Development of Health-Based Screening

Levels for Use in State- or Local-Scale

Water-Quality Assessments

Water-Resources Investigations Report 03-4054

U.S. Department of the Interior

U.S. Geological Survey

NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

Prepared in cooperation with the Oregon Health & Science University
U.S. Department of the Interior

U.S. Geological Survey

Development of Health-Based Screening

Levels for Use in State- or Local-Scale

Water-Quality Assessments

By Patricia Toccalino, Lisa Nowell, William Wilber, John Zogorski, Joyce Donohue,

Catherine Eiden, Sandra Krietzman, and Gloria Post

Water-Resources Investigations Report 03-4054

Prepared in cooperation with the Oregon Health & Science University
U.S. Department of the Interior

GALE A. NORTON, Secretary

U.S. Geological Survey

Charles G. Groat, Director

The use of firm, trade, and brand names in this report is for identification purposes only and does not constitute endorsement by the U.S. Government.

Rapid City, South Dakota: 2003

For additional information write to:

District Chief

U.S. Geological Survey

1608 Mt. View Road

Rapid City, SD 57702

Copies of this report can be purchased from:

U.S. Geological Survey

Information Services

Building 810

Box 25286, Federal Center

Denver, CO 80225-0286
Foreword III

FOREWORD

The U.S. Geological Survey (USGS) is 

committed to providing the Nation with accurate and 

timely scientific information that helps 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 quality of the Nation’s water resources is 

critical to assuring the long-term availability of water 

that is safe for drinking and recreation and suitable for 

industry, irrigation, and habitat for fish and wildlife. 

Population growth and increasing demands for 

multiple water uses make water availability, now 

measured in terms of quantity and quality, even more 

essential to the long-term sustainability of our 

communities and ecosystems.

The USGS implemented the National WaterQuality Assessment (NAWQA) Program in 1991 to 

support national, regional, and local information needs 

and decisions related to water-quality management 

and policy (http://water.usgs.gov/nawqa). Shaped by 

and coordinated with ongoing efforts of other Federal, 

State, and local agencies, the NAWQA Program is 

designed to answer: What is the condition of our 

Nation’s streams and ground water? How are the 

conditions changing over time? How do natural 

features and human activities affect the quality of 

streams and ground water, and where are those effects 

most pronounced? By combining information on 

water chemistry, physical characteristics, stream 

habitat, and aquatic life, the NAWQA Program aims to 

provide science-based insights for current and 

emerging water issues and priorities. 

From 1991-2001, the NAWQA Program 

completed interdisciplinary assessments in 51 of the 

Nation’s major river basins and aquifer systems, 

referred to as Study Units (http://water.usgs.gov/ 

nawqa/studyu.html). Baseline conditions were 

established for comparison to future assessments, and 

long-term monitoring was initiated in many of the 

basins. During the next decade, 42 of the 51 Study 

Units will be reassessed so that 10 years of 

comparable monitoring data will be available to 

determine trends at many of the Nation’s streams and 

aquifers. The next 10 years of study also will fill in 

critical gaps in characterizing water-quality 

conditions, enhance understanding of factors that 

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.

The USGS aims to disseminate credible, timely, 

and relevant science information to inform 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 a fully 

integrated understanding of watersheds and for costeffective management, regulation, and conservation of 

our Nation’s water resources. The NAWQA Program, 

therefore, depends on advice and information from 

other agencies—Federal, State, interstate, Tribal, and 

local—as well as nongovernmental organizations, 

industry, academia, and other stakeholder groups. 

Your assistance and suggestions are greatly 

appreciated.

Robert M. Hirsch

Associate Director for Water

Contents V

CONTENTS

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

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

Acknowledgments ................................................................................................................................................................. 3

History of Pilot Effort and Reaching Consensus................................................................................................................... 3

USEPA Human Health-Based Benchmarks and Those Used by USGS ............................................................................... 4

Health-Based Screening Level (HBSL) Approach................................................................................................................ 6

Acceptable Sources of Cancer Classifications and Toxicity Values for HBSL Development .................................... 6

Developing HBSL Concentrations for Unregulated Noncarcinogens......................................................................... 7

Methodology for Unregulated Noncarcinogens ................................................................................................ 7

Examples: Unregulated Noncarcinogens.......................................................................................................... 9

Developing HBSL Concentrations or Concentration Ranges for Unregulated Carcinogens ..................................... 9

Methodology for “Known,” “Likely,” Group A, and Group B Carcinogens..................................................... 10

Methodology for “Suggestive Evidence” and Group C (Possible) Carcinogens .............................................. 11

Examples: Unregulated Carcinogens ............................................................................................................... 11

Benefits of Using HBSL Benchmarks......................................................................................................................... 12

Use of HBSL Values as Tools in Water-Quality Assessments .............................................................................................. 14

Summary................................................................................................................................................................................ 20

References ............................................................................................................................................................................. 20

FIGURES

1. Flowchart showing development of HBSL concentrations for unregulated noncarcinogens

at the State or local scale............................................................................................................................................. 8

2. Flowchart showing development of HBSL ranges for unregulated “known,” “likely,” Group A,

and Group B carcinogens at the State or local scale................................................................................................... 10

3. Flowchart showing development of HBSL concentrations or ranges for unregulated Group C and

“suggestive evidence” carcinogens at the State or local scale .................................................................................... 12

4. Graph showing number of NAWQA analytes, including VOCs, pesticides, pesticide degradation

products, nutrients, and trace elements and major ions, with health-based benchmarks

before and after HBSL calculations............................................................................................................................ 14

5. Flowchart for hypothetical ground-water example showing the number of detected compounds that

are regulated and unregulated, and the availability of HBSL values for unregulated compounds ............................. 15

TABLES

1. Descriptions of published USEPA human-health benchmark concentrations applicable to drinking water.............. 5

2. Examples of development of HBSL concentrations for unregulated noncarcinogens ............................................... 9

3. Examples of development of HBSL concentrations and ranges for unregulated carcinogens ................................... 13

4. Regulated versus unregulated compounds in hypothetical ground-water example.................................................... 16

5. Comparison of measured concentrations with USEPA and State drinking-water standards for

regulated compounds in hypothetical ground-water example .................................................................................... 16

6. Comparison of measured concentrations with State guideline values and HBSL concentrations and

ranges for unregulated compounds in hypothetical ground-water example ............................................................... 17

7. Examples of factors to consider when interpreting results of water-quality analyses................................................ 18

8. What are HBSL values, and what does it mean if HBSL values are exceeded? ........................................................ 19
VI Contents

ABBREVIATIONS AND ACRONYMS

μg/L ...................................micrograms per liter

mg/L ..................................milligrams per liter

L ........................................liter

kg body wt.........................kilograms of body weight

mg/kg/day..........................milligrams of contaminant per kilogram of body weight per day

(mg/kg/day)-1 ....................inverse of milligrams of contaminant per kilogram of body weight per day

ACT...................................Action Level

ASDWA.............................Association of State Drinking Water Administrators

AWQC...............................Ambient Water Quality Criteria

DWEL ...............................Drinking Water Equivalent Level

DWLOC ............................Drinking Water Level of Comparison

GW ....................................Ground Water

GWQC ..............................Ground-Water Quality Criteria

HBSL ................................Health-Based Screening Level

HEAST..............................Health Effects Assessment Summary Tables

IRIS ...................................Integrated Risk Information System

Lifetime HA ......................Lifetime Health Advisory

LINJ ..................................Long Island – New Jersey

MCL ..................................Maximum Contaminant Level

MCLG ...............................Maximum Contaminant Level Goal

MI......................................Major Ion

MOE..................................Margin of Exposure

NA .....................................Not Available

NAWQA............................U.S. Geological Survey National Water-Quality Assessment Program

NJDEP...............................New Jersey Department of Environmental Protection

NWQMC...........................National Water Quality Monitoring Council

OAR ..................................Office of Air and Radiation (USEPA)

OGWDW...........................Office of Ground Water and Drinking Water (USEPA)

OHSU................................Oregon Health & Science University

OPP ...................................Office of Pesticide Programs (USEPA)

ORD .................................Office of Research and Development (USEPA)

OSWER.............................Office of Solid Waste and Emergency Response (USEPA)

OW ...................................Office of Water (USEPA)

PRG...................................Preliminary Remediation Goal

q1* .....................................see SF

RBC...................................Risk Based Concentration

RfD....................................Reference Dose

RSC ...................................Relative Source Contribution

RSD...................................Risk-Specific Dose

SDWR ...............................Secondary Drinking Water Regulation

SF ......................................Slope Factor

TE......................................Trace Element

USEPA ..............................U.S. Environmental Protection Agency

USGS ................................U.S. Geological Survey

VOC ..................................Volatile Organic Compound
Contents VII

DEFINITIONS

Term Definition

Health-Based Screening Level 

(HBSL)

Estimates of benchmark concentrations (for noncarcinogens) or 

concentration ranges (for carcinogens) in water that (1) may be of potential 

human-health concern; (2) can be used as threshold values against which 

measured concentrations of contaminants in ambient water samples can be 

compared; and (3) are consistent with USEPA Office of Water 

methodologies (U. S. Environmental Protection Agency, 1988; U. S. 

Environmental Protection Agency, 1993b) for setting drinking-water 

Lifetime Health Advisory and Risk-Specific Dose values.

Lifetime Health Advisory (Lifetime 

HA)

The concentration of a chemical in drinking water that is not expected to 

cause any adverse noncarcinogenic effects over a lifetime of exposure (70 

years). The Lifetime HA is based on exposure of a 70-kg adult consuming 2 

L of water per day, and assumes that only a portion (generally 20 percent) of 

the total exposure to a contaminant is from drinking water (U. S. 

Environmental Protection Agency, 1993b; U. S. Environmental Protection 

Agency, 2002a).

Regulated Compounds As used in this report, compounds for which Federal and (or) State drinkingwater standards have been established. 

Reference Dose (RfD) An estimate (with an uncertainty of perhaps one order of magnitude) of the 

daily exposure that is likely to be without appreciable risk of deleterious 

health effects in the human population (including sensitive subgroups) over 

an individual's lifetime (70 years) (U. S. Environmental Protection Agency, 

1989b). 

