levels of fluvial deposits are present at NSA MidSouth, a lower level having a basal altitude of about 220 feet 

above NGVD 29 and ranging from about 5 to 80 feet in thickness, and an upper level having a basal altitude of about 300 feet 

above NGVD 29 and ranging from about 10 to 20 feet in thickness. At NSA Mid-South, the lower-level fluvial deposits are 

located beneath most of the facility outside the stream valleys 

where land-surface altitudes generally range from about 265 to 

300 feet above NGVD 29, and the upper-level fluvial deposits 

are located beneath the northernmost former part of the Northside where land-surface altitudes generally range from about 

320 to 350 feet above NGVD 29. The transition between the 

two levels of fluvial deposits is thought to be present in the 

north-central part of the airfield area, northeast of the northwest-southeast trending runway (fig. 2), and where land-surface 

altitudes range between about 300 and 320 feet above 

NGVD 29. An erosional scarp in the underlying Cockfield Formation also is thought to be present in the transition area across 

which the two levels of fluvial deposits may be hydraulically 

connected. The lower-level fluvial deposits generally are saturated and ground water locally is under artesian pressure. The 

upper-level fluvial deposits commonly are dry or contain only a 

thin perched-water zone at their base. 

Saturated sand and gravel in the fluvial deposits constitute 

the fluvial deposits aquifer in the Memphis area. Where hydraulically connected, saturated sand and gravel in the lower alluvium and the fluvial deposits constitute the alluvial-fluvial 

deposits aquifer, the primary part of the “shallow” aquifer at 

NSA Mid-South (Carmichael and others, 1997). Within the 

AOC A study area, however, the alluvial-fluvial deposits aquifer is composed of only the fluvial deposits because the coarsegrained lower part of the alluvium is not known to be present 

beneath the middle to upper reaches of the alluvial valleys of 

North Fork Creek and its tributary Lateral A located in the western part of the area (fig. 2).

The Cockfield Formation of Eocene age is the uppermost 

unit in the Claiborne Group in the Memphis and NSA MidSouth area (table 1). The Cockfield Formation consists of interbedded clay, silt, and sand. Thickness of the Cockfield Formation generally ranges from about 25 to 185 feet in the NSA MidSouth area, but locally the Cockfield Formation may be absent 

as a result of structural relief from faulting and subsequent erosion (Carmichael and others, 1997). The thickest preserved section of the Cockfield Formation is located in the northernmost

part of the NSA Mid-South area beneath the upper-level fluvial 

deposits. Because of its primarily fine-grained texture, the 

Cockfield Formation and the underlying predominantly clayrich Cook Mountain Formation together serve as the lower confining unit for the alluvial-fluvial deposits aquifer and the upper 

confining unit for the Memphis aquifer throughout most of the 

Memphis area. In western Tennessee, including the Memphis 

area, the Cockfield Formation locally contains discontinuous 

sand lenses that supply water to wells and compose the Cockfield aquifer (Parks and Carmichael, 1990). At NSA Mid-South, 

sand lenses in the upper part of the Cockfield aquifer that lie 

beneath the upper-level fluvial deposits are thought to grade laterally into and thus be hydraulically connected to the alluvialfluvial deposits aquifer across and southwest of the erosional 

scarp area. Together, the alluvial-fluvial deposits aquifer and 

the upper part of the Cockfield aquifer lying northeast of the 

erosional scarp comprise the shallow aquifer at NSA Mid-South 

and are referred to as the A1 aquifer (Robinson and others, 

1997). The A1 aquifer is confined from below by clay and silt 

lenses of the Cockfield Formation that form the Cockfield confining unit.

Since the publication of Carmichael and others (1997) and 

Robinson and others (1997), additional hydrogeologic data 

have been collected at and near NSA Mid-South as a result of 

the installation of new monitoring and production wells and the 

identification of existing domestic wells at both on- and offbase locations. On the basis of these additional data, geologic 

structure maps presented in those reports have been revised, and 

a potentiometric-surface map of the A1 aquifer has been constructed for the northern part of the NSA Mid-South area for 

February and March 2000 water-level conditions. Additional 

hydraulic-conductivity data also have been collected for the 

alluvial-fluvial deposits aquifer beneath the airfield area from a 

multiple-well, 24-hour, constant-rate-withdrawal aquifer test 

conducted by EnSafe, Inc., in 1999 as part of the RFI at AOC A 

(EnSafe, Inc., written commun., 2000). The implications of 

these new data to this study are discussed later in this report.
8 Hydrogeology and Ground-Water-Flow Simulation...Naval Support Activity Mid-South, Millington, Tennessee

Structure

Since publication of Carmichael and others (1997) and 

Robinson and others (1997), hydrogeologic data collected from 

several new monitoring and existing domestic wells screened in 

the alluvial-fluvial deposits aquifer or the underlying Cockfield 

Formation in the NSA Mid-South area, along with data from a 

new production well screened in the Memphis aquifer in the 

southeastern part of the NSA Mid-South Northside, were used 

to revise two of the geologic structure maps presented in those 

reports. Revisions were made to the map that shows the altitude 

of the base of the loess or silt and clay in the upper alluvium and 

the map that shows the altitude of the base of sand and gravel in 

the lower alluvium or fluvial deposits. The revised maps were 

prepared at the same scale as those shown in the 1997 reports, 

including all of NSA Mid-South and the surrounding area. For 

this report, maps are presented that show revisions made within 

the smaller AOC A study area only (figs. 4 and 5). The revised 

map of the altitude of the base of the loess or silt and clay in the 

upper alluvium for the AOC A study area shows minor adjustments for this area, mostly as a result of additional geologic 

control points (fig. 4). The most significant change in the map 

of the altitude of the base of sand and gravel in the lower alluvium or fluvial deposits for the AOC A study area is a linear 

depression in this surface located in the northwestern part of the 

study area between Lateral A and North Fork Creek (figs. 5 and 

6a). This depression defines an area where the alluvial-fluvial 

deposits aquifer thickens and the Cockfield confining unit 

appears to thin. These revised maps were used to define the top 

and bottom altitudes and thickness of the alluvial-fluvial deposits aquifer for that part of the area covered by the AOC A 

ground-water-flow model.

