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OCR for page 21
Development and Testing of Permanent Isolation Surface
Barriers at the Hanford Site
Glendon W. Gee, Pacific Northwest National Laboratory, Richiand, Washington;
N. Richard Wing, Richard; and Anderson L. Ward, Pacific Northwest National
Laboratory, RichIand
ABSTRACT
Engineered barriers are being developed to isolate wastes disposed of near the earthts
surface at the U.S. Department of Energy's (DOE) Hanford Site, near Richland, Washington. The
surface barriers use engineered layers of natural materials to create an integrated structure with
redundant protective features. For example, one current design incorporates a capillary barrier as
well as a low-permeability asphalt component. The natural construction materials (e.g., fine soil,
sand, gravel, nprap, asphalt) have been selected to optimize barrier performance and longevity.
The objective of current designs is to use natural materials to develop a maintenance-free surface
barrier that isolates wastes for a minimum of 1,000 years by limiting water drainage to near-zero
amounts; reducing the likelihood of plant, animal, and human intrusion; controlling the
exhalation of noxious gases; and minimizing erosion-related problems.
A multiyear barrier development program was started at the Hanford Site in 1985 to
develop, test, and evaluate the effectiveness of various barrier designs. A team of engineers and
scientists have directed the barrier development effort. ICE Kaiser Hanford Company (KH) has
provided design support for barrier-related projects, and Westinghouse Hanford Company
(WHC), Bechtel Hanford Incorporated (BHI), and the Pacific Northwest National Laboratory
(PNNL) have provided engineering and scientific support to the development effort. A prototype
barrier, incorporating all essential elements of a long-term surface barrier, was constructed at the
Hanford Site in 1994 and is currently being monitored.
This paper provides an overview of the barrier development work being conducted at the
Hanford Site and the functional performance of the permanent isolation surface barrier. The
paper focuses on the control of water movement into and through the barrier and discusses how
. . . ~
various aspects of the barrier have been purposely designed to minimize water intrusion into
underlying buried wastes. Field tests conducted on Individual components of the barrier and
more recently on the completed prototype barrier show that the combination of a capillary barrier
(designed to store water and subsequently enhance near-surface water loss
ev~notransDiration) and an asphalt sublayer (to shed water from sideslope drainages can be
near-surface water loss via
.
. . . · . ~. ~. . .
effective in keeping water from draining into underlying wastes. Extreme precipitation evens,
including 1,000-year storms, are accommodated by use of the multiple-layer design. Eight years
of testing of individual components and 2 years of testing of a full-scale prototype surface barrier
are providing engineering parameters needed for design of extensive cover systems planned for
the Hanford Site.
D-3
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D-4
BARRIER TECHNOLOGIES FOR ENVIRONMENTAL MANAGEMENT
INTRODUCTION
The In-Place Remediaiton Alternative
Permanent isolation surface barriers have been proposed for use at the U.S. Department
of Enernr's (DOE) Hanford Site' near Richiand, Washington, to isolate and dispose of certain
A.` ~ , , , _
~ ~ · ~ ~1 _ 1 ~ 1. ._ . fir ~ ~ ~1~ ~ ~ ~_~1
types ot waste In place. 1 he exhumation ana Irealmem OI wastes may not always oe me paid
alternative in the remediation of a waste site. Tn-place disposal alternatives, In certain
circumstances, may be the most desirable alternative to use in the protection of human health and
the environment. The implementation of an Emplace disposal alternative probably will require
some type of protective covering that will provide long-term isolation of the wastes from the
accessible environment. (Even if the wastes are exhumed and treated, a iong-term barrier may
still be needed to dispose of the wastes adequately.) Currently, no "proven" long-term bamer is
available. However, the Hanford Site Permanent Isolation Surface Bamer Development Program
(BDP), which is described below, was organized to develop the technolc~v needed to provide a
long-term surface bamer capability for the Hanford Site and elsewhere.
- - - O ~- 1-
The Hanford Site Permanent Isolation Surface Barrier Development Program
The Hanford Site Permanent Isolation Surface BaITier Development Program (BDP) was
organized In 1985 to develop, test, and evaluate the effectiveness of various barrier designs. The
BDP was supported by DOE and consisted of a team of engineers and scientists from
Westinghouse Hanford Company (WHC) and the Pacific Northwest National Laboratory
(PEAL), which directed the barrier development effort. ICE Kaiser Hantord (company (ate
provided design support for numerous barrier-related projects.
Fifteen groups of tasks were identified by the barrier development team to resolve the
technical concerns and complete the development and design of protective barriers (Wing 19931.
These major barrier development task groups are water infiltration control, biointrusion control,
erosion/deposition control, physical stability testing, human interference control. barrier
construction materials procurement, prototype barrier designs and testing, mode! applications
and validation, natural analog studies, long-term climate change effects, interface with regulatory
agencies, Resource Conservation and Recovery Act of 1976 (RCRA) equivalency, technology
integration and transfer, project management, and final design. Figure 1 illustrates how the
information and data generated within each of the task groups are input into the final designts) of
the barrier.
