U.S. Department of the Interior

U.S. Geological Survey

National Water-Quality Assessment Program

Quality of Shallow Groundwater and Drinking Water 

in the Mississippi Embayment-Texas Coastal Uplands 

Aquifer System and the Mississippi River Valley Alluvial 

Aquifer, South-Central United States, 1994–2004

Scientific Investigations Report 2009–5091

Quality of Shallow Groundwater and Drinking 

Water in the Mississippi Embayment-Texas 

Coastal Uplands Aquifer System and the 

Mississippi River Valley Alluvial Aquifer, 

South-Central United States, 1994–2004

By Heather L. Welch, James A. Kingsbury, Roland W. Tollett, and Ronald C. Seanor

National Water-Quality Assessment Program

Scientific Investigations Report 2009–5091

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2009

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Suggested citation:

Welch, H.L., Kingsbury, J.A., Tollett, R.W., and Seanor, R.C., 2009, Quality of shallow groundwater and drinking water 

in the Mississippi embayment-Texas coastal uplands aquifer system and the Mississippi River Valley alluvial aquifer, 

south-central United States, 1994–2004: U.S. Geological Survey Scientific Investigations Report 2009–5091, 51 p.
iii

Foreword

The U.S. Geological Survey (USGS) is committed to providing the Nation with reliable scientific 

information that helps to enhance and protect the overall quality of life and that facilitates 

effective management of water, biological, energy, and mineral resources (http://www.usgs.

gov/). Information on the Nation’s water resources is critical to ensuring long-term availability 

of water that is safe for drinking and recreation and is suitable for industry, irrigation, and fish 

and wildlife. Population growth and increasing demands for water make the availability of that 

water, measured in terms of quantity and quality, even more essential to the long-term sustainability of our communities and ecosystems.

The USGS implemented the National Water-Quality Assessment (NAWQA) Program in 1991 

to support national, regional, State, and local information needs and decisions related to 

water-quality management and policy (http://water.usgs.gov/nawqa). The NAWQA Program is 

designed to answer: What is the quality of our Nation’s streams and ground water? How are 

conditions changing over time? How do natural features and human activities affect the quality of streams and ground water, and where are those effects most pronounced? By combining 

information on water chemistry, physical characteristics, stream habitat, and aquatic life, the 

NAWQA Program aims to provide science-based insights for current and emerging water issues 

and priorities. From 1991 to 2001, the NAWQA Program completed interdisciplinary assessments 

and established a baseline understanding of water-quality conditions in 51 of the Nation’s river 

basins and aquifers, referred to as Study Units (http://water.usgs.gov/nawqa/studyu.html). 

In the second decade of the Program (2001–2012), a major focus is on regional assessments 

of water-quality conditions and trends. These regional assessments are based on major river 

basins and principal aquifers, which encompass larger regions of the country than the Study 

Units. Regional assessments extend the findings in the Study Units by filling critical gaps in 

characterizing the quality of surface water and ground water, and by determining water-quality 

status and trends at sites that have been consistently monitored for more than a decade. In 

addition, the regional assessments continue to build an understanding of how natural features 

and human activities affect water quality. Many of the regional assessments employ modeling 

and other scientific tools, developed on the basis of data collected at individual sites, to help 

extend knowledge of water quality to unmonitored, yet comparable areas within the regions. 

The models thereby enhance the value of our existing data and our understanding of the hydrologic system. In addition, the models are useful in evaluating various resource-management 

scenarios and in predicting how our actions, such as reducing or managing nonpoint and point 

sources of contamination, land conversion, and altering flow and (or) pumping regimes, are likely 

to affect water conditions within a region.

Other activities planned during the second decade include continuing national syntheses of 

information on pesticides, volatile organic compounds (VOCs), nutrients, trace elements, and 

aquatic ecology; and continuing national topical studies on the fate of agricultural chemicals, 

effects of urbanization on stream ecosystems, bioaccumulation of mercury in stream ecosystems, effects of nutrient enrichment on stream ecosystems, and transport of contaminants 

to public-supply wells.
iv

The USGS aims to disseminate credible, timely, and relevant science information to address 

practical and effective water-resource management and strategies that protect and restore 

water quality. We hope this NAWQA publication will provide you with insights and information 

to meet your needs, and will foster increased citizen awareness and involvement in the 

protection and restoration of our Nation’s waters. 

The USGS recognizes that a national assessment by a single program cannot address all 

water-resource issues of interest. External coordination at all levels is critical for cost-effective 

management, regulation, and conservation of our Nation’s water resources. The NAWQA 

Program, therefore, depends on advice and information from other agencies—Federal, State, 

regional, interstate, Tribal, and local—as well as nongovernmental organizations, industry, 

academia, and other stakeholder groups. Your assistance and suggestions are greatly 

appreciated.

 Matthew C. Larsen

Associate Director for Water
v

Contents

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

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

Description of Study Area ............................................................................................................................4

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

Groundwater Networks .......................................................................................................................7

Water Use ..............................................................................................................................................8

Land Use.................................................................................................................................................8

Methods.........................................................................................................................................................14

Data Compilation and Quality-Control Data ...................................................................................14

Ancillary Data......................................................................................................................................15

Water-Quality Standards and Human Health Benchmarks.........................................................15

Quality of Shallow Groundwater in the Mississippi Embayment-Texas Coastal Uplands 

and Mississippi River Valley Alluvial Aquifer Systems ............................................................16

Inorganic Constituents and Physical Properties...........................................................................16

Nutrients...............................................................................................................................................21

Pesticides.............................................................................................................................................28

Volatile Organic Compounds.............................................................................................................32

The Quality of Drinking Water in the Mississippi Embayment-Texas Coastal Uplands 

Aquifer System................................................................................................................................34

Inorganic Constituents.......................................................................................................................34

Nutrients...............................................................................................................................................34

Pesticides and Volatile Organic Compounds .................................................................................35

Vulnerability of Drinking-Water Aquifers to Shallow Groundwater Leakage—

An Example.............................................................................................................................38

Summary........................................................................................................................................................41

References....................................................................................................................................................43

Appendix 1. Detection frequencies of volatile organic compounds, 1994–2004, in the 

Mississippi embayment-Texas coastal uplands aquifer system and 

Mississippi River Valley alluvial aquifer with and without the 0.2 microgram 

per liter assessment level.................................................................................................47

Appendix 2. Water-quality standards and human-health benchmarks for selected 

constituents analyzed for in the Mississippi embayment-Texas coastal uplands 

aquifer system and Mississippi River Valley alluvial aquifer, 1994–2004..................49

Figures

1. Map showing locations of National Water-Quality Assessment Program study 

units, the Mississippi embayment-Texas coastal uplands aquifer system, and 

the Mississippi River Valley alluvial aquifer, south-central United States..........................3

2. Stratigraphic column of the Mississippi embayment-Texas coastal uplands 

aquifer system...............................................................................................................................5

3. Map showing hydrogeologic units and locations of wells sampled from 1994 to 

2004 in the Mississippi embayment-Texas coastal uplands aquifer system and 

Mississippi River Valley alluvial aquifer....................................................................................6
vi

4. Idealized hydrogeologic sections of the study area: (A–A’) west to east 

in Arkansas and Mississippi; (B–B’) north to south in Arkansas and Louisiana; 

and (C–C’) north to south in southwestern Texas ...................................................................9

5. Maps showing total groundwater withdrawals for public supply and domestic 

use and irrigation use by county from the Mississippi embayment-Texas coastal 

uplands aquifer system and Mississippi River Valley alluvial aquifer, 

south-central United States, 2000............................................................................................10

6. Map showing land use and location of wells sampled in the Mississippi embaymentTexas coastal uplands aquifer system and Mississippi River Valley alluvial aquifer, 

south-central United States......................................................................................................12

7. Box plots showing concentrations of major inorganic constituents in samples from 

four shallow well groups in the Mississippi embayment-Texas coastal uplands 

aquifer system and Mississippi River Valley alluvial aquifer..............................................17

8. Box plots showing tritium concentrations in samples from four shallow well 

groups in the Mississippi embayment-Texas coastal uplands aquifer system and 

Mississippi River Valley alluvial aquifer and graph showing decay-corrected tritium 

concentrations in precipitation ................................................................................................18

9. Graphs showing total dissolved solids and tritium concentrations in samples from 

four shallow well groups in the Mississippi embayment-Texas coastal uplands 

aquifer system and Mississippi River Valley alluvial aquifer..............................................19

10. Box plots showing concentrations of redox-sensitive constituents in samples 

from four shallow well groups in the Mississippi embayment-Texas coastal 

uplands aquifer system and Mississippi River Valley alluvial aquifer...............................20

11. Pie charts showing percentage of samples that were classified as oxic, suboxic, 

anoxic, or mixed redox state in four shallow well groups in the Mississippi 

embayment-Texas coastal uplands aquifer system and Mississippi River Valley 

alluvial aquifer.............................................................................................................................21

12. Box plots showing differences in well depth, depth to water, percentage of soils 

in hydrologic group B, percentage of soils in hydrologic group D, and average clay 

content in four shallow well groups in the Mississippi embayment-Texas 

coastal uplands aquifer system and Mississippi River Valley alluvial aquifer.................22

13. Graph showing relation between dissolved iron and nitrate concentrations in 

samples from four shallow well groups in the Mississippi embayment-Texas 

coastal uplands aquifer system and Mississippi River Valley alluvial aquifer.................24

14. Graph showing relation between the ratio of ammonia and nitrate to total nitrogen 

in samples from four shallow well groups in the Mississippi embayment-Texas 

coastal uplands aquifer system and Mississippi River Valley alluvial aquifer.................25

15. Box plots showing concentrations of dissolved phosphorus in samples from four 

shallow well groups in the Mississippi embayment-Texas coastal uplands aquifer 

system and Mississippi River Valley alluvial aquifer............................................................25

16. Graphs showing relation of agricultural land use in buffer areas surrounding 

wells to dissolved phosphorus concentrations in water from wells completed 

in the Holocene alluvium and Pleistocene valley trains ......................................................26

17. Box plots showing dissolved phosphorus concentrations by streamflow quartile 

for samples collected from the Bogue Phalia near Leland, Mississippi (1995–2008), 

and the Tensas River at Tendal, Louisiana (1996–2000)........................................................27

18. Map showing number of pesticides and volatile organic compounds detected in 

shallow wells in the Mississippi embayment-Texas coastal uplands aquifer system 

and Mississippi River Valley alluvial aquifer..........................................................................30
vii

19. Graphs showing median estimated agricultural use (1997) of detected pesticides 

in buffer areas and number of detections in samples from wells in the Holocene 

alluvium, Pleistocene valley trains, Pleistocene terrace deposits, and shallow 

Tertiary wells in the Mississippi embayment-Texas coastal uplands aquifer system 

and Mississippi River Valley alluvial aquifer..........................................................................31

20. Box plots showing distribution of soil hydrologic group and average organic matter 

content for samples with and without pesticide detections from four shallow well 

groups in the Mississippi embayment-Texas coastal uplands aquifer system and 

the Mississippi River Valley alluvial aquifer...........................................................................32

21. Graph showing relation between nitrate and dissolved-oxygen concentrations in 

drinking-water wells in the Mississippi embayment-Texas coastal uplands aquifer 

system...........................................................................................................................................35

22. Map showing dissolved phosphorus concentrations in samples from 55 drinkingwater wells in the Mississippi embayment-Texas coastal uplands aquifer system.......36

23. Graphs showing relation between dissolved phosphorus and total dissolved solids 

and fluoride concentrations in drinking-water wells in the Mississippi embaymentTexas coastal uplands aquifer system....................................................................................37

24. Map showing thickness of the middle Claiborne confining unit and number of 

pesticides and volatile organic compounds detected in samples from the 

Pleistocene terrace deposits and shallow Tertiary wells in the Memphis, 

Tennessee, area..........................................................................................................................39

25. Map showing potentiometric surface of the middle Claiborne aquifer, 1995, and 

water levels for Pleistocene terrace deposits and shallow Tertiary wells, 1997.............40

Tables

1. Regional aquifers and hydrogeologic units sampled in the Mississippi embaymentTexas coastal uplands aquifer system and Mississippi River Valley alluvial aquifer .......7

2. Application rates for agricultural pesticides in the Mississippi embayment-Texas 

coastal uplands aquifer system and Mississippi River Valley alluvial aquifer, 1997 .......13

3. Volatile organic compounds detected in more than 10 percent of field blanks...............15

4. Summary of selected nutrient concentrations in samples from four shallow well 

groups in the Mississippi embayment-Texas coastal uplands aquifer system and 

Mississippi River Valley alluvial aquifer, 1994–2004..............................................................23

5. Sources of nitrogen to the buffer areas surrounding wells in four shallow well 

groups in the Mississippi embayment-Texas coastal uplands aquifer system 

and Mississippi River Valley alluvial aquifer..........................................................................24

6. Detection frequency and maximum concentration for pesticides and pesticide 

degradates detected in water from four shallow well groups in the Mississippi 

embayment-Texas coastal uplands aquifer system and Mississippi River Valley 

alluvial aquifer, 1994–2004.........................................................................................................28

7. Detection frequency and maximum concentration for volatile organic compounds 

detected in water from four shallow well groups in the Mississippi embaymentTexas coastal uplands aquifer system and Mississippi River Valley alluvial aquifer, 

1994–2004 .....................................................................................................................................33

8. Detection frequency and maximum concentrations for pesticides, pesticide 

degradates, and volatile organic compounds detected in water from 55 drinkingwater wells greater than 200 feet deep in the Mississippi embayment-Texas coastal 

uplands aquifer system, 1994–2004 .........................................................................................38
viii

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 0.004047 square kilometer (km2

)

square foot (ft2) 0.09290 square meter (m2)

square mile (mi2) 2.590 square kilometer (km2) 

Volume

gallon (gal) 0.003785 cubic meter (m3

) 

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

)

cubic foot (ft3) 0.02832 cubic meter (m3) 

Flow rate

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

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

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

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

cubic foot per day (ft3/d) 0.02832 cubic meter per day (m3/d)

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

gallon per day (gal/d) 0.003785 cubic meter per day (m3

/d)

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

/s)

Radioactivity

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

Application rate

pounds per acre per year 

[(lb/acre)/yr]

 1.121 kilograms per hectare per year

[(kg/ha)/yr]

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

°F = (1.8 × °C) + 32

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

(NGVD of 1929).

Horizontal coordinate information is referenced to the North American Datum of 1927 (NAD27). 

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

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 either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L).
ix

Abbreviations and Acronyms

CWA Clean Water Act

DO dissolved oxygen

GIS Geographic Information System

HA health advisory

HBSL health-based screening level

MCL maximum contaminant level

MEK methyl ethyl ketone

MMM multimedia mitigation

MRVA Mississippi River Valley alluvial aquifer

MTBE methyl tert-butyl ether

NAWQA National Water-Quality Assessment Program

NWQL National Water Quality Laboratory

PVC polyvinyl chloride

QC quality control

RL reporting limit

SDWA Safe Drinking Water Act

SMCL secondary maximum contaminant level

TDS total dissolved solids

TU tritium unit

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

UST underground storage tank

VOC volatile organic compound

Abstract

The Mississippi embayment-Texas coastal uplands 

aquifer system is an important source of drinking water, providing about 724 million gallons per day to about 8.9 million 

people in Texas, Louisiana, Mississippi, Arkansas, Missouri, 

Tennessee, Kentucky, Illinois, and Alabama. The Mississippi 

River Valley alluvial aquifer ranks third in the Nation for 

total withdrawals of which more than 98 percent is used for 

irrigation. From 1994 through 2004, water-quality samples 

were collected from 169 domestic, monitoring, irrigation, 

and public-supply wells in the Mississippi embayment-Texas 

coastal uplands aquifer system and the Mississippi River 

Valley alluvial aquifer in various land-use settings and of 

varying well capacities as part of the U.S. Geological Survey’s 

National Water-Quality Assessment Program. Groundwater 

samples were analyzed for physical properties and about 

200 water-quality constituents, including total dissolved 

solids, major inorganic ions, trace elements, radon, nutrients, 

dissolved organic carbon, pesticides, pesticide degradates, and 

volatile organic compounds.

The occurrence of nutrients and pesticides differed 

among four groups of the 114 shallow wells (less than or equal 

to 200 feet deep) in the study area. Tritium concentrations in 

samples from the Holocene alluvium, Pleistocene valley trains, 

and shallow Tertiary wells indicated a smaller component of 

recent groundwater than samples from the Pleistocene terrace 

deposits. Although the amount of agricultural land overlying 

the Mississippi River Valley alluvial aquifer was considerably 

greater than areas overlying parts of the shallow Tertiary and 

Pleistocene terrace deposits wells, nitrate was rarely detected 

and the number of pesticides detected was lower than other 

shallow wells. Nearly all samples from the Holocene alluvium 

and Pleistocene valley trains were anoxic, and the reducing 

conditions in these aquifers likely result in denitrification of 

nitrate. In contrast, most samples from the Pleistocene terrace 

deposits in Memphis, Tennessee, were oxic, and the maximum 

nitrate concentration measured was 6.2 milligrams per liter. 

Additionally, soils overlying the Holocene alluvium and 

Pleistocene valley trains, generally in areas near the wells, had 

lower infiltration rates and higher percentages of clay than 

soils overlying the shallow Tertiary and Pleistocene terrace 

deposits wells. Differences in these soil properties were 

associated with differences in the occurrence of pesticides. 

Pesticides were most commonly detected in samples from 

wells in the Pleistocene terrace deposits, which generally had 

the highest infiltration rates and lowest clay content.

Median dissolved phosphorus concentrations were 0.07, 

0.11, and 0.65 milligram per liter in samples from the shallow 

Tertiary, Pleistocene valley trains, and Holocene alluvium, 

respectively. The widespread occurrence of dissolved 

phosphorus at concentrations greater than 0.02 milligram per 

liter suggests either a natural source in the soils or aquifer 

sediments, or nonpoint sources such as fertilizer and animal 

waste or a combination of natural and human sources. 

Although phosphorus concentrations in samples from the 

Holocene alluvium were weakly correlated to concentrations 

of several inorganic constituents, elevated concentrations 

of phosphorus could not be attributed to a specific source. 

Phosphorus concentrations generally were highest where 

samples indicated anoxic and reducing conditions in the 

aquifers. Elevated dissolved phosphorus concentrations in 

base-flow samples from two streams in the study area suggest 

that transport of phosphorus with groundwater is a potential 

source contributing to high yields of phosphorus in the lower 

Mississippi River basin.

Water from 55 deep wells (greater than 200 feet deep) 

completed in regional aquifers of Tertiary age represent a 

sample of the principal aquifers used for drinking-water 

supply in the study area. The wells were screened in both 

confined and unconfined parts of these aquifers, and tritium 

samples collected from 11 wells did not indicate a significant 

component of recent water. The quality of the water generally 

was good, and concentrations of most inorganic constituents 

were less than water-quality criteria for drinking water. 

Relatively few pesticides or volatile organic compounds were 

detected. Secondary drinking-water standards were exceeded 

Quality of Shallow Groundwater and Drinking Water in the 

Mississippi Embayment-Texas Coastal Uplands Aquifer 

System and the Mississippi River Valley Alluvial Aquifer, 

South-Central United States, 1994–2004

By Heather L. Welch, James A. Kingsbury, Roland W. Tollett, and Ronald C. Seanor
2 Quality of Shallow Groundwater and Drinking Water in Aquifers, South-Central United States, 1994–2004

for iron, manganese, and total dissolved solids in a total of 20, 

17, and 10 wells, respectively. Fluoride was detected in one 

sample at a concentration greater than its secondary maximum 

contaminant level. Radon activities were greater than 

300 picocuries per liter in samples from 10 wells screened 

in the middle Wilcox aquifer in northwestern Louisiana. 

While not a drinking-water concern, elevated phosphorus 

concentrations also were measured in the drinking-water 

supply aquifers. A relation between phosphorus concentrations 

and dissolved solids and fluoride suggests a geologic source of 

phosphorus in these deep wells. 

Groundwater withdrawals from these regional aquifers 

have lowered water levels, which increases the potential 

of movement of shallow groundwater to deep aquifers. 

Water-level data for a few wells in the Pleistocene terrace 

deposits in the Memphis area suggest a hydraulic connection 

between this shallow aquifer and the deeper middle Claiborne 

aquifer, locally named the Memphis aquifer, that is used for 

public supply. Additionally, pesticides and volatile organic 

compounds were detected in samples from wells screened in 

shallow and deeper portions of the middle Claiborne aquifer, 

further illustrating the potential vulnerability of this source of 

drinking water.

Introduction

Groundwater is one of the Nation’s most important 

resources and is the source of drinking water for about 50 percent of the population, or about 130 million U.S. residents 

(U.S. Geological Survey, 1999). In 1991, the U.S. Geological 

Survey began full implementation of the National WaterQuality Assessment (NAWQA) Program to describe the 

status and trends in the quality of the Nation’s surface water 

and groundwater resources and to determine the natural and 

human-related factors that affect water quality (Hirsch and 

others, 1988; Gilliom and others, 1995). More than 50 major 

river basins or aquifer systems were identified for investigation as part of the first decade (1991–2001) of sampling for the 

NAWQA Program. Together, these basins and aquifer systems 

include water resources available to more than 60 percent 

of the Nation’s population and encompass about one-half of 

the land area in the conterminous United States. Knowledge 

of the quality of the Nation’s surface-water and groundwater 

resources is important for the protection of human and aquatic 

health and for the management of land and water resources 

and the conservation and regulation of those resources. 

