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Ecology, Design, and Long-Term Performance of Surface
Barriers: Applications at a Uranium Mill Tailings Site
W. I. Waugh, Roy F. Weston, Inc., DOE Grand Junction Office, Grand Junction,
Colorado; and G. N. Richardson, G.N. Richardson and Associates, Inc., Raleigh, North
Carolina
ABSTRACT
Conventional engineering approaches for designing surface barriers for uranium mill
tailings repositories fail to fully consider ecological processes that can have either beneficial or
deleterious effects on long-term performance. The U.S. Department of Energy developed an
alternative design for the semiarid Monticello, Utah, Superfimd site that combines fundamental
ecological principles with the required engineered barriers (e.g., geomembranes, compacted soil
layers). The design relies on soil water retention enhanced by a capillary barrier and soil/plant
evapotranspiration to seasonally return precipitation to the atmosphere. A compacted soil layer'
which can fail because of desiccation cracking and biointrusion, is included only as a secondary
infiltration barrier. The design relies on a combination of vegetation and a simulated desert
pavement to limit soil loss without influencing the soil-water balance. Rock riprap (coarse gravel
used to prevent erosion), which can increase water infiltration and create habitat for deep-rooted
plants, is used only on clean-filled side slopes. The design also controls radon releases,
biointrusion, and protects critical layers from disturbance by frost. Preliminary analog studies of
climate change, ecological change, and pedogenesis suggest that this design may improve with
time. Field performance data and quantitative evaluations of analogs are needed before this
alternative design, without the redundant engineered barriers, is used at other sites. Analog
studies are needed to understand and evaluate possible long-term changes in the ecology of
surface barriers that do not occur during short-term laboratory and field tests or that cannot be
modeled numerically.
INTRODUCTION
The U.S. Department of Energy (DOE) is in the midst of cleaning up more than 20
million metric tons of low-level radioactive and sometimes chemically toxic tailings at
abandoned uranium mills in the Four Corners region (Portillo, 1992). The accepted remedial
action is to cover tailings and other contaminated materials either in place or in landfill
repositories. DOE faces the unprecedented legislative and engineering requirements that these
tailings repositories persist for 200 to 1,000 years (USEPA, 1983). Engineered surface barriers or
covers for tailings repositories typically consist of compacted soil layers, sand drains, and rock
riprap intended to function as physical barriers to radon releases, water infiltration, and erosion
(USDOE, 1989). This conventional engineering approach fails to fully consider the ecology of
cover environments. After only a few years, biological disturbances threaten cover integrity at
many sites (USDOE, 1992).
DOE developed an alternative cover design for the disposal of uranium mill tailings at
the Monticello, Utah, millsite. This design is the product of unique combinations of regulatory
D-36
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P P E N D ~ ~ P A P E R S P ~ S E N ~ E D
D-37
and technical drivers. The Monticello repository design must satisfy both minimum technology
guidance (MTG) for hazardous waste disposal facilities (USEPA, 1989) tinder Subtitle C of the
Resource Conservation and Recovery Act of 1976 (RCRA), and design guidance for radon
attenuation and 1,000-year longevity (USDOE, 1989) under the Uranium Mill Tailings Radiation
Control Act of 1978 (UMTRCA). This required engineering guidance was refined by
incorporating some fundamental ecological principles. Our goal was to design a cover that will
improve rather than degrade over the long term as inevitable natural processes act on the
repository.
We summarize contaminant release mechanisms at uranium mill tailings repositories and
then compare the design and intended functional performance of the Monticello cover with
conventional RCRA and UMTRCA covers. Recommendations for design improvements, cost
reductions, and assessment of long-term performance issues are also presented.
CONTAMINANT RELEASE MECHANISMS
Several concomitant release mechanisms acting on the cover potentially could cause
environmental transport of tailings contaminants.
