United States
Environmental Protection
Agency
Office of Radiation Programs
Las Vegas Facility
PO Box 15027
Las Vegas NV 89114
ORP LV-78-5
June 1978
Radiation
v>EPA
Study of Engineering and
Water Management Practices
that will Minimize the
Infiltration of Precipitation
into Trenches Containing
Radioactive Waste
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EPA REVIEW NOTICE
This report has been reviewed by the EPA and approved for publication.
Approval does not signify that the contents necessarily reflect the vtews and
policies of the EPA, nor does mention of trade names, or commercial products
constitute endorsement or recommendation for use.
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PREFACE
The Office of Radiation Programs of the U.S. Environmental Protection
Agency carries out a national program designed to evaluate population exposure
to ionizing and non-ionizing radiation, and promote the development of controls
necessary to protect the public health and safety.
This report was prepared to summarize current engineering and water
management practices that could minimize water infiltration from precipitation
into trenches containing buried radioactive waste.
Problems have occurred at existing commercial radioactive burial sites
located in humid zones due to the formation of leachates containing radioactive
nuclides from the waste materials. The leachates in turn provide a source,
and in some cases, a driving mechanism for the migration of radioactive
materials from the trenches into the burial site environs.
The present study examines methods by which burial site containment could
be improved through the application of improved trench cover designs to exist-
ing and future burial sites. The work is based primarily upon a review of
existing engineering practices as reported in the literature. The study has
reviewed a number of materials and methods that could be considered for
application at radioactive waste burial sites.
Readers of this report are encouraged to inform the Office of Radiation
Programs - Las Vegas Facility of any omissions or errors. Comments or re-
quests for further information are also invited.
r
.(
>~c?
Donald W. Hendricks
Director, Office of
Radiation Programs, LVF
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Final Report
STUDY OF ENGINEERING AND WATER MANAGEMENT
PRACTICES THAT WILL MINIMIZE THE INFILTRATION
OF PRECIPITATION INTO TRENCHES CONTAINING
RADIOACTIVE WASTE
CONTRACT NO. 68-03-2452
Prepared for:
Office of Radiation Programs
U.S. Environmental Protection Agency
National Environmental Research Center
P. 0. Box 15027
Las Vegas, Nevada 89114
By:
SCS Engineers
4014 Long Beach Boulevard
Long Beach, California 90807
September 17, 1977
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CONTENTS
Page
List of Tables iv
ListofFigures v
Acknowledgements vi
1. Summary and Recommendations 1
Summary 1
Recommendations 7
Remedial Steps 8
New or Improved Practices 8
Post-Closure Practices 9
2. Introduction 10
Background 10
The Problem 10
Waste Types & Characteristics 11
Disposal Practices 15
Project Objectives 19
Method of Approach 20
3. Water Balance Considerations at Land Burial Sites 21
Introduction 21
Water Balance Factors 21
Surface Water Inputs 24
Groundwater and Waste Moisture Content 24
Site Factors and Water Balance 25
Soil Characteristics 25
Topography 25
Hydrology 27
Vegetation 27
Climatology 27
Water Balance 27
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CONTENTS (continued)
Page
4. Trench Caps and Covers 33
Introduction 33
Routine Trench Covering 33
Trench Caps 33
Concrete 36
Application 36
Disadvantages 42
Advantages 42
Asphaltics 43
Application 43
Disadvantages 44
Advantages 44
Soil Cement 45
Synthetic Polymer Membranes 46
Types 46
Applications 48
Clay 49
Applications 50
Advantages and Disadvantages 51
SoilSealants 51
Applications 51
Disadvantages 51
Advantages 51
Trench Cap Covers 52
5. Alternative Trench and Site Construction Methods 55
Introduction 55
Trench Construction 58
Trench Siting 58
Diversion Trenches 60
Trench Liners 63
Advantages and Disadvantages 63
Grout Curtains 63
In-SituEncapsulation 66
Advantages and Disadvantages 67
i i
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6. Burial Trench Maintenance and Monitoring 68
Introduction 68
Surface Maintenance 69
Vegetation and Landscaping 71
Monitoring 73
Trench Monitoring Systems 73
Monitoring Frequency 76
Bibliography 77
Glossary 85
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TABLES
Number Page
1 Summary of Concepts for Control of Water
Infiltration into Radwaste Disposal Trenches 2
2 Projected Average Annual Low-Level Radwaste
Generation Volumes 15
3 Total Volumes and Quantities of Low-Level
Radioactive Wastes Buried at Commercial
Sites Through 1975 16
4 Generalized Water Balance Equation at a
Waste Disposal Site 23
5 Water Transmission under Saturation 26
6 Runoff and Infiltration for a 2.5 cm Rainfall 28
7 Approximate Seasonal Consumption of Water
by Example Types of Vegetation 29
8 Summary of Example Water Balance Calculations
for Three Areas 31
9 Summary of Trench Caps and Covers 37
10 Costs of Synthetic Polymer Membrane Liners 47
11 Summary of Trench and Site Construction
Alternatives 56
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FIGURES
Number Page
1 ERDA Radwaste Disposal Sites 13
2 Commercial Radwaste Disposal Sites 14
3 Radwaste Disposal Practices 17
4 A Typical Waste Disposal Trench and
Water Balance Pathways 22
5 Typical Trench Completion 35
6 Concrete Capping Concepts 41
7 Depth of Frost Penetration 53
8 Conceptual Completed Low-Level Radwaste
Disposal Trench 54
9 Horizontal Flow of Water into a Backfilled
Trench from Topsoil Located Uphill from
the Trench 59
10 Burial Trench Arrangement to Impede
Horizontal Water Infiltration through
the Topsoil 61
11 Radwaste Disposal Trench with Diversion
Trenches 62
12 Radwaste Disposal Trench with Chemical
Grout Curtain 65
13 Grass-Covered Radwaste Disposal Site 72
14 Cross Section of Moisture Monitoring Cells
for a Burial Trench 75
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ACKNOWLEDGEMENTS
This manual and supporting literature and case study
reports are the result of cooperation between EPA, industry,
university, and SCS personnel. The guidance and assistance
of Mr. Michael O'Connell, Project Officer, Office of Radiation
Programs, U.S. EPA, Las Vegas, Nevada, is gratefully acknow-
ledged. Keros Cartwright, Ph.D., hydrogeologist, provided
consulting assistance to the project team.
SCS project participants were David E. Ross, Project
Director, Messrs. Rodney Marsh and Thomas Wright, Project
Engineers, and Dallas Weaver, Ph.D., Technical Advisor. Mr.
Robert P. Stearns served as Reviewing Principal.
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CHAPTER 1
SUMMARY AND RECOMMENDATIONS
SUMMARY
Low-level radioactive solid wastes (radwastes) are gen-
erated by virtually every facet of the nuclear industry and
are conventionally buried in shallow trenches at several land
disposal sites. Burial practices are straightforward,
involving placement of wastes into the trench, backfilling
with soil, some compacting, and mounding of the surface soil
to minimize ponding.
Disposal of low-level radwastes by these procedures has
generally been effective. Escape from disposal sites of low-
level radioactive material that could cause environmental
damage is rare. However, the few problems that have developed at
several disposal sites have spawned investigations into methods
for improving the safety of low-level radwaste disposal practices
For example, water has infiltrated through soil covers and trench
walls and filled trenches at several sites, and trenches have
overflowed and leaked into underlying bedrock formations thus
contaminating local ground and surface water with measurable
radioactive material. So far, levels of contamination from such
incidents have not exceeded safe limits, but suggest potentially
more significant emissions.
Conventional low-level radwaste disposal practices can be
improved. In some instances, improper sites have been selected,
and operations have been inadequate. Disposal trenches are
not always water secure, and infiltration and leakage have
occurred. However, problem situations may be avoided in the
future and are largely correctable at existing sites.
The purpose of this project is to identify methods by which
infiltration of water into disposal trenches can be controlled
or eliminated. Table 1 summarizes a variety of practices and
concepts based on experience and research into the prevention of
water movement at sanitary landfills and other waste disposal
facilities on land, reservoirs, canals, dams, and other situa-
tions where water infiltration control is necessary:
• Barriers to minimize or eliminate infiltration of
precipitation into radwaste disposal trenches;
• Methods to stabilize the surface at disposal sites; and
• Procedures to minimize perpetual site care requirements.
1
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TABLE 1. SUMMARY OF CONCEPTS FOR CONTROL OF WATER INFILTRATION INTO
RADWASTE DISPOSAL TRENCHES
Concept Description
Approximate
Expected Cost/Trench
Longevity3 Installed"
Reference
No.
Comments
Trench and Site Construction
Alternatives
Situate disposal site on a
slope
Situate disposal site on
level ground
Construct system of berms/
drainage ditches on the
site
Construct trenches with
level bottoms
Construct trenches with
sloped bottoms
Line trench bottom with
permeable soil or other
material
Construct narrow gravel -
filled diversion trenches
around the entire site
and/or individual
disposal trenches
NAV
NA
NA
NA
Indefinite $10/1inear
with meter of berm
regular or ditch
maintenance
NA
NA
NA
NA
NA
$500-1,000
Indefinite $100/1inear
with regu- meter of
lar main- diversion
tenance trench
42 Applicable for new trenches only
42 Applicable for new trenches only
Should be implemented in all cases
42 Applicable for new trenches only
42 Applicable for new trenches only
42 Applicable for new trenches only
36 Applicable for new trenches or as a
remedial measure
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TABLE 1 (Continued)
Concept Description
Trench and Site Construction
Alternatives
Trench liners
Grout curtains around
entire site and/or
trench periphery
Injection of a slurry
of impermeable material
into each trench to
fill void spaces
Approximate
Expected Cost/Trench Reference
Longevity3 Installed" No. Comments
Varies $40,000 to 28» 39, Applicable for new trenches only.
with mater- 275,000, 75
ial and depending
exposure on liner
conditions; material
at least
5 to 40
years
Variable; Up to 2, 7, 8 Applicable for new trenches or as
grout $70,000 a remedial measure.
curtains depending
for dams on the
and grout
buildings material
have expec- used
ted lives
of at least
10 to 50
years
5 to 50 $140,000 -- Applicable for new trenches or as a
years, to 700,000 remedial measure.
depending depending on
on the the material
filler
material
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TABLE 1 (Continued)
Concept Description
Trench Caps and Covers
Concrete caps (in general)
-thin layer cap (10 cm)
-thick concrete mound
cap (100 cm)
-concrete encapsulation
and cover
Asphaltics
-normal asphalt
concrete (10 cm)
-hydraulic asphalt
-soil asphalt
-catalytically blown
bituminous seal
Soil cement
Expected
Longevity3
at least
40 years
at least
15 years
at least
25 years
Approximate
Cost/Trench Reference
Installed0 No. Comments
see below 36, 52, Applicable for new trenches or as
80, 28 a remedial measure
$ 16,000
$140,000
$500,000
see below 28, 37, Applicable for new trenches or as
39, 55 a remedial measure
$3,600-5,400
$5,400-7,600
$2,300 .
$2,700-3,600°
see below 5, 39, Applicable for new trenches or as
74 a remedial measure.
-15 to 20 cm layer
with a bituminous seal
coal
-carbonate bonding (15 cm)
$2,300
$2,400
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TABLE 1 (Continued)
Concept Description
Trench Caps and Covers
Synthetic polymer membranes
-butyl rubber
-polyethylene
-polyvinyl chloride
-ethyl ene-propyl ene
diene
-chlorinated
polyethylene
-hypalon
Clay
Expected
Longevity
at least
20 years
at least
5-10
years
at least
5-10
years
at least
5-10
years
at least
5-10
years
at least
10 years
1 ,000 +
years in
absence of
mechanical
damage
Approximate
Cost/Trench
Install edb
$5,000a
$8,000°
$1 ,800-;
$3,500°
$2,300-,
$4,500°
$5,000-,
$7,500°
$4,300->
$6,500°
$4,300-,
$6,500°
$1 ,300
(37)-
$10,000
(35)
Reference
No . Comments
Applicable for new trenches or as
a remedial measure.
JZ?
28, 39,
40
28, 39
28
28
28
1,3, Applicable for new trenches or as a
5, 36, remedial measure.
39, 69,
82
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TABLE 1 (Continued)
Concept Description
Expected
Longevity
Approximate
Cost/Trench
Installed
Reference
No.
Comments
Trench Caps and Covers
Soil sealants
at least
5 years
$1,000
and up,
depending
on the
sealant
material
33, 35, Applicable for new trenches or
86, 87 as a remedial measure.
o>
a Expected longevity is the estimated time for the conceptual method to remain intact with
minimum maintenance. A minimum of 40 years is considered essential.
b Costs approximate for a "typical" trench 100 m long by 15 m wide by 6 m deep. Costs
include materials and installation. The costs do not include normal trench excavation
and filling or maintenance. Maintenance costs can vary widely depending on what type
of trench cap cover is used (soil, gravel, grass).
c NA - not applicable.
d Includes a 15 to 30 cm soil cover.
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In general, these concepts have not been applied at radwaste
disposal sites, although they have been proven elsewhere.
Surface infiltration can be controlled with a multilayer
cover:
1. A compacted soil cover over the wastes, mounded;
2. An impervious cap of clay, concrete, asphalt, plastic,
or similar material ;
3. A protecting layer of soil; and
4. A 1ayer of gravel .
Covers of this type would protect the wastes from surface
infiltration and are themselves protected from exposure to
environmental extremes and root damage from vegetative growth.
Infiltration of precipitation through the surface soil
between trenches and thence horizontally into the trenches can
be minimized through the use of berms and drainage ditches to
divert surface runoff; construction of impermeable curtains
or other barriers could be used to inhibit horizontal water
flow.
A series of moisture cells placed in the trench bottom and
below the soil cover can detect the presence of water in the
trenches. Any water that has entered a disposal trench can be
accumulated in a sump and removed by pumping via an access pipe
before it saturates deposited wastes or flows offsite. If
water is allowed to remain in contact with the wastes for
more than a few days, it can become contaminated and thus re-
quire further treatment. Frequent monitoring and pumping can
avoid water contamination problems. Pumping will be necessary
until any danger of radioactive contamination has passed.
