United States Science Advisory Board EPA-SAB-EPEC-95-020
Environmental 1400 September 1995
Protection Agency Washington, DC
&EPA AN SAB REPORT: REVIEW
OF THE AGENCY'S
APPROACH FOR
DEVELOPING SEDIMENT
CRITERIA FOR FIVE
METALS
PREPARED BY THE SEDIMENT
QUALITY CRITERIA
SUBCOMMITTEE OF THE
ECOLOGICAL PROCESSES AND
EFFECTS COMMITTEE
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September 29, 1995
EPA-SAB-EPEC-95-020
Honorable Carol M. Browner
Administrator
U.S. Environmental Protection Agency
401 M Street, SW
Washington, DC 20460
RE: Science Advisory Board (SAB) Review of the Agency's Approach for Developing
Sediment Criteria for Five Metals (cadmium, copper, lead, nickel, and zinc)
Dear Ms. Browner:
Contaminated sediments are of major concern to the Agency, because in some instances,
such as parts of the Great Lakes or New Bedford Harbor, fish accumulate chemicals from the
sediments and, in turn, they may be eaten by humans. A fundamental problem in evaluating the
toxic effects of chemicals in sediments is deriving a meaningful estimate of the amount of the
chemical in sediment that is available to cause toxic effects (often referred to as the biologically
available portion). In the past, the Agency has applied the theory of Equilibrium Partitioning to
predict biological availability of organic chemicals. In this review, the SAB is evaluating the
Agency's approach for assessing the toxic effects of metals in sediments. As you are aware, the
assessment of these toxic effects is a key consideration in the management of contaminated
sediments in regulations affecting wetlands, surface water discharges, drinking water supplies, and
solid wastes.
The Sediment Quality Criteria Subcommittee of the Ecological Processes and Effects
Committee (EPEC) of the Science Advisory Board met in Washington, D.C. on January 4-6,
1995 to review research and data developed to support the use of the Equilibrium Partitioning
(EqP) approach to predict availability of five metals in sediments. In this Agency approach, the
five metals are separated from sediments by the "simultaneously extracted metal" procedure
(SEM). The amount of the five metals (cadmium, copper, lead, nickel, and zinc) that is
presumably biologically available to organisms that live and feed in the sediments is the difference
between the SEM and the Acid Volatile Sulfide (AVS), a binding factor for metals in sediment.
The attached report discusses the findings and recommendations of the Subcommittee.
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The Office of Water asked the SAB to evaluate the adequacy of the proposed
methodology for establishing sediment quality criteria for the five metals. In particular, OW asked
the SAB to:
a) Consider the adequacy of an existing method (bulk chemical analysis) and each of
four new methods proposed by the agency to establish a plausible cause and effect
relationship between metals concentrations and organism responses.
b) Determine whether data from lab, field, and colonization experiments demonstrate
a plausible cause and effect link between the simultaneously extracted metal to acid
volatile sulfide ratio (SEM/AVS) and/or interstitial water metals concentration (the
concentration of metals in water between the sediment particles) and organism
response.
c) Judge whether the four approaches to derive sediment quality criteria for the five
metals are sufficiently sound to proceed with criteria development.
d) Identify any further documentation and research that is needed to develop
sediment criteria.
Based on its review and knowledge of the literature, the Subcommittee feels that the
SEM/AVS is the best technology developed to date for assessing the significance of the five
metals in sediments. The proposed methodology will help to identify sediments that are likely to
cause adverse effects and other sediments that are unlikely to cause such effects due to metals.
The SEM/AVS approach may significantly reduce the variability currently associated with
predictions based on the analysis of bulk sediment samples. Moreover, in its effort to produce
sediment quality criteria that are supported by a strong theoretical foundation, the Agency has
engaged in thoughtfully targeted research and has provided much useful information with
potential wide applicability outside the Agency.
The proposed SEM/AVS methodology is based on sound theory and has been verified by
considerable experimental evidence. The Agency has provided strong experimental evidence to
support a plausible cause and effect link between SEM/AVS and acute toxicity.
We wish to point out, however, significant limitations to its application currently exist:
a) Considerable uncertainty remains, however, regarding the levels of SEM/AVS that
cause no adverse chronic effects.
b) Substantial effort is needed to improve the predictive model for bioaccumulation.
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c) In addition, before the techniques are ready to use as criteria, information is
needed to guide the collection of biologically representative samples and samples
that represent the spatial variability of AVS in a variety of environmental
situations.
Once standardized protocols for determining criteria are developed, the Subcommittee
recommends that the Agency produce a consolidated user's manual for use by the EPA and State
regulators; industrial, municipal, and federal parties responsible for managing sediments; and by
concerned scientists and citizens . The guidance manual should provide estimates of the cost of
this analysis and describe the appropriate use of the criteria (various other screening levels) in a
variety of environments as well as in a variety of regulatory contexts. In this regard, we
recommend that the proposed SEM/AVS approach be used as part of a tiered assessment system
(Please refer to Figure 1 on page 6 in this report as an example). We also suggest the guidance
discuss the strengths and weaknesses of each of the methods, techniques for collecting
representative samples, quality assurance, and quality control procedures (including estimates of
interlaboratory variability).
A guidance manual is needed to assure the best use of these and other methods for
assessing sediment quality. Such a manual should describe the context in which peer reviewed
methods could be used (including any tiered approach). Guidance should also discuss the
strengths and weaknesses of the methods, quality assurance and quality control procedures for
sediment sampling and analyses. The manual should also receive technical review.
Again, the Subcommittee commends the achievements of the research team supported by
the Office of Water. While there are still important research questions to be answered, the studies
which have been completed are of exceptionally high quality and the foundation is very good. We
look forward to reviewing future results from this research program, its implementation guidance,
and to receiving the Agency responses to our recommendations and comments.
Sincerely,
Dr. Genevieve Matanoski, Chair
Executive Committee
Science Advisory Board
k.
Dr. Frederick Pfaender, cU-Chair
Sediment Quality Criteria Subcommittee
Ecological Processes and Effects
Committee
Dr. Mark Harwell, Chair
Ecological Processes and Effects
Committee
Dr.
Sedfrn£#f Qutyty Ciiteri/ Sub committee
Ecological Processes and Effects
Committee
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US ENVIRONMENTAL PROTECTION AGENCY
SCIENCE ADVISORY BOARD
ECOLOGICAL PROCESSES AND EFFECTS COMMITTEE
SEDIMENT CRITERIA SUBCOMMITTEE
CHAIRS
DR. FREDERIC K. PFAENDER, Director, Institute of Environmental Studies, Dept. of
Environ. Sciences & Engineering, University North Carolina, Chapel Hill, NC
[MEMBER]
DR. TERRY F. YOUNG, Environmental Defense Fund, Oakland, CA [MEMBER]
MEMBERS. CONSULTANTS, and FEDERAL EXPERTS
DR. WILLIAM J. ADAMS, ABC Laboratories, Columbia, MO [MEMBER]
DR. PETER CHAPMAN, EVS Consultants, Vancouver, British Columbia, Canada
[CONSULTANT]
DR. ROLF HARTUNG, Professor, School of Public Health, University of Michigan, Ann
Arbor, MI [CONSULTANT]
DR. CHRISTOPHER G. INGERSOLL, Midwest Research Center, National Biological
Survey, Columbia, MO [FEDERAL EXPERT]
DR. EDWARD R. LONG, National Oceanographic and Atmospheric Administration, Seattle,
WA [FEDERAL EXPERT]
DR. SAMUEL N. LUOMA, United States Geological Survey, Menlo Park, CA
[FEDERAL EXPERT]
DR. HERBERT WINDOM, Acting Director, Skidaway Institute of Oceanography, Savannah,
GA [CONSULTANT]
SCIENCE ADVISORY BOARD STAFF
DR. EDWARD S. BENDER, Designated Federal Official, US EPA, Science Advisory Board,
401 M Street, SW (1400F), Washington, DC 20460
MRS. CONNIE VALENTINE, Subcommittee Secretary, US EPA, Science Advisory Board,
401 M Street, SW (1400F), Washington, DC 20460
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NOTICE
This report has been written as a 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 a
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 or
other agencies in Federal government. Mention of trade names or commercial products does not
constitute a recommendation for use.