Water Infiltration
R~in``r~tPr and In melt not lost hv runoff and evaporation will enter the rock and soil
my ~ ~ ~ ~ ~ ~ ^~_~~ CAN J ~ r -
layers overlying the tailings and become distributed in the these materials in response to various
water potential gradients (Hillel, 19801. Depending on the properties and thicknesses of these
layers, soil water could evaporate from the cover surface, be extracted by plants and returned to
the atmosphere as transpiration, remain stored in the soil, pass into and remain stored in the
tailings, or drain from the tailings and potentially mobilize and release contaminants.
Radon Release
Residual radioactive materials (radium-226) in uranium mill tailings emit radon gas.
Rates of radon escape into the atmosphere above the repository will depend on the physical,
hydrological, and radiological properties of the tailings and overlying soil layers. The properties
that most influence radon release are the soil moisture content of the cover, the radon diffusion
coefficient for the cover, radium-226 concentrations in the tailings, and the emanating fraction
for radon in the tailings (Smith, Nelson, and Baker, 19851.
Erosion
Removal of fine-grained material by sheet-flow erosion, rifling, Sullying, and wind
deflation could expose and disperse tailings under extreme conditions or, more likely, reduce the
thickness of overlying layers leading to contaminant transport by other pathways (e.g., water
infiltration). Soil loss by sheet-flow erosion involves the detachment of soil particles from the
cover by raindrop splash and overland flow. If storm runoff is intense, flow may concentrate and
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D-38
BARRIER TECHNOLOGIES FOR ENVIRONMENTAL MANA CEMENT
cut rills and gullies deep into the cover (Walters and Skaggs, 1986). Wind transports soil
particles by surface creep, saltation, and resuspension and may be particularly rapid leeward of
topographic highs formed by mounded repositories (Ligotke, 1994).
Frost Penetration
As temperatures drop and soil layers within the cover freeze, water drawn toward the
freezing front can cause desiccation cracking (Chamberlain and Gow, 1979), freeze/thaw
cracking, and frost heaving (Miller, 1980), particularly in compacted soil layers. Desiccation and
frost cracking may lead to increased permeability and gas diffusion in compacted soil layers
within the frost zone (Kim and Daniel, 19921. Frost heaving may also cause distinct engineered
soil layers to become mixed, thereby disrupting the integrity of critical layer interfaces
(Bjornstad and Teel, 19931.
Plant Root Intrusion
Plants growing in the cover potentially could root into tailings, actively translocating and
`, ~.
disseminating contaminants In above-ground tissues troxx, ~ Jersey, and rams, ~ Act, ~v~-~
and Fraley, 1989; Markose, Bhat, and Pillai, 19931. Roots may also alter tailings chemistry,
potentially mobilizing contaminants (Cataldo et al., 19871. Macropores left by decomposing
plant roots act as channels tor water and gases to bypass compacted soil barriers effectively
(Hillel, 1980; Passioura, 19911. Plant roots may concentrate In and extract water from buried
clay layers, causing desiccation and cracking (Reynolds, 19901. This water extraction can occur
even when overlying soils are nearly saturated (Hakonson, 1986), indicating that the rate of
water extraction by plants may exceed the rehydration rate of the buried clay. Roots can also
clog lateral drainage layers ~JSDOE, 1992), potentially increasing infiltration rates.
Animal Intrusion
~ ~ ~ an,, ~
nllrrowin~ animals can mobilize contaminants by vertical displacement of tailings or by
altering erosion, water balance, and radon-release processes (Hakonson., Lane, and Springer,
19921. Vertical displacement results as animals excavate burrows and ingest or transport
contamination on skin and fur (Hakonson, Martinez, and White, 1982). Once in the surface
environment, contaminants may then be transferred through higher trophic levels and carried off
site (Arthur and Markham, 19831. Loose soil cast to the surface by burrowing animals is
vulnerable to wind and water erosion (Winsor and Whicker, 19801. Burrowing influences
soil-water balance and radon releases by decreasing runoff, increasing rates of water infiltration
and gas diffusion, and Increasing evaporation because of natural drafts (Landeen, 19941.
, ~. ~id. /Y 1
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APPENDIX~PAPERS PRESENTED
D-39
Cover Design and Performance
The Monticello cover (Figure I) is structurally similar to the RCRA Subtitle C design for
hazardous-waste disposal facilities (USEPA, 19891. The seemingly subtle structural differences,
however, represent salient conceptual and functional differences in performance. Table 1
compares components of the Monticello and RCRA designs.
