**Monitoring the Performance of an Alternative Cover**

Using Caisson Lysimeters

**ABSTRACT**

The U.S. Department of Energy (DOE) office in Grand Junction, Colorado,
and the\
U.S. Environmental Protection Agency (EPA), Region 8, collaborated on a
series of field lysimeter studies to design and monitor the performance
of an alternative cover for a uranium mill tailings disposal cell at the
Monticello, Utah, Superfund Site. Because groundwater recharge is
naturally limited at Monticello in areas with thick loess soils, DOE and
EPA chose to design a cover for Monticello using local soils and a
native plant community to mimic this natural soil-water balance.

Two large drainage lysimeters fabricated of corrugated steel culvert
lined with high-density polyethylene were installed to evaluate the
hydrological and ecological performance of an alternative cover design
constructed in 2000 on the disposal cell. Unlike conventional,
low-permeability designs, this cover relies on (1) the water storage
capacity of a 163-cm soil "sponge" layer overlying a sand-and-gravel
capillary barrier to retain precipitation while plants are dormant and
(2) native vegetation to remove precipitation during the growing season.
The sponge layer consists of a clay loam subsoil compacted to 1.65
g/cm^2^ in one lysimeter and a loam topsoil compacted to 1.45 g/cm^2^ in
the other lysimeter, representing the range of as-built conditions
constructed in the nearby disposal cell cover.

About 0.1 mm of drainage occurred in both lysimeters during an average
precipitation year and before they were planted, an amount well below
the EPA target of \<3.0 mm/yr. However, the cover with less compacted
loam topsoil sponge had a 40% greater water storage capacity than the
cover with overly compacted clay loam subsoil sponge. The difference is
attributable in part to higher green leaf area and water extraction by
plants in the loam topsoil. The lesson learned is that seemingly subtle
differences in soil types, sources, and compaction can result in salient
differences in performance. Diverse, seeded communities of predominantly
native perennial species were established on both lysimeters during an
extended 3-yr drought, highlighting the importance of a sound
understanding of the local ecology and of implementing the science and
methods of disturbed-land revegetation.

**\_\_\_\_\_**

^\*^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.

**INTRODUCTION**

The Monticello, Utah, mill, built in 1942 to provide vanadium for World
War II, later processed nearly 1.0 million tons of uranium ore and
produced more than 2.5 million m^3^ of tailings before its closure in
the early 1960s. The U.S. Department of Energy (DOE) office in Grand
Junction, Colorado, Region 8 of the U.S. Environmental Protection Agency
(EPA), and the State of Utah Department of Environmental Quality
collaborated on the design and construction of a disposal cell to
contain tailings and tailings-contaminated materials at Monticello.
Remedial actions were regulated under the Comprehensive Environmental
Response, Compensation, and Liability Act (CERCLA). The disposal cell
design satisfied both (1) minimum technology guidance for hazardous
waste disposal facilities \[1\] under subtitle C of the Resource
Conservation and Recovery Act of 1976 (RCRA) and (2) design guidance for
radon attenuation and 1,000-yr longevity \[2\] under the Uranium Mill
Tailings Radiation Control Act of 1978 (UMTRCA).

Early cover designs constructed for UMTRCA disposal cells typically
consist of compacted soil layers (CSLs), sand drains, and rock riprap
intended to function as physical barriers to radon releases, water
infiltration, and erosion.\[2, 3\] Typical RCRA cover designs also
include prescribed physical barriers.\[1\] These conventional engineered
covers, which attempt to resist natural processes rather than work with
them, will likely require increasing maintenance.\[4\] After only a few
years, CSLs have desiccated and cracked under routine wetting and drying
conditions \[5\], and biological disturbances threaten cover integrity
at many sites.\[6, 7, 8, 9, 10\] The goal at Monticello was to design an
engineered cover system that enhances beneficial natural processes to
help make long-term containment possible.\[11\]

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.\[12, 13,
14, 15\] But simple P/PET relationships inadequately predict recharge in
arid regions that can approach 60% of precipitation in coarse-textured
soils denuded of vegetation.\[16\] At arid and semiarid waste disposal
sites, recharge can be minimized with thick, fine-textured soil layers
that store precipitation in the root zone where evapotranspiration (ET)
seasonally removes it.\[17, 18, 19, 20, 21\] Capillary barriers
consisting of coarse-textured sand and gravel placed below this soil
"sponge" layer can enhance water storage and limit unsaturated
flow.\[22, 23\] To be accepted by regulators, end users must demonstrate
that the water balance of these *alternative* cover designs is at least
equivalent to conventional designs.

Weighing and drainage lysimeters offer the most direct and reliable
means for evaluating soil-water balance of alternative cover
designs.\[24\] Lysimeters have been used for many years to evaluate
irrigation needs \[25\] and have been used more recently to test the
hydrologic performance of waste landfill cover designs.\[26, 27, 28,
29\] DOE and EPA conducted a series of field lysimeter experiments at
Monticello beginning in 1990 to help design and then monitor the
performance of a disposal cell cover that would rely in part on a high
soil water-storage capacity and high ET to limit infiltration and
leaching of contaminants from tailings.\[30\]

This paper presents the status of a lysimeter study of an alternative
cover design at Monticello, Utah. The purposes of this study are (1)
monitor the hydrological and ecological performance of the Monticello
design for as-built conditions in the actual cover and (2) evaluate the
general application of the design as an alternative to EPA design
guidance for arid and semiarid sites.

**STUDY AREA: CLIMATE, SOILS, AND VEGETATION**

The study area is adjacent to the tailings disposal cell 2 km south of
Monticello, Utah. Monticello is semiarid with cold, windy winters and
mild summers. The 30-yr average (1961−1990) annual precipitation is 39
cm. The average minimum January temperature is\
-10.5 °C and the average July maximum temperature is 28.9 °C. The year
can be characterized as three seasons with respect to soil-water
balance: November through March, the season of deep infiltration and
moisture accumulation in soils (average precipitation equals 16 cm);
April through June, a moisture-depletion period when plants become water
stressed (average precipitation equals 6 cm); and July through October,
a season of variable shallow moisture accumulation and depletion
resulting from monsoonal convection storms (average precipitation equals
17 cm). Annual snowfall averages 160 to 170 cm.

The soils within the footprint of the Monticello disposal cell formed in
Pleistocene loess.\[31\] The taxonomic classification is *Monticello
very fine sandy loam*.\[32\] The soil texture varies from clay loam to
sandy loam. The natural vegetation of the *Monticello very fine sandy
loam* at the disposal cell site consists primarily of Western wheatgrass
(*Pascopyrum smithii* \[Rydb.\] Á. Löve), Sandberg bluegrass (*Poa
secunda* J. Presl), blue grama (*Bouteloua gracilis* \[Kunth\] Lag.),
mountain big sagebrush (*Artemisia tridentata* Nutt. subsp*. vaseyana*
\[Rydb.\] Beetle), and rubber rabbitbrush (Ericameria nauseosa \[Pallas
ex Pursh\] Nesum & Baird) with a canopy coverage of 50% to 60%.\[32\]

**COVER DESIGN AND LYSIMETER INSTALLATION**

Installation of a cover on the Monticello disposal cell similar to the
RCRA Subtitle C design \[1\] began in fall 1998. The disposal cell has a
double liner and a leachate collection system with a design permeability
of 1 x 10^-9^ cm/s. EPA accepted a design with a geomembrane and a CSL
in the cover to ensure that water flux through the cover will not exceed
flux through the liner---at least in the short term. Because of the
uncertain durability of the geomembrane and the CSL, the design
incorporates an *alternative* cover system as the primary means for
limiting percolation for the long term.