A major focus for groundwater studies in the NAWQA 

Program during the second decade (2002–2012) of sampling 

is for regional- and national-scale assessments of groundwater 

status and trends in 19 of 62 principal aquifers identified by 

the U.S. Geological Survey Office of Groundwater (U.S. Geological Survey, 2003). A principal aquifer is a regionally 

extensive aquifer or aquifer system that has the potential of or 

currently is being used as a primary source of drinking water. 

Three quarters of the estimated withdrawals for drinking-water 

supply in 1990 came from the 19 principal aquifers. 

The Mississippi embayment and Texas coastal uplands 

regional aquifers (fig. 1) were selected as principal aquifers 

for additional study in the NAWQA Program. Water-quality 

data collected by three NAWQA study units, the AcadianPonchartrain, Mississippi embayment, and Trinity River Basin 

study units, in the Tertiary aquifers are included in this report. 

Water-quality data collected from the Mississippi River Valley 

alluvial (MRVA) aquifer and the Pleistocene terrace deposits 

near Memphis, TN, also are included in this report because 

locally both aquifers are hydraulically connected with the 

underlying Tertiary aquifers. 

In general, the NAWQA Program focuses on compounds 

with well-established analytical methods for constituents such 

as pesticides, nutrients, volatile organic compounds (VOCs), 

and trace elements that have been or are currently being used 

in agricultural and urban areas across the Nation. Analytes 

are measured at very low concentrations, many of them 10 

to 100 times lower than Federal or State standards and health 

advisories. Detection of compounds does not necessarily 

translate to risks to humans, but detection of compounds at 

low concentrations can be useful for evaluating groundwater 

trends and for identifying and assessing movement of contaminants. Detections are compared to human-health benchmarks 

to determine if a potential health concern exists and whether 

additional monitoring may be warranted. 

This report describes the quality of shallow (less than 

or equal to 200 feet [ft]) water in parts of the Mississippi 

embayment-Texas coastal uplands aquifer system and the 

MRVA aquifer. The occurrence of nutrients, selected pesticides, and VOCs in shallow groundwater from wells in two 

subunits of the MRVA aquifer, Pleistocene terrace deposits, 

and shallow parts of three aquifers in sediments of Tertiary age 

is characterized with respect to differences in hydrogeology, 

groundwater geochemistry and residence time, selected 

properties of the overlying soils, and land use. This report also 

compares the quality of the water in deeper parts of the aquifer 

systems to shallow groundwater quality because in much of 

the study area, groundwater gradients are such that water from 

shallow aquifers can move to deeper units that are used as 

drinking-water sources.

Data included in this report were collected as part of the 

NAWQA Program from 1994 to 2004. Samples were analyzed 

for major and trace inorganic constituents, nutrients, VOCs, 

and pesticides, and a subset of wells were sampled for tritium 

analysis. Inorganic constituents and tritium data were used to 

characterize groundwater geochemistry and residence time 

in the aquifer to aid in the interpretation of occurrence of 

nutrients, VOCs, and pesticides. Data were collected from 

114 shallow wells and 55 deep wells that are used primarily 

for drinking water, either domestic or public supply. The 

shallow well dataset includes several different well types, but 

largely consists of domestic, irrigation, and monitoring wells.
Introduction 

3

Figure 1. Locations of National Water-Quality Assessment (NAWQA) Program study units, the Mississippi embayment-Texas coastal uplands aquifer system, and the Mississippi River Valley alluvial aquifer, south-central United States (Miller, 1990; Trapp and Horn, 1997).

KANSAS

OKLAHOMA

0 50 100 150 MILES

0 50 100 150 KILOMETERS

Mississippi River Valley alluvial aquiferMississippi embayment aquifer systemTexas coastal uplands aquifer systemCoastal lowlands principal aquiferNAWQA study-unit boundary

EXPLANATION

Figure 1.

100 95 90 85

353025

Base map modified fromU.S. Geological Surveydigital data, 1:250,000

MISSOURI

ILLINOIS

MISSISSIPPI

GEORGIA

FLORIDA

ARKANSAS

TEXAS

ALABAMA

TENNESSEE

KENTUCKY

LOUISIANA

MEXICO

^

^

^

^

^

^ ^

^

^

^

^

Little Rock

Memphis

Jackson

New Orleans

Baton Rouge Mobile

Shreveport

Dallas

Austin

Houston

San Antonio

Mobile

River

Basin

Mississippi

Embayment

Acadian-Ponchartrain

Trinity

River

Basin

South-Central

Texas

Location of study area in the United States

Mississippi

River
4 Quality of Shallow Groundwater and Drinking Water in Aquifers, South-Central United States, 1994–2004

Description of Study Area

The study area covers approximately 197,000 square 

miles and includes parts of Alabama, Arkansas, Illinois, 

Kentucky, Louisiana, Mississippi, Missouri, Tennessee, and 

Texas (fig. 1) and includes three principal aquifers or aquifer 

systems: the Mississippi embayment aquifer system, the Texas 

coastal uplands aquifer system, and the Mississippi River 

Valley alluvial aquifer (MRVA). Geologic units of Tertiary 

age in the Mississippi embayment and Texas coastal uplands 

aquifer systems are stratigraphically equivalent and, in this 

report, these aquifer systems are referred to collectively as the 

Tertiary aquifers. A large regional aquifer, the MRVA, overlies 

most of the central part of the Mississippi embayment and 

Texas coastal uplands aquifer systems (fig. 1). The MRVA 

is about 70 to 120 miles wide and covers approximately 

33,000 square miles in parts of Arkansas, Louisiana, Mississippi, and Tennessee. Locally, shallow aquifers composed 

of sediments of Pleistocene and Holocene age overlie the 

regional aquifers. For example, in the Memphis area, terrace 

deposits of Pleistocene age contain a shallow water-table 

aquifer that overlies and in most locations is separated by a 

confining unit from the Tertiary aquifers. Hydrogeologic units 

of Cretaceous age crop out in a small area of Alabama, and, 

because no wells were sampled in these units, these aquifers 

are not discussed in this report. 

Hydrogeology 

Hydrogeologic units of Tertiary age consist of 

predominantly unconsolidated to semiconsolidated sand, 

silt, and clay and underlie the entire study area (fig. 1). In 

the western part of the study area, these units dip coastward 

(southward) beneath the coastal lowlands aquifer system 

and range in thickness from a feathered edge to more than 

6,000 ft (Renken, 1998). In the eastern part of the study area, 

the Tertiary units are thickest near the Mississippi River, 

which approximates the axis of the Mississippi embayment, a 

trough-like structure in which these sediments were deposited. 

The units thin to the west and east, upgradient from the center 

of the Mississippi embayment. 

The Tertiary aquifers consist of several regional aquifers 

and confining units, which have been described in detail by 

Lloyd and Lyke (1995), Ryder (1996), and Renken (1998). 

The regional aquifers consist of one or more local aquifers, the 

names of which may vary between States and, in some cases, 

within a State (fig. 2). The regional aquifers sampled as part of 

this study include the upper and middle Claiborne aquifer, the 

lower Claiborne-upper Wilcox aquifer, and the middle Wilcox 

aquifer (figs. 2 and 3). The lower Wilcox aquifer underlies the 

middle Wilcox aquifer; however, this unit was not sampled 

for this study. The aquifers in the regional aquifer system are 

bounded at the top by the Vicksburg-Jackson confining unit 

and at the bottom by the Midway confining unit throughout 

most of the study area (Hosman and Weiss, 1991). Regional 

confining units that separate some of the aquifers throughout 

the study area are the middle Claiborne and lower Claiborne 

confining units (figs. 2 and 3). In Texas, Louisiana, Mississippi, and southern Arkansas, clays in the lower Claiborne 

serve as a regional confining unit that is not present in 

Tennessee, Missouri, and northeastern Arkansas. 

Groundwater generally moves from outcrop areas, which 

are the primary source of recharge for these aquifers, downdip 

to areas of discharge. In the outcrop areas, groundwater 

is unconfined, and generally, the aquifers are confined 

downgradient from the recharge areas. In the southern part of 

the study area, groundwater generally moves southward from 

outcrop bands downdip to the deeper parts of the system. In 

the northern part of the study area, groundwater flow direction 

is generally eastward and westward toward the axis of the 

Mississippi embayment. Groundwater discharge is to pumping 

wells, incised rivers and streams, and as upward leakage to 

shallower aquifers (Lloyd and Lyke, 1995). In the area of 

the Sabine uplift along the Texas and Louisiana State line, 

groundwater flow paths differ from the regional flow paths; 

groundwater flow is generally radial, away from the center 

of the uplift. Additionally, faulting in the northern half of 

the Sabine uplift has further affected local groundwater flow 

(Rapp, 1996). 

The hydrogeologic units of Quaternary age are largely 

alluvial deposits of unconsolidated sand and gravel, silt, and 

clay (fig. 1). The largest deposit of Quaternary age in the 

study area is the Mississippi River alluvium, which includes 

the MRVA aquifer. The MRVA aquifer consists of a braided 

sequence of gravel and coarse sand and overlies the Tertiary 

aquifers in the east-central part of the study area. Thickness 

of the MRVA aquifer ranges from 25 to 150 ft and is more 

than 75 ft in most locations. The MRVA aquifer is divided into 

two subunits based on different lithologic characteristics: the 

Pleistocene valley trains and Holocene alluvium. As a result 

of different depositional environments, the Pleistocene valley 

trains generally contain coarser sediments than the Holocene 

alluvium. Both units have a surficial layer of silt and clay; 

however, the clay layer overlying the Holocene alluvium is 

thicker. These fine-grained sediments are an upper confining 

unit for much of the MRVA aquifer (Renken, 1998).

The sources of recharge for the MRVA aquifer include 

infiltration of precipitation through the overlying confining 

unit, downward movement of water from streams that incise 

the aquifer (the Mississippi River and smaller tributaries), and 

locally, upward flow from underlying aquifers. The contribution to recharge to the MRVA aquifer from these sources has 

changed over time as a result of withdrawals from both the 

MRVA and the underlying Tertiary aquifers. Prior to development, water levels (hydraulic head) in the Tertiary aquifers 

generally were higher than those in the MRVA aquifer (Arthur, 

2001). In areas where withdrawal from the Tertiary aquifers 

has lowered groundwater levels and a hydraulic connection 

exists, groundwater may move downward from the MRVA 

to the Tertiary aquifers. Conversely, where these aquifers are 

connected and heads are lower in the MRVA aquifer, water 
Description of Study Area 5 Figure 2. Stratigraphic column of the Mississippi embayment-Texas coastal uplands aquifer system (modified from Hosman and Weiss, 1991; Ryder, 1996; Warwick and others, 1997; Renken, 1998; MRVA, Mississippi River Valley alluvial aquifer; Fm, formation; Mbr, member). Figure 2.

Era Cenozoic

Tertiary

Eocene Upper Paleocene

Undifferentiated

Undifferentiated

Undifferentiated

Undifferentiated Undifferentiated

Dolet Hills Fm

Naborton Fm

Jackson Group Wilcox Group

System Quarternary

Series Holocene Pleistocene

South

Texas

Southeast

and

Northeast

Arkansas

South

Northeast

Mississippi

North

Central

and

SouthernLouisiana Missouri

Deposits of Oligocene and Miocene ages outcrop locally in the extreme southeastern part of the study area and are not discussed in this report.

There are no deposits of Pliocene age located in this principal aquifer.

Illinois Kentucky Tennessee Alabama

Hydrogeologic

units used

in this report

Holocene alluvium

(part of the MRVA)

(25 wells)

Pleistocene valley trains

(part of the MRVA) (29 wells)

Pleistocene terrace deposits

(24 wells)

Miocene

Oligocene

Alluvium

and Terrace

Deposits

Jackson Group

Yegua Fm

Laredo

 Fm

El Pico

Clay

Sparta Sand

Weches Fm

Queen City

Sand

Reklaw Fm

Bigford

 Fm

Cockfield Fm

Cook Mountain Fm

Sparta sand

Carrizo Sand

Cane River Fm

Memphis

Sand

Not

Present

Memphis

Sand

Sparta

Sand

Sparta

Sand

Cook Mountain Fm

Cockfield Fm

Jackson Group

Zilpha Clay

Winona

Sand

Tallahatta

Fm

Tallahatta

Fm

Sparta

Sand

Gosport

Sand

Zilpha Clay

Tallahatta

Fm

Hatchetigbee

Fm

Bashi Fm

Tuscahoma

Fm

Nanafalia

Fm

Bashi Fm

Tuscahoma Sand

Bells Landing

Marl Mbr

Greggs Landing

Marl Mbr

Nanafalia Fm

Grampian Hills

Mbr

Meridian sand

Mbr

Tallahatta

Fm

Lower

Claiborne

confining

unit

Upper Claiborne aquifer

(2 wells)

Middle Claiborne aquifer

(25 wells)

Lower Claiborneupper Wilcox aquifer

(26 wells)

Middle Wilcox aquifer

(38 wells)

Lower Wilcox aquifer Gravel Creek Sand Mbr

Midway confining unit

Middle Claiborne

confining unit

Vicksburg-Jackson

confining unit

Lisbon

Fm

Flour Island Fm Flour Island Fm Wilcox Fm

Fort Pillow

Sand

No Wilcox

deposits

of Paleocene

age

No Wilcox

deposits

of Paleocene

age

Old Breastworks Fm

Fort Pillow

Sand

Old Breastworks Fm

Midway Group
6 Quality of Shallow Groundwater and Drinking Water in Aquifers, South-Central United States, 1994–2004

Figure 3. Hydrogeologic units and locations of wells sampled from 1994 to 2004 in the Mississippi embayment-Texas coastal uplands 

aquifer system and Mississippi River Valley alluvial aquifer.

X

X

X

X

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97 92

Base map modified from

U.S. Geological Survey

digital data, 1:250,000

KANSAS

MISSOURI

TENNESSEE

KENTUCKY

ARKANSAS

A

A’

B

B’

C

C’

Memphis

37

32

OKLAHOMA

TEXAS

MISSISSIPPI

LOUISIANA

C C’

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Y

.

0 50 100 150 MILES

EXPLANATION 0 50 100 150 KILOMETERS

Hydrogeologic units

!.

!.

!

!

!

!

.

.

.

.

.

.

. .

. .

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H

H HH HH

HH

H H

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^

Little Rock

Memphis

Dallas

Shreveport Jackson

TENNESSEE

MISSISSIPPI

Location of wells near Memphis, Tennessee

Holocene alluvium

Pleistocene valley trains

Vicksburg-Jackson confining unit

Upper Claiborne aquifer

Middle Claiborne confining unit

Middle Claiborne aquifer

Lower Claiborne confining unit

Lower Claiborne-upper Wilcox aquifer

Middle Wilcox aquifer

Lower Wilcox aquifer

Trace of hydrogeologic section shown in figure 4

Wells less than or equal to 200 feet below land surface;

 color represents hydrogeologic unit in figure 2

Wells greater than 200 feet below land surface; color

 represents hydrogeologic unit in figure 2

Figure 3.
Description of Study Area 7

may move from the Tertiary aquifers into the MRVA aquifer. 

In addition to movement into deeper aquifers, groundwater 

discharge from the MRVA is to wells, streams, and rivers. 

The general direction of groundwater flow in the MRVA 

aquifer is southward, down the Mississippi River valley, and 

locally, toward cones of depression created by large volumes 

of water being pumped from wells screened in the aquifer 

(Renken, 1998).

The Pleistocene terrace deposits aquifer is a shallow 

water-table aquifer that overlies the Tertiary aquifers in parts 

of western Tennessee. The terrace deposits are predominantly 

sand and gravel of Pleistocene age, and equivalent deposits 

are present in much of the study area and locally are shallow 

water-table aquifers. In the Memphis area, this unit generally 

is between 10 and 100 ft thick and consists of sand and gravel. 

Locally, the lower part of the Pleistocene terrace deposits 

is cemented with iron oxide to form ferruginous sandstone. 

Throughout much of the Memphis area, the middle Claiborne 

confining unit separates this shallow water-table aquifer 

from the underlying middle Claiborne aquifer. Recharge to 

the Pleistocene terrace deposits aquifer is predominantly 

from the infiltration of precipitation and locally from losing 

reaches of streams where the streambed intersects the aquifer. 

Groundwater moves from recharge areas to either streams or 

seeps and, where the lower confining unit is thin or absent, to 

the underlying Tertiary aquifers (Parks, 1990). 

Groundwater Networks 

From 1994 through 2004, a total of six groundwater 

network studies were completed in the Tertiary aquifers, 

MRVA, and Pleistocene terrace deposits (table 1). Samples 

were collected from existing wells in five of these networks 

(Tertiary aquifers and MRVA) to characterize the occurrence 

and distribution of natural and anthropogenic constituents in 

major aquifers, many of which are used as sources of drinking 

Table 1. Regional aquifers and hydrogeologic units sampled in the Mississippi embayment-Texas coastal uplands aquifer system and 

Mississippi River Valley alluvial aquifer.

[LSD, land-surface datum; N, number of wells; median values are in parentheses; N/A, not applicable]

Aquifer system Regional aquifer 

sampled

Primary 

well types

Well depth,

 in feet 

Water level, 

in feet 

below LSD

Source 

Mississippi River Valley 

alluvial aquifer

Holocene alluvium

N=25 Irrigation 60 – 200(100)

8 – 46.8

(24) Gonthier, 2003

Mississippi River Valley 

alluvial aquifer

Pleistocene valley trains

N=29

Irrigation and 

public supply

25 – 180

(110)

6 – 95

(20) Gonthier, 2003

Mississippi embayment

Upper Claiborne aquifer

Middle Claiborne aquifer

Lower Claiborne-upper Wilcox aquifer

Middle Wilcox aquifer

N=30

Public supply 208 – 1,466

(675)

13.5 – 242

(55)a Gonthier, 2000

Mississippi embayment 

and Texas coastal 

uplands

Middle Wilcox aquifer

N=30 Domestic 65 – 330(210)

6.5 – 130.6

(48.2) N/A

Texas coastal uplands Lower Claiborne-upper Wilcox aquifer

N=23

Domestic and 

public supply

21 – 490

(160)

6.1 – 130.6

(28.9)b

Reutter and Dunn, 

2000

Water-table aquifer 

Mississippi 

embayment

Pleistocene terrace deposits

Middle Claiborne aquifer

N=32

Monitoring 33 – 109

(67)

8.3 – 73.1

(25.2) Gonthier, 2002

a

 Values and median are based on 19 of 30 wells. b Values and median are based on 14 of 23 wells. 
8 Quality of Shallow Groundwater and Drinking Water in Aquifers, South-Central United States, 1994–2004

water. In the Tertiary aquifers, the predominant sampled well 

types were domestic and public-supply wells. In the MRVA 

networks, irrigation wells were the principal well type; 

however, 10 public-supply wells sampled were completed 

in the Pleistocene valley trains. The well capacities varied 

considerably for these networks, with domestic wells having 

the lowest yields, typically less than 100 gallons per minute 

(gal/min) and public-supply and irrigation wells producing as 

much as 2,000 to 5,000 gal/min. Samples from a sixth network 

were collected from 2-inch observation wells screened in 

the Pleistocene terrace deposits and upper part of the middle 

Claiborne aquifer to characterize the quality of shallow 

groundwater in an urban setting. Of the 32 wells in the network, 22 were completed in the Pleistocene terrace deposits, 

8 were completed in the middle Claiborne aquifer, and 2 wells 

were completed in more recent alluvium in the flood plain of a 

stream in the Memphis, TN, area. The two wells completed in 

the local alluvium were included with the wells in the terrace 

deposits because of the well type and location, which did not 

correspond to wells completed in the MRVA. The monitoring 

wells generally produced less than 1 gal/min. 

Generalized hydrogeologic sections through parts of the 

study area show the approximate locations of wells sampled in 

the aquifers (fig. 4). Most of the wells completed in the lower 

Claiborne-upper Wilcox and middle Wilcox in Texas and 

Louisiana were sampled in areas without an overlying regional 

confining unit, and conditions can range from confined to 

unconfined in these aquifers depending on local geology; 

whereas, most wells in the upper Claiborne and middle 

Claiborne in Tennessee, Arkansas, Mississippi, and Missouri 

were below regional confining units and represented confined 

parts of these aquifers. The Pleistocene terrace deposits in the 

Memphis area represent a water-table aquifer and generally 

are unconfined, but locally, the overlying loess deposits create 

semiconfined to confined conditions. Three networks sampled 

a single regional aquifer or hydrogeologic unit, and three 

networks sampled wells completed in two or more aquifers 

(table 1). In this report, wells are grouped based on the aquifer 

in which they are completed and by depth: shallow (less than 

or equal to 200 ft deep) and deep (greater than 200 ft deep). 

Wells greater than 200 ft deep generally are representative of 

water that is used for drinking-water purposes and are classified as Tertiary drinking-water aquifers in this report. 

Water Use

About 1,327 million gallons per day (Mgal/d) of water 

were withdrawn from the Tertiary aquifers for public supply, 

irrigation, and self-supplied industrial uses in 2000 (Maupin 

and Barber, 2005). Groundwater withdrawals of 9,290 Mgal/d 

from the MRVA aquifer accounted for about 12 percent of the 

total groundwater withdrawals in the Nation and placed the 

MRVA aquifer as third largest in withdrawals from the 66 principal aquifers across the Nation (Maupin and Barber, 2005). 