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ABSTRACT
This report represents the conclusions and recommendations of the U.S. Environmental
Protection Agency's Science Advisory Board regarding the Agency's proposed use of the
simultaneously extracted metal to acid volatile sulfide ratio (SEM/AVS) to develop sediment
criteria for five metals (cadmium, copper, lead, nickel, and zinc). The review was held January 4-
6, 1995 in Arlington, VA by the Sediment Quality Criteria Subcommittee of the Ecological
Processes and Effects Committee of the SAB.
The Subcommittee commends the Agency for its high-quality research and for developing
a valuable new tool for assessing sediment quality. Use of SEM/AVS to predict the absence of
acute toxicity due to the presence of the five metals in sediments is supported by experimental
evidence and scientific theory. The Subcommittee cautions, however, that significant limitations
to the application of SEM/AVS currently exist. For example, considerable uncertainly remains
regarding the levels of SEM/AVS that cause no adverse chronic effects. Similarly, a substantial
effort is needed to improve the predictive model for bioaccumulation. In addition, before the
techniques are ready to apply as criteria, information is needed to guide the collection of
biologically representative samples and samples that represent the spatial variability of AVS in a
variety of environmental situations.
The Subcommittee further recommends that SEM/AVS be used as part of a tiered system
of sediment quality assessments and that a guidance manual be developed to describe the context
in which this and other assessment tools should be used. The manual should discuss the strengths
and weaknesses of the methods as well as the quality assurance and quality control procedures for
sediment sampling and analyses. The report also provides a list of research priorities, including
the investigation of other naturally-occurring binding factors..
KEY WORDS: AVS, Sediment Quality Criteria, Metals, Uncertainty, Binding factors.
m
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TABLE OF CONTENTS
1. EXECUTIVE SUMMARY 1
2. INTRODUCTION 3
2.1 Statement of the Charge 3
2.2 Subcommittee Review Procedures 4
3. COMMENTS ON THE APPROACH 5
3.1 Proposed Uses of the SEM/AVS method 5
3.2 Sample Tiered Screening System 5
4. SEM/AVS THEORETICAL FOUNDATION 8
4.1 SEM/AVS Theory 8
4.2 Limitations 8
5. EXPERIMENTAL EVIDENCE: LABORATORY AND FIELD VALIDATION 10
5.1 Acute Studies 10
5.2 Chronic Tests 11
5.3 Bioaccumulation Tests 11
5.4 Field Verification 13
6. METHODOLOGICAL REFINEMENTS 16
6.1 Metal Sulfide Oxidation Kinetics 16
6.2 Large- and Small-Scale Spatial Variability 16
6.3 Applicability 18
6.4 Standardized Methodology 18
6.5 Additional Methodological Refinements 19
7. SEM/AVS: SUMMARY 20
8. SEDIMENT INTERSTITIAL WATER CRITERIA: THEORY, LABORATORY AND
FIELD VERIFICATION
22
9. OTHER BINDING PHASES 23
10. USE OF DRY WEIGHT METALS MEASUREMENTS IN CRITERIA 24
11. RESEARCH NEEDS AND PRIORITIES 25
REFERENCES R-l
APPENDIX A. SPECIFIC COMMENTS A-l
IV
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1. EXECUTIVE SUMMARY
The Sediment Quality Criteria Subcommittee of the Ecological Processes and Effects
Committee of the Science Advisory Board (SAB) met in Washington, DC on January 4-6, 1995
to review research and data developed to support the use of the Equilibrium Partitioning (EqP)
approach to predict metal availability in sediments. In the Agency approach, metals are separated
from sediments by the "simultaneously extracted metal" procedure (SEM). The amount of metal
that is presumed biologically available is the difference, on a molar basis, between the SEM and
the Acid Volatile Sulfide (AVS), a binding factor for metal, in the sediment. This method is
currently restricted to evaluating five metals: cadmium, copper, lead, nickel, and zinc.
The Office of Water asked the SAB to evaluate the adequacy of the proposed
methodology for establishing sediment quality criteria for the five metals. Specifically, OW asked
the SAB to evaluate the capabilities of each of four related methods to predict adverse effects
(acute, chronic and bioaccumulation) based on data from lab, field, and colonization experiments;
to judge whether any of the four approaches to derive sediment quality criteria for the five metals
were sufficiently sound to proceed with criteria development; and to identify any documentation
and research needed to develop sediment criteria.
The Subcommittee commends the Agency for the outstanding work that has been
accomplished to date. In its effort to produce sediment quality criteria that are supported by a
strong theoretical foundation, the Agency has engaged in thoughtfully targeted research and has
provided much useful information with wide applicability outside the Agency. The proposed
methodologies will be an important component of the ultimate system for identifying sediments
that are likely to cause adverse effects and other sediments that are unlikely to cause such effects
due to metals.
The criteria are being developed for two uses: (a) assessing the safety of in-place
sediments contaminated with particular metals; and (b) predicting how much metal can be added
to a sediment without causing adverse environmental effects. Several empirical approaches for
assessing toxicity of in-place metals are currently available. Although many of these approaches
appear to be useful, they all lack a strong theoretical basis, which impairs their potential predictive
ability. One of the strong points of the SEM/AVS approach is that it is based on a solid
theoretical foundation, using equilibrium partitioning and metal activity to predict when metals are
effectively sequestered and not bioavailable. As a result, the SEM/AVS approach promises
ultimately to provide a more reliable and predictive tool than those currently available.
As currently conceived, the SEM/AVS approach would be used as a "no-effect" criterion
to identify sediments not expected to cause adverse effects due to the presence of the five metals.
In view of this limitation, the Subcommittees concurs with the Agency's intent to use the
SEM/AVS method as part of a part of a tiered testing approach, and a general scheme is offered
in the report.
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The basis for deriving sediment criteria is strongest for "no effects levels" for acute
toxicity in marine and fresh waters. Accordingly, the Subcommittee supports the conclusion of
Agency scientists that the absence of acute toxicity could be predicted using SEM/AVS data.
However, more research is needed - of the quality presented by this research team — to establish
a chronic toxicity threshold, to determine why bioaccumulation occurs below the putative "no-
effect level", and to establish appropriate thresholds for bioaccumulation. Moreover, before the
SEM/AVS approach is ready for use as a criterion, additional research is required to guide the
collection of biologically representative samples and samples that represent the spatial variability
of AVS in a variety of environmental situations. Recommendations regarding field verification,
investigation of oxidation kinetics, development of a standardized protocol, and additional testing
of sensitive life stages of benthic organisms, fish, and plankton are also provided in the report.
Should the Agency wish to extend the SEM/AVS paradigm to other binding phases, the
Subcommittee recommends that clay minerals, metal oxides, carbonates, and related phases be
considered in addition to organic carbon.
The Subcommittee also recommends that a guidance manual be developed to assure the
best use of these and other methods for assessing sediment quality. Such a manual should
describe the context in which peer reviewed methods could be used (including any tiered
approach). Guidance should also discuss the strengths and weaknesses of the methods, quality
assurance and quality control procedures for sediment sampling and analyses. The manual should
also receive technical review.
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2. INTRODUCTION
Since 1988, the Office of Water has served as the technical manager and focal point for an
in-depth research program to develop methods for evaluating the quality of sediments. During
this time, the Science Advisory Board (SAB) has reviewed the Apparent Effects Threshold
Method (SAB, 1988), the Equilibrium Partitioning method (SAB, 1990a), the Sediment Methods
Compendium (SAB, 1990b), and the application of the Equilibrium Partitioning method to the
derivation of criteria for non-ionic organic contaminants (SAB, 1992). .
More recently, the Agency has attempted to extend the application of Equilibrium
Partitioning theory to estimating effects of biologically available metals in sediments. In this
review, the SAB evaluated two methodologies that have been investigated by the Agency: the
Simultaneous Extraction of Metals/Acid Volatile Sulfide method (SEM/AVS), and the closely-
related Interstitial Water method. The SEM/AVS method recognized that many metal sulfides are
not biologically available and that the amount of naturally-occurring sulfide therefore governs the
amount of metal that is effectively sequestered. In theory, therefore, if the AVS concentration
exceeds the SEM concentration, no adverse effects should occur.
