Augusts, 1997
EPA-SAB-EPEC-LTR-97-009
Honorable Carol M. Browner
Administrator
U.S. Environmental Protection Agency
401 M Street, SW
Washington, D.C. 20460
Subject: Evaluation of Superfund Ecotox Threshold Benchmark Values for
Water and Sediment
Dear Ms. Browner:
On May 13, 1997, the Sediment Quality Subcommittee (Subcommittee) of the
Ecological Processes and Effects Committee of the Science Advisory Board (SAB)
reviewed the Office of Solid Waste and Emergency Response's Ecotox Threshold (ET)
benchmark values to be used within the Superfund program to determine the need for
site-specific ecological risk assessment involving water and sediments. Ecotox
Threshold (ET) benchmark values are defined as media-specific contaminant
concentrations above which there is sufficient concern regarding the potential for
adverse ecological effects to warrant further site investigation. The Subcommittee's
review and comments are based on information contained in an Agency bulletin, ECO
Update (January 1996, volume 3, number 2), on the development and use of ET
benchmark values for water and sediment in the Superfund Ecological Risk
Assessment process. (Agency guidance on ET benchmark values for soils is not yet
available and was not included in this SAB review.) The ECO Update bulletin,
hereafter referred to as the ET document, states that the ET values: 1) are intended to
provide a screening-level process that determines the need for additional ecological
risk assessment for a given site for specific contaminants of concern at that site; and 2)
are not proposed or intended to be used as regulatory criteria, site-specific cleanup
standards, or remediation goals.
In short, the Subcommittee treated the current ET document as limited in scope
(addressing only ecological toxicity-not fate and transport-and sediment and water
levels, not terrestrial levels) and intent (for screening purposes only).
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The Charge to the Subcommittee included four questions, which are presented
and discussed below:
1) Recognizing the continued scientific debate over the accuracy of the
equilibrium partitioning theory, are the sediment thresholds derived
using this model accurate enough to use as screening levels?
Yes. The Subcommittee continues to support the conclusions of previous SAB
reviews of the equilibrium partitioning (EqP) approach, which is based on a theory
describing the partitioning of chemical contaminants between sediment and interstitial
(or pore) water; these reviews found the EqP to be a scientifically sound approach for
assessing sediment quality if the uncertainties associated with assumptions of
equilibrium, bioavailability, routes of exposure, and so forth, are considered and
described (SAB, 1992; SAB, 1990). While we recognize that the equilibrium approach
to establishing sediment quality values has both strengths and weaknesses, the
Subcommittee agrees that it is satisfactory for use in a screening-level risk assessment
that recognizes these uncertainties. As discussed in our response to Charge Question
3, however, the choice to use values derived using the EqP, rather than values derived
from other methods, should be based on a scientific rationale. Additionally, the ET
document indicates that "Superfund has elected to use the lower limit of the 95 percent
confidence interval presented in the criteria documents as the ET, rather than the
central tendency value." This decision adds an additional level of conservatism for
screening purposes.
2) Should the dissolved water column concentration of heavy metals be
used to compare to the thresholds or should the total concentrations
be used to add more conservatism?
The Subcommittee recommends that dissolved concentrations of metals in
surface water samples collected at Superfund sites be used to compare to threshold
concentrations for screening-level risk assessments; this is the accepted approach for
assessing exposure for organisms that dwell in the water column. It has been shown,
as the state-of-the-science has evolved over the past decade, that the dissolved
fraction more appropriately reflects the bioavailable portion of the total metal present in
a sample. Considerable study in this area shows that free metal ions are most often
responsible for the toxic effects observed from metals. The dissolved fraction appears
to provide a much better measure of the free metal ion concentration than total metal,
which includes metals that are sorbed or bound to various other particles, bound to
dissolved organic matter, or incorporated in various matrices (Bergman and Dorward-
King, 1997). Using this rationale, the Agency's Office of Water established the same
position for the National Pollutant Discharge Elimination System (NPDES) (Prothro,
1993).
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The Subcommittee recognizes, however, that the state-of-the-science is evolving
regarding the assessment of suspended particles and associated sorbed metals. We
recommend, therefore, that the Agency revisit this issue at an appropriate time in the
future. Consistent with the state-of-the-science, the ET document evaluates potential
exposure to metals from sediment ingestion by comparison of sediment concentration
against ET values for sediment.
