DRC-1988-001002 - 0901a068807b2ffe

2037707
3rd International Symposium on Environmental Geotechnclogy, June 10-12, 1996
THE DESIGN OF FINAL COVERS SYSTEMS
FOR ARID AND SEMI-ARID REGIONS OF THE WEST
Gregory N. Richardson, Ph.D., P.E.
G.N. Richardson & Associates
Raleigh, North Carolina
W. Joseph Waugh, Ph.D.
Rust Geotech Inc.
Grand Junction, Colorado
ABS1RACI: Conventional engineering approaches for designjng covers for uranium mill tailings
repositories fail to fully consider ecological processes that can impact long-term performance. The
U.S. Department ofEnergy (DOE) developed an alternative design for the semiarid Monticello,
Utah, Superfund site that combines ecological infiltration contols with engineered barriers (e.g.,
geomembranes, compacted soil layers). The design does not soley rely on compacted soils to control
water infiltration, which can fail because of desiccation and cracking, but provides a soil water­
balance ecological system to limit infiltration. The design relys on a combination of vegetation and
a simulated desert pavement to limit soil loss without influencing the soil water balance. Additionally:
the design controls radon releases, biointrusion, and protects critical layers from disturbanceoy frost. Preliminary analog studies of climate change, ecological change, and pedogenesis suggest that the performance of this design may ?mProve with time.
INTRODUCTION
DOE is in the midst of cleaning up more than 20 million metric tons of low-level radioactive and
chemically toxic tailings at abandoned uranium mills in the Four Comers region (PoitiiiO 1992). The
accepted remedial action is to cover tailings and other contaminated materi$ �er-in place or in
landfill repositories. DOE mces the unprecedented legislative and engineering requirements that these
tailings repositories have design lives of200 to 1,000 years (EPA 1983}. Engineered 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 (DOE f989}. 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 po�_1992}.
DOE developed an ahemative cover design for the disposal of uranium mill tailings at the Monticello,
Utah, millsite The Monticello repository design must satisfy both (1) minimum technology guidance
(MTG) for hazardous waste disposal facilities (EPA 1989) under Subtitle C of the Resource
Conservation and Recovery Act of 1976 (RCRA) and (2) design guidance for radon attenuation and
1,000-year longevity (DOE 1989) under the Uranium Mill Tailings Radiation Control Act of 1978
(UMTRCA).
CONTAMINANT RELEASE MECHANISMS
Several concomitant release mechanisms acting on the cover could potentially cause environmental
transport of tailings contaminants.
..
-------·--
Water Infiltration --- Rain water and snow melt not lost by runoff and evaporation Will enter the
rock and soil layers overlying the tailings and become distn"buted in response to various water
potential gradients (Hille11980). Depending on the properties and thicknesses ofthese 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. The �ates of radon gas release will depend on the physical, hydrological, and radiological
properties of the tajlings 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 et al. 1985).
Erosion ----Loss of cover material by erosion could expose tailings under extreme conditions or,
more likely, reduce the cover thickness 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 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 l992). Frost heaving may also cause distinct engineered soil layers to become
mixed, thereby disrupting the integrity of critical layer interfaces (Bjornstad and Teell993).
Plant Root Intrusion --Plants growing in the cover could potentially root into tailings, actively
translocating and disseminating contaminants in aboveground tissues (Foxx et al1984, Morris and
Fraley 1989; Markose et al 1993). Roots may also alter tailings chemistry potentially mobilizing
contaminants (Cataldo et a1 1987). Macropores left by decomposing plant roots act as channels for
water and gases to effectively bypass compacted soil barriers (Hillel1980; Passioura 1991). Plant
roots may concentrate in and extract water from buried clay layers, causing desiccation and cracking
(Reynolds 1990). This water extraction can occur even when overlying soils are nearly saturated
(Hakanson 1986), indicating that the rate of water extraction by plants may exceed the rehydration
rate ofthe buried clay. Roots can also clog lateral drainage layers (DOE 1992}, potentially increasing
infiltration rates.
Animal Tntmsion ---Burrowing animals can mobilize contaminants by vertical displacement of
tailings or by altering erosion, water balance, and radon-release processes (Hakanson et al. 1992).
