Prepared in cooperation with the Tennessee Valley Authority

Effects of Groundwater Withdrawals Associated with 

Combined-Cycle Combustion Turbine Plants in West 

Tennessee and Northern Mississippi

Scientific Investigations Report 2012–5072

U.S. Department of the Interior

U.S. Geological Survey
Photographs. Front cover: Gleason combustion turbine plant, Weakley County, Tennessee; title page: Southaven

combined-cycle plant, Desoto County, Mississippi. Photographs courtesy of Tennessee Valley Authority.
Effects of Groundwater Withdrawals 

Associated with Combined-Cycle 

Combustion Turbine Plants in West 

Tennessee and Northern Mississippi

By Connor J. Haugh

Prepared in cooperation with the Tennesse Valley Authority

Scientific Investigations Report 2012–5072

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012

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reproduce any copyrighted materials contained within this report.

Suggested citation:

Haugh, C.J., 2012, Effects of groundwater withdrawals associated with combined-cycle combustion turbine plants in 

west Tennessee and northern Mississippi: U.S. Geological Survey Scientific Investigations Report 2012–5072, 22 p.
iii

Contents

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

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

Purpose and Scope ..............................................................................................................................2

Approach................................................................................................................................................2

Study Area..............................................................................................................................................2

Regional Model ..............................................................................................................................................2

Water-Use Trends..........................................................................................................................................2

Effects of Groundwater Withdrawals.........................................................................................................4

Gleason...................................................................................................................................................7

Tenaska...................................................................................................................................................7

Jackson ..................................................................................................................................................7

Southaven ..............................................................................................................................................7

Magnolia.................................................................................................................................................7

Discussion of Results.........................................................................................................................18

Model Limitations.........................................................................................................................................18

Summary........................................................................................................................................................22

Selected References...................................................................................................................................22

Figures

Maps showing—

 1. Locations of potential combined-cycle turbine plants in Tennessee 

and Mississippi ......................................................................................................................3

 2. Simulated potentiometric surface change in the Memphis/Sparta 

aquifer from combined-cycle-plant withdrawals at proposed 

combined-cycle turbine plants ...........................................................................................5

 3. Simulated potentiometric surface change in the Lower Wilcox/Fort 

Pillow aquifer from combined-cycle-plant withdrawals at proposed 

combined-cycle turbine plants ...........................................................................................6

 4. Local grid refinement area at the Gleason site, Weakley County, Tennessee ...........8

 5. Simulated potentiometric surface change in the Memphis aquifer from 

combined-cycle-plant withdrawals at the Gleason site, Weakley 

County, Tennessee ................................................................................................................9

 6. Simulated potentiometric surface change in the Fort Pillow aquifer 

from combined-cycle-plant withdrawals at the Gleason site, 

Weakley County, Tennessee..............................................................................................10

 7. Local grid refinement area at the Tenaska site, Haywood County, Tennessee........11

 8. Simulated potentiometric surface change in the Memphis aquifer from 

combined-cycle-plant withdrawals at the Tenaska site, Haywood 

County, Tennessee ..............................................................................................................12

 9. Simulated potentiometric surface change in the Memphis aquifer from 

combined-cycle-plant withdrawals at the Jackson site, Madison 

County, Tennessee ..............................................................................................................13

 10. Simulated potentiometric surface change in the Fort Pillow aquifer 

from combined-cycle-plant withdrawals at the Jackson site, Madison 

County, Tennessee ..............................................................................................................14
iv

Figures—Continued

Maps showing—

 11. Simulated potentiometric surface change in the Lower Wilcox/Fort 

Pillow aquifer from combined-cycle-plant withdrawals at the Southaven 

site, DeSoto County, Mississippi.......................................................................................15

 12. Simulated potentiometric surface change in the Upper Wilcox aquifer 

from combined-cycle-plant withdrawals at the Magnolia site, Benton 

County, Mississippi..............................................................................................................16

 13. Simulated potentiometric surface change in the Lower Wilcox aquifer

from combined-cycle-plant withdrawals at the Magnolia site, Benton 

County, Mississippi..............................................................................................................17

 14. Simulated potentiometric surface change in the Cockfield aquifer from 

withdrawals by all users at the end of the combined-cycle-plant 

withdrawal scenario ...........................................................................................................19

 15. Simulated potentiometric surface change in the Memphis/Sparta aquifer 

from withdrawals by all users at the end of the combined-cycle-plant 

withdrawal scenario ...........................................................................................................20

 16. Simulated potentiometric surface change in the Fort Pillow/Lower 

Wilcox aquifer from withdrawals by all users at the end of the 

combined-cycle-plant withdrawal scenario...................................................................21

Tables

1. Generalized correlation chart of units of Tertiary age of the Claiborne and 

Wilcox Groups in Tennessee and northern Mississippi.........................................................4

2. Annual average and 30-day maximum water demand at proposed 

combined-cycle turbine plants...................................................................................................4

3. Annual average and 30-day maximum water demand by well at the 

Gleason site ...................................................................................................................................7

4. Annual average and 30-day maximum water demand by well at the 

Tenaska site...................................................................................................................................7

5. Summary of simulated potentiometric surface changes resulting from 

groundwater withdrawals at potential combined-cycle-plant sites..................................18
Effects of Groundwater Withdrawals Associated with 

Combined-Cycle Combustion Turbine Plants in West 

Tennessee and Northern Mississippi

By Connor J. Haugh

Abstract

The Mississippi Embayment Regional Aquifer Study 

groundwater-flow model was used to simulate the potential 

effects on future groundwater withdrawals at five powerplant sites—Gleason, Weakley County, Tennessee; Tenaska, 

Haywood County, Tennessee; Jackson, Madison County, 

Tennessee; Southaven, DeSoto County, Mississippi; and 

Magnolia, Benton County, Mississippi. The scenario used in 

the simulation consisted of a 30-year average water-use period 

followed by a 30-day peak water-demand period. Effects 

of the powerplants on the aquifer system were evaluated by 

comparing the difference in simulated water levels in the 

aquifers at the end of the scenario (30 years plus 30 days) with 

and without the combined-cycle-plant withdrawals. Simulated 

potentiometric surface declines in source aquifers at potential 

combined-cycle-plant sites ranged from 56 feet in the upper 

Wilcox aquifer at the Magnolia site to 20 feet in the Memphis 

aquifer at the Tenaska site. The affected areas in the source 

aquifers at the sites delineated by the 4-foot potentiometric 

surface-decline contour ranged from 11,362 acres at Jackson 

to 535,143 acres at Southaven. The extent of areas affected 

by potentiometric surface declines was similar at the Gleason 

and Magnolia sites. The affected area at the Tenaska site was 

smaller than the affected areas at the other sites, most likely 

as a result of lower withdrawal rates and greater aquifer 

thickness. The extent of effect was smallest at the Jackson 

site, where the nearby Middle Fork Forked Deer River may 

act as a recharge boundary. Additionally, the Jackson site lies 

in the Memphis aquifer outcrop area where model-simulated 

recharge rates are higher than in areas where the Memphis 

aquifer underlies less permeable deposits.

