U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2018–1097

Prepared for the Tennessee Valley Authority in cooperation with the University of Memphis, 

Center for Applied Earth Science and Engineering Research

Preliminary Evaluation of the Hydrogeology and Groundwater 

Quality of the Mississippi River Valley Alluvial Aquifer and 

Memphis Aquifer at the Tennessee Valley Authority Allen 

Power Plants, Memphis, Shelby County, Tennessee
Cover. Groundwater discharge from an aquifer test at the Tennessee Valley Authority Allen Combined Cycle Plant in 

Memphis, Tennessee, October 3–4, 2017.
Preliminary Evaluation of the Hydrogeology 

and Groundwater Quality of the Mississippi 

River Valley Alluvial Aquifer and Memphis 

Aquifer at the Tennessee Valley Authority 

Allen Power Plants, Memphis, Shelby 

County, Tennessee

By John K. Carmichael, James A. Kingsbury, Daniel Larsen, and 

Scott Schoefernacker

Prepared for the Tennessee Valley Authority in cooperation with the University of 

Memphis, Center for Applied Earth Science and Engineering Research

Open-File Report 2018–1097

U.S. Department of the Interior

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

RYAN K. ZINKE, Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2018

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Suggested citation:

Carmichael, J.K., Kingsbury, J.A., Larsen, Daniel, and Schoefernacker, Scott, 2018, Preliminary evaluation of the 

hydrogeology and groundwater quality of the Mississippi River Valley alluvial aquifer and Memphis aquifer at the 

Tennessee Valley Authority Allen Power Plants, Memphis, Shelby County, Tennessee: U.S. Geological Survey 

Open-File Report 2018–1097, 66 p., https://doi.org/10.3133/ofr20181097.

ISSN 2331-1258 (online)
iii

Contents

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

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

Purpose and Scope ..............................................................................................................................3

Background............................................................................................................................................3

Geology and Hydrogeology of the Study Area..........................................................................................6

Methods...........................................................................................................................................................8

Subsurface Hydrogeologic Data Collection.....................................................................................8

Water-Quality Sample Collection.......................................................................................................8

Aquifer Test............................................................................................................................................9

Water-Level Monitoring..............................................................................................................9

Data Analysis..............................................................................................................................11

Results ...........................................................................................................................................................11

Subsurface Hydrogeologic Interpretations....................................................................................11

Anthropogenic Fill......................................................................................................................11

Mississippi River Valley Alluvium............................................................................................11

Upper Alluvium..................................................................................................................11

Blue Clay Interval..............................................................................................................12

Channel Unit III..................................................................................................................12

Channel Unit II...................................................................................................................13

Channel Unit I ....................................................................................................................13

Cook Mountain Formation ........................................................................................................13

Transition of Cook Mountain Formation to Memphis Sand.................................................14

Memphis Sand............................................................................................................................14

Geologic Structure..............................................................................................................................14

 Water Quality......................................................................................................................................20

Aquifer Test..........................................................................................................................................28

Memphis Aquifer........................................................................................................................28

MRVA Aquifer.............................................................................................................................33

Summary and Conclusions.........................................................................................................................43

References....................................................................................................................................................45

Appendix 1. SeriesSEE Water-Level Model Hydrographs—Allen Combined Cycle 

Plant Monitoring Wells..................................................................................................................48

Appendix 2. SeriesSEE Water-Level Model Hydrographs—Allen Fossil Plant 

Monitoring Wells ............................................................................................................................51

Appendix 3. SeriesSEE Water-Level Model Hydrographs—Memphis Aquifer 

Observation Wells ..........................................................................................................................65
iv

Figures

1. Maps showing locations of the Tennessee Valley Authority Allen Combined Cycle 

and Allen Fossil Plants in southwestern Memphis, Shelby County, Tennessee................2

2. Map showing locations of production wells, Memphis aquifer observation wells, 

and Mississippi River Valley alluvial aquifer monitoring wells used in the preliminary 

evaluation of the hydrogeology and groundwater conditions at the Tennessee Valley 

Authority Allen Combined Cycle and Allen Fossil Plants, southwest Memphis, Shelby 

County, Tennessee........................................................................................................................4

3. Schematic cross section showing the hydrostratigraphy of the northern Mississippi 

embayment east and west of Memphis, Tennessee ..............................................................5

4. Lithologic chart showing geologic units underlying Memphis, Shelby County, 

Tennessee, and surrounding area .............................................................................................7

5. Graph and table showing pumping rates, durations, and volume of water pumped 

for the Tennessee Valley Authority Allen Combined Cycle Plant aquifer test, 

October 3–4, 2017........................................................................................................................10

6. Map showing locations of cross sections and inferred faults, Tennessee Valley 

Authority Allen Combined Cycle and Allen Fossil Plants area............................................16

7. Geologic section A–A’, Tennessee Valley Authority Allen Combined Cycle and 

Allen Fossil Plants area .............................................................................................................17

8. Geologic section B–B’, Tennessee Valley Authority Allen Combined Cycle and 

Allen Fossil Plants area .............................................................................................................18

9. Geologic section C–C’, Tennessee Valley Authority Allen Combined Cycle and 

Allen Fossil Plants area .............................................................................................................19

10. Piper diagram showing chemical characteristics of groundwater samples 

collected from wells at the Tennessee Valley Authority Allen Combined Cycle 

Plant, September 2017 ...............................................................................................................26

11. Graph showing tritium concentrations in groundwater samples in relation to the 

depths of the tops of the screens for Memphis aquifer production wells at the 

Tennessee Valley Authority Allen Combined Cycle Plant, September 2017 .....................27

12. Graphs showing hypothetical mixing line for groundwater samples collected from 

Memphis aquifer production wells PW 1 through PW 4 that are unaffected by 

leakage to production well PW 5 where alluvial water has moved into the 

Memphis aquifer at the Tennessee Valley Authority Allen Combined Cycle Plant, 

September 2017...........................................................................................................................28

13. Map showing maximum drawdown measured in the Memphis aquifer production

wells and maximum estimated drawdown in Memphis aquifer observation 

wells, Tennessee Valley Authority Allen Combined Cycle Plant aquifer test, 

October 3–4, 2017........................................................................................................................30

14. Graph showing measured drawdown and cumulative pumping rates in Memphis 

aquifer production wells, Tennessee Valley Authority Allen Combined Cycle Plant 

aquifer test, October 3–4, 2017 .................................................................................................31

15. Graphs showing measured water levels and estimated drawdown in Memphis 

aquifer observation wells Sh:H-001, Sh:H-013, and Ar:H-002, Tennessee Valley 

Authority Allen Combined Cycle Plant aquifer test, October 3–4, 2017 .............................32
v

16. Graphs showing measured water levels and estimated drawdown in selected 

Mississippi River Valley alluvial aquifer monitoring wells MW 1A, MW 3A, MW 5A 

and MW 5B at Allen Combined Cycle Plant; MW 202, MW 202A and MW 202B, 

MW 210 and MW 210A, and MW 204A and MW 204B at Allen Fossil Plant; and stage 

of the Mississippi River at Memphis, Tennessee Valley Authority Allen Combined 

Cycle Plant aquifer test, October 3–4, 2017............................................................................35

17. Map showing estimated drawdown in selected monitoring wells screened in the 

Mississippi River Valley alluvial aquifer, Tennessee Valley Authority Allen Combined 

Cycle Plant aquifer test, October 3–4, 2017............................................................................41

18. Map showing pre-test water-level altitudes in the Mississippi River alluvial 

aquifer and stage of Lake McKellar, Tennessee Valley Authority Allen Combined 

Cycle Plant aquifer test, October 3–4, 2017............................................................................42

19. Map showing late-pumping period water-level altitudes in the Mississippi River 

Valley alluvial aquifer and stage of Lake McKellar, Tennessee Valley Authority 

Allen Combined Cycle Plant aquifer test, October 3–4, 2017...............................................43

Tables

1. Radiocarbon ages for wood samples collected from the Mississippi River 

Valley alluvium at the Tennessee Valley Authority Allen Combined Cycle and 

Allen Fossil Plants.......................................................................................................................12

2. Concentrations of major inorganic constituents and trace elements in groundwater 

samples collected from selected Tennessee Valley Authority Allen Combined Cycle 

Plant wells, September 2017.....................................................................................................21

3. Results from analyses of water-quality samples collected from selected production 

wells during the Allen Combined Cycle Plant aquifer test, October 3–4, 2017.................25

4. Concentrations of tritium in groundwater samples collected from selected Allen 

Combined Cycle Plant wells, September and October 2017................................................27

5. Pumping rates, drawdown values, and specific capacities for Memphis aquifer 

production wells during the aquifer test, October 3–4, 2017, and from performance 

tests, June 15–21, 2016, Tennessee Valley Authority Allen Combined Cycle Plant.........29

6. Model drawdown estimates for monitoring wells screened in the Mississippi 

River Valley alluvial aquifer at the Tennessee Valley Authority Allen Fossil and 

Allen Combined Cycle Plants, Tennessee Valley Authority Allen Combined Cycle 

Plant aquifer test, October 3–4, 2017.......................................................................................34
vi

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square foot (ft2

) 929.0 square centimeter (cm2)

square foot (ft2

) 0.09290 square meter (m2)

square mile (mi2

) 259.0 hectare (ha)

square mile (mi2

) 2.590 square kilometer (km2) 

Volume

gallon (gal) 3.785 liter (L) 

gallon (gal) 0.003785 cubic meter (m3

) 

gallon (gal) 3.785 cubic decimeter (dm3

) 

million gallons (Mgal) 3,785 cubic meter (m3

)

Flow rate

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

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

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

/s)

Pressure

pound per square inch (lb/in2

) 6.895 kilopascal (kPa) 

Radioactivity

picocurie per liter (pCi/L) 0.037 becquerel per liter (Bq/L) 

Specific capacity

gallon per minute per foot ([gal/

min]/ft)

0.2070 liter per second per meter ([L/s]/m)

Hydraulic conductivity

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

Transmissivity

foot squared per day (ft2

/d) 0.09290 meter squared per day (m2/d) 

Conversion Factors

U.S. customary units to International System of Units

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F = (1.8 × °C) + 32.

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

°C = (°F – 32) / 1.8.
vii

Datum

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 1983 (NAD 83).

Altitude, as used in this report, refers to distance above the vertical datum.

Supplemental Information

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 

25 °C).

Concentrations of chemical constituents in water are given in either milligrams per liter (mg/L) 

or micrograms per liter (μg/L).

Abbreviations

ACC Allen Combined Cycle Plant (TVA)

ALF Allen Fossil Plant (TVA)

CAESER Center for Applied Earth Science and Engineering Research (University of Memphis)

DO Dissolved oxygen

MERAS Mississippi Embayment Regional Aquifer Study

MLGW Memphis Light, Gas and Water Division

MRVA Mississippi River Valley alluvial (aquifer)

MW Monitoring well

NWIS National Water Information System (USGS)

PW Production well

QAPP Quality-assurance project plan

RCYBP Radiocarbon years before present

RI Remedial investigation

SC Specific conductance

TDEC Tennessee Department of Environment and Conservation

TDS Total dissolved solids

TU Tritium units

TVA Tennessee Valley Authority

USGS U.S. Geological Survey

Abstract 

The hydrogeology, groundwater quality, and potential for 

hydraulic connection between the Mississippi River Valley 

alluvial aquifer and the Memphis aquifer in the area of the 

Tennessee Valley Authority (TVA) Allen Combined Cycle and 

Allen Fossil Plants in southwestern Memphis, Tennessee, were 

evaluated from September through December 2017. The study 

was designed as a preliminary assessment of the potential for 

leakage of groundwater from the Mississippi River Valley 

alluvial aquifer through the underlying upper Claiborne 

confining unit into the underlying Memphis aquifer at the 

plants. A short-term aquifer test of four of the five Memphis 

aquifer production wells installed at the Allen Combined 

Cycle Plant induced drawdown in water levels in the 

Mississippi River Valley alluvial aquifer, locally. The largest 

drawdown occurred in the eastern and southeastern parts 

of the TVA plants area, and generally was coincident with 

locations where stratigraphic data show increased thickness of 

and depth to the base of the alluvium and decreased thickness 

and inferred offset in the base of the confining unit relative 

to nearby locations. In contrast, stratigraphic data for most 

other locations at the site indicate shallower depths to the 

base of the alluvium and more consistent thickness of and 

depth to the base of the confining unit, which corresponds 

with areas where less drawdown was observed during the test. 

Water-quality results for samples from the production wells 

and from monitoring wells screened in the Mississippi River 

Valley alluvial aquifer indicate that groundwater with higher 

dissolved-solids concentrations and tritium from this shallow 

aquifer has mixed with water in the upper part of the Memphis 

aquifer at one of the production wells. Results of the study 

collectively confirm that the Mississippi River Valley alluvial 

and Memphis aquifers are hydraulically connected near the 

TVA plants area.

Introduction

Understanding the extent and magnitude of hydraulic 

connection between aquifers is essential for efficient and 

sustainable use of groundwater resources. Key factors 

regarding this issue include the purposes and magnitude 

of groundwater use, and the contrasts in water quality and 

the potential connection between aquifers. In the Memphis 

area in Shelby County, Tennessee, large withdrawals for 

public and industrial supply have resulted in movement of 

groundwater from the shallow, “water-table” aquifers into 

the underlying Memphis aquifer in areas where the upper 

Claiborne confining unit is thin or absent (Graham and Parks, 

1986; Parks, 1990). Proposed withdrawals from the Memphis 

aquifer at the newly constructed Tennessee Valley Authority 

(TVA) Allen Combined Cycle (ACC) Plant in southwestern 

Memphis (fig. 1) has caused public concern about the 

potential for degradation of the drinking-water supply. Those 

concerns were heightened by the identification of elevated 

concentrations of arsenic and other trace metals in the 

overlying Mississippi River Valley alluvial (MRVA) aquifer at 

the TVA Allen Fossil (ALF) Plant (Charlier, 2017). In 2017, 

the U.S. Geological Survey (USGS), in cooperation with the 

TVA and the University of Memphis, Center for Applied Earth 

Science and Engineering Research (CAESER), conducted a 

preliminary investigation of the hydrogeology, groundwater 

quality, and potential for hydraulic connection between the 

MRVA and Memphis aquifers in the area.

Preliminary Evaluation of the Hydrogeology and 

Groundwater Quality of the Mississippi River Valley 

Alluvial Aquifer and Memphis Aquifer at the Tennessee 

Valley Authority Allen Power Plants, Memphis, Shelby 

County, Tennessee

By John K. Carmichael,1 James A. Kingsbury,1 Daniel Larsen,2 and Scott Schoefernacker2

1

U.S. Geological Survey.

2

University of Memphis.
2 Hydrogeology and Groundwater Quality, Tennessee Valley Authority Allen Power Plants

Bluffs

Bluffs

EXPLANATION

Memphis Light, Gas and 

 Water Division well 

 field and name

Known or suspected 

 window locations

 (Parks, 1990; Parks 

 and others, 1995)

0 2.5 5 MILES

0 2.5 5 KILOMETERS

0 0.25

0.25

0.5 MILE

0 0.5 KILOMETER

Memphis

Allen Combined Cycle (ACC) Plant

Allen Fossil (ALF) Plant

Lake McKellar

40

40

240

240

55

President’s 

Island

Pidgeon Industrial Park

Allen

Allen

Sheahan

Mccord

Davis

Morton

Lichterman

Mallory

Palmer

Allen 

Plants

site

Wolf River

Loosahatchie River

Mississippi River

Nonconnah Creek

SHELBY

DE SOTO

ARKANSAS

MISSISSIPPI

TENNESSEE

90°15' 90° 89°45'

35°15'

35°

Base from U.S. Geological Survey digital data,

variously dated, various scales

CRITTENDEN

ST FRANCIS

Source: Esri, DigitalGlobe, GeoEye, 

Earthstar Geographics, CNES/Airbus DS, 

USDA, U.S. Geological Survey, AeroGRID, 

IGN, and the GIS User Community

90°09' 90°08'

35°04'30"

35°04

Figure 1. Locations of the Tennessee Valley Authority Allen Combined Cycle and Allen Fossil 

Plants in southwestern Memphis, Shelby County, Tennessee.
Introduction 3

Purpose and Scope

This report presents the results of a preliminary 

evaluation of the hydrogeology and groundwater conditions 

of the MRVA and Memphis aquifers at and near the TVA ALF 

and ACC Plants. The evaluation includes (1) interpretation 

of core samples and geophysical logs to assess and correlate 

the hydrogeologic units locally, including the extent and 

thickness of the confining unit separating the MRVA and 

Memphis aquifers; (2) characterization of water quality in 

the MRVA and Memphis aquifers at the TVA plants area to 

evaluate whether water from the MRVA aquifer has moved 

into the Memphis aquifer; (3) a short-term aquifer test of 

the Memphis aquifer production wells at the ACC Plant to 

determine the potential for hydraulic connection between 

the MRVA and Memphis aquifers; and (4) development of 

interpretations to support refinement of a regional conceptual 

model for application to the local groundwater system. All 

field investigations for this evaluation were conducted during 

September and October 2017.

Background

In 2016, the USGS produced a numerical simulation of 

groundwater withdrawals from proposed new production wells 

screened in the Memphis aquifer at the ACC Plant (Haugh, 

2016). The Mississippi Embayment Regional Aquifer Study 

(MERAS) groundwater-flow model (Clark and Hart, 2009) 

was used for the simulation. The simulation scenario was 

designed to evaluate the effects of pumping from the Memphis 

aquifer at the ACC Plant for 30 years at the average expected 

rate of withdrawal of 2,500 gallons per minute (gal/min), 

followed by a 30-day period at the maximum expected rate 

of withdrawal of 5,000 gal/min. Simulated declines in the 

potentiometric surface of the Memphis aquifer at the plant site 

were 7 feet (ft) at the end of the 30-year average withdrawal 

period and 11 ft at the end of the scenario. Simulated declines 

in the underlying Fort Pillow aquifer and overlying MRVA 

aquifer were less than 1 ft at the end of the 30-year average 

period and at the end of the scenario. Based on the results of 

the simulation, TVA decided to install wells in the Memphis 

aquifer to supply process water for the ACC Plant.

In 2017, following installation of five Memphis aquifer 

production wells at the ACC Plant (fig. 2), groundwater 

samples collected from two MRVA aquifer monitoring wells 

at the ALF Plant, located north of the ACC Plant (fig. 2), 

contained elevated levels of arsenic and other trace metals 

(Charlier, 2017). Subsequently, concerns were raised by the 

Tennessee Department of Environment and Conservation 

(TDEC) as to whether operation of the Memphis aquifer 

production wells at the ACC Plant could potentially cause 

downward movement of groundwater and contaminants 

from the overlying MRVA aquifer at the ALF Plant. Because 

of these concerns, TDEC requested that the TVA conduct 

a Remedial Investigation (RI) of the nature and extent of 

contamination in the MRVA aquifer, including the potential for 

contaminant migration to the Memphis aquifer. TVA retained 

Stantec Consulting Services Inc. (Stantec) to conduct the RI.