Vertical clisplacement results as animals excavate burrows and ingest or transport contamination on
skin and fur (Hak:onson et al. 1982). Once in the surface environment, contaminants may then be
carried off site {Arthur and Markham 1983). Loose soil cast to the swface by burrowing animals is
wlnerable to wind and water erosion {Winsor and Whicker 1980). Burrowing influences soil-water
Typeol Cover System
Water Storage/Frost
ProtecUon (Hne soiQ
150 ern---:;-,i1�!1it---li �
Animal Intrusion Layer
260 ern
m
(native pediment gravels)
Geotextlle Filter
Topsoil Layer
30cm
t
Sand Layer and Capmary Break
60 MD Geomembrane (high-density polyelhylana)
60c:m
Figure 1. DOE Cover Design for the Monticello Repository
Table 1. Comparison of RCRA Subtitle C (EPA 1989) and DOE MRAP Cover Designs
"Kut • saturated llydtauDc; c;onducllvlty.
balance and radon releases by decreasing runoft; increasing rates of water infiltration and gas
diffusion, and increasing evaporation because of natural drafts (Landeen 1994).
COVER DESIGN
The Monticello cover (Figure 1) is structurally similar to the RCRA subtitle C design (EPA 1989).
The seemingly subtle structural differences, however, represent important differences to enhance
water balance performance. Table 1 compares components of the Monticello and RCRA design.
Water lnflltratio� Control
Water Balance System---Water infiltration and leakage through the cover must not exceed the
leakage rate of the. repository liner (EPA 1989). The Monticello repository liner includes two
composite liners, each having an HOPE liner and a geosynthetic clay layer. The Monticello cover is
essentially a RCRA MTG design, but with a thicker topsoil layer. The reliance of RCRA and
UMTRCA designs on low-permeability compacted soil layers is well documented (Daniel1994; DOE
1989), and the failure of compacted soil layers to achieve performance objectives because of
desiccation �and shrinkage is also documented (Melchoir et al. 1994 ).
At the semiarid Monticello site, infiltration is limited by a natural water-balance mechanism Thick
loess soils at the surface store precipitation until soil evaporation and plant transpiration seasonally
return it to the atmosphere (Waugh and Link 1992). The Monticello water-balance cover design uses
the sand drainage layer typical to RCRA covers as a capillary break to enhance this natural water
conservation. In accordance with the "outflow law" of soil physics (Richards 1950), the capillary
barrier limits downward water movement and increases water storage capacity of the topsoil layer.
High pore tensions (suction) in the topsoil impede movement of water into the larger pores of the
underlying sand layer. Leakage into the sand occurs only if water accumulation at the topsoil/sand
layer inter.filce approaches saturation (Hillel l980). A geotextile filter maintains the :fine/coarse layer
discontinuity until soil aggregation occurs by natural pedogenic processes (Bjornstad and Teel1993).
Evapotranspiration can prevent exces$ye water accumulation above the textural break (Waugh et al.
1991; Anderson et a1 1993; Link et a1 1994). In short, the topsoil stores water while plants are
dormant, then plants extract stored water during the growing season and return it to the atmosphere.
. ..
Leakage from the water-balance system occurs if water accnmnlation rates exceed evapotranspiration
and, eventuall.y, the water storage capacity ofthe 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; Wmg and Gee 1993;
Anderson et al. 1993) suggest that, with plants present, water accumulation at Monticello will not
likely exceed the topsoil storage capacity, even during higher than record precipitation years. Field
and modeling studies are ongoing at Monticello to test this hypothesis.
Revegetation--The increased thickness of the Monticello topsoil and presence of a capillary break
provides an optimum water-balance system and creates a habitat more suitable for vegetation. A
diverse mixture of native plants on the cover will maximize water removal by evapotranspiration
(Link et a1. 1994) and remain more resilient to catastrophes and fluctuations in the environment
(Begon et all986). 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(Tausch et a1. 1993). Similarly, biological diversity in the cover
vegetation will be important to communitY stability and resilience. 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 Attenuatio.n
The 60-cm compacted soil radonfmfiltration barrier, Figure 1), satisfies the requirement for an -
average surface flux of radon-222 o� less than 20 pCi m-2 s1 (EPA 1983). The thickness was
calculated with the U.S. Nuclear Regulatory Commission (NRC) model RADON (NRC 1989). This
design apptoach is documented eJsewhere in DOE (1989). As required for UMTRCA sites (NRC
1989), only the compacted soil layer (radonfmfiltration barrier) of the cover was included in this
calculation. An overlying layers were omitted. Further analysis suggests that the compacted soil layer
may be unnecessary. RADON model results show a satisfactory radon flux from a cover consisting
of onlv the water-balance system ·
Erosion Control
The primaiy erosion control issue is \Wether vegetation alone adequately limits soil loss or are gravel
mulches, gravel admixtures, or rock rlprap 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 altemative RCRA designs include cobble or rock rlprap to control erosion in and
environments with sparse vegetation (DOE 1989; EPA 1989). However, these designS reduce
evaporation (Groenevelt et a1. 1989; Kemper et a1. 1994), possibly increasing leakage through compacted soil layers and creating habitat for undesirable plants that root into the radon/infiltration
barrier (DOE 1992).