The information and insights gained from the development tasks previously mentioned
have enabled the barrier program to progress to the point where design, construction, and testing
of a full-scale prototype battier has been possible. The full-scale prototype barrier is providing
engineers and scientists with insights and experience on barrier design, construction, and
performance that have not been possible with the individual tests and experiments conducted to
date in the program. Construction of the prototype was completed In August of 1994, and testing
and monitoring was initiated at that time. The testing and monitoring of the prototype battier is
planned to last for a minimum of 3 years. A comparison of the Hanford barrier design and testing
program with other DOE sponsored surface barrier designs has been reported recently (Daniel et
al., 19961. One of the major differences between the Hanford battier development activities and
those at other sites has been the emphasis at Hanford on design and testing of a surface barrier
OCR for page 23
APPENDIXD~PAPERS PRESENTED
D-5
that has a high probability of lasting for 1,000 years or more. For example, tests for the prototype
barrier have been designed to evaluate barrier surface and sidesioperesponse to 1,000-year storm
events. These and other tests will be described In the following sections.
-L
Regulatory ~ \ ~/ ~Biointrusion
~Agencie \ 5,_]
1 - 1
Resource
Conservation
and Recovery _
Act Equivalency
Technology
Integration
and Transfer
ProJect
Management
~1
Model
Appilcatlons ~_
and Validation ~/ Final
i ~- I ~ Design
Long-Term
Cllmate Change
Effects
1 ' --- 1
Natural Ba rier ~ /
Analogs ~/
.~
\ \rF 'hysical
\ Stability
\ Testing
Prototype
Barrier Designs
and Testing
FIGURE 1 Barrier Development Tasks.
Barrier
Construction
Materials
Procurement
Human
Interference
Control
FllNCTIONAL REQUIREMENTS FOR THE BARRIER
.
Water
Infiltration
Control
Erosion/
Deposition
Control
Much of the waste that would be disposed of by using anyplace isolation techniques is
located In subsurface structures, such as solid waste burial grounds, tanks? vaults, and cribs.
Unless protected In some way, the wastes could be transported to the accessible environment via
the following pathways (Figure 2~.
Water infiltration. The infiltration and percolation of water through the waste
zone, resulting in the leaching and subsequent transport of mobile radionuclides and other
contaminants to the water table.
· Biointrusion. The penetration of deep-rooting plants and burrowing animals into
the waste zone below. The deep-rooting plants could draw radionuclides and other contaminants
into their root systems and subsequently transIocate the contaminants to the above-grade portion
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D-6
)~) ~ Imate
Transpiratlon
BARRIER TECHNOLOGIESFORENFIRONMENTALMANAGEMENT
Precip ~ :7 ;
OCR for page 25
APPENDIX~PAPERS PRESENTED
accessible environment.
D-7
Gaseous release. The diffusion of noxious gases from the waste zone to the
Permanent isolation surface barriers have been proposed to protect wastes, disposed of in
place, from the transport pathways identified. Surface markers, used to inform future generations
of the nature and hazards of the buried wastes, are being considered for placement around the
periphery of the waste sites. in addition, throughout the protective barrier, subsurface markers
could be placed to warn any inadvertent human intruders of the dangers of the wastes below.
The protective barrier design consists of a fine-soi! layer overlying other layers of
coarser materials such as sands, gravels, and basalt riprap (Figure 31. Each of these layers serves
a distinct purpose. The fine-soi! layer acts as a medium in which moisture is stored until the
processes of evaporation and transpiration recycle any excess water back to the atmosphere. The
fine-soil layer also provides the medium for establishing plants that are necessary for
transpiration to take place. The coarser materials placed directly below the fine-soil layer create
a capillary break that inhibits the downward percolation of water through the barner. The
placement of fine soils directly over coarser matenals also creates a favorable environment that
encourages plants and animals to limit their natural biological activities to the upper, fine-soi!
Erosion
Resistant
Gravel
Admix
(a)
FIGURE 3 Barrier Cross Section.
~1
1~ /N
_ ~-A - ~ Lateral Drainge (D )
Existing Grade \<
Evapotranspiration
(ET)
L ~ Upper Neutron Probe
, t A ccess Tube
\ Jvertic~_ ~J
/ Drainge(Dv, Waste Crib I/
/Clean Fill Side Slope
~(pit run gravel)
/ ~ Neutron Probe
/ ~ Access Tube
/ 10 ~ ~ ~
~Basalt Rock
_ Rip Rap 1.5 m
Drainage Gravel ~///
_ 0.3 m mint ~ ,` I/
Composite Asphalt / /
(aspthdait~c/f~cuoidncrete Top Course /
applied asphalt) 0.1 m min.
_ - 0.15 m min. / /
Sandy soil /
(structural) Fill /
In Situ Soil
Upper Silt~
w / Admix 1.0 m ~/ 5o 1
Lower Silt 1.0 m \\ / . .L
Ba71t Side Slope \
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D-8
BARRIER TECHNOLOGIES FOR ENVIRONMENTAL MANAGEMENT
portion of the barrier, thereby reducing biointrusion into the lower layers. The coarser materials
also help to deter inadvertent human intruders from digging deeper into the barrier profile. Low-
permeability layers, placed In the balkier profile below the capillary break, also are used in the
protective barriers. The purpose of the low-permeability layers is (1) to divert away from the
waste zone any percolating water that gets through the capillary break and (2) to limit the
upward movement of noxious gases from the waste zone. The coarse materials located above the
low-permeability layers also serve as a drainage medium to channel any percolating water to the
edges of the battier.
Because of the need for the barrier to perform for at least 1,000 years without
maintenance, natural construction materials (e.g., fine soil, sand, gravel, cobble, crushed basalt
riprap, asphalt) have been selected to optimize barrier performance and longevity. Most of these
natural construction materials are available in large quantities on the Hanford Site and are known
to have existed In place for thousands of years or longer (e.g., basalt). In contrast to the natural
construction materials, the ability of synthetic construction materials to survive and function
properly for 1,000years is not known. Because of this uncertainty, synthetic construction
materials cannot be relied upon to perform satisfactorily (or even exist) over centuries or
millennia, so were not given any credit in the design.
