mtg summary - designing, building, & regulating ... · rock then asked for a show of hands from...

SUMMARY OF THE REMEDIATION TECHNOLOGIES DEVELOPMENT FORUM ... of 19

SUMMARY OF THE REMEDIATION TECHNOLOGIES DEVELOPMENT FORUMPHYTOREMEDIATION OF ORGANICS ACTION TEAM MEETING

Adams Mark HotelDenver, Colorado SFUND RECORDS CTRMarch 9-10, 2004 2065264

WELCOME AND INTRODUCTIONS

Opening RemarksSteve Rock, U.S. Environmental Protection Agency (EPA)

Steve Rock, co-chair of the Remediation Technologies Development Forum's (RTDF's)Phytoremediation of Organics Action Team, welcomed meeting attendees (see Attachment A, PDF, 15pp., 134 KB) and thanked the organizing committee, session chairs, and speakers for making themeeting possible. Rock explained that the meeting would discuss the state of development ofevapotranspiration (ET) landfill cover systems and focus on the design, construction, and regulation ofsuch systems.

Regulatory Acceptance of Alternative Landfill Covers (see Attachment B, PDF, 14 pp., 264 KB)Gary W. Baughman, Colorado Department of Public Health and Environment

Baughman indicated that the acceptance of alternative landfill covers presents a challenge to theregulatory world due to the increased time, cost, and risk associated with reviewing and approving suchnew technologies. He explained that the Interstate Technology and Regulatory Council (ITRC) focuseson these challenges and noted that one of ITRC's main objectives is to develop guidance and training forstate regulators on how to address and confront regulatory barriers. Baughman added that of the 15,000people who have participated in ITRC classroom or Internet training, 87 percent said that theinformation presented would help them save either time or money in the process of reviewing andaccepting innovative technologies.

Baughman emphasized that there is a certain amount of regulatory flexibility in accepting alternativelandfill covers. He said that regulatory language exists that allows one to circumvent explicit statutes ifan alternative technology is proven to be equally effective as a conventional remedy. The challenge liesin proving equal effectiveness. Baughman indicated that numerous demonstration projects have beeninitiated to study the effectiveness of alternative landfill covers, including six covers in Colorado thathave gained regulatory approval. He added that few regulatory barriers have been encountered whenimplementing these demonstration projects, noting that review and approval of alternative covers underthe current regulatory structure is not as difficult as one might think.

Landfill ET Covers - Past Myth, Current Fact, Possible Future (see Attachment C, PDF, 56 pp.,1,945KB)Louis Licht, Ecolotree® Inc.

Licht explained that an ET landfill cover can be considered a subset of plant-augmented bioremediation,also called phytoremediation. He noted that this field is relatively new; yet a critique of the early-promised benefits and fears can now be made based on the information gleaned from research andinstrumented-prototype ET covers. Since 1990, Licht said, approximately 20 sites have used a treeoverstory and grass understory design for permitted final closure. Data from these demonstration sitesand EPA's Alternative Cover Assessment Program (ACAP) are being used to help inform the ET cover

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rSUMMARY OF THE REMEDIATION TECHNOLOGIES DEVELOPMENT FORUM ... of 19 „ '

design process. These data, in conjunction with the original projected outcomes of the demonstrationinstallations, can be reviewed in retrospect to evaluate which ofthe conceptions about ET landfill coversare myth and which are fact. Licht's presentation focused on two questions:

• Based on these measured "facts," what cover options exist and are possible for a finished landfillcell closure?

• Based on the ongoing research, what are possible future scenarios for a landfill now operating buteventually going to ET closure?

Licht emphasized that an ET cover uses plants as an engineered system that speeds up contaminantcapture. The functions of plants within such a system include (1) water removal, (2) microbestimulation, (3) decomposer stimulation (i.e. worms, microflaura, etc.), and (4) soil stabilization. If doneproperly, this system will result in contaminant sequestration, pollutant uptake, and pollutantmineralization. As a result, ET covers prevent greenhouse gases (GHGs) from emitting into the air andprevent volatile organic compounds (VOCs) from being released into the groundwater.

Licht presented information about an ET landfill cover that had been installed at a construction debrisdemonstration project in Oregon. He noted that after more than a decade of strong growing seasons(using poplar trees) the ET cover at this site failed due to poor fertility, fungus, drought stress, andpossible gas toxicity. As a result, a different plant mix was added (conifer/poplar blend) and is currentlyunder study.

Licht concluded that the use of ET covers is far from a mature science, noting that these covers are noteffective for all sites, and that operators are still learning how to prioritize and manage stress.Nevertheless, the use of ET covers will gain acceptance as pioneering plants give way to diversity andmaturity that better protects the environment and human health.

PerspectivesSteve Rock, U.S. EPA

Rock outlined three generations of landfill technology. The first generation of waste technology wasopen and uncontrolled dumping, with burning waste. The second set of technology was codified inRCRA, with liners, leachate control systems, daily cover requirements, restrictions on what isacceptable, and final closure guidance. The third generation is currently in the research demonstrationstage and includes bioreactors and other forms of leachate recirculation, semi-permeable covers, and ETcovers.'

Rock then asked for a show of hands from regulators, teachers/students, consultants, and site owners toemphasize the diversity in the crowd and indicate that people will have differing opinions for eachpresentation. Each group has their own needs and organizational goals, and they all have to be taken intoaccount. He asked that the group keep this in mind throughout the meeting.

SESSION 1: DESIGN AND CONSTRUCTIONSession co-Chairs: Glendon Gee and Jim Norstrom

Design Guidance (see Attachment D, PDF, 26 pp., 422 KB)Craig Benson, University of Wisconsin-Madison

Benson presented information about a five-step sequential methodology that can be used to implementan alternative landfill cover. He said that this methodology has been developed based on more than a

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decade of experience with alternative covers. The five steps are:

• Site characterization. Site characterization includes identifying meteorological conditions,defining potential borrow sources for soils, and determining the type of vegetation that is native tothe area. Meteorological data are compiled from historical records and soil samples are collected

, and tested to define their mechanical and hydraulic properties (i.e. compaction, saturatedhydraulic conductivity, soil water characteristic curve, and shear strength). Information about thecoverage, growing season, and rooting characteristics ofthe vegetation is also collected and thedesign meteorological period is selected.