Water Infiltration Control
Water Balance System
Water infiltration and leakage through the cover must not exceed the leakage rate of the
repository liner (USEPA, 19891. The Monticello repository liner includes a geosynthetic clay
layer with a design permeability of ~ x 10~9 calls. The Monticello cover design for controlling
water infiltration is essentially an MTG RCRA design (sand drainage layer, geomembrane, and
compacted soil layer) but with a thicker topsoil layer. The reliance of RCRA and UMTRCA
designs on low-permeability compacted soil layers is well documented (Daniel, 1994; USDOE,
1989), and the failure of compacted soil layers to achieve performance objectives because of
desiccation and shrinkage is also documented (Melchoir et al., 1994~. The sand drainage layer,
geomembrane, and compacted soil layer in the Monticello design serve as a backup for what we
call a water-balance system. The water-balance system is the primary means for limiting
infiltration over the long term.
Type of is ~
'. 32~. ~-~
_ 30 cm _
~:~11 1 ~ ~
FIGURE 1 DOE Cover Design for the Monticello Repository.
Vegetation
Soil/Gravel Admixture
Water Storage/Frost
Protection (fine soil)
Animal Intrusion Layer
(native pediment gravels)
Geotextile Filter
Sand Layer and Capillary Break
60 Mil Geomembrane
(high-density polyethylene)
Radon/lnfiltration Barrier
(compacted soil)
Topsoil Layer
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Representative terms from entire chapter:
mill tailings
D-40
TABLE 1
Designs
BARRIER TECHNOL O GIES FOR ENVIR ONMENTAL MANA GEMENT
Comparison of RCRA Subtitle C (USEPA, 1989) and DOE MRAP Cover
-}~(~(i~275!~ aide; (Yd3I~^~^ ~t ~ ~ ; - - ~;~ ~ ~ 1
Vegetation consists of locaay adapted perennial Vegetation consists of locally adapted perennial plants
plants selected for erosion control selected for erosion control and soil-water extraction
I No-~} Hi-D;; 5 > jay } Y 'no: 18(~ ~ i
1~:·~f~ :: :~:~ ' .~: :::: I:~:~:.~f~ ~I.l.g,..~ 1~ ~ ~ I t: ~ :.
Top slope between 3% and 5% Tope slope between 3% and 5%
.
| =: ! ~ ill: r ~ ~ ~ ~ ;~ ~ 3~:S <.;c~:
APPENDIXI - PAPERS PRESENTED
D-41
Leakage from the water-balance system is evaluated as the probability that water
accumulation rates will exceed evapotranspiration and, eventually, the water storage capacity of
the topsoil layer. Soil-water storage capacity is the difference between the upper storage limit
(before leakage occurs), sometimes referred to as the field capacity, and the lower storage limit
(after removal of plant extractable water) (Ritchie, 1981). Field-plot and lysimeter tests
conducted at other DOE sites (Waugh et al., 1991; Wing and Gee, 1993; Anderson et al., 1993)
suggest that, with plants present to seasonally dry the Monticello cover, water accumulation
likely will not exceed the topsoil storage capacity, even during higher than record precipitation
years. Field and modeling studies are ongoing at Monticello to test this hypothesis. Preliminary
results corroborate results of the previous studies. For the next generation of DOE cover designs,
a water-balance system without redundant geomembranes and compacted soil layers may be
adequate to control water infiltration at arid and semiarid sites.
Revegetation
The calculated thickness of the Monticello topsoil not only provides an optimum
water-balance system but also creates a habitat more suitable for desirable vegetation. A thinner
layer would encourage the establishment of a woodland plant community consisting of
undesirable deep-rooted species. A diverse mixture of native plants on the cover will maximize
water removal by evapotranspiration (Link et al., 1994) and remain more resilient to catastrophes
and fluctuations in the environment (Begon et al., 1986).