Shallow Aquifer 

The shallow or A1 aquifer, as defined by Robinson and 

others (1997) for their “basewide” ground-water-flow model, is 

the primary hydrogeologic unit of interest for this study. Southwest of the erosional scarp in the Cockfield Formation located 

beneath the north-central part of the former NSA Mid-South 

Northside (fig. 5) , the A1 aquifer consists of the alluvial-fluvial 

deposits aquifer; northeast of the erosional scarp, the A1 aquifer 

consists of the upper part of the Cockfield aquifer (fig. 6). As 

described previously, the alluvial-fluvial deposits and upper 

part of the Cockfield aquifers are interpreted to be hydraulically 

connected across the scarp because the fluvial deposits thin and 

the basal altitude of this unit rises from about 220 feet above 

NGVD 29 to about 300 feet above NGVD 29 from southwest to 

northeast across this feature. Because of the rise in basal altitude 

across the scarp, the upper-level fluvial deposits commonly are 

dry or contain only a thin perched water zone in the area northeast of the scarp; thus, the water table in this area is in the upper 

part of the Cockfield aquifer. For this study, the top of that part 

of the A1 aquifer that consists of the alluvial-fluvial deposits 

aquifer was defined by the revised structure map of the base of 

the loess or silt and clay in the upper alluvium (fig. 4). The bottom of that part of the A1 aquifer that consists of the alluvialfluvial deposits aquifer was defined by the revised structure 

map of the base of sand and gravel in the lower alluvium or fluvial deposits (fig. 5). The bottom of that part of the A1 aquifer 

that consists of the upper part of the Cockfield aquifer has been 

defined by the altitude of the top of a clay unit in the Cockfield 

Formation, designated the Cockfield confining unit by Robinson and others (1997). The top of the A1 aquifer in this area is 

the water table as defined by water levels measured in a couple 

of wells screened in the upper part of the Cockfield Formation 

in February and March 2000. Ground water in the A1 aquifer in 

the study area exists under artesian conditions in most of the 

part consisting of the alluvial-fluvial deposits aquifer, and 

under water-table conditions in the section consisting of the

upper part of the Cockfield aquifer where water levels are below 

the base of the loess, with the transition in conditions occurring 

near the erosional scarp.

The basewide flow model presented in Robinson and others (1997) was constructed using data from the investigation 

described by Carmichael and others (1997). A horizontal 

hydraulic-conductivity value of 5.3 feet per day (ft/d) was used 

as input to the basewide model for that part of the A1 aquifer 

that consists of the alluvial-fluvial deposits aquifer, and a value 

of 1 ft/d was used as input for that section of the A1 aquifer that 

consists of the upper part of the Cockfield aquifer. The value of 

hydraulic conductivity used for the alluvial-fluvial deposits 

aquifer was obtained from a multiple-well, 24-hour constantrate-withdrawal aquifer test conducted by the USGS in 1995 at 

a location south of the western end of the airfield apron and was 

thought at the time to be the most reliable value available for 

calibrating the model. The value of hydraulic conductivity used 

for the upper part of the Cockfield aquifer was estimated from 

the results of specific-capacity tests conducted by EnSafe, Inc., 

in wells screened in this unit (Robinson and others, 1997). In 

1999, as part of the AOC A RFI, EnSafe, Inc., conducted a 

multiple-well, 24-hour constant-rate-withdrawal aquifer test in 

the alluvial-fluvial deposits aquifer within AOC A, at a location 

about 2,500 feet north of the USGS aquifer-test site. This test 

resulted in horizontal hydraulic conductivities for the pumping 

and for selected observation wells that range from 44.6 to 

68.4 ft/d, with a geometric mean of 59.1 ft/d (EnSafe, Inc., written commun., 2000). These horizontal hydraulic conductivity 

values are about an order of magnitude greater than the value 

from the USGS test and show that the hydraulic conductivity of 

the alluvial-fluvial deposits aquifer in the airfield area is variable and higher in some areas than the value of 5.3 ft/d used uniformly for this aquifer in the basewide flow model. In addition 

to spatial variations in horizontal hydraulic conductivity, quantitative hydraulic data from slug tests and borehole-flowmeter 

measurements made in selected wells, as well as qualitative data 

from pumping rates, drawdown, and water-level recovery during well sampling, also indicate that horizontal hydraulic conductivities generally are lowest in the upper part of the fluvial 

deposits and increase with depth.
Hydrogeology 9
10 Hydrogeology and Ground-Water-Flow Simulation...Naval Support Activity Mid-South, Millington, Tennessee
Simulation of Ground-Water Flow 11

Water-level data were collected from wells in the AOC A 

study area in February and March 2000 and a potentiometricsurface map of the A1 aquifer was constructed (fig. 7). The 

potentiometric-surface map of the A1 aquifer for February and 

March 2000 is similar to the previous potentiometric-surface 

maps of the alluvial-fluvial deposits aquifer for April and October 1996 (Carmichael and others, 1997). Overall, water-levels 

for the February and March 2000 map are about 3 feet lower 

than the average water levels for April and October 1996. The 

February and March 2000 map also shows steep gradients and 

highest water-levels in the scarp area located in the northeastern 

part of the study area where the A1 aquifer transitions from the 

alluvial-fluvial deposits aquifer to the upper part of the Cockfield aquifer. This transition occurs near the 260-foot potentiometric contour (fig. 7). A ground-water divide occurs southeast 

of the airfield area. Ground-water flow directions generally are 

to the northwest in most of the model area. A depression in the 

potentiometric surface occurs in an off-base location in the 

northwestern part of the area where water levels in the A1 aquifer are at an altitude of less than 225 feet. This depression coincides with an area where data collected during monitoring-well 

installations show that the alluvial-fluvial deposits aquifer 

thickens and, on the basis of relatively flat topography in the 

area, the Cockfield Formation consequently thins. Downward 

leakage of water locally through the Cockfield and Cook Mountain confining units is indicated by observed water levels in the 

A1 aquifer in this depression that are approximately equal in 

altitude to water levels in the Memphis aquifer in the area (Robinson and others, 1997, fig. 13). Downward leakage of water 

from the alluvial-fluvial deposits aquifer to the Memphis aquifer has been identified on the basis of depressed water levels in 

the alluvial-fluvial deposits aquifer at various locations in the 

Memphis area (Parks, 1990).

