UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D.C. 20460
JUN 13, 2000
OFFICE OF
WATER
Chair
Science Advisory Board
U.S. Environmental Protection Agency
1200 Pennsylvania Ave., NW
Washington, D.C. 20460
Dear Chair:
The Office of Water received your letter on March 2, 2000, transmittal of: An SAB
Report: Review of an Integrated Approach to Metals Assessment in Surface Waters and
Sediment., EPA-SAB-00-005, February 28, 2000. Thank you for the comprehensive review of
the Agency's research into metals bioavailability and its use as the technical foundation for
setting guidelines for sediment assessment and protection. The SAB provided both high praise
for the research conducted to date and recommendations for future research that would further
our understanding of metals bioavailability. It is the goal of the Agency to provide sound science
for use in regulatory programs, by States and Tribes, and by the regulated community. The
commendations reassure us that we are providing a sound scientific basis for our decisions and
the recommendations define new paths to improvement.
It is the Agency's intent to move forward with the current guidelines, adjusted to reflect
the SAB's concerns and cautions, along with existing sediment assessment tools in regulatory
programs in line with the SAB's recommendation that"... SEM-AVS be incorporated into
sediment assessment guidelines in a way that assures that SEM-AVS will continue to be used in
conjunction with other assessment tools to characterize the safety of sediments, rather than being
used as stand-alone tests." The Implementation Framework for Equilibrium Partitioning
Sediment Guidelines has been very carefully crafted to promote this implementation strategy.
The Agency also intends to investigate the research recommendations presented by the SAB so
that the guidelines' underpinnings will become even stronger.
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Enclosed is a detailed response from the contaminated sediment research team addressing
the SAB's major concerns and recommendations. If you have any questions or outstanding
concerns, please contact Jeanette Wiltse, Director, Health and Ecological Criteria Division, at
202-260-7317.
Sincerely,
/Signed/
J. Charles Fox
Assistant Administrator
Enclosure
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Contaminated Sediment Research Team
Response to:
An SAB Report:
Review of an Integrated Approach to Metals Assessment in
Surface Waters and Sediments
Dietary Exposure/Bioaccumulation (pp.11 - 14)
The SAB expressed concern that"... the importance of dietary accumulation [of metals] and the
potential for toxicity has not been systematically examined." The Agency agrees. However, to
date the data collected demonstrate that although benthic organisms may accumulate a body
residue of metal when the AVS concentration exceeds the SEM concentration no toxic effects are
realized. The database supporting the metal mixtures ESG includes a number of sediment
ingesting organisms that could be expected to have significant dietary exposure. That these
organisms do not show toxicity lends support to the expectation that bioaccumulation of metals
under these circumstances (SEM-AVS < 0) does not lead to acute or chronic toxicity. Several
researchers are actively exploring factors controlling accumulation of metals from sediments and
the Agency will continue to keep abreast of these research efforts.
To further evaluate these assumptions the Agency is scoping a research plan for a systematic
investigation of dietary exposure to metals and potential for toxic effects. This research includes
the use of sediment ingesting organisms (exposed to metals through sediment) as prey. An initial
Agency workshop on this topic was held in March 2000. The Agency research team is also co-
chairing a session at the upcoming Society of Toxicology and Chemistry Annual Meeting
(November 2000) on the topic of dietary exposure and bioaccumulation.
Nonetheless, the striking uniformity of the lexicological data with SEM-AVS based analysis,
even in organisms showing accumulation of metals, compels the Agency to move forward with
releasing the current metal ESG, particularly when considering the current absence of guidance.
Biological and Ecological Processes (pp 14-17)
Exposure/Ecological Processes (pp 14 -16)
The SAB expressed the concern that the sediment environment is very complicated, with high
variability and gradients both in the horizontal and (especially) in the vertical, and called into
question whether a simple sampling protocol (such as sampling the top 2 centimeters) can
account for this variability. The Agency acknowledges that sampling the top strata of a sediment
column may not fully describe the sediment column, especially in some cases where
contaminated sediments are overlain with more recent, cleaner sediments. As would be true with
any sediment guideline, the most robust approach would be to take multiple cores to the
maximum penetration of the benthos at any particular site, and then to measure the SEM and
AVS at a number of different horizons, providing a complete characterization of the sediments in
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a particular area. This would seldom be practical, unless there is reason to believe that there may
be vertical stratification of metals concentration at a particular site. Although some
understanding of the variability in a particular area is essential, there is no reason to believe that
the SEM-AVS guideline will be more susceptible to this variability than any other. The same
problem is also faced with toxicity testing of field-collected sediments. The surface layer is
generally chosen because this is the zone of highest biological activity, and the zone most prone
to oxidation. In the few cases where there have been apparent mismatches between measured
SEM and AVS concentrations and biological effects, the mismatches can be explained if only the
measurements from the surface sediments are used. In general, however, sampling and analysis
based on the top 2 cm has shown good correspondence with measured effects, suggesting that it
is a reasonable guideline for the general case.
Effects/Biology (pp 16 -17), e.g. life stage sensitivity, feeding strategy.
The SAB expressed concern that there are many different kinds of benthic organisms, and that
the ESG, especially if it is applied to the surface sediments, may not be protective of all species
and life stages. As stated in the previous section, sampling of the surface sediments theoretically
may not show a problem that may manifest itself with organisms that are deeper burrowing.
However, as the SAB points out, the data from the array of colonization and other toxicity tests
done to date are impressive. Biological effects have not been found in any of the colonization
tests done to date in sediments with an excess of AVS, even though these tests expose many
different species, many in their most sensitive life stages. Of course these tests generally only run
for several months, and some of the large, deep-dwelling species may not have had time to
develop during the course of these experiments. The Agency has not compiled a list of
organisms tested in the course of validating the ESG. The intent is, as it is for all ambient water
quality documents, to test a sufficient number of species to verify that it is reasonable to expect
that the theory is applicable to a variety species and feeding types.
