Historical Water-Quality Data for the 

High Plains Regional Ground-Water 

Study Area in Colorado, Kansas, 

Nebraska, New Mexico, Oklahoma, 

South Dakota, Texas, and Wyoming, 

1930–98

By David W. Litke

Denver, Colorado

2001

NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 00–4254
District Chief

U.S. Geological Survey

Box 25046, Mail Stop 415

Denver Federal Center

Denver, CO 80225–0046

For additional information write to:

District Chief

U.S. Geological Survey

Box 25046, Mail Stop 415

Denver Federal Center

Denver, CO 80225-0046

U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

The use of firm, trade, and brand names in this report is for identification purposes only and does 

not constitute endorsement by the U.S. Geological Survey.

Copies of this report can be purchased 

from:

U.S. Geological Survey

Information Services

Box 25286

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Denver, CO 80225
 III

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 sciencebased 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. 

The NAWQA Program recognizes that a national assessment by a single program cannot address all waterresource 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.

FOREWORD

Robert M. Hirsch

Associate Director for Water
CONTENTS III

CONTENTS

Foreword................................................................................................................................................................................ III

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

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

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

Acknowledgments ....................................................................................................................................................... 2

Description of High Plains Regional Ground-Water Study Area .......................................................................................... 2

Summary of Previous Water-Quality Studies ........................................................................................................................ 6

Sources and Characteristics of Available Water-Quality Data .............................................................................................. 8

Data-Selection Criteria and Screening Procedures...................................................................................................... 8

Quality Assurance of Data........................................................................................................................................... 9

Content of Retrospective Data Base ............................................................................................................................ 9

Site Characterization.................................................................................................................................................... 13

Data-Analysis Methods ......................................................................................................................................................... 18

Analysis of Historical Water-Quality Data............................................................................................................................ 22

Nitrate .......................................................................................................................................................................... 35

Pesticides ..................................................................................................................................................................... 41

Dissolved Solids .......................................................................................................................................................... 45

Other Constituents ....................................................................................................................................................... 49

Summary................................................................................................................................................................................ 54

References Cited.................................................................................................................................................................... 55

Appendix A. Description of data on the CD-ROM and instructions on the use of the data base ......................................... 61

Appendix B. Diagrams showing content and structure of the data base ............................................................................... 61

1. Tables included in the High Plains retrospective water-quality data base............................................................... 62

2. Data fields included in SITE table of High Plains retrospective water-quality data base ....................................... 63

3. Data fields included in QW_RESULT table of High Plains retrospective water-quality data base ........................ 64

4. Relationship diagram for High Plains retrospective water-quality data base.......................................................... 65

Appendix C. High Plains retrospective data base on CD-ROM (in pocket)

FIGURES

1–3. Maps showing:

1. Location of the High Plains Regional Ground-Water Study Area .................................................................... 3

2. Hydrogeologic units within the High Plains study area.................................................................................... 5

3. Location of sampling sites in the High Plains retrospective data base ............................................................. 11

4. Graphs showing temporal distribution of water-quality observations for selected parameter groups, 

1950–98..................................................................................................................................................................... 12

5. Map showing geographic distribution of observations for nutrients, pesticides, trace elements, 

and volatile organic compounds................................................................................................................................ 14

6. Graph showing number of sites with selected site-specific information .................................................................. 16

7–10. Maps showing:

7. Depth to water during 1997 in the High Plains study area................................................................................ 17

8. Percentage of land irrigated during 1980 in the High Plains study area ........................................................... 19

9. Nitrogen application rates for manure and fertilizer in the High Plains study area, by county, 1992 .............. 20

10. Soil infiltration rates in the High Plains study area........................................................................................... 21

11. Boxplot showing distribution of nitrate concentrations, by decade.......................................................................... 37

12. Graph showing temporal variability of nitrate concentrations, by hydrogeologic unit ............................................ 37

13. Map showing distribution of nitrate concentrations by county................................................................................. 38

14. Graph showing relation between nitrate concentration and depth of sample below water table,

by hydrogeologic unit ............................................................................................................................................... 40

15. Map showing locations of sites with pesticide data and pesticide detections for atrazine, metolachlor, 

simazine, and prometon ............................................................................................................................................ 44

16. Boxplot showing distribution of dissolved-solids concentrations, by decade .......................................................... 46
IV CONTENTS

17. Graph showing temporal variability of dissolved-solids concentrations, by hydrogeologic unit............................. 46

18. Map showing geographic distribution of dissolved-solids concentration data prior to 1980 and 

data after 1980........................................................................................................................................................... 48

19. Boxplots showing major ion chemistry of High Plains ground water, by hydrogeologic unit................................. 50

20. Map showing distribution of the percentage of iron, manganese, and fluoride observations that exceeded

drinking-water standards........................................................................................................................................... 52

21. Map showing distribution of the number of radon and uranium observations that exceeded

drinking-water standards........................................................................................................................................... 53

TABLES

1. Selected characteristics of hydrogeologic units within the High Plains study area................................................ 6

2. Summary of selected water-quality studies conducted within the High Plains study area ..................................... 7

3. Criteria for rating data quality................................................................................................................................. 9

4. Data sources for the retrospective data base ........................................................................................................... 10

5. Geographic characteristics assigned to sites by using a geographic information system ....................................... 18

6. Parameter codes for nitrate...................................................................................................................................... 22

7. Descriptive statistics for selected constituents or properties by hydrogeologic unit, 1980–98 .............................. 23

8. Selected constituents from the retrospective data base compared to drinking-water standards ............................. 34

9. Kendall’s tau correlation coefficients between nitrate concentrations and ancillary variables,

by hydrogeologic unit ............................................................................................................................................. 40

10. Most commonly used and detected pesticides in the High Plains study area ......................................................... 42

11. Kendall’s tau correlation coefficients between dissolved-solids concentrations and ancillary variables,

by hydrogeologic unit ............................................................................................................................................. 49

ABBREVIATIONS USED IN THIS REPORT

GIRAS Geographic Information Retrieval and Analysis System

GIS Geographic Information System

MCL maximum contaminant level

μg/L micrograms per liter

mg/kg milligrams per kilogram

mg/L milligrams per liter

Mgal/d million gallons per day

NAWQA National Water-Quality Assessment Program

MCL maximum contaminant level

NURE National Uranium Resource Evaluation

pCi/L picocuries per liter

PMCL proposed maximum contaminant level

RASA Regional Aquifer-System Analysis

SS secondary standard

STORET U.S Environmental Protection Agency Storage and Retrieval System

USGS U.S. Geological Survey

USEPA U.S. Environmental Protection Agency

WWW World Wide Web

Sea level: In this report "sea level" refers to the National Geodetic Vertical Datum of 1929—

a geodetic datum derived from a general adjustment of the first-order level nets of the United States 

and Canada, formerly called Sea Level Datum of 1929.

Altitude, as used in this report, refers to distance above or below sea level.
Abstract 1

Historical Water-Quality Data for the High Plains 

Regional Ground-Water Study Area in Colorado, 

Kansas, Nebraska, New Mexico, Oklahoma,

South Dakota, Texas, and Wyoming, 1930–98

By David W. Litke

Abstract

The High Plains aquifer underlies 174,000 

square miles in parts of eight States and includes 

eight primary hydrogeologic units, including the 

well-known Ogallala Formation. The High Plains 

aquifer is an important resource, providing water 

for 27 percent of the Nation’s irrigated agricultural lands in an otherwise dry landscape. Since 

the 1980’s there has been concern over the 

sustainability of the aquifer due to water-level 

declines caused by substantial pumping. Water 

quality of the aquifer is a more recent concern. As 

part of the U.S. Geological Survey’s National 

Water-Quality Assessment Program, historical 

water-quality data have been gathered for the 

High Plains Regional Ground-Water Study Area 

into a retrospective data base, which can be used 

to evaluate the occurrence and distribution of 

water-quality constituents of concern.

Data from the retrospective data base verify 

that nitrate, pesticides, and dissolved solids 

(salinity) are important water-quality concerns in 

the High Plains study area. Sixteen percent of all 

measured nitrate concentrations were larger than 

the U.S. Environmental Protection Agency 

drinking-water standard of 10 milligrams per liter. 

In about 70 percent of the counties within the 

High Plains study area, nitrate concentrations for 

1980–98 were significantly larger than for 1930–

69. While nitrate concentrations are largest where 

depth to water is shallow, concentrations also 

have increased in the Ogallala Formation where 

depth to water is large. Pesticide data primarily 

are available only in the northern half of the study 

area. About 50 pesticides were detected in the 

High Plains study area, but only four pesticides 

(atrazine, alachlor, cyanazine, and simazine) had 

concentrations exceeding a drinking-water standard. The occasional detection of pesticides in 

deeper parts of the Ogallala Formation indicates 

that contamination pathways exist. Dissolved 

solids, which are a direct measure of salinity, had 

29 percent of measured concentrations in excess 

of the secondary drinking-water standard of 500 

milligrams per liter. Comparison of dissolvedsolids concentrations prior to 1980 to concentrations after 1980 indicates dissolved-solids 

concentrations have increased in the alluvial 

valleys of the Platte, the Republican, and the 

Arkansas Rivers, as well as in the Ogallala 

Formation–South hydrogeologic unit.

Water-quality results indicate that human 

activities are affecting the water of the High 

Plains aquifer. Because there is a potential for 

water quality to become impaired relative to the 

historical uses of the aquifer, water quality needs 

to be considered when evaluating the sustainability of the High Plains aquifer. Data collected 

as part of the High Plains Regional Ground-Water 

Study will help to fill in gaps in water-quality 

information and provide additional information 

for understanding the factors that govern ambient 

water quality.
2 Historical Water-Quality Data for the High Plains Regional Ground-Water Study Area in Colorado, Kansas, Nebraska, New Mexico, 

Oklahoma, South Dakota, Texas, and Wyoming, 1930–98

INTRODUCTION

The High Plains Regional Ground-Water Study 

Area (also referred to in this report as the High Plains 

study area) encompasses the High Plains aquifer, 

which underlies 174,000 square miles in parts of 

eight States (fig. 1). The High Plains aquifer includes 

the well-known Ogallala Formation and is an important national resource, providing water for agriculture 

in an otherwise dry landscape. About 27 percent of the 

irrigated land in the United States is in the High Plains 

study area, and about 30 percent of the ground water 

used for irrigation in the United States is pumped from 

the High Plains aquifer (Dennehy, 2000). Irrigation is 

the dominant water use, with irrigation withdrawals 

during 1995 totaling more than 15 billion gallons 

per day (U.S. Geological Survey National Water 

Information System data base). However, the aquifer 

also provides drinking water for 82 percent of the 

2.3 million people who live within the study area 

boundary (Dennehy, 2000).

 Substantial pumping of the High Plains aquifer 

for irrigation since about the 1940’s has resulted in 

water-level declines in some parts of the aquifer of 

more than 100 feet (McGuire and Sharpe, 1997). 

Concern about these declines and the sustainability of 

the aquifer led the U.S. Congress in 1984 to mandate a 

water-level monitoring program for the aquifer. Water 

quality of the aquifer is a more recent concern. States 

and local governmental entities have done local 

studies on water quality, but no comprehensive assessment has been made of the entire aquifer. The U.S. 

Geological Survey (USGS) National Water-Quality 

Assessment (NAWQA) Program is an ongoing effort 

to document water-quality conditions across the 

United States, and the High Plains Regional GroundWater Study was instituted as part of NAWQA in 

1998. One of the first elements of NAWQA studies is 

a retrospective assessment of available water-quality 

information. This report summarizes the results of the 

compilation and assessment of historical water-quality 

data for the study area.

Purpose and Scope

The purpose of this report is to describe and 

assess historical (1930–98) water-quality data for the 

High Plains study area. This retrospective assessment 

gives an indication of where data are present or 

lacking and helps define the temporal and geographic 

extent of water-quality concerns, thereby providing a 

framework for the NAWQA study design. In this 

report, the compilation of historical data is described, 

including sources of data, data-base design, screening 

procedures and quality assurance of data, and characteristics of the resulting data base. The data base is 

included in digital form as a companion to this report. 

Statistical summaries are presented for selected 

constituents in the data base. Finally, for some constituents, temporal and spatial variability in concentrations is described, and variability is examined relative 

to human and environmental factors that might affect 

concentrations.

Acknowledgments

The task of locating and assembling ancillary 

and water-quality data for the eight-State study area 

would not have been possible without the assistance of 

many individuals. The author would like to thank the 

employees of the various governmental agencies who 

provided information used in this report. A complete 

listing of these agencies is provided (see table 4), but 

the author would especially like to acknowledge 

Patrick Larson and Mary Exner of the University of 

Nebraska and Janie Hopkins of the Texas Water 

Development Board for their assistance in making 

available large water-quality data sets from the States 

of Nebraska and Texas.The author also would like to 

thank USGS employees for their assistance: Christi V. 

Hansen and Joan Kenny in Kansas; Judith A. Boohar 

and Jennifer Stanton in Nebraska; Denis F. Healy in 

New Mexico; Mark F. Becker and Lyn K. Osburn in 

Oklahoma; Michael J. Moran and Michael L. Strobel 

in South Dakota; Dana L. Barbie, Freeman L. 

Andrews, and Dee Lurry in Texas; and Deb Trefren 

and Melanie Clark in Wyoming.

DESCRIPTION OF HIGH PLAINS 

REGIONAL GROUND-WATER STUDY 

AREA

 The High Plains occupy the higher elevation 

parts of the much larger Great Plains physiographic 

province, which lies between the Rocky Mountains on 

the west and the Central Lowland on the east. This 

high area, with elevations ranging from 7,800 feet on
DESCRIPTION OF HIGH PLAINS REGIONAL GROUND-WATER STUDY AREA 3

Scottsbluff

North

Platte

Wayne

Garden

City

Colby

Guymon

Wichita

Amarillo

Lubbock

Midland

Odessa

Cheyenne

SOUTH DAKOTA

NEBRASKA

KANSAS

OKLAHOMA

TEXAS

WYOMING

COLORADO

NEW MEXICO

N. Platte River

S. Platte River

Platte River

EXPLANATION

0

0 50 100 150 200 KILOMETERS

50 100 150 200 MILES

35°

40°

105° 100°

Location of study area

Elevation, in

 feet above

 sea level

7,800

1,100

Figure 1. Location of the High Plains Regional Ground-Water Study Area.
4 Historical Water-Quality Data for the High Plains Regional Ground-Water Study Area in Colorado, Kansas, Nebraska, New Mexico, 

Oklahoma, South Dakota, Texas, and Wyoming, 1930–98

the west to 1,100 feet on the east, is a remnant of 

a vast plain formed by sediments of Tertiary age 

that were deposited by streams flowing eastward from 

the ancestral Rocky Mountains. The boundary of the 

High Plains is marked in some areas by an escarpment 

formed by ledges of the resistant cap rock of the 

Ogallala Formation. The land within the High Plains is 

characterized by gently sloping, smooth plains with a 

local topographic relief of less than 300 feet, which is 

ideal for agricultural use. The climate is dry: mean 

annual precipitation ranges from 16 inches in the 

western part of the study area to about 28 inches in the 

east. Because precipitation is small and slopes are flat, 

little runoff is available to produce streamflow. Most 

principal rivers in the study area flow through the 

High Plains but derive their water primarily from the 

Rocky Mountains to the west. Natural vegetation in 

this shortgrass prairie area was grama-buffalo grass, 

and the natural precipitation supports only droughttolerant crops such as wheat.

A vast ground-water reservoir underlying the 

High Plains was described as early as 1901 (Johnson, 

1901), but at that time the well technology was lacking 

to bring this deep water to the surface. Beginning in 

the 1930’s, advances in well technology made deeper 

wells feasible. Development of the High Plains aquifer 

started in Texas, where depths to water were small, 

and expanded northward. During 1949–78, irrigated 

acreage in the High Plains increased from about 

2 million acres to 13 million acres, and ground-water 

pumpage increased from about 4 million acre-feet to 

23 million acre-feet (Gutentag and others, 1984). The 

High Plains aquifer currently (2000) sustains the 

economy and population of the region by providing 

water for irrigation and public water supplies.

These large changes in land use and water use 

have increased the complexity of the water budget of 

the High Plains study area. Prior to development, 

water fluxes were relatively small and in dynamic 

equilibrium. The total volume of drainable water in 

storage in the High Plains aquifer was equivalent to a 

column of water about 30 feet deep across the 

174,000-square-mile extent of the aquifer (Gutentag 

and others, 1984). Precipitation recharged the aquifer 

at a rate of about 0.6 inch per year (Weeks and others, 

1988). Small quantities of water left the aquifer in the 

form of discharge to springs and streams. Subsequent 

to development, ground-water pumpage has removed 

an amount of water equivalent to about 2 feet of water 

from the aquifer, or about 7 percent of the original 

total (Dugan and Cox, 1994). However, in addition to 

removing water from the aquifer, ground-water 

pumpage has increased the internal cycling of water 

within the aquifer, with about 2 inches of water 

pumped from the aquifer each year (U.S. Geological 

Survey National Water Information System data base, 

1999) and as much as 10 percent of that water 

returning to the aquifer each year (Dugan and Zelt, 

2000). Agricultural practices also can increase the rate 

of recharge to the aquifer; for example, water can 

more easily percolate downward from fallow fields 

and from fields where crop growth is at an early stage, 

as compared to areas covered with natural vegetation. 

Estimated recharge to the High Plains aquifer in Oklahoma is almost three times as large as it was prior to 

development of the aquifer (Luckey and Becker, 

1999).

The High Plains study area is identical in areal 

extent to the High Plains water-table (unconfined) 

aquifer as defined by the U.S. Geological Survey’s 

Regional Aquifer-System Analysis Program (Weeks 

and others, 1988). The principal geologic unit of the 

High Plains aquifer is the Ogallala Formation, which 

consists of a sequence of unconsolidated clays, silts, 

sands, and gravels of Tertiary age (2 to 65 million 

years old). In addition to the Tertiary-age Ogallala 

Formation, the High Plains aquifer also includes older 

materials (sandstones and siltstones of the Arikaree 

and Brule Formations of early Tertiary age in the north 

part of the study area) and younger Quaternary materials deposited on top of or adjacent to the Ogallala 

Formation (windblown sands primarily in the Sand 

Hills of Nebraska; alluvial deposits in central Kansas 

and along present-day river valleys; and glacial 

deposits in eastern Nebraska). For summarizing water 

quality in this report, the High Plains aquifer has been 

subdivided into eight hydrogeologic units: Ogallala 

Formation–North, Ogallala Formation–Central, Ogallala Formation–South, Arikaree-Brule Formations, 

Nebraska Sand Hills, Kansas Quaternary Deposits, 

Nebraska Quaternary Deposits, and Valley-Fill Alluvium (fig. 2). The Ogallala Formation was subdivided 

into three hydrogeologic units due to its large 

geographic extent within the study area. Although the 

High Plains aquifer has been subdivided into these 

hydrogeologic units, it is recognized that within the 

individual units there could be sediments of varying 

age (Gutentag and others, 1984; Smith, 1940). The 

individual hydrogeologic units differ in several ways 

(table 1). The most-used units, as measured by water
DESCRIPTION OF HIGH PLAINS REGIONAL GROUND-WATER STUDY AREA 5

Area of bedrock or no saturated thickness

Arikaree-Brule Formations

Kansas Quaternary Deposits

Nebraska Quaternary Deposits

Nebraska Sand Hills

Ogallala Formation—North

Ogallala Formation—Central

Ogallala Formation—South

Valley-Fill Alluvium

EXPLANATION

0

0 50 100 150 200 KILOMETERS

105°

40°

32°

100°

50 100 150 200 MILES

SOUTH DAKOTA

NEBRASKA

KANSAS

OKLAHOMA

TEXAS

WYOMING

COLORADO

NEW MEXICO

Figure 2. Hydrogeologic units within the High Plains study area.
6 Historical Water-Quality Data for the High Plains Regional Ground-Water Study Area in Colorado, Kansas, Nebraska, New Mexico, 

Oklahoma, South Dakota, Texas, and Wyoming, 1930–98

use and percentage of irrigated area, are the Nebraska 

Quaternary Deposits, the Valley-Fill Alluvium, and 

the three Ogallala Formation units in which the intensity of use increases from north to south. Median depth 

to water is small and saturated thickness is large in the 

Nebraska Sand Hills, but water use in this unit is small 

due to rough topography and sandy soils. Water use is 

large and median depth to water is small in the ValleyFill Alluvium. In contrast, water use is large and 

median depth to water is largest for the three Ogallala 

Formation units.

