U.S. Department of the Interior

U.S. Geological Survey

Circular 1357

Water Quality in the Denver Basin Aquifer System,

Colorado, 2003–05

National Water-Quality Assessment Program

The Quality of Our Nation’s Waters
Cover. View of Castle Rock, Colorado, with the Rocky Mountains to the west (photograph by Steve Krull, copyright istockphoto.com).
The Quality of Our Nation’s Waters

Water Quality in the Denver Basin Aquifer System, 

Colorado, 2003–05

By Nancy J. Bauch, MaryLynn Musgrove, Barbara J. Mahler, and Suzanne S. Paschke

National Water-Quality Assessment Program

Circular 1357

U.S. Department of the Interior

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

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2014

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as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

Bauch, N.J., Musgrove, MaryLynn, Mahler, B.J., and Paschke, S.S., 2014, The quality of our Nation’s waters —

Water quality in the Denver Basin aquifer system, Colorado, 2003– 05: U.S. Geological Survey Circular 1357, 

100 p., http://dx.doi.org/10.3133/cir1357.

Library of Congress Cataloging-in-Publication Data

Bauch, Nancy J.

 The quality of our nation’s waters---water quality in the Denver Basin aquifer system, Colorado, 2003--05 /

by Nancy J. Bauch, MaryLynn Musgrove, Barbara J. Mahler, and Suzanne S. Paschke, National Water Quality 

Assessment Program.

 pages cm. -- (Circular / U.S. Geological Survey ; 1357)

 Other title: Water quality in the Denver Basin aquifer system, Colorado, 2003--05

 Includes bibliographical references.

 ISBN 978-1-4113-3755-8

 1. Water quality--Denver Basin. 2. Water quality--United States. 3. Groundwater--Quality--Denver Basin. 

4. Groundwater--Quality--United States. I. Musgrove, MaryLynn, 1964- II. Mahler, B. J. (Barbara June), 1959- 

III. Paschke, S. S. (Suzanne Smith) IV. Geological Survey (U.S.) V. Title. VI. Title: Water quality in the Denver Basin 

aquifer system, Colorado, 2003--05. VII. Series: U.S. Geological Survey circular ; 1357.

 TD223.B375 2014

 553.7’909788--dc23

 2014029709

ISSN 1067-084X (print)

ISSN 2330-5703 (online)

ISBN 978-1-4113-3755-8
iii

The National Water-Quality Assessment Program

“The USGS NAWQA Program provides the power of knowledge. 

Results of NAWQA studies are an invaluable resource for public health 

officials, water and land use planners, and the community that relies 

on Denver Basin groundwater as a vital source of drinking water.”

 Hope Dalton

Water Specialist

Tri-County Health Department

Greenwood Village

Copyright photograph by Habich Enterprises, from istockphoto.com
iv

Foreword

The United States has made major investments in assessing, managing, regulating, and conserving 

natural resources, such as water and a variety of ecosystems. Sustaining the quality of the 

Nation’s water resources and the health of our diverse ecosystems depends on the availability 

of sound water-resources data and information to develop effective, science-based policies. 

Effective management of water resources also brings more certainty and efficiency to important 

economic sectors. Taken together, these actions lead to immediate and long-term economic, 

social, and environmental benefits that make a difference to the lives of millions of people 

(http://water.usgs.gov/nawqa/applications/).

Two decades ago, Congress established the U.S. Geological Survey’s National Water-Quality 

Assessment (NAWQA) Program to meet this need. Since then NAWQA has served as a primary 

source of nationally consistent information on the quality of the Nation’s streams and groundwater, 

on ways in which water quality changes over time, and on the natural features and human activities 

affecting the quality of streams and groundwater. Objective and reliable data, systematic scientific 

studies, and models are used to characterize where, when, and why the Nation’s water quality 

is degraded—and what can be done to improve and protect the water for human and ecosystem 

needs. This information is critical to our future because the Nation faces an increasingly complex 

and growing need for clean water to support people, economic growth, and healthy ecosystems. For 

example, NAWQA findings for public-supply wells, which provide water to about 105 million people, 

showed that 22 percent of source-water samples contained at least one contaminant at levels of 

potential health concern. Similarly, 23 percent of samples from domestic (or privately owned) wells, 

which supply untreated water to an additional 43 million people, also had contaminant levels of 

potential concern.

This report is one of a collection of publications that describe water-quality conditions in selected 

Principal Aquifers of the United States (http://water.usgs.gov/nawqa/studies/praq/). The collection 

is part of the series “The Quality of Our Nation’s Waters,” which describes major findings of the 

NAWQA Program on water-quality issues of regional and national concern and which provides 

science-based information for assessing and managing the quality of our groundwater resources. 

Other reports in this series focus on occurrence and distribution of nutrients, pesticides, and volatile 

organic compounds in streams and groundwater, the effects of contaminants and streamflow 

alteration on the condition of aquatic communities in streams, and the quality of untreated water 

from private domestic and public-supply wells. Each report builds toward a more comprehensive 

understanding of the quality of regional and national water resources (http://water.usgs.gov/nawqa/

nawqa_sumr.html). All NAWQA reports are available online at http://water.usgs.gov/nawqa/bib/.

The information in this series primarily is intended for those interested or involved in resource 

management and protection, conservation, regulation, and policymaking at regional and national 

levels. In addition, the information should be of interest to those at a local level who wish to know 

more about the general quality of streams and groundwater in areas near where they live and how 

that quality compares with other areas across the Nation. We hope this 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.

Jerad Bales 

Acting Associate Director for Water

U.S. Geological Survey
http://pubs.usgs.gov/sir/2014/1360

v

California Coastal Basin aquifers

Central Valley aquifer system

Basin and Range basin-fill aquifers

Rio Grande aquifer system

Snake River Plain basin-fill and 

basaltic-rock aquifers

Columbia Plateau basin-fill and

basaltic-rock aquifers 

Mississippi embayment–Texas coastal

uplands aquifer system—Circular 1356

Mississippi River Valley alluvial aquifer

Denver Basin aquifer system—Circular 1357

Piedmont and Blue Ridge 

carbonate-rock aquifers

Piedmont and Blue Ridge 

crystalline-rock aquifers

Valley and Ridge carbonaterock aquifers

High Plains aquifer system—Circular 1337

Upper Floridan aquifer and overlying

surficial aquifers—Circular 1355

Northern Atlantic Coastal Plain 

surficial aquifer system—Circular 1353

Glacial aquifer system—Circular 1352

Hawaiian volcanic-rock aquifers

Early Mesozoic basin aquifers

Valley and Ridge siliciclasticrock aquifers

Southwest Principal Aquifers—Circular 1358 Piedmont, Blue Ridge, and Valley and 

Ridge aquifers—Circular 1354

Western Volcanics—Circular 1359

Principal Aquifers and Water-Quality Summary Reports

National summary of groundwater quality in Principal Aquifers—Circular 1360

High Plains aquifer system—Circular 1337

Glacial aquifer system—Circular 1352

Northern Atlantic Coastal Plain surficial

aquifer system—Circular 1353

Piedmont, Blue Ridge, and Valley and 

Ridge aquifers—Circular 1354

Piedmont and Blue Ridge 

carbonate-rock aquifers

Piedmont and Blue Ridge 

crystalline-rock aquifers

Valley and Ridge siliciclasticrock aquifers

Valley and Ridge carbonaterock aquifers

Early Mesozoic basin aquifers 

Upper Floridan aquifer and overlying

surficial aquifers—Circular 1355

Mississippi embayment–Texas coastal 

uplands aquifer system—Circular 1356

Mississippi River Valley alluvial aquifer

Denver Basin aquifer system—Circular 1357

Southwest Principal Aquifers—Circular 1358

California Coastal Basin aquifers

Central Valley aquifer system

Basin and Range basin-fill aquifers

Rio Grande aquifer system 

Western Volcanics—Circular 1359

Hawaiian volcanic-rock aquifers

Snake River Plain basin-fill and 

basaltic-rock aquifers

Columbia Plateau basin-fill and 

basaltic-rock aquifers
vi

Contents

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

Chapter 2: NAWQA Approach to Assessing Groundwater Quality......................................7

Chapter 3: Environmental Setting and Water-Resource Characteristics ........................15

Chapter 4: Natural and Human Factors that Affect Groundwater Flow and Quality.......27

Chapter 5: Quality of Groundwater Used for Drinking and Irrigation.................................37

Chapter 6: Understanding Where and Why Key Contaminants Occur 

 in Groundwater........................................................................................................45

Chapter 7: Vulnerability of Groundwater to Contamination.................................................71

References Cited .........................................................................................................................77

Glossary ...............................................................................................................................83

Appendixes ...............................................................................................................................87
vii

Introduction to This Report

This report contains the major findings of a regional assessment of water quality in the Denver 

Basin aquifer system. It is one of a series of reports by the U.S. Geological Survey (USGS) National 

Water-Quality Assessment (NAWQA) Program that present major findings for Principal Aquifers, 

other aquifers, and major river basins across the Nation. In these reports, water quality is discussed 

in terms of local, State, regional, and national issues. Water-quality conditions in the Denver Basin 

aquifer system are compared to conditions found elsewhere and to selected national water-quality 

benchmarks, such as those for drinking water. 

This report is intended for individuals working with water-resource issues in local, State, or Federal 

agencies, universities, public interest groups, or the private sector. The information will be useful in 

addressing current issues, such as drinking-water quality, source-water protection, and monitoring 

and sampling strategies. This report also will be useful for individuals who wish to know more about 

the quality of groundwater in areas near where they live and how that quality of water compares to 

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

Water-quality conditions in the Denver Basin aquifer system summarized in this report are discussed 

in greater detail in other reports listed in the references. 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 Principal Aquifers can be 

accessed from the national NAWQA Web site (http://water.usgs.gov/nawqa/).

Companion studies of the groundwater quality in the Denver Basin aquifer system are discussed in 

the following reports:

Quality of Groundwater in the Denver Basin Aquifer System, Colorado, 2003–5

U.S. Geological Survey Scientific Investigations Report 2014–5051

By MaryLynn Musgrove, Jennifer A. Beck, Suzanne S. Paschke, Nancy J. Bauch, 

and Shana L. Mashburn

(Also available at http://pubs.usgs.gov/sir/2014/5051/)

Occurrence of Selected Organic Compounds in Groundwater Used for 

Public Supply in the Plio-Pleistocene Deposits Near East-Central Nebraska 

and the Dawson and Denver Aquifers Near Denver, Colorado, 2002–2004

U.S. Geological Survey Scientific Investigations Report 2008–5243 

By Jeffrey B. Bails, Benjamin J. Dietsch, Matthew K. Landon, and Suzanne S. Paschke

(Also available at http://pubs.usgs.gov/sir/2008/5243/)
An April dusting of snow covers the south metropolitan area of Denver and the foothills of the Front Range of the Rocky Mountains at sunrise.

Photograph by Lynn Bashaw, used with permission
Denver

COLORADO

Denver

Basin

Chapter 1

The conversion of undeveloped or rural land to agricultural and developed land in areas along the Front 

Range of the Rocky Mountains in Colorado has affected water quality in the Denver Basin aquifer system.

Chapter 1: Overview of Major Findings 

and Implications

Groundwater from the Denver Basin aquifer system is a critical water 

resource for the semiarid Front Range urban corridor of Colorado. 

Population in the Denver metropolitan area has more than doubled in 

the last 40 years as a result of Colorado’s reputation for a high quality of life and 

favorable business climate.(1) Much of this growth has taken place in areas that 

rely primarily or solely on Denver Basin groundwater for supply. Pumping of 

groundwater from the Denver Basin bedrock aquifers has more than quadrupled 

since 1970,(2) and Denver Basin groundwater provides about 70 percent of the 

water supply in the south Denver metropolitan area.(3) Population in the Denver 

Basin study area is projected to continue to grow, particularly in areas that 

rely on groundwater for drinking water and other municipal, industrial, and 

domestic uses, further straining surface-water and groundwater resources.

 Development of agricultural and urban areas has adversely affected 

groundwater quality in the alluvial aquifer and shallow bedrock aquifer system, 

but the quality of water produced by private (domestic) and public-supply wells 

completed in deeper parts of the Denver Basin aquifer system remains suitable 

for drinking. However, the presence of contaminants from human sources and 

other evidence of recently recharged water from the shallow system in some parts 

of the deeper aquifer system indicate that deeper groundwater may be vulnerable 

to contamination from the land surface as a result of high-volume pumping.

 Water managers in the Denver Basin area are concerned primarily with 

the quantity of groundwater available for human use because high-volume 

pumping of deep parts of bedrock aquifers has caused water-level declines 

and removal of water from storage in some parts of the system. The concern 

is that the availability of groundwater is insufficient for meeting long-term 

demands. However, there also are concerns about the quality of Denver Basin 

groundwater, which could limit water availability in the future.
Denver

Denver aquifer

Dawson aquifer

Laramie–Fox Hills aquifer

Arapahoe aquifer

Confining units

Public-supply

wells

Monitoring wells,

urban setting Domestic

wells

Direction of

 groundwater 

 flow

EXPLANATION

NOT TO SCALE

West

TX

OK

KS

NE

SD

MT ND

WY

CO

AZ NM

UT

The quality of groundwater used for drinking 

water generally is very good

Most constituents at a concentration of concern 

for human health were from geologic sources

Only 1 in 10 

drinking-water wells had a 

contaminant detected at a 

concentration of potential 

human-health concern. 

Exceedances of humanhealth benchmarks in 

samples from wells 

used for drinking water 

were measured for 

just six constituents. 

Only benchmarks for 

manganese and radon 

were exceeded in 

more than one sample. 

Management practices that limit the migration of poor-quality 

shallow groundwater into deeper parts of the aquifer system 

where groundwater is used for drinking will help maintain the 

high quality of the drinking-water resource.

Overview of Major Findings and Implications for the Denver 

Availability and sustainability of groundwater in the Denver Basin aquifer system 

depend on water quantity and water quality. The Denver Basin aquifer system underlies 

about 7,000 square miles of the Great Plains in eastern Colorado and is the primary 

or sole source of water for domestic and public supply in many areas of the basin. Use 

of groundwater from the Denver Basin sandstone aquifers has been instrumental for 

development of the south Denver metropolitan area and other areas, but has resulted 

in a decline in water levels in some parts of the system. Human activities in many areas 

have adversely affected the quality of water in the aquifer system, especially the shallow 

parts. Groundwater in deeper parts of the system used for drinking water, once considered 

isolated from the effects of overlying land use, is increasingly vulnerable to contamination 

from human activities and geologic materials. Availability and sustainability of highquality groundwater for supply is vital for private well owners, water managers, and 

other stakeholders and is important for the economic health of the Denver Basin area.

Five of the six contaminants with potential human-health 

concerns were from geologic sources. Granitic sediments in 

the Dawson Arkose, for example, shown at right, are a source 

of radon in the Dawson aquifer. Hydrologic and geochemical 

factors, such as groundwater age and dissolved oxygen content, 

and human activities affect the distribution of constituents of 

concern in Denver Basin groundwater.

1

2

Exceedances of humanhealth benchmarks

Percentage of drinking-water wells

with a concentration greater than

a human-health benchmark

Manganese

Radon

Uranium

Nitrate

Arsenic

Selenium

0 10 20

All

sources

Geologic

sources

Manmade

sources

30 40 50
Monitoring wells, 

agricultural setting

Alluvial

aquifer

Pierre Shale

Water

table

East

South Platte River

Irrigation of agricultural and urban lands has adversely affected the quality of shallow groundwater

Basin Aquifer System

The interaction of infiltrating irrigation water with 

minerals in the subsurface increases the amount of 

dissolved constituents in water that recharges the shallow 

aquifer system. Concentrations of dissolved solids and 

dissolved constituents, such as nitrate, were greater in wells 

tapping shallow groundwater than those tapping deeper 

groundwater used for drinking. Shallow groundwater 

typically is not used as a drinking-water resource, but 

high concentrations of dissolved solids, nitrate, and other 

contaminants resulting from human activities on the land 

surface could limit even nonpotable uses of the resource.

Pumping of groundwater from the Denver Basin 

alluvial and sandstone bedrock aquifers has steadily 

increased since the 1930s and 1950s, mirroring increased 

production of irrigated crops in agricultural parts of the 

basin, residential growth in rural areas, and large increases 

in population in urban areas. One effect of higher pumping 

rates, as indicated by groundwater-flow models, chemistry, 

and groundwater-age tracers, is the potential for shallow 

groundwater of poor quality to migrate downward into 

deeper parts of bedrock aquifers that are tapped by domestic 

and public-supply wells, such as the one shown at the left.

High-volume pumping of groundwater can 

increase the vulnerability of the Denver 

Basin bedrock aquifers to contamination

3

4

1910

1930

1950

1970

1990

2010

Year

Estimated pumping,

in million gallons per day

0

20

40

60

80

100

120

Bedrock

aquifers

Alluvial

aquifer

Estimated pumping from Denver Basin aquifers

EXPLANATION

Shallow aquifer system

Deeper aquifer system used

 for drinking water

Well depth, in feet below land surface

Concentration, in milligrams per liter

0 2,000 4,000 6,000

2,500

2,000

1,500

1,000

500

0

Concentration of dissolved solids by well depth

Secondary drinkingwater standard 

(500 mg/L)
1 The quality of groundwater used for drinking water generally

is very good.

Contaminants were present at a concentration of potential concern for human health 

in water from only 10 percent of drinking-water wells sampled. Only two constituents—

manganese and radon—were measured at a concentration that exceeded a human-health 

benchmark in more than one sample. These constituents, both from geologic sources, each 

exceeded their benchmark in less than 5 percent of samples from drinking-water wells. 

Human-health benchmarks for arsenic, nitrate, selenium, and uranium were exceeded in 

only one sample each. Manmade contaminants, such as pesticides and volatile organic 

compounds (VOCs), were detected in some samples from drinking-water wells but at low 

concentrations (near laboratory reporting levels).

The quality of the resource could be affected in the future by the downward movement 

of poor-quality shallow groundwater into the deeper parts of the aquifer system where 

groundwater is used for drinking. Management practices that limit this downward migration 

will help maintain drinking-water quality.

2 Most constituents at a concentration of concern for human 

health were from geologic sources.

Rocks and minerals in Denver Basin soils and aquifer sediments are natural geologic 

sources of arsenic, manganese, radon, selenium, and uranium—five of the six constituents 

detected at a concentration that exceeded a human-health benchmark in at least one sample 

from a drinking-water well. Other constituents from geologic sources, such as dissolved 

solids, fluoride, iron, manganese and sulfate, can affect the use of drinking water because 

they impair the taste or odor of the water or cause other non-health based concerns. 

Concentrations of these five constituents exceeded a non-regulatory and non-human health 

benchmark (called a Secondary Maximum Contaminant Level (SMCL) in almost half 

(42 percent) of the samples from drinking-water wells.

The sources and depositional environments of soils and aquifer sediments, groundwaterflow patterns, and groundwater age contribute to variations in the chemical composition of 

the Denver Basin groundwater. Chemical processes that typically occur in the Denver Basin 

aquifer system, such as precipitation/dissolution reactions, evaporative concentration, and 

oxidation/reduction, can increase concentrations of dissolved constituents in groundwater. 

Understanding how hydrologic and geochemical conditions affect groundwater quality 

and the effect that human activities have on groundwater quality can help identify which 

contaminants might affect the quality of current or future water supplies.

4 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05

Photographs from top to bottom: Steve Krull, copyright 

istockphoto.com; USGS; copyright istockphoto.com; USGS

Overview of Major Findings 
3 Irrigation of agricultural and urban lands has adversely affected 

the quality of shallow groundwater.

Irrigation in agricultural and urban areas of the semiarid Denver Basin has increased 

groundwater recharge. As irrigation water infiltrates the subsurface, the water reacts with 

minerals in soils and underlying aquifer sediments, increasing the amount of dissolved 

solids in water that recharges the shallow aquifer system. As a result, about two-thirds of the 

samples of shallow groundwater contained dissolved solids at a concentration that exceeded 

the SMCL. The interaction of dissolved oxygen in recharge water with uranium-rich 

sediments has released uranium to shallow groundwater at levels that exceeded the humanhealth benchmark in about 27 percent of the samples from shallow groundwater. Fertilizers 

and pesticides that are applied to the land surface and VOCs that are generated from human 

activities also are transported by irrigation recharge water to shallow groundwater. 

Although the shallow aquifer system typically is not used as a source of drinking water, 

high concentrations of dissolved solids and dissolved constituents in shallow groundwater 

could limit even nonpotable uses of the water, such as for irrigation.

4 High-volume pumping of groundwater can increase

the vulnerability of the Denver Basin bedrock aquifers 

to contamination.

Pumping of water from the alluvial and bedrock aquifers has steadily increased since 

the 1930s and 1950s to meet increasing supply needs for agricultural irrigation, domestic 

purposes in rural areas, and recently developed urban areas in the basin. About 30 times 

more water was pumped from the alluvial aquifer in 2003 than in 1930, and about 25 times 

more water was pumped from bedrock aquifers in 2003 than in 1950. Denver Basin groundwater-flow models indicate that pumping has caused the flow of shallow ground-water from 

the alluvial aquifer down to the bedrock aquifers to increase by 50 percent from predevelopment conditions. This downward migration of groundwater can cause constituents in the 

shallow aquifer system to migrate into the deeper system used for drinking water. There is 

some evidence that this migration already is occurring—dissolved oxygen, nitrate, tritium, 

and other constituents associated with shallow oxic groundwater or human activities at the 

land surface were found in deeper parts of the aquifer system, and young groundwater has 

mixed with older groundwater at intermediate aquifer depths. Continued development of 

land and water resources likely will result in additional movement of constituents in shallow 

groundwater to deeper parts of the Denver Basin bedrock aquifers, including uranium, 

selenium, and other constituents that are stable in oxic groundwater. 

Treating groundwater to decrease contaminant concentrations is costly, so management 

practices that prevent migration of poor-quality shallow groundwater are likely to be a more 

effective way to maintain the current high quality of drinking water in the Denver Basin. 

Understanding the effects of continued development of the Denver Basin area on groundwater resources provides insight on how human activities might affect groundwater quality 

and availability in other less developed basins in similar hydrogeologic settings.

Chapter 1

Overview of Major Findings and Implications 5

Photographs from top to bottom: Jon Mullen, copyright

istockphoto.com; Scott Bauer, USDA ARS; USGS; USGS

Water-Quality Issues for the Denver Basin Aquifer System
Sampling of groundwater included the use of a mobile laboratory and the 

collection of water samples in bagged chambers to protect against airborne 

contaminants. Samples to be analyzed for dissolved constituents were 

filtered in the field before collection in sample bottles.

In the south Denver metropolitan area, about 70 percent 

of the water supply is from wells that tap Denver Basin 

bedrock aquifers, such as the public-supply well shown 

here. Collecting water samples from public-supply wells 

is important for assessing the vulnerability of groundwater 

used for drinking to contamination from geologic sources 

and human activities on the land surface. 

6 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05
Chapter 2

Installation of shallow water-table 

monitoring wells and sampling of those 

wells were part of the systematic 

evaluation of groundwater quality in 

the Denver Basin.

Chapter 2: NAWQA Approach to Assessing 

Groundwater Quality

Groundwater from the sandstone bedrock aquifers of the Denver Basin 

is the primary or sole source of water for domestic use in rural areas 

and public supply in the south Denver metropolitan area and other 

locations with growing population. Availability and sustainability of high-quality 

groundwater for supply are vital for private well owners, water managers, and 

other stakeholders and for the overall economic health of the Denver Basin area. 

The studies conducted as part of the USGS NAWQA Program were designed to 

answer broad questions about the occurrence, fate, and transport of contaminants 

in these bedrock aquifers used as sources of drinking water and the effects of 

human activities in agricultural and urban areas on groundwater quality. 

This chapter summarizes 

the study design used 

to investigate the 

quality of Denver Basin 

groundwater.
Understanding study results

Important aspects of the NAWQA Principal Aquifer assessments:

• Water samples were collected at the wellhead (see sidebar, Anatomy of a well, p. 12) prior to any treatment. 

They represent the quality of the groundwater resource but not necessarily the quality of tap water. 

• The focus of the assessments is the condition of the total resource, including groundwater in a wide range 

of hydrologic and land-use settings across the Nation, rather than conditions at specific sites with known 

water-quality concerns.

• The assessments are guided by a nationally consistent study design, and all assessments use the same 

methods of sampling and analysis. Findings apply to water quality of a particular aquifer but also contribute 

to the larger picture of how and why water quality varies regionally and nationally. This consistent approach 

helps to determine if a water-quality issue is isolated or widespread. (See http://water.usgs.gov/nawqa/

about.html for more information.)

• The assessments focus on aquifers used for water supply or on shallow groundwater that underlies an 

area with a particular type of land use. Because the NAWQA groundwater study areas do not cover the full 

spatial extent of the targeted Principal Aquifer, the findings might not represent the effects of the full range 

of geology, climate, and land use present.

• Analytical methods used by USGS chemists for assessments of water quality in Principal Aquifers are 

designed to measure constituents at as low a concentration as possible. As a result, constituents frequently 

are detected at concentrations far below human-health benchmarks for drinking water (see sidebar, 

Human-health benchmarks and other guidelines used in this assessment, p. 40). Low-level detections 

allow scientists to identify and evaluate emerging issues and to track contaminant levels over time.

Assessing Water Quality in the Denver Basin 

Aquifer System 

How does one go about characterizing the quality 

of groundwater over an area as large as that covered by 

the Denver Basin, let alone the whole United States? The 

approach taken by the USGS is to use different types of 

groundwater studies to gain a better understanding of how and 

why water quality varies (see sidebar, Understanding study 

results, below). These groundwater studies are the building 

blocks of NAWQA’s water-quality assessment of Principal 

Aquifers. Many studies have been conducted in each Principal 

Aquifer, each with a different focus on information needs 

about groundwater quality and the natural and human-related 

factors that affect the aquifer. Groundwater studies designed 

to broadly assess water-quality conditions in parts of aquifers 

used for drinking-water supply focused on sampling networks 

of domestic (private) and public-supply wells (see sidebar, 

What types of wells were sampled?, p. 9). Agricultural and 

urban land-use studies designed to characterize and explain the 

quality of young groundwater recently recharged (in the last 

50 years or so) in these land-use settings focused on sampling 

shallow monitoring (nonpumping) wells that were installed as 

part of the NAWQA Program. Other studies focused on public 

water supplies or groundwater quality along individual flow 

paths (appendix 1).(4–7 ) The design of the Principal Aquifer 

assessments enables comparisons of groundwater quality to 

be made at regional and national scales for a wide suite of 

contaminants in different aquifer and environmental settings.

This Principal Aquifer assessment brings together and 

interprets results from all of the NAWQA groundwater studies 

in the Denver Basin conducted during 2003– 05 (appendix 1). 

Throughout this assessment, the results are grouped as 

those that characterize the quality of deep groundwater 

used as a drinking-water supply and those that characterize 

the quality of shallow groundwater that was recharged in 

either an agricultural or an urban land-use setting (fig. 2–1; 

table 2–1). Groundwater in deep parts of the aquifer system 

was recharged many years ago, in some cases long before 

any manmade chemicals were used on the land surface. The 

results of the land-use studies allow us to evaluate the effect 

of recent human activities on groundwater quality and can 

provide an early warning for issues that might adversely affect 

drinking-water resources in the future. Land-use studies also 

can provide an indication of degradation or improvement in 

water quality related to changes in chemical use.

8 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05
What types of wells were sampled?

Two types of wells used for drinking water were sampled 

in the Denver Basin: domestic (private) wells and public-supply 

wells. Domestic wells typically are shallower than public-supply 

wells and, therefore, pump water that is nearer to sources of 

manmade contaminants, such as fertilizers and pesticides, applied 

at the land surface. Domestic wells commonly are located in 

rural areas, so they are more likely than public-supply wells to be 

vulnerable to contamination from agricultural chemicals. Publicsupply wells pump water from deeper in aquifers, have high 

pumping rates, and commonly are located in urban and suburban 

areas and so are more likely than domestic wells to be vulnerable to contamination from chemicals associated with urban 

activities. Public-supply wells have larger well diameters and 

longer screened intervals than domestic wells and are pumped 

for longer periods of time. As a result, public-supply wells pump 

much larger volumes of water than domestic wells, and so have 

much larger capture zones and thus are more vulnerable than 

domestic wells to manmade contamination from distant sources. 

If the amount of water withdrawn is large enough, it can change 

the flow direction and velocity of groundwater, which in turn 

can affect the groundwater geochemistry and the constituents 

contained in the water. Routine testing of water from domestic 

wells is not required, and homeowners are responsible for testing, 

maintenance, or treatment of the water from their private well. 

Water from public-supply wells is required to be tested by the well 

operator on a routine basis to help assure that the water provided 

to consumers meets Federal and State water-quality standards. 

Some groundwater samples were collected from monitoring 

wells. Monitoring wells are not pumped regularly—they are used 

for measuring water levels or occasionally for collecting water 

samples but are not used for drinking water, irrigation, or other 

purposes. Monitoring wells sampled in the NAWQA groundwater 

studies were installed by the USGS to monitor the quality of 

recently recharged water and were purged several borehole 

volumes prior to collection of water-quality samples. 

Top, wells, such as this domestic well that taps the Arapahoe 

aquifer, were sampled for water quality and measurements 

related to water quality, such as pH and dissolved-oxygen 

concentration. Bottom, a hydrologist carefully measures 

the water level in a monitoring well in an agricultural area. 

Information on groundwater sampling methods used by the 

USGS can be found in the USGS National Field Manual.(51) 

Groundwater beneath agricultural and urban areas was the focus of land-use studies in NAWQA Principal Aquifer assessments 

because of the potential contaminant sources associated with activities in these areas. For the Denver Basin assessment, samples 

were collected from shallow monitoring wells underlying non-irrigated wheat fields (left) and suburban residential areas developed 

primarily after about 1970 in the south metropolitan area of Denver (right) (fig. 2–1).

Chapter 2

Photograph by Scott Bauer, USDA ARS

Chapter 2: NAWQA Approach to Assessing Groundwater Quality 9
Table 2–1. The objectives of the different studies determined the types of wells sampled in the Denver Basin. Some studies 

focused on sampling wells screened at deeper depths that represent the part of the aquifers used for drinking water, whereas 

others (termed land-use studies) focused on sampling water from monitoring wells screened just below the water table to assess 

the quality of shallow groundwater underlying agricultural and urban areas.

