r-o A U.S. Environmental Washington, DC
•Hfc ^HF •_»
Cr*^ Protection Ageficy EPA*SAB-EP6C-§0-O06
Report of The Sediment
Criteria Subcommittee of The
Ecological Processes and
Effects Committee
Evaluation of The Equilibrium
Partitioning (EqP) Approach for
Assessing Sediment Quality
A SCIENCE ADVISORY BOARD REPORT February 1990
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I UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, O,C. 20460
February 26, 1990 EP&-SAB-EPEC-90-OQ6
OFFICE Of
THE AOWI«|ST«**TOR
The Honorable William Reilly
Administrator
U.S. Environmental Protection Agency
401 M. street, S.W.
Washington, D.C. 20460
Dear Mr. Reilly;
The Sediment Criteria Subcommittee of the science Advisory
Board has completed its review of the Equilibrium Partitioning
(Eqp) approach for judging sediment quality. This approach was
developed by or under the auspices of EP&'s Office of Water,
Criteria and standards Division, which requested this review.
The SAB was asked to review the sediment assessment approaches
in order toi
1) Evaluate the scientific and technical foundations for each
methodology,*
2) Evaluate the feasibility of using each method for
determining the extent of contamination and risk posed to the
environment and man;
3) Identify research directions that would provide
information to strengthen each methodology? and
4) Compare and contrast the strengths and weaknesses of the
methodologies available to EPA based on Agency provided documents.
The review was conducted by the Sediment Criteria Subcommittee
which was established to review the Agency's technical
methodologies for sediment quality assessment. This is their
second report. The committee met five times over the past twelve
months for this review,
The EqP approach assumes that the critical parameter for
gauging sediment quality is the concentration of a chemical in the
water that will not cause significant adverse affects•to aquatic
species of interest. This water concentration can then be used to
estimate the corresponding concentration of the chemical in the
underlying sediment. The approach, as developed and reviewed
to date, pertains only to non-ionic organic; compounds.
The 2qP approach is considered by the Subcommittee to have
major strengths in its foundation in chemical theory, its ease
of calculation, and its ability to make use of existing data
(e.g., water quality criteria values). In addition, the
developers of the approach have begun to explore the uncertainty
surrounding the approach, which is to be commended.
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The conceptual basis of the approach is supported by the
Subcommittee, however its application at this time is limited,
This is because a better understanding of the uncertainty around
the assumptions inherent in the approach, including assumptions
of equilibrium, bioavailability, and kinetics, all critical
to the application of the EgP approach, is needed.
Additional concerns of the subcommittee relate to the
limited number of existing water quality criteria which the
Eqp approach can use, the compound-specific nature of the
approach, and the questionable ability of the method to protect
sediment-ingesting organisms. The exposure to organisms by food
chain transfer is also not considered and the amount of field data
available to validate the approach is limited. Some suggestions
for research to explore the sources of uncertainty and the
Subcommittee's other concerns are provided in the attached report.
It should be noted that our charge was to evaluate the method
relative to its ability to gauge sediment quality. We did not
therefore, evaluate its applicability for establishing sediment
quality criteria per se. We were unable to compare the strengths
and weaknesses of the EqP with other methods as identified in our
charge, because appropriate documents were not available at this
time.
The Subcommittee appreciates the opportunity to conduct this
scientific review, we request that the Agency formally respond to
the scientific advice transmitted in the attached report.
Sincerely,
r,
Executive Committee
Science Advisory Board
Dr. Kenneth DicKson, Chairman
Ecological Processes and
Effect Committee
Dr. Robert Huggett,defcairman
Sediment Criteria Subcommittee
cc: LaJuana wilcher, Martha Prothro, James M.
Catherine Krueger, Donald Barnes
conlon,Chris Zarba
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ABSTRACT
This report presents the conclusions and recommendations of the
U.S. Environmental Protection Agency's Science Advisory Board
summarizing a review of the Equilibrium Partitioning (EqP) approach
for estimating sediment quality. The EqP approach relates the
level of a chemical that has been found to be acceptable from a
biological standpoint to the corresponding concentration of the
chemical sorbed to sediments. The value that results, the sediment
quality value, is based on the assumption that only the portion
of the chemical dissolved in the water surrounding
sediment particles is available to exert toxicity on resident
biota, The approach, as developed and reviewed to date,
pertains only to non-ionic organic compounds. The EP
approach is considered by the Subcommittee to have major
strengths in its foundation in chemical theory, its ease of
calculation, and its ability to make use of existing data. In
addition, the researchers responsible for its development have
begun to explore the uncertainty surrounding the approach,
which is to be commended. The conceptual basis of the
approach is supported by the Subcommittee, however its
application at this time is limited. This is because a better
understanding of the uncertainty around the assumptions inherent
in the approach, including assumptions of equilibrium!
bioavailability, effect levels, and kinetics, all critical to the
application of the IP approach, is needed. Additional concerns
expressed by the Subcommittee relate to the limited number of
existing water quality criteria which the EP approach can use,
the compound-specific nature of the approach, and the
questionable ability of the method to protect sediment-ingesting
organisms. The exposure to organisms by food chain transfer is
also not considered and the amount of field data to validate the
EqP approach is limited. Some suggestions for research to
explore the sources of uncertainty and the subcommittees other
concerns a,re provided in the report.
Key Words: Sediment, EqPi Equilibrium Partitioning approach: Non-
ionic organic compounds
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U.S, ENVIRONMENTAL PROTECTION AGENC¥
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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U.S. ENVIRONMENTAL PROTECTION AGENCY
SCIENCE ADVISORY BOARD
ENVIRONMENTAL EFFECTS, TRANSPORT AND PATE COMMITTEE
SEDIMENT CRITERIA SUBCOMMITTEE
ROSTER
CHAIRMAN
Dr. Robert Huggett
Virginia Institute of Marine Science
School of Marine Sciences
College of William and Maisy
Gloucester point, Virginia 23062
VICE CHAIRMAN
Dr. Rolf Hartung
University of Michigan
3125 Fernwood Avenue
Ann Arbor, Michigan 48108-1955
MEMBERS
Dr. William J. Adams
Monsanto Company (U4G)
800 N. Lindbergh Blvd.
St. Louis, Missouri 63167
Dr. Kenneth L. Dickson
University of North Texas
Institute of Applied Sciences
P.O. BOX 13078
Denton, Texas 76203
Dr» Benjamin C, Dysart III
Environmental and Water
Resources Engineering
401 Rhodes Engineering Research center
Clemson University
eiemson, south Carolina 29634-0919
Dr, Eugene Kenaga
Consultant (Ret./Dow)
1584 E. Pine River Road
Midland, Michigan 48640
Dr. Frederic K. Pfaende-r
Department of Environmental sciences
and Engineering
University of North Carolina
Chapel Hill, North Carolina 27599-7400
111
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Dr. Y. Peter Sheng
Professor, Department of Coastal and
Oceanographic Engineering
336 Weil Hall
University of Florida
Gainesville, Florida 32611
SUBCOMMITTEE LIAISONS
Dr. Robert M. Engler
(CEWESEP-D)
Waterways Experiment station
U.S. Amy Corps of Engineers
P.O. Box 631
Vicksburg, Mississippi 39180
Dr, Chris Ingersoll
National Fisheries contaminant
Research Center
U.S. Fish and wildlife Service
Route 2
Columbia, Missouri 65201
Dr, H. Suzanne Bolton
NQAA-DQC
Office of Legislative Affairs
(LAX-2)