The potentiometric surface decline in aquifers overlying 

or underlying a source aquifer was generally 2 feet or less at 

all the sites except Gleason. At the Gleason site, withdrawals 

from the Memphis aquifer resulted in declines of as much as 

9 feet in the underlying Fort Pillow aquifer. The simulated 

potentiometric surface change occurring in the Fort Pillow 

aquifer appears to be the result of leakage through the Flour 

Island Formation separating the Memphis and Fort Pillow 

aquifers where this confining unit is thin, sandy, or absent.

Introduction

As demand for electrical power increases, so does the 

need for water to operate electrical generation facilities. To 

meet rising electrical demands, the Tennessee Valley Authority 

(TVA) is considering the use of existing combined-cycle 

plants in West Tennessee and northern Mississippi and the 

conversion of simple-cycle plants to combined-cycle operation. The plants use groundwater for emission control and 

for cooling. Simple-cycle plants can use as much as about 

300 gallons per minute (gal/min) of water, and combinedcycle plants can use as much as 4,000 gal/min, depending 

on plant megawatt capacity. Depending on location, local 

groundwater conditions, and the aquifers used by the plants, 

the use of groundwater for the combined-cycle plants could 

affect groundwater levels in nearby domestic and municipal 

wells. Given these considerations, the effects of groundwater 

withdrawals at combined-cycle turbine plants on the aquifers 

and on groundwater levels are being evaluated.

In 2008, the U.S. Geological Survey (USGS), in cooperation with TVA, began an investigation to define the potential 

effects of groundwater withdrawals associated with combinedcycle-turbine-plant operation on the Mississippi embayment 

aquifer system in West Tennessee and northern Mississippi. 

In these areas, groundwater is the sole source of water for 

municipal and industrial supply. The primary sources of 

groundwater are the Memphis and Fort Pillow aquifers, which 

are the two principal aquifers of the Mississippi embayment 

aquifer system in West Tennessee (Parks and Carmichael, 

1989, 1990). Self-supplied domestic groundwater typically is 

produced from shallow zones including the terrace deposits 

or Cockfield aquifer, which constitute the “water-table” 

aquifer(s) at many locations. The Mississippi Embayment 

Regional Aquifer Study (MERAS) was recently completed as 

part of the Groundwater Resources Program of the USGS to 

assess groundwater availability within the Mississippi embayment. The MERAS groundwater-flow model was the primary 

tool used in the assessment of groundwater availability (Clark 

and Hart, 2009). 
2 Effects of Groundwater Withdrawals Associated with Combined-Cycle Combustion Turbine Plants

Purpose and Scope

This report presents an analysis of the potential effects of 

groundwater withdrawals at the following five sites: Gleason, 

Weakley County, Tennessee; Tenaska, Haywood County, 

Tennessee; Jackson, Madison County, Tennessee; Southaven, 

DeSoto County, Mississippi; and Magnolia, Benton County, 

Mississippi (fig. 1). The effects of groundwater withdrawals 

at plant sites, in conjunction with existing withdrawals, were 

analyzed using the MERAS regional groundwater-flow model 

(Clark and Hart, 2009). Local grid refinement (LGR; Mehl 

and Hill, 2005) was added to the MERAS model for a more 

detailed analysis at two of the sites. Water-use estimates for 

the plant sites were provided by TVA. Water-use trends and 

population-growth projections were used to estimate future 

water demands from existing withdrawals. 

Approach

The effects of groundwater withdrawals were simulated 

for the locations of five prospective combined-cycle plants for 

a 30-year period of average water use followed by a 30-day 

period of peak water use. Groundwater withdrawals at each 

of the sites were simulated using the MERAS groundwaterflow model beginning in 2007. Water use by the combinedcycle combustion plants was assumed to be constant over the 

30-year period. Projections of future water-supply withdrawals 

were estimated assuming a linear growth of 2 percent per year. 

Six stress periods of 5 years each plus a 30-day stress period 

were used for model simulations. At the end of the 30-year 

simulation period, an additional short-term stress period of 30 

days followed using the projected 30-day peak water withdrawals at the plant sites. Effects of the powerplants on the 

aquifer system were evaluated by comparing the difference in 

simulated water levels in the aquifers at the end of the scenario 

(30 years plus 30 days) with and without the combined-cycleplant withdrawals. 

The grid-cell size of the MERAS flow model is 1 square 

mile (mi2

) [1 mile (mi) by 1 mi]. A more detailed local evaluation was simulated for the Gleason and Tenaska sites using a 

refined model grid generated with the LGR package (Mehl and 

Hill, 2005). The grid-cell size of the local models is 0.04 mi2

(0.2 mi by 0.2 mi). 

Study Area

The simulated sites in West Tennessee and northern 

Mississippi lie in the gently rolling terrain of the Gulf Coastal 

Plain Physiographic Province (Fenneman, 1938). Land-surface 

altitudes range from about 200 feet (ft) near the Mississippi 

River to more than 500 ft in the upland hills in the eastern part 

of the study area. Average annual precipitation ranges from 

about 50 to 54 inches and is uniformly distributed throughout 

the year. The mean annual temperature is about 59 degrees 

Fahrenheit (o

F). Mean summer temperature is about 79 oF, and 

mean winter temperature is about 40 o

F (National Oceanic and 

Atmospheric Administration, 2002). 