Most of the water withdrawn from the Tertiary aquifers was 

used for public supply, whereas most of the water withdrawn 

from the MRVA was used for irrigation.

Groundwater represents the primary source of drinking 

water for most of the population in the study area. In 2000, 

the total groundwater withdrawals for public supply in the 

study area were about 794 Mgal/d (Maupin and Barber, 2005). 

About 90 percent (724 Mgal/d) of these withdrawals were 

from the Tertiary aquifers. Some of the largest withdrawals in 

the Tertiary aquifers are from the middle Claiborne and lower 

Claiborne-upper Wilcox aquifers. Specifically, large withdrawals for public supply occur in western Tennessee, particularly 

the Memphis area, and parts of northwestern Mississippi, 

north-central Louisiana, and southern and eastern Arkansas 

(fig. 5A). In rural areas, both Tertiary aquifers and alluvial 

aquifers are used for domestic supply. Domestic supply use 

is relatively evenly distributed throughout the study area, and 

withdrawals are relatively small (about 80 Mgal/d) compared 

to public-supply use. 

In 2000, about 9,533 Mgal/d were withdrawn from the 

Tertiary aquifers and MRVA aquifer for irrigation (Maupin 

and Barber, 2005). In contrast to public-supply use, about 

96 percent (9,150 Mgal/d) of the irrigation withdrawals were 

from the MRVA aquifer, most of which was in Arkansas and 

Missouri and counties and parishes that lie along the Mississippi River in Mississippi and Louisiana (fig. 5B). 

Withdrawals of groundwater from both the Tertiary 

aquifers and the MRVA aquifer have lowered water levels 

in these regional aquifers, particularly near pumping centers 

(Criner and Parks, 1976; Edds and Fitzpatrick, 1984; Renken, 

1998). Prior to these withdrawals, groundwater levels in parts 

of the Tertiary aquifers were higher than in the overlying 

MRVA aquifer, and groundwater moved upward into the 

MRVA. Locally, areas of high chloride concentration and 

elevated dissolved solids concentrations in the MRVA aquifer 

in parts of Arkansas, Louisiana, and Mississippi have been 

interpreted to be the result of movement into the MRVA of 

more mineralized groundwater from deep aquifers as leakage 

through the lower confining unit, along faults, or through 

abandoned oil and gas wells (Morris and Bush, 1986). Large 

groundwater withdrawals from the middle Claiborne aquifer in 

the Memphis area, northern Louisiana, and southern Arkansas 

have lowered groundwater levels as much as 100 ft and 

induced downward leakage from the MRVA to the Tertiary 

aquifers (Renken, 1998) throughout the study area. 

Land Use

Agriculture accounts for 41 percent of the land use in the 

study area with much of the use occurring in areas along the 

Mississippi River (fig. 6). Sixty percent of the area overlying 

the MRVA is agricultural land used mostly to cultivate soybeans, cotton, corn, and rice (Coupe, 2000). These row crops 

may require large amounts of pesticides to control grasses, 

broadleaf weeds, and insects; however, the intensity of pesticide use differs. Propanil, a herbicide used to control weeds 
Description of Study Area 9

Figure 4. Idealized hydrogeologic sections of the study area: (A–A’) west to east in Arkansas and Mississippi; (B–B’) north to south 

in Arkansas and Louisiana; and (C–C’) north to south in southwestern Texas. Wells projected onto section represent generalized 

National Water-Quality Assessment Program sampling zones (black, less than or equal to 200 feet below land surface; white, greater 

than200 feet below land surface); key aquifers and formations are identified in parentheses. The lines of sections are shown in figure 3.

500

0

–500

–1,000

–1,500

–2,000

–2,500

500

0

–500

–1,000

–1,500

–2,000

500

0

–500

–1,000

Pleistocene

valley 

trains

Holocene

alluvium

Middle Claiborne confining unit

Middle Claiborne

Upper Claiborne

Pleistocene

terrace deposits

ARKANSAS

MISSISSIPPI

A A’

Middle Claiborne

Locally significant

lignite deposits

Lower Claiborne confining unit 

Lower Claiborne - Upper Wilcox 

B B’

Lower Claiborne

confining unit

Rocks of

Cretaceous

age

Vicksburg-Jackson

confining unit

Upper Claiborne 

Middle Claiborne confining unit

Middle Claiborne

Lower Claiborne - Upper Wilcox 

Wilcox (undifferentiated)

Midway Group

Coastal lowlands

principal aquifer system

C C’

Lower Claiborne - Upper Wilcox

Middle Wilcox

Lower Wilcox

Wilcox

(undifferentiated)

Midway Group

Sabine Uplift

Bull Bayou

Dome

Vertical scale greatly exaggerated Modified from Renken (1998)

Vertical scale greatly exaggerated Modified from Page and May (1964), Payne (1975), Snider (1982), Rapp (1996)

Vertical scale greatly exaggerated Modified from Ryder (1996)

Midway Group

Rocks of

Paleozoic

age

Rocks of

Cretaceous

age

ARKANSAS

LOUISIANA

ALTITUDE, IN FEET ABOVE OR BELOW NGVD OF 1929

Figure 4.
10 Quality of Shallow Groundwater and Drinking Water in Aquifers, South-Central United States, 1994–2004

Figure 5A. Total groundwater withdrawals for public supply and domestic use by county from the Mississippi embayment-Texas 

coastal uplands aquifer system and Mississippi River Valley alluvial aquifer, south-central United States, 2000.

!

!

!

!

!

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X

^

Little Rock

Memphis

Dallas

Jackson

TENNESSEE

MISSISSIPPI

Location of wells near Memphis, Tennessee

Groundwater withdrawals, in million gallons per day,

in 2000, for public supply and domestic water use

Tertiary aquifers boundary

Wells with depths less than or equal to 200 feet below land surface

Wells with depths greater than 200 feet below land surface

Domestic wells

Public-supply wells

Other wells

Figure 5a.

97 92

Base map modified from

U.S. Geological Survey

digital data, 1:250,000

MISSOURI

TENNESSEE

KENTUCKY

ARKANSAS Memphis

37

32

OKLAHOMA

TEXAS

MISSISSIPPI

LOUISIANA

Shreveport

^

0 50 100 150 MILES

0 50 100 150 KILOMETERS

EXPLANATION

^ ^

^

0 – 5

5 – 10

10 – 50

50 – 209
Description of Study Area 11

Figure 5B. Total groundwater withdrawals for irrigation use by county from the Mississippi embayment-Texas coastal uplands 

aquifer system and Mississippi River Valley alluvial aquifer, south-central United States, 2000.

!

!

!

!

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!

!

!

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! X X

X X

X

X

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Tertiary aquifers boundary

Wells with depths less than or equal to 200 feet below land surface

Wells with depths greater than 200 feet below land surface

Domestic wells

Public-supply wells

Other wells

!

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!!!!

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X

X

^

Little Rock

Memphis

Dallas

Jackson

TENNESSEE

MISSISSIPPI

Location of wells near Memphis, Tennessee

Groundwater withdrawals, in million gallons per day,

in 2000, for irrigation water use

Figure 5b.

97 92

Base map modified from

U.S. Geological Survey

digital data, 1:250,000

MISSOURI

TENNESSEE

KENTUCKY

ARKANSAS Memphis

37

32

OKLAHOMA

TEXAS

MISSISSIPPI

LOUISIANA

Shreveport

^

0 50 100 150 MILES

0 50 100 150 KILOMETERS

EXPLANATION

^ ^

^

0 – 5

5 – 10

10 – 50 209 – 451

50 – 209
12 Quality of Shallow Groundwater and Drinking Water in Aquifers, South-Central United States, 1994–2004

Figure 6. Land use and location of wells sampled in the Mississippi embayment-Texas coastal uplands aquifer system and 

Mississippi River Valley alluvial aquifer, south-central United States (land-use data from Vogelmann and others, 1998a, 1998b).

X X

X X

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X X

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XX

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^

Little Rock

Memphis

Dallas

Jackson

TENNESSEE

MISSISSIPPI

Location of wells near Memphis, Tennessee

Land-use category and percentage of area

Barren (1%)

Grasslands (2%)

Urban (2%)

Lakes and rivers (3%)

Shrubland (5%)

Wetlands and coastal marshes (9%)

Forest (37%)

Agriculture (41%)

Tertiary aquifers boundary

Wells with depths less than or equal to 200 feet

 below land surface

Wells with depths greater than 200 feet

 below land surface

Figure 6.

97 92

Base map modified from U.S. Geological Survey

digital data, 1:250,000

MISSOURI

TENNESSEE

KENTUCKY

ARKANSAS Memphis

37

32

OKLAHOMA

TEXAS

MISSISSIPPI

LOUISIANA

Shreveport

^

0 50 100 150 MILES

0 50 100 150 KILOMETERS

EXPLANATION

^ ^

^
Description of Study Area 13

for rice cultivation, was the most heavily applied (more than 

6,000,000 pounds) pesticide in the study area; however, its use 

was largely in rice-growing areas overlying the MRVA aquifer. 

In contrast, a lower total amount of the herbicide glyphosate 

was used, but it was applied over a much broader area than 

propanil (table 2). Other herbicides with relatively large 

amounts of use and intensity of use include atrazine, molinate, 

and fluometuron—herbicides used primarily on corn, rice, and 

cotton, respectively. Malathion, methyl parathion, and aldicarb 

are the only insecticides included in the 25 most heavily used 

pesticides in the study area. Malathion and methyl parathion 

are used predominantly on cotton but may have use on other 

crops, and aldicarb is incorporated in the soil with cotton seeds 

at planting to control nematodes. 

Forests cover large parts of northern Louisiana, east 

Texas, south-central Arkansas, and east-central Mississippi, 

totaling about 37 percent of the study area (fig. 6). Forested 

land represents about 60 percent of the land use in the outcrop 

areas of the Tertiary aquifers. In contrast, about 20 percent of 

the area overlying the MRVA aquifer is forest. 

Urban areas account for about 2 percent of the land use 

in the study area (fig. 6). In 2000, the total population of 

parishes or counties within or partially within the study area 

boundary was about 8,900,000 (U.S. Census Bureau, 2002). 

The largest metropolitan area is Memphis, TN, with a population of about 650,000 in 2000. Other major urban areas are 

Shreveport, Louisiana (population about 200,000); BryanCollege Station, Texas (population about 134,000); Tyler, 

Table 2. Application rates for agricultural pesticides in the Mississippi embayment-Texas coastal uplands aquifer system and 

Mississippi River Valley alluvial aquifer, 1997.

[H, herbicide; I, insecticide; MSMA, monosodium methanearsonate]

Pesticide 

Amount 

applied, in 

thousands 

of pounds

Acres applied, in 

thousands 

of acres

Amount 

applied 

per acre, 

in pounds

Type Principal crop or use

Propanil 6,000 1,800 3.3 H rice

Glyphosate 4,800 20,400 0.24 H soybeans and corn

Metolachlor 3,100 14,200 0.22 H soybean, peanuts, cotton

Atrazine 3,000 2,700 1.1 H corn

Methyl parathion 2,900 10,100 0.29 I rice, cotton (mites)

2,4-D 2,900 38,100 0.08 H pasture, corn, soybean, rice, cotton

Pendimethalin 2,800 14,500 0.19 H soybean, cotton, rice, corn

Trifluralin 2,400 11,900 0.20 H soybean, cotton

Malathion 2,300 8,700 0.26 I general use on cotton and rice; boll weevil control

Fluometuron 2,200 3,500 0.63 H cotton

Sodium chloratea 2,100 9,600 0.22 H defoliant on cotton and soybean

Ethephona 1,800 3,500 0.51 H cotton, rice

 MSMAa 1,800 3,500 0.51 H cotton

Tribuphos 1,700 3,500 0.49 H defoliant used for cotton crops

Cyanazine 1,500 4,700 0.32 H corn, cotton

Molinate 1,400 1,800 0.78 H rice

Bentazon 900 10,200 0.09 H soybean, corn, rice

Paraquata 780 13,800 0.06 H non-selective defoliant for total vegetation control

Norflurazon 710 3,500 0.20 H cotton

Aldicarb 700 3,500 0.20 I roundworms, aphids, spider mites (cotton)

Alachlor 680 4,500 0.15 H rice, cotton, corn, soybean

Acifluorfen 640 9,800 0.07 H soybeans, rice

Thiobencarb 640 1,800 0.36 H rice

Acetochlor 580 2,000 0.29 H corn

Prometryn 520 3,500 0.15 H cotton

a

 Pesticide not analyzed in this study.
14 Quality of Shallow Groundwater and Drinking Water in Aquifers, South-Central United States, 1994–2004

Texas (population about 84,000); Monroe-West Monroe, 

Louisiana (population about 66,000); and LufkinNacogdoches, Texas (population about 63,000). 

Methods

Data Compilation and Quality-Control Data

From 1994 through 2004, water samples were collected 

from 169 wells and were analyzed for 5 physical properties, 

total dissolved solids, major and trace inorganic constituents, 

nutrients, radon, dissolved organic carbon, 75 pesticides, 

7 pesticide degradates, and as many as 84 volatile organic 

compounds (VOCs). A subset of the 169 wells sampled 

included the analysis of tritium: 90 wells 200 ft deep or less 

and 11 wells greater than 200 ft deep were sampled. This 

report focuses on major and trace inorganic constituents, 

tritium, selected nutrients, pesticides, and VOCs. 

During the period of sample collection, some changes in 

analytical methods occurred, such as the addition or removal 

of constituents in an analytical method or, for some analytical 

methods, changes in the laboratory minimum reporting levels. 

For example, samples collected before 1998 were analyzed 

for both dissolved phosphorus and orthophosphate and 

after 1998 only for orthophosphate. In this report, data for 

orthophosphate for the network without dissolved phosphorus 

analyses were included with the dissolved phosphorus data 

for the remaining networks. The analytical method for VOCs 

had a minimum laboratory reporting level of 0.2 microgram 

per liter (μg/L) for most compounds analyzed prior to 1996 

(Moran and others, 2006). Samples collected subsequently 

were analyzed with lower detection and reporting levels; 

however, in this report, detection frequencies of VOCs are 

reported at a common assessment level of 0.2 μg/L. Detection 

frequencies determined with and without an assessment level 

are summarized in Appendix 1.

Concentrations of many pesticides and VOCs were 

estimated (“E”) by the USGS National Water Quality Laboratory (NWQL). Childress and others (1999) documented 

several reasons that justify an “E” code: (1) the concentration 

was greater than or equal to the long-term method detection 

level but less than the laboratory reporting limit; (2) the 

concentration was greater than or equal to the long-term 

method detection level but less than the lowest calibration 

standard; (3) the result was extrapolated above the calibration 

curve; (4) data quantification was not performed according to 

method-specific criteria; (5) performance of the analyte did 

not meet acceptable method-specific criteria; (6) deviation 

from the standard operating procedure was required; (7) some 

moderate losses occurred in sample preparation but were not 

quantifiable; and (8) moderate matrix interference conditions 

occurred. Values coded with a remark code of “E” were treated 

as detections in this report. 

Quality-control (QC) data were collected to ensure that 

sample-collection, sample-processing, and laboratory-analysis 

procedures did not introduce bias into results and to determine 

the variability associated with collection and analysis of 

samples. QC samples collected included field-equipment 

blanks, replicate samples, and field-spiked samples. Replicate 

environmental samples were collected to assess the effects 

of sample collection and laboratory analysis on measurement 

variability. The spike samples were environmental samples to 

which known concentrations of the analytes of interest were 

added to determine the accuracy and precision of organic 

analyses, the stability of analytes during typical holding times, 

and whether characteristics of the environmental sample might 

interfere with the analysis. Previous reports (Gonthier, 2000, 

2002, and 2003; Reutter and Dunn, 2000; Roland W. Tollett, 

U.S. Geological Survey, written commun., 2005) summarizing 

results for specific networks included in this report indicated 

that recoveries were within expected ranges (60 to 140 percent) and that the results were reproducible. 

Field-equipment blank samples were collected to verify 

that decontamination procedures were sufficient and that 

collection and analysis procedures did not contaminate the 

samples. Several compounds that were present in one field 

blank also were detected in a water sample collected on 

the same day and in a sample collected the previous day. 

Results for these compounds in the samples and field blanks 

were interpreted to represent contamination of the sampling 

equipment or during sample shipment, and results for these 

compounds were removed from the dataset. Ten VOC compounds were detected in 10 percent or more of the field blanks 

(table 3). Field blanks collected prior to 1997 may have had 

detection of a few compounds because of their presence in the 

organic-free water used for blanks (Halde and others, 1998). 

Organic-free water used after July 1997 was purged by the 

laboratory supplier with nitrogen gas to remove any low-level 

VOCs that might be present in the blank water. Many of the 

detections were in field blanks collected using the unpurged 

blank water; however, toluene, 1,2,4-trimethylbenzene, and 

methyl ethyl ketone (MEK) were detected in field blanks that 

used nitrogen-purged blank water. Additionally, many of the 

detections in field blanks were less than the 0.2 μg/L assessment level used in this report, and the potential bias from 

sample contamination was considered to be low. Methyl ethyl 

ketone was detected in two field blanks and eight samples at 

concentrations between 0.5 and 1.6 μg/L. The similar range 

in concentration in blanks and environmental samples, lack 

of other VOC or pesticide detections, and no detections of 

MEK in subsequent samples (2002 and 2005) from one of 

these wells suggested MEK results were affected by sample 

contamination, and these data were not included in this report. 

Malathion was the only pesticide detected in a field 

blank, but several agricultural pesticides were detected in a 

groundwater sample collected in 1997 in an urban area. One of 

the pesticides detected was acetochlor, which was introduced 

for use in 1994 primarily as a herbicide for corn. Tritium was 
Methods 15

not detected in this sample, suggesting that most of the water 

was older than the early 1950s. These results combined with 

results for a second sample collected during 2006 without 

detections of acetochlor and several other agricultural pesticides suggest that the data for the original sample may have 

been switched with a surface-water sample from another area, 

either physically in the laboratory or electronically with data 

entry. The data for 1997 for this sample were not included in 

the dataset; however, the resample from 2006 was used. 

Ancillary Data

The occurrence of many of the constituents analyzed 

is related to the presence of a source in the contributing 

area of a well, the physical and chemical properties of the 

constituent, and environmental factors such as soil properties 

in the contributing area of a well. Contributing areas for 

wells, however, are rarely delineated. An approach used by 

the NAWQA Program to relate occurrence of compounds to 

environmental factors, particularly for shallow observation 

wells, has been to characterize land use and other data, such 

as soil properties, within a buffer area with a 1,640-ft radius 

around each well. This approach for characterizing potential 

sources of contaminants and factors that may affect their 

movement likely is less applicable for deep or high-capacity 

wells, because the contributing (recharge) areas could be very 

different from the 1,640 ft around the well.

Ancillary data used in this report for buffer areas around 

wells include land use, soil properties, nitrogen loading, 

and pesticide use. Land use was estimated using a 30-meter 

(98-ft) resolution digital dataset based on the 1992 National 

Land Cover Data (Vogelmann and others, 1998a, 1998b) with 

selected classifications from the USGS aerial photographbased Land Use Land Cover Data of the mid-1970s to 

mid-1980s. A 1,640-ft radius circular buffer was delineated 

around each site, and the fraction of the 

total buffer represented by each land-use 

category was calculated (U.S. Geological 

Survey, 2008). Soil organic matter, 

average clay content, and soil hydrologic 

group data were compiled for buffer 

areas from State Soil Geographic Data 

(U.S. Department of Agriculture, 1995). 

Nitrogen input data for well 

buffers were prepared (Kerie J. Hitt, 

U.S. Geological Survey, written commun., 2005) from county-level estimates 

of nutrient inputs (Ruddy and others, 

2006). Fertilizer and manure inputs 

for each buffer area were based on the 

amount of agricultural land from the 

Enhanced National Land Use Data 1992 

and fertilizer and manure application 

rates (Nakagaki and others, 2007). 

Nitrogen from atmospheric deposition 

was assumed to be independent of land 

use. A weighted average for the buffer area for nitrogen from 

fertilizer, manure, and atmospheric deposition was calculated 

for each well for each year of compilation (1987 through 

2001), and data were used from the year that corresponded to 

the year the well was sampled.

County-wide agricultural pesticide use data were used 

to create a use estimate in the buffer area for 1992 and 1997 

(Majewski, 1997). Use estimates for more than 200 compounds were derived by applying county areal weights of 

agricultural land use in the buffer area and prorating county 

pesticide use to determine the estimate of chemical use in the 

buffer, in kilograms, geoprocessed at the 30-meter resolution 

(Majewski, 1997). For wells sampled in 1994, the 1992 data 

were used, but for all other wells, the 1997 data were used to 

interpret water-quality data. 

Water-Quality Standards and Human 

Health Benchmarks

In 1972, Congress passed the Clean Water Act (CWA) to 

better assess water-quality issues primarily in surface water 

(U.S. Environmental Protection Agency, 2003). Because the 

CWA did not directly address groundwater or water-quantity 

issues, the Safe Drinking Water Act (SDWA) was passed in 

1974 to regulate the Nation’s public drinking-water supply and 

its sources: rivers, lakes, reservoirs, springs, and groundwater 

wells. The SDWA gives the U.S. Environmental Protection 

Agency (USEPA) the authority to set drinking-water standards 

in the form of primary (MCLs) and secondary maximum 

contaminant levels (SMCLs). Primary drinking-water 

regulations are legally enforceable on finished water (both 

surface water and groundwater) used for public drinking-water 

supplies, and the secondary regulations are non-enforceable 

guidelines that relate mainly to cosmetic and aesthetic effects 

(U.S. Environmental Protection Agency, 2006). 

