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Points of Contact and Additional Information

The companion Web site for NAWQA reports:

http://water.usgs.gov/nawqa/nawqa_sumr.html

High Plains Regional Groundwater Study contact and Web site:

USGS State Representative

U.S. Geological Survey

Colorado Water Science Center

Denver Federal Center, M.S. 415

Denver, CO 80225

http://co.water.usgs.gov/nawqa/hpgw/HPGW_home.html

National Water-Quality Assessment (NAWQA) Program:

Chief, NAWQA Program

U.S. Geological Survey

Water Resources Discipline

12201 Sunrise Valley Drive, M.S. 413

Reston, VA 20192

http://water.usgs.gov/nawqa/

Cover photograph: 

Aerial view of irrigated and nonirrigated cropland and rangeland of the High Plains (photograph courtesy of John Charlton, Kansas Geological Survey). 
Water Quality in the High Plains Aquifer, 

Colorado, Kansas, Nebraska, New Mexico, 

Oklahoma, South Dakota, Texas, and 

Wyoming, 1999–2004

By Jason J. Gurdak, Peter B. McMahon, Kevin Dennehy, and Sharon L. Qi

Circular 1337

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Suzette Kimball, Acting Director

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

For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, 

natural hazards, and the environment, visit http://www.usgs.gov or call 1-888-ASK-USGS

For an overview of USGS information products, including maps, imagery, and publications, 

visit http://www.usgs.gov/pubprod

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Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

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

reproduce any copyrighted materials contained within this report.

Suggested citation:

Gurdak, J.J., McMahon, P.B., Dennehy, K.F., and Qi, S.L., 2009, Water quality in the High Plains aquifer, Colorado, 

Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas, and Wyoming, 1999–2004: U.S. Geological Survey 

Circular 1337, 63 p.
 iii

—Jim Goeke

Professor and Research Hydrogeologist

University of Nebraska Cooperative Extension

“The rigorous scientific approach of the USGS NAWQA Program and 

the resulting improved understanding of groundwater quality in 

the High Plains aquifer will be invaluable for local and statewide management toward sustainable groundwater resources of 

this important aquifer system.”
iv

Foreword

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

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

that facilitates effective management of water, biological, energy, and mineral resources 

(http://www.usgs.gov/). Information on the Nation’s water resources is critical to ensuring longterm availability of water that is safe for drinking and recreation and is suitable for industry, 

irrigation, and fish and wildlife. Population growth and increasing demands for water make the 

availability of that water, now measured in terms of quantity and quality, even more essential to 

the long-term sustainability of our communities and ecosystems.

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

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

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

designed to answer: What is the condition of our Nation’s streams and groundwater? How are 

conditions changing over time? How do natural features and human activities affect the quality 

of streams and groundwater, and where are those effects most pronounced? By combining 

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

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

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

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

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

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

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

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

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

in characterizing the quality of surface water and ground water, and by determining status and 

trends at sites that have been consistently monitored for more than a decade. In addition, the 

regional assessments continue to build an understanding of how natural features and human 

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

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

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

models thereby enhance the value of our existing data and our understanding of the hydrologic 

system. In addition, the models are useful in evaluating various resource-management scenarios 

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

contamination, land conversion, and altering flow and (or) pumping regimes, are likely to affect 

water conditions within a region. Other activities planned during the second decade include 

continuing national syntheses of information on pesticides, volatile organic compounds (VOCs), 

nutrients, selected trace elements, and aquatic ecology; and continuing national topical studies 

on the fate of agricultural chemicals, effects of urbanization on stream ecosystems, bioaccumulation of mercury in stream ecosystems, effects of nutrient enrichment on stream ecosystems, 

and transport of contaminants to public-supply wells.

NAWQA National Water-Quality Assessment Program
v

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

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

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

to meet your needs, and will foster increased citizen awareness and involvement in the protection and restoration of our Nation’s waters. The USGS recognizes that a national assessment by 

a single program cannot address all water-resource issues of interest. External coordination at 

all levels is critical for cost-effective management, regulation, and conservation of our Nation’s 

water resources. The NAWQA Program, therefore, depends on advice and information from 

other agencies—Federal, State, regional, interstate, Tribal, and local—as well as nongovernmental organizations, industry, academia, and other stakeholder groups. Your assistance and 

suggestions are greatly appreciated.

Matthew C. Larsen, 

USGS Associate Director for Water

Basin and Range

 Basin-fill aquifers

 Carbonate-rock aquifers

Biscayne aquifer

California Coastal Basin aquifers

Cambrian-Ordovician aquifer system

Central Valley aquifer system

Coastal Lowlands aquifer system

Columbia Plateau

 Basin-fill aquifers

 Basaltic-rock aquifers

Denver Basin aquifer system

Edwards-Trinity aquifer system

Floridan aquifer system

Surficial aquifer system 

 (overlying the Floridan)

Glacial aquifer system

Hawaiian volcanic-rock aquifers—Locally overlain by

 sedimentary deposits

New England crystalline-rock aquifers

High Plains aquifer

Mississippi Embayment–Texas Coastal Uplands

 aquifer system

Northern Atlantic Coastal Plain aquifer system

Piedmont and Blue Ridge

 Carbonate-rock aquifers

 Crystalline-rock aquifers

Early Mesozoic aquifers

Rio Grande aquifer system

Snake River Plain

 Basaltic-rock aquifers

 Basin-fill aquifers

Valley and Ridge aquifers–Carbonate-rock

 aquifers are patterned

EXPLANATION

1

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Figure 1. Locations of NAWQA regional assessments in principal (or other) aquifers.
vi

Contents

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

Introduction to this Report .........................................................................................................................vii

Chapter 1 — Overview of Major Findings and Implications...................................................................1

Chapter 2 — Background on the Water Resource Beneath the High Plains......................................7

Chapter 3 — NAWQA Approach to Assessing Water Quality in the High Plains Aquifer...............13

Chapter 4 — Quality of Water Near the Water Table—What Was Found, Where, and Why ........18

Chapter 5 — Natural Processes and Human Activities Affecting the Quality of the Water 

as it Moves From the Water Table to the Resource Used for Drinking and Irrigation........30

Chapter 6 — Quality of the Resource Used for Drinking and Irrigation—What Was Found, 

Where, and Why.............................................................................................................................39

References Cited..........................................................................................................................................47

Glossary.........................................................................................................................................................50

Appendix 1 – Abbreviations and Acronyms ............................................................................................52

Appendix 2 – Study-Unit Design................................................................................................................53

Appendix 3 – High Plains Findings in a National Context .....................................................................58

Acknowledgments.......................................................................................................................................63

 Boxes

A—Unique Features of the NAWQA Approach.......................................................................14

B—How Stable Isotopes Provide Insight Into the Sources and Sinks of 

Nitrate in the Aquifer.......................................................................................................22

C—Understanding Vertical Directions of Water Movement in the Aquifer 

using Groundwater-Level Measurements...................................................................32

D—Short Circuiting of Multiscreened Wells Increases Groundwater 

Vulnerability to Contamination.......................................................................................38
 vii

Introduction to this Report

This report contains the major findings of a 1999–2004 assessment of water 

quality in the High Plains aquifer. It is one of a series of reports by the National 

Water-Quality Assessment (NAWQA) Program that present major findings 

for principal and other aquifers and major river basins across the Nation. In 

these reports, water quality is discussed in terms of local, regional, State, 

and national issues. Conditions in the aquifer system are compared to conditions found elsewhere and to selected national benchmarks, such as those for 

drinking-water quality.

This report is intended for individuals working with water-resource issues 

in Federal, State, or local agencies, universities, public interest groups, or the 

private sector. The information will be useful in addressing a number of current 

issues, such as drinking-water quality, the effects of agricultural practices on 

water quality, source-water protection, and monitoring and sampling strategies. 

This report is also for individuals who wish to know more about the quality of 

ground water in areas near where they live and how that water quality compares 

to the quality of water in other areas across the region and the Nation.

The water-quality conditions in the High Plains aquifer summarized in this 

report are discussed in greater detail in other reports that can be accessed in 

Appendix 1 of http://pubs.usgs.gov/pp/1749/. Detailed technical information, 

data and analyses, collection and analytical methodology, models, graphs, and 

maps that support the findings presented in this report in addition to reports in 

this series from other basins can be accessed from the national NAWQA Web 

site (http://water.usgs.gov/nawqa). This report accompanies the detailed and 

technical report of water-quality conditions in the High Plains aquifer “Waterquality assessment of the High Plains aquifer, 1999–2004” 

(http://pubs.usgs.gov/pp/1749/).

Detailed description of water quality in the High Plains aquifer from 

1999 to 2004 is in the following report: 

Water-quality assessment of the High Plains aquifer, 1999–2004

by P.B. McMahon, K.F. Dennehy, B.W. Bruce, J.J. Gurdak, and S.L. Qi:

U.S. Geological Survey Professional Paper 1749 available at

http://pubs.usgs.gov/pp/1749/
viii Water Quality in the High Plains Aquifer, 1999–2004

Irrigated agriculture is the dominant use of groundwater from the High Plains aquifer and can have 

substantial effects on groundwater quantity and quality in this important aquifer system. (Photograph by 

Jason J. Gurdak, USGS.)
Overview of Major Findings and Implications 1

Chapter 1 Overview of Major Findings and Implications

Today, improved groundwater 

pumping technologies that 

support agriculture have 

transformed the High Plains 

aquifer into the most heavily 

pumped aquifer in the 

United States. 

Photograph by Jason J. Gurdak, USGS. 

Agricultural activities and 

groundwater pumping 

can affect the quality of 

groundwater in the High 

Plains aquifer in many ways. 

The NAWQA assessment of water-quality conditions of the High Plains 

aquifer for the period 1999–2004 provides the first systematic and comprehensive regional assessment of water quality in this nationally important aquifer. NAWQA results show where, when, why, and how specific water-quality 

conditions occur in groundwater across the High Plains aquifer and yield 

science-based implications for assessing and managing the quality of this water 

resource. Understanding groundwater quality conditions and the natural and 

human factors that control water quality in this aquifer is important because of 

the implications to human health, the sustainability of rural agricultural economies, and the substantial costs associated with land and water management, 

conservation, and regulation.

Windmills, symbolic of the nineteenth-century 

American West, were used to harness the winds of 

the High Plains to pump groundwater from the aquifer 

for people, animals, and crops. 

Photograph by John Charlton

Kansas Geological Survey.

This chapter provides 

a broad overview of 

NAWQA findings about 

the occurrence and 

distribution, trends, 

and factors controlling 

water-quality conditions 

in the High Plains aquifer 

and summarizes the 

major implications of 

these findings for waterquality assessment and 

management. Detailed 

discussions of each major 

topic are provided in 

subsequent chapters.

Chapter 1
2 Water Quality in the High Plains Aquifer, 1999–2004

Chemical transport to the water table follows fast 

and slow paths – Throughout much of the High Plains 

aquifer, the transport of chemicals from the land surface to the 

water table is slow, taking from decades to centuries. However, 

local variations in rates of chemical transport do occur, such as 

beneath natural depressions in the land surface that collect 

precipitation and return flow from irrigated fields, which can 

reduce the transport time of chemicals to the water table to 

months or years. See Chapter 4. 

Overview of Major Findings 

NITRATE CONCENTRATION,

 IN MILLIGRAMS PER LITER AS NITROGEN

Drinking-water 

standard

0 5 10 15 20 25 30

DEPTH OF WELL SCREEN 

BELOW WATER TABLE, IN FEET

0

100

200

300

400

500

600

700

800

Important differences exist between the quality of shallow 

groundwater and deep groundwater – Water in deeper parts of the 

aquifer was recharged under natural conditions during the past several 

thousand years. In contrast, shallow groundwater near the water table 

under irrigated cropland was mostly recharged during the past 50 years. 

The introduction of modern agricultural recharge near the water table has 

increased concentrations of dissolved solids, nitrate (see figure (left) of 

nitrate concentration relative to depth below water table), pesticides, and 

other constituents in shallow groundwater compared to deeper groundwater. See Chapters 4 and 6. (Data compiled from all land-use, majoraquifer, and regional-transect study wells. See Chapter 3.) 

Conversion of rangeland to irrigated cropland 

affects groundwater quality – Irrigation enhances 

recharge in this semiarid area and has altered the quality of 

water recharging the aquifer by increasing the transport of 

agricultural chemicals and natural salt deposits to the water 

table. Concentrations of dissolved solids, nitrate, pesticides, 

and other constituents were elevated at the water table, 

reflecting the change in quality of recharge in areas converted 

from rangeland to cropland with applied agricultural chemicals. 

See Chapter 4. 

Availability and sustainability of groundwater in the High 

Plains aquifer are a function of water quantity and quality – 

The High Plains aquifer underlies parts of eight States and is one of the 

largest aquifers in the world. The current availability and future sustainability of a high-quality groundwater supply is central to the overall 

health of the High Plains agricultural economy, viability of its cities and 

rural communities, and environmental well-being of the landscape. Use 

of the High Plains aquifer for irrigation water has transformed this area 

into an important agricultural region that sustains more than one-fourth 

of the Nation’s agricultural production. 
Overview of Major Findings and Implications 3

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

2

1

0

Natural Attenuation of Nitrate Requires

Hundreds to Thousands of Years 

The quality of most water produced by private, publicsupply, and irrigation wells is suitable for the intended 

use – Concentrations of chemical constituents generally were low and 

exceeded U.S. Environmental Protection Agency (USEPA) drinkingwater standards (Maximum Contaminant Level or Secondary Maximum 

Contaminant Level) in 37.5 percent of sampled private wells used for 

drinking. Most of the exceedances of primary (health) and secondary 

(esthetics) drinking-water standards were for dissolved solids, nitrate, 

fluoride, iron, manganese, and arsenic. See Chapter 6. 

Range of times required to 

lower nitrate concentrations

The aquifer has a limited ability to naturally attenuate nitrate – 

The natural rate of nitrate attenuation is slow in most parts of the High 

Plains aquifer. Hundreds to thousands of years could be required to lower 

nitrate concentrations by just 1 milligram per liter (mg/L) (as nitrogen). 

Therefore, nitrate that enters the aquifer is likely to remain in the aquifer 

well into the foreseeable future. See Chapter 5. 

Mixing of groundwater by high-capacity wells with long or 

multiple screens adversely affects water quality – In many 

parts of the High Plains aquifer, high-capacity water-supply wells with 

long or multiple screens can mix water from near the water table with 

water from deeper in the aquifer where most of the wells screens extend. 

As a result, the quality of water produced by water-supply wells can be 

adversely affected by elevated concentrations of dissolved constituents 

that are normally present in water near the water table. See Chapter 5. 

Photographs by USGS staff: Breton W. Bruce, Kevin F. Dennehy, 

Jason J. Gurdak, and Peter B. McMahon. 

Groundwater quality of the High Plains aquifer – In general, 

groundwater in the High Plains aquifer currently meets Federal and State 

guidelines for drinking-water quality. However, many factors, such as 

water use and chemical use, can affect water quality over time. An 

understanding is therefore needed on the timing and magnitude of the 

transport of chemicals from the land surface, through the unsaturated 

zone, and to groundwater for informed future management and development of this limited resource. 

for the High Plains aquifer 

NITRATE CONCENTRATION,

 IN MILLIGRAMS PER LITER 

AS NITROGEN

TIME (YEARS)

Chapter 1
4 Water Quality in the High Plains Aquifer, 1999–2004

Availability and Sustainability of Groundwater in the High Plains Aquifer are a Function of 

Water Quantity and Quality 

Groundwater is the primary source of water used in the High Plains; thus, knowledge about groundwater availability and 

sustainability are essential for the informed management of this limited resource. Groundwater availability and sustainability are 

functions of many factors, one of which is water quality. Understanding groundwater quality is important because it directly affects how water can be used. Water quality generally has been overlooked in the High Plains because the primary focus has been 

on obtaining a sufficient water supply, and it has been broadly assumed that the aquifer contains high-quality water. For the most 

part, results from the NAWQA study support that assumption. See Chapters 4, 5, and 6 for more details.

Conversion of Rangeland to Irrigated Cropland Affects Water Quality 

NAWQA findings indicate that the nitrogen and pesticide applications on irrigated cropland result in substantially more 

nitrate and pesticides being transported to the water table at irrigated cropland settings than at rangeland settings. In some locations, however, the change in recharge chemistry following conversion of rangeland to cropland results from the mobilization by 

irrigation water of large natural nitrate and chloride deposits in the unsaturated zone (above the water table). See Chapter 4 for 

more details.

•	Groundwater quality, particularly regarding elevated nitrate or dissolved-solids concentrations, may be a limiting factor for some

intended uses such as drinking- or irrigation-water supply at local and, in some instances, subregional scales.

•	Groundwater quality, particularly regarding elevated nitrate concentrations in recently recharged groundwater, is changing over

time and, because of the slow rates of water movement in the aquifer, could affect groundwater availability for decades and

even millennia.

Implications

•	Reducing the acreage of rangeland that is converted to irrigated cropland in areas that contain large natural subsurface nitrate

and chloride deposits (currently not delineated across the entire High Plains aquifer) is likely to be an effective way to reduce

the dissolution and transport of natural chemical constituents in recharge water and resulting adverse effects on water quality

in the aquifer.

•	Implementing efficient agricultural-chemical application and irrigation technologies in areas not currently implemented to reduce

deep percolation of irrigation water and chemicals under cropland is likely to be an effective way to reduce the adverse effects

on the quality of the recently recharged groundwater.

Implications

•	Reducing the amount of untreated agricultural and urban runoff to topographic depressions or other fast-path zones is likely

to reduce the rapid transport of contaminants to the water table and be an effective management strategy toward minimizing

groundwater contamination.

Implications

Chemical Transport to the Water Table Follows Fast and Slow Paths 

NAWQA findings indicate that chemical transport to the water table occurs by fast and slow paths through the unsaturated 

zone. In most locations studied, estimated chemical transit times from land surface to the water table exceeded the period of 

agricultural activity and imply that agricultural chemicals should not yet be present at the water table. In fact, agricultural chemicals are commonly detected at the water table beneath irrigated cropland. This apparent discrepancy is explained by localized 

fast or preferential flow paths that enable water and chemicals to move quickly through the unsaturated zone to the water table. 

Fast paths are most likely to be present beneath topographic depressions in the land surface in which surface runoff from 

irrigation or precipitation collects. Slow paths are most likely to be present in areas of fine-grained sediments or beneath flat 

terrain where surface water does not collect. Along fast paths, water and chemicals from land surface may reach the water table 

in months to decades. Along slow paths, water and chemicals from land surface are likely to reach the water table in centuries to 

millennia. See Chapter 4 for more details.
Overview of Major Findings and Implications 5

Important Differences Exist Between the Quality of Shallow Groundwater and Deep Groundwater 

NAWQA findings indicate that changes in water quality have occurred over time that may affect the sustainability of the 

groundwater resource in the High Plains aquifer. Important spatial differences in the concentrations of dissolved solids, nitrate, 

pesticides, arsenic, and other constituents were observed between the quality of shallow and relatively young groundwater and 

the quality of deep and relatively old groundwater. Dissolved-solids concentrations in shallow groundwater generally increased 

from north to south in the High Plains aquifer and were significantly greater in the southern part (median of 800 milligrams per 

liter [mg/L]) than in the northern and central parts (medians of about 450 to 500 mg/L). Nitrate above local background concentrations (4 mg/L as nitrogen [Chapter 4]) in shallow groundwater was detected throughout the High Plains aquifer. Nitrate 

concentrations in shallow groundwater were greater than the background concentration in 90 percent of samples from the northern, 60 percent of samples from the central, and 55 percent of samples from the southern High Plains. Although widely detected, 

pesticide concentrations in shallow groundwater were less than USEPA drinking-water standards in all but 2 of 119 samples. 

Atrazine and its degradate deethylatrazine were the most commonly detected pesticide compounds. Concentrations of arsenic 

in shallow groundwater were significantly greater in samples from the southern High Plains than in samples from the northern 

High Plains. About 40 percent of the samples in the southern High Plains exceeded the USEPA arsenic drinking-water standard 

of 10 μg/L, whereas none exceeded the standard in the northern High Plains. See Chapter 4 for additional information about the 

quality of shallow groundwater that was mostly recharged during the past 50 years. 