2.1 Statement of the Charge
The Office of Water asked the SAB to evaluate the adequacy of the proposed
methodology for establishing sediment quality criteria for the five metals (cadmium, copper, lead,
nickel, and zinc). After discussion with the Sediment Criteria Subcommittee, the following
specific questions were developed to help focus the review:
a) Can metals concentrations on a dry weight basis be used to establish a plausible cause and
effect relationship between metals concentrations and organism responses?
b) Are the data presented from lab, field, and colonization experiments sufficient to
demonstrate a plausible cause and effect link between the "simultaneously extracted metal"
to acid volatile sulfide ratio (SEM/AVS) and/or interstitial water metals concentration and
organism response?
c) Does the Subcommittee believe that the basis for each of the following four approaches to
derive sediment quality criteria for the five metals (presented in Chapter 11, EPA 1994a,b)
is sufficiently sound to proceed with criteria development?
1) AVS Criteria: (No Effect Criteria)
2) Interstitial Water Criteria: (No Effect Criteria)
3) AVS and Organic Carbon Criteria: (Effect Criteria)
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4) AVS and Minimum Partition Coefficient Criteria: (No Effect Criteria)
d) For any that are not sufficient to proceed with criteria development, what type of research
or analysis needs to be conducted to fully support criteria development, or what other
direction needs to be pursued?
e) Are there any major shortcomings associated with the approach?
1) Are there chemical or physical characteristics other than organic carbon that
should be considered in order to derive sediment criteria that address both no
effect and effect organism responses?
2) Are there biological or ecological factors that should be considered in order to
derive sediment criteria that address both no effect and effect organism responses?
2.2 Subcommittee Review Procedures
The Sediment Criteria Subcommittee of the Ecological Processes and Effects Committee
of the Science Advisory Board met on January 4-6, 1995 to review research and data developed
by the Agency to support the use of the equilibrium partitioning approach to predict metal
bioavailability in sediments, as applied to cadmium, copper, lead, nickel, and zinc. The
Subcommittee was composed of members of the Ecological Processes and Effects Committee,
Consultants, and Federal Experts all of whom had specialized knowledge and experience with
evaluating metals in sediments. The Federal Experts served as scientists who offered comments
based on their professional knowledge and judgement. Prior to the review, each Subcommittee
member received copies of the two volume briefing document (EPA, 1994a,b). During the
meeting, the Subcommittee received public comments, including comments from industry and
from Corps of Engineers scientists who represented the position of the Corps.
The Subcommittee received briefings on the results of field and laboratory research
conducted in freshwater and marine environments. Experiments included data on
bioaccumulation, colonization and studies of the correlation between AVS and sediment organic
carbon, the oxidation of metal sulfides and pore water sampling. Finally, the Agency staff
discussed proposed sediment quality criteria for several metals.
During the meeting, the Subcommittee developed an outline of this report and discussed
its preliminary findings with the Agency in public session. A draft document was compiled by the
Co-Chairs and reviewed by the Subcommittee prior to its release to the Agency and the public.
This final report reflects reactions of the Subcommittee to oral and written comments from the
public and from a review of the draft document by the Ecological Processes and Effects
Committee and the Executive Committee of the Science Advisory Board.
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3. COMMENTS ON THE APPROACH
We compliment the EPA's study team on the extent and quality of the work they have
conducted to develop the Equilibrium Partitioning (EqP) and SEM/AVS approaches. In an effort
to produce sediment quality standards that are supported by a strong theoretical foundation, the
Agency has engaged in original and thoughtfully targeted research, has produced numerous peer-
reviewed publications, and has provided much useful information with potential wide applicability
outside the Agency. We commend the team for laying the groundwork to ensure that
regulatory decisions are based on sound science.
As a result of this research, the Agency has identified a potentially useful new
methodology which complements the relatively few tools currently available for sediment
assessment. The Subcommittee supports the Agency's continued efforts to develop this
methodology. However, we also want to point out its limitations and indicate where additional
research is needed to improve the technical basis of the approach.
3.1 Proposed Uses of the SEM/AVS method
The criteria are being developed for two uses: (a) assessing the safety of in-place
sediments contaminated with particular metals; and (b) predicting how much metal can be added
to a sediment without causing adverse environmental effects. Three types of adverse effects are
considered: acute effects, chronic effects, and bioaccumulation. Several empirical approaches for
assessing toxicity of in-place metals are currently available. Although many of these approaches
appear to be useful, they all lack a strong theoretical basis, and therefore have limited predictive
ability. One of the strong points of the SEM/AVS approach is that it is based on a solid
theoretical foundation, using equilibrium partitioning and metal activity to predict when metals are
effectively sequestered and not bioavailable. As a result, the SEM/AVS approach promises
ultimately to provide a more reliable predictive tool than those currently available, and one which
may allow more informed environmental management decisions.
In its present state of development, the SEM/AVS approach, would provide a "no effect"
criterion for acute and chronic toxicity (but not necessarily for bioaccumulation). In other words,
the method would be used to identify sediments that are not expected to cause acute toxicity due
to the presence of the five metals if physical and chemical conditions in the sediment do not
change. The method cannot at present predict when metals would be toxic or bioavailable in
sediment moreover, because other binding phases may also limit the bioavailability of metals in
sediment. Research on additional binding phases might allow this methodology to be used to
predict when sediments are likely to cause adverse effects.
3.2 Sample Tiered Screening System
As the Agency notes in its briefing document (EPA, 1994a, page 1-5): "A likely
application of EqP sediment criteria would be in a tiered approach." We strongly concur, both
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because the currently proposed methodology provides a no-effect criterion (and only for five
metals) and because other screening methods should be used in combination with EqP-derived
chemical criteria to provide an overall approach that is both streamlined and defensible.
In Figure 1 we suggest a possible tiered testing approach for toxicity. This is a conceptual
approach, and is intended only to provide the context for the comments that follow. However,
this approach is consistent with that provided in the manual entitled"Evaluation of Dredged
Materials for Proposed Ocean Disposal-Testing Manual" (commonly referred to as the "Green
Book") (EPA and USAGE, 1991) and the Inland Testing Manual for disposal of dredged material
in waters of the U.S. (EPA and USAGE, 1994).
Figure 1 - A Possible Tiered Testing Approach for Toxicity
Tier I takes advantage of readily-available screening tools to identify "safe" sediments.
Possible screening tools include: existing effects-based empirical approaches (e.g., Long et. al.
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1995); and to some extent, using data on the co-occurrence of organisms and contaminants
(whose predictability improves if data are normalized to account for naturally-occurring metals,
such as aluminum in bulk sediment samples); and/or comparisons with reference areas and
comparisons with cores from pre-anthropogenic times.
Tier II includes chemical screens, such as those derived from EqP methodologies.
The biological effects testing for in-place sediments at Tier III may take different forms
depending on the site and circumstances, but must involve appropriately sensitive organisms.
Biological effects testing should not be restricted to laboratory toxicity testing with benthic
organisms, because other biological effects measures (e.g., benthic community structure) or
bioaccumulation may also be appropriate in some instances, and because sediment metals may
have adverse effects on other aquatic organisms (e.g., fish). Note that the box labelled
"Prioritize/Other Analyses" is purposefully not specified, nor are the arrows leading from that box
qualified. This box represents research that should be done, to determine when biological effects
testing or bioaccumulation testing are necessary, and the form of such testing.
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4. SEM/AVS THEORETICAL FOUNDATION
4.1 SEM/AVS Theory
One of the most attractive attributes of the SEM/AVS method is its strong theoretical
underpinning. The theory not only explains experimentally-determined toxicity in a satisfying
way, but also may eventually provide the capability to predict adverse effects.