A properly located, designed, and operated low-level
radwaste disposal site can be essentially water secure for several
decades, if not centuries. Such security will require more
effort'and care than has been committed toward low-level rad-
waste disposal management in the past,and it will not be free.
But,considering the potential damage from inadequate practices,
control of water infiltration is neither infeasible nor pro-
hibitively expensive.
RECOMMENDATIONS
The following recommendations apply to low-level radwaste
disposal in general. A specific recommendation may not be
applicable at all sites, but universal application is not the
intent, nor is it expected that any one site would benefit from
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implementation of all of the recommendations. Finally, it
should be noted that these recommendations are not intended
to be requirements.
The recommendations are divided into three groups --
remedial steps that could be taken at existing sites and ongoing
burial operations, new or improved practices that could be
implemented during the design and operation of future sites,
and post-closure practices.
Remedial Steps
• Where not already available, access wells should be
placed at the low end of each trench and any standing
water removed.
t Trench covers should be leveled, the cover soil recom-
pacted, a new soil mound placed, and an impermeable
cap and cover added.
t Moisture probes should be placed in each trench and
monitored regularly.
• A series of berms and drainage ditches should be con-
structed and regularly maintained to handle surface
runoff and prevent site flooding.
• At sites where trenches are leaking or leaking is sus-
pected, or that are underlain by fractured bedrock,
grouting should be considered as a means to prevent
offsite movement of radioactive material.
• Consideration should be given to injection of an imper-
meable material into trenches where water contact or
waste settlement has been or is now a problem.
New or Improved Practices
t Greater emphasis should be given to the site selection
aspects of disposal activities. Over a span of several
hundred years, leaks are possible at virtually any
site; thus burial activities should be situated
to minimize the impacts of'such leaks on the
local and areal environment. The newer low-level rad-
waste burial sites, which were selected after
relatively extensive site surveys and hydrogeological
investigations, are generally more secure than the older
sites.
8
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• Radwastes should never be placed in a trench contain-
ing standing water nor should they be dumped during
a rainstorm. The trench should be drained ahead of
time or a separate wet-weather trench with covers for
contingency use provided.
• Temporary berms should be constructed around each open
trench to prevent entrance of surface runoff.
t Wastes should be compacted as much as possible and,
to minimize settlement as the organic waste fraction
decomposes, void spaces filled with granular or other
sui table material .
• Covers should be placed as noted in Remedial Steps,
above.
• Moisture cells should be placed both immediately under
the mounded cover and in the trench bottom. Any
water detected in the trenches should be removed
immediately. Immediate removal may avoid further
treatment and disposal problems for the infiltrate.
Post-Closure Practices
• Vegetative growth over closed trenches should be
carefully controlled or eliminated to prevent dis-
ruption of the cover's integrity by root propagation.
t Ground and surface water should be monitored regularly;
soil and vegetation samples from the area should be
analyzed for possible contaminants.
• Site inspections should be conducted regularly,
especially following any major climatic or geological
event.
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CHAPTER 2
INTRODUCTION
BACKGROUND
In the early days of the Manhattan Project, little thought
was given to radioactive waste disposal; it was secondary to
the main task at hand. Solid wastes were usually buried in
the most accessible and convenient vacant place without much
thought for long-term consequences. Initially, this presented
no problem because of the small waste quantities involved and
the isolated nature of the research facilities. However, with
the passing of time, nuclear research and production expanded
and wastes accumulated. The commercialization of nuclear power
and isotope technology led to commercial waste generation and
the establishment of two types of waste burial facilities -
federal (Atomic Energy Commission (AEC) or Energy, Research and
Development Administration (ERDA)) for wastes generated by
government research and production, and commercial, for wastes
generated by industrial and private use of radioisotopes. The
commercial sites were limited to low-level (<1 microcurie (uCi)/
ft3 or gal.) radwastes. The federal government segregated high-
and low-level wastes; most of their attention focused on the
disposal problems of the more dangerous high-level wastes. Low-
level waste land burial was not considered particularly
hazardous.
THE PROBLEM
In November 1973, a group of researchers for the State of
Kentucky identified radioactive tritium (hydrogen -3), cobalt
-60, strontium -89 and -90, cesium -134 and -137, and plutonium
-238 and -239 in the soils and surface waters below the Maxey
Flats low-level radwaste burial site. A December 1974 report
concluded that the disposal site was responsible for the unusual
offsite radioactivity (46). At the West Valley, New York low-
level radwaste burial site, water has been seeping out of two
trenches and running off of the site into some nearby streams.
The runoff contains elevated tritium levels (Richard Cunningham,
57). The levels of radioactivity in a Clinch River, Tennessee,
tributary creek periodically exceed the maximum permissible EPA
limits (See 10 CFR 20 and 40 CFR 158 for drinking water regu-
lations). The radioactive materials are leaching out of
several Holifield National Laboratory, Oak Ridge radioactive
10
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waste burial trenches which are flooded by a rising water table
several months each year (46).
These and similar problems at the other low-level radwaste
burial, sites are among the formidable obstacles facing
continued nuclear reactor development. Safe waste burial
a necessity. If the leaching of wastes cannot be prevented,
then an alternate disposal method must be developed; if there
are none, waste generation, and consequently, nuclear techno-
logy development must cease.
The problems are not insurmountable, however. Radwaste
burial is similar in principle to many hazardous waste control
situations. The disposal technique must be such as to prevent
any possibility of the hazardous material coming in contact
with any part of the environment which could affect man. Tradi-
tionally, burial, or more generally land disposal, has been
considered the best method for achieving this end. But, as the
recent experiences at Maxey Flats, West Valley, and Holifield
indicate, environmental contamination is still a possibility.
A basic problem with land burial is water: rainwater,
surface runoff, groundwater. Water, leaking into or percolat-
ing through a burial site, can dissolve certain constituents
of the waste. As the water percolates out of the site, it
carries some of these constituents with it. If the contaminated
water flows into a groundwater aquifer or surface water, it
will contaminate parts of the environment directly affecting man
When water attacks radwastes, the resulting leachate can
contain not only dissolved organics and heavy metals, but also
radioactive material. This radioactive leachate, because of
its potential carcinogenic and mutagenic properties, is much
more insidious in its effects than other hazardous leachates.
Although such radioactive leachates are in one sense less
hazardous than other toxic leachates because the radioactivity
will eventually decay, the immediate health effects can be much
more devastating. It should be noted at this point that none
of the radioactively contaminated leachates thus far identified
has had serious environmental or public health effects; the
leachates have been low level discharges further diluted in the
environment (Richard Cunningham, 57). The potential for serious
damage in the future is nonetheless real.
WASTE TYPES AND CHARACTERISTICS
Every operation involved with radioactive material produces
wastes which either are, or may be, contaminated with radio-
active isotopes. To date, nuclear weapons manufacturing has
accounted for 83 percent of the radwastes generated; this is
expected to decrease to 6 percent by the year 2000 (if nuclear
11
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power plant construction continues at the predicted rate)
(Gerald H. Daly, 57). Most of the remainder of radwastes is
from power plants and associated fuel reprocessing. A small
fraction of the radwastes comes from hospitals, schools, and
industries using radioisotopes. The AEC and ERDA have classi-
fied these wastes as "high-level" and "other" or "low-level."
This report is limited to a discussion of low-level radwastes,
defined as having a radioactivity of less than or equal to
lyCi/gal (3.785 liters) or cubic ft (0.028 cubic meters).
Physically, low-level radwastes may range from paper and
rubber gloves to a contaminated 25-ton semitrailer (44).
Commercial burial sites were originally licensed for low-level
radwastes from hospitals, research facilities, and industry
(68). However, the burial sites have been used increasingly
for the disposal of contaminated wastes from the nuclear power
industry. The radwastes buried at ERDA sites are wastes
from nuclear weapons manufacturing.
Weapons-related low-level radwastes, to date, have totaled
about 1.2 million cu m (43 million cu ft) (Gerald H. Daly,
57). These wastes are buried in five principal ERDA sites
(Oak Ridge, Tennessee; Los Alamos, New Mexico; Richland,
Washington; Savannah River, South Carolina; and the Idaho
National Engineering Laboratory (INEL); and six minor ERDA
sites (Sandia, New Mexico; Amarillo, Texas; Fernald, Ohio;
Paducah, Kentucky; Portsmouth, Ohio; and the Nevada Test Site)
(Figure 1). One million cu m (36 million cu ft) containing
1.8X10? Ci have been buried at the five principal sites,
including 740 kg (1,632 Ib) of plutonium at INEL and 212 kg
(467 Ib) of plutonium at Richland (46). The low-
level radwastes from weapons manufacturing are currently
being generated at a rate of about 37,000 cu m (1.3 million
cu ft) per year, a rate not expected to change significantly
before the end of the century (46).
There are six commercial burial sites operated by three
private companies - Nuclear Engineering Company (Hanford,
Washington; Beatty, Nevada; Sheffield, Illinois; and Maxey
Flats, Kentucky), Chem Nuclear Systems, Inc. (Barnwell, South
Carolina), and Nuclear Fuel Services (West Valley, New York),
(Figure 2). To date, 258,000 cu m (9.1 million cu ft), or
about 17 percent of the total low-level radioactive wastes
generated, have been buried at these sites (Gerald H. Daly,
57). With the projected increase in nuclear power production,
the commercial sites are expected to contain 94 percent of the
low-level radwastes by the year 2000 (Gerald H. Daly, 57).
Table 2 presents the projected average annual volume of low-
level radwastes through the year 2000.
12
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1 OAK RIDGE, TN
2 LOS ALAMOS, NM
3 IDAHO NATIONAL
ENGINEERING LABS
4 RICHLAND.WA
5 SAVANNAH RIVER, SC
6 SANDIA, NM
7 AMARILLO, TX
8 NEVADA TEST SITE
9 FERNALD, OH
10 PADUCAH, KY
11 PORTSMOUTH, OH
Figure 1. ERDA radwaste disposal sites
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1 MOREHEAD, KY
(MAXEY FLATS)
2 HANFORD, WA
3 BARNWELL, SC
4 SHEFFIELD, IL
5 BEATTY, NV
6 WEST VALLEY, NY
Figure 2. Commercial radwaste disposal sites
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TABLE "2. PROJECTED AVERAGE ANNUAL LOW-LEVEL
RADWASTE GENERATION VOLUMES (68)
(103 cu m/yr)
Type
Fuel cycl
Non-fuel
1976-1980
e waste 57
cycle waste 45
Total 102
1981-1990
300
110
410
1991-2000
1 ,926
311
2,237
Table 3 presents the total volumes and quantities of
each type of waste buried at the six commercial sites through
1975.
DISPOSAL PRACTICES
Essentially all low-level radwastes generated in the
United States are buried in trenches. Typical trench dimen-
sions are about 90 m (300 ft) long by 12 m (40 ft) wide by
6 m (20 ft) deep, with 6 m (20 ft) between trenches (66). In
practice, however, trench sizes vary somewhat: the trenches
at Maxey Flats are 110 m (360 ft) by 21 m (69 ft) by 6 m
(20 ft), while those at West Valley are 180 m (590 ft) long by
6 m (20 ft) deep (4). Each trench bottom is designed to be
sloped at about 1 degree from end to end (10). A stone-filled
sump at the low end of the trench permits collection and
removal of any water that accumulates during filling and after
the trench is completed.
The trenches are filled from the high end. The wastes may
either be dumped randomly or stacked neatly, depending on
their nature and a given site's operational procedures (Figure
3). Wastes generally arrive at the site in cardboard or
wooden boxes, 55-gal drums, bulk (for equipment, demolition
wastes, reactor shielding, etc.), or containerized liquid form.
Much of the liquid radwaste is solidified with newspaper and
concrete before disposal. Increasingly, paper trash and
other loose or boxed wastes are solidified or encapsulated
in concrete. One unofficial estimate is that up to 90
percent of all low-level radwastes being buried at present are
solidi fied.
The trenches are backfilled as needed to keep radiation
levels below federal government-set limits (100 mRem/hr around
open trenches) (10). A minimum of approximately one meter of
15
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TABLE 3 . TOTAL VOLUMES AND QUANTITIES OF LOW-LEVEL RADIOACTIVE
WASTES BURIED AT COMMERCIAL SITES THROUGH 1975a (Nuclear Regulatory Commission, 57)
Beatty,
Site
Nevada
Maxey Flats, Kentucky
Hanford,
Washington
Sheffield, Illinois
West Val
Barnwell
Total
ley, New York0
, South Carolina
Volume
(1000 Cu m)
52
126
12
55
68
60
373
By-product
Material
(1000 Ci)
128
1,912
440
40
538
253
3,311
Source
Material
(1000 Ib)
110
343
17
180
605
283
1,538
SNM
(1000 g)
178
375
33
72
55
343
1,056
Plutonium
(1000 g)
14b
69
13
13
4
0
113
Numbers rounded to nearest thousand
Does not include 1974
GClosed March 1975; does not include 1975
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K if
Figure 3. Radwaste disposal practices
! 1
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soil is required to prevent alpha and beta radiation leaks
from the trenches. The radwastes themselves generally are not
compacted. Thus, as much as 30 percent of the trench volume
is void space (85).
As will be noted shortly, lack of compaction can lead to
problems, which are largely avoidable, since the wastes are
compactible and could be reduced in volume by factors of 2 to
10 (4, 12). As each trench is filled, the surface soil
cover is compacted and shaped to form an "umbrella" to seal
out groundwater and divert surface water (10). A standpipe
is left in the sump to facilitate removal of any water which
might collect in the trench bottom.
Meyer (60) has described what he calls a typical trench
life cycle:
1. The trench is excavated from soils with relatively
low permeability.
2. The trench is filled with high porosity, permeable,
compressible wastes which contain organics and a
wide range of chemical forms.
3. The wastes are covered with an earthen cap, which
is often more permeable than the original pre-
trench soil and rock, in effect creating an
infiltration gallery.
4. Some of the precipitation which falls on the
cap infiltrates into the trench and soaks
the wastes.
