v>EPA
United States
Environmental
Protection Agency
Science Advisory Board
(1400A)
Washington. DC
EPA-SAB-EPEC-00-005
February 2000
www.epa.gov/s
ab
AN SAB REPORT: REVIEW
OF AN INTEGRATED
APPROACH TO METALS
ASSESSMENT IN SURFACE
WATERS AND SEDIMENTS
PREPARED BY THE ECOLOGICAL
PROCESSES AND EFFECTS
COMMITTEE OF THE SCIENCE
ADVISORY BOARD
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February 28, 2000
EPA-SAB-EPEC-00-005
Honorable Carol M. Browner
Administrator
U.S. Environmental Protection Agency
1200 Pennsylvania Ave., NW
Washington, DC 20460
Subject: Review of An Integrated Approach to Metals in Surface Waters and Sediment
Dear Ms. Browner:
At the request of the EPA Office of Water, the Ecological Processes and Effects Committee
(EPEC) of the Science Advisory Board (SAB) met on April 6-7, 1999 to review the Agency's
proposal for assessing the bioavailability and toxicity of metals in surface waters and sediments. In
addition to its current water quality criteria, the Agency has worked with outside researchers to develop
the Biotic Ligand Model to predict the acute toxicity of metals to aquatic organisms and has developed
sediment quality guidelines based on the Acid Volatile Sulfide (AVS) approach. The Committee's
comments on the Biotic Ligand Model are contained in a companion SAB document (EPA-SAB-
EPEC-00-006). The focus of the present report is on the AVS-based sediment guidelines and on
other aspects of an integrated approach to the management of metals in the aquatic/sediment
environment.
It has long been recognized that contaminated sediments can cause adverse effects even where
waters are meeting water-quality based criteria. As a result, the Agency has been working to develop
sediment quality guidelines that can be applied in conjunction with water quality criteria to protect
human health and aquatic life. The SAB strongly supports the Agency's development of sediment
quality guidelines to fill this existing gap in environmental protection.
The approach that the Agency has selected for developing sediment guidelines is based on the
theory that chemical equilibrium principles can be used to predict the partitioning of sediment
contaminants among the sediment phases (e.g., mineral sediments, sediment organic carbon, and
interstitial water). The Agency has sponsored work demonstrating that when the concentration of acid-
volatile sulfide, a binding agent for metals in sediments, exceeds that of simultaneously extracted metal,
metals toxicity is not observed. The Agency has also presented data on the observed relationship
between metals concentrations in sediment interstitial (pore)
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water and toxicity. The proposed sediment guidelines for metals mixtures rely on these two
components to predict which sediments are "unacceptably contaminated" with respect to metals.
In the attached report, the Committee reviews the overall approach being proposed for
sediment guidelines. We commend the Agency for developing an important body of scientific work
with broad practical application and for developing a powerful predictive tool that is suitable for use in
sediment assessment. The Committee also, however, provides advice regarding the appropriate use of
the AVS method in the field and discusses some important limitations of the method, many of which
were outlined in a 1995 SAB review of the subject. Recognizing these limitations, the Committee
recommends that the language in the Guidelines Statement be refined.
The Committee supports the Agency's quest to integrate approaches to management of surface
waters and sediments. In the attached review, we suggest possible refinements to the array of tools that
the Agency is developing for this purpose.
The Committee has appreciated its long working relationship with the Office of Water on the
important topic of sediment quality guidelines and hopes that the attached report will contribute to the
development of scientifically defensible guidelines. We look forward to a reply from the Assistant
Administrator for Water.
Sincerely,
/signed/
Dr. Joan M. Daisey, Chair
Science Advisory Board
/signed/
Dr. Terry F. Young, Chair
Ecological Processes and
Effects Committee
Science Advisory Board
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NOTICE
This report has been written as part of the activities of the Science Advisory Board, a public
advisory group providing extramural scientific information and advice to the Administrator and other
officials of the Environmental Protection Agency. The Board is structured to provide balanced, expert
assessment of scientific matters related to problems facing the Agency. This report has not been
reviewed for approval by the Agency and, hence, the contents of this report do not necessarily
represent the views and policies of the Environmental Protection Agency, nor of other agencies in the
Executive Branch of the Federal government, nor does mention of trade names or commercial products
constitute a recommendation for use.
Distribution and Availability: This Science Advisory Board report is provided to the EPA
Administrator, senior Agency management, appropriate program staff, interested members of the
public, and is posted on the SAB website (www.epa.gov/sab^. Information on its availability is also
provided in the SAB's monthly newsletter (Happenings at the Science Advisory Board). Additional
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copies and further information are available from the SAB Staff.
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U.S. ENVIRONMENTAL PROTECTION AGENCY
SCIENCE ADVISORY BOARD
ECOLOGICAL PROCESSES AND EFFECTS COMMITTEE
Review of the Integrated Approach for Predicting Metals Toxicity in Surface Waters and
Sediments (Including the Equilibrium Sediment Guidelines)
April 6-7, 1999
CHAIR
Dr. Terry F. Young, Environmental Defense Fund, Oakland, CA
MEMBERS
Dr. Miguel F. Acevedo, University of North Texas, Denton, TX
Dr. William J. Adams, Kennecott Utah Copper Corp., Magna, UT
Dr. Lisa Alvarez-Cohen, University of California-Berkeley, Berkeley, CA
Dr. Steven M. Bartell, Cadmus Group, Inc., Oak Ridge, TN
Dr. Calvin Chien, E.I. DuPont Co., Wilmington, DE (Liaison from the SAB's Environmental
Engineering Committee)
Dr. Kenneth W. Cummins, Tarpon Bay Environmental Lab, Sanibel, FL
Dr. Carol A. Johnston, Natural Resources Research Institute, Duluth, MN
Dr. Paul A. Montagna, Marine Science Institute, University of Texas at Austin, Port Aransas, TX
Dr. Charles A. Pittinger, Procter and Gamble Co., Ivorydale Technical Center, Cincinnati, OH
Dr. Frieda B. Taub, School of Fisheries, University of Washington, Seattle, WA
FEDERAL EXPERTS
Dr. Samuel N. Luoma, U.S. Geological Survey (MS 465), Menlo Park, CA
SCIENCE ADVISORY BOARD STAFF
Ms. Stephanie Sanzone, Designated Federal Officer, EPA Science Advisory Board (1400A), 1200
Pennsylvania Ave., NW, Washington, D.C.
Ms. Mary L. Winston, Management Assistant, EPA Science Advisory Board (1400A), 1200
Pennsylvania Ave., NW, Washington, D.C.
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TABLE OF CONTENTS
1. EXECUTIVE SUMMARY 1
2. BACKGROUND AND CHARGE 5
2.1 Background 5
2.2 Charge 6
3. STRENGTHS OF THE PROPOSED SEM-AVS METHODOLOGY 8
3.1 Theoretical Foundation 8
3.2 Experimental Verification 8
4. LIMITATIONS OF THE PROPOSED SEM-AVS METHODOLOGY 10
4.1 Meeting Key Assumptions 10
4.2 Dietary Exposure/Bioaccumulation 11
4.3 The Importance of Biological and Ecological Processes 14
5. APPROPRIATE USE OF THE PROPOSED SEM-AVS METHODOLOGY 18
5.1 Accounting for Spatial and Temporal Variability 18
5.2 When the Methodology Does Not Apply 20
6. AGENCY-PROPOSED MODIFICATIONS TO THE SEM-AVS METHODOLOGY 21
6.1 Organic Carbon Normalization 21
6.2 Application of the Biotic Ligand Model (BLM) to Sediment Guidelines 23
6.3 Inclusion of Chromium and Silver in the Sediment Guidelines 24
7. AN INTEGRATED APPROACH TO METALS 26
8. SUMMARY 30
REFERENCES CITED R - 1
APPENDIX A: ACRONYMS AND ABBREVIATIONS A - 1
APPENDIX B. RESEARCH ON THE SIGNIFICANCE OF DIETARY BIO ACCUMULATION
OF METALS B - 1
IV
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1. EXECUTIVE SUMMARY
The Ecological Processes and Effects Committee of the Science Advisory Board met in April
1999 to review the Agency's proposed approach to assessing the bioavailability, and hence toxicity, of
metals in sediments. The approach, which is based on equilibrium partitioning theory, assumes the
bioavailable fraction of total sediment metals to be the difference between the Simultaneously Extracted
Metal (SEM) and the Acid Volatile Sulfide (AVS), a binding factor for metals in sediments. The
Agency also presented recent work demonstrating the utility of normalizing the SEM-AVS to fraction
organic carbon in the sediment. The Agency proposes that the SEM-AVS approach be incorporated
into sediment quality guidelines for a mixture of metals (cadmium, copper, lead, nickel, silver, and zinc).
The Committee commends the Agency for developing an important body of scientific work with
broad practical application far beyond the specific subject of sediment assessment guidelines. The
SEM-AVS methodology is soundly grounded in chemical theory and has been verified by a wide range
of convincing acute toxicity studies. Recent studies of longer-term effects, including chronic toxicity
studies, have added substantially to the body of evidence suggesting that organisms will not be
adversely affected by metals when SEM-AVS < 0. Additional work regarding the influence of organic
carbon on metal bioavailability has allowed the Agency to further refine the method. In short, the
SEM-AVS methodology provides a valuable addition to our understanding of the bioavailability (or
lack thereof) of metals in certain sediments. It is also a powerful predictive tool, suitable for
incorporation into sediment assessment guidelines.