The equilibrium partitioning conceptual model is a basic element of the proposed
SEM/AVS (and SEM-AVS and interstitial water) protocol. The model assumes that metal (i.e.,
Cd, Cu, Ni, Pb, and Zn) activity controls metal bioavailability, that metal activity in all reactive or
biologically available phases is equilibrated, and that overall metal activity can be predicted from
pore water metal concentrations. In its application to SEM/AVS protocols, the theory also
assumes that metal activity is controlled by the most unreactive phase, which is assumed to be the
metal sulfide (although the theory, in principle, allows for consideration of other phases).
It is often assumed that pore water is the only route of exposure considered by equilibrium
partitioning method. In fact, the method assumes that the pore water and the sediment are in
equilibrium, therefore it does not matter whether the chemical is taken up rapidly via the gut and
slowly via the water or vice versa. In short, the theory implies that the toxicity of metals to
organisms is the same regardless of whether metal exposure is from pore water or particles (i.e.,
due to ingestion).
4.2 Limitations
An alternative possibility discussed by several critics of equilibrium partitioning is that
metal activity may differ in different exposure phases. If so, the logic continues, then complete
exposure to metals can only be predicted from a sum of the contributions from the different
routes. Ram and Gillett (1993) point out that the importance of the diet in total uptake of
chlorinated organics varies significantly based on different additive models. The authors reviewed
three possible models describing the uptake of chlorinated organics from sediments by benthic-
dwelling organisms: a) sediment pore water via the body surface and in particular oxygen
exchange surfaces; b) sediment ingestion; and c) the sum of pore water and sediment ingestion.
They concluded that uptake from overlying water and ingested sediment is the most likely model
and that "It is inadvisable to apply the equilibrium partitioning approach to sediment quality
criteria...if the aim is to predict possible concentrations in benthic organisms." These and other
experiments have demonstrated that for some chemicals ( i.e., those with high Log P) or for some
organisms (e.g., clams and oligochaetes) the diet may actually contribute more to body burden
than uptake across respiratory membranes. None of the referenced experiments have shown that
toxicity is increased as a result of considering the diet separate from the water exposure route.
Mackay and co-workers have recently modelled total body burden, irrespective of the route of
uptake, as a key parameter in predicting toxicity. In short, studies that use only pore water as a
basis for estimating free metal activity may underestimate total exposures.
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The issue of whether the equilibrium partitioning model or the additive model controls
bioavailability remains scientifically controversial. The literature indicates that the diet is an
important route of exposure but there is a good deal of controversy as to how this route of
exposure should be integrated into the overall equilibrium model. In addition, the passage of
sediment through the gut may in fact change equilibrium partitioning and metal binding to sulfides
by changing the pH, redox potential, and forming metal-organo ligands (Woodward et al., 1993).
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5. EXPERIMENTAL EVIDENCE: LABORATORY AND FIELD
VALIDATION
5.1 Acute Studies
Ten-day lethality tests were conducted with both field-collected and laboratory-spiked
sediments using a variety of freshwater and marine organisms. Table 6-3 of the briefing document
(EPA, 1994a) summarizes the accuracy of prediction of the toxicity of sediments as a function of
SEM/AVS and Interstitial Water Toxic Units (IWTUs). Predictions of sediment not likely to be
toxic because of metals based on SEM/AVS < 1 or IWTUs <0.5, were extremely accurate (96 to
98%). However, predictions of sediments likely to be toxic based on SEM/AVS > 1 or IWTUs
>0.5 were less accurate (57 to 79%).
The Subcommittee supports the conclusion of Agency scientists that the absence of acute
toxicity could be predicted using SEM/AVS and IWTU data. However, the accuracy of these
predictions may have been overestimated by using a critical value of 24% mortality as the basis
for differentiating between "toxic" and "nontoxic" categories. This critical value was published by
Mearns et al. (1986) following an interlab oratory comparison of the performance of a particular
test with one species of amphipod. It was not based upon power analyses of the data from each
of the species to which the value now has been applied by the Agency. The apparent concordance
between predicted and actual toxicity may change if critical values are used that are specific to
each of the tests. The Subcommittee recommends that the Agency reevaluate these predictions
using a more conservative estimate of toxicity. For example, results from the EPA (1994a,b)
round-robin testing with Hyalella azteca and Chironomus tentans indicate about 15% mortality
may be a more appropriate value for these two species. Twenty percent mortality would be
appropriate for Ampelisca abdita.1
Notwithstanding this recommended adjustment to the Agency's data presentation, we
agree with the briefing document's conclusion ( EPA, 1994a, p. 6-23) that "SEM/AVS ratios of
< 1.0 can accurately predict field sediments not likely to be acutely toxic" to benthic invertebrates
as a result of Cd, Cu, Ni, Pb, and/or Zn contamination. However, the Subcommittee would like
to point out that the no-effects level for SEM/AVS<1 has not yet been determined
Personal Communication with Dr. Glenn Thursby, EPA, Naragansett.
10
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5.2 Chronic Tests2
The utility of SEM/AVS and IWTUs was also evaluated in a chronic laboratory test that
exposed the estuarine amphipod Leptocheirus plumosus to cadmium-spiked sediments.3 In this
test, toxic effects on survival, growth, and reproduction were not observed at Cd SEM/AVS < 2
or Cd IWTUs < 1. These data also support the conclusion that sediment is not likely to be toxic
due to contamination by the five metals at SEM/AVS < 1 or IWTUs < 0.5; however, insufficient
data were available to determine if sediments would be consistently toxic at higher SEM/AVS or
IWTUs. Indeed, no chronic responses were recorded because mortality occurred.
The Subcommittee believe that there is a need for additional chronic toxicity studies with
exposure periods of at least 28 days. Longer exposure periods are needed to insure that the test
organism tissues have come to steady state with sediment-metal concentrations. In the briefing
document, several studies demonstrated the occurrence of bioaccumulation in circumstances
where acute toxicity was not observed (see below). Hence, longer exposure to the same sediment
might result in sublethal or lethal effects.
5.3 Bioaccumulation Tests
Chapter 8 of the briefing document (EPA, 1994a) describes results of bioaccumulation
studies for both field-collected and lab oratory-spiked sediments with a variety of freshwater and
marine organisms. According to EqP theory, metals should not be bioavailable if pore-water
concentrations of metals are low or if SEM/AVS ratios are < 1. However, several of the
bioaccumulation studies raised the possibility that metals may be bioavailable even when
SEM/AVS<1, because bioaccumulation was often observed in this range. For example, results
summarized in Table 8-2 indicate accumulation of Ni and Cd at SEM/AVS ratios < 1 for all
exposures. Similarly, Carlson et al. (1991), Pesch et al. (1995), Ingersoll et al. (1994), and Hare
et al. (1994) report significant accumulation of metals at SEM/AVS ratios < 1. Note in particular,
Pesch et al. (1995) demonstrated significant accumulation of metals at SEM/AVS ratios < 1 with
polychaetes which were depurated;4 Besser et al. [in review and in press] have demonstrated
accumulation of metals at SEM/AVS ratios < 1 with midges that were depurated; and Hare et al.
(1994) found depurated organisms accumulated metals at ratios < 1 (e.g., Procladius and
Sergentia). Detectable bioaccumulation was also shown in the Canadian Lake Study reported at
the briefing between SEM/AVS ratios of 0.1 and 1.0.
Results of long-term exposures on colonization are discussed in Section 5. 4 of this report.
This information was presented by Dr. Rick Swartz at the meeting but was not included in the Agency briefing document.
4
See Figures 8-2 and 8-3 in the briefing document.
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Similarly, numerous samples at SEM/AVS > 1 or IWTUs > 0.5 were not toxic in
laboratory exposures,5 yet bioaccumulation was consistently elevated at SEM/AVS > I.6 The
lack of toxicity at the higher concentrations was explained by the binding of metals by sources
other than sulfide (i.e., organic carbon). These data suggest, however, that the lack of observed
toxic effects may still be accompanied by exposure to metals through either pore water or whole
sediment. A key factor that is missing is the ability to relate body residue levels with observed
effects. The question arises when elevated tissue levels are observed at SEM/AVS either greater
or less than 1.0, whether or not the body burden is approaching the point where effects might be
seen.
