Suthersan, Suthan S. “Engineered Vegetative Landfill Covers” Natural and Enhanced Remediation Systems Edited by Suthan S. Suthersan Boca Raton: CRC Press LLC, 2001 ©2001 CRC Press LLC CHAPTER 7 Engineered Vegetative LandÞll Covers CONTENTS 7.1 Historical Perspective on Landfill Practices 7.2 The Role of Caps in the Containment of Wastes 7.3 Conventional Landfill Covers 7.4 Landfill Dynamics 7.5 Alternative Landfill Cover Technology 7.6 Phyto-Cover Technology 7.6.1 Benefits of Phyto-Covers over Traditional RCRA Caps 7.6.2 Enhancing In Situ Biodegradation 7.6.3 Gas Permeability 7.6.4 Ecological and Aesthetic Advantages 7.6.5 Maintenance, Economic, and Public Safety Advantages 7.7 Phyto-Cover Design 7.7.1 Vegetative Cover Soils 7.7.2 Nonsoil Amendment 7.7.3 Plants and Trees 7.8 Cover System Performance 7.8.1 Hydrologic Water Balance 7.8.2 Precipitation 7.8.3 Runoff 7.8.4 Potential Evapotranspiration — Measured Data 7.8.5 Potential Evapotranspiration — Empirical Data 7.8.6 Effective Evapotranspiration 7.8.7 Water Balance Model 7.9 Example Application 7.10 Summary of Phyto-Cover Water Balance 7.11 General Phyto-Cover Maintenance Activities 7.11.1 Site Inspections 7.11.2 Soil Moisture Monitoring ©2001 CRC Press LLC 7.11.2.1 Drainage Measurement 7.11.3 General Irrigation Guidelines 7.11.4 Tree Evaluation 7.11.4.1 Stem 7.11.4.2 Leaves 7.11.5 Agronomic Chemistry Sampling 7.11.6 Safety and Preventative Maintenance 7.11.7 Repairs and Maintenance 7.12 Operation and Maintenance (O&M) Schedule 7.12.1 Year 1 — Establishment 7.12.2 Years 2 and 3 — Active Maintenance 7.12.3 Year 4 — Passive Maintenance 7.13 Specific Operational Issues 7.13.1 Irrigation System Requirements 7.13.2 Tree Replacement References Maintaining and enhancing the closed landfill as a bioreactor requires modifi- cation of design and operational criteria normally associated with traditional landfill closure… 7.1 HISTORICAL PERSPECTIVE ON LANDFILL PRACTICES The practice of using shallow earth excavations, or landfills, for disposal of liquid and solid waste has a very long history. Landfill practices basically followed the design philosophy of “out of sight, out of mind” in that a pit or trench was excavated into the ground, waste was placed into the excavation, and, when it was full, the excavation was covered with soil and abandoned. If thought was ever given to the matter, it was likely assumed that the soil surrounding the waste effectively prevented contaminant migration from the burial zone. It was not until 1976, with the passage of the Resource Conservation and Recovery Act (RCRA), and 1980, with the passage of Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) that federal and state regu- lations mandated much improved methods for disposal of waste in landfills. Today there are a plethora of federal and state regulations controlling all aspects of landfill disposal of municipal, radioactive, and hazardous waste. The problem in the U.S., however, is that hundreds of thousands of landfills were operated and then decom- missioned prior to the requirements of current regulations. Many of these old landfills now come under the closure requirements of RCRA or CERCLA, depending on the agreements between the responsible parties. In 1989, U.S. Environmental Protection Agency (USEPA) stated that there are 226,000 sanitary landfills in the U.S. requiring evaluation for potential risks to human and environmental receptors. 1 Regardless of the corrective action imposed on these old sites, almost all of them will require installation of a new cover as a final step in the closure process. The ©2001 CRC Press LLC design of most landfill covers in the U.S. has been based on criteria developed by EPA for use in closing either RCRA subtitle C (hazardous waste) or subtitle D (municipal solid waste) landfills. Two major themes emerge in reviewing recent work in landfill cover design: 2 1) there has been an overemphasis on regulatory compliance, thus inhibiting innovative and creative design that looks at the entire landfill system as a holistic biogeochemical environment, and 2) there are few published data on field performance of constructed cover systems and their impacts on the biogeochemistry of the groundwater within the footprint of the landfill. 