The quality of deeper groundwater that is used by many private, public-supply, and irrigation wells in the High Plains 

generally is of suitable quality for most uses, although some spatial differences in water quality are observed across the region. 

Based on USEPA drinking-water standards, the quality of deeper groundwater decreases from the northern High Plains to the 

southern High Plains where the water contains greater concentrations of dissolved solids, chloride, nitrate, fluoride, manganese, 

arsenic, and uranium. These elevated constituent concentrations in deeper groundwater from the southern High Plains are the 

result of natural processes, such as water/rock interactions, and human activities, such as the mixing of high-quality groundwater 

with lower-quality water from underlying hydrogeologic formations, induced by pumping of high-capacity wells. See Chapters 5 

and 6 for more details.

•	The quality of groundwater near the water table may not be currently suitable for human consumption in some locations of the

aquifer because of elevated concentrations of salts, nitrate, and (or) pesticides. However, the quality of groundwater in deeper

parts of the aquifer, with some exceptions, currently is generally suitable for human consumption.

•	The quality of groundwater that is influenced by mixing with brackish surface water or water from underying hydrogeologic

formations may not be suitable for irrigation or drinking-water supply in some locations of the aquifer because of the elevated

concentrations of dissolved solids.

•	Groundwater remediation is expensive, slow, and impractical across regional-scale aquifers. Therefore, management practices

that prevent groundwater contamination are likely to be a more effective way to maintain the availability and sustainability of

groundwater in the High Plains aquifer for the intended uses for human consumption and agricultural practices.

•	Collecting long-term monitoring data is likely to be an effective way to detect gradual temporal trends and to provide early warning of water-quality problems (nitrate, for example) for which the aquifer may have limited natural-attenuation capacity.

Implications

Mixing of Groundwater by High-Capacity Wells with Long or Multiple Screens Adversely Affects 

Water Quality

NAWQA findings indicate that the quality of deeper groundwater from private, public-supply, and irrigation wells, which 

typically are completed using one long or multiple screens across much of the saturated thickness of the High Plains aquifer, is 

sometimes affected by mixing with poorer-quality water from the water table. The mixing is caused by leakage through long or 

multiple well screens and long-term pumping of high-capacity public-supply and irrigation wells. In other instances, pumping of 

high-capacity wells causes water in the aquifer to be affected by mixing with saline water from underlying bedrock aquifers or 

with water from streams that is of poorer quality based on USEPA drinking-water standards. See Chapter 5 for more details. 

Chapter 1
6 Water Quality in the High Plains Aquifer, 1999–2004

The Aquifer Has Limited Ability to Naturally Attenuate Some Contaminants

NAWQA findings indicate that the High Plains aquifer is limited in its ability to naturally attenuate some contaminants, 

such as nitrate. Denitrification, the primary natural process for removing nitrate in groundwater, generally occurs very slowly in 

the aquifer and would require hundreds to thousands of years to lower nitrate concentrations by just 1 mg/L as nitrogen (N) in 

many locations. Additionally, because water residence times in the aquifer are long (several thousand years in some locations), 

simply flushing nitrate from the aquifer could take many years. These results highlight the importance of managing land use in 

the High Plains to minimize the amount of nitrate entering the aquifer. See Chapter 5 for more details.

The Quality of Most Water Produced by Private, Public-Supply, and Irrigation Wells is Suitable 

for the Intended Uses 

NAWQA findings indicate that the quality of groundwater from deeper in the High Plains aquifer, where most private, 

public-supply, and irrigation wells are screened, is generally suitable for drinking and as irrigation water. Comparison of privatewell water quality to USEPA national primary and secondary drinking-water standards indicates that water from the Ogallala 

Formation in the northern and central High Plains had the best water quality, whereas water from the Ogallala Formation in the 

southern High Plains had the poorest quality. Most exceedances of primary and secondary drinking-water standards were those 

for dissolved solids, nitrate, arsenic, fluoride, iron, manganese, and nitrate. The most frequently detected pesticide compounds 

were atrazine and deethylatrazine, and the most frequently detected volatile organic compound was chloroform. None of the pesticide compounds or volatile organic compounds exceeded a primary drinking-water standard. See Chapter 6 for more details. 

•	The following management strategies are likely to help reduce adverse effects on the quality of water in deep zones of the

aquifer caused by mixing processes:

•	 reduce the screen length of public-supply and irrigation wells, and optimize the spatial and temporal distribution of pumping

to minimize mixing and water-table drawdown;

•	 eliminate the practice of screening wells across confining layers in the aquifer; and

•	 practice screening wells below confining layers in the aquifer where contaminant sources at the land surface are a concern,

and practice screening wells above confining layers in the aquifer where contaminant sources in underlying bedrock are

a concern.

•	Proper construction of wells to limit the movement of low-quality groundwater above and (or) below a well screen will result in

maintaining a higher-quality groundwater supply for a variety of uses.

Implications

•	In the indefinite future, the aquifer will continue to be subject to the effects of mobilized agricultural chemicals on groundwater

quality from ongoing irrigation, conversion of rangeland to cropland, or climate variability.

•	The long transit times of nitrate through the unsaturated zone will undoubtedly delay future improvements in water quality from

implementation of best management land-use practices.

Implications

•	If contaminated, the deep zones in the aquifer, in which production wells are screened, are not likely to be remediated quickly

because of slow recharge rates, long water residence times in the aquifer, and slow rates of contaminant degradation.

Implications
Chapter

Background on the Water Resource Beneath the High Plains 7

2 Background on the Water Resource Beneath 

the High Plains

Photograph by Lynne S. Fahlquist, USGS.

Groundwater from the High Plains aquifer sustains 

more than one-fourth of the Nation’s agricultural 

production and serves as the primary source of 

drinking water for most residents of the region.

“The enormous Ogallala [High Plains] aquifer is groundwater 

trapped below 174,000 square miles of fertile but otherwise dry plains 

farmland. Unlike most of the world’s water supplies, Ogallala groundwater is largely nonrenewable because its sources were cut off thousands of years ago. It is essentially “fossil water” that was generated 

ten thousand to twenty-five thousand years ago from the glacier-laden 

Rockies....However, more than three billion acre-feet (an acre-foot is 

a foot of water across one square acre, or 325,851 gallons) had been 

deposited under the plains.” 

—John Opie (2000, p. 3) 

Ogallala: water for a dry land 

This chapter summarizes 

background information 

for the High Plains 

aquifer, thus providing the 

context for understanding 

findings about water 

quality in this hydrologic 

system. Topics include 

the environmental setting, 

population and land use, 

water use, hydrogeologic 

setting, water-quality 

issues, and availability 

and sustainability of 

groundwater.

Chapter 2
8 Water Quality in the High Plains Aquifer, 1999–2004

Figure 2–1. The High 

Plains aquifer underlies 

174,000 square miles and 

eight States. NAVD, North 

American Vertical Datum 

of 1988.

Midland

Odessa

Lubbock

Amarillo

Guymon

Wichita

Garden

City

North

Platte

Scottsbluff

Cheyenne

TX

CO

KS

NM

NE

OK

WY

SD

Base information from U.S. Geological Survey digital data, 1:100,000

Albers Equal-Area projection

Standard Parallels 29°30' and 45°30', central meridian –96°

108°W 106°W 104°W 102°W 100°W 98°W

32°N

34°N

36°N

40°N

42°N

0 50 100 MILES

0 50 100 KILOMETERS

EXPLANATION

Elevation, in feet

above sea level (NAVD88)

8,000

1,000

Loup River

RepublicanRiver

Platte River

Elkhorn River

North Platte River

South

PlatteRiver

Arkansas

River

Cimarron River

Canadian River

Red

River

Environmental Setting

The High Plains aquifer underlies about 174,000 square 

miles (mi2) in parts of eight Western States (Colorado, Kansas, 

Nebraska, New Mexico, Oklahoma, South Dakota, Texas, and 

Wyoming) (fig. 2–1). In 2000, the High Plains aquifer had an 

estimated 2,980 million acre-feet of drainable water in storage, making it one of the largest aquifers in the world (McGuire, 2007). 

Elevations of the High Plains range from about 1,000 feet 

along the eastern boundary to about 8,000 feet along the north

western boundary (fig. 2–1). The topography is characterized 

by flat to gently rolling terrain. 

The High Plains has a middle-latitude dry continental climate (average annual air temperature ranges from 40 to 65°F), 

characterized by abundant sunshine, moderate precipitation 

(average annual precipitation ranges from 12 to 33 inches), frequent winds, low humidity, and a high rate of annual evaporation (60 to 109 inches) (Dennehy and others, 2002). The large 

areal extent of the High Plains results in relatively large northto-south temperature gradients (cooler in the north and warmer 

in the south) and east-to-west precipitation gradients (more 

precipitation in the east and less precipitation in the west). 
Background on the Water Resource Beneath the High Plains 9

Population and Land Use

The population of the study area in 2000 was approximately 2.3 million people. The majority (77 percent) of this 

population resides in rural areas and smaller towns and cities, 

while only 23 percent of the residents live in the four largest 

cities: Lubbock, Tex. (199,564), Amarillo, Tex. (173,627), 

Midland, Tex. (94,996), and Cheyenne, Wyo. (53,011). The 

dominant land use/land cover is rangeland (about 56 percent) 

and agriculture (about 38 percent, in cultivated crops, small 

grains, fallow, and pasture/hay); urban land use accounts for 

only 3 percent (fig. 2–2A). About one-third of the agricultural 

land is irrigated, mostly concentrated in eastern Nebraska, 

southwestern Kansas, and the west-central part of the Texas 

Panhandle (fig. 2–2B).

A

0 50 100 MILES

0 50 100 KILOMETERS

TX

CO

KS

NM

NE

OK

WY

SD

108°W 106°W 104°W 102°W 100°W 98°W

32°N

34°N

36°N

40°N

42°N

38°N

EXPLANATION

Urban

Agriculture

Rangeland

Forest

Water

Wetland

Barren

Land use/Land cover

B

0 50 100 MILES

0 50 100 KILOMETERS

IA

TX

CO

KS

NM

NE

OK

WY

SD

EXPLANATION

Irrigated cropland

108°W 106°W 104°W 102°W 100°W 98°W

32°N

34°N

36°N

40°N

42°N

38°N

Base information from U.S. Geological Survey digital data, 1:100,000

Albers Equal-Area projection

Standard Parallels 29° 30' and 45° 30', central meridian -96°

Base information from U.S. Geological Survey digital data, 1:100,000

Albers Equal-Area projection

Standard Parallels 29° 30' and 45° 30', central meridian -96°

1

2

3

4

1 2

3 4

Rangeland Nonirrigated cropland

Irrigated cropland Urban setting – Lubbock, Texas

Figure 2–2. (A) The land 

use/land cover of the High 

Plains is dominated by natural 

rangeland and agricultural 

land (1–3), both of which 

can affect the quality of 

groundwater (National Land 

Cover Database, 2001). Urban 

areas (4) are relatively small 

in land area but represent 

a substantial demand for a 

high-quality groundwater 

supply. (B) Although irrigated 

cropland accounts for about 

30 percent of the agricultural 

land (Qi and others, 2002a, 

b), the high rates of water 

use and chemical application 

associated with irrigated 

cropland can substantially 

affect water and chemical 

movement to the water table. 

Photographs by USGS staff 

(Breton W. Bruce, Kevin 

F. Dennehy, and Peter B. 

McMahon) and Wikipedia 

(accessed December 30, 

2008, at http://en.wikipedia.

org/wiki/Lubbock,_TX).

Chapter 2
10 Water Quality in the High Plains Aquifer, 1999–2004

Figure 2–3. Irrigated agriculture is the dominant use of 

groundwater from the High Plains aquifer (data from the year 

2000 and modified from Maupin and Barber, 2005).

Water Use

Use of groundwater from the High Plains aquifer as 

a source of irrigation water has transformed the study area 

into one of largest and most productive agricultural regions, 

earning it the nickname “breadbasket of the world” (Opie, 

2000). The High Plains aquifer is the most intensively used 

aquifer in the United States for irrigation, public supply, and 

self-supplied industry, producing almost two-times more 

water than any other U.S. aquifer (Maupin and Barber, 2005). 

Groundwater withdrawals from the High Plains aquifer in the 

year 2000 accounted for about 20 percent of total groundwater 

withdrawn in the United States (fig. 2–3). Most (97 percent) of 

the water withdrawn from the High Plains aquifer is used for 

irrigation (fig. 2–3). Although withdrawals for drinking water 

account for a relatively small percentage of the total groundwater use, they provide drinking water for about 82 percent of 

the 2.3 million people who live within the study area boundary. 

0 2,000 4,000 6,000 8,000 10,000 12,000 14,000 16,000 18,000

GROUNDWATER WITHDRAWALS, IN MILLION GALLONS PER DAY

Irrigation

Public supply

Self-supplied industrial

USE

Hydrogeologic Setting

The High Plains aquifer consists of sedimentary deposits 

that form six hydraulically connected hydrogeologic units. The 

most extensive of these hydrogeologic units is the Ogallala 

Formation, which makes up about three-fourths of the total 

High Plains study area (fig. 2–4). 

The depth to water below land surface (unsaturated-zone 

thickness) ranges from 0 to approximately 500 feet, averages 

about 100 feet, and is generally greatest in the central and 

southern High Plains. The saturated thickness of the High 

Plains aquifer ranges from less than 1 to more than 1,000 feet 

and averages about 200 feet. The saturated thickness varies 

geographically and is greatest in the northern High Plains. 

Evaporation rates exceed precipitation rates across much 

of the High Plains, so little water is available to recharge 

the aquifer. Recharge to the High Plains aquifer occurs by 

infiltration of irrigation water, areally diffuse infiltration from 

precipitation, focused infiltration of storm- and irrigationwater runoff through streambeds and other topographic 

depressions (Gurdak and others, 2008), and upward movement 

of water from underlying aquifers. Discharge from the High 

Plains aquifer is primarily to irrigation well pumping, streams 

and underlying aquifers, groundwater flow across the eastern 

boundary of the aquifer, and evapotranspiration. Regional 

groundwater flow is generally from west to east; however, 

local variability in hydraulic gradients can result in different 

directions of groundwater flow, particularly near high-capacity 

pumping wells and major rivers like the Platte, Republican, and 

Arkansas Rivers (fig. 2–1).

Water levels have declined (fig. 2–5) substantially since 

predevelopment times (approximately the mid-1950s) because 

groundwater withdrawals have greatly exceeded recharge 

across much of the aquifer (McGuire and others, 2003). This 

is particularly true in the central and southern High Plains. 

The largest water-level declines range from 50 to more than 

150 feet, primarily across parts of Kansas, Oklahoma, New 

Mexico, and Texas (fig. 2–5). The saturated thickness of the 

aquifer has decreased by more than 50 percent in some parts 

of Kansas and Texas. This groundwater depletion has led to 

increased pumping costs and a reduction of water discharging 

to streams, among other things. Ecosystems along riparian corridors that rely on groundwater discharge are adversely affected 

by even small volume changes in the groundwater system 

(Alley, 2006). 
Background on the Water Resource Beneath the High Plains 11

Figure 2–4. The Ogallala Formation is the largest and most important hydrogeologic unit of 

the High Plains aquifer. 

Base information from U.S. Geological Survey digital data, 1:100,000

Albers Equal-Area projection

Standard Parallels 29°30' and 45°30', central meridian –96°

TX

CO

KS

NM

NE

OK

WY

SD

IA

108°W 106°W 104°W 102°W 100°W 98°W

32°N

34°N

36°N

40°N

42°N

38°N

0 50 100 MILES

0 50 100 KILOMETERS

EXPLANATION

Alluvial systems

Quaternary deposits

Sand Hills eolian deposits

Plio-Pleistocene deposits

Ogallala Formation

Arikaree/Brule Formations

Aquifer not present

Hydrogeologic units

Chapter 2
12 Water Quality in the High Plains Aquifer, 1999–2004

Figure 2–5. The most substantial declines in water levels since predevelopment are in 

parts of Texas, New Mexico, Oklahoma, and Kansas (modified from McGuire, 2007) and the 

High Plains water-level monitoring study (http://ne.water.usgs.gov/ogw/hpwlms/, accessed 

January 16, 2009).

TX

KS

NM

NE

SD

WY

CO

IA

Base information from U.S. Geological Survey digital data, 1:100,000

Albers Equal-Area projection

Standard Parallels 29°30' and 45°30', central meridian –96°

108°W 106°W 104°W 102°W 100°W 98°W

32°N

34°N

36°N

40°N

42°N

38°N

0 50 100 MILES

0 50 100 KILOMETERS

OK

EXPLANATION

Water-level change, in feet

More than 150

100 to 150

50 to 100

25 to 50

10 to 25

+10 to –10

10 to 25

25 to 50

More than 50

Declines

No substantial change

Rises

Area of little or no

saturated thickness
Chapter

NAWQA Approach to Assessing Water Quality in the High Plains Aquifer 13

This chapter summarizes 

the primary features of 

the NAWQA study design, 

providing the context for 

understanding findings 

about water quality in 

the High Plains aquifer 

that are presented in 

subsequent chapters.

NAWQA Approach to Assessing Water Quality in 

the High Plains Aquifer

Sampling water from installed monitoring wells (above 

photograph) and existing private wells is part of NAWQA’s 

systematic evaluation of groundwater quality in the High 

Plains aquifer.

Photograph by Lynne S. Fahlquist, USGS.

3

This NAWQA study (see Box A) provides the first systematic assessment 

and most comprehensive regional analysis of water quality in the High Plains 

aquifer. An approach was developed that assessed overall water quality in the 

aquifer and also enabled an understanding of the linkages between the quality 

of water recharging the aquifer, the effect of transport through the hydrologic 

system on water quality, and the quality of the resource used for human consumption and irrigation. 

Chapter 3
14 Water Quality in the High Plains Aquifer, 1999–2004

Unique Features of the NAWQA Approach BoxA

Water-quality assessments by NAWQA, which is a single program among many local, 

State, and Federal programs, are not designed to address all of the Nation’s waterresource information needs and issues. Listed below are several characteristics and 

limitations of the NAWQA approach that are important to consider when interpreting the 

findings presented in this report.

•	NAWQA assessments characterize the quality of the available, untreated water

resources. The quality of drinking water from water-treatment plants or from household

taps is not monitored. By focusing on the quality of streams and groundwater in their

present condition (ambient water quality), NAWQA complements many Federal, State,

and local drinking-water monitoring programs.

•	NAWQA assessments do not focus on specific sites with known water-quality concerns

or narrowly defined “issues of the day” but rather on the condition of the total resource,

including streams and groundwater in a wide range of hydrologic and land-use settings

across the country.

•	NAWQA assessments are guided by a nationally consistent study design and uniform

methods of sampling and analysis. Findings thereby pertain not only to water quality of

a particular stream or aquifer but also contribute to the larger picture of how and why

water quality varies regionally and nationally. This consistent, multiscale approach helps

to determine if a water-quality issue is isolated or pervasive.

•	NAWQA assessments target specific land-use settings that are most extensive or

important to water quality in a wide range of hydrologic and environmental settings

across the Nation. This targeted approach gives priority to understanding the most critical factors influencing water quality. Extension of results to national analysis, however,

requires careful definition of each type of water resource and environmental setting for

which conclusions are drawn and the use of statistical models to extrapolate results to

resources that have not been measured.

•	USGS analytical methods are designed to measure concentrations as low as economically and technically feasible. Studies of contaminant occurrence and behavior benefit

from the most information possible at all concentration levels, and such data help to

identify emerging issues and to track changes in concentrations over time. By this

approach, however, many constituents, such as pesticides and volatile organic compounds, are commonly detected at concentrations far below Federal or State standards

and guidelines for protecting water quality.
NAWQA Approach to Assessing Water Quality in the High Plains Aquifer 15

Study Components of the High Plains

Water-Quality Assessment 

The High Plains water-quality assessment was divided 

into four study components: unsaturated-zone studies, landuse studies, major aquifer studies, and regional-transect 

studies (fig. 3–1). The objectives of the unsaturated-zone 

studies were to measure the amount of chemicals stored in the 

subsoil above the water table and determine their transit times 

from land surface to the water table in rangeland and irrigated 

settings. The objective of the land-use studies was to measure 

and explain the quality of recently recharged groundwater in 

Unsaturated-zone 

study sites

Land-use 

study wells

Regional-transect study wells

Major aquifer 

study wells

Figure 3–1. The four distinct study components enabled a systematic understanding of the quality of water as it moves from land 

surface, through the unsaturated zone, to the water table, and finally to deeper depths in the aquifer that are commonly used to supply 

water for drinking and irrigation.

irrigated cropland and urban settings by sampling water-table 

monitoring wells installed as part of the NAWQA Program. 

