Ground-Water Quality Beneath Irrigated 

Agriculture in the Central High Plains Aquifer, 

1999–2000

By Breton W. Bruce, Mark F. Becker, Larry M. Pope, and Jason J. Gurdak

Water-Resources Investigations Report 03–4219

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:

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endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.
 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 long-term 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 water-resource 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
Contents v

Foreword. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .iii

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

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

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

Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

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

Hydrogeologic Setting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Land Use. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Fertilizer and Pesticide Use. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

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

Study Design and Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Site Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Well Installation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Sample Collection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Land-Use Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Quality Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Ground-Water Quality Beneath Irrigated Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Major Dissolved Ions and Water Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Nutrients and Pesticides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Tritium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Dissolved Organic Carbon. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Relation Between Land Use and Water Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Nutrients, Pesticides, and Tritium. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Nitrogen Isotopes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Dissolved Solids and Major Ions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Land-Use and Hydrogeologic Factors Affecting Ground-Water Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Summary and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

References Cited. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Supplemental Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Figures

1. Map showing the location of the High Plains aquifer system and the boundaries of the 

central High Plains agricultural land-use study, 1999–2000. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2

2. Map showing the areal distribution of primary land-use types in the central High Plains, 

nominal 1992. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4

3. Graph showing the number of acres planted in the primary agricultural crops in the 

central High Plains in 2001, by State. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5

4. Map showing the distribution of nitrogen application rates, by county, from manure (1997) 

and commercial fertilizer (1999) in the central High Plains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6

5. Map showing the distribution of atrazine application rates, by county, in the central 

High Plains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

 6. Graph showing the primary ground-water uses in the central High Plains in 1995. . . . . . . . . . . . . . . . . . . . . .8

7. Map showing the distribution of irrigated agriculture greater than 30 percent in areas 

where ground water is less than 200 feet below land surface, and locations of wells installed 

to evaluate ground-water quality beneath irrigated fields in the central High Plains 

land-use study, 1999–2000 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
vi 

8–10. Graphs showing the:

8.Detection frequency for pesticides and pesticide metabolites in ground water 

beneath irrigated agricultural fields in the central High Plains, 1999–2000 . . . . . . . . . . . . . . . . . . . 16

9. Relation between nitrate and tritium concentrations and the detection of 

pesticides beneath irrigated agricultural fields in the central High Plains, 1999–2000 . . . . . . . . 18

10. Relation between nitrate concentration and nitrogen isotope ratio in water 

samples from beneath irrigated agricultural fields in the central High Plains, 

1999–2000. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

11. Boxplots showing the distribution of dissolved-solids concentrations for sites affected 

and unaffected by agricultural land use in the central High Plains, 1999–2000. . . . . . . . . . . . . . . . . . . . . . . . 21

12. Trilinear diagram of the relative major ion composition of ground-water samples 

collected beneath irrigated agricultural fields and nonagricultural areas in the 

central High Plains, 1999–2000 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

13. Boxplots showing the distribution of sulfate concentrations (A) for sites affected and 

unaffected by irrigated agriculture in the central High Plains, and (B) relative to 

dissolved solids concentrations for 30 samples collected beneath irrigated fields in the 

central High Plains, 1999–2000 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

14. Boxplots showing the comparison between sites affected and unaffected by agricultural 

land use and for selected factors that might contribute to aquifer vulnerability in the 

central High Plains. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Tables

1. Ten most frequently applied pesticides over the central High Plains aquifer between 

1990 and 1995. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

2. Selected well-construction information for monitoring wells installed in the central 

High Plains aquifer and sampled as part of the agricultural land-use study, 1999–2000 . . . . . . . . . . . . . . . 10

3. Water-quality constituents and parameters analyzed in water samples collected in the 

central High Plains agricultural land-use study, 1999–2000. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4. Laboratory analysis methods for measured water-quality constituents in the central 

High Plains agricultural land-use study, 1999–2000 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

 5. Constituents detected in quality-control blank samples in the central High Plains agricultural 

land-use study, 1999–2000 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

6. Summary of relative percent differences for replicate samples, and percent recoveries 

for field-matrix spike analytes, in the central High Plains agricultural land-use study, 

1999–2000. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

7. Summary statistics for major dissolved ions and water properties for 30 ground-water 

samples collected as part of the central High Plains agricultural land-use study, 1999–2000. . . . . . . . . . 15

Tables in Supplemental Information Section

8. Median percent recoveries for individual compounds in pesticide field spike mixtures in 

the central High Plains agricultural land-use study, 2000. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

9. Results of physical and chemical analyses of water samples from wells completed as part of 

the central High Plains agricultural land-use study, 2000. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
CONVERSION FACTORS, VERTICAL DATUM, AND ABBREVIATIONS

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

°C = 5/9 (°F –32)

°F = 9/5 (°C) +32

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

(NGVD 29); horizontal coordinate information is referenced to the North American Datum of 

1983 (NAD83).

OTHER ABBREVIATIONS:

micrometer, μm

milliequivalent, meq

Multiply By To obtain

inch 2.54 centimeter

foot (ft) 0.3048 meter

mile (mi) 1.609 kilometer

meter (m) 3.281 foot

acre 640 square mile

milligram per liter (mg/L) 1.0 part per million

microgram per liter (μg/L) 1.0 part per billion

milliliter (mL) 0.0338 ounce, fluid

pound per acre (lb/acre) 1.121 kilogram per hectare

pound per square inch (lb/in2) 6.895 kilopascal

square mile (mi2) 2.590 square kilometer

million gallons per day (Mgal/d) 2.629 cubic meters per minute
Ground-Water Quality Beneath Irrigated Agriculture in the 

Central High Plains Aquifer, 1999–2000

By Breton W. Bruce, Mark F. Becker, Larry M. Pope, and Jason J. Gurdak

Abstract

In 1999 and 2000, 30 water-quality monitoring wells were 

installed in the central High Plains aquifer to evaluate the 

quality of recently recharged ground water in areas of irrigated 

agriculture and to identify the factors affecting ground-water 

quality. Wells were installed adjacent to irrigated agricultural 

fields with 10- or 20-foot screened intervals placed near the 

water table. Each well was sampled once for about 100 waterquality constituents associated with agricultural practices. 

Water samples from 70 percent of the wells (21 of 30 sites) contained nitrate concentrations larger than expected background 

concentrations (about 3 mg/L as N) and detectable pesticides. 

Atrazine or its metabolite, deethylatrazine, were detected with 

greater frequency than other pesticides and were present in all 

21 samples where pesticides were detected. The 21 samples 

with detectable pesticides also contained tritium concentrations 

large enough to indicate that at least some part of the water 

sample had been recharged within about the last 50 years. These 

21 ground-water samples are considered to show water-quality 

effects related to irrigated agriculture. The remaining 9 groundwater samples contained no pesticides, small tritium concentrations, and nitrate concentrations less than 3.45 milligrams per 

liter as nitrogen. These samples are considered unaffected by 

the irrigated agricultural land-use setting. Nitrogen isotope 

ratios indicate that commercial fertilizer was the dominant 

source of nitrate in 13 of the 21 samples affected by irrigated 

agriculture. Nitrogen isotope ratios for 4 of these 21 samples 

were indicative of an animal waste source. Dissolved-solids 

concentrations were larger in samples affected by irrigated 

agriculture, with large sulfate concentrations having strong 

correlation with large dissolved solids concentrations in these 

samples. A strong statistical correlation is shown between samples affected by irrigated agriculture and sites with large rates 

of pesticide and nitrogen applications and shallow depths to 

ground water.

Introduction

Knowledge of the quality of the Nation’s water resources 

is important because of the implications to human and aquatic 

health and because of the significant costs associated with decisions involving land and water management, conservation, and 

regulation. In 1991, the U.S. Geological Survey (USGS) began 

full implementation of the National Water-Quality Assessment 

(NAWQA) Program. The long-term goals of the NAWQA Program are to describe the status and trends in the quality of the 

Nation’s surface- and ground-water resources and determine 

the natural and anthropogenic factors affecting water quality 

(Gilliom and others, 1995). More than 50 major river basins and 

aquifer systems are targeted for investigation under the 

NAWQA Program. Together, these include water resources 

available to more than 60 percent of the population and cover 

about one-half of the land area in the conterminous United 

States. 

The High Plains Regional Ground Water study began in 

October 1998 and represents a modification of the traditional 

NAWQA design in that the ground-water resource is the primary focus. The High Plains aquifer is a nationally important 

water resource that underlies about 174,000 mi2

 in parts of eight 

Western States (fig. 1). About 27 percent of agricultural land in 

the United States is in the High Plains, and about 30 percent of 

all the ground water used for irrigation in the United States is 

pumped from this aquifer (Dennehy, 2000). 

Water quality in the High Plains aquifer might be vulnerable to effects from land-surface activities; however, large 

depths to ground water in some areas could provide a buffer 

from these influences. The lack of a regionally extensive 

geologic barrier to impede downward migration of contaminants contributes to the potential for water-quality degradation 

from land-surface activities. Similarly, applications of large 

amounts of water at the land surface can be a driving force for 

downward migration of chemicals. Residential and urban 

settings, agricultural activities, and oil and gas exploration/

development are potential sources of contaminants to this 

drinking-water aquifer.
2 Ground-Water Quality Beneath Irrigated Agriculture in the Central High Plains Aquifer, 1999–2000

Northern

High Plains

Central

High Plains

Southern

High Plains

CO

NM

TX

OK

KS

WY NE

SD

Dodge

City

Garden

City

Liberal

Guymon

Pampa

Canadian R

Arkansas R

Cimarron R

70

40

35

54

40

Amarillo

106° 102° 98°

32°

34°

36°

38°

40°

42°

EXPLANATION

Quaternary deposits

No aquifer present

Central High Plains aquifer

Area of little or no saturation

0 50 100 MILES

0 50 100 KILOMETERS

35°

36°

37°

38°

County, State, and hydrography base information are from U.S. Geological Survey

1:100,000 Digital Line Graphs (DLG). Roads are from 1:2,000,000 DLG.

104° 102° 100° 98°

Figure 1. Location of the High Plains aquifer system and the boundaries of the central High Plains 

agricultural land-use study, 1999–2000.
Description of Study Area 3

NAWQA ground-water studies include a component 

designed to assess the relation between regionally significant 

land-use settings and the quality of recently recharged ground 

water. These land-use studies investigate the natural and human 

factors affecting ground-water quality in these settings. In a 

reconnaissance study of the effect of irrigated agriculture on 

water quality in the central High Plains aquifer, McMahon 

(2000) demonstrated that recently recharged water was present 

in the High Plains aquifer in selected areas with depths to water 

as great as 200 ft and that the quality of ground water in these 

areas was affected by agricultural land-use practices. Based on 

these findings, a more comprehensive and areally extensive 

NAWQA land-use study (LUS) was completed in 2000 to evaluate the regional effects of irrigated agriculture on groundwater quality in the central High Plains aquifer.

Purpose and Scope

This report describes the quality of recently recharged 

ground water beneath irrigated agricultural land in the central 

High Plains aquifer. Thirty water-quality monitoring wells were 

installed in proximity to irrigated fields in areas where depth to 

water was less than 200 ft. The 30 wells had short (generally 

10 ft) well screens that were installed near the water table. The 

wells were sampled once between February 1999 and September 2000 and analyzed for approximately 100 constituents 

including major dissolved ions, nutrients (compounds of nitrogen and phosphorus), nitrogen isotopes, pesticides and pesticide 

metabolites, tritium, and dissolved organic carbon (DOC). Land 

use within about 500 meters of each well was characterized, and 

correlations between ground-water quality and the overlying 

land-use were explored.

Acknowledgments

The authors thank each landowner that allowed a monitoring well to be installed on their property. Without their openminded, proactive attitude, this study would not have been possible. We also thank the following USGS employees: Carol 

Carlson, Mike Carlson, Chad Milligan, Marty Phillips, and 

Martin Schneider for their assistance with field activities; Cristi 

Hansen for her review and processing of site land-use data; and 

Sharon Qi for assisting with the preparation of graphics shown 

in this report. We also thank the following reviewers and editors 

for their contributions to this manuscript: Mary Kidd, Suzanne 

Paschke, and Andrew Ziegler.

Description of Study Area

 For logistical reasons, the High Plains aquifer was subdivided into three regions of study (fig. 1). This land-use study 

was done within the central High Plains region in geologic units 

of Tertiary age or younger. Nonglacial deposits of Quaternary 

age lie east of the study area in south-central Kansas and, 

although part of the central High Plains aquifer, they were not 

investigated by this study. The boundaries used for the central 

High Plains region and the extent of the central High Plains 

aquifer are consistent with those described by Gutentag and others (1984). The central High Plains region covers an area of 

about 48,500 mi2 in parts of Colorado, Kansas, New Mexico, 

Oklahoma, and Texas (fig. 1). The central High Plains aquifer 

studied here underlies about 38,000 mi2

 in all but the eastern 

part of the central High Plains (fig. 1).

Hydrogeologic Setting

The hydrogeologic setting of the High Plains aquifer has 

been described in detail by Gutentag and others (1984) and 

Luckey and others (1986, 1988). Other localized studies 

describe the hydrogeology of subareas of the High Plains 

(Gutentag and others, 1981; Peckenpaugh and Dugan, 1983; 

Peckenpaugh and others, 1987; Nativ, 1988). The central High 

Plains aquifer is primarily composed of the Ogallala Formation 

(of Tertiary age) and generally can be described as interbedded 

sand, gravel, silt, and clay deposited by fluvial (stream) and 

eolian (windblown) processes. The sediments are poorly sorted 

and generally unconsolidated but in some locations are 

cemented by calcium carbonate and, to a lesser degree, silica. 

The Tertiary-age sediments are locally overlain by Quaternaryage alluvial gravel, dune sand, and loess deposits and are underlain by older bedrock formations.

Sand and gravel zones in the central High Plains aquifer 

are the primary source of water to wells and commonly yield 

between 100 and 2,000 gallons per minute (Gutentag and others, 1981). Ground water exists predominantly under unconfined conditions, though confined conditions might be present 

locally. Overlying Quaternary-age deposits and underlying 

bedrock formations can, where saturated, contribute water and 

solutes to the central High Plains aquifer. The regional water 

table slopes eastward with a gradient of about 0.002 (10.5 ft per 

mi; Luckey and Becker, 1998), and water movement generally 

is in that direction. The saturated thickness of the central High 

Plains aquifer is as much as 600 ft with the greatest saturated 

thickness in areas that overlie the deepest channels in the bedrock surface (McGuire and Fischer, 1999). Some areas in the 

western part of the central High Plains are underlain by Tertiary-age sediments but contain no water or are minimally saturated (fig. 1). Wells in this western part of the central High 

Plains commonly withdraw water from deeper bedrock units 

rather than the High Plains aquifer.

Water levels in the central High Plains aquifer have been 

declining since the area underwent extensive ground-water irrigation development in the 1950’s. Luckey and others (1981) 

reported water-level declines of more than 100 ft in parts of the 

central High Plains for the period between predevelopment 

(1950) and 1980. McGuire (2001) calculated an average areaweighted decline rate of 0.33 ft per year for southwestern Kansas between predevelopment and 1980 and a decline rate of 

about 0.44 ft per year from 1980 to 1999.
4 Ground-Water Quality Beneath Irrigated Agriculture in the Central High Plains Aquifer, 1999–2000

Land Use

The central High Plains region is sparsely populated, and 

the area is dominated by an agricultural economy. About 

57 percent of the study area is rangeland, 41 percent is active 

agricultural land, and less than 1 percent is considered an urban 

environment (U.S. Geological Survey, National Land Cover 

Data Set, nominal 1992). Figure 2 shows the distribution of 

land-use types in the central High Plains region. In 2000, the 

estimated population in the study area was about 340,000. 

Amarillo, Tex., is the largest city in the study area and had a 

population of 173,627 in 2000. Other major cities (and their 

2000 populations) are Garden City, Kans. (28,000); Dodge 

City, Kans. (25,000); Liberal, Kans. (20,000); Pampa, Tex. 

(18,000); and Guymon, Okla. (11,000). Counties in the study 

area that do not encompass one of these major cities generally 

have rural populations of about 5,000 or less. (U.S. Census 

Bureau, 2000). 

Figure 2. Areal distribution of primary land-use types in the central High Plains, nominal 1992.