Resource targeted

Number of wells Well depth range,

in feet below

land surface

Domestic Aquifer

supply

Public

supply Monitoring

Drinking water 75 15 0 130 –2,149 Dawson, Denver, Arapahoe, 

Laramie–Fox Hills

Shallow groundwater in 

agricultural areas

0 0 31 18 –113 Alluvial, Denver

Shallow groundwater in 

urban areas

0 0 29 18 –82 Alluvial, Dawson

Total 75 15 60 8–2,149 Alluvial, Dawson, Denver, 

Arapahoe, Laramie–Fox Hills

Figure 2–1. The wells sampled by the NAWQA Program for the study of groundwater quality in 

the Denver Basin were deep drinking-water wells used for domestic and public supply in rural 

and urban settings or shallow monitoring wells near the water table underlying agricultural and 

urban areas.

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

DOUGLAS

DENVER

Limon

Parker

Greeley

Monument

Brighton

Franktown Castle Rock

Highlands Ranch

Colorado

Springs

Denver Aurora

Littleton

Henderson

Fort Morgan

Denver

COLORADO

Denver

Basin

Base from U.S. Geological Survey

digital data, 2012, 1:100,000 

105° 104°

40°

39°

0 10 20 MILES

0 10 20 KILOMETERS

76

25

25

70

70

BROOMFIELD

Domestic

Public supply

EXPLANATION

Figure 2-1.

Alluvial aquifer

Dawson aquifer

Denver aquifer

Arapahoe aquifer

Laramie–Fox Hills aquifer

Drinking-water well

Agriculture

Urban

Land-use monitoring well

Denver Basin aquifer system

10 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05
Denver

Monitoring wells, 

agricultural setting

Denver aquifer

Dawson aquifer

Laramie–Fox Hills aquifer

Arapahoe aquifer

Confining units

Alluvial

aquifer

Pierre Shale

Public-supply

wells

Monitoring wells,

urban setting Domestic

wells

Water

table

Direction of

 groundwater 

 flow

EXPLANATION

NOT TO SCALE

West

East

South Platte River

Groundwater-Quality Assessment Design 

To assess the water-quality conditions of the groundwater 

used as a drinking-water resource in the Denver Basin, one 

sample was analyzed from each of 90 randomly selected 

existing domestic and public-supply wells in the Dawson, 

Denver, Arapahoe, and Laramie–Fox Hills bedrock aquifers 

(fig. 2–1; table 2–1). The samples were analyzed for major 

and trace inorganic constituents, nutrients, volatile organic 

compounds (VOCs), and pesticides (see sidebar, NAWQA 

assessments use a wide range of geochemical data and site 

information, p. 13; appendix 2). The domestic and publicsupply wells (drinking-water wells) are distributed across 

much of the area overlying the aquifers, but because the 

wells sampled are located only in NAWQA groundwater 

study areas, only some parts of the Denver Basin are 

represented. About 83 percent of the wells sampled were 

domestic drinking-water wells and about 17 percent were 

public-supply wells. Because of the asymmetrical bowl 

shape of the bedrock aquifers (fig. 2–2), wells in the bedrock 

aquifers sampled as part of the NAWQA study range from 

very shallow (tens of feet) to very deep (thousands of feet). 

To assess the quality of shallow groundwater underlying 

agricultural and urban areas, one groundwater sample was 

collected from each of 60 monitoring wells installed as part of 

the NAWQA Program (fig. 2–1; table 2–1). The agricultural 

setting for the Denver Basin assessment was non-irrigated wheat

fields. In this setting, no supplemental irrigation water is used, 

but fertilizers and pesticides are applied as needed. The urban 

land-use setting where samples were collected was similar 

to that where many Americans work and live—single and 

multifamily residential and commercial developments built in 

suburban areas of large cities where agricultural or undeveloped 

land is being converted to urban land uses. The setting for the 

urban study in the Denver Basin was the south metropolitan area 

of Denver. Since the early 1970s, residential and commercial 

developments in this area have been built on previously 

undeveloped rangeland. The monitoring wells in the two landuse settings were relatively shallow—from 18 to 113 feet (ft) 

deep — and tap groundwater just below the water table in the 

alluvial, Dawson, and Denver aquifers. This water typically is 

not used as a source of drinking-water supply but is expected to 

move into deeper parts of the aquifer system. The water quality 

of these samples is an indication of how the chemicals we use

in day-to-day life—fertilizers, pesticides, solvents, gasoline—

might affect the quality of a future drinking-water resource.

Figure 2–2. The quality of groundwater in the Denver Basin was assessed as it moves from the land surface to the water 

table and then to deeper parts of the aquifer system that are commonly used to supply water for drinking. Because the 

Denver Basin has an asymmetrical bowl shape, some aquifers have both shallow and deep parts. The alluvial aquifer and 

shallow parts of the bedrock (Dawson and Denver) aquifers less than 125 feet below land surface are referred to as the 

shallow aquifer system. The deeper parts (130–2,149 feet below land surface) of the bedrock aquifers used for drinkingwater supply are referred to as the deep aquifer system.

Chapter 2

Chapter 2: NAWQA Approach to Assessing Groundwater Quality 11
Anatomy of a well

A well is simply a hole in the ground (well bore or borehole) from 

which water can be removed. The well bore is lined with a well 

casing, such as a pipe, to prevent the well bore from collapsing. 

The casing, along with a sealant (called grout), also prevents water 

from flowing into the well from the land surface or from parts of 

the aquifer where the water quality may be less desirable. The 

casing can be open at the bottom or perforated at a specific depth 

with a screen, to allow water to flow into the well where it can be 

pumped to the surface. Coarse sand or gravel (called sand pack or 

gravel pack) can be placed around the well screen to help improve 

the flow of water into the well. Some wells are cased only near the 

land surface, allowing water to flow into the well from nearly the 

entire length of the well bore.

When pumped, groundwater flows through the 

screened interval and 

up and out of the well

Unsaturated

Saturated

Water table

Screen

Well bore

Water level

in well

Wellhead

Well casing

Pump

Grout

Gravel pack

Unconfined aquifer

Wells

An underground layer of saturated permeable materials (rock, gravel, sand, or silt) that will yield 

a useful quantity of water to a well.

Sedimentary aquifer

Groundwater storage and 

flow between grains of 

sediment

Bedrock aquifer

Groundwater storage

and flow in fractures

An unconfined aquifer is bounded at its top by the 

water table, below which water fills all the pore spaces 

in the rock. Water from the land surface can move 

down into an unconfined aquifer. 

A confining layer is a layer of material (often clay)

through which water does not easily flow, creating a 

boundary between aquifers.

A confined aquifer is bounded at its top by a confining 

layer. Water enters or “recharges” confined aquifers 

where the confining layer is not present. Where the

confining layer is not continuous or is breached (for

example, by a well), flow between the unconfined and

confined aquifer can occur. 

The pressure within a confined aquifer can be greater

than that in the overlying unconfined aquifer if the

source of the water in the confined aquifer is at a higher

elevation than the unconfined aquifer. In that case, 

water in a well in a confined aquifer will rise to a higher

level than that in the overlying unconfined aquifer. 

The unsaturated zone is the area below the land 

surface and above an aquifer. In addition to soil, 

rocks, and air, the unsaturated zone contains water 

from the land surface (such as rain) that is slowly 

moving downward to the water table of the aquifer. 

Aquifer (aq.ui.fer)—  ̆

Confining layer

Confined aquifer

Unconfined aquifer

Unsaturated zone

Water table

Surface water

Screen

12 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05
Chemists at the USGS National Water Quality Laboratory analyze 

groundwater samples using an array of sophisticated techniques.

Additional information (often called “ancillary information”) about 

the well and surrounding environment complements the chemical 

data measured. This additional information often is key to making 

sense of the chemical data. For example, the information might be 

used to determine that shallow groundwater is more (or less) vulnerable to contamination than deep groundwater, that domestic wells 

are more (or less) vulnerable to contamination than public-supply 

wells, or that urban land use is associated with different types of 

groundwater contamination than is agricultural land use. Chemical 

data without accompanying ancillary data are much less useful for 

understanding factors that affect groundwater quality. 

Constituents measured in samples from most wells

Constituent group Examples

Water-quality properties pH, specific conductance, dissolved 

oxygen, temperature

Major ions (filtered) Bromide, calcium, chloride, magnesium, sodium, sulfate

Trace elements (filtered) Arsenic, boron, iron, manganese, 

selenium, uranium

Nutrients (filtered) Ammonia, nitrate, phosphorus

Pesticides (filtered) Herbicides, insecticides, fungicides

Volatile organic 

compounds

Solvents, gasoline hydrocarbons, 

refrigerants, trihalomethanes, 

fumigants

Organic carbon (filtered)

Additional constituents measured in samples from some wells

Constituent group Examples

Radionuclides Radon

Groundwater age tracers Tritium, chlorofluorocarbons

Stable isotopes Oxygen-18, hydrogen-2

Microorganisms Escherichia coli and total coliforms

Additional site information

Use of well

Well depth

Depth to water

Well-construction data

Principal Aquifer

Land-surface elevation at well

Land use within a 500-meter 

(1,640-foot) radius buffer

Estimates of nutrient inputs

Estimates of pesticide use

NAWQA assessments use a wide range of geochemical data and site information

Chapter 2

Chapter 2: NAWQA Approach to Assessing Groundwater Quality 13
Colorado

Springs

Denver 

metropolitan 

area

WELD

EL PASO

ELBERT

LINCOLN

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

BROOMFIELD

DENVER

DOUGLAS

LARIMER

COLORADO

Denver

Basin

Base from Environmental Systems Research Institute

Landsat imagery 1990–2010, 30-meter resolution

0 10 20 MILES

0 10 20 KILOMETERS

Figure 2-3. Unnumbered in layout.

EXPLANATION

Landsat color descriptions

Bright orange to red: Vigorous vegetation

Darker oranges and browns: Less vigorous vegetation and 

 natural vegetation

Greens and browns: Cured out (brown) grass or rangeland

Dark purple and grey to white: Urban

Black: Water

N

Most of the land that overlies the Denver Basin aquifer system is grassland and rangeland or irrigated 

and non-irrigated agricultural land. Developed areas, primarily the Denver metropolitan area and the 

Colorado Springs area, are concentrated on the western side of the basin near the mountain foothills.

14 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05
Chapter 3

The Denver Basin forms the western boundary of the Great Plains 

Physiographic Province where it abuts the Front Range of the 

Rocky Mountains. Small towns, rural residences, and farms are 

spread across less-populated parts of the Denver Basin. 

Photograph by Ann Cantelow, dreamstime.com

Chapter 3: Environmental Setting and 

Water-Resource Characteristics

This chapter summarizes

background information

for the Denver Basin 

aquifer system, thus 

providing the context for 

understanding findings 

about water quality in 

this Principal Aquifer. 

This chapter covers the 

physical setting, climate, 

hydrogeologic setting, 

land use, population, and 

water use.

The Denver Basin is a land of contrasts, with its semiarid rangelands 

and green expanses of irrigated agricultural areas, lawns, and parks, 

its fast-growing cities and suburbs, dense urban core of Denver, and 

rural residences, and its use of groundwater and surface water for supply. All 

these features—climate, land use, population growth, and water use—affect 

the hydrogeologic system and groundwater quality. Understanding these key 

background features is essential to assessing the vulnerability of the Denver 

Basin aquifer system to contamination from human and geologic sources.
Physical Setting and Climate

The Denver Basin extends eastward from the Rocky 

Mountains, underlying about 7,000 square miles of the Great 

Plains in eastern Colorado (fig. 3–1). The basin extends from 

Greeley in the north to Colorado Springs in the south and from 

the Front Range east to Limon. The Palmer Divide forms the 

topographic high in the basin and is the drainage divide between 

the tributaries of the South Platte River Basin to the north and 

those of the Arkansas River to the south. Ponderosa pine forests 

cover the highest altitudes, shrubs and mixed prairie grasslands 

grow from mid-altitude to low-altitude areas of the plains, and 

cottonwood trees and willows line low-lying riparian areas.

The climate of the Denver Basin is semiarid. The 

long-term average annual precipitation of the Denver Basin 

is only about 15 inches per year (in/yr). Weather conditions 

vary dramatically from one season to the next. Summer high 

temperatures range from the 80s to the low 100s (degrees 

Fahrenheit, °F), and summer thunderstorms that develop 

over the mountains and Palmer Divide can produce several 

inches of rain or hail in a few hours or less. During the 

summer, evapotranspiration rates are high, so only a small 

fraction of precipitation infiltrates to groundwater. Winter low 

temperatures can range from 0 to –30 °F or lower, and winter 

storms can deposit several feet of snow in eastern Colorado. 

Long-term precipitation records indicate that average annual 

precipitation increased over the period 1978 –2000, after 

which a regional drought began. Drought conditions persisted 

through 2004; precipitation was near average during 2005 –11, 

after which drought conditions again set in.

Figure 3–1. The highest point in the Denver Basin is about 8,000 feet and is located along 

the Palmer Divide, from which streams flow to the north and south. The topographic low of 

approximately 4,300 feet lies along the South Platte River on the northeastern edge of the basin. 

Topography has a strong effect on climate in the Denver Basin—precipitation falls primarily in 

the mountains to the west and at higher altitudes along the Palmer Divide.

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

BROOMFIELD

DOUGLAS

DENVER

Middle Bijou Creek

Bijou Creek

Cherry Creek

Big S andy Creek

Sand Creek West Bijou Creek

Black Squirrel Creek

Kiowa Creek

Kiowa Creek

East Bijou Creek

Lost Creek

South Platte River

Toll Gate

Cr

Limon

Parker

Greeley

Monument

Brighton

Franktown Castle

Rock

Highlands

Ranch

Colorado

Springs

Denver Aurora

Littleton

Henderson

Fort Morgan

Castlewood

Canyon

State Park

Front Range

Denver

GREAT PLAINS 

PHYSIOGRAPHIC 

PROVINCE

Continental

Divide

South Platte

River

Palmer Divide

COLORADO

Denver

Basin

Base from U.S. Geological Survey

digital data, 2012, 1:100,000 

0 10 20 MILES

0 10 20 KILOMETERS

105° 104°

40°

39°

76

25

25

70

70

Arkansas River

EXPLANATION

Figure 3-1.

Land-surface altitude, 

 in feet

7,992

7,000

6,000

5,000

4,347

16 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05
More than two-thirds of the average annual precipitation of 15 inches in the Denver Basin area falls from April through September; 

several inches of rain can fall in a few hours or less during summer thunderstorms. Because of the semiarid climate and lack of 

surface-water supply, recently developed (since about 1970) suburban areas south of Denver rely heavily on groundwater from the 

Denver Basin bedrock aquifers for their water needs. 

e Denver Basin at a glance

 2,700,000 Number of people living within the basin (2005 data)

 8,000 Elevation of the highest point (feet) 

 3,200 Maximum thickness of the aquifers (feet)

 600 Water-level decline in some wells south of Denver since the 1980s (feet) 

70 Percentage of annual precipitation that falls during the spring and summer

15 Average amount of annual precipitation (inches)

CH 3. Aquifers by the numbers

Chapter 3

Photographs copyright istockphoto.com

Chapter 3: Environmental Setting and Water-Resource Characteristics 17
Hydrogeologic Setting

The Denver Basin aquifer system is composed of upper 

Cretaceous- to Tertiary-age bedrock sandstones that overlie 

the Cretaceous-age Pierre Shale (fig. 3–2). The four primary 

sedimentary rock aquifers are (from oldest to youngest) the 

Laramie–Fox Hills aquifer, Arapahoe aquifer, Denver aquifer, 

and Dawson aquifer. The deepest part of the aquifer system is 

in Douglas County, at an altitude of approximately 3,410 ft.(2) 

In parts of the basin, the bedrock aquifers are overlain by the 

alluvial aquifer, which is primarily along stream channels 

(fig. 3–3).

Water

level

Pierre Shale

Denver

Confining units

Denver aquifer

Alluvial fan, swamp, overbank deposits

Andesitic sandstone, volcanic ash deposits, coal, 

 lignite, mudstone/claystone, 

Source of selenium and uranium 

Median hydraulic conductivity=0.45 ft/d

Arapahoe aquifer

Fluvial environment, alluvial fan deposits

 near mountain front

Conglomerates, sandstone, siltstone, shale

Pebbles and cobbles with granite, chert, 

 metamorphic rocks, and quartzite

Median hydraulic conductivity=1.8 ft/d

Dawson aquifer

Channel sandstones, overbank mudstone/claystone deposits

Upper sequence of coarse-grained arkosic sandstones

Granitic sediments source of uranium 

 and radon in groundwater

Median hydraulic conductivity=0.80 ft/d 

South Platte River

Laramie–Fox Hills aquifer

Laramie aquifer

 Swamps, deltas, overbank deposits 

 Claystone, coal, fluvial channel sandstones

Fox Hills aquifer

 Marine beach and delta-front environment

 Sandstone, thin siltstone and claystone beds

Median hydraulic conductivity of Laramie–

 Fox Hills aquifer=0.40 ft/d

Alluvial aquifer

Unconsolidated sand and gravel, interbedded clay

Primarily along present-day stream channels

Igneous and sedimentary rock fragments

West Median hydraulic conductivity=479 feet per day (ft/d)

East

Figure 3-2.

NOT TO SCALE

Figure 3–2. Conceptual diagram illustrating 

hydrogeologic features of the Denver Basin. The 

aquifer layers dip steeply on the west side of the 

basin and dip gently in the center and on the 

eastern side, resembling asymmetrical stacked 

bowls. Because of this shape, the oldest rocks of 

the Laramie–Fox Hills aquifer are exposed around 

the outer margins of the basin, and the youngest 

rocks of the Dawson aquifer are exposed in the 

center of the basin (fig. 3–3). Characteristics of 

the aquifer sediments, including their depositional 

environment, lithologic composition, and capacity to 

transmit water, are described in the colored boxes.

18 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05
To understand the hydrogeology of the Denver Basin, it 

is important to understand how the depositional environment 

of the sediments that form the bedrock aquifers changed over 

geologic time. In the mid-Cretaceous period, an ancient sea 

(the Western Interior Seabed) occupied much of Colorado. 

The Pierre Shale, the geologic unit forming the base of the 

aquifer system, consists of mudstones and other rocks that 

were deposited on the ancient sea floor. As the sea retreated 

to the east, sands were deposited along beaches, forming 

the Fox Hills Sandstone. With additional sea level change, 

claystones, coal beds, and sandstones that now make up the 

Laramie Formation accumulated in swamps and deltas. The 

Laramie–Fox Hills aquifer includes the Fox Hills Sandstone 

and lowermost part of the Laramie Formation. As mountains 

rose up during the late Cretaceous period, rivers began draining 

the mountains, and alluvial sediments were carried eastward. 

On the western side of the basin, sandstones and conglomerates 

were deposited as coarse-grained alluvial fans, forming the 

lowermost sediments of the Arapahoe aquifer; farther to the 

east, the Arapahoe aquifer sediments are finer grained. Denver 

aquifer sediments also were deposited as alluvial fans near the 

mountain front where they are composed largely of volcanic 

debris, but to the east are made up of fine-grained sediment and 

coal that accumulated near streams and in swamps. Sediments 

of the lowermost part of the Dawson aquifer are similar to 

sediments of the Denver aquifer, but the uppermost part of the 

Dawson aquifer is composed of debris weathered from Front 

Range granitic rock. Confining units, primarily interbedded 

claystone and shale, separate the Denver Basin aquifers, and 

confining units also separate the lowermost and uppermost 

parts of the Arapahoe and Dawson aquifers in some parts of 

the basin. Because depositional environments varied spatially 

and through geologic time, aquifer sediments throughout the 

basin exhibit substantial horizontal and vertical differences in 

lithologic composition and hydrogeologic characteristics. 

Unconsolidated, coarse-grained Quaternary-age sand and 

gravel deposits that are saturated form an alluvial aquifer that 

overlies parts of the bedrock aquifers (fig. 3–3). The alluvial 

aquifer lies primarily along modern stream channels, such 

as the South Platte River, and is composed of granitic and 

gneissic rock fragments, sedimentary rock fragments, quartz, 

and feldspar, with interbedded clay in some areas.

Chapter 3

Middle Bijou Creek

Bijou Creek

Cherry Creek

Big S andy Creek

Sand Creek West Bijou Creek

Black Squirrel Creek

Kiowa Creek

Kiowa Creek

East Bijou Creek

Lost Creek 76

25

25

70

70

Limon

Parker

Greeley

Monument

Brighton

Franktown Castle

Rock

Highlands

Ranch

Colorado

Springs

Aurora

Littleton

Henderson

Fort Morgan WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

BROOMFIELD

DOUGLAS

DENVER

Palmer Divide

Denver

Basin

Base from U.S. Geological Survey

digital data, 2012, 1:100,000 

0 10 20 MILES

0 10 20 KILOMETERS

105° 104°

40°

39°

Denver

South Platte River

EXPLANATION

Figure 3-3.

Alluvial aquifer

Dawson aquifer

Denver aquifer

Arapahoe aquifer

Laramie–Fox Hills aquifer

Confining unit

Denver Basin aquifer system

Figure 3–3. The bedrock 

units of the Denver Basin 

aquifer system crop out in a 

ring pattern around the basin. 

Groundwater is generally confined 

at depth in bedrock aquifers 

and unconfined in the alluvial 

aquifer and in bedrock aquifers 

where they are exposed at the 

land surface. Unconfined parts 

of the aquifer system are more 

susceptible to contamination from 

human activities and chemical 

applications at the land surface 

than are confined parts.

Chapter 3: Environmental Setting and Water-Resource Characteristics 19
The amount and quality of groundwater in the Denver 

Basin reflect the depositional setting of the aquifer sediments 

(fig. 3–2). Course-grained sediments that were deposited 

on the west side of the basin near the mountain front form 

more productive aquifers than do fine-grained sediments on 

the east side of the basin. The sandstones and conglomerates 

of the lower part of the Arapahoe aquifer on the west side 

of the basin, for example, form a highly productive aquifer 

that is heavily used for municipal and domestic supply in 

Douglas and El Paso Counties. The minerals that make up the 

sediments affect groundwater quality—granite clasts contain 

uranium, and volcanic ash and lignite contain selenium and 

other trace elements. 

Most groundwater recharge that occurs naturally is from 

precipitation that infiltrates unconsolidated and porous sand 

and gravel alluvial aquifers at the land surface. Additionally, 

bedrock aquifers are recharged directly where the bedrock 

is exposed at land surface. Infiltration to bedrock aquifers 

primarily is to the upper part of the Dawson aquifer at higher 

altitudes. Recharge to the alluvial and bedrock aquifers 

naturally is low—about 7 percent of annual precipitation 

(or about 1 inch) is estimated to recharge the alluvial aquifer 

and from 1 to 2 percent recharges the bedrock aquifers.(2)

Regional groundwater flow in the bedrock and alluvial 

aquifers generally is away from the topographic high and 

recharge area of the Palmer Divide (fig. 3–1) toward the 

north and south.(2, 8) More locally, water moves from upland 

recharge areas through the near-surface parts of the aquifers 

to discharge areas in nearby stream valleys. Groundwater also 

moves vertically down to underlying bedrock aquifers.

In the bedrock aquifers, groundwater exists primarily 

under confined conditions, although conditions are unconfined 

Alluvial fans, such as the example shown on the left in 

Death Valley, Calif., were deposited on the west side of 

the Denver Basin. Near the mountain front, the deposits 

are coarse-grained, such as those for the Arapahoe 

Formation (above, 12-inch ruler for scale). Remnants of 

prehistoric alluvial fans, such as those near Highlands 

Ranch and northwest of Littleton, Colo. (fig. 3 –1), can be 

productive sources of groundwater.

Sediments of the Dawson Arkose are exposed along a stream 

bank in Castlewood Canyon State Park near Franktown, Colo. The 

geologic unit contains coarse-grained arkosic sandstones and 

granitic sediments, a source of uranium and radon in the upper 

part of the Dawson aquifer.

where the bedrock crops out or is near the land surface. 

Although downward water movement through confining 

units is very slow (3.5×10 –5 feet per day [ft/d]) for the 

Denver-Dawson confining unit, in contrast to 0.4–1.8 ft/d for 

the bedrock aquifers,(2, 8) this vertical leakage is an important 

source of recharge to underlying bedrock aquifers. Compared 

to the rate of lateral flow in the bedrock aquifers, the rate of 

downward flow is small. However, the downward flow takes 

place over a large area and likely results in large volumes of 

water moving between aquifers.(2, 8) The estimated downward 

flow in 2003 was about 148 million gallons per day (Mgal/d).(2)

Photograph by David M. Miller, USGS

20 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05
Land Use and Population

Although some settlers came to Denver hoping to strike 

it rich during the gold rushes of 1849 and 1859, many found 

that they could turn a greater profit by developing commercial 

businesses and farms to support the growing population. 

Horace Greeley, the famous editor of the New York Tribune 

newspaper and promoter of Colorado, said “...the hardest way 

to obtain gold is to mine for it. A good farmer, or merchant, 

will usually make money faster... by sticking to his business 

than by deserting it for gold digging.” (9) Historically, native 

grassland and rangeland in the Denver Basin were converted 

to agricultural and urban land; more recently, grassland, rangeland, and agricultural land are being converted to suburban 

(commercial and residential) land use. Land use overlying the 

Denver Basin aquifer system ranges from high-density urban 

development in downtown Denver to irrigated agriculture 

along the South Platte River (fig. 3–4). Urban and suburban 

development is mostly along the west side of the Denver Basin 

in and around the metropolitan areas of Denver and Colorado 

Springs. Agricultural land is primarily in the northern part 

of the basin, and grassland and rangeland cover much of 

the central and southern parts of the basin. Human activities 

in urban, suburban, and agricultural areas can be sources 

of contaminants, such as fertilizers, gasoline compounds, 

solvents, and pesticides, to shallow groundwater.

The population of the metropolitan areas overlying the 

Denver Basin aquifer system more than doubled from 1970 

to 2005, from 1.3 million to about 2.7 million (T.L. Arnold, 

U.S. Geological Survey, written commun., 2009). Development has been especially rapid and extensive on the west 

side of the basin between Denver and Colorado Springs. 

From 2000 to 2009, Douglas County (fig. 3–1) was among 

the fastest growing counties in the United States(10) and also 

is the area in the Denver Basin with the greatest water-table 

declines. Three general areas of recent development rely 

heavily on Denver Basin groundwater for municipal water 

supply. The area south of the Denver metropolitan area that 

encompasses Littleton, Parker, Franktown, and Castle Rock 

has been transformed from a rural residential area to one 

with sprawling suburbs as part of this population increase 

(fig. 3– 4). A second area includes former rangeland and ruralresidential areas around Monument and northeast of Colorado 

Springs that have undergone similarly rapid development. 

Suburbs north and northeast of Denver in Weld and Adams 

Counties represent the third area of intense development.

Chapter 3

Middle Bijou Creek

Bijou Creek

Cherry Creek

Big S andy Creek

Sand Creek West Bijou Creek

Black Squirrel Creek

Kiowa Creek

Kiowa Creek

East Bijou Creek

Lost Creek 76

25

25

70

70

WELD

EL PASO

ELBERT

LINCOLN

LARIMER

ADAMS

MORGAN

BOULDER

ARAPAHOE

TELLER

JEFFERSON

WASHINGTON

BROOMFIELD

DOUGLAS

DENVER

Brighton

Parker

Colorado

Springs

Monument

Franktown

Castle

Rock

Highlands

Ranch

Littleton

Denver Aurora

Henderson

Greeley

Fort Morgan

Limon

Palmer Divide

Denver

Basin

Base from U.S. Geological Survey

digital data, 2012, 1:100,000 

0 10 20 MILES

0 10 20 KILOMETERS

105° 104°

40°

39°

South Platte River

EXPLANATION

Figure 3-4.

Urban/suburban development

Forest

Grassland and rangeland

Irrigated pasture or row crops

Non-irrigated small grains or fallow

Water

Land-use category

Figure 3–4. About 60 percent of 

the Denver Basin area is nonirrigated grassland and rangeland; 

16 percent is non-irrigated 

agricultural areas, primarily wheat 

and small grains; 11 percent is 

irrigated agricultural areas, primarily 

corn; 9 percent is developed urban 

and suburban areas; and 3 percent 

is forested.(90, 91) The remaining 

1 percent of land cover is open 

water or urban recreational areas.

Chapter 3: Environmental Setting and Water-Resource Characteristics 21
Rangeland in the Parker, Colo., area has rapidly been converted to suburban residential development. Parker and other areas with similar 

recent development tap the Denver Basin bedrock aquifers for public water supply. Extensive pumping from bedrock aquifers has resulted 

in a lowering of groundwater levels in some parts of the aquifers. (Imagery for 1999, 2002, and 2008 from U.S. Geological Survey Digital 

Orthophoto Quarter Quads; imagery for 2011 from U.S. Department of Agriculture National Agricultural Imagery Program).

1999

2008 2011

2002

22 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05
Water Use

Public water supply along the Front Range urban corridor 

is a combination of surface water from the Rocky Mountains 

and groundwater from alluvial and bedrock aquifers (fig. 3 –5). 

In rural areas, domestic water users and irrigators commonly 

rely on groundwater pumped from private wells. Water use in 

the State of Colorado is governed by extensive water law. Use 

of surface water and tributary groundwater— groundwater that 

is hydraulically connected to surface water— is governed by 

a system of prior appropriation. Simply, prior appropriation 

means “first come, first served”— the first user to divert water 

for beneficial use has a senior water right. Older municipalities, such as Denver and Colorado Springs, and irrigation 

companies generally hold water rights that are senior to more 

recently established municipalities and suburbs such as Parker 

and Castle Rock. Much of the water supply for older 

municipalities is from surface water. Water for agricultural 

irrigation along the South Platte River is supplied by surface 

water delivered through irrigation diversion ditches and by 

tributary groundwater pumped from the alluvial aquifer.

