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OCR for page 79
Natural Physical and Biological Processes Compromise the
Long-Term Performance of Compacted Soil Caps
Ellen D. Smith, Robert I. Luxmoore, arid Glenn W. Suter Il. Environmental Sciences
Division, Oak Ridge National Laboratory, Oak Ridge, Tenn.
Compacted soil barriers are components of essentially all caps placed on closed waste
disposal sites. The intended functions of soil barriers in waste facility caps include restricting
infiltration of water and release of gases and vapors, either independently or in combination with
synthetic membrane barriers, and protecting other manmade or natural barrier components. Review
of the performance of installed soil barriers and of natural processes affecting their performance
~. ~i. ,,
(Suter, Luxmore, and Smith, 1993) indicates that compacted soil caps may Unction effectively tor
relatively short periods (years to decades), but natural physical and biological processes can be
expected to cause them to fail in the long term (decades to centuries). This paper addresses natural
physical and biological processes that compromise the performance of compacted soil caps and
suggests measures that may reduce the adverse consequences of these natural failure mechanisms.
PHYSICAL PROCESSES INFLUENCING BARRIER INTEGRITY
Physical phenomena that affect the integrity of compacted soil barriers in shallow land
burial facilities and remediation sites are related to natural cycles of temperature and precipitation.
Cycling occurs over three distinct hme scales: diurnal (hourly changes), precipitation-
evapotranspiration (few days to months), and annual (seasonal). The diurnal cycle determines
temperature gradients through the soil surface and influences root water uptake and gradients of soil
metric potential. Wetting and drying cycles associated with precipitation and evapotranspiration
directly change soil water content and metric potential. The annual climatic cycle provides the
greatest range of temperature and moisture gradients. Long-term climatic cycles have much smaller
amplitude and lower frequency and therefore are less important, although low-frequency extreme
weather events can plumage caps.
Effects of Wetting and Drying Cycles
A wet soil may shrink as it dries, forming cracks that penetrate from the surface to some
depth in the soil. Soils vary in their propensity to foam cracks, due largely to the type and quantity
of clay minerals In the soil. Cracking may occur predominantly at the surface and may result In the
formation of a crust. A mechanically strong crust will crack while maintaining its integrity, whereas
in other soils a profusion of fine cracks may result in a Liable surface. Crusts reduce the rate of soil
drying by creating a resistance to liquid and vapor flux.
Expanding-lattice clay minerals such as montmorillonite have a large propensity for
shrinldng and swelling with drying and wetting cycles, and soils with these clays have the greatest
problems with cracking. Expanding-lattice clay minerals are present in soils through much of the
continental United States (Lutton, Regan and [ones, 19791. When montmor~llonite in the form of
bentonite is added to soil barriers, the potential for problems with cracking is increased. Fixed
D-61
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BARRIER TECHNOLOGIES FOR ENVIRONMENTAL MANAGEMENT
lattice clays (e.g., kaol~nite) may develop small cracks, so subsoils composed of such minerals
usually form aggregates, with small cracks between aggregates. Repeated annual wetting and
drying cycles lead to clay aggregation in the subsoil through annual shrink and swell changes.
Cracks tend to remain as planes of wealmess that become enhanced in subsequent shrink-swell
cycles (Brady, 1974~. Several clay minerals (e.g., illite and vermiculite) exhibit partial lattice
expansion and are intermediate between montmorillonite and kaolinite in their propensity to shrink
and swell.
If the water-content of a soil balTier could be maintained in a narrow range near the
optimum moisture content for compaction of the soil, problems with shrinkage and swelling might
be avoided. A modeling study for landfills in a mesic eastern Tennessee environment predicted that
a well vegetated soil cover of 0.6 m thickness over a clay cap could provide a stable environment
for the cap. Very little variation in cap water content was found through several multi-year
simulations, although baIrier water contents were lower for years with lower rainfall than normal
(Luxmoore and Tharp, 1992~. An experimental study of compacted soil caps In a mesic
environment in Germany suggests, however, that field performance would be less than predicted by
the model. Melchior et al. (1994) found that, even with a 1-m thickness of topsoil and sand over a
compacted 0.6-m soil cap with a low (17 percent) clay content, after five summer seasons
desiccation had caused sufficient soil shrinkage that Infiltration through a flat cap increased from an
initial value of 0.007 to 0.1 mlyr.