Dodge

City

Garden

City

Liberal

Guymon

Pampa

Amarillo

Canadian R

Arkansas R

Cimarron R

104° 102° 100° 98°

35°

36°

37°

38°

EXPLANATION

Land use

Water

Urban

Barren

Rangeland

Agriculture

Wetlands

0 25 50 75 100 KILOMETERS

0 25 50 75 100 MILES Base information from U.S. Geological Survey 1:100,000 Digital Line Graphs (DLG).

Land cover information from the National Land Cover Dataset.
Description of Study Area 5

Of the 41 percent of central High Plains region that is 

under cultivation, about 68 percent is dry-land farming and 

about 32 percent is irrigated crop land (U.S. Geological Survey, 

National Land Characteristics Data Set, nominal 1992). Wheat, 

corn, and sorghum are the primary agricultural crops in the central High Plains region; other small grains and hay are minor 

crops. Generally, corn is an irrigated crop, whereas wheat and 

sorghum generally are not irrigated. Figure 3 shows the acres of 

land, by State, under the major crop types in the central High 

Plains region (U.S. Department of Agriculture, 2001).

Livestock production also is an important commodity in 

the central High Plains region with more than 4.5 million cattle 

and calves in inventory in 1997 (U.S. Department of Agriculture, 2001). Of the top 20 cattle-producing counties in the 

United States, 3 are in the central High Plains region (Finney 

County, Kansas; Texas County, Oklahoma; and Hansford 

County, Texas). Swine production, centered in the Guymon, 

Okla., area also is an important livestock commodity and is 

rapidly increasing. Though actual numbers of swine in inventory are not reported for every county in the central High Plains 

region, Texas County in Oklahoma ranked third in the Nation in 

1997 with about 907,000 hogs in inventory (U.S. Department of 

Agriculture, 2001). The counties in Kansas, New Mexico, and 

Texas that border the Oklahoma panhandle are the largest producers of hogs and pigs in those States.

Fertilizer and Pesticide Use

The use of fertilizers and agricultural chemicals on crop 

land is of importance to a study of ground-water quality in the 

agriculturally dominated central High Plains region. Fertilizers 

provide nutrients (primarily nitrogen) needed by crops to 

optimize growth and production. Excess nitrogen applied to 

fields can leach to the ground-water system and cause elevated 

nitrate concentrations. Nitrate concentration in drinking water 

is a human health concern and is regulated by the U.S. Environmental Protection Agency. Manure and commercial fertilizer 

(generally in the form of anhydrous ammonia) are the primary 

forms of nitrogen applied to fields in the central High Plains 

region. Figure 4 shows the agricultural application rates (by 

county) for both forms of nitrogen in the study area.

Naturally occurring organic nitrogen accumulating in the 

soil zone due to plant decomposition also can be an important 

secondary source of nitrogen in agricultural settings. This soil 

organic nitrogen can be mobilized when the sod is first tilled, 

and this mobilization can create a pulse of nitrogen to the 

ground-water system. The largest nitrogen concentrations associated with this soil nitrogen pulse generally decrease over time; 

however, it might take several years (or decades) for the initial 

nitrogen pulse to migrate to the water table in areas with large 

depths to ground water. The concentrations of ground-water 

nitrate derived from soil organic nitrogen are likely to be 

smaller than those derived from annual fertilizer applications on 

crop lands with long agricultural histories.

Pesticides are chemicals used to control unwanted plants 

and insects in crops. They also are used to control weeds in 

lawns, rights-of-way, ponds, and ditches. The 10 most frequently applied pesticides in the central High Plains region (by 

pounds applied and acres applied) are listed in table 1. Different 

pesticides exhibit different levels of persistence and mobility in 

the environment. Some heavily applied compounds are rarely 

detected in ground water either because they quickly degrade in 

the near surface or they become strongly adsorbed to soil and 

sediment. The herbicide atrazine is heavily used for broadleaf

Figure 3. Number of acres planted in the primary agricultural crops in the 

central High Plains in 2001, by State.

500,000

1,000,000

1,500,000

2,000,000

2,500,000

IDENTIFIED CROP, IN ACRES

0

KANSAS COLORADO OKLAHOMA TEXAS NEW MEXICO

Wheat

Corn

Sorghum

Hay

Other
6 Ground-Water Quality Beneath Irrigated Agriculture in the Central High Plains Aquifer, 1999–2000

Figure 4. Distribution of nitrogen application rates, by county, from manure (1997) and commercial fertilizer (1999) in the central High Plains.

Canadian

R

Arkan

as s R

Cimarron R

70

40

35

54

40

DodgeCity

GardenCity

Liberal

Guymon

Pampa

Amarillo

Canadian

R

Arkan

as s R

CimarronR

70

40

35

54

40

DodgeCity

GardenCity

Liberal

Guymon

Pampa

Amarillo

104o 102o 100o 98o

34o36o38o

34o36o38o

104o 102o 100o 98o

EXP

LANATION

Nitrogen from fertilizer,

in pounds per acre per yar

0 - 1718 - 4041 - 6869 - 109

0 50 100 MILES

0 50 100 KILOMETERS

EXP

LANATION

Nitrogen from manure,

in pounds per acre per year

0 - 89 - 1718 - 3233 - 62

Base information from U.S. Geological Survey1:100,000 Digital Line Graphs (DLG). Roadsare from 1:2,000,000 DLG. 
Description of Study Area 7

weed and grass control in corn, sorghum, and other crops. Atrazine has been detected in ground water in many parts of the 

United States and is indicative of the effect of agricultural activities (particularly corn production) on ground-water quality. 

Figure 5 shows atrazine application rates, in pounds per acre by 

county, for the central High Plains region.

Table 1. Ten most frequently applied pesticides over the central High Plains aquifer between 1990 and 1995.

[Pesticide-use rates compiled by the National Center for Food and Agricultural Policy (NCFAP); applied to crop acreage from 1997 Census of 

Agriculture]

Rank Weighted by pounds applied Weighted by acres applied

1 Atrazine 2,4-D

2 2,4-D Atrazine

3 Metolachlor Metolachlor

4 Alachlor Chlorpyrifos

5 Butylate Dimethoate

6 Chlorpyrifos Carbofuran

7 Dimethoate Alachlor

8 Carbofuran Terbufos

9 Terbufos Cyanazine

10 Cyanazine Butylate

Figure 5. Distribution of atrazine application rates, by county, in the central High Plains.

Pesticides (pounds per acre)

0 - 0.05

0.05 - 0.2

0.2 - 0.4

Greater than 0.4

Dodge

City

Garden

City

Liberal

Guymon

Pampa

Arkansas R

Cimarron R

Amarillo

CanadianR

EXPLANATION

104° 102° 100° 98°

35°

36°

37°

38°

39°

40

54

70

35

40

0 25 50 KILOMETERS

0 25 50 MILES
8 Ground-Water Quality Beneath Irrigated Agriculture in the Central High Plains Aquifer, 1999–2000

Irrigation

95.96 percent

Other

4.04 percent Public supply

1.53 percent

Domestic selfsupplied

0.17 percent

Thermoelectric

power

0.20 percent

Mining

0.63 percent

Livestock

1.51 percent

Water Use

Ground water provides about 93 percent of the water used 

in the central High Plains region (U.S. Geological Survey, 

Aggregate Water-Use Data System, 1995). Few rivers cross the 

central High Plains, and surface water generally is not accessible away from river courses. Counties near the Arkansas River 

in Colorado and Kansas, the Canadian River in Texas, and the 

three counties in northeastern New Mexico that are part of the 

central High Plains region (fig. 1) have greater access to surface 

water and use the largest amount of this resource in the study 

area. Surface-water use in the central High Plains region is 

almost entirely for irrigation of crops with the exception of the 

city of Amarillo, Tex., which gets a large percentage of its public supply from Lake Meredith in Hutchinson County, Texas 

(about 44 Mgal/d). Many parts of the central High Plains region 

rely entirely upon ground water for human consumption and all 

other uses. Irrigated agriculture uses about 96 percent of this 

ground-water supply (U.S. Geological Survey, Aggregate 

Water-Use Data System, 1995). Other major uses of ground 

water in the central High Plains region are public supply, livestock, mining, thermoelectric power, and domestic self-supplied water (fig. 6). About 85 percent of the population in the 

central High Plains region is served by public-supply systems 

operated by the towns and cities; about 79 percent of these people are receiving ground water.

Study Design and Methods

Site Selection

Because McMahon (2000) showed that ground water in 

the central High Plains region with a depth less than 200 ft 

below land surface was vulnerable to water-quality effects 

from agricultural land use, and because agriculture is such a 

dominant land use in this area, site selection for this land-use 

study focused on irrigated agricultural areas where depth to 

ground water was less than 200 ft. Areas of irrigated agriculture 

(nominal 1980) were determined using a Geographic Information System (GIS) raster data set (2-kilometer cell size) showing the percentage of irrigated lands in the central High Plains 

region (Thelin and Heimes, 1987). If more than 30 percent of a 

raster cell was identified as irrigated agriculture, the cell was 

classified as irrigated land. Depth to water was determined by 

surface interpolation of 1997 water-level point data measured 

from wells in the High Plains Water Level Monitoring Program 

(V.L. McGuire, U.S. Geological Survey, written commun., 

1998). The irrigated lands and depth-to-water data sets were 

intersected to delineate areas that were greater than 30 percent 

irrigated and had depths to water less than 200 ft (fig. 7). 

Within these areas, a grid-based site-selection computer program (Scott, 1990) was used to select 30 randomly distributed 

primary sites for well installation. The site-selection program 

also selected a paired alternate site for each primary site. The 

alternate site was used only in the event that permission could 

not be acquired to install a well within a 2-mi radius of the primary site. Local landowners were contacted near each site, and 

permission to install and sample wells at 30 sites was granted; 

22 primary sites and 8 alternate sites were used. The locations 

of the installed wells are shown in figure 7 along with the well 

numbers used to identify individual wells in this report.

Well Installation

Wells were installed using a hollow-stem auger system so 

that no drilling fluids would be introduced during the drilling 

process. Boreholes were drilled to a depth approximately 20 ft 

below the water table, and a 2-inch-inside-diameter polyvinylchloride (PVC) well was installed through the center of the 

auger. The well included a bottom well point and 5-ft blank-casing sump attached to the bottom of a 10- or 20-ft screened interval. The top of the screened interval was placed about 5 ft below 

the estimated water table and blank riser-casing extended to 

land surface. The borehole annular space opposite the screened 

interval was backfilled during auger removal with 10–20 sieve 

silica sand to a level a few feet above the water table and a 5-ft 

bentonite-pellet plug was installed above the silica sand to seal 

the annular opening. Once the sand pack and bentonite plug 

were in place, the remaining augers were removed from the hole 

and the remaining annular space was filled to land surface with 

bentonite/cement grout. The surface was completed with either 

a 5-ft steel protective casing with locking cap or a flush-mount 

manhole cover even with land surface. Both types of surface 

completions were set in concrete pads designed to drain surface 

runoff away from the borehole. The construction details for the 

30 monitoring wells installed for this study are listed in table 2.

Sample Collection

The 30 monitoring wells installed for this study were each 

sampled once between February 1999 and September 2000. 

Samples were collected by lowering a portable submersible 

pump into each well to about the middle of the screened interval

Figure 6. Primary ground-water uses in the central High Plains 

in 1995.
Study Design and Methods 9

and pumping water to a truck-mounted mobile water-quality 

laboratory where water samples were collected and processed. 

All equipment in contact with the water during sampling was 

composed of either stainless steel or Teflon.

Sampling procedures are described in detail in Koterba 

and others (1995). Prior to sample collection, at least three well 

volumes of water were purged from each well to remove any 

stagnant water. During purging, measurements of specific conductance, pH, water temperature, and dissolved oxygen were 

recorded every 5 minutes inside a closed-cell flow-through 

chamber until three consecutive stable readings were obtained. 

Turbidity also was measured every 5 minutes using a portable 

turbidity meter. Once stable readings for these water properties 

were obtained, the flow of water was redirected to a clean sampling chamber inside the mobile laboratory where water samples were immediately collected for analysis. Constituents analyzed for in the water samples collected for this study are listed 

in table 3.

Samples collected for the analysis of major ions, nutrients, 

and nitrogen isotopes were filtered through a 0.45-μm disposable capsule filter and captured in precleaned plastic bottles that 

were rinsed onsite with filtered ground water. Samples for 

major cation analysis were preserved with 2 mL of 7.5–7.7N 

nitric acid dispensed from individual polypropylene vials. Samples collected for pesticide and pesticide metabolite analyses 

were filtered through a methanol-rinsed stainless-steel filter 

stand containing a 0.7-μm glass-fiber filter. The pesticide samples were collected in cleaned and baked amber-glass bottles. 

Unfiltered samples for tritium analysis were collected in the 

sampling chamber directly from the pump discharge line in a 

manner that minimized agitation. Tritium sample bottles were 

filled and capped leaving no headspace, and the caps were 

sealed with plastic electrical tape. Finally, in an effort to avoid 

contact with methanol-rinsed sampling equipment, samples collected for DOC analysis were collected directly from the well 

after the portable submersible pump was removed. A singleFigure 7. Distribution of irrigated agriculture greater than 30 percent in areas where ground water is less than 200 

feet below land surface, and locations of wells installed to evaluate ground-water quality beneath irrigated fields in 

the central High Plains land-use study, 1999–2000.

19

9

5

28 11

10

12

27 13

22

26

14 4

21

30 20

6

18

29

17

7

3 24

25 8

2

1

23

15

16

23

Canadian R

Arkansas R

Cimarron R

104° 102° 100° 98°

35°

36°

37°

38°

EXPLANATION

Well location and site ID

Irrigated land greater than 30 percent

Base information from U.S. Geological

Survey 1:100,000 Digital Line Graphs

(DLG). Irrigated land data from Qi and

others, 2002.

0 25 50 75 100 MILES

0 25 50 75 100 KILOMETERS
10 Ground-Water Quality Beneath Irrigated Agriculture in the Central High Plains Aquifer, 1999–2000

Table 2. Selected well-construction information for monitoring wells installed in the central High Plains aquifer and sampled as 

part of the agricultural land-use study, 1999–2000.

[BLS, below land surface]

Map index

number

(fig. 7)

Date of

installation

Elevation1

of installation

land surface

(feet)

Depth of

completed

well BLS

(feet)

Depth to water

BLS at time

of sampling

(feet)

Screened

interval BLS

(feet)

Sand-pack

interval BLS

(feet)

1 09–23–1999 3,460 198 186.8 187–197 177–198

2 08–17–2000 3,965 182 160.0 169–178 151–182

3 07–22–2000 3,623 113 96.7 101–110 86–114

4 06–11–2000 3,442 178 159.6 166–174 153–178

5 03–28–2000 3,655 199 186.5 186–196 180–199

6 06–21–2000 3,266 198 173.3 186–196 177–198

7 07–24–2000 3,324 197 181.7 184–194 166–197

8 06–20–1999 3,100 145 132.8 120–140 100–145

9 11–13–1998 2,904 133 122.7 107–127 90–135

10 03–24–2000 3,217 174 163.4 164–174 153–174

11 04–09–2000 3,181 182 171.3 167–176 161–182

12 04–04–2000 2,920 139 121.2 122–131 116–139

13 11–12–1998 2,853 94 70.4 68–88 57–101

14 06–30–2000 2,921 193 177.1 182–190 169–194

15 03–22–2000 2,890 170 151.4 157–167 139–170

16 04–01–2000 2,844 156 151.8 143–153 136–156

17 05–23–2000 2,520 31 13.0 16–25 11–31

18 06–05–2000 2,580 157 137.7 145–154 138–158

19 05–29–2000 2,520 115 99.2 102–112 96–115

20 11–14–1998 2,797 190 164.1 164–184 156–195

21 05–16–2000 2,740 149 137.7 136–146 130–149

22 05–13–2000 2,857 187 174.9 175–186 168–188

23 07–19–2000 4,463 180 169.6 170–178 156–184

24 07–27–2000 3,190 158 133.7 144–152 129–158

25 08–19–2000 3,531 113 99.8 102–110 85–114

26 06–27–2000 3,019 106 92.2 96–104 85–108

27 07–08–2000 2,977 162 150.9 150–160 141–162

28 04–13–2000 3,216 174 156.2 161–170 154–173

29 05–25–2000 2,470 149 134.4 138–146 127–149

30 05–11–2000 2,595 108 92.2 94–102 89–114

1

Accuracy of elevation was within ±10 feet.
Study Design and Methods 11

Table 3. Water-quality constituents and parameters analyzed in water samples collected in the central High Plains agricultural 

land-use study, 1999–2000.

