Cover. The Great Falls of the Potomac River is located at the Fall Line, 14 miles upstream from Washington, D.C. 

Photograph by U.S. Geological Survey.
Anthropogenic Organic Compounds in 

Source and Finished Groundwater of 

Community Water Systems in the Piedmont 

Physiographic Province, Potomac River 

Basin, Maryland and Virginia, 2003–04

By William S.L. Banks and Betzaida Reyes

National Water-Quality Assessment Program

Source-Water-Quality Assessments

Scientific Investigations Report 2009–5064

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Suzette Kimball, Acting Director

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

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

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reproduce any copyrighted materials contained within this report.

ISBN 978 1 4113 2504 3

Suggested citation:

Banks, W.S.L., and Reyes, B., 2009, Anthropogenic organic compounds in source and finished groundwater of 

community water systems in the Piedmont Physiographic Province, Potomac River Basin, Maryland and Virginia, 

2003–04: U.S. Geological Survey Scientific Investigations Report 2009–5064, 33 p.
iii

Foreword

The U.S. Geological Survey (USGS) is committed to providing the Nation with reliable 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, measured in terms of quantity and quality, even 

more essential to the long-term sustainability of our communities and ecosystems.

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

National, regional, State, and local information needs and decisions related to water-quality management 

and policy (http://water.usgs.gov/nawqa). The NAWQA Program is designed to answer: What is the 

quality 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 to 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). 

National and regional assessments are ongoing in the second decade (2001–2012) of the NAWQA 

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

in the Study Units by determining water-quality 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 studies 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. In addition, national syntheses of information on pesticides, volatile organic 

compounds (VOCs), nutrients, 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. 

The USGS recognizes that a national assessment by a single program cannot address all water-resource 

issues of interest. External coordination at all levels is critical for cost-effective management, regulation, 

and conservation of our Nation’s water resources. The NAWQA Program, therefore, depends on advice 

and information from other agencies — Federal, State, regional, interstate, Tribal, and local — as well as 

nongovernmental organizations, industry, academia, and other stakeholder groups. Your assistance and 

suggestions are greatly appreciated.

 Matthew C. Larsen

 Associate Director for Water

v

Contents

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

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

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

Purpose and Scope ..............................................................................................................................4

Location and Description of Study Area...........................................................................................4

Hydrologic Setting .......................................................................................................................4

Groundwater Withdrawals and Distribution ...........................................................................4

Study Design and Methods..........................................................................................................................5

Quality Assurance and Quality Control .............................................................................................5

Anthropogenic Organic Compounds in Source Water............................................................................6

Volatile Organic Compounds.............................................................................................................11

Pesticides.............................................................................................................................................13

Other Anthropogenic Organic Compounds ....................................................................................15

Comparison of Source Water and Finished Groundwater....................................................................15

Volatile Organic Compounds.............................................................................................................15

Pesticides.............................................................................................................................................21

Other Anthropogenic Organic Compounds ....................................................................................24

Summary........................................................................................................................................................24

Acknowledgments.......................................................................................................................................25

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

Appendix A. Comparison Between Source Water and Associated Finished Water 

(Non-Blended and Blended) for Detected Anthropogenic Organic Compounds 

in Community Water-Supply Systems in the Piedmont Physiographic Province, 

Potomac River Basin, Maryland and Virginia, 2004..................................................................29
vi

Tables

1. Maximum concentration, detection frequency, and maximum benchmark 

quotient for regulated and unregulated compounds detected in source water 

from 15 community water-supply wells in the Piedmont Physiographic 

Province, Potomac River Basin, Maryland and Virginia, 2003 ..............................................7

2. Maximum concentration, detection frequency, and maximum benchmark 

quotient for regulated and unregulated compounds detected in source water 

from 10 community water-supply wells and the associated finished water 

in the Piedmont Physiographic Province, Potomac River Basin, Maryland and 

Virginia, 2004................................................................................................................................17

Figures

1–2. Maps showing—

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

units and groundwater source-water-quality assessment ..........................................2

2. Location of Potomac River Basin, Piedmont Physiographic Province, and 

location and names of wells in the Piedmont source-water assessment 

groundwater network, 2003–04 .........................................................................................3

3–10. Graphs showing—

3. Volatile organic compounds detected in source water from 15 community 

water-supply wells: (A) concentrations and occurrence, and (B) benchmark 

quotient values, Maryland and Virginia, 2003 ...............................................................12

4. Distribution of maximum concentrations of detected anthropogenic organic 

compounds in source water from 15 community water-supply wells, 

Maryland and Virginia, 2003.............................................................................................13

5. Pesticides detected in source water from 15 community water-supply wells: 

(A) concentrations and occurrence, and (B) benchmark quotient values, 

Maryland and Virginia, 2003.............................................................................................14

6. Other anthropogenic organic compounds detected in source water from 

15 community water-supply wells: (A) concentrations and occurrence, 

and (B) benchmark quotient values, Maryland and Virginia, 2003............................16

7. Volatile organic compounds detected in source water from 10 community 

water-supply wells and associated finished water (non-blended and 

blended): (A) concentrations and occurrence, and (B) benchmark 

quotient values, Maryland and Virginia, 2004 ...............................................................20

8. Concentrations of bromodichloromethane and chloroform detected in 

source water from 10 community water-supply wells and associated 

finished water (non-blended and blended), Maryland and Virginia, 2004 ...............21

 9. Comparison of concentrations of pesticides, volatile organic compounds, 

and other anthropogenic compounds in source water from 10 community 

water-supply wells and associated finished water (non-blended and 

blended), Maryland and Virginia, 2004...........................................................................22

10. Pesticides detected in source water from 10 community water-supply 

wells and associated finished water (non-blended and blended): 

(A) concentrations and occurrence, and (B) benchmark quotient values, 

Maryland and Virginia, 2004.............................................................................................23
vii

Conversion Factors

Multiply By To obtain

square mile (mi2) 2.590 square kilometer (km2) 

gallon per minute (gal/min) 0.06309 liter per second (L/s)

gallon per day (gal/d) 0.003785 cubic meter per day (m3/d)

million gallons per day (Mgal/d) 0.04381 cubic meter per second (m3/s)

micrograms per liter (μg/L) 1.0 parts per billion (ppb)

pounds per year (lbs/yr) 0.0004536 metric tons per year (tons/yr)

million pounds per year (Mlbs/yr) 453.59237 metric tons per year (tons/yr)

Symbol Explanation

AOC Anthropogenic organic compound

BQ Benchmark Quotient

BQmax Ratio of the maximum concentration to a drinking-water benchmark

CCR Consumer confidence report

CERCLA Comprehensive Environmental Response, Compensation, and Liability Act

CWS Community water system

DBP Disinfection by-product

DEET N,N,-diethyl-meta-toluamide

ESA Ethanesulfonic acid

HBSL Health-Based Screening Level

MCL Maximum Contaminant Level

MTBE Methyl tert-butyl ether

NAWQA National Water-Quality Assessment

NWQL National Water-Quality Laboratory

OA Oxanilic acid

OAOC Other anthropogenic organic compound

QA and QC Quality assurance and quality control

SWQA Source-Water-Quality Assessment

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

VOC Volatile organic compound

Abbreviations and Acronyms
viii

Definitions

Term Definition

Benchmark 

Quotient (BQ)

Ratio of the concentration of a contaminant to its Maximum Contaminant 

Level (MCL) value for a regulated compound or to its Health-Based 

Screening Level (HBSL) value for an unregulated compound. BQs greater 

than 1 identify concentrations of potential human-health concern. BQs 

greater than 0.1 identify compounds that may warrant inclusion in a lowconcentration, trends-monitoring program.

Blended Water In this report, finished water that has been blended with one or more 

different untreated groundwater sources. 

BQmax Ratio of the maximum concentration of a contaminant to its MCL or HBSL 

value.

Community 

Water System 

(CWS)

A public water system with 15 or more connections and serving 25 or more 

year-round residents, making it subject to U.S. Environmental Protection 

Agency (USEPA) regulations that enforce the Safe Drinking Water Act. A 

CWS serves a residential population, such as a municipality, mobile home 

park, or nursing home.

Drinking-Water 

Guidelines

As used in this report, a threshold concentration that has no regulatory 

status, but is issued in an advisory capacity by the USEPA or State 

agencies.

Drinking-Water 

Standards

As used in this report, a threshold concentration that is legally enforceable 

(such as MCLs) by the USEPA or State agencies.

Finished Water Water is “finished” when it has passed through all the processes in a water 

treatment plant and is ready to be delivered to consumers.

Health-Based 

Screening 

Level (HBSL)

Benchmark concentrations of contaminants in water that may be 

of potential concern for human health, if exceeded. HBSLs are nonenforceable benchmarks that were developed by the U.S. Geological 

Survey (USGS) in collaboration with the USEPA and others using USEPA 

methodologies for establishing drinking-water guidelines and the most 

current, USEPA peer-reviewed, publicly available human-health toxicity 

information.

Human-Health 

Benchmarks

As used in this report, these include USEPA and Florida Department of 

Environmental Protection maximum contaminant level values and HBSL 

values developed collaboratively by the USGS, USEPA, New Jersey 

Department of Environmental Protection, and Oregon Health & Science 

University.

Maximum 

Contaminant 

Level (MCL)

As used in this report, a USEPA drinking-water standard that is legally 

enforceable, and that sets the maximum permissible level of a contaminant 

in water that is delivered to any user of a public water system at which no 

known or anticipated adverse effect on the health of persons occurs, and 

which allows an adequate margin of safety.

Source Water Source water is the raw (ambient) water collected at the supply well or 

surface-water intake prior to water treatment. Following water treatment, 

source water is finished water.

Unregulated 

Compound

As used in this report, a compound for which no Federal and (or) State 

drinking-water standard has been established. Note that a compound that 

is unregulated under the Safe Drinking Water Act may be regulated in other 

contexts and under other statutes.


Anthropogenic Organic Compounds in Source and 

Finished Groundwater of Community Water Systems 

in the Piedmont Physiographic Province, Potomac River 

Basin, Maryland and Virginia, 2003–04

By William S.L. Banks and Betzaida Reyes

Abstract

A source- and finished-water-quality assessment of 

groundwater was conducted in the Piedmont Physiographic 

Province of Maryland and Virginia in the Potomac River 

Basin during 2003–04 as part of the U.S. Geological Survey’s 

National Water-Quality Assessment Program. This assessment 

used a two-phased approach to sampling that allowed investigators to evaluate the occurrence of more than 280 anthropogenic organic compounds (volatile organic compounds, 

pesticides and pesticide degradates, and other anthropogenic 

organic compounds). Analysis of waters from 15 of the largest 

community water systems in the study area were included in 

the assessment. Source-water samples (raw-water samples 

collected prior to treatment) were collected at the well head. 

Finished-water samples (raw water that had been treated 

and disinfected) were collected after treatment and prior to 

distribution. Phase one samples, collected in August and 

September 2003, focused on source water. Phase two analyzed 

both source and finished water, and samples were collected in 

August and October of 2004. 

The results from phase one showed that samples 

collected from the source water for 15 community water 

systems contained 92 anthropogenic organic compounds 

(41 volatile organic compounds, 37 pesticides and pesticide 

degradates, and 14 other anthropogenic organic compounds). 

The 5 most frequently occurring anthropogenic organic compounds were detected in 11 of the 15 source-water samples. 

Deethylatrazine, a degradate of atrazine, was present in all 

15 samples and metolachlor ethanesulfonic acid, a degradate 

of metolachlor, and chloroform were present in 13 samples. 

Atrazine and metolachlor were present in 12 and 11 samples, 

respectively. All samples contained a mixture of compounds 

with an average of about 14 compounds per sample. 

Phase two sampling focused on 10 of the 15 community 

water systems that were selected for resampling on the basis 

of occurrence of anthropogenic organic compounds detected 

most frequently during the first phase. A total of 48 different 

anthropogenic organic compounds were detected in samples 

collected from source and finished water. There were a similar 

number of compounds detected in finished water (41) and 

in source water (39). The most commonly detected group 

of anthropogenic organic compounds in finished water was 

trihalomethanes — compounds associated with the disinfection of drinking water. This group of compounds accounted 

for 30 percent of the detections in source water and 44 

percent of the detections in finished water, and were generally 

found in higher concentrations in finished water. Excluding 

trihalomethanes, the number of total detections was about the 

same in source-water samples (33) as it was in finished-water 

samples (35).