In an experimental study of water budgets of two landfill cover designs in the semiarid
environment of Los Alamos, New Mexico, Nyhan, Hakonson, and Drennon (1990) found that the
combination of a thick topsoil layer (0.7 m) and a coarse subsurface capillary barrier (0.5 m of
gravel and 0.9 m of river cobble stones) over a compacted soil layer limited seepage through the
soil cover, relative to a more conventional cover design including just 0.2 m of topsoil over
compacted soil. Perennial grass vegetation was established on both covers. Deep seepage was
monitored over three years including two years with higher precipitation than normal. The thicker
battier had greater evapotranspiration from the vegetative cover and much less infiltration through
the soil cover.
Freeze-Thaw Effects
Freezing and subsequent thawing can destroy the integrity of a soil baITier by creating
cracks and by reducing soil stability on sloping surfaces. The depth of freezing is highly dependent
on the soil material, water content, and surface cover. Fine-textured soils with high water content
are most susceptible to structural modification by freezing (Gillott, 19681. Frost heaving results to a
small extent from the 9.1 percent volume increase as liquid water becomes ice. Most heaving is due
to the upward movement of soil water into the frozen layer contributing to the formation of ice
lenses (Miller, 19801. Clay-based caps are highly susceptible to frost heaving due to fine grain size
and high water content.
Thawing of frozen soil can reduce its bulk density, and in the melting process, the
sidesiopes of a landfill can be vulnerable to slumping due to the formation of a saturated layer with
reduced cohesive strength above the frozen layer. Cycles of freezing and thawing also can lead to
particle sorting with coarse materials rising above fine-textured particles, changing the hydraulic
attributes of the soil. Frost damage to a landfill cap will increase percolation of water into the
buried wastes.
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APPENDIX JO PAPERS PRESENTED
D-63
The potential for frost (lamage to a soil cap can be reduced if the cap is protected by a
vegetated topsoil layer that is thicker than the local depth of frost penetration. Frost penetration
depths in much of the northern United States range from 0.6 to 1.8 m (Suter et al., 1993), which
exceeds the recommended 0.6-m thickness of topsoil layers on hazardous waste facility caps JO SA
Environmental Protection Agency (1989~.
Soil Erosion
Wind and water erosion can contribute to cap failure. Both can be largely prevented with
vegetative cover. In semiarid sites in the western United States, maintenance of a vegetative cover
may be difficult (Webb and Voorhees, 19841. In this situation, a rock riprap may be the most
effective means for preventing wind and water erosion (Voorhees et al., 19831. However, this
method of long-term erosion control may encourage burrowing animals and may increase
infiltration of water to the waste by slowing runoff and decreasing evapotranspiration.
Subsidence
Continuing changes in wastes due to decomposition, consolidation, and settling can result
In differential subsidence, which can produce depressions and cracking of the cap. The amount of
subsidence depends on the characteristics and density of the waste. Poorly compacted municipal
refuse can settle as much as 33 percent (Brunner and Keller, 19721. Most municipal landfill
subsidence occurs during the first few years after a landfill has been closed. The hazardous and
radioactive wastes typically found at remedial sites are less susceptible than municipal waste to
subsidence due to decomposition, but poor initial compaction of waste can lead to cap subsidence
unless wastes are stabilized prior to cap placement.
BIOLOGICAL PROCESSES INFLUENCING BARRIER INTEGRITY
Without continual human intervention, ecological succession will occur on waste sites
resulting in the establishment of a series of biotic comm unities that will modify soil barriers. Two
biological processes that are particularly significant with respect to soil caps are the intrusions of
plant roots and burrowing animals.
Root Intrusion
Plant roots can have several effects on soil baITiers and the underlying waste: (~) plants can
decrease drainage through the baIrier by taking up water; (2) roots may penetrate the soil cap;
(3) decomposing roots leave channels for movement of water and vapors through the cap; (4) roots
may dry clay layers, causing shrinking and cracking; (5) uprooted trees may leave depressions in
the cap, thinning the cap and allowing water to collect; (6) roots may enter the wastes, take up
constituent chemicals, and transport them above ground; and (7)roots may increase waste
decomposition rates and release exudates that mobilize metals (Browning and Molz, 1978; Cataldo
et al., 19871. Vegetation is planted on soil caps routinely to prevent erosion of the cap and because
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BARRIER TECHNOLOGIES FOR ENVIRONMENTAL MANAGEMENT
water uptake by plants can reduce the quantity of water available for leaching (Lutton et al., 1979).
However, the other effects of roots reduce the long-term protection actually provided by vegetated
soil caps.