Herbert c. Hoover Building
Room 5222
Washington, D.C. 20230
SCIENCE ADVISORY BOARD STAFF
Dr. Edward s. Bender
Biologist & Executive Secretary*
U.S. Environmental Protection Agency
Science Advisory Board
401 M street, S.W.
Washington, D.C. 20460
*Ms. Janis Kurtz, currently assigned to 1RL Gulf Breeze, FL,
served as the Executive Secretary through September, 1989,
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1.0 Executive Summary ................ l
2.0 Introduction ......... . . . 2
2.1 Request for Science Advisory Board Review . . ' . 2
2.1.1 Charge to the Subcommittee . 2
2„2 Subcommittee Review Procedures ...... 2
2.3 Expected Future Activities ........ 3
3.0 Evaluation of the Equilibrium Partitioning Approach 5
3.1 Chemical Considerations ... .... 6
3.1.1 Kinetics •.,.... 6
3.1.2 Particle Size Distribution ....... 7
3.1.3 Analysis ....... a
3.1.4 Sorption .......... 8
3.1.5 organic Carbon Normalization . 10
3.1.6 Particle Concentration Effect ..... 10
3.1.7 spiked versus Natural Sediment Adsorption 11
3.2 Biological Considerations ........... 12
3.2.1 Dependency on Water Quality Criteria
and Advisories '..........*..... 12
3.2.2 Exposure Routes-Water Versus Sediments . 13
3.2,3 Relevance of Water Quality Criteria to
Benthic Organisms .............. 14
3.3 Inappropriate Use of Water Quality criteria to
Determine Effect Levels in the EP Approach . . 15
4.0 Summary of Subcommittee Conclusions, Recommendations 16
5.0 References ... ....... 18
Appendix A - Briefing Report to the EPA Science Advisory Board
on the Equilibrium Partitioning approach to
Generating Sediment Quality Criteria
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1.0 EXECUTIVE SUMMARY
The Equilibrium Partitioning (EqP) approach to
assessing sediment quality is based on an assumed set of
relationships between chemical contaminants sorbed to bottom
sediments and living organisms residing in, on or above
those sediments. Some of the relationships are supported by
thermodynaiaic principles, others by limited observations.
The approach assumes that only the fraction of contaminant
that is dissolved in interstitial water is biologically
available, and that this dissolved fraction is inversely
proportional to the organic carbon content of the sediment.
Additionally, it is postulated that the distribution of the
contaminant between pore water and sediment organic carbon
is comparable to the distribution of material between water and
the organic solvent n-octanol (i.e., the octanol/water partition
coefficient).
To describe the distribution of a substance between water and
bulk sediments, the sediment partitioning coefficiv it, VL, is used.
The organic carbon normalized sediment partitioning coefficient,
Kt is used to describe the distribution of the chemical between
the organic fraction of the sediments and the associated water and
it is approximated by K^, the octanol/water partition coefficient.
Therefore, if the assumptions are valid, if the coefficients are
known and if the biologically acceptable water concentration of a
persistent contaminant is known (e*g., from the establishment of
a water quality criterion), the acceptable amount of contaminant
in the solid sediment phase can be calculated.
The IqP approach relies on a fundamental chemical parameter,
fugacity. The Subcommittee considers this foundation to be a major
strength, and supports the conceptual basis of the approach. The
relative ease of calculation and the approach's reliance on the
use of existing data also contribute to the approach's appeal
and potential for application.
The EqP approach shows promise but, in the opinion of the
subcommittee, should not be broadly applied without an
understanding of the uncertainty around the assumptions inherent
in the approach. Areas of uncertainty that must be explored
prior to generic applicability include assumptions of
equilibrium, bioavailability, and kinetics.
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For instance, the Subcommittee recommends that uncertainty
associated with the following facets of the EqP Approach be
critically examined:
o Uncertainty associated with equating K^ and K^;
o Uncertainty associated with the use of water quality
criteria, water quality advisories and other estimates of
effect levels;
It is recommended that additional research be conducted to
evaluate the influence of particle size distribution on sorption
and desorption and the role of organic carbon in sediments in
regulating the bioavailability of non-ionic organic sediment
contaminants,
Finally, the Subcommittee expressed reservations about the
limited number of water quality criteria, the higher levels of
uncertainty associated with advisories, the inability of the
approach to account for mixtures of chemicals, and the
questionable ability of the method to protect sediment-ingesting
organisms. The Subcommittee was also concerned with the limited,
amount of data available to validate the EqP approach.
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2*1 laottftitt for Science Advisory Board Review
At the request of the Office of Water, Criteria and
Standards Division, the Science Advisory Board (SAB) agreed to
conduct a scientific review of the Equilibrium Partitioning
(EqP) approach. The SAB's Ecological Processes and Effects
Committee ( formerly the Environmental Effects, Transport and
Fate committee) authorized the formation of a Subcommittee to
perform a series of tasks related to the technical aspects of
sediment quality assessment and criteria development, with the
approval of the Board's Executive Committee,
2.1.1 Charge to tb« Subcommittee
This review is the second to be completed in a series of SAB
reviews related to sediment quality criteria. Preliminary
reviews focused on approaches to examining non-ionic organic
contaminants. The first report of the Subcommittee, entitled
"Evaluation of the Apparent Effects Threshold (AJ~?) Approach for
Assessing Sediment Quality" July, 1989, presented recommendations
and conclusions concerned with the AET Approach. The charge
accepted by the Subcommittee for the review of the AIT and EqP
approaches was to;
a) Evaluate individually the scientific and technical
foundations for the various methodologies available to the Agency
to estimate sediment toxicity and biological impact of contam-
inated sediments insitu.
b) Evaluate the feasibility of utilizing each methodology
to determine extent of contamination and risk posed to the envi-
ronment and human health.
c) Identify research directions that would provide informa-
tion to strengthen each methodology.
d) Conduct a technical review of documents provided by the
Agency that compare and contrast the scientific and technical
strengths and weaknesses of the methodologies available to the
Agency to estimate sediment toxicity and biological impact of
contaminated sediments in situ*
2.2 Subcommittee Review Procedures
The Sediment Criteria subcommittee met on August 8 and 9,
1988, in Denver, Colorado, to assess the Agency's activities
regarding contaminated sediment and to explore avenues for
providing oversight. A second meeting was held on October 27 and
28, 1988, in Seattle, Washington. Informative briefings were
provided on the objectives, historical perspective and technical
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components of the AIT approach. A third meeting was held on
February 2 and- 3, 1989 in Washington, D.C. to begin the review of
the EqP approach. The briefings provided to the Subcommittee
were thorough and were supported by documentation which was
provided to Subcommittee members prior to the sneering. This
preparation and support was provided by Office of Water, office
of Research and Development and associated contractors.
Select members of the Subcommittee were convened for writing
sessions in Gloucester Point, VA on May 8 and 9, 1989 and in
Washington, DC on June 1 and, 2 and again on August 17, 1989. The
final report has been reviewed and approved by the Subcommittee,
the Ecological Processes and Effects committee, and the Executive
Committee of the SAB.
2*3 Expected Future Activities
The Sediment Criteria Subcommittee is reviewing the
technical appropriateness of a series of methods that may be
applied to assessing sediment quality. These methods are contained
in a manual under development by EPA/QW's contaminated Sediment
Technical committee. The subcommittee will comment on this
manual in the near future.
other sediment quality assessment methods, including methods
for assessing metal contaminants, are expected to be developed
and existing approaches refined, and as this occurs, they will
be transmitted to the Subcommittee for review.
During the course of these critical evaluation processes, it
is likely that areas for additional or future research will be
targeted. To facilitate the incorporation of these recommenda-
tions into EPA research planning, the Subcommittee nay conduct a
review of the Office of Research and Development's proposed Sedi-
ment Initiative. The time sequence of these proposed events is
contingent on their completion by Agency staff.
3.0 EVALUATION OF THE EQUILIBRIUM PARTITIOHIKG APPROACH
The EqP approach focuses on the partitioning of chemicals
between particulate or solid (bulk) sediments and interstitial
water (i.e., the water between sediment particles), and water
quality criteria established for contaminants to indicate their
toxicity. since the contaminants partitioning to the interstitial
water appear to be more available to biota than contaminants bound
to bulk sediment, the EqP method for generating sediment quality
values is based gn predicted contaminant concentrations in
interstitial K».')t.*fr...'plee Appendix A). Chemically contaminated
sediments are expected to cause adverse biological effects if the
predicted interstitial water concentration for a given contaminant
exceeds the water quality criterion established , for that
contaminant.
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The EqP approach for assessing the quality of bottom
sediments relative to chemical contamination relies on a
fundamental thermodynamic parameter, fugacity. Pugacity is the
tendency of a substance to escape or flow from one phase of a
system to another until equilibrium is established. Equilibrium
is attained when the energy of that particular system is at its
lowest, and under this condition, all concentrations in the system
are independent of time.