Regional aquifers in the area are part of the Mississippi 

embayment aquifer system. The regional aquifers are formed 

by deposits of Tertiary age that make up the Claiborne and 

Wilcox Groups (table 1). In Tennessee, the aquifer system 

includes, in descending order, the following units: the 

Cockfield Formation (Cockfield or upper Claiborne aquifer) 

of the Claiborne Group, the Memphis Sand (Memphis or 

middle and lower Claiborne aquifer) of the Claiborne Group, 

and the Fort Pillow Sand (Fort Pillow aquifer) of the Wilcox 

Group. In northern Mississippi, the aquifer system includes, in 

descending order, the Cockfield Formation (Cockfield aquifer), the Sparta Sand (Sparta aquifer) of the Claiborne Group, 

and underlying sands of the Claiborne and Wilcox Groups that 

make up the lower Claiborne–upper Wilcox aquifer and the 

lower Wilcox aquifer (Hosman and Weiss, 1991). 

Regional Model

The MERAS model covers 97,000 mi2 and consists of 13 

model layers with grid cells of 1 mi2

. The model code used for 

the MERAS model is MODFLOW 2005 (Harbaugh, 2005). 

Model layers correspond to aquifers and confining units from 

land surface down to the top of the Midway Group. In Tennessee these layers include the following aquifers: the fluvial 

deposits aquifer, the Cockfield aquifer, the Memphis aquifer, 

and the Fort Pillow aquifer. The MERAS model simulations 

span more than 130 years from 1870 to 2007 and incorporate 

the most current water-use data available (Clark and Hart, 

2009). 

Water-Use Trends

Groundwater use for the next 30 years was estimated on 

the basis of historic trends and population-growth projections. 

Nationwide, withdrawals for irrigation increased from 1950 

to 1980 but have stabilized since 1985. This stabilization in 

irrigation withdrawals can be attributed to climate, crop type, 

advances in irrigation efficiency, and higher energy costs 

(Hutson and others, 2004). Groundwater use for public supply 

in Tennessee for the 30-year period from 1970 through 2000 

increased at an average annual rate of 2.4 percent. During 

this time, the average annual rate of growth of groundwater 

withdrawal for public supply slowed, with average annual 

rates of 2.8 percent from 1970 to 1980, 2.5 percent from 1980 

to 1990, and 1.9 percent from 1990 to 2000 (Webbers, 2003). 

Future groundwater withdrawals for public supply also were 

estimated using population projections (University of Tennessee, 2003). Population projections for 5-year increments for 

the period 2010 to 2040 for each municipality in West Tennessee were multiplied by the current per capita water use of each 

public water-supply system to estimate future groundwater 
CLAY

LEE

SHELBY

DYER

OBION

TATE

HENRY

FAYETTE

GIBSON

MISSISSIPPI

HARDIN

MARSHALL

TIPPAH

BUTLER

CARROLL

DUNKLIN

GREENE

TIPTON

MCNAIRY

WEAKLEY

MADISON

HARDEMAN

DE SOTO

BENTON

POINSETT

TUNICA

CROSS

HAYWOOD

CRITTENDEN

BENTON

PEMISCOT

ALCORN

NEW MADRID

CRAIGHEAD

PRENTISS

HENDERSON

UNION

LAUDERDALE

LAKE

DECATUR

TISHOMINGO

ST FRANCIS

GRAVES

CHESTER

CALLOWAY

FULTON

STODDARD

CROCKETT

STEWART

HICKMAN

RIPLEY

PHILLIPS LEE PANOLA LAFAYETTE ITAWAMBA

MISSISSIPPI

TIPTON

FULTON

CARTER

Figure 1.

TENNESSEE

ARKANSAS

MISSISSIPPI

MISSOURI

KENTUCKY

ALABAMA

Tenaska Jackson

Gleason

Magnolia Southaven

0 9 18 Miles

0 9 18 Kilometers

EXPLANATION

MERAS boundary

Plant site

MISSOURI

ILLINOIS

ALABAMA

ARKANSAS

LOUISIANA

MISSISSIPPI

KENTUCKY

TENNESSEE

Map area

N

Figure 1. Locations of potential combined-cycle turbine plants in Tennessee and Mississippi.

Water-Use Trends 3
4 Effects of Groundwater Withdrawals Associated with Combined-Cycle Combustion Turbine Plants 

Table 1. Generalized correlation chart of units of Tertiary age of the Claiborne and Wilcox Groups in Tennessee 

and northern Mississippi.

[Fm, Formation]

System Series Group Tennessee Northern Mississippi Regional hydrogeologic 

unit

Tertiary

Eocene

Claiborne

Cockfield Fm

(Cockfield aquifer)

Cockfield Fm

(Cockfield aquifer)

Upper Claiborne aquifer

Cook Mountain Fm Cook Mountain Fm

Middle Claiborne 

confining unit

Memphis Sand

(Memphis aquifer)

Sparta Sand

(Sparta aquifer) Middle Claiborne

Zilpha Clay

Lower 

Claiborne 

confining 

unit

aquifer

Lower sands in the 

Claiborne Group Lower ClaiborneUpper Wilcox aquifer

Wilcox

Flour Island Fm Upper sands in the 

Wilcox Group

Paleocene

Fort Pillow Sand

(Fort Pillow aquifer) Middle Wilcox aquifer

Lower sands in the 

Wilcox Group Old Breastworks Fm Lower Wilcox aquifer

Midway

Porters Creek Clay Porters Creek Clay

Midway confining unit

Clayton Fm Clayton Fm

Modified from Hosman and Weiss, 1991.

demand. The average annual rate of growth for all public 

groundwater supplies in West Tennessee was 0.7 percent with 

most individual public-supply systems ranging from 0 to 

1.2 percent. Combining the two approaches, withdrawals for 

public groundwater use were assumed to increase at an average annual rate of 2 percent. Groundwater withdrawals for 

irrigation were assumed to remain constant at 2007 levels.

Effects of Groundwater Withdrawals

The effects of groundwater withdrawals associated with 

the operation of combined-cycle turbine plants were evaluated by comparing simulations with and without pumping at 

the five plant sites (table 2). The differences in water levels 

between these two simulations were contoured to provide an 

overall measure of effects (figs. 2 and 3; table 3).