During both phases of the study, two measurements for 

human-health assessment were used. The first, the Maximum 

Contaminant Level for drinking water, is set by the U.S. 

Environmental Protection Agency and represents a legally 

enforceable maximum concentration of a contaminant permitted in drinking water. The second, the Health-Based Screening 

Level, was developed by the U.S. Geological Survey, is not 

legally enforceable, and represents a limit for more chronic 

exposures. Maximum concentrations for each detected compound were compared with either the Maximum Contaminant 

Level or the Health-Based Screening Level when available. 

More than half of the compounds detected had either a 

Maximum Contaminant Level or a Health-Based Screening 

Level. A benchmark quotient was set at 10 percent (greater 

than or equal to 0.1) of the ratio of the detected concentration of a particular compound to its Maximum Contaminant 

Level, or Health-Based Screening Level. This was considered 

a threshold for further monitoring. During phase one, when 

only source water was sampled, seven compounds (chloroform, benzene, acrylonitrile, methylene chloride, atrazine, 

alachlor, and dieldrin) met or exceeded a benchmark quotient. 

No detected compounds in source or finished water exceeded a 

benchmark quotient during phase two.
2 Anthropogenic Organic Compounds in Groundwater, Piedmont Physiographic Province, Potomac River Basin, MD and VA, 2003–04

Introduction

Anthropogenic organic compounds (AOCs) are 

chemicals that are associated with human activities that range 

from manufacturing and agriculture to domestic-use products 

discharged through the waste stream. Virtually all of these 

compounds are manmade, and many are released into the 

environment as a function of their use. Some AOCs are known 

to affect human health. Recent research has focused on the 

fate and transport (and in some instances, the human-health 

effects) of many AOCs in shallow groundwater (Barbash and 

Resek, 1996; Herberer, 2002; Ternes and others, 2002; and 

Zogorski and others, 2006). Their occurrence and distribution 

in source water and fate and transport through public 

drinking-water systems is not clearly understood. Further 

study would benefit the understanding of these processes. As 

an example, in one sampling survey investigators showed that 

AOCs were detected in the raw water of approximately one 

quarter of the nearly 1,100 public water-supply wells sampled 

(Herberer, 2002). Additionally, Loraine and Pettigrove (2006), 

and Stackelberg and others (2004) have shown that AOCs 

often occur in finished water at concentrations similar to those 

found in source water, and that some compounds are created 

through the source-water disinfection process. 

The U.S. Geological Survey (USGS) has designed 

Source-Water-Quality Assessment (SWQA) studies to 

characterize the water quality of major rivers and aquifers 

used as a source of drinking water for larger community water 

systems (CWSs). These studies are intended to complement 

the drinking-water monitoring currently performed by 

public water suppliers and required by Federal, State, and 

local programs. These programs tend to focus primarily on 

post-treatment compliance monitoring. The SWQA studies 

provide a mechanism for comparing the results obtained from 

both regional NAWQA monitoring and required drinkingwater monitoring and can help determine whether water used 

for human consumption is meeting appropriate human-health 

guidelines (fig. 1). This study looks at three broad classes of 

AOCs in 15 CWSs in the Piedmont Physiographic Province 

of the Potomac River Basin (fig. 2) (U.S. Geological Survey, 

2007a, 2007b, 2007c, and 2007d).

Figure 1. Location of National Water-Quality Assessment (NAWQA) Program study units and groundwater sourcewater-quality assessment.
Introduction 3

Figure 2. Location of the Potomac River Basin, Piedmont Physiographic Province, and location and names of wells in the 

Piedmont source-water assessment groundwater network, 2003–04.
4 Anthropogenic Organic Compounds in Groundwater, Piedmont Physiographic Province, Potomac River Basin, MD and VA, 2003–04

Volatile organic compounds (VOCs) are organic chemicals that have been produced and used in a variety of commercial, industrial, and household applications for many decades, 

and are major components or additives to gasoline, paints, varnishes, solvents, glues, dyes, and plastics. The second class of 

AOCs, pesticides, are chemicals used extensively throughout 

the United States to increase crop yields, enhance the aesthetics of lawns, gardens, golf courses, and recreational areas, and 

protect the public from insect-associated diseases. This class 

also includes herbicides and insecticides, fungicides, and some 

of the degradates (breakdown products) that form when pesticides are exposed to the environment. Compounds in the last 

class, other AOCs (OAOCs), are present in a wide range of 

products commonly used in homes, industry, and agriculture, 

including personal care and domestic-use products, plant- or 

animal-derived biochemicals, and fumigants. 

Purpose and Scope

This report describes the occurrence of AOCs in source 

water obtained from 15 of the largest CWS wells serving small 

and medium communities in the Piedmont Physiographic 

Province in Maryland and Virginia. The report also describes 

the occurrence of selected AOCs in samples from 10 of the 

same 15 CWS wells and the associated AOCs in finished 

water from each of the 10 selected sites. Concentrations of 

detected compounds are compared to the U.S. Environmental 

Protection Agency (USEPA) Maximum Contaminant Levels 

(MCLs) or USGS Health-Based Screening Levels (HBSLs) 

to evaluate the potential relevance of the findings to human 

health.

Location and Description of Study Area

The Potomac River Basin drains 14,670 mi2 (square 

miles) within the states of Maryland, Virginia, West Virginia, 

Pennsylvania, and Washington, D.C. Within the basin, the 

Piedmont Physiographic Province is roughly oriented northeast to southwest along the Atlantic coast and is the second 

largest province in the study area, covering about 12 percent 

of the Basin in Maryland, Pennsylvania, Virginia, and 

Washington, D.C. The area is characterized by rolling topography and low to moderate relief underlain by crystalline, metamorphic, and igneous rocks of Precambrian to Ordovician age 

(Milici and others, 1963; Cardwell and others, 1968; Cleaves 

and others, 1968; King and Beikman, 1974; Berg 1980). 

Hydrologic Setting

Groundwater in the Piedmont Physiographic Province 

is under water-table conditions in saprolitic material 

(a chemically weathered, in-place rock material overlying 

consolidated crystalline rock), and occurs in the joints and 

fractures of the underlying crystalline rock. Water-supply 

wells in the Piedmont region are generally designed to intercept the fractures and joints of the crystalline bedrock below 

the overlying unconsolidated material. Wells are drilled by 

isolating the overburden and drilling into the bedrock until one 

or multiple zones yielding satisfactory volumes of water are 

encountered. Water-bearing fracture zones tend to be located 

in topographic low areas where multiple fractures or joints 

occur and where the overburden has higher hydraulic head and 

permeability. Nutter and Otton (1969) noted that this localization is partly because valleys in the Piedmont tend to develop 

along fracture zones. The interconnected nature of joints 

and fractures in the Piedmont Physiographic Province often 

obscures the source and direction of groundwater flow and 

thus, the ultimate source of contaminants (Cleaves and others, 1968; Nutter and Otton, 1969; Heath, 1984; and Bolton, 

1998).

Groundwater Withdrawals and Distribution

About 20 percent of public water used in the study area is 

supplied through groundwater withdrawals, and the remaining 

80 percent comes from various surface-water supplies (W.P 

McPherson and J.P. Pope, USGS, written commun., 2006). 

The ratio of groundwater to surface water used for public 

supply remained consistent between 2000 and 2004 in the 

four counties that comprise most of the study area (Fauquier 

and Loudon Counties in Virginia and Carroll and Frederick 

Counties in Maryland) (fig. 2). Although the proportions 

of water used from different sources have remained fairly 

constant, the volume of groundwater consumed in the study 

area has risen dramatically. In Fauquier County, Virginia, 

groundwater usage more than doubled between 2000 and 

2004, from 0.69 to 1.66 Mgal/d (million gallons per day). In 

Loudon County, Virginia (the second fastest growing county 

in the United States between 2000 and 2005), groundwater 

use increased from 0.92 to 1.38 Mgal/d, or nearly 50 percent, from 2000 through 2004 (U.S. Census Bureau, 2006). 

From 2000 through 2004, groundwater usage in Carroll and 

Frederick Counties, Maryland, increased by 20 and 5 percent, or 0.51 and 0.16 Mgal/d, respectively. This increase 

in resource consumption is in response to a 10-year general 

increase in population in the region as a result of a rapidly 

expanding technology industry centered in Washington, D.C. 

These new communities are served by over 3,600 public supply wells throughout the study area in Maryland and Virginia 

(the Washington, D.C. area is supplied solely by surface water 

from the Potomac River). 

Water from CWS wells may be used for a variety of purposes, including domestic, commercial, industrial, and agricultural. These supply wells may be publicly or privately owned 

and operated. Water from these community wells and all 

CWS wells in the study area is treated before distribution by a 

variety of methods including disinfection using chlorine gas, 

coagulation and filtration, adsorption, lime softening, addition 
Study Design and Methods 5

of corrosion and pH control additives, and reverse osmosis 

(Participating CWS managers, written commun., 2003). The 

wells that were monitored as part of this SWQA represent a 

cross section of the variety of CWS wells in the study area.

Study Design and Methods

Study design was based on a two-phase site selection and 

sampling process during 2003–04 to characterize the occurrence of anthropogenic compounds that may exist in publicly 

supplied groundwater. In phase one, 15 of the largest CWSs 

in the study area were identified and the owners and operators 

were contacted and asked to participate in this study. Each 

CWS received information on the objectives of the program 

and was asked to grant USGS scientists permission to collect 

samples from one randomly selected well in their system. 

Selected wells were at least one km (kilometer) apart and produced between about 10 and 200 gal/min (gallons per minute) 

(fig. 2).

During phase one, 15 CWS wells were initially sampled 

before treatment (at the source) in August and September 2003 

and analyzed for 284 VOCs, pesticides and pesticide degradates (in this report, the term degradate refers to any herbicide 

transformation product formed by biotic or abiotic processes), 

and OAOCs. Source-water samples (water that is untreated or 

raw), were collected at the well head, prior to any treatment.

During the second phase of the study, 10 of the original 

15 wells were selected for resampling on the basis of occurrence of AOCs detected most frequently during the first phase 

(during the 2003 sampling) (fig. 2). Source-water samples 

were collected from each of the 10 public supply wells along 

with samples of the associated finished water (treated source 

water that is ready to be delivered to consumers) prior to entry 

to transport pipelines. All systems were chlorinated, however; 

other treatments prior to delivery were not documented. Each 

of the 10 source-water samples collected during phase two 

were associated with either a blended or non-blended finishedwater sample. Seven source-water samples were associated 

with non-blended finished water and three were associated 

with blended finished water. At sites where blending occurred, 

the finished water was the product of mixing sampled source 

water with water from one or more additional groundwater 

sources. All samples for finished water were collected approximately 1 hour after the source-water sample was collected. 

The suite of constituents for each well varied on the basis of 

phase one results. During phase two, samples of both source 

water and the associated finished water from 10 wells were 

analyzed for VOCs, 6 wells were analyzed for pesticides, and 

2 wells were analyzed for OAOCs. 

Samples were collected using established USGS protocols described in Koterba and others (1995), and on-line 

at http://pubs.usgs.gov/of/1995/ofr-95-399/. Water samples 

were filtered at the time of collection to remove suspended 

particulate matter. Organic chemicals were sorbed onto a 

polystyrene-divinylbenzene resin using a disposable solidphase cartridge. All samples were shipped overnight to the 

USGS National Water-Quality Laboratory (NWQL) in Denver, 

Colorado for analysis. Upon arrival in Denver, samples were 

eluted and compounds were analyzed by capillary-column gas 

chromatography/mass spectrometry. Analytical methods are 

documented in Zaugg and others (1995), Lindley and others 

(1996), Connor and others (1998), Furlong and others (2001), 

Sandstrom and others (2001), Zaugg and others (2002), and 

Madsen and others (2003).

Finished-water samples were treated with a dechlorination reagent (ascorbic acid) to remove free chlorine typically 

introduced as part of the treatment process. Free chlorine has 

been shown to be a potential interference in the analysis of 

some organic compounds. pH buffers were added to finishedwater samples during sample collection to stabilize them prior 

to shipment and analyses. Current research indicates that the 

addition of these reagents in a laboratory setting does not 

interfere with analytical performance (Mark Sandstrom, USGS 

NWQL, oral commun., 2004).