A major factor contributing to questions about the 

potential for downward leakage of groundwater from the 

MRVA aquifer to the Memphis aquifer stems from an inferred 

breach or “window” in the clay confining unit that separates 

the two aquifers beneath President’s Island (fig. 1), north of 

the TVA plants area (Parks, 1990). Here, clay-rich units of 

the upper Claiborne Group (fig. 3) that make up the upper 

Claiborne confining unit have been inferred to be sandy, thin, 

or absent (0 thickness), creating a potential for exchange of 

water between the two aquifers. A window has been identified 

at the south end of the Pidgeon Industrial Park (fig. 1), about 

3 miles (mi) south of the TVA plants area and west of the 

Memphis Light, Gas and Water Division (MLGW) Davis well 

field where downward leakage of water from the MRVA to 

the Memphis aquifer has been documented (Parks and others, 

1995; Koban and others, 2011). Several other locations in the 

Memphis area have been identified where leakage of water 

from the shallow, “water-table” (alluvial and [or] fluvial 

deposits) aquifers to the Memphis aquifer occurs. Most of 

these locations were identified based on variations in thickness 

of clay that makes up the upper Claiborne confining unit, 

local declines in water levels in the shallow aquifers near 

locations of withdrawal from the Memphis aquifer, changes 

in water-quality characteristics in the Memphis aquifer related 

to groundwater withdrawals, and (or) results of hydrologic 

tracer studies, all of which indicate movement of water from 

the shallow aquifers to the Memphis aquifer (Criner and 

others, 1964; Bell and Nyman, 1968; Parks and Lounsbury, 

1976; Graham and Parks, 1986; Parks and Mirecki, 1992; 

Larsen and others, 2003; Gentry and others, 2005; Gentry and 

others, 2006; Ivey and others, 2008; Larsen and others, 2013; 

Larsen and others, 2016). Because of the proximity of known 

or inferred areas of leakage near the power plants, the TVA 

requested that the USGS and CAESER jointly conduct a study 

to evaluate the hydrogeology and groundwater conditions at 

the TVA plants site, with focus on the potential for hydraulic 

connection between the MRVA and Memphis aquifers.
4 Hydrogeology and Groundwater Quality, Tennessee Valley Authority Allen Power Plants

EXPLANATION

Memphis aquifer observation well 

 and name

MRVA aquifer monitoring well 

 and name

Production well and name

MW 210

Sh:H-027

PW 4

Sh:H-013

Mississippi River

President’s Island

Hennington Avenue

ALF Plant

ACC Plant

Horn Lake Cutoff

Pidgeon 

Industrial 

Park

MW 3A MW 1A

MW P-4S

MW 5A

MW 5B

MW 201A

MW 201B

MW 202A

MW 202B

MW 203A

MW 203B MW 204A

MW 204B MW 205A

MW 205B

MW 210A

MW 212A

MW 214

MW 214A

MW 214B

MW 215

MW P-4

MW 206

MW 213 Harsco

MW 212

MW 202

MW 201

MW 205

MW 203 MW 204

MW 210

MW 216 MW 3A MW 1A

MW P-4S

MW 5A

MW 5B

MW 201A

MW 201B

MW 202A

MW 202B

MW 203A

MW 203B MW 204A

MW 204B MW 205A

MW 205B

MW 210A

MW 212A

MW 214

MW 214A

MW 214B

MW 215

MW P-4

MW 206

MW 213 Harsco

MW 212

MW 202

MW 201

MW 205

MW 203 MW 204

MW 210

Sh:H-031

PW 1

Sh:H-025

PW 2

Sh:H-026

PW 3

Sh:H-027

PW 4

Sh:H-001

Sh:H-032

PW 5

Sh:H-013

Ar:H-002

Sh:H-031

PW 1

Sh:H-025

PW 2

Sh:H-026

PW 3

Sh:H-027

PW 4

Sh:H-001

Sh:H-013

Ar:H-002 Sh:H-032

PW 5

T.O. Fuller 

State Park

90°12' 90°08'

35°06'

35°03'

MW 216

Mississippi River

Source: Esri, DigitalGlobe, GeoEye, 

Earthstar Geographics, CNES/Airbus DS, 

USDA, U.S. Geological Survey, AeroGRID, 

IGN, and the GIS User Community

T.O. Fuller 

State Park ACC Plant

ALF Plant

Horn Lake Cutoff

Hennington Avenue

Pidgeon 

Industrial 

Park

President’s Island

0 1 2 KILOMETERS

0 1 2 MILES

Lake McKellar

Figure 2. Locations of production wells, Memphis aquifer observation wells, and Mississippi River Valley alluvial aquifer 

monitoring wells used in the preliminary evaluation of the hydrogeology and groundwater conditions at the Tennessee 

Valley Authority Allen Combined Cycle and Allen Fossil Plants, southwest Memphis, Shelby County, Tennessee.
Introduction 5

(Ripley F ormation and equivalents)

Owl Creek Formation, and equivalents)

Porters Creek Clay, Clay ton Formation,

Midway confining unit (Old Breastworks Formation,

Confining unit (Flour Island Formation and equivalents)

McNairy-Nacatoch aquifer

 Confining unit (Undifferentiated pre-Ripley and McNairy

Cretaceous formations

Fort Pillow aquifer

(Fort Pillow Sand and equivalent)

VERTICAL SCALE GREATLY EXAGGERATED

0 10 20 30 40 50 MILES

0 10 20 30 40 50 KILOMETERS

1,000

2,000

3,000

4,000

NGVD 29

FEET

WEST

Memphis

EAST

Alluvium

Mississippi River Valley

alluvial aquifer

Crowleys Ridge

Approximate axis

of Mississippi

embayment 

ARKANSAS 

TENNESSEE

TENNESSEE

Loess and

fluvial

deposits

MISSISSIPPI

Confining unit (Jackson Formation 

and upper part of Claiborne Group)

Memphis aquifer 

(Memphis Sand and equivalents)

Undifferentiated Paleozoic rocks

Figure 3. Schematic cross section showing the hydrostratigraphy of the northern Mississippi embayment east and 

west of Memphis, Tennessee.
6 Hydrogeology and Groundwater Quality, Tennessee Valley Authority Allen Power Plants

Geology and Hydrogeology of the 

Study Area

The TVA ALF and ACC Plants are located in the 

Mississippi River Valley within the northern Mississippi 

embayment. Numerous investigations have been conducted 

in the past regarding the geology and hydrogeology of 

the Memphis and Shelby County, Tennessee, area. The 

general geology and hydrogeology of the area and of the 

hydrostratigraphic units present in in the TVA study area are 

briefly described below to provide context for the site-specific 

investigation at the TVA plants site.

Memphis and Shelby County lie in the center of the 

northern Mississippi embayment, a trough-shaped basin that 

plunges southward along an axis that approximately overlies 

the Mississippi River (fig. 3). The Mississippi embayment 

is filled with more than 3,000 ft of sediments of Cretaceous, 

Tertiary, and Quaternary age (Moore and Brown, 1969; 

Brahana and Broshears, 2001). The upper-Cretaceous- through 

middle-Tertiary-age geologic units beneath Shelby County 

dip to the west and include unconsolidated sand, silt, and 

clay with minor lignite. The Tertiary and older units form the 

Mississippi embayment aquifer system, a system of aquifers 

and confining units that are important sources of water for 

most uses in the Mississippi embayment (Lloyd and Lyke, 

1995). Loess (windblown silt and clay) of Pleistocene age 

and fluvial (terrace) deposits primarily composed of sand and 

gravel deposits of Pliocene(?) and Pleistocene age mantle 

the middle Tertiary units in the upland parts of the Memphis 

area outside the Mississippi River Valley alluvial plain and 

the alluvial plains of the major tributaries that drain the area. 

Alluvium of Holocene age makes up the surficial unit in the 

Mississippi River alluvial valley and the present-day valleys of 

the primary streams in the Memphis area. The alluvium in the 

Mississippi River alluvial valley makes up the MRVA aquifer, 

an important regional aquifer in the Mississippi embayment 

(Lloyd and Lyke, 1995).

In the TVA plants area, the hydrogeologic units that are 

present and of importance to this evaluation are, in descending 

order, the MRVA aquifer, the upper Claiborne confining unit, 

and the Memphis aquifer (figs. 3 and 4). Loess and fluvial 

deposits underlie the Mississippi River bluffs area to the 

southeast of the TVA plants site (fig. 1). These units were not 

investigated during the evaluation.

The Mississippi River Valley alluvium in the Memphis 

area consists primarily of sand, gravel, silt, and clay in the 

upper parts and sand and gravel in the lower parts. The 

alluvium ranges from 0 to about 200 ft in thickness in the 

Memphis area, but commonly is 100 to 150 ft thick (Parks, 

1990). Horizontal hydraulic conductivity of the MRVA aquifer 

ranges from 90 to 390 feet per day (ft/d), with a geometric 

mean range of 205 to 210 ft/d (Krinitzsky and Wire, 1964; 

Newcome, 1971; Ackerman, 1996). 

The Cockfield and Cook Mountain Formations are 

the uppermost Tertiary units in the Memphis area and are 

composed predominantly of clay, silt, and fine sand. The 

thicknesses of the Cockfield and Cook Mountain Formations 

in the Memphis area range from 0 to 250 ft and 0 to 110 ft, 

respectively (Kingsbury and Parks, 1993). The Cockfield 

and Cook Mountain Formations are the primary components 

of the upper Claiborne confining unit, which is the lower 

confining unit for the shallow aquifers and upper confining 

unit for the Memphis aquifer in the Memphis area (figs. 3 

and 4). The thickness of the upper Claiborne confining unit 

varies within Shelby County from about 375 ft in northwestern 

Shelby County to 0 ft in the southeastern part of the county 

(Parks, 1990). Although fine-grained strata constitute much 

of both formations, the Cook Mountain Formation is the most 

persistent clay layer in the upper Claiborne confining unit in 

the Memphis area (Parks, 1990). An aquifer is present locally 

in the Cockfield Formation in Millington in the northern 

part of Shelby County (Robinson and others, 1997) and in 

other locations in West Tennessee (Parks and Carmichael, 

1990b). The Cockfield Formation is absent or has only limited 

thickness beneath the Mississippi River alluvial plain; at 

the TVA plants site, the Cook Mountain Formation is the 

primary component of the upper Claiborne confining unit. 

Slug and single-well aquifer tests in wells screened in the 

upper Claiborne confining unit at a site in the eastern part of 

Memphis yielded horizontal hydraulic conductivity values of 

2.0×10–5 to 5.0×10–5 ft/d (Gentry and others, 2006). Vertical 

hydraulic conductivity values for clay samples collected 

from the upper Claiborne confining unit near Millington, 

Tennessee, ranged from 5.0×10–6 to 2.5×10–3 ft/d (Robinson 

and others, 1997). The thickness of clay beds in the confining 

unit is as much as 250 ft in the Memphis area but thins to 0 ft 

at some locations (Parks, 1990), suggesting that hydrologic 

connectivity exists between the underlying Memphis aquifer 

and the overlying shallow aquifers where the clay beds are 

thin or absent.

The Memphis aquifer is a thick, sand-dominated aquifer, 

ranging in thickness from about 650 to 900 ft in the Memphis 

area (Kingsbury and Parks, 1993). Lenses of clay and silt 

are present in the aquifer at various horizons (Parks, 1990). 

The test hole for production well PW 4 at the ACC Plant 

penetrated the full thickness of the Memphis aquifer; thickness 

at this location was about 840 ft. Based on results from about 

60 aquifer tests in the Memphis area, transmissivities range 

from about 6,700 to 53,500 feet squared per day (ft2

/d), with 

an average of 33,400 ft2

/d, and storativity (storage coefficients) 

values range from 0.0001 to 0.003 for the Memphis aquifer 

(Parks and Carmichael, 1990a). Assuming an average 

thickness of 700 ft for the Memphis aquifer in the Memphis 

area, the average hydraulic conductivity based on the average 

transmissivity from the tests is about 48 ft/d. The Memphis 

aquifer is the primary source of water for all uses at Memphis 

and throughout most of West Tennessee. Average withdrawals 

from the Memphis aquifer for public-water supply in Shelby 

County were reported at 187 million gallons per day (Mgal/d) 

in 2005 (Robinson and Brooks, 2010).
Geology and Hydrogeology of the Study Area 7

System and series Group Stratigraphic 

unit

Hydrostratigraphic 

unit

Thickness 

(feet) Lithology and hydrologic significance

Quaternary

Holocene

and

Pleistocene

Alluvium Alluvial aquifer 0–200 Sand and gravel that grades upward to silt and clay. 

Underlies the Mississippi River alluvial plain and 

alluvial plains of tributary streams in West Tennessee. Thickness beneath the Mississippi River alluvial plain is as much as 230 feet; generally, less 

than 50 feet thick in tributary valleys. Provides 

water to domestic, farm, industrial, and irrigation 

wells in the Mississippi alluvial plain.

Quaternary

Pleistocene

Loess Leaky confining 

layer 

0–65 Silt, silty clay, and minor sand. Principal unit at 

the surface in upland areas of the Mississippi 

embayment. Thickest on the bluffs that border the 

Mississippi alluvial plain; thinner and finer grained 

eastward from the bluffs. Tends to retard downward percolation of water recharging the fluvial 

deposits aquifer.

Quaternary and 

Tertiary

Pleistocene

and Pliocene (?)

Fluvial deposits (terrace 

deposits)

Fluvial deposits 

aquifer

0–100 Sand, gravel, minor clay and ferruginous sandstone. 

Generally, underlies the loess in upland areas, but 

locally absent. Thickness varies greatly because of 

erosional surfaces at top and base. Provides water 

to domestic and farm wells in rural areas.

Tertiary

Eocene

Claiborne

Upper part of 

Claiborne 

Group, includes Cockfield and Cook 

Mountain 

Formations 

(capping clay)

Upper Claiborne 

confining unit

0–360 Clay, silt, sand, and lignite. The Cook Mountain 

Formation is mainly silt and clay with minor finegrained sand and rare lignite beds. The Cockfield 

Formation comprises fine-grained sand that commonly grades upward to silt and clay with common lignite beds. The Cook Mountain Formation 

serves as the primary upper confining unit for the 

Memphis aquifer. Saturated sands in the Cockfield 

Formation locally serve as aquifers in western 

Tennessee.

Memphis Sand 

(“500-foot” 

sand)

Memphis aquifer 500–900 Sand, clay, and minor lignite. Thick body of fine- to 

very coarse-grained 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. Main 

source of water for the City of Memphis.

Paleocene

Wilcox

Flour Island 

Formation

Flour Island confining unit140–310 Clay, silt, sand, and lignite. Consists primarily of 

silty clay and sandy silt with lenses of fine sand 

and lignite. Serves as the lower confining bed for 

the Memphis aquifer and the upper confining bed 

for the Fort Pillow aquifer.

Fort Pillow Sand 

(“1,400-foot” 

sand)

Fort Pillow aquifer 98–300 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. Second principal aquifer supplying 

water to the City of Memphis and some industry. 

Principal aquifer providing water for municipal and 

industrial supplies west of the Mississippi River.

Old Breastworks 

Formation

Old Breastworks 

confining unit

180–310 Clay, silt, sand, and lignite. Consists primarily of 

silty clay and clayey silt with lenses of fine sand 

and lignite. Serves as the lower confining bed for 

the Fort Pillow aquifer, along with the underlying 

Porters Creek Clay.

Figure 4. Geologic units underlying Memphis, Shelby County, Tennessee, and surrounding area (from Graham and Parks, 1986; Parks, 

1990; Kingsbury and Parks, 1993).
8 Hydrogeology and Groundwater Quality, Tennessee Valley Authority Allen Power Plants

Methods

The preliminary evaluation of the hydrogeology, 

groundwater quality, and potential for hydraulic connection 

between the MRVA and Memphis aquifers at the TVA plants 

site consisted of three main components of work: subsurface 

hydrogeologic data collection and interpretation; collection 

of groundwater samples from selected MRVA and Memphis 

aquifer wells at the ACC Plant for analyses of major inorganic 

constituents, trace metals, and tritium; and a short-term aquifer 

test to evaluate hydraulic connection between the MRVA and 

Memphis aquifers.

Subsurface Hydrogeologic Data Collection

To better understand the local hydrogeology at the ACC 

and ALF Plants, the USGS and CAESER assisted the TVA and 

Stantec in data collection during the installation of monitoring 

wells in the MRVA aquifer and drilling of four deep 

stratigraphic test holes into the top of the Memphis aquifer in 

September 2017 as part of the RI at the ALF Plant. Prior to 

September 2017, a network of monitoring wells was in place 

at the ALF Plant, with most wells screened in the upper part 

of the MRVA aquifer (“shallow” wells designated with “200” 

series numbers [for example, MW 203], and well MW P-2 

which was not used for this study), and two wells screened 

in the middle part of the aquifer, only one of which was used 

for this study (well MW P-4) (fig. 2). In September 2017, 

the network was expanded by the installation of additional 

monitoring wells at locations at both TVA plants. The new 

wells were drilled and continuously cored using rotasonic 

methods into deeper parts of the MRVA aquifer. The new 

monitoring wells are screened in the middle (or intermediate) 

parts (wells designated with a “B” [for example, MW 203B 

and MW 5B]) and lower (or deep) parts (wells designated 

with an “A” [for example, MW 203A and MW 5A]) (fig. 2) 

of the MRVA aquifer. An additional shallow monitoring well 

(MW P-4S) also was installed with a screened interval in 

the upper part of the MRVA aquifer as part of the drilling 

work at the location of monitoring well MW P-4 (fig. 2). 

During the drilling program, the four stratigraphic test holes 

(designated by adding “c” to existing monitoring well series 

location numbers [205c, 212c, 214c, and P-4c]) were drilled 

into the upper part of the Memphis aquifer to determine the 

thickness of the confining unit and the altitude of the top of the 

Memphis Sand at selected locations at the ALF Plant. Natural 

gamma-ray geophysical logs were made in these test holes 

before they were filled and abandoned. Geophysical logs and 

core samples from the four test holes and core samples from 

borings for selected monitoring wells screened in the lower 

part of the MRVA aquifer were used along with geophysical 

logs made in the five Memphis aquifer production wells 

installed at the ACC Plant for stratigraphic correlation of the 

contacts between the base of the MRVA aquifer and top of the 

upper Claiborne confining unit, and the base of the confining 

unit and the top of the Memphis Sand at the TVA plants 

site. In addition, geophysical logs from two nearby offsite 

wells screened in the Memphis aquifer (fig. 2; Sh:H-001 and 

Sh:H-013) were used to aid in correlating the stratigraphy in 

the TVA plants area. 

Cuttings from selected borings for monitoring 

wells (MW 1A, MW 3A, MW 5B, and MW 216) and the 

stratigraphic test holes (205c, 212c, 214c, and P-4c) were 

described in the field. Analysis of the drilling core samples 

included visual inspection of grain size and composition 

and characterization of fines (silt and clay), sedimentary 

structures, and other features useful for correlating the 

borehole strata to regional stratigraphic units and interpreting 

the sedimentary depositional environments. Samples of 

wood were collected from the cores at different depths in 

the MRVA aquifer in stratigraphic test holes 205c, 212c, and 

214c and from the borings for monitoring wells MW 1A and 

MW 5B for carbon-14 (14C) analysis to identify intervals of 

sediment of similar age from site to site. The wood samples 

were submitted to the University of Arizona Accelerator Mass 

Spectrometer Laboratory in Tucson, Arizona, for analysis. 

