Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 78
Deep Burial of Toxic Wastes
INTRODUCTION
STANLEY N. DAVIS
University of Arizona
AB STRACT
Deep burial of toxic wastes provides several advantages over disposal in surface structures or by shallow
burial. The primary advantage of deep burial is the high degree of physical isolation that it provides. Some
of the hydrogeologic advantages of deep burial are (1) increase in length of flow path of contaminants that
may become dissolved in groundwater; (2) increased protection of waste against weathering and erosion; (3)
for some wastes and waste containers, elimination of free oxygen that may mobilize certain constituents; and
(4) for plutonic rocks and to some extent all rocks, reduction of permeability with depth. Primary disadvantage
of deep burial is the high cost of exploration, development, and monitoring of deep disposal systems.
Reduction of permeability with depth in metamorphic and platonic igneous rocks is well defined to depths
of about 300 m. Further reduction of permeability with depth probably takes place but is difficult to quantify
using present data. Most of the reduction in permeability with depth in sedimentary rocks is within the
upper 100 m. The reduction of permeability is generally at least three orders of magnitude from near the
land surface to depths of 100 m for all types of indurated rocks. Based on hydrogeologic criteria alone, many
different rock types should provide safe waste repositories at depths greater than 100 m.
Deep repositories in most locations will eventually fill with water. However, if zones of significant ground-
water circulation are either avoided or grouted, repositories at depths of more than 300 m in granitic rocks
should take several hundred years to fill with groundwater once they are closed. In a well-placed repository,
several thousand years may be needed to accomplish a simple piston-flow displacement of all water in the
flooded repository. Even this length of isolation may not allow enough time for the degradation of all wastes
to take place, but a long isolation time will most commonly mean a slow release of mobile contaminants into
the biosphere, which, in turn, suggests that dilution will be more effective than in the case of a fast release
of contaminants from shallow burial sites.
Humans have produced toxic wastes since before recorded
history. At least four factors, however, have made the problem
of toxic waste disposal far more acute within the past 100 yr
than at any previous period. First, and most obvious, is the
78
human population explosion and the fact that most of the ad-
ditional people have accumulated in urban areas. The second
is the development of numerous industrial processes that have
toxic wastes as a by-product. The third is the development of
the ability to concentrate and produce radionuclides. The fourth
is the public awareness of the toxic waste problem, which has,
OCR for page 79
Deep Burial of Toxic Wastes
paradoxically, made the solution both possible and at the same
time more difficult.
Public awareness has forced the formation of the legal frame-
work to control the wastes and has stimulated funding to help
solve the technical problems of disposal. However, concern
about the hazards has given rise to an almost universal resis-
tance to the creation of repositories for hazardous wastes. Com-
monly, the same groups that have lobbied for the formation of
strict laws that force the creation of special waste repositories
join forces with local citizens to prevent the actual construction
of repositories at any specific site. Partly as a consequence of
this resistance, in some regions the actual disposal of waste is
so difficult, owing to great distances to approved disposal sites
or to excessive disposal costs, that illegal and highly dangerous
dumping of wastes has been practiced.
Even though the location, authorization, and preparation of
disposal sites for hazardous wastes have been slow partly owing
to the public's "not in my backyard" reaction, the additional
time made available due to the delay is being used beneficially.
Analytical methods are being developed that will allow a more
precise characterization of hazardous wastes; alternative meth-
ods of packaging, transportation, and disposal are being stud-
ied; legal and administrative procedures are being perfected;
and time for a balanced public education program is available.
The present production of hazardous wastes in the United
States is measured in several tens of millions of cubic meters
per year (United States Congress, 19797. The exact amount
varies according to the classification system used. Unfortu-
nately, only a small fraction of this waste is currently treated
or disposed of in a satisfactory manner (Council on Environ-
mental Quality, 19811. Compaction, evaporation, and chemical
treatment of the waste may eventually transform much of the
waste to an innocuous form and reduce the volume of the
remaining hazardous material to only a few million cubic meters
per year, which will still require disposal in highly engineered
structures. Even if this optimistic goal is achieved, the disposal
of this volume of material still requires a major national effort.
Because the principal mechanism for escape of hazardous waste
to the biosphere is through groundwater flow, involvement of
the geologic profession in this effort is essential for the proper
solution of the problem (NRC Committee on the Geological
Aspects of Industrial Waste Disposal, 1982)
Wastes can be hazardous because of the presence of (a) toxic
chemicals, (b) chemicals that are initially innocuous but turn
toxic on degradation or reaction with other chemicals, (c) ex-
plosive or combustible materials, (d) sharp objects that will cut
or puncture, (e) large objects that will collapse or tumble, (f)
radioactivity, and (g) pathogenic organisms. Unless noted
otherwise, the remainder of this chapter will be confined to a
discussion of only the more stable toxic chemicals and materials
contaminated with radionuclides having half-lives in excess of
several decades. Although the broad topic of the disposal of
radioactive waste is not treated in detail, the similarity of the
programs of disposing of transuranic and some low-level ra-
dioactive material to problems of hazardous chemical wastes
makes it convenient to combine all hazardous chemical and
radioactive wastes in the discussion.
79
METHODS OF DISPOSAL
A large number of methods of disposing of toxic wastes exist.
Large concrete mausoleums, slurry injection in deep wells,
trenches in desert regions, and repositories mined in bedrock
are some of the options of disposal. From a purely hydrogeo-
logic standpoint, trench burial in dry permafrost, which would
receive prefrozen wastes, would be the most acceptable method
of disposal. The delicate nature of the Arctic environment, the
physical difficulties of operating in such a harsh climate, and
its remoteness from points of origin of the waste, however,
suggest that extensive use will not be made of dry permafrost
for the disposal of hazardous wastes. The eventual choice of a
method and geographic location will be a function of waste
form, level of hazard present, duration of the hazardous char-
acteristics, availability of a proper natural setting, and complex
economic and legal factors. Although a number of disposal
methods will be used eventually, this chapter will discuss only
deep repositories mined in consolidated rocks. For this chap-
ter, "deep burial" will be considered to start at depths of 100
m. The existence of alternative disposal methods is assumed,
so that only highly compacted wastes that will remain hazardous
for thousands of years are considered for deep burial.
Actual burial of waste will be assumed to be in mined cavities
within rocks of low porosity and permeability. Although ex-
.