[μg/L, micrograms per liter; pCi/L, picocuries per liter; mg/L, milligrams per liter; na, not applicable; USGS, U.S. Geological Survey; LC, lab code; SC, 

schedule]

Constituent Analytical method

reporting limit Constituent Analytical methodreporting limit

Pesticides, USGS SC2001 by gas chromatography/mass spectrometry,

in micrograms per liter

2,6-Diethylaniline 0.003 Acetochlor 0.002

Alachlor 0.002 Atrazine 0.001

Azinphos-methyl 0.001 Benfluralin 0.002

Butylate 0.002 Carbaryl 0.003

Carbofuran 0.003 Chlorpyrifos 0.004

Cyanazine 0.004 DCPA 0.002

Deethylatrazine 0.002 Diazinon 0.012

Dieldrin 0.001 Disulfoton 0.02

EPTC 0.002 Ethalfluralin 0.004

Ethoprop 0.003 Fonofos 0.003

Lindane 0.004 Linuron 0.002

Malathion 0.005 Metolachlor 0.002

Metribuzin 0.004 Molinate 0.004

Napropamide 0.003 Parathion 0.007

Parathion-methyl 0.006 Pebulate 0.004

Pendimethalin 0.004 Permethrin 0.005

Phorate 0.002 Pronamide 0.003

Prometon 0.02 Propachlor 0.007

Propanil 0.004 Propargite 0.01

Simazine 0.005 Tebuthiuron 0.01

Terbacil 0.007 Terbufos 0.01

Thiobencarb 0.002 Triallate 0.001

Trifluralin 0.002 alpha-HCH 0.002

p,p'-DDE 0.006

Pesticide metabolites, USGS custom method Liquid Chromatography Acetamide Analysis,

in micrograms per liter

Acetochlor ESA 0.05 Acetochlor oxanilic acid 0.05

Alachlor ESA 0.05 Alachlor oxanilic acid 0.05

Metolachlor ESA 0.05 Metolachlor oxanilic acid 0.05

Major ions, USGS SC2750,

in milligrams per liter (unless noted)

Bromide 0.01 Calcium 0.02

Chloride 0.29 Fluoride 0.1

Iron 10.0 μg/L Magnesium 0.014

Potassium 0.24 Silica 0.09

Sodium 0.09 Solids, residue 10.0

Sulfate 0.31

Nutrients, USGS SC2752,

in milligrams per liter

Nitrogen, ammonia 0.02 Nitrogen, ammonia plus organic 

nitrogen

0.1

Nitrogen, nitrite 0.01 Nitrogen, nitrite plus nitrate 0.05

Phosphorus 0.06 Orthophosphate 0.01

Radionuclides and other constituents

(units as noted)

Radon, USGS LC1369 26 pCi/L Tritium, USGS LC1565 1.0 pCi/L

Dissolved organic carbon, 

USGS SC2085

0.33 mg/L Nitrogen-15/Nitrogen-14, per mil na
12 Ground-Water Quality Beneath Irrigated Agriculture in the Central High Plains Aquifer, 1999–2000

use, precleaned disposable plastic bailer was lowered into the 

well and filled with water. The bailer was rinsed three times 

with ground water, and then ground water was poured directly 

from the bailer into a clean, stainless-steel, pressure-filtration 

funnel equipped with a 0.45-μm silver filter. The ground water 

was forced through the silver filter using purified nitrogen gas 

at a pressure not exceeding 15 lb/in2

. The DOC sample was collected in a cleaned and baked amber-glass bottle. All nutrient, 

nitrogen isotope, pesticide, and DOC samples were immediately placed on ice and chilled until delivered to the laboratory 

for analysis.

With the exception of certain pesticide metabolites, all 

water-quality samples were analyzed at the USGS National 

Water-Quality Laboratory (NWQL) in Lakewood, Colo. Six 

pesticide metabolites were analyzed at the USGS Organic 

Geochemistry Research Laboratory in Lawrence, Kans. 

References for the analytical methods used for each analysis are 

provided in table 4. The results for all water-quality analyses 

performed during this study are provided in table 9 in the 

“Supplemental Information” section at the back of this report.

Land-Use Classification

To document the land-use characteristics in the vicinity of 

sampled wells, land-use and land-cover information was collected using the procedures described in Koterba (1998). 

Detailed land-use/land-cover information was recorded in both 

written and map format for a 50-m buffer and a 500-m buffer 

around each well. Characterization of the 50-m buffer was 

intended to document activities in proximity to the well and 

identify potential contaminant point sources that might directly 

affect ground-water quality near the well. Characterization of 

the 500-m buffer was intended to describe the overall land use/

land cover most likely to be associated with the recharge area 

around the well. Though some wells might not receive significant recharge from within the 500-m buffer zone, characterization of the 500-m zone is assumed to represent the average landuse setting for a much larger area around the well (Koterba, 

1998).

Land use/land cover within the 50-m buffer zone was 

recorded in tabular form on the Land-Use Land-Cover Field 

Sheet (LULCFS) presented in Koterba (1998) and also on a 

scale drawing of the buffer zone. The LULCFS includes information regarding precise well location, land-use practices, local 

land-use features, soil characteristics, and well characteristics. 

The scale drawing of the 50-m buffer zone shows the location 

of the sampled well and any buildings, fields, equipment, chemical, fuel, and feed storage, or other potential contaminant 

sources near the well. Each of the wells installed for this study 

was purposely located next to an irrigated field and away from 

farm operations. No point sources for potential contamination 

were identified in the land-use coding of the 50-m buffer for any 

of the 30 wells sampled.

Land use/land cover within the 500-m buffer zone also was 

recorded in tabular form on the LULCFS and included local features, crop types, irrigation and drainage practices, and apparent 

fertilizer and pesticide practices. The location and types of field 

crops and other land cover in the 500-m buffer zone were drawn 

on mylar overlays using black-and-white aerial photographs as 

templates. The aerial photographs (nominal 1990) were 

obtained from the National Aerial Photography Program and 

had an average scale of about 1:20,000. The overlays provide a 

means to update and correct the images on the aerial photographs and allow an estimate of the percentage of identified 

land covers within the 500-m buffer zone. All land-use/landcover data are archived in the NAWQA National Data Warehouse maintained by the USGS.

Table 4. Laboratory analysis methods for measured water-quality constituents in the central High Plains agricultural land-use 

study, 1999–2000.

[USGS, U.S. Geological Survey; GC/MS, gas chromatography/mass spectrometry; LCAA, Liquid Chromatography Acetamide Analysis; HPLC, highperformance liquid chromatography; UV, ultraviolet]

Constituent or constituent group Analysis method Method reference

Major ions

 (USGS schedule 2750)

Atomic absorption spectrometry Fishman (1993)

Nutrients

(USGS schedule 2752)

Various methods Fishman (1993)

Nitrogen isotopes

(USGS lab code 1718)

Mass spectrometry of nitrogen gas Flipse and Bonner (1985)

Pesticides

(USGS schedule 2001)

Solid phase extraction using a C-18 

cartridge and GC/MS

Zaugg and others (1995)

Pesticide metabolites

(USGS analysis LCAA - Kansas District)

Solid phase extraction and HPLC Hostetler and Thurman (2000)

Dissolved organic carbon

(USGS schedule 2085)

UV-promoted persulfate oxidation and 

infrared spectrometry

Brenton and Arnett (1993)

Tritium

(USGS lab code 1565)

Electrolytic enrichment with gas 

counting

Ostlund and Dorsey (1975)
Study Design and Methods 13

Quality Control

Quality-control (QC) samples were collected to evaluate 

the effectiveness of equipment cleaning and variability of 

water-quality data as a result of sample collection, processing, 

and analysis. The QC samples included field-blank samples, 

replicate environmental samples, and field-matrix spike samples. The QC samples were collected at a frequency of about 

30 percent of the environmental samples collected from wells; 

4 field-blank samples, 4 replicate environmental samples, and 3 

field-matrix spike samples were collected in addition to the 

30 environmental samples. 

Field-blank samples were collected to document thorough 

decontamination of sampling equipment and that field 

procedures did not contaminate samples. Quality-assured 

organic-free or inorganic-free water is provided for blanksample processing by the USGS for the NAWQA Program. The 

field-blank water was passed through all sampling equipment, 

including filters, and collected using the same procedures as 

those for environmental samples. Field-blank samples were 

analyzed for concentrations of major ions, nutrients, pesticides, 

and DOC. Few water-quality constituents were detected in the 

field-blank samples (table 5), and no nutrient or pesticide constituents were detected in field-blank samples. The detection of 

only a few constituents in the field-blank samples indicates that 

decontamination procedures were adequate and that field procedures allowed minimal contamination of environmental samples.

Replicate samples were collected to assess the combined 

effects of field and laboratory procedures on measurement variability. Sequential replicate samples (samples collected consecutively as opposed to simultaneously) were collected in the 

same manner as the environmental samples for major ions, 

nutrients, and DOC analyses. Replication of pesticide sampling 

was accomplished during the field-matrix spike sampling process. The relative percent difference (RPD) between reported 

concentrations of replicate samples was calculated using the 

formula:

Perfect replication between paired analyses would yield an 

RPD of 0 percent. The RPD was not calculated if one or both 

analyses for a replicate pair were less than the analytical method 

reporting limits (MRL). Generally, about 90 percent of the 

RPD’s were less than 5 percent, which indicated an acceptable 

level of field and laboratory effect on measurement variability 

(table 6). Four constituents had relatively high average RPD’s 

(bromide, 128.6 percent; terbacil, 22.6 percent; deethylatrazine, 

21.8 percent; and DOC, 9.9), which is attributed to the relatively 

large differences in concentrations close to the analytical MRL 

for these constituents.

Field-matrix spike samples for pesticides were collected to 

determine bias and variability from matrix interference on analyte recovery and to evaluate the degradation of constituent concentrations during sample processing, shipping, and analysis. 

Field-matrix spikes involve injecting replicate environmental 

pesticide samples with known concentrations of multiple pesticides at the sampling site and submitting the samples to the laboratory for analysis. A spike recovery of 100 percent would 

indicate no matrix interference or degradation for that constituent. Replicate field-matrix spike samples also were collected for 

two of the three field-matrix spike samples, and the RPD 

between replicate field-matrix spike samples was calculated 

(table 6).

The percent recovery for each spiked pesticide compound 

can be found in table 8 in the “Supplemental Information” 

section at the back of this report. A summary of percent recoveries for spiked samples is provided at the bottom of table 6. 

Median percent recoveries for pesticide compounds were very 

close to 100 percent (table 6). The median percent recoveries 

for 25 of the 47 spiked pesticides were greater than 100 percent. 

The median percent recoveries for approximately 55 percent 

(26 of 47) of the pesticides was between 90 and 110 percent,

RPD Sample1 – Sample2

( ) Sample1 + Sample2

2 --------------------------------------------------------

= -------------------------------------------------------------- × 100

Table 5. Constituents detected in quality-control blank samples in the central High Plains agricultural land-use study, 

1999–2000.

[mg/L, milligrams per liter; <, less than]

Constituent Number of detections/

number of samples

Median concentration

in blank samples

(mg/L)

Maximum concentration

in blank samples

(mg/L)

Major ions

Bromide 1/4 <0.01 0.02

Calcium 3/4 0.05 0.06

Other constituents

Dissolved organic carbon 1/4 <0.33 0.49
14 Ground-Water Quality Beneath Irrigated Agriculture in the Central High Plains Aquifer, 1999–2000

including most of the commonly used pesticides for agricultural 

practices within the study area. Although the median percent 

recovery for atrazine was 132 percent, it is still within the range 

(60 to 140 percent recovery) of acceptable laboratory precision 

(Mark Sandstrom, U.S. Geological Survey National Water 

Quality Laboratory, written commun., 2001). The median percent recoveries for carbaryl, methylazinphos, and carbofuran 

were unusually high (greater than 140 percent); however, these 

pesticides were not detected in ground water during this study. 

An acceptable percent recovery for pesticides indicates sample 

processing and analytical methods provided a reasonable representation of pesticide concentrations in environmental samples.

Ground-Water Quality Beneath Irrigated 

Fields

The results of all water analyses performed during this 

study are provided in table 9 in the “Supplemental Information” 

section at the back of this report. The following discussion provides an overview of ground-water quality beneath irrigated 

agricultural fields in the central High Plains.

Major Dissolved Ions and Water Properties

A summary of the concentrations of the major dissolved 

ions analyzed in the 30 water samples and the range of the water 

properties measured onsite during sample collection are given 

in table 7. Though none of the wells sampled are used as a 

drinking-water source, applicable national drinking-water standards are provided as a frame of reference for the quality of the 

collected samples. Major dissolved ions account for the bulk of 

dissolved constituents in natural water samples. These dissolved constituents generally are derived from rock/water interactions; however, reuse of water can increase constituent concentrations through repeated exposure to weathered rock 

minerals or through evaporation that removes water but leaves 

the minerals behind. Evaporative concentration would tend to 

increase the concentration of all dissolved ions, whereas exposure to weathered rock would affect only selected constituents.

In general, the major-ion chemistry of the collected water 

samples indicates a resource of acceptable quality. Though 

some water samples had sufficient dissolved ions to be considered “hard” water, the drinking-water standards shown in table 

7 were, for the most part, not exceeded. One notable exception 

is the large concentrations of sulfate. The sulfate concentrations 

for several samples exceeded the National Secondary DrinkingWater standard (250 mg/L). The large sulfate concentrations 

(>250 mg/L) are primarily responsible for the large dissolved 

solids concentrations in the same samples. Also of note (table 7) 

are the data on dissolved-oxygen concentrations in ground 

water of this study area. Generally, the ground water is well 

oxygenated; however, the dissolved oxygen concentration of 

one sample can be considered suboxic (less than 1.0 mg/L). 

Low dissolved-oxygen conditions (<1 mg/L) can affect the concentration of other dissolved constituents by altering the oxidation/reduction reactions in the ground-water environment. This 

condition will be discussed in greater detail later in this report.

Nutrients and Pesticides

Each water sample was analyzed for 6 nutrient compounds 

(compounds of nitrogen and phosphorus), and 53 pesticides and 

pesticide metabolites (table 3). Nutrient compounds can be 

derived from natural processes such as mineral weathering or 

decay of organic material in the soil zone, or from anthropogenic influences like septic waste disposal or agricultural

Table 6. Summary of relative percent differences (RPD’s) for replicate samples, and percent recoveries for field-matrix spike 

analytes, in the central High Plains agricultural land-use study, 1999–2000.

Compound class

Percentile

Count Minimum Maximum

10 25 50 75 90

Environmental sample replication

(relative percent difference, in percent)

Dissolved organic carbon 2.4 5.7 9.9 12.3 12.5 4 0.1 12.6

Major ions 0.2 0.5 1.4 2.6 5.5 44 0.0 128.6

Nutrients 0.0 0.2 1.8 4.1 5.4 11 0.0 9.5

Field-matrix spike replication

(relative percent difference, in percent)

Pesticides 0.2 0.9 2.4 5.3 10.8 94 0.0 22.6

Field-matrix spike recoveries

(in percent)

Pesticides 71.4 90.8 100.9 109.5 131.0 235 47.8 194.4
Ground-Water Quality Beneath Irrigated Fields 15

practices. Large nutrient concentrations in ground-water samples generally are related to anthropogenic influences on water 

quality. The occurrence of pesticides in ground water is solely 

the result of the use, handling, or storage of these compounds at 

the land surface and subsequent migration to the ground-water 

system.

The inorganic compounds of phosphorus have relatively 

low solubilities in natural waters and, unless influenced by a 

substantial local source, usually occur in small concentrations 

(Hem, 1989). The concentrations of dissolved phosphorus and 

orthophosphate measured in the 30 water samples collected for 

this study were small; median concentrations were 0.008 mg/L 

and less than 0.01 mg/L, respectively. The maximum concentration of dissolved phosphorus measured in the 30 water samples was 0.047 mg/L.