• Preliminary design based on storage. During the preliminary design phase, analytical calculationsare performed to determine the soil water storage capacity of the soil and the cover thickness thatis required to meet a performance goal. These calculations also indicate whether a monolithiccover design is satisfactory or if a capillary barrier is required.

• Design refinement using numerical models. Benson said that analytical calculations must bechecked against more realistic numerical models during the design refinement process, noting thatthe cover profile may need to be refined based on these numerical models to ensure efficiency.

• Design details (runoff, erosion, desiccation, frost, biota intrusion, etc.). Benson indicated that thedesign details of alternative landfill covers are similar to those encountered when designingconventional covers. Design details are aspects other than the barrier system that are needed tosustain adequate performance and/or to satisfy regulatory requirements. In some cases, theoutcome of this step may require additional analytical calculations and numerical modeling.

• Performance evaluation and monitoring. The final step is performance evaluation and monitoring.In this step, a method is selected to confirm that the design goal has been achieved. The methodmay consist of water content monitoring, lysimetry, physical observations, or a combinationthereof. Of these methods, lysimetry is preferred.

Benson concluded that when using this five-step approach, you must (1) be realistic about sitesuitability, (2) locate soil with sufficient storage capacity that satisfies all engineering and agronomiccriteria, (3) account for scaling (since laboratory results may not transfer to field testing), and (4) checkthe design using verified models with justifiable input parameters and reasonable output.

Ecological Design and Revegetation (see Attachment E, PDF, 23 pp., 959 KB)Amy Forman, S.M. Stoller Corporation

Forman explained that ET covers have two primary and equally important components: (1) a soil capsufficient to store precipitation while plants are dormant, and (2) a plant community sufficient to depletesoil moisture during the growing season. She noted that the configuration ofthe soil cap generallyreceives more consideration than the elements of the vegetation community during the design process,yet emphasized that the plant community is at least as important to the long-term effectiveness of an ETcover as the soil cap.

Forman supported her position by presenting information that has been collected from the ProtectiveCap/Biobarrier Experiment (PCBE) at the Idaho National Engineering and Environmental Laboratory(INEEL). The PCBE consists of four soil cap configurations planted in two vegetation types andsubjected to three precipitation regimes. Ultimately, the PCBE demonstrated that with a thoughtful andcomprehensive revegetation design, native plant species can be quickly established on ET covers in

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semi-arid regions and they typically perform better than exotic monocultures. Results from the PCBEillustrate the importance of carefully considering revegetation as an integral part of a complete capdesign. Three aspects of revegetation are especially important for designing a plant community for afinal cover: (1) choosing appropriate plant material, (2) implementing effective planting andestablishment techniques, and (3) considering long-term plant community change and associated wateruse.

Forman declared that the ability of a landfill cap to function effectively and contribute to the long-termland management goals of a particular site is largely a consequence ofthe materials planted there.Therefore, decisions pertaining to plant materials should address (1) the use of seed compared toseedlings, (2) the genetic makeup ofthe plant material used, (3) the compilation of a species mix withdesired root distributions, and (4) the choice of species with growth habits that contribute to thefunctional stability ofthe cover.

Forman stressed that an effective revegetation design should also incorporate strategies to increaseplanting success and facilitate establishment. These strategies include: (1) planting in densities anddistributions similar to adjacent plant communities, (2) using mulch, (3) controlling undesirable weeds,and (4) using supplemental irrigation. Forman added that factors, such as global climate change,invasion of undesirable species, and catastrophic disturbances should be considered when developing arevegetation design.

Borrow Source Considerations (see Attachment F, PDF, 31 pp., 1,929 KB)Patrick McGuire, Earth Tech

McGuire noted that the Resource Conservation Recovery Act (RCRA) indicates that a regulated landfillmust have a hydrologic barrier cover that complies with prescribed design criteria. However, alternativedesigns are allowed as long as their performance is equivalent to that which is exhibited by conventionalprescribed-design covers. In arid and semi-arid climates, alternative covers rely on soil water storage,establishment of vegetation, and soil water loss through evapotranspiration to restrict deep drainage. Toexplore these points in more detail, McGuire provided information about a 6.1 hectare (15 acre) ETcover located at the U.S. Army-Fort Carson site in Colorado Springs, Colorado.

At this site, McGuire said, soil characterization ofthe borrow area was conducted to (1) inventorysuitable and unsuitable soils based on hydraulic and productivity properties, (2) develop numericalmodel input values, and (3) establish a target soil compaction range based on the undisturbed borrowarea conditions. He noted that the borrow soil is predominantly clay loam, formed in alluvial and aeoliandeposits, with a dry bulk density that is typically less than 1.3 grams per cubic centimeter (g/cm3). Headded that a numerical water balance model (UNSAT-H) predicted that annual drainage through a 122-cm (48-inch) thick clay loam, based on four continuous years of high annual precipitation at 53 cm (20.8in), was near or less than 0.1 mm (0.004 in).

McGuire explained that the ET cover at the U.S. Army-Fort Carson site was constructed by placementof four soil lifts, each of which was 30 cm (12 in) thick. During construction, haul routes were definedto reduce the cover impact area, and low ground pressure dozers were used to work the soil. Followinglift placement, cover areas were tilled, when necessary, to achieve a compaction that did not exceed 80percent ofthe Proctor test maximum dry density. Management practices that were used to establish thepermanent plant cover included: (1) incorporation of biosolids, (2) soil fertilization, (3) straw mulching,(4) use of erosion blankets, and (5) irrigation. McGuire added that post-construction analysis of thecover indicates a relatively uniform soil type that is consistent with the borrow soil characterization.Other aspects include: (1) a predominantly clay loam ET cover, (2) a measured dry bulk density, basedon limited sampling; typically less than 1.50 g/cm3 (or 90 percent ofthe Proctor test maximum dry



density); (3) a measured clay loam water storage capacity of about 43 cm (17 in); (4) suggested upwardunsaturated flow of soil water from the ET cover surface; and (5) an established dominant westernwheatgrass on the ET cover.

Experiences with Placement of Alternative Final Covers (see Attachment G, PDF, 25 pp., 659 KB)Leonard Butler, Waste Management of Colorado, Inc.

Butler indicated that Waste Management of Colorado, Inc. has three ongoing projects involving theplacement of alternative final covers (AFC) at municipal waste landfills in Colorado. He focused hispresentation on the Denver Arapaho Disposal Site (DADS) and highlighted seven lessons that werelearned when installing the AFC at DADS:

• Designate the soil borrow areas to be used for cover material and conduct testing to determine thepercent weight that is finer than the #200 Sieve (according to design specifications).