Quality Assurance and Quality Control

Quality-assurance and quality-control (QA and QC) samples were periodically collected during sampling events using 

the procedures described above. Samples included blanks, 

replicates, and spikes. Blanks can be used to detect contamination that may be introduced during sampling, shipping, and 

(or) the analytical process. Replicate samples measure variance within sampling groups and are used to control sample 

collection, processing, and analysis. Sample spikes are used to 

detect bias caused by analyte degradation or interference from 

the sampling media. In keeping with the NAWQA Program’s 

scope and scale, QA and QC efforts for each project were 

focused at the national level. Thus, many individual NAWQA 

studies did not collect a sufficient number of QA and QC 

samples to adequately assess individual studies without being 

combined with other QA and QC samples from other SWQA 

studies. These samples were, however, aggregated nationally 

and are reviewed annually to assess systemic contamination 

from either sample-collection equipment or sampling procedures and sample handling or shipping protocol. Any recurrent 

detection (more than 10 percent of ambient concentrations) 

of one or more analytes in QA and QC samples indicates that 

there is a potential for bias for those compounds resulting 

from one or more of the aforementioned sources. All samples 

that were collected and analyzed for the uncontrolled analytes 

are therefore considered suspect. Environmental sample data 

for the affected analytes were removed from the database and 

were not used in any comparative or quantitative analysis 

(Greg Delzer, USGS, written commun., 2007). 

Eight compounds were potentially biased nationwide 

and were subsequently eliminated from the national project 

data inventory. For this study, a sufficiently robust set of 

QA and QC samples were collected so that only those 
6 Anthropogenic Organic Compounds in Groundwater, Piedmont Physiographic Province, Potomac River Basin, MD and VA, 2003–04

compounds that showed systemic contamination at the project 

and national levels were eliminated. Specifically, five of the 

eight compounds were found to be biased in more than 5 

percent of the source solution and field blanks nationally, 

and in blank samples collected at the project level. This 

pattern may indicate that contamination comes from point 

sources such as sampling equipment, field personnel (insect 

repellents and fragrances), field environment (dust), and (or) 

sample handling procedures or sample containers rather than 

from environmental, nonpoint sources. The five compounds 

excluded from the data inventory were the common insect 

repellent N,N diethyl-meta-toluamide (DEET), and all the 

phenol-based compounds such as phenol, para-nonylphenol, 

benzophenone, and bisphenol A. These compounds were not 

included in the current study.

Anthropogenic Organic Compounds in 

Source Water

Each source-water sample was analyzed for up to 284 

AOCs, with a total of 92 AOCs detected among the 15 wells 

(not every compound was analyzed at each site): 41 VOCs, 

37 pesticides and pesticide degradates, and 14 OAOCs. All 

samples contained a mixture of compounds. Two samples contained over 30 compounds, another only 4, and all 15 samples 

averaged about 14 AOCs per sample (table 1). The majority 

of concentrations for all AOCs detected in source water were 

generally low and ranged from 0.005 (estimated) μg/L 

(micrograms per liter) for the fungicide propiconazole to 13.5 

μg/L for chloroform, a naturally occurring chemical and a byproduct of disinfecting water through the use of chlorine (table 

1). Two common industrial solvents, methyl ethyl ketone and 

tetrahydrofuran, were detected at concentrations of 330 and 

1,020 μg/L, respectively, and were higher than concentrations 

found for other AOCs.

The five most frequently occurring AOCs were detected 

in 11 (or 73 percent) of the 15 source waters sampled during 

phase one. Deethylatrazine (an atrazine degradate) was 

the only compound present in all 15 samples. Similarly, 

metolachlor ethanesulfonic acid (metolachlor ESA), a 

degradate of the herbicide metolachlor, and chloroform, a 

VOC, were present in 13 of 15 wells, or 87 percent of the 

samples. Atrazine and metolachlor were present in 12 (80 

percent), and 11 (73 percent), respectively, of the 15 wells 

sampled.

Different AOCs have different minimum reporting levels. 

For the purposes of this report, comparisons of AOCs are 

Consumer Confidence Reports and Source-Water-Quality Assessments

Since 1999, the U.S. Environmental Protection Agency (USEPA) has required water suppliers to provide annual drinkingwater-quality reports called Consumer Confidence Reports (CCRs) to their customers (see http://www.epa.gov/safewater/

ccr/whereyoulive.html, accessed May 18, 2009). CCRs are the centerpiece of the right-to-know provisions of the 1996 

Amendments to the Safe Drinking Water Act. Each CCR provides consumers with fundamental information about their drinking water including (1) the source of the drinking water, (2) a brief summary of the susceptibility to contamination of the local 

drinking-water source, (3) the concentrations (or range of concentrations) of any contaminants found in local drinking water, 

as well as their USEPA Maximum Contaminant Levels (MCLs), which are legally enforceable drinking-water standards and 

are the highest allowed concentrations of contaminants in drinking water, for comparison, and (4) phone numbers for additional sources of information.

Information in CCRs is specific to a particular water utility. Water utilities analyze finished-water samples primarily for 

regulated contaminants (that is, those with MCLs) using USEPA analytical methods for the purpose of compliance monitoring. In contrast, Source-Water-Quality Assessments (SWQAs) performed by the USGS are not conducted for compliance 

monitoring and encompass data from multiple water utilities spatially distributed across the Nation. As part of SWQAs, both 

source- and finished-water samples are analyzed using USGS analytical methods, where source water is the raw (ambient) 

water collected at the surface-water intake or supply well prior to water treatment, and finished water is the treated water 

sampled prior to entering the distribution system. USGS analytical methods used in SWQAs typically have lower analytical 

reporting levels than those used in compliance monitoring; contaminant detection frequencies reported in SWQA reports may 

therefore be higher than detection frequencies for the same contaminants reported in CCRs. In SWQAs, concentrations of 

regulated and unregulated contaminants in source and finished water are compared to MCLs and Health-Based Screening 

Levels (HBSLs), respectively. HBSLs are estimates of benchmark concentrations of contaminants in water that may be of 

potential human-health concern, and are consistent with USEPA Office of Water methodologies for setting non-enforceable 

drinking-water guideline values. HBSLs are not legally enforceable regulatory standards, and water utilities are not required 

to compare contaminant monitoring results to HBSLs.
Anthropogenic Organic Compounds in Source Water 

7

Table 1. Maximum concentration, detection frequency, and maximum benchmark quotient for regulated and unregulated compounds detected in source water from 15 community water-supply wells in the Piedmont Physiographic Province, Potomac River Basin, Maryland and Virginia, 2003.—Continued [HBSL, Health-Based Screening Level; MCL, Maximum Contamination Level; BQmax, Benchmark Quotient = ratio of maximum compound concentration to MCL or HBSL value; E, estimated value; μg/L, micrograms per liter; MRL, Minimum Reporting Level; --, not available; VOC, volatile organic compound; OAOC, other anthropogenic organic compounds; Bold type under compound heading indicates “unregulated,” a compound that has no Federal and (or) State drinking-water standard; ESA, ethanesulfonic acid; OA, oxanilic acid.]

Compound 

(Regulated or unregulated)

Chemical 

Abstracts Service 

Registry Number

Number of 

samples 

collected

Number of 

samples with 

detections

Frequency 

of detection 

(percent)

MRL 

(μg/L)

Maximum detected 

concentration 

(μg/L)

MCL or HBSL1

concentration 

(μg/L)

BQMax2

Volatile Organic Compounds

Chloroform 67-66-3 15 13 87 0.024 13.5 380 0.2Methyl tert-butyl ether 1634-04-4 15 10 67 0.1 1.88 -- --Toluene 108-88-3 15 6 40 0.02 0.224 1,000 0.00021,1,1-Trichloroethane 71-55-6 15 3 20 0.032 0.122 200 0.0006tert-Amyl Methyl Ether 994-05-8 15 3 20 0.04 0.281 -- --Tetrachloroethene 127-18-4 15 3 20 0.03 E 0.050 5 0.011,1-Dichloroethane 75-34-3 15 2 13 0.035 0.238 -- --1,1-Dichloroethene 75-35-4 15 2 13 0.024 0.115 7 0.02Benzene 71-43-2 15 2 13 0.021 0.631 5 0.1Bromodichloromethane 75-27-4 15 2 13 0.028 E 0.058 380 0.0007Trichloroethene 79-01-6 15 2 13 0.038 0.112 5 0.02Trichlorofluoromethane 75-69-4 15 2 13 0.08 E 0.065 62,000 0.000031,2-Dichloroethane 107-06-2 15 1 7 0.13 E 0.113 5 0.021,2-Dichloropropane 78-87-5 15 1 7 0.029 E 0.092 5 0.021-Ethyl-2-methylbenzene 611-14-3 15 1 7 0.06 0.666 -- --1,2,3,4-Tetramethylbenzene 488-23-3 15 1 7 0.14 0.921 -- --1,2,3,5-Tetramethylbenzene 527-53-7 15 1 7 0.18 1.480 -- --1,2,3-Trimethylbenzene 526-73-8 15 1 7 0.09 1.510 -- --1,2,4-Trimethylbenzene 95-63-6 15 1 7 0.056 2.370 -- --1,3,5-Trimethylbenzene 108-67-8 15 1 7 0.044 0.942 -- --Acetone 67-64-1 15 1 7 6 7.990 66,000 0.001Acrylonitrile 107-13-1 15 1 7 0.8 E 0.504 10.06 8.4Bromoform 75-25-2 15 1 7 0.1 E 0.037 380 0.0005Carbon disulfide 75-15-0 15 1 7 0.038 12.700 6700 0.02Carbon Tetrachloride 56-23-5 15 1 7 0.06 E 0.017 5 0.003cis-1,2-Dichloroethene 156-59-2 15 1 7 0.024 0.700 70 0.01
8 Anthropogenic Organic Compounds in Groundwater, Piedmont Physiographic Province, Potomac River Basin, MD and VA, 2003–04 Table 1. Maximum concentration, detection frequency, and maximum benchmark quotient for regulated and unregulated compounds detected in source water from 15 community water-supply wells in the Piedmont Physiographic Province, Potomac River Basin, Maryland and Virginia, 2003.—Continued 

[HBSL, Health-Based Screening Level; MCL, Maximum Contamination Level; BQmax, Benchmark Quotient = ratio of maximum compound concentration to MCL or HBSL value; E, estimated value; μg/L, micrograms per liter; MRL, Minimum Reporting Level; --, not available; VOC, volatile organic compound; OAOC, other anthropogenic organic compounds; Bold type under compound heading indicates “unregulated,” a compound that has no Federal and (or) State drinking-water standard; ESA, ethanesulfonic acid; OA, oxanilic acid.]