**Alternative Cover Design**

The Monticello alternative cover design (Figure 1) is fundamentally an
ET cover with a capillary barrier.\[33\] The design relies on the
water-storage capacity of a 163-cm fine-textured soil layer (sponge)
overlying a 38-cm sand capillary barrier layer to retain precipitation
until it is seasonally removed by vegetation (solar pumps). Leakage into
the sand should occur only if water accumulation at the sponge/sand
layer interface approaches saturation and tensions decrease sufficiently
for water to enter the larger pores of the sand layer. Hydraulic
performance can be evaluated as the probability that, over time, ET is
sufficient to prevent water accumulation in the soil sponge from
exceeding the storage capacity.\[11\]

> ![](media/image1.wmf){width="4.679861111111111in"
> height="3.5965277777777778in"}Figure 1. Alternative cover design
> tested in caisson lysimeters\
> and constructed on a uranium mill tailings disposal cell at the\
> Monticello Superfund Site.

Other components of the Monticello design either facilitated
construction or were included to enhance long-term performance. A
geotextile fabric maintains the fine-grained soil/coarse sand layer
discontinuity during construction and until soil aggregation occurs by
natural pedogenic processes.\[34\] The combination of vegetation and
gravel admixture controls erosion. Vegetation and organic litter
disperse raindrop energy, shield underlying fine soils, increase
infiltration, reduce water flow and surface wind velocity, bind soil
particles, and filter sediment from runoff.\[35\] 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 \[36, 37\] and, functioning as a mulch, can
enhance seedling emergence and plant growth.\[20\]

The Monticello design includes frost protection, deterrents for
biointrusion, and other attributes for plant growth. The depth is more
than adequate to isolate the underlying RCRA components (CSL and
geomembrane) from frost damage.\[38\] The soil sponge thickness is also
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 rock layer, preventing it from
behaving like a second capillary barrier. The topsoil layer, obtained
from the root zone of the borrow area, has physical and hydraulic
properties similar to the rest of the soil sponge, but also contains
available nutrients, propagules, and microorganisms (e.g., mycorrhizae)
needed for the establishment of a sustainable plant community.

**Lysimeter Installation and Soil Materials**

We installed two large drainage lysimeters to evaluate the range of
as-built conditions in the actual Monticello alternative cover.
Lysimeter 1 closely matches the materials and compaction as built during
the latter stages of construction. Lysimeter 2 mimics less desirable
materials and compaction as built during the early stages of
construction.

Pleistocene eolian soils from the lysimeter excavation were used for the
fine-grained sponge layers in Lysimeter 1. The upper 61 cm was
stockpiled topsoil that had been separated from subsoils. Lysimeter 1
sponge soil layers were compacted to about 1.45 g/cm^3^, closely
matching the dry-weight bulk density of native *Monticello very fine
sandy loam*. Fine-grained sponge soils for Lysimeter 2, taken from the
disposal cell stockpile, consisted of a mixture of loess soils and
underlying pediment paleosols, the material used during early stages of
the cover construction. Lysimeter 2 sponge layers were compacted to
about 1.65 g/cm^3^, closely matching the bulk density achieved during
the early stages of construction.

A combination of sieve and hydrometer methods \[39\] was used for soil
particle-size analysis (Table I). The U.S. Department of Agriculture
(USDA) soil textural classes for Lysimeters 1 and 2 sponge soils are
loam and clay loam, respectively. Because sample volumes were inadequate
for accurate analysis of gravel splits, representative sampling and
analysis of all materials will be conducted at the conclusion of the
study.

Table I. Soil Particle Size^a^ and Dry-Weight Bulk Density^b^ for
Lysimeters 1 and 2.

<table style="width:100%;">
<colgroup>
<col style="width: 21%" />
<col style="width: 7%" />
<col style="width: 6%" />
<col style="width: 7%" />
<col style="width: 7%" />
<col style="width: 7%" />
<col style="width: 7%" />
<col style="width: 7%" />
<col style="width: 7%" />
<col style="width: 7%" />
<col style="width: 7%" />
<col style="width: 7%" />
</colgroup>
<tbody>
<tr class="odd">
<td><strong>Layer</strong></td>
<td><strong>Class<sup>c</sup></strong></td>
<td><p><strong>CG</strong></p>
<p><strong>(%)</strong></p></td>
<td><p><strong>FG</strong></p>
<p><strong>(%)</strong></p></td>
<td><p><strong>VCS</strong></p>
<p><strong>(%)</strong></p></td>
<td><p><strong>CS</strong></p>
<p><strong>(%)</strong></p></td>
<td><p><strong>MS</strong></p>
<p><strong>(%)</strong></p></td>
<td><p><strong>FS</strong></p>
<p><strong>(%)</strong></p></td>
<td><p><strong>VFS</strong></p>
<p><strong>(%)</strong></p></td>
<td><p><strong>Si</strong></p>
<p><strong>(%)</strong></p></td>
<td><p><strong>Cl</strong></p>
<p><strong>(%)</strong></p></td>
<td><p><strong>B.D.<sup>b</sup></strong></p>
<p><strong>(g/cm<sup>3</sup>)</strong></p></td>
</tr>
<tr class="even">
<td colspan="12"><strong>Lysimeter 1</strong></td>
</tr>
<tr class="odd">
<td>Gravel Admixture</td>
<td>GL</td>
<td>13</td>
<td>2</td>
<td>−</td>
<td>−</td>
<td>1</td>
<td>7</td>
<td>23</td>
<td>31</td>
<td>23</td>
<td>−</td>
</tr>
<tr class="even">
<td>Fine-Grain Topsoil</td>
<td>L</td>
<td>−</td>
<td>−</td>
<td>−</td>
<td>1</td>
<td>1</td>
<td>5</td>
<td>24</td>
<td>45</td>
<td>24</td>
<td>−</td>
</tr>
<tr class="odd">
<td>Fine-Grain Soil A</td>
<td>L</td>
<td>3</td>
<td>2</td>
<td>2</td>
<td>2</td>
<td>2</td>
<td>7</td>
<td>23</td>
<td>36</td>
<td>27</td>
<td>1.44</td>
</tr>
<tr class="even">
<td>Biointrusion Barrier<sup>d</sup></td>
<td>GL</td>
<td>38</td>
<td>11</td>
<td>2</td>
<td>3</td>
<td>4</td>
<td>6</td>
<td>9</td>
<td>15</td>
<td>12</td>
<td>−</td>
</tr>
<tr class="odd">
<td>Fine-Grain Soil B</td>
<td>L</td>
<td>3</td>
<td>9</td>
<td>5</td>
<td>5</td>
<td>3</td>
<td>7</td>
<td>15</td>
<td>29</td>
<td>24</td>
<td>1.45</td>
</tr>
<tr class="even">
<td>Capillary Barrier</td>
<td>S</td>
<td>−</td>
<td>37</td>
<td>9</td>
<td>15</td>
<td>21</td>
<td>12</td>
<td>3</td>
<td>2</td>
<td>1</td>
<td>−</td>
</tr>
<tr class="odd">
<td>Compacted Soil Layer</td>
<td>L</td>
<td>4</td>
<td>1</td>
<td>1</td>
<td>1</td>
<td>2</td>
<td>7</td>
<td>18</td>
<td>39</td>
<td>27</td>
<td>1.75</td>
</tr>
<tr class="even">
<td colspan="12"><strong>Lysimeter 2</strong></td>
</tr>
<tr class="odd">
<td>Gravel Admixture</td>
<td>GClL</td>
<td>13</td>
<td>2</td>
<td>3</td>
<td>4</td>
<td>4</td>
<td>6</td>
<td>15</td>
<td>29</td>
<td>24</td>
<td>−</td>
</tr>
<tr class="even">
<td>Fine-Grain Soil A<sup>e</sup></td>
<td>ClL</td>
<td>−</td>
<td>1</td>
<td>1</td>
<td>1</td>
<td>1</td>
<td>8</td>
<td>30</td>
<td>28</td>
<td>30</td>
<td>1.64</td>
</tr>
<tr class="odd">
<td>Biointrusion Barrier<sup>d</sup></td>
<td>GL</td>
<td>49</td>
<td>16</td>
<td>2</td>
<td>1</td>
<td>2</td>
<td>3</td>
<td>5</td>
<td>13</td>
<td>9</td>
<td>−</td>
</tr>
<tr class="even">
<td>Fine-Grain Soil B<sup>e</sup></td>
<td>ClL</td>
<td>−</td>
<td>10</td>
<td>3</td>
<td>4</td>
<td>5</td>
<td>6</td>
<td>15</td>
<td>31</td>
<td>26</td>
<td>1.66</td>
</tr>
<tr class="odd">
<td>Capillary Barrier</td>
<td>S</td>
<td>−</td>
<td>38</td>
<td>13</td>
<td>9</td>
<td>12</td>
<td>20</td>
<td>3</td>
<td>3</td>
<td>2</td>
<td>−</td>
</tr>
</tbody>
</table>

