U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2008–5243

Occurrence of Selected Organic Compounds in Groundwater 

Used for Public Supply in the Plio-Pleistocene Deposits in 

East-Central Nebraska and the Dawson and Denver Aquifers 

near Denver, Colorado, 2002–2004
Cover. Location of National Water-Quality Assessment (NAWQA) study units, High Plains Regional Groundwater and South Platte 

River Basin study units, and Source Water-Quality Assessment study areas. 
Occurrence of Selected Organic 

Compounds in Groundwater Used for 

Public Supply in the Plio-Pleistocene 

Deposits in East-Central Nebraska and the 

Dawson and Denver Aquifers near Denver, 

Colorado, 2002–2004

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

Suzanne S. Paschke

Scientific Investigations Report 2008–5243

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

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

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

reproduce any copyrighted materials contained within this report.

Suggested citation:

Bails, J.B., Deitsch, B.J., Landon, M.K., Paschke, S.S., 2009, Occurrence of selected organic compounds in groundwater used for public supply in the Plio-Pleistocene deposits of east-central Nebraska and the Dawson and Denver 

aquifers near Denver, Colorado, 2002–2004: U.S. Geological Survey Scientific Investigations Report 2008–5243, 29 p. 

with appendixes.

ISBN 978-1-4113-2513-5
iii

Foreword

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

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

effective management of water, biological, energy, and mineral resources (http://www.usgs.

gov/). Information on the Nation’s water resources is critical to ensuring long-term availability 

of water that is safe for drinking and recreation and is suitable for industry, irrigation, and fish 

and wildlife. Population growth and increasing demands for water make the availability of that 

water, now measured in terms of quantity and quality, even more essential to the long-term 

sustainability of our communities and ecosystems.

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

support national, regional, State, and local information needs and decisions related to waterquality management and policy (http://water.usgs.gov/nawqa). The NAWQA Program is 

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

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

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

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

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

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

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

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

Multiple national and regional assessments are ongoing in the second decade (2001–2012) of 

the NAWQA Program as 42 of the 51 Study Units are reassessed. These assessments extend the 

findings in the Study Units by determining status and trends at sites that have been consistently 

monitored for more than a decade, and filling critical gaps in characterizing the quality of surface 

water and groundwater. For example, increased emphasis has been placed on assessing the 

quality of source water and finished water associated with many of the Nation’s largest community water systems. During the second decade, NAWQA is addressing five national priority 

topics that build an understanding of how natural features and human activities affect water 

quality, and establish links between sources of contaminants, the transport of those contaminants through the hydrologic system, and the potential effects of contaminants on humans 

and aquatic ecosystems. Included are topics on the fate of agricultural chemicals, effects of 

urbanization on stream ecosystems, bioaccumulation of mercury in stream ecosystems, effects 

of nutrient enrichment on aquatic ecosystems, and transport of contaminants to public-supply 

wells. These topical studies are conducted in those Study Units most affected by these issues; 

they comprise a set of multi-Study-Unit designs for systematic national assessment. In addition, 

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

selected trace elements, and aquatic ecology are continuing. 

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

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

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

to meet your needs, and will foster increased citizen awareness and involvement in the protection and restoration of our Nation’s waters. 
iv

The USGS recognizes that a national assessment by a single program cannot address all waterresource issues of interest. External coordination at all levels is critical for cost-effective management, regulation, and conservation of our Nation’s water resources. The NAWQA Program, 

therefore, depends on advice and information from other agencies—Federal, State, regional, 

interstate, Tribal, and local—as well as nongovernmental organizations, industry, academia, and 

other stakeholder groups. Your assistance and suggestions are greatly appreciated.

Matthew C. Larsen

Associate Director for Water
v

Contents

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

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

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

Description of Study Areas ..........................................................................................................................5

Land and Water Use.............................................................................................................................7

Groundwater Hydrology.......................................................................................................................8

Nebraska.......................................................................................................................................8

Colorado ........................................................................................................................................8

Study Design and Methods..........................................................................................................................9

Regulatory and Non-Regulatory Human-Health Benchmarks.............................................................10

Occurrence of Selected Organic Compounds in Groundwater Used for Public Supply .................12

Nebraska..............................................................................................................................................12

Colorado ...............................................................................................................................................12

Comparison of the Two Source Water-Quality Assessment Studies.........................................12

Source and Finished Waters.............................................................................................................15

Comparison to Other Community Water Systems Well Data ...............................................................15

Summary........................................................................................................................................................17

Acknowledgments.......................................................................................................................................18

References....................................................................................................................................................19

Appendixes 1–3............................................................................................................................................21

Figures

1–3. Maps showing—

1. Location of National Water-Quality Assessment (NAWQA) study units, 

High Plains Regional Groundwater and South Platte River Basin study 

units, and Source Water-Quality Assessment study areas..........................................3

2. Location and land use of the High Plains Regional Groundwater Source 

Water-Quality Assessment study area and sampled wells..........................................4

3. Location and land use of the South Platte River Basin Source WaterQuality Assessment study area and sampled wells ......................................................6

4. Pie chart showing water use in counties included in the A, High Plains 

Regional Groundwater and B, South Platte River Basin Source WaterQuality Assessment study areas, water year 2000 .................................................................8
vi

Tables

1. Human-health benchmarks used for comparison to source water-quality 

data for public-supply wells in the High Plains Regional Groundwater study 

unit and the South Platte River Basin study unit sampled in 2003–2004 ...........................11

2. Summary of concentrations, detections frequencies, minimum reporting 

levels, and maximum contaminant levels for pesticides, volatile organic 

compounds, and other selected organic compounds detected in 15 publicsupply wells in the Plio-Pleistocene aquifer, east-central Nebraska, 2003......................13

3. Summary of concentrations, detections frequencies, minimum reporting 

levels, and maximum contaminant levels for pesticides, volatile organic 

compounds, and other selected organic compounds detected in 12 publicsupply wells in the Dawson and Denver aquifers near Denver, Colorado, 2003..............14

4. Comparison of detected contaminants from source and finished waters from 

selected public-supply wells in the Plio-Pleistocene aquifer, east-central 

Nebraska, 2003 and 2004 ...........................................................................................................16

5. Comparison of selected organic compounds in source-water samples 

collected in the High Plains Regional Groundwater, the South Platte River 

Basin, the Nevada Basin and Range, and the Great Salt Lake Basins study 

units of the National Water-Quality Assessment Program..................................................17

Appendixes

1. Volatile organic compounds analyzed in the Source Water-Quality 

Assessment studies in the South Platte River Basin and High Plains 

Regional Groundwater National Water-Quality Assessment study areas 

during 2003–2004.........................................................................................................................22

2. Pesticide compounds analyzed in the Source Water-Quality Assessment 

studies in the South Platte River Basin and High Plains Regional Groundwater 

National Water-Quality Assessment study areas during 2003–2004 .................................25

3. Other selected organic compounds analyzed in the Source Water-Quality 

Assessment studies in the South Platte River Basin and High Plains Regional 

Groundwater National Water-Quality Assessment study areas during 2003–2004.........28
vii

Conversion Factors and Datums

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

mile, nautical (nmi) 1.852 kilometer (km)

yard (yd) 0.9144 meter (m)

Area

acre 4,047 square meter (m2

)

square foot (ft2

) 0.09290 square meter (m2)

square inch (in2

) 6.452 square centimeter (cm2)

square mile (mi2

) 2.590 square kilometer (km2) 

Volume

ounce, fluid (fl. oz) 0.02957 liter (L) 

pint (pt) 0.4732 liter (L) 

quart (qt) 0.9464 liter (L) 

gallon (gal) 3.785 liter (L) 

gallon (gal) 0.003785 cubic meter (m3

) 

million gallons (Mgal) 3,785 cubic meter (m3

)

cubic inch (in3

) 16.39 cubic centimeter (cm3) 

cubic inch (in3

) 0.01639 liter (L)

cubic foot (ft3

) 0.02832 cubic meter (m3) 

acre-foot (acre-ft) 1,233 cubic meter (m3

)

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

 °F=(1.8×°C)+32

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

 °C=(°F-32)/1.8

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L).

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

(NGVD 29).

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

Elevation, as used in this report, refers to distance above the vertical datum.
viii
Occurrence of Selected Organic Compounds in 

Groundwater Used for Public Supply in the PlioPleistocene Deposits in East-Central Nebraska and the 

Dawson and Denver Aquifers near Denver, Colorado, 

2002–2004

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

Abstract

The National Water-Quality Assessment Program of the 

U.S. Geological Survey has an ongoing Source Water-Quality 

Assessment program designed to characterize the quality of 

water in aquifers used as a source of drinking-water supply for 

some of the largest metropolitan areas in the Nation. In addition to the sampling of the source waters, sampling of finished 

or treated waters was done in the second year of local studies 

to evaluate if the organic compounds detected in the source 

waters also were present in the water supplied to the public.

An evaluation of source-water quality used in selected 

groundwater-supplied public water systems in east-central 

Nebraska and in the south Denver metropolitan area of 

Colorado was completed during 2002 through 2004. Fifteen 

wells in the Plio-Pleistocene alluvial and glacial deposits in 

east-central Nebraska (the High Plains study) and 12 wells in 

the Dawson and Denver aquifers, south of Denver (the South 

Platte study), were sampled during the first year to obtain 

information on the occurrence and distribution of selected 

organic chemicals in the source waters. During the second 

year of the study, two wells in east-central Nebraska were 

resampled, along with the associated finished water derived 

from these wells, to determine if organic compounds detected 

in the source water also were present in the finished water. 

Selection of the second-phase sampling sites was based on 

detections of the most-frequently occurring organic compounds from the first-year Source Water-Quality Assessment 

study results. The second-year sampling also required that finished waters had undergone water-quality treatment processes 

before being distributed to the public.

Sample results from the first year of sampling groundwater wells in east-central Nebraska show that the most-frequently 

detected organic compounds were the pesticide atrazine and 

its degradate, deethylatrazine (DEA, otherwise known as 

2-chloro-4-isopropylamino-6-amino-s-triazine or CIAT), which 

were detected in 9 of the 15 wells (60 percent of the samples). 

The second most frequently detected organic compound was 

tetrachloroethylene, detected in 4 of the 15 wells (27 percent of 

the samples), followed by chloroform, trichloroethylene, and 

2-hydroxyatrazine (2-hydroxy-4-isopropylamino-6-ethylaminos-triazine, or OIET), present in 3 of the 15 wells (20 percent 

of the samples). The pesticide compounds deisopropylatrazine (2-chloro-6-ethylamino-4-amino-s-triazine, or CEAT), 

metolachlor, and simazine and the volatile organic compound 

cis-1,2-dichloroethylene were detected in 2 of the 15 wells, and 

the compounds diuron and 1,2-dichloroethane were detected 

in only 1 of the 15 wells during the first-year sampling. Most 

detections of these compounds were at or near the minimum 

reporting levels, and none were greater than their regulatory 

maximum contaminant level.

There were few detections of organic compounds during the first year of sampling groundwater wells in the South 

Platte study area. The compounds atrazine, deethylatrazine, picloram, tetrachloroethylene, methyl-tert-butyl-ether 

(MTBE), tris(2-butoxyethyl)phosphate, and bromoform were 

detected only once in all the samples from the 12 wells. Most 

detections of these compounds were at or near the minimum 

reporting levels, and none were greater than their regulatory 

maximum contaminant level.

Second-year sampling, which included the addition of 

paired source- and finished-water samples, was completed at 

two sites in the High Plains study area. Source-water samples 

from the second-year sampling had detections of atrazine and 

deethylatrazine; at one site deisopropylatrazine and chloroform also were detected. The finished-water samples, which 

represent the source water after blending with water from 

other wells and treatment, indicated a decrease in the concentrations of the pesticides at one site, whereas concentrations 

remained nearly constant at a second site. The trihalomethanes 

(THMs or disinfection byproducts), chloroform, bromoform, 

bromodichloromethane, and dibromochloromethane were 

detected in all finished-water samples and are a result of the 
2 Occurrence of Selected Organic Compounds in Groundwater in the Plio-Pleistocene Deposits in Nebraska and Colorado, 2002–2004

chlorine added during treatment reacting with organics in the 

source waters. Finished waters at one site also contained concentrations of organic compounds not present in the sourcewater sample, indicating that other wells may be contributing 

compounds to the finished waters.

When comparing the source-water data from both studies, 

the more frequent detection and higher concentrations of pesticides and pesticide degradation byproducts in the High Plains 

wells are an indication that the agricultural land-use setting 

near these wells has affected source-water quality to a greater 

extent than the rangeland setting historically surrounding the 

Colorado wells. Land use near the Colorado public-supply 

wells is shifting from rangeland to an urban environment 

that may eventually affect groundwater quality; however, the 

potential effects of this transition on water quality currently 

(2004) are not expressed in the sampled source water.