For all of its merits, however, the SEM-AVS methodology also has limitations: first, it is still
unclear whether adverse effects on biota are necessarily prevented when AVS exceeds SEM; and
second, environmental conditions in many locations will be unsuitable for SEM-AVS use because
underlying assumptions of the approach will be violated. For example, AVS does not persist in aerobic
conditions, whereas the oxidized zone is where most species concentrate their interaction with their
environment. For these reasons, the Committee recommends that SEM-AVS be incorporated into
sediment assessment guidelines in a way that assures that SEM-AVS will continue to be used in
conjunction with other assessment tools to characterize the safety of sediments, rather than being used
as a stand-alone test. The SEM-AVS method may be particularly useful to prioritize sites requiring
attention and to explain situations when bioassays show a lack of toxicity even though metal
concentrations in sediments are high.
The Committee strongly recommends that the Agency now turn its attention to the appropriate
application of SEM-AVS in the field and thereafter to other methods of assessing sediment quality that
can be used when SEM-AVS cannot. To facilitate this effort, the Committee provides the following
specific observations and recommendations:
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a) The SEM-AVS methodology relies on the observed correlation between toxicity and
metals concentrations in the interstitial water under certain chemical conditions. It does
not consider explicitly the role of dietary exposures to metals. The Committee
recommends, therefore, that the Agency begin at the earliest possible date to assess the
currently available literature on the importance of dietary exposure to metal uptake
(particularly for cadmium, silver, copper, and zinc) and that the Agency also initiate
studies that evaluate the significance of dietary uptake compared to SEM-AVS
predictions of metal exposure and effect. These studies are required to determine
under what circumstances AVS-SEM can most accurately be used as a "no-effect"
test, as the Agency envisions.
b) Bioaccumulation of metals occurs in circumstances where the SEM-AVS methodology
would predict no effect. The potential adverse effects of this bioaccumulation on the
organisms themselves is uncertain at this time; adverse effects of bioaccumulated metals
on consumer organisms also may be significant. The Committee therefore would like to
re-emphasize the importance of incorporating bioaccumulation measures, particularly
the effects on consumer organisms, in the Agency's overall sediment assessment
approach.
c) The SEM-AVS methodology is based on equilibrium partitioning theory which, in turn,
assumes a steady-state system in which chemistry can be used to predict bioavailability.
In nature, however, sediments are not steady-state systems and exposure of organisms
to metals (as well as adverse effects) is influenced by biological and ecological
processes. The SEM-AVS predictions of toxicity should be verified for a greater range
of test organisms, representing a more complete range of behaviors and functional
groups. This work can be undertaken in conjunction with the studies recommended in
(a) above, that would investigate exposure of organisms to metals via sediment
ingestion (particularly targeting organisms whose behavior and ecology maximize their
exposure in the field).
d) Accurate characterization of SEM and AVS in the field is as essential as it is complex.
Because sediments are so variable in both space and time, using the right sampling
protocols is just as important as using the right toxicity test protocols. Recognizing that
any sampling strategy must be both practical and affordable, the Committee
recommends that the Agency investigate and then provide clear guidelines for assessing
SEM and AVS that consider the biologically active zone (e.g., the vertical distribution
of organisms in the sediment) as well as temporal and spatial variability (including
vertical and horizontal gradients) of sediment chemical parameters..
e) There are many sediments for which the SEM-AVS methodology does not apply. In
addition to aerobic sediments, these include anaerobic sediments whose redox
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conditions are likely to change as a result of, for example, seasonal changes, periodical
exposures associated with tidal fluctuations, or resuspension during storms, floods, or
dredging operations. The Committee recommends that these limitations be
incorporated into the sediment guidelines to assure that the SEM-AVS methodology is
not mistakenly used where it is inappropriate.
f) While the draft "Equilibrium Partitioning Guidelines (ESG) for the Protection of Benthic
Organisms" incorporates caveats with respect to most of the concerns above, the
Committee suggests that these be emphasized to a greater degree in the final
"Guidelines Statement" (pp. 1-108, 109), to help assure appropriate use of the SEM-
AVS method. For example, the circumstances in which the SEM-AVS method is
clearly inapplicable could be explained in the initial paragraph of the Guidelines
Statement. In addition, it would be useful to elaborate on the statement that the ESG is
not designed to protect organisms from sediment ingestion or ingestion of contaminated
benthos and refer to a discussion within the document that assesses the relative
importance of these exposure pathways under a variety of conditions.
Finally, the Committee notes that the format of the Guidelines Statement is reminiscent of a
water quality criterion (generally used as a necessary and sufficient test of designated use protection).
For example, the guidelines states that if the SEM-AVS component is violated but the IW component
is satisfied, "then the sediment meets the guideline and benthic organisms are acceptably protected from
metals-induced sediment toxicity." The Committee recommends that the guideline language be
modified to reflect the fact that the ESG is an assessment tool designed to be used in concert with other
tools, rather than a stand-alone pass/fail test.
The Committee understands that a "Sediment Guidelines User's Guide" is being developed by
the Agency to assist potential users of the ESG to apply it in real world applications. This User's Guide
should undergo careful peer review to ensure that the various sediment assessment tools are not applied
outside their validated conditions.
Given both the strengths and the limitations of the SEM-AVS methodology, it is important to
consider how the Agency intends to use the technique in concert with other water quality standards and
assessment tools. The Committee urges the Agency to develop a refined conceptual model that
incorporates all partitioning phases and routes of exposure in order to guide the Agency's long-term
efforts to integrate water column and sediment standards and to assist users to apply current standards
and guidelines appropriately. A conceptual model of exposure of organisms to metals in aquatic
environments, coupled with a parallel diagram of the Agency's approaches for evaluating that exposure
and the resulting effects, would be a very useful tool for the Agency as it attempts to build an integrated
water/sediment protection system. An example of this type of presentation generated by the Agency
for the Committee highlighted potential gaps in the Agency's currently proposed approach.
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The Agency requested the Committee's advice on a series of questions that relate to
refinements of the SEM-AVS methodology. A brief summary of the Committee's findings in response
to these charge questions follows:
Charge Question 1: By incorporating the fraction organic carbon into the
bioavailability equation, has the Agency retained the protective features of the
guidelines and improved its predictiveness of toxic effects?
The normalization of SEM-AVS to fraction organic carbon reduced the variability in the
exposure estimates and therefore improved the predictive capability of the method, particularly for
laboratory experiments. The results for the field experiments are less clear. There may be a
mechanistic reason why the fraction organic carbon normalization would do less to improve precision
under field conditions; there is some evidence that the effect of organic carbon on bioavailability
depends upon the nature of the organic carbon. In any case, the Agency may wish to weigh the
benefits of the normalization procedure against the additional variability that must be captured in the
sampling design.
Incorporation of the organic carbon normalization should not reduce the degree of protection
afforded by the SEM-AVS method, unless the organic carbon is present as biological complexes that
tend to increase bioavailability.
Charge Question 2: If the Biotic Ligand Model (BLM) is used to derive or adjust a
water quality criterion, is the revised criterion appropriate for use in the interstitial
water component of the Metals Mixtures ESG?
Because the chemistry of interstitial water (IW) is not the same as the chemistry in the water
column, it would be inappropriate to substitute the BLM-adjusted water column criterion for the water
quality criterion in the ESG equation at this time. While the Committee is optimistic that the BLM will
be a useful new tool for assessing bioavailability in the water column, we recommend that specific
validation experiments be performed before applying the method to interstitial (pore) water. The
currently proposed IW component of the ESG relies on comparison of the IW metals concentration to
the water quality criteria Final Chronic Value for each metal, corrected only for site-specific hardness;
the BLM, if validated for application to interstitial water, would allow consideration of additional site-
specific chemistry conditions that affect metals bioavailability.
Charge Question 3: Are the data presented from lab and field experiments with
chromium and silver sufficient to support their addition to the Metals Mixtures ESG?
Although the results from acute toxicity tests are promising, further research is required to
support the addition of either chromium or silver to the ESG. In addition to undertaking chronic toxicity
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tests, the Committee recommends that the Agency address selected questions regarding the chemistry
of chromium in the field and other factors affecting the bioavailability of chromium (m) in nature. With
regard to silver, the data presented do not provide the same clear demonstration that AVS binding
eliminates silver toxicity as has been shown for other metals.
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2. BACKGROUND AND CHARGE
2.1 Background
In recent years, the Ecological Processes and Effects Committee has commented on Agency
proposals to use Equilibrium-Partitioning (EqP) to predict the availability (and hence, toxicity) of
chemicals in sediments, including non-ionic organic chemicals (SAB, 1992) and metals (SAB, 1995).
The EqP approach, in which chemicals are assumed to be in equilibrium between sediments and pore
water, was proposed as a means of predicting the extent to which sediment chemicals are biologically
available, and thus may produce toxic effects. With regard to metals, the bioavailable fraction of
sediment metals is assumed to be the difference between the Simultaneously Extracted Metal (SEM)
and the Acid Volatile Sulfide (AVS), a binding factor for metals in sediments. In other words, toxicity
is not expected for sediments in which SEM-AVS<0. In the 1995 review, the Committee concluded
that the SEM-AVS methodology was "based on sound theory and [had] been verified by considerable
experimental evidence." The Committee noted, however, the significant limitations to application of the
methodology and identified a number of remaining research questions associated with its use.
