Geohydrologic Aspects for 

Siting and Design of Low-Level 

Radioactive-Waste Disposal 
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Geohydrologic Aspects for 

Siting and Design of Low-Level 

Radioactive-Waste Disposal 

By M.S. BEDINGER 

U.S. GEOLOGICAL SURVEY CIRCULAR 1034 
DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Any use of trade, product, or firm names in this publication 

is for descriptive purposes only and does not imply 

endorsement by the U.S. Government 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1989 

Free on Application to the 

Books and Open-File Reports Section, 

U.S. Geological Survey 

Federal Center, Box 25425 

Denver, CO 80225 

Library of Congress Cataloging-in-Publication Data 

Bedinger, M.S. 

Geohydrologic aspects for siting and design of low-level radioactive-waste 

disposal I by M.S. Bedinger. 

p. cm.-(U.S. Geological Survey circular; 1034) 

Includes bibliographic references. 

1. Radioactive waste sites-Design and construction. 2. Radioactive waste 

disposal in the ground. 3. Hydrology. I. Title. II. Series. 

TD898.155.D47B43 1989 89-28810 

621.48'38-dc20 CIP 
CONTENTS 

Abstract 1 

Introduction 1 

Classification of low-level radioactive waste 1 

Purpose and scope 2 

Performance of shallow low-level radioactive-waste repositories, siting guidelines, 

and engineered barriers 2 

Historical perspective of low-level radioactive-waste disposal and repository 

siting 2 

Studies at selected low-level radioactive-waste repository sites 3 

Oak Ridge National Laboratory, Tennessee 3 

Maxey Flats, Kentucky 6 

West Valley, New York 7 

Barnwell, South Carolina 8 

Sheffield, Illinois 9 

Beatty, Nevada 10 

Development of siting criteria 11 

Site-suitability requirements of the U.S. Nuclear Regulatory 

Commission 15 

Engineered barrier systems 17 

Hydrology of shallow-land disposal of low-level radioactive waste 19 

Climate 20 

Hydraulic barriers to flow 21 

Hydrology of repository systems 23 

Trench repository in the unsaturated zone 23 

Trench cap 23 

Waste form and backfill 24 

Host medium 25 

Conceptual flow in a simple trench repository system 25 

Waste repository in the saturated zone 27 

Buffer zone and flow system 27 

Hydrologic modeling 28 

Conclusions 29 

Lessons learned 29 

Geohydrologic screening and site selection 30 

Repository construction and maintenance 31 

Subsurface monitoring and modeling at low-level radioactive-waste repository 

sites 31 

Engineered barrier systems 31 

References cited 31 

Metric conversion factors 36 

FIGURES 

1. Map showing locations of commercial and major Department of Energy 

low-level radioactive-waste burial sites in the United States 3 

2. Map showing locations of burial grounds at Oak Ridge National Laboratory, Tenn. 4 

Contents Ill 
IV Contents 

3-6. Photographs of: 

3. Trench at burial ground 6 at Oak Ridge National Laboratory, Tenn., 

partially filled with randomly deposited low-level radioactive waste 5 

4. Burial ground 3 at Oak Ridge National Laboratory, Tenn. 6 

5. Low-level radioactive-waste burial trench at Oak Ridge National Laboratory, Tenn., intersecting water table 7 

6. Leachate from burial ground 5 seeping from depression formed by 

uprooted tree in Melton Valley, Oak Ridge National Laboratory, 

Tenn. 8 

7. Cross section showing geologic section and location of typical burial trench 

at Maxey Flats, Ky. 9 

8. Vertical section through trench burial area near West Valley, N.Y., showing weathered zone, glacial deposits, and direction of ground-water 

flow 10 

9. Cross section showing potentiometric-head distribution and flow directions 

at experimental trenches in May 1984 at burial grounds near Barnwell, 

S.C. 11 

10. Photograph of trench at burial ground near Sheffield, Ill., showing placement of waste packages 12 

11. Oblique aerial photograph of low-level radioactive-waste burial grounds 

near Sheffield, Ill. 13 

12. Map of burial ground near Sheffield, Ill., showing areas where greater than 

background tritium concentrations were detected in ground water 14 

13. Photograph of collapse cavity, approximately 0.8 m wide, at burial ground 

near Sheffield, Ill. 15 

14. Aerial photograph of part of northern Amargosa Desert showing Bare 

Mountain, alluvial fans and dry channel of Amargosa River, and waste 

burial site near Beatty, Nev. 16 

15. Photograph of active waste burial trench at site near Beatty, Nev. 17 

16. Photograph of alluvial materials in waste burial trench at site near Beatty, 

Nev. 18 

17-22. Sketches showing: 

17. Cutaway view of earth-mounded concrete bunker 19 

18. Below-ground vault design 19 

19. Above-ground vault design 20 

20. Trench design incorporating engineered barriers 20 

21. Shaft disposal design above water table 21 

22. Shaft disposal of low-level radioactive waste in zone of saturation 21 

23. Diagrammatic cross section of simple trench repository showing repository 

cap, waste, backfill, and host medium 22 

24. Diagram of laboratory tank model showing capillary barriers 23 

25. Graph of water content, unsaturated hydraulic conductivity, and moisture 

tension for three hypothetical examples of porous media 23 

26. Diagrammatic trench repository and idealized relative unsaturated and 

saturated hydraulic conductivity and median grain size of trench cap, host 

medium, and backfill 26 

27. Diagrammatic sketch of hydrology of trench repository 26 

28. Graph showing predicted tritium concentrations beneath trench after 10 

and 100 years 28 

TABLE 

1. Comparison of hydrologic conditions at humid, subhumid, and arid 

sites 22 
Geohydrologic Aspects for Siting and Design of 

Low-Level Radioactive-Waste Dis1posal 

By M.S. Bedinger 

Abstract 

The objective for siting and design of low-level 

radioactive-waste repository sites is to isolate the waste from 

the biosphere until the waste no longer poses an unacceptable hazard as a result of radioactive decay. low-level 

radioactive waste commonly is isolated at shallow depths 

with various engineered features to stabilize the waste and to 

reduce its dissolution and transport by ground water. The 

unsaturated zone generally is preferred for isolating the 

waste. Low-level radioactive waste may need to be isolated 

for 300 to 500 years. Maintenance and monitoring of the 

repository site are required by Federal regulations for only 

the first 100 years. Therefore, geohydrology of the repository 

site needs to provide natural isolation of the waste for the 

hazardous period following maintenance of the site. Engineering design of the repository needs to be compatible with 

the natural geohydrologic conditions at the site. Studies at 

existing commercial and Federal waste-disposal sites provide information on the problems encountered and the basis 

for establishing siting guidelines for improved isolation of 

radioactive waste, engineering design of repository structures, and surveillance needs to assess the effectiveness of 

the repositories and to provide early warning of problems that 

may require remedial action. 

Climate directly affects the hydrology of a site and 

probably is the most important single factor that affects the 

suitability of a site for shallow-land burial of low-level radioactive waste. Humid and subhumid regions are not well 

suited for shallow isolation of low-level radioactive waste in 

the unsaturated zone; arid regions with zero to small infiltration from precipitation, great depths to the water table, and 

long flow paths to natural discharge areas are naturally well 

suited to isolation of the waste. The unsaturated zone is 

preferred for isolation of low-level radioactive waste. The 

guiding rationale is to minimize contact of water with the 

waste and to minimize transport of waste from the repository. 

The hydrology of a flow system containing a repository is 

greatly affected by the engineering of the repository site. 

Prediction of the performance of the repository is a complex 

problem, hampered by problems of characterizing the natural and manmade features of the flow system and by the 

limitations of models to predict flow and geochemical processes in the saturated and unsaturated zones. Disposal in 

Manuscript approved for publication, September 1, 1988. 

low-permeability unfractured clays in the saturated zone may 

b~ feasible where the radio nuclide transport is controlled by 

diffusion rather than advection. 

IHTRODUCTION 

The isolation of low-level radioactive waste is in its 

formative stage with respect to siting, design, and consttuction of repositories, handling of the waste, and 

predictability of site performance. Successful isolation of 

lqw-level radioactive waste depends on application of 

~any scientific and engineering disciplines to provide 

a¢ceptable site selection and repository design and effective repository construction and operation at the site. 

:Erxamination of the performance of low-level radioactivewaste repository sites indicates that, although many sites 

are satisfactory with respect to the isolation of the 

radioactive waste, the coordination among scientific and 

engineering disciplines has not been accomplished to the 

point where acceptable siting, design, and operation of a 

l~w-level radioactive-waste repository site is a predictable or routine procedure. 

· The most likely path of radionuclides from a shall~w radioactive-waste repository constructed on land to 

tpe biosphere is transport by water. However, shallow 

r~positories also are subject to intrusion by humans, 

plant roots, and burrowing animals, and to exposure by 

erosion. Migration of radionuclides as gases in the unsaturated zone also is of concern at shallow repositories. 

Classification of Low-Level 

Radioactive Waste 

The U.S. Nuclear Regulatory Commission (1982) 

classifies low-level radioactive waste into three categories 

~n the basis of radionuclide type and radiation emitted. 

Class A wastes are relatively innocuous, trash-type 

wastes that contain low activities of radionuclides. Class 

l3 wastes are those that have high activities and contain 

short-lived radionuclides. They require stable waste 

(orms-that is, waste forms that are designed to maintain 

Introduction 1 
gross physical properties and identity for about 300 years. 

Class C wastes are those that contain the highest activities of radionuclides. Class C wastes require not only the 

stable waste form of class B wastes but also additional 

measures at the repository site to protect against inadvertent intrusion for about 500 years. 

The isolation time required for a radioactive waste 

is a function of the half-lives of the radionuclides and 

their decay products present in the waste. A rule of 

thumb commonly used in the nuclear industry is that 

radioactive waste should be confined at least 10 times the 

half-life of the longest lived, dominant radionuclides. The 

half-lives of the principal components of low-level radioactive waste range from about 3 to 30 years. For waste 

containing primarily strontium-90 and cesium-137, 300 

years of isolation (about 10 half-lives) is required to 

decrease the radiation to one-thousandth of the initial 

radiation, and 500 years of isolation is required to 

decrease the radiation to one-hundred thousandth of its 

initial activity. Thus, on the basis of half-lives and radiation limits, the waste needs to be isolated from the 

biosphere for 300 to 500 years. The preparation of longer 

lived radionuclides, such as carbon-14 and plutonium239, within the waste should not exceed the limits established by regulatory authorities. 

Purpose and Scope 

This report reviews the problems and experiences 

at selected low-level radioactive-waste repository sites, 

the development of the current technical rationale for the 

isolation of low-level radioactive waste, and the site 

suitability requirements of the U.S. Nuclear Regulatory 

Commission. Additionally, this circular presents the 

application of hydrologic principles to engineering design 

of repositories and includes conclusions reached from 

studies of low-level waste disposal. This report is concerned only with hydrologic and related geologic factors 

of waste isolation and repository siting. Social, economic, 

demographic, political, and other nongeohydrologic factors are significant in repository siting, but these factors 

are beyond the scope of this report. 

PERFORMANCE OF SHALLOW LOW-LEVEL 

REPOSITORIES, SITING GUIDELINES, AND 

ENGINEERED BARRIERS 

Historical Perspective of Low-Level 

Radioactive-Waste Disposal and 

Repository Siting 

Disposal of waste from radioactive-element refining operations in the early 1900's was commonly at the 

processing site with little or no regard for the health and 

environmental risks posed by the waste. An example of 

early disposal of waste was revealed in 1979 when 

radioactive waste from pre-1920 radium-refining operations was discovered in the Denver, Colo., area. Beginning with the discovery of radioactive waste at the site of 

the long defunct National Radium Institute in February 

1979 (Strain, 1979a, p. 1 ), 30 additional radioactivewaste dumps from the pre-1920 refining era were discovered by mid-June 1979. Popular belief prevailing prior to 

1920, as revealed in a Denver Post article (Strain, 1979b, 

p. 42), was that the radioactive element radium was 

beneficial in the treatment of many health problems, and, 

concomitantly, there was an apparent total ignorance of 

the health risks associated with radioactivity. Realization 

of the health risks associated with radioactivity was slow; 

recognition of the need for isolation of wastes was even 

slower. 

The earliest repositories for radioactive waste were 

established by the Federal Government for waste from 

national defense and research operations. Until 1962, 

nuclear waste from commercial operations was commonly disposed on federally operated sites. The method 

of disposal was commonly burial at shallow depths in 

trenches, as at the Savannah River Plant, S.C., Palos 

Forest Preserve, Ill., Oak Ridge National Laboratory, 

Tenn., and Nevada Test Site, Nev. (fig. 1). Apparently, 

problems of waste migration were not anticipated; little 

concern during the first disposal operations was given to 

methods of packaging the waste, to the geologic and 

hydrologic characteristics of the burial site, and to the 

backfill and capping materials of the waste trenches. 

Recognition that radioactive waste was not being 

completely contained in many of the early disposal sites 

and increased awareness of the hazardous nature of 

radioactive waste brought about concern for the manner 

in which such waste was disposed. Between 1962 and 

1971, six commercially operated land burial sites commenced operation (Maxey Flats, Ky., West Valley, N.Y., 

Richland, Wash., Beatty, Nev., Sheffield, Ill., and Barnwell, S.C., fig. 1). Siting requirements were not rigorous, 

but efforts to provide greater confinement of radioactive 

waste at most of the humid sites involved the excavation 

of the burial trenches in fine-grained low-permeability 

formations and also capping the trenches and backfilling 

of the waste packages with fine-grained material, such as 

silt and clay. The prevailing rationale was to cover the 

waste with a material that would inhibit infiltration of 

water into the trenches and to bury the waste in a 

medium that would retard transport of the radionuclides 

from the waste site by ground water. 

