Exposure Assessment Methods Handbook: Draft


EPA/600
September 1989
DRAFT
EXPOSURE ASSESSMENT METHODS HANDBOOK
Exposure Assessment Group
Office of Health and Environmental Assessment
U.S. Environmental Protection Agency
Washington, DC 20460

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DISCLAIMER
This document has been reviewed in accordance with U.S. Environmental
Protection Agency policy and approved for publication. Mention of trade
names or commercial products does not constitute endorsement or
recommendation for use.

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TABLE OF CONTENTS
Page No.
Tables 		vii
Foreward 		xi
Preface 		xii
Abstract 		xi i i
Authors, Contributors, and Reviewers 		xiv
PART I
1.	INTRODUCTION 		2-1
1.1	Background 		1-1
1.2	Purpose and Scope 		1-2
1.3	Organization of the Handbook 		1-3
2.	OVERVIEW OF THE EXPOSURE ASSESSMENT PROCESS 		2-1
2.1	Methodological Framework 		2-1
2.2	Elements of Exposure Assessment 		2-1
2.2.1	General Information 		2-1
2.2.2	Estimation of Release 		2-3
2.2.3	Environmental Fate Analysis/Determination of
Environmental Concentrations 		2-4
2.2.4	Development of Exposure Scenarios 		2-5
2.2.5	Estimation of Exposure/Dose 		2-5
2.2.6	Estimation of Integrated Exposure 		2-9
3.	ASSESSMENT OF EXPOSURE VIA SURFACE WATER 		3-1
3.1	Release Analysis - Surface Water 		3-1
3.1.1	Screening of Release Potential 		3-1
3.1.2	Quantitative Release Estimation 		3-2
3.2	Fate Analysis - Surface Water 		3-15
3.2.1	Screening of Surface Water Fate Processes 		3-15
3.2.2	Quantitative Fate Estimation 		3-25
3.3	Estimation of Concentrations - Surface Water 		3-38
3.4	Exposure Estimation - Surface Water 		3-40
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TABLE OF CONTENTS (continued)
Page No.
4.	ASSESSMENT OF EXPOSURE VIA GROUND WATER 		4-1
4.1	Release Analysis - Ground Water 		4-1
4.1.1	Screening of Release Potential 		4-1
4.1.2	Quantitative Release Estimation 		4-2
4.2	Fate Analysis - Ground Water 		4-10
4.2.1	Screening of Fate Processes 		4-10
4.2.2	Quantitative Fate Estimation 		4-19
4.3	Estimation of Concentration - Ground Water 		4-40
4-4 Exposure Estimation - Ground Water 		4-47
5.	ASSESSMENT OF EXPOSURE VIA FISH/SHELLFISH 		5-1
5.1	Release Analysis - Fish/Shellfish 		5-1
5.1.1	Screening of Release Potential 		5-1
5.1.2	Quantitative Release Estimation 		5-1
5.2	Fate Analysis - Fish/Shellfish 		5-1
5.2.1	Screening of Fate Processes 		5-1
5.2.2	Quantitative Fate Estimation 		5-2
5.3	Estimation of Concentrations - Fish/Shellfish 		5-10
5.4	Exposure Estimation - Fish/Shellfish 		5-10
6.	ASSESSMENT OF EXPOSURE VIA TERRESTRIAL PLANT AND ANIMAL FOOD
SOURCES 		6-1
6.1	Release Analysis - Terrestrial Food Sources 		6-1
6.1.1	Screening of Release Potential 		6-1
6.1.2	Quantitative Release Estimation 		6-1
6.2	Fate Analysis - Terrestrial Food Sources 		6-1
6.2.1	Screening of Fate Processes 		6-1
6.2.2	Quantitative Fate Estimation 		6-2
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TABLE OF CONTENTS (continued)
Page No.
6.3	Estimation of Concentrations - Terrestrial Food
Sources 		6-8
6.4	Exposure Estimation - Terrestrial Food Sources 		6-12
7.	ASSESSMENT OF EXPOSURE VIA AMBIENT AIR 		7-1
7.1	Release Analysis - Ambient Air 		7-1
7.1.1	Screening of Release Potential 		7-1
7.1.2	Quantitative Release Estimation 		7-2
7.2	Fate Analysis - Ambient Air 		7-22
7.2.1 Atmospheric Degradation 		7-23
7.3	Estimation of Concentrations - Ambient Air 		7-32
7.4	Exposure Estimation - Ambient Air 		7-43
8.	ASSESSMENT OF EXPOSURE VIA DIRECT CONTACT WITH CONTAMINATED
SOILS 		8-1
8.1	Release Analysis - Soil 		8-1
8.1.1	Screening of Release Potential 		8-2
8.1.2	Quantitative Release Estimation 		8-5
8.2	Fate Analysis - Soil 		8-7
8.2.1	Screening of Soil Fate Processes 		8-7
8.2.2	Quantitative Fate Estimation 		8-11
8.3	Estimation of Concentrations - Soil 		8-11
8.3.1	Estimation of Airborne Concentrations Resulting
from Contaminated Soil 		8-11
8.3.2	Estimation of Concentrations in Surface Water
Resulting from Contaminated Soil 		8-16
8.3.3	Estimation of Concentrations in Ground Water
Resulting from Contaminated Soil 		8-16
8.4	Exposure Estimation - Soil 		8-18
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TABLE OF CONTENTS (continued)
Page No.
PART II
1.	EXPOSED POPULATIONS 		1-1
2.	CHARACTERIZATION AND ANALYSIS OF UNCERTAINTIES IN THE
SAMPLING, MONITORING, AND MODELING OF ENVIRONMENTAL
CONCENTRATIONS OF CONTAMINANTS 					2-1
2.1	Causes of Uncertainty 		2-1
2.2	Qualitative Analysis of Uncertainties 		2-3
2.3	Quantitative Analysis 		2-4
2.4	Presentation of Uncertainty Analysis Results 		2-16
3.	REFERENCES 		3-1
APPENDIX A - Glossary of Terms 		A-l
APPENDIX B - Exposure Assessment Worksheets 		B-l
APPENDIX C - Completed Exposure Assessment Worksheets for An
Example Chemical 		C-l
APPENDIX D - Transfer Coefficients for Assessing Exposures to
Contaminants in Animal Tissues and Animal Products .	D-l
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LIST OF TABLES
Page No.
PART I
Table 3-1. Runoff Curve Numbers 	:	 3-4
Table 3-2. Magnitude of Soil Erodibility Factor K 	 3-6
Table 3-3. "C" Values for Permanent Pasture, Rangeland, and
Idle Land 	 3-10
Table 3-4. "C" Values for Woodland 	 3-11
Table 3-5. Values of P for Agricultural Land 	 3-12
Table 3-6. Values of P for Construction Sites 	 3-13
Table 3-7. Relative Importance of Processes Influencing Aquatic
Fate of Organic Priority Pollutants 	 3-17
Table 3-8. Estimated Adsorption of Chemicals to Suspended
Solids in Surface Waters 	 3-21
Table 3-9. Classes of Organic Functional Groups Potentially
Susceptible to Hydrolysis 	 3-23
