Water Movement Through Thick 

Unsaturated Zones Overlying the Central 

High Plains Aquifer, Southwestern Kansas, 

2000–2001

By P.B. McMahon, K.F. Dennehy, R.L. Michel, M.A. Sophocleous, K.M. Ellett, and 

D.B. Hurlbut

National Water-Quality Assessment Program

Water-Resources Investigations Report 03–4171

U.S. Department of the Interior

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

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2003

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

Box 25286, Denver Federal Center

Denver, CO 80225

For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS

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

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

endorsement by the U.S. Government.

Although this report is in the public domain, it contains copyrighted materials that are noted in the text. 

Permission to reproduce those items must be secured from the individual copyright owners.
iii

Foreword

The U.S. Geological Survey (USGS) is committed to serve the Nation with accurate and timely scientific information that helps enhance and protect the overall quality of life, and facilitates effective management of water, biological, energy, and mineral resources. (http://www.usgs.gov/). 

Information on the quality of the Nation’s water resources is of critical interest to the USGS 

because it is so integrally linked to the long-term availability of water that is clean and safe 

for drinking and recreation and that is suitable for industry, irrigation, and habitat for fish and 

wildlife. Escalating population growth and increasing demands for the multiple water uses make 

water availability, now measured in terms of quantity and quality, even more critical to the longterm sustainability of our communities and ecosystems.

The USGS implemented the National Water-Quality Assessment (NAWQA) Program to support 

national, regional, and local information needs and decisions related to water-quality management and policy. (http://water.usgs.gov/nawqa/). Shaped by and coordinated with ongoing 

efforts of other Federal, State, and local agencies, the NAWQA Program is designed to answer: 

What is the condition of our Nation’s streams and ground water? How are the 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. 

NAWQA results can contribute to informed decisions that result in practical and effective waterresource management and strategies that protect and restore water quality.

Since 1991, the NAWQA Program has implemented interdisciplinary assessments in more 

than 50 of the Nation’s most important river basins and aquifers, referred to as Study Units. 

(http://water.usgs.gov/nawqa/nawqamap.html). Collectively, these Study Units account for 

more than 60 percent of the overall water use and population served by public water supply, 

and are representative of the Nation’s major hydrologic landscapes, priority ecological 

resources, and agricultural, urban, and natural sources of contamination. 

Each assessment is guided by a nationally consistent study design and methods of sampling 

and analysis. The assessments thereby build local knowledge about water-quality issues and 

trends in a particular stream or aquifer while providing an understanding of how and why 

water quality varies regionally and nationally. The consistent, multi-scale approach helps to 

determine if certain types of water-quality issues are isolated or pervasive, and allows direct 

comparisons of how human activities and natural processes affect water quality and ecological 

health in the Nation’s diverse geographic and environmental settings. Comprehensive assessments on pesticides, nutrients, volatile organic compounds, trace metals, and aquatic ecology 

are developed at the national scale through comparative analysis of the Study-Unit findings. 

(http://water.usgs.gov/nawqa/natsyn.html).

The USGS places high value on the communication and dissemination of credible, timely, and 

relevant science so that the most recent and available knowledge about water resources can be 

applied in management and policy decisions. We hope this NAWQA publication will provide you 

the needed insights and information to meet your needs, and thereby foster increased awareness and involvement in the protection and restoration of our Nation’s waters. 
iv

The NAWQA Program 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 a 

fully integrated understanding of watersheds and for cost-effective management, regulation, 

and conservation of our Nation’s water resources. The Program, therefore, depends extensively on the advice, cooperation, and information from other Federal, State, interstate, Tribal, 

and local agencies, non-government organizations, industry, academia, and other stakeholder 

groups. The assistance and suggestions of all are greatly appreciated.

 Robert M. Hirsch

 Associate Director for Water
v

Contents

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

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

Description of Study Area .................................................................................................................. 3

Purpose and Scope ............................................................................................................................. 4

Acknowledgments............................................................................................................................... 5

Study Methods .............................................................................................................................................. 5

Drilling and Sediment Sample Collection ........................................................................................ 5

Analysis of Sediment Cuttings and Core.......................................................................................... 7

Physical Properties .................................................................................................................... 7

Chemistry ................................................................................................................................... 10

Instrument Installation...................................................................................................................... 12

Water-Level Measurements ............................................................................................................ 14

Matric-Potential Measurements..................................................................................................... 14

Collection of Ground Water and Soil Water.................................................................................. 14

Analysis of Ground Water and Soil Water..................................................................................... 15

Water Movement Through the Unsaturated Zone ................................................................................ 15

Total Water Potentials....................................................................................................................... 15

Tritium Profiles.................................................................................................................................... 16

Chloride Profiles................................................................................................................................. 24

Nitrate and Pesticide Profiles.......................................................................................................... 27

Summary and Conclusions........................................................................................................................ 28

References Cited ........................................................................................................................................ 29

Figures

 1. Map showing location of the High Plains aquifer and the unsaturated-zone 

and ground-water monitoring sites in the central High Plains aquifer area ..................... 2

 2. Lithology of unsaturated-zone sediment, locations of monitoring devices 

in the unsaturated zone, and locations of monitoring-well screens at CNG, 

CAL–121, and CAL–122................................................................................................................ 4

 3. Map showing location of irrigated land in the central High Plains..................................... 5

 4. Aerial photograph showing locations of CAL–121 and CAL–122 in areas 

of intensive irrigation .................................................................................................................. 6

 5. Long-term hydrographs for irrigation wells located within 1.6 kilometers 

of (A) CNG, (B) CAL–121, and (C) CAL–122 .............................................................................. 7

 6. Hydrographs for the monitoring wells at CNG, CAL–121, and CAL–122 ............................. 8

 7. Photographs showing the (A) drilling operation, (B) collection of sediment 

cuttings, and (C) and (D) preservation of sediment cores. ................................................... 9

 8. Photograph showing unsaturated-zone monitoring equipment being prepared 

for installation............................................................................................................................. 13
vi

 9–15. Graphs showing:

 9. Matric potentials measured using heat-dissipation probes at depths of 

 (A) 23.6 and 41.5 meters below land surface at CNG, (B) 20.1 and 35.4 meters 

 below land surface at CAL–121, and (C) 9.9 and 35.4 meters below land 

 surface at CAL–122 ............................................................................................................ 17

 10. Total water potential as a function of depth below land surface at (A) CNG, 

 (B) CAL–121, and (C) CAL–122 .......................................................................................... 18

 11. Relation between depth and (A) lithology, (B) tritium concentrations in 

 ground water and cores, (C) chloride concentrations in core and cuttings, 

 and (D) nitrate concentrations in ground water and cuttings at CNG....................... 20

 12. Relation between depth and (A) lithology, (B) tritium concentrations in 

 ground water and cores, (C) chloride concentrations in core and cuttings, 

 and (D) nitrate concentrations in cuttings, soil water, and ground water 

 at CAL–121 ........................................................................................................................... 21

 13. Relation between depth and (A) lithology, (B) tritium concentrations in 

 ground water and cores, (C) chloride concentrations in core and cuttings, 

 and (D) nitrate concentrations in cuttings, soil water, and ground water 

 at CAL–122 ........................................................................................................................... 22

 14. Relation between depth and (A) chloride concentrations and water fluxes 

 and (B) chloride mass-balance age at CNG .................................................................. 26

 15. Relation between depth and pesticide concentrations in core, soil water, 

 and ground water from (A) CNG, (B) CAL–121, and (C) CAL–122 ............................... 28

Tables

 1. History of agronomic practices at CAL–121 and CAL–122, Finney County, 

Kansas ........................................................................................................................................... 8

 2. Physical properties of sediment cores .................................................................................. 11

 3. Gravimetric water content, bulk density, and volumetric water content of 

sediment cores........................................................................................................................... 23

 4. Gravimetric water content and chloride concentrations in sediment cores 

from CNG..................................................................................................................................... 25
vii

Abbreviations used in this report

φp porosity

ρb bulk density

ρp particle density

θ volumetric moisture content

Ψ matric potential

Θ effective saturation

°C degrees Celsius

ASTM American Society for Testing and Materials

AT advanced tensiometer

cm centimeters

cm3 cubic centimeters

g grams

g/cm3

 grams per cubic centimeter

g/m2

/yr grams per square meter per year

g/m3 grams per cubic meter

HDP heat-dissipation probe

HRL Hydrologic Research Laboratory

kg kilograms

km kilometers

km2

 square kilometers

Ksat saturated hydraulic conductivity

K(θ)

 unsaturated hydraulic conductivity

K(θ)

eff effective unsaturated hydraulic conductivity

L liter

m meter

m3

 cubic meters

MCL maximum contaminant level

mg/L milligrams per liter

mg/L as N milligrams per liter as nitrogen

mg/L as P milligrams per liter as phosphorus

mL milliliters

mm millimeters

mm/yr millimeters per year

NAWQA National Water-Quality Assessment

ng/g nanograms per gram

PVC polyvinyl chloride

q water flux

QC blank quality control

TU tritium units

USGS U.S. Geological Survey

μg/g micrograms per gram

μg/L micrograms per liter

μL microliters

μm micrometers

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).
Abstract

The role of irrigation as a driving force for water and 

chemical movement to the central High Plains aquifer is 

uncertain because of the thick unsaturated zone overlying 

the aquifer. Water potentials and profiles of tritium, chloride, 

nitrate, and pesticide concentrations were used to evaluate 

water movement through thick unsaturated zones overlying 

the central High Plains aquifer at three sites in southwestern 

Kansas. One site was located in rangeland and two sites 

were located in areas dominated by irrigated agriculture. 

In 2000–2001, the depth to water at the rangeland site was 

50 meters and the depth to water at the irrigated sites was 

about 45.4 meters. Irrigation at the study sites began in 

1955–56.

Measurements of matric potential and volumetric water 

content indicate wetter conditions existed in the deep unsaturated zone at the irrigated sites than at the rangeland site. Total 

water potentials in the unsaturated zone at the irrigated sites 

systematically decreased with depth to the water table, indicating a potential existed for downward water movement from 

the unsaturated zone to the water table at those sites. At the 

rangeland site, total water potentials in the deep unsaturated 

zone indicate small or no potential existed for downward 

water movement to the water table.

Postbomb tritium was not detected below a depth of 

1.9 meters in the unsaturated zone or in ground water at the 

rangeland site. In contrast, postbomb tritium was detected 

throughout most of the unsaturated zone and in ground water 

at both irrigated sites. These results indicate post-1953 water 

moved deeper in the unsaturated zone at the irrigated sites 

than at the rangeland site. The depth of the interface between 

prebomb and postbomb tritium and a tritium mass-balance 

method were used to estimate water fluxes in the unsaturated 

zone at each site. The average water fluxes at the rangeland 

site were 5.4 and 4.4 millimeters per year for the two methods, 

which are similar to the average water flux (5.1 millimeters 

per year) estimated using a chloride mass-balance method. 

Tritium profiles in the unsaturated zone at the irrigated sites 

were complicated by the presence of tritium-depleted intervals 

separating upper and lower zones containing postbomb tritium. If the interface between prebomb and postbomb tritium 

was at the top of the tritium-depleted interval and postbomb 

tritium detected beneath that interval was from the declining 

water table in the area, then the average water flux at the irrigated sites was estimated to be 21 to 54 millimeters per year. 

If postbomb tritium detected beneath the tritium-depleted 

interval was from bypass or preferential water movement 

through the local unsaturated zone instead of the declining 

water table, then the minimum water flux at the irrigated 

sites was estimated to be 106 to 116 millimeters per year. 

In either case, water fluxes at the irrigated sites were at least 

4 to 12 times larger than the flux at the rangeland site, indicating irrigation was an important driving force for water movement through the unsaturated zone.

The presence of postbomb tritium and large nitrate and 

total pesticide concentrations (24 milligrams per liter as nitrogen and 0.923 microgram per liter, respectively) in ground 

water at the irrigated sites indicates irrigation water also was 

an important driving force for chemical movement to the water 

table. The persistence of a downward hydraulic gradient from 

the deep unsaturated zone to the water table at the irrigated 

sites, in addition to large nitrate and atrazine concentrations 

in deep soil water (34 milligrams per liter as nitrogen and 

0.79 microgram per liter, respectively), indicate that the deep 

unsaturated zone will be a source of nitrate and atrazine to the 

aquifer in the future.

Introduction

The High Plains aquifer is a nationally important water 

resource that underlies about 450,700 km2

 in parts of eight 

States (fig. 1). The aquifer supplies drinking water to 82 percent of the people who live within the aquifer boundary and 

about 30 percent of the ground water used for irrigation in the 

United States (Dennehy, 2000). McGuire and Sharpe (1997) 

reported that of the 20.4 billion m3

 of water pumped from the 

Water Movement Through Thick Unsaturated Zones 

Overlying the Central High Plains Aquifer, Southwestern 

Kansas, 2000–2001

By P.B. McMahon,1 K.F. Dennehy,1 R.L. Michel,1 M.A. Sophocleous,2 K.M. Ellett,1 and D.B. Hurlbut3

1

U.S. Geological Survey

2

Kansas Geological Survey

3

Colorado Department of Agriculture
2 Water Movement Through the Central High Plains Aquifer, Southwestern Kansas, 2000–2001

High Plains aquifer in 1990, 19.4 billion m3

, or about 95 percent, was used for irrigation. Several water-quality studies 

have reported on the presence of agricultural chemicals in the 

unsaturated zone and at the water table in irrigated areas with 

shallow (less than 15 m) depths to ground water (Spalding and 

Kitchen, 1988; Komor and Emerson, 1994; Townsend and others, 1996; Spalding and others, 2001; Costa and others, 2002). 

These findings indicate irrigation water was the driving force 

for transporting agricultural chemicals through the unsaturated 

zone to the water table in those shallow environments. It is 

likely that irrigation water plays a similar role in the High 

Plains because of its long-term use in the area. In some 

areas of the central High Plains, which is the focus of this 

study, irrigation began in the late 1950’s. Historical waterquality data indicate agricultural chemicals are found in the 

central High Plains aquifer (Litke, 2001); however, the unsaturated zone overlying the central High Plains aquifer is relatively thick so the relation between land use and water quality 

in the aquifer is not always clear. In 2000, the average thickness of the unsaturated zone in the central High Plains was 

43.3 m. Recent studies have reported the presence of agricultural chemicals at the water table under some irrigated fields 

Figure 1. Location of the High Plains aquifer and the unsaturated-zone and ground-water monitoring 

sites in the central High Plains aquifer area.