Results from the High Plains and South Platte studies in 

Nebraska and Colorado also were compared to similar studies 

done in Utah and Nevada to provide a larger-scale perspective 

on the quality of source waters and the effects from other landuse settings. Whereas groundwater-quality data from the Utah 

and Nevada studies were similar to each other, frequency of 

detections and concentrations for a majority of the compounds 

were intermediate to those of the High Plains and South Platte 

studies. These results indicate that the long history of agricultural land use and the more permeable alluvial deposits in 

Nebraska are affecting the source-water quality, whereas the 

potential effects of developing urbanization on source-water 

quality in the deep Dawson and Denver bedrock aquifers near 

Denver, Colorado, are not yet apparent.

Introduction

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

implementation of the National Water-Quality Assessment 

(NAWQA) Program as a way to provide nationally consistent information on the physical, chemical, and biological 

condition of a representative set of the Nation’s most densely 

populated river basins. The NAWQA program also has the 

goals of understanding the long-term trends in water quality 

and the primary factors that control the observed water-quality 

conditions (Gilliom and others, 1995). The emphasis during 

the first decade of the NAWQA Program (1991–2000) was on 

assessing the current status of water-quality conditions within 

the United States. Beginning in 2001, the NAWQA program 

began its second decade of intensive water-quality assessments and shifted to a greater emphasis on the study of trends 

in water quality and the primary processes controlling waterquality conditions (Mallard and others, 1999). During this second decade of the NAWQA program, 42 of the initial 52 study 

units evaluated during the first 10 years will be studied.

One of the investigative components for the second 

decade of the NAWQA program is the study of the occurrence and fate of natural and anthropogenic contaminants in 

groundwater aquifers and streams used as primary drinkingwater sources for municipal suppliers in major metropolitan 

areas. These studies, termed “Source Water-Quality Assessments (SWQA),” evaluated the status of source-water quality 

of Community Water Systems (CWS) on a local and regional 

scale in areas suspected or known to have degraded water quality because of natural or anthropogenic factors. Source water 

in the context of this report is defined as raw (untreated) water 

taken directly from the local groundwater aquifer or stream. 

Source-water quality information was collected to develop a 

better understanding of the quality of the drinking-water supplies and to increase the monitoring of drinking-water sources 

to benefit local suppliers and regulatory authorities. The 

water-quality data and interpretation of the results will aid local 

and state policy makers with a more comprehensive data set 

representing these important groundwater resources.

Groundwater SWQA studies were conducted in 

15 NAWQA study units across the country (fig. 1). The CWS 

wells sampled in each of these study areas produce water from 

what the USGS considers “principal aquifers”; hydrogeologic 

units that are significant contributors to the regional water 

supply (U.S. Geological Survey, 2003). To enhance the usefulness of data collected by the SWQA study, all studies were 

nested within the boundaries of other studies being conducted 

by the NAWQA program. The SWQA studies also fit within 

the NAWQA framework for large-scale comparisons of water 

quality within the Nation.

Two of the study units within the NAWQA program 

include the High Plains Regional Groundwater study, which 

started in 1998, and the South Platte River Basin study, which 

began its first phase of studies in 1991 (fig. 1). The High 

Plains study is unique in the NAWQA program as it focuses 

solely on groundwater resources and encompasses a much 

larger spatial scale than the typical NAWQA groundwater 

study. Studies of the chemical transport and transformation 

processes of potential groundwater contaminants also were 

a substantial part of the High Plains study. More information 

on the NAWQA program is given at the web address (http://

water.usgs.gov/nawqa/), along with specific information 

on the High Plains Regional study at (http://co.water.usgs.

gov/nawqa/hpgw/) and the South Platte River Basin study 

at (http://co.water.usgs.gov/nawqa/splt/). Implementation of 

SWQA studies in the High Plains and South Platte study areas 

(hereinafter referred to as the “High Plains study” and the 

“South Platte study,” respectively) began in 2002 and were 

completed over 2 years.

Fifteen supply wells (sites) located within the boundary 

of the Plio-Pleistocene alluvial and glacial deposits in eastcentral Nebraska (fig. 2) were sampled as part of the High 

Plains study. In the second year of the High Plains study, 2 of 

the 15 wells were resampled, and 2 additional sites representing finished waters from the 2 resampled wells also were 

sampled to determine if compounds in the source waters were 

present in the finished waters. The two sites were selected 

because organic compounds detected in the source-water wells 

were the most frequently detected in SWQA studies across 
Introduction 3

COOK

OAHU

PUGT

WILL

SANJ

SACR

NVBR

NVBR

SOCA

USNK

CCYK

YELL

CAZB RIOG

GRSL

UCOL

SCTX

TRIN

CACI

KANS

CNBR

OZRK

UMIS

EIWA

WMIC

LIRB

UIRB LERI

ACAD

SOFL

GAFL

MISE

MOBL

TENN

WHMI 

SANT

ALBE

ACFB

PODL

HDSN

DELR

CONN

NECB

LINJ

[NAWQA study units selected for 

Source Water-Quality Assessments]

SPLT and HPGW Groundwater Source Water-Quality Study Units

Other Groundwater Source Water-Quality Study Units

Non-Source Water-Quality Study Unit

HPGW

SPLT

CACI

ACAD Acadian-Pontchartrain

ACFB Apalachicola-Chattahoochee-Flint River Basin

ALBE Albermarle-Pamlico Drainage

CACI Canadian-Cimarron River Basins

CAZB Central Arizona Basins

CCYK* Central Columbia Plateau/Yakima River Basin

CNBR Central Nebraska Basins

CONN* Connecticut, Housatonic and Thames River Basins

COOK Cook Inlet Basin

DELR Delaware River Basin

EIWA Eastern Iowas Basins

GAFL* Georgia-Florida Coastal Plain

GRSL Great Salt Lake Basins

HDSN Hudson River Basin

HPGW* High Plains Regional Groundwater

KANS Kansas River Basin

LERI Lake Erie-Lake St. Clair Drainage

LINJ Long Island and New Jersey Coastal Drainages

LIRB Lower Illinois River Basin

MISE* Mississippi Embayment

MOBL Mobile River and Tributaries

NECB New England Coastal Basins

NVBR* Nevada Basin and Range

OAHU Island of Oahu

OZRK Ozark Plateaus

PODL* Potomic River Basin and Delmarva Peninsula

PUGT* Puget Sound Basin

RIOG* Rio Grande Valley

SACR Sacramento River Basin

SANJ* San Joaquin-Tulare Basins

SANT Santee Basin and Coastal Drainages

SCTX* South Central Texas

SOCA Southern California Coastal Drainages

SOFL Southern Florida

SPLT* South Platte River Basin

TENN Tennessee River Basin

TRIN* Trinity River Basin

UCOL Upper Colorado River Basin

UMIS* Upper Mississippi River Basin

URIB Upper Illinois River Basin

USNK Upper Snake River Basin

WHMI* White River and Great and Little Miami River Basins

WILL Willamette River Basin

WMIC Western Lake Michigan Drainage

YELL Yellowstone River Basin

NOTE: * Indicates Groundwater Source Water-Quality study.

EXPLANATION

Figure 1. Location of National Water-Quality Assessment (NAWQA) study units, High Plains Regional Groundwater and South Platte 

River Basin study units, and Source Water-Quality Assessment study areas.
4 Occurrence of Selected Organic Compounds in Groundwater in the Plio-Pleistocene Deposits in Nebraska and Colorado, 2002–2004

HALL

BUFFALO

YORK

CLAY

BOONE

PLATTE

POLK

VALLEY

BUTLER

ADAMS

CUSTER

SALINE

SEWARD

NANCE

HOWARD

GREELEY

SHERMAN

HAMILTON

MERRICK

COLFAX

SAUNDERS

LANCASTER

GAGE

DODGE

JEFFERSON

KEARNEY

FILLMORE

MADISON

LOUP GARFIELD WHEELER

STANTON

PHELPS

CUMING

ANTELOPE

GAGE

Index map

-97°

99° 98° 97°

41°

Loup River

Kearney

Grand Island York

EXPLANATION

Land cover

Urban

Extent of Plio-Pleistocene deposits

Agriculture

Rangeland

Forest

Water

Wetlands

Barren

Supply well—One well unless otherwise noted

Supply well—Where source and finished waters sampled

Platte River

TX

NM

OK

CO KS

WY NE

SD

Area of

 detail

High

Plains

aquifer

0 10 20 MILES

0 10 20 KILOMETERS

Base information from U.S. Geological Survey digital data, 1:24,000 and 1:100,000

Land-cover data are 30-meter resolution from National Land-Cover Dataset (NLCD) 1992 data

Albers Equal-Area projection

Standard Parallels 29o30'N and 45o30'N, central meridian -96o00'W

8 sites

Figure 2. Location and land use of the High Plains Regional Groundwater Source Water-Quality Assessment study area and 

sampled wells.
Description of Study Areas 5

Selected Inorganic Constituents

In addition to the organic data collected at all the Source Water-Quality Assessment 

sites in this study, selected inorganic constituents also were part of the sampling regimen. Constituents of interest included arsenic, uranium, and nitrate. Arsenic and uranium 

are primarily derived by reaction processes between groundwater and the aquifer material itself, whereas elevated levels of nitrate in groundwater are most commonly derived 

from the application of fertilizers and from septic-tank leachate (McMahon and others, 

2008). Data from the 15 source-water samples collected in the High Plains study area show 

one site with an arsenic concentration above the U.S. Environmental Protection Agency 

(USEPA) Maximum Contaminant Level (MCL) of 10 micrograms per liter (μg/L), three sites 

with uranium concentrations above the 30 μg/L MCL, and one site with a nitrate concentration above the 10 milligrams per liter (mg/L) MCL (U.S. Environmental Protection Agency, 

2003, 2004). Data from the 12 source-water samples collected in the South Platte study 

area indicate one site with uranium detected above the USEPA MCL. No inorganic analyses 

were completed on the finished-water samples in the High Plains study area, so no conclusion can be derived on the effectiveness of treatment for the removal of these contaminants; however, most community water systems monitor these compounds regularly and 

blend waters from many sources to keep concentrations less than any health-based risk 

level.

the Nation. In the context of this report, finished waters are 

defined as water that has passed through the CWS treatment 

process and are representative of the water being distributed 

to the community. Treatment at CWS may include blending 

of waters from multiple sources, in-line chlorination, and 

filtration.

The South Platte study was conducted in the south 

metropolitan area of Denver, Colorado (fig. 3). In this area 

many municipalities draw their water from the Dawson and 

Denver aquifers (S.S. Paschke, U.S. Geological Survey, written commun., 2006) which are the uppermost bedrock units 

of the Denver Basin aquifer system. Potential sites for this 

study spanned multiple municipalities, and because not all 

wells screened in the Dawson and Denver aquifers were being 

actively used during the period of the study, only 12 sites were 

available for sampling. Of the 12 sites, 8 sites are in Douglas 

County, 3 sites are in Arapahoe County, and the remaining site 

is in Elbert County. Because of the low number of detections 

and concentrations measured in the first year of sampling, 

there were no second-year samples collected for the South 

Platte study.

The purpose of this report is to present results from the 

High Plains and South Platte SWQA studies. Results characterize the occurrence and concentrations of several classes of 

organic compounds in groundwater resources that are used as 

primary drinking-water sources for municipalities in eastcentral Nebraska and the south Denver, Colorado, metropolitan area. This report describes, where available, observed 

differences between the qualities of untreated (source) and 

treated (finished) waters. Where applicable, comparisons 

between the studies outlined in this report are made in an 

effort to understand how the differences in current and 

historical land use and aquifer properties are reflected in 

groundwater quality.

Description of Study Areas

The High Plains and South Platte SWQA study areas 

are located within the Great Plains physiographic province 

(Fenneman and Johnson, 1946) of the west-central United 

States. The High Plains study area is within the boundaries of the Plio-Pleistocene alluvial and glacial deposits in 

east-central Nebraska, with sampled wells located in the 

following eight counties: Adams, Butler, Fillmore, Hamilton, 

Kearney, Madison, Seward, and York (fig. 2). The High Plains 

SWQA study area is in the northern part of the much larger 

High Plains aquifer system being studied by the High Plains 

Regional Groundwater study of the NAWQA program. Historically, the area in east-central Nebraska was used predominantly for irrigated agriculture. Data from the 2002 agricultural census showed that, for the total of 2.3 million acres of 

farmland in the eight Nebraska counties that contain wells 

included in this SWQA study, 58 percent of the area was used 

for growing corn, 35 percent was used to grow soybeans, 

5 percent for hay, and 2 percent for wheat, oats, sorghum, 

and sunflower seeds (U.S. Department of Agriculture, 2004). 

