U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2005-3133

April 2006

U.S. Geological Survey Studies Quality of Drinking-Water 

Supplies in the Mississippi Embayment–Texas Coastal 

Uplands Principal Aquifer

Printed on recycled paper

Introduction

The quality and quantity of ground 

water can affect the health and well-being 

of citizens who depend upon ground 

water for drinking-water supply, as well 

as have important consequences to the 

economic well being of a community. 

For many Americans, ground water is the 

primary source of drinking water. It also 

replenishes our streams and rivers, and 

provides freshwater for irrigation and 

industry. 

An aquifer is a geologic formation, a 

group of formations, or part of a formation that contains sufficient saturated 

permeable material to yield substantial 

quantities of water to wells and springs. 

Currently 16 of the Nation’s 62 principal aquifers are being studied by the 

NAWQA Program. In aggregate, these 

16 principal aquifers accounted for about 

three quarters of the estimated withdrawals for drinking-water supply in 1990 

from the Nation’s 62 principal aquifers. 

The METX principal aquifer was ranked 

9th (with 1st being the largest use) of the 

62 principal aquifers for 1990 estimated 

drinking-water use.

Mississippi Embayment–Texas 

Coastal Uplands Regional Aquifer

The METX principal aquifer has 

an approximate areal extent of about 

197,190 square miles and includes parts 

of the Cockfield, Sparta, Memphis, and 

Wilcox aquifers in Alabama, Arkansas, 

Illinois, Kentucky, Louisiana, Missouri, 

Mississippi, Tennessee, and Texas. A 

detailed description of the aquifers and 

corresponding geohydrologic units can 

be found in the Ground Water Atlas of 

the United States (Ryder, 1996; Renken, 

One of the primary missions of the U.S. Geological 

Survey (USGS) is to evaluate the quantity and quality of 

the Nation’s water resources. To this end, since 1991 the 

National Water Quality Assessment (NAWQA) Program 

of the USGS has been collecting data on many of the 

Nation’s principal aquifers used as a source for drinking 

water, including the Mississippi Embayment-Texas Coastal 

Uplands (METX) principal aquifer. There have been three 

separate studies of the aquifer by the NAWQA Program 

from 1994 until 2004; these studies have been local in 

nature and covered only small parts of the principal aquifer. 

In 2005, a synthesis of the results from the three studies was 

begun to evaluate the quality of water and to understand 

how it changes throughout the principal aquifer.

Millions of Americans receive high quality drinking water 

A principal aquifer is defined as a 

regionally extensive aquifer or aquifer 

system that has the potential to be used 

as a source of potable water. 

Geohydrologic Unit

Local aquifer name 

(number of wells 

sampled)

Mississippi River 

Valley alluvial 

aquifer

Mississippi River 

alluvial aquifer

Shallow watertable aquifer (24)

Upper Claiborne 

aquifer

Cockfield aquifer (2)

Middle Claiborne 

aquifer

Memphis-Sparta 

aquifer (25)

Lower ClaiborneUpper Wilcox 

aquifer

Carrizo aquifer (23)

Meridian-upper 

Wilcox aquifer (2)

Middle Wilcox 

aquifer

Fort Pillow 

aquifer (1)

Wilcox aquifer (8)

Geohydrologic units and corresponding local 

aquifer names
sippi. Along with the Winona-Tallahatta 

aquifer, the Cane River formation, and 

the Carrizo aquifer, the Sparta aquifer is 

considered the equivalent of the Memphis 

aquifer in Tennessee.

Meridian-Upper Wilcox Aquifer

The Meridian Sand in Mississippi 

is situated at the base of the Claiborne 

Group and is the stratigraphic equivalent to the Carrizo Sand in Arkansas. 

Thickness of the Meridian-upper Wilcox 

aquifer is variable and ranges from about 

50 to 400 ft. Well yields range from 100 

to 1,000 gallons per minute (gal/min).

Fort Pillow-Wilcox Aquifer

The Wilcox Group includes at least 

two major sand units separated by clays 

that are collectively called the Wilcox 

aquifer. The Fort Pillow Sand, also 

named the “1,400-foot “ sand in the 

Memphis area, is included as part of 

the Wilcox aquifer. The Wilcox aquifer 

ranges in thickness from 0 to 1,100 ft. 