TEXAS

COLORADO

NEW

MEXICO

KANSAS

NEBRASKA

OKLAHOMA

WYOMING SOUTH DAKOTA

Guymon

Elkhart Liberal

Amarillo

Dodge

City

Garden City

CNG

CNG

CAL-122

CAL-121

UNION

QUAY

COLFAX

BACA

LAS ANIMAS

TEXAS

LINCOLN

PUEBLO

BENT

KIOWA

EL PASO

KAY

HARDING

ELLIS

BEAVER

RENO

CURRY

NESS

FORD

DALLAM

GOVE

HALL

CADDO

OTERO

FINNEY

OLDHAM

CIMARRON WOODS

ELLIS

GRAY

CHEYENNE

PROWERS

HARTLEY

GRADY KIOWA

GRAY

LOGAN

RICE

CLARK

GRANT

BARBER MEADE

DEWEY

LANE

MAJOR

SUMNER

BLAINE

RUSH

TREGO

HARPER

DEAF SMITH

CUSTER

MOORE

MARION

POTTER

DONLEY

PRATT

WASHITA

NOBLE

KIOWA

BARTON

CARSON

CASTRO

LOGAN

GARVIN

GARFIELD

KEARNY

SCOTT

PARMER

PAYNE

ALFALFA

SALINE

RANDALL

WOODWARD

RUSSELL

HAMILTON

SWISHER

HARPER

WALLACE

ROBERTS

COMANCHE

KINGMAN

SEDGWICK

WHEELER

SHERMAN

BECKHAM

HEMPHILL

GREER

LIPSCOMB

CANADIAN

PAWNEE

JACKSON

WICHITA

OTTAWA

ROGER

MILLS

LINCOLN

GREELEY

GRANT

MORTON

HANSFORD

OCHILTREE

CROWLEY

STEVENS

DICKINSON

KINGFISHER

STAFFORD

HODGEMAN

SEWARD

STANTON

ARMSTRONG

MCPHERSON

COMANCHE

HUTCHINSON

HARVEY

MCCLAIN

HASKELL

OKLAHOMA

EDWARDS

HARMON

ELLSWORTH

COLLINGSWORTH

0 50 100 KILOMETERS

0 50 100 MILES

Arkansas R.

Cimarron R.

Canadian R.

110° 105° 100° 95°

35°

40°

NORTHERN HIGH

PLAINS

CENTRAL HIGH

PLAINS

SOUTHERN HIGH

PLAINS

EXPLANATION

Unsaturated-zone and ground-water

monitoring site, and site identification

Area

of

Interest
Introduction 3

in the central High Plains (McMahon, 2000, 2001). Although 

these results imply agricultural chemicals were transported 

through the thick unsaturated zone in these irrigated areas, 

water and chemical movement in these settings have not been 

systematically evaluated.

This report evaluates water movement through thick 

unsaturated zones in areas of southwestern Kansas where 

rangeland and irrigated agriculture were the dominant land 

uses. Comparisons of hydraulic and chemical data collected 

from three study sites (fig. 1) provide insights into the processes and rates of water movement in the thick unsaturated 

zone overlying the central High Plains aquifer. Ongoing 

studies in Colorado, Nebraska, and Texas will provide similar 

information on water movement in the unsaturated zones in 

the southern and northern portions of the High Plains aquifer.

Description of Study Area

The three study sites are within the boundaries of the 

central High Plains aquifer in southwestern Kansas (fig. 1). 

Details on the climate, topography, geology, and hydrology 

of the central High Plains region are in Weeks and others 

(1988), Dennehy (2000), and Dennehy and others (2002). 

Details on the climate, topography, geology, and hydrology of 

the unsaturated-zone study sites are presented in this section. 

One site (CNG) is located in Morton County, in the Cimarron 

National Grassland, and two sites (CAL–121 and CAL–122) 

are located in Finney County, within about 8 km of each other 

(fig. 1). Morton and Finney Counties have semiarid continental 

climates with frequent winds, low humidity, and high rates of 

evaporation. The average (1900–2000) air temperatures during 

the winter, spring, summer, and fall in Elkhart, Kans., about 

29 km west of CNG, were 1.8, 12.3, 24.7, and 13.8 °C, respectively. The average (1900–2000) annual precipitation in Elkhart 

was 453 mm, with 56 percent of the precipitation occurring 

during May through August. The average (1948–2000) air temperatures during the winter, spring, summer, and fall in Garden 

City, Kans., about 23 km north of the CAL sites, were –0.6, 

11.0, 24.1, and 12.3 °C, respectively (High Plains Regional 

Climate Center, 2001). The average (1948–2000) annual 

precipitation in Garden City was 487 mm, with 62 percent of 

the precipitation occurring during May through August. Pan 

evaporation in southwestern Kansas ranged from about 2,290 

to 2,410 mm/yr (Weeks and others, 1988).

Topography at CNG is undulating, with slopes less than 

1 percent. The site is in the Vona-Tivoli soil association, which 

is composed of loamy, fine sand (Dickey and others, 1963). 

Vegetation at CNG is sparse and consists of sagebrush, yucca, 

and short grasses. Topography at the CAL sites is characterized by gently sloping (less than 1 percent) plains. These 

sites are within the Manter-Keith soil association, which is 

composed of sandy and loamy soils (Harner and others, 1965). 

Vegetation at the CAL sites is discussed below. In 2000, the 

thickness of the unsaturated zone at CNG was about 50.0 m, 

and at the CAL sites it was about 45.4 m. The lithology of 

sediment in the unsaturated zone at each site is shown in 

figure 2. Unsaturated-zone sediment at all of the sites consists 

primarily of sand; however, intervals of clay, silt, and gravel 

were present at several depths at each site. The most significant clay layer, a brown silty clay grading into a plastic blue 

clay, was below the water table at CAL–122. The top of the 

layer was about 45.7 m below land surface and extended to at 

least 67.1 m (drilling was terminated before reaching the lower 

boundary of the clay). 

The unsaturated-zone monitoring sites are underlain by 

the central High Plains aquifer. The Ogallala Formation is the 

largest geologic unit of the aquifer in the study area (Smith, 

1940; Gutentag and others, 1984). The Ogallala Formation 

consists primarily of unconsolidated clay, silt, sand, and gravel 

with scattered cemented zones consisting of calcium carbonate. Younger sedimentary deposits that comprise a lesser proportion of the aquifer in the study areas include alluvial and 

windblown deposits of Quaternary age. The contact between 

the Ogallala Formation and younger deposits was not mapped 

at the study sites, and it is possible that all or a portion of the 

unsaturated zone at the study sites consists of sediments of 

Quaternary age (Olson and others, 1997).

Large-scale development of the central High Plains 

aquifer for irrigation began in the 1950’s (Luckey and others, 1986); since that time, water levels in some areas of the 

aquifer have declined more than 30 m (McGuire and Sharpe, 

1997). The CAL sites are in one of the most intensively irrigated areas in the central High Plains (figs. 3 and 4). On the 

basis of long-term water-level measurements made in irrigation wells located within 1.6 km of the CAL sites, water levels 

at CAL–121 declined by about 18.3 m from 1962 to 1999 and 

water levels at CAL–122 declined by about 27.4 m from 1961 

to 1999 (fig. 5). The area within a 1-km radius of CNG was 

never irrigated and has been part of the Cimarron National 

Grassland since the 1930’s. An irrigation well located about 

1.6 km west of the CNG site had a 2.1-m net decline in water 

level from 1984 to 1999. Water levels measured in the watertable monitoring wells installed at each site fluctuated less 

than 0.3 m during the 2000–2001 measurement period (fig. 6). 

Sophocleous and others (2002) discussed possible reasons 

why the water-level hydrographs in the monitoring wells 

were essentially flat during the period of measurement.

Water was applied by furrow irrigation directly over 

the CAL–121 borehole location from 1956 to 1989. In 1990, 

the irrigation practice in that quarter section was changed to 

center-pivot sprinkler irrigation. After the change to centerpivot irrigation in 1990, the CAL–121 borehole location 

was 75 m from the edge of the irrigated field and no longer 

received applications of irrigation water. The irrigation history 

at CAL–122 is similar to that at CAL–121 except that furrow irrigation began in 1955. Corn is the primary irrigated 

crop at both sites. Vegetation at the CAL boreholes consisted 

of nonirrigated wheat and sorghum from 1990 to 1999 and 

annual grasses in 2000. Nitrogen fertilizer has been applied to 

the irrigated fields since about 1956. Atrazine has been applied 

to the irrigated fields since about 1960. Specific histories of 

agronomic practices at both CAL sites are presented in table 1. 
4 Water Movement Through the Central High Plains Aquifer, Southwestern Kansas, 2000–2001

Purpose and Scope

The purpose of this report is to describe the results of a 

study that evaluated water movement through thick unsaturated zones overlying the central High Plains aquifer in southwestern Kansas. In 2000, boreholes were drilled at one site in 

Morton County, Kans., at which rangeland was the dominant 

land use, and at two sites in Finney County, Kans., at which 

irrigated agriculture was the dominant land use. Sediment 

cuttings and cores were collected during drilling for the analysis of physical properties (particle size, bulk density, porosity, gravimetric and volumetric water contents, and matric 

potentials) and chemical concentrations (tritium, chloride, 

nitrate, and six pesticides and two pesticide degradates). The 

boreholes were instrumented with monitoring wells and pressure transducers to measure ground-water elevations, advanced 

0

Lithology Well construction Lithology Well construction Lithology Well construction

5

10

15

20

25

30

35

40

45

50

55

60

0 20 40 60

PERCENT CLAY, SILT,

SAND, AND GRAVEL

80 100 0 20 40 60

PERCENT CLAY, SILT,

SAND, AND GRAVEL

80 100 0 20 40 60

PERCENT CLAY, SILT,

SAND, AND GRAVEL

80 100

0

5

10

15

20

25

30

35

40

45

50

55

60

0

5

10

15

20

25

30

35

40

45

50

55

60

0

5

10

15

20

25

30

35

40

45

50

55

60

0

5

10

15

20

25

30

35

40

45

50

55

60

0

5

10

15

20

25

30

35

40

45

50

55

60

DEPTH BELOW LAND SURFACE, IN METERS

CNG

Lithology

EXPLANATOIN

Well construction

CAL–121 CAL–122

Clay

Silt

Sand

Gravel

Sand

Bentonite

Silica flour

Native material

Advanced tensiometer

Heat-dissipation probe

Suction lysimeter

Gas-sampling port

Well screen

Well screen

Well screen

Water table

Water table Water table Water table

Water table

Water table

Figure 2. Lithology of unsaturated-zone sediment, locations of monitoring devices in the unsaturated zone, and locations of 

monitoring-well screens at CNG, CAL–121, and CAL–122.
Study Methods 5

tensiometers (AT’s) and heat-dissipation probes (HDP’s) to 

measure soil-water matric potentials, suction lysimeters were 

installed in the unsaturated zone to collect soil-water samples, 

and ground-water samples were collected from the monitoring wells. Soil water collected in 2000–2001 was analyzed 

for chloride, nitrate, and atrazine. Ground water collected in 

2000–2001 was analyzed for tritium, chloride, nitrate, and 

47 pesticides and pesticide degradates.

Acknowledgments

This study was part of the U.S. Geological Survey 

(USGS) National Water-Quality Assessment (NAWQA) 

Program, High Plains Regional Ground-Water Study. The 

authors thank the Kansas Geological Survey for providing M.A. Sophocleous to assist us during this study, and the 

Colorado Department of Agriculture for providing pesticide 

analyses of sediment core samples. The cooperation of the 

private landowner who agreed to the installation and longterm operation of monitoring equipment on his land and 

who provided detailed historical information on the agronomic practices used at those sites is gratefully acknowledged. The U.S. Forest Service granted permission to install 

and maintain a monitoring facility in the Cimarron National 

Grassland. The authors would like to recognize John Izbicki, 

USGS, for his guidance during the planning and design of this 

study. Geoffrey Delin (U.S. Geological Survey, Minnesota), 

John Izbicki (U.S. Geological Survey, California), and Bridget 

Scanlon (Texas Bureau of Economic Geology) provided colleague reviews of this report.

Study Methods

The following section of the report describes the 

methods used to drill boreholes and collect sediment samples, 

analyze sediment for physical properties and chemistry, instrument the boreholes, measure water levels, measure matric 

potentials, collect ground-water and soil-water samples, and 

analyze the chemistry of soil water and ground water.

Drilling and Sediment Sample Collection

Each borehole was drilled using the under-reamer 

method known as the ODEX air-hammer drilling method 

(Driscoll, 1986; Hammermeister and others, 1986; Izbicki 

and others, 2000) (fig. 7A). The boreholes were drilled 

during March and April 2000. The ODEX method uses a 

downhole percussion hammer to drill and ream at the bottom of a casing. A pilot bit, in conjunction with an eccentric 

reamer, drills a hole slightly larger than the outside diameter 

of the casing. The percussion hammer also impacts on the 

Figure 3. Location of irrigated land in the central High Plains. Locations based on satellite imagery collected in 1992 

(Qi and others, 2002).

CNG

CAL-122

CAL-121

0 50 100 MILES

0 50 100 KILOMETERS

EXPLANATION

Unsaturated-zone

 and ground-water

 monitoring site

Irrigated land
6 Water Movement Through the Central High Plains Aquifer, Southwestern Kansas, 2000–2001

casing through a shoe attached to the bottom joint of the 

casing. Thus, the casing is advanced downward as the hole is 

drilled deeper. Steel casing with an inside diameter of 20 cm 

was used in this study.

Sediment cuttings from each borehole were collected 

by advancing the drill bit in 30-cm increments from land 

surface to the water table and removing the cuttings between 

drilled intervals using compressed air (fig. 7B). Comparison of 

chemical-concentration data from cuttings and core collected 

over the same intervals indicated the cuttings were representative of the drilled interval. Cuttings were used to analyze physical and chemical properties of the sediment. Core 

samples (10 cm in diameter by 60 cm long) were collected 

continuously from land surface to a depth of 3.1 m using a piston core barrel and at incrementally larger spacings between 

cores below a depth of 3.1 m. The core barrel was lined with 

Figure 4. Locations of CAL–121 and CAL–122 in areas of intensive irrigation. Circular areas indicate the locations of fields 

irrigated using center-pivot sprinklers.

CAL-121 CAL-121

CAL-122 CAL-122

Sprinkler-irrigated

field

Furrow-irrigated

field

Sprinkler-irrigated

field

0 1 2 MILES

0 1 2 KILOMETERS
Study Methods 7

four aluminum core liners, each about 15 cm long. For 

each 60-cm core run, one 15-cm-long section was used for 

physical-parameter analysis, one section was used for tritium 

analysis, one section was used for the analysis of chloride, 

nitrate, and pesticides, and one section was held in reserve 

for future analyses. Except for the section used for chloride, 

nitrate, and pesticide analyses, each core section was capped, 

labeled, wrapped in plastic, and sealed in individual aluminum pouches immediately after being removed from the core 

barrel following the procedures of Hammermeister and others 

(1986) and Izbicki and others (2000) to protect the core from 

evaporation and contamination (figs. 7C and 7D). The section 

of core used for chemical analysis was subsampled for pesticide analyses in the field before sealing the core according 

to the procedure previously described. For pesticide analysis, 

approximately 500 g of sediment was removed from the liner 

using a stainless-steel spatula that had been cleaned in a detergent solution and rinsed with deionized water and methanol. 

The 500-g sample was placed in a precleaned amber glass jar, 

sealed with a Teflon-lined cap, and kept chilled until analyzed.

Analysis of Sediment Cuttings and Core

Characterization of the physical properties of the sediment was accomplished through a variety of field and laboratory analyses. Textural analysis of drill cuttings was done 

at the USGS Colorado District laboratory to determine the 

general stratigraphy of the sites and to select core samples 

representing the full range of geologic units contacted 

throughout the unsaturated zone for additional measurements at the USGS Hydrologic Research Laboratory (HRL) 

in Sacramento, Calif. Fifteen core samples were analyzed by 

the HRL. Analyses performed at the HRL included particlesize analysis, bulk density, porosity, gravimetric and volumetric water content, percent saturation, and matric potential. 