. .
cavat~on anct Placement ot wastes may be accomplished by
remotely controlled methods, all shafts and cavities will be large
enough for human entry for various purposes including testing,
inspection, and monitoring. Deep burial will require consid-
erable investment of effort and money for site preparation and
sinking of shafts; consequently, large volumes of waste, perhaps
as much as 105 m3 of radioactive waste or 106 m3 of chemical
waste, would probably need to be buried in a single repository
in order to justify the overall investment in such an undertak-
ing. Even with multilevel placement of drifts, the excavation
alone would most likely underlie at least 100 hectares.
The concept of multibarriers to the migration of wastes, which
has been discussed widely in connection with radioactive wastes
(U.S. Department of Energy, 1979; Davis, 1982; NRC Board
on Radioactive Waste Management, 1983), should be applied
to all repositories. An attempt should be made to convert the
waste into a chemically inert, nonpermeable form. The waste
should then be packaged in strong, resistant containers to fa-
cilitate handling and to retard contact with groundwater after
burial. Drifts and shafts should be backfilled with material
having a low permeability combined with a high capacity to
sorb water-transported contaminants. The host rock should be
grouted along fractures to reduce further the already low nat-
ural permeability. The host rock should be chosen for its me-
chanical stability, for its low permeability, and for its uniformity
of mechanical and hydrogeologic properties. The repository
should be placed in an area that would favor a long flow path
of the groundwater prior to the groundwater's emerging at the
surface or into a large body of water such as the ocean (Bre-
dehoeft and Maini, 1981~. This placement would allow time
for chemical and radioactive decay as well as dilution of the
contaminants in the groundwater. Thus, the multiple barriers
OCR for page 80
80
would be waste form, packaging material, backfill, engineering
modification of host rock, host rock, and, finally, the long mi-
gration path prior to the emergence of contaminants at the land
surface.
ADVANTAGES AND DISADVANTAGES OF
DEEP BURIAL
Deep burial of toxic wastes has several advantages over shallow
burial or surface storage systems. One of the most important
advantages is that contaminants that may become dissolved in
groundwater will not migrate directly to the land surface. The
increased length of the groundwater flow path will allow time
for the decay of radionuclides, the decomposition of unstable
chemical compounds, and the dilution of toxic materials by
dispersion. Deep burial will commonly place wastes in zones
near or adjacent to natural saline or brackish water, which, in
contrast with surface water, has little or no practical value and,
if contaminated, would not represent a large loss. Deep burial
will also afford protection against the possibility that the haz-
ardous materials will be exposed at the surface through slow
processes of erosion. Finally, and perhaps most important,
deep burial will make human intrusion less likely.
The depth of burial that may be chosen is related generally
to rock permeability so that the deeper the burial, the lower
the permeability. As will be shown, however, the reduction of
permeability is small beyond depths of about 300 m. The re-
duction in the first 50 m, in contrast, is most commonly at least
one order of magnitude.
Generally, pH, salinity, and alkalinity of natural water in-
crease with depth, but concentrations of nitrate and dissolved
oxygen decrease (Davis, 1981~. For some wastes, the chemical
characteristics of deep groundwater may have some advantages
over the chemical characteristics of shallow groundwater. For
example, many of the transuranic elements will be less mobile
in the deeper groundwater, where chemically reducing con-
ditions prevail.
The most serious disadvantage of deep burial is economic.
Deep repositories will be expensive to construct; they will be
difficult to monitor; and if errors are made or unexpected flaws
in the repository are uncovered, the removal of the waste in
order to place it in a better location will be costly.
Estimates of costs (in 1982 dollars) of constructing deep re-
positories for high-level nuclear waste generally range from
about $1.5 billion to $1.7 billion (Waddell et at., 1982~. Such
repositories would be designed to receive about 7.0 x 104
metric tons of high-level waste in about 1.25 x 105 containers
of various types having a total volume of about 1.1 x 105 m3.
The cost of the repository includes site acquisition, site im-
provements, receiving facilities, excavation of underground
workings, and ventilation systems. It does not include opera-
tion costs nor waste preparation facilities. If one assumes equal
costs for a mined repository for hazardous chemical wastes,
then it becomes clear that such a structure can be justified only
for the most hazardous wastes. For packages of nuclear waste,
the cost will be in excess of $1O,OOO per m3 of packaged waste.
If operating and processing costs are added, the total costs of
STANLEY N. DAVIS
disposal will be more than $30,000 per m3 of waste (Waddell
et al., 1982~. Similar cost estimates are not available for a hy-
pothetical deep repository constructed to receive only chemical
wastes. However, the costs should be considerably less because
the heat dissipation problem of high-level waste, which pre-
vents close packing of the waste, will not be present. Therefore,
the volume of chemical waste accommodated should be much
larger than for high-level wastes, perhaps by an order of mag-
nitude. A guess of possible costs might range from $5000 to
$10,000 per m3 for chemical wastes, which would include con-
struction of the repository and operating costs. Even with extra
close packing of waste containers and relaxed problems of waste
handling, it is hard to visualize a total cost of less than $1000
per m3 (or $1 per liter) for deep burial with isolation require-
ments similar to that of nuclear waste.
Costs of burial in shallower mined cavities would be much
less. Estimates of costs in mined space in salt together with
conventional facilities to handle packages of low-level radio-
active waste suggest costs of about $100 (1978 dollars) per m3
for a volume of 1.5 x 10 m3 of waste to be placed in the
repository each year (Wacks, 1979~.
In conclusion, the cost of waste disposal will probably range
from $100 to $200 per m3 for not-so-hazardous materials to a
few thousand dollars per cubic meter for very hazardous chem-
ical wastes and as much as several tens of thousands of dollars
for high-level radioactive waste. Exact costs are most sensitive
to variations in the density of waste placement in the subsur-
face; requirements for special containers and packaging pro-
cedures; and methods of transporting, storing, and handling
the materials prior to subsurface disposal (Clark and Cole, 1982~.
Total costs for repositories for high-level wastes are not highly
sensitive to variations of depth and geology, which help de-
termine the costs of the excavation of subsurface space, because
processing, packaging, and handling together comprise the largest
part of the total cost.