Table 3-10. Classes of Organic Functional Groups Potentially
Resistant to Hydrolysis 	 3-24
Table 4-1. Parameter Values for Permeation Equation (at 25°C) . 4-6
Table 4-2. Polymer Categorization for Permeation of Water 	 4-7
Table 4-3. Permachor Values of Some Organic Liquids in
Polyethylene and PVC 	 4-8
Table 4-4. Water Permachor Value for Dry Polymers 	 4-9
Table 4-5. The General Relationship Between the Soil/Solution
Partition Coefficient Kj and Soil Mobility 	 4-15
Table 4-6. Representative Values for Saturated Moisture
Contents and Field Capacities of Various Soil
Types 	 4-25
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LIST OF TABLES (continued)
Page No.
Table 4-7. Representative Values of Hydraulic Parameters
(Standard Deviation in Parentheses) 	 4-26
Table 4-8. Representative Values of Saturated Hydraulic
Conductivity 	 4-27
Table 4-9. Saturated Hydraulic Conductivity Ranges for Selected
Rock and Soil Types 	 4-28
Table 4-10. Suggested Value for £et Relating Evaporation from a
U.S. Class A Pan to Evapotranspiration from 8- to
15-cm Tall, Well-Watered Grass Turf 	 4-31
Table 4-11. Crop Coefficients for Estimating
Evapotranspiration 	 4-32
Table 4-12. Features of Unsaturated Zone and Ground-Water Models 4-43
Table 4-13. Data Requirements for Unsaturated Zone and Ground-
Water Models 	 4-46
Table 5-1. Reasonable Worst-Case Bioconcentration Factors
(BFCs) for Metals in Fish/Shellfish 	 5-9
Table 6-1. Relative Accumulation of Heavy Metals into Edible
Plant Parts of Vegetable Crops 	 6-9
Table 6-2. Parameter Values for Estimation of Concentrations in
Animal Tissues/Products Using Transfer
Coefficients 	 6-13
Table 7-1. Environmental Variables and Model Parameters for the
Wind Erosion Equation 	 7-4
Table 7-2. Diffusion Coefficients of Selected Organic Compounds 7-13
Table 7-3. Approximate Absorption Regions for Organic Molecules 7-25
Table 7-4. Oxidant Concentrations in Water and Air 	 7-28
Table 7-5. Rate Constants for Oxidation by HO Radical in the
Atmosphere at 25°C 	 7-29
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LIST OF TABLES (continued)
Page No.
Table 7-6. Rates of Oxidation by Ozone in the Atmosphere at
25°C 	 7-31
Table 7-7. Capabilities of Reviewed Models 	 7-33
Table 7-8. Preferred Models for Selected Applications in Simple
Terrain 	 7-36
Table 7-9. Resource Requirements and Information Sources:
Atmospheric Fate Models 	'	 7-37
Table 7-10. Features of Atmospheric Fate Models 	 7-41
Table 7-11. Data Requirements for Atmospheric Models 	 7-42
Table 8-1. Factors Necessary to Determine Releases from
Contaminated Soils 	 8-3
Table 8-2. Importance of Volatility from Dry Soils 	 8-10
Table 8-3. Default Assumptions for Calculation of Short-Term
Maximum Concentrations in Air 	 8-15
PART II
Table 2-1. Relative Errors and Sensitivity Coefficients for the
Gaussian Plume Concentration Model of Examples 1
& 2: C = (8.65xl07Q/1000ua)exp_1/2(H2/a2j 	 2_8
Table 2-2. Uncertainty Analysis Report for Example 3
Concentration Estimation 	 2-18
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LIST OF FIGURES
Page No.
PART I
Figure 2-1. Framework and Components of an Exposure Assessment . 2-2
Figure 2-2. Representations of Microenvironment Contributions to
Total Inhalation Exposure to Nitrogen Dioxide 	 2-12
Figure 3-1. Slope Effect Chart Applicable to Areas A-l in
Washington, Oregon, and Idaho, and All of A-3 	 3-7
Figure 3-2. Soil Moisture-Soil Temperature Regimes of the
Western United States 	 3-8
Figure 3-3. Slope Effect Chart for Areas Where Figure 3-2 is Not
Applicable 	 3-9
Figure 4-1. Examples of the Range of Hydrolysis Half-Lives for
Various Types of Organic Compounds in Water at pH 7
and 25°C 	 4-17
Figure 7-1. Mean number of Days per Year With >0.01 Inches of
Precipitation (i.e., "Wet Days") 	 7-7
Figure 7-2. Decision Tree for Selection of Air Transport
Models 	 7-35
Figure 8-1. Horizontal Dispersion Coefficient as a Function of
Downwind Distance from the Source 	 8-13
Figure 8-2. Vertical Dispersion Coefficient as a Function of
Downwind Distance from the Source 	 8-14
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FOREWORD
The Exposure Assessment Group (EAG) of EPA's Office of Research and
Development has three main functions: (1) to conduct exposure assess-
ments, (2) to review assessments and related documents, and (3) to develop
guidelines for exposure assessments. The activities under each of these
functions are supported by and respond to the needs of the various program
offices. In relation to the third function, EAG sponsors projects aimed
at developing or refining techniques used in exposure assessments.
The purpose of this document is to provide guidance to exposure
assessors regarding accepted methodologies for estimating or modeling
concentrations of contaminants in various environmental media. This is
presented as a companion document to the Exposure Factors Handbook (EPA
Publication No. 600/8-89/043) which provides algorithms and exposure
parameter values for estimating exposures based on contaminant concentra-
tions, which are the output of the methods described in this document.
The methodologies presented in this document are generic and are based
upon our evaluation of the available literature and accepted methods
employed by most exposure assessors. Some modification of these methods
may be necessary for scenarios that differ significantly from those repre-
sented here. The document is published in the three-ring binder format so
that it can be easily updated as new methodologies and information become
available.
Michael A. Callahan
Director
Exposure Assessment Group
xi