5. Leaching of the wastes begins, aided by the
presence of organic matter, bacterial action, for-
mation of organic and inorganic acids, and chelat-
ing agents.
6. Trench leachate begins to (a) migrate downward and
laterally, because of the hydraulic head imposed
by the leachate in the trench and/or (b) to over-
flow at the land surface in springs and seeps at
some low point between the cap and the undisturbed
soi 1 .
7. As the wastes are soaked and leached, they compact,
undermine the cap, cause surface cracking, increase
infiltration into the trench, and thereby increase
leachate generation.
18
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Furthermore, at the time the six low-level radwaste
burial sites were selected, systematic site selection criteria
had not been established. The geological, geochemical, hydro-
logical, soil, climatological, and other criteria necessary to
ensure that a given site would retain its radioactivity had not
been clearly formulated. Tentative site selection criteria
are now being developed, and it is becoming evident that many
of the sites currently in use are not suitable as radwaste
burial sites (George D. DeBuchananne, 57).
With the increase in solidified wastes and the decrease
in loose, compressible, biodegradable organics, much of this
settlement/trench cover collapse problem has been overcome.
There may still be a problem of settlement in the loose backfill
between and around the solidified wastes.
Among the other potential problems that can be encountered
at radwaste burial sites are gas generation and nearby new
trench construction. Any organic, biodegradable materials
will generate methane and C0£ upon biodegradation. Unless the
gas is allowed to escape through the cover cap or vents,
pressure buildup can rupture a cap. Furthermore, the methane
can present an explosion hazard if allowed to accumulate
in enclosed spaces. Gas hazards are also minimized by increas-
ing waste solidification, but older burial trenches may still
present problems.
The construction of new burial trenches can also pose a
threat to completed trenches because of nearby heavy equip-
ment traffic. Unless care and planning are devoted to new
trench construction, it is possible for heavy equipment to
inadvertently drive over completed trenches and damage covers.
Also, construction of new trenches adjacent to previously
filled ones may affect the integrity of trench side walls,
depending on soil conditions and separation distance.
PROJECT OBJECTIVES
Radioactive contaminants are reportedly leaving the
burial trenches at several disposal sites. It is generally
conceded that these contaminants are being (or at least may
be) leached out of the buried wastes by infiltrating surface
water from rainfall and runoff. The principal objective of
this report is to investigate methods for selectively improving
current and future land disposal operations for low-level
radioactive wastes by:
1. Minimizing infiltration of incident rain water
and surface runoff;
2. Maximizing surface stabilization to prevent
rupture of the trench cap; and
19
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3. Minimizing perpetual care requirements.
Secondary objectives include an investigation of methods to
evaluate the water budget at low-level radwaste disposal sites
and an evaluation of the economics for implementing improved
infiltration control practices.
METHOD OF APPROACH
The core of this report was based on an extensive search
of the literature. Because of the nature of the problem,
not only literature on radioactive waste disposal, but also
literature on solid and hazardous waste disposal, sanitary
landfills, soil hydrology, channel and reservoir construction,
seepage and mine drainage control was reviewed. The bibli-
ography at the end of this report is by no means exhaustive,
but it does present an excellent cross section of the
pertinent literature available on these subjects.
Interviews with acknowledged experts in related fields
were used to verify and update information from the literature
These individuals brought to the preparation of this report
expertise in radioactive waste disposal, soil hydrology
dynamics, and water infiltration prevention. Experience in
solid waste and hazardous waste disposal was used to relate
and evaluate all of the concepts and suggestions from these
sources.
20
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CHAPTER 3
WATER BALANCE CONSIDERATIONS AT
LAND BURIAL SITES
INTRODUCTION
The various physical, chemical, and biological processes
that occur within a land disposal trench produce compounds
that are susceptible to solution or suspension in water perco-
lating through the disposed wastes. This percolating water
containing contaminants derived from the solid waste is
leachate. The volume of leachate produced at any particular
site is dependent on many factors but generally is determined
by surface water infiltration and/or interception of ground-
water. The relationship between precipitation/groundwater
and leachate generation is not necessarily linear, nor does
the presence of precipitation/groundwater necessarily result
in leachate. Leachate generation is more directly related to
the quantity of water which actually reaches the buried waste.
The quantity of water, in turn, depends not only on precipita-
tion but also on such factors as the surface and subsurface
conditions and climatological characteristics in the area.
Water balance concepts have been developed to assess the
potential leachate problem for a given area, in light of the
variety of factors influencing leachate production. The
usefulness of water bal'ance in regard to radwaste disposal
sites is twofold: (1) water balance evaluations can be used
to help evaluate a candidate site's suitability in terms of
leachate generation potential; and (2) water balance assess-
ments can help establish the extent of any infiltration
problem which might exist at a site in use and thus help
determine what type(s) of controls, if any, are necessary.
WATER BALANCE FACTORS
Water balance is based principally upon the relationships
among precipitation, evapotranspiration, surface runoff, and
soil moisture storage. Also, regional geological factors,
such as the site's location with respect to recharge and dis-
charge areas, must be known to properly assess a site's water
balance. Figure 4 shows a schematic of a typical waste
disposal trench and the various water pathways available.
Table 4 presents the equations relating the various water
balance terms.
21
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PRECIPITATION
EVAPOTRANSPIRATION
EVAPOTRANSPIRATION
SURFACE
RUN-OFF
COMPACTED
SOIL COVER
LOW-LEVEL
RADIOACTIVE
WASTES
LEACHATE
POSSIBLE
GROUNDWATER—\
CONTOURS )
Figure 4. A typical waste disposal trench
and water balance pathways.
22
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TABLE 4. GENERALIZED WATER BALANCE EQUATION
AT A WASTE DISPOSAL SITE (46)
where
and :
where
wl
w
w
w
I
R
E
W
SR
W
GW
W
IR
R
SR
GW
'IR
Input water from precipitation
Input water from surrounding surface runoff
Input water from groundwater
Input water from irrigation
Percolation
Surface runoff
Evapotranspiration
W
Ss = Change in moisture storage in soil
R = Change in moisture storage in solid waste
LG = Leachate flow to groundwater
LS = Leachate flow to surface waters
Wn = Water contributed by waste decomposition
23
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Surface Hater Inputs
The principal water source at most burial sites is pre-
cipitation (WR). Irrigation water (WIR) can also be a signifi-
cant source, if the closed trenches have a vegetative cover
which must be maintained by artificial means. In fact, irri-
gation has been implicated as a primary leachate source in
semi-arid areas, where rainfall is insufficient to produce
much leachate (22). Vegetation will be discussed later. At
this point it should be noted that leachate from radwaste
disposal sites is a sufficiently serious problem to prohibit
irrigation, if such added water appears likely to increase the
leachate production potential. The only other likely water
source is site flooding (W$R). Given proper site selection,
flooding is a remote possibility; it is mentioned only because
it happened at the INEL disposal site (Carl Kuhlman, 57).
Water applied to the surface of a burial site has four
possible destinies: evapotranspiration (E), runoff (R),
percolation (I), and retention (Ss). Evapotranspiration is
the combination of evaporation from the soil and transpira-
tion by the vegetative cover. Transpiration is usually the
greater of the two (22). Surface runoff is that fraction of
the added water which is lost to overland flow before it has
a chance to infiltrate. Retention is the fraction of the
water that is retained by the soil. The total amount of water
that can be stored by a given soil is referred to as its field
capacity and consists of two components - hygroscopic water
(zero soil moisture to the wilting point) and available water
(the wilting point to field capacity). Percolation water is
that fraction of the soil moisture which exceeds the field
capacity; it is this fraction which is responsible for saturat-
ing wastes in trenches and causing leaching.
Groundwater and Waste Moisture Content
Most water balance discussions dismiss groundwater (WQ^)
and the initial moisture content of the wastes (Wo) as negligible
relative to precipitation. There are exceptions, however.
The groundwater table at the Holifield low-level radwaste
disposal site rises periodically and floods several disposal
trenches for a few months; radioactive material leaches out of
the wastes during these periods and contaminates the nearby
Clinch River. To date, this is the only reported incident of
leachate directly related to groundwater infiltration.
Much of the low-level radwaste is metal, plastic, concrete,
or other material with virtually no free moisture. Most liquid
wastes are encapsulated; some wet wastes, such as filter sludges,
are sealed in steel drums before disposal. Disposal trenches
full of these drums could be a major leachate source if the
drums rupture due to corrosion. Several ERDA low-level radwaste
24
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disposal sites are presently exhuming drums to encapsulate the
wastes (80).
SITE FACTORS AND WATER BALANCE
Precipitation, groundwater, irrigation, soil moisture,
evapotranspiration, runoff, retention, and percolation constitute
a site's water budget. In addition, there are a number of
variables which can affect a site's water budget and thus
influence the quantity of leachate produced. These variables
include site characteristics (soil types, topography, hydrology,
ecology, meteorology), waste characteristics, waste disposal
practices, and trench and site construction and management.
Site characteristics are perhaps the most important
variables. It is conceivable that site characteristics could
be such that no leachate would ever be produced, regardless of
how poorly controlled the other variables might be (as in
certain arid regions where annual rainfall seldom exceeds a
few millimeters). It is sometimes possible to adjust these other
variables to overcome otherwise prohibitive site characteristics,
but ultimately, the natural site characteristics will determine,
to a large degree, how much leachate a disposal trench can
produce.
Soil Characteristics
The three soil characteristics which most strongly
influence the water budget are (1) water intake (infiltration),
(2) hydraulic conductivity, and (3) evaporation. Water intake
is influenced by the amount of water applied and surface con-
dition. Hydraulic conductivity (under saturated conditions)
is influenced by particle size, compaction, aggregation,
organic matter content, fracturing, etc. Evaporation is
influenced by water availability, temperature, wind speed,
and surface conditions. Soils with small particles (clays,
silts) have a low hydraulic conductivity; large particle soils
(sands) have a high hydraulic conductivity and poor water
retention properties. This relationship is shown quantitatively
in Table 5.
Topography
Topography most strongly affects runoff both onto and off
of a disposal site. A site situated on a hillside would lose
more incident precipitation to runoff than would a valley
site, which might actually gain water from upstream runoff.
For sites on a slope, steeper slopes have greater runoffs.
As runoff increases, the amount of water available for in-
filtration decreases. The potential for local soils replaced
as trench cover to erode and form gullies is also influenced
by topography at the site. Such erosion may disrupt the
25
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TABLE 5. WATER TRANSMISSION UNDER SATURATION (82)
Soil description
Hydraulic3
conductivity
cm/sec
Hourly transmitted
volume
m3/ha
Well -graded gravels or gravel-
sands, no fines
> 10
~
> 3.6 x 10
Poorly-graded gravels or gravel-
sands, little or no fines
> 10
> 3.6 x 10
Silty gravels, gravel -sand-silt
mixtures
~ .
10~J to 10"°
. .
3.6 x NT to 3.6 x 10"'
Clayey gravels, gravel -sand-
clay mixtures
Well -graded sands or gravelly
sands, little or no fines
fi „
10"° to 10~°
> 10
3
"
, . ,
3.6 x 10"' to 3.6 x 10
> 3.6 x 10
Poorly-graded sands or gravelly
sands, little or no fines
Silty sands, sand-silt mixtures
Clayey sands, sand-clay
mixtures
,
> 10
10"3 to 10"5
10"° to 10
a
~°
> 3.6 x 10
3.6 x 102 to 3.6 x 10"1
3.6 x 10"' to 3.6 x 10
Inorganic silts, very fine
sands, rock flour, silty or
clayey fine sands, clayey
silts with slight plasticity
Inorganic silts and organic
silts of low plasticity
Inorganic silts, micaceous or
diatomaceous fine sandy or
silty soils, elastic silts
Inorganic clays of high
plasticity
Inorganic clays of medium to
high plasticity, organic silts
Inorganic clays of low to medium
plasticity, gravelly clays,
silty clays-, sandy clays *
lean clays
K
"°
10 to 10
, fi
10 to 10"°
, fi
10 to 10
AS
10"° to 10
fi o
10~° to 10"°
AS
10"° to 10"°
3.6 x 10 to 3.6 x 10
, -,
3.6 x 10* to 3.6 x 10"'
, ,
3.6 x 10 to 3.6 x 10"
3.6 x 10" to 3.6 x 10
-t 7
3.6 x 10"' to 3.6 x 10"J
3.6 x 10"' to 3.6 x IP
aAssumes that soils are saturated and uniform,
and that sufficient water is available
26
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integrity of trenchcaps. Table 6 gives typical runoff and
infiltration characteristics for different topographical
conditions.
Hydrology
Hydrology is the study of water in natural systems. Such
study includes consideration of groundwater location, quantity,
and movement and the hydraulic characteristics of the soil
and the disposal strata. Characterization of the hydrological
regimen for site selection will preclude groundwater interr
ception of the disposal trenches throughout seasonal variations
or intense periods of precipitation. USGS recommended site
selection criteria include stipulations that piezometric levels
be several meters below the burial site (68). The site's
hydrological system, especially the groundwater, will play a
major role in determining the pathways available to any
leachates emanating from the fill.
Vegetation
Vegetative cover at a disposal site can influence runoff,
infiltration, and evapotranspiration. Table 6 demonstrates
how vegetation decreases runoff under all soil and slope
conditions. Vegetation also increases infiltration by provid-
ing channels for water flow into the soil. However, vegeta-
tion does consume large quantities of soil moisture (Table
7), and this, in turn, increases evapotranspiration. Vegeta-
tion can also prevent site erosion, but long-rooted flora may
intercept the trenches and carry radioactive material to the
surface (51).
C1imatology
Climatology includes precipitation and those weather
factors which affect evapotranspiration or infiltration. Pre-
cipitation is the major water source at virtually all disposal
sites. The quantities and seasonality of the precipitation
strongly influence leachate generation: 125 cm of rainfall
annually is more likely to lead to leaching than is 10 cm
annually; 100 cm yearly falling in one month will have a diffe-
rent impact than 8 cm monthly for 12 months. The relative
humidity of the air above the site will affect evaporation
rates; dry air favors increased evaporation. Freezing tempera-
tures prevent infiltration during the winter, but may disrupt
natural or artificial infiltration barriers.