Risk-Specific Dose (RSD) The drinking-water concentration associated with a specified cancer risk 

level (typically 1 in 10,000, 1 in 100,000, or 1 in 1,000,000), under certain 

exposure conditions: consumption of 2 L per day of drinking water by a 

70-kg body weight individual over a lifetime (70 years) (U. S. 

Environmental Protection Agency, 1993b).

Slope Factor (SF) The cancer potency estimate for a compound as derived from the slope of 

the dose-response (carcinogenicity) data extrapolated to zero using an 

appropriate mathematical model. If the model selected for extrapolation 

from dose-response data is the linearized multistage model, the SF value is 

also known as the q1* value (U. S. Environmental Protection Agency, 

1989a; U. S. Environmental Protection Agency, 1993b). 

Unregulated Compounds As used in this report, compounds for which no Federal and (or) State 

drinking-water standards have been established. Note that a compound that 

is unregulated in drinking water (federally, under the Safe Drinking Water 

Act) may be regulated in other contexts and under other statutes.

Abstract 1

Development of Health-Based Screening

Levels for Use in State- or Local-Scale

Water-Quality Assessments

By Patricia Toccalino1, Lisa Nowell2, William Wilber3, John Zogorski4, Joyce Donohue5, Catherine Eiden6, 

Sandra Krietzman7

, and Gloria Post7

ABSTRACT

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

need to communicate the significance of the waterquality findings of its National Water-Quality 

Assessment (NAWQA) Program in a human-health 

context. Historically, the USGS has assessed 

water-quality conditions by comparing water concentration data against established drinking-water 

standards and guidelines. However, because drinking-water standards and guidelines do not exist for 

many of the contaminants analyzed by the 

NAWQA Program and other USGS studies, this 

approach has proven to be insufficient for placing 

USGS data in a human-health context. To help 

meet this need, health-based screening level 

(HBSL) concentrations or ranges are being determined for unregulated compounds (that is, those 

for which Federal or State drinking-water standards have not been established), using a consensus 

approach that was developed collaboratively by the 

USGS, U.S. Environmental Protection Agency

(USEPA), New Jersey Department of Environmental Protection, and Oregon Health & Science 

University. USEPA Office of Water methodologies 

for calculating Lifetime Health Advisory and 

Risk-Specific Dose values for drinking water are 

being used to develop HBSL concentrations (for 

unregulated noncarcinogens) and HBSL concentration ranges (for most unregulated carcinogens). 

This report describes the methodologies used to 

develop HBSL concentrations and ranges for 

unregulated compounds in State- and local-scale 

analyses, and discusses how HBSL values can be 

used as tools in water-quality assessments. Comparisons of measured water concentrations with 

Maximum Contaminant Level values and HBSL 

values require that water-quality data be placed in 

the proper context, with regard to both hydrology 

and human health. The use of these HBSL concentrations and ranges by USGS will increase by 

27 percent the number of NAWQA contaminants 

for which health-based benchmarks are available 

for comparison with USGS water-quality data.

__________

1

Oregon Health & Science University, Department of Environmental and Biomolecular Systems, Beaverton, Oregon.

2

U.S. Geological Survey, Pesticides National Synthesis Project, Sacramento, California.

3

U.S. Geological Survey, NAWQA National Synthesis Coordinator, Reston, Virginia.

4

U.S. Geological Survey, VOC National Synthesis Project, Rapid City, South Dakota.

5

U.S. Environmental Protection Agency, Office of Water, Washington, D.C. (The opinions expressed in this article represent those of 

the author and are not necessarily those of the USEPA.)

6U.S. Environmental Protection Agency, Office of Pesticide Programs, Washington, D.C.

7New Jersey Department of Environmental Protection, Trenton, New Jersey.
2 Development of Health-Based Screening Levels: State and Local Water-Quality Assessments

USGS can use HBSL values to assist the USEPA 

and State and local agencies by providing them 

with comparisons of measured water concentrations to scientifically defensible human healthbased benchmarks, and by alerting them when 

measured concentrations approach or exceed these 

benchmarks.

INTRODUCTION

The National Water-Quality Assessment 

(NAWQA) Program of the U.S. Geological Survey 

(USGS) is well known for its work in describing the 

status and trends in the quality of the Nation’s surfaceand ground-water resources, and for providing a sound, 

scientific understanding of the primary natural and 

human factors affecting the quality of these resources. 

NAWQA has increasingly been asked about the publichealth implications of its findings. The NAWQA 

Program was originally designed to characterize the 

Nation’s water resources and was not designed to 

answer questions about the potential safety of surface 

water and ground water for drinking-water use. Nonetheless, if interpreted carefully, NAWQA data can 

provide valuable information to State agencies, the 

U.S. Environmental Protection Agency (USEPA), and 

others interested in drinking-water quality.

To date, NAWQA has assessed water-quality 

conditions using two approaches: (1) ranking of waterquality conditions at individual surface-water sites and 

among networks of ground-water wells within a study 

unit in relation to surface-water sites and ground-water 

wells in other study units; and (2) comparing waterconcentration data against established drinking-water 

standards and guidelines for the protection of human 

health (Gilliom and others, 1998). However, the former 

approach provides no information on whether the levels 

detected are of potential concern to human health, and 

the latter approach is limited by the fact that drinkingwater standards and guidelines have not been established for many of the contaminants analyzed by 

NAWQA. 

In June 1999, the USGS and Oregon Health & 

Science University (OHSU), in collaboration with the 

USEPA, began a pilot effort to develop, test, and refine 

concepts to more effectively communicate waterquality information in a human-health context. The 

USGS-NAWQA study area in Glassboro, New Jersey, 

was selected for a pilot effort for State-level analyses 

and reports. The Glassboro, New Jersey, study area 

overlaps with NAWQA’s Long Island–New Jersey 

(LINJ) Coastal Drainages Study Unit, and the pilot 

effort focuses on ground-water data collected in the 

LINJ Study Unit. Representatives from the USGS 

LINJ Study Unit and the New Jersey Department of 

Environmental Protection (NJDEP), which has the 

responsibility for regulating drinking water in the State 

of New Jersey, then joined the interagency pilot effort. 

After a series of workshops and written 

exchanges, the USGS, USEPA, NJDEP, and OHSU 

representatives participating in the pilot project agreed 

to develop health-based screening level (HBSL) concentrations and ranges for those compounds without 

existing drinking-water standards. HBSL values are 

defined as estimates of benchmark concentrations (for 

noncarcinogens) or concentration ranges (for carcinogens) in water that (1) may be of potential humanhealth concern; (2) can be used as threshold values 

against which measured concentrations of contaminants in ambient water samples can be compared; and 

(3) are consistent with USEPA Office of Water methodologies (U.S. Environmental Protection Agency, 1988, 

1993b) for setting drinking-water Lifetime Health 

Advisory (HA) and Risk-Specific Dose (RSD) values.

This report describes the methodologies used to 

develop HBSL concentrations and ranges for unregulated compounds for use in State or local-scale waterquality assessments. Specifically, this report: 

(1) documents the history of this pilot effort and how 

consensus was reached on the HBSL approach; 

(2) defines the systematic procedures followed in the 

development of HBSL concentrations and ranges for 

contaminants analyzed by NAWQA; (3) describes the 

benefits of using the HBSL approach; and (4) discusses 

how HBSL values can be used as tools in water-quality 

assessments.

The scope of this pilot effort applies only to data 

analyzed and reported at the State or local (or studyunit) scales. Expansion of this effort to a national or 

regional (multi-State) scale will be more complex and 

is the subject of a second, national-scale pilot project. 

The additional complexity at the national (or multiState) scale arises because the approach described in 

this report builds upon a framework of USEPA and 

State drinking-water standards. However, different 

States may not use the same drinking-water standard 

values, or even regulate the same contaminants in 

drinking water. The systematic development of HBSL 

concentrations and ranges will enable the USGS to 
History of Pilot Effort and Reaching Consensus 3

present water-quality data in a human-health context 

for a larger suite of contaminants than have drinkingwater standards and guidelines, thus providing an 

increased basis for interpreting the significance of the 

data.

ACKNOWLEDGMENTS

The authors wish to acknowledge the collaboration and cooperation provided by several USEPA 

offices (Office of Water (OW), Office of Pesticide 

Programs (OPP), Office of Solid Waste and Emergency 

Response (OSWER), Office of Air and Radiation 

(OAR), and Office of Research and Development 

(ORD)) and several NJDEP divisions (Division of 

Science, Research and Technology, Water Supply 

Administration, Risk Communication, and Bureau of 

Safe Drinking Water), without which this study could 

not have been completed. The authors also acknowledge and thank Bruce K. Hope of the Oregon Department of Environmental Quality, and Michael J. Focazio 

and Gregory C. Delzer of the USGS, for their insightful 

technical reviews of this report. The authors are 

grateful to Robyn H. Phillips and Julia E. Norman of 

OHSU for their careful editorial and illustration assistance. Finally, the authors also thank Ella Decker and 

Connie Ross who prepared this report for publication.

HISTORY OF PILOT EFFORT AND REACHING 

CONSENSUS

Reaching consensus on the development of 

HBSL concentrations and ranges was a 2.5-year process. In 1998, the USGS sought assistance and advice 

from the USEPA, the Centers for Disease Control, and 

the National Academy of Sciences on how to communicate NAWQA water-quality data in a human-health 

context more effectively. One suggestion by representatives of these agencies was that NAWQA should conduct a pilot study in conjunction with USEPA and other 

appropriate agencies.

In June 1999, the USGS met with representatives 

of USEPA’s OW, OPP, and ORD to reach consensus on 

the goals of a pilot effort, and to identify one or more 

pilot projects of mutual interest to the USGS and 

USEPA. The Glassboro, New Jersey, study area, which 

overlaps with NAWQA’s LINJ Study Unit, was selected 

as an appropriate pilot study for State-level analyses. 