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PREFACE
The Exposure assessment Group (EAG) of the Office of Health and Envi-
ronmental Assessment (OHEA) has prepared this handbook as a companion doc-
ument to the Exposure Factors Handbook (EPA Publication No. 600/8-89/043).
It was prepared in response to requests from many EPA program and regional
offices for guidance on how estimated concentrations of chemicals in
various environmental media should be calculated.
The purpose of this Handbook is to provide a summary of the available
and accepted methods for estimating and modeling concentrations of chemi-
cals in ambient air, surface water, ground water, fish and shellfish,
terrestrial food sources, and soil. In addition, the Handbook provides,
where appropriate, default values for parameters used for making these
calculations. While originally intended for assessing environmental
concentrations resulting from releases from uncontrolled hazardous waste
sites, the Handbook will also provide a common resource that exposure
assessors in all Agency programs can use. Thus, it should help improve
the consistency with which exposure assessments are conducted across the
Agency, but still allow different approaches that may be appropriate based
on considerations of policy, precedent, or other factors.
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ABSTRACT
This document provides a summary of the available algorithms and
parameter values that can be used to determine estimated or modeled con-
centrations of chemicals in various environmental media, including ambient
air, surface water, ground water, fish and shellfish, terrestrial food
sources, and soil. These methods are organized and presented by environ-
mental media for the convenience of the user. The chapters on ambient
air, surface water, and ground water include quantitative treatment of
losses in concentrations of chemicals due to environmental fate processes.
This Handbook also provides an overview of the exposure assessment process
and how these methods are integrated within that process, a discussion of
methods for characterizing and enumerating exposed populations, and a dis-
cussion of methods for quantitating the statistical uncertainties related
to estimation of environmental concentrations. In addition, the Handbook
provides a series of worksheets that contain the step-by-step procedure
for employing the methods described.
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AUTHORS, CONTRIBUTORS, AND REVIEWERS
The Exposure Assessment Group (EAG) within EPA's office of Health and
Environmental Assessment was responsible for the preparation of this
handbook. The document was prepared by Versar Inc. under EPA Contract
No. 68-02-4254, Work Assignment No. 249. Seong Hwang of EAG served as
the EPA Task Manager, providing overall direction and guidance.
AUTHORS
Gary Whitmyre	RiskFocus Division
Versar Inc.
Springfield, Virginia
James Konz	Exposure Assessment Division
Clay Carpenter	Versar Inc.
Dan Arrenholz	Springfield, Virginia
Arthur Clarke
Norman Gabel
Karen Li si
Willi am Myers
Sanjeev Aggarwal
Among the authors, Mr. Gary Whitmyre was the Task Manager responsible
for coordinating the overall effort; determining the scope and technical
content of the manual; providing assistance in development of the chapters
related to estimation of concentrations in terrestrial food sources and
in fish/shellfish and to analysis of uncertainties; and creation of the
exposure assessment worksheets.
Mr. James Konz prepared chapters related to terrestrial food sources
and ground water. Mr. Clay Carpenter prepared chapters related to over-
view of the exposure assessment process and characterizing and enumerating
exposed populations. Mr. Dan Arrenholz was responsible for preparation of
the chapter on estimation of contaminant concentrations in surface water.
Dr. Arthur Clarke contributed to the chapter on analysis of uncertainties.
Dr. Norman Gabel and Ms. Karen Lisi researched and prepared sections deal-
ing with quantitative assessment of environmental fate. Mr. William Myers
and Mr. Sanjeev Aggarwal contributed to chapters that addressed estimation
of contaminant concentrations in soil and ambient air, respectively.
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REVIEWERS
The following individuals within EPA reviewed an earlier draft of
this document and provided valuable comments:
Mr. Randall Bruins
Environmental Criteria and Assessment Office
Cincinnati, Ohio
Ms. Lynn Del pi re
Office of Toxic Substances
Washington, D.C.
Mr. Charles Ris
Office of Research and Development
Washington, D.C.
Ms. Terri Knonza
Office of Research and Development
Washington, D.C.
The following individuals outside of EPA also reviewed this document
and provided helpful comments and suggestions:
Mr. Mitchell Small
U.S. Army Biomedical Research and Development Laboratory
Fort Dietrick, Maryland
Dr. Thomas McLaughlin
CH2M HILL
Herndon, Virginia
ACKNOWLEDGEMENTS
Quality assurance was provided by Mr. Clay Carpenter, Mr. James Konz,
and Ms. Cindy Lewis. Computer-assisted literature searches and informa-
tion retrieval were provided by Ms. Mimi Faha. Technical editing support
was provided by Ms. Barbara Malczak, Ms. Martha Martin, and Ms. Juliet
Crumrine; word processing was provided by Ms. Lynn Maxfield and Ms. Kammi
Johannsen; and graphics were prepared by Ms. Kathy Bowles. The dedicated
assistance of these Versar Inc. employees allowed the successful comple-
tion of this project and is sincerely appreciated.
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1.	INTRODUCTION
1.1 Background
Over the past few years, exposure assessment has played an increasing-
ly important role in many environmental programs at the federal, regional,
and state levels. Exposure assessment has been recognized as an important
component of the risk assessment of chemical releases. Because an expo-
sure assessment requires examination of the entire history of a chemical
in the environment, from release at the site to arrival at the receptor,
a multidisciplinary approach is needed that encompasses the expertise of
engineers, hydrologists, geologists, meteorologists, biologists, chemists,
and toxicologists. Exposure assessment can also be a valuable tool for
examining different options for remediation based on exposure impacts, and
therefore it is also useful in the context of risk management.
Approaches to exposure assessment that have been applied over the past
decade include (USEPA 1988a) the following:
•	Direct measurement methods, which attempt to estimate the
concentrations of pollutants in environmental media to which an
individual or receptor population is exposed;
•	Predictive methods, which attempt to estimate exposure based on
available knowledge of source characterization, the behavior of
pollutants in the environment, and exposure parameters relevant
to receptors (e.g., inhalation rates and frequency and duration
of exposures); and
•	Reconstructive methods, which attempt to reconstruct exposure
information from evidence within an organism such as concentra-
tions in body tissues, toxicological indicators, or other inter-
nal evidence.
All three of these approaches are potentially useful in exposure
assessment. While this Handbook focuses primarily on predictive methods,
it contains elements of the other two approaches as well.
The Exposure Assessment Methods Handbook is intended to serve as a
support document to EPA's Guidelines for Estimating Exposures (USEPA
1986a) and the Proposed Guidelines for Exposure-Related Measurements
(USEPA 1988a) by providing data on standard methods that are used to
1-1