Table 3. Volatile organic compounds detected in more than 10 percent of field blanks.

[μg/L, microgram per liter; E, estimated]

Compound

Laboratory 

reporting level, 

in μg/L

Number 

of blanks 

(N)

Detection 

frequency 

in blanks, 

in percent

Maximum

concentration, 

in μg/L

1,2,4-Trimethylbenzene 0.05 15 26.7 6.8

Acetone 5.0 13 38.5 19.1

Carbon disulfide 0.05 13 15.4 E 0.027

Chlorobenzene 0.05 15 13.3 E 0.009

Dichloromethane 0.05 15 26.7 0.70

Methyl ethyl ketone 5.0 13 23.1 1.6

Tetrahydrofuran 8.79 13 15.4 E 0.7

Toluene 0.05 15 33.3 0.18

Trichlorofluoromethane 0.05 15 13.3 0.20

Trichloromethane 0.05 15 40.0 0.30
16 Quality of Shallow Groundwater and Drinking Water in Aquifers, South-Central United States, 1994–2004

For those compounds with Federal drinking-water 

standards, concentrations in samples from Tertiary aquifer 

wells were compared to USEPA MCLs, SMCLs, or health 

advisories (HAs) listed in Appendix 2 (U.S. Environmental 

Protection Agency, 2004). Some concentrations of unregulated 

compounds were compared to health-based screening levels 

(HBSLs), which were developed by the USGS in collaboration 

with the USEPA, New Jersey Department of Environmental 

Protection, and the Oregon Health & Science University 

(Appendix 2). Although HBSLs are non-enforceable regulatory standards, they are concentrations of contaminants in 

water that may be of potential human-health concern. The 

HBSLs are based on health effects alone and have been 

calculated for unregulated contaminants analyzed by the 

NAWQA Program. Screening levels were developed using 

standard USEPA Office of Water methodologies and the 

most current, USEPA peer-reviewed, publicly available 

human-health toxicity information. The HBSLs are regularly 

reviewed and, as needed, revised to incorporate the most 

recent toxicity information and research findings (Toccalino 

and others, 2003). 

Quality of Shallow Groundwater in the 

Mississippi Embayment-Texas Coastal 

Uplands and Mississippi River Valley 

Alluvial Aquifer Systems

Because of the large range in depths (21 to 1,466 ft) of 

the wells sampled in this study, a subset of 114 wells less 

than or equal to 200 ft deep was selected to characterize the 

occurrence of many of the constituents analyzed. This depth 

was used to subset the data because it included the greatest 

depth of wells completed in the MRVA; it included the majority of the wells in the dataset, and many of the constituents 

analyzed have sources related to human activities at land 

surface. In this report, shallow is used to describe wells and 

samples collected from wells 200 ft deep or shallower. Four 

groups of shallow wells were defined: (1) shallow Tertiary 

aquifer wells, (2) wells completed in the Holocene alluvium 

and (3) Pleistocene valley trains of the MRVA, and (4) wells 

completed in the Pleistocene terrace deposits (the shallow 

water-table aquifer in the Memphis, TN, area). Of the 36 wells 

in the shallow Tertiary group, 8 were completed in the middle 

Claiborne, 14 in the lower Claiborne-upper Wilcox, and 14 in 

the middle Wilcox aquifers. 

Inorganic Constituents and Physical Properties 

Concentrations of major inorganic constituents and total 

dissolved solids typically reflect the composition of the aquifer 

material and the length of time that groundwater is in contact 

with the aquifer. The composition of groundwater often 

changes with residence time and along regional flow paths. In 

addition to changes along flow paths, regional differences in 

inorganic water quality in both the Tertiary aquifers and the 

MRVA have been delineated (Pettijohn, 1996). In the updip 

and northern parts of the Tertiary aquifers, groundwater is 

predominantly a calcium or sodium bicarbonate type with 

dissolved solids concentrations less than 1,000 milligrams per 

liter (mg/L). Total dissolved solids concentrations generally 

increase in downdip areas and in the southern part of the 

study area where chloride becomes the dominant anion and 

sodium concentrations increase (Pettijohn, 1996). Some of the 

changes in water quality in downgradient areas are a result of 

groundwater evolution along regional flow paths, but upward 

leakage from deeper aquifers also may affect water quality in 

these aquifers (Pettijohn, 1996). 

Groundwater in the two subunits of the MRVA aquifer 

generally is more mineralized than groundwater in the 

Pleistocene terrace deposits and shallow Tertiary wells. The 

median dissolved solids concentrations for samples from the 

Holocene alluvium and Pleistocene valley trains were about 

350 and 325 mg/L, respectively (fig. 7). The dominant ions 

in these aquifers were calcium, magnesium, and bicarbonate. 

The dissolved solids concentrations for samples from the 

Pleistocene terrace deposits and Tertiary wells were somewhat 

lower (170 and 200 mg/L, respectively); however, the shallow 

Tertiary wells had the greatest variability of all four shallow 

well groups (fig. 7). The large variability for dissolved solids, 

bicarbonate, sodium, and chloride concentrations observed in 

the Tertiary wells may be a result of wells being completed in 

three different Tertiary aquifers, as well as the wide geographic distribution of wells. In general, samples from Tertiary 

wells screened in the middle Wilcox aquifer in the southern 

part of the study area (located in Louisiana and Texas) had 

higher dissolved solids concentrations than other parts of the 

Tertiary aquifers. 

 Most of the shallow wells were sampled for tritium, 

which can be used as a qualitative indicator of groundwater 

age or residence time. The presence of tritium in groundwater 

indicates that a component of the water includes recent 

recharge; that is, recharge after the initial atmospheric tritium 

bomb peak in 1953. The absence of tritium in a sample 

indicates that most of the water was recharged prior to 1953 

(fig. 8) or that the contribution of recent recharge represents a 

small fraction of the sample. 

Of the 90 samples from shallow wells analyzed for 

tritium, 73 percent had tritium concentrations greater than 

1 tritium unit (TU) indicating a component of recent recharge; 

however, tritium concentrations were less than 1 TU in 

samples from at least one well from each group (fig. 8A). 

Based on decay-corrected concentrations for precipitation in 

the study area (Michel, 1989), samples with tritium concentrations between 3 and 10 TUs likely indicate a large component 

of recharge to wells after about 1975 (fig. 8B). Using this 

range in concentration as a qualitative indicator of average 

groundwater residence time, groundwater from the Pleistocene 

terrace deposits generally has the largest component of recent 
Quality of Shallow Groundwater in Mississippi Embayment-Texas Coastal Uplands and MRVA Aquifer Systems 17

Figure 7. Concentrations of major inorganic constituents in samples from four shallow well groups in the Mississippi embaymentTexas coastal uplands aquifer system and Mississippi River Valley alluvial aquifer.

Total dissolved solids Bicarbonate

1,200

800

400

0

800

600

400

200

0

50

40

30

20

10

0

Calcium

150

100

50

0

Magnesium

600

400

200

0

Sodium

300

200

100

0

Holocene

alluvium

Figure 7.

Pleistocene

valley

trains

Pleistocene

terrace

deposits

Shallow

Tertiary

wells

Chloride

Holocene

alluvium

Pleistocene

valley

trains

Pleistocene

terrace

deposits

Shallow

Tertiary

wells

CONCENTRATION, IN MILLIGRAMS PER LITER

75th percentile

Median

25th percentile

Outlier data value more than 1.5 times

 the IQR above the 75th percentile

Data value less than or equal to

 1.5 times the IQR outside the

 quartile

Data value less than or equal to

 1.5 times the IQR outside the

 quartile

EXPLANATIONInterquartile range (IQR)

25 29

NUMBER OF SAMPLES

24 36 25 29

NUMBER OF SAMPLES

24 36
18 Quality of Shallow Groundwater and Drinking Water in Aquifers, South-Central United States, 1994–2004

recharge in samples among the four shallow groups with more 

than half of the groundwater in the wells having an apparent 

age less than about 20 years. Tritium concentrations were 

substantially higher in the Pleistocene valley trains than in 

the Holocene alluvium (Gonthier, 2003), suggesting younger 

water in this aquifer than in the Holocene alluvium (fig. 8A). 

Water samples from 10 of the 25 wells in the Holocene 

alluvium had tritium concentrations less than 1 TU. Median 

concentrations of both the Holocene alluvium and shallow 

Tertiary wells were less than 3 TUs, indicating a relatively 

small component of recharge in the last 30 years. Eight of 

the 12 samples from the shallow Tertiary wells completed in 

the middle Claiborne and middle Wilcox aquifers had tritium 

concentrations less than 1 TU, indicating little to no recent 

recharge to these wells. The four samples containing tritium 

in the Tertiary group also were from wells completed in the 

middle Claiborne and middle Wilcox aquifers. In contrast, 

median tritium concentrations greater than 3 TU suggest 

that most of the samples from wells completed in the 

Pleistocene valley trains and Pleistocene terrace deposits 

typically included a substantial amount of water younger 

than 30 years. 

Figure 8. (A) Tritium concentrations in samples from four shallow well groups in the Mississippi embayment-Texas coastal uplands 

aquifer system and Mississippi River Valley alluvial aquifer and Figure 8. (B) decay-corrected tritium concentrations in precipitation.

A

B

16

12

8

4

0

Holocene

alluvium

1955

Estimated tritium in

 precipitation

Decay corrected to

 1997 (Michel, 1989)

1960 1965 1970 1975

YEAR

1980 1985 1990

Pleistocene

valley

trains

Pleistocene

terrace

deposits

Shallow

Tertiary

wells

25 29

NUMBER OF SAMPLES

TRITIUM CONCENTRATION, IN TRITIUM UNITS

10,000

1,000

100

10

1

0

24 12

75th percentile

Median

25th percentile

Outlier data value more than 1.5 times

 the IQR above the 75th percentile

Data value less than or equal to

 1.5 times the IQR outside the

 quartile

Data value less than or equal to

 1.5 times the IQR outside the

 quartile

Note: Shaded areas include tritium

 concentrations between 3 and 

 10 tritium units

EXPLANATIONInterquartile range (IQR)
Quality of Shallow Groundwater in Mississippi Embayment-Texas Coastal Uplands and MRVA Aquifer Systems 19

Tritium concentrations were inversely related to the 

concentration of some inorganic constituents in two of the 

shallow aquifers (fig. 9). In samples from both the Pleistocene 

valley trains and shallow Tertiary wells, total dissolved solids 

concentrations were highest when tritium concentrations were 

low, which likely reflects an increase in the concentration of 

inorganic constituents in groundwater as the residence time 

increases in these aquifers. This relation did not exist for 

wells completed in the Holocene alluvium and Pleistocene 

terrace deposits. The lack of a relation between the inorganic 

water quality and apparent groundwater age in these aquifers 

may reflect greater heterogeneity of the sediments that make 

up these aquifers, or samples may represent more complex 

mixtures of groundwater with varying residence times.

In addition to water-rock interactions along a groundwater flow path, microbial activity, primarily microbial 

respiration, also affects groundwater geochemistry. Concentrations of dissolved oxygen, nitrate, manganese, iron, and sulfate 

are affected by the oxidation-reduction (redox) conditions 

in the aquifer. Dissolved oxygen (DO) that enters an aquifer 

with recharge plays an important role in redox conditions 

by influencing the solubility of redox-sensitive constituents. 

As water moves along a flow path, DO can be consumed by 

subsurface microorganisms that use available organic carbon 

in the aquifer matrix or in groundwater. Generally, DO values 

less than 0.5 mg/L in groundwater indicate anoxic or reducing 

conditions in an aquifer (McMahon and Chapelle, 2008). As 

DO is consumed in an aquifer, redox conditions typically 

progress sequentially from an oxygen-reducing environment 

(oxic) to nitrate-, manganese-, iron-, sulfate-reducing conditions (anoxic), and finally to methanogenic conditions under 

which organic carbon is reduced to form methane (Chapelle, 

1993) given the presence of each of these electron acceptors 

in the aquifer. The presence or absence of DO, nitrate, iron, 

manganese, and sulfate in groundwater can be used to characterize the redox conditions in the aquifer (fig. 10). 

Dissolved-oxygen concentrations varied considerably 

among the four aquifers, ranging from predominantly oxic to 

anoxic conditions (fig. 10). Groundwater in the Pleistocene 

terrace deposits generally is oxic (DO >0.5 mg/L) with a 

Figure 9. Total dissolved solids and tritium concentrations in samples from four shallow well groups in the Mississippi embaymentTexas coastal uplands aquifer system and Mississippi River Valley alluvial aquifer.

0 200 400 600 800 1,000 1,200 0 200 400 600 800 1,000 1,200

TOTAL DISSOLVED SOLIDS, IN MILLIGRAMS PER LITER

Figure 9.

16

12

8

4

0

16

12

8

4

0

Holocene alluvium Pleistocene valley trains

Pleistocene terrace deposits Shallow Tertiary wells

TRITIUM CONCENTRATION, IN TRITIUM UNITS
20 Quality of Shallow Groundwater and Drinking Water in Aquifers, South-Central United States, 1994–2004

median DO concentration of about 3 mg/L. Median DO 

concentrations for wells in the shallow Tertiary, Pleistocene 

valley trains, and Holocene alluvium were all less than 

0.5 mg/L, indicating more than half of the groundwater 

samples from these aquifers were anoxic. However, a number 

of shallow Tertiary wells had samples that were classified as 

oxic, with a maximum DO concentration of 5.8 mg/L. Nearly 

all of the wells sampled in the MRVA (Holocene alluvium 

and Pleistocene valley trains) were anoxic. Only one well 

completed in the Holocene alluvium was classified as oxic, 

with the maximum DO concentration of 0.7 mg/L, and three 

wells in the Pleistocene valley trains had DO concentrations 

greater than 0.5 mg/L (fig. 10). A relation between well depth 

and water level to DO concentrations did not exist, in part 

because of the narrow range of these characteristics for each of 

the aquifers (table 1).

The overall redox conditions differ among the shallow 

well groups (fig. 11). Using redox categories defined by 

McMahon and Chapelle (2008), concentrations of DO, 

nitrate, manganese, iron, and sulfate were used to classify the 

parts of the aquifers sampled by the wells as oxic, suboxic, 

anoxic, or mixed redox conditions. In anoxic aquifers, 

nitrate, manganese, iron, and sulfate are the terminal electron 

acceptors, rather than oxygen (McMahon and Chapelle, 

2008). Wells completed in the Holocene alluvium indicated 

the most reducing conditions, with all but one well being 

anoxic (fig. 11). Dissolved manganese and iron concentrations, in addition to the generally low sulfate concentrations 

when iron concentrations are high, suggest iron-reducing to 

sulfate-reducing redox conditions in this aquifer. Four samples 

indicated methanogenic conditions in parts of the Holocene 

alluvium. Redox conditions in the Pleistocene valley trains 

generally are similar to the Holocene alluvium in that most 

wells sampled anoxic groundwater, but lower iron and higher 

sulfate concentrations suggest conditions in this aquifer are not 

as reducing as in the Holocene alluvium. Redox conditions are 

more variable in the Pleistocene terrace deposits and Tertiary 

wells than in the MRVA wells (fig. 11), with redox conditions 

ranging from oxic to anoxic in both of these groups. 

Figure 10. Concentrations of redox-sensitive constituents in samples from four shallow well groups in the Mississippi embayment-Texas 

coastal uplands aquifer system and Mississippi River Valley alluvial aquifer.

Dissolved oxygen Manganese

8

6

4

2

0

200

150

100

50

0

Iron

Holocene

alluvium

Pleistocene

valley

trains

Pleistocene

terrace

deposits

Shallow

Tertiary

wells

Sulfate

Holocene

alluvium

Pleistocene

valley

trains

Pleistocene

terrace

deposits

Shallow

Tertiary

wells

Figure 10.

CONCENTRATION, IN MICROGRAMS PER LITER CONCENTRATION, IN MILLIGRAMS PER LITER

CONCENTRATION, IN MICROGRAMS PER LITER CONCENTRATION, IN MILLIGRAMS PER LITER

75th percentile

Median

25th percentile

Outlier data value more than 1.5 times

 the IQR above the 75th percentile

Data value less than or equal to

 1.5 times the IQR outside the

 quartile

Data value less than or equal to

 1.5 times the IQR outside the

 quartile

EXPLANATIONInterquartile range (IQR)

25 29

NUMBER OF SAMPLES

24 32 25 29

NUMBER OF SAMPLES

24 36

25 29 24 36 25 29 24 36

10,000

1,000

100

10

1

0

100,000

10,000

1,000

100

10

1

0
Quality of Shallow Groundwater in Mississippi Embayment-Texas Coastal Uplands and MRVA Aquifer Systems 21

Given that the depth to water was less than 50 ft for most 

of the wells in each of the shallow wells groups and that, with 

the exception of the Pleistocene terrace deposits, well depths 

are not substantially different (fig. 12A and B), differences in 

apparent groundwater age or residence time likely are a result 

of differences in recharge and shallow hydrogeology of these 

aquifers. Detailed lithologic information from sources such as 

drillers’ or geophysical logs was not available for most wells 

to characterize subsurface geology; however, soil survey data 

were available. Soil data were used to generally characterize 

differences in the near surface hydrology of these aquifers 

(fig. 12C–E). Soil properties that may affect recharge to these 

aquifers, such as soil hydrologic groups and average clay 

content, are shown in figure 12. Soil hydrologic group is a 

soil grouping based on infiltration and runoff characteristics: 

soils in group A have the lowest runoff potential and highest 

infiltration rate, soils in group D have the highest runoff 

potential and lowest infiltration rates, and groups B and 

C have intermediate runoff potential and infiltration rates 

(U.S. Department of Agriculture, 2007). The average clay 

content was calculated from the lowest and highest percentages of soil particles that were less than 2 millimeters in size 

(U.S. Department of Agriculture, 2007). 

The soils in the buffer areas around wells in the Pleistocene terrace deposits had higher infiltration rates (based on the 

soil hydrologic group) and lower average clay content than the 

remaining aquifers (fig. 12). Soils in hydrologic groups B and 

D accounted for much of the buffer areas around the wells. 

Hydrologic group B includes soils that 

have moderate to high infiltration rates 

with moderately low runoff potential, 

and hydrologic group D includes soils 

with very low infiltration rates and 

high runoff. Differences in these soil 

properties generally were reflected in the 

amount of recent water in these shallow 

well groups. The highest percentage of 

group B soils for the Pleistocene terrace 

deposits corresponded to the highest 

tritium concentrations (fig. 12C; fig. 8A) 

among the four aquifers. In turn, the 

redox conditions also were associated 

with the overlying soils. The most 

reducing conditions were in samples 

from the Holocene alluvium, where 

the buffer areas included the lowest 

median percentage of hydrologic group 

B, the highest median percentage of 

hydrologic group D, and the highest 

median average clay content (fig. 12). In 

contrast, samples from the Pleistocene 

terrace deposits, which were largely 

oxic (fig. 11), were characterized by 

soils with higher infiltration rates and a 

smaller clay content.

Nutrients 

Nutrients, such as nitrogen and phosphorus, are essential 

elements for plant and animal growth, and various forms of 

nitrogen and phosphorus occur naturally in the environment. 

Concentrations of these nutrients generally are low in groundwater; for example, estimated national background concentrations for nitrate and orthophosphate in shallow groundwater 

are 2.0 and 0.02 mg/L, respectively (Fuhrer and others, 1999). 

Nitrogen is not a common element in rocks, but phosphorus 

can be present in accessory minerals in igneous rocks or in 

sedimentary rocks generally representing less than 0.1 percent 

of the composition of elements in the rocks (Hem, 1985). 

Elevated concentrations of both nitrogen and phosphorus in 

groundwater may be the result of application of fertilizers 

in agricultural and urban areas, animal waste, leaking sewer 

lines or septic systems, or the infiltration of treated wastewater 

into aquifers. The atmosphere is also a nonpoint source for 

nitrogen, depositing more than 3 million tons nationally each 

year from chemical reactions or from the combustions of fossil 

fuels (Fuhrer and others, 1999). 

Nitrate usually is the species of nitrogen that is of concern in groundwater systems because of its solubility, mobility, 

and potential health effects. Nitrate concentrations in shallow 

wells in the Tertiary aquifers and MRVA aquifers generally 

were low and less than the national background concentration 

of 2 mg/L as nitrogen (N), a level that might indicate effects 

from anthropogenic activities (Mueller and Helsel, 1996). No 

Figure 11. Percentage of samples that were classified as oxic, suboxic, anoxic, or mixed 

redox state in four shallow well groups in the Mississippi embayment-Texas coastal 

uplands aquifer system and Mississippi River Valley alluvial aquifer.

Holocene alluvium Pleistocene valley trains

Pleistocene terrace deposits Shallow Tertiary wells

EXPLANATION

Anoxic

Mixed

Suboxic

Oxic
22 Quality of Shallow Groundwater and Drinking Water in Aquifers, South-Central United States, 1994–2004

Figure 12. Differences in (A) well depth, (B) depth to water, (C) percentage of soils in hydrologic 

group B, (D) percentage of soils in hydrologic group D, and (E) average clay content in four shallow 

well groups in the Mississippi embayment-Texas coastal uplands aquifer system and Mississippi 

River Valley alluvial aquifer.