Problems with the containment of waste in the 

trenches developed at many of these sites (Fischer, 

1986). The capping layers of many of the trenches 

developed collapse structures resulting from differential 

compaction of trench materials, caused in part by the 

2 Geohydrologic Aspects for Siting and Design of Low-Level Radioactive-Waste Disposal 
deterioration and compaction of the waste packages and 

backfill material. In addition, desiccation cracks developed in clay-rich caps during extended periods of dry 

weather. The cracks allowed water to enter the trenches; 

water accumulated in many of the trenches, thus saturating and promoting the leaching of the waste. Radionuclides were transported from the trenches by groundwater flow and by overflow of water from the trenches at 

the surface. These problems and other experiences at 

selected low-level radioactive-waste sites are reviewed in 

the following sections. 

Studies at Selected Low-Level 

Radioactive-Waste Repository Sites 

Hydrologic studies at five of the commercial lowlevel radioactive-waste sites and one federally operated 

site are reviewed in this section. The sites include a range 

of distinctly different geohydrologic settings and climates 

that range from arid to humid. The review of conditions 

at the sites provides a background of useful information 

that should be considered in establishing a rationale for 

site selection and design and for evaluating the performance of future repository sites. 

Hanfqrd~t 

Reserv"\~n• 

~ *Richland. 

~Wash. 

Idaho National 

: Engineering ' 

! Laboratory, ld: 

Beatty, Nev. * ' 

• 

. , __ . . . 

·Nevada Test She · 

EXPLANATION 

• ' Los Alamos 

National 

Laboratory, 

N.Mex. 

• DEPARTMENT OF ENERGY SITES 

* COMMERCIAL SITES 

Oalic. Ridge National Laboratory, Tennessee 

Burial of radioactive waste at Oak Ridge National 

La~oratory began in 1944 and has continued to the 

pr~sent (1988). The earliest disposal of low-level radioactive waste at Oak Ridge National Laboratory was not 

giv~n special siting, design, or operational considerations 

(Mlezga, 1984). As problems of waste containment 

beqame obvious, more rigorous disposal procedures have 

be¢n introduced. Oak Ridge National Laboratory, in the 

VaUey and Ridge province (Webster, in press), is underlai* by consolidated Paleozoic formations of shale and 

limestone. Burial sites are excavated in the regolith or 

zoJjle of weathered material overlying the consolidated 

ro¢k. Regolith of limestone consists of silt and clay; the 

limestone regolith ranges in thickness from zero to 7 m. 

R~golith of the shale consists of silt, clay, pebbles, and 

ro¢k fragments and has the deformed structure of the 

bedding still clearly visible. The shale regolith ranges in 

thlckness from about 1 to 12m. Hydraulic conductivity of 

th¢ shale regolith ranges from 1.1 X 10-1 to 1.8X 10-2 m/d; 

th~.t of the shale from 1.1 X 10-2 to 3.0X 10-3 m/d (Webst~r, in press). The first burial sites were developed in 

Bethel Valley (burial grounds 1, 2, and 3), which is 

underlain by limestone (fig. 2). Because the movement of 

Palos Forest/ 

Preserve, IlL 

• 

--Sheffield, 

. Ill. 

-~ •. Frenald, 

eOhio, 

Weldon * 

Spring, Mo. , _'Maxey Flats,Ky., 

-oak Ridge National 

• .Laboratory •. Term. 

, , West Valley; 

:*N.Y. -

Savannah ·• } 

River Plant, *J¥irnwell 

S.C. >!:>.C. 

0 200 400 600 800 KILOMETERS 

I I II II I II I II I 

0 100 200 300 400 MILES 

Figure 1. Locations of commercial and major Department o~ Energy low-level radioactive-waste burial sites in the 

United States. 

Performance of Shallow Low-Level ~epositories, Siting Guidelines, and Engineered Barriers 3 
water and contaminants was considered unpredictable in 

the cavernous limestone, subsequent burial sites ( 4, 5, 

and 6) were located in Melton Valley, which is underlain 

by shale. Average annual precipitation at Oak Ridge 

National Laboratory is about 130 cm/yr (Webster, in 

press), greater than that at any other low-level 

radioactive-waste-repository site in the United States. 

Early waste-disposal operations at Oak Ridge 

National Laboratory are described by Webster (1979; in 

press) as being relatively simple. After clearing an area of 

0-tk 

./-€.~~ ~··· 

j 

0-€k 

0~ 

• 

.. ·~. :/ 

col>' Base from U.S.Geological Survey and 

Tennessee Valley Authority, 

/ .Yso 

Geology by W.M. McMaster, 1962 O's, 

Bethel Valley, Tennessee, 1:24,000, 1968 

METERS A 

366 

274 

183 

A' METERS 

366 

274 

183 

91 

Sea level 

91 

~l<.-.i:~__..:..~~>....L:~~~~~:::::2:~~~~~~~~~~~::::;:L Sea level 

Vertical exaggeration X 2 Detail of Gc from Haase, Walls, 

and Farmer (1985) 

0 500 1000 METERS 

I I I 

0 3000 FEET 

CONTOUR INTERVAL 20 FEET 

DATUM IS SEA LEVEL 

I Och I CHICKAMAUGA LIMESTONE 

I O£k I KNOX GROUP 

~CONASAUGA GROUP 

~ROME FORMATION 

EXPLANATION 

CONTACT 

............ THRUST FAULT--Sawteeth on upthrown 

block 

rzJ BURIAL GROUND AND NUMBER 

A-A' LINE OF SECTION 

Figure 2. Locations of burial grounds at Oak Ridge National Laboratory, Tenn. 

4 Geohydrologic Aspects for Siting and Design of Low-Level Radioactive-Waste Disposal 
trees, trenches were cut to depths of as great as 4.5 m and 

to widths of 3 m. Lengths have been variable; the longest 

trenches exceed 150 m and are oriented down the 

topographic slope. The common practice was to dump 

low-level radioactive waste in the open trenches (fig. 3) 

until the trenches were nearly filled and to fill the 

remaining space with spoil material from the excavation. 

As burial areas have been phased out, they have been 

seeded with grass. Burial ground 3 was closed in 1951 

and trees were allowed to reestablish. In addition, 

radioactive-waste burial equipment was stored above 

ground within the burial ground. Subsequently, the area 

was cleared of trees and contaminated equipment, and 

grass was sown in 1979. Figure 4 shows burial ground 3 

in 1987. The trenches have not been mounded nor have 

monuments been emplaced to mark them, although 

access to the burial grounds is controlled. In the earlier 

years of disposal, the presence of water in the trenches 

was not considered a problem and trenches commonly 

were constructed within the fluctuating zone of the water 

table. Waste in some trenches is perennially saturated 

(Webster, in press). 

A photograph of a trench in burial ground 5 that 

intersects the water table is shown in figure 5. Webster 

(in press) observed that the depth to water table has risen 

1.8 to 2.7 m above predisposallevels in one area because 

of the greater permeability of the trench-cap material 

than the natural ground and because of reduced transpiration as a result of removal of deep-rooted trees. 

Duguid (1979) and Webster (in press) note that water in 

many trenches, especially the long downslope-oriented 

trenches, has accumulated at a rate greater than it can 

flow out to the undisturbed regolith. The trenches have 

become filled with water at the lower ends and seeps have 

formed by overflow along the regolith-spoil contact (fig. 

6). Stueber and others (1978) determined that the discharge of strontium-90 from burial ground 5 was 0.45 

curie (Ci) in 1978 and remained fairly constant from year 

to year despite variability in annual precipitation. In a 

subsequent study, Stueber and others (1981) determined 

that the discharge of strontium-90 from the burial 

ground 3 area averaged 6.4 p.Ci per month for a 7-month 

period, with the monthly discharges from this site being 

related to precipitation. The principal mode of transport 

Figure 3. Trench at burial ground 6 at Oak Ridge National Laboratory, Tenn., partially filled with randomly deposited 

low-level radioactive waste. Structure of bedrock is retained in wall of trench excavated in regolith of shale. Photograph by 

David Webster, 1982. 

Performance of Shallow Low-Level Repositories, Siting Guidelines, and Engineered Barriers 5 
of radionuclides from burial ground 3 appears to be by 

ground water moving through a solution-cavity system 

(Stueber and others, 1981). At burial ground 4 transport 

is both by ground water and trench overflow (Huff and 

Farrow, 1983), whereas, at burial ground 5 transport is 

by ground water moving through the weathered regolith 

(Cerling and Spalding, 1982). Considerable evidence has 

been collected to show that radioactive contaminants 

have been transported by ground water in both the 

regolith and the bedrock to the local streams (Webster, 

in press). 

Maxey Flats, Kentucky 

Maxey Flats (fig. 1) is an isolated plateau in northeastern Kentucky. The radioactive-waste burial site, 

which was operated from 1963 to 1977, is located on the 

plateau, about 90 to 120 m above the surrounding valleys. 

The rocks in the Maxey Flats area are almost flat lying 

and consist of fractured shale and sandstone of Mississippian to Silurian age. The rock units, in descending 

order, are the lower part of the Nancy Member (shale) 

and Farmers Member (sandstone), including its basal 

Henley Bed (shale), of the Borden Formation (Mississippian), the Sunbury, Bedford, and Ohio Shales (Mississippian and Devonian), and the upper shale part of the 

Crab Orchard Group (Silurian) (fig. 7). 

The waste burial trenches at Maxey Flats are 

excavated in the regolith, the weathered zone, of the 

Nancy Member. Two sandstone beds of variable thickness, fracturing, and occurrence are encountered within 

the depth excavated for the burial trenches. The lower 

bed is at the level of buried waste in the trenches and 

appears to control the movement of leachate (Lyverse, in 

press). Most trenches are about 100m long, 17m wide, 

and 7 to 8 m deep. Waste was covered with excavated 

regolith material as the trench was progressivly filled. A 

compacted clay and crushed shale cap at least 1 m thick 

covers the trenches. 

Most of the waste was buried in solid form, except 

for tritium, which was buried typically as tritiated water 

enclosed in glass containers packed in steel drums 

(Lyverse, in press). Each trench was designed with sump 

pipes at a low point of the trench to provide for waterquality monitoring and removal of water that might 

infiltrate and accumulate in the trenches. In 1977, radionuclides were detected having migrated from a closed 

disposal trench to an adjacent trench. Subsequent 

ground-water studies of the site have indicated that 

contaminated water has migrated from the site through 

the fractured sandstone (Lyverse, in press). Water containing tritium concentrations as large as 3.5 ~-tCi/mL has 

been sampled from wells as far as 70 m from the trench 

area. Wet-weather seeps on hillsides surrounding the 

trench area have contained tritium concentrations as 

large as 2,500 pCi/L. Zehner (1983) estimated cobalt-60 

and manganese-54 in water in the lower sandstone bed to 

migrate about 17m per year. 

Clay layers were compacted over the trenches to 

minimize infiltration. However, infiltration through the 

Figure 4. Burial ground 3 at Oak Ridge National Laboratory, Tenn. Photograph by David Webster, 1987. 

6 Geohydrologlc Aspects for Siting and Design of Low-Level Radioactive-Waste Disposal 
clay caps has been recognized as a continuing problem at 

Maxey Flats burial site. Water accumulating in the 

trenches is pumped out and evaporated. To reduce 

infiltration into the trenches, 0.38- to 0.51-mm-thick 

PVC (polyvinylchloride) covers were placed over the 

trenches. Because of deterioration of PVC by exposure 

to ultraviolet light, the covers are replaced at intervals of 

18 to 30 months. Infiltration after emplacement of the 

PVC covers was 2 to 7 percent of precipitation, a 

decrease of from 5 to 37 percent compared to before 

emplacement of covers (Lyverse, in press). Because the 

impermeable trench covers increased runoff from the 

trench areas, erosion has increased downslope from the 

trenches. 

The geohydrologic conditions at Maxey Flats are 

particularly unsuitable for disposal of radioactive waste. 

The fractured sandstone and shale provides localized 

pathways for rapid migration of leachate from the waste. 

Where the host medium is less permeable, the trenches 

accumulate water. The effort to provide an impermeable 

clay cover by compacting natural materials failed; infiltration into the trenches from precipitation, which averFigure 5. Low-level radioactive-waste burial trench at Oak 

Ridge National Laboratory, Tenn., intersecting water table. 

Photograph by David Webster, 1979. 

ages 117 cm/yr (Lyverse, in press), is a significant problem addressed by pumping of the accumulated water 

from the trenches. The water is then evaporated. 

West Valley, New York 

The radioactive-waste burial grounds near West 

Valley are in western New York (fig. 1). The Statelicensed burial ground was opened in 1963, and burial 

ceased in 1975. Twelve of the 14 trenches filled with 

waste are between 115 and 215 m long, 11 m wide at the 

top and 8 m wide at the bottom, and about 6 m deep. A 

gravel-filled sump was constructed at the lower end of 

some of the trenches and equipped with a riser pipe for 

pumping out water, if necessary. The trenches were 

capped with compacted and graded till. Bedrock is a 

thick, monotonous sequence of shale and minor siltstone. 

Overlying the bedrock is a relatively thick sequence of 

fine-grained lake-bottom deposits alternating with clayey 

tills rich in reworked lacustrine sediment, with few sand 

or gravel units (fig. 8). The burial trenches are excavated 

in an upper unit of silty clay till that overlies a unit of silt 

and clay. The upper 2 to 3 m of till are oxidized and 

contain a network of abundant intersecting fractures. Till 

at greater depth is gray, plastic, and unoxidized, but 

fractures having firm, oxidized borders a few millimeters 

wide extend downward into the unoxidized till to a depth 

of about 5 m. 

Ground water moves slowly downward through the 

till to the unit of lacustrine clay and silt (fig. 8), which acts 

as an underdrain. In the silt and clay unit, flow is lateral 

toward Buttermilk Creek, which is the local discharge 

area. Prudic (1986, p. 44) estimates that ground-water 

travel time from a trench to Buttermilk Creek would be 

about 800 to 2,800 years. 