At that point, representatives from the USGS LINJ 

Study Unit and the NJDEP were asked to collaborate 

in the pilot project. The scope of the initial pilot effort 

was limited to human-health issues involving potential 

exposure to individual contaminants in ground water 

because (1) ground water is the principal drinkingwater source in the area, and (2) procedures for 

assessing potential risks or hazards resulting from 

exposure to multiple contaminants are still under 

development by USEPA, and they are not considered 

sufficiently developed for incorporation into the pilot 

project at this time. 

OHSU personnel were asked to identify and 

describe currently available methods that could be 

used to express the Glassboro ground-water data in a 

human-health risk context, and to provide recommendations on which methods to use in the Glassboro pilot 

project (Toccalino and Binder, 2000). These methods 

and recommendations as well as alternative 

approaches were discussed at a workshop with USGS, 

USEPA, and NJDEP representatives in May 2000. 

The workshop participants agreed that the USGS 

should not conduct a baseline risk assessment on 

NAWQA data using standard deterministic 

approaches to place the concentration data in a risk 

context. Instead, the USGS should pursue a twotiered, HBSL-based methodology for placing waterquality data in a human-health context. The participants in the May 2000 workshop did not reach consensus on exactly how the HBSL values would be 

developed, but there was general agreement that, for 

USGS analyses and reports at the State or study-unit 

scale, it was appropriate to use methodologies and 

assumptions from USEPA’s OW to develop HBSL 

values in the absence of Maximum Contaminant Level 

(MCL) values. 

Toccalino and others (2001) proposed a possible HBSL approach to USGS, USEPA, and NJDEP. 

In May 2001, USGS and OHSU personnel held a 

series of meetings with USEPA and NJDEP representatives to negotiate a consensus approach to developing HBSL values. Consensus was reached on an 

HBSL approach during these May 2001 meetings, and 

the approach was refined and iterated over several 

months (Toccalino, 2001). The consensus approach 

was presented to representatives of the Association of 

State Drinking Water Administrators (ASDWA) in 

May 2001, at the National ASDWA meeting in 

October 2001, at the National Water Quality Monitoring Council (NWQMC) meeting in May 2002, the 
4 Development of Health-Based Screening Levels: State and Local Water-Quality Assessments

USEPA Office of Ground Water and Drinking Water 

(OGWDW) in December 2002, and the national conference on USGS health-related research in April 2003. 

A few technical details (such as how to resolve differences between toxicity values or cancer classifications 

from different USEPA sources) were resolved by 

March 2002. The HBSL approach described in this 

report is the result of the meetings and negotiations 

described above.

USEPA HUMAN HEALTH-BASED BENCHMARKS AND THOSE USED BY USGS

The USEPA has developed a variety of numerical 

water-quality standards and guidelines related to 

drinking water. These standards and guidelines have 

been used in numerous applications, including 

screening pollutants in environmental media, designing 

monitoring programs, evaluating the need for detailed 

water-quality assessments, conducting remedial investigations, and providing an initial cleanup goal, if applicable. Table 1 provides descriptions of 10 humanhealth benchmark concentrations published by the 

USEPA that are related to drinking water.

USGS has historically used 5 of the 10 types of 

USEPA human-health benchmarks presented in table 1 

to evaluate NAWQA data: (1) MCL, (2) Action Level 

(ACT), (3) Lifetime HA, (4) RSD, and (5) Secondary 

Drinking Water Regulation (SDWR) (Gilliom and 

others, 1998). In all five cases, the benchmark levels 

are concentrations pertaining to long-term average 

exposure through drinking water. For some contaminants, more than one of these five benchmarks are 

available. For these contaminants, either the MCL or 

the ACT (for two trace elements that are regulated by 

treatment techniques rather than MCL values) is used, 

if available; otherwise, either the Lifetime HA (for noncarcinogens) or the RSD at a 10-5 cancer risk (for 

potential carcinogens) is used. For a few constituents 

with both a Lifetime HA and an RSD value, the lower 

value is used. In some analyses, the USGS also has 

used the SDWR for dissolved solids. In addition to 

these USEPA benchmarks, USGS also compares waterquality data to State standards and guidelines, where 

they exist, for State- and local-scale water-quality 

studies.

The remaining 5 of the 10 types of USEPA 

human-health benchmarks are not typically used to 

evaluate NAWQA data for a variety of reasons: 

• The Drinking Water Equivalent Level (DWEL) is 

not used because it is an intermediate term that is 

calculated in the process of developing the Lifetime HA value. 

• USEPA’s OPP derives Drinking Water Level of 

Comparison (DWLOC) values only for pesticides, 

so these values are not available for other 

NAWQA analytes. The DWLOC considers multiple exposure routes to pesticides, in addition to 

ingestion of water, so the exposure assumptions 

are not comparable with those used by the OW in 

developing drinking-water MCL values and Lifetime HA values.

• The Maximum Contaminant Level Goal (MCLG) is 

identical to the MCL for most noncarcinogens. 

For potential carcinogens, the MCLG is set at 

zero, implying that there is no safe level of this 

chemical (under the conservative, protective 

model used by USEPA). A benchmark of zero 

has limited utility for comparison with measured 

contaminant concentrations. In addition, all compounds that have MCLG values also have MCL 

values. MCL values are enforceable standards, 

whereas MCLG values are unenforceable health 

goals.

• Both Risk Based Concentrations (RBCs) and Preliminary Remediation Goals (PRGs) are regional 

USEPA values. Because NAWQA is a national 

program, regional health-based benchmarks have 

less utility than national benchmarks. RBCs and 

PRGs for residential tap water use are calculated 

in a manner similar to HBSL values. However, 

both tap water RBCs in Region 3 and tap water 

PRGs in Region 9 account for inhalation of volatile organic compounds (VOCs) present in a drinking-water source (U.S. Environmental Protection 

Agency, 1999a, 2002f), and inhalation exposure is 

not considered as part of this pilot effort. Note that 

Region 9 tap water PRGs are presented in three 

ways: (1) ingestion exposure, (2) inhalation exposure, and (3) combined ingestion and inhalation 

exposure (U.S. Environmental Protection Agency, 

2002f).
USEPA Human Health-Based Benchmarks and Those Used by USGS 5

Table 1. Descriptions of published USEPA human-health benchmark concentrations applicable to drinking water

[Maximum Contaminant Levels (MCLs) are enforceable standards; the remainder of the listed benchmarks are unenforceable guidelines or public-health 

goals. USEPA, U.S. Environmental Protection Agency; OW, Office of Water (USEPA); OPP, Office of Pesticide Programs (USEPA); ORD, Office of 

Research and Development (USEPA); mg/L, milligrams per liter]

Human-health

benchmark Acronym USEPA 

office Description References

Action Level ACT OW The concentration of a contaminant, which, if 

exceeded, triggers treatment or other requirements 

that a water system must follow. USEPA has 

action levels for lead and copper only.

(U.S. Environmental Protection 

Agency, 2001b, 2002a, 

2002c)

Drinking Water Equivalent LevelDWEL OW A lifetime exposure concentration protective of 

adverse, noncancer health effects, that assumes all 

of the exposure to a contaminant is from drinking 

water. 

(U.S. Environmental Protection 

Agency, 2002a)

Drinking Water Level 

of Comparison

DWLOC OPP A theoretical maximum concentration of a pesticide 

in drinking water that, when combined with exposures through food, residential, and other uses, 

would not exceed the maximum allowable dose 

(from toxicity studies) for that pesticide. 

(U.S. Environmental Protection 

Agency, 2000a) 

Lifetime Health 

Advisory

Lifetime HA OW The concentration of a contaminant in drinking 

water that is not expected to cause any adverse 

noncarcinogenic effects over a lifetime of exposure (70 years). The Lifetime HA assumes that 

only a portion (generally 20 percent) of the total 

exposure to a contaminant is from drinking water. 

(U.S. Environmental Protection 

Agency, 1993b, 2002a)

Maximum Contaminant LevelMCL OW The highest concentration of a contaminant that is 

allowed in drinking water. MCLs are set as close 

to MCLGs as feasible using the best available 

treatment technology and taking cost into consideration. 

(U.S. Environmental Protection 

Agency, 2002a, 2002b)

Maximum Contaminant Level GoalMCLG OW The concentration of a contaminant in drinking 

water at which no known or anticipated adverse 

health effects occur and which allows an adequate 

margin of safety. 

(U.S. Environmental Protection 

Agency, 2002a, 2002b)

Preliminary Remediation Goals (for tap 

water)

PRG Region 9 Contaminant concentrations that correspond to fixed 

levels of risk (that is, a noncarcinogenic hazard 

quotient of 1, or lifetime cancer risk of 1 in 

1,000,000) in soil, air, and water.

(U.S. Environmental Protection 

Agency, 2002f)

Risk Based Concentrations (for tap 

water)

RBC Region 3 Contaminant concentrations that correspond to fixed 

levels of risk (that is, a noncarcinogenic hazard 

quotient of 1, or lifetime cancer risk of 1 in 

1,000,000, whichever occurs at a lower concentration) in water, air, fish tissue, and soil.

(U.S. Environmental Protection 

Agency, 1999a)

Risk-Specific Dose RSD OW, ORD The concentration of a contaminant in drinking 

water that corresponds to a specific estimated lifetime cancer risk (typically 1 in 10,000, 1 in 

100,000, or 1 in 1,000,000). The RSD is always 

associated with a specific cancer risk level. A 

related benchmark is the unit drinking-water risk, 

which is risk per mg/L concentration.

(U.S. Environmental Protection 

Agency, 2001a, 2002a)

Secondary Drinking 

Water Regulation

SDWR OW Contaminant concentrations that may cause cosmetic effects (such as skin or tooth discoloration) 

or aesthetic effects (such as taste, odor, or color) 

in drinking water. 

(U.S. Environmental Protection 

Agency, 2002a, 2002b)
6 Development of Health-Based Screening Levels: State and Local Water-Quality Assessments

HEALTH-BASED SCREENING LEVEL (HBSL) 

APPROACH

USGS, USEPA, NJDEP, and OHSU participants 

unanimously agreed to pursue the following two-tiered, 

screening level-based methodology for placing 

NAWQA water-quality data in a human-health context 

for State- and local-scale analyses and reports:

• Tier 1: For regulated contaminants (compounds for 

which Federal and (or) State drinking-water standards have been established), compare measured 

NAWQA concentration data with existing USEPA 

MCL values and applicable State standards. 