Table 2. Annual average and 30-day maximum water demand 

at proposed combined-cycle turbine plants.

[gal/min, gallons per minute]

Plant Source 

aquifer

Number 

of wells

Annual 

average 

water 

demand 

(gal/min)

30-day 

maximum 

water 

demand 

(gal/min)

Gleason Memphis 4 2,460 3,473

Tenaska Memphis 5 1,643 2,315

Jackson Memphis — 2,460 3,473

Southaven Lower Wilcox 6 2,460 3,473

Magnolia Upper Wilcox 12 1,968 2,778

Lower Wilcox 3 492 695
Effects of Groundwater Withdrawals 5

CLAY

LEE

SHELBY

DYER

OBION

TATE

HENRY

FAYETTE

GIBSON

MISSISSIPPI

HARDIN

MARSHALL

TIPPAH

BUTLER

CARROLL

DUNKLIN

GREENE

TIPTON

MCNAIRY

WEAKLEY

MADISON

HARDEMAN

DE SOTO

BENTON

POINSETT

TUNICA

CROSS

HAYWOOD

CRITTENDEN

BENTON

PEMISCOT

ALCORN

NEW MADRID

CRAIGHEAD

PRENTISS

HENDERSON

UNION

LAUDERDALE

LAKE

DECATUR

TISHOMINGO

ST FRANCIS

GRAVES

CHESTER

CALLOWAY

FULTON

STODDARD

CROCKETT

STEWART

HICKMAN

RIPLEY

PHILLIPS LEE PANOLA LAFAYETTE ITAWAMBA

MISSISSIPPI

TIPTON

FULTON

CARTER

Figure 2.

TENNESSEE

ARKANSAS

MISSISSIPPI

MISSOURI

KENTUCKY

ALABAMA

Tenaska Jackson

Gleason

Magnolia

Southaven

0 9 18 Miles

0 9 18 Kilometers

EXPLANATION

Potentiometric surface change 

contour—Shows simulated 

change in potentiometric surface.

Contour interval is 2 feet from –2

to –10, then every 10 feet

MERAS boundary

Plant site

N

-2 -4

-6

-8

-2

-4

-6

-4

-6

-8

-2

-4

-6

–4

Figure 2. Simulated potentiometric surface change in the Memphis/Sparta aquifer from combined-cycle-plant withdrawals at 

proposed combined-cycle turbine plants.
6 Effects of Groundwater Withdrawals Associated with Combined-Cycle Combustion Turbine Plants

CLAY

LEE

SHELBY

DYER

OBION

TATE

HENRY

FAYETTE

GIBSON

MISSISSIPPI

HARDIN

MARSHALL

TIPPAH

BUTLER

CARROLL

DUNKLIN

GREENE

TIPTON

MCNAIRY

WEAKLEY

MADISON

HARDEMAN

DE SOTO

BENTON

POINSETT

TUNICA

CROSS

HAYWOOD

CRITTENDEN

BENTON

PEMISCOT

ALCORN

NEW MADRID

CRAIGHEAD

PRENTISS

HENDERSON

UNION

LAUDERDALE

LAKE

DECATUR

TISHOMINGO

ST FRANCIS

GRAVES

CHESTER

CALLOWAY

FULTON

STODDARD

CROCKETT

STEWART

HICKMAN

RIPLEY

PHILLIPS LEE PANOLA LAFAYETTE ITAWAMBA

MISSISSIPPI

TIPTON

FULTON

CARTER

Figure 3.

TENNESSEE

ARKANSAS

MISSISSIPPI

MISSOURI

KENTUCKY

ALABAMA

Jackson Tenaska

Gleason

Magnolia Southaven

0 9 18 Miles

0 9 18 Kilometers

EXPLANATION

Potentiometric surface change 

contour—Shows simulated 

change in potentiometric surface.

Contour interval is 2 feet from –2

to –10, then every 10 feet

MERAS boundary

Plant site

N

-2

-4

-6

-8

-10

-20

-2

-6

-2

-4

-6

-8

-10

–10

-2

-4

-6

-8

-10

-20

-2

-6

-2

-4

-6

-8

-10

–10

Figure 3. Simulated potentiometric surface change in the Lower Wilcox/Fort Pillow aquifer from combined-cycle-plant 

withdrawals at proposed combined-cycle turbine plants.
Effects of Groundwater Withdrawals 7

Gleason

The Gleason site (fig. 1) is estimated to have an annual 

average groundwater withdrawal of 2,460 gal/min and a 

30-day maximum water withdrawal of 3,473 gal/min (table 2). 

For the simulation, four wells at the site pumped water from 

the Memphis aquifer (table 3). To obtain more detailed 

resolution in the simulation, local grid refinement was used 

at the Gleason site (fig. 4). At the end of the TVA withdrawal 

scenario, the simulated potentiometric surfaces at the plant 

site in the model cell containing well G5 declined by 40 ft in 

the Memphis aquifer (fig. 5) and 9 ft in the Fort Pillow aquifer 

(fig. 6). The simulated changes in the Fort Pillow aquifer 

appear to be the result of leakage through the Flour Island Formation, which separates the Memphis and Fort Pillow aquifers 

where the confining unit is thin, sandy, or absent.

Table 3. Annual average and 30-day maximum water demand by 

well at the Gleason site.

[gal/min, gallons per minute]

Well Source aquifer

Annual average 

water demand 

(gal/min)

30-day maximum 

water demand 

(gal/min)

G-1 Memphis 516 1,000

G-2 Memphis 48 473

G-3 Memphis 896 1,000

G-5 Memphis 1,000 1,000

Tenaska

The Tenaska site (fig. 1) is estimated to have an annual 

average water withdrawal of 1,643 gal/min and a 30-day 

maximum water withdrawal of 2,315 gal/min (table 2). For the 

simulation, five wells at the site pumped water from the Memphis aquifer (table 4). To obtain more detailed resolution in the 

simulation, local grid refinement was used at the Tenaska site 

(fig. 7). At the end of the TVA withdrawal scenario, simulated 

potentiometric surfaces at the plant site in the model cell 

containing well T4 declined by 20 ft in the Memphis aquifer 

(fig. 8), about 1 ft in the Cockfield aquifer, and 2 ft in the Fort 

Pillow aquifer.