The average annual precipitation at West Valley is 

about 100 cm/yr (Randall, in press). The potential for 

accumulation of water in the trenches was recognized 

immediately (Kelleher, 1979) and, in fact, did occur. In 

three trenches completed by 1969, water rose above the 

top of the undisturbed till and into the cover by 1975, and 

seepage out to the land surface was observed. From 1975 

through 1983, water accumulations in several trenches 

were removed by periodic pumping, and the water was 

treated chemically. Efforts to eliminate or lessen the 

accumulations of water in the trenches included increasing the cover thickness from 1.2 to 2.4 m, imposing a 

surcharge load to the filled and capped trenches by 

temporarily piling spoil, and grading land surface to drain 

water away from the trenches. Infiltration into the 

trenches was decreased but not eliminated (Randall, in 

press). 

Over the long term, improved cap design and 

reconstruction could do no more than delay the effects of 

decay, collapse, and desiccation cracks that provide a 

means for infiltration into the trenches (Randall, m 

Performance of Shallow Low-Level Repositories, Siting Guidelines, and Engineered Barriers 7 
press). Matuszek (1986) and Matuszek and Robinson 

(1983) argue that only incineration would yield a product 

that could be buried without the prospect of collapse. 

Even with a stable waste form, means for infiltration may 

be provided by desiccation cracks after a long dry period, 

as determined by Prudic (1979) following the summer of 

1978. 

The natural travel time of flow from the trench 

area to Buttermilk Creek through the ground-water flow 

system would afford adequate time for low-level radioactive waste to decay to innocuous levels. However, the 

trench caps admitted infiltration of precipitation into the 

trenches at rates that exceeded the quantity of flow into 

the till at the base of the trenches. This caused water 

levels to rise in several trenches until water seeped to the 

land surface or was pumped out. 

Barnwell, South Carolina 

The radioactive-waste disposal facility near Barnwell, S.C., is one of three commercially active sites in the 

United States (fig. 1). Disposal began in 1971 and is 

scheduled to continue until 1992. Average annual precipitation at Barnwell is about 117 cm/yr (McMahon and 

Dennehy, in press). The disposal facility located in the 

Coastal Plain is underlain by a thick sequence of unconsolidated deposits ranging in age from Cretaceous to 

Quaternary. The burial trenches are excavated in a 

sequence of sand intermixed with clay (zone 1 of Cahill, 

1982a) that is overlain by a sand layer as much as 1 m 

thick. Underlying zone 1 is a sequence of very fine to 

medium sand (zone 2 of Cahill, 1982a). Local recharge in 

the repository area moves downward through zone 1 to 

zone 2, where it moves to the south and discharges into 

Marys Branch 330 m south of the site. Cahill (1982a) 

estimated the minimum travel time of ground water 

migrating from the burial site to Marys Branch to be 

about 50 years. 

The following description of trench construction is 

from McMahon and Dennehy (in press). The initial step 

during trench construction is to excavate and remove a 

3-m-wide perimeter of the surface sand, including about 

0.3 m of underlying clay, around the designated trench 

area. Clay is placed in the excavation and compacted. The 

compacted clay serves as an upper barrier wall (fig. 9A) 

to prevent the upper wall of the trench from caving and to 

Figure 6. Leachate from burial ground 5 seeping from depression formed by uprooted tree in Melton Valley, Oak Ridge 

National Laboratory, Tenn. Photograph by David Webster, 1975. 

8 Geohydrologlc Aspects for Siting and Design of Low-Level Radioactive-Waste Disposal 
limit infiltration into the trench. The trench is excavated 

within the clay barrier perimeter to a depth of about 7 m, 

but the depth may vary to maintain a minimum of 1.5 m 

between the bottom of the trench and the water table. 

Trench floors are sloped to one side and along the long 

axis of the trench to promote drainage. Monitoring pipes 

and sumps are placed at regular intervals along the drain. 

Sand is placed in the trench floor and is used to backfill 

between waste packages. A minimum of 0.6 m of clay is 

added to the top of the trench and compacted with a 

vibrating compactor. At least 1 m of material is added 

over the clay, and the trench cap is contoured and sowed 

with grass seed. Pre-1976 trenches were constructed 

similarly except for their having shorter surface dimensions and no barrier of compacted clay around the trench 

perimeter. 

Infiltration of water through the trenches has been 

studied by Dennehy and McMahon (1985). They determined that the compacted clay barrier (fig. 9A) reduced 

the water infiltrating the trenches compared to the 

trenches without the barrier (fig. 9B). Water ponded in 

the bottom of a trench with a compacted clay barrier to a 

depth of 0.3 m and in a trench without a clay barrier to a 

depth of 2.3 m (fig. 9). Cahill (1982b) reported tritium 

East 

290 

260 

and organic constituents greater than background concentrations in water at a well 3 m from a trench with no 

barrier and traces of cobalt-60 and tritium greater than 

background concentrations in water beneath the trench 

floor. 

Sheffield, Illinois 

The Sheffield disposal site is about 5 km southeast 

of Sheffield, Ill. (fig. 1). The trenches are excavated in 

glacial deposits including lacustrine sediments, till, outwash deposits, and aeolian silt and sand. A few of the 

trenches were constructed above the original grade with 

berms of surficial material. The glacial deposits are 

underlain by shale and mudstone of Pennsylvanian age. 

Trenches excavated in the glacial deposits range from 10 

to 170 m long, 2.6 to 23 m wide, and 2.6 to 8.6 m deep 

(fig. 10). The trenches were bottomed at least 3 m above 

the saturated zone, although seasonal water level fluctuations may be at levels of the waste in some trenches. The 

trenches were covered with a layer of silty clay that was 

capped by a layer of clayey silt to silt. The cap was 

compacted by operating heavy machinery over the 

West 

COLLUVIUM AND SOIL 

OR ALLUVIUM 

SHALE 

SANDSTONE 

CONTACT 

230 ----------------------------

200 

------------------------------

------------------------------

DATUM IS SEA LEVEL 

0 

0 50 

30 60 METERS 

100 150 FEET 

=--~~---.J 

Figure 7. Geologic section and location of typical burial trench at Maxey Flats, Ky. 

Performance of Shallow Low-Level Repositories, Siting Guidelines, and Engineered Barriers 9 
trenches. The cap supports a growth of brome grass. The 

disposal site and a strip mine lake near Sheffield, Ill., is 

shown in figure 11. 

Precipitation at the site averages about 88 cm/yr 

(Healy, in press). Recharge to the saturated zone is 

estimated to be about 5 cm/yr, runoff from the site is 

about 23 cm/yr, and evapotranspiration is about 60 

em/yr. Infiltration in the burial area is primarily along the 

periphery of the caps and secondarily through the center 

of the caps as indicated by data collected from instruments installed to indicate water movement into the 

trenches (Healy, in press). Thus, the intent of the compacted clay cap to limit infiltration is in part successful. 

The outwash deposits, a pebbly sand unit, underlie 

about two-thirds of the burial site. The pebbly sand unit 

is a permeable zone that drains water which infiltrates 

into the trenches; the unit minimizes the water-level 

fluctuations and reduces the risk of water accumulation 

in the trenches. Ground-water velocity in the pebbly sand 

was measured at about 750 m/yr; the unit conveys 

tritiated water from the burial site to a strip mine lake 

(Healy, in press) (fig. 12). Organic contamination of 

ground water at the Sheffield site has been described by 

Goode (in press). 

West 

METERS 

450 

430 

410 

390 

370 

0 

Runoff from the burial site was 23 cm/yr compared 

to 4 cm/yr for an undisturbed area. The increased runoff 

is attributed to the compacted clay caps over the trenches 

and to the shorter, less dense vegetation than in the 

undisturbed areas. Sediment yields from the undisturbed 

area were about two orders of magnitude smaller than 

yields from the burial site (Gray, in press). 

Collapse cavities in trench covers have been documented from October 1978 through September 1985. A 

total of 302 collapse cavities (fig. 13), having a cumulative 

volume of 497 m3

, occurred during this period. Most 

collapses were recorded following periods of rainfall 

when soil moisture was near maximum (Gray, in press). 

Beatty, Nevada 

Low-level radioactive solid wastes have been buried in trenches at a site near Beatty, Nev., since 1962 (fig. 

1 ). The waste burial facility is in the Amargosa River 

valley, a desert valley in southern Nevada (fig. 14). An 

active (1988) waste burial trench is shown in figure 15. 

This trench is about 17m deep, 90 m wide, and 180m 

long. The mean annual precipitation is 11.4 em at Beatty, 

18 km north of the site, and 7.4 em at Lathrop Wells, 30 

East 

60 120 METERS 

0 100 200 300 FEET 

D 

D 

SATURATED DEPOSITS 

UNSATURATED DEPOSITS 

EXPLANATION 

CONTACT--Dashed where approximately located 

MAGNITUDE AND DIRECTION OF WATER MOVEMENT--Length of arrows indicates relative 

magnitude of movement and orientation of arrow indicates relative direction of movement 

Figure 8. Vertical section through trench burial area near West Valley, N.Y., showing weathered zone, glacial 

deposits, and direction of ground-water flow. 

10 Geohydrologic Aspects for Siting and Design of Low-Level Radioactive-Waste Disposal 
km southeast of the site (Nichols, 1985). The site is 

underlain by more than 170m of unconsolidated alluvialfan, fluvial, and playa deposits. The upper 30 m of which 

are an unconsolidated mixture of poorly sorted cobbles, 

gravels, sands, and silts. Burial trenches are excavated in 

the unconsolidated alluvial deposits (fig. 16). The waste 

is backfilled and covered with the material removed from 

the trenches. The depth to water near the site ranges 

from 85 to 115 m below the surface. Ground-water 

movement beneath the site is to the southeast; the 

nearest downgradient natural discharge is in Ash meadows, about 40 km southeast of the site. The dry channel 

of the Amargosa River passes about 3 km west of the 

burial site; surface runoff in the channel is rare and 

limited to a few days following major storms. 

The study of the site by the U.S. Geological Survey 

has concentrated on the unsaturated zone and the potential for recharge (Nichols, 1985; Fischer, in press). It is 

68 

67 

66 

through the unsaturated zone that any radionuclides 

leached from the waste must pass before reaching the 

saturated zone. Water budget studies by Nichols (1985) 

indicate that the potential for percolation to depths 

greater than 2 m does exist despite high evaporation 

demands. Measurements of soil-moisture tension adjacent to the waste site during a 2.5-yr period indicate no 

percolation deeper than 2m (Fischer, in press). Studies 

to date (1988) indicate that recharge, if it occurs at the 

site, is episodic and probably small. The extremely small 

potential for downward percolation suggests small potential for transport of radionuclides. 

Development of Siting Criteria 

Problems of water accumulation in waste trenches, 

collapse of trench caps, and leaching and migration of 

B 

1-----]3-----

72 

71 

70 

69 I 

t 

68 

67 

66 

Water table 

65~~~~-------------~ 

DATUM IS SEA LEVEL 0 2 3 4 METERS 

0 5 10 FEET 

EXPLANATION 

~ COMPACTED CLAYEY SAND-Ksat = 1.5 x 10"6 centimeters per second 

[J SURFICAL SAND AND BACKFILLED SAND-Ksat = 4.0 x 10"3 centimeters per second 

0 UNDISTURBED CLAYEY SAND-Ksat = 1.4 x 10"7 centimeters per second 

'l/~ ~//. . SATURATED MATERIAL 

f DIRECTION OF WATER MOVEMENT 

-76- WATER-LEVEL CONTOUR-Shows altitude of total hydraulic head . Contour interval 1 meter. 

Dashed line is supplemental contour 

Figure 9. Cross section showing potentiometric-head distribution and flow directions at experimental trenches 

in May 1984 at burial grounds near Barnwell, S.C. (from McMahon and Dennehy, in press). A, Trench design 

with 3-m-wide clay barrier around perimeter. 8, Trench design without clay barrier. 

Performance of Shallow Low-Level Repositories, Siting Guidelines, and Engineered Barriers 11 
waste have been documented at each of the burial sites in 

humid and subhumid climates discussed in the preceeding sections. From a geohydrologic perspective, the most 

direct and potentially suitable solution to the problem of 

low-level radioactive-waste isolation would be to locate 

suitable burial sites in arid climates. However, isolation 

of low-level waste is approached as a problem which will 

be resolved regionally. This has fostered development of 

measures to mitigate the problems experienced at sites in 

humid and sub humid climates. Mitigating measures have 

taken two forms: (1) the development of various geohydrologic repository siting criteria and, (2) the design of 

engineered barrier systems to prevent migration of the 

waste. These two avenues of approach to the problem are 

discussed in the following sections. Geohydrologic siting 

guidelines have been proposed by investigations sponsored by many groups and organizations (Cherry and 

others, 1973; Papadopulos and Winograd, 1974; Cherry 

and others, 1979; International Atomic Energy Agency, 

1981, 1982; Falconer and others, 1982; Fischer and 

Robertson, 1984) to overcome many of the problems 

associated with existing sites. The following paragraphs 

discuss many of the applications of guidelines that have 

been recommended; many of the specific guidelines have 

been adopted by the U.S. Nuclear Regulatory Commission in site-suitability requirements (U.S. Nuclear Regulatory Commission, 1982) and in their technical position 

on the siting requirements (Siefken and others, 1982). 

The first generation of commercial waste burial 

grounds was excavated in deposits of low permeability, 

except for the sites near Barnwell, S.C., and Beatty, Nev. 

Meyer (1979, p. 661) questioned if a more permeable 

burial medium would not avoid the problems of the 

bathtub effect. Fischer and Robertson (1984) recommended that the burial medium be permeable and that it 

be overlain by a clay cap to provide a capillary barrier to 

downward movement of water through the waste zone. 