All analyses were performed in accordance with standards 

developed by the American Society for Testing and Materials 

(ASTM) or other approved technical procedures.

Physical Properties

Particle-size analysis of the core at the HRL was done 

through a combination of sieve and hydrometer analyses. 

Determination of gravel- and sand-sized fractions on a mass 

basis was accomplished through sieve analysis in accordance 

with ASTM procedure C136 (American Society for Testing 

and Materials, 1997). Hydrometer analysis for determination 

of silt- and clay-sized fractions was done according to the 

HRL procedure HP–263. Classification of textural size 

fractions was in accordance with the U.S. Department of 

Agriculture system (Tindall and Kunkel, 1999). In this system, 

sand-sized particles range from 0.05 to 2.00 mm, silt-sized 

particles range from 0.002 to 0.05 mm, and clay-sized particles are smaller than 0.002 mm. All particles larger than 

2.00 mm in diameter are considered gravel-sized particles.

Textural analysis of the cuttings at the USGS Colorado 

District laboratory was done through a combination of sieve and 

hydrometer analyses. Determination of gravel fractions (larger 

than 2.00 mm) on a mass basis was accomplished through 

sieve analysis in accordance with ASTM procedure C136. 

Figure 5. Long-term hydrographs for irrigation 

wells located within 1.6 kilometers of (A) CNG, 

(B) CAL–121, and (C) CAL–122. DEPTH TO WATER, IN METERS BELOW LAND SURFACE DEPTH TO WATER, IN METERS BELOW LAND SURFACE DEPTH TO WATER, IN METERS BELOW LAND SURFACE

1960's

60

50

40

30

20

60

50

40

30

20

60

50

40

30

20

1970's 1980's 1990's

DECADE

1960's 1970's 1980's 1990's

DECADE

1960's 1970's 1980's 1990's

DECADE

A

B

C

Irrigation well located

one Section west of CNG

Irrigation well located

one Section east of CAL–121

Irrigation well located

in same Section as CAL–122
8 Water Movement Through the Central High Plains Aquifer, Southwestern Kansas, 2000–2001

52

5/1/2000 8/1/2000 11/1/2000 2/1/2001 5/1/2001 8/1/2001 11/1/2001

50

48

46

44

DATE

DEPTH TO WATER, IN METERS BELOW LAND SURFACE

CAL–121

CAL–122

CNG

Figure 6. Hydrographs for the monitoring wells at CNG, CAL–121, and CAL–122.

Table 1. History of agronomic practices at CAL–121 and CAL–122, Finney County, Kansas.

Agronomic history CAL–121 CAL–122

Year field was first cultivated approximately 1900 approximately 1915

Year field was first irrigated 1956 1955

Primary crop before irrigation wheat/fallow wheat/fallow

Primary crop during irrigation corn corn

Irrigation method furrow irrigation, 1956–89; 

sprinkler irrigation 1990–2001

furrow irrigation, 1955–89; 

sprinkler irrigation, 1990–2001

Year nitrogen applications began 1956 1955

Year atrazine applications began 1960 1958
Study Methods 9

A

B

C

D

Figure 7. Photographs showing the (A) drilling operation, (B) collection of sediment cuttings, and (C) and 

(D) preservation of sediment cores.
10 Water Movement Through the Central High Plains Aquifer, Southwestern Kansas, 2000–2001

Hydrometer analysis for determination of sand, silt, and 

clay-sized fractions was done in accordance with the method 

described by Gee and Bauder (1986). 

Physical properties of the core samples were determined 

according to the HRL procedure HP–229. Bulk density (ρb

, 

in grams per cubic centimeter) was measured in the following 

way: a sample of known volume was ovendried at 105 °C and 

the dried sample was weighed. To measure porosity (φp

, in 

cubic centimeters per cubic centimeter), the cores were fully 

saturating under a vacuum and the volume of pore space was 

calculated as the difference in mass between the saturated 

and ovendried sample (mass = volume because the density 

of water = 1.0 g/cm3

). This direct measurement is considered 

preferable to calculating porosity from the equation φp

 = 

1 – (ρb/ρp) because particle density (ρp) was not measured. 

Gravimetric (g/g) and volumetric (θ, in cubic centimeters 

per cubic centimeter) water contents were measured as the 

difference between the initial (field) sample weight and the 

oven-dry weight. Percent saturation (%) was calculated as the 

ratio of the water-filled pore space (θ) to the total porosity of 

the sample. The accuracy of these measurements was dependent upon the precision of the balances used for weighing the 

samples (±0.01 g) as well as sample storage (preservation 

of field moisture conditions) and sample handling (loss of 

sample throughout measurement process). Calibration of all 

measurement devices to National Institute of Standards and 

Technology traceable standards and a documented Quality 

Assurance program at the HRL ensured minimal error in the 

measurements.

A matric potential was measured in one core from each 

site to supplement matric-potential measurements made in 

the field. The measurements were accomplished by installing 

tensiometers and HDP’s in the laboratory. The tensiometer 

method provided a direct measurement of soil suction, and the 

HDP method used the thermal properties of a calibrated sensor 

for indirect measurement of the soil matric potential (Phene 

and others, 1971). In accordance with the HRL procedure, a 

microtensiometer and a calibrated HDP (Campbell Scientific 

Inc., model 229) were inserted into the center of the core. 

HDP’s were saturated before installation, and measurements 

were collected on a 4-hour interval by use of an automated 

data-logging system. Due to capillary forces, water from the 

saturated ceramic of the HDP, as well as the sealed reservoir 

of the tensiometer, drained into the soil matrix until equilibrium was reached. The accuracy of the tensiometer measurement depended on the sensor used to measure pressure in the 

reservoir (2 millibars in this case). The accuracy of the HDP 

matric-potential measurement depended on the porous ceramic 

material from which the sensor is constructed and the method 

of calibration. Campbell and Gee (1986) reported an accuracy 

of 0.1 bar or better in the range of matric potentials measured 

on these cores. In this study, matric potentials are reported 

in centimeters (1 bar = 1,021 cm). Results of the laboratory 

analysis of physical properties are given in table 2.

Chemistry

Sediment samples were analyzed for tritium, inorganic 

anions, and pesticides and pesticide degradates to assess the 

movement of water and chemicals in the unsaturated zone at 

the CNG and CAL sites.

Tritium. Tritium was measured in water extracted from 

about 1 kg of core material by vacuum distillation at 80 °C and 

analyzed by liquid scintillation with electrolytic enrichment at 

the USGS Tritium Laboratory in Menlo Park, Calif. (Thatcher 

and others, 1977). The 1-sigma precision of the analyses 

ranged from 0.3 to 2.8 tritium units (TU) and was better for 

moist cores than for dry cores. 

Inorganic anions. Sediment cuttings were analyzed 

for their bromide, chloride, nitrite, nitrate, orthophosphate, 

and sulfate contents. Only the chloride and nitrate data are 

presented in this report. These inorganic anions were extracted 

from sediment using the method described by Lindau and 

Spalding (1984) and Herbel and Spalding (1993), with slight 

modification. Briefly, cuttings were ovendried at 50 °C for 

24 hours, and 10 g of the dried sediment was combined with 

100 mL of deionized distilled water. The sediment/water 

mixture was hand stirred for 1 minute and then placed on 

an orbital shaker for 1 hour at 170 revolutions per minute. 

After shaking, the sediment/water mixture was centrifuged 

for 10 minutes, and then the liquid layer was separated and 

filtered through a 0.45-μm filter. Anion concentrations in 

the filtered solution were quantified using ion chromatography with conductivity detection. Anion concentrations were 

initially reported on a dry-weight basis (micrograms per 

gram dry sediment) and converted to a volume basis (milligrams per liter water) using the gravimetric water content 

of the sediment.

A method blank was run after every sixth environmental sample by subjecting distilled deionized water 

to the extraction process to verify that the process did not 

contaminate samples. Analyses of method blanks indicated 

the extraction procedure was not a substantial source of dissolved inorganic anions. A replicate sample was run after 

every fifth environmental sample to assess the effects of 

field and laboratory procedures on measurement variability. 

The median percent difference in chloride concentrations 

in 38 pairs of replicate samples was 5 percent. The median 

percent difference in nitrate concentrations in 38 pairs of 

replicate samples was 1.5 percent. Detection limits for chloride and nitrate were about 0.8 μg/g and 0.2 μg of nitrogen/g, 

respectively.

Pesticides and pesticide degradates. Sediments were 

analyzed for atrazine and two of its degradates, deisopropylatrazine and deethylatrazine, along with prometon, alachlor, metolachlor, simazine, and cyanazine at the Colorado Department of 

Agriculture Ground-Water Laboratory in Denver, Colo. Three 

types of sediment samples were analyzed: environmental, 

blank quality control (QC), and fortified QC samples. For 
Study Methods 11

Table 2. Physical properties of sediment cores. (Matric potentials were measured in the field using heat-dissipation probes, unless oth erwise noted.)[m, meters; g/cm3, grams per cubic centimeter; cm3/cm3, cubic centimeters per cubic centimeter; g/g, grams per gram; –cm, negative centimeters; --, not measured or not at equilibrium; 

*, measured in laboratory]

Sample 

identification

Depth

below land

surface

(m)

Bulk

density

(g/cm3)

Porosity

(cm3/cm3)

Gravimetric 

water 

content

(g/g)

Volumetric

water

content

(cm3/cm3)

Saturation

(percent)

Matric

potential

(–cm)

Gravel

(percent)

Sand

(percent)

Silt

(percent)

Clay

(percent)

CNG 26–26.5 7.9 1.67 0.390 0.025 0.041 11 -- 0 94 1 5CNG 37.5–38 11.4 1.69 .384 .099 .167 43 -- 2 60 22 16CNG 77.5–78 23.6 1.99 .237 .087 .173 73 1,583 to 1,788 7 59 18 16CNG 136–136.5 41.5 1.72 .381 .041 .071 19 725 0 89 5 6CNG 157.5–158 48.0 1.56 .430 .249 .387 90 633* 0 35 39 26CAL–121 21–21.5 6.4 1.60 .401 .054 .085 21 -- 5 74 12 9CAL–121 46–46.5 14.0 1.71 .413 .143 .243 59 -- 0 43 46 11CAL–121 66–66.5 20.1 1.88 .310 .037 .070 23 235 25 69 3 3CAL–121 86–86.5 26.2 2.04 .258 .028 .058 22 286* 34 62 3 1CAL–121 116–116.5 35.4 1.83 .334 .044 .081 24 255 9 87 2 2CAL–122 32.5–33 9.9 1.81 .332 .065 .118 36 654 3 83 6 8CAL–122 46–46.5 14.0 1.96 .261 .016 .031 12 -- 46 52 1 1CAL–122 77.5–78 23.6 1.76 .345 .022 .038 11 -- 11 87 0 2CAL–122 97.5–98 29.7 1.70 .392 .205 .348 89 153* 0 22 47 31CAL–122 116–116.5 35.4 1.89 .270 .048 .090 33 225 to 286 30 64 2 4
12 Water Movement Through the Central High Plains Aquifer, Southwestern Kansas, 2000–2001

environmental samples, 100 g of sediment was weighed into a 

250-mL screw-top Nalgene bottle, a 50-μL aliquot of surrogate 

(bromacil) solution was added, and the sample was mixed and 

allowed to sit for 15 minutes before being extracted. For blank 

QC samples, 100 g of sediment that did not contain any of the 

target analytes was weighed into a 250-mL screw-top Nalgene 

bottle. A 50-μL aliquot of the surrogate solution was added, 

and the sample was mixed and allowed to sit for 15 minutes 

before being extracted. For the fortified QC samples, 100 g 

of sediment that did not contain any of the target analytes 

was weighed and placed into a 250-mL screw-top Nalgene 

bottle, a 50-μL aliquot of the surrogate solution and a 

100-μL aliquot of a 1-mg/L fortification solution (solution 

containing each target analyte) were added, and the sample 

was mixed and allowed to sit for 15 minutes before being 

extracted. 

To each field and QC sample, 40 mL of 0.3 millinormal 

sodium hydroxide solution was added and thoroughly mixed 

throughout the soil. Methanol, 60 mL, was added and the 

sealed container was placed on a wrist shaker for 15 minutes. 

The container was transferred to a suitable centrifuge and spun 

at 3,000 revolutions per minute for 5 minutes. The liquid layer 

was carefully decanted into a 500-mL flat-bottomed round 

flask. The extraction process was repeated two additional 

times using 20-mL aliquots of the 0.3-millinormal sodium 

hydroxide solution. These extracts also were added to the 

500-mL flat-bottomed round flask. The resulting solution 

was concentrated to a volume of 25 to 60 mL under vacuum 

by connecting the collection flask to a Roto-Vap and placing 

the flask in a 45 °C water bath. The concentrate was poured 

into the reservoir of a previously prepared solid-phase extraction column (packed with 3 cm3

, 60 mg Oasis HLB, Waters 

Corporation), and the evaporation flask was rinsed with three 

10-mL aliquots of water. The rinses also were added to the 

reservoir. Using a vacuum pump, the concentrate/rinsate 

solution was eluted through the solid-phase extraction column at a rate of one to two drops per second. After sample 

loading, the sorbent bed of the extraction column was washed 

with three 3-mL aliquots of a 5-percent methanol in water 

(volume/volume) solution. The vacuum pump was set to 

produce a 12-mm mercury vacuum, and the column was dried 

for 30 minutes to remove any residual liquid. After drying, a 

15-mL centrifuge tube containing 0.5 mL of n-butyl acetate 

was placed beneath the sample outlet port connecting the 

extraction column to the vacuum manifold. The target analytes 

and surrogate compound were removed from the sorbent bed 

by eluting three 1-mL aliquots of methanol through the extraction column and collecting the eluates in the centrifuge tube. 

The vacuum pump was used to remove any residual solvent 

from the extraction column after each methanol elution. With 

the methanol eluates collected, the centrifuge tube was placed 

in an ambient temperature water bath and the sample was 

evaporated to approximately 0.5 mL by using a gentle stream 

of nitrogen gas. The concentrate was transferred to a 1-mL 

Type A volumetric tube. The centrifuge tube was rinsed with 

three 100-μL aliquots of n-butyl acetate, which then were 

added to the volumetric tube. A 50-μL aliquot of the internal 

standard solution (phenanthrene-d10) was added and the solution was then diluted to volume with n-butyl acetate and 

mixed. The final solution was transferred to a screw-top amber 

glass vial and analyzed by capillary gas chromatography/mass 

spectrometry. Ninety-five percent of the samples were analyzed in duplicate.

The median detection level for each compound 

was 0.049 ng/g (atrazine), 0.054 ng/g (deethylatrazine), 

0.073 ng/g (deisopropylatrazine), 0.049 ng/g (alachlor), 

0.096 ng/g (cyanazine), 0.034 ng/g (metolachlor), 0.073 ng/g 

(prometon), and 0.046 ng/g (simazine). The mean recovery 

efficiency for each compound in fortified QC samples was 

103 percent (atrazine), 98.4 percent (deethylatrazine), 123 percent (deisopropylatrazine), 109 percent (alachlor), 110 percent 

(cyanazine), 107 percent (metolachlor), 110 percent (prometon), and 103 percent (simazine). The median percent difference in atrazine and deethylatrazine concentrations, based on 

analyses of four and eight replicate samples, respectively, was 

8 and 14 percent.