A HYPOTHETICAL REPOSITORY
Deep geologic repositories of various designs have been pro-
posed for the disposal of radioactive wastes (St. John, 1982~. If
the economic factors become favorable, similar repositories will
undoubtedly be proposed also for highly toxic chemical wastes.
From the standpoint of scientific factors, no general reason
exists that properly packaged chemical wastes, particularly if
they are in solid form, could not be placed in the same repos-
itory with transuranic and low-level radioactive wastes. In the
United States, however, institutional arrangements for the su-
pervision and control of hazardous chemical wastes are gen-
erally separated from radioactive wastes. Construction of a joint-
use repository would seem to be unlikely for the next few
decades. Consequently, the hypothetical repository described
will be assumed to be only for the purpose of receiving toxic
chemical wastes.
The selection of a repository site will undoubtedly follow
many of the criteria developed for locating sites for nuclear
waste (Bredehoeft et al., 1978; NRC Panel on Geologic Site
OCR for page 81
OCR for page 83
OCR for page 84
OCR for page 85
OCR for page 86
OCR for page 87
OCR for page 88
OCR for page 89
OCR for page 90
Representative terms from entire chapter:
chemical wastes
Deep Burial of Toxic Wastes
81
Criteria, 1978; U. S. Department of Energy, 1979, 1983~. The erosion would generally be slower than with most other rocks
following are general criteria as adapted from St. John (1982~: given equivalent geographic locations.
(a) Site geometry
(b) Host-rock
properties
Adequate depth, thickness, and lateral
extent of host rock
Low permeability and mechanical
strength sufficient to allow for stable
excavations; chemical composition of
waste will not react adversely with
rock
(c) Hydrology Low groundwater velocities; long dis-
tance from points of potential contami-
nation to points of use of potable
groundwater
Uplift or subsidence at rates low enough
not to be a threat to the longer-term
stability of the site
Located away from active faults that
would threaten operational safety or
long-term containment; not located in
areas of major historical earthquakes
Site would avoid areas of Quaternary
(d) Tectonic
stability
(e) Seismic activ-
ity and faulting
82
~1
Ventilation
shelf t
(Exhaust)
i
MAT
a, .
(around - water
f low
,
11
~1
//
/
Waste receiving,
inspection, and
final packaging ~ I _
Monitoring ~ n n ~ hi/ ~
~ ~ , ~~,,~-,,^~l -~ 7~~ -' ~ !
_ .
_
· Double liner on shaft
to be filled and
, grouted on closure
. ~o~
Water bearing
fractures grouted
~~ Bedrock /)<
~ a/
\
Massive metamorphic or
platonic igneous rocks
3uikhcad
closure
Zeolite tuft ~ Grout <=
backfill
~ bib
~1
Of'
Shaft
t~for waste
_ _ _
· . ~
Alluvial
aquifer
a
Inner casing
into upper
part of
bedrock
/
STAN LE Y N. DAVI S
Service
shaft
l
_£ i,~
_ - - O
Ground-watcr
f low
.4 .
~°
Grout back of
inner casing
~ //
/'
,/
/
Subsurface
control
station
S ~ ''
containers for
solid waste
- Lr~
Inclined drif ~ for sea led
canIstcre of liquid waste
FIGURE 5.1 Type of repository development that may be possible for toxic chemical wastes. Features shown are not to scale, and all shafts and
drifts are not shown. Full development would involve one or more levels having a gridlike network of drifts. Mining, placement of waste, and
eventual closure would follow a sequence that is not indicated in the figure.
Deep Burial of Toxic Wastes
o
20
40
, - 60
-
I
c:~80 _
100 _
120 _
\38 `"
\ ""
_ ~ `_
I ~
I_""~to~ \
\ ~ 26
20. \
';
_
140
0 20 40 60 80 100
PERCENT UNWEaTHERED GRANITIC ROCK
FIGURE 5.2 Percentage of unweathered rock as a function of depth
at the Folsom Dam site in California. The total number of drill holes
that supplied data for each depth interval is given next to each plotted
point. Data are from unpublished reports of the U.S. Army Corps of
Engineers and were compiled by Davis (1981).
bias in favor of a greater permeability at depth. However, the
location of the dam or tunnel most commonly favors areas of
sound rock, so permeabilities of rocks within the area in general
would tend to be lower than adjacent areas. The general con-
clusion is that test-hole data also show a decrease of permea-
bility with depth that is similar to water-well data (Figures 5.4,
5.5, and 5.6~. In general, the well-defined decreases in perme-
ability are caused by at least the following five factors (Davis
and Turk, 1964~:
1. A decrease in the effects of surficial weathering with an
increase in depth (Figure 5.2~.
2. An increase in distance between joints, particularly sheet-
ing joints, with depth.
3. An increase of lithostatic pressures with depth that tends
to close fractures at depth.
4. A decrease with depth of fractures related to mass wast-
ing.
5. A decrease with depth of the effect of topography on those
stress patterns that might help contribute to localized rock
failure.
83
,000_0 ~
~ ,oO.O
-
o
-
~ at _
~ - _
,.0
0.001 0.01 0.1 1.0 10.0 100.0
l
W.~°.\o
\~ 0
x Statesville Area, North Carolina
o Howard and Montgomery Counties, Maryland
~ Satpura Region, Central India
——Llano Area, Central Texas
—· Sierra Nevada, California
~ Eastern United Status
~~~
of
0 \,.~~~;'
a---. ·.
\
WELL YIELD (liters per minute per motor)
FIGURE 5.3 Depth-yield relationship for water wells in metamor-
phic and platonic igneous rocks. With the exception of the one line
labeled "median," data used are from mean specific capacities of wells
of different depths. Diagram by Johnson (1981).
Even though a large number of mines in consolidated rocks
are virtually dry, the fact that many actually produce large
amounts of water (Cook, 1982) suggests that those sites where
proper conditions exist for geologic isolation of wastes may be
difficult to locate. As Cook has pointed out, much of the perme-
ability, which allows an initial inflow to excavated openings, is
local and has been caused by strains associated with the mining
itself. If this permeability is potentially too large, repository
extraction ratios must be kept low. Nevertheless, it should be
remembered that most mines from which data are available are
in hydrologically anomalous regions where various types of
geologic discontinuities should favor natural zones of locally
high permeabilities that contain water and would be inter-
cepted by zones of artifically high permeability near the mines.