> ^a^USDA soil particle-size classes: CG = coarse gravel (10−70 mm); FG
> = fine gravel (2−10 mm); VCS = very coarse sand (1−2 mm); CS = coarse
> sand (0.5−1 mm); MS = medium sand (0.25−0.5 mm); FS = fine sand
> (0.1−0.25 mm); VFS = very\
> fine sand (0.05−0.1 mm); Si = silt (0.002−0.05 mm); Cl = clay (\<0.002
> mm).
>
> ^b^Dry-weight bulk density (B.D.) was determined using a nuclear
> gauge. Sample size *n* ≥ 3.
>
> ^c^USDA soil texture classes: L = loam, GL = gravely loam, ClL = clay
> loam, GClL = gravely clay loam, S = sand. Only fine-grained splits
> (VCS and finer) were used for soil textural classification.
>
> ^d^Gravel percentages in the biointrusion layer are inaccurate because
> of inadequate sample volumes.
>
> ^e^Gravel percentages in the fine-grain layers in Lysimeter 2 will be
> determined at the conclusion of the study.

Construction of the first of two caisson lysimeters (Lysimeter 1) began
in fall 1998. We excavated a pit approximately 8 m in diameter by 3 m in
depth, using a track hoe, at an existing lysimeter test facility \[30\]
adjacent to the Monticello tailings disposal cell. Eolian and pediment
soils excavated from the pit were segregated in stockpiles. A corrugated
steel culvert, 3.05 m in diameter by 2.75 m in depth, forms the walls of
Lysimeter 1. Access to instrumentation is through an adjacent caisson,
1.52 m in diameter by 3.66 m in depth. We bolted the steel culverts
(lysimeter caisson and instrument access caisson) together, lowered them
into the pit using a track hoe bucket, and aligned them vertically. We
installed Lysimeter 2 adjacent to Lysimeter 1 in fall 1999 (Figure 2).
Lysimeter 2 is 2.44 m deep. Caissons were partially backfilled to create
a compacted floor with a 20:1 slope and a drainage port at the low end.

The caisson lysimeters were lined with 40-mil high-density polyethylene
(HDPE), filled with water, covered with plastic, and leak tested using a
manometer. HDPE tubes, welded to drainage holes cut into the lower end
of the HDPE floor liner, were inserted through ports into the access
caisson. We constructed cover layers in lysimeters by marking soil lift
heights on the interior walls, hauling and dumping stockpiled materials
into the lysimeters, spreading and wetting lift materials, and then
tamping lifts to achieve soil bulk-density specifications. Bulk density
was measured with a nuclear density gauge (Troxler Inc.).
Instrumentation was installed as the cover layers were constructed.
Twenty-cm wide HDPE flaps were heat-welded to the HDPE walls, at a depth
of about 60 cm below the lips of the lysimeters, to divert any
preferential flow of water along the sidewall back into the soil mass.

**Plant Establishment**

Revegetation goals for ET covers include plant communities that (1) are
well-adapted to the engineered soil habitat, (2) are capable of high
transpiration rates, (3) limit soil erosion, and\
(4) are structurally and functionally resilient.\[40\] Seeding of
monocultures or low-diversity mixtures on engineered covers is common.
Instead, revegetation should attempt to emulate the structure, function,
diversity, and dynamics of native plant communities in the area. Diverse
mixtures of native and naturalized plants will maximize water removal
and remain more resilient given variable and unpredictable changes in
the environment resulting from pathogen and pest outbreaks, disturbances
(overgrazing, fire, etc.), and climatic fluctuations. Local indigenous
ecotypes that have been selected over thousands of years are usually
best adapted. In contrast, the exotic grass plantings common on
engineered covers are genetically and structurally rigid, are more
vulnerable to disturbance or eradication by single factors, and will
require continual maintenance. Successful establishment of a diverse and
resilient plant community requires the enlistment of practitioners
knowledgeable in the science and methods of disturbed land revegetation.

Revegetation of the lysimeters matched the specifications and methods
used for the adjacent tailings disposal cell.\[41\] We seeded the
lysimeters in September 2000 with a mixture of grasses, forbs, and
shrubs in an attempt to mimic the potential natural vegetation for the
borrow soils and local climate (Table II). Seed was hand-broadcasted,
raked, and tamped into the soil surface, and the surface was mulched
with straw. Several species were also transplanted. Plants selected for
the lysimeters are perennial, native, cool-season species except as
indicated in Table II.

![](media/image2.jpeg){width="4.653472222222222in" height="3.00625in"}

![](media/image3.jpeg){width="5.061111111111111in"
height="4.346527777777778in"}

Figure 2. Plan view (top) and cross section (bottom) of instrumentation
in the lysimeter and access caissons. Water Content Reflectometers,
thermocouples, and root observations tubes, shown separately for
purposes of illustrating layouts and depths, were all installed in both
lysimeters.

**PERFORMANCE MONITORING METHODS AND RESULTS**

Evaluating the performance of alternative covers depends on careful
analysis of climate, soil hydrology, and plant ecology. Lysimeters
enable us to evaluate performance of the cover as a system---an
integrated whole---over diurnal, seasonal, and yearly time scales. Our
monitoring instrumentation and methods focused on the components of the
soil-water balance (precipitation, changes in water storage, drainage,
and evapotranspiration) and on plant community composition and relative
abundance. Monitoring began during the 1999 growing season for Lysimeter
1 and a year later for Lysimeter 2. This section presents the
meteorological, soil-water balance, and vegetation monitoring methods
and results as of October 2003.

Table II. Species Planted on the Lysimeters.