Instrument Installation

A water-table monitoring well, AT’s, HDP’s, suction 

lysimeters, and gas-sampling ports were installed in each 

borehole (fig. 2). The AT’s and HDP’s were provided and 

maintained by the Kansas Geological Survey. Monitoring 

wells were constructed using flush-threaded, 5-cm-diameter, 

schedule-40 polyvinyl chloride (PVC) casing and screen. 

Each well had a 0.9- to 3-m-long sump at the bottom and a 

3-m-long screen with 0.25-mm-wide slots. For the CNG and 

CAL–121 sites, the top of the screened interval was placed 

2 to 3.4 m below the water table at the time of drilling. At 

CAL–122, however, the bottom of the screen was placed 

1.2 m below the water table because of the presence of a clay 

layer below the water table. Generally, wells were installed by 

placing the PVC well screen and casing in the borehole after 

drilling to the target depth. A sand filter was then placed in the 

annular space around the well screen with a bentonite plug on 

top of the sand filter. As the 20-cm-diameter outer steel casing was removed from the hole, granular bentonite was used 

to seal the annular space from the top of the plug to the next 

instrumented interval.

The porous ceramic cup of each AT was epoxied to the 

bottom of a 3.2-cm-diameter, flush-threaded, schedule-80 

PVC casing, which also provided the support for the HDP’s, 

suction lysimeters, and gas-sampling ports. Hubbell and 

Sisson (1998) provided a detailed discussion of the AT’s. The 

suction lysimeters (Model 1940, Soil Moisture Equipment 

Corporation, Goleta, California) were 4.8 cm in diameter and 

60 cm in length, with 2-bar ceramic cups. The sample chamber was made of schedule-40 PVC. The gas-sampling ports 

consisted of 3.3-cm-diameter by 30-cm-long, stainless-steel 

wire-wrapped screens with 0.25-mm-wide openings attached 
Study Methods 13

to 0.6-cm-diameter copper tubing (Intermountain Drilling 

Supply, Salt Lake City, Utah). HDP’s, suction lysimeters, 

and gas-sampling ports (with associated wiring or tubing) 

were attached to the outside of the PVC casing with plastic 

ties so the HDP, ceramic cup of the suction lysimeter, and 

midpoint of the gas-sampling port were located 0.3, 0.9, and 

1.5 m, respectively, above the porous cup of the AT (fig. 8). 

The ceramic cup of each AT was saturated by soaking it in a 

beaker of deionized water before placing it in the borehole. 

Each HDP was embedded in silica flour, wrapped in cheesecloth, and saturated with deionized water before placing it in 

the borehole. Wires and tubing leading from the instrumentation to the land surface, together with data-logging devices (to 

be discussed in following sections), were housed at land surface in a weather-proofed structure constructed by the Kansas 

Geological Survey.

Gas port 

Suctioncup

lysimeter

Heatdissipation

probe Advanced

tensiometer

Figure 8. Photograph showing unsaturated-zone monitoring equipment being prepared 

for installation.
14 Water Movement Through the Central High Plains Aquifer, Southwestern Kansas, 2000–2001

Different materials were used to backfill the borehole, 

depending on the instrumentation. Silica flour was used to 

backfill around the AT’s, HDP’s, and suction-cup lysimeters. 

Silica sand (10 to 20 mesh) was used to backfill around the 

gas-sampling ports. Granular bentonite was used to seal the 

annular space between instruments (fig. 2).

Water-Level Measurements

Water levels in the monitoring wells were measured 

manually using an electric tape during each site visit and 

automatically on a daily basis using a pressure transducer 

(Model PDX–261, In-Situ, Inc., Laramie, Wyo.) and data 

logger (Model 21X, Campbell Scientific, Inc., Logan, Utah) 

that were provided and maintained by the Kansas Geological 

Survey.

Matric-Potential Measurements

Matric potentials were measured in the field by using 

HDP’s. The AT’s used in this study proved ineffective because 

of their limited range of measurement (less than –817 cm), 

apparent damage during installation, and difficulties with the 

transducer; therefore, AT’s will not be discussed further in 

this report. The reader is referred to Sophocleous and others 

(2002) for additional information regarding performance of 

the AT’s. The HDP’s used in this study (Model 229, Campbell 

Scientific, Inc., Logan, Utah) were calibrated at the HRL in 

Sacramento, Calif. The HDP consists of a thermocouple wire 

pair and heater wire enclosed in a hypodermic needle that is 

embedded in a cylindrical, porous ceramic body. The sensor operates by passing an electric current through the heater 

wire and measuring the thermocouple temperature rise with 

time. The rate of heat dissipation through the ceramic from 

the needle is dependent on the thermal conductivity of the 

ceramic, which will change with θ. When placed in contact 

with the soil, the HDP should equilibrate to the same matric 

potential as the soil. In this study, temperatures were measured 

1 second after heating started and again 20 seconds after heating started (Sophocleous and others, 2002). The difference in 

temperature between the early and late reading was used to 

compute matric potential. The HDP’s were controlled using 

the 21X data logger.

Collection of Ground Water and Soil Water

Water was collected from the monitoring wells and 

analyzed for tritium, major inorganic ions, nutrients, and 

pesticides and pesticide degradates. Only the tritium, chloride, 

nitrate, and pesticide data are presented and discussed in this 

report. Water from the monitoring wells was collected using 

a disposable plastic bailer or a submersible pump made of 

stainless steel and Teflon. For both sampling devices, three 

casing volumes of water were removed from the well before 

measuring pH, water temperature, specific conductance, 

alkalinity, and dissolved oxygen in the field. To minimize 

the risk of sample contamination, all sample collection and 

preservation took place in dedicated environmental chambers consisting of clear polyethylene bags supported by PVC 

frames. Sampling equipment was either disposed of (bailers) 

or thoroughly cleaned (submersible pump) after sampling each 

well. Unfiltered 1-L water samples were collected in unrinsed, 

high-density polyethylene bottles and capped with polyseal 

caps for the tritium analysis. The bottle caps were secured 

with electrical tape and stored at room temperature until delivery to the laboratory for analysis. Water samples collected for 

the analysis of chloride and nitrate were filtered using capsule 

filters (0.45 μm), collected in field-rinsed plastic bottles, and 

stored on ice with no other preservation. Water samples collected for the analysis of pesticides and pesticide degradates 

were passed through a methanol-rinsed stainless-steel filter 

chamber containing a baked 0.7-μm glass fiber filter. Pesticide 

samples were collected in clean baked amber-colored glass 

bottles and immediately chilled on ice until delivery to the 

laboratory for analysis. 

Ground water also was collected from the largecapacity wells used to supply irrigation water at the study 

sites and analyzed for the same constituents as water from the 

monitoring wells. Irrigation water at each site was supplied by 

a single large-capacity well that pumped continuously for at 

least 1 week before sampling. Each well was equipped with 

a sampling port upstream from any chemical injection port. 

Procedures for measurement of field parameters and collection 

of water samples were the same as described for the monitoring wells.

Water in the unsaturated zone (soil water) was collected 

using the suction-cup lysimeters installed at each site and analyzed for inorganic anions and atrazine. At least 1 week before 

sampling, the lysimeters were purged of residual soil water 

from previous sampling events by using a hand pump to apply 

pressure to the sample chamber through the pressure/vacuum 

line. The lysimeters were equipped with one-way valves so 

water in the lysimeters could not reenter the formation when 

pressure was applied. Once the sample chamber was purged 

of residual water, a 60-centibar vacuum was applied to the 

sample-chamber/ceramic-cup assembly to induce the movement of new soil water into the lysimeter. The newly collected 

soil water was removed from the sample chamber as previously described. Soil water was initially collected in graduated 

polyethylene beakers to determine the volume of collected 

water. Aliquots of water from the beakers were then filtered 

through 0.45-μm syringe filters and stored in precleaned 

polyethylene or amber-glass bottles for anion and atrazine 

analyses.
Water Movement Through the Unsaturated Zone 15

Analysis of Ground Water and Soil Water

Chemical analyses of water collected from monitoring and 

irrigation wells were done at the USGS National Water Quality 

Laboratory in Lakewood, Colo., using the methods of Fishman 

(1993) and Zaugg and others (1995). Tritium analyses were done 

at the USGS Tritium Laboratory in Menlo Park, Calif. (Thatcher 

and others, 1977), and analyses for chloroacetanilide herbicide 

degradation products were done at the USGS Pesticide Research 

Laboratory in Lawrence, Kans. (Hostetler and Thurman, 1999). 

Soil water was analyzed for inorganic anions using ion 

chromatography with conductivity detection at the USGS 

Colorado District Laboratory. Detection limits for chloride 

and nitrate were about 0.1 mg/L and 0.05 mg/L as nitrogen 

(N), respectively. Soil water was analyzed for atrazine by 

immunoassay at the USGS Pesticide Research Laboratory 

in Lawrence, Kans. The detection limit for atrazine using the 

immunoassay method was 0.1 μg/L. The immunoassay may 

cross-react with other triazine herbicides beside atrazine. The 

most significant cross-reactivity is against the triazine herbicides 

ametryn, prometryn, and propazine (Thurman and others, 1990). 

However, ametryn was not used in the study area, prometryn’s 

only row-crop usage is on cotton, and propazine is not used on 

row crops (Lisa Zimmerman, U.S. Geological Survey, written 

commun., 2000). The cross-reactivities of deethylatrazine and 

deisopropylatrazine are negligible; therefore, neither of these 

atrazine degradates was quantified using the immunoassay technique. This is not of concern for deisopropylatrazine because 

it is a minor degradate in soil, but deethylatrazine is a major 

degradation product of atrazine in soil (Adams and Thurman, 

1991). A third atrazine degradate, hydroxyatrazine, also was 

not analyzed for in this study. Thus, the immunoassay analyses 

could underestimate the concentration of atrazine plus atrazine 

degradates in soil water. 

Water Movement Through the 

Unsaturated Zone

Several approaches were used to evaluate water movement through the thick unsaturated zone at the study sites. 

Matric potentials and total water potentials were used to evaluate moisture conditions and directions of water movement, 

respectively. Tritium and chloride profiles were used to estimate water fluxes, and nitrate and pesticide profiles were used 

to evaluate the movement of agricultural chemicals through 

the unsaturated zone. Results of these analyses are presented 

in the following sections.

Total Water Potentials

One of the most fundamental relations in hydrology is 

Darcy’s Law. This relation states that the rate of water flow 

in a rigid, saturated, porous medium is equal to the product 

of the material’s saturated hydraulic conductivity and the 

applied total hydraulic-head gradient. This relation can be 

expressed mathematically as

Q

A

--- –Ksat

∆H

∆X------- © 1 § · = (1) 

where

Q is the volume of water flowing per unit time,

A is the cross-sectional area, and

and

∆H/∆X is the total hydraulic-head gradient, where ∆H

is the total head difference between the two 

points of interest, and ∆X is the total distance 

between the same two points.

The quantity Q/A is referred to as the water flux and is denoted 

by the symbol q.

In the case where the porous material is unsaturated, the 

pathway for water flow becomes substantially reduced and 

the hydraulic conductivity of the material becomes a function 

of the water content. The governing equation (known as the 

Darcy-Buckingham equation) for such a system is a modification of equation 1 above, yielding

q K– ( ) θ ∂H

∂z

------- –K( ) θ ∂

∂z = = -----( ) h z + (2)

where K(θ) represents the unsaturated hydraulic conductivity and H represents the total water potential. Assuming 

osmotic and pneumatic potentials are negligible, the total 

water potential can be separated into two components, the 

matric potential, h, and the gravitational potential, z. Water 

in the unsaturated zone moves from large to small total 

water potentials.

Matric potential is a function of water content and 

sediment texture, so both factors need to be considered 

when using matric potential to evaluate moisture conditions 

in the unsaturated zone. For example, the matric potentials at 

41.5 m (–725 cm) and 48.0 m (–633 cm) at CNG were similar 

(table 2), given the ±102 cm accuracy of the HDP. However, 

the volumetric water content at 48.0 m was five times larger 

than the water content at 41.5 m (table 2). This difference in 

water content is attributed, in part, to the much larger clay 

plus silt content of sediment at 48.0 m (65 percent clay plus 

silt) compared to sediment at 41.5 m (11 percent clay plus 

silt). Sediment from 41.5 m at CNG, 35.4 m at CAL–121, and 

35.4 m at CAL–122 had similar textures (table 2); therefore, 

matric potentials measured at those depths may be useful for 

comparing moisture conditions at those depths. The matric potentials measured by the 35.4-m HDP’s at CAL–121 

and CAL–122 were about three times larger than the matric 

potential measured by the 41.5-m HDP at CNG (table 2), 

indicating deep unsaturated-zone sediment at the CAL sites 

was wetter than deep sediment at CNG. In fact, the volumetric 
16 Water Movement Through the Central High Plains Aquifer, Southwestern Kansas, 2000–2001

water contents of the two CAL cores from 35.4 m (0.081 and 

0.090) were larger than the water content of the CNG core 

from 41.5 m (0.071). The wetter conditions at the CAL sites 

probably resulted from a combination of downward movement 

of irrigation water during the 34- to 35-year period of furrow 

irrigation and the declining water table at those sites (fig. 5). 

The smallest measured volumetric water content at each site 

occurred in sediment with at least 94 percent sand plus gravel 

(table 2). These small water contents may result from rapid 

drainage of water in these very coarse-grained sediments. 

The HDP data indicate the instruments required several months to reach equilibrium with the formation (fig. 9). 

Although the ceramic of the HDP’s was thoroughly saturated 

before borehole installation, a dry backfill material was used 

to minimize disturbance of the initial hydrologic conditions 

and to avoid introducing fluids that might alter the soil-water 

chemistry. If the geologic formation surrounding the instrument zone is relatively dry, the HDP will typically desaturate 

in the backfill material before the formation itself reaches 

potential equilibrium with the backfill material. After the 

initial equilibration period, the matric potentials remained relatively constant at most of the probe depths and did not change 

in response to recharge-event-based or seasonal fluctuations 

in matric potential that might have occurred near the land surface. Scanlon and others (1999) also observed little temporal 

fluctuation in matric potentials below a depth of about 7 m 

during more than 2 years of monitoring in the Chihuahuan 

Desert of Texas. Walvoord and others (2002) concluded that 

matric potentials in deep unsaturated zones under vegetated 

desert landscapes respond so slowly to surface transients, 

perhaps over thousands of years, that they appear to be at 

steady state. The apparent steady-state conditions shown in 

figure 9 also may result from the deep unsaturated zone having 

response times to surface transients longer than the measurement period (about 1 year).