The Agency currently has ambient aquatic life criteria for 11 individual metals and draft
sediment guidance based on the EqP approach for a mixture of five metals (cadmium, copper, lead,
nickel, and zinc), the latter of which was reviewed by the SAB in 1995. The proposed Metals
Mixtures ESG reviewed by EPEC in April 1999 contains two components: an AVS guideline and an
Interstitial Water Guideline. The AVS guideline requires that the molar sum of the SEM for the six
metals (silver, copper, lead, cadmium, zinc, and nickel) in sediment not exceed the sediment AVS
concentration. The Interstitial Water Guideline requires that the sum of the ratios of each metal to its
Final Chronic Value (FCV) not exceed 1. The Guidelines Statement (Section 6 of the proposed ESG)
states that "if both of these conditions are violated, or if the AVS Guideline is violated and the sediment
is contaminated with silver [for which there is no FCV], then there is reason to believe that the sediment
may be unacceptably contaminated by these metals." If only one of the two conditions is violated,
however, this does not mean that the sediment violates the ESG. The Guidelines Statement also states
that, "except possibly where a locally important species is very sensitivie, benthic organisms should be
acceptably protected in freshwater and saltwater sediments if any one or both of the ...conditions is
satisfied." With the exception of the proposed inclusion of silver, the components in the draft document
are the same as those presented to the SAB in 1995.
The Guidelines Statement in the proposed ESG does not include organic carbon partitioning
because, in the words of the review document, the Agency feels that "the Organic Carbon and
Minimum Partitioning Approaches as proposed to the SAB and in Ankley et al. (1996) require
additional research prior to their implementation" (p. 1-87). However, in the Charge to the Committee,
the Agency asks whether the organic carbon normalization of the AVS guideline will improve its ability
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to predict toxic effects, in addition to its intended role as a guideline for predicting "no effects"
concentrations.
2.2 Charge
For the current SAB review, the Committee was asked to review the Agency's proposed
refinements to the approaches for deriving aquatic life criteria for metals and sediment guidelines for
metals mixtures. The proposed modifications include: use of the Biotic Ligand Model (BLM) to
improve prediction of bioavailability of metals both in the water column and in interstitial (pore) water,
inclusion of organic carbon normalization in the calculation of sediment metals bioavailability, and
inclusion of chromium and silver in the Metals Mixtures ESG. In addition to requesting Committee
comments on the proposed refinements, the Charge to the Committee also included a request for
comment on whether the linkages between the water column criteria and sediment guidelines would
improve the Agency's ability to integrate predictions of metals toxicity in aquatic environments.
The Committee met in Washington, DC on April 6-7, 1999 to review the following documents:
a) Biotic Ligand Model of the Acute Toxicity of Metals (U.S. EPA, 1999b), proposed for
incorporation into the Agency's approach for deriving aquatic life criteria for metals in the water column
and b) the Agency's draft guidance, Equilibrium Partitioning Guidelines (ESG) for the Protection
ofBenthic Organisms: Metals Mixtures-Cadmium, Copper, Lead, Nickel, Silver, and Zinc (U.S.
EPA, 1999a), referred to as the Metals Mixtures ESG. The Charge to the Committee included the
following questions:
Overall Charge Question: Integrated Methodology
a) Does this integrated metals methodology improve the Agency's ability to make both
protective and predictive assessments of toxicity due to copper, silver and other
selected metals in the water column and sediment?
Charge Questions on the Metals Mixtures ESG
b) By incorporating the fraction organic carbon into the bioavailability equation, has the
Agency retained the protective features of the guidelines and improved their
predictiveness of toxic effects:
c) If the Biotic Ligand Model (BLM) is used to derive or adjust a water quality criterion,
is the revised criterion appropriate for use in the interstitial water component of the
Metals Mixtures ESG?
d) Are the data presented from laboratory and field experiments with chromium and silver
sufficient to support their addition to the Metals Mixtures ESG?
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Charge Questions Pertaining to the BLM:
e) Does the BLM improve the Agency's ability to predict toxicity to water column
organisms due to metals (copper and silver) in comparison to the currently applied
dissolved metal concentration criterion?
f) Is the scientific and theoretical foundation of the model sound?
g) In comparison to the current WER adjustment for aquatic life criteria, will the
application of the BLM as a site-specific adjustment reduce uncertainty associated with
metals bioavailability and toxicity?
h) Are the data presented for validation of the BLM sufficient to support the incorporation
of the BLM directly into copper and silver documents?
This report contains the Committee's comments on the Metals Mixtures ESG, as well as the
inter-relationship between the metals assessment approaches proposed for protection of water column
and benthic organisms. The Committee's comments on the Biotic Ligand Model (questions e through
h, above) are contained in a companion report (EPA-SAB-EPEC-00-006).
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3. STRENGTHS OF THE PROPOSED SEM-AVS METHODOLOGY
3.1 Theoretical Foundation
The EPA and collaborating scientists deserve praise for working to develop a means of
incorporating bioavailability principles into guidelines for management of contaminated sediments.
Assessment techniques based upon total metal concentrations have limitations that have long been
recognized, but the techniques to deal with these limitations have been missing. The research
accomplishments presented in this review are notable considering that the endeavor was extremely
challenging and the factors that affect bioavailability in sediment are complex. The attempt to find a
simplified and unifying approach to incorporate these complexities for managers is admirable.
The proposed ESG for metal mixtures is based upon a decade of study of the
influence of acid volatile sulfide (AVS) on metal bioavailability in sediments. The theory behind the
proposal is that AVS in sediments controls metal concentrations in interstitial waters, limits
bioavailability of metals in bedded sediments through the formation of insoluble sulfides, and thereby
controls overall metal availability and toxicity to benthic organisms. Metal exchange among interstitial
water, bedded sediments, and sulfide phases is assumed to be controlled by equilibrium partitioning of
the metal, which can be described by the physico-chemical properties of each metal. The authors have
developed an elegant and convincing set of results supporting the use of AVS theory to explain
bioavailability of metals in sediments and their potential to cause toxicity. They have also developed an
impressive and extensive publication list, and this work has been very influential with scientists interested
in contaminated sediments and with regulatory/water management organizations all over the world.
The most recent work presented in this review provides information that incorporates metal
partitioning to organic carbon as an additional binding ligand that influences metal bioavailability. The
addition of fraction organic carbon (foe) to the overall conceptual model for assessing bioavailability of
metals in sediments is seen as a natural extension of the EqP theory for metals. As demonstrated by
data presented to the Committee, toxicity is not always observed when SEM exceeds AVS. This lack
of toxicity has been attributed to metal binding to various phases such as iron and manganese oxides as
well as organic carbon. The development of an approach to incorporate organic carbon into the metals
EqP methodology is an appropriate and timely step toward developing a methodology that can be used
to identify those sediments that have the greatest potential to cause environmental effects.
3.2 Experimental Verification
EPEC acknowledges the long-term and chronic sediment studies that were performed in
response to previous concerns raised by the Committee that the ESG data consisted primarily of results
from acute toxicity studies. Chronic toxicity studies were performed with cadmium (28-days;
Leptocheirus plumulosus) and zinc (56 days; Chironomus tentans). Colonization studies were
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performed with cadmium (118 days; laboratory study), cadmium (120-days; field study), equimolar
ratios of cadmium, copper, nickel and zinc (120-day; saltwater field study); and zinc (1-year,
freshwater field study). The results of these long-term studies demonstrated a lack of toxic effects in all
cases where SEM-AVSSEM has not been
ruled out due to the limited number of studies performed, and due to the mechanisms discussed in
Section 4.
Agency subsequently provided information that lead (Pb) had been spiked into the test
sediment in the marine field colonization chronic test, but that lead had been inadvertently left out of
Table 3-2 in the review document.
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4. LIMITATIONS OF THE PROPOSED SEM-AVS METHODOLOGY
4.1 Meeting Key Assumptions
Although the SEM-AVS work has been influential, it is also controversial. The elegance and
consistency of the results have convinced some scientists that potential applications are widespread.
However, a growing body of scientists are skeptical. The skepticism is not about the theory and
experimental evidence that AVS is important for metal behavior in sediments. It is the extent of the
application that is in question; and this is very relevant to its use as a regulatory tool. Concerns with the
SEM-AVS approach have been raised with respect to its biological limitations and based on studies of
the geochemistry of AVS in sediments (e.g., in Meyer et al., 1994), yet the proposed Metals Mixtures
ESG document does not present much in the way of these alternative views.
It is complicated, but important, for EPA to understand why the work so elegantly and
convincingly presented in this proposal is also controversial. There is a legitimate concern that the
Agency and its researchers have not demonstrated that the SEM-AVS approach is applicable as a
predictor of sediment toxicity beyond a given set of circumstances. Although it is not discussed in the
reports submitted to the SAB, there is also a body of literature that questions application of equilibrium
partitioning models, because of their limited capability for dealing with important environmental
complexities (Landrum et al., 1992; Farrington, 1989). These limitations are especially important for
metals and, at least by implication, affect the applicability of SEM-AVS (Luoma and Fisher, 1997).
Many of the limitations of the proposed SEM-AVS approach relate to the ability of key
assumptions to be met under field conditions. For example, the approach assumes equilibrium
conditions, that the effects of the metals are no more than additive, and that toxicity can be predicted
from metal concentrations in interstitial water. The Metals Mixtures ESG specifically states that ESG
approaches "are not designed to protect aquatic systems from metal release associated, for example,
with sediment suspension, or the transport of metals into the food web either from sediment ingestion or
the ingestion of contaminated benthos" (EPA, 1999b). However, there may be few or no aquatic
systems where these processes do not occur.
In addition, biogeochemical processes controlling vertical profiles of sulfides in sediments are
complex and can promote nonequilibrium conditions, yet the implications of these processes on
sampling strategies and data interpretation have not been addressed adequately. Different metals and
different sulfide complexes likely have different solubility constants. The importance of diagenetic
processes in sediments and the different vertical horizons of the processes in different environments
should be considered.