+----------------------------------+-----------------------+-----------+
| # Scientific Name                | # Common Name         | # PL      |
|                                  |                       | S/Acre^a^ |
+----------------------------------+-----------------------+-----------+
| ## **Shrubs**                    |                       |           |
+----------------------------------+-----------------------+-----------+
| *Ericameria* *nauseosa*          | Rubber rabbitbrush    | 1.5       |
+----------------------------------+-----------------------+-----------+
| *Purshia tridentata*             | Antelope bitterbrush  | 1.0       |
+----------------------------------+-----------------------+-----------+
| *Artemisia tridentate var.       | Mountain big          | 0.5       |
| tridentata*                      | sagebrush^b^          |           |
+----------------------------------+-----------------------+-----------+
| *Artemisia tridentata var        | Basin big sagebrush   | 0.1       |
| vaseyana*                        |                       |           |
+----------------------------------+-----------------------+-----------+
| *Artemisia tridentate var.       | Wyoming big sagebrush | 0.05      |
| wyomingensis*                    |                       |           |
+----------------------------------+-----------------------+-----------+
| ## **Forbs**                     |                       |           |
+----------------------------------+-----------------------+-----------+
| *Linum lewisii*                  | Blue flax^b^          | 2.0       |
+----------------------------------+-----------------------+-----------+
| *Astragalus cicer*               | Cicer milkvetch^c^    | 1.6       |
+----------------------------------+-----------------------+-----------+
| *Sphaeralcea coccinea*           | Scarlet               | 0.5       |
|                                  | globemallow^d^        |           |
+----------------------------------+-----------------------+-----------+
| *Sphaeralcea grossulariifolia*   | Gooseberry            | 0.5       |
|                                  | globemallow^d^        |           |
+----------------------------------+-----------------------+-----------+
| *Erigeron speciosus*             | Aspen daisy           | 0.15      |
+----------------------------------+-----------------------+-----------+
| *Achillea millefolium var.       | Common yarrow         | 0.12      |
| lanulosa*                        |                       |           |
+----------------------------------+-----------------------+-----------+
| *Aster tanacetifolius*           | Prairie aster^e^      | 0.05      |
+----------------------------------+-----------------------+-----------+
| ## **Grasses**                   |                       |           |
+----------------------------------+-----------------------+-----------+
| *Bromus carinatus*               | Mountain brome^b^     | 4.0       |
+----------------------------------+-----------------------+-----------+
| *Elymus lanceolatus*             | Thickspike wheatgrass | 3.0       |
+----------------------------------+-----------------------+-----------+
| *Pascapyrum smithii*             | Western wheatgrass^b^ | 3.0       |
+----------------------------------+-----------------------+-----------+
| *Stipa comata*                   | Needle and thread     | 2.0       |
|                                  | grass                 |           |
+----------------------------------+-----------------------+-----------+
| *Stipa hymenoides*               | Indian ricegrass      | 2.0       |
+----------------------------------+-----------------------+-----------+
| *Bouteloua gracilis*             | Blue grama^d^         | 1.0       |
+----------------------------------+-----------------------+-----------+
| *Hilaria jamesii*                | Galleta grass^d^      | 1.0       |
+----------------------------------+-----------------------+-----------+

> ^a^PLS/acre = pure live seed per acre.
>
> ^b^Plants seeded and transplanted onto small lysimeters.
>
> ^c^Annual or biennial.
>
> ^d^Warm season (C^4^) species.
>
> ^e^Not native.

**Soil-Water Balance**

The primary objective of the caisson lysimeter study is to evaluate the
soil-water balance of the as-built cover. The overall soil-water balance
of a cover would include inputs of precipitation (*P*) and run-on (*Ri*)
and outputs of evapotranspiration (*ET*), drainage past plant roots in
the soil profile (*D*), and runoff (*Ro*). Soil-water storage changes
(Δ*S*) can be expressed as

Δ*S* = *P* + *Ri* − *ET* − *D* − *Ro.* (Eq. 1)

The caisson lysimeter soil surfaces are isolated from *Ri* and *Ro*,
thus *ET* can be estimated using a simplified water balance equation

*ET* = *P* − *D* − Δ*S,* (Eq. 2)

where *ET*, *P,* and Δ*S* are recorded as linear units (mm or cm) of
water.

Direct measurement of all water-balance terms was not possible. For the
Monticello study, precipitation, drainage, and water storage changes are
monitored, and actual ET is estimated by difference. Potential ET can be
estimated by calculation of the energy budget (Penman-Montieth equation)
using the field parameters of wind speed, relative humidity, solar
radiation, and air temperature.

##### Meteorological Conditions. A weather station was installed to monitor meteorological conditions in July 2000. The meteorological station consists of a 3-m-high tripod anchored in concrete with supports for sensors and enclosures. The weather station was configured to provide data for the following parameters:

+-------------------+--------------------------------------------------+
| **[Paramet        | ## **Sensor**                                    |
| er]{.underline}** |                                                  |
+-------------------+--------------------------------------------------+
| Wind speed        | Cup anemometer                                   |
+-------------------+--------------------------------------------------+
| Wind direction    | Vanes that use precision potentiometer           |
+-------------------+--------------------------------------------------+
| Solar radiation   | Thermopile pyranometer                           |
+-------------------+--------------------------------------------------+
| Air temperature   | Platinum resistance temperature detector         |
+-------------------+--------------------------------------------------+
| Relative humidity | Capacity sensors that use integral signal        |
|                   | conditioning                                     |
+-------------------+--------------------------------------------------+
| Precipitation     | Tipping-bucket rain gauge                        |
+-------------------+--------------------------------------------------+

The weather station uses a programmable data logger (CR23X, Campbell
Scientific, Inc., Logan, Utah) that records sensor measurements and
stores data. Sensory measurements are stored hourly and daily.
Conditional outputs, such as rainfall intensity, are also processed and
stored. The data logger is wired to alternating current and data are
downloaded via phone modem.

Total annual precipitation has been less than the 30-yr average (39 cm)
since the lysimeters were planted in 2000 (Table III). The 2002 growing
season was particularly dry, with winter and spring precipitation about
50% and 15% of normal, respectively. Precipitation was only 57% of
normal between November 2000 and June 2002, the critical period for
plant establishment.

##### Table III. Annual and Seasonal Precipitation (cm) at Monticello.

  ------------- ------------ ----------------------- ------------------- ---------------------
  **Year^a^**   **Annual**   **November−March^b^**   **April−June^b^**   **July−October^b^**

  Average       39.0         16.0                    6.0                 17.0

  2000                                                                   17.4

  2001          25.7         10.6                    5.7                 9.4

  2002          22.9         8.0                     0.9                 14.0

  2003                       21.0                    11.0                
  ------------- ------------ ----------------------- ------------------- ---------------------

^a^Annual precipitation calculated for November through October of the
current year.

> ^b^Yearly precipitation characterized as three seasons with respect to
> soil-water balance: November through March is the season of deep
> infiltration and moisture accumulation in soils; April through June is
> a moisture-depletion period when plants become water stressed; and
> July through October is a season of variable shallow moisture
> accumulation and depletion resulting from monsoonal convection storms.

##### 

##### Soil-Water Storage and Drainage Measurements. Soil moisture and water storage are monitored with CS-615 water content reflectometers (WCRs) manufactured by Campbell Scientific, Inc. (Logan, Utah). WCRs consist of two, 30-cm-long parallel rods attached to an electronic signal generator. A pulsed wavelength traveling down a coax, or waveguide, is influenced by the type of material surrounding the conductors. If the dielectric constant of the material is high, the signal propagates slower. Because the dielectric constant of water is much higher than most other materials, a signal within a wet medium propagates slower than in the same medium when dry. The reflectometer measures the effective dielectric as a pulse transit time, which in turn is calibrated against water content. Changes in soil moisture are determined by reading the WCR probes hourly.

In each lysimeter, 18 WCRs monitor water content within the cover layers
with three replicate measurements at six depths:

  --------------------- ---------------------------------------------------
  **[Depth              **[Material]{.underline}**
  (cm)]{.underline}**   

  10                    Gravel admixture layer

  45                    Topsoil layer of soil sponge

  90                    Fine-grain soil sponge layer

  120                   Animal intrusion barrier layer

  160                   Soil sponge layer just above capillary interface

  200                   Capillary barrier layer, near the bottom
  --------------------- ---------------------------------------------------

Additional WCRs monitor water content in Lysimeter 1 with three
replicate measurements at a depth of 240 cm in the compacted soil layer
(radon barrier) and three replicate measurements at a depth of 290 cm in
native soil below the engineered cover.