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calculate environmental concentrations of toxic chemicals. The guidelines
were developed to promote consistency among the various exposure assess-
ment activities that are carried out by the various EPA program offices.
This Handbook should assist in this goal by providing a consistent frame-
work by which to calculate environmental concentrations. This Handbook
serves as a companion document to the Exposure Factors Handbook (USEPA
1989) which contains the standard factors needed to calculate exposure.
1.2 Purpose and Scope
This Handbook provides a compendium of selected available approaches
to assessment of multimedia exposures from waste disposal facilities.
However, while the release portions of this report are specifically
tailored to waste disposal settings, many of the approaches relating to
fate and exposure are more generic and could be applied to other types of
exposure settings.
The purpose of this Handbook is not only to acquaint exposure
assessors with the release, fate, and exposure algorithms necessary to
carry out an exposure assessment for waste disposal facilities, but also
to provide a step-by-step procedure for performing such an assessment.
This step-by-step procedure is presented in the worksheets that accompany
this report (see Appendix B); these worksheets in a sense represent the
most practical output of this effort. The worksheets and the information
contained within this report do not assume that the user has prior
knowledge of specific models, algorithms, or terminology. However, by
using this document, exposure assessors and others should be able to:
•	Understand the key assumptions, methodology, and limitations
relating to exposure assessment;
•	Understand the basic differences between options for approaching
exposure assessment;
•	Identify sources of information on exposure assessment methodolo-
gies and relevant parameter values; and
•	Comprehend the degree of uncertainty associated with exposure
estimates.
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Although this document contains a step-by-step procedure for perform-
ing exposure assessments of uncontrolled hazardous waste sites, it should
not be used as a substitute for review and consultation with more experi-
enced exposure assessors, environmental fate specialists, modelers, and
other environmental professionals.
1.3 Organization of the Handbook
Following several introductory sections, this Handbook is primarily
divided according to exposure pathways; these sections cover specific
exposure pathways and present the environmental release, fate, and concen-
tration considerations and estimation algorithms relevant to each pathway.
Part I contains media-specific information to be used in the estimation
of environmental concentrations. Part II provides discussions of popula-
tions and uncertainties, and presents the references cited. The specific
sections are as follows for Part I: Section 2 provides an overview of the
exposure assessment process; Section 3 presents methods for quantifying
environmental concentrations in surface water; Section 4 discusses
methodologies for assessing environmental concentrations in ground water;
Section 5 describes quantitative approaches for assessing chemical concen-
trations in fish and shellfish; Section 6 addresses methods for assessing
chemical concentrations in the terrestrial food chain; Section 7 discusses
methodologies for assessing chemical concentrations in ambient air; and
Section 8 presents approaches to determination of chemical concentrations
in soil. In Part II, Section 1 provides information for characterizing
and enumerating exposed populations; Section 2 discusses the sources of
uncertainty in estimates of environmental concentrations and methods for
quantifying uncertainty; and Section 3 presents cited references.
Several appendices are also included in this document. Appendix A
provides a glossary of terms used throughout the report. The format of
the Exposure Assessment Worksheets, which incorporate the environmental
release, fate, concentration, and exposure estimation algorithms is
provided in Appendix B. Completed Exposure Assessment Worksheets for an
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example chemical are presented in Appendix C. Transfer coefficients that
can be used to assess exposures to contaminants in animal-derived food
products are listed in Appendix D.
1-4

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2.	OVERVIEW OF THE EXPOSURE ASSESSMENT PROCESS
Planning should be the first step in any exposure assessment; deter-
mining the purpose, scope, depth, and approach will ultimately affect the
quality and usefulness of the resulting exposure estimates. By addressing
these elements in the planning stage, the assessor will focus on the most
critical aspects of the exposure assessment and will conserve resources by
eliminating unnecessary scenarios and inappropriate exposure pathways.
This will allow the assessor to identify the procedures to be used, to
establish the boundaries of the assessment, to facilitate its interfacing
with other assessments (e.g., human health risk assessments), and to
determine and control the final form of the assessment.
2.1	Methodological Framework
While each exposure assessment may be different, certain elements are
common to most exposure assessments. In general, the following major
topics are usually addressed: background information about the chemical;
sources of the chemical's releases; environmental pathways and fate
analysis (transport and transformation); development of the exposure
scenarios that are specific to the individual exposure assessment; summary
of monitoring data and/or prediction of environmental concentrations;
estimation of the exposure/dose levels; evaluation of the exposed human
populations; and integration of the route-specific exposure analyses.
The relationships among these components is shown in Figure 2-1.
2.2	Elements of Exposure Assessment
2.2.1 General Information
The exposure assessment process will usually be initiated by gathering
of information on the chemical of concern and compiling a brief descrip-
tion of these data. Each chemical should be identified by as many of the
following characteristics as appropriate: molecular formula and struc-
ture; synonyms; Chemical Abstracts Service (CAS) Registry Number; and
description of likely contaminants. The available data on chemical and
physical properties should also be tabulated; where data are lacking but
2-1

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ro
i
ro
FIGURE 2-1. FRAMEWORK AND COMPONENTS OF AN EXPOSURE ASSESSMENT