• Develop workable construction specifications that allow compaction between 80 to 90 percent ofmaximum density (dry of optimum moisture content) as determined by Standard Proctor (ASTMD 698). The ability to construct AFCs by reducing the compaction range (e.g., 85 percent to 88percent) is more difficult and expensive and does not necessarily model natural in situ soilplacement.

• Retrain heavy equipment operators to ensure the AFC is lightly compacted at full thickness.

• Select a vegetative mix that includes both cool- and warm-weather germinating grasses anddeeper- rooted plant species. Periodic inspection ofthe seeded area is critical to AFC success.

• No adjustment is necessary to accommodate overland flow on side slopes with a ratio less than4:1.

• Use a diverse vegetative mix, with deeper rooted plant species that hold soil, to minimize slopestability issues.

• Utilize a Construction Quality Assurance (CQA) Program to help streamline construction andprovide documentation of compliance with construction specifications.

Butler said that the experience gained at DADS confirms that the keys to successful AFC placement are(1) identification of borrow areas, (2) workable specifications, (3) heavy equipment operator retrainingfrom past cover projects, (4) careful selection of seed mix, and (5) a CQA program that will comply withconstruction specifications and record documentation requirements. Butler reiterated that thisinformation should be taken into consideration during the planning and construction of AFCs.

Session 1 - Panel Discussion

Discussion topics included:

• Release date of EPA's Final Cover Guidance Document for RCRA/CERCLA Sites—Thedocument is currently under review by EPA's Office of Solid Waste. Comments should beincorporated and the document should be released within 3 to 6 months.

• Installation costs of poplar trees—Licht indicated that it takes 3 to 4 years to obtain an effective

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poplar canopy. The cost to plant, irrigate, and solidify the trees during this time period isapproximately $8,000 to $12,000 per acre.

• Consideration of thicker lifts—An attendee noted that thicker lifts lead to less compaction andquestioned what the maximum lift thickness is for a successful ET cover. Butler said that theDADS site utilized 18-inch lift thickness and that he recommends a lift between 12 to 18 inches.Licht added that global position systems (GPS) allow equipment to cover the same continuoustrack and in turn minimize compaction.

• Distinction between construction and maintenance—An attendee asked when constructionends and maintenance begins. Butler and Carl Mackey (of RMA) noted that construction endswith placement ofthe seed or seedling.

• Use of invasive weeds—An attendee asked why invasive weeds cannot be beneficial. Formanresponded by saying that weeds grow without absorbing a lot of moisture, noting that adequateabsorption potential is key to the success of ET covers. In addition, she added that stateregulations mandate the control of invasive species, including weeds.

• Interest in engineered vegetation for remediation—Licht indicated that while engineeredvegetation may prove to be useful in ET cover systems, uncertainties associated with geneticallymodified organisms lead him to question their immediate use.

• Natural correction of soil density—An attendee noted that even with freezing and thawing, soilcompaction may not naturally correct itself. He pointed to a study in Minnesota, in which acompaction affect can be seen 100 years after the initial stress, and warned that ET cover systemsuccess is dependant on root depth, which is partially dependant on soil density. Site owners needto be aware of this and avoid excessive soil compaction during construction activities.

SESSION 2: MODELINGSession co-Chairs: Bridget Scanlon and Beth Gross

ET Cover Modeling Introduction (see Attachment H, PDF, 4 pp., 54 KB)Beth Gross, GeoSyntec Consultants

Gross introduced those who would be presenting during this session and provided an overview ofthetopics that would be discussed. She indicated that several numerical models exist, but that there aremany issues that affect the accuracy of their simulations. For instance, current models do not accuratelyaccount for certain processes (e.g. runoff) or attributes (e.g. plant growth, snow accumulation, and snowmelt). In addition, input parameters change over time and are oftentimes difficult to measure.

Monitoring versus Modeling ET Covers for Performance Evaluation (see Attachment I, PDF, 36pp., 759 KB)Bridget Scanlon, University of Texas-Austin

Scanlon indicated that assessing the performance of ET covers is complicated; therefore, it is importantto apply different approaches, including monitoring and modeling. She described how detailedmonitoring and modeling of ET covers in both Texas (with l . lm of silty sand) and New Mexico (with2.0 m of thick silty clay loam) have provided valuable information on different approaches for assessingET cover performance.



In monitoring the water balance at the Texas and New Mexico sites, Scanlon found that 3-dimensional(3D) variability in water storage is related to topography. Specifically, low-water storage occurs inupland areas while high-water storage occurs at the base ofthe slopes. In addition, temporal variabilityin water storage is controlled by the presence of vegetation, precipitation, and ET. At both sites, Scanlonsaid that her team found that drainage was zero. Scanlon noted that the two sites are particularly suitablefor ET covers because ofthe dominance of summer precipitation, which coincides with the time of yearwhen ET is maximized. The team also found that long-term model simulations (30 years) yielded similarresults to the short-term (4-5 years), a finding that leads investigators to believe that ET covers shouldperform adequately over the long term.

Scanlon emphasized that the performance studies at these sites provide valuable insights that can beused to guide future monitoring and modeling studies. Important factors with respect to monitoringinclude (1) drainage monitoring, (2) length of monitoring record, (3) spatial variability, and (4) emphasison vegetation monitoring. Scanlon added that the capillary barrier effects created by lysimeters result inunderestimation of drainage and overestimation of soil water storage. In addition, it is important to notethat short term monitoring is dominated by the effects of initial conditions (i.e. construction effects etc).In closing, Scanlon recommended:

• Expanding vegetation monitoring because ofthe dominant role vegetation plays in controlling thewater budget of ET covers.

• Expanding modeling analysis beyond the traditional 1-dimensional analysis to capture 3D floweffects.

• Developing codes that better simulate vegetative effects on the water balance.

• • Referencing the Texas and New Mexico ET covers when designing future monitoring andmodeling analyses.

Fact or Fiction: Comparing Model Predictions and Field Data from ACAP (see Attachment J, PDF,32pp., 1,104KB)Craig Benson, University of Wisconsin-Madison

Benson noted that numerical models are often used during alternative cover design to evaluate thesufficiency of a cover profile or to demonstrate that an alternative cover meets an equivalency criterion.He indicated that a variety of models exist that can be used in this manner; the most common beingUNSAT-H, HYDRUS-2D, Vadose/W, and LEACHM. Each of these models has been (to some degree)evaluated with field data, but none has been subjected to an evaluation where all ofthe input parametersand output quantities have been measured.