In those same eight counties, the aggregated population data 

between 1990 and 2000 indicated a population increase of 4.7 

percent, resulting in a population of 129,157 in 2000 (U.S. 

Census Bureau, 2006a, 2006b). County-specific population 

data between 1990 and 2000 indicated a population change 

ranging from a 6.6 percent decrease in Fillmore County to a 

7.9 percent increase in Madison County. Incorporating 2004 

population estimates, the average population change for all 

eight counties between 1990 and 2004 was a 4.8 percent 

increase. The 2004 estimates show one-half of the counties in 

Nebraska having decreasing populations from 2000 to 2004 

(U.S. Census Bureau, 2006a).
6 Occurrence of Selected Organic Compounds in Groundwater in the Plio-Pleistocene Deposits in Nebraska and Colorado, 2002–2004

105° 104°30'

39°30'

DOUGLAS

ELBERT

ARAPAHOE

DENVER

CO

WY NE

KS Area of

detail

South Platte

River Basin

EXPLANATION

Extent of Dawson aquifer

Supply well

Land cover

Urban

Agriculture

Rangeland

Forest

Water

Barren

Plum

Creek

South

Platte

River

0 3 6 MILES

0 3 6 KILOMETERS

Base information from U.S. Geological Survey digital data, 1:24,000 and 1:100,000

Land-cover data are 30-meter resolution from National Land-Cover Dataset (NLCD) 1992 data

Albers Equal-Area projection

Standard Parallels 29o30'N and 45o30'N, central meridian -96o00'W

Cherry

Creek

Coal Creek

R

un

ning

Creek

Index map

Figure 3. Location and land-use of the South Platte River Basin Source Water-Quality Assessment study area and sampled wells.
Description of Study Areas 7

The South Platte study area in the southern Denver 

metropolitan area of Colorado is within the boundaries 

of the Tertiary-aged Dawson aquifer and the underlying 

Cretaceous to Tertiary-aged Denver aquifer, which are the 

uppermost bedrock units of the larger Denver Basin aquifer 

system (fig. 3). This area, which includes parts of Arapahoe, 

Douglas, and Elbert Counties, has experienced a substantial 

population increase during the past decade and a subsequent 

shift from the predominantly native rangeland to highdensity urban and suburban land use. In the three counties 

where SWQA sites were located, aggregated population data 

indicated a 48 percent population increase between 1990 

and 2000, resulting in a population in the three Colorado 

counties of 683,605 (U.S. Census Bureau, 2006a, 2006b). 

Individual county data indicated population increases ranging from 25 percent in Arapahoe County to 191 percent in 

Douglas County. Incorporating 2004 estimated population 

data indicated a 70 percent increase in the population in all 

three counties between 1990 and 2004 (U.S. Census Bureau, 

2006a, 2006b). Although both SWQA study areas lie within 

the same physiographic province, the substantial difference in land-use settings and population change allows for 

a comparison of the relative effects of different land uses on 

source-water quality.

Land and Water Use

Because the study areas as well as some of the well 

sites within each study area are separated by large distances, 

land-use statistics were calculated for each site to provide 

meaningful information on land-use differences. Land-use 

data were compiled for a circular buffer with a radius of 

500 meters (1,640 feet) around each SWQA sample site. This 

area was expected to represent the land-use conditions likely 

to affect water quality in each well (Koterba, 1998). The 

most recent land-use data available in electronic format has 

a nominal date of 1992 and, therefore, cannot be considered 

current (U.S. Geological Survey, 1992). However, 1992 data 

give a historic assessment of land use, that when considered 

with more current population information, provides general 

indications of the recent transition in land-use settings. Generally, local land-use conditions will have a more pronounced 

effect on the water quality in a given well than regional landuse data (Koterba, 1998).

Land-use data around the wells in the High Plains study 

area were aggregated to give a percentage of total land use 

for each land-use type within the 500-meter buffer around 

the well sites. Data indicated that urban settings were the 

most prominent land use around the High Plains wells (about 

63 percent). The study area in east-central Nebraska primarily is agricultural; however, public-supply wells are generally 

located within city boundaries, and the 500-meter buffers 

reflect local urban land use. Agricultural land use represented 

about 25 percent of the land use around each well in the High 

Plains study area. This agricultural setting includes small 

grains, row crops, and hay fields. Rangeland (6 percent) 

and forested areas (4 percent) were the next most common. 

The remaining 2 percent of land use was split between open 

water, bare rock, and wetlands (U.S. Geological Survey, 

1992). 

In the South Platte study area, land use within the 500-

meter buffer around the wells sites predominantly was rangeland, which accounted for 64 percent of the total land-use 

setting. Urban land use accounted for about 22 percent of the 

total area within all 500-meter buffers. Agricultural land-use 

accounted for about 12 percent of the area around the South 

Platte wells, with the remaining 2 percent represented by 

open water, forest, and bare rock (U.S. Geological Survey, 

1992). The counties in the South Platte study area have been 

experiencing some of the largest population growth rates in 

the country with Douglas County having the largest percentage increase in population of any county in the United States 

between 1990 and 2000 (U.S. Census Bureau, 2001). The 

land use around each of the sites in the South Platte Basin 

has been shifting from a historically rangeland setting to a 

more urbanized setting.

Water-use data were aggregated at the county level 

to determine usage in the eight Nebraska counties of the 

High Plains study area (U.S. Geological Survey, 2006). All 

surface water used in the Nebraska counties was for irrigation, livestock, and mining purposes and accounted for about 

6 percent of all water use in 2000. All public and domestic 

water supplies during 2000 were derived from groundwater 

sources. Primary use of groundwater in the counties was for 

irrigation, which accounted for 96.6 percent (1.44 million 

acre-feet) of all groundwater used (fig. 4A). Public-water 

supplies accounted for 1.9 percent (29,000 acre-feet) of the 

total. Domestic water supplies, or wells that supply single 

family homes or small housing developments, accounted 

for only 0.39 percent (5,800 acre-feet) of all groundwater 

use in 2000. Industrial water use accounted for 0.43 percent 

(6,500 acre-feet) of groundwater use, livestock watering 

accounted for 0.45 percent (6,700 acre-feet), and thermoelectric uses accounted for 0.14 percent (2,100 acre-feet).

Water-use data for the three South Platte study area 

counties (Arapahoe, Douglas, and Elbert) also were aggregated for 2000. Surface-water sources accounted for about 

75 percent (146,660 acre-feet) of all water use in the three 

Colorado counties, and surface-water resources were 

used predominantly (84 percent) for public supply. Irrigation accounted for about 13 percent of surface-water use, 

and industrial uses accounted for the rest. The remaining 

25 percent (48,000 acre feet) of all water used in the South 

Platte study area was derived from groundwater sources 

with its uses almost equally distributed between domestic supply (35 percent, or 17,000 acre-feet), public supply 

(31 percent, or 14,700 acre-feet), and irrigation (30 percent, 

or 14,400 acre-feet) (fig. 4B). The remaining 4 percent 

(1,900 acre-feet) of the groundwater used in the South Platte 

study area was for industrial purposes (U.S. Geological 

Survey, 2006).
8 Occurrence of Selected Organic Compounds in Groundwater in the Plio-Pleistocene Deposits in Nebraska and Colorado, 2002–2004

Groundwater Hydrology

Groundwater sources for the studies discussed in this 

report are unconfined (or water table) to confined aquifers 

that have the potential for contamination from the infiltration of contaminants. The movement of contaminants derived 

from land-surface activities to the aquifer is increased by 

unconfined aquifer conditions, relatively shallow water tables, 

permeable aquifer materials at land surface, and enhanced 

infiltration from irrigation practices.

Nebraska

The surficial High Plains aquifer in east-central Nebraska 

mainly consists of unconsolidated Plio-Pleistocene alluvial 

and glacial deposits of sand, gravel, silt, and clay. Groundwater occurs under unconfined conditions in the glacial sands, 

although the interbedded deposits can result in locally confined conditions (Landon and Turco, 2007). The saturated 

thickness of the aquifer ranges from about 50 feet in the 

northwestern part of the aquifer to about 350 feet near York, 

Nebraska. The average saturated thickness of the aquifer is 

about 210 feet.

Well depths for the High Plains study range from about 

100 to 400 feet below land surface, with an average depth of 

285 feet below land surface. Water levels measured at the time 

of sampling ranged from 6 to 213 feet below land surface. The 

distance from the water level to the top of the screened interval ranged from 15 feet to about 200 feet. These minimum 

and maximum values were extremes in the data set; removing 

these two extreme values yields an average distance of about 

85 feet from the water surface to the top of the screened interval. Regional groundwater flow is from northwest to southeast. Aquifer recharge in this area predominantly is from the 

infiltration of irrigation return flows but also is supplemented 

by regional groundwater flow and precipitation (Landon and 

Turco, 2007).

Colorado

Wells in the South Platte study area are completed in the 

Dawson and Denver aquifers, which comprise the uppermost 

units in the Denver Basin aquifer system (Robson and Banta, 

1995). The Denver Basin aquifer system is a synclinal structure of sedimentary rocks that dip sharply to the east on the 

western basin margin. The Dawson aquifer and underlying 

Denver aquifer are composed of Cretaceous to Tertiary-aged 

deposits consisting of coarse grained, poorly to well-consolidated sandstones interbedded with conglomerate, siltstone, 

and shale. The Cretaceous-aged Arapahoe and Laramie-Fox 

Hills sandstone aquifers underlie the Denver aquifer and 

outcrop around the edges of the Denver Basin. The Cretaceous Pierre Shale underlies the Laramie-Fox Hills aquifer 

and is considered the base of the aquifer system (Robson and 

Banta, 1995). The aquifer system, as a whole, is a primary 

supplier of groundwater for many areas in the Front Range. 

The Dawson aquifer generally is exposed at the land 

surface in the South Platte study area and reaches depths to 

1,100 feet below land surface in the deepest part of the Denver 

Basin south of the study area (S.S. Paschke, U.S. Geological 

Survey, written commun., 2008). The Dawson aquifer generally has unconfined conditions in its uppermost parts with the 

deeper parts of the aquifer displaying confined conditions. 

The Denver Formation underlies the Dawson aquifer and 

consists of a 600 to 1,100-foot thick sequence of interbedded sandstone, siltstone, shale, claystone, coal, and volcanic 

ash (Robson, 1987). The Denver aquifer is composed of 

Thermoelectric, 0.14 percent

A. High Plains Regional Groundwater

B. South Platte River Basin

Public supply,

 31 percent

(14,700 acre-feet)

Domestic supply, 35 percent

(17,000 acre-feet)

Industrial, 4 percent

(1,900 acre-feet)

Irrigation, 30 percent

(14,400 acre-feet)

Public supply, 1.9 percent (29,000 acre-feet)

Domestic Supply, 0.39 percent

(5,800 acre-feet)

Industrial, 0.43 percent (6,500 acre-feet)

Irrigation, 96.6 percent

(1.44 million acre-feet)

Livestock, 0.45 percent

(6,700 acre-feet)

(2,100 acre-feet)

Figure 4. Water use in counties included in the (A) High Plains 

Regional Groundwater and (B) South Platte River Basin Source 

Water-Quality Assessment study areas, water year 2000.
Study Design and Methods 9

permeable sandstones within the Denver Formation (Robson, 1987). Recharge to the Denver Basin aquifers is derived 

from infiltration of incident precipitation and irrigation with 

discharge to evapotranspiration, streams, springs, and wells. 

Regional groundwater movement in the bedrock aquifers 

generally is from south to north. Wells included in this study 

range from 250 to 1,000 feet deep, with an average depth of 

640 feet below land surface. Water levels obtained at the time 

of sampling ranged from 60 to 350 feet below land surface and 

depth from water surface to the top of the screened interval 

ranged from 0 to 680 feet, although water-level information 

was not collected at all sites.

Study Design and Methods

The SWQA study, as developed by the NAWQA program, is intended to provide a better understanding of the 

prevalence of selected organic compounds in streams and 

aquifers that serve as primary contributors to municipal water 

supplies and to characterize the frequency and concentrations 

in the treated water for the most-frequently detected compounds in source water.

Selection criteria for study areas included in the first year 

of this nationwide NAWQA effort were based on six factors. 