Erosion control for the Monticello design consists of mixing gravel and sand in the top 20 em of the
topsoil (Figure 1) to mimic conditions leading to the formation of desert pavement. The method of
Temple et a1. (1987) was used to size the gravel mulch (Table 1). The sand component was sized
relative to the topsoil and gravel with Stephanson's {1979) method. Several erosion studies (Finely
et al1985;ligotke 1994) and soil-water balance studies (Waugh et a1. 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 sur.filce, some loss of fines from the admi:xtw;e is expected. The remaining sand "filter" and root
cohesion of fines will impede continued soil loss beneath this pavement (Styczen and Morgan 1995).
Rilling and gullying is controlled by maintaining top-slope gradients equal to surrounding terrain
(which lack rills and futermittent gullies) and by limiting lengths of overland flow paths.
Frost Protection
The 170-cm composite topsoil layer (Figure I) provides more than adequate depth to isolate the
capillary break layer, drainage layer, geomembrane, and compacted soil layer (radonfm.filtration
barrier) from frost damage. The estimated maximum frost depth for a 200-year return interval in the
topsoil layer is 115 em 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 (DOE 1989}.
Biointrusion Control
The· Monticello cover includes baniers 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 et al 1980; Ha.konson 1986). Root growth is generally limited to regions
within the soil where extractable water is available.
.. The composite topsoil layer thickness is 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 topsoil layer is an added deterrent Loosely aggregated gravel and rock have been shown to deter
burrowing mammals (Cline et al 1980; Hakonson 1986). This layer protects the capillary break from
bioturbation, a primary long-term threat to layer systems (Bjornstad and Teell993).
Cover Long Term Performance
· The greatest uncertainties in the Monticello cover stem from the need to extrapolate the resuhs of
short-term tests to the required 1,000-year performance period. Standard engineering approaches
implicitly assume that initial conditions of material properties and of processes that drive contaminant
transport w!Jl persist. Jn contrast, long-tenn covers are evolving components of dynamic ecosystems.
Longevity -Natural analogs provide clues from past environments to possible long-term changes
in engineered covers (Waugh et al 1994a). Natural analogs are natural and archaeological
occurrences of materials, conditions, or processes that are simi1iar to those known or predicted to
occur in some part of the cover system. As such, natural analogs can be thought of as uncontrolled,
long-term experiments. Long-term performance issues at Monticello that can be assessed with the
use of analogs include climate change, ecological change, and pedogenesis (soil development).
C)jmate Change--Climate influences the performance of the cover designed to isolate tailings at
Monticello. With evidence of relatively rapid past climate change (Crowley and North 199 1) and
model predictions of global climatic variation exceeding the historical record (Ramanathan 1988),
DOE recognizes a need to incorporate possible ranges of future climatic and ecological change in the
repository design process (Petersen et al 1993). Past climate change for Monticello were constructed
using available proxy data from tree rings, packrat maddens, lake sediment pollen, and archaeological
records (Waugh and Petersen 1995). Interpretation of proxy paleoclimatic records was based on
present-day relationships between plant distn'bution, precipitation, and temperature along. a
'generalized �leVatloiW 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 em, respectively,
corresponding to late glacial and Altithennal periods.
Pedogenesis and Ecological Change --Pedogenic processes will gradually change the physical and
hydraulic properties of earthen materials used to COJ!stluct the Monticello cover (e.g., McFadden et
a1 1987; Hillel1980). 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 and
Link 1992). Monolithic lysim.eters 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 em of leakage was �easured 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 capillaiy break would have retained all the excess soil water.
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 UM1RCA designs. These typical 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 the same natural processes to isolate tailings and to control the release of contaminants but is expected to improve over time. .
The Monticello design should be considered a more desireable alternative to RCRA Subtitle C and
UMTRCA designs at arid and semiarid sites based on the following advantages:
• Compacted soil layers, as required for RCRA and UMTRCA ·designs to control water infiltration, are wlnerable to damage by desiccation and biointrusion. In contrast, the Monticello water-balance cover relies on soil-water retention, capillary barriers, and
soil-water extraction by plants.
