**DESIGN, PERFORMANCE, AND SUSTAINABILITY OF**

**ENGINEERED COVERS FOR URANIUM MILL TAILINGS**

Final remedies at most uranium mill tailings sites include engineered
covers designed to contain metals and radionuclides in the subsurface
for hundreds of years. Early cover designs rely on compacted soil layers
to limit water infiltration and release of radon, but some of these
covers inadvertently created habitats for deep-rooted plants. Root
intrusion and soil development increased the saturated hydraulic
conductivity several orders of magnitude above design targets. These
covers may require high levels of maintenance to sustain long-term
performance. Relatively low precipitation, high potential
evapotranspiration, and thick unsaturated soils favor long-term
hydrologic isolation of buried waste at arid and semiarid sites. Later
covers were designed to mimic this natural soil-water balance with the
goal of sustaining performance with little or no maintenance. For
example, the cover for the Monticello, Utah, Superfund site relies on a
thick soil-sponge layer overlying a sand-and-gravel capillary barrier to
store precipitation while plants are dormant and on native vegetation to
dry the soil sponge during the growing season. Measurements of both
off-site caisson lysimeters and a large 3-ha lysimeter built into the
final cover show that drainage has been well below a U.S. Environmental
Protection Agency target of less than 3.0 mm/yr. Our stewardship
strategy combines monitoring precursors to failure, probabilistic
risk-based modeling, and characterization of natural analogs to project
performance of covers for a range of possible future environmental
scenarios. Natural analogs are needed to understand how ecological
processes will influence cover performance, processes that cannot be
predicted with short-term monitoring and existing numerical models.

**Introduction**

The U.S. Department of Energy Office of Legacy Management (DOE−LM) is
responsible for long-term stewardship of disposal sites for uranium mill
tailings (www.gjo.doe.gov/LM/). Final remedies at most sites include
engineered covers. Cover design and performance evaluation guidelines
are prescriptive in nature and fail to consider consequences of
inevitable changes in ecological settings (1,2). In contrast, the DOE−LM
Environmental Sciences Laboratory (ESL)[^1] in Grand Junction, Colorado,
combines monitoring, modeling, and natural analog studies to evaluate
the long-term performance of covers. Below are examples and lessons
learned over many years of experience monitoring existing covers,
designing alternative covers that accommodate ecological change, and
using natural analog studies in combination with monitoring and modeling
to project the long-term performance of covers for uranium mill
tailings.

**Monitoring Existing Covers**

Disposal cell covers designed to satisfy the Uranium Mill Tailings
Radiation Control Act of 1978 (UMTRCA) have evolved in response to
regulatory changes and lessons learned (3). Early designs focused on
radon attenuation and a 1,000-year longevity standard (4). Early designs
basically consisted

of three layers: 1) a compacted soil layer (CSL) overlying the tailings
for radon attenuation, 2) a surface layer of durable rock for erosion
protection, and 3) a lateral drainage layer consisting of coarse sand or
gravel sandwiched between the CSL and the surface layer (2). The CSLs
were later advocated as low-permeability barriers (2).

Plants began growing in the rock-armored, low-permeability covers within
a few years after construction (5). Plant encroachment should have been
anticipated. Surface layers of rock reduce evaporation (6), increase
soil water storage (7), and, consequently, create habitat for
deep-rooted plants. Deep-rooted plants may either decrease or increase
the likelihood of contaminant releases from disposal cells, thus
long-term maintenance has become problematic. Extraction of soil water
by plants (transpiration) may significantly decrease recharge through
covers. Even in humid climates where precipitation exceeds potential
evapotranspiration (ET), water extraction by plants may account for more
than half of soil water loss from disposal cell covers (8). Woody
vegetation has also been shown to improve the stability of
riprap-armored slopes (9).

Conversely, plants rooted in uranium mill tailings may contain elevated
levels of U, Mo, Se, ^226^Ra, ^230^Th, and ^210^Po (10,11,12). Radon-222
can be transported into the atmosphere through transpiration water (13).
Roots may also alter waste chemistry, potentially mobilizing
contaminants (14). Root intrusion can physically degrade covers. CSLs
are vulnerable to desiccation and cracking from wet−dry cycles,
freeze−thaw cycles, and biointrusion (8,15). Macropores left by
decomposing plant roots may act as channels for water and gases to
bypass the soil mass in CSLs. Plant roots also tend to concentrate in
and extract water from CSLs high in clay, causing desiccation and
cracking, even when overlying soils are nearly saturated (16).
Furthermore, roots can clog lateral drainage layers (5), potentially
increasing percolation rates.

