Nazih K Shammas

CONTENTS

25.1 Introduction 1058

25.2 Requirements for Landfill Covers 1059

25.2.1 Landfill Remediation Requirements 1059

25.2.2 Site-Specific Requirements for Landfill Covers 1059

25.2.3 Conventional Covers 1059

25.2.4 ET Cover Definition 1061

25.2.5 ET Cover Concept Verification 1061

25.2.6 Requirements for ET Landfill Covers 1061

25.3 Description of ET Cover Systems 1062

25.4 Limitations 1063

25.5 Design Issues and Requirements 1064

25.5.1 Hydrologic Water Balance 1065

25.5.2 Climate 1066

25.5.3 Evapotranspiration 1067

25.5.4 Surface Runoff 1068

25.5.5 Soil-Water Storage and Movement 1068

25.5.6 Deep Percolation 1069

25.5.7 Soil Type 1070

25.5.8 Soil Thickness 1070

25.5.9 Vegetation Types 1070

25.5.10 Soil and Organic Properties 1071

25.5.11 Control Layer Types 1072

25.5.12 Safety Factor 1072

25.6 Hydrologic Models 1074

25.6.1 Important Model Characteristics 1074

25.6.2 Environmental Policy Integrated Climate Model 1075

25.6.3 HELP Model 1076

25.6.4 UNSAT-H (Version 3.0) Model 1077

25.6.5 HYDRUS 1078

25.6.6 Model Comparisons 1078

25.7 Performance and Monitoring 1080

25.7.1 Monitoring Systems 1080

25.7.2 Numerical Models 1081

25.8 Cost 1082

25.9 Technology Status 1082

25.10 Examples of ET Covers

25.10.1 Example 1: Monolithic ET Cover at Lopez Canyon Sanitary Landfill,

1082

Los Angeles, California

1082

25.10.2 Exercise 2: Capillary Barrier ET Cover at Lake County Landfill,

Acronyms . . Nomenclature References .

25.10.3 Example 3: ALCD

25.10.4 Example 4: ACAP

Polson, Montana

1084

1084

1085 1087 1087 1087

25.1 INTRODUCTION

The technology available for landfill remediation is changing. New remediation methods are becoming available, and some are now being accepted by regulators. The old notion of entombment and preservation of waste is giving way to new concepts for managing wastes that may remove the threat to human health and the environment. Both old and new technology should meet the critical goal of landfill remediation, which is to protect human health and the environment.

Leading new technologies include the landfill as a bioreactor,1 and the use of natural attenuation processes to enhance effectiveness of remediation at reduced cost by naturally renewable and continuing processes.2,3

Alternative final cover systems, such as the innovative evapotranspiration (ET) cover systems, are increasingly being considered for use at waste disposal sites, including municipal solid waste (MSW) and hazardous waste landfills when equivalent performance to conventional final cover systems can be demonstrated. Unlike conventional cover system designs that use materials with low hydraulic permeability (barrier layers) to minimize the downward migration of water from the cover to the waste (percolation), ET cover systems use water balance components to minimize percolation. These cover systems rely on the properties of soil to store water until it is either transpired through vegetation or evaporated from the soil surface.

The ET landfill cover is a complete cover, not a cover component. It offers opportunities for improved performance and lower construction and maintenance cost. In addition, the new cover may be beneficial for use with bioreactor landfills because ET covers can be designed to pass a controlled amount of precipitation through the cover and into the waste.4

Design, construction, and use of ET landfill covers are dependent upon the following:

1. Definition of requirements for the cover.

2. Decision that an ET cover meets site cover requirements.

The ET landfill cover design problem includes complex relationships between climate, soil, and vegetation and is best solved with the aid of a computer model.4-6 ET landfill covers have different design requirements than do conventional covers; therefore, model requirements for design and evaluation differ from conventional practice. This chapter provides the bases of ET cover systems, including general considerations in their design, performance, monitoring, cost, current status, limitations on their use, and project-specific examples. It is intended to provide basic information to site owners and operators, regulators, consulting engineers, and other interested parties about these potential design alternatives. United States Environmental Protection Agency (U.S. EPA) has developed an on-line database that provides information about specific projects using ET covers.7

25.2 REQUIREMENTS FOR LANDFILL COVERS

This section is an overview of the requirements and definitions for landfill covers. Additional details are available in Refs. 8-13.