To assess the applicability of the Metals Mixtures ESG, one must assess the extent to which the
method's underlying assumptions can be met in the field and assess the significance of exposure
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pathways other than exposure to interstitial water. A number of these concerns were raised by the
SAB in 1995, and they continue to be relevant to the Agency's proposed application of a Metals
Mixtures ESG as a regulatory tool. Accordingly, the Committee suggests that the Agency consider the
following questions (which are discussed in greater detail in the following sections) as it determines how
to apply the SEM-AVS method and the ESG:
a) Dietary Exposure/Bioaccumulation —Is internal metal exposure of the organism
independent of the route of uptake, as asserted by EqP, or can dietary exposure
increase the internal metal dose received by the organism so that toxicity is expressed
even when AVS exceeds SEM? Does bioaccumulation at SEM-AVS < 0 indicate the
possibility of chronic toxicity?
b) Biology and Ecology—Do differences in behavioral and ecological processes among
organisms (e.g., strategies for feeding and obtaining oxygen in sediments) influence the
applicability of the SEM-AVS approach? What organisms are or are not protected by
SEM-AVS?
c) Sediment Dynamics—Would regulatory applications of SEM-AVS be affected by
the dynamic nature of sediments and sediment biogeochemistry? Are there times when
the method should or should not be used?
d) Sampling and Variability—How should variability resulting from biogeochemistry of
AVS in sediments, including vertical and horizontal AVS gradients, be accounted for in
sampling protocols?
These questions are testable and, in some cases, tests are underway or published. It is
important that the Agency understand the basis and implications of these potential limitations before
determining how to apply the guidelines in a regulatory context. In the following sections, the
Committee will discuss each of these concerns and consider the extent to which the work conducted by
the Agency or outside researchers since 1995 has addressed them.
4.2 Dietary Exposure/Bioaccumulation
Equilibrium Partitioning (EqP) theory was originally developed for non-ionic chemicals,
following the suggestion by Mackay (1982) that a single chemical potential determines equilibrium
between an organism and its environment. Organic chemical bioaccumulation from sediments is
determined from knowledge of the hydrophobicity of the chemical and prediction of pore water
concentrations (DiToro et al., 1991). Studies by Swartz et al. (1985) and Kemp and Swartz (1988)
demonstrated that pore water concentrations controlled cadmium toxicity or cadmium bioaccumulation
by benthos. These findings, together with several subsequent studies discussed in the Metals Mixtures
ESG, are cited as the evidence that knowledge of pore water metal concentrations is sufficient to
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determine metal exposures and toxicity in sediments. In the body of SEM-AVS literature cited in the
documents submitted to the SAB, toxicity is consistently correlated with pore water concentrations of
metals. Again, this supports the validity of equilibrium partitioning.
The criticisms of the EqP model (when it is applied to metals) stem from an alternative
conceptual view of how exposure to metals occurs in nature. The most important aspect of this
alternative model is the assumption that the internal dose of metal experienced by an animal is
determined by the sum of the contributions from different uptake routes (from diet and from water) and
that these are not necessarily in equilibrium. Stated another way, route of exposure does matter, and
total internal exposure should be derived from the sum of uptake from each route. As noted by the
SAB (1995), if this alternative model is accurate, then studies that use only pore waters as a basis for
estimating bioavailability will underestimate under some conditions the total exposures of animals that
ingest contaminants in their food (Luoma, 1995; Meador et al, 1995). The multi-pathway conceptual
model, described by Clark et al. (1990), McKim and Nichols (1994), Thomann et al. (1995) and
Luoma and Fisher (1997), is also supported by direct experimental evidence. Since the late 1970's,
experiments have been conducted that show that bioaccumulation pathways by benthic invertebrates
can be additive. When water exposures are combined with food exposures, bioaccumulation can
exceed uptake from either source alone (see review by Luoma, 1983; Young, 1975; Harvey and
Luoma, 1985; Borchardt, 1983; van Hattum et al., 1989; Warnau et al., 1996). In all these
experimental studies, prediction of metal exposure from dissolved concentrations alone would have
underestimated the total metal burden experienced by the organism.
According to both EqP theory and the additive model: a) toxicity of metals is correlated with
the total exposure to metals (i.e., exposure from dietary sources, including sediment ingestion, and from
cutaneous and respiratory exposures to pore water or overlying water); b) the empirical finding that
toxicity is correlated with interstitial water concentrations of metals would be expected for experimental
conditions under which pore water exposure is the dominant (though not necessarily the only) route of
exposure; and c) in experiments with metal spiked sediments, pore water concentrations of metals may
be higher than contaminated sediments in field conditions and so a stronger correlation with toxicity
would be expected. However, it is in field scenarios where pore water:sediment metal ratios are
smaller (i.e., lower pore water metal concentrations)—and hence dietary exposures might constitute a
greater proportion of the total exposure— that the toxicity predicted by the two models might be
expected to diverge.
In Kemp and Swartz (1988), for example, experimental conditions facilitated high
concentrations of cadmium in pore waters compared to sediments (conditions that could cause such an
effect include extremely high metal concentrations and short equilibration times). Luoma and Fisher
(1997) showed that the Kd (ratio between sediment and water concentrations) in the Kemp and
Swartz (1988) experiment was approximately 15. The Kd for cadmium between sediment and water
in an estuary is more typically 5000. When a bioaccumulation model was applied, it predicted that
pore waters would dominate uptake in the experiment (as was observed), but uptake from food would
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dominate the more natural condition. In the latter case, predictions based upon pore waters alone
would underestimate exposures by nine fold. The Kemp and Swartz (1988) experiment may represent
an unusual circumstance, but it behooves the EPA to be certain that SEM-AVS predictions are not just
applicable to the most extreme contamination conditions and sediments only recently contaminated with
metals. Most regulatory situations are more complex than this. Verification of SEM-AVS predictions
should include experiments with sediments that are contaminated to levels typical of many contaminated
environments and protocols that involve times of equilibration and exposure typical of nature.
Luoma and Fisher (1997) concluded that there are mechanistic reasons why organic chemical
exposures should be better explained than metal exposures by equilibrium partitioning. The first is that
tissue bioaccumulation of metals is not driven by any single principle analogous to hydrophobicity. The
chemical potential of metals in food and within tissues is controlled by a myriad of biochemical reactions
and a variety of metal forms. They also concluded that a variety of surface and geochemical processes
control metals in sediments and this results in controls on metal bioavailability that are complex. For
example, Luoma and Bryan (1982) showed that simple normalizations were not sufficient to predict
metal bioavailability from field sediments, when a diverse array of sediments were considered.
In addition, some studies (including those presented to the SAB in 1995) have shown that
bioaccumulation of metals may occur even when AVS exceeds SEM. Although bioaccumulation by
itself is not proof of an adverse effect or toxicity—indeed, bioaccumulation of low levels of metals can
occur with no adverse effects—it does provide a measure of the presence and bioavailability of metals
in sediments. Further, tissue residue measurements can be indicative of both metal exposure and
internal dose (see Figure 1 in Chapman, 1995), except in cases where the metal is regulated by the
organism. Since bioaccumulated metals can contribute to an organism's body burden, it is plausible
that toxicity could result from uptake via the dietary route. A body of literature exists and, is rapidly
growing, that shows the importance of dietary exposure to metal uptake in a variety of species, for a
variety of metals, in a variety of circumstances (e.g., see Appendix B). However, the importance of
dietary accumulation and the potential for toxicity has not been systematically examined. Experiments
designed to separate exposure pathways and evaluate the potential for toxicity to occur via the diet
were suggested previously by the SAB (1995), but have not yet been undertaken. The Committee
recommends, therefore, that the Agency incorporate research of this nature in future testing programs.
As the Agency moves toward an integrated water/sediment assessment and management
scheme, including consistent criteria and guidelines for the different environmental compartments, it will
be important to relate the various threshold levels to total exposure (and associated effects) for aquatic
and benthic organisms. Bioaccumulation and food chain exposures will be an important link between
water column and sediment criteria, and ultimately wildlife criteria. This fact is recognized by the
Agency's inclusion in its integrated vision (Figure 1) of possible future tissue-residue based criteria. We
encourage the Agency to evaluate the potential to develop tissue residue thresholds that could be used
to evaluate the significance of metals bioaccumulation. Moreover, until this question can be resolved,
the Committee
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recommends that the Agency seriously consider including bioaccumulation in the suite of measures that
would be evaluated in a weight-of-evidence approach to sediment assessment.
Sediment Ingestion—A subset of dietary exposure is that which occurs with sediment
ingestion. Deposit feeders are benthic invertebrates that ingest whole sediment and receive nutrition by
stripping or digesting the scant amounts of organic matter present. The AVS component of the
proposed ESG assumes that metal sulfides are not bioavailable. However, benthic deposit feeders can
ingest sediment whole into an acidic stomach, where bioavailability of metal sulfides may be altered by
changes in pH, oxygen conditions, and digestive enzymes. Deposit feeders can dominate sediment
communities. In the Chesapeake Bay, for example, deposit feeders represent 70% of the tidal
freshwater communities, and constitute 9-38% of the community in all benthic habitats (Weisberg et al,
1997).
While little is known about the effect of ingestion on mobilizing consumed sediment metals,
except for those that are required as micronutrients, low pH, oxidizing gut conditions, long gut residence
times (days in bivalves; Decho and Luoma, 1991), and presence of surfactants and strong ligands in gut
fluids (Chen and Mayer, 1998) all affect extraction of metals from sediments within the digestive
system. These changes in thermodynamic potential in the gut relative to pore water are thought to
account for observed bioaccumulation in benthic organisms when sediment AVS exceeds SEM.
To assess the protectiveness of the proposed ESG, one must understand how animals that feed
on sediment (rather than just being in contact with interstitial water) accumulate metals and express
toxicity to AVS-bound metals. How does the variety of sediment ingesting behavior (e.g., surface
versus deep deposit feeding, and selective versus non-selective deposit feeding) alter the predictive
nature of the ESG? Some discussion of this issue would be useful because the goal of the Metals
Mixtures ESG is to protect benthic organisms, for many of whom sediment ingestion may be a major
exposure pathway.
