United States Office of EPA 520/1-87-011
Environmental Protection Radiation Programs June 1988
Agency Washington, DC 20460
Radiation
&EPA Sediment Monitoring
i
Parameters and Rationale
for Characterizing
Deep-Ocean Low-Level
Radioactive Waste
Disposal Sites
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EPA 520/1-87-011
Sediment Monitoring Parameters and Rationale
for Characterizing
Deep Ocean Low-Level Radioactive Waste Disposal Sites
by
James Neiheisel
June 1988
Office of Radiation Programs
U.S. Environmental Protection Agency
Washington, DC 20460
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FOREWORD
The Environmental Protection Agency (EPA) was given a
Congressional mandate to develop criteria, standards, and
regulations governing the ocean disposal of all forms of wastes
pursuant to Public Law 92-532, the Marine Protection, Research,
and Sanctuaries Act of 1972. Within this Congressional
mandate, EPA has initiated a specific program to develop
regulations and criteria to control the ocean disposal of
low-level radioactive wastes (LLW).
The EPA Office of Radiation Programs (ORP) has conducted
feasibility studies to determine whether current technologies
could be applied toward determining the fate of radioactive
wastes dumped in the past. After successfully locating
radioactive waste containers in three of the primary LLW
disposal sites previously used by the United States, ORP
initiated disposal site characterization studies to: (1)
determine the biological, chemical, and physical parameters,
(2) identify and ascertain the distribution of radionuclides
within the sites, and (3) evaluate previous packaging
techniques and materials.
The purpose of this document is to provide a rationale for
the sediment measurements that will indicate radionuclide
retention characteristics at potential LLW disposal sites in
the deep-ocean environment. Prior to sediment sampling, the
site selection shall have been directed to specific areas by
site selection criteria supported by geophysical data to
indicate site stability- Sediment sampling is conducted to
meet the sediment monitoring and criteria needs for site
characterization.
The Agency invites all readers of this report to send any
comments or suggestions to Mr. David E. Janes, Director,
Analysis and Support Division, Office of Radiation Programs
(ANR-461), Washington, DC 20460.
[ichata1 J. GuimorftJ, Director
Office of Rad/iation Programs
111
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ACKNOWLEDGMENTS
The assistance of Dr. James Booth of the U.S. Geological
Survey, Branch of Atlantic Marine Geology, Woods Hole,
Massachusetts, and his staff in conducting mineralogical and
geotechnical measurements, and guidance in specific testing
needs is gratefully acknowledged. Appreciation is also
extended to Dr. Mark Fuhrmann and Mr. Peter Colombo of
Brookhaven National Laboratory and Dr. Fred Sayles of Woods
Hole Oceanographic Institution for discussions regarding the
capabilities and limitations of the Kd method and Eh meter
method for sediment redox determinations. The comparison of
mineralogy techniques and textural methods by Mr. Ray
Willingham of the U.S. Army Corps of Engineers, South Atlantic
Division Laboratory on sediment samples from the Atlantic and
Pacific low-level radioactive waste disposal sites is also
appreciated.
The critical review and comments provided by the special
interagency Technical Subcommittee on Ocean Disposal of
Low-Level Radioactive Waste is gratefully acknowledged and
appreciated. The comments by Dr. William Forster of the
Department of Energy and Drs. Larry Brush, Rip Anderson and Mel
Marietta of Sandia National Laboratories were especially
valuable in the review and final preparation of this document,
and their contribution is acknowledged with special
apprec iation.
The author also wishes to thank Mr. Robert S. Dyer,
Mr. William R. Curtis and Ms. Marilyn Varela of this Office for
critical review of this report, and for the many helpful
suggestions and discussions during its preparation. In
addition, the typing assistance provide by Ms. Phoebe Suber is
also gratefully acknowledged.
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TABLE OF CONTENTS
Page
Foreword i11
Acknowledgments v
1.0 Sediment Monitoring Parameters and Rationale 1
1.1 Introduction 1
1.2 Special Considerations for Low-Level Radioactive Wastes .. 2
1.3 Parameters Already in the Ocean Disposal Regulations 4
1.3.1 Organic Carbon 5
1.3.2 Sediment Texture Grain Size 5
1.3.3 Recommendations 7
1.3,4 Mineral Composition of Sediment 7
1.3.5 Recommendations 8
1.4 Consideration of Parameters Not In Existing Regulations .. 9
1.4.1 Sorption Distribution Coefficient (Kd) 9
1.4.2 Recommendat ions 10
1.4.3 Sediment Redox Considerations 11
1.4.4 Recommendations 14
1.4.5 Determination of pH 15
1.4.6 Recommendations 15
1.5 Geotechnical Parameters 15
1.5.1 Recommendations 16
VII
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TABLE OF CONTENTS (Continued)
Page
1.6 X-radiographs 16
1.6.1 Recommendations 16
1.7 Correlation with Geological Stability 16
1.7.1 Recommendations 17
1.8 Sampling Density 17
1.9 Summary of Recommendations 18
1.10 Future Considerations:
Implementation of Section 424, Public Law 97-424 .. 19
References 21
Vlll
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1.0 SEDIMENT MONITORING PARAMETERS AND RATIONALE
1.1 INTRODUCTION
The purpose of this document is to discuss the need and
present the rationale for special sediment monitoring
parameters and criteria to characterize LLW disposal sites
meeting the International Atomic Energy Agency (IAEA)
recommended minimum disposal depth of 4000m. The IAEA disposal
depth limitation was developed pursuant to the London Dumping
Convention, to which the United States is a signatory.