25.2.1 Landfill Remediation Requirements

The application of containment—the presumptive remedy—often requires the design and installation of a landfill cover. Other common components, such as landfill gas management, groundwater treatment or containment, and collection and disposal of leachate, may also be required. Landfill covers may offer several environmental benefits (minimize erosion, prevent occurrence of disease vectors and other nuisances, and meet aesthetic and other end-use purposes), but they are based on three primary goals.14,15

1. Minimize infiltration of precipitation into the waste to control potential leaching of contaminants from the waste.

2. Isolate the wastes to prevent direct contact with potential receptors at the surface and to control movement of waste by wind or water.

3. Control landfill gases to minimize risks from toxic or explosive gases that may be generated within the landfill.

Final cover systems are intended to remain in place and maintain their functions for an extended period of time.

In addition, cover systems are also used in the remediation of hazardous waste sites. For example, cover systems may be applied to source areas contaminated at or near the ground surface or at abandoned dumps. In such cases, the cover system may be used alone or in conjunction with other technologies to contain the waste (e.g., slurry walls and groundwater pump and treat systems).

25.2.2 Site-Specific Requirements for Landfill Covers

The site-specific requirements for landfill remediation should be developed before beginning design or selection of cover type. Site-specific requirements depend on numerous site-specific factors, including landfill history; waste type, quantity, and age; climate; geologic setting; local surface water and groundwater use; and regulatory requirements.

After a performance requirement has been established for remediating a particular landfill, any remedial alternative meeting that requirement can be selected and applied. Site-specific requirements are discussed in more detail in Refs. 8 and 13.

25.2.3 Conventional Covers

The dominant feature of covers currently in use is one or more barrier layers that are intended to stop the natural downward movement of water through the profile of the cover. Conventional and barrier-type covers include several layers, including grass for surface cover. These covers typically include one or more barrier layers made of compacted clay, geomembranes, or geosynthetic clay. Barrier-type covers are more completely described in Refs. 9, 13, and 16-19. The Subtitle D cover is a simplified barrier-type cover with a single barrier layer of compacted clay. It is less expensive than other barrier-type covers and is used in dry climates.20,21

The design of cover systems is site-specific and depends on the intended function of the final cover—components can range from a single-layer system to a complex multilayer system. To minimize percolation, conventional cover systems use low-permeability barrier layers. These barrier layers are often constructed of compacted clay, geomembranes, geosynthetic clay liners, or combinations of these materials.

Depending on the material type and construction method, the saturated hydraulic conductivities for these barrier layers are typically between 1 x 10-5 and 1 x 10-9 cm/s. In addition, conventional cover systems generally include additional layers, such as surface layers to prevent erosion; protection layers to minimize freeze/thaw damage; internal drainage layers; and gas collection layers.6,22

Although barrier layers are sometimes referred to as impermeable, in practice this is seldom true. Suter et al.23 reviewed failure mechanisms for compacted soil covers in landfills; they concluded that natural, physical, and biological processes can be expected to cause clay barriers to fail in the long term. Melchior24 reported the results of a German study in a cool, wet climate; he found that clay barriers were already leaking 150-200 mm/yr in the eighth year of operation. Geomembrane barriers are also prone to leak. Board and Laine25 traced most leaks in geomembranes to holes left by construction. Melchior24 reported that three composite covers, containing more than one barrier, leaked, on average, between 1 and 4 mm/yr with annual leakage as high as 5.2 mm/yr. Albright and Benson26 reported that conventional clay-barrier covers at two sites leaked 5.5% and 37% of the precipitation into the waste.