A B

C

E

D

0

50

100

150

200

0

40

80

120

100

80

60

40

20

0

80

60

40

20

0

60

40

20

0

Holocene

alluvium

Figure 12.

Pleistocene

valley

trains

Pleistocene

terrace

deposits

Shallow

Tertiary

wells

AVERAGE SOIL CLAY CONTENT IN BUFFER AREAS,

IN PERCENT WELL DEPTH, IN FEET BELOW LAND SURFACE

DEPTH TO WATER, IN FEET BELOW LAND SURFACE PERCENTAGE OF SOILS IN HYDROLOGIC GROUP D

IN BUFFER AREAS

PERCENTAGE OF SOILS IN HYDROLOGIC GROUP B

IN BUFFER AREAS

75th percentile

Median

25th percentile

Outlier data value more than 1.5 times

 the IQR above the 75th percentile

Outlier data value more than 1.5 times

 the IQR below the 25th percentile

Data value less than or equal to

 1.5 times the IQR outside the

 quartile

Data value less than or equal to

 1.5 times the IQR outside the

 quartile

EXPLANATIONInterquartile range (IQR)

25 29

NUMBER OF SAMPLES

24 36 25 29

NUMBER OF SAMPLES

24 36
Quality of Shallow Groundwater in Mississippi Embayment-Texas Coastal Uplands and MRVA Aquifer Systems 23

concentrations were greater than the maximum contaminant 

level of 10 mg/L for drinking water; the maximum concentration measured was 6.8 mg/L in the shallow Tertiary aquifer 

(table 4). Only samples from the Pleistocene terrace deposits 

had a median nitrate concentration greater than the reporting 

level (0.05 mg/L) for nitrate (table 4), and samples from only 

six wells had nitrate concentrations greater than 2 mg/L. 

Nitrate concentrations in groundwater are affected by 

redox conditions. In anoxic groundwater, nitrate may be 

reduced as a result of microbial activity to nitrogen gas (N2) 

or to ammonia (Stumm and Morgan, 1996). In the Holocene 

alluvium and Pleistocene valley trains, iron concentrations are 

high, whereas dissolved oxygen values are low (<0.5 mg/L) 

indicating a strongly reducing environment (fig. 10). 

Dissolved oxygen was not measured for all samples, and 

dissolved iron concentrations are used to illustrate the relation 

between redox conditions and nitrate concentrations (fig. 13). 

Generally, when iron concentrations were greater than about 

50 g/L (indicating anoxic conditions), nitrate was not 

detected or was present at concentrations less than 0.5 mg/L. 

The low nitrate concentrations in the Holocene alluvium and 

Pleistocene valley trains likely reflect the redox conditions in 

these aquifers. In these largely anoxic aquifers, nitrate was 

detected infrequently; only one sample from the Holocene 

alluvium and eight samples from the Pleistocene valley 

trains had nitrate concentrations greater than 0.1 mg/L. The 

highest nitrate concentrations in the Pleistocene valley trains 

were in samples in which iron concentrations were less than 

20 g/L (fig. 13). Any nitrate that moves with recharge into 

these aquifers likely is subject to denitrification because of the 

reducing conditions within these aquifers. Ten samples from 

the shallow Tertiary wells had nitrate concentrations greater 

than 0.5 mg/L and were associated with oxic conditions (iron 

<50 μg/L); however, nitrate was not detected in more than half 

of the samples from the shallow Tertiary wells. In contrast, 

under the largely oxic conditions in the Pleistocene terrace 

deposits, nitrate was detected at a concentration greater than 

0.5 mg/L in more than half of the samples. 

Ammonia is a reduced form of nitrogen and may be 

present in groundwater naturally as a result of degradation of 

organic matter (ammonification), or in some cases it may be 

a result of contamination from sources such as leaking septic 

systems, infiltration of wastewater, or agricultural sources 

(Chapelle, 1993). Denitrification is another possible source 

of ammonia to groundwater. Nitrate is typically converted 

to nitrogen gas (N2) during denitrification but also may be 

converted to ammonia (Chapelle, 1993). 

In contrast to nitrate, ammonia concentrations in samples 

from the Holocene alluvium and Pleistocene valley trains 

generally were higher than in samples from the Pleistocene 

terrace deposits and shallow Tertiary aquifer (table 4). The 

highest concentration of ammonia was 2.6 mg/L in a sample 

from the Holocene alluvium. Ammonia concentrations in the 

Holocene alluvium and Pleistocene valley trains were correlated with specific conductance, suggesting concentrations 

increase with longer average groundwater residence time, 

particularly in the Pleistocene valley trains samples where a 

relation between residence time (tritium concentrations) and 

concentration of inorganic constituents was observed (fig. 9). 

The relation between ammonia and specific conductance 

suggests that ammonia likely is stable in these anoxic aquifers. 

Although the median amount of nitrogen contributed 

from various sources within the buffer areas of the Holocene 

alluvium and Pleistocene valley trains wells was three times 

greater than the loading around the Pleistocene terrace 

deposits wells and the Tertiary aquifer wells (table 5), in 

general most of the total nitrogen concentrations ranged 

from about 0.05 to 3 mg/L in samples from all four aquifers 

(fig. 14). Total nitrogen concentrations were somewhat higher 

in samples from the Pleistocene terrace deposits and shallow 

Tertiary wells where nitrate was the predominant species of 

nitrogen in groundwater, likely reflecting the greater solubility 

and mobility of nitrate compared to other species of nitrogen. 

The overall lower total nitrogen concentrations in samples 

from the Holocene alluvium and Pleistocene valley trains, 

despite the greater input of nitrogen largely from fertilizer use, 

Table 4. Summary of selected nutrient concentrations in samples from four shallow well groups in the Mississippi embayment-Texas 

coastal uplands aquifer system and Mississippi River Valley alluvial aquifer, 1994–2004. 

[mg/L, milligram per liter; N, nitrogen; P, phosphorus; min, minimum; max, maximum; <, less than]

Aquifer

Nitrate as N,

 in mg/L

Ammonia as N, 

in mg/L

Total nitrogen,

 in mg/L

Dissolved phosphorus as P, 

in mg/L

min max median min max median min max median min max median

Holocene alluvium <0.06 0.35 <0.06 0.14 2.6 0.58 0.17 2.7 0.65 <0.01 1.3 0.65

Pleistocene valley 

trains <0.06 2.6 <0.06 0.024 1.5 0.14 0.06 2.7 0.27 <0.01 1.0 0.11

Pleistocene terrace 

deposits <0.06 6.2 0.73 <0.015 0.94 <0.015 <0.01 6.2 0.84 <0.01 0.08 <0.01

Shallow Tertiary 

wells <0.06 6.8 <0.06 <0.015 1.4 <0.04 <0.01 6.8 0.46 <0.01 0.87 0.07
24 Quality of Shallow Groundwater and Drinking Water in Aquifers, South-Central United States, 1994–2004

is likely the result of several factors. The fine-grained sediments and clay-rich soils that overlie these units may slow the 

movement of water and nitrogen into the aquifer. Additionally, 

much of the nitrate that may be contributed with recharge to 

these aquifers likely is converted to N2 or ammonia through 

denitrification. Dissolved gases were not analyzed in these 

samples, so the extent of denitrification or the amount of N2

in these aquifers is not known. Ammonia in 

groundwater typically is present as ammonium (NH4

+

), and its transport may be slowed 

because of sorption to aquifer materials 

(Freeze and Cherry, 1979; DeSimone and 

Howes, 1998). Total nitrogen concentrations 

in groundwater in the MRVA may be low, 

in part because of sorption of ammonia, 

the dominant species of nitrogen in these 

aquifers, to aquifer sediments.

Concentrations of dissolved phosphorus 

in groundwater typically are low because 

phosphorus tends to sorb to soil and aquifer 

sediments and is not readily transported in 

groundwater (Hem, 1985). For example, 

the estimated background concentration of 

orthophosphate for 47 shallow wells across 

the country was 0.02 mg/L as phosphorus 

(P) (Fuhrer and others, 1999). Wetzel (1983) 

suggested average groundwater concentrations also were about 0.02 mg/L. Samples 

from the Pleistocene terrace deposits reflect 

the general occurrence of phosphorus in 

groundwater; dissolved phosphorus was 

detected in only seven samples and the 

maximum concentration was 0.083 mg/L (table 4). However, 

dissolved phosphorus concentrations in the three remaining 

shallow well groups were considerably higher than 0.02 mg/L 

and higher than concentrations in the Pleistocene terrace 

deposits (fig. 15). The highest concentrations of dissolved 

phosphorus were in samples from the Holocene alluvium 

with a median concentration of 0.65 mg/L and maximum of 

Figure 13. Relation between dissolved iron and nitrate concentrations in samples 

from four shallow well groups in the Mississippi embayment-Texas coastal uplands 

aquifer system and Mississippi River Valley alluvial aquifer.

Table 5. Sources of nitrogen to the buffer areas surrounding wells in four shallow well groups in the Mississippi embayment-Texas 

coastal uplands aquifer system and Mississippi River Valley alluvial aquifer.

[Median input from 1,640-foot radius around wells in each shallow aquifer group. Nitrogen, in kilograms per square kilometer per year; <, less than]

Shallow well 

group

Septic 

systemsa

Atmospheric 

depositionb

Non-farm 

fertilizer 

useb

Farm 

fertilizer 

useb

Animal 

manure, 

confinedb

Animal 

manure, 

unconfinedb

Total nitrogen 

input, 

all sourcesc

Median nitrate 

concentrationd

Holocene alluvium 18 610 0 4,500 3.6 41 5,200 <0.05

Pleistocene valley 

trains 23 620 0 5,700 8.8 73 6,600 <0.05

Pleistocene terrace 

deposits 0 500 1,030 110 2.1 23 1,900 0.73

Shallow Tertiary 

wells 37 460 0 510 18 290 1,750 <0.05

a

 Calculated from Ns = Ce x Ve x n, where Ns is the mass of nitrogen input from septic effluent, Ce is concentration of nitrogen in septic effluent (85 milligrams per liter; Miller, 1980), Ve is the volume of septic effluent per person per day (170 liters per day; Miller, 1980), and n is the number of people using 

septic systems in the areal extent of the zone of contribution to the well (assuming four persons per household and using septic-system density [Kerie J. Hitt, 

U.S. Geological Survey, written commun., 2005]).

b

 Kerie J. Hitt, U.S. Geological Survey, written commun., 2005.

c

 Totals are medians from study area and do not sum across table.

d

 Concentration in milligrams per liter, as nitrogen.

Holocene alluvium

Pleistocene valley trains

Pleistocene terrace deposits

Shallow Tertiary wells

Figure 13.

8

6

4

2

0

NITRATE AS NITROGEN, IN MILLIGRAMS PER LITER

1 10 100 1,000 10,000 100,000

DISSOLVED IRON, IN MICROGRAMS PER LITER
Quality of Shallow Groundwater in Mississippi Embayment-Texas Coastal Uplands and MRVA Aquifer Systems 25

Figure 14. Relation between the ratio of ammonia and nitrate to total nitrogen in 

samples from four shallow well groups in the Mississippi embayment-Texas coastal 

uplands aquifer system and Mississippi River Valley alluvial aquifer.

Holocene alluvium

Pleistocene valley trains

Pleistocene terrace deposits

Shallow Tertiary wells

100

10

1

0.1

0.01

0.001

RATIO OF AMMONIA TO NITRATE

0.001 0.01 0.1 1 10

TOTAL NITROGEN AS NITROGEN, IN MILLIGRAMS PER LITER

Ammonia greater than nitrate

Nitrate greater than ammonia

Figure 15. Concentrations of dissolved phosphorus in samples from four shallow well groups in the Mississippi 

embayment-Texas coastal uplands aquifer system and Mississippi River Valley alluvial aquifer.

10

1

0.1

0.01

0.001

Figure 15.

Holocene

alluvium

Pleistocene

valley

trains

Pleistocene

terrace

deposits

Shallow

Tertiary

wells

25 29

NUMBER OF SAMPLES

24 36

75th percentile

Median

25th percentile

Outlier data value more than 1.5 times

 the IQR above the 75th percentile

Data value less than or equal to

 1.5 times the IQR outside the

 quartile

Data value less than or equal to

 1.5 times the IQR outside the

 quartile

EXPLANATIONInterquartile range (IQR)

DISSOLVED PHOSPHORUS CONCENTRATION, IN MILLIGRAMS PER LITER
26 Quality of Shallow Groundwater and Drinking Water in Aquifers, South-Central United States, 1994–2004

1.3 mg/L. Three-quarters of the samples from this aquifer had 

phosphorus concentrations greater than 0.2 mg/L (fig. 15). 

Phosphorus concentrations in samples from the Pleistocene 

valley trains were somewhat lower than in samples from the 

Holocene alluvium with a median concentration of 0.11 mg/L 

and a maximum of 1 mg/L (table 4). In the samples from 

shallow Tertiary wells, phosphorus concentrations ranged from 

less than 0.01 to 0.87 mg/L with a median concentration of 

0.07 mg/L. The occurrence of phosphorus in the three Tertiary 

aquifers sampled differed somewhat. Phosphorus concentrations were lowest in the middle Claiborne aquifer where it 

was detected in only four of eight wells in the Memphis area, 

and the maximum concentration was 0.07 mg/L. The lower 

Claiborne-upper Wilcox and middle Wilcox aquifers had 

median concentrations of 0.1 and 0.12 mg/L, respectively, and 

maximum concentrations of 0.27 and 0.87 mg/L, respectively. 

The principal sources of phosphorus to groundwater 

systems include the overlying soils, dissolution of minerals 

that contain phosphate in aquifer sediments, agricultural 

fertilizer, animal waste, and leaking septic systems or infiltration of wastewater (Fuhrer and others, 1999). The widespread 

occurrence of elevated phosphorus concentrations (greater 

than 0.02 mg/L) in three of the shallow well groups suggests 

either a natural source in the soils or aquifer sediments, or 

nonpoint sources such as fertilizer and animal waste, or a 

combination of natural and human sources. The elevated 

concentrations of phosphorus in these aquifers could not be 

attributed to a specific source; however, phosphorus concentrations in samples from the Holocene alluvium and shallow 

Tertiary wells were weakly correlated to several inorganic 

constituents. Dissolved phosphorus concentrations were 

weakly related to the amount of agricultural land use in buffer 

areas around wells in the Holocene alluvium (Spearman’s 

rho = 0.34, p = 0.05) but were not related in samples from the 

Pleistocene valley trains (fig. 16). Phosphorus concentrations 

were not correlated to soil properties in the surrounding buffer 

areas for any of the shallow well groups. 

Weak correlations between specific conductance and 

dissolved phosphorus in samples from the Holocene alluvium 

and shallow Tertiary wells could be interpreted as indicating 

a contribution of phosphorus from the dissolution of aquifer 

sediments. In the shallow Tertiary wells, the occurrence of 

elevated phosphorus in the lower Claiborne-upper Wilcox 

and middle Wilcox aquifers, in areas of largely forested 

land use, suggests a geologic source for phosphorus in these 

aquifers in the absence of a manmade source; however, this 

interpretation is not conclusive. The relation between specific 

conductance and phosphorus concentrations also may represent geochemical conditions favorable to phosphorus transport 

in ground-water, whether the source of phosphorus is natural 

or manmade. 

Although transport in groundwater generally has not been 

considered to be an important pathway in the environment for 

phosphorus, its occurrence in groundwater at elevated concentrations has been documented in a few studies (Atkinson, 

1974; Burkart and others, 2004). In a number of groundwater 

studies, the source and transport of phosphorus were localized 

and related to sewage disposal or animal waste lagoons and 

downgradient plumes (Walter and others, 1996; Spruill, 1997). 

In a plume emanating from a sewage lagoon, phosphorus 

transport occurred in both anoxic and suboxic zones in the 

aquifer. Phosphorus transport in the anoxic zone was thought 

to be a result of the reducing conditions in the aquifer. As iron 

solubility increased and iron oxyhydroxides on the aquifer 

Figure 16. Relation of agricultural land use in buffer areas (1,640-foot radius) surrounding wells to dissolved phosphorus 

concentrations in water from wells completed in the Holocene alluvium and Pleistocene valley trains.

Holocene alluvium Pleistocene valley trains

Figure 16.

1.5

1.0

0.5

0

DISSOLVED PHOSPHORUS AS PHOSPHORUS,

IN MILLIGRAMS PER LITER

0 20 40 60 80 100 0 20 40 60 80 100

PERCENTAGE OF AGRICULTURAL LAND USE IN BUFFER AREA AROUND WELL
Quality of Shallow Groundwater in Mississippi Embayment-Texas Coastal Uplands and MRVA Aquifer Systems 27

matrix dissolved, the number of sorption sites decreased 

(Walter and others, 1996). Continued downgradient transport 

in the suboxic zone was interpreted to be the result of a lack 

of available sorption sites in the aquifer matrix. That is, over 

time phosphorus transported with groundwater had occupied 

sorption sites and limited the capacity of the sediments to 

sorb additional phosphorus (Walter and others, 1996). In this 

dataset, a general association between elevated phosphorus 

concentrations and dissolved iron suggests that reducing 

conditions that mobilize iron also may facilitate transport 

of phosphorus. Although a statistically significant relation 

between iron and phosphorus concentrations in individual 

samples did not exist, the relative distribution of iron and 

phosphorus concentrations was somewhat similar among the 

shallow well groups (figs. 10 and 15). Both maximum iron 

and phosphorus concentrations were in samples from the 

Holocene alluvium and both were lowest in samples from the 

Pleistocene terrace deposits (figs. 10 and 15). This general 

association between iron and phosphorus concentrations 

in the Holocene alluvium, Pleistocene valley trains, and 

shallow Tertiary wells could be the result of 

a decrease in the number of sorption sites 

on the aquifer matrixes as a result of iron 

dissolution, or iron colloids in groundwater

may serve as sorption sites for phosphorus 

transport with groundwater, or a combination 

of both processes may account for the dissolved phosphorus concentrations observed. 

The elevated concentrations of 

phosphorus in these aquifers may represent 

a source of phosphorus to streams and rivers 

in the study area. Nutrient enrichment of 

streams in the southeastern United States is a 

topic of concern because of the hypoxic zone 

in the Gulf of Mexico (U.S. Environmental 

Protection Agency, 2007a) as well as the 

potential effects on local streams and stream 

ecology. Dissolved phosphorus results for 

samples from the Tensas River and Bogue 

Phalia were grouped into streamflow 

quartiles to generally characterize whether 

elevated groundwater concentrations 

may affect surface-water phosphorus 

concentrations (fig. 17). These basins are 

390 and 484 square miles, respectively and 

do not receive any major point sources of 

phosphorus (Coupe, 2002). Samples collected during periods where the streamflow 

consisted primarily of groundwater inflow to 

the streams are represented by samples in the 

25th quartile (low base flow) and probably 

some of the samples in the 50th quartile (high 

base flow). Median dissolved phosphorus 

concentrations for low base-flow samples 

collected from the Tensas River were about 

0.15 mg/L and higher than concentrations in 

Figure 17. Dissolved phosphorus concentrations by streamflow quartile for samples 

collected from the Bogue Phalia near Leland, Mississippi (1995–2008), and the Tensas 

River at Tendal, Louisiana (1996–2000).

0.30

0.25

0.20

0.15

0.10

0.05

0

Tensas River at Tendal, LA

Q25 Q50 Q75 Q100

Q25 Q50 Q75

STREAMFLOW QUARTILES

Q100

Bogue Phalia near Leland, MS

Figure 17.

DISSOLVED PHOSPHORUS AS PHOSPHORUS, IN MILLIGRAMS PER LITER

75th percentile

Median

25th percentile

Outlier data value more than 1.5 times

 the IQR above the 75th percentile

Data value less than or equal to

 1.5 times the IQR outside the

 quartile

Data value less than or equal to

 1.5 times the IQR outside the

 quartile

EXPLANATION Interquartile range (IQR)

0.4

0.3

0.2

0.1

0

most of the samples collected when streamflow was greater 

(fig. 17). The maximum measured dissolved phosphorus value 

at this site was in the lowest streamflow quartile. Base-flow 

concentrations in samples from the Bogue Phalia were 

lower than in samples from the Tensas River, but the median 

concentration of about 0.04 mg/L during base-flow conditions 

and concentrations between 0.05 and 0.3 mg/L are greater than 

are usually attributed to groundwater. Although this analysis 

does not specifically identify groundwater as a major source 

of phosphorus to these streams, the elevated phosphorus 

concentrations during base-flow conditions indicate the 

potential for groundwater discharge to be a contributing source 

of phosphorus to these streams (fig. 17). Additional study is 

needed to better understand the extent to which phosphorus 

in groundwater is a contributing factor to watersheds in the 

study area having some of the highest phosphorus yields in the 

Mississippi River basin (Kleiss and others, 2000; Alexander 

and others, 2008). 
28 Quality of Shallow Groundwater and Drinking Water in Aquifers, South-Central United States, 1994–2004

Pesticides 

A total of 24 pesticides were detected at least once in 

shallow groundwater; however, one-quarter of these pesticides 

were detected only once (table 6). Comparatively few 

pesticides were detected in samples for the Holocene alluvium 

(6 pesticides) and Pleistocene valley trains (7 pesticides). 