Compound 

(Regulated or unregulated)

Chemical 

Abstracts Service 

Registry Number

Number of 

samples 

collected

Number of 

samples with 

detections

Frequency 

of detection 

(percent)

MRL 

(μg/L)

Maximum detected 

concentration 

(μg/L)

MCL or HBSL1

concentration 

(μg/L)

BQMax2

Dichlorodifluoromethane 75-71-8 15 1 7 0.18 E 0.145 61,000 0.0001Deisopropyl ether 108-20-3 15 1 7 0.1 0.159 -- --Ethylbenzene 100-41-4 15 1 7 0.03 0.517 700 0.0007Isopropylbenzene 98-82-8 15 1 7 0.038 E 0.074 6700 0.0001m- and 

p-Xylene m=108-38-3 p=106-42-3 15 1 7 0.06 3.280 410,000 0.0003

Methyl ethyl ketone 78-93-3 15 1 7 2 330 64,000 0.08Methyl isobutyl ketone 108-10-1 15 1 7 0.37 E 0.766 -- --Methylene chloride 75-09-2 15 1 7 0.06 0.994 5 0.2Naphthalene 91-20-3 15 1 7 0.52 0.956 6100 0.01n-Butylbenzene 104-51-8 15 1 7 0.14 E 0.069 -- --n-Propylbenzene 103-65-1 15 1 7 0.042 0.108 -- --o-Xylene 95-47-6 15 1 7 0.038 2.840 410,000 0.0003p-Isopropyltoluene 99-87-6 15 1 7 0.08 E 0.063 -- --sec-Butylbenzene 135-98-8 15 1 7 0.06 E 0.065 -- --Tetrahydrofuran 109-99-9 15 1 7 1.2 1020 -- --Total number of VOC detections 79

Pesticides

Deethylatrazine 6190-65-4 15 15 100 0.014 E 0.276 -- --Metolachlor ESA -- 15 13 87 0.05 2.720 -- --Atrazine 1912-24-9 15 12 80 0.007 0.439 3 0.1Metolachlor 51218-45-2 15 11 73 0.006 3.580 700 0.005Alachlor ESA -- 15 7 47 0.05 0.640 -- --Metolachlor OA -- 15 7 47 0.05 2.960 -- --Simazine 122-34-9 15 5 33 0.005 0.114 4 0.032-Hydroxyatrazine 2163-68-0 13 4 31 0.032 E 0.032 670 0.0005Prometon 1610-18-0 15 4 27 0.01 E 0.011 6100 0.0001Carbofuran 1563-66-2 13 3 23 0.016 0.006 40 0.0002
Anthropogenic Organic Compounds in Source Water 

9

Table 1. Maximum concentration, detection frequency, and maximum benchmark quotient for regulated and unregulated compounds detected in source water from 15 community water-supply wells in the Piedmont Physiographic Province, Potomac River Basin, Maryland and Virginia, 2003.—Continued [HBSL, Health-Based Screening Level; MCL, Maximum Contamination Level; BQmax, Benchmark Quotient = ratio of maximum compound concentration to MCL or HBSL value; E, estimated value; μg/L, micrograms per liter; MRL, Minimum Reporting Level; --, not available; VOC, volatile organic compound; OAOC, other anthropogenic organic compounds; Bold type under compound heading indicates “unregulated,” a compound that has no Federal and (or) State drinking-water standard; ESA, ethanesulfonic acid; OA, oxanilic acid.]

Compound 

(Regulated or unregulated)

Chemical 

Abstracts Service 

Registry Number

Number of 

samples 

collected

Number of 

samples with 

detections

Frequency 

of detection 

(percent)

MRL 

(μg/L)

Maximum detected 

concentration 

(μg/L)

MCL or HBSL1

concentration 

(μg/L)

BQMax2

Alachlor 15972-60-8 15 3 20 0.005 0.249 2 0.1Bentazon 25057-89-0 13 2 15 0.024 E 0.491 6200 0.002Terbacil 5902-51-2 13 2 15 0.026 E 0.060 690 0.0007Alachlor OA -- 15 2 13 0.05 1.230 -- --Aldicarb sulfone 1646-88-4 13 1 8 0.018 E 0.007 57 0.001Clopyralid 1702-17-6 13 1 8 0.067 E 0.013 -- --Deisopropylatrazine 1007-28-9 13 1 8 0.08 E 0.116 -- --Flumetsulam 98967-40-9 13 1 8 0.04 E 0.139 67,000 0.00002Oryzalin 19044-88-3 13 1 8 0.023 E 0.138 14 0.03Picloram 1918-02-01 13 1 8 0.032 0.085 500 0.0002Propiconazole 60207-90-1 13 1 8 0.01 E 0.005 670 0.00007Siduron 1982-49-6 13 1 8 0.02 E 0.012 -- --3,4-dichloroaniline 95-76-1 15 1 7 0.005 0.008 -- --Acetochlor 34256-82-1 15 1 7 0.006 0.037 11 0.04Acetochlor ESA -- 15 1 7 0.05 0.070 -- --Actochlor OA -- 15 1 7 0.05 0.120 -- --Carbaryl 63-25-2 15 1 7 0.018 E 0.010 140 0.0003Chlorpyrifos, oxygen analog 5598-15-2 15 1 7 0.056 E 0.017 -- --Dieldrin 60-57-1 15 1 7 0.009 0.024 10.002 12.1Fipronil 120068-37-3 15 1 7 0.016 E 0.008 -- --Fipronil sulfide 120067-83-6 15 1 7 0.013 0.007 -- --Fipronil sulfone 120068-36-2 15 1 7 0.024 0.020 -- --Hexazinone 51235-04-2 15 1 7 0.026 E 0.008 6400 0.00002Metalaxyl 57837-19-1 15 1 7 0.007 0.037 6600 0.00006Metribuzin 21087-64-9 15 1 7 0.028 0.010 690 0.0001Myclobutanil 88671-89-0 15 1 7 0.033 0.011 6200 0.00006Tebuthiuron 34014-18-1 15 1 7 0.026 E 0.013 61,000 0.00001Total number of pesticide detections 113
10 Anthropogenic Organic Compounds in Groundwater, Piedmont Physiographic Province, Potomac River Basin, MD and VA, 2003–04 Table 1. Maximum concentration, detection frequency, and maximum benchmark quotient for regulated and unregulated compounds detected in source water from 15 community water-supply wells in the Piedmont Physiographic Province, Potomac River Basin, Maryland and Virginia, 2003.—Continued 

[HBSL, Health-Based Screening Level; MCL, Maximum Contamination Level; BQmax, Benchmark Quotient = ratio of maximum compound concentration to MCL or HBSL value; E, estimated value; μg/L, micrograms per liter; MRL, Minimum Reporting Level; --, not available; VOC, volatile organic compound; OAOC, other anthropogenic organic compounds; Bold type under compound heading indicates “unregulated,” a compound that has no Federal and (or) State drinking-water standard; ESA, ethanesulfonic acid; OA, oxanilic acid.]

Compound 

(Regulated or unregulated)

Chemical 

Abstracts Service 

Registry Number

Number of 

samples 

collected

Number of 

samples with 

detections

Frequency 

of detection 

(percent)

MRL 

(μg/L)

Maximum detected 

concentration 

(μg/L)

MCL or HBSL1

concentration 

(μg/L)

BQMax2

Other Anthropogenic Organic Compounds

4-octylphenol, diethoxylate 26636-32-8 15 2 13 1 E 0.120 -- --4-octylphenol, monoethoxylate 26636-32-8 15 2 13 1 E 0.590 -- --1-Methylnaphthalene 90-12-0 15 1 7 0.5 E 0.12 -- --2-Methylnaphthalene 91-57-6 15 1 7 0.5 E 0.23 630 0.0084-Cumylphenol 599-64-4 15 1 7 1 E 0.18 -- --Acetophenone 98-86-2 15 1 7 0.5 E 0.15 6700 0.0002Anthracene 120-12-7 15 1 7 0.5 E 0.024 62,000 0.00001Anthraquinone 84-65-1 15 1 7 0.5 E 0.11 -- --Carbazole 86-74-8 15 1 7 0.5 E 0.081 -- --Fluoranthene 206-44-0 15 1 7 0.5 E 0.036 6300 0.0001p-Cresol 106-44-5 15 1 7 1 E 0.1 -- --Phenanthrene 85-01-8 15 1 7 0.5 E 0.015 -- --Pyrene 129-00-0 15 1 7 0.5 E 0.018 6200 0.0001Triphenyl phosphate 115-86-6 15 1 7 0.5 E 0.095 -- --Total number of OAOC detections 16

1 The HBSL is a range that is based on United States Environmental Protection Agency’s (USEPA) risk-specific dose at a 10-6 to 10-4 cancer risk level. The lowest value of this range was used in this 

assessment (Toccalino and others, 2004).

2 BQmax for unregulated compounds calculated using HBSL concentration at the low end of the the HBSL range, and was rounded to one significant figure.3 1998 Final Rule for Disinfectants and Disinfection By-Products: The total for trihalomethanes is 80 micrograms per liter.4 MCL is for the sum of concentrations for mixed isomers of xylene.5 The concentration of any two or more of the three aldicarb compounds should not be greater than 7 μg/L because of similar mode of action.6 Value calculated using USEPA’s formula for Lifetime Health Advisory.
Anthropogenic Organic Compounds in Source Water 11

made among AOCs and between sites regardless of varying 

reporting levels in order to characterize general occurrence 

rates and similarities, or the lack thereof, between these sites.

Human-health benchmarks are available for slightly 

more than half (52 of 92) of the detected compounds (table 

1). About 40 percent of these benchmarks are set by the 

USEPA in the form of MCLs and are established for regulated 

chemicals (U.S. Environmental Protection Agency, 2006). 

The remaining compounds with benchmarks had HBSLs 

developed by the USGS to place water-quality data in a 

human-health context (Toccalino and others, 2004). In order to 

determine whether the concentration of a detected compound 

had potential adverse human-health effects, a benchmark 

quotient (BQ) was calculated by creating a ratio between the 

concentration of a compound and the appropriate humanhealth benchmark (either the MCL for regulated compounds 

or HBSL for unregulated compounds). When applied as a fraction to the highest concentration of a contaminant in a group 

or network of samples, the ratio is called the BQmax. This 

ratio can be used as a conservative estimate of the potential 

negative impact of water consumption on human health. In 

general, a BQmax ratio greater than or equal to 0.1 identifies 

concentrations of compounds that may warrant inclusion in 

a low-concentration trends-monitoring program, whereas a 

BQmax greater than or equal to 1.0 identifies concentrations of 

potential human-health concern (Toccalino and others, 2004). 

The concentration of compounds in source-water samples 

that had human-health benchmarks were generally between 

one and five orders of magnitude below the 0.1 benchmark. 

Seven compounds (chloroform, benzene, acrylonitrile, methylene chloride, atrazine, alachlor, and dieldrin), however, met or 

were greater than the 0.1 benchmark in one or more sourcewater samples. Without similar metrics for the remaining compounds (compounds having neither an MCL nor an HBSL), 

the potential human-health relevance is unknown. Two of the 

most frequently occurring compounds, deethylatrazine and 

metolachlor ESA, occurred in more than 80 percent of the 

samples analyzed, and both lack a human-health benchmark 

(table 1).

Volatile Organic Compounds

Forty-one different VOCs were detected in the sourcewater samples with an average of about 5 compounds per 

sample (table 1, fig. 3a). Chloroform, the most frequently 

detected compound, was detected in 13 of 15 wells or 87 

percent of the samples. Chloroform and several other compounds are common disinfection by-products (DBPs) and are 

produced when chlorine is added to either drinking or wastewater. The detection of chloroform in groundwater can be an 

indicator of chlorinated water entering the hydrologic system 

and the input of other chloroform sources (such as direct 

releases) to the environment (Ivahnenko and Barbash, 2004). 

Chloroform can enter the hydrologic system in the form of 

recharge from chlorinated water. Sources include chlorinated 

water used to irrigate, leaking distribution lines for finished 

water and wastewater, and leakage from septic fields. The use 

of sodium hypochlorite to disinfect wells, the dehalogenation 

of tetrachloromethane, and a variety of natural sources can 

also contribute to its occurrence in groundwater (Ivahnenko 

and Barbash, 2004). 

Two other compounds that are associated with the 

chlorination of drinking water were detected — bromodichloromethane at a detection frequency of 13 percent (2 of 15 samples), and bromoform at a detection frequency of 7 percent (1 

of 15 samples). Chloroform occurred in 87 percent of sampled 

wells (13 of 15 samples) and in all samples where 4 or fewer 

compounds were detected (table 1, fig. 3a). The common use 

of free chlorine in the drinking-water decontamination process 

produces one or more trihalomethanes (THMs) in addition to 

chloroform (Ivahnenko and Zogorski, 2006). Further, when a 

naturally occurring bromide ion is present during chlorination, 

the bromide forms halogenated DBPs. Therefore, in the presence of bromide, the detection frequency of the four principal 

THMs (bromoform, dibromochloromethane, bromodichloromethane, and chloroform) will decrease sequentially, inversely 

proportional to the concentration of bromide ion (Thiros, 

2000; Grady and Casey, 2001; Moran and others, 2002). Thus, 

the relatively high detection frequency of chloroform in the 

study area and the relatively low occurrence of other DBPs 

indicates the use of chlorination for the disinfection of drinking water. Ivahnenko and Zogorski (2006) suggested that this 

pattern is indicative of the use of chlorine to disinfect domestic 

and, to a lesser degree, public groundwater supplies.