The environmental setting of the High Plains 

study area has been described in detail in previous 

reports (Gutentag and others, 1984; Weeks and others, 

1988). For the purposes of this retrospective analysis, 

selected environmental characteristics were compiled 

for comparison with water quality. Selection of relevant environmental characteristics was accomplished 

through a review of previous water-quality studies.

SUMMARY OF PREVIOUS WATERQUALITY STUDIES

There are more than 100 references to water 

quality cited in a bibliography of water-related studies 

for the High Plains study area that was compiled as 

part of the NAWQA study (Gerhard Kuhn, U.S. 

Geological Survey, written commun., 2000). In this 

bibliography, the most common constituent keyword 

was nitrate (70 references), followed by dissolved 

solids (45 references), pesticides (37 references), 

radiological constituents (23 references), bacteria (13 

references), organic compounds (11 references), and 

trace elements (8 references). The frequency of citation is indicative of the most common water-quality 

concerns within the High Plains study area.

Water-quality studies have been conducted at 

scales ranging from regional to site-specific. Because 

the retrospective data base is aquifer wide, the results 

of regional studies are more pertinent to this analysis 

than are site-specific studies. Regional studies are 

listed in table 2 along with environmental factors 

found to be related to water quality. Early studies 

focused on salinity (dissolved solids) because high 

salinity can affect the use of water for irrigation and 

because extensive irrigation can lead to increased 

salinity. Krothe and others (1982) estimated that 

dissolved-solids concentrations were in excess of 500 

milligrams per liter (mg/L) over 19 percent of the area 

of the High Plains aquifer. Areas of large dissolvedsolids concentrations were primarily related to the 

underlying bedrock type but also occurred in irrigated 

alluvial valleys. A study in Texas (Reeves and Miller, 

1978) related high salinity to bedrock type and to 

seepage from playa lakes. This Texas study also was 

among the first to document large nitrate concentrations, which were related to irrigation intensity and 

soil properties. A study of ground-water quality in 

Nebraska (Engberg, 1984) focused on major ions and

Table 1. Selected characteristics of hydrogeologic units within the High Plains study area

[Mgal/d, million gallons per day]

Hydrogeologic unit

Area 

(square 

miles)

Estimated 

water use in 

1995

(Mgal/d)1

Median 

saturated 

thickness

(feet)2

Median 

depth to 

water

(feet)2

Percentage 

of area 

irrigated3

Ogallala Formation–North 37,700 2,700 110 110 10

Ogallala Formation–Central 35,100 4,100 120 160 15

Ogallala Formation–South 27,000 4,200 60 120 19

Arikaree-Brule Formations 19,400 480 220 64 5

Nebraska Sand Hills 22,500 700 540 20 5

Kansas Quaternary Deposits 5,600 360 87 28 9

Nebraska Quaternary Deposits 10,800 2,100 170 79 34

Valley-Fill Alluvium 8,000 41,000 4120 18 26

1Estimated using data from USGS National Water Information System data base. 2

Estimated using 1997 well data (Sharon Qi, U.S. Geological Survey, written commun., 1999). 3Estimated using 1980 irrigated lands data (Thelin and Heimes, 1987). 4Water use and saturated thickness in the Valley-Fill Alluvium may include contributions from underlying High Plains hydrogeologic units.
SUMMARY OF PREVIOUS WATER-QUALITY STUDIES 7

dissolved solids but listed nitrate as the most pervasive 

potential problem constituent. In the late 1980’s, 

water-quality surveys began to document widespread 

occurrences of elevated nitrate concentrations. A 

survey of farmstead wells in Kansas (Steichen and 

others, 1988) reported nitrate in excess of the U.S. 

Environmental Protection Agency drinking-water 

maximum contaminant level (MCL) in 28 percent of 

wells sampled; and a compilation of data in Nebraska 

(Exner and Spalding, 1990) reported the nitrate MCL 

exceeded in 20 percent of the wells sampled. These 

studies attributed large nitrate concentrations both to 

site-specific factors (depth to water, well construction, 

age of well) and to broader geographic factors (irrigation intensity, land use, soil properties). However, 

statistical correlations between nitrate and explanatory 

variables often were poor due to the large variability in 

environments across large geographic areas and due to 

a mixed population of wells affected by point sources 

and of wells affected by nonpoint sources. A study 

restricted to nonpoint-source contamination in the 

High Plains aquifer in Nebraska (Druliner and others, 

1996) found significant relations between nitrate 

concentrations and age of well (likely indicative of 

longer histories of chemical application), irrigation 

intensity, and soil permeability. A study restricted to 

nonpoint-source contamination in the High Plains 

aquifer of Kansas (Helgesen and others, 1994) found 

land use to be a statistically significant variable relative to nitrate concentrations. Recent regional studies 

(Center for Disease Control and Prevention, 1998; 

Nolan and others, 1997) have confirmed that ground 

water in the High Plains area is susceptible to nitrate 

contamination.

Surveys of the occurrence of pesticides in 

ground water began appearing in about 1990 for 

Table 2. Summary of selected water-quality studies conducted within the High Plains study area

Reference

(Area of study in parentheses)

Environmental characteristic related to water quality

Site-specific characteristic Geographic characteristic

Well 

depth

Depth to 

water

Age

of well

Well

construction

Land 

use

Chemical 

application 

rate

Irrigation 

intensity

Soil

properties

Bedrock 

type

REGIONAL STUDIES

Center for Disease Control and 

Prevention, 1998 

(Midwestern States)

X X X

Krothe and others, 1982 

(High Plains)

X X X

Nolan and others, 1997 

(Nationwide)

X X

STATEWIDE WITHIN HIGH PLAINS

Engberg, 1984 (Nebraska) X X X

Exner and Spalding, 1990 

(Nebraska)

X X X X

Druliner and others, 1996 

(Nebraska)

X X X X X

Gosselin and others, 1997 

(Nebraska)

X X X X X X

Helgesen and others, 1994 

(Kansas)

X

Hopkins, 1993 (Texas) X

Nativ, 1988 (Texas, New Mexico) X X

Perry and others, 1988 (Kansas) X X

Reeves and Miller, 1978 (Texas) X X X

Spalding, 1991 (Nebraska) X X X

Steichen and others, 1988 

(Kansas)

X X
8 Historical Water-Quality Data for the High Plains Regional Ground-Water Study Area in Colorado, Kansas, Nebraska, New Mexico, 

Oklahoma, South Dakota, Texas, and Wyoming, 1930–98

Kansas (Steichen and others, 1988), Texas (Aurelius, 

1989), and Nebraska (Exner and Spalding, 1990). 

Pesticide studies historically have been constrained by 

analytical considerations (high cost of analysis, large 

detection limits, and lack of analytical techniques for 

pesticides of interest). These early studies generally 

reported pesticide detections in only a small proportion of sampled wells, with large concentrations of 

pesticides occurring rarely; but the documentation of 

pesticide occurrence in ground water was important 

because it showed that synthetic chemicals did migrate 

into aquifers. Pesticide properties such as water solubility, degradation half-life, and soil sorption index 

were recognized to cause the mobility of pesticides in 

the environment to differ from nitrate and from each 

other. Atrazine, one of the most heavily used pesticides, has chemical properties favorable for mobility 

in the environment; as a result, atrazine has documented occurrence rates as large as 13 percent of wells 

sampled (Exner and Spalding, 1990). However, occurrence rates in Kansas and Nebraska were found to be 

highly variable, based on regional environmental 

conditions. Given the complexity of pesticide mobility 

in the environment, many recent studies have focused 

on farm-field scale examinations of pesticide movement (see, for example, Sophocleous and others, 

1990).

Despite the smaller geographic scope of 

previous water-quality studies, the findings of these 

studies can help guide the analysis of High Plains 

retrospective data. The historical studies show that 

water quality has changed in the High Plains NAWQA 

study area; the studies also indicate which waterquality constituents are of most concern. Historical 

studies suggest that stratification of study areas into 

smaller, more homogeneous areas is useful for data 

analysis and provide insights into selection of environmental factors that may affect water quality.

SOURCES AND CHARACTERISTICS OF 

AVAILABLE WATER-QUALITY DATA

Data compiled for this retrospective analysis 

include ground-water-quality data available in digital 

format for 1930–98. A data survey was conducted 

during the summer of 1998. Employees of the USGS 

in each of the eight States within the High Plains study 

area contacted Federal, State, and local agencies to 

identify available data. As a result of this effort, 67 

ground-water-quality data sets were inventoried. 

Water-quality data sets ranged in scope from small 

spreadsheet data sets for specific projects maintained 

by local agencies to large, comprehensive data sets 

from multiple agency sources compiled by State and 

Federal agencies. The large, comprehensive data sets 

were preferentially acquired because they included 

many of the smaller agency data sets and because data 

quality was previously evaluated. Data sets were not 

acquired if they were not available in digital format.

Data-Selection Criteria and Screening 

Procedures

Minimum requirements were established for 

inclusion of data into the retrospective data base to 

ensure compatibility of data for analysis. Location of 

the well, aquifer source, sampling date, and description of each water-quality constituent were required.

Location of the well, in the form of latitude and 

longitude, was required to enable geographic analysis 

of water-quality data. The accuracy of location varied 

depending on the method of determination, and when 

known, an accuracy code was assigned to each well 

location. About 75 percent of the sites have good location accuracy (accurate within 300 feet). 

An indication of the aquifer source of the 

sampled water was required. The most straightforward 

indication of water source was an assigned aquifer for 

the well. For wells without an assigned aquifer, welldepth information was required. The aquifer source of 

sampled water was then deduced on the basis of the 

location of the well relative to the eight defined hydrogeologic unit boundaries, supplemented by comparison of well depth with geographic information system 

(GIS) information about elevation of the bottom of the 

aquifer.

For water-quality measurements, a sampling 

date was required. Measurements in the data base go 

back as far as 1930. In some cases, only the year of the 

sampling was known, and these were arbitrarily 

assigned a date of January 1 for the year of sampling. 

In other cases, only a year and month were known; 

these samples were arbitrarily assigned a sampling 

date of the 15th of the month. A time of sampling was 

not required; where a time was missing, a value of 

–9999 was coded for time. For water-quality measurements, a specific description for each constituent also 

was required so that each measurement could be 
SOURCES AND CHARACTERISTICS OF AVAILABLE WATER-QUALITY DATA 9

assigned a parameter code from the USGS parameter 

code dictionary (Garcia and others, 1997).

Additional data fields were included in the database design to provide more complete information 

about sites and water-quality measurements, but information for these fields was not required for inclusion 

of data into the data base. Additional information 

available about sites includes land-surface datum, 

primary water use, date of well completion, casing 

material, and aquifer and well openings information. 

Additional information available about water-quality 

measurements includes data quality, data project, and 

data agency. A more complete description of the tables 

and fields included in the data base is provided in the 

Appendix of this report. Detailed information also is 

provided about the design and use of the retrospective 

data base. The data base is provided as part of this 

report on CD-ROM, in both proprietary (Microsoft 

Access) and nonproprietary (tab-delimited ASCII 

files) formats.

Quality Assurance of Data

Each contributing data source is the primary 

agency providing for the quality assurance of data, and 

where available, references to these quality-assurance 

procedures are included in the data base. A dataquality-rating scheme was developed (table 3); in this 

scheme, each water-quality measurement was 

assigned a quality rating from 0 to 4, based on criteria 

about sampling procedures, field quality-assurance 

practices, and laboratory analytical methods. Similar 

schemes are used by the Nebraska Water Sciences 

Laboratory (Mary Exner, University of Nebraska, 

written commun., 1999) and the Texas Water Development Board (Nordstrom and Quincy, 1997). In some 

cases water-quality measurements were assigned a low 

data-quality rating only because information about 

field and laboratory methods used was not available.

In addition to quality-assurance checks done by 

source agencies, additional simple data-verification 

checks were made on selected data fields in the retrospective data base. Descriptive statistics for constituents were calculated to check for outliers or 

inconsistent values (for example, large negative water 

temperatures); where these types of data values were 

discovered, data were verified or excluded from the 

data base. Geographic checks were made on ancillary 

well information by using a GIS; for example, county 

codes were verified by comparison with a county 

boundary map, and sites with conflicting location 

information were excluded from the data base.

 Content of Retrospective Data Base

Eleven data sets were incorporated into the 

retrospective data base (table 4) representing data 

collected by 43 agencies. Sixty percent of the data 

were acquired from Federal agencies (USGS, USEPA, 

Center for Disease Control and Prevention, U.S. 

Department of Energy), 30 percent from State agencies (primarily the Nebraska Water Sciences Laboratory and the Texas Water Development Board), and 

about 10 percent from local agencies (notably the 

Equus Beds Groundwater Management District 

No. 2). After screening and quality assurance were 

completed, the retrospective data base contained 

29,041 water-sampling sites, 58,731 ground-waterquality samples, and 1,003,868 observations (in this 

report, the word observation is used to refer to any 

physical measurement or analytical result available for 

a sampled well). About 22,500 of the sites had only

Table 3. Criteria for rating data quality

[USEPA, U.S. Environmental Protection Agency]

Quality

rank

Data ranking criteria

Sampling procedure Field-quality assurance Laboratory-analytical method

0 Unknown Unknown Unknown

1 Grab sample None Not a USEPA-accepted method

2 Follows accepted sampling procedures None or unknown A USEPA-accepted method

3 Follows accepted sampling procedures Field duplicates or blanks A USEPA-accepted method

4 Strict sampling protocols Field duplicates and blanks A USEPA-accepted method
10 Historical Water-Quality Data for the High Plains Regional Ground-Water Study Area in Colorado, Kansas, Nebraska, New Mexico,

Oklahoma, South Dakota, Texas, and Wyoming, 1930–98

one sample collected, but 6,545 sites had more than 

one sample collected, including 716 sites that had 10 

or more samples collected. The number of observations per sample ranged from 1 to 253, with an average 

of 17. The average number of observations per sample 

ranged from 3 for sites from the Nebraska Water 

Sciences Laboratory data set (a pesticide and nitrate 

data set) to 30 for samples from the New Mexico 

Environment Department Drinking Water Bureau data 

set (a finished drinking-water data set) and from the 

Central Nebraska NAWQA study data set (a comprehensive ambient water-quality data set).

The locations of water-quality sampling sites are 

shown in figure 3. Of the 236 counties that intersect 

with or are included within the study area, 222 have at 

least one water-quality site. The largest number of 

sites within a county is 796 for Gaines County, Texas, 

which is located near the southern boundary of the 

study area. About one-half the counties in the study 

area have at least 100 water-quality-sampling sites. 

Areas with few sampling sites include the Sand Hills 

area of Nebraska and areas in Colorado, New Mexico, 

Kansas, and Oklahoma where the saturated thickness 

of the aquifer is small. Patterns in the location of 

sampling sites sometimes are indicative of specific 

projects. For example, the high density of sites in the 

Wichita, Kansas, area (east-central Kansas) is due to 

projects investigating salinity in ground water used for 

water supply. The grid-like pattern of sites in parts of 

Nebraska and Kansas is due to sampling done as part 

of the National Uranium Resource Evaluation 

(NURE) program (Smith, 1999) of the Department of 

Energy.

The retrospective data base includes results for 

733 parameters. These can be grouped according to 

constituent or measurement type, and the number of 

observations in the data base for each group for the 

years 1950 to 1998 is shown in figure 4. The most 

common parameter group is the major ions (77 parameters; 369,111 observations). This group includes the 

primary substances dissolved in water that contribute 

to dissolved solids (salinity), such as sodium, calcium, 

chloride, and sulfate. There are at least 2,000 observations for major ions for every year from 1950 to 1998. 

The second most common parameter group is physical 

measurements (28 parameters; 136,040 observations), 

which include properties of water such as dissolved 

oxygen, pH, specific conductance, water temperature, 

and suspended solids. The temporal pattern for physical measurements is similar to that of major ions but 

with fewer observations because there are fewer 

parameters in this group. The third most common 

group is trace elements (58 parameters; 116,072 observations). However, the temporal pattern of traceelement observations is different than the pattern for 

major ions and physical measurements; most observations occurred after 1960 and during only a few years, 

in particular during 1977–79, when data were 

collected as part of the NURE program. Data for the

Table 4. Data sources for the retrospective data base

[NAWQA, National Water-Quality Assessment Program; USEPA, U.S. Environmental Protection Agency]

Source of data Number of 

agencies

Number of

sites

Number of 

samples

Number of

observations

Center for Disease Control and Prevention 1 553 553 2,212

Colorado Department of Public Health and Environment 1 210 210 8,494

Equus Beds Groundwater Management District No. 2 1 342 3,483 30,268

Nebraska Water Sciences Laboratory 17 6,302 11,967 41,467

New Mexico Environment Department Drinking Water Bureau 1 234 2,386 70,484

Oklahoma Department of Environmental Quality 1 212 347 10,766

Texas Water Development Board 13 6,399 9,612 206,869

U.S. Geological Survey National Water Information System 1 8,453 23,284 468,894

U.S. Geological Survey, Central Nebraska NAWQA Study 4 1,254 1,263 38,008

U.S. Department of Energy by way of U.S. Geological Survey, 

Geologic Division

1 3,865 3,865 82,662

USEPA Storage and Retrieval (STORET) data base 2 1,217 1,761 43,744

TOTAL 43 29,041 58,731 1,003,868
SOURCES AND CHARACTERISTICS OF AVAILABLE WATER-QUALITY DATA 11

nutrients group, which includes nitrate, have generally 

increased from about 1960 to the present, except for 

the large number of observations in the late 1970’s. 

The pesticides and volatile organic compounds groups 

include the largest number of parameters and have 

similar numbers of total observations (about 90,000); 

however, few data were available for these groups 

prior to 1985. Few data exist for radiological parameters, probably because few laboratories do these analyses and the cost is high. Few data exist for bacteriological parameters in the data base. This group 

includes coliform bacteria counts, which commonly 

are determined by public health department laboratories for domestic water-supply wells. However, these 

data were not incorporated into the retrospective data

Figure 3. Location of sampling sites in the High Plains retrospective data base.

EXPLANATION

Sampling site

0

0 50 100 150 200 KILOMETERS

105°

40°

32°

100°

50 100 150 200 MILES
12 Historical Water-Quality Data for the High Plains Regional Ground-Water Study Area in Colorado, Kansas, Nebraska, New Mexico,

Oklahoma, South Dakota, Texas, and Wyoming, 1930–98

1950 1960 1970 1980 1990 2000

0

5,000

10,000

15,000

1950 1960 1970 1980 1990 2000

0

5,000

10,000

15,000

NUMBER OF OBSERVATIONS

NUMBER OF OBSERVATIONS

1950 1960 

0

5,000

10,000

15,000

1950 1960 1970 1980 1990 2000

0

5,000

10,00015,000

1950 1960 

0

5,000

10,000

15,000

1950 1960 1970 1980 1990 2000

0

5,000

10,00015,000

NUMBER OF OBSERVATIONS 1950 1960 1970 1980 1990 2000

0

5,000

10,000

15,000

1950 1960 1970 1980 1990 2000

0

5,000

10,00015,000

MAJOR IONS(77 parameters)NUTRIENTS(20 parameters)RADIOLOGICAL(45 parameters)

PHYSICAL MEASUREMENTS(28 parameters)PESTICIDES(248 parameters)

TRACE ELEM(58 parameteVOLATILE OR(199 paramet

BACTERIOLOGICAL(7 parameters)

NOTE: Observations iare not shown. Data fo

Figure 4. Temporal distribution of water-quality observations for selected parameter groups, 1950–98.
SOURCES AND CHARACTERISTICS OF AVAILABLE WATER-QUALITY DATA 13

base because ancillary information about the sampled 

well generally was not available.

The geographic distribution of water-quality 

observations varies among parameter groups (fig. 5). 

For example, nutrient data are available throughout the 

study area, but pesticide data exist primarily for 

Nebraska. Nutrient data generally consist of fewer 

than 10 observations at a site and often consist of a 

single nitrate observation. In contrast, pesticide data 

generally consist of more than 10 observations at a site 

because pesticides commonly are analyzed in the laboratory as a suite of compounds. For trace-element data, 

observations are distributed throughout the study area, 

but sites with 10 or more observations are in areas 

where NURE data are available. Observations for 

volatile organic compounds are sparse in the study 

area and generally are clustered in areas where local 

studies have been done or where drinking-water monitoring programs exist.