Sediment monitoring requirements for ocean disposal of all
wastes are contained in Part 228 of the current Ocean Disposal
Regulations (42 FR 2462, January 11, 1977). Part 228 contains
general requirements and does not specifically address sediment
monitoring requirements for disposal of packaged LLW. Part 228
does acknowledge, however, that there may be special
requirements for deep-sea monitoring. Paragraph
228.13(e)(3)(ii) states that "additional parameters may be
selected based on the materials likely to be in the wastes
dumped at the site." The sediment measurement parameters
required for all wastes are: organic carbon, texture, particle
size distribution, major mineral constituents, and settling
rate. For LLW, all of these parameters, with the exception of
settling rate, will be used. Some modifications of methods to
characterize sediments are necessary because of the sediment
conditions in the deep ocean and the nature of the wastes.
This document recommends additional baseline sediment
monitoring parameters and criteria to address the special
considerations applicable to ocean disposal of packaged LLW.
The existing EPA waste package performance criteria require
that LLW must radiodecay to environmentally acceptable levels
within the expected immobilization period of the waste package
and the surrounding sediment. Using a multibarrier approach
for containment of the LLW at the site, the engineered waste
package represents the primary barrier. The natural retention
afforded by the physicochemical characteristics of deep ocean
sediments constitutes an additional barrier. Some of these
factors can be measured. Estimates of the ability of the
physicochemical environment to increase radionuclide retention
in the sediment, or to restrict radionuclide movement to the
water column after release from a waste package, are considered
as part of the sediment monitoring recommendations.
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This document addresses geotechnical parameters which may
be used to estimate the extent of penetration of a free-falling
waste package into the sediment and to provide baseline data
for evaluating the option of subseabed disposal of LLW. The
correlative value of sediment monitoring criteria to (a)
geologic stability of the disposal site and (b) interactions of
the sediment and its environment with the waste package is
presented. It also presents a discussion of specific sediment
parameters applicable to a deep-ocean LLW disposal site.
Numerical limits are proposed where applicable. A Methods
Manual, with recommended testing procedures, will be provided
as a separate document (EPA 520/1-87-011, June 1988).
The Methods Manual addresses the use of geophysical
methods including bathymetric and side-scan sonar techniques to
determine the geologic stability of a candidate site prior to
sediment monitoring. It was developed to provide details of
the recommended sediment testing techniques for the sediment
monitoring parameters discussed in this report. It is an
important ancillary supplement to the sediment monitoring
parameters and criteria. It is intended to assure uniformity
in analyses of sediment samples and in reporting data.
1.2 SPECIAL CONSIDERATIONS FOR LOW-LEVEL RADIOACTIVE WASTE
The present ocean disposal regulations look to the
engineering of the waste package (the container and the
solidification agent containing the waste) as the primary
barrier to prevent direct dispersion of any LLW into ocean
waters. In a multibarrier protection system, the sediment
constitutes an additional natural barrier to the migration of
radionuclides if the waste container has been breached by
natural or accidental causes. The sediment can only be an
effective barrier, however, if the waste container is in
contact with the sediment. Release of radionuclides from any
seawater-exposed area of the container could "short circuit"
the sediment sorption mechanism by moving radionuclides
directly into the water column. However, some scavenging of
radionuclides by suspended particulates in the benthic boundary
layer will probably occur. Biological activity of organisms
living in or on the sediment (bioturbation) could also "short
circuit" radionuclide retention by increasing the rate of
dispersion; however, this activity can also carry radionuclides
deeper into the sediment and thereby provide greater
immobility. Any site selected for LLW should have
comparatively low biological activity.
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A waste container in contact with deepsea sediment creates
an environment where information on the sediment composition
and the oxidation-reduction (redox) state of the sediment may
be used to predict both the time of release of waste from a
container and the potential for immobilization of some of the
radionuclides upon their release to the sediment environment.
The redox environment immediately adjacent to and beneath a
waste container, however, is altered by the weight of the
container on the sediment. Therefore, knowledge of the
geotechnical properties (e.g. compressive strength) of the
sediment and sediment surface will facilitate assessment of
this effect and will allow prediction of the depth of
penetration of the waste package into the sediment upon impact
at terminal velocity. This will establish the initial extent
of sediment-container physicochemical interactions. Any
additional burial of the drum by sedimentation at the site is
considered desirable and will increase the amount of sediment
retention afforded if a waste package were to release its
contents.
To assess the impact of radionuclide release at a LLW
site, it is important to know the radionuclides in the waste
that are most likely to be detrimental to biota or man and the
sediment retention potential of those radionuclides. The
radionuclides of importance are those that are (a) primary
constituents of LLW, (b) enduring or persistent (e.g. have a
half-life of 5 years or more) and, (c) biologically toxic.
According to Wild et al (1981), the radionuclides in LLW that
meet one or more of these criteria include tritium, carbon-14,
cobalt-60, nickel-59 and-63, strontium-90, niobium-94,
technetium-99, iodine-129, cesium-137, uranium-235, and
uranium-238. Those radionuclides that meet all three criteria
include cesium, cobalt, and strontium. Therefore, focus will
be on cesium, cobalt and strontium for assessment of the
predicted retention by sediment. Other radionuclides may be
considered during any further development or revisions to
regulations and criteria; and site characterization will
include analysis of parameters sensitive to prediction of
retention for all the radionuclides of importance in a LLW
package.