A cover constructed at Shiprock, New Mexico, in 1986, before the U.S.
Environmental Protection Agency (EPA) proposed groundwater quality
standards for UMTRCA sites, exemplifies the dilemma. The Shiprock area
receives an average of about 15 cm precipitation per year. The Shiprock
cover consists of three layers: a 198-cm silt loam CSL for radon
attenuation, a 15-cm sand drainage layer overlying the CSL, and a 30-cm
layer of large, durable cobble sized to prevent erosion. Early
laboratory tests suggested that the Shiprock CSL had a saturated
hydraulic conductivity (*K~sat~*) between 6.4 x 10^-8^ and\
2.3 x 10^-6^ cm/s (17). After groundwater standards for UMTRCA sites
were promulgated, DOE became concerned that potentially deep-rooted
plants observed growing on the cover, including tamarisk, rabbitbrush,
and Russian thistle, could increase the permeability of the CSL.

Soil moisture monitoring and in situ measurements of *K~sat~* suggest
that recharge through the cover is higher than previously thought (18).
Neutron hydroprobe measurements from June 1999 through September 2000
show that the CSL and upper tailings were saturated. In situ *K~sat~*
was measured using air-entry permeameters (19) in pits where tamarisk,
rabbitbrush, and Russian thistle rooted into the CSL and in adjacent
pits without plant roots. Results were highly variable with a mean
*K~sat~* equal to 4.4 x 10^-5^ cm/s. Given saturation of the CSL and
tailings and a higher *K~sat~* than previously assumed, higher than
expected recharge through the cover is likely. ESL is investigating
methods for direct measurement of water flux from the cover and, as a
contingency, for retrofitting the cover to accelerate plant
establishment and ET (see "Alternative Cover Design"). Regulatory
agreements currently require spraying cover vegetation with herbicides.

Effects of root intrusion on the performance of the cover at the
Burrell, Pennsylvania, site were also evaluated (20). Annual
precipitation at Burrell averages more than 100 cm/yr. The Burrell cover
consists of a 90-cm CSL overlying tailings materials, a 30-cm sand and
gravel drainage layer, and a 30-cm rock riprap layer. Within 3 years
after construction, woody plants, including sycamore, box elder, black
locust, tree-of-heaven, and Japanese knotweed, began emerging from the
rock cover. Within 10 years, Japanese knotweed had rooted through the
CSL. At Burrell, *K~sat~* averaged 3.0 x 10^-5^ cm/s at locations where
Japanese knotweed roots penetrated the CSL, but only 2.9 x 10^-7^ cm/s
at locations without plants. The weighted-average *K~sat~,* calculated
using the leaf area index (LAI) (21) for Japanese knotweed, was 4.4 x
10^-6^ cm/s. At a nearby site with a subsoil similar to the material
used to construct the CSL, the *K~sat~* averaged 1.3 x 10^-4^ cm/s.
Earthworm holes, root channels, and pedogenic structure all contributed
to macropore flow. The nearby site was considered to be a reasonable
analog of a long-term ecological scenario for the Burrell cover (see
"Natural Analogs of Long-Term Performance").

**Alternative Cover Design**

Lessons learned from monitoring early UMTRCA covers contributed to
design improvements. DOE and EPA Region 8 collaborated on an alternative
design for a uranium mill tailings disposal cell at the Monticello,
Utah, Superfund site (22). The goal at Monticello was to design an
engineered cover system that enhances beneficial natural processes to
help make long-term containment possible (23).

At semiarid sites such as Monticello, relatively low precipitation (P),
high potential evapotranspiration (PET), and thick unsaturated soils
seem to favor long-term hydrologic isolation of buried waste (24). But
simple P/PET relationships inadequately predict recharge that can
approach 60 percent of precipitation in arid-land soils denuded of
vegetation (25). Recharge can be minimized if disposal cells are covered
with thick, fine-textured soil layers that store precipitation in the
root zone where ET seasonally removes it (26,27). Capillary barriers
consisting of coarse-textured sand and gravel placed below this
soil-sponge layer can enhance water storage and limit unsaturated flow
(28,29).