Site Characterization

Ancillary information about the sampled well 

sites is included in the data base. In some cases, site 

information is available that was gathered onsite at the 

time of sampling or that was acquired by cross reference with well-permit files. This site-specific information generally is the most accurate type of ancillary 

information, but often is unavailable. The number of 

sites having various types of site-specific ancillary 

information is shown in figure 6. Well depth information was available for 94 percent of the wells; depths 

ranged from less than 10 feet to 1,703 feet and had a 

median value of 160 feet. Well-type information was 

available for 88 percent of the wells; among these, 

93 percent were installed for water withdrawal and 

only 7 percent (1,772 wells) were installed as observation wells. Water-use-type information was available 

for 90 percent of the wells; among these, 43 percent 

were used for irrigation, 28 percent were used for 

domestic water supply, 11 percent were unused, 9 

percent were used for livestock, 7 percent were used 

for public supply, and 2 percent were used for other 

water uses. About one-third of the sites had information about the age of the well; completion dates ranged 

from the year 1850 to 1997. Only about 22 percent of 

the wells had information about screened interval; 

therefore, for most water-quality observations there is 

a lack of information about the precise depth of 

sampled water.

Two important well characteristics are the 

aquifer source of water and depth to water, but only 57 

percent of the wells had aquifer information and only 

33 percent of the wells had a measured depth to water. 

Ancillary map information was therefore used to 

provide a more complete data base. Although the High 

Plains aquifer is the aquifer source, individual hydrogeologic units were assigned to the aquifer data field 

in the data base. Well-site locations were compared to 

a map of hydrogeologic units (fig. 2) to assign units to 

wells that had no site-specific information; this 

process was automated using an overlay process 

within a GIS. Assignment of units using this process is 

reasonable where one unit does not underlie another 

unit. However, in some parts of the study area the 

Valley-Fill Alluvium unit or the Nebraska Sand Hills 

unit is underlain by Ogallala Formation units. Wells in 

these areas were assumed to be completed in the surficial unit. About half of the wells located in the ValleyFill Alluvium area (fig. 2) were assigned to the ValleyFill Alluvium unit in this way; the median depth of 

these wells was 60 ft. About two-thirds of the wells in 

the Nebraska Sand Hills area were assigned to the 

Nebraska Sand Hills unit in this way; the median 

depth of these wells was 135 ft. A GIS process also 

was used to check whether wells were completed at a 

level deeper than the bottom of the High Plains geohydrologic units. A depth-to-bedrock surface was generated by the GIS, using data for wells from the High 

Plains Water-Level Monitoring Network (Sharon Qi, 

USGS, written commun., 1999). Wells whose depths 

were substantially greater than the depth-to-bedrocksurface value were assigned to bedrock aquifers and 

excluded from the water-quality data analysis.

Using similar GIS techniques, depth to water 

was assigned to the 67 percent of wells that had no 

site-specific information. A depth-to-water surface 

(fig. 7) was generated in the GIS by using 1997 waterlevel information for wells from the High Plains 

Water-Level Monitoring Network (Sharon Qi, U.S. 

Geological Survey, written commun., 1999). To verify 

the accuracy of this method, site-specific depth-towater information (available for 33 percent of the 

wells) was compared with the 1997 GIS depth-towater surface; the median difference between these 

values was 22 percent. Depths to water estimated in 

this way ranged from less than 10 feet to 495 feet, with 

a median value of 67 feet.
14 Historical Water-Quality Data for the High Plains Regional Ground-Water Study Area in Colorado, Kansas, Nebraska, New Mexico,

Oklahoma, South Dakota, Texas, and Wyoming, 1930–98

10 or more

Less than 10

Number of observations

EXPLANATION

NUTRIENTS PESTICIDES

105°

40°

32°

100°

105°

40°

32°

100°

00 50 100 150 200 KILOMETERS

50 100 150 200 MILES

Figure 5. Geographic distribution of observations for nutrients, pesticides, trace elements, and volatile organic compounds.
SOURCES AND CHARACTERISTICS OF AVAILABLE WATER-QUALITY DATA 15 00 50 100 150 200 KILOMETERS

105°

40°

32°

100°

105°

40°

32°

50 100 150 200 MILES

10 or more

Less than 10

EXPLANATION

Number of observations

TRACE ELEMENTS VOLATILE ORGANIC COMPOUNDS

Figure 5. Geographic distribution of observations for nutrients, pesticides, trace elements, and volatile organic compounds—Continued.
16 Historical Water-Quality Data for the High Plains Regional Ground-Water Study Area in Colorado, Kansas, Nebraska, New 

Mexico, Oklahoma, South Dakota, Texas, and Wyoming, 1930–98

Characteristics determined by previous studies 

to be important to water quality (see tables 2 and 5) 

were assigned to wells by using GIS map information. 

Land use was mapped for the United States during the 

early 1980’s at a resolution of about 10 acres for agricultural land (Fegeas and others, 1983) and remains 

the only detailed nationwide data set for land use for 

the United States. These land-use data, called GIRAS 

(Geographic Information Retrieval and Analysis 

System) data, were available for most of the study 

area, and 75 percent of the wells were assigned a land 

use by using this data set; among these wells, 73 

percent were in agricultural land use, 19 percent were 

in rangeland, 6 percent were in urban land use, and 

2 percent were in other land uses. Irrigation is an especially relevant part of agricultural land use, and the 

location of irrigated lands in the study area was most 

recently determined in 1980 as part of the USGS High 

Plains Regional Aquifer-System Analysis (RASA) 

project (Thelin and Heimes, 1987), in which irrigation 

was reported as the percentage of irrigated land in 

1-square-mile cells across the study area (fig. 8); the 

total irrigated area is 22,800 square miles (14.5 million 

acres). Irrigation intensity was assigned to retrospective wells by use of this RASA data set, and values 

ranged from no irrigated lands at a well to 99 percent 

irrigated land, with a median value of 19 percent irrigated land.

Agricultural land use is an important source of 

nitrogen to the land surface, with inputs from manure 

and fertilizer applications (fig. 9). County manure 

nitrogen-application rates can be estimated (Puckett 

and others, 1998) based on agricultural census counts 

of the number of animals in each county. Manure is 

generally applied to the land surface within 50 miles 

of where it is generated due to transportation costs. 

Manure application rates are relatively uniform 

because farm animals are generally widespread across 

the High Plains; counties with the largest rates generally are those where large cattle feedlots are located. 

County fertilizer nitrogen-application rates have been 

estimated (David Lorenz, U.S. Geological Survey, 

written commun., 1999) based on agricultural census 

data about fertilizer sales. In the High Plains study 

area, fertilizer nitrogen application rates are larger 

than manure nitrogen application rates. Fertilizer is 

applied to large contiguous areas in the High Plains 

study area, which relate to areas of high irrigation 

intensity. Fertilizer application rates are large in areas 

where corn is grown because corn has the largest 

nitrogen requirement among crops grown in the High 

Plains. To construct a variable representative of these 

nitrogen inputs, the manure and fertilizer input rates 

for each county were summed, and this total input rate 

was assigned to each sampled well in the county. Total 

nitrogen application rates among sites in the retrospective data base ranged from 1.2 pounds per acre to 142 

pounds per acre, with a median value of 46 pounds per 

acre. These rates are normalized to total county area 

rather than county crop area, so the application rate on 

fields may be several times larger, depending on the 

percentage of the county that is cultivated.

Soil type is an important part of site characterization because soil properties can affect the rates at 

which chemicals move though soils into the underlying aquifers. A nationwide soils data set (called 

STATSGO) from the U.S. Department of Agriculture 

(U.S. Department of Agriculture, 1991) includes a 

variable called Soil Hydrologic Group, which is a 

measure of infiltration rates through soils and drainage 

characteristics. To characterize retrospective data sites, 

the Soil Hydrologic Group variable was transformed 

into an area-weighted numeric variable ranging in 

value from 1 to 4 where 1 represents soils with low 

infiltration rates and 4 represents soils with high infiltration rates. Among retrospective sites, the infiltration 

variable ranged from 1.0 to 4.0 with a median value of 

2.94, which indicates that High Plains soils generally 

have moderately high infiltration rates. The 

geographic distribution of infiltration rates is shown in 

figure 10; for this figure, an infiltration variable value 

of 1 to 2 is classified as low, a value from 2.01 to 3 is 

classified as medium, and a value from 3.01 to 4 is 

classified as high.

Figure 6. Number of sites with selected site-specific 

information.

0 5,000 10,000 15,000

NUMBER OF SITES

20,000 25,000 30,000

Screened interval

Casing material

Depth to water

Age of well

Aquifer

Well type

Water use

Well depth

Well location
SOURCES AND CHARACTERISTICS OF AVAILABLE WATER-QUALITY DATA 17

0

0 50 100 150 200 KILOMETERS

50 100 150 200 MILES

Less than 10

10 to 100

100 to 250

More than 250

No data 

EXPLANATION

Depth to water, in feet

105°

40°

37°

32°

100°

Figure 7. Depth to water during 1997 in the High Plains study area.
18 Historical Water-Quality Data for the High Plains Regional Ground-Water Study Area in Colorado, Kansas, Nebraska, New Mexico,

Oklahoma, South Dakota, Texas, and Wyoming, 1930–98

Underlying bedrock type is an important site 

characteristic because water from underlying bedrock 

can migrate into High Plains aquifers and affect water 

quality. Information on bedrock age is available from 

maps published by the High Plains RASA project 

(Weeks and Gutentag, 1981). Bedrock age ranges from 

Permian to Tertiary, with older rocks generally in the 

southern half of the study area. Detailed information 

has not yet been compiled about the soluble-mineral 

content of the many bedrock units that underlie the 

study area, but in general the older rocks contain more 

soluble minerals than the younger rocks (Krothe and 

others, 1982). The older rocks generally contain water 

of moderate to high salinity, in particular the Permianage rocks in central Kansas and in the Texas and Oklahoma panhandles, and the Lower Cretaceous-age 

rocks in Texas at the southern end of the study area 

(Krothe and others, 1982). Salinity (dissolved-solids 

concentration) as large as 33,800 mg/L has been 

reported in water from Permian-age rocks in southwest Kansas (Gutentag and others, 1980). Among 

retrospective sites, about 70 percent are underlain by 

bedrock containing lower salinity water, while the 

remaining 30 percent are underlain by Permian- and 

Lower Cretaceous-age bedrock containing higher 

salinity water.

DATA-ANALYSIS METHODS

This report contains a descriptive summary of 

historical water-quality data. The extent of data analysis for individual constituents varies with the amount 

of data available and with the relevance of the individual constituent to human health. Nonparametric 

descriptive statistics are presented for all the common 

or relevant constituents represented in the data base. 

Data for a smaller subset of constituents are compared 

with drinking-water regulatory limits. Finally, data for 

a few constituents historically of primary concern 

(nitrate, pesticides, dissolved solids) are analyzed in 

more detail.

Methods used for selecting data from the data 

base vary depending on the type of data analysis. For 

each data analysis presented in this report, the selection criteria are listed. Data are selected on the basis of 

date of sample, parameter code, number of samples 

per well, location of site, or data quality. The date of 

sample is relevant depending on whether historical 

information or recent information is needed. Parameter code is important because for some constituents 

there are numerous parameter codes which may not be 

equivalent. For example, there are six parameter codes 

for nitrate in the data base (table 6), and for a general 

descriptive analysis these can be aggregated (after 

adjustment of reporting units) using the assumption 

that nitrate concentrations in the solid phase are small 

and nitrite concentrations are negligible. Multiple 

observations from a single well might be used in a data 

analysis of temporal variation of a constituent, but 

only the most recent observation might be used if an 

average concentration for the aquifer is being calculated. Site location might also be considered when 

summarizing aquifer water quality to ensure that 

multiple wells clustered in a small location are not 

overrepresented relative to the entire aquifer. Finally, 

observations with a poorer data-quality rating might 

be included when doing descriptive nonparametric 

statistics (for which outliers are given less weight) but 

not included when doing a comparison of concentrations with a water-quality limit (for which individual 

observations exceeding a limit are important). Dataselection criteria are documented and discussed for 

each of the data analyses presented in this report.

Table 5. Geographic characteristics assigned to sites by using a geographic information system

Geographic

characteristic Data source type Reference

Hydrogeologic unit 1:500,000 scale map Weeks and others (1988).

Depth to water in 1997 Interpolated surface Sharon Qi, U.S. Geological Survey, written commun., 1999.

Land use 1:250,000 scale map Fegeas and others (1983).

Irrigation intensity LANDSAT imagery Thelin and Heimes (1987).

Nitrogen application rate County data Fertilizer: David Lorenz, U.S. Geological Survey, written commun., 1999; 

manure: Puckett, and others (1998).

Soil infiltration rate 1:250,000 scale map U.S. Department of Agriculture (1991).

Underlying bedrock 1:1,000,000 scale map Weeks and Gutentag (1981).
DATA-ANALYSIS METHODS 19

0

0 50 100 150 200 KILOMETERS

105°

40°

32°

100°

50 100 150 200 MILES

Less than 10

10 to 30

31 to 60

More than 60

No data

EXPLANATION

Percentage of 1-square-mile

 cell that is irrigated

Figure 8. Percentage of land irrigated during 1980 in the High Plains study area.
20 Historical Water-Quality Data for the High Plains Regional Ground-Water Study Area in Colorado, Kansas, Nebraska, New Mexico,

Oklahoma, South Dakota, Texas, and Wyoming, 1930–98

Less than 55 to 25More than 25

EXPLANATION

Nitrogen application rate, in pounds per acre

0

0 50 100 150 200 KILOMETERS

105 

40 

32 

100 

105 

40 

32 

100 

50 100 150 200 MILES

MANURE FERTILIZER

Figure 9. Nitrogen application rates for manure and fertilizer in the High Plains study area, by county, 1992.
DATA-ANALYSIS METHODS 21
22 Historical Water-Quality Data for the High Plains Regional Ground-Water Study Area in Colorado, Kansas, Nebraska, New 

Mexico, Oklahoma, South Dakota, Texas, and Wyoming, 1930–98

Statistical techniques used for analyses in this 

report are nonparametric techniques that are suitable 

for environmental data because environmental data 

generally are not normally distributed. Nonparametric 

techniques also are suitable for data with varying data 

quality because less weight is given to outliers. 

Nonparametric techniques used for this report include 

boxplots, which show the distribution of data; the 

Kruskal-Wallis (Iman and Conover, 1983),Wilcoxon 

rank-sum test (Wilcoxon, 1945), and Tukey multiple 

comparison (Helsel and Hirsch, 1992) tests, which are 

used to detect differences between groups of data; 

Lowess smooths (Helsel and Hirsch, 1992), which 

graphically depict relations between two variables; 

and rank correlation and regression tests, which 

measure relations between multiple variables (Helsel 

and Hirsch, 1992; SAS Institute, 1990). An alpha 

value of 0.05 was used to evaluate the significance of 

test results, and in some cases the p-value is also 

reported to provide information on the level of significance of the statistical test.

ANALYSIS OF HISTORICAL WATERQUALITY DATA

From the 733 parameters contained in the

retrospective data base, 48 common or relevant 

constituents were selected for summarization by 

hydrogeologic unit (table 7). For this table, data were 

selected for the period 1980–98 so as to be representative of more recent conditions. Multiple parameter 

codes representing constituents were aggregated, 

including both filtered- and unfiltered-sample results, 

except for trace elements, where only filtered-sample 

results were selected. For some parameter groups, 

such as pesticides, trace elements, and volatile organic 

compounds, multiple detection limits were present in 

the data base. For these constituents, the most 

common detection limit was selected; observations 

less than this limit were coded to be nondetections, 

and nondetections at larger detection limits were not 

used in the analysis. Where wells had multiple observations for a constituent, only the most recent observation was used. To evaluate the geographic 

representation of data within each hydrogeologic unit, 

a grid of cells 10 miles square was overlaid on the 

study area, and the proportion of cells in each hydrogeologic unit that had data for each constituent was 

calculated; this proportion is reported in table 7 in the 

column labeled “Percentage of unit represented.” 

Results in table 7 give a general picture of water 

quality in the study area. Variability between hydrogeologic units is commonly quite large, which underscores the need to stratify the High Plains aquifer into 

subunits when describing water quality. The table indicates that data are lacking for many constituents and 

hydrogeologic units, which validates the need for 

aquiferwide representative water-quality sampling to 

better characterize water quality in the aquifer.

Water quality can be evaluated by comparing 

constituent concentrations to USEPA drinking-water 

standards (table 8). These standards are relevant 

because High Plains aquifer water is used as a 

drinking-water source for 82 percent of the people 

living within the study area. Water-quality standards 

generally exist for constituents that have been identified as being a concern for human health. Constituents 

of concern also have been identified as part of the 

High Plains Regional Ground Water Study by polling 

interested agencies within the High Plains study area; 

constituent groups of concern, ranked by number of 

replies, were: nutrients, trace elements, pesticides, 

bacteria, radiological parameters, dissolved solids 

(salinity), and volatile organic compounds. To 

compare data from the retrospective data base to 

water-quality standards, data were extracted from the 

retrospective data base that had a data quality rating of 

at least 2.

Within this selected data set, 16 percent of all 

nitrate observations were larger than the drinkingwater standard of 10 mg/L. Nitrite is the only other 

nutrient with a drinking-water standard, but nitrite 

observations in the data set did not exceed the standard 

of 1 mg/L. Only four pesticides (atrazine, alachlor, 

Table 6. Parameter codes for nitrate

Parameter 

code1 Parameter name

P00618 Nitrate, water, filtered, as nitrogen, milligrams per liter.

P00620 Nitrate, water, unfiltered, as nitrogen, milligrams per liter.

P00630 Nitrite plus nitrate, water, unfiltered, as 

nitrogen, milligrams per liter.

P00631 Nitrite plus nitrate, water, filtered, as 

nitrogen, milligrams per liter.

P71850 Nitrate nitrogen, total, as nitrate, milligrams per liter.

P71851 Nitrogen, nitrate, dissolved, as nitrate, 

milligrams per liter.