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Since the physicochemical processes controlling sorption
of radionuclides onto the sediment are complex and are not brie
same for all radionuclides of concern, the determination of the
sorption distribution coefficient (Kd) oa site-specific
sediments gives the best state-of-the-art assessment of
radionuclide retention. The test is performed under similar
environmental conditions (both oxidizing and reducing) as exist
in the prototype sediment. Tritium, carbon-14, a-ndjiodine-129,
however, have little or no retention and little purpose is
served in performing laboratory analyses on them. One of the
radionuclides in which solubilitiy changes enormously with
slight changes in redox potential is tec.hnetium-99. Some of
the other radionuclides; e.g., cobalt-60, and uranium isotopes,
are also affected to some degree by redox state. The ,Kd of
cesium isotopes, strontium-90, and other cations exhibiting a
single oxidation state are most affected by the amount and type
of sorptive minerals and surface area of the sediment. Some
radionuclides form soluble complexes with
organics that might occur in the waste form or sediment; e.g.,
cobalt-60 is known to be mobilized by organics such as EDTA.
Some radionuclides also coprecipitate with iron and manganese
oxyhydroxides. ,~j
It is feasible to predict, within limits, the potential
retention of radionuclides of concern if adequate measurements
for characterization of the sediment are performed at a
prospective LLW site. Such an approach might eventually be
extended to the heavy metal analogs in hazardous waste.
"•?£', ,
The sediment parameters relating to site selection and
baseline monitoring of a LLW site will include: (a) those
parameters already in place in paragraph 228.13(e)(3) of the
Regulations, where applicable, and (b) recommended .parameters
that relate to measurement of radionuclide retention (e.g. Kds)
or which predict the retention of radionuclides in the sediment.
1.3 PARAMETERS ALREADY IN THE OCEAN DISPOSAL REGULATIONS
The parameters required by the current EPA Ocean Disposal
Regulations are applicable to LLW disposal sites. Paragraph
228.13(e)(3) lists organic carbon, texture, particle size
distribution, major mineral constituents, and settling rate as
required measurements for sediment analyses. These sediment
parameters, however, must be addressed by methods applicable to
the special nature of low-level radioactive waste and the deep-
ocean environment. Settling rate, listed in paragraph
228.13 (e) (3), is not applicable to the disposal of LLW,
however, because radioactive wastes are presumed to be packaged
in containers as specified in paragraph 227.11(b)(1).
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1.3.1 ORGANIC CARBON
The concentration of organic carbon (regardless of source)
in sediments within the deep-ocean environment is an index to
the total organic matter in sediments, and to the biomass of
living organisms in the overlying water-column and adjacent
areas. It is important in siting considerations since
relatively higher levels of organic carbon in sediments at a
prospective LLW disposal site would make the site less suitable
for disposal of wastes due to a higher probability -of having
biological pathways to the surface waters. The presence of
organic matter in sediment can also be detrimental for
radionuclides that form complex chemical bonds, in that their
retention by sediment is reduced. Radiocobalt, for example, is
known to form organic complexes in shallower ocean areas,
resulting in virtually no retention by geologic media.
Information about the organic chemical complexes formed by
radionuclides is incomplete, making prediction of radionuclide
retention by the sediments more difficult. According to Emery
and Uchupi (1972), the concentration of organic carbon in the
deeper waters of the Western Atlantic Ocean is generally less
than 0.5 percent. Typical results for organic carbon, measured
in surficial sediment samples, in the vicinity of LLW disposal
sites are as follows: NW Atlantic (2,800m site), 0.6 percent;
NE Atlantic, 0.2-0.3 percent; Pacific (Farallon Islands), 0.6
percent.
Determination of organic carbon involves high temperature
combustion using a carbon analyzer on samples from which the
inorganic carbon (carbonate carbon) has been removed; the
latter is accomplished by either wet or dry leach techniques.
1.3.2 SEDIMENT TEXTURE GRAIN SIZE
The sediment texture grain size is one of the most
important and versatile of the sediment monitoring parameters
at a prospective LLW site. This parameter (a) provides a basis
for classification of the sediment by grain size (b) correlates
with geotechnical parameters (Keller et al, 1979) and the
sorptive properties of the sediment (Onishi et al, 1981), (c)
allows for mathematical computation of overall mineral
percentages in a sediment sample that was analyzed for mineral
content in specific sand, silt and clay-size fractions
(Neiheisel, 1983) and, (d) provides an indication of site
stability.
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The sediment texture classification is based on the
percentage of sand, silt, and clay-size fractions in accordance
with the nomenclature of Shepard (1954). A color description
precedes the sediment size classification. The triangular
textural diagram used for sediment classification is depicted
in Figure 1 of the Methods Manual (EPA, 1988). Ten categories
of sediment are classified based on the percentage of sand,
silt and clay-size materials in the sediment. These are sand,
clayey sand, sandy clay, clay, silt, silty sand, clayey silt,
sand-silt-clay, silty clay, and sandy silt. In this
classification, the sand-size material is sediment greater than
0.062 mm, silt-size material is sediment less than 0.062 mm but
greater than 0.002 mm (2 micron size) and clay-size material is
all sediment less than 2 micron size. The 62-micron size is
recommended (Galehouse, 1971) for the boundary between silt and
sand because (a) most investigators use Stokes1 law up to this
size-limit of sand and (b) the 62-micron stainless steel sieve
is convenient for removing coarser size materials from the
finer material to be analyzed by sedimentation (silt and clay)
techniques. The 2-micron size is the preferred dividing-line
between silt and clay-size materials as a mineral composition
change usually occurs at this size-boundary. Using the above
texture size classification, the typical sediment description
for the Atlantic 2,800m LLW disposal site is clayey silt, with
an olive grey color.