Because probe-to-probe variability is insignificant \[42\], we
calibrated WCRs after the lysimeters were filled. WCR calibrations were
determined for all soil materials using 30-cm-diameter by 80-cm-deep
polyvinyl chloride (PVC) calibration cells. Soil materials were
compacted in 10-cm lifts to match the dry-weight bulk density attained
in the lysimeters. Calibrations for each material were obtained for soil
moisture contents ranging between air dry and saturation, from
approximately 8% to 40% by volume. Calibrations were repeated for three
different WCRs to check probe-to-probe variability. WCRs were inserted
vertically near the center of a filled PVC cell until the rods were
completely covered with soil, five readings were taken for each
soil/moisture combination with the probe embedded in a different
location each time, and then five volumetric samples were collected
spanning the depth of the WCR rods.

Water-content reflectometers, thermocouples, and root observations tubes
were placed in shallow trenches cut into the soil lifts. Root
observation tubes consist of 5-cm-diameter by 3-m-long clear Lexan
cylinders placed horizontally from sealed ports in the access caisson
through the center of the lysimeter caisson. Table IV presents layer
materials and depths, WCR and thermocouple placement depths, mean WCR
calibration coefficients, and depths of root observation tubes.
Drainage, soil water content, and soil temperature data are monitored
hourly, stored in the microprocessor on site, and downloaded
periodically via phone modem.

Table IV. Cover Layers, WCR Placement Depths and Calibration, and Root
Tube Depths.

+------------+--------+--------+--------+-------+-------+-----------+
| **Layer\   | **     | **WCR\ | **WCR  |       |       | **Root    |
| Material** | Layer\ | Depth  | Calib  |       |       | Tube\     |
|            | Depth  | (cm)** | ration |       |       | Depth     |
|            | (cm)** |        | Co     |       |       | (cm)**    |
|            |        |        | effici |       |       |           |
|            |        |        | ents** |       |       |           |
+------------+--------+--------+--------+-------+-------+-----------+
|            |        |        | **C0** | *     | *     |           |
|            |        |        |        | *C1** | *C2** |           |
+------------+--------+--------+--------+-------+-------+-----------+
| ####### L  |        |        |        |       |       |           |
| ysimeter 1 |        |        |        |       |       |           |
+------------+--------+--------+--------+-------+-------+-----------+
| Gravel     | 0−20   | 10     | -      | -0    | 0     |           |
| Admix      |        |        | 0.1001 | .0069 | .1958 |           |
+------------+--------+--------+--------+-------+-------+-----------+
| Topsoil    | 20−61  | 46     | -      | -0    | 0     | 30        |
|            |        |        | 0.1001 | .0069 | .1958 |           |
+------------+--------+--------+--------+-------+-------+-----------+
| Sponge     | 61−102 | 90     | -      | 0     | -0    | 76        |
| Soil       |        |        | 0.4544 | .6998 | .1272 |           |
+------------+--------+--------+--------+-------+-------+-----------+
| Biobarrier | 1      | 120    | -      | 0     | 0     |           |
|            | 02−133 |        | 0.2166 | .2362 | .0815 |           |
+------------+--------+--------+--------+-------+-------+-----------+
| Sponge     | 1      | 160    | -      | 0     | -0    | 147       |
| Soil       | 33−163 |        | 0.4544 | .6998 | .1272 |           |
+------------+--------+--------+--------+-------+-------+-----------+
| Sand       | 1      | 200    | 0.4161 | -1    | 0     | 173       |
|            | 63−201 |        |        | .2964 | .9926 |           |
+------------+--------+--------+--------+-------+-------+-----------+
| Compacted  | 2      | 240    | -      | 0     | -0    |           |
| Soil       | 01−274 |        | 0.4544 | .6998 | .1272 |           |
+------------+--------+--------+--------+-------+-------+-----------+
| Native     | 274+   | 290    | -      | 0     | -0    |           |
| Soil       |        |        | 0.4544 | .6998 | .1272 |           |
+------------+--------+--------+--------+-------+-------+-----------+
| ####### L  |        |        |        |       |       |           |
| ysimeter 2 |        |        |        |       |       |           |
+------------+--------+--------+--------+-------+-------+-----------+
| Gravel     | 0−20   | 10     | -      | 0     | -0    |           |
| Admix      |        |        | 0.4544 | .6998 | .1272 |           |
+------------+--------+--------+--------+-------+-------+-----------+
| Sponge     | 20−61  | 45     | -      | 0     | -0    | 30        |
| Soil       |        |        | 0.4544 | .6998 | .1272 |           |
+------------+--------+--------+--------+-------+-------+-----------+
| Sponge     | 61−102 | 90     | -      | 0     | -0    | 76        |
| Soil       |        |        | 0.4544 | .6998 | .1272 |           |
+------------+--------+--------+--------+-------+-------+-----------+
| Biobarrier | 1      | 120    | -      | 0     | 0     |           |
|            | 02−133 |        | 0.2166 | .2362 | .0815 |           |
+------------+--------+--------+--------+-------+-------+-----------+
| Sponge     | 1      | 160    | -      | 0     | -0    | 147       |
| Soil       | 33−163 |        | 0.4544 | .6998 | .1272 |           |
+------------+--------+--------+--------+-------+-------+-----------+
| Sand       | 1      | 200    | 0.4161 | -1    | 0     | 173       |
|            | 63−201 |        |        | .2964 | .9926 |           |
+------------+--------+--------+--------+-------+-------+-----------+

***Soil Water Responses.*** Time series of drainage and water storage
changes show conspicuous seasonal variability and an overall drying
trend (Figure 3). Drainage did not exceed 0.1 mm/yr, well below the EPA
target of \<3.0 mm/yr. The only drainage occurred in spring 2000. The
lysimeters were not planted until 2000 to allow water storage to build
to the maximum limit for each soil type. No measurable drainage occurred
during the dry years while vegetation was maturing.

Time series of soil-water storage are displayed as millimeters of water
in Figure 3. In both lysimeters, seasonal high and low water storage
occurred in mid-to-late spring and mid-to-late fall, respectively,
depending on the amount and seasonality of precipitation, the soil type
and

> ![](media/image4.wmf){width="5.620138888888889in"
> height="4.277083333333334in"}Figure 3. Soil-water storage time series
> in Lysimeter 1 (less compacted loam\
> soil sponge) and Lysimeter 2 (more compacted clay loam soil sponge)
> between\
> July 1999 and August 2003.

compaction, and the maturity of vegetation. The maximum storage in both
lysimeters occurred in spring 2000 before plants became established.
Because drainage also occurred at that time, water storage may have
reached the maximum limit for each soil type: about 440 mm in Lysimeter
1 and 400 mm in Lysimeter 2. The lower maximum storage limit for
Lysimeter 2 as compared with Lysimeter 1 may be attributable to a lower
porosity in the more compacted clay loam than in the less compacted
loam. Once vegetation established during the dry years, the seasonal
peak storage did not approach the limit in either lysimeter and no
drainage occurred. The peak in spring 2002 was virtually nonexistent
because precipitation was virtually nonexistent.

