United States
Environmental Protection
Agency
Great Lakes
National Program Office
77 West Jackson Boulevard
Chicago, Illinois 60604
EPA 905-R93-007
December 1993
&EPA Assessment and
Remediation
Of Contaminated Sediments
(ARCS) Program
RISK ASSESSMENT AND
MODELING OVERVIEW
DOCUMENT
United States Areas of Concern
ARCS Priority Areas of Concern
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ASSESSMENT AND REMEDIATION OF CONTAMINATED SEDIMENTS
(ARCS) PROGRAM
RISK ASSESSMENT AND
MODELING OVERVIEW DOCUMENT
Great Lakes National Program Office
U.S. Environmental Protection Agency
77 West Jackson Boulevard
Chicago, Illinois 60604-3590
U.S. Environmental Protection Agency
Region 5, Library (PL-12J)
77 West Jackson Boulevard, 12th Floor
Chicago, IL 60604-3590
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DISCLAIMER
The information in this document has been funded wholly
or in part by the U.S. Environmental Protection Agency
(USEPA) under USEPA Contract Numbers 68-C1-0012 and
68-CO-0054 to AScI Corporation and under USEPA Con-
tract Number 68-C2-0134 to Battelle Ocean Sciences.
Mention of trade names or commercial products does not
constitute endorsement or recommendation for use by
USEPA.
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A CKNO WLEDGMENTS
The initial draft of this report was prepared through support from U.S. Environmental
Protection Agency (USEPA) to AScI Corporation of Athens, Georgia, under USEPA
Contract Numbers 68-C1-0012 and 68-CO-0054, and administered by USEPA's Environ-
mental Research Laboratory of Athens, Georgia. The primary authors of the initial draft
report were Dr. James Martin of AScI and Dr. Judy Crane, formerly of AScI, now of
E.V.S. Consultants of Vancouver, Canada.
This report was edited and produced by PTI Environmental Services (PTI) of Bellevue,
Washington, for Battelle Ocean Sciences of Duxbury, Massachusetts, under USEPA
Contract Number 68-C2-0134. Additions to the text of the initial draft were made by
Dr. Robert Pastorok and Ms. Lisa Yost of PTI.
This report was prepared for the Risk Assessment/Modeling Work Group as part of the
Assessment and Remediation of Contaminated Sediments (ARCS) Program administered
by USEPA's Great Lakes National Program Office (GLNPO) in Chicago, Illinois.
Dr. Marc Tuchman of GLNPO served as chairman of the Risk Assessment/Modeling
Work Group. Mr. David Cowgill of GLNPO and Dr. Tuchman served as the ARCS
Program managers.
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ABSTRACT
This document provides an overview of risk assessment and modeling methods as applied
to areas with contaminated sediments in the Great Lakes region. The document was
prepared under the Assessment and Remediation of Contaminated Sediments (ARCS)
Program, administered by the U.S. Environmental Protection Agency's (USEPA) Great
Lakes National Program Office (GLNPO), in Chicago, Illinois.
The goal of the risk assessment and modeling studies was to develop and demonstrate a
comprehensive human health and ecological risk assessment framework for use in the
evaluation of alternative remedial actions for contaminated sediments. As part of that
effort, risk assessment and modeling studies were performed at selected Areas of Con-
cern in the Great Lakes region. The goal of those studies was to provide estimates of
potential changes in exposure and risk that may occur either under a no-action alternative
or following implementation of various remedial alternatives for contaminated sediments.
The risk estimates may then be used to aid in the selection of an appropriate remedial
action. This document does not provide detailed guidance on risk assessment and model-
ing methods, but refers the reader to pertinent source documents for further information.
IV
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CONTENTS
Page
DISCLAIMER ii
ACKNOWLEDGMENTS iii
ABSTRACT iv
LIST OF FIGURES vii
LIST OF TABLES viii
ACRONYMS AND ABBREVIATIONS ix
1. INTRODUCTION 1
BACKGROUND 1
RISK MANAGEMENT FRAMEWORK 2
2. HUMAN HEALTH AND ECOLOGICAL RISK ASSESSMENTS FOR
CONTAMINATED SEDIMENTS 8
HUMAN HEALTH RISK ASSESSMENT 8
Data Review 10
Identifying Contaminants of Concern 11
Exposure Assessment 11
Toxicity Assessment 15
Risk Characterization 16
Uncertainty Analysis 18
Applications 19
ECOLOGICAL RISK ASSESSMENT 20
General Framework 21
Problem Formulation 24
Exposure Assessment 29
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Page
Ecological Effects Assessment 32
Risk Characterization 35
3. MASS BALANCE MODELING APPROACH FOR ASSESSING
REMEDIAL ALTERNATIVES AT CONTAMINATED SEDIMENT SITES 38
OVERVIEW 38
COMPLEXITY OF THE MASS BALANCE MODELING STUDY 40
COMPONENTS OF THE MASS BALANCE MODELING STUDY 42
Water Transport Models 42
Sediment Transport Models 44
Contaminant Exposure Model 45
Food Chain Model 46
REQUIRED FIELD DATA 47
Water Transport Data 48
Sediment Transport Data 48
Contaminant Exposure Data 49
Food Chain Data 52
MODEL APPLICATION 53
4. COMPARATIVE RISK ASSESSMENT 55
5. SUMMARY 58
6. GLOSSARY 60
7. REFERENCES 69
VI
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LIST OF FIGURES
Page
Figure 1-1. Overview of the comprehensive risk management process 4
Figure 2-1. Components of a human health risk assessment 9
Figure 2-2. Ecological risk assessment framework 22
Figure 2-3. Ecological assessment tools for contaminated sediments 28
Figure 3-1. Components of the mass balance modeling study 43
Figure 4-1. Comparative risk assessment in the risk management
process 56
VII
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LIST OF TABLES
Page
Table 2-1. Generic equation for calculating chemical intake levels 14
Table 2-2. Estimated carcinogenic and noncarcinogenic risks to
individuals residing in the lower Saginaw River Area of
Concern 17
Table 3-1. Examples of parameters measured for the ARCS RAM
mass balance modeling studies 51
VIII
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ACRONYMS AND ABBREVIATIONS
AOC
ARCS
Corps
CSF
CSO
GLNPO
IRIS
LOAEL
NOAEL
PCB
QA/QC
RAM
RAP
RfC
RfD
STORET
USEPA
USGS
Area of Concern
Assessment and Remediation of Contaminated Sediments
U.S. Army Corps of Engineers
cancer slope factor
combined sewer overflow
Great Lakes National Program Office
Integrated Risk Information System
lowest-observed-adverse-effect level
no-observed-adverse-effect level
polychlorinated biphenyl
quality assurance and quality control
Risk Assessment/Modeling
Remedial Action Plan
reference concentration
reference dose
storage and retrieval
U.S. Environmental Protection Agency
U.S. Geological Survey
IX
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1. INTRODUCTION
This document provides an overview of risk assessment and modeling methods as applied
to areas with contaminated sediments in the Great Lakes region. It was prepared under
the Assessment and Remediation of Contaminated Sediments (ARCS) Program, admini-
stered by the U.S. Environmental Protection Agency's (USEPA) Great Lakes National
Program Office (GLNPO) in Chicago, Illinois.
BACKGROUND
Although toxic discharges in the Great Lakes and elsewhere have been reduced in the last
20 years, persistent contaminants in sediments continue to pose a potential risk to human
health and the environment. High concentrations of contaminants in bottom sediments
and associated adverse effects have been well documented throughout the Great Lakes
and associated connecting channels. The extent of sediment contamination and its associ-
ated adverse effects have been the subject of considerable concern and study in the Great
Lakes community and elsewhere. Contaminated sediments can have direct toxic effects
on aquatic life, such as the development of cancerous tumors in fish exposed to polycyc-
lic aromatic hydrocarbons in sediments. The bioaccumulation of toxic contaminants in
the food chain can also pose a risk to humans, wildlife, and aquatic organisms. As a
result, advisories against consumption of fish are in place in many areas of the Great
Lakes. These advisories have also had a negative economic impact on the affected areas.
To address concerns about the deleterious effects of contaminated sediments in the Great
Lakes, Annex 14 of the Great Lakes Water Quality Agreement between the United States
and Canada stipulates that the cooperating parties will identify the nature and extent of
sediment contamination in the Great Lakes, develop methods to assess impacts, and eval-
uate the technological capability of programs to remedy such contamination. The 1987
amendments to the Clear Water Act, in § 118(c)(3), authorized GLNPO to coordinate
and conduct a 5-year study and demonstration projects relating to the appropriate treat-
ment of toxic contaminants in bottom sediments. Five areas were specified in the Act
as requiring priority consideration in conducting demonstration projects: Saginaw Bay,
Michigan; Sheboygan Harbor, Wisconsin; Grand Calumet River, Indiana; Ashtabula
River, Ohio; and Buffalo River, New York. To fulfill the requirements of the Act,
GLNPO initiated the ARCS Program. In addition, the Great Lakes Critical Programs
Act of 1990 amended the section, now § 118(c)(7), by extending the program by 1 year
and specifying completion dates for certain interim activities. ARCS is an integrated
program for the development and testing of assessment techniques and remedial action
alternatives for contaminated sediments. Information from ARCS Program activities will
help address contaminated sediment concerns in the development of Remedial Action
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Chapter 1. Introduction
Plans (RAPs) for all 43 Great Lakes Areas of Concern (AOCs), as identified by the
United States and Canadian governments, as well as lakewide management plans.
To accomplish the ARCS Program objectives, the following work groups were estab-
lished:
• The Toxicity/Chemistry Work Group was responsible for assessing the
current nature and extent of contaminated sediment problems in the five
priority AOCs by studying the chemical, physical, and biological charac-
teristics of contaminated sediments and their biotic communities, and for
demonstrating cost-effective assessment techniques that can be used at
other Great Lakes AOCs.
• The Risk Assessment/Modeling (RAM) Work Group was responsible for
assessing the current and future risks presented by contaminated sediments
to human and ecological receptors under various remedial alternatives
(including the no-action alternative) at the five priority AOCs,
• The Engineering/Technology Work Group was responsible for evaluating
and testing available removal and remedial technologies for contaminated
sediments, for selecting promising technologies for further testing, and for
performing field demonstrations at each of the five priority AOCs.
• The Communication/Liaison Work Group was responsible for facilitating
the flow of information from the technical work groups and the overall
ARCS Program to the interested public and for providing feedback from
the public to the ARCS Program on needs, expectations, and perceived
problems.
This document is intended to provide an overview of the risk assessment and modeling
methods developed by the ARCS RAM Work Group and to provide general guidance on
their application to other Great Lakes AOCs.
RISK MANAGEMENT FRAMEWORK
Sediment contamination is of concern primarily because of the potential risks it poses to
humans, wildlife, and aquatic organisms. Therefore, the management of contaminated
sediments includes the overall process of risk management. For this project, risk man-
agement is defined as the process of integrating findings from a risk assessment with
engineering, policy, and nontechnical concerns to make decisions about sediment reme-
diation at a specific site or to set remediation priorities among sites. Risk management
should be distinguished from risk assessment, which is the process of producing qualita-
tive or quantitative estimates of the potential risks associated with exposure to specific
concentrations of contaminants under specific current or future exposure conditions at a
site.
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Chapter 1. Introduction
The general objective of the ARCS RAM Work Group was to develop and demonstrate
a comprehensive risk management framework for: 1) identifying existing risks to human
health and ecological receptors at sites with contaminated sediments, 2) estimating the
potential impact of various sediment remedial alternatives on contaminant concentrations
in various media and their associated risks, and 3) comparing existing and potential
future risks to aid in the selection of sediment remedial alternatives.
Steps in the overall risk management process are illustrated in Figure 1-1. A general
discussion of each of the steps is provided below, followed by a more detailed descrip-
tion of the use of risk assessment and modeling in the ARCS RAM studies.
Step 1. Initial Screening of Potential AOCs: The first step in the risk man-
agement process involves the use of screening-level assessments to
identify sites that may pose a potential threat to human health or
ecological receptors based, in part, on sediment contamination. The
Great Lakes states, the U.S. and Canadian governments, and the
International Joint Commission have designated 43 AOCs around the
Great Lakes on the basis of impairment of beneficial uses. All but
one of these AOCs have been identified as having sufficient sediment
contamination to pose potential threats to human health or ecological
receptors.
Step 2. Risk Assessment Planning: In this step, existing information is first
compiled to describe the physical features of the AOC, the general
distribution of sediment contaminants and their potential sources, and
the human and ecological receptor populations likely to be present.
Contaminants of concern, biological species, endpoints (measured bio-
logical or ecological qualities), and primary exposure pathways for
human and ecological receptors are then identified for use in the risk
assessment. This information is used to develop preliminary remedial
action objectives, which are general descriptions of what remedial
actions should accomplish, including the reduction of risks associated
with exposure to contaminated sediments. Potential remedial actions
may then be identified. As part of risk assessment planning, deficien-
cies in the available data that might preclude an adequate baseline risk
assessment should be identified. Supplementary field sampling
(Step 3) may then be conducted if necessary. The risk assessment
planning step provides the organizational framework for the subse-
quent steps in the risk management process.
Step 3. Supplementary Field Sampling: If data gaps were identified as part
of the previous step, supplementary field sampling efforts may be
required to collect the information necessary for a detailed site
assessment. Additional information may need to be gathered on the
physical, biological, and chemical conditions of the system to further
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Modeling of Contaminant
Transport and Fate
f
Initial Screening of
Potential Areas of Concern
(AOCs)
f
Risk Assessment
Planning
t
Baseline Risk Assessment
f
Ranking of Subareas
within the AOC
••
I
Initial Screening of
Remedial Alternatives
-r
Comparative Risk
Assessment
f
F
Supplementary Field
Sampling
Selection and Implementation
of Final Remedial Action Plan
Post-Remediation Monitoring
Figure 1-1. Overview of the comprehensive risk management process.
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Chapter 1. Introduction
characterize the nature and extent of the sediment contamination prob-
lem. The data are also used to develop appropriate sediment remedial
alternatives, to support mass balance modeling, and to conduct the
comparative risk assessment of the remedial alternatives. A separate
ARCS Assessment Guidance Document (USEPA 1993) describing field
sampling methods is being prepared as part of the ARCS Program.
Step 4. Baseline Risk Assessment: A baseline risk assessment estimates
current risks to humans, wildlife, and aquatic organisms resulting
from direct and indirect exposure to contaminated sediments in the
absence of any sediment remediation. The baseline risk estimates,
developed using conservative, or health protective, assumptions are
used to determine which contaminants and exposure pathways pose
the greatest risk, to determine whether remediation is likely to be
required, and to provide a baseline against which any future remedial
action can be evaluated.
Step 5. Ranking of Subareas Within the AOC: Within a particular AOC,
there will be spatial variations in the concentrations and types of
sediment contaminants; variations in the risks the sediment contami-
nants pose to humans and ecological receptors resulting from varying
exposure potential, bioavailability, or toxicity; and variations in the
costs associated with sediment remediation. Available information on
sediment chemistry, toxicity tests, and benthic community structure
may be combined in a numerically based ranking system to prioritize
specific subareas within an AOC for remedial action. Additional
detail on sediment ranking procedures developed under the ARCS
Program is provided in the ARCS Assessment Guidance Document
(USEPA 1993). The results of the human health and ecological risk
assessments may be qualitatively considered along with the numerical
sediment ranking in this prioritization process.
Step 6. Initial Screening of Remedial Alternatives: There is a wide variety
of possible sediment remedial alternatives, only a few of which may
be practical at a particular site. This step in the risk management
process involves the selection of a limited number of possible reme-
dial alternatives (e.g., no action, in situ treatment, or removal alter-
natives) for further evaluation. Additional field sampling may be
required following the selection of the sediment remedial alternatives
to be evaluated. The selection of candidate subareas for sediment
remediation and possible remedial alternatives is based on a detailed
site assessment, which delineates the nature and extent of sediment
contamination within subareas of the AOC.
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Chapter 1. Introduction
Step 7. Modeling of Contaminant Transport and Fate: To assess the human
health and ecological risks posed by various sediment remediation
scenarios, contaminant releases must be estimated for each of the
remedial alternatives. Previous steps of the risk management process
provide information concerning the nature and extent of existing
sediment contamination and estimates of baseline human health and
ecological risks. However, those steps provide little information that
can be used directly to estimate changes that may occur as a result of
remediation. In this step, transport and fate models are used with
physical, chemical, and biological data for the AOC to evaluate the
effectiveness of the various remedial alternatives in reducing contam-
inant concentrations in environmental media of concern. Outputs
from these models may include predictions of contaminant concentra-
tions in air, water, soil, sediments, and biota based on present or
projected contaminant loadings or expected changes in contaminant
concentrations over time following remediation.