The process of determining the percentages of grains
representative of particular sizes is known as grain-size
analysis. The grain-size distribution of the sediment sample
is determined by combining the analytical results from the
sand, silt, and clay fractions. The grain-size distribution of
the sand-size fraction is determined by sieve analysis or by
using the rapid sediment analyzer, in which sediments of less
than 0.062 mm size are analyzed by sedimentation techniques.
The methods used depend on the type of sediment encountered.
The technique to be used can normally be determined by visual
inspection of the sediment cores. Some of the methods that are
acceptable, widely used, and available, are described in detail
in a Methods Manual (EPA, 1988).
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1.3.3 RECOMMENDATIONS
Sediment analyzed for texture should be representative of
the top 30 cm of sediment core from a LLW site. Based upon
observations from the deep submersible ALVIN ( Hanselman and
Ryan, 1983) during an EPA Office of Radiation Programs survey
at the Atlantic 3,800m LLW disposal site, and similar
observations during EPA submersible surveys at the Atlantic
2,800m site and the Pacific Ocean Farallon Islands site, we do
not expect waste drums to penetrate deeper than 30 cm into the
sediments. This depth is also the sediment recovery depth
obtained by conventional sediment box corers. Subcore samples
obtained from the box core will thus provide relatively
undisturbed sediment for the various tests to be made on the
sediment.
The proposed disposal site should be free of exposed
bedrock and display uniformity of sediment texture and relative
geologic stability across the site. To ensure uniformity of
sediment texture over a proposed site, we recommend that the
average sediment texture to 30cm depth for each sample location
should fall in a close pattern on the triangular diagram that
constitutes the Shepard Texture Classification Chart; and the
number of different types of sediment should not exceed 3 of
the possible 10 textural descriptions displayed on the chart.
Such uniformity of texture across the proposed site will
reflect on uniform, stable, geologic processes in the area.
1.3.4 MINERAL COMPOSITION OF SEDIMENT
The current Regulations cite "major mineral constituents"
as one of the sediment parameters. However, an important
consideration in the assessment of a LLW site in a deep-ocean
environment is a reasonably precise evaluation of the mineral
composition of both major and minor mineral components
(sorptive minerals may occur in the minor component). This
parameter indicates (a) the potential retention of
radionuclides by the sediment (b) the geologic stability of the
site (c) presence of economic mineral deposits, and (d) dynamic
factors and sediment source considerations.
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An accurate assessment of mineral composition (+_10 percent)
would reflect on the potential retention of radionuclides that
are known to exhibit sorption primarily by cation exchange and
sediment surface area phenomena. According to Onishi et al
(1981), the radionuclides in LLW that are most strongly
affected by cation exchange and surface area include cesium,
strontium, uranium and radium. The minerals most capable of
affecting cation exchange and having the greatest surface area
are the clay minerals (generally less than 2-micron size in
diameter) and zeolites. The sorptive minerals most responsive
to cation exchange, in order of priority, are smectite
(montmorillonite), zeolites, illite, kaolinite, and chlorite.
The greater surface area occurs in smectite (montmorillonite)
and zeolites which are generally confined to the clay-size
(less than 2 microns). The general distribution of sediment
composition in relation to grain-size distribution is given by
Neiheisel (1983) for ocean sediment at the Atlantic 3,800 m LLW
disposal site. This study demonstrates the fact that a major
portion of the highly sorptive minerals and those with greatest
surface area occur in the less than 2-micron size (clay)
fraction. Since a composition change commonly occurs between
silt and clay-size materials and most of the sorptive materials
occur in clay-size fraction, it is desirable to separate these
size fractions for individual chemical and X-ray diffraction
analysis. Average mineral composition (prior to separation)
may be computed using the texture grain-size curve. An
accuracy of +_10 percent is considered desirable if the mineral
data is to be of assistance in the assessment of radionuclide
retention or geologic stability of site. The methods for
recommended detailed mineral analyses are included in a Methods
Manual (EPA, 1988).
1.3.5 RECOMMENDATIONS
Evaluation of the potential for radionuclide retention
requires a detailed mineral analysis of the sediment that
exceeds the "major minerals constituents" cited in paragraph
228.13(e) (3) of the current regulation. It is recommended that
a method with at least +10 percent mineral accuracy be
specified for the potential prediction of radionuclide
retention such mineral precision would provide.
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1.4 CONSIDERATION OF PARAMETERS NOT IN EXISTING REGULATIONS
The current regulations do not contain sediment monitoring
criteria that address the parameters which (a) measure
radionuclide retention and (b) provide prediction potential of
radionuclide retention by the sediment. These considerations
are addressed below with recommendations.
1.4.1 SORPTION DISTRIBUTION COEFFICIENT (Kd)
The sorption distribution coefficient (Kd) of a
radionuclide is a measure of its retention potential determined
by laboratory analysis of site-specific sediment. The Kd is
used to determine the degree of partitioning between
radionuclides in solution and the same radionuclides in the
solid phase. The Kd, in effect, represents the relative length
of time a radionuclide, released to the sediment environment in
solution, is impeded from movement toward the water column by
sorption onto solid surfaces. The factors that affect the Kd
of a radionuclide vary with the sorptive minerals of a
particular sediment, particle size, pH, Eh (redox), and
chemical speciation of the radionuclide released from the waste
container.