The objective of the major aquifer studies was to broadly 

assess water-quality conditions in the aquifer by sampling 

networks of randomly selected, areally distributed, existing 

private wells. The objectives of the regional-transect studies 

were to characterize vertical gradients in groundwater chemistry and apparent age in the thickest areas of the aquifer and to 

identify major biogeochemical reactions affecting the quality 

of water along groundwater flow paths leading from recharge 

areas to downgradient wells. Details of each study component 

are explained by McMahon, Dennehy, and others (2007).

Chapter 3
16 Water Quality in the High Plains Aquifer, 1999–2004

The study area was divided into three geographic subregions: northern High Plains (NHP) (96,500 mi2

); central 

High Plains (CHP) (48,500 mi2); and southern High Plains 

(SHP) (29,000 mi2) (fig. 3–2). These subdivisions are based 

on considerations of the aquifer boundaries; hydrogeology; 

regional temperature gradients that affect agricultural practices, land use/land cover, and recharge, and thus water quality; 

and logistics of conducting field activities across such a large 

area (fig. 3–3). 

An important concept of the High Plains aquifer 

assessment design was the nesting of the studies described 

in figure 3–3 and Appendix 2, which resulted in improved 

understanding of assessment results and supported extrapolation of results to unmonitored areas of the aquifer. An example 

of the nested design implemented in the central High Plains 

is shown in figure 3–3. Nested within the major aquifer study 

was a land-use study targeting irrigated corn agriculture and 

a regional-transect study characterizing vertical gradients in 

water chemistry and apparent groundwater age between the 

Figure 3–2. A stratification scheme based on climate, hydrogeology, and land use/land cover was used to identify important areas for 

groundwater quality sampling.

Climate

Hydrogeology Land Use/Land cover

EXPLANATION

Rangeland

Irrigated cropland

IA

TX

CO

KS

NM

NE

OK

WY

SD

EXPLANATION

Mean annual air temperature,

in degrees Fahrenheit 

65 

40 

Base information from U.S. Geological Survey digital data, 1:100,000

Albers Equal-Area projection

Standard Parallels 29°30' and 45°30', central meridian –96°

108°W 106°W 104°W 102°W 100°W 98°W

32°N

34°N

36°N

40°N

42°N

0 50 100MILES

0 50 100KILOMETERS

38°N

Southern High

Plains

Central High

Plains

Northern High

Plains

EXPLANATION

Quaternary deposits

Ogallala Formation

Aquifer not present

Subareas

water table and base of aquifer. Nested within the land-use 

study was an unsaturated-zone study of agricultural-chemical 

storage and transit times in the unsaturated zone. 

Monitoring water and chemical movement from the 

land surface to the water table and from the water table to the 

base of the aquifer along flow paths leading from recharge to 

discharge areas resulted in an improved understanding of the 

timescales at which that movement occurred. This enabled 

interpretation of local-scale data over a broad range of variability contained within the regional aquifer. Thus, a beneficial 

aspect of the High Plains aquifer study design is that it bridges 

the interpretation gap between local-scale studies and the aggregation to regional and national-scale synthesis. Additional 

information on sampling locations, sampling frequency, and 

data collected is detailed in Appendix 2. Groundwater-quality 

data collected by local, State, and other Federal agencies were 

compiled to help assess the quality of groundwater used for 

irrigation and public supply, which is described in Chapter 6. 
NAWQA Approach to Assessing Water Quality in the High Plains Aquifer 17

Major aquifer studies

(assessment study)

Agricultural

land-use study

(assessment study)

Regional-transect

study

(process study)

Unsaturated-zone

study

(process study)

EXPLANATION

Unsaturated-zone study monitoring site – irrigated cropland

Unsaturated-zone study monitoring site – rangeland

Regional-transect location

Agricultural land-use study monitoring well

Domestic (major aquifer study) well

Alluvial systems

Quaternary deposits

Ogallala Formation

Aquifer not present

Irrigated cropland

Hydrogeologic units

Land use/Land cover

Central High Plains 

Figure 3–3. The regional assessment used four nested study components (unsaturated-zone studies, land-use studies, 

major aquifer studies, and regional-transect studies) to better assess groundwater quality conditions and controlling 

processes at local and regional scales. 

Chapter 3
Chapter

18 Water Quality in the High Plains Aquifer, 1999–2004

Quality of Water Near the Water Table—

What Was Found, Where, and Why

Land-use

study wells

Regional-transect study wells

Major aquifer

study wells

Unsaturated-zone

study sites

4

A substantial part of the High Plains aquifer assessment focused on the 

quality of recently recharged groundwater near the water table beneath irrigated 

cropland used mainly for the production of corn and cotton. For this study, 

recently recharged groundwater refers to water recharged in the past 50 years. 

This recently recharged groundwater is an important part of the resource to 

study because it is likely to contain chemicals from modern agricultural practices that represent a major land use in the High Plains. Moreover, the recently 

recharged groundwater eventually moves deeper in the aquifer where watersupply wells typically are screened. The unsaturated-zone and land-use studies 

are used to assess this component of the resource and determine if chemicals 

introduced at land surface by human activities are reaching the water table. 

This chapter summarizes 

the quality of water near 

the water table. Topics 

include the occurrence 

and distribution of 

selected constituents, the 

factors affecting water 

quality, extrapolation to 

unmonitored locations, and 

trends in water quality near 

the water table. 
Quality of Water Near the Water Table—What Was Found, Where, and Why 19

General Findings — Quality of Recently 

Recharged Groundwater

In general, NAWQA findings show that dissolved solids, 

nitrate, pesticides, and arsenic were present in relatively large 

concentrations in recently recharged groundwater beneath 

irrigated cropland. The quality of that recently recharged 

groundwater, however, was not uniform across the High Plains 

aquifer. Details of the urban land-use study are described by 

Pope and others (2002). 

Dissolved-solids concentrations in recently recharged 

groundwater beneath the irrigated cropland generally increased from north to south in the High Plains, and were 

significantly greater in the southern part (median of 800 mg/L) 

than in the northern and central parts (medians of about 450 to 

500 mg/L, respectively). Sodium, magnesium, chloride, and 

sulfate are important contributors to the significant increase in 

dissolved solids from north to south in the High Plains, all of 

which can have natural and human sources. 

Nitrate was above background concentrations in each 

of the land-use studies. Nitrate concentrations in shallow 

groundwater were greater than the background concentration 

of 4 mg/L as nitrogen in 90 percent of samples from the northern, 60 percent of samples from the central, and 55 percent 

of samples from the southern High Plains. Natural nitrate in 

the High Plains aquifer can be derived from processes such 

as mineral weathering, decay of organic material, and atmospheric deposition. The background concentration of 4 mg/L 

as nitrogen is based on age-dated groundwater samples that 

were recharged more than 50 years ago. Thus, concentrations 

greater than 4 mg/L as nitrogen are likely to include nitrate 

from other sources, such as fertilizer and manure from agricultural practices. 

 Pesticides were more commonly detected in shallow 

groundwater in the northern High Plains than in the central 

and southern High Plains. Atrazine and its degradate deethylatrazine were the most commonly detected pesticide compounds. Although widely detected, pesticide concentrations 

generally were low; concentrations of the pesticide compounds were less than established drinking-water standards in 

all but two of 119 samples. Generally, pesticide compounds 

were detected most frequently in groundwater that also contained nitrate above background concentrations or that was 

recharged in the past 50 years. 

 Concentrations of arsenic in shallow groundwater were 

significantly greater in the southern High Plains than in the 

northern High Plains. About 40 percent of the samples from 

the southern part exceeded the arsenic drinking-water standard 

of 10 μg/L, whereas none from the northern part exceeded 

the standard.

Detailed descriptions of water quality of recently recharged groundwater beneath irrigated cropland are in

the following reports:

Ground-water quality beneath irrigated agriculture in the northern and southern High Plains 

aquifer, Nebraska and Texas, 2003–04

by J.S. Stanton and L. Fahlquist:

U.S. Geological Survey Scientific Investigations Report 2006–5196 available at

http://pubs.usgs.gov/sir/2006/5196

Ground-water quality beneath irrigated agriculture in the central High Plains aquifer, 1999–2000

by B.W. Bruce, M.F. Becker, L.M. Pope, and J.J. Gurdak:

U.S. Geological Survey Water-Resources Investigations Report 2003–4219 available at

http://pubs.usgs.gov/wri/wri034219

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2006–5196

National Water-Quality Assessment Program

Ground-Water Quality Beneath Irrigated Cropland

of the Northern and Southern High Plains Aquifer,

Nebraska and Texas, 2003−04

0 50 100 150 200 MILES

0 50 100 150 200 KILOMETERS

Colorado

Wyoming

South Dakota

Nebraska

Kansas

New Mexico

Texas

Oklahoma

Iowa

Canadian R

Southern High Plains

agricultural land-use

study area

Northern High Plains

agricultural land-use

study area

NORTHERN

CENTRAL

SOUTHERN

Boundary of

High Plains

aquifer

Chapter 4
20 Water Quality in the High Plains Aquifer, 1999–2004

Factors Affecting Quality of Recently Recharged 

Groundwater

Several factors may control the observed regional differences in water quality of recently recharged groundwater 

beneath irrigated cropland, including land-management practices involving chemical use and water use, natural reservoirs 

of nitrate in the subsoil, storage of chemicals in the unsaturated zone, chemical transit times in the unsaturated zone, 

and climate.

The north-to-south decrease in nitrate concentrations 

and frequency of pesticide-compound detections are consistent with patterns of chemical use in those regions. Fertilizer 

and pesticide application rates are larger in the northern High 

Plains than in the southern High Plains. The amount of water 

used for irrigation is smaller in the northern part, but natural 

recharge is greater and depths to the water table are less than 

in the central and southern parts, which can result in shorter 

transit times of chemicals from the land surface to the water 

table in the northern High Plains.

Processes that promote recharge of agricultural chemicals 

such as irrigation and certain other farming practices can increase the potential for groundwater contamination by increasing chemical movement, thereby decreasing chemical transit 

times to the water table. Recharged water is being degraded 

by infiltration of water from repeated irrigation applications 

in which concentrations of dissolved solids have increased 

because of evaporative concentration on land surface and in 

the near subsurface (McMahon, Dennehy, and others, 2007). 

Elevated concentrations of dissolved solids in groundwater beneath irrigated cropland are derived in part from 

application of agricultural fertilizers at the land surface, which 

can increase the salt load moving to the water table. Subsoil 

salt deposits, including perchlorate salts (Rajagopalan and others, 2006), also can occur naturally. NAWQA findings suggest 

that about 80 percent of chloride in the subsoil beneath one 

irrigated field may be from natural sources. Natural subsoil 

chloride deposits in the High Plains probably were derived 

from long-term evaporative concentration of chloride from 

atmospheric deposition near the base of the root zone in rangeland settings. This evapoconcentration process is dependent 

on climate, vegetation, topography, and soil type. Subsoil salt 

deposits increase from the north to the south in the High Plains 

along with air temperatures; warmer air temperatures tend 

to increase evaporation and reduce recharge, both of which 

can result in the accumulation of atmospheric chloride in the 

unsaturated zone (McMahon and others, 2006). 

Conversion of rangeland to irrigated cropland could 

result in increasing groundwater chloride concentrations over 

time in the southern part of the High Plains aquifer as natural subsoil salt deposits are mobilized and transported to the 

water table by irrigation return flow. Mobilization of natural 

salt deposits has already occurred in some areas of the High 

Plains, but slow chemical transit times indicate that many of 

those salts generally have not reached the water table. Thus, 

the amount of salts (chemical mass) entering the aquifer could 

increase in the future as the stored chemicals, which currently 

reside in the unsaturated zone, are mobilized, transported 

downward in the unsaturated zone, and reach the water table.

For the most part, less nitrate is stored in the subsoil 

beneath rangeland than beneath irrigated cropland in the High 

Plains (fig. 4–1), which is consistent with the much larger 

nitrogen application rates on cropland compared to rangeland. 

Large natural nitrate deposits, however, have been detected 

in the subsoil beneath rangeland in some locations (fig. 4–2). 

Some nitrate could be mobilized in the unsaturated zone by 

irrigation return flow if the rangeland is converted to cropland, 

which could lead to increased nitrate concentrations in the 

aquifer. Nitrogen isotopic data (see Box B) indicate that nitrate 

from both fertilizer and natural sources are mobilized under 

the high recharge conditions that occur beneath irrigated fields 

(fig. 4–2). 
Quality of Water Near the Water Table—What Was Found, Where, and Why 21

0

30

60

90

120

150

180

(2.4 mg/L as N)

(8.4 mg/L as N)

CONCENTRATION OF NITRATE, 

IN MICROGRAMS OF NITROGEN PER GRAM OF SEDIMENT

B

DEPTH BELOW LAND SURFACE, IN FEET

0

30

60

90

120

150

180

0 5 10 15 20 0 5 10 15 20 

(2.4 mg/L as N) Concentration of nitrate in groundwater,

in milligrams per liter as nitrogen 

Water table for 2002 sampling

EXPLANATION

Rangeland setting

A Irrigated-corn setting

Sediment cuttings

Monitoring well

EXPLANATION

Sediment cuttings

Monitoring well

EXPLANATION

CONCENTRATION OF NITRATE, 

IN MICROGRAMS OF NITROGEN PER GRAM OF SEDIMENT

Figure 4–1. Generally, less nitrate is stored in the subsoil beneath (A) natural rangeland than beneath (B) irrigated 

cropland, such as corn fields, in the High Plains. The subsoil nitrate can be derived from natural and agricultural sources. 

Photographs by Breton W. Bruce and Kevin F. Dennehy (USGS). 

Chapter 4
22 Water Quality in the High Plains Aquifer, 1999–2004

Isotopes of a particular element have the same number of protons in the atomic nucleus but different numbers of neutrons, resulting in different atomic masses. Stable isotopes do not undergo radioactive decay. The relative abundance of isotopes in chemicals 

of environmental concern, such as the isotopes of nitrogen and oxygen in nitrate, can be precisely measured using mass spectrometry. The element nitrogen has seven protons and either seven or eight neutrons. Thus, nitrogen has two isotopes (14N and 15N), both of which are stable. The element oxygen has 8 protons and either 8, 9, or 10 neutrons. Thus, oxygen has three isotopes 

(

16O, 17O, and 18O), all of which are stable. The most abundant isotopes of nitrogen and oxygen in nature are 14N and 16O (Coplen and 

others, 2002).

The isotopic composition of nitrate, and of most other natural materials, is measured relative to standards of known iso-topic 

composition. Isotope compositions are reported as delta (δ) values, in units of per mil (‰). Delta values for the N and O isotopes 

in nitrate are calculated as indicated in the following equations.

How Stable Isotopes Provide Insight into the Sources and

Sinks of Nitrate in the Aquifer

J.K. Böhlke1 and Peter B. McMahon2

Denitrification is a biological process that reduces nitrate to N2 gas, and it is the most important natural sink for nitrate in 

groundwater.

Assimilation is a biochemical or abiotic process that uses nitrate or ammonium to form plant proteins and other nitrogencontaining compounds. 

Many chemical and physical processes can produce large variations in the relative abundances of N and O isotopes in nitrate. 

This isotopic variability can be useful for distinguishing sources of nitrate and for detecting nitrate-consuming reactions (nitrate 

sinks). In general terms, the O isotopic composition of nitrate indicates how the nitrate was formed, whereas the N isotopic 

composition provides clues to the source of the N (figure B–1). The N and O isotopic compositions of nitrate may be altered, or 

fractionated, by subsequent reactions that consume nitrate, such as assimilation and denitrification (figure B–1).

Oxygen isotope data indicate that most nitrate in groundwater from the High Plains aquifer is biogenic and not from synthetic 

(commercial nitrate fertilizer) or atmospheric sources (figure B–1). Biogenic nitrate is formed by the process of nitrification in 

which ammonium is oxidized to nitrate. Nitrification in the High Plains commonly occurs in the soil zone above the water table. 

Sources of ammonium during nitrification include fertilizers, natural soils (including plant material), and manure (shown as blue 

arrows in figure B–1). Chemical and isotopic data show that nitrate from fertilizer and manure was most commonly detected in 

groundwater from agricultural settings such as beneath irrigated cropland. Nitrate from natural soils was detected in groundwater beneath rangeland and cropland, but concentrations generally were small compared to those for nitrate from fertilizer and 

manure sources.

Evidence for the occurrence of denitrification in the High Plains aquifer is recorded in the N and O isotopic composition of nitrate 

in groundwater that has moved through denitrification zones (McMahon and Böhlke, 2006). Under denitrifying conditions, the 

delta N and O values in nitrate systematically increase as the process consumes nitrate (shown as black arrows in figure B–1). 

Thus, N and O isotopes help to define the spatial distribution of denitrification in the aquifer. Understanding the spatial distribution of denitrifying zones in the aquifer is important because those areas could be less vulnerable to long-term nitrate contamination compared to zones in which nitrate is not actively consumed. In general, denitrification is not widespread in the aquifer, and 

rates at which it occurs are slow compared to rates measured in many other aquifers. 

1

 U.S. Geological Survey, Reston, Virginia.

 2 U.S. Geological Survey, Lakewood, Colorado.

,

,

BoxB
How Stable Isotopes Provide Insight into the Sources andSinks of Nitrate in the Aquifer 23

Figure B–1. Nitrogen and oxygen isotopic composition indicates that most nitrate is biogenic in groundwater beneath 

rangeland and irrigated cropland of the High Plains.

-20 0 20 40 60 80 100

-10

0

10

20

30

δ18O[nitrate], in per mil

δ15N[nitrate], in per mil

Atmospheric

nitrate Synthetic

nitrate

Biogenic 

nitrate

Fractionation trends 

(assimilation and denitrification)

manure

fertilizer

natural soils

Groundwater samples beneath:

 irrigated cropland

 rangeland
24 Water Quality in the High Plains Aquifer, 1999–2004

160

120

80

40

0

0 100 200 300

DEPTH BELOW LAND SURFACE, IN FEET

-3 0 3 6 9 12 15 0 5 10 15 20 -3 0 3 6 9 12 15

NITRATE CONCENTRATION, 

IN MICROGRAMS OF NITROGEN 

PER GRAM OF SEDIMENT

δ15N[NO3

–

], IN PER MIL RELATIVE TO AIR

Historical high

water table at

irrigated site

Fertilized agriculture Manure

Natural soils

Fertilized agriculture Manure

Natural soils

Rangeland Irrigated corn

NITRATE CONCENTRATION, 

IN MICROGRAMS OF NITROGEN 

PER GRAM OF SEDIMENT

δ15N[NO3

–

], IN PER MIL RELATIVE TO AIR

Figure 4–2. Chemical and isotopic data indicate that natural nitrate deposits exist in relatively large concentrations in the unsaturated 

zone beneath rangeland in some parts of the High Plains (McMahon and others, 2006). When that rangeland is converted to irrigated 

cropland, some of the nitrate can be mobilized by irrigation return flow, thereby increasing the amount of nitrate moving to the water 

table. In many locations, these large nitrate concentrations in unsaturated-zone pore water have not yet reached the water table 

(McMahon and others, 2006). Photographs by Breton W. Bruce and Kevin F. Dennehy (USGS).
Quality of Water Near the Water Table—What Was Found, Where, and Why 25

Chapter 4

Estimated transit times of water and chemicals 

through the unsaturated zone beneath irrigated cropland at 

the unsaturated-zone study sites are highly variable, ranging 

from 49 to 373 years (McMahon and others, 2006). A major 

finding is that although the estimated chemical transit times 

are only approximations, the times exceeded the irrigation 

period at all of the study sites (28 to 46 years), implying that 

agricultural chemicals should not have reached the water 

table, when in fact they have (McMahon and others, 2006). 