These results are significant for two reasons. First, bioaccumulation is a sensitive
indicator of exposures to bioavailable metal from all sources (pore water, ingestion, surface
binding). The observation of dose-response relationships below the purported SEM/AVS=1
"switch" indicates that determining how and why bioaccumulation occurs at these concentrations
is critical to defining the mechanistic underpinnings of the SEM/AVS theory. This mechanism
should be defined for both invertebrates and sensitive vertebrates for all routes of exposure.
Second, although bioaccumulation does not necessarily indicate an adverse effect on the
test organism, it does expose consumers of the organism to elevated doses of metals that may
become bioavailable. For example, potentially bioavailable metals ingested by fish may include
metals in the gut and tissues of invertebrates (Woodward et al., 1993; Farag et al., 1994).
Research should investigate the significance of such exposures to the consumer and its predators.
Sublethal whole organism toxicity tests, longer term studies of population dynamics (for
example with amphipods), or expansion of the colonization studies are examples of sensitive tests
that might be employed to further understand the significance of the bioaccumulation response
indicated to date. Such studies can be elaborate, expensive and/or time consuming. Therefore,
they would be best considered after the completion of a series of detailed bioaccumulation studies
to benefit from generalizations that may result from a less elaborate bioaccumulation approach.
While the results of the acute and chronic toxicity studies tend to substantiate the SEM/AVS
method, the occurrence of bioaccumulation at SEM/AVS<1 raises the possibility that effects may
occur but have not been observed because of the limited sensitivity or duration of the toxicity
tests.
The Subcommittee concurs with the Agency's conclusion that additional bioaccumulation
studies should be conducted to determine more clearly the true thresholds of bioavailability and
toxicity of metals in the AVS extracts and pore water. We recommend that such studies be
designed with the following in mind:
See Figures 6-13 and 6-14 in the briefing document.
6 See Figures 8-1, 8-2, 8-3, 8-4, 8-6, 8-7 and Table 8-2 in the briefing document.
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a) Defining appropriate sediment horizons for sampling pore-water metals and
SEM/AVS to better define appropriate exposures of organisms to metals in
aerobic zones of sediment. Laboratory tests of the bioaccumulation and toxicity of
the five metals necessarily caused some oxidation of sulfides. However, the degree
and effects of oxidation upon test results were not quantified, leading to some
degree of uncertainty.
b) Determining contributions of adsorbed or digested materials to the total body
burden of metals in test organisms. The role that kinetics play in the uptake of
metals across the gut walls of benthic invertebrates needs to be investigated.
Equilibrium theory indicates that when sediment metal concentrations are at
equilibrium with the interstitial water, the route of uptake is not of major
importance. However, if the thermodynamic potential is altered in the gut of the
organism such that uptake from sediment is favored over that from interstitial
water, there is a potential for the equilibrium approach to underestimate the
potential for bioaccumulation.
c) Further evaluation of the potential effects of dietary metals on aquatic organisms.
5.4 Field Verification
The field verification experiments performed thus far consist of colonization experiments
at two freshwater sites and a colonization experiment with marine sediments using natural
unfiltered sea water in a laboratory environment. These experiments represent a good first effort
to determine whether the SEM/AVS method accounts for the bioavailability of elevated
concentrations of the five metals on a chronic basis in natural sediments. The evidence is
consistent with laboratory toxicity studies which indicate that sulfide-metal binding is an important
process and may be a dominant factor controlling the bioavailability of select metals in freshwater
and marine sediments.
The Subcommittee believes that additional field verification would provide greater
confidence in various aspects of the overall approach. The areas of concern include the following:
a) The sensitivity and accuracy of the overall method needs further evaluation.
b) There has been insufficient field testing of sensitive early life stages of benthic
invertebrates and offish with benthic life stages. Greater emphasis on chronic
testing is needed.
c) The applicability of the method to different sediment types in both saltwater and
freshwater environments requires additional clarification.
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d) The proposed SEM/AVS method may not adequately explain the lack of chronic
toxicity in highly contaminated sediments. The ability to explain and/or understand
the lack of toxicity to organisms for sediments with very high concentrations of
metals is key to establishing the credibility of the approach.
Further field verification of the SEM/AVS approach should make it possible to address
many of the concerns raised above. Two approaches are recommended although we recognize
that other options exist. The first approach we recommend is a suite of additional experiments.
For example, the studies of bioaccumulation and toxicity in spiked sediments transplanted into
inland ponds provided useful information under controlled conditions with the advantage of
environmental realism attained in a field setting. It would be worthwhile for the Agency to
conduct similar studies in saltwater to verify bioaccumulation and toxicity. Additional
experiments might include site specific colonization experiments (chronic evaluation) at sites
where the SEM/AVS ratio is greater than or equal to 1.0 and at sites where the SEM/AVS ratio
may be less than 1.0, but the site is known to contain "high" levels of metals.
Second, additional field data sets can be used to evaluate the sensitivity and accuracy of
the SEM/AVS method. Because the proposed SEM/AVS approach and several empirical
approaches used to derive numerical guidelines differ considerably in methodology, it is important
to compare their relative predictability, reliability, and accuracy in field verification studies. In
addition, a large number of state, provincial and federal agencies have prepared or are preparing
sediment quality guidelines for total concentrations of metal in bulk sediments. The differences in
proposed no-effect levels obtained from the SEM/AVS and empirical approaches could be
quantified for similar field data sets with a view towards explaining the differences, comparing the
accuracy of the predictions, and establishing the conditions under which each can be reliably used.
Existing EMAP data sets also can be further evaluated. The Agency's recent efforts to
evaluate SEM/AVS ratios and species abundance data using the EMAP data set discussed at the
meeting provided insights that were not presented in the briefing document. The Agency's intent
was to demonstrate adverse impacts at the population level, which is admirable. Comparisons of
measures of benthic community structure with SEM/AVS concentrations would be powerful
confirmation of the impacts of excess SEM. However, the data trends that one would expect on
the basis of SEM/AVS normalization of the data were not consistently observed. We recommend
further evaluation of the data, especially for those sites where the SEM/AVS ratio is greater than
or equal to 1.0 and where toxicity tests indicate the sediments are toxic.7 A sediment toxicity
identification evaluation may be needed to confirm that the toxicity is due to metals. At sites
where the SEM/AVS ratio may be less than 1.0, but no toxicity is observed, the site should be
The subcommittee cautions, however, that use of pre-existing data sets — even those of excellent quality such as EMAP — requires considerable attention
to detail. As the Agency recognizes, its preliminary attempt to correlate population impacts with SEM/AVS using EMAP data collected from numerous
different estuaries for this purpose was flawed. The EMAP-E data set is composed of one to several samples collected at randomly chosen stations within
many different estuaries in the northeast and Gulf regions. The data were not collected along a single pollution gradient in which all other variables were
constant. Although the data suggest a correspondence between the SEM/AVS ratios and species richness, this apparent relationship may well be a function
of covariance with numerous natural factors such as salinity, depth, flow velocity, slope, grain size, and predatory pressure that are known to have major
impacts upon species richness.
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evaluated in light of species abundance measures, sensitive toxicity tests and bioaccumulation
measurements. Additionally, EMAP data from other locations, especially in the Southeastern
United States, and freshwater systems, should be evaluated in light of the benefit of SEM/AVS
normalization of the data.
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6. METHODOLOGICAL REFINEMENTS
The above discussion addresses the strengths and weaknesses of the research to date, and
suggests additional work that may be undertaken to further validate the SEM/AVS method. In
this section, we suggest additional work that the Agency should consider in order to refine the
method and make it usable, and to address other issues related to criteria and standards designed
to protect against metal toxicity.
6.1 Metal Sulfide Oxidation Kinetics
The long-term potential for sediment bound Cu, Cd, Ni, Pb, and Zn to exert toxicity to
benthic and epibenthic species depends on more than metal association with sulfide (AVS) phases.