The process by which the radionuclides are removed from
the dissolved phase by particulate matter or sediment is
generally referred to as scavenging (IAEA, 1986). If the
release of the dissolved phase is into the sediment surrounding
the waste package, the scavenging is essentially the sorption
distribution coefficient (Kd) that is measured from specific
sediment at the site. Any release of the dissolved phase
directly to the water column from the waste package, however,
is more difficult to estimate for retention of the
radionuclide(s) because of the dynamic nature of the
particulate matter in the benthic nepheliod layer (BNL).
Nyffeler and Godet (1986) describe the nature of the BNL for a
basin in the NE Atlantic. Since the BNL particulate matter is
recognizably higher in organic matter than the consolidated
sediment at a candidate disposal site, the sorption
distribution coefficient (Kd) of this material may differ from
the consolidated sediment. While this probable difference is
recognized, it is as yet not determined whether it is
significant enough to warrant a separate test. If such testing
were warranted, it would require enough sample to perform the
test; the "fluff" or unconsolidated sediment layer from
sediment cores might be the most practical representative
sample of this material.
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The laboratory batch tests for Kd are described in a
Methods Manual (EPA, 1988) in terms of Rd which is the
distribution ratio, instead of the distribution coefficient
(Kd). The use of Kd implies sufficient knowledge of the
chemical systems under investigation to ensure that a final
reversible equilibrium is attained. In natural systems this is
not always the case, so the value Rd is determined. This is
calculated in the same manner as Kd but does not carry with it
the thermodynamic requirements of a system at equilibrium. The
distribution ratio (Rd) and the distribution coefficient (Kd)
are defined identically, but, in actuality, Rd is only equal to
Kd at equilibrium.
The Kds of radionuclides are site specific, and laboratory
measurements are only as accurate as the ability exists to
duplicate deep-ocean conditions. Onishi et al (1981) cites Kd
measurements from a wide range of environments, and data bases
have been developed for various programs which indicate the
range of values possible. Radionuclides with virtually no
retention by any environment are the most predictable; these
include tritium, carbon-14, and iodine-129. However, for other
radionuclides of interest, such predictability is not possible,
and few measurements are available to compare potential Kds
that exist in ocean environments. Since Kd measurements for
radionuclide retention are expensive (approximately $1,000 per
radionuclide per sample), the number of tests performed should
be limited to those radionuclides of most concern.
The methods commonly employed include the batch test and
sediment column method. Since the redox state constitutes a
primary factor for redox sensitive nuclides, this condition
must also be considered in the laboratory testing. The redox
conditions in the deep-ocean environment are addressed, in
Section 1.4.3.
1.4.2 RECOMMENDATIONS
It is recommended that at least one sediment core from
each prospective LLW disposal site be measured for Kd's of
radionuclides of concern including, as a minimum, cobalt,
cesium, and strontium. The measurements should be conducted on
samples representative of the sediment to 30 cm depth.
10
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1.4.3 SEDIMENT REDOX CONSIDERATIONS
The retention of some of the radionuclides in the LLW at a
deepsea disposal site is sensitive to the oxidation-reduction
(redox) state of the sediments. The redox state at most
disposal sites will range from oxygenated (aerobic) conditions
on the surface to reducing (anaerobic) conditions with depth.
This decrease of oxygen with depth is indicative of oxygen
consumption in the sediment below the sediment-water interface
and reflects input of organic carbon (Murray and Grundmanis,
1980). In a qualitative sense this is related to distance from
the continental margins where production occurs. Both the
Atlantic and Pacific are similar in this respect. The depth to
oxygen depletion in the sediments in the Atlantic Ocean has
been reported by several investigators (Schmidt 1979, Carpenter
et al 1983, Wilson et al 1983, and Van der Loeff and Lavaleye
1984), who indicate typical ranges from 10 to 30 cm depth to
oxygen depletion.
At the NE Atlantic LLW disposal site Van der Loeff and
Waijers (1985) report that average depths of oxygen penetration
extend to 50 cm and as much as 100 cm. Wilson et al (1983)
reports the greatest depth to oxygen depletion (10 m) in the
Atlantic in the Cape Verde Abyssal Plain. In the Pacific Ocean
investigations, Grundmanis and Murray (1982) report oxygen
depletion as relatively rapid in the first 10 to 15 cm of
sediment depth in the equatorial region, where the
concentrations of surface organic carbon are also high.
Elsewhere in the Pacific, investigations indicate the aerobic
zone extends to several meters in depth in the sediment.
The range of the oxygenated sediment environment may vary
in depth from less than the diameter of a 55-gallon waste drum
to as much as several meters. The retention of several
radionuclides, including technetium, uranium, cobalt, and
nickel, is affected by the redox state. This parameter
requires special attention in the assessment of radionuclide
retention in the sediment of a deep ocean LLW site.
Remobilization at lowered redox potentials below the depth
where oxygen reaches zero also causes an upward diffusion
transport of manganese.
11
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Concentrations of sulfide sulfur, iron, and manganese
observed in the sediment profile can be used for understanding
variations in redox sensitive radionuclides. These parameters
also provide a rough correlation with redox conditions obtained
by the more precise measurement of NC>3 in sediment pore water
for determination of oxygen depletion. Care must be exercised
in the use of these parameters; e.g., if detrital pyrite or
relict conditions occur in the sediment, interpretations might
be misleading. However, the use of these parameters in a
well-characterized sediment is considered of interpretative
value in correlating with the redox condition of the sediment
column. In order to be effective, measurements should be made
at 5 cm intervals for the full 30 cm of sediment core. Thus
the thickness of the aerobic layer to oxygen depletion could,
in a thoroughly characterized site, be estimated to within
approximately 5 cm if the redox boundary occurs within the
sediment core.