WATER BALANCE
Several mathematical models have been prepared incorporat-
ing water components and site characteristics (22, 79, 82).
These models can be used to predict the percolation of water
27
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TABLE 6 . RUNOFF AND INFILTRATION FOR
A 2.5 cm RAINFALL (82)
ro
00
Surface
Condition
Cover Crop
(Pasture or
meadow)
Flat
Rolling
Hilly
Cultivated
(no vegeta-
tion, not
compacted)
Flat
Rolling
Hilly
Rational Runoff
Slope Coefficient.
% Sandy
loam
0- 5 0.05-0.
5-10 0.10-0.
10-30 0.15-0.
0- 5 0.30
5-10 0.40
10-30 0.52
Clay or Clay
silt loam
10 0.30 0.40
16 0.36 0.55
22 0.42 0.66
0.50 0.60
0.60 0.70
0.72 0.82
Sandy
loam
25.7
41
56.3
76.7
102.5
132.5
Runoff (m3/ha)
Clay or Clay
silt loam
77.1 102.5
91.1 141.0
107.2 155.1
127.8 59.2
153.2 179.5
184.2 209.6
Infi
Sandy
loam
230.3
215.3
199.3
179.5
153.2
123.1
Itration (m
Clay or
silt loam
179.5
163.6
148.5
127.8
102.5
71.7
/ha)a
Clay
153.2
114.7
102.5
102.5
76.7
46.1
Hydraulic movement directed downward
-------
TABLE 7. APPROXIMATE SEASONAL CONSUMPTION OF WATER
BY EXAMPLE TYPES OF VEGETATION (82)
mm
Coniferous trees 102-229
Deciduous trees 177-254
Potatoes 177-280
Rye >457
Wheat 509-560
Grapes >_152
Corn 509-1910
Oats 711-1020
Meadow grass 560-1525
Lucern grass 660-1400
29
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into disposal trenches. Fenn, e_t aj_. (22) actually calculated
water balances on a monthly basis for case study (nonradio-
active) municipal refuse disposal sites in Cincinnati, Orlando,
and Los Angeles. The calculations included actual and
potential evapotranspiration, precipitation, surface turn-off,
as well as infiltration, soil moisture storage, and changes in
soil moisture storage. Historical climatological data was
used as a basis on which to calculate expected leachate
generation rates. Table 8 summarizes the results of these
calculations. Similar calculations could be employed to
determine the potential for leachate production at any site.
Radwaste characteristics, burial practices, and site
management method influence percolation and leachate genera-
tion. Dry or anhydrous wastes will take longer to begin
leaching than saturated wastes. Wastes sealed in steel drums
or concrete will not leach until the casings begin to corrode
and/or disintegrate. Radwaste characteristics will have
little direct impact on percolation if the wastes are loosely
packed in the trenches. However, decomposition of organic
wastes can produce gas that may, in turn, affect the trench
soil or membrane cover.
Current radwaste burial practices were discussed in
Chapter 2. Many of the more serious radionuclide migration
problems could possibly have been prevented with better place-
ment practices. As the photographs in Figure 1 indicate, under
past practices wastes remained in water-filled trenches for
some time. Although now largely controlled, this is still a
potential problem area and could be overcome by more efficient
trench pumping operations and/or diversion of off-site runoff.
At the very least, if there is a possibility that a trench
may accumulate standing water, any exposed wastes could be
covered to inhibit radwaste-water contact.
Another problem is the lack of waste compaction. As
noted, filled trenches may be as much as 30 percent void space
(68). This allows water to collect in a trench to such an
extent that the wastes are often sitting in water. Tech-
nologies currently exist which could reduce the volume of
radwastes by a factor of about 2.5, minimizing the problem
(64). Compaction would help inhibit the settlement and
cover-undermining disc'ussed in Chapter 1. Trench life and
disposal site life could be extended by compacting the low-
level radwastes.
Compaction is not without problems. Aside from the
added costs, there is increased risk of exposure to site
personnel engaged in the disposal and compaction operations
because of closer and longer contact with unshielded radwastes
in open trenches. Shielding and shorter working periods could
30
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TABLE 8. SUMMARY OF EXAMPLE WATER BALANCE CALCULATIONS
FOR THREE AREAS9 (22)
Mean
annual
precipi-
tation
Location (mm)
Mean
annual
runoff
(mm)
Mean annual
infiltration
(mm)
Annual mean
actual evapo-
transpi ration
(mm)
Mean annual
percolation
(mm)
Time
of first
appearance
of leachate
(yrs)
Average
annual
leachate
quantity
expected
(liters?
Cincinnati, Ohio
1,025
154
872
658
213
11
40
Orlando, Florida 1,342
100
1,243
1,172
70
15
30
Los Angeles,
California
378
44
334
334
Not specifically related to existing radwaste burial sites.
5 2
Landfill areas assumed: Cincinnati - 2 xlO c m
Orlando - 4 x 101 m2
Los Angeles - 5 x 10 m2
-------
probably overcome this objection. Another possible solution
to both the hazards and the compaction problem is encapsulation
of the radwastes before disposal. Although beyond the scope
of this study, encapsulation does appear to avoid many of the
problems which plague current disposal operations.
When filled, each trench is designed to be covered with
an umbrella shaped cap to encourage runoff of the trench
surface. There are some advantages to placing disposal sites
on slopes to discourage surface ponding and encourage runoff;
this requires some sort of upslope ditch or berm to prevent
the upslope runoff from draining into the disposal site.
Ultimately, the entire problem revolves around surface
infiltration control. If no water percolates into the
trenches, no hazardous leachate will percolate out. While
easy to state in concept, such control is not so easy to
accomplish in the field. A variety of barriers is available
to prevent infiltration, but none is universally applicable,
and all have serious flaws under some conditions. There have
been few, if any, long-term (i.e., hundreds of years) studies
done on these barriers, so their lifetimes are largely
conjectural. However, proper coordination of barrier(s) and
site conditions does hold promise as a leachate control method.
The different types of barriers available and their
advantages and disadvantages are discussed in the following
chapters.
32
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CHAPTER 4
TRENCH CAPS AND COVERS
INTRODUCTION
Meyer (60), in his description of a typical low-level rad-
waste disposal trench "life cycle" (discussed in Chapter 1),
attributes the presence of water in trenches to rainwater infil-
tration through the trench cap. While this may not always be
true, infiltration through the cap must be considered a serious
potential pathway: a highly permeable cap will pass water
readily. Consequently, it would be beneficial to consider
concepts by which the trench caps may be made impermeable to
infiltrating rain water.
Before discussing the concepts in depth, it should be noted
that the integrity of many types of caps is dependent on the
material in the trench. Meyer (60) describes virtual cap
collapse due to settlement in the trench; this would occur with
consolidation and decomposition of uncompacted paper and
other organic, biodegradable wastes. Much of the radioactive
waste in place at Maxey Flats, for example, is paper (60).
Researchers (4, 68) have indicated that compaction of low-
level radwastes in the trenches is, at best, poor and in some
cases may be absent altogether. Under such conditions settle-
ment is virtually inevitable, and the probability of trench
cover collapse increases greatly. When the cover collapses,
any water infiltration control methods previously implemented
become superfluous.
Essentially, there are two ways to avoid this problem:
1) construct a trench cap of inherent structural integrity, or
2) bury only wel1-compacted or nonsettl eabl e wastes. The first
method calls for a rigid trench cap, such as a roof, steel
sheet, or reinforced concrete. However, this approach has
several disadvantages:
• The prohibitive costs of materials and construction,
t The questionable ability of such covers to last
several decades because of corrosion or structural
failure, and
• The possibly difficult access to the wastes in the
event removal should become necessary.
33
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Advantages include:
• Greatly decreased susceptibility to damage from
vegetation, erosion, or nearby waste-handling
activities; and
• Positive monitoring control.
The second option involves the nature of the wastes them-
selves. Much low-level radwaste (e.g., demolition rubble,
concrete-encapsulated and/or metallic wastes) has the necessary
structural stability to support a trench cap, especially in the
absence of appreciable corrosion or water damage from infiltra-
tion. Biodegradable organic wastes (paper, boxes, carcasses,
protective clothing, etc.) would require some sort of baling or
dense compaction before disposal to inhibit settling. Even
with the best compaction methods currently available, some
settlement is-likely, but uncompacted radwastes could settle by
as much as 20 to 30 percent, assuming all void space is
eventually filled by settlement.
ROUTINE TRENCH COVERING
As part of all trench completion procedures, approximately
the top one meter of trench space would be filled with excavated
soil, compacted, mounded, and sloped (Figure 5). It is doubt-
ful that mechanical compaction will return the excavated soil
to its original, undisturbed density, but compaction will help
minimize infiltration. Mounding and sloping is necessary to
facilitate runoff from the trench covers; the optimum slope
is about 5 percent. A lesser slope could soon settle and create
depressions for ponding of water, while a greater slope may be
subject to erosion by surface runoff.
TRENCH CAPS
Frequently the soil of the trench cover is more permeable
than the undisturbed soil between the trenches, even after
compaction. Under certain circumstances, soil can be com-
pacted to a lower permeability than the undisturbed soil,
especially for porous soils. For most soils, however, the
trench cover soil is invariably more permeable. Consequently,
a further barrier may be necessary to completely inhibit
surface water infiltration. This barrier, or cap, should be
placed over the soil cover. There are a variety of materials
available which can provide a relatively impermeable trench cap,
but not all are equally suited for low-level radwaste disposal
trenches. The materials discussed here are concrete, asphalt,
soil cement, synthetic polymer membranes, clay, and various
combinations of these materials with each other and the soil.
34
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COMPACTED SOIL
FROM THE TRENCH
EXCAVATION
Figure 5. Typical trench completion
35
-------
Table 9 lists these materials with a summary of their
characteristics. It should be noted that these cap concepts
are largely untested in regard to radwaste disposal. Some of
the concepts are almost hypothetical, in that no large-scale
testing has been performed with any waste. They are pre-
sented because they are potentially valid concepts, not because
they are proven solutions.
In evaluating and comparing the various trench cap concepts
several generalizations and assumptions are made. It is
unlikely that any burial system can be kept intact and water-
tight for thousands of years, time enough for total decay of
some low-level radwaste. However, perpetual maintenance may
be unnecessary. If a burial trench can be kept tight for at
least 40 to 50 years, most short-lived isotopes will have
decayed, and infiltration will result only in the slow release
of any radioactive isotopes remaining. Consequently, many
experts consider that a burial trench concept life of 40 to 50
years is sufficient.
Concept costs given are for materials and installation
costs above regular trench construction costs. They do not
include backfilling, compacting, or maintenance. Maintenance
costs can add significantly to the overall costs of some con-
cepts, but these costs can be controlled through the proper
selection of final cover materials.
Concrete
The usefulness .of concrete as a water barrier is well
documented. Concrete structures are expected to be watertight
and, on the whole, they have performed well. Countless dams,
canals, and water tanks are constructed of concrete. Properly
designed and bui 11concrete canal linings have been known to
last 40 years or longer (52). A concrete cap is probably the
strongest structurally of the cap types under consideration
here.
Application--
In practice, a concrete cap could be applied several ways
(Figure 6); the simplest is to lay a concrete cap several
inches deep over the compacted soil mound. Given the expense
of installing concrete (about $9/yd2 for a 4-in layer, adjusted
to January 1977 (28)), this is the least expensive application,
as it uses the least amount of concrete, but it is also the most
susceptible to damage from either waste or soil settling or
freeze/thaw. Frost heave or freeze/thaw damage possibly could
be averted by covering the cap with a layer of soil or gravel,
while damage due to settlement of buried wastes could be
36
-------
TABLE 9. SUMMARY OF TRENCH CAPS AND COVERS'
Trench Cap
Description
Concrete (in general)
Advantages
Longevity.
Structural strength.
Di
Expense
to crac
sadvantages
Susceptible
king due to
Expected
Longevity
40+ years
Approximate
Cost/Trench,
Installed13
Reference
No.
52
36, 8D
- 10 cm (4-in) layer
100 cm (40-in)
layer extending
below the top of
the trench
Concrete encapsu-
lation/trench
•filling/cover
Proven success as a
water barrier and
radwaste container.
Cost, as compared
to other concrete
caps.
Less susceptible to
cracking than the
thin concrete layer.
Least damageable of
the concrete caps.
settling or frost heave
of buried waste and/or
soil cap in certain en-
vironments; chemical
attack in acid, alka-
line, and high-sulfate
soils; erosion due to
freeze/thaw cycles.
Difficulty of access
in the event that relo-
cation of wastes be-
comes necessary.
Most expensive con-
crete cap.
Wastes are least
accessible
$16,200
$140,000
$500,000
28
-------
TABLE 9 (continued)
Trench Cap
Description
Advantages
Disadvantages
Expected
Longevity
Approximate
Cost/Trench,
Installed^
Reference
No.
Asphaltics (in
general)
Ease of placement.
Cost.
Proven worth as rad-
waste encapsulation
material.
Versatility.
CJ
00
- Normal asphalt
concrete, 4-in
layer
- Hydraulic asphalt
concrete, 4-in
layer
- Soil asphalt
Decreased perme-
ability over normal
asphalt concrete.
Cost.
Flexibility.
Catalytically
blown bituminous
seal (0.5-1 cm)
Flexibility.
Susceptible to chem-
ical degradation.
Photosensitive.
Susceptible to
cracking due to
settling or frost
heave.
Difficulty of access
in the event that
removal of the
wastes becomes
necessary.
15+ years
More difficult to
apply than normal
asphalt concrete.
Increased perme-
ability.
Questionable quality
control in installa-
tion.
Temperature sensitive.
Questionable homogen-
eity of cap.
Become brittle at low
temperatures.
Little structural
strength.
28, 37, 55
$3,600-
5,400
$5,400-
7,600
$2,250
39
39
39
$2,700-
3,600b
39
-------
TABLE 9 (continued)
OJ
IO
Trench Cap
Description Advantages
Soil Cement Costs.