Continuous water-level data are available for about 9 years 

(1995 to 2004) at well Sh:U-101 screened in the fluvial deposits 

located south of the western end of the airfield apron (fig. 7) at 

the site of the aquifer test conducted by the USGS in 1995 (Carmichael and others, 1997). These data are indicative of the timing and magnitude of water-level fluctuations in the alluvialfluvial deposits aquifer, the primary part of the A1 aquifer in the 

AOC A study area. The hydrograph for well Sh:U-101 (fig. 8) 

shows short- and long-term water-level fluctuations in response 

to seasonal and annual changes in recharge. Water levels have 

fluctuated between 3 and 8 feet annually in this well during the 

period of record (1995-2004). On the basis of periodic measurements, water levels throughout the A1 aquifer in the study area 

generally fluctuate by about 1 to 5 feet annually (Robinson and 

others, 1997). The hydrograph for well Sh:U-101 shows no 

obvious trends, increasing or decreasing, in water levels, and no 

significant effects from pumping on the A1 aquifer within the 

study area are indicated.

Simulation of Ground-Water Flow

The physical system, described in the hydrogeology section of this report, provides the framework for design and calibration of a numerical ground-water-flow model of AOC A. 

Models that simulate the flow of water through aquifers are useful tools to test the understanding and conceptualization of a 

flow system. Although a model is necessarily a simplification of 

the physical system, the model should be consistent with all 

known hydrogeologic observations. The ground-water-flow 

model code used in this study, MODFLOW-2000, was developed by McDonald and Harbaugh (1988) and was recently 

updated (Harbaugh and others, 2000). MODFLOW uses the

finite-difference technique to solve the ground-water-flow 

equation for three-dimensional, steady or non-steady flow in a

heterogeneous and anisotropic medium.

A steady-state model of the A1 aquifer in the AOC A study 

area was constructed and calibrated to conditions of February 

and March 2000. Following model calibration, a particletracking simulation was used to analyze ground-water-flow 

paths from selected locations within AOC A.

Conceptual Model

The modeled A1 aquifer consists of the alluvial-fluvial 

deposits aquifer within most of the modeled area and the upper 

part of the Cockfield aquifer in the scarp area and to the northeast. As described in the hydrogeology section of this report, the 

alluvial-fluvial deposits are a coarsening downward sequence 

that generally is composed of silty sand or sandy silt in the 

upper part, and sand or sand and gravel through the remaining 

part, but with gravel more common in the lower part. Additionally, hydraulic conductivity generally is lowest in the upper part 

of the formation and increases with depth. Recharge to the modeled aquifer occurs as leakage from the overlying loess. Ground 

water does not discharge to streams or springs anywhere within 

the model area. Ground water leaves the modeled area through 

a constant-head boundary at the 225-foot potentiometric contour that defines a depression in the potentiometric surface in 

the alluvial-fluvial deposits aquifer (fig. 7). This constant-head 

boundary is near an area where the Cockfield confining unit is 

thought to thin and where water moves downward and out of the 

A1 aquifer.

Model Assumptions

The following assumptions were made in the development 

of the flow model of the hydrologic system in the AOC A study 

area:

1. The A1 aquifer is assumed to be in steady-state conditions 

because water levels in February-March 2000 were relatively stable (fig. 8).
12 Hydrogeology and Ground-Water-Flow Simulation...Naval Support Activity Mid-South, Millington, Tennessee
Simulation of Ground-Water Flow 13
14 Hydrogeology and Ground-Water-Flow Simulation...Naval Support Activity Mid-South, Millington, Tennessee
Simulation of Ground-Water Flow 15

2. The top boundary of the A1 aquifer model is assumed to 

be the bottom of the loess in the area southwest of the 

scarp where artesian conditions predominantly exist and 

the water table in the scarp area and to its northeast 

where water levels are below the base of the loess.

3. The bottom boundary of the A1 aquifer model is assumed 

to be the base of the alluvial-fluvial deposits aquifer 

southwest of the scarp and the base of the upper part of 

the Cockfield aquifer northeast of the scarp. This 

boundary, which corresponds to the top of the Cockfield 

confining unit throughout the model area, is assumed to 

be a no-flow boundary.

4. The hydraulic properties of the hydrogeologic units are 

homogeneous within a block of the finite-difference grid.

5. The hydraulic properties are isotropic within a layer.

6. Flow within a layer is horizontal; flow between layers is 

vertical.

Model Boundaries

The lateral boundaries of the AOC A model correspond to 

a local ground-water divide, ground-water flow-path lines, and 

a potentiometric contour (fig. 7). The east-southeastern boundary corresponds to a local ground-water divide and is simulated 

as a no-flow boundary. The north-northeastern and southsouthwestern boundaries correspond to ground-water flow-path 

lines as defined by the February and March 2000 potentiometric 

surface of the A1 aquifer and are simulated as no-flow boundaries. The northwestern boundary corresponds to the 225-foot 

potentiometric contour of February and March 2000 and is simulated as a constant-head boundary. The upper boundary of the 

model ranges between altitudes of 220 and 285 feet above 

NGVD 29 and corresponds to the base of the loess southwest of 

the scarp where artesian conditions exist and to the water table 

northeast of the scarp where water levels are below the base of 

the loess (fig. 9). A recharge flux simulating leakage from the 

loess was applied to the upper boundary. The bottom boundary 

of the A1 aquifer is the top of the Cockfield confining unit 

throughout the model area which corresponds to the base of the 

alluvial-fluvial deposits aquifer southwest of the scarp and to 

the base of the upper part of the Cockfield aquifer northeast of 

the scarp (fig. 10). This boundary ranges between altitudes of 

160 and 250 ft and is simulated as a no-flow boundary.

Model Construction

The AOC A model grid is approximately a 1.8- by 1.9-mi 

rectangle consisting of 100- by 100-ft grid cells (fig. 11). The 

grid is composed of 92 columns and 103 rows. About 2.2 mi2 of 

the 3.4-mi2 model grid is active. Vertically, the total thickness 

of the A1 aquifer (fig. 12) was divided into three equal layers to 

model vertical variations in aquifer texture and resulting 

hydraulic conductivity.

Model parameters, as discussed by Harbaugh and others 

(2000), were defined for recharge and hydraulic-conductivity 

zones (table 2). Recharge to the model is from downward leakage from the overlying loess and is constant throughout the 

area.