7.2 THE ROLE OF CAPS IN THE CONTAINMENT OF WASTES Because of the expense and risk associated with treating or removing large volumes of landfill wastes, remediation usually relies upon containment, which requires the construction of a suitable cover. Both regulators and the public usually accept covers as part of the presumptive remedy for final landfill remediation; therefore, covers are likely to be included in the optimal remedial actions for closure of most landfills. The intent of landfill remediation is to protect the public health and the environ- ment. In keeping with this intent, a modern philosophy has evolved requiring con- taminants in the waste to be isolated from receptors and contained within the landfill. As a result, landfills have become warehouses in which wastes are stored for an indefinite time, possibly centuries. There are fundamental scientific and technical reasons for placing a cover on landfill sites. Although regulations are often the most apparent influence governing the selection and design of landfill covers today, these regulations were drafted because of specific environmental concerns and were based upon scientific and technical understandings. The three primary requirements for landfill covers are to: • Minimize infiltration: water that percolates through the waste may dissolve contaminants and form leachate, which can pollute both soil and groundwater as it travels from the site. • Isolate wastes: a cover over the wastes prevents direct contact with potential receptors at the surface and prevents movement by wind or water. • Control landfill gas: landfills may produce explosive or toxic gases, which, if allowed to accumulate or to escape without control, can be hazardous. Landfills have been covered by barriers for years, usually built with little regard for the monetary and environmental costs associated with constructing and main- taining them. A typical landfill cover design consists of a sequence of layered materials to control landfill gas infiltration and promote internal lateral drainage. The uppermost layer of a landfill cover consists of a vegetative soil layer to prevent erosion, promote runoff, and insulate deeper layers from temperature changes. The landfill cover is not a single element but a series of components functioning together. 3 ©2001 CRC Press LLC Landfill covers are designed to minimize infiltration of rainfall and melting snow into the landfill in order to minimize postclosure leachate production. This objective is achieved by converting rainfall into surface runoff and infiltration into evapotrans- piration and lateral drainage without compromising cover integrity. Secondary per- formance objectives of landfill cover design include the following: 3 minimize post- closure maintenance; return the site to beneficial use as soon as possible; make the site aesthetically acceptable to adjacent property owners; accommodate post-closure settlement of the waste; address gas and vapor issues; provide stability against slumping, cracking, and slope failure; provide resistance to disruption by animals and plants; and comply with landfill closure regulations. The design features of a landfill cover are varied to affect changes in the overall water balance within the landfill to meet primary landfill cover objectives. The design adopted must take into account numerous other considerations, including costs, long term maintenance implications, and construction risks. The relatively large areas that landfill covers protect, and the thickness and number of individual layers within them, make covers a cost-intensive component of landfill facility design. 7.3 CONVENTIONAL LANDFILL COVERS Nearly all conventional landfill covers in current use incorporate a barrier within the cover. The “impermeable” barrier layer is intended to prevent water from moving downward in response to the force of gravity. In effect, these covers are designed to oppose the forces of nature. Barrier-type covers commonly include five layers above the waste (Figure 7.1). 1 The top layer consists of cover soil typically two feet thick and supports a grass cover that provides erosion control. The barrier layer consists of either a single low-permeability barrier or two or more barriers in combination. The fourth layer is designed to remove landfill gases as they accumulate underneath the barrier layer. The bottom layer is a foundation layer of variable thickness and material; its purpose is to separate the waste from the cover and to establish sufficient gradient to promote rapid and complete surface drainage from the finished cover. The barrier layer is the defining characteristic of conventional landfill covers. It may be composed of compacted clay, a geomembrane, a clay blanket, or two or more layers of materials in combination. A compacted clay layer is frequently specified to have a maximum saturated hydraulic conductivity (K) £ 1 ¥ 10 –7 cm/sec. In contrast, both the drainage and gas collection layers are constructed to enhance flow and commonly contain washed and selectively sieved sand, gravel, or specially designed synthetic materials. The soil in the top layer of barrier-type covers is usually too thin or has inadequate water holding capacity to store infiltrating precipitation during a large storm. These covers rely on barrier layers and rapid drainage through lateral drainage layers to prevent precipitation from reaching the waste. Barrier-type covers must accommo- date specific site conditions, and supplemental components are sometimes added. For example, gravel may be added to the surface soil in desert regions to control ©2001 CRC Press LLC wind erosion, or a layer of cobbles may be used with the cover to discourage animal burrowing into the waste. 