The identified study should (1) overlie 1 of the 19 principal 

aquifers prioritized by NAWQA (Lapham and others, 2005; 

U.S. Geological Survey, 2003) or an aquifer of regional 

importance as a drinking-water resource; (2) be nested within 

the boundaries of existing NAWQA studies slated for periodic resampling as part of the NAWQA water-quality trends 

network; (3) contain an aquifer that contributes a significant 

amount of the drinking water to local communities; (4) have 

existing documentation that the aquifer is vulnerable to anthropogenic and natural contamination; (5) have local waterquality issues related to human activities; and (6) include 

site-specific criterion in the selection of wells. Wells selected 

for inclusion in the study should be at least 1 kilometer 

(3,281 feet) away from any other well in the study, have a 

large local production rate or population served, and have an 

unlikely chance to have a direct connection with a surfacewater source (G.C. Delzer, U.S. Geological Survey, written 

comm., 2002).

The High Plains and South Platte SWQA studies started 

in October 2002 and ended October 2004. Initial project 

work included study-area characterization and well selection, which required USGS personnel to work closely with 

local CWS managers. During the first year of the study, the 

sampling effort was intended to establish the occurrence 

and distribution of organic compounds in source water from 

15 of the highest-producing municipal wells in the aquifer 

selected for the study.

Sampling sites for the High Plains SWQA study area 

were within the boundary of the Plio-Pleistocene alluvial and 

glacial deposits located in east-central Nebraska (fig. 2). Wells 

completed in the Plio-Pleistocene alluvial and glacial deposits 

were selected for the study because this near-surface aquifer is 

considered extremely vulnerable to contaminants introduced 

by human activities and also is a primary drinking-water 

supply for the city of York, Nebraska, and its surrounding 

communities (Landon and Turco, 2007; Lapham and others, 

2005). From the available wells in the targeted study area, 

the eight wells with the largest annual water production in 

the York, Nebraska, CWS were chosen for inclusion in this 

study. The remaining seven wells for the High Plains SWQA 

study were chosen from the highest-producing wells in nearby 

communities.

Sites selected for the South Platte study area were all 

completed in the Dawson or Denver aquifers, the upper most 

units of the Denver Basin aquifer system. These aquifers were 

chosen because of their use in numerous CWS, their exposure 

at the land surface, and, therefore, their potential vulnerability 

to contaminants introduced at the land surface. The selection 

of wells for the South Platte study was hampered by the lack 

of available wells during the study. Although the Dawson 

aquifer contains numerous CWS wells, these wells commonly 

are used only seasonally or intermittently as a back-up to the 

more productive wells in the deeper formations of the Denver Basin aquifer system. With the assistance of local CWS 

managers, 12 active production wells were identified that met 

the study criteria.

The design for the second year of the study was to resample a subset of the first-year sample sites where contaminants 

were detected and focus on the most commonly detected compounds from the first-year nationwide SWQA study results. 

The second-year study design also included the sampling of 

finished (treated) water, which represents water supplied to 

municipal customers that was derived from the parcels of 

source water sampled before treatment. These finished-water 

samples were collected after water-quality treatment and may 

have been subjected to blending of waters from other sources, 

filtration and chemical treatment to remove particulates or dissolved constituents, and microbial disinfection.

Wells that were sampled in the High Plains and South 

Platte studies usually had in-line chlorination and water 

blending occurring just “downstream” from the well head. 

To sample source water, existing sampling taps were located 

before any treatment or blending, or new sampling taps were 

installed by the CWS technicians to facilitate collecting 

the water sample. All samples were collected following the 

USGS protocols outlined by the National Field Manual for the 

Collection of Water-Quality Data (U.S. Geological Survey, 

variously dated). The VOC samples were collected unfiltered, 

all other samples were filtered through 0.45-micron filters and 

are considered dissolved (vs. total) concentrations. Sampling 

equipment consisted primarily of Teflon tubing with stainless 

steel fittings and filters.

All samples collected for both studies were analyzed 

at the USGS National Water Quality Laboratory (NWQL) 

in Lakewood, Colorado. The various analytical methods for 

the organic compounds are outlined in Rose and Sandstrom 
10 Occurrence of Selected Organic Compounds in Groundwater in the Plio-Pleistocene Deposits in Nebraska and Colorado, 2002–2004

(2003); Zaugg and others (2002); Sandstrom and others 

(2001); Furlong and others (2001); and Conner and others (1998). Many of the analytical results for the selected 

organic compounds were qualified with an estimated (E) 

designation. These results are reliable detections but are 

determined to be outside the acceptable laboratory determined calibration curve and have an uncertainty greater than 

the laboratory average.

Wells resampled in the High Plains study area during 

the second year of the SWQA were selected on the basis of 

five criteria: (1) a first-year sample from the well was determined to contain anthropogenic compounds that were detected 

frequently on a National scale; (2) first-year sample results 

revealed two or more local wells that had a detection of a 

single compound not included in the National target list; (3) an 

individual well had detections of 6 or more of the 289 organic 

compounds analyzed; (4) a first-year sample had a significant 

concentration (greater than 10 micrograms per liter (μg/L)) of 

any compound; or (5) there were important local concerns that 

could be addressed by additional sampling (G.C. Delzer, U.S. 

Geological Survey, written commun., 2003). Applying these 

criteria to the 15 High Plains sites identified 2 sites that were 

selected for resampling during the second year of the SWQA 

study. 

Compound classes analyzed in each water sample 

included volatile organic compounds (VOC), pesticides and 

pesticide degradation products, and other organic compounds 

(OOC) (appendixes 1–3). Because of possible natural sources 

and suspected sample contamination, the occurrence of carbon 

disulfide, isophorone, phenol, and only in the case of the 

South Platte samples, chloroform, could not be related directly 

to human effects and are not discussed further in this report 

(Carter and others, 2007). Because of the presence of free 

chlorine in the finished waters caused by microbial disinfection practices, a dechlorinating reagent (ascorbic acid) and a 

neutral pH buffer were added to the finished-water samples. 

A neutral pH buffer was not added to VOC samples because 

sample preservation protocols for VOC samples include 

preserving samples with hydrochloric acid, and pH buffers can 

create interference with the preservation method. Protocols for 

the collection of the source and finished waters also stipulated 

that source waters be collected before finished waters and that 

the sampling team determine, with help from the CWS personnel, the retention time of water passing through the treatment 

process. This collection process ensured that the best effort 

possible was made to sample the same parcel of water before 

and after treatment.

Quality assurance and quality control (QA/QC) of the 

SWQA sample data for both studies were conducted as part 

of the larger, NAWQA QA/QC program. The standardization of sampling protocols, sampling equipment, all sample 

analysis at the NWQL, and consistent quality-assurance 

sample collection of blanks, replicates, and spikes, allowed 

for the pooling of quality-assurance data for SWQA studies nationwide. Results from the pesticide analysis indicate 

there were no reportable systematic sample contaminations 

or bias in detected concentrations (Hopple and others, 2005). 

Analysis of VOC QA/QC data collected for all SWQA studies 

also indicate concentrations for carbon disulfide were suspect 

and are not included in this report (Carter and others, 2007). 

Within the South Platte study, systematic contamination was 

found in blank samples for chloroform, and results were not 

included in this report for this study unit. Analysis of the QA/QC 

data for the OOCs from all SWQA studies show there was 

some systematic contamination for phenol and that isophorone 

detections were linked to the preservation methods (Carter and 

others, 2007).

Regulatory and Non-Regulatory 

Human-Health Benchmarks

Water–quality results presented in this report are compared to regulatory and non-regulatory health-based waterquality standards to provide a reference for understanding the 

relative quality of the source waters used for municipal supplies (table 1). Sources of human-health benchmarks include 

the U.S. Environmental Protection Agency (USEPA) drinkingwater standards (U.S. Environmental Protection Agency, 2003, 

2004), Nebraska and Colorado State-mandated drinking-water 

standards (if different from USEPA standards) (Nebraska 

Health and Human Services, 2005; Colorado Department of 

Public Health and Environment, 2005), and USGS-developed 

Health-Based Screening Levels (HBSL) for compounds 

currently (2004) unregulated on the Federal or State level 

(Toccalino and others, 2003, 2004). Generally, these Federal 

and State drinking-water-quality criteria apply only to publicly 

supplied drinking water at the point of consumption and not to 

raw source water at the well head. The HBSLs were developed 

to provide a reference for frequently detected constituents that 

do not have a regulatory water-quality standard. Comparison 

of water-quality data to applicable human-health standards, 

or benchmarks, is useful for local, county, State, and Federal authorities responsible for maintaining drinking-water 

resources.

The USEPA regulations on drinking-water contaminants 

most commonly are referred to as “Maximum Contaminant 

Levels (MCLs)” and represent the maximum allowable 

concentration of a contaminant allowed in publicly supplied 

drinking water. These enforceable regulations are applied 

to the finished waters supplied to the users of the municipal 

water utility but do not apply to source waters collected before 

treatment or to self-supplied drinking water from a private 

domestic source. The MCLs are referenced in this report 

because they are useful in understanding the relative quality 

of the source waters before being treated and distributed to the 

community. Another non-enforceable regulatory benchmark 

used in this report is the Lifetime Health Advisory (LHA), 

which is defined as “the concentration of a chemical in drinking water that is not expected to have any adverse noncarcinogenic effects for a lifetime of exposure” (U.S. Environmental 
Regulatory and Non-Regulatory Human-Health Benchmarks 11

Protection Agency, 2004). Nebraska and Colorado State 

drinking-water regulations also were assessed where applicable to this study. State drinking-water standards usually are set 

at the USEPA MCLs; however, individual states may establish 

drinking-water MCLs at concentrations lower than the USEPA 

MCL. 

Many of the organic compounds sampled for in this 

study do not have an associated Federal or State drinkingwater standard. To assess these unregulated chemicals in a 

health-based framework, the USGS, in collaboration with the 

USEPA, New Jersey Department of Environmental Protection, and the Oregon Health and Science University developed 

benchmarks called the Health-Based Screening Level (HBSL) 

(Tocallino and others, 2003, 2004). These screening levels 

are non-enforceable and represent estimates of a benchmark 

concentration that can have potential human-health concerns. 

The HBSLs were developed from the protocols of the USEPA 

Office of Water determinations of LHA and Risk-Specific 

Dosage (RSD). Applications of HBSLs are useful in interpreting water-quality data containing non-regulated contaminants 

and in prioritizing contaminants and assessing the need for 

further monitoring (Tocallino and others, 2003, 2004).

To quantify potential human-health concerns from the 

organic data collected and assessed against USEPA, State, or 

HBSL drinking-water standards or screening levels, a Benchmark Quotient (BQ) was used. The BQ is defined as “the ratio 

of a contaminant concentration to its corresponding MCL, 

LHA, or HBSL.” Maximum BQ (BQmax) values represent the 

maximum measured concentration of a contaminant divided 

by the corresponding MCL (for regulated compounds) or 

Table 1. Human-health benchmarks used for comparison to source water-quality data for public-supply wells in the High Plains 

Regional Groundwater study unit and the South Platte River Basin study unit sampled in 2003–2004.

[Federal and State maximum contaminant levels are enforceable standards; the remainder of the listed benchmarks are unenforceable guidelines or publichealth goals; USEPA, U.S. Environmental Protection Agency; MCL, Maximum Contaminant Level; OW, Office of Water (USEPA); MCLG, Maximum 

Contaminant Level Goal; LHA, Lifetime Health Advisory; NHHS, Nebraska Health and Human Services; CDPHE, Colorado Department of Public Health and 

Environment; HBSL, Health-Based Screening Level; USGS, U.S. Geological Survey; NJDEP, New Jersey Department of Environmental Protection; OHSU, 

Oregon Health and Science University]

Human–health 

benchmark Acronym Agency Description Reference

Maximum Contaminant 

Level (Federal)

USEPA MCL USEPA (OW) The maximum permissible level of a contaminant in 

water that is delivered to any user of a public-water 

system. MCLs are set as close to Maximum Contaminant Level Goals (MCLGs) as feasible using the best 

available treatment technology and taking cost into 

consideration. The MCLG is the concentration of a 

contaminant in drinking water at which no known or 

anticipated adverse health effects occur, and which 

allows an adequate margin of safety.

(USEPA, 2003, 2004)

Lifetime Health Advisory LHA USEPA (OW) The concentration of a contaminant in drinking water 

that is not expected to cause any adverse noncarcinogenic effects over a lifetime of exposure (70 years). 

The LHA is based on exposure of a 70-kilogram (154 

pound) adult consuming 2 liters (0.5 gallon) of water 

per day, and assumes that only part (generally 20 

percent) of the total exposure to a contaminant is from 

drinking water.

(USEPA, 1993, 2004)

Maximum Contaminant 

Level (State)

Nebraska MCL NHHS The MCLs for organic chemical contaminants apply to 

community and non-transient, non-community water 

systems.