Because of the desire for the barrier to remain maintenance free, an understanding ot
how natural processes affect baITier performance enables a design to be developed that meets
performance objectives passively. This paper discusses the natural processes acting on the
permanent isolation barrier, as well as the engineered features of the barrier that have been
designed to protect buried wastes from the natural processes. Specifically, this paper focuses on
how various battier components are used to protect buried wastes from water has been
incorporated into the design of the barrier.
WATER INFILTRATION AND PERCOLATION CONTROL
The control of water infiltration and percolation through the barrier depends on the
amount of water available. The amount of water available depends on the climate. Because of the
long time frame during which permanent isolation surface barriers must function (1,000+ years),
the climatic conditions acting on the barrier may change.
Current Climatic Conditions
Since 1912, the amount of precipitation collected at the Hanford Meteorological Station
(HMS) has averaged 160 mm (6.30 in.) annually (Stone et al., 19831. Most of this precipitation
(67 percent) is received in the winter months (October through March), while only 13 percent is
received July through September. About 38 percent of all precipitation is in the form of snow
during the months of December through February. Total annual snowfall averages 335 mm (13.2
in.~. Based on extreme-value analysis of Hanford Site climatological records from 1947 through
1969, the 60-min, 100-yr storm would result in 20.6 mm (0.81 in.) of precipitation, and the 24-
hour, 1,000-yr storm would result in 68.1 mm (2.68 in.~. No records have been kept for time
periods less than 60 min. However, the rain gauge chart for Jude 12, 1969, shows that 14.0 mm
(0.55 in.) of precipitation was collected during a 20-min period. In addition, an afternoon
thunderstorm on June 29, 1991, dumped 11.2 mm (0.44 in.) of rain at the HMS in only 10 min.
fat
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APPENDIX~PAPERS PRESENTED
D-9
The average monthly temperature at the HMS is 11.7 °C (53.0 OF). However, January
monthly temperatures average -~.5 °C (29.3 OF), and July monthly temperatures average 24.7 °C
(76.4 OF). Temperatures reach 32.2 °C (90 OF) or above an average of 55 days/yr, while
minimum temperatures of 2 ~ .1 °C (70 OF) or above occur only an average of ~ days/yr.
The prevailing wind direction at the Hanford Site is either WNW or NW In every month
of the year. The strongest winds are from the SSW, SW, and WSW. June, the month of highest
average speed, has fewer instances of hourly averages exceeding 13.9m/s (31 mph) than
December, which has the lowest average speed. When extreme value analysis of peak gusts is
performed on data from 1945 through 1980 (collected at an elevation of 15.2 m t50 ft] at the
HMS), the 100-year return period for a peak wind gust is estimated to be 38 m/s (85 mph). The
maximum gust recorded In the data set was measured in January 1972 at 35.S m/s (80 mph). The
I,000-year peak gust is estimated to be 44 m/s (99 mph).
Projected Climatic Conditions
Projections of the long-term variability in the Hanford Site's climate have been
developed so that Lamer performance over its projected design life (l,000+yr) could be
predicted (Petersen, Chatters, and Waugh, ~ 993~. One of many activities that has been performed
as part of the climate-change task is the extraction of a pollen record from the lake-bottom
sediments of Carp Lake, located near Goldendale, Washington, southwest of the Hanford Site
(Wing et al., 1995~. This pollen record, dating back 75,000 years or more, enables scientists to
determine the types of vegetation that once grew in the vicinity of the lake. With an
understanding of the vegetation species that once grew, scientists then are able to predict the
climatic conditions that had to exist to support the growth of the types of vegetation determined
from the pollen record.
Refening to the climatic conditions of the Columbia Basin inferred from the Carp Lake
pollen record, Petersen et al. (1993) states the following:
Throughout the record, mean annual precipitation
ranged from 25 to 50% below modern levels...to 28% above...At
no time did precipitation levels reach three times that of present
day. Three times modern precipitation has been taken as an
upward bounding condition of precipitation to be used in barrier
performance assessment...
The three times average annual precipitation (3X) projection has been used since November
1990 as the upper bound when applying supplemental precipitation to field test plots.
Designing a Barrier for Drainage and Percolation Control
Based on the climatological conditions and projections discussed previously, three
methods are described for controlling the Infiltration and percolation of water through a
protective baITier: (~) engineering the baIlier surface to maximize runoff, while at the same time
minimizing erosion, (2) incorporating a capillary break (or capillary barrier) within the
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D-10
BARRIER TECHNOLOGIES FOR ENVIRONMENTAL MANAGEMENT
integrated barrier system, and (3) incorporating a low-permeability, umbrella-like layer below
the capillary break to shed any infiItrating/percolat~ng water away from the waste zone.
Runoff
The amount of water available for infiltration and percolation is a Unction of the amount
of precipitation that falls on the balkier surface, minus the amount of water that runs off the
barrier surface and away from the structure. The surface of the protective barrier has been
designed with a slight slope or crown to maximize the runoff of water from the barrier surface
while minimizing the erosion of the fine-soil layer. Tests have been conducted to aid in the
design of this feature (Gilmore and Walters, 19931. The current barrier design uses a 2 percent
sloped surface.