Water-Quality Sample Collection

In September 2017, the USGS, CAESER, and Stantec 

collected groundwater samples from the five Memphis 

aquifer production wells (PW 1 through PW 5) and five 

monitoring wells (MW 1A, MW 3A, MW 5A, MW 5B, and 

MW 216) at the ACC Plant. Water samples were analyzed 

onsite for temperature, dissolved oxygen (DO), pH, and 

specific conductance (SC). Filtered and unfiltered samples 

were collected for laboratory analyses of major inorganic 

constituents (ions), trace elements, and tritium. Major 

inorganic constituents and trace elements analyses were 

completed at the USGS National Water Quality Laboratory in 

Denver, Colorado. Samples were analyzed for tritium at the 

University of Miami Tritium Laboratory in Miami, Florida. 

Water-quality data collected by the USGS as part of this study 

are provided in this report and are publicly available through 

the USGS National Water Information System (NWIS; U.S. 

Geological Survey, 2018).

Samples from the production wells were collected after 

the turbine pumps were run for a minimum of 30 minutes and 

field parameters (temperature, SC, pH, DO, and turbidity) 

were stable. Monitoring wells were purged and sampled 

using a stainless-steel bladder pump and low-flow sampling 

techniques, and samples were collected when field parameters 

were stable. Sampling equipment that was reused, such as the 

bladder pump, was cleaned between wells with a soap wash 

and tap- and deionized-water rinse. Tubing used for sampling 

at all of the wells was single use. Samples were filtered 

with disposable 0.45-micron capsule filters. Alkalinity was 

measured in the field by titration.

Additional groundwater samples were collected by 

the CAESER from four of the five production wells (PW1, 
Methods 9

PW3, PW4, and PW5) at the ACC Plant during the aquifer 

test in October 2017. The samples were analyzed for selected 

dissolved major anions at the University of Memphis, and for 

tritium at the University of Miami Laboratory. Water-quality 

data collected and analyzed by the CAESER as part of this 

study are provided in this report.

Aquifer Test

A 24-hour aquifer test of four of the five Memphis 

aquifer production wells at the ACC Plant was conducted 

during October 3–4, 2017. The purpose of the test was to 

evaluate drawdown in the Memphis aquifer and the potential 

for hydraulic connection between the Memphis and MRVA 

aquifers local to the ACC and ALF Plants. The pumping 

scenario consisted of a step test with a maximum production 

rate of 5,000 gal/min designed to simulate groundwater 

withdrawals at the ACC Plant for an extremely hot summer 

day in which an estimated 5.4 Mgal of water would be needed 

to meet local power demands. Water levels were measured in 

the ACC Plant production wells to determine the maximum 

drawdown for each pumping step and rate of water-level 

recovery in the Memphis aquifer following pumping cessation. 

Water levels also were monitored in three offsite Memphis 

aquifer observation wells to evaluate the magnitude and 

extent of drawdown in the aquifer from the test. To evaluate 

hydraulic connection between the Memphis aquifer and 

the MRVA aquifer, water levels were monitored in selected 

network monitoring wells screened in the MRVA aquifer 

across the TVA plants site (fig. 2). No substantial rainfall 

occurred in the Memphis area from October 1 through the end 

of the test.

The aquifer test began at 10:00 a.m. on Tuesday, October 3, 

2017, and ended at 10:00 a.m. on Wednesday, October 4, 

2017. The test comprised three pumping steps, starting with 

two production wells (PW 1 and PW 5) pumping for 8 hours, 

followed by the addition of a third well (PW 3) pumping 

for the second 8 hours, and ending with the addition of a 

fourth well (PW 4) pumping for the last 8 hours (fig. 5). 

Production well PW 2 was not pumped and served as an 

onsite observation well during the test. Prior to the start of 

the test, a 5- to 30-minute pre-test (fig. 5) of the production 

wells was conducted in a stepped sequence similar to that 

of the planned test to evaluate the temporary piping, pump 

controls, and discharge point. Discharge from the wells was 

routed by temporary piping from the permanent production 

well discharge points at the ACC Plant cooling tower basin 

to a large concrete-lined stormwater ditch along Hennington 

Avenue that drains eastward along the southern side of the 

ACC Plant and empties into the Horn Lake Cutoff southeast of 

the ACC Plant (fig. 2).

Water-Level Monitoring

Airline water-level measurements were made in the 

four pumping production wells and in the one nonpumped 

production well during the test. Each production well is 

equipped with an analog pressure gage attached to the upper 

end of a small-diameter copper-tube airline that displays in 

units of pounds per square inch and feet of water. Airline 

water-level readings were accurate to approximately + 1 ft 

(+ 0.4 pound per square inch). All readings were recorded on 

field sheets specific for each well. Periodic measurements of 

static water levels were made in each well starting on October 2 

and prior to the pre-test pumping on the morning of October 3. 

Measurements also were made in each well during and 

immediately following the pre-test to determine response to 

this short-duration pumping and how quickly water levels 

recovered to static conditions prior to start of the aquifer 

test (water levels in all wells recovered to static levels based 

on the accuracy of air-line readings). Once the test started, 

periodic water-level measurements were made in all wells 

at maximum time intervals of 1 hour. Measurements were 

made more frequently during the early time of each pumping 

step when drawdown rates were greatest and less frequently 

as drawdowns began to stabilize. Water-level measurements 

continued to be made in each well for approximately 2 hours 

after the pumps were turned off or until water levels recovered 

to within about 90 percent of pre-pumping levels. Final 

measurements were made in each production well during midmorning on October 5, 2017.

Three offsite Memphis aquifer observation wells were 

monitored during the test (fig. 2): well Ar:H-002 (USGS 

350344090130000) located in Crittenden County, Arkansas, 

about 4 mi west of the ACC Plant; well Sh:H-001 (USGS 

350342090073201) located at T.O. Fuller State Park, about 

0.75 mi east-southeast of the ACC Plant; and well Sh:H-013 

(USGS 350452090075901) located at the Cargill Memphis 

Plant on President’s Island, about 1 mi north-northeast of the 

ACC Plant. Absolute (nonvented) pressure transducers were 

deployed in these wells to collect water-level data at 5-minute 

intervals for several days prior to, during, and several days 

following the test. In addition, a few periodic measurements 

were made using either a steel tape and chalk or an electric 

tape in wells Sh:H-001 and Sh:H-013 to verify the continuous 

data collected from these wells during the test. The transducers 

were retrieved from the wells several days following the 

test, and the raw data were downloaded and compensated 

for barometric pressure using data recorded by a transducer 

deployed at land surface in well Sh:H-001.

A subset of the network of monitoring wells located at the 

ALF and ACC Plants was used for evaluating water levels in the 

MRVA aquifer during the test. The subset included 27 wells at 

the ALF Plant and the 5 wells at the ACC Plant (fig. 2). Absolute 
10 Hydrogeology and Groundwater Quality, Tennessee Valley Authority Allen Power Plants

pressure transducers installed by TVA in each monitoring well 

were used to collect water-level data at 5-minute intervals 

before, during, and following the test. During the test, periodic 

measurements were made in the wells by Stantec using an 

electric tape to compare to and verify the accuracy of the 

continuous transducer data. Following completion of the test, 

the transducer data were compensated for barometric pressure 

using data collected from a TVA-maintained barometer located 

at the ALF Plant, and the data were provided to the USGS and 

CAESER by Stantec for use in the aquifer-test analysis.

Water-level data were collected by the USGS, CAESER, 

and Stantec during the test. The periodic water-level data 

collected by the USGS and CAESER for the Memphis 

aquifer from the ACC Plant production wells and the offsite 

observation wells are stored in the USGS NWIS (U.S. 

Geological Survey, 2018). The continuous water-level data 

collected using transducers from the offsite Memphis aquifer 

observation wells were processed by and are available from 

the CAESER. TVA collects, stores, and maintains the waterlevel data collected from the network of MRVA aquifer 

monitoring wells at the ALF and ACC Plants as part of the RI. 

Water-level data collected from these monitoring wells that 

were furnished to the USGS by Stantec for use in this study 

are the property of the TVA and will be made available with 

results of the RI.

0

1,000

2,000

3,000

4,000

5,000

6,000

10/3/2017 

0:00

10/3/2017 

12:00

10/4/2017 

0:00

10/4/2017 

12:00

10/5/2017 

0:00

Cumulative pumping rate, in gallons per minute

Average pumping rate per 8-hour step

Cumulative pumping

EXPLANATION

Add PW 4

Pumps off

Add PW 3

Start PW 1 and PW 5 All 5 wells

pumped

Pre-test

Start/step

times*

Step/finish

times*

Duration,

hours 

Flow rate,

in gallons

per minute 

Production

wells 

Water volume, 

million gallons

Start 10:00 am

October 3 

6:00 pm 8 2,500 1, 5 1.2

6:00 pm 2:00 am 8 3,750 1, 5, +3 1.8

2:00 am Finish 10:00 am

October 4

8 5,000 1, 5, 3, +4 2.4

Total 5.4

*Note: Production well PW 2 was not pumped and served as an observation well/emergency

backup well during test 

PW 1 PW 2 PW 3 PW 4 PW 5 Total

Total Gallons 1,790,189 4,903 1,199,572 616,140 1,817,733 5,428,537

Date and time

Figure 5. Pumping rates, durations, and volume of water pumped for the Tennessee Valley Authority Allen Combined Cycle Plant 

aquifer test, October 3–4, 2017.
Results 11

Data Analysis

Water-level data collected during the test were evaluated 

using different methods. Drawdown values for the five 

Memphis aquifer production wells were calculated by 

comparing pre-test (static) and late-pumping period water 

levels. Recovery-period data were used to verify that water 

levels in the production wells returned to near-static conditions 

once pumping stopped. Water-level data collected from the 

Memphis aquifer observation wells and the MRVA aquifer 

monitoring wells were evaluated by comparing pre- and latepumping period continuous transducer-recorded data and by 

modeling the transducer data using the program SeriesSEE 

(Version 1.20), which is a Microsoft Excel add-in (Halford and 

others, 2012), to remove “environmental fluctuations” or the 

effects of natural and anthropogenic water-level changes in 

wells. Monitoring wells screened in the intermediate and deep 

parts of the MRVA aquifer primarily were used to evaluate 

results of the aquifer test; water-level data from some shallow 

MRVA aquifer monitoring wells that are co-located with 

intermediate and deep wells were not used because the water 

levels in the shallow wells appear to represent a perched water 

table or zones of limited hydraulic connectivity to the deeper 

parts of the aquifer. Stage data from the U.S. Army Corps of 

Engineers streamgage (USGS 07032000, Mississippi River 

at Memphis, TN) operated by the USGS on the Mississippi 

River about 5.5 mi upstream of the ALF Plant also were used 

as a baseline and surrogate for estimating water-level trends 

in the MRVA aquifer for separation of environmental effects 

during analysis of the aquifer-test data using the SeriesSEE 

program because of general correlation of the stage data with 

the MRVA aquifer water-level data locally. Input data for the 

model and model results for the transducer data are available 

from Carmichael and Kingsbury (2018).

Results

The data collected for this investigation were used 

to provide a preliminary evaluation of the potential for 

leakage from the MRVA aquifer to the Memphis aquifer in 

the TVA plants area. Descriptions of findings from the three 

main components of the evaluation provide various lines of 

evidence that support the conclusions of the study.

Subsurface Hydrogeologic Interpretations

Stratigraphic and lithologic data collected in the field 

as part of drilling of monitoring wells and stratigraphic 

test holes for the RI were used to develop a framework 

for the hydrogeologic units from land surface to the 

upper part of the Memphis aquifer at the TVA plants site. 

Lithologic descriptions of the geologic units encountered 

and interpretations of the depositional environments and 

hydrologic characteristics of the sediments composing each 

unit are discussed below.

Anthropogenic Fill

The upper intervals in each of the monitoring-well 

borings and test holes contain non-native materials, such 

as coal, asphalt, and angular limestone gravel, and include 

clumped soil. These intervals are interpreted to consist of 

anthropogenic-fill material mixed with tilled soil and floodplain sediment. The fill materials extend from the surface to as 

much as 11 ft below land surface and commonly are layered 

in 0.5- to 2-ft-thick beds. The fill materials in stratigraphic test 

holes P-4c and 205c commonly have gypsum crusts and pore 

lining, especially where limestone gravel is present. 

Mississippi River Valley Alluvium

Below the fill material, the uppermost stratigraphic unit 

present in the TVA plants area is the Mississippi River Valley 

alluvium. Thickness of the alluvium beneath the TVA plants 

site ranges from about 100 to 200 ft. Excluding the mainly 

fine-grained upper part of the alluvium, sand and gravel 

deposits that make up the MRVA aquifer range from about 

85 to 180 ft in the area. The subsurface deposits that make up 

the Mississippi River Valley alluvium beneath the TVA plants 

site are described in terms of five units, in descending order: 

upper alluvium, blue clay interval, and channel units III, II, 

and I. The channel units are designated based on intervals with 

gravel and sand at the base that fine upward to medium- to 

fine-grained sand. 

Upper Alluvium

The upper alluvium is fine-grained tan to gray sand and 

olive-gray to dark-gray silty clay in beds approximately 0.5 

to 5 ft thick. The total thickness varies from 13 to 26 ft across 

the TVA plants site. The sand beds are horizontal to crossbedded or ripple cross-laminated. Silt and clay intervals are 

commonly laminated or finely bedded. Fining upward sand 

to silty clay beds commonly have blocky soil structure and 

extensive root traces or preserved roots. The upper parts of the 

rooted intervals commonly are oxidized. Dark-brown to black 

organic matter is common in the rooted and soil structured 

intervals. Silt and clay intervals commonly have slight to 

moderate effervescence to dilute hydrochloric acid, indicating 

the presence of minor calcium carbonate. 

Interpretation: The upper alluvium represents overbank 

deposits on the flood plain of the Mississippi River. The 

fining upward beds represent proximal deposits of individual 

flooding periods followed by drying and oxidation. 

Similar deposits are observed in flood-plain deposits near 

Loosahatchie Bar across from downtown Memphis (Koontz 

and others, 2014; Koban, 2017). Silty clay intervals as 

much as 5 ft thick represent more distal overbank deposits 

in back swamp settings. The lithologic characteristics of 

the upper alluvium at the TVA plants site is consistent with 

the Mississippi River Valley confining unit described by 

Ackerman (1996). Although the upper alluvium deposits are 
12 Hydrogeology and Groundwater Quality, Tennessee Valley Authority Allen Power Plants

generally fine grained, they are unlikely to provide complete 

confinement because of extensive macroporosity resulting 

from soil-forming processes (rooting and drying). The 

upper alluvium likely acts as a leaky confining unit over the 

underlying MRVA aquifer.

Blue Clay Interval

The blue clay interval is composed of blue-gray silty clay 

and interbedded very fine-grained sand. The thickness of this 

interval ranges from less than 1 ft in the boring for monitoring 

well MW 3A and in stratigraphic test hole P-4c to 27 ft in test 

hole 212c. The designation of the blue clay interval is based 

on the presence of laminated to massive blue-gray silty clay 

that contains calcium carbonate and, in most cases, weathered 

millimeter-sized calcium carbonate gastropod shells. Finingupward beds that are 1 to 4 ft thick and composed of very 

fine-grained sand to silt are interbedded with the clay. Fine 

root traces and preserved root material are common, but 

evidence for drying and oxidation is rare. Organic matter 

and less common lignite are present in the thicker intervals, 

such as that in stratigraphic test hole 212c. The blue clay is 

not definitively identified in the boring for well MW 5A, but 

a similar clay interval is present at the appropriate depth. A 

root from the upper part of the blue clay interval at 21 ft in 

stratigraphic test hole 212c yielded a 14C age of about 150 

radiocarbon years before present (RCYBP; table 1). A small 

twig collected from the base of the blue clay interval at 35 ft 

below land surface in stratigraphic test hole 205c yielded a 

discordant 14C age of about 3,860 RCYBP.

Interpretation: The blue clay interval represents a 

shallow lake environment, possibly an oxbow cut-off lake. 

The thickest sections of the blue clay intervals (stratigraphic 

test holes 212c and 214c) are present where the upper 

alluvium is the thinnest (13 and 14 ft thick, respectively). This 

suggests that the lake co-existed in time with neighboring 

fluvial overbank areas. The species of gastropod shells was 

not identifiable, but visual assessment indicates that they are 

similar to pond species identified in Pleistocene lake deposits 

near Fulton, Tennessee (Grimley and others, 2009). The lower 

contact of the blue clay interval is commonly abrupt, with clay 

deposits overlying gravelly sand with or without root traces. 

This observation suggests a sudden change in the depositional 

environment, such as abrupt channel cutoff and isolation of 

the soon-to-be lake from the coarse-grained channel sediment. 

The approximately 150-year age of the root from the upper 

part of the blue clay interval suggests that the inferred oxbow 

lake may have existed until recent times. The discordant age 

of the twig in stratigraphic test hole 205c likely represents 

reworking of old wood. 

The blue clay interval is likely to impede vertical 

movement of water locally. The degree of rooting is not 

extensive, nor is the interval particularly broken or disrupted. 

The interval is likely to cause local perching of water in the 

overlying upper alluvium and possibly confinement of water 

in the underlying MRVA aquifer during high water-level 

conditions where the blue clay interval is thickest. Thinner 

intervals of blue clay, such as at the location of monitoring 

well MW 3A, are more rooted and may be locally removed 

by erosion, allowing for hydraulic connection between the 

overlying and underlying alluvium intervals.

Channel Unit III

Channel unit III is the uppermost recognizable finingupward gravel to sand interval observed in stratigraphic test 

holes and monitoring-well borings at the TVA plants site. 

The interval is identified in stratigraphic test holes 212c and 

205c, but may also be present in the boring for monitoring 

well MW 216 and the upper gravel-bearing intervals of the 

borings for wells MW 1A and MW 5A. The most complete 

preservation is in stratigraphic test hole 212c, where a basal 

gray gravel and sand interval at 93 to 76 ft below land surface 

abruptly fines upward to very fine-grained laminated and 

cross-laminated gray to olive-gray sand and silt that persists 

from 76 to 44 ft below land surface. Calcium carbonate and 

sparse fine gastropod fragments are present in the upper part 

of the silt interval. Fining-upward gravel to sand intervals 

with minor interbedded silt are observed in stratigraphic test 

hole 205c and in the borings for monitoring wells MW 1A 

and MW 5A. A partially oxidized gravel and gravelly sand 

interval is observed from 57 to 26 ft below land surface in 

stratigraphic test hole P-4c and may be the youngest part of 

the channel unit III complex or a separate young channel. The 

gravel is composed mainly of tan to white chert and white 

quartzite. The gravel and sand are notably rich in lignite 

Table 1. Radiocarbon ages for wood samples collected from 

the Mississippi River Valley alluvium at the Tennessee Valley 

Authority Allen Combined Cycle and Allen Fossil Plants.

[USGS, U.S. Geological Survey; MW, monitoring well; c, stratigraphic test 

hole; 14C, carbon-14; RCYBP, radiocarbon years before present; see figure 2 

for locations of monitoring wells and figure 6 for locations of stratigraphic 

test holes]

Stratigraphic 

test hole/

monitoring 

well field ID1

USGS 

well 

number

Sample 

depth (feet 

below 

land 

surface)

Fraction 

modern 14C

14C age

(RCYBP)

Precision 

of 14C age

(RCYBP)

205c Sh:H-043 35 0.6186 3,858 21

212c Sh:H-041 21 0.9817 149 19

212c Sh:H-041 116 0.6868 3,018 22

214c Sh:H-042 112 0.6242 3,786 22

MW 1A Sh:H-035 36 0.6271 3,748 20

MW 1A Sh:H-035 56 0.7386 2,433 19

MW 1A Sh:H-035 116 0.3331 8,830 25

MW 1A Sh:H-035 166 0.1437 15,586 56

MW 5B Sh:H-037 46 0.8509 1,297 18

1

Fraction modern carbon is relative to atmospheric carbon in 1950. 14C 

age is the uncalibrated carbon-14, in years before 1950. The precision is 

±1-sigma standard deviation of multiple sample analyses.
Results 13

chips. Small branches to chips of wood are present in many 

intervals. The larger pieces of wood have 14C ages that range 

from about 2,430 RCYBP in basal gravels in the boring for 

monitoring well MW 1A to 1,300 RCYBP in upper sand-rich 

beds in the boring for MW 5B (table 1). A discordant age of 

about 3,750 RCYBP was obtained for fine woody material at 

the top of the unit in the boring for monitoring well MW 1A; 

however, this likely is reworked older wood.