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are important for characterizing the chemical itself or its potential
transport or tranformation in the environment, the critical properties
should be estimated by the use of appropriate calculations such as those
available in the CHEMEST system (part of GEMS, NUMERICA) and presented by
Lyman et al. (1982). These properties include the molecular weight,
density, melting point, boiling point, vapor pressure, vapor density,
water solubility, partition coefficients, soil sorption coefficient,
chemical half-lives in environmental media, and other properties, as
appropriate.
2.2.2 Estimation of Release
The estimation of the environmental release of a chemical involves the
identification of all known or suspected sources, accounting for specific
releases to each of the environmental media or compartments (e.g., air,
surface water, ground water, soil, biota). The releases should be
characterized in as much detail as possible concerning the amounts of the
chemical and known or estimated rates of the release.
A detailed exposure assessment may include or be based on a previous
"materials balance." The materials balance is a thorough accounting of
the source locations, production, uses, destruction/disposal, and environ-
mental release of the chemical, quantifying the mass that was produced,
incorporated into products, and lost to the environment. This materials
balance may attempt to quantify chemical releases as a function of time,
especially if it is expected that releases are intermittent or sporadic,
as well as to describe the physical characteristics and magnitude of the
source and the physical and chemical form of the chemical being released.
Aqueous releases to receiving waters are often referred to as effluents or
discharges; atmospheric releases are often termed emissions; and releases
to soil may be described as leachate, spills, or leaks. In the absence
of complete mass-balance information, methods for estimating releases of
a given chemical into environmental media are provided in the following
sections.
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2.2.3 Environmental Fate Analysis/Determination of Environmental
Concentrations
This component of the exposure assessment is a description of the
route the chemical follows as it travels from the source to the receptor
population and of how much of the chemical degrades and/or transforms to
other chemicals during transport through the environment. The environmen-
tal fate analysis provides information on the extent and magnitude of
environmental contamination, including the concentrations of the contami-
nant in each of the specific compartments of the environment. In the ab-
sence of direct data on pathways and fate of a given chemical, this infor-
mation can often be predicted based on the physical-chemical properties
of the chemical. In addition, chemical fate can be predicted by the use
of a number of available mathematical models. The degree of specificity
of the environmental pathways and fate estimates is dependent on the level
of detail required of the overall exposure assessment for the individual
chemical. Some environmental routes of exposure or some transformation
processes may be judged to be insignificant and can be excluded from
further consideration.
For a detailed exposure assessment, the fate analysis should describe
each of the transport and transformation processes of the chemical,
because the ultimate environmental distribution and concentrations at the
receptors will differ considerably from concentrations in the initial
releases and levels in the immediate area of the release. The transport
analysis should include hydrological, atmospheric, and/or other physical
processes that describe the routes of the contaminant through each com-
partment and across intercompartmental boundaries from the source to the
receptor. The behavior of the chemical should be assessed at each bound-
ary, including the air/water, the soil/air, and the water/soil interfaces.
This environmental partitioning assessment is important because transport
processes may dilute the chemical or may serve to concentrate the chemi-
cal in certain environmental compartments.
Transformation processes also must be assessed for the chemical in
each of the environmental compartments of interest. Chemical alteration
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may occur via (1) biodegradation; (2) photodegradation; (3) hydrolysis,
oxidation/reduction, and other chemical degradation processes; and
(4) reactions with other chemicals.
Once the detailed transport and transformation pathways have been
identified, one can focus on the individual exposure pathways for more
thorough analyses, depending on the relative importance of different expo-
sure pathways and routes for the chemical.
2.2.4	Development of Exposure Scenarios
Information on the environmental fate and exposure pathways of the
chemical allows the assessor to predict the types of human receptor popu-
lations that could be exposed and the routes by which exposure could
occur. For some chemicals, the exposure routes to be studied may have
been selected at the beginning of the exposure assessment process in order
to focus on specific concerns.
Exposure scenarios should be developed for each relevant major expo-
sure pathway; these may include contact via air, ground water, surface
water, food, and. contaminated soil. Exposure scenarios account for the
route of contact (i.e., dermal, inhalation, and ingestion) and important
exposure parameters such as frequency of contact, duration of each expo-
sure event, and rate of contact (i.e., rate of inhalation of ambient air,
rate of ingestion of drinking water, and rate of ingestion of contaminated
food). In addition, other types of exposure parameters may need to be
specified as part of the development of exposure scenarios. For example,
development of an exposure scenario for inhalation exposure to volatile
organic compounds released from indoor home use of contaminated surface
water would require information or assumptions regarding room volume and
ventilation rate.
2.2.5	Estimation of Exposure/Dose
There are two basic methods of determining concentrations of chemicals
in environmental media, from which an environmental exposure value can be
determined; these are monitoring data and estimates based on models.
2-5

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Monitoring is the repeated sampling and analysis of different locations
within selected compartments of the environment to determine the levels of
the chemical and how those levels change with time. At best, most moni-
toring concentration values represent short-term averages. Thus, monitor-
ing data must be carefully reviewed to evaluate their precision and
accuracy and whether the sampling locations and durations used provide
representative values. Because monitoring data are usually neither com-
plete nor comprehensive for each environmental compartment, it is often
desirable to supplement the concentration data with modeling estimates.
In estimating exposure, the exposure assessor should distinguish
between exposure and dose. For some chemicals, the external exposure
resulting from the concentration in the environment to which the receptor
is exposed may be substantially greater than the internal exposure, or
absorbed dose incurred by the receptor. The absorbed dose is a function
of the degree of penetration through and chemical transfer across barriers
(e.g., skin) and membranes (e.g., the lining of the lung) that it con-
tacts; thus, different exposure routes are often associated with different
rates of absorption.
For most exposure assessments, the dose is estimated based on external
exposure, because the human health/animal toxicological data used in com-
bination with exposure data to calculate risk are most frequently based on
administered dose or ambient exposure rather than internal dose. In order
to add exposures incurred from different exposure routes, as discussed in
Section 2.2.6, the absorbed dose is the desired common denominator for
different routes; however, calculation of exposure route-specific absorbed
doses calls for a level of sophistication that is well beyond (1) the
limited available data on pharmacokinetic factors for most chemicals and
(2) the screening-level purpose of most exposure assessors using this
document. Therefore, this document will be directed at external exposure
rather than absorbed dose.
EPA's Exposure Assessment Guidelines (USEPA 1986a; 1988a) define expo-
sure as the contact with a chemical or physical agent. The magnitude of
2-6