Capillary Barrier
The protective barrier is designed and constructed with a fine-soi! layer overlying a layer
of coarser materials (e.g., sands and/or gravels). The differences in textures between the barrier
materials at this interface provide a capillary barrier for percolating water (Figure 3~.
In an unsaturated system, the capillary pressures are much less than atmospheric
pressure. For significant quantities of water to flow into and through the coarser sublayers, the
water pressure must be raised almost to atmospheric pressure. The overlying f~ne-textured soils
must become nearly saturated for the water pressure to approach atmospheric pressure and allow
water to flow into the sublayers. This resistance to drainage increases the storage capacity of the
overlying fine-textured soil. Keeping the water in the fine-textured layer provides time for the
processes of evaporation and transpiration to remove it.
The critical component of the capillary barrier is the fine-soi! layer. The fine-soi] layer
must be able to retain infiltrating precipitation until the processes of evaporation and
transpiration can recycle the water back to the atmosphere. The removal of water from a barrier's
f~ne-soi! layer is increased significantly by the presence of vegetation. After the construction of a
battier, desired stands of vegetation on the battier surface will be engineered and cultivated.
However, during a barr~er's design life, the engineered vegetative cover may be disturbed at
times by range fires, drought, disease, or some other phenomenon. Because of the design
objective to create a maintenance-free battier, revegetat~ng the barrier surface with the desired
plant species may not always be possible. In these circumstances, a climax community of
vegetation may not reestablish itself on the barrier surface for a long time (Waugh et al., 1994;
Link et al., 19951. Although the presence of vegetation on the barrier surface is ideal, the results
of lysimeter tests, presented in the following paragraphs, provide interesting evidence that the
capillary barrier concept performs effectively, even in the absence of vegetation.
The capillary balkier concept has been tested for several years at the Field Lysimeter
Test Facility (FI,TF) (Figure 41. Results from these tests indicate that the capillary barrier
functions as designed. During the first 3 years of testing, twice the annual average precipitation
(320 mm, or 2X) was added to lysimeters simulating a wetter climate. During the next 2 years,
three times the annual average precipitation (480 mm, or 3X) was added to the same lysimeters.
During this entire 5-yr testing period, water losses from evaporation and transpiration exceeded
water gains by precipitation and irrigation-even for the lysimeters receiving treatments
OCR for page 29
APPENDIX~PAPERS PRESENTED
D-11
representative of wetter climatic conditions. These results were observed for both vegetated and
unvegetated lysimeters. Although the vegetated Tysimeters were most effective at removing soil
water, even the soil water stored in the unvegetated Tysimeters decreased during the 5-year test
period. No drainage was collected from any of these lysimeters.
The capillary barrier concept does have its limits, however. During the commencement
of the sixth year of testing, drainage was observed (during the unusually wet winter of 1992-
~ 993, when record snowfalls occurred) from several unvegetated lysimeters receiving
supplemental precipitation (Campbell et al., ~ 9901. The routine supplemental irrigation
treatments, when combined with the unusually large amount of precipitation received during that
winter, caused more than 3X (>520 mm) precipitation to be added to these lysimeters. The net
result was that the storage capacity of the fine-soi! reservoir was exceeded and the unvegetated
lysimeters began draining. The Tysimeters with vegetation did not drain, even though they
received the same amount of moisture (520 mm).
'at
.~ ~ N~
_~-If.. "-' ~ ·
. ~1. .1 ~
~ ~,1 ~q ~-~'u · ~
~ ,,,
1
'am
FIGURE 4 The Field Lysimeter Test Facility: Schematic View.
tW~''~
~.....
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D-12
BARRIER TECHNOLOGIES FOR ENVIRONMENTAL MANAGEMENT
Because of earlier tests conducted on two of the lysimeters at the FLTF, some
understanding existed of the limits of the capillary barrier's performance. In two of the drainage
lysimeters at the FETE, enough water was added to force water to break through the capillary
barrier. As expected, virtually no water passes through the capillary barrier until the soil
approaches saturation and pore pressure approaches zero. Once breached, the capillary barriers in
the {ysimeters drained only slowly until they reached a stable water content, resulting in a
storage of over 500 mm, which was almost twice as high as that normally held (~250 mm) by the
silt loam soil against gravity (Gee et al., 1993a).
Observations at the FLTF indicate that both vegetated and unvegetated barrier systems
are able to store and evapotranspire at least three times the annual average precipitation --
simulating the upper bound of projected climate changes at the Hanford Site during the next
1,000 years. Vegetated battier systems are able to accommodate even greater amounts of
precipitation because of the water extraction capabilities of plants, thereby providing increased
storage capacity.
Low-Permeability Layers
The basic premise of the capillary barrier concept is that most, if not all, of the meteoric
water that infiltrates the barrier surface can be returned to the atmosphere by surface evaporation
and plant transpiration. However, for periods of unusually heavy, intense, andlor prolonged
precipitation, the water-holding capacity of the fine soils may be exceeded, allowing water to
break through the capillary battier before it can be recycled back to the atmosphere. Unless
checked in some way, the water would be free to migrate through the barrier and into the waste
zone. Also, coarse-textured, sparsely vegetated side slopes will allow significant water
infiltration. As a means of restricting the percolating water from the waste zone, a Tow-
permeability component is placed strategically within the Lamer profile below the capillary
bander to divert percolating water away from the buried waste. This diversion barrier is
constructed of a materialist with low permeability. The BDP is using asphaltic concrete and
polymer-modified asphalt to create the low-permeability layer.