Disadvantages
Variable permeabili-
Expected
Longevity
25+ years
Approximate
Cost/Trench,
Installed b
Reference
No-
74
15-20 cm (6-8 in)
layer with a
bituminous seal
coat
Carbonate bonding
(15 cm (6-in)
layer)
Synthetic polymer
membranes
- Butyl rubber
- Polyethylene
- Polyvinyl
chloride
Proven worth as a
water barrier.
More flexible than
concrete.
Works better in
heavy clay soils
than normal soil
cement.
Very permeable.
Cost
Strength, chemical
resistance.
Impermeability.
ties.
Susceptible to chem-
ical attack and
freeze/thaw erosion.
Reduced longevity
compared to concrete.
Cost.
Poor quality.
Poor weatherability.
Poor puncture resist-
ance.
Stability.
Temperature sensi-
tive.
20+ years
10+ years
10+ years
$2,250
$2,400
$5,000-
7,700C
$1,800-
3.400C
$2,300-
4,500C
39
39
28, 39, 40
28, 39
-------
TABLE 9 (continued)
-P.
o
Trench Cap
Description
- Ethyl ene-propy-
lene diene
- Chlorinated
polyethylene
- Hypalon
Clay
Advantages
Resistant to weather-
ing and temperature
deterioration.
- - see ethyl ene propyl
Resistant to chemical
attack, puncture,
temperature deteri-
oration.
Proven success as a
water barrier.
High impermeability.
Expected
Disadvantages Longevity
Poor chemical resist-
ance.
ene diene - -
Cost.
Low Tensile Strength.
Susceptibility to 1,000+
mechanical damage years
(animals, plant
Approximate
Cost/Trench,
Installed b
$4,900-
7,600C
$4,300-
6,300C
$4,300-
6,300C
$1,300
$10,000+
Reference
No.
28
28
28
39
36
1, 36
Soil sealants
Self-sealing proper-
ties.
Flexible.
Relatively low
cost. Ease of
application.
roots).
Susceptibility to
chemical deteriora-
tion.
Must be kept wet.
Lack of control over
polymerization or
sealing process.
Subject to chemical
and biological attack.
unknown $l,100+c 33, 35,
86, 87
a All trench caps can be used either as remedial measures on old trenches or on new trenches.
b Costs approximate for a trench 100 m long by 15 m wide by 6 m deep. Includes only the materials
and installation of the cap itself.
c Includes a 15 to 30 cm soil cover.
-------
COMPACTED
SOIL
RADIOACTIVE /,
THIN CAP OVER
COMPACTED SOIL
COVERING
CONCRETE CAP, NO
MOUNDED SOIL COVER
/^RADIOACTIVE
r*
CONCRETE USED TO FILL
VOID SPACE AND CAP THE
TRENCH (NO SOIL)
Figure 6. Concrete capping concepts.
-------
controlled by efficient waste and cover compaction (although
some repair to the concrete cap may be necessary if
settlement is severe).
The second type of concrete cap would be much less
susceptible to mechanical damage due to frost heave or settling,
although the exposed concrete surface is susceptible to chipping
and flaking from freeze/thaw cycles. Because the quantity of
concrete is increased, the overall cost for capping the trench
is increased likewise (Table 9).
The third type of cap is actually a combination of void-
filler and cap. The concrete is poured on top of the wastes,
filling voids and essentially encapsulating the wastes. The
trench is filled with concrete, and the surface mounded.
Structually, only freeze/thaw, which can be largely overcome
by covering the concrete with soil or gravel, presents any
problems to this cap (see Chapter 6). This concept does have
the disadvantage of requiring a complete, filled trench for
a homogenous concrete cover; filling the trench in sections
creates joints which are more prone to failure than a contiguous
cover. Leaving the trench open until completely filled may
violate radwaste disposal safety regulations (100 m Rem/hr
(10)). This concept is also the most expensive by a wide
margi n (Table 9).
Di sadvantages--
Concrete is easily the most expensive of the capping
materials under discussion (28). It is completely inflexible
and will tend to crack under certain kinds of environmental
stress. Low temperatures can freeze interstitial water in
the concrete, expanding and cracking it. Concrete is
susceptible to chemical attack under both acid or alkaline
conditions, while concrete exposed to soils containing
sulfates will gradually dissolve (36).
In one sense, the permanence of concrete may become a
problem. If, in the future, it becomes necessary to remove
any transuranic radwastes and recover or relocate them, a
concrete cap would be a liability; removal would be costly
and time consuming.
Advantages--
On the positive side, concrete has a long record of
successful application as a watertight barrier with a long
lifetime. A variety of concrete casks.and boxes has been and
is being used at several low-level radwaste disposal sites
(Los Alamos, Oak Ridge, Hanford, Savannah River) to store
Plutonium and other transuranic wastes (80). These casks
and boxes are usually buried in burial trenches, and, to date,
42
-------
they have held up well with no leakage or mechanical failure
reported. At slightly higher cost, concrete can be im-
pregnated with one of several polymers and made even more
impermeable. A properly designed and constructed concrete
trench cap, protected from the environment, should, with
minimum maintenance, be able to provide an effective barrier
to water infiltration for at least several decades, based
on the apparent success of concrete dams, for example.
Asphaltics
Asphaltics include a variety of natural or refined liquid,
semi-sol id,or solid hydrocarbon mixtures. Asphaltic linings
are used regularly in canals and reservoirs, and asphaltic
patches are used to repair cracks in concrete or other linings.
There are several types of asphaltic materials which could be
used as a trench cap - asphalt concrete, hydraulic asphalt
concrete, soil asphalt, and catalytically blown bituminous seals.
Applicat ions--
Normally, asphalt concrete is applied as a heated mixture
of asphalt, aggregate, and filler; hydraulic asphalt concrete
differs in the asphalt content and aggregate gradation. Both
are mixed and laid using the same equipment and techniques,
although the hydraulic asphalt concrete is somewhat more diffi-
cult to handle (39). The major differences in the two types
of asphalt concrete are permeability and expense. Hydraulic
asphalt concrete, at $3.00 to $4.20/yd2, 4 in thick, installed,
runs about 30 percent higher than normal asphalt concrete (39).
The added expense buys a decreased permeability - from 1.2 x
10~8 cm/sec for normal asphalt concrete to 3.3 x 10~9 cm/sec
for hydraulic asphalt concrete (37).
Soil asphalt is a mixed-in-place surfacing made by mixing
a liquid asphalt into the soil. Compared to other asphaltic
caps, it is relatively inexpensive - $1.25/yd2 installed (39).
The permeability of a soil asphalt can be controlled by the
amount and type of asphalt used. One study used a soil asphalt
of an initial permeability of 1.7 x 10-3 cm/sec, which decreased
to 2.8 x 10~8 cm/sec after one year in the field (37).
Catalytically blown bituminous seals are produced by air-
blowing hot asphalt in the presence of a catalyst. One to one
and one-half gal/yd2 are required to form a film 3/16 to 5/16
in thick (39). This type of membrane is supposedly impervious
to water, but it is prone to mechanical damage. A soil cover
is usually necessary, bringing the total cost to $1.50 to
$2.00/yd2 installed.
43
-------
The use of asphaltics as water barriers is well es-
tablished. The main canal of the Boise Project, Idaho, has a
14-yr-old asphalt concrete surface which has shown no cracks
or signs of failures (55). Experience with asphaltics as
sanitary landfill liners is not very extensive. One of the
first such liners was constructed in 1971 at Montgomery
County, Pennsylvania. This liner was a 3-in tar cement with
a 1/8-in hot tar sealer coat covered with crushed rock and
incinerator ash (28). Similar systems have been constructed
elsewhere, but their relatively short time in place does
not allow a full evaluation of their success or failure as
long-term sanitary landfill liners.
Disadvantages--
Organic acids attack asphaltics to a limited extent, and
oily materials can cause failures. Neither of these should
present a problem at radwaste burial sites. Asphaltics are
generally photosensitive and will decay in direct sunlight
(39). Due to settlement or freezing, asphalt concretes are
susceptible to cracking. Soil asphalts and bituminous seals
are more flexible, but can become embrittled at extremely
low temperatures (-350Q). Bituminous seals lack sufficient
structural integrity to withstand much surface traffic,
animal or plant damage, or tearing due to settlement. Soil
asphalts may be the best compromise as trench caps in terms of
flexibility and strength, but they also are the most permeable
of the asphaltics (37).
Advantages--
Asphaltics are relatively inexpensive and, while placement
requires a certain expertise, this expertise is readily avail-
able nationwide. A structurally sound asphaltic membrane is
almost impermeable to water. Its sensitivities and weaknesses
can be overcome through the proper use of cover materials over
the membrane. Bituminous seals or soil asphalt could even
be used to seal each section of trench as it is filled. This,
in conjunction with an overall trench cap, could ensure that
a cap failure would at least be localized in only one section
of a given trench. At best, the double seal could eliminate
leachate due to surface damage, or retard it until the cap
could be repai red.
It should be noted that several nations, including the
United States and U.S.S.R., are seriously considering using
asphalt or bitumen as an encapsulating material for high-
level radwastes. Preliminary tests have been favorable, thus
establishing its potential applicability to low-level radwaste
si tes .
44
-------
The asphaltics also have the same disadvantage as concrete
in that their permanency precludes easy removal of the wastes,
should this ever become necessary. Otherwise, asphaltics could
be a good trench cap material for preventing water infiltration
into the trenches.
Soil Cement
Soil cement is a mixture of soil and Portland cement,
compacted at optimum moisture content and cured to hydrate the
cement. It is generally used for strength or seepage control,
where the expense or longevity of concrete is unnecessary.
The installed cost of a 6 to 8-in soil cement layer with a bitumv
nous seal coat is $1.25/yd2 (39) as compared to $7.90/yd2 for
a 4-in concrete layer (5). However, the reduced cost reflects
a somewhat reduced quality. Soil cements are seldom used
where the 50+ year longevity of concrete is required. Further-
more, depending on the quantity of cement and the soil type,
soil cements vary widely in permeability. Reported permeability
coefficients range from a high of 10 cm/sec (39) to 10~6 cm/sec
(74) to 1.5 x TO'8 cm/sec (37).
Soil cements are susceptible to attack by acids. They are
readily degraded by environmental factors such as wet-dry
or freeze-thaw cycles (39). Soil cements are more flexible than
concrete and thus could withstand some differential settling and
frost heave to a greater degree, although they too will crack
under sufficient stress.
Soil cements have a long history of successful applications
as low-cost liners for reservoirs, canals, ponds, sewage lagoons,
and dam facings. Rogers (74) lists a variety of actual applica-
tions of soil cement, including site descriptions and general
specifications. The longest lifetime he reports is 23 years for
a soil cement-lined reservoir in Port Isabel, Texas. At the
end of that period, the lining, with minimum maintenance needs,
was still performing satisfactorily.
A variation on traditional soil cement is carbonate bonding
(5), which incorporates lime hydrate instead of Portland cement;
it requires carbon dioxide gas to set up properly, and it is
cost competitive with normal soil cement ($1.30/yd2 for a 6-in
layer installed (5), and $1.25/yd2 for a 6-in layer installed
(39)). Its permeability to water is comparable to that of
concrete, and it resembles normal soil cement in other respects.
It might find its greatest application in heavy clay soils
where the carbonate bonds form best and where good soil cements
are most difficult to attain. Carbonate bonding has not yet
been evaluated in full-scale applications.
45
-------
Synthetic Polymer Membranes
Flexible polymeric membranes are assuming increased im-
portance as liner materials because of their low permeability
to water. The polymeric materials used in the manufacture of
these liners includes synthetic plastics and rubbers. The
membrane sheeting is usually available in rolls about 2 meters
wide and 60 meters long. Several sheets can be factory seamed
by the fabricator to form larger panels. The sheets can be
heat sealed, cemented, or solvent welded in the field. Often
two sheets are plied together to increase strength and reduce
the effects of pinholes and other defects. Sometimes a nylon
or polyester scrim fabric reinforcement is sandwiched between
the plies to give added strength to the liner.
The use of plastics and synthetic rubbers as watertight
liners is a relatively recent innovation. In fact, most of
the synthetic polymers under consideration today did not even
exist commercially 25 years ago. Their use as sanitary land-
fill or reservoir liners is even more recent. Consequently, it
is difficult to estimate, longevities for many of the membrane
types simply because there has not been sufficient time to
test them.
Types--
There are at least seven types of synthetic polymer
membranes currently used or under consideration for landfill
liners - butyl rubber, Hypalon, chlorinated polyethylene (CPE),
polyethylene (PE), polyvinyl chloride (PVC), Neoprene, and
ethylene-propylene diene monomer (EPDM). These membranes vary
widely in characteristics and costs. Table 10 lists the costs
for each commonly used synthetic membrane.
Butyl rubber is one of the older synthetic polymers. As
such, it has demonstrated longevities in excess of 20 years
(38, 39). It is one of the least permeable membrane materials,
but is also one of the more expensive. Butyl rubber has ex-
cellent resistance to permeation of water and swelling in water.
Butyl liners age well although they are susceptible to cracking
in the presence of ozone. They are difficult to splice in the
field.
Polyethylene is the least expensive membrane material. It
is probably the least durable as well. Membranes of this
material are often full of pinholes and blisters which can go
undetected (39). Polyethylene weathers poorly and has poor
puncture resistance (28). Despite these drawbacks, there are
examples of effective PE liners with lifetimes exceeding 10 years
(40). These liners are usually multi-ply and are often reinforced
46
-------
TABLE 10. COSTS OF SYNTHETIC POLYMER MEMBRANE LINERS
Li ner
Butyl rubber
Hypalon
Neoprene
Chlorinated polyethylene
Polyethyl ene
Ethylene-propyl ene diene
monomer
Polyvinyl chloride
2a
Installed Cost/yd^
$2.70-3.80
2.30-3.00
4.40-5.40
2.30-3.00
0.90-1.40
2.60-3.70
1 .20-2.00
Reference
No.
28
28
39
28
39
28
28, 40
aSoil cover over membrane not included, cost of which could
range from $0.10 to $0.50/yd /ft of depth.