In the case of chemically contaminated sediments, under
conditions which approach ideal, fugacities can be used to predict
how a chemical contaminant will be distributed among the
various phases making up the system. This system may be in
equilibrium, but equilibrium does not imply that the chemical
concentrations are equal among the phases. Those phases include:
inorganic material, non-living organic material, pore water,
dissolved gases, biota, and the overlying water column.
Therefore, if one knows the equilibrium concentration of a
chemical in any one phase, then the concentrations in the
remaining phases can be calculated if the distribution or
partition coefficients among the phases are known.
The EgP method assumes relatively ideal conditions and
equilibrium. This allows one to estimate the pore water
concentration by knowing the concentration of the substance in
the solid phases. If the assumption is made that only the pore
water fraction is biologically available, and if the aqueous
toxicity is known, exposure and hazard can be determined.
Ideal conditions are seldom approached under real-world
conditions, and approximations and assumptions must be made. The
EqP methodology relies on a number of empirically derived
estimates to describe the interactions or relationships among
hazardous chemicals, sediments, and indigenous biota.
To describe the distribution of a substance between water
and bulk sediments, the sediment partition coefficient, Kp, is
used. The organic carbon normalized sediment partitioning
coefficient, K^, is used to describe the distribution of the
chemical between the organic fraction of the sediments and the
associated water and it is approximated by K^, the octanol/water
partition coefficient. Each of these estimates has an
associated uncertainty resulting from experimental errors and,
in some cases, assumptions for which accuracy is not known with
certainty. There is uncertainty about the impact of non-ideal
conditions on partitioning and the estimate of sediment quality
derived from partitioning values. The Subcommittee recommends
that these assumptions, including assumptions of relatively
ideal conditions and equilibrium, be investigated to bettor
characterize th« uncertainty that pair axiat Around them.
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3.1 Chemical Considerations
The EqP approach for setting sediment quality criteria, like
the Agency's water quality criteria approach, focuses on the
effects caused by single chemicals. This is recognized as a
limitation in both approaches, since the sy_nergistic,
additive, and/or antagonistic effects posed by mixtures of
contaminants are not taken into account. The following is a
discussion of the EqP methodology's strengths and weaknesses
from a chemical standpoint, based on the methodology's focus on
single chemical compounds. Ongoing studies by "the Agency (SAB,
1989) and completed work by others (NRC, 1988 and NRC, 1989) should
be examined relative to their usefulness here.
3*1.1 Kinetics
In idealized situations, where thermodynamie equilibrium
exists between the solid phase and the aqueous phase of a
particular chemical, and when cause/effect relationships are
known, the EqP approach is valid and can be used to estimate
sediment quality from the partitioning coeffi ients and the
water quality criteria for that chemical. In such situations,
the method is very straightforward. However, in field
conditions it is necessary to examine whether and how well the
assumption of thermodynamic equilibrium is satisfied before
applying this"methodology. Although the limited data presented in
the draft briefing document appeared to support the application
of EqP, a few recent studies have indicated that deviation from
thermodynamic equilibrium is not uncommon in real world
situations,
Witkowski, et al. (1988) reported on the sorption and
desorption dynamics of ARQCLQR 1242 to natural sediment and found
that a two-stage kinetic model, rather than the equilibrium
model, was more appropriate for representing sediment uptake and
release processes in long-term (up to 16 days) simulations. The
data presented show that the partition coefficient can deviate by
a factor of 7 from the one corresponding to the equilibrium
conditions. Thus, if one uses the sediment quality estimate
determined from the EqP approach to set a limit on the solid
phase, the limit may be in error. Likewise, calculations of
interstitial water concentrations based on measured solid phase
concentrations may be in error.
Several studies have strongly indicated that kinetic control
rather than equilibrium partitioning, dictates the extent of
accumulation of sediment contaminants. Oliver (1984, 1987)
suggested that the importance of kinetic control in affecting the
accumulation of contaminants by benthos. Klump et. al. (1987)
measured the assimilation coefficient of oligochaetes and
demonstrated the importance of ingestion in affecting the
accumulation of contaminants. In a recent study by Landrum et. al.
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(1989), Pontoporei^ hovi were exposed to sediments contaminated
with TCB (tetrachlorobiphenol) and BaP (benzoapyrene), Although
the partition coefficient of TCB is only twice that of BaP, the
uptake rate of TCB was found to be 10 times that o£ Bap. Landrum
et. al. (1989)) and Landrum (1S89) concluded that, in their
experiment, the accumulation of contaminants is dictated by the
rate of desorption of contaminants from sediments and the
assimilation of contaminants by benthos instead of partitioning.
In the same study, Landrum et. al. (1989) also found that the
time for equilibrium (via adsorption)to be established between
contaminants on sediments and contaminants in interstitial water
is on the order of one month or longer* The time to approach
equilibrium via desorption, which occurs in most depositional
areas, is liJcely to be even longer.
The data of Landrum, et al. (1989) suggest that the time
scale depends on rates of desorption, ingestion, and elimination,
instead of the simple equilibrium partitioning. Furthermore,
physical and biological factors (e.g. temperature, salinity,
turbulence level, sediment type, sediment size distribution,
degree of flocculation, bioturbation, and decomposition) can
also be expected to affect the kinetics. These factors are
extremely difficult to reproduce in a laboratory experiment.
Moreover, laboratory experiments can only mimic "snap shots" in the
field, but cannot simulate the variability in physical and
biological factors associated with episodic events. Extrapolation
of laboratory results to field conditions thus contains significant
uncertainty.
Because of uncertainties produced by the problems
mentioned above, and because of uncertainties associated with the
determination of partition coefficients due to various other
factors (e.g, particle concentration effect, flocculation effect,
complex mixtures effect). it is recommendedthat a more
sophisticated uncertainty analysis b» carried out for the Egg
approach to setlimita/bounds on its applicability* Uncertainty
analysis may point to needs for additional research in various
aspects of the methodology. The uncertainty analysis in the draft
briefing document fails to adequately address all these important
factors, A rnor* rigorous approach would b» to perform
uncertainty analyses on both tha watar quality criteria and the
partition coefficient toyield a combineduncertainty on the
sediment quality criteria. This point should be addressed in the
technical support manual that EPA is to prepare.
3*1.2 Particle Size Distribution (PSD)
Specific to the EgP Approach is the fact that PSD can be an
important factor controlling chemical adsorption in certain
areas. Bottom sediment may be a mixture of coarse sand-size primary
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particles, finer primary particles such as the silts and clays,
and organic material, as well as aggregates that may contain both
inorganic particles, organic material, and living organisms of
assorted sizes. The active benthic zone of bottom sediments may
be cohesive or non-cohesive and may well be mixed vertically or
stratified by density or particle size. Composition can vary
greatly both among sites and within sites. These facts are well
known and necessitate a very thorough sampling plan to properly
characterize a particular area and document the sample
heterogeneity,
It is recognized that the surface area and organic content
of sediments often increase as the particle size decreases. In
such cases organic carbon and increased surface area produce
interdependent effects on sorption. Organic carbon is thought
to normalize about 70% of the variability in chemical
concentrations on sediments (for sediments with an organic carbon
content of 0.5% or more). Based on these observations. it is
recommended that additionalresearch be 'conducted to further
evaluate the importance of PSD In controlling or influencing
adsorption and description in combination vith organic carbon. To
date much of the published literature reports results in which
the two factors are separated and not integrated.
3*1.3 Analysis
The draft briefing document supplied by the office of Water
did not address specific procedures for the analysis of
non-ionic organic compounds in sediments or pore water.
Detailed analyses of sediment and pore water characteristics such
as dissolved organic carbon (DOC), total organic carbon (TOC), and
sediment particle sizes were not presented. While not all these
factors must be known to carry out a sediment quality criteria
calculation by the EqP method, they should be known to be
more certain that its application is appropriate.