Several types of asphalt have been used in tests conducted by the BDP. Based on
recommendations supported by laboratory test results, lysimeter studies at the Small-Tube
Lysimeter Facility (STEP) have used two asphalt formulations: (1) hot rubberized asphalt and (2)
an admixture of cationic asphalt emulsion and concrete sand containing 24 wt% residual thick
asphalt. These asphalt formulations have been effective in limiting percolation (Freeman and
Gee, 19891. A third type of asphalt formulation is being used In the prototype barrier. This
formulation consists of a composite layer of asphaltic concrete (with A% asphalt and very low
voids) overlain by polymer-modified asphalt (5.1-mm thick). There are two major advantages to
this third asphalt formulation. The first advantage is its high mechanical strength. The second
advantage is that composite layers have been shown to provide much lower permeabilities than
one layer alone (Daniel and Trautwein, 1991~. Tests of the permeability of the asphaltic
components indicate that the hydraulic conductivity of the combined components will be less
that lx10-~ ~ cm/s (Petersen, Link, and Gee, 19951.
The low-permeability layers, in concert with (1) the engineered surface that maximizes
runoff and (2) the capillary barrier, which blocks the downward movement of percolating water,
is expected to perform in such a way that near-zero drainage rates through the barrier can be
achieved.
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APPEND~PAPERS PRESENTED
BIOINTRUSION EFFECTS ON WATER INFILTRATION CONTROL
D-13
The presence of animal burrows (for both small and large mammals) on the surface of
the barrier has been a concern for scientists and engineers on the BDP. The presence of animal
burrows could provide preferential pathways or conduits through which infiltrating water could
bypass the fine-soi! layer of the protective battier and subsequently migrate deeper into the
barrier profile or possibly into the waste zone below. Tests have been conducted to assess the
impact of burrowing animals on the infiltration and percolation of water through protective
barriers (Cadwell, Eberhardt, and Simmons, 1989; 1,andeen et al., 1990; Landeen, 1990, 1991,
19941. The results ofthese tests have provided interesting results.
From the results of lysimeter tests performed at the Animal Intrusion Lysimeter Facility
(Figure 5), the presence of small-mammal burrows did not appear to have a significant influence
on the deep percolation of water through the battier (Cadwell et al., 1989; Landeen et al., 1990;
Landeen, 1990, 1991, 1994~. During the summer months, more water was lost from plots with
animal burrows than from plots where no animal burrows were present. During the winter
months, both the plots with animal burrows and the control plots gained water. In addition, water
did not infiltrate below ~1 m (36 in.), even though burrow depths always exceed ~1.2 m (48 ink.
The lack of significant water infiltration at depth and the overall water loss in the lysimeter plots
occurred despite the following worst-case conditions:
.
.
.
In Natural settings;
no vegetative cover (no water loss through transpiration);
no water runoff (all incipient precipitation is contained);
the burrow densities In the Tysimeters are greater than the burrow densities found
· extreme rainfall events are applied frequently (three 100-year storm events in
3 months); and
.
animals burrow deeper in the Tysimeters than in "natural" settings.
. ~
The overall water loss from soils with small-mammal burrows appears to be enhanced by a
combination of soil turnover and subsequent Crying, ventilation effects from open burrows, and
high ambient temperatures.
Similar water loss results have been observed for experiments conducted on existing
large-mammal burrows found in a natural setting on the Arid Land Ecology Reserve at the
Hanford Site. The large-mammal burrows studied were excavated by coyotes and badgers in
search of prey. The soils into which the burrows were excavated consist of a silt loam similar to
the sediments that will be used to construct surface baITiers.
Large mammals do appear to cause increased deep penetration of water in the fine-soi!
layer, but much of this water was later removed by a dense stand of vegetation (primarily
mustards) that grew vigorously in the vicinity of the burrow. The density of the vegetation near
the badger burrow was significantly greater than in adjacent undisturbed soils away from the
burrows. The soil under the burrows was actually drier in midsummer than the adjacent soils
away from the burrows.
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D-14
BARRIER TECHNOLOGIES FOR ENVIRONMENTAL MANAGEMENT
Three Apes of Animals Used
To~rnsend Pocket Pocket
Ground Squt'Tel blouse Gopher
· Animals Introduced into and Allowed To Barlow in the
Lysimeters
· Supplemental Precipitation Added to Some of the
Lysimeters
· Water Movement through the Lysimetera Is Monitored
· After month Period, the Animals Are Released
and the Burrow Systems Are Mapped
· The LysiFnotere Are Refilled and l4owTreatment
Combinations Initiated (ThreeTreatrnents per Year)
Enhanced Precipitation-100 Year
[lanford Stonn Added Once a Donut
~... , ; ~
A. 41~411~ ,:,~!.:21
..................... ~...............
:::::: :::::. ::::::::::::
............. .... , ~
... ....
Example Treatment
V
~ 66 ~ Six Lysimeters
I ~` ~-Backfilled with
Barber (:onat~uction
Matenale
Lysimeters Fit into
Outer Boxes
Burled-The Top
of the Boxes Are at the
Same Elevation as
the Original Grade
Amblent Precipitation
(1 5 ,1<-5'-~
6' (1.8 m)
FIGURE 5 Animal intrusion Lysimeter Facility: Experimental Design.