The occurrence of plant root penetration into caps depends on the plants established by
revegetation and subsequent ecological succession, the rooting depth of the plants, and the roots'
ability to penetrate the cap material. Radioactive waste sites provide strong evidence for root
penetration of soil caps because monitoring readily detects radionuclides in above-ground plant
tissues. Suter et al. (1-993) reviewed reports of operating experiences at several U.S. Department of
Energy sites indicating that plant roots have taken up radionuclides from wastes buried 0.6 to 2.5 m.
Lys~meter studies also demonstrate that grasses, herbs, and shrubs can penetrate soil caps and
extend roots into wastes (Browning and Mo~z, 1978; Molz and Browning, 1977; Nyhan, 1989;
Reynolds, 1990; Melchior et al., 1994~. These results demonstrate that concern about Increased
mobilization of buried wastes by plant roots is not academic.
Ecological succession is the process by which the vegetation of a site changes through the
establishment of plant species, competition among plants, herbivory, modification of the soil, and
the actions of physical agents such as Cost. It is possible to estimate the course of vegetation
succession at a particular site, but because many factors affect succession, one cannot predict
accurately what the vegetation of a site might be at any specific date after closure. For example, the
succession from bare ground to shrub-steppe at Hanford Reservation, Washington, normally
requires 30-40 years, but at some waste sites the ground was still bare after 35 years (Fitzner et al.,
1979).
A major consequence of succession with respect to soil caps is that longer-lived and
deeper-rooted species become established. In the eastern United States, this occurs by succession
from grasses and weedy annual herbs through a brushy stage of woody perennial herbs, vines and
pioneer trees, to forest trees. At Oak Ridge, Tennessee, succession from a planted grassland to a
brushy stage would require 10-25 years, and succession to mature forest would occur in 65-150
years, absent intervention. The average depth of grass roots is less than 1 m, but tree roots average
over 3 m, with maximum depth exceeding 60 m (Table 11. Once trees are established, they may
grow up to 0.3 m/year in height with roughly equivalent rooting depths for about the first 10 years.
Assuming that neither the waste nor the soil cap significantly inhibits plant growth, a soil cap in the
eastern United States is in jeopardy within 25 years after abandonment, and after 100 years, wind-
thrown trees could begin tearing holes in the cap.
On arid and semiarid sites, there is not such a regular successional series. The changes in
species composition are often slow, and pioneer species are often the permanent residents of the
site. However, growth rates on arid sites tend to be low so that it may take as Tong to establish a
"mature" desert shrub vegetation as a "mature" mesic forest. Shrubs are the major threat to soil caps
in these environments (Webb and Voorhees, 1984), but perennial grasses that can extend roots to
significant depths (Table 1) also pose a threat. Growth rates vary considerably, but soil caps in arid
to semiarid environments could be breached by plant roots in 10-50 years if the sites are amenable
to plant growth.
The soil atmosphere above buried wastes may inhibit plant growth. Root inhibition by
methane and other gases generated by decomposing municipal waste hinders, but does not
necessarily prevent, revegetation of closed sanitary landfills (Gilman, Leone, and Flower, l9Sl).
However, most hazardous wastes do not produce such gases or produce them for a relatively short
period of time because they contain relatively little decomposable organic matter.
OCR for page 83
APPENDIXD~PAPERS PRESENTED
TABLE 1 Mean and maximum rooting depth of ten plant life forms (from Foxx et al.,
1984)
.
Number of Depth (m)
Life FormObservations
Mean Maximum
Evergreen trees 403.4 61.0
Deciduous trees 1073.3 30~0
Shrubs 873.5 17.0
Subshrubs 361.4 6.4
Vines 41.7 2.8
Perennial fortes 3701.7 39.0
Biennial fortes 91.1 1.5
Annual fortes 810.8 3.0
Perennial grasses 3051.4 8.2
Annual grasses 500.5 1.1
D-65
Once deep-rooting vegetation is established on the site, the only remaining condition that
must be met before the cap is breached is that the material of the cap be penetrable by roots. Clay
provides some inhibition of root growth. Foxx, Tierney, and Williams, (1984) found that plants
rooted less deeply on adobe clays, but half of 82 specimens growing in clay soils rooted to depths
greater than 0.6 m and 7 percent rooted to below 4.5 m. Reynolds (1990) found that three species of
grass and one scrub planted in Tysimeters had all penetrated 0.6-m clay barriers after three years.
The clay barrier was less effective on average than equal thicknesses of scoria or of gravel and
cobble.