47
-------
Polyvinyl chloride is one of the more common synthetic
liners. It is strong, puncture and chemical resistant, and
very impermeable. Its stability is variable, however,
depending on the particular plasticizer used. As a result,
PVC may be subject to deterioration from wind, sunlight or
heat (39). Some plasticizers are biodegradable or water
soluble. PVC tends to stiffen and become brittle at sub-
freezing temperatures (28). It is probably the most popular
1i ner material.
Ethylene-propylene diene monomer is very resistant to
weathering or temperature deterioration. It is heat resistant
and retains its flexibility at low temperatures, but it has
poor chemical resistance.
Chlorinated polyethylene is a relatively recent develop-
ment. It makes durable linings for wastewater, or chemical
storage pits, ponds, or reservoirs. It withstands ozone,
weathering, ultraviolet light, and microbial attack.
Hypalon is a high cost membrane material. It is highly
chemical resistant with good puncture and temperature
resistance. It has a low tensile strength, however, which
makes it somewhat susceptible to breakage due to settling or
frost heave in the underlying soil or wastes. It is the
second most popular liner material.
Most of these membranes have been used as liners in
sanitary landfills, reservoirs, sewage lagoons, or similar
applications. Some, such as PVC, PE, and butyl rubber have
demonstrated longevities of at least 10 to 20 years (39, 40).
For further information on the properties of these membranes,
with examples of their applications, see Geswein (28), Haxo
(37, 38, 39), Mickey (40), Goldstein (30), C.luff (11), or . .
Moeller and Ryffel (62).
Applications--
As trench caps, the membrane liner would be laid on over
the mounded soil cover. In general, the membranes come in
rolls and may need to be seamed to provide a continuous
surface. Large panels are available for some types of
membranes. Seaming can be a construction problem for some
polymer .types and generally is the weak link in the liner.
One-yr leachate tests on PVC, CPE, and Hypalon have shown that
the seams may fail even when the membranes remain intact
(37). Under mechanical stress, the seams often given way
first. .
As trench caps, the membranes would generally be exposed
only to infiltrating rain water which is relatively weak
48
-------
chemically. As a result, membranes susceptible to chemical
attack might still be suitable as trench caps. The major
threats to synthetic membranes as caps are microbial attack,
sunlight, burrowing animals, plant roots, or weathering.
The membrane could be covered with a layer of soil to
prevent mechanical damage or deterioration from sunlight.
The proper synthetic polymer membrane correctly emplaced can
provide an impermeable trench cap of at least 30 yr duration,
with the added advantage of providing a good cap while being
easily removable in the event that the trench contents need
to be removed. Reclosing the trench would require a new
membrane, but this is much less expensive or troublesome than
replacing a concrete or asphalt trench cap.
Clay is a natural, polymeric hydrous alumino-silicate
mineral. Structurally it is formed of layers of connected
tetrahedral or octahedral molecular units. Because of the
charge distribution of these units, the surfaces of the
sheets are populated by cations, usually sodium or calcium.
The addition of water causes the cations to hydrate, generating
electrical charges that repel the sheets. Physically, the
clay particles "swell.." A layer of the "swollen" particles
is virtually impermeable to water because of the negligible
pore space between particles.
This sealing property makes clay an ideal liner for water-
containing structures - reservoirs, ponds, canals, etc. Clay
layers have also been used, with varying success, to line
sani tary landfi11s.
Clays are available from several sources. Often, natural
clay deposits in the vicinity of disposal sites are used.
These clays are generally combinations of illite, kaolinite,
attapulgite, and montmori11onite in varying quantities (1).
Most native clays contain substantial nonclay portions con-
sisting of sand and silt, which diminishes the value of the
clay as a sealant. The alternative is to buy proprietary clay
mixtures (e.g., bentonite, Volclay) which adds to the expense
of the clay, but ensures chemical and physical quality.
Most successful clay liners have relied on commercial clays.
The chemical nature of the clay can strongly affect its
permeability. Sodium bentonite (standard Wyoming bentonite),
for instance, is several times more "swellable" than calcium
bentonite; that is, calcium bentonite swells to a much lesser
degree. Consequently, sodium bentonite is generally less
permeable. It is also less stable. If sodium bentonite comes
in contact with water containing divalent cations (e.g.,
49
-------
calcium, magnesium), the sodium will be replaced by the
divalent cations. This, in turn, affects the structural
integrity of the clay and shortens its usable lifetime
dramatically. Calcium bentonites are generally unaffected by
this type of situation and are chemically more stable.
Applications--
As a trench cap, a clay layer would be placed over the
mounded soil cover and covered with another layer of excavated
soil. The soil layer must be thick enough to prevent the
clay from drying out; when the clay dries out, it cracks.
Swelling will close the cracks when the clay is wet again,
but sand or soil particles will undoubtedly have gotten into
the cracks, making the closure incomplete. After several such
wet and dry cycles, the incompletely closed cracks could go
through the clay and breach the layer. The thickness of the
soil cover required to prevent such drying out will vary,
depending on local climatic conditions. Hawkins and Horton
(36) found that a 1-ft soil layer was insufficient to protect
a clay layer during a 3-mo drought, whereas 2 ft of soil pro-
tected the clay and prevented cracking.
Often, especially in sanitary landfill applications, clay
is used in conjunction with another liner, such as plastic
membranes. While more costly than either liner type separately,
the combination is stable, very impermeable, and not subject
to many of the disadvantages which hamper the use of clay alone.
Advantages and Disadvantages--
Clay liners are extremely susceptible to biological
damage. Insects and animals can burrow through them with
little difficulty. Plant roots can be especially damaging.
Use of pesticides or plastic membranes along with the clay
can prevent much of this type of damage.
Clay liners are generally flexible and can tolerate some
deformity due to differential settling. The clay must be
protected from freezing temperatures, as the expansion of
the water of hydration can crack the liner and threaten its
integrity. A soil or gravel cover of sufficient thickness
can prevent this.
Cost estimates for bentonite clay vary. Haxo (39) gives
an installed cost for a 9-lb mixture of soil and bentonite of
$0.72/yd2. Hawkins and Horton (39) state that the installed
cost of a bentonite cover in 1967 was about equal to that of
concrete. Since clay has not increased in cost at the same
rate as Portland cement, concrete covers would be slightly
more costly than clay in 1977. In either case, it has been
50
-------
estimated that, in the absence of mechanical damage, a clay
liner has an expected longevity of thousands of years (36).
Soil Sealants
Appli cations--
Soil sealants are chemicals, usually polymers of some
sort, which can be admixed with the soil or clay of a trench
cover to form a relatively impermeable layer. They work by
polymerizing or swelling between the soil particles and
forming a sheet effect around the particles, sealing the
surface of the trench cover. They are applied as liquids to
the trench cover and generally penetrate several millimeters
or centimeters into the soil. They are polymerized either
through drying or with steam heat. These sealers are fairly
new and relatively untested in full-scale applications.
There are a wide variety of soil sealers: calcium
1ignosulfonate, alumino-si 1icate gels, elastomeric polymers,
latex, asphalt emulsions, and styrenes. Gulf South Research
Institute (33) has done considerable research with these
materials and reported that bentonite alone worked better than
any of them.
Di sadvantages--
The major problem with the sealers is lack of control over
the polymerization or sealing process. Consequently, incomplete
seals are rather common. Soil sealants are of marginal
stability, subject, in many cases, to both chemical and biologi-
cal attack. These defects have led Uniroyal, for instance, to
conclude that latex will not work as a soil sealant (87).
In general, soil sealants have found their greatest appli-
cability in the stabilization of mine tailings piles (35, 86).
Sealants provide structural stability even when they do not
form impermeable surface layers.
Advantages--
The greatest advantage of soil sealants is their cost.
Many soil sealants can be formulated from waste polymers and
consequently, are quite inexpensive. Havens and Dean (35)
estimate that either DCA-70, an elastomeric polymer, or Norlig A
a calcium 1ignosulfonate, would cost less than $0.10/yd2
installed. In conjunction with a better cap, such as bentonite
or synthetic membranes, they would be a good backup system. It
might be possible to mix some of these sealants in with the fill
and cover soils in the trenches before capping to provide an
additional safety factor against water infiltration, in the
event that the basic material cap does fail.
51
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TRENCH CAP COVERS
None of the trench cap concepts mentioned so far is
completely resistant to environmental stresses: concrete
will flake under freeze-thaw or crack due to frost heave;
asphalt will crack from frost heave or degrade in sunlight;
soil cements will degrade like concrete; many synthetic mem-
branes will degrade in sunlight or suffer mechanical damage.
Consequently, some sort of cover is required to protect the
trench cap. This cover will prevent mechanical damage,
protect the cap from sunlight, freeze/thaw and frost heave,
and keep clays damp.
The two principal cover materials are gravel and soil.
Excavated trench soil material could be used for the soil cover;
gravel would usually need to be purchased and brought in from
off site. The placement of gravel directly on top of some
trench caps (e.g., synthetic polymer membranes) may damage the
cap material. For those caps most susceptible to mechanical
damage, a thin layer of soil can be laid down first and covered
with gravel .
The depth of the cover is largely dependent on the local
climate. Since one of the purposes of the cover is to protect
the cap from frost heave, it must be deep enough to thwart frost
penetration to the cap. Figure 7 shows the average frost
penetrations in the United States. For some caps, such as clay,
the cover must also be deep enough to prevent drying. A depth
of 2 ft was previously cited as being sufficient to prevent clay
drying and cracking under severe conditions. In this context,
a soil cover would probably work better than gravel. A gravel
cover would allow freer air circulation at the cap surface
with probable drying. Of course, vegetative damage to a clay
cap would be more likely with a soil cover than with gravel.
A layered cover, half soil and half gravel, might be the best
compromise for a clay cap cover.
Soil covers are susceptible to erosion or mechanical
damage. Normally, a vegetative cover could be used to prevent
such damage, but there may be the threat of root damage
to the cap. It would be possible to use some of the soil
sealants mentioned earlier to stabilize the soil surface,
prevent erosion, and inhibit vegetation growth. Herbicide
application to the surface may be necessary, where excessive
weed growth is expected.
In general, for most cap materials, the multi-layer cover
concept appears best. The soil cover protects the cap, and the
gravel protects the soil cover. An average trench would then
resemble Figure 8: waste material, soil fill and mounded
cover, trench cap, cover soil, and gravel top cover. Use of
this type of system can reduce maintenance costs dramatically.
52
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on
Co
Depth of
Frost Penetration
Contours represent averoge depth of frost
penetration in inches where data permit
generalization. Elsewhere, depths at
individual stations are shown.
(Based on 40 years of record)
Source: U.S. Dept of Agriculture
Source: The Clow Corporation. Pipe Economy, 1975.
Figure 7. Depth of frost penetration.
-------
COMPACTED SOIL (0.3 TO 0.6 M)
GRAVEL
(15 TO 40
CM)
TRENCH CAP
(MEMBRANE, CLAY* ETC.)
COMPACTED
SOIL
BACKFILL
WASTE MATERIAL
Figure 8. Conceptual completed low-level radwaste disposal trench,
54
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CHAPTER 5
ALTERNATIVE TRENCH AND SITE CONSTRUCTION METHODS
INTRODUCTION
Two concepts will be discussed in this chapter:
1. Methods that can be implemented prior to the deposit
of radioactive wastes in a disposal trench, i.e.,
during the initial trench excavation and generally
applicable only at future or partially completed
di sposal si tes.
2. Methods or procedures that can be implemented during
or after disposal operations (other than trench
covers, which are discussed in Chapter 4) and as
remedial measures at existing sites where trenches
are completed.
The trench and site construction concepts discussed in
this chapter are not intended to be considered as infiltration
controls independent of the trench covers discussed in Chapter
4; rather, the concepts should be viewed as complementary.
Optimally constructed disposal trenches depending on particular
infiltration control needs at a given site would probably
include features presented in both chapters. Whereas the con-
cepts developed in Chapter 4 are concerned primarily with
infiltration through the cover, this chapter addresses such
problems as surface runoff control and curtailment of hori-
zontal groundwater percol ation in saturated soils.
Table 11 presents a summary of the concepts addressed in
this chapter. Some of these concepts are theoretical, and
thus largely untested, but potentially usable; others have been
used, but not at low level radwaste disposal sites. In most
cases, the estimated longevities are conjectural, since the
methods have not been evaluated over the time spans considered
applicable for radwaste decay and/or degradation. Even those
concepts of proven utility in management of other types of
wastes have not been evaluated in the radwaste disposal
environment, although such methods are valid concepts for
potential radwaste disposal applications. However, direct
application of methods developed for other wastes to radwaste
disposal conditions may not be possible without modifications or
adjustments in design, construction technique, and/or monitoring
procedures .
55
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TABLE 11. SUMMARY OF TRENCH AND SITE CONSTRUCTION ALTERNATIVES
en
Concept
Description Comments
Expected
Longevity
Approxi-
mate
Cost/
Trench,
Instal-
led3
Refer-
ence
No.
Situate disposal
site on a slope
or hillside
Situate disposal
site on flat
ground
Construct system of
berms/drainage
ditches on the site
Construct trenches
with level bottoms
Construct trenches
with sloped bottoms
Line trench bottom
with permeable soil
or other material
Applicable for new sites only
Applicable for new sites only
Applicable as a remedial measure and at
new site construction
Applicable for new trenches only
Applicable for new trenches only
Applicable for new trenches only
NA
NA
Indefinite with $10/linear
regular mainte- meter of
nance berm or
ditch
NA
NA
NA
$500-1,000
42
42
42
42
42
Costs are for a "typical" 100 m by 15 m x 6 m trench
-------
TABLE 11. (continued)
Ul
--J
Concept
Description
Construct narrow gravel-
filled diversion trenches
around entire site and/or
individual disposal
trenches
Comments
Applicable at new sites or as
a remedial measure
Expected Longivity
Indefinite with
regular maintenance
Approxi-
mate
Cost/
Trench, Refer-
Instal- ence
led3 No.