In more recently developed areas, however, there is 

little surface water, limited tributary groundwater, or limited 

available water rights. As a result, municipalities in these 

areas have tapped the bedrock aquifers for a readily available 

source of water.(11) Groundwater in the bedrock aquifers is 

administratively recognized as nonrenewable because the 

aquifers receive little recharge from precipitation and primarily 

are confined.(12) A water law passed in 1973 recognized the 

existence of nontributary groundwater— groundwater with little 

physical connection to surface water—in the Denver Basin and 

established criteria for withdrawal of water from the bedrock 

aquifers. A 1985 water law ruled that a landowner (private or 

public) could appropriate groundwater from bedrock aquifers 

beneath the land at a rate that insured at least a 100-year aquifer 

life.(11) Groundwater in the Denver Basin bedrock aquifers is 

considered mostly nontributary and therefore is not subject to 

the prior appropriation doctrine. Because allocation is based 

on land ownership, the bedrock aquifers tapped by wells on 

publicly owned land are heavily used for supply by recently 

established municipalities. Additional information on groundwater law for the Denver Basin is available in Hobbs.(13) 

Chapter 3

Figure 3–5. About 94 percent of the water supply for eight 

counties that overlie the Denver Basin aquifer system is surface 

water from the Front Range mountains or diversions from mountains 

west of the Continental Divide. Although only 6 percent of the water 

used in the eight counties is from groundwater, groundwater is an 

important source of supply for recently developed suburbs between 

Denver and Colorado Springs and areas north and northeast of 

Denver and in rural areas. (Water-supply data are estimated 

values from Ivahnenko(92) and Ivahnenko and Flynn.(93))

Denver Basin groundwater: A source of supply for 130 years

Development of Denver Basin groundwater began in the 

spring of 1883 when water began to flow from a well that 

accidentally tapped the Laramie Formation. In the first report 

on Denver Basin groundwater, published in June 1884, 

Frederic F. Chisolm of the Colorado Scientific Society wrote:

“This [artesian] water was characterized by 

its extreme purity and its superiority over water 

furnished by the Denver Water Company from 

the Platte, and the interest created by this 

discovery was very great.”(52)

By late 1886, more than 130 artesian wells had been 

drilled in Denver.(9) The Denver historian Louisa Ward Arps 

later wrote: 

“The newspapers were full of the benefits of 

artesian water...Learned treatises were printed,

discussing the mineral content of each well. 

The opinions of doctors were sought as to

which well was best to cure which ailment.”(9)

Artesian well.

Photograph by C.A. Fisher, U.S. Geological Survey Photographic Library, ID. Fisher, C.A. 16 fca00016

From east of the Continental Divide

Groundwater (92 Mgal/d)

Surface water (1,420 million gallons

 per day [Mgal/d])

2005 water supplies for selected counties

that overlie the Denver Basin

EXPLANATION

Fig. 3-5

Transmountain diversion, from 

 west of the Continental Divide

Chapter 3: Environmental Setting and Water-Resource Characteristics 23
Figure 3–6. A, Groundwater withdrawals from the alluvial aquifer to irrigate crops has been the largest 

component of pumping from Denver Basin wells since substantial withdrawals began around 1925. 

Although pumping from the alluvial aquifer for irrigation continues to exceed the amount of water pumped 

from the bedrock aquifers for non-irrigation (municipal, domestic, and commercial, or industrial) uses, 

pumping from bedrock aquifers is increasing rapidly. B, Pumping of groundwater from Denver Basin 

bedrock aquifers has increased in response to rapid growth in population. Bedrock aquifers provide 

much of the water supply for those areas with the most rapid growth. 

Extensive development of the bedrock aquifers began in 

the 1950s to support population growth and increased demand 

for water supplies by suburban communities in the south 

metropolitan area, northern El Paso County, and areas north 

and northeast of Denver (fig. 3– 6). Water from bedrock aquifers is used for municipal and commercial/industrial purposes 

and domestic supply in rural areas, in contrast to water from 

the alluvial aquifer, which is used mostly for agricultural irrigation (fig. 3 –7). Large-volume pumping from deep, confined 

parts of the bedrock aquifers for public water supply is causing 

groundwater levels to decline and shallow groundwater in 

some parts of the aquifer system to move downward. In the 

south Denver metropolitan area in Douglas County, water 

levels in the Arapahoe aquifer have declined as much as 600 ft 

since the 1980s.(14) The drawdown of water (localized decline 

in the water table) around a well can decrease well production 

and increase the cost of pumping. Recent levels of pumping 

from the bedrock aquifers are not indefinitely sustainable. The 

continued increase in population, development, and associated 

groundwater pumping in the Denver Basin since the 1980s has 

caused renewed concern for groundwater availability and the 

sustainability of the Denver Basin aquifer system.(2, 11)

0 50 100 150

Commercial/

industrial

Domestic

Irrigation

Municipal

Total

Estimated pumping for 2003,

in million gallons per day

Water use

Alluvial aquifer

Bedrock aquifers

EXPLANATION

Water use by aquifer type

Mdified from Paschke(2)

Figure 3-7.

Figure 3–7. Groundwater 

pumped from bedrock aquifers is 

used extensively for public supply 

in areas north and south of Denver 

and near Colorado Springs and 

for domestic supply in rural areas. 

Groundwater pumped from the 

alluvial aquifer primarily is used 

for agricultural irrigation.

1970

1975

1980

1985

1990

1995

2000

2005

1.0

1.5

2.0

2.5

3.0

10

30

50

70

90

Year

Population

Bedrock aquifer 

pumping

Population overlying the Denver

Basin aquifer system, in millions

Estimated pumping from bedrock

aquifers, in million gallons per day

EXPLANATION

Irrigation use, 

 by aquifer

Non-irrigation use,

 by aquifer

Alluvial

Bedrock

Alluvial

Bedrock

1910

1920

1930

1940

1950

1960

1970

1980

1990

2000

2010

Year

Estimated pumping, in million gallons per day

0

20

40

60

80

100

120

A. Estimated pumping from alluvial and bedrock aquifers by use

B. Population and estimated pumping

 from bedrock aquifers

Modified from Paschke(2)

Fig. 3-6

24 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05
Chapter 3

Evaluating groundwater availability from the Denver Basin aquifer system

As populations continue to grow and more people rely on the Denver Basin aquifer system for their water 

supply, it is increasingly important to continually monitor and reassess the availability of groundwater resources.(2)

In 2004, the U.S. Geological Survey initiated a large regional study of the Denver Basin aquifer system to provide an 

updated assessment of groundwater availability. The study evaluated the effects of continued pumping on Denver 

Basin groundwater resources and documented an updated groundwaterflow model useful for assessing hydrologic conditions.(2) The updated 

model is a fully three-dimensional groundwater-flow model that uses the 

MODFLOW-2000 computer program(53 –56) to simulate transient groundwater 

flow in the bedrock aquifers and overlying alluvial aquifer from prior to 

development (pre-1880) through 2003. Model predictions for 2004–2053 were 

included to provide estimates of how water levels might respond to different 

scenarios for managing the groundwater resources in the future. Estimated 

groundwater-flow data and pumping amounts presented in this circular are 

results from the calibrated groundwater-flow model simulations.(2)

Results of the groundwater-flow simulations indicate that water use 

at the land surface has increased with development in the basin and that 

pumping of confined bedrock aquifers has lowered the hydraulic head in 

some areas and reduced aquifer storage.(2) With a lowered hydraulic head, 

there is less upward flow from bedrock aquifers to the alluvial aquifer, more 

downward flow between bedrock aquifers, and a change from confined 

to unconfined conditions in some parts of the system. Simulating the groundwater-flow system provides a tool for 

understanding past and present groundwater conditions for the Denver Basin aquifer system, assessing the effects 

of continued development and pumping on groundwater resources, and predicting future aquifer response to 

management decisions on the use of the finite groundwater resource.

Chapter 3: Environmental Setting and Water-Resource Characteristics 25
Processes that affect the quality of Denver Basin 

groundwater include the infiltration of chemicals 

applied on the land surface to shallow groundwater 

along with recharge water (upper left), the 

weathering of trace elements, such as iron 

(upper right) from aquifer sediments, and 

transpiration of water from crops, such as 

sunflowers (lower left), which can concentrate 

dissolved solids in the subsurface. 

Photograph copyright istockphoto.com

Photograph by Andrew Silver, Brigham Young University

26 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05
This chapter explains 

and discusses the hydrologic and geochemical 

processes and human 

activities that affect the 

movement and quality 

of groundwater in the 

Denver Basin. 

Chapter 4

Chapter 4: Natural and Human Factors 

That Affect Groundwater Flow and Quality

For more than 100 years, 

water fountains in the 

Brown Palace Hotel in 

Denver have been supplied 

by a well tapping the 

Arapahoe aquifer 750 feet 

below land surface. The 

first wells to tap the bedrock 

aquifers were drilled in 

downtown Denver in the 

1880s; by 2003, about 

8,000 alluvial wells and 

44,000 bedrock wells were 

completed in the Denver 

Basin aquifer system. 

(Well data from Paschke.(2))

The quality of groundwater used for drinking and irrigation in the Denver 

Basin is determined by natural factors, such as evapotranspiration and 

geochemical conditions in the aquifer, and human activities, such as 

urban development, farming, and groundwater pumping. Human activities can 

introduce a diverse array of contaminants to the land surface and subsurface 

environment and, equally important, have extensively modified the hydrology 

in the Denver Basin. Increases in groundwater recharge resulting from 

irrigation and urban development and increased pumping for water supply 

have altered groundwater flow paths and geochemistry in the Denver Basin 

and increased the volume of water moving through the aquifer system. These 

modifications have the potential to transport contaminants derived from 

geologic materials and human activities from shallow parts of the aquifer to 

deep parts of the Denver Basin that are used for drinking-water supply.
Million gallons per day

Recharge

Discharge

Recharge

Discharge

Natural predevelopment Modern developed

EXPLANATION

*Discharge to springs and 

the alluvial aquifer and flow 

out of the basin (<4 percent 

of discharge)

Sources of

groundwater recharge

Sources of

groundwater discharge

Irrigation and

 precipitation

Precipitation

Streams

Pumping

Streams and reservoirs

Evapotranspiration

Other*

Natural predevelopment groundwater flow

Denver Basin water budget

0

200

400

600

800

Fig. 4-1

Irrigation and Groundwater Pumping Associated 

With Development Have Nearly Doubled the

Amount of Groundwater Moving Through Shallow

Parts of the Denver Basin Aquifer System

Recharge to the Denver Basin aquifer system under 

natural, predevelopment conditions was primarily from 

precipitation (fig. 4 –1). Under modern developed conditions, 

recharge from precipitation is enhanced by recharge from 

irrigation. Irrigation is primarily in agricultural areas, but in 

a sense urban areas also are irrigated. About 50 percent of 

residential water use in urban areas along the Front Range 

is for outdoor watering, most of it on turf (for example, on 

lawns, golf courses, and parks).(15)

Discharge under predevelopment conditions was 

primarily evapotranspiration and outflow to streams. Under 

modern conditions, discharge is greatly enhanced by pumping 

from wells and, to a lesser extent, by increases in evapotranspiration from irrigated agricultural and urban areas 

and flow of groundwater to streams and reservoirs. Almost 

one-third of the modern discharge is groundwater withdrawn 

through wells. The amount of water removed from the aquifer 

system is now greater than recharge, which results in a net loss 

of groundwater and a decline in groundwater levels in some 

parts of the Denver Basin. 

The largest decline in groundwater levels has been in the 

Arapahoe aquifer in the south Denver metropolitan area.(14)

The greatest pumping of groundwater has been from the 

lowermost part of the Arapahoe aquifer because it is made up 

of relatively porous and extensive sandstones and is a highly 

productive source of groundwater for supply. Water levels 

also have declined in some parts of the Dawson, Denver, and 

Laramie–Fox Hills aquifers because of pumping. With the 

Figure 4 –1. Hydrologic conditions in the Denver Basin have changed substantially over the last 

130 years because of land and water-resource development. The natural predevelopment hydrologic 

system was dominated by recharge from precipitation, which was balanced by discharge associated 

with evapotranspiration. The modern developed hydrologic system includes additional recharge 

from irrigation and additional discharge from pumped wells. The amount of water moving through 

the aquifer system has increased by about 160 percent from predevelopment to modern conditions. 

Currently, discharge exceeds recharge, resulting in a net loss of groundwater, as demonstrated by 

continued groundwater-level declines in several parts of the Denver Basin. (Simulated recharge and 

discharge values are from Paschke.(2))

28 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05
Chapter 4

increased pumping and corresponding decline in water levels, 

the downward flow of water between aquifers has increased, and 

shallow groundwater is moving to deeper groundwater in larger 

quantities.(2) Simulated flow from the alluvial aquifer downward 

to the bedrock aquifers increased by 50 percent from predevelopment conditions, from 5.2 Mgal/d (8 cubic feet per second 

[ft3/s]) before 1880 to 7.8 Mgal/d (12 ft3/s) in 2003. Simulated 

downward flow from some of the bedrock units to deeper parts 

of the system has more than doubled since the early 1950s 

because of increased pumping from the Arapahoe aquifer.(2)

With a decrease in the hydraulic head or water pressure 

in bedrock aquifers in some areas, upward flow to the alluvial 

aquifer has been reduced. As a result, lateral groundwater flow 

in some areas has been redirected toward cones of depressions 

around supply wells rather than towards natural discharge 

points in streams, and some formerly perennial (year-round) 

streams may be dry at certain times of the year. These changes 

“About noon on that day we reached Cherry 

Creek...The stream, however, like most of the 

others which we passed, was dried up in the heat 

and we had to dig holes in the sand to find water 

for ourselves and our horses.” 

 Francis Parkman from

 “The Oregon Trail” (1846)

As noted by early settlers traveling through the Denver 

Basin area, many streams in the region dried up during 

summer months. Because the amount of water applied 

to the land surface has increased as the Denver Basin 

area has been developed, groundwater discharge to 

some streams, such as Cherry Creek shown here, has 

increased, and streamflow has changed from flowing 

only during wet seasons to flowing year round.

have implications for water quality and biota living in the 

streams. In addition, parts of some confined aquifers have 

become unconfined, and artesian conditions have been lost. 

A change to unconfined conditions can increase the possibility 

of contamination of groundwater from human activities.

In contrast to declining water levels in some parts of the 

Denver Basin, water levels in some parts of the alluvial and 

shallow bedrock aquifers have risen as a result of enhanced 

recharge from irrigation in agricultural and urban areas. Some 

streams, such as the South Platte River, Cherry Creek, Sand 

Creek, and Toll Gate Creek, that historically were dry part of 

the year before development now have year-round streamflow. 

A change in streamflow conditions can alter the susceptibility 

and vulnerability of streams to contamination from groundwater (see sidebar, Effects of groundwater/surface-water 

interaction on water quantity and quality and ecosystem 

health in the Denver Basin, p. 30).

Chapter 4: Natural and Human Factors That Affect Groundwater Flow and Quality 29
Effects of groundwater/surface-water interaction on water quantity and 

quality and ecosystem health in the Denver Basin 

The interaction of groundwater and surface water affects water supply, water quality, and the health 

of aquatic ecosystems.(58) Groundwater that discharges to streams, lakes, reservoirs, and wetlands helps 

sustain the health of the aquatic environment but may also be harmful to aquatic life if groundwater 

is contaminated.

In the Denver Basin, the amount of water in a stream can be increased by the discharge of excess 

irrigation water or decreased by a decline in the water table resulting from the pumping of wells. As shown 

elsewhere in this report (see chapter 6), recharge of oxic irrigation water can mobilize contaminants 

(uranium, selenium) from geologic sources present in aquifer materials, which, in turn, can be delivered 

to streams. In Aurora, Colo., for example, groundwater transports selenium to Toll Gate Creek, where the 

stream aquatic-life standard for selenium of 4.6 micrograms per liter (μg/L) has been exceeded for the past 

several years (see chapter 6).(20) Groundwater discharges to the stream when the water table near the 

stream is higher than the stream surface.(58) 

Surface water seeps into groundwater through the streambed when the stream surface is higher than 

the nearby water table.(58) In such situations, the stream is called a “losing stream.” Groundwater quality 

can be adversely affected by losing streams if water in the stream is of poorer quality than water found 

in the shallow aquifer. Contaminants known to be found at high concentrations in Denver Basin streams 

include dissolved solids, selected trace elements, and, for some streams heavily affected by human 

activities, nitrate or organic compounds derived from fertilizers, pesticides, or other sources.

During summer, no water flows in this streambed in Castle Rock, Colo., because the water table is lower 

than the streambed.

30 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05
Not All Groundwater in Bedrock Aquifers 

Is Old Water

The “age” of groundwater is the time elapsed since the 

recharge water reached the water table and became isolated 

from the atmosphere. The term “age” typically is qualified 

with the word “apparent” to signify that the accuracy of the 

estimated age depends on many variables and the determined 

age is not certain; (16, 17) however, the term “groundwater age” 

is used in this report for simplicity. Estimates of groundwater 

age can be made by analyzing manmade or radioactive 

compounds that enter the subsurface with recharge water and 

travel with the water to a sampling point. Groundwater that 

was recharged within the last 60 or so years contains tritium, 

a radioactive element released during the testing of atomic 

bombs in the 1950s. Prior to atomic bomb testing, recharge 

water contained small amounts of tritium produced naturally; 

these amounts would have decayed to levels near or below 

detection levels in present day water samples. Groundwater 

age estimates are useful for understanding selected aspects of 

hydrogeology, such as recharge rates, rates of geochemical and 

microbiological processes, aquifer susceptibility and vulnerability to contamination, and water-resource management.(17) 

On the basis of groundwater age estimates, samples 

collected from the Denver Basin aquifer system were grouped 

into three age categories: (1) young water (post-1950s), (2) old 

water (pre-1950s recharge and, for many samples, recharge 

that is likely thousands of years old), and (3) mixed-age water 

(mixture of young and old water) (fig. 4–2).(18) Most samples of 

shallow groundwater from wells in the urban area were young; 

most of the shallow agricultural well samples were of mixed 

age. Not all groundwater from deeper parts of bedrock aquifers 

was old. Samples from drinking-water wells that tap the 

Dawson aquifer mostly were of mixed age, indicating the likely 

mixing of old water at depth with young recharge. Mixtures 

of young and old groundwater at depth in two samples from 

the Denver aquifer and one sample of young groundwater 

from the Arapahoe aquifer indicate that groundwater mixing 

has occurred locally in the two aquifers. Some of the bedrockaquifer wells used for drinking water can have long screen 

intervals. These wells likely draw water from multiple depths

within the aquifer, such that mixing can occur in the well 

bore during sampling. With high-volume pumping from deep

supply wells, though, there can be a mixture of young and 

old groundwater at intermediate depths as the downward 

movement of young groundwater increases due to pumping.

Figure 4–2. Three age categories were identified for Denver Basin groundwater samples—

young (post-1950s), old (pre-1950s, some samples likely thousands of years old), and mixedaged samples (mixtures of young and old groundwater). Mixed-age groundwater at 

intermediate depths, particularly in the Dawson aquifer but also in a few samples from 

the Denver aquifer, indicates that young, shallow groundwater has moved into underlying 

bedrock aquifers where it has mixed with older groundwater. This mixing suggests that 

the quality of deep groundwater used for drinking-water supply could be affected by 

contaminants found in poor quality shallow groundwater.

Shallow monitoring well, 

 agricultural area

Deep well used for 

 drinking water

Shallow monitoring well, 

 urban area

EXPLANATION

Alluvial

Dawson

Denver

Arapahoe

Laramie–Fox Hills

Groundwater-age interpretation

0 20 40 60 80 100

Estimated young water in mix, in percent

2,500

2,000

1,500

1,000

500

0

Depth of well, in feet below land surface

Old groundwater

10 samples

Young groundwater

Well type

Aquifer

Mixture of young and old groundwater

Fig. 4-2

Chapter 4

Chapter 4: Natural and Human Factors That Affect Groundwater Flow and Quality 31
Geologic Materials and Human Activities Are 

Sources of Contaminants to Groundwater

Aquifer sediments in the Denver Basin contain geologic 

materials (rock and mineral fragments and particles; fig. 3–2) 

that are sources of several naturally occurring constituents of 

concern. The Denver aquifer, for example, contains extensive 

volcanic ash deposits that contain easily leached sulfate and 

carbonate minerals that can contribute to high dissolved-solids 

concentrations in shallow groundwater. Some of the minerals 

also contain trace elements, such as selenium and uranium, that

are released to groundwater when the minerals dissolve during

periods of elevated recharge or irrigation.(19, 20) As described 

in chapter 3, each of the major aquifers of the Denver Basin 

has varying depositional histories and different sources of 

sediment. Some of the aquifers are composed of source rocks 

that are enriched in specific elements; for example, the granitic 

sediments in the Dawson and Arapahoe aquifers can be a 

source of uranium to Denver Basin groundwater.

The greatest manmade factor affecting the quality of 

Denver Basin groundwater is the change in land cover and 

land uses from natural grassland to agriculture and urbanization. Excess recharge water from irrigation and other 

sources (leaky water mains and sewer lines, for example) can 

mobilize contaminants from geologic and manmade sources. 

Agricultural irrigation can contribute contaminants to the 

land surface through use of commercial fertilizers, pesticides 

(herbicides, insecticides, and fungicides), manure, and other 

substances (fig. 4 –3). The predominant source of nitrogen to 

watersheds in areas heavily used for agriculture is the application of commercial fertilizers.(4, 21) In urban and suburban 

areas, contaminants from manmade sources, including volatile 

organic compounds (VOCs), can be added to groundwater 

through the use of fertilizers, pesticides, commercial and 

household products, and gasoline. Contaminants are released 

to the environment from septic tanks, leaking underground 

storage tanks, chemical spills, leaky sewer lines, treated wastewater, biosolids, urban runoff, and commercial and industrial 

processes. Pesticides and VOCs can degrade (be transformed) 

to other chemical compounds through abiotic or biotic 

processes. Detections of pesticides and VOCs in groundwater 

samples are dependent on a variety of factors, such as type and 

source of pesticide and VOC, land use, depth of groundwater, 

reactions that occur during transport from the land surface to 

the water table, and geochemical reactions in the aquifer. These 

factors and others are discussed in more detail in chapter 6.

Figure 4–3. Water entering the Denver Basin 

aquifer system is exposed to different sources of 

contaminants during and after recharge and as it 

travels along different flow paths at different rates. 

Groundwater from irrigation, domestic, or publicsupply wells, therefore, can be a mix of ages and 

chemistries. Constituents from geologic sources, 

such as dissolved solids and uranium, are derived 

from soil and aquifer sediments, and their transport 

may be affected by human activities, such as irrigation. Sources of contaminants in groundwater from human 

activities on the land surface include point sources, for example, spills and leaking underground storage or septic 

tanks, and non-point sources, such as fertilizers or pesticides applied to cropland or lawns.

Public-supply well

Irrigation

well

NOT TO SCALE

Domestic well

Agricultural

 irrigation

Urban

irrigation

Groundwater movement

Water table

Days to years

Days to 

decades

Years to 

decades

Centuries to

Centuries to millennia

millennia

Contaminant movement

Leaky

tank

Septic

tank

Spills

tractor

Chemical

application

Manure

Percolation

EXPLANATION

Fig 4-3.

Alluvial aquifer

Dawson aquifer

Denver aquifer

Arapahoe aquifer

Laramie–Fox Hills aquifer

Confining unit

Denver Basin aquifer system Unsaturated soil

32 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05
Increased interest in aquifer storage and recovery programs in the Denver Basin

As water levels and aquifer storage have declined in response to high-volume pumping, interest has increased in the south 

Denver metropolitan area in obtaining additional sources of water. Aquifer storage and recovery (ASR) has been identified as one 

process for storing water to meet supply needs. For more than two decades, the Centennial Water and Sanitation District (CWSD), 

the water supplier for Highlands Ranch in the south Denver metropolitan area, has used ASR to augment water withdrawn from wells 

completed in Denver Basin bedrock aquifers.(59) During ASR, high-quality treated surface water from the South Platte River is injected 

into wells in the Denver Basin aquifer system and stored when the supply is available. Water is withdrawn from the aquifers when 

the demand for water is greater than the supply. Through February 2012, CWSD has injected more than 4.5 billion gallons of water into 

19 wells.(60) In Colorado, supplemental water for injection is treated and must meet water-quality standards, and injection wells require 

permits from the U.S. Environmental Protection Agency and Colorado State Engineer’s Office. A major concern about the use of ASR 

has been potential contamination and (or) plugging of an aquifer as a result of geochemical reactions between the injected water 

and native aquifer water.(61) For example, injection of oxic river water into anoxic aquifer water could potentially mobilize naturally 

occurring constituents, such as uranium, present in aquifer sediments, resulting in changes in the quality of water produced by nearby 

supply wells. Treated water typically contains disinfection byproducts formed during chlorination, for example, trihalomethanes, such 

as chloroform. An ASR study on injection of treated drinking water found that oxygen in the CWSD-stored water was quickly depleted, 

and disinfection byproducts in the City of Centennial water-distribution system were removed naturally from the stored water in the 

aquifer over a period of several weeks.(62) ASR projects also have been implemented by other water suppliers in the Denver Basin area, 

including Colorado Springs Utilities and the Consolidated Mutual Water Company in Lakewood, Colo. Research on the use of aquifer 

storage and recovery in the Denver Basin is ongoing.

During aquifer storage and recovery, the aquifer essentially acts as a “water bank”—oxygen-rich water is injected into the 

aquifer and stored during wet periods when there is excess supply and is recovered through pumping during dry periods 

when additional supply is needed. Contaminants bound to aquifer sediments could potentially dissolve from aquifer sediments when oxic (oxygen-rich) and anoxic (oxygen-reduced) water are mixed.

Chapter 4

Injection of oxygen-rich water into

well and storage in the aquifer 

Modified from Reese and Alvarez-Zarikian(63)

Confining

layer

Confining

layer

Aquifer Mixing

zone

Zone

of 

native 

anoxic

water

Storage

zone

Recovery of water 

from well 

Fig. 4-ASR

NOT TO SCALE

Chapter 4: Natural and Human Factors That Affect Groundwater Flow and Quality 33
Natural Processes That Affect Water Quality

in the Denver Basin Can Be Accelerated by 

Human Activities

The chemical composition of recharge water changes as 

it moves through the unsaturated zone and subsequently 

through aquifers in the Denver Basin. The composition 

is controlled by hydrologic and geochemical processes, 

including precipitation/dissolution reactions, evaporative 

concentration, and reduction/oxidation reactions that occur 

naturally in the unsaturated and saturated zones. Each of these 

processes, however, can be enhanced by human activities, 

such as irrigation in agricultural and urban areas. 

The interaction between water and subsurface materials 

can add or remove dissolved constituents to water. Precipitation along the Front Range typically contains less than 

5 milligrams per liter (mg/L) of dissolved solids.(23) In the 

same way that water dissolves table salt, recharge water from 

precipitation dissolves soluble minerals in the unsaturated 

zone, increasing the concentration of dissolved solids in the 

water. Once water enters the aquifer, continued reaction of 

groundwater with aquifer sediments can further increase the 

dissolved-solids concentration of water, but at a slower rate, 

as these minerals are less soluble than the salts that accumulate 

in soils. The dissolution of materials in the subsurface of 

the Denver Basin, along with evapotranspiration, results in 

dissolved-solids concentrations that are 100 to 1,000 times 

more than those of precipitation. 

Dissolved solids in the unsaturated zone are concentrated 

by evapotranspiration as part of annual and long-term (decades 

to millennia) wetting and drying cycles. As water in the 

subsurface evaporates or is taken up by plants, soluble salts are 

left behind. As a result, salts, nitrate, and some trace elements 

and other dissolved constituents accumulate in the unsaturated 

zone over time.(24, 25) When recharge water from precipitation 

reaches zones of salt accumulation, it dissolves the salts and 

other soluble constituents and transports them downward 

to the water table. Whether or not these salts actually reach 

the water table depends on the amount of recharge. Because 

potential evapotranspiration is five times as high as annual 

precipitation, the transport of salts to the water table under 

natural conditions is slow, taking place over hundreds to 

thousands of years, except in the case of extreme precipitation 

events or during wet climatic periods.(26) The amount of 

evaporative concentration, based on results of stable isotope 

analysis, is estimated to be as high as 36 percent.(20) In practical

terms, this means that water with an initial dissolved-solids 

concentration of 400 mg/L would have a concentration greater 

than the Secondary Maximum Contaminant Level (SMCL) of 

500 mg/L after evaporative concentration. Accumulation of 

dissolved solids is enhanced with evapotranspiration of excess 

recharge water from irrigation.

The salt encrustations on the surface of a Denver 

Formation claystone bedrock outcrop exposed along 

Toll Gate Creek, Aurora, Colo., are salt deposits formed 

from ongoing evaporative processes that concentrate 

selenium and other dissolved materials from underlying 

bedrock and sediments.(20, 27) An increase in streamflow 

levels and the water table during storm events and 

groundwater recharge dissolves the salts, which 

releases selenium to the stream water. Tonsteins, thin 

distinct clay deposits of weathered volcanic ash in 

the Denver Formation, commonly have high selenium 

concentrations.(22) Bank height is approximately 5 feet; 

tonstein outcrop is about 3 inches thick.

 Photograph from Paschke and others(20)

Claystone

Encrustations on 

surface of claystone

Tonstein

34 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05
The presence or absence of oxygen in 

groundwater affects the occurrence of 

some contaminants 

The amount of dissolved oxygen in groundwater 

has a substantial effect on geochemical reactions in the 

subsurface and on water quality (see sidebar, How do redox 

reactions work?, p. 36). Some naturally occurring trace 

elements, such as uranium, dissolve when groundwater 

is oxic (dissolved-oxygen concentration of 0.5 mg/L or 

more) and precipitate (become solid) when groundwater is 

anoxic (dissolved-oxygen concentration less than 0.5 mg/L). 

Conversely, other trace elements, such as iron, precipitate 

under oxic conditions and dissolve under anoxic conditions 

(fig. 4 – 4). Redox (reduction-oxidation) conditions evolve 

from mostly oxic water in the alluvial aquifer and shallow 

parts of bedrock aquifers to mostly reducing (anoxic) water 

in deeper parts of the bedrock aquifers (fig. 4 –5). Samples 

from the Dawson aquifer were mostly oxic; samples from 

the other bedrock aquifers were mostly anoxic. As oxic water 

moves deeper along groundwater flow paths, redox conditions 

commonly change because of biological use of dissolved 

oxygen and other redox-sensitive constituents by microbes. 

The downward movement of young, oxic groundwater from 

Figure 4– 4. Arsenic, selenium, and uranium are 

readily soluble when groundwater is oxic, so higher 

concentrations of these constituents are more common 

in oxic groundwater than anoxic groundwater. The 

opposite is true for iron. Iron is more soluble when 

anoxic conditions are present, and concentrations of 

iron are higher in anoxic groundwater. 

Figure 4–5. Groundwater samples collected

from the Denver Basin aquifer system 

primarily were oxic (dissolved-oxygen 

concentration of at least 0.5 milligram 

per liter) at shallow depths and anoxic 

(dissolved-oxygen concentrations less than 

0.5 milligram per liter) at deeper depths.

the shallow system has resulted in the presence of dissolved 

oxygen deeper in the aquifer system. Excess irrigation 

water, heavy pumping, and the injection of oxic surface 

water during aquifer storage and recovery operations can 

accelerate this downward movement, which is very slow 

under natural conditions.