Bentazon was the only pesticide detected in more than 

two samples from these aquifers. In both subunits of the 

MRVA aquifer, agricultural herbicides were the predominant 

pesticides detected and generally reflected compounds used on 

the principal crops grown: rice, cotton, and soybeans (table 2). 

In contrast, 16 different pesticides were detected in samples 

from both the Pleistocene terrace deposits and shallow Tertiary 

Table 6. Detection frequency and maximum concentration for pesticides and pesticide degradates detected in water from four shallow 

well groups in the Mississippi embayment-Texas coastal uplands aquifer system and Mississippi River Valley alluvial aquifer, 1994–2004.

[N, number of wells sampled; detection frequency, in percent; concentration, in micrograms per liter; —, not detected; E, estimated]

Pesticide

Number of 

detections 

in shallow 

wells

Holocene alluvium

N=25

Pleistocene valley 

trains

N=29

Pleistocene terrace 

deposits

N=24

Shallow Tertiary 

aquifers

N=36

Detection 

frequency

Maximum 

concentration

Detection 

frequency

Maximum 

concentration 

Detection 

frequency

Maximum 

concentration

Detection 

frequency

Maximum 

concentration

Herbicides

2,4-Da 2 — — 3 E 0.02 — — 5b E 14.8

Acetochlor 1 — — — — — — 3 0.023

Atrazine 14 — — 3 0.019 29 0.072 17 0.029

Bentazona 18 24 3.17 28 0.31 8 0.08 9b 0.12

Bromacila 2 — — — — 8 0.27 — —

DCPA 2 — — — — 4 E 0.001 3 0.002

Dicambaa 1 — — — — — — 5b E 1.46

Diurona 2 — — — — 4 0.19 5b E 0.001

EPTC 1 — — — — 4 0.02 — —

Fenurona 3 4 E 0.02 — — 8 0.57 — —

Fluometurona 4 4 0.28 7 1.22 4 0.11 — —

Metolachlor 10 — — 3 0.011 21 0.013 11 0.008

Metribuzin 3 — — — — 4 0.010 6 0.007

Molinate 4 8 0.038 7 0.053 — — — —

Pendimethalin 1 — — — — — — 3 E0.017

Piclorama 3 — — — — 13 E 3.91 — —

Prometon 2 — — — — 4 0.051 3 E 0.008

Propanil 2 4 E 0.003 — — — — 3 0.005

Simazine 12 — — — — 29 0.413 14 0.384

Tebuthiuron 4 — — — — 12 1.76 3 0.014

Insecticides

Diazinon 1 — — — — — — 3 0.004

Dieldrin 2 — — — — 8 3.20c — —

Degradates

Deethylatrazine 10 — — 3 E 0.013 25 E 0.046 8 E 0.021

p,p'-DDE 1 4 E 0.002 — — — — — —

a

 Pesticide not analyzed for during 2004 sampling of the middle Wilcox aquifer. 

b

 Number of wells used to calculate detection frequency was 22. 

c

 Exceeds the U.S. Environmental Protection Agency Health Advisory of 0.2 microgram per liter for dieldrin (U.S. Environmental Protection Agency, 2004). 
Quality of Shallow Groundwater in Mississippi Embayment-Texas Coastal Uplands and MRVA Aquifer Systems 29

wells, and several pesticides were detected in more than 

two samples (table 6). The most commonly detected pesticides 

in samples from the Pleistocene terrace deposits and shallow 

Tertiary wells were the herbicides atrazine (29 and 17 percent), simazine (29 and 14 percent), and metolachlor (21 and 

11 percent). The degradate deethylatrazine also was detected 

frequently (25 percent) in samples from the Pleistocene terrace 

deposits but at low, estimated concentrations. Pesticides 

detected in samples from these aquifers included herbicides 

with both agricultural and nonagricultural use. For example, 

atrazine and simazine are used for both agriculture and in 

urban settings, but tebuthiuron and prometon are not used in 

agriculture. Generally, the concentrations at which pesticides 

were detected were low. Only seven pesticides were detected 

at a concentration greater than 1 μg/L, and a sample from 

each shallow aquifer had at least one detection greater than 

1 μg/L (table 6). Dieldrin was the only pesticide detected at a 

concentration greater than a human-health benchmark with a 

maximum concentration of 3.2 μg/L compared to the healthbased screening level of 0.2 μg/L. The presence of dieldrin 

in groundwater at this elevated concentration, 10 years after 

being banned for use as a termite treatment in 1987, highlights 

its persistence in the environment.

The occurrence of pesticides was more widespread in 

samples from the Pleistocene terrace deposits than in samples 

from the three other shallow well groups (fig. 18). Two-thirds 

of the samples from wells in the Pleistocene terrace deposits 

contained at least one pesticide in contrast to between 30 and 

43 percent of samples from the remaining shallow aquifers. 

The maximum number of pesticides detected in a single 

sample from the Holocene alluvium, Pleistocene valley trains, 

and shallow Tertiary wells were 3, 4, and 8, respectively. 

One sample from the Pleistocene terrace deposits contained 

10 pesticides. 

Agricultural use of pesticides accounts for about 76 percent of the total estimated use across the Nation and about 

24 percent is nonagricultural use (Kiely and others, 2004). 

Agricultural pesticide use of specific pesticides was estimated 

using average or recommended application rates for crops and 

the acreage or amount of the crop grown in the buffer area 

around each well. Data are not available, however, to reliably 

estimate pesticide use for nonagricultural purposes. For the 

agricultural pesticides detected in two or more samples, 

the amount applied in the buffer area around each well was 

estimated. The median pesticide amount applied in the buffer 

area for each aquifer is shown in figure 19. Because of the 

predominance of forest and urban land in areas overlying the 

Pleistocene terrace deposits and shallow Tertiary wells, the 

median estimated use of agricultural pesticides is considerably 

less than in areas overlying the MRVA aquifer because the 

application rates in buffer zones around the wells for nonagricultural purposes cannot be estimated (fig. 19). 

Although the estimated use of several agricultural 

pesticides applied was larger in buffer areas overlying the 

MRVA aquifer, most of these herbicides were detected more 

commonly in samples from the Pleistocene terrace deposits 

and shallow Tertiary wells. Several herbicides, for example, 

atrazine, metolachlor, and 2,4-D, may have been used in 

urban areas and may not have been accounted for by the use 

estimates; however, it is likely that the systematic and areally 

extensive estimated use for agricultural purposes is greater 

than local commercial or residential use of these compounds 

in buffer areas. Bentazon was the only herbicide detected in 

all four aquifers that was detected more frequently in samples 

from the MRVA aquifer. The factors contributing to a relatively high detection frequency of bentazon compared to other 

herbicides with greater amounts of use in the MRVA is not 

clear, but its frequent occurrence may be related to its physical 

properties such as its octanol-water partition and soil sorption 

coefficients that are lower than those for atrazine and metolachlor. It is also possible that the use of bentazon is greater than 

the estimated amount applied. Although bentazon is not commonly used in urban areas, it was detected in two wells in the 

Memphis area. Both of these wells are located near a stream, 

and bentazon may have been transported downstream from 

agricultural areas in the watershed. Propanil had the highest 

estimated use, but was detected in only one sample from the 

Holocene alluvium and one from the shallow Tertiary aquifer. 

Propanil was detected in samples from three streams that 

overlie the MRVA aquifer (Coupe, 2000); however, its detection frequency and the concentrations at which it was detected 

were lower than other herbicides with smaller amounts of use. 

Propanil has a short half life (1–3 days; Kamrin, 1997), which 

may account for its low detection frequency in both groundwater and surface-water samples compared to herbicides with 

lower amounts of estimated use. 

Several herbicides with both agricultural and nonagricultural use, such as simazine, 2,4-D, atrazine, and metolachlor, 

were detected in samples from the Pleistocene terrace deposits 

and shallow Tertiary wells more commonly than in the MRVA 

(fig. 19). Herbicides, such as tebuthiuron and prometon, used 

for weed control along rights-of-way and other nonagricultural 

areas were detected in groundwater from both the Pleistocene 

terrace deposits and shallow Tertiary wells. Most of the detections (about 86 percent) in wells completed in shallow parts 

of the Tertiary aquifers were in samples from wells screened 

in the middle Claiborne aquifer with the largest number of 

different pesticides detected in individual wells in the Memphis area (fig. 18). The remaining 14 percent of the detections 

in the shallow Tertiary wells was for atrazine, deethylatrazine, 

and diazinon in samples from the lower Claiborne-upper 

Wilcox aquifer in Texas, and all concentrations were less than 

0.02 μg/L. No pesticides were detected in the middle Wilcox 

aquifer in the area around Shreveport, Louisiana. 

The potential of a pesticide to affect water quality is 

influenced by several factors including the type and intensity 

of use, environmental factors such as soil properties and 

aquifer hydrology, as well as chemical and physical properties 

that determine mobility and persistence of a pesticide (Gilliom 

and others, 2006). Estimated use of pesticides in the buffer 

areas around shallow wells was not a good predictor of 

occurrence, in part because only pesticides with agricultural 
30 Quality of Shallow Groundwater and Drinking Water in Aquifers, South-Central United States, 1994–2004

Figure 18. Number of pesticides and volatile organic compounds detected in shallow wells in the Mississippi embayment-Texas 

coastal uplands aquifer system and Mississippi River Valley alluvial aquifer (land-use data from Vogelmann and others, 1998a, 1998b).

!

! !

! !

!

!

!!

!

^

^

Little Rock

Memphis

Dallas

Jackson

TENNESSEE

MISSISSIPPI

Location of wells near Memphis, Tennessee

(1,0/2,0 and 2,1/3,0 are nested wells)

Land-use category and percentage of area

Barren (1%)

Grasslands (2%)

Urban (2%)

Lakes and rivers (3%)

Shrubland (5%)

Wetlands and coastal marshes (9%)

Forest (37%)

Agriculture (41%)

Tertiary aquifers boundary

Shallow wells with number of pesticides and volatile 

 organic compounds detected

Shallow wells without detections of pesticides

 or volatile organic compounds

Figure 18.

97 92

MISSOURI

TENNESSEE

KENTUCKY

ARKANSAS Memphis

37

32

OKLAHOMA

TEXAS

MISSISSIPPI

LOUISIANA

Shreveport

^

0 50 100 150 MILES

0 50 100 150 KILOMETERS

EXPLANATION

^ ^

^

1,0

2,0

3,0

1,0

1,0

1,0

1,1

1,1

2,0

1,0

1,2

1,0 0,1

1,0

4,4 0,2

1,0

1,0 1,0

1,0

1,0

0,1

2,0

0,1 0,1

1,0

2,0

0,1 0,1

0,1

0,1

0,1

0,1 0,1

1,0

1,0

0,1

1,0

2,0

0,1

1,0

4,1 0,1 6,0

1,0

3,3 2,1 3,0

1,0 3,0

1,0 2,0

10,0 1,0/2,0

4,0

1,0

1,0

8,0 1,0

7,0

2,1/3,0

3,1

Base map modified from U.S. Geological Survey

digital data, 1:250,000
Quality of Shallow Groundwater in Mississippi Embayment-Texas Coastal Uplands and MRVA Aquifer Systems 31

use could be estimated. Pesticide occurrence generally was 

not correlated to other water-quality constituents, apparent 

residence time (tritium concentrations), well depth, or depth 

to water in the shallow groundwater. Differences in pesticide 

occurrence between aquifers and, in some cases, wells 

screened in the same aquifer generally were associated with 

differences in the overlying soils (fig. 20). Pesticides were 

detected in about two-thirds of samples from the Pleistocene 

terrace deposits compared to about one-third of samples from 

the other shallow well groups (fig. 20). The soils overlying 

the Pleistocene terrace deposits aquifer had the highest 

infiltration rates (lowest percentage of hydrologic group D 

soils) and lowest organic matter content. Although the relation 

between soil properties and occurrence do not apply in all 

of the shallow well groups, generally, pesticides were more 

commonly detected in samples from wells where the soils had 

higher infiltration rates based on a lower median percentage of 

hydrologic group D soils in the buffer area and lower organic 

matter content (fig. 20). For example, with the exception of the 

Pleistocene valley trains, the median percentage of the buffer 

areas around wells where pesticides were detected was lower 

than in those wells without detections. 

Figure 19. Median estimated agricultural use (1997) of detected pesticides in buffer areas and number of detections in samples from wells 

in the (A) Holocene alluvium, (B) Pleistocene valley trains, (C) Pleistocene terrace deposits, and (D) shallow Tertiary wells in the Mississippi 

embayment-Texas coastal uplands aquifer system and Mississippi River Valley alluvial aquifer.

A B ATRAZINE

DEETHYL ATRAZINE

BENTAZON

FLUOMETURON

METOLACHLOR

METRIBUZIN

MOLINATE

PROPANIL

SIMAZINE

2,4-D

DIURON

DCPA

BROMACIL

FENURON

PICLORAM

PROMETON

TEBUTHIURON

DIELDRIN

0 2 4

NUMBER OF DETECTIONS

6 8 10

NUMBER OF DETECTIONS

0 2 4 6 8 10

C D ATRAZINE

DEETHYL ATRAZINE

BENTAZON

FLUOMETURON

METOLACHLOR

METRIBUZIN

MOLINATE

PROPANIL

SIMAZINE

2,4-D

DIURON

DCPA

BROMACIL

FENURON

PICLORAM

PROMETON

TEBUTHIURON

DIELDRIN

0 500 1,000 1,500 2,000 2,500

EXPLANATION

0 500 1,000 1,500 2,000 2,500

MEDIAN PESTICIDE USE FOR AGRICULTURE IN BUFFER AREAS AROUND WELLS, IN POUNDS

ACTIVE INGREDIENT APPLIED, IN POUNDS

NUMBER OF DETECTIONS

Holocene alluvium

Pleistocene terrace

deposits Shallow Tertiary wells

Pleistocene valley

trains
32 Quality of Shallow Groundwater and Drinking Water in Aquifers, South-Central United States, 1994–2004

Volatile Organic Compounds 

Volatile organic compounds are highly soluble, mobile, 

and persistent in groundwater, and many are suspected 

carcinogens (Lapham and Tadayon, 1996). Commonly used 

products, such as gasoline, refrigerants, paints, solvents, and 

plastics, contain VOCs. VOCs are used as fumigants in agriculture to control insects, worms, and other pests (Zogorski 

and others, 2006) and are also found in fuels used in farming 

equipment. Leaking underground storage tanks (USTs), supply 

pipelines, and improper waste disposal are potential point 

sources of VOCs to groundwater. 

A relatively small number of VOCs were detected in each 

of the shallow well groups at a common assessment level of 

0.2 g/L (table 7; fig. 18). The number of detections of VOCs 

in each shallow well group ranged from 3 in samples from 

the Holocene alluvium to 12 in samples from the Pleistocene 

valley trains (table 7). The most frequently detected VOC was 

dichlorodifluoromethane, which was detected in five samples 

collected from the Pleistocene valley trains and Pleistocene 

terrace deposits. Tetrahydrofuran was detected four times, 

Figure 20. Distribution of soil hydrologic group and average organic matter content for samples with and without 

pesticide detections from four shallow well groups in the Mississippi embayment-Texas coastal uplands aquifer 

system and the Mississippi River Valley alluvial aquifer.

Figure 20.

17 9

NUMBER OF SAMPLES

17 11 8 16 24 12

Holocene

alluvium

Pleistocene

valley

trains

Pleistocene

terrace

deposits

Shallow

Tertiary

wells

75th percentile

Median

25th percentile

Outlier data value more than 1.5 times

 the IQR above the 75th percentile

Data value less than or equal to

 1.5 times the IQR outside the

 quartile

Data value less than or equal to

 1.5 times the IQR outside the

 quartile

No pesticides detected

One or more pesticides detected

EXPLANATIONInterquartile range (IQR)

PERCENTAGE OF BUFFER AREA WITH SOILS IN

HYDROLOGIC GROUP D

AVERAGE SOIL ORGANIC MATTER CONTENT IN

BUFFER AREAS, IN PERCENT

100

80

60

40

20

0

1.5

1.0

0.5

0

Outlier data value more than 1.5 times

 the IQR below the 25th percentile
Quality of Shallow Groundwater in Mississippi Embayment-Texas Coastal Uplands and MRVA Aquifer Systems 33

and methyl tert-butyl ether (MTBE) and chloroform were 

each detected three times overall (table 7). Given the variety 

and localized nature of potential sources (for example, USTs, 

machine shops, and dry cleaners), relating the occurrence of 

these few detections to explanatory factors is not feasible. 

However, one general observation is that five of the nine 

VOCs detected in samples from wells in the Holocene alluvium and Pleistocene valley trains were compounds that are 

associated with motor fuels, either gasoline oxygenates such 

as MTBE, or hydrocarbons in gasoline such as 2-ethyltoluene 

(table 7). One sample from a domestic well completed in the 

Pleistocene valley trains accounted for two of the highest 

concentrations of VOCs measured. Detections of MTBE, 

methyl tert-pentyl ether, and diisopropyl ether in this sample 

may be related to the storage and use of fuels near the well. 

Tetrahydrofuran is a solvent that is used in polyvinyl chloride 

(PVC) glues, and its occurrence in four samples may be from 

PVC plumbing at the well head rather than its occurrence 

in groundwater. 

The occurrence of chlorinated solvents in samples from 

the Pleistocene terrace deposits (table 7) is likely a consequence of the overlying land use. Chlorinated solvents are 

Table 7. Detection frequency and maximum concentration for volatile organic compounds detected in water from four shallow well 

groups in the Mississippi embayment-Texas coastal uplands aquifer system and Mississippi River Valley alluvial aquifer, 1994–2004.

[N, number of wells sampled; detection frequency, in percent; concentration, in micrograms per liter; —, not detected; E, estimated]

Volatile organic 

compound

Number of 

detections 

in shallow 

wells

Holocene alluvium

N=25

Pleistocene valley 

trains

N=29

Pleistocene terrace 

deposits

N=24

Shallow Tertiary 

aquifers

N=35

Detection 

frequency

Maximum 

concentration

Detection 

frequency

Maximum 

concentration 

Detection 

frequency

Maximum 

concentration

Detection 

frequency

Maximum 

concentration

Trihalomethanes

Trichloromethane 

(Chloroform) 

3 — — — — 4 2.06 6 0.91

Chlorinated solvents

1,1,-Dichloroethane 1 — — — — 4 0.20 — —

1,2-Dichloroethane 1 — — 3 2.68 — — — —

cis-1,2-Dichloroethene 1 — — — — 4 0.57 — —

Tetrachloroethene (PCE) 1 — — — — 4 0.44 — —

Gasoline hydrocarbons

1,2,4-Trimethylbenzene 1 — — 3 0.25 — — — —

2-Ethyltoluene 1 4 0.38 — — — — — —

Benzene 1 — — — — — — 3 0.36

Toluene 2 — — — — 4 0.27 3 0.28

Gasoline oxygenates

Diisopropyl ether 2 — — 7 22 — — — —

Methyl tert-butyl ether 

(MTBE)

3 — — 10 6.4 — — — —

Methyl tert-pentyl ether 1 — — 3 0.79 — — — —

Refrigerants

Dichlorodifluoromethane 5 — — 14 E 1.18 4 E 0.3 — —

Trichlorofluoromethane 2 — — — — 4 0.23 3 0.30

Other solvents – ether based

Acetonea 1 — — — — — — 5b 0.6

Diethyl ether 1 4 1.11 — — — — — —

Tetrahydrofurana 4 4 3.58 — — — — 13b 1.35

a

 Volatile organic compound not analyzed for during 1994 sampling of the lower Claiborne-upper Wilcox aquifer. 

b Number of wells used to calculate detection frequency was 22. 
34 Quality of Shallow Groundwater and Drinking Water in Aquifers, South-Central United States, 1994–2004

used in industrial, commercial, and domestic applications, and 

the wells in the Pleistocene terrace deposits are in a predominantly urban setting (table 7). Wells in which chlorinated 

solvents were detected were oxic with DO concentrations 

between 1.9 and 4.9 mg/L, and chlorinated solvents generally 

are stable in oxic groundwater (Wiedemeier and others, 1998). 

The Quality of Drinking Water in the 

Mississippi Embayment-Texas Coastal 

Uplands Aquifer System

Fifty-five wells completed in the Tertiary aquifers and 

sampled as part of these studies were deeper than 200 ft and 

generally are representative of drinking-water resources in 

the study area. These wells were screened in several different 

aquifers: the upper Claiborne (2 wells), middle Claiborne 

(17 wells), lower Claiborne-upper Wilcox (12 wells), and 

middle Wilcox (24 wells) aquifers. Fifty-four of these wells 

were used for domestic or public supply, and one well was 

used for stock supply; however, all 55 wells are referred to 

as drinking-water wells for the purposes of this report. Well 

depths ranged from 208 to 1,466 ft and the median depth was 

395 ft. The wells were screened in both confined and unconfined parts of these aquifers, and tritium samples collected at 

11 wells did not indicate a significant component of recent 

water at those 11 wells.

Inorganic Constituents

Concentrations of most inorganic constituents were 

less than available water-quality criteria for drinking water. 