Step 8. Comparative Risk Assessment: The objective of this step is to esti-
mate changes in risks, relative to the baseline risk, that would result
from implementation of the various remedial alternatives evaluated.
For example, the comparative risk assessment can be used to estimate
the impacts of various remedial alternatives on human health risks
from consumption of contaminated fish over time. This assessment
integrates data from all previous steps into a risk assessment frame-
work. Ideally, this comparative risk assessment should include an
estimation of both the changes in risks at the AOC following sediment
remediation and the changes in risks at the site of disposal of the
contaminated sediments. The remedial action objectives that had been
developed under Step 2 are then refined during this step.
Step 9. Selection and Implementation of Final Remedial Action Plan: In this
step, information from the comparative risk assessment is used in
conjunction with other factors (e.g., economic, political) to select the
most appropriate remedial alternative(s) to implement.
Step 10. Post-Remediation Monitoring: The last step in the risk management
process is to monitor the AOC following sediment remediation to
demonstrate successful reductions in sediment contamination and
associated risks to human health and ecological receptors. Monitoring
should focus on parameters that have the greatest influence on risk
estimates and remedy selection. For example, if the human health
risk estimates are predominantly based on concentrations of polychlor-
inated biphenyls (PCBs) in fish, this parameter should be used as the
indicator of remedial effectiveness.
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Chapter 1. Introduction
The studies of the ARCS RAM Work Group have provided support for Steps 4-8 of the
risk management process. Other ARCS studies deal specifically with other aspects of the
decision-making process. The final results of the ARCS RAM studies are estimates of
contaminant concentrations and potential risks associated with various sediment remedial
alternatives that may then be used, along with other information collected at a site, to
select the appropriate remedial action from among the various alternatives. The
following sections of this document provide an overview of the use of risk assessment
and modeling in the ARCS RAM studies.
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2. HUMAN HEAL TH AND ECOL OGICAL RISK
ASSESSMENTS FOR CONTAMINATED
SEDIMENTS
As part of the ARCS RAM studies, baseline risk assessments were performed to estimate
the current health risks to humans and wildlife exposed to sediment-derived contaminants
in the absence of any remediation. The results of these assessments can be used by
ARCS risk managers in prioritizing sites and making decisions concerning the need for
sediment remediation. The risk assessment approach developed and used in the ARCS
Program is intended to produce conservative estimates of risk in order to ensure adequate
protection of human health and the environment. This approach to risk assessment is
specifically designed not to underestimate risks and, therefore, is likely to overestimate
risks at many sites. The following sections provide an overview of the risk assessment
approach used under the ARCS Program. This approach may be used to assess potential
human and ecological risks at other sites with contaminated sediments.
HUMAN HEAL TH RISK ASSESSMENT
Individuals in the Great Lakes region may be exposed to sediment contaminants through
various activities that result in intake of contaminants through dermal, ingestion, and/or
inhalation pathways. For the ARCS risk assessments, human health risk estimates were
determined for both carcinogenic effects (i.e., increased probability of an individual
developing cancer over a lifetime) and noncarcinogenic effects (i.e., chronic or
subchronic effects other than cancer) over a range of exposure scenarios. The risk
estimates were calculated by using conservative exposure assumptions and USEPA-
verified toxicity values called cancer slope factors (CSFs) and reference doses (RfDs)
(for noncarcinogenic effects). The primary guidance used to conduct these risk
assessments was obtained from USEPA's Risk Assessment Guidance for Superjund—
Volume I: Human Health Evaluation Manual (Part A) (USEPA 1989c), although the use
of additional USEPA guidance for risk assessment (USEPA 1988b, 1989a,b, 1991) is
also described herein. The following sections describe the main components used in
performing the ARCS human health risk assessments (Figure 2-1), including specific
examples and recommendations. Baseline human health risk assessments were conducted
under the ARCS Program for the five priority AOCs: Saginaw River, Michigan (Crane
1992b); Sheboygan River, Wisconsin (Crane 1993a); Grand Calumet River, Indiana
(Crane 1993b); Ashtabula River, Ohio (Crane 1992a); and Buffalo River, New York
(Crane 1993c). The same human health risk assessment framework can be applied to any
site with contaminated sediments.
8
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Example of an Exposure Pathway for
Recreational Fishing
Sediments
Basis for Selection of
Contaminants of Concern
Frequency of detections
Comparison with background
concentrations
Consideration of potential
laboratory contamination
Exposure Assessment
• Populations
• Pathways
• Exposure point
Concentrations
• Intake rates
Data Review
and Identification
of Contaminants
of Concern
Toxicity, persistence, and mobility
Carcinogens
Risk Characterization
• Carcinogenic and
noncarcinogenic risks
• Uncertainty assessment
Toxicity Assessment
Risk =
Intake x Cancer Slope Factor
Noncarcinogens
Hazard Index =
Site-specific Intake
Reference Dose
| Carcinogenic Effects
Use cancer slope factors
I Noncarcinogenic Effects
Use reference doses
Figure 2-1. Components of a human health risk assessment.
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Chapter 2. Human Health and Ecological Risk Assessment
Data Review
Prior to beginning the human health risk assessment, site data must be collected and
analyzed to determine contaminant concentrations in the media of interest (e.g., air,
water, sediments, biota) and potential routes of exposure to contaminated media. Avail-
able historical data, including information on site use and possible contaminant sources,
should be reviewed to focus sampling efforts on contaminants known or likely to be pres-
ent. Such data may include analytical data from previous sampling efforts, descriptions
of the past uses of the site or other site records, and interviews with site personnel that
may suggest what contaminants may be present. For example, RAPs contain information
about monitoring and scientific studies that have been conducted at Great Lakes sites.
USEPA's storage and retrieval (STORET) database is also a good source of water quality
data that are routinely collected at U.S. Geological Survey (USGS) and statewide gaging
stations. In addition, to determine current or future uses of the site, personnel at various
local, State, and Federal agencies that deal with public health, natural resource, and fish
and wildlife issues should also be contacted for information about the site. In particular,
applicable zoning regulations, land use plans, and restrictions on site uses (e.g., fishing
or hunting bans) should be described in the risk assessment. The risk assessor should
visit the site, preferably during the period of greatest activity or during several seasons,
to observe recreational and business uses of the AOC. It may also be helpful to inter-
view game wardens, lifeguards, and local officials regarding site use.
A thorough sampling program for sediments, water, fish, and other important media
should be conducted using appropriate quality assurance and quality control (QA/QC)
procedures, such as those identified in Guidance for Data Usability in Risk Assessment
(USEPA 1990). Water, sediment, and fish samples should be collected preferentially in
areas where people are known to be using the site, such as public beaches, or are likely
to use the site in the future, such as areas near shorelines or along access roads. Samples
should also be collected at one or more reference sites (i.e., areas that are unlikely to be
influenced by sediment contaminants within the AOC or by other anthropogenic sources).
In addition, several species of fish should be sampled, including species that feed on the
bottom, such as carp and catfish, and species that feed in the water column, such as
walleye. Both whole body and skin-on or skin-off fillets should be analyzed for various
organic and inorganic chemicals, especially those chemicals detected in the sediments and
known to bioaccumulate in fish tissue.
The need for air sampling should be evaluated on a case-by-case basis. Exposures to
contaminants in air are likely to contribute much less to overall site risks than exposures
via other pathways such as consumption of fish. Thus, air sampling is not generally
required for screening-level evaluations and was not conducted as part of the ARCS risk
assessments. However, the air pathway may be important at sites with volatile
contaminants of concern, where upland soils are exposed, or where areas of sediments
are dry for a substantial proportion of the time and thus may be a source of airborne
particulates. At some such sites, simple models may be used to estimate exposures to
contaminants via the air pathway.
10
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Chapter 2. Human Health and Ecological Risk Assessment
Once data from current and historical sources have been reviewed, the most appropriate
data available for the media of interest should be selected for use in the risk assessment.
The adequacy of QA/QC procedures followed in generating the analytical data should be
a key criterion in selecting data. However, a complete QA/QC review may not be
possible, particularly when analyses are conducted using historical data. For example,
the ARCS risk assessments relied primarily on historical data, and in many cases, little
QA/QC information was supplied with the data. Thus, risk assessment staff should work
with the regulatory agency's project manager in determining whether a data set is
adequate for a specific risk assessment application. In addition, the implications of any
limitations in available data should be discussed in the risk assessment document. For
example, unsuitable detection and quantification limits are often a major limitation in the
use of historical data sets.
Identifying Contaminants of Concern
A list of all the contaminants detected in the media of interest at the site should be made.
Inorganic chemicals present at concentrations near background levels and chemicals that
are infrequently detected or that may be present as laboratory contaminants may be
excluded. Where the list of contaminants of concern is extensive, a screening step can
be conducted to exclude contaminants that only contribute a minimal amount to the
overall site risk. For example, risks associated with the maximum detected concentration
can be calculated using toxicity data available in USEPA's Integrated Risk Information
System (IRIS) database and exposure assumptions that assume a higher degree of
exposure than is likely to occur at the site. Such an approach is considered to be
conservative because it incorporates assumptions that may overestimate risks in order to
ensure that risks are not underestimated. Using this approach, contaminants can then be
excluded when they contribute an individual risk of less than 1 x 10~7 (for carcinogens)
or a hazard quotient of less than 0.1 (for noncarcinogens). A carcinogenic risk of
1 x 10~7 corresponds to a one-in-ten-million chance of an individual developing cancer
during their lifetime. Use of these conservative target risk levels and worst-case
exposure assumptions (e.g., use of the maximum detected concentration) generally
ensures that chemicals with significant risks due to the cumulative effects of multiple
contaminants and multiple exposure pathways are not prematurely excluded from the risk
assessment.
Exposure Assessment
In the exposure assessment, the magnitude, frequency, duration, and route of direct and
indirect exposures of individuals to sediment-derived contaminants from an AOC are
determined. Populations that may be exposed (i.e., receptor populations) should first be
identified by considering the site's proximity to population centers, the accessibility of
the site, and any features such as beaches or fishing piers that would attract visitors. The
predominant types of receptor populations to consider are residents, workers, and recrea-
tional visitors. Recreational uses of AOCs may include fishing, swimming, boating, or
11
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Chapter 2. Human Health and Ecological Risk Assessment
beach activities. Current and potential future exposures should be evaluated. In particu-
lar, future exposures should be evaluated when future uses of a site may increase the
potential for exposure to site contaminants. For example, if a site may be used for resi-
dential purposes in the future, exposure to site soils and sediment could be greatly
increased in comparison with a current recreational scenario.
Exposures to contaminants can potentially occur via three exposure routes: ingestion,
dermal contact, and inhalation, each of which is in turn part of numerous exposure
pathways. Ingestion of contaminants can result from inadvertent consumption of
contaminated soils or sediment, or through consumption of drinking water, surface water,
or wildlife. Dermal contact involves direct contact of the skin with either contaminated
sediments, riverplain soils, or overlying water. Inhalation of airborne vapors or dust
may introduce contaminants of concern into the respiratory system. The ingestion
exposure pathways often result in higher exposure estimates than the dermal or inhalation
pathways because of the greater absorption of contaminants through the gastrointestinal
tract as compared with absorption through the skin, and the relatively high levels of
intake of contaminants in soil, water, and food as compared with inhalation of contami-
nants.
The potential pathways by which people may be exposed to contaminants from an AOC
are then examined to determine whether they are complete or incomplete. An exposure
pathway is complete if there is: 1) a source and mechanism of chemical release, 2) a
retention or transport medium (or media) whereby chemicals are transferred between
media, 3) an exposure point where contact occurs, and 4) an exposure route by which
contact occurs (USEPA 1989c). An exposure pathway is incomplete if any of these
conditions is not met. The exposure pathways that were complete for most of the five
priority ARCS sites included: 1) consumption of contaminated fish, 2) dermal contact
with contaminated water, 3) limited dermal contact with contaminated sediments, and
4) limited ingestion of surface water while swimming. Incidental ingestion of sediment
may also be of concern at some sites.
All complete exposure pathways should be evaluated in the exposure assessment unless
certain criteria apply. These criteria include: 1) the potential magnitude of exposure
from a pathway is low, or 2) the probability of the exposure occurring is very low and
the risks associated with the occurrence are not high (USEPA 1989c). For example, at
the Saginaw River AOC, there are no beaches along the river and swimming may occur
only infrequently when people jump off recreational boats into the water. In addition to
contacting the water, these people could ingest some water while swimming. In this
case, the risk from ingesting surface water was considered insignificant, and an assump-
tion was made that the health risk from dermal contact would be even lower than the
health risk associated with ingestion (Crane 1992b). At some sites, it may be reasonable
to assume that no fishing takes place because of the absence of edible fish or shellfish
or because sites are physically inaccessible or remote.
Once the exposure pathways to be quantitatively evaluated are selected for a site,
contaminant concentrations and exposure parameters are used to calculate the chronic or
12
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Chapter 2. Human Health and Ecological Risk Assessment
subchronic intake level of each contaminant (in mg of chemical per kg body weight per
day) (Table 2-1). For each current and potential future exposure scenario, exposure
parameters may be selected to represent typical, reasonable maximum, and, in some
instances, worst-case exposure conditions. Typical, or average, exposures and reason-
able maximum exposures (i.e., the maximum exposure that is reasonably expected to
occur at a site) are usually evaluated for each complete pathway. In general, an average
exposure case is calculated using site concentrations and exposure parameters that best
represent the central tendency of the data. Under the reasonable maximum exposure
case, 95th or 90th percentile values are used for contact rates, intake rates, and exposure
frequency and duration variables, and the upper 95 percent confidence limit on the
average concentration is used for the exposure point concentration in the contaminated
media. (See also US EPA [1992c] for further clarification of calculation of exposure
point concentrations.)
Site-specific information is often not available for many exposure parameters; thus,
assumptions about the types and frequencies of exposure may be made based on recom-
mended USEPA values or on professional judgment. The following documents provide
useful information on estimating exposure parameters and conducting the exposure
assessment: Superfund Exposure Assessment Manual (USEPA 1988b); Exposure Factors
Handbook (USEPA 1989b); Assessing Human Health Risks from Chemically Contami-
nated Fish and Shellfish: A Guidance Manual (USEPA 1989a); and Standard Default
Exposure Factors: Interim Final (USEPA 1991), which is a supplement to the Superfund
risk assessment manual (USEPA 1989c); and Dermal Exposure Assessment: Principles
and Applications (USEPA 1992a).
In some cases, it may be appropriate to determine the fractional intake of exposure that
occurs at a site. The fractional intake, which is the proportion of all exposure of a given
type (e.g., the fraction of all fish consumed) that comes from the site, is generally esti-
mated based on best professional judgment of factors such as the site size and accessibil-
ity and any restrictions on site use (e.g., warning signs, fishing bans, or barriers to the
site). In calculating fractional intake for fish consumption, the abundance of edible fish
and shellfish at a site should be considered. Some sites may not have any fish or may
not have edible aquatic species. However, although it is important to consider these
limits on site use, they may not be sufficient to prevent access at a site, and thus the risk
assessment should not assume that exposure will not occur. For example, although all
of the five priority AOCs examined for the ARCS Program had fish advisories in effect,
some people continued to fish from the river.
Because recreational fishing is very popular in the Great Lakes region and consumption
of contaminated fish is an important exposure pathway, several researchers have gathered
data on consumption rates of fish by Great Lakes populations. A survey of the angler
population in the AOC should be made to obtain a better estimate of local fish con-
sumption rates and patterns. If these data cannot be obtained, the results of a survey of
Michigan anglers and their families by researchers at the University of Michigan (West
et al. 1989) may be used to estimate more "localized" consumption patterns. The survey
results can be obtained from Patrick West at the University of Michigan. An important
13
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TABLE 2-1. GENERIC EQUATION FOR CALCULATING
CHEMICAL INTAKE LEVELS
C x CR x EFD
BW x AT
where:
I
Intake =
the amount of chemical at the exchange
boundary (mg chemical/kg body weight-day)
Chemical-Related Variables
C Chemical
concentration = the average concentration contacted over the
exposure period (e.g., mg/L)
Variables that Describe the Exposed Population
Contact rate =
CR
EFD
the amount of contaminated medium contacted
per unit time or event (e.g., L/day)
Exposure frequency
and duration = how long and how often exposure occurs;
often calculated using two terms, EF and ED,
where:
BW
Body weight =
EF = exposure frequency (days/year)
ED = exposure duration (years)
the average body weight (kg) over the
exposure period
Assessment-Determined Variables
AT Averaging time = period over which exposure is averaged (days)
Source: U.S. EPA (1989c).