The solubility, transport, concentration, and chemical 

form of many water-quality constituents in groundwater are 

affected by pH. Some redox-sensitive trace elements, such as 

iron and manganese, are more soluble under low pH and (or) 

anoxic conditions. Other trace elements, such as arsenic and 

selenium, are more mobile when pH is higher. The formation 

of arsenic minerals, for example, is inhibited as groundwater 

pH increases. Most (94 percent) pH values in Denver Basin 

groundwater were between 6.5 and 8.5 (7.0 is neutral). Values 

of pH outside of this range can affect the concentration of 

many constituents.

The median concentration of uranium in oxic water 

was more than 100 times as high as the median 

concentration of uranium in anoxic water.

Chapter 4

18 to 124

>700

125 to 400

Percentage of oxic and anoxic samples

0 20 40 60 80 100

Oxic Anoxic

EXPLANATION

Redox conditions by well depth

Figure 4-5.

Well depth, in feet

below land surface 401 to 700

Arsenic

Selenium

Uranium

0

Median concentration, in micrograms per liter

Iron

1 2 43 5 0 10 20 30

<8 μg/L

<0.4 μg/L

<0.04 μg/L

<0.2 μg/L

Figure 4-4.

Contaminant concentrations in 

oxic and anoxic groundwater

Oxic

Anoxic

EXPLANATION

Chapter 4: Natural and Human Factors That Affect Groundwater Flow and Quality 35
How do redox reactions work? 

Reduction/oxidation (redox) processes require one chemical species that donates electrons and another chemical species that 

accepts those electrons. As a chemical species donates electrons it is “oxidized,” and as the other species accepts electrons it is 

“reduced.” Redox processes typically are facilitated by microbes (bacteria), which use the energy produced by the processes. In 

groundwater, organic carbon is the most common electron donor. If dissolved oxygen is present, it is the preferred electron acceptor, 

because reduction of dissolved oxygen produces more energy than reduction of other chemical species that commonly occur in 

groundwater. The atmosphere is the source of the dissolved oxygen, so the redox conditions in an aquifer near where recharge 

occurs usually are oxic (defined here as having a concentration of dissolved oxygen of at least 0.5 mg/L).

As groundwater moves through the aquifer along a flow path, the dissolved oxygen in the groundwater gradually is consumed by 

redox processes. Once all of the dissolved oxygen is consumed, other chemical species can accept electrons and become reduced. 

If nitrate is present, it will become the preferred electron acceptor until it in turn is completely consumed. This pattern continues, 

with manganese, iron, sulfate, and finally carbon dioxide acting as electron acceptors until they are consumed, in that order. This 

order of use of electron acceptors has important implications for the preservation, degradation, and even production of contaminants 

in groundwater. Because redox reactions occur in a sequence, it can take a long time for strongly reducing conditions to develop. 

For this reason, anoxic groundwater commonly is older than oxic groundwater, and, within the anoxic category, strongly reducing 

groundwater commonly is older than mildly reducing groundwater. 

From a water-quality perspective, denitrification—the reduction of nitrate to nitrogen gas—is one of the most important redox 

processes that occurs in groundwater. Nitrate is a concern for human health and, where it discharges to surface water, can impair 

aquatic communities. Conversion of nitrate by denitrification to harmless nitrogen gas, the same gas we breathe in the atmosphere, 

is the primary way that nitrate is removed from water.

Oxygen Nitrate Manganese Iron Sulfate Carbon dioxide

Oxic Anoxic

Oxic Mildly

reducing

Strongly

reducing

Very strongly

reducing

Order of consumption of electron acceptors

36 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05
Chapter 5: Quality of Groundwater 

Used for Drinking and Irrigation 

Chapter 5

Glass photograph copyright istockphoto.com;

irrigation photograph by Jon Mullen, copyright istockphoto.com

This chapter identifies

and discusses constituents

that were detected at a 

concentration greater 

than a human-health 

or non-health-based 

benchmark in water from 

drinking-water supply 

wells or irrigation wells 

tapping the Denver 

Basin aquifer system. 

Drinking water and irrigation are critical uses of groundwater from the 

Denver Basin aquifer system. Does the quality of the groundwater 

support these uses? What are the constituents that might present 

human-health concerns when the water is used for drinking? What other 

constituents might adversely affect use of the water? For the Denver Basin 

aquifer system, chemical constituents of concern for groundwater use originate 

from geologic sources (uranium, radon, arsenic, selenium, manganese, 

iron, fluoride, sulfate), human activities (nitrate, pesticides, VOCs), or a 

combination of both geologic and human sources (dissolved solids). 
The Quality of Groundwater Used for Drinking 

Generally Is Very Good

Only about 1 in every 10 Denver Basin wells used for 

drinking water yielded groundwater that had a concentration 

of a contaminant that exceeded a human-health benchmark. 

Concentrations of contaminants from geologic sources 

exceeded a human-health benchmark seven times more 

frequently than did concentrations of contaminants from 

human sources (fig. 5–1). Concentrations of manganese, 

radon, arsenic, uranium, and selenium—contaminants 

with geologic sources—each exceeded their human-health 

benchmark in only a small percentage (less than 5 percent) of 

drinking-water wells sampled (fig. 5–2; table 5–1). Concentrations of nitrate, which is most commonly associated with 

human activities, exceeded its human-health benchmark in 

only 1 percent of drinking-water wells. 

Figure 5 –1. Ten percent of wells in the Denver 

Basin used for drinking water had at least one 

contaminant with a concentration greater than 

a human-health benchmark. This was about 

one-half the frequency of exceedances for 

drinking-water wells that tap sandstone aquifers 

nationwide. In the Denver Basin and nationally, 

most exceedances of human-health benchmarks 

were for contaminants from geologic sources. 

In the Denver Basin and nationally, most 

exceedances of human-health benchmarks 

were for contaminants from geologic sources.

Figure 5– 2. Six constituents—five from geologic 

sources and one from primarily human sources— 

in Denver Basin groundwater could be a drinkingwater concern because concentrations in some 

wells used for drinking water exceeded humanhealth benchmarks. Exceedances for arsenic and 

selenium were slightly more common for Denver 

Basin groundwater than for other sandstone aquifers 

across the United States (appendix 3). *Shown here 

for the proposed Alternative Maximum Contaminant 

Level (MCL) of 4,000 picocuries per liter (pCi/L); 

radon values were greater than the proposed MCL 

of 300 pCi/L in 93 percent of samples.

Percentage of drinking-water wells with

 one or more contaminants at a concentration

greater than a human-health benchmark

All

Human

Geologic

0 5 10 15 20 25

All

Human

Geologic

Denver Basin bedrock aquifers

Other sandstone aquifers 

 assessed by NAWQA

Sources of contaminants in water 

used for drinking

Fig. 5–1

Percentage of drinking-water wells

with concentrations greater than

human-health benchmarks

Manganese

Radon*

Uranium

Nitrate

Arsenic

Selenium

0 5 10 15 20 25

Geologic sources

Figure 5-2.

Human sources

Contaminants from geologic and 

human sources with exceedances

of human-health benchmarks

All

38 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05
What is a contaminant? 

Contaminants have a wide range of sources, both manmade and geologic. Most organic chemicals in groundwater that are 

of concern for human health are manmade. In contrast, most inorganic constituents in groundwater have geologic or other natural 

sources, although their concentrations in groundwater may be altered by human activities, such as irrigation and groundwater 

pumping. Some contaminants have both manmade and natural sources. For example, nitrate in groundwater has many natural 

sources, but nitrate concentrations in groundwater underlying agricultural and urban areas commonly are higher than in other 

areas because of contributions from sources associated with human activities.

But what exactly is a contaminant? The word means different things to different people. For example, a contaminant is defined 

by the Safe Drinking Water Act (SDWA) as “any physical, chemical, biological, or radiological substance or matter in water” 

(see http://www.epw.senate.gov/sdwa.pdf). This broad definition of contaminant includes every substance that may be found 

dissolved or suspended in water— everything but the water molecule itself. This is not a very practical definition because this 

would imply that all water is “contaminated.” Pure water that has nothing dissolved in it does not occur naturally—not even rainfall 

is pure water, because it contains, at a minimum, some dissolved gases.

The U.S. Environmental Protection Agency defines a contaminant as “Any physical, chemical, biological, or radiological 

substance or matter that has an adverse effect on air, water, or soil” (see http://epa.gov/region04/superfund/qfinder/glossary/html). 

This definition is more practical and allows both manmade constituents and those with geologic sources in water to be defined 

as contaminants. However, it does not define what “adverse” means, and what may be adverse in one way might be beneficial 

in another. In this circular, a contaminant is defined as any physical, chemical, biological, or radiological substance or matter in 

groundwater that is manmade or that impairs the use of water for its intended purpose. Impairment is determined by comparing a 

measured concentration to benchmarks or guidelines. By this definition, all manmade compounds, such as pesticides and volatile 

organic compounds, are contaminants because they do not occur naturally in groundwater. If a constituent with a geologic 

source, such as arsenic, occurs in drinking water at a concentration above its human-health benchmark, it also is considered a 

contaminant.

Table 5–1. Only six constituents were measured in drinking-water wells at a concentration that exceeded a human-health benchmark.

[μg/L, microgram per liter; HBSL, Health-Based Screening Level; pCi/L, picocuries per liter; AMCL, alternative Maximum Contaminant Level; MCL, 

Maximum Contaminant Level; nitrate, nitrate plus nitrite as nitrogen; mg/L, milligram per liter; N, nitrogen]

Constituent

Human-health benchmark*

Number of

wells sampled

Percentage of sampled wells with

concentrations greater than 

Value and unit Type Benchmark One-tenth of

benchmark

Contaminants from geologic sources

Manganese 300 μg/L HBSL 90 4.4 28

Radon 4,000 pCi/L Proposed AMCL58†

3.4 78

(300 pCi/L) (Proposed MCL) 93 100

Arsenic 10 μg/L MCL 90 1.1 27

Uranium 30 μg/L MCL 90 1.1 13

Selenium 50 μg/L MCL 90 1.1 8.9

Contaminant from human sources

Nitrate 10 mg/L as N MCL 90 1.1 11

*Proposed radon regulations from U.S. Environmental Protection Agency,(76) HBSLs from Toccalino and others,(67) and MCLs from 

U.S. Environmental Protection Agency.(64)

†Drinking-water wells from the Dawson and Arapahoe aquifers. Samples from the alluvial and shallow bedrock aquifers, drinking-water 

wells in the Denver and Laramie–Fox Hills aquifers, and samples from most public-supply wells were not analyzed for radon. 

Chapter 5

Chapter 5: Quality of Groundwater Used for Drinking and Irrigation 39
“Nuisance” constituents in groundwater 

used for drinking water might impair taste, 

odor, and other qualities

“Nuisance” constituents—those that can affect 

the use of water because they impair the taste or odor 

of drinking water or cause other non-health-based 

concerns—were more common in drinking-water wells 

than were those constituents that can affect human 

health. The SMCL for at least one nuisance constituent 

was exceeded in 42 percent of drinking-water wells 

sampled. An SMCL was exceeded most frequently 

for manganese and dissolved solids, and somewhat 

less frequently for iron, fluoride, and sulfate (fig. 5 –3; 

table 5–2). Percentages of exceedances were similar 

for public-supply wells (40 percent of samples) and 

domestic wells (43 percent of samples). 

Figure 5–3. Secondary 

Maximum Contaminant 

Levels were exceeded 

more commonly for 

samples from drinkingwater wells than were 

human-health benchmarks (shown in 

figure 5–1).

Manganese

Dissolved

solids

Iron

Fluoride

Sulfate

All

Percentage of drinking-water wells

with concentrations greater than SMCLs

0 10 20 30

Nuisance constituents with 

exceedances of SMCLs

Figure 5-3.

40 50

Human-health benchmarks and other guidelines used in this assessment 

Concentrations of constituents measured for this assessment were compared to human-health benchmarks to place study 

findings in the context of human health. The benchmarks are threshold concentrations in water above which the concentration of a 

contaminant in drinking water could adversely affect human health. Human-health benchmarks were available for about two-thirds of 

the 292 constituents and properties measured for this assessment (appendix 2). Two types of human-health benchmarks were used: 

U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Levels (MCLs) (http://water.epa.gov/drink/contaminants/index.cfm)

and U.S. Geological Survey (USGS) Health-Based Screening Levels (HBSLs). MCLs are legally enforceable drinking-water standards 

that specify the maximum permissible level of a constituent in water that is delivered to any user of a public water system(64) (values 

used in this report were current as of February 2012). Although MCLs are used to regulate the quality of drinking water only from publicsupply sources, they also are useful for evaluating the quality of water from domestic and monitoring wells. An MCL was available for 

53 of the constituents measured. For some constituents for which an MCL has not been established, the USGS, in collaboration with the 

USEPA and others, developed non-enforceable HBSLs by using standard USEPA methods for establishing drinking-water guidelines and 

current toxicity information(65–67) (values used in this report were current as of February 2012; see http://water.usgs.gov/nawqa/HBSL). 

An HBSL was available for 135 constituents measured. Radon has neither an MCL nor an HBSL, but two MCLs have been proposed. 

Copper and lead have USEPA action levels rather than an MCL.

In addition to human-health benchmarks, non-health-based guidelines—Secondary Maximum Contaminant Levels (SMCLs)—

were available for some of the constituents measured in this assessment. The SMCLs are non-enforceable guidelines for concentrations of “nuisance” constituents in drinking water that can cause unwanted cosmetic effects, such as skin or tooth discoloration; 

aesthetic effects, such as unpleasant taste, odor, or color; or technical effects, such as corrosion or sedimentation of plumbing or 

reduced effectiveness of water treatment.(68) 

Concentrations greater than one-tenth of a human-health benchmark were used in this assessment to indicate which contaminants 

occurred, either individually or as mixtures, at concentrations that approach those of potential concern for human health, and to identify 

contaminants that might warrant additional monitoring and study. The criterion of one-tenth of a benchmark is consistent with various 

State and Federal practices for reporting contaminant occurrence in groundwater and for identifying contaminants of potential humanhealth concern (for example, see U.S. Environmental Protection Agency;(69) New Jersey Department of Environmental Protection(70)).

Screening-level assessments, such as this one, provide perspective on the potential relevance of detected contaminants to 

human health and can help in planning future studies.(66) They are not designed to evaluate specific effects of contaminants on human 

health and are not a substitute for comprehensive risk assessments. It is important to note that occurrence of a contaminant at a 

concentration greater than its benchmark does not mean that adverse effects are certain to occur, because the benchmarks are 

conservative (protective) and source-water samples were collected prior to any treatment or blending that could alter contaminant 

concentrations in finished drinking water. There are water-treatment options, such as charcoal filtration, that can be used to lower 

the concentration of the contaminant to below the benchmark before the water is consumed.

40 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05
Why is it important for domestic well owners to understand concentrations 

of contaminants from geologic and human sources?

Seventy-five of the drinking-water wells sampled in 

the Denver Basin were domestic wells. Almost all (eight 

of the nine) samples with at least one exceedance of 

a human-health benchmark were from domestic wells. 

Samples from the eight domestic wells had at least one 

exceedance for contaminants from geologic sources, and 

one also had an exceedance for a contaminant (nitrate) 

from human sources. Unlike public-supply wells, routine 

testing of the quality of water from domestic wells is not 

required, and domestic well owners rarely test for these 

contaminants. Domestic well owners in the Denver Basin 

should be aware of the potential occurrence of contaminants in their well water, especially those from geologic 

sources, and the potential health risks if contaminants 

are present at concentrations greater than human-health 

benchmarks. Information on water testing for domestic 

wells owners is available from the Colorado Department 

of Public Health and Environment at http://www.colorado.

gov/cs/Satellite/CDPHE-Lab/CBON/1251594535218.

Water that contains iron, a 

nuisance constituent, can have 

an unpleasant metallic taste 

and can cause reddish-brown 

staining of plumbing fixtures, 

appliances, and laundry. 

Treatment techniques, including 

water conditioning and filtration, 

can remove iron from water.

Table 5 –2. Manganese, dissolved solids, and other constituents from 

geologic sources exceeded values recommended for drinking water for 

quality reasons other than human health.

[μg/L, microgram per liter; mg/L, milligram per liter. All non-health guidelines are 

Secondary Maximum Contaminant Levels from the U.S. Environmental Protection 

Agency.(68) Ninety wells were sampled for each constituent]

Constituent Non-health 

guideline

Percentage of 

drinking-water wells 

with concentrations

greater than

the guideline

Manganese 50 μg/L 20

Dissolved solids 500 mg/L 19

Iron 300 μg/L 12

Fluoride 2.0 mg/L 11

Sulfate 250 mg/L 11

Photograph by Gerardo Alvarez, copyright istockphoto.com

Photograph courtesy of Culligan

Chapter 5

Chapter 5: Quality of Groundwater Used for Drinking and Irrigation 41
Exceedances of benchmarks were common in 

the shallow aquifer system, which could have 

implications for water use and the quality of the 

water in the deeper system used for drinking

Water from wells tapping the shallow aquifer system 

underlying agricultural and urban areas is less potable 

than water from deeper wells underlying mixed land uses 

(figs. 5– 4, 5–5). Shallow groundwater is more strongly 

influenced than deeper groundwater by the release of 

constituents to water from the interaction of recharge water 

with minerals in soils and aquifer sediments, by the release 

of trace elements to groundwater under oxidizing conditions, 

and by short travel times for contaminants from human 

sources to reach the water table. Although water from the 

shallow system typically is not used for drinking, any potential 

future use of this groundwater for drinking might require 

treatment or blending with high-quality water from other 

sources.(28) The deeper drinking-water wells are more isolated 

from reactions of recharge water with subsurface materials and 

from potential sources of contamination at land surface than 

are the shallow monitoring wells. But because of the increase 

in the downward movement of water with excess recharge 

from irrigation and high-volume pumping for public supply, 

the water quality of the shallow system can eventually affect 

the water quality of the deep system. 

Almost 60 percent or more of samples from monitoring 

wells in the alluvial aquifer and shallow parts of the Dawson 

and Denver aquifers had concentrations of nuisance constituents (chloride, dissolved solids, fluoride, iron, manganese, and 

sulfate) that exceeded SMCLs (fig. 5–5), which could affect 

use of the shallow groundwater. The most common exceedances for the shallow groundwater were for dissolved solids, 

sulfate, and manganese. Exceedances for manganese and 

dissolved solids also were most common for the deep groundwater (table 5 –2). Fluoride was the only constituent with a 

low exceedance rate (about 2 percent of wells) of its SMCL 

for shallow groundwater and a more common exceedance rate 

(about 11 percent of wells) for deep groundwater.

Figure 5–5. The alluvial aquifer and 

shallow parts of the Dawson and Denver 

aquifers have poorer water quality 

relative to benchmarks than do the 

deeper aquifers used for drinking-water 

supply. Exceedances of SMCLs for 

nuisance constituents (chloride, dissolved 

solids, fluoride, iron, manganese, 

and sulfate) in all aquifers are much 

more common than are exceedances 

for constituents with human-health 

benchmarks (arsenic, manganese, 

nitrate, selenium, and uranium).

Figure 5–4. Exceedances of humanhealth benchmarks for five contaminants 

were common in samples from shallow 

monitoring wells in agricultural and 

urban areas but were rare in samples 

from deeper Denver Basin wells used 

for drinking water. Human sources of 

contaminants at or near the land surface 

and geochemical reactions between 

infiltrating water and soluble minerals in 

the unsaturated zone both can contribute 

contaminants to shallow groundwater but 

not to deeper groundwater.

Uranium

Manganese

Selenium

Arsenic

Percentage of wells tapping the Denver Basin aquifer system

with concentrations greater than human-health benchmark

0 20 40 60 80 100

Nitrate

Contaminants from geologic and human sources with exceedances 

of human-health benchmarks

Contaminant from human sources

Contaminants from geologic sources

Monitoring wells in the 

 shallow aquifer system

Deep bedrock wells 

 used for drinking water

EXPLANATION

Figure 5–4.

Percentage of wells tapping the Denver Basin aquifer system

with concentrations greater than indicated benchmark

0 20 40 60 80 100

Dawson

Denver

Arapahoe

Laramie–Fox Hills

Alluvial

Dawson

Denver

Human-health benchmark

Secondary Maximum Contaminant Level (SMCL)

Aquifers with exceedances of benchmarks 

EXPLANATION

Figure 5-5.

Monitoring wells in the 

shallow aquifer system

Deep bedrock wells used

for drinking water

42 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05
Table 5–3. Selected constituents of potential concern for 

agricultural irrigation.

[mg/L, milligram per liter; —, not applicable; <, less than; >, greater 

than. General water-quality criteria values are rounded values from 

milliequivalents per liter. Criteria values from Ayers and Wescot (29)]

Potential

irrigation problem

General 

water-quality 

criteria

Percent criteria

exceedance for

South Platte River

alluvial aquifer

underlying

irrigated cropland

Salinity hazard: Decrease in crop productivity and yield,

reduction in water infiltration

Dissolved solids concentration mg/L —

None < 450 0

Slight to moderate 450– 2,000 70

Severe >2,000 30

Excessive bicarbonate: Unsightly deposits on fruits

 or leaves from overhead sprinklers

Bicarbonate concentration mg/L —

None < 90 0

Slight to moderate 90 –520 97

Severe >520 3

Boron toxicity: Toxic to certain crops, can cause

crop damage and reduce crop yield

Boron concentration mg/L —

None < 700 100

Slight to moderate 700–3,000 0

Severe >3,000 0

Chloride toxicity: Toxic to certain crops, can cause

crop damage and reduce crop yield

Chloride concentration mg/L —

None < 140 83

Slight to moderate 140 –350 17

Severe >350 0

Sodium toxicity: Toxic to certain crops, can cause

crop damage and reduce crop yield

Sodium concentration mg/L —

None < 70 7

Slight to moderate 70–210 56

Severe >210 37

Quality of water in the alluvial aquifer and 

potential implications for irrigation 

Pumping of water from the alluvial aquifer for irrigation 

of crops has been the greatest use of shallow groundwater in 

northeastern Colorado, including parts of the Denver Basin area. 

The quality of irrigation water can have beneficial or adverse 

effects on crops, depending on the particular constituents in the 

water. Use of poor-quality irrigation water in agricultural areas 

can result in decreased crop production, reduction in water 

infiltration, scaling of some irrigation systems, ion toxicity 

(table 5 –3), and excessive nutrients.(29, 30) Measurements to 

evaluate the quality of irrigation water include the total salt 

content and concentrations of specific constituents in the water.

In Colorado, excessive amounts of salts in many areas 

of irrigated agriculture are an ongoing concern. Potential 

implications from use of Denver Basin alluvial groundwater 

for irrigation can be evaluated by assessing the quality of 

water withdrawn from monitoring wells tapping the alluvial 

aquifer in irrigated crop areas of the South Platte River Basin 

(see sidebar, Groundwater-quality data from the NAWQA 

study of the South Platte River Basin, p. 44). On the basis of 

data for shallow monitoring wells sampled in 1994, salinity 

hazard and sodium toxicity are potential concerns for farmers 

irrigating their crops with groundwater pumped from the 

alluvial aquifer (table 5–3). Water from the monitoring wells 

was not used for irrigation but can be used to assess potential 

implications if the water was used for crop irrigation. In 

some areas of northeastern Colorado, water-use practices 

have changed because of the poor quality of the alluvial 

groundwater (see sidebar, The quality of shallow groundwater 

in agricultural areas has changed water-use practices, below). 

Chapter 5

The quality of shallow groundwater in agricultural areas has changed water-use practices

• In far northeastern Colorado, crops that are sensitive to salinity, such as dry beans, are no longer grown. Crop production 

has changed to crops that are more tolerant of saline soils and water, such as corn (T.A. Bauder, Colorado State University 

Extension, oral commun., 2013). 

• Different sources of water are used on agricultural fields depending on the time of the year. Early in the growing season when crops 

are most sensitive to saline soils and water, high-quality water from surface-water supplies is used for irrigation. Use of saline 

groundwater for irrigation increases later in the growing season when plants are more developed and less sensitive to salinity.

• The city of Brighton, Colo., (fig. 2–1), is in a highly productive agricultural area of the Denver Basin. Because of fertilizer used 

in farming, high concentrations of nitrate can be found in the city’s alluvial groundwater supply. In 1993, the city constructed a 

Reverse Osmosis Water Treatment Plant to reduce the amount of nitrate in the supply water because of health concerns.(71)

Water with a low salt content is used to irrigate 

crops early in the growing season. As plants grow 

and become less sensitive to salinity, water with a 

greater amount of salt can be used for irrigation.

Chapter 5: Quality of Groundwater Used for Drinking and Irrigation 43
Groundwater-quality data from the NAWQA study of the South Platte River Basin 

Water quality of the South Platte River Basin, which partially overlies the Denver 

Basin bedrock aquifers but also extends north and east into western Nebraska, 

was assessed during 1992–95 as part of the NAWQA Program.(32, 41) Results from 

two studies of water quality of the alluvial aquifer underlying agricultural and urban 

areas of the South Platte River Basin are included in this circular to supplement 

water-quality data for the Denver Basin. The two study areas for the South Platte 

assessment were (1) the primary irrigated agricultural areas (corn was the major 

crop in 1994) that overlie the alluvial aquifer along the South Platte River downstream 

from Denver to the mouth of the basin in North Platte, Nebraska, and (2) the older 

urban area of Denver and nearby suburbs that overlie the alluvial aquifer along major 

tributaries to the South Platte River.

 

Groundwater-quality studies in the South Platte River Basin in 1993–94 and the Denver Basin 

in 2003–05 both included land-use studies in agricultural and urban areas. Assessing the 

quality of groundwater that underlies different types of land uses (irrigated corn and nonirrigated wheat in agricultural areas, older urban development and suburban areas developed 

primarily after about 1970) contributes to the understanding of groundwater resources in the 

Denver Basin. The quality of groundwater in the South Platte River Basin and Denver Basin 

are shown in a national context in appendix 4.

South Platte

COLORADO River

WYOMING NEBRASKA

Parker

Greeley

Cheyenne

Henderson

Denver

Castle Rock

Fort

Morgan

North

Platte

25

80

80

76

70

EXPLANATION

Groundwater-quality study area

South Platte study (1994), groundwater

 underlying irrigated agriculture

South Platte study (1993), groundwater

 underlying urban area

Denver Basin study (2003), groundwater

 underlying non-irrigated agriculture

Denver Basin study (2003), groundwater

 underlying urban area

Denver Basin boundary

Base from U.S. Geological Survey digital data, 2012

101°

104°

105°

41°

40°

103°

39°

102°

South Platte River Basin

0 25 50 MILES

0 50 KILOMETERS 25

CH 5-SP.map.sidebar 

Map

area

science for a changing world

Modified from Bruce and McMahon (41)

44 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05
Chapter 6: Understanding Where and Why 

Key Contaminants Occur in Groundwater

This chapter describes 

the sources of and natural

and human factors that 

affect the occurrence and 

distribution of uranium, 

radon, arsenic, selenium, 

manganese, dissolved 

solids, nitrate, pesticide 

compounds, and VOCs in

Denver Basin groundwater.

Source of contaminants to Denver Basin groundwater 

include geologic materials and human activities in 

agricultural and urban areas. Background, outcrop of 

the Dawson Arkose. Inset, Castle Rock, Colo., a growing 

bedroom community south of Denver, relies primarily on 

groundwater for water supply.

Inset photograph by Steve Krull, copyright istockphoto.com

Chapter 6

The quality of Denver Basin groundwater used for drinking is generally 

good, with very few of the samples collected containing a contaminant 

at a concentration of concern for human health. The quality of shallow 

groundwater, however, is quite different— the water is poor quality, with high 

concentrations of several contaminants making it unsuitable for drinking 

without costly treatment. Pumping and irrigation have changed the hydrology 

of the Denver Basin, increasing the downward movement of water from 

shallow to deep parts of the aquifer system. This change raises a number of 

questions about Denver Basin groundwater quality. Will human alteration 

of the hydrology impair the quality of the groundwater in the deep bedrock 

aquifers used for drinking water? Is degradation of the drinking-water resource 

already present in some parts of the aquifer system and, if so, where? If not, 

how soon might such degradation occur and how might such degradation be 

prevented? Are there geochemical processes that will prevent contamination in 

shallow groundwater from reaching the deeper groundwater used for drinking? 

Answering these questions may be critical for water-resource managers who 

need to forecast how changes in water and land use will affect groundwater 

quality in the Denver Basin and similar basins along the Front Range. 
Constituents from Geologic Sources: Uranium and Radon 

Concentrations of uranium in drinking-water samples rarely exceeded the MCL of 

30 μg/L, but uranium concentrations in one of every four samples of shallow groundwater were greater than the MCL. Two factors control concentrations of uranium in 

Denver Basin groundwater: (1) whether the soils and aquifer sediments contain uranium 

and (2) whether the groundwater contains dissolved oxygen. Irrigation in agricultural 

and urban areas has increased the volume of oxic recharge water that can react with 

shallow uranium-rich materials in the subsurface, leading to elevated concentrations 

of uranium in shallow groundwater. Pumping can move oxic recharge water deeper into the

aquifer system, potentially releasing uranium in aquifers tapped by drinking-water wells. 

Consumption of drinking water with high concentrations of uranium can pose a risk to 

human health. Radon, produced by the radioactive decay of uranium, was measured only 

in samples from drinking-water wells in the Dawson and Arapahoe aquifers—concentrations rarely exceeded the proposed Alternative Maximum Contaminant Level (AMCL) 

of 4,000 pCi/L but exceeded the proposed MCL of 300 pCi/L in almost every sample. The 

use of well water that contains radon, a carcinogen, can be a source of radon detected 

in indoor air. 