The nitrogen compounds measured during this study were 

ammonium, ammonium plus organic nitrogen, nitrite, and 

nitrite plus nitrate. Only 4 of 30 samples had detectable ammonium (concentrations larger than the analytical method reporting limit of <0.02 mg/L) and the concentrations were small, 

from 0.025 to 0.534 mg/L. Because ammonium concentrations 

in the water samples were small, the concentrations reported for 

ammonium plus organic nitrogen were primarily due to the 

presence of organic nitrogen. Ammonium plus organic nitrogen 

concentrations exceeded the MRL (0.1 mg/L) in 16 of 30 samples, and these concentrations ranged from 0.1 to 0.66 mg/L.

Nitrite exceeded the laboratory MRL (0.01 mg/L) in only 

6 of 30 samples. The concentrations of nitrite detected were 

small, ranging from 0.011 to 1.09 mg/L, which indicates that the 

concentrations reported for nitrite plus nitrate were composed 

almost entirely of nitrate. Hereinafter, nitrite plus nitrate will be 

referred to as the “nitrate concentration.”

Nitrate concentrations exceeded the MRL (0.05 mg/L) in 

all 30 water samples ranging from 0.248 to 61.1 mg/L as nitrogen with a median concentration of 7.05 mg/L. The national 

drinking-water standard maximum contaminant level (MCL) 

for nitrate as nitrogen is 10 mg/L (U.S. Environmental Protection Agency, 2003). The MCL for nitrate was exceeded in 10 of 

the 30 (33 percent) water samples collected.

Of the 53 pesticides and pesticide metabolites analyzed in 

water samples from this study (see table 3), 8 different pesticides and the metabolites for 4 of these pesticides were detected 

in one or more samples (fig. 8). Atrazine (detected in 20 of 

30 wells) and its metabolite, deethylatrazine (21 of 30 wells), 

were by far the most frequently detected pesticide compounds 

occurring in about 70 percent of the water samples. In all, 

21 different wells had at least 1 or more detectable pesticide, 

and as many as 11 separate pesticide or metabolite compounds 

were present in an individual well (well 19; table 9). Atrazine

Table 7. Summary statistics for major dissolved ions and water properties for 30 ground-water samples collected as part of the 

central High Plains agricultural land-use study, 1999–2000.

[mg/L, milligrams per liter; SMCL, secondary maximum contaminant level; MCL, maximum contaminant level; μS/cm, microsiemens per centimenter; 

°C, degrees Celsius; NTU, nephelometric turbidity units; >, greater than]

Constituent

Major ion concentration

(mg/L)

Applicable

drinking-water

Minimum Median Maximum standards

Alkalinity 97 193 363

Bromide 0.07 0.23 1.43

Calcium 37.1 78.8 410

Chloride 6.30 30.9 223 250 (SMCL)

Fluoride 0.3 0.8 2.4 4.0 (MCL)

Magnesium 9.04 22.4 220

Potassium 1.99 5.54 14.9

Silica 10.8 29.7 64.5

Sodium 8.98 31.7 503

Sulfate 10.6 113 2,360 250 (SMCL)

Sum of dissolved solids 277 491 4,512 500 (SMCL)

Water properties

Field parameters Applicable

drinking-water

standards Minimum Median Maximum

Dissolved oxygen (in mg/L) 0.6 6.7 7.9

pH (in standard units) 6.8 7.4 7.9 6.5–8.5 (SMCL)

Specific conductance (in μS/cm at 25°C) 385 740 3,770

Temperature (in °C) 12.1 17.7 21.0

Turbidity (in NTU) 0.57 5.3 >1,000
16 Ground-Water Quality Beneath Irrigated Agriculture in the Central High Plains Aquifer, 1999–2000

and deethylatrazine were detected in all wells containing other 

pesticide compounds. For the pesticides whose metabolites also 

were detected, the metabolites were detected more frequently 

than the parent compounds.This finding indicates that sampling 

for these pesticide metabolites in ground water might be the best 

method to evaluate the water-quality effect of using the parent 

compound at the land surface.

Tritium

Tritium is an isotope of hydrogen that has two hydrologically important sources: the interaction of nitrogen with cosmic 

radiation in the upper atmosphere, and nuclear fusion primarily 

associated with the atmospheric detonation of hydrogen bombs 

in the 1950’s and 1960’s. Hydrologists have used tritium for 

several years to estimate the relative age of ground water; that 

is, to determine approximately when the water was recharged to 

the aquifer and isolated from the atmospheric source of tritium. 

Once the water is isolated from the tritium source, tritium concentrations gradually are reduced by radioactive decay with a 

half-life of 12.43 years. The concentrations of natural “prebomb” tritium in precipitation that recharged the aquifer before 

1952 have decreased to very low concentrations (less than 

1.5 picocuries per liter, pCi/L) in ground water (Thatcher, 

1962). The atmospheric tritium produced by hydrogen bombs, 

beginning about 1952, reached concentrations thousands of 

times greater than those produced naturally. This “bomb peak” 

was incorporated into rainfall and recharged to aquifers worldwide. Due to its large initial concentration, some of the post1952 tritium is still present after decades of isolation. Tritium 

analysis provides a relative age for ground water; samples generally are determined to be of pre-1952 or post-1952 age. The 

presence of more than 1.5 pCi/L tritium in ground water indicates that at least some part of the water sample was recharged 

after 1952.

The tritium concentrations determined for water samples 

collected during this study ranged from less than 1.0 to 

39.7 pCi/L with a median of 7.0 pCi/L (table 9). The tritium 

concentrations for 23 of 30 samples were greater than 

1.5 pCi/L and are considered to have some component of 

recently recharged water. Tritium concentrations for the 

remaining seven water samples were less than 1.5 pCi/L and are 

probably composed primarily of water from earlier (pre-1952) 

recharge events.

Dissolved Organic Carbon

Dissolved organic carbon (DOC) in water samples can be 

a result of natural sources or an indicator of anthropogenic 

effects on water quality (Hem, 1989). Natural sources can 

include decaying plant material from near the land surface or 

interaction of ground water with petroleum deposits in the subsurface. Possible anthropogenic sources can include improper 

disposal or spills of hydrocarbons at the land surface, oil-lubricated ground-water pumping systems, petroleum exploration 

and development, septic system discharges, and use of oilbased dispersants during application of agricultural chemicals. 

Substantial amounts of DOC in the ground-water system can 

provide a food source for microbial communities. Microbial 

metabolism of DOC can affect the oxidation/reduction (redox) 

conditions in the subsurface, which in turn can affect concentrations of dissolved oxygen and many other water-quality constituents.

The DOC concentrations determined for the samples 

collected during this study generally were small with concentrations ranging from 0.45 to 24.2 mg/L and a median concentration of 2.31 mg/L (table 9). DOC concentrations of only two 

samples were larger than 6.5 mg/L (well 2, 14.0 mg/L; and well 

19, 24.2 mg/L). These water samples were well oxygenated (7.4 

and 7.6 mg/L dissolved oxygen, respectively). One sample in 

this study (from well 16) did have a dissolved-oxygen concentration small enough to be considered suboxic (0.61 mg/L). The 

small nitrate (0.248 mg/L) and large manganese (744 μg/L)

Figure 8. Detection frequency for pesticides and pesticide 

metabolites in ground water beneath irrigated agricultural 

fields in the central High Plains, 1999–2000.

Metribuzin

EPTC

Alachlor

Acetochlor

Metolachlor

Chlorpyrifos

Simazine

Atrazine

Alachlor OA

Metolachlor OA

Acetochlor ESA

Alachlor ESA

Metolachlor ESA

Deethylatrazine

PESTICIDE METABOLITES DETECTED

PESTICIDE METABOLITES DETECTED

NUMBER OF DETECTIONS IN 30 WELLS SAMPLED

0 2 4 6 8 10 12 14 16 18 20 22

NUMBER OF DETECTIONS IN 30 WELLS SAMPLED

0 2 4 6 8 10 12 14 16 18 20 22

Pesticide Metabolites Detected in Ground Water in the

Central High Plains Irrigated Agricultural Setting

Pesticide Compounds Detected in Ground Water in the

Central High Plains Irrigated Agricultural Setting
Relation Between Land Use and Water Quality 17

concentrations in this water sample suggest a low redox condition with probable denitrification and manganese reduction. 

The DOC concentration in this water sample was not especially 

large (2.54 mg/L) indicating that large DOC concentrations are 

not correlated with reducing conditions in this study. Evidence 

of an organic-rich shale contacted at the bottom of the borehole 

during well installation is considered the primary reason for the 

suboxic conditions at this sampling site.

Relation Between Land Use and Water 

Quality

Land-use characteristics within a study area can be one of 

the most important factors affecting ground-water quality. 

Agricultural chemicals and irrigation water applied at the land 

surface in the central High Plains provide the source and transport mechanism for constituents that might affect ground-water 

quality. All the wells sampled for this study were installed in 

proximity to irrigated fields where corn was the primary longterm crop. Because the land-use setting was very similar around 

each well, no attempt was made to correlate slight differences 

in land use near individual wells with minor differences in water 

quality between samples collected from these wells. Alternatively, this report identifies those sampling sites where water 

quality was affected (and unaffected) by agricultural activities 

and evaluates the differing land-use and hydrologic factors 

associated with these locations. Correlating land-use and hydrologic factors with water-quality effects can help identify other 

areas of the central High Plains that might be vulnerable to these 

effects.

Nutrients, Pesticides, and Tritium

The presence of agricultural chemicals in ground water 

collected beneath irrigated fields provides direct evidence of the 

effect of irrigated agriculture on ground-water quality. Pesticides and nutrients, especially nitrogen, commonly are used on 

agricultural fields to increase crop yields. Selected pesticides 

are widely used with certain crop types, and their presence 

would be expected in ground water if the local land-use setting 

was affecting the water quality. For example, though atrazine 

has numerous agricultural and nonagricultural applications, the 

primary use of atrazine in many parts of the country is seasonlong control of weeds in corn, sorghum, and other grain crops— 

the predominant agricultural products of the central High Plains 

(fig. 3). At the time of this study, atrazine was the most heavily 

used herbicide in the central High Plains (in pounds applied to 

fields) and ranks second in number of acres to which it is 

applied (table 1). The presence of atrazine in ground-water samples is direct evidence of water-quality effects from the overlying land-use setting.

Nitrate can be produced by the natural decomposition of 

organic material in the soil and, under oxygenated conditions, is 

likely to be present in most natural waters (Mueller and Helsel, 

1996). Large concentrations of nitrate, however, are generally 

considered indicative of anthropogenic effects. To estimate 

what nitrate concentration would be indicative of anthropogenic 

effects, the USGS NAWQA Program has analyzed nitrate data 

from ground-water samples collected throughout the Nation 

from areas having minimal effects from agriculture, cities, or 

other human activities (Mueller and Helsel, 1996). This analysis indicates that the national average background-nitrate concentration for water unaffected by human sources generally was 

less than 2.0 mg/L. McMahon (2000) and Bruce and Oelsner 

(2001), however, suggest that the natural background concentration of nitrate in the central High Plains aquifer is slightly 

larger than the published USGS national average. Information 

also is available on the chemistry of paleorecharge conditions in 

the central High Plains (Peter McMahon and others, U.S. Geological Survey, unpub. data, 2002) that shows nitrate concentrations in paleorecharge water (water with no tritium present) 

range from 0.43 to 3.5 mg/L with a median concentration of 

2.5 mg/L. Similarly, Becker and others (2002) report nitrate 

concentrations for 74 domestic well samples in the central High 

Plains aquifer. The nitrate concentrations for 32 wells in Becker 

and others (2002) with no detectable tritium range from 1.07 to 

3.98 mg/L with a median nitrate concentration of 2.04 mg/L.

Nitrate, several pesticides, and selected pesticide metabolites were detected in the water samples collected for this study. 

Figure 9 shows the concentrations of nitrate and tritium in the 

30 water samples, arranged in order of increasing nitrate concentration. Nitrate concentrations for 19 of the 30 water samples 

were larger than the 3.5-mg/L maximum concentration calculated for paleorecharge. Figure 9 also indicates water samples 

with detectable concentrations of pesticides or pesticide metabolites; these samples are represented by red triangles in this 

figure. Of the 30 samples collected, every sample with a nitrate 

concentration larger than the 3.45 mg/L (20 samples) contained 

detectable quantities of pesticides. The only other sample with 

detectable pesticides (well 16) had the smallest nitrate concentration measured during the study (0.248 mg/L). This sample 

had a dissolved-oxygen concentration of 0.61 mg/L, indicating 

that suboxic conditions exist at well number 16. The low nitrate 

concentration observed at well 16 is likely the result of denitrification processes.

The relation between pesticide detection, large (greater 

than 2.5 mg/L) nitrate concentrations, and tritium concentrations indicative of recently recharge ground water is strong 

(fig. 9). Recently recharged (post-1952) ground water would be 

expected in samples showing water-quality effects of the overlying agricultural setting, and this is indeed the case. Even the 

sample that has apparently undergone denitrification contains 

sufficient tritium (4.8 pCi/L) to indicate a component of recent 

recharge. A sample that has undergone denitrification would 

have the nitrate concentration reduced by microbial processes, 

but tritium and pesticide compounds generally would remain 

unaffected. This sample does contain pesticides and tritium, 

verifying the occurrence of recently recharged water and agricultural effects. For the remainder of this report, the 21 water
18 Ground-Water Quality Beneath Irrigated Agriculture in the Central High Plains Aquifer, 1999–2000

samples (70 percent of those sampled) with detectable pesticides, large nitrate concentrations (with the exception of the 

sample affected by denitrification), and tritium concentrations 

indicating recent recharge, are considered to be affected by the 

overlying irrigated agricultural setting.

Of the 30 samples collected during this study, 9 did not 

contain detectable pesticides and had relatively small concentrations of nitrate. These samples are indicated by the triangular 

blue symbols in figure 9. Generally, these nine samples also had 

small concentrations of tritium, indicating a lessor component 

of recent ground-water recharge. Two of these water samples 

(from wells 2 and 4) had tritium concentrations above the 

1.5-pCi/L cutoff between pre-1952 and post-1952 water ages. 

These tritium concentrations, 2.2 pCi/L in well 2 and 3.2 pCi/L 

in well 4, were large enough to consider these samples as having 

some component of recent recharge, but no pesticides were 

detected in these water samples. The nitrate concentrations for 

these samples were 3.08 mg/L in well 2 and 2.85 mg/L in well 

4, which are less than the 3.5 mg/L maximum paleorecharge 

concentration from McMahon and others (unpub. data, 2002). 

Although two of these nine water samples have slightly elevated tritium concentrations, they do not display the other possible effects of the overlying irrigated agriculture. For the purposes of interpretation, these nine samples are considered to be 

unaffected by agricultural activities.

Nitrogen Isotopes

The 15N/14N nitrogen isotope ratio in dissolved nitrate can 

be indicative of the source of the nitrate in ground water 

(Edwards, 1973; Kreitler, 1975). Although the exact boundary 

between 15N/14N isotope ratios for different nitrogen sources 

can vary from one study area to another, generally speaking, 

15N/14N isotope ratios between –2 and +4.0 per mil (per thousand, ‰) are related to commercial fertilizers (for example, 

anhydrous ammonia) produced from atmospheric nitrogen 

sources; nitrogen isotope ratios between +4.0‰ and +9.0‰ are 

the result of the decay of organic material in the soil; and nitrogen isotope ratios greater than +9.0‰ are indicative of animal 

waste (Edwards, 1973). Animal waste sources include human 

and wildlife sources as well as livestock manure.

Identifying nitrate sources using the 15N/14N ratio does 

have some limitations. The boundary between 15N/14N ratios 

for commercial fertilizer and soil nitrogen is the least precise, 

and the ranges probably overlap (Kreitler, 1975). Isotope ratios 

as large as +6.5‰ have been observed under fields that received 

only commercial fertilizers (Flipse and Bonner, 1985). Isotope 

ratios in uncultivated soils with only natural organic nitrogen 

have been measured as low as +2.0‰. The larger ratios 

observed in areas of commercial fertilizer applications might be 

partly due to isotopic fractionation during volatile loss of 

Figure 9. Relation between nitrate and tritium concentrations and the detection of pesticides beneath 

irrigated agricultural fields in the central High Plains, 1999–2000.