Well yields range from 500 to 2,000 

gal/min.

Texas Coastal Uplands Aquifer System

Carrizo-Wilcox Aquifer

The Carrizo-Wilcox aquifer includes 

the Carrizo Sand of the Claiborne Group. 

Municipal and irrigation withdrawals 

from the aquifer account for about 35 and 

51 percent, respectively, of total pumpage. The largest metropolitan areas that 

are dependent on the aquifer for drinking 

water are Bryan-College Station, LufkinNacagdoches, and Tyler, Texas (Texas 

Water Development Board, 2005). The 

wells in this aquifer were sampled as part 

of the Carrizo-Wilcox well network.

Data Collection

Ground-water samples have been 

collected by NAWQA from parts of the 

METX principal aquifer since 1994. 

Sampling throughout the principal 

aquifer is ongoing, but varies throughout 

the aquifer in a cyclical manner. Samples 

have been collected from 85 wells in 

studies of the Mississippi River alluvial 

aquifer, the Mississippi Embayment 

aquifer system, and the Texas Coastal 

Uplands aquifer system. Data were summarized based on location within the 

principal aquifer. 

Areal extent of Mississippi Embayment - Texas Coastal Uplands aquifer system and location of 

wells sampled.

1998). For the purpose of this discussion, 

aquifers are referred to here using local 

nomenclature.

Mississippi River Alluvial Aquifer

Shallow Water Table Aquifer

Although not part of the METX principal aquifer, there is direct connection 

between the shallow water-table aquifer 

and the upper part of the Memphis aquifer in some areas east of Memphis, Tennessee. The potential exists for water and 

contaminants in this shallow aquifer to 

move into the lower part of the Memphis 

aquifer.

The shallow water table aquifer is not 

used as a primary ground-water resource 

within the Memphis vicinity. In rural 

areas near the Memphis metropolitan 

area, however, some small domestic and 

livestock wells pump from the shallow 

water-table aquifer. 

Mississippi Embayment Aquifer System

The Mississippi Embayment aquifer 

system consists of the aquifers that are 

in the Claiborne and Wilcox Groups of 

Tertiary age. Aquifers that were sampled 

include the Cockfield, Memphis-Sparta, 

the Meridian-upper Wilcox, and the Fort 

Pillow-Wilcox.

Cockfield Aquifer

The Cockfield aquifer provides water 

mainly for public and industrial supply 

(some domestic and livestock wells) in 

the southern part of the study area in Mississippi and Louisiana. Thickness of the 

aquifer in the outcrop area ranges from 

0 to 700 feet (ft) . Within the study area, 

the aquifer supplies water to low-yield 

domestic or stock wells. Just outside the 

study area in Mississippi, the aquifer is 

pumped for public supply in some parts 

of the Jackson metropolitan area.

Memphis-Sparta Aquifer

Ground water from the lower part 

of the Memphis aquifer is the primary 

source of drinking water for the Memphis 

metropolitan area, including Collierville, 

Bartlett, Germantown, and Memphis. In 

1995, ground-water pumpage was 190 

million gallons per day from the lower 

part of the Memphis aquifer for Shelby 

County, in the vicinity of Memphis. 

The Sparta aquifer is a major drinking 

water source in southern Arkansas, northern Louisiana, and west-central Missis-
Why are these constituents 

important?

Major Ions

Major ions affect some basic 

chemical characteristics of water 

such as taste, color, and hardness. 

Calcium and magnesium contribute 

to hardness of water, which is an 

indicator of the water’s ability to 

form insoluble residues with soaps 

and to form scale in plumbing fixtures associated with heating water. 

High dissolved solids concentrations 

can affect suitability of water for 

drinking purposes. High concentrations of iron and manganese can 

cause discoloration of water.

Trace Elements

The human body needs iron, 

copper, manganese, and zinc in 

trace quantities, but larger amounts 

of these elements in the body may 

be toxic. Other elements, such as 

arsenic and cadmium, are toxic even 

at trace quantities in the human 

body. Some of the more toxic trace 

elements are cumulative, systemic 

poisons that can be categorized as 

carcinogens. Chronic exposure to 

certain trace elements can cause damage to the digestive, respiratory, and 

nervous systems.