Other observations were made with the large-mammal burrows. Link et al. (1995)
reported that characterization of existing marked badger burrows indicated that abandoned
burrows are only temporary surface features that soon fill with soil and organic debris. Many of
the badger burrows also connect with small-mammal burrows. The small mammals appear to be
instrumental in filling the larger burrows by casting soil into the openings. More importantly, the
smaller burrows provide an opportunity for runoff that enters large burrows to drain.
The current barrier design does not include features to reduce the hazards of deep water
penetration through large-mammal burrows because there has been no demonstrated need, based
on work conducted to date. In addition, the presence of the low-permeability asphalt layers lower
in the barrier profile will act like an umbrella to shed any percolating water away from the waste.
OCR for page 33
APPENDIX~PAPERS PRESENTED
D1 C
WIND AND WATER EROSION CONTROL
Protective barriers are being designed to minimize the effects of wind and water erosion
of the surface cover, side slopes, and toe of a protective barrier. Understanding the effects that
erosive forces (and the techniques being considered to stabilize the surface soils from the erosive
forces) have on barrier performance with regard to water infiltration is important. For example,
the erosive forces acting on the barrier could be strong enough to reduce the thickness of the
fine-soil reservoir such that the moisture retention capability of the barrier is reduced. With
reduced capacity for moisture retention, the infiltration of water through the barrier may become
a greater concern. Also, the types of erosion control techniques used to stabilize the surface soils
from erosive forces also may affect water infiltration through the barrier adversely. The
following paragraphs describe the results of wind-and-water erosion studies with regard to water
infiltration concerns.
Throughout the majority of its design life, vegetation will be growing on the surface of
the protective barrier (Waugh et al., 19941. The presence of vegetation on the barrier surface will
reduce the amount of fine soil lost from the barrier by wind and water erosion significantly.
However, to protect the barrier surface during periods when the vegetative cover is disturbed by
range fires, drought, disease, or some other phenomenon, surface gravels will be admixed into
the surface of the protective barrier.
Studies conducted in the PNNT~ Aerosol Wind Tunnel Research Facility have shown that
field wind erosion stresses and surface conditions can be replicated in the wind tunnel. These
studies have provided significant input for the design of protective barriers (Ligotke and Klopfer,
1990; Ligotke, 19931. For example, wind-t~'nne! tests have demonstrated that admixtures and
layers of gravels (with partial sizes of 3- to 7-mm in diameter) provided superior surface
protection. The best gravel admixtures reduced surface deflation rates by greater than 96%
(compared to unprotected soil). Also, rounded river rock and gravel-sized, angular crushed rock
provided equal surface protection, expanding the possibilities of finding adequate source
materials for the least expense.
In addition to the wind-erosion studies, other studies have been conducted to optimize
the design of the barrier surface to resist water erosion (Gilmore and Walters, 19931. Gilmore
and Walters ( 1993) have stated the following:
...Ethe] most dominant factor in reducing runoff and
sediment yield was the presence of vegetation cover...Another
factor that has significance is the amount of antecedent moisture
in the soil. For very dry conditions representative of the Hanford
summer climate, runoff is greatly reduced...The dry soil
conditions coupled with the presence of vegetation can reduce
surface runoff to a minimal amount, less than 1% of the applied
rainfall, with very little sediment yield. Grave! admix with the
natural vegetation cover and dry soil conditions reduced the
sediment yield to the lowest observed levels for these tests. An
established vegetation cover with grave! admix could possibly
reduce sediment yield by 10-100 times for equivalent storms.
From the studies cited previously, the presence of grave! admix has been demonstrated
to be effective in reducing the deflation of fine soils from the barrier surface by wind and water
OCR for page 34
D-16
BARRIER TECHNOLOGIES FOR ENVIRONMENTALMANAGEMENT
erosion. The amount of gravel used to stabilize the surface of the protective barrier is a critical
design consideration from a water infiltration perspective. If too much gravel is mixed into or
spread onto the fine-soil surface, plant transpiration and surface evaporation could be reduced
significantly, which would increase the potential for water drainage through the barrier.
Conversely, if too little gravel is used, the ability of the gravel admix to reduce wind and water
erosion may be limited severely.
At the Small-Tube Lysimeter Facility (STLF), the water storage and evapotranspiration
In a permanent isolation barrier were determined to be affected significantly by the types of
materials used on the barrier surface (Sackschewsky et al., 19951. The lysimeters at the STLF
were backfilled with materials to test how various erosion-control surface treatments affect soil
moisture balance (Figure 6~. Data collected at the STLF show that when gravel is spread onto a
fine-soil surface instead of being tilled or pug milled into it, plant transpiration and surface
evaporation are reduced significantly, which increases the amount of water available for drainage
through the barrier. Similar results were observed for lysimeters with a layer of dune sand
overlying fine-textured soils. Drainage has occurred only in irrigated gravel- and sand-covered
lysimeters. Because of these results, the use of admix gravels rather than gravel mulches is
recommended to avoid water infiltration.
ASSESSING WATER INFILTRATION T~OUGII TlIE PROTOTYPE BARRIER
The prototype surface barrier constructed at the Hanford Site in 1994 is shown in cross
section and plan view in Figure 3. In addition to testing the performance of a capillary barrier
design (fine soil over coarse), the prototype is being used to test two different sideslope designs:
(~) a relatively flat apron (10:1, horizontal:vertical) of clean-fill materials (commonly called a
clean-fill dike) and (2) a relatively steep (2:~) embankment of fractured basalt riprap (Gee et al.,
1993b). From November 1994 through October 1996, soil (capillary barrier) plots on the
northern half of the prototype battier were subjected to a 3X Legation regime. This treatment
included application of sufficient irrigation water on March 24, 1995, and March 25' 1996, to
mimic a 1000-year storm event (70 mm of water) and periodic applications to achieve a test
design of 480 mm/yr for the entire water year (November 1-October 311.