Total water potentials in the unsaturated zone at the CAL 

sites systematically decreased with depth to the arbitrary value 

of zero at the water table (figs. 10B and 10C). Water in the 

unsaturated zone moves from large to small potentials, so the 

data indicate a potential existed for downward water movement to the aquifer at the CAL sites. Total water-potential 

gradients across three of the depth intervals at the CAL sites 

were close to one (figs. 10B and 10C), which is the maximum 

gradient under gravity-driven flow. Total water potentials at 

CNG may not have systematically decreased with depth. The 

negative total water potential at 48.0 m indicates a potential 

existed for upward water movement from the water table 

(fig. 10A). The measurement at 48.0 m was made on sediment 

in the laboratory, so it is possible that the measurement was 

not representative of field conditions. Even if that measurement is excluded from the analysis, the total water-potential 

gradient between the 41.5-m HDP and water table was close to 

zero and was much smaller than the gradients above the water 

table at the CAL sites (fig. 10). This result indicates a much 

smaller potential existed for downward water movement to the 

aquifer at CNG compared to the CAL sites. 

The preceding analysis of total water-potential gradients 

does not consider components of horizontal flow that might 

have developed because of the proximity of irrigated fields 

to the CAL boreholes and textural layering in the unsaturated 

zone. Horizontal flow could occur along contacts between 

sediment layers of contrasting texture. The contact between 

an upper gravelly sand and lower sandy clay loam at a depth 

of 27 m at CAL–122 may be an example of this (fig. 2). Gravity drainage of water in the coarse-grained sediment could be 

impeded by the fine-grained sediment, causing water to move 

laterally along the contact between the two layers. A different depositional sequence existed at a depth of about 20 m 

at CAL–121 where a gravelly sand was overlain by loamy 

sand (fig. 2). Because of the larger water content of the loamy 

sand (0.243 at 14.0 m) compared to the gravelly sand (0.058 

at 26.2 m), the loamy sand could have the larger unsaturated 

hydraulic conductivity. As a consequence, the coarse-grained 

interval could act as a barrier to vertical water movement, in 

which case, water would move laterally along the contact of 

the gravelly sand and below the loamy sand. Matric-potential 

measurements at different locations along these contacts are 

needed to verify this potential for horizontal flow. 

Tritium Profiles

Tritium is a radioactive isotope of hydrogen with a 

half-life of about 12.3 years. It is an excellent tracer of water 

movement because it substitutes for stable hydrogen isotopes 

in the water molecule. Because tritium is radioactive, its concentration in the unsaturated zone decreases over time due to 

radioactive decay. In general, tritium in the unsaturated zone 

originates from precipitation. Before the onset of atmospheric 

testing of nuclear weapons in 1953 (prebomb), the tritium content of precipitation in the study area was probably about 8 TU 

(Thatcher, 1962). As a result of radioactive decay, moisture in 

the unsaturated zone derived from precipitation that fell before 

1953 would contain less than 0.5 TU in 2000. The tritium content of precipitation increased substantially after the onset of 

atmospheric nuclear weapons testing but has slowly decreased 

from its peak in about 1963. Even with the variability in the 

tritium content of precipitation over time, moisture totally 

derived from precipitation that fell since 1953 (postbomb) 

contained more than 0.5 TU in 2000.

Tritium profiles for each site are shown in figures 11B, 

12B, and13B. The vertical dashed line in each plot indicates 

the estimated maximum concentration of prebomb tritium in 

the year 2000, which is the year samples for this study were 

collected and analyzed. At CNG, postbomb tritium was not 

detected below a depth of 1.9 m in the unsaturated zone or 

in ground water. In contrast, postbomb tritium was detected 

throughout most of the unsaturated zone and in ground water 

at both CAL sites. These data indicate post-1953 water moved 

more rapidly through the unsaturated zones at the CAL sites 

than at CNG. 
Water Movement Through the Unsaturated Zone 17

Figure 9. Matric potentials measured using heat-dissipation probes at depths of (A) 23.6 and 

41.5 meters below land surface at CNG, (B) 20.1 and 35.4 meters below land surface at CAL–121, 

and (C) 9.9 and 35.4 meters below land surface at CAL–122.

3,500

3,300

2,500

2,000

1,500

1,000

500

5/1/2000 11/1/2000 5/1/2001

DATE

11/1/2001

5/1/2000 11/1/2000 5/1/2001

DATE

11/1/2001

5/1/2000 11/1/2000 5/1/2001

DATE

11/1/2001

500

400

300

200

100

1,800

1,500

1,200

900

600

300

0

MATRIC POTENTIAL, IN NEGATIVE CENTIMETERS MATRIC POTENTIAL, IN NEGATIVE CENTIMETERS MATRIC POTENTIAL, IN NEGATIVE CENTIMETERS 

A

B

C

Missing data

23.6 m

41.5 m

35.4 m

20.1 m

9.9 m

35.4 m
18 Water Movement Through the Central High Plains Aquifer, Southwestern Kansas, 2000–2001

Two methods were used to estimate water fluxes from the 

tritium profiles. In the case where the interface between prebomb and postbomb tritium may be present in the unsaturated 

zone or where the peak concentration of postbomb tritium 

in the unsaturated zone is attributed to the 1963 peak in the 

tritium concentration in precipitation, the water flux (q) would 

be calculated as

q = θz t ⁄ (3)

where

z is the depth of the interface or postbomb peak 

below land surface,

and

t is the elapsed time since 1953 or 1963.

Water fluxes calculated on the basis of equation 3 assume 

piston movement of the wetting front, with no lateral spreading with depth, or preferential flow (Izbicki and others, 2002). 

These assumptions may not be met at the study sites. 

Water flux also was calculated using a tritium massbalance approach (Allison and Hughes, 1978)

q WT ( ) ⁄ t T TT = ( ) ⁄ (4)

where

WT is the total water (precipitation plus irrigation) 

applied at the land surface since 1953,

t is the elapsed time since 1953,

T is the measured amount of tritium in the unsaturated zone above the prebomb/postbomb 

interface,

and

TT is the total amount of tritium that could have been 

added to the profile since 1953.

The value of T is obtained by integrating the tritium profile 

from land surface to the prebomb/postbomb interface

A B C

TOTAL WATER POTENTIAL,

IN CENTIMETERS

–600 0

50

45

40

35

30

25

20

15

10

5

600 1,200 1,800 2,400 3,000

TOTAL WATER POTENTIAL,

IN CENTIMETERS

–600 0 600 1,200 1,800 2,400 3,000

TOTAL WATER POTENTIAL,

IN CENTIMETERS

–600 0 600 1,200 1,800 2,400 3,000

Water table Water table

Water table

DEPTH BELOW LAND SURFACE, IN METERS

50

45

40

35

30

25

20

15

10

5

50

45

40

35

30

25

20

15

10

5

–0.463 

± 0.171

–0.858 ± 0.314

2.16 ± 0.510 –0.147 ± 0.120

–0.704 ± 0.104

–0.966 ± 0.222

–1.08 ± 0.334 –0.747 

± 0.103

–1.18 ± 0.410

–0.749 

± 0.130

Figure 10. Total water potential as a function of depth below land surface at (A) CNG, (B) CAL–121, and (C) CAL–122. 

Error bars represent the ±102 centimeter accuracy of the heat-dissipation probes plus uncertainty in the equilibrium value 

of matric potential measured by the probes. The numbers on the plotted lines are total water-potential gradients between 

adjacent measurements. Negative total water-potential gradients indicate a downward flow direction.
Water Movement Through the Unsaturated Zone 19

T Tzθzdz

0

z

3 = (5)

where Tz and θz are the tritium concentration and volumetric 

water content measured at depth z. The total amount of tritium, 

corrected for decay, that could have been added to the profile 

is calculated as

TT wnTnWn exp( ) nλ

n = 1

t

= ¦ (6)

where

n is years before time of sampling,

t is elapsed time since 1953,

wn is a weighting function that takes into account 

year-to-year variations in recharge,

Tn is the tritium content of precipitation,

Wn is the amount of precipitation,

and

λ is the rate constant for tritium decay (0.0563/yr).

The record of tritium in precipitation at Waco, Tex., was used 

for Tn (International Atomic Energy Agency, 1992). Precipitation data from Elkhart and Garden City, Kans. (High Plains 

Regional Climate Center, 2001) were used for Wn

. The weighting function, wn

, was calculated as

wn

 = (precipitation plus irrigation water applied 

 in year n)/(average annual water application). (7)

Assumptions made in using equation 4 are that all 

postbomb tritium still resided in the unsaturated zone at the 

time of sampling (no breakthrough of postbomb tritium at 

the water table), irrigation water did not contain postbomb 

tritium, and recharge only occurred during the winter and 

irrigation seasons. Water from both irrigation wells contained 

small amounts of tritium in 2001 (1.1 to 2.1 TU). If postbomb 

tritium arrived at the irrigation wells after 1990 then this 

small amount of tritium would not be a factor in the analysis 

because the borehole locations did not receive irrigation water 

after 1989. If postbomb tritium was present in irrigation water 

prior to 1990 then the water flux calculated using equation 4 

is an overestimate. According to Allison and others (1994), q

estimated using the mass-balance method is independent of 

the tritium distribution in the profile; thus, it is unaffected by 

nonpiston flow.

A clearly defined postbomb peak was not present in any 

of the tritium profiles. An interface between prebomb and 

postbomb tritium was present in the unsaturated zone at CNG 

(fig. 13B), however, so the interface method was used to calculate a water flux in the interval containing postbomb tritium. 

The value assigned to z was somewhat subjective because of 

the lack of tritium data in the depth interval from 1.9 to 5.3 m, 

in which tritium concentrations decreased from postbomb to 

prebomb levels (fig. 11B). Therefore, three values of z were 

chosen: 1.9 m—the largest depth at which postbomb tritium 

was measured; 5.3 m—the shallowest depth at which prebomb 

tritium was measured; and 3.6 m—the midpoint between those 

two depths. A value of 47 years was used for t. None of the 

CNG core samples collected for measurement of volumetric water content were in the interval containing postbomb 

tritium; therefore, values of volumetric water content for that 

interval were calculated from measurements of gravimetric 

moisture content and bulk density (table 3). The weightedaverage volumetric water content of the three samples from 

the top 5.1 m was 0.071. This weighting was based on the 

distances between adjacent samples. Water fluxes calculated 

using these values of z, t, and θ ranged from 2.9 to 8.0 mm/yr, 

with an average of 5.4 mm/yr. The water flux calculated using 

the mass-balance method was 4.4 mm/yr. These fluxes are in 

the lower end of the range in recharge fluxes to the central 

High Plains aquifer estimated using a two-dimensional areal 

simulation of ground-water flow (1.4 to 21 mm/yr) (Luckey 

and Becker, 1999). They are about in the middle of the range 

in values estimated using two-dimensional cross-sectional 

simulations of ground-water flow calibrated using carbon-14 

ages of ground water (0.8 to 8 mm/yr) (P.B. McMahon, unpub. 

data, on file in Colorado District Office of the U.S. Geological 

Survey). The water fluxes at CNG probably overestimate current recharge fluxes (defined as the water flux below the root 

zone) because the postbomb tritium was confined to depths 

within the root zone. The base of the root zone was assumed to 

be at a depth of 2.5 to 3 m, which corresponds to the depth at 

which the peak chloride concentration in the unsaturated zone 

occurred (fig. 11C).

Interfaces between prebomb and postbomb tritium may 

have been present in the intervals from 14.4 to 17.4 m below 

land surface at CAL–121 and 11.3 to 14.4 m below land surface at CAL–122 (figs. 12B and 13B). At both sites, postbomb 

tritium was detected again deeper in the profile at a depth that 

was close to the estimated depth of the early 1960’s water 

table. The water-table depths in the early 1960’s were estimated on the basis of 2000–2001 water levels in the monitoring wells and historical water-level declines in those areas 

(figs. 5B and 5C). These findings indicate that ground water 

in the irrigated areas may have contained postbomb tritium in 

the early 1960’s and that postbomb tritium detected below the 

tritium-depleted zones in 2000 was from the declining water 

table and not from movement through the local unsaturated 

zone. The weighted-average volumetric water content in the 

upper 17.3 m at CAL–121 was 0.160 and it was 0.141 in the 

upper 14.3 m at CAL–122. Using equation 3, calculated water 

fluxes for 1953 to 2000 ranged from 49 to 59 mm/yr, with an 

average of 54 mm/yr, at CAL–121 and from 34 to 43 mm/yr, 

with an average of 39 mm/yr, at CAL–122. Water fluxes 

estimated using the mass-balance method, and assuming 

postbomb tritium below the interfaces was not from the local 

unsaturated zone, were 34 mm/yr at CAL–121 and 21 mm/yr 

at CAL–122. The factor of 1.6 to 1.9 difference in waterflux estimates using two different techniques is relatively 

small given the uncertainty associated with each approach. 
20 Water Movement Through the Central High Plains Aquifer, Southwestern Kansas, 2000–2001 Water table Core Cuttings Less than detection level Greater than detection level Ground water Ground water Water table Sand Clay Silt Gravel Water table Water table Estimated maximum concentration of prebomb tritium in year 2000 0 20 40 60 80 100 –2.5 0 2.5

PERCENT CLAY, SILT, SAND, 

AND GRAVEL

DEPTH BELOW LAND SURFACE, IN METERS

TRITIUM CONCENTRATION IN

CORE AND GROUND WATER,

IN TRITIUM UNITS

CHLORIDE CONCENTRATION IN

CORE AND CUTTINGS, 

IN MICROGRAMS PER GRAM

NITRATE CONCENTRATION IN

CUTTINGS, IN MICROGRAMS

PER GRAM AS NITROGEN, AND

GROUND WATER, IN MILLIGRAMS

PER LITER AS NITROGEN

5.0 7.5 10.0 0 8 16 24 32 40 0 10 20 30 40 100 200 300

05

1015202530354045505560

05

1015202530354045505560

05

1015202530354045505560

05

1015202530354045505560

A BCD

Figure 11. Relation between depth and (

A) lithology, (

B) tritium concentrations in ground water and cores, (

C) chloride concentrations in core and cuttings, 

and (

D) nitrate concentrations in ground water and cuttings at CNG. Error bars associated with the tritium data represent the 1 standard-deviation precision 

of the analysis.
Water Movement Through the Unsaturated Zone 21

0

5

Core

Cuttings

Less than detection levelGreater than detection level

Ground water

Water table in 2000–2001

Estimated water

table in 1960's

Ground water

Soil water

Soil water Sand

Clay

Silt

Gravel Estimated maximum

concentration ofprebomb tritiumin year 2000

0 20 40 60 80 100 –2.5 0 2.5

PERCENT CLAY, SILT, SAND, 

AND GRAVEL

DEPTH BELOW LAND SURFACE, IN METERS

TRITIUM CONCENTRATION IN

CORE AND GROUND WATER,

IN TRITIUM UNITS

CHLORIDE CONCENTRATION IN

CORE AND CUTTINGS, 

IN MICROGRAMS PER GRAM

NITRATE CONCENTRATION IN

CUTTINGS, IN MICROGRAMS

PER GRAM AS NITROGEN, AND

SOIL WATER AND GROUND WATER,

IN MILLIGRAMS PER LITER

AS NITROGEN

5.0 7.5 10.0 0 70 60 50 40 30 20 10 0 10 20 30 40

05

10152025303540455055

05

10152025303540455055

05

10152025303540455055

05

10152025303540455055

0 A BCD

Figure 12. Relation between depth and (

A) lithology, (

B) tritium concentrations in ground water and cores, (

C) chloride concentrations in core and cuttings, 

and (

D) nitrate concentrations in cuttings, soil water, and ground water at CAL–121. Error bars associated with the tritium data represent the 1 standarddeviation precision of the analysis.
22 Water Movement Through the Central High Plains Aquifer, Southwestern Kansas, 2000–2001 Ground water Water table in 2000–2001 Estimated water table in 1960's Ground water Soil water Soil water Sand Silt Clay Estimated maximum concentration of prebomb tritium in year 2000 0 20 40 60 80 100 –2.5 0 2.5

PERCENT CLAY, SILT, SAND, 

AND GRAVEL

DEPTH BELOW LAND SURFACE, IN METERS

TRITIUM CONCENTRATION IN

CORE AND GROUND WATER,

IN TRITIUM UNITS

CHLORIDE CONCENTRATION IN

CORE AND CUTTINGS, 

IN MICROGRAMS PER GRAM

NITRATE CONCENTRATION IN

CUTTINGS, IN MICROGRAMS

PER GRAM AS NITROGEN, AND

SOIL WATER AND GROUND WATER,

IN MILLIGRAMS PER LITER

AS NITROGEN

5.0 7.5 10.0 0 50 40 30 20 10 0 10 20 55 60 65

05

101520253035404550

05

101520253035404550

05

101520253035404550

101520253035404550

05

A BCD

Gravel

Core

Cuttings

Less than detection levelGreater than detection level

Figure 13. Relation between depth and (

A) lithology, (

B) tritium concentrations in ground water and cores, (

C) chloride concentrations in core and cuttings, 

and (

D) nitrate concentrations in cuttings, soil water, and ground water at CAL–122. Error bars associated with the tritium data represent the 1 standarddeviation precision of the analysis.
Water Movement Through the Unsaturated Zone 23

Most of the water movement through the unsaturated zones at 

the CAL sites probably occurred from 1955–56 to 1989 when 

the borehole locations received direct applications of irrigation 

water; therefore, these average fluxes may underestimate water 

fluxes that occurred during the period of furrow irrigation.