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the exposure is the amount of the agent available at human exchange bound-
aries (skin, lungs, gut) during some specified time. Starting with a
general integral equation for exposure (USEPA 1988a), several exposure
equations can be derived depending upon boundary assumptions. One of the
more useful of these derived equations for dealing with lifetime exposures
to agents with linear non-threshold responses (i.e., our current assump-
tions about many carcinogens) is the Lifetime Average Daily Exposure
(LADE) discussed below. Exposure assessments are usually done to support
risk assessments; only exposure calculations used to support cancer risk
assessments and repeated and prolonged (chronic) exposures to noncarcino-
gens will be covered in this Handbook. The Proposed Guidelines for
Exposure-Related Measurements (USEPA 1988a) contain an expanded discussion
of some of the other equations that can be used. The Exposure Factors
Handbook (USEPA 1989) discusses exposure parameters and factors used to
estimate exposure.
For cancer risk assessments, exposure is averaged over the body weight
and lifetime:
iAr\c	Total Exposure		(? n
Body Weight x Lifetime	* '
The total exposure can be expanded as follows:
Media
Total ExDosure = Chemical Contact Exposure (?
p	Concentration Rate Duration ' '
The chemical concentration term above is the concentration of the
contaminant in the medium (air, food, soil, etc.) contacting the body and
has units of mass/volume or mass/mass.
The contact rate refers to the rate of inhalation, ingestion, or
dermal contact depending on the route of exposure. For ingestion, the
contact rate is simply the amount of food, soil, or water containing the
chemical of interest that an individual ingests during some specific time
period (units of mass/time).
2-7

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The exposure duration is the length of time that contact with the con-
taminated environmental media lasts. The time a person lives in an area,
frequency of activities leading to exposure (e.g., bathing), time spent
indoors vs. outdoors, etc., may all affect the exposure duration. When
the above parameters remain constant over time, duration may be substi-
tuted directly into the exposure equation. When they change with time, a
summation approach is needed to calculate exposure. In either case, the
exposure duration is the length of time exposure occurs at the concentra-
tion and contact rate specified by the other parameters in the equation.
Exposure (sometimes called "administered dose") can be expressed as a
total amount (with units of mass, e.g., mg), as an exposure rate in terms
of mass/time (e.g., mg/day), or as a rate normalized to body mass (e.g.,
with units of mg of chemical per kg of body weight per day (mg/kg-day).
The LADE is usually expressed in terms of mg/kg-day or other mass/mass-
time units..
The lifetime term in Equation (2-1) is the total period of time over
which the administered dose is averaged. For carcinogens, this should
represent the average life expectancy of the exposed population. For
exposure estimates to be used for assessments other than carcinogenic
risk, different averaging periods are frequently used. For acute expo-
sures, the administered doses are usually averaged over a day or single
exposure event. For chronic noncancer effects, the time period used is
often the actual period of exposure (e.g., length of occupational expo-
sure). The objective in selecting the averaging time is to express the
exposure in a way that makes it comparable to the dose-response relation-
ship used in conjunction with the exposure estimate to calculate risk.
The body weight used to calculate the total exposure in the above
equation should reflect the average weight of the exposed population
during the time when the exposure actually occurs. If the exposure occurs
continuously throughout an individual's life or only during the adult
ages, using an adult average weight should provide sufficient accuracy.
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However, when the exposure is limited to childhood, the weight represent-
ing those ages should be used. More detailed information for these expo-
sure parameters is presented in the Exposure Factors Handbook (USEPA
1989).
2.2.6 Estimation of Integrated Exposure
Integration of exposures is the final step in a multimedia exposure
assessment. It usually involves summation of exposures within a given
exposure route (dermal, inhalation, ingestion) across n relevant
exposure pathways (e.g., ambient air, surface water, ground water, food).
In other words,
n
Total route-specific exposure = £ Exposure ^	(2-3)
i=l
Multimedia exposures are frequently not integrated across multiple
exposure routes. This is true for the following reasons:
•	Some combinations of exposure routes are not realistic; for
example, some persons exposed via ambient air near the site may not
be the same individuals who are exposed via drinking water further
away.
•	Different exposure routes for a chemical are often associated
with different absorption rates.
•	Some risk assessments to which exposure estimates provide inputs
must be performed on a route-specific basis (because of differences
in the potency and toxicity endpoint of some compounds according
to exposure route).
•	EPA's regulatory needs often require separate exposure-pathway-
specific estimates to facilitate formulation and evaluation of
control options for specific environmental media.
In other words, the potential application of the exposure numbers dictates
the manner in which exposures are to be integrated.
Assessment of total human exposure is an emerging science that focuses
on the activities of populations and individuals that bring them into
contact with the chemicals of concern. The methodology used in this
2-9

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Handbook can be used to estimate the concentrations of chemicals and
resulting exposures at human physical boundaries (e.g., skin, lungs,
gastrointestinal tract), regardless of whether the chemicals arrive at the
receptor via air, water, food, or other exposure pathways.
Total human exposure assessment methodologies are often limited by
the simplistic nature of developed exposure scenarios and frequently-used
default values for parameters. Such integrated approaches may not always
account specifically for variations in exposure parameters or for the
multitude of microenvironments in which people are exposed via a given
exposure pathway and a given exposure route on a daily basis. Prioritiza-
tion of microenvironments for an exposure assessment is dependent upon
human activities (i.e., durations and frequencies of exposures in differ-
ent microenvironments), pollutant concentrations encountered in those
microenvironments, and exposure parameters such as inhalation rate that
vary according to microenvironments and associated activities.
Initially, examination of human activity patterns can identify candi-
date microenvironments. Knowledge of the amount of time individuals spend
performing various activities in specific microenvironments during the
course of a day can provide estimates of the duration and frequency of
exposure. Thus, time-use studies (see Exposure Factors Handbook (USEPA
1989)) provide the type of information needed to initially prioritize
microenvironments for selecting exposure pathways to be integrated. When
activity data are combined with pollutant concentration data, a clear
profile of the relative importance of various exposure pathways becomes
apparent; an illustration of this is shown for inhalation exposure to
nitrogen dioxide (NC^) in Figure 2-2. When such information is not
readily available, a conservative approach that accounts for all potential
exposure routes should be taken.
Equations to be used in calculating exposure for each exposure medium
and route in the final integrated assessment are contained in the Exposure
Factors Handbook (USEPA 1989). The exposure assessor is referred to the
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MICRO ENVIRONMENTS
TOTAL HUMAN EXPOSURE —
TO AIR POLLUTION ~~
POLLUTANT EXPOSURE
IN RESIDENCES
POLLUTANT EXPOSURE
IN OUTDOOR AIR
POLLUTANT EXPOSURE
IN BUILDINGS
(OFFICE. SCHOOL, ETC.)
POLLUTANT EXPOSURE
IN VEHICLES
(CAR, PLANE, TRAIN, ETC )
POLLUTANT
CONCENTRATION
(ppb)
140
I 20- —
100
rv>
i
-120
-100
,4°	POLLUTANT
CONCENTRATION
(PPt>)
0000 0300 0600 0900 1200 1500 1800 2100 2400
TIME (HOUR OF THE DAY)
24-HOUR EXPOSURE PROFILE
SUMMARY OF MICROENVIRONMENT
CONTRIBUTION TO TOTAL EXPOSURE
(SHADED AREA IS TOTAL INDOOR AIR CONTRIBUTION)
Figure 2-2. Representations of microenvironment contributions to total inhalation exposure to nitrogen dioxide.
Source: Adapted from Sexton and Ryan (1987); exposure data are for N02-