Procedures for these analyses are reasonably well known and
understood, and have been addressed by previous documents. The
use and application of the EqP approach will depend on
high-quality data with known levels of uncertainty for the K^,
chemical concentration in the sediment, sediment organic carbon,
and sediment characteristics. J|» part of th« aevglopnont of
the approach, appropriate mothoda for nil necessary analyses need
to be assembled, criteria for uae of various procedures
defined, quality Assurance/quality control evaluations specified,
and methods for quantifying tineartaiatv in thai analysis stated.
fhe SttbeoHiiaittaa therefore rsc/T***?^?1 that tk* preparation of a
procedures and methods manual b« undertaken to address both
chemical and biological sampling.
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3.1.4 Sorption
"Sorption" is the generic and non-committal term which
encompasses both adsorption and absorption processes. The
distinction between these two processes is sometimes difficult to
demonstrate experimentally, but mechanistically they represent
two unique processes. Adsorption is an interfacial phenomenon
which results in an increase in the concentration of a sorbant in
the interfacial layer between the bulk aqueous and solid phases,
In contrast, absorption is defined as the transfer of a component
from the aqueous phase into the solid phase. The use of
partition coefficients does not readily distinguish between these
mechanistically different processes, and in practice it is likely
that adsorption and absorption occur sequentially, and that their
relative importance can have significant consequences for the
extent of sorption (Mingelgrin and Gerstl, 1§83).
The basic hypotheses related to the sorption and desorption
of non-ionic organic chemicals in the EqP methodology state that
the sediment/water partition coefficient(FL) is controlled by
the organic carbon fraction (£«) and the sediment organic
carbon/water partition coefficient (K^), and that the Koc is, in
turn, directly related by use of regression equations to the n-
octanol/water partition coefficient (K^) . The extent to which
these hypotheses operate in real world situations affects
the fidelity with which carbon normalization and partition
coefficients can reflect sorption phenomena in nature*
K_ values for a given chemical, when run under standardized
laboratory conditions are constant and reproducible. K^ values
for a given chemical tested under standardized laboratory
conditions are fairly reproducible where the organic carbon is
0,5 % or more, although the confidence limits are greater than for
Kow calculations. K,. values however, appear unreliable when
correlated with K^ Below that percentage. While sediment
organic matter appears to be the most important sorbent for non-
ionic chemicals it is not the only sorbent. Furthermore,
different types of organic carbon are lilcely to have different
potentials for partitioning, especially when their surface areas
differ (Amidon and Anik, 1980).
The partitioning approach based on the organic carbon content
of sediments does not take sorption to sedimentary components
other than organic carbon into account. It is well known that
major sedimentary constituents, such as SiO2 and complex
silicates are poor substrates for the adsorption of non-ionic
organic chemicals, because these common substrates form hydrated
ionic double layers about them that pre-empt interactions with
hydrophobia chejniie^||js. However, other sedimentary constituents,
such as sulf idea Jr £ jj%ric oxide (Fe2Q3), rutile (TiO2) and elemental
sulfur do not form hydrated ionic double layers and can readily
sorb non-ionic organic chemicals. In addition, polymers can
negate the influence of double layers (Sturom and Morgan, 1981)*
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3'1«5 Organic Carbon Normalization
Data show the organic portion of sediments to be usually less
than 10% on a dry-weight basis, but higher values .have been
reported. Experimental evidence indicates that the organic carbon
fraction of sediment is the principal sorbent for many
hazardous organic compounds, especially non-ionic chemicals;
therefore normalizing the sediment partition coefficient for the
carbon content of a particular sediment adjusts, in
principle, the coefficient to better approximate actual
environmental conditions.
In many instances sediments may be polluted with high
concentrations of natural and/or man-made hydrophobia organic
contaminants. At high concentrations, multi-phase systems exist
with multiple mass transport interfaces between the interstitial
water and sediment, interstitial water and an organic liquid
phase, and the organic liquid and the particles. In these
instances the non-aqueous material can be both a source of
soluble material to the water and another phase for sorbing
pollutants. The draft briefing document does not address this
situation, which may be common for certain kinds of pollutants and
sites (e.g., PCBs, petroleum hydrocarbons, creosote, etc.)- Several
questions should be addressed. Can the EqP method be used in
these circumstances to establish sediment criteria? Are these
situations of enough significance to warrant generation of a new
or modified methodology? Could a modification to the existing
approach incorporate a third phase? Can it be assumed that the
biota will not interact with the non-aqueous liquid and therefore
that it is not important from a toxicological perspective? is
this an instance of a very complex mixture where we have no
adequate method for assessing sediment quality?
3*3"* Particle concentration Effect
The use of the IqP approach is based on the empirical
observation that in most laboratory experiments K^ is
approximately equivalent to K^. Many of the laboratory
measurements used to estimate !£„ have been made at relatively
low suspended particle concentrations but are applied to bedded
sediments. If the sorption properties of high particle
concentration aqueous suspension or intact sediments are not the
same as the low concentration suspensions used to derive K^,
then there is a basic problem in the application of this
method. This observed particle concentration effect suggests that
some factor may cause sorption to solids to decrease as
the particle concentration increases, A question is therefore
raised as to proper ways to estimate bed sediment sorption and
desorption,
10
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Additional data exist that indicate the apparent effect of
particle concentration on sorption isotherms is an artifact of
the way the laboratory experiment is performed. The data of
Gschwend and wu (1985) suggest that apparent particle effects
are due to the fact that soluble macromolecules (humic, fulvic
acids, proteins, etc.) and colloidal particles are not removed
from the liquid phase of isotherm studies using existing
centrifugation techniques. The change in the K, is then a
function of the amount of soluble macromolecules and particles
left in liquid phase which is directly related to the
concentration of solids used in the isotherm study. These
conflicting viewpoints indicate that the mechanisms of adsorption
and desorption are not fully understood and that there is not yet
a universally accepted and applied approach for measuring
sorption. Since tho relationship batir»yp T^ and K^ is aueh am
important aspect of thu application of th« EqP approach, and since
it may impact hovfuture measurements *r«mada,the Subcommittee
recommends that additional research into particle concentration
effects be conducted.
3.1*7 Spiked versus Natural Sediment Adsorption
The scientific literature clearly indicates that there are
major differences in the sorption characteristics of
sediment-bound chemicals depending on whether the chemical was
spiked onto the sediment or occurred as a result of a release
into the natural environment. Major differences in the
desorption rates have been reported. These differences are
thought to be due to the "aging process" that occurs after
sediment and chemical contact has been wade. Laboratory
experiments support the idea that the longer a chemical has been
adsorbed to a sediment, the slower will be the overall
desorption/diffusion rate. This implies that chemicals may be
more bioavailable in toxicity studies with freshly spiked sediment
than with sediments collected from the natural environment with
the same chemical.
Much of the data generated to date to test the EqP method has
been collected using freshly spiked sediments, while this
approach is satisfactory for understanding the basic approach and
evaluating whether carbon is a reasonable normalizing factor, there
may be considerable error associated with field applications of
these data.
conducted using natural and/or aq«d apifcad sediment» to determine
the relationship betv««n duration of contact of sediment and
chemical, and gorption/deaorption proportion of th
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values..
3-2 Biological considerations
The following is a discussion of the EqP methodology's
strengths and weaknesses from a biological standpoint.
3,2.1 Dependency on WaterQuality crit«ri« and Advisories
The EqP method for generating sediment quality criteria
appropriates for non-ionic organic chemicals uses water quality
criteria (WQC) as the effects concentration in calculating the
sediment quality value. Thus the approach is dependent on the
availability of a water quality criterion or some surrogate
before it can be used. At the present time water quality
criteria have not been developed for many non-ionic organic
compounds, and the data base for water quality criteria does not
contain many chemicals with very high K^ values* In
addition, of the chemicals for which water quality criteria do
exist, only a relatively small number (approximately 30) have
been developed following the National Guidelines for the
Development of Water Quality Criteria. Thus/ of the available
water quality criteria, there are varying degrees of uncertainty
regarding their ability to protect aquatic life.