In the preceding analysis, it was assumed that postbomb 

tritium reached the water table somewhere in the vicinity of 

the CAL sites by the early 1960’s. Downward water velocities 

(equal to distance traveled divided by time of travel) of at least 

2 to 3 m/yr would be required for postbomb tritium to reach 

a water table 18 to 27 m below land surface by 1962. Such 

rapid water movement seems unlikely unless it occurred as 

preferential flow in the unsaturated zone, downward leakage through the annular space of improperly sealed wells 

(wellbore leakage), or spatially variable recharge with most 

recharge occurring in focused recharge zones such as ditches 

and depressions. If one or more of these processses did occur 

during the late 1950’s or early 1960’s, then there is no apparent reason to believe they did not still occur at the time of this 

study.

If postbomb tritium did not reach the water table 

before the start of its decline in the early 1960’s, then postbomb tritium detected beneath the tritium-depleted zones 

must have resulted from bypass or preferential flow in the 

local unsaturated zone. As discussed in the section “Total 

Water Potentials,” the gravelly sand from about 20 to 26.5 m 

below land surface at CAL–121 was drier than the overlying 

loamy sand, which could result in the gravelly sand being 

a barrier to water movement because of its reduced unsaturated hydraulic conductivity at small water contents. Postbomb 

tritium could either have moved laterally around the gravelly 

sand or possibly downward by gravity drainage, once 

Table 3. Gravimetric water content, bulk density, and volumetric 

water content of sediment cores. Bulk densities were estimated 

using bulk-density measurements listed in table 2 and measured 

sediment textures. Volumetric water contents were calculated 

using gravimetric water contents and bulk densities.

[m, meter; g/g, grams of water per gram of dry sediment; g/cm3, grams of dry 

sediment per cubic centimeter of sediment; cm3/cm3, cubic centime ters of 

water per cubic centimeter of sediment]

Site Depth

(m)

Gravimetric 

water 

content

(g/g)

Bulk 

density

(g/cm3

)

Volumetric 

water 

content

(cm3

/cm3)

CNG 0.23 0.064 1.86 0.119

 1.75 .044 1.86 .082

 5.11 .017 1.86 .032

 8.15 .021 1.86 .039

 11.20 .203 1.84 .374

 14.10 .019 1.71 .032

 17.30 .047 1.86 .087

 20.35 .021 1.86 .039

 23.39 .080 1.84 .147

 29.49 .101 1.84 .186

 35.28 .026 1.86 .048

 47.78 .247 1.84 .454

CAL–121 .23 .212 1.84 .390

 .84 .082 1.71 .140

 1.75 .033 1.71 .056

 2.51 .026 1.71 .044

 3.20 .033 1.71 .056

 4.95 .083 1.71 .142

 6.60 .157 1.86 .292

 8.15 .098 1.86 .182

 9.68 .109 1.71 .186

 11.20 .072 1.71 .123

 14.24 .126 1.84 .232

 17.30 .067 1.84 .123

 20.34 .020 1.86 .037

 23.24 .029 1.86 .054

 26.44 .043 1.86 .080

 29.49 .027 1.86 .050

 41.68 .146 1.86 .272

Table 3.—Continued

Site Depth

(m)

Gravimetric 

water 

content

(g/g)

Bulk 

density

(g/cm3

)

Volumetric

water

content

(cm3

/cm3)

CAL–122 .99 .139 1.70 .236

 1.75 .089 1.70 .151

 2.51 .027 1.86 .050

 3.28 .009 1.86 .017

 5.11 .208 1.86 .387

 6.63 .050 1.71 .086

 8.15 .115 1.84 .212

 9.68 .063 1.71 .108

 11.20 .050 1.71 .086

 14.25 .012 1.71 .021

 17.30 .035 1.86 .065

 20.35 .016 1.86 .030

 23.24 .019 1.86 .035

 26.44 .023 1.86 .043

 29.49 .180 1.64 .295

 41.68 .019 1.86 .035

 44.73 .035 1.86 .065
24 Water Movement Through the Central High Plains Aquifer, Southwestern Kansas, 2000–2001

sufficient water had accumulated at the top of the interval 

to allow water entry into the coarse sediment. In the latter 

scenario, most of the interval could contain old water but 

be interspersed with vertical pathways containing postbomb 

tritium during periods of drainage. This is analogous to the 

theory of fingering during infiltration described by Hillel 

and Baker (1988) and Baker and Hillel (1990). A minimum 

estimate of water flux can be made by assuming postbomb tritium arrived at the CAL water tables in 2000 when they were 

45.2 m (CAL–121) and 45.6 m (CAL–122) below land surface. The weighted-average volumetric water contents of the 

sediment above the 2000 water tables were 0.121 and 0.109 

at CAL–121 and CAL–122, respectively. Substituting these 

values in equation 3 gives water fluxes of 116 and 106 mm/yr 

at CAL–121 and CAL–122, respectively.

Analysis of the tritium profiles shows that water fluxes 

at the CAL sites were at least 4 to 12 times larger than at 

CNG, assuming postbomb tritium detected beneath the 

tritium-depleted zones at the CAL sites was from the declining 

water table. If postbomb tritium detected beneath the tritiumdepleted zones at the CAL sites resulted from bypass or preferential flow in the local unsaturated zone, then water movement at the CAL sites may have been at least 20 times faster 

than at CNG. In either case, these results indicate irrigation 

was an important driving force for water movement through 

the unsaturated zone at the CAL sites. 

Chloride Profiles

Chloride profiles for each site are shown in figures 11C, 

12C, and13C. The similarity between core and cuttings data 

shows there was little mixing of cuttings from different depths 

as they were transported up the borehole during drilling. 

The chloride profile at CNG was similar to profiles reported 

from arid and semiarid areas in New Mexico, Texas, and 

California (Phillips, 1994; Scanlon, 1991; Izbicki and others, 2000). The profile was characterized by small chloride 

concentrations (1 to 5 μg/g) at depths less than 2.5 m below 

land surface that increased to maximum concentrations (28 

to 37 μg/g) at depths 2.5 to 3 m below land surface before 

decreasing to consistently small concentrations (less than 

1.5 μg/g) below a depth of 14.5 m. The small chloride concentrations in the upper 2.5 m may be the result of dilution 

by precipitation that occurred shortly before the samples 

were collected. Chloride enrichment immediately below this 

surficial zone was attributed to concentration in the root zone 

by evapotranspiration. The chloride enrichment also may be 

related to lithology because the two primary zones of chloride enrichment (2.5 to 3 m and 9 to 13.5 m) corresponded to 

zones with large clay and silt contents (figs. 11A and 11C). 

Decreases in chloride concentrations below the root zone 

have been attributed to preferential flow of water (Scanlon, 

1991), which could dilute chloride concentrations deeper 

in the profile, or to larger water fluxes in the past associated 

with cooler, wetter climates (Phillips, 1994; Scanlon and 

Goldsmith, 1997). If water preferentially moved past the 

root zone to dilute chloride concentrations deeper in the 

profile, it also is likely postbomb tritium would have been 

detected deeper in the profile. However, postbomb tritium 

was not detected deeper than 1.9 m at CNG. Thus, the tritium 

profile is consistent with piston-type flow and does not indicate preferential flow occurred in the unsaturated zone 

at CNG.

The chloride peak in the unsaturated zone at CAL–121 

was 15 to 20 m below land surface (fig. 12C). The absence 

of a chloride peak and presence of postbomb tritium in the 

upper 15 m at CAL–121 indicates that recent water movement 

in that interval may have flushed out chloride that had accumulated before the onset of irrigation. A water-flux estimate 

can be made using equation 3 and the measured downward 

displacement of the chloride peak at CAL–121 relative to 

the chloride peak at the rangeland site (figs. 11C and 12C). 

This estimate assumes that the depth of the chloride peak at 

CAL–121 prior to the onset of irrigation was the same as at 

CNG. The centers of mass of the chloride peaks at CNG and 

CAL–121 were at depths of about 2.8 and 17.5 m below land 

surface in 2000, representing a downward displacement of 

14.7 m. The elapsed time since irrigation began at CAL–121 

was 44 years, and the weighted-average volumetric water 

content in the upper 17.3 m at CAL–121 was 0.160. Substituting these values in equation 3 results in a water flux of 

53 mm/yr. This water flux compares well with the tritiumbased water-flux estimate of 54 mm/yr that assumed postbomb 

tritium in the deep unsaturated zone was from a declining 

water table.

A chloride peak occurred about 2 m below land surface 

at CAL–122 (fig. 13C), even though the upper 11 m at 

CAL–122 also contained postbomb tritium. That peak might 

represent incompletely flushed chloride that accumulated 

before the onset of irrigation, a past agricultural-chemical 

application containing chloride (for example, potassium chloride), or enrichment by evapotranspiration after direct applications of irrigation water were stopped in 1990.

Chloride profiles have been used by several researchers 

to calculate water fluxes in the unsaturated zone (Allison 

and Hughes, 1978, 1983; Johnston, 1987; Cook and others, 

1989; Scanlon, 1991; Scanlon and Goldsmith, 1997). Chloride 

is a useful tracer of water movement in the unsaturated zone 

because it is chemically conservative, nonvolatile (no vapor 

transport), and generally is not taken up by plants. Under 

steady-state conditions, one-dimensional chloride transport 

in the unsaturated zone is described by the following equation 

(Scanlon and Goldsmith, 1997)

qCl cClq Dh

∂cCl

∂z = – --------- (8)
Water Movement Through the Unsaturated Zone 25

where

qCl is the chloride flux at the land surface,

cCl is the chloride concentration in soil water,

q is the water flux,

and

Dh is the hydrodynamic dispersion coefficient.

The value of qCl is approximated by the product of the annual 

precipitation rate and chloride concentration in wet plus dry 

deposition. The first term on the right represents the advective 

flux of chloride and the second term represents the dispersive 

flux of chloride. In semiarid regions like southwestern Kansas, 

the dispersive flux of chloride is assumed negligible (Allison 

and Hughes, 1983; Scanlon and Goldsmith, 1997); therefore, 

equation 4 is simplified to

q qCl cCl = ⁄ (9)

The primary assumptions associated with the use of 

chloride profiles to calculate water fluxes are that precipitation 

was the only source of chloride, water and chloride movement 

are represented by one-dimensional downward piston-type 

flow, and the chloride flux at the land surface was constant 

over time (Allison and Hughes, 1978; Johnston, 1987; Scanlon, 

1991). There were no strong mineralogic sources of chloride 

in the unsaturated zone at CNG, as indicated by the small 

chloride concentration of 13 mg/L in ground water at that site; 

therefore, wet plus dry deposition at the land surface likely 

was the only substantial source of chloride. Irrigation water 

and potassium-chloride applications may have been additional 

sources of chloride at the CAL sites; therefore, the chloride 

profiles at the CAL sites were not used to estimate water 

fluxes. As previously discussed, the tritium profile at CNG is 

consistent with piston-type flow. It is not possible to evaluate 

the validity of the third assumption with the available data.

Equation 9 was used to calculate water fluxes at CNG. 

Chloride concentrations measured in the cores were used 

for cCl rather than concentrations measured in cuttings 

because the cores had associated gravimetric water-content 

data needed to convert chloride concentrations from micrograms per gram to grams per cubic meter (table 4). The 

chloride profile at CNG, in grams per cubic meter, is shown in 

figure 14A. The chloride flux at the land surface was approximated as the product of the mean annual precipitation rate 

and the concentration of chloride in wet plus dry deposition. 

The mean annual precipitation rate, 452 mm/yr, was based 

on data collected from 1900 to 2000 at a weather station in 

Elkhart, Kans. (fig. 1), about 29 km west of CNG (High Plains 

Regional Climate Center, 2001). Measurements of chloride 

concentrations in wet plus dry deposition were not made in the 

vicinity of CNG. However, Wood and Sanford (1995) reported 

a chloride concentration of 0.6 g/m3

 in wet plus dry deposition 

collected in 1984–85 in Amarillo, Tex., located about 210 km 

south of CNG. Scanlon and Goldsmith (1997) calculated a 

chloride concentration of 0.3 g/m3

 in wet plus dry deposition near Amarillo on the basis of prebomb 36Cl/Cl ratios 

in fallout, which may be more representative of long-term 

chloride inputs than recent measurements (Bridget Scanlon, 

Texas Bureau of Economic Geology, written commun., 2002). 

Therefore, the chloride concentration in wet plus dry deposition reported by Scanlon and Goldsmith (1997), 0.3 g/m3

, was 

used in this study. For comparison, the mean chloride concentration (±1 standard deviation) was 0.117±0.026 g/m3

 in only 

wet deposition collected from 1984 to 2000 at three National 

Atmospheric Deposition Program sites that geographically 

bracketed CNG and the city of Amarillo (sites in Scott County, 

Kansas, Texas County, Oklahoma, and Bailey County, Texas) 

(National Atmospheric Deposition Program, 2001). If the 

chloride concentration in wet plus dry deposition exhibited a 

proportional amount of variability, the concentration would 

be about 0.3±0.07 g/m3. The chloride flux at the land surface 

calculated on the basis of the precipitation rate and chloride 

concentrations would be 0.14±0.03 g/m2

/yr. The lack of sitespecific information on chloride concentrations in wet plus dry 

deposition represented the largest uncertainty in the water-flux 

calculations by the chloride mass-balance method. 