Interpretation: Channel unit III represents channel 

deposits of the Mississippi River. The overall fining-upward 

grain-size trend and thickness in stratigraphic test hole 212c 

are consistent with the channel of a large meandering stream. 

Cross-laminated very fine-grained sandy silt is present in the 

unit and likely represents point-bar deposits over the channel 

gravel. However, the uppermost calcium carbonate- and 

gastropod-bearing silt represents a transition to the overlying 

blue clay interval. An interval of greenish- to olive-gray 

horizontal and cross-bedded very fine- to medium-grained 

sand in the boring for monitoring well MW 216 likely 

represents a nonvegetated channel bar similar to those that 

exist in the Mississippi River near Memphis (Koban, 2017). 

The position of point-bar deposits in stratigraphic test hole 

212c and the channel bar in the boring for monitoring well 

MW 216 suggest that the channel migrated west with time. 

Thus, if the gravel-rich interval from 57 to 26 ft below land 

surface in stratigraphic test hole P-4c is related to channel unit 

III, it represents the youngest part of this channel system.

The gravel-rich intervals of channel unit III represent 

a hydraulically conductive upper zone in the MRVA aquifer 

at the TVA plants site. The hydrologic effect of the sand and 

gravel within this interval is likely to be most pronounced in 

the area of stratigraphic test holes P-4c and 205c. 

Channel Unit II

Channel unit II is recognized in the borings for all 

monitoring wells and stratigraphic test holes that penetrated 

through the entire Mississippi River Valley alluvium. The 

base of the interval begins between 135 and 115 ft below 

land surface. Channel unit II is composed of gray sand and 

gravel that fine upward to fine- to medium-grained sand. 

Bedding structure is rarely present, but where present, 

includes horizontal bedding and various types of cross 

bedding. The gravel is composed mainly of tan to white chert, 

with lesser quantities of white quartzite, dark-gray to green 

metasedimentary clasts, red volcanic tuff, and various granite 

and gneiss clasts. Lignite, gravel, and sand are common in 

many beds, and boulder-size blocks of Tertiary mudstone are 

interpreted as locally present. Wood logs and branches are 

preserved in several beds. A log at the base of channel unit II 

at 116 ft below land surface in the boring for monitoring 

well MW 1A has a 14C age of about 8,800 RCYBP (table 1). 

However, wood logs near the base of channel unit II in 

stratigraphic test holes 212c and 214c have ages of about 

3,020 at 116 ft below land surface and 3,790 RCYBP at 112 ft 

below land surface, respectively.

Interpretation: Channel unit II represents channel 

deposits of the modern Mississippi River and is a hydraulically 

conductive intermediate zone of the MRVA aquifer at the TVA 

plants site. The most complete fining-upward sequences are 

from the boring for monitoring well MW 3A at the western 

side of the ACC Plant and from stratigraphic test hole 214c, 

the southwestern-most of the four test holes drilled at the 

ALF Plant. The gravel clast composition notably contains 

red volcanic tuff, which is likely to have originated from the 

St. Francis Mountains in Missouri and indicates Mississippi 

River sediment sources (Ward and others, 2017). The age of 

the wood log from the base of the unit also corresponds well 

to similar sediments and radiocarbon results from modern 

Mississippi River deposits near Marion, Arkansas (Ward and 

others, 2017); however, the wood logs from above the base of 

the channel unit suggest a late Holocene age for much of the 

channel fill. 

Channel Unit I

Channel unit I is identified in the borings for monitoring 

wells MW 1A and MW 5A and in stratigraphic test hole 212c. 

The base of the unit ranges from 228 ft below land surface in 

stratigraphic test hole 212c to 140 ft below land surface in the 

boring for well MW 5A. Channel unit I is composed of gray 

sand and gravel that fines upward to fine- to medium-grained 

sand with sparse pebbles. The gravel is composed mainly of 

tan to white chert, with lesser quantities of white quartzite, 

gray metasedimentary clasts, red volcanic tuff, and granite 

clasts. Lignite and lignitic mudstone, gravel, and sand are 

common in the boring for monitoring well MW 1A, but not in 

the other monitoring-well borings. Small branches of wood are 

rare. A branch at the base of channel unit I at 166 ft below land 

surface in the boring for monitoring well MW 1A has an age 

of about 15,600 RCYBP (table 1). 

Interpretation: Channel unit I represents the deepest 

channel deposits of the Mississippi River and is the lowermost 

hydraulically conductive zone of the Mississippi River Valley 

alluvium at the TVA plants site. Overall downward incision 

into underlying Eocene strata and thickening of the channel 

deposits toward the eastern margin of the site suggest that 

channel scour occurred adjacent to the Mississippi River 

bluffs. The radiocarbon age obtained from the base of the 

Mississippi River Valley alluvium in the boring for monitoring 

well MW 1A is consistent with the Kennett or Blodgett 

alluvium of the Pleistocene Mississippi River (Rittenour 

and others, 2007; Ward and others, 2017). The gravel clast 

composition is generally similar to that of channel unit II, 

suggesting similar origins and deposition from integration of 

the Mississippi and Ohio Rivers that occurred beginning about 

18,000 years ago (Blum and others, 2000).

Cook Mountain Formation

The Cook Mountain Formation underlies the Mississippi 

River Valley alluvium at the TVA plants site. The contact is 
14 Hydrogeology and Groundwater Quality, Tennessee Valley Authority Allen Power Plants

disconformable and has as much as 110 ft of erosional relief. 

The preserved Cook Mountain Formation ranges from 26 to 

about 75 ft thick at the TVA plants site. The Cook Mountain 

Formation is composed primarily of very fine-grained sandy 

to silty mudstone with interlaminated silty very fine-grained 

sand. The color is generally olive-gray to gray, but is locally 

light gray directly beneath the contact with the Mississippi 

River Valley alluvium. The mudstone is generally laminated 

to finely cross-laminated. Inclined beds with more very finegrained sand than mudstone are present in the lower 12 ft of 

the Cook Mountain Formation in stratigraphic test hole P-4c. 

The sand is commonly micaceous, but is mainly composed of 

quartz. Chips of lignitic plant debris and leaves are common 

along partings and in sandy beds. The mudstone locally has 

fractures at 30 degrees to bedding with slickensides. The 

fractures are most prominent in stratigraphic test hole 212c, 

but are present in minor amounts in test hole P-4c. Sand-filled 

vein-fractures are present in stratigraphic test hole 212c.

Interpretation: The Cook Mountain Formation at the TVA 

plants site represents ancient delta-front and prodelta deposits. 

The inclined sandy beds at the base of the unit in stratigraphic 

test hole P-4c are similar to those observed in outcrop of 

the Cook Mountain Formation along the Mississippi River 

near Fulton, Tennessee, where the beds represent delta-front 

deposits into an ancient coastal lake (Tye and Coleman, 1989). 

The ripple cross-lamination commonly shows aggradation and 

downgradient migration, typical of deposits from decelerating 

turbid flows. The pervasive lamination observed is common 

for distal deposits of turbid flows from a delta into shallow 

water. The abundance of lignitic material is interpreted to 

derive from densely vegetated ancient coastal swamps and 

forests that bordered the deltaic distributary channels. 

The sand-filled veins are interpreted to reflect local sanddike formations, perhaps from ancient earthquakes or dilation 

resulting from incipient delta slope failure. Paleoliquefaction 

features have been observed in upper Claiborne Group 

strata in northwestern Shelby County (Ferreira and others, 

2013). The fractures are probably the result of more recent 

phenomena, mainly because of their fresh appearance and 

preservation of slickensides. The prominence of fractures in 

the unit in stratigraphic test hole 212c may reflect slumping 

processes along the bluff or disruption from nearby faults.

The Cook Mountain Formation is the primary and most 

clay-rich component of the upper Claiborne confining unit 

for the Memphis aquifer in the Memphis area (Parks, 1990). 

At the TVA plants site, the Cook Mountain Formation is 

composed primarily of clay and is a low-permeability unit that 

restricts movement of water between the MRVA and Memphis 

aquifers.

Transition of Cook Mountain Formation to 

Memphis Sand

Based on observations from the four stratigraphic test 

holes drilled at the ALF Plant, the contact of the Memphis 

Sand with the overlying Cook Mountain Formation is 

conformable and gradational. Seven to 25 ft of interbedded 

white very fine-grained sand and light-gray sandy silt are 

present between the sand-dominated Memphis Sand and 

mud-dominated Cook Mountain Formation and represent a 

transitional interval. Applying stratigraphic standards for the 

Cook Mountain Formation from Parks (1990) and Kingsbury 

and Parks (1993), the transitional interval with roughly equal 

content of sand and mud is ascribed to the Cook Mountain 

Formation so as to maintain similar formation thickness across 

the area. The transitional interval is composed of interbedded 

white to light-gray very fine-grained sand and laminated silty 

mud. Intraclasts of mud are common in the sand, and some 

mud intervals are broken. Lignite flakes are common in the 

silty mud. The sand is mostly quartz with a trace of silt or clay. 

Grain size increases and sorting improves with depth in the 

interval.

Memphis Sand

The four stratigraphic test holes drilled at the ALF Plant 

penetrated at least 13 ft of the upper Memphis Sand. Samples 

of the Memphis Sand from the borings consist of white very 

fine- to fine-grained sand with a few mud beds that are up to 

0.5 ft thick. The sand is well sorted and is composed of quartz 

and dispersed lignite flakes.

Interpretation: The transitional interval of the Cook 

Mountain Formation to the Memphis Sand and the upper 

part of the Memphis Sand penetrated in the four test holes 

at the ALF Plant are interpreted as mouth bar and deltaic 

distributary channel deposits. The interbedded mud and sand 

likely reflect sedimentation of turbid waters at the terminus of 

the distributary channel, where flow becomes unconfined (for 

example, a delta mouth bar). The prominence of intraclasts 

in the sand is the result of caving of fine-grained flood-plain 

deposits, similar to channel deposits observed elsewhere in the 

Memphis Sand (Gentry and others, 2006; Larsen and Brock, 

2014). The moderate sorting of the sands and inclusion of fine 

lignitic material are consistent with hydrodynamic processes 

in river and distributary channels.

The transitional interval between the mud of the Cook 

Mountain Formation and the well-sorted sands of the 

Memphis Sand are likely to impede vertical groundwater flow, 

but less so than the mud of the Cook Mountain Formation. 

The well-sorted sand is part of the Memphis aquifer; however, 

the most productive groundwater zones are deeper in the 

Memphis Sand.

Geologic Structure

Correlation of stratigraphic units at the TVA plants site 

and comparison with geophysical logs from nearby wells 

were used to produce cross sections spanning the study area 

(figs. 6–9). These correlations suggest that stratigraphic units 

are offset along faults, one cutting southwest to northeast 
Results 15

across the TVA site between the ALF and ACC Plants, and 

another following the Mississippi River bluff line east of 

the TVA plants site (fig. 6). The upper contact of the Cook 

Mountain Formation with the Mississippi River Valley 

alluvium is disconformable, and the variation in the depth of 

the contact reflects variable depth of erosion by the Mississippi 

River during late Pleistocene and Holocene time. The lower 

contact of the Cook Mountain Formation is considered the 

base of the transitional interval to the underlying Memphis 

Sand. Information is available about the depth of the upper 

contact of the Cook Mountain Formation from rotasonic core 

data from most of the borings for monitoring wells screened 

in the lower part of the MRVA aquifer and from the four 

stratigraphic test holes drilled at the ALF Plant as part of the 

RI. The depth of the lower contact is less certain because 

the top of the Memphis Sand was only penetrated in the four 

stratigraphic test holes and because the surface casing of the 

production wells drilled at the ACC Plant prevented data 

collection using electric geophysical-log methods and limited 

definition and resolution of natural gamma-ray log data. 

Furthermore, geologic correlations of data for offsite Memphis 

aquifer wells (wells Sh:H-001, Sh:H-002, and Sh:H-013; 

fig. 6) are based solely on geophysical logs and, as a result, 

lack the additional resolution available from core data. 

Interpretation of faulting throughout the Memphis area based 

on apparent offsets of the Cook Mountain Formation and top 

of the Memphis Sand is difficult because of the gradational 

transition across these units. Deep stratigraphic information 

(for example, the base of the Memphis aquifer) used in 

concert with shallow data allows for greater confidence in 

interpretation of the stratigraphy and the apparent offsets.

The inferred fault with a trace passing from southwest to 

northeast between the TVA ACC and ALF Plants is interpreted 

to have up-to-the-north offset (figs. 6–8). The lower contact 

of the Cook Mountain Formation has a shallow northerly 

dip north of the ACC Plant in cross-sections A–A′ and B–B′ 

(figs. 7 and 8); however, the lower contact of the Cook 

Mountain Formation in cross-section C–C′ (fig. 9) is nearly 

horizontal within the Mississippi River Valley at the TVA 

plants site. Between production well PW 5 and stratigraphic 

test hole 214c, and between stratigraphic test holes 212c 

and 205c (figs. 7 and 8), the Cook Mountain Formation is 

interpreted to have upward offset on the northern side of 

the inferred fault trace. Additionally, marker beds in the 

Mississippi River Valley alluvium (blue clay interval) may 

be offset along this inferred fault across the TVA plants area. 

The inferred fault follows the southwest-to-northeast trend of 

several identified faults in the area, including the MeemanShelby Fault (Hao and others, 2013; Ward and others, 2017) 

and the Big Creek Fault (Van Arsdale and Cupples, 2013). 

Another offset of the stratigraphic units is interpreted 

along the eastern border of the TVA plants site based on a 

natural gamma-ray log made in the test hole for observation 

well Sh:H-001 at T.O. Fuller State Park (fig. 9). The 

stratigraphic units overlying the Cook Mountain Formation 

at this well location are the Pliocene-Pleistocene-age fluvial 

(terrace) deposits and Pleistocene-age loess, both of which 

are verified by surface exposures in the area. Based on 

interpretation made for this study, as much as 130 ft of offset 

may be present on the lower contact of the Cook Mountain 

Formation at this well location, with the relative motion up 

to the east along the fault. Martin and Van Arsdale (2017) 

identified a fault in this area based on a regional study of 

offset of the lower contact of the Cook Mountain Formation. 

Previous interpretations of the natural gamma-ray log of 

observation well Sh:H-001 in the context of the hydrogeology 

of the Memphis area (Parks, 1990; Kingsbury and Parks, 

1993) indicate a lower altitude base and greater thickness 

of the Cook Mountain Formation in this well. The alternate 

interpretations of the thickness and depth to the base of 

the Cook Mountain Formation illustrate the difficulty in 

conclusively characterizing the contact between the upper 

Claiborne confining unit and the Memphis Sand and potential 

offsets in these units without deep stratigraphic control.

The inferred faults have implications for groundwater 

movement between the MRVA and Memphis aquifers where 

offset may provide for hydraulic connection between the two 

units. However, uncertainty in the character and thickness 

of the Cook Mountain Formation away from the observation 

wells and constraint in determining the amount of offset along 

the inferred faults in the absence of deep geologic control 

preclude definitive interpretation of a hydraulic connection 

along the faults.
16 Hydrogeology and Groundwater Quality, Tennessee Valley Authority Allen Power Plants

Up

Down

Down

Up Sh:H-031

PW 1

Sh:H-025

PW 2

Sh:H-026

PW 3

Sh:H-027

PW 4

Sh:H-032

PW 5

Sh:H-001

Sh:H-013

Sh:H-002

ACC Plant

Horn Lake Cutoff

ALF Plant

P-4c

205c

212c

214c

A

A',B'

B

C

C'

Pidgeon Industrial Park

T.O. Fuller 

State Park

90°09' 90°08'

35°04'30"

35°03'30"

Pidgeon Industrial Park

T.O. Fuller 

State Park

Up

Down

Down

Up Sh:H-031

PW 1

Sh:H-025

PW 2

Sh:H-026

PW 3

Sh:H-027

PW 4

Sh:H-032

PW 5

Sh:H-001

Sh:H-013

Sh:H-002

ACC Plant

Cutoff Horn Lake

ALF Plant

P-4c

205c

212c

214c

A

A',B'

B

C

C'

Source: Esri, DigitalGlobe, GeoEye, 

Earthstar Geographics, CNES/Airbus DS, 

USDA, U.S. Geological Survey, AeroGRID, 

IGN, and the GIS User Community

0 0.4 0.8 MILE

0 0.4 0.8 KILOMETER

EXPLANATION

Fault—Existence inferred by geophysical 

 log and lithologic correlation, approximately 

 located. Ball and bar on downthrown side

Stratigraphic test hole and name

Memphis aquifer production 

 well and name

Memphis aquifer observation 

 well and name

Line of geologic section 

Sh:H-031

PW 1

Sh:H-001

212c

A A'

Note: Geophysical logs for wells and test holes

shown in figures 7–9

Lake McKellar

Figure 6. Locations of cross sections and inferred faults, Tennessee Valley Authority Allen Combined Cycle and Allen 

Fossil Plants area.
Results 17

50150200

100300400500600

Sh:H-013

SOUTH NORTH

214c PW 5 PW 2

Mississippi River

Valley Alluvium

Memphis Sand

Cook Mountain Formation

P-4c

A' A

Blue clay (pond)

14C = 3,786 RCYBP

50100150

100

LSD = 218 ft

NGR

LSD = 217 ft

NGR

LSD = 238 ft

NGR SPR

LSD = 216 ft

300

400

500

600

Depth, in feetbelow LSD

200

700

LSD = 219 ft

300400500600

Depth, in feetbelow LSD

200700800

-100

-200

-300

-400

Altitude, in feet above NGVD 29

-500

-600

Depth, in feetbelow LSD

Depth, in feetbelow LSD

NGR NGR SPR

SPR

Depth, in feetbelow LSD

100

SP

100

0

100

200

200

14C = 1,297 RCYBP*

* 14C samples from boring for monitoring well MW 5B

200

0

Lake McKellar Approximate land surface

?

? ? ?

?

?

?

050 FEET

Known contact—Identified by geophysical log correlation and supported by lithologic evidence Approximate contact—Based mainly on geophysical log correlation and supported by lithologic evidenceProbable fault—Existence established by geophysical log correlation, but only approximately located. Arrows show direction of displacementGeophysical log—Trace of geophysical log made in test hole or well; NGR, Natural gamma ray; SPR, single point resistivity;  SP, spontaneous potential

PW 5

?