When applying the EqP method to chemicals for which no water
quality criteria exist, the user must develop surrogate
effects concentration. Such a surrogate will have additional
uncertainty associated with it and the degree of uncertainty
depends on the approach used to develop the surrogate. For
example, Water Quality Advisories might be used to derive
an effects concentration, yet all water Quality Advisories are not
equal in terms of amount and quality of data used in their
development. Thus, varying degrees of uncertainty exist
regarding their ability to protect aquatic life. It is
conceivable that an effects concentration for a chemical could
be derived from acute toxicity data by applying an application
factor, in such a case, the uncertainty of the effects
concentration used in the EqP method may be very large.
12
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Because of the critical role the estimate of an effects
concentration plays in the EqP approach for the development of
sediment quality values- and the varying degrees of uncertainty
for the various estimates of an effects concentration, tlxa
Subcommittee T«*^^i"nenda that th* developers of the EqP . roethod
include a atatamant of confidence with »ach sediment quality
value developed bv th« EqP Approach. This quality designation
should indicate the certainty or confidence attached to the
sediment quality value. Quality designations could take the fora
of descriptive statements or numerical indicators. Quality
designations should also be included as part of any sediment
criterion value, regardless of the method that is used to develop
the sediment criterion.
3.2.2 Exposure Routes^Water Versus S«dinaflta
The data presented in the IqP support document, on both
bioaccumulation factors and sediment toxieity, support the
overall contention that carbon normalization does a reasonable
job of explaining bioavailability differences among different
sediments. However, providing data on selected biota is not
equivalent to providing evidence that WQC can be used to establish
sediment quality values protective of all benthic organisms
regardless of the route of exposure.
A key question surrounding the IqP method has to do with the
method's ability to adequately protect species which are detrital
feeders, those that may not receive most of their chemical
body burden from interstitial water. Data are emerging that
indicate some benthic organisms are exposed primarily via detrital
feeding (Boese, Ii88? with hexachlorobenzene and Macoma sp.)* It
also has been shown that for certain PAHs (high pK) uptake via the
gut can exceed uptake across the gill in freshwater
amphlpods (Landrum, 1989). It is unknown at this time whether or
not there are a large number of benthic species to which this
applies, and whether or not these species are more sensitive
than other benthic organisms.
Equilibrium partitioning theory, per se, does not assume
that the interstitial water is the primary or only uptake route.
It is recognized that the thermodynamic potential for transport
to the organism (i.e., effective exposure concentration) is the
same from the sediment or the water when sediment and
interstitial water are at equilibrium. Field data generated to
date are not capable of discriminating between the two routes
of exposure. However, the bulk of the experimental data have
shown that, for many organisms, effect levels attributed to
sediment interstitial water concentration match traditional
aquatic toxieity values generated without the presence of
sediment. This does not prove the route of exposure, it only says
that the weight of evidence is that for many organisms the WQC
13
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can be used to estimate sediment effect levels. This approach is
vulnerable in that it is not known whether or not organisms which
derive their body burden primarily via the gut are affected at
the same sediment chemical concentration as those which are
affected by the interstitial water. Do water quality criteria
adequately protect these organisms?
conducted to determine vh«th«r or not vater quality values and
sediment qualitycriteria d«v«lot>«d bv th« Eap method are
protective of benthic organisms vho»« primary route of exposure
in via detrital feeding.
3.2.3 Relevance of water Quality Critoriato Batnthic Organisms
One of the basic assumptions in the development of sediment
quality criteria is that water quality criteria (or some
surrogate) are adequate estimates of the effects concentration
for benthic organisms. The developers of the Eqp approach have
attempted to validate this assumption by analyiing the relative
sensitivity of marine water column species and benthic species by
segregating the acute toxicity data base for 30 chemicals for
which water quality criteria exist* In addition, data from
benthic colonization experiments for six chemicals were examined.
The existing method of data analysis relies on the use of
pooled data and comparisons of pereentile ranges. This approach
does not allow review of data for specific species. The
marina and fresbvat+r WQC also be reviewed to
determine thai two or three most aenaitiv* benthic and vater
column species in th«dataset. The data used for the species
sensitivity comparisons should be based on studies with animals:
a) at a similar life stage, b) tested under similar conditions
(e.g. hardness, pH, temperature), and c) exposed for the same
period of time. A series of sensitivity ratios could then be
developed by dividing the acute values for a benthic species into
the acute value for a water column organism for the same chemical.
This could help determine if benthic species are as acutely
sensitive, on the average, as certain water column organisms*
To date only acute data for benthic and water column marine
species has been analyzed. It is important to determine if
benthic and water column species have similar chronic
sensitivities to chemicals, since chronic toxicity data are
essential to the development of WQC. The subcommittee
recommends that the d«v«lop»ra of th« BqP approach expand the
analysis of the Wafrer Quality criteria data base toinclude a
broader range of' ap«ci«a, including macrophytea and deposit
faadera, and include considerations and comparisons of both
chronic and acute toxicitv data.
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3.3 Inappropriate use of Water Qualitycriteria (WQO to
Determine Effect Ldvals in th«BaP Approach
The subcommittee is concerned that the water quality
criteria documents could be used inappropriately to determine
effect levels for the EqP approach. For instance, some of the WQC
are based on residue considerations, i.e., PCBs. It would be
inappropriate to use the overall WQC in these cases for deriving
sediment quality values. The residue derived WQC are usually
estimated based on measured bioconcentration coefficients.
Potential user of WQC should not accept these numbers directly
without careful reference to the technical support documentation
for the WQC to establish the limits of applicability of values
that have been selected.
When the expected residue concentrations are to be inferred from
the concentrations of contaminants in sediments using the EqP
method (or any other method), then the additional extrapolation
of sediment-water-biota will introduce an additional uncertainty,
making the entire process even less reliable. Use of the data in
the WQC document that pertains to chronic effect levels would be
more appropriate, such as the final chronic value (FCV). The
Subcommittee recommends that vary caraful uaa of tha WQC
documenta be made and that guidelines be prepared for using WQC
and vater quality advisoryvalues inth» calculation of a sediment
quality values by the EaP approach.
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4'° SUMMARY OF SUBCOMMITTEE CONCLUSIONS AUD RECOMMENDATIONS
a. The Subcommittee recommends that the assumptions used in
the EqP approach, including assumptions of relatively ideal
conditions and equilibrium, be investigated to better characterize
the uncertainty that may exist around them.
b. It is suggested that a more sophisticated uncertainty
analysis be carried out for the IqP approach to set limits/bounds
on its applicability, A more rigorous approach would be to
perform uncertainty analyses on both the water quality criteria
and the partition coefficients, K^. and K^ to yield a combined
uncertainty on the sediment quality values.
c. It is recommended that additional research be conducted to
further evaluate the importance of particle size distribution in
sediments in controlling or influencing sorption and desorption
on mineral soil particles in combination with organic carbon. It
is recommended that various inorganic sediment constituents such
as Sio2 and sulfides and various forms of organic carbon be
investigated to determine the extent that they influence the
sorption and desorption of non-ionic chemicals.
d. As part of the development of the approach, the
appropriate" methods for all the necessary analyses need to be
assembled, criteria for use of various procedures defined,
quality assurance/quality control evaluations specified, and
methods for quantifying uncertainty in the analysis stated. The
Subcommittee therefore recommends that the preparation of a
procedures and methods manual be undertaken to address both
chemical and biological sampling.
e. Since the relationship between Kx and K^ is such an
important aspect of the application of the EqP approach, and since
it may impact how future measurements are made, the Subcommittee
recommends that additional research into particle concentration
effects be conducted,
f. The Subcommittee recommends that validation experiments be
conducted using natural and/or aged spiked sediments to
determine the relationship between duration of contact of sediment
and chemical, and sorption/desorption properties of the chemical,
sediment, and biological effects. It is also recommended that
consideration be given to the use of aged sediment "bound
chemical desorption partition coefficients in deriving sediment
quality values.
g. The Subcommittee recommends that the developers of the IqP
method include a statement of confidence with each sediment
quality value developed by the IqP Approach.