The Monticello cover design (Figure 1) relies on the water-storage
capacity of a 163-cm fine-textured soil- sponge layer overlying a 38-cm
capillary barrier of coarse sand to retain precipitation until it is
seasonally removed by vegetation. Gravel mixed into the surface helps
control erosion when vegetation is sparse (following construction,
fires, drought, etc.), mimicking conditions that lead to the formation
of gravel pavements. The gravel admixture can control both wind and
water erosion (30,31) and can enhance

.![](media/image1.wmf){width="3.946527777777778in"
height="3.5388888888888888in"}

Figure 1. Alternative cover constructed at Monticello, Utah.

seedling emergence and plant growth by functioning as a mulch. The
soil-sponge thickness is the primary biointrusion deterrent. Water
retention in the soil sponge creates habitat for relatively
shallow-rooted plants, and the thickness of the sponge exceeds the depth
of most burrowing vertebrates in the Monticello area. A layer of
cobble-size rock 30.5 cm above the capillary barrier is an added
deterrent should deeper burrowers, such as prairie dogs, move into the
area in response to climate change. Fine-textured sponge soil fills the
interstices of the cobble layer, preventing it from behaving like a
second capillary barrier. Physical and hydraulic properties of the
topsoil layer, obtained from the root zone of the borrow area, match the
rest of the soil sponge. However, the topsoil also contains available
nutrients, propagules, and microorganisms (e.g., mycorrhizae) needed to
sustain a diverse and resilient plant community.

ESL personnel conducted a series of field lysimeter experiments to help
design and monitor the performance of the Monticello cover. The
lysimeter test facility evolved as a sequence of installations, first to
test the concept of using an ET/capillary barrier cover design at
Monticello, next to evaluate the soil-water balance of the design, and
finally to monitor the hydrologic performance of a large facet of the
completed cover. In 1990, ESL installed small weighing lysimeters
containing intact, 100-cm-deep columns of undisturbed native silt loam
soil (monoliths) overlying a pea-gravel capillary barrier and supporting
mature native grasses (32). Leaf water potential, leaf transpiration,
and whole-plant gas exchange of plants growing in and adjacent to the
lysimeters were compared to test the physiological responses of plants
to confinement in the small lysimeters. Favorable results led to
construction of an array of 15 additional small weighing lysimeters in
1993 to compare drainage, ET, and water-storage capacities of cover
designs with varying soil types and soil layer thickness (32). Favorable
results this time led to a final cover design for Monticello.

In 1999, ESL teamed with EPA Region 8 on a study using large caisson
lysimeters to evaluate the hydrological and ecological performance of
the Monticello cover as built. Covers constructed inside the caissons
matched the range of conditions as built in the actual cover (33).
Finally, in 2000, ESL and the EPA Alternative Cover Assessment Program
(ACAP) collaborated on installation of a large drainage lysimeter under
a 3-ha facet of the 14-ha disposal cell cover at Monticello. Both the
caisson and ACAP lysimeter studies show that plant transpiration has
kept drainage flux levels well below the EPA target of 3.0 mm/yr.

**Natural Analogs of Long-Term Performance**

Understanding how inevitable ecological processes may alter the
long-term performance of engineered covers is crucial to designing,
constructing, and maintaining cover systems (34). Effective performance
modeling requires both current and possible future ecological scenarios
(35). Natural analog studies are needed to identify and evaluate likely
shifts in cover environments (36), to design covers that mimic favorable
natural systems, to bound possible future conditions for input to
models, and to provide clues about the possible evolution of engineered
covers as a basis for monitoring precursors of failure. Natural analogs
also provide tangible evidence of the reliability of numerical
predictions. ESL and its partners have collaborated on studies of
natural and archaeological analogs to discern likely long-term changes
in the environmental setting of cover systems, including climate change,
pedogenesis (soil development), and ecological succession (37).

Reasonable projections of possible future changes in climate states and
extreme events are needed to design sustainable covers. ESL and its
partners are demonstrating methods based on global change models and
paleoecological evidence to establish a first approximation of possible
future climatic states at DOE−LM sites. A preliminary analysis of
paleoclimate data for Monticello yielded average annual temperature and
precipitation ranges of 2 to 10 °C and 80 to 60 cm, respectively,
corresponding to late glacial and mid-Holocene periods (38).
Instrumental records were used as a basis for selecting soil and
vegetation analog sites that span a reasonable range of future climate
scenarios for Monticello (37).