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Chapter 2. Human Health and Ecological Risk Assessment
result of this survey was that Michigan anglers and their families had an average fish
consumption rate of 19.2 g/person-day (West et al. 1989), nearly 3 times the average fish
consumption rate of people in the United States as a whole (USEPA 1989c). Anglers in
other Great Lakes states may be consuming fish at a comparable rate to Michigan
anglers. Additional data on fish consumption rates for sport anglers on Lake Michigan
are provided as part of a nationwide survey reported in Rupp et al. (1980).
In selecting appropriate consumption rates, subsistence fishing or hunting should be
considered (in addition to the average and reasonable maximum exposure cases) at sites
with special subgroups of people who rely on locally caught fish, waterfowl, or other
aquatic-related wildlife as their main source of protein. Examples may include members
of a particular ethnic community who traditionally rely on fish as an important part of
their diet (e.g., the southeast Asian community of Hmong in Sheboygan, Wisconsin) or
indigent people who spend time in the area and may rely on locally caught fish for their
main source of protein.
Toxicity Assessment
In a toxicity assessment, available data are reviewed to determine and quantify the
relationship between the level of exposure to a contaminant (dose or intake level) and the
increased likelihood and/or severity of adverse effects. This relationship is termed the
dose-response relationship and provides the basis for deriving quantitative toxicity values
used in the risk assessment. For carcinogenic health effects, CSFs are used to estimate
the risk of developing cancer that corresponds to estimated exposure concentrations.
This risk is in addition to the risk of developing cancer due to other causes and thus is
often termed excess cancer risk.
The potential for noncarcinogenic health effects from oral exposures is typically evaluated
by comparing estimated daily intake levels with RfDs, which represent daily intake levels
at which no adverse effects are expected to occur. For assessment of inhalation
exposures, USEPA has recently begun issuing reference concentrations (RfCs) that
represent exposure concentrations at which no adverse effects are expected to occur.
Carcinogens and systemic toxicants are treated differently, because according to current
scientific theory it is plausible that for any dose of a carcinogen there could be some
finite increase in cancer risk. Systemic toxicants are considered to act via a threshold
mechanism, which allows for the identification of a safe dose. Hazard identification and
dose-response evaluations for more than 600 chemicals have been conducted and verified
by USEPA work groups; additional chemicals are awaiting review. USEPA-verified
toxicity values can be obtained by accessing USEPA's IRIS database. The IRIS User
Support group can provide technical assistance and information on how to access IRIS
and can be reached at (513) 569-7254.
Brief toxicity profiles on contaminants of concern should be prepared as part of the
toxicity assessment. At a minimum, such profiles should contain information on the
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Chapter 2. Human Health and Ecological Risk Assessment
derivation of toxicity values for the contaminants of concern and should describe any
uncertainties associated with the toxicity values. The following data should be gathered,
to the extent available, for all contaminants of concern that have been shown to be carci-
nogenic in experimental animals or in human populations:
• Current CSFs from IRIS
• Weight-of-evidence classifications, which characterize the degree to which
the available evidence indicates that an agent is a human carcinogen
• Type of cancer for Class A carcinogens (i.e., contaminants that have been
shown to cause cancer in humans).
Pertinent data to be identified and discussed in the baseline risk assessment for contami-
nants associated with noncarcinogenic effects include the following:
• Current RfDs (and RfCs, if applicable) from IRIS
• Confidence level in the overall database and the critical study on which the
toxicity value is based, including identification of the critical effects
• Effects that occur at doses higher than those required to elicit the critical
effect
• Uncertainty factors used by USEPA in deriving the toxicity value
• 1- and 10-day health advisories for shorter-term oral exposures.
Inclusion of these background data in the toxicity assessment assists risk managers in
interpreting the findings of the risk assessment.
Risk Characterization
The purpose of the risk characterization step is to combine the exposure and toxicity
estimates into an integrated expression of human health risk. Three means of expressing
carcinogenic and noncarcinogenic risks are presented in the risk assessment. First,
chemical-specific risks are estimated for each exposure pathway. Second, these
chemical-specific risks are added to estimate a cumulative path way-specific risk. Finally,
risks are added across all chemicals and relevant pathways to estimate the total human
health risks to individuals exposed to contaminants from the AOC. Table 2-2 illustrates
how a summary table of risk estimates may be arranged. The approaches used to
quantify carcinogenic and noncarcinogenic health risks are described below.
Carcinogenic risk is expressed as the upper-bound excess probability of an individual
developing cancer over their lifetime following exposure to a given chemical concentra-
tion for a specified period of time. Carcinogenic risk estimates are computed by multi-
plying the chronic daily intake prorated over a lifetime of exposure by the CSF for each
carcinogen of interest. Carcinogenic effects are summed for all chemicals in an exposure
16
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TABLE 2-2. ESTIMATED CARCINOGENIC AND NONCARCINOGENIC RISKS TO
INDIVIDUALS RESIDING IN THE LOWER SAGINAW RIVER AREA OF CONCERN
Type of
Risk and Exposure3
Individual Risks
Walleye
Carp
Waterfowl
Additive Risks
Walleye + Carp +
Waterfowl Waterfowl
Carcinogenic
Typical
Reasonable Maximum
Subsistence
1x10~5 1x10~4 6x10~6
2x10~4 3x10~3 2x10~4
2x10~3 2x10~2 1x10"3
2xicr5 ixicr4
4x10"4 3x10~3
Noncarcinogenic
(hazard index)
Typical
Reasonable Maximum
Subsistence
0.02
0.2
1
0.08
0.5
4
0.001
0.02
0.08
0.02
0.2
0.08
0.5
Source: Crane (1992b)
a Noncarcinogenic risks were averaged over the same period as the exposure duration, while
carcinogenic risks were averaged over a period of 70 years (i.e., average lifetime of an individual).
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Chapter 2. Human Health and Ecological Risk Assessment
pathway (e.g., consumption of fish, incidental ingestion of sediments). This summation
of carcinogenic risks assumes that there are no synergistic or antagonistic chemical inter-
actions and that all chemicals produce the same effect. USEPA believes it is prudent
public health policy to consider actions to mitigate or minimize exposures to contami-
nants when estimated excess lifetime cancer risks exceed the 10~5 to 10~6 range
(USEPA 1988a).
Noncarcinogenic effects are evaluated by calculating the ratio, otherwise known as the
hazard quotient, of a site-specific exposure level for a specified time period to an RfD
derived from a similar exposure period. Unlike cancer risk estimates, hazard quotients
are not expressed as a probability. A hazard quotient of less than 1 indicates that
exposures are not likely to be associated with adverse noncarcinogenic effects. As the
hazard quotient approaches or exceeds 10, the likelihood of adverse effects is increased
to the point where action to reduce human exposure should be considered (although the
magnitude of the uncertainty factors used to derive the RfD should also be considered).
Because of the uncertainties involved with these estimates, values between 1 and 10 may
be of concern, particularly when additional significant risk factors are present. However,
because RfDs do not have equal accuracy or precision and they are not based on the
same severity of toxic effects, evaluation of hazard indices (the sum of two or more
hazard quotient values for multiple substances and/or multiple exposure pathways) should
take into account the uncertainties associated with specific RfDs.
The consumption of contaminated fish resulted in the greatest human health risk at the
five priority AOCs examined in the ARCS Program: Saginaw River, Michigan (Crane
1992b); Sheboygan River, Wisconsin (Crane 1993a); Grand Calumet River, Indiana
(Crane 1993b); Ashtabula River, Ohio (Crane 1992a); and Buffalo River, New York
(Crane 1993c). Locally caught fish were assumed to accumulate contaminants primarily
through the food chain, and in-place contaminated sediments were assumed to be the
major source of contaminants to the food chain and water column. In most cases, PCB
contamination contributed the greatest degree of carcinogenic risk. Noncarcinogenic risk
levels were usually not of concern except for the subsistence exposure case; however,
some chemicals (e.g., PCBs) lacked a verified RfD, and thus the noncarcinogenic effects
of these contaminants could not be evaluated in the risk assessments. In addition, the
consumption of bottom-feeding fish species, like carp, usually resulted in carcinogenic
risks greater than 10~6, whereas the consumption of water column-feeding fish species,
like walleye, did not always result in significant carcinogenic risks.
Uncertainty Analysis
A number of assumptions and estimated values are used in baseline risk assessments that
contribute to the level of uncertainty about possible human health risks. As with most
environmental risk assessments, the uncertainty about the risk estimate is generally at
least an order of magnitude or greater (USEPA 1989c). Thus, at a minimum, the risk
assessment should include a qualitative uncertainty analysis that identifies the key
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Chapter 2. Human Health and Ecological Risk Assessment
site-related variables and assumptions that contribute most to the uncertainty inherent in
the risk estimates.
Some of the major uncertainties in the ARCS risk assessments arose from the following
factors:
• Use of contaminant burdens in fish based on uncooked fish, and in some
instances, whole fish
• Exclusion of some complete exposure pathways (e.g., dermal exposure to
water and sediments)
• Use of default exposure frequency and duration variables, body weight,
life expectancy, and population characteristics
• Use of RfDs and CSFs that are usually based on animal studies and that
may be based on only one form of a chemical (e.g., Aroclor® 1260 was
used to derive the CSF for PCBs)
• Assuming additive health risks for both carcinogenic and noncarcinogenic
effects
• Natural variability (e.g., small-scale spatial and temporal variability in
sediment and hydrological conditions)
• Inherent approximations of physical, chemical, and biological processes in
the models.
For each of these assumptions, the level of uncertainty associated with the final risk
estimates was estimated as low, moderate, or high. Additional site-specific sources of
uncertainty are likely to be important for risk assessments conducted at other con-
taminated sediment sites. Calibration and fine-tuning of model results after field testing
can greatly reduce uncertainties associated with risk estimates at specific sites.
Applications
The results of the baseline risk assessment can be used by risk managers for several
purposes. First, the baseline risk assessment provides a quantitative way to identify the
exposure pathways and contaminants that contribute to carcinogenic and noncarcinogenic
human health risks at a site. However, the calculated human health risks are not actual
values, but are instead estimates that must be interpreted in the context of all the
uncertainties associated with each step in the risk assessment process. Second, the
baseline risk assessment can be used to identify sensitive subgroup populations (e.g.,
children, subsistence anglers) within the AOC. Third, the results of the baseline risk
assessment can be used to compare the estimated risks of different sediment remedial
alternatives with the impact of the no-action alternative during the comparative risk
assessment. Additional applications of the baseline risk assessment are discussed in
subsequent chapters.
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Chapter 2. Human Health and Ecological Risk Assessment
ECOLOGICAL RISK ASSESSMENT
The wildlife and aquatic organisms of the Great Lakes region may be exposed to sedi-
ment contaminants through various mechanisms, resulting in ingestion, inhalation, or
dermal uptake of potentially toxic chemicals. For the Great Lakes AOCs, ecological risk
assessments may be conducted to evaluate the likelihood of acute and chronic adverse
effects of sediment contaminants on wildlife, aquatic plants, benthic invertebrates, and
fish that rely on lakes and streams for habitat, food, and drinking water. Under the
ARCS Program, wildlife risk assessments were conducted for two of the priority AOCs,
Buffalo River and Saginaw River.
Ecological assessments may involve empirical measurements of realized effects using the
retrospective approach and theoretical modeling to estimate the probability of effects
using the predictive approach. The balance of empirical and modeling approaches
depends on the objectives of the assessment, the practicality of measurement methods for
the receptors of concern, and data availability. For example, empirical approaches are
commonly used to evaluate the effects of existing contamination on species populations
and communities that are easily sampled, and on endpoints such as population abundances
that can be easily quantified and interpreted. Theoretical models are used to estimate
exposure of large-bodied wildlife species and rare species for which direct measurement
of population or community endpoints is impractical, and to predict the effects of future
conditions.
USEPA is currently developing guidelines for conducting ecological risk assessments
(Norton et al. 1992) and recently issued their Framework for Ecological Risk Assessment
(USEPA 1992b). This framework report describes the elements of an ecological risk
assessment and provides the basis for conducting ecological risk assessments within
USEPA. Development of specific guidelines for ecological risk assessment is in
progress, but will require considerable time (Norton et al. 1992). USEPA Region V has
issued its own framework document entitled Regional Guidance for Conducting
Ecological Assessments. In addition, the State of Wisconsin recently issued their
Guidance for Assessing Ecological Impacts and Threats from Contaminated Sediments
(WDNR 1992a,b). In addition to the guidance offered herein, these other guidance
documents should be consulted in planning an ecological risk assessment for con-
taminated sediments. Past ecological risk assessments that have been performed for
contaminated sediment sites in Region V are available from USEPA Region V.
Because of the variety of habitats and species associated with sediments and interactions
between biota and physical-chemical conditions, diverse techniques may be used in
ecological risk assessments. The physical and chemical structure of an ecosystem
influences the bioavailability and toxicity of contaminants to resident species. Biological
interactions may determine the transport and fate of contaminants in the environment as
well as species exposure patterns. Thus, the risk assessment process cannot be easily
standardized to a "cookbook" approach.
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Chapter 2. Human Health and Ecological Risk Assessment
The remainder of this chapter presents an overview of ecological risk assessment methods
as they might be applied to contaminated sediments. The elements of the approach
described below are consistent with USEPA's framework report and other USEPA
reports (USEPA 1989d,e; Warren-Hicks et al. 1989). Other related activities in the
development of ecological risk assessment guidance for aquatic habitats include:
1) formation of an Ecorisk Group within the USEPA Office of Water to develop a
paradigm for ecological risk assessment, 2) development of sediment quality criteria by
USEPA based on the equilibrium partitioning model (DiToro et al. 1991), 3) issuance
of a review of aquatic risk assessment methods by Parkhurst et al. (1990) for the Water
Environment Federation (formerly the Water Pollution Control Federation), and
4) USEPA's development of proposed methods for the derivation of ambient water qual-
ity criteria that would be protective of human health, wildlife, and aquatic organisms
under the Great Lakes Initiative (40 CFR Parts 122, 123, 131, and 132; 58 Fed.
Reg. 20802).
Because of the varied nature of contaminated sediment sites and the objectives of
individual ecological risk assessments, only general guidance is offered here. Ecological
risk assessors must still rely on their own judgment and expertise when evaluating poten-
tial risks to wildlife and/or aquatic organisms. Thus, any ecological risk assessment
should include a clear statement of assumptions and an uncertainty analysis.
General Framework
The risk assessment process developed for estimating human health risks generally
applies to determination of ecological risks. However, the complexity of ecological
systems requires consideration of multiple species and other physical-chemical stressors
in addition to toxic chemicals. Ecological endpoints may also differ from those used in
human health risk assessment. For example, survival, growth, and reproduction may be
emphasized as ecological endpoints, instead of cancer or more subtle sublethal effects.
In ecological risk assessment, risks to populations, communities, and ecosystems are
often considered more relevant than individual risk. Except in the case of rare,
threatened, and endangered species, individual plants and animals are not highly valued
because compensatory mechanisms in ecological systems may preclude higher-level
effects even if individuals are eliminated from a population. In ecological risk assess-
ment, the ability of the ecosystem to recover from the stress may also be considered.
USEPA (1992b) defines ecological risk assessment as "a process that evaluates the
likelihood that adverse ecological effects may occur or are occurring as a result of
exposure to one or more stressors." In general, both wildlife and aquatic risk assess-
ments follow the basic framework shown in Figure 2-2 (USEPA 1992b):
• Problem Formulation: This planning and scoping step defines the objec-
tives, approach, and data needs for the assessment. It includes: 1) a
qualitative evaluation of contaminant release, transport, and fate; 2)
identification of contaminants of concern, receptors, exposure pathways,
21
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IV)
Problem Formulation
Exposure Assessment
Ecological Effects Assessment
Sediment/Water Analysis
Bioaccumulation
Toxicity Tests
Community Analysis
Food Web Models
Risk Characterization
Figure 2-2. Ecological risk assessment framework.
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Chapter 2. Human Health and Ecological Risk Assessment
and known ecological effects of the contaminants; 3) selection of endpoints
for further study; and 4) integration of the preceding information into a
conceptual model. Assessment endpoints should represent ecological
"values" to be protected. Measurement endpoints are the observed or
measured variables related to assessment endpoints. The lack of standard-
ized ecological risk assessment procedures and the complexity of ecosys-
tems make the initial planning of the assessment extremely important.