16
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h. The Subcommittee recommends that further research be
conducted to determine whether or not water quality criteria and
sediment quality value developed by the EgP method are
protective of bentliie organisms whose primary route of exposure
may be via detrital feeding.
i. The Subcommittee recommends that the species sensitivity
data (30 marine and freshwater WQC documents) also be reviewed
to determine the two or three most sensitive benthic and water
column species in the data set.
j. The Subcommittee recommends that the developers of the EqP
approach expand the analysis of Water Quality Criteria data base
to include a broader range of species, include considerations and
comparisons of both chronic and acute toxicity data.
k. The Subcommittee recommends that very careful use of the WQC
documents be made and that guidelines be prepared for using WQC
values in the calculation of a sediment quality values by the EqP
approach.
1« The Subcommittee recommends that field experiments be
conducted to validate the EqP approach.
17
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5*0 References
Adams, W.J., R.A. Kimerle and R.G. Mosher. (1982) An approach for
assessing the environmental safety of chemical sorbed to
sediments. In: Aquatic Toxicology and Hazard Assessment: 7th
Volume, ASTM STP 854, R.D. Cardwell, R, Purdy, and R.C. Bahner,
Eds, American Society for Testing and Materials, Philadelphia,
pp. 429-453.
Amidon, G.L. and S.T. Anik. (1980) Hydrophobieity of polycyclic
aromatic compounds . Thermodynamic partitioning analysis.
J. Phvs. Chem. 84 : 970-974%
Boese, B.L. et al. (in press) Comparison of aqueous and solid phase
uptake for Hexachlorobenzene in the tellinid clam, Macoma nasuta
(Conrad) t A mass balance approach. Environ. Toxi col. Chem..
DiToro, D.M. and L.M. Horzempa, (1982) Reversible and Resistant
components of PCB adsorption-desorption isotherms. Environ ,
Sci. Techn. 16(9) ;594~602.
Gschwend, P.M. and S-C Wu. (1985) On the constancy of sediment-
water partition coefficients of hydrophobia organic
pollutants* Environ . Sci . Techn . 19:90-96.
Karickhofff S.w. and K.R, Morris. (1985) Sorption dynamics of
hydrophobic pollutants in sediment suspensions. Environ.
ToxicQl . Chem. ,4:469-479.
Klump, et al. (1987) Dual tracer studies of the accumulation of
an organic contaminant from sediments by deposit feeding
oligochaetes, J^_ ............. Fish.. Aguat . ............. _Sci . JJ.J 1574-1583.
Lake, J.L. , N.R. Rubenstein and S. Pavignano. (1987) Predicting
bioaccumulation: development of a simple partitioning model for
use as a screening tool for regulating ocean disposal. Im Fate
and Effects of Sediment-Bound Chemicals in Aquatic Systems « K.L.
DicKson, A.W. Maki and W.A. Brungs. Eds. SETAC Special
Publication Series, Society of Toxicology and Chemistry,
Washington, D.C*, pp. 151-166,
Landrum, P.F. and D. Scavia. (1983) Influence of sediment on
Anthracene uptake, depuration, and biotransformation in the
amphipod Hvalella azteca. Can. J. Fish. Aquat. Sci. 40:298-305.
Landrum, P.F., W.R. Faust and B.J. Eadie. (1989) Bioavailability
and toxicity of a mixture of sediment associated chlorinated
hydrocarbons to the amphipod, Pontoporeia hov_i. In; Aquatic
Toxicology and Hazard Assessment 12th Volume, ASTM STP 1027 U.K.
Cargill and L.R. Williams. Eds. American Society for Testing and
Materials, Philadelphia* pp. 315-329.
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Landrum, P.F, (1989) Bioava liability and toxieoJcinetics of
polycyclic aromatic hydrocarbons sorbed to sediments for the
amphipod, Fontoporeia hoyi. Environ. Sci. Techn. 23.;588-595.
Mingelgrin, U. and z. Gerstl. (1983) Reevaluation of partitioning
as a mechanism of nonionic chemicals adsorption in soils, i-.
Environ. Oual. 12(l):l-ll.
National Research Council. (1988) Complex Mixtures; Methods for
invivo toxicity testing. National Academy Press* Washington, D.C.
National Research Council, (1989). Drinking water and health.
Volume IX. Part II. Selected issues in risk assessment. National
Academy Press. Washington, D.C,
Nebeker, A.V., G.S, Schuytema, W.L. Griffis, J»A. Barbitta and
L.A. Carey. (1989) Effects of sediment organic carbon on
survival of Hyalella azteca exposed to DDT and endrin*
Environ. Toxicol, Chero. 8_(8) :7Q5-718.
Oliver, B.C. (1987) Biouptake of chlorinated hydrocarbons from
laboratory-spiked and field sediments by oligochaetes. Environ.
Sci. Techn. 21;785-790.
Oliver, B.G. (1984) Uptake of chlorinated organics from
anthropogenically contaminated sediments by oligochaete worms.
Can. J> Fish. A_qiiat, Sci. 4^:878-883.
Science Advisory Board. (1989). Preliminary review of misture
issues relating to some Phase II drinking water regulations of the
Office of Drinking Water. EPA-SAB-IHC-89-036.
Stumm, W. and J.J. Morgan (1981) Aquatic Cheatistry. Chapt. 10; The
Solid-Solution Interface. J. Wiley & sons, New York.
Swartz, R.C., D,W. Schults, T.H, Dewitt, G.R. Ditsworth and J.O.
Lamberson. (1987) Toxicity of fluoranthene in sediment to
marine amphipods: a test of the equilibrium partitioning
approach to sediment quality criteria. Presented at the 8th
Annual Meeting; Society of Toxicology and Chemistry,
Pensacola, Pla.
Witkowski, et al. (1988) Sorption and desorption dynamics of
Arachlor 1242 to Natural Sediments. J * Contaain^ Hydro!.
2:249-269.
Word, et al. (1987) Evaluation of the equilibrium partitioning
theory for est.l.S^.tr&sg the toxicity of the nonpolar organic compound
DDT, to the sediment dwelling amphipod Rhepoxynius abrpnius*
Sediment Criteria Document ill USEPA Office of Water Regulations
and Standards, Washington, D.C.
19
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APPENDIX
Briefing Report to the EPA Science Advisory Board on the
The Equilibrium Partitioning Approach
20
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DRAFT
BRIEFING REPORT
to the
EPA SCIENCE ADVISORY BOARD
on the
EQUILIBRIUM PARTITIONING APPROACH
TO GENERATING SEDIMENT QUALITY CRITERIA
December 1988
U.S. ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF MATER
OFFICE OF WATER REGULATIONS AND STANDARDS
CRITERIA AND STANDARDS DIVISION
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Fag* 1
SUMMARY
This report has been prepared to assist the EPA Science Advisory Soard wish
its evaluation of the Equilibrium Partitioning method far generating sedisusnc
quality criteria. Sediment quality criteria as used in this report refer to
numerical values for individual chemicals that ar* applicable across the range
of sediments encountered in practis*. They ar* intended to b* predictive af
biological effects and protective of the prtsene* and uses of benchic
organisms. As a consequence they could b*.ased in ouch the same way as water
quality criteria * as the concentration of a chemical which is protective of
th« intended use,
The specific regulatory uses of sediment quality criteria have not been
established. However, the range of potential application* is quite large since
the need for the evaluation of potentially contaminated sediments arises in
many contexts. Sediment quality criteria ar* not meant to replace direct
toxieity testing of sediments as a method of evaluation, but rather to provide
a chemical by chemical specification of what sediment concentrations would be
protective of aquatic life and their uses.
TOXICITY AND IIOAVAllABILITY OF CHEMICALS IN SEDIMSTTS
The principal technical difficulty that oust b* overcome in establishing
sedinent quality criteria is to determine the *xtent of bioavallability of
sediment associated chemicals. It has frequently been observed that similar
concentrations of a chemical in units of mass of chemical per mass of sediment
dry weight (e.g. j*g eh«mie*l/g s*dl«enc) can produce widely different
biological effects In different sediments. If the purpose of sediment quality
criteria la to establish chemical concentrations that apply across sediments of
differing types it is essential that the reasons for this varying
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Page 2
bioavailability be understood and that they be explicitly included in the
criteria. Otherwise the criteria cannot be presumed to b« applicable across
sediments of differing properties.