Revegetation activities will attempt to emulate the structure, function, diversity, and
dynamics of native plant communities in the area. The native sagebrush-grass vegetation at
Monticello is a mosaic of many species that structurally and functionally changes in response to
disturbances and environmental fluctuations (Tausch, Wigand, and Burkhardt, 1993). Similarly,
biological diversity in the cover vegetation will be important to community stability and
resilience, given variable and unpredictable changes in the environment resulting from pathogen
and pest outbreaks, disturbances (overgrazing, fire, etc.), and climatic fluctuations. Local
indigenous genotypes that have been selected over thousands of years are best adapted to
climatic and biological perturbations. In contrast, exotic grass plantings, common on waste sites,
are genetically and structurally monotonous (Harper, 1987) and, thus, more vulnerable to
disturbance or eradication by single factors.
Radon Attenuation
The 60-cm compacted soil layer (radon/infiltration barrier in Figure 1) satisfies the
requirement for a radon barrier that limits the average surface flux of radon-222 to less than 20
psi my s~' (USEPA, 1983). The thickness was calculated with the standard method-the U.S.
Nuclear Regulatory Commission (USNRC) model RADON (USNRC, 1989). This design
approach is documented elsewhere (USDOE, 1989). As required for UMTRCA sites (USNRC,
1989), only the compacted soil layer (radon/infiltration barrier) of the cover was included in this
calculation. All overlying layers were omitted. Further analysis suggests that the compacted soil
layer may be unnecessary. RADON model results show a lower radon flux from a cover
consisting of only a water-balance system.
D-42
BARRIER TECHNOLOGIES FOR ENVIRONMENTAL MANAGEMENT
Erosion Control
The primary erosion control issue is: Will vegetation alone adequately limit soil loss, or
are gravel mulches, gravel admxitures, or rock riprap necessary to armor the soil when
vegetation is sparse or less dependable? Vegetation and organic litter disperse raindrop energy,
slow flow velocity, bind soil particles, filter sediment from runoff, increase infiltration, and
reduce surface wind velocity (Wischmeier and Smith, 1978). Vegetation may be inadequate in
the first years after construction. UMTRCA and alternative RCRA designs include cobble or
rock riprap to control erosion in arid environments with sparse vegetation (USDOE, 1989;
USEPA, 1989). However, these designs reduce evaporation (Groenevelt et al., 1989; Kemper,
Nicks, and Corey, 1994), possibly increasing leakage through compacted soil layers and creating
habitat for undesirable plants that root into the radon/infiltration bamer (USDOE, 1992).
Erosion control for the Monticello design consists of mixing gravel and sand in the top
20 cm of the topsoil (Figure 1) to mimic conditions leading to the formation of desert pavement.
The method of Temple et al. (1987) was used to size the gravel (Table 1). The sand component
was sized relative to the topsoil and gravel with Stephanson's (1979) method. Several erosion
studies (Finley, Harvey, and Watson, 1985; Ligotke, 1994) and soil-water balance studies
(Waugh et al., 1994b; Sackschewsky et al., 1995) suggest that moderate amounts of gravel mixed
into the cover topsoil will control both water and wind erosion with little effect on plant habitat
or soil-water balance. As wind and water pass over the surface, some winnowing of fines from
the admixture is expected, leaving a vegetated erosion-resistant pavement. The sand "filter" and
root cohesion of fines will impede continued soil loss beneath this pavement (Styczen and
Morgan, 1995). The combination of vegetation and gravel pavement will control sheet flow,
minor rifling, and wind erosion by decreasing tractive sheer stresses. Rilling and Bullying is
controlled by maintaining top-slope gradients equal to surrounding terrain (which lack rills and
intermittent gullies) and by limiting lengths of overland flow paths.
Frost Protection
The 170-cm composite topsoil layer (Figure 1) provides more than adequate depth to
isolate the capillary break layer, drainage layer, geomembrane, and compacted soil layer
(radon/infiltration barrier) from frost damage. The estimated maximum frost depth for a
200-year return interval in the topsoil layer is 115 cm. This value was extrapolated from soil
physical properties for the loess soil and Monticello weather data by using the modified
Berggren equation presented in DOE's Technical Approach Document (USDOE, 1989).