• Tier 2: For contaminants that are not regulated in 

drinking water at either the Federal or State level, 

compare measured NAWQA concentration data 

with HBSL concentrations or ranges developed 

using USEPA OW methodologies (U.S. Environmental Protection Agency, 1988, 1993b).

This two-tiered approach for State- and localscale analyses entails listing regulated compounds separately from unregulated compounds. For regulated 

compounds, USGS will continue to compare measured 

concentration data with Federal and (or) State standard 

values as has been done historically. Standard values 

are threshold values that are legally enforceable by 

Federal or State government agencies. HBSL concentrations or ranges will be used only for compounds for 

which no drinking-water standards have been established (that is, unregulated compounds). 

In determining how to derive HBSL values for 

the Tier 2 analyses, several criteria were considered. To 

meet USGS needs, HBSL values must be: (1) health 

based, (2) systematically defined, (3) consistent for a 

given analyte (that is, only one HBSL value or range 

can exist for each analyte), (4) scientifically defensible, 

and (5) simple to use and interpret. The consensus 

approach described in this report meets all of these criteria. Note that, with respect to the last criterion, care 

must be taken that HBSL values are not used or interpreted inappropriately. This last caveat applies equally 

well, however, to comparison of measured concentrations with drinking-water standards and guidelines as it 

does to comparison with HBSL values, and it would 

apply regardless of how HBSL values were developed. 

The procedures used to develop HBSL values 

differ for noncarcinogens and carcinogens. For unregulated compounds, the HBSL concentrations for 

noncarcinogens and HBSL concentration ranges for 

most carcinogens will be developed using (1) OW 

methodologies (U.S. Environmental Protection 

Agency, 1988, 1993b) for calculating Lifetime HA and 

RSD values, respectively, and (2) toxicity values from 

USEPA. As such, the HBSL values always will be consistent with the information about drinking water 

already coming from USEPA. 

Toxicity values are available for many compounds analyzed by the NAWQA program that do not 

have USEPA standards (MCLs) or guidelines (Lifetime 

HA and RSD values). These toxicity values can be 

used to calculate HBSL values using the OW methodologies (U.S. Environmental Protection Agency, 1988, 

1993b) described in this report. Drinking-water guidelines have not been established for these compounds 

with available toxicity values because the development 

of new guidelines is typically initiated when requested 

by States or regions for particular unregulated contaminants that frequently appear in drinking water. In 

recent years, requests to OW for new or updated 

drinking-water guidelines have been infrequent 

(J. Donohue, U.S. Environmental Protection Agency, 

written commun., 2003).

Acceptable Sources of Cancer 

Classifications and Toxicity

Values for HBSL Development

Because the procedures used to develop HBSL 

values differ for noncarcinogens and carcinogens, the 

cancer classification of a given chemical is a critical 

piece of information. Also, the procedures rely on the 

availability of toxicity information at USEPA; that is, 

oral Reference Dose (RfD) or oral slope factor (SF) 

values for each chemical. The RfD value is an estimate 

(with an uncertainty of perhaps one order of magnitude) of the daily exposure that is likely to be without 

appreciable risk of deleterious health effects in the 

human population (including sensitive subgroups) over 

an individual's lifetime (70 years) (U.S. Environmental 

Protection Agency, 1989b). The oral SF value is the 

cancer potency estimate for a compound as derived 

from the slope of the dose-response (carcinogenicity) 

data extrapolated to zero using an appropriate mathematical model. If the model selected for extrapolation 

from dose-response data is the linearized multistage 

model, the SF value is also known as the q1* value 

(U.S. Environmental Protection Agency, 1989a, 

1993b). The SF is used to estimate an upper-bound 

probability of an individual developing cancer as a 
Health-Based Screening Level (HBSL) Approach 7

result of a lifetime (70 years) of exposure to a particular 

level of a potential carcinogen (U.S. Environmental 

Protection Agency, 1989b).

Exactly what constitutes an acceptable cancer 

classification or an acceptable toxicity value is an 

important technical detail in the derivation of HBSL 

values. The USGS, USEPA, NJDEP, and OHSU representatives involved in the pilot effort to develop HBSL 

values agreed that three primary sources of cancer 

classifications and toxicity values are acceptable to use 

in HBSL development. These acceptable sources of 

toxicity values are: 

1. USEPA’s Integrated Risk Information System 

(IRIS) database (U.S. Environmental Protection 

Agency, 2001a), 

2. Publications from OW (U.S. Environmental 

Protection Agency, 2002a), and 

3. Publications from OPP (U.S. Environmental 

Protection Agency, 1997b, 2002d, 2002e). 

A fourth potential source of cancer classifications and toxicity values is the USEPA OSWER Health 

Effects Assessment Summary Tables (HEAST) (U.S. 

Environmental Protection Agency, 1997a). Neither 

OW nor OPP personnel use toxicity values listed in 

HEAST. However, the NJDEP does use toxicity information from HEAST. Consensus agreement with 

regard to HEAST data was that, if HEAST data have 

been peer reviewed as determined by consulting with 

ORD, then HEAST toxicity data may be used to 

develop HBSL values. 

For some compounds, multiple USEPA cancer 

classifications or toxicity values for the same compound exist in different USEPA publications. The hierarchy of cancer classifications or toxicity sources to use 

in the development of HBSL values when toxicity 

values differ between various USEPA sources of information is shown in equation 1:

(1)

HBSL values will be derived using IRIS toxicity 

data when these data exist, otherwise the most recent of 

OW and OPP toxicity data will be used. For example, 

if a compound has two RfD values, one published in 

IRIS and one published by OPP, the HBSL value will be 

calculated by using the RfD value published in IRIS. 

USGS and OHSU will consult with OW and OPP in 

cases when OW and OPP toxicity values differ from 

each other. 

IRIS > most recent of OW & OPP >> HEAST

HBSL values will not be developed for compounds without acceptable toxicity values. The USGS 

will notify the USEPA when compounds that have no 

acceptable toxicity values are detected frequently in 

water resources.

Developing HBSL Concentrations for 

Unregulated Noncarcinogens

Unregulated noncarcinogens are defined here as 

contaminants without USEPA MCL values and (or) 

State standard values and having USEPA cancer classifications of Group D, Group E, “not likely to be 

carcinogenic to humans,” “inadequate data or information,” or “cannot be determined.” These cancer 

classifications are described in USEPA's 1986 guidelines for carcinogen risk assessment (U.S. Environmental Protection Agency, 1986), USEPA's 1996 

proposed guidelines for carcinogen risk assessment 

(U.S. Environmental Protection Agency, 1996), 

USEPA's 1999 draft revisions to these proposed 

guidelines (U.S. Environmental Protection Agency, 

1999b), and USEPA’s draft final guidelines for carcinogen risk assessment (U.S. Environmental Protection 

Agency, 2003). Note that OW considers Group C carcinogens to be noncarcinogens when developing Lifetime HA values, and introduces a risk management 

factor of 10 to account for potential carcinogenicity. 

HBSL procedures for Group C carcinogens are discussed below in the section called “Methodology for 

‘Suggestive Evidence’ and Group C (Possible) 

Carcinogens.”

Methodology for Unregulated Noncarcinogens

For unregulated noncarcinogens evaluated at 

the State or local scale, USEPA’s Lifetime HA value 

will be used as the HBSL concentration, if available. 

If no Lifetime HA value is available for a noncarcinogen, but there is an acceptable RfD value, then the 

RfD value will be converted into a concentration using 

OW procedures and assumptions for calculating the 

Lifetime HA (U.S. Environmental Protection Agency, 

1988, 1993b), and this calculated concentration will 

be used as the HBSL concentration. For noncarcinogens with no available Lifetime HA or RfD values, 

HBSL concentrations cannot be calculated (fig. 1). 
8 Development of Health-Based Screening Levels: State and Local Water-Quality Assessments

Figure 1. Flowchart showing development of HBSL concentrations for unregulated noncarcinogens at the State or

local scale.

The OW’s Lifetime HA is established for that part of an individual's lifetime exposure that is attributed to 

drinking water and is considered protective of noncarcinogenic adverse health effects over a lifetime of exposure 

(70 years). The Lifetime HA is calculated from the DWEL, which is based on the RfD value (eq. 2) (U.S. 

Environmental Protection Agency, 1989b). 

(2)

Lifetime HA value available? Use Lifetime HA

value as HBSL concentration.

Acceptable* oral RfD value available?

Do not develop HBSL concentration.

Calculate HBSL concentration using OW

procedures for calculating

Lifetime HA values.

no

yes

no

yes

* IRIS > most recent of OW & OPP >> HEAST

[Lifetime HA, Lifetime Health Advisory; HBSL, health-based screening level; RfD, reference dose; USEPA, U.S. Environmental

Protection Agency; OW, Office of Water (USEPA); IRIS, Integrated Risk Information System; OPP, Office of Pesticide Programs

(USEPA); HEAST, Health Effects Assessment Summary Tables]

DWEL μg/L ( ) ( ) RfD mg/kg/day [ ] × ( ) 70 kg body wt × ( ) 1,000 μg/mg

(2 L water consumed/day) = ---------------------------------------------------------------------------------------------------------------------------------------

Like the DWEL, the Lifetime HA also assumes a 

body weight of 70 kg (kilograms) and consumption of 

2 L/day (liters per day) of drinking water. OW accounts 

for the percentage of the total exposure typically 

accounted for by ingestion to drinking water using a 

Relative Source Contribution (RSC) factor. In the 

absence of actual exposure data, the RSC factor for the 

ingestion of drinking water is generally assumed to be 

20 percent (80 percent of exposure is assumed to come 

from other sources) (U.S. Environmental Protection 

Agency, 1993b). OW now has a new RSC policy that 

accounts for exposures from other media (for example, 

nondrinking-water ingestion exposures, inhalation and 

(or) dermal exposures) when setting an Ambient Water 

Quality Criteria (AWQC) value (U.S. Environmental 

Protection Agency, 2000b). For the purpose of HBSL 

development, however, the RSC factor will remain 

20 percent because only potential drinking-water exposure is considered. The Lifetime HA is calculated as 

shown in equation 3 (U.S. Environmental Protection 

Agency, 1988, 1993b). With few exceptions, OW 

rounds Lifetime HA values to one significant figure 

(U.S. Environmental Protection Agency, 2002a). HBSL 

values also are rounded to one significant figure.