The discordant finding of long chemical transit times beneath 

irrigated cropland and the presence of agricultural chemicals 

at the water table cannot be explained as a single flow process 

in the unsaturated zone and indicates that both fast and slow 

paths for water and chemical movement are present in the unsaturated zone (fig. 4–3) (McMahon and others, 2006; Gurdak, 

Hanson, and others, 2007). 

Figure 4–3. Water and chemical movement from land surface to the 

water table may be slow (decades to millennia) or fast (months to decades) 

depending on the natural hydrogeology conditions in the unsaturated zone, 

land-management practices, and climate variability (McMahon and others, 

2006; Gurdak, Hanson, and others, 2007; Gurdak and others, 2008). Therefore, 

it is important to understand the rate of water and chemical movement and 

the time scales in which best-management practices may affect groundwater 

quality for determining the implications on the sustainability of a high-quality 

groundwater supply in the High Plains aquifer. 

Chemical flux

EXPLANATION

Water table

FAST PATH SLOW PATH

Not to scale

Unsaturated 

zone

Saturated

zone

Months to decades

 chemical traveltime

Decades to millennia

chemical traveltime

chemical 

application chemical 

application

chemicals reach 

groundwater

chemicals reach 

groundwater

surface water

Fast paths (with transit times of months to decades) 

through the unsaturated zone are most likely to occur beneath topographic depressions in the land surface in which 

surface runoff from irrigation or precipitation collects (such 

as seasonal ponds or in playas in the southern High Plains) 

because of the increased hydraulic head by the ponded water 

(fig. 4–3) (McMahon and others, 2006). Fast paths also could 

occur through wellbore leakage and through cracks, burrows, 

fissures, and other soil structural features. Climate variability 

that occurs on interannual to multidecadal timescales also has 

been shown to be an important control on transit times beneath 

fast paths (Gurdak, Hanson, and others, 2007). Slow paths 

(with transit times of decades to centuries) may occur in finegrained sediments or beneath flat terrain, and possibly in other 

environments. Evapotranspiration can restrict water movement 

in both fast and slow path settings (fig. 4–3) (McMahon and 

others, 2006).
26 Water Quality in the High Plains Aquifer, 1999–2004

Areas of the High Plains containing fast paths for chemical 

transport to the water table are thought to be a small part of the 

total High Plains area. For example, previous studies estimated 

that greater than 50 percent of the recharge to the southern 

High Plains occurred through playa basin floors that occupied 

only about 6 percent of the total land area (Claborn and others, 

1985; Wood and others, 1997). NAWQA findings indicate that 

seasonal ponding in irrigated fields (fig. 4–4) can contribute 

to water and chemical transit times ranging from 7 to 50 years 

(fig. 4–4). These findings have practical implications for waterresource managers interested in best-management practices 

designed to reduce nonpoint-source groundwater contamination. 

For example, limiting the occurrence of seasonal surface ponds 

caused by leaking chemigation irrigation plumbing is a manageable solution toward reducing elevated concentrations of nitrate 

and pesticides that reach the groundwater. CHEMICAL TRANSIT TIME (YEARS)

10,000

1,000

100

10

Fast path 

Slow path 

A Northern High Plains B Central High Plains

C Southern High Plains D

Figure 4–4. (A–C) Seasonal ponding near irrigation wells and in irrigated-agricultural fields creates 

conditions which can lead to fast paths for water and chemical movement. (D) Simulations indicate 

that chemical transit times to the water table beneath areas having seasonally ponded conditions (fast 

paths) are orders of magnitude faster than from field estimates from areas having diffuse recharge 

settings (slow paths) (Gurdak and others, 2008; and McMahon and others, 2006). Photographs by Jason 

J. Gurdak and Peter B. McMahon (USGS). 
Quality of Water Near the Water Table—What Was Found, Where, and Why 27

Chapter 4

Extrapolation of Findings to Unmonitored Areas

Understanding the connection between chemical use at 

the land surface and the quality of recently recharged groundwater is important but costly to measure across the entire 

High Plains aquifer. NAWQA sampling provides a basis for 

extrapolating results to unmonitored areas of the aquifer. Using the monitoring data, a probability map of elevated nitrate 

concentrations at the water table (fig. 4–5) was developed to 

evaluate the chemistry of recharge in unmonitored areas of 

the High Plains aquifer. Output from these types of predictive 

models can be used to identify potentially vulnerable areas for 

enhanced monitoring and protection.

“USGS research regarding the vulnerability of the 

High Plains aquifer to contamination from nitrate 

is immensely important to protect this valuable 

resource and has greatly improved our understanding of the movement and fate of nitrate and helped 

us better target our limited monitoring and outreach 

resources to areas where it is most needed.”

—Troy Bauder, Extension Specialist 

Water Quality Department of Soil and Crop Sciences

Colorado State University

Measured nitrate concentrations in groundwater were used 

in a model to predict the probability of occurrence of nitrate 

concentrations greater than background (4 mg/L) in recently 

recharged groundwater in unmonitored areas of the aquifer 

(fig. 4–5). The analysis showed that the features of the aquifer 

affecting vulnerability to nitrate concentrations greater than 

background include nonirrigated agriculture, irrigated agriculture, organic matter in the soil, depth to the water table, and 

average percentage of clay in the unsaturated zone (Gurdak, 

2008; Gurdak, McCray, and others, 2007). 

Simulation findings suggest that more than half (53 percent) of the aquifer area has less than a 40-percent predicted 

probability of containing recently recharged groundwater with 

nitrate concentrations greater than 4 mg/L as N. The areas 

of low vulnerability typically are areas that have a relatively 

small percentage of land in agriculture and relatively thick 

unsaturated zones that help to protect the aquifer. Approximately 21 percent of the study area has relatively high (greater 

than 60 percent) predicted probability of containing recently 

recharged groundwater with nitrate concentrations greater than 

4 mg/L as N. These areas tend to have greater percentages 

of agricultural land and shallower depths to water table than 

low-vulnerability areas. The most vulnerable areas (greater 

than 80-percent predicted probabilities) are scattered across the 

eastern and southern areas of the northern High Plains and the 

southernmost areas of the aquifer (Gurdak, McCray, and others, 

2007). The vulnerability map (fig. 4–5) is intended for regional, 

subregional, or county-scale use and might not adequately 

represent local conditions. Additionally, the vulnerability map 

(fig. 4–5) was created using data collected from 1990 to 2004 

and therefore might not be appropriate for forecasting future 

conditions in the aquifer (Gurdak and Qi, 2006). 

Detailed descriptions of the vulnerability of groundwater in the High Plains aquifer are in the

following report:

Vulnerability of recently recharged ground water in the High Plains aquifer to nitrate 

contamination

by J.J. Gurdak and S.L. Qi:

U.S. Geological Survey Scientific Investigations Report 2006–5050 available at

http://pubs.usgs.gov/sir/2006/5050
28 Water Quality in the High Plains Aquifer, 1999–2004

Figure 4–5. The probability of nitrate concentrations greater than 4 milligrams per liter as 

nitrogen in recently recharged groundwater in the High Plains aquifer is predicted to be much 

less in the north-central parts of the northern High Plains and the central High Plains than 

in other areas (Gurdak, 2008; Gurdak and Qi, 2006; Qi and Gurdak, 2006). NHP, northern High 

Plains; CHP, central High Plains; SHP, southern High Plains. 

TX

KS

NM

NE

SD

WY

CO

IA

Base information from U.S. Geological Survey digital data, 1:100,000

Albers Equal-Area projection

Standard Parallels 29°30' and 45°30', central meridian –96°

OK

108°W 106°W 104°W 102°W 100°W 98°W

32°N

34°N

36°N

40°N

42°N

38°N

Percent probability of 

nitrate concentration 

greater than 4 milligrams

per liter as nitrogen

EXPLANATION

0 – 20

21 – 40

41 – 60

61 – 80

81 – 100

No data/Aquifer 

not present

NHP

model

CHP 

and 

SHP 

model

0 50 100 MILES

0 50 100 KILOMETERS
Quality of Water Near the Water Table—What Was Found, Where, and Why 29

Chapter 4

15,000 10,000 5,000 0

0

10

20

30

40

80

120

APPARENT GROUNDWATER AGE, IN YEARS

Nitrate concentrations in

agricultural recharge

Nitrate concentrations in

pre-agricultural recharge

Drinkingwater 

standard

Approximate trend line

1940 1950 1960 1970 1980 1990 2000 2010

0

300

600

900

1,200

1,500

FERTILIZER USE IN THE HIGH PLAINS,

IN MILLIONS OF KILOGRAMS OF NITROGEN

YEAR

NITRATE CONCENTRATION IN RECHARGE, IN MILLIGRAMS PER LITER AS NITROGEN

Land-use study well sample

(generally represents agricultural recharge)

Regional-transect study well sample

(generally represents pre-agricultural recharge)

Figure 4–6. The nitrate concentrations in pre-agricultural recharge water during the past ~12,000 years was relatively stable at 

approximately 2.2 milligrams per liter as nitrogen. A substantial increase in nitrate concentrations in recharge water since the 1940s 

and 1950s is the result of agricultural application of nitrogen and mobilization of natural sources of nitrogen in the subsoil by irrigation 

return flow (McMahon, Böhlke, and Christenson, 2004; McMahon and Böhlke, 2006).

Trends in the Quality of Groundwater Recharge

Long-term trends in concentrations of nitrate in recharge 

were reconstructed using age-dated groundwater samples. The 

results showed that during the past approximately 12,000 years, 

nitrate concentrations in recharge water ranged from 0.8 to 

4.2 mg/L as N, with a median concentration of 2.2 mg/L as 

N (fig. 4–6). However, during the past 50 to 60 years, nitrate 

concentrations in recharge water have substantially increased 

compared to concentrations in older recharge water (fig. 4–6). 

An analysis of existing data collected from 1930 to 1999 

confirms the relatively recent increase in nitrate concentrations. 

The recent increase in recharge nitrate concentrations appears 

to be related to the increased use of agricultural fertilizer during the past 50 to 60 years (fig. 4–6), although contributions of 

nitrate from other human sources such as waste from animals 

and humans, and from natural sources in the subsoil that were 

mobilized by irrigation returnflow, also may be important. 
Chapter

30 Water Quality in the High Plains Aquifer, 1999–2004

Natural Processes and Human Activities 

Affecting the Quality of the Water as it Moves 

From the Water Table to the Resource Used for 

Drinking and Irrigation

Unsaturated-zone

study sites

Land-use

study wells

Regional-transect study wells

Major aquifer

study wells

Unsaturated-zone

study sites Major aquifer

study wells

Private, public-supply, and irrigation wells in the High Plains aquifer generally are not screened near the water table but are screened deeper in the aquifer and collect water that has moved deeper within the aquifer. The chemistry 

of the “used” resource therefore depends on more than the quality of recently 

recharged groundwater, as described in Chapter 4. Understanding processes that 

affect the chemistry of water as it moves along flow paths from recharge areas 

to points deeper in the aquifer is critical to the understanding of water quality in 

private, public-supply, and irrigation wells. Such processes include water/rock 

interactions, mixing, and reduction-oxidation (redox) reactions. The regionaltransect study wells are used to assess this component of the resource.

5

This chapter summarizes 

the natural processes and 

human activities that affect 

recently recharged groundwater as it moves deeper 

in the system toward wells 

used for private, publicsupply, and irrigation uses. 
Natural Processes and Human Activities Affecting the Quality of the Water 31

Chapter 5

General Findings — Natural Processes and 

Human Activity

The relation between the chemistry of the recently 

recharged groundwater (Chapter 4) and the quality of the 

resource used for water supplies (Chapter 6) is controlled 

by multiple natural processes and human activities, including temporal variability in recharge chemistry, groundwater 

residence time, mixing of water from different sources, and 

biogeochemical reactions occurring along groundwater flow 

paths to the private, public-supply, and irrigation wells. 

Vertical Gradients in Groundwater Age 

and Chemistry

Vertical movement of water in the High Plains aquifer 

affects the distribution of apparent groundwater ages and 

chemistry with depth below the water table. Groundwater 

moves downward in recharge areas and upward in discharge 

areas, but vertical flow directions can be altered by pumping 

wells (see Box C) (McMahon and others, 2003; McMahon, 

Böhlke, and Lehman, 2004). 

NAWQA findings indicate that apparent groundwater 

ages in recharge areas systematically increase with depth 

below the water table in areas with relatively small amounts 

of pumping for irrigation or water supply (fig. 5–1A). In areas 

with relatively large amounts of pumping for irrigation or 

water supply, apparent-age gradients are less evident or nonexistent at depth, most likely resulting from vertical mixing 

of water in the aquifer by pumping wells or vertical leakage 

in wells having long or multiple well screens. Vertical mixing 

at the more heavily irrigated and developed regional-transect 

sites affected at least 50 to 65 percent of the saturated thickness of the aquifer (fig. 5–1B).

One important consequence of mixing shallow and deep 

groundwater is that widespread contamination of the deep 

groundwater production zones within the aquifer could occur. 

Once contaminated, deep zones in the aquifer are not likely to 

remediate quickly because of slow recharge rates, long water 

residence times in the aquifer, and slow rates of contaminant 

degradation (for example, slow denitrification).

Detailed descriptions of vertical gradients in water chemistry for the northern High Plains are in the

following report:

Vertical gradients in water chemistry and age in the northern High Plains aquifer, Nebraska, 2003

by P.B. McMahon, J.K. Böhlke, and C.P. Carney:

U.S. Geological Survey Scientific Investigations Report 2006–5050 available at

http://pubs.usgs.gov/sir/2006/5050

Detailed descriptions of vertical gradients in water chemistry and age in the southern High Plains are in the

following report:

Vertical gradients in water chemistry and age in the southern High Plains aquifer, Texas, 2002

by P.B. McMahon, J.K. Böhlke, and T.M. Lehman:

U.S. Geological Survey Scientific Investigations Report 2004–5053 available at

http://pubs.usgs.gov/sir/2004/5053

U.S. Department of the Interior

U.S. Geological Survey

Scienti c Inv estigations Rep ort 200 6–5294

National Water-Quality Assessment Program

VerticalGradients in Water Chemistry and Age in the

Northern High PlainsAquifer, Nebraska, 2003
32 Water Quality in the High Plains Aquifer, 1999–2004

Understanding Vertical Directions of Water Movement in the Aquifer

using Groundwater-Level Measurement

Groundwater levels are determined by measuring the depth to water in a well relative to a measuring point of known elevation. 

Water levels from different wells must be referenced to a common datum, such as sea level, to make them comparable. Thus, 

water levels usually are reported as elevations above sea level and are referred to as hydraulic heads. Hydraulic heads measured 

in closely spaced wells screened at different depths in the same aquifer can be used to determine vertical directions of water 

movement because water moves from areas of high head (or elevation) to areas of low head.

The difference in hydraulic head between two depths in the aquifer, normalized to the vertical distance between the well screens, 

is referred to as vertical hydraulic gradient. Positive hydraulic gradients indicate the potential for downward water movement and 

negative gradients indicate the potential for upward water movement. The potential for vertical water movement is directly proportional to the magnitude of the hydraulic gradient.

In general terms, contaminants enter the aquifer at the water table as recharge from the land surface or at the bottom of the 

aquifer as recharge from underlying geologic formations. Contaminated water at the top or bottom of the aquifer may then move 

toward the middle of the aquifer, depending on the vertical hydraulic gradient in the aquifer. A downward gradient near the water 

table would allow contaminants from the land surface to move deeper into the aquifer. An upward gradient near the bottom of the 

aquifer would permit contaminants from deeper formations to move upward into the aquifer.

Pumping effects on hydraulic gradient are particularly important with respect to contaminant movement in the aquifer. Seasonal 

development of large downward gradients over many years could be an important mechanism for drawing recently recharged 

water deeper into the aquifer more quickly than would happen under undisturbed flow conditions. Similarly, a large decrease 

in water level near the base of the aquifer could induce or enhance upward movement of mineralized water from underlying 

geologic units.

Why is it important to measure groundwater levels and understand vertical hydraulic gradients

in the High Plains aquifer? 

(hydraulic head)shallow well – (hydraulic head)deep well

vertical distance between well screens

Peter B. McMahon1 and Jason J. Gurdak1

1

 U.S. Geological Survey, Lakewood, Colorado. 

BoxC

Vertical hydraulic gradient =
Understanding Vertical Directions of Water Movement in the Aquiferusing Groundwater-Level Measurement 33

3,460

3,480

3,500

3,520

Shallow well

Deep well

–0.10

–0.05

0.00

0.05

0.10

F M A M J J A S O N D

Upward gradient

Downward gradient

J

MONTH

Shallow well

Deep well

F M A M J J A S O N D

Upward gradient

Downward gradient

J

MONTH

Shallow well

Deep well

F M A M J J A S O N D

Upward gradient

Downward gradient

J

MONTH

Irrigation season

VERTICAL HYDRAULIC

GRADIENT 

GROUNDWATER ELEVATION, 

IN FEET ABOVE SEA LEVEL 

Uplands—Regional Recharge Area

Irrigation well Monitoring wells

Monitoring wells

River Valley—

Regional Discharge 

Area

Unsaturated zone

Saturated zone Vertical distance

between well

screens

Direction of groundwater flow

downward gradients seasonal responses upward gradients

Figure C–1. Three general patterns in vertical hydraulic gradient were observed near the water table 

in the High Plains aquifer; downward gradients associated with recharge, seasonal responses to nearby 

pumping wells such as irrigation wells, and upward gradients associated with discharge. Modified from 

McMahon, Dennehy, and others (2007). 
34 Water Quality in the High Plains Aquifer, 1999–2004

Groundwater recharged more than 50 years ago accounted for at least 44 to 95 percent (median 70 percent) 

of the saturated thickness of the High Plains aquifer at the 

selected regional-transect study locations (fig. 5–1). The apparent groundwater ages at the base of the aquifer for selected 

regional-transect study locations (fig. 5–1) ranged from about 

6,500 years to about 12,500 years. The apparent groundwater 

ages at the base of the aquifer at all 11 transect sites ranged 

600

500

400

300

200

100

0

0 3,000 6,000 9,000 12,000 15,000

 (More than 10 high-capacity wells in 3-mile radius)

 SCREEN DEPTH BELOW WATER TABLE, IN FEET

APPARENT GROUNDWATER AGE, 

IN RADIOCARBON YEARS

Recharge rate = 0.02 foot per year

Recharge rate = 0.05 foot per year

A Rangeland B Irrigated cropland

 (Fewer than 3 high-capacity wells in 3-mile radius)

0 3,000 6,000 9,000 12,000 15,000

APPARENT GROUNDWATER AGE, 

IN RADIOCARBON YEARS

EXPLANATION

Northern High Plains 

Central High Plains

Southern High Plains

Exponential age distribution

Base of aquifer 

Figure 5–1. Vertical gradients in apparent groundwater age illustrate how the construction and operation of high-capacity 

irrigation wells with long or multiple screened intervals can mix waters from different depths in the aquifer. For example, 

in (A), rangeland settings that have few high-capacity wells and a relatively small amount of pumping, the apparent 

groundwater ages characteristically increase with depth in the aquifer and indicate that relatively young water remains 

near the water table. However, in (B), irrigated-cropland settings that have numerous high-capacity wells and relatively 

large amount of pumping, the apparent groundwater ages are more vertically mixed and indicate a greater potential for 

recently recharged water to move deeper in the aquifer where water-supply wells are more typically screened. Modified 

from McMahon, Dennehy, and others (2007). 

from about 3,400 years in the southern High Plains to 

15,600 years in the northern High Plains (McMahon, Böhlke, 

and Christenson, 2004; McMahon, Böhlke, and Lehman, 

2004). These apparent groundwater age estimates indicate that 

most of the water in the High Plains aquifer at the transect 

sites consisted of old groundwater that was recharged prior to 

the onset of modern human activity in the region.
Chapter 5

Natural Processes and Human Activities Affecting the Quality of the Water 35

Figure 5–3. Vertical gradients of dissolved-solids 

concentrations at (A) an undisturbed location generally 

increase with depth, but can be (B) reversed from what would 

be expected in undisturbed areas because of agricultural 

recharge. Natural and pumping-enhanced upwelling of 

brackish water from underlying units results in (C) substantial 

increases in dissolved-solids concentrations near the base of 

the aquifer.