Hydraulic-conductivity zones for the AOC A flow model 

were determined on the basis of geology and well hydraulic-test 
16 Hydrogeology and Ground-Water-Flow Simulation...Naval Support Activity Mid-South, Millington, Tennessee
Simulation of Ground-Water Flow 17
18 Hydrogeology and Ground-Water-Flow Simulation...Naval Support Activity Mid-South, Millington, Tennessee
Simulation of Ground-Water Flow 19
20 Hydrogeology and Ground-Water-Flow Simulation...Naval Support Activity Mid-South, Millington, Tennessee

data including aquifer tests, slug tests, and borehole-flowmeter 

measurements, as well as on supplemental data from pumping 

rates, drawdown, and water-level recovery during well sampling. Each of the layers contains four hydraulic-conductivity 

zones (fig. 13). The highest conductivity zone (HK_High) is in 

the northwestern part of the model area where slug tests and the 

multiple-well aquifer test conducted by EnSafe, Inc., (written 

commun., 2000), indicate the highest horizontal hydraulic conductivities for the alluvial-fluvial deposits aquifer at AOC A. 

Based on 12 measured values, hydraulic conductivities in this 

zone range from 45 to 68 ft/d with a median value of 61 ft/d. The 

lowest conductivity zones (HK_ScarpA and HK_ScarpB) are 

located in the scarp area in the eastern part of the model area. In 

the HK_ScarpA zone, the aquifer consists of the upper part of 

the Cockfield aquifer. In the HK_ScarpB zone, the aquifer consists of both the alluvial-fluvial deposits aquifer and the upper 

part of the Cockfield aquifer. Based on 14 measured values, horizontal hydraulic conductivities in the upper part of the Cockfield Formation range from 0.5 to 3 ft/d with a median value of 

1.0 ft/d (Robinson and others, 1997). The other conductivity 

zone (HK_Average) is located in the center and southern part of 

the model area where slug tests and a multiple-well aquifer test 

indicate hydraulic conductivities to be about an order of magnitude lower than the HK_High zone. Based on 10 measured values, horizontal hydraulic conductivities in this zone range from 

5 to 40 ft/d (excluding one outlier of 150 ft/d) with a median 

value of 13 ft/d (Robinson and others, 1997).

Each model layer contains four hydraulic-conductivity 

zones. The layers are set up such that hydraulic conductivity 

increases with depth by a factor of two between layers in zones 

HK_High and HK_Average. Hydraulic conductivity is constant 

in all layers in zones HK_ScarpA and HK_ScarpB (table 3).

Transmissivity for each cell was calculated by the model 

using hydraulic-conductivity and layer or saturated thickness 

data, both of which vary areally. The model layers were 

assumed to be hydraulically well connected and not separated 

by confining material. The vertical hydraulic conductivity was 

modeled as a constant ratio of the horizontal hydraulicconductivity value.

Model Calibration

The process of adjusting the input variables to produce the 

best match between simulated and observed water levels and 

flows is known as calibration. The model developed for this 

study was calibrated to steady-state conditions as defined by the 

potentiometric-surface map of the A1 aquifer for February and 

March 2000 (fig. 7). Because no ground water discharges to surface features within the modeled area, ground-water discharge 

fluxes could not be used to aid in model calibration. The 

recharge flux into the model (leakage from the overlying loess) 

is difficult to measure in the field, so no independent measurements of this flux are available. Therefore, data to calibrate the 

model are limited to matching observed water levels and measured hydraulic-conductivity values. To provide additional 

information to help calibrate the model, an advective-flow 

observation was added (Anderman and Hill, 1997; Hill and others, 2000). An advective-flow observation requires specifying 

two observation points, the advective travel time between the 

points, and the aquifer porosity. The starting and ending points 

for the advective-flow observation were at the southeastern end 

of the main TCE plume and the northwestern edge of the former 

Northside property boundary where the TCE plume crosses this 

boundary (fig. 3). An advective travel time of 40 years was estimated based on ground-water velocities of between 30 and 

140 ft/yr determined for two different zones by Ensafe, Inc. 

(written commun., 2000). This travel time is consistent with the 

estimated site disposal history. An aquifer effective porosity of 

25 percent was assumed (Robinson and others, 1997).

The AOC A model was calibrated using a combination of 

automated and manual methods to minimize the difference 

Table 2. Recharge and hydraulic conductivity parameters defined in the Area of Concern A flow model, Naval Support Activity Mid-South

Model

parameter Description

RECH Recharge rate to the A1 aquifer from leakage from the overlying loess

HK_Average Hydraulic conductivity in layer 3 for the model area outside the scarp area and southeast of the main runway. The A1 aquifer in this area consists of the alluvial-fluvial deposits aquifer. Hydraulic conductivity of layers 2 and 1 in this area is calculated by multiplying the parameter HK_Average by 0.5 and 0.25, respectively.

HK_High Hydraulic conductivity in layer 3 for the area outside the scarp area and northwest of the main runway. The A1 aquifer in 

this area consists of the alluvial-fluvial deposits aquifer. Hydraulic conductivity of layers 2 and 1 in this area is calculated 

by multiplying the parameter HK_High by 0.5 and 0.25, respectively.

HK_ScarpA Hydraulic conductivity in layers 1, 2, and 3 for the scarp area. The A1 aquifer in this area consists of the upper part of the 

Cockfield aquifer.

HK_ScarpB Hydraulic conductivity in layers 1, 2, and 3 for a transition area near the edge of the scarp. The A1 aquifer in this area consists of both the alluvial-fluvial deposits aquifer and the upper part of the Cockfield aquifer.

VANI Ratio of the horizontal to vertical hydraulic conductivity
Simulation of Ground-Water Flow 21
22 Hydrogeology and Ground-Water-Flow Simulation...Naval Support Activity Mid-South, Millington, Tennessee

between simulated and measured water levels, and the simulated and estimated advective travel time. These observations 

were weighted, as described by Hill (1998), to account for the 

greater accuracy in the measured water levels and lesser accuracy in the estimated advective travel time. The four most sensitive parameters (HK_High, HK_Average, HK_ScarpA, and 

RECH) were estimated using the parameter-estimation process 

in MODFLOW-2000. The two least sensitive parameters, 

HK_ScarpB and the ratio of horizontal to vertical hydraulic 

conductivity (VANI), were calibrated manually during the calibration process.