• Exposure Assessment: The exposure assessment uses chemical measure-
ments and chemical transport and fate models to estimate the magnitude,
duration, and frequency of exposure to the contaminants of concern. It
involves the following steps: 1) quantification of contaminant release,
transport, and fate including information on temporal and spatial variabil-
ity; 2) characterization of exposure pathways and receptors; 3) measure-
ment or estimation of exposure point concentrations (or chemical intake
rates); and 4) evaluation of the quality of the data available for the
exposure assessment.
• Ecological Effects Assessment: The ecological effects assessment deter-
mines the relationship between the levels of exposure and the levels and
types of effects. It involves an evaluation of literature reviews, field
studies, and toxicity tests that link contaminant concentrations to effects on
ecological receptors. The effects assessment often uses models to extrapo-
late toxicity test data to different species, life stages, levels of biological
organization, and exposure conditions.
• Risk Characterization: The risk characterization documents existing
chemical effects and estimates the likelihood of adverse ecological effects
by integrating the exposure and ecological effects assessments. It also
provides narrative explanations of underlying assumptions, the nature and
magnitude of uncertainties, and the quality of the data.
In ecological risk approaches, assessment endpoints are defined as environmental
characteristics or values that are to be protected, such as wildlife population abundance,
species diversity, or ecosystem productivity (Suter 1989). For example, maintenance or
restoration of valuable natural resources is typically a goal of the remediation process.
If protection of a valuable commercial fish stock is the goal, the assessment endpoint may
be recruitment rate for the species population. Measurement endpoints are quantitative
expressions of an observed or measured biological response related to the valued
environmental characteristic chosen as the assessment endpoint. In some cases, the
measurement endpoint is the same as the assessment endpoint. When these endpoints
differ, a model must be used to express their relationship.
The process of estimating ecological risk based on chemical, lexicological, and ecological
data is called a forward-mode assessment. Risk assessment procedures may also be used
to back-calculate exposure guidelines from an allowable risk level or a no-observed-
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Chapter 2. Human Health and Ecological Risk Assessment
adverse-effect level (NOAEL). A reverse-mode assessment may be used to derive
cleanup levels (e.g., maximum allowable concentrations of contaminants in sediment).
The level of detail required for a given risk assessment depends on remedial action
objectives, the complexity of the site, and the difficulty in adequately describing
exposure, toxicity, and other properties of the contaminants of concern. An ecological
risk assessment can be conducted in tiers with the most basic analyses conducted first.
For example, an initial screening-level risk assessment is conducted that uses available
data and conservative assumptions about exposure and toxicity. From the results of this
screening-level assessment, areas, contaminants, and species of concern are identified and
decisions are made about additional data collection. In the next tier, more realistic
models are used and additional data may be collected that will better define the
relationship between chemical concentrations and adverse effects at the site.
Problem Formulation
The conceptual model developed during the planning phase of an ecological risk
assessment illustrates how exposure to sediment contaminants may cause ecological
effects. The results of the problem formulation stage clarify the scope of an ecological
risk assessment and how the results will be used in developing RAPs for the AOCs.
Based on the results of a screening-level assessment, chemicals, species, and endpoints
are selected for a detailed assessment that may involve collection of additional field data
and risk modeling.
Selection of Contaminants of Concern
Contaminants of concern are selected for the risk assessment based on available data and
the preliminary evaluation of releases, transport, and fate of sediment contaminants
relative to their potential toxicity. Sequential criteria for selection of contaminants of
concern for an ecological risk assessment may include:
1. Detection in sediments within the AOC
2. Presence in sediments or tissues at concentrations significantly above
reference concentrations
3. Relationship to human activities
4. Presence at concentrations above screening toxicity criteria.
The last step in the selection process is to compare measured or estimated environmental
concentrations with threshold concentrations such as NOAELs. This analysis should be
conservative by incorporating plausible worst-case assumptions regarding bioavailability,
exposure, and sensitivity of ecological receptors.
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Chapter 2. Human Health and Ecological Risk Assessment
The list of contaminants of concern for an ecological risk assessment may differ from
that used for a human health risk assessment because of differences in exposure
pathways, uptake, and sensitivities between humans and ecological receptors. Therefore,
the initial list of contaminants considered for evaluation in the selection process described
above should be comprehensive rather than simply the list selected for evaluation in the
human health risk assessment.
Selection of Species
Because of the complexity of the food web in the Great Lakes basin, not all of the
trophic levels and species can be evaluated. Thus, a few species or species groups may
be selected as ecosystem indicators of environmental conditions. The Ecosystem Objec-
tives Committee of the International Joint Commission developed the following criteria
(as cited in Kubiak and Best [1991]) for selecting indicator species:
• Displays a broad distribution within the AOC
• Maintains itself through natural reproduction and is indigenous
• Interacts directly with many components of its ecosystem
• Maintains well-documented and quantifiable niche dimensions
• Exhibits a gradual response to a variety of human-induced stresses
• Responds to stresses in a manner that is both identifiable and quantifiable
• Represents an important species to humans.
These criteria for selection of indicator species were adopted for use in the ARCS
Program. In addition, species selected for the assessment should be sensitive to effects
of the contaminants of concern and, if possible, should be representative of a group of
valuable species.
The U.S. Fish and Wildlife Service is evaluating the bald eagle, mink, otter, colonial
waterbird group, and lake trout (salmonids) as possible indicator species for Great Lakes
water quality (Kubiak and Best 1991). These species can also be used to evaluate the
effects of contaminated sediments. In addition, other aquatic biota that should be
considered in most risk assessments include benthic macroinvertebrates and bottom-
feeding fish.
Any of the species or groups just discussed could be considered in a predictive as-
sessment. For empirical assessments of existing conditions, the following ecosystem
indicators are recommended:
• Bottom-feeding fish populations
• Higher tropic level fish (if AOC is very large)
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Chapter 2. Human Health and Ecological Risk Assessment
• Benthic macroinvertebrate communities
• Locally important species of amphipods and chironomids.
For a specific risk assessment, the selection of species to be evaluated also depends on
the contaminants of concern and the scale of resolution needed to define problem areas
within AOCs. For example, assessment of bioconcentratable contaminants such as PCBs
and dioxins over a wide portion of Lake Michigan might consider wide-ranging fish and
wildlife species that feed at high trophic levels. Relatively fine-scale resolution of
problem areas within a tributary might consider benthic macroinvertebrates and localized
bottom-feeding fish species like bullhead. The selection of species and endpoints for an
assessment should consider whether the contaminants biomagnify and whether they cause
direct toxicity to receptors at lower trophic levels.
Practical methods for field and/or laboratory measurements must be available for
retrospective assessments. Predictive and retrospective assessments both require adequate
data on contaminant distributions or appropriate transport and fate models to estimate
exposure. Limitations in data or models may influence the final selection of species for
the assessment.
Selection of Endpoints
Both assessment endpoints and measurement endpoints are used as indicators of
ecological risk. When the measurement endpoint differs from the assessment endpoint,
a model must be used to express their interrelationship. The primary measurement
endpoints for an ecological risk assessment should be related to the survival, growth, and
reproduction of exposed organisms. These endpoints are used in most standardized
toxicity tests and in the development of USEPA ambient water quality criteria, wildlife
criteria, and sediment quality criteria. Moreover, such endpoints can be quantitatively
related to changes in population numbers and structure. For example, PCBs are known
to be accumulated in gull eggs in the Great Lakes region and have been linked to repro-
ductive failure. Here, the measurement endpoint might be the proportion of nonviable
gull eggs, as a predictor of effects at the population level. Although other endpoints,
such as enzymatic responses and histological lesions in individual organisms, may
indicate chemical exposure and response, they do not necessarily indicate adverse effects
on populations, communities, or ecosystems.
Various endpoints may be used for predictive assessments, but their final selection is
often affected by the availability of toxicity data in the literature and the quality of the
data. Because prediction of community-level responses from survival, growth, and
reproductive endpoints involves substantial uncertainties, wherever practical, effects on
selected communities should be directly observed in the field. For example, population
and community analyses of benthic macroinvertebrate communities may be used to
evaluate toxic effects of sediment contamination.
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Chapter 2. Human Health and Ecological Risk Assessment
Ecological Risk Assessment Objectives
The objectives of an ecological risk assessment are developed from conceptual models
of chemical transport and fate, exposure pathways for selected receptors, and potential
mechanisms for adverse ecological effects. The objectives should specify the selected
contaminants, receptors, and endpoints to be included in the assessment. An example
conceptual model of a site near a tributary might show the transport of contaminants of
concern from soil to groundwater, which is then discharged to the river, where
contaminants are absorbed or ingested by fish. Contaminants may be transferred to the
terrestrial environment when a waterbird eats fish from an affected portion of the
tributary.
Assessment techniques appropriate for the receptors and contaminants of concern and the
level of complexity of the risk assessment are determined on a site-by-site basis. The
selection of ecological assessment techniques to be applied at a site depends on the
objectives of the risk assessment, site-specific receptor species and contaminants of
concern, and the extent of available data. The primary techniques are:
• Chemical analysis of samples of sediment, surface water, and organism
tissues from the site
• Toxicity testing of sediments
• Community analysis based on measurements of the types and number of
benthic macroinvertebrates at the site
• Exposure models to predict chemical concentrations and bioavailability in
environmental media and to estimate uptake by key receptors
• Ecological models to extrapolate from measurement endpoints to assess-
ment endpoints in receptor groups for which community analysis is not a
primary tool.
Each combination of tools selected for an AOC should provide adequate data for the
assessment and facilitate risk predictions. Figure 2-3 summarizes some of the candidate
tools for ecological risk assessments according to habitat, media, and receptors.
The problem formulation stage should include development of a strategy for integrating
the results of individual assessment tools into the overall approach to risk charac-
terization. Moreover, the risk assessment objectives should be clearly related to remedial
action objectives and the decision-making process. The overall assessment strategy may
involve both empirical and theoretical approaches.
Empirical approaches involve direct measurement of biological effects or derivation of
relationships between chemical and biological variables from field data, or toxicity testing
of field-collected samples. Empirical approaches rely heavily on observed relationships
without attempting to describe theoretical cause-effect relationships. Warren-Hicks et al.
(1989) describe empirical assessment approaches used to quantify the ecological effects
27
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Riparian and Upland
Chemical Analyses
Sediment/Water
Tissue
Plants
Birds
Small Animals
Large Animals
Toxicity Tests
O
IV)
00
Community Analysis
Models
Exposure Models'
Ecological Models
a Includes transport and fate models and food web
models to estimate exposure.
b Includes models to extrapolate measurement
endpoints (e.g., organism-level effects) to
assessment endpoints (e.g., population- or
community-level effects).
• Primary tool - used at most sites
O Secondary tool - used at selected
sites based on specific conditions
Figure 2-3. Ecological assessment tools for contaminated sediments.
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Chapter 2. Human Health and Ecological Risk Assessment
of contaminants at hazardous waste sites, including: 1) toxicity testing, 2) use of bio-
markers, including analysis of tissue contaminants, and 3) community analysis based on
field surveys.
Theoretical models are mainly derived from theoretical principles and include explicit
mechanistic (cause-effect) relationships. Modeling may be used to support empirical risk
assessments and to make risk predictions. Mathematical exposure models are used for
dynamic systems (such as river water), for long-term predictions in more stable systems
(such as sediment), and for transfer of chemicals through the food web to receptors
higher up the food chain. Ecological models are used primarily to extrapolate from
measurement endpoints to assessment endpoints in receptor groups such as amphibians,
birds, and large mammals. Both exposure models and ecological models may vary from
relatively simple extrapolation models with few data requirements to complex mechanistic
models with substantial data requirements. Whenever practical, models should be based
on site-specific data and validated.
Exposure Assessment
In the exposure assessment phase, measurements or estimates are made of the concentra-
tions of contaminants of concern in the environment or the rate of chemical intake by
organisms. Analysis of the magnitude, duration, and frequency of exposure is based on
information or assumptions about:
• Chemical sources and pathways
• Chemical distributions in water, sediment, and organisms
• Spatial/temporal distributions of key receptors.
For empirical assessments, tissue concentrations of contaminants in key species may be
measured as indicators of exposure. To develop estimates of exposure using models,
exposure scenarios are developed from the conceptual site model to describe the
pathways a chemical may take through various environmental media to reach an
organism. For each site, the analysis of several exposure scenarios helps to identify data
gaps for transport pathways and key exposure processes, such as chemical transforma-
tions or biological uptake. Data gaps for specific chemical forms or processes related
to important pathways are filled through estimations from predictive chemical models or
the collection of additional site-specific data.
The distributions and seasonal activity patterns of receptors are described relative to
contaminant distributions in various habitats at a site. Habitats, concentrations of
contaminants of concern, species distributions, and exposure variables related to species
activities may be mapped and spatial patterns investigated using a mapping/database
system such as a geographic information system.
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Chapter 2. Human Health and Ecological Risk Assessment
Information from the transport and fate analysis for each exposure scenario is used to
develop quantitative estimates of exposure that serve as inputs to the risk characteriza-
tion. Summaries of data for the exposure assessment may include:
• Contaminant sources
Mapping of source locations
Contaminant release data for outfalls, landfills, combined sewer
overflows (CSOs), and other sources
• Sediment
Mapping of contaminant distributions
Comparison of contaminant concentrations in sediments at the
site with reference area values
Comparison of contaminant concentrations in sediments with
levels associated with biological effects (based on available
toxicological literature and field surveys)
Pattern analysis of contaminant data to evaluate potential sources
of contamination
Evaluation of the suitability of the reference area
• Surface water
Comparison of concentrations of contaminants in water with
USEPA ambient water quality criteria
Pattern analysis of contaminant data to evaluate potential sources
of the contamination
Evaluation of the degree of chemical contamination in water
collected from stations near the site relative to reference values
Evaluation of the suitability of the reference area
• Bioaccumulation
Evaluation of the degree of chemical contamination in fish tissue
collected from the site relative to reference values
Evaluation of contaminant concentration gradients in tissue
collected from the site
Evaluation of the suitability of the reference area.
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Chapter 2, Human Health and Ecological Risk Assessment
Wildlife
The principal routes of chemical uptake for terrestrial wildlife are ingestion, inhalation,
and dermal absorption. For riparian or terrestrial wildlife species, some routes of
exposure, particularly ingestion, may involve many different media. Scientists from the
U.S. Fish and Wildlife Service have conducted risk assessments for wildlife indicator
species (e.g., bald eagle, other fish-eating birds, mink) for the ARCS Program at the
Buffalo and Saginaw River AOCs. The approaches used by these researchers can
generally be applied to other areas with sediments contaminated by bioconcentratable
chemicals. Such chemicals are typically the primary contaminants of concern for
assessment of sediment-associated risk to wildlife.
The exposure assessment for indicator species can be approached in two different ways
(Kubiak and Best 1991). The first approach requires site-specific information or
estimates of the types of forage items commonly eaten, contaminant concentrations in
forage items, and grams of food eaten per kilogram of wildlife predator body weight.
This information is used to calculate an ingested dose for contaminants of concern in the
indicator species, similar to food web model approaches currently being used for
sediments and other media at hazardous waste sites (e.g., Fordham and Reagan 1991;
Menzie et al. 1992). The calculated ingested dose is then compared to a NOAEL
developed from a model feeding study where known adverse toxicological endpoints were
measured. There is considered to be a significant risk to the indicator species if the
calculated ingested dose exceeds the NOAEL. Unfortunately, the database necessary for
applying this approach is not sufficiently developed for Great Lakes species.
The second approach can be conducted for contaminants that bioaccumulate through the
food chain (e.g., DDE, PCBs, dieldrin). In this approach, the concentration of a
contaminant of interest is measured in a specific tissue of the indicator species. Because
the exposure estimate is expressed as a contaminant concentration in tissue, the ecological
effects assessment includes an estimate of the NOAEL expressed as a contaminant
concentration in the tissue rather than as an ingested dose. The measured tissue
contaminant concentration is compared with the NOAEL to evaluate existing risk. Based
on the NOAEL and on biomagnification factors calculated from actual field data, Kubiak
and Best (1991) also applied this approach to backcalculate the contaminant concentra-
tions in forage fish that would be necessary to result in a contaminant concentration equal
to the NOAEL in the tissue of the indicator species.