UMTRCA rock riprap covers have essentially no frost protection for the radon infiltration
barrier, and the 60 cm of frost protection offered by the RCRA cover is inadequate for
Monticello.
Biointrusion Control
The Monticello cover includes barriers to biological intrusion by plant roots and
burrowing vertebrates. By retaining soil water close to the surface, the combined topsoil and
capillary barrier create a habitat for relatively shallow-rooted plant species and, thus, function as
a de facto root-intrusion barrier (Cline, Gano' and Rogers' 1980; Hakonson' 1986). Root growth
APPENDIX~PAPERS PRESENTED
D-43
generally is limited to regions within the soil where extractable water is available. The
compacted soil layers in RCRA and UMTRCA covers may offer some protection. Agronomists
have long observed that highly compacted soils cause stubby and gnarled root growth (Passioura,
1991) and can reduce rooting depths (Foxx et al., 19841. However, plants vary greatly in their
ability to penetrate compacted soils (Materechera, Dexter, and Alston, 19911. At arid and
semiarid sites, root densities can be higher in buried clay layers and cause seasonal desiccation
(Hakonson, 1986; Reynolds, 19901.
The composite topsoil layer thickness is also the primary barrier to burrowing; it exceeds
the maximum burrow depths of most vertebrates at Monticello. The 30-cm layer of native
pediment gravel within the composite topsoil layer is an added deterrent. Loosely aggregated
gravel and rock have been shown to deter burrowing mammals (Cline et al., 1980; Hakonson,
19861. This layer is above and protects the capillary break from bioturbation, a primary
long-term threat to layer systems (Bjornstad and Teel, 1993~. The native pediment gravels
contain enough fines to prevent this layer from behaving like a secondary capillary barrier.
Longevity
The greatest uncertainties in designing the Monticello cover stem from the scientifically
challenging need to extrapolate the results of short-term tests to the
required 200- to 1,000-year
performance period. Standard engineering approaches that are based on laboratory tests,
short-term field demonstrations, and numerical predictions implicitly assume that initial
conditions of material properties and of processes that drive contaminant transport will persist. In
contrast, engineered covers must be viewed as evolving components of larger, dynamic
ecosystems.
Natural analogs provide clues from past environments to possible long-term changes in
engineered covers (Waugh et al., 1994a). Logical analogy is used to investigate natural and
archaeological occurrences of materials, conditions, or processes that are similar to those known
or predicted to occur in some part of the engineered cover system. As such, analogs can be
thought of as uncontrolled, long-term experiments. Analogs may also have a role in
communicating the results of the performance assessment to the public. Evidence from natural
systems can help demonstrate that numerical predictions have real-world complements.
Long-term performance issues at Monticello that can be assessed with the use of analogs include
climate change, ecological change, and pedogenesis (soil development).
Climate Change
Climate greatly influences the release of hazardous materials from buried tailings at
Monticello and the performance of the engineered cover designed to isolate tailings. With
evidence of relatively rapid past climate change (Crowley arid North, 1991) and model
predictions of global climatic variation exceeding the historical record (Ramar~athan, 1988),
DOE recognizes a need to incorporate possible ranges of future climatic and ecological change in
the repository design process (Petersen, Chatters, and Waugh, 1993). Paleoclimatic records may
be useful not only as a window on the past, but also as analogs of possible local responses to
future global change.
D-44
BARRIER TECHNOLOGIES FOR ENVIRONMENTAL MANAGEMENT
We reconstructed past climate change for Monticello by using available proxy data from
tree rings, packrat middens, lake sediment pollen, and archaeological records (Waugh and
Petersen, 1995). Interpretation of proxy paleoclimatic records was based on present-day
relationships between plant distribution, precipitation, and temperature along a generalized
elevational gradient for the region. For Monticello, this first approximation yielded mean annual
temperature and precipitation ranges of 2 to 10° C, and 38 to 80 cm, respectively, corresponding
to late glacial and Altithermal periods. These data are considered to be reasonable ranges of
future climatic conditions that can be input to evaluations of water infiltration, radon-gas escape,
erosion, frost penetration, and biointrusion.