7.4 LANDFILL DYNAMICS Landfills that contain a large amount of organic, putrescible materials (such as municipal solid waste) literally function as bioreactors. Most “landfill bioreacters” in general contain anaerobic and/or facultative microorganisms. Landfill leachate is gen- erated as a result of the percolation of water or other liquids through the waste and also due to the accumulation of moisture generated as a result of microbial degradation of waste. Leachate is a concentrated fluid containing a number of dissolved and suspended materials, specifically, high concentrations of organic compounds (organic acids, hydrocarbons, etc.) and certain inorganic compounds (ammonia, sulfates, dis- solved metals, etc. characteristic of the parent waste materials) from which it is derived. In addition, natural microbial activity in landfills also results in the generation of gases such as methane, carbon dioxide, ammonia, and hydrogen sulfide, a fraction of which will be dissolved in the leachate and may be introduced into the groundwater. Numerous landfill investigation studies 4 have suggested that the stabilization of waste proceeds in sequential and distinct phases. The rate and characteristics of leachate produced and biogas generated from a landfill vary from one phase to another and reflect the processes taking place inside the landfill. These changes are depicted in Figure 7.2. Figure 7.1 Typical single barrier cover system. Waste Foundation Layer Protective Cover Layer Vegetative Layer 0'-6" 1'-18" 2'-30" (min.) Vegetation Topsoil Common Borrow Material Geocomposite (Textile-Net-Textile) 40-mil LLDPE ©2001 CRC Press LLC The initial phase is associated with initial placement of waste and accumulation of moisture within landfills. Favorable biochemical conditions are created for the decomposition of waste. During the next phase, transformation from an aerobic to anaerobic environment occurs, as evidenced by the depletion of oxygen trapped within and introduced to landfill media and continuous consumption of nitrates and sulfates. Subsequent phases involve the formation of organic acids and methane gas. During maturation phase, the final state of landfill stabilization, available organic carbon and nutrients become limiting, and microbial activity shifts to very low levels of activity. Gas production dramatically drops and leachate strength remains constant at much lower concentrations than in earlier phases. Biochemical decomposition of putrescible solid waste is shown below by an example (Equation 7.1). Typical landfill gas composition during peak activity as a bioreactor is: 60% methane, 40% carbon dioxide, 5–10% other gases, and 0.3–1.0% VOCs and non-monitored organic compounds. Gas generation rates during peak activity typically fall within the ranges of 5–15 ft 3 per pound of refuse per year. 9 (7.1) Due to very high gas pressures generated at the source areas within the landfill (up to 4 atmospheres), migration of dissolved contaminants into the gaseous phase could be a serious concern. Contaminants transferred into the gas phase could be readsorbed in the waste above the water table, or dissolve in the moisture, condense in the waste zone, or migrate away from the landfill. The potential for contaminant migration from the dissolved phase into the landfill gas can be evaluated as shown in Equation 7.2, and Figure 7.3. Figure 7.2 Description of leachate and gas concentration changes during landÞll lifecycle. CH OHO CH CO 6 10 5 242 33 Anaerobic Bacteria Æ+ ©2001 CRC Press LLC Under non-equilibrium conditions: (7.2) where = transfer rate from gas to water K=phase transfer coefficient H=Henry’s Law Constant of VOC C g =gas phase concentration of VOC C w =water phase concentration of VOC A=gas/liquid contact area The progress toward final stabilization of any landfill and the organic waste in it is subject to physical, chemical, and biological factors within the landfill environ- ment, age and characteristics of the waste, operation and management controls applied, as well as site-specific external conditions. Although barrier layers are sometimes referred to as “impermeable” layers, in practice this is seldom true. An extensive review of failures and failure mechanisms for compacted soil covers in landfills was performed and emphasized that “…natural physical and biological processes can be expected to cause [clay] barriers to fail in the long term.” 5 Another study discussed a field test conducted in Germany in which, Figure 7.3 Equilibrium mass transfer conditions of contaminants into landÞll gas. dm dt KC HC A gw =- () dm dt ©2001 CRC Press LLC seven years after construction, percolation through the compacted clay was almost 200 mm/yr and increasing. Geomembrane barriers are also prone to leak. 6 Others have traced most leaks in geomembranes to holes left by construction. 