Either approach, appropriately used, could be applied to the calculation of "safe
concentrations" or remedial action goals for sediments. The goal would be to determine
the contaminant concentrations in sediments that would not result in exceedances of the
NOAELs, expressed as an ingested dose to the indicator species in the first approach or
as a tissue contaminant concentration in the second approach. For the ARCS Program,
the second approach was applied for bald eagle, other fish-eating birds, and mink in the
wildlife risk assessments conducted for the Buffalo River AOC (Mann-Klager, in prep.)
and the Saginaw River AOC (Kubiak, in prep.).
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Chapter 2. Human Health and Ecological Risk Assessment
Aquatic Life
The principal routes of chemical uptake for aquatic organisms are gill and body-surface
absorption of chemicals in surface water or sediment pore water, and ingestion of water,
sediments, and food. Exposure estimates for aquatic life may be based on one or more
of the following:
• Contaminant concentration in bulk sediment
• Contaminant concentration in sediment pore water
• Contaminant concentration in the water column
• Contaminant concentration in organism tissue.
In most cases, the exposure estimate will be expressed as the contaminant concentration
in the bulk sediment, pore water, or water column, which is obtained from empirical
measurements. Few toxicity data are available to interpret exposure estimates expressed
as contaminant concentrations in tissue (e.g., Dillon 1984).
Ecological Effects Assessment
Ecological effects assessment includes a hazard identification step to identify the potential
effects of chemicals and an exposure-response assessment to characterize the relationship
between each stressor and the biological or ecological endpoints. Confounding effects
of physical stressors such as currents or sediment grain size must be addressed by using
models, reference-area measurements, or experimental designs to separate the effects of
physical factors from those of chemicals.
Techniques for ecological effects assessment may include the following:
• Laboratory/field toxicity tests
• Observational field studies
• Interspecies extrapolation of effects
• Interchemical extrapolation based on knowledge of their modes of action,
such as quantitative structure-activity relationships
• Biological or ecological modeling to extrapolate from measurement
endpoints to assessment endpoints.
Data from direct field measurements or from laboratory analyses of field samples should
be used whenever possible to derive exposure-response relationships. Site-specific prop-
erties of sediment and water may modify the bioavailability of chemicals, and literature
data may not be appropriate for ecological effects assessment.
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Chapter 2. Human Health and Ecological Risk Assessment
Wildlife
Although site-specific toxicity data are preferred for exposure-response assessment, use
of available data from the general literature may be appropriate in certain cases. For
example, literature estimates of stress thresholds such as NOAELs for wildlife species
might be used with conservative assumptions about bioavailability in a screening-level
assessment.
An estimate of the NOAEL for each contaminant of concern can be derived from the
literature or from a model feeding study on a surrogate laboratory species. NOAELs can
be expressed as chemical intake rates, which typically result from ingestion of chemicals,
or chemical concentrations in tissue. The units for the NOAEL must correspond to the
units used for the exposure estimate. When developing NOAELs, the following
guidelines developed by PTI (1992) are relevant:
• Data from the receptor species of concern or a representative surrogate
species that is closely related to the receptor should be used to derive
NOAELs
• Ecologically relevant endpoints should be selected or a quantitative uncer-
tainty analysis, which delimits the probable range of the NOAELs, should
be performed when endpoint extrapolations are required
• The mode of administration of chemicals in laboratory exposures must be
evaluated, and inter-route extrapolations should be avoided if possible.
An endpoint for a relatively sensitive tissue, organ, or life stage should be determined
to derive a conservative estimate of a NOAEL. Adverse effects on reproductive organs
and early life stages are typically good endpoints for risk assessment because they are
likely relevant to changes at the population level. For birds, the most sensitive stages
are the egg and the developing embryo, at least for chlorinated organic compounds and
methylmercury. In contrast, the liver is the most sensitive tissue known for mink
(Kubiakand Best 1991).
Aquatic Life
Ecological effects assessments for aquatic life may be based on theoretical approaches
such as the use of sediment criteria developed from equilibrium partitioning models (e.g.,
DiToro et al. 1991) and bioaccumulation models (e.g., Thomann et al. 1992). An
ecological epidemiological approach is recommended to enhance sediment risk assess-
ments by taking into account factors other than source information. In this empirical
approach, summaries of data for the aquatic ecological effects assessment may include:
• Sediment toxicity
Estimation of mean and variance of percent response for each
toxicity test endpoint at each station
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Chapter 2. Human Health and Ecological Risk Assessment
Mapping of bioassay response data
Correlations of bioassay response with contaminant concentra-
tions and ancillary variables to evaluate potential cause-effect
relationships
Evaluation of the suitability of the reference area (if available)
and negative controls
Pairwise statistical comparison of mean percent response at each
potentially contaminated site with the reference area (if available)
or negative control responses
• Benthic macroinvertebrate communities
Estimation of mean and variance of taxon abundances or commu-
nity indices at each station
Mapping of benthic invertebrate data
Correlations of community parameters with contaminant concen-
trations and ancillary variables to evaluate potential cause-effect
relationships
Evaluation of the suitability of the reference area (if available)
Pairwise statistical comparison of community parameters at each
potentially contaminated site with the reference area value (if
available)
• Fish histopathology
Correlations of the prevalence of tumors and other abnormalities
with contaminant concentrations and ancillary variables to
evaluate potential cause-effect relationships
Evaluation of the suitability of the reference area (if available)
Pairwise statistical comparison of the prevalence of tumors and
other abnormalities at each potentially contaminated site with the
reference area value (if available).
Guidance on the use of toxicity tests, benthic macroinvertebrate community surveys, and
fish histopathology investigations in support of ecological risk assessments is provided
in the ARCS Assessment Guidance Document (USEPA 1993).
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Chapter 2. Human Health and Ecological Risk Assessment
Risk Characterization
In risk characterization, the exposure and ecological effects assessments are combined
to estimate the probability of adverse ecological effects. Final risk estimates may be
expressed in simple narrative terms or as quantitative values. The risk characterization
should summarize:
• Results of the exposure and ecological effects assessments
• Risk estimates for aquatic and wildlife receptors of concern
• Potential ecological consequences
• Major sources of uncertainty.
Mitigating factors, such as reduced bioavailability of contaminants incorporated into
sediment particles and mechanisms for possible wildlife avoidance of hot spots, should
be discussed. The importance of mitigating factors should be confirmed by field
measurements or laboratory experiments on samples from the site. Also, compensatory
mechanisms that preclude population- or community-level effects should be acknowl-
edged, even though effects on individuals may be predicted.
Approaches to develop quantitative risk estimates (or hazard indices) include the quotient
method, joint probability analysis, model uncertainty analysis, and integrated analysis of
site-specific empirical data (Barnthouse et al. 1986; Suter et al. 1992; Ginn and Pastorok
1992; Chapman et al. 1992). The quotient method uses a ratio of the value of an end-
point at the site to a toxicity reference value such as a NOAEL as an approximate risk
index. The quotient method is useful mainly for screening-level analyses because it does
not provide a complete characterization of the magnitude of risk and uncertainties
(Bascietto et al. 1990). Joint probability analysis (Barnthouse et al. 1986) can be applied
to estimate the risk that exposure exceeds toxicity thresholds or criteria where probability
distributions are available for the variables being compared. Model uncertainty analysis
(Barnthouse et al. 1986; Suter et al. 1992) may be used to develop risk estimates for a
species based on statistical analysis of growth, survival, and reproduction of individuals.
Approximate risk estimates may be derived for an ecological system associated with
sediments by combining site-specific data for chemicals, toxicity tests, community
indices, and possibly other risk indicators (Chapman et al. 1992).
Wildlife
The quotient method and joint probability analysis will likely continue to be the primary
methods for expressing estimates of risk to wildlife receptors. An estimated chemical
intake by a wildlife receptor may be compared with a NOAEL. In interpreting hazard
quotients, Kubiak and Best (1991) propose that to be ecologically protective, the ratio of
the exposure to the NOAEL should be less than 1, because this provides a reasonable
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Chapter 2. Human Health and Ecological Risk Assessment
level of assurance that adverse effects would not occur as a result of excessive contami-
nant exposures. However, hazard quotients must be interpreted relative to the assump-
tions on which the assessment is based, the assessment endpoints, and the degree of con-
fidence in the relationship between the assessment endpoints and the measurement end-
point used in the hazard quotient. For example, for reasonable maximum exposure scen-
arios, hazard quotient values between 1 and 10 do not necessarily indicate a significant
risk. For most exposure scenarios, hazard quotients of greater than 10 are generally con-
sidered to represent a significant ecological risk. Prior to interpreting any hazard
quotient, agreement must be reached on the toxicity value analyzed (e.g., dose of a
substance that results in 50-percent mortality in a population of test organisms [LD50],
lowest-observed-adverse-effect level [LOAEL], NOAEL, reference dose).
The joint action of contaminants should also be considered. Hazard quotients for
individual contaminants with similar modes of action may be summed to yield a hazard
index.
Aquatic Life
The quotient method, joint probability analysis, and site-specific integrated data analysis
will likely be the approaches that are commonly applied to assess risks to aquatic life
associated with contaminated sediments. Use of the quotient method may involve
comparison of a measured assessment endpoint to a threshold value considered indicative
of toxicity or to a value indicative of reference area conditions.
Uncertainty Analysis
Possible sources of uncertainty include natural variation, missing information, and errors
associated with measurements, extrapolations of data, or models. Uncertainties may be
related to selection of contaminants of concern, selection of species, estimates of
exposure concentrations or doses, the quality of the toxicological data used for NOAELs
or LOAELs, or differences in exposure-response relationships or bioaccumulation of
chemicals among species. The most important sources of uncertainty identified in the
exposure and ecological effects assessments should be evaluated and quantified to the
extent possible. Model uncertainty analysis may include sensitivity analysis and Monte
Carlo simulation.
The baseline aquatic risk assessments for the ARCS Program were designed to comple-
ment the baseline human health and wildlife risk assessments so that the exposure
pathways leading from sediments to fish to humans and wildlife could be quantified.
However, these aquatic risk assessments were difficult to perform for different trophic
levels of the aquatic food chain because of data gaps. Conservative assumptions and
published data from other studies were used to fill missing information. The problem
of data gaps affects most aquatic and wildlife risk assessments and constitutes a major
source of uncertainty in the risk estimates.
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Chapter 2. Human Health and Ecological Risk Assessment
Uncertainty analysis should be conducted during the problem formulation stage to
identify data gaps and plan the approach for the ecological risk assessment. When
significant data gaps exist, the assessment should typically include several tiers of
analysis, with use of available data in the early screening tier to help define critical data
needs to be addressed in further field sampling or laboratory studies and subsequent
analyses in the next tier.
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3. MA SS BALANCE MODELING APPRO A CH
FOR ASSESSING REMEDIAL AL TERN A TIVES
A T CONTAMINA TED SEDIMENT SITES
Mass balance modeling studies were conducted at the Buffalo and Saginaw River AOCs
to demonstrate the use of this approach in evaluating remedial alternatives for con-
taminated sediments. Mass balance modeling studies were applied at these AOCs to
estimate changes in contaminant concentrations in water, sediments, and biota that may
occur following sediment remediation. The estimated concentrations can then be used
to compute changes in human health and ecological risks and to aid in the selection of
remedial alternatives. The mass balance modeling studies, as described below, are based
on established models and methods and are considered applicable to other sites with
contaminated sediments.
OVERVIEW
While there are many possible remedial alternatives for contaminated sediments, only a
few may be feasible at a particular site. After the range of feasible remedial alternatives
is identified for a site, the potential for reduction of contaminant concentrations in water,
sediments, and biota must be considered when selecting a remedial alternative. This
selection process requires some method to estimate changes in contaminant concentrations
that would result from each remedial alternative. The preferred approach for estimating
these changes, and the approach used in the ARCS RAM studies, is based on the
application of mass balance models.
Mass balance models attempt to describe each of the underlying mechanisms causing
change in the system, and are termed mechanistic. In the mass balance modeling
approach, the law of conservation of mass is applied in the evaluation of the sources,
transport, and fate of contaminants. This approach requires that the quantities of
contaminants entering the system (i.e., contaminant loading) equal the quantities leaving
the system, less the quantities stored, transformed, or degraded. Thus, the mass balance
is simply a bookkeeping of all of the processes affecting the mass of contaminants in a
system. After the mass balance has been established for each contaminant of concern,
quantitative changes in contaminant concentrations can be estimated. For example, the
mass balance can be used to estimate the change that may be expected following removal
of some portion of the contaminant mass.
Mass balance models have been successfully applied to the Great Lakes and elsewhere
in the regulation of toxic and conventional pollutants. Properly applied, a mass balance
38
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Chapter 3. Mass Balance Modeling Approach
model can serve as a surrogate for the natural system that is easily manipulated to
estimate the system's response to change.
Management questions that can be addressed with a mass balance model include:
1. What are the consequences of leaving the contaminated sediments in place
(the no-action alternative) under present conditions or where contaminant
loadings are reduced?
a. Is the system in equilibrium with present loadings? For exam-
ple, will sediment contaminant concentrations increase or
decrease over time or remain the same under present loading
conditions?
b. What are the relative contributions of loadings from point and
nonpoint sources? What are the major loss mechanisms (e.g.,
outflow, burial)?
c. What is the effect of changes in loadings? For example, if
loadings are reduced or eliminated, what is the effect on contam-
inant concentrations in the water, sediments, and biota?
d. How long does it take for the system to respond to changes in
loadings? For example, if the loads are reduced, how long
would it take for the concentrations to reach acceptable levels in
water, sediments, and biota?
e. If left in place, will contaminated sediments pose a threat to
downstream areas or will they become more widely dispersed?
2. What are the consequences of alternative remedial/mitigative actions, such
as removing, immobilizing, or treating the contaminated sediments?
a. What are the expected benefits of alternative remedial/mitigative
actions in terms of contaminant reductions in water, sediment,
and biota?
b. What is the probability of recontamination following remedial/
mitigative actions under present loading conditions? How long
would recontamination take?
c. What risks are associated with implementation of alternative
remedial/mitigative actions?
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Chapter 3. Mass Balance Modeling Approach
3. What are the estimated loadings from the AOC to downstream water
bodies? For example, the Buffalo River may serve as a source of contami-
nants to Lake Ontario, whereas contaminants from the Saginaw River may
be transported to Saginaw Bay. What changes can be expected in these
loadings following the implementation of selected remedial/mitigative
actions?
COMPLEXITY OF THE MASS BALANCE MODELING STUDY
Mass balance modeling studies vary widely in their complexity. The modeling approach
can vary from simple screening calculations to applications of complex computer
programs. Modeling studies also vary in the complexity of the field studies used to
support the mass balance calculations, from studies relying solely on historical data to
large and expensive field efforts. The degree of complexity required depends on the
physics of the system, factors affecting the transport and transformations specific to the
contaminants of concern, and the management questions the mass balance modeling study
will address. The degree of complexity used in particular studies is often dictated by the
time and funding available.
Modeling studies of contaminants in the Great Lakes have typically been cataloged into
groups or study levels depending on the level of effort and complexity in the modeling
and supporting field studies. Studies have been categorized as either "screening" or
"detailed" studies, as well as "Level 0" through "Level III" studies. The study levels
can generally be described as:
• Level 0—Application of simple manual or graphical methods based on
statistical or deterministic equations to obtain rough estimates of contami-
nant concentrations over extensive areas or to identify trouble spots for
more detailed analyses. These analyses rely solely on available data to
obtain a preliminary assessment of management options and to identify
deficiencies in the database when planning more detailed evaluations.
• Level I—Application of simple computerized models to obtain rough esti-
mates of contaminant concentrations over extended periods of time.
Model equations are generally mechanistic in nature but only approximate
the basic processes. As a result, model projections used to address the
management questions involve considerable uncertainty. Data collection
is usually limited to one preliminary data collection study. Qualitative
estimates are usually based on experience in interpreting the results. A
formal uncertainty analysis is generally not included.
• Level n—Application of a computerized model of intermediate complexity
as a planning model, or as a rough engineering design or resource manage-
ment model. Extensive areas and periods of time can be simulated but at
significant cost in data collection and preparation. Data collection involves
acquisition of at least two independent data sets to bracket important
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Chapter 3. Mass Balance Modeling Approach
environmental conditions (e.g., high-flow vs. low-flow). All contaminant
loadings to the system must be well characterized. Some simplifications
and approximations limit the applicability of the model for remedial design
and management. Thus, uncertainty analyses are generally included as
part of the model application.
• Level in—Application of advanced mechanistic computerized models for
detailed remedial design and management. The modeling approach would
typically include descriptions of the hydrodynamics and sediment transport
in the system, as well as detailed computerized models of water quality
processes. Data collection involves at least two surveys to provide input
for both model calibration and model evaluation. The surveys may be
coupled with data collection over longer periods to establish trends and the
range of environmental conditions.