Data on rock permeabilities from mines in general should,
therefore, be considered as representing the higher extremes
to be expected.
.e solo
0
E
-
1.0
Various locations in Swedish bedrock.
~ Crystalline rock in Oroville, California.
—-— Auburn Dam site, California. Metamorphic rocks.
lose 10-5
HYDRAULIC CONDUCTlVlTY (mesers per second)
FIGURE 5.4 Results of packer tests in drill holes penetrating met-
amorphic and platonic igneous rocks. Trend lines are based on the
mean hydrualic conductivities at various depths Johnson, 1981).
84
500
-
-
100
50 _
1 ,/,l~
o f.
o /
·/
~ o
a/
/-
/o
/
10 50 100
PERCENT ZERO WATER TAKE
FIGURE 5.5 Diagram showing the increase in nonpermeable zones
with depth in metamorphic (open circles) and platonic igneous rocks
(black dots) of the Sierra Nevada, California. Data are from U. S. Bureau
of Reclamation drill holes. Individual points represent data from at
least 30 packer tests at various depth intervals. The percentage of
packer tests that were unable to inject water are plotted for each depth
interval (Davis and Turk, 1964).
n
1 two
Ann
500
, I HI 1
.
,,, 200 _ ,,
~ .RM
/ 7(170)/
~ 1.0a ''
7(168,: 7(171'
; /6 11(237) '''
/. 11 (244) '''
11(240)/ ,,
_ 1 1(2~3)/ ~~ ~
/ 11(244)/
0 10 20 30 40 50
l
7(59)
8(187) /
PERCENTAGE OF 2-m PACKER TESTS INDICATING
HYDRAULIC CONDUCTIVITY OF 1 X 10-8 m/s
(PERMEABI LITY OF 1.03 MD at 20°C) OR GREATER
FIGURE 5.6 Diagram showing the reduction in the number of
permeable zones with depth in metamorphic and plutonic igneous
rocks of Sweden. Percentage values for each 50-m depth interval are
plotted. Small numbers indicate the number of test holes in which the
packer tests were performed, and the larger number in parentheses
gives the total number of packer tests for each depth interval (Davis,
1981).
STANLEY N. DAVIS
Many permeable zones encountered in mines and tunnels
drain rather rapidly, and little water is produced from these
zones after a few days even though the zones may be a long
distance beneath the regional water table. This general obser-
vation suggests that many permeable fractures in metamorphic
and platonic igneous rocks have a limited extension. This qual-
itative conclusion is supported by aquifer tests and other tests
completed on test holes in metamorphic rocks underlying the
Savannah River Plant (Marine, 1981~.
Geochemical Evidence
Several aspects of water chemistry suggest that much of the
water in the deep subsurface is almost static, and, under natural
gradients, the movement of potential contaminants in this water
toward the surface would be so slow that it would be negligible.
One of the most important aspects of repository design is to
ensure that this remains so. The most basic argument for the
conclusion that the water is nearly static comes from the high
chloride content of almost all deep water. Because significant
amounts of chloride do not come from the dissolution of min-
erals except in obvious cases where evaporites are present, the
most important reason for the high chloride content in the
water would be either the presence of ancient formation water
or the concentration of chlorides through ion filtration. In the
case of ancient formation water, if an active circulation system
is connected with the land surface, the chloride would have
been flushed out long ago by infiltrating surface water that has
a chloride concentration from three to four orders of magnitude
less than would be present in today's subsurface brines.
In contrast, if subsurface brines are the end product of chlo-
ride concentrating by means of ion filtration, water circulation
would need to be quite vigorous in order to account for the
total mass of chloride present in the aquifer. For example, if
a sandstone outcrop, which is also the intake area of an aquifer,
is 3 km wide and 1 m of water having 10 mg/L of chloride
enters the aquifer each year, then for every 1.0 m along the
strike of the aquifer, a total of 3 x 107 mg of chloride enters
the subsurface each year. If the same sandstone has a porosity
of 20 percent and extends downdip 50 km with an average
thickness of 20 m and is saturated with water having 20,000
mg/L of chloride, then a total of 2 x 10~2 mg of chloride is
present in each 1-m strip perpendicular to the strike. Thus,
about 7 x 104 yr would be needed to accumulate this amount
of chloride if the ion filtration is 100 percent efficient. The
velocity of groundwater needed (750 m/yr) in this example
would be very large for a natural system that might also be
near a waste repository. The correct interpretation of the origin
of the chloride in the groundwater is, therefore, critical to the
safety evaluation of the waste-disposal system.
Studies of the quality of water from deep drill holes and
mines in metamorphic and plutonic igneous rocks generally
show an increase of salinity with depth (Jacks, 1973, 1978;
Marine, 1976; Davis, 1981; Frape and Fritz, 1981). In general,
water contains less than 1000 mg/L total of dissolved solids at
depths of less than 100 m. Concentrations increase to more
than 10,000 mg/L as bedrock is penetrated to depths of more
than 1000 m. Isotopic composition of the deeper water suggests
Deep Burial of Toxic Wastes
that the major dissolved constituents are unrelated to modern
surface water and that they probably are not derived from
ancient seawater (Fritz and Frape, 1981; Nordstrom et al.,
1982~. The salinity may be caused by the slow diffusion of small
amounts of ions from original metamorphic and magmatic water
in micropores in the dense rock into larger fractures that pen-
etrate into the subsurface (Nordstrom et al., 1982~. The ability
of minor amounts of interstitial brines to increase significantly
the salinity of water in fractures has been demonstrated in the
Hot Dry Rock Energy Extraction test in New Mexico. Here,
water of low salinity was injected into large artificial fractures
made within nonpermeable geologically young plutonic rock.
The first return water from this injection showed large increases
in total dissolved solids as well as a number of key ions. For
example, chloride concentrations in one of the injection cycles
increased from less than 50 mg/L in the injected water to 1750
mg/L in the initial water circulated back to the surface from
the fresh fracture (Smith and Ponder, 1982~.
To summarize the chemical evidence, in metamorphic rocks
and plutonic igneous rocks the salinity of water in fractures
increases with depth. If ion filtration can be discounted as a
mechanism for concentrating these dissolved solids, then it is
suggested that the chemical composition reflects water within
fractures in these rocks, which approaches stagnation at depths
of from 300 to 1000 m in most regions. This is, then, a further
argument for the safety of deep burial of wastes within these
rocks.