Sampling and Handling Protocols (18-20)
The SAB expressed the need for properly designed sampling and handling protocols so that a
number of sediment characteristics can be addressed including their temporal and spatial
variability, non-equilibrium conditions, and oxidizing conditions. The Agency agrees that
establishing the proper sampling and handling protocols and strategies are critical to conducting
a robust and meaningful site assessment. In light of this, the Agency is preparing a sediment
sampling and handling protocol manual outlining its recommendations. This document is
scheduled for completion (inclusive of peer review) in March of 2001. The Agency is also
beginning discussions on what research would help answer questions about non-equilibrium
conditions.
Organic Carbon Normalization (pp. 21 -22)
The SAB recommended that the organic carbon normalized metals mixture ESG not be
implemented as a stand alone value but rather along with other supporting biological data
because of concerns about the variability of it's usefulness across metals and its spatial and
temporal variability. By way of clarification, the Agency is not proposing that any of the ESGs
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be used as stand alone determinations. The Implementation Framework for Equilibrium
Partitioning Sediment Guidelines, which will be available for public comment in the near future,
outlines the Agency recommendations for ESG implementation which hinges on the use of both
ESGs and bioassays in regulatory applications.
The Agency recognizes that the organic carbon normalization is a relatively new evaluation
approach and, while promising, has not had opportunity for extensive evaluation. For that
reason, the metals ESG has been structured such that the SEM-AVS and interstitial water metals
guidelines are the primary basis for the ESG. The organic-carbon normalization has been
included in the document in section 3.4 (and removed from the guidelines statement in chapter 6)
as a supplementary analysis that can be used to further evaluate the potential consequences of an
excess of SEM.
Application of the Biotic Ligand Model (ELM) to Sediment Guidelines
The SAB does not recommend that the BLM be incorporated into the ESGs at this time. This
recommendation comes from concerns about the complexity of sediment and pore water
chemistry. The SAB recommended that the ability of the BLM to appropriately predict pore
water toxicity under a variety of chemical conditions be evaluated prior to its incorporation in the
sediment guideline. The Agency accepts the SAB's recommendation and will postpone the
incorporation of the BLM into the ESG until these complexities can be more thoroughly
assessed.
Inclusion of Chromium and Silver in the Metals Mixture ESG
The SAB expressed reservations about the applicability of the EqP approach to chromium and
silver. The SAB established a number of areas of needed research for chromium in particular:
additional acute and chronic toxicity data, oxidizing conditions of Chromium (m) to Chromium
(VI), conditions when MnO2 are expected in sediments, types of food, and biokinetics. The
Agency agrees with the SAB that chromium is not yet ready for inclusion in the guidelines. The
data presented by the Agency was designed to demonstrate what research had been conducted
and to solicit recommendations from the SAB for additional investigations.
The SAB also identified concerns for the incorporation of silver in the guideline. These included
from page 25 (g-j): questions about the bioavailability of silver, quantity of chronic data,
elimination of toxicity by AVS binding, and acute mortality of >24% when IWTU are less than
0.5. The Agency has reviewed the SAB's concerns and the data presented in the review package
and has concluded that it is appropriate to include silver in the metals mixture guidelines.
Specifically the Agency found that for the concerns expressed on page 25:
g) though laboratory and field data demonstrate that silver may be.bioaccumulated
by some organisms, there is no demonstrated toxicity.
h) it is correct that no chronic sediment testing has been done with silver-spiked
sediments to date. Chronic sediment tests have been done using all of the other
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metals covered under the metals ESG. It was never the Agency's intention to
perform chronic tests with all of the metals before the ESG was released because
it would not be possible to check all effects to all organisms. Rather, it was the
Agency's intention to conduct a sufficient number of chronic tests with a variety
of metals and species to show that AVS normalization worked as would be
predicted by theory and that it would be reasonable to apply it to metals of similar
characteristics. To our knowledge there are no published data which show
directly that chronic effects occur with silver or any of the other metals when
AVS is in excess in sediments.
i) the data presented in Figure 8, p. 2-36 demonstrate clearly that AVS binding
eliminates silver toxicity, but is easily misinterpreted, because the X axis does not
contain 0.0. This is because the data are plotted on a logarithmic scale to make a
point about TOC normalization. Note that for all points plotted on the graph
metals are in excess of sulfide, and therefore the sediments are likely candidates
for silver toxicity.
j) the interstitial water toxic units guideline is not applicable to silver, only to the
other metals in the mixture guideline. This is specifically stated in sections 4.2.2,
5.2.2, and in the guidelines statement (chapter 6).
Integrated Approach to Metals
The Agency presented to the SAB an example conceptual model of how metals assessment may
be integrated to provide more holistic aquatic ecosystem. The SAB made a number of
recommendations for the Agency to consider as the model moves from an example to an actual
model that will assist the Agency is determining research and guidance needs. The Agency will
incorporate the SAB's recommendations in the model as the model matures.
ESG Implementation
The SAB has stated at the conclusion of each of four reviews of ESGs that to accommodate the
uncertainties about the methodology the guidelines not be used as stand alone, pass/fail criteria
for determining whether a sediment is toxic or not. The Agency is preparing to release for public
comment the proposed Implementation Framework for Equilibrium Partitioning Sediment
Guidelines. The Framework is very specific in its recommendation that the guidelines not be
used as stand alone, pass/fail criteria. Rather the Agency recommends the ESGs be used in
conjunction with whole sediment bioassays, and when necessary with benthic assessments as
well, when a determination on sediment toxicity must be made.
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