Sulfur compounds in marine sediments range from 0.02 to
2.0 percent and reduction of sulfate to sulfide will not occur
in the aerobic layer. In fact, sulfide formation in sediments
occurs well below the sediment depth of measurable oxygen
depletion. For example, Schmidt (1979) estimated the depth of
sulfide formation would occur at 50 cm depth at a Gulf of Maine
2,500m depth location in the Western North Atlantic where pore
water chemistry and Eh meter studies indicated the depth to
oxygen depletion at 30 cm. For sulfides to form in the
anaerobic layer, both iron and sulfate-reducing bacteria must
be present. The presence of both sulfate-reducing bacteria and
iron in the sediment will generate pyrite (FeS2) in the
anaerobic layer. The sulfide sulfur test is a relatively
simple and inexpensive test that has limited application and in
some cases may be used to correlate with the approximate
boundary to the anaerobic layer.
The redox state and sulfide formation are also of interest
in waste container corrosion. For example, a steel drum, well
below the depth of oxygen depletion and in the presence of
anaerobic, sulfate-reducing bacteria, would be corroded by a
process called cathodic depolarization (Uhlig, 1971). As
Dexter (1982) indicates, the bacteria utilize hydrogen in their
metabolism to reduce sulfates to sulfides. The sulfides, in
turn, form an iron-sulfide scale on the steel which is cathodic
to a bare steel surface. Thus, this mechanism of corrosion
occurs in anaerobic conditions where there is a source of
sulfate.
12
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The manganese and iron in sediments correlate with the
redox state and radionuclide retention potential of sediments.
Under oxidizing conditions, iron is highly insoluble. However,
under mildly reducing conditions, Fe++ may be solubilized and
precipitate in sulfide-bearing sediments. Technetium from a
LLW waste drum, released to a reducing environment, has been
reported to be immobilized in the presence of Fe++ (Van der
Graff et al, 1984). Manganese is also controlled by the redox
state, and the factors which cause remobilization and
deposition of manganese influence the migration potential of
cobalt and nickel.
The process of manganese reduction and mobilization in
marine sediment is well documented. Froleich et al (1979)
describe the accumulation of manganese at the surface of the
anaerobic boundary layer in marine sediment. A manganese
"spike" at this sediment depth essentially marks the point of
oxygen depletion. This spike of manganese can be used to
estimate the depth at which anaerobic conditions prevail in the
sediment; however, because of the sharpness of the boundary,
care must be exercised in the sample spacing requirements. The
manganese profile is also a mirror image of the iron profile;
manganese increases as iron decreases in the sediment core.
The inflection points on a concentration-versus-depth plot of
manganese and iron are thus also of value for correlation with
the redox boundary which is determined by more precise,
methods. The interpretation of these parameters is difficult
unless the point of precipitation lies well below the zone of
bioturbation.
The inductively coupled plasma (ICP) spectrometer method
and atomic adsorption spectrophotometry (AAS) methods are
commonly used to determine manganese and iron in marine
sediment. These should be conducted on sediment samples at
5 cm intervals to assist in determining redox trends with depth
for the 30 cm of core length.
An electrometric method that reflects on the redox state
of marine sediment is the Eh meter, using a combination of
platinum and reference electrodes. Measurements are read in
millivolts. The measurements, however, are rather qualitative
because of the complex electrochemistry involved if more than
one mineral system or valence state is present. According to
Meyer (1982), the use of the Eh probe will not indicate whether
oxidation or reduction will occur but will give an approximate
indication of oxidizing or reducing conditions. Lindberg and
Runnells (1984) also question the analysis of redox state by
simple electrochemical means.
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Any valid Eh measurement of sediment redox conditions must
be performed as soon as the core sample is collected aboard
ship. A plastic liner with holes drilled at 5 cm intervals is
inserted into the box corer to prepare the sample for prompt Eh
measurements. As an example, samples obtained in this manner
at the Atlantic 2,800m LLW disposal site in the fall of 1984
indicated oxygenated conditions for the top 16.5 cm of sediment
core. At 16.5 cm depth, a measurement of 229 mV was obtained,
indicating that the redox boundary was probably being
approached and could be expected perhaps within several more
centimeters. Schmidt (1979) used both the Eh meter and pore
water chemistry analysis for oxygen depletion in an
investigation of sediment from a 2,500m depth in the NW
Atlantic and found that the point of oxygen depletion coincided
with an Eh reading of 225 mV.
The redox state of the sediment is best determined by
analysis of the oxygen depletion in the pore water of the
sediment with depth. The most quantitative measurement for the
determination of oxygen depletion is the nitrate test of the
sediment pore water. Sayles and Livingston (1984) discuss this
method for marine sediment cores. The extraction of pore water
from the sediment for nitrate analysis is relatively complex;
however, this method of approach is the most reliable technique
available for determination of redox conditions with depth in
the sediment.
1.4.4 RECOMMENDATIONS
The redox state of the sediment at a prospective LLW site
should be measured because of its requirement in (a) Kd
measurement of sediment involving redox sensitive radionuclides
and (b) providing data that has direct application to
prediction of radionuclide retention. The measurement of redox
as a sediment monitoring criteria parameter should include:
(1) Measurement of oxygen depletion in pore water by
nitrate analysis as a primary technique for
determination of redox state of the sediment.
(2) Measurement of (a) Eh (by Eh meter), (b) sulfide
sulfur, and (c) manganese and iron in the sediment at
5 cm intervals to 30 cm depth for sediment
characterization and interpretative values relative to
sorption of redox sensitive radionuclides and
correlation with the redox conditions measured in (1)
above.