Generally, the level of uncertainty is reduced as the studies increase in complexity from
Level 0 to Level III, while the time and costs associated with each study level increase.
Increases in modeling complexity need not always correspond to increases in data
collection, so various combinations of the levels mentioned above are possible. For
example, simple models usually predict average conditions, so sufficient data must be
collected to provide an accurate estimate of the average condition for comparison with
predictions. However, more complex models may predict conditions at a specific point
(in time and space), thus reducing the need for averaging.
In the ARCS RAM studies, both near-field and far-field modeling studies were con-
ducted. The near-field modeling studies concentrated on the lower portions of the
Buffalo and Saginaw Rivers where sediment contamination was the greatest and where
remedial actions will likely be implemented. The far-field modeling studies were used
to estimate the impact of remediation of the Buffalo and Saginaw Rivers on their
receiving waters—Lake Ontario and Saginaw Bay, respectively.
While the general mass balance modeling approaches used in the near-field and far-field
studies were similar, they differed in the resolution of the models applied and in the level
of supporting field studies. The ARCS near-field studies were categorized as Level I or
a "Mini-Mass Balance" modeling effort. The primary criterion restricting the modeling
studies to Level I was the limited level of supporting field studies.
The far-field modeling was more typical of Level 0, in terms of data collection, and
Level I, in terms of modeling. These studies were designed to estimate the long-term
impact of changes in loadings from the rivers on their respective receiving water bodies.
As such, hydrodynamics and sediment transport were described rather than predicted,
and the resolution was not as precise as in the near-field studies. The far-field modeling
studies of Lake Ontario and Saginaw Bay relied exclusively on historical data.
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Chapter 3. Mass Balance Modeling Approach
COMPONENTS OF THE MASS BALANCE MODELING STUDY
The application of the mass balance modeling approach involves the quantification of the
sources, transport, and fate of contaminants. The components of the modeling approach
are illustrated in Figure 3-1. The typical steps in a mass balance modeling study are:
1) predict water and sediment transport, 2) use the predicted water and sediment trans-
port, along with estimates of contaminant loadings from point and nonpoint sources, to
estimate the changes in chemical concentrations in water and sediments, and 3) use the
predicted contaminant concentrations in water and sediments to estimate the transfer of
contaminants through the food chain and their accumulation in fish. The models used
for each step of this process are described below.
Water Transport Models
The first step in characterizing the transport of dissolved contaminants is to characterize
the transport of the water, or its motion. Often, much of the variability of contaminant
concentrations in the water column can be explained by water transport alone. Water
transport models may be descriptive (i.e., based on a balance of the water's mass) or
hydrodynamic (i.e., based on a balance of the water's momentum).
Characterization of water transport may be qualitative or quantitative, depending on the
study level. In a qualitative approach, flow patterns are either measured directly or
inferred from measurements of related parameters. The qualitative approach is often
adequate where the system is very simple (hydraulically) or where only long-term,
relatively rough estimates of water transport are required. A qualitative approach is most
often used in Level 0 and Level I studies and is occasionally used in Level II studies.
Hydrodynamic models are used to quantitatively predict changes in volumes, depths, and
velocities in response to changes in flow or water surface elevations. Hydrodynamic
models require data on boundary conditions (i.e., flows or water surface elevations, wind
speed and direction), which are applied to predict the resulting flows within the modeled
system. Flows are often measured at gaging stations on many Great Lakes tributaries,
and water surface elevations are routinely measured at many locations within the Great
Lakes. Where such direct measurements of flows in the tributaries of the Great Lakes
are problematical because of the complex interactions and relationships between upstream
inflows and lake effects, hydrodynamic models can be used to quantitatively predict
flows. Hydrodynamic models can also be used to estimate changes in flows that may
occur under future conditions, such as in evaluating the effects of changes in dredging
patterns.
The hydrodynamic models used in the ARCS RAM modeling studies included
HYDRO-3D, a 3-dimensional hydrodynamic and transport model, and RIVMOD, a
1-dimensional model. Both models are maintained and distributed by the Center for
Exposure Assessment Modeling at Athens, Georgia. Multidimensional models were
required where it was necessary to resolve variations in water transport and chemical
42
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Hydrodynamic Model
Sediment Transport
00
Contaminant Transport
Loading Study
Food Chain Model
Figure 3-1. Components of the mass balance modeling study.
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Chapter 3. Mass Balance Modeling Approach
characteristics along the longitudinal, vertical, and lateral axes of the rivers. For
example, a model that was capable of estimating variations in transport with depth was
required in the Buffalo River because of water column stratification during low-flow
periods.
A qualitative approach was taken for the far-field studies. For Lake Ontario, historical
measurements of inflows and outflows were used to compute water transport, while for
Saginaw Bay, water transport was computed from measured flows and chemical data.
Sediment Transport Models
Because contaminant concentrations in surface sediments are a primary concern in the
AOCs, adequate characterization of the movement of surface sediments is a critical step
in the mass balance modeling process. There are two primary goals of the sediment
transport model: 1) to predict the movement of the sediments to estimate changes that
may occur in patterns of erosion, deposition, and transport, and 2) to estimate the
transport of the paniculate contaminant mass.
Sediment transport models are based on a sediment mass balance. As with water
transport, sediment transport may be described either qualitatively or quantitatively in
mass balance modeling studies.
Qualitative approaches have commonly been used in Level 0 and Level I studies. In
studies of this type, measured or estimated settling, resuspension, and sedimentation rates
are used to compute the transport of sediments and their associated contaminants.
Although it is generally assumed that there is no net sedimentation or erosion, the
different contaminant concentrations on suspended and settled paniculate matter make the
quantification of these processes important. The qualitative approach has proven useful
in providing rough estimates of the effects of sediment transport on contaminant
distributions. However, sediment transport is a very dynamic process and the assumption
of no net sedimentation or erosion is a gross simplification.
In more quantitative sediment transport models, resuspension and transport are computed
using the output of a hydrodynamic model and the measured characteristics of the
sediments. The sediment types of primary importance are silts and clays, rather than
sands. Silts and clays are classified as cohesive sediments, while sand is classified as a
noncohesive sediment. The sediment transport model is used to predict changes in
sediment resuspension, deposition, and transport and the effect of these processes on
particulate concentrations in the water column. The sediment transport model is also
used to estimate changes in the structure of the sediment bed, such as the impact of
erosion or deposition on the channel shape and sediment composition.
As with hydrodynamic models, sediment transport models can be used to interpolate
between existing measurements or estimate sediment transport for conditions for which
data are not available. The majority of sediment transport occurs during extreme (rare)
44
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Chapter 3. Mass Balance Modeling Approach
events, such as storms on lakes and large runoff events in rivers. Because data are rarely
available for these events, sediment transport models must be used to estimate transport
under these conditions. For example, these models may be used to estimate whether con-
taminated sediments may be buried or exposed by a 10-year or 100-year flood. This
information can be used in the evaluation of remedial alternatives as well as the no-action
alternative. Sediment transport models may also be used to evaluate the impact of
removing or immobilizing sediments on subsequent erosion and deposition patterns. For
example, if sediments are removed from a particular area, sediment transport models
may be used to estimate how long it may take for the area to fill in, as well as changes
that may occur in deposition and erosion areas.
In the ARCS RAM modeling studies, a model of the transport of cohesive sediments was
applied to both of the near-field study sites (i.e., Buffalo and Saginaw Rivers) to predict
the interactions between the transport, deposition, and resuspension of sediments under
various meteorological and hydrological conditions. The model applied was a
2-dimensional (longitudinal and lateral), time-variable, hydrodynamic and sediment
transport model developed by Wilbert Lick at the University of California, Santa
Barbara. This model has been applied at various locations around the Great Lakes,
including the Detroit River, Fox River, Green Bay, Lake Erie, and elsewhere. The
sediment transport model was coupled with a 3-dimensional, time-variable model of the
sediment bed and its properties, the Water Quality Analysis Program, WASP4 (Ambrose
et al. 1990). WASP4 integrates predictions from the hydrodynamic and sediment trans-
port model to estimate contaminant concentrations in the water and sediment. The
WASP4 model, maintained and distributed by the Center for Exposure Assessment
Modeling at Athens, Georgia, has been widely distributed. WASP4 is currently the
framework used for modeling studies in Green Bay, Lake Michigan, Lake Ontario, and
elsewhere around the Great Lakes. It is generally assumed that the water column is
completely mixed vertically because sediment resuspension typically occurs at flows
where this would be the case. This model was used to predict variations in suspended
solids concentrations, resuspension and deposition rates, and variations in the sediment
bed as a function of flows and loadings. The model provided predictions for use in
determining the transport of sorbed contaminants and resuspension of toxic sediments.
Application of the sediment transport model was of particular importance in these studies
because of the lack of historical sediment data. Therefore, these studies relied heavily
on the sediment transport model to supply estimates of sediment transport, resuspension,
and deposition.
Contaminant Exposure Model
The contaminant exposure model is the mass balance model for contaminants. The
contaminant exposure model used as a framework for the ARCS RAM mass balance
modeling studies was WASP4. In the application of the contaminant exposure model,
the rate of change in contaminant mass (accumulation) is a function of the transport of
a contaminant into, out of, and within the system via water transport or sediment
transport for those materials that sorb to sediments; the mass added to the system via
45
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Chapter 3. Mass Balance Modeling Approach
point and nonpoint loadings minus the outputs; and the quantities transformed and
degraded within the system via processes such as volatilization, biodegradation, and
photodegradation. The output expected from the contaminant exposure model includes
estimated contaminant concentrations of both paniculate and dissolved forms in water and
sediments, as well as estimated changes in mass fluxes due to inflows and loadings,
outflows, and degradation and transformation processes. Depending on the level of
complexity, the transport via water and sediments may be described or predicted using
hydrodynamic and sediment transport models, which are then coupled with the con-
taminant exposure model.
In the ARCS RAM near-field studies, WASP4 was applied to predict the effects of water
transport, sediment transport, sorption, and transformation processes on the concen-
trations of selected contaminants of concern. The WASP4 model was linked to the out-
put of the hydrodynamic and sediment transport models, which together provided the
necessary transport information, using data collected during ARCS field studies. The
output of the contaminant exposure model included temporally and spatially varying
estimates of contaminant concentrations in water (both particulate and dissolved) and
sediments for comparison to field data and for projections of the effectiveness of various
remedial alternatives. In addition, the output included estimates of the magnitude of all
processes that result in gains or losses of contaminants, so that their relative importance
could be evaluated. The contaminant exposure model also provided information that
could be incorporated into the food chain model to estimate the contaminant body
burdens in fish due to varying contaminant concentrations in water and sediment.
The same contaminant exposure model used in the near-field studies (WASP4) was
applied in the far-field studies. However, in the far-field studies the resolution was not
as precise as in the near-field studies. Therefore, there was no need to apply the
hydrodynamic and sediment transport models in the far-field studies. WASP4 was
applied to predict steady-state (long-term average) conditions over large spatial scales.
The output of the model included load-response relationships. For example, given a
particular contaminant loading, the model predicted the average contaminant con-
centrations in water [both particulate and dissolved] and sediments. This relationship was
then used to evaluate the impact of changes in loadings from the two AOCs on Lake
Ontario and Saginaw Bay.
Food Chain Model
The food chain model is a mass balance model for contaminants where the rate of change
in contaminant mass in each component of the food chain is a function of the transport
of a contaminant into and out of that component (e.g., via ingestion, gill exchange,
excretion), as well as internal changes that may occur due to growth. The food chain
model supports evaluation of the impact of various remedial alternatives including the no-
action alternative on contaminant concentrations within the food chain, given variations
in contaminant concentrations in water and sediments derived from the contaminant
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Chapter 3. Mass Balance Modeling Approach
exposure model. Outputs from the food chain model include time-variable estimates of
contaminant concentrations in each component of the food chain.
The food chain model applied to both the Buffalo and Saginaw Rivers as part of the
ARCS RAM studies was the Water Quality Analysis Simulation for Toxics, WASTOX
(USEPA 1985), a predecessor of WASP4 that includes a food chain component. In the
Buffalo River, the only fish species simulated was carp, because it is a bottom-feeder
with a high fat content and would be expected to have an appreciable contaminant body
burden. In the Saginaw River, the food chain model included forage fish and walleye,
the latter due to its high importance to recreational anglers. Data collected as part of the
ARCS RAM studies were used to construct a simple food chain model.
REQUIRED FIELD DATA
Field data are required to apply any mass balance model. For model application, field
data are required to define the characteristics of the site and the mass fluxes of water,
sediment, and the associated contaminants into and out of the site. In addition, data are
often required to estimate the site-specific values of model parameters and to assess the
uncertainty associated with model projections. The confidence that can be placed on
those projections is dependent upon both the integrity of the model and how well the
model is calibrated to that particular water body.
While models can be run with minimal data, the resulting predictions are subject to large
uncertainty. Models are best used to interpolate between existing conditions but may be
used to extrapolate from existing to future conditions, such as in the evaluation of the
effects of remedial or mitigative alternatives.
The types of data required and the necessary frequency of measurement vary with the
level of model complexity, the characteristics of the system, and the contaminants of
concern. Generally, three kinds of data are required: 1) boundary conditions, 2) initial
conditions, and 3) data for calibration/evaluation. Boundary conditions are external to
the model (i.e., the model does not predict them). Instead, they are used to "drive" the
model. Initial conditions are used to aid in the design of the model application (e.g., to
determine segmentation) and to provide a starting point for model predictions. For
example, the initial, or existing, sediment contaminant concentrations are required to
provide a starting point for model predictions. Temperature data may be used to
determine the need for predicting water-column stratification and its effects on contami-
nant transport. Model calibration/evaluation data include the parameters that the model
is designed to predict (e.g., contaminant concentrations and fluxes). These data are
compared to the model's predictions to aid in determining the values of site-specific
parameters and in estimating the uncertainty associated with those predictions. The types
of data required for each component of a mass balance modeling study are described
below.
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Chapter 3. Mass Balance Modeling Approach
Water Transport Data
Boundary condition data for the water transport models include the system's bathymetry,
point source and nonpoint source flows and loadings, upstream and downstream flows
and/or water surface elevations, and weather data (e.g., wind speed and direction,
temperature). These data can often be obtained from the National Oceanic and Atmos-
pheric Administration, U.S. Army Corps of Engineers (Corps), USEPA, USGS, and
local government agencies. Both historical data and data collected during the course of
field studies of the AOC are required. Model calibration/evaluation data may include
measurements of flows, velocities, or water surface elevations within the system under
investigation for comparison to model predictions.
The frequency of measurement of hydrodynamic parameters varies with the complexity
of the system under investigation. However, for most Great Lakes rivers and harbors,
their dynamic nature requires that data be available on an hourly basis for major tributary
flows, water surface elevations, and weather data. Water surface elevation and weather
data are usually readily available at this frequency.
In the ARCS RAM modeling studies, historical data were available on flows, water sur-
face elevations at the mouths of the Buffalo and Saginaw Rivers, meteorological condi-
tions, and the values of some conventional parameters, such as temperature and conduc-
tivity. Additional measurements of water surface elevations, water velocities and dis-
charges, and wind velocities and directions were also obtained concurrently with the
ARCS field studies.
Sediment Transport Data
Because sediment transport models are driven by hydrodynamic models, their application
requires the same data described above. The data for the system's bathymetry should be
the same as that used for the hydrodynamic model. However, because a sediment trans-
port model may be used to predict changes in the bathymetry in response to storm or
flow events, additional bathymetric information collected periodically (e.g., every
3 months or after large storm events) is highly desirable for evaluating the model's
performance.
Data on the initial conditions of sediment properties include particle size distributions,
bulk densities, porosity (water content), and resuspension potential. Because sediment
properties may vary spatially, characterization or mapping of these properties over the
study area is usually necessary. This mapping may be quantitative or qualitative,
depending on the objectives of the particular study.
Suspended solids concentrations at the boundaries over time are required to drive the
model, and suspended solids data within the modeled system are required for model
calibration/evaluation. Because sediment transport is nonlinear and highly dynamic,
these data are required as frequently as possible (i.e., continuously if feasible) at least
48
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Chapter 3. Mass Balance Modeling Approach
during the course of high-flow events. Collection of these data may often be readily
accomplished using automated sampling techniques or using some surrogate measurement
(e.g., water transparency) to estimate suspended solids concentrations where a relation-
ship can be established between the surrogate measurement and the suspended solids
concentration.