Seasonal low water-storage levels also differed between the two
lysimeters. The difference is most likely attributable to plant water
extraction (evapotranspiration). During the 2000 growing season, before
plants established, the seasonal low remained at about 280 mm; only
about a\
5-mm difference was observed between lysimeters. After plants became
established, water storage in the less compacted loam (Lysimeter 1)
dropped below 200 mm, about 30 mm below water storage in the compacted
clay loam (Lysimeter 2). The water storage capacity of a soil layer can
be calculated as the difference between the maximum storage limit and
the lowest measured water storage level after the plant water potential
reaches the wilting point. If this definition is used, the water storage
capacity for the less compacted loam soil in Lysimeter 1 (about 250 mm)
is more than 40% greater than the more compacted clay loam in Lysimeter
2 (about 175 mm). Soil type and layer construction practices can have
significant effects on the performance of alternative covers.

**Vegetation Composition and Abundance**

The water storage data in Figure 3 show that the hydrologic performance
of the Monticello cover relies, in part, on the establishment and
resilience of a diverse plant community. Plant canopy structure plays a
fundamental role in processes involving the interaction of plant
communities and their environment, such as ET \[43\] and biomass
productivity.\[44\] Plant canopy structure can be thought of as the
amount and organization of aboveground plant material. Canopy structure
may include the size, shape, orientation, and distribution of various
plant parts such as leaves, stems, branches, flowers, and fruits. The
amount of leaf and stem material in a canopy can be represented by
measurements of leaf area index (LAI) and productivity. LAI, the leaf
area per unit of ground area, is one of the more useful measures of
plant canopy structure because it is also an index of the relative
transpiration potential of the canopy. Productivity is the rate of
change in biomass per unit time, such as from one year to the next.
Percent cover, a more common measure of plant abundance, is the
percentage of a unit area (1 m^2^, for example) beneath the canopy of a
given species.

***Methods.*** Plant species composition and abundance were measured on
the caisson lysimeters near the end of the growing seasons in 2002 and
in 2003. Species composition and percent cover were measured over the
entire 7.3-m^2^ lysimeter surface. The lysimeter surface was divided
into 50- by 100-cm quadrats delineated with string. A quadrat is an area
of ground surface delimited for plant measurement. All plant species in
each quadrat were recorded. We used an ocular point-intercept sampling
method \[45\] to measure percent cover in each quadrat.

LAI and productivity of green vegetation (current year's growth) were
sampled in half of the quadrats by harvesting green leaf material and
running the leaves through a Licor, Inc. LI-3100 Area Meter
([www.licor.com](http://www.licor.com/)). Green leaf material was
harvested by hand or cut with shears, placed in paper bags, and
processed soon after returning to the laboratory. *Artemisia tridentata*
green leaves were not removed because defoliation can stress or kill the
plant. Accuracy of the LI-3100 Area Meter was assessed with a 10-cm^2^
calibration disk periodically passed through the meter. Green LAI was
computed by dividing the total green leaf area in a quadrat by the
quadrat area. Green leaf material was then air dried in paper bags and
weighed to estimate productivity. We considered that samples were dry if
we measured no change after repeated weighings for\
several days.

**Species Composition, Cover, LAI, and Productivity**. Species growing
on the lysimeters were grouped based on revegetation acceptance criteria
(Table V). Total percent cover for all plants growing in lysimeters,
when averaged over years and lysimeters (37.1%, S.E.=0.6%, n = 4), is
close to the minimum 40% cover criterion.\[41\] However, as much as
20.6% cover or 56% relative cover consisted of species either not listed
as a permissible species or listed as noxious and non-noxious weeds.
Only about 16.5% of the cover consisted of permissible species, well
below the requirement. Western wheatgrass (*Pascopyrum smithii*), the
dominant native grass growing on Monticello very fine sandy loam, was
also the most abundant species on the lysimeters. Six native species not
listed as permissible in the acceptance criteria

Table V. Plant Species Composition and Percent Cover.

+----------------------------+---------------------------+-------------+
| **Scientific Name^a^**     | #### Common Name^a^       | **% Cover** |
+----------------------------+---------------------------+-------------+
| **Permissible Species^b^** |                           | **16.5**    |
+----------------------------+---------------------------+-------------+
| *                          |                           | [15.1]{     |
| **[Grasses]{.underline}*** |                           | .underline} |
+----------------------------+---------------------------+-------------+
| *Bromus inermis*           | Smooth brome              | 2.2         |
+----------------------------+---------------------------+-------------+
| *Pascopyrum smithii*       | Western wheatgrass        | 10.0        |
+----------------------------+---------------------------+-------------+
| *Thinopyrum intermedium*   | Intermediate wheatgrass   | 2.9         |
+----------------------------+---------------------------+-------------+
| ***[Forbs]{.underline}***  |                           | [1.4]{      |
|                            |                           | .underline} |
+----------------------------+---------------------------+-------------+
| *Astragalus spp*           | Milk vetch                | 0.4         |
+----------------------------+---------------------------+-------------+
| *Sphaeralcea spp*          | Globemallow               | \*          |
+----------------------------+---------------------------+-------------+
| ***[Shrubs]{.underline}*** |                           | 1.0         |
+----------------------------+---------------------------+-------------+
| *Artemisia tridentata*     | Big sagebrush             | 0.7         |
+----------------------------+---------------------------+-------------+
| *Ericameria* *nauseosa*    | Rubber rabbitbrush        | 0.3         |
+----------------------------+---------------------------+-------------+
| **Non-Noxious Weed         |                           | **0.5**     |
| Species**^b^               |                           |             |
+----------------------------+---------------------------+-------------+
| *Kochia scoparia*          | Mexican fireweed          | \*          |
+----------------------------+---------------------------+-------------+
| *Salsola kali*             | Russian thistle           | 0.5         |
+----------------------------+---------------------------+-------------+
| **Not Listed as            |                           | **20.1**    |
| Permissible or Not         |                           |             |
| Permissible^b^**           |                           |             |
+----------------------------+---------------------------+-------------+
| *                          |                           | [16.0]{     |
| **[Grasses]{.underline}*** |                           | .underline} |
+----------------------------+---------------------------+-------------+
| [*Achnatherum              | Indian ricegrass          | 0.3         |
| hymenoides*](http://pl     |                           |             |
| ants.usda.gov/cgi_bin/plan |                           |             |
| t_profile.cgi?symbol=ACHY) |                           |             |
+----------------------------+---------------------------+-------------+
| *Agropyron cristatum*      | Crested wheatgrass        | 1.1         |
+----------------------------+---------------------------+-------------+
| *Bromus tectorum*          | Cheatgrass                | 0.5         |
+----------------------------+---------------------------+-------------+
| *Elymus lanceolatus*       | Streambank wheatgrass     | 0.7         |
+----------------------------+---------------------------+-------------+
| *Elymus trachycaulus*      | Slender wheatgrass        | 2.8         |
+----------------------------+---------------------------+-------------+
| *Pseudoroegneria spicata*  | Bluebunch wheatgrass      | 3.1         |
+----------------------------+---------------------------+-------------+
| *Hesperostipa* *comata*    | Needle and thread         | 0.3         |
+----------------------------+---------------------------+-------------+
| Unidentifiable perennial   |                           | 7.2         |
| grasses                    |                           |             |
+----------------------------+---------------------------+-------------+
| ***[Forbs]{.underline}***  |                           | [4.1]{      |
|                            |                           | .underline} |
+----------------------------+---------------------------+-------------+
| *Achillea millefolium*     | Common yarrow             | \*          |
+----------------------------+---------------------------+-------------+
| *Amaranthus blitoides*     | Mat amaranth              | \*          |
+----------------------------+---------------------------+-------------+
| *Chenopodium album*        | Lambsquarters             | \*          |
+----------------------------+---------------------------+-------------+
| *Linum perenne*            | Blue flax                 | 2.6         |
+----------------------------+---------------------------+-------------+
| *Medicago sativa*          | Alfalfa                   | 1.5         |
+----------------------------+---------------------------+-------------+
| *Taraxacum* *officinale*   | Common dandelion          | \*          |
+----------------------------+---------------------------+-------------+
| # Ground Surface           |                           | **68.3**    |
+----------------------------+---------------------------+-------------+
| #  Soil                    |                           | 30.8        |
+----------------------------+---------------------------+-------------+
| #  Rock                    |                           | 10.4        |
+----------------------------+---------------------------+-------------+
| #  Litter                  |                           | 27.1        |
+----------------------------+---------------------------+-------------+