Jackson

The Jackson site (fig. 1) is estimated to have an annual 

average water withdrawal of 2,460 gal/min and a 30-day 

maximum water withdrawal of 3,473 gal/min (table 2). For the 

simulation, wells at the site pumped water from the Memphis 

aquifer. At the end of the TVA withdrawal scenario, simulated 

potentiometric surfaces in the 1-mi2

 model cell located at the 

 7

plant site declined by 35 ft in the Memphis aquifer (fig. 9) and 

4 ft in the Fort Pillow aquifer (fig. 10). The areal extent of the 

decline at the Jackson site appears to be limited by a nearby 

recharge boundary at the Middle Fork Forked Deer River, 

which may provide a source of water to the aquifer. Additionally, the Jackson site lies in the Memphis aquifer outcrop 

where model-simulated recharge rates are higher than in areas 

where the Memphis aquifer underlies less permeable deposits.

Table 4. Annual average and 30-day maximum water demand by 

well at the Tenaska site.

[gal/min, gallons per minute]

Well Source aquifer

Annual average 

water demand 

(gal/min)

30-day maximum 

water demand 

(gal/min)

T-1 Memphis 538 758

T-2 Memphis 0 0

T-3 Memphis 373 526

T-4 Memphis 690 972

T-5 Memphis 42 59

Southaven

The Southaven site (fig. 1) is estimated to have an annual 

average water withdrawal of 2,460 gal/min and a 30-day 

maximum water withdrawal of 3,473 gal/min (table 2). For the 

simulation, six wells at the site pumped water from the lower 

Wilcox aquifer (Fort Pillow aquifer equivalent). At the end of 

the TVA withdrawal scenario, simulated potentiometric surfaces in the 1-mi2

 model cell located at the plant site declined 

by 38 ft in the lower Wilcox aquifer (fig. 11) and about 1 ft in 

the overlying Memphis aquifer.

Magnolia

The Magnolia site (fig. 1) is estimated to have an annual 

average water withdrawal of 2,460 gal/min and a 30-day 

maximum water withdrawal of 3,473 gal/min (table 2). For the 

simulation at this site, 12 wells pumped water from the upper 

Wilcox aquifer and 3 wells from the lower Wilcox aquifer. 

Pumping was apportioned between the two aquifers on the 

basis of the number of wells in each aquifer. At the end of the 

TVA withdrawal scenario, simulated potentiometric surfaces in 

the 1-mi2 model cell located at the plant site declined by 56 ft 

in the upper Wilcox aquifer (fig. 12) and 51 ft in the lower 

Wilcox aquifer (fig. 13). The spatial extent of change in both 

aquifers is constrained to the south and east by the boundary 

of the aquifers.
8 Effects of Groundwater Withdrawals Associated with Combined-Cycle Combustion Turbine Plants

WEAKLEY

HENRY

CARROLL

GIBSON

OBION

Gleason

0 2 4 Miles

0 2 4 Kilometers

EXPLANATION

Area of local grid refinement

MERAS boundary

Well

N

Figure 4. Local grid refinement area at the Gleason site, Weakley County, Tennessee.
Effects of Groundwater Withdrawals 9

WEAKLEY

HENRY

CARROLL

GIBSON

OBION

Gleason

0 2 4 Miles

0 2 4 Kilometers

EXPLANATION

Potentiometric surface change 

contour—Shows simulated 

change in potentiometric surface.

Contour interval is 2 feet from –2

to –10, then every 10 feet

MERAS boundary

Well

N

–6

–2

–4

–6

–8

–10

–20

–2

–4

–6

–8

–10

–20

Figure 5. Simulated potentiometric surface change in the Memphis aquifer from combined-cycle-plant withdrawals at the 

Gleason site, Weakley County, Tennessee.
10 Effects of Groundwater Withdrawals Associated with Combined-Cycle Combustion Turbine Plants

WEAKLEY

HENRY

CARROLL

GIBSON

OBION

Gleason

–2

–4

–6

–8

0 2 4 Miles

0 2 4 Kilometers

EXPLANATION

Potentiometric surface change 

contour—Shows simulated 

change in potentiometric surface.

Contour interval is 2 feet from –2

to –10, then every 10 feet

MERAS boundary

Well

N

–6

Figure 6. Simulated potentiometric surface change in the Fort Pillow aquifer from combined-cycle-plant withdrawals at the 

Gleason site, Weakley County, Tennessee.
Effects of Groundwater Withdrawals 11

HAYWOOD

LAUDERDALE

CROCKETT

TIPTON

DYER

DYER

GIBSON

MISSISSIPPI

Tenaska

0 2 4 Miles

0 2 4 Kilometers

EXPLANATION

Area of local grid refinement

Well

N

Figure 7. Local grid refinement area at the Tenaska site, Haywood County, Tennessee.
12 Effects of Groundwater Withdrawals Associated with Combined-Cycle Combustion Turbine Plants

HAYWOOD

LAUDERDALE

CROCKETT

TIPTON

DYER

DYER

GIBSON

MISSISSIPPI

-2

-4

-6

-8

-10

0 2 4 Miles

0 2 4 Kilometers

EXPLANATION

Potentiometric surface change 

contour—Shows simulated 

change in potentiometric surface.

Contour interval is 2 feet from –2

to –10, then every 10 feet

Well

N

–6

–2

–4

–6

–8

–10

Figure 8. Simulated potentiometric surface change in the Memphis aquifer from combined-cycle-plant withdrawals at the 

Tenaska site, Haywood County, Tennessee.
Effects of Groundwater Withdrawals 13

Jackson

0 0.9 1.8 Miles

0 0.9 1.8 Kilometers

EXPLANATION

Potentiometric surface change 

contour—Shows simulated 

change in potentiometric surface.

Contour interval is 2 feet from –2

to –10, then every 10 feet

Plant site

Middle Fork Forked Deer River

N

–6

–2

–4

–6

–8

–10

–20

–30

–2

–4

–6

–8

–10

–20

–30

Jackson site, Madison County, Tennessee.