Benson described (1) a comparison between water-balance measurements made at four sites in ACAPand (2) predictions made with the two most commonly used models, UNSAT-H and HYDRUS-2D. Thefour sites are in semi-arid and sub-humid climates ranging from seasonal without snow to widelyvarying conditions (including hot summers combined with freezing, snowy winters). Bensonemphasized that input to the models was measured to the greatest extent possible, meteorological datawere collected, and the properties ofthe soil and vegetation were extensively characterized.

Benson's team found that UNSAT-H generally provided more accurate predictions ofthe water balancethan HYDRUS-2D. However, predictions generally were in poor agreement with field water-balancedata. The team determined that surface runoff generally is over-predicted, which results in under-predictions in ET, soil water storage, and percolation.



Benson noted that sensitivity analyses show that the three most influential parameters are (1) thesaturated hydraulic conductivity ofthe cover soils, (2) the n-parameter in van Genuchten's equation, and(3) the intensity ofthe precipitation relative to the saturated hydraulic conductivity. More reasonablepredictions can be obtained by increasing the saturated hydraulic conductivity by a factor between 10and 20 and by ensuring the intensity ofthe precipitation closely resembles that occurring in the field.Nevertheless, even with these changes, Benson added that percolation can only be predicted within anaccuracy of ±10 mm/yr.

Overland Flow Implications on Surface Cover (see Attachment K, PDF, 18 pp., 1,425 KB)EarlMattson, INEEL

Mattson explained that water flow and solute transport on a hill slope are complex nonlinear issues.Rainwater initially infiltrates at a rate equal to the rainfall rate, however once the soil infiltrationcapacity is reached, surface runoff is generated and water is redistributed along sloped surfaces. As aresult, water usually infiltrates at the lower parts of a hill slope, where there are generally longer surfaceponding times and vegetation density. Mattson added that variable infiltration along a hill slope hassignificant consequences for plant growth and the overall water balance of ET covers.

To describe these complex interactions, Mattson's group coupled the HYDRUS-2D software packagewith a newly developed overland flow routine, simulating water flow and solute transport in variablysaturated porous media. The overland flow solver uses a fully implicit, four-point, finite differencemethod to numerically solve the one-dimensional kinematic wave equation (with overland fluxesevaluated using Manning's hydraulic resistance law). Mattson noted that a Picard iterative solutionscheme, similar to the one used for solution ofthe Richard's equation, is invoked to solve the resultingsystem of nonlinear equations. The subsurface flow module determines the main time step for thecoupled system, and, if required for numerical stability, the overland flow module can use multiplesmaller time steps. This type of time management considers the fact that overland flow and variably-saturated subsurface flow often run at quite different time scales.

Mattson presented several ET cover examples ofthe updated HYDRUS-2D program and showed thedevelopment of overland flow as a function of storm intensity and slope angle. He explained that simpleexamples verify the accuracy ofthe numerical implementation against an analytical solution, while morecomplex examples examine infiltration with and without the overland flow modifications along a hillslope.

Mattson also discussed the potential of positive feedback loops between the interaction of (1) run-on, (2)vegetative growth, and (3) permeability changes. Additional infiltration will occur along slope breaks,such as the toe of a landfill, due to overland flow from precipitation events. In arid and semi-aridclimates, this additional infiltration will result in enhanced plant growth. Mattson presented the resultsfrom several studies that illustrated the positive relationship between plant growth density and saturatedhydraulic conductivity. The increased hydraulic conductivity will lead to greater amounts of infiltrationand enhance the feedback loop. Mattson ended his talk by suggesting that these mechanisms areresponsible for the variation in subsurface moisture contents and vegetation seen at landfills illustratedby Dr. Scanlon. Currently available numerical models using Richard's equation for landfill cover designdo not account for overland flow using Manning's equation and incorporate the hydrologic feedbackloop processes.

Prediction of Water and Energy Balance in Surface Covers and Protective Side-slopes Using theSTOMP Simulator (see Attachment L, PDF, 16 pp., 809 KB)Andy Ward, Battelle



Ward indicated that surface barriers are being considered for final closure within most U.S. Departmentof Energy (DOE) waste sites. At DOE's Hanford (Washington) site alone, some 200 barriers will bedeployed to cover over 1,000 acres. Ward explained that existing tools are limited in their ability torepresent the multidimensional, non-isothermal, multi-phase transport of mass and energy that governsthe performance of field-scale cover systems. However, the STOMP simulator was recently extended forapplication to water- and energy-balance predictions in surface barriers and their protective side-slopes.

Ward shared unique features ofthe STOMP simulator including:

• Use of the Shuttleworth-Wallace sparse canopy ET model to account for the potential ET rate(ETP) of multiple plant species.

• Independent computation of root uptake and transpiration rate using evaporation rate as a functionof plant type and atmospheric condition.

• Use of spatially and temporally variable plant area index to partition ETP into potentialevaporation (Ep) and potential transpiration (Tp).

• Explicit calculation ofthe evaporating surface depth to circumvent arbitrary depth selection.

• Evaluation ofthe impact of storm intensity and leaf area index on condensation and evaporation inthe plant canopy.

Ward added that the model has been coupled with UCODE to facilitate automatic calibration andsensitivity analysis. He noted that calibration and validation exercises show solid agreement between thesimulated and observed water/energy balance in potential Hanford Site designs.

Session 2 - Panel Discussion


• Use of a ditch or culvert to account for excess water infiltration at the foot of slopes.

• Underestimation of drainage by shallow lysimeters.

• Measuring flow in a manner that replicates reality.

• Reducing landfill slopes to ensure all water percolates into the ET system.