Root penetration of soil caps is affected by the inherent strength of the soil, which is
=-ha-~=A h`T r~mn~ti~n V`~ihm~v~r ~nr1 Hendrickson (19481 found the maximum bulb density
permitting root penetration could vary from 1.46 Mg/m3 in clays to 1.75 Mg/m3 in sands. The
compacted density of clay materials in soil barriers can be in the vicinity of 1.75 Mg/m3 (Goldman
et al.. 19881. sufficient to provide a significant battier to root growth. Roots do not enter pores with
~141~1~ V y w~-~ ~^~ ~ _~ _ ~ _ _ _ ~
diameters smaller than that of the root, and typically roots become thickened in dense soils
(Russell, 1977~. Lateral roots with smaller diameters than the main root may be able to enter some
small pores in a compacted layer. Numerous studies have shown a linear decline in root density
with increase In mechanical impedance.
At high water content soil penetration resistance is low, but a soil with high bulk density
and high water content may be unsuitable for root growth due to poor aeration. Oxygen diffusion to
roots may limit root respiration In dense clay layers with high water content due to low gas filled
porosity. There is evidence that roots become less effective in penetrating a dense soil layer as soil
aeration is diminished (Russell, 1977~.
Zones of poor compaction may permit roots to penetrate a compacted soil barrier. It is very
difficult to ensure complete compaction of barriers with field-scale compaction equipment (Suter et
al., 1993~. Most compacted soil barriers have some form of preferential flow paths, and roots tend
to proliferate in these zones once intersected. With time, old root channels, worm holes, and cracks
between soil aggregates will increase (Edwards, Fehrenbacher, and Vavra., 1964; Ehlers et al.,
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BARRIER TECHNOLOGIES FOR ENVIRONMENTAL MANAGEMENT
1983; Wang, Hesketh, and Woolley, 1986). Root exploration eventually will lead to penetration of
a compacted soil barrier, initially through zones where compaction was ineffective.
The fate of tombs and ceremonial mounds, which are built of soil and often contain clay
layers, may be analogous to that of soil caps at waste sites. In the eastern United States, the
Mississippian culture built thousands of mounds. The soil of the mounds is compacted and often
consists of clay or clay loess, apparently in a deliberate attempt to reduce erosion (Lindsey et al.,
1982~. However, those mounds that have not been kept cleared now support trees and shrubs.
Presence of trees has encouraged slumping and other erosional processes, but grasses seem to help
protect the structures if trees are suppressed (Lindsey et al., 19821.
Animal Intrusion
Animals can have several effects on soil caps and the underlying waste. (1) They may
burrow through the caps resulting in direct channels for movement of water, vapors, roots, and
other animals. (2) Even when they do not penetrate to the waste? burrows may increase the porosity
of the soil, thereby increasing rates of infiltration and movement of water and vapors. (3) Animals
may become externally contaminated or consume the waste, thereby spreading the waste in their
feces, unne, and flesh and increasing decomposition of the waste. (4) They may carry the waste
directly to the surface dining excavation. (5) By working the soil and by transporting seeds, they
may hasten establishment of deep-rooted plants on the cap (Cadwell, Eberhardt, and Simmons.,
19891. (6) They cast soil on the surface thereby increasing erosion of the cap (Day, 1931; Winsor
and Whicker, 19801. (7) Animals construct their burrows for natural ventilation, which may dry the
soil thereby decreasing water intrusion (Vogel, Ellington, and Kilgore, 1973; Cadwell et al., 1989;
Landeen and Kemp, 19911.
Animal intrusions into buried wastes have been documented to occur at waste sites. At
Grand Junction, Colorado, prairie dogs (Cynomys ;ludovicianus) have burrowed through soil caps
and brought uranium mill tailings to the surface (McKenzie et al., 19821. At Hanford, a large
mammal believed to be a coyote (Cants latrans) or badger (Taxidea taxus) burrowed Into a waste
trench, exposing radiocontaminated salt that was consumed and distributed by other wildlife
(O'Farrell and Gilbert, 1975~. At Los Altos National Laboratory, pocket gophers (Thomomys
bottoe) burrowed through a 0.25-m soil cover and brought the bioba~Tier, crushed luff, to the
surface QIakonson, Martinez, and White, 19821. At the Idaho National Engineering Laboratory,
several species of rodents and their surface castings have been contaminated by burrowing into
trenches of radioactive waste even in areas with 1.2-m caps (Arthur and Markham, 19834.
The extent to which burrowing damages soil barriers depends on the depth of burrows, and
in particular, whether the burrows extend through the soil cover of a compacted layer or even
further through the cap. Depths of burrows for vertebrate animals present at major Department of
Energy sites are shown in Table 2 (Suter et al., 19931.