We recommend, however, that guidance be provided or referenced in the ET
document on how to obtain reliable data on dissolved metal concentrations. For
example, considerable care must be taken in both the sample collection and analysis to
avoid contamination or loss of the analyte.
3) The document currently presents a tiered approach to the use of
preferred methods. For sediments, for example, a draft Sediment
Quality Criterion is the first choice, then Sediment Quality
Benchmarks using the equilibrium approach, and then the NOAA
Effects Range Low values. Should the document continue with this
approach, should different priorities be recommended, or should it
present all available screening levels, including other values
developed by states (e.g., New York), other Agencies (e.g., Ontario)
and let the regional risk assessors decide which one is most
appropriate for their site?
The current ET document does not provide a scientific rationale supporting the
sequence (tiers) for the selection of the preferred values. While the Subcommittee has
supported the Agency's use of the EqP approach for deriving threshold values, we also
recognize that other approaches can provide useful numbers and might be preferred in
certain circumstances. We note, however, that there are only a few chemicals for
which the ET document provides more than one value. We recommend that the
Agency update the existing document to reflect the Agency's current thinking on the
calculation and use of EqP-based sediment quality criteria (e.g., for polycyclic aromatic
hydrocarbons), and to provide a scientific rationale for the choice of methods used to
derive a threshold when values are available from more than one approach. The
published literature provides some insight on the strengths and weaknesses of the
various methods, which vary depending on the class of chemical and sometimes the
individual constituent in question. These studies should guide the rationale for
selecting among the threshold values. For example, the EqP is applicable to non-ionic
organics, but not to polar organics or metals. For these chemicals, the threshold
values (ERL - effects range low) developed by Long et al. (1995) may be the best
choice for marine sediments. These authors have indicated chemicals for which this
approach also has limitations. As noted in the ET document, a scientific rationale for
selecting ET values should allow consideration of site-specific characteristics that
affect the extent to which critical assumptions would be met, and would provide a basis
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for selecting among validated and peer-reviewed ET values. In general, the
Subcommittee believes that less weight should be given to methods that require the
use of safety factors to derive threshold values (e.g., the method used to calculate
Sediment Quality Benchmarks, or SQB).
The Subcommittee views the question of who should decide which ET value is
most appropriate for the limited purpose of screening at a specific site to be a matter of
policy, rather than science.
4) We recognize that there could be some situations where all
chemicals at a site could "pass" the screen, but due to unpredicted
cumulative effects or the presence of very sensitive species or
ecological processes, adverse ecological effects could occur, but we
feel the likelihood of this happening is relatively low (i.e., <5%) and is
acceptable. Do these thresholds and the use of the maximum
environmental concentrations provide sufficient conservatism to
make it unlikely that sites posing a "real" risk would pass the
screening test?
The Subcommittee believes that the threshold concept is valid and useful and
supports the use of the ET values in a decision-making pass/fail screening system.
Insufficient information was available to the Subcommittee at the review meeting to
make a quantitative determination of the degree of protection provided by individual ET
benchmarks. Based on past briefings to the Subcommittee and upon its expert opinion,
however, the Subcommittee believes that ET values used in conjunction with maximum
environmental concentrations are sufficiently conservative for screening to identify sites
that require further study.
Additional Comments
The Subcommittee recommends that the ET document be revised to contain
more explicit information as to where in the Superfund process the ET benchmarks are
most appropriately used. Although the document discusses the limited scope and
intent of ETs as benchmark screening values for baseline (preliminary) risk
assessment, this guidance does not explicitly relate the comparatively crude ET
procedure to the fuller Risk Assessment Guidance for Superfund (RAGS)(EPA, 1990)
or to the overall ecological risk assessment process. Indeed, the RAGS is mentioned
only in a footnote on the first page of the ET document. While we recognize that the
level of detail in the RAGS cannot be duplicated in the ET document, additional
information should be included in the ET document regarding the connection between
ET benchmarks and the overall ecological risk assessment process. An explicit
discussion of the relationship between ET and RAGS will make the document more
informative and useful to the reader. At a minimum, the various ecorisk guidance
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documents should be cross-referenced. As part of an integration into the ecological
risk assessment process, we recommend that the Agency's recently proposed
Guidelines for Ecological Risk Assessment (EPA, 1996) be utilized as the principal
guidance for performing ecological risk assessments and that the Agency reexamine
the RAGS to ensure consistency with these proposed guidelines.