CONCENTRATION OF DISSOLVED

NITRATE, IN MILLIGRAMS 

PER LITER AS NITROGEN

Southern High Plains

DEPTH OF SCREEN MIDPOINT BELOW WATER TABLE, IN FEET

MCL

0 5 10 15 20 25

Central High Plains

Northern High Plains

0

200

400

600

800

0

200

400

600

800

0

200

400

600

800

Regional-transect well

Land-use and major aquifer-study wells

EXPLANATION

LOWESS of all plotted data

USEPA Maximum Contaminant Level (MCL) 

Figure 5–2. Nitrate concentrations decrease 

with depth below the water table and could be 

indicative of increasing concentrations of nitrate 

in recharge over time and (or) degradation along 

flow paths to deeper locations in the aquifer.

Dissolved-solids concentrations in the aquifer in rangeland or undeveloped areas typically increase gradually 

with depth below the water table as the residence time of 

groundwater increases and water/rock interactions progress 

(fig. 5–3A). However, concentrations of dissolved solids generally decreased with depth below the water table in the vicinity of agricultural land and indicate increased concentrations 

of dissolved solids in recharge in agricultural areas. Thus, 

agricultural activity at land surface can increase concentrations 

of dissolved solids in recharge above what would be expected 

under natural conditions, resulting in a reversed gradient in 

dissolved-solids concentration near the water table (fig. 5–3B).

Natural upwelling of saline groundwater from underlying 

geologic units at the base of the aquifer in regional-discharge 

zones can cause large increases in dissolved-solids concentrations with depth below the water table (fig. 5–3C). The 

upwelling could be enhanced by pumping in the aquifer that 

lowers hydraulic heads and increases the upward-hydraulic 

gradient across the base of the aquifer (McMahon, Dennehy, 

and others, 2007). These results indicate that agricultural 

inputs, natural upwelling, and pumping-enhanced upwelling 

all have the potential to alter groundwater quality at different 

depths in the High Plains aquifer. 

CONCENTRATION OF DISSOLVED SOLIDS, 

IN MILLIGRAMS PER LITER

Base of aquifer

Water table

Agricultural 

recharge

at water table

Saline recharge

at base of aquifer

No inputs from agriculture 

or saline groundwater

100 200 300 400 500 4,000 4,100

0

0.2

0.4

0.6

0.8

1.0

A

C B

RELATIVE DEPTH OF WELL SCREEN

BELOW WATER TABLE, IN FEET

Concentrations of some important water-quality constituents, such as nitrate and dissolved solids, also systematically change with depth below the water table. For example, 

nitrate concentrations in groundwater generally decrease with 

depth below the water table (fig. 5–2). Understanding the 

relative importance of changing source strength of chemical 

constituents in recharge and biogeochemical reactions, such 

as denitrification, in the aquifer is necessary to assess the vulnerability of the deep groundwater to effects on water quality 

from human activities. 
36 Water Quality in the High Plains Aquifer, 1999–2004

Processes Affecting Water Quality as it Moves 

Within the Aquifer

Reduction-Oxidation (Redox)

Denitrification is the redox process that is most relevant 

to drinking-water quality in the High Plains aquifer because 

of widespread high nitrate concentrations (see Chapter 6). 

Denitrification cannot be relied upon to substantially attenuate nitrate concentrations because it occurs very slowly in 

most parts of the aquifer. Locally, however, denitrification can 

be important. Local reducing conditions, possibly associated 

with organic matter in the aquifer or at clay/sand interfaces, 

are known to occur in heterogeneous sedimentary aquifers. 

Specifically, chemical and isotopic data indicate that the 

Plio-Pleistocene deposits of eastern Nebraska (fig. 2–4) may 

have sufficient denitrification potential to be an important 

mechanism for attenuating nitrate concentrations in deeper 

parts of the aquifer where many of the water-supply wells are 

screened.

In most parts of the High Plains aquifer, apparent denitrification rates are slow compared to the rates reported for some 

other aquifer systems and attest to the general lack of nitrateattenuating capacity in the High Plains aquifer (fig. 5–4). The 

slow denitrification rates have important implications with 

respect to nitrate concentrations in groundwater recharge 

that exceed the drinking-water standard; it is estimated that 

it would take hundreds to thousands of years to lower nitrate 

concentrations by just 1 mg/L as N (McMahon, Dennehy, and 

others, 2006). The results highlight the importance of bestmanagement practices in the High Plains to minimize nitrate 

concentrations in recharge water. Slow recharge rates and the 

lack of natural attenuation indicate that once the water in the 

High Plains aquifer contains elevated nitrate concentrations, 

those concentrations could remain for a long time. 

Water/Rock Interactions

In general, the primary water/rock interactions that affect the quality of water as it moves through the aquifer are 

carbonate and silicate mineral dissolution, precipitation of kaolinite and silica, and cation exchange. Such reactions can increase concentrations of dissolved solids in water as it moves 

along flow paths from recharge areas to points deeper in the 

aquifer (fig. 5–3A). In addition to contributing major ions such 

as calcium, sodium, inorganic carbon, sulfate, and silica to 

solution, these reactions can also contribute trace elements to 

solution if they are present in the dissolving mineral phases. 

For example, pyrite commonly contains arsenic, and its dissolution in the presence of oxygen or other oxidants such as 

nitrate could be a source of arsenic or sulfate in groundwater. 

The presence of arsenic in groundwater and the possibility of a 

related drinking-water concern are discussed in Chapter 6. 

Figure 5–4. The apparent denitrification rates of the 

High Plains aquifer are substantially less than rates in 

many other aquifer systems (summarized from Green and 

others [2008] and Groffman and others [2006]) and indicate 

that nitrate, once present, will remain in the High Plains 

aquifer for a long time. These findings support the need 

for emphasis on best-management practices that reduce 

the concentration of nitrate in recharge water.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 

 AQUIFER

APPARENT DENITRIFICATION RATE,

IN MILLIGRAMS OF NITROGEN PER LITER PER YEAR

EXPLANATION

102

101

100

10–1

10–2

10–3

10–4

10–5

Aquifer

Maximum reported value

Range indicating a “less than” value

(1) High Plains aquifer, Ogallala Formation

(2) Confined sandstone aquifer, Kalahari Desert, Africa

(3) Unconfined alluvial aquifer, Nebraska

(4) North Atlantic Coastal Plain aquifer system, Maryland

(5) Unconfined sand aquifer, Central Valley aquifer, California

(6) Unconfined alluvial aquifer, Colorado

(7) Sand aquifer, Yakima River watershed, Washington

(8) Glacial outwash sand aquifer, Minnesota

(9) Sand aquifer, Delmarva Peninsula, Maryland

(10) North Atlantic Coastal Plain aquifer system, North Carolina

(11) Sand aquifer, Elkhorn River watershed, Nebraska

(12) Sand aquifer, San Joaquin River watershed, California

(13) Floridan aquifer system, Florida

(14) Shale confining unit, Colorado

(15) Unconfined sand aquifer, flood plain, Nebraska

(16) Unconfined sand aquifer, Germany

(17) Sand aquifer, Central Valley aquifer, California

(18) Clayey till, Nebraska

(19) Confined limestone aquifer, United Kingdom

(20) Unconfined glacial outwash sand aquifer, Canada
Chapter 5

Natural Processes and Human Activities Affecting the Quality of the Water 37

Mixing

Although redox processes and water/rock interactions 

likely occur throughout the High Plains aquifer, mixing can 

also occur, particularly at local or subregional scales. Mixing 

is likely the most important process in areas with the greatest density of irrigation or other supply wells and in regional 

discharge areas. Mixing with surface water, mixing with 

groundwater from geologic units underlying the High Plains 

aquifer, and vertical mixing within the aquifer because of well 

pumping or leakage through long or multiple well screens all 

have the potential to affect water quality in the aquifer. 

Groundwater/surface-water mixing is likely a common 

occurrence in major river valleys of the High Plains, such 

as the Arkansas River valley (fig. 2–1) (Whittemore, 2002), 

because the alluvial sediments are important sources of water 

for irrigation and public supply. Pumping groundwater from 

alluvial sediments can draw river water into the High Plains 

aquifer and affect concentrations of sulfate, pesticides, and 

other constituents in the aquifer, as indicated in regionaltransect studies in Kansas and Nebraska.

Substantial introduction of surface water into alluvial 

aquifers also occurs in areas receiving surface-water irrigation. For example, the alluvial aquifer along the Platte River 

in western Nebraska is recharged primarily by infiltration 

of river-derived water beneath unlined canals (low nitrate, 

high uranium concentrations) and beneath the irrigated fields 

(high nitrate, high uranium concentrations) (Böhlke and 

others, 2007).

Mixing by wells may also accelerate the upward movement of saline water from underlying formations. In the 

central High Plains, for example, halite-dissolution brines 

with dissolved-chloride concentrations up to 35,000 mg/L 

were present in geologic units underlying the aquifer and 

contribute about 5 percent of the water at the base of the High 

Plains aquifer in the regional-discharge area of the Cimarron 

River valley (McMahon, Böhlke, and Christenson, and 

others, 2004).

Mixing is also caused by leakage through long or multiple well screens and by long-term pumping of high-capacity 

public-supply and irrigation wells. Mixing caused by leakage (or short-circuiting, see Box D) and pumping is a major 

process for moving chemical constituents from near the water 

table to deeper zones more rapidly than would occur otherwise 

under natural hydraulic gradients. Thus, human activity is 

not only a source of chemical constituents to the water table, 

but it also appears to enhance their transport once they are in 

the aquifer.
38 Water Quality in the High Plains Aquifer, 1999–2004

Short Circuiting of Multiscreened Wells Increases

Groundwater Vulnerability to Contamination

Well-construction methods used in development of the High Plains aquifer to supply large quantities of irrigation and drinking 

water have, in locations such as York, Nebraska (figure D–1), introduced short-circuit pathways that adversely affect the quality 

of groundwater produced by public-supply wells.

Multiscreened wells are irrigation, commercial, and public-supply wells that have well screens across the unconfined and confined aquifers to maximize yields (see irrigation well in figure D–1). Multiscreened wells offer conduits for downward movement 

of water and contaminants when the well is not in use (see inactive well in figure D–1). The movement along this short-circuit 

path is years to centuries faster than flow of water through the confining unit. During periods when a multiscreened well is not 

pumped (usually during the fall, winter, and spring for irrigation wells), contaminants that flow down the well and out of the well 

screen in the confined aquifers may travel beyond the capture zone of the well before pumping is resumed and affect the quality 

of water produced by downgradient public-supply wells (see public-supply well in figure D–1).

Additional details on the human activities, natural processes, and pathways that affect public-supply well vulnerability to contamination are outlined by Clark and others (2008), Jagucki and others (2008), Landon and others (2008), Landon and Turco (2007), 

and Eberts and others (2005).

High Plains aquifer near York, Nebraska

Shale

Not to scale

Inactive well

Irrigation well

Public-supply well

(sand and gravel)

Loess

Unconfined aquifer

Confining unit

(sand) (clayey silt till)

Upper confined aquifer

Lower confining unit

Lower confined aquifer

(sand and silt)

Recharge

Discharge

Water table

Generalized 

groundwater

flow

EXPLANATION

York, Nebraska

High Plains

aquifer

Figure D–1. Conceptual model of groundwater flow and short circuits for contaminants to public-supply wells near York, Nebraska 

(modified from Jagucki and others, 2008).

Sandra Eberts,1 Brian Clark,2 Matthew Landon,3 and Martha L. Jagucki1

1

 U.S. Geological Survey, Columbus, Ohio.

 2 U.S. Geological Survey, Little Rock, Arkansas.

 3 U.S. Geological Survey, San Diego, California.

BoxD
Chapter

Quality of the Resource Used for Drinking and Irrigation—What Was Found, Where, and Why 39

Quality of the Resource Used for Drinking and 

Irrigation—What Was Found, Where, and Why 

Unsaturated-zone

study sites

Land-use

study wells

Regional-transect study wells

Unsaturated-zone

study sites

Land-use

study wells

Regional-transect study wells

Major aquifer

study wells

6

The quality of deeper water in the High Plains aquifer, where private, 

public-supply, and irrigation wells typically are screened, directly affects the 

“used” resource. The used resource assessed in this chapter includes water 

consumed by humans and crops and, to a lesser extent, livestock. Irrigated 

crops receive the largest amount of water from the High Plains aquifer. Data 

from the major-aquifer studies and existing data compiled from public-supply 

and irrigation wells are used to assess the used groundwater resource.

This chapter summarizes 

the quality of water that 

might be consumed by 

humans and irrigated 

crops, and to a lesser 

extent livestock, through 

their use of water from 

private, public-supply, and 

irrigation wells.

Chapter 6
40 Water Quality in the High Plains Aquifer, 1999–2004

General Findings—Quality of the Used Resource

The water produced by private, public-supply, and irrigation wells in the High Plains generally is of suitable quality relative to the U.S. Environmental Protection Agency’s 

Maximum Contaminant Level (MCL) or Secondary Maximum 

Contaminant Level (SMCL) (U.S. Environmental Protection 

Agency, 2009) for drinking water. However, some spatial 

differences in water quality are observed. The northern part of 

the Ogallala Formation has the best water quality (fig. 6–1), 

based on data from private wells that were sampled for the 

major-aquifer studies and in terms of various water-quality 

standards. The southern part of the Ogallala Formation has the 

poorest water quality (fig. 6–1), with elevated concentrations 

of chloride, fluoride, manganese, dissolved solids, arsenic, 

nitrate, and uranium greater than the MCL or SMCL. The observed water-quality differences between hydrogeologic units 

are attributed to many human and natural factors, including 

pumping rates, irrigation density, pesticide and nitrogen applications, climate, depth to water (below land surface), saturated 

thickness of the aquifer, well screen depth below the water 

table, redox conditions, and mixing.

The changing quality of groundwater recharge during the 

past 50 to 60 years and transport processes in the High Plains 

aquifer have resulted in small but measurable increases in 

concentrations of human contaminants in water-supply wells. 

This is illustrated by comparing nitrate concentrations in High 

Plains groundwater since the 1930s. Median nitrate concentrations in groundwater were similar from the 1930s to the 1960s, 

somewhat larger in the 1970s, and significantly larger yet in 

the 1980s and 1990s (fig. 6–2). The trend becomes even more 

apparent when viewed in the context of a millennial timeframe 

(fig. 4–6). This type of long-term gradual increase in groundwater nitrate concentrations is an example of the concept 

called “creeping normalcy” (Fogg and LaBolle, 2006) because 

the gradual increase generally is hard to separate from the 

background when viewed in the context of short timeframes. 

An implication of the trend data observed in water from 

supply wells of the High Plains aquifer is that some constituent concentrations in the used resource will continue to increase in the future as the fraction of recent recharge captured 

by supply wells increases. The upward nitrate trend could limit 

groundwater sustainability given current uses and therefore is 

an important factor for water-availability planning and future 

management.

0

10

20

30

40

50

60

70

80

90

100

OGALLALA FORMATION, NHP

OGALLALA FORMATION, CHP

SAND HILLS EOLIAN DEPOSITS, NHP

PLIO-PLEISTOCENE DEPOSITS, NHP

QUATERNARY DEPOSITS, CHP

PERCENTAGE OF DOMESTIC-WELL SAMPLES WITH

AT LEAST ONE MCL OR SMCL EXCEEDANCE

OGALLALA FORMATION, SHP

 EXPLANATION

114 67

20

49

27

30

MCL = Maximum Contaminant Level

SMCL = Secondary Maximum Contaminant 

 Level

NHP = Northern High Plains

CHP = Central High Plains

SHP = Southern High Plains

114 = Number of observations

CONCENTRATION OF NITRATE,

IN MILLIGRAMS PER LITER AS NITROGEN

1930 1940 1950 1960 1970 1980 1990 

16

14

12

10

8

6

4

2

0

433 1,171

972

3,483

5,332

7,562

7,711

EXPLANATION

90th percentile

10th percentile

75th percentile

Median (50th percentile)

25th percentile

7,711 Number of observations

Maximum Contaminant Level (MCL)

Figure 6–2. A “creeping normalcy” over time of nitrate 

concentrations in water from supply wells may limit the 

sustainability of the aquifer as a source of drinking water 

(modified from Litke, 2001). 

Figure 6–1. Comparison of overall water quality in assessed 

hydrogeologic units of the High Plains aquifer based on water 

samples collected from private wells and in terms of various 

water-quality standards (modified from McMahon, Dennehy, 

and others, 2007). Note: The bar colors correspond to the 

hydrogeologic units in figure 2–4. 
Quality of the Resource Used for Drinking and Irrigation—What Was Found, Where, and Why 41

Chapter 6

Private Wells

Water-quality conditions in the High Plains aquifer 

reported for private wells are based on samples collected from 

307 randomly distributed private wells as part of the majoraquifer studies. The results for the samples from private wells 

are an indicator of the broad range of water-quality conditions 

within each hydrogeologic unit. 

Water quality generally is good, as indicated by comparisons to SMCLs and MCLs (fig. 6–1). Median concentrations 

did not equal or exceed the SMCLs in any of the hydrogeologic units except for dissolved solids in the southern part of 

the Ogallala Formation. The fact that water in this part of the 

Ogallala Formation had a median concentration of dissolved 

solids that exceeded the SMCL highlights the general northto-south increase in concentrations of several of the major ions 

and focuses some concern regarding water-quality issues in 

the southern High Plains. Many of the factors controlling the 

high dissolved-solids concentrations occur naturally, including smaller saturated thicknesses than many other units in 

the High Plains aquifer and regional climate gradients in the 

High Plains that resulted in larger evapotranspiration rates in 

the south. Those conditions, coupled with the fact that some 

bedrock units underlying the southern part of the High Plains 

(and parts of the central part) have large amounts of soluble 

minerals that could contribute dissolved solids to water near 

the bottom of the aquifer, contribute to the observed regional 

differences in dissolved-solids concentrations. Superimposed 

on these largely natural factors are regional differences in 

land-use practices that affect recharge chemistry.

Arsenic may present a drinking-water concern because of 

its detection frequencies and concentrations. Arsenic concentrations equaled or exceeded the 10 micrograms per liter 

(μg/L) MCL in at least one water sample from all hydrogeologic units with the exception of the central Ogallala Formation. The greatest percentage of arsenic MCL exceedances 

occurred in the southern part of the Ogallala Formation (nearly 

30 percent) (fig. 6–3). The sources of arsenic in the High 

Plains aquifer are, in large part, naturally occurring; possible 

natural sources include marine shale in underlying bedrock 

units, volcanic-ash deposits in the aquifer, iron-oxide grain 

coatings on aquifer sediments, pyrite in the aquifer, and saline 

lakes. A possible human source of arsenic in the southern High 

Plains could be the historical use of arsenic acid as a cotton 

defoliant along with the application of arsenical pesticides or 

other arsenic-bearing fertilizers (Scanlon, 2006). However, 

based on land-use/land-cover distribution and arsenic profiles 

in the unsaturated zone, Scanlon (2006) concluded that surface 

application of arsenic was not the primary source of arsenic to 

groundwater in the southern High Plains.

Private wells in the aquifer generally supply suitable 

drinking water with respect to nitrate. Overall, only 5 percent 

of the samples from private wells had nitrate concentrations 

that equaled or exceeded the MCL of 10 mg/L as N. 

Nitrate concentrations in water from private wells increased significantly from north to south in the High Plains, 

which is opposite the pattern that was observed for nitrate 

concentrations in recently recharged water beneath irrigated 

cropland. Higher concentrations of nitrate in water from 

private wells in the central and southern parts (fig. 6–4) most 

likely relate to the greater presence of dissolved oxygen in the 

vicinity of measured wells (less denitrification); more intensive well pumping for irrigation or public supply that vertically mixes the water and accelerates the movement of recent 

recharge, presumably containing relatively large nitrate concentrations, deeper in the aquifer; smaller saturated thickness 

of the aquifer; well screens closer to the water table; smaller 

depth to water; and a lack of fine-grained confining layers to 

isolate deep water-bearing layers from surface contamination.