Pedogenic processes will change the soil physical and hydraulic
properties of engineered covers. Pedogenesis includes processes such as
formation of macropores and preferential flow associated with root
growth, animal holes, and soil structural development; secondary
mineralization, deposition, and illuviation of fines, colloids, soluble
salts, and oxides that can alter water storage and movement; and soil
mixing caused by freeze−thaw activity, animal burrows, and the
shrink−swell action of expansive clays (37). ESL and its partners are
characterizing natural and archaeological soils considered
representative of pedogenic changes in engineered cover soils. For
example, key soil physical and hydraulic properties at natural and
archaeological soil profiles at climate analog sites were measured to
infer possible future pedogenic changes in the performance of the
Monticello cover.

Plant communities will establish and change on soil covers in response
to climate, soil development, and disturbances such as fire, grazing, or
noxious plant invasion. Changes in plant abundance, ET rates, root
instrusion, and animal habitat may alter the soil water balance and
stability of a cover. ESL and its partners draw evidence of possible
future ecological changes using successional chronosequences (a mosaic
of plant communities that represent different stages of recovery
following a disturbance). For example, at the Lakeview, Oregon, uranium
mill tailings disposal site, possible future responses of plant
community composition and LAI to fire were evaluated using a regional
chronosequence. Similarly, possible future vegetation responses to
climate change scenarios were evaluated at regional global-change analog
sites. LAI, an index of plant transpiration, ranged from 0.15 to 1.28
for the fire chronosequence and from 0.43 to 1.62 for dry and wet
climate analog sites.

**Conclusions**

The DOE office in Grand Junction, Colorado, has learned several lessons
from monitoring, designing, and evaluating the long-term performance of
engineered covers constructed to contain uranium mill tailings in the
subsurface that could be of benefit to designers of the next generation
of covers.

Early rock-armored covers that rely on CSLs to limit water movement into
tailings may fall short of permeability targets, and many inadvertently
created habitats for deep-rooted plants. Root intrusion and soil
development may have increased the *K~sat~* several orders of magnitude
above design targets in several covers. At Shiprock, New Mexico, ESL is
evaluating methods for measuring flux directly to ensure that ongoing
efforts to remediate groundwater are not compromised by contaminants
seeping from the disposal cell. Saturated flow into tailings is likely
occurring in the Burrell, Pennsylvania, disposal cell. But because of
low contaminant concentrations, root intrusion and increased saturated
flow are not adversely impacting human health or the environment at the
Burrell site. Overall, these low-permeability covers attempt to resist
natural processes, rather than work with them, and will likely require
increasing levels of maintenance or retrofitting to sustain long-term
performance.

Relatively low precipitation, high potential ET, and thick unsaturated
soils favor long-term hydrologic isolation of buried waste at arid and
semiarid sites. The cover constructed at the Monticello site mimics this
natural soil-water balance. The Monticello cover relies on a thick soil
sponge layer overlying a sand and gravel capillary barrier to store
precipitation while plants are dormant and on native vegetation to dry
the sponge layer during the growing season. Lysimeter results show that
less than 0.05 mm of drainage has occurred since 2000, an amount well
below the EPA target of less than 3.0 mm/yr.

Given unprecedented longevity requirements, a stewardship objective
should be to design or retrofit covers to accommodate long-term
ecological processes with the goal of sustaining performance with as
little maintenance as possible. Investigations of natural analogs can
provide insights into how ecological processes may influence the
performance of engineered covers. Evidence from natural analogs can
improve our understanding of meteorological variability associated with
possible long-term changes in climate; vegetation responses to climate
change and disturbances; effects of vegetation dynamics on ET, soil
permeability, soil erosion, and animal burrowing; and effects of soil
development processes on water storage and permeability.

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This document was authored by the following individuals on behalf of the
Office of Legacy Management:

W. Jody Waugh

[^1]: ^\*^ ^\*^The Environmental Sciences Laboratory is operated by S.M.
    Stoller Corporation for the U.S. Department of Energy, Grand
    Junction, Colorado, under DOE Contract Number DE−AC01−02GJ79491.