Aquifer sediments and rocks are the sources of uranium, but human activities

have increased concentrations of uranium in shallow groundwater 

The reaction between uranium-rich aquifer sediments and oxic recharge water causes 

uranium, a radionuclide, to be released into groundwater. As a result, high concentrations 

(greater than 30 μg/L) in the Denver Basin aquifer system were detected about 25 times more 

frequently in samples of shallow groundwater than in samples from deeper groundwater used for 

drinking-water supply (fig. 6 –1). Exceedances of the MCL for uranium of 30 μg/L were found 

only in samples from monitoring wells in the alluvial and Dawson aquifers and one relatively 

shallow drinking-water well, also tapping the Dawson aquifer. Samples from the shallow aquifer 

system contained concentrations of dissolved uranium as high as 941 μg/L (the maximum 

concentration measured in all Denver Basin groundwater samples), and almost all of the samples 

were oxic. In contrast, concentrations of uranium in groundwater samples collected from deeper 

parts of the aquifer system typically were low and only one contained uranium at a concentration 

exceeding the MCL; most of these samples were anoxic. High concentrations of uranium in 

drinking water can lead to increased risk of cancer and kidney toxicity in humans.(31) 

Granitic rock eroded from the Rocky Mountains, volcanic ash, claystone, and shale are the main 

sources of uranium in Denver Basin groundwater. Materials rich in uranium make up parts of the 

alluvial, Dawson, and Arapaho aquifers, and, to a lesser extent, the Denver aquifer (fig. 6 –2).(32, 33)

Figure 6–1. Concentrations 

of dissolved uranium in 

Denver Basin groundwater 

were greatest in samples 

from monitoring wells in the 

alluvial and shallow bedrock 

aquifers. Only samples from 

the alluvial and Dawson 

aquifers had concentrations 

that exceeded the MCL for 

uranium of 30 μg/L.

Uranium is a radioactive 

element found in aquifer 

materials. Drinking water 

with uranium at levels 

above the MCL has 

been associated with 

increased risk of cancer 

and kidney toxicity.

0 20 40 60 80 100

2,500

2,000

1,500

1,000

500

USEPA MCL (30 μg/L)

0

Depth of well, in feet below land surface

Concentration, in micrograms per liter

1,000

Uranium in Denver Basin aquifers

Figure 6-1.

10 30 50 70 90

EXPLANATION

Alluvial

Aquifer

Dawson

Denver

Arapahoe

Laramie–Fox Hills

Monitoring wells in the 

shallow aquifer system

Deep bedrock wells used

for drinking water

46 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05
Boxplots 

Boxplots are used to illustrate how results are distributed within a 

group. The “box” ranges from the 25th to the 75th percentile and represents 

50 percent of the data. The horizontal line in the middle of the box is the median 

value—one-half of the 

values in the group are 

greater than the median 

and one-half are less. 

Percentiles describe 

the percentage of values 

in a group that are less 

than the given value: 

25 percent of the values in 

a group are less than the 

25th percentile; 75 percent 

of the values in a group are 

less than the 75th percentile. The median is also the 

50th percentile.

If, for example, the 

75th percentile for the 

measured concentration 

of a contaminant in a group of wells is equal to the human-health benchmark 

for that contaminant, then 75 percent, or three-fourths, of the wells have a 

concentration of that contaminant less than the benchmark, and 25 percent, or 

one-fourth, have a concentration greater than the benchmark.

The “whiskers” (vertical lines) in these figures extend to the 10th and 

90th percentiles; box and whiskers together represent 80 percent of the data. 

Values greater than the 90th or less than the 10th percentiles are shown as 

individual points (outliers).

EXPLANATION

25th percentile

10th percentile

75th percentile

50th percentile

 (median)

90th percentile

Greater than 

 90th percentile

Less than 

 10th percentile

50 percent of data

fall within the box 

80 percent of data fall 

 between endpoints of whiskers 

Pikes Peak photograph by Steve Krull; granite photograph by alphtran, both copyright istockphoto.com

Figure 6–2. Uranium-rich 

minerals in the sediments that 

make up the alluvial and Dawson 

aquifers release uranium to 

shallow groundwater. Sediments 

that make up the Denver aquifer 

contain less uranium, and 

concentrations in groundwater 

are lower than those in the 

alluvial and Dawson aquifers.

Chapter 6

Alluvial and shallow

bedrock aquifers

Alluvial Dawson Denver

0.1

1.0

10

1,000

100

Concentration, in micrograms per liter 

Uranium in shallow groundwater

Figure 6-2.

USEPA MCL

(30 μg/L)

See sidebar, Boxplots, at right

Chapter 6: Understanding Where and Why Key Contaminants Occur in Groundwater 47

Sediments from the 

Pikes Peak granite that 

have been weathering 

for millions of years 

are one source of 

uranium and radon 

in the Denver Basin 

aquifer system. 
Uranium is most soluble in water that contains dissolved oxygen. At shallow depths, oxic

water releases uranium from the aquifer material. Where water is anoxic, uranium adheres to 

mineral surfaces or forms insoluble minerals, removing the uranium from groundwater (fig. 6 –3). 

Recharge to groundwater has increased since development of water resources for 

irrigation began in earnest around 1925. The amount of recharge in the basin is greatest in 

irrigated cropland (10 in/yr), intermediate in urban and residential areas (2.5 in/yr), and lowest 

in non-irrigated areas (0.1 to 1.3 in/yr).(2) Irrigation delivers a large volume of oxic water to 

uranium-rich sediments in the subsurface. In the alluvial aquifer, median uranium concentrations and related MCL exceedances were highest for groundwater underlying irrigated 

cropland and lowest for groundwater underlying non-irrigated wheat fields (fig. 6 – 4). The 

lithology of aquifer materials also affects uranium concentrations. Monitoring wells in the 

irrigated cropland and irrigated urban areas tap the alluvial and Dawson aquifers, which are 

rich in uranium-containing minerals, whereas monitoring wells in the non-irrigated agricultural 

area tap the Denver aquifer, which contains less uranium (fig. 6 –2).(32, 33) Both the amount of 

uranium in soils and aquifer sediments and oxic recharge control uranium concentrations in 

Denver Basin groundwater.

Figure 6–3. Uranium was detected most commonly 

in samples of shallow groundwater that were oxic. 

As water moves deeper in the aquifer system, conditions

become more anoxic, and dissolved uranium can 

adsorb to aquifer sediments or precipitate as insoluble 

uranium minerals and become undetectable in a water 

sample. [n=number of samples]

Figure 6–4. Irrigation of agricultural 

and urban areas provides oxic 

recharge water to the alluvial 

aquifer, which can cause uranium in 

sediments that make up the aquifer 

to be released to groundwater. Less 

uranium is found in groundwater 

beneath agricultural areas that are 

not irrigated.

Uranium detected

Uranium not detected

EXPLANATION

Figure 6.3.

Oxic

Anoxic

Uranium Redox condition

Well depth

(feet below 

and surface)

18 to 124

(n=60)

125 to 400

(n=26)

401 to 700

(n=39)

>700

(n=25)

Percentage of samples

Water and land use

0.1

1.0

10

1,000

100

Concentration, in micrograms per liter

Irrigated

agriculture

Nonirrigated

agriculture

Irrigated

urban

Figure 6-4.

Uranium in the alluvial aquifer

See sidebar, Boxplots, p. 47

USEPA MCL

(30 μg/L) 

48 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05
A cautionary tale: Irrigation and pumping have increased uranium concentrations in 

groundwater in the Central Valley of California

Elevated concentrations of uranium in Denver Basin groundwater are not unique in the western United States. The 

effects of development in agricultural and urban areas on the presence of uranium in groundwater are illustrated in the 

Central Valley of California,(7) one of the most productive agricultural areas in the world. In the eastern San Joaquin Valley, 

which makes up the southern two-thirds of the Central Valley, irrigation and pumping during 100 years of agricultural and 

urban development have changed the chemistry and magnitude of recharge water and increased the downward flow of 

water in the basin-fill aquifer system.(49) High-alkalinity (primarily bicarbonate ions) water resulting from irrigation recharge 

has reacted with sediments in the shallow part of the basin-fill aquifer to dissolve uranium into the groundwater—a process 

that has increased uranium concentrations in shallow groundwater since agricultural activities began. Additional downward 

movement of water from pumping and irrigation has caused young shallow groundwater with high alkalinity and uranium 

concentrations to migrate to deeper parts of the aquifer system tapped by public-supply wells.(49) This downward movement 

and additional dissolution of uranium likely will increase uranium concentrations in shallow and deeper public-supply 

wells in the future. Such increases could potentially occur in the Denver Basin and other semiarid to arid regions that have 

similar alkalinity and uranium occurrence, irrigation recharge, and groundwater pumping as those found in the eastern 

San Joaquin Valley. 

Chapter 6

In the eastern San Joaquin Valley near Modesto, Calif., concentrations of alkalinity and uranium were highest in shallow

groundwater, which has been altered geochemically by irrigation

in agricultural and urban areas.(7) Groundwater in the deepest 

part of the aquifer system represents water recharged under 

more natural conditions,(72) and alkalinity and uranium are low. 

At intermediate depths, shallow groundwater has been drawn 

downward by pumping and irrigation, and alkalinity and uranium 

concentrations are characteristic of a mixture of both shallow 

water and deep groundwater. Elevated concentrations of dissolved 

uranium have been detected in a public-supply well near Modesto, 

Calif., and in other public-supply wells in the San Joaquin Valley 

because the deep wells draw in uranium-rich shallow groundwater.

Uranium in groundwater in the 

San Joaquin Valley of California

USEPA MCL (30 μg/L)

0

20

50

40

Concentration, in micrograms per liter 10

30

Alkalinity, in milligrams per liter

as calcium carbonate

EXPLANATION

Monitoring well

Public-supply well

Shallow

Deep

Intermediate

0 100 200 300 400 500

CH. 6-ur.CentralValley.sidebar

Modified from Eberts and others(7)

Photograph from NRCS, California

Chapter 6: Understanding Where and Why Key Contaminants Occur in Groundwater 49
Radon, a natural decay product of uranium, is found at high concentrations 

in Denver Basin groundwater 

Radon was measured only in samples from drinking-water wells in the Dawson and 

Arapahoe aquifers. Only 3 percent of those samples had concentrations that exceeded the 

proposed AMCL of 4,000 pCi/L, but almost all of the samples (93 percent) had concentrations 

that exceeded the lower proposed MCL of 300 pCi/L (table 5 –1). Radon was not measured in 

samples from the drinking-water wells in the Denver and Laramie–Fox Hills aquifers or the 

shallow monitoring wells in areas of agricultural and urban land use.

Radon, a radionuclide, is found naturally in rocks, soil, water, and air as a dissolved gas 

and results from the radioactive decay of uranium and radium. Concentrations of radon in 

groundwater depend on the uranium and radium content of soils and aquifer sediments, aquifer 

properties such as porosity, and the proximity of radon to its source materials.(34) Radon decays 

rapidly in groundwater, and high radon concentrations are not likely to be found in groundwater 

that is relatively distant from the natural radioactive source.(35) Because radon is a gas, it can 

dissolve into groundwater and be released into the air of a home when water is used (see 

sidebar, Facts about radon, p. 51). The primary health risk from radon in water is the inhalation 

of radon gas, which can cause lung cancer.

Radon concentrations in water from wells in the Dawson aquifer were substantially 

greater than those for Arapahoe aquifer wells and for alluvial wells from the South Platte 

River Basin study; the median concentration for the Dawson aquifer was about two or three 

times more than that for the alluvial and Arapahoe aquifer wells, respectively (fig. 6 –5). High 

concentrations of radon in Dawson aquifer wells are associated with uranium-bearing granitic 

sediments in the aquifer.

Figure 6–5. Concentrations of radon in samples from drinkingwater wells in the Dawson aquifer were greater than in those 

from the Arapahoe aquifer and from monitoring wells in the 

alluvial aquifer sampled in 1993 during the South Platte River 

Basin study. Exceedances of the proposed MCL for almost all 

samples indicate that radon concentrations are of potential 

concern for human health.

Radon is an element 

that results from the 

radioactive decay of 

uranium in aquifer 

materials. Radon 

in drinking water is 

associated with lung 

cancer and stomach 

cancer.

Concentration, in picocuries per liter 

0

1,000

5,000

4,000

3,000

25,000

2,000

Dawson

Arapahoe

30,000

Alluvial

Aquifer and study

Denver Basin South Platte

River Basin

Radon in selected Denver Basin 

aquifers

Figure 6-5.

See sidebar, Boxplots, p. 47

USEPA proposed MCL

 (300 pCi/L) 

USEPA proposed AMCL

 (4,000 pCi/L)

50 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05
Facts about radon

Why is radon a concern?

Radon is the second-leading cause of lung cancer in the United States after smoking. High concentrations of radon in the 

air have been linked to an estimated 20,000 lung cancer deaths in the United States each year.(73, 74) Ingestion of radon through 

drinking water contributes to about 168 cancer deaths 

each year.(74, 75)

Are there human-health benchmarks for radon?

Because of the radiation emitted by radon and the 

associated human-health risks, two benchmarks have 

been proposed by the U.S. Environmental Protection 

Agency (USEPA) to limit the acceptable amount of radon 

in drinking water. The proposed MCL of 300 pCi/L and 

proposed AMCL of 4,000 pCi/L provide two options for 

the amount of radon that will be allowed in community 

drinking-water resources.(74) The lower proposed MCL for 

radon applies to States and public water systems that do 

not develop programs to address health risks from radon 

in indoor air, whereas the higher proposed AMCL applies 

to States and public water systems that have established 

such programs.(76)

Where can I find more information on radon?

• Colorado Department of Public Health and Environment: Radon outreach, http://www.colorado.

gov/cs/Satellite/CDPHE-HM/CBON/1251617274212

(Hazardous Materials and Waste Management 

Division, Radiation Program)

• USEPA: Radon information, including “A Citizen’s 

Guide to Radon,” http://www.epa.gov/radon/

• USGS: The geology of radon, http://energy.cr.

usgs.gov/radon/georadon/1.html

Chapter 6

Radon and Colorado homeowners

The Colorado Department of Public Health and Environment (CDPHE) has estimated that about 50 percent of homes in 

Colorado have indoor radon gas levels that exceed the USEPA recommended action level of 4 pCi/L, a non-enforceable health 

advisory.(78) In the Denver Basin, indoor air radon levels were tested in more than 120,000 homes and businesses from 1995 

through 2011 (data by zip code, provided by Chrys Kelley, CDPHE). The median concentration for all tests was about 3.6 pCi/L. 

About 46 percent of the concentrations were greater than 4.0 pCi/L. The CDPHE recommends that all homes in Colorado be tested 

for the presence of indoor air radon gas and that homeowners in Colorado using a domestic well test their well water for radon if 

the concentration of radon gas in indoor air is greater than the action level. The CDPHE Web site(78) addresses radon issues in the 

State and provides information on radon risks and radon test kits.

Groundwater

 use

Ways for radon to enter a home

Ch6, radon sidebar

Water table

Loosefitting 

pipe Floor

drain

Crack in

foundation

Modified from Otton and others (77) 

Two sources of radon gas in a home are groundwater use and soil 

gas. The red dots represent radon. Red arrows represent the movement of radon dissolved in groundwater toward and in the well, 

and the yellow arrows represent upward diffusion of radon gas 

from soil, rock, and groundwater beneath the house. 

Chapter 6: Understanding Where and Why Key Contaminants Occur in Groundwater 51
Other Constituents from Geologic Sources: Arsenic, Selenium, and Manganese

Similar to uranium, exceedances of human-health benchmarks for arsenic, 

selenium, and manganese were more common in samples from alluvial and shallow 

bedrock wells than in those from drinking-water wells. Concentrations of these three 

trace elements in Denver Basin groundwater depend in part on the composition of 

geologic source material, redox conditions, and pH of groundwater. Arsenic and 

selenium in Denver Basin sediments dissolve more readily with recharge of oxic 

groundwater. In contrast, manganese in Denver Basin sediments dissolves more 

readily into anoxic groundwater, particularly if the groundwater has low pH, but 

also can dissolve in oxic groundwater. High concentrations of arsenic, selenium, and 

manganese in drinking water can adversely affect human health. 

Concentrations of dissolved arsenic, selenium, and manganese that exceeded their 

respective human-health benchmark primarily were detected in groundwater from wells that 

tap the alluvial aquifer and (or) shallow parts of the Dawson and Denver aquifers but were 

rare in groundwater from drinking-water wells (fig. 6 – 6). There was only one exceedance of 

benchmarks for arsenic and selenium and only four exceedances for manganese in wells used 

for drinking water, and all of these wells were in the Dawson or Arapahoe aquifers.

At high concentrations, all three trace elements can affect human and aquatic health and 

(or) water use. Arsenic is a known human carcinogen and can harm the gastrointestinal, cardiovascular, and nervous systems.(36) Selenium has been linked to circulatory system dysfunction in 

humans, and skeletal deformities, decreased reproduction, and mortality in animals, birds, and 

fish.(37, 38) Manganese can impair neurological function.(39) High concentrations of manganese 

in water also can cause drinking water to taste bad, increase water treatment costs, and cause 

staining and mineral deposits in plumbing systems.

Figure 6–6. Concentrations of dissolved arsenic, selenium, and manganese were greater than MCLs, the HBSL, 

and the SMCL in samples from all aquifers except the Laramie–Fox Hills aquifer. Exceedances of the human-health 

benchmarks (MCLs and HBSL) for the three trace elements and the SMCL for manganese were less common for 

samples from drinking-water wells than for those from monitoring wells that tap the shallow aquifer system. Humanhealth benchmark exceedances for drinking-water wells only were found in samples from the Dawson and Arapahoe 

aquifers. These wells were relatively shallow—the maximum well depth for a sample from a drinking-water well with 

an exceedance was 400 feet below land surface.

Arsenic is a trace element found 

in aquifer materials and can also 

come from pesticide application 

or industrial waste. Long-term 

exposure to arsenic in drinking 

water is related to elevated risks 

of cancer and skin damage.

In small amounts, the trace 

element selenium is essential 

for human and animal health, 

but excessive concentrations 

can be toxic. Some aquatic biota 

are particularly sensitive to the 

amount of selenium in water.

Manganese is a trace element 

found in aquifer materials 

and some industrial wastes. 

Manganese has a human-health 

benchmark of 300 μg/L because

of neurological effects.

2,500

2,000

1,500

1,000

500

0

Depth of well, in feet below land surface

0

20

80

100

Concentration, in micrograms per liter

Arsenic

EXPLANATION

Figure 6-6.

Alluvial

Aquifer

Dawson

Denver

Arapahoe

Laramie–Fox Hills

10

0

250

500

750

1,000

0

500

1,000

2,000

3,000

USEPA SMCL

for manganese

(50 μg/L)

USEPA MCL

for arsenic 

(10 μg/L)

USEPA MCL

for selenium 

(50 μg/L)

HBSL for 

manganese

(300 μg/L)

Monitoring wells in the 

shallow aquifer system

Deep bedrock wells used

for drinking water

Selenium Manganese

52 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05
Figure 6–7. Manganese concentrations in Denver Basin groundwater 

tended to be greater in anoxic 

groundwater than in oxic groundwater, regardless of pH. This finding 

is consistent with results from a study 

of trace elements in groundwater from 

across the Nation that reported that 

some trace elements, such as iron 

and manganese, are more likely to be 

released from aquifer sediments to 

groundwater under low pH and (or) 

anoxic conditions.(87)

Geologic source material, redox, and pH account for differences in the 

occurrence and distribution of arsenic, selenium, and manganese in 

Denver Basin groundwater

Concentrations of arsenic, selenium, and manganese in Denver Basin groundwater, which 

ranged from less than laboratory reporting levels to exceedances of human-health benchmarks, 

depend on many factors including the composition of aquifer rock and sediment, redox conditions, pH, chemical adsorption and desorption, mineral precipitation, and biological activity. 

Understanding the occurrence of these trace elements in groundwater and the sources and 

mechanisms that affect their release into groundwater is important for human health and the 

health of animals and aquatic biota.

Arsenic, selenium, and manganese are found naturally in many minerals, rocks, and 

soils. Possible sources of arsenic in the Denver Basin include volcanic-ash deposits, shales, 

pyrite, and iron-oxide grain coating on sediment. Organic-rich bentonite claystones, lignite 

deposits, and tonsteins in the Denver Formation are sources of selenium.(22) Some rocks and 

sediment of the Denver Basin contain high concentrations of manganese, which, under the 

right geochemical conditions, can dissolve into groundwater. 

Similar to patterns observed for uranium, the occurrence and distribution of arsenic and 

selenium in shallow groundwater have been affected by increased recharge of oxic irrigation 

waters and minerals present in subsurface sediments. For samples from all wells, median 

arsenic and selenium concentrations were greater in oxic groundwater than in anoxic groundwater (fig. 4 –4). Five of the six arsenic samples and all selenium samples with exceedances of 

the respective MCLs were oxic. Most samples with detectable concentrations of arsenic were 

from Dawson aquifer wells (fig. 6 – 6). 

Manganese typically dissolves more readily in groundwater when dissolved-oxygen 

concentrations are low; therefore, manganese concentrations commonly are greater in anoxic 

groundwater than in oxic groundwater. In the Denver Basin, the median manganese concentration in samples of anoxic groundwater was about five times more than that in samples of 

oxic groundwater. Concentrations of manganese, however, exceeded benchmarks with similar 

frequencies for anoxic and oxic samples — 6 to 10 percent for the Health-Based Screening 

Level (HBSL) and 20 to 21 percent for the SMCL. The similarity in frequency of exceedances 

likely results from the widespread occurrence of manganese in aquifer sediments and mixed 

redox conditions; more than 50 percent of samples with benchmark exceedances had chemical 

signatures suggestive of mixed redox conditions. For anoxic samples, pH also affected 

manganese solubility. All anoxic samples with pH less than 7 had manganese concentrations 

greater than 50 μg/L; most anoxic samples with pH of 7 or more had manganese concentrations 

less than 50 μg/L (fig. 6 –7).

Chapter 6

Concentration, in micrograms per liter

pH<7

Oxic Anoxic

pH≥7

Oxic Anoxic

10

100

10,000

1.0

0.1

1,000

Manganese in Denver Basin 

groundwater by redox and pH

Figure 6-7.

HBSL (300 μg/L) 

USEPA SMCL 

 (50 μg/L)

Laboratory

reporting

level (0.2 μg/L)

See sidebar, Boxplots, p. 47

Chapter 6: Understanding Where and Why Key Contaminants Occur in Groundwater 53
Selenium in Denver Basin groundwater can impair water quality in streams 

In the Denver Basin, the interaction of recharge water from agricultural and urban irrigation with selenium-bearing 

rocks and soils can cause selenium to dissolve and move down with the recharge water to the water table. Samples from 

wells in the shallow aquifer system commonly had selenium concentrations that exceeded the CDPHE aquatic-life standard 

of 4.6 μg/L. Where Denver Basin groundwater provides base flow to streams, such as the South Platte River and Toll Gate 

Creek in Aurora, Colo., high concentrations of selenium in groundwater pose a threat to aquatic life. This threat is more 

important in streams where the streamflow has changed from ephemeral to perennial because of increased discharge of 

groundwater associated with a higher water table.

Since the early 2000s, selenium concentrations in Toll Gate Creek (fig. 3–1) have consistently been greater than the 

CDPHE aquatic-life standard of 4.6 μg/L. Previous studies of selenium in Toll Gate Creek provide specific examples of the 

effects of groundwater on surface-water quality and ecosystem health.(20, 27) The primary geologic source of selenium to 

Toll Gate Creek is organic-rich claystone contained in the Denver Formation.(22) Selenium in the claystone dissolves when 

selenium-bearing minerals react with oxic water, and selenium in evaporative salt deposits dissolves during groundwater 

recharge and high streamflow events. Concentrations of selenium in more than 84 percent of groundwater samples from 

wells in the Toll Gate Creek watershed were greater than the aquatic-life standard, and some concentrations were greater 

than the MCL of 50 μg/L. Most concentrations of selenium in inflows to Toll Gate Creek that represent groundwater discharge 

to the stream were greater than the aquatic-life standard, as were concentrations in almost all downstream reaches of 

Toll Gate Creek. Data for Toll Gate Creek graph and other information from Paschke and others.(20)

Photographs by Andrew Silver, Brigham Young University

Many contaminants from geologic sources originate from the weathering and dissolution of rocks and minerals that make up Denver Basin 

soils and aquifer sediments. Example sources include realgar, an arsenic sulfide; native selenium; and pyrolusite, a manganese oxide. 

High concentrations of selenium in surface 

water and groundwater can adversely 

affect the health of wildlife and aquatic 

biota. About 45 to 72 percent of samples 

from the alluvial aquifer and shallow 

parts of the Dawson and Denver aquifers 

had selenium concentrations that were 

greater than the Colorado Department of 

Public Health and Environment aquaticlife standard for selenium of 4.6 μg/L. 

Concentrations in about 72 to 97 percent 

of samples collected from groundwater, 

inflows, or streams in the Toll Gate Creek 

watershed in Aurora, Colo., during July and 

August 2007 also were greater than the 

aquatic-life standard.

1.0

10

100

1,000

0.1

Concentration, in micrograms per liter

Alluvial 

Dawson

Denver 

Denver Basin

shallow aquifer system

Selenium in Denver Basin shallow groundwater

and the Toll Gate Creek watershed

Groundwater 

Toll Gate Creek 

Toll Gate Creek watershed

Inflows 

Chapter 6 selenium sidebar

USEPA MCL

 for selenium 

 (50 μg/L)

CDPHE aquatic-life

 standard for selenium 

 (4.6 μg/L)

See sidebar, Boxplots, p. 47

54 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05
Dissolved Solids 

Two of every three samples of shallow groundwater contained dissolved solids at concentrations that exceeded the SMCL of 500 mg/L, which could limit use of shallow groundwater 

as a future source of drinking water or even irrigation supply without costly treatment or 

blending. One of every five drinking-water wells sampled, which pump water from deep parts 

of the aquifer system, had dissolved-solids concentrations that exceeded the SMCL. Dissolved 

solids that result naturally from mineral dissolution and evaporative concentration in the 

subsurface are carried to the water table with precipitation recharge. In areas of urban and 

agricultural development, excess recharge water associated with irrigation leads to further 

dissolution of soluble salts and minerals and high concentrations of dissolved solids in 

shallow groundwater underlying these areas. Concentrations of dissolved solids in deeper 

parts of bedrock aquifers likely result from natural interactions between the groundwater and 

the rock that are found with distance along with flow paths over long residence times. 

What are dissolved solids?

The term “dissolved solids” is a measure of all substances dissolved in water. Calcium, magnesium, sodium, potassium, 

bicarbonate, sulfate, chloride, and silica typically constitute most of the dissolved solids in water. The concentration of 

dissolved solids is reported in milligrams per liter and represents the total amount of dry solids, in milligrams, that remain 

after all the water in a 1-liter sample is evaporated at a temperature of 180 degrees Celsius. Water with a concentration of 

dissolved solids less than 1,000 mg/L is referred to as “fresh.” Slightly or moderately saline water typically is considered to 

have 1,000 –10,000 mg/L dissolved solids.(79) Whether water with high dissolved solids tastes good or bad depends on personal 

preference, but water containing more than 3,000 mg/L dissolved solids generally is too salty to drink.(80)

Dissolved solids in the Denver Basin, High Plains, and Southwest basin-fill aquifers 

The decrease in dissolved-solids concentrations that occurs with depth in the Denver Basin also occurs in the High 

Plains aquifer system, which underlies a large area of the Great Plains. Although dissolved- solids concentrations typically 

increase with depth in rangeland and undeveloped areas of the Great Plains,(25) dissolved-solids concentrations in groundwater underlying agricultural areas are high in shallow groundwater because of agricultural recharge. In Southwest basinfill aquifers—the Basin and Range basin-fill aquifers, California Coastal Basin, Central Valley aquifer system, and Rio Grande 

aquifer system—salts in the subsurface are concentrated by evapotranspiration in the semiarid to arid regions.(81) As in the 

Denver Basin, dissolved-solids concentrations are high in shallower parts of Southwest basin-fill aquifers, and the SMCL for 

dissolved solids was exceeded more commonly in shallow groundwater beneath agricultural and urban areas than in deeper 

parts used for domestic and public supply.(81)

Chapter 6

Ch06 TDS sidebar

Modified from Thiros and others(81)

0 1,000 2,000 3,000 4,000 5,000

Dissolved-solids concentration in Southwest basin-fill aquifers, in milligrams per liter

Public-supply wells

Domestic wells

Agricultural monitoring wells

Urban monitoring wells

19% 

31% 

45% 

64% 

SMCL for dissolved solids is 500 mg/L. Percentage values 

are the percentage of samples that exceeded the SMCL

Direction of

generally

increasing

well depth

NOTE: Twelve samples with 

concentrations greater than 

5,000 mg/L are not shown. 

See sidebar, Boxplots, p. 47

Chapter 6: Understanding Where and Why Key Contaminants Occur in Groundwater 55
Figure 6–8. Concentrations 

of dissolved solids were 

greatest in samples from the 

alluvial and shallow bedrock 

aquifers of the Denver Basin 

and decreased as well depth 

increased. There were fewer 

exceedances of the SMCL in 

samples from deeper parts 

of bedrock aquifers than in 

samples from the shallow 

aquifer system.

Dissolution and evaporative concentration are two natural processes that can cause the 

dissolved-solids concentrations in shallow groundwater of the Denver Basin to increase. 

Precipitation dissolves minerals and other materials in the soil. When some of that water 

evaporates, the dissolved solids are concentrated in the water that remains. Under natural 

conditions, further evaporation in the unsaturated zone results in the formation of soluble 

salts that can eventually be flushed down to the water table during wet periods. In the 

Denver Basin, irrigation water applied to the land surface in agricultural and urban areas 

increases the dissolution of soluble salts that have been accumulated from evaporation in 

the unsaturated zone under natural, semiarid conditions. The amount of soluble salts in the 

unsaturated zone increases when excess recharge water from irrigation evaporates. Movement 

of accumulated salts to the water table occurs over time in response to repeated irrigation 

cycles or periods of above normal precipitation; however, the rate of downward movement 

is substantially greater in irrigated parts of the basin relative to natural processes. 

Concentrations of dissolved solids in Denver Basin groundwater were greatest in samples 

from the alluvial and shallow parts of bedrock and decreased with depth below the water 

table (fig. 6–8). Concentrations in samples of groundwater from alluvial and shallow bedrock 

aquifers were as high as 5,200 mg/L, whereas the maximum concentration in samples from 

drinking-water wells in deeper parts of bedrock aquifers was about 1,200 mg/L. Exceedances 

of the dissolved solids SMCL mirror the difference in concentrations — 66 percent exceedance 

for shallow aquifer system samples and 20 percent exceedance for the deep aquifer system 

samples. In a national context, shallow groundwater in the Denver Basin was of poorer 

quality than was shallow groundwater underlying other agricultural and urban areas across the 

Nation; only about one in three samples from shallow aquifers nationally had dissolved-solids 

concentrations that were greater than the SMCL. For the bedrock aquifers, the frequency of 

exceedances of the SMCL was the same as that for similar settings nationwide. Depth of well, in feet below land surface

Concentration, in milligrams per liter

0 1,000 2,000 3,000 4,000 5,000 6,000

2,500

2,000

1,500

1,000

500

0

Dissolved solids in Denver Basin aquifers

EXPLANATION

Figure 6-8.