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1.4.5 DETERMINATION OF pH
Measurement of pH is important to characterizing sediment
in a LLW disposal site. According to Zobell (1946), the pH
range of recent marine sediments is 6.4 to 9.5 and that of Eh
is from +0.350 to -0.500 mV. In the deepsea environment, pH
will be more constant whereas the Eh will vary widely in
relation to changes in redox conditions. Typical pH values of
sediment samples taken in the vicinity of LLW disposal sites in
the Atlantic and Pacific Oceans are 7.2-7.8 (NW Atlantic),
7.9-8.1 (NE Atlantic), and 7.9 at the Farallon Islands
(Pacific).
The most common method for pH measurement is the
electrometric determination of the activity of the hydrogen ion
by potentiometric measurement using a glass electrode and a
reference electrode. Determinations of pH should be made
aboard ship soon after receiving the sediment aboard and as
soon as possible at the laboratory where it is sent for the
other tests.
1.4.6 RECOMMENDATIONS
It is recommended that pH be included as a sediment
monitoring requirement for any proposed low-level radioactive
waste site. This inexpensive parameter has application in
radionuclide retention prediction for most radionuclides of
interest.
1.5 GEOTECHNICAL PARAMETERS
Analysis of sediment geotechnical properties provides
insight into the penetration potential of waste packages into
the bottom sediments and probable subsequent settlement of
waste packages. The analysis may provide information on the
past geological stability of a site and show if active erosion
or rapid sediment loading is occurring. The engineering
information will aid in evaluating the sites for waste package
insertion and burial should subseabed disposal be considered a
viable option in the future for LLW.
The parameters to be measured will focus on establishing
the general engineering characteristics of the sediment and
permit some prediction of behavior under various loading
conditions (e.g., waste package emplacement or impact). These
parameters may include those measured or determined from index
properties (liquid limit, plastic limit, and water content),
strength, and consolidation tests. The U.S. Geological Survey
is currently investigating the geotechnical test parameters
which are appropriate for deepsea disposal sites.
15
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1.5.1 RECOMMENDATIONS
It is recommended that basic index properties (liquid
limit, plastic limit and water content) be included in sediment
monitoring criteria. This test can be used to (a) compare
bottom penetration of the waste drum and (b) correlate with
geologic stability of the site. This and other geotechnical
tests are relatively simple and inexpensive, and provide
fundamental engineering information.
Under consideration is the emplacement of LLW beneath the
sediment surface. For this proposed option, additional
geotechnical parameters attendant to deep-sea penetration and
burial (e.g., consolidation, triaxial, and vane shear tests)
are being evaluated.
1.6 X-RADIOGRAPHS
X-radiographs of sediment cores allow the nondestructive
observation of certain textural and structural features of the
sediment. The x-radiographs reveal biological activity
(bioturbation) in the sediment.
X-radiographs are made of each core to 30 cm depth to
determine the extent of bioturbation in the sediment. The
bioturbation process could accelerate the transport of
radionuclides from within the sediment to the sediment-water
interface as well as acting as a means of transporting
radionuclides sorbed at the surface to depth. The depth of
bioturbation is correlative with redox conditions in that it
relates to activity in the oxygenated layer of sediment.
However, the depth to the base of bioturbation activity is not
to be considered the point of oxygen depletion since other
factors are involved.
1.6.1 RECOMMENDATIONS
It is recommended that x-radiographs be required in the
evaluation of LLW disposal sites. Since bioturbation is
considered an undesirable entity, the degree of bioturbution
could be indexed in some manner, based on density of biological
structures. The index number could be instrumental in picking
the most acceptable site.
1.7 CORRELATION WITH GEOLOGICAL STABILITY
Some of the sediment monitoring parameters may be used to
correlate with geologic stability of a site as well as provide
predictive capabilities for radionuclide retention. These
include (a) detailed mineral composition, (b) x-radiographs,
(c) index properties, and (d) texture.
16
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The detailed mineralogy of the sediment used to predict
sorption capabilities of the sediment also reflects on the
geologic stability of the site. As an example, Van der Loeff
et al (1984), interprets the uninterrupted and constant
composition of the clay fraction in the NE Atlantic LLW
dumpsite as indicative of stable geologic conditions in the top
layer of sediment. They also correlate turbidite (rapid
deposition layers) and nonunifprm conditions with changes in
mineralogy laterally at deeper depth in cores.
The x-radiographs of sediment cores, in addition to
indexing biological activity, reveal sediment structures and
layers in the sediment. Any disruption of sediment pattern not
related to biological activity would relate to geologic
stability of the site. Uninterrupted sediment layers would
correlate with geologic stability of the site.
The index properties also relate to the geologic stability
of an area. As an example, Keller et al (1979), reporting on
the geotechnical properties of the upper continental slope off
the Atlantic coast, effectively correlates the physical
properties with depositipnal stability of the region. In
addition, any major variations of sediment texture reflect on
the geologic stability of the site.
1.7.1 RECOMMENDATIONS
It is recommended that sediment monitoring criteria
parameters that serve as predictors of radionuclide retention
also serve as a monitor of geological stability at the site.
1.8. SAMPLING DENSITY
The size of a deep-ocean LLW disposal site will not exceed
100 square miles (360 km2). The density of sampling
locations within a proposed LLW site will be in accordance with
the specifications provided in the existing Regulations (par.