SESSION 3: CASE STUDIESSession co-Chairs: Bill Albright and Craig Benson

Coast to Coast: Performance Data from the ACAP Field Sites (see Attachment M, PDF, 44 pp.,1,136KB)Bill Albright, University of Nevada, Desert Research Institute (DRI)

Albright noted that landfill covers constitute a major expense to landfill operators, yet performance ofspecific cover designs has not been well documented and seldom compared in side-by-side testing. In

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1998, EPA initiated the Alternative Cover Assessment Program (ACAP), a comprehensive studydesigned to evaluate conventional and alternative covers over a range of climates (humid to arid).Albright added that ACAP tests the ability to control landfill water-balance and minimize drainagethrough the cover. At 11 field sites in 7 states, Albright's group monitored conventional coversemploying (1) resistive barriers (i.e. soil layers with low saturated hydraulic conductivity or compositebarriers consisting of a geomembrane over a soil barrier), and (2) alternative covers relying on water-storage principles. His team found that:

• Surface runoff from the covers was a small fraction ofthe water balance (0-10 percent, and 4percent on average) and was nearly insensitive to the cover slope, barrier type, or climate.

• Lateral drainage from internal drainage layers was also a small fraction ofthe water balance (0-5percent, and 2 percent on average), but was typically a larger fraction ofthe water balance athumid sites.

• Average percolation rates for the conventional covers with a composite barrier typically were lessthan 1.4 percent of precipitation (12 mm/yr) at humid locations and 0.4 percent of precipitation(1.5 mm/yr) at sites in arid/semi-arid/sub-humid locations.

• Conventional covers with soil barriers in humid climates had percolation rates ranging between 6-17 percent of precipitation (52 and 195 mm/yr). These high rates were attributed to flow throughcracks and other defects in the soil barrier.

• Average percolation rates for alternative covers ranged between 6 and 18 percent of precipitation(33 and 160 mm/yr) in humid climates and generally less than 0.4 percent of precipitation (2.2mm/yr) in arid/semi-arid/sub-humid climates.

• Fifty percent ofthe alternative covers (5 total) in arid/semi-arid/sub-humid climates transmittedless than 0.1 mm of percolation.

• Two ofthe alternative covers had percolation rates much higher than anticipated due to inadequatestorage or limited transpiration capacity.

Albright indicated that successful cover design requires careful attention to many technical details.Observation is a step toward understanding, but there is still need for model improvement. He concludedthat there is much to be gained from destructive sampling of ACAP covers.

Design and Construction of an ET Cover in the Eastern United States (U.S.) (see Attachment N,PDF, 29 pp., 2,083 KB)Beth Gross, GeoSyntec Consultants

Gross compared ET cover systems in the western United States to those in the eastern United States. Sheadded that due to problems with long-term compacted clay barrier performance and potential costsavings, ET barriers (rather than compacted clay barriers) are being increasingly used in cover systemsat semi-arid and arid sites. ET barriers are also used in humid climates, but to a lesser extent than theyare used in drier climates and generally only when a relatively high level of percolation is acceptable.

Gross presented on a series of ET cover systems designed for sludge impoundments at an industrialfacility in the eastern United States. She indicated that average annual precipitation at the site isapproximately 860 mm. The cover systems consist of (from top to bottom): (1) 15 cm topsoil, (2) 45 cmon-site soil, and (3) 60 cm flyash. To accommodate site conditions and eventual end-use, three types of



planting schemes were developed: (1) upland forest plants for higher elevations, (2) marsh edge plantsfor lower elevations, and (3) short upland forest plants along a utility easement. Gross evaluated long-term average annual cover system percolation using UNSAT-H and found a percolation range of 90 to130 mm/yr. Cover system construction began in 2003 with hopes of converting the site into a 100-hectare (ha) park.

Field Performance Monitoring of ET Cover Systems at Mine Sites in Australia, Canada, and theUnited States (see Attachment O, PDF, 36 pp., 2,582 KB)Mike O 'Kane, O 'Kane Consultants, Inc.

O'Kane presented ET cover system field performance monitoring data from sites in Australia, Canada,and the United States. He focused on (1) fundamental processes controlling performance of ET coversystems, (2) lessons learned through monitoring of full-scale and large-scale field trial ET coversystems, and (3) research required to develop defensible predictions of long-term performance. Areas ofdiscussion included:

• Appropriate conceptual, analytical, and numerical modeling tools.

• Appropriate field performance monitoring systems for research and full-scale ET cover systems.

• Physical, chemical, and biological processes affecting the long-term performance of an ET coversystem.

• Negative impact of successive above-average wet climate years on ET cover system performance.

• Influence of precipitation characteristics (i.e. duration, intensity, form, and date) on ET coversystem performance.

• Presence of vegetation and its positive influence on ET cover system performance.

• Segregation that occurs .during the placement of cover material.

O'Kane concluded that applying a successful design from one site to the next is a potentially fatal coversystem design flaw, especially when material properties, slope angles, slope lengths, and climateconditions differ between the sites. He suggested that the design methodology, not the actual design, betransferred from one site to the next, and that methodologies be updated as new information becomesavailable.

Session 3—Panel Discussion


• Cost of installing an ET system in the eastern United States.

• Consideration of hysteresis to explain the difference in site release curves.

• Tendency of composite covers to undergo ET during the summer months.

• Confidence in the use of short-term lysimeter studies to provide data for long-term modeling.


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• The need to revisit the monitoring of moisture storage during the dry season.

• The need to study natural recharge rates within the region that a site is located.

SESSION 4: MONITORING, LONG-TERM STABILITY, AND THE REGULATORYDILEMMA

Session co-Chairs: Kerry Guy and Bill Albright

The Quest for Consistency Among Regulatory, Design, and Post-Closure Monitoring Frameworks(see Attachment P, PDF, 30 pp., 1,731 KB)Jorge Zornberg, University of Texas-Austin

Zornberg explained that although ET cover systems are becoming acceptable alternatives for hazardousand municipal waste landfills located in arid climates, design methods and post-closure monitoringapproaches are not yet well established. This is partly due to the lack of consensus on how to translateregulatory requirements (i.e. an equivalence demonstration) into criteria for design and post-closuremonitoring. Zornberg noted that equivalence demonstration approaches have included the use of eithernumerical simulations or field monitoring demonstrations. Numerical simulation strategies haveinvolved comparison ofthe performance of an ET cover with that of a prescriptive cover (i.e.comparative criterion), while field monitoring demonstration approaches have involved the definition ofa maximum acceptable percolation for the ET cover (i.e. quantitative criterion).

The design of ET covers should quantify the parameters that (1) minimize the infiltration of liquids intothe cover soils, (2) enhance the storage of moisture during the rainy season, and (3) promote thesubsequent release of moisture during the dry season. Zornberg emphasized that there is a lack ofconsensus regarding the design parameters that govern the performance ofthe system. This often arisesfrom the need to compromise between soil conditions that correspond with enhanced hydraulicproperties and those that correspond with enhanced vegetation development.