Other frequently detected VOCs were the gasoline additives methyl tert-butyl ether (MTBE) (10 of 15 samples, or 67 

percent), and toluene (6 of 15 samples, or 40 percent) (table 

1). Toluene is known to be toxic to a wide range of biota and 

MTBE is classified as a potential human carcinogen (U.S. 

Environmental Protection Agency, 1999). Both are relatively 

hydrophilic and commonly co-occur in gasoline-contaminated 

groundwater (Weaver and others, 1999). In a study of gasoline components in urban areas, Delzer and others (1996) 

and Zogorski and others (2006) suggested that contaminated 

stormwater could be a source for MTBE (and possibly 

toluene) in shallow groundwater. These findings support the 

results of a study by Grady (2003), which included a national 

review of MTBE and other VOCs, and showed that MTBE 

and chloroform were the most frequently detected compounds 

in source-water supplies (groundwater and surface water) 

nationwide. 

Twenty-five of the VOCs detected had human-health 

benchmarks (table 1). Three of these compounds (methylene 

chloride, chloroform, and benzene) had BQmax values greater 

than or equal to 0.1. One compound, acrylonitrile, had a 

BQmax value (8.4) above the 1.0 level of potential humanhealth concern (fig. 3b). All four compounds are listed by the 

USEPA as high production volume (HPV) chemicals, which 

means that their production in the United States exceeds 

1 Mlbs/yr (million pounds per year); however, their use is not 

restricted to manufacturing (U.S. Environmental Protection 
12 Anthropogenic Organic Compounds in Groundwater, Piedmont Physiographic Province, Potomac River Basin, MD and VA, 2003–04

Figure 3. Volatile organic compounds detected in source water from 15 community water-supply wells: (A) concentrations and 

occurrence, and (B) benchmark quotient values, Maryland and Virginia, 2003.
Anthropogenic Organic Compounds in Source Water 13

Agency, 1996). Benzene is used as an organic solvent and 

dilution agent in many industries including the manufacturing 

of insecticides and herbicides. Acrylonitrile is an acrylic fiber 

(monomer) used in the production of nylon and synthetic rubber. Chloroform, as previously described, is associated with 

the disinfection of drinking water. In addition, all four compounds appear on USEPA’s Comprehensive Environmental 

Response, Compensation, and Liability Act (CERCLA) 

Hazardous Substances list (U.S. Environmental Protection 

Agency, 2001). The relatively high BQmax values for acrylonitrile, methylene chloride, and benzene, but relatively low 

frequency of occurrence (one sample or 7 percent for acrylonitrile and methylene chloride, and two samples or 13 percent 

for benzene) indicates that there is a need for continued monitoring of these compounds. For VOCs that were detected, the 

median concentration of all maximum concentrations of VOCs 

was 0.51 μg/L (fig. 4). 

Pesticides 

Thirty-seven different pesticide and pesticide degradate 

compounds were detected in the source water, which represents about 8 compounds per sample (with a maximum of 27 

compounds in one sample and a minimum of 1 compound per 

sample) (table 1, fig. 5a). Although the number of individual 

pesticides and pesticide degradates detected (37) was roughly 

the same as for VOCs (41), pesticides were detected more 

frequently in comparison to VOCs (113 pesticide detections 

compared to 79 VOC detections). The median value of 

maximum concentrations of detected pesticides was 0.04 μg/L 

(fig. 4). The majority of the pesticides that were found are 

designated as “unrestricted” with respect to use and application. The USEPA designates a pesticide as “restricted” for use 

based on the compound’s acute (not chronic) toxicity (U.S. 

Environmental Protection Agency, 2007). 

Nearly half (about 45 percent) of the 113 pesticide detections resulted from the quantifiable presence of atrazine and 

metolachlor — two common, unrestricted-use agricultural 

herbicides and their degradates, deethylatrazine and 

metolachlor ESA. Almost one quarter (27 detections) were 

either from atrazine (12 detections) or its degradate deethylatrazine (15 detections). Metolachlor ESA and its parent 

compound metolachlor accounted for 21 percent of the total 

number of detections (13 and 11 detections, respectively). In 

addition, when more than five compounds were detected at a 

specific site, those compounds were likely to be one or more 

of the six most common pesticides found, either atrazine or an 

atrazine degradate (deethylatrazine), metolachlor or metolachlor degradates [metolachlor ESA or metolachlor OA (metolachlor oxanilic acid)], or alachlor ESA (fig. 5a). 

After it is applied to crops (usually corn to control 

weeds), atrazine can pass through the soil zone and into the 

shallow groundwater. As it moves, it undergoes microbial 

degradation, forming deethylatrazine, among other 

compounds. In a study of commonly used pesticides, Gilliom 

and others (2006) determined that atrazine was the single most 

used pesticide nationwide — 75 Mlbs/yr. This same study 

noted that atrazine and deethylatrazine were the most commonly detected compounds in groundwater nationwide — up 

to about 44 percent for deethylatrazine and 42 percent for atrazine, with detection frequency dependent on the surrounding 

land use. Atrazine is water-soluble and relatively persistent in 

soil with a half life (the time needed for one half of the applied 

compound to degrade or metabolize into one or multiple 

other compounds) of 146 days. This high-intensity use and 

relatively long half life may contribute to the high detection 

frequency among the 15 CWSs sampled in this study. Adams 

and Thurman (1991), in attempting to quantify the source term 

of atrazine and its degradates in groundwater, determined that 

deethylatrazine was the major degradate of atrazine and that it 

could enter the aquifer at higher concentrations than the parent 

compound. Other independent research has indicated that the 

half life for atrazine in soils can be more than 100 days, and 

that detectable concentrations of atrazine and deethylatrazine 

can persist for up to 6 years and more than 25 years, respectively (Barbash and others, 1999; Denver and Sandstrom, 

1991; Gaus, 2000).

Figure 4. Distribution of maximum concentrations of detected 

anthropogenic organic compounds in source water from 15 

community water-supply wells, Maryland and Virginia, 2003.
14 Anthropogenic Organic Compounds in Groundwater, Piedmont Physiographic Province, Potomac River Basin, MD and VA, 2003–04

Figure 5. Pesticides detected in source water from 15 community water-supply wells: (A) concentrations and occurrence, and (B) 

benchmark quotient values, Maryland and Virginia, 2003. [ESA, ethanesulfonic acid; OA, oxanilic acid]
Comparison of Source Water and Finished Groundwater 15

Metolachlor is a common pre-emergent herbicide used 

extensively to control weeds in corn and soybeans. Its estimated usage in the United States in 1997 was 67 million lbs/yr 

(Gianessi and Marcelli, 2000) — the second most used herbicide after atrazine. Like atrazine, metolachlor is water soluble 

and persistent in the environment with a half life of 114 days 

(Kollman and Segawa, 2000). Metolachlor was detected in 

groundwater nationwide at a frequency of up to 17 percent, 

depending on surrounding land use (Gilliom and others, 2006). 

Detection frequency in the current study indicates a substantially higher frequency of occurrence — about 73 percent. 

Once applied, metolachlor metabolizes; the two most common degradates of metolachlor are metolachlor OA and ESA. 

Eckhardt and others (1999) and Phillips and others (1999) 

found that these degradates are more soluble than their parent 

compounds. Both studies also indicated that the degradates are 

more persistent, found at higher concentrations, and detected 

more frequently in groundwater and surface water than the 

parent compounds. Similarly, Phillips and others (1999) found 

that the ratio of metolachlor ESA to metolachlor was higher 

in groundwater when compared to surface water. In a study of 

groundwater on the Delmarva Peninsula, Debrewer and others 

(2007) detected the ESA degradate of metolachlor in as many 

as 88 percent of shallow (less than 100 feet) wells sampled, 

and metolachlor OA was detected in 49 percent of the wells 

sampled. The current study found higher detection frequencies 

for both pesticides and their degradates than those found by 

Debrewer and others (2007). Deethylatrazine had a 100-percent detection frequency and atrazine had an 80-percent 

(12 out of 15) detection frequency. The detection frequency 

for metolachlor was 73 percent (11 out of 15) whereas metolachlor ESA and metolachlor OA were detected at 87 percent 

(13 out of 15) and 47 percent (7 out of 15), respectively, in 

source water. Other pesticides and degradates occurring in 

more than one third of the source-water samples collected 

were metolachlor OA (47 percent, or 7 of 15 samples), alachlor ESA (alachlor ethanesulfonic acid) — a degradate of the 

unrestricted herbicide alachlor (47 percent), and simazine 

(33 percent, or 5 of 15 samples). Maximum concentrations 

of pesticides detected in source water ranged from 0.005 

(estimated) μg/L for propiconazole, an unrestricted fungicide, 

to 3.58 μg/L for metolachlor, a common unrestricted herbicide 

(table 1, fig. 5b). 

Human-health benchmarks were available for 22 of the 

37 pesticide compounds detected (fig. 5b). The BQmax values 

for these 22 compounds were generally one to several orders 

of magnitude below the benchmark quotient value of 0.1. 

Three compounds had BQmax values greater than or equal 

to the 0.1 threshold, however. Atrazine and alachlor, both 

unrestricted, widely used herbicides, had a BQmax value of 

about 0.1, indicating that further monitoring in a low-concentration trends-monitoring program may be warranted. Dieldrin 

had a BQmax greater than 1.0. The BQmax for dieldrin, an 

unregulated insecticide and known carcinogen, was 12.1 based 

on an HBSL of 0.002. Dieldrin is an organochlorine pesticide, is hydrophobic, tends to bioaccumulate, and was used 

extensively throughout the United States in the agricultural 

and building industries from the 1940s until it was completely 

banned in 1987 (Agency for Toxic Substances and Disease 

Registry, 1993). 

Other Anthropogenic Organic Compounds

The most frequently detected OAOCs in source water 

were 4-octylphenol diethoxylate and 4-octylphenol monoethoxylate, both with a detection frequency of 13 percent (2 out 

of 15). The maximum concentrations were 0.120 (estimated) 

μg/L for 4-octylphenol diethoxylate, and 0.590 (estimated) 

μg/L for 4-octylphenol monoethoxylate (table 1, fig. 6a). 

The median concentration of all maximum concentrations of 

OAOCs was approximately 0.10 μg/L (fig. 4). Only one occurrence of each of the remaining 12 compounds was detected in 

the 15 samples. Of these 12 compounds, 2-methylnaphthalene 

had the highest concentration, 0.23 (estimated) μg/L, and 

1-methylnaphthalene had a concentration of 0.12 (estimated) 

μg/L. Both 2-methylnaphthalene and 1-methylnaphthalene are 

HPV chemicals used in industrial manufacturing. Five of the 

14 OAOCs had human-health benchmarks — none generated 

a BQmax value that exceeded 0.1 (fig. 6b).

Comparison of Source Water and 

Finished Groundwater

The source and finished water from 10 of the original 

15 CWS wells was resampled in August and October 2004. 

For the 10 finished-water samples, 3 were blended with 

groundwater from other CWS wells and 7 were not blended. 

Since these CWSs were sampled only for those AOCs that 

occurred most frequently during the first phase of sampling, 

not all pairs of CWS wells and associated finished water were 

sampled for the same AOCs. That is, 10 pairs of source water 

and the associated finished water were analyzed for VOCs, 6 

for pesticides, and 1 for OAOCs. Concentrations of each AOC 

detected in either source water and (or) the associated blended 

or non-blended finished water are presented in Appendix A.

Volatile Organic Compounds

Samples from 10 CWS sites and the associated finished 

water were analyzed for a suite of 85 VOCs. Three of the 

finished-water samples were blended with other sources of 

groundwater prior to treatment. Samples from the remaining 

seven CWSs were non-blended and came from a single well. 