The recommendation that the host medium be permeable has not been explicitly included as siting requirements of the U.S. Nuclear Regulatory Commission 

(1982), though permeable media are not precluded as 

host media. Siefken and others (1982) point out that the 

backfill should be granular material that drains freely and 

that water percolating through the disposal unit must 

drain readily from the bottom of the disposal unit to 

avoid the bathtub effect. Meanwhile, some current 

screening studies for low-level radioactive-waste sites 

have targeted low-permeability formations as the preFigure 10. Trench at burial ground near Sheffield, Ill., showing placement of waste packages. Photograph by James 

Foster, 1977. 

12 Geohydrologlc Aspects for Siting and Design of Low-Level Radioactive-Waste Disposal 
ferred terrain for isolation of low-level radioactive waste. 

Experience has shown that low-permeability material as 

a burial medium has produced many problems with 

low-level radioactive-waste containment; satisfactory 

performance of such a medium cannot be expected 

without adequate consideration of the geohydrologic and 

design factors involved in the dynamics of the site performance. 

The unsaturated zone is favored for shallow-land 

repositories (Cherry and others, 1973; Papadopulos and 

Winograd, 1974; U.S. Nuclear Regulatory Commission, 

1982; Fischer and Robertson, 1984) because of the 

opportunity to minimize the contact of water with the 

waste. However, in many geohydrologic settings in humid 

regions, the unsaturated zone may be unsuitable for 

shallow-land repositories because of the insufficient 

thickness of the unsaturated zone and the short travel 

times from the prospective burial site to the discharge 

area. Cherry and others (1979) determined that in large 

areas of Canada there is insufficient thickness of unsaturated zone for burial of low-level radioactive waste. 

Cherry and others (1979) proposed constructing augered 

holes at depths of a few tens of meters below the 

permanent water table. The waste would be placed in the 

augered holes and backfilled with bentonite and clay to 

provide a very stable, low-permeability barrier above the 

waste. They further proposed that the permeability of the 

burial medium be minimal, such that its capacity to 

transport waste radionuclides by advection would be less 

than that by diffusion. Burial below the water table is 

allowed by the U.S. Nuclear Regulatory Commission 

(1982) where it can be shown that the predominant 

means of radionuclide movement will be by molecular 

diffusion. 

Extremely important attributes which a flow system should possess are geochemical and hydraulic charFigure 11. Oblique aerial photograph of low-level radioactive-waste burial grounds near Sheffield, Ill., looking southwestward. 

Strip mine lake is in foreground; boundary of low-level radioactive-waste disposal site is delineated by dashed line. Photograph 

by Jerry Abbey, 1985. 

Performance of Shallow Low-Level Repositories, Siting Guidelines, and Engineered Barriers 13 
actenstlcs that will afford a long residence time for 

radionuclides by virtue of geochemical processes that 

retard the transport of radionuclides and long flow paths 

and slow ground-water velocities in the ground-waterflow system. These factors come to the forefront of 

importance in the later stages of waste isolation, after the 

engineered barriers are no longer effective and institutional control of the site has ceased. This concept of 

employing multiple barriers that are naturally present by 

the geochemical and physical nature of the flow system is 

considered in demonstrating compliance with the U.S. 

Nuclear Regulatory Commission's (1982) performance 

objectives. Advocates of the multiple-barrier concept in 

disposal of low-level waste include Cherry and others 

(1973), Falconer and others (1982), and Siefken and 

others (1982). The advocacy of the multiple-barrier flow 

system concept is a conservative principle that provides 

added confidence in a disposal system which provides 

isolation extending in time and space beyond the probable capabilities of the manmade waste-isolation system. 

The geologic and hydrologic conditons at the site should 

89' 47'45" 40" 35" 30" 25" 

allow the repository site to be characterized with a degree 

of confidence to provide for defensible estimates of 

facility performance. 

Characterization of the site provides the basis for 

determination of ground-water-flow paths, estimation of 

rates of radio nuclide migration, and design of monitoring 

networks to evaluate the performance of the disposal 

facility. Characterization includes determination of the 

variations in physical and hydraulic properties of the 

saturated and unsaturated zones, the hydrologic balance 

of the surface and subsurface system, diffusion and 

dispersion properties of the system, and the geochemical 

properties of the system. Heterogeneous systems of 

geologic units that are discontinuous and variable in 

character and systems in which fracture permeability is a 

significant component are difficult to characterize with 

confidence and may not be amenable to prediction of 

ground-water flow and radionuclide migration. Systems 

dominated by fracture flow are particularly difficult to 

characterize. Fractured media have provided relatively 

rapid migration routes for radionuclides in the near20" 

Il l 

" 

15" 10" 89°47'05" 

---·-. 1111 - e I Ground -water-basin boundary II 

41 ' 20'30" 

25" -

20" 

15" 

41'20'10" 

-~i r- ·--·~ ~- ~ ~ i 1 c/l' ::r· \ ;; ~. ·:<>·'>' . 11 

-g ~ 0 ~ sl f/.. :: · 6 .g I ~··· II --....__ , 

CD ro' . ~· _ D.!,J: :;:1 

1

,___...__ - -r _l __ -.~ "'V --- ·-r -.o.. ---

. ! ~· .. r-. .Y;JJ ;; < 

__ -/J I \iiio "> J",., 

~~ i-? C\ 'ic ~ :: }/ 

~/3 ·.""" ........... -r .... .:}' Boundar y of low-level ;( '-- ~ · 11 

rad1oact1ve-waste s1te .. . · . :: 

--........ ... ...---·· ·: II 

) II 

II . II 

I :: 

0 50 100 METERS 

I II 2 0 200 FEET 

• 

2 \ 

r--: ~ ... 

I 

) 

I 

f 

~ 

> 

EXPLANATION 

BASIN NUMBER 

WELL 

AREA WHERE TRITIUM CONCENTRATIONS 

IN GROUND WATER ARE GREATER THAN 

BACKGROUND CONCENTRATIONS 

Figure 12. Burial ground near Sheffield, Ill., showing areas where greater than background tritium concentrations 

were detected in ground water (from Healy, in press). 

14 Geohydrologic Aspects for Siting and Design of Low-Level Radioactive-Waste Disposal 
surface weathered zone of fine-grained till at West 

Valley, N.Y. (Randall, in press); in consolidated rock at 

Maxey Flats, Ky. (Lyverse, in press), and Palos Forest 

Preserve, Ill. (J.R. Nicholas, U.S. Geological Survey, oral 

commun., 1987); and in weathered rock at Oak Ridge 

National Laboratory, Tenn. (D.A. Webster, U.S. Geological Survey, oral commun., 1987). Siting guidelines by 

Falconer and others (1982) recommend that geologic 

units in which the ground-water flow principally occurs in 

fractures, joints, or solution voids be excluded from 

consideration as shallow-land burial sites. 

Site-Suitability Requirements of the 

U.S. Nuclear Regulatory Commission 

The site-performance objectives, site-suitability 

requirements, and the technical position of the U.S. 

Nuclear Regulatory Commission on the suitability of 

sites will be preeminent in influencing the next generation of low-level radioactive-waste repository sites in the 

United States. The performance objectives and sitesuitability requirements were published in the Federal 

Register, vol. 47, no. 248, Monday, December 27, 1982, 

p. 57463-57482, and are referred to as document 

10CFR61 (U.S. Nuclear Regulatory Commission, 1982). 

The technical position of the U.S. Nuclear Regulatory 

Commission on implementing the suitability requirements is given in Siefken and others (1982). Weber (in 

press) discusses the hydrogeologic information needed to 

demonstrate compliance with the regulations of the U.S. 

Nuclear Regulatory Commission. 

The performance objectives of the U.S. Nuclear 

Regulatory Commission (1982, Subpart C, 10CFR61.41) 

state, in part, that 

concentration of radioactive material which may be 

released to the general environment in ground water, 

surface water, air, soil, plants, or animals must not result 

in an annual dose exceeding an equivalent of 25 millirems to the whole body, 75 millirems to the thyroid, and 

25 millirems to any other organ of any member of the 

public. Reasonable effort should be made to maintain 

release of radioactivity in effluents to the general environment as low as is reasonably achievable. 

The site-suitability requirements of the U.S. Nuclear Regulatory Commission (1982, Subpart D, 

10CFR61.50) basically incorporate the siting guidelines 

as discussed in the section "Development of Siting Criteria." The site-suitability requirements as given in 

10CFR61 are as follows: 

Figure 13. Collapse cavity, approximately 0.8 m wide, at burial ground near Sheffield, Ill. Photograph by John Gray, 1984. 

Performance of Shallow Low-Level Repositories, Siting Guidelines, and Engineered Barriers 15 
1. The disposal site shall be capable of being characterized, 

modeled, analyzed and monitored. 

2. Within the region or state where the facility is to be 

located, a disposal site should be selected so that projected 

population growth and future developments are not likely to 

affect the ability of the disposal facility to meet the performance 

objectives .... 

3. Areas must be avoided having known natural resources 

which, if exploited, would result in failure to meet the performance objectives .... 

4. TI1e disposal site must be generally well-drained and free 

of areas of flooding or frequent ponding. Waste disposal shall 

not take place in a 100-year floodplain, coastal high-hazard area 

or wetland .... 

5. Upstream drainage must be minimized to decrease the 

amount of runoff which could erode or inundate waste disposal 

units. 

6. The disposal site must provide sufficient depth to the 

water table that ground-water intrusion, perennial or otherwise, 

into the waste will not occur. The Commission will consider an 

Figure 14. Aerial photograph of part of northern Amargosa Desert showing Bare Mountain, alluvial fans and dry channel 

of Amargosa River, and waste burial site near Beatty, Nev. Photograph taken June 6, 1976. 

16 Geohydrologic Aspects for Siting and Design of Low-Level Radioactive-Waste Disposal 
exception to this requirement to allow disposal below the water 

table if it can be conclusively shown that disposal-site characteristics will result in molecular diffusion being the predominant means of radionuclide movement and the rate of movement will result in the performance objectives ... being met. In no 

case will waste disposal be permitted in the zone of fluctuation 

of the water table. 

7. The hydrogeologic unit used for disposal shall not 

discharge ground water to the surface within the disposal site. 

8. Areas must be avoided where tectonic processes such as 

faulting, folding, seismic activity, or vulcanism may occur with 

such frequency and extent to significantly affect the ability of 

the disposal site to meet the performance objectives ... or may 

preclude defensible modeling and prediction of long-term 

impacts. 

9. Areas must be avoided where surface geologic processes 

such as mass wasting, erosion, slumping, landsliding, or weathering occur with such frequency and extent to significantly affect 

the ability of the disposal site to meet the performance objectives ... or may preclude defensible modeling and prediction of 

long-term impacts. 

10. The disposal site must not be located where nearby 

facilities or activities could adversely impact the ability of the 

site to meet the performance objectives ... or significantly mask 

the environmental monitoring program. 

Engineered Barrier Systems 

The design and use of facilities utilizing extensive 

engineered features have been prompted by the desire to 

provide greater confinement of waste than has been 

experienced at some shallow burial sites. Engineered 

barriers and repositories, such as below-ground or 

above-ground vaults, earth-mounded bunkers, tile holes 

grouted with cement or other low-permeability matrix, 

high-integrity waste containers, and other designs have 

been proposed with the objective of providing greater 

confinement and protection of waste from weathering 

and transport. 

The U.S. Nuclear Regulatory Commission considers engineered structures as part of a redundant barrier 

system to waste isolation-not a substitute for a suitable 

site. Thus, site-suitability requirements must be met for 

any site regardless of engineered structures proposed. 

Such a requirement is necessary but may not be sufficient 

alone to assure waste isolation. Engineered features need 

to be designed with the natural conditions in mind in 

order to be effective and not to be counterproductive. 

Similar engineering features, such as simple trench construction and backfill of waste and capped with the 

excavated material, were employed at the burial sites 

near Beatty, Nev., and the trenches at West Valley, N.Y. 

The design is quite adequate at Beatty, but problems with 

waste containment have been experienced at West Valley. The lesson to be learned is that the design of the 

engineered features of a repository must be compatible 

with the natural geohydrologic conditions. 

A chief distinction of an engineered facility is one 

which provides one or more of the following: (1) Mechanical support and stability to the structure, (2) barriers for 

containment of radioactive waste, and (3) facilities to 

collect and monitor migration of water within and beyond 

the facility. A review of engineered near-surface facilities 

has been made by Schwarz (in press), and the subject has 

Figure 15. Active waste burial trench at site near Beatty, Nev. Photograph taken in 1987. 

Performance of Shallow Low-Level Repositories, Siting Guidelines, and Engineered Barriers 17 
been treated in a series of reports by Bennett and others 

(1984), Bennett (1985), Miller and Bennett (1985), and 

Warriner and Bennett (1985). Engineered near-surface 

repositories have been in use for less than 20 years. 

Earth-mounded concrete bunkers for disposal of 

low- and intermediate-level radioactive waste were first 

used in France (Van Kote, 1982). In the French design of 

the earth-mounded concrete bunker, low-level radioactive waste with appropriate packaging is stored over the 

monoliths above the original ground surface in earth 

mounds. The monoliths rest on a concrete pad containing 

a drainage system to collect any infiltration that may 

occur during construction and initial operation (fig. 17). 

Below-ground vault repository refers to any 

enclosed engineered structure constructed below the 

surface of the earth by cut-and-cover construction or 

built above ground and then covered with earth. Belowground vaults constructed of reinforced, cast-in-place 

concrete walls, roof, and floor have been used at Oak 

Ridge National Laboratory, Tenn., for retrievable storage of transuranic waste. Below-ground vaults have been 

used for low-level radioactive-waste storage in Ontario, 

Canada, at the Chalk River Nuclear Laboratory and in 

Manitoba, Canada, at Whiteshell Nuclear Research 

Establishment (Morrison, 1974; Charlesworth and 

Carter, 1982). 

A below-ground vault design is shown in figure 18. 

Above-ground vaults are similar to below-ground vaults 

but, as the name implies, are above ground and include 

no earth cover or earth mound (fig. 19). 