(NHHS, 2005)

Maximum Contaminant 

Level (State)

Colorado MCL CDPHE The maximum permissible level of a contaminant in 

water, which is delivered to any user of a public-water 

system.

(CDPHE, 2005)

Health-Based Screening 

Level

HBSL USGS,

USEPA,

NJDEP,

OHSU

Estimates of benchmark concentrations (for noncarcinogens) or concentration ranges (for carcinogens) 

in water that (a) may be of potential human-health 

concern; (b) can be used as threshold values against 

which measured concentrations of contaminants in 

ambient water samples can be compared; and (c) are 

consistent with USEPA OW methodologies for setting 

drinking-water LHA and Risk-Specific Dose values 

(USEPA, 1988, 1993).

(Toccalino and others, 

2003, 2004)
12 Occurrence of Selected Organic Compounds in Groundwater in the Plio-Pleistocene Deposits in Nebraska and Colorado, 2002–2004

HBSL (for unregulated compounds) (Toccalino and others, 

2003, 2004). A BQmax value greater than or equal to 1.0 is 

used to identify compound concentrations of potential humanhealth concern, and additionally, to determine if further study 

on a specific compound is needed. A BQmax value greater 

than or equal to 0.1 is used to identify compounds that may 

warrant additional monitoring to analyze trends in their occurrence. Additional monitoring may provide an early indication 

of contaminant levels that approach human-health benchmarks 

and, consequently, contaminant concentrations of potential 

human-health concern.

Occurrence of Selected Organic 

Compounds in Groundwater Used for 

Public Supply

Nebraska

During the first year of sampling (October 2002 through 

January 2003), water-quality samples were collected at 

15 sites in the High Plains study area (fig. 2). The frequency of 

detection for all constituents analyzed in the samples is shown 

in table 2. Of the constituents analyzed, two regulated pesticides and five unregulated pesticides or pesticide degradation 

products were detected. The most-frequently detected regulated pesticide was atrazine, which had measurable concentrations in 9 of 15 wells (60 percent of the samples). Simazine, 

another commonly used pesticide, was in 2 of 15 wells 

(13 percent of the samples) at concentrations estimated 

below the minimum reporting level (MRL) of 0.005 μg/L. 

Unregulated compounds also were present in the samples. The 

atrazine degradate, deethylatrazine (DEA or otherwise known 

as 2-chloro-4-isopropylamino-6-amino-s-triazine or CIAT) 

was detected in the same 9 of 15 wells as atrazine (60 percent 

of the samples). The pesticide 2-hydroxyatrazine (2-hydroxy4-isopropylamino-6-ethylamino-s-triazine or OIET) was 

detected at concentrations estimated below the MRL of 

0.032 μg/L in 3 of 15 wells (20 percent of the samples), 

and the atrazine degradate deisopropylatrazine (2-chloro6-ethylamino-4-amino-s-triazine or CEAT) was detected in 

2 of 15 wells (13 percent of the samples). Two other unregulated pesticides detected include metolachlor in 2 of 15 wells 

(13 percent of the samples) and diuron in 1 of 15 wells (7 percent of the samples).

Volatile organic compounds (VOCs) were detected in a 

few of the source-water samples, and all of the detected VOCs 

are regulated as drinking-water contaminants. The mostfrequently detected VOC was tetrachloroethylene (PCE), a dry 

cleaning and industrial solvent, which was detected in 4 of the 

15 High Plains wells (27 percent of the samples). Chloroform 

(trichloromethane), a byproduct of water disinfection and a 

solvent, and the solvent trichloroethylene (TCE) were detected 

in 3 of the 15 wells (20 percent of the samples). The most 

likely source of the chloroform is the chemical reaction of 

chlorine used to disinfect drinking water with organic matter 

found in the source-water aquifer (Ivahnenko and Zogorski, 

2006). The detection of chloroform in source water indicates 

finished waters (treated) are recharging the aquifer, possibly 

through lawn and garden irrigation, recharge of treated wastewaters, or leakage from distribution lines of treated drinking 

water (Zogorski and others, 2006). Also detected, but at lesser 

frequencies, were cis-1,2-dichloroethylene in 2 of 15 wells 

(13 percent of the samples) and 1,2-dichloroethane in 1 of 

15 wells (7 percent of the samples). None of the organic compounds detected in the High Plains study had concentrations 

greater than their regulatory MCL and none had a BQmax 

greater than the benchmark value of 0.1.

Colorado

During the first year of the South Platte study (March 

through October 2003), water-quality samples were collected 

at 12 sites south of Denver, Colorado (fig. 2). Organic compounds were generally not detected in the deep bedrock wells 

of the South Platte study areas except at two sites. Compounds 

detected in 1 of the 12 wells (8 percent of the samples) were 

the regulated pesticides atrazine, its degradation product DEA 

(unregulated), and picloram, as well as the VOCs PCE, a 

solvent, methyl-tert-butyl ether (MTBE), a gasoline additive, 

and bromoform, a byproduct of the chlorination of water 

(table 3). Also detected was the OOC tris(2-butoxyethyl)

phosphate, a plasticizer used in rubber products and as a flame 

retardant. None of the organic compounds detected in the 

South Platte study had concentrations greater than their regulatory MCL and none had a BQmax greater than the benchmark 

value of 0.1 (table 3).

Comparison of the Two Source Water-Quality 

Assessment Studies

Comparison of the SWQA water-quality data from the 

High Plains and South Platte study areas indicate distinct 

differences in the type of compounds detected and their 

frequency of occurrence. A greater number of pesticides were 

detected in the High Plains study (seven) as compared to 

the South Platte study (three), and the frequency of detected 

pesticides was greater in the High Plains study than the South 

Platte study. The pesticide atrazine and its degradate DEA 

were detected in both study areas, although the frequency and 

concentrations were greater in the High Plains study (nine 

atrazine detections with a mean concentration of 0.03 μg/L) 

than the South Platte study (one atrazine detection with an 

estimated concentration of 0.003 μg/L). The greater number 

of detected pesticides and the greater frequency of atrazine 

detections in the High Plains study area than the South Platte 

study area indicate long-term agricultural land use in the High 

Plains study area is having a more pronounced effect on the 
Table 2. Summary of concentrations, detections frequencies, minimum reporting levels, and maximum contaminant levels for pesticides, volatile orgother selected organic compounds detected in 15 public-supply wells in the Plio-Pleistocene aquifer, east-central Nebraska, 2003.[CASRN, Chemical Abstracts Service Registry Number; MRL, Minimum Reporting Level; μg/L, micrograms per liter; USEPA, U.S. Environmental Protection Agency; MCL, MLHA, Lifetime Health Advisory; HBSL, Health-Based Screening Level; BQmax, Benchmark Quotient (maximum detected concentration divided by corresponding MCL); U, udata; N/A, not available; R, regulated]

Compound

Regulated 

or 

unregulated

CASRN1 Number of samples Number of samples with detection Percentage of samples with detection MRL (μg/L) Maximum detected concentration (μg/L) USEPA MCL2 or LHA3 (μg/L) Nebraska MCL4 (μg/L) 

Pesticides

2-HydroxyatrazineAtrazineDeethylatrazineDeisopropylatrazineDiuron

MetolachlorSimazine

U

R

UUU

U

R

2163-68-01912-24-96190-65-41007-28-9330-54-1

51218-45-2122-34-9

1515151515

1515

39921

22

20606013

7

1313

0.0320.0070.0060.0480.014

0.0060.005

E 0.005

0.190

E 0.209E 0.06

0.04

E 0.008E 0.003

--

3

----

610

6100

4

--

3

--------

4

Volatile organic compounds

TetrachloroethyleneTrichloroethyleneChloroform1,2-dichloroethanecis-1,2-dichloroethylene

RRRRR

127-18-479-01-667-66-3107-06-2156-59-2

1515151515

43312

272020

7

13

0.0300.0380.0240.130.024

0.270.26

E 0.03E 0.1

0.18

55

780

5

70

55

--

5

70

Other selected organic compounds

Isophorone U 78-59-1 15 1 7 0.5 E 0.3 6100 -- 

1 This report contains CAS Registry Numbers®, which is a Registered Trademark of the American Chemical Society. CAS recommends the verification of the CASRNs throug2 U.S. Environmental Protection Agency, 2003, 2004.3 U.S. Environmental Protection Agency, 1993, 2004.4 Nebraska Health and Human Services, 2005.5 Toccalino and others, 2003, 2004.6 Lifetime Health Advisory.7 The MCL is for sum of all trihalomethanes (chloroform, bromodichloromethane, dibromochloromethane, and bromoform).
Table 3. Summary of concentrations, detections frequencies, minimum reporting levels, and maximum contaminant levels for pesticides, volatile organd other selected organic compounds detected in 12 public-supply wells in the Dawson and Denver aquifers near Denver, Colorado, 2003. [CASRN, Chemical Abstracts Service Registry Number; MRL, Minimum Reporting Level; μg/L, micrograms per liter; USEPA, U.S. Environmental Protection Agency; MCL, MLevel; LHA, Lifetime Health Advisory; HBSL, Health-Based Screening Level; BQmax, Benchmark Quotient (maximum detected concentration divided by corresponding MCLestimated, --, no data; U, unregulated; N/A, not available] Compound Regulated or unregulated CASRN1 Number of samples Number of samples with detection Percentage of samples with detection MRL (μg/L) Maximum detected concentration (μg/L) USEPA MCL2 or LHA3 (μg/L) Colorado MCL4 (μg/L) 

Pesticides Atrazine Picloram Deethylatrazine RRU 1912-24-9 1918-02-1 6190-65-4 121212 111 888 0.007 0.032 0.006 E 0.003 0.07 E 0.003 3 500-- 3 500-- 

Volatile organic compounds Tetrachloroethylene Methyl tert-butyl ether Bromoform RRR 127-18-4 1634-04-4 75-25-2 121212 111 888 0.03 0.1 0.1 E 0.02 1.2 E 0.9 5 620–40 80 5---- 

Other anthropogenic organic compounds Tris(2-butoxyethyl) phosphate U 78-51-3 12 1 8 0.5 E 0.1 -- -- 

1 This report contains CAS Registry Numbers®, which is a Registered Trademark of the American Chemical Society. CAS recommends the verification of the CASRNs throug2 U.S. Environmental Protection Agency, 2003, 2004. 3 U.S. Environmental Protection Agency, 1993, 2004. 4 Colorado Department of Public Health and Environment, 2005. 5 Toccalino and others, 2003, 2004. 6 Taste and odor threshold.
Comparison to Other Community Water Systems Well Data 15

water quality than in the more recently urbanized South Platte 

study area.

The frequency and concentration of VOCs also were different between the two studies. In the High Plains study, five 

VOCs were detected in the first-year source-water samples 

(table 2), and three VOCs were detected in the South Platte 

first-year source-water samples (table 3). Only one compound, 

PCE, was detected in wells from both studies. The detection 

of a VOC compound in both study areas indicate urban land 

use in both areas is affecting water quality. The greater detection frequencies and maximum concentrations of VOCs in 

the High Plains study area likely reflect the shallow depths to 

water and more permeable alluvial deposits in the High Plains 

study area, which allow more rapid transport of compounds in 

urban-affected recharge as compared to the South Platte study 

area.

Source and Finished Waters 

The second phase of the SWQA study was to resample 

wells from phase one that had significant contaminant detections and compare these data to samples collected from finished waters derived from the raw-water sources. Second-year 

sampling was conducted only in the High Plains study area. 

No second-year samples were collected for the South Platte 

study because of the relatively low number of detections and 

concentrations of organics, and the organics detected were not 

analytes of National or local interest.

Two of the original 15 sampling sites in the High Plains 

study area were selected to be sampled in the second year 

based on the total number of contaminant detections, detections of compounds of a National interest, and the relative large 

concentrations present in the first-year samples. These two new 

sampling sites, hereinafter referred to as “sites A and B,” represent finished (or treated) waters whose origins were the sourcewater wells tested in the first year. To determine the frequency 

and concentrations of contaminants in the finished waters, 

source-water samples were again collected and, depending on 

the residence time of the source water in the water-treatment 

process, a finished-water sample was collected at the estimated 

time that the same parcel of source water was exiting the treatment facility for distribution to users. Treatment processes at 

both sites included blending of source waters, in-line chlorination/fluoridation, aeration, and filtration. Site A also included 

reverse osmosis in its treatment processes. 

Results from the second-year SWQA study in the High 

Plains study area are given in table 4. Although results 

between the first and second year of source-water sampling 

at site A indicate variability in the measured constituents and 

concentrations, the results for site B indicate no change in 

pesticide concentrations. In source waters from sites A and B, 

the pesticide atrazine and its degradate DEA (deethylatrazine) 

were detected in samples from both sampling events. Atrazine 

and DEA also were detected in the associated finished waters. 