Overall, simulated water levels (fig. 14) agree reasonably 

well with measured water levels (fig. 7). Water levels were 

available for comparison at 68 wells. The root mean square 

error (RMSE) was calculated to compare simulated and measured water levels. The RMSE, in feet, is calculated by 

(1)

where

N is the number of observations;

hi

m is the measured water level, in feet; and

hi

c is the simulated water level, in feet.

The RMSE for water levels is 1.1 ft. The average difference between measured and simulated water levels for the calibration model simulation is 0.04 ft. Sixty-two percent of the 

simulated water levels are within 1 ft of the observed water levels and 96 percent are within 2 ft. The errors or residuals in simulated water levels show no significant spatial pattern; however, 

in a small area in the southeastern corner of the model, simulated water-levels tended to be consistently higher than 

observed water levels.

Calibrated horizontal hydraulic conductivities for the 

AOC A flow model range from 1.2 to 65 ft/d (table 4). These 

values are comparable to measured values from well hydraulic tests (Robinson and others, 1997, table 3; Ensafe, Inc., written commun., 2000). Calibrated transmissivities for layer 1 vary 

from 7.9 to 430 ft2

/d (fig. 15) with an average of 110 ft2/d and 

a median of 61 ft2

/d. The highest transmissivities in layer 1 are 

in the northwestern part of the model area where a thick section 

of the alluvial-fluvial deposits aquifer coincides with higher 

than average hydraulic-conductivity values. Calibrated transmissivities for layer 2 vary from 7.9 to 870 ft2

/d (fig. 16) with 

an average of 220 ft2

/d and a median of 120 ft2/d. Calibrated 

transmissivities for layer 3 vary from 7.9 to 1,700 ft2/d (fig. 17) 

with an average of 430 ft2/d and a median of 240 ft2/d. The calibrated hydraulic conductivities used in this model are greater 

than the conductivities used by Robinson and others (1997) for 

the basewide flow model. The higher calibrated values for the 

AOC A model are the result of data collected from additional 

wells and the EnSafe, Inc., aquifer test that indicates hydraulic 

conductivities of the alluvial-fluvial deposits aquifer at AOC A 

are higher than the value of about 5 ft/d assumed to be representative of this aquifer by Robinson and others (1997). Additionally, the areal and vertical distribution of hydraulic conductivities at AOC A are now better understood on the basis of 

additional data collected since 1997.

The ratio of horizontal to vertical hydraulic conductivity 

(VANI) for the calibrated AOC A flow model is 100. This ratio 

is supported by horizontal and vertical hydraulic-conductivity 

data presented in Robinson and others (1997, tables 2 and 3).

The AOC A flow model calibrated recharge rate is 

1.82 in/yr. This recharge rate is higher than the natural recharge 

rate of 0.32 in/yr used in the basewide ground-water model by 

Robinson and others (1997). The basewide model also included 

anthropogenic recharge in some areas at rates between 0.67 and 

1.8 in/yr. The AOC A area, however, had no anthropogenic 

recharge applied in the basewide model. Differences in these 

recharge rates are discussed in the section “Model Limitations.” 

This recharge rate of 1.82 in/yr results in a ground-water flux of 

0.31 ft3/s through the model. This is the total flux through the 

model because recharge is the only source of water for the 

model. All water leaves the model through the constant-head 

boundary near the northwestern edge of the model area.

Many of the estimated parameters for the calibrated AOC 

A flow model show high correlation coefficients (table 5). 

RMSE hi

m hic 〈 〉 – 2

i = 1

N

∑⎝ ⎠ ⎜ ⎟ ⎛ ⎞ = ⁄ N

Table 3. Relation between hydraulic conductivity parameters and hydraulic conductivity by layers 

for the Area of Concern A flow model, Naval Support Activity Mid-South

Hydraulic conductivity zone

HK_Average HK_High HK_ScarpA HK_ScarpB

Layer 1 0.25 * HK_Average 0.25 * HK_High HK_ScarpA HK_ScarpB

Layer 2 0.5 * HK_Average 0.5 * HK_High HK_ScarpA HK_ScarpB

Layer 3 HK_Average HK_High HK_ScarpA HK_ScarpB

Table 4. Calibrated hydraulic conductivities of the Area of Concern A 

flow model, Naval Support Activity Mid-South

Calibrated hydraulic conductivity, in feet per day

HK_Average HK_High HK_ScarpA HK_ScarpB

Layer 1 3.6 16 1.2 8.0

Layer 2 7.3 32 1.2 8.0

Layer 3 15 65 1.2 8.0
Simulation of Ground-Water Flow 23
24 Hydrogeology and Ground-Water-Flow Simulation...Naval Support Activity Mid-South, Millington, Tennessee
Simulation of Ground-Water Flow 25
26 Hydrogeology and Ground-Water-Flow Simulation...Naval Support Activity Mid-South, Millington, Tennessee
Simulation of Ground-Water Flow 27

When parameters are correlated, the parameter-estimation process may not have enough information to estimate parameters 

individually and may estimate only the ratio or sums of parameters. To determine if the parameters were uniquely estimated, 

the parameter estimation was run from several different sets of 

starting parameter values. In each case, the regression converged to the same final values. This result indicates that the 

final values probably were estimated individually. Testing during model calibration indicates that without the advective-flow 

observation, parameters could not have been estimated individually.

Sensitivity Analysis

Composite scaled sensitivities were calculated for the calibrated AOC A flow model by using the sensitivity process in 

MODFLOW-2000 for all the hydraulic-conductivity and 

recharge parameters (fig. 18). Hill and others (2000) describe 

how sensitivities can be calculated for any of the model parameters discussed by Harbaugh and others (2000). Composite 

scaled sensitivities can be used to compare the importance of 

different parameters to the calculation of model simulated water 

levels and flows (Hill, 1998). Parameters with greater 

composite sensitivities have greater importance and 

influence on the model solution. The most sensitive 

parameter in the AOC A flow model is the recharge rate 

(RECH). The next most sensitive parameter is the 

hydraulic conductivity for the average zone 

(HK_Average), followed by the hydraulic conductivity 

for the high zone (HK_High). The model is least sensitive to the hydraulic conductivity in the scarp area 

(HK_ScarpA and HK_ScarpB) and the ratio of horizontal to vertical hydraulic conductivity (VANI).