Pedogenesis and Ecological Change
Pedogenic processes gradually will change the physical and hydraulic properties of
earthen materials used to construct the Monticello cover (e.g., McFadden, Wells, and
Jercinovich, 1987; Hillel, 1980). Plant and animal communities inhabiting the cover will also
change in response to climate and disturbances. As the ecology of the cover changes, so also will
performance factors such as water infiltration, evapotranspiration, water retention, soil loss,
radon diffusion, and biointrusion.
Weighing lysimeters encasing 100-cm-deep soil monoliths were installed near the
proposed Monticello repository site to measure the water balance of analog soils and vegetation
(Waugh arid Link, 1992). Monolithic lysimeters preserve, as well as possible, native soil profiles
and vegetation. All precipitation received during the 1991 and 1992 bioclimatic years
(November through October) was retained (no leakage occurred); close to normal precipitation
was received for both years. Approximately 2.8 cm of leakage was measured during spring of
1993, indicating that soil-water accumulation exceeded the storage capacity that year. The 1992-
1993 winter (December-February) was one of the wettest on record (315 percent of normal);
Monticello experienced the wettest February of this century. The increased storage capacity of a
170-cm soil layer over a capillary break would likely have retained all the excess soil water.
These results suggest that with plants present to seasonally dry the topsoil layer of the cover,
water accumulation likely will not exceed the topsoil storage capacity, except during years with
higher than record precipitation.
SUMMARY
.
DOE plans to construct a lined landfill for disposal of tailings from an abandoned
uranium mill at Monticello, Utah. The cover design, although similar in appearance, represents a
departure from typical RCRA and UMTRCA designs. These designs are vulnerable to natural
processes that will degrade the cover over the long term. In contrast, the DOE design for the
Monticello cover relies on natural processes to isolate tailings and to control the release of
contaminants and is expected to improve over time.
The Monticello design should be considered as an alternative to RCRA Subtitle C and
UMTRCA designs at other arid and semiarid sites:
- Compacted soil layers, as required for RCRA and UMTRCA designs to control
water infiltration, are vulnerable to damage by desiccation and biointrusion. In contrast, the
APPENDIX~PAPERS PRESENTED
D-45
Monticello water-balance cover relies on soil-water retention, capillary barriers, and soil-water
extraction by plants.
· Rock riprap layers, as recommended for UMTRCA designs, control erosion but
enhance water infiltration and biointrusion. The Monticello design includes a topsoil and gravel
admixture. The admixture is designed to control erosion, much like a desert pavement, without
adversely influencing desirable vegetation and the soil-water balance.
· The Monticello design includes a geomembrane and a compacted soil layer as
redundant infiltration barriers and to control radon release. These layers are also required to meet
RCRA and UMTRCA design requirements. Results of small-scale field tests and numerical
modeling suggest that the water-balance cover will satisfy perfo~ance standards for water
infiltration and radon releases without the engineered barriers.
· Field monitoring of water balance, erosion, and biointrusion are needed to evaluate
the performance of the Monticello design under realistic conditions, before the design is used at
other sites without the redundant engineered barriers. Similar measurements in natural analog
environments may provide clues about long-term performance.
Engineered covers that are intended to last hundreds and thousands of years must be
designed as evolving components of larger dynamic ecosystems. Four tenets accompany this
principle: (~) cover components will not function and, thus, cannot be designed independently;
(2) physical and ecological conditions will change over time; therefore, initial conditions cannot
be extrapolated as tests of long-term performance; (3) designs should not rely on man-made
materials of unknown durability; and (4) the design should not rely on physical barriers to
natural processes but on the use of natural processes.
ACKNOWLEDGMENT
This work was performed under DOE Contract No. DE-AC13-95GI87335 for the U.S.
Department of Energy.
D-46
BARRIER TECHNOLOGIES FOR ENVIRONMENTAL MANAGEMENT
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APPENDIX~PAPERS PRESENTED
D-47
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