Atrazine and DEA concentrations were less in the finished 

waters than the source waters at site A, whereas pesticide 

concentrations measured at site B were similar between source 

and finished water. BQmax for all the samples were well 

below 0.1 indicating that atrazine is not a high-priority compound at the time of sampling (2004).

Data from the second-year sampling of the source and 

finished water at site A indicate additional compounds were 

present in the finished waters (chloroform, bromodichloromethane, dibromodichloromethane, metsulfuron-methyl, 

m/p xylene, total o-xylene, chlorobenzene, and ethylbenzene) 

that were not detected in the source waters. These compounds 

are not likely byproducts of any water-treatment process, but 

possibly an artifact of source-water blending during the treatment process. The multiple source waters used in the blending 

process were not sampled for this study.

Common to finished-water samples from sites A and 

B was the detection of chlorinated organic compounds not 

detected in source-water samples. The compounds detected 

in the finished waters include chloroform, bromodichloromethane, dibromochloromethane and bromoform, which are 

more commonly known as disinfection byproducts (DBPs) 

or trihalomethanes (THMs). Although these compounds may 

be present in the other source waters blended with the site 

A and B water during treatment, it is more likely that these 

compounds were produced by chlorination of the water for 

microbial disinfection during the water-treatment process. 

These THMs most commonly are regulated as the sum of their 

concentrations and termed total trihalomethanes, or TTHMs. 

These compounds are a common byproduct of the disinfection 

process in waters that contain dissolved organic carbon (DOC) 

(Ivahnenko and Zogorski, 2006). Although samples were not 

analyzed for DOC by this study, the detection of THMs in the 

finished waters indicates the source water has some level of 

DOC that is being oxidized by the addition of chlorine during 

the treatment processes at sites A and B. TTHM concentrations at site A, and the resulting BQmax value of the TTHMs, 

are at levels below regulatory concern. At site B, the TTHMs 

BQmax value of 0.18 is above the concern threshold of 

0.1 and is an indication that the TTHMs in the finished waters 

at site B are at concentrations that warrant closer attention 

and further monitoring. None of the finished-water samples 

contained any measured compound concentration above applicable water-quality standards.

Comparison to Other Community Water 

Systems Well Data

The completion of all SWQA studies conducted by the 

NAWQA program is still in its final stages; however, data from 

other NAWQA study units are available for comparison with the 

data collected for these studies. Data from the SWQA sampling of 15 public-supply wells in the Nevada Basin and Range 

study unit (Stockton and others, 2003), as well as the sampling 

of 15 wells in a similar study in the Great Salt Lake NAWQA 
16 Occurrence of Selected Organic Compounds in Groundwater in the Plio-Pleistocene Deposits in Nebraska and Colorado, 2002–2004

study unit (Thiros and Manning, 2004), are available for 

comparison. Comparing data collected in different study units 

allows for the regional synthesis of the data and helps place the 

SWQA data from the High Plains and South Platte study areas 

into a regional context. Although data comparisons from these 

areas are limited because of differences in climate, hydrogeology, land use, and water use, a general comparison can be made 

based on the similarities of relatively young aquifers, study 

design, and protocols. A comparison of the four commonly 

detected contaminants in source waters— atrazine, DEA, chloroform, and PCE—from these four studies is shown in table 5.

The detection frequency of atrazine, DEA, chloroform, 

and PCE and their concentrations in source water sampled for 

the Nevada Basin and Range, Great Salt Lake Basins, High 

Plains, and South Platte study areas varied by study and are 

best attributed to differences in land-use practices and hydrogeologic setting. Results from the Great Salt Lake Basin and 

Nevada Basin and Range study units show frequency of detections and concentrations for a majority of the compounds were 

intermediate between those in the High Plains and South Platte 

Studies. Of the four study areas, the High Plains study has the 

most extensive agricultural land use overlying an unconfined 

shallow aquifer (Landon and Turco, 2007) and consequently 

has the highest detection frequency and concentrations of 

the pesticide atrazine and its degradate DEA. The Nevada 

Basin and Range and Great Salt Lake Basins study areas also 

have a history of agricultural practices, but these areas have 

seen recent (2004) transitions from agricultural to urban land 

use (Bevans and others, 1998; Thiros and Manning, 2004) 

The South Platte study area had the lowest overall detection 

frequency and concentrations of the selected compounds, most 

likely attributed to the recent (2004) urbanization of historically rangeland areas in Colorado overlying unconfined to 

confined deep bedrock aquifers. Samples from all study areas 

except the South Platte contained measurable concentrations 

of chloroform, with unusually high detection frequencies and 

concentrations in the Great Salt Lake Basins study, attributed 

to the injection of chlorinated water into the aquifer during 

Table 4. Comparison of detected contaminants from source and finished waters from selected public-supply wells in the PlioPleistocene aquifer, east-central Nebraska, 2003 and 2004.

[μg/L, micrograms per liter; BQmax, Benchmark Quotient (maximum detected concentration divided by corresponding Maximum Contaminant Level); --, no 

data; E, estimated; ND, not detected]

Source water 2003 Source water 2004 Finished water 2004

Well Compounds Concentration BQmax μg/L

Concentration BQmax μg/LConcentration BQmax μg/L

High Plains Regional 

Groundwater study site A

High Plains Regional 

Groundwater study site B

Atrazine

Deethylatrazine

Deisopropylatrazine

Chloroform

Bromodichloromethane

Dibromochloromethane

Bromoform

Metsulfuron-methyl

m/p Xylene

Total o-xylene

Chlorobenzene

Ethylbenzene

Atrazine

Deethylatrazine

2-Hydroxyatrazine

Metolachlor

Simazine

Chloroform

Bromoform

Bromodichloromethane

Dibromochloromethane

0.129

0.209

E 0.04

ND

ND

ND

ND

ND

ND

ND

ND

ND

0.028

0.013

E 0.004

E 0.004

E 0.002

ND

ND

ND

ND

0.04

--

--

--

--

--

--

--

--

--

--

--

0.009

--

--

0.00004

0.0005

--

--

--

--

0.083

E 0.118

E 0.01

E 0.02

ND

ND

ND

ND

ND

ND

ND

ND

0.034

E 0.012

ND

ND

ND

ND

ND

ND

ND

0.03

--

--

0.0003

--

--

--

--

--

--

--

--

0.01

--

--

--

--

--

--

--

--

0.018

E 0.026

E 0.01

1

0.28

1

0.73

1

1.20

E 1

0.4

E 0.01

E 0.05

E 0.02

E 0.01

E 0.02

0.039

E 0.013

ND

ND

ND

E 1

0.43

E 1

6.7

1

1.98

1

4.9

0.006

--

--

1

0.03

1

0.03

1

0.03

1

0.03

--

0.000005

0.000002

0.0001

0.00003

0.01

--

--

--

--

1

0.18

1

0.18

1

0.18

1

0.18

1

 The Maximum Contaminant Level for total trihalomethanes is for the sum of the concentrations of chloroform, bromodichloromethane, dibromochloromethane, and bromoform; therefore, the BQmax is the sum of the concentrations divided by the 80 μg/L Maximum Contaminant Level.
Summary 17

certain periods of the year (Thiros and Manning, 2004). The 

VOC PCE was detected at about the same frequency in sam-

ples from all four study units, with the exception of the South 

Platte study, which had the least number of detections and 

the lowest concentrations. This may be attributed to the more 

recent (1994–2004) change toward urbanization in the South 

Platte study area, the affects of which may not be apparent for 

many years (if at all) because of the slow migration of shallow 

water to the deep aquifer. It should be noted that the BQmax 

for chloroform and PCE in the Great Salt Lake Basins study 

were greater than the benchmark 0.1 threshold indicating 

future monitoring of these two compounds may be warranted.

Summary

The National Water-Quality Assessment Program of the 

U.S. Geological Survey has an ongoing Source Water-Quality 

Assessment (SWQA) program designed to characterize the 

quality of water in aquifers used as a primary drinking-water 

sources for municipal suppliers in major metropolitan areas. 

Groundwater SWQA studies focus on municipal supply 

systems that produce water from large-scale regional aquifers 

that are considered vulnerable to contamination from anthro-

pogenic sources.

Source Water-Quality Assessment studies were conducted 

in the High Plains (east-central Nebraska region) and South 

Platte (south Denver, Colorado, metropolitan region) study 

units of the National Water-Quality Assessment Program 

during 2002–2004. The first year of this study included the 

sampling of source waters, which in the context of this report 

are defined as “the raw water pumped directly from aquifers 

before any water treatment.” In the second year of the study, 

source water from the first-year wells was resampled where 

contaminant detections occurred most frequently, and samples 

were collected from the finished waters derived from those 

same sites. Finished waters, in the context of this report, are 

defined as “waters that have passed through the water-quality 

treatment process and represent the post-treatment source 

water.” The goal of these SWQA studies was to determine the 

occurrence and distribution of selected organic compounds in 

the source and finished waters distributed as public drinkingwater supply in selected areas.

The High Plains study area is in east-central Nebraska in 

an area of Plio-Pleistocene alluvial and glacial deposits. The 

South Platte study area overlies the Dawson and Denver aqui-

fers of the Denver Basin aquifer system and is on the southern 

edge of the Denver metropolitan area. Data from these two 

studies were evaluated individually and with data from other 

NAWQA study units to better understand the effects land use 

and hydrogeologic settings have on groundwater quality.

Results from the source-water-quality sampling were 

compared to Federal and State water-quality standards or 

screening levels to present the results in a health-based frame-

Table 5.

work. Sources of water-quality standards and screening levels 

Comparison of selected organic compounds in source-water samples collected in the High Plains Regional Groundwater, the South Platte River Basin, the Nevada 

Basin and Range, and the Great Salt Lake Basins study units of the National Water-Quality Assessment Program.[N, number of samples; μg/L, micrograms per liter; BQmax, Benchmark Quotient (maximum detected concentration divided by corresponding maximum contaminant level); E, estimated; --, no data]

Compounds

High Plains Regional Groundwater

N=15

South Platte River Basin

N=12

Nevada Basin and Range

N=15

Greater Salt Lake Basins

N=31

Number of 

detections

Maximum 

concentration 

(μg/L)

BQmax Number of detections Maximum concentration (μg/L) BQmax Number of detections Maximum concentration (μg/L) BQmax Number of detections Maximum concentration (μg/L) BQmax

Atrazine 9 0.190 0.06 1 E 0.003 0.001 3 0.01 0.003 7 0.034 0.01

Deethylatrazine 9 E 0.209 -- 1 E 0.003 -- 4 E 0.04 -- 10 E 0.07 --Tetrachloroethylene 4 0.27 0.05 1 E 0.02 0.004 2 0.4 0.08 3 1.0 0.2

Chloroform 3 E 0.03 0.0004 -- -- -- 5 0.2 0.003 17 19.7 0.2
18 Occurrence of Selected Organic Compounds in Groundwater in the Plio-Pleistocene Deposits in Nebraska and Colorado, 2002–2004

include the U.S. Environmental Protection Agency Maximum 

Contaminant Levels (MCL) and Lifetime Health Advisories (LHA), State Maximum Contaminant Levels, and U.S. 

Geological Survey derived Health-Based Screening Levels 

(HBSL). 

Of the 15 sample sites in the High Plains study area, 

13 organic compounds were detected at varying frequencies in the first year of the study. These organic compounds 

included pesticides, volatile organic compounds (VOC), and 

other organic compounds of concern. The most-frequently 

detected compounds were the pesticide atrazine and its degradate deethylatrazine (detected in 9 of the 15 wells), although 

concentrations measured in samples were well below MCL 

standards. The second most frequently detected organic compound was tetrachloroethylene, detected in 4 of the 15 wells, 

followed by chloroform, trichloroethylene, and 2-hydroxyatrazine (2-hydroxy-4-isopropylamino-6-ethylamino-s-triazine, or 

OIET), present in 3 of the 15 wells. The pesticide compounds 

deisopropylatrazine (2-chloro-6-ethylamino-4-amino-s-triazine, or CEAT), metolachlor, and simazine and the VOC cis1,2-dichloroethylene were detected in 2 of the 15 wells, and 

the compounds diuron and 1,2-dichloroethane were detected 

in only 1 of the 15 wells during the first-year sampling. Volatile organic compounds detected in the first year of the study 

included tetrachloroethylene (PCE), chloroform, trichloroethylene, cis-1,2-dichloroethylene, and 1,2-dichlrorethane. Most 

detections of these compounds were at or near the minimum 

reporting levels, and none were greater than their regulatory 

maximum contaminant level.