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Exposure Assessment Worksheets in Appendix B for the step-by-step
procedure for estimating exposures using algorithims contained within
this Handbook and to Appendix C for completed worksheets for an example
chemical.
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3.	ASSESSMENT OF EXPOSURE VIA SURFACE WATER
3.1 Release Analysis - Surface Water
People can be exposed to chemical contaminants in surface water that
result from many sources. Contaminants may be delivered to surface water
in precipitation or through dry deposition (i.e., dust) from the
atmosphere. Contaminated ground water may discharge into the surface
water body of concern. Surface runoff from contaminated sites may carry
contaminated soils or dissolved contaminants to surface water. On-site
treatment processes, where they exist, may result in direct discharge of
contaminants, via the treated effluent, into the surface water body.
This section will emphasize surface runoff releases as the major source
of contaminant entry into surface water.
3.1.1 Screening of Release Potential
In order for chemical contaminants to be available for surface runoff,
they must first be present in the upper layer (1 cm) of soil; if they are
deeper than 1 cm, the contaminants will likely adsorb in various amounts
to the soil or be leached to ground water. Contaminants may be present
in soil because of (1) unintentional releases, such as spills, leaks, or
container failure; (2) intentional surface application of chemical sub-
stances, such as occurs during landfarming of wastes; or (3) deposition
from atmospheric sources. Hydrophobic substances, in particular, can be
expected to readily sorb to soils on site; substances adsorbed to environ-
mental matrices such as soil make up the largest proportion of substances
of concern found at uncontrolled hazardous waste sites.
Once the contaminants are present in the upper soil layer, they are
subject to a variety of fate processes (e.g., volatilization, photolysis)
that may reduce their concentrations in the soil (see Section 8.2). The
remaining contaminants, however, will be available for runoff during
storm events. They will be present in the runoff stream either adsorbed
to suspended soils or dissolved in the runoff, depending on the
individual properties of the chemicals involved.
3-1

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3.1.2 Quantitative Release Estimation
Once it has been determined that the contaminants are present in the
upper soil layer and are subject to removal via runoff, it is necessary to
estimate quantitatively the amount of contaminants that will migrate off
site.
The amount of a given hydrophobic compound released in surface runoff
can be estimated using the Modified Universal Soil Loss Equation (MUSLE)
and sorption partition coefficients derived from the compound's
octanol-water partition coefficient. An estimation of the amount of soil
eroded in a storm event of given intensity can be obtained using MUSLE.
By using the sorption partition coefficients, one can estimate the
amounts of contaminants carried along with the soil and the amounts
carried in dissolved form.
(1) Soil loss calculation. The Modified Universal Soil Loss
Equation (Williams 1975), as presented in Mills et al. (1982), is given
in Equation 3-1. Equations 3-2 through 3-5 are provided to guide calcula-
tion of certain input parameters required to utilize Equation 3-1.
Y(S)e = a(Vrqp)°-56KLSCP	(3-1)
where
Y(S)f = sediment yield (tons per event, metric tons per event)
a = conversion constant (95 English, 11.8 metric)*
Vr = volume of runoff (acre-feet, m3)
qc = peak flow rate (cubic feet per second, nr/sec)
K = the soil-erodibility factor; a value of K for a given site
can be obtained from the local Soil Conservation Service
office. This should be treated as a dimensionless number.**
L = the slope-length factor (dimensionless ratio)
S = the slope-steepness factor (dimensionless ratio)
* Metric conversions presented in the following runoff contamination
equations are from Mills et al. (1982).
** In other situations, K is commonly expressed in terms of tons per
acre per dimensionless rainfall erodibility unit. In this equation,
the dimensionless number is treated the same regardless of whether
other parameters are expressed in English or metric units.
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C = the cover factor (dimensionless ratio = 1.0 for bare soil;
see the following discussion for vegetated site "C" values)
P = the erosion control practice factor (dimensionless ratio =
1.0 for uncontrolled hazardous waste sites).
Storm runoff volume is calculated as follows (Mills et al. 1982):
Vr = aAQr	(3-2)
where
Vr = storm runoff volume
a = conversion constant (0.083 English, 100 metric)
A = contaminated area (acres, ha)
Qr = depth of runoff (in, cm).
Depth of runoff, Qr, is determined by (Mockus 1972):
Qr = (Rt - 0.2Sw)2/(Rt + 0.8SW)	(3-3)
where
Qr = the depth of runoff from the watershed area (in, cm)
Rt = the total storm rainfall (in, cm)
Sw = water retention factor (in, cm).
The value of Sw, the water retention factor, is obtained as follows
(Mockus 1972):
Sw = poafi - lOj a	(3-4)
where
Sw = water retention factor (in, cm)
CN = the SCS Runoff Curve Number (dimensionless, see Table 3-1)
a = conversion constant (1.0 English, 2.54 metric).
The CN factor is determined by the type of soil at the site, its
condition, and other parameters that establish a value indicative of the
tendency of the soil to absorb and hold precipitation or to allow precipi-
tation to run off the surface. The analyst can obtain CN values of uncon-
trolled hazardous waste sites from Table 3-1. If further information on
CN values is desired, particularly for agricultural situations, the
analyst can refer to Novotny and Chesters (1981) for additional CN values
3-3