Even if, for a given sediment at a given time, AVS represents the major binding phase, attention
must be paid to the fate of these metals upon oxidation of their sulfides. Seasonal variations in
oxidation conditions at the sediment-water interface is the rule, so that metal sequestered during
one season may be released during another. Likewise environmental remediation of contaminated
water bodies typically includes improvements in DO which implies that associated contaminated
sediments would be expected to become more oxic, potentially releasing sulfide-bound metals. In
addition, benthic infaunal deposit feeders typically create micro-oxidized environments as they
migrate through sediments. This again may liberate metals from the sulfide phase.
Sediment oxidation has not been significantly addressed by the program, nor has the
operational distinction between oxic and anoxic sediments been clearly defined. Some of the
presentations clearly demonstrated that metals are apparently released from sulfide phases during
long term experiments but the rates of release have not been investigated. Rates of oxidation of
the metal sulfide phase would be expected to vary. Some metals may be systematically released
(or become more available) relative to others during periodic (e.g., seasonal) oxidation events.
Such information is necessary to evaluate the long-range biological significance of metal
contaminated sediments.8
6.2 Large- and Small-Scale Spatial Variability
Eventual field application of the SEM/AVS technique will require a systematic resolution
of questions about SEM and AVS profiles in natural sediments. A crucial question is how to
collect samples. Studies to date show that, in general, AVS follows the depth profile expected
from the principles of diagenetics. However, it is also expected that such profiles could vary with
sediment type, water chemistry, physical and biological processes. They also vary seasonally and
could vary on small spatial scales. A defensible strategy for maximizing geographic consistency in
implementation of the SEM/AVS tool requires an understanding of such variability. Field studies
are necessary to systematically demonstrate differences in SEM and AVS profiles, for example
EPA should identify sediment sampling procedures that minimize the release of AVS from samples during collection, packaging and shipment.
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among sediments of different pore size, in different flow regimes, and in environments with
different sulfate concentrations. The effects of re-suspension on profiles, as well as other
processes that might change seasonally, could be studied by comparing different field sites.
Comparisons of profiles between different types of rivers, in lakes, in estuaries and in coastal
(intertidal and subtidal) environments will eventually be necessary.
The scale of variability is a critical question in developing a standardized approach for
investigating biologically effective AVS. Currently recommended methods homogenize sediments
from a consistent depth (2 cm). Data show that the gradient of AVS is steep within the 0-2 cm
layer in the few sediments carefully studied. Thus criteria for determining the depth of sampling
must be carefully detailed, especially if a site-specific method is recommended. A very difficult
question that requires additional study is evaluation of the bioavailability of metals from oxidized
microzones created by organisms burrowing in sediment. Specific studies might consider the
difference between availability from the microzone and availability from a homogenized sediment
around the microzone.
In addition to vertical profiles, horizontal variation in SEM and AVS should also be
discussed. For example, Brumbaugh et al. (1994) reported that SEM/AVS ratios for individuals
cores within a 10 by 10 meter station varied by almost 100 fold.9 However, although variability
between cores was quite high, the authors also reported the average SEM or AVS in composite
samples was similar to the average SEM or AVS in individual cores. Similar investigations should
address the possibility of under- or over-estimating exposure in the field as a result of compositing
sediment samples.
See also Besser et al. in press.
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6.3 Applicability
The above-mentioned concerns regarding oxidation conditions and other factors that
result in spatial and temporal variability are a subset of the broader issue of applicability: where
and when will the SEM/AVS method be useful?
Some additional effort will be required to evaluate the range of sediment types to which
the method is applicable. Field validation of the method in different sediment types (including
more sandy sediments, recently disturbed sediments, and sediments recently deposited from a
dredge) in a broader range of locations would help to define the appropriate conditions for
applying (or not) the SEM/AVS method. We note that most of the field verification data used
thus far were collected in the northeastern USA or in the Midwest; a range of sediment types
should be used in the various follow-up studies to better represent the southeast, the Gulf coast,
the Pacific coast, Hawaii, and Alaska. In this way, the Agency can also begin to assess the
geographic extent of sediments that are contaminated with excess SEM and verify that these
sediments represent toxicological problems.
Another issue relating to applicability of the method is its optimum use as one among a
suite of tools for assessing mixtures of contaminants within sediments of various types. One of
the difficulties in evaluating the data presented, particularly the field studies, is that multiple
contaminants and sorptive phases may have been present. It is clear that metals do sorb to
organic material (and other phases) in addition to AVS, although AVS binding is theoretically
strong and therefore should be the dominant mechanism. The presence of other toxic compounds,
i.e. organic contaminants such as pesticides and aromatic hydrocarbons, in some of the samples
would create toxicity not predicted or accounted for by the SEM/AVS equilibrium partitioning
measurements. Since it is often not known or not clear what contaminants are present, the
conditions under which this methodology can be appropriately used or not used need to be
carefully defined. The Agency must remind those who would use this method that
SEM/AVS is intended to determine when metals present on sediments are not toxic.
6.4 Standardized Methodology
Standardized analytical methods and QA/QC techniques are needed for sampling and
measuring SEM and AVS. In addition, if the Agency intends to propose that interstitial water
metals concentrations be measured simultaneously, in order to increase the reliability of the
SEM/AVS methodology, standard protocols must be developed for extraction and analysis of
pore waters. Such methods should improve the exclusion of microparticulates and account for
the effects of precipitation and dissolution of metals that may occur with changes in pH or redox.
Although the briefing document recommends centrifugation under nitrogen or carefully-controlled
use of a peeper, the Agency recognizes that current pore water extraction methods are
controversial, that some or all may selectively exclude toxicants (including metals), and that
rigorous testing will be required to develop a reliable method. Adding to this challenge, the
preferred method must also be practical for widespread use (see, for example, the briefing
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document's description (EPA, 1994a, p. 9-47) of the potential difficulties of using a glove box to
extract pore water).
Once standardized protocols are developed, and issues with regard to spatial
variability, temporal variability, and applicability of the method to various sediment types
have been examined, the Subcommittee recommends that the Agency develop a
consolidated user's manual.
6.5 Additional Methodological Refinements
The Subcommittee concurs with the Agency's suggestion to investigate using SEM-AVS
calculations (differences) as opposed to SEM/AVS calculations (ratios), because of the
information that is lost in the latter conversion. Re-assessing the results of past experiments using
differences appears useful.
We also note that organism behavior affects toxicity. This was clearly shown in the
briefing document and presentations, and has been shown by other authors (e.g., Landrum et al
1994) as a factor which can result in EqP overestimating toxicity. The consequences of organism
behavior need to be included in uncertainty analyses.
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7. SEM/AVS: SUMMARY
The SEM/AVS method determines the amount of cadmium, copper, nickel, lead and zinc
in the sediment that is potentially not bioavailable. It is therefore a threshold test; contamination
lower than the threshold amount should not cause adverse effects and contamination higher than
the threshold amount may cause adverse effects (unless the metal is not bioavailable for some
other physical or chemical reason). In short, the logical use of SEM/AVS is as a "no-effect"
criterion, as the Agency has proposed. The Subcommittee is not yet entirely satisfied, however,
that the experimental evidence warrants the assumption that there are no effects below a ratio of
1.0.
The most convincing experimental evidence presented in support of the SEM/AVS
method is the relationship between acute toxicity and SEM/AVS (or pore water metal activity). It
appears that the method can be used to predict the lack of acute toxicity in sediments at
SEM/AVS ratios <1, if spatially and biologically representative samples have been collected.
Above rations of 1, however, toxicity cannot be reliably predicted, because other phases in
addition to AVS may control the bioavailability of metals. Additionally, it was noted that the
accuracy of acute toxicity predictions in the SEM/AVS<1 "quadrant" may have been
overestimated by using the 24% mortality benchmark to classify samples as toxic.
The experiments that assessed the relationship between chronic toxicity or
bioaccumulation and SEM/AVS produced results that are not as clear as the acute toxicity
experiments. The colonization study, although elegant in conception and implementation, lacked
exposures between ratios of 0.1 and 1.5. Preliminary bioaccumulation studies raise the possibility
that the SEM/AVS ratio <1 is not a "switch", and indicate that bioavailable metal may occur in
circumstances not expected from the development of the SEM/AVS theory to date. Detectable
bioaccumulation has been shown in a number of experiments, both above and below
SEM/AVS=1. Additional work is needed to determine whether the SEM/AVS approach can be
used to define conditions where sediment sorbed metals can be bioaccumulated by benthic
organisms. These investigations also could be designed to provide additional insight into the
contributions of adsorbed or digested material to total exposure, thereby strengthening
understanding of the theoretical basis for the EqP and SEM/AVS approaches.