A total of 21 different VOCs were detected in either the source 

water and (or) the finished water — 16 in source water, and 19 

in finished water (table 2, fig. 7a). All BQmax values in source 

and finished water were well below the benchmark quotient 
16 Anthropogenic Organic Compounds in Groundwater, Piedmont Physiographic Province, Potomac River Basin, MD and VA, 2003–04

Figure 6. Other anthropogenic organic compounds detected in source water from 15 community water-supply wells: (A) 

concentrations and occurrence, and (B) benchmark quotient values, Maryland and Virginia, 2003.
Comparison of Source Water and Finished Groundwater 17

Table 2. Maximum concentration, detection frequency, and maximum benchmark quotient for regulated and unregulated compounds detected in source water from 10 community water-supply wells and the associated finished water in the Piedmont Physiographic Province, Potomac River Basin, Maryland and Virginia, 2004.—Continued[HBSL, Health-Based Screening Level; MCL, Maximum Contaminant Level; BQmax, Benchmark Quotient = ratio of maximum compound concentration to MCL or HBSL value; E, estimated value; μg/L, micrograms per liter; MRL, Minimum Reporting Level; --, not available; VOC, volatile organic compound; OAOC, other anthropogenic organic compounds; Bold type under compound heading indicates “unregulated,” a compound that has no Federal and (or) State drinking-water standard; ESA, ethanesulfonic acid; OA, oxanilic acid.]

Compound 

(Regulated or unregulated)

Chemical 

Abstracts 

Service 

Registry 

Number

Number 

of 

samples 

collected

Number of 

sourcewater 

samples 

with 

detections

Number of 

finishedwater 

samples 

with 

detections

MRL 

(μg/L)

Maximum 

concentration 

for source 

water 

(μg/L)

Maximum 

concentration 

for finished 

water 

(μg/L)

MCL or HBSL1

concentration

(μg/L)

BQMax2

for 

source 

water 

BQMax2

for 

finished 

water 

Volatile Organic Compounds

Chloroform 67-66-3 10 9 10 0.024 1.569 1.574 380 0.02 0.021,4-Dichlorobenzene 106-46-7 10 8 8 0.034 E 0.048 E 0.056 75 0.0006 0.0007Methyl tert-butyl ether 1634-04-4 10 7 8 0.1 2.671 2.565 -- -- --1,1,1-Trichloroethane 71-55-6 10 3 3 0.032 0.105 0.106 200 0.0005 0.0005Bromodichloromethane 75-27-4 10 3 7 0.028 0.204 0.725 380 0.003 0.009Tetrachloroethene 127-18-4 10 3 2 0.03 E 0.037 E 0.020 5 0.007 0.0041,1-Dichloroethene 75-35-4 10 2 2 0.024 0.112 E 0.096 7 0.02 0.01Deisopropyl ether 108-20-3 10 2 2 0.1 0.165 0.148 -- -- --tert-Amyl Methyl Ether 994-05-8 10 2 2 0.04 0.231 0.212 -- -- --Trichloroethene 79-01-6 10 2 1 0.038 E 0.094 E 0.035 5 0.02 0.0071,1-Dichloroethane 75-34-3 10 1 1 0.035 0.240 0.238 -- -- --Bromoform 75-25-2 10 1 5 0.1 0.396 0.520 380 0.005 0.006 Dibromochloromethane 124-48-1 10 1 6 0.1 0.433 1.109 380 0.005 0.01 cis-1,2-Dichloroethene 156-59-2 10 1 1 0.024 0.506 2.188 70 0.007 0.03 Isopropylbenzene 98-82-8 1 1 0 0.5 E 0.007 -- 5700 0.00001 --Tetrahydrofuran 109-99-9 10 1 0 1.2 E 1.10 -- -- -- --Carbon tetrachloride 56-23-5 10 0 1 0.06 -- E 0.025 5 -- 0.005Methylene chloride 75-09-2 10 0 1 0.06 -- E 0.019 5 -- 0.004Ethylbenzene 100-41-4 10 0 1 0.03 -- E 0.023 700 -- 0.00003m- and p-Xylene m=108-38-3 

p=106-42-3

10 0 1 0.06 -- E 0.085 410,000 -- 0.000009

o-Xylene 95-47-6 10 0 1 0.038 -- E 0.027 410,000 -- 0.000003Total number of VOC detections

 47 63 
18 Anthropogenic Organic Compounds in Groundwater, Piedmont Physiographic Province, Potomac River Basin, MD and VA, 2003–04 Table 2. Maximum concentration, detection frequency, and maximum benchmark quotient for regulated and unregulated compounds detected in source water from 10 community water-supply wells and the associated finished water in the Piedmont Physiographic Province, Potomac River Basin, Maryland and Virginia, 2004.—Continued

[HBSL, Health-Based Screening Level; MCL, Maximum Contaminant Level; BQmax, Benchmark Quotient = ratio of maximum compound concentration to MCL or HBSL value; E, estimated value; μg/L, micrograms per liter; MRL, Minimum Reporting Level; --, not available; VOC, volatile organic compound; OAOC, other anthropogenic organic compounds; Bold type under compound heading indicates “unregulated,” a compound that has no Federal and (or) State drinking-water standard; ESA, ethanesulfonic acid; OA, oxanilic acid.]

Compound 

(Regulated or unregulated)

Chemical 

Abstracts 

Service 

Registry 

Number

Number 

of 

samples 

collected

Number of 

sourcewater 

samples 

with 

detections

Number of 

finishedwater 

samples 

with 

detections

MRL 

(μg/L)

Maximum 

concentration 

for source 

water 

(μg/L)

Maximum 

concentration 

for finished 

water 

(μg/L)

MCL or HBSL1

concentration

(μg/L)

BQMax2

for 

source 

water 

BQMax2

for 

finished 

water 

Pesticides

Atrazine 1912-24-9 6 6 5 0.007 0.230 0.203 3 0.08 0.07Deethylatrazine 6190-65-4 6 6 5 0.014 E 0.146 0.111 -- -- --Deisopropylatrazine 1007-28-9 6 6 4 0.080 E 0.138 E 0.058 -- -- --Metolachlor ESA -- 5 5 4 0 3.950 2.890 -- -- --2-Hydroxyatrazine 2163-68-0 6 4 4 0.032 E 0.017 E 0.021 570 0.0002 0.0003Alachlor ESA -- 5 4 3 0.05 0.680 0.470 -- -- --Metolachlor OA -- 5 4 3 0.05 3.770 2.990 -- -- --Alachlor OA -- 5 3 3 0.05 0.840 0.750 -- -- --Benomyl 17804-35-2 6 2 2 0.022 E 0.026 E 0.026 640 0.0007 0.0006Deethyldeisopropylatrazine 3397-62-4 6 2 0 0.04 E 0.029 -- -- -- --Acetochlor ESA -- 5 1 1 0.05 0.090 0.080 -- -- --Actochlor OA -- 5 1 1 0.05 0.140 0.120 -- -- --Alachlor ESA–second amide -- 5 1 2 0.05 0.070 0.040 -- -- --Bentazon 25057-89-0 6 1 1 0.024 E 0.748 E 0.370 5200 0.004 0.002Flumetsulam 98967-40-9 6 1 2 0.040 E 0.122 E 0.098 57,000 0.00002 0.00001Metolachlor 51218-45-2 1 1 1 0.006 E 0.004 E 0.005 700 0.000006 0.000007Nicosulfuron 111991-09-4 6 1 0 0.04 E 0.006 -- 59,000 0.000001 --Simazine 122-34-9 1 1 1 0.005 0.047 0.044 4 0.01 0.01Tebuthiuron 34014-18-1 6 1 1 0.026 E 0.020 E 0.018 51,000 0.00002 0.00002Terbacil 5902-51-2 6 1 0 0.026 E 0.023 -- 590 0.0003 --Triclopyr 55335-06-3 6 1 1 0.026 0.032 0.026 5400 0.00008 0.00007Acetochlor/Metolachlor ESA–second amide

-- 5 0 1 0.05 -- 0.120 -- -- --

Imazethapyr 81335-77-5 6 0 1 0.038 -- E 0.042 52,000 -- 0.00002Metsulfuron methyl 74223-64-6 6 0 3 0.067 -- E 0.105 52,000 -- 0.00005
Comparison of Source Water and Finished Groundwater 19

Table 2. Maximum concentration, detection frequency, and maximum benchmark quotient for regulated and unregulated compounds detected in source water from 10 community water-supply wells and the associated finished water in the Piedmont Physiographic Province, Potomac River Basin, Maryland and Virginia, 2004.—Continued[HBSL, Health-Based Screening Level; MCL, Maximum Contaminant Level; BQmax, Benchmark Quotient = ratio of maximum compound concentration to MCL or HBSL value; E, estimated value; μg/L, micrograms per liter; MRL, Minimum Reporting Level; --, not available; VOC, volatile organic compound; OAOC, other anthropogenic organic compounds; Bold type under compound heading indicates “unregulated,” a compound that has no Federal and (or) State drinking-water standard; ESA, ethanesulfonic acid; OA, oxanilic acid.]

Compound 

(Regulated or unregulated)

Chemical 

Abstracts 

Service 

Registry 

Number

Number 

of 

samples 

collected

Number of 

sourcewater 

samples 

with 

detections

Number of 

finishedwater 

samples 

with 

detections

MRL 

(μg/L)

Maximum 

concentration 

for source 

water 

(μg/L)

Maximum 

concentration 

for finished 

water 

(μg/L)

MCL or HBSL1

concentration

(μg/L)

BQMax2

for 

source 

water 

BQMax2

for 

finished 

water 

Picloram 0918-02-1 6 0 1 0.032 -- 0.151 500 -- 0.0003Total number of pesticide detections

53 50

Other Anthropogenic Organic Compounds

4-octylphenol, diethoxylate 26636-32-8 1 1 0 1 E 0.070 -- -- -- --4-octylphenol, monoethoxylate 26636-32-8 1 1 0 1 E 0.32 -- -- -- --Total number of OAOC detections

2 0

1 HBSL range corresponds to a 10-6 to 10-4 cancer risk range for unregulated carcinogens.2 BQmax for unregulated compounds was calculated using HBSL concentration at the low end of the HBSL range, and was rounded to one significant figure.3 1998 Final Rule for Disinfectants and Disinfection By-Products: The total for trihalomethanes is 80 micrograms per liter.4 MCL is for the sum of concentrations for mixed isomers of xylene.5 Value calculated using United States Environmental Protection Agency’s (USEPA) formula for Lifetime Health Advisory.6 HBSL calculated using USEPA’s Lifetime Health Advisory approach for Group C carcinogens.
20 Anthropogenic Organic Compounds in Groundwater, Piedmont Physiographic Province, Potomac River Basin, MD and VA, 2003–04

Figure 7. Volatile organic compounds detected in source water from 10 community water-supply wells and associated finished water 

(non-blended and blended): (A) concentrations and occurrence, and (B) benchmark quotient values, Maryland and Virginia, 2004.
Comparison of Source Water and Finished Groundwater 21

value of 0.1. The compound with the highest BQmax values 

in source and finished water was chloroform, with a BQmax 

value of 0.02. In many instances, a compound’s BQmax value 

was several (and, in the case of o-Xylene, four) orders of 

magnitude below the threshold (table 2, fig. 7b).

As a group, the THMs contributed about 30 percent 

and 44 percent of all detections in source and finished water, 

respectively. This group of compounds (chloroform, bromodichloromethane, bromoform, and dibromochloromethane — all 

of which were found in both source and finished water) can be 

formed when chlorine, used to disinfect drinking water, reacts 

with organic and inorganic matter in the source water. As 

previously discussed, these compounds are collectively known 

as DBPs and are regulated by the USEPA at a maximum 

allowable, annual average of 80 μg/L for the sum of the four 

compounds (U.S. Environmental Protection Agency, 2006). 

The differences in median concentrations of chloroform and 

bromodichloromethane between finished and source water, 

when these compounds were detected above their minimum 

reporting levels, were 0.15 μg/L and 0.25 μg/L, respectively 

(fig. 8). Collectively, there were 28 detections of DBPs in 

finished water compared to half that number in source water 

(table 2, Appendix A). Chloroform had some of the highest 

concentrations of DBPs found, and was detected in 9 of the 

source-water samples and all 10 of the finished-water samples 

(Appendix A). If the maximum concentrations of all four 

THMs are added together, however, the total concentration 

of 3.9 μg/L is still well below the 80 μg/L MCL set by the 

USEPA.