0

10

20

30

40

50

60

70

SAMPLES ARRANGED IN ORDER OF INCREASING NITRATE CONCENTRATION

NITRATE CONCENTRATION, IN MILLIGRAMS PER LITER,

AND TRITIUM, IN PICOCURIES PER LITER

Median nitrate concentration in paleorecharge:

2.5 milligrams per liter, (P.B. McMahon,

U.S. Geological Survey, written commun., 2002)

Nitrate concentration for samples not

containing pesticides

 Tritium concentration

Nitrate concentration for samples

containing pesticides
Relation Between Land Use and Water Quality 19

applied ammonia. Another possibility is a mixing of nitrogen in 

commercial fertilizer with possibly large amounts of soil nitrogen in newly tilled fields. The large release of nitrogen due to 

the breakup of natural soils has not received much attention but 

is an important source of nitrogen to shallow ground water 

(Kreitler, 1975). Additionally, some natural processes, such as 

microbially mediated denitrification in anaerobic water, can 

alter the initial isotope composition of nitrate. Denitrification 

reduces the nitrate concentration and increases the nitrogen isotope ratio as microbial processes preferentially remove the 

lighter nitrogen molecules.

Nitrogen isotopes were useful for identifying nitrate 

sources in this study. Figure 10 is a scatterplot of nitrate concentration relative to nitrogen isotope ratio for the 30 samples collected during this study. The colored areas of the graph depict 

the three generalized ranges of isotope values that identify commercial fertilizer, soil nitrogen, and animal waste. The 30 symbols plotted on figure 10 representing individual samples are 

color coded to indicated those samples identified as affected 

(red) and unaffected (blue) by agricultural surface activities. 

The unaffected samples generally had the smallest nitrate concentrations (median nitrate concentration of 2.85 mg/L), and 

their nitrogen isotope ratios fall in the range indicative of soil 

nitrogen. These results support the earlier conclusion that these 

nine samples are not affected by the overlying irrigated agriculture. This result does not mean that these samples will never 

show the effects of the overlying land-use practices. It simply 

means that, at the time of this sampling, there was no evidence 

of agricultural effects.

Other data shown in figure 10 support the conclusion that 

irrigated agriculture has affected water quality, and the figure 

indicates possible sources for the observed nitrate concentrations. Eight of the samples in figure 10 plot within the nitrogen 

isotope range indicative of commercial fertilizer (less than 

+4.0‰). These samples also contain at least one pesticide, have 

nitrate concentrations larger than the suggested maximum value 

for paleorecharge, and have relatively large tritium concentrations (table 9). Nitrate concentrations in these water samples 

apparently are the result of commercial nitrogen fertilizer leaching to the water table. The depth to water in one of these wells 

was 181 feet below land surface, indicating that agricultural 

chemicals may have migrated through at least that thickness of 

overlying sediment to reach the ground water. The five samples 

that have relatively large nitrate concentrations and pesticide 

detections, but have nitrogen isotope ratios between +4.0 and 

+5.0, also are considered to be affected by agricultural activities, with the primary source of the nitrate being commercial 

fertilizer. There may be some component of soil (or possibly 

animal waste) nitrogen in these five samples or a slight enrichment in the heavier isotope due to volatilization or microbial 

activity. Their isotopic ratios, however, are only slightly larger 

than the generally accepted range for commercial fertilizer and 

well within the maximum upper bound for commercial fertilizer 

discussed previously. In all, 13 of the 21 affected water samples

Figure 10. Relation between nitrate concentration and nitrogen isotope ratio in water samples from beneath irrigated agricultural fields in the central High Plains, 1999–2000.

0

10

20

30

40

50

60

70

NITRATE AS NITROGEN, IN MILLIGRAMS PER LITER

–2 0 2 4 6 8 12 10 14 16

15N / 14N ISOTOPE RATIO (per mil)

Possible mixture

of nitrogen

sources

Low dissolved

oxygen

(denitrification)

Isotope range

for soil nitrogen

source

Ground-water

 sample affected

 by agricultural

 activities

Ground-water

 sample unaffected

 by agricultural

 activities Isotope range

for commercial

fertilizer source

Isotope range

for animal waste

source

EXPLANATION
20 Ground-Water Quality Beneath Irrigated Agriculture in the Central High Plains Aquifer, 1999–2000

are considered to have commercial fertilizer as the primary 

source of anthropogenic nitrate.

Nitrate concentrations in four samples were greater than 

6.5 mg/L (fig. 10) and nitrogen isotope ratios were greater than 

+9, indicating an animal waste source. All four of these samples 

also contained pesticide compounds and relatively large tritium 

concentrations. These results suggest that these wells are 

affected by irrigated agriculture where manure has been applied 

as the primary nitrogen fertilizer. The large nitrate concentration observed in well 12 (61.1 mg/L), however, could be related 

to domestic waste disposal at this site. Well 12 is located at the 

boundary between an irrigated corn field and a residential farmyard, and the leach field for domestic waste disposal was 

approximately 245 ft from the well. The water sample from well 

12 did contain agricultural pesticide compounds, indicating at 

least some effect from agricultural activities, but the very large 

nitrate concentration in the isotope range of animal waste could 

indicate local contamination from the domestic leach field. The 

other relatively large nitrate concentration shown in figure 10 

(54.4 mg/L in well 9) is probably related to manure applications. Well 9 is adjacent to a corn field that has, for many years, 

been fertilized with manure from a nearby feedlot. The nitrogen-isotope data indicate that this manure application is the 

probable source of the large nitrate concentration in ground 

water.

More difficult to interpret are the samples plotted in figure 

10 that contain pesticides and elevated nitrate concentrations 

(therefore, considered affected by agricultural activities) but 

have nitrogen isotope ratios indicative of a soil nitrogen source. 

One sample is identified as affected by denitrification (well 16). 

The sample from well 16 had the smallest nitrate concentration 

measured during this study but also contained recently 

recharged water and pesticide compounds (table 9). This water 

sample was collected at a site where the ground water was suboxic (dissolved oxygen was 0.61 mg/L). Denitrification is likely 

to occur under these conditions and would reduce the nitrate 

concentration and cause the nitrogen isotope ratio to increase 

but would not affect the tritium or pesticide concentrations. The 

nitrate in the water sample from well 16 may have originally 

come from commercial fertilizer applications, but the denitrification process shifted the nitrogen isotope ratio into the range 

indicative of soil nitrogen.

Three other samples in figure 10 contain recently 

recharged water, have relatively large nitrate concentrations, 

and detectable pesticides, but also plot in a cluster in the isotopic range indicative of a soil nitrogen source. It is not clear 

why these samples show evidence of agricultural effects yet 

have nitrogen isotope ratios indicative of natural soil nitrogen. 

It is possible that a mixture of waters with both large and small 

isotope ratios could result in the observed ratios, but a mixing 

scenario cannot be verified with the data collected during this 

study. These three affected wells (numbers 13, 28, and 30) are 

not geographically near one another (fig. 7); therefore, this 

water-quality result cannot be attributed to hydrologic conditions or land-use practices that are unique to a specific part of 

the study area. Although the source of the relatively large nitrate 

concentrations is not clear, these samples have been affected by 

the overlying agricultural land use, as indicated by the detection 

of pesticide compounds.

Dissolved Solids and Major Ions

The dissolved-solids content (similar to salt content) of 

ground water used for irrigation is of concern to farmers 

because it can affect crop health and yields. Large dissolvedsolids concentrations (greater than 500 mg/L) also constrain the 

usefulness of water for drinking and other purposes. It is, therefore, also of interest to this study whether agricultural practices 

might be increasing dissolved-solids concentrations in the 

ground water. 

The dissolved-solids concentration in a water sample is 

composed of the sum of the major dissolved ion concentrations 

in solution. The major ions in water generally are the constituents calcium (Ca), magnesium (Mg), sodium (Na), potassium 

(K), chloride (Cl), sulfate (SO4), carbonate alkalinity (as 

CaCO3), and to a minor extent fluoride (F). Bruce and Oelsner 

(2001) and Becker and others (2002) provide a broad-scale 

assessment of water-quality conditions from randomly distributed public-supply and domestic-supply wells across the central 

High Plains. These studies reported little difference in majorion chemistry across the study area but provide a baseline 

assessment of major-ion chemistry against which wells affected 

by agricultural activities in this study can be compared.

In the agricultural land-use study reported here, the water 

quality at 21 of 30 wells has been identified as affected by agricultural land use as evidenced by relatively large concentrations 

of nitrate and tritium, the detection of pesticides, and nitrogen 

isotope ratios indicating agricultural sources for much of the 

nitrate. The concentrations of dissolved solids in these agriculturally affected samples were significantly larger than those 

from the unaffected samples (fig. 11). The range of dissolvedsolids concentrations for the affected water samples was from 

300 to 4,510 mg/L with a median concentration of 675 mg/L 

(table 9). The range of dissolved-solids concentrations for the 

unaffected water samples was from 277 to 542 mg/L with a 

median concentration of 374 mg/L (table 9). Statistical significance of these data was determined by the Wilcoxon rank-sum 

test p-value (Helsel and Hirsch, 1995). The Wilcoxon rank-sum 

test p-value is used to evaluate whether one group of data tends 

to have larger values than a second group of data. The calculated p-value of 0.005 shown in figure 11 indicates that, at a 

99.5-percent confidence level, the dissolved-solids concentrations in the agriculturally affected water samples were significantly larger than the dissolved-solids concentrations in the 

unaffected water samples.

Trilinear diagrams allow the simultaneous graphical comparison of the relative percentages of major-ion concentrations 

(in terms of total milliequivalents per liter) for numerous wholewater samples and provide a means to determine which of the 

major ions in this data set were most affected by agricultural 

activities. Figure 12 shows the major-ion compositions for the 

30 agricultural water samples collected for this study and for
Relation Between Land Use and Water Quality 21

randomly distributed samples from the 15 domestic-supply 

wells and 15 public-supply wells sampled in the central High 

Plains as reported by Bruce and Oelsner (2001) and Becker and 

others (2002). Most of the samples collected from the domesticsupply, public-supply, and unaffected agricultural wells 

(fig. 12, in blue) cluster in the same part of the trilinear diagram 

indicating similar major-ion chemistry dominated by calciumbicarbonate type water. Water samples affected by agriculture 

(fig. 12, red circles) range from calcium-carbonate to calciumsulfate type water and exhibit a shift in the dominant anion from 

bicarbonate to sulfate.

Comparing sulfate concentrations in more detail, sulfate 

concentrations in water samples affected by agricultural activities were larger than those that were unaffected by agriculture 

(fig. 13A). The range of sulfate concentrations for the affected 

water samples was from 24.5 to 2,360 mg/L with a median concentration of 152 mg/L (table 9). The range of sulfate concentrations for the unaffected water samples was from 10.6 to 

161 mg/L with a median concentration of 78 mg/L (table 9). 

The Wilcoxon rank-sum test p-value shown in figure 13A 

(p = 0.07) indicates that the sulfate concentrations in the 

affected water samples were significantly larger (at the 93-percent confidence level) than those from the unaffected samples. 

Sulfate concentrations in this study also are highly correlated 

with dissolved-solids concentrations (p < 0.0001, fig. 13B). 

Sulfate concentrations in the ground water of this study area 

were apparently affected by irrigated agricultural land-use practices and play an important part in the increase in dissolved solids in the ground-water resource. The increase in sulfate concentrations beneath irrigated fields could be caused by sulfate 

leaching from selected sedimentary deposits during infiltration 

of irrigation water or the application of sulfur-containing agricultural chemicals that migrate to the ground-water system.

Land-Use and Hydrogeologic Factors Affecting 

Ground-Water Quality

A number of land-use and hydrogeologic factors have the 

potential to control the relative effect of agricultural activities 

on ground-water quality. All wells in this study, however, were 

located in very similar land-use settings (adjacent to irrigated 

corn fields), and the detailed information was not available to 

determine how minor differences in land-use practices near 

individual wells related to minor differences in ground-water 

quality between these locations. Instead, the relative magnitude 

of five selected land-use and hydrogeologic factors were correlated with water-quality samples determined to be either 

affected (21 samples) or unaffected (9 samples) by irrigated 

agriculture. The five factors evaluated were (1) the percentage 

of irrigated land within the 500-m circular buffer zone coded 

around each well, (2) the number of years the land near each 

well had been irrigated, (3) application rate of atrazine in the 

county containing each well, (4) application rate of nitrogen in 

the county containing each well, and (5) depth to ground water 

at time of sample collection.

The percentage of irrigated agriculture in the areas around 

sampled wells may correlate with land-use effects on groundwater quality; that is, the more irrigated land near a well, the 

more likely agricultural chemicals will be present in the ground 

water. The estimated amount of irrigated land within the 500-m 

circular buffer around the sampled wells (as diagramed in the 

field during water sampling, nominal 2000) was relatively similar for both the unaffected and affected subsets of wells. The 

minimum, median, and maximum percentages of irrigated land 

within 500 meters of the nine unaffected wells were 17 percent, 

45 percent, and 95 percent, respectively. The minimum,

Figure 11. Distribution of dissolved-solids concentrations for sites affected 

and unaffected by agricultural land use in the central High Plains, 1999–2000.

UNAFFECTED SITES

(9)

1,400

1,200

1,000

800

600

400

200

DISSOLVED SOLIDS, IN MILLIGRAMS PER LITER

0

Wilcoxon p-value = 0.005

AFFECTED SITES

(21) 

EXPLANATION

90

75

50

25

10

Percentiles

(21) Number of samples
22 Ground-Water Quality Beneath Irrigated Agriculture in the Central High Plains Aquifer, 1999–2000

C A T I O N S A N I O N S percent milliequivalents

Na+K HCO3 +CO3 Cl

Mg SO4

Ca

Calcium (Ca) Chloride (Cl)

Sulfate(SO4)+Chloride(Cl)

Calcium(Ca)+Magnesium(Mg)

Carbonate(CO3)+Bicarbonate(HCO3)

Sodium(Na)+Potassium(K)

Sulfate(SO4)

Magnesium(Mg)

80 60 40 20 20 40 60 80

80

60

40

20

20

40

60

80

20

40

60

80

80

60

40

20

20

40

60

80

20

40

60

80

80

60

40

20

80

60

40

20

Irrigated sites affected by agricultural land use

EXPLANATION

Domestic-supply wells (Becker and others, 2002)

Public-supply wells (Bruce and Oelsner, 2001)

Irrigated sites unaffected by agricultural land use

Figure 12. Relative major ion composition of ground-water samples collected beneath irrigated agricultural fields and 

nonagricultural areas in the central High Plains, 1999–2000.
Relation Between Land Use and Water Quality 23

median, and maximum percentage of irrigated land within 

500 meters of the 21 affected wells was 20 percent, 70 percent, 

and 90 percent, respectively. Because of the similarity in the 

amount of irrigated land between the unaffected and affected 

wells, irrigation density did not strongly correlate with whether 

ground-water quality was affected by agricultural activities. 

The large p-value for this comparison (0.4972) indicates a low 

level of confidence that differences in irrigation density exist 

between the affected and unaffected wells.

Figure 14 shows statistical boxplots, oriented horizontally, 

that depict the percentile values determined for the four remaining land-use and hydrogeologic factors previously indicated. 

For each factor there are two boxplots: one representing the 

values for the subset of unaffected wells (blue), and one representing the values for the subset of affected wells (red). The vertical line near the center of each boxplot represents the median 

for that subset of data. The left and right edges of the box itself 

represent the 25th and 75th percentiles, respectively, and the 

left and right ends of the whiskers on each box represent the 5th 

and 95th percentiles, respectively. Associated with each category is a statistical p-value calculated using the Mann-Whitney 

rank-sum test (Helsel and Hirsch, 1995, p. 118). The MannWhitney test assesses the likelihood that the two sets of data 

come from the same population. Because the Mann-Whitney 

test is nonparametric, it is resistant to the effects of extreme 

outliers in either of the groups compared. The p-value represents the level of confidence at which the two groups being 

tested are thought to come from different populations. For 

example, a p-value of 0.05 indicates there is a 95-percent confidence level that the groups being compared are different; a 

p-value of 0.10 indicates a 90-percent confidence level that the 

groups are different, and so on.