Model Limitations

The AOC A flow model is calibrated as a steadystate model to water levels measured in the study area in 

February and March 2000. These measurements are 

assumed to be representative of long-term average conditions; however, not enough long-term water-level data 

exist for the study area to define the actual long-term average 

conditions. The flow paths investigated in the study area are 

estimated to be about 40 years old, but continuous water-level 

data are available for only about 9 years at well Sh:U-101 

screened in the fluvial deposits at AOC A (fig. 8). If conditions 

in the aquifer have changed significantly over the past 40 years, 

the model flow field may not be representative of long-term 

conditions in the aquifer. The hydrograph shows no obvious 

trends, increasing or decreasing, in water levels for the period 

of record. Overall, water levels for the February and 

March 2000 map are about 3 ft lower than the average of the 

water levels for April and October 1996 that were used by Robinson and others (1997) to calibrate the basewide model, but the 

horizontal gradients across the study area are similar. Therefore, 

the assumption that the aquifer is at steady state is considered 

adequate to investigate flow paths within the study area.

The AOC A model is consistent with the current conceptual model of ground-water flow and assumed boundary conditions. However, the model, by necessity, is a simplified approximation of the actual ground-water system at AOC A. For 

example, the model simulates known variations in hydraulic 

conductivity, both areally and with depth, by using four 

hydraulic-conductivity zones and three layers, but this representation of the aquifer still is a simplification of the actual spatial variations and patterns of aquifer properties. Similarly, the 

model uses a uniform recharge rate to simulate water leakage 

through the loess. The actual recharge rate probably varies spatially, but the processes that control water movement through 

the loess are poorly understood; therefore, the spatial pattern of 

recharge was assumed to be uniform. Additionally, the model 

cannot provide simulations on a scale finer than the grid resolution.

The model provides a reasonable match to measured water 

levels and gradients (figs. 7 and 14), but no independent check 

on the model flux (recharge or discharge) is available. This lack 

of an independent check on model flux is the most important 

limitation of the flow model. Robinson and others (1997, 

Table 5. Correlation coefficients between estimated parameters of 

the Area of Concern A flow model, Naval Support Activity Mid-South

Estimated 

parameters

Correlation coefficients

RECH HK_High HK_Average HK_ScarpA

RECH 1.00

HK_High 0.97 1.00

HK_Average 0.98 0.91 1.00

HK_ScarpA 0.98 0.95 0.96 1.00
28 Hydrogeology and Ground-Water-Flow Simulation...Naval Support Activity Mid-South, Millington, Tennessee

p. 45-46) clearly explain this limitation with the basewide 

model. With no independent measurement of the recharge flux, 

any constant ratio of recharge flux and hydraulic conductivity 

will produce the same simulated potentiometric surface. Therefore, in both of these models, the calibrated value of recharge is 

directly dependent on the values and distribution of hydraulic 

conductivity. This explains why the AOC A model presented in 

this report has a higher value of recharge (1.82 in/yr) than that 

used for the AOC A area of the basewide model (0.32 in/yr) of 

Robinson and others (1997). The basewide model assumed a 

constant hydraulic conductivity of about 5 ft/d for the alluvialfluvial deposits aquifer based on the aquifer test conducted 

south of the western end of the airfield apron. Additional data 

and well tests indicated that the hydraulic conductivity in parts

of the alluvial-fluvial deposits aquifer at AOC A is higher than 

5 ft/d. The basewide model underestimated the hydraulic conductivity at AOC A and therefore underestimated the recharge 

rate. Given that the hydraulic-conductivity values and distribution have been better defined at AOC A since 1997, the calibrated recharge rate of 1.82 in/yr should be a more realistic estimate of recharge for the area, but this estimate still is limited by 

the confidence in the hydraulic-conductivity data.

The advective-flow observation helps overcome the lack 

of ground-water flux data and allows the conductivity and 

recharge parameters to be estimated individually. Although the 

mapped contaminant plume clearly defines the advective flow 

path, the estimate of the advective travel time is uncertain. The 

exact time when the contaminants were released to the aquifer 

is unknown. Other factors such as dispersion, retardation, and 

degradation affect travel times, causing the contaminants to 

move faster or slower than the ground water. An advective 

travel time of 40 years used to calibrate the AOC A model is 

consistent with all known data and results in estimates of 

parameters that are within expected ranges. If the advective 

travel time is actually 30 years, then the model would converge 

to higher parameter values for hydraulic conductivity and 

recharge, but the ratio of conductivity to recharge remains constant. Similarly, if the advective travel time is actually 50 years, 

then the model would converge to lower parameter values for 

hydraulic conductivity and recharge (table 6).

Advective Flow Particle Tracking

Ground-water-flow paths were simulated from two sites at 

AOC A suspected as being contaminant-plume source areas. 

The sites simulated as source areas are the “grassy area” and the 

“north edge of apron” (fig. 19). MODPATH, a particle-tracking 

program (Pollock, 1994), was used to simulate ground-waterflow directions and times-of-travel from these areas. The 

particle-tracking analysis does not account for physical and 

chemical processes such as dispersion, sorption, or degradation 

that would cause dissolved contaminants to move at velocities 

different from the average ground-water velocity. A contaminant plume, as the result of dispersion, also would spread out 

more than the advective flow paths indicated. A uniform effective porosity of 25 percent was assumed for the particle-tracking analysis conducted for this study (Robinson and others, 

1997). The flow paths indicated by the particle tracking (fig. 19) 

agree reasonably well with the interpreted maps of TCE plumes 

(fig. 3). The time-of-travel plots show that travel times from the 

north edge of the apron area source to the model boundary are 

faster than travel times from the grassy area source to the same 

boundary. Particles originating at the north edge of the apron 

area source enter the high hydraulic conductivity zone of the A1 

aquifer sooner than particles originating from the grassy area 

source, which decreases their travel time to the boundary. This 

simulation illustrates how the distribution of hydraulic conductivities can affect travel times.