Inorganic constituents were compared to available MCLs, 

secondary drinking-water standards, and health-based 

screening levels to generally assess the quality of water in 

the drinking-water wells. No constituents were detected at 

concentrations greater than an available MCL. Ten of the 

55 samples had total dissolved solids concentrations greater 

than the SMCL of 500 mg/L (U.S. Environmental Protection 

Agency, 2004), with a maximum concentration of 1,000 mg/L. 

High concentrations of total dissolved solids can limit the use 

of groundwater for drinking water, irrigation, and industry 

because the water can be corrosive, have a salty or brackish 

taste, or result in scale formation in pipes. Most of the total 

dissolved solids concentrations (80 percent) in the deep 

drinking-water wells were between 27 and 492 mg/L, with an 

overall median of 291 mg/L. Dissolved iron and manganese 

concentrations were greater than their SMCLs of 300 and 

50 μg/L (U.S. Environmental Protection Agency, 2004) in 

samples from 20 and 17 wells, respectively. High levels of 

iron and manganese are largely an aesthetic issue related to 

staining of fixtures and plumbing. Concentrations in the drinking-water wells ranged from 3 to 8,100 μg/L, with a median 

of 78 μg/L for iron, and ranged from 2.2 to 410 μg/L, with a 

median value of 15 μg/L for manganese. In many samples, 

the reducing conditions in the aquifer resulted in elevated 

iron and manganese concentrations. Samples from 13 wells 

had both iron and manganese concentrations that exceeded 

their SMCL. One sample had a concentration of fluoride of 

3.8 mg/L, which exceeded the SMCL of 2 mg/L. Radon was 

the only other inorganic constituent that had concentrations 

greater than a health-related benchmark. The USEPA has 

proposed that public water supplies treat groundwater that 

has radon concentrations greater than 300 picocuries per liter 

(pCi/L) if a multimedia mitigation (MMM) program is not 

undertaken by a State (U.S. Environmental Protection Agency, 

2007b). The recommended level of radon in drinking water is 

4,000 pCi/L or less if a MMM program that addresses indoor 

air, the primary exposure route, is in place. Radon concentrations ranged from 46 to 518 pCi/L, with a median value of 

98 pCi/L. Samples from 10 domestic wells screened in the 

middle Wilcox aquifer of northeastern Texas and northwestern 

Louisiana accounted for all radon concentrations greater than 

300 pCi/L. 

Nutrients

When detected, nitrate concentrations in samples from 

the deep drinking-water wells were low. Nitrate was detected 

in only 19 of the 55 wells with concentrations ranging from 

0.05 to 3.8 mg/L and most of these wells were less than 500 ft 

deep (fig. 21). Concentrations of nitrate were low, in part, 

because DO concentrations were low, and nitrate is subject 

to denitrification in anoxic groundwater. All wells greater 

than 500 ft deep were anoxic (DO less than 0.5 mg/L), and 

overall, more than 75 percent of the samples were anoxic. 

The maximum nitrate concentration was in a sample that had 

a DO concentration of 7.40 mg/L (fig. 21). Ammonia was the 

predominant form of nitrogen in the deep wells. Ammonia was 

present in 52 of the 55 samples, and concentrations ranged 

from 0.02 to 1.8 mg/L. Similar to results for shallow Tertiary 

wells, concentrations were positively correlated to dissolved 

solids (Spearman’s rho=0.75, p<0.001). Although this relation is relatively strong and might suggest a geologic source 

of nitrogen, sediments that compose these aquifers are not a 

likely source of nitrogen. A possible source of ammonia is 

from ammonification of organic matter in the aquifer resulting 

in increases in ammonia concentration with residence time in 

the aquifer. 

Phosphorus was detected in 50 of the 55 wells, and 46 of 

those concentrations were greater than 0.02 mg/L. Dissolved 

phosphorus concentrations in the drinking-water wells ranged 

from 0.008 to 1.45 mg/L. The median concentration was 

0.16 mg/L, which is greater than the median concentration 

measured in samples from the shallow Tertiary wells (table 4). 

Although elevated phosphorus concentrations occurred 

throughout the Tertiary aquifers, high concentrations were 

prevalent in parts of the study area. For example, some of the 
The Quality of Drinking Water in the Mississippi Embayment-Texas Coastal Uplands Aquifer System 35

highest phosphorus concentrations were in wells completed 

in the middle Wilcox aquifer wells in northwestern Louisiana 

(fig. 22). In addition, deep Tertiary wells in the southern part 

of the Mississippi Valley generally had higher phosphorus 

concentrations than wells in the northern part of the study 

area (fig. 22). Elevated concentrations of phosphorus were 

not limited to one of the Tertiary aquifers; concentrations 

greater than 0.1 mg/L occurred in several samples from each 

aquifer, with the exception of the upper Claiborne aquifer. 

However, only two wells from the upper Claiborne aquifer 

were sampled. 

Dissolved phosphorus concentrations were correlated 

to total dissolved solids, and three depth ranges of wells had 

comparable ranges in phosphorus concentrations (fig. 23). 

This relation, the widespread occurrence of phosphorus, the 

absence of recent water in wells with tritium samples, and 

the fact that concentrations were not related to the depth of 

these wells (fig. 23), suggest that the aquifer sediments may 

be the predominant source of phosphorus in these samples. 

Relatively few minerals contain phosphorus, and a relation 

between fluoride and phosphorus concentrations (Spearman’s 

rho=0.6, p<0.001) suggests that fluoroapatite is a possible 

source of phosphorus in these aquifers (fig. 23); however, apatite was not identified in heavy mineral separates from several 

of the units of Tertiary age (Lonsdale and 

others, 1931; Blankenship, 1956). In general, a 

relation between phosphorus and fluoride did 

not exist in samples from the shallow wells with 

the exception of a weak relation in the shallow 

Tertiary wells (Spearman’s rho=0.27, p=0.06). 

Fluoride is relatively reactive in groundwater; 

for example, if a sufficient amount of calcium 

is present, the water may be in equilibrium with 

respect to fluorite and limit dissolved fluoride 

concentrations (Hem, 1985). 

Pesticides and Volatile Organic 

Compounds

Pesticides and VOCs were detected 

infrequently in samples from deep drinkingwater wells. When detected, both pesticide and 

VOC concentrations in most instances were 

considerably less than 1 μg/L (table 8), and 

no concentrations were greater than available 

drinking-water criteria. Two pesticides and 

two pesticide degradates were detected in 5 of 

the 55 drinking-water wells. Diazinon, an 

insecticide used predominantly in residential 

areas (discontinued in 2004), was detected in 

three samples from wells screened in the lower 

Claiborne-upper Wilcox aquifer in eastern 

Texas at concentrations of 0.017 μg/L or less 

(table 8). No other pesticide was detected more than once 

in samples from the deep wells. The compound p,p'-DDE, 

a degradate of the insecticide DDT, was detected in water 

from a lower Claiborne-upper Wilcox well in Texas. The use 

of DDT was banned in 1972. Bromacil and deethyl atrazine 

were both detected in one sample from a well screened in the 

middle Wilcox aquifer in eastern Arkansas. The five wells 

with pesticide detections were less than 500 ft deep. These 

pesticides also were detected in shallow wells, generally at 

comparable concentrations (table 6). Three of the four wells 

where pesticides were detected were in areas where the 

aquifers were unconfined. 

Volatile organic compounds were detected at concentrations above 0.2 μg/L in samples from four of the drinkingwater wells. Two of the five VOC detections were in a sample 

from a well completed in the middle Claiborne aquifer at a 

depth of 737 ft. The two VOCs, m- plus p-xylene and isobutyl 

methyl ketone, were not detected in any shallow samples. 

Chloroform was detected twice and was among the most 

commonly detected VOCs in shallow wells. The highest VOC 

concentration, 10.1 μg/L of tetrahydrofuran, was collected 

from a domestic well. Tetrahydrofuran may have been from 

the PVC plumbing at the wellhead, similar to the relatively 

high concentrations detected in shallow well samples. 

Figure 21. Relation between nitrate and dissolved-oxygen concentrations in 

drinking-water wells in the Mississippi embayment-Texas coastal uplands 

aquifer system.

Less than 500

500 – 1,000

Greater than 1,000

Figure 21.

1

0.1

0.01

NITRATE AS NITROGEN, IN MILLIGRAMS PER LITER

0 2 4 6 8

DISSOLVED-OXYGEN CONCENTRATION, IN MILLIGRAMS PER LITER

Well depth, in feet

Laboratory reporting level
36 Quality of Shallow Groundwater and Drinking Water in Aquifers, South-Central United States, 1994–2004

Figure 22. Dissolved phosphorus concentrations in samples from 55 drinking-water wells in the Mississippi embayment-Texas 

coastal uplands aquifer system (land-use data from Vogelmann and others, 1998a, 1998b).

0.1

0.2

0.02

0.05

0.46

0.35

0.02

0.36

0.13

0.17

0.31

0.14 0.19 0.27

0.2

0.16

0.17 0.56

0.39

0.61

0.19

0.33

0.79

0.1

0.32

0.38

0.12

0.01

0.01

0.14

0.14

0.15

0.14

0.06

0.16

0.02

0.22

0.09

0.12

0.15 0.54

0.14

0.96 0.9

1.45 0.87

0.05

0.25

0.17 0.18

0.06

^

Little Rock

Dallas

Jackson

Land-use category and percentage of area

Barren (1%)

Grasslands (2%)

Urban (2%)

Lakes and rivers (3%)

Shrubland (5%)

Wetlands and coastal marshes (9%)

Forest (37%)

Agriculture (41%)

Tertiary aquifers boundary

Wells with dissolved phosphorus concentrations

Wells without detections of dissolved phosphorus

 concentrations

Figure 22.

97 92

MISSOURI

TENNESSEE

KENTUCKY

ARKANSAS Memphis

37

32

OKLAHOMA

TEXAS

MISSISSIPPI

LOUISIANA

Shreveport

^

0 50 100 150 MILES

0 50 100 150 KILOMETERS

EXPLANATION

^ ^

^

Base map modified from U.S. Geological Survey

digital data, 1:250,000
The Quality of Drinking Water in the Mississippi Embayment-Texas Coastal Uplands Aquifer System 37

Figure 23. Relation between dissolved phosphorus and (A) total dissolved solids and (B) fluoride 

concentrations in drinking-water wells in the Mississippi embayment-Texas coastal uplands aquifer system.

Less than 500

500 – 1,000

Greater than 1,000

Figure 23.

1

0.1

0.01

DISSOLVED PHOSPHORUS AS PHOSPHORUS, IN MILLIGRAMS PER LITER

0 400 800

1

0.1

0.01

0.1 1

1,200

FLUORIDE CONCENTRATIONS, IN MILLIGRAMS PER LITER

TOTAL DISSOLVED SOLIDS CONCENTRATIONS, IN MILLIGRAMS PER LITER

Well depth, in feet

Phosphorus and fluoride detected

Fluoride not detected

Phosphorus and fluoride not detected

5 samples

A

B
38 Quality of Shallow Groundwater and Drinking Water in Aquifers, South-Central United States, 1994–2004

Vulnerability of Drinking-Water Aquifers to 

Shallow Groundwater Leakage—An Example

Although the quality of deep drinking water throughout 

the study area is good, the occurrence of pesticides and VOCs 

at low concentrations locally indicates that the deeper aquifers, 

particularly where unconfined, are vulnerable to contamination 

if contaminant sources are present. Groundwater withdrawals 

for water supply have lowered water levels in the Tertiary 

aquifers throughout much of the study area such that gradients 

in the areas of water-level decline are downward from surficial 

aquifers to deeper aquifers that are used as sources of drinking 

water. Increases in water use in the future may increase the 

potential for downward movement of water and contaminants 

from shallow sources. 

The groundwater hydrology of the Memphis, TN, area 

has been characterized in a number of studies. Throughout 

most of the area, the middle Claiborne confining unit separates 

the shallow water-table aquifer (Pleistocene terrace deposits) 

from the underlying middle Claiborne aquifer, locally called 

the Memphis aquifer. Hydrogeologic studies by Graham and 

Parks (1986) and Parks (1990) characterized the potential for 

leakage in the Memphis area, documented the occurrence of 

recent water in parts of the middle Claiborne aquifer (Memphis aquifer) using tritium data, and identified areas where 

hydraulic heads between the water-table 

aquifer and the middle Claiborne aquifer 

were similar, indicating a hydraulic connection between the aquifers. Parks (1990) 

constructed a map of the thickness of the 

confining unit separating the two aquifers 

based on geophysical logs and inferred areas 

where the confining unit was thin or absent. 

Changes in water quality were characterized 

in local-scale studies at a landfill (Bradley, 

1991) and a well field (Parks and others, 

1995) in the Memphis area. These changes 

were associated with the downward 

movement of water from the water table in 

both the western and eastern parts of the 

Memphis area.

In this study, pesticides and VOCs were 

detected in samples from 16 and 5 wells, 

respectively, in the Pleistocene terrace 

deposits in the Memphis area (fig. 24). Only 

six wells in this shallow aquifer did not have 

a detection of either a pesticide or VOC, 

highlighting the widespread occurrence of 

these organic compounds in shallow groundwater in the Memphis area. In addition, 

pesticides also were detected in samples 

from five wells completed in the Pleistocene 

terrace deposits located in proximity to areas 

where the underlying confining unit is thin 

or absent (fig. 24), representing the potential 

for contaminants to move downward into the upper part of the 

middle Claiborne aquifer. 

In the southeastern part of the Memphis area, samples 

from each of the eight wells completed in the upper part of 

the middle Claiborne aquifer contained at least one pesticide, 

and samples from two wells also contained VOCs (fig. 24). 

The occurrence of these organic compounds in the upper part 

of the middle Claiborne aquifer indicates that this aquifer is 

vulnerable to contamination, corroborating conclusions of 

previous studies of largely point sources of contamination in 

the area (Parks, 1990; Bradley, 1991; Parks and others, 1995). 

Water-level data collected from 24 shallow Pleistocene 

terrace deposits wells and 8 shallow Tertiary middle 

Claiborne wells during the spring 1997 were compared to a 

potentiometric-surface map of the middle Claiborne aquifer 

(Kingsbury, 1996; fig. 25). The potentiometric-surface 

map was drawn using water-level data collected during 

September 1995 when water levels in the Memphis area 

were generally at their lowest for the year. Water levels in 

the eight shallow Tertiary aquifer wells in the southeastern 

part of the Memphis area generally were 4 to 10 ft higher 

than nearby water-level contours (fig. 25). The difference 

likely is because of the different seasons in which the water 

levels were measured. Most of the water levels from shallow 

Pleistocene terrace deposits wells were considerably higher, 

in much of the area more than about 50 to 140 ft, than water 

Table 8. Detection frequency and maximum concentrations for pesticides, 

pesticide degradates, and volatile organic compounds detected in water from 

55 drinking-water wells greater than 200 feet deep in the Mississippi embaymentTexas coastal uplands aquifer system, 1994–2004.

[N, number of wells; detection frequency, in percent; concentration, in micrograms per liter; E, 

estimated. There were no exceedances of a U.S. Environmental Protection Agency drinking-water 

standard (U.S. Environmental Protection Agency, 2004) or U.S. Geological Survey health-based 

screening level (Toccalino and others, 2003)]

Compound

Number of 

detections in 

drinking-water 

wells

Drinking-water wells

N=55

Detection 

frequency

Maximum 

concentration

Herbicides

Bromacil 1 2 0.16

Insecticides

Diazinon 3 5 0.017

Degradates

Deethyl atrazine 1 2 E 0.004

p,p'-DDE 1 2 E 0.001

Volatile organic compounds

Isobutyl methyl ketone 1 2 E 0.20

m- plus p-xylene 1 2 0.43

Tetrahydrofuran 1 2 10.1

Trichloromethane (Chloroform) 2 4 0.72
The Quality of Drinking Water in the Mississippi Embayment-Texas Coastal Uplands Aquifer System 39 Figure 24. Thickness of the middle Claiborne confining unit and number of pesticides and volatile organic compounds detected in samples from the Pleistocene terrace deposits and shallow Tertiary wells in the Memphis, Tennessee, area (modified from Parks, 1990).

100

100

150

150

200

1,0

0,0 2,0

10,0

0

50

200

250 2,0

0,0

200

150

100

200

150

6,0

4,1

1,0

1,0

3,0

0,0

0,0

1,0 0,1

100

100

150

150

200

50

0

3,3 3,0 (UR–18) 2,1 (UR–29)

0,1(UR–30)

0,0

100

50

0

1,0

3,1 1,0

4,0

3,0

0,0

2,1

7,0 1,0

2,0

1,0

8,0

50

100

100

50

50

100

0

0

50

0

50

50

150

100

0

150

200

250

150

Areas where the middle Claiborne confining unit is inferred to be thin or absentLine of equal thickness of the middle Claiborne confining unit—Interval is 50 feetWell screened in Pleistocene terrace deposits and number of pesticides and volatile organic compounds detectedShallow Tertiary well and number of pesticides and volatile organic compounds detected

50

3,38,0

EXPLANATION

3515’

35

90 8945’

Base map modified from U.S. Geological Survey, 1:24,000

Figure 24.

0 5 MILES

0 5 KILOMETERS

ARKANSAS

Location of study area in Arkansas, Tennessee,

and Mississippi

TENNESSEE

MISSISSIPPI
40 Quality of Shallow Groundwater and Drinking Water in Aquifers, South-Central United States, 1994–2004

Figure 25. Potentiometric surface of the middle Claiborne aquifer, 1995, and water levels in the Pleistocene terrace deposits and shallow Tertiary wells, 1997 (modified from Kingsbury, 1996).

170

160

150

140

130

120

180

294

282

3515’

35

90 8945’

Base map modified from U.S. Geological Survey, 1:24,000 0 5 MILES

0 5 KILOMETERS

ARKANSAS

TENNESSEE

MISSISSIPPI

140

140

234 (UR–30)

286

247

238

259

270

282 221

266

220

343

219 (UR–29) 203 (UR–18)

227

210

240

218

276

278

194 207

208 203

254

230

214

294 229

190

220

210

200

190

180

170

160

150

140

130

224

247

256

207

150

Approximate eastern

limit of middle

Claiborne confining unit

250

240

230 180

250

343

230

Well fieldPotentiometric contour shows altitude at which water level would have stood in tightly cased wells. Dashed where approximately located. Hatches indicate depression. Contour interval 10 feet. Datum is National Geodetic Vertical Datum of 1929Pleistocene terrace deposits and water-level altitude, in feetShallow Tertiary wells and water-level altitude, in feet

EXPLANATION
Summary 41

levels in the middle Claiborne aquifer (as indicated by nearby 

contours). Two well pairs in this dataset, with one well in 

Pleistocene terrace deposits and one middle Claiborne well, 

in the southeastern Memphis area, indicated a difference of 

about 50 ft in water levels with between 20 to 30 ft of middle 

Claiborne confining unit present at these locations (Strom, 

1997). Although these differences in head favor the downward 

movement of water, where the confining unit is present, the 

potential for leakage is considered to be less than in areas 

where the confining unit is thin or absent.

The difference between water levels at two Pleistocene 

terrace deposits wells (UR-18 and UR-29) and the middle 

Claiborne aquifer, however, was less than 10 ft (fig. 25). The 

water level at a third well (UR-30), located in the same general 

area, was only about 25 ft higher than water levels in the 

middle Claiborne aquifer. These shallow wells are located near 

two cones of depression in the potentiometric surface of the 

middle Claiborne aquifer formed by groundwater withdrawals. 

The low water levels in wells UR-18, UR-29, and UR-30 in an 

area where the middle Claiborne confining unit is interpreted 

as being thin or absent (fig. 24) suggest an area of potential 

leakage from the shallow aquifer to the middle Claiborne 

aquifer. Additionally, the pesticides and VOCs detected in 

these shallow wells have the potential to move into the deeper 

drinking-water aquifer in this area. 

Summary 

Water-quality data collected for 169 wells as part of the 

National Water-Quality Assessment (NAWQA) Program from 

1994 to 2004 were aggregated to better understand factors 

that affect the quality of groundwater in the Mississippi 

embayment-Texas coastal uplands aquifer system and Mississippi River Valley alluvial (MRVA) aquifer. The wells sampled 

included public-supply, domestic use, irrigation, and monitoring wells and were completed in four regional aquifers of the 

Mississippi embayment-Texas coastal uplands aquifer system, 

two subunits of the MRVA aquifer (Holocene alluvium and 

Pleistocene valley trains), and a shallow water-table aquifer in 

the Memphis, TN, area (Pleistocene terrace deposits). 

 Because of the large range in depths (21 to 1,466 ft) 

of the wells sampled for this study and because many of the 

constituents analyzed likely originate from activities at land 

surface, a subset of 114 wells 200 ft deep or less was selected 

to characterize the occurrence of many of the constituents in 

shallow groundwater. These wells were subdivided into 

four groups—the Holocene alluvium and Pleistocene valley 

trains units of the Mississippi River Valley alluvial aquifer; 

the Pleistocene terrace deposits in Memphis, TN; and the 

shallow Tertiary wells, which included wells in the middle 

Claiborne, lower Claiborne-upper Wilcox, and middle 

Wilcox aquifers. 