Nutrients

Excessive nutrients can have 

adverse effects on human health. 

Nitrate and phosphorus in water can 

promote the growth of pathogens that 

may cause gastrointestinal-related 

illness. Elevated concentrations of 

nitrate in drinking water have been 

associated with methemoglobinemia 

or “blue-baby” syndrome and with 

increased rates of stomach cancer. 

What kind of data Chemical constituents of interest Expected in 

ground water?

Constituents detected in water sample

(number in parentheses is number of detections)

Major Ions Bicarbonate, bromide, calcium, chloride, 

fluoride, iron, magnesium, manganese, potassium, silica, sodium, and total dissolved 

solids

Yes Bicarbonate, sodium, calcium, magnesium, manganese, and iron were the most commonly detected

Trace elements Aluminum, antimony, arsenic, barium, beryllium, cadmium, chromium, cobalt, copper, lead, molybdenum, nickel, selenium, 

silver, uranium, and zinc

Yes Aluminum, barium, chromium, nickel, and zinc 

were the most commonly detected.

Nutrients Ammonia, ammonia plus organic nitrogen, 

nitrite plus nitrate, nitrite, organic carbon, 

orthophosphate, and phosphorus

Yes Ammonia (40); ammonia plus organic nitrogen 

(31); nitrite plus nitrate (46); nitrite (3); organic 

carbon (49); orthophosphate (56); and phosphorus 

(55)

Pesticides 75 pesticides and 8 pesticide degradates 

including atrazine, bromacil, deethyl atrazine, diazinon, metolachlor, prometon, and 

simazine

No 33 wells had pesticide detections. Atrazine (13); 

bromacil (3); deethyl atrazine (10); diazinon (4); 

metolachlor (10); prometon (2); and simazine (12)

Volatile 

organic 

compounds 

(VOCs)

87 VOCs including carbon disulfide, chloroform, vinyl chloride, benzene, toluene, 

ethyl-benzene, m- and p-xylenes, tetrachoroethen, and methyl tert-butyl ether 

No 67 wells had detections. Carbon disulfide (24); 

chloroform (12); m- and p-xylenes (14); tetrachoroethene (9); and toluene (7)

Summary of selected constituents.

Elevated nutrients in ground-water 

discharge can contribute to nuisance 

algae and aquatic plant growth in 

streams or lakes.

Pesticides

Pesticides in water supplies 

have been of interest since 

before 1972 when the manufacture and usage of DDT was 

banned in the United States. 

Many pesticides used today to 

control weeds, insects, and fungi 

that inhibit the growth of crops 

can threaten human health. Pesticides have varying chemical 

and physical properties and have 

a tendency to reside in the soil. 

Herbicides generally are more 

soluble than other pesticides and 

are more likely to infiltrate to

the water table.

Volatile Organic Compounds

Volatile organic compounds 

(VOCs) are soluble, mobile, and 

persistent in ground water, and many 

are suspected carcinogens. Many 

commonly used substances such as 

gasoline, refrigerants, paints, and 

plastics contain VOCs.
Additional Information

For additional information about 

the Mississippi Embayment–Texas 

Coastal Uplands Principal Aquifer 

NAWQA study contact:

U.S. Geological Survey

Mississippi Water Science

 Center

308 South Airport Rd.

Jackson, MS 39208-6649

http://ms.water.ugsg.gov/

U.S. Geological Survey

Texas Water Science Center

8027 Exchange Dr.

Austin, TX 78754-4733

http://tx.water.usgs.gov/

U.S. Geological Survey 

Louisiana Water Science Center

3535 S. Sherwood Forest Blvd.,

 Suite 120

Baton Rouge, LA 70816

http://la.water.usgs.gov/

Significant Findings

Although the quality of water from 

the METX principal aquifer generally 

is good, there were a few detections 

of some anthropogenic (non-naturally 

occurring) compounds such as pesticides 

and volatile organic compounds (VOCs). 

The presence of these compounds could 

indicate degradation of the quality of 

water in the aquifer and requires further 

investigation. 