Table 6. Estimated parameter values using alternate advective 

travel times, Area of Concern A flow model, Naval Support 

Activity Mid-South

Parameter

Advective travel times

30 years 40 years 50 years

RECH (inches) 2.47 1.86 1.49

HK_High (feet/day) 86.3 65.0 52.0

HK_Average (feet/day) 19.3 14.6 11.6

HK_ScarpA (feet/day) 1.57 1.17 0.929
Simulation of Ground-Water Flow 29
30 Hydrogeology and Ground-Water-Flow Simulation...Naval Support Activity Mid-South, Millington, Tennessee

Summary and Conclusions

Naval Support Activity (NSA) Mid-South is a Department 

of the Navy base located in Millington, Tennessee. During environmental investigations at the base, plumes of dissolved chlorinated solvents resulting from past aircraft maintenance and 

training operations were identified in shallow ground water 

beneath the airfield area. The area containing the plumes has 

been designated as Area of Concern (AOC) A. Chlorinated solvents, primarily trichloroethene, are the principal contaminants 

in the ground water at AOC A. The nature and extent of the primary plumes at AOC A were addressed during a Resource Conservation and Recovery Act (RCRA) Facility Investigation 

(RFI), and selected options for remediation currently (2004) are 

being implemented under a corrective action program.

Geologic units of primary importance to hydrogeologic 

investigations conducted under the RFI at AOC A include the 

alluvium, loess, fluvial deposits, and Cockfield Formation. Saturated and hydraulically connected sections of the lower, 

coarse-grained part of the alluvium and the fluvial deposits constitute the alluvial-fluvial deposits aquifer, the primary part of 

the shallow aquifer at NSA Mid-South. Within the AOC A 

study area, the alluvial-fluvial deposits aquifer is interpreted as 

comprising only the fluvial deposits because the coarse-grained 

lower part of the alluvium is absent beneath the middle and 

upper reaches of the alluvial valleys of North Fork Creek and its 

tributary Lateral A located in the western part of the area. The 

upper, fine-grained sections of the alluvium and loess generally 

retard recharge to and serve as the upper confining unit for the 

alluvial-fluvial deposits aquifer, and clay in the Cockfield Formation and underlying Cook Mountain Formation serve as the 

lower confining unit for the aquifer and the upper confining unit 

for the Memphis aquifer. Discontinuous saturated sand lenses 

are present locally in the Cockfield Formation and constitute the 

Cockfield aquifer. Within the AOC A study area, the alluvialfluvial deposits and Cockfield aquifers are interpreted as 

hydraulically connected across an erosional scarp in the Cockfield Formation located in the northern part of the area.

Recent investigations at and near the facility have produced new data prompting updates to two USGS reports published in 1997. The updates consist primarily of (1) refinements 

to geologic structure maps presented in the 1997 reports that 

show the altitude of the base of the loess or silt and clay in the 

upper alluvium, and the altitude of the base of sand and gravel 

in the lower alluvium or fluvial deposits at AOC A on the basis 

of data collected from new wells at on- and off-base locations; 

(2) additional hydraulic-conductivity data for the alluvialfluvial deposits aquifer at AOC A from a multiple-well, 24-hour

constant-rate-withdrawal aquifer test conducted by EnSafe, 

Inc., in 1999; and (3) construction of a potentiometric-surface 

map of the A1 aquifer for the northern part of the NSA MidSouth area for February and March 2000 water-level conditions. As part of the study, the USGS also developed and calibrated a numerical ground-water-flow model of AOC A.

A steady-state model of the shallow aquifer at AOC A was 

constructed and calibrated to conditions of February and March 

2000. Following model calibration, a particle-tracking simulation was used to analyze ground-water-flow paths and time-oftravel from selected locations suspected as being contaminant 

source areas within AOC A. The modeled aquifer, designated as 

the A1 aquifer for a previous modeling study at NSA MidSouth, consists of the alluvial-fluvial deposits aquifer within 

most of the modeled area and the upper part of the Cockfield 

aquifer in the scarp area and to the northeast. The A1 aquifer in 

this study was divided into three layers to simulate vertical variations in lithology and hydraulic conductivity in the fluvial 

deposits. Horizontal hydraulic conductivity of the alluvialfluvial deposits aquifer at AOC A increases with depth; therefore, in areas where the A1 aquifer consists of the alluvialfluvial deposits aquifer, layer 1 has the lowest hydraulic conductivity and layer 3 has the highest hydraulic conductivity. In 

the scarp area and to the northeast, where the alluvial-fluvial 

deposits are unsaturated and the A1 aquifer consists of the upper 

part of the Cockfield aquifer, hydraulic conductivity is assumed 

to be constant with depth. Recharge to the modeled aquifer 

occurs as leakage from the overlying loess. Ground water does 

not discharge to streams or springs anywhere in the model area. 

Ground water leaves the modeled area through a constant head 

boundary at the 225-ft potentiometric contour that defines a 

depression in the potentiometric surface in the alluvial-fluvial 

deposits aquifer. This constant-head boundary is near an area 

where the alluvial-fluvial deposits aquifer thickens and the 

Cockfield Formation thins. Downward leakage of water locally 

occurs from the alluvial-fluvial deposits aquifer through the 

Cockfield and Cook Mountain confining units and is indicated 

by observed water levels in the alluvial-fluvial deposits aquifer 

in this depression that are approximately equal in altitude to 

water levels in the Memphis aquifer in this area.

The model developed for this study was calibrated to 

steady-state conditions as defined by the potentiometric-surface 

map for February and March 2000. The model was calibrated to 

observed water levels and measured hydraulic-conductivity 

values. To provide additional information to help calibrate the 

model, an advective-flow observation was added. Overall, simulated water levels agree reasonably well with measured water 

levels. Water levels were available for comparison at 68 wells. 

The RMSE for water levels is 1.1 ft. The average difference 

between measured and simulated heads for the calibrated model 

is 0.04 ft.

Calibrated horizontal hydraulic conductivities for the 

AOC A flow model range from 1.2 to 65 ft/d. The calibrated 

hydraulic conductivities used in the AOC A model are greater 

than the value of 5.3 ft/d used for the “basewide” model of NSA 

Mid-South calibrated in 1996. The higher calibrated values for 

the AOC A model are the result of data from the 1999 aquifer 

test that indicate hydraulic conductivities in parts of the alluvial-fluvial deposits aquifer at AOC A are higher than previously determined.