1Parameter codes are from U.S. Geological Survey National 

Water Information System.
ANALYSIS OF HISTORICAL WATER-QUALITY DATA 23

Table 7. Descriptive statistics for selected constituents or properties, by hydrogeologic unit, 1980–98

 [--, data not available; <, less than]

Constituent or property and

hydrogeologic unit1

Percentile value Number 

of

observations

Percentage 

of unit 

represented 

(see p. 22)

10th 25th 50th 75th 90th

PHYSICAL MEASUREMENTS

Specific conductance, in microsiemens per centimeter, at 25o Celsius

Ogallala Formation—North 292 339 468 590 776 626 70

Ogallala Formation—Central 407 461 526 620 869 1,287 58

Ogallala Formation—South 540 622 906 1,610 3,000 2,134 80

Arikaree-Brule Formations 310 365 540 828 1,070 414 37

Nebraska Sand Hills 97 142 200 381 515 286 38

Kansas Quaternary Deposits 270 350 526 832 1,970 267 63

Nebraska Quaternary Deposits 410 500 600 731 935 776 99

Valley-Fill Alluvium 514 719 1,000 1,390 2,510 1,143 91

pH, in standard units

Ogallala Formation—North 7.0 7.2 7.4 7.7 7.9 691 72

Ogallala Formation—Central 7.2 7.4 7.6 7.9 8.2 1,327 63

Ogallala Formation—South 7.2 7.4 7.7 8.1 8.3 2,185 80

Arikaree-Brule Formations 7.3 7.4 7.6 7.9 8.1 454 40

Nebraska Sand Hills 6.6 7.0 7.3 7.6 7.8 287 38

Kansas Quaternary Deposits 6.8 7.1 7.4 7.7 8.2 204 61

Nebraska Quaternary Deposits 6.8 6.9 7.1 7.3 7.5 809 99

Valley-Fill Alluvium 6.9 7.1 7.4 7.8 8.2 1,030 92

Dissolved oxygen, in milligrams per liter

Ogallala Formation—North 0.2 2.9 5.4 7.2 8.6 72 13

Ogallala Formation—Central -- -- -- -- -- 6 1

Ogallala Formation—South -- -- -- -- -- 0 0

Arikaree-Brule Formations 1.2 4.2 6.1 7.5 9.7 270 21

Nebraska Sand Hills 0.3 0.4 1.8 5.8 9.2 90 8

Nebraska Quaternary Deposits 0.4 2.2 5.9 7.8 9.0 284 52

Kansas Quaternary Deposits -- -- -- -- -- 0 0

Valley-Fill Alluvium 0.1 0.2 1.7 5.0 6.7 310 47

Water temperature, in degrees Celsius

Ogallala Formation—North 12.5 13.5 14.9 15.5 17.0 509 65

Ogallala Formation—Central 16.0 17.0 18.0 19.0 20.0 949 57

Ogallala Formation—South 17.0 18.0 19.0 20.0 21.0 1,190 75

Arikaree-Brule Formations 12.0 12.5 13.5 15.0 16.5 380 35

Nebraska Sand Hills 11.5 12.0 12.5 13.2 14.0 260 36

Kansas Quaternary Deposits 14.4 15.0 15.5 16.5 19.3 156 61

Nebraska Quaternary Deposits 12.0 12.5 13.0 14.0 15.0 796 99

Valley-Fill Alluvium 12.0 12.5 13.5 15.0 16.5 744 91

NUTRIENTS

Nitrite, in milligrams per liter as N

Ogallala Formation—North <0.01 <0.01 <0.01 <0.01 <0.01 56 11
24 Historical Water-Quality Data for the High Plains Regional Ground-Water Study Area in Colorado, Kansas, Nebraska, New 

Mexico, Oklahoma, South Dakota, Texas, and Wyoming, 1930–98

NUTRIENTS—Continued

Ogallala Formation—Central <0.01 <0.01 <0.01 <0.01 <0.01 181 20

Ogallala Formation—South <0.01 <0.01 <0.01 <0.01 <0.01 226 47

Arikaree-Brule Formations <0.01 <0.01 <0.01 <0.01 <0.01 152 12

Nebraska Sand Hills <0.01 <0.01 <0.01 <0.01 <0.01 19 5

Kansas Quaternary Deposits <0.01 <0.01 <0.01 0.012 0.020 10 3

Nebraska Quaternary Deposits <0.01 <0.01 <0.01 <0.01 <0.01 62 51

Valley-Fill Alluvium <0.01 <0.01 <0.01 <0.01 0.030 264 38

Nitrite plus nitrate, in milligrams per liter as N

Ogallala Formation—North 1.1 1.9 2.8 4.6 8.9 1,787 91

Ogallala Formation—Central 0.6 1.3 2.1 3.2 5.6 1,468 65

Ogallala Formation—South 0.6 1.2 2.5 5.4 11.1 2,249 80

Arikaree-Brule Formations 0.8 1.6 3.6 7.1 12.0 939 59

Nebraska Sand Hills 0.3 0.9 2.4 7.8 15.1 840 82

Kansas Quaternary Deposits 0.1 2.2 5.4 9.3 15.7 302 67

Nebraska Quaternary Deposits 0.5 1.9 4.3 7.8 12.9 3,325 100

Valley-Fill Alluvium 0.1 0.8 4.0 10.6 20.0 2,544 100

Ammonia, in milligrams per liter as N

Ogallala Formation—North <0.010 <0.010 0.015 0.045 0.244 93 16

Ogallala Formation—Central <0.010 <0.010 0.020 0.020 0.030 353 36

Ogallala Formation—South <0.010 <0.010 <0.010 0.020 0.140 435 54

Arikaree-Brule Formations <0.010 0.015 0.015 0.027 0.055 183 15

Nebraska Sand Hills <0.010 <0.010 <0.010 <0.010 0.040 39 7

Kansas Quaternary Deposits <0.010 <0.010 <0.010 0.063 0.138 38 28

Nebraska Quaternary Deposits <0.010 <0.010 0.020 0.090 0.542 63 51

Valley-Fill Alluvium <0.010 0.015 0.020 0.044 0.130 364 51

Ammonia plus organic nitrogen, in milligrams per liter as N

Ogallala Formation—North <0.20 <0.20 0.40 0.50 0.60 29 9

Ogallala Formation—Central <0.20 <0.20 <0.20 <0.20 <0.20 306 23

Ogallala Formation—South <0.20 <0.20 <0.20 0.24 0.50 494 55

Arikaree-Brule Formations <0.20 <0.20 0.22 0.35 0.52 64 4

Nebraska Sand Hills 0.24 0.30 0.50 0.60 2.28 43 7

Kansas Quaternary Deposits -- -- -- -- -- -- --

Nebraska Quaternary Deposits <0.20 0.25 0.40 0.60 1.22 13 11

Valley-Fill Alluvium <0.20 <0.20 0.40 0.80 1.30 118 22

Phosphorus, in milligrams per liter as P

Ogallala Formation—North <0.01 <0.01 0.02 0.05 0.12 68 13

Ogallala Formation—Central <0.01 <0.01 <0.01 0.03 0.09 139 24

Ogallala Formation—South -- -- -- -- -- -- --

Arikaree-Brule Formations <0.01 <0.01 0.02 0.05 0.07 78 12

Nebraska Sand Hills 0.03 0.05 0.09 0.14 0.21 57 8

Kansas Quaternary Deposits <0.01 0.04 0.06 0.09 0.13 59 39

Nebraska Quaternary Deposits -- -- -- -- -- 5 6

Valley-Fill Alluvium 0.02 0.03 0.06 0.14 0.28 196 47

Table 7. Descriptive statistics for selected constituents or properties, by hydrogeologic unit, 1980–98—Continued

 [--, data not available; <, less than]

Constituent or property and

hydrogeologic unit1

Percentile value Number 

of

observations

Percentage 

of unit 

represented 

(see p. 22)

10th 25th 50th 75th 90th
ANALYSIS OF HISTORICAL WATER-QUALITY DATA 25

NUTRIENTS—Continued

Orthophosphorus, in milligrams per liter as P

Ogallala Formation—North <0.01 <0.01 0.05 0.12 0.19 71 15

Ogallala Formation—Central <0.01 <0.01 <0.01 <0.01 0.06 191 28

Ogallala Formation—South <0.01 <0.01 0.07 0.12 0.16 186 41

Arikaree-Brule Formations <0.01 0.02 0.03 0.05 0.20 92 11

Nebraska Sand Hills <0.01 0.08 0.12 0.17 0.31 14 5

Kansas Quaternary Deposits <0.01 0.03 0.05 0.08 0.12 97 43

Nebraska Quaternary Deposits 0.02 0.06 0.17 0.23 0.30 63 51

Valley-Fill Alluvium <0.01 0.02 0.04 0.08 0.16 312 47

Organic carbon, in milligrams per liter as C

Ogallala Formation—North 0.2 0.4 0.7 4.5 7.4 16 5

Ogallala Formation—Central 0.2 0.3 0.5 1.3 4.3 16 4

Ogallala Formation—South -- -- -- -- -- -- --

Arikaree-Brule Formations -- -- -- -- -- 2 1

Nebraska Sand Hills 0.6 0.9 1.3 2.2 3.0 125 9

Kansas Quaternary Deposits 0.3 0.4 0.9 2.4 11.0 25 20

Nebraska Quaternary Deposits -- -- -- -- -- -- --

Valley-Fill Alluvium 0.7 1.1 1.7 2.5 3.8 110 27

PESTICIDES

Alachlor, in micrograms per liter

Ogallala Formation—North <0.5 <0.5 <0.5 <0.5 <0.5 534 60

Ogallala Formation—Central <0.5 <0.5 <0.5 <0.5 <0.5 85 15

Ogallala Formation—South <0.5 <0.5 <0.5 <0.5 <0.5 216 20

Arikaree-Brule Formations <0.5 <0.5 <0.5 <0.5 <0.5 115 14

Nebraska Sand Hills <0.5 <0.5 <0.5 <0.5 <0.5 293 63

Kansas Quaternary Deposits <0.5 <0.5 <0.5 <0.5 <0.5 81 38

Nebraska Quaternary Deposits <0.5 <0.5 <0.5 <0.5 <0.5 576 100

Valley-Fill Alluvium <0.5 <0.5 <0.5 <0.5 <0.5 528 84

Atrazine, in micrograms per liter

Ogallala Formation—North <0.1 <0.1 <0.1 <0.1 <0.1 540 57

Ogallala Formation—Central <0.1 <0.1 <0.1 0.7 1.6 26 4

Ogallala Formation—South -- -- -- -- -- 5 1

Arikaree-Brule Formations <0.1 <0.1 <0.1 <0.1 <0.1 127 17

Nebraska Sand Hills <0.1 <0.1 <0.1 <0.1 <0.1 362 63

Kansas Quaternary Deposits <0.1 <0.1 <0.1 <0.1 <0.1 74 32

Nebraska Quaternary Deposits <0.1 <0.1 <0.1 <0.1 0.3 637 100

Valley-Fill Alluvium <0.1 <0.1 <0.1 0.3 1.2 574 77

Carbaryl, in micrograms per liter

Ogallala Formation—North <2.5 <2.5 <2.5 <2.5 <2.5 474 50

Ogallala Formation—Central -- -- -- -- -- 9 1

Ogallala Formation—South -- -- -- -- -- -- --

Arikaree-Brule Formations <2.5 <2.5 <2.5 <2.5 <2.5 59 11

Table 7. Descriptive statistics for selected constituents or properties, by hydrogeologic unit, 1980–98—Continued

 [--, data not available; <, less than]

Constituent or property and

hydrogeologic unit1

Percentile value Number 

of

observations

Percentage 

of unit 

represented 

(see p. 22)

10th 25th 50th 75th 90th
26 Historical Water-Quality Data for the High Plains Regional Ground-Water Study Area in Colorado, Kansas, Nebraska, New 

Mexico, Oklahoma, South Dakota, Texas, and Wyoming, 1930–98

PESTICIDES—Continued

Nebraska Sand Hills <2.5 <2.5 <2.5 <2.5 <2.5 273 61

Kansas Quaternary Deposits -- -- -- -- -- 1 1

Nebraska Quaternary Deposits <2.5 <2.5 <2.5 <2.5 <2.5 532 100

Valley-Fill Alluvium <2.5 <2.5 <2.5 <2.5 <2.5 326 66

Carbofuran, in micrograms per liter

Ogallala Formation—North <1.0 <1.0 <1.0 <1.0 <1.0 482 54

Ogallala Formation—Central <1.0 <1.0 <1.0 <1.0 <1.0 59 14

Ogallala Formation—South <1.0 <1.0 <1.0 <1.0 <1.0 171 8

Arikaree-Brule Formations <1.0 <1.0 <1.0 <1.0 <1.0 60 11

Nebraska Sand Hills <1.0 <1.0 <1.0 <1.0 <1.0 256 60

Kansas Quaternary Deposits -- -- -- -- -- 6 6

Nebraska Quaternary Deposits <1.0 <1.0 <1.0 <1.0 <1.0 432 100

Valley-Fill Alluvium <1.0 <1.0 <1.0 <1.0 <1.0 319 73

Cyanazine, in micrograms per liter

Ogallala Formation—North <1.0 <1.0 <1.0 <1.0 <1.0 538 56

Ogallala Formation—Central <1.0 <1.0 <1.0 <1.0 <1.0 18 2

Ogallala Formation—South -- -- -- -- -- 3 1

Arikaree-Brule Formations <1.0 <1.0 <1.0 <1.0 <1.0 125 16

Nebraska Sand Hills <1.0 <1.0 <1.0 <1.0 <1.0 330 63

Kansas Quaternary Deposits <1.0 <1.0 <1.0 <1.0 <1.0 74 32

Nebraska Quaternary Deposits <1.0 <1.0 <1.0 <1.0 <1.0 596 100

Valley-Fill Alluvium <1.0 <1.0 <1.0 <1.0 <1.0 536 73

Chlorpyrifos, in micrograms per liter

Ogallala Formation—North <0.5 <0.5 <0.5 <0.5 0.9 462 43

Ogallala Formation—Central -- -- -- -- -- -- --

Ogallala Formation—South -- -- -- -- -- -- --

Arikaree-Brule Formations <0.5 <0.5 0.9 0.9 0.9 123 25

Nebraska Sand Hills <0.5 <0.5 <0.5 <0.5 <0.5 276 66

Kansas Quaternary Deposits -- -- -- -- -- -- --

Nebraska Quaternary Deposits <0.5 <0.5 <0.5 <0.5 <0.5 595 100

Valley-Fill Alluvium <0.5 <0.5 <0.5 <0.5 0.9 368 71

Fonofos, in micrograms per liter

Ogallala Formation—North <0.2 <0.2 <0.2 <0.2 0.4 443 43

Ogallala Formation—Central -- -- -- -- -- -- --

Ogallala Formation—South -- -- -- -- -- -- --

Arikaree-Brule Formations <0.2 <0.2 <0.2 <0.2 <0.2 124 25

Nebraska Sand Hills <0.2 <0.2 <0.2 <0.2 <0.2 278 66

Kansas Quaternary Deposits -- -- -- -- -- -- --

Nebraska Quaternary Deposits <0.2 <0.2 <0.2 <0.2 <0.2 616 100

Valley-Fill Alluvium <0.2 <0.2 <0.2 <0.2 <0.2 375 72

Metolachlor, in micrograms per liter

Ogallala Formation—North <0.2 <0.2 <0.2 <0.2 <0.2 143 21

Table 7. Descriptive statistics for selected constituents or properties, by hydrogeologic unit, 1980–98—Continued

 [--, data not available; <, less than]

Constituent or property and

hydrogeologic unit1

Percentile value Number 

of

observations

Percentage 

of unit 

represented 

(see p. 22)

10th 25th 50th 75th 90th
ANALYSIS OF HISTORICAL WATER-QUALITY DATA 27

PESTICIDES—Continued

Ogallala Formation—Central <0.2 <0.2 <0.2 <0.2 0.4 20 3

Ogallala Formation—South <0.2 <0.2 <0.2 <0.2 <0.2 186 13

Arikaree-Brule Formations <0.2 <0.2 <0.2 <0.2 <0.2 70 9

Nebraska Sand Hills <0.2 <0.2 <0.2 <0.2 <0.2 26 6

Kansas Quaternary Deposits <0.2 <0.2 <0.2 <0.2 <0.2 68 30

Nebraska Quaternary Deposits <0.2 <0.2 <0.2 <0.2 <0.2 84 26

Valley-Fill Alluvium <0.2 <0.2 <0.2 <0.2 <0.2 266 48

Terbufos, in micrograms per liter

Ogallala Formation—North <0.9 <0.9 <0.9 <0.9 <0.9 462 43

Ogallala Formation—Central --- -- -- -- -- -- --

Ogallala Formation—South -- -- -- -- -- -- --

Arikaree-Brule Formations <0.9 <0.9 <0.9 <0.9 <0.9 123 25

Nebraska Sand Hills <0.9 <0.9 <0.9 <0.9 <0.9 276 66

Kansas Quaternary Deposits -- -- -- -- -- -- --

Nebraska Quaternary Deposits <0.9 <0.9 <0.9 <0.9 <0.9 595 100

Valley-Fill Alluvium <0.9 <0.9 <0.9 <0.9 <0.9 368 71

Trifluralin, in micrograms per liter

Ogallala Formation—North <0.25 <0.25 <0.25 <0.25 <0.25 374 38

Ogallala Formation—Central -- -- -- -- -- 9 1

Ogallala Formation—South -- -- -- -- -- 3 1

Arikaree-Brule Formations <0.25 <0.25 <0.25 <0.25 <0.25 61 11

Nebraska Sand Hills <0.25 <0.25 <0.25 <0.25 <0.25 285 63

Kansas Quaternary Deposits <0.25 <0.25 <0.25 <0.25 <0.25 68 30

Nebraska Quaternary Deposits <0.25 <0.25 <0.25 <0.25 <0.25 571 100

Valley-Fill Alluvium <0.25 <0.25 <0.25 <0.25 <0.25 384 67

MAJOR IONS

Calcium, in milligrams per liter as Ca

Ogallala Formation—North 33.0 41.7 52.0 78.3 97.1 530 64

Ogallala Formation—Central 14.1 16.4 19.6 26.4 55.7 1,217 58

Ogallala Formation—South 13.6 17.2 24.4 42.4 71.9 2,181 80

Arikaree-Brule Formations 19.2 38.0 50.0 75.0 110.0 351 33

Nebraska Sand Hills 13.1 18.2 35.0 63.8 77.7 202 37

Kansas Quaternary Deposits 35.0 49.0 69.0 100.0 149.0 211 62

Nebraska Quaternary Deposits 55.1 69.0 80.4 96.0 120.0 431 87

Valley-Fill Alluvium 54.0 74.2 97.0 125.0 169.0 850 82

Magnesium, in milligrams per liter as Mg

Ogallala Formation—North 7.0 10.0 12.6 16.8 23.0 530 64

Ogallala Formation—Central 1.5 2.5 3.1 3.8 12.0 1,217 58

Ogallala Formation—South 2.8 3.7 5.5 10.6 19.2 2,181 80

Arikaree-Brule Formations 1.8 5.3 8.8 16.0 26.8 351 33

Nebraska Sand Hills 2.0 3.0 5.1 9.5 11.6 202 37

Kansas Quaternary Deposits 3.7 5.0 7.5 13.0 24.7 211 62

Nebraska Quaternary Deposits 8.5 10.4 12.1 15.0 19.2 430 87

Table 7. Descriptive statistics for selected constituents or properties, by hydrogeologic unit, 1980–98—Continued

 [--, data not available; <, less than]

Constituent or property and

hydrogeologic unit1

Percentile value Number 

of

observations

Percentage 

of unit 

represented 

(see p. 22)

10th 25th 50th 75th 90th
28 Historical Water-Quality Data for the High Plains Regional Ground-Water Study Area in Colorado, Kansas, Nebraska, New 

Mexico, Oklahoma, South Dakota, Texas, and Wyoming, 1930–98

MAJOR IONS—Continued

Valley-Fill Alluvium 10.0 14.0 20.0 28.0 40.0 850 82

Sodium, in milligrams per liter as Na

Ogallala Formation—North 7.3 9.2 13.0 20.1 32.9 530 64

Ogallala Formation—Central 14.0 19.6 26.0 39.0 62.0 1,222 61

Ogallala Formation—South 28.7 38.0 61.0 120.0 266.0 2,210 80

Arikaree-Brule Formations 5.1 12.0 41.0 67.0 110.0 351 33

Nebraska Sand Hills 5.0 5.9 7.6 10.4 13.1 202 37

Kansas Quaternary Deposits 10.0 16.0 27.0 46.8 76.5 212 62

Nebraska Quaternary Deposits 12.0 16.9 24.0 37.0 52.0 430 87

Valley-Fill Alluvium 25.1 42.0 84.0 156.0 400.0 850 82

Potassium, in milligrams per liter as K

Ogallala Formation—North 3.4 4.7 6.3 9.1 12.0 520 63

Ogallala Formation—Central 2.8 4.0 5.0 6.0 6.9 978 58

Ogallala Formation—South 4.5 6.0 8.4 14.0 21.0 1,964 80

Arikaree-Brule Formations 5.0 6.5 8.8 12.0 16.0 281 33

Nebraska Sand Hills 3.6 4.2 4.9 5.8 7.0 190 36

Kansas Quaternary Deposits 1.4 2.0 2.7 3.3 5.0 206 61

Nebraska Quaternary Deposits 4.2 5.0 6.6 8.8 11.6 430 87

Valley-Fill Alluvium 2.6 3.4 5.6 11.0 18.0 823 80

Silica, in milligrams per liter as SiO2

Ogallala Formation—North 37.8 48.0 55.0 60.0 65.0 267 45

Ogallala Formation—Central 23.6 27.0 30.0 36.0 47.0 1,041 56

Ogallala Formation—South 29.0 39.0 49.0 58.0 65.0 1,902 74

Arikaree-Brule Formations 36.3 51.0 58.0 64.0 68.0 202 28

Nebraska Sand Hills 30.0 38.0 47.0 52.0 58.0 93 30

Kansas Quaternary Deposits 17.6 21.0 23.0 28.7 36.3 194 59

Nebraska Quaternary Deposits 27.0 30.5 35.0 44.0 49.6 133 69

Valley-Fill Alluvium 17.0 19.7 24.0 33.5 52.0 677 71

Alkalinity, in milligrams per liter as CaCO3

Ogallala Formation—North 103 122 151 222 280 457 62

Ogallala Formation—Central 164 185 200 217 238 1,331 63

Ogallala Formation—South 178 205 239 274 310 2,147 80

Arikaree-Brule Formations 138 157 180 220 301 238 33

Nebraska Sand Hills 41 49 65 81 106 123 32

Kansas Quaternary Deposits 83 129 180 233 290 214 62

Nebraska Quaternary Deposits 154 192 222 260 308 133 69

Valley-Fill Alluvium 150 186 227 270 322 727 72

Sulfate, in milligrams per liter as SO4

Ogallala Formation—North 5.5 12.9 27.2 55.8 95.7 530 64

Ogallala Formation—Central 13.4 22.0 37.0 59.5 99.6 1,353 64

Ogallala Formation—South 25.8 36.2 89.0 226.0 464.0 2,134 80

Arikaree-Brule Formations 3.0 10.0 28.0 150.0 230.0 343 33

Table 7. Descriptive statistics for selected constituents or properties, by hydrogeologic unit, 1980–98—Continued