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/ uccn
Table 3-1. Runoff Curve Numbers
	Site type	
Overall Road/right of May Meadow	Woods
Soil group	Description	site3
Lowest runoff potential: Includes	59	74	30	45
deep sands with very little silt and
clay; also deep, rapidly permeable
loess (infiltration rate = 8-12 mn/h).
B	Moderately low runoff potential:	74	84	58	66
Mostly sandy soils less deep than A,
and loess less deep or less aggregated
than A. but the group as a whole has
above-average infiltration after
thorough wetting (infiltration
rate = 4-8 mm/h).
C	Moderately high runoff potential:	82	90	71	77
Conprises shallow soils and soils
containing considerable clay and
colloids, though less than those of
group D. The group has below-
average infiltration after presatura-
tion (infiltration rate = 1-4 m/h).
0	Highest runoff potential: Includes	86	92	78	83
mostly clays of high swelling percent,
but the group also includes scrae
shallow soils with nearly impermeable
subhorizons near the surface
(infiltration rate = 0-1 nq/h).
Source: Adapted from Schwab et al. 1966.
aValues taken from farmstead category, which is a conposite including buildings, farmyard, road, etc.
3-4

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and more background information. The local Soil Conservation Service
office may also be helpful.
The peak runoff rate, qp, is determined as follows (Haith 1980):
a - aARtQr	<3-5)
p VRt-°-2V
where
qp = the peak runoff rate (ft3/sec, m3/sec)
a = conversion constant (1.01 English, 0.028 metric)
A = contaminated area (acres, ha)
Rj- = the total storm rainfall (in, cm)
Qr = the depth of runoff from the watershed area (in, cm)
Tr = storm duration (hr)
Sw = water retention factor (in, cm).
The soil erodibility factor, K, is indicative of the erosion potential
of given soil types; it is, therefore, highly site specific. Values for
soil erodibility factors are given in Table 3-2. Values for K for sites
under study are available from the local Soil Conservation Service
office. Additional information on determining values for K can also be
obtained from Wischmeier et al. (1971).
The slope length factor, L, and slope steepness factor, S, are common-
ly entered into the MUSLE as a combined topographic factor, LS. This can
be obtained from Figures 3-1 through 3-3.
The vegetative cover factor, C, is determined by the amount and type
of vegetative cover present at the site. For bare soils a value of "1"
is used. Tables 3-3 and 3-4 should be used to obtain C values for sites
with vegetative covers.
The factor, P, refers to any erosion control practices that may be
used on the site. These practices include contouring, compacting, estab-
lishing sediment basins, and other control practices. Some representative
values of P for agricultural land and for construction sites are given in
Tables 3-5 and 3-6, respectively. A conservative, worst-case value of "1"
should be used for abandoned or uncontrolled hazardous waste sites, where
erosion control practices are unlikely to be used.
3-5

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7622H
Table 3-2. Magnitude of Soil Erodibility Factor Ka
K for organic matter content (X)
Textural class	<0.5	2	4
Sand	0.05	0.03	0.02
Fine sand	0.16	0.14	0.10
Very fine sand	0.42	0.36	0.28
Loamy sand	0.12	0.10	0.08
Loamy fine sand	0.24	0.20	0.16
Loamy very fine sand	0.44	0.38	0.30
Sandy loam	0.27	0.24	0.19
Fine sandy	loam 0.35	0.30	0.24
Very fine sandy loam	0.47	0.41	0.33
Loam	0.38	0.34	0.29
Silt loam	0.48	0.42	0.33
Silt	0.60	0.52	0.42
Sandy clay	loan 0.27	0.25	0.21
Clay loam	0.28	0.25	0.21
Silty clay	loam 0.37	0.32	0.26
Sandy clay	0.14	0.13	0.12
Silty clay	0.25	0.23	0.19
Clay	0.13-0.2
a The values shown are estimated averages of broad ranges of
specific soil values. When a texture is near the borderline
of two texture classes, use the average of the two K values
(Novotny and Chesters 1981).
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9uti
e J-i
«O0 S/^
6°0
°°
not?.
So,
Urce: he das^
^'°Pe
'**£ »s
Hins
3.7
fo.

shi.
et
^u]
n9t,
("Lbev
on.
*1. '¦ »ey0nd
Or,
'e9on.
<32)
"
-------
Figure 3-2. Soil moisture-soil temperature regimes of the Western United States.
Source: USDA (1974).
3-8
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Slope Length, Meters
3.5 6.0 10 20 40 60 100 200 4O0 6O0
Slope Length, Feet
Figure 3-3. Slope effect chart for areas where Figure 3-2 is not applicable.
NOTE: The dashed lines represent estimates for slope dimensions beyond the range of lengths
and steepnesses for which data are available.
Source: USDA (1974).
3-9
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Table 3-3. "C" Values for Permanent Pasture. Rangeland, and Idle Land
Vegetation canopy Canopy Cover that contacts the surface
(type and height coverc Percent ground cover
of raised canopy)b (X) Type 0 20 40 60 80 95-100
No appreciable canopy

G
0.45
0.20
0.10
0.042
0.013
0.003

W
0.45
0.24
0.15
0.090
0.043
0.011
Canopy of tall weeds
25
G
0.36
0.17
0.09
0.038
0.012
0.003
or short brush

W
0.36
0.20
0.13
0.082
0.041
0.011
(0.5 m fall height)
50
G
0.26
0.13
0.07
0.035
0.012
0.003

U
0.26
0.16
0.11
0.075
0.039
0.011

75
G
0.17
0.10
0.06
0.031
0.011
0.003

U
0.17
0.12
0.09
0.067
0.038
0.011
Appreciable brush
25
G
0.40
0.18
0.09
0.040
0.013
0.003
or brushes

W
0.40
0.22
0.14
0.085
0.042
0.011
(2 m fall height)
50
G
0.34
0.16
0.085
0.038
0.012
0.003

W
0.34
0.19
0.13
0.081
0.041
0.011

75
G
0.28
0.14
0.08
0.036
0.012
0.003