Data used in the ARCS RAM studies of the Buffalo and Saginaw Rivers included histori-
cal data, such as Corps dredging records. In addition: 1) data on sediment characteris-
tics (e.g., grain size, water content) were collected during the ARCS sediment surveys,
2) periodic bathymetric surveys were conducted to estimate changes in the system's
morphometry, 3) suspended solids concentrations were measured concurrently with the
river sampling, 4) suspended solids concentrations were measured either during high-flow
events (Buffalo River) or hourly during periods of the year (Saginaw River) to support
the sediment transport model, and 5) studies were conducted to estimate the resuspension
characteristics of the sediments.
Contaminant Exposure Data
The computation of contaminant exposure also requires information on water and sedi-
ment transport from the hydrodynamic and sediment transport models. When not avail-
able from models, these data must be derived from field measurements, similar sites, or
general guidelines.
Data on boundary conditions include measured concentrations of the contaminant(s) of
concern or loadings (mass per unit time) from all significant sources (e.g., tributaries,
point and nonpoint sources). Data on initial conditions include existing contaminant
concentrations in the water and surface sediments. Contaminant concentration profiles
of sediments are required if the impact of exposing deep sediments is to be evaluated.
For example, if dredging to a certain depth is proposed, then measurements of contami-
nant concentrations in the sediment layer expected to be exposed are required. Data
should be collected during both high- and low-flow events because the relative impor-
tance of processes affecting contaminant transport and fate vary under different flow
conditions.
The analytical approach used to support mass balance modeling may differ from that used
to support other studies. For example, for regulatory purposes it may be sufficient to
know that contaminant concentrations are below some criterion. However, to compute
contaminant mass the contaminant concentrations must be accurately known. Appreciable
numbers of concentrations below analytical detection limits, "non-detects," may make
the data unsuitable for use in mass balance modeling studies. In addition, excessive
"noise" in the data (random high or low detections due to sampling or analytical
variability) may make it difficult to distinguish trends. Specialized sampling and
analytical methods for trace metal and organic analyses are often required to reduce
analytical noise and obtain sufficiently low analytical detection limits.
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Chapter 3. Mass Balance Modeling Approach
Data are also required for materials that may affect the transport and transformation of
contaminants. For example, sorption onto solids affects the transport and transformation
of some organic and inorganic chemicals. The prediction of sorption requires that data
be available on the fraction of organic carbon in sediments and suspended solids,
dissolved organic carbon concentrations, and phytoplankton concentrations. Data on pH
are required to predict the speciation of metals and ionization of some organic chemicals.
Data on sulfides are required to predict metal precipitation. If data are not available for
constituents that affect contaminant transport and transformations, large uncertainties may
be introduced in the modeling study. Knowledge of the geochemistry of the contami-
nant(s) of concern and of the site to be modeled is essential.
In the ARCS RAM near-field studies, both historical and field data collected as part of
the ARCS Program for ambient water, sediment, contaminant loading, and food chain
relationships were used for the calibration of the contaminant exposure model. For the
far-field studies, only historical data were used.
The ARCS RAM near-field studies concentrated on the lower Buffalo and Saginaw
Rivers. Although there were some differences, the field sampling plans for the two sites
were similar in design. Synoptic surveys were conducted at the Buffalo and Saginaw
Rivers to identify spatial variability in the systems during certain low- and high-flow
periods. Synoptic surveys provide a "snapshot" of the system, or data at a particular
point in time. Six sampling stations were selected to allow estimates to be made of
contaminant fluxes into, and out of, the AOCs. Samples were integrated over the width
of the system and, in some cases, over depth. Where significant stratification was
encountered, samples were collected at several depths. Ambient data for particulate and
dissolved contaminants, as well as conventional parameters, were obtained over six
sampling days during 1990 and 1991. Selected conventional parameters were measured
at a greater frequency to aid in calibration of the hydrodynamic and sediment transport
models and to aid in estimating yearly loadings. Examples of the parameters measured
for the ARCS RAM mass balance modeling studies are listed in Table 3-1. Sediment
contaminant concentrations were measured during separate field sampling studies.
ARCS studies were also conducted to identify contaminant loadings and concentrations
in water, sediments, and biota. Contaminant loadings were estimated and/or measured
from both point and nonpoint sources. Historical data were used to estimate loadings
from point sources. Loading measurements were also acquired concurrently with the
ambient water quality studies. Loadings from CSOs in the Buffalo River were estimated
based on a limited field sampling program (24 samples at 10 CSOs) and storm water
modeling. For the Saginaw River study, CSOs were not identified as a significant source
of contaminant loadings and were, therefore, not sampled. Loadings of contaminants and
suspended solids from upstream tributaries were based on six daily measurements during
the fall of 1990 and the spring of 1991. Historical contaminant, suspended solids, and
flow data, as well as data from a suspended solids survey, were collected to extrapolate
these measurements to annual loading rates.
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TABLE 3-1. EXAMPLES OF PARAMETERS MEASURED FOR THE ARCS RAM
MASS BALANCE MODELING STUDIES
Parameter
Dissolved PCBs
Participate PCBs
Total PCBs
Dissolved PAHs
Paniculate PAHs
Total PAHs
(benz[a]anthracene, benzo[b]fluoranthene,
benzo[k]fluoranthene, benzo[a]pyrene.
chrysene)
Dissolved metals
Paniculate metals
Total metals
(lead, copper, zinc3)
Paniculate iron
Sulfides8
Dissolved oxygen
Conductivity
Temperature
PH
Alkalinity
Suspended solids
Dissolved organic carbon
Paniculate organic carbon
Chlorophyll a and phaeophytin
Hardness
Total incident radiation
Light extinction
Wind velocity and direction
Water surface and elevation
Flow
Lipid content
River
X
X
S
X
X
S
X
X
S
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
Tributaries,
Point Sources
X
X
X
S
X
X
S
X
X
X
X
X
X
X
X
X
X
Biota
X
X
X
X
Note: PAH - polycyclic aromatic hydrocarbon
PCB - polychlorinated biphenyl
S - total computed from sum of
paniculate
and dissolved concentrations
Saginaw River study only.
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Chapter 3. Mass Balance Modeling Approach
Food Chain Data
The predictions of the food chain model are based on the contaminant concentration data
obtained from the contaminant exposure model. That is, the food chain model is the
final link in the chain of models relating transport and contaminant transformations to
concentrations of contaminants in biota. The exposure concentrations of chemicals are
used along with information on the bioenergetics of organisms to estimate concentrations
in the biota.
Generally, the species that are of interest in food chain modeling are the higher preda-
tors, such as walleye. However, to estimate the variation of contaminant concentrations
in these higher predators, and the effects of changing conditions on them, it is necessary
to estimate the contaminant concentrations in the components of the food chain leading
up to the higher predators. Therefore, one of the initial steps in the food chain modeling
is the determination of the components of the food chain for the species of interest. The
food chain may not only include different species of fish and forage but may be further
subdivided into different life stages because of variations in food or feeding patterns. In
addition, many higher predators may migrate into and out of the study area. The prefer-
ence for a particular forage species may also change seasonally. It is often desirable to
select target organisms that have limited and well-defined food chains and migration pat-
terns to minimize the complexity of the food chain model and the extent of the supporting
field data that will be required.
In addition to food chain relationships, it is necessary to have information on the
bioenergetics of the components of the food chain. This information may include growth
rates, reproduction rates, gill characteristics, ingestion rates and uptake efficiencies,
swimming rates, and excretion rates. Although site-specific information is preferred,
much of these data can be obtained from available literature and databases.
Contaminant concentrations in the various components of the food chain are required to
provide a starting point for the model (the initial conditions) as well as to provide for
model calibration/evaluation. In addition to contaminant concentrations, data should be
collected on other characteristics affecting uptake. For example, data on lipid content
are required because it affects the uptake of many hydrophobic contaminants. One diffi-
culty with obtaining data for calibration/evaluation of food chain models, as with
contaminant exposure models, is that changes in contaminant concentrations may occur
slowly. Particularly in higher predators, changes may occur over seasons and over years
so that, ideally, data should be available on those time scales. Therefore, a complete
characterization requires either an adequate historical database or long-term field studies.
The lack of these data may result in greater uncertainty in model predictions.
Data were collected in ARCS studies to support food chain modeling studies. In the
Buffalo River, contaminant concentrations in carp and in their stomach contents were
analyzed to establish a relationship between contaminant concentrations in carp tissue and
contaminant concentrations in their benthic forage. Carp were selected because there are
currently advisories in effect against consumption of carp from the Buffalo River. Data
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Chapter 3. Mass Balance Modeling Approach
were collected for nine carp composited into three age classes for analysis. Sampling in
the Saginaw River concentrated on walleye and its food chain, because of the importance
of the walleye fishery in this area.
MODEL APPLICATION
A typical model application involves characterization of the system followed by an
evaluation of model predictions. Upon completion of the evaluation, the model is then
used to address management questions.
The characterization step involves establishing the boundary conditions for the model
(those things outside of the model that affect its predictions, such as inflows and
bathymetry), determining the initial conditions of the system (such as initial sediment
contaminant concentrations), and determining site-specific parameter values. Some site-
specific parameters may be known from previous studies or may be easily measured.
Some parameters may be unknown or difficult to measure. Unknown parameters can be
estimated by calibrating the model to field data, by applying values that have been
established as representative of similar sites, or by using general guidelines to establish
values.
Model calibration using site-specific field data is likely to yield the most accurate esti-
mates for unknown parameters, but can only be applied when existing data (e.g., particle
distributions, contaminant distributions in water and solids, or contaminant distributions
in species) are available. In addition, as the number of unknown parameters increases,
so does the difficulty of estimating them by calibrating the model to a single data set.
With even two unknown parameters, it is possible that values could be selected so that
the model prediction matches a data set used for calibration, but these values may not be
intrinsically accurate. Therefore, the values may lead to biased predictions under other
conditions, such as a change in contaminant loading.
Parameter values that have been established as representative of a site similar to the one
being modeled may be used if those values are the result of direct observation, calibra-
tion of the model at that other site, or the prediction of a different model that in turn has
been calibrated and verified. Two sites need not be similar in all respects for some
parameters to apply equally well to both. For example, similarity of particle settling
rates might be established on the basis of bottom type, particle type, particle supply rate,
and current velocities; other site characteristics such as size and boundary configuration
could be very different.
General guidelines for establishing parameter values may consist of published tables of
values, statistical distributions or regressions, or rule-of-thumb calculations. A literature
search may be needed to locate such general guidelines, if they exist. One potentially
useful compendium of guidelines is Bowie et al. (1985). General guidelines usually
suggest a range of parameter values that may be appropriate for a given situation. Such
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Chapter 3. Mass Balance Modeling Approach
ranges may be used to estimate the potential error of the model's prediction, or they can
be used to select an environmentally protective value.
The evaluation step involves comparisons of model predictions to field data. These data
are separate from those used in the calibration step. The comparison of predictions to
field data in the evaluation step allows an estimation of model uncertainty.
The final step of model application is the use of the model for its intended purpose, that
is to address management questions concerning an AOC. Some of the management
questions a model may be used to address were listed at the beginning of this chapter.
The model applications at the Buffalo and Saginaw River AOCs resulted in estimated
concentrations of selected contaminants in sediment, water, and biota as a function of
contaminant loadings. In addition, these models provided a means of estimating the
consequences of various remedial or mitigative alternatives. Specific model outputs for
each AOC included:
• Loading/response curves for each selected contaminant, relating external
loadings to contaminant concentrations in water, sediment, and specific
fish species by age group, for each river reach. Uncertainty estimates
were provided for loading/response relationships.
• For each selected contaminant, estimated loadings from the AOC to the
receiving water body for a variety of flow conditions and as affected by
selected remedial or mitigative alternatives.
• Estimated time to recovery in order to assess the no-action alternative as
well as the relative benefits of selected remedial or mitigative alternatives.
• Estimates of the relative importance of various processes affecting the
transport and transformations of contaminants, such as losses due to
volatilization, burial, and outflows.
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4. COMPARA TIVE RISK ASSESSMENT
The comparative risk assessment is the final step in the comprehensive risk management
process (Figure 1-1) that was addressed in the ARCS RAM studies. The comparative
risk assessment integrates information from all previous steps in the process to estimate
changes in risk, relative to the baseline risk, that would result from implementation of
the various sediment remedial alternatives evaluated. Thus, the comparative risk assess-
ment provides a prognostic framework that can aid in addressing management questions.
Management questions that may be addressed by the comparative risk assessment, such
as the potential impacts of proposed remedial alternatives, generally coincide with those
addressed by mass balance models. However, in the comparative risk assessment, con-
taminant concentration estimates generated by mass balance modeling (Step 7 of
Figure 1-1) are used to derive risk estimates for various remedial alternatives being
considered within an AOC. The remedial action objectives that had initially been
developed during risk assessment planning (Step 2 of Figure 1-1) are then refined during
the comparative risk assessment.
The approach used in the comparative risk assessment integrates the results from the
baseline risk assessment, field studies, and mass balance modeling studies to provide
estimates of the potential impact of remedial actions on human health, wildlife, and
aquatic organisms. The output of the baseline risk assessment and modeling studies
serves as input to the comparative risk assessment (Figure 4-1). For example, output
from the baseline risk assessment includes algorithms, exposure parameters, and toxicity
values used for deriving conservative estimates of risks based on current site conditions.
Output from the field studies includes measured contaminant concentrations in water,
sediments, and biota. Output from the mass balance modeling studies includes estimated
contaminant concentrations in water, sediments, and selected fish species following the
implementation of proposed remedial alternatives. The comparative risk assessment
integrates these outputs to produce estimates of risks for all remedial alternatives under
consideration. Thus, the risks associated with each remedial alternative can be compared
with the risks associated with the other remedial alternatives, as well as with the baseline
risks.
Comparative risk assessment studies were conducted for the Buffalo and Saginaw River
AOCs to demonstrate methods for estimating potential changes in risks to humans, wild-
life, and aquatic organisms exposed to sediment-derived contaminants under selected
remedial alternatives. The comparative risk assessments resulted in estimates of potential
risks that may be used in the selection of the most appropriate remedial alternatives.
In the ARCS RAM studies, potential remedial alternatives considered included no action
and several dredging and capping scenarios. No action was defined as no change in
existing sediment management practices (e.g., continued maintenance dredging).
-------
Site Characterization
Evaluation
of Available Data
Mass Balance
Modeling Studies
Field Studies
Baseline Risk Assessment
(human health, wildlife,
aquatic organisms)
Human Health
COMPARATIVE RISK ASSESSMENT
Wildlife
CJl
t
t
B
No
Action
REMEDIAL
ALTERNATIVE
B C No
Action
REMEDIAL
ALTERNATIVE
Selection and
Implementation of
Remedial Alternative
Aquatic Organisms
t
B C No
Action
REMEDIAL
ALTERNATIVE
Figure 4-1. Comparative risk assessment in the risk management process.
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Chapter 4. Comparative Risk Assessment
Dredging options that were considered included complete dredging of contaminated
sediments, dredging of hot spots, dredging to a selected depth, and cessation of dredging
in part or all of a channel. The comparative risk assessment produced estimates of the
potential changes in risk that may result from each of these remedial alternatives. These
estimates may then be compared to the existing (baseline) risk to evaluate the relative
benefits of remediation and aid in the selection of the most appropriate remedial alterna-
tive.
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5. SUMMARY
There are currently large numbers of sites in the Great Lakes and elsewhere where con-
taminant concentrations in sediments are at levels that are of concern to environmental
scientists, regulatory agencies, and the general public. Many of these contaminants, such
as heavy metals and chlorinated organic compounds, are resistant to degradation by
physical, chemical, or biological processes. Therefore, the impact of contaminated
sediments may continue well into the future.
Among the primary concerns associated with contaminated sediments are the risks they
pose to humans, wildlife, and aquatic organisms. To protect human health, local authori-
ties in many areas have posted fishing and swimming bans, issued fisheries advisories,
or ceased drawing drinking water from areas with contaminated sediments. Sediment
contamination has made many areas uninhabitable for benthic organisms or has resulted
in bioaccumulation through the food chain, adversely affecting both fish and wildlife.
The risks posed by contaminated sediments result from the exposure of organisms to the
chemicals through a number of pathways, including direct adsorption from water or sedi-
ments or through feeding. However, myriad processes are known to affect that
exposure. For example, factors affecting the degree of contamination and exposure may
include the degree of ongoing contaminant loading, hydraulic and sedimentation patterns,
the physical characteristics of the sediments, the degree of sediment/water interaction,
and the characteristics of the chemicals.