The chloride mass-balance method assumes the chloride 

flux at the land surface was constant; therefore, the variability 

in soil-water chloride concentrations is attributed to variations 

in the water flux. Water fluxes calculated for depths below 

the root zone ranged from 2.4 to 10 mm/yr, with a average of 

Table 4. Gravimetric water content and chloride concentrations in 

sediment cores from CNG.

[m, meter; g/g, grams of water per gram of dry sediment; μg/g, micrograms of 

chloride per gram of dry sediment; g/m3, grams of chloride per cubic meter of 

water]

Sample

depth

(m)

Gravimetric

water

content

(g/g)

Concentration

of chloride

(μg/g)

Concentration

of chloride

(g/m3

)

 0.23 0.064 0.85 13

 1.75 .044 .79 18

 5.11 .017 3.06 184

 8.15 .021 5.22 253

 11.2 .203 12.03 59

 14.1 .019 1.15 61

 17.3 .047 .99 21

 20.35 .021 1.18 56

 23.39 .080 1.59 20

 29.49 .101 2.11 21

 35.28 .026 1.35 52

 47.78 .247 3.31 13
26 Water Movement Through the Central High Plains Aquifer, Southwestern Kansas, 2000–2001

Figure 14. Relation between depth and (A) chloride concentrations and water fluxes and 

(B) chloride mass-balance age at CNG. The error bars on the chloride concentrations represent the plus or minus 5 percent precision of the chloride analyses. The error bars on the 

water fluxes represent the combined uncertainties in the chloride analyses and chloride flux 

at the land surface.

5.1 mm/yr. This average flux was within the range in recharge 

fluxes to the central High Plains aquifer estimated using the 

ground-water flow simulations discussed earlier (Luckey 

and Becker, 1999; P.B. McMahon, unpub. data, on file in 

Colorado District Office of the U.S. Geological Survey). Wood 

and Sanford (1995) used a form of equation 9 and concentrations of choride in ground water to estimate recharge fluxes to 

the southern High Plains aquifer. Using their approach and the 

average chloride concentration (13 mg/L) in water from the 

CNG monitoring well yielded a recharge flux of 11±2 mm/yr, 

which is similar to the fluxes estimated using chloride concentrations in the unsaturated zone. 

The length of time since water moved through the deep 

unsaturated zone can be estimated using equation 10 (Scanlon 

and Goldsmith, 1997; Izbicki and others, 2002)

t θcCl∂z qCl ⁄

0

z

3 = (10)

where

t is chloride age,

and

z is depth below land surface.

This equation states that the chloride age equals the total 

mass of chloride present in the unsaturated zone above a 

given depth divided by the chloride flux at the land surface. 

Use of equation 10 assumes precipitation was the only source 

of chloride in the sediments and the chloride flux at the land 

surface remained constant over time. Chloride ages are shown 

in figure 14B. The chloride age at the base of the unsaturated 

zone was about 1,600 years, implying that it has been several 

hundred years since water moved through the deep unsaturated zone at CNG. This age is considerably younger than the 

chloride ages reported for sites in the Chihuahuan and Mojave 

Deserts where chloride ages were in excess of 10,000 years 

(Scanlon, 1991; Izbicki and others, 2000). This age difference 

Concentration

Water flux

0

0 5 10 15 20

60

50

40

30

20

10

0

100

CHLORIDE CONCENTRATION,

IN GRAMS PER CUBIC METER

WATER FLUX, IN MILLIMETERS PER YEAR

200 300 0 600

CHLORIDE MASS-BALANCE AGE,

IN YEARS

1,200 1,800

DEPTH BELOW LAND SURFACE, IN METERS

60

50

40

30

20

10

0

DEPTH BELOW LAND SURFACE, IN METERS

A B
Water Movement Through the Unsaturated Zone 27

results from smaller long-term water fluxes in those settings, 

generally less than 0.2 mm/yr, compared to the average flux 

at CNG of 5.1 mm/yr. Chloride ages of several hundred years 

below the root zone indicate that chloride mass-balance 

estimates of water flux do not represent current recharge 

fluxes.

Nitrate and Pesticide Profiles

Elevated nitrate and pesticide concentrations in the 

unsaturated zone and ground water at the irrigated sites are 

further evidence of relatively rapid water movement through 

unsaturated zones in irrigated areas of the central High Plains. 

Nitrate was detected in unsaturated-zone sediment from each 

of the three sites (figs. 11D, 12D, and 13D); however, nitrate 

in sediment from CNG was confined to the upper 11.5 m 

of the section. The relatively large nitrate concentration 

(259 μg/g) measured in sediment at a depth of 3.2 m probably resulted from evaporative concentration in the root zone. 

The profile of nitrate concentrations in the unsaturated zone 

at CNG is consistent with the relatively small nitrate concentrations (1.1 to 1.2 mg/L as N) measured in ground water at 

that site. In a recent study of water quality in the central High 

Plains aquifer, a median nitrate concentration of 2.04 mg/L 

as N was reported in 32 water samples containing less than 

0.3 TU tritium (Becker and others, 2002). Litke (2001) 

reported a median nitrate concentration of 2.1 mg/L as N in 

the central High Plains aquifer on the basis of 1,468 analyses. 

These findings indicate nitrate concentrations in water from 

the CNG monitoring well were typical of background concentrations in the aquifer and do not indicate any input of nitrate 

from anthropogenic sources. Water from the CNG monitoring well contained more than 6 mg/L dissolved oxygen so it 

is unlikely denitrification, which largely occurs in suboxic or 

anoxic settings, could account for the small nitrate concentrations in ground water. 

The pattern of nitrate detections in the unsaturated 

zone at CAL–121 (fig. 12D) was very similar to the pattern 

of postbomb-tritium detections (fig. 12B). Nitrate was detected 

above and below an interval from 17.4 to 26.5 m in which 

essentially no postbomb tritium or nitrate was detected. This 

pattern of detections indicates nitrate-enriched, post-1953 

water moved to a depth of at least 14.4 to 17.4 m below land 

surface at CAL–121. Depending on the role played by the 

declining water table in leaving residual nitrate in the deep 

unsaturated zone, it is possible nitrate moved through the 

local unsaturated zone to a depth greater than 17.4 m.

Nitrate concentrations in water from the CAL–121 

monitoring well ranged from 20 to 24 mg/L as N and were 

about 10 times larger than the estimated background nitrate 

concentrations in the aquifer. Water from the deep lysimeter 

at CAL–121 contained from 27 to 34 mg/L of nitrate N, which 

is not very different from the nitrate concentrations in ground 

water at that site. This similarity in concentrations is consistent with nitrate in the deep unsaturated zone coming from 

a declining water table or from downward water movement 

through the unsaturated zone at CAL–121. In either case, the 

nitrate concentrations in ground water at CAL–121 indicate 

relatively large amounts of nitrate reached the water table in 

this area dominated by irrigated agriculture.

The source of nitrate in ground water at CAL–121 

cannot be distinguished on the basis of data presented in 

this report. Some possible sources of the nitrate include N 

applied to nearby fields and nitrate that accumulated in the 

unsaturated zone prior to the onset of irrigation. The nitrate 

data from CNG indicate substanial amounts of nitrate could 

have accumulated in the unsaturated zone before the onset 

of irrigation. Later, deep percolation of irrigation water 

could have mobilized that nitrate. Measurements of N isotopes in nitrate might provide insights into the sources 

of nitrate at the study sites and could be the subject of 

future investigations.

Nitrate concentrations in ground water from CAL–122 

were small, ranging from 0.03 to 0.4 mg/L as N (fig. 13D). 

These small concentrations probably were not due to a lack of 

recent recharge because ground water at CAL–122 contained 

tritium and pesticides. Instead, the small nitrate concentrations 

probably were the result of denitrification. Concentrations of 

dissolved oxygen were less than 1 mg/L in ground water from 

CAL–122 and indicate geochemical conditions at CAL–122 

were conducive to denitrification. 

The use of preemergent herbicides, like atrazine, to 

control weeds on corn and other row crops began in the 

mid-1950’s (Plummer and others, 1993), so their presence in ground water is a good indicator of recent recharge. 

Concentrations of selected pesticides in sediment and water 

at the CNG and CAL sites are shown in figure 15. Pesticides 

or their degradation products were detected in sediment, soil 

water, and ground water at both CAL sites. In contrast, only 

a single pesticide degradation product (deisopropylatrazine) 

was detected in the shallowest sediment sample at CNG. No 

pesticides were detected in ground water from CNG. The 

maximum total pesticide concentration in any single groundwater sample from the CAL sites was 0.923 μg/L and at 

CNG it was less than 0.01 μg/L. The maximum atrazine concentration detected in deep soil water at the CAL sites was 

0.79 μg/L (fig. 15B). Sediments at CNG were too dry to yield 

water to lysimeters installed at that site, so pesticide analyses 

of soil water could not be performed. Records of atrazine 

use provided by the landowner indicate it was first applied 

on the CAL–121 and CAL–122 fields in 1960 and 1958, 

respectively; therefore, recent recharge containing atrazine 

may have moved through the unsaturated zone to the water 

table at both sites during a period of 40 to 42 years. However, the possibility that the pesticide detections in the deep 

unsaturated zone and ground water at the CAL sites were 

the result of the declining water table cannot be ruled out. 

A minimum estimate of water flux can be made using equation 3 and assuming atrazine reached the water table in 2000. 

Based on this assumption, minimum water fluxes at the CAL 

sites were 118 to 137 mm/yr. These water fluxes are larger 
28 Water Movement Through the Central High Plains Aquifer, Southwestern Kansas, 2000–2001

than the tritium-based estimates because the time available 

for atrazine transport (40 to 42 years) was less than the time 

available for tritium transport (about 47 years).

The large nitrate and pesticide concentrations in ground 

water at the CAL sites indicate relatively large amounts of 

nitrate and pesticides reached the water table in these areas 

dominated by irrigated agriculture. Although elevated nitrate 

concentrations at the CAL sites cannot be directly linked to 

agricultural N applications, agricultural pesticide applications 

are the most likely source of the pesticides detected in the 

unsaturated zone and ground water. The persistence of a downward hydraulic gradient from the deep unsaturated zone to the 

water table at the CAL sites, coupled with the large nitrate and 

atrazine concentrations in deep soil water, indicates the deep 

unsaturated zone will be a source of nitrate and atrazine to the 

aquifer in the future.

Summary and Conclusions

The High Plains aquifer is the principal source of water 

for irrigated agriculture in the study area, transforming it into 

one of the most productive agricultural regions of the world. 

This transformation, however, has come at the cost of declining water levels in the aquifer. Water-level declines were greatest in those parts of the High Plains aquifer where irrigation 

was developed first and where it was most intense. Chemical 

data indicate agricultural activities have affected the quality 

of water in the aquifer. Understanding the movement of water 

and chemicals from land surface to the aquifer is crucial 

to developing management plans that ensure future water 

supplies (aquifer sustainability) and to assessing the water 

quality of the High Plains aquifer. This report evaluates water 

0

60

55

50

45

40

35

30

25

20

15

10

5

0

60

55

50

45

40

35

30

25

20

15

10

5

0

60

55

50

45

40

35

30

25

20

15

10

5

0

0.4 0.8

PESTICIDE CONCENTRATION IN CORE, IN NANOGRAMS PER GRAM, AND SOIL WATER AND GROUND WATER, IN MICROGRAMS PER LITER

DEPTH BELOW LAND SURFACE, IN METERS

1.2 1.6 0 0.4 0.8 1.2 1.6 0 0.4 0.8 1.2 1.6

Water table in 2000–01

Water table in 2000–01 Water table in 2000–01

Ground water

Soil water

Estimated water table in early 1960's

Estimated water table in early 1960's

Soil water

Soil water

Ground water

Ground water

Soil water

Atrazine

Deethylatrazine

Metolachlor Simazine

EXPLANATION

Metolachlor ESA

Deisopropylatrazine

AB C

Figure 15. Relation between depth and pesticide concentrations in core, soil water, and ground water from (A) CNG, (B) CAL–121, 

and (C) CAL–122.
References Cited 29

movement through thick unsaturated zones overlying the 

central High Plains aquifer. Chemical, hydraulic, and physical 

data were collected from throughout the unsaturated zone at 

one site where rangeland was the dominant land use and at two 

sites where irrigated agriculture was the dominant land use. In 

2000–2001, the depth to water at the rangeland site was 50 m 

and the depth to water at the irrigated sites was about 45.4 m. 

Irrigation at the study sites first began in 1955–56.

Measurements of matric potential and volumetric water 

content indicate wetter conditions existed in the deep unsaturated zone at the irrigated sites than at the rangeland site. Total 

water potentials in the unsaturated zone indicate a potential 

existed for downward water movement from the unsaturated 

zone to the water table at the irrigated sites. At the rangeland 

site, little or no potential existed for downward water movement from the unsaturated zone to the water table. 

The depth of the interface between prebomb and postbomb tritium and a tritium mass balance were used to estimate 

water fluxes in the unsaturated zone at each site. The average 

water fluxes at the rangeland site were 5.4 and 4.4 millimeters 

per year for the interface and mass-balance methods. Postbomb tritium at the rangeland site was confined to the root 

zone, so actual recharge fluxes to the aquifer probably were 

less than the estimated water fluxes. Tritium profiles in the 

unsaturated zone at the irrigated sites were complicated by 

the presence of tritium-depleted intervals separating upper 

and lower zones containing postbomb tritium. If the interface 

between prebomb and postbomb tritium was at the top of 

the tritium-depleted interval and postbomb tritium detected 

beneath that interval was from the declining water table in 

the area, then the average water flux at the irrigated sites 

was estimated to be 39 to 54 millimeters per year. The water 

fluxes estimated using the mass-balance method were 21 to 

34 millimeters per year. If postbomb tritium detected beneath 

the tritium-depleted interval was from bypass or preferential 

water movement through the local unsaturated zone instead of 

the declining water table, then the minimum water flux at the 

irrigated sites was estimated to be 106 to 116 millimeters per 

year. In either scenario, water fluxes at the irrigated sites were 

at least 4 to 12 times larger than the flux at the rangeland site, 

indicating irrigation was an important driving force for water 

movement through the unsaturated zone.

At the irrigated site CAL–121, an independent waterflux estimate of 53 mm/yr, which was based on the downward 

displacement of chloride by deep percolation, compares well 

with the tritium-based water-flux estimate of 54 mm/yr that 

assumed postbomb tritium in the deep unsaturated zone was 

from a declining water table, supporting this hypothesis for the 

origin of postbomb tritium in the deep unsaturated zone. This 

scenario for tritium movement at the irrigated sites assumes 

postbomb tritium reached the water table somewhere in the 

vicinity of the those sites by the early 1960’s. Downward 

water velocities (equal to distance traveled divided by time of 

travel) of at least 2 to 3 meters per year would be required for 

postbomb tritium to reach a water table 18 to 27 meters below 

land surface by 1962. Such rapid water movement seems 

unlikely unless it occurred as preferential flow in the unsaturated zone, downward leakage through the annular space 

of improperly sealed wells (wellbore leakage), or spatially 

variable recharge with most recharge occurring in focused 

recharge zones such as ditches and depressions. If these processses did occur during the late 1950’s or early 1960’s, then 

there is no apparent reason to believe they did not occur at the 

time of this study.