 [--, data not available; <, less than]

Constituent or property and

hydrogeologic unit1

Percentile value Number 

of

observations

Percentage 

of unit 

represented 

(see p. 22)

10th 25th 50th 75th 90th
ANALYSIS OF HISTORICAL WATER-QUALITY DATA 29

MAJOR IONS—Continued

Nebraska Sand Hills 2.4 3.9 7.6 13.1 19.6 182 36

Kansas Quaternary Deposits 9.0 12.0 21.0 36.0 78.5 214 63

Nebraska Quaternary Deposits 16.0 24.8 37.5 67.6 122.0 431 87

Valley-Fill Alluvium 23.0 49.0 93.0 185.0 300.0 850 82

Chloride, in milligrams per liter as Cl

Ogallala Formation—North 1.4 2.4 4.5 11.8 29.5 530 64

Ogallala Formation—Central 7.0 9.1 15.0 30.0 83.0 1,390 63

Ogallala Formation—South 12.0 21.0 66.0 193.0 464.0 2,244 80

Arikaree-Brule Formations 1.7 3.8 6.6 14.0 20.0 356 33

Nebraska Sand Hills 0.7 0.9 1.3 4.1 9.9 193 36

Kansas Quaternary Deposits 3.2 6.1 18.0 71.0 297.0 270 63

Nebraska Quaternary Deposits 2.6 5.5 11.6 20.8 35.2 431 87

Valley-Fill Alluvium 12.0 22.0 50.0 200.0 632.0 851 83

Fluoride, in milligrams per liter as F

Ogallala Formation—North 0.2 0.3 0.4 0.7 1.2 266 45

Ogallala Formation—Central 0.4 0.8 1.2 1.7 2.1 1,299 58

Ogallala Formation—South 1.3 1.9 2.7 3.9 5.0 2,064 78

Arikaree-Brule Formations 0.2 0.3 0.4 0.6 1.0 218 30

Nebraska Sand Hills 0.1 0.2 0.3 0.3 0.4 93 30

Kansas Quaternary Deposits 0.2 0.2 0.3 0.4 0.5 200 61

Nebraska Quaternary Deposits 0.2 0.2 0.3 0.3 0.4 133 69

Valley-Fill Alluvium 0.2 0.3 0.4 0.6 0.7 712 71

Dissolved solids, in milligrams per liter

Ogallala Formation—North 219 275 315 371 461 457 62

Ogallala Formation—Central 254 281 320 379 538 1,264 63

Ogallala Formation—South 329 384 517 896 1,620 2,077 80

Arikaree-Brule Formations 206 239 295 412 561 277 33

Nebraska Sand Hills 118 130 155 247 317 112 32

Kansas Quaternary Deposits 173 211 290 468 701 222 62

Nebraska Quaternary Deposits 260 330 382 467 572 132 69

Valley-Fill Alluvium 314 412 603 862 1,350 469 72

TRACE ELEMENTS

Dissolved arsenic, in micrograms per liter

Ogallala Formation—North <5 <5 <5 9 12 31 9

Ogallala Formation—Central <5 <5 <5 <5 6 197 27

Ogallala Formation—South <5 <5 9 14 28 366 53

Arikaree-Brule Formations <5 <5 7 11 24 114 18

Nebraska Sand Hills <5 <5 <5 6 9 29 8

Kansas Quaternary Deposits <5 <5 <5 10 20 30 22

Nebraska Quaternary Deposits -- -- -- -- -- 4 5

Valley-Fill Alluvium <5 <5 <5 8 13 148 41

Dissolved boron, in micrograms per liter

Ogallala Formation—North 20 35 50 90 130 265 36

Table 7. Descriptive statistics for selected constituents or properties, by hydrogeologic unit, 1980–98—Continued

 [--, data not available; <, less than]

Constituent or property and

hydrogeologic unit1

Percentile value Number 

of

observations

Percentage 

of unit 

represented 

(see p. 22)

10th 25th 50th 75th 90th
30 Historical Water-Quality Data for the High Plains Regional Ground-Water Study Area in Colorado, Kansas, Nebraska, New 

Mexico, Oklahoma, South Dakota, Texas, and Wyoming, 1930–98

TRACE ELEMENTS—Continued

Ogallala Formation—Central 63 97 134 200 267 302 26

Ogallala Formation—South 134 197 318 500 900 586 57

Arikaree-Brule Formations 20 40 70 110 230 130 22

Nebraska Sand Hills 20 20 30 130 131 28 14

Kansas Quaternary Deposits -- -- -- -- -- 7 1

Nebraska Quaternary Deposits 30 40 50 80 121 98 61

Valley-Fill Alluvium 52 70 93 120 184 202 27

Dissolved copper, in micrograms per liter

Ogallala Formation—North <5 <5 6 10 35 18 6

Ogallala Formation—Central <5 <5 <5 <5 9 189 24

Ogallala Formation—South <5 <5 <5 <5 8 315 52

Arikaree-Brule Formations <5 <5 <5 <5 9 26 4

Nebraska Sand Hills <5 <5 <5 <5 <5 27 7

Kansas Quaternary Deposits <5 7 10 20 28 30 20

Nebraska Quaternary Deposits -- -- -- -- -- 2 2

Valley-Fill Alluvium <5 <5 8 11 18 218 36

Dissolved iron, in micrograms per liter

Ogallala Formation—North <3 <3 5 20 60 115 25

Ogallala Formation—Central 7 11 22 35 54 184 27

Ogallala Formation—South 6 10 21 37 62 280 48

Arikaree-Brule Formations <3 <3 <3 5 15 127 19

Nebraska Sand Hills <3 4 9 30 67 39 13

Kansas Quaternary Deposits <3 <3 10 30 144 122 53

Nebraska Quaternary Deposits <3 <3 10 47 604 92 57

Valley-Fill Alluvium <3 4 40 545 2,200 384 58

Dissolved lead, in micrograms per liter

Ogallala Formation—North <5 <5 <5 <5 <5 22 7

Ogallala Formation—Central <5 <5 <5 <5 <5 197 27

Ogallala Formation—South <5 <5 <5 <5 <5 309 52

Arikaree-Brule Formations <5 <5 <5 <5 10 35 5

Nebraska Sand Hills <5 <5 <5 <5 <5 27 7

Kansas Quaternary Deposits <5 <5 <5 <5 10 24 19

Nebraska Quaternary Deposits -- -- -- -- -- 2 2

Valley-Fill Alluvium <5 <5 <5 <5 <5 112 34

Dissolved lithium, in micrograms per liter

Ogallala Formation—North -- -- -- -- -- 6 2

Ogallala Formation—Central 13 31 44 66 85 176 21

Ogallala Formation—South 39 53 75 114 155 308 52

Arikaree-Brule Formations 15 21 31 44 51 59 11

Nebraska Sand Hills -- -- -- -- -- 1 1

Kansas Quaternary Deposits -- -- -- -- -- -- --

Nebraska Quaternary Deposits -- -- -- -- -- -- --

Valley-Fill Alluvium 16 20 33 48 73 15 9

Table 7. Descriptive statistics for selected constituents or properties, by hydrogeologic unit, 1980–98—Continued

 [--, data not available; <, less than]

Constituent or property and

hydrogeologic unit1

Percentile value Number 

of

observations

Percentage 

of unit 

represented 

(see p. 22)

10th 25th 50th 75th 90th
ANALYSIS OF HISTORICAL WATER-QUALITY DATA 31

TRACE ELEMENTS—Continued

Dissolved manganese, in micrograms per liter

Ogallala Formation—North <3 <3 <3 10 40 134 28

Ogallala Formation—Central <3 <3 <3 <3 10 223 30

Ogallala Formation—South <3 <3 <3 <3 26 325 52

Arikaree-Brule Formations <3 <3 <3 <3 9 129 19

Nebraska Sand Hills <3 <3 <3 <3 45 44 15

Kansas Quaternary Deposits <3 <3 <3 10 313 118 52

Nebraska Quaternary Deposits <3 <3 <3 62 402 117 66

Valley-Fill Alluvium <3 5 40 243 681 388 60

Dissolved selenium, in micrograms per liter

Ogallala Formation—North <5 <5 <5 <5 <5 35 11

Ogallala Formation—Central <5 <5 <5 <5 8 336 30

Ogallala Formation—South <5 <5 11 24 53 530 55

Arikaree-Brule Formations <5 <5 <5 <5 <5 126 21

Nebraska Sand Hills <5 <5 <5 <5 <5 29 8

Kansas Quaternary Deposits <5 <5 <5 <5 <5 25 20

Nebraska Quaternary Deposits -- -- -- -- -- 4 5

Valley-Fill Alluvium <5 <5 <5 <5 8 122 40

Dissolved zinc, in micrograms per liter

Ogallala Formation—North <5 6 10 30 124 26 8

Ogallala Formation—Central <5 14 40 106 211 239 26

Ogallala Formation—South <5 6 18 59 134 361 53

Arikaree-Brule Formations <5 <5 8 16 38 85 14

Nebraska Sand Hills <5 7 10 18 42 29 8

Kansas Quaternary Deposits 10 10 30 72 140 32 23

Nebraska Quaternary Deposits -- -- -- -- -- 2 2

Valley-Fill Alluvium <5 8 36 99 233 238 41

RADIOLOGICAL PARAMETERS

Radium, in picocuries per liter

Ogallala Formation—North -- -- -- -- -- 6 2

Ogallala Formation—Central -- -- -- -- -- 2 1

Ogallala Formation—South 0.05 0.08 0.20 0.40 1.06 181 38

Arikaree-Brule Formations 0.06 0.10 0.13 0.19 0.22 25 8

Nebraska Sand Hills -- -- -- -- -- -- --

Kansas Quaternary Deposits -- -- -- -- -- -- --

Nebraska Quaternary Deposits -- -- -- -- -- -- --

Valley-Fill Alluvium 0.15 0.19 0.32 0.38 0.45 31 8

Radon, in picocuries per liter

Ogallala Formation—North -- -- -- -- -- 8 2

Ogallala Formation—Central -- -- -- -- -- -- --

Ogallala Formation—South 108 155 220 390 838 92 16

Arikaree-Brule Formations 257 310 433 648 1,200 124 10

Table 7. Descriptive statistics for selected constituents or properties, by hydrogeologic unit, 1980–98—Continued

 [--, data not available; <, less than]

Constituent or property and

hydrogeologic unit1

Percentile value Number 

of

observations

Percentage 

of unit 

represented 

(see p. 22)

10th 25th 50th 75th 90th
32 Historical Water-Quality Data for the High Plains Regional Ground-Water Study Area in Colorado, Kansas, Nebraska, New 

Mexico, Oklahoma, South Dakota, Texas, and Wyoming, 1930–98

RADIOLOGICAL PARAMETERS—Continued

Nebraska Sand Hills -- -- -- -- -- -- --

Kansas Quaternary Deposits -- -- -- -- -- -- --

Nebraska Quaternary Deposits -- -- -- -- -- 2 2

Valley-Fill Alluvium 225 292 403 542 606 57 27

Uranium, in micrograms per liter

Ogallala Formation—North -- -- -- -- -- 9 3

Ogallala Formation—Central -- -- -- -- -- -- --

Ogallala Formation—South -- -- -- -- -- -- --

Arikaree-Brule Formations 3.9 6.8 12.0 18.0 25.6 77 13

Nebraska Sand Hills -- -- -- -- -- 1 1

Kansas Quaternary Deposits -- -- -- -- -- -- --

Nebraska Quaternary Deposits -- -- -- -- -- 2 2

Valley-Fill Alluvium 4.3 8.6 20.0 26.0 43.0 51 22

VOLATILE ORGANIC COMPOUNDS2

Benzene, in micrograms per liter

Ogallala Formation—North <0.5 <0.5 <0.5 <0.5 <0.5 33 7

Ogallala Formation—Central <0.5 <0.5 <0.5 <0.5 <0.5 62 12

Ogallala Formation—South -- -- -- -- -- -- --

Arikaree-Brule Formations -- -- -- -- -- 6 2

Nebraska Sand Hills -- -- -- -- -- 1 1

Kansas Quaternary Deposits <0.5 <0.5 <0.5 <0.5 <0.5 13 13

Nebraska Quaternary Deposits <0.5 <0.5 <0.5 <0.5 <0.5 14 16

Valley-Fill Alluvium <0.5 <0.5 <0.5 <0.5 <0.5 105 41

Carbon tetrachloride, in micrograms per liter

Ogallala Formation—North <0.7 <0.7 <0.7 <0.7 <0.7 39 9

Ogallala Formation—Central <0.7 <0.7 <0.7 <0.7 <0.7 62 12

Ogallala Formation—South -- -- -- -- -- 2 0

Arikaree-Brule Formations -- -- -- -- -- 6 2

Nebraska Sand Hills -- -- -- -- -- 7 3

Kansas Quaternary Deposits <0.7 <0.7 <0.7 <0.7 <0.7 13 13

Nebraska Quaternary Deposits <0.7 <0.7 <0.7 <0.7 <0.7 67 40

Valley-Fill Alluvium <0.7 <0.7 <0.7 <0.7 <0.7 118 44

Ethylbenzene, in micrograms per liter

Ogallala Formation—North <0.7 <0.7 <0.7 <0.7 <0.7 33 7

Ogallala Formation—Central <0.7 <0.7 <0.7 <0.7 <0.7 62 12

Ogallala Formation—South -- -- -- -- -- -- --

Arikaree-Brule Formations -- -- -- -- -- 6 2

Nebraska Sand Hills -- -- -- -- -- 1 1

Kansas Quaternary Deposits <0.7 <0.7 <0.7 <0.7 <0.7 13 13

Nebraska Quaternary Deposits <0.7 <0.7 <0.7 <0.7 <0.7 14 16

Valley-Fill Alluvium <0.7 <0.7 <0.7 <0.7 <0.7 105 41

Table 7. Descriptive statistics for selected constituents or properties, by hydrogeologic unit, 1980–98—Continued

 [--, data not available; <, less than]

Constituent or property and

hydrogeologic unit1

Percentile value Number 

of

observations

Percentage 

of unit 

represented 

(see p. 22)

10th 25th 50th 75th 90th
ANALYSIS OF HISTORICAL WATER-QUALITY DATA 33

VOLATILE ORGANIC COMPOUNDS—Continued

Methyl tert-butyl ether (MTBE), in micrograms per liter

Ogallala Formation—North -- -- -- -- -- 3 1

Ogallala Formation—Central -- -- -- -- -- -- --

Ogallala Formation—South -- -- -- -- -- -- --

Arikaree-Brule Formations -- -- -- -- -- 6 2

Nebraska Sand Hills -- -- -- -- -- -- --

Kansas alluvial aquifer -- -- -- -- -- -- --

Nebraska Quaternary Deposits -- -- -- -- -- 2 2

Valley-Fill Alluvium <0.2 <0.2 <0.2 <0.2 <0.2 61 25

Vinyl chloride, in micrograms per liter

Ogallala Formation—North <0.8 <0.8 <0.8 <0.8 <0.8 33 7

Ogallala Formation—Central <0.8 <0.8 <0.8 <0.8 <0.8 62 12

Ogallala Formation—South -- -- -- -- -- -- --

Arikaree-Brule Formations -- -- -- -- -- 6 2

Nebraska Sand Hills -- -- -- -- -- 1 1

Kansas Quaternary Deposits <0.8 <0.8 <0.8 <0.8 <0.8 13 13

Nebraska Quaternary Deposits <0.8 <0.8 <0.8 <0.8 <0.8 14 16

Valley-Fill Alluvium <0.8 <0.8 <0.8 <0.8 <0.8 105 41

1

Except for trace elements, data summarized include both filtered and unfiltered sample observations. In general, filtered and unfiltered samples 

should yield similar results for most constituents in ground water. For trace elements, only filtered samples are included and results are for the dissolved 

phase, because substantial concentrations of trace elements can occur in both the dissolved and the suspended phases.

Table 7. Descriptive statistics for selected constituents or properties, by hydrogeologic unit, 1980–98—Continued

 [--, data not available; <, less than]

Constituent or property and

hydrogeologic unit1

Percentile value Number 

of

observations

Percentage 

of unit 

represented 

(see p. 22)

10th 25th 50th 75th 90th

2Data from finished drinking-water data sets not included in volatile organic compound summary.
34 Historical Water-Quality Data for the High Plains Regional Ground-Water Study Area in Colorado, Kansas, Nebraska, New 

Mexico, Oklahoma, South Dakota, Texas, and Wyoming, 1930–98

Table 8. Selected constituents from the retrospective data base compared to drinking-water standards

[MCL, maximum contaminant level; SS, secondary standard; PMCL, proposed maximum contaminant level; ns, no standard; na, not applicable; --, insufficient data; mg/L, milligrams per liter; μg/L, micrograms per liter; pCi/L, picocuries per liter; %, percent; <, less than]

Constituent1

1Except for trace elements, data summarized include both filtered and unfiltered samples. In general, filtered and unfiltered samples should yield 

similar results for most constituents in ground water. For trace elements, only filtered samples are included and results are for the dissolved phase because 

substantial concentrations of trace elements can occur in both the dissolved and the suspended phases.

Drinking-water

standard2

Number of 

observations

Exceedances Detections

Numerical 

standard

Standard 

type

Number of 

exceedances

Percentage of 

total 

observations

Number 

of 

detections

Percentage of 

total

 observations

NUTRIENTS

Nitrate 10 mg/L MCL 37,256 5,812 16 35,444 95

Nitrite 1 mg/L MCL 2,826 0 0 701 25

PESTICIDES

2,4-D 70 μg/L MCL 1,477 0 0 8 0.5

Alachlor 2 μg/L MCL 3,872 3 0.1 50 1.3

Atrazine 3 μg/L MCL 4,387 62 1.4 1,090 25

Chlorpyrifos 20 μg/L SS 2,323 0 0 6 0.3

Cyanazine 1 μg/L SS 3,228 2 0.1 47 1.5

Diazinon 0.6 μg/L SS 546 0 0 4 0.7

Metolachlor 70 μg/L SS 3,380 0 0 146 4.3

Prometon 100 μg/L SS 1,362 0 0 111 8.1

Simazine 4 μg/L MCL 2,082 1 0 85 4.1

Trifluralin 5 μg/L SS 2,481 0 0 20 0.8

MAJOR IONS

Chloride 250 mg/L SS 26,889 2,256 8 24,689 92

Fluoride 2 mg/L SS 14,611 1,885 13 14,504 99

Sulfate 250 mg/L SS 22,983 1,701 7 22,091 96

Dissolved solids 500 mg/L SS 15,621 4,518 29 15,620 100

TRACE ELEMENTS

Dissolved arsenic 50 μg/L MCL 5,580 28 <0.1 4,532 81

Dissolved iron 300 μg/L SS 11,035 1,147 10 6,326 57

Dissolved lead 15 μg/L MCL 2,235 105 5 379 17

Dissolved manganese 50 μg/L SS 9,563 1,962 21 4,635 48

Dissolved selenium 50 μg/L MCL 5,590 55 1 4,061 73

RADIOLOGICAL

Radium-226 ns ns 399 na na 295 74

Radium-228 ns ns 175 na na 62 35

Radium-226 plus 

Radium-228

5 pCi/L MCL 173 10 6 129 74

Radon 300 pCi/L PMCL 545 375 69 542 99

Uranium 20 μg/L PMCL 5,089 460 9 4,855 96

VOLATILE ORGANIC COMPOUNDS3

Benzene 5 μg/L MCL 445 0 0 7 2

Carbon tetrachloride 5 μg/L MCL 568 3 0.5 6 1

Ethylbenzene 700 μg/L MCL 445 0 0 6 1

Vinyl chloride 2 μg/L MCL 483 3 <0.1 3 1

BACTERIOLOGICAL

Total coliforms 5.0%4 MCL 307 -- -- 54 18

2

U.S. Environmental Protection Agency, 1996.

3Finished drinking-water data not included in volatile organic compound data set.