The Subcommittee strongly recommends that additional research be performed to
evaluate the potential for chronic effects and bioaccumulation to occur where SEM/AVS<1
(or SEM-AVS<0). The evaluation is needed to support the Agency's contention that the
approach can be used to predict no-effect levels. In view of this recommendation, the Agency
may wish to investigate whether a no-effect criterion at some level less than 1 (or difference less
than zero) would be a more defensible approach. The lower 90% or 95% confidence interval
from an uncertainty analysis might be used for this purpose.
In addition to the above concerns, significant questions regarding oxidation kinetics,
temporal and spatial variability, and other factors governing appropriate application of the
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methodology remain to be answered . Despite the above, however, the Subcommittee concludes
that the SEM/AVS methodology is a promising tool in sediment quality assessment and that
targeted research should proceed to further develop the method. For example, the SEM/AVS
method provides a basis for concluding that when SEM is less than AVS, observed toxicity may
be due to materials other than metals. It also seems clear that in situations where the AVS
concentrations are high, the metal concentrations low, and other contaminants are not present,
that SEM/AVS provides a strong indication of no probable acute toxic effect.
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8. SEDIMENT INTERSTITIAL WATER CRITERIA: THEORY,
LABORATORY AND FIELD VERIFICATION
The use of sediment interstitial water metal concentrations as a predictor of acute and
chronic metal toxicity provides a useful approach to predict when sediment metal concentrations
may be toxic to aquatic life. This approach allows for a direct comparison with toxicity values
obtained from traditional water exposure toxicity tests and allows for the use of water quality
criteria. Use of interstitial water toxicity estimates in combination with SEM/AVS estimates of
free metal concentration appears to provide a potentially useful tool that will be more robust than
either approach alone for predicting when sediments will not be toxic to aquatic organisms.
There are four major drawbacks to the interstitial water approach for performing sediment
quality assessments or deriving criteria. The first drawback is that the interstitial metal-water
concentration must be measured. Current approaches for measuring sediment interstitial water
concentrations have not been standardized and a variety of approaches are being used. This is an
area that needs additional research, as mentioned earlier (Bufflap and Allen, 1995). The second
drawback is that sophisticated non-routine techniques are required in order to accurately measure
low concentrations (a few ug/1) of metals in interstitial water. It is also difficult to account for the
effects of DOC, hardness, and salinity on the pore water matrix, as pointed out in the briefing
document. The third drawback is that multiple chemicals are present at any one time in sediment
interstitial water and an additive model is used to sum the toxicity units. This approximation is
correct most, but not all of the time. There are only limited chronic toxicity data to support the
additive model. The fourth drawback is that there are some non-equilibrium circumstances where
interstitial water does not predict all routes of uptake.
Additional field verification of the interstitial approach is needed. Such measurements
could be built into a larger sediment research program aimed at evaluating the EMAP sediment
data or field colonization studies. An important part of this field verification is collecting
matching data for SEM/AVS, interstitial water-metals concentrations, and chronic toxicity
measurements with planktonic early life stages of benthic invertebrates and fish. An additional
important aspect of field verification is the need for bioaccumulation measurements when the
interstitial water toxic units are greater than or equal to 1.0 and SEM/AVS is less than or equal to
1.0.
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9. OTHER BINDING PHASES
To date, the Agency's program has emphasized AVS as the major binding phase which
controls the bioavailability of metals (Cu, Ni, Cd, Pb, Zn) in sediments. To extend the paradigm,
the Agency is tentatively considering investigation of binding by organic carbon as its next
research priority. While binding by organic carbon is undoubtedly significant — indeed, Fu et al.
(1992) make the case that "humic substances are the major absorbent for cadmium, and perhaps
other metals, in oxic sediments" — other phases in sediments also play a significant role in binding
metals, especially under aerobic conditions.10 The presence of other metal binding phases is
mentioned in the Briefing Document11 and referenced in relevant studies by the principal
investigators (e.g., Ankley et al., 1993). In particular, metal oxides (e.g., Fe, Mn, Al) are
important in binding metals (Jenne, 1968); cadmium and other metals are bound by Fe and Mn
oxyhydroxides in oxic sediments; interactions with hydrous metal oxides and carbonates are also
important in some geographic regions.
The suite of sediment assessment tools envisioned by the Agency will ultimately include
methods that address lack of bioavailability due to these other binding phases. The
Subcommittee recommends that the role of the array of additional binding phases such as
clay minerals, oxides and carbonates be investigated before settling on organic carbon as
the next research priority.12 Initial, theoretical sensitivity analyses comparing the potential
control by other phases could be carried out assuming the necessary chemical thermodynamic data
are available.
10 In this regard, we note that the correlation between SEM and organic carbon might be equally significant if clay minerals (using Al as a proxy) were considered the
additional binding phase.
11 See line 8, page 5-9; line 1, page 5-16; lines 20-22, page 6-10 for examples.
12 Such studies could be conducted in the context of establishing minimum partitioning coefficient by using end member sediment (i.e. sediments dominated by, or composed
solely of, one solid phase) rather than by using sand (quartz is not an important binding phase).
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10. USE OF DRY WEIGHT METALS MEASUREMENTS IN CRITERIA
The Subcommittee believes that in-place bed sediments should be assessed with
multiple tools, including the SEM/AVS criterion for the five metals where appropriate
There is evidence that the envisioned SEM/AVS approach correctly identifies non-toxic samples
in many cases. However, since this approach is applicable thus far to only five metals, EPA
should recognize that other tools are useful in assessing and estimating the potential hazard of
already-contaminated sediments.
Background concentrations of total metals normalized to dry weight can be used with
SEM/AVS as a complementary screening tool. Currently, normalized background values are
available for the same five metals addressed by the SEM/AVS criterion, plus a number of other
metals. Background concentrations have been determined from relatively clean areas, from pre-
anthropogenic sedimentary strata as metal/aluminum ratios, and as effects-based guidelines
calculated from empirical observations of toxicity. Often they are conservative and frequently
they correctly predict non-toxicity in sediments. (For example, a similar percentage of samples is
correctly classified as toxic or not toxic in Figure 6-7 and Figure 6-8 of the briefing document if
10-100 wmol/g dry weight or if an SEM/AVS ratio of 1, respectively, are used to predict toxicity.)
Since background concentrations often are lower than SEM concentrations in AVS, they can be
used as an initial screen in assessments of bed sediments. SEM/AVS analyses would not be
warranted if trace metals failed to equal or exceed background levels.
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11. RESEARCH NEEDS AND PRIORITIES
In the previous sections of this review, the Subcommittee has provided numerous
suggestions regarding additional research that would provide a stronger foundation for sediment
quality criteria based on the SEM/AVS method. These recommendations are summarized in
priority order in Table 1 below. We strongly recommend that the Agency begin to pursue
research required to complete the rest of a tiered sediment assessment system.
For example, the currently-proposed use of the SEM/AVS method is to predict sediments
that are not likely to be toxic. Methods are also required that can be used to predict toxic
sediments. The SEM/AVS method correctly predicted toxicity in acute tests in only 7 of 26
(27%) samples in which SEM>AVS in saltwater studies. Many samples expected to be toxic
when SEM>AVS were not toxic, possibly due to the binding of metals by other phases.
Moreover, SEM/AVS theory predicts that toxicity will occur only when excess SEM occurs and
metals are found in the pore water. It would be useful if EPA confirmed that when SEM>AVS
metals dissolved in the pore water actually were toxic. Toxicity tests with sensitive taxa such as
daphnids and sea urchin gametes should be performed on fresh and saline pore waters,
respectively, to confirm their toxicity.
Similarly, tools that can be used in low AVS and oxic sediments are required. Finally,
many toxic metals (such as mercury, silver, chromium, and selenium) are not addressed by the
SEM/AVS method, yet they merit substantial environmental concern.