? Contact uncertain

Radiocarbon ages for wood samples, in radiocarbon years before present (RCYBP) 

14C

EXPLANATION

LSD

NGVD 29

Land surface datum National Geodetic Vertical Datum of 1929

Figure 7. Geologic section A–A’, Tennessee Valley Authority Allen Combined Cycle and Allen Fossil Plants area.
18 Hydrogeology and Groundwater Quality, Tennessee Valley Authority Allen Power Plants

050 FEET

Known contact—Identified by geophysical log correlation and supported by lithologic evidence Approximate contact—Based mainly on geophysical log correlation and supported by lithologic evidenceProbable fault—Existence established by geophysical log correlation, but only approximately located. Arrows show direction of displacementGeophysical log—Trace of geophysical log made in test hole or well; NGR, Natural gamma ray; SPR, single point resistivity;  SP, spontaneous potential

PW 1

?

? Contact uncertain

Radiocarbon ages for wood samples, in radiocarbon years before present (RCYBP) 

14C

EXPLANATION

LSD

NGVD 29

Land surface datum National Geodetic Vertical Datum of 1929

SOUTH NORTH

Sh:H-013 212c PW 1 205c

B B'

Memphis Sand

14C = 2,433 RCYBP*

14C = 8,830 RCYBP*

14C = 3,018 RCYBP

14C = 3,858 RCYBP

14C = 15,586 RCYBP*

Mississippi River

Mississippi River

Valley Alluvium

Valley Alluvium

Cook Mountain Formation

Blue clay (pond) Blue clay (pond)

Cook Mountain

LSD = 211 ft LSD = 236 ft LSD = 216 ft LSD = 238 ft

NGR

Depth, in feetbelow LSD Depth, in feet

below LSD

Depth, in feetbelow LSD

SPR NGR

Depth, in feetbelow LSD

NGR NGR SP SPR

Altitude, in feet above NGVD 29

50100150200

100200300400500600

100

0

-100

-200

-300

-400

-500

-600

100200

Formation

300400500600700800

100150200250

200 C = 149 RCYBP 14

50

0

14C = 3,748 RCYBP*

Memphis Sand

Approximate land surface Lake McKellar

?

? ?

? ?

?

?

?

?

?

?

* 14C samples from boring for monitoring well MW 1A

Figure 8. Geologic section 

B–

B’, Tennessee Valley Authority Allen Combined Cycle and Allen Fossil Plants area.
Results 19

Figure 9. Geologic section C–C’, Tennessee Valley Authority Allen Combined Cycle and Allen Fossil Plants area.

WEST EAST

Sh:H-001

PW 2 PW 3 PW 1

C C'

Memphis Sand

MemphisSand

Mississippi River

Mississippi River

Valley bluff line

Valley Alluvium

Loess

Fluvial deposits

Cook Mountain Formation

Cook MtnFm

Fm

Blue clay (pond)

14C = 15,586 RCYBP*

14C = 3,748 RCYBP*

14C = 2,433 RCYBP*

14C = 8,830 RCYBP*

0

LSD = 312 ft

NGR

Depth, in feetbelow LSD

100200300

NGR

100

0

-100-200-300-400-500

-600

SP SPR

Depth, in feetbelow LSD

Depth, in feetbelow LSD

SPR SP NGR

LSD = 216 ft LSD = 219 ft LSD = 211 ft

Depth, in feetbelow LSD

SPR

NGR SP

Altitude, in feet above NGVD 29

100 100

200

300400500

600

700

800

100200300400500600700800

200300400500600700800

200

Approximate land surface

Cockfield(?) and Cook MountainFormations, undifferentiated

* 14C samples from boring for monitoring well MW 1A

?

?

?

?

? ?

?

??

? Known contact—Identified by geophysical log correlation

 and supported by lithologic evidence Approximate contact—Based mainly on geophysical log correlation and supported by lithologic evidenceProbable fault—Existence established by geophysical log correlation, but only approximately located. Arrows show direction of displacementGeophysical log—Trace of geophysical log made in test hole or well; NGR, Natural gamma ray; SPR, single point resistivity;  SP, spontaneous potential

PW 5

?

? Contact uncertain

Radiocarbon ages for wood samples, in radiocarbon years before present (RCYBP) 

14C

EXPLANATION

LSD

NGVD 29

Land surface datum National Geodetic Vertical Datum of 1929Formation

050 FEET
20 Hydrogeology and Groundwater Quality, Tennessee Valley Authority Allen Power Plants

 Water Quality

SC and total dissolved solids (TDS) data for samples 

collected in September 2017 (table 2) and SC data for samples 

collected in October 2017 (table 3) indicate a substantial 

difference in water quality between the wells screened in the 

MRVA aquifer and wells screened in the Memphis aquifer, 

with the MRVA aquifer having higher SC and TDS than the 

Memphis aquifer. Values of SC and TDS for samples collected 

from the MRVA aquifer monitoring wells at the ACC Plant in 

September 2017 ranged from 550 to about 890 microsiemens 

per centimeter at 25 degrees Celsius (μS/cm at 25 oC) and 282 

to 537 milligrams per liter (mg/L), respectively (table 2). In 

contrast, the ranges of SC and TDS for samples collected from 

the Memphis aquifer production wells in September were 

171 to 391 μS/cm at 25 oC and 100 to 233 mg/L, respectively 

(table 2). SC values for samples collected from the Memphis 

aquifer in October 2017 ranged from about 150 to 393 μS/cm 

at 25 oC (table 3). The ranges of SC and TDS for both aquifers 

are comparable to those measured in groundwater samples 

collected from wells at and west of the MLGW Davis well 

field (Parks and others, 1995).

The relative proportion of major ions for the September 

2017 samples, as illustrated on a Piper diagram (fig. 10), is 

similar for waters in both aquifers and is predominantly a 

calcium-magnesium-bicarbonate type. Concentrations of these 

major ions, however, were generally two to five times greater 

in samples from the MRVA aquifer than in samples from the 

Memphis aquifer (table 2). Cation and anion charge balances 

were 5 percent or less for all samples collected in September 

2017, and concentrations of filtered and unfiltered constituents 

generally were comparable (table 2). 

Samples from both aquifers indicate predominantly 

anoxic conditions—generally defined as DO concentrations 

less than about 0.5 mg/L (tables 2 and 3). As the result of 

these reducing conditions, nitrate was not detected in samples 

collected from either aquifer in September 2017 (table 2) and 

was detected in only one Memphis aquifer production well at 

a concentration equal to its reporting level in October 2017 

(table 3). Also as a result of the reducing conditions, iron 

concentrations were about 1,000 micrograms per liter (μg/L) 

in samples from the Memphis aquifer and about 3,000 to 

25,000 μg/L in samples from the MRVA aquifer (table 2).

Similar to results for major ions, concentrations of trace 

elements that were detected in both aquifers were higher 

in samples from the MRVA aquifer than in those from the 

Memphis aquifer (table 2). Results for filtered samples are 

discussed herein because these results were not affected 

by differences in sample turbidity and, overall, the filtered 

and unfiltered results were comparable (table 2). None of 

the trace elements analyzed were detected at concentrations 

exceeding drinking-water criteria (U.S. Environmental 

Protection Agency, 2018). Arsenic was detected in samples 

from the five monitoring wells screened in the MRVA aquifer 

at the ACC Plant, with concentrations ranging from 0.51 to 

5.44 μg/L. Arsenic commonly is detected in samples across 

the extent of the MRVA aquifer and likely is present in the 

groundwater because of the sediments that make up the 

aquifer (Kresse and Fazio, 2003; Kingsbury and others, 2014). 

Arsenic concentrations in samples from the Memphis aquifer 

production wells (0.08–0.14 μg/L) were about an order of 

magnitude less than the concentrations in samples from the 

MRVA aquifer wells. 

Tritium analyses were used to qualitatively evaluate 

groundwater age. The presence of tritium in groundwater 

samples indicates that groundwater in an aquifer includes a 

component of young water (post-1950). Tritium is a naturally 

occurring isotope of hydrogen, and since about 1950, 

atmospheric concentrations of tritium have varied over time, 

peaking during the late 1950s and 1960s when testing of 

nuclear weapons added substantial amounts of tritium to the 

atmosphere. Tritium has been proven to be a useful tool in 

the Memphis area for identifying areas of downward leakage 

of groundwater from the shallow aquifers to the Memphis 

aquifer (Graham and Parks, 1986; Larsen and others, 2016). 

As part of the September 2017 sampling event, groundwater 

samples also were collected from all of the production and 

monitoring wells at the ACC Plant for analysis of tritium to 

confirm the presence of young water in the MRVA aquifer and 

to determine if a component of young water is present in the 

Memphis aquifer. Tritium was present in the samples collected 

from the MRVA aquifer and ranged from 2.13 to 5.59 tritium 

units (TU) (table 4). Three of the Memphis aquifer production 

wells (PW 1, PW 2, and PW 4) did not have detectable tritium 

concentrations. Production well PW 3 had a low concentration 

(0.25 TU), and PW 5 had a tritium concentration of 1.24 TU. 

These data indicate a general relation between tritium 

occurrence and depth of the screened interval in the wells 

(fig. 11); production well PW 5, the well with the shallowest 

depth to the top of the screen, contained water with the highest 

tritium concentration. The presence of tritium at production 

wells PW 3 and PW 5 indicates that a component of young 

water is present in the upper part of the Memphis aquifer at the 

depths and locations of these wells.
Results 21

Table 2. Concentrations of major inorganic constituents and trace elements in groundwater samples collected from selected Tennessee Valley Authority Allen Combined Cycle Plant wells, September 2017.[See figure 2 for locations of wells; PW, production well; MW, monitoring well; USGS, U.S. Geological Survey; μS/cm, microsiemens per centimeter at 25 degrees Celsius; mg/L, milligrams per liter; <, less than; --, no data; N, nitrogen; NO2, nitrite; NO3, nitrate; μg/L, micrograms per liter; ROE, residue on evaporation; °C, degrees Celsius; TDS, total dissolved solids]

Well field ID USGS well number USGS site number Date Time pH (laboratory) Specific conductance (μS/cm) Dissolved oxygen (mg/L) Ammonia, as N (mg/L) Nitrite, as N (mg/L) NO2 + NO3, as N (mg/L) Orthophos- phate (mg/L)

MW 1A Sh:H-035 350355090082102 9/27/2017 1615 7.2 550 -- 0.043 <0.001 <0.04 <0.004

MW 5B Sh:H-037 350404090083302 9/21/2017 1100 7.3 872 0.4 0.41 0.0012 <0.04 0.0174

MW 5A Sh:H-038 350404090083303 9/20/2017 1630 7.4 564 0.5 0.030 0.0018 <0.04 <0.004

MW 3A Sh:H-036 350400090085302 9/27/2017 1200 7.4 830 0.4 0.70 0.0077 <0.04 0.18

MW 216 Sh:H-045 350401090081901 9/28/2017 1030 7.3 888 -- 0.19 0.011 <0.04 0.0329

PW 1 Sh:H-031 350355090082101 9/22/2017 1230 7.1 171 0.6 0.030 <0.001 <0.04 0.177

PW 2 Sh:H-025 350356090084001 9/22/2017 1030 7.2 183 0.3 0.035 0.0012 <0.04 0.467

PW 3 Sh:H-026 350400090085301 9/21/2017 1800 7.2 195 0.3 0.03 <0.001 <0.04 0.206

PW 4 Sh:H-027 350404090084701 9/21/2017 1530 7.3 210 0.5 0.027 <0.001 <0.04 0.523

PW 5 Sh:H-032 350404090083301 9/21/2017 1230 7.2 391 0.3 0.036 <0.001 <0.04 0.0104

Well field ID USGS well number USGS site number Total nitro- gen, filtered, as N (mg/L) Calcium, filtered (mg/L) Calcium, unfiltered (mg/L) Magnesium, filtered (mg/L) Magnesium, unfiltered (mg/L) Sodium, filtered (mg/L) Sodium, unfiltered (mg/L) Potassium, filtered (mg/L)

MW 1A Sh:H-035 350355090082102 0.063 61.1 66 31.1 33.9 10.9 11.4 2.8MW 5B Sh:H-037 350404090083302 0.48 143 136 35.5 34.7 7.55 7.23 2.3

MW 5A Sh:H-038 350404090083303 <0.05 66.2 60.5 27.7 23.2 19.8 19.7 1.9

MW 3A Sh:H-036 350400090085302 0.78 115 121 39.3 42.4 11.7 12.4 3.4MW 216 Sh:H-045 350401090081901 0.23 134 143 38.2 40.9 7.25 7.59 2.9

PW 1 Sh:H-031 350355090082101 <0.05 18.8 18.2 7.39 6.96 7.42 7.15 0.9

PW 2 Sh:H-025 350356090084001 <0.05 20.9 19.9 7.72 7.21 7.66 7.23 1

PW 3 Sh:H-026 350400090085301 0.08 21.6 20.7 9.37 8.58 7.81 7.46 0.9

PW 4 Sh:H-027 350404090084701 0.08 24 23.1 10 9.22 8.2 7.87 1

PW 5 Sh:H-032 350404090083301 0.104 48 45.9 19.9 19.3 8.6 8.17 1.2
22 Hydrogeology and Groundwater Quality, Tennessee Valley Authority Allen Power Plants Table 2. Concentrations of major inorganic constituents and trace elements in groundwater samples collected from selected Tennessee Valley Authority Allen Combined Cycle Plant wells, September 2017.—Continued [See figure 2 for locations of wells; PW, production well; MW, monitoring well; USGS, U.S. Geological Survey; μS/cm, microsiemens per centimeter at 25 degrees Celsius; mg/L, milligrams per liter; <, less than; --, no data; N, nitrogen; NO2, nitrite; NO3, nitrate; μg/L, micrograms per liter; ROE, residue on evaporation; °C, degrees Celsius; TDS, total dissolved solids] Well field ID USGS well number USGS site number Chloride, filtered (mg/L) Sulfate, filtered (mg/L) Fluoride, filtered (mg/L) Silica, filtered (mg/L) Silica, unfiltered (mg/L) Arsenic, filtered (μg/L) Arsenic, unfiltered (μg/L) Barium, filtered (μg/L) Barium, unfiltered (μg/L) MW 1A Sh:H-035 350355090082102 6.39 2.14 0.17 22.6 23.8 1.73 1.9 596 592 MW 5B Sh:H-037 350404090083302 2.95 33.7 0.14 28.2 27.3 1.83 1.7 463 467 MW 5A Sh:H-038 350404090083303 13.4 75.7 0.26 21.3 19.5 0.51 0.67 169 149 MW 3A Sh:H-036 350400090085302 6.55 12.5 0.12 32 33.6 0.59 0.7 896 914 MW 216 Sh:H-045 350401090081901 3.15 47.6 0.095 26.1 27 5.44 4.7 691 698 PW 1 Sh:H-031 350355090082101 2.24 2.74 0.11 10.5 10.2 0.13 0.22 75.7 72.6 PW 2 Sh:H-025 350356090084001 2.27 2.69 0.11 10.8 10.7 0.12 0.2 77.3 75.8 PW 3 Sh:H-026 350400090085301 2.48 2.87 0.11 10.4 10.3 0.08 0.1 104 98.5 PW 4 Sh:H-027 350404090084701 2.56 3.15 0.11 10.5 10.6 0.14 0.1 95.3 94.8 PW 5 Sh:H-032 350404090083301 4.64 26.7 0.11 10.5 10.1 0.1 0.1 177 172 Well field ID USGS well number USGS site number Beryllium, filtered (μg/L) Beryllium, unfiltered (μg/L) Boron, filtered (μg/L) Boron, unfiltered (μg/L) Cadmium, filtered (μg/L) Cadmium, unfiltered (μg/L) Chromium, filtered (μg/L) Chromium, unfiltered (μg/L) MW 1A Sh:H-035 350355090082102 <0.01 <0.19 15 13.1 <0.03 <1.60 <0.5 <0.60 MW 5B Sh:H-037 350404090083302 <0.01 <0.19 68 73 <0.03 <1.20 <0.5 0.98 MW 5A Sh:H-038 350404090083303 <0.01 <0.19 20 22.5 <0.03 <0.400 <0.5 <0.60

MW 3A Sh:H-036 350400090085302 <0.01 <0.19 97 128 <0.03 <2.00 <0.5 <0.60

MW 216 Sh:H-045 350401090081901 <0.01 <0.19 77 109 <0.03 <1.60 <1.5 <0.60

PW 1 Sh:H-031 350355090082101 <0.01 <0.19 <5 4.45 <0.03 <0.40 <0.5 <0.60

PW 2 Sh:H-025 350356090084001 0.01 <0.19 <5 3.92 <0.03 <0.40 <0.5 <0.60

PW 3 Sh:H-026 350400090085301 0.01 <0.19 <5 2.65 <0.03 <0.40 <0.5 <0.60

PW 4 Sh:H-027 350404090084701 0.08 <0.19 <5 4.93 <0.03 <0.40 <0.5 <0.60

PW 5 Sh:H-032 350404090083301 <0.01 <0.19 <5 3.91 <0.03 <0.40 <0.5 <0.60
Results 23

Table 2. Concentrations of major inorganic constituents and trace elements in groundwater samples collected from selected Tennessee Valley Authority Allen Combined Cycle Plant wells, September 2017.—Continued[See figure 2 for locations of wells; PW, production well; MW, monitoring well; USGS, U.S. Geological Survey; μS/cm, microsiemens per centimeter at 25 degrees Celsius; mg/L, milligrams per liter; <, less than; --, no data; N, nitrogen; NO2, nitrite; NO3, nitrate; μg/L, micrograms per liter; ROE, residue on evaporation; °C, degrees Celsius; TDS, total dissolved solids]

Well field ID USGS well number USGS site number Cobalt, filtered (μg/L) Cobalt, unfiltered (μg/L) Copper, filtered (μg/L) Copper, unfiltered (μg/L) Iron, unfiltered (μg/L) Iron, filtered (μg/L) Lead, filtered (μg/L) Lead, unfiltered (μg/L)

MW 1A Sh:H-035 350355090082102 0.93 <0.70 <0.2 <5.60 16,900 15,500 <0.02 0.05MW 5B Sh:H-037 350404090083302 2.89 2.34 <0.2 <5.60 2,850 3,270 <0.02 0.025MW 5A Sh:H-038 350404090083303 0.84 <0.70 <0.2 <2.8 4,530 5,990 <0.02 0.09MW 3A Sh:H-036 350400090085302 0.07 <3.50 <0.2 <7.00 21,600 21,600 <0.02 0.029MW 216 Sh:H-045 350401090081901 0.75 <0.70 <0.6 <8.40 24,500 23,600 <0.02 <0.02

PW 1 Sh:H-031 350355090082101 0.07 <0.70 <0.2 <1.40 1,360 1,300 0.25 0.053

PW 2 Sh:H-025 350356090084001 0.09 <0.70 <0.2 <1.40 1,090 1,030 <0.02 0.04PW 3 Sh:H-026 350400090085301 0.05 <0.70 <0.2 <1.40 1,400 1,310 <0.02 <0.02

PW 4 Sh:H-027 350404090084701 0.19 <0.70 <0.2 <1.40 1,350 1,120 <0.02 0.04

PW 5 Sh:H-032 350404090083301 0.06 <0.70 <0.2 <1.40 2,080 2,130 <0.02 <0.02

Well field ID USGS well number USGS site number Manganese, filtered (μg/L) Manganese, unfiltered (μg/L) Thallium, filtered (μg/L) Molybdenum, filtered (μg/L) Nickel, filtered (μg/L) Nickel, unfiltered (μg/L) Silver, filtered (μg/L) Silver, unfiltered (μg/L)