4.3 The Importance of Biological and Ecological Processes
Exposure—The SEM-AVS approach to predicting a "no effect" guideline assumes
bioavailabilty is predicted by equilibrium partitioning. Equilibrium partitioning theory is based on several
assumptions as well, e.g., a closed system in steady state where reactions are reversible. In nature,
these assumptions frequently are violated.
Recent advances in benthic ecology, biogeochemistry, geology, and benthic boundary physics
have revealed the central role of vertical profiles of solutes, organisms, and sediment types in regulating
all benthic processes (Meyer et al., 1994). Vertical profiles are heterogeneous in 3-dimensional space,
and organism-sediment relationships cause most of this heterogeneity. Organismal bioturbation
ventilates, oxygenates, and moves sediments against vertical gradients. Bioturbation can enhance
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biological activity, solute transport, and diagenetic rates associated with organismal tubes and burrows
(Aller, 1983).
The basic biogeochemical nature of sediments is well known and one of the most important
concepts is the vertical distribution of electron acceptors for the biological process of respiration
(Fenchel and Jorgensen, 1977). There is a vertical gradient of diagenetic reactions for organic matter
degradation in recent sediments based on the thermodynamics of the reactions. Oxidation is the
primary reaction in surface sediments, but reduction is the primary reaction in anaerobic sediments,
beneath the sediment surface. The denitrification zone probably occurs within the first few cm from the
surface, followed by a sulfate reduction zone from 5 to 50 cm beneath the surface, and the
methanogenesis zone occurs in the deepest sediments. This simple, yet powerful, view of sediment
biogeochemistry has fueled a generation of studies. Yet, the gradients of increasing sulfide in sediments
are not accounted for in the SEM-AVS methodology. As a result, the recent history of metal
deposition will have a strong influence on how the method might under- or over-estimate excess sulfide
at different sediment horizons.
Sediments are not closed systems and have enormous variability due to the many behavioral
and ecological processes operating simultaneously in natural systems. Ecological processes that can
affect exposure include migration and emigration, exposure time, life history or life cycle strategies,
habitat effects (i.e., benthic and pelagic species have very different exposure histories), and feeding
modes.
The question, then, is for what range of benthos would ESG be protective? Would the ESG
protect animals whose exposures to sediments and pore waters occur via oxidized burrows or micro-
environments, where AVS does not persist? Macrofauna that live in sediments employ diverse
strategies for obtaining oxygen; these strategies are implemented at scales very different than those used
to sample sediments for AVS. AVS studies have not systematically evaluated the relevance of different
sampling methodologies to different fauna. Of paramount importance is the question of whether
protocols for sampling a fixed layer of surface sediments (e.g., 2 or 3 cm) replicate the AVS
experienced by, for example, animals that live on or near the oxidized sediment surface, animals that
irrigate sediment burrows with oxidized water from the surface, or animals that migrate out of the
sediment periodically. This difficulty is compounded by the fact that the oxidized layer of sediments
varies widely in depth among sediments. The concern with the relevance of sediment sampling is
supported by studies by Hare et al. (1994) and Warren et al. (1998). These studies directly showed
that the response of lake benthos to SEM-AVS predictions of bioavailability varied greatly among
several species. The methodology was predictive of metal bioavailability for chironomid species that
lived within the deeper sediments. It was not applicable to other benthos. The latter group had life
strategies that avoided the anoxic horizons of the sediments, a common biological strategy.
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The biogeochemical issues are likely different in coastal marine systems and freshwater systems.
Coastal systems are relatively more open compared to more closed freshwater systems. In addition,
the chemical milieu of coastal systems is more complex with mixing of fresh and sea water, various
sources and sinks of organic matter, widespread availability of sulfate, and complex interactions of
various anaerobic respiration pathways. The presence of sulfate is especially important because it will
limit the production of sulfide, and thus the availability of excess sulfides. The interaction between metal
deposition gradients and spatially and temporally variable geochemical gradients in estuaries needs to
be considered in the design of sediment sampling protocols.
In summary, ecological processes limit the applicability of equilibrium partitioning to predict
bioavailability, particularly in coastal systems. In each case, the net effect appears to be an
underestimation of exposure. In addition, the interactions of these complex ecological processes
explain the high variability of geochemical measurements made in sediments and lead to methodological
difficulties in applying the SEM-AVS guidelines. As a result, the Agency should amend its conceptual
model for water column and sediment criteria to incorporate ecological processes and resulting
exposure pathways. In addition it is crucial that the Agency develop a sampling methodology (for
SEM, AVS, and other sediment assessment measurements) that accounts for the complex vertical
gradients caused by ecological processes.
Effects—It is also time to turn greater attention to the role of biology, including differential
sensitivities for different life stages of a species and inter-species differences, in predicting toxic effects
from sediment metals. The Metals Mixtures ESG document notes that mortality in laboratory and
spiked field samples as a function of SEM-AVS was organism independent (p. 1-42). However, in
Figure 3-1 it appears that midges always had low mortality and the polychaete always had high
mortality. Only the amphipods appear to exhibit the full range of mortality across the full range of
concentrations. Part of the problem is fewer tests at low ranges for polychaetes and high ranges for
midges. The document should discuss more fully the importance of differences in species sensitivity and
the implications of these differences for application of the ESG.
There is a complex explanation for differential species responses based on ecological
processes. Table 3-1 of the Metals Mixtures ESG document contains data that show toxicity in oxic
environments when AVS is non-detectable and SEM is high, and in contrast that anaerobic
environments are not toxic when AVS is high and SEM is low. Therefore, it follows that infaunal or
burrowing species would have adaptations such that they are tolerant to high AVS, but sensitive to
metals. Epibenthic or surface crawlers would be the opposite. This is supported by the observations
that Capitella, a burrowing deposit feeder, has high mortality (Figs. 3-1 to 3-6) compared to
Neanthes, a surface-dwelling polychaete (pages 1-74, 75). Because of different feeding strategies,
burrowers tend to be deposit feeders and surface dwellers tend to be omnivores, burrowers can have
higher metal loads due to direct sediment ingestion, whereas surface dwellers may be exposed to metals
primarily from ingestion of water and contaminated prey. In addition, metals may bind to organic
particles, which deposit feeders select by an unknown mechanism. So, it is possible that the net result
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is burrowers have the highest exposure to metals through gut linings, which are acidic and could release
metals for absorption, and are more sensitive to exposure.
The data presented on predicting organism toxicity by SEM-AVS normalized for organic
matter in the laboratory, field, and colonization experiments is convincing. The selection of test
organisms and the confinement in field experiments, however, is always a worry in toxicity testing.
Getting around the confinement problem in chronic tests by using colonization substrates raises the
question about representativeness of the colonizing organisms. In the case of the selected metals, a
functional approach to the selection of test organisms would seem useful. That is, the organisms that
have been used or would be potentially considered for use in laboratory or field testing to support the
ESG could be categorized functionally on the basis of the expected site of action of the metal as
predicted from their morphology (e.g., exposed gills, poorly protected cutaneous surfaces) or feeding
behavior (e.g., ingestors of fine paniculate organic matter [FPOM]). Among the invertebrates, ingestion
categories of most direct importance would be filter or suspension feeders (FPOM filtering collectors)
and sediment ingestors or deposit feeders (FPOM gathering collectors). Other functional groups, such
as coarse particulate organic matter (CPOM) detritivores (shredders), benthic algal grazers (scrapers),
and predators would experience exposure indirectly through food chain accumulation. With regard to
digestive surface exposure, obligate sediment ingestors would be the choice to represent maximum
effects.
A case in point concerning the appropriateness of test organisms to support the ESG would be
the comparison between the freshwater amphipod Hyalella, which is a facultative periphytic algal
scraper, and the snail Helisoma, which is an obligate algal scraper. Neither would be expected to
ingest much sediment, except that associated with algal colonies. Most larval midges in the tribe
Chironomini (e.g., Chironomus tentans) would be expected to ingest primarily sediments because they
are benthic deposit feeders. The obligate filter feeders, such as bivalve mollusks and some marine
polychaetes, would be predicted to exhibit maximum response to all metal-organic complexes in the
interstitial water. A further important separation of test organisms would be their ability to withstand
anaerobic conditions, which would significantly effect metal bioavailability. For example, the ability of
many Chironomini midge larvae to withstand anaerobic conditions should be considered in their
selection of test organisms to support ESG. The above considerations should be given at least equal
status with ease of collection and/or culture when selecting test organisms.
It would be useful to include in the ESG document a table summarizing the functional attributes
of the test organisms that were used in the laboratory and field studies supporting the ESG.
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5. APPROPRIATE USE OF THE PROPOSED SEM-AVS
METHODOLOGY
5.1 Accounting for Spatial and Temporal Variability
If the SEM-AVS method is to be used to help classify sediments as toxic or not toxic, then
such use of this measure will depend upon the accurate characterization of the SEM-AVS for the
sediments of interest. This characterization probably will be statistical. For any system, SEM-AVS
values will vary in time and space as the integration of spatial-temporal variation in inputs of metals to
surface waters and the variability or periodicity (e.g., seasonal) in the physical (i.e., advection,
dispersion, sedimentation, sediment resuspension, sediment particle size), chemical (i.e., chelation,
complexation, precipitation, sediment type), and biological (i.e., bioaccumulation, sequestration, trophic
interactions) processes that determine the transport, distribution, and fate of metals in sediments. AVS
concentrations in sediments vary vertically due to oxidation of surficial sediments, bioturbation of
sediments, seasonal changes in the concentration of oxygen in overlying waters, and varying activity of
sulfur-reducing microbes. Microbial activity also varies with depth in sediments depending on supply of
organic material, availability of sulfate, and sediment substrate type.