Figure 9. Simulated potentiometric surface change in the Memphis aquifer from combined-cycle-plant withdrawals at the 
14 Effects of Groundwater Withdrawals Associated with Combined-Cycle Combustion Turbine Plants

Jackson

0 0.9 1.8 Miles

0 0.9 1.8 Kilometers

EXPLANATION

Potentiometric surface change 

contour—Shows simulated 

change in potentiometric surface.

Contour interval is 2 feet

Plant site

Middle Fork Forked Deer River

N

–2

–2

–4

Jackson site, Madison County, Tennessee.

Figure 10. Simulated potentiometric surface change in the Fort Pillow aquifer from combined-cycle-plant withdrawals at the 
Effects of Groundwater Withdrawals 15

SHELBY

DE SOTO

TATE

CRITTENDEN

TUNICA

MARSHALL

LEE

FAYETTE

ST FRANCIS

ST FRANCIS

–4

–6

–2

–8

–10

–20

–30

–4

–6

–2

–8

–10

–20

–30

EXPLANATION

Potentiometric surface change 

contour—Shows simulated 

change in potentiometric surface.

Contour interval is 2 feet from –2

to –10, then every 10 feet

Plant site

0 5 Miles

0 5 Kilometers

N

–2

Figure 11. Simulated potentiometric surface change in the Lower Wilcox/Fort Pillow aquifer from combined-cycle-plant 

withdrawals at the Southaven site, DeSoto County, Mississippi.
16 Effects of Groundwater Withdrawals Associated with Combined-Cycle Combustion Turbine Plants

MARSHALL

BENTON

UNION

FAYETTE

TIPPAH

LAFAYETTE

TATE

HARDEMAN

SHELBY

PANOLA

DE SOTO

–2

–4

–8

–10

–20

–30

–40

–50

–2

–4

–8

–10

–20

–30

–40

Magnolia –50

Figure 12.

0 5 Miles

0 5 Kilometers

EXPLANATION

Potentiometric surface change 

contour—Shows simulated 

change in potentiometric surface.

Contour interval is 2 feet from –2

to –10, then every 10 feet

MERAS boundary

Plant site

–2

Figure 12. Simulated potentiometric surface change in the Upper Wilcox aquifer from combined-cycle-plant withdrawals at the 

Magnolia site, Benton County, Mississippi.
Effects of Groundwater Withdrawals 17

MARSHALL

BENTON

UNION

FAYETTE

TIPPAH

LAFAYETTE

TATE

HARDEMAN SHELBY

PANOLA

DE SOTO

Figure 13.

–4

–6

–8

–10

–30

–4

–2

–4

–6

–8

–10

–20

–30

–4

0 5 Miles

0 5 Kilometers

EXPLANATION

Potentiometric surface change 

contour—Shows simulated 

change in potentiometric surface.

Contour interval is 2 feet from –2

to –10, then every 10 feet

MERAS boundary

Plant site

–2

Magnolia

Magnolia site, Benton County, Mississippi.

Figure 13. Simulated potentiometric surface change in the Lower Wilcox aquifer from combined-cycle-plant withdrawals at the 
18 Effects of Groundwater Withdrawals Associated with Combined-Cycle Combustion Turbine Plants

Discussion of Results

Simulated potentiometric surface declines in source 

aquifers attributed to plant withdrawals at potential combinedcycle-plant sites ranged from a maximum of 56 ft in the 

upper Wilcox aquifer at the Magnolia site to a minimum of 

20 ft in the Memphis aquifer at the Tenaska site (table 5). 

The areas encompassed by the 4 ft potentiometric surfacedecline contour (affected area) ranged from a minimum of 

11,362 acres at Jackson to a maximum of 535,143 acres at 

Southaven. The magnitude of change at these sites and the 

spatial extent of affected areas vary with the transmissivity and 

storativity of the aquifers, the degree of confinement above 

and below the aquifers, the modeled withdrawal rates, and 

the effects of nearby boundary conditions. The overall extents 

of affected areas are similar at the Gleason and Magnolia 

sites. The more limited extent of effect at the Tenaska site 

is most likely a result of the relatively low withdrawal rates 

and greater aquifer thickness at Tenaska. The spatial extent 

of effect was least at the Jackson site where the presence 

of the Middle Fork Forked Deer River nearby may act as a 

recharge boundary. Additionally, the Jackson site lies in the 

Memphis aquifer outcrop where modeled recharge rates are 

higher than in areas where the Memphis aquifer underlies less 

permeable deposits. 

The simulated potentiometric surface in aquifers overlying or underlying source aquifers generally declined less 

than 2 ft at all the sites except Gleason. At the Gleason site, 

withdrawals from the Memphis aquifer resulted in a decline of 

about 9 ft in the underlying Fort Pillow aquifer—perhaps the 

result of leakage through the Flour Island Formation separating the Memphis and Fort Pillow aquifers where this confining 

unit is thin, sandy, or absent. 

Simulated declines in potentiometric surfaces from TVA 

withdrawals were similar in magnitude and extent to larger 

patterns of decline associated with all users over the simulation period. Simulated declines in the Cockfield aquifer from 

all groundwater use ranged from less than 10 ft to about 70 ft 

with the largest declines occurring in Shelby County, Tenn. 

(fig. 14). Simulated declines in the Memphis/Sparta aquifer 

from all groundwater use ranged from less than 10 ft to about 

80 ft with the largest declines occurring in Shelby County, 

Tenn. (fig. 15). Simulated potentiometric surface declines in 

the Fort Pillow/lower Wilcox aquifer from all groundwater 

use ranged from less than 10 ft to about 90 ft with the largest 

declines occurring in Crittenden County, Ark. (fig. 16).

Model Limitations

Models are simplifications of natural systems. Factors 

that affect how well a model represents a given natural system 

include the model scale; the accuracy and availability of 

hydraulic property data; the accuracy of pumping, water level, 

and streamflow data; and appropriately defined boundary 

conditions. The MERAS model, used for the analysis presented in this report, is consistent with the conceptual model 

and hydrologic data of the MERAS study area. The MERAS 

model uses a grid-cell size of 1 mi2. The local grid refinement 

used in the Tenaska and Gleason areas use a grid-cell size 

of 0.04 mi2. A model will not provide an accurate prediction 

on a scale smaller than the grid resolution. The hydraulicconductivity zones used in the model represent large-scale 

variation in hydraulic properties; the actual spatial variations 

of hydraulic properties of the aquifer system occur on a much 

smaller scale and are poorly defined. Further discussion of the 

limitations of the MERAS model are reported by Clark and 

Hart (2009, p. 56).