Engineered features of near-surface repositories 

that include various procedures to stabilize the waste are: 

sorbent barriers (Freeman and others, 1984; Freeman 

and Buelt, 1986) to reduce radionuclide migration; plastic sheeting, clay caps, and bioengineered barriers to 

reduce infiltration into and exfiltration from the repository; and concrete roofs and walls around the waste to 

enhance structural stability. The extent of engineered 

facilities may range from a single feature incorporated 

into a trench repository to a completely engineered 

structure buried, partly buried, or even wholly above the 

land surface. A design trench incorporating engineered 

barriers is shown in figure 20. 

The long-term durability and performance of concrete and other materials used in engineered barriers and 

in completely engineered facilities are not fully known 

(Denson, 1985). There is no long-term experience or 

basis for predicting the performance of engineered facilFigure 16. Alluvial materials in waste burial trench at site near Beatty, Nev. Photograph taken in 1987. 

18 Geohydrologic Aspects for Siting and Design of Low-Level Radioactive-Waste Disposal 
ities over periods of 300 years or longer. Nevertheless, 

engineered low-level radioactive-waste repositories are 

being planned in humid areas where experience has 

revealed problems with simple trench-burial techniques. 

For example, Illinois is opting for an engineered facility, 

and Lavallee (1986) proposes an earth-mounded concrete bunker for low-level radioactive-waste disposal in 

Maine. 

Below ground construction 

Above ground construction 

In-situ material 

{ 

Pervious layer 

rt1::J=l,~.IJI1'--t 

Disposal of low-level radioactive waste in shafts has 

been practiced above the water table and has been 

proposed for placement below the water table. A design 

for disposal of low-level waste in a shaft above the water 

table is shown in figure 21 (Bennett, 1985). The waste 

packages are backfilled with non cohesive, freely draining 

materials and capped by concrete. The shaft is free to 

drain through the host medium to the water table. The 

Figure 17. Cutaway view of earth-mounded concrete bunker (modified from Van Kote, 1982). 

Vegetation 

Topsoil 

Pervious backfill 

Low-permeability membrane 

In- situ material 

Figure 18. Below-ground vault design (from Bennett, 1985). 

Low-permeability 

clay layer 

Under-drainage blanket 

Performance of Shallow Low-Level Repositories, Siting Guidelines, and Engineered Barriers 19 
below-water-table shaft disposal concept is illustrated in 

figure 22. The waste is disposed at 5 m or more below the 

water table; the shaft is sealed by a bentonite or cement 

grout. Hydraulic conductivity of the host medium is very 

low and migration of waste is principally by molecular 

diffusion. 

HYDROLOGY OF SHALLOW-LAND DISPOSAL 

OF LOW-LEVEL RADIOACTIVE WASTE 

The site criteria previously discussed, as advanced 

by various investigators, and the site-suitability requirements of the U.S. Nuclear Regulatory Commission have 

not yet been applied to shallow repositories. It remains to 

Slope under gravel to single drain 

Figure 19. Above-ground vault design (from Bennett and 

Warriner, 1985). 

be seen by the next generation of repositories if the 

lessons afforded by the first generation of repositories 

have been learned and if the principles of hydrology will 

be adequately applied. This section of the report reviews 

hydrologic principles that are pertinent to analysis of the 

flow dynamics of a repository, addresses problems of 

screening data for large regions for potential repository 

sites, and discusses identification of potentially suitable 

sites. This section also examines application of the U.S. 

Nuclear Regulatory Commission's requirements with 

regard to performance assessment of repositories, and it 

recommends long-term monitoring for assessing site 

performance and continued revision of hydrologic models of the flow system. 

Climate 

Climate and climate related factors are undoubtedly the most important hydrologic factors affecting the 

suitability of a region for selection of a favorable site for 

shallow-land isolation of low-level radioactive waste. The 

quantity and seasonal distribution of precipitation and 

evapotranspiration influence the infiltration of water into 

a repository and also the contact of moisture with the 

waste and the flow of water for transporting waste. 

Climate affects the thickness of the unsaturated zone, the 

stream density, and consequently the distance a repository can be located from the ground-water-discharge 

point. Areas of great aridity are naturally well suited for 

Engineered trench wall barrier 

Component Material 

As required to develop 

cover slope of 6 percent Gravel floor 

A Soil cover Native soil 

B Filter layer Geotextile layer and 19-millimeterdiameter stone 

C Biointrusion barrier Cobbles 

D Drainage layer 19-millimeter-diameter stone 

E Infiltration barrier 19-millimeter-diameter stone, 

granular bentonite filler, 

geotextile layer, 

19-millimeter-diameter stone , 

and granular bentonite filler 

F Soil backfill Native soil 

Figure 20. Trench design incorporating engineered barriers (from Funk and Mills, 1986). 

20 Geohydrologlc Aspects for Siting and Design of Low-Level Radioactive-Waste Disposal 
disposal of low-level radioactive waste. A comparison of 

the hydrologic conditions of the arid site at Beatty, Nev., 

with the subhumid site at Sheffield, Ill., and the humid 

site at Barnwell, S.C. (table 1), shows that estimates of 

several important hydrologic parameters in these varied 

environments are significant for low-level radioactivewaste repository siting. 

Monitoring wells 

Permeable backfill- 1---

Waste packages 

Open bottom 

to liner 

Concrete 

~ ., 

'~ 

.·~ ,;-. . 

,. ~ r-- Dra1nage layer 

I' Liner 

Figure 21. Shaft disposal design above water table (from 

Bennett, 1985). 

:: Weathered till -

5 meters 

Unweathered till 

Figure 22. Shaft disposal of low-level radioactive waste in 

zone of saturation (after Prudic, in press). 

In arid regions, potential evaporation greatly 

exceeds precipitation; recharge is small or nonexistent. 

Ground-water flow paths from ground-water divides to 

discharge areas are long; time of travel to discharge areas 

is correspondingly great. Depths to ground water in arid 

regions are commonly great. In the arid Basin and Range 

province, where annual precipitation averages about 280 

mm/yr, depths to water are greater than 30 m for more 

than 90 percent of the province (Bedinger and Langer, 

1986). Depths to water are greater than 150m for more 

than 20 percent of the province. Similarly, large tracts of 

land in the Colorado Plateaus, Columbia Plateaus, and 

other parts of the western conterminous United States 

have depths to water greater than 30 m (Bedinger and 

Langer, 1986). 

In contrast, in the humid and subhumid eastern 

and midcontinental United States recharge is relatively 

great, the density of perennial streams is large, and the 

depth to ground water is small. Because of these conditions, sites having sufficient depth to water for disposal in 

the unsaturated zone and having long ground-water flow 

paths to discharge areas are of very limited occurrence 

(Ichimura, in press). 

Hydraulic Barriers to Flow 

Hydraulic barriers are basically low-permeability 

beds that retard the flow of water. In the unsaturated 

zone, flow may be retarded by capillary barriers in which 

a fine-grained layer overlies a coarse-grained layer. Capillary barriers may be created by the lower permeability 

of a coarse-grained bed under high moisture tension 

underlying a fine-grained bed. The fine-grained bed 

absorbs and holds more water by its greater capillary 

attraction; hydraulic conductivity of the fine-grained bed 

with large moisture content is greater than the underlying 

coarse-grained bed with very low moisture content. A 

diagrammatic representation of a trench repository 

incorporating a capillary barrier is shown in figure 23. 

The proposed use of capillary barriers in the design 

of radioactive-waste repositories is based on classical 

theory of fluid flow in porous media, laboratory and field 

experiments of unsaturated flow, and mathematical modeling of flow in the unsaturated zone. The hydraulic 

laboratory tank models of Palmquist and Johnson (1962) 

demonstrated that the interface between a homogeneous 

layer of silt-sized particles overlying a homogeneous 

layer of sand-sized particles acted as a barrier to downward infiltration of water in the unsaturated zone. The 

tank model of Palmquist and Johnson is shown in figure 

24. 

It has been suggested by some investigators that 

moisture breakthrough of the capillary barrier does not 

occur until saturation of the overlying fine-grained layer 

occurs. However, Palmquist and Johnson (1962) conHydrology of Shallow-Land Disposal of Low-Level Radioactive Waste 21 
Table 1. Comparison of hydrologic conditions at humid, subhumid, and arid sites 

[mm/yr, millimeter per year; m, meter; km, kilometer; yr, year] 

Annual precipitation (mm/yr) ----------------

Annual lake evaporation (mm/yr) ------------

Depth to water (m)----------------------------

Recharge (mm/yr) -----------------------------

Distance to ground-water discharge (km)-----

Ground-water travel time, site to discharge 

area (yr) -----------------------------------

Arid site1 

Beatty, Nev. 

114 

1,676 

85-115 

.04 

40 

500-1,000 

Subhumid site2 

Sheffield, ill. 

880 

787 

6--11 

50 

.28 

.3 

Humid site3 

Barnwell, S.C. 

1,170 

1,067 

8.5 

380 

.33 

50 

I Data from Nichols (1985) and Bedinger and others (1984). 

2Data from Gray (in press) and Healy (in press). 

3Data from Cahill (1982a) and Dennehy and McMahon (1985). 

eluded that a breakthrough of flow from a silt bed to a 

sand bed occurred before saturation of the upper layer 

when the liquid pressure on the interface was less than 

atmospheric, a fact that is concurred with by Richards 

(1950), Hillel and Talpaz (1977), and Johnson and others 

(1983). 

Sand tank experiments indicate that the initial flow 

through the coarse-grained layer occurs in narrow columns where the moisture content is relatively great 

(Palmquist and Johnson, 1962). Thus, after the capillary 

barrier initially ceases to function as a flow barrier, the 

sand tank models indicate that greater moisture contents 

are limited to small channels, drainage is relatively rapid, 

and the bulk of the coarse-grained unit has low moisture 

content. 

The capillary barrier effect of a boundary between 

two materials is a function of their respective moisture 

tension versus hydraulic conductivity relations. Consider 

a layer of clay overlying sand having moisture tension 

versus hydraulic conductivity as shown in figure 25. 

These relations are modified from Reed (in press) as 

computed from the relation between relative hydraulic 

conductivity and moisture tension expressed by Ripple 

and others (1972, p. 6). At moisture tensions from about 

6 to 46 m of water at the clay-sand interface, the sand has 

a smaller hydraulic conductivity than the clay, and the 

sand will retard downward percolation. If the terrain at 

the interface decreases to below a moisture tension of 

about 6 m of water, the sand will be more permeable than 

the clay and the contact will not act as a capillary barrier. 

The importance of the relation between moisture tension 

and hydraulic conductivity is further emphasized in the 

work of Goode (1986). 

Herzog and others (1982) reviewed the literature 

on trench-cap design and discussed several designs 

employing capillary barriers. However, few capillary barriers have been constructed and reported in the literature. Experimental capillary barriers were constructed in 

11------10 to 30 meters ------1 

Host medium Host medium 

Stable waste fo rm 

Figure 23. Diagrammatic cross section of simple trench repository showing repository cap, waste, backfill, and 

host medium. 

22 Geohydrologic Aspects for Siting and Design of Low-Level Radioactive-Waste Disposal 
~ 

1:>:<<:::1 

I ><.LI 

Infiltration crib (inflow 15 milliliters per minute I 

EXPLANATION 

AREAS REMAINING DRY AFTER 38 HOURS 

OF INFLOW 

BED COMPOSED OF 0.036-MILLIMETER 

GLASS BEADS 

BED COMPOSED OF 0.47-MILLIMETER 

GLASS BEADS 

SHOWS POSITION OF WETTING FRONT 

-..._14- AFTER SELECTED TIMES OF 

INFILTRATION, IN HOURS 

Figure 24. Laboratory tank model showing capillary barriers 

(from Palmquist and Johnson, 1962}. 

>-

<( 3X 10-1 

trenches by Rancon (1979) to test the applicability of 

these barriers in shallow-land disposal repositories. The 

experimental trench was excavated in fine soil, filled 

two-thirds with gravel, and then covered with argillaceous sand forming a dome over the structure. After 5 

years of regular monitoring, no appreciable moisture 

transport was detected across the interface of the argillaceous sand toward the gravel. 

The effectiveness of the full-scale trench repositories with capillary barriers are dependent on many factors, including the dimensions and components of the 

trench, the lithology and unsaturated hydraulic conductivity of the host medium, the climate and related factors 

such as the infiltration rate into the fine-grained layer and 

the depth to the water table, and, as has been shown 

above, the relation between soil moisture tension and 

hydraulic conductivity of the layers. In addition, the 

effectiveness of a capillary barrier could be adversely 

affected by collapse of the waste packages, desiccation of 

the clay cap, and the penetration of the cap by burrowing 

animals and plant roots. 

Hydrology of Repository Systems 

Trench Repository in the Unsaturated Zone 

Components of a simple trench repository include 

the repository cap, the waste, waste packaging, backfill, 

and host media (fig. 23). As a system designed to provide 

stability of the repository, lessen infiltration, and minimize moisture contact with the waste, each component 

1.0 0 ............. 

------- CI: 

""' UJ 

UJ 

~ L LEFT SCALE u a.. \ <( 

R RIGHT SCALE a.. (/) :-\, ~0 (/) CI: 

UJ UJ 

~ CI: UJ ,~;.. 0 ::?: \"- ~ a.. 

~ u.. 

\ ", 0 >--

~ z ~ " 0 

> i= 

i= " u u ~ ~ <( 

=:> ~ CI: 

0 '<~ u.. 

z ~ 05 <( 

0 ( (/) 

u " <( 

u ~ :::J 

___ , z =:> \ ~~ UJ 

<( ~ 

CI: z 0 "' '\~ 0 

>- ~ "~ u I 

~!!_vs<J " -~ UJ 

0 CI: 

UJ -.....::._fJ50J :\._ =:> ~ ~ 

<( - '\ __ _:__"--: (/) 

CI: 6 =:> ::?: ~ 

<( 3X10-g (/) 

z 0.015 0.305 3.05 

=:> MOISTURE TENSION, IN METERS OF WATER 

Figure 25. Water content, unsaturated hydraulic conductivity, and moisture tension for 

three hypothetical examples of porous media (modified from Reed, in press). 