The first pair of boxplots in figure 14 compares the approximate number of years that the fields adjacent to the sampled 

wells had been irrigated. Due to changes in ownership of these 

fields and incomplete records, the year in which irrigation water 

was first applied to these fields was not available for all sampling sites. The approximate year that water was first applied to 

each field was provided by the current landowners for all 

9 unaffected wells and for 17 of 21 affected wells. The data 

were not weighted depending on whether the fields historically 

were flood/furrow irrigated, sprinkler irrigated, or a combination; however, some of the fields in this study were never flood 

irrigated.

A correlation is observed between the number of years a 

site was irrigated and whether the ground water was affected by 

irrigated agricultural activities. The p-value indicates an 84-percent confidence level that the agriculturally affected wells had 

a longer history of irrigation than the unaffected sites. An 

84-percent confidence level is generally not considered statistiFigure 13. Distribution of sulfate concentrations (A) for sites affected and 

unaffected by irrigated agriculture in the central High Plains, and (B) relative to 

dissolved solids concentrations for 30 samples collected beneath irrigated fields 

in the central High Plains, 1999–2000.

500

400

300

200

100

0

EXPLANATION

Percentiles

AFFECTED SITES

SULFATE, IN MILLIGRAMS PER LITER

(21)

DISSOLVED SOLIDS, IN MILLLIGRAMS 

PER LITER

Wilcoxon p-value = 0.07

SULFATE, IN MILLIGRAMS PER LITER

R2 = 0.75

p = less than 0.0001

2,500

2,250

500

250

0

0 250 500 750 1,000 1,250 4,000 5,000

B

A 90

75

50

25

10

(21) 

UNAFFECTED SITES

(9)

Number of samples
24 Ground-Water Quality Beneath Irrigated Agriculture in the Central High Plains Aquifer, 1999–2000

cally definitive but may indicate that ground water beneath 

areas of long-term irrigation is vulnerable to water-quality 

effects from this agricultural land-use practice.

Data generated by this study indicate that there is better 

than a 95-percent confidence level (p less than 0.05) that counties that apply large amounts of nitrogen and(or) atrazine to 

crops will have relatively large concentrations of nitrate and(or) 

atrazine or deethylatrazine present in ground-water samples 

(second and third box-plot pairs in fig. 14). Although the application rates only were available at the county-level scale, the 

correlations between these regional application rates and effects 

on ground-water quality were strong.

The final pair of boxplots in figure 14 indicate the importance of depth-to-water on the effect of agricultural activities on 

ground-water quality. Smaller depth to water in this study was 

strongly correlated (about a 97-percent confidence level) with 

water samples determined to be affected by agricultural activities. However, as previously mentioned, the water level in one 

of the affected wells exceeded 180 ft below land surface.

Figure 14. Comparison between sites affected and unaffected by agricultural land use and for selected factors that might contribute to aquifer vulnerability in the central High Plains

EXPLANATION

Percentiles

Affected sites

Unaffected sites 90 75 50 25 10

APPROXIMATE NUMBER OF YEARS UNDER IRRIGATION

ATRAZINE APPLIED, BY COUNTY, IN THOUSANDS OF POUNDS

DEPTH TO GROUND WATER, IN FEET BELOW LAND SURFACE

NITROGEN APPLIED, BY COUNTY, IN THOUSANDS OF POUNDS

0 10 20 30 40 50

0 20 40 60 80 100 120 140

50 100 150 200

p = 0.1611

p = 0.0087

p = 0.0465

p = 0.0316

0 20,000 40,000 60,000 80,000
Summary and Conclusions 25

Summary and Conclusions

In 1999 and 2000, 30 water-table monitoring wells were 

installed near irrigated agricultural fields in the central High 

Plains of Colorado, Kansas, New Mexico, Oklahoma, and 

Texas to evaluate the effect of irrigated agriculture on recently 

recharged ground-water quality. The wells were located adjacent to irrigated fields in areas where the depth to ground water 

was 200 ft or less. Each well was constructed with a short (usually 10 ft long) well screen placed just below the water table. 

Each well was sampled once during 1999–2000 and the samples 

were analyzed for more than 100 water-quality constituents.

Water-quality data from the sampled wells indicate that 

70 percent of the ground-water samples (21 of 30) were affected 

by agricultural land use. Nitrate concentrations in water samples exceeded the estimated maximum paleorecharge concentration of 3.5 mg/L as nitrogen in samples from 19 of 30 wells. 

The pesticide atrazine and its metabolite, deethylatrazine, were 

detected in samples from 20 and 21 of 30 wells, respectively. 

Atrazine and deethylatrazine were detected in all water samples 

that contained other pesticide compounds. Every water sample 

that had detectable deethylatrazine also had a nitrate concentration larger than 3.45 mg/L as nitrogen with the exception of one 

sample from a low-dissolved-oxygen environment that indicated the probable effects of microbial denitrification. This 

denitrified sample had the smallest nitrate concentration measured in any of the 30 water samples collected. Tritium data, 

which provide the relative age of each ground-water sample, 

indicate that every sample containing pesticides had at least 

some component of recently recharged (post-1952) water, 

including the denitrified sample. The 21 water samples with 

detectable pesticides, large nitrate concentrations (with the 

exception of the sample affected by denitrification), and tritium 

concentrations indicating recent recharge are considered to be 

affected by the overlying irrigated agricultural setting.

Two water samples contained enough tritium to have some 

component of recent recharge (2.2 and 3.2 pCi/L) but did not 

contain any detectable pesticides. The nitrate concentrations in 

these samples were 3.08 and 2.85 mg/L, respectively. Although 

the nitrate concentrations in these water samples were slightly 

larger than the 2.5-mg/L median paleorecharge concentration, 

the lack of detectable pesticides and relatively small tritium 

concentrations suggest that these samples were unaffected by 

agricultural land use. The remaining seven samples contained 

low concentrations of nitrate and tritium and no detectable pesticides. A total of nine water samples collected for this study 

were considered unaffected by agricultural land-use activities.

Nitrogen isotope ratios indicated the possible source of 

nitrate detected in ground-water samples. The nine water samples that were unaffected by agricultural land use had nitrogen 

isotope ratios typical of a natural organic soil-nitrogen source. 

The 21 water samples that were affected by agricultural activities had a range of isotopic ratios: 13 samples had ratios indicative of a commercial fertilizer source of nitrate (probably anhydrous ammonia), 4 samples had isotopic ratios indicative of an 

animal waste source (probably manure as fertilizer), and the 4 

remaining samples, though they contained nitrate, tritium, and 

pesticides, had isotopic ratios in the soil-nitrogen range. One of 

these four affected samples with isotope ratios in the soil-nitrogen range showed the effects of denitrification, which would 

increase the isotopic ratio and shift it into the soil-nitrogen 

range. The other three affected samples with isotope ratios in 

the soil-nitrogen range might represent a mixture of two or more 

sources of nitrogen.

A number of land-use and hydrogeologic factors might 

control the relative effect of agricultural activities on groundwater quality. The relative magnitude of five selected factors 

were correlated with water-quality samples determined to be 

either affected (21 samples) or unaffected (9 samples) by irrigated agricultural. Data for the percentage of irrigated land 

within 500 meters of each well, the number of years the land 

near each well was irrigated, the amount of atrazine and nitrogen applied in the county where the well was located, and the 

depth to ground water were compared between the two subsets 

of sites. The difference in percentage of irrigated land between 

affected and unaffected sites was not found to be statistically 

significant. Although irrigated agriculture probably has affected 

the water quality at 70 percent of the sites sampled, the percentage of irrigated land associated with a well did not predict 

agricultural effects on ground-water quality. Because of uncertainties in delineating local ground-water flow directions, irrigation density was summarized for a circular buffer around 

each well rather than determining irrigation density for upgradient areas directly contributing recharge to the sampled wells. 

Correlations between the density of irrigated agriculture and the 

concentration of water-quality constituents may have been 

weakened by not limiting irrigation-density analysis to upgradient areas.

The data indicate some relation between the number of 

years irrigation had been taking place at a sampling site and 

whether ground-water quality was affected by the land-use 

activities. It appears (at an 84-percent confidence level) that 

sites with longer irrigation histories were more likely to show 

water-quality effects. Additionally, counties with larger application rates of atrazine and nitrogen, and well sites with smaller 

depths to ground water, correlated strongly (greater than a 

95-percent confidence level) with water samples affected by 

agricultural activities.

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Supplemental Information
30 Hydrology and Water-Quality Characteristics of Muddy Creek and Wolford Mountain Reservoir near Kremmling, Colorado, 

1990 Through 2002
Supplemental Data 31

Table 8. Median percent recoveries for individual compounds in pesticide field-spike mixtures in the central High Plains 

agricultural land-use study, 1999–2000

Compound Percent recovery Compound Percent recovery

Carbaryl 168 Fonofos 100

Methylazinphos 149 Methyl parathion 99

Carbofuran 146 Pebulate 99

Atrazine 132 Butylate 98

Terbacil 129 Alpha-BHC 96

Tebuthiuron 125 Ethoprop 96

Propanil 116 Molinate 94

Linuron 113 Triallate 93

2,6-diethylaniline 112 Chlorpyrifos 91

Cyanazine 109 Parathion 91

Propachlor 108 Dieldrin 88

DCPA 106 Ethalfluralin 84

Metolachlor 106 Terbufos 84

Prometon 106 Trifluralin 79

Simazine 103 Pendimethalin 78

Thiobencarb 103 Benfluralin 76

Acetochlor 103 Phorate 76

Pronamide 103 Propargite 76

Lindane 103 Deethylatrazine 70

Napropamide 103 p,p’-DDE 65

EPTC 102 Disulfoton 63

Alachlor 101 Permethrin 61

Malathion 101

Diazinon 101

Metribuzin 101
32 Ground-Water Quality Beneath Irrigated Agriculture in the Central High Plains Aquifer, 1999–2000 Table 9. Results of physical and chemical analyses of water samples from wells completed as part of the central High Plains agricultural land-use study, 1999–2000

[μS/cm, microsiemens per centimeter at 25°C; Deg. C, degrees Celsius; NTU, nephelometric turbidity units; mg/L, milligrams per liter; CaCO3, calcium carbonate; μg/L, micrograms per liter; <, less than; >, greater than; E, estimated; --, no data; G, gallon; min, minute; mm, millimeters; Hg, mercury; NGVD, National Geodetic Vertical Datum of 1929; ESA, ethanesulfonic acid; OA, oxanilic acid; N, nitrogen; P, phosphorus; NO3, nitrate as nitrogen; pCi/L, picocuries per liter]

Site

number

(fig. 7)

Site

identification Date Time

Water

level

(feet)

Depth

of well

(feet)

Elevation of 

land surface 

(feet above

NGVD)

Turbidity

(NTU)

Air

pressure

(mm of Hg)

Dissolved

oxygen

(mg/L)

pH,

field

(standard

units)

Specific

conductance

(μS/cm at 25°C)

1 363518101515101 09/12/00 1000 186.85 198.00 3,460 42 675 7.9 7.5 6112 364846102161201 09/17/00 1000 160.00 181.72 3,965 3.5 661 7.4 7.7 8193 371303102003001 09/14/00 1700 96.70 113.18 3,623 1.2 673 7.4 7.5 9484 374750101553501 08/09/00 0900 159.55 178.00 3,442 47 673 1.1 7.7 6525 384422101434201 07/14/00 1200 186.48 199.00 3,655 6.6 675 7.3 7.8 3856 374005101433501 08/08/00 0800 173.27 198.18 3,266 330 676 4.6 7.6 6107 371909101413101 09/16/00 1000 181.70 196.60 3,324 180 677 7.9 7.7 1,1008 370130101180904 09/14/00 0800 132.80 145.00 3,100 37 681 7.7 7.8 6229 375734100452301 02/02/99 1130 122.69 133.00 2,877 -- 694 6.5 7.0 1,640

10 383334101131301 08/08/00 1500 163.37 174.50 3,217 24 676 5.6 7.7 545

11 383803101084701 07/13/00 1100 171.34 182.00 3,181 2.0 688 5.1 7.4 548

12 381201100495101 07/11/00 1200 121.21 139.00 2,920 5.3 689 5.5 7.0 1,22013 375855100484001 06/27/00 1200 70.45 94.00 2,853 2.0 697 5.1 6.8 3,77014 375106100544301 09/26/00 1400 177.07 193.20 2,921 14 689 7.0 7.6 47015 374625100490701 06/29/00 1100 151.37 170.00 2,890 3.4 695 7.2 6.9 99616 374412100453201 07/10/00 1100 151.77 155.50 2,844 180 689 0.6 7.4 86717 372511100155601 09/27/00 0900 12.99 31.10 2,520 0.57 700 5.6 7.3 1,45018 373346100005701 09/27/00 1400 137.74 157.32 2,580 5.0 698 3.9 7.6 50419 373904099545701 09/28/00 1300 99.16 115.00 2,520 5.1 699 7.6 7.4 60720 374220100422501 06/28/00 1100 164.12 190.00 2,797 1.2 697 6.7 6.9 77421 374642100291701 06/30/00 1200 137.70 149.00 2,740 2.1 695 6.2 7.2 55622 375502100363601 09/26/00 1000 174.87 187.40 2,857 >1,000 690 6.7 7.9 70523 360159103044401 09/11/00 1500 169.64 179.80 4,463 1.4 646 7.0 7.3 49824 371728101311401 09/15/00 1100 133.69 158.00 3,190 3.8 685 7.4 7.4 1,12025 370941101540001 09/16/00 1300 99.81 113.30 3,531 1.0 673 5.6 7.8 1,38026 375251101135301 08/11/00 1000 92.18 105.81 3,019 5.7 685 7.7 7.3 1,66027 380256101035901 08/10/00 0800 150.89 162.20 2,978 65 686 6.2 7.2 1,10028 383936101120801 07/12/00 1500 156.16 173.50 3,216 9.1 686 5.8 7.4 58029 373234099430201 09/25/00 1300 134.40 149.00 2,470 1.3 705 7.0 7.7 46030 374430100093801 09/28/00 0900 92.24 108.50 2,595 6.0 699 7.8 7.4 792
Supplemental Data 33

Table 9. Results of physical and chemical analyses of water samples from wells completed as part of the central High Plains agricultural land-use study, 1999–2000—Continued[μS/cm, microsiemens per centimeter at 25°C; Deg. C, degrees Celsius; NTU, nephelometric turbidity units; mg/L, milligrams per liter; CaCO3, calcium carbonate; μg/L, micrograms per liter; <, less than; >, greater than; E, estimated; --, no data; G, gallon; min, minute; mm, millimeters; Hg, mercury; NGVD, National Geodetic Vertical Datum of 1929; ESA, ethanesulfonic acid; OA, oxanilic acid; 

N, nitrogen; P, phosphorus; NO3, nitrate as nitrogen; pCi/L, picocuries per liter]

Site

number

(fig. 7)

Water

temperature

(Deg. C)

Calcium,

dissolved

(mg/L)

Magnesium,

dissolved

(mg/L)

Potassium,

dissolved

(mg/L)

Sodium,

dissolved

(mg/L)

Alkalinity,

field

(mg/L as 

CaCO3)

Bromide,

dissolved

(mg/L)

Chloride,

dissolved

(mg/L)

Fluoride,

dissolved

(mg/L)

Silica,

dissolved

(mg/L)

Sulfate,

dissolved

(mg/L)

1 18.0 43.7 30.4 5.38 25.6 146 0.55 69.5 2.4 36.5 46.22 19.3 56.7 54.2 5.73 26.0 166 0.71 69.1 2.2 37.2 1493 18.2 57.5 63.0 6.37 43.8 205 0.60 70.8 2.2 32.1 1524 19.0 76.5 21.1 6.29 26.0 168 0.15 10.5 0.7 64.5 1515 18.3 37.1 14.2 4.63 23.8 154 0.11 7.46 1.3 28.5 38.66 17.0 55.5 22.7 5.03 31.3 112 0.17 6.30 1.2 18.2 1227 15.8 86.6 53.3 5.73 47.6 130 1.43 113 2.4 24.4 2028 17.0 38.5 11.5 8.42 74.8 216 0.15 18.0 0.5 28.9 60.29 12.1 145 69.6 8.89 81.6 340 0.25 101 0.7 46.8 163