PROBLEMS OF EVALUATING THE
HYDROLOGIC HAZARDS
OF A WASTE REPOSITORY
Flow of groundwater through a waste repository is the most
likely natural mechanism for the transportation of hazardous
materials from the subsurface into the biosphere. Several ques-
tions must be posed concerning this potential source of con-
tamination. These can be generalized as follows:
1. If contamination reaches the surface, where will this take
place?
2. What will be the predicted concentrations of these con-
taminants in the water when it reaches a point of use?
3. What will be the total amount of contaminants to reach
the surface?
4. How long will the contaminants take to reach the surface?
Each of these questions will be discussed in a general way in
relation to the hypothetical repository.
Trajectory of Contaminants
The trajectory of a contaminated plume of groundwater from
the repository will be particularly difficult to predict within a
few hundred meters of the repository. After an initial non-
steady-state flow, which will be controlled by the repository
geometry and construction and may last a few months, the
subsequent pseudo-steady-state flow- of water will be controlled
primarily by major fracture zones in the bedrock. Within the
85
major fractures, flow could be locally almost at right angles to
the trajectory of the water, which might be predicted on the
basis of regional hydraulic head measurements in the overlying
aquifer. The location of the major fractures that form potential
conduits is aided by geologic and geophysical methods but
probably will never be precise enough to allow an exact defi-
nition of the details of groundwater flow in the vicinity of the
repository. Once the contaminated water reaches the more
permeable upper part of the bedrock and the base of the over-
lying aquifer, however, it will move downgradient in the same
general direction as the bulk of the groundwater in the aquifer.
After a few kilometers of migration, transverse dispersion should
work the contaminated water into the upper part of the aquifer
where it will eventually reach the biosphere through down-
gradient wells, springs, or diffuse seepage into large bodies of
water. The assumed upward diffusion into the aquifer, never-
theless, could be inhibited almost indefinitely if the density of
the contaminated water is much larger than normal ground-
water and if the aquifer is horizontal with abundant clay lenses,
which would have the effect of producing a strongly anisotropic
flow in the horizontal direction.
Concentration of Contaminants
No method exists to predict concentrations of contaminants in
groundwater passing through a repository because of the un-
certainties in the source term. Containers for waste will be
built to last as long as practical, and their rate of failure will
be unknown. Once the container is breached, the contents will
be removed slowly by water. Some rough estimates of the rates
of this removal can be made based on the results of laboratory
tests. Backfill around containers, however, will be designed to
sorb as many of the contaminants as is practical. The rate of
migration of contaminants through the irregularly shaped back-
ffll probably can only be bounded in a broad way by generalized
calculations.
The usual method of handling the source-term problem of
predicting concentrations is to assume some physically reason-
able rate of removal of the repository contents and arbitrarily
inject this hypothetical amount into a mathematical model of
the moving body of groundwater. Sophisticated transport models
then are used to predict downgradient migration rates and
concentrations. The transport models, unfortunately, lend an
air of authenticity that is rarely justified because of the arbitrary
assumptions made in the source terms.
The movement and dispersion of contaminants once the
groundwater leaves the artificial cavities and enters the sur-
rounding host rock will be very difficult to define in great detail.
As the water moves into the regional flow field, nevertheless,
useful approximations of contaminant concentrations can be
made for any hypothetical source term.
Total Amount of Contaminants that Reach the Surface
A concern for the total amount of contaminants that eventually
reach the surface is particularly acute in relation to the burial
of radioactive wastes. This concern exists because, for lack of
better evidence, adverse health effects from radiation are as-
86
sumed to be linearly related to the radiation dose received by
humans. Thus, even very low levels of radiation widely dis-
persed in the environment, if extended over a long period of
time and if enough people are exposed, may cause the same
total number of health effects as would a highly concentrated
dose to which only a few people were exposed. One stategy
for reducing the total amount of radionuclides reaching the
biosphere is, therefore, to increase the time of isolation to allow
for radioactive decay.
The problem of isolation of hazardous chemical wastes has
not been handled in the same way as radioactive wastes. The
long-term stability of many compounds under conditions of
burial are poorly understood. Certainly, some complex com-
pounds will break down with time so that isolation renders the
wastes less hazardous. Many compounds and elements such as
cadmium and arsenic, in contrast, are stable for an infinite
period, so that the time of isolation alone will not affect the
total amount of contaminants that will eventually reach the
surface. The seriousness of the eventual movement of trace
amounts of these stable materials to the Earth's surface has not
been studied in detail. Can we assume a linear relationship
between the hazardous chemical and resultant adverse health
effects? Probably not. For example, arsenic, in milligram
amounts, is hazardous. However, current evidence suggests
that arsenic in nanogram amounts is essential to human health
(Mertz, 19811. Thus, simple dilution of wastes containing ar-
senic could eliminate all adverse health effects from this con-
taminant. In this example, the total amount of a contaminant
reaching the surface is, therefore, of much less importance than
the projected maximum concentrations, which would control
the dose to individuals.
Travel Time
Estimates of the time that might be taken for a contaminant
to be transported by groundwater from a subsurface repository
to the land surface should be made for most repositories. In
general, the longer the travel time is, the more favorable the
site will be; although some hydrogeologic situations certainly
exist where additional travel time does not necessarily mitigate
potential hazards from toxic chemicals. Additional time in most
settings will allow for dilution by molecular diffusion, decay of
radiaoctive components, and decomposition of hazardous com-
pounds. Most important, long travel times will help with the
public acceptance of a site. Contaminants that might reach the
land surface after thousands of years may be of little direct
concern to the average citizen.
The calcuation of travel time, unfortunately, is commonly
accomplished only by assuming "reasonable" values for a num-
ber of critical factors. Using different assumptions, hydrogeol-
ogists can calculate travel times for waste from the same re-
pository that may vary from one another by an order of magnitude
or more. This is particularly true of irregularly fractured met-
amorphic and plutonic igneous rocks.