Lifetime HA μg/L ( ) = = DWEL RSC × DWEL 0.2 × (3)
Health-Based Screening Level (HBSL) Approach 9

Examples: Unregulated Noncarcinogens

The examples in table 2 illustrate the procedures 

used to develop HBSL concentrations for four unregulated noncarcinogens analyzed by NAWQA. All four 

of these NAWQA analytes are considered noncarcinogens because their USEPA cancer classifications are 

Group D (not classified—inadequate or no human or 

animal evidence of carcinogenicity), Group E (evidence of noncarcinogenicity), or not available. Compounds with no available cancer classifications are 

equivalent to Group D compounds (not classified), and 

the default procedure here, and for OW when developing Lifetime HA values (U.S. Environmental Protection Agency, 1988, 1993b), is to treat these compounds 

like noncarcinogens. For noncarcinogens, Lifetime 

HA values are used as the HBSL concentrations, if 

Lifetime HA values exist for those compounds (fig. 1).

One of the NAWQA analytes in table 2, terbacil, 

has a Lifetime HA value, and therefore, this Lifetime 

HA value is adopted as the HBSL concentration. 

Neither ethyl ether nor isopropyl benzene has a Lifetime HA value, but they both have RfD values, which 

are used as the basis for the HBSL concentration using 

the Lifetime HA approach (eq. 3). Fenuron, the fourth 

compound listed in table 2, does not have a Lifetime 

HA value or an RfD value, so an HBSL concentration 

cannot be developed for this compound. 

Developing HBSL Concentrations or 

Concentration Ranges for Unregulated 

Carcinogens 

Unregulated carcinogens are defined here as 

contaminants without USEPA MCL values or State 

standards and having cancer classifications of “known” 

(Group A), “probable” (Group B1 or B2), and “possible” (Group C) carcinogens for contaminants classified under USEPA's 1986 guidelines for carcinogen 

risk assessment (U.S. Environmental Protection 

Agency, 1986). Unregulated carcinogens also include 

those contaminants having cancer classifications of 

“known/likely,” “likely to be carcinogenic to humans,” 

“carcinogenic to humans,” and “suggestive evidence of 

carcinogenic potential” for contaminants classified 

under USEPA's 1996 proposed guidelines for carcinogen risk assessment (U.S. Environmental Protection 

Agency, 1996), USEPA's 1999 draft revisions to these 

proposed guidelines (U.S. Environmental Protection 

Agency, 1999b), and USEPA’s draft final guidelines for 

carcinogen risk assessment (U.S. Environmental 

Protection Agency, 2003). Regulated carcinogens have 

these same cancer classifications, but they have USEPA 

MCL and (or) State standard values. 

Table 2. Examples of development of HBSL concentrations for unregulated noncarcinogens

[Lifetime HA, Lifetime Health Advisory; RfD, oral reference dose; HBSL, health-based screening level; USEPA, U.S. Environmental Protection Agency; 

OW, Office of Water (USEPA); OPP, Office of Pesticide Programs (USEPA); IRIS, Integrated Risk Information System; HEAST, Health Effects Assessment 

Summary Tables; μg/L, micrograms per liter; mg/kg/day, milligrams of contaminant per kilogram of body weight per day; NA, not available]

Analyte Cancer class, 

source

Lifetime HA

(μg/L)

RfD, source

(mg/kg/day)

HBSL

concentration

(μg/L)

Basis for HBSL

concentration

Terbacil Group E, OW 

and OPP

90 0.013, IRIS and OPP; 

0.01, OW

90 Lifetime HA

Ethyl ether Not Classified NA 0.2, IRIS and HEAST 1,000 Lifetime HA approach using 

IRIS RfD

Isopropyl benzene Group D, IRIS 

and OW

NA 0.1, IRIS, OW, and 

HEAST

700 Lifetime HA approach using 

IRIS RfD

Fenuron Not Classified NA NA NA Do not develop HBSL. Lack of 

toxicity values.
10 Development of Health-Based Screening Levels: State and Local Water-Quality Assessments

Methodology for “Known,” “Likely,” Group A, and 

Group B Carcinogens

For “known,” “likely,” Group A, or Group B carcinogens without USEPA MCL and (or) State standard 

values, an HBSL concentration range (10-6 to 10-4) will 

be calculated in lieu of a single HBSL concentration for 

those carcinogens using an acceptable oral SF value. 

HBSL concentration ranges cannot be calculated for 

“known,” “likely,” Group A, or Group B carcinogens 

with no available SF values (fig. 2). 

The HBSL concentration range will correspond 

to an acceptable cancer risk range of 10-6 to 10-4, and 

will be calculated using the OW’s procedures and 

default assumptions for deriving RSD values. An RSD 

is the drinking-water concentration associated with a 

specified cancer risk level, under certain exposure conditions: consumption of 2 L/day of drinking water by a 

70-kg body weight individual over a lifetime (70 years) 

(U.S. Environmental Protection Agency, 1993b). The 

low end of the HBSL concentration range will be 

equivalent to the RSD value associated with a cancer 

risk level of 10-6, and the high end of the HBSL concentration range will be equivalent to the RSD value 

associated with a cancer risk level of 10-4. This range 

is consistent with drinking-water information currently 

reported by USEPA. In the IRIS database, USEPA 

reports RSD values at risk levels of 10-4, 10-5, and 10-6

for carcinogens that have quantitative estimates of the 

carcinogenic risk from oral exposure (see, for example, 

dieldrin (U.S. Environmental Protection Agency, 

1993a)). RSD values are called drinking-water concentrations at specified risk levels in the IRIS database. 

OW reports RSD values at a cancer risk level of 10-4 in 

its “2002 Edition of the Drinking Water Standards and 

Health Advisories” (U.S. Environmental Protection 

Agency, 2002a).

An RSD value is always associated with a specific cancer risk level, and this cancer risk level represents the upper-bound estimate of the excess cancer 

risk attributed to a lifetime of consumption of drinking 

water that contains the contaminant at the RSD concentration. RSD values are calculated from the cancer risk 

level and the oral SF value, also called the q1*. The 

mathematical model used to derive the SF value is conservative (that is, protective), and is typically a nonthreshold model, so the resulting cancer risk is an 

upper-limit estimate (U.S. Environmental Protection 

Agency, 1989a, 1993b). The RSD is calculated as 

shown in equation 4:

(4)

Figure 2. Flowchart showing development of HBSL ranges for unregulated “known,” “likely,” Group A, and Group B

carcinogens at the State or local scale.

RSD μg/L ( ) ( ) 70 kg body wt × ( ) risk level

( ) 2 L water consumed/day SF [mg/kg/day]–1× ( ) × ( ) mg/1,000 μg

= -----------------------------------------------------------------------------------------------------------------------------------------------------------------

yes Acceptable* oral SF value available?

Calculate HBSL concentration range using

OW's RSD approach for a 10-6 to 10-4

risk-based concentration range.

Do not develop HBSL concentration range.

no

* IRIS > most recent of OW & OPP >> HEAST

[SF, slope factor; HBSL, health-based screening level; USEPA, U.S. Environmental Protection Agency; OW, Office of Water

(USEPA); RSD, Risk-Specific Dose; IRIS, Integrated Risk Information System; OPP, Office of Pesticide Programs (USEPA);

HEAST, Health Effects Assessment Summary Tables]
Health-Based Screening Level (HBSL) Approach 11

An RSD value at a 10-4 or 10-6 cancer risk level 

is the concentration of a potential carcinogen in 

drinking water that is estimated to result in an excess 

cancer risk of one in ten thousand, or one in one million, 

respectively. For carcinogens in drinking water, 

USEPA considers risk levels of 10-6 (and for some compounds, risk levels as high as 10-4) to be protective of 

public health, provided these levels also are protective 

of noncancer adverse effects (U.S. Environmental 

Protection Agency, 1988). Selection of an acceptable 

risk level is a risk management decision (U.S. Environmental Protection Agency, 1995b). USEPA reviews 

individual State policies on cancer risk levels as part of 

its water-quality standards oversight function under the 

Clean Water Act. USEPA's policy is to accept cancer 

risk policies from the States in the range of 10-6 to 10-4

(U.S. Environmental Protection Agency, 1992, 1995a), 

although USEPA prefers risk levels of 10-6 and generally accepts only those State standards for the general 

public that are in the 10-5 to 10-6 range. The risk for 

highly exposed populations should not exceed 10-4

(U.S. Environmental Protection Agency, 2000b). 

Therefore, developing an HBSL concentration range 

for unregulated carcinogens that represents a cancer 

risk range of 10-6 to 10-4 is consistent with USEPA 

procedures and acknowledges the uncertainty of the 

estimate. 

The importance of keeping pace with any 

changes in toxicity information, cancer classifications, 

or OW methodologies for quantifying cancer risks is 

acknowledged. For example, new cancer classifications (such as “likely” at high doses and “unlikely” at 

low doses) and methods for quantification of human 

risk (such as the Margin of Exposure (MOE) approach) 

are anticipated to be used more often by USEPA in 

response to revisions to USEPA’s guidelines for carcinogen risk assessment (which were proposed in 1996, 

revised in 1999 and 2003, but were not final as of April 

2003). Currently, OPP recommends that the USGS not 

deviate from OW methodologies (for example, using 

the RSD approach), even when OPP has recommended 

quantification of potential human cancer risk using an 

approach other than a linear, nonthreshold model, such 

as the MOE approach. OPP has not replaced the RSD 

approach with the MOE approach, although it has 

sometimes used both approaches (C. Eiden and 

E. Doyle, U.S. Environmental Protection Agency, oral 

commun., 2001). When USEPA recommends that the 

MOE approach be used for carcinogens, HBSL concentration ranges for such carcinogens will be developed in 

consultation with USEPA. 