At least one of the 44 measured pesticide compounds 

was detected in water from each of the hydrogeologic units, 

with detection frequencies ranging from 5.3 to 30 percent 

(fig. 6–5). One or more pesticide compounds were detected in 

about 14 percent of private wells. Atrazine, a herbicide commonly used on corn, and its degradation product, deethylatrazine, were the most frequently detected compounds in each of 

the hydrogeologic units. Concentrations were low; all of the 

pesticide-compound concentrations were less than 2 μg/L, and 

none of them equaled or exceeded an MCL, although only 2 

of the 12 detected compounds had MCLs. Nine of the 12 detected pesticide compounds in water were among the 14 most 

commonly detected compounds in the Nation’s groundwater 

beneath agricultural land-use areas (Gilliom and others, 2006). 

This group of compounds listed for the Nation is characterized by extensive agricultural use and relatively high mobility 

and (or) persistence in the environment. 

Volatile organic compounds (VOCs) were detected in 

about 5 percent of the sampled wells, which is about three 

times less than the detection frequency for pesticides. This 

finding is consistent with the predominance of agriculture over 

urban land uses in the High Plains, assuming that most VOC 

sources occur in urban settings. Ten of the 85 measured VOCs 

were detected. All of the detected concentrations were low, 

and none exceeded an MCL. The trihalomethane compound 

chloroform was one of the most commonly detected VOCs, 

which is consistent with results for private-well samples from 

across the Nation (Zogorski and others, 2006). The relatively 

common occurrence of chloroform in the private-well samples 

may be related in part to redox conditions in the aquifer and 

the fact that chloroform is relatively stable in oxic groundwater (McMahon, Dennehy, and others, 2007).
42 Water Quality in the High Plains Aquifer, 1999–2004

TX

KS

NM

NE

SD

WY

CO

IA

Base information from U.S. Geological Survey digital data, 1:100,000

Albers Equal-Area projection

Standard Parallels 29°30' and 45°30', central meridian –96°

108°W 106°W 104°W 102°W 100°W 98°W

32°N

34°N

36°N

40°N

42°N

38°N

0 50 100 MILES

0 50 100 KILOMETERS

OK

x

EXPLANATION

Concentration of dissolved arsenic in 

domestic wells, in micrograms per 

liter

Less than 1

Greater than or equal to 1 and

less than 10

Greater than or equal to 10

No data

Alluvial systems

Quaternary deposits

Sand Hills eolian deposits

Plio-Pleistocene deposits

Ogallala Formation

Arikaree/Brule Formations

Aquifer not present

Hydrogeologic units

x

Figure 6–3. At least one groundwater sample from private wells screened in each 

hydrogeologic unit, except the central Ogallala Formation, Arikaree/Brule Formation, and 

Alluvial systems, contained arsenic concentrations that equaled or exceeded the U.S. 

Environmental Protection Agency’s Maximum Contaminant Level (MCL) of 10 micrograms 

per liter for drinking water (U.S. Environmental Protection Agency, 2009). 
Quality of the Resource Used for Drinking and Irrigation—What Was Found, Where, and Why 43

Chapter 6

Figure 6–4. About 5 percent of water samples from private wells contained nitrate 

concentrations that equaled or exceeded the U.S. Environmental Protection Agency’s 

Maximum Contaminant Level (MCL) of 10 milligrams per liter as nitrogen for drinking water 

(U.S. Environmental Protection Agency, 2009). A much larger percentage of water samples 

from private wells contained nitrate greater than 4 milligrams per liter as nitrogen, which is 

considered the natural background concentration for the High Plains aquifer. 

TX

KS

NM

NE

SD

WY

CO

IA

0 50 100 MILES

0 50 100 KILOMETERS

OK

EXPLANATION

Concentration of dissolved nitrate

in domestic wells, in milligrams per

liter as nitrogen

Less than or equal to 4

Greater than 4 and less than 10

Greater than or equal to 10

Alluvial systems

Quaternary deposits

Sand Hills eolian deposits

Plio-Pleistocene deposits

Ogallala Formation

Arikaree/Brule Formations

Aquifer not present

Hydrogeologic units

Base information from U.S. Geological Survey digital data, 1:100,000

Albers Equal-Area projection

Standard Parallels 29°30' and 45°30', central meridian –96°

108°W 106°W 104°W 102°W 100°W 98°W

32°N

34°N

36°N

40°N

42°N

38°N
44 Water Quality in the High Plains Aquifer, 1999–2004

Figure 6–5. About 14 percent of water samples from private wells contained at least one 

pesticide compound. Atrazine and its degradation product deethylatrazine were the most 

frequently detected pesticide compounds in water from the private wells. 

TX

KS

NM

NE

SD

WY

CO

IA

Base information from U.S. Geological Survey digital data, 1:100,000

Albers Equal-Area projection

Standard Parallels 29°30' and 45°30', central meridian –96°

108°W 106°W 104°W 102°W 100°W 98°W

32°N

34°N

36°N

40°N

42°N

38°N

0 50 100 MILES

0 50 100 KILOMETERS

OK

EXPLANATION

Number of pesticide compounds 

detected in domestic wells

No detections

One detected

More than one detected

Alluvial systems

Quaternary deposits

Sand Hills eolian deposits

Plio-Pleistocene deposits

Ogallala Formation

Arikaree/Brule Formations

Aquifer not present

Hydrogeologic units
Quality of the Resource Used for Drinking and Irrigation—What Was Found, Where, and Why 45

Chapter 6

Public-Supply Wells

Water-quality conditions reported for public-supply wells 

are based on existing data from 101 wells located in the PlioPleistocene deposits in Nebraska and throughout the Ogallala Formation.

The quality of water from the public-supply wells is generally similar to that of private wells, including north-to-south 

variations in major-ion chemistry that generally are attributed 

to regional differences in underlying bedrock geology, climate, 

aquifer mineralogy, land use, and hydrologic features such as 

saline lakes (McMahon, Dennehy, and others, 2007).

Significant differences were noted only in dissolvedsolids concentrations; specifically, water samples from publicsupply wells in the northern part of the aquifer contained 

significantly greater concentrations than samples from the 

private wells. Data collected from this study does not support 

a conclusive reason for the differences in dissolved-solids 

concentrations between public-supply and private wells. 

Based on a small subset of paired public-supply and 

private wells (15 pairs in the Ogallala Formation of the central 

High Plains), water from public-supply wells had more pesticide detections and more tritium concentrations greater than 

0.5 tritium unit (indicating younger water), probably because 

of mixing resulting from longer or multiple screens, higher 

pumping rates, and longer pumping periods for the publicsupply wells. This finding strongly supports the conclusion 

that leakage through wells with long or multiple screens and 

long-term pumping of high-capacity wells such as publicsupply and irrigation wells are important mechanisms for 

moving contaminants downward from the water table more 

quickly than would occur naturally in the High Plains aquifer 

(McMahon, Böhlke, and others, 2008; McMahon, Burow, and 

others, 2008) (see Box D).

Irrigation Wells

Irrigated crops receive the largest amount of water 

pumped from the High Plains aquifer. Groundwater quality 

from irrigation wells can have positive or adverse effects on 

crop yield. Salinity, reduced water infiltration at the land surface, and boron toxicity have the potential to reduce crop yield. 

Nitrogen in irrigation water potentially can increase crop yield 

and (or) reduce production costs. Water-quality data reported 

by other agencies for irrigation wells were assembled for 

131 wells in the Plio-Pleistocene deposits in Nebraska and the 

Ogallala Formation (McMahon, Dennehy, and others, 2007).

Elevated salinity in irrigation water is greatest in the 

southern part of the Ogallala Formation, in which 3 percent 

of samples had dissolved-solids concentrations in excess of 

2,000 mg/L, indicating severe salinity restrictions if the water 

is used for irrigation. No salinity restrictions were exceeded 

for samples from the northern part of the Ogallala Formation. 

About half of the samples from the central and southern parts 

could be characterized as having slight to moderate salinity 

with restrictions on the use of irrigation water (McMahon, 

Dennehy, and others, 2007).

Low salinity can also be a water-quantity issue in irrigation water because it can ultimately reduce water infiltration at 

the land surface. Specifically, water with low salinity can leach 

surface soils of soluble minerals and salts that stabilize soil 

aggregates and soil structure, allowing finer soil particles to fill 

pore spaces in the upper soil zones and reduce rates of water 

infiltration. Soil mineral leaching did not seem to be an issue 

in the High Plains; only one sample was identified to have the 

potential to severely limit infiltration (table 1 and fig. 6–6). 

Most of the assessed irrigation water was classified as having 

slight to moderate potential to reduce infiltration rates. 

Potential irrigation problem General water-quality 

criteria1

Ogallala Formation (percentage of samples in each category)

Northern High Plains Central High 

Plains

Southern High 

Plains

Salinity

Dissolved solids mg/L n=13 n=34 n=30

None <450 100 59 40

Slight to moderate 450–2,000 0 41 57

Severe > 2,000 0 0 3

Boron toxicity

Boron concentration μg/L n=9 n=29 n=21

None 0–700 100 100 100

Slight to moderate 700–3,000 0 0 0

Severe >3,000 0 0 0

[mg/L, milligrams per liter; μg/L, micrograms per liter; n, number of samples; <, less than; >, greater than]

Table 1. Salinity and boron toxicity are not a concern from irrigation well water of the Ogallala Formation.

1

From Ayers and Wescot (1994).
46 Water Quality in the High Plains Aquifer, 1999–2004

SPECIFIC CONDUCTANCE OF IRRIGATION WATER,

IN MICROSIEMENS PER CENTIMETER AT 25 DEGREES CELSIUS

Severe 

reduction 

in rate of

 infiltration

Slight to moderate 

reduction in rate 

of infiltration

No reduction in rate

of infiltration

SODIUM ADSORPTION RATIO (SAR)

10

8

6

4

2

0

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

EXPLANATION

Ogallala Formation, northern High Plains

Ogallala Formation, central High Plains

Ogallala Formation, southern High Plains

Figure 6–6. Most of the irrigation waters sampled have a slight to moderate potential to 

reduce water infiltration (modified from Ayers and Wescot, 1994). 

Toxicity effects occur in plants if certain constituents, 

such as boron, in irrigation water are present in concentrations 

large enough to cause crop damage or reduce yield (Ayers and 

Wescot, 1994). None of the irrigation-water samples in this 

study contained boron concentrations that are toxic to crops 

commonly grown in the High Plains (table 1).

Nitrogen in most of the samples represents less than 

20 percent of the nitrogen requirement for irrigated corn in the 

High Plains. Nevertheless, the amount could be important from 

a water-quality perspective if producers were to reduce applications at the land surface by a similar amount. Nitrate concentrations deeper in the High Plains aquifer are likely to increase 

over time as nitrate in the thick unsaturated zones reaches the 

water table and as irrigation pumping vertically redistributes 

nitrate throughout the aquifer. Thus, nitrogen concentrations (or 

credits) in irrigation water could increase over time and help to 

increase crop yield and (or) reduce production costs.
References Cited 47

References Cited

The High Plains aquifer study publications are shown in bold type. 

Alley, W.M., 2006, Tracking U.S. ground water—Reserves for 

the future?: Environment, v. 49, p. 11–29.

Alley, W.M., Reilly, T.L., and Franke, O.H., 1999, Sustainability of ground-water resources: U.S. Geological Survey 

Circular 1186, 79 p.

Ayers, R.S., and Wescot, D.W., 1994, Water quality for agriculture: Food and Agriculture Organization irrigation and 

drainage paper no. 29 Rev. 1, ISBN 92-5-102263-1, 174 p.

Böhlke, J.J., Verstraeten, I.M., and Kraemer, T.F., 2007, 

Effects of surface-water irrigation on sources, fluxes, and 

residence times of water, nitrate, and uranium in an alluvial 

aquifer: Applied Geochemistry, v. 22, p. 152–174. 

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

2003, Ground-water quality beneath irrigated agriculture in the central High Plains aquifer, 1999–2000: 

U.S. Geological Survey Water-Resources Investigations 

Report 2003–4219, 39 p.

Claborn, B. J., Urban, L.V., and Oppel, S.E., 1985, Frequency 

of significant recharge to the Ogallala Aquifer from playa 

lakes: Water Resource Center, Project Number G–935–03, 

24 p. 

Clark, B.R., Landon, M.K., Kauffman, L.J., and Hornberger, 

G.Z., 2008, Simulations of ground-water flow, transport, 

age, and particle tracking near York, Nebraska, for a study 

of transport of anthropogenic and natural contaminants 

(TANC) to public supply wells: U.S. Geological Survey 

Scientific Investigations Report 2007–5068, 48 p. 

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P., 2002, Compilation of minimum and maximum isotope 

ratios of selected elements in naturally occurring terrestrial 

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High Plains aquifer; USA – Groundwater development 

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

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P.A., Martin, J.D., Nakagaki, N., Nowell, L.H., Scott, 

J.C., Stackelberg, P.E., Thelin, G.P., and Wolock, D.M., 

2006, Pesticides in the Nation’s streams and ground water, 

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H.M., 2008, Limited occurrence of denitrification in 

four shallow aquifers in agricultural areas of the United 

States: Journal of Environmental Quality 37, 994–1009, 

doi:10.2134/jeq2006.0419. 

Groffman, P.M., Altabet, M.A., Böhlke, J.K., ButterbachBahl, K., David, M.B., Firestone, M.K., Giblin, A.E., 

Kana, T.M., Nielson, L.P., and Voytek, M.A., 2006, Methods for measuring denitrification—Diverse approaches to 

a difficult problem: Ecological Applications, v. 16, no. 6, 

p. 2091–2122.

Gurdak, J.J., 2008, Groundwater vulnerability: Nonpoint-source contamination, climate variability, and 

the High Plains aquifer: Saarbrucken, Germany, VDM 

Verlag Publishing, ISBN: 978-3-639-09427-5, 223 p. 

 Gurdak, J.J., Hanson, R.T., McMahon, P.B., Bruce, 

B.W., McCray, J.E., Thyne, G.D., and R.C. Reedy, 

2007, Climate variability controls on unsaturated water 

and chemical movement, High Plains aquifer, USA: 

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Gurdak, J.J., McCray, J.E., Thyne, G.D., and Qi, 

S.L., 2007, Latin hypercube approach to estimate 

uncertainty in groundwater vulnerability: Groundwater, v. 45, no. 3, p. 348–361, doi: 10.1111/j.1745-

6584.2006.00298.x.

Gurdak, J.J., and Qi, S.L., 2006, Vulnerability of recently 

recharged ground water in the High Plains aquifer to 

nitrate contamination: U.S. Geological Survey Scientific Investigations Report 2006–5050, 39 p.

Gurdak, J.J., Walvoord, M.A., and McMahon, P.B., 2008, 

Susceptibility to enhanced chemical migration from 

depression-focused preferential flow, High Plains aquifer: Vadose Zone Journal, v. 7, no. 4, p. 1172–1184,

doi: 10.2136/vzj2007.0145.

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48 Water Quality in the High Plains Aquifer, 1999–2004

Jagucki, M.L., Landon, M.K., Clark, B.R., and Eberts, 

S.M., 2008, Assessing the vulnerability of public supply wells to contamination—High Plains aquifer near 

York, Nebraska: U.S. Geological Survey Fact Sheet 

2008–3025, 6 p.

Landon, M.K., Clark, B.R., McMahon, P.B., McGuire, 

V.L., and Turco, M.J., 2008, Hydrogeology, chemical 

characteristics, and transport processes in the zone of 

contribution of a public-supply well in York, Nebraska: 

U.S. Geological Survey Scientific Investigations Report 

2008–5050, 149 p.

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S.S., ed., Hydrogeologic settings and ground-water 

flow simulations for regional studies of the transport 

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regional Assessments of aquifers: U.S. Geological Survey 

Fact Sheet 2005–3013, 4 p.

Litke, D.W., 2001, Historical water-quality data for the 

High Plains Regional Groundwater study area in 

Colorado, Kansas, Nebraska, New Mexico, Oklahoma, 

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in the High Plains aquifer, predevelopment to 2005: U.S. 

Geological Survey Fact Sheet 2007–3029, 2 p.

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Sebree, S.K., and Verstraeten, I.M., 2003, Water in storage 

and approaches to ground-water management, High Plains 

aquifer, 2000: U.S. Geological Survey Circular 1243, 51 p.

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in the isotopic composition of natural and anthropogenic 

nitrate in groundwater, High Plains, USA: Environmental Science & Technology, v. 40, p. 2965–2970.

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Geochemistry, radiocarbon ages, and paleorecharge 

conditions along a transect in the central High Plains 

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K.L., Landon, M.K., Crandall, C.A., Burow, K.R., and 

Brown, C.J., 2008, Source and transport controls on the 

movement of nitrate to public supply wells in selected 

principal aquifers of the United States: Water Resources 

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Vertical gradients in water chemistry and age in the 

southern High Plains aquifer, Texas, 2002: U.S. Geological Survey Scientific Investigations Report 2004–5053, 

53 p.

McMahon, P. B., Burow, K.R., Kauffman, L.J., Eberts, 

S.M., Böhlke, J.K., and Gurdak, J.J., 2008, Simulated 

response of water quality in public supply wells to land 

use change: Water Resources Research, v. 44, W00A06, 

doi:10.1029/2007WR006731.

McMahon, P.B., Dennehy, K.F., Bruce, B.W., Böhlke, J.K., 

Michel, R.L., Gurdak, J.J., and Hurlbut, D.B., 2006, 

Storage and transit time of chemicals in thick unsaturated zones under rangeland and irrigated cropland, 

High Plains, United States: Water Resources Research, 

v. 42, W03413, doi:10.1029/2005WR004417.

McMahon, P.B., Dennehy, K.F., Bruce, B.W., Gurdak, 

J.J., and Qi, S.L., 2007, Water-quality assessment of the 

High Plains aquifer, 1999–2004: U.S. Geological Survey 

Professional Paper 1749, 136 p.

McMahon, P.B., Dennehy, K.F., Michel, R.L., Sophocleous, M.A., Ellett, K.M., and Hurlbut, D., 2003, Water 

movement through thick unsaturated zones overlying 

the central High Plains aquifer, southwestern Kansas, 

2000-2001: U.S. Geological Survey Water-Resources 

Investigations Report 2003–4171, 30 p.

National Land Cover Database, 2001: accessed January 30, 

2007, at http://www.mrlc.gov/mrlc2k_nlcd.asp

Nolan, B.T., and Stoner, J.D., 2000, Nutrients in groundwaters 

of the conterminous United States: Environmental Science 

& Technology, v. 34, p. 1156–1165.
References Cited 49

Opie, John, 2000, Ogallala—Water for a dry land: Lincoln, 

University of Nebraska Press, 475 p.

Plummer, L.N., and Friedman, L.C., 1999, Tracing and dating young ground water: U.S. Geological Survey Fact 

Sheet 134–99, 4 p.

Plummer, L.N., Michel, R.L., Thurman, E.M., and Glynn, 

P.D., 1993, Environmental tracers for age-dating young 

groundwater, in Alley, W.M., ed., Regional Groundwater 

Quality: Van Nostrand Reinhold, New York, p. 255–294.

Pope, L.M., Bruce, B.W., Rasmussen, P.P., and Milligan, 

C.R., 2002, Quality of shallow ground water in areas 

of recent residential and commercial development, 

Wichita, Kansas, 2000: U.S. Geological Survey WaterResources Investigations Report 2002–4228, 75 p.

Qi, S.L., and Gurdak, J.J., 2006, Percentage of probability 

of nonpoint source nitrate contamination of recently 

recharged ground water in the High Plains aquifer: 

U.S. Geological Survey Data Series DS–192, available at 

http://water.usgs.gov/lookup/getspatial?ds192_hp_npctprob 

(accessed 01/21/08).

Qi, S.L., Konduris, A., Litke, D.W., and Dupree, J., 2002a, 

Classification of irrigated land using satellite imagery, 

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2002–4236, 31 p.