Given these sources of variability, appropriate sampling designs will be necessary to apply the
SEM-AVS methodology in specific locations. For example, the biologically active zone of the surficial
sediments should be included in sampling; the depth to which sediment samples are routinely taken must
be determined based on data and good science, rather than an arbitrary depth such as 2 cm. It is likely
that no single depth value will be appropriate for all sediments across the nation because the redox-
discontinuity zone varies in space and time, and with sediment texture, circulation patterns, and supply
of organic material. Samples should be taken periodically, in relation to the physical mixing
characteristics (e.g., stratification, turn-over) and seasonal changes of production dynamics. Spatial
and temporal variability in the oxidation-reduction (redox) potential of the sediments should also be
quantified in developing sediment sampling designs on a site-specific basis; typically, the deeper the
sample, the more reduced (anoxic) sediment is included. The characteristic variability in space and time
of environmental processes that determine SEM-AVS measures, combined with the area and depth, of
the sediment system of interest, will likely constrain the statistical power of practical and affordable
sampling designs. An important implication of such variability is that sediments might only be classified
as toxic or non-toxic in probabilistic terms with an associated risk of incorrect classification.
One possible way to address such concerns would be to attempt to characterize the nature of
the variance of the physical, chemical, and biological processes for different types of aquatic systems.
Such information might be used to develop a general model for quantifying the variability of SEM-AVS
measures and using this information to a) estimate the performance characteristics for statistical
designation of sediments as toxic or not toxic, and b) identify which of the controlling processes
contributed the major components of variation in SEM-AVS measures. The results of the latter
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"sensitivity" analysis might be used to effectively and economically allocate resources toward sufficiently
powerful sampling designs that would result in sediment classifications that met pre-specified
(risk-based) criteria for accuracy and reliability. Such quantification of characteristic variability in
sediment environments might require substantial investments in basic measurements and monitoring if
existing data prove inadequate.
In addition to the inherent variability of AVS concentrations in the field, AVS and the
geochemical milieu of sediments are notoriously difficult to sample, preserve, and measure correctly.
Sulfide is volatile and easily oxidized, and sediment disruption and oxidation results from the act of
sampling and analysis.
In its 1995 review, the SAB raised concerns about the applicability of the SEM-AVS method
in the field, where it will be applied in a regulatory context. The current Metals Mixtures ESG
document does not provide additional confidence in the method with regard to these concerns. For
example, it does not appropriately reflect the limitations of the method when it specifies that the ESG
"are intended to apply to sediments permanently inundated with water, intertidal sediment, and to
sediments periodically inundated for durations sufficient to permit development of benthic assemblages"
(p.l -15); sediments that are periodically exposed would experience oxidation of sulfides, with potential
release of bound metals. Similarly, guidance regarding the collection of samples does not reflect the
fact that AVS and SEM can vary considerably within the top 2 cm. The oxidized zone is where most
species concentrate their interaction with their environment and where AVS concentrations are lowest;
the implications of fixed depth sampling, relative to concentrated sampling in the oxidized zones should
be evaluated. Development of the protocol also should address variability of AVS concentrations as
measured by recommended sampling protocols, as compared to variability in oxidized sediments
carefully collected from the sediment-water interface.
The Committee recognizes that sampling protocols must be practical. Simplifying assumptions,
however, should be applied only with full recognition of the potential to over- or under-estimate
adverse biological effects. With this in mind, we suggest that the following questions be considered in
the design of sampling protocols: Where does one sample? How does vertical sampling, or pooling
sediments over different sediment horizons, affect the applicability of the SEM-AVS model? Does it
matter if there are excess sulfides deep in sediments, but not in surface sediments? How is
bioavailability influenced by bioturbation or sediment turnover? How does the SEM-AVS model
perform in the context of temporally varying salinity gradients in the same locations over temporal scales
relevant to tidal cycles, and long-term hydrological cycles (e.g., floods and droughts, and stochastic
storm events)? The answers to these questions would incorporate the complexity introduced by
ecological processes and increase the ability of the SEM-AVS method to predict metals bioavailability.
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In short, it is most important that a highly specific sampling protocol be developed to guide
regulatory applications of SEM-AVS methodologies. If sampling protocols result in inconsistencies or
inappropriate estimates of AVS, then inaccurate predictions of "non-toxic" or "toxic" sediments may
occur. It does not appear that systematic studies necessary to develop such a protocol have been
conducted in environments where the protocol would potentially be implemented for regulatory
purposes.
5.2 When the Methodology Does Not Apply
Using the SEM-AVS methodology, perhaps normalized to sediment organic carbon content, to
classify sediments as toxic or non-toxic may prove ill advised under certain circumstances. As
discussed above, the method might not be effective in assessing sediment toxicity where the primary
route of exposure for organisms of concern is ingestion of contaminated prey, although the SEM-AVS
measures might assist in characterizing prey contamination from direct exposure to metals in sediments
or pore water. In addition, if the spatial distributions of different sediment types (e.g., sand, silts, clays),
redox potentials, and fractions organic content are highly variable within the system of concern, the
SEM-AVS methodology may prove impractical because of the necessary sampling design required to
accurately describe such variability. This limitation might be further compounded by spatial and
temporal variability in the distribution and abundance of sediment dwelling organisms (i.e., bioturbation),
including sulfate reducing bacteria. Finally, if the sediments of concern are typically well oxidized, the
proposed methodology also will be of little use in assessing bioavailability and toxicity of sediment
metals.
The ESG states that the proposed guideline approaches "are not designed to protect aquatic
systems from metal release associated, for example, with sediment suspension." The Committee agrees
that the Metals Mixtures ESG should not be used as a "no effect" threshold in aquatic environments
where sediment resuspension or transport is expected because sediment resuspension greatly increases
the potential for re-oxidation of AVS-bound metals. This limitation in the ESG is important because
sediment suspension, whether associated with extreme storm events, tidal or riverine transport,
dredging operations, or other activities, is quite common in aquatic systems. We recommend that the
Agency consider how sediments should be assessed in high energy zones and provide further guidance
in the future. A similar caveat should be added to the guideline statement for sediments whose redox
conditions are likely to change due to periodic exposures associated with tidal fluctuations or seasonal
changes. The Committee recommends that these limitations of the SEM-AVS approach be
incorporated into both the Metals Mixtures ESG and the sediment user's guide to assure that the SEM-
AVS methodology is not mistakenly used where it is inappropriate.
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6. AGENCY-PROPOSED MODIFICATIONS TO THE SEM-AVS
METHODOLOGY
6.1 Organic Carbon Normalization
Charge Question 1: By incorporating the fraction organic carbon into the bioavailability
equation, has the Agency retained the protective features of the guidelines and improved
its predictiveness of toxic effects?
The Metals Mixtures ESG is based on the assumption that bioavailability and any subsequent
toxicity are predicted by relationships among metal concentrations in sediments, pore water, and tissue.
Although equilibrium partitioning provides a mechanistic basis for predicting the partitioning of metals
between particulate and dissolved compartments (i.e., pools), the associated relationships between
partitioning and metal toxicity are based on empirical observations that are characterized by large
variances. The fraction organic carbon (foe) normalized approach is an extension of the EqP
methodology developed for non-ionic organic chemicals. It recognizes that divalent metals in anaerobic
sediments will bind to sulfides first until the labile sulfides are exhausted and then will bind to other
phases such as organic carbon. The incorporation of this binding phase into the bioavailability equation
therefore improves the overall ability of the SEM-AVS method to identify sediments that are toxic.
The primary limitations identified by the Committee include potential implications of the foe
normalized SEM-AVS methodology for accurately assessing biological exposure to metals and
translating exposures to estimates of toxic response, both acute and chronic. Regarding exposure
estimation, the foe normalization provides a largely theoretical explanation for metal toxicity data that
were not consistent with previous equilibrium-based analyses of sediment exposure and toxicity. It has
been proposed that organic carbon reduces the bioavailability of metals. As suggested by results
shown in Figure 3-1 of the Metals Mixtures ESG document, the normalization of SEM-AVS to fraction
organic content reduced the variability in the exposure estimates. However, there remain some
important issues that might realistically constrain the general applicability of the foe normalization. We
point out that some studies have shown inconsistent results, indicating that foe normalization does not
appear to reduce bioavailability of metals consistently. Additionally, test results appear to be
dependent in part on the nature of the organic carbon (Lee and Luoma, 1998). Such inconsistencies
may lead to inappropriate applications of the foe normalization with subsequent incorrect conclusions
that particular sediments are probably not toxic.
Further, while the foe normalization generally might increase the precision of exposure estimates
for metals, there was not necessarily a corresponding improvement in the precision of the estimated
toxic response to exposure, particularly in the analysis of field sediment samples. Mortalities for many
of the organisms of interest ranged between nearly 0 and 100% across foe normalized estimates of
exposure to metals. Such variability in toxic response reduces the power of this methodology for
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developing the exposure-response functions that are essential for risk assessment. Similarly, estimates
of acute toxicity (lower bound exposures adjusted to organic carbon) did not correctly identify many
sediments as nontoxic as determined from sediment bioassay results. Nevertheless, the Committee
recognized that the foe normalization did seem to improve the relationship between exposure and
response in the analysis of limited field data for aquatic midges. Additionally, the methodology also
improved the ability to quantify the upper bound exposure that defined a 95% chance that amphipod
mortality would exceed 50% under laboratory test conditions. However, the Committee was
concerned that the overall methodology is primarily empirical and that the foe normalization might
improve the effectiveness of the SEM-AVS method only for those conditions where the collected data
pertain to the actual form of the metals that are biologically available and toxic.