$100/1 inear 36
meter of
diversion
trench
Trench liners
Grout curtains around
sites and/or trench
periphery
In-situ encapsulation-
injection of a slurry
of impermeable material
into each trench to fill
void spaces
Applicable for new trenches
only
Applicable primarily as a
remedial measure, but can be
used at new sites
Applicable primarily as a
remedial measure in closed
trenches
Varies with
material and expo-
sure conditions;
at least 5 to 40
years
$40,000 to 28, 39,
$275,000. 75
depending on
linear
material
Variable, depend-
ing on the filler
material; 10 to
50 years
Up to
$70,000,
depending on
grout mate-
rial used
$140,000
to $700,000,
depending on
material
2, 8, 75
Costs are for a "typical" 100 m by 15 m x 6 m trench
-------
TRENCH CONSTRUCTION
The typical trench configuration described in Chapter 2
shows each trench with a sloped (3 to 5 percent) bottom. This
method of construction allows any water entering the trench
during fill operations (e.g., direct rainfall and undiverted
runoff) to drain to one end of the trench where it can be
conveniently pumped out. However, a sloped bottom may actually
contribute to the presence of water in the trench, as noted by
Horton (42). In his study of soil moisture flow in relation to
radwaste disposal, he suggests that radwaste disposal trenches
be constructed with level trench bottoms. However, flat bottoms
do not prevent rain water or runoff from entering trenches
during fill operations, and removal of any water is difficult,
if it is in a relatively shallow layer. If a trench can be
filled in such a manner as to avoid standing water during the
operation (e.g., by location of sites in arid climates; rapid
excavation and filling of trenches during dry spells; dewater-
ing at the site during excavation; a system of berms and
temporary tarps to cover open trenches), then the level bottom
probably is less likely to allow horizontal water movement into
the trench. Trenches with level bottoms may cost more to
monitor and remove water from, than trenches with sloped bottoms,
The potential for horizontal percolation of water in the
unsaturated zone contacting radwaste through sloped bottoms
can be mitigated somewhat by providing a 30- to 60-cm thick
gravel floor in the trench before placement of wastes, prevent-
ing contact between the wastes and any standing water. Frequent
monitoring would be necessary to ensure that the depth of the
standing water did not rise above the depth of the gravel layer.
TRENCH SITING
Trench siting, whether on flat ground or slopes, can
significantly affect the potential for infiltration of surface
runoff into disposal trenches. If a trench is located on an
incline, horizontal water percolation in the unsaturated zone
into the trench is possible, because the original ground surface
slopes toward and intersects the trench. Water infiltrating
in homogenous soils through the zone of aeration will naturally
tend to move toward the area of lower potential, in this case
the trench, depending upon the nature of the backfill (Figure 9)
One way to avoid this is to place trenches only on a relatively
flat ground. However, surface drainage at flat sites is often
poor, allowing rainfall or spilled water to pond between the
mounded trench covers. In such cases, a series of drainage
ditches would be necessary to maintain water-free surfaces
between the trenches. Normally, however, percolating waters
will flow in a vertical pathway until reaching the ground-
water table.
58
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PRECIPITATION
PRECIPITATION
IMPERMEABLE CAP
BACKFILLED
TRENCH
PATHWAYS OF
PERCOLATING WATER
Figure 9. Possible horizontal flow of water into a
backfi11ed trench.
59
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Placement of trenches on grades should not be ruled out.
Figure 10 depicts a slope site, with the topsoil removed,
protected by a series of berms and an upslope diversion ditch.
The natural slope allows precipitation runoff with little
danger of ponding, while the ditch and berms divert most
surface water away from the trenches. This configuration will
inhibit both horizontal movement in the saturated zone and
surface runoff into the trenches.
DIVERSION TRENCHES
Deep narrow trenches filled with a permeable medium (e.g.,
gravel) are sometimes used to prevent or control horizontal
groundwater movement and divert water away from the individual
disposal trenches or even the entire site. Groundwater
encountering one of these diversion trenches will follow the
diversion trench around the disposal trench or site if there
is a sufficient gradient within the trench. This system, which
has been successfully applied to groundwater control at sanitary
landfills, has been proposed by other researchers for use at
radwaste disposal sites (36).
The narrow trenches provide a high permeability path for
the water to flow around the site. This type of approach is
analogous to electronic shielding problems where a circuit
is isolated from outside voltage gradients by putting the
circuit in question inside a conducting box. There is no need
to dewater the trench as long as the hydraulic potential is
less than in the surrounding soil. If groundwater periodically
rises into the wastes, then falls again, the diversion trench
system will not prevent contamination from this pumping action.
In practice, the trenches should be about 50 cm wide,
deeper than the level of the disposal trench bottom, and
usually filled with gravel or other permeable material to
within one meter of the surface and then backfilled with the
excavated soil. Such diversion trenches down to the aquifer
will prevent the water table, in areas of high water table,
from rising into the disposal trenches from beneath.
In arid regions, it might be possible to limit construction
of diversion trenches to the periphery of the site, rather than
around each trench, while for sites in wetter regions or in
high water table areas, diversion trenches throughout the site,
or possibly around each trench (Figure 11) would provide better
protection. Economics might be a factor for this concept;
construction of diversion trenches will cost about $100/1inear
meter.
60
-------
DIVERSION
DITCH
BERM
BERM
TRENCH 2
Source: 40
Figure 10. Burial trench arrangement to impede horizontal water infiltra-
tion through the topsoil
61
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TRENCH PLAN
LINED TRENCHES
TYPICAL
no
0. 0%
2'-0 TYP
LONGITUDINAL SECTION
NOTE: ALL ELEVATIONS ARE ASSUMED
^x o
o "o
0.0%
CROSS SECTION
GRAVEL TRENCH
(TYP)
GRAVEL
TRENCH
(TYP)
Figure 11 . Radwaste disposal trench with diversion trenches.
-------
TRENCH LINERS
Nearly all of the materials previously discussed as being
applicable for trench caps could also be used as trench liners;
most information referenced in the discussion of trench cap
materials concerns use of these materials as liners for sanitary
landfills, ponds, reservoirs, and canals. Most of the informa-
tion on material properties, costs, advantages, and disadvan-
tages presented in Chapter 4 are, therefore, largely applicable
here as well. In general, since materials used as liners are
generally buried, damages from frost heave, freeze/thaw,
settling and sunlight are less a problem for liners than for
caps. However, chemical damage (due to soil alkalinity,
acidity, sulfates, and leachate) may be more severe.
Advantages and Disadvantages
Aside from the behavior of individual liners, the concept
of trench lining has numerous advantages and disadvantages. A
good liner can prevent groundwater infiltration into the
trenches through both the bottom and the sides. If water
infiltrates into the trench through the surface, any leachate
formed will be retained for extended periods and should not
enter the environment, as long as the liner remains intact.
However, the ability to hold water can also be viewed as a
major disadvantage of trench liners. Lined trenches are
essentially bathtubs; they trap and hold any water that may
have entered the wastes, leaving it in contact with the wastes
for an indefinite time. Under such conditions, more highly
concentrated leachates than might otherwise be encountered can
be produced. As a result, lined trenches may require more
rigorous monitoring and more frequent remedial pumping than
unlined trenches. This, in turn, could create a radioactive
liquid waste disposal problem. In general, most experts have
been unenthusiastic about the applicability of the trench liner
concept to low-level radwaste disposal situations.
GROUT CURTAINS
Grouting is the process of injecting appropriate material
into soils and rocks thus reducing the strata's permeability
and/or increasing its strength. As a water sealant, grouting
has a history of successful application in mines and tunnels,
and under dams (7, 8). Grouting materials include cement,
clays, asphalts, bitumens, silicates, 1ignochromes, 1ignosulfates
epoxy resins, acrylamide, polyester resins, polyphenolics,
resorcinolformaldehyde, and other chemical polymers (6, 7).
63
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At a low-level radwaste disposal site, grout curtains
could be placed around a burial trench (Figure 12) at any
time during the life of a trench site in such a way as to
prevent the horizontal movement of subsurface water, either
into or out of the trench. Grout curtains would work better
as a remedial measure as new trench construction could damage
adjacent grout curtains around existing trenches.
The longevity of grout curtains can be expected to vary
with the materials used; in general, they are expected to
last the life of the dam (7). References 7 and 8 contain
lists of many sites where old grout curtains are still intact.
The soil or rock beneath a trench could conceivably be
grouted as well, but such practice could lead to the formation
of a "bathtub," as discussed above. In areas of fractured
bedrock grout could be used to seal some of the fissures, thus
providing a more secure and better delineated foundation for
a disposal site. Grout injections could also be used to seal
leaks in the other liner systems.
The variety of grout materials and injection techniques
available allows' considerable versatility in how and where grouts
can be used. In general, grouting is easiest in loose, dry
soils, since wet soils can inhibit and slow grout gelling,
sometimes to the extent that the grout material can dissolve
or be washed away before it has a chance to set. Fine grain
soils require low viscosity grouting materials to ensure
proper spreading. Despite these and similar problems, enough
different materials are available to provide sufficient sealing
in almost every soil environment to be expected at a disposal
si te.
Recently, Applied Nucleonics Company, Inc., Los Angeles,
studied radwaste relocation at Hanford, Washington; the
feasibility of grouting around radwaste trenches was investi-
gated^). Although final results of this study are not yet
available, indications are that grouting does have potential
for controlling water infiltration at this site.
Grouting has several disadvantages. Considerable research
is required to determine precise site soil conditions and to
select suitable grout material. Skilled personnel are needed
to properly place the grout, and it is difficult to assess
whether the grout curtain is working unless it fails. Costs
are generally in the range of $400/m3 installed, or up to
$70,000 per typical trench for a complete curtain (around all
four sides) .
For more complete information, Reference 2 contains an
extensive bibliography on all aspects of grouting.
64
-------
CTl
cn
~^
TRENCH PLAN
0.0 %
2' TYP
LONGITUDINAL SECTION
/«
CHEMICAL
GROUT
LINED CHANNEL
(TYPICAL )
'-
o.o
^
CROSS SECTION
NOTE: ALL ELEVATIONS ARE ASSUMED
Figure 12. Radwaste disposal trench with chemical grout curtain.
-------
IN-SITU ENCAPSULATION
Because of the loose packing of radwastes in many burial
trenches, there may be up to 2,000 m3 of void space per typical
trench of 7,000 m3 total volume. If water does enter a trench,
the amount of waste surface area available for contact suggests
that highly concentrated leachates could be generated. Thus,
it would be advantageous to prevent or minimize infiltration
into the wastes (and thereby reduce potential leachate
production) by filling (encapsulating) the void space with a
relatively impermeable material. Water contact with the wastes
would be reduced and, as an added benefit, settlement of the
waste as it decomposes could be decreased significantly,
especially if the void filler material had some structural
strength.
The same basic methodology would be used for encapsulation
as in grouting. In general the nature of the wastes in the
trenches is such that less care is needed in selecting a
suitable void fill material, and, in fact, it might be possible
to use certain wastes, such as fly ash or spent drilling muds.
Fly ash will set up like concrete and provide a nearly imper-
meable fill material, and, being a waste product of combustion
the major costs would be in transport to the disposal site and
application. Materials costs would be very low or zero,
although the use of fly ash for encapsulation would be limited
to sites near facilities producing such a waste. Drilling
muds can provide a seal of permeability comparable to bentonite.
Being fluid, drilling muds should readily fill void spaces in
radwaste trenches, and the water in drilling muds is such an
integral part of the mud structure that little excess water
(and thus, little leachate production) need be expected from the
mud/radwaste contact. Drilling muds being inorganic and
basically clay, would not degrade in the anaerobic environment
of a trench. There are examples of drilling mud linings which
have successfully withstood mechanical and chemical assault
for many years (8).
Due to the variety of materials that could be used, costs
of in-situ encapsulation would vary widely. For instance,
costs for fly ash filler would be limited almost exclusively to
the installation. In general, costs will range from $2 to
$10/ft3, or up to $140,000 to $700,000 per trench.
66
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Advantages and Disadvantages
Filling the void spaces with a material of some structural
strength can inhibit the waste settling which undermines trench
caps, causes cracks, and allows surface water infiltration.
Many fillers have indefinite life spans and employ relatively
inexpensive waste materials.
On the other hand, there might be an initial danger of
leachate production from the injection of some slurried fillers
Furthermore, some of the filler materials (such as fly ash) may
themselves contain constituents that are capable of producing
hazardous leachate. There would likely be a lack of quality
control when injecting into a closed trench; void filling may
be incomplete. The skilled personnel necessary for application
would make the costs high even though low cost waste materials
are used as fi1lers.
67
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CHAPTER 6
BURIAL TRENCH MAINTENANCE AND MONITORING
INTRODUCTION
No trench burial method can be completely secure indefi-
nitely. However, it is questionable whether truly indefinite
security is really necessary. Unlike other hazardous materials,
radwastes will decay to innocuous forms. And, although
centuries may be necessary for complete decay, much shorter
time spans will yield relatively safe wastes. The issue, then,
becomes one of maintaining tight control over water infiltra-
tion for a relatively short period of time, fifty years for
instance, after which limited infiltration and leakage can
be tolerated, since any leachate that would then be generated
would not be radioactive.
How soon and how much leakage and leachate can be tolerated
is dependent largely on the hydrogeology and concomitant attenu-
ation mechanisms of the disposal site and regional environment.
In some respects, the true working life of a disposal site includes
not only the secure lifetime of the trench itself but also the
movement time of contaminants from the trench to the biosphere.
The factors affecting this movement time include:
• Distance to groundwater;
• Direction and rate of groundwater movement;
• Size and use of aquifer (volume of water);
• Hydraulic conductivity, porosity, and mineralogy
of the sub-trench lithology;
t lon-exchange/adsorption capacity of the subsurface
soils;
• Position and extent of any fractures in the bedrock;
and
• Distribution coefficients for nuclide movement
through the hydrostratigraphy.
68
-------
Theoretically, without extensive trench modification or
sealing, a radioactive waste requiring X years to decay to a
safe level could be placed in ordinary trenches in an environ-
ment which had an X-year delay between trench and biosphere.