Hydrology of Shallow-Land Disposal of Low-Level Radioactive Waste 23 
must be designed as a part of the whole, with consideration given to the hydrologic interactions of each component with the others. 

Trench Cap 

Trench caps should function as a stable cover over 

the repository and, in humid and subhumid regions, limit 

infiltration into the repository. The low permeability of 

the trench cap also may lessen gaseous radioactive emissions into the atmosphere. In extremely arid regions 

where infiltration is low, there may be no need to reduce 

infiltration. If potential infiltration is significant, as in a 

humid region, the trench cap needs to be designed to 

limit infiltration to the repository. Historically, repositories have been plagued by problems of trench-cap instability, which result in infiltration to the repository. As 

discussed in an earlier section, these problems have 

resulted from poor drainage of trench-cap areas, unstable waste forms that collapse by weathering and overburden pressure, and desiccation and cracking of trench-cap 

material. Other processes which may increase infiltration 

of precipitation in trench caps include weathering and 

biologic activity, such as growth of plant roots and 

burrowing of animals. Structural instability of the trench 

cap caused by weathering and collapse of waste forms 

and trench components often are detected soon after 

construction. Collapse features will presumably decrease 

in frequency over time. However, weathering and biologic activity in the trench cap will be a long-term, 

continuing process causing progressive deterioration of 

the trench-cap effectiveness in inhibiting infiltration. 

Reed (in press) shows that in a region of potentially 

large infiltration the clay cap functions as a retardant to 

infiltration as well as an integral part of the capillary 

barrier. The effectiveness of the clay cap is subject to 

severe deterioration by processes that increase the cap's 

permeability. Effectiveness of the capillary barrier 

decreases as the infiltration increases and, consequently, 

moisture content of the clay cap and coarse-grained layer 

increases. There are many unknown factors concerning 

the long-term effectiveness of capillary barriers. 

Several trench-cap designs have been proposed to 

reduce infiltration and provide trench-cap stability. 

These include capillary barriers to reduce infiltration, 

admixtures or layers of bentonite (swelling clays) to 

reduce infiltration, stable waste forms to prevent their 

collapse, and noncompressible backfill to prevent compaction in the repository. The results of trench-cap 

experiments at Los Alamos National Laboratory are 

reported by Hakonson and others (in press). The plastic 

sheets that have been used to cover the trench caps at 

Maxey Flats, Ky., have lessened infiltration greatly, but 

also have facilitated a great quantity of overland runoff, 

which has caused erosion problems downslope from the 

trenches. In addition, such designs require perpetual 

maintenance. 

In addition, a technique referred to as "bioengineering" has been proposed to reduce infiltration. This 

technique attempts to cover the cap with impermeable 

panels separated by vegetated rows. The panels increase 

runoff from the trench cap and the vegetation intercepts 

and transpires infiltrating moisture, preventing deep percolation to the repository. Such experiments in bioengineering have been conducted to develop and test techniques for control of infiltration into waste burial 

trenches by vegetative cover and runoff control in a 

humid environment at Maxey Flats, Ky. (Schulz and 

Ridky, 1986; Schulz and others, 1985). The long-term 

performance of such covers, however, remains to be 

proven. 

The bioengineered trench-cap cover described 

above may present problems from the following standpoint: 

1. If compaction and collapse problems occur, the 

impermeable panels will fail to efficiently drain runoff 

and hamper action to fill depressions in the trench. 

2. The bioengineered trench-cap cover requires 

active, continual maintenance to be effective. 

Studies in an arid environment with experimental 

trench-cap designs indicate that deep percolation can be 

reduced by the combined effects of a capillary barrier 

impeding infiltration and increased evapotranspiration 

(Hakonson, 1986). A major drawback to control of the 

water balance is the permanency of controls established 

to reduce infiltration. Experience shows that deterioration occurs with time and causes increased capacity of the 

trench cap to accept infiltration by collapse features, 

desiccation cracks, and intrusion by plant roots and 

burrowing animals. 

A capillary barrier may be a component of trenchcap design for a shallow repository. Bentonite additions 

to trench -cap materials have been used in experiments at 

the U.S. Army Corps of Engineers Waterways Experiment Station, Vicksburg, Miss., and at Los Alamos 

National Laboratory, Los Alamos, N. Mex. Long-term 

experimental data on prototype trench-cap designs are 

not available for evaluation. Such data and experience 

are necessary for adequate evaluation of cover design. 

The swelling properties of bentonite that effect its low 

permeability are complemented by its shrinkage during 

drying, causing potential stability problems within the cap 

and possibly rapid infiltration before the clays swell. 

Bioengineering includes barriers to prevent intrusion of roots and burrowing animals and a vegetative 

cover to reduce deep percolation. A biointruder barrier 

is a layer of boulders that inhibits or prevents root 

penetration and animal burrowing activities (Hakonson 

and others, in press). The barrier also may operate as a 

capillary barrier. However, biologic activity in the over24 Geohydrologic Aspects for Siting and Design of Low-Level Radioactive-Waste Disposal 
lying soil layer would be expected to develop permeability by root and animal penetration and to reduce the 

efficiency of the capillary barrier, thus permitting increased infiltration. 

To summarize the status of trench-cap effectiveness, trench caps at existing repositories in humid areas 

have experienced problems of instability. As a result, the 

caps have permitted infiltration of water into low-level 

radioactive-waste repositories. Long-term field experiments with careful monitoring are needed to test and 

refine trench-cap and capillary-barrier designs. 

Waste Form and Backfill 

The waste form greatly affects the stability of the 

trench cap. Efforts to enhance the physical stability of the 

waste form include incineration, compaction, incorporating in a solidification medium such as concrete or bitumen, and enclosing in steel, fiberglass, and plastic 

containers. Stabilization of the waste form by compaction, incineration, or incorporation in a stable solid form 

can reduce the magnitude of problems related to instability of the waste form experienced at many low-level 

radioactive-waste repository sites. 

The systematic placement of waste packages in the 

repository and careful backfilling of voids between packages can greatly improve the stability of the repository. 

The physical as well as the hydraulic and geochemical 

properties of the backfill are important in efficient design 

of a repository. Clay silt or soil backfill has been used in 

many existing repositories and is commonly included in 

proposed repository designs. Fine-grained backfill tends 

to retain a greater moisture content and tends to be 

unstable because of its compressibility. Backfilling with a 

cohesionless backfill, such as sand, facilitates filling of 

void spaces. Sand is less compressible than clay and 

facilitates drainage of water that penetrates into the 

repository. 

The low-level radioactive-waste facilities at Barnwell, S.C., Beatty, Nev., and Richland, Wash., currently 

accept waste in stable packages, systematically place the 

packages in the trenches (as opposed to random dumping), and backfill voids between the waste packages with 

sand. Such practices enhance the stability of the repository zone by reducing deterioration of the waste form and 

reducing potential settlement and compaction of the 

waste and backfill. 

Host Medium 

Monitoring at existing repositories indicates that 

repositories in clay deposits tend to accumulate water in 

the trench from infiltration at a rate exceeding the rate of 

drainage from the trench. Water accumulates in the 

trench, promotes leaching of the waste, and may overflow 

at the surface or flow away from the trench through the 

weathered zone near the surface. The so-called bathtub 

effect has prompted the recommendation that the host 

media be moderately permeable in order to allow seepage from the trench. The permeability of the host medium would ideally be of sufficient permeability to permit 

drainage of infiltration through the repository, but not 

excessively permeable, so as to provide for slow movement of water beyond the trench. The flow rate at which 

water will drain through the host media is greatest when 

the host media is saturated. An unsaturated buffer zone 

above the base of the trench and below the base of the 

waste could provide a storage area for intermittent pulses 

of recharge without saturating the waste. 

Conceptual Flow in a Simple Trench Repository System 

The objective of a simple trench repository is to 

provide a stable system and minimize the contact of 

water with the waste. The hydrology of the trench is not 

simple. The construction of the trench is referred to as 

"simple" because it is contructed of natural materials 

with none of the enhanced engineered barriers. 

The simple trench system considered consists of a 

shallow excavation constructed in a host medium, filled 

with waste packages and backfill, capped, and mounded 

(fig. 26). Idealized relations between the relative grain 

size of the materials of the major components of the 

trench repository and their unsaturated hydraulic conductivity at moisture tensions where the capillary barrier 

is effective are shown in figure 26. The relative saturated 

hydraulic conductivity is shown for contrast to be the 

inverse of the relative unsaturated hydraulic conductivity. 

The relative unsaturated hydraulic conductivity in 

figure 26 is simplified. As shown in figure 26, unsaturated 

hydraulic conductivity is a function of moisture tension, 

and a capillary barrier is only effective for a specific range 

of moisture tensions at the interface. Likewise, the relative grain size and saturated hydraulic conductivity are 

based on granular materials. Goode (1986) has shown 

that these relations may not be valid for some types of 

materials. For example, a coarse-grained crushed tuff is 

shown by Goode (1986) to be ineffective as the coarsegrained layer in a capillary barrier. 

A diagrammatic representation of the water balance at the trench and the unsaturated flow of water 

through the clay cap, host media, and backfill is shown in 

figure 27. The relative unsaturated hydraulic conductivity 

of each of these components is critical in effecting a 

minimum of flow through and moisture accumulation in 

the backfill. The relative unsaturated hydraulic conductivity is also very complex because each material has a 

unique relation between moisture content and hydraulic 

conductivity, and these factors change constantly in response to climatic changes and water-level fluctuations. 

The water balance is represented in figure 27 A. The 

infiltration of water from precipitation through the clay 

Hydrology of Shallow-Land Disposal of Low-Level Radioactive Waste 25 
cap is the residual, J, in the following water balance 

equation: 

sowed with short-rooted plants to decrease erosion and 

increase evapotranspiration. Design features to decrease 

infiltration largely through the use of natural materials 

have been tested in experimental trenches at the Las 

Alamos National Laboratory, New Mexico (Hakanson 

and others, in press). It is anticipated that efforts to 

decrease infiltration through design of trench cap and 

cover will meet with greater success in arid and semiarid 

regions than in humid regions. 

where 

s 

p 

0 

E 

c 

I= (S + P- 0-E) - C 

antecedent moisture content of clay cap, 

= precipitation, 

surface runoff, 

evapotranspiration, and 

moisture storage capacity of the clay cap. 

Infiltration into the clay cap could be reduced by 

increasing the runoff and the evapotranspiration. Trench 

caps are commonly mounded to increase runoff and are 

Trench-cap material of compacted nonswelling 

clay will reduce infiltration. The clay cap needs to be of 

sufficient thickness that desiccation cracks will not penetrate its full thickness. Runoff can only be increased so 

much before adverse effects of increased erosion become 

excessive. Vegetative cover, though consuming water, at 

Cap 

Waste and backfill 

--~~table __________________ __ 

IDEALIZED RELATIVE HYDRAULIC CONDUCTIVITY (KJ 

AND MEDIAN GRAIN SIZE (D) OF CAP, C, HOST 

MEDIUM, H, AND BACKFILL, B. 

CAP HOST BACKFILL 

UNSATURATED (U) Kuc > KuH > Kus HYDRAULIC CONDUCTIVITY 

SATURATED (S) Ksc < HYDRAULIC CONDUCTIVITY KsH < Kss 

MEDIAN GRAIN SIZE De < DH < Ds 

Figure 26. Diagrammatic trench repository and idealized relative unsaturated and saturated hydraulic 

conductivity and median grain size of trench cap, host medium, and backfill. 

B 

J{/lt""' 

Host medium Host medium 

Waste and backfill 

Figure 27. Diagrammatic sketch of hydrology of trench repository. (A) Net infiltration into clay cap is 

balance of precipitation minus runoff and evapotranspiration, plus or minus change in storage; (B) 

downward flow from clay cap to backfill may be impeded if backfill is at great moisture tension and low 

hydraulic conductivity; (C) lateral unsaturated flow of water in trench cap to host medium; (D) water may 

flow into or out of backfill depending on relative unsaturated hydraulic conductivities of backfill and host 

medium; (E) moisture in backfill must percolate downward into host medium or •bathtub effect" may 

occur. 

26 Geohydrologic Aspects for Siting and Design of Low-Level Radioactive-Waste Disposal 
the same time tends to retard runoff. Vegetative cover 

should be shallow rooted to avoid deep disturbance of 

the cap and penetration into the waste. Short-rooted 

vegetation is limited in capacity to transpire large quantities of water. Shallow-rooted plants in arid regions are 

much more efficient in consuming precipitation in excess 

of runoff than shallow-rooted plants native to humid 

regions. 

Infiltration through the clay cap will be impeded by 

the coarse-grained backfill, when according to the moisture tension-hydraulic conductivity relation, the backfill 

is of lower hydraulic conductivity than the fine-grained 

cap layer (fig. 27B). If moisture content builds up in the 

clay cap, the permeability contrast between the clay cap 

and the host medium (fig. 27C) should permit lateral 

flow of water into the host medium, rather than flow 

between the cap and the backfill. A coarse-grained host 

medium of comparable unsaturated hydraulic conductivity to the backfill might, at times, create a capiiiary 

barrier between the cap and the host medium. The 

potential for flow of water through the near-vertical 

surface between the host medium and the backfill (fig. 

270) will depend largely on the relative hydraulic conductivity of the host medium and the backfill. 

At the base of the trench, moisture in the backfill 

must percolate downward into the host medium or the 

so-called bathtub effect will occur. A sump of backfill at 

the base of the trench and beneath the lowermost waste 

package would provide a space for accumulation of 

moisture and provide sufficient hydraulic head to drive 

water from the base of the trench into the host medium 

(fig. 27E). At the base of the trench, ideally the host 

medium would be as permeable as the backfill. However, 

because the host medium should be fine-grained near the 

surface, the ideal fine-grained material at the surface and 

overlying coarse-grained material could tend to create a 

capillary barrier within the host medium. 