4

No more than 5.0 percent of samples total coliform-positive in a month.
ANALYSIS OF HISTORICAL WATER-QUALITY DATA 35

cyanazine, and simazine) had observations exceeding 

a water-quality standard, and except for atrazine, rates 

of exceedance were smaller than 0.1 percent of observations. Atrazine, the most commonly used pesticide, 

had the most exceedances (62 exceedances; 1.4 

percent of observations) and was detected in 25 

percent of all observations. A large number of observations are available for dissolved solids, which is a 

direct measure of salinity. The secondary standard of 

500 mg/L for dissolved solids was exceeded in 29 

percent of the 15,621 observations. Two constituents 

that contribute to salinity, chloride and sulfate, had 

about 10 percent of observations exceeding their 

secondary standard (250 mg/L). Among trace 

elements, iron and manganese most often exceeded a 

standard: 10 percent of iron observations exceeded the 

secondary standard of 300 micrograms per liter 

(μg/L), and 21 percent of manganese observations 

exceeded the secondary standard of 50 μg/L. Among 

trace elements with an MCL, lead had the largest 

exceedance rate (5 percent), and arsenic and selenium 

had exceedance rates of 1 percent or less. These trace 

elements were commonly detected, with lead having 

the smallest detection rate of 17 percent. Among 

radiological parameters in the data set, only radium226 plus radium-228 currently has a drinking-water 

standard. Although 10 of the 173 observations in the 

data set exceeded the MCL of 5 picocuries per liter, an 

insufficient number of observations are in the data set 

to evaluate the status of radium as a water-quality 

concern. Similarly, although 69 percent of observations for radon exceeded a proposed standard of 300 

picocuries per liter, there are insufficient observations 

to evaluate radon as a water-quality concern. Among 

radiological parameters, the most observations are 

available for uranium because uranium analyses were 

done as part of the NURE program; however, as 

discussed in the “Content of Retrospective Data Base” 

section, the NURE sites are not uniformly distributed 

across the study area. Among sites with uranium data, 

the proposed water-quality standard of 20 μg/L was 

exceeded for 9 percent of the observations. Insufficient data are available to evaluate volatile organic 

compounds as a water-quality concern. Only about 

500 observations are available for 4 volatile organic 

compounds having MCL’s listed in table 8; the 

frequency of detections and exceedances for volatile 

organic compounds as a group is somewhat less than 

that for pesticides as a group. There were insufficient 

observations in the data set for total coliforms to evaluate the status of bacteria as a water-quality concern; 

however, coliform bacteria were detected in 18 percent 

of the 307 observations in the data set.

The data from table 8 indicate that the retrospective data base has sufficient data to corroborate that 

nitrate and dissolved-solids (salinity) concentrations 

are substantial water-quality concerns. A moderate 

amount of available retrospective data also indicate 

that among pesticides, atrazine detections are a 

concern. The retrospective data indicate that iron, 

manganese, fluoride, uranium, and radon might 

warrant concern in some areas. In the sections that 

follow, retrospective data for these constituents are 

examined in more detail. Data for the other constituents listed in table 8 are insufficient to warrant detailed 

data analysis.

Nitrate

Nitrate is a form of nitrogen that is one among a 

group of substances called nutrients, which are 

substances essential to the growth of plants. However, 

excessive nutrients can be deleterious to the environment, supporting excessive growths of algae and 

aquatic plants in surface waters, and nitrate can accumulate in ground water and become a human-health 

concern (Mueller and Helsel, 1996). Nitrate tends to 

accumulate in ground water where there are substantial sources of nitrogen applied to the land surface 

because nitrate is highly soluble in water and is stable 

over a wide range of aerobic environmental conditions.

Sources of nitrogen to the land surface in the 

High Plains study area include natural, atmospheric, 

agricultural, and urban sources. The principal natural 

source of nitrogen is provided by plants converting 

atmospheric nitrogen gas to organic nitrogen (nitrogen 

fixing). Organic nitrogen in plants can then be 

converted (mineralized) to nitrate by natural degradation processes. This natural source of nitrogen generally leads to a small, stable background nitrate 

concentrations in ground water, although temporary 

increases may occur when natural vegetation is 

converted to cropland.

The remaining sources of nitrogen are anthropogenic (human-derived) sources that result from agricultural and urban land use. Primary among these is 

the application of manure and fertilizer to the land 

surface (fig. 9). Nationwide, the application of 

nitrogen in fertilizer has increased twentyfold from
36 Historical Water-Quality Data for the High Plains Regional Ground-Water Study Area in Colorado, Kansas, Nebraska, New Mexico,

Oklahoma, South Dakota, Texas, and Wyoming, 1930–98

1945 to 1993 (Puckett, 1995) and is widespread in the 

High Plains, where cropland now amounts to an estimated 50 million acres (U.S. Department of Agriculture, 1999). Manure from livestock is a second 

important agricultural source of nitrogen. Together, 

manure and fertilizer sum to an average application 

rate across the entire High Plains study area of 34 

pounds of nitrogen per acre per year. Anthropogenic 

and natural sources of nitrogen also can be recycled by 

uptake into the atmosphere and reapplied to the land 

surface in the form of nitrate and ammonia dissolved 

in rainfall; the average annual nitrogen load in the 

High Plains study area from precipitation is about 3.5 

pounds per acre, equivalent to a concentration in rainfall of about 0.7 mg/L nitrogen (National Atmospheric 

Deposition Program, 2000). Additional sources of 

nitrate occur from residential and urban wastewater, 

which is conveyed to stream channels through wastewater-treatment plants or to the ground through septic 

systems. Due to the low population density in the High 

Plains, wastewater is not a large component of the 

total nitrogen loading but can be an important factor 

locally in stream channels downstream from wastewater-treatment plants and in areas where septic 

systems are concentrated or are functioning improperly.

Because nitrogen sources are common in the 

High Plains study area, it might be expected that a 

portion would be transported to aquifers. The bulk of 

the nitrogen applied is incorporated into the tissue of 

growing plants, but residual nitrogen can accumulate 

in soils beneath fields and move into aquifers when 

transport mechanisms are present. Data from the 

retrospective data base indicate that nitrate is the 

dominant form of nitrogen in the High Plains study 

area (table 7). While the median nitrite plus nitrate 

concentration in the High Plains retrospective data 

base was 3.0 mg/L, the median ammonia concentration was 0.02 mg/L, the median nitrite concentration 

was less than the detection limits of 0.01 mg/L, 

and the median organic nitrogen concentration was 

0.3 mg/L. Nitrite can be an important nitrogen species 

where dissolved oxygen concentrations are small. In 

the retrospective data set, the median concentration of 

dissolved oxygen was 4.8 mg/L, indicating overall 

aerobic conditions in the High Plains aquifer. The 

smallest median dissolved oxygen concentrations 

(1.7 and 1.8 mg/L) occurred in shallower Valley-Fill 

Alluvium and Nebraska Sand Hills units, while larger 

concentrations (medians of 5.4 mg/L or larger) 

occurred in the deeper Ogallala Formation–North, 

Arikaree-Brule Formations, and Nebraska Quaternary 

Deposits units. In the retrospective data set nitrite 

concentrations are inversely related to dissolved 

oxygen concentrations, with nitrite concentrations 

exceeding 0.1 mg/L found only where dissolved 

oxygen concentrations are less than 1 mg/L. In the 

70 wells in which nitrite concentrations exceeded 

0.1 mg/L, nitrite was still a minor constituent, averaging 16 percent of the nitrite plus nitrate concentration in those wells. Because nitrite concentrations 

were small, in the discussion that follows, nitrate 

observations and nitrite plus nitrate observations will 

be grouped together and referred to as nitrate.

 All nitrate data in the retrospective data base 

were selected and grouped by decade to examine 

temporal changes in nitrate concentrations for the 

study area. To ensure that spatial variability was not a 

factor in this analysis, sites sampled during each 

decade were mapped, and a visual verification was 

made that the study area had a reasonable distribution 

of sampling sites during each decade. So that single 

wells were not overrepresented where multiple 

samples were taken, a median concentration was 

selected for each well for each decade. Boxplots of the 

decadal data (fig. 11) indicate that nitrate concentrations in the study area started to increase beginning in 

the 1970’s. Differences in median nitrate concentrations were verified using the Wilcoxon rank-sum test 

(p=0.0001), and Tukey’s multiple comparison test 

(α = 0.05) indicated that median concentrations weresimilar from the 1930’s to the 1960’s, significantly 

larger in the 1970’s, and larger yet in the 1980’s and 

1990’s. The median nitrate concentration from the 

1930’s to the 1960’s was 1.7 mg/L and may be representative of background levels. The median nitrate 

concentration in the 1980’s and 1990’s was 3.2 mg/L. 

To examine temporal changes in nitrate within 

each hydrogeologic unit, the decadal data set was 

stratified by hydrogeologic unit, and a LOWESS 

smooth line was calculated for each unit (fig. 12). Data 

distribution was sufficient for each decade within each 

hydrogeologic unit except for the Nebraska Sand 

Hills, where few data were available prior to the 

1970’s. These data indicate that nitrate concentrations 

are increasing at varying rates in most of the hydrogeologic units in the study area. Units with a large 

percentage of irrigated lands (Valley-Fill Alluvium, 

Nebraska Quaternary Deposits) have the largest nitrate 

concentrations and the largest increases in nitrate 

concentrations. Large increases have also occurred in
ANALYSIS OF HISTORICAL WATER-QUALITY DATA 37

EXPLANATION

Number of observations

90th percentile

75th percentile

Median

25th percentile

10th percentile

7,711

1930 1940 1950 1960 1970 1980 1990

433 1,171

972

3,483

5,332

7,562

7,711

0

2

4

6

8

10

12

NITRATE CONCENTRATION, IN MILLIGRAMS PER LITER 14

16

the Arikaree-Brule Formations unit, which has a 

moderate depth to water. Nitrate concentrations in the 

shallow Kansas Quaternary Deposits unit have generally been large throughout the period of record. 

Among the Ogallala Formation units, the two with 

shallower depth to water (Ogallala Formation–North 

and Ogallala Formation–South units) have had larger 

increases in nitrate concentrations than the deeper 

Ogallala Formation–Central unit. To test for statistical 

significance of these changes, the Wilcoxon rank-sum 

test was used to test for nitrate concentration differences between grouped 1930–69 data and grouped 

1980–98 data for each unit (except for the Nebraska 

Sand Hills which had insufficient data). Data from the 

transitional 1970’s decade were excluded from this 

analysis. Significant differences were found for all 

units except for the Kansas Quaternary Deposits.

To examine temporal variability of nitrate 

concentrations at a more local geographic scale, differences between grouped 1930–69 data and grouped 

1980–98 data nitrate concentrations were evaluated for 

each county. Where more than one observation was 

available for a well during a grouped time period, a 

median value was used for that well before a median 

was calculated for the county. The change in county 

median concentration between the two periods is 

displayed in figure 13A. There were 167 counties that 

had an increase in median nitrate concentrations 

between these two time periods and 41 counties that 

had no change or a decrease. The Wilcoxon rank-sum 

test indicated that county increases were statistically 

significant for 93 counties, while decreases were 

statistically significant for only 4 counties. The 

remaining counties had insufficient data (generally 

fewer than 20 observations) for the statistical test. 

Counties with increases in nitrate concentrations are 

distributed throughout the study area. Largest

Figure 12. Temporal variability of nitrate concentrations, by 

hydrogeologic unit.

1930 1940 1950 1960

YEAR OF SAMPLE

NITRATE CONCENTRATION, IN MILLIGRAMS PER LITER

1970 1980 1990 2000

0

1

2

3

4

5

6

Ogallala Formation—North

Ogallala Formation—Central

Ogallala Formation—South

Arikaree-Brule Formations

Nebraska Sand Hills

Kansas Quaternary Deposits

Nebraska Quaternary Deposits

Valley-Fill Alluvium

Figure 11. Distribution of nitrate concentrations, by decade.
38 Historical Water-Quality Data for the High Plains Regional Ground-Water Study Area in Colorado, Kansas, Nebraska, New Mexico,

Oklahoma, South Dakota, Texas, and Wyoming, 1930–98

Perce exc per 

EXPLANATION

Change in median concentration, in milligrams per literCrosshatch indicateschange is statisticallysignificant at α = 0.05

No change or decreaseLess than 2Greater than 2No data

105°

40°

32°

105

40°

32°

00 50 100 150 200 KILOMETERS

50 100 150 200 MILES

A. Change in median nitrate concentration(1930–69 median compared to 1980–98 median)

B. Percentage of observations exceeding10 milligrams per liter (1980–98)

Figure 13. Distribution of nitrite concentrations by decade.
ANALYSIS OF HISTORICAL WATER-QUALITY DATA 39

increases have occurred in counties along the major 

river valleys, in eastern Nebraska and central Kansas, 

and in the southern part of the study area in Texas. The 

county changes show that there is some variability 

within hydrogeologic units. For example, although 

concentrations have increased significantly in the 

Ogallala Formation–South unit, the changes are 

largest in the southern part of this unit; and although 

changes in the Ogallala Formation–Central unit have 

been small, there are counties within this area where 

changes have been large. These differences within 

hydrogeologic units indicate that there are factors 

within units that affect nitrate concentrations. Areas 

where nitrate increases have been largest generally 

were in contiguous groups of counties, which might 

indicate that causative factors are fairly widespread, as 

might be expected with nonpoint-source factors.

Among the 1980–98 data, the percentage of 

nitrate observations exceeding the 10 mg/L MCL for 

each county (fig. 13B) indicates that exceedance rates 

greater than 10 percent are widespread and occur 

throughout Nebraska, in central Kansas, and in the 

southern part of the study area in Texas. Exceedance 

rates larger than 30 percent are more scattered and 

occur in 19 counties. In some areas, such as the 

southern part of the study area in Texas and in the 

eastern part of the study area in Nebraska, multicounty 

areas have both large increases in nitrate concentrations and large rates of exceedance of the MCL; these 

characteristics have been attributed to high nitrogen 

application rates and sandy soils (Exner and Spalding, 

1990; Reeves and Miller, 1978). In contrast, in some 

areas nitrate increases were small, but rates of exceedance of the 10-mg/L MCL were large. In one such 

area, Holt County in north-central Nebraska, this has 

been attributed to point-source contamination of 

poorly constructed wells (Exner and Spalding, 1990).

Data from the retrospective data base were next 

evaluated for relations between nitrate concentrations 

and well-site characteristics. Because there is temporal 

variability in the data, only data from the recent period 

(1980–98) were used, and because there is variability 

based on hydrogeologic unit, tests were done separately for each unit. Where multiple observations were 

available for a well, only the most recent observation 

was used. 

Differences in median values for categorized 

data were tested using the Kruskal-Wallis test, and 

groupings were identified using the Tukey multiple 

comparison test. Significant differences in nitrate 

concentrations were determined based on well type. 

Wells constructed for water withdrawal had significantly larger nitrate concentrations than did observation wells in hydrogeologic units where depth to water 

was small (Valley-Fill Alluvium, Nebraska Sand Hills, 

Kansas Quaternary Deposits, and Arikaree-Brule 

Formations). However, there was no significant difference in nitrate concentrations between these two well 

types in the deeper units. This may indicate that 

construction techniques are more variable for shallow 

water wells, with poorly constructed wells susceptible 

to surface contamination, while deeper wells may be 

more likely to be better constructed regardless of the 

well type. Water use of a well was not found to be a 

significant factor for nitrate concentrations except in 

the Ogallala Formation units, where water from livestock wells had smaller nitrate concentrations than did 

water from wells used for irrigation or domestic 

purposes. Land use was a significant factor for nitrate 

concentrations in five of the units (Kansas Quaternary 

Deposits, Arikaree-Brule Formations, and the three 

Ogallala Formation units); in four of these hydrogeologic units, nitrate concentrations were smaller under 

rangeland than under other land uses. However, in the 

Ogallala Formation–South unit, nitrate concentrations 

were larger under urban land use and rangeland than 

under agricultural land use.

Nitrate concentrations were tested for relations 

with continuous numerical variables by using the 

Kendall’s tau correlation coefficient (table 9). Each of 

the correlation coefficients shown in this table is 

significant (p=0.0001), but the correlations are weak, 

which is indicative of the large amount of variability in 

the data, perhaps due to natural variation in the environmental setting within the large geographic extent of 

the study area. Well depth was inversely correlated 

with nitrate concentrations in five of the eight hydrogeologic units, indicating that deeper wells are less 

likely to have large nitrate concentrations. However, 

estimated depth to water was inversely correlated with 

nitrate concentrations in only three of the eight hydrogeologic units, with the stronger correlations in the 

Ogallala Formation–Central and Ogallala Formation–

South. This may indicate that depth to water is important only for the deeper aquifers, while in the shallow 

aquifers, depths to water are everywhere small enough 

so as to be susceptible to contamination. For the 

Nebraska Sand Hills, nitrate concentrations were 

directly correlated with estimated depth to water; the 

reason for this is not known.
40 Historical Water-Quality Data for the High Plains Regional Ground-Water Study Area in Colorado, Kansas, Nebraska, New Mexico,

Oklahoma, South Dakota, Texas, and Wyoming, 1930–98

Table 9. Kendall’s tau correlation coefficients between nitrate concentrations and ancillary variables by, hydrogeologic unit

[Correlations shown only if the magnitude is greater than 0.10; all correlations shown are significant with p=0.0001; --, no correlation]

Hydrogeologic unit Estimated

depth to water

Well

depth

Depth of 

sample 

below 

water table

Irrigation 

intensity

Nitrogen 

input 

Soil

permeability

Age of

well

Ogallala Formation—North -- -- –0.17 -- -- -- --

Ogallala Formation—Central –0.27 –0.23 -- -- 0.17 0.26 --

Ogallala Formation—South –0.38 –0.34 -- –0.20 –0.13 0.32 --

Arikaree-Brule Formations –0.13 –0.19 –0.26 0.26 0.31 0.15 --

Nebraska Sand Hills 0.19 -- -- 0.30 0.26 -- --

Kansas Quaternary Deposits -- -- -- 0.11 –0.16 -- --

Nebraska Quaternary Deposits -- –0.15 –0.24 -- -- -- 0.14

Valley-Fill Alluvium -- –0.16 –0.17 0.25 -- 0.14 --

For 5,300 wells, information was available 

about the perforated interval of the well, and a depth of 

sample below the water table could be calculated by 

subtracting the depth of the midpoint of the open 

interval from the depth to water. In general, larger 

nitrate concentrations might be expected where the 

depth of the sample below the water table is small 

(near the top of the aquifer) because this is the first 

portion of the aquifer to be in contact with substances 

transported down from the land surface. This variable 

inversely correlated with nitrate concentrations in four 

hydrogeologic units, but correlation coefficients were 

still relatively small (maximum magnitude of –0.26). 

The strength of this correlation might be weakened by 

inaccuracies in calculating the depth of the sample 

below the water table: some of the open intervals were 

quite large, and for many wells the depth-to-water 

measurement may not have been taken at the time of 

sampling. The relation between nitrate concentrations 

and depth of the sample below the water table is 

shown graphically in figure 14. For six of the hydrogeologic units, nitrate concentrations decrease as 

depth of the sample below the water table increases. 

The slope of this relation is quite large for the 

Nebraska Quaternary Deposits, Valley-Fill Alluvium, 

and Arikaree-Brule Formations units and smaller for 

the Ogallala Formation–North, Ogallala Formation–

Central, and Nebraska Sand Hills units. In contrast, 

nitrate concentrations appear to increase and then 

decrease with depth of sample below the water table 

for the Kansas Quaternary Deposits and Ogallala 

Formation–South units. This pattern might be due to 

processes such as denitrification in shallower environments or flushing of nitrates deeper into the aquifer, or 

it might be a relic of the data base due to the relatively 

small number of points used in the analysis (only 51 

data points are available for the Kansas Quaternary 

Deposits, but 875 points are available for the Ogallala 

Formation–South). 

Irrigation intensity was directly correlated with 

nitrate concentrations in four hydrogeologic units 

where depths to water were shallow. Irrigation intensity was not correlated with nitrate concentrations in 

the deeper Ogallala Formation units, except in the 

Ogallala Formation–South unit where nitrate concen

0 100 200 300 400 500

0

1

2

3

4

5

6

7

8

DEPTH OF SAMPLE BELOW WATER TABLE, IN FEET

NITRATE CONCENTRATION, IN MILLIGRAMS PER LITER

Ogallala Formation—North

Ogallala Formation—Central

Ogallala Formation—South

Arikaree-Brule Formations

Nebraska Sand Hills

Kansas Quaternary Deposits

Nebraska Quaternary Deposits

Valley-Fill Alluvium

Figure 14. Relation between nitrate concentration and 

depth of sample below water table, by hydrogeologic unit.
ANALYSIS OF HISTORICAL WATER-QUALITY DATA 41

trations were larger where irrigation intensity was less; 

this finding seems in agreement with the land-use 

results for the Ogallala Formation–South where nitrate 

concentrations were smaller in agricultural land use 

than in other land uses. In this unit other factors such 

as the location of focused recharge from playa lakes 

may affect contamination of the aquifer (Mullican and 

others, 1994).