Regulations under the Resource Conservation and Recovery Act (RCRA)27,28 for the design and construction of final cover systems are based on using a barrier layer (conventional cover system). Under RCRA Subtitle D, the minimum design requirements for final cover systems at MSW landfills depend on the bottom liner system or the natural subsoils, if no liner system is present. The final cover system must have permeability less than that of the bottom liner system (or natural subsoils) or <1 x 10-5 cm/s, whichever is less. This design requirement was established to minimize the "bathtub effect," which occurs when the landfill fills with liquid because the cover system is more permeable than the bottom liner system. This "bathtub effect" greatly increases the potential for generation of leachate. Figure 25.1a shows an example of an RCRA Subtitle D cover at an MSW landfill with a 6-in. (0.15 m) soil erosion layer, a geomembrane, and an 18-in. (0.45 m) barrier layer of soil that is compacted to yield a hydraulic conductivity <1 x 10-5 cm/s.29

For hazardous waste landfills, RCRA Subtitle C provides certain performance criteria for final cover systems. While RCRA does not specify minimum design requirements, U.S. EPA has issued guidance for the minimum design of these final cover systems. Figure 25.1b shows an example of an RCRA Subtitle C cover at a hazardous waste landfill.30

0.15 m Erosion layer

—p tawattMattMa

Compacted clay layer

Compacted clay layer

!■ Composite barrier

Geomembrane

!■ Composite barrier

As required Topsoil layer '-¿y^

> Frost penetration Cover soil layer

Geomembrane

, ; Sand drainage layer X

Compacted clay layer

As required Gas drainage layer

FIGURE 25.1 Examples of final cover systems. (a) MSW landfill and (b) hazardous waste landfill.15

The design and construction requirements, as defined in the RCRA regulations, may also be applied under cleanup programs, such as Superfund31 or state cleanup programs, as part of a remedy for hazardous waste sites such as abandoned dumps. In these instances, the RCRA regulations for conventional covers are usually identified as applicable or relevant and appropriate requirements for the site.

Under RCRA, an alternative design, such as an ET cover, can be proposed in lieu of an RCRA design if it can be demonstrated that the alternative provides equivalent performance with respect to reduction in percolation and other criteria, such as erosion resistance and gas control.

25.2.4 ET Cover Definition

Because of the water-holding properties of soils and the fact that most precipitation returns to the atmosphere via ET, it is possible to devise a landfill cover to meet remediation requirements, and yet contain no barrier layer. The ET cover consists of a layer of soil covered by native grasses; it contains no barrier or "impermeable" layers. The ET cover uses two natural processes to control infiltration: (1) soil provides a water reservoir and (2) natural evaporation from the soil plus plant transpiration (ET) empties the soil water reservoir.32-38 The ET cover is an inexpensive, practical, and easily maintained biological system that will remain effective during extended periods of time—perhaps centuries—at low cost.

The ET cover contains selected soil and requires correct placement to maintain desirable soil properties. Successful performance by the ET cover requires robust plant growth and good soil properties. It should be designed for the site to ensure that it meets the cover requirements.

25.2.5 ET Cover Concept Verification

The technology that forms the basis for ET landfill covers was developed, tested, and well understood years ago, and field data are available from water balance measurements in both natural and constructed soil layers similar to those required for ET covers. The concept was corroborated in the field by both long- and short-term measurements that were collected during the past century. The long-term measurements established the water balance under grass during time periods from three decades to several centuries in length, and included unusually wet periods, fires, and other natural disasters. These data demonstrate that the ET cover can minimize movement of precipitation through soil covers by using natural forces and the soil's water-holding capacity.11

25.2.6 Requirements for ET Landfill Covers

The ET cover has the following minimum criteria:

1. Support a robust, healthy, vegetative cover.

2. The soil should allow rapid and prolific root growth in all parts of the soil cover.

3. The soil should hold enough water to minimize water movement below the cover during extreme or critical design periods.

In keeping with the requirements for all landfill remediation, the ET cover must meet the requirements for a landfill cover.