TABLE 1. SUMMARY OF RESEARCH NEEDS
(Listed in Decreasing Order of Priority and Urgency)
A. General Needs for Sediment Criteria
1. Procedures for obtaining a representative sample of contaminated sediment.
2. Conceptual framework for integrating the SEM methods into a Tiered Approach.
B. Specific Needs for the SEM/AVS Methodology
1. Improving the pore water preparation to account for method-induced changes in
biologically available metals.
2. Establishing (confirming) a toxicity threshold in terms of SEM/AVS (or SEM-AVS).
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3. Establishing a bioaccumulation threshold in terms of SEM/AVS (or SEM-AVS)
and the route of exposure.
4. Evaluating spatial and temporal variability in AVS and SEM.
5. Evaluating if factors other than AVS and organic carbon control metals
bioavailability.
6. Field verification of the criteria, especially with sensitive life stages of
benthic organisms, benthic fish, and algae.
7. Field data to evaluate the sensitivity and accuracy of SEM/AVS method.
8. Cumulative effects measures of contaminants on toxicity and bioaccumulation.
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Manual EPA 503/8-91/001. Prepared by the Office of Science and Technology, Office of
Water, EPA, and the U.S. Army Corps of Engineers, Department of the Army, Washington,
DC.
EPA and USAGE. 1994. Evaluation of Dredged Material Proposed for Discharge in Waters of the
U.S. - Testing Manual (Draft) (Inland Testing Manual) EPA 823-F-94-002. Prepared by the
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Office of Science and Technology, Office of Water, EPA, and the U.S. Army Corps of
Engineers, Department of the Army, Washington, DC.
Farag, A.M., C. Boese, D.F. Woodward, and H.L. Bergman. 1994. Physiological changes and tissue
metal accumulation in rainbow trout exposed to food borne and waterborne metals. Environ.
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binding in sediments. Environ. Toxicol. Chem. 11:1363-1372.
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accumulation of benthic invertebrates; implications for the acid volatile sulfide (AVS) model.
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significant role of hydrous Mn and Fe oxides. Adv. Chem. 73: 337-387.
Ingersoll, C.G., W.G. Brimbaugh, FJ. Dwyer, and N.E. Kemble. 1994. Bioaccumulation of metals
by Hyalella azteca exposed to contaminated sediments from the upper Clark Fork River,
Montana. Environ. Toxicol. Chem. 13:2013-2020.
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Woodward. 1994. Toxicity of metal-contaminated sediments from the upper Clark Fork
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APPENDIX A. SPECIFIC COMMENTS
The term "dialysis" should be replaced by "passive diffusion" when referring to pore water sampling
because dialysis is generally reserved for membranes with much smaller pore sizes. "Peepers" should
be described as diffusion samplers not dialysis samplers.
In the briefing document, Chapters 1, 10 and 11 should state at SEM/AVS ratios < 1 toxicity is
seldom observed; however, bioaccumulation may still occur. The qualifier at the end of Chapter 8
(page 8-21) regarding the limitations of AVS to predicted metal bioaccumulation from sediment
should also be included in Chapters 1, 10, and 11 (e.g., top of page 11-1 and bottom of page 11-2).
The document needs to more clearly differentiate between bioavailability as measured by toxicity vs
bioavailability as measured by bioaccumulation.
Figure 4-1. What do the three lines represent?
Page 5-3. Why discuss Rhepoxynius hudsoni! Are not only data for Ampelisca presented in this
chapter? Is this in reference to the statement on page 5A1, 2nd paragraph? If R. hudsoni data are
presented in this chapter, then relabel the figure legends.
Page 5-4, 2nd paragraph. The statement is made that AVS, SEM, and dry weight concentrations
varied but were typically within 20%. Was there any consistent trend? Ankley et al. [5] report a fairly
sizable drop in AVS over 10 days (> 20%; Table 6.2). Note: Kemble et al. (1994) reported consistent
SEM and AVS in 28-day tests with amphipods.
Figure 5-1 (and others), Define "ND".
Figure 5-8. Any ideas why there was high mortality at low Cd IWTUS?
Page 5-27, 2nd paragraph. Ankley et al. [22] is not listed in the references. Did this study evaluate
Cd or Cu?
Page 6-1 5, 3rd paragraph. IWTUs are discussed for amphipods. What about for oligochaetes?
Page 6A-2, 1st paragraph, last sentence. Why were 30 volume additions/day used? Could this not
reduce AVS or overlying water concentrations?
Table 7-6. The statement is made that chronic lethality or sublethal effects are possible at SEM/AVS
ratios < 1. This point should also be made in Chapters 1, 10, and 11.
Could some of the temporal variability discussed in Chapters 7 and 9 (e.g., page 7-2, last paragraph)
have resulted from horizontal variation in AVS?
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Page 9-1 2. The description of sulfide oxidation rates should include additional detail. How were
the sediments placed into the test chambers? Are these rates for very thin layers of sediment without
overlying water? The rate of oxidation would probably depend on depth and overlying water amount.
Page 9-46 and page 9-47. Suggesting minimum partition coefficients based on sorption of metals to
sand is questionable. Would not the presence of organisms alter this partitioning, themselves being
organic? Partitioning is not as absolute as AVS binding.
Page 9-51, top. The ion-electrode procedure is only glossed over. A recommendation can be made
for widely ranging AVS concentrations with this detection method.
Page 10-3, top. "Pore water and SEM/AVS should be considered a snapshot measure" - However,
the SEM/AVS would be a greater time interval, perhaps weeks unless a turnover or major
precipitation event occurs.
Page 10-6, last sentence. Bioaccumulation tests should be recommended in addition to toxicity tests.
Page 11-6, last sentence; page 3-1, 3rd paragraph, last sentence; page 5- 1, 1st paragraph; page 5-27,
last paragraph. Dissolved organic carbon is suggested as an important binding phase for metals. Yet,
equation 11-7 only accounts for TOC, hardness, and pH.
Page 11-8. Should SEM Co, Hg, and Ag be included in the sum molar SEM concentration? Are
there data to support this approach? Further discussion of Hg, Ag, Co are needed, especially because
Hg and Ag are the least soluble sulfides. Are these metals considered less reliable or is it assumed
that they will be much lower in concentration and therefore insignificant?
Page 11-10. Are the FCVs for metals protective of benthos?
Page 11-19, 1st paragraph. WQC are established to protect 95 % of the species, WQC are not
intended to protect for "no effects". However, the statement on page 1-2, 2nd paragraph is consistent
with the used of criteria (to prevent "unacceptably affects" on benthic organisms).
A discussion of "reactive" versus "refractory" organic carbon as it relates to AVS formation begs for
further discussion. How does one measure these two forms? Is metal sorption different? Are there
examples of each type of organic carbon?
Page 3-11, 2nd paragraph. Comparisons are made in this chapter between pore-water and water-only
exposures regarding toxicity. Have these comparisons been made for bioaccumulation?
Citation #1 5 at the end of Chapter 1 5 should be cited as: Ingersoll, C.G., W.G. Brumbaugh, FJ.
Dwyer, and N.E. Kemble. Bioaccumulation of metals by Hyalella azteca exposed to contaminated
sediments from the upper Clark Fork River, Montana. Environ. Toxicol. Chem. 13:2113-2020.
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Page 6-30, 3rd paragraph. The statement is made that sampling problems occurred with the spiking
of Cd in freshwater sediments. Please explain.
Page 6-31, 2nd paragraph. How accurately could IWTU alone predict toxicity? What if IWTU were
normalized to hardness, pH, and DOC?
Deletion of samples from the evaluation of metal toxicity because of the potential for effects of
organic contaminant should be more quantitative that the approach described in Chapter 6. For
example, on Page 6-9 of the briefing document, the statement is made that organic contaminants
contributed to the toxicity of Bear Creek sediments. However, insufficient data are presented to
support this conclusion. Were these or other samples analyzed for organics?
TOC measurements are critical and require QA/QC to ensure their reliability. Also, note that not all
TOC is the same. How is this factored into the calculations?
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