Alluvial

Aquifer

Dawson

Denver

Arapahoe

Laramie–Fox Hills

USEPA SMCL (500 mg/L)

Monitoring wells in the 

shallow aquifer system

Deep bedrock wells used

for drinking water

56 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05
Figure 6–9. Concentrations of dissolved 

solids in groundwater underlying irrigated 

agricultural and urban areas exceeded the 

SMCL of 500 mg/L in almost every sample, 

because excess recharge water from crop 

and landscape irrigation increased the 

movement of dissolved solids to the water 

table. In contrast, about half of the samples 

from non-irrigated agricultural areas 

exceeded the SMCL level.

The use of brackish water to meet supply needs can be costly

To address overuse of Denver Basin groundwater and fill future gaps in water supply, water providers in the Denver Basin 

area are increasingly looking to use poor-quality alluvial groundwater to meet supply needs. The East Cherry Creek Valley Water 

and Sanitation District (ECCV), working with the United Water and Sanitation District, developed alluvial wells in Beebe Draw 

(a paleochannel of the South Platte River in the northern reaches of the Denver Basin) for withdrawal of brackish groundwater to 

develop additional supply. In order to meet water-quality goals, concentrations of dissolved solids and other constituents in the 

groundwater need to be reduced. The ECCV has constructed a 10-million-gallon per day Reverse Osmosis (RO) treatment plant 

for treating the brackish water. Estimated cost for the RO treatment system, and deep well/brine handling facilities is $35 million 

(Chris Douglass, East Cherry Creek Valley Water and Sanitation District, written commun., 2013; estimated costs as of January 2013).

Irrigation in agricultural areas and landscape watering in urban areas has increased 

dissolved-solids concentrations in the underlying groundwater. Dissolved-solids concentrations 

in all samples from the alluvial aquifer underlying areas of irrigated agriculture in the South 

Platte River Basin exceeded the SMCL, as did most samples from urban areas (fig. 6–9). The 

occurrence of dissolved-solids concentrations greater than the SMCL was greatest for samples 

of young (recharged since the 1950s) groundwater collected from shallow aquifers. Leaching 

of chemicals applied to the land surface, such as fertilizers and road deicing salts in agricultural 

and urban areas, and reaction between some applied chemicals and minerals in the subsurface 

also contribute to increased concentrations of dissolved solids near the water table.(2, 40, 41)

Chapter 6

Photographs from East Cherry Creek Valley Water and Sanitation District

Concentration, in milligrams per liter

10,000

100

1,000

Dissolved solids in the

alluvial aquifer

Water and land use

Irrigated

agriculture

Non-irrigated

agriculture

Irrigated

urban

USEPA SMCL

(500 mg/L)

See sidebar, Boxplots, p. 47

Chapter 6: Understanding Where and Why Key Contaminants Occur in Groundwater 57
Contaminants Derived Primarily from Human Activities: Nitrate

Concentrations of nitrate exceeded the MCL of 10 mg/L as nitrogen in one of 

every five samples collected from the alluvial and shallow bedrock wells but rarely 

exceeded the MCL in samples of groundwater from deep bedrock wells. At high 

concentrations, nitrate in drinking water can cause health problems. The presence 

of nitrate is controlled by the presence or absence of sources of nitrogen, irrigation 

practices, and groundwater redox conditions. In a given area, nitrate is most prevalent 

and is detected at higher concentrations when more nitrogen and water are applied to 

the land surface and when groundwater is oxic. 

Nitrate is a form of nitrogen, an essential nutrient for human health and plant and animal 

growth. Of the different chemical forms of nitrogen in groundwater, only nitrate typically is 

found at concentrations that can be harmful to human health and impair aquatic systems. Excess 

nitrate in drinking water is associated with adverse human-health effects, including methemoglobinemia (“blue-baby syndrome”), which can cause infants to become severely ill or die.(42)

Only 1 of the 90 drinking-water wells sampled in the Denver Basin had a nitrate concentration greater than the MCL of 10 mg/L as nitrogen. The highest concentrations of nitrate were 

detected in samples of shallow groundwater underlying agricultural and urban areas in the 

basin (fig. 6 –10): concentrations in about 18 percent of samples from the alluvial aquifer and 

shallow parts of the Dawson and Denver aquifers in these areas were greater than the MCL. 

The discharge of nitrogen-enriched groundwater to surface water increases the likelihood of 

eutrophication, including harmful algal blooms. Too much algal growth can adversely affect 

water quality and human, aquatic-biota, and ecosystem health. 

Figure 6–10. Concentrations of nitrate were greatest in samples from the 

shallow aquifer system and decreased with increasing well depth. Only one 

sample from a drinking-water well had a nitrate concentration that exceeded 

the MCL of 10 mg/L as nitrogen. At 140 feet deep, that well, in the Arapahoe 

aquifer near Brighton, Colo. (an area prone to high nitrate concentrations 

in groundwater), is one of the shallowest drinking-water wells sampled for 

this study. About two-thirds of the shallow groundwater samples had nitrate 

concentrations greater than 1 mg/L, generally considered the threshold 

for contamination from human activities.(46) The threshold was exceeded in 

10 percent of the wells used for drinking water, at depths of as much as 500 feet.

0 5 10 15 20 25 30 35

2,500

2,000

1,500

1,000

500

0

Concentration, in milligrams per liter as nitrogen

Depth of well, in feet below land surface

Nitrate in Denver Basin aquifers

Figure 6-10.

EXPLANATION

Alluvial

Aquifer

Dawson

Denver

Arapahoe

Laramie–Fox Hills

USEPA MCL (10 mg/L) 

Monitoring wells in the 

shallow aquifer system

Deep bedrock wells used

for drinking water

58 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05
Photograph by Mike Dabell, 

copyright istockphoto.com

Importance of nitrogen sources and excess irrigation to nitrate 

concentration in shallow groundwater

Nitrogen is found naturally in plants, animals, soils, the atmosphere, and in some rocks 

and minerals. Human activity and sources, however, are the primary cause of elevated concentrations of nitrate in groundwater, including application of chemical fertilizers and manure, 

feedlots, animal waste, biosolids disposal, urban runoff, wastewater-treatment effluent, leaky 

sewer lines, and septic systems. 

When grassland and rangeland are first converted to agriculture, excess recharge water 

from irrigation can flush nitrate that has accumulated naturally in the unsaturated zone down to 

the water table.(25) Once agricultural land is established, nitrogen-containing fertilizers routinely 

applied to the land surface are an additional source of nitrate to shallow groundwater. The 

amount of nitrogen applied depends on crop type: recommended application rates range from 

75 pounds of nitrogen per acre for dryland winter wheat (expected yield, 50 bushels per acre) 

to 210 pounds of nitrogen per acre for irrigated corn (expected yield, 175 bushels per acre), 

assuming similar soil nitrogen organic matter content.(43, 44) The effect of nitrogen applications 

rates and irrigation practices on nitrate concentrations in shallow groundwater is illustrated 

by results for the Denver Basin and South Platte River Basin studies (fig. 6 –11). The median 

concentration of nitrate in shallow groundwater underlying irrigated areas where corn is grown 

and fertilizer application rates are high was about 50 percent more than in shallow groundwater 

underlying non-irrigated areas where dryland wheat is grown and fertilizer application rates 

generally are low. Thus, application rates of nitrogen and the intensity of irrigation are important controls on the occurrence of nitrate in shallow groundwater.

Animal manure applied as a fertilizer to 

agricultural fields is a source of nitrogen to the 

subsurface. Nitrate, a form of nitrogen, moves 

down to the water table with excess recharge 

water from irrigation.

Chapter 6

Concentration, in milligrams per liter

Nitrate in groundwater

EXPLANATION

Figure 6-11.

0

10

60

50

30

40

20

Oxic

Anoxic

Redox condition

Redox condition

Median estimated nitrogen 

input for 500-meter buffer area

around well, in kilograms

Nitrogen input

0

2,000

6,000

4,000

Deep wells

used for

drinking

water

Ag:

dryland

wheat

Ag:

irrigated

corn

Urban:

older

Urban:

newer

Shallow wells

Denver Basin study

(2003–05)

South Platte

River Basin

study (1993–94)

No

input

Modified from K.J. Hitt, unpublished data, 2008

USEPA MCL (10 mg/L)

Estimated national 

background concentration 

of nitrate (1 mg/L)

See sidebar, Boxplots, p. 47

Figure 6–11. Nitrate concentrations are highest where 

application rates of nitrogen 

are high and groundwater 

primarily is oxic, for example in 

shallow groundwater underlying 

irrigated agricultural areas (Ag). 

As water moves deeper into the 

aquifer system where water is 

anoxic, nitrate concentrations 

decrease through oxidation/

reduction processes. 

Chapter 6: Understanding Where and Why Key Contaminants Occur in Groundwater 59
Fertilization and watering of landscaping in urban areas also can deliver nitrogen to shallow 

groundwater. Fertilizers are commonly applied to turf grass and other landscaping, and these 

areas are irrigated in the sense that they are watered regularly—outdoor watering, mostly of 

lawns, accounts for about 50 percent of the water used by urban residents along the Front Range. 

Most nitrate concentrations in shallow groundwater underlying urban areas of metropolitan 

Denver were greater than 1 mg/L, indicating contamination from human activities. Although 

estimated nitrogen inputs for older developed areas and more recently developed areas were 

similar, the median concentration of nitrate in shallow groundwater was about twice as high 

for older parts of the Denver metropolitan area than for areas of newer suburban development 

(fig. 6 –11). This difference probably reflects the relatively long time—years to decades—it 

takes for water and contaminants to move from the land surface down to the water table. 

As a result, nitrate associated with recent urban development likely has not yet reached the 

groundwater sampled for this assessment.(45, 46) Many of the communities in the south Denver 

metropolitan area with recent development, such as Castle Rock and Parker, have landscape 

and irrigation standards or recommendations to reduce water use, including planting droughttolerant plants, which need less fertilization than lawns (fig. 6 –12). Reducing the use of 

irrigation water and fertilizers could decrease the amount of nitrate that reaches the water table.

Figure 6–12. About half of the water used in urban residential areas of the Front Range is for outdoor watering, 

primarily of lawns.(15) Depending on the type of grass grown, some lawns require large amounts of water and fertilization 

to grow properly. Communities throughout the Denver metropolitan area stress the importance of using water efficiently 

for landscaping—the choice of plants and irrigation system can greatly reduce water use. Traditional lawns (left) require 

more water and fertilizer applications than do drought-tolerant plants (right) on drip irrigations systems. Average total season (April–September) water use, in gallons

Water and nitrogen use

0

100,000

20,000

0

400

200

800

1,000

Application rate of nitrogen,

in kilograms of nitrogen per

500 square meters of lawn area

Bluegrass Drought-tolerant plants

Landscape

40,000

60,000

80,000 EXPLANATION

Figure 6-12.

Water use (Modified from 

Waskom and others(94))

Nitrogen application

 (Modified from Koski 

and Skinner(95))

*

*Buffalograss

600

Left photograph copyright istockphoto.com

60 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05
Denitrification decreases nitrate concentrations in groundwater

Input of nitrogen is one factor that controls concentrations of nitrate in groundwater, but 

redox conditions also are important for nitrate occurrence. Once nitrate is in groundwater, the 

only way that it can be removed naturally is by denitrification, a process that reduces nitrate 

to harmless nitrogen gas (see sidebar, How do redox reactions work?, p. 36). Denitrification 

occurs only when conditions are anoxic—it does not occur in oxic groundwater. As a result, for 

areas where amounts of nitrogen inputs are similar, nitrate concentrations are lower in anoxic 

groundwater than in oxic groundwater; where there are no nitrogen inputs, nitrate concentrations 

in groundwater are negligible regardless of redox condition (fig. 6 –13).

Nitrate concentrations in samples from drinking-water wells decreased as well depth 

and the percentage of wells with anoxic water increased (fig. 6 –14). The long amount of 

time required for water to travel from the water table to deeper parts of the aquifer system 

provides more time for nitrate to be transformed to nitrogen gas by denitrification reactions. 

In samples from the deepest wells, where anoxic conditions were most common, nitrate was 

rarely detected. In addition to redox processes and denitrification, the decrease in nitrate with 

well depth might also reflect older groundwater (centuries to millennia old) that recharged the 

aquifer before extensive agricultural or urban development occurred in the Denver Basin.(18)

Figure 6–13. Nitrate concentrations are controlled 

by nitrogen inputs and redox condition. Where there 

are no nitrogen inputs, such as in deep groundwater 

used for drinking, nitrate concentrations are negligible, 

in both oxic and anoxic groundwater. Where nitrogen 

inputs are similar, such as in shallow groundwater, 

nitrate concentrations are consistently greater when 

conditions are oxic and decrease when conditions are 

anoxic. (Moderate nitrate input: 1,000–2,000 kilograms 

for 500-meter buffer area around well[K.J. Hitt, unpub. 

data, 2008].)

Figure 6–14. Nitrate was most commonly 

detected in monitoring wells that tap groundwater at the shallowest depths, where oxic 

conditions were most dominant. Detections 

of nitrate decreased as well depth and the 

occurrence of anoxic conditions increased. 

Only 8 percent of samples from the deepest 

drinking-water wells, most of which had anoxic 

water, contained detectable concentrations of 

nitrate. [n=number of samples] 

Chapter 6

Nitrate detected

Nitrate not detected

EXPLANATION

Figure 6-14.

Oxic

Anoxic

Well depth

(feet below 

land surface)

18 to 124

(n=60)

125 to 400

(n=26)

401 to 700

(n=39)

>700

(n=25)

Nitrate Redox

condition

Percentage of samples

Drinking-water wells—Well depth >124 feet

Shallow well, agricultural areas,

 Denver Basin study (2003)

Deep wells used for drinking water

EXPLANATION

Figure 6-13.

Shallow well, urban areas,

 Denver Basin study (2003) and

 South Platte River Basin study (1993)

Well type

Median concentration,

in milligrams per liter

Nitrate concentration relative

to nitrogen input and redox 

0

2

6

8

4

Estimated nitrogen input

No

input

Moderate

Oxic

Anoxic

Redox condition

Chapter 6: Understanding Where and Why Key Contaminants Occur in Groundwater 61
Trends in nitrate concentrations

Twenty-nine agricultural land-use monitoring wells and 15 groundwater flow-path study wells that tap the South Platte 

alluvial aquifer were sampled in the early 1990s and again in the early 2000s to investigate near-decadal changes in groundwater 

quality in this irrigated agricultural area, some of which overlies the Denver Basin aquifer system.(4) Results of groundwater 

flow-path studies in 1993 and 2004 indicate that dissolved-oxygen concentrations decreased and groundwater age increased with 

distance along groundwater flow paths and with depth of wells. Every flow-path well had a detectable concentration of nitrate. 

Because denitrification has been identified 

as an important mechanism for mitigating nitrate 

concentrations in the alluvial aquifer,(4, 82) samples from 

the 1994 and 2002 agricultural land-use monitoring 

wells were categorized by dissolved-oxygen content 

before nitrate concentrations from the same wells 

were compared for temporal differences. Nitrate 

concentrations in oxic samples (defined in the study 

as dissolved-oxygen concentration greater than 

or equal to 2 mg/L) from the alluvial aquifer were 

substantially greater in 2002 than in 1994, but there 

was no substantial difference in nitrate concentrations 

in reduced samples (dissolved-oxygen concentration 

less than 2 mg/L) (see figure at right; note different 

definition of oxic and reducing conditions than in 

the Denver Basin study, which used a breakpoint of 

0.5 mg/L for defining oxic and anoxic samples). This 

near-decadal increase in nitrate concentrations in oxic 

groundwater that underlies the agricultural area is 

attributed to the increased use of synthetic fertilizers 

and manure since the 1950s.(4)

Will nitrate concentrations in Denver Basin 

shallow groundwater underlying urban areas and 

deeper groundwater used for drinking water be 

different in the future? The answer will depend on 

changes in water and fertilizer use and movement of 

oxic groundwater. In response to the onset of drought 

conditions in 2001 and an increased emphasis on 

water conservation, water use for municipal, domestic, and other purposes has decreased throughout the Front Range. Since the 

drought, customers of Denver Water, for example, have used about 20 percent less water,(83) and outdoor use of water decreased 

by more than 50 percent between 1994 and 2004.(84) More and more homeowners and communities are planting drought-tolerant 

plants with a lower need for nitrogen in place of lawn grasses with higher nitrogen requirements. In contrast to increased water 

and fertilizer use and higher nitrate concentrations in irrigated agricultural areas, a reduction in water and fertilizer use in urban 

areas could decrease future nitrate concentrations in urban shallow groundwater. As observed in the near-decadal agricultural 

land-use studies, though, redox conditions also need to be considered in assessing trends in nitrate concentrations. Concentrations in groundwater used for drinking were highest (1 mg/L or greater) in oxic samples collected at well depths of less than 

500 ft below land surface (fig. 6 –10). Nitrate concentrations in all anoxic drinking-water samples were at least 10 times lower than 

in the oxic samples. If nitrate-laden oxic groundwater was to move into parts of the aquifer system that currently are anoxic, the 

groundwater would lose its capacity to assimilate nitrate by denitrification, and nitrate concentrations in some drinking-water 

supplies could increase.

Nitrate concentrations in the alluvial aquifer

underlying agricultural land use in the

South Platte River Basin, 1994 and 2002

USEPA MCL 

Concentration, in milligrams per liter

(10 mg/L)

60

10

20

Redox and year of sample

EXPLANATION

50

40

30

0

Reduced

2002

Oxic

1994

Reduced

1994

Oxic

2002

Modified from Paschke and others(4)

Ch. 6-nitrate.sidebar

Oxic: Dissolved-oxygen concentration ≥2 mg/L 

Reduced: Dissolved-oxygen concentration <2 mg/L

Redox condition

See sidebar, Boxplots, p. 47

62 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05
Left photograph by R. Carner, copyright istockphoto.com

Contaminants Derived Primarily from Human Activities: Pesticides and 

Volatile Organic Compounds

Eight pesticide compounds and 17 VOCs were detected in at least one sample from 

shallow monitoring wells and drinking-water wells in the Denver Basin, but almost 

all concentrations were 10 or more times lower than human-health benchmarks. 

Similar to nitrate and other constituents, pesticides and VOCs were more common in 

the shallow groundwater system than in the deeper system used for drinking water. 

Pesticides and VOCs are tracers of the movement of groundwater from shallow to 

deeper depths in the aquifer system—their occurrence and distribution in groundwater, 

even at low concentrations, indicates that Denver Basin groundwater is vulnerable to 

contamination from human activities. 

Pesticides and VOCs are organic chemicals that can be present in groundwater because of 

their release at or near the land surface. As a result, pesticides and VOCs were detected more 

frequently in samples of shallow groundwater than in samples of deeper groundwater used for 

drinking. Pesticides are chemicals used to prevent or control unwanted plants, insects, fungi, 

and other pests. VOCs are carbon-based compounds that can move between air and water 

(hence the word “volatile”) and are used or produced in a variety of industrial, commercial, and 

domestic applications. The occurrence of pesticide compounds and VOCs in groundwater can 

be of concern because of potential human-health effects. Effects of exposure can be short term, 

such as irritation to skin, eyes, and respiratory system, and (or) long term, such as impairment 

to the reproductive, immune, and nervous systems and increased risk of cancer. Human-health 

benchmarks have been established for some but not all pesticides and VOCs. No standards or 

benchmarks have been established for mixtures or degradates of these compounds in water—

the effects of such mixtures or degradates on human health are not well known.

Left, Pesticide applications are common in Denver Basin agricultural areas—here, a helicopter sprays pesticides 

on a corn field. Detections of atrazine and other pesticide compounds were greatest in shallow groundwater 

underlying agricultural and urban areas, particularly irrigated agricultural areas where pesticide use is extensive. 

Concentrations of pesticides in Denver Basin shallow groundwater and deeper groundwater used for drinking 

were 10 or more times lower than human-health benchmarks. Right, Chloroform, a VOC, is formed by the reaction 

of chlorine used to disinfect drinking water and organic matter in the water. When treated drinking water is used 

to water lawns, the chloroform in the water can be carried down to the water table. Leakage from pipes used to 

deliver treated drinking water is another potential source of chloroform to shallow groundwater. 

Detections of pesticide 

compounds and VOCs 

in well samples, even 

though at concentrations 

well below human-health 

benchmarks, indicate 

vulnerability of Denver 

Basin groundwater to 

contamination.

Assessment levels

The occurrence of pesticide compounds and VOCs in water samples is evaluated in two ways. Detections at

any concentration are assessed without regard to differences in reporting levels for individual pesticide compounds

or VOCs. Detections are assessed at a common assessment level, greater than 0.1 μg/L for pesticide compounds 

(fig 6–16, p. 65) and 0.2 μg/L for VOCs (fig 6–18, p. 69), when detection frequencies of two or more compounds 

with different laboratory reporting levels are being compared. Evaluating detections at a common assessment

level facilitates comparison of detection frequencies among compounds with different reporting levels.

Chapter 6

Chapter 6: Understanding Where and Why Key Contaminants Occur in Groundwater 63
Occurrence of pesticide compounds

A total of 130 samples of shallow groundwater underlying agricultural and urban areas 

and deeper groundwater from wells that tap the Dawson, Denver, and Arapahoe aquifers for 

drinking-water supply were analyzed for 152 pesticide compounds (pesticides and degradates, 

table A1–1, appendix 2) — samples from domestic wells in the Denver aquifer and all wells 

in the Laramie–Fox Hills aquifer were not analyzed for pesticide compounds; these aquifers 

are not as heavily used for supply as are the Dawson and Arapahoe aquifers. At least one 

pesticide or pesticide degradate was detected in about one-third of samples from monitoring 

wells tapping shallow groundwater underlying urban and non-irrigated agricultural areas of 

the Denver Basin (fig. 6 –15). In contrast, only 4 percent of samples (3 of 70 samples) from the 

deep bedrock aquifers used for drinking water contained a pesticide or a pesticide degradate. 

All three samples with detections were from the Dawson aquifer at depths of less than 500 ft; 

in two of these samples more than one pesticide was detected. No pesticides or degradates were 

detected in samples from the Arapahoe aquifer or from the one sampled public-supply well in 

the Denver aquifer. Although all concentrations were at least 10 times lower than human-health 

benchmarks, detections of pesticide compounds in shallow groundwater and deeper groundwater used for drinking indicate the vulnerability of Denver Basin groundwater to contamination from human activities.

Figure 6–15. Pesticides were detected infrequently in samples from wells used for 

drinking water but were much more commonly detected in shallow monitoring wells in 

agricultural and urban areas. Pesticides applied at the land surface are transported with 

recharge water to shallow groundwater. Detections of pesticides in the drinking-water 

wells indicate that the bedrock aquifers used for drinking water are vulnerable to the 

downward movement of contaminants from shallow groundwater. Mixtures of pesticide 

compounds (two or more pesticides or degradates detected in the same sample) were 

present in about one-third of the agricultural and urban wells with detections of pesticides 

and in two drinking-water wells with pesticide detections. The presence of multiple 

pesticides or degradates in a drinking-water sample, even at low concentrations, is 

of concern because the health effects of such mixtures are not well understood.(96)

500

0

Number of pesticide and

pesticide degradate detections

per well sample and depth

Shallow monitoring well, 

 agricultural area

Deep well used for 

 drinking water

EXPLANATION

Figure 6-15.

Shallow monitoring well, 

 urban area

Not

detected

Alluvial

Dawson

Denver

Arapahoe

Well type Aquifer

Depth of well, in feet below land surface

0

500

1,000

1,500

2,000

2,500

Detected

Pesticides and 

pesticide degradates

Occurrence of pesticide 

and pesticide degradates

by well depth

300

200

100

0

1 2 3 7

Number of pesticide and 

pesticide degradates detected

400

500

Monitoring wells in the 

shallow aquifer system

Deep bedrock wells used

for drinking water

64 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05
Eight pesticide compounds were detected

Eight pesticide compounds were detected in at least one sample from the study wells: seven 

herbicides (acetochlor, atrazine, metolachlor, metribuzin, picloram, prometon, and simazine) 

and one herbicide degradate (deethylatrazine) (fig. 6 –16; apendix A2). One or more herbicide 

compound was detected in 41 percent of samples of shallow groundwater underlying urban areas 

of the Denver Basin but in only about 26 percent of samples of shallow groundwater underlying 

agricultural areas (non-irrigated [dryland] winter wheat fields). In the 1994 study of the quality 

of groundwater in the South Platte River alluvial aquifer underlying irrigated fields of corn 

and other crops in northeastern Colorado, herbicide compounds were detected in 97 percent of 

the samples (29 of 30 samples). The large difference in herbicide detections between the two 

studies likely reflects less intensive use of herbicides on winter wheat fields than on corn and 

other crops, and less recharge to the water table underlying the wheat fields because of the lack 

of irrigation. Herbicides also were detected much more frequently (93 percent of samples) in 

shallow groundwater underlying older, more urbanized areas of the Denver metropolitan area 

studied during 1993 than in shallow groundwater underlying the newer suburban areas included 

in the Denver Basin study. Because watering of lawns and other outdoor plants has occurred for 

decades longer in older urban areas than in areas with newer development, there has been more 

time in older areas for contaminated water to reach the water table.

Chapter 6

Shallow wells, urban area

Figure 6-16.

Most frequently detected pesticides

Denver Basin study

(2003–05)

Atrazine

Prometon

Deethylatrazine

Metolachlor

Simazine

Atrazine

Prometon

Deethylatrazine

Metolachlor

Simazine

Shallow wells, agricultural area

South Platte River Basin study

(1993– 94)

Deeper bedrock wells used for drinking water

Atrazine

Prometon

Deethylatrazine

Metolachlor

Simazine

Non-irrigated Irrigated

Percentage of samples in

which pesticide was detected

Percentage of samples in which pesticide was detected

0 20 40 8060 100

0 20 40 8060 100 0 20 40 8060 100

Older urban core/

nearby suburbs

Newer suburbs

(since about 1970)

ND

ND

EXPLANATION

Assessment levels, in

micrograms per liter

Any > 0.1

Well type

Shallow wells, urban area

Shallow wells, agricultural area

Deeper bedrock wells used for 

 drinking water

Not detected

Italic indicates pesticide degradate

ND

Deethylatrazine

Figure 6–16. Only four herbicides—atrazine, 

prometon, metolachlor, and simazine—and the 

herbicide degradate deethylatrazine each were 

detected in two or more samples of Denver 

Basin groundwater. These compounds were 

detected much less frequently in samples of 

shallow groundwater underlying newer suburban 

areas in the south Denver metropolitan area 

and non-irrigated agricultural areas than in 

shallow groundwater underlying the older 

urban core of Denver and nearby suburbs and 

irrigated agricultural areas. Individual pesticide 

compounds were detected in only three samples 

(4 percent of wells sampled) of deep groundwater 

from Denver Basin drinking-water wells.

Chapter 6: Understanding Where and Why Key Contaminants Occur in Groundwater 65
Chemical use and geochemical conditions control chemical occurrence in groundwater

Some chemicals are used more commonly in some land-use 

settings than others. The herbicides atrazine and metolachlor are 

used far more heavily in agricultural areas than in urban areas 

(see figure below), and consequently they were detected more 

frequently in shallow groundwater underlying agricultural areas 

than in shallow groundwater underlying urban areas in the South 

Platte River Basin study (fig. 6–16). 

Once a chemical has been applied at the land surface, it 

can be carried with recharge water down to the water table. 

Whether or not a chemical persists in soil and groundwater 

or whether it is broken down by microbial processes depends 

on the geochemical characteristics of the chemical and of 

the groundwater. Chloroform, for example, is more readily 

broken down by the microbial communities that exist in anoxic 

groundwater, but carbon disulfide is more readily broken down 

in oxic groundwater and can form naturally in anoxic conditions. 

These contrasting behaviors are mirrored by the more frequent 

detection and higher concentrations of chloroform in oxic 

groundwater and the more frequent detection and higher 

concentrations of carbon disulfide in anoxic groundwater (see figure at upper right).

Agricultural areas Urban areas

Ch 6-Chemical and geochemical sidebar

Estimated average pesticide use in 1992 in 500-meter buffer around shallow wells

in the South Platte River Basin Estimated average pesticide use, in kilograms

0.0001

0.001

100

1.0

10

0.1

0.01

0.0001

0.001

100

1.0

10

0.1

0.01

Data from Nakagaki(85)

Atrazine Metolachlor Atrazine Metolachlor

Photograph by USDA ARS

80

60

40

Frequency of detection in samples with

chloroform or carbon disulfide, in percent

Chloroform

100

20

0

Carbon

disulfide

0.24

0.15

0.02

0.07

Detection frequencies and median concentrations 

of chloroform and carbon disulfide in Denver Basin

groundwater by redox condition 

EXPLANATION

Chapter 6. Chemical and geochemical sidebar

Anoxic

Oxic

Number at top

of bar is median

concentration,

in micrograms per 

liter

66 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05
Occurrence of VOCs

A total of 99 samples of shallow groundwater underlying urban areas and of deeper 

groundwater from wells that tap the Dawson, Denver, and Arapahoe aquifers were analyzed 

for 86 VOCs (tables A1–1, A2–1)—samples from shallow groundwater underlying agricultural 

areas, domestic wells in the Denver aquifer, and all wells in the Laramie–Fox Hills aquifer 

were not analyzed for VOCs. At least one VOC was detected in 62 percent of samples from 

shallow monitoring wells in urban areas and 24 percent of samples from the deeper groundwater used for drinking. All detections for drinking-water samples were from the Dawson and 

Arapahoe aquifers at well depths from 130 to 1,225 ft (fig. 6–17). The concentrations measured 

in all samples were very low —only one concentration (for chloroform) exceeded one-tenth 

its human-health benchmark. The common occurrence of VOCs in shallow groundwater and 

detections of VOCs in samples of deeper groundwater are additional indicators of the vulnerability of Denver Basin groundwater to contamination. 

Figure 6–17. Volatile 

organic compounds (VOCs) 

were detected in about 

one-third of Denver Basin 

groundwater samples, 

more commonly in samples 

of shallow groundwater 

underlying urban areas than 

in deep groundwater used for 

drinking. Mixtures of VOCs 

(two or more VOCs detected in 

a single sample) were present 

in 15 percent of the sampled 

wells. The health effects 

associated with consuming 

mixtures of VOCs are not well 

understood.

Because VOCs in air can readily 

dissolve in water, groundwater 

samples for VOC analysis are 

collected from Teflon tubing 

within a bagged chamber 

to reduce the possibility of 

contamination. The tubing is 

directly connected at its far 

end to the wellhead (out of 

view) so that the water sample 

does not come in contact with 

the air while it is being placed 

in the sample bottle.