Waste Repository in the Saturated Zone 

Shallow-land disposal in the saturated zone may be 

preferable in some environments. In many humid and 

subhumid regions, the saturated zone commonly is no 

more than 5 or 10 m below the land surface. Cherry and 

others (1979) have proposed that low-level radioactive 

waste be disposed in unfractured, low-permeability clays 

in the saturated zone well below the zone of water-table 

fluctuation. In southern Canada, Cherry and others 

(1979) recommend burial of low-level radioactive waste 

in the bottom of large holes augered below the water 

table in dense, relatively unfractured clayey till and 

glaciolacustrine clay. In such an environment, contaminant movement would be predominantly by molecular 

diffusion. Cherry and others (1979) propose that in 

hydrologic studies of prospective repository sites in 

clayey deposits below the water table, the determination 

of the age of the ground water by means of naturally 

occurring isotopes-tritium, o:xygen-18, deuterium, and 

carbon-14-be a key factor. The burial zones need to be 

determined to have pore water that is very old, thereby 

indicating extremely slow ground-water velocity and favorable natural conditions for containment of the waste. 

If such a site is used, as seen by the preceding example, 

the design criteria are considerably different than for a 

repository in the unsaturated zone. In the saturated zone, 

which is not subject to alternate wetting and drying, 

swelling clay such as bentonite provides a stable, lowpermeability, high-sorption barrier to radionuclide migration. 

The concept of Cherry and others (1979) has been 

applied hypothetically to the geohydrologic conditions at 

the low-level radioactive-waste burial site near West 

Valley, N.Y. (Prudic, in press) (fig. 8). This concept may 

be feasible at the site because ground-water velocity in 

the unweathered till is less than 6 cm/yr, which results in 

diffusion controlling radionuclide migration (Prudic, 

1986). Analyses of radionuclides in water from till samples collected beneath three trenches indicate that detectable concentrations of tritium had migrated less than 

3m in 7 to 11 years. Assuming the water levels in a trench 

and the concentration of tritium in the trench water 

remains constant for 100 years, Prudic (in press) projected detectable concentrations of tritium to migrate only 

about 10m beneath the trenches after 100 years (fig. 28), 

mostly by diffusion. Prudic considered projection beyond 

100 years to be unreasonable because radioactive decay 

of tritium in the tenches probably would result in much 

smaller tritium concentrations in the trench water. 

Buffer Zone and Flow System 

The buffer zone, as defined in 10CFR61 is a 

"portion of a disposal site that is controlled by the 

licensee and that lies under the site and between the 

boundary of the site and any disposal unit." The repository, including the disposal units and the buffer zone, is 

thus part of the flow system. As a part of the flow system 

under control of the licensee, it is the zone in which 

remedial actions are most likely to be taken, if necessary. 

Major natural barriers to radionuclide migration 

need to be present in the ground-water flow system in the 

buffer zone and beyond the repository to the natural 

discharge area. Factors that present major barriers to 

radionuclide transport include (1) those that result in 

long ground-water travel time-long flow paths, low 

hydraulic gradient, and large effective porosity, and (2) 

those that decrease the concentration of radionuclides in 

solution-such as decay, sorption, and minimal solubility 

of waste. Dispersion, diffusion, and dilution are processes that also decrease point concentrations of contaminants but do not decrease the quantity of contaminant in 

solution. 

Hydrology of Shallow-Land Disposal of Low-Level Radioactive Waste 27 
Site-suitability requirements of the U.S. Nuclear 

Regulatory Commission specify that the geohydrologic 

conditions of the area allow reliable performance predictions and that the geohydrologic system must be characterized to enable confident predictions of contaminant 

movement. Adequate characterization and reliable performance predictions are most readily achieved in areas 

with simple geohydrologic settings. Many problems of 

contaminant transport experienced at repository sites 

stem from complex geologic conditions and hydraulic 

properties that were not adequately defined; consequent 

investigations were unable to predict or anticipate the 

contaminant transport that occurred. The complex distribution of the pebbly sand unit at the burial site near 

Sheffield, Ill., of the fractured nature of the sandstone 

beds at Maxey Flats, Ky., and of the heterogeneity of the 

regolith at Oak Ridge National Laboratory, Tenn., can be 

a: 

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100 

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DISTANCE BELOW BURIAL-TRENCH FLOOR, 

IN METERS 

Figure 28. Predicted tritium concentrations beneath 

trench after 10 and 100 years assuming: constant 

concentration of 1.44 microcuries per milliliter in 

trench water, constant water level in trench, porosity 

of 0.3, specific flux of 0. 7 centimeter per year, tortuosity factor of 1.6, diffusion coefficient of water of 475 

square centimeters per year, distribution coefficient of 

0.0 liter per gram (from Prudic, 1986). 

cited as instances where the rapid transport of radionuclides to the surface environment was not predicted 

accurately because of complexities of the ground-water 

flow system and inadequacies in characterization and 

analysis of the system. 

Containment of radioactive waste within the repository for the hazardous lifetime of the waste, with or 

without engineered barriers, is not considered practicable. The site needs to be selected so the natural geohydrologic flow system affords isolation of the waste from 

the surface. Monitoring and maintenance of the· repository site will be provided for an indefinite period generally considered not to exceed 100 years. Beyond this time, 

natural processes need to afford adequate isolation time 

for the waste. Engineered features of the site need to not 

interfere with the natural isolation afforded by the geohydrologic setting. This principle is emphasized because 

many engineered barrier systems do not appear to be 

effective unless continually maintained. For example, 

drains and sumps collecting infiltration on concrete or 

impermeable bases require continual maintenance. If 

infiltration into the repositories occurs and the sumps are 

not emptied periodically, the repository will emulate the 

"bathtubs" of the current generation of trench repositones. 

The ground-water flow system needs to provide 

long flow paths with long travel times from the repository. Long travel times may be afforded by low to moderately permeable materials, low hydraulic gradients, or 

geochemical environments which retard migration of 

radionuclides. 

Hydrologic Modeling 

The technical position of the U.S. Nuclear Regulatory Commission (1982) requires that the site can be 

characterized adequately to model the site to demonstrate compliance with performance objectives and that 

the natural processes affecting the site be defined such 

that the modeling of the site represent both present and 

anticipatable site conditions after closure (Siefken and 

others, 1982). From the considerations reviewed in this 

report, it is concluded that modeling of a repository site 

cannot be done with sufficient confidence for it alone to 

be used to confirm that a site will be acceptable as a 

repository site. 

The practice customarily followed in design, verification, and analysis of a ground-water model includes, 

initially, definition of the geology, geochemistry, and 

hydrology of the ground-water flow system from previous 

studies and field exploration studies designed to complete the characterization of the system. This first step 

provides data for definition of the hydraulic and 

geochemical parameters of the flow system-the framework of the model. The second step is model simulation 

28 Geohydrologlc Aspects for Siting and Design of Low-Level Radioactive-Waste Disposal 
of stress and response of the system, providing a basis for 

refinement of the model, a process called calibration of 

the model. In practice, the historical data on stress and 

response of the system may be divided by use of part of 

the data to revise the hydraulic and geochemical parameters of a model and use of the other part of the data to 

test the ability of the model to simulate the response to 

stress. 

The accuracy of models to simulate the response to 

stress has recently been examined by Konikow (1987), 

who examined several cases in which deterministic 

ground-water flow or solute-transport models were used 

to make predictions. Konikow (1987) found 

In general, the results of these postaudits did not yield a 

high correlation between observed and predicted changes. In one case, a 1-year period of detailed observation 

provided an inadequate basis to predict longer term (10 

year) changes in ground-water salinity in an irrigated 

stream-aquifer system. In another example, it was shown 

that a 40-year calibration period, in itself, did not provide 

a reliable basis for predicting changes in ground-water 

levels for a 10-year period. Although these examples are 

neither exhaustive in scope nor firmly conclusive in 

implications, they at least call into question the credibility and validity of predictions of heads and contaminant 

transport in ground water for perhaps tens, hundreds, or 

thousands of years in areas where there may be no 

historical observations, as is required for nuclear-waste 

isolation. 

Konikow (1987) concludes that a large component 

of the error arises from an overly simplistic approximation of a complex three-dimensional heterogeneous 

world, but points out that much of the predictive error is 

attributable to error in the assumed future stresses. It is 

noted that among the unknowns in the case of a low-level 

radioactive-waste repository are the future stress on the 

system and the hydraulic and geochemical properties of 

the future system as the site is excavated and reassembled as waste, backfill, and trench cap. 

Modeling of flow and transport in the unsaturated 

zone is complex, and models to simulate such conditions 

are limited. The problems presented in modeling 

geochemical reactions in an aquifer system are discussed 

by Thorstenson (in press), who pointed out that none of 

the modeling efforts he reviewed have produced a completely definitive model in spite of large expenditures of 

time, effort, and dollars. He concludes that models based 

solely on aqueous geochemistry are nonunique. Estimates of radionuclide transport are commonly based on 

flow models and empirical values for sorption of radionuclides based on laboratory experiments rather than 

data on the chemical nature of the aquifer system under 

examination. 

Thus, from experience with postaudits of groundwater flow models and considering the status of unsaturated flow, geochemical reaction, and transport models, it 

is apparent that modeling of a potential site is not feasible 

as the sole means of providing a definitive presite operational performance assessment. Models, however, can 

play an important and essential role in the overall evaluation of the site during the site selection phase and in 

evaluation of the operational and postoperational performance of the repository. Such use would need continued 

revision and refinement of the models. The process of 

model revision needs to be based not only on routine 

surveillance but also data collected for the purpose of 

revising the model periodically during the operational 

and postoperational phases of the repository. 

CONCLUSIONS 

Lessons Learned 

Experience at low-level radioactive-waste repository sites provides many lessons that can be applied in 

evaluation of future waste sites, in application of geologic 

and hydrologic principles to the design of low-level waste 

facilities, and in monitoring of low-level waste sites in 

order to evaluate site performance. Some lessons learned 

are given below. 

1. Simple trench repositories filled with waste and 

backfilled and capped by excavated materials have been 

constructed at low-level radioactive-waste disposal sites 

in arid, subhumid, and humid climates in the United 

States. The performance of repositories in these environments differs markedly in effectiveness of waste containment. The simple engineering design at Beatty, Nev., in 

an arid climate is apparently adequate, but similar designs at sites in the eastern part of the United States in 

humid and subhumid climates generally have not performed as well in containing waste. 

2. Backfill of waste containers and the integrity of 

waste containers in resisting compaction plays a large 

role in the integrity of the capping materials. The collapse 

problems at the site near Sheffield, Ill., are more completely documented than the other sites, although subsidence and settlement cavities have presented problems 

also at West Valley, N.Y., Maxey Flats, Ky., and Oak 

Ridge National Laboratory, Tenn. At each of these sites, 

trenches have been backfilled with materials containing a 

large clay fraction. In contrast, current practice at the site 

near Barnwell, S.C., is to backfill with sand, a material 

having much less compactibility than clay. Collapse features at Barnwell, S.C., are reported by the operator to be 

of minor occurrence; data on frequency and volume of 

collapse are not available. 

It was determined from the review of site histories 

that the construction of clay caps has undergone changes 

in development in attempts to create a cap which limits 

infiltration and surface runoff. Caps at the disposal site 

near Sheffield, Ill., greatly reduced infiltration even 

Conclusions 29 
though compaction of the capping material was largely a 

byproduct of operating heavy machinery over the trenches in backfilling the waste and laying the caps. At West 

Valley, N.Y., compaction of the clay caps was employed 

specifically to decrease infiltration; compaction methods 

included load surcharging of the trench cap with material 

excavated in constructing successive trenches. Infiltration 

into the trenches at West Valley, N.Y., was decreased, 

but not to the extent required, and water accumulation in 

the trenches has been periodically pumped out to prevent 

overflow. Deterioration of the clay caps has been effected 

by compaction of the waste, backfill, and shrinkage 

cracks developed during prolonged periods of dry weather. 

3. The hydrologic regime of a site is profoundly 

effected by construction and engineered features of the 

waste facility. Studies of the presite operational hydrology are useful and necessary but do not provide direct 

information on all significant aspects of the hydrology of 

the site during operation and after closure. For example, 

disturbance of the site during operation has been shown 

to affect infiltration at the site and that infiltration may 

increase or decrease. The effect of trench-cap materials 

and construction practices on infiltration at the waste site 

is a question that cannot be answered definitively prior to 

construction. 

4. Suitability of a repository is a function of both 

the site characteristics and the engineering design and 

construction of the repository and the waste packages. 

With the possible exception of the extremely arid environments, there are no intrinsically well-suited geohydrologic environments; likewise, there are no intrinsically 

well-suited engineered barrier systems. 

5. Low-permeability materials are severely limited 

as suitable host media for low-level radioactive waste in 

the unsaturated zone. Both the scientist and the layman 

seem predisposed to select rocks of low permeability as 

host media for low-level waste. What would be better 

than to isolate waste in a low-permeability matrix? The 

problem is that the low-permeability host medium in the 

coventional low-level radioactive-waste repository does 

not enclose the waste. The capping materials are not 

impermeable; experience has shown that they permit 

infiltration and will become more permeable with time. 

Moreover, experience indicates that infiltration capacity 

of a trench cap will increase with time due to biological 

activity and weathering. The resulting phenomenon is 

increased infiltration and accumulation of water in the 

trench, commonly referred to as the "bathtub effect," 

because the low-permeability host media will not permit 

the repository to drain as rapidly as water infiltrates the 

repository. Saturation of the waste promotes its weathering and leaching. Filling of the trench with water results 

in either overflow of the trench and rapid overland 

transport of leachate or subsurface migration of radionuclides in the weathered zone of the host rock. 