As part of the ARCS Program, studies were conducted by the RAM Work Group to
demonstrate an integrated approach to evaluating the processes that affect exposure and
risks resulting from sediment contamination. This approach is potentially applicable to
other areas with contaminated sediments, both in the Great Lakes region and elsewhere.
The first step in the approach is the identification of the degree of existing sediment
contamination, potential exposure pathways, chemical toxicity, and the resulting potential
risks posed to humans, wildlife, and aquatic organisms. The second step, if required,
is to conduct field studies to further characterize the distributions and concentrations of
contaminants of concern and to aid in identifying factors affecting their transport and
transformations. Based on those field studies and risk assessments, priorities can be
established for areas that need remediation and potential remedial alternatives can be
identified. Next, mass balance models can be used to evaluate the processes affecting
exposures and risks and to predict potential changes in conditions that may occur follow-
ing implementation of the selected remedial alternative(s). Finally, these predicted
changes in conditions can be used to estimate potential changes in risks that may occur
following remediation.
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Chapter 5. Summary
The integrated approach used in the ARCS RAM studies and described in this report
provides a means of evaluating the potential consequences of remediation, in terms of
both exposure and risks. The approach can be used to aid in addressing management
questions, such as "How long will it take for the problem to go away through natural
processes in the absence of active remediation?," "What happens if the sediments are
dredged to a particular depth?," or "Will the sediments become recontaminated following
remediation?" Like all such approaches, there are various limitations that result from
deficiencies in the current understanding of factors affecting the transport and transfor-
mations of contaminants, chemical characteristics, or chemical toxicity. However, prop-
erly used, the approach may provide a viable means of assessing the nature and extent
of sediment contamination and aid in the evaluation and selection of appropriate remedial
alternatives.
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6. GLOSSARY
Acute—Characterized by a time period that is relatively short in comparison to the life
span of an organism. Acute toxicity is the characteristic of a chemical to cause a toxic
response in organisms immediately or shortly after exposure to the chemical.
Adverse effect—An impairment of biological functions or description of ecological
processes that results in unfavorable changes in an ecological system.
Ambient water quality criterion (plural: criteria)—An estimate of how much of a
chemical could be present in the water without harming human health or aquatic life.
Aquatic—Living or growing in water.
Area of Concern (AOC)—A waterbody (e.g., river, harbor, bay) within the Great Lakes
basin that has been identified as having impairment of beneficial uses attributable to
chemical contamination. In most of the Great Lakes AOCs, sediment contamination is
a significant contributor to the impairment of beneficial uses.
Assessment endpoint—An ecological value to be protected (e.g., trout population
abundance or community structure that indicates a "healthy" biological community).
Baseline risk assessment—An assessment that estimates risks associated with existing
environmental conditions.
Benthic—Pertaining to, or associated with, the bottom of a body of water.
Bioaccumulation—Net uptake of a chemical into the tissues of an organism as a result
of direct contact with a medium, such as water or soil, or through the diet.
Biodegradation—The decomposition of a chemical substance by natural biological
processes.
Biomagnification factor—A measure of the degree of increase in the tissue concentration
of a chemical with each trophic step in a food chain. For example, a biomagnification
factor of 5.0 indicates that the concentration of a given chemical in the tissues of a
predator is 5 times the concentration of that chemical in the tissues of its primary prey
species.
Bioavailability—The degree to which a chemical can be taken into the tissues of an
exposed organism.
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Chapter 6. Glossary
Biomass—The total weight of live organisms in a sampled population or community.
Cancer slope factor (CSF)—Plausible upper-bound estimate of the probability of a
response per unit of intake of a chemical over a lifetime. The cancer slope factor is used
to estimate an upper-bound probability of an individual developing cancer as a result of
a lifetime of exposure to a particular level of a particular carcinogen.
Carcinogenic—Capable of causing cancer in an organism.
Chronic—Characterized by a time period that represents a substantial portion of the life
span of an organism (e.g., chronic toxicity is the characteristic of a chemical to produce
a toxic response when an organism is exposed over a long period of time).
Chronic intake level—Exposure expressed as the mass of a substance contacted per unit
body weight over a long-term exposure period, often expressed as mg/kg-day over a
lifetime.
Community—Interacting populations of species (plants or animals) living in the same
habitat.
Comparative risk assessment—An evaluation of the changes in human health and/or
ecological risks resulting from a range of candidate remedial alternatives. Ideally, the
comparative risk assessment should include evaluation of the risks associated with all
components (e.g., removal, pretreatment, treatment, disposal) of each remedial
alternative under consideration.
Compensatory mechanism—A biological process that offsets or counteracts an adverse
effect (e.g., increased survival of young fish related to reduced competition because egg
hatching success was reduced).
Concentration—The amount of a chemical substance expressed relative to the amount
of environmental medium (e.g., jug/L [micrograms of chemical per liter of water] or
/ug/g [micrograms of chemical per gram of soil]).
Conceptual model—A simplified description of important functional or structural
relationships in an ecosystem, including working hypotheses of how chemicals might
affect populations or communities.
Contaminant exposure model—A mass balance model for contaminants, in which the
rate of change in contaminant mass (accumulation) is a function of the transport of a
contaminant into, out of, and within the system (via water transport or sediment transport
for those materials that sorb to sediments); the mass added to the system (via point and
nonpoint loadings) minus the outputs; and the quantities transformed and degraded within
the system (via processes such as volatilization, biodegradation, and photodegradation).
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Chapter 6. Glossary
Contaminant of concern—A chemical or specific form of a chemical suspected of being
present in concentrations in the environment that may cause adverse effects to humans
or ecological receptors.
Dose—The amount of chemical taken into an organism per unit of time.
Dose-response relationship—The relationship between the dose of a contaminant
administered or received and the incidence of adverse effects in the exposed population.
From the quantitative dose-response relationship, toxicity values are derived that are used
in the risk characterization step to estimate the likelihood of adverse effects occurring in
humans at different exposure levels.
Ecological effects assessment—An assessment conducted to determine the relationship
between the levels of contaminant exposure (or other stressors) and the levels and types
of ecological effects.
Ecological epidemiological approach—An empirical assessment approach based on an
evaluation of existing ecological effects, especially the establishment of the causes of
reduced population abundances and alterations of community structure.
Ecological risk assessment—Evaluation of the likelihood of adverse effects on organ-
isms, populations, and communities from chemicals present in the environment.
Ecosystem—An ecological community of plants and animals together with its physical
environment, regarded as a unit.
Endpoint—The biological or ecological unit or variable being measured or assessed (see
measurement endpoint and assessment endpoint).
Equilibrium partitioning model—A mathematical expression that describes the
distribution of a chemical between sedimentary organic carbon and interstitial water
based on the assumption of thermodynamic equilibrium.
Exposure—Contact between a human or ecological receptor and a chemical in the
environment.
Exposure assessment—The portion of the risk assessment that describes the frequency,
magnitude, and duration of exposure of human or ecological receptors to contaminants
of concern.
Exposure duration—In human health risk assessment, the estimated number of years
over which exposure to contaminated media may occur.
Exposure frequency—In human health risk assessment, the number of days per year that
a person may contact contaminated media.
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Chapter 6. Glossary
Exposure parameters—Values used to estimate exposure in a risk assessment, such as
the number of days per week that exposure is expected to occur, or the amount of
contaminated media that a person might incidentally ingest per day.
Exposure pathway—The path a chemical takes or could take from a source to exposed
organisms. Exposure pathways include the source, the mechanism of release and
transport, a point of contact, and the means of contact (e.g., ingestion or inhalation).
Exposure point concentration—The concentration of a chemical at the point of exposure
to an organism. For example, the concentration in the soil in which a plant is growing.
Exposure-response assessment—A description of the relationship between the concentra-
tion (or dose) of the chemical that causes adverse effects and the magnitude of the
response of the receptor.
Exposure route—The means of contact between an organism and a toxic chemical (e.g.,
eating [ingestion], breathing [inhalation], or touching [dermal contact]).
Exposure scenario—A conceptual model of how exposure takes place, including specific
combinations of exposure media, pathways, and receptors and organism activities that
may lead to exposure.
Food chain—A sequence of species at different trophic (feeding) levels that represent a
single path of energy within a food web. For example, grasses and seeds are eaten by
a mouse which is then eaten by an owl. The owl is higher up the food chain (at a higher
trophic level) than the mouse.
Food web—Interconnected food chains that describe the pathways of energy and matter
flow in nature.
Forward-mode assessment—The use of ecological risk assessment techniques to estimate
risk (see reverse-mode assessment).
Fractional intake—The fraction of total contaminant intake that occurs at an AOC. For
example, an exposure assessment might estimate that an individual ingests 56 g of fish
per day from all sources, but that the fractional intake of fish consumed from the specific
AOC is only 0.1, indicating that only one-tenth of the total fish consumption (or 5.6 g
per day) is from the AOC in question.
Habitat—The place where animals and plants normally live, often characterized by a
dominant plant form or physical characteristic.
Hazard—The ability of a physical, chemical, or biological agent to harm plants, animals,
or humans under a particular set of circumstances.
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Chapter 6. Glossary
Hazard identification—The stage of the toxicity assessment that defines the qualitative
relationship between chemicals and adverse effects to receptors.
Hazard index (plural: indices)—The sum of more than one hazard quotient for multiple
substances and/or multiple pathways. The hazard index is calculated separately for
chronic, subchronic, and shorter duration exposures.
Hazard quotient—The ratio of a single substance exposure level over a specified time
period (e.g., subchronic) to a reference dose for that substance derived from a similar
exposure period.
Human health risk assessment—Prediction of the likelihood of adverse effects in human
populations through calculations combining quantitative estimates of the toxicity of
chemical contaminants in the environment with quantitative estimates of the potential for
human populations to be exposed to those contaminants.
Hydrodynamic model—A description of the transport of water, or its motion, based on
a balance of the water's momentum.
Intake—A measure of exposure expressed as the mass of a substance in contact with the
exchange boundary per unit body weight per unit time (e.g., mg chemical/kg body
weight-day)
Joint probability analysis—A statistical technique used to estimate the likelihood of
chemical concentrations exceeding toxicity criteria.
LD50—Dose of a substance that results in 50-percent mortality in a population of test
organisms.
Life stage—A developmental stage of an organism (e.g., juvenile, adult, egg, pupa,
larva).
LOAEL (lowest-observed-adverse-effect level)—The lowest concentration or dose at
which significant adverse effects were observed in experimental trials.
Macroinvertebrate—An invertebrate organism visible to the naked eye. Often refers to
animals such as insects, worms, and snails.
Mass balance model—A quantitative description of the sources, transport, and fate of
the mass of a substance (e.g., water, sediment, or chemical contaminants), such that the
mass entering the system equals the mass leaving the system, less the mass stored,
transformed, or degraded.
Measurement endpoint—An ecological variable that is measured to quantify the
response of an organism, population, community, or ecosystem to chemicals. Each
measurement endpoint is related to an assessment endpoint.
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Chapter 6. Glossary
Medium (plural: media)—The substance in which a chemical may exist, such as air,
soil, sediments, and water.
No-action alternative—The alternative in which no remedial action is taken.
Noncarcinogenic—Capable of causing chronic or subchronic effects other than cancer
in an organism.
NOAEL (no-observed-adverse-effect level)—The highest concentration or dose at which
no significant adverse effects were observed in experimental trials.
Organism—An individual plant or animal.
Photodegradation—The decomposition of a chemical substance by radiant energy,
generally natural sunlight.
Population—A group of individuals of the same species interacting within a given
habitat.
Predictive approach—Any assessment approach that estimates risks based on assumed
scenarios (e.g., future conditions), extrapolation models, or theory rather than direct
measurement.
Probability—The likelihood of an event occurring, expressed as a numerical ratio,
frequency, or percent.
Quotient method—The process of comparing a concentration or dose (estimated or
measured) with a concentration or dose known to have adverse effects on organisms.
Receptor—The organism, population, or community that might be affected by exposure
to a contaminant of concern.
Reference area—An area that has similar physical characteristics to a site being
evaluated, but is unaffected by contaminants of concern. The reference area is compared
to the site to assess the effects of contaminants of concern.
Reference concentration (RfC)—For assessment of inhalation exposures, the con-
taminant concentrations in the air at which no adverse effects are expected to occur.
Reference dose (RfD)—For an individual chemical, an estimate of an exposure level for
the human population, including sensitive subpopulations, that is likely to be without an
appreciable risk of noncarcinogenic effects. There are chronic, subchronic, and
developmental reference doses, but when used without a modifier, reference dose is
generally understood to mean the chronic reference dose, or the acceptable daily
exposure level that is likely to be without an appreciable risk of noncarcinogenic effects
during a lifetime.
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Chapter 6. Glossary
Remedial action objectives—The general descriptions of what remedial actions should
accomplish (e.g., to reduce risks to particular species of plants and animals at a site).
Remedial action goals—A subset of remedial action objectives consisting of medium-
specific chemical concentrations that are protective of human health and the environment.
Remedial Action Plan—A detailed description of the activities selected for the remedia-
tion of contamination (especially sediments) within a given AOC.
Remedial action alternative (or remedial alternative)—A combination of technologies
used in series and/or parallel to isolate contaminated sediments or to alter the con-
centrations of sediment contaminants in order to achieve specific project objectives. In
the simplest case, a remedial alternative may employ a single technology, such as in situ
capping. In more complex cases, a remedial alternative may involve several tech-
nologies, such as dredging, pretreatment, treatment, and confined disposal.
Remediation—Action taken to control the sources of contamination and/or to clean up
contaminated media (e.g., sediments).
Retrospective approach—Any empirical assessment approach based on evaluation of
existing ecological effects and stressors.
Reverse-mode assessment—The use of ecological risk assessment techniques to derive
criteria or cleanup levels corresponding to a specified risk level (e.g., acceptable risk
level set by policy) (see forward-mode assessment).
Riparian—The land and habitat along the bank of a stream, river, or lake. The riparian
area of a river or stream includes the active flood plain (contrasted with upland).
Risk assessment planning—A step in an ecological risk assessment that evaluates
physical, chemical, and biological characteristics of a site to provide a preliminary risk
characterization, to determine whether an ecological risk assessment is warranted, and,
if so, to develop risk assessment objectives.
Risk characterization—The step in an ecological risk assessment in which information
on exposure and toxicity are combined to estimate the probability of adverse effects on
organisms, populations, or communities.
Risk index—An expression of the potential for adverse effects to the biological com-
munity derived from endpoints. For example, the quotient of exposure concentrations
to species toxicity values.
Risk management—The process of integrating findings from a risk assessment with
engineering, policy, and nontechnical concerns to make decisions about the need for
remediation at a specific site or to set remediation priorities among sites.
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Chapter 6. Glossary
Screening level—A process or criterion that separates sites that pose no apparent risk
from those for which further analysis is necessary.
Sediment transport model—A description of the physical transport of sediments in
natural systems, in either bed-load or suspended forms.
Speciation—Refers to the various forms in which metals occur.
Stressor—A physical, chemical, or biological agent that can induce an adverse response
in organisms or other components of ecosystems.
Subchronic intake level—Exposure expressed as the mass of a substance contacted per
unit body weight over an exposure period of less than a lifetime, often expressed as
mg/kg-day over 1-10 years.
Terrestrial—Living or growing on land.
Threatened or endangered species—Species that are at risk of becoming extinct.
Threshold—The chemical concentration (or dose) at which physical or biological effects
begin to be produced.
Toxicity—The property of a chemical substance manifested as its ability to cause a
harmful effect (e.g., death, disease, reduced growth, modified behavior) on an organism.
Toxicity assessment—The stage of a risk assessment that describes the potential effects
of a chemical on organisms and the quantitative exposure-response relationship.
Toxicity test—A test in which organisms are exposed to chemicals in a test medium
(e.g., waste, sediment, soil) to determine the effects of exposure.
Transport and fate—A description of how a chemical is carried through the environ-
ment. This may include transport through biological as well as physical parts of the
environment.
Uncertainty analysis—An evaluation, qualitative or quantitative, of parameters or
assumptions used in a risk assessment that are not completely known or cannot be
precisely estimated, which is used to help place quantitative risk estimates in perspective.
Upland—Land usually above the floodplain of a river or stream (contrasted with
riparian).
Volatilization—The conversion of a chemical substance from a liquid or solid state to
a gaseous or vapor state by the application of heat, by reducing pressure, or by a
combination of these processes.
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Chapter 6. Glossary
Water transport model—A description of the transport of water, or its motion, in
natural systems that may be descriptive (i.e., based on a balance of the water's mass) or
hydrodynamic (i.e., based on a balance of the water's momentum).
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