The following equation can be used to calculate the incre-
ment of time, At, for the average time that a particle of ground-
water takes to traverse a given distance, AL:
STANLEY N. DAVIS
Ne(/`L)2
At =
(5.1)
in which K is a measure of hydraulic conductivity of the rock,
Ah is the head drop over the distance AL, and Ne is the effective
(interconnected) porosity. The value of K can be estimated from
field tests, and the values of AL and Ah are commonly deter-
mined to some extent by known boundary conditions. The
value of Ne however, for fractured rocks is rarely determined
with accuracy and may easily vary from about 0.5 for weathered
rock to 0.05 for highly fractured rock to less than 0.005 for
dense, sparsely fractured rock. Calculated travel times would
vary in the same way and are more often based on porosity
values that are assumed rather than measured.
Chemical variables introduce even greater uncertainties in
travel-time estimates than do problems of defining the effective
porosities of the rocks. Most chemical species dissolved in water
will be sorbed to some extent on the solid matrix of the water-
bearing materials. Even though Resorption takes place, most
chemical species will partition strongly onto the solid matrix.
This means that the chemical species that is a potential con-
taminant will usually travel at only a small fraction of the ve-
locity of the groundwater. Theoretically, the relative velocities
of the chemical species and the groundwater can be measured
by laboratory experiments. In practice, however, only an order-
of-magnitude estimate is commonly possible. The chemical
processes involved in the transport phenomena are a complex
function of pH, chemistry of the solid surfaces of the rocks,
nature of the dissolved species, temperature, water velocity,
total volume of water flowing, other dissolved species in the
water, and the relative concentrations of those species. These
variables must be specified in time and space for the complex
natural setting in order to estimate the velocities of contaminant
migration. An early error of chemists dealing with this problem
was to conduct laboratory experiments with artificially crushed
rock and consider only mineralogical properties of the rocks,
whereas the thin natural coatings along fractures in the rocks,
which would not show up in bulk analyses, will actually be the
most important control in the sorption process.
A process similar to sorption in fractured media is that of
molecular diffusion into the micropores in the otherwise solid
rock. This process is important if the migration of the contam-
inant is slow, which would allow time for diffusion to take place.
Molecular diffusion would, therefore, serve to slow down the
velocity of contaminant movement by allowing these contam-
inants to migrate through the rock as well as through the more
open fractures.
FLOW OF GROUNDWATER THROUGH A
CLOSED REPOSITORY
Initial Inflow
After the repository is filled with waste and access shafts are
sealed, the repository should become saturated with water if
it is below the water table. The rate of inflow of groundwater,
however, should be quite small, particularly if fractures leaking
Deep Burial of Toxic Wastes
significant amounts of water have been grouted during exca-
vation of the repository. The process of saturation may take
from decades to thousands of years, depending on the perme-
ability of the rock and whether gases are easily expelled from
the closed repository and also depending on the depth of the
repository beneath the water table.
Once the repository is saturated, water will move slowly into
the structure and then drift out the downgradient side. If waste
is in an insoluble form or if waste containers are watertight,
the initial water flowing through the repository should not
become contaminated. However, containers in the repository
will ultimately fail, and "insoluble" material will dissolve to
some extent so that water flowing through the repository will
eventually be contaminated. The length of time taken for con-
taminated water to start to move out from the repository after
the repository has become saturated could be in the range of
hundreds to thousands of years, depending primarily on the
time that it takes to saturate the repository, waste form, and
construction of waste containers.
Steady-State Flow Conditions
The quantity of water moving through the repository per unit
time under steady-state conditions will be very small as illus-
trated by the following example. The excavated region in the
repository is assumed to measure 1500 m x 1500 m x 10 m
and has an internal permeability much greater than the sur-
rounding bedrock so that it intercepts a steam of water twice
the width and height of the repository (Figure 5.7~. Assuming
a hydraulic conductivity of 1O-4 m/day (see Figure 5.4—Col-
orado Front Range and Auburn data) and a regional hydraulic
gradient of 10-3, then only 6 x 10-3 m3/day will move through
the repository each day.
If the repository has an overall porosity of 10 percent when
pillars and other nonexcavated portions as well as backfill are
considered, then the total pore volume in the respository is
2.25 x 106 m3. If the flow through the system is 6 x 10-3/
._ ~
-
Repository
`'=~ - ,`~3
-
E:
-
-
-
E
o
o
o
or
K (bedrock) ~ 10-4 m/day
Gradient ( L ) = 10-3
AN 6 x 104 m2
0=10-4(6 x 104)10-3-6 x 10~3m3/day
0~6 1 iters /day
FIGURE 5.7 Map of a hypothetical repository showing groundwater
flowing through the repository. The width of the zone of groundwater
diversion is only approximate. The height of the repository, measured
perpendicularly to the map, has been assumed to be 10 m.
87
day, then a once-through, pistonlike displacement of all the
water in the repository would take 3.7 x 1O# days or about 106
yr. Even if the assumed grouting of larger cracks in the host
rock is not effective and the average permeability is an order
of magnitude larger, the time taken for piston displacement
will still probably be at least 104 yr. This is hardly the picture
one obtains when reading the literature where calculations of
the dissolution rates of hazardous materials are based on lab-
oratory batch tests or flow-through tests where simulated waste
is exposed to periodically replaced water or to a constant stream
of water greatly undersaturated with respect to the waste. A
more exact field analogy of these types of laboratory experi-
ments would be the placement of waste canisters directly in
the Mississippi River rather than in a body of groundwater
moving at velocities of much less than a millimeter/day and
water that is already close to chemical equilibrium with respect
to the repository contents. To be sure, a repository will not
have pure piston displacement of water. However, simple cal-
culations such as those given above but taking into account
various degrees of dispersion will all show that the water moves
very slowly in the repository and that thousands, if not hundreds
of thousands, of years will be needed to flush out the water
saturating the repository just once.
Isolated Leaks
The slow drift of small amounts of groundwater through the
entire repository is not expected to present a significant health
problem, at last for periods of many thousands of years. A more
likely problem would be created by an isolated leak that could
develop (1) along a major fracture, (2) through a poorly sealed
shaft, or (3) by a borehole drilled into the repository. Stress
release associated with the construction of the repository could
also open existing small fractures enough to.provide significant
leakage. The maximum amount of water cloning through an
isolated leak is limited by the low permeability of the rock as
a whole, which would not transmit much water to a permeable
zone. It is further limited by the very low permeability of the
backfill around waste cannisters, which would not allow sig-
nificant water circulation past the Bannisters. Under extreme
conditions, a properly constructed repository might be able to
feed a maximum of 1 or 2 L/day to a fracture from a groundwater
system driven by a normal hydraulic gradient. This water could
then contaminate an aquifer, or, if an aquifer does not overlie
the bedrock, the water moving upward could form a small
contaminated spring. The fact that only a small volume of con-
taminated water is involved in these processes is important. A
well penetrating the contaminated aquifer overlying the re-
pository simply would not be developed unless it had a yield
of at least several hundred or, more commonly, several thou-
sand liters per day. Therefore, at least a thousandfold dilution
of the water would be expected before it would be used for
drinking (Figure 5.8~.