Table 5. Summary of simulated potentiometric surface changes 

resulting from groundwater withdrawals at potential combinedcycle-plant sites.

[TVA, Tennessee Valley Authority]

Simulated potentiometric surface change from 

the TVA withdrawal scenario 

at potential combined-cycle-plant sites (in feet)

Memphis 

Sand

Upper 

Wilcox

Fort Pillow/

lower Wilcox

Gleason –40 — –9

Tenaska –20 — –2

Jackson –35 — –4

Southaven –1 — –38

Magnolia — –56 –51
Effects of Groundwater Withdrawals 19

CLAY

LEE

SHELBY

DYER

OBION

TATE

HENRY

FAYETTE

GIBSON

MISSISSIPPI

HARDIN

MARSHALL

TIPPAH

BUTLER

CARROLL

DUNKLIN

GREENE

TIPTON

MCNAIRY

WEAKLEY

MADISON

HARDEMAN

DE SOTO

BENTON

POINSETT

TUNICA

CROSS

HAYWOOD

CRITTENDEN

BENTON

PEMISCOT

ALCORN

NEW MADRID

CRAIGHEAD

PRENTISS

HENDERSON

UNION

LAUDERDALE

LAKE

DECATUR

TISHOMINGO

ST FRANCIS

GRAVES

CHESTER

CALLOWAY

FULTON

STODDARD

CROCKETT

STEWART

HICKMAN

RIPLEY

PHILLIPS LEE PANOLA LAFAYETTE ITAWAMBA

MISSISSIPPI

TIPTON

FULTON

CARTER

TENNESSEE

ARKANSAS

MISSISSIPPI

MISSOURI

KENTUCKY

ALABAMA

Tenaska Jackson

Gleason

Magnolia Southaven

0 9 18 Miles

0 9 18 Kilometers

EXPLANATION

Potentiometric surface change 

contour—Shows simulated 

change in potentiometric surface.

Contour interval is 10 feet

MERAS boundary

Cockfield Formation—Shows approximate

extent of formation

Plant site

N

–20 

–10

–20

–30

–40

–50

–60 –30

–50

–40

–20

–10

–40

–30

–50

–40

–40

–20

–10

–20

–30

–40

–50

–60 –30

–50

–40

–20

–10

–40

–30

–50

–40

–40

–10

–20

Figure 14. Simulated potentiometric surface change in the Cockfield aquifer from withdrawals by all users at the end of the 

combined-cycle-plant withdrawal scenario.
20 Effects of Groundwater Withdrawals Associated with Combined-Cycle Combustion Turbine Plants

CLAY

LEE

SHELBY

DYER

OBION

TATE

HENRY

FAYETTE

GIBSON

MISSISSIPPI

HARDIN

MARSHALL

TIPPAH

BUTLER

CARROLL

DUNKLIN

GREENE

TIPTON

MCNAIRY

WEAKLEY

MADISON

HARDEMAN

DE SOTO

BENTON

POINSETT

TUNICA

CROSS

HAYWOOD

CRITTENDEN

BENTON

PEMISCOT

ALCORN

NEW MADRID

CRAIGHEAD

PRENTISS

HENDERSON

UNION

LAUDERDALE

LAKE

DECATUR

TISHOMINGO

ST FRANCIS

GRAVES

CHESTER

CALLOWAY

FULTON

STODDARD

CROCKETT

STEWART

HICKMAN

RIPLEY

PHILLIPS LEE PANOLA LAFAYETTE ITAWAMBA

MISSISSIPPI

TIPTON

FULTON

CARTER

TENNESSEE

ARKANSAS

MISSISSIPPI

MISSOURI

KENTUCKY

ALABAMA

Tenaska Jackson

Gleason

Magnolia

Southaven

0 9 18 Miles

0 9 18 Kilometers

EXPLANATION

Potentiometric surface change 

contour—Shows simulated 

change in potentiometric surface.

Contour interval is 10 feet

MERAS boundary

Memphis/Sparta boundary—Shows approximate

extent of the Memphis/Sparta aquifer

Plant site

N

–10 

–30 –20

–10

–40

–50

–60

–70

–80

–80

–10

–10

–30

–30

–60

–80

–60

–40

–30 –20

–10

–40

–50

–60

–70

–80

–80

–10

–10

–30

–30

–60

–80

–60

–40

Figure 15. Simulated potentiometric surface change in the Memphis/Sparta aquifer from withdrawals by all users at the end of the 

combined-cycle-plant withdrawal scenario.
Effects of Groundwater Withdrawals 21

CLAY

LEE

SHELBY

DYER

OBION

TATE

HENRY

FAYETTE

GIBSON

MISSISSIPPI

HARDIN

MARSHALL

TIPPAH

BUTLER

CARROLL

DUNKLIN

GREENE

TIPTON

MCNAIRY

WEAKLEY

MADISON

HARDEMAN

DE SOTO

BENTON

POINSETT

TUNICA

CROSS

HAYWOOD

CRITTENDEN

BENTON

PEMISCOT

ALCORN

NEW MADRID

CRAIGHEAD

PRENTISS

HENDERSON

UNION

LAUDERDALE

LAKE

DECATUR

TISHOMINGO

ST FRANCIS

GRAVES

CHESTER

CALLOWAY

FULTON

STODDARD

CROCKETT

STEWART

HICKMAN

RIPLEY

PHILLIPS LEE PANOLA LAFAYETTE ITAWAMBA

MISSISSIPPI

TIPTON

FULTON

CARTER

TENNESSEE

TENNESSEE

ARKANSAS

MISSISSIPPI

MISSOURI

KENTUCKY

ALABAMA

Tenaska Jackson

Gleason

Magnolia Southaven

0 9 18 Miles

0 9 18 Kilometers

EXPLANATION

Potentiometric surface change 

contour—Shows simulated 

change in potentiometric surface.