• Riprap layers, as recommended for UMTRCA designs, control erosion, but enhance water infiltration, and biointrusion. The Monticello design includes a topsoil and gravel admixture.
Over time, 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 baniers 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 performance standards for water infiltration
and radon releases without these additional engineered barriers.
Engineered covers that are intended to last thousands of years must be designed as evolving
components of larger dynamic ecosystems. Four tenets accompany this principle: (1) cover
components will not function and cannot be designed independently; (2) physical and ecological
conditions will change over time, therefore, initial conditions cannot be extrapolated as tests of
long-term perfo�ce; (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.
References
Anderson, J.E., RS. Nowak, T.D. RatzJafl: and O.D. Markham, 1993. "Managing soil moisture on waste burial
sites in and regions," J. Environ. Qual., 22:62-69.
Arthur, W J. 111, and O.D. Markham, 1983. "Small mammal soil burrowing as a radionuclide transport vector
at a radioactive waste disposal area in southeastern," J. Environ. Qtial.,
12:117-122.
Begon,M, JL. Harper, and C.R Townsend, 1986. Ecology: Individuals, Populations, and Communities, Sinauer
Associates, Sunderland, MA.
Bjornstad, B.N., and S.S. Teel, 1993. Natural Analog Study ofEngineered Protective Barriers at the Hanford
Site, PNL-8840, Pacific Northwest Laboratory, Richland, W A
Cataldo, D.A, C.E. Cowan K.M McFadden, T.R. Garland, and R.E. Wtldung, 1987. Plant Rhizoshpere
Processes Influencing Radionuclide Mobility in Soil, PNL-6277, Pacific Northwest Laboratory, Richland, WA
Chamberlain, E.J., and AJ. Gow 1979. "Effects of freezing and thawing on the permeability and structure of
soils," Eng. Geol., 13:73-92.
Cline, J.F., K.A Gano, and L.E. Rogers, 1980. "Loose rock as biobarriers in shallow land burial," Health
Physics, 39:497-504.
Crowley, T.J., and G.R North, 1991. Paleoclimatology, Oxford Monographs on Geology and Geophysics No.
16, Oxford University Press, NY.
Daniel, DE., 1994. "Surface barriers: Problems, solutions, and future needs," In G. W. Gee and N.R Wing ( eds.),
In-Situ Remediation: Scientific Basisfor Current and Future Technologies, pp. 441-487, Battelle Press, Richland,
WA
DOE (U.S. Department of Energy), 1989. Technical Approach Document, Revision IT, UMTRA-DOEIAL
050425.0002, Uranium Mill Tailings Remedial Action Program, U.S. Department ofEnergy, Albuquerque, NM
DOE, 1992. Vegetation Growth Patterns on Six Rock-Covered UMrRA Project Disposal Cells,
UMI'RA-DOEI AL 400677.0000, Uranium Mill Tailings Remedial Action Project, U.S. Department of Energy, -Albuquerque, NM.
EPA (U.S. Environmental Protection Agency), 1983. Standards for the Disposal of Uranium Mill Tailings, 40
CFR 192, Washington, DC.
EPA, 1989. Technical Guidance Document, Final Caps on Hazardous Waste LandfilJs and Surface Impoundments, EP A/530-SW-89-047, Washington, DC.
Finely, J.B.,MD. Harvey, and C. C. Watson, 1985. "Experimental Study: Erosion ofOverb�rden 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, CO.
Foxx, T.S., G.D. Tierney, and J.M Williams, 1984. Rooting Depths of Plants Relative toBiological and Environmental Factors, LA-I 0254-MS, Los Alamos National Laboratory, Los Alamos, NM
Groenevelt, P.H., P. van Straaten, V. Rasiah, and J. Simpson,l989. "Modification in evaporation parameters by rock," Soil TechnoJ., 2:279-285.
Hakanson, T.E., 1986. Evaluation of Geologic Materials to Limit Biological InbUsion into Low-Level Radioactive Waste Disposal Sites, LA-10286-MS, Los Alamos National Laboratory, Los Alamos, NM.
Hakanson, T.E., J.L. Martinez, and G.C. White, 1982. "Disturbance of low-level waste burial site covers by pocket gophers, u Health Physics, 42:868-871.