Methodology for “Suggestive Evidence” and Group C 

(Possible) Carcinogens

For unregulated “suggestive evidence of carcinogenic potential” or Group C (possible) carcinogens, 

USEPA’s Lifetime HA value will be used as the HBSL 

concentration, when available (fig. 3). For those “suggestive evidence” or Group C carcinogens without a 

Lifetime HA and for which USEPA has recommended 

that an RfD approach be used for quantification of 

risk, the HBSL concentration will be calculated using 

the OW’s procedures for calculating a Lifetime HA 

value, using an additional uncertainty factor of 10 to 

account for potential carcinogenicity (eq. 5); this is 

OW’s standard procedure for calculating the Lifetime 

HA for Group C carcinogens (U.S. Environmental 

Protection Agency, 1993b).

(5)

For those Group C carcinogens without a Lifetime HA and for which USEPA has recommended 

quantification of potential human cancer risk using a 

linear low-dose extrapolation model, and that do have 

oral SF values, an HBSL concentration range will be 

calculated using OW’s RSD approach (eq. 4), instead 

of a single HBSL concentration calculated using 

OW’s Lifetime HA approach. HBSL values cannot be 

calculated for Group C carcinogens with no available 

Lifetime HA, RfD, or SF values (fig. 3). 

Examples: Unregulated Carcinogens 

The examples in table 3 illustrate the procedures used to develop HBSL concentrations and 

ranges for unregulated carcinogens analyzed by 

NAWQA. All four of the NAWQA analytes in table 3 

are considered carcinogens because their cancer classifications are Group B2 (probable human carcinogen), Group C (possible human carcinogen), or 

“suggestive evidence.”

Each of the three Group C or “suggestive evidence” carcinogens in table 3 is treated differently. 

Cyanazine is a Group C carcinogen with a Lifetime 

HA value, so the Lifetime HA value is adopted as the 

HBSL concentration. Dicrotophos is a “suggestive 

evidence” carcinogen with no Lifetime HA value.

Lifetime HAGroup C (μg/L) DWEL RSC ×

10 = -----------------------------------

DWEL 0.2 ×

10 = -------------------------------
12 Development of Health-Based Screening Levels: State and Local Water-Quality Assessments

Figure 3. Flowchart showing development of HBSL concentrations or ranges for unregulated Group C and “suggestive 

evidence” carcinogens at the State or local scale.

Lifetime HA value available? Use Lifetime HA value as

HBSL concentration.

Acceptable* oral RfD value available?

Acceptable* oral SF value available?

Calculate HBSL concentration using

OW procedures for calculating Lifetime

HA values using an additional factor of 10,

unless USEPA recommends using the

SF approach.

Calculate HBSL concentration range

using OW's RSD approach for a 10-6 to

10-4 risk-based concentration range.

no

yes

no

yes

yes

Do not develop HBSL concentration

or concentration range.

no

* IRIS > most recent of OW & OPP >> HEAST

[Lifetime HA, Lifetime Health Advisory; HBSL, health-based screening level; RfD, reference dose; OW, Office of Water (USEPA);

USEPA, U.S. Environmental Protection Agency; SF, slope factor; RSD, Risk-Specific Dose; IRIS, Integrated Risk Information

System; OPP, Office of Pesticide Programs (USEPA); HEAST, Health Effects Assessment Summary Tables]

However, dicrotophos does have an RfD value, which is 

used as the basis for the HBSL concentration using the 

Lifetime HA approach for Group C compounds (which 

includes an extra 10-fold uncertainty factor). Note that 

dicrotophos has two different RfD values—one published in the IRIS database, and one published by OPP. 

The IRIS RfD value was used to calculate the HBSL 

value because IRIS toxicity values take precedence in 

the development of HBSL values when toxicity values 

differ between USEPA sources of information (eq. 1). 

Beta-benzene hexachloride (beta-BHC) is a Group C 

carcinogen that does not have either a Lifetime HA 

value or RfD value, but does have an SF value. Therefore, the SF value is used as the basis for the HBSL concentration range using the RSD approach. The fourth 

compound in table 3, p,p'-DDE, is a Group B2 carcinogen with an available SF value; therefore, its SF value 

is used as the basis for the HBSL concentration range 

using the RSD approach. HBSL values are rounded to 

one significant figure.

Benefits of Using HBSL Benchmarks

The most useful USEPA benchmarks for comparison with contaminant concentrations in water measured by NAWQA are the MCL, Lifetime HA, and 

RSD values. However, these drinking-water standards 

and guidelines do not exist for 207 out of 329 analytes 

measured by the NAWQA program. Nonetheless, 

acceptable toxicity values are available for nearly onehalf of the NAWQA analytes that do not currently (as of 

April 2003) have MCL, Lifetime HA, or RSD values, 

and these toxicity values can be used to calculate HBSL 
Health-Based Screening Level (HBSL) Approach 13

values using OW methodologies (U.S. Environmental 

Protection Agency, 1988, 1993b). The calculation of 

HBSL values will increase the percentage of NAWQA 

analytes that have health-based benchmarks from 

37 percent to 64 percent (fig. 4). Approximately 90 

new HBSL concentrations or concentration ranges can 

be calculated for unregulated VOCs, pesticides, pesticide degradation products, nutrients, and trace elements. Most (72) of the new HBSL values are for 

pesticides (fig. 4). The majority of NAWQA analytes 

are included in figure 4, although additional compounds are occasionally analyzed by some of the 

NAWQA study units. Note that because different 

States may not use the same drinking-water standard 

values, the number of compounds for which HBSL 

values are developed may vary from State to State. 

Therefore, HBSL concentrations and concentration 

range values are not included in this report, but will be 

published after the completion of the LINJ and national 

pilot projects referred to in the “Introduction” section.

Utilizing the HBSL values also will simplify 

data analysis in State-level reports. The USGS will 

continue to compare measured concentrations of regulated compounds with Federal and State MCL values. 

Concentrations of unregulated compounds will be 

compared with HBSL values. As described above, 

HBSL values for noncarcinogens and Group C carcinogens will be equivalent to OW’s Lifetime HA values, 

where Lifetime HA values exist. HBSL values for 

carcinogens typically are equivalent to published RSD 

values, unless more recent toxicity values are used to 

develop the HBSL concentration ranges for carcinogens than were used in RSD development. Therefore, 

HBSL concentrations and concentration ranges are 

consistent with the information about drinking water 

already published by the USEPA.

Finally, the HBSL values have utility beyond the 

USGS NAWQA program. HBSL values and the rationale used to develop these values may have potential 

applications to other projects and studies conducted by 

USGS, as well as by other agencies. Several factors 

must be considered, however, when applying HBSL 

values to studies conducted by NAWQA or by others. 

First, the methodologies described in this report apply 

only to State- and local-scale water-quality assessments. Multi-State water-quality assessments would 

require additional considerations, which are being 

investigated in a second, national-scale pilot project. 

Second, comparisons of raw water concentrations with 

HBSL values, as with any other drinking-water benchmarks such as MCL values, require careful interpretation. Lastly, HBSL values are new benchmarks that 

will not initially be familiar to State drinking-water 

agencies, water utilities, or other interested parties, so 

it is important that water-quality findings be communicated in consultation with the appropriate drinkingwater agencies.

Table 3. Examples of development of HBSL concentrations and ranges for unregulated carcinogens

[Lifetime HA, Lifetime Health Advisory; RfD, oral reference dose; SF, oral slope factor; HBSL, health-based screening level; USEPA, U.S. Environmental 

Protection Agency; OPP, Office of Pesticide Programs (USEPA); HEAST, Health Effects Assessment Summary Tables; OW, Office of Water (USEPA); 

IRIS, Integrated Risk Information System; RSD, Risk-Specific Dose; μg/L, micrograms per liter; mg/kg/day, milligrams of contaminant per kilogram of 

body weight per day; NA, not available]

Analyte Cancer class,

source

Lifetime HA

(μg/L)

RfD, source

(mg/kg/day)

SF, source

(mg/kg/day)-1

HBSL

concentration

or range

(μg/L)

Basis for HBSL

concentration

or range

Cyanazine Group C, OPP and 

HEAST

1 0.002, OW, OPP,

and HEAST

0.166, OPP; 

0.84, HEAST

1 Lifetime HA

Dicrotophos “Suggestive Evidence,” 

OPP

NA 0.0001, IRIS; 

0.00002, OPP

NA 0.07 Lifetime HA 

approach for 

Group C 

carcinogens using 

IRIS RfD

Beta-BHC Group C, IRIS and 

HEAST

NA NA 1.8, IRIS 0.02 to 2 RSD approach using 

IRIS SF

p,p'-DDE Group B2, IRIS, OPP, 

and HEAST

NA NA 0.34, IRIS and 

HEAST

0.1 to 10 RSD approach using 

IRIS SF
14 Development of Health-Based Screening Levels: State and Local Water-Quality Assessments

Figure 4. Number of NAWQA analytes, including VOCs, pesticides, pesticide degradation products, nutrients, and

trace elements and major ions, with health-based benchmarks before and after HBSL calculations. The health-based

benchmarks before HBSL calculations include MCL, Lifetime HA, RSD, and ACT values.

5

22

211

56

120

4 5

26

4

48 43

122

0

50

100

150

200

250

All

compounds

VOCs Pesticides Pesticide

degradates

Nutrients TE & MI

NUMBER OF COMPOUNDS

Before HBSL calculations

After HBSL calculations

total n = 329 total n = 88 total n = 9 total n = 39 total n = 139 total n = 54

[NAWQA, National Water-Quality Assessment Program; VOCs, volatile organic compounds; HBSL, health-based screening level;

MCL, maximum contaminant level; Lifetime HA, Lifetime Health Advisory; RSD, Risk-Specific Dose; ACT, action level; TE & MI,

trace elements & major ions; n, number of compounds}

USE OF HBSL VALUES AS TOOLS IN WATERQUALITY ASSESSMENTS

This section provides step-by-step procedures, 

followed by a general hypothetical ground-water 

example, for the application of HBSL concentrations 

and ranges in water-quality assessments. A real, more 

detailed example of how HBSL values can be used to 

analyze water-quality data in State- or local-scale 

reports in a human-health context, is being prepared 

using data from the Glassboro-LINJ State-level pilot 

project. The USGS, USEPA, NJDEP, and OHSU 

agreed that, in State and local-scale reports and analyses, water-quality concentration data will be presented 

in two tables. The first table will list USEPA MCL 

values and (or) State drinking-water standards for compounds that are regulated at the Federal and (or) State 

level. The second table will list HBSL concentrations 

(for noncarcinogens) and HBSL concentration ranges 

(for most carcinogens) for compounds that are not 

regulated. 