Chapter 6

Figure 6-17.

Depth of well, in feet below land surface

0

500

1,000

1,500

2,000

2,500

Number of VOCs detected*

Number of VOCs detected

per well sample and depth

0 1 2 3 4

Deep well used for 

 drinking water

EXPLANATION

Shallow monitoring 

 well, urban area

Well type

Alluvial

Dawson

Denver

Arapahoe

Aquifer

Monitoring wells in the 

shallow aquifer system

Deep bedrock wells used

for drinking water

*Carbon disulfide detections not

 included for samples from the

 deep bedrock aquifers

Photograph by Stephen S. Anthony, USGS

Chapter 6: Understanding Where and Why Key Contaminants Occur in Groundwater 67
Seventeen VOCs were detected

Seventeen VOCs were detected in at least one sample from the study wells (appendix A2). 

The VOCs most commonly detected were trihalomethanes (THMs), organic synthesis compounds,

and solvents, but gasoline oxygenates, hydrocarbons, and refrigerants also were detected. 

Trihalomethanes, such as chloroform and bromodichloromethane, are disinfection by-products 

formed during the chlorination of drinking water. Organic synthesis compounds, such as carbon 

disulfide, are used to form other organic compounds. Solvents, such as tetrachloroethene (PCE) 

and methylene chloride, are used to dissolve or disperse other substances and are common in 

industrial, commercial, and domestic products. Gasoline oxygenates, such as methyl tert-butyl 

ether (MTBE), are added to gasoline to comply with USEPA air-quality regulations; MTBE 

was banned for use by the State of Colorado in April 2002.(47) Gasoline hydrocarbons, such as 

toluene, are organic compounds that are common in gasoline and other petroleum products. 

Refrigerants, such as dichlorodifluoromethane, are used for cooling and air conditioning. Of 

the 17 VOCs detected in Denver Basin groundwater, the only one with a natural source that 

is important for this study is carbon disulfide, which forms from the reaction between organic 

matter and dissolved sulfide under anoxic conditions. In anoxic groundwater, carbon disulfide 

likely is naturally occurring; in oxic groundwater, it likely is from manmade sources. 

Seven VOCs each were detected in two or more samples from Denver Basin groundwater 

wells (fig. 6 –18); an additional 10 each were detected in a single sample (appendix A2). Chloroform

was the VOC most commonly detected in samples of shallow groundwater underlying urban 

areas, and carbon disulfide was the VOC most commonly detected in samples of deep groundwater from wells used for drinking. Less frequent detections and lower concentrations in newer 

urban areas than older areas likely result because, in newer areas, there are fewer sources of 

VOCs, and there has been less time for contaminants in recharge to reach the water table.

In the 1993 South Platte River Basin study of shallow groundwater underlying older, more 

urbanized areas of the Denver metropolitan area, MTBE was detected in 79 percent of samples, 

more than twice as frequently as any other VOC (fig. 6 –18).(41) In contrast, MTBE was detected 

in only 7 percent of samples from Denver Basin wells in newer suburban areas. The difference 

in detection frequency is consistent with a greater abundance of MTBE sources in densely 

developed urban areas in 1993 and a statewide ban on MTBE use implemented in 2002.

Chloroform was the VOC most frequently detected in groundwater from Denver 

Basin wells and in groundwater across Principal Aquifers sampled nationwide as part of the 

NAWQA Program.(35, 48) Chloroform and other THMs can infiltrate to aquifers by the widespread application of chlorinated water to the land surface (for example, by lawn irrigation) and 

by releases of chlorinated water to the subsurface from leaky water-supply and sewer lines or 

septic systems. Once in the subsurface, chloroform can be degraded by microorganisms under 

anoxic conditions. The three samples from drinking-water wells with detectable concentrations 

of chloroform were oxic.

Some household 

products contain 

VOCs or chemicals 

that form VOCs when 

added to water. 

Photograph by Joel Beamer, professional photographer

68 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05
Figure 6–18. Seven volatile organic compounds (VOCs) each were 

detected in two or more samples of Denver Basin groundwater. Of 

these, only chloroform and carbon disulfide were detected in more 

than 10 percent of all samples. Chloroform was detected in 45 percent 

of samples of shallow groundwater underlying newer suburbs of 

the south Denver metropolitan area. The most frequently detected 

VOCs in deeper groundwater were carbon disulfide, chloroform, 

and dichlorodifluoromethane. Only carbon disulfide was detected in 

more than 10 percent of the deep groundwater samples—most of 

these samples were anoxic, which is consistent with a natural source 

for this compound. MTBE, a gasoline additive, was detected about 

10 times less frequently in samples of shallow groundwater collected 

in newer suburban areas in 2003 than in shallow groundwater 

samples from older urban areas collected in 1993. The use of MTBE 

as a gasoline additive was discontinued in 2002.

Chapter 6

Chloroform

Carbon disulfide**

Bromodichloromethane

Tetrachloroethene (PCE)

Methylene chloride

Dichlorodifluoromethane

 Methyl tert-butyl ether (MTBE)

Chloroform

Carbon disulfide

Bromodichloromethane

Tetrachloroethene (PCE)

Methylene chloride

Dichlorodifluoromethane

 Methyl tert-butyl ether (MTBE)

Shallow wells, urban area

Most frequently detected VOCs

Denver Basin study

(2003–05)

South Platte River Basin study

(1993)

Deeper bedrock wells used for

drinking water

*

*

**

ND

ND

Older urban core/

nearby suburbs

Newer suburbs

(since about 1970)

EXPLANATION

Figure 6-18.

Assessment level, in 

micrograms per liter

Any ≥0.2

Well type

Shallow wells, urban area

Deeper bedrock wells used 

 for drinking water

Not detected

No analysis

ND

0 20 40 8060

Percentage of samples in which VOC was detected

0 20 40 8060 100 100

0 20 40 8060

Percentage of samples in

which VOC was detected

100

Carbon disulfide in samples

 from deeper bedrock aquifers

 may be naturally occurring

Chapter 6: Understanding Where and Why Key Contaminants Occur in Groundwater 69
70 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05

Monitoring wells that tap the South Platte River alluvial aquifer and shallow parts of bedrock aquifers were 

sampled to study the effects of land and water resource development on shallow groundwater quality. 

(Above, the South Platte River near Kersey, Colorado.)
Chapter 7: Vulnerability of Groundwater 

to Contamination 

This chapter describes 

the vulnerability of the

alluvial aquifer and

Denver Basin bedrock

aquifers to contamination. 

Human activities in the Denver Basin have visibly changed the land 

surface, as natural grasslands have been converted to rangeland, 

cropland, small towns, residential developments, and large cities. The 

related changes to groundwater quality and the vulnerability of groundwater to 

contamination are less visible, but no less real. Development of the land and of 

water resources has altered the amount and quality of groundwater recharge, 

leading to the degradation of water quality in the alluvial aquifer and shallow 

parts of bedrock aquifers. Deeper groundwater—the groundwater resource—is 

vulnerable to contamination by downward-migrating shallow groundwater. 

Chapter 7

Converting rangeland to residential suburbs and other human 

activities increases the vulnerability of Denver Basin groundwater to 

contamination. As other areas along the Front Range are developed, 

groundwater in those areas may undergo a decrease in quality similar 

to the changes observed in the Denver Basin. 

Photograph by Jon Wark, Wark Photography, Inc.
Human Activities Have Degraded the Quality of Shallow Groundwater— 

the Source of Recharge to the Deeper Aquifers

The effects of human activities—alteration of groundwater flow by irrigation and pumping 

and use of chemicals—are evident in the degraded quality of groundwater in the alluvial and 

shallow bedrock aquifers. The alluvial aquifer and shallow unconfined parts of the bedrock 

aquifers are most vulnerable to contamination because of proximity to the land surface, shallow 

depths to water, porous aquifer materials, lack of a confining layer, and increased recharge from 

excess irrigation water. 

In this semiarid climate, the addition of large quantities of recharge water, primarily 

excess irrigation water, either from agriculture or landscape watering, has markedly changed 

the hydrology from its natural state. Aquifer sediments in the Denver Basin are naturally rich in 

minerals that contain the trace elements arsenic, selenium, and uranium, which dissolve when they 

come into contact with oxic recharge water. As a result, each of these constituents was detected 

in most or all samples of shallow groundwater (fig. 7–1). More than one-third of the shallow 

Water-table monitoring

wells in the shallow aquifer 

system: alluvial aquifer

 and shallow parts of

Dawson and Denver

aquifers (n=60)

Denver

aquifer

(n=11)

Dawson

aquifer

(n=40)

Arapahoe

aquifer

(n=29)

LaramieFox Hills

aquifer

(n=10)

Wells in bedrock aquifers used for drinking-water supply

Dissolved

oxygen

(DO)

Arsenic Selenium Uranium

Tritium

(Hydrogen-3 in 

tritium units)

(n=12)

(n=12)

(n=10)

Redox

chemistry

Contaminants from

geologic sources

Groundwater

age

Oxic: DO

≥0.5 mg/L

Anoxic: DO

<0.5 mg/L

Detected

Not

detected

Detected

Not

detected

Detected

Not

detected

3

H≥0.3 TU 

3

H<0.3 TU

Well characteristic

and aquifer

(n=number

of samples)

Nitrate

(NO3

)

Pesticides

and (or)

VOCs*

Contaminants from

manmade sources

Figure 7-1.

NO3

≥1 mg/L

NO3

<1 mg/L

Detected

Not

detected

Not

sampled

Not

sampled

Increasing stratigraphic age

*Samples from water-table monitoring wells in the agricultural land-use study were not analyzed for VOCs. 

 Carbon disulfide detections not included for samples from the deep bedrock aquifers.

(n=1)

Figure 7–1. Detections of natural and human contaminants and other tracers of shallow, young groundwater indicate 

vulnerability of the groundwater to contamination. Contamination of groundwater by chemicals from geologic and 

manmade sources is most evident for the shallow aquifer system, but some contamination is present in the deeper 

bedrock aquifers used for drinking-water supply. Tracers of shallow, young groundwater (dissolved oxygen and 

tritium) in the bedrock aquifers were most common in the Dawson aquifer and decreased as aquifer age increased.

72 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05
groundwater samples (22 of 60 samples) had a detection of arsenic, selenium, and (or) uranium 

at a concentration of concern for human health if the groundwater were to be used for drinking. 

Historically low recharge and high evapotranspiration in the semiarid climate of the 

Denver Basin resulted in the natural accumulation of soluble salts in the unsaturated zone. 

The relatively recent introduction of irrigation water, with repeated cycles of application of 

freshwater to the land surface, has dissolved some of these salts and increased their downward 

movement to the water table—dissolved-solids concentrations were 10 or more times higher 

in some shallow groundwater than in deep groundwater. Dissolved-solids concentrations 

are high enough in some parts of the Denver Basin to preclude use of shallow groundwater 

even for irrigation.

Nitrate, like trace elements and dissolved solids, has accumulated naturally in the 

unsaturated zone. As grassland and rangeland were converted to agricultural use and to urban 

and suburban landscapes, this nitrate was gradually transported down to the water table. Most 

nitrate in shallow groundwater, however, probably is derived from human sources—septic 

systems, manure, municipal and industrial wastewater, and fertilizers for crops, lawns, golf 

courses, and landscaping. Nitrate concentrations greater than 1 mg/L, associated with human 

contamination, were detected in two-thirds of shallow groundwater samples (fig. 7–1). 

Concentrations in some samples exceeded the MCL of 10 mg/L as nitrogen by a factor 

of 2 or more.

Other chemicals that are applied deliberately or released accidentally at the land surface 

also can be carried down to the water table. Almost half of the samples of shallow groundwater 

in agricultural and urban areas contained at least one pesticide and (or) VOC (fig. 7–1). 

Overall, four of every five monitoring wells sampled in the alluvial aquifer and shallow 

unconfined parts of bedrock aquifers had a detection of nitrate, a pesticide compound, and (or) 

a VOC that indicated contamination from human activities (fig. 7–2). These aquifers are used 

largely to supply irrigation water for agriculture, but are nonetheless potential future sources 

of drinking water in rapidly growing areas of the Front Range. Further, because shallow 

groundwater is vulnerable to water-quality changes within relatively short time periods (years 

to decades), it can indicate how human activities might, in time, affect older (centuries to 

millennia), deeper groundwater.

Chapter 7

Figure 7–2. Irrigation and pumping have altered the natural groundwater flow system in the Denver Basin, which has 

brought manmade contaminants down to shallow and deep groundwater.

0 20 40 60 80 100

Percentage of wells with nitrate

concentrations greater than 1 milligram

per liter, pesticide compound detection,

and (or) VOC detection

>700

401 to 700

125 to 400

<125

Depth of well, 

in feet below land surface

Detections of chemicals from human

sources in Denver Basin aquifers

Figure 7-2.

*Percentages for drinking-water

 wells do not include domestic wells

 in the Denver aquifer and all wells 

 in the Laramie–Fox Hills aquifer

Monitoring wells in the 

shallow aquifer system

Deep bedrock wells used

for drinking water*

Photograph copyright istockphoto.com

Chapter 7: Vulnerability of Groundwater to Contamination 73
In other Principal Aquifers in the United States, alteration of the hydrologic system has 

affected the deep drinking-water resource, sometimes in unexpected ways 

Findings from other Principal Aquifer assessments conducted in similar hydrogeologic and climatic settings 

provide insights on the vulnerability of Denver Basin groundwater 

San Joaquin Valley, California

In the San Joaquin Valley, California, pumping of groundwater from wells for agricultural and urban development and 

recharge of excess irrigation water have altered the movement of groundwater and contaminants from geologic sources (see 

sidebar, A cautionary tale: Irrigation and pumping have increased uranium concentrations in groundwater in the Central Valley of 

California, p. 49). Shallow groundwater with high uranium concentrations has moved downward into parts of the aquifer system 

used for drinking water that historically have had low uranium concentrations.(49)

More unexpectedly, though, wells not being pumped also can alter groundwater movement by providing pathways for movement of water and contaminants from one part of the aquifer to another.(50) Where hydraulic gradients are downward, shallow 

groundwater and the contaminants it contains can move down through and along wells into deeper parts of the aquifer commonly 

used for public supply. In the San Joaquin Valley, natural groundwater flow paths were “short circuited” by a public-supply well 

and contaminant movement downward increased. During winter, when pumping from the public-supply well was minimal, 

a substantial volume of shallow water migrated downward through the public-supply well into deep groundwater. VOCs and 

pesticides that were consistently detected in shallower parts of the aquifer also were detected in water from a deep monitoring 

well near the public-supply well. 

High Plains Aquifer, Nebraska

In York, Nebraska, pumping from the High Plains aquifer mixes water from shallow and deep parts of the aquifer, changing 

redox conditions and mobilizing uranium in the deep aquifer.(50, 87) In this area of east-central Nebraska, the High Plains aquifer 

is a layered sequence of saturated sands separated by confining layers. The aquifer is heavily pumped to provide water for 

irrigation, and the pumping wells commonly are screened in more than one aquifer layer. Hydraulic head in the upper aquifer is 

higher than in the lower aquifer, so that groundwater would flow downward if the confining unit were not present. When wells 

are not pumping, water can flow from the shallow, unconfined aquifer down through the well and into the deep, confined aquifer. 

The mixing of oxic, slightly acidic shallow groundwater with anoxic, slightly alkaline deep groundwater causes uranium to be 

released from deep aquifer sediments. A public-supply well that is screened in the deep aquifer produces water with elevated 

uranium concentrations.

Data from the Denver Basin study suggest that mixing of groundwater with different ages and redox conditions is present 

in some parts of the Denver Basin such that contaminants present in poor-quality shallow groundwater have the potential to 

move to deeper bedrock aquifers used for drinking-water supply.

Groundwater flow can be “short circuited” 

through a well that is open to more than 

one depth in an aquifer. When the well is 

not being pumped, if the vertical hydraulic 

gradient is downward, contaminants can 

move in and along the well to deeper parts 

of the aquifer. Downward flow in a well can 

occur even if there is a confining layer of 

clay or shale within an aquifer if the well 

penetrates the confining layer.

Water table

Confining layer Flow

in

aquifer

Higher 

hydraulic 

head

Lower 

hydraulic 

head

Flow in well under 

pumping conditions

Open interval

NOT TO SCALE

Flow in well under 

nonpumping conditions

Well

CH7 sidebar other PAs

Modified from Bexfield and others(86)

74 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05
Geochemistry and Groundwater Ages Suggest Deep Parts of the Denver 

Basin Might Be Vulnerable to Contamination From Shallow Groundwater

Shallow groundwater is more likely than deeper groundwater to contain fertilizers, 

pesticides, solvents, gasoline oxygenates, tritium, and other manmade chemicals. As shallow 

water moves downward by natural and (or) human-influenced flow processes, it can affect the 

water quality of the deeper groundwater. Historically, groundwater in deep parts of Denver 

Basin bedrock aquifers was isolated from the effects of human activities at the land surface. 

In some areas, however, the movement of shallow groundwater into the deeper parts of the 

aquifer system is now apparent. Manmade contaminants (nitrate, pesticide compounds, and 

VOCs) were detected in 24 percent of the samples from deep bedrock wells analyzed for these 

chemicals (figs. 6–10, 6–15, 6–17). The detections primarily were for samples collected at 

depths of less than 500 ft, but VOCs were present in some samples from depths of more than 

500 ft (fig. 6–17).

Tritium, a product of atomic bomb testing, is an indicator of recharge that has occurred since 

the early 1950s. Detections of tritium in deep groundwater also can provide an indication that 

shallow, young groundwater has moved downward. In the Denver Basin, tritium was detected 

in the Dawson, Denver, and Arapahoe aquifers at well depths as great as 500 ft (fig. 7–1).

The presence of dissolved oxygen at depth also can indicate the downward movement 

of young, shallow groundwater. Dissolved oxygen is brought in from the atmosphere with 

recharge water. As the groundwater moves through the aquifer along a flow path, the dissolved 

oxygen in the groundwater commonly is consumed by biological and chemical reactions. 

Because water in an aquifer is isolated from the atmosphere, the dissolved oxygen, once 

consumed, is not replenished by natural processes (see sidebar, How do redox reactions work?, 

p. 36). Oxygen in the deep groundwater, however, can be replenished as a result of human 

activities. Heavy pumping from deep bedrock wells and the short circuiting of groundwater can 

cause shallow, oxic groundwater to move downward.(2, 49, 50) Aquifer storage and recovery also 

can provide a direct pathway for shallow, oxic water to reach deep groundwater (see sidebar, 

Increased interest in aquifer storage and recovery programs in the Denver Basin, p. 33). Oxic 

water was present in several deep wells in the Denver Basin (figs. 4 – 5, 7–3). In addition to 

indicating the vulnerability of deep groundwater to contamination from the shallow part of the 

system, the mixing of shallow, oxic water with deeper, anoxic groundwater can cause constituents associated with the aquifer rocks and sediment to dissolve.

Chapter 7

Anoxic groundwater—Dissolved oxygen <0.5 mg/L

Oxic groundwater—Dissolved oxygen ≥0.5 mg/L

EXPLANATION

Figure 7-3.

Young

(post-1950s)

Estimated age of water

0

2,000

1,500

1,000

500

Depth of well, in feet below land surface

Groundwater age and redox

condition by well depth

Mixed Old

(pre-1950s)

Increasing groundwater age

and anoxic condition with well depth

Increasing groundwater age

Increasing 

well depth

Figure 7–3. There are at least three 

ways that young, oxic water might 

move deeper into the aquifer system—

through high-volume pumping of deep 

groundwater, short circuiting of shallow 

groundwater through deep wells, and 

direct injection of water into the deeper 

aquifer during aquifer storage and 

recovery. The downward migration of 

young groundwater and any associated 

contaminants increases the vulnerability 

of deep groundwater to contamination. 

Chapter 7: Vulnerability of Groundwater to Contamination 75
The quality of the deep groundwater of the Denver Basin still is very good — only one 

drinking-water well contained at least one constituent at a concentration of concern for human 

health, and that well was relatively shallow. The results of this study indicate, however, that 

the deep groundwater is vulnerable to contamination from human activities at the land surface, 

whether from application or releases of chemicals or from increased concentrations of trace 

elements resulting from downward movement of shallow, oxic groundwater. Of the four 

bedrock aquifers used as a drinking-water resource, the Dawson aquifer is the most vulnerable 

to contamination. A plentiful, high-quality source of drinking water is an enormous economic 

benefit to the Denver Basin area. Understanding the factors that control groundwater quality is 

essential to managing this vital resource.

Sustainability of Denver Basin groundwater

The sustainability of Denver Basin groundwater depends not only on the quality of water available but also 

on the quantity of that water. Studies by Paschke(2) and others (57, 88) indicate that high rates of pumping are not 

sustainable indefinitely. Of concern for many water users and managers is the removal of water from storage 

in the bedrock aquifers and subsequent decrease in groundwater availability. Changes in land and water uses 

since the 1950s have altered the groundwater-flow system and groundwater availability. More groundwater 

is available in some parts of the alluvial aquifer and shallow parts of the hydrologic system, but less water is 

available in some parts of the bedrock aquifers.(2) Increased pumping has resulted in a decline in water levels, 

less discharge from bedrock aquifers to streams and to the alluvial aquifer, and a decrease in the amount of 

water in storage in the bedrock aquifers. One action water managers can consider to address these concerns 

is to pump less water from bedrock aquifers. To demonstrate the possible effects from such a decrease, 

Paschke used different predictive simulations in a Denver Basin groundwater-availability model.(2) The first 

simulation assumed recharge and pumping conditions did not change from 2003 through 2053, and the second 

simulation assumed recharge did not change but pumping was discontinued from the lowermost part of the 

Arapahoe aquifer. Results of the first simulation indicated continued water-level declines, continued decrease 

in discharge from the bedrock aquifers to the surficial system, and continued loss of water from storage. When 

pumping from the lowermost part of the Arapahoe aquifer was stopped in the second simulation, however, 

there was an increase in water levels, and the rate of aquifer depletion slowed. A reduction in groundwater 

pumping, along with other management actions such as water conservation, water reuse, increased use of 

surface water for supply, and aquifer-storage and recovery projects, might extend the usable life of the Denver 

Basin aquifer system. Many municipalities that rely on Denver Basin groundwater for supply have instituted 

these and other management actions to decrease the volume of groundwater withdrawals.

Groundwater sustainability can be defined as the achievement of an acceptable 

tradeoff between groundwater use and the long-term effects of that use.(89)

76 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05
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Nevada, New Mexico, Utah, 1993–2009: U.S. Geological Survey Circular 1358, 113 p. (Also available at 

http://dx.doi.org/10.3133/cir1358.)

82.	 McMahon, P.B., and Böhlke, J.K., 1996, Denitrification

and mixing in a stream-aquifer system—Effects on nitrate 

loading to surface water: Journal of Hydrology, v. 186, 

p. 105–128.

83. Denver Water, 2011, Drought response plan: Denver, 

Denver Water, 34 p., accessed February 2013 at http://

www.denverwater.org/docs/assets/CECFBC95-E611-

03E5-FD2B05E1B8A6B497/DroughtResponsePlan.pdf.

84. Aquacraft, Inc., 2006, Post drought changes in residential 

water use: Aquacraft, Inc., 41 p.

85. Nakagaki, Naomi, 2011, NAWQA GIS ancillary data—

Estimates of 1992 agricultural pesticide use for 500-meter 

radius buffers around NAWQA sampling sites: U.S. 

Geological Survey Web site, http://water.wr.usgs.gov/pnsp/

gis/data/gwancil/GWanc_pest.htm.

86.	 Bexfield, L.M., Thiros, S.A., Anning, D.W., Huntington, J.M.,

and McKinney, T.S., 2011, Effects of natural and human 

factors on groundwater quality of basin-fill aquifers in

the southwestern United States—Conceptual models for 

selected contaminants: U.S. Geological Survey Scientific

Investigations Report 2011–5020, 90 p. (Also available 

at http://pubs.usgs.gov/sir/2011/5020/.)

87.	 Ayotte, J.D., Gronberg, J.M., and Apodaca, L.E., 2011,

Trace elements and radon in groundwater across the 

United States, 1992–2003: U.S. Geological Survey 

Scientific Investigations Report 2011–5059, 115 p.

(Also available at http://pubs.usgs.gov/sir/2011/5059.)

88. Colorado Division of Water Resources, 1998, Denver 

basin and South Platte River basin technical study, final

report: Colorado Senate Bill 96 –074, [variously paged].

89. Alley, W.M., 2006, Tracking U.S. groundwater—Reserves 

for the future?: Environment: Science and Policy for 

Sustainable Development, v. 48, no. 3, p. 10–25.

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http://www.mrlc.gov/nlcd.php.

91.	 Homer, Collin, Huang, Chengquan, Yang, Limin,

Wylie, Bruce, and Coan, Michael, 2004, Development of 

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v. 70, no. 7, p. 829–840, accessed December 27, 2012, 

at http://www.mrlc.gov/nlcd2001.php.

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irrigation use, 2005: U.S. Geological Survey Open-File 

Report 2008 –1267, 25 p., accessed November 2013 at 

http://pubs.usgs.gov/of/2008/1267/.

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withdrawals and use of water in Colorado, 2005: 

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Report 2010 –5002, 61 p., accessed November 2013

at http://pubs.usgs.gov/sir/2010/5002/.

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2011, Homeowner’s guide to household water conservation:

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garden/07202.html.

96.	 Toccalino, P.L., and Hopple, J.A., 2010, The quality of

our Nation’s waters—Quality of water from public-supply 

wells in the United States, 1993–2007—Overview of major

findings: U.S. Geological Survey Circular 1346, 58 p.

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assessment (SWQA) program: U.S. Geological Survey 

Web site, http://water.usgs.gov/nawqa/swqa/.

References Cited 81
Before the invention of the combustion engine, windmills, such as this antique windmill near 

Denver, supplied a limited amount of groundwater for people and domestic animals in agricultural 

areas of the Great Plains, including eastern Colorado. 

Photograph by Dudikoffski, copyright istockphoto.com

82 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05
Glossary

A

alluvial aquifer An aquifer composed of 

unconsolidated material, such as sand and 

gravel, deposited by a river or other 

flowing water.

alluvial fan A low, outspread, relatively flat 

to gently sloping mass of loose rock material, 

shaped like an open fan or a segment of a 

cone, deposited by a stream (especially in a 

semiarid region) at the place where the rock 

mass issues from a narrow mountain valley 

on a plain or broad valley.

anoxic Water with no dissolved oxygen 

or a very low concentration (less than 

0.5 milligram per liter) of dissolved oxygen.

aquifer A geologic formation, group 

of formations, or part of a formation that 

contains a sufficient amount of saturated 

permeable material (for example, soil, sand, 

gravel and (or) rock) to yield substantial 

quantities of water to wells and springs. 

arkosic sandstone A feldspar-rich sandstone that generally forms from the rapid 

breakdown of granite or granitic rock.

artesian Referring to confined groundwater. 

If the potentiometric surface in a confined 

aquifer is higher than the land surface, 

water discharges spontaneously from a 

well tapping the aquifer; this is called a 

“flowing artesian well.”

B

base flow Groundwater seepage into a 

stream or river. The continual contribution 

of groundwater to streams and rivers is an 

important source of streamflow between 

rain events.

bedrock aquifer As used in this report, 

refers to the sandstone aquifers that make up 

the Denver Basin aquifer system, including 

the Dawson, Denver, Arapahoe, and Laramie–

Fox Hills aquifers. Bedrock is a general term 

for consolidated (solid) rock that underlies 

soils or other unconsolidated material.

C

confined aquifer (artesian aquifer) An 

aquifer in which the groundwater is bounded 

between layers of relatively impermeable 

material, such as clay or dense rock. When 

tapped by a well, water in a confined aquifer 

is forced up, sometimes above the land 

surface, by pressure within the aquifer.

confining unit A hydrogeologic unit of 

impermeable or distinctly less permeable 

material within an aquifer or bounding one 

or more aquifers.

constituent A chemical or biological 

substance in water, sediment, or biota that 

can be measured by an analytical (laboratory) 

method.

contaminant For the purposes of this report, 

any manmade compound at any concentration, or any constituent with a geologic source 

measured at a concentration exceeding the 

designated human-health benchmark.

D

denitrification The bacterial reduction of 

dissolved nitrate to nitrogen gas. Denitrification is the primary process by which nitrate 

can be eliminated naturally in groundwater.

discharge The rate of flow of surface 

water or groundwater past a given point at a 

given moment, expressed as volume per unit 

of time. Also, the outflow from an aquifer, 

spring, or well or up through a streambed.

dissolution The process of dissolving a 

solid (mineral) into a homogeneous solution 

(water). Dissolution reactions result in the 

addition of ions to water as minerals react 

with water. Common dissolution reactions 

include dissolution of carbonate rock 

(limestone or dolomite) and incongruent 

dissolution of silicate minerals (feldspar) 

by carbonic acid (H2

CO3).

Glossary

Glossary 83
domestic well A privately owned well 

that typically serves one home and supplies 

water for human consumption and other 

homeowner uses. 

drawdown The amount that the level of a 

reservoir, water level in a well, or head in an 

aquifer is lowered by the withdrawal of water.

drinking-water well As used in this 

report, a domestic or public-supply well that 

produces water used for drinking; the water 

sample is collected before any treatment. 

E

evaporative concentration An increase in 

the concentration of dissolved solids in 

solution caused by the removal of water 

through evapotranspiration. 

evapotranspiration Loss of water from soil by 

evaporation and plant transpiration combined.

excess irrigation water As used in this 

report, the part of irrigation water applied to 

the surface that is not taken up by plants or 

lost by evaporation and that migrates to an 

aquifer or surface-water body. Also known 

as irrigation return flow.

F

flow path The route or pathway of water 

flowing through the hydrologic system. Typically refers to subsurface (groundwater) flow.

Front Range urban corridor Urban and 

suburban area extending north from Denver to 

Fort Collins (northwest of Greeley) and south 

to Colorado Springs along the eastern face of 

the Rocky Mountains, in Colorado.

G

groundwater Water that exists beneath the

land surface, but most commonly refers to water 

in fully saturated soils and geologic formations.

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 

depends on many variables.

groundwater discharge The flow of water

from the saturated zone, for example, from a 

spring or a well or as seepage to surface water.

groundwater flow path See flow path. 

groundwater recharge The infiltration 

of water to the saturated zone. Also refers 

to water that reaches the water table by 

infiltration of precipitation or irrigation water 

through the unsaturated zone or by seepage 

of water from surface-water bodies, such as 

streams and lakes. 

groundwater source-water-quality assessment A study by the U.S. Geological 

Survey National Water-Quality Assessment 

(NAWQA) Program that characterizes the 

quality of groundwater from aquifers used as 

a source of public drinking-water supply in 

the United States.