We hope these comments will be useful to the Superfund program in
implementing preliminary ecological risk screening for water column and sediments.
The Subcommittee has been very supportive of the Agency's efforts to develop uniform
guidance for the performance of ecological risk assessment across the Agency and we
appreciate the opportunity to review program-specific guidance that brings the ecorisk
process to the operational level of Agency activities.
Sincerely,
/signed/
Dr. Genevieve M. Matanoski, Chair
Executive Committee
/signed/ /signed/
Dr. Mark A. Harwell, Chair Dr. William J. Adams, Chair
Ecological Processes and Sediment Quality Subcommittee
Effects Committee
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LITERATURE CITED
Bergman, H.L. and Dorward-King, Elaine. 1997. Reassessment of Metals Criteria for
Aquatic Life Protection. Society of Environmental Toxicology and Chemistry
(SETAC), SETAC Press, Pensacola, Florida
Long, E.R., D.D. MacDonald, S.L. Smith, and F.D. Calder. 1995. Incidence of Adverse
Biological Effects Within Ranges of Chemical Concentrations in Marine and
Estuarine Sediments. Environmental Management 19:81-97.
Prothro, M.G. 1993. Office of Water Policy and Technical Guidance on Interpretation
and Implementation of Aquatic Life Metals Criteria. Memorandum to EPA Water
Management Division Directors, Environmental Services Division Directors, and
Regions. October 1, 1993.
SAB. 1990. Evaluation of the Equilibrium Partitioning (EqP) Approach for Assessing
Sediment Quality (EPA-SAB-EPEC-90-006). February 1990. U.S. EPA Science
Advisory Board, Washington, D.C.
SAB. 1992. Review of Sediment Criteria Development Methodology for Non-Ionic
Organic Contaminants (EPA-SAB-EPEC-93-002). November 1992. U.S. EPA
Science Advisory Board, Washington, D.C.
EPA. 1990. Risk Assessment Guidance for Superfund, Volume II: Environmental
Evaluation Manual, U.S. Environmental Protection Agency. EPA/540-1-89/001.
EPA. 1996. Proposed Guidelines for Ecological Risk Assessment. U.S. Environmental
Protection Agency. EPA/630/R-95/002B, August 1996.
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US ENVIRONMENTAL PROTECTION AGENCY
SCIENCE ADVISORY BOARD
ECOLOGICAL PROCESSES AND EFFECTS COMMITTEE
Sediment Quality Subcommittee
MAY 13, 1997
CHAIR
Dr. William J. Adams, Kennecott Utah Copper Corp., Magna, UT
MEMBERS
Dr. Steven M. Bartell, SENES Oak Ridge, Inc., Oak Ridge, TN
Dr. Kenneth W. Cummins, South Florida Water Management District, Palm Beach
Gardens, FL
Dr. Mark A. Harwell, Rosenstiel School of Marine and Atmospheric Science, University
of Miami, Miami, FL
Dr. Carol A. Johnston, Natural Resources Research Institute, Univ. of Minnesota,
Duluth, MN
Dr. Alan W. Maki, Exxon Company, USA, Houston, TX
Dr. Frederic K. Pfaender, Carolina Federation for Environmental Programs, University
of North Carolina, Chapel Hill, NC
Dr. William H. Smith, School of Forestry and Environmental Studies, Yale University,
New Haven, CT
CONSULTANTS
Dr. J. Walter Milon, Department of Food and Resource Economics, University of
Florida, Gainesville, FL
SCIENCE ADVISORY BOARD STAFF
Ms. Stephanie Sanzone, Designated Federal Official, EPA Science Advisory Board
(1400), 401 M Street, SW, Washington, D.C. 20460
Ms. Wanda Fields, Staff Secretary, EPA Science Advisory Board (1400), 401 M Street,
SW, Washington, D.C. 20460
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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.
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