Hakanson. T.E., L.J. Lane, and E.P. Springer, 1992. "Biotic and abiotic processes," In: Deserts as Dumps? The
Disposal of Hazardous Materials in Arid Ecosystems, C.C. Reith and B.M Thompson (eds.), pp. 101-146,
University of New Mexico Press, Albuquerque, NM.
Hatper, JL., 1987. "The heuristic value of ecological restoratioo." In: W.R. Jordon ill, ME. Gilpin, and J.B. Aber (eds.), Restoration Ecology: A Synthetic Approach to Ecological Research, pp. 35-45, Cambridge University
Press, New York. NY.
Hillel, D., 1980. Fundamentals of Soil Physics, Academic Press, Inc., San Diego, CA.
Kemper, W.D., AD. Nicks, and AT. Corey, 1994. "Accumulation of water in soils under gravel and sand mulches," Soil Sci. Soc. Am. J., 58:56-63.
Kim, W .H., and D.E. Daniel, 1992. aEffects of freezing on the hydraulic conductivity of compacted clay," J. Geotech. Eng., 118:1083-1097. ·
Landeen. D.S., 1994. "The influence ofsmall-man-imal burrowing activity on water storage at the Hanford Site,"
In G.W. Gee and N.R. Wmg (eds.), In-Situ Remediation: Scientific Basisfor Current and Future Technologies,
pp. 523-543, Battelle Press, Richland, W A.
Ligotke, MW., 1994. "Control of eolian soil erosion from waste-site surface barriers," pp. 545-559, In G.W. Gee
and N.R Wmg (eds.), In-Situ Remediation: Scientific Basisfor Current and Fitture Technologies, Battelle Press,
Richland. W A
Link, S.O., W.J. Waugh, and JL. Downs, 1994. "The Role of Plants in Isolation Barrier Systems," In G.W. Gee
and N.R. Wing (eds.), In-Situ Remediation: Scientific Basisfor Current and Future Technologies, pp. 561-592,
Battelle Press, Richland, W A
Markose, P.M, IS. Bhat, and K.C. Pillai, 1993. "Some characteristics of226Ra transfer from soil and uranium
mill tailings to plants," J. Environ. Radioactivity, 21:131-142.
Materechera, S.A, A.R. Dexter, and AM Alston, 1991. "Penetration of very strong soils by seedling roots of
different plant species," Plant Soil, 135:3 1-4 1.
McFadden, LD., S.G. Wells, and MJ. Jercinovicb, 1987. "Influences of eolian and pedogenic processes on the
origin and evolution of desert pavements," Geology, 15:504-508.
Melchior, K. Berger, B. Vielhaber, and G. Miehlich, 1994. "Multilayer Landfill Covers: Field Data on the Water
Balance and Liner Performance," In G.W. Gee and N.R Wmg (eds.), In-Situ Remediation: Scientific Basisfor
Current and Future Technologies, pp. 411-425, Battelle Press, Richland, WA
Miller, RD., 1980. "Freezing Phenomena in Soils," In: Applications in Sqjl_Physics, pp. 254-259, D. Hillel (ed.),
Academic Press, Inc., San Diego, CA.
Morris, RC., and L. Fraley, Jr., 1989. "Effects of vegetation, a clay cap, and envirorunental variables on Rn-222
fluence rate from reclaimed U mill tailings," Health Physics, 56:431-440.
NRC (U.S. Nuclear Regulatoly Commission), 1989. Calculation ofRadon Flux Attenuation by Earthen Uranium
Mill Tailings Covers, Regulatory Guide 3.64 (Task WM 503-4), U.S. Nuclear Regulatory Commission,
Washington, DC.
Passioura, J.B., 1991. "Soil structure and plant growth: Australian Journal of Soil Research, 29:717-728.
Peters� K.L., J.C. Chatters, and W.J. Waugh, 1993. Long-Term Climate Change Assessment Study Plan for
·the Hanford .Site Permanent Isolation Barrier Development Program, WHC-EP-0569, Westinghouse Hanford
Company, Richland, W A. ·
Portillo, RS.,1992. "Mill Tailings Remediation: The UMTR.A Project," In: Deserts as Dumps? The Disposal of
Hazardous Materials in Arid Ecosystems, pp. 281-302, C.C. Reith and B.M Thompson (eds.), University of
New Mexico Press, Albuquerque, NM ·
Ramanathan, V., 1988. "The Greenhouse Theory of Climate Change: A Test by an Inadvertent Global
Experiment," Science, 240:293-299. . ,
Reynolds, T.D., 1990. "Effectiveness of three natural biobarriers in reducing root intrusion by four semi-arid
plant species," Health Physics, 59:849-852.