H

Health-Based Screening Level (HBSL)

An estimate of concentration (for a 

noncarcinogen) or concentration range 

(for a carcinogen) in water that (1) may be 

of potential human-health concern, (2) can 

be used as a threshold value against which 

measured concentrations of contaminants 

in ambient groundwater samples can be 

compared, and (3) is consistent with U.S. 

Environmental Protection Agency Office 

of Water methodologies. 

human-health benchmark A threshold 

concentration above which the concentration 

of a contaminant in drinking water could have 

adverse effects on human health. Treatment or 

other measures can be used before the water 

is consumed to lower the concentration of the 

contaminant below the benchmark. 

hydraulic conductivity The volume of 

water that will move in a porous medium in a 

given amount of time under a given hydraulic 

gradient through a given area measured 

at right angles to the direction of flow. A 

measure of the ease with which a fluid moves 

through a porous or fractured material.

hydraulic gradient In an aquifer, the rate 

of change of total head (water-level altitude 

in a well) per unit of distance of flow at a 

given point and in a given direction. Water 

will flow from higher hydraulic head to lower 

hydraulic head.

hydraulic head The height above a datum 

plane (such as sea level) of the column of 

water that can be supported by the hydraulic 

84 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–2005
pressure at a given point in a groundwater 

system. For a well, the hydraulic head is equal 

to the distance between the water level in the 

well and the datum plane. I

intrinsic susceptibility A measure of the 

ease with which a contaminant in water enters 

and moves through an aquifer; a characteristic 

of the aquifer and overlying material and 

hydraulic conditions independent of the 

chemical characteristics of the contaminant 

and its sources. L

laboratory reporting level Concentration 

determined in the laboratory at which the risk 

of a false negative (not detecting an analyte 

when it is present) is not more than 1 percent.

land-use study A study by the U.S. 

Geological Survey National Water-Quality 

Assessment (NAWQA) Program to assess the 

effects of a specific land-use type (generally 

agricultural or urban) on groundwater quality, 

in most cases by sampling groundwater from 

monitoring wells that tap water from or near 

the water table.

lithology The physical character of a rock on

the basis of color, structure, mineralogical 

composition, grain size, and other characteristics. M

major aquifer study A study by the U.S. 

Geological Survey National Water-Quality 

Assessment (NAWQA) Program that involves 

sampling of water at 20 to 30 domestic and 

(or) public-supply wells that withdraw water 

from major aquifers. The major aquifer 

studies represent a mix of land uses and target 

water that is used for drinking-water supply.

Maximum Contaminant Level (MCL)

Maximum permissible level of a contaminant 

in water that is delivered to any user of a 

public water system. MCLs are enforceable 

standards established by the U.S. Environ-

mental Protection Agency. 

median The middle or central value in a 

distribution of data ranked in order of magni-

tude such that half of the data are higher than 

the median and half are lower. The median is 

also called the 50th percentile.

methemoglobinemia A health condition 

characterized by reduced ability of the blood to

carry oxygen. Infants are most affected. One of

the most common causes is nitrate in drinking 

water. Also called “blue baby syndrome.” 

milligrams per liter (mg/L) A unit expressing 

the concentration of a chemical constituent 

as weight (milligrams) of constituent per 

unit volume (liter) of water; equivalent to 

one part per million in most streamwater and 

groundwater. One thousand micrograms per 

liter (μg/L) is equivalent to 1 mg/L.

monitoring well A well used to measure 

water quality or groundwater levels continu-

ously or periodically. Not typically used as a 

source of drinking water. Sometimes referred 

to as an “observation well.” N

nitrate An ion consisting of one nitrogen 

atom and three oxygen atoms (NO

3

–

). Nitrate 

is a plant nutrient and is very mobile in soils. O

oxic Water with a concentration of 

dissolved oxygen greater than or equal to 

0.5 milligram per liter.

P

pesticide compounds A term used to 

refer collectively to parent pesticides, their 

degradates and, where applicable, their 

manufacturing by-products. 

predevelopment The time prior to substan-

tial groundwater development by humans or 

effects of agricultural, urban, suburban, or 

other human-related land uses.

Principal Aquifer A regionally extensive 

aquifer or aquifer system that has the potential 

to be used as a source of potable water. A 

Principal Aquifer can be composed of one or 

more major aquifers.

public-supply well A privately or publicly 

owned well that provides water for public 

use to (1) a community water system, (2) a

transient noncommunity water system, such 

as a campground, or (3) a nontransient, 

noncommunity system, such as a school. 

Glossary

Glossary 85
R

recharge The addition of water to the 

saturated zone naturally by precipitation or 

runoff or artificially by spreading or injection. 

Also, the water that is added. 

reduction-oxidation (redox) Chemical 

reactions that involve the transfer of electrons 

from one chemical species to another, 

resulting in a change in the valence state of 

the species. Redox processes in groundwater 

often are microbially facilitated.

S

saturated The condition in which all the 

pores (voids, interstices) within a material are 

filled with a liquid, typically water.

saturated zone The region in the subsurface 

in which all the spaces (pores and fractures) 

are filled with water and are under pressure 

greater than atmospheric pressure. 

Secondary Maximum Contaminant Level 

(SMCL) Guidelines set by the U.S. Environmental Protection Agency for concentrations 

of “nuisance” constituents in drinking water 

that may cause unwanted effects, such as 

unpleasant taste, color, or odor; discoloration 

of skin or teeth; or corrosion or staining of 

plumbing fixtures. Public drinking-water 

systems are recommended but not required to 

comply with these guidelines. 

sedimentary rocks Rocks composed of 

particles derived from the erosion or weathering of preexisting rocks or from chemical 

precipitation from water. Sandstone and 

limestone are examples of sedimentary rocks. 

shallow aquifer system As used in this 

report, the alluvial aquifer and parts of the 

Dawson, and Denver aquifers less than 

125 feet below the land surface. 

subsurface The region of earth materials 

beneath the land surface that encompasses the 

soil and unsaturated and saturated zones.

susceptibility See intrinsic susceptibility.

T

tritium unit (TU) A measure of the 

concentration of tritium (3H), equal to one 3

H atom in 1,018 atoms of hydrogen (H), or 

3.24 picocuries per liter.

tonstein Sedimentary rock composed 

mainly of the clay mineral kaolinite. 

Tonsteins are formed by the alteration and 

leaching of volcanic-ash layers deposited in 

coal-forming swamps.

U

unconfined aquifer An aquifer that has a 

water table; an aquifer containing unconfined 

groundwater.

unconsolidated material Deposit of loosely 

bound sediment that typically fills topographically low areas.

unsaturated zone A subsurface zone 

containing both water and air. The unsaturated 

zone is limited above by the land surface and 

below by the water table.

V

volatile organic compound (VOC) An 

organic chemical that has a high vapor 

pressure relative to its water solubility. VOCs 

include components of gasoline, fuel oils, 

lubricants, organic solvents, fumigants, some 

inert ingredients in pesticides, and some 

by-products of chlorine disinfection.

vulnerability The tendency or likelihood for 

contaminants to reach a specified position in 

the groundwater system after introduction at 

some location above the uppermost aquifer. 

The vulnerability of a groundwater resource 

to contamination depends both on the intrinsic 

susceptibility of the resource and on the 

locations and types of human and geologic 

sources of contaminants, locations of wells, 

and the characteristics of the contaminant(s).

W

water table The upper surface of the 

saturated zone below which all voids (spaces) 

are filled with water.

water-table monitoring well As used in this 

report, a well in the alluvial, Dawson, and 

Denver aquifers with a depth of less than 

125 feet below land surface, constructed for 

the collection of hydrologic data, such as 

water levels and water quality, and not used 

as a source for drinking water.

86 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–2005
Appendixes 1–4

Appendixes
Appendix 1. Study Components of the Denver Basin 

Aquifer System Water-Quality Assessment

How does land use affect groundwater quality? How does water quality change as it moves 

through an aquifer? What is the quality of the drinking-water resource? NAWQA groundwater 

assessments include different types of studies, specifically designed to answer questions such 

as these. 

• Land-use studies sampled water from water-table monitoring wells installed in urban

and agricultural areas to assess the effects of these land uses on the quality of the

underlying groundwater. Although not usually used for drinking, this shallow groundwater system supplies recharge water to the deeper aquifer system.

• Major aquifer studies provide a broad overview of the quality of the deeper aquifer

system used for drinking-water supply. Most of the wells sampled were domestic wells

that were distributed across a large area in a mixture of land uses.

• Source-water-quality assessment studies sampled water from public-supply wells

to understand occurrence of unregulated manmade chemicals in the groundwater

resources that serve large numbers of people.(97)

Each Denver Basin study involved sampling water from a network of 10 to as many as 31 wells. 

Results of these studies were reinforced by locating some of the studies within the area sampled

for the major aquifer studies. This nesting of wells was designed to provide information on how 

the quality of the recharge water affects that of deeper groundwater.

88 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05
Appendixes

Table A1–1. Study components of the Denver Basin aquifer system water-quality assessment.

[All wells were sampled once. Samples for radionuclides and (or) stable isotopes were not collected at all wells in a study]

Study component Study description Well type Year 

sampled

Number 

of wells 

sampled

Water-quality

parameters*

Land-use study Characterize the quality of recently 

recharged water in the alluvial and 

Denver aquifers underlying nonirrigated wheat fields 

Water-table 

monitoring 

wells

2003 31 PP, M, N, D, R, TE, 

P-PD, SI-HO

Land-use study Characterize the quality of recently 

recharged water in the alluvial and 

Dawson aquifers underlying newer 

(since about 1970) urban setting in 

the south Denver metropolitan area 

Water-table 

monitoring 

wells

2003† 29 PP, M, N, D, R, TE, 

P-PD, SI-HO, V

Major aquifer study Broadly characterize the regional 

drinking-water resource of the 

Dawson aquifer

Domestic 2004–

2005

29 PP, M, N, D, R, TE, 

P-PD, SI-HOC, V

Major aquifer study Broadly characterize the regional 

drinking-water resource of the 

Denver aquifer

Domestic 2005 10 PP, M, N, D, R, TE, 

SI-HOC

Major aquifer study Broadly characterize the regional 

drinking-water resource of the 

Arapahoe aquifer

Domestic and 

public supply

2005 29 PP, M, N, D, R, TE, 

P-PD, V, SI-HOC

Major aquifer study Broadly characterize the regional 

drinking-water resource of the 

Laramie–Fox Hills aquifer

Domestic 2005 10 PP, M, N, D, R, TE, 

SI-HOC

Groundwater sourcewater-quality

assessment

Characterize the quality of water in 

the Dawson and Denver aquifers 

used for public supply

Public supply 2003 12 PP, M, N, D, R, TE, 

P-PD, V

*Water-quality parameters

 D, dissolved organic carbon

M, major ions

 N, nutrients

P-PD, pesticides and pesticide degradates

PP, physical properties including water temperature, specific conductance, pH, and dissolved oxygen

R, radiochemical including radon and (or) carbon14 and tritium

SI-HO, stable isotopes of hydrogen and oxygen

SI-HOC, stable isotopes of hydrogen, oxygen, and carbon

TE, trace elements

V, volatile organic compounds

†Samples for stable isotopes of hydrogen and oxygen were collected in 2005.

Appendix 1 89
Figure A1–1. Location of water-table monitoring wells in the Denver Basin aquifer system sampled 

during 2003– 05 for land-use studies in agricultural and urban settings.

Brighton

Parker

Colorado

Springs

Monument

Franktown

Castle

Rock

Highlands

Ranch

Littleton

Denver Aurora

Henderson

Greeley

Fort Morgan

Limon

Palmer Divide

Denver

Base from U.S. Geological Survey

digital data, 2012, 1:100,000 

0 10 20 MILES

0 10 20 KILOMETERS

105° 104°

40°

39°

COLORADO

Denver

Basin

South Platte River

EXPLANATION

Figure App1-1

Urban/suburban development

Forest

Grassland

Irrigated pasture or row crops

Non-irrigated small grains or fallow

Water

Land-use category

Water-table monitoring wells

 sampled for land-use studies

Denver Basin study area boundary

Agricultural setting

Urban setting

90 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05
Figure A1–2. Location of domestic and public-supply wells in the Denver Basin aquifer system sampled 

during 2003–05 to assess the quality of the drinking-water resource.

Appendixes

Denver

Denver

105° 104°

40°

39°

Base from U.S. Geological Survey

digital data, 2012, 1:100,000 

0 10 20 MILES

0 10 20 KILOMETERS

COLORADO

Denver

Basin

South Platte River

Alluvial aquifer

Dawson aquifer

Denver aquifer

Arapahoe aquifer

Confining unit

Laramie–Fox Hills aquifer

Denver Basin aquifer system

EXPLANATION

Figure App1-2

Wells sampled for major aquifer

 studies and groundwater source- 

 water-quality assessment study

Denver Basin study area boundary

Domestic

Public supply

Appendix 1 91
Appendix 2. Groundwater-Quality Properties and 

Constituents Measured, a Summary of the Data, and 

Complete Data Archive for 2003– 05

Groundwater-quality properties and constituents measured and a summary of the data for 

the 2003–05 Denver Basin water-quality assessment, including laboratory reporting levels, 

human-health benchmarks for drinking water, and non-health guidelines, are presented 

only online. The data summary and a complete data archive are available for download at 

http://pubs.usgs.gov/circ/1357/.

92 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05
Appendix 3. Constituents in Sandstone Aquifers and 

Shallow Groundwater of the Denver Basin and Across 

the United States

Concentrations of manganese, radon, nitrate, arsenic, selenium, and uranium exceeded humanhealth benchmarks in samples collected during 2003–05 from drinking-water wells in the 

Denver Basin bedrock aquifers, none with an exceedance frequency greater than 5 percent 

(fig. A3–1). Exceedances for the six constituents were less than 8 percent for drinking-water 

wells in sandstone aquifers sampled throughout the United States by the NAWQA Program 

from 1991 to 2010.(35) Concentrations of manganese, dissolved solids, iron, fluoride, and sulfate in 

Denver Basin drinking-water samples exceeded SMCLs. Only the exceedance for fluoride was 

more common in groundwater from Denver Basin bedrock aquifers than from other sandstone 

aquifers nationally. The overall quality of water in Denver Basin bedrocks aquifers and in other 

sandstone aquifers across the United States is mostly suitable for drinking water. 

No VOCs or pesticide compounds in Denver Basin drinking-water samples were detected at 

concentrations greater than human-health benchmarks. Concentrations of only four manmade 

organic compounds in drinking water from other sandstone aquifers were greater than humanhealth benchmarks: the insecticides diazinon and dieldrin and the VOC solvents methylene 

chloride and trichloroethene (TCE). Of these four compounds, only methylene chloride was 

detected in Denver Basin groundwater from wells used for drinking water.

For both the Denver Basin and other locations in the United States, exceedances of humanhealth benchmarks for manganese, nitrate, arsenic, selenium, and uranium were more common 

in samples from shallow groundwater wells in agricultural and urban areas than in groundwater 

from deeper drinking-water wells (figs. A3–1, A3–2). The major difference in exceedances for 

wells in agricultural and urban areas in the Denver Basin and across the United States was a 

much greater frequency of exceedances of the MCL for selenium and uranium in both Denver 

Basin land-use studies and arsenic and nitrate in the agricultural study (fig. A3–2). Exceedances 

of SMCLs for sulfate, manganese, dissolved solids, and chloride also were more common 

for shallow groundwater underlying agricultural and urban areas of the Denver Basin than 

groundwater underlying similar areas nationally. 

The VOC chloroform was detected in 45 percent of samples from shallow groundwater wells 

in urban areas of the Denver Basin (fig. 6–18) and in about 28 percent of similar samples 

collected in other urban areas of the United States. The herbicide atrazine and its degradate 

deethylatrazine were the most common pesticides detected in shallow groundwater wells in 

agricultural areas of the Denver Basin and nationally; however, detection frequencies varied—

about 20 percent (atrazine) and 13 percent (deethylatrazine) in the Denver Basin (fig. 6–16) and 

40 percent (atrazine) and 43 percent (deethylatrazine) across the United States. Prometon, the 

most common herbicide detected in shallow groundwater from urban areas of the Denver Basin 

(fig. 6–16), was the third most common herbicide detected in groundwater collected in urban 

areas of the United States. 

Differences in exceedances of human-health benchmarks and SMCLs in Denver Basin 

groundwater, as well as in other sandstone aquifers and in shallow groundwater underlying 

agricultural and urban areas of the United States, result from differences in geology, recharge 

rates, and geochemical conditions.(35) Human activities that alter the natural groundwater-flow 

system can mobilize naturally occurring constituents in shallow parts of aquifers and transport 

nitrate and other manmade compounds from the land surface to the water table.(35)

Appendixes

Appendix 3 93
Percentage of drinking-water wells

with concentrations greater than indicated benchmark

0 10 20 30 40 50 60 70

Manganese

Dissolved solids

Iron

Fluoride

Sulfate

MCL or HBSL Manganese

Radon*

Uranium

Nitrate

Arsenic

Selenium

SMCL

Constituents in drinking-water wells with 

exceedances of benchmarks 

EXPLANATION

Denver Basin bedrock aquifers

Other sandstone aquifers 

 assessed by NAWQA

Figure A3–1. Exceedances of human- and non-human 

health benchmarks for constituents of concern in 

groundwater wells that tap parts of aquifers used for 

drinking water in the Denver Basin and other sandstone 

aquifers across the United States.(35) (*Proposed radon 

regulation of 4,000 picocuries per liter.)

94 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05
Appendixes

Figure A3–2. Exceedances of human- and non-human health benchmarks for constituents of concern in shallow 

groundwater wells beneath (A) agricultural and (B) urban areas of the Denver Basin and similar land-use settings 

across the United States.(35)

0 10 20 30 40 50 60 70

Percentage of shallow groundwater wells with concentrations greater than indicated benchmark

0 10 20 30 40 50 60 70

Manganese

Dissolved solids

Iron

Fluoride

Sulfate

Manganese

Uranium

Nitrate

Arsenic

Selenium

Chloride

MCL or HBSL

SMCL

MCL or HBSL

SMCL

A. Constituents in shallow groundwater wells beneath

 agricultural areas with exceedances of benchmarks

B. Constituents in shallow groundwater wells beneath

 urban areas with exceedances of benchmarks

EXPLANATION

Denver Basin aquifers

Other aquifers assessed by NAWQA

Appendix 3 95
Appendix 4. Groundwater Quality of the Denver Basin Aquifer System 

in a National Context

This appendix shows graphical comparisons of chemical concentrations (1991 to 2010) for selected inorganic and 

organic constituents of potential health or aquatic-life concern in selected Principal Aquifers of the United States. For 

each constituent, the concentration data are grouped according to five well types: agricultural land-use study wells 

(includes shallow agricultural monitoring wells), urban land-use study wells (includes shallow urban monitoring wells), 

major aquifer study wells, domestic wells, and public-supply wells. For each well type, the aquifers also are grouped 

according to aquifer lithology: basalt and volcanics,** crystalline, carbonate, sandstone, semiconsolidated sand and 

gravel, glacial unconsolidated sand and gravel, and unconsolidated sand and gravel (nonglacial). Data for a particular 

compound were not plotted if there were fewer than 10 samples for a particular well network in a Principal Aquifer; 

not all Principal Aquifers for which data were available are shown. Note that analytical detection limits varied among 

the constituents and that the number of samples for a constituent can vary greatly between Principal Aquifers. The data 

used in this appendix and boxplots for additional constituents are available at http://pubs.usgs.gov/circ/1360/.

Principal Aquifer (number of samples)

10th

percentile

25th

percentile

75th

percentile

Median

(50th percentile)

Lithology

Basalt and volcanics** Crystalline Carbonate Sandstone Semiconsolidated

 sand and gravel

Glacial unconsolidated sand and gravel Unconsolidated sand and gravel (nonglacial)

MCL Maximum Contaminant Level

U.S. Environmental Protection Agency

 contaminant levels for drinking water 

(NA) Wells of this type were not 

sampled for this constituent

** Note: Two of the Principal Aquifers in this group include limited samples from basin-fill aquifers within the extent of the basaltic aquifer. 

90th

percentile

Data above

90th percentile

Data below

10th percentile*

SMCL Secondary Maximum Contaminant Level

PMCL Proposed Maximum Contaminant Level

AMCL Proposed Alternative Maximum Contaminant Level

* Data below laboratory reporting levels are not shown.

96 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05
Dissolved solids

Snake River Plain basin-fill–basalt-rock (110)

Columbia Plateau basin-fill–basalt-rock (127)

Surficial (136)

Mississippi River Valley alluvial (NA)

High Plains (85)

Alluvial–Pleistocene terrace deposits (NA)

Alluvial–Denver Basin (11)

Rio Grande (119)

Central Valley (140)

California Coastal Basin (NA)

Basin and Range basin-fill (37)

Glacial–West (25)

Glacial–West Central (153)

Glacial–Central (282)

Glacial–East (54)

Texas coastal uplands (NA)

Northern Atlantic Coastal Plain (83)

Mississippi embayment (NA)

Valley and Ridge siliciclastic (22)

Early Mesozoic (NA)

Denver Basin (11)

Cambrian-Ordovician (NA)

Valley and Ridge carbonate (93)

Piedmont and Blue Ridge carbonate (30)

Upper Floridan (6)

Castle Hayne (NA)

Piedmont and Blue Ridge crystalline (NA)

New England crystalline (NA)

Hawaiian volcanic (NA)

Snake River Plain basin-fill–basalt-rock (103)

Columbia Plateau basin-fill–basalt-rock (62)

Surficial (30)

Mississippi River Valley alluvial (5)

High Plains (311)

Alluvial–Pleistocene terrace deposits (NA)

Alluvial–Denver Basin (7**)

Rio Grande (25)

Central Valley (138)

California Coastal Basin (18)

Basin and Range basin-fill (131)

Glacial–West (54)

Glacial–West Central (115)

Glacial–Central (208)

Glacial–East (98)

Texas coastal uplands (50)

Northern Atlantic Coastal Plain (66)

Mississippi embayment (27)

Valley and Ridge siliciclastic (80)

Early Mesozoic (68)

Denver Basin (75)

Cambrian-Ordovician (71)

Valley and Ridge carbonate (117)

Piedmont and Blue Ridge carbonate (29)

Upper Floridan (128)

Castle Hayne (6)

Piedmont and Blue Ridge crystalline (152)

New England crystalline (113)

Hawaiian volcanic (NA)

(28)

(NA)

(30)

(27)

(39*)

(44)

(26)

(25)

(115)

(NA)

(74)

(129)

(150)

(NA)

(94)

(10)

(NA)

(NA)

(20)

(NA)

(20)

(NA)

(25)

(NA)

(51)

(NA)

(NA)

(NA)

(NA)

Domestic wells

Agricultural land-use study wells Urban land-use study wells

Dissolved-solids concentration, in milligrams per liter

SMCL=500 mg/L

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

Dissolved-solids concentration,

in milligrams per liter

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

SMCL=500 mg/L

SMCL=500 mg/L

Principal Aquifer

Dissolved solids

*Includes 30 wells in 1993 South Platte River Basin study

 located within the Denver Basin boundaries.

**Wells in 1993 South Platte River Basin study 

 located within the Denver Basin boundaries.

Appendixes

Appendix 4 97
Manganese

Domestic wells

Manganese concentration, in micrograms per liter

Public-supply wells

SMCL=50 μg/L

0.1 1.0 10 100 1,000 10,000 100,000 0.1 1 10 100 1,000 10,000 100,000

Snake River Plain basin-fill–basalt-rock (149)

Columbia Plateau basin-fill–basalt-rock (63)

Surficial (30)

Mississippi River Valley alluvial (5)

High Plains (313)

Alluvial–Pleistocene terrace deposits (NA)

Alluvial–Denver Basin (7**)

Rio Grande (25)

Central Valley (138)

California Coastal Basin (18)

Basin and Range basin-fill (131)

Glacial–West (55)

Glacial–West Central (114)

Glacial–Central (210)

Glacial–East (99)

Texas coastal uplands (50)

Northern Atlantic Coastal Plain (66)

Mississippi embayment (27)

Valley and Ridge siliciclastic (80)

Early Mesozoic (68)

Denver Basin (75)

Cambrian-Ordovician (71)

Valley and Ridge carbonate (117)

Piedmont and Blue Ridge carbonate (29)

Upper Floridan (128)

Castle Hayne (6)

Piedmont and Blue Ridge crystalline (152)

New England crystalline (113)

Hawaiian volcanic (NA)

(NA)

(12)

(31)

(NA)

(NA)

(42)

(15)

(94)

(101)

(11)

(28)

(37)

(49)

(8)

(35)

(41)

(1)

(2)

(15)

(81)

(4)

(NA)

(35)

(5)

(5)

(1)

(25)

(19)

(34)

SMCL=50 μg/L

Principal Aquifer

Manganese

**See footnote, p. 97.

98 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05
Nitrate plus nitrite (nitrate)

Appendixes

Snake River Plain basin-fill–basalt-rock (111)

Columbia Plateau basin-fill–basalt-rock (129)

Surficial (138)

Mississippi River Valley alluvial (NA)

High Plains (85)

Alluvial–Pleistocene terrace deposits (NA)

Alluvial–Denver Basin (19)

Rio Grande (119)

Central Valley (140)

California Coastal Basin (NA)

Basin and Range basin-fill (37)

Glacial–West (25)

Glacial–West Central (154)

Glacial–Central (282)

Glacial–East (54)

Texas coastal uplands (NA)

Northern Atlantic Coastal Plain (83)

Mississippi embayment (NA)

Valley and Ridge siliciclastic (23)

Early Mesozoic (NA)

Denver Basin (11)

Cambrian-Ordovician (NA)

Valley and Ridge carbonate (92)

Piedmont and Blue Ridge carbonate (30)

Upper Floridan (6)

Castle Hayne (NA)

Piedmont and Blue Ridge crystalline (NA)

New England crystalline (NA)

Hawaiian volcanic (NA)

Snake River Plain basin-fill–basalt-rock (151)

Columbia Plateau basin-fill–basalt-rock (63)

Surficial (30)

Mississippi River Valley alluvial (5)

High Plains (314)

Alluvial–Pleistocene terrace deposits (NA)

Alluvial–Denver Basin (7**)

Rio Grande (25)

Central Valley (137)

California Coastal Basin (18)

Basin and Range basin-fill (123)

Glacial–West (55)

Glacial–West Central (96)

Glacial–Central (210)

Glacial–East (97)

Texas coastal uplands (50)

Northern Atlantic Coastal Plain (66)

Mississippi embayment (27)

Valley and Ridge siliciclastic (81)

Early Mesozoic (69)

Denver Basin (75)

Cambrian-Ordovician (71)

Valley and Ridge carbonate (116)

Piedmont and Blue Ridge carbonate (29)

Upper Floridan (127)

Castle Hayne (6)

Piedmont and Blue Ridge crystalline (151)

New England crystalline (113)

Hawaiian volcanic (NA)

(28)

(NA)

(30)

(27)

(39*)

(44)

(26)

(24)

(116)

(NA)

(74)

(129)

(153)

(NA)

(95)

(10)

(NA)

(NA)

(19)

(NA)

(20)

(NA)

(25)

(NA)

(50)

(NA)

(NA)

(NA)

(NA)

Domestic wells

Agricultural land-use study wells Urban land-use study wells

MCL=10 mg/L

0.01 0.1 1.0 10 100 0.01 0.1 1.0 10 100

0.01 0.1 1.0 10 100

MCL=10 mg/L

Principal Aquifer

Nitrate concentration, in milligrams per liter as nitrogen

Nitrate concentration, in 

milligrams per liter as nitrogen

MCL=10 mg/L

Nitrate plus nitrite (nitrate)

**See footnote, p. 97.

*See footnote, p. 97.

Appendix 4 99
Radon

Domestic wells

Snake River Plain basin-fill–basalt-rock (115)

Columbia Plateau basin-fill–basalt-rock (59)

Surficial (30)

Mississippi River Valley alluvial (5)

High Plains (307)

Alluvial–Pleistocene terrace deposits (NA)

Alluvial–Denver Basin (6**)

Rio Grande (24)

Central Valley (111)

California Coastal Basin (17)

Basin and Range basin-fill (112)

Glacial–West (48)

Glacial–West Central (58)

Glacial–Central (153)

Glacial–East (97)

Texas coastal uplands (40)

Northern Atlantic Coastal Plain (55)

Mississippi embayment (17)

Valley and Ridge siliciclastic (79)

Early Mesozoic (48)

Denver Basin (55)

Cambrian-Ordovician (47)

Valley and Ridge carbonate (112)

Piedmont and Blue Ridge carbonate (29)

Upper Floridan (88)

Castle Hayne (6)

Piedmont and Blue Ridge crystalline (150)

New England crystalline (109)

Hawaiian volcanic (NA)

Radon concentration, in picocuries per liter

PMCL=

300 pCi/L AMCL=4,000 pCi/L

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

Principal Aquifer

Radon

**See footnote, p. 97.

100 Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05
Special thanks to the following individuals and organizations for

their contributions:

NAWQA personnel in the Denver Basin study area for their 

contributions of data and research results.

Jean Dupree of the USGS Colorado Water Science Center for 

production of maps used in this circular.

NAWQA’s many partners in the governmental and non-governmental 

sectors who have helped guide scientific efforts and ensure that 

NAWQA information meets the needs of local, State, Tribal, regional, 

and national stakeholders.

USGS reviewers:

• Geoff Delin

• Wayne Lapham

• Barbara Mahler

• Gary Rowe

Other reviewers:

• Ray Alverez, Colorado Water Conservation Board 

• Lee Pivonka, Colorado Department of Public Health and Environment

• Reagan Waskom, Colorado Water Institute

Prepared by the USGS Science Publishing Network

Raleigh Publishing Service Center

Edited by Kay P. Naugle

Illustrations and layout by Caryl J. Wipperfurth

For more information concerning this report, contact:

Chief, National Water-Quality Assessment Program

U.S. Geological Survey

413 National Center

12201 Sunrise Valley Drive 

Reston, VA 20192

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

Basin

aquifer

system

Printed on recycled paper

ISSN 1067-084X (print)

ISSN 2330-5703 (online)

http://dx.doi.org/10.3133/cir1357
Bauch and others—Water Quality in the Denver Basin Aquifer System, Colorado, 2003–05—Circular 1357