The AOC A flow model calibrated recharge rate is 

1.82 in/yr. This recharge rate is higher than the natural recharge 

rate of 0.32 in/yr used in the basewide ground-water model. The 

higher recharge rate also is the result of hydraulic conductivity 
Selected References 31

values being higher in the AOC A study area than previously 

determined.

The model provides a reasonable match to measured water 

levels and gradients, but no independent check on the model 

flux (recharge or discharge) is available. This lack of an independent check on model flux is the most important limitation of 

the flow model. With no independent measurement of the 

recharge flux, any constant ratio of recharge flux and hydraulic 

conductivity will produce the same simulated potentiometric 

surface. Given that the hydraulic conductivity values and distribution have been better defined within AOC A, the calibrated 

recharge rate of 1.82 in/yr should be a more realistic estimate of 

recharge for the area, but this estimate still is limited by the confidence in the hydraulic-conductivity data. 

Many of the estimated parameters for the calibrated AOC 

A flow model show high correlation coefficients. Testing during model calibration indicates that without the advective-flow 

observation, parameters could not have been estimated individually. The advective-flow observation helps overcome the lack 

of ground-water flux data and allows the hydraulic conductivity 

and recharge parameters to be estimated individually. The most 

sensitive parameter in the AOC A flow model is the recharge 

rate (RECH). The next most sensitive parameter is the hydraulic 

conductivity for the average zone (HK_Average), followed by 

the hydraulic conductivity for the high zone (HK_High). 

Ground-water-flow paths were simulated from two suspected contaminant-plume source areas at AOC A. The sites 

simulated as source areas are the “grassy area” and the “north 

edge of apron” area. The flow paths indicated by the particle 

tracking agree reasonably well with the interpreted maps of 

TCE plumes. The time-of-travel plots show that travel times 

from the north edge of the apron source to the model boundary 

are faster than travel times from the grassy area source to the 

same boundary.

Selected References

Anderman, E.R., and Hill, M.C., 1997, Advective-transport 

observation (ADV) package, a computer program for adding 

advective-transport observations of steady-state flow fields 

to the three-dimensional ground-water flow parameterestimation model MODFLOWP: U.S. Geological Survey 

Open-File Report 97-14, 67 p. 

Carmichael, J.K., Parks, W.S., Kingsbury, J.A., and Ladd, D.E., 

1997, Hydrogeology and ground-water quality at Naval Support Activity Memphis, Millington, Tennessee: U.S. Geological Survey Water-Resources Investigations Report 97-4158, 

64 p.

Frederiksen, N.O., Bybell, L.M., Christopher, R.A., Crone, 

A.J., Edwards, L.E., Gibson, T.G., Hazel, J.E., Repetski, J.E., 

Russ, D.P., Smith, C.C., and Ward, L.W., 1982, Biostratigraphy and paleoecology of lower Paleozoic, upper Cretaceous, 

and lower Tertiary rocks in U.S. Geological Survey New 

Madrid test wells, southeastern Missouri: Tulane Studies in 

Geology and Paleontology, v. 17, no. 2, p. 23-45.

Harbaugh, A.W., Banta, E.R., Hill, M.C., and McDonald, M.G., 

2000, MODFLOW-2000, The U.S. Geological Survey modular ground-water model—User guide to modularization 

concepts and the ground-water flow process: U.S. Geological 

Survey Open-File Report 00-92, 121 p.

Hill, M.C., 1998, Methods and guidelines for effective model 

calibration: With application to UCODE, a computer code 

for universal inverse modeling, and MODFLOWP, a computer code for inverse modeling with MODFLOW: U.S. 

Geological Survey Water-Resources Investigations Report 

98-4005, 90 p.

Hill, M.C., Banta, E.R., Harbaugh, A.W., and Anderman, E.R., 

2000, MODFLOW-2000, The U.S. Geological Survey modular ground-water model—User guide to the observation, 

sensitivity, and parameter-estimation processes and three 

post-processing programs: U.S. Geological Survey OpenFile Report 00-184, 209 p.

Kingsbury, J.A., and Carmichael, J.K., 1995, Hydrogeology of 

post-Wilcox Group stratigraphic units in the area of Naval 

Air Station Memphis, near Millington, Tennessee: U.S. Geological Survey Water-Resources Investigations Report 95-

4011, 1 sheet.

Kingsbury, J.A., and Parks, W.S., 1993, Hydrogeology of the 

principal aquifers and relation of faults to interaquifer leakage in the Memphis area, Tennessee: U.S. Geological Survey 

Water-Resources Investigations Report 93-4075, 18 p.

McDonald, M.G., and Harbaugh, A.W., 1988, A modular threedimensional finite-difference ground-water flow model: 

Techniques of Water-Resources Investigations of the United 

States Geological Survey, book 6, chap. A1, 576 p.

Moore, G.K., and Brown, D.L., 1969, Stratigraphy of the Fort 

Pillow test well, Lauderdale County, Tennessee: Tennessee 

Division of Geology Report of Investigations 26, 1 sheet.

Parks, W.S., 1990, Hydrogeology and preliminary assessment 

of the potential for contamination of the Memphis aquifer in 

the Memphis area, Tennessee: U.S. Geological Survey 

Water-Resources Investigations Report 90-4092, 39 p.

Parks, W.S., and Carmichael, J.K., 1989, Geology and groundwater resources of the Fort Pillow Sand in western Tennessee: U.S. Geological Survey Water-Resources Investigations 

Report 89-4120, 20 p.

Parks, W.S., and Carmichael, J.K., 1990, Geology and groundwater resources of the Cockfield Formation in western Tennessee: U.S. Geological Survey Water-Resources Investigations Report 88-4181, 17 p.

Pollock, D.W., 1994, User’s guide for MODPATH/MODPATH-PLOT, version 3: A particle tracking post-processing 

package for MODFLOW, the U.S.Geological Survey finitedifference ground-water flow model: U.S. Geological Survey 

Open-File Report 94-464, variously paginated.

Robinson, J.L., Carmichael, J.K., Halford, K.J., and Ladd, D.E., 

1997, Hydrogeologic framework and simulation of groundwater flow and travel time in the shallow aquifer system in 

the area of Naval Support Activity Memphis, Millington, 

Tennessee: U.S. Geological Survey Water-Resources Investigations Report 97-4228, 56 p.