The importance of this issue cannot be underestimated. For example, if L
ppm of kapone is the LCjQ for an organisa in on* sediaent and 40 ppa is che
LCgo in another sediment, then unless the cause of this diff stance can be
associated with some explicit sediment properties it is not possible to decide
what the LCjQ would be of a third sediment without a direct toxicity test.
An additional difficulty is rhat the' results of toxic ley cests used to
establish che toxicicy of chemicals in s«dim«ncs would net be generalizmble co
other sedimencs. Imagine the situation if the results of coxicity tests in
vacer depended strongly on che particular water source - e.g.. Lake Superior
versus well water. Until the source of the differences were understood, i~
would be fruitless te attempt to establish water quality criteria. It is for
this reason that the issue of bioavai lability is a principal focus of chis
report.
Th* observation which provided eh« key insight co the problem of
quantifying the bioavailability of chtaieals tn sediments was chat the
concentration- response curve for the biological effect ef concern could be
correlated not to the total sediment chemical concentration (MS cheaical/g
sediaent) but to the interstitial water (i.e., pore water) concentration (Mg
chemical/liter pore water). Organisa mortality, growth rate, and
bioaecunulation data were used to demonstrate this correlation, which is a.
critical part of the logic behind che equilibrium partitioning approach co
developing sediment quality criteria. A substantial amount of data is
presented in the report co Illustrate the generality of this finding (Sections
3,1 through 3.3).
This correlation can be interpreted in a number of ways. In particular i:
is premature to conclude that the route of exposure for the organisa is onl:
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t
I
I
I
I
I
I
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Page 3
via the pore water. The reason is that the solid phase is in equilibrium with
the liquid phase and ch* effective exposure concentration is likely to b* the
saae via either roue*. However from * purely empirical point of view the
correlation suggests that if it were possible to either (1) measure che pore
water chemical concentration or (2) predict it from the total sediment:
concentration and the relevant sediment properties, then that concentration
could be used to quantify the exposure concentration for *n organism. Thus,
the partitioning of chemicals between th« solid and the liquid phase In a
sediment becomes a necessary component of sediment quality criteria. It is for
this reason that the methodology is called the equilibrium partitioning (£?)
method.
In addition, if it vere true that benthie organisms are as sensitive as
water column organisms - and as shown in Section 5 the evidence appears to
support this supposition * then, a sediment quality criteria could be
established using the water quality criteria, cyqc, as the effects
concentration, and th« partition coefficient,, Kp, to relate the pore water
concentration to the sediment quality criteria concentration, rsqc vi
partitioning equation. The calculation procedure is as follows. If
(jig/L) is the water quality criteria for the cheaieal of interest, then the
sediment quality criteria, rgqc C/ngAg sediment) is computed using the
partition coefficient, Kp (LAg sediaenc) between sediment and water;
rSQC
This is the fundamental equation from which sediment quality criteria are
generated. Its utility depends upon the existence of a methodology for
quantifying partition,coefficients.
PARTITIONING OF SOU-IONIC ORGANIC CHEHICALS
The partitioning of non-ionic organic chemicals between particles and water
is reasonably well understood and a standard model exists for describing che
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?age 4
CSQC
foc
If u* d*fin«:
I
I
process. The chemical propercy of importance is cha octanol-Bracer parti-ion _
coefficient, K^w. the important particle property is the ntas,s fraction of p
organic carbon, foc. For particles with foc > 0,51 th* organic carbon appears
Co b* the predoninant sorpeion phase, The partition coefficient, Kp, th* ratio •
of s*di»ent to pore water concentration is given by:
K - f K
p oc oc
I
wh«re See is th* partition coefficient for particle organic carbon.
The only other environmental variable that has a dramatic effect on
partitioning appears to b* Che particle concentration itself. There is I
considerable controversy regarding th* aechanisa responsible for th* particle
concentration effect and a number of explanation* have been offered. However, •
all th* interpretations yield the saa* result for s*diaent*pore water •
partitioning, namely that K^e - KQV for sediments. Hence sediment quality
criteria are calculated from:
tSQG
I
This equation is linear In ch* organic carbon fraction, foc. As a consequence, *
the relationship can b* expressed as: .
rSQC,OC * f
oc
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Page 5
aa the organic carbon normalized sediment concentration (pig chemical/kg organic
carbon) then:
rSQC,QC
Hence we arrive at the following important conclusion: for a specific chemical,
with a specific KOVl the organic carbon normalized total sediment concentration
is proportional to the dissolved free effects concentration, . eyqc, for any
sediment for foc > 0.5X.
Hydrophobic chemicals tend to partition to colloidal sized organic carbon
particles (DOC) .as well. A1chough DOC *ffaces the apparent pore water
concentracions of highly hydrophobic chemicals the DOC-bound chemical appears
not to be bioavaitable and the above equation still applies (Sections 4.2
through 4.4). The available field data for sediment partitioning is relieved
and related to the model presented abova.
The above discussion suggests that toxicicy and bioaccumulation data for
sediments should be normalized by the sediment organic carbon concentration. It
is found that responses which are quite variable on a dry weight normalised
basis are either statistically equivalent or the differences are significantly
reduced on an organic carbon basis. The low carbon sediments are seen to
depart from the normalized results as is expected (Section 4.6),
FIELD VALISATIOH OF SEOIMBJT QUALITY CRITEIIA
The most convincing demonstration that sediment quality criteria are sound
would be a demonstration that they can predict che degree of toxic icy of
natural sediments. There are three technical difficulties that apply to all
field data based approaches; bioavailabiliey, cheaical mixtures and control
sediments.
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Page S
B toava, i,labiJ. t tg
Contaminated sediaenc* contain many chemicals. la order to use the
magnitude of the chemical concentration «s a measure of its potential to
have biological effects, it is necessary that its bioavailability in chac
particular sediment b« assessed, For toxic metals and ionic organic
chemicals there is as yec no comprehensive partitioning theory thac
id*ntifi«s the normalization quantities and provides the parameters Co
predict fr«« dissolved concentration. H*nc« btoavailmbility cannot b*
directly ass«Aa«d.
Hiieurea and Causa Ittr
If the bioavmilability problem vert solv*4 ch*re remains a difficulty vith
using naturally contaminated sediments. Just a* wish water quality criteria it
is always possible that, there is present another cheaical or chemicals thac are
biologically very active but, which h«v« yet Co b« identified. If this
cheaical is the cause of significant coxicity chen it would cause a biological
effect chat would not be predicted froa the application of sediment quality
criteria.
Control Sedtaenta and Non Toxic
\
i
Variations in sediment coxicity test results and community structure
can be caused by variations in sediment characteristics other than
cheaical contamination. Grain six* distribution and organic carbon I
content ar* well known examples. In order to judge the toxicity of a sediment
it is necessary that a comparative control response be obtained. The perfect I
control is the aante sediment without any chemical contamination. Sine* this is '
not available, sediments fraa an unimpacted site are assumed to approximate the •
response of the perfect control. The degree to which this approximation is I
inappropriate limits the assessment of comparative toxicity.
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Page 7
• *
These three major difficulties 'appear to render a direct figId validation
of sediment criteria beyond current capabilities, Nevertheless ic would be
helpful if some evidence thac criteria developed from laboratory coxicological
data are at least reasonable, A methodology is presented that can b* used to
established lower bounds for sediment quality critaria from field observations
of organism presence and sediment chemistry.
EFFECTS CONCENTRATION
The other principal assumption in the development of sediment quality
criteria is that the water quality criteria are adequate estimates of the
affects concentrations for benthic organisms. Two sets of analyses are
pr*senc«d to examine this question. The acute toxicity data bas« from the
water quality criteria are segregated into benthic and water column species and
th* relative sensitivity of each group are compared for the 30 water quality
criteria chemicals. In addition benthic colonization experiments for six
chemicals ar« examined.
The conclusion from this examination is that the sensitivities of benthic
species ar* sufficiently similar to those of water column species to
tentatively permit the use of watar quality criteria for th« derivation of
sediment quality criteria in the equilibrium partitioning approach. Th* acute
toxicity database derived from 30 water quality criteria documents suggests
that the most sensitive infaunal species is typically less sensitive than the
most sensitive water column (epibenthic and water column) species. When both
infauna and epibenthie specie* are classed as "benthic*, the sensitivities of
the most sensitive benthie and water column species are on th* average similar
(Section 5).