Shrub and grass cover was established on the prototype surface successfully. Shrubs
were planted at a density of approximately two plants per square meter in November 1994.
Sagebrush (Artemsia tridentata) and rabbitbrush (Chrysothamnus nauseosus) were planted in a
ratio of 4:1, sagebrush to rabbitbrush. Survival rate of the transplanted shrubs has been
remarkably high; 97 percent for sagebrush and 57 percent for rabbitbrush (Gee et al., 1996~. A
heavy invasion of tumbleweed (Salsola Bali) occurred in 1995 but was virtually absent in 1996.
Grass cover, consisting of twelve varieties of annuals and perennials (including cheatgrass,
several bluegrasses, and bunch grasses), dominated the surfaces, particularly those that were
irrigated. Approximately 75 percent of the surface was covered by vegetation; a cover value
typical of shrub-steppe plant communities. In all respects, the vegetated cover appeared to be
healthy and normal. There was a surface response to irrigation, with nearly twice as much grass
cover on the irrigated surfaces compared to the non-irrigated surfaces (Gee et al., 19961.
Prototype water-storage data are shown In Figure 7. All irrigation and natural
precipitation plus all available stored soil water was removed via evapotranspiration (i.e.,
combined evaporation from plant and soil surfaces) during the first year of surface battier
operation. Water was removed from the entire soil profile so that by late summer (September) of
OCR for page 35
APPENDIX~PAPERS PRESENTED
Gantry Crane
| Crane
-1~/ Scale
Small-Tube
l~ysimeter
\,.
D-17
· Lysimeter Constructed of 169 cm-
by~O cm diameter ABS Plastic
· Lysimaters Provide Both Weighing
and Drainage Capability
· Lysimeters Are Relatively Low Cost - -
En,able Statistically Designed Testing
I///
1~= it,,
......
......
__
me
At,,
I 1~41 1 ~ Pea Gravel
[ml] Bimodal Gravel
V
FIGURE 6 Small-Tube Lysimeter Facility: Experimental Design.
Treatment
Combinations
~3 Gravel Admix
Dune Sand
Surface Gravel
McGee Soil
No. 20/30 Sand
No. 8 Sand
r'~o~o~o.'~
1995, water contents in both Agates and non-irr~gated plots had reached a relatively uniform
Tower-limit of about 5 volume-percent throughout the soil profile. Correspondingly, water
storage was reduced to levels near 100 mm (i.e., lower-limit of plant-available water), for both
the irngated and non-imgated soil surfaces. This is about one-fifth the amount of water required
for drainage. Based on these observations and considering the irrigation treatment to represent
the extreme in wet climate, the soil cover would not be expected to drain, even under the wettest
Hanford climate conditions.
Figure 7 shows that in 1996, water from the second 1000-year storm was also removed
from the soil profile by evapotranspiration, thus demonstrating the continued positive benefits of
having vegetation on the barrier surface. Evapotranspiration for the irrigated surfaces was nearly
double that for the non-irr~gated (ambient) surfaces (Figure 8), suggesting that vegetation is
capable of adjushug to rate limiting processes, such as water applications. It is apparent that the
capacity of vegetation for water consumption has not been exceeded even at the 3X precipitation
rates, even after the second year of testing. This further supports the hypothesis that the
combination of vegetation and soil storage capacity is more than sufficient to remove all applied
water under the imposed test conditions.
OCR for page 36
D-18
800
700
600
~ 500
O 400
~Q
a,
300
200
100
o
BARRIER TECHNOLOGIES FOR ENVIRONMENTAL MANAGEMENT
1 1 1 ~1 ~ ~ 1 1 1
· Irrigated
O Nonirrigated
Des~n Storage Ca~acity
~ _ ~ ~
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
- 1~ 1 1 1 1 1 1 1 1 1 1 , . . . 1 1 1 1 1 1 1 1 1
S O N D J F M A M J J A S O N D J F M A M J J A S O
1994 1995 1996
Time (mo)
FIGURE7 Temporal Variation ~n Soil-Water Storage at the Prototype Barr~er S~nce
September 1994.
1200
1000
~ 800
E~
:; 600
V 400
200
o
O Nonirrigated f
~Irrigated ~r
~ *F ~
~,
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
I I T I ~ I I I I I I I I I I I I I I I I I I I
S O N D J F M A M J J A S O N D J F M A M J J A S O
1994 1995 1996
Time (mo)
FIGURE ~Cumulative Evapotransipiration at the Prototype Barr~er Since September 1994.
OCR for page 37
APPENDIX~PAPERS PRESENTED
D-19
Drainage did not occur from the soil-covered part of the prototype barrier, even under
the extreme conditions of 3X precipitation. These observations from the prototype agree with the
extensive lysimeter testing of capillary barriers (Campbell et al., 1990; Gee et al., 1993a) and
suggest that water storage capacity of the soil is well in excess of the 3X (480 mm) precipitation.