Zornberg indicated that post-closure monitoring programs have been evaluated for the assessment oflong-term ET cover performance as well as for extended equivalence demonstration. Since the overallobjective of any type of cover system is to minimize liquid percolation, he noted that post-closuremonitoring programs often involve flux rate monitoring. The overall performance of an ET cover relieson its ability to store moisture, which can be assessed by monitoring changes in moisture profiles.Zornberg concluded by discussing the need to achieve consistency among regulatory requirements,design methods, and post-closure monitoring.

Challenges in Monitoring ET Covers (see Attachment Q, PDF, 36 pp., 1,352 KB)Glendon Gee, Battelle

Gee defined an ET cover as a vegetated soil that acts like a sponge by storing excess precipitation duringwet periods and removing water via ET during dry periods. He noted that an effective ET cover greatlylimits drainage through the soil and minimizes leaching ofthe landfill waste. If and when leachate isgenerated, it drains to the water table, where by law it is monitored in down-gradient wells. In contrast,actual monitoring of ET covers is not required by law, but is needed to prove that ET covers areequivalent in performance to more conventional resistive-layer covers.



Gee explained that in arid and semi-arid locations, where the water table is deep and contaminant traveltimes are long, cover monitoring can be used as an early warning of potential groundwatercontamination. While drainage rates through the cover and resulting leachate production rates may notbe as important as actual risk, some regulatory groups have set target limits for ET cover drainageranging from 1 to 3 mm/yr.

Gee noted that verifying low drainage fluxes from ET covers and demonstrating equivalency toconventional covers is a topic of debate. He added that water content and water potential sensors aregenerally inadequate because they do not measure flux rates directly. In addition, water sensing datamust be coupled with estimates ofthe soil's unsaturated hydraulic conductivity, giving rise to drainageestimates that are uncertain (often by more than an order of magnitude). Similarly, large uncertaintiesexist with water-balance models used to predict drainage, particularly at low flux rates. Tracer tests offersome promise for indirectly estimating drainage flux, but the only direct way to verify drainage rates isby lysimetry. Test sections with drainage collection systems have proven useful for evaluating ET coverperformance with drainage rates of less than 0.2 mm/yr. Gee added that with proper care, drainage canbe measured directly on the ET cover using a water fluxmeter or "drain gage," a device capable ofmeasuring drainage rates of 0.2 mm/yr or less.

Sustainability of Conventional and Alternative Landfill Covers (see Attachment R, PDF, 58 pp.,2,800 KB)Jody Waugh, Environmental Sciences Laboratory

Waugh indicated that conventional covers rely on the low permeability of a compacted soil layer (CSL)to limit water movement into landfills. By contrast, ET covers rely on (1) a thick soil sponge to storeprecipitation while plants are dormant, and (2) ET to dry the sponge during the growing season. Henoted that regulators may allow ET covers as an alternative to low-permeability covers if performanceequivalency can be demonstrated. However, current cover design approaches and evaluations ofequivalency fail to address effects of near-term and long-term ecological processes on performance. Inaddition, conventional covers often fall short of permeability requirements and some cover designsinadvertently create habitat for deep-rooted plants and burrowing animals. Waugh noted that biologicalintrusion and soil development can increase the saturated hydraulic conductivity of CSLs by severalorders of magnitude above the design targets. Therefore, the low-permeability requirements forconventional covers may not be achievable (or may require high levels of maintenance or retrofitting tosustain long-term performance).

Waugh explained that alternative ET covers can be designed and constructed to accommodate ecologicalprocesses, and thereby sustain a high level of performance with little maintenance. He emphasized thatdesigning sustainable ET covers will require an ecosystem engineering approach that addresses thefollowing types of issues:

• Meteorological variability that is representative of possible long-term changes in climate.

• Vegetation responses to climate change and to disturbances such as fire, grazing, pests, andinvasion of exotic plants.

• Effects of vegetation patterns and dynamics on ET, soil permeability, soil erosion, and animalburrowing.

• Effects of soil development processes on water storage, permeability, and ecology.

Waugh noted that natural analogs can provide insight about how ecological processes may influence the



performance of both conventional and alternative covers. He stressed that investigations of naturalanalogs can identify and evaluate likely changes in cover environments that cannot be addressed withshort-term field tests and existing numerical models.

Session 4—Panel Discussion


• Inconsistencies between lysimeter data and model replication.

• Ability to use the fluxmeter device (as discussed by Gee) on soil types other than sand and gravel.

• Industry concerns with direct and indirect means of monitoring and error of accuracy.

• Issues related to fluxmeter or macro-lysimeter installation.

• Internal and external suction profiles resulting in lysimeters at varying depths.

• Cost and accuracy of fluxmeter use.

SESSION 5: LANDFILL GAS ISSUESSession co-Chairs: Charles Johnson and Mark Ankeny

Landfill Gas Interactions with ET Covers (see Attachment S, PDF, 43 pp., 2,336 KB)Mark Ankeny, INEEL

Ankeny noted that well-established vegetation and deep root penetration are often critical to the successand effectiveness of vegetated landfill covers. Poor vegetative stands can result in reduced transpiration,increased percolation, and increased erosion regardless ofthe thickness ofthe cover. He noted thatbecause landfill gas (LFG) inhibits plant growth on landfill covers, it is important to evaluate thepotential effects LFG may have on cover performance.

Ankeny stressed that bare (vegetation-free) areas are not uncommon on landfill covers, and shallowdigging in these areas often shows reducing conditions that are not present in vegetated areas. Heexplained that methane and carbon dioxide ascend from waste into overlying soil and displace oxygen,which is essential to maintaining healthy root activity. In addition, the presence of methane causes soilmicrobes to consume oxygen thereby reducing the amount of oxygen available for plant root respiration.Typically, even low methane levels indicate minimal oxygen concentrations. The magnitude of theseeffects can vary dramatically with changes in barometric pressure.

In addition, LFG directly affects landfill cover water budgets, because biological activity in landfillcovers can consume, produce, and release water. Degradation of waste typically occurs in two steps: (1)anaerobic fermentation followed by (2) oxidation. Biological activity can result in biogenic waterproduction on the order of centimeters of water per year. This amount of water is often larger than thatcalculated for percolation by standard cover water-balance models. The implication is that standardhydrologic models that ignore both water production and consumption may result in significant water-balance errors.