In order to present water-quality concentration 

data in two tables, one each for regulated and unregulated compounds, follow four general steps:

1. Determine which of the detected compounds are 

regulated and which are unregulated by locating the 

USEPA MCL and State drinking-water standard 

values for each detected compound. Current (as of 

April 2003) USEPA MCL values are published in 

the USEPA drinking-water standards and health 
Use of HBSL Values as Tools in Water-Quality Assessments 15

advisories document (U.S. Environmental Protection Agency, 2002a). State MCL values are typically published on State drinking-water program 

Internet sites (for example see, Association of State 

Drinking Water Administrators, 2002). Regulated 

compounds have established Federal and (or) State 

drinking-water standards, and unregulated compounds do not. Standards are threshold values that 

are legally enforceable by agencies of the Federal 

or State governments.

2. List regulated compounds in a table along with the 

applicable Federal and State drinking-water standard values. For these regulated compounds, compare measured concentrations for the regulated 

compounds with all applicable standards.

3. List compounds that are unregulated at both the 

Federal and State levels in a second table along 

with HBSL values and any applicable State guideline values. Guidelines are threshold values that 

were issued in an advisory capacity and are not 

legally enforceable. Then, compare measured concentrations for the unregulated compounds with 

State drinking-water guidelines values (if available) and with HBSL concentrations or concentration ranges. 

4. Interpret the results, bearing in mind the limitations 

of comparing measured concentrations in 

untreated water resources with drinking-water 

standards and HBSL values. Data should be interpreted and published in collaboration or consultation with the appropriate State and (or) local 

drinking-water agencies.

An example demonstrating these four steps follows. In the following hypothetical example, assume 

that 10 compounds were analyzed in 10 ground-water 

samples collected from 10 different wells, and that 8 of 

these 10 compounds (arbitrarily named AAA through 

HHH) were detected at various frequencies in the samples. Two of the eight detected compounds are regulated, and six are unregulated. Five of the six 

unregulated compounds have available toxicity data, 

and therefore HBSL values, and one of the unregulated 

compounds does not have an HBSL value (fig. 5). 

Further assume that State standards and guidelines 

apply to this example. 

Figure 5. Flowchart for hypothetical ground-water example showing the number of detected compounds that are 

regulated and unregulated, and the availability of HBSL values for unregulated compounds.

10 compounds analyzed in

ground-water samples

2 regulated compounds

(with drinking-water standards)

5 compounds

with

HBSL values

1 compound

without

HBSL value

6 unregulated compounds

(without drinking-water standards)

8 compounds detected 2 compounds not detected

[HBSL, health-based screening level]
16 Development of Health-Based Screening Levels: State and Local Water-Quality Assessments

In the first step, list the eight compounds 

detected in ground-water samples, along with all available USEPA MCL and State standard values (table 4). 

Compounds GGG and HHH are considered regulated 

compounds because they have USEPA MCL values 

and (or) State MCL values.

In the second step, compare measured groundwater concentrations for the regulated compounds with 

USEPA MCL values and State MCL values (table 5). 

The measured concentrations may be presented in a 

variety of ways, depending on the objectives of the 

analyses. Table 5 presents median concentrations and 

maximum concentrations along with the frequency of 

detection of each regulated compound. In this hypothetical example, the maximum concentration of compound HHH exceeds both the USEPA and State MCL 

values. However, the median concentration of compound HHH does not exceed either the USEPA or State 

MCL values. Neither the maximum concentration nor 

median concentration of compound GGG exceeds 

USEPA or State MCL values. Because compound 

GGG was detected infrequently (1 out of 10 samples), 

the median concentration for compound GGG is presented as less than the method detection limit (in this 

case, 0.002 μg/L (micrograms per liter)). Tables like 

table 5 may include additional information (such as the 

number of samples that exceed each standard), if 

desired. Note that table 5 follows a format similar to 

that currently used by the USGS in reporting its waterquality data.

In the third step, compare measured groundwater concentrations (for example, median concentrations, maximum concentrations, etc.) for the unregulated compounds with HBSL concentrations and 

concentration ranges, and with any applicable State 

guideline values (table 6). Not all States have drinkingwater guidelines, but some State guideline values are 

called ground-water quality criteria (GWQC) that 

specify the concentrations of constituents in the 

untreated ground-water resource, concentrations above 

which the ground-water would pose an unacceptable 

risk for drinking water. In this ground-water example, 

the median concentration and maximum concentration 

for compound BBB exceeds both the State GWQC and 

the low end of the HBSL concentration range. All of 

the remaining unregulated compounds were detected at 

concentrations below HBSL values. Table 6 also indicates the frequency of detection of each unregulated 

compound, as well as the basis for the HBSL (the 

approach and the source and type of toxicity data used 

to calculate the HBSL concentrations and ranges). For 

compounds that were detected infrequently (compounds AAA, DDD, and FFF), the median concentration is presented as less than the method detection limit 

(in these cases, 0.002 μg/L). Again, additional information (such as the number of samples that exceed the 

HBSL or State GWQC guideline values) could be 

included in the table, if desired.

Table 4. Regulated versus unregulated compounds in 

hypothetical ground-water example

[USEPA, U.S. Environmental Protection Agency; OW, Office of Water 

(USEPA); MCL, Maximum Contaminant Level; μg/L, micrograms per 

liter; NA, not available]

Contaminant

name

USEPA

OW MCL

(μg/L)

State MCL

(μg/L)

Regulated/ 

unregulated

AAA NA NA Unregulated

BBB NA NA Unregulated

CCC NA NA Unregulated

DDD NA NA Unregulated

EEE NA NA Unregulated

FFF NA NA Unregulated

GGG 2 2 Regulated

HHH 5 1 Regulated

Table 5. Comparison of measured concentrations with USEPA and State drinking-water standards for regulated compounds 

in hypothetical ground-water example

[Bold type indicates measured concentration exceeds standard. GW, ground water; USEPA, U.S. Environmental Protection Agency; OW, Office of Water 

(USEPA); MCL, Maximum Contaminant Level; μg/L, micrograms per liter; <, less than]

Contaminant

name

Number of samples 

with detections

(out of 10)

Median GW

concentration

(μg/L)

Maximum GW

concentration

(μg/L)

USEPA OW MCL

(μg/L)

State MCL

(μg/L)

GGG 1 <0.002 0.07 2 2

HHH 7 0.06 7 5 1
Use of HBSL Values as Tools in Water-Quality Assessments 17

Table 6. Comparison of measured concentrations with State guideline values and HBSL concentrations and ranges for 

unregulated compounds in hypothetical ground-water example

[Bold type indicates measured concentration exceeds State guideline value and the low end of the HBSL range. GW, ground water; GWQC, Ground-Water 

Quality Criteria; HBSL, health-based screening level; RSD, Risk-Specific Dose; USEPA, U.S. Environmental Protection Agency; OPP, Office of Pesticide 

Programs (USEPA); q1*, cancer potency factor; IRIS, Integrated Risk Information System; SF, oral slope factor; Lifetime HA, Lifetime Health Advisory; 

RfD, oral reference dose; μg/L, micrograms per liter; NA, not available; <, less than]

Contaminant

name

Number of

samples

with detections

(out of 10)

Median GW

concentration

(μg/L)

Maximum GW

concentration

(mg/L)

State

guideline

value

(GWQC)

(μg/L)

HBSL concentration

or range

(μg/L)

Basis for HBSL

AAA 1 <0.002 0.06 NA 2 to 200 RSD approach using

OPP q1*

BBB 8 0.03 0.05 0.002 0.002 to 0.2 RSD approach, using

IRIS SF

CCC 10 0.008 0.02 NA NA No HBSL because there 

are no toxicity data

DDD 1 <0.002 0.01 NA 1 Lifetime HA approach for 

Group C carcinogens 

using IRIS RfD

EEE 9 0.009 0.7 NA 100 Lifetime HA value 

FFF 2 <0.002 0.03 NA 700 Lifetime HA approach 

using IRIS RfD

In the fourth step, interpreting the water-quality 

results, any exceedances (or lack of exceedances) of 

MCL values and HBSL values should be put in the 

proper context, with regard to both hydrology and to 

human health. Detection frequency, contaminant 

sources, the physical-chemical properties of the contaminants, and potential exposure considerations 

(table 7) should all be taken into account when interpreting the results. When a sampled water resource is 

currently used as a source of drinking water, it is particularly important to consider whether the water will be 

treated prior to consumption, and to acknowledge the 

effects that treatment processes are likely to have on 

contaminant concentrations. It is also important to 

consider how changes within the distribution system 

may affect contaminant concentrations. Note that 

although the hypothetical example discussed in this 

section is for a ground-water resource, most of the factors identified in table 7 to consider when interpreting 

water-quality data also apply to surface-water 

resources. A detailed example that provides guidance 

on how to consider the factors listed in table 7 (detection frequency, etc.) will be presented in a planned 

report for the Glassboro-LINJ pilot project.