7,8 A modification of the typical barrier cover is the subtitle D cover (Figure 7.4) that relies on compaction to create a layer of soil with reduced K value. Used primarily for municipal landfills in dry regions, its use and components are specified in subtitle D of RCRA (40 CFR, Part 258.60), hence the name. From the surface downward, the cover includes an erosion control layer and a layer of compacted soil. A major advantage of the subtitle D cover is that its construction cost is lower than for an RCRA subtitle C cover. Even though it has gained regulatory and public acceptance, the subtitle D cover cannot ensure long-term protections against infil- tration of water into the waste, even in dry regions, because 1) the topsoil layer has limited water holding capacity, 2) there is no drainage layer, 3) few roots can grow in the barrier layer to remove water, and 4) soil freezing and root activity are likely to increase the K value of the barrier soil layer over time. Figure 7.4 Subtitle D cover for municipal solid waste landÞlls. 0.47 m 0.15 m Waste Soil Barrier K ≤1 x 10-5 cm/sec Topsoil Precipitation Foundation - Gravel Runoff ©2001 CRC Press LLC 7.5 ALTERNATIVE LANDFILL COVER TECHNOLOGY Alternative covers to the RCRA subtitle C or D design include evapotranspiration (ET) covers and capillary barriers. The ET cover uses no barrier or horizontal drainage layers; it is designed to work with the forces of nature rather than attempting to control them. An ET cover in its simplest form is a vegetated soil cover with a sufficiently deep soil profile so that infiltrated water is stored until removal by evaporative losses from the soil surface and by plant roots at depth in the profile. A capillary barrier also relies on water removal by ET, but is designed such that water storage near the surface is enhanced to promote the efficient removal of infiltrated water by the ET process. Optimization of material types and thicknesses for capillary barriers is critical to their effective performance. The use of sands or clays as the fine-soil component in the capillary barrier has proven to be less effective in storing water than silt loams. Capillary barriers can be thought of as enhanced ET covers — alternative cover systems that work best in semi- and/or arid environments where high ET rates and low precipitation make it possible to remove all infiltrated water by ET. However, even in arid environments there are situations where ET covers and capillary barriers can allow excessive percolation, particularly where the soil used in the cover design has insufficient storage capacity to accommodate winter snow melt events. 7.6 PHYTO-COVER TECHNOLOGY The phyto-cover is the most popular application of the ET cover and is an engineered agronomic system that harnesses the natural transpiration process of plants to limit percolation to the groundwater. A phyto-cover relies on shallow- and deep-rooted plants to create a thick root zone from which the plants can extract available moisture. In effect, the plants serve as natural, solar-powered “pumps” to withdraw soil moisture and either convert it into biomass or evaporate it through their leaves. The withdrawal rate of the botanical pumps is limited by the available energy (sunlight), rate of growth, and available soil moisture; withdrawal virtually ceases during winter dormancy. Accordingly, the depth and composition of the root zone must be sufficient to store accumulated water like a sponge and hold it until the plants remove it. Properly designed, this “sponge and pump” water removal system (Figure 7.5) can limit water from percolating below the root zone and can be equally protective of groundwater as a RCRA cap. Thus, a phyto-cover serves as a functional alternative to natural clay, geocomposite, or geosynthetic membrane cap, yet offers several advantages over those technologies. The effectiveness of poplars in maintaining low soil moisture levels was first documented by data collected from a phyto-cover application in Iowa. 10 The phyto- cover consistently maintained soil moisture levels substantially below the soil’s field capacity (i.e., the amount of water that soil can retain without allowing percolation) of 40–45%. Soil dryness was maintained by the trees’ prodigious water extracting ability. The capacity of certain trees such as hybrid poplar and willow trees to extract soil moisture has been demonstrated by monitoring data from landfill at many sites. [...]