6. Humid and subhumid regions characterized by 

moderate to large infiltration of precipitation, shallow 

depths to the water table, and relatively short flow paths 

to discharge areas are not well suited for the shallow 

isolation of low-level radioactive waste in the unsaturated 

zone; arid regions characterized by zero or small infiltration from precipitation, great depths to the water 

table, and long flow paths to natural discharge areas are 

naturally well suited to the isolation of low-level radioactive waste. 

7. Consolidated rocks of sedimentary, metamorphic, or igneous origin, weathered zones or regoliths of 

consolidated rocks, and a thin veneer of unconsolidated 

sedimentary deposits overlying consolidated rocks at 

shallow depths are poorly suited for the trench disposal 

of low-level radioactive waste. Thick sequences of unconsolidated deposits having interstitial porosity and permeability are better suited to the disposal of low-level 

radioactive waste at shallow depths. 

Geohydrologic Screening and Site Selection 

The following possible alternatives are listed concerning the geohydrologic screening of regions for prospective low-level radioactive-waste repository sites and 

selection of sites for characterization: 

1. Distinguish between geohydrologic criteria for 

unsaturated and saturated zone burial of waste. For 

trench disposal in the unsaturated zone, it is desirable 

that depths to ground water be 15m or greater. 

2. Potential host media in the saturated zone are 

low-permeability materials in which the rate of transport 

of radionuclides by diffusion will be greater than that by 

advection. Host media essentially need to be homogeneous, relatively thick (greater than 20 m) clay. The 

burial depth needs to be a few meters beneath the 

weathered zone. Low-permeability capping material 

needs to extend to below the weathered zone. Consolidated sedimentary, igneous, and metamorphic rocks are 

not considered suitable host media because of the likelihood of fractured or channel-like flow paths. 

3. Potential host media in the unsaturated zone 

needs to be unconsolidated, granular, bedded, primary 

sedimentary material with moderate permeability. Host 

media could be beneficially overlain by low-permeability 

beds of clay. Consolidated rocks, weathered material, 

and regolith are considered undesirable host media. 

4. Flow system characteristics need to include long 

flow paths and travel times from the repository to the 

discharge area. Flow direction needs to be vertically 

downward in the repository area. Radionuclide isolation 

time in the repository and the buffer zones needs to be 

300 to 500 years. 

5. Natural materials that will provide adsorption of 

radionuclides need to be present in the flow system 

downgradient from the waste units. 

30 Geohydrologlc Aspects for Siting and Design of Low-Level Radioactive-Waste Disposal 
Repository Construction and Maintenance 

The following recommendations are made concerning repository construction and maintenance: 

1. Repositories need to be excavated to depths 

allowing burial either fully above the highest fluctuation 

of the water table or below the lowest fluctuation of the 

water table. 

2. Repositories need to be constructed to provide 

for burial of waste in unweathered sections of the host 

media. 

3. Trench or auger hole caps above the water table 

need to be of low-permeability, nonswelling, compacted 

clay of sufficient thickness to extend below the weathered 

section of the host medium. Maintenance of the repository needs to provide for prompt filling and tamping of 

cavities or general subsidence as they occur for the 

hazardous life of the waste. 

4. Backfill of waste packages in the unsaturated 

zone needs to be with noncohesive, coarse-grained material (that is, medium to coarse sand), free from clay and 

silt. The backfill needs to fill a thickness of from 1 to 5 m 

of the trench below the lowest level and above the highest 

level of the waste. 

5. Vegetation of the trench caps needs to be limited 

to shallow-rooted plants such as grasses. 

Subsurface Monitoring and Modeling at 

Low-Level Radioactive-Waste 

Repository Sites 

Subsurface monitoring at low-level radioactivewaste repository sites may be thought of as being part of 

the three major phases of a repository site, namely, (1) 

the characterization phase, (2) the constructional and 

operational phase, and (3) the postoperational closure 

phase. As the name implies, monitoring consists of 

repeated or periodic measurements of hydrologic conditions at selected locations. A subsurface monitoring 

network is a number of discrete points selected to 

provide a basis for evaluating changes in processes that 

occur in a continuum throughout the study region. Monitoring at low-level radioactive-waste repository sites has 

been comprehensively discussed in a series of reports 

prepared for the U.S. Nuclear Regulatory Commission. 

The first report (Lutton and others, 1982b) identifies (1) 

the parameters that should be investigated and evaluated 

to determine the suitability of a site, (2) the parameters 

needed for design and construction of a repository, and 

(3) the parameters needed to be monitored to evaluate 

the performance of a disposal facility. The second report 

(Lutton and others, 1982a) identifies recommended laboratory and field tests for determining the parameters 

identified in the first report. The third report (Lutton and 

others, 1983) recommended a program for monitoring 

during site characterization, construction, operation, and 

closure. In each of the phases of site activity, the monitoring network can be implemented by hydrologic and 

geochemical models to assist in evaluating the magnitude 

and rates of changes occurring throughout the region of 

interest and to predict the future performance of phenomena within the region. Neither monitoring nor modeling activity provides the whole picture; together these 

activities can be used conjunctively for mutual enhancement and refinement. 

Collection of hydrologic information on the repository site and the flow system needs to continue from the 

characterization phase through the operational and postoperational phases. Although it is concluded that definitive models cannot be made during the characterization 

phase, hydrologic models of the flow system are an 

essential continuing part of the analysis of site performance and an essential tool in early detection and 

projection of radionuclide migration from the disposal 

units. The U.S. Nuclear Regulatory Commission (1982) 

requires monitoring to provide an early warning of 

radionuclide migration. It is here suggested that hydrologic models are needed, and they need to be maintained 

and refined concurrently with the monitoring program. 

Hydrologic models, if revised and refined on the basis of 

a carefully designed monitoring program can provide 

increasingly accurate projections of rates of radio nuclide 

movement and a basis for anticipating the need for 

mitigating or remedial action. 

Engineered Barrier Systems 

There are neither experimental nor experiential 

real-time bases for long-term projections regarding the 

effectiveness of engineered barriers for long-term containment of radionuclides. Engineered barriers may 

serve a useful purpose for a limited period of time, but 

they need not be designed in any way that will ultimately 

reduce the effectiveness of the natural hydrologic system 

in isolating the waste from the environment. Engineering 

barriers including those designed to isolate the waste, 

drain the repository, stabilize the waste or the repository, 

or prevent the waste from coming in contact with moisture cannot be relied upon to provide long-term (300 to 

500 years) isolation for the radioactive life of the waste. 

The inherent limitations in the life of engineered 

barriers for waste isolation and limitations of modeling 

capability to predict the rate of release and migration of 

the waste emphasize the need for independent engineered and natural barriers for waste isolation. 

Institutional control and maintenance of low-level 

radioactive-waste repositories are expected to cease by 

100 years following closure of the waste site (U.S. 

Nuclear Regulatory Commission, 1982). Beyond this 

time, the repository and the geohydrologic conditions 

Conclusions 31 
need to act passively to effectively isolate the radionuclides for an additional 300 to 500 years. Most engineered barrier systems appear to have been designed 

independently of the natural geohydrologic conditions. 

Furthermore, many engineered systems need maintenance in order to perform as designed. 

For example, systems that are impermeable containers with drainage sumps could adversely affect the 

repository if not maintained. Moisture continuing to 

accumulate will saturate the waste and overflow, releasing radionuclides to the environment. Such problems 

need to be addressed in evaluation of engineering design 

of repository facilities. 

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Regulatory Commission NUREG/CR-2478, v. 1, 245 p. 

Hillel, Daniel, and Talpaz, H., 1977, Simulation of soil water 

dynamics in layered soils: Soil Science, v. 123, no. 1, p. 

54-62. 

Huff, D.D., and Farrow, N.D., 1983, Solid waste storage area-4 

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Development Administration, Oak Ridge National Laboratory Report ORNL-6009, p. 84-85. 

lchimura, V.T., in press, Difficulties with above water-table 

disposal of low-level radioactive waste in areas having 

high accretion rates, in Bedinger, M.S., and Stevens, 

P.R., eds., U.S. Geological Survey Workshop on LowLevel Radioactive-Waste Disposal, Big Bear Lake, Calif., 1987, Proceedings: U.S. Geological Survey Circular 

1036. 

International Atomic Energy Agency, 1981, Shallow ground 

disposal of radioactive wastes-A guidebook: Vienna, 

Safety Series 53, 52 p. 

---1982, Site investigations for repositories for solid radioactive wastes in shallow ground: Vienna, Technical Report Series 216, 72 p. 

Johnson, T.M., Larson, T.H., Herzog, B.L., Cartwright, Keros, 

Stohr, C.J., and Klein, S.J., 1983, Study of trench covers 

to minimize infiltration at waste disposal sites-Task II 

report, Laboratory evaluation and computer modeling of 

trench cover design: U.S. Nuclear Regulatory Commission NUREG/CR-2478, v. 2, 94 p. 

Kelleher, W.J., 1979, Water problems at the West Valley burial 

site, in Carter, M.W., Moghissi, A.A., and Kahn, Bernd, 

eds., Management of low-level radioactive waste, v. 2: 

New York, Pergamon Press, p. 843-852. 

Konikow, L.F., 1987, Current issues-Predictive modeling of 

ground water: The Hydrogeologist, 4 p. 

Lavallee, Fred, 1986, Design, siting, criteria, and cost analysis 

for a small low-level waste disposal facility, in Seventh 

Annual Participants' Information Meeting, Las Vegas, 

Nev., 1985, Proceedings: U.S. Department of Energy 

Low-Level Waste Management Program CONF8509121,p. 176-196. 

Lutton, R.J., Butler, D.K., Meade, R.B., Patrick, D.M., Strong, 

A.B., and Taylor, H.M., Jr., 1982a, Tests for evaluating 

sites for disposal of low-level radioactive waste: U.S. 

Nuclear Regulatory Commission NUREG/CR-3038, 

164 p. 

Lutton, R.J., Malone, P.G., Meade, R.B., and Patrick, D.M., 

1982b, Parameters for characterizing sites for disposal of 

low-level radioactive waste: U.S. Nuclear Regulatory 

Commission NUREG/CR-2700, 74 p. 

Lutton, R.J., Strohm, W.E., and Strong, A.B., 1983, Subsurface 

monitoring programs at sites for disposal of low-level 

radioactive waste: U.S. Nuclear Regulatory Commission 

NUREG/CR-3164, 89 p. 

Lyverse, M.A., in press, Recent investigations and brief description of site hydrology at the Maxey Flats low-level 

radioactive-waste disposal site, Fleming County, Kentucky, in Bedinger, M.S., and Stevens, P.R., eds., U.S. 

Geological Survey Workshop on Low-Level 

Radioactive-Waste Disposal, Big Bear Lake, Calif., 

1987, Proceedings: U.S. Geological Survey Circular 

1036. 

Matuszek, J .M., 1986, Safer than sleeping with your 

spouse-The West Valley experience: American Chemical Society, New York, 1986 (Preprint). 

Matuszek, J.M., and Robinson, L.W., 1983, Respiration of 

References Cited 33 
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1983, Proceedings: Tucson, Ariz., LaGrange, Ill., American Nuclear Society, p. 423-430. 

McMahon, P.B., and Dennehy, K.F., in press, Geohydrology of 

the low-level radioactive-waste disposal facility near 

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Stevens, P.R., eds., U.S. Geological Survey Workshop on 

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Nichols, W.D., 1985, Geohydrology of the unsaturated zone at 

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Palmquist, W.N., Jr., and Johnson, A.I., 1962, Vadose flow in 

layered and nonlayered materials, in Short papers in 

geology, hydrology, and topography: U.S. Geological 

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Papadopulos, S.S., and Winograd, I.J., 1974, Storage of lowlevel radioactive wastes in the ground, hydrogeologic and 

hydrochemical factors, with an appendix on the Maxey 

Flats, Kentuck.J', radioactive-waste storage site-Current 

knowledge and data needs for a quantitative hydrogeologic evaluation: U.S. Geological Survey Open-File Report 74-344, 49 p. 

Prudic, D.E., 1979, Recharge to low-level radioactive-waste 

burial trenches 11 through 14, West Valley, New York: 

U.S. Geological Survey Open-File Report 79-990, 12 p. 

---1986, Ground-water hydrology and subsurface migration of radionuclides at a commercial radioactive-waste 

burial site, West Valley, Cattaraugus County, New York: 

U.S. Geological Survey Professional Paper 1325, 83 p. 

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humid northeast, in Bedinger, M.S., and Stevens, P.R., 

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International Agency of Atomic Energy, OECD Agency 

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Geological Survey Workshop on Low-Level 

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Thorstenson, D.C., in press, Chemical modeling of regional 

aquifer systems-Implications for chemical modeling of 

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P.R., eds., U.S. Geological Survey Workshop on Low34 Geohydrologic Aspects for Siting and Design of Low-Level Radioactive-Waste Disposal 
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156 p. 

References Cited 35 
Metric Conversion Factors 

For readers who wish to convert measurements from the metric system of units to the inch-pound 

system of units, the conversion factors are listed below. 

Multiply metric unit By To obtain inch-pound unit 

millimeter (mm) 0.03937 inch 

centimeter (em) 0.3937 inch 

centimeter per year (cm/yr) 0.3937 inch per year 

meter (m) 3.281 foot 

meter per day (m/d) 3.281 foot per day 

meter per year (m/yr) 3.281 foot per year 

cubic meter (m3

) 35.31 cubic foot 

kilometer (km) 0.6214 mile 

The following terms and abbreviations also are used in this report: 

microcurie per milliliter (p.Ci/mL) 

picocurie per liter (pCi!L) 

Sea level: In this report "sea level" refers to the National Geodetic Vertical Datum of 1929 

(NGVD of 1929)-a geodetic datum derived from a general adjustment of the first-order level nets of 

both the United States and Canada, formerly called Sea Level Datum of 1929. 

36 Geohydrologlc Aspects for Siting and Design of Low-Level Radioactive-Waste Disposal 
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