On the other hand, an environment allowing rapid transport of
contaminants to ground or surface water would require a trench
with a water-secure lifetime of at least X years. In practice,
to account for unknowns and emergencies, safety factors dictate
a combination of trench life and movement rate controls which
exceed the X years. Consequently, site factors play a large
role in determining not only site acceptability but also the
nature and duration of the trench seal required to prevent
water infiltration. It will likely be beneficial to incorporate
some combination of sealing, capping, or encapsulation at all
burial sites, regardless of the natural attenuation charac-
teristics present.
A good example of this type of reasoning is found in the
design of the Barnwell , South Carolina, burial site (43).
Although in a high rainfall area, this site possesses a highly
favorable groundwater hydrology. A hydrostatic head reversal
prevents downward flow into an aquifer; thus, migration of
nuclides is confined to the direction of surface streams. Given
the depth to the water table, the short flow path to a surface
stream, and the groundwater movement rate, the estimated travel
time for subsurface water from the solid radioactive waste
storage site to this stream is about 70 years. Consequently,
little attempt is made to prevent infiltration into the trenches.
Trenches are pumped regularly to produce a short residence time
and a less contaminated leachate. The 70 years is considered
sufficient to allow for complete attenuation of any radionuclides
in the leachate.
SURFACE MAINTENANCE
Regardless of the type of cap, cover, trench, or berm,
some degree of site maintenance will undoubtedly be required
to ensure cap integrity and to minimize infiltration. Any
system, especially a poorly-designed system, is subject to
failure from settlement or cover erosion. Even the best systems
will require periodic inspection to identify problems and to
indicate repairs if problems are found.
The causes and effects of settlement have been noted
previously. If waste settlement within a trench threatens the
integrity of a cover or cap, the only alternatives available
are to replace the cover and/or arrest the settling. In severe
cases, where settlement has caused cover failure, it may be
necessary to remove the existing cover, recompact the wastes,
add soil or additional waste to fill the void, and place a new
cover. For the non-permanent cover types (soil sealants,
69
-------
synthetic membranes, clays, etc.), it might be possible to
simply compact the cover material well and place a new cover on
top of the old one. However, further settlement could cause a
repetition of the problem.
It might be possible to arrest settling by injecting a non-
settleable material, such as spent drilling mud, bentonite
slurry, concrete, or similar grouting substances, into the
trenches. The major objection to such injection is that these
all contain water and water is the liquid to be kept out of
the trenches. However, for some injectable materials, such
as drilling muds, the water is such an integral part of the
slurry that leaching will, at most, be minimal. Significant
leaching will not likely occur with any of the materials.
Erosion of the trench cover can be caused by wind, water,
or temperature extremes. In the earlier discussion of trench
caps, it was determined that a multi-layer system where the
cap was covered with soil and gravel layers could effectively
control erosion. Gravel covers for upstream runoff diversion
berms would also help prevent erosion of the berm material.
Berms and trench covers can still be damaged by severe
weather incidents or changes (e.g., torrential rains, tornados,
hurricanes, heavy freezes, spring thaws) or earthquakes. Close
inspection following such incidents is necessary to locate and
assess any damage. In the event of collapsed berms, rebuilding
might be necessary. Damaged gravel and soil covers would need
to be replaced where necessary. Cracks in permanent caps (e.g.,
concrete, asphalt) could be patched with a suitable synthetic
or asphaltic patching compound.
In extreme cases where a cap or berm is too extensively
damaged to repair, replacement would be necessary. This is
not likely if the disposal site is properly designed, con-
structed, and maintained.
Maintenance also includes keeping the trenches water free.
A sloped-bottom trench with a stone-filled sump at the low end
and a permanent standpipe in place can be checked periodically
to see if, in spite of all precautions, water has infiltrated.
If it has, the trenches can be pumped out. If trenches are
allowed to fill or if water is allowed to stand in the trenches
for very long, opportunities for the formation and movement of
hazardous leachate are present. The pumped water could be
examined for radioactivity and, if necessary, routed to holding
ponds for further treatment before disposal. There should be
a series of standpipes or a length of perforated collection
pipe along the bottom of flat-bottomed trenches. Construction
70
-------
of the sump and standpipe are considered routine. Thus, they
add nothing to the trench construction costs over the normal.
A portable generator and pump could be used to dewater the
trenches. Lengths of flexible hose would be needed to route the
water into temporary holding ponds. A capital outlay of less
than $1,000 should be sufficient.
Because of the nature of the wastes, it may be necessary
to monitor and pump indefinitely. Leachate from radwastes
could be hazardous at any time during the hazardous lifetime
of the wastes.
VEGETATION AND LANDSCAPING
Vegetation and landscaping can serve a dual purpose at a
disposal site: erosion control and aesthetic improvement.
A nicely landscaped site is far more pleasing to look at than a
series of mounds covered with concrete, asphalt, or gravel.
Figure 13 shows a closed burial site which has been contoured
and planted with grass at Oak Ridge National Laboratory.
Shallow-rooted grasses are used for erosion control at Maxey
Flats (60).
Vegetation serves a variety of purposes other than aesthetics
Roots hold the soil and prevent water and wind erosion of covers
and the soil between the trenches. Fenimore's studies show
that once a grass cover is established, very little further care
and maintenance is required to maintain the cover. Vegetation
also plays a significant part in water infiltration control
through evapotranspiration.
The issue of landscaping is not simple, however. Kenny
(51) reports that deep-rooted plants on the site of buried waste
from a nuclear reactor accident are radioactively contaminated
due to the raising of radionuclides through the plant. Hawkins
and Morton (36) reported similar aftereffects in Bermuda grass
and small plants. They further concluded that plant growth
must be completely prevented if bentonite caps are used, since
the clay has little resistance to root penetration. Also, plant
growth could conceivably aggravate minute cracks or holes in
other types of caps. Such root penetration into the cap will
tend to increase the potential for water infiltration. Fenimore
examined several grasses for use as possible covers at radwaste
burial sites (21). He found that Carpet grass roots could pene-
trate to 46 cm (18 inches), Bahia and Dallis grass roots to
16 inches, Centipede grass roots to 30 cm (12 inches), and
Bermuda grass roots to 26 cm (10 inches).
Overall, the benefits derived from planting cover vegetation
on completed burial trenches (erosion control, aesthetics) may
be more significant than the costs of continuing surface
maintenance. There are no published reports of any vegetation-
related problems at the Oak Ridge or Maxey Flats disposal areas,
for example.
71
-------
Figure 13. Grass covered radwaste disposal site,
72
-------
Alternatively, gravel cover can provide erosion control
similar to that provided by vegetation, without the potential
side-effects of cap penetration and radionuclide trans!ocation.
Burial sites prone to root damage could be kept free of unwanted
vegetation through the use of gravel covers or the regular use
of herbicides. If, in the future, aesthetics prove to be a
major issue, the site could be screened with a line of trees,
hedges, walls, or similar screening devices.
Although gravel and other non-vegetative covers can prevent
erosion and vegetation growths on the actual trench cover, the
uncovered areas between the trenches are subject to both. In
fact, it is not unusual for deep-rooted vegetation to become
established in the uncovered soil or for severe erosion to occur,
both of which can undermine cap integrity. Non-vegetative covers
would require periodical herbicide treatments to keep down all
vegetation growth. Erosion control would be considerably more
difficult. Gravel covers have a further advantage in that they
discourage burrowing animals far more than vegetation would.
MONITORING
In the past, most solid waste land disposal sites have
incorporated a total site monitoring system consisting of
monitoring devices situated along site property boundaries.
These systems have a serious flaw in that they detect leachate
only after it has been forming for some time, possibly long
after the site is closed. Remedial measures at that time can
only stop the further formation of leachate; the leachate
already formed may continue to transport hazardous contaminants
off the site. The site boundary system serves only to provide
an indication of the direction and rate of leachate movement
so that precautions can be taken ahead of the flow. For a more
complete discussion of total site monitoring systems, the reader
is referred to the several studies on monitoring in the biblio-
graphy (15, 24, 25, 27, 73, 90).
A proper monitoring system must anticipate leachate and
its probable rate and flow path and detect it before contami-
nation (if any) has progressed very far. If leachate can be
detected while still in the immediate vicinity of a burial trench,
remedial steps can be implemented which will prevent or signifi-
cantly delay off-site movement of hazardous materials. To
this end, a monitoring system involving detectors in and around
each burial trench is also desirable.
Trench Monitoring Systems
In the context of this report, individual trench monitoring
involves the detection of moisture in the trench and is primarily
meant to:
73
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• Determine when field capacity of the trench
cover and walls is reached;
• Determine the efficiency of the infiltration
control systems of each trench; and
• Establish when pumping might be necessary to
reduce water levels in trenches with inadequate
or faulty infiltration controls.
Ideally, the monitoring system needed to achieve these goals
includes moisture cells in the compacted soil backfill, in the
trench bottom, to the sides of the trench, and in the soil
beneath the trench. In practice, field conditions may necessi-
tate variations in the placement, but the overall purpose of the
system - to detect infiltration into the trench and leachate
flow out of the trench - must be kept in mind.
Commercial electro-couple moisture cells, such as the
Soiltest® MC-310A, are the simplest means for remotely monitor-
ing soil moisture. Use of this type of cell requires prelimi-
nary engineering analysis to determine the field capacity of
the soils around the trench and the backfill material. The
moisture cells can then detect any changes in soil moisture
or simply the approach of saturation and flow. Cells in the
soils around and below the trench can monitor groundwater and
soil moisture levels and flow; cells in the radwastes and
backfill soils can monitor moisture levels in the trench.
The number and placement of the cells is largely dependent
on local moisture conditions; higher water tables, more rain-
fall, more permeable soils would require greater numbers of
cells, for example. For the worst case, cells should be.placed
in the mounded soil cover, below the trench bottoms, and in
two horizontal planes about 2 and 5 m deep (Figure 14). Cells
placed about 2 to 3 m apart can cover most normal soil moisture
conditions. It is not necessary to maintain vertical planes
of eel Is.
Although easier to place in new trenches, moisture cells
can be added to filled trenches by simply drilling holes into
the fill. If it is undesirable to drill through the cover,
slant drilling can be used to place probes in the trench.
If it appears impossible to place a cell beneath each filled
trench without rupturing covers, liners, or radwaste containers,
cells can be placed between trenches below the plane of touch
bottoms.
The cells are connected to the surface by labelled leads.
These leads can be gathered into groups at appropriate places
74
-------
CAP
MOISTURE
PROBE
HOUSING
MOISTURE
PROBE
HOUSING
COMPACTED
SOIL
COVER
Figure 14.
Cross section of moisture monitoring cells for
a burial trench
75
-------
and routed to a common moisture probe housing at either side
of the trench. Monitoring would be a simple matter of attaching
a lead to a portable moisture meter and reading the electrical
resistance. Graphs could be kept for each cell to maintain
"moisture profiles" of each trench. At current (1977) prices,
moisture cells cost about $10 to $12 each and another $10 to
$20 to place in a filled trench, depending on the equipment
used. Placing cells as a trench is filled can reduce placement
costs substantially. Complete costs for placing cells in a filled
trench could run as high as $9,000 per trench in a wet area.
Costs would be less in arid regions or for placement during
filling. A portable moisture meter can be purchased for under
$300. Maintenance would consist of periodic inspection of leads
or replacement of faulty cells.
Monitoring Frequency
The frequency of monitoring is a function of site climato-
logy and hydrogeology, and the completeness of the hydrogeologic
data; it will probably vary somewhat from site to site. As a
general rule, from a practical, cost point of view, monthly
monitoring intervals will likely suffice. In extremely arid
regions (e.g., Beatty, Nevada), bimonthly sampling with weekly
sampling following major rains might be preferable. For
excessively rainy or high-watertable sites, weekly or biweekly
sampling might be necessary to ensure safe trench operation.
Whatever monitoring method is selected, it should be noted
that monitoring is useless without an authority to react and
a set of criteria specifying when or how to react. The lack
of criteria or authority has plagued environmental control
attempts from the beginning and could serve to undermine any
attempts to establish safe radwaste land burial sites.
75
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84
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GLOSSARY
(C i )
Curiv ' - Arbitrary unit for specifying the amount
of radioactivity. It was originally defined
as the rate of decay of Ig of radium, but
it has more recently been redefined as
3.70 x I0l° disintegrations/sec.
(yCi - microcurie (IQ-^Ci),
nCi - nanocurie (10-9Ci))
Radwaste - Radioactive wastes
Rem - Roentgen equivalent man; radioactive dose
unit which produces a biological effect
equivalent to the absorption of one
roentgen of X'radiation
Transuranics - Those elements having atomic numbers
greater than 92. In practice, plutonium
is essentially the only element in this
category.
85
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
ORP/LV-78-5
2.
I. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
Study of Engineering and Water Management Practices
that will Minimize the Infiltration of Precipitation
into Trenches Containing Radioactive Waste
5. REPORT DATE
September 1977
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Stearns, Conrad and Schmidt
Consulting Engineers, Inc.
4014 Long Beach Blvd.
Long Beach, CA 90807
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
68-03-2452
12. SPONSORING AGENCY NAME AND ADDRESS
Office of Radiation Programs
U.S. Environmental Protection Agency
P.O. Box 15027
Las Vegas, NV 89114
13. TYPE OF REPORT AND PERIOD COVERED
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
16. ABSTRACT
This report is the final output of a U.S. Environmental Protection Agency
contract which was funded to review present engineering and water management practices
to minimize the infiltration of precipitation through trench caps. The objective of
this effort was to evaluate and compare the existing practices in use at sanitary
landfills, hazardous waste disposal facilities and experimental burial sites, and to
apply these practices to the commercial low-level radioactive waste sites.
The report is based on a review of the literature and general knowledge of
the state-of-the-art in sanitary engineering developments. The report describes
presently available techniques which may be applicable to current and future shallow
land burial operations.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Radioactive waste (1406)
Sanitary engineering (1302)
Disposal facilities
Management practices
18-G
13-B
18. DISTRIBUTION STATEMENT
Unlimited
19. SECURITY CLASS (This Report)
21. NO, OF PAGES
20. SECURITY CLASS (This page)
NA
22. PRICE
EPA Form 2220-1 (Rev. 4-77) PREVIOUS EDITION is OBSOLETE
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