Qi, S.L., Konduris, A., Litke, D.W., and Dupree, J., 

2002b, HPIRRLND_92—Location of irrigated land 

classified from satellite imagery, High Plains area, 

nominal date 1992: U.S. Geological Survey Open-File 

Report 2002–441 (GIS dataset).

Rajagopalan, S., Anderson, T.A., Fahlquist, L., Rainwater, 

K.A., Ridley, M., and Jackson, W.A., 2006, Widespread 

presence of naturally occurring perchlorate in High Plains 

of Texas and New Mexico: Environmental Science & Technology, v. 40, p. 3156–3162.

Reilly, T.E., Dennehy, K.F., Alley, W.M., and Cunningham, 

W.L., 2008, Ground Water availability in the United States: 

U.S. Geological Survey Circular 1323, 70 p., available at 

http://pubs.usgs.gov/circ/1323/ (accessed 01/21/08).

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http://www.beg.utexas.edu/environqlty/vadose/arsenic.htm.

Stanton, J.S., and Fahlquist, Lynne, 2006, Ground-water 

quality beneath irrigated agriculture in the northern 

and southern High Plains aquifer, Nebraska and Texas, 

2003–04: U.S. Geological Survey Scientific Investigations 

Report 2006–5196, 95 p.

U.S. Environmental Protection Agency, 2009, National primary (and secondary) drinking water standards: accessed 

January 9, 2009, at http://www.epa.gov/safewater/

mcl.html#mcls

Whittemore, D.O., 2002, Groundwater recharge in the upper 

Arkansas river corridor in southwest Kansas: Kansas Geological Survey Open-File Report 2002–30, 29 p.

Wood, W.W., Rainwater, K.A., Thompson, D.B., 1997, Quantifying macropore recharge: examples form a semi-arid 

area: Groundwater, v. 35, no. 6, p. 1097–1106.

Zogorski, J.S., Carter, J.M., Ivahnenko, T., Lapham, W.W., 

Moran, M.J., Rowe, B.L., Squillace, P.J., and Toccalino, 

P.L., 2006, Volatile organic compounds in the Nation’s 

ground water and drinking-water supply wells: U.S. Geological Survey Circular 1292, 101 p.

References Cited
50 Water Quality in the High Plains Aquifer, 1999–2004

Glossary

(Terms in definitions that are defined elsewhere in the Glossary are shown in boldface within the definition.)

apparent groundwater age The time elapsed since the 

recharge water became isolated from the atmosphere. The term 

“age” is normally qualified with the word “apparent” to signify 

that the accuracy of the determined age is dependant on many 

variables (Plummer and others, 1993; Plummer and Friedman, 1999)

aquifer A geologic formation, group of formations, or part of 

a formation that contains sufficient saturated permeable material 

to yield significant quantities of water to springs and wells.

assimilation A biochemical or abiotic process that uses 

nitrate or ammonium to form plant proteins and other nitrogencontaining compounds. 

biogenic nitrate Nitrate (NO3–) that is formed by the process 

of nitrification in which ammonium is oxidized to nitrate. 

chemigation An agricultural practice of injecting chemicals 

(such as fertilizers) directly into the irrigation water for application of the chemicals through the irrigation system to the crop 

of field.

confining unit A hydrogeologic unit of impermeable or 

distinctly less permeable material within an aquifer or bounding 

one or more aquifers.

degradate A new compound formed by transformation of a 

pesticide by chemical, photochemical, or biological reactions. 

denitrification A microbially facilitated process of nitrate 

reduction to nitrogen gas (N2) through a series of intermediate 

steps. It is the most important natural sink for nitrate in groundwater. 

diffuse recharge A type of recharge that occurs when precipitation or melting snow infiltrates across a uniform area of an 

aquifer and percolates relatively uniformly through the unsaturated zone, eventually intercepting the water table.

discharge See groundwater discharge. 

drawdown The vertical distance the water-table elevation is lowered because of the withdrawal or pumping of 

groundwater.

focused recharge A type of recharge that occurs by rapid 

movement of water through the soil and unsaturated zone that 

bypasses a large portion of the soil and unsaturated-zone matrix. 

groundwater Water beneath land surface in the saturated zone.

groundwater age See apparent groundwater age. 

groundwater availability A function not only of the quantity 

and quality of groundwater in an aquifer but also the physical structures, laws, regulations, and socioeconomic factors that 

control the demand and use of groundwater (Reilly and others, 2008). 

groundwater discharge Flow of water from the saturated zone.

groundwater recharge The flux of water to groundwater. 

As used in this report, recharge is the vertical, volumetric 

flux of water across the water table or saturated zone of 

an aquifer. Rates of recharge are often expressed in units of 

length per time (mm yr–1). 

groundwater sustainability The amount of groundwater

that will be available to support future uses of a particular 

aquifer or groundwater resource. Alley and others (1999) state 

that groundwater sustainability is the development and use of 

groundwater in such a manner that can be maintained for an 

indefinite time without causing unacceptable environmental, 

economic, or social consequences.

hydraulic properties Those properties of rock or sediment 

that govern the entrance of water and the capacity to hold, 

transmit, and deliver water. 

hydrogeologic unit A distinct soil or rock unit or zone which 

by virtue of its hydraulic properties has a distinct influence 

on the storage or movement of groundwater.

hydrogeology The geologic and hydrologic features characteristic of a hydrogeologic unit that control the movement of 

water, solutes, and small particles through the subsurface.

hydrologic system The assemblage of pathways by which 

water travels as it circulates beneath, at, and above the Earth’s 

surface through various processes such as precipitation, runoff, 

evaporation, infiltration, transpiration, and groundwater 

recharge and discharge.

infiltration The process of precipitation or melting snow 

entering soil or rock across its interface with the atmosphere. 

irrigation return flow The part of artificially applied water 

that is not consumed by evapotranspiration and that migrates 

to an aquifer or surface-water body. 

land-use study The land-use studies are used to measure 

and explain the quality of recently recharged groundwater in 

irrigated cropland and urban settings by sampling water-table 

monitoring wells installed as part of the NAWQA Program. 

major-aquifer study The major-aquifer studies are used 

to broadly assess water-quality conditions in the aquifer by 

sampling networks of randomly selected, areally distributed 

existing private wells.

Maximum Contaminant Level (MCL) A drinking-water standard that is legally enforceable and that sets the permissible 

concentration of a specific compound delivered to any user 

of a public-water system. In this report, only values established by the U.S. Environmental Protection Agency (USEPA) 

are used. 

milligrams per liter (mg/L) A unit expressing the concentration of chemical constituents in solution as weight (milligrams) of solute per unit volume (liter) of water; equivalent 

to one part per million in most surface water and groundwater. 

One thousand micrograms per liter equals 1 mg/L. 
Glossary 51

natural attenuation The reduction in constituent concentration in the environment that is caused by naturally occurring 

physical, chemical, and (or) biological processes. 

nonpoint sources A diffuse and widely dispersed location at 

which one or more contaminants are known to be released into 

the hydrologic system, such as agricultural runoff or atmospheric deposition. 

pesticide A chemical applied to crops, rights-of-way, lawns, 

or residences to control weeds, insects, fungi, nematodes, 

rodents, and other “pests.” 

point source A specific location at which one or more 

contaminants are known to be released into the hydrologic 

system. 

predevelopment The time prior to substantial groundwater 

development for irrigation. McGuire and others (2003) estimate that 1957 was the median year defining predevelopment 

across the High Plains.

public-supply well A privately or publicly owned well 

that provides water for public use to: (A) community water 

systems, (B) transient noncommunity water systems, such as 

campgrounds, or (C) nontransient, noncommunity systems, 

such as schools. 

recently recharged groundwater In this report, refers to 

groundwater that was recharged during the past approximate 50 years, as indicated by tritium concentrations in 

groundwater greater than 0.5 tritium unit. 

recharge See groundwater recharge. 

reduction-oxidation (redox) Chemical reactions that involve 

a change in the oxidation number (state) of atoms. Redox processes in groundwater often are microbially facilitated. 

regional-transect study The regional-transect studies are 

used to characterize vertical gradients in groundwater chemistry and apparent groundwater age in the thickest areas of 

the High Plains aquifer and to identify major biogeochemical 

reactions affecting the quality of water along groundwater flow 

paths leading from recharge areas to down gradient wells.

saturated zone The subsurface region in the aquifer in which 

all the interstices or voids are filled with water under a pressure equal to or exceeding that of the atmosphere. 

Secondary Maximum Contaminant Level (SMCL) A drinkingwater guideline that is not legally enforceable for “secondary” 

contaminants, which is set by the U.S. Environmental Protection Agency (USEPA). A secondary contaminant is a concern 

for the esthetic qualities of drinking water. 

sodium adsorption ratio (SAR) A measure of irrigation 

water sodium hazard. It is the ratio of sodium to calcium plus 

magnesium adjusted for valence. The SAR value of water is 

considered along with specific conductance in determining 

suitability for irrigation.

Study Unit A major hydrologic system of the United States, 

geographically defined by surface-water or groundwater features, in which NAWQA sampling studies are focused. 

subsurface The region of material beneath the land surface 

that encompasses the soil, unsaturated, and saturated zones. 

transit time The time required for water or a chemical to 

travel between two locations. 

tritium unit (TU) A measure of the concentration of tritium 

(3H), equal to 1 3H atom in 1018 atoms of hydrogen (H), or 

3.24 picocuries per liter (pCi/L). 

unsaturated zone The subsurface zone between land surface 

and the water table, characterized as containing water under 

pressure less than that of the atmosphere. The matrix of the 

unsaturated zone is not completely filled with water; thus, 

gases exist in the pore spaces of the unsaturated zone. 

unsaturated-zone study The unsaturated-zone studies are 

used to measure the amount of chemicals stored in the subsoil 

above the water table of the High Plains aquifer and determine their transit times from land surface to the water table in 

rangeland and irrigated settings.

water table The point below land surface at which groundwater is first present and below which the aquifer material 

is saturated. 

Glossary
52 Water Quality in the High Plains Aquifer, 1999–2004

Appendix 1

 Abbreviations and Acronyms

δ delta value of an isotopic composition

km kilometer

LOWESS locally weighted scatterplot smoothing

mg/L milligrams per liter

MCL Maximum Contaminant Level

N nitrogen

NAWQA U.S. Geological Survey National Water-Quality Assessment Program

NO3

– nitrate

SAR sodium adsorption ratio

SDWA Safe Drinking Water Act

SMCL Secondary Maximum Contaminant Level

TU tritium unit

μg/L micrograms per liter

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

VOC volatile organic compound
Appendix 2 53

Appendix 2 

 Study-Unit Design

TX

CO

KS

NM

NE

OK

WY

SD

Base information from U.S. Geological Survey digital data, 1:100,000

Albers Equal-Area projection

Standard Parallels 29°30' and 45°30', central meridian –96°

108°W 106°W 104°W 102°W 100°W 98°W

32°N

34°N

36°N

40°N

42°N

0 50 100 MILES

0 50 100 KILOMETERS

EXPLANATION

Elevation, in feet

above sea level (NAVD88)

8,000

1,000

EXPLANATION

Major aquifer study, 

domestic wells

Quaternary deposits

Sand Hills eolian deposits

Plio-Pleistocene deposits

Ogallala Formation

Unsaturated-zone study 

 irrigated cropland site

 rangeland site

Agricultural land-use study 

 monitoring well

Regional-transect study

 monitoring well

Figure A2–A. Location of all sampled wells. 

Appendixes
54 Water Quality in the High Plains Aquifer, 1999–2004

TX

KS

NM

NE

SD

WY

CO

IA

Base information from U.S. Geological Survey digital data, 1:100,000

Albers Equal-Area projection

Standard Parallels 29°30' and 45°30', central meridian –96°

0 50 100 MILES

0 50 100 KILOMETERS

OK

108°W 106°W 104°W 102°W 100°W 98°W

32°N

34°N

36°N

40°N

42°N

38°N

EXPLANATION

Unsaturated-zone study monitoring site – 

irrigated cropland

Unsaturated-zone study monitoring site – 

rangeland

Agricultural land-use study monitoring well

Irrigated cropland

Figure A2–B. Location of unsaturated-zone study monitoring wells, agricultural land-use study 

monitoring wells, and distribution of irrigated cropland in the High Plains aquifer area, nominal date 1992 

(Qi and others, 2002a). 
Appendix 2 55

IA

TX

CO KS

NM

NE

OK

WY

SD

MN

Base information from U.S. Geological Survey digital data, 1:100,000Albers Equal-Area projectionStandard Parallels 29 ̊30' and 45 ̊30', central meridian –96 ̊

A

A'

B B'

C

C'

0 50 100 KILOMETERS

0 50 100 MILES

EXPLANATION

Transect-study siteGeohydrologic sectionWater-table elevation,in feet. Contour intervalis 500 feet 

No aquifer present

A A'

150

1

0

500

2000

52

00

03

00

53

00

0004

0

0

54

05

00

5500

051 0002 0

2500

3000

53

00 04

00

54

00

2500 3500

4000

3000

3500

4000

2500

Rolla

Hugoton

Liberal

Cimarron

Castro

McPherson Co. #1

Hale

Lincoln Co. #4 Lincoln Co. #2

Lincoln Co. #1 Lincoln Co. #3

3,5003,0002,5002,0003,5003,0002,5002,000

0 20 40 60 80

4,0003,5003,0002,500

Platte River Lincoln Co. #3

Lincoln Co. #1 Lincoln Co. #2

ELEVATION,

IN FEET

Northern High Plains, Ogallala Formation

Land surface

Monitoring-well screen

Lincoln Co. #4

Cimarron River

Cimarron Liberal Hugoton Rolla

Screen in Upper Permian-Pennsylvanian confining unit

ELEVATION,

IN FEET

Central High Plains, Ogallala Formation

Base of aquifer

Water table

McPherson Co. #1

Hale

Castro

ELEVATION,

IN FEET

DISTANCE, IN MILES

Southern High Plains, Ogallala Formation

Screen in Dockum Formation

A A’

B

B’

C

C’

Figure A2–C. Location of regional transect study wells (V.L. McGuire, U.S. Geological Survey, written commun., 2002). 

Appendixes
56 Water Quality in the High Plains Aquifer, 1999–2004

TX

KS

NM

NE

SD

WY

CO

IA

Base information from U.S. Geological Survey digital data, 1:100,000

Albers Equal-Area projection

Standard Parallels 29°30' and 45°30', central meridian –96°

108°W 106°W 104°W 102°W 100°W 98°W

32°N

34°N

36°N

40°N

42°N

38°N

0 50 100 MILES

0 50 100 KILOMETERS

OK

EXPLANATION

Major aquifer-study domestic wells

Hydrogeologic units

Quaternary deposits

Sand Hills eolian deposits

Plio-Pleistocene deposits

Ogallala Formation

Alluvial systems

Quaternary deposits

Sand Hills eolian deposits

Plio-Pleistocene deposits

Ogallala Formation

Arikaree/Brule Formations

Aquifer not present

Figure A2–D. Location of major aquifer-study wells and distribution of hydrogeologic units
Appendixes

Appendix 2 57

Well 

network Well type Network purpose Water-quality parameters Sampling frequency Network locations Number of wells

Unsaturatedzone 

studies

Monitoring wells with 10-foot-long 

screens located near water table. Various instruments in the unsaturated zone to measure water content and chemistry

Measure chemical storage and transit 

times in the unsaturated zone 

Groundwater:

Physical properties; dissolved oxygen; major ions; nutrients; trace elements; multiple stable isotopes and dissolved gases; Tritium; dissolved organic carbon; pesticide compounds

Unsaturated zone:

Chloride; nitrate; tritium; multiple stable isotopes and gases; pesticide compounds

Multiple

times

NHP, Ogallala Formation, irrigated 

corn and rangeland; 

CHP, Ogallala Formation, irrigated 

corn and rangeland; 

SHP, Ogallala Formation, irrigated 

corn and rangeland

332

Land-use 

studies

Monitoring wells with 10-foot-long 

screens placed near water table 

Characterize the quality of recently 

recharged water 

Physical properties; dissolved oxygen; 

major ions; nutrients; trace elements; stable isotopes of nitrate; tritium; dissolved organic carbon; pesticide compounds; volatile organic compounds (urban LUS only) 

1 time

NHP, Plio-Pleistocene deposits, 

irrigated corn/soybeans; 

CHP, Ogallala Formation, 

irrigated corn; 

CHP, Alluvial system, urban; SHP, Ogallala Formation, 

irrigated cotton

30 

 30

30

 29

Regionaltransect 

studies 

Monitoring wells with 10-foot-long 

screens nested between the water table and base of aquifer

Characterize vertical gradients in 

water chemistry and age. Identify major biogeochemical reactions affecting water quality along flow paths from recharge areas to downgradient wells 

Physical properties; dissolved oxygen; 

major ions; nutrients; trace elements; multiple stable isotopes and dissolved gases; tritium; carbon-14; radon; dissolved organic carbon; pesticide compounds

1 time

NHP, Ogallala Formation and Sand 

Hills eolian deposits; 

CHP, Ogallala Formation; SHP, Ogallala Formation

19 14

 8

Major

aquifer 

studies Private wells Broadly characterize water quality 

Physical properties; dissolved oxygen; 

major ions; nutrients; trace elements; radon; tritium (selected wells); dissolved organic carbon; pesticide compounds; volatile organic compounds 1 time

NHP, Sand Hills eolian deposits; NHP, Plio-Pleistocene deposits; NHP, Ogallala Formation; CHP, Quaternary deposits; CHP, Ogallala Formation; SHP, Ogallala Formation

27 30 

114 

20 67 49

Table A2–1. Study components of the High Plains NAWQA study.[LUS, land-use study; NHP, northern High Plains; CHP, central High Plains; SHP, southern High Plains]
58 Water Quality in the High Plains Aquifer, 1999–2004

Appendix 3 

EXPLANATION

(organic constituents)

ND No data

MCL USEPA Maximum Contaminant Level

for drinking water

SMCL USEPA Secondary Maximum Contaminant

Level for drinking water

n=30 Number of observations

d=10

f=33

Number of detections

Frequency of detections, in percent

75th percentile

25th percentile

MEDIAN

90th percentile

10th percentile

EXPLANATION

(inorganic constituents)

ND No data

n=96 Number of observations

MCL USEPA Maximum Contaminant Level

for drinking water

SMCL USEPA Secondary Maximum Contaminant

Level for drinking water

75th percentile

25th percentile

MEDIAN

90th percentile

10th percentile

High Plains Findings in a National Context

This appendix includes graphical comparisons of chemical concentrations for some of the most commonly detected 

inorganic and organic constituents in the High Plains aquifer 

and other principal aquifers of the United States (Nolan and 

Stoner, 2000; Lapham and others, 2005; Gilliom and others, 

2006; Zogorski and others, 2006). For each constituent, the 

concentration data are grouped according to four well types: 

shallow agricultural monitoring wells (agricultural land-use 

study wells), private wells (major-aquifer study wells), shallow urban monitoring wells (urban land-use study wells), 

and public-supply wells. For each well type, the aquifers also 

are grouped according to aquifer lithology: basalt and volcanics, carbonate/crystalline, carbonate/sandstone, sand and 

gravel, and semiconsolidated sand. For organic compounds, 

graphs show only concentrations detected above a common 

assessment level, and data for a particular compound were 

not plotted if there were fewer than five detections of the 

compound. Note that analytical detection limits varied among 

the constituents; thus, detection frequencies are not comparable between constituents. For a given constituent, however, 

detection frequencies are comparable between aquifers. The 

data used in this report for the High Plains aquifer are available at URL: http://co.water.usgs.gov/nawqa/hpgw/index.html