The next most commonly occurring compounds were 

1,4-dichlorobenzene and MTBE. 1,4-dichlorobenzene is 

one of three common dichlorobenzene isomers and does not 

occur naturally. As a class of chemicals, dichlorobenzenes 

are hydrophobic, and tend to volatize easily. 1,4-dichlorobenzene is a HPV industrial chemical and a white solid that will 

sublimate at room temperature and is commonly used in toilet 

deodorizer products, thus finding its way into the wastewater 

stream. It was detected in eight of the source- and finishedwater samples. The gasoline oxygenate MTBE was found in 

seven source-water and eight finished-water samples (table 2, 

Appendix A).

Overall, there were 16 fewer VOC detections in source 

water than in finished water (47 and 63, respectively). 

However, DBPs account for most of the additional detections 

in finished water, and the difference is reduced to two when 

the DBPs are excluded. Concentrations of detected non-DBP 

VOCs were very similar in source water and in finished water 

(table 2, fig. 9).

Pesticides 

Source- and finished-water samples for pesticides were 

collected from one to six CWS wells and analyzed for a suite 

of 141 common pesticides. Not all compounds were analyzed 

in all samples. Twenty-five pesticides were detected among the 

CWS samples in either the source or finished water, or both. 

Two of the six sites had blended finished water, indicating that 

those sites received water from groundwater sources other 

than the one analyzed (table 2, fig. 10a). 

The most commonly detected pesticides in both source 

and finished water were atrazine and its degradate, 

deethylatrazine. These two compounds were detected in six 

of six source-water samples and five of six finished-water 

samples. When detections from two other atrazine degradates, 

deisopropylatrazine and 2-hydroxyatrazine are included, 

atrazine and its degradates account for about 42 percent of the 

detections in source-water samples and 36 percent of all detections in finished-water samples. If detected in source water, 

these four compounds often were detected in the associated 

finished-water product, whether it was blended or non-blended 

(fig. 10a). 

The next most commonly detected pesticides were the 

OA and ESA degradates of alachlor and metolachlor. Five 

samples were collected and analyzed for each of the four 

compounds. One sample was analyzed for the parent compound metolachlor. Finished-water detection frequencies were 

80 percent for metolachlor ESA and 60 percent for alachlor 

ESA and both metolachlor OA and alachlor OA. When taken 

collectively, metolachlor and its degradates and the degradates 

of alachlor accounted for about 32 percent of the detections in 

source water and 28 percent of the detections in finished water. 

The two metolachlor degradates were detected at the highest 

concentrations of all pesticides detected in source water (metolachlor ESA, 3.950 μg/L, and metolachlor OA, 3.770 μg/L) 

Figure 8. Concentrations of bromodichloromethane and 

chloroform detected in source water from 10 community watersupply wells and associated finished water (non-blended and 

blended), Maryland and Virginia, 2004.
22 Anthropogenic Organic Compounds in Groundwater, Piedmont Physiographic Province, Potomac River Basin, MD and VA, 2003–04

— concentrations almost four orders of magnitude greater than 

that of the parent compound (metolachlor) in source water, 

0.004 (estimated) μg/L (table 2, fig. 10a). 

Detection frequencies for the remaining 16 compounds 

were two or fewer detections in the well(s) sampled for 

both source and finished water. Concentrations in source 

water ranged from 0.006 (estimated) μg/L for nicosulfuron 

to 0.748 (estimated) μg/L for bentazon — both herbicides. 

Finished-water concentrations ranged from 0.018 (estimated) 

μg/L for tebuthiuron (a herbicide) to 0.370 (estimated) μg/L 

for bentazon (table 2, fig. 10a). The herbicide metsulfuron 

methyl occurred only in finished water at three CWSs. One 

of these sites blended the sampled source water with other 

sources. Other compounds that were detected only in finished water were picloram (one detection), imazethapyr (one 

detection), and acetochlor/metolachlor ESA–second amide 

(one detection). The picloram and imazethapyr came from a 

single CWS that was not blending multiple sources for the finished product. Acetochlor/metolachlor ESA — second amide 

(a degradate common to both acetochlor and metolachlor) 

occurred in separate, non-blended CWSs (Appendix A).

Although no BQmax value equaled or exceeded 0.1, 

atrazine neared the threshold with a BQmax of 0.08 in source 

water and about 0.07 in finished water. All other BQmax 

values were between 0.000001 for nicosulfuron (in source 

water) and 0.01 for simazine (in source water and finished 

water) (fig. 10b).

The detection frequency and concentrations of pesticides 

in source water can indicate the potential for certain pesticides 

to be present in some finished waters, but not all (fig. 9). In 

addition, the introduction of multiple unquantifiable source 

terms (blended finished water) can decrease the concentration 

of a given compound in the finished product (dilution). 

However, blending with contaminated source waters may 

have actually increased the concentration of five pesticides 

(metolachlor ESA, atrazine, 2-hydroxyatrazine, metsulfuron 

methyl, and flumetsulam) at two different sites 

(Appendix A).

Figure 9. Comparison of concentrations of pesticides, volatile organic compounds, and 

other anthropogenic compounds in source water from 10 community water-supply wells and 

associated finished water (non-blended and blended), Maryland and Virginia, 2004
Comparison of Source Water and Finished Groundwater 23

Figure 10. Pesticides detected in source water from 10 community water-supply wells and associated finished water (non-blended 

and blended): (A) concentrations and occurrence, and (B) benchmark quotient values, Maryland and Virginia, 2004. 

[ESA, ethanesulfonic acid; OA, oxanilic acid]
24 Anthropogenic Organic Compounds in Groundwater, Piedmont Physiographic Province, Potomac River Basin, MD and VA, 2003–04

Other Anthropogenic Organic Compounds

Samples from a single CWS well and the associated 

blended finished water were analyzed for OAOCs. Only two 

OAOCs, 4-octylphenol monoethoxylate and 4-octylphenol 

diethoxylate, were detected in the source water, and none were 

detected in finished water (table 2, fig. 9). Both are degradates 

of common detergents. The highest detected concentration 

of 4-octylphenol monoethoxylate was 0.32 (estimated) μg/L. 

The absence of the compound in the finished product may 

be attributed to dilution as the analyzed source water was 

blended with water from one or more other wells. Conversely, 

chlorination (as part of the disinfection process) may alter 

some compounds so that they may be present, but are in a 

chlorinated form that is not quantifiable (Appendix A). Due 

to the lack of health guidelines (MCLs or HBSLs) for these 

OAOCs, no BQmax values were obtained (table 2).

Summary

A source-water-quality assessment was performed in the 

Piedmont Physiographic Province of the Potomac River Basin 

in 2003 and 2004. The assessment included two sampling 

phases. The first phase focused on determining the occurrence 

of 284 anthropogenic organic compounds in the source water 

of some of the highest-producing community water systems in 

the study area. The anthropogenic organic compounds monitored included volatile organic compounds, pesticides and 

pesticide degradates, and other anthropogenic organic compounds. The second phase of the program focused on those 

anthropogenic organic compounds that were detected most 

frequently during the first phase of sampling, and characterized their occurrence in source water as well as associated 

finished water prior to distribution.

During phase one, 92 of the 284 anthropogenic organic 

compounds were detected (41 volatile organic compounds, 

37 pesticides and pesticide degradates, and 14 other 

anthropogenic organic compounds) in source-water samples 

collected in August and September 2003. The five most 

frequently occurring anthropogenic organic compounds were 

detected in more than 70 percent of the 15 source-water 

samples. Deethylatrazine (an atrazine degradate) was present 

in all 15 samples. Metolachlor ethanesulfonic acid, a degradate of the herbicide metolachlor, and chloroform, a volatile 

organic compound and known disinfection by-product, were 

present in 87 percent of the samples. Atrazine and metolachlor 

were present in 80 percent and 73 percent of the samples, 

respectively. All samples contained a mixture of compounds 

with an average of about 14 compounds per sample. 

Concentrations for compounds detected in source water 

were generally low, ranging from 0.005 (estimated) 

micrograms per liter for the fungicide propiconazole to 13.51 

micrograms per liter for chloroform. Two industrial solvents, 

methyl ethyl ketone and tetrahydrofuran, were detected at 

concentrations that exceeded the next highest concentration by 

an order of magnitude. Chloroform and methyl tert-butyl ether 

were the most frequently detected volatile organic compounds 

(87 and 67 percent, respectively). They were most often found 

at sites that contained at least four other volatile organic compounds, which is consistent with other studies that indicate 

that these are the two most frequently detected volatile organic 

compounds nationwide. 

Two measurements for human-health assessment — the 

Maximum Contaminant Levels set by the U.S. Environmental 

Protection Agency and Health-Based Screening Levels 

developed by the U.S. Geological Survey — were used to 

place water-quality data in a human-health context. Slightly 

more than half (56 percent) of the detected compounds had 

either a Maximum Contaminant Level or a Health-Based 

Screening Level that was used to create a ratio of the maximum detected concentration of a compound to one of these 

two human-health standards; this unitless ratio is called 

the benchmark quotient or BQmax. One tenth of this ratio 

(BQmax greater than or equal to 0.1), was used as a threshold 

value, allowing investigators to determine which compounds 

may warrant further investigation. Samples collected in 

phase one yielded concentrations for some compounds 

that had BQmax values between one and five orders of 

magnitude below a BQmax of 0.1. Seven compounds (chloroform, benzene, acrylonitrile, methylene chloride, atrazine, 

alachlor, and dieldrin) met or exceeded a BQmax of 0.1, 

however. Benchmark quotient values were not calculated for 

compounds that had neither Maximum Contaminant Levels 

nor Health-Based Screening Levels.

Pesticides were detected more frequently but at lower 

median concentrations than volatile organic compounds. 

Nearly half of the detections (51 of 113) resulted from the 

presence of the herbicides atrazine and metolachlor and 

two degradates of these compounds (deethylatrazine and 

metolachlor ethanesulfonic acid). Maximum concentrations 

of pesticides detected in source water ranged from 0.005 

(estimated) micrograms per liter for propiconazole to 3.58 

micrograms per liter for metolachlor. Benchmark quotient 

values were available for 22 pesticides and were generally 

one to several orders of magnitude below the 0.1 threshold 

that would warrant further monitoring. Three compounds 

— atrazine, alachlor, and dieldrin — were at or significantly 

above the threshold in one or more samples. Dieldrin had a 

BQmax of 12.1 — the maximum concentration of dieldrin was 

12 times higher than the health standard for that compound in 

one sample. 

In August and October 2004, 10 of the 15 community 

water supply wells, along with their associated finished 

water, were selectively resampled for the anthropogenic 

organic compounds that occurred most frequently during 

phase one. Three of the 10 sites blended source water with 

other sources of groundwater or surface water. Seven sites 

were non-blended. Ten sample pairs (source and finished 

water) were analyzed for volatile organic compounds, six 

pairs for pesticides, and one pair for other anthropogenic 
References Cited 25

organic compounds. In total, 21 volatile organic compounds, 

25 pesticides and pesticide degradates, and two other 

anthropogenic organic compounds were detected. Overall, 

there were 113 anthropogenic organic compounds detected in 

finished water and 102 detected in source water.

The most common group of volatile organic compounds 

that were detected was the trihalomethanes, a group of 

disinfection by-products. Trihalomethanes accounted for about 

30 percent of the detections in source water and 44 percent 

of the detections in finished water. Other commonly detected 

volatile organic compounds included 1,4-dichlorobenzene, 

detected in 8 of 10 source-water and finished-water samples, 

and the gasoline oxygenate methyl tert-butyl ether, found in 

7 source-water samples and 8 finished-water samples. This 

indicates that blended finished water may introduce additional 

contaminant sources or the creation of compounds through the 

disinfection process. No volatile organic compound exceeded 

the 0.1 BQmax threshold.

Twenty-five pesticides were detected in either source 

water, finished water, or both among the community water 

systems sampled in phase two. In total, there were slightly 

more pesticide detections in source water (53) than in 

finished water (50). The most commonly detected pesticide 

compounds in both source water and finished water were 

atrazine and its degradate, deethylatrazine. With the inclusion 

of deisopropylatrazine and 2-hydroxyatrazine, atrazine and its 

degradates accounted for about 42 percent of the detections 

in source-water samples and 36 percent of all detections in 

finished-water samples. The next most commonly detected 

pesticide compounds were the parent compounds and 

degradates of alachlor and metolachlor. These compounds 

accounted for about 32 percent of detections in source 

water and 28 percent of the detections in finished water. 