Another concern lies in the spatial and temporal variability of sediment organic carbon content.
Physical, chemical, and biological processes that determine the input, transport, and ultimate distribution
of carbon exhibit characteristic scales of variability (e.g., seasonal) that must be considered in the design
of sampling programs aimed at accurately and precisely measuring organic carbon in sediments.
Similarly, the foe normalization does not improve the ability of the SEM-AVS method to characterize
the toxicity of aerobic (or oxidized) sediments. This limitation also applies to oxic microclimates often
created by sediment dwelling organisms and to variations of AVS caused by bioturbation in surficial
sediments (e.g., top 2 centimeters). Finally, the foe normalized SEM-AVS method does not address
dietary routes of exposure to metals in the sediment environment; many sediment dwelling organisms
direct their feeding activities on available carbon when they ingest sediments.
In summary, the incorporation of the foe into the SEM-AVS methodology improves the overall
ability of the approach to predict when sediments are likely to be toxic to benthic invertebrates. With
foe normalization, the method has a precision of about one order of magnitude, which is an
improvement of a factor of 10. The Committee believes the SEM-AVS normalized to foe content is an
appropriate method for screening and establishing priorities among contaminated sediments. This
method, however, should be applied with caution and used in combination with other approaches
including sediment bioassays and or field bioassessments. The Committee does not support the use of
the foe normalized SEM-AVS as a stand-alone, definitive method for identifying "toxic" and/or "non-
toxic" sediments without supporting biological data.
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6.2 Application of the Biotic Ligand Model (BLM) to Sediment Guidelines
Charge Question 2: If the BLM is used to derive or adjust a water quality criterion, is
the revised criterion appropriate for use in the interstitial water component of the Metals
Mixtures ESG?
As noted in a companion SAB report (EPA-SAB-EPEC-00-006) on the Biotic Ligand Model,
the Committee is optimistic that the BLM represents a technical improvement in the state-of-the-
science for assessing bioavailability of metals in water over a wider array of environmental conditions
than previously used. The BLM methodology also offers advantages over the currently used Water-
Effect Ratio (WER) approach in the derivation of site-specific acute water quality criteria. It appears
that a logical next step would be to evaluate/validate the BLM for application to sediments via the
interstitial water component of the Metals Mixtures ESG. If this approach is successful, it would allow
for additional variables, such as dissolved organic carbon (DOC), to be accounted for in the
assessment of bioavailability of metals in sediment pore waters.
In theory, the BLM would be applicable to sediment interstitial water because hardness, pH,
and DOC are important toxicity modifying factors for interstitial waters, as they are in overlying waters.
However, several important issues require additional attention before the Committee would recommend
the general application of the BLM to sediments or sediment pore waters. First, the BLM currently has
been validated to varying degrees for only two metals, with few benthic species and/or pore water test
procedures (EPA-SAB-EPEC-00-006). The Metals Mixtures ESG, on the other hand, includes a
number of other metals for which the BLM has not been sufficiently validated. Second, the BLM was
originally developed for conditions characteristic of the water column. By comparison, the chemistry of
interstitial water, including the relationships between particulate and dissolved organic carbon and the
binding of metals, is less well understood and characterized. Pore waters may contain different kinds
and higher concentrations of constituents than the water column immediately above. A critical
component of verifying the model, therefore, will be demonstrating its ability to appropriately predict
binding to pore water DOC under a variety of different conditions. It would be useful to initiate further
testing of the BLM with mixtures of metals in pore water matrices, compared against SEM predictions,
in order to determine its effectiveness for predicting net toxicity of pore water under varying chemical
conditions. An interesting experiment would be to physically concentrate overlying water above
sediments to achieve the same approximate DOC concentration as in the pore water below, then test
each in bioassay in the absence of sediment to see if pore water is "just more concentrated overlying
water." In short, experiments are needed over much wider concentration ranges and with interactions
of various DOC constituents found in pore water before adopting the use of the BLM for sediments.
A BLM-adjusted WQC would not be appropriate for use in the interstitial water component of
the Metals Mixtures ESG because the site-specific water chemistry used to derive the adjusted criterion
would not be the same as the water chemistry of the pore water (even from the same site). The BLM-
adjusted WQC is specific to a given site, as is a WER-adjusted value. Application of the BLM to
24
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interstitial water would require measurements of the site interstitial water chemistry (pH, DOC,
hardness, etc).
Alternatively, if the BLM were used to derive a metal-specific (e.g., copper) equation or
algorithm which would replace the existing acute hardness equation in a national water quality criterion,
the new equation/algorithm would, in theory, be applicable to sediment interstitial water; however, site-
specific pore water chemistry data would still be needed to apply the algorithm to interstitial water. The
currently proposed IW component of the Metals Mixtures ESG relies on comparison of the IW metals
concentration to the water quality criteria Final Chronic Value for each metal, corrected only for site-
specific hardness; the BLM, if validated for application to interstitial water, would allow consideration
of additional site-specific chemistry conditions that affect metals bioavailability.
In summary, the Committee is encouraged by the BLM 's performance for predicting toxicity of
metals in the water column, and believes there is potential to apply it to assessing the toxicity of metals
in pore waters. At the same time, the Committee believes further research is needed to demonstrate
the model's performance for interstitial water applications.
6.3 Inclusion of Chromium and Silver in the Sediment Guidelines
Charge Question 3: Are the data presented from lab and field experiments with
chromium and silver sufficient to support their addition to the Metals Mixtures ESG?
The information presented to the Committee indicates that when sufficient AVS is present in
sediment, the binding/precipitation of chromium and silver will prevent acute toxicity from occurring
much the same as has been demonstrated for divalent metals. The Committee is encouraged by these
data and supports the Agency's desire to further evaluate the EqP approach to determine its application
to silver and chromium. However, at the present time the Committee has reservations about including
either of these metals in the ESG without further research. Committee concerns are the following:
a) Most of the data available to assess the methodology are based on acute toxicity and
the data sets are not extensive. For example, the chromium test results presented in
Table 3-1 (p. 1-58) did not provide sufficient insight into acute toxicity at lower IWTUs
(less than 100).
b) Additional chronic toxicity studies are needed to verify model application to assessing
chronic toxicity for chromium;
c) Conditions under which Chromium (HI) can be oxidized to Chromium (VI) have not
been fully explored and this oxidation could provide a means whereby a more toxic
form of chromium is available to cause effects. Cr(VT) appears to be the
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most bioavailable form of dissolved chromium, but uptake is slow enough that long term
studies are needed to evaluate potential effects.
d) Conditions under which MnO2 are expected in sediments should be identified since this
chemical can oxidize Cr(m) to Cr(VI).
e) Cr (HI) is absorbed by bivalves and polychaetes from some food types that might
occur in sediments (Wang et al., 1997).
f) Biokinetic studies indicate that low assimilation rate of Cr (IE) from highly contaminated
sediments represents a significant route of exposure (Wang et al., 1997).
g) SEM-AVS and EqP theory predict that silver should almost never be bioavailable and
cause toxicity when found in sediments with [AVS] > [Ag]/2. However, both
laboratory and field studies indicate this is not the case. As a result, application of the
SEM-AVS approach to silver may not be protective of benthic organisms.
h) There are insufficient chronic data for silver.
i) The data presented in Figure 8, p. 2-36, are not entirely consistent with the theory that
AVS binding/reactions with silver should eliminate all toxicity when sufficient AVS is
present. In the graphs presented, the AVS normalization of the data does not provide
the same clear demonstration that AVS binding eliminates silver toxicity as has been
shown for other metals.
j) Figure B on p. 2-63 shows acute mortality of >24% at IWTU < 0.5. This appears
inconsistent with the Guidelines and with the interpretive table on p. 2-60.
These concerns suggest that additional research is needed before incorporating chromium and silver
into the ESG.
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7. AN INTEGRATED APPROACH TO METALS
In presentation materials and briefing documents supplied to the Committee, the Agency
defined its vision for an integrated metals methodology that would lead to a consistent set of metals
criteria for the "total aquatic environment" (i.e., both sediments and overlying waters). This integrated
methodology included the proposed ESG and proposed applications of the Biotic Ligand Model for
water column and sediment pore water assessments of bioavailability, as well as possible future
development of tissue residue-based criteria. In addition to specific comments on the BLM and the
Metals Mixtures ESG, the Charge to the Committee asked:
Does this integrated metals methodology improve our ability to make both protective
and predictive assessments of toxicity due to copper, silver and other selected metals
in the water column and sediment?
The Committee supports the Agency's quest for an integrated approach to assessment and
regulation of metals in the environment. A useful approach for understanding the relationships among
environmental compartments, drivers, exposure pathways, and the various water and sediment criteria
measures is to construct a conceptual model. During the presentation to the Committee, such a model
was used to highlight the Agency's vision for the inter-relationships among the various criteria and to
highlight areas where additional work will be needed (Figure 1). In addition to guiding the Agency's
own efforts to integrate management of water and sediments, a conceptual model of this type would be
a critical component of the Sediment Guidelines User's Guide and the technical guidance documents
such as the Metals Mixtures ESG, so that users are informed of the interrelationships among various
standards and guidelines.
The Committee noted, however, that the conceptual model presented does not include some
potentially important considerations. The labeling of suspended metals as "not bioavailable" raises
particular concern because suspended solids in the water column bind metals and are a major source of
food for filter feeders. In addition, the conceptual model appears to assume that SEM-AVS can be
used as a stand-alone test of the bioavailability of metals in sediments. We recommend that the Agency
consider carefully the limitations discussed in Sections 4 and 5 of this review when revising its
conceptual model.