The soil and plants employed on the cover are critical to success. A mixture of grasses native to the area is preferred to provide effective water removal from the cover in all years in spite of temporary changes in local conditions. Native grasses have already proven their ability to withstand local climate variations, insects, plant disease, periodic fire, and other factors. A mixture of native grasses assures an active vegetative cover during years when insects, plant disease, or other factors reduce the vigor of one or more species.11,38

The soil cover construction process is important because it has the power to assure success or cause poor performance of the cover. The ET cover uses a different mechanism to control water from that of conventional covers; therefore, the design and construction methods also differ. The soil of the cover should provide adequate plant nutrients, plant-available water-holding capacity, aeration, soil strength, and other factors critical to rapid and robust plant growth, including the highly essential root system. Soil strength is particularly important because it limits the rate of plant root growth. Soil strength may be optimized by control of soil density during and after cover soil construction. These and other requirements are discussed by Hauser et al.11-13

25.3 DESCRIPTION OF ET COVER SYSTEMS

ET cover systems use one or more vegetated soil layers to retain water until it is either transpired through vegetation or evaporated from the soil surface. These cover systems rely on the water storage capacity of the soil layer, rather than low hydraulic conductivity materials, to minimize percolation. ET cover system designs are based on using the hydrological processes (water balance components) at a site, which include the water storage capacity of the soil, precipitation, surface runoff, ET, and infiltration. The greater the storage capacity and evapotranspirative properties, the lower the potential for percolation through the cover system. ET cover system designs tend to emphasize the following6,39,40:

1. Fine-grained soils, such as silts and clayey silts, that have a relatively high water storage capacity.

2. Native vegetation to increase ET.

3. Locally available soils to streamline construction and provide for cost savings.

In addition to being called ET cover systems, these types of covers have also been referred to in the literature as water balance covers, alternative earthen final covers, vegetative landfill covers, soil-plant covers, and store-and-release covers.

Two general types of ET cover systems are

1. Monolithic barriers41

2. Capillary barriers.15

Monolithic covers, also referred to as monofill covers, use a single vegetated soil layer to retain water until it is either transpired through vegetation or evaporated from the soil surface. A conceptual design of a monolithic cover system is shown in Figure 25.2.

Capillary barrier cover systems consist of a finer-grained soil layer (like that of a monolithic cover system) overlying a coarser-grained material layer, usually sand or gravel, as shown conceptually in Figure 25.3. The differences in the unsaturated hydraulic properties between the two layers minimize percolation into the coarser-grained (lower) layer under unsaturated conditions.

Vegetation Fine-grained layer

Interim cover

Waste

Vegetation Fine-grained layer

Interim cover

Waste

FIGURE 25.2 Conceptual design of a monolithic ET final cover.15

FIGURE 25.2 Conceptual design of a monolithic ET final cover.15

Vegetation Fine-grained layer

Coarse-grained layer Interim cover

Waste

FIGURE 25.3 Conceptual design of a capillary barrier ET final cover.15

The finer-grained layer of a capillary barrier cover system has the same function as the monolithic soil layer; that is, it stores water until it is removed from the soil by evaporation or transpiration mechanisms. The coarser-grained layer forms a capillary break at the interface of the two layers, which allows the finer-grained layer to retain more water than a monolithic cover system of equal thickness. Capillary forces hold the water in the finer-grained layer until the soil near the interface approaches saturation. If saturation of the finer-grained layer occurs, the water will move relatively quickly into and through the coarser-grained layer and to the waste below.