The presence of animals on a waste site depends on the habitat quality of the site. Habitat
quality depends in turn on the management of the site and, in the absence of vegetation
management, on ecological succession. Many waste sites that are not too arid are maintained as
mowed lawns. Frequent mowing not only minimizes the rooting depth of the vegetation but also
makes the site unattractive to burrowing vertebrates. In the eastern United States, the vertebrate that
is most likely to burrow through a cap, the woodchuck (Marmota moncuc), occurs primarily in the
earliest successional stages, 10-25 years after site abandonment. The abundance of burrowing small
rodents is also highest during that period. At arid and semiarid sites, faunal succession is
unimportant; all burrowing animals that are likely to occur at a site are likely to be present as soon
as vegetation is established. Although rock armoring would seemingly protect caps from burrowing
OCR for page 85
APPEND~PAPERS P=SENTED
D-67
animals, large rocks attract burrowing mammals, apparently because they provide dry sites for den
chambers and discourage den excavation by large predators.
TABLE 2 Maximum reported depth of burrows created by some animals occurring at or In
the vicinity of DOE waste sites (from Suter et al., 1993)
Species or Taxon
Depth
(m)
Earthworms (Lumbricidae)
Harvester ants (Pogonomyrmex spy.)
Mole (Scalopus aquaticus)
Shorttailed shrew (Blarina brevicaudata)
Pine vole (Microtus pinetorum)
Montane vole (Micro tus montanus)
Pocket mouse (Perognathus parvus)
Pocket gopher (Thomomys talpoid~es)
Deer mouse (Peromyscus maniculatus)
Kangaroo rat (Dipodomys ordii)
Ground squirrel (Spermophilis townsendii)
Chipmunk (Tamias striates)
Woodchuck (Marmota monax)
Box turtle (Terrapene Carolina)
2
3
0.6
0.5
0.3
0.5
1.4
>1
0.5
0.7
1 4
1
1.5
0.1
Earthworms and other soil invertebrates, which simply require a vegetation cover to
provide plant detritus for food, are not discouraged by mowing. in humid areas, earthworms
probably cause more significant damage to soil barriers than vertebrates because they are abundant
and may burrow deeply (Table 21. In addition, the extensive burrowing of earthworms provides
OCR for page 86
D-68
BARRIER TECHNOLOGIES FOR ENVIRONMENTAL MANAGEMENT
channels for root penetration of clay layers; the roots provide more food for worms resulting in
more and deeper channels.
CONCLUSIONS AND RECOMMENDATIONS
Clearly, natural physical and biological phenomena may cause failure of earthen barriers.
Site closure or in-situ remediation designs that depend solely on compacted soil caps or composite
caps therefore cannot be depended upon for long-term isolation of waste, but should be used only in
combination with effective stabilization of the underlying waste materials. Additionally, the
following design features and maintenance measures should be considered to delay hairier failure
or reduce its potential adverse consequences:
(~) Infiltration barriers should be covered by a soil layer sufficiently thick to extend below
the frost line, to accommodate the typical rooting depths of native plants expected to invade the
site, and to extend below the probable depth of animal burrows (i.e., at least 3 m in most areas).
(2) Composite caps (i.e., including both compacted clay soils and synthetic membranes)
may be superior to those constructed with either material alone, as they provide some defense
against the problems that can occur when sole reliance is placed on either earthen or synthetic
harriers. If composite cans are to be used effectively, additional research may be needed to
, .
~ .< . . .. ~ ,. , ~ ~ 1 ~ -~ 1~ ~ T:~1~ ~+ ~1 {1 OMAN
understand the interaction of the two types or earners. nor example, vlemauer at al. ~-'
reported formation of desiccation cracks in a compacted soil layer below an intact synthetic
membrane battier, probably due to natural thermal cycles and the exothermic decomposition of
underlying wastes.
(3) Post-closure land uses should not only avoid disturbance to the final cover, but should
also discourage biotic disturbances. For example, land uses that involve regular lawn maintenance
or maintenance of a paved area over a waste site are likely to limit root penetration and discourage
most burrowing animals.
(4) Monitoring and maintenance of inactive hazardous waste burial sites should be
continued indefinitely (not just for the 30-year period often mandated by regulation).
ACKNOWLEDGMENT
Based on work performed at Oak Ridge National Laboratory, managed for the U.S.
Department of Energy under contract DE-AC05-960R22464 with Lockheed Martin Energy
Research Corporation, Environmental Sciences Division publication number 4607.
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OCR for page 87
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D-69
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Representative terms from entire chapter:
compacted soil