Nitrogen inputs were most strongly correlated 

with nitrate concentrations in the two hydrogeologic 

units (Arikaree-Brule Formations and Nebraska Sand 

Hills) where agricultural land use is minimal; correlations were poorer and inconsistent in intensive agricultural areas. This may indicate that county-wide 

estimates of nitrogen inputs are not site-specific 

enough or variable enough to be useful predictors of 

nitrate concentrations within areas where there is 

abundant agricultural activity. Soil permeability was 

correlated with nitrate concentrations in four hydrogeologic units, with correlations large in the Ogallala 

Formation-Central and Ogallala Formation-South 

units. Age of well was weakly correlated with nitrate 

concentrations in one hydrogeologic unit, the 

Nebraska Quaternary Deposits.

In general, ancillary variables were best correlated with nitrate concentrations in the Ogallala 

Formation–Central, Ogallala Formation–South, 

Arikaree-Brule Formations, and Nebraska Sand Hills. 

Correlations were generally poor for the remaining 

units, especially for the Ogallala Formation-North 

unit. These poor correlations probably reflect the large 

variability of geographic and site-specific well characteristics even within the individual hydrogeologic 

units. Error also is contributed by assignment of 

geographic characteristics using generalized maps to 

make up for the lack of site-specific information. 

Additional site-specific information, such as information about potential contamination pathways and 

processes, might be needed to accurately predict which 

areas are susceptible to nitrate contamination.

Pesticides

During the past 40 years, the use of pesticides to 

enhance crop growth has become widespread. For 

example, the nationwide use of atrazine has been estimated to have increased ninefold between 1964 and 

1974 (Barbash and others, 1999). The use of some 

pesticides has declined as improved products are 

developed; for example, the use of alachlor decreased 

fourfold between 1975 and 1992. Also, health and 

environmental concerns have caused the use of some 

pesticides, such as dieldrin, to be canceled.

Each of 43 pesticides were applied to crops at an 

estimated annual rate of 100,000 pounds or more of 

active ingredient in the High Plains study area during 

1992 (Gail Thelin, U.S. Geological Survey, written 

commun., 1999). The most commonly used pesticides, 

ranked by pounds of active ingredient applied and by 

number of acres treated, are listed in table 10. The 

herbicide atrazine is the most commonly used pesticide by both measures, with about 9.5 million pounds 

applied to 8.8 million acres. Atrazine use is widespread in the High Plains study area because it is used 

on three (corn, wheat, and sorghum) of the four principal crops in the study area. Other common herbicides 

are metolachlor, 2,4-D, alachlor, cyanazine, and trifluralin. Common insecticides are chlorpyrifos, terbufos, 

methyl parathion, and carbofuran. Pesticides are most 

heavily applied in areas where irrigation is most 

intense in eastern Nebraska, southwest Kansas, and the 

panhandle of Texas. At least 30 pesticides are applied 

in most counties, and more than 60 pesticides are 

applied in counties in the panhandle of Texas and in 

areas of western Nebraska where crop diversity is 

large. A few types of pesticides (such as permethryn, 

which is produced by the chrysanthemum plant and 

which has a commonly used synthetic derivative called 

permethrin) have natural sources, but the pesticides 

discussed in this report are synthetic compounds with 

no natural sources; therefore, their presence in the 

environment may be attributed to anthropogenic 

sources.

Over 94,000 pesticide observations are in the 

retrospective data base with 2,436 pesticide detections, 

and the overall detection rate is about 2.6 percent. 

About 50 pesticides were detected in the High Plains 

study area, but only 11 pesticides were detected at 10 

or more sites. The most commonly detected pesticides, 

based on number of sites having detections, are listed 

in table 10. A comparison of the most commonly used 

pesticides with those most commonly detected indicates that use and occurrence in ground water generally are correlated. Atrazine was the most commonly 

used pesticide and also is by far the most commonly 

detected pesticide (detected in 24.8 percent of all 

tested samples and detected at 578 sites). Metolachlor,
42 Historical Water-Quality Data for the High Plains Regional Ground-Water Study Area in Colorado, Kansas, Nebraska, New Mexico,

Oklahoma, South Dakota, Texas, and Wyoming, 1930–98

alachlor, cyanazine, and trifluralin also are ranked high 

on the lists of pesticides applied and number of acres 

treated and are ranked high on the list of pesticides 

detected. Each of these is a herbicide used on corn and 

other crops in the study area. Three of the most 

commonly detected pesticides (prometon, simazine, 

propazine) are triazine-type herbicides like atrazine 

but do not appear on either list of most commonly used 

pesticides in table 10. Prometon and simazine are 

commonly used for bare-ground weed control on 

noncropped areas in agricultural and other land-use 

settings, but few data are available on the total 

amounts used. Among nationwide pesticide studies, 

prometon was the second most commonly detected 

pesticide after atrazine, and simazine was the fourth 

most commonly detected pesticide (Barbash and 

Resek, 1996). Propazine is a commonly used herbicide 

on sorghum, but its use was canceled during 1988–98; 

therefore, its use was not tabulated in the 1992 estimates. The herbicide 2,4,5-TP was commonly used to 

control weeds on rangeland and pastureland, but its 

use was canceled in 1985 due to health concerns; its 

detection in the High Plains study area is limited to a 

small area in one county in New Mexico. Tebuthiuron 

(used to control weeds in noncropland areas) and 

metribuzin (used to control weeds on cropland and 

fallow land) are ranked 98th and 38th in pounds 

applied in the High Plains study area, respectively; but 

due to their high solubility in water and low sorption 

potential to soils, they have a high ground-water 

contamination potential. Their detection in the High 

Plains study area is limited to a scattered few counties. 

The herbicide 2,4-D is the second most commonly 

applied pesticide in the study area but only the twelfth 

most commonly detected pesticide. This may be 

because 2,4-D rapidly degrades in soil and aquatic 

environments. Chlorpyrifos, diazinon, and dieldrin are 

the only insecticides on the most-detected list in table 

10. Chlorpyrifos is commonly used but sorbs strongly 

to soils and is not readily soluble in water. Diazinon is 

not a commonly used pesticide by amount applied or 

acres treated but is the most commonly used household 

insecticide (Whitmore and others, 1992). The use of 

dieldrin wascanceled in 1987, but it is occasionally 

detected in ground water because it is persistent in the 

environment.

Pesticide data have been commonly collected in 

the High Plains study area only since the mid-1980’s 

(fig. 4). Since 1985, the annual percentage rate of 

pesticide detections has varied from 1 percent to 8 

Table 10. Most commonly used and detected pesticides in the High Plains study area

[Insecticides appear in bold, other chemicals are herbicides. Source of use data: Gail Thelin, U.S. Geological Survey, written commun., 1999; 

detection rank is based on number of sites having a detection]

Rank

Pesticide use in 1992

based on pounds active

ingredient applied

Pesticide use in 1992

based on number of

acres treated

Pesticide detected in the High Plains 

retrospective data base

(number of sites having detections is 

in parentheses)

1 Atrazine Atrazine Atrazine (578)

2 2,4-D Metolachlor Metolachlor (70)

3 Metolachlor 2,4-D Simazine (68)

4 Dicamba Alachlor Prometon (63)

5 Alachlor Cyanazine Alachlor (38)

6 Metsulfuron Trifluralin Propazine (35)

7 Trifluralin Chlorpyrifos Cyanazine (35)

8 Chlorpyrifos Terbufos Trifluralin (20)

9 Methyl parathion Acetochlor 2,4,5-TP (17)

10 Chlorsulfuron Methyl parathion Tebuthiuron (16)

11 Cyanazine Butylate Metribuzin (10)

12 Permethrin EPTC 2,4-D (8)

13 Terbufos Carbofuran Chlorpyrifos (6)

14 Carbofuran Pendimethalin Diazinon (4)

15 Picloram Propachlor Dieldrin (3)
ANALYSIS OF HISTORICAL WATER-QUALITY DATA 43

percent of measurements made, but there is no pattern 

of increasing rates of pesticide detections. This is 

probably due to the variety of pesticide analytical 

methods that have become available over the years, 

which includes highly sensitive methods (such as high 

pressure liquid chromatography) and less sensitive 

field-screening techniques (such as immunoassay 

analysis). Over the decades, the median pesticide 

detection limit in the retrospective data base has 

increased, probably due to the increased use of immunoassay field methods. For these reasons it is not 

possible to evaluate temporal trends in pesticide 

concentrations or detections using the High Plains 

retrospective data base.

Sites with pesticide data, sites with detections, 

and sites with concentrations exceeding drinkingwater standards are shown in figure 15 for the four 

most commonly detected pesticides. For each of the 

four pesticides, data primarily are available only in the 

northern half of the study area. Sixty percent of sites 

with atrazine detections, 59 percent of sites with metolachlor detections, 80 percent of sites with simazine 

detections, and 86 percent of sites with prometon 

detections were in the Valley-Fill Alluvium hydrogeologic unit. The frequency of detection of pesticides 

was also largest in the Valley-Fill Alluvium unit; for 

example, atrazine was detected at 49 percent of the 

sites sampled in the Valley-Fill Alluvium unit. The 

number of sites with pesticide detections and the 

frequency of detection generally were second largest 

in the Nebraska Quaternary Deposits unit. Pesticides 

were detected much less commonly in the deeper 

Ogallala Formation units. In the Ogallala Formation–

North unit, atrazine was detected at 24 of 677 sites 

sampled; in the Ogallala Formation–Central unit, atrazine was detected at 11 of 90 sites sampled; and in the 

Ogallala Formation–South unit, atrazine was detected 

at 4 of 170 sites sampled. However, the detection of 

pesticides in deeper aquifers indicate that contamination pathways exist.

Atrazine concentrations exceeded the USEPA 

drinking-water maximum contaminant level at 37 

sites. Nineteen of these sites were in the Valley-Fill 

Alluvium hydrogeologic unit, where 2.7 percent of the 

sites sampled had an atrazine exceedance. Twelve sites 

in the Nebraska Quaternary Deposits unit had atrazine 

exceedances (1.7 percent of the sites sampled). Two 

sites in the deeper Ogallala Formation units had atrazine concentrations exceeding the standard. There 

were no sites where metolachlor or prometon concentrations exceeded a standard and only one site where 

simazine exceeded a standard. The pesticide data from 

the retrospective data base indicate that among hydrogeologic units the shallow Valley-Fill Alluvium is the 

most susceptible to pesticide contamination. Each of 

the four most commonly detected pesticides was 

detected in the Valley-Fill Alluvium unit along the 

Platte River. Atrazine also was detected in the ValleyFill Alluvium unit along the South Platte River and the 

Republican River. Corn is the predominant crop in the 

Nebraska Quaternary Deposits unit where atrazine 

also was commonly detected. However, the atrazine 

detection and exceedance rates are about half as large 

in the Nebraska Quaternary Deposits unit as in the 

Valley-Fill Alluvium unit; this difference might be 

related to the median depth to water for these two units 

(table 1).

About 75 percent of the pesticide data in the 

retrospective data base are from sites in Nebraska and 

central Kansas, so relations identified in the data are 

not representative of the High Plains study area as a 

whole. Nebraska pesticide data have been discussed at 

some length by Exner and Spalding (1990), and 

Kansas pesticide data have been discussed by 

Helgesen and others (1994). In the retrospective data 

base, the percentage of atrazine detections in a county 

was significantly correlated (r = 0.324; p < 0.001) to 

the percentage of nitrate exceedances in the county. 

Atrazine detections were significantly larger in areas 

of agricultural land use than in rangeland use and were 

significantly larger where the principal crop was corn 

or sorghum. About 30 percent of the variance in atrazine detections was explained by well depth and pesticide application rate.

Recent reconnaissance sampling in the central 

and northern High Plains (McMahon, 2000; Kolpin 

and others, 1998) has indicated that pesticide degradate compounds are often more prevalent in the environment than are parent compounds. The principal 

degradate compounds available in the retrospective 

data base are by-products of atrazine, namely deethylatrazine and deisopropylatrazine. In the retrospective 

data base, deethylatrazine had a higher rate of detection (43 percent) than did atrazine (24.8 percent), but 

deisopropylatrazine had a smaller rate of detection (6 

percent). As with atrazine, the by-product detections 

occurred almost exclusively in the Valley-Fill Alluvium hydrogeologic unit.
44 Historical Water-Quality Data for the High Plains Regional Ground-Water Study Area in Colorado, Kansas, Nebraska, New Mexico,

Oklahoma, South Dakota, Texas, and Wyoming, 1930–98

105 

40 

32 

100 

105 

40 

32 

100 

105 

40 

32 

100 105 

40 

32 

100 

0

0 100 200 KILOMETERS

100 200 MILES

Site with pesticide data

Site where pesticide is detected

Site where drinking-water

 standard is exceeded

EXPLANATION

SIMAZINE (MCL = 4) PROMETON (SS = 100)

ATRAZINE (MCL = 3) METOLACHLOR (SS = 70)

Figure 15. Locations of sites with pesticide data and pesticide detections for atrazine, metolachlor, simazine, and prometon.
ANALYSIS OF HISTORICAL WATER-QUALITY DATA 45

Dissolved Solids

Dissolved solids, or salinity, refers to the quantity of mineral salts dissolved in water. Excessive 

salinity impairs the use of water both for irrigation and 

for human consumption. A common measure of 

salinity is dissolved-solids concentration. The USEPA 

secondary drinking-water standard for dissolved solids 

is 500 mg/L (U.S. Environmental Protection Agency, 

1996), although water with concentrations as large as 

2,000 to 3,000 mg/L is sometimes tolerated. Similarly, 

in agricultural use, most crops can flourish in water 

containing as much as 500 mg/L dissolved solids and 

with proper management practices may tolerate water 

with concentrations as large as 2,250 mg/L (U.S. Environmental Protection Agency, 1976). Also relevant to 

irrigation and domestic use of water are the concentrations of the chemical constituents that together constitute dissolved solids. The major chemical constituents 

that constitute dissolved solids are calcium, magnesium, sodium, potassium, bicarbonate, chloride, and 

sulfate. Chloride and sulfate have secondary drinkingwater standards of 250 mg/L, and fluoride has a 

secondary drinking-water standard of 2 mg/L (U.S. 

Environmental Protection Agency, 1996). Excessive 

concentrations of sodium relative to concentrations of 

calcium and magnesium are harmful to plants and soil 

and are quantified using a measure called the sodiumadsorption ratio.

The constituents that constitute dissolved solids 

are all natural substances. They occur in varying 

amounts in soils and rocks and are sometimes naturally concentrated in the form of minerals such as 

gypsum (calcium sulfate). Ground water naturally 

increases in dissolved solids as it moves through earth 

materials by partially dissolving the minerals, and the 

natural salinity of water can vary greatly depending on 

whether the material through which it flows is easily 

soluble (for example, gypsum) or insoluble (for 

example, sandstone). The sand, gravel, and clay that 

compose the High Plains aquifer generally do not 

contain saline minerals, so the salinity of High Plains 

ground water is generally small. However, older, 

underlying bedrock units such as Permian-age formations may contain considerable amounts of saline 

minerals; the ground water that flows through these 

older rocks can discharge upward into the High Plains 

aquifer and thereby increase the salinity of High Plains 

water.

Land-use activities, in particular irrigated agriculture, can have a large effect on dissolved-solids 

concentrations. As crops grow, their roots selectively 

uptake nearly pure water and leave behind in the 

residual water all of the salts present in the original 

water; this more saline water can then infiltrate downward into ground water. In addition, as unused irrigation water infiltrates downward into ground water, 

minerals present in the soil are dissolved. The salinity 

of water generally increases as the exposure of water 

to rocks and soils increases. Irrigation increases water 

exposure by the annual recycling process of pumping 

and infiltration of excess water back into the ground. 

Additional sources of salinity are pumped oil-field 

brines and human and animal wastewater.

As was done for the nitrate data, all dissolvedsolids data in the retrospective data base were selected 

and grouped by decade to examine temporal changes 

in dissolved-solids concentrations for the study area as 

a whole. Boxplots of the decadal data (fig. 16) indicate 

that dissolved-solids concentrations have not changed 

greatly within the High Plains aquifer as a whole. 

Concentrations were largest during the 1930’s, but the 

number of observations during the decade was relaTo examine temporal changes in dissolvedsolids concentrations within each hydrogeologic unit, 

the decadal data set was stratified by hydrogeologic 

unit, and a LOWESS smooth line was calculated for 

each unit (fig. 17). Dissolved-solids concentrations 

historically have been large in the Valley-Fill Alluvium unit and in the Ogallala Formation–South unit 

and have been less than the secondary drinking-water 

standard in the remaining units. Dissolved-solids 

concentrations have increased slightly in all of the 

hydrogeologic units except for the Ogallala Formation-South, where concentrations appear to have 

decreased slightly. However, when the data are tested 

for significance using the Wilcoxon rank-sum test, 

differences between 1930–69 and 1980–98 are significant only for the Valley-Fill Alluvium, Ogallala 

Formation–North, and Ogallala Formation–South 

units. The apparent trends for the other units cannot be 

verified statistically due to the large variability in 

concentrations among samples. The increase in 

dissolved-solids concentrations is largest in the ValleyFill Alluvium unit, where the median concentration of
46 Historical Water-Quality Data for the High Plains Regional Ground-Water Study Area in Colorado, Kansas, Nebraska, New 

Mexico, Oklahoma, South Dakota, Texas, and Wyoming, 1930–98

EXPLANATION

Number of

 observations

90th percentile

75th percentile

Median

25th percentile

10th percentile

1,212

0

1930

761

1940

1,212

1950

1,019

1960

3,448

DISSOLVED-SOLIDS CONCENTRATION,

IN MILLIGRAMS PER LITER

1970

3,234

1980

2,898

1990

2,740

200

400

600

800

1,000

1,200

1,400

1,600

Figure 16. Distribution of dissolved-solids concentrations, by decade.

dissolved solids for 1980–98 (603 mg/L) is in excess 

of the secondary drinking-water standard. The ValleyFill Alluvium unit is susceptible to increasing salinity 

because irrigation water is recycled along alluvial 

valleys in a downstream direction as water is pumped 

from the aquifer or diverted from the river, applied to 

fields, and then returned to the aquifer as irrigation 

drainage. During each irrigation/return flow cycle, 

salinity is increased as water is transpired by crops and 

as return-flow water dissolves additional minerals 

from the soil. Whittemore (Kansas Geological Survey, 

oral commun., 1999) has calculated that sulfate 

concentrations in the Arkansas River and its alluvial 

aquifer could increase by a factor of 2 in the next 

40 years due to this recycling process. Dissolvedsolids concentrations historically have been in excess 

of the drinking-water standard in the Ogallala Formation–South unit. In this area, excessive concentrations 

1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

100

150

200

250

300

350

400

450

500

550

600

650

700

Ogallala Formation—North

Ogallala Formation—Central

Ogallala Formation—South

Arikaree-Brule Formations

Nebraska Sand Hills

Kansas Quaternary Deposits

Nebraska Quaternary Deposits

Valley-Fill Alluvium

YEAR OF SAMPLE

DISSOLVED-SOLIDS CONCENTRATION,

IN MILLIGRAMS PER LITER

EXPLANATION

Figure 17. Temporal variability of dissolved-solids concentrations, by hydrogeologic unit.
ANALYSIS OF HISTORICAL WATER-QUALITY DATA 47

have been attributed both to saline water discharging 

to the aquifer from underlying bedrock units and to the 

effects of agricultural land use (Reeves and Miller, 

1978). The decrease in dissolved-solids concentrations 

in this unit might partly be an artifact of more dense 

sampling of wells during 1940–50 in the southern part 

of the unit (where salinity is larger) than in the 

northern part of the unit (where salinity is smaller).

To examine temporal variability of dissolvedsolids concentrations at a more local geographic scale, 

a dissolved-solids concentration map was constructed 

from the High Plains retrospective data base by using 

data from 1980–98; this concentration map was 

compared to a similar map constructed by Krothe and 

others (1982) using data prior to 1980 (fig. 18). The 

concentration maps indicate that there is considerable 

variation of dissolved-solids concentrations within 

hydrogeologic units. Comparison of these maps 

suggests that dissolved-solids concentrations have 

increased in the vicinities of the alluvial valleys of the 

Platte River, the Republican River, and the Arkansas 

River. In the Ogallala Formation–South hydrogeologic 

unit, the size of areas having dissolved-solids concentrations larger than 500 mg/L has increased. Also, 

dissolved-solids concentrations have increased in the 

Colorado portion of the Ogallala Formation–North 

unit.