10 16.0 46.7 20.6 6.70 28.1 97 0.19 23.0 2.4 56.8 59.2

11 18.6 61.5 25.1 4.76 18.2 234 0.15 11.8 1.2 34.6 31.9

12 19.5 113 43.5 8.88 105 202 0.53 70.2 2.1 56.9 20913 18.9 410 220 14.9 503 325 1.38 223 0.7 38.6 2,36014 18.0 64.7 9.04 3.09 17.9 152 0.08 9.36 0.4 20.7 25.715 17.4 121 17.4 4.35 66.6 206 0.46 104 0.3 22.0 10516 17.3 126 22.0 6.77 27.5 145 0.23 13.0 0.5 10.8 32417 15.0 127 39.9 4.65 163 363 0.32 40.3 1.8 49.5 34418 17.5 63.3 19.7 3.97 8.98 198 0.20 31.5 0.7 29.7 14.119 16.0 77.9 15.3 4.49 22.1 234 0.13 22.8 0.3 28.0 28.420 17.9 97.3 14.8 1.99 52.2 250 0.18 30.3 0.3 22.9 64.221 20.0 79.8 12.8 3.70 25.0 180 0.10 24.6 0.4 22.6 24.522 15.0 83.3 29.2 4.93 21.0 156 0.27 22.8 0.8 35.3 16123 18.7 48.3 22.2 4.57 38.7 188 0.20 19.4 1.1 24.3 78.024 18.0 84.4 63.0 7.06 62.4 262 0.31 47.6 1.4 30.9 27225 16.8 86.9 78.0 11.9 82.6 182 1.19 132 2.1 22.5 34326 17.5 226 35.4 4.75 102 127 0.31 55.3 0.4 17.2 60927 21.0 129 43.1 7.22 46.5 252 0.54 54.3 0.6 29.7 21828 17.8 57.8 21.0 10.3 32.1 178 0.23 27.6 1.6 30.7 64.929 16.6 71.3 12.1 3.66 9.33 202 0.07 7.79 0.4 31.0 10.630 16.0 108 16.1 5.70 25.8 262 0.11 35.5 0.3 26.8 42.2
34 Ground-Water Quality Beneath Irrigated Agriculture in the Central High Plains Aquifer, 1999–2000 Table 9. Results of physical and chemical analyses of water samples from wells completed as part of the central High Plains agricultural land-use study, 1999–2000—Continued

[μS/cm, microsiemens per centimeter at 25°C; Deg. C, degrees Celsius; NTU, nephelometric turbidity units; mg/L, milligrams per liter; CaCO3, calcium carbonate; μg/L, micrograms per liter; <, less than; >, greater than; E, estimated; --, no data; G, gallon; min, minute; mm, millimeters; Hg, mercury; NGVD, National Geodetic Vertical Datum of 1929; ESA, ethanesulfonic acid; OA, oxanilic acid; 

N, nitrogen; P, phosphorus; NO3, nitrate as nitrogen; pCi/L, picocuries per liter]

Site

number

(fig. 7)

Dissolved

solids

(mg/L)

Ammonium,

dissolved

(mg/L as N)

Ammonium

plus organic

nitrogen,

dissolved

(mg/L as N)

Nitrate

plus nitrite,

dissolved

(mg/L as N)

Nitrite,

dissolved

(mg/L as N)

Phosphorus,

dissolved

(mg/L as P)

Orthophosphate,

dissolved

(mg/L as P)

Carbon, 

organic,

dissolved

(mg/L)

Iron,

dissolved

1 373 <0.02 <0.10 3.48 <0.010 <0.050 <0.01 0.7 <302 542 <0.02 0.13 3.08 <0.010 <0.050 <0.01 14.5 <103 624 <0.02 0.13 9.27 <0.010 0.006 <0.01 4.2 <104 493 <0.02 <0.10 2.85 <0.010 0.008 <0.01 0.4 335 277 0.03 <0.10 2.36 <0.010 <0.050 0.02 1.2 <106 400 <0.02 <0.10 3.07 <0.010 0.047 0.01 0.8 <107 720 <0.02 0.17 14.9 0.016 0.022 <0.01 3.0 <108 388 0.53 0.66 3.75 0.091 0.018 <0.01 0.8 <109 1,060 <0.02 0.65 54.4 <0.010 0.009 0.02 -- <10

10 374 <0.02 0.20 3.42 <0.010 0.012 <0.01 1.5 <10

11 382 <0.02 0.11 8.31 <0.010 <0.050 <0.01 2.9 <10

12 1,040 <0.02 0.35 61.1 <0.010 <0.05 <0.01 2.6 <1013 4,510 0.03 0.23 9.60 <0.010 0.016 0.01 2.4 <3014 300 <0.02 <0.10 11.6 <0.010 0.015 0.01 6.5 <1015 680 <0.02 0.20 24.1 <0.010 0.007 <0.01 1.9 <1016 675 <0.02 0.10 0.25 0.012 <0.05 <0.01 2.5 <1017 1,090 <0.02 0.28 21.5 <0.010 0.022 0.02 5.3 <1018 300 <0.02 <0.10 1.27 0.011 <0.006 <0.01 6.3 1019 380 <0.02 <0.10 6.57 <0.010 0.012 0.01 24.2 <1020 554 <0.02 0.13 22.0 <0.010 0.007 <0.01 1.5 <1021 396 <0.02 <0.10 19.3 <0.010 0.019 0.02 1.1 <1022 485 <0.02 <0.10 3.21 <0.010 0.012 <0.01 0.6 <1023 366 <0.02 <0.10 1.29 <0.010 <0.050 <0.01 0.7 <1024 782 <0.02 <0.10 5.91 <0.010 <0.050 <0.01 1.7 <3025 964 0.06 0.35 6.62 1.09 0.011 <0.01 3.6 <1026 1,310 <0.02 <0.10 22.9 <0.010 0.006 0.01 1.1 <1027 782 <0.02 0.20 15.8 0.011 0.009 <0.01 1.9 <1028 407 <0.02 <0.10 7.56 <0.010 <0.050 <0.01 3.1 <1029 287 <0.02 <0.10 2.71 <0.010 <0.050 <0.01 2.3 <1030 489 <0.02 0.10 7.48 <0.010 0.032 0.03 3.0 <10
Supplemental Data 35

Table 9. Results of physical and chemical analyses of water samples from wells completed as part of the central High Plains agricultural land-use study, 1999–2000—Continued[μS/cm, microsiemens per centimeter at 25°C; Deg. C, degrees Celsius; NTU, nephelometric turbidity units; mg/L, milligrams per liter; CaCO3, calcium carbonate; μg/L, micrograms per liter; <, less than; >, greater than; E, estimated; --, no data; G, gallon; min, minute; mm, millimeters; Hg, mercury; NGVD, National Geodetic Vertical Datum of 1929; ESA, ethanesulfonic acid; OA, oxanilic acid; N, nitrogen; P, phosphorus; NO3, nitrate as nitrogen; pCi/L, picocuries per liter]

Site

number

(fig. 7)

Manganese,

dissolved

(μg/L)

2-6-

Diethylaniline

(μg/L)

Acetochlor,

dissolved

(μg/L)

Acetochlor,

ESA

(μg/L)

Acetochlor,

OA

(μg/L)

Alachlor,

dissolved

(μg/L)

Alachlor,

ESA

(μg/L)

Alachlor,

OA

(μg/L)

Alpha BHC,

dissolved

(μg/L)

Atrazine

(μg/L)

Benfluralin

(μg/L)

1 2.7 <0.003 <0.002 <0.05 <0.05 <0.002 <0.05 <0.05 <0.002 <0.005 <0.0022 2.7 <0.003 <0.002 <0.05 <0.05 <0.002 <0.05 <0.05 <0.002 <0.001 <0.0023 3.5 <0.003 <0.002 <0.05 <0.05 <0.002 <0.05 <0.05 <0.002 1.71 <0.0024 44.2 <0.003 <0.002 <0.05 <0.05 <0.002 <0.05 <0.05 <0.002 <0.001 <0.0025 3.0 <0.003 <0.002 <0.05 <0.05 <0.002 <0.05 <0.05 <0.002 <0.001 <0.0026 45.7 <0.003 <0.002 <0.05 <0.05 <0.002 0.07 <0.05 <0.002 <0.01 <0.0027 7.8 <0.003 <0.002 <0.05 <0.05 <0.002 <0.05 <0.05 <0.002 0.310 <0.0028 35.0 <0.003 <0.002 <0.05 <0.05 <0.002 <0.05 <0.05 <0.002 0.065 <0.0029 <3.0 <0.003 <0.002 <0.05 <0.05 <0.002 <0.05 <0.05 <0.002 5.01 <0.002

10 40.7 <0.003 <0.002 <0.05 <0.05 <0.002 <0.05 <0.05 <0.002 <0.010 <0.002

11 43.4 <0.003 <0.002 <0.05 <0.05 <0.002 0.16 <0.05 <0.002 E4.78 <0.002

12 5.4 <0.003 <0.002 <0.05 <0.05 <0.002 <0.05 <0.05 <0.002 0.038 <0.00213 <3.0 <0.003 <0.002 0.47 <0.05 <0.002 <0.05 <0.05 <0.002 0.008 <0.00214 2.5 <0.002 <0.004 <0.05 <0.05 <0.002 0.23 <0.05 <0.005 0.016 <0.01015 <3.0 <0.003 <0.002 <0.05 <0.05 <0.002 <0.05 <0.05 <0.002 0.270 <0.00216 744 <0.003 <0.010 <0.05 <0.05 <0.002 <0.05 <0.05 <0.002 0.117 <0.00217 <2.2 <0.002 0.009 1.84 <0.05 <0.002 <0.05 <0.05 <0.005 0.438 <0.01018 76.1 <0.002 <0.004 <0.05 <0.05 <0.002 <0.05 <0.05 <0.005 <0.007 <0.01019 <3.0 <0.002 <0.004 <0.05 <0.05 0.049 0.52 0.06 <0.005 2.56 <0.01020 <2.2 <0.003 <0.002 0.80 <0.05 <0.002 <0.05 <0.05 <0.002 1.28 <0.00221 <3.0 <0.003 <0.002 <0.05 <0.05 <0.002 <0.05 <0.05 <0.002 0.050 <0.00222 <2.2 <0.002 <0.004 <0.05 <0.05 <0.002 <0.05 <0.05 <0.005 <0.007 <0.01023 <3.0 <0.003 <0.002 <0.05 <0.05 <0.002 <0.05 <0.05 <0.002 <0.001 <0.00224 <3.0 <0.003 <0.002 <0.05 <0.05 <0.002 <0.05 <0.05 <0.002 0.128 <0.00225 160 <0.003 <0.002 <0.05 <0.05 <0.002 <0.05 <0.05 <0.002 0.009 <0.00226 <3.0 <0.003 <0.002 <0.05 <0.05 <0.002 <0.05 <0.05 <0.002 0.288 <0.00227 13.2 <0.003 <0.002 <0.05 <0.05 <0.002 1.24 <0.05 <0.002 0.782 <0.00228 4.3 <0.003 <0.002 <0.05 <0.05 <0.002 0.07 <0.05 <0.002 0.013 <0.00229 <2.2 <0.002 <0.004 <0.05 <0.05 <0.002 <0.05 <0.05 <0.005 <0.010 <0.01030 <2.2 <0.002 <0.004 <0.05 <0.05 <0.002 0.13 <0.05 <0.005 0.232 <0.010
36 Ground-Water Quality Beneath Irrigated Agriculture in the Central High Plains Aquifer, 1999–2000 Table 9. Results of physical and chemical analyses of water samples from wells completed as part of the central High Plains agricultural land-use study, 1999–2000—Continued

[μS/cm, microsiemens per centimeter at 25°C; Deg. C, degrees Celsius; NTU, nephelometric turbidity units; mg/L, milligrams per liter; CaCO3, calcium carbonate; μg/L, micrograms per liter; <, less than; >, greater than; E, estimated; --, no data; G, gallon; min, minute; mm, millimeters; Hg, mercury; NGVD, National Geodetic Vertical Datum of 1929; ESA, ethanesulfonic acid; OA, oxanilic acid; N, nitrogen; P, phosphorus; NO3, nitrate as nitrogen; pCi/L, picocuries per liter]

Site

number

(fig. 7)

Butylate

(μg/L)

Carbaryl

(μg/L)

Carbofuran

(μg/L)

Chlorpyrifos

(μg/L)

Cyanazine

(μg/L)

DCPA

(dacthal)

(μg/L)

Deethylatrazine

(μg/L)

Diazinon

(μg/L)

Dieldrin

(μg/L)

Disulfoton

(μg/L)

EPTC

(eptam)

(μg/L)

Ethalfluralin

(μg/L)

1 <0.002 <0.003 <0.003 <0.004 <0.004 <0.002 E0.074 <0.002 <0.001 <0.02 <0.002 <0.0042 <0.002 <0.003 <0.003 <0.004 <0.004 <0.002 <0.006 <0.002 <0.001 <0.02 <0.002 <0.0043 <0.002 <0.003 <0.003 <0.004 <0.004 <0.002 E2.10 <0.002 <0.001 <0.02 <0.002 <0.0044 <0.002 <0.003 <0.003 <0.004 <0.004 <0.002 <0.002 <0.002 <0.001 <0.02 <0.002 <0.0045 <0.002 <0.003 <0.003 <0.004 <0.004 <0.002 <0.002 <0.002 <0.001 <0.02 <0.002 <0.0046 <0.002 <0.003 <0.003 <0.004 <0.004 <0.002 <0.002 <0.002 <0.001 <0.02 <0.002 <0.0047 <0.002 <0.003 <0.003 0.007 <0.004 <0.002 E0.029 <0.002 <0.001 <0.02 <0.002 <0.0048 <0.002 <0.003 <0.003 0.018 <0.004 <0.002 E0.012 <0.002 <0.001 <0.02 <0.002 <0.0049 <0.002 <0.003 <0.003 <0.004 <0.004 <0.002 E1.70 <0.002 <0.001 <0.02 <0.002 <0.004

10 <0.002 <0.003 <0.003 <0.004 <0.004 <0.002 <0.002 <0.002 <0.001 <0.02 <0.002 <0.004

11 <0.002 <0.003 <0.003 <0.004 <0.004 <0.002 E0.532 <0.002 <0.001 <0.02 <0.002 <0.004

12 <0.002 <0.003 <0.003 <0.004 <0.004 <0.002 E0.172 <0.002 <0.001 <0.02 <0.002 <0.00413 <0.002 <0.003 <0.003 <0.004 <0.004 <0.002 E0.049 <0.002 <0.001 <0.02 <0.002 <0.00414 <0.002 <0.041 <0.020 <0.005 <0.018 <0.003 E0.187 <0.005 <0.005 <0.02 <0.002 <0.00915 <0.002 <0.003 <0.003 <0.004 <0.004 <0.002 E0.407 <0.002 <0.001 <0.02 <0.002 <0.00416 <0.002 <0.003 <0.003 <0.004 <0.004 <0.002 E0.074 <0.007 <0.001 <0.02 <0.002 <0.00417 <0.002 <0.041 <0.020 <0.005 <0.018 <0.003 E0.726 <0.005 <0.005 <0.02 <0.002 <0.00918 <0.002 <0.041 <0.020 <0.005 <0.018 <0.003 <0.006 <0.005 <0.005 <0.02 <0.002 <0.00919 <0.002 <0.041 <0.020 <0.005 <0.018 <0.003 E1.00 <0.005 <0.005 <0.02 0.018 <0.00920 <0.002 <0.003 <0.003 <0.004 <0.004 <0.002 E0.549 <0.002 <0.001 <0.02 <0.002 <0.00421 <0.002 <0.003 <0.003 <0.004 <0.004 <0.002 E0.097 <0.002 <0.001 <0.02 <0.002 <0.00422 <0.002 <0.041 <0.020 <0.005 <0.018 <0.003 <0.006 <0.007 <0.005 <0.02 <0.002 <0.00923 <0.002 <0.003 <0.003 <0.004 <0.004 <0.002 <0.002 <0.002 <0.001 <0.02 <0.002 <0.00424 <0.002 <0.003 <0.003 <0.004 <0.004 <0.002 E0.439 <0.002 <0.001 <0.02 <0.002 <0.00425 <0.002 <0.003 <0.030 0.012 <0.004 <0.002 E0.249 <0.002 <0.001 <0.02 <0.010 <0.00426 <0.002 <0.003 <0.003 <0.004 <0.004 <0.002 E1.20 <0.002 <0.001 <0.02 <0.002 <0.00427 <0.002 <0.003 <0.003 <0.004 <0.004 <0.002 E1.30 <0.002 <0.001 <0.02 <0.002 <0.00428 <0.002 <0.003 <0.003 <0.004 <0.004 <0.002 E0.037 <0.002 <0.001 <0.02 <0.002 <0.00429 <0.002 <0.041 <0.020 <0.005 <0.018 <0.003 <0.006 <0.005 <0.005 <0.02 <0.002 <0.00930 <0.002 <0.041 <0.020 <0.005 <0.018 <0.003 E1.04 <0.005 <0.005 <0.02 <0.002 <0.009
Supplemental Data 37