Source-water collected for the South Platte study had 

7 different organic compounds detected in the 2 of the 

12 groundwater samples collected. Compounds detected only 

once in the 12 samples include the pesticides atrazine, its degradate deethylatrazine, picloram, PCE, methyl-tert-butyl ether 

(MTBE), bromoform, and tris(2-butoxyethyl)phosphate. Most 

detections of these compounds were at or near the minimum 

reporting levels, and none were greater than their regulatory 

maximum contaminant level.

Comparison of the water-quality data from the High 

Plains and South Platte study areas indicate distinct differences in the type of compounds detected and their frequency 

of occurrence. A greater number of pesticides were detected 

in the High Plains study (seven) as compared to the South 

Platte study (three) and the frequency of detected pesticides 

indicate long-term agricultural land use in the High Plains 

study area is having a more pronounced effect on the water 

quality than found in the more recently urbanized South Platte 

study area. The frequency and concentration of VOCs were 

different between the two studies. More VOCs were detected 

in the High Plains study area than in the South Platte first-year 

source-water samples. The detections of VOC compounds in 

both study areas indicate urban land use in both areas is affecting water quality. The greater detection frequencies and maximum concentrations of VOCs in the High Plains study area 

likely are a reflection of the shallow depths to water and more 

permeable alluvial deposits in the High Plains study area, 

which allow more rapid transport of compounds in urbanaffected recharge as compared to the South Platte study area.

Second-year sampling, which included the addition of 

paired source- and finished-water samples, was completed at 

two sites in the Nebraska study. Source-water samples from 

the second-year sampling had detections of atrazine and deethylatrazine; at one site deisopropylatrazine and chloroform also 

were detected. The finished-water samples, which represent 

the source water after blending with water from other wells 

and treatment, indicated a decrease in the concentrations of the 

pesticides at one site, whereas concentrations remained nearly 

constant at a second site. The trihalomethanes (THMs or disinfection byproducts), chloroform, bromoform, bromodichloromethane, and dibromochloromethane were detected in all 

finished-water samples and are a result of the chlorine added 

during treatment reacting with organics in the source waters. 

Finished waters at one site also contained concentrations of 

organic compounds not present in the source-water sample, 

indicating that other wells may be contributing compounds 

to the finished waters. Results show that all detected organic 

compounds were well below any applicable standard.

The data collected for this study also were compared to 

data collected in two similar study areas in Nevada and Utah 

in order to place the results in a more regional context. Results 

from the Great Salt Lake Basin and Nevada Basin and Range 

study units show that frequency of detections and concentrations for a majority of the compounds were intermediate 

between those in the High Plains and South Platte studies. 

Although data comparisons from these areas are limited 

because of difference in climate, hydrogeology, land use, and 

water use, a general comparison can be made based on the 

similarities of relatively young aquifers, study design, and 

protocols. Of all the study areas, the most extensive irrigated 

agriculture occurs in the High Plains study area, which corresponds with the largest concentrations and frequency of 

detections of pesticides. The study areas in the Nevada Basin 

and Range and Great Salt Lake Basins study units have a history of agricultural land uses that are transitioning to a more 

urban land use. The lowest frequency of organic compound 

detections and concentrations occurred in the South Platte 

study area, which is going through a more recent (1994–2004) 

change from rangeland to urban land use. The affects of the 

conversion to urban land use may not be apparent for many 

years (if at all) because of the slow migration of shallow water 

to the deep aquifer in the South Platte study area.

Acknowledgments

The authors thank the numerous community water systems for providing information on water use and water treatment and for allowing the water-quality sample collection.
References 19

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20 Occurrence of Selected Organic Compounds in Groundwater in the Plio-Pleistocene Deposits in Nebraska and Colorado, 2002–2004

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spectrometry: U.S. Geological Survey Water-Resources 

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Appendixes 1–3 21

Appendixes 1–3
22 Occurrence of Selected Organic Compounds in Groundwater in the Plio-Pleistocene Deposits in Nebraska and Colorado, 2002–2004

Appendix 1. Volatile organic compounds analyzed in the Source Water-Quality Assessment studies in the South Platte River Basin 

and High Plains Regional Groundwater National Water-Quality Assessment study areas during 2003–2004.—Continued

[CASR, Chemical Abstracts Service Registry Number; μg/L, micrograms per liter; shading represents compound detected in the study; Bold type under compound heading indicates unregulated compound, for which no Federal and (or) State drinking-water standards have been established; regulated is a compound 

for which Federal and (or) State drinking-water standards have been established]

Regulated or unregulated 

compound CASRN1

Minimum 

reporting level

μg/L

Remark

1,2,3,4-Tetramethylbenzene 488-23-3 0.14

1,2,3,5-Tetramethylbenzene 527-53-7 0.14

2-Butanone 78-93-3 2

trans-1,4-Dichloro-2-butene 110-57-6 0.7

2-Hexanone 591-78-6 0.4

4-Methyl-2-pentanone 108-10-1 0.37

Acetone 67-64-1 6

Acrylonitrile 107-13-1 0.8

Benzene 71-43-2 0.021

1,2,3-Trichlorobenzene 87-61-6 0.18

1,2,3-Trimethylbenzene 526-73-8 0.06

1,2,4-Trichlorobenzene 120-82-1 0.12

Bromobenzene 108-86-1 0.028

Chlorobenzene 108-90-7 0.028 Used as a solvent for some pesticide formulations, to degrease automobile 

parts, and as a chemical intermediate to make several other chemicals.

Ethylbenzene 100-41-4 0.03 Used primarily to make another chemical, styrene. Other uses include 

as a solvent, in fuels, and to make other chemicals.

1,3-Dichlorobenzene 541-73-1 0.03

Butylbenzene 104-51-8 0.12

n-Propylbenzene 103-65-1 0.042

1,2-Dichlorobenzene 95-50-1 0.048

1,4-Dichlorobenzene 106-46-7 0.034

sec-Butylbenzene 135-98-8 0.06

tert-Butylbenzene 98-06-6 0.06

Bromoethene 593-60-2 0.1

Bromoform 75-25-2 0.1 Byproduct when chlorine is added to water-supply systems.

Hexachlorobutadiene 87-68-3 0.14

Carbon disulfide 75-15-0 0.038 Occurs naturally; manufactured chemical is used as a solvent in industrial processes.

Tetrachloromethane 56-23-5 0.06

Chloroform 67-66-3 0.024 Also known as trichloromethane, byproduct when chlorine is added to 

water-supply systems; used as a solvent.

Isopropylbenzene 98-82-8 0.038

1,1,1,2-Tetrachloroethane 630-20-6 0.03

1,1,1-Trichloroethane 71-55-6 0.032

1,1,2-Trichlorotrifluoroethane 76-13-1 0.038

1,2-Dibromoethane 106-93-4 0.036

1,2-Dichloroethane 107-06-2 0.13 The most common use of 1,2-dichloroethane is in the production of vinyl 

chloride which is used to make a variety of plastic and vinyl products 

including polyvinyl chloride (PVC) pipes. It is also used as a solvent and 

is added to leaded gasoline to remove lead.

Hexachloroethane 67-72-1 0.14

1,1,2,2-Tetrachloroethane 79-34-5 0.08

Chloroethane 75-00-3 0.12
Appendixes 1–3 23

Appendix 1. Volatile organic compounds analyzed in the Source Water-Quality Assessment studies in the South Platte River Basin 

and High Plains Regional Groundwater National Water-Quality Assessment study areas during 2003–2004.—Continued

[CASR, Chemical Abstracts Service Registry Number; μg/L, micrograms per liter; shading represents compound detected in the study; Bold type under compound heading indicates unregulated compound, for which no Federal and (or) State drinking-water standards have been established; regulated is a compound 

for which Federal and (or) State drinking-water standards have been established]

Regulated or unregulated 

compound CASRN1

Minimum 

reporting level 

μg/L

Remark

Diethyl ether 60-29-7 0.08

Ethyl tert-butyl ether 637-92-3 0.03

cis-1,2-Dichloroethylene 156-59-2 0.024 Used as a solvent for waxes and resins; used as a refrigerant.

Tetrachloroethylene 127-18-4 0.03 Also known as tetrachloroethene and PCE, primarily used as dry cleaning 

agent.

trans-1,2-Dichloroethylene 156-60-5 0.032

Trichloroethylene 79-01-6 0.038 Also known as trichloroethane and TCE, solvent.

1,1-Dichloroethane 75-34-3 0.035

Tetrahydrofuran 109-99-9 1

Diisopropyl ether 108-20-3 0.1

m- and p-Xylene m=108-38- 

p=106-42-3

0.06 Occurs naturally in petroleum; manufactured chemical is used in 

chemical processes and pesticides.

1,3,5-Trimethylbenzene 108-67-8 0.044

Ethyl methacrylate 97-63-2 0.18

Methyl methacrylate 80-62-6 0.2

Methyl acrylonitrile 126-98-7 0.4

Bromochloromethane 74-97-5 0.12

Bromodichloromethane 75-27-4 0.028 Also known as dichlorobromomethane; byproduct when chlorine is 

added to water-supply systems.

Dibromochloromethane 124-48-1 0.1 Also known as chlorodibromomethane; byproduct when chlorine is 

added to water-supply systems.

Dichlorodifluoromethane 75-71-8 0.18

Trichlorofluoromethane 75-69-4 0.08

Methyl acrylate 96-33-3 1

Bromomethane 74-83-9 0.26

Chloromethane 74-87-3 0.17

Methyl iodide 74-88-4 0.5

Methyl-tert-butyl ether 1634-04-4 0.1 Gasoline additive.

Dibromomethane 74-95-3 0.05

Dichloromethane 75-09-2 0.06

Naphthalene 91-20-3 0.52

o-Xylene 95-47-6 0.038 Used in the production of plastics, dyes, and insecticides.

4-Isopropyl-1-methylbenzene 99-87-6 0.08

1,2,3-Trichloropropane 96-18-4 0.18

1,3-Dichloropropane 142-28-9 0.06

2,2-Dichloropropane 594-20-7 0.05

1,2-Dibromo-3-chloropropane 96-12-8 0.51

1,1-Dichloropropene 563-58-6 0.026

3-Chloropropene 107-05-1 0.5

cis-1,3-Dichloropropene 10061-01-5 0.05

trans-1,3-Dichloropropene 10061-02-6 0.09

1,2-Dichloropropane 78-87-5 0.029

1,2,4-Trimethylbenzene 95-63-6 0.056

tert-Pentyl methyl ether 994-05-8 0.04
24 Occurrence of Selected Organic Compounds in Groundwater in the Plio-Pleistocene Deposits in Nebraska and Colorado, 2002–2004

Appendix 1. Volatile organic compounds analyzed in the Source Water-Quality Assessment studies in the South Platte River Basin 

and High Plains Regional Groundwater National Water-Quality Assessment study areas during 2003–2004.—Continued

[CASR, Chemical Abstracts Service Registry Number; μg/L, micrograms per liter; shading represents compound detected in the study; Bold type under compound heading indicates unregulated compound, for which no Federal and (or) State drinking-water standards have been established; regulated is a compound 

for which Federal and (or) State drinking-water standards have been established]

Regulated or unregulated 

compound CASRN1

Minimum 

reporting level 

μg/L

Remark

Toluene 108-88-3 0.02

2-Chlorotoluene 95-49-8 0.04

o-Ethyl toluene 611-14-3 0.06

4-Chlorotoluene 106-43-4 0.05

Vinyl chloride 75-01-4 0.08

1,1,2-Trichloroethane 79-00-5 0.04

1,1-Dichloroethylene 75-35-4 0.024

1

 This report contains CAS Registry Numbers®, which is a Registered Trademark of the American Chemical Society. CAS recommends the verification of the 

CASRNs through CAS client ServicesSM.
Appendixes 1–3 25

Appendix 2. Pesticide compounds analyzed in the Source Water-Quality Assessment studies in the South Platte River Basin and High 

Plains Regional Groundwater National Water-Quality Assessment study areas during 2003–2004.—Continued

[CASR, Chemical Abstracts Service Registry Number; μg/L, micrograms per liter; shading represents compound detected in the study; Bold type under compound heading indicates unregulated compound, for which no Federal and (or) State drinking-water standards have been established; regulated is a compound 

for which Federal and (or) State drinking-water standards have been established]

Regulated or unregulated

compound CASRN1

Minimum 

reporting 

level 

(μg/L)

Remark

2,4-D 94-75-7 0.038

2,4-D methyl ester 1928-38-7 0.016

2,4-DB 94-82-6 0.02

2-Hydroxy-4-isopropylamino-6-

ethylamino-s-triazine (OIET) 

2163-68-0 0.032 Breakdown product of atrazine; also referred to as 2-Hydroxyatrazine.