Richards, L.A., 1950. "Laws of soil moisture," Trans.·Am. Geophys. U., 31:750-756.
Ritchie, J.T., 198 1. n Soil water availability," Plant and Soil, 58:327-338.
Sackshewsky,MR, C.J. Kemp, S.O. Link, and W.J. Waugh, 1995. "Soil water balance changes in engineered
soil smfaces," J. Environ. Qual., 24:352-359.
Smith, W.J., RA Nelson, and K.R Baker, 1985. "Sensitivity analysis of parameters affecting radon. barrier cover
thickness,'' In: Proceedings of the Seventh Syniposium on Management OfUranium Mill Tailings, Low-Level Waste, and Hazardoits Waste, Colorado State University, Ft. Collins, CO.
Stephenson, D., 1979. Rocldill in Hydraulic Engineering, Elsevier Scientific Publishing Company, Amsterdam.
Styczen, ME., and RP.C. Morgan, 1995. ''Engineering Properties ofVegetation," In: R.P.C. Morgan and R.J. Rickson (eds.), Slope Stabilization and Erosion Control: A Bioengineering Approach, pp. 5-58, E & FN Spon, London .
. Tausch, RJ., P .E. Wigand, and J.W. Burkhardt, 1993. "Plant co�unity thresholds, multiple steady states, and multiple succession pathways: Legacy of the Quaternary," J. Range. Manage., 46:439-447 .
. ...
Temple, D.M, K.M Robinson, R.M Ahring, and AG. Davis, 1987. Stability Design of Grass-Lined Open Channels, Agricultural Handbook 667, Agricultural Research Service, U.S. Department of Agriculture, Washington, DC.
Walters, W.H., and RL. Skaggs, 1986. The Protection of Uranium Mill Tailings Impoundments Against Overland Erosion, NUREG/CR-4323, U.S. Nuclear Regulatory Commission, Washington, DC.
Waugh, W.J., KL. Petersen, S.O. Link, BN. Bjornstad, and G.W. Gee, I994a "Natural analogs of the long-term perfonnance of engineered covers," In G.W. Gee and N.R. Wmg (eds.), In-Situ Remediation: Scientific Basisfor Current and Future Technologies, pp. 379-409,Battelle Press, Richland, WA
Waugh W.J., ME. Thiede, D.J. Bates, L.L. Cadwell, G. W. Gee, and C.J. Kemp, 1994b. "Plant cover and water balance in gravel admixtures at an and waste-burial site," J. Environ. Qual., 23:676-685.
Waugh, W J., and K.L. Petersen, 1995. "Paleoclimatic data application: long-term performance of uranium mill tailings repositories," In W.J. Waugh, K.L. Petersen, P.E. Wigand, and B.Louthan (eds.), Proceedings of the Workshop, Climate Change in the Four Comers and Adjacent Regions: Irnplicationsfor Envirorunental Rest<?ration and Land� Use Planning, CONF-9409325, U.S. Department of Energy, Grand Junction, CQ,
Waugh, W.J., and S.O. Link. 1992. Small Lysimetersfor Evaluating the Water Balance ofEngineered Covers: Interim Report, GlPO-TMS-7, U.S. Department ofEnergy Grand Junction Projects Office, Grand Junction, CO.
Waugh, W.J., ME. Thiede, LL. Cadwell, G.W. Gee, liD. Freeman, MR. Sackschewsky, and J.F. Relyea, 1991. "Small lysimeters for documenting and site water balance," In: Lysimetersfor Evapotranspiration and Environmental Measurements,pp. 151-159, R.G. ADen, T.A Howell, W.O. Pruitt, LA Walter, and ME. Jensen (eds.), American Society of Civil Engineers (ASCE), New York, NY.
Wing, N.R., and G.W. Gee, 1993. The Development of Permanent Isolation Surface Barriers: Hanford Site,
Richland, Washington, U.S.A, WHC-SA�1799-FP, Westinghouse Hanford Company, Richland, WA
Wmsor, T.F., andF.W. Whicker, 1980. "Pocket gophers and redistribution of plutonium in soil," Health Physics,
39:257-262.
Wiscluneier, W.H., and D.D. Smith, 1978. Predicting Rainfall Erosion Losses-A Guide to Conservation Planning, Agricultural Handbook No. 537, U.S. Department of Agriculture, Washington, DC.