> ^a^Scientific and common names are consistent with the USDA Plants
> National\
> Database (http://plants.usda.gov).
>
> ^b^Plant categories are from revegetation acceptance criteria for the
> Monticello cover.\[41\]

###### (Achnatherum hymenoides, Elymus lanceolatus, Elymus trachycaulus, Elymus trachycaulus, Pseudoroegneria spicata, Hesperostipa comata, and Achillea millefolium) should be included as acceptable. The total cover of all native and permissible species, at least 23.7%, is better than expected considering that it occurred during the driest 3 consecutive years on record for Monticello.

Total plant cover remained consistent between lysimeters and years
(Figure 4). Green LAI, a better measure of the transpiration potential
than percent cover, was significantly greater in 2002 on the less
compacted loam (Lysimeter 1) than on the overly compacted clay loam
(Lysimeter 2). Greater transpiration loss may partially explain the
seasonally lower water storage values and consistently greater water
storage capacity of the less compacted loam. As an apparent anomaly,
productivity was highest on Lysimeter 2 in 2003, possibly attributable
to the combination of a wet late summer, different species composition,
and a later sampling date in 2003. Much of the high 2003 biomass on
Lysimeter 2 is thick-stemmed alfalfa (*Medicago sativa*) that re-greened
following late summer rains.

> ![](media/image5.wmf){width="5.442361111111111in"
> height="3.4479166666666665in"}Figure 4. Percent cover, green LAI, and
> annual dry-weight biomass (productivity) comparing Lysimeters 1 and 2
> in 2002 and 2003. Within-parameter bars with the\
> same letter are not significantly different (*P* \<0.05).

**SUMMARY**

Two large drainage lysimeters fabricated of steel caissons lined with
HDPE were installed to evaluate the hydrological and ecological
performance of an alternative engineered cover constructed in 2000 on a
uranium mill tailings disposal cell at the Monticello, Utah, Superfund
Site. The hydrological performance of the Monticello design relies on
the water storage capacity of a 163-cm soil "sponge" layer overlying a
capillary barrier of fine gravel and sand, to retain precipitation while
plants are dormant, and on native vegetation to seasonally remove
precipitation during the growing season. The sponge layer consists of a
clay loam subsoil compacted to 1.65 g/cm^2^ in one lysimeter and a loam
topsoil compacted to 1.45 g/cm^2^ in the other lysimeter, the range of
conditions existing in the nearby disposal cell cover.

About 0.1 mm of drainage occurred in both lysimeters before they were
planted during an average precipitation year, an amount well below the
EPA target of \<3.0 mm/yr. However, the cover with the less compacted
loam sponge had a 40% greater water storage capacity than the cover with
the overly compacted clay loam sponge. The difference is attributable in
part to higher green leaf area and water extraction by plants in the
loam topsoil. The lesson learned is that seemingly subtle differences in
soil types, sources, and compaction can result in salient differences in
performance, especially during wet years. Diverse, seeded communities of
predominantly native perennial species were established on both
lysimeters during an extended 3-yr drought, highlighting the importance
of a sound understanding of the local ecology and of implementing the
science and methods of disturbed-land revegetation.

**REFERENCES**

1.  U.S. Environmental Protection Agency, "Technical Guidance Document,
    Final Covers\
    on Hazardous Waste Landfills and Surface Impoundments,"
    EPA/530--SW--89--047, Washington, DC (1989).

2.  U.S. Department of Energy, "Technical Approach Document," Revision
    II, UMTRA−DOE/\
    AL 050425.0002, Uranium Mill Tailings Remedial Action Program,
    Albuquerque,\
    New Mexico (1989).

3.  W. J. WAUGH, G. M. SMITH, D. BERGMAN-TABBERT, and D. R. METZLER,
    "Evolution of Cover Systems for the Uranium Mill Tailings Remedial
    Action Project, USA," Mine Water and the Environment, 20:190−197
    (2001).

4.  J. H. CLARKE, M. M. MACDONELL, E. D. SMITH, R. J. DUNN, and W. J.
    WAUGH. "Engineered Containment and Control Systems: Nurturing
    Nature," Risk Analysis (in press).

5.  C. H. BENSON, D. E. DANIEL, and G. P. BOUTWELL, "Field Performance
    of Compacted Clay Liners," Journal of Geotechnical and
    Geoenvironmental Engineering, 125:390 (1999).

6.  U.S. Department of Energy, "Vegetation Growth Patterns on Six
    Rock-Covered UMTRA Project Disposal Cells," UMTRA--DOE/AL
    400677.0000, Uranium Mill Tailings Remedial Action Project,
    Albuquerque, New Mexico (1992).

7.  G. W. SUTER, II, R. J. LUXMOORE, and E. D. SMITH, "Compacted Soil
    Barriers at\
    Abandoned Landfill Sites are Likely to Fail in the Long Term,"
    Journal of Environmental Quality, pp. 217−226 (1993).

8.  W. J. WAUGH, "Ecology of Uranium Mill Tailings Covers," pp. 199−212,
    In: T. Reynolds and R. Morris (editors), Landfill Capping in the
    Semi-Arid West: Problems, Perspectives, and Solutions, ESRF--019,
    Environmental Science and Research Foundation, Idaho Falls, Idaho
    (1997).

9.  A. G. BOWERMAN, and E. F. REDENTE, "Biointrusion of Protective
    Barriers as Hazardous Waste Sites," Journal of Environmental
    Quality, 27:625−632 (1998).

10. W. J. WAUGH, S. J. MORRISON, G. M. SMITH, M. KAUTSKY, T. R.
    BARTLETT,\
    C. E. CARPENTER, and C. A. JONES, "Plant Encroachment on the
    Burrell, Pennsylvania, Disposal Cell: Evaluation of Long-Term
    Performance and Risk," GJO--99--96--TAR, Environmental Sciences
    Laboratory, U.S. Department of Energy, Grand Junction,\
    Colorado (1999).

11. W. J. WAUGH, and G. N. RICHARDSON, "Ecology, Design, and Long-Term
    Performance of Surface Barriers: Applications at a Uranium Mill
    Tailings Site," pp. 36−49, In: Barrier Technologies for
    Environmental Management, National Research Council, National
    Academy Press, Washington, DC (1997).

12. I. J. WINOGRAD, "Radioactive Waste Storage in Thick Unsaturated
    Zones," Science, 212:1457−1464 (1981).

13. I. J. WINOGRAD, "Archaeology and Public Perception of a
    Transscientific Problem Disposal of Toxic Waste in the Unsaturated
    Zone," U.S. Geological Survey Circulation 990, U.S. Geological
    Survey, Denver, Colorado (1986).

14. E. H. ROSEBOOM, "Disposal of High-Level Radioactive Waste Above the
    Water Table\
    in Arid Regions," U.S. Geological Survey Circulation 903, U.S.
    Geological Survey, Washington, DC (1983).

15. C. C. REITH, and B. M. THOMPSON (eds.), "Deserts as Dumps? The
    Disposal of Hazardous Materials in Arid Ecosystems," University of
    New Mexico Press, Albuquerque, New Mexico (1992).