Installation of Low Permeability Covers and the Coincidental Effects on Gas Contamination ofGroundwater at Solid Waste Landfills (see Attachment T, PDF, 30 pp., 672 KB)John Baker, Alan Environmental

Baker provided a detailed review of pre-Subtitle D landfills and the coincidental timing of landfill coverinstallation with the occurrence of LFG migration and/or groundwater contamination. He reviewed thetypes of interim landfill covers before gas/groundwater contamination occurred and showed the types offinal low permeability caps that were installed after gas/groundwater contamination was confirmed.

Baker explained that LFG contains numerous types of chlorinated and non-chlorinated VOCs at a rangeof 10-500 parts per million (ppm) depending on the age ofthe waste. In certain permeable geology andsite conditions, gas can migrate under or adjacent to the landfills that are unlined or lined with littleknowledge of quality assurance/quality control (QA/QC) information. When a low permeability cap isinstalled, it forces the gas to the path of least resistance and can diffuse VOCs into groundwater.Typically gas/groundwater contamination is seen as chlorinated VOCs at the 10-100 parts per billion(ppb) range. Baker briefly discussed how to confirm the origin of VOC contamination and reviewedfield techniques used to assess the method of VOC diffusion. He also showed the effects of cappermeability on gas migration from an unlined landfill.

Methane Degradation in a Vegetated Cover Test System (see Attachment U, PDF, 29 pp., 667 KB)Steve Rock, U.S. EPA

Rock indicated that the goal of any waste containment system is to protect human health and theenvironment by eliminating direct contact with waste and preventing contamination of air andgroundwater. He noted that when properly designed and installed, ET covers prevent direct contact withwaste and limit infiltration, but research into LFG escape in ET covers has been limit«d. He noted thatEPA has constructed a test facility in Cincinnati, Ohio, that studies the rate and extent of gasconsumption by unconsolidated soils with plants.

Rock's presentation focused on two identical 12 ft by 12 ft by 12 ft, polished stainless steel, insulatedenvironmental chambers, located at the Cincinnati municipal sewer district treatment plant. Thechambers are used to replicate a wide range of climate conditions and grow a variety of grasses andtrees. The system utilizes 16 light fixtures containing a total of 32 light bulbs. Each fixture contains onemetal halide and one sodium vapor bulb that omit the photosynthetically active radiation (PAR) portionofthe sunlight spectrum (wavelengths between 400 nm and 700 nm). The chamber system is equippedwith a powerful heating, ventilation, and air conditioning (HVAC) system that includes a 10-ton chiller,electric heater, and humidifier.

Four 100-gallon stainless steel tanks, 35-inch diameter by 34-inch tall, are located in the chamber. A gasdistribution diffuser placed within a 4-inch layer of gravel at the bottom of each tank feeds methaneand/or carbon dioxide into the soil via copper tubing. A manual control valve and rotometer are used tocontrol the flow of methane into the tank. Felt is placed above the gravel to prevent soil from enteringthe gravel layer and to aid in dispersing the gas. Gas samples are collected from slotted PVC pipespositioned at different depths within the soil, and from static control chambers on the soil surface.Finally, ambient air samples are collected above the tank. All samples are analyzed by direct injection ofa gas chromatography/flame ionization detector (GC/FID) located in the adjacent control room.

Rock focused on the comparison of methane degradation in three treatments: sand, soil, and soil withgrass and poplar trees. His group used two gas flow rates over a five-month study. They ran intodifficulties simulating winter, but were able to achieve a natural oxidation rate. Rock concluded by

2i«p://www.rtdf.org/public/phyto/minutes/031004/phyto_agenda_9mar04.htm 5/12/04


offering an open invitation for attendees to use the environmental chamber system to test their own sitesoils.

Biological Function of a Vegetative/Compost Landfill Cap (see Attachment V, PDF, 10 pp., 290 KB)Lori Miller, U.S. Department of Agriculture

Miller indicated that the USDA Beltsville Agricultural Research Center (BARC), in Beltsville,Maryland is on the National Priorities List as a Superfund site. She explained that BARC's College ParkLandfill is a 30-acre municipal landfill that was active from 1955 to 1978 and has never been capped.Although the presumptive remedy is a standard RCRA cap, BARC's Environmental Unit has opted toinvestigate the use of a sustainable vegetative/compost cap.

Miller explained that in order to show that the vegetative/compost cap will perform as well as a standardcap, BARC's Environmental Unit is performing a three-year pilot study with an emphasis on:

• Plant water usage.

• Plant carbon sequestration.

• The role of microbes in carbon sequestration.

• The optimal compost composition to maximize water holding capacity and support plant andmicrobial growth.

Miller's group intends to optimize these aspects thereby maximizing the performance ofthevegetative/compoSt cap.

SESSION 6: OUTLOOK, OPPORTUNITIES, AND RESEARCH NEEDSSession co-Chairs: Craig Benson, Jorge Zornberg, and Kelly Madalinski

Madalinski thanked the session chairs for organizing the sessions and he applauded the speakers forsharing such a diverse array of knowledge and information. Given the bulk of information presentedduring the meeting, Madalinski said, it would be helpful to regroup and summarize the information in aclear and concise manner.

Summaries of Sessions by Session Chairs

Session 1: Design and Construction—Glendon Gee

Gee summarized each speaker's major points and posed questions that attendees should consider whenreviewing the presentations. He asked Benson about the universal use ofthe 0.7 reference, Forman aboutthe effects of plant disease, McGuire about using thicker lifts to guard against preferential flow, andButler about how to determine appropriate storage capacity and rooting media.

Session 2: ET Cover Modeling—Beth Gross (PDF, 10 pp., 165 KB)

Gross focused on a broader perspective and summarized a number of lessons learned from Session 2.She identified issues with various model aspects and provided recommendations to ensure model



evolution and progress. Issues discussed include:

• Codes—Available codes are often 1-dimensional and many codes do not incorporate importantprocesses such as (1) snow accumulation and melt, (2) plant growth based on positive feedback,and (3) nutrient feedback monitoring. In addition, since codes are continuously updated, it is oftendifficult to know which one to use.

• Process—Certain processes (e.g. runoff) have not been developed at a high enough level to givegood agreement with monitoring results. In addition, more information is needed on the effect ofLFG on vegetation.