In order to better characterize the 

water quality of the METX principal 

aquifer, an additional 30 wells were 

sampled in the Shreveport, Louisiana, 

area in 2004, and those data will be part 

of the regional assessment.

Additional Reading

Much of the material included here 

has been previously presented in the following publications.

Gonthier, G.J., 2000, Water quality in the 

deep Tertiary aquifers of the Mississippi 

Embayment, 1996: U.S. Geological Survey 

Water-Resources Investigations Report 

99-4131, 91 p.

Gonthier, G.J., 2002, Quality of shallow 

ground water in recently developed residential and commercial area, Memphis 

vicinity, Tennessee, 1997: U.S. Geological 

Survey Water-Resources Investigations 

Report 02-4294, 105 p.

Compiled by

Carol P. Moss and Heather L. Welch

USGS Data are Publicly 

Available

The National Water Information 

System (NWISweb) is the USGS 

database for water-quality conditions 

and other information. NWISweb is 

publicly available and can be readily 

accessed via the Internet (http://water.

usgs.gov/nwis). 

Aquifers sampled Well depths

(in feet) Sampling period Significant findings

Mississippi River Valley 

 alluvial aquifer

 Shallow water table 

 Upper Memphis aquifer 

33 to 109 April –May 1997 Atrazine was the only compound in this study detected at a 

concentration that exceeded a U.S. Environmental Protection 

Agency primary drinking-water standard.

Mississippi Embayment aquifer

 system

 Cockfield aquifer 

 Memphis-Sparta aquifer 

 Meridian-upper Wilcox aquifer 

 Fort Pillow-Wilcox aquifer

208 to 1,460 April – May 1996 Water from 3 wells had total dissolved solids concentrations 

higher than the Secondary Drinking Water Regulation (SDWR; 

U.S. Environmental Protection Agency); 13 wells had iron concentrations higher than the SDWR; and 9 wells had manganese 

concentrations higher than the SDWR.

Texas Coastal Uplands aquifer

 system

 Carrizo-Wilcox aquifer

21 to 462 February – August 1994 Concentrations of dissolved solids and iron were higher than 

the SDWR established for drinking water by the U.S. Environmental Protection Agency in four wells; manganese concentrations were higher than the SDWR in eight wells. 

Summary of sampled wells and significant findings.

Kleiss, B.A., Coupe, R.H., Gonthier, G.J., 

and Justus, B.J., 2000, Water quality in 

the Mississippi Embayment: Mississippi, 

Louisiana, Arkansas, Missouri, Tennessee, 

and Kentucky, 1995-98: U.S. Geological 

Survey Circular 1208, 36 p.

Land, L.F., Moring, J.B., Van Metre, P.C., 

Reutter, D.C., Mahler, B.J., Shipp, A.A., 

and Ulery, R.L., 1998, Water quality in the 

Trinity River Basin: Texas, 1992-95: U.S. 

Geological Survey Circular 1171, 39 p.

Renken, R.A, 1998, Ground water atlas of 

the United States-Segment 5, Arkansas, 

Louisiana, Mississippi: :U.S. Geological 

Survey Hydrologic Investigations Atlas 

730-F, 28 p.

Reutter, D.C., and Dunn, D.D., 2000, Waterquality assessment of the Trinity River 

Basin, Texas--Ground-water quality of the 

Trinity, Carrizo-Wilcox, and Gulf Coast 

aquifers, February-August 1994: U.S. Geological Survey Water-Resources Investigations Report 99-4233, 56 p. 

Ryder, P.D., 1996, Ground water atlas of the 

United States-Segment 4, Oklahoma and 

Texas: U.S. Geological Survey Hydrologic 

Atlas 730-E, 30 p.

Texas Water Development Board, 2005, 

Carrizo-Wilcox aquifer, accessed July 26, 

2005, at http://www.twdb.state.tx.us/publications/reports/GroundWaterReports/

GWReports

U.S. Environmental Protection Agency, 

Understanding the Safe Drinking Water 

Act, accessed May 10, 2005, at 

http://www.epa.gov/safewater/sdwa/30th/

factsheets/understand.html