16. G. W. GEE, and S. W. TYLER (eds.), "Symposium: Recharge in Arid and
    Semiarid Regions," Soil Science Society of America Journal, 58:5−72
    (1994)

17. W. J. WAUGH, M. R. ADAMS, and R. C. ROUTSON, "Demonstration of a
    Design for Shallow-Land Burial of Low-Level Radioactive Waste," In:
    Proceedings of the 1983 Meetings of the Soil Science Society of
    America, Washington, DC, American Society of Agronomy, Madison,
    Wisconsin (1983).

18. J. E. ANDERSON, R. S. NOWAK, T. D. RATZLAFF, and O. D. MARKHAM,
    "Managing\
    Soil Moisture on Waste Burial Sites in Arid Regions," Journal of
    Environmental Quality,\
    22:62−69 (1993).

19. S. O. LINK, W. J. WAUGH, and J. L. DOWNS, "The Role of Plants on
    Isolation\
    Barrier Systems," In: Proceedings of the 33rd Hanford Symposium on
    Health and the Environment---In-Situ Remediation: Scientific Basis
    for Current and Future Technologies, Battelle Press, Columbus, Ohio
    (1994).

20. W. J. WAUGH, M. E. THIEDE, D. J. BATES, L. L. CADWELL, G. W. GEE,
    and\
    C. J. KEMP, "Plant Cover and Water Balance in Gravel Admixtures at
    an Arid Waste\
    Burial Site," Journal of Environmental Quality, 23:676−685 (1994).

21. A. WARD and G. GEE, "Performance Evaluation of a Field-Scale Surface
    Barrier," Journal of Environmental Quality 26:694−705 (1997).

22. J. STORMONT and C. ANDERSON, "Method to Estimate Water Storage
    Capacity of Capillary Barriers," Journal of Geotechnical and
    Geoenvironmental Engineering 124:297−302 (1998).

23. M. V. KHIRE, C. H. BENSON, and P. J. BOSSCHER, "Capillary Barriers:
    Design\
    Variables and Water Balance," Journal of Geotechnical and
    Geoenvironmental Engineering 126:695−708 (2000).

24. G. W. GEE, and D. HILLEL, "Groundwater Recharge in Arid Regions:
    Review and Critique of Estimation Methods," Hydrological Processes,
    2:255−266 (1988).

25. H. KOHNKE, F. R. DRIEBELBIS, and J. M. DAVIDSON, "A Survey and
    Discussion of\
    Lysimeters and a Bibliography on Their Construction and Use,"
    Miscellaneous Publication No. 372, U.S. Department of Agriculture,
    Washington, DC (1940).

26. J. W. NYHAN, T. E. HAKONSON, and B. J. DRENNON, "A Water Balance
    Study of\
    Two Landfill Cover Designs for Semiarid Regions," Journal of
    Environmental Quality, 19:281−288 (1990).

27. M. R. SACHSCHEWSKY, C. J. KEMP, S. O. LINK, and W. J. WAUGH, "Soil
    Water Balance Changes in Engineered Soil Surfaces," Journal of
    Environmental Quality, 24:352−359 (1995).

28. A. L. WARD, and G. W. GEE, "Performance Evaluation of a Field-Scale
    Surface Barrier," Journal of Environmental Quality, 26:694−705
    (1997).

29. A. C. Roesler, C. H. Benson, and W. H. Albright, "Field Hydrology
    and Model\
    Predictions for Final Covers in the Alternative Cover Assessment
    Program -- 2002,"\
    Geo Engineering Report No. 02−08, Geo Engineering Program,
    University of Wisconsin-Madison, Madison, Wisconsin (2002).

30. W. J. Waugh, "Monticello Field Lysimetry: Design and Monitoring of
    an Alternative Cover," Proceedings of the Waste Management 2002
    Symposium, University of Arizona, Tucson, Arizona (2002).

31. C. B. HUNT, "Cenozoic Geology of the Colorado Plateau," U.S.
    Geological Survey Professional Paper 279 (1956).

32. U.S. Department of Agriculture, "Soil Survey: San Juan Area, Utah,"
    Series 1945, No. 3, Soil Conservation Service, Washington, DC.

33. U.S. Environmental Protection Agency, "Evapotranspiration Landfill
    Cover System\
    Fact Sheet," EPA 542--F--03--015, Solid Waste and Emergency Response
    (5102G), Washington, DC (2003).

34. B. N. Bjornstad and S. S. Teel, "Natural Analog Study of Engineered
    Protective Barriers at the Hanford Site," PNL--8840, Pacific
    Northwest National Laboratory, Richland, Washington (1993).

35. W. H. WISCHMEIER and D. SMITH, "Predicting Rainfall Erosion
    Losses---A Guide to Conservation Planning," Agricultural Handbook
    No. 537. U.S. Department of Agriculture, Washington, DC (1978).

36. M. W. LIGOTKE, "Control of Eolian Soil Erosion From Waste-site
    Surface Barriers,"\
    pp. 545--559 in G.W. Gee and N.R. Wing, (editors), "In-Situ
    Remediation: Scientific Basis for Current and Future Technologies,"
    Battelle Press, Richland, Washington (1994).

37. J. B. FINELY, M. D. HARVEY, and C. C. WATSON, "Experimental Study:
    Erosion of Overburden Cap Material Protected by Rock Mulch," pp.
    273--282 In: Proceedings of Seventh Symposium on Management of
    Uranium Mill Tailings, Low-Level Waste, and Hazardous Waste,
    Colorado State University, Ft. Collins, Colorado (1985).

38. G. M. SMITH and R. E. REGER, "Protective Layer Design in Landfill
    Covers Based\
    on Frost Protection," Journal of Geotechnical and Geoenvironmental
    Engineering 128:794−799 (2002).

39. G. W. GEE and D. OR, "Particle-Size Analysis," pp. 255−293 In: J. H.
    Dane and G. C. Topp (editors), Methods of Soil Analysis, Part 4,
    Physical Methods, Book Series Number 5, Soil Science Society of
    America, Madison, Wisconsin (2002).

40. W. J. WAUGH, G. VANREYPER, and S. L. VANCE, "Characterization of
    Vegetation Within the 3-Hectare ACAP Lysimeter at the Monticello,
    Utah, Superfund Site,"\
    GJO--2002--302--TAR, Environmental Sciences Laboratory, U.S.
    Department of Energy, Grand Junction, Colorado (2002).

41. M. KASTENS and J. WAUGH, "Methodology for Determining Revegetation
    Success at the Monticello Repository," GJO--2002--325--TAR, U.S.
    Department of Energy, Grand Junction, Colorado (2002).

42. K. C. KIM and C. H. BENSON, "Water Content Reflectometer
    Calibrations for Final Cover Soils," Geo Engineering Report No.
    02--12, Geo Engineering Program, University of Wisconsin-Madison,
    Madison, Wisconsin (2002).

43. K.G. McNaughton and P. G. Jarvis, "Predicting Effects of Vegetation
    Changes on Transpiration and Evaporation," pp. 1−47 in T. T.
    Kozlowski (editor), Water Deficits and Plant Growth, Vol. VII,
    Academic Press, New York (1983).

44. C. T. deWit, "Photosynthesis of Leaf Canopies," Agriculture Research
    Report No. 663, Centre Agr. Publ. Doc., Wageningen, The Netherlands
    (1965).

45. D. A. FLOYD and J. E. ANDERSON, "A New Point Interception Frame for
    Estimating Cover of Vegetation," Vegetation 50: 185−186 (1982).

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

W.J. Waugh

G.M. Smith

P.S. Mushovic