MW 1A Sh:H-035 350355090082102 295 311 <0.02 0.96 1.8 1.8 <1 2.04

MW 5B Sh:H-037 350404090083302 1,390 1,300 <0.02 1.42 5.2 5.4 <1 2.23

MW 5A Sh:H-038 350404090083303 363 302 <0.02 3.06 1.4 1.7 <1 1.2

MW 3A Sh:H-036 350400090085302 360 375 <0.02 1.06 0.4 0.52 <1 2.43

MW 216 Sh:H-045 350401090081901 1,220 1,270 <0.02 0.42 0.9 0.9 <1 2.63

PW 1 Sh:H-031 350355090082101 26.5 27.4 <0.02 0.16 0.3 0.37 <1 <0.600

PW 2 Sh:H-025 350356090084001 22.5 22.8 <0.02 0.12 0.4 0.4 <1 <0.600PW 3 Sh:H-026 350400090085301 24.3 24.7 <0.02 0.07 0.3 0.3 <1 0.644

PW 4 Sh:H-027 350404090084701 25.5 26.2 <0.02 0.38 0.4 0.6 <1 <0.600

PW 5 Sh:H-032 350404090083301 46.6 43.8 <0.02 <0.05 0.2 0.2 <1 0.898
24 Hydrogeology and Groundwater Quality, Tennessee Valley Authority Allen Power Plants Table 2. Concentrations of major inorganic constituents and trace elements in groundwater samples collected from selected Tennessee Valley Authority Allen Combined Cycle Plant wells, September 2017.—Continued [See figure 2 for locations of wells; PW, production well; MW, monitoring well; USGS, U.S. Geological Survey; μS/cm, microsiemens per centimeter at 25 degrees Celsius; mg/L, milligrams per liter; <, less than; --, no data; N, nitrogen; NO2, nitrite; NO3, nitrate; μg/L, micrograms per liter; ROE, residue on evaporation; °C, degrees Celsius; TDS, total dissolved solids] Well field ID USGS well number USGS site number Strontium, filtered (μg/L) Strontium, unfiltered (μg/L) Vanadium, filtered (μg/L) Vanadium, unfiltered (μg/L) Zinc, filtered (μg/L) Zinc, unfiltered (μg/L) Antimony, filtered (μg/L) MW 1A Sh:H-035 350355090082102 353 354 <0.1 0.85 2 2.54 0.09 MW 5B Sh:H-037 350404090083302 520 511 <0.1 0.81 <2 <2.0 0.06 MW 5A Sh:H-038 350404090083303 489 429 <0.1 0.65 <2 49.6 0.14 MW 3A Sh:H-036 350400090085302 479 474 <0.1 0.65 <2 2.23 0.04 MW 216 Sh:H-045 350401090081901 311 309 <0.3 0.68 <6 <8.0 <0.03 PW 1 Sh:H-031 350355090082101 79.8 82.8 <0.1 <0.6 <2 <2.0 <0.03 PW 2 Sh:H-025 350356090084001 104 105 <0.1 <0.6 <2 <2.0 <0.03 PW 3 Sh:H-026 350400090085301 77.1 77.9 <0.1 <0.6 <2 <2.0 <0.03 PW 4 Sh:H-027 350404090084701 89.2 354 <0.1 0.85 2 2.54 <0.03 PW 5 Sh:H-032 350404090083301 182 511 <0.1 0.81 <2 <2.0 <0.03 Well field ID USGS well number USGS site number Aluminum, unfiltered (μg/L) Aluminum, filtered (μg/L) Lithium, filtered (μg/L) Selenium, filtered (μg/L) Uranium, filtered (μg/L) ROE, filtered, at 180 oC (TDS) (mg/L) Bromide, filtered (mg/L) MW-1A Sh:H-035 350355090082102 30.7 <3 1.34 <0.05 0.41 309 0.045 MW-5B Sh:H-037 350404090083302 <25.0 <3 18.6 <0.05 1.63 282 0.021 MW-5A Sh:H-038 350404090083303 67.4 <3 5.34 <0.05 0.35 370 0.049

MW-3A Sh:H-036 350400090085302 <25.0 <3 6.74 <0.10 0.16 415 0.075

MW 216 Sh:H-045 350401090081901 <25.0 <3 17.2 <0.15 0.81 537 <0.02

PW 1 Sh:H-031 350355090082101 32.5 <3 0.93 <0.05 <0.01 100 0.026

PW 2 Sh:H-025 350356090084001 <25.0 <3 2.01 <0.05 <0.01 109 0.027

PW 3 Sh:H-026 350400090085301 <25.0 <3 1.08 <0.05 <0.01 119 0.028

PW 4 Sh:H-027 350404090084701 33.8 9 1.92 <0.05 0.01 132 0.032

PW 5 Sh:H-032 350404090083301 <25.0 <3 1.07 <0.05 0.02 233 0.019
Results 25

Table 3. Results from analyses of water-quality samples collected from selected production wells during the Allen Combined Cycle Plant aquifer test, October 3–4, 2017. [Temp., water temperature; oC, degrees Celsius; mg/L, milligrams per liter; μS/cm at 25 oC, microsiemens per centimeter at 25 degrees Celsius; BD, below detection]

USGS well 

number/

well field ID 

(see figure 2 

for location)

Date Time

Field data Laboratory data

Temperature

(°C)

Dissolved 

oxygen

(mg/L)

Specific 

conductance 

(μS/cm at 

25 oC)

pH Fluoride (mg/L) Chloride (mg/L) Bromide (mg/L) Nitrate (mg/L) Phosphate (mg/L) Sulfate (mg/L)

Sh:H-032/PW 5 10/3/2017 1115 18.4 0.70 393 6.2 0.07 4.6 BD BD BD 26.8

1640 18.4 0.75 341 6.4

2005 18.4 0.61 368 6.4

10/4/2017 0935 18.4 0.57 340 6.6 0.08 4.4 BD BD BD 24.0

Sh:H-031/PW 1 10/3/2017 1205 18.9 0.64 163 6.4 0.08 2.2 0.02 BD BD 2.8

1700 18.8 0.63 159 6.4

2045 18.8 0.55 150 6.5

10/4/2017 0800 18.8 0.56 161 6.6 0.07 2.1 0.02 BD BD 2.7

Sh:H-026/PW 3 10/3/2017 2135 18.8 0.56 194 6.6 0.07 2.3 0.01 BD BD 2.8

10/4/2017 0120 18.8 0.56 180 6.5

0840 18.8 0.64 188 6.6 0.07 2.3 0.02 BD BD 2.8

Sh:H-027/PW 4 10/4/2017 0350 19.1 0.56 183 6.5 0.08 2.4 0.02 0.01 0.1 2.9

0910 19.1 0.55 198 6.6 0.07 2.4 0.01 BD BD 2.9

Precision: 0.01 0.1 0.01 0.01 0.1 0.2
26 Hydrogeology and Groundwater Quality, Tennessee Valley Authority Allen Power Plants

Memphis aquifer

EXPLANATION

Percent

Percent

Percent

MRVA aquifer

Calcium (Ca) Chloride (Cl) + 

Fluoride (F) 

100 80 60 40 20 0 0

0

0

0 0

0

20

20

20 20

20

40

40

40 40

40

60

60

60 60

60

80

80

20

40

60

80

0

20

40

60

80

20

40

60

80

100 100

80 80

80

20

40

60

80

100

100 100

100 100

100 100

Sulfate (SO4)

Magnesium (Mg)

Calcium (Ca) +

Potassium (K)

Sodium (Na) +

Carbonate (CO3)

Bicarbonate (HCO3)

Chloride (CI) +

Fluoride (F) +

Sulfate (SO4)

Magnesium (Mg)

Figure 10. Chemical characteristics of groundwater samples collected from wells at the Tennessee Valley Authority Allen 

Combined Cycle Plant, September 2017.
Results 27

The elevated SC and the corresponding elevated 

concentrations of major inorganic constituents in the sample 

collected from production well PW 5 in September 2017 

reflect a mixture of high SC and TDS from water in the MRVA 

aquifer and lower SC and TDS from water in the Memphis 

aquifer. The SC and low tritium concentration at production 

well PW 3 also indicate a component of young water, but 

the proportion of young to old water is substantially smaller 

than that at well PW 5. Water-quality samples collected from 

the four pumping production wells during the aquifer test in 

October 2017 (table 3) are similar to the set of water-quality 

samples collected in September 2017. The most notable 

difference is the absence of tritium in the sample collected 

from production well PW 3 in October 2017 (table 4), 

suggesting that little if any young water was present in 

the Memphis aquifer at this location when the sample was 

collected. Water-quality samples collected by the Layne 

Christensen Company during performance testing of the 

production wells in 2016 and by TVA in June–July 2017 (TVA, 

written commun., 2017) showed elevated concentrations of 

inorganic constituents in production well PW 5, indicating that 

mixing of MRVA aquifer water with water in the upper part of 

the Memphis aquifer may predate production well installation. 

Although the water-quality data indicate that young water 

with higher TDS concentrations is moving from the MRVA 

aquifer into the Memphis aquifer in the area near the ACC 

Plant, they also indicate that the source of the young water 

may differ from that in the MRVA aquifer directly beneath 

the ACC Plant. The water quality in the Memphis aquifer at 

production well PW 5 is interpreted to reflect a binary mixture 

of Memphis aquifer and MRVA aquifer waters and, as such, 

should lie along a mixing line between points that represent 

end-member concentrations for the two aquifers (for example, 

production wells PW 1 and PW 2 and MRVA aquifer samples). 

Samples from two of the five MRVA monitoring wells fall near 

a hypothetical mixing line for magnesium and calcium for the 

Memphis aquifer samples, but the other three samples do not 

(fig. 12A). None of the MRVA monitoring well samples fall on 

the mixing line for calcium and boron (fig. 12B), suggesting 

that the source of the water that represents the MRVA aquifer 

end member differs from that in the MRVA aquifer beneath the 

ACC Plant, and that the component of MRVA aquifer water 

present in the samples collected from the Memphis aquifer 

production wells originated from locations away from the 

ACC Plant.

Table 4. Concentrations of tritium in groundwater samples 

collected from selected Allen Combined Cycle Plant wells, 

September and October 2017. 

[Critical values (approximate detection level) for these samples ranged from 

0.13 to 0.25 picocurie per liter (pCi/L); USGS, U.S. Geological Survey; TU, 

tritium units; --, no data; <, less than]

Well field 

ID (see 

figure 2 for 

location)

USGS 

well 

number

September 2017 October 2017

Tritium 

(pCi/L)

Tritium 

(TU)

Tritium 

(pCi/L)

Tritium 

(TU)

Mississippi River Valley alluvial aquifer

MW 1A Sh:H-035 17.5 5.44 -- --

MW 5B Sh:H-037 6.87 2.13 -- --

MW 5A Sh:H-038 8.37 2.60 -- --

MW 3A Sh:H-036 18 5.59 -- --

MW 216 Sh:H-045 15.5 4.81 -- --

Memphis aquifer

PW 1 Sh:H-031 0 0 <0.14 0

PW 2 Sh:H-025 0 0 -- --

PW 3 Sh:H-026 0.79 0.25 0 0

PW 4 Sh:H-027 <0.14 0 0 0

PW 5 Sh:H-032 4.01 1.24 3.51 1.09

400

420

440

460

480

500

520

540

560

580

600

0 0.20 0.40 0.60 0.80 1.00 1.20

Top of well screen, in feet below land surface

PW 5

PW 3

PW 1

PW 2, PW 4

Tritium, in tritium units

Figure 11. Tritium concentrations in groundwater samples in 

relation to the depths of the tops of the screens for Memphis 

aquifer production wells at the Tennessee Valley Authority 

Allen Combined Cycle Plant, September 2017.
28 Hydrogeology and Groundwater Quality, Tennessee Valley Authority Allen Power Plants

Aquifer Test

Results of the ACC Plant aquifer test indicate that a 

hydraulic connection exists between the Memphis aquifer 

and the MRVA aquifer near the TVA plants site. The apparent 

location of hydraulic connection is to the east-southeast of 

the plants and is coincident with a general eastward thinning 

of the confining unit and lowering in the altitude of the base 

of the MRVA aquifer as shown in the geophysical logs for 

production well PW 1 and stratigraphic test hole 212c (fig. 8). 

The following sections describe the effects of the test on water 

levels in the observation wells screened in the MRVA and 

Memphis aquifers.

Memphis Aquifer

Pre-test water-level measurements made in ACC Plant 

monitoring wells MW 1A, MW 3A, and MW 5A and MW 

5B, and in co-located production wells PW 1, PW 3, and 

PW 5 indicate a downward vertical gradient of about 5 ft 

between the MRVA and Memphis aquifers at the locations 

of wells PW 3 and PW 5, and a downward gradient of about 

3 ft at the location of well PW 1 prior to the start of the test. 

Production well PW 1 is located about 2,700 ft east of well 

PW 3 and about 1,300 ft southeast of well PW 5 (fig. 2). In 

1995, a hydraulic gradient in the potentiometric surface of the 

Memphis aquifer of about 2.5 ft per mile downward to the 

1

10

100

1,000

0 20 40 60 80 100 120 140 160

0

5

10

15

20

25

30

35

40

45

0 20 40 60 80 100 120 140 160

Calcium, in milligrams per liter

Calcium, in milligrams per liter

Magnesium, in milligrams per liter Boron, in micrograms per liter

Mixing line

Mixing line

A

B

PW 5

PW 5

MRVA aquifer

MRVA aquifer

Memphis

aquifer

Memphis

aquifer

Figure 12. Hypothetical mixing line for groundwater samples collected from 

Memphis aquifer production wells PW 1 through PW 4 that are unaffected 

by leakage to production well PW 5 where alluvial water has moved into the 

Memphis aquifer at the Tennessee Valley Authority Allen Combined Cycle Plant, 

September 2017.
Results 29

east was present in the TVA plants area (Kingsbury, 1996). 

Assuming the current gradient in the area is similar, about 

0.6 ft and 1.2 ft of downward slope would be present in the 

Memphis aquifer from well PW 5 to PW 1 and from well PW 

3 to PW 1, respectively, under nonpumping conditions locally. 

The primarily easterly slope in the potentiometric surface of 

the Memphis aquifer in the TVA plants area and an easterly 

gradient in the MRVA aquifer toward the apparent area of 

leakage to the east of the ACC Plant likely are the cause of 

the smaller difference between pre-test (static) water levels in 

wells PW 1 and MW 1A compared to the other well locations.

Maximum drawdown values observed in the Memphis 

aquifer in the five production wells and the three offsite 

observation wells during the aquifer test are shown in figure 

13. Drawdowns ranged from 76 to 107 ft (figs. 13 and 14), and 

specific capacities ranged from 12 to 16.4 gallons per minute 

per foot ([gal/min]/ft) (table 5) in the four pumped production 

wells. Drawdown values from the test were greater and 

specific-capacity values were less than values reported for the 

wells from initial well performance tests (Layne Christensen 

Company, written commun., 2017), but values were consistent 

among wells between the tests (table 5). The maximum 

drawdown measured in production well PW 2, which served 

as an onsite observation well during the test, was about 18 ft, 

and the drawdown appeared to stabilize in this well for the last 

5 hours of the pumping period (fig. 14). The rate of declines 

measured in the four pumped production wells also slowed 

near the end of the pumping period.

Drawdown from the aquifer test was observed in two of 

the three offsite Memphis aquifer observation wells (wells 

Sh:H-001 and Sh:H-013), indicating that the effects of the 

test extended beyond 1 mi from the ACC Plant (figs. 13 and 

15). The maximum estimated drawdowns induced by the test 

in wells Sh:H-001 and Sh:H-013 were about 1.6 and 1.3 ft, 

respectively (figs. 15A and 15B). In addition to the drawdown 

in well Sh:H-013 resulting from the test, the effects of daily 

pumping of a Memphis aquifer production well at an industrial 

site on President’s Island, located about 2,500 ft west of 

observation well Sh:H-013, were seen in the water-level data 

collected from this well during the test. Based on separation 

of test-induced drawdown observed in well Sh:H-013 from 

that produced by pumping at the industrial site on President’s 

Island during the test using the SeriesSEE program, an 

additional 1.5 ft of drawdown attributed to the industrial 

site was estimated in well Sh:H-013 (fig. 15B). Water levels 

declined about 0.03 ft in well Ar:H-002 from the start to 

the end of the test pumping period based on the continuous 

water-level data collected in this well (fig. 15C). Water levels 

monitored in well Ar:H-002 show a downward trend for at 

least several days prior to the start of the test. Given that no 

significant withdrawals from the Memphis aquifer are known 

to occur near well Ar:H-002, the water-level data collected 

from this well probably are indicative of ambient conditions 

in the Memphis aquifer in the TVA plants area. The waterlevel data for this well also indicate the effect of semi-diurnal 

Earth tides. Given the distance from the ACC Plant to this well 

(about 4 mi) and the downward trend in water levels observed 

in this well before and during the test, effects of the test are 

unlikely to have extended to this well.

Table 5. Pumping rates, drawdown values, and specific capacities for Memphis aquifer production wells 

during the aquifer test, October 3–4, 2017, and from performance tests, June 15–21, 2016, Tennessee Valley 

Authority Allen Combined Cycle Plant. 