Table 9. Results of physical and chemical analyses of water samples from wells completed as part of the central High Plains agricultural land-use study, 1999–2000—Continued[μS/cm, microsiemens per centimeter at 25°C; Deg. C, degrees Celsius; NTU, nephelometric turbidity units; mg/L, milligrams per liter; CaCO3, calcium carbonate; μg/L, micrograms per liter; <, less than; >, greater than; E, estimated; --, no data; G, gallon; min, minute; mm, millimeters; Hg, mercury; NGVD, National Geodetic Vertical Datum of 1929; ESA, ethanesulfonic acid; OA, oxanilic acid; N, nitrogen; P, phosphorus; NO3, nitrate as nitrogen; pCi/L, picocuries per liter]

Site

number

(fig. 7)

Ethoprop

(μg/L)

Fonofos

(μg/L)

Lindane

(μg/L)

Linuron

(μg/L)

Malathion

(μg/L)

Methylazinphos

(μg/L)

Methyl,

parathion

(μg/L)

Metolachlor,

dissolved

(μg/L)

Metolachlor,

ESA

(μg/L)

Metolachlor,

OA

(μg/L)

Metribuzin

(μg/L)

Molinate

(μg/L)

1 <0.003 <0.003 <0.004 <0.002 <0.005 <0.001 <0.006 <0.002 <0.05 <0.05 <0.004 <0.0042 <0.003 <0.003 <0.004 <0.002 <0.005 <0.001 <0.006 <0.002 <0.05 <0.05 <0.004 <0.0043 <0.003 <0.003 <0.004 <0.002 <0.005 <0.001 <0.006 <0.002 <0.05 <0.05 <0.004 <0.0044 <0.003 <0.003 <0.004 <0.002 <0.005 <0.001 <0.006 <0.002 <0.05 <0.05 <0.004 <0.0045 <0.003 <0.003 <0.004 <0.002 <0.005 <0.001 <0.006 <0.002 <0.05 <0.05 <0.004 <0.0046 <0.003 <0.003 <0.004 <0.002 <0.005 <0.001 <0.006 <0.002 <0.05 <0.05 <0.004 <0.0047 <0.003 <0.003 <0.004 <0.002 <0.005 <0.001 <0.006 <0.002 <0.05 <0.05 <0.004 <0.0048 <0.003 <0.003 <0.004 <0.002 <0.005 <0.001 <0.006 <0.002 <0.05 <0.05 <0.004 <0.0049 <0.003 <0.003 <0.004 <0.002 <0.005 <0.001 <0.006 0.097 2.80 1.20 <0.004 <0.004

10 <0.003 <0.003 <0.004 <0.002 <0.005 <0.001 <0.006 <0.002 <0.05 <0.05 <0.004 <0.004

11 <0.003 <0.003 <0.004 <0.002 <0.005 <0.001 <0.006 0.021 0.26 0.09 <0.004 <0.004

12 <0.003 <0.003 <0.004 <0.002 <0.005 <0.001 <0.006 <0.002 0.11 <0.05 <0.004 <0.00413 <0.003 <0.003 <0.004 <0.002 <0.005 <0.001 <0.006 <0.002 <0.05 <0.05 <0.004 <0.00414 <0.005 <0.003 <0.004 <0.035 <0.027 <0.050 <0.006 <0.013 0.09 <0.05 <0.006 <0.00215 <0.003 <0.003 <0.004 <0.002 <0.005 <0.001 <0.006 <0.002 0.24 <0.05 <0.004 <0.00416 <0.003 <0.003 <0.004 <0.002 <0.005 <0.001 <0.006 <0.002 <0.05 <0.05 <0.004 <0.00417 <0.005 <0.003 <0.004 <0.035 <0.027 <0.050 <0.006 <0.013 0.47 <0.05 <0.006 <0.00218 <0.005 <0.003 <0.004 <0.035 <0.027 <0.050 <0.006 <0.013 <0.05 <0.05 <0.006 <0.00219 <0.005 <0.003 <0.004 <0.035 <0.027 <0.050 <0.006 0.679 0.92 0.62 0.017 <0.00220 <0.003 <0.003 <0.004 <0.002 <0.005 <0.001 <0.006 <0.004 <0.05 <0.05 <0.004 <0.00421 <0.003 <0.003 <0.004 <0.002 <0.005 <0.001 <0.006 <0.002 <0.05 <0.05 <0.004 <0.00422 <0.005 <0.003 <0.004 <0.035 <0.027 <0.050 <0.006 <0.013 <0.05 <0.05 <0.006 <0.00223 <0.003 <0.003 <0.004 <0.002 <0.005 <0.001 <0.006 <0.002 <0.05 <0.05 <0.004 <0.00424 <0.003 <0.003 <0.004 <0.002 <0.005 <0.001 <0.006 <0.002 <0.05 <0.05 <0.004 <0.00425 <0.003 <0.003 <0.004 <0.002 <0.005 <0.001 <0.006 <0.002 <0.05 <0.05 <0.004 <0.00426 <0.003 <0.003 <0.004 <0.002 <0.005 <0.001 <0.006 <0.002 <0.05 <0.05 <0.004 <0.00427 <0.003 <0.003 <0.004 <0.002 <0.005 <0.001 <0.006 <0.002 <0.05 <0.05 <0.004 <0.00428 <0.003 <0.003 <0.004 <0.002 <0.005 <0.001 <0.006 <0.002 <0.05 <0.05 <0.004 <0.00429 <0.005 <0.003 <0.004 <0.035 <0.027 <0.050 <0.006 <0.013 <0.05 <0.05 <0.006 <0.00230 <0.005 <0.003 <0.004 <0.035 <0.027 <0.050 <0.006 <0.013 0.08 <0.05 <0.006 <0.002
38 Ground-Water Quality Beneath Irrigated Agriculture in the Central High Plains Aquifer, 1999–2000 Table 9. Results of physical and chemical analyses of water samples from wells completed as part of the central High Plains agricultural land-use study, 1999–2000—Continued

[μS/cm, microsiemens per centimeter at 25°C; Deg. C, degrees Celsius; NTU, nephelometric turbidity units; mg/L, milligrams per liter; CaCO3, calcium carbonate; μg/L, micrograms per liter; <, less than; >, greater than; E, estimated; --, no data; G, gallon; min, minute; mm, millimeters; Hg, mercury; NGVD, National Geodetic Vertical Datum of 1929; ESA, ethanesulfonic acid; OA, oxanilic acid; 

N, nitrogen; P, phosphorus; NO3, nitrate as nitrogen; pCi/L, picocuries per liter]

Site

number

(fig. 7)

Napromide

(μg/L)

p,p’ DDE

(μg/L)

Parathion

(μg/L)

Pebulate

(μg/L)

Pendimethalin

(μg/L)

Permethrin

(μg/L)

Phorate

(μg/L)

Prometon

(μg/L)

Pronamide

(μg/L)

Propachlor

(μg/L)

Propanil

(μg/L)

1 <0.003 <0.006 <0.004 <0.004 <0.004 <0.005 <0.002 <0.02 <0.003 <0.007 <0.0042 <0.003 <0.006 <0.004 <0.004 <0.004 <0.005 <0.002 <0.02 <0.003 <0.007 <0.0043 <0.003 <0.006 <0.004 <0.004 <0.004 <0.005 <0.002 <0.02 <0.003 <0.007 <0.0044 <0.003 <0.006 <0.004 <0.004 <0.004 <0.005 <0.002 <0.02 <0.003 <0.007 <0.0045 <0.003 <0.006 <0.004 <0.004 <0.004 <0.005 <0.002 <0.02 <0.003 <0.007 <0.0046 <0.003 <0.006 <0.004 <0.004 <0.004 <0.005 <0.002 <0.02 <0.003 <0.007 <0.0047 <0.003 <0.006 <0.004 <0.004 <0.004 <0.005 <0.002 <0.02 <0.003 <0.007 <0.0048 <0.003 <0.006 <0.004 <0.004 <0.004 <0.005 <0.002 <0.02 <0.003 <0.007 <0.0049 <0.003 <0.006 <0.004 <0.004 <0.004 <0.005 <0.002 <0.02 <0.003 <0.007 <0.004

10 <0.003 <0.006 <0.004 <0.004 <0.004 <0.005 <0.002 <0.02 <0.003 <0.007 <0.004

11 <0.003 <0.006 <0.004 <0.004 <0.004 <0.005 <0.002 <0.02 <0.003 <0.007 <0.004

12 <0.003 <0.006 <0.004 <0.004 <0.004 <0.005 <0.002 <0.02 <0.003 <0.007 <0.00413 <0.003 <0.006 <0.004 <0.004 <0.004 <0.005 <0.002 <0.02 <0.003 <0.007 <0.00414 <0.007 <0.003 <0.007 <0.002 <0.010 <0.006 <0.011 <0.01 <0.004 <0.010 <0.01115 <0.003 <0.006 <0.004 <0.004 <0.004 <0.005 <0.002 <0.02 <0.003 <0.007 <0.00416 <0.003 <0.006 <0.004 <0.004 <0.004 <0.005 <0.002 <0.02 <0.003 <0.007 <0.00417 <0.007 <0.003 <0.007 <0.002 <0.010 <0.006 <0.011 <0.01 <0.004 <0.010 <0.01118 <0.007 <0.003 <0.007 <0.002 <0.010 <0.006 <0.011 <0.01 <0.004 <0.010 <0.01119 <0.007 <0.003 <0.007 <0.002 <0.010 <0.006 <0.011 <0.01 <0.004 <0.010 <0.01120 <0.003 <0.006 <0.004 <0.004 <0.004 <0.005 <0.002 <0.02 <0.003 <0.007 <0.00421 <0.003 <0.006 <0.004 <0.004 <0.004 <0.005 <0.002 <0.02 <0.003 <0.007 <0.00422 <0.007 <0.003 <0.007 <0.002 <0.010 <0.006 <0.011 <0.01 <0.004 <0.010 <0.01123 <0.003 <0.006 <0.004 <0.004 <0.004 <0.005 <0.002 <0.02 <0.003 <0.007 <0.00424 <0.003 <0.006 <0.004 <0.004 <0.004 <0.005 <0.002 <0.02 <0.003 <0.007 <0.00425 <0.003 <0.006 <0.004 <0.004 <0.004 <0.005 <0.002 <0.02 <0.003 <0.007 <0.00426 <0.003 <0.006 <0.004 <0.004 <0.004 <0.005 <0.002 <0.02 <0.003 <0.007 <0.00427 <0.003 <0.006 <0.004 <0.004 <0.004 <0.005 <0.002 <0.02 <0.003 <0.007 <0.00428 <0.003 <0.006 <0.004 <0.004 <0.004 <0.005 <0.002 <0.02 <0.003 <0.007 <0.00429 <0.007 <0.003 <0.007 <0.002 <0.010 <0.006 <0.011 <0.01 <0.004 <0.010 <0.01130 <0.007 <0.003 <0.007 <0.002 <0.010 <0.006 <0.011 <0.01 <0.004 <0.010 <0.011 
Supplemental Data 39

Table 9. Results of physical and chemical analyses of water samples from wells completed as part of the central High Plains agricultural land-use study, 1999–2000—Continued[μS/cm, microsiemens per centimeter at 25°C; Deg. C, degrees Celsius; NTU, nephelometric turbidity units; mg/L, milligrams per liter; CaCO3, calcium carbonate; μg/L, micrograms per liter; <, less than; >, greater than; E, estimated; --, no data; G, gallon; min, minute; mm, millimeters; Hg, mercury; NGVD, National Geodetic Vertical Datum of 1929; ESA, ethanesulfonic acid; OA, oxanilic acid; N, nitrogen; P, phosphorus; NO3, nitrate as nitrogen; pCi/L, picocuries per liter]

Site

number

(fig. 7)

Propargite

(μg/L)

Simazine

(μg/L)

Tebuthiuron

(μg/L)

Terbacil

(μg/L)

Terbufos

(μg/L)

Thiobencarb

(μg/L)

Triallate

(μg/L)

Trifluralin

(μg/L)

N-15/N-14

NO3 fraction

(ratio per mil)

Tritium

total

(pCi/L)

Tritium

2 sigma

(pCi/L)

1 <0.01 <0.005 <0.01 <0.007 <0.01 <0.002 <0.001 <0.002 4.80 7 1.02 <0.01 <0.005 <0.01 <0.007 <0.01 <0.002 <0.001 <0.002 4.70 2 1.03 <0.01 <0.005 <0.01 <0.007 <0.01 <0.002 <0.001 <0.002 3.80 6 1.04 <0.01 <0.005 <0.01 <0.007 <0.01 <0.002 <0.001 <0.002 8.50 3 1.05 <0.01 <0.005 <0.01 <0.007 <0.01 <0.002 <0.001 <0.002 5.30 <1 1.06 <0.01 <0.005 <0.01 <0.007 <0.01 <0.002 <0.001 <0.002 6.60 <1 1.07 <0.01 <0.005 <0.01 <0.007 <0.01 <0.002 <0.001 <0.002 3.80 15 1.38 <0.01 <0.005 <0.01 <0.007 <0.01 <0.002 <0.001 <0.002 3.40 37 2.69 <0.01 0.038 <0.01 <0.007 <0.01 <0.002 <0.001 <0.002 15.80 16 1.3

10 <0.01 <0.005 <0.01 <0.007 <0.01 <0.002 <0.001 <0.002 7.80 <1 1.0

11 <0.01 0.033 <0.01 <0.007 <0.01 <0.002 <0.001 <0.002 10.90 24 1.9

12 <0.01 <0.005 <0.01 <0.007 <0.01 <0.002 <0.001 <0.002 10.50 11 1.013 <0.01 <0.005 <0.01 <0.007 <0.01 <0.002 <0.001 <0.002 6.80 24 1.614 <0.02 <0.011 <0.02 <0.034 <0.02 <0.005 <0.002 <0.009 2.10 11 1.015 <0.01 0.006 <0.01 <0.007 <0.01 <0.002 <0.001 <0.002 1.90 16 1.316 <0.01 <0.005 <0.01 <0.007 <0.01 <0.002 <0.001 <0.002 6.80 5 1.017 <0.02 <0.011 <0.02 <0.034 <0.02 <0.005 <0.002 <0.009 4.50 10 1.018 <0.02 <0.011 <0.02 <0.034 <0.02 <0.005 <0.002 <0.009 4.60 <1 1.019 <0.02 0.021 <0.02 <0.034 <0.02 <0.005 <0.002 <0.009 4.10 14 1.320 <0.01 0.023 <0.01 <0.007 <0.01 <0.002 <0.001 <0.002 3.00 16 1.321 <0.01 <0.005 <0.01 <0.007 <0.01 <0.002 <0.001 <0.002 4.40 7 1.022 <0.02 <0.011 <0.02 <0.034 <0.02 <0.005 <0.002 <0.009 7.80 <1 1.023 <0.01 <0.005 <0.01 <0.007 <0.01 <0.002 <0.001 <0.002 4.30 <1 1.024 <0.01 <0.005 <0.01 <0.007 <0.01 <0.002 <0.001 <0.002 4.20 7 1.025 <0.01 <0.005 <0.01 <0.300 <0.01 <0.002 <0.001 <0.002 9.90 4 1.026 <0.01 <0.005 <0.01 <0.007 <0.01 <0.002 <0.001 <0.002 –0.30 40 2.627 <0.01 <0.005 <0.01 <0.007 <0.01 <0.002 <0.001 <0.002 2.20 7 1.028 <0.01 <0.005 <0.01 <0.007 <0.01 <0.002 <0.001 <0.002 6.80 16 1.329 <0.02 <0.011 <0.02 <0.034 <0.02 <0.005 <0.002 <0.009 6.10 1 1.030 <0.02 <0.011 <0.02 <0.034 <0.02 <0.005 <0.002 <0.009 6.40 20 1.3