UNCERTAINTY
The s«dim«nt quality criteria methodology employed above relies on an
empirical modal to compute the free interstitial water concentration from the
solid phase measurements. As a consequence there it an uncertainty associated
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with the us* of the model- In addition there Is uncertainty with rtspect to
the Kav Associated vith the specific chemical sine* it is an experimentally
determined quantity. Finally th* assumption that water eoluati and bunthic
organisms have siailar sensitivities has a level of uncertainty.
Th* quantification of the level of uncertainty for sediment quality
criteria has only b««n accomplished in a preliminary way (Section 7.1), It is
antic If ated that a complete uncertainty analysis will accompany a sediment
quality criteria and that, for example, 951 confidence limits will be specified
as w*ll as the most probable value,
PRELIMINARY SEDIMENT QUALITY CRITERIA '
An initial attempt to compute equilibrium partitioning based sediment
quality criteria for 13 chemical* is presented in Section 7.2. The 951
confidence limits are computed from a method vhich is known to exaggerate the
uncertainty. For chemicals where either field data derived lower bounds or
sediment toxicity experiments are available th* results art reasonable.
TOXIC METALS
The rationale for establishing sediment quality criteria for toxic aetais
is siailar to that developed for non-ionic organic chemicals, the bioavailable
fraction is identified and a partitioning modal will be investigated in order
to predict the bioavailable fraction, Hater column experiments point to the
fact that biological effects can be correlated to the divalent metal activity
(Me-*). The implication to be drawn from chase experiments is that the
partitioning model required for establishing sediment quality criteria should
predict {Me^+j in th* pore water (Section 6.1 - S.2).
METAL SORfTION MODELS AJID EXTRACTIONS
The state of the art, of modeling metal sorption in laboratory systems is
well developed. Models for natural soil and sediment particles are less well
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However, recent applications suggest chat similar models can be
applied co soil systems, An approach Is presented which envisions three
sorpcion phases in aerobic sediments (Section 6.3).
In addition to the sorpcion phase concentrations it is necessary to
quantify the fraction of total sediment metal that is chemically interacting
with che pore water. A substantial effort is needed over several years to
determine the bioavailable portion of trace metals in soils and sediments using
chemical extractions. Initial results are reviewed and preliminary directions
are suggested {Section 6.4).
CONCLUSION
Methodologies are being developed to establish sediment quality criteria
using sediment equilibrium partitioning. The importance of bioavailability and
the role of partitioning between sediment and pore water is clarified. The
effects concentration for benthic organisms can be estimated from the water
quality criteria. For non*ionic organic chemicals an adequate partitioning
model exists and is presented in this document, AM a result sediment quality
criteria can be computed. For metals initial studies indicate that the same
rationale can be used. The development of sediment criteria for aetal
contaminants using equilibrium partitioning is the focus of future sediment
criteria development activities.
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SECTION 1.
IHtlODUCTIOH
Under the Clean Wat«r Act (CUA) the Environmental Protection Agency (E?A)
is responsible for protecting the chemical, physical, and biological integrity
of th« Nation's waters. In keeping with this responsibility, E?A has published
ambient water quality criteria (WQC) in 1980 for 64 of the S3 priority
pollutants and pollutant categories listed a* toxic in the OTA, Additional
water quality documents that update criteria foe selected consent deer**
chemicals and new criteria have also been published line* 1980. These water
quality crit«ria ar* numerical concentration liaita that are protective of
human health and aquatic life. While these criteria play an important role in
assuring a healthy aquatic environment, they alone- ar* not sufficient to ensure
appropriate levels of environmental and human health protection.
Toxic contaminants in bottom sediments of the Nation's lakes, rivers, wet
lands, and coastal water* create the potential for continued environmental
degradation even where water-column contaminant levels comply vith established
water quality criteria. In addition, contaminated sediments can lead to water
quality degradation, even when pollutant sources ate stopped. The absence of
defensible sediment quality criteria makes it difficult to accurately assess
the extent of the contaminated sediment problem and to identify and implement
appropriate remediation activities when needed. A* a result of the need for a
procedure to assist regulatory agencies in making decisions concerning
contaminated sediment problems, a EfA Office of Uater Regulations and
Standards, Criteria and Standards Division (QWRS/CSD) research team was
established to review alternative approaches. Each approach had both strengths
and weaknesses and no single approach was found to be most applicable in all
situations. The equilibrium partitioning method w« selected, because it
appeared to provide the most practical and effective regulatory tool for
addressing contaminated sediments on a national basis. The three principal
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observations chat underlay the equilibrium partitioning method of establishing
sediment quality criteria arc:
1, 'for sediaenc dwelling organisms, the pore water concentration of 3
chemical correlates to observed biological effects across a range of
sediaencs,
2. eh* range of sensitivities of benthle organisms co chemicals are
similar Co wacer coluisn organism* so chat eh* currencly established
wattr quality criteria can b* used to define acceptable por* water
levels; and,
3. partitioning models which relat* pore water concentrations to bulk
s«dia*nt concentrations alther exist (for non-ionic organic chemicals)
or can be developed (for toxic metals and, perhaps, for ionic organic
chemicals).
The data that support these observations will b« examined in subsequent
sections of this report.
Sediment quality criteria generated using the equilibrium partitioning
method, are suitable for use in providing guidance to regulatory agencies
because they are;
1. Numerical values
2, Chemical specific
3. Applicable across sediments
4. Predictive of biological effects
5, Protective of the presence and uses of benthic organisms
As Is the ease with vater quality criteria, the sediment quality criteria
reflect the use of available scientific data to: (1) assess the likelihood of
significant environmental effects frott contaminants in sediments and to (2)
derive regulatory requirements which will protect against these effects.
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Over the past several years research activities have focused on the
•valuation and development , of the equilibrium partitioning methodology for
generating sediment quality criteria for use in providing guidance to
regulatory agencies. Ic is the purpose of this report; to describe results that
support the equilibrium partitioning method for establishing sediment quality
criteria. This report is structured in the following way:
The historical framework and statutory basis for developing sediment
quality criteria are discussed in Section 2. Toxicitv and bioavailability of
chemicals in sediments art discussed in Section 3 where th« importance of pore
water concentration is established, this leads so a discussion of partitioning
behavior of chemicals and their division into two major classes: non-ionic
organic chemicals and metals, for which partitioning models have been proposed.
Non-tonic organic chemicals art discussed in Section 4, Sections 4,1
through 4,5 concentrate on partitioning and she role of particulate and
dissolved organic carbon. The models available to evaluate the partitioning of
chemicals in sediments are presented in Section 4.1 for particle suspensions
and Sections 4.2 through 4.4 for tn-place sediments, including a discussion of
'he effect; of DOC completing. Field data, related to partitioning in
sediments, are analyzed In Section 4.5. The results of organic carbon
normalization of toxicity and bioaccujoulatton experiments are presented in
Section 4.6, The issue of pore water versus sediment as the route of exposure
is addressed in Section 4.7. This section concludes with a review of the field
validation of sediment criteria in Section 4,8, where a screening level
methodology is presented.
The applicability of using water quality criteria for the effects
concentration in sediaents is examined in Section 5. 'A discussion of the
overall similarity of the sensitivities of benthic and water column species is
included in this section.
Section 6 reviews the current status of sediment quality criteria
development efforts related to toxic metals. The difficulties In using pore
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water metal concentration directly ar« discussed in Section 6,1, this leads co
3 discussion of the data demonstrating the correlation of toxicity Co divalent
m*tal activity which is presented in S*ecion 6,2. 'Th* scat*-of-the-art of
metal sorptien models is discuss*d in Sec'ton 6.3. th* suitability of
extraction methodologies to quantify th* bio*vailabl« fraction is exaained in
Seccioti €.4, The remainder of Section 6 describes the Initial approach*! that
are being pursued in order to establish s«dio*nr metal criteria.
Finally, Section 7 describes che generation of interim sediment quality
criteria for non-ionic organic chemicals, th* uncertainty associated vieh the
criteria Is discussed (Section 7,1} and preliminary valu*a ar* presented
(Section 7.2).
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