Three degradates (metolachlor ethanesulfonic acid, alachlor 

ethanesulfonic acid, and metolachlor oxanilic acid) were found 

more often in source-water than finished-water samples. Two 

of the metolachlor degradates were detected at the highest 

concentrations of all pesticides detected in source water — 

metolachlor ethanesulfonic acid at 3.950 micrograms per 

liter, and metolachlor oxanilic acid at 3.770 micrograms per 

liter. Metolachlor ethanesulfonic acid and oxanilic acid were 

detected at frequencies of 100 and 80 percent, respectively, in 

source water, and 80 and 60 percent in finished water. 

Acknowledgments

The authors are grateful to employees of the water 

utilities for their participation in this study. Without their 

cooperation, this study would not have been possible. The 

authors also acknowledge Linda Debrewer, USGS, Baltimore, 

MD for her extensive help with data retrievals, and Matthew 

Ferrari, USGS, Sacramento, CA for his contribution to the 

study design. 

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Appendix A 29

Appendix A
30 Anthropogenic Organic Compounds in Groundwater, Piedmont Physiographic Province, Potomac River Basin, MD and VA, 2003–04

Appendix A. Comparison between source water and associated finished water (non-blended and blended) for detected 

anthropogenic organic compounds in community water-supply systems in the Piedmont Physiographic Province, Potomac River 

Basin, Maryland and Virginia, 2004.—Continued

[E, estimated value; --; no detection; μg/L, micrograms per liter; red type indicates blended water; Bold type under compound heading indicates 

“unregulated,” a compound that has no Federal and (or) State drinking-water standard; ESA, ethanesulfonic acid; OA, oxanilic acid.]

Well identifier Compound 

(Regulated or unregulated)

Source-water 

concentration

(μg/L)

Finished-water concentration 

(non-blended and blended) 

(μg/L)

Volatile Organic Compounds

49X 10 1,1,1-Trichloroethane E 0.027 E 0.029

CL Bd 179 1,1,1-Trichloroethane 0.105 0.106

FR Dc 62 1,1,1-Trichloroethane E 0.032 E 0.030

49X 10 1,1-Dichloroethane 0.240 0.238

49X 10 1,1-Dichloroethene 0.112 E 0.096

CL Bd 179 1,1-Dichloroethene E 0.044 E 0.044

49X 10 1,4-Dichlorobenzene E .040 --

49X 11 1,4-Dichlorobenzene E 0.038 E 0.056

CL Bd 179 1,4-Dichlorobenzene E 0.025 E 0.053

CL Bd 180 1,4-Dichlorobenzene E 0.044 E 0.024

FR Dc 62 1,4-Dichlorobenzene E 0.048 E 0.039

FR Ec 43 1,4-Dichlorobenzene -- E 0.047

FR Ef 54 1,4-Dichlorobenzene E 0.026 E 0.029

FR Eg 36 1,4-Dichlorobenzene E 0.039 E 0.041

FR Eg 37 1,4-Dichlorobenzene E 0.042 E 0.033

49X 10 Bromodichloromethane -- 0.725

49X 11 Bromodichloromethane -- E 0.062

CL Bd 179 Bromodichloromethane 0.109 0.167

FR Dc 69 Bromodichloromethane -- 0.454

FR Ef 54 Bromodichloromethane -- 0.569

FR Eg 36 Bromodichloromethane 0.204 0.187

FR Eg 37 Bromodichloromethane E 0.059 0.358

49X 10 Bromoform -- 0.520

FR Dc 69 Bromoform -- E 0.150

FR Ef 54 Bromoform -- 0.121

FR Eg 36 Bromoform 0.396 0.417

FR Eg 37 Bromoform -- E 0.085

49X 10 Carbon tetrachloride -- E 0.025

49X 10 Chloroform E 0.028 0.236

49X 11 Chloroform -- 0.141

CL Bd 179 Chloroform 1.569 1.574

CL Bd 180 Chloroform E 0.090 0.107

FR Dc 62 Chloroform 1.058 1.085

FR Dc 69 Chloroform E 0.018 0.245

FR Ec 43 Chloroform E 0.027 E 0.048

FR Ef 54 Chloroform E 0.043 0.573

FR Eg 36 Chloroform 0.144 E 0.079

FR Eg 37 Chloroform 0.415 0.871

49X 11 cis-1,2-Dichloroethene 0.506 2.188
Appendix A 31

Appendix A. Comparison between source water and associated finished water (non-blended and blended) for detected 

anthropogenic organic compounds in community water-supply systems in the Piedmont Physiographic Province, Potomac River 

Basin, Maryland and Virginia, 2004.—Continued

[E, estimated value; --; no detection; μg/L, micrograms per liter; red type indicates blended water; Bold type under compound heading indicates 

“unregulated,” a compound that has no Federal and (or) State drinking-water standard; ESA, ethanesulfonic acid; OA, oxanilic acid.]

Well identifier Compound 

(Regulated or unregulated)

Source-water 

concentration

(μg/L)

Finished-water concentration 

(non-blended and blended) 

(μg/L)

49X 10 Dibromochloromomethane -- 1.109

CL Bd 179 Dibromochloromomethane -- 0.183

FR Dc 69 Dibromochloromomethane -- 0.511

FR Ef 54 Dibromochloromomethane -- 0.431

FR Eg 36 Dibromochloromomethane 0.433 0.397

FR Eg 37 Dibromochloromomethane -- 0.206

49X 10 Deisopropyl ether 0.165 0.148

CL Bd 180 Deisopropyl ether E 0.044 E 0.051

FR Ef 54 Ethylbenzene -- E 0.023

FR Ef 54 Isopropylbenzene E 0.007 --

FR Ef 54 m- and p-Xylene -- E 0.085

49X 10 Methyl tert-butyl ether 0.583 0.540

49X 11 Methyl tert-butyl ether 0.752 1.076

CL Bd 179 Methyl tert-butyl ether 0.329 0.292

CL Bd 180 Methyl tert-butyl ether 2.671 2.565

FR Dc 62 Methyl tert-butyl ether -- E 0.084

FR Dc 69 Methyl tert-butyl ether E 0.058 E 0.058

FR Eg 36 Methyl tert-butyl ether E 0.161 E 0.152

FR Eg 37 Methyl tert-butyl ether 0.193 0.193

CL Bd 179 Methylene chloride -- E 0.019

FR Ef 54 o-Xylene -- E 0.027

49X 10 tert-Amyl methyl ether E 0.087 E 0.081

CL Bd 180 tert-Amyl methyl ether 0.231 0.212

49X 10 Tetrachloroethene E 0.020 E 0.020

CL Bd 179 Tetrachloroethene E 0.015 E 0.013

FR Dc 62 Tetrachloroethene E 0.037 --

FR Dc 69 Tetrahydrofuran E 1.096 --

49X 10 Trichloroethene E 0.041 E 0.035

49X 11 Trichloroethene E 0.094 --

Pesticides

49X 10 2-Hydroxyatrazine E 0.017 E 0.021

FR Dc 62 2-Hydroxyatrazine E 0.005 E 0.005

FR Dc 69 2-Hydroxyatrazine E 0.015 E 0.014

FR Ef 54 2-Hydroxyatrazine E 0.016 E 0.020

49X 10 Acetochlor ESA 0.090 0.080

49X 10 Acetochlor/Metolachlor ESA–second amide -- 0.120

49X 10 Actochlor OA 0.140 0.120

FR Ef 54 Alachlor ESA–second Amide 0.070 0.040

FR Eg 36 Alachlor ESA–second Amide -- 0.020

49X 10 Alachlor OA 0.120 0.100
32 Anthropogenic Organic Compounds in Groundwater, Piedmont Physiographic Province, Potomac River Basin, MD and VA, 2003–04

Appendix A. Comparison between source water and associated finished water (non-blended and blended) for detected 

anthropogenic organic compounds in community water-supply systems in the Piedmont Physiographic Province, Potomac River 

Basin, Maryland and Virginia, 2004.—Continued

[E, estimated value; --; no detection; μg/L, micrograms per liter; red type indicates blended water; Bold type under compound heading indicates 

“unregulated,” a compound that has no Federal and (or) State drinking-water standard; ESA, ethanesulfonic acid; OA, oxanilic acid.]

Well identifier Compound 

(Regulated or unregulated)

Source-water 

concentration

(μg/L)

Finished-water concentration 

(non-blended and blended) 

(μg/L)

FR Ef 54 Alachlor OA 0.020 0.020

FR Eg 36 Alachlor OA 0.840 0.750

49X 10 Alachlor ESA 0.060 0.050

49X 11 Alachlor ESA 0.030 --

FR Ef 54 Alachlor ESA 0.680 0.470

FR Eg 36 Alachlor ESA 0.330 0.290

49X 10 Atrazine 0.230 0.203

49X 11 Atrazine 0.013 --

FR Dc 62 Atrazine 0.053 0.050

FR Dc 69 Atrazine 0.074 0.060

FR Ef 54 Atrazine 0.106 0.108

FR Eg 36 Atrazine 0.066 0.060

49X 10 Benomyl E 0.026 E 0.026

FR Eg 36 Benomyl E 0.005 E 0.008

49X 10 Bentazon E 0.748 E 0.370

49X 10 Deethylatrazine 0.048 0.040

49X 11 Deethylatrazine 0.049 --

FR Dc 62 Deethylatrazine 0.129 0.111

FR Dc 69 Deethylatrazine E 0.129 E 0.087

FR Ef 54 Deethylatrazine E 0.146 E 0.073

FR Eg 36 Deethylatrazine E 0.024 0.078

FR Dc 62 Deethyldeisopropylatrazine E 0.027 --

FR Dc 69 Deethyldeisopropylatrazine E 0.029 --

49X 10 Deisopropylatrazine E 0.053 E .042

49X 11 Deisopropylatrazine E 0.010 --

FR Dc 62 Deisopropylatrazine E 0.022 E 0.016

FR Dc 69 Deisopropylatrazine E 0.009 --

FR Ef 54 Deisopropylatrazine E 0.138 E 0.058

FR Eg 36 Deisopropylatrazine E 0.016 E 0.030

49X 10 Flumetsulam E 0.122 E 0.098

FR Ef 54 Flumetsulam -- E 0.046

FR Eg 36 Imazethapyr -- E 0.042

FR Ef 54 Metolachlor E 0.004 E 0.005

49X 10 Metolachlor ESA 2.570 2.150

49X 11 Metolachlor ESA 0.960 --

FR Dc 62 Metolachlor ESA 0.230 0.250

FR Ef 54 Metolachlor ESA 3.950 2.890

FR Eg 36 Metolachlor ESA 1.040 0.860

49X 10 Metolachlor OA 3.770 2.990

49X 11 Metolachlor OA 0.020 --
Appendix A 33

Appendix A. Comparison between source water and associated finished water (non-blended and blended) for detected 

anthropogenic organic compounds in community water-supply systems in the Piedmont Physiographic Province, Potomac River 

Basin, Maryland and Virginia, 2004.—Continued

[E, estimated value; --; no detection; μg/L, micrograms per liter; red type indicates blended water; Bold type under compound heading indicates 

“unregulated,” a compound that has no Federal and (or) State drinking-water standard; ESA, ethanesulfonic acid; OA, oxanilic acid.]

Well identifier Compound 

(Regulated or unregulated)

Source-water 

concentration

(μg/L)

Finished-water concentration 

(non-blended and blended) 

(μg/L)

FR Ef 54 Metolachlor OA 0.460 0.450

FR Eg 36 Metolachlor OA 0.720 0.630

49X 10 Metsulfuron methyl -- E 0.053

FR Ef 54 Metsulfuron methyl -- E 0.105

FR Eg 36 Metsulfuron methyl -- E 0.031

FR Dc 69 Nicosulfuron E 0.006 --

FR Eg 36 Picloram -- 0.151

FR Ef 54 Simazine 0.047 0.044

49X 10 Tebuthiuron E 0.020 E 0.018

49X 10 Terbacil E 0.023 --

FR Eg 36 Triclopyr 0.032 0.026

Other Anthropogenic Organic Compounds

FR Ef 54 4-octylphenol diethoxylate E 0.070 --

FR Ef 54 4-octylphenol monoethoxylate E 0.320 --

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Edited by Valerie M. Gaine.

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For additional information, contact:

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