3(4-Chlorophenyl)-1-methyl urea 5352-88-5 0.036

3-Ketocarbofuran 16709-30-1 0.02

Acifluorfen 50594-66-6 0.028

Aldicarb 116-06-3 0.04

Aldicarb sulfone 1646-88-4 0.018

Aldicarb sulfoxide 1646-87-3 0.022

Chloramben, methyl ester 7286-84-2 0.024

Atrazine 1912-24-9 0.007 Herbicide.

Bendiocarb 22781-23-3 0.02

Benomyl 17804-35-2 0.022

Bensulfuron-methyl 83055-99-6 0.018

Bentazon 25057-89-0 0.012

Bromacil 314-40-9 0.018

Bromoxynil 1689-84-5 0.028

Carbaryl 63-25-2 0.018

Carbofuran 1563-66-2 0.016

3-Hydroxycarbofuran 16655-82-6 0.008

Chlorimuron-ethyl 90982-32-4 0.032

Chlorothalonil 1897-45-6 0.035

Clopyralid 1702-17-6 0.024

Cycloate 1134-23-2 0.014

Dacthal monoacid 887-54-7 0.028

2-Chloro-4-isopropylamino-6-

amino-s-triazine (CIAT) 

6190-65-4 0.006 Breakdown product of atrazine; also referred to as Deethylatrazine.

Chlordiamino-s-triazine 

(CAAT)

3397-62-4 0.022

2-Chloro-6-ethylamino-4-amino-s-triazine (CEAT) 1007-28-9 0.08 Breakdown product of atrazine; also referred to as Deisopropylatrazine.

Dicamba 1918-00-9 0.036

Dichlorprop 120-36-5 0.028

Dinoseb 88-85-7 0.038

Diphenamid 957-51-7 0.01

Diuron 330-54-1 0.014 Herbicide.

Fenuron 101-42-8 0.018

Flumetsulam 98967-40-9 0.04

Fluometuron 2164-17-2 0.016

Imazaquin 81335-37-7 0.036

Imazethapyr 81335-77-5 0.038

Imidacloprid 138261-41-3 0.02

Linuron 330-55-2 0.014
26 Occurrence of Selected Organic Compounds in Groundwater in the Plio-Pleistocene Deposits in Nebraska and Colorado, 2002–2004

Appendix 2. Pesticide compounds analyzed in the Source Water-Quality Assessment studies in the South Platte River Basin and High 

Plains Regional Groundwater National Water-Quality Assessment study areas during 2003–2004.—Continued

[CASR, Chemical Abstracts Service Registry Number; μg/L, micrograms per liter; shading represents compound detected in the study; Bold type under compound heading indicates unregulated compound, for which no Federal and (or) State drinking-water standards have been established; regulated is a compound 

for which Federal and (or) State drinking-water standards have been established]

Regulated or unregulated 

compound CASRN1

Minimum 

reporting 

level 

(μg/L)

Remark

MCPA 94-74-6 0.03

MCPB 94-81-5 0.01

Metalaxyl 57837-19-1 0.0051

Methiocarb 2032-65-7 0.01

Methomyl 16752-77-5 0.02

Metsulfuron-methyl 74223-64-6 0.025

Neburon 555-37-3 0.012

Nicosulfuron 111991-09-4 0.04

Norflurazon 27314-13-2 0.02

Oryzalin 19044-88-3 0.012

Oxamyl 23135-22-0 0.03

Picloram 2/1/1918 0.032 Herbicide.

Prometon 1610-18-0 0.01

Propham 122-42-9 0.03

Propiconazole 60207-90-1 0.01

Propoxur 114-26-1 0.008

Siduron 1982-49-6 0.02

Simazine 122-34-9 0.005 Herbicide.

Sulfometuron-methyl 74222-97-2 0.038

Tebuthiuron 34014-18-1 0.016

Terbacil 5902-51-2 0.016

Tribenuron-methyl 101200-48-0 0.0088

Triclopyr 55335-06-3 0.026

1-Naphthol 90-15-3 0.0882

2-Chloro-2,6-diethylacetanilide 6967-29-9 0.005

2-Ethyl-6-methylaniline 24549-06-2 0.0045

3,4-Dichloroaniline 95-76-1 0.0045

4-Chloro-2-methylphenol 1570-64-5 0.0057

Acetochlor 34256-82-1 0.006

Alachlor 15972-60-8 0.005

2,6-Diethylaniline 579-66-8 0.006

Azinphos-methyl 86-50-0 0.05

Azinphos-methyl-oxon 961-22-8 0.07

Benfluralin 1861-40-1 0.01

Chlorpyrifos 2921-88-2 0.005

Chlorpyrofos, oxygen analog 5598-15-2 0.0562

cis-Permethrin 54774-45-7 0.006

Cyfluthrin 68359-37-5 0.008

Cypermethrin 52315-07-8 0.0086

Dacthal 1861-32-1 0.003

Diazinon 333-41-5 0.005

Diazinon, oxygen analog 962-58-3 0.006

Dichlorvos 62-73-7 0.0118

Dicrotophos 141-66-2 0.0843
Appendixes 1–3 27

Appendix 2. Pesticide compounds analyzed in the Source Water-Quality Assessment studies in the South Platte River Basin and High 

Plains Regional Groundwater National Water-Quality Assessment study areas during 2003–2004.—Continued

[CASR, Chemical Abstracts Service Registry Number; μg/L, micrograms per liter; shading represents compound detected in the study; Bold type under compound heading indicates unregulated compound, for which no Federal and (or) State drinking-water standards have been established; regulated is a compound 

for which Federal and (or) State drinking-water standards have been established]

Regulated or unregulated 

compound CASRN1

Minimum 

reporting 

level 

(μg/L)

Remark

Dieldrin 60-57-1 0.009

Dimethoate 60-51-5 0.0061

Ethion 563-12-2 0.004

Ethion monoxon 17356-42-2 0.002

Fenamiphos 22224-92-6 0.029

Fenamiphos sulfone 31972-44-8 0.0491

Fenamiphos sulfoxide 31972-43-7 0.0387

Desulfinylfipronil amide 0.029

Fipronil sulfide 120067-83-6 0.013

Fipronil sulfone 120068-36-2 0.024

Desulfinylfipronil 0.012

Fipronil 120068-37-3 0.016

Fonofos 944-22-9 0.003

Fonofos, oxygen analog 944-21-8 0.0029

Hexazinone 51235-04-2 0.0129

Iprodione 36734-19-7 0.387

Isofenphos 25311-71-1 0.0034

Malaoxon 1634-78-2 0.0298

Malathion 121-75-5 0.027

Methidathion 950-37-8 0.0058

Parathion-methyl 298-00-0 0.015

Metolachlor 51218-45-2 0.006 Herbicide.

Metribuzin 21087-64-9 0.006

Myclobutanil 88671-89-0 0.008

Paraoxon-methyl 950-35-6 0.0299

Pendimethalin 40487-42-1 0.022

Phorate 298-02-2 0.011

Phorate, oxygen analog 2600-69-3 0.1048

Phosmet 732-11-6 0.0079

Phosmet oxon 3735-33-9 0.0511

Prometryn 7287-19-6 0.0054

Propyzamide 23950-58-5 0.004

Terbufos 13071-79-9 0.017

Terbufos, oxygen analog sulfone 56070-15-6 0.0676

Terbuthylazine 5915-41-3 0.0102

Trifluralin 1582-09-8 0.009

1

 This report contains CAS Registry Numbers®, which is a Registered Trademark of the American Chemical Society. CAS recommends the verification of the 

CASRNs through CAS client ServicesSM.
28 Occurrence of Selected Organic Compounds in Groundwater in the Plio-Pleistocene Deposits in Nebraska and Colorado, 2002–2004

Appendix 3. Other selected organic compounds analyzed in the Source Water-Quality Assessment studies in the South Platte River 

Basin and High Plains Regional Groundwater National Water-Quality Assessment study areas during 2003–2004.—Continued

[CASR, Chemical Abstracts Service Registry Number; μg/L, micrograms per liter; shading represents compound detected in the study; Bold type under compound heading indicates unregulated compound, for which no Federal and (or) State drinking-water standards have been established; regulated is a compound 

for which Federal and (or) State drinking-water standards have been established]

Regulated or unregulated

compound CASRN1

Minimum 

reporting level 

μg/L

Remark

Cotinine 486-56-6 1

5-Methyl-1H-benzotriazole 136-85-6 2

Anthraquinone 84-65-1 0.5

Acetophenone 98-86-2 0.5

Acetyl hexamethyl 

tetrahydronaphthalene (AHTN) 

21145-77-7 0.5

Anthracene 120-12-7 0.5

Benzo[a]pyrene 50-32-8 0.5

Benzophenone 119-61-9 0.5

3-tert-Butyl-4-hydroxy anisole (BHA) 25013-16-5 5

Caffeine 58-08-2 0.018

Camphor 76-22-2 0.5

Carbazole 86-74-8 0.5

Cholesterol 57-88-5 2

3-beta-Coprostanol 360-68-9 2

N,N,-diethy-meta-toluamide 

(DEET)

134-62-3 0.5

Bisphenol A 80-05-7 1

Triethyl citrate (ethyl citrate) 77-93-0 0.5

Fluoranthene 206-44-0 0.5

Hexahydrohexamethylcyclopentabenz

opyran (HHCB) 

1222-05-5 0.5

Indole 120-72-9 0.5

Isoborneol 124-76-5 0.5

Isophorone 78-59-1 0.5 Used as a solvent in some printing inks, paints, lacquers, and 

adhesives.

Isoquinoline 119-65-3 0.5

d-Limonene 5989-27-5 0.5

Menthol 89-78-1 0.5

1-Methylnaphthalene 90-12-0 0.5

2,6-Dimethylnaphthalene 581-42-0 0.5

2-Methylnaphthalene 91-57-6 0.5

Nonylphenol, diethoxy- (total) 26027-38-2 5

Octylphenol, diethoxy 26636-32-8 1

Octylphenol, monoethoxy- 26636-32-8 1

p-Cresol 106-44-5 1

4-Cumylphenol 599-64-4 1

para-Nonylphenol (total) 84852-15-3 5

4-n-Octylphenol 1806-26-4 1

4-tert-Octylphenol 140-66-9 1

Phenanthrene 85-01-8 0.5

Phenol 108-95-2 0.5 Used primarily in the formation of phenolic resins. It is also used in 

the manufacture of nylon and other synthetic fibers.

Pentachlorophenol 87-86-5 2
Appendixes 1–3 29

Appendix 3. Other selected organic compounds analyzed in the Source Water-Quality Assessment studies in the South Platte River 

Basin and High Plains Regional Groundwater National Water-Quality Assessment study areas during 2003–2004.—Continued

[CASR, Chemical Abstracts Service Registry Number; μg/L, micrograms per liter; shading represents compound detected in the study; Bold type under compound heading indicates unregulated compound, for which no Federal and (or) State drinking-water standards have been established; regulated is a compound 

for which Federal and (or) State drinking-water standards have been established]

Regulated or unregulated 

compound CASRN1

Minimum 

reporting level 

μg/L

Remark

Tributyl phosphate 126-73-8 0.5

Triphenyl phosphate 115-86-6 0.5

Tris(2-butoxyethyl)phosphate 78-51-3 0.5 Used mainly as a component in floor polishes, a solvent in some 

resins, a viscosity modifier in plastisols, an antifoam and also as a 

plasticizer in synthetic rubber, plastics and lacquers.

Tris(2-chloroethyl)phosphate 

(TCEP)

115-96-8 0.5

Pyrene 129-00-0 0.5

Methyl salicylate 119-36-8 0.5

3-Methyl-1(H)-indole (Skatole) 83-34-1 1

beta-Sitosterol 83-46-5 2

beta-Stigmastanol 19466-47-8 2

Triclosan 3380-34-5 1

Tris(dichloroisopropyl)phosphate 

(TCPP)

13674-87-8 0.5

1

 This report contains CAS Registry Numbers®, which is a Registered Trademark of the American Chemical Society. CAS recommends the verification of the 

CASRNs through CAS client ServicesSM.
30 Occurrence of Selected Organic Compounds in Groundwater in the Plio-Pleistocene Deposits in Nebraska and Colorado, 2002–2004

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Appendixes 1–3 31
32 Occurrence of Selected Organic Compounds in Groundwater in the Plio-Pleistocene Deposits in Nebraska and Colorado, 2002–2004
Appendixes 1–3 33
Bails and others—Occurrence of Selected Organic Compounds in Groundwater in the Plio-Pleistocene Deposits in Nebraska and Colorado, 2002–2004—SIR 2008–5243

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