In addition to the fracture leakage shown in Figure 5.8, a
poorly sealed shaft leading into a repository is also a possible
source of leakage (Figure 5.9~. If a natural hydraulic gradient
is assumed to be driving water through the repository and even
if the repository offers no resistance to water movement, it is
Municipal = More than
supply wel 1 200,000 L /day
Domestic well= 2,OOO L/day
i l C =~`J' Vl ~ U-
''.1 Contaminated water . is, ~
x~: I ~~ >;~=
of// Dilution by a factor
any' of at least 103.
/
\~3
Waste Repository
L
FIGURE 5.8 Even though small amounts of contaminated water may
leave the repository through isolated fractures, this water will most
probably be diluted before use, because water wells and springs are
rarely developed unless they can yield several hundred to several
thousand gallons per day. Although a leak of as much as 2 L/day, as
shown in this figure, is considered unlikely, the contaminated water
would probably be diluted by a factor of at least 103 before use.
difficult to imagine more than a liter per day being circulated
through the filled shafts. In fact, if the shafts were sealed by
packing them only with silt having a hydraulic conductivity of
10-2 m/day (Davis, 1969>, the total leakage would be proably
much less than 1.0 L/day (Figure 5.9~. Shafts will certainly be
sealed with material much less permeable than silt, so, in gen-
eral, shaft sealing is not a major problem unless unusual hy-
draulic gradients exist between the repository and waters in
the more accessible parts of the environment.
DISCUSSION AND CONCLUSIONS
The Federal Nuclear Waste Policy Act of 1982 and subsequent
documents such as the U. S. Department of Energy's proposed
general guidelines for the location of repositories for high-level
radioactive waste (U.S. Department of Energy, 1983) have
assumed that the first permanent storage of high-level waste
will be in geologic repositories specifically mined for that pur-
pose. If political and social problems can be overcome, there
seems to be little doubt that such repositories will be con-
structed. I have tried to make the case that similar repositories
for highly toxic chemical wastes should be considered.
Given identical geologic settings and construction methods,
mined repositories for chemical wastes will probably be cheaper
per unit volume of waste stored than for high-level radioactive
STANLEY N. DAVIS
//r/ / /}
Fil led
shafts
Q Ah=1 m
L
by
_ - - - Q
it ~ ~
. , ~ ~ _ _ ~ ~
A=10 mama
~ ,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,/,
777 K=~ crepes tOrY ;;t
~ A=IOm2
c
0
Q = K A L
L= (500 + 500) m
A= 10 m2
Ah 1 -3
= = 10
L 1000
K= 10~2m/day
Q = (10-2)(10)(10 3)=10~4 m3/day
Q = 100 ml /day
1 //~/
/ Fi l led
r shaft
FIGURE 5.9 The small potential erect of poor shaft sealing is shown
in this hypothetical example of a repository that has an infinite hydraulic
conductivity so that the head drop is entirely within the 1000 no of the
two filled shafts. Even though the shaft is "sealed" with a semiperme-
able material, only 100 mL/day of contaminated water flows out of the
respository.
wastes because most chemical wastes will not generate large
amounts of heat after being packaged for the repository. Wastes
that might have strong exothermic chemical reactions after
closure of the repository are assumed to be excluded from the
repository. In contrast, all high-level wastes will generate sig-
nificant amounts of heat, and close packing of waste in repos-
itories must be avoided in order to prevent very high tem-
peratures from building up in the storage area. In general,
however, extraction ratios, mining methods, rock stability, and
other factors may be more important than heat dissipation in
determining overall construction costs.
Even with the lower costs, however, storage of chemical
wastes in large mined repositories will still probably range from
a few hundred to a few thousand dollars per cubic meter of
waste. Clearly, waste storage in a mined repository will be cost
effective only for exceptionally hazardous materials.
As is also true of repositories for high-level radioactive wastes,
the long-term confinement of chemical waste in deep reposi-
tories will be threatened most by human intrusion and by the
transport of chemicals in solution through groundwater migra-
tion. Deep burial in metamorphic and platonic igneous rocks
probably will provide the most protection of any geologic ma-
terials against human intrusion. The same geologic material
will probably also be hydrogeologically satisfactory at depths
greater than 300 m, although salt and shale are two geologic
materials that may have lower permeabilities (Davis, 1969).
Deep Burial of Toxic Wastes
AC KN OWLE D G M E NTS
The presentation in this paper has been improved significantly
by many constructive suggestions of N. G. W. Cook. The work
of L. l. Turk and K. L. Johnson, former students and associates,
has been most useful. These individuals, however, should be
disassociated from my snore simplistic calculations and free-
wheeling remarks.
REFERENCES
Bredehoeft, J. D., and T. Maini (1981). Strategy for radioactive waste
disposal in crystalline rocks, Science 213, 293-296.
Bredehoeft, J. D., et al. (1978). Geologic disposal of high-level radio-
active wastes, U.S. Geol. Sure. Circ. 779, 15 pp.
Clark, L. L., and B. M. Cole (1982). An analysis of the cost of mined
geologic repositories in alternative media, Richland, Washington,
Battelle Pacific Northwest Laboratory, Publ. PNL-3949, UC-70, 57
PP
Cook, N. G. W. (1982). Groundwater problems in open-pit and un-
derground mines, Geol. Soc. Am. Spec. Pap. 189, pp. 397-405.
Council on Environmental Quality (1981). Contamination of Ground-
water by Toxic Organic Chemicals, Executive Office of the Presi-
dent, U.S. Government Printing Office, Washington, D.C., 84 pp.
Davis, S. N. (1969). Porosity and permeability of natural materials, in
Flow Through Porous Media, R. J. M. DeWiest, ea., Academic
Press, New York, pp. 53-86.