Regardless of the possible roles played by preferential 

flow, wellbore leakage, and focused recharge, the large nitrate 

and pesticide concentrations in ground water at the irrigated 

sites indicate relatively large amounts of nitrate and pesticides 

reached the water table in these areas dominated by irrigated 

agriculture. Although elevated nitrate concentrations in the 

irrigated sites cannot be directly linked to agricultural N 

applications, agricultural pesticide applications are the most 

likely source of the pesticides detected in the unsaturated zone 

and ground water. The persistence of a downward hydraulic 

gradient from the deep unsaturated zone to the water table 

at the irrigated sites, coupled with the large nitrate and atrazine concentrations in deep soil water, indicate that the deep 

unsaturated zone will be a source of nitrate and atrazine to the 

aquifer in the future. Extrapolation of these study results to the 

entire central High Plains area indicates that under 2000–2001 

climatic conditions, most recharge and chemical loading to the 

aquifer probably occurred in irrigated areas.

References Cited

Adams, C.D., and Thurman, E.M., 1991, Formation and transport of deethylatrazine in the soil and vadose zone: Journal 

of Environmental Quality, v. 20, p. 540–547.

Allison, G.B., Gee, G.W., and Tyler, S.W., 1994, Vadose-zone 

techniques for estimating ground-water recharge in arid and 

semiarid regions: Soil Science Society of America Journal, 

v. 58, p. 6–14.

Allison, G.B., and Hughes, M.W., 1978, The use of environmental chloride and tritium to estimate total recharge to an 

unconfined aquifer: Australian Journal of Soil Research, 

v. 16, p. 181–195.

Allison, G.B., and Hughes, M.W., 1983, The use of natural 

tracers as indicators of soil-water movement in a temperate 

semi-arid region: Journal of Hydrology, v. 60, p. 157–173.

American Society for Testing and Materials, 1997, Standard 

test method for sieve analysis of fine and coarse aggregates, 

C 136–96a: Annual Book of ASTM Standards.

Baker, R.S., and Hillel, D., 1990, Laboratory tests of a theory 

of fingering during infiltration into layered soils: Soil Science Society of America Journal, v. 54, p. 20–30.
30 Water Movement Through the Central High Plains Aquifer, Southwestern Kansas, 2000–2001

Becker, M.F., Bruce, B.W., Pope, L.M., and Andrews, W.J., 

2002, Ground-water quality in the central High Plains aquifer, Colorado, Kansas, New Mexico, Oklahoma, and Texas, 

1999: U.S. Geological Survey Water-Resources Investigations Report 02–4112, 63 p.

Campbell, G.S., and Gee, G.W., 1986, Water potential—

Miscellaneous methods, in Klute, A., ed., Methods of 

soil analysis, pt. 1 (2d ed.): p. 625–628.

Cook, P.G., Walker, G.R., and Jolly, I.D., 1989, Spatial variability of groundwater recharge in a semiarid region: Journal of Hydrology, v. 111, p. 195–212.

Costa, J.L., Massone, H., Martinez, D., Suero, E.E., Vidal, 

C.M., and Bedmar, F., 2002, Nitrate contamination of a 

rural aquifer and accumulation in the unsaturated zone: 

Agricultural Water Management, v. 57, p. 33–47.

Dennehy, K.F., 2000, High Plains regional ground-water 

study: U.S. Geological Survey Fact Sheet 091–00, 6 p.

Dennehy, K.F., Litke, D.W., and McMahon, P.B., 2002, 

The High Plains aquifer, USA—Ground-water development and sustainability, in Hiscock, K.M., Rivett, M.O., 

and Davison, R.M., eds., Sustainable groundwater development: London Geological Society, Special Publication 193, 

p. 99–119.

Dickey, H.P., Swafford, W.R., and Markley, Q.L., 1963, Soil 

survey of Morton County, Kansas: U.S. Department of 

Agriculture, Soil Conservation Service Series 1960, no. 8, 

51 p.

Driscoll, F.G., 1986, Groundwater and wells: Johnson Filtration Systems, Inc., Saint Paul, Minn., 1089 p.

Fishman, M.J., 1993, Methods of analysis by the U.S. 

Geological Survey National Water Quality Laboratory—

Determination of inorganic and organic constituents in 

water and fluvial sediments: U.S. Geological Survey 

Open-File Report 93–125, 217 p.

Gee, G.W., and Bauder, J.W., 1986, Particle-size analysis, in

Klute, A., ed., Methods of soil analysis, physical and mineralogical methods, 2d ed.: Soil Science Society of America 

Monograph 9, pt. 1, p. 383–412.

Gutentag, E.D., Heimes, F.J., Krothe, N.C., Luckey, R.R., 

and Weeks, J.B., 1984, Geohydrology of the High Plains 

aquifer in parts of Colorado, Kansas, Nebraska, New 

Mexico, Oklahoma, South Dakota, Texas, and Wyoming: 

U.S. Geological Survey Professional Paper 1400–B, 

63 p.

Hammermeister, D.P., Blout, D.O., and McDaniel, J.C., 1986, 

Drilling and coring methods that minimize the disturbance 

of cuttings, core, and rock formations in the unsaturated 

zone, Yucca Mountain, Nevada: Proceedings of the National 

Water Well Association Conference on Characterization 

and Monitoring of the Vadose (Unsaturated) Zone, National 

Water Well Association, Worthington, Ohio, p. 507–541.

Harner, R.F., Agell, R.C., Lobmeyer, M.A., and Jantz, D.R., 

1965, Soil survey of Finney County, Kansas: U.S. Department of Agriculture, Soil Conservation Service Series 1961, 

no. 30, 91 p.

Herbel, M.J., and Spalding, R.F., 1993, Vadose zone fertilizerderived nitrate and δ15N extracts: Ground Water, v. 31, 

p. 376–382.

High Plains Regional Climate Center, 2001, Data on the World 

Wide Web at http://hpccsun.unl.edu/wrcc/states/ks.html

(accessed 09/26/01).

Hillel, D., and Baker, R.S., 1988, A descriptive theory of 

fingering during infiltration into layered soils: Soil Science, 

v. 146, p. 51–56.

Hostetler, K.A., and Thurman, E.M., 1999, Determination 

of chloroacetanilide herbicide metabolites in water using 

high-performance liquid chromatography-diode array detection and high-performance liquid chromatography/mass 

spectrometry, in Morganwalp, D.W., and Buxton, H.T., eds., 

1999, U.S. Geological Survey Toxic Substances Hydrology 

Program Proceedings of the technical meeting, Charleston, 

S.C., March 8–12, 1999—v. 2, Contamination of hydrologic 

systems and related ecosystems: U.S. Geological Survey 

Water-Resources Investigations Report 99–4018B, 482 p.

Hubbell, J.M., and Sisson, J.B., 1998, Advanced tensiometer 

for shallow or deep soil water potential measurements: Soil 

Science, v. 163, p. 271–277.

International Atomic Energy Agency, 1992, Statistical treatment of data on environmental isotopes in precipitation: 

Technical Report Series 331, 781 p.

Izbicki, J.A., Radyk, J., and Michel, R.L., 2000, Water 

movement through a thick unsaturated zone underlying an 

intermittent stream in the western Mojave Desert, southern 

California, USA: Journal of Hydrology, v. 238, p. 194–217.

Izbicki, J.A., Radyk, J., and Michel, R.L., 2002, Movement 

of water through the thick unsaturated zone underlying Oro 

Grande and Sheep Creek Washes in the western Mojave 

Desert, USA: Hydrogeology Journal, v. 10, p. 409–427.

Johnston, C.D., 1987, Distribution of environmental chloride 

in relation to subsurface hydrology: Journal of Hydrology, 

v. 94, p. 67–88.
References Cited 31

Komor, S.C., and Emerson, D.G., 1994, Movements of water, 

solutes, and stable isotopes in the unsaturated zones of 

two sand plains in the upper Midwest: Water Resources 

Research, v. 30, p. 253–267.

Lindau, C.W., and Spalding, R.F., 1984, Major procedural 

discrepancies in soil extracted nitrate levels and nitrogen 

isotopic values: Ground Water, v. 22, p. 273–278.

Litke, D.W., 2001, Historical water-quality data for the High 

Plains Regional Ground-Water study in Colorado, Kansas, 

Nebraska, New Mexico, Oklahoma, South Dakota, Texas, 

and Wyoming: U.S. Geological Survey Water-Resources 

Investigations Report 00–4254, 65 p.

Luckey, R.R., Gutentag, E.D., Heimes, F.J., and Weeks, J.B., 

1986, Digital simulation of ground-water flow in the High 

Plains aquifer in parts of Colorado, Kansas, Nebraska, New 

Mexico, Oklahoma, South Dakota, Texas, and Wyoming: 

U.S. Geological Survey Professional Paper 1400–D, 57 p.

Luckey, R.R., and Becker, M.F., 1999, Hydrogeology, water 

use, and simulation of flow in the High Plains aquifer in 

northwestern Oklahoma, southeastern Colorado, southwestern Kansas, northeastern New Mexico, and northwestern 

Texas: U.S. Geological Survey Water-Resources Investigations Report 99–4104, 68 p.

McGuire, V.L., and Sharpe, J.B., 1997, Water-level changes 

in the High Plains aquifer—Predevelopment to 1995: 

U.S. Geological Survey Water-Resources Investigations 

Report 97–4081, 2 pls.

McMahon, P.B., 2000, A reconnaissance study of the effect of 

irrigated agriculture on water quality in the Ogallala Formation, central High Plains aquifer: U.S. Geological Survey 

Fact Sheet FS–009–00, 6 p.

McMahon, P.B., 2001, Vertical gradients in water chemistry 

in the central High Plains aquifer, southwestern Kansas and 

Oklahoma Panhandle, 1999: U.S. Geological Survey WaterResources Investigations Report 01–4028, 47 p.

National Atmospheric Deposition Program, 2001, Data on 

the World Wide Web at http://nadp.sws.uiuc.edu/nadpdata/

(accessed 09/26/01).

Olson, C.G., Nettleton, W.D., Porter, D.A., and Brasher, B.R., 

1997, Middle Holocene aeolian activity on the High Plains 

of west-central Kansas: The Holocene, v. 7, p. 255–261.

Phene, C.J., Hoffman, G.J., and Rawlins, S.L., 1971, Measuring soil matric potential in situ by sensing heat dissipation 

within a porous body—I. Theory and sensor construction: Soil Science Society of America Proceedings, v. 35, 

p. 27–32.

Phillips, F.M., 1994, Environmental tracers for water movement in desert soils of the American southwest: Soil Science 

Society of America Journal, v. 58, p. 15–24.

Plummer, L.N., Michel, R.L., Thurman, E.M., and Glynn, 

P.D., 1993, Environmental tracers for age dating young 

ground water, in Alley, W.M., ed., Regional ground-water 

quality: New York, Van Nostrand Reinhold, p. 255–294.

Qi, Sharon, Konduris, A., and Litke, D.W., 2002, Classification and mapping of irrigated lands using satellite imagery—High Plains aquifer area, 1990: U.S. Geological 

Survey Water-Resources Investigations Report 02–4236, 

30 p.

Scanlon, B.R., 1991, Evaluation of moisture flux from 

chloride data in desert soils: Journal of Hydrology, 

v. 128, p. 137–156.

Scanlon, B.R., and Goldsmith, R.S., 1997, Field study of spatial variability in unsaturated flow beneath and adjacent to 

playas: Water Resources Research, v. 33, p. 2239–2252.

Scanlon, B.R., Langford, R.P., and Goldsmith, R.S., 1999, 

Relationship between geomorphic settings and unsaturated 

flow in an arid setting: Water Resources Research, v. 35, 

p. 983–999.

Smith, H.T.U., 1940, Geological studies in southwestern Kansas: Kansas Geological Survey Bulletin 34, 244 p.

Sophocleous, M.A., Kluitenberg, G., and Healy, J., 2002, 

Southwestern Kansas High Plains unsaturated zone pilot 

study to estimate Darcian-based ground-water recharge 

at three sites: Kansas Geological Survey Open-File 

Report 2002–11, 46 p.

Spalding, R.F., and Kitchen, L.A., 1988, Nitrate in the intermediate vadose zone beneath irrigated cropland: Ground 

Water Monitoring Review, Spring Issue, p. 89–95.

Spalding, R.F., Watts, D.G., Schepers, J.S., Burbach, M.E., 

Exner, M.E., Poreda, R.J., and Martin, G.E., 2001, Controlling nitrate leaching in irrigated agriculture: Journal of 

Environmental Quality, v. 30, p. 1184–1194.

Thatcher, L.L., 1962, The distribution of tritium fallout in precipitation over North America: Bulletin of the International 

Association of Scientific Hydrology, v. 7, p. 48–58.

Thatcher, L.L., Janzer, V.J., and Edwards, K.W., 1977, Methods for determination of radioactive substances in water and 

fluvial sediments: U.S. Geological Survey Techniques of 

Water-Resources Investigations, chap. A–5, 95 p.
32 Water Movement Through the Central High Plains Aquifer, Southwestern Kansas, 2000–2001

Thurman, E.M., Meyer, M., Pomes, M., Perry, C.A., and 

Schwab, A.P., 1990, Enzyme-linked immunosorbent assay 

compared with gas chromatography/mass spectrometry for 

the determination of triazine herbicides in water: Analytical 

Chemistry, v. 62, p. 2043–2048.

Tindall, J.A. and Kunkel, J.R., 1999, Unsaturated zone hydrology for scientists and engineers: Upper Saddle River, New 

Jersey, Prentice-Hall, 624 p.

Townsend, M.A., Sleezer, R.O., and Macko, S.A., 1996, 

Effects of agricultural practices and vadose zone stratigraphy on nitrate concentrations in ground water in Kansas, 

USA: Water Science and Technology, v. 33, p. 219–226.

Walvoord, M.A., Plummer, M.A., Phillips, F.M., and 

Wolfsberg, A.V., 2002, Deep arid system hydrodynamic—

1. Equilibrium states and response times in thick desert 

vadose zones: Water Resources Research, v. 38, 

p. 44.1–44.15.

Weeks, J.B., Gutentag, E.D., Heimes, F.J., and Luckey, 

R.R., 1988, Summary of the High Plains regional aquifersystem analysis in parts of Colorado, Kansas, Nebraska, 

New Mexico, Oklahoma, South Dakota, Texas, and Wyoming: U.S. Geological Survey Professional Paper 1400–A, 

30 p.

Wood, W.W., and Sanford, W.E., 1995, Chemical and isotopic methods for quantifying ground-water recharge in 

a regional, semiarid environment: Ground Water, v. 33, 

p. 458–468.

Zaugg, S.D., Sandstrom, M.W., Smith, S.G., and Fehlberg, 

K.M., 1995, Methods of analysis by the U.S. Geological 

Survey National Water Quality Laboratory—Determination of pesticides in water by C–18 solid-phase extraction 

and capillary-column gas chromatography/mass spectrometry with selected-ion monitoring: U.S. Geological Survey 

Open-File Report 95–181, 60 p.