... cost effective and providing additional benefits The final design of a phyto-cover often includes provisions for monitoring soil moisture levels to ensure that performance criteria are met Two-year Old Stand of Poplars Four-year Old Poplar Trees Figure 7. 6 Phyto-covers: comparison of two-year-old and four-year-old growth of a phytocover (courtesy of Licht, 1998) Engineered phyto-cover systems have been... variables in Equations 7. 9 and 7. 10 are evaluated using the following expressions: D= 4098e s (2 37. 3 + T)2 (7. 11) where T es = temperature (˚C) = saturation vapor pressure (kPa) È 17. 27 T ù e s = 0.6108 exp Í ú Î (2 37. 3 + T) û g P (7. 12) P l = atmospheric pressure (kPa) estimated from the site elevation using the relationship = 0.0016286 P = 101.0 – 0.011 5E + 5.44 c10 7 E2 E l (7. 13) = land surface elevation... Silt Loam Lt Clay Loam Loam Fine Sandy Loam Sandy Loam Fine Sand Sand 0 Figure 7. 16 Water holding characteristics of soils abandoned landfills, and routinely develop roots deeper than 8 feet below the soil surface Accordingly, the engineered phyto-cover system is designed to root into the waste to capture additional water holding capacity According to Table 7. 1, 17. 78 inches is the total water holding... phyto-cover systems have been applied to contain spilled petrochemicals and cover landfills, as well as buffers to remove nitrogen from industrial and municipal wastewater Sites where phyto-covers have been installed and recent research and demonstration sites for phyto-cover systems include the following:2,1 0-1 3 • A 15-acre construction debris landfill in Beaverton, OR was covered with trees in 1990 as an alternative... dQ(t ) PET =dt Q(t ) Qi Ú (7. 24) 0 t Q lnQ Q2 == 1 PET t Qi 0 (7. 25) Substituting and exponentiation of both sides of Equation 7. 22 yields È PET ù Q2 = exp Ítú Q1 Î Qi û ©2001 CRC Press LLC (7. 26) The moisture content at time t (Q2) can therefore be computed from the initial moisture content (Q1) using the formula: È PET ù Q 2 = Q1 exp Ítú Î Qi û (7. 27) Equations 7. 21 and 7. 27 describe the change in... Aesthetic Advantages Both a phyto-cover and an RCRA cap are designed to be vegetated on the surface, but vegetation on a phyto-cover has the appearance of a tree farm and, eventually, ©2001 CRC Press LLC Figure 7. 7a Biogeochemical conditions and mass balance for a presumptive remedy Figure 7. 7b Biogeochemical conditions and mass balance for a holistic remedy a forest, and serves the same ecological function... phyto-cover functions as a sponge and pump system, with the root zone acting as the sponge, and trees acting as the solar-driven pumps In contrast to restrictive permeability barrier design, the engineered phyto-cover design involves the storage of free water in soil pores and the extraction of stored water by the tree roots The effectiveness of engineered phyto-cover systems as landfill closure systems. .. landfill, which facilitates stabilization of the waste By contrast, the single-barrier cap would admit only an estimated 75 pounds of oxygen or about one tenth of one percent of the influx that could support the aerobic natural attenuation mechanisms (Figures 7. 7a and b).15 7. 6.3 Gas Permeability Unlike RCRA caps, which are essentially impermeable to gases and therefore require elaborate gas venting systems. .. installation of a liner, and then recovering with a geomembrane The phyto-cover is serving to protect groundwater cost-effectively The owner has continued to expand the cover as new areas are closed ©2001 CRC Press LLC • From 1992 to 1993, the Riverbend Landfill in McMinneville, OR planted a 17acre phyto-cover to manage landfill leachate water and soluble compounds All nutrient and water cycling results... tractor and mechanical planter These trees are typically planted with an in-row spacing of 3 feet and a row spacing of 10 to 13 feet They are planted in rows positioned along the land elevation contours, perpendicular to slopes to aid in reducing sheet flow velocities and surface erosion 7. 8 COVER SYSTEM PERFORMANCE The engineered phyto-cover system should be designed to meet the post-closure and remediation . 7. 6.5 Maintenance, Economic, and Public Safety Advantages 7. 7 Phyto-Cover Design 7. 7.1 Vegetative Cover Soils 7. 7.2 Nonsoil Amendment 7. 7.3 Plants and Trees 7. 8 Cover System Performance 7. 8.1. closed. Figure 7. 6 Phyto-covers: comparison of two-year-old and four-year-old growth of a phyto- cover (courtesy of Licht, 1998). Four-year Old Poplar Trees Two-year Old Stand of Poplars ©2001. Vegetative Landfill Covers” Natural and Enhanced Remediation Systems Edited by Suthan S. Suthersan Boca Raton: CRC Press LLC, 2001 ©2001 CRC Press LLC CHAPTER 7 Engineered Vegetative LandÞll