In addition to being potentially less costly to construct, ET covers have the potential to provide equal or superior performance compared to conventional cover systems, especially in arid and semiarid environments. In these environments, they may be less prone to deterioration from desiccation, cracking, and freezing/thawing cycles. ET covers also may be able to minimize side slope instability, because they do not contain geomembrane layers, which can cause slippage.5,42,43

Capillary barrier ET cover systems may also eliminate the need for a separate biointrusion and/ or gas collection layer. The coarser-grained layer can act as a biointrusion layer to resist root penetration and animal intrusion, due to its particle size and low water content. The coarser-grained layer can also act as a gas collection layer, because the soil properties and location within the cover system are comparable with a typical gas collection layer in a conventional cover system.39,44

25.4 LIMITATIONS

ET cover systems are generally considered potentially applicable only in areas that have arid or semiarid climates; their application is generally considered limited to the western United States. In addition, site-specific conditions, such as site location and landfill characteristics, may limit the use or effectiveness of ET cover systems. Local climatic conditions, such as amount, distribution, and form of precipitation, including amount of snow pack, can limit the effectiveness of an ET cover at a given site. For example, if a large amount of snow melted when vegetation was dormant, the cover may not have sufficient water storage capacity, and percolation might occur.6,45

Further, landfill characteristics, such as production of landfill gases, may limit the use of ET covers. The cover system may not adequately control gas emissions since typical ET cover designs do not have impermeable layers to restrict gas movement. If gas collection is required at the site, it may be necessary to modify the design of the cover to capture and vent the gas generated in the landfill. In addition, landfill gas may limit the effectiveness of an ET cover, because the gases may be toxic to the vegetation.13,45

Limited data are available to describe the performance of ET cover systems in terms of minimizing percolation, as well as the covers' ability to minimize erosion, resist biointrusion, and remain effective for an extended period of time. While the principles of ET covers and their corresponding soil properties have been understood for many years, their application as final cover systems for landfills has emerged only within the past 10 years. Limited performance data are available on which to base applicability or equivalency decisions.39,43,46

Vegetation Fine-grained layer

Coarse-grained layer Interim cover

Waste

FIGURE 25.3 Conceptual design of a capillary barrier ET final cover.15

Numerical models are used to predict the performance and assist in the design of final cover systems. The availability of models used to conduct water balance analyses of ET cover systems is currently limited, and the results can be inconsistent. For example, models such as Hydrologic Evaluation of Landfill Performance (HELP) and Unsaturated Soil Water and Heat Flow (UNSAT-H) do not address all of the factors related to ET cover system performance. These models, for instance, do not consider percolation through preferential pathways; may underestimate or overestimate percolation; and have different levels of detail regarding weather, soil, and vegetation. In addition, HELP does not account for physical processes, such as matric potential, that generally govern unsaturated flow in ET covers.39,42,47

25.5 DESIGN ISSUES AND REQUIREMENTS

The ET cover cannot be tested at every landfill site; so it is necessary to extrapolate the results from sites of known performance to specific landfill sites. The factors that affect the hydrologic design of ET covers encompass several scientific disciplines and there are numerous interactions between factors. As a consequence, a comprehensive computer model is needed to evaluate the ET cover for a site.48 The model should effectively incorporate soil, plant, and climate variables, and include their interactions and the resultant effect on hydrology and water balance. An important function of the model is to simulate the variability of performance in response to climate variability and to evaluate cover response to extreme events. Because the expected life of the cover is decades, possibly centuries, the model should be capable of estimating long-term performance. In addition to a complete water balance, the model should be capable of estimating long-term plant biomass production, need for fertilizer, wind and water erosion, and possible loss of primary plant nutrients from the ecosystem.

Model needs are best met by an "engineering design model." In addition to requirements discussed here, an engineering model should require site parameters that can be measured or are available in historical records. Because adequate site-specific data are almost never available, the engineering design model should not require calibration.

The properties of the ET cover and its design are different from those of conventional covers. Because model evaluation should include all of the important elements required in design, this section provides a review of important elements of the ET cover that influence its design and should be evaluated before selecting and using a particular model. The reader may find additional important details in Refs. 4, 8-12, and 42.