[Performance-test data from Layne Christensen Company, written commun., 2017; ft, feet; gal/min, gallons per minute; (gal/min)/ft, 

gallons per minute per foot]

Production well 

(see figure 2 

for location)

Aquifer test, October 3–4, 2017 Performance tests, June 15–21, 2016

Average 

pumping rate 

(gal/min)

Drawdown 

(ft)

Specific 

capacity 

([gal/min]/ft)

Pumping 

rate 

(gal/min)

Drawdown 

(ft)

Specific 

capacity 

([gal/min]/ft)

PW 1 1,243 93 13.4 1,280 82 15.6

PW 2 * 18* * 1,303 74 17.6

PW 3 1,250 76 16.4 1,304 65 20.1

PW 4 1,284 107 12.0 1,338 95 14.1

PW 5 1,262 89 14.2 1,260 76 16.6

*

Well not pumped during test.
30 Hydrogeology and Groundwater Quality, Tennessee Valley Authority Allen Power Plants

Source: Esri, DigitalGlobe, GeoEye, EXPLANATION

Earthstar Geographics, CNES/Airbus DS, 

USDA, U.S. Geological Survey, AeroGRID, 

IGN, and the GIS User Community

0 1 2 KILOMETERS

0 1 2 MILES

T.O. Fuller 

State Park

1

10

50

100

0.1

Horn Lake Cutoff

Lake McKellar

1.3

Sh:H-031

PW 1

Sh:H-025

PW 2

Sh:H-026

PW 3

Sh:H-027

PW 4

Sh:H-032

PW 5

Ar:H-002

8 9

93

1 8

7 6

Sh:H-001

Sh:H-013

<0.05 1.6

107

ACC Plant

ALF 

Plant

Pidgeon 

Industrial 

Park

Mississippi River

90°12' 90°08'

35°06'

35°03'

T.O. Fuller 

State Park

Mississippi River

Sh:H-031

PW 1

Sh:H-025

PW 2

Sh:H-026

PW 3

Sh:H-027

PW 4

Sh:H-032

PW 5

Ar:H-002

107

98

93

1 8

7 6

Sh:H-001

Sh:H-013

<0.05 1.6

1.3

10

Sh:H-031

PW 1

93

Sh:H-001

1.6

Well name and maximum measured 

 drawdown value, in feet

Well name and maximum model-estimated 

 drawdown value, in feet

Maximum drawdown in the Memphis 

 aquifer—In feet. Dashed where 

 estimated

ACC Plant

ALF 

Plant

Pidgeon 

Industrial 

Park

Figure 13. Maximum drawdown measured in the Memphis aquifer production wells and maximum estimated drawdown in 

Memphis aquifer observation wells, Tennessee Valley Authority Allen Combined Cycle Plant aquifer test, October 3–4, 2017.
Results 31

EXPLANATION

PW 1

PW 2

PW 3

PW 4

PW 5

Cumulative pumping

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

9,000

10,000

11,000

12,000

20 13,000

30

40

50

60

70

80

90

100

110

120

130

140

150

10/2/17 10/2/17 10/3/17 10/4/17 10/4/17 10/5/17

Cumulative pumping rate, in gallons per minute

Depth to water, in feet below land surface

Date

Figure 14. Measured drawdown and cumulative pumping rates in Memphis aquifer production 

wells, Tennessee Valley Authority Allen Combined Cycle Plant aquifer test, October 3–4, 2017.
32 Hydrogeology and Groundwater Quality, Tennessee Valley Authority Allen Power Plants

Cumulative pumping rate for test

EXPLANATION

Model-estimated drawdown

Measured water levels

Pumping period

EXPLANATION

Measured water level 

 full period of record

Drawdown ~1.6 feet

Water-level altitude, in feet Pumping rate, in gallons per minute

Depth to water, in feet 

below land surface

A. Sh:H-001 Located ~1 mile ESE of ACC Plant

Drawdown, in feet

0

1,000

2,000

3,000

4,000

5,000

6,000

-2.0

-1.5

-1.0

-0.5

0

0.5

1.0

10/1/2017 10/2/2017 10/3/2017 10/4/2017 10/5/2017 10/6/2017

Date

176

177

178

179

132 180

133

134

135

136

9/25/2017 9/30/2017 10/5/2017 10/10/2017 10/15/2017

Drawdown = ~1.3 feet

Cumulative pumping rate for test

EXPLANATION

Model-estimated drawdown

Industry pumping

Pumping period

EXPLANATION

Measured water level 

 full period of record

Measured water levels

Depth to water, in feet 

below land surface

B. Sh:H-013 Located ~1 mile N of ACC Plant

Water-level altitude, in feet Pumping rate, in gallons per minute

Drawdown, in feet

175

176

177

178

59 179

60

61

62

63

9/29/2017 10/4/2017 10/9/2017 10/14/2017

0

1,000

2,000

3,000

4,000

5,000

6,000

-2.5

-2.0

-1.5

-1.0

-0.5

0

0.5

1.0

1.5

10/1/2017 10/2/2017 10/3/2017 10/4/2017 10/5/2017 10/6/2017

Date

Figure 15. Measured water levels and estimated drawdown in Memphis aquifer observation wells (A) Sh:H-001, (B) Sh:H-013, 

and (C) Ar:H-002, Tennessee Valley Authority Allen Combined Cycle Plant aquifer test, October 3–4, 2017.
Results 33

The magnitude of drawdown observed in offsite Memphis 

aquifer observation well Sh:H-001 (about 1.6 ft at end of 

pumping) provides a reliable estimate of drawdown resulting 

from the aquifer test at a distance of about 1 mi from the 

ACC Plant. Drawdown of this magnitude at about 1 mi from 

the plant is similar to the results of the scenario simulation 

conducted for the TVA with the MERAS groundwater model 

(Haugh, 2016). The extent and uninterrupted character of 

drawdown recorded in well Sh:H-001 suggest that the flux of 

groundwater from the MRVA aquifer to the Memphis aquifer 

in the area of leakage was not a substantial component of the 

water pumped during the test. No accurate estimate of the flux 

of water draining from the MRVA aquifer under natural or 

pumped conditions can be made until the nature and extent of 

the area of leakage is better known.

MRVA Aquifer

Although drawdown from the aquifer test was observed 

in several of the MRVA aquifer monitoring wells at both 

TVA plants, continuous data collected from monitoring wells 

unaffected by the test show that water levels in the MRVA 

aquifer were declining at a rate of about 0.2 ft/d from the 

beginning of October through completion of the test. The 

SeriesSEE program was used to remove this trend of waterlevel decline in the MRVA aquifer to produce estimates of 

drawdown in monitoring wells in the aquifer resulting solely 

from the test.

Estimated values of drawdown in the MRVA aquifer from 

the test ranged from about 0.5 ft at the ALF Plant to less than 

0.1 ft in wells at multiple locations at both plants (table 6; 

figs. 16 and 17). The largest drawdown was to the southeast 

of the ALF Plant and along the eastern side of the ACC Plant 

(figs. 16A, 16E, and 17; monitoring wells MW 1A, MW 202A, 

MW 202B, and MW 212A). Estimated drawdown generally 

decreased to the north and west. Water-level data collected 

from wells along the northern side of the ALF Plant show 

little to no obvious response to pumping during the test, and 

water levels are similar to and fluctuate in unison with the 

stage of Lake McKellar estimated from streamgage 07032000 

(figs. 16G, 16H, and 17; wells MW 204A and MW 204B).

Figure 15. Measured water levels and estimated drawdown in Memphis aquifer observation wells (A) Sh:H-001, (B) Sh:H-013, 

and (C) Ar:H-002, Tennessee Valley Authority Allen Combined Cycle Plant aquifer test, October 3–4, 2017.—Continued

Any drawdown <0.05 foot

Pumping period

EXPLANATION

Measured water level 

 full period of record

Cumulative pumping rate for test

EXPLANATION

Model-estimated drawdown

Measured water levels

Depth to water, in feet 

below land surface

C. Ar:H-002 Located ~4 miles W of ACC Plant

Water-level altitude, in feet Pumping rate, in gallons per minute

Drawdown, in feet

183.1

183.3

183.5

183.7

183.9

26.9 184.1

27.1

27.3

27.5

27.7

27.9

9/25/2017 9/30/2017 10/5/2017 10/10/2017 10/15/2017

0

1,000

2,000

3,000

4,000

5,000

6,000

-0.10

-0.05

0

0.05

0.10

0.15

10/1/2017 10/2/2017 10/3/2017 10/4/2017 10/5/2017 10/6/2017

Date
34 Hydrogeology and Groundwater Quality, Tennessee Valley Authority Allen Power Plants

Table 6. Model drawdown estimates for monitoring wells screened in the 

Mississippi River Valley alluvial aquifer at the Tennessee Valley Authority Allen 

Fossil and Allen Combined Cycle Plants, Tennessee Valley Authority Allen 

Combined Cycle Plant aquifer test, October 3–4, 2017.

[Not detected (ND), inconclusive (I), and detected (D) indicate relative certainty of drawdown 

greater than 0.05 foot. DP, deep; I, intermediate; S, shallow; <, less than]

Well 

field 

ID (see 

figure 2 for 

location)

Interval 

screened

Estimated 

drawdown 

from aquifer 

test 

(feet)

Estimated 

drawdown 

from other 

pumping 

(feet)

Remarks

MW 1A DP 0.33 (D) 0 (ND)

MW 3A DP 0.15 (I) 0 (I) Possible delayed drawdown

MW 5A DP 0.2 (D) 0.1 (D)

MW 5B I 0.15 (D) 0.1 (D) 

MW 201 S 0.25 (D) 0.15 (D)

MW 201A DP 0.3 (D) 0.15 (D)

MW 201B I 0.3 (D) 0.15 (D)

MW 202 S 0.02 (I) 0.05 (I) Possible perched water

MW 202A DP 0.5 (D) 0.15 (D)

MW 202B I 0.5 (D) 0.15 (D)

MW 203 S 0.02 (I) 0.08 (D)

MW 203A DP 0.02 (I) 0.15 (D)

MW 203B I 0.02 (I) 0.15 (D)

MW 204 S 0.08 (D) 0.05 (I) Possible perched water

MW 204A DP 0.05 (I) 0.05 (I)

MW 204B I 0.05 (I) 0.07 (D)

MW 205 S 0.07 (D) 0.15 (D)

MW 205A DP 0.07 (D) 0.08 (D)

MW 205B I 0.07 (D) 0.08 (D)

MW 206 S 0.04 (I) 0.05 (I)

MW 210 S 0.02 (I) 0.1 (D) Possible delayed drawdown

MW 210A DP 0.02 (I) 0.14 (D) Possible delayed drawdown

MW 212 S 0.01 (I) 0.03 (I) Possible perched water

MW 212A DP 0.49 (D) 0.08 (D)

MW 213 S 0.25 (D) 0.08 (D)

MW 214 S 0.1 (D) 0.15 (D)

MW 214A DP 0.06 (D) 0.15 (D)

MW 214B I 0.07 (D) 0.15 (D)

MW 215 S 0 (ND) 0.02 (I)

MW 216 S 0.28 (D) 0.1 (D)

MW P-4 I <0.05 (I) 0.2 (I)

MW P-4S S <0.05 (I) 0.15 (I)
Results 35

Drawdown ~0.33 foot

A. MW 1A

Well located near SE corner 

of ACC Plant property Measured water level 

 full period of record

EXPLANATION

Pumping period

EXPLANATION

Cumulative pumping rate for test

Model-estimated drawdown

Measured water levels

0

1,000

2,000

3,000

4,000

5,000

6,000

184.6

185.1

185.6

186.1

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1.0

Date

Date

Water-level altitude, in feet

Pumping rate, in gallons per minute

Drawdown, in feet

10/1/2017 10/3/2017 10/5/2017 10/7/2017 10/9/2017 10/11/2017 10/13/2017

10/1/2017 10/2/2017 10/3/2017 10/4/2017 10/5/2017 10/6/2017

Figure 16. Measured water levels and estimated drawdown in selected Mississippi River Valley alluvial aquifer 

monitoring wells (A) MW 1A, (B) MW 3A, (C) MW 5A and MW 5B at Allen Combined Cycle Plant; (D) MW 202, 

(E) MW 202A and MW 202B, (F) MW 210 and MW 210A, and (G) MW 204A and MW 204B at Allen Fossil Plant; and 

(H) stage of the Mississippi River at Memphis (USGS station number 07032000, Mississippi River at Memphis, TN), 

Tennessee Valley Authority Allen Combined Cycle Plant aquifer test, October 3–4, 2017.

B. MW 3A

Well located near SW corner 

of ACC Plant property

Possible delayed drawdown

Measured water level 

 full period of record

EXPLANATION

Pumping period

EXPLANATION

Cumulative pumping rate for test

Model-estimated drawdown

Measured water levels

0

1,000

2,000

3,000

4,000

5,000

6,000

-0.2

0

0.2

0.4

0.6

0.8

1.0

185.3

185.8

186.3

186.8

Date

Water-level altitude, in feet

Pumping rate, in gallons per minute

Drawdown, in feet

10/1/2017 10/3/2017 10/5/2017 10/7/2017 10/9/2017 10/11/2017 10/13/2017

10/1/2017 10/2/2017 10/3/2017 10/4/2017 10/5/2017 10/6/2017
36 Hydrogeology and Groundwater Quality, Tennessee Valley Authority Allen Power Plants

Pumping period

EXPLANATION

Measured water levels

Full period of record

C. MW 5A and MW 5B

Wells located near N central part

of ACC Plant property

Intermediate

Deep

Slight downward gradient

Drawdown ~0.2 foot

Drawdown ~0.15 foot

MW 5A

MW 5B

EXPLANATION

Cumulative pumping rate for test

Model-estimated drawdown

Measured water levels

EXPLANATION

Cumulative pumping rate for test

Model-estimated drawdown

Measured water levels

0

1,000

2,000

3,000

4,000

5,000

6,000

0

1,000

2,000

3,000

4,000

5,000

6,000

184.8

185.3

185.8

186.3

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1.0

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1.0

Date

Pumping rate, in gallons per minute

Drawdown, in feet

Pumping rate, in gallons per minute

Drawdown, in feet

10/1/2017 10/3/2017 10/5/2017 10/7/2017 10/9/2017 10/11/2017 10/13/2017

10/1/2017 10/2/2017 10/3/2017 10/4/2017 10/5/2017 10/6/2017

10/1/2017 10/2/2017 10/3/2017 10/4/2017 10/5/2017 10/6/2017

Water-level altitude, in feet

Figure 16. Measured water levels and estimated drawdown in selected Mississippi River Valley alluvial aquifer 

monitoring wells (A) MW 1A, (B) MW 3A, (C) MW 5A and MW 5B at Allen Combined Cycle Plant; (D) MW 202, 

(E) MW 202A and MW 202B, (F) MW 210 and MW 210A, and (G) MW 204A and MW 204B at Allen Fossil Plant; and 

(H) stage of the Mississippi River at Memphis (USGS station number 07032000, Mississippi River at Memphis, TN), 

Tennessee Valley Authority Allen Combined Cycle Plant aquifer test, October 3–4, 2017.—Continued
Results 37

Figure 16. Measured water levels and estimated drawdown in selected Mississippi River Valley alluvial aquifer 

monitoring wells (A) MW 1A, (B) MW 3A, (C) MW 5A and MW 5B at Allen Combined Cycle Plant; (D) MW 202, 

(E) MW 202A and MW 202B, (F) MW 210 and MW 210A, and (G) MW 204A and MW 204B at Allen Fossil Plant; and 

(H) stage of the Mississippi River at Memphis (USGS station number 07032000, Mississippi River at Memphis, TN), 

Tennessee Valley Authority Allen Combined Cycle Plant aquifer test, October 3–4, 2017.—Continued

D. MW 202

Well located S central of ALF ponds

Water level

~20 feet higher

than deeper

wells in cluster

Drawdown inconclusive

Drawdown inconclusive

Pumping period

EXPLANATION

Measured water levels

EXPLANATION

Cumulative pumping rate for test

Model-estimated drawdown

Measured water levels

0

1,000

2,000

3,000

4,000

5,000

6,000

203.7

203.8

203.9

204.0

204.1

-0.10

-0.05

0

0.05

0.10

0.15

0.20

0.25

Date

Water-level altitude, in feet

Pumping rate, in gallons per minute

Drawdown, in feet

10/1/2017 10/3/2017 10/5/2017 10/7/2017 10/9/2017 10/11/2017 10/13/2017

10/1/2017 10/2/2017 10/3/2017 10/4/2017 10/5/2017 10/6/2017
38 Hydrogeology and Groundwater Quality, Tennessee Valley Authority Allen Power Plants

Wells located S central 

of ALF ponds

Drawdown ~0.5 foot

Upward gradient

Deep

Intermediate

Drawdown ~0.5 foot

E. MW 202A and MW 202B

MW 202A

MW 202B

Pumping period

EXPLANATION

Measured water levels

Full period of record

EXPLANATION

Cumulative pumping rate for test

Model-estimated drawdown

Measured water levels

EXPLANATION

Cumulative pumping rate for test

Model-estimated drawdown

Measured water levels

1,000

2,000

3,000

4,000

5,000

6,000

0

0

1,000

2,000

3,000

4,000

5,000

6,000

-1.0

-0.5

0

0.5

1.0

183.2

183.7

184.2

184.7

-1.0

-0.5

0

0.5

1.0

Date

Water-level altitude, in feet

Pumping rate, in gallons per minute

Drawdown, in feet

Pumping rate, in gallons per minute

Drawdown, in feet

10/1/2017 10/3/2017 10/5/2017 10/7/2017 10/9/2017 10/11/2017 10/13/2017

10/1/2017 10/2/2017 10/3/2017 10/4/2017 10/5/2017 10/6/2017

10/1/2017 10/2/2017 10/3/2017 10/4/2017 10/5/2017 10/6/2017

Figure 16. Measured water levels and estimated drawdown in selected Mississippi River Valley alluvial aquifer 

monitoring wells (A) MW 1A, (B) MW 3A, (C) MW 5A and MW 5B at Allen Combined Cycle Plant; (D) MW 202, 

(E) MW 202A and MW 202B, (F) MW 210 and MW 210A, and (G) MW 204A and MW 204B at Allen Fossil Plant; and 

(H) stage of the Mississippi River at Memphis (USGS station number 07032000, Mississippi River at Memphis, TN), 

Tennessee Valley Authority Allen Combined Cycle Plant aquifer test, October 3–4, 2017.—Continued
Results 39

Figure 16. Measured water levels and estimated drawdown in selected Mississippi River Valley alluvial aquifer 

monitoring wells (A) MW 1A, (B) MW 3A, (C) MW 5A and MW 5B at Allen Combined Cycle Plant; (D) MW 202, 

(E) MW 202A and MW 202B, (F) MW 210 and MW 210A, and (G) MW 204A and MW 204B at Allen Fossil Plant; and 

(H) stage of the Mississippi River at Memphis (USGS station number 07032000, Mississippi River at Memphis, TN), 

Tennessee Valley Authority Allen Combined Cycle Plant aquifer test, October 3–4, 2017.—Continued

Possible delayed drawdown 

Possible delayed drawdown 

F. MW 210 and MW 210A

MW 210

MW 210A

Wells located S of 

ALF Plant building

Drawdown

not apparent

Little vertical gradient

Gray -shallow

Blue - deep

Pumping period

EXPLANATION

Full period of record

Measured water levels

EXPLANATION

Cumulative pumping rate for test

Model-estimated drawdown

Measured water levels

EXPLANATION

Cumulative pumping rate for test

Model-estimated drawdown

Measured water levels

Pumping period

0

1,000

2,000

3,000

4,000

5,000

6,000

0

1,000

2,000

3,000

4,000

5,000

6,000

183.2

183.7

184.2

184.7

185.2

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1.0

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1.0

Date

Water-level altitude, in feet

Pumping rate, in gallons per minute

Drawdown, in feet

Pumping rate, in gallons per minute

Drawdown, in feet

10/1/2017 10/3/2017 10/5/2017 10/7/2017 10/9/2017 10/11/2017 10/13/2017

10/1/2017 10/2/2017 10/3/2017 10/4/2017 10/5/2017 10/6/2017

10/1/2017 10/2/2017 10/3/2017 10/4/2017 10/5/2017 10/6/2017
40 Hydrogeology and Groundwater Quality, Tennessee Valley Authority Allen Power Plants

Drawdown ~0.05 foot

Drawdown ~0.05 foot

G. MW 204A and MW 204B

MW 204A

MW 204B

Deep

Intermediate

Upward gradient

Wells located N central of ALF ponds

Water levels slightly higher and 

lagged in time compared to 

Mississippi River (see figure 16H)

Pumping period

EXPLANATION

204A

204B

EXPLANATION

Cumulative pumping rate for test

Model-estimated drawdown

Measured water levels

EXPLANATION

Cumulative pumping rate for test

Model-estimated drawdown

Measured water levels

0

1,000

2,000

3,000

4,000

5,000

6,000

0

1,000

2,000

3,000

4,000

5,000

6,000

177.3

177.8

178.3

178.8

179.3

-0.2

0

0.2

0.4

0.6

0.8

1.0

-0.2

0

0.2

0.4

0.6

0.8

1.0

Date

Water-level altitude, in feet

Pumping rate, in gallons per minute

Drawdown, in feet

Pumping rate, in gallons per minute

Drawdown, in feet

10/1/2017 10/3/2017 10/5/2017 10/7/2017 10/9/2017 10/11/2017 10/13/2017

10/1/2017 10/2/2017 10/3/2017 10/4/2017 10/5/2017 10/6/2017

10/1/2017 10/2/2017 10/3/2017 10/4/2017 10/5/2017 10/6/2017

Figure 16. Measured water levels and estimated drawdown in selected Mississippi River Valley alluvial aquifer 

monitoring wells (A) MW 1A, (B) MW 3A, (C) MW 5A and MW 5B at Allen Combined Cycle Plant; (D) MW 202, 