228.13(e) (1) (ii). The maximum number of replicate samples at
each station of a proposed site will coincide with the number
indicated in paragraph 228.13(e)(1)(i). The actual number of
samples to be tested to a minimum 30 cm depth will depend on
the homogeneity of the sediment parameters at the site.
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1.9 SUMMARY OF RECOMMENDATIONS
The existing Ocean Disposal Regulations contain provisions
for measurement of sediment parameters in paragraph
228.13(e)(3), which support the sediment monitoring needs for
LLW at deep ocean sites. These parameters include organic
carbon, texture, particle size distribution, major mineral
constituents, and settling rate. Because LLW will be
containerized, as specified in paragraph 227.11(a)(l) of the
Regulations, settling rate will not apply. The special
requirement for predicting sorption potential of the sediment
will require modification of existing parameters listed in
paragraph 228.13(e)(3) to include (a) detailed mineral analyses
(b) texture grain-size, (c) measurement of sorption
distribution coefficient (Kd) of Cs, Co, and Sr and (d) site
characterization parameters that include the following: redox
measurements (nitrate analysis of pore water for oxygen
depletion), pH, Eh, manganese, iron, and sulfide sulfur.
The cost of parameters specified for measurement would
approximate the following:
Measurement Average Cost Per Analysis
pn diiu JLU ^ mi j.pu
$
$
J-J . UU
200.00
1,000.00
400.00
75.00
120.00
45.00
25.00
25.00
75.00
300.00
These are the basic costs of a normal operating
laboratory and do not include ship costs, collection
apparatus, storage, or personnel costs.
Additional parameters that relate to physical response of
the sediment to impact of the waste container are still being
evaluated.
L8
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1.10 FUTURE CONSIDERATIONS: IMPLEMENTATION OF SECTION 424,
PL 97-424
The purpose of the proposed criteria is to determine
acceptability of a specific site that meets the general site
selection criteria by evaluating and quantifying its
geochemical and geotechnical immobilization characteristics.
The evaluation of specific wastes proposed for disposal at a
specific site will require the preparation of a Radioactive
Material Disposal Impact Assessment (RMDIA). When such an
assessment is prepared, a critical determination will be
whether the radionuclides will be immobilized at the site. If
a satisfactory determination is made that immobilization of the
specific wastes will occur, then an important consideration
arises. If radioactive isotopes are released by a waste
package and retained by the sediment at a site, rather than
dispersed, the radionuclide concentrations in the sediment will
increase. It will become important to establish sediment
quality criteria or concentration limits linked to effects on
indigenous marine organisms. The primary effects on marine
organisms would occur through ingestion of materials. The
availability of radionuclides to infauna and sessile epifauna
is directly linked to the sediment dynamics (sediment-water
interaction) .
Although the proposed criteria are directed at the
sediment dynamics, particularly physicochemical immobilization
processes, the establishment of sediment criteria will also be
necessary to determine the acceptable amount of radioactive
material that can be disposed of at a site.
While it is beyond the scope of the proposed criteria to
consider specific permit evaluation requirements, early
consideration of some of the questions attendant to setting
sediment quality criteria would be useful.
The need for sediment monitoring are different for ocean
disposal of hazardous waste than for LLW. These differences
are apparent if a comparison is made of the spectrum of
approaches available for setting sediment quality criteria for
hazardous waste versus LLW.
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*At a national perspective workshop on the scientific
approaches for establishing sediment quality criteria, convened
by EPA in November 1984, a review was made of the pros and cons
of the various methodologies available. The status report on
this effort, released by EPA in March 1985, identified four
general approaches for development of sediment quality
criteria. These were the (a) background concentration
approach, (b) equilibrium partitioning approach, (c) water
quality criteria approach, and (d) bioassay approach. Within
each of these approaches, hazardous compounds included polar
organics, nonpolar organics and heavy metals. Of these
approaches, all but the background concentration approach and
bioassay approach require the use of water quality criteria.
Virtually no water quality criteria exist for radionuclides
except for the Interim Drinking Water Standards. Therefore,
the background approach seems most feasible since, unlike the
other approaches, it does not rely on water quality criteria to
set sediment quality criteria. The background approach relies
on the establishment of a reference concentration against which
increasing concentrations at a site can be compared in order to
determine the extent and magnitude of effects from disposal
operations. The reference concentration reflects the ambient
concentrations existing at a site (e.g., from nuclear fallout
prior to its use for any disposal operation). While it may be
assumed that the reference concentration for radionuclides
represents a "No-effects Level," setting sediment quality
criteria requires (1) cytogenetic and other measurement and
evaluation techniquesJ(see Prepermit Testing Protocols to
establish the "No-effects Level") and (2) assessment of the
factors controlling radionuclide retention and bioavailability
at deep ocean disposal sites. As a starting point, the
radionuclides in the waste could be divided into groups that
respond to similar predictive factors. One group might be
iodine, carbon, and tritium, which have virtually no retention
in any known sediment. Another waste group might contain
cationic species affected by type and amount of sorptive
minerals. Still 'another group that is sensitive to oxidation
conditions would require assessment in both oxidizing and
reducing environments. Radionuclide groups responsive to
chelating agents, ligands, or other chemical factors
(coprecipitation, etc) would require further consideration.
Although seemingly complex, this approach is feasible because
the deep ocean environments have more stable conditions
(r.r stant pH, salinity, etc) than other more variable
environments such as fresh water or shallower marine water
disposal sites.
With the above information, sediment quality criteria
could be established for radionuclides, and applicable
requirements of an ~~lMr'Tn could be evaluated.
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