Data from the retrospective data base were next 

evaluated to test for relations between dissolved-solids 

concentrations and site characteristics. Because there 

is temporal variability in the data, only data from the 

recent period (1980–98) were used, and because there 

is variability based on hydrogeologic unit, tests were 

done separately for each unit. Where multiple observations were available for a well, only the most recent 

observation was used. Differences were verified using 

the Wilcoxon rank-sum test and identified using the 

Tukey multiple comparison test. Well type (observation or water-withdrawal) was not a significant factor 

relative to dissolved-solids concentrations. Water-use 

type was a significant factor in two units (Ogallala 

Formation–Central and Ogallala Formation–South), 

where domestic wells had significantly larger 

dissolved-solids concentrations than did wells used for 

other purposes. Land use was a significant factor in 

four units (Kansas Quaternary Deposits and the three 

Ogallala Formation units), where dissolved-solids 

concentrations were larger for wells in urban land use 

than for wells in other land uses. Underlying bedrock 

was a significant factor in four units. In three units 

(Nebraska Sand Hills, Ogallala Formation–North, and 

Ogallala Formation–Central), dissolved-solids 

concentrations were larger at sites underlain by Late 

Cretaceous-age rocks than at sites underlain by other 

geologic units. In the Ogallala Formation–South, 

dissolved-solids concentrations were larger at sites 

underlain by Early Cretaceous-age rocks than at sites 

underlain by other geologic units.

Dissolved-solids concentrations were tested for 

relations with continuous numerical variables by using 

the Kendall’s tau correlation coefficient (table 11). For 

wells having sampled-interval information, two additional variables were calculated. Depth of the sample 

below the water table was calculated as was done with 

nitrate, and depth of the sample above bedrock was 

calculated using information about depth to bedrock. 

Each of the correlation coefficients shown in this table 

is significant (α = 0.05), but correlations are generally 

weak. As with nitrate, dissolved-solids concentrations 

commonly were inversely correlated with well depth 

and with depth of the sample below the water table. A 

significant inverse relation between dissolved-solids 

concentration and depth of sample above bedrock was 

observed for three units (Ogallala Formation–South, 

Nebraska Sand Hills, and Kansas Quaternary 

Deposits); that is, the closer to bedrock a sample was 

taken, the larger the dissolved-solids concentration. 

This may indicate that these three units are susceptible 

to increased salinity from saline water discharging 

upward from underlying bedrock units. In contrast 

with nitrate, dissolved-solids concentrations generally 

were inversely correlated with soil permeability; that 

is, dissolved-solids concentrations were larger where 

soil permeability was low. This might indicate that 

low-permeability soils are more likely to contain 

mineral salts.

To examine the composition of dissolved solids 

in samples from High Plains wells, data were extracted 

from the High Plains data base for wells that had 

complete major ion analyses. All available historical 

data were used in order to evaluate the major ion 

chemistry of each hydrogeologic unit. For wells that 

were sampled more than once, the best-quality data 

were selected; data quality was evaluated by calculating a charge balance for each water analysis and by 

using the analysis with the best charge balance. 

Boxplots of major ion composition for each hydrogeologic unit (fig. 19) indicate that all of the units are 

calcium bicarbonate type waters but that there is some 

variability in water types among the units. The 
48 Historical Water-Quality Data for the High Plains Regional Ground-Water Study Area in Colorado, Kansas, Nebraska, New Mexico,

Oklahoma, South Dakota, Texas, and Wyoming, 1930–98

0

105

40 40 

32 32 

105 

100

50 100 150 200 MILES

Less than 250250 to 500501 to 1,000More than 1,000

Dissolved-solids concentration, in milligrams per liter

EXPLANATION

Figure 18. Geographic distribution of dissolved-solids concentrations: data prior to 1980 and data after 1980.
ANALYSIS OF HISTORICAL WATER-QUALITY DATA 49

Nebraska Sand Hills water is the simplest water type, 

consisting almost entirely of calcium and bicarbonate, 

and Ogallala Formation–South water is most complex, 

having substantial proportions of all the major ions 

except potassium. Sulfate concentration is large in 

Ogallala Formation–South water and Valley-Fill Alluvium water but also is a substantial component of 

Ogallala Formation-North and Kansas Quaternary 

water. Chloride concentration is largest in Ogallala 

Formation–South water but also is a substantial 

component of Kansas Quaternary and Valley-Fill 

Alluvium water. Chloride is more common than 

sulfate only in Kansas Quaternary water.

Median concentrations of major ions for each 

unit as depicted in figure 19 might be considered 

typical values for background major ion chemistry for 

the High Plains aquifer waters except for the ValleyFill Alluvium unit where dissolved-solids concentrations have changed substantially over time. Many of 

the boxplots in figure 19 are symmetrical in shape, 

which suggests normal distributions of data and water 

derived under a stable set of natural processes. Water 

contamination or mixing of water types might be 

expected where data distributions are skewed, such as 

for sulfate in the Valley-Fill Alluvium and Ogallala 

Formation–South units, and sodium and chloride in 

the Kansas Quaternary Deposits unit. Individual well 

analyses can be compared to the median values to 

investigate whether shifts in major ion chemistry have 

occurred. This type of analysis can be used to detect 

contamination from human and animal wastes (large 

chloride concentrations), contamination from agricultural land use (large sulfate concentrations), or 

contamination from oil-field brines (large sodium and 

chloride concentrations).

Other Constituents

Other constituents in the retrospective data base 

found to have large percentages of exceedances of 

water-quality standards are iron, manganese, fluoride, 

radon, and uranium (table 8). Iron is the most abundant metallic element in the Earth’s outer crust, 

manganese is the second-most abundant metal 

(although it only is about one-fiftieth as common as 

iron), and fluoride is the most common of the halogen 

group of elements in rocks. These three constituents 

are usually considered major solutes in water and will 

be discussed together. Radon and uranium are radioactive elements, which are not abundant in the Earth’s 

crust, and they will be discussed together.

Iron and manganese are required micronutrients 

for plants and animals, and fluoride is used by animals 

in building bones and teeth (Hem, 1992). However, 

secondary drinking-water standards have been established by the USEPA (U.S. Environmental Protection 

Agency, 1996) for these constituents (300 μg/L for 

iron, 50 μg/L for manganese, and 2 mg/L for fluoride) 

because higher concentrations cause stain or taste 

problems in water. In addition, fluoride has a 

maximum contaminant level of 4 mg/L in drinking

Table 11. Kendall’s tau correlation coefficients between dissolved-solids concentrations and ancillary variables, by 

hydrogeologic unit

[Correlations shown only if the magnitude is greater than 0.10; all correlations shown are significant at α = 0.05; --, no correlation]

Hydrogeologic

unit

Estimated 

depth to 

water

Well 

depth

Depth of 

sample 

below 

water 

table

Depth of 

sample 

above 

bedrock

Irrigation 

intensity

Nitrogen 

input 

Soil 

permeability

Age of

well

Ogallala Formation—North -- –0.27 –0.17 -- -- -- –0.16 0.11

Ogallala Formation—Central -- -- -- -- -- -- -- --

Ogallala Formation—South –0.45 –0.46 –0.06 –0.13 –0.24 –0.18 0.40 --

Arikaree-Brule Formations -- –0.10 –0.14 -- -- –0.14 –0.15 --

Nebraska Sand Hills -- –0.24 -- –0.45 0.33 0.39 –0.30 --

Kansas Quaternary Deposits -- -- –0.12 –0.17 -- -- –0.38 --

Nebraska Quaternary Deposits -- -- -- -- 0.19 0.15 –0.18 --

Valley-Fill Alluvium -- –0.12 –0.34 0.24 -- –0.12 -- --
50 Historical Water-Quality Data for the High Plains Regional Ground-Water Study Area in Colorado, Kansas, Nebraska, New 

Mexico, Oklahoma, South Dakota, Texas, and Wyoming, 1930–98

 

0

2

4

6

8

10

0

2

4

6

8

10

0

2

4

6

8

10

CONCENTRATION, IN MILLIEQUIVALENTS PER LITERCONCENTRATION, IN MILLIEQUIVALENTS PER LITER

0

2

4

6

8

10

0

2

4

6

8

10

0

2

4

6

8

10

0

2

4

6

8

10

0 Ca Mg Na K HCO3 Cl SO4 Ca Mg Na K HCO3 Cl SO4

2

4

6

8

10

90th percentile

EXPLANATION

75th percentile

Median

25th percentile

10th percentile

Ogallala Formation—North

Valley-Fill Alluvium Arikaree-Brule Formations

Nebraska Sand Hills

Nebraska Quaternary Deposits

Kansas Quaternary Deposits

Ogallala Formation—Central

Ogallala Formation—South

Figure 19. Major ion chemistry of High Plains ground water, by hydrogeologic unit.
ANALYSIS OF HISTORICAL WATER-QUALITY DATA 51

water because it can cause bone disease or mottled 

teeth in children. Although each of these three constituents has multiple industrial, agricultural, or other 

human uses, their presence in ground water is generally from natural sources. Human activities can affect 

iron and manganese concentrations through altering 

the geochemical environment and causing the trace 

elements to become more mobile. Well construction 

can affect concentrations by allowing waters from 

different aquifers to mix. Iron concentrations in well 

water can be increased by corrosion of the metallic 

well casings.

The geographic distributions of exceedances of 

drinking-water standards for iron, manganese, and 

fluoride are shown in figure 20. Exceedances for iron 

were most common in the central part of the study 

area. Iron concentrations were not significantly 

different based on well casing type. Manganese 

exceedances occurred most often in the northern part 

of the study area, while fluoride exceedances occurred 

most often in the southern part of the study area.

Radon, a water-soluble gas formed by the radioactive decay of the element radium, is common in 

small concentrations in ground water. Uranium occurs 

naturally throughout the Earth’s outer crust and generally is present in small concentrations in ground water; 

elevated concentrations generally are associated with 

uranium ore deposits. The human intake of radon and 

uranium is a health concern because both are radioactive substances; uranium also is of concern as a 

possible toxic chemical substance. Humans are 

exposed to radon primarily through the air and secondarily through drinking water. Humans are exposed to 

uranium in drinking water and in food. The USEPA 

has established proposed drinking-water maximum 

contaminant levels for radon (300 pCi/L) and uranium 

(20 μg/L); these levels are under review, and final 

standards are expected in late 2000. A new proposed 

rule for radon gas would establish an alternative 

maximum contaminant level for radon of 4,000 pCi/L 

for community water systems that have in place a 

multimedia mitigation program that includes control 

of radon in air (U.S. Environmental Protection 

Agency, 1999).

The proposed drinking-water standard for radon 

(300 pCi/L) was exceeded in 375 observations, or 69 

percent of the total observations in the retrospective 

data base. The proposed drinking-water standard for 

uranium (20 μg/L) was exceeded in 460 observations, 

or 9 percent of the total observations in the retrospective data base. The occurrence of radon and uranium 

drinking-water standard exceedances by county is 

shown in figure 21. Most radon data are from New 

Mexico, Texas, and the Platte River valley of 

Nebraska. Within these areas, most exceedances were 

in New Mexico and in three counties in western 

Nebraska. The exceedances in Nebraska may be 

related to volcanic ash in the Brule Formation 

(Verstraeten and others, 1995). Uranium exceedances 

were primarily along the Platte River in Nebraska and 

in one county in southwest Kansas. The large values 

along the Platte River in western Nebraska have been 

attributed to oxidizing conditions that favor uranium 

solubility (Verstraeten and others, 1995). In the Platte 

River valley in central Nebraska, uranium exceedances have been related to leachates from evaporite 

deposits in bottomland soils (Spalding and Druliner, 

1981). In both areas, the ultimate source of the 

uranium has been attributed to sediments derived from 

the ancestral Rocky Mountains and transported downstream by alluvial and colluvial processes. Uranium in 

the Arkansas River valley in Colorado has been linked 

to phosphate fertilizers, which can contain trace 

amounts of uranium (Zielinski and Asher-Bolinder, 

1996).
52 Historical Water-Quality Data for the High Plains Regional Ground-Water Study Area in Colorado, Kansas, Nebraska, New Mexico,

Oklahoma, South Dakota, Texas, and Wyoming, 1930–98 

105 

40 

32 

100 

105 

40 

32 

100 

105 

40 

32 

100 

IRON (SS = 300 μg/L) MANGANESE (SS = 50 μg/L) FLOURIDE (SS = 2 μg/L)

Figure 20. Distribution of the percentage of iron, manganese, and fluoride observations that exceeded drinking-water standards.
ANALYSIS OF HISTORICAL WATER-QUALITY DATA 53 Figure 21. Distribution of the number of radon and uranium observations that exceeded drinking-water standards. EXPLANATION None 1 to 10 More than 10 No data 00 50 100 150 200 KILOMETERS

105°

40°

32°

100°

105°

40°

32°

100°

50 100 150 200 MILES

RADON (PMCL = 300 pCi/L) URANIUM (PMCL = 20 μg/L)
54 Historical Water-Quality Data for the High Plains Regional Ground-Water Study Area in Colorado, Kansas, Nebraska, New Mexico,

Oklahoma, South Dakota, Texas, and Wyoming, 1930–98

SUMMARY

The High Plains aquifer underlies 174,000 

square miles in parts of eight States and includes eight 

primary hydrogeologic units. It includes the wellknown Ogallala Formation and is an important 

resource, providing water for 27 percent of the 

Nation’s irrigated agricultural lands in this otherwise 

dry landscape. Since the 1980’s there has been concern 

over the sustainability of the aquifer due to water-level 

declines caused by substantial pumping. Water quality 

of the aquifer is a more recent concern. States and 

local governmental entities have done local studies on 

water quality, but few aquiferwide assessments have 

been made. The U.S. Geological Survey National 

Water-Quality Assessment Program (NAWQA) is an 

ongoing effort to document water-quality conditions 

across the United States, and the High Plains Regional 

Ground-Water Study Area was created as part of 

NAWQA in 1998.

One of the first elements of the High Plains 

Regional Ground-Water NAWQA Study was to gather 

existing historical water-quality data into a retrospective data base, which can be used to evaluate occurrence and distribution of water-quality constituents of 

concern. Data from 43 agencies were compiled into 

this data base. In addition to water-quality information, the data base includes well-site information such 

as well depth, aquifer, underlying bedrock, land use, 

and agricultural practices. The data base contains 

water-quality information collected at 29,041 sites. 

Over one million water-quality observations are 

present in the data base, with information about 733 

different water-quality parameters. Considerable 

historical information for nitrate and salinity is available for the High Plains study area. However, data for 

important constituents such as pesticides is more 

sparse and has poorer spatial and temporal distribution. The historical data base is included in digital 

form as a companion to this report. 

Data from the retrospective data base verify that 

nitrate, pesticides, and salinity are important waterquality concerns in the High Plains study area. Additional constituents in the retrospective data base found 

to have large percentages of exceedances of U.S. Environmental Protection Agency drinking-water standards were iron, manganese, fluoride, radon, and 

uranium. 

Sixteen percent of all nitrate observations were 

larger than the drinking-water standard of 10 mg/L. In 

six of the eight geohydrologic units and in about 70 

percent of the counties within the High Plains study 

area, nitrate concentrations for the period 1980–98 

were significantly larger than for the period 1930–69. 

While nitrate concentrations are largest in the geohydrologic units with the shallowest depth to water, 

nitrate concentrations also have increased in the units 

of the Ogallala Formation where depth to water is 

large. Relations between nitrate concentrations and 

well-site characteristics varied among geohydrologic 

units. Well depth, depth to water, and depth of sample 

below the water table were related to nitrate concentrations in at least four of the geohydrologic units. 

Correlations were generally poor and probably reflect 

the large variability of geographic and site-specific 

well characteristics even within the individual geohydrologic units. Additional site-specific information, 

such as information about potential contamination 

pathways and processes, might be needed to accurately predict which areas are susceptible to nitrate 

contamination.

About 50 pesticides were detected in the High 

Plains study area, but only 11 pesticides were detected 

at 10 or more sites. Only four pesticides (atrazine, 

alachlor, cyanazine, and simazine) had observations 

exceeding a water-quality standard. For each of the 

four pesticides, data primarily are available only in the 

northern half of the study area. Sixty percent of sites 

with atrazine detections, 59 percent of sites with metolachlor detections, 80 percent of sites with simazine 

detections, and 86 percent of sites with prometon 

detections were in the Valley-Fill Alluvium hydrogeologic unit. Atrazine, the most commonly used pesticide, had the most exceedances of drinking-water 

standards (62) and was most commonly detected 

(about 25 percent of all observations). Atrazine detections were significantly larger in agricultural land use 

than in rangeland use and were significantly larger 

where the principal crop was corn or sorghum. 

Although pesticides were not commonly detected in 

the deeper Ogallala Formation units, the detection of 

pesticides in deeper aquifers indicates that contamination pathways exist.

Dissolved-solids concentrations, which are a 

direct measure of salinity, had 29 percent of observations in excess of the secondary drinking-water standard of 500 mg/L. Two constituents that contribute to 

salinity, chloride and sulfate, had 8 and 7 percent of 

observations exceeding their respective secondary 

drinking-water standards. Comparison of dissolved-
REFERENCES CITED 55

solids concentrations prior to 1980 to concentrations 

after 1980 indicates dissolved-solids concentrations 

have increased in the vicinities of the alluvial valleys 

of the Platte River, the Republican River, and the 

Arkansas River, as well as in the Ogallala Formation–

South hydrogeologic unit. Among site characteristics, 

dissolved-solids concentrations generally were 

inversely correlated with well depth, depth to water 

below the water table, and soil permeability.

These water-quality results indicate that human 

activities are affecting the water of the High Plains 

aquifer. Because there is a potential for water quality 

to become impaired relative to the historical uses of 

the aquifer, water quality needs to be considered when 

evaluating the sustainability of the High Plains aquifer. 

Historical data has limitations for evaluating water 

quality: data are lacking for some constituents and in 

some areas, and data are lacking for ancillary site characteristics that are needed to understand the processes 

that affect water quality. Data collected as part of the 

High Plains Regional Ground-Water Study will help to 

fill in gaps in water-quality information and provide 

additional information for understanding the factors 

that govern ambient water quality.

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APPENDIX

APPENDIX 61

Appendix A. Description of data on the CDROM and instructions on the use of the data 

base

The retrospective data base is on the CD-ROM 

in two formats: Microsoft Access mdb file and ASCII 

files. The Access mdb file Data.mdb is a single selfcontained file that includes the data base. Microsoft 

Access software is required to enable use of this file. 

ASCII versions of files can be used as they are or 

loaded into any data-base software that reads standard 

ASCII format; there is a separate ASCII file for each 

table in the data base. If ASCII files are loaded into a 

relational data base, the tables should be linked as 

depicted in the relationship diagram in Appendix B-4.

Files on CD-ROM are read only. Therefore, to 

make changes to files (for instance, when making a 

query in a MS Access data base), the files should first 

be copied to a hard drive, and the read-only access 

restrictions should be removed.

Additional information on the use of the data 

base in either Microsoft Access or ASCII formats can 

be found in the README_ASCII.txt files and 

README_ACCESS.txt files on the CD-ROM. 

Appendix B. Diagrams showing content and 

structure of the data base

The following four tables give information 

about the content of the High Plains retrospective 

water-quality data base. Table B-1 shows a listing of 

the tables in the data base as presented in the Access 

data-base container. Users loading data from ASCII 

format need to create these same tables by loading 

separate ASCII files. Table B-2 shows the fields 

included in the SITE data-base table, and table B-3 

shows the fields in the QW_RESULT data-base table. 

Table B-4 shows a relationship diagram for the High 

Plains retrospective data base. The data base is a relational data base, and for optimal use, the individual 

tables need to be linked in the manner depicted in 

table B-4 to allow full querying capabilities of the 

data.
62 Pesticides in Surface Water in Agricultural Areas of the South Platte River Basin, from Denver, Colorado, to North Platte, 

Nebraska, 1993–94

Appendix B-1. Tables included in the High Plains retrospective water-quality data base.
APPENDIX 63

Appendix B-2. Data fields included in SITE table of High Plains retrospective water-quality data base.
64 Pesticides in Surface Water in Agricultural Areas of the South Platte River Basin, from Denver, Colorado, to North Platte, 

Nebraska, 1993–94

Appendix B-3. Data fields included in QW_RESULT table of High Plains retrospective water-quality data base.
APPENDIX 65

Appendix B-4. Relationship diagram for High Plains retrospective water-quality data base.