(accessed January 13, 2009).
Appendix 3 59

Appendixes

Basalt and volcanics

Carbonate/Crystalline

Carbonate/Sandstone Sand and Gravel Semiconsolidated sand Basalt and volcanics

Carbonate/Crystalline

Carbonate/Sandstone Sand and Gravel Semiconsolidated sand

Basalt and volcanics

Carbonate/Crystalline

Carbonate/Sandstone Sand and Gravel Semiconsolidated sand Basalt and volcanics

Carbonate/Crystalline

Carbonate/Sandstone Sand and Gravel Semiconsolidated sand

DOMESTIC WELLS

PUBLIC-SUPPLY WELLS SHALLOW URBAN MONITORING WELLS

SHALLOW AGRICULTURAL MONITORING WELLS

n=46 n=37 n=23

n=203

n=59

n=26 n=48 ND

ND

ND ND

ND

ND

ND

ND

ND n=125 n=76

n=239

n=306

n=72 n=52 n=5

n=5

ND

n=58

n=112

n=79

n=68

n=67

n=97

ND n=89 ND

n=184

n=30

n=28 n=25

n=31

ND

ND

ND

n=64

ND

n=30

n=24

n=23 n=103 ND

n=72

n=35

n=56

n=11

n=23

n=8

ND n=5

n=27

n=26

CONCENTRATION OF DISSOLVED ARSENIC, 

IN MICROGRAMS PER LITER

0

755025

100

COLUMBIA PLATEAU

SNAKE RIVER PLAIN

FLORIDAN

NEW YORK AND NEW ENGLAND

PIEDMONT AND BLUE RIDGE

CAMBRIAN-ORDOVICIAN

EDWARDS-TRINITY

BASIN AND RANGE

CALIFORNIA COASTAL BASIN

CENTRAL VALLEY

GLACIAL DEPOSITS

HIGH PLAINS

COASTAL LOWLANDS

MISSISSIPPI EMBAYMENT

NORTHERN ATLANTIC

COASTAL PLAIN

HAWAIIAN VOLCANIC

MCL=10 μg/L

0

755025

100

MCL=10 μg/L

0

755025

100

MCL=10 μg/L

0

755025

100

MCL=10 μg/L

n=3

n=1

n=4 n=4 n=1

COLUMBIA PLATEAU

SNAKE RIVER PLAIN

FLORIDAN

NEW YORK AND NEW ENGLAND

PIEDMONT AND BLUE RIDGE

CAMBRIAN-ORDOVICIAN

EDWARDS-TRINITY

BASIN AND RANGE

CALIFORNIA COASTAL BASIN

CENTRAL VALLEY

GLACIAL DEPOSITS

HIGH PLAINS

COASTAL LOWLANDS

MISSISSIPPI EMBAYMENT

NORTHERN ATLANTIC

COASTAL PLAIN

HAWAIIAN VOLCANIC

COLUMBIA PLATEAU

SNAKE RIVER PLAIN

FLORIDAN

NEW YORK AND NEW ENGLAND

PIEDMONT AND BLUE RIDGE

CAMBRIAN-ORDOVICIAN

EDWARDS-TRINITY

BASIN AND RANGE

CALIFORNIA COASTAL BASIN

CENTRAL VALLEY

GLACIAL DEPOSITS

HIGH PLAINS

COASTAL LOWLANDS

MISSISSIPPI EMBAYMENT

NORTHERN ATLANTIC

COASTAL PLAIN

HAWAIIAN VOLCANIC

COLUMBIA PLATEAU

SNAKE RIVER PLAIN

FLORIDAN

NEW YORK AND NEW ENGLAND

PIEDMONT AND BLUE RIDGE

CAMBRIAN-ORDOVICIAN

EDWARDS-TRINITY

BASIN AND RANGE

CALIFORNIA COASTAL BASIN

CENTRAL VALLEY

GLACIAL DEPOSITS

HIGH PLAINS

COASTAL LOWLANDS

MISSISSIPPI EMBAYMENT

NORTHERN ATLANTIC

COASTAL PLAIN

HAWAIIAN VOLCANIC

CONCENTRATION OF DISSOLVED ARSENIC, 

IN MICROGRAMS PER LITER

CONCENTRATION OF DISSOLVED ARSENIC, 

IN MICROGRAMS PER LITER

CONCENTRATION OF DISSOLVED ARSENIC, 

IN MICROGRAMS PER LITER

Figure A3–A. Arsenic concentrations from the High Plains aquifer in a national context. 
60 Water Quality in the High Plains Aquifer, 1999–2004 ND ND ND ND ND ND ND ND ND n=60 d=23 f=38 n=37 d=7 f=19 n=441 d=143 f=32 n=86 d=40 f=47 MCL=3 μg/L; common assessment level (cal) = 0.007 μg/L ND ND ND n=72 d=15 f=21 n=126 d=11 f=9 n=75 d=12 f=16 n=348 d=27 f=8 n=304 d=38 f=13 MCL=3 μg/L; cal = 0.007 μn=144 g/Ld=41 f=28n=111 d=2 f=2 n=181 d=56 f=31 n=71 d=23 f=32 n=66 d=0 f=0

ND ND ND

n=112

d=33

f=29

n=277

d=30

f=11

n=30d=14

f=47

MCL=3 μg/L;

cal=0.007 μg/L

n=61d=11

f=18

n=30d=18

f=60

ND ND

ND ND

ND n=31

d=8

f=26

n=131

d=13

f=10

n=33d=15

f=45

n=16d=11

f=69

n=26

d=0

f=0

ND n=117

d=28

f=24

1.51.00.50.0

1.51.00.50.0

1.51.00.50.0

1.51.00.50.0

n=23

d=4

f=17

n=27

d=5

f=19

ND ND n=60

d=14

f=23

ND

MCL=3 μg/L;

cal=0.007 μg/L

n=3d=3

f=100

n=22

d=2

f=9

n=25

d=1

f=4

n=97

d=3

f=3

n=69

d=1

f=1

n=5d=1

f=20

n=20

d=1

f=5

n=6d=0

f=0

n=4d=0

f=0

n=39

d=0

f=0

n=23

d=1

f=4

n=26

d=2

f=8

COLUMBIA PLATEAU

SNAKE RIVER PLAIN

FLORIDAN

NEW YORK AND NEW ENGLAND

PIEDMONT AND BLUE RIDGE

CAMBRIAN-ORDOVICIAN

EDWARDS-TRINITY

BASIN AND RANGE

CALIFORNIA COASTAL BASIN

CENTRAL VALLEY

GLACIAL DEPOSITS

HIGH PLAINS

COASTAL LOWLANDS

MISSISSIPPI EMBAYMENT

NORTHERN ATLANTIC

COASTAL PLAIN

HAWAIIAN VOLCANIC

COLUMBIA PLATEAU

SNAKE RIVER PLAIN

FLORIDAN

NEW YORK AND NEW ENGLAND

PIEDMONT AND BLUE RIDGE

CAMBRIAN-ORDOVICIAN

EDWARDS-TRINITY

BASIN AND RANGE

CALIFORNIA COASTAL BASIN

CENTRAL VALLEY

GLACIAL DEPOSITS

HIGH PLAINS

COASTAL LOWLANDS

MISSISSIPPI EMBAYMENT

NORTHERN ATLANTIC

COASTAL PLAIN

HAWAIIAN VOLCANIC

COLUMBIA PLATEAU

SNAKE RIVER PLAIN

FLORIDAN

NEW YORK AND NEW ENGLAND

PIEDMONT AND BLUE RIDGE

CAMBRIAN-ORDOVICIAN

EDWARDS-TRINITY

BASIN AND RANGE

CALIFORNIA COASTAL BASIN

CENTRAL VALLEY

GLACIAL DEPOSITS

HIGH PLAINS

COASTAL LOWLANDS

MISSISSIPPI EMBAYMENT

NORTHERN ATLANTIC

COASTAL PLAIN

HAWAIIAN VOLCANIC

COLUMBIA PLATEAU

SNAKE RIVER PLAIN

FLORIDAN

NEW YORK AND NEW ENGLAND

PIEDMONT AND BLUE RIDGE

CAMBRIAN-ORDOVICIAN

EDWARDS-TRINITY

BASIN AND RANGE

CALIFORNIA COASTAL BASIN

CENTRAL VALLEY

GLACIAL DEPOSITS

HIGH PLAINS

COASTAL LOWLANDS

MISSISSIPPI EMBAYMENT

NORTHERN ATLANTIC

COASTAL PLAIN

HAWAIIAN VOLCANIC

DOMESTIC WELLS

PUBLIC-SUPPLY WELLS SHALLOW URBAN MONITORING WELLS

SHALLOW AGRICULTURAL MONITORING WELLS

CONCENTRATION OF DISSOLVED ATRAZINE, 

IN MICROGRAMS PER LITER

Basalt and volcanics

Carbonate/Crystalline

Carbonate/Sandstone Sand and Gravel Semiconsolidated sand

Basalt and volcanics

Carbonate/Crystalline

Carbonate/Sandstone Sand and Gravel Semiconsolidated sand Basalt and volcanics

Carbonate/Crystalline

Carbonate/Sandstone Sand and Gravel Semiconsolidated sand

Basalt and volcanics

Carbonate/Crystalline

Carbonate/Sandstone Sand and Gravel Semiconsolidated sand

CONCENTRATION OF DISSOLVED ATRAZINE, 

IN MICROGRAMS PER LITER

CONCENTRATION OF DISSOLVED ATRAZINE, 

IN MICROGRAMS PER LITER

CONCENTRATION OF DISSOLVED ATRAZINE, 

IN MICROGRAMS PER LITER

Figure A3–B. Atrazine concentrations from the High Plains aquifer in a national context. 
Appendix 3 61

Appendixes

n=61 n=37 n=23

n=483

n=89

n=26 n=72 ND

ND

ND ND

ND

ND

ND

ND

n=62 n=127 n=76

n=413

n=304

n=71 n=56 n=5

n=5

ND

n=99

n=113

n=182

n=70

n=67

n=97

ND n=116 ND

n=305

n=30

n=28 n=65 n=25

n=31

ND

ND

ND

n=64

ND

n=30

n=24

n=34 n=103 ND

n=78

n=76

n=63 n=74

n=40

n=23

n=12

ND n=6

n=27

n=26

SMCL = 500 mg/L

n=3

n=1

n=4

SMCL = 500 mg/L

SMCL = 500 mg/L SMCL = 500 mg/L

5,0004,0003,0002,0001,000

0

5,0004,0003,0002,0001,000

0

5,0004,0003,0002,0001,000

0

5,0004,0003,0002,0001,000

0

DOMESTIC WELLS

PUBLIC-SUPPLY WELLS SHALLOW URBAN MONITORING WELLS

SHALLOW AGRICULTURAL MONITORING WELLS

CONCENTRATION OF DISSOLVED SOLIDS, 

IN MILLIGRAMS PER LITER COLUMBIA PLATEAU

SNAKE RIVER PLAIN

FLORIDAN

NEW YORK AND NEW ENGLAND

PIEDMONT AND BLUE RIDGE

CAMBRIAN-ORDOVICIAN

EDWARDS-TRINITY

BASIN AND RANGE

CALIFORNIA COASTAL BASIN

CENTRAL VALLEY

GLACIAL DEPOSITS

HIGH PLAINS

COASTAL LOWLANDS

MISSISSIPPI EMBAYMENT

NORTHERN ATLANTIC

COASTAL PLAIN

HAWAIIAN VOLCANIC

COLUMBIA PLATEAU

SNAKE RIVER PLAIN

FLORIDAN

NEW YORK AND NEW ENGLAND

PIEDMONT AND BLUE RIDGE

CAMBRIAN-ORDOVICIAN

EDWARDS-TRINITY

BASIN AND RANGE

CALIFORNIA COASTAL BASIN

CENTRAL VALLEY

GLACIAL DEPOSITS

HIGH PLAINS

COASTAL LOWLANDS

MISSISSIPPI EMBAYMENT

NORTHERN ATLANTIC

COASTAL PLAIN

HAWAIIAN VOLCANIC

COLUMBIA PLATEAU

SNAKE RIVER PLAIN

FLORIDAN

NEW YORK AND NEW ENGLAND

PIEDMONT AND BLUE RIDGE

CAMBRIAN-ORDOVICIAN

EDWARDS-TRINITY

BASIN AND RANGE

CALIFORNIA COASTAL BASIN

CENTRAL VALLEY

GLACIAL DEPOSITS

HIGH PLAINS

COASTAL LOWLANDS

MISSISSIPPI EMBAYMENT

NORTHERN ATLANTIC

COASTAL PLAIN

HAWAIIAN VOLCANIC

COLUMBIA PLATEAU

SNAKE RIVER PLAIN

FLORIDAN

NEW YORK AND NEW ENGLAND

PIEDMONT AND BLUE RIDGE

CAMBRIAN-ORDOVICIAN

EDWARDS-TRINITY

BASIN AND RANGE

CALIFORNIA COASTAL BASIN

CENTRAL VALLEY

GLACIAL DEPOSITS

HIGH PLAINS

COASTAL LOWLANDS

MISSISSIPPI EMBAYMENT

NORTHERN ATLANTIC

COASTAL PLAIN

HAWAIIAN VOLCANIC

Basalt and volcanics

Carbonate/Crystalline

Carbonate/Sandstone Sand and Gravel Semiconsolidated sand Basalt and volcanics

Carbonate/Crystalline

Carbonate/Sandstone Sand and Gravel Semiconsolidated sand

Basalt and volcanics

Carbonate/Crystalline

Carbonate/Sandstone Sand and Gravel Semiconsolidated sand Basalt and volcanics

Carbonate/Crystalline

Carbonate/Sandstone Sand and Gravel Semiconsolidated sand

CONCENTRATION OF DISSOLVED SOLIDS, 

IN MILLIGRAMS PER LITER

CONCENTRATION OF DISSOLVED SOLIDS, 

IN MILLIGRAMS PER LITER

CONCENTRATION OF DISSOLVED SOLIDS, 

IN MILLIGRAMS PER LITER

Figure A3–C. Dissolved solids concentrations from the High Plains aquifer in a national context. 
62 Water Quality in the High Plains Aquifer, 1999–2004 n=62 n=37 n=23n=484n=89 n=26 n=73 ND ND ND ND ND ND ND ND n=74 n=119 n=76n=399n=307n=73 n=56 n=5 n=5 ND n=146n=113n=182n=70 n=67 n=97

ND n=116 ND

n=306

n=30

n=28 n=65 n=24

n=31

ND

ND

ND

n=64

ND

n=30

n=24

n=34 n=103 ND

n=77

n=94

n=63 n=74

n=40

n=23

n=20

ND n=6

n=27

n=24

0

302010

0

302010 MCL = 10 mg/L as N MCL = 10 mg/L as N

0

302010 MCL = 10 mg/L as N

0

302010 MCL = 10 mg/L as N

n=3

n=1

n=4

DOMESTIC WELLS

PUBLIC-SUPPLY WELLS SHALLOW URBAN MONITORING WELLS

SHALLOW AGRICULTURAL MONITORING WELLS

CONCENTRATION OF DISSOLVED NITRITE PLUS

NITRATE, IN MILLIGRAMS PER LITER AS NITROGEN COLUMBIA PLATEAU

SNAKE RIVER PLAIN

FLORIDAN

NEW YORK AND NEW ENGLAND

PIEDMONT AND BLUE RIDGE

CAMBRIAN-ORDOVICIAN

EDWARDS-TRINITY

BASIN AND RANGE

CALIFORNIA COASTAL BASIN

CENTRAL VALLEY

GLACIAL DEPOSITS

HIGH PLAINS

COASTAL LOWLANDS

MISSISSIPPI EMBAYMENT

NORTHERN ATLANTIC

COASTAL PLAIN

HAWAIIAN VOLCANIC

COLUMBIA PLATEAU

SNAKE RIVER PLAIN

FLORIDAN

NEW YORK AND NEW ENGLAND

PIEDMONT AND BLUE RIDGE

CAMBRIAN-ORDOVICIAN

EDWARDS-TRINITY

BASIN AND RANGE

CALIFORNIA COASTAL BASIN

CENTRAL VALLEY

GLACIAL DEPOSITS

HIGH PLAINS

COASTAL LOWLANDS

MISSISSIPPI EMBAYMENT

NORTHERN ATLANTIC

COASTAL PLAIN

HAWAIIAN VOLCANIC

COLUMBIA PLATEAU

SNAKE RIVER PLAIN

FLORIDAN

NEW YORK AND NEW ENGLAND

PIEDMONT AND BLUE RIDGE

CAMBRIAN-ORDOVICIAN

EDWARDS-TRINITY

BASIN AND RANGE

CALIFORNIA COASTAL BASIN

CENTRAL VALLEY

GLACIAL DEPOSITS

HIGH PLAINS

COASTAL LOWLANDS

MISSISSIPPI EMBAYMENT

NORTHERN ATLANTIC

COASTAL PLAIN

HAWAIIAN VOLCANIC

COLUMBIA PLATEAU

SNAKE RIVER PLAIN

FLORIDAN

NEW YORK AND NEW ENGLAND

PIEDMONT AND BLUE RIDGE

CAMBRIAN-ORDOVICIAN

EDWARDS-TRINITY

BASIN AND RANGE

CALIFORNIA COASTAL BASIN

CENTRAL VALLEY

GLACIAL DEPOSITS

HIGH PLAINS

COASTAL LOWLANDS

MISSISSIPPI EMBAYMENT

NORTHERN ATLANTIC

COASTAL PLAIN

HAWAIIAN VOLCANIC

Basalt and volcanics

Carbonate/Crystalline

Carbonate/Sandstone Sand and Gravel Semiconsolidated sand Basalt and volcanics

Carbonate/Crystalline

Carbonate/Sandstone Sand and Gravel Semiconsolidated sand

Basalt and volcanics

Carbonate/Crystalline

Carbonate/Sandstone Sand and Gravel Semiconsolidated sand Basalt and volcanics

Carbonate/Crystalline

Carbonate/Sandstone Sand and Gravel Semiconsolidated sand

CONCENTRATION OF DISSOLVED NITRITE PLUS

NITRATE, IN MILLIGRAMS PER LITER AS NITROGEN

CONCENTRATION OF DISSOLVED NITRITE PLUS

NITRATE, IN MILLIGRAMS PER LITER AS NITROGEN

CONCENTRATION OF DISSOLVED NITRITE PLUS

NITRATE, IN MILLIGRAMS PER LITER AS NITROGEN

Figure A3–B. Nitrate concentrations from the High Plains aquifer in a national context. 
Acknowledgments 63

Acknowledgments

The cooperation of landowners who agreed to the installation and long-term operation of monitoring wells on their 

property and owners of private and public-supply wells for 

allowing access is gratefully acknowledged. Coordination and 

scientific collaboration with agencies and organizations in the 

High Plains were integral to the success of this water-quality 

assessment. We thank those who served as members of our 

liaison committee: 

• Colorado Department of Agriculture (Daniel Hurlbut)

• Kansas Geological Survey (Marios Sophocleous and 

Margaret Townsend)

• Texas Bureau of Economic Geology (Bridget Scanlon 

and Robert Reedy)

• Nebraska Conservation and Survey Division 

(Jim Goeke)

• Nebraska Cooperative Hydrology Study (Clint Carney)

We thank the following individuals within the USGS 

for contributing to this study. Personnel from several USGS 

offices were instrumental in providing guidance and scientific 

and field support during the 6-year field effort: 

• Bret Bruce (USGS Regional Executive’s Office, Rocky 

Mountain Area),

• Larry (Mike) Pope and Cristi Hansen (USGS Kansas 

Water Science Center), 

• Jennifer Stanton and Virginia (Ginny) McGuire (USGS 

Nebraska Water Science Center), 

• Mark Becker and Scott Christenson (USGS Oklahoma 

Water Science Center), 

• Lynn Fahlquist (USGS Texas Water Science Center), 

• the USGS Central Region Drilling Team, and 

• Kevin Ellett (USGS California Water Science Center). 

Amber Swallow prepared the block-diagram figures 

of the High Plains aquifer. Tracy Hancock (USGS, Virginia 

Water Science Center), Suzanne Paschke (USGS, Colorado 

Water Science Center), Gary Rowe (USGS, Central Region), 

Mike Woodside (USGS, Eastern Region), Pixie Hamilton 

(USGS), and Wayne Lapham (USGS) provided constructive 

reviews of earlier versions of this report. 
Publishing support provided by: 

Denver Publishing Service Center

For more information concerning this publication, contact:

Director, USGS Colorado Water Science Center

Box 25046, Mail Stop 415

Denver, CO 80225

(303) 236-4882

Or visit the Colorado Water Science Center Web site at:

http://co.water.usgs.gov/