Although the ranges in well depth and depth to water 

generally were similar among the four shallow well groups, 

differences in water quality were observed. Groundwater 

in the two subunits of the MRVA aquifer generally is more 

mineralized than groundwater in the Pleistocene terrace 

deposits and shallow Tertiary wells. In addition, tritium 

concentrations in samples from the Holocene alluvium, 

Pleistocene valley trains, and shallow Tertiary wells indicated 

a smaller component of recent groundwater than samples from 

the Pleistocene terrace deposits. Samples from the Pleistocene 

terrace deposits had the highest median and maximum tritium 

concentrations, likely indicating that this group had the largest 

component of recent recharge. In samples from the Pleistocene 

valley trains and shallow Tertiary wells, tritium concentrations 

were inversely related to total dissolved solids, suggesting 

increases in inorganic constituent concentrations with longer 

groundwater residence times in these aquifers.

The redox conditions, as indicated by concentrations 

of dissolved oxygen, nitrate, manganese, iron, and sulfate, 

differed considerably among the four shallow well groups. 

The majority of the samples from the Holocene alluvium and 

Pleistocene valley trains were anoxic (dissolved oxygen less 

than 0.5 mg/L). Redox conditions in the Holocene alluvium 

were strongly reducing, and several samples had iron concentrations as high as 14,000 μg/L. Samples from the Pleistocene 

valley trains indicated somewhat less reducing conditions with 

a few oxic wells. In contrast, more than half of the samples 

from the Pleistocene terrace deposits were oxic, as were about 

one-third of the samples from the shallow Tertiary wells. 

Iron and manganese concentrations in the Pleistocene terrace 

deposits and shallow Tertiary wells were considerably lower 

than in the subunits of the MRVA. 

Differences in apparent age of water or residence time 

and redox conditions were associated with differences in soil 

properties within 1,640-ft (500-meter) buffer areas around the 

wells. Soils near the Pleistocene terrace deposits wells had 

higher infiltration rates and lower amounts of clay than the 

other three shallow well groups. Soils overlying wells in the 

two subunits of the MRVA aquifer had the highest average 

clay content and lowest infiltration rates. 

The estimated loading of nitrogen from various sources, 

including farm and non-farm fertilizer use, animal manure, 

septic systems and atmospheric deposition, was about three 

times higher in the buffer areas surrounding the Holocene 

alluvium and Pleistocene valley trains than in the buffer areas 

surrounding the remaining shallow well groups. Nitrate, 

however, was not detected in most samples from the Holocene 

alluvium and Pleistocene valley train aquifers. Nitrate was 

more commonly detected in samples from the shallow 

Tertiary and Pleistocene terrace deposits, which both had 

maximum concentrations of about 6 mg/L. Reducing conditions in the MRVA aquifer likely result in denitrification of 

most nitrate that moves with recharge, and soils overlying 

these aquifers likely slow the movement of water and 

facilitate denitrification.

Dissolved phosphorus concentrations in samples from 

the Holocene alluvium, Pleistocene valley trains, and shallow 

Tertiary wells were considerably higher than 0.02 mg/L, a 
42 Quality of Shallow Groundwater and Drinking Water in Aquifers, South-Central United States, 1994–2004

concentration typical of many groundwater systems. The highest concentrations (maximum 1.3 mg/L; median 0.65 mg/L) 

were in samples from the Holocene alluvium. Median 

concentrations of dissolved phosphorus were 0.11 mg/L in 

the Pleistocene valley trains and 0.07 mg/L in the shallow 

Tertiary wells. Phosphorus was detected in only seven samples 

from the Pleistocene terrace deposits and had a maximum 

concentration of 0.08 mg/L. 

The widespread occurrence of elevated phosphorus 

concentrations in three of the four shallow well groups 

suggests a natural source and(or) a nonpoint source and also 

indicates groundwater is a potential source of phosphorus to 

streams in these areas. Although phosphorus concentrations 

in samples from the Holocene alluvium showed a weak 

correlation to several inorganic constituents and to the 

amount of agricultural land use in the buffer area, the elevated 

concentrations could not be attributed to a specific source. The 

highest phosphorus concentrations in the shallow wells tended 

to be associated with water that had high iron concentrations, 

suggesting reducing conditions that mobilize iron might 

facilitate the transport of phosphorus in these aquifers. Data 

for dissolved phosphorus concentrations and stream discharge 

for two streams in the study area indicate that groundwater 

discharge may be a contributing source of phosphorus to these 

streams. Additional study is needed to better understand the 

sources and transport of phosphorus in these shallow aquifers. 

A total of 24 pesticides were detected at least once in 

shallow groundwater; however, one-quarter of these pesticides 

were detected only once. Relatively few pesticides were 

detected in samples from the Holocene alluvium and Pleistocene valley trains, although the estimated use for agricultural 

purposes was considerably higher in areas overlying wells in 

these aquifers than near the shallow Tertiary and Pleistocene 

terrace deposits wells. Bentazon was the only herbicide 

detected in more than two samples from wells completed in 

the MRVA aquifer, although its estimated use was lower than 

other herbicides. The most commonly detected pesticides in 

samples from the Pleistocene terrace deposits and shallow 

Tertiary wells were the herbicides atrazine, simazine, and 

metolachlor. The degradate, deethylatrazine, also was detected 

in about 25 percent of the samples from the Pleistocene terrace 

deposits. Generally, the concentrations at which pesticides 

were detected were low. Only seven compounds were detected 

at a concentration greater than 1 μg/L, and a sample from each 

shallow aquifer had at least one detection greater than 1 μg/L. 

Dieldrin was the only pesticide detected at a concentration 

greater than a human-health benchmark with a maximum 

concentration of 3.2 μg/L, compared to the health-based 

screening level of 0.2 μg/L. Differences in the number of 

pesticides detected among the shallow well groups generally 

corresponded to differences in the soil properties near the 

wells. Pesticides were detected more commonly in samples 

from wells in the Pleistocene terrace deposits where soils had 

higher infiltration rates and lower organic matter content. 

Volatile organic compounds were not detected commonly 

at an assessment level of 0.2 μg/L in shallow groundwater. 

The number of detections of volatile organic compounds 

in shallow groundwater ranged from 3 in samples from the 

Holocene alluvium to 12 in samples from the Pleistocene 

valley trains. The most commonly detected VOC was 

dichlorodifluoromethane, which was detected in a total of 

five samples collected from the Pleistocene valley trains and 

Pleistocene terrace deposits aquifers. Methyl tert-butyl ether 

(MTBE), chloroform, and tetrahydrofuran were each detected 

three times overall.

Fifty-five wells deeper than 200 ft completed in the 

Mississippi embayment-Texas coastal uplands aquifer system 

were sampled as part of this study. Tritium samples collected 

at 11 of these wells did not indicate a component of recent 

recharge. Nearly all of these deep wells were used for public 

supply or were domestic wells. The quality of the water 

generally was good, and concentrations of most inorganic 

constituents were less than water-quality criteria for drinking 

water. Secondary maximum contaminant levels (SMCLs) 

were exceeded for total dissolved solids in 10 wells, iron in 

20 wells, and manganese in 17 wells. Samples from 13 wells 

had both iron and manganese concentrations that exceeded 

their SMCL. Radon activities in samples from 10 wells 

located in the middle Wilcox aquifer of northeastern Texas 

and northwestern Louisiana were greater than 300 picocuries 

per liter (pCi/L). Nitrate concentrations were low, only two 

samples had nitrate concentrations that were greater than 

1 mg/L. Low nitrate concentrations corresponded to the anoxic 

conditions sampled in most wells. Ammonia was the predominant form of nitrogen in drinking-water wells. Few pesticides 

(six) and volatile organic compounds (five) were detected, and 

concentrations were generally less than 1 μg/L, except for one 

detection of tetrahydrofuran that was 10.1 μg/L. Tetrahydrofuran is found in glues used for polyvinyl chloride pipes, and its 

occurrence may be from plumbing at the wellhead.

Phosphorus was detected in 91 percent of the drinkingwater wells, and 84 percent of the concentrations were greater 

than 0.02 mg/L. The highest concentrations generally were 

in samples from the middle Wilcox aquifers in northwestern 

Louisiana and the middle Claiborne aquifer in the southern 

part of the Mississippi Valley. Dissolved phosphorus was 

correlated with total dissolved solids as well as fluoride. These 

relations and the fact that phosphorus was detected at depth 

suggest a geologic source may be the predominant source of 

phosphorus in drinking-water wells.

Groundwater withdrawals from the Tertiary aquifers 

have lowered water levels, which increases the potential of 

movement of shallow groundwater to deep aquifers. Waterlevel data for a few wells in the Pleistocene terrace deposits 

in the Memphis area suggest a hydraulic connection between 

this shallow aquifer and the deeper middle Claiborne aquifer, 

locally named the Memphis aquifer, that is used for public 

supply. Pesticides and volatile organic compounds were 

detected in samples from Pleistocene terrace deposits wells 

and middle Claiborne aquifer wells, further illustrating the 

potential for the downward migration of the compounds to the 

drinking-water aquifer. Eight shallow Tertiary wells screened 
References 43

in the upper part of the middle Claiborne aquifer contained 

at least one pesticide, and two of these wells contained a 

volatile organic compound. The occurrence of these organic 

compounds in the upper part of the middle Claiborne aquifer 

indicate that, in some locations, this aquifer is vulnerable to 

contamination, corroborating conclusions of previous studies. 

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Appendixes 47

Appendix 1. Detection frequencies of volatile organic compounds, 1994–2004, in the Mississippi embayment-Texas coastal uplands 

aquifer system and Mississippi River Valley alluvial aquifer with and without the 0.2 microgram per liter assessment level.—Continued

[μg/L, micrograms per liter; MCL, maximum contaminant level; —, none; N/A, not applicable; HBSL, health based screening level]

Name

Detection 

frequency for 

concentrations 

greater than or 

equal to 0.2 μg/L 

(percent)

Number of 

analyses

Maximum 

concentration 

(μg/L)

Benchmark 

level 

(μg/L)

Benchmark 

typea

Detection 

frequency with 

no assessment 

level (percent)

Shallow wells

Chloroform 2.7 113 2.06 80b MCL 15.9

Tetrahydrofuran 4.0 100 3.58 — N/A 34.0

Tetrachloroethene (PCE) 0.9 113 0.44 5 MCL 7.0

Toluenec 1.8 113 0.28 1,000 MCL 7.0

Acetone 1.0 100 0.60 6,000 HBSL 1.0

Methyl tert-butyl ether (MTBE) 2.7 113 6.40 — N/A 3.5

Trichloroethene (TCE) 0.0 113 0.05 5 MCL 1.8

1,1,1-Trichloroethane 0.0 113 0.03 200 MCL 1.8

1,2,4-Trimethylbenzened 0.9 113 0.25 — N/A 10.6

cis-1,2-Dichloroethene 0.9 113 0.57 — N/A 2.7

Carbon disulfide 0.0 100 0.18 700 HBSL 16.0

Dichlorodifluoromethane (CFC-12) 4.4 113 1.18 1,000 HBSL 6.2

Bromodichloromethane 0.0 113 0.17 80b MCL 1.8

1,1-Dichloroethane 0.9 113 0.20 — N/A 5.3

Dichloromethane 0.0 113 0.15 0 MCL 0.9

Benzene 0.9 113 0.36 5 MCL 7.1

Trichlorofluoromethane (CFC-11) 1.8 113 0.30 2,000 HBSL 1.8

Diisopropyl ether (DIPE) 2.0 100 22.0 — N/A 5.0

1,1-Dichloroethene 0.0 113 0.07 7 MCL 2.7

1,3,5-Trimethylbenzene 0.0 113 0.01 — N/A 5.3

Chlorobenzene 0.0 113 0.003 100 MCL 0.9

Ethylbenzene 0.0 113 0.01 700 MCL 0.9

1,2,3-Trimethylbenzene 0.0 100 0.01 — N/A 3.0

2-Ethyltoluene 0.9 100 0.38 — N/A 1.0

m- + p-Xylene 0.0 100 0.05 — N/A 12.0

o-Xylene 0.0 100 0.01 — N/A 2.0

Styrene 0.0 100 0.02 100 MCL 1.0

1,2-Dichloroethane 0.9 113 2.68 5 MCL 0.9

Chloroethane 0.0 113 0.02 — N/A 0.9

Chloromethane 0.0 113 0.06 — N/A 5.3

Diethyl ether 0.9 100 1.11 — N/A 0.9

Methyl tert-pentyl ether 0.9 100 0.79 — N/A 1.0

trans-1,2-Dichloroethene 0.0 113 0.01 100 MCL 0.9
48 Quality of Shallow Groundwater and Drinking Water in Aquifers, South-Central United States, 1994–2004

Appendix 1. Detection frequencies of volatile organic compounds, 1994–2004, in the Mississippi embayment-Texas coastal uplands 

aquifer system and Mississippi River Valley alluvial aquifer with and without the 0.2 microgram per liter assessment level.—Continued

[μg/L, micrograms per liter; MCL, maximum contaminant level; —, none; N/A, not applicable; HBSL, health based screening level]

Name

Detection 

frequency for 

concentrations 

greater than or 

equal to 0.2 μg/L 

(percent)

Number of 

analyses

Maximum 

concentration 

(μg/L)

Benchmark 

level 

(μg/L)

Benchmark 

typea

Detection 

frequency with 

no assessment 

level (percent)

Drinking-water wells

Methyl ethyl ketone 15.3 46 1.30 — N/A 15.3

Chloroform 3.7 54 0.72 80b MCL 18.5

m- + p-Xylene 2.2 46 0.43 — N/A 4.3

Isobutyl methyl ketone 2.2 46 0.20 — N/A 2.2

Tetrahydrofuran 2.2 46 10.11 — N/A 2.2

1,2,4-Trimethylbenzened 0.0 54 0.12 — N/A 13.0

Acetone 0.0 46 0.30 6,000 HBSL 2.2

Bromodichloromethane 0.0 54 0.02 80b MCL 1.9

Carbon disulfide 0.0 46 0.18 700 HBSL 28.3

Chloromethane 0.0 54 0.10 — N/A 3.7

Diisopropyl ether (DIPE) 0.0 46 0.10 — N/A 2.2

Ethylbenzene 0.0 54 0.16 700 MCL 1.9

o-Xylene 0.0 46 0.16 — N/A 2.2

Tetrachloroethene (PCE) 0.0 54 0.18 5 MCL 3.7

Trichloroethene (TCE) 0.0 54 0.05 5 MCL 1.9

a

 U.S. Environmental Protection Agency, 2004; Toccalino and others, 2003.

b

 Maximum contaminant level of 80 μg/L for total trihalomethanes used for comparison.

c

 Included concentrations greater than or equal to 0.03 μg/L.

d

 Included concentrations greater than or equal to 0.05 μg/L.
Appendixes 49

Appendix 2. Water-quality standards and human-health benchmarks for selected constituents analyzed 

for in the Mississippi embayment-Texas coastal uplands aquifer system and Mississippi River Valley 

alluvial aquifer, 1994–2004.—Continued

[MCL, maximum contaminant level; SMCL, secondary maximum contaminant level; HA, health advisory; HBSL, healthbased screening level; —, not available; mg/L, milligrams per liter; μS/cm, microsiemen per centimeter; °C, degree Celsius; 

μg/L, microgram per liter; pCi/L, picocurie per liter; AMCL, alternative maximum contaminant level. Compounds in italics 

were compared to U.S. Geological Survey health-based screening levels (Toccalino and others, 2003). All other compounds 

were compared to U.S. Environmental Protection Agency drinking-water standards (U.S. Environmental Protection Agency, 

2004) as a frame of reference]

Property or constituent MCL SMCL HA HBSL

Physical properties

pH, field, in standard 

units

— 6.5 – 8.5 — —

Dissolved solids and major inorganic ions, in mg/L

Dissolved solids, residue 

on evaporation, 180 °C

— 500 — —

Sodium, as Na — 30 – 60 (taste) — —

Chloride, as Cl — 250 — —

Fluoride, as Fl 4 2 — —

Sulfate, as SO4 — 250 — —

Trace elements, in μg/L

Aluminum, as Al — 50–200 — —

Antimony, as Sb 6 — — —

Arsenic, as As 10 — — —

Barium, as Ba 2,000 — — —

Beryllium, as Be 4 — — —

Cadmium, as Cd 5 — — —

Chromium, as Cr 100 (total) — — 10,000

Copper, as Cu 1,300 (action level) 1,000 — —

Iron, as Fe 300 — —

Lead, as Pb 15 (action level) — — —

Manganese, as Mn — 50 300 300

Molybdenum, as Mo — — 40 40

Nickel, as Ni — — 100 100

Selenium, as Se 50 — — —

Uranium, as Ur 30 — 20

Zinc, as Zn — 5,000 2,000 2,000

Radon, in pCi/L

Radona 300 (AMCL 4,000 pCi/L) — — —

Nutrients, in mg/L

Ammonia, as N — — 30 —

Nitrite plus nitrate, as N 10 — — —

Nitrite, as N 1 — — —
50 Quality of Shallow Groundwater and Drinking Water in Aquifers, South-Central United States, 1994–2004

Appendix 2. Water-quality standards and human-health benchmarks for selected constituents analyzed 

for in the Mississippi embayment-Texas coastal uplands aquifer system and Mississippi River Valley 

alluvial aquifer, 1994–2004.—Continued

[MCL, maximum contaminant level; SMCL, secondary maximum contaminant level; HA, health advisory; HBSL, healthbased screening level; —, not available; mg/L, milligrams per liter; μS/cm, microsiemen per centimeter; °C, degree Celsius; 

μg/L, microgram per liter; pCi/L, picocurie per liter; AMCL, alternative maximum contaminant level. Compounds in italics 

were compared to U.S. Geological Survey health-based screening levels (Toccalino and others, 2003). All other compounds 

were compared to U.S. Environmental Protection Agency drinking-water standards (U.S. Environmental Protection Agency, 

2004) as a frame of reference]

Property or constituent MCL SMCL HA HBSL

Pesticides, in μg/L

Herbicides 

2,4-D 70 — —

Acetochlor — — 1–100

Alachlor 2.0 — 10 —

Atrazine 3.0 — — —

Bentazon — — 200 200

Bromacil — — 70 70

DCPA — — 70 70

Dicamba — — 4,000 3,000

Diuron — — 200 2–200

EPTC — — — 200

Fluometuron — — 500 4

Metolachlor — — 700 70

Metribuzin — — 70 90

Molinate — — — 0.7

Pendimethalin — — — 70

Picloram 500 — — —

Prometon — — 100 100

Propanil — — — 6

Simazine 4 — — —

Tebuthiuron — — 500 1,000

Thiobencarb — — — 70

Trifluralin — — 10 20

Insecticides

Carbofuran 40 — — —

Diazinon — — 1 1

Dieldrin — — 0.2 0.002–0.2

Oxamyl 200 — — —
Appendixes 51

Appendix 2. Water-quality standards and human-health benchmarks for selected constituents analyzed 

for in the Mississippi embayment-Texas coastal uplands aquifer system and Mississippi River Valley 

alluvial aquifer, 1994–2004.—Continued

[MCL, maximum contaminant level; SMCL, secondary maximum contaminant level; HA, health advisory; HBSL, healthbased screening level; —, not available; mg/L, milligrams per liter; μS/cm, microsiemen per centimeter; °C, degree Celsius; 

μg/L, microgram per liter; pCi/L, picocurie per liter; AMCL, alternative maximum contaminant level. Compounds in italics 

were compared to U.S. Geological Survey health-based screening levels (Toccalino and others, 2003). All other compounds 

were compared to U.S. Environmental Protection Agency drinking-water standards (U.S. Environmental Protection Agency, 

2004) as a frame of reference]

Property or constituent MCL SMCL HA HBSL

Volatile organic compounds, in μg/L

1,1,1-Trichloroethane 200 — — —

1,2-Dichloroethane 5 — — —

Acetone — — — 6,000

Benzene 5 — — —

Bromodichloromethane 80 — — —

Carbon disulfide — — — 700

Chloromethane (Methyl 

chloride)

— — 30 30

Dichlorodifluoromethane — — 1000 1,000

Dichloromethane (Methylene chloride)5 — — —

Methyl ethyl ketone 

(2-Butanone)

— — 4,000 4,000

Ethylbenzene 700 — — —

Methyl tert-butyl ether 

(MTBE)

— 20 (odor)

40 (taste)

— —

Styrene 100 — — —

Tetrachloroethene (PCE) 5 — — —

Toluene 1,000 — — —

trans-1,2-Dichloroethylene100 — — —

Trichloroethene (TCE) 5 — — —

Trichlorofluoromethane — — 2,000 2,000

Trichloromethane (Chloroform)80 — — —

a

 The U.S. Environmental Protection Agency has a proposed MCL for radon in ground water of 300 pCi/L for States 

without a multimedia mitigation program and an alternate maximum contaminant level of 4,000 pCi/L for States with a 

multimedia mitigation program. 

Prepared by:

USGS Publishing Network

Raleigh Publishing Service Center

3916 Sunset Ridge Road

Raleigh, NC 27607

For additional information regarding this publication, contact:

Heather L. Welch, Hydrologist

USGS Mississippi Water Science Center

308 South Airport Road

Jackson, MS 39208-6649

phone: 601-933-2900

email: hllott@usgs.gov

Or visit the NAWQA Web site at:

http://water.usgs.gov/nawqa/
Mississippi embayment-Texas coastal

uplands aquifer system

Mississippi River Valley

alluvial aquifer

Welch and others—Quality of Shallow Groundwater and Drinking Water in Aquifers, South-Central United States, 1994–2004—Scientific Investigations Report 2009–5091