Davis, S. N., ed. (1981). Workshop on hydrology of crystalline base-
ment rocks, Los Alamos National Laboratory, Rep. LA-8912-C, 63
PP
Davis, S. N. (1982). Hydrogeology of radioactive waste isolation, the
challenge of a rational assessment, Geol. Soc. Am. Spec. Pap. 189,
pp. 389-396.
Davis, S. N., and L. J. Turk (1964). Optimum depth of wells in crys-
talline rocks, Ground Water 2(2), 6-11.
Davison, C. C. (1981). Physical hydrogeologic measurements in frac-
tured crystalline rock- summary of 1979 research programs at WNRE
and CRNL, Inland Waters Directorate, Environment Canada, Publ.
TR-161, 108 pp.
Frape, S. K., and P. Fritz (1981). A preliminary report on the occur-
rence and geochemistry of saline groundwaters on the Canadian
shield, Atomic Energy of Canada Ltd., Tech. Rep. TR-136, 68 pp.
Fritz, P., and S. K. Frape (1981). Saline groundwaters in the Canadian
shield, a first review, Chem. Geol.
Gale, J. E. (1982). Assessing the permeability characteristics of frac-
tured rock, Geol. Soc. Am. Spec. Pap. 189, pp. 163-181.
Jacks, G. (1973). Chemistry of some groundwaters in igneous rocks,
Nordic Hydrol. 4, 207-236.
Jacks, G. (1978). Groundwater chemistry at depth in granites and
gneisses, KBS Tech. Rep. 88, 28 pp.
Johnson, K. L., (1981). Permeability-depth relationships in crystalline
rocks with applications to low-level waste repositories, unpublished
M.S. thesis, U. of Arizona, Tucson, 112 pp.
Landers, R. A., and L. J. Turk (1973). Occurrence and quality of
groundwater in crystalline rocks of the Llano area, Texas, Ground
Water 11(1), 5-10.
Lindblon, U. E. (1977). Geological and geotechnical conditions in ground-
water movements around a repository, in KBS Tech. Rep. 54(1), H.
Stille et al., eds., pp. 1-49.
Longmire, P. A., B. M. Gallaher, and J. W. Hawley (1981). Geological,
geochemical and hydrological criteria for disposal of hazardous wastes
in New Mexico, New Mexico Geol. Soc. Spec. Publ. No. 10, pp. 93-
102.
Marine, I. W. (1967). The permeability of fractured crystalline rock at
the Savannah River Plant near Aiken, South Carolina, U.S. Geol.
Surv. Prof. Pap. 575-B, pp. 203-211.
Marine, I. W. (1976). Geochemistry of groundwater at the Savannah
River Plant, Savannah River Laboratory Publ. DP-1356, 102 pp.
Marine, I. W. (1981). Comparison of laboratory, in situ, and rock mass
measurements of the hydraulic conductivity of metamorphic rock at
the Savannah River Plant near Aiken, South Carolina, Water Resour.
Res. 17, 637-640.
Mertz, W. (1981). The essential trace elements, Science 213, 1332-
1338.
Mundi, E. K., and J. R. Wallace (1973). On the permeability of some
fractured crystalline rocks, Assoc. Eng. Geol. Bull. 10, 299-312.
NRC Board on Radioactive Waste Management (1983). A Study of the
Isolation System for Geologic Disposal of Radioactive Wastes, Na-
tional Research Council, National Academy Press, Washington, D.C.,
345 pp.
NRC Committee on the Geological Aspects of Industrial Waste Dis-
posal (1982). Geological Aspects of Industrial Waste Disposal, Na-
tional Research Council, National Academy Press, Washington, D.C.,
44 pp.
NRC Panel on Geologic Site Criteria (1978). Geological Criteria for
Repositories for High-Level Radioactive Wastes, National Research
Council, National Academy Press, Washington, D.C., 19 pp.
Nordstrom, D. K., P. Fritz, R. J. Donahoe, and J. Ball (1982). Recent
investigations of the major element, trace element, and isotopic
geochemistry of deep granitic groundwaters at the Stripa test site,
Sweden, Geol. Soc. Ann. Abstr. Programs 14, 577.
St. John, C. M. (1982). Repository design, Underground Space 6, 247-
258.
Smith, M. C., and G. M. Ponder, eds. (1982). Hot dry rock geothermal
energy development program, Annual Report Fiscal Year 1981, Los
Alamos National Lab. Publ. LA-9287-HDR, 140 pp.
Snow, D. T. (1968). Hydraulic character of fractured metamorphic
rocks of the front range and implications for the Rocky Mountain
arsenal wells, Q. J. Colo. School Mines 63(1), 167-199.
Stewart, G. W. (1966). Drilled water wells in New Hampshire, New
Hampshire Mineral Resources Survey, Part 20, 58 pp.
Uhl, V. W., Jr. (1976). The occurrence of groundwater in the Satpura
Region of Central India, unpublished M.S. thesis, U. of Arizona,
Tucson, 135 pp.
U. S. Congress (1979). Hazardous waste disposal, U. S. House of Rep-
resentatives, 96th Congress, 1st Session, Committee on Interstate
- and Foreign Commerce Print 96-lFC31, 82 pp.
U. S. Department of Energy (1979). Management of commercially gen-
erated radioactive waste, draft environmental impact statement, U.S.
Department of Energy Publ. DOE/EIS-0046-D.
U. S. Department of Energy (1983). Nuclear Waste Policy Act of 1982;
proposed general guidelines for recommendation of sites for nuclear
waste repositories, Federal Register 48(26), 5670-5682.
Wacks, M. E. (1979). Alternatives to shallow land burial for the disposal
of low-level wastes, U. of Arizona Engineering Experiment Station,
Quarterly Rep. on Contract L 28-9766F-1, 137 pp.
Waddell, J. D., D. G. Dippold, and T. I. McSweeney (1982). Projected
costs for mined geologic repositories for disposal of commercial nu-
clear wastes, Columbus, Office of NWTS Integration, Publ. ON1-
3, Battelle Memorial Institute, Columbus, Ohio, 55 pp.
Winograd, I. J. (1974). Radioactive waste storage in the arid zone, EOS
55, 884-894.
Winograd, I. J. (1981). Radioactive waste disposal in thick unsaturated
zones, Science 212, 1457-1464.