• Data—Input parameters are sometimes difficult to measure due to the effects of scale, timing,and/or preferential flow. In addition, limited databases exist that allow access to parameter data.

Gross indicated that while there has been significant progress in recent years, there is still room forimprovement in research and data collection. She recommended that researchers focus more onsensitivity analysis, especially on that ofthe most critical input parameters.

Session 3: Case Studies—Bill Albright (PDF, 3 pp., 48 KB) and Craig Benson (PDF, 8 pp., 82 KB)

Albright and Benson revisited the case studies from Session 3 and summarized a number of lessonslearned:

• Alternative covers can be designed, permitted, and constructed in humid regions.

• Composite covers work well, but not as well as currently believed. Meanwhile, alternative covershave proven to meet their objectives.

• Clay barriers fail quickly and need to be phased out.

• The group has learned a great deal from ACAP, but there is still a great deal of missing data.

• There is a need to develop a better understanding of vegetation, especially the role of vegetation insequential "wet" years.

• Cover designs need to consider that material properties evolve over time.

Session 4: Monitoring, Long-Term Stability, and the Regulatory Dilemma—Kerry Guy

Guy indicated that covers have been designed based on modeling and field demonstrations. Designers ofboth conventional and ET covers need to carefully consider factors such as (1) risk, (2) regulatoryrequirements, (3) long-term stability, and (4) design issues. In addition, engineers need to understandthat cover systems will change in the long-term due to plant succession and/or climate change.

Session 5: Landfill Gas Issues—Charles Johnson

Johnson summarized the LFG session and noted that the key to understanding LFG issues is determiningwhat is going on inside the cover system. There is a fine balancing act that must occur between leachatepercolation from the bottom and gas emission from the top. Vegetation helps support this balance andthe group should take advantage of Rock's offer to study and test LFG oxidation rates.



Panel and Audience Discussion

Madalinski asked the audience to consider the next steps in ET cover evolution. He opened thediscussion to the group who discussed a number of important issues.

• Zornberg indicated that ET covers have evolved significantly over the past 5 to 10 years, but thereis still a long way to go.

• Benson emphasized the need to answer the same recurring issues: (1) understanding vegetation,(2) determining how soils change over time, and (3) developing more effective modelingtechniques and/or guidance documents.

• Waugh stressed the importance of stewardship and asked designers/builders to take intoconsideration ways to reduce future maintenance and monitoring costs early during the designprocess.

• Licht discussed the potential to move ET covers to the eastern United States and presented thepossibility of regulatory easement to ensure a successful transformation from conventional coverdesign. Regardless of regulatory change, climate factors will be a major obstacle for eastern ETcover implementation.

• An attendee stressed the urgent need to consider the system from a biological perspective (ratherthan the traditional engineering view). In addition, he noted the importance of public informationdissemination.

Madalinksi concluded by thanking the speakers for presenting, the audience for attending, and the groupmembers for organizing the two-day session.

ATTACHMENTS A THROUGH V

SUMMARY OF THE REMEDIATION TECHNOLOGIES DEVELOPMENT FORUMPHYTOREMEDIATION OF ORGANICS ACTION TEAM MEETING

Adams Mark HotelDenver, ColoradoMarch 9-10, 2004

Attachment A: List of Speakers and Attendees (PDF, 15 pp., 134 KB)

Attachment B: Regulatory Acceptance of Alternative Landfill Covers (Gary W. Baughman)(PDF, 14 pp., 264 KB)

Attachment C: Landfill ET Covers - Past Myth, Current Fact, Possible Future (Louis Licht) (PDF,56pp., 1,945KB)

Attachment D: Design Guidance (Craig Benson) (PDF, 27 pp., 422 KB)

Attachment E: Ecological Design and Revegetation (Amy Forman) (PDF, 23 pp., 959 KB)

Attachment F: Borrow Source Considerations (Patrick McGuire) (PDF, 31 pp., 1,929 KB)


• SUMMARY OF THE REMEDIATION TECHNOLOGIES DEVELOPMENT FORU... of 19

Attachment G: Experiences with Placement of Alternative Final Covers (Leonard Butler) (PDF,25 pp., 659 KB)

Attachment H: ET Cover Modeling Introduction (Beth Gross) (PDF, 4 pp., 54 KB)

Attachment F: Monitoring versus Modeling ET Covers for Performance Evaluation (BridgetScanlon) (PDF, 36 pp., 759 KB)

Attachment J: Fact or Fiction: Comparing Model Predictions and Field Data from ACAP (CraigBenson) (PDF, 32 pp., 1,104 KB)

Attachment K: Overland Flow Implications on Surface Cover (Earl Mattson) (PDF, 18 pp., 1,425KB)

Attachment L: Prediction of Water and Energy Balance in Surface Covers and Protective Side-slopes Using the STOMP Simulator (Andy Ward) (PDF, 16 pp., 809 KB)

Attachment Coast to Coast: Performance Data from the ACAP Field Sites (Bill Albright)M: (PDF, 44 pp., 1,136KB)

Attachment N: Design and Construction of an ET Cover in the Eastern United States (Beth Gross)(PDF, 29 pp., 2,083 KB)

Attachment O: Field Performance Monitoring of ET Cover Systems at Mine Sites in Australia,Canada, and the United States (Mike O'Kane) (PDF, 36 pp., 2,582 KB)

Attachment P: The Quest for Consistency Among Regulatory, Design, and Post-ClosureMonitoring Frameworks (Jorge Zornberg) (PDF, 30 pp., 1,731 KB)

Attachment Q: Challenges in Monitoring ET Covers (Glendon Gee) (PDF, 36 pp., 1,352 KB)

Attachment R: Sustainability of Conventional and Alternative Landf i l l Covers (Jody Waugh)(PDF, 58 pp., 2,800 KB)

Attachment S: Landfill Gas Interactions with ET Covers (Mark Ankeny) (PDF, 43 pp., 2,336 KB)

Attachment T: Installation of Low Permeability Covers and the Coincidental Effects on GasContamination of Groundwater at Solid Waste Landfills (John Baker) (PDF, 30pp., 672 KB)

Attachment U: Methane Degradation in a Vegetated Cover Test System (Steve Rock) (PDF, 29pp., 667 KB)

Attachment V: Biological Function of a Vegetative/Compost Landfill Cap (Lori Miller) (PDF, 10pp., 290 KB)


mtg summary - designing, building, & regulating ... · rock then asked for a show of hands from...

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