Contour interval is 10 feet

MERAS boundary

Fort Pillow/Lower Wilcox extent—Shows

approximate extent of the Fort Pillow/Upper

Wilcox aquifer

Plant site

N

–20 

–40

–10

–30

–20

–50

–60

–70

–80

–80

–50

–50

–50

–60

–60

–60

–60

–20

–70

–60

–50

–30

–40

–10

–30

–20

–50

–60

–70

–80

–80

–50

–50

–50

–60

–60

–60

–60

–20

–70

–60

–50

–30

Figure 16. Simulated potentiometric surface change in the Fort Pillow/Lower Wilcox aquifer from withdrawals by all users at the 

end of the combined-cycle-plant withdrawal scenario.
22 Effects of Groundwater Withdrawals Associated with Combined-Cycle Combustion Turbine Plants

Summary

Increases in population have brought about corresponding increases in demand for electric power. The Tennessee 

Valley Authority is considering the use of one or more existing combined-cycle plants in West Tennessee and northern 

Mississippi and the potential conversion of simple-cycle plants 

to combined-cycle operation. The combined-cycle plants, 

depending on electrical generation capacity, may require 

relatively large amounts of water for operation. Given these 

considerations, the effects of groundwater withdrawals at 

combined-cycle turbine plants on the aquifers and on groundwater levels are being evaluated.

The potential effects of increased groundwater withdrawals were simulated using the MERAS regional groundwaterflow model at five sites: Gleason, Weakley County, Tennessee; 

Tenaska, Haywood County, Tennessee; Jackson, Madison 

County, Tennessee; Southaven, DeSoto County, Mississippi; 

and Magnolia, Benton County, Mississippi. The scenario 

evaluated consisted of a 30-year average water-use period 

followed by a 30-day peak water-demand period. Projections 

of future water use by public utilities were estimated assuming 

a linear growth in demand of 2 percent per year. Effects of the 

powerplants on the aquifer system were evaluated by comparing the difference in simulated water levels in the aquifers 

at the end of the scenario (30 years plus 30 days) with and 

without the combined-cycle-plant withdrawals.

Simulated declines in the potentiometric surface in source 

aquifers at potential combined-cycle-plant sites ranged from 

56 ft in the upper Wilcox aquifer at the Magnolia site to 20 ft 

in the Memphis aquifer at the Tenaska site. The spatial extent 

of declines as delineated by the minus 4-ft potentiometric 

surface change contour ranged from 11,362 acres at Jackson to 

535,143 acres at Southaven. 

The simulated potentiometric surface declines in aquifers 

overlying or underlying source aquifers generally were less 

than 2 ft at all the sites except Gleason. At the Gleason site, 

withdrawals from the Memphis aquifer result in potentiometric 

surface declines of about 9 ft in the underlying Fort Pillow 

aquifer. Overall, declines in potentiometric surfaces resulting 

from combined-cycle powerplants would not appear to be 

unusual compared to those associated with other projected 

uses in the region.

Selected References

Clark, B.R., and Hart, R.M., 2009, The Mississippi Embayment Regional Aquifer Study (MERAS)—Documentation 

of a groundwater-flow model constructed to assess water 

availability in the Mississippi Embayment: U.S. Geological 

Survey Scientific Investigations Report 2009–5172, 61 p.

Fenneman, N.M., 1938, Physiography of the eastern United 

States: New York and London, McGraw-Hill Book 

Company, 714 p.

Harbaugh, A.W., 2005, MODFLOW-2005, the U.S. Geological Survey modular ground-water model—The 

ground-water flow process: U.S. Geological Survey 

Techniques and Methods 6–A16, variously paged.

Hosman, R.L., and Weiss, J.S., 1991, Geohydrologic units 

of the Mississippi embayment and Texas coastal uplands 

aquifer systems, south-central United States: U.S. Geological Survey Professional Paper 1416–B, 19 p.

Hutson, S.S., Barber, N.L., Kenny, J.F., Linsey, K.S. Lumia, 

D.S., and Maupin, M.A., 2004, Estimated use of water 

in the United States in 2000: U.S. Geological Survey 

Circular 1268, 46 p.

Mehl, S.W., and Hill, M.C., 2005, MODFLOW-2005, 

the U.S. Geological Survey modular ground-water 

model—Documentation of shared node Local Grid 

Refinement (LGR) and the Boundary Flow and Head 

(BFH) Package: U.S. Geological Survey Techniques and 

Methods 6–A12, 68 p.

Mehl, S.W., and Hill, M.C., 2007, MODFLOW-2005, the U.S. 

Geological Survey modular ground-water model—

Documentation of the multiple-refined-areas capability of 

Local Grid Refinement (LGR) and the Boundary Flow and 

Head (BFH) Package: U.S. Geological Survey Techniques 

and Methods 6–A21, 13 p.

National Oceanic and Atmospheric Administration, 2002, 

Divisional normals and standard deviations of temperature, precipitation, and heating and cooling degree days 

1971–2000 (and previous normals periods): National 

Oceanic and Atmospheric Administration Climatography 

of the United States no. 85.

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

Report 89–4120, 20 p.

Parks, W.S., and Carmichael, J.K., 1990, Geology and groundwater resources of the Memphis Sand in Western Tennessee: 

U.S. Geological Survey Water-Resources Investigations 

Report 88–4182, 30 p.

University of Tennessee, Center for Business and Economic 

Research, 2003, Population projections for the State of 

Tennessee 2005 to 2025: University of Tennessee, 66 p., 

accessed April 29, 2008, at http://www.state.tn.us/tacir/

population.htm.

Webers, Ank, 2003, Public water-supply systems and associated water use in Tennessee, 2000: U.S. Geological Survey 

Water-Resources Investigations Report 03–4264, 90 p.
Prepared by:

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Raleigh Publishing Service Center

3916 Sunset Ridge Road

Raleigh, NC 27607

For additional information regarding this 

publication, contact:

 USGS Tennessee Water Science Center

 640 Grassmere Park, Suite 100

 Nashville, TN 37211

 (615) 837-4700

Or visit the USGS Tennessee Water Science 

Center Web site at:

 http://tn.water.usgs.gov 
Haugh—Effects of Groundwater Withdrawals Associated with Combined-Cycle Combustion Turbine Plants—Scientific Investigations Report 2012–5072