In interpreting the water-quality results, care 

must be taken that the HBSL values are not used or 

interpreted inappropriately. Table 8 summarizes what 

HBSL values are and what they are not, and what it 

means if measured concentrations exceed HBSL 

values. Health-based benchmarks (including MCL and 

HBSL values) are based on a lifetime (70 years) of 

exposure (table 8). Therefore, it is important to determine which measured concentration (for example, 

median concentration, maximum concentration, etc.) 

to compare with these benchmarks in a given hydrologic system. In surface-water bodies, contaminant 

concentrations can change significantly as a function of 

time, and comparing time-weighted average concentrations with health-based benchmarks may be most 

appropriate. In ground water, contaminant concentrations change more slowly as a function of time, and, 

depending on the objectives of the analysis, it may be 
18 Development of Health-Based Screening Levels: State and Local Water-Quality Assessments

appropriate to compare a variety of concentrations with 

health-based benchmarks (examples include median 

and maximum concentrations, a median-detected concentration (which excludes nondetected concentrations), and (or) a range of percentiles such as the 25th, 

50th, and 75th percentiles). Note that the mediandetected concentration may be biased towards the high 

end of the concentration distribution, particularly when 

there are a significant number of samples with nondetected concentrations. It is also important to recognize 

that comparisons between maximum concentrations 

and health-based benchmarks are highly conservative 

and may overestimate the hazard or risk associated 

with potential exposure to the water resource. Exposure over the long-term, as measured by average or 

median concentrations (as opposed to 1-day maximumdetected concentrations), is more relevant to benchmarks based on a lifetime of exposure, particularly for 

surface-water bodies. 

It is important to consult with the appropriate 

State or local drinking-water agencies before presenting or publishing interpretive statements about 

drinking-water quality or exposure. These agencies 

have regulatory responsibilities in the area of drinking 

water, they will be stakeholders in the results, and they 

have significant technical expertise in risk assessment 

and risk communication that can be brought to bear in 

assessing the results. In some cases, it may be appropriate for the USGS to issue a joint press release with 

appropriate State or local agencies. 

Table 7. Examples of factors to consider when interpreting results of water-quality analyses

[MCL, Maximum Contaminant Level; HBSL, health-based screening level]

Detection frequency and contaminant

source considerations

Physical-chemical property

considerations Potential exposure considerations

• How frequently is the contaminant 

detected in an area?

• How often were MCL or HBSL values 

exceeded and not exceeded?

• How far above or below the MCL or 

HBSL values are the measured 

concentrations? 

• Are there possible natural and 

anthropogenic sources of the contaminant 

in the vicinity of the sampled sites?

• What is the land use in the vicinity of the 

sampled sites (for example, agricultural, 

residential, etc.)?

• What do the physical and chemical 

properties of the contaminant indicate 

about its fate and transport?

• How biodegradable is the contaminant?

• How long is the contaminant likely to 

persist in surface water or ground water? 

• Is the contaminant likely to migrate to 

other water resources?

• What were the hydrologic conditions at 

the time of sample collection?

• If ground water is sampled, what were the 

important geochemical conditions in the 

aquifer at the time of sample collection?

• Were filtered or whole water samples 

analyzed?

• Is the sampled surface water or ground 

water a current drinking-water source, or 

a potential future drinking-water source? 

• Is the surface water or ground water 

treated before it is consumed? If so, what 

is the potential for common treatment 

practices to reduce or remove the 

contaminant?

• Are contaminant concentrations expected 

to change in distribution systems?

• If ground water is sampled, what types of 

wells were sampled (for example, 

monitoring wells versus domestic wells 

and public-supply wells)? 

• If ground water is sampled, is it shallow 

ground water that is not directly 

consumed but that may be both a 

contributor to, and a predictor of, 

contaminant concentrations in deeper 

aquifers at some later period of time? 

• If surface water is sampled, what type of 

surface water was sampled (for example, 

agricultural drainage ditch, creek, stream, 

river, reservoir, etc.) and what is the 

distance (if known or can be estimated) to 

a drinking-water intake (if any)?
Use of HBSL Values as Tools in Water-Quality Assessments 19

Table 8. What are HBSL values, and what does it mean if HBSL values are exceeded?

[HBSL, health-based screening level; USEPA, U.S. Environmental Protection Agency; OW, Office of Water (USEPA); Lifetime HA, Lifetime Health Advisory; RSD, Risk-Specific Dose; RSC, Relative Source Contribution; MCL, Maximum Contaminant Level]

What HBSL values are, and what it means

if an HBSL is exceeded

What HBSL values are not, and what HBSL

exceedances do not mean

• HBSL values provide human health-based benchmarks, against 

which measured contaminant concentrations can be compared, 

that are consistent with OW methodologies for setting drinkingwater Lifetime HA and RSD values.

• The use of HBSL values expands the number of contaminants for 

which human health-based benchmarks (such as drinking-water 

standards or guidelines) are available for comparison with waterquality data.

• HBSL values are a useful trigger or threshold for notifying the 

appropriate State drinking-water agencies and the USEPA when 

measured concentrations meet or exceed the HBSL 

concentrations or ranges.

• HBSL values are defined systematically. HBSL values for 

noncarcinogens are consistent with Lifetime HA values and 

HBSL ranges for carcinogens are consistent with RSD values at 

risk levels of 10-6 to 10-4. 

• HBSL values entail the same assumptions as USEPA Lifetime 

HA and RSD values for humans: consumption of 2 liters of water 

per day over a 70-year lifetime by the general adult population 

(70-kilogram body weight). For noncarcinogens, HBSL 

concentrations and (in the absence of data to support a different 

RSC) Lifetime HA values also assume that a default 20 percent 

of exposure comes from drinking water.

• For potential carcinogens, the HBSL range merely indicates 

concentrations that are associated with worst-case estimates 

of cancer risk between 10-6 and 10-4. For carcinogens in 

drinking water, USEPA considers risk levels of 10-6 (and for 

some compounds, risk levels as high as 10-4) to be protective 

of public health, provided these levels also are protective of 

noncancer adverse effects (U.S. Environmental Protection 

Agency, 1988). Exceedance of the lower end of the HBSL 

range indicates that the worst-case cancer risk associated with 

a lifetime consumption of drinking water at that 

concentration would exceed one in a million.

• HBSL values can be used as a planning tool to identify water 

resources with contaminant concentrations of potential concern, 

to support science-based decisions to manage these water 

resources, to identify locations for future water-supply 

development, and to help direct future scientific research.

• Exceedance of HBSL values does not indicate any violation of 

drinking-water standards, State or Federal. 

• HBSL concentrations do not necessarily indicate safe drinkingwater concentrations.

• Exceedance of HBSL values does not necessarily mean that 

people who potentially ingest the water are adversely affected 

because (1) the models used by OW are intentionally 

conservative (protective) and include safety factors; (2) the 

models assume lifetime exposure to these concentrations, 

whereas the concentrations of contaminants may change 

seasonally or over time; (3) raw water is often treated prior to 

consumption and contaminant concentrations may be 

substantially reduced by treatment methods, particularly in 

surface water.

• HBSL values, like USEPA drinking-water standards, Lifetime 

HA values, and RSD values, do not take into account the effects 

of mixtures of contaminants. 

• HBSL values are not the same as State or USEPA MCL values. 

MCL values sometimes consider factors such as treatment 

feasibility and analytical detection limits, as well as health 

effects.

• HBSL values, as developed in this report, are appropriate to use 

at the State and local scales, but not in national and multi-State 

studies, which are more complex because multiple States are 

affected.
20 Development of Health-Based Screening Levels: State and Local Water-Quality Assessments

SUMMARY

Drinking-water standards and guidelines do not 

exist for many of the contaminants analyzed by the U.S. 

Geological Survey (USGS) National Water-Quality 

Assessment (NAWQA) Program and in other USGS 

water-quality studies. Therefore, an interagency pilot 

effort was begun in June 1999 in New Jersey to 

develop, test, and refine concepts to improve the communication of USGS water-quality findings in Stateand local-scale reports. After a series of workshops and 

written exchanges over 2.5 years, the USGS, U.S. 

Environmental Protection Agency (USEPA), New 

Jersey Department of Environmental Protection, and 

Oregon Health & Science University representatives 

participating in the New Jersey pilot project agreed to 

develop health-based screening level (HBSL) concentrations and ranges for compounds without existing 

drinking-water standards. HBSL values are defined as 

estimates of benchmark concentrations (for noncarcinogens) or concentration ranges (for carcinogens) in 

water that (1) may be of potential human-health concern; (2) can be used as threshold values against which 

measured concentrations of contaminants in ambient 

water samples can be compared; and (3) are consistent 

with USEPA Office of Water (OW) methodologies for 

setting drinking-water Lifetime Health Advisory (HA) 

and Risk-Specific Dose (RSD) values.

The pilot project representatives agreed to pursue 

a two-tiered, screening level-based methodology for 

placing NAWQA water-quality data in a human-health 

context for State- and local-scale analyses and reports. 

In Tier 1 analysis, ground-water concentration data for 

regulated compounds will continue to be compared 

with USEPA and (or) State standard values, as has been 

done historically. In Tier 2 analysis, ground-water concentration data for unregulated compounds will be 

compared with newly established HBSL concentrations or ranges. HBSL concentrations (for unregulated 

noncarcinogens) and HBSL concentration ranges (for 

most unregulated carcinogens) are being developed 

using USEPA OW methodologies for calculating Lifetime HA and RSD values for drinking water. Therefore, HBSL concentrations and concentration ranges 

are consistent with the information about drinking 

water already coming from USEPA. 

HBSL values can be used as tools in waterquality assessment and their use will expand the 

number of NAWQA contaminants for which healthbased benchmarks are available for comparison with 

USGS water-quality data from 122 to 211 out of 329 

compounds, a 27 percent increase. Because the 

systematic development of HBSL concentrations and 

ranges will enable the USGS to present its waterquality data in a human-health context for a larger suite 

of contaminants than have existing drinking-water 

standards and guidelines, the use of HBSL values will 

increase the basis for interpreting the significance of 

water-quality data. When interpreting water-quality 

results, comparison of measured water concentrations 

with Maximum Contaminant Level values and HBSL 

values needs to be placed in the proper context, both 

hydrologically and with regard to human health. USGS 

can use HBSL values to assist the USEPA and State and 

local agencies by providing them with comparisons of 

measured water concentrations to scientifically defensible human health-based benchmarks, and by alerting 

them when measured concentrations approach or 

exceed these benchmarks.

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