Other components that the Agency should consider incorporating in a conceptual model
include a) the biological role of organic matter complexes as partitioning phases in both the water
column and sediment components of the model, and b) the distinction between the sites of biological
action of the metal toxicity, namely respiratory, cutaneous, and digestive surfaces. The dissolved
organic complexes are undoubtedly important in interstitial sediment water. In particular,
sediment-ingesting invertebrates will engulf both organic and mineral particles in the correct size range
and, therefore, adsorption of metals to mineral sediment and complexing with dissolved organic matter
27
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to form fine paniculate organic matter (FPOM)~and the equilibria between these components and the
pore water- are important to factor into the general model. The distinction between the sites of
biological action that are relevant for water column vs. sediment exposures will bear directly on the
choice of organisms to be used to evaluate effects of metal contamination. If complexing with dissolved
organic matter is introduced, then food chain effects can be considered in both water column and
sediment environmental compartments. Similarly the free solution (non-complexed) form of the metals
can directly affect respiratory and cutaneous surfaces in both compartments. It is likely that the food
chain effects shown in Figure 1 would almost always be mediated through uptake from (microbial) or
ingestion of (invertebrates) organic particulate complexes.
A possible means of incorporating these additional considerations into a conceptual model is
illustrated in Figures 2 and 2a. The scientific picture of water/sediment/biota interactions presented in
these figures could be supplemented with notations relating to Agency standards and guidelines, as was
done in Figure 1.
RELEASED TO WATER
^Dissolved Metal ^.
Partitioning Model
^jf Particulate Metal j
< Aquatic Life ^v
Water Column j^-
Criteria ^r
Indirect Foodchain
Toxicity (Hg, Se)
Bioaccumulatio |
AVS-SEM/F,,C
Figure 1. Agency Briefing Slide
28
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As part of its attempt to integrate water and sediment criteria and guidelines, the Agency has
suggested that the Biotic Ligand Model (BLM) methodology could be incorporated into both. While
the use of the BLM with the ESG shows promise and clearly warrants additional research, our theory
and understanding of complex geochemical and biological interactions in sediment/water systems is not
yet sufficiently advanced to enable adoption of a single unified set of criteria to ensure protection of
both pelagic and benthic organisms. There is no evidence to suggest that this cannot be achieved over
time by gaining further experience with the BLM in pore water tests, but at present the BLM cannot
replace the need for sediment and interstitial water evaluation by empirical bioassay procedures (EPA-
SAB-EPEC-00-006). The ESG and BLM do not yet provide a sufficient basis upon which to assess
and manage water quality concerns associated with contaminated sediment/water systems. The BLM
is a predictive, but not a definitive, tool for setting acute lexicological criteria for either matrix.
Furthermore, it does not address important additional considerations for sound water quality
management, including chronic effects mediated by non-dissolved metal species, and acute mechanisms
of action and routes of exposure not directly related to impairment of physiological function or uptake at
the external surface of the organism.
EPEC applauds the Agency's efforts to produce an integrated water/sediment management
system. We suggest that a revised conceptual model is essential to this endeavor. The conceptual
model can be used to assess whether environmental compartments or routes of exposure are being
addressed and also to assess whether the Agency's guidance is inadvertently providing incentives to
accumulate metals in one compartment versus another. It may not be necessary to use the same
assessment method (such as the BLM) in both water and sediments to achieve these purposes. While
the BLM is not yet ready for application in both water and sediment methodologies, this does not mean
that the Agency has failed to improve its integration of water and sediment management. Appropriate
use of the SEM-AVS methodology, along with development of assessment methods for the elements of
the conceptual model that are still unaddressed, will greatly improve the Agency's ability to protect the
"total aquatic environment" from toxic effects due to metals.
29
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Water Column
Bulk Sediment
Respi
1 ation
V
W
W;
exti i nal
coi i act
EqP
Sed
exti
V
co i act
i lent
inal
ing*
Sed i lent
V
Resp
tion
Biota
s
F
cli
exp
>d
in
lure
Biota
Figure 2. Multipathway Exposure Model
1 ation
Bulk Sediment
Exposure to
Cutaneous Surfaces
(Aerobic)
Exposure to
Digestive Surfaces
(Anaerobic)
Exposure to
Respiratory Surfaces
(Aerobic)
Figure 2a. Detail of Bulk Sediment Compartment
30
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8. SUMMARY
The Agency and its funded scientists have developed an elegant body of scientific work
showing that the availability of metals in sediments, as modeled by the excess of SEM over AVS,
correlates well with observed toxicity in a range of laboratory studies. The development of a Metals
Mixtures ESG that incorporates the SEM-AVS theory is an important endeavor and one that the
Committee supports. Work conducted since the previous SAB review of the methodology—including
field manipulation experiments and a limited number of chronic studies of metals toxicity-is a step in the
right direction, and the work relating organic carbon normalization of the SEM-AVS to observed
toxicity is well done and enhances the ESG approach. Moreover, promoting the routine measurement
of AVS in addition to total organic carbon (TOC) is sound advice. However, there are underlying
limitations to the applicability of the SEM-AVS approach that are not addressed by organic carbon
normalization. The Agency has not yet convincingly shown that the SEM-AVS model adequately
describes the realities of metals exposure in the environment and there are reasons to believe that
premature application of the ESG could yield inconsistent results, especially in the absence of a peer-
reviewed sampling protocol. The most common result may be to incorrectly identify sediments as "not
toxic" when that is not the case.
In this report, the Committee comments on the proposed modifications to the ESG approach
and suggests that the Agency focus future research efforts on better understanding the biological and
ecological—to complement chemical—aspects of metals toxicity in the aquatic environment. The
report reflects the Committee's conclusion that, although introduction of "chemical corrections" such as
organic carbon normalization into the ESG calculations might reduce the variability in toxicity as a
function of SEM-AVS, such corrections do not address the underlying question of the method's
applicability in a variety of field conditions. In order to further validate the applicability of the ESG
model in the field, greater attention needs to be given to biological questions, e.g., the possibility of
chronic effects of sediment metals in sediments with low metal-binding capacity or less than extreme
metal concentrations; the relative importance of dietary exposures to metals and metal sulfides; drivers
of bioavailability in the gut of benlhic organisms; differential toxicities among benthic species; and the
role of behavior and microhabitat in moderating exposure to sediment metals. These research questions
primarily relate to the use of the SEM-AVS method to identify "no effects" levels of sediment
contamination. For this reason, the Committee recommends that additional experimental work be
undertaken to verify the applicability of the proposed methods in the field and/or to develop
complementary tools with which to predict toxicity of metals in environments where the assumptions of
the ESG approach cannot be met. Specific concerns exist for bioaccumulation and chronic toxicity in
aerobic sediments with low AVS and low organic carbon.
The Committee reiterates a number of concerns with the EqP approach that were outlined in
the 1995 SAB review of the proposed EqP-based criteria for five metals, and emphasizes that a
number of these concerns remain valid today. While the ESG document may clearly acknowledge the
31
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limitations of the method's applicability, these limitations make it vital that the Agency provide clear
guidance to potential users regarding real world applications. The Committee understands that such
guidance (including applicability of the ESG method to the dredged material program) will be included
in a "Sediment Guidelines User's Guide" under
preparation by the Agency. This User's Guide should also undergo careful peer review to ensure that
the various sediment assessment tools are not applied outside their validated conditions.
In response to the Charge questions, the Committee concludes that: a) the incorporation of the
fraction organic carbon (foe) into the SEM-AVS methodology improves the overall ability of the
approach to predict when sediments are likely to be toxic to benthic invertebrates, but does not render
the method suitable for use as a stand-alone method for identifying "toxic" and/or "non-toxic" sediments
without supporting biological data; b) the Biotic Ligand Model should be validated for pore water
applications before being used in the Interstitial Water component of the Metals Mixtures ESG; and c)
further research is required before adding either chromium or silver to the Metals Mixtures ESG.
For these reasons, the Committee recommends that SEM-AVS be incorporated into sediment
assessment guidelines in a way that assures that SEM-AVS will continue to be used in conjunction
with other assessment tools to characterize the safety of sediments, rather than being used as a stand-
alone test. The SEM-AVS method may be particularly useful to prioritize sites requiring attention and
to explain situations when bioassays show a lack of toxicity even though metal concentrations in
sediments are high. The Committee strongly recommends that the Agency now turn its attention to the
appropriate application of SEM-AVS in the field and thereafter to other methods of assessing sediment
quality that can be used when SEM-AVS cannot.
The Committee applauds the Agency's efforts to integrate its approaches to the management of
water column and sediment metals, while noting that the conceptual model presented to the Committee
describing the relationships among environmental compartments, exposure pathways, and criteria
measures was incomplete. The Committee urges the Agency to develop a refined conceptual model
that incorporates all partitioning phases and routes of exposure in order to guide the Agency's long-
term efforts to integrate water column and sediment standards and to assist users to apply current
standards and guidelines appropriately.
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APPENDIX A: ACRONYMS AND ABBREVIATIONS
AVS Acid Volatile Sulfide
BLM Biotic Ligand Model
CPOM Coarse Paniculate Organic Matter
DOC Dissolved Organic Carbon
EqP Equilibrium Partitioning
ESG Equilibrium Sediment Guidelines
FCV Final Chronic Value
foe fraction organic carbon
FPOM Fine Paniculate Organic Matter
IW Interstitial Water
IWTU Interstitital Water Toxic Unit
SEM Simultaneously Extracted Metal
TOC Total Organic Carbon
WER Water-Effect Ratio
WQC Water Quality Criteria
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APPENDIX B. RESEARCH ON THE SIGNIFICANCE OF DIETARY
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