In contrast, the sideslope plots all drained (Figure 91. Sideslope drainage was expected since the
surfaces are coarse and bare, with no vegetation growing on the rock rewrap and only a sparse
(less than 10 percent) cover growing on the cleanfill gravel. Surprisingly, the elevated drainage
from the clean-fi11 sideslope was greater than that from the basalt r~prap sideslopes. We speculate
that the lower drainage on the r~prap sideslopes may be in part due to advective drying similar to
that described by Stormont, Anxemy, and Tansey, (1994) and Rose and Guo (19951. Additional
testing will better document the effect of advective drying on the sideslopes.
50
40
<, 30
bid
Ct
· ~
Ct
50
20
10
.
[1
Irrigated Gravel
Irrigated Basalt
Nonirrigated Gravel
Nonirrigated Basalt
05~
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
l
l
\//:
W/N
/ \~
_
S O N D 1995 F M A M J J A
Time (mo)
O N- D 1996 F M A M J J A S O
FIGURE 9 Monthly Drainage From Sideslope at the Prototype Barrier Since September
1994.
OCR for page 38
D-20
BARRIER TECHNOLOGIES FOR ENVIRONMENTAL MANAGEMENT
The rapid establishment of vegetation on the soil surface was thought to be responsible
for at least three positive benefits to surface barrier performance. First, the vegetation was
dominant in the water removal process from the soil surfaces. Second, the surface was stabilized
against water erosion and runoff. Runoff from the 1000-year storm in 1995 was 1.8 mm (about 2
percent of the 70 mm applied). There was no runoff in ~ 996. The improvement was attributed to
vegetative growth and plant establishment. Root growth caused a lowering In soil bulk density'
resulting in an increase in hydraulic conductivity and an increase in water infiltration capacity of
the soil surface. Finally, there has been a positive benefit in controlling wind erosion. After plant
establishment In November 1994, there have been no measurable Tosses of soil from the surface
of the prototype by wind erosion. This is attributed to the vegetation and lack of surface
disturbance during the past 2 years.
A minimum of 3 years of testing is planned for the prototype barrier. Because only a
finite amount of time exists to test a battier that is intended to function for a minimum of 1,000
years, the testing program has been designed to "stress" the prototype so that barrier performance
can be determined within a reasonable hme frame. Continued monitoring of prototype barrier
performance for extended periods is desirable because the succession of vegetation types, the full
development of root profiles, and the natural colonization of the barrier surface by burrowing
animals will occur over a longer time period. Long-term monitoring of the prototype barrier
would be a valuable asset for hydrologic model validation studies and in the assessment of the
long-term performance of cover systems at the Hanford Site.
CONCLUSIONS
The study of surface baIriers at the Hanford Site has evolved into an integrated
demonstration of key features of barriers designed to minimize water intrusion, erosion, and
biointrusion. The results of field tests, experiments, and lysimeter studies are providing a
defensible foundation on which barrier designs can be based. Test results show that for the
Hanford Site's arid climate, a well-designed capillary barrier limits drainage to near-zero
amounts. A subsurface asphalt layer provides additional redundancy. The data collected under
extreme events (excess precipitation) are building confidence that the barrier has the ability to
meet its performance objectives for the 1,000-year design life. A prototype battier, constructed
In 1994 is providing data for evaluating both cover and sideslope performance data needed for
final design of surface barriers at the Hanford Site. Data from the prototype confirm earlier
observations with lysimeters and field plots and show that all available water can be removed
from the soil surfaces by evapotranspiration, even under elevated precipitation conditions.
Sidesiopes, in contrast, drain because they are barren. The sidesiope drainage is less than
predicted because of wind action and possibly advective heating. Asphalt sublayers can be
successfi~1 in extending the area of surface protection and can divert drainage water away from
underlying wastes.
ACKNOWLEDGMENTS
This work is supported by the U.S. Department of Energy under contract DE-AC06-
87RL10930.
OCR for page 39
APPENDIX~PAPERS PRESENTED
D-21
REFERENCES
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Protective Barriers: Status Report for FY 1 98S, PNL-6869, Pacific Northwest
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Facility: Second Year (FY 1989) Test Results, PNL-7209, Pacific Northwest Laboratory,
Richiand, Washington.
Daniel, D. E. and S. Trautwe~n. 1991. Clay Liners and Covers for Waste Disposal Facilities,
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Gee, G. W., M. I. Payer, M. L. Rockhold, and M. D. Campbell. 1992. Variations in Recharge at
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Gee, G. W., D. G. Felmy, J. C. Ritter, M. D. Campbell, J. L. Downs, M. I. Payer, R. R. Kirkham,
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89 ~ 1, Pacific Northwest Laboratory, RichIand, Washington.
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Landeen, D. S. 1990. Animal Intrusion Status Report for Fiscal Year 1989, WHC-EP-0299,
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OCR for page 40
D-22
BARRIER TECHNOLOGIES FOR ENVIRONMENTAL MANAGEMENT
Ligotke, M. W. 1993. Soil Erosion Rates Caused by Wind and Saltat~ng Sand Stresses in a Wind
Thinned, PNL-8478, Pacific Northwest Laboratory, RichIand, Washington.
Ligotke, M. W., and D. C. Klopfer. 1990. Soil Erosion Rates from Mixed Soil and Grave]
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Link. S. O.. N. R. Wince and G. W. Gee. 1995. The Development of Permanent Isolation Barriers
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Westinghouse Hanford Company, Richiand, Washington.
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Fiscal Year ~ 989 Highlights, WHC-EP-0318, Westinghouse Hanford Company,
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Effects Task for the Hanford Site Permanent Isolation Barrier Development Program:
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, A,
Representative terms from entire chapter:
water infiltration
