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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

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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 21
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

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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.

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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

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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

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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.

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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.

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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.

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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.

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APPENDIX~PAPERS PRESENTED D-21 REFERENCES Cadwell, L. L., E. L. Eberhardt, and M. A. Simmons. 1989. Animal Intrusion Studies for Protective Barriers: Status Report for FY 1 98S, PNL-6869, Pacific Northwest Laboratory, Richiand, Washington. Campbell, M. D., G. W. Gee, M. I. Kanyid, and M. L. Rockhold. 1990. Field Lysimeter Test 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, Short Course, the University of Texas at Austin, College of Engineering, Austin, Texas. Daniel, D. E., B. A. Gross, R. C. Bachcus, C. H. Benson, I. Boschuk, Ir., S. Dutta, L. G. Everett, H. Freeman, G. W. Gee, E. Kavazanjian, Ir., R. M. Koerner, R. E. Landreth, W. E. Limbach, S. Melchior, R. W. Risky, P. R. Schroeder, K.Skahn, J. C. Stormont, A. Street, F. C. Walberg, and T. F. Zimmie. 1996. Caps. In R. R. Rumer and J. K. Mitchell (eds.~. pp 119- 140. Assessment of Barrier Containment Technologies. International Containment Technology Workshop. August 1995. Baltimore, Maryland, U. S. Department of Energy, U. S. Environmental Protection Agency, and DuPont Company. NTIS Springfield, Virginia, Publication #PB96-180583. Freeman, H. D. and G. W. Gee. 1989, Hanford Protective Barriers Program: Status of Asphalt Bamer Study - FY 1989, PNL-7513, Pacific Northwest Laboratory, Richland' Washington. Gee, G. W., M. I. Payer, M. L. Rockhold, and M. D. Campbell. 1992. Variations in Recharge at the Hanford Site. Northwest Sci. 66: 237-250. Gee, G. W., D. G. Felmy, J. C. Ritter, M. D. Campbell, J. L. Downs, M. I. Payer, R. R. Kirkham, and S. O. Link. 1993a. Field Lysimeter Test Facility Status Report IV: FY 1993, PNL- 89 ~ 1, Pacific Northwest Laboratory, RichIand, Washington. Gee,G.W.,L.L.Cadwell,H.D.Freeman,M.W.Ligotke, S.O.Link,R.A.Romine,andW.H. Walters, Jr. 1993b. Testing and Monitoring Plan for the Permanent Isolation Surface Barrier Prototype, PNI,-8391, Pacific Northwest Laboratory, RichIand, Washington. Gee, G. W., A. L. Ward, B. G. Gilmore, S. O. Link, G. W. Dennis, and T.K. O'Neil. 1996. Hanford Protective Battier Status Report. FY 1996. PNNL- 11367. Pacific Northwest National Laboratory, Richiand, Washington. Gilmore, B. G. and W. H. Walters. 1993. Water Erosion Field Tests for Hanford Protective Barriers: FY 1992 Status Report, PNL-8949, Pacific Northwest Laboratory, Richiand, Washington. Landeen, D. S. 1990. Animal Intrusion Status Report for Fiscal Year 1989, WHC-EP-0299, Westinghouse Hanford Company, RichIand, Washington. Landeen, D. S. 1991. Animal Intrusion Status Report for Fiscal Year 1990, VVHC-EP-0398, Westinghouse Hanford Company, Richiand, Washington. Landeen, D. S. 1994. The Influence of Small Mammal Burrowing Activity on Soil Water Storage at the Hanford Site, WHC-EP-0730, Westinghouse Hanford Company, RichIand, Washington. Landeen, D. S., L. L. Cadwell, L. E. Eberhardt, R. E. Fitzner, and M. A. Simmons. 1990. Animal Intrusion Field Test Plan, WHC-EP-0253, Westinghouse Hanford Company, Richiand, Washington.

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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] Surfaces in a Wind Thinned, PNL-7435, Pacific Northwest Laboratory, RichIand, Washington. Link. S. O.. N. R. Wince and G. W. Gee. 1995. The Development of Permanent Isolation Barriers for Buried Wastes in Cool Deserts: Hanford Washington. J. Arid Land Studies 4:215- 224. Link, S. O., L. L. Cadwell, K. L. Petersen, M. R. Sackschewsky, and D. S. Landeen. 1995. The Role of Plants and Animals In Isolation Barriers at Hanford Washington. PNL-10788. Pacific Northwest National Laboratory, Richiand, Washington. Petersen, K. L., I. C. Chatters, and W. I. Waugh. 1993. Long-Term Climate Change Assessment Study Plan for the Hanford Site Penmanent Isolation Battier Development Program, WHC-EP-0569, Revision 1, Westinghouse Hanford Company, Richiand, Washington. Petersen, K. L., S. O. Link, and G. W. Gee. 1995. Hanford Site Long-Term Surface Barrier Development Program: Fiscal Year 1994 Highlights. PNL-10605. Pacific Northwest Laboratory, Richiand, Washington. Rose, A. W., and W. Guo. 1995. Thermal Convection of Soil Air on Hillsides. Environ. Geology 25:258-262. Sackschewsky, M. R., C. I. Kemp, S. O. Link, and W. I. Waugh. 1995. Soil Water Balance Changes in Engineered Soil Surfaces, I. Environ.Quality 24: 352-359. Stone, W. A., I. M. Thorp, O. P. Gifford, and D. I. Hoitink. 1983. Climatological Summary for the Hanford Area, PNL-4622, Pacific Northwest Laboratory, RichIand, Washington. Stormont, I. C., M. D. Ankeny, and M. K. Tansey. 1994. Water Removal from A Dry Barner Cover System, In G. W. Gee and N. R. Wing (eds.~. pp. 325-346. In-Situ Remediation: Scientific Basis for Current and Future Technologies, Parts 1-2. Thirty-Third Hanford Symposium on Health and the Environment. November 7- 11, ~ 994, Pasco, Washington, Battelle Press, Columbus, Ohio. Waugh, W. I., M. E. Thiede, D.~. Bates, L.L. Cadwell, G. W. Gee, and C. I. Kemp. 1994. Plant Cover and Water Balance in Grave! Admixture at an Arid Waste-Burial Site, I. Environ. Qual. 23:676-685. Wing, N. R. 1993. Permanent Isolation Surface Barner Development Plan, WHC-EP-0673, Westinghouse Hanford Company, Richiand, Washington. Wing, N. R., and G. W. Gee (eds.~. 1990. Hanford Site Protective Barrier Development Program: Fiscal Year ~ 989 Highlights, WHC-EP-0318, Westinghouse Hanford Company, Richiand, Washington. Wing, N. R., and G. W. Gee. 1994. Quest for the Perfect Cap. Civil Engr. 64~10~:38-41. Wing, N. R., K. L. Petersen, C. Whitiock, and R. L. Burk. 1995. Long-Term Climate Change Effects Task for the Hanford Site Permanent Isolation Barrier Development Program: Final Report. BHI-00144, Bechtel Hanford, Tnc., Richiand, Washington. , A,