(E) MW 202A and MW 202B, (F) MW 210 and MW 210A, and (G) MW 204A and MW 204B at Allen Fossil Plant; and 

(H) stage of the Mississippi River at Memphis (USGS station number 07032000, Mississippi River at Memphis, TN), 

Tennessee Valley Authority Allen Combined Cycle Plant aquifer test, October 3–4, 2017.—Continued
Results 41

Figure 16. Measured water levels and estimated drawdown in selected Mississippi River Valley alluvial aquifer 

monitoring wells (A) MW 1A, (B) MW 3A, (C) MW 5A and MW 5B at Allen Combined Cycle Plant; (D) MW 202, 

(E) MW 202A and MW 202B, (F) MW 210 and MW 210A, and (G) MW 204A and MW 204B at Allen Fossil Plant; and 

(H) stage of the Mississippi River at Memphis (USGS station number 07032000, Mississippi River at Memphis, TN), 

Tennessee Valley Authority Allen Combined Cycle Plant aquifer test, October 3–4, 2017.—Continued

H. USGS Gage 07032000

Mississippi River at Memphis

Gage located ~5.5 miles NE of ACC Plant

Zero gage height = 183.91 feet

Mississippi River begins to rise while water level

in MW 204A and MW 204B still declining (see figure 16G)

Pumping period

EXPLANATION

Water surface

175.91

176.91

177.91

178.91

179.91

180.91

-8

-7

-6

-5

-4

-3

Date

Water-surface altitude, in feet 

Stage, in feet

10/1/2017 10/3/2017 10/5/2017 10/7/2017 10/9/2017 10/11/2017 10/13/2017

MW 1A

0.33

MW 3A

0.15

MW P-4

<0.05

MW 5A

0.2

MW 5B

0.15

MW 201A

0.3

MW 201B

0.3

MW 202A

0.5

MW 202B

0.5

MW 203A

0.02

MW 203B

0.02 MW 204A

0.05

MW 204B

0.05 MW 205A

0.07 MW 205B

0.07

MW 210A

0.02

MW 212A

0.49

MW 214A

0.06

MW 214B

0.07

MW 216

0.28

MW P-4S

<0.05

MW 206

0.04

MW 213

0.25

Harsco

MW 1A

0.33

MW 3A

0.15

MW P-4

<0.05

MW 5A

0.2

MW 5B

0.15

MW 201A

0.3

MW 201B

0.3

MW 202A

0.5

MW 202B

0.5

MW 203A

0.02

MW 203B

0.02 MW 204A

0.05

MW 204B

0.05 MW 205A

0.07 MW 205B

0.07

MW 210A

0.02

MW 212A

0.49

MW 214A

0.06

MW 214B

0.07

MW 216

0.28

MW P-4S

<0.05

MW 206

0.04

MW 213

0.25

Harsco

0.1

0.2

0.3

0.4

0.5

0.6

0.1

0.2

0.3

0.4

0.5

0.6

Source: Esri, DigitalGlobe, GeoEye, EXPLANATION

Earthstar Geographics, CNES/Airbus DS, 

USDA, U.S. Geological Survey, AeroGRID, 

IGN, and the GIS User Community

0 0.2 0.4 MILE

0 0.2 0.4 KILOMETER

90°09' 90°08'

35°04'30"

35°04' ACC Plant

Horn Lake Cutoff

MW 1A

0.33

0.4

Well name and maximum 

 model-estimated drawdown, in 

 feet. Larger yellow symbols 

 indicate wells with hydrographs 

 shown in figure 16

Drawdown in the Mississippi 

 River Valley alluvial aquifer—In 

 feet. Dashed where inferred

Lake McKellar

Pidgeon 

Industrial 

Park

ALF 

Plant

Industrial 

Figure 17. Estimated drawdown in selected monitoring wells screened in the Mississippi River Valley alluvial aquifer, Tennessee 

Valley Authority Allen Combined Cycle Plant aquifer test, October 3–4, 2017.
42 Hydrogeology and Groundwater Quality, Tennessee Valley Authority Allen Power Plants

The largest drawdown in the MRVA aquifer during the 

test occurred in monitoring wells MW 202A (deep) and MW 

202B (intermediate) located in the southeastern part of the 

ALF Plant (figs. 16E and 17). Continuous water-level data 

collected from these wells indicate an upward gradient from 

the lower to the middle part of the MRVA aquifer at this 

location before, during, and after the test. Although the upward 

hydraulic gradient between the wells persisted during the test, 

the gradient decreased during the pumping period (fig. 16E), 

indicating that the effects of pumping extended to and altered 

the existing gradient between the lower and middle parts of the 

MRVA aquifer at this location.

Pre-test and late pumping period water-level data indicate 

that hydraulic gradients in the intermediate and deep parts of 

the MRVA aquifer beneath most of the TVA plants area were 

downward to the north toward Lake McKellar before the test 

and remained so throughout the test (figs. 18 and 19). These 

results, along with upward vertical gradients between water 

levels in paired monitoring wells screened in the intermediate 

and deep parts of the MRVA aquifer at some locations 

at the ALF Plant (figs. 16E and 16G), indicate that Lake 

McKellar likely was the point of discharge of groundwater 

in the aquifer beneath most of the TVA plants area prior to, 

during, and after the test. These observations indicate that the 

induced drawdown from the test did not substantially alter 

the northward direction of groundwater flow in the MRVA 

aquifer at most locations at the TVA plants site. Water-level 

data collected for the test from monitoring wells in the eastern 

and southeastern parts of the TVA plants area indicate an 

easterly and convergent component of flow in the MRVA 

aquifer under both pre-test (nonpumping) and late pumpingperiod conditions in the Memphis aquifer (figs. 18 and 19). It 

should be noted that the test was conducted under low-stage 

conditions in Lake McKellar and presumably corresponding 

low water-level conditions in the MRVA aquifer, and that 

the results are not necessarily representative of how the flow 

regime in the aquifer would respond under higher water-level 

conditions, with or without pumping from the Memphis 

aquifer locally.

~177

MW 1A

185.71

MW 3A

186.49

MW 4

177.9

MW 5A

186

MW 5B

186

MW 201A

184.71

MW 201B

184.62 MW 202A

184.45

MW 202B

184.08

MW 203A

179.14

MW 203B

179.15

MW 204A

178.14

MW 204B

178 MW 205A

177.94

MW 205B

177.74

MW 210A

184.3

MW 212

183.19

MW 214A

185.14 MW 214B

185.17

MW 216

186.24

MW P-4S

178.59

MW 206

177.94

MW 213

180.91

Harsco

Horn Lake Cutoff

ACC Plant MW 1A

185.71

MW 3A

186.49

MW 4

177.9

MW 5A

186

MW 5B

186

MW 201A

184.71

MW 201B

184.62 MW 202A

184.45

MW 202B

184.08

MW 203A

179.14

MW 203B

179.15

MW 204A

178.14

MW 204B

178 MW 205A

177.94

MW 205B

177.74

MW 210A

184.3

MW 212

183.19

MW 214A

185.14 MW 214B

185.17

MW 216

186.24

MW P-4S

178.59

MW 206

177.94

MW 213

180.91

Harsco

~177

186

184

182

180

178

Source: Esri, DigitalGlobe, GeoEye, EXPLANATION

Earthstar Geographics, CNES/Airbus DS, 

USDA, U.S. Geological Survey, AeroGRID, 

IGN, and the GIS User Community

0 0.2 0.4 MILE

0 0.2 0.4 KILOMETER

90°09' 90°08'

35°04'30"

35°04'

MW 1A

185.71

186

Well name and water-level altitude, in 

 feet above NGVD 29 

 

Water-level altitude in the Mississippi 

 River Valley alluvial aquifer—In feet 

 above NGVD 29. Dashed where 

 estimated Horn Lake Cutoff

ACC Plant

ALF 

Plant

Pidgeon 

Industrial 

Park

Lake McKellar

Figure 18. Pre-test water-level altitudes in the Mississippi River alluvial aquifer and stage of Lake McKellar, Tennessee Valley 

Authority Allen Combined Cycle Plant aquifer test, October 3–4, 2017.
Summary and Conclusions 43

Summary and Conclusions

Prior to this study, geophysical and drillers’ logs from 

newly installed production wells at the ACC Plant indicated 

that the upper Claiborne confining unit between the MRVA 

and Memphis aquifers was at least 45 ft thick beneath the 

plant, and was thinnest in the southeastern part of the plant 

at the location of production well PW 1. Work conducted for 

the RI and for this evaluation shows that the confining unit 

also thins to about 26 ft in thickness in the southeastern part 

of the ALF Plant, and that increasing depth to the base of 

the unit in this area compared to other locations in the TVA 

plants area indicates potential offset from faulting. A general 

east-southeasterly thinning of the confining unit toward the 

eastern margin of the Mississippi River Valley alluvial plain 

and the bluff along the east side of the Pidgeon Industrial Park 

at both TVA plants; the existence of a window in the confining 

unit in a similar setting west of the MLGW Davis well field, 

about 3 mi south of the plants; and an inferred window on 

President’s Island, about 1 mi north of the plants; indicate a 

potential for leakage from the MRVA aquifer to the Memphis 

aquifer in the general vicinity of the TVA plants site.

Results of this evaluation provide evidence for 

downward leakage of water from the MRVA aquifer to the 

Memphis aquifer near the TVA plants site. The aquifer test 

and groundwater-quality sample data indicate that the effects 

of downward leakage of water from the MRVA aquifer are 

limited to the upper part of the Memphis aquifer in the area 

of ACC Plant production well PW 5, and possibly production 

well PW 3, and imply that changes in water quality in the 

Memphis aquifer at these locations predate any leakage 

induced during the test. This interpretation is further supported 

by the results from the second round of tritium analyses for 

samples collected from the production wells during the aquifer 

test. Slight declines in tritium concentrations in the second 

samples collected from production wells PW 5 and PW 3 

suggest that the proportion of young water in the Memphis 

aquifer may have decreased during the test as the contribution 

of unaffected Memphis aquifer water pulled toward and 

withdrawn by the pumping wells increased.

186

184

182

180

178

MW 1A

185.34

MW 3A

MW P-4

177.67

MW 5A

185.77 MW 5B

185.77

MW 201A

184.33

MW 201B

184.21 MW 202A

183.6

MW 202B

183.52

MW 203A

179

MW 203B

179

MW 204A

178.06

MW 204B

177.91 MW 205A

177.9

MW 205B

177.7

MW 210A

184.18

MW 212A

182.7

MW 214A

184.92

MW 214B

184.97

MW 216

185.9

MW P-4S

178.4

MW 206

177.88

MW 213

180.62

Harsco

~177

Horn Lake Cutoff

ACC Plant MW 1A

185.34

MW 3A

MW P-4

177.67

MW 5A

185.77 MW 5B

185.77

MW 201A

184.33

MW 201B

184.21 MW 202A

183.6

MW 202B

183.52

MW 203A

179

MW 203B

179

MW 204A

178.06

MW 204B

177.91 MW 205A

177.9

MW 205B

177.7

MW 210A

184.18

MW 212A

182.7

MW 214A

184.92

MW 214B

184.97

MW 216

185.9

MW P-4S

178.4

MW 206

177.88

MW 213

180.62

Harsco

Source: Esri, DigitalGlobe, GeoEye, 

Earthstar Geographics, CNES/Airbus DS, 

USDA, U.S. Geological Survey, AeroGRID, 

IGN, and the GIS User Community

0 0.2 0.4 MILE

0 0.2 0.4 KILOMETER

90°09' 90°08'

35°04'30"

35°04'

~177

EXPLANATION

186

MW 1A

185.34

Well name and water-level altitude, in 

 feet above NGVD 29

Water-level altitude in the Mississippi 

 River Valley alluvial aquifer—In feet 

 above NGVD 29. Dashed where 

 estimated Horn Lake Cutoff

ACC Plant

ALF 

Plant

Pidgeon 

Industrial 

Park

Lake McKellar

Figure 19. Late-pumping period water-level altitudes in the Mississippi River Valley alluvial aquifer and stage of Lake McKellar, 

Tennessee Valley Authority Allen Combined Cycle Plant aquifer test, October 3–4, 2017.
44 Hydrogeology and Groundwater Quality, Tennessee Valley Authority Allen Power Plants

The results of the evaluation provide an update to the 

general conceptual model and hydrogeologic framework of 

the area. Downward leakage of groundwater from the MRVA 

aquifer to the Memphis aquifer near the MLGW Davis well 

field and in the TVA plants area has not been simulated using 

the regional MERAS groundwater-flow model because the 

hydrogeologic framework used to construct the model did not 

include windows in the confining unit between the MRVA and 

Memphis aquifers in these areas. The results of the aquifer 

test and other newly acquired hydrogeologic data are critical 

components needed for refining the hydrogeologic framework 

and for incorporation into a numerical model that produces 

meaningful simulations of groundwater flow and solute 

transport in the area.

A number of conclusions can be drawn from this 

evaluation. These include the following:

• The upper Claiborne confining unit is present beneath 

all parts of the TVA plants site investigated to date, 

but the confining unit thins and the base deepens in 

an east-southeasterly direction across the area where 

possible offset along one of two inferred faults may 

put the MRVA and Memphis aquifers in hydraulic 

connection.

• The aquifer-test results indicate that the MRVA and 

Memphis aquifers are hydraulically connected in the 

TVA plants area.

• The greatest drawdown from the aquifer test in the 

MRVA aquifer occurred in the eastern-southeastern 

part of the TVA plants area, in the direction of the bluff 

along the eastern side of the Mississippi River Valley 

alluvial plain, suggesting that an area of leakage exists 

in this general direction.

• The area of leakage appears to lie in a similar setting 

as the window identified about 3 mi south of the TVA 

plants site and west of the MLGW Davis well field.

• Analysis of the aquifer-test results indicates that 

withdrawals by at least one other Memphis aquifer 

production well in the area appear to affect water levels 

in the MRVA aquifer near the TVA plants site.

• Inorganic constituent water-quality data collected 

from at least one of the production wells at the ACC 

Plant during the evaluation indicate mixing of water 

from the MRVA aquifer with Memphis aquifer water; 

similar results for the original samples collected from 

the production wells imply that this mixing likely 

pre-dates the aquifer test.

• Tritium was not detected in the ACC Plant production 

wells where mixing of water from the MRVA aquifer 

with the Memphis aquifer is not inferred based on inorganic constituent concentrations; conversely, tritium 

was detected in the production well where mixing of 

water from the two aquifers is indicated.

Along with these findings, this evaluation raises 

additional questions that need to be considered when planning 

for future data collection efforts and analysis. These questions 

include the following:

• Are observed offsets in the confining unit the result of 

faulting, erosion, or a combination of both?

• What methods can be used to better map and verify the 

location and extent of the window(s) in the confining 

unit?

• What are the lithologic and hydraulic characteristics of 

sediments that make up the window area(s)?

• How do the gradients between the two aquifers vary 

spatially and temporally around the window area(s), 

particularly in relation to changes in the stage of Lake 

McKellar? 

• How can the relative proportions of young water moving from the MRVA to the Memphis aquifer and the 

potential effects on water quality be better quantified, 

and how do fluxes and mixing vary under different 

pumping conditions?

• How do other groundwater withdrawals from elsewhere in Shelby County affect water levels and leakage in this area?

• Does the presence of a window(s) in proximity to 

development on President’s Island and at the Pidgeon 

Industrial Park increase the potential for contamination 

of the Memphis aquifer?

Answering these questions will provide valuable 

information needed to (1) better characterize the hydraulic 

connection between the MRVA and Memphis aquifers in 

the TVA plants area, (2) conduct model-based simulation 

of groundwater flow and solute transport in the area, and 

(3) increase overall understanding of the hydrogeology of 

the aquifer system in order to better manage and protect the 

groundwater resources in the Memphis area. 
References 45

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48 Hydrogeology and Groundwater Quality, Tennessee Valley Authority Allen Power Plants

Appendix 1. SeriesSEE Water-Level Model Hydrographs—Allen Combined 

Cycle Plant Monitoring Wells

These Graphic Interchange Format (.gif) images show the synthetic (modeled) data that were created by the SeriesSEE 

models from the continuous water-level data collected during the aquifer test. The residuals between the synthetic and measured 

data are shown as the differences. [MW, monitoring well; ft, feet; RMS, root-mean-square error; discharge, pumping rate; Q_

ALL.gpm, cumulative pumping rate by the TVA ACC Plant production wells for the test]
Appendixes 49
50 Hydrogeology and Groundwater Quality, Tennessee Valley Authority Allen Power Plants
Appendixes 51

Appendix 2. SeriesSEE Water-Level Model Hydrographs—Allen Fossil Plant 

Monitoring Wells

These Graphic Interchange Format (.gif) images show the synthetic (modeled) data that were created by the SeriesSEE 

models from the continuous water-level data collected during the aquifer test. The residuals between the synthetic and measured 

data are shown as the differences. [MW, monitoring well; ft, feet; RMS, root-mean-square error; discharge, pumping rate; Q_

ALL.gpm, cumulative pumping rate by the TVA ACC Plant production wells for the test]
52 Hydrogeology and Groundwater Quality, Tennessee Valley Authority Allen Power Plants
Appendixes 53
54 Hydrogeology and Groundwater Quality, Tennessee Valley Authority Allen Power Plants
Appendixes 55
56 Hydrogeology and Groundwater Quality, Tennessee Valley Authority Allen Power Plants
Appendixes 57
58 Hydrogeology and Groundwater Quality, Tennessee Valley Authority Allen Power Plants
Appendixes 59
60 Hydrogeology and Groundwater Quality, Tennessee Valley Authority Allen Power Plants
Appendixes 61
62 Hydrogeology and Groundwater Quality, Tennessee Valley Authority Allen Power Plants
Appendixes 63
64 Hydrogeology and Groundwater Quality, Tennessee Valley Authority Allen Power Plants
Appendix 3. SeriesSEE Water-Level Model Hydrographs—Memphis Aquifer Observation Wells 65

Appendix 3. SeriesSEE Water-Level Model Hydrographs—Memphis Aquifer 

Observation Wells

These Graphic Interchange Format (.gif) images show the synthetic (modeled) data that were created by the SeriesSEE 

models from the continuous or periodic (well PW 2) water-level data collected during the aquifer test. The residuals between 

the synthetic and measured data are shown as the differences. [MW, monitoring well; ft, feet; RMS, root-mean-square error; 

discharge, pumping rate; Q_ALL.gpm, cumulative pumping rate by the TVA ACC Plant production wells for the test]
66 Hydrogeology and Groundwater Quality, Tennessee Valley Authority Allen Power Plants
Publishing support provided by

 Lafayette Publishing Service Center

For more information concerning this publication, contact

 Director, USGS Lower Mississippi-Gulf Water Science 

Center—Tennessee (LMGWSC)

 640 Grassmere Park, Suite 100

Nashville, TN 37211

 (615) 837-4700

Or visit the LMGWSC website at

 https://www.usgs.gov/centers/lmg-water/
Carmichael and others—Hydrogeology and Groundwater Quality, Tennessee Valley Authority Allen Power Plants—Open-File Report 2018–1097

ISSN 2331-1258 (online)

https://doi.org/10.3133/ofr20181097