Because borrow soils will be mixed and modified during placement, the cover soil for an ET landfill cover, as constructed, will be unique to the site. However, the soil properties may be easily described. The design process requires an evaluation of whether or not the proposed soil and plant system can achieve the goals for the cover. Numerous factors interact to influence ET cover performance. A mathematical model is needed for design that is capable of (1) evaluating the site water balance that is based on the interaction of soil, plant, and climate factors and (2) estimating the performance of an ET landfill cover during extended future time periods.

Future predictions of ET cover performance require a sophisticated model. A suitable model should include the following14:

1. Contain a stochastic climate generator capable of simulating daily precipitation and other weather parameters that are similar in amount and statistical variability to historical weather records for the site.

2. Realistically estimate daily plant and soil response to variable generated climate.

3. Realistically estimate daily water balance including deep percolation (PRK).

These requirements are similar to those required for flood flow estimates before designing a bridge or culvert on a roadway. In both cases, the future climate and resulting water balance are unknown, but an estimate of the critical future event and its probability of occurrence are needed to guide the design. These needs can be satisfied for ET landfill cover design or evaluation by a suitable hydro-logic computer model.

25.5.1 Hydrologic Water Balance

A major requirement of a landfill cover is to control the amount of precipitation that enters the waste. The amount of water that percolates through the cover and may enter the waste is called PRK. PRK is a part of a much bigger hydrologic system and must be assessed in parallel with the other parts. Therefore, it is necessary to estimate the entire hydrologic water balance for the cover in order to assess its behavior.49

Because the quantity of water on or near the earth is believed to be constant, the hydrologic water balance for a landfill cover may be expressed by the statement14:

where P is the precipitation, I is the irrigation, if applied, ET is the evapotranspiration (the actual amount, not potential amount), Q is the surface runoff, L is the lateral flow, ASW is the change in soil water (SW) storage, and PRK is the deep percolation (below the cover or root zone).

The site water balance for an ET landfill cover is illustrated in Figure 25.4. The incoming water (P + I) should equal the outgoing water (ET + Q + L + ASW + PRK). Where all terms are measured, for example, lysimeter measurements—the difference or lack of balance is an expression of measurement error.

Lateral flow (L) within the soil layer containing plant roots is small for most landfill cover situations and is zero for lysimeters with sidewalls. During the course of a hydrologic year, ASW is usually small in comparison to the other terms, but it may be large on a daily basis. A primary focus for the design is PRK below the ET landfill cover as represented by the rearranged equation:

Here, Error = lack of balance in the measured terms.

PRK is the primary design criterion for landfill covers that are expected to limit and control the amount of precipitation that enters the waste of the landfill. As a result, the primary focus of model

Incoming water = Outgoing water P + I = ET + Q + L + A SW + PRK,

Precipitation and irrigation

Storage in soil

Deep percolation

Precipitation and irrigation

Runoff

Lateral movement

Root depth

Runoff

Lateral movement

Foundation

Waste

FIGURE 25.4 Water balance terms for an ET landfill cover.14

evaluation is the accuracy with which a model estimates PRK. However, the model estimate of PRK is strongly affected by errors in measured inputs P and I, and by errors in estimating ET, Q, and ASW. ET is the largest term of the outgoing water balance for almost all sites. Q is often the second-largest term; it is substantial at many, but not all, sites. Therefore, in addition to the accuracy of PRK estimates, it is also important to assess the accuracy of the model estimates of both ET and Q because errors in these estimates contribute directly to the error in PRK.

In a natural system, soil-water content changes in response to water removal by plants, soil evaporation, and gravitational drainage. During and immediately after rainfall or snowmelt, soil-water storage may change rapidly in response to the influx of water from the rain or snowmelt and the removal of water due to drainage by gravitational forces and plant use. While gravitational drainage can be a significant removal mechanism, it is effective for a short time and is near zero most of the time. Soil evaporation is important for one to a few days after precipitation; then it rapidly declines to near-zero amounts. Plant use is the primary mechanism for change in soil-water content and continues for long time periods or until the soil becomes dry.

Because soil-water content strongly affects daily values of ET, Q, and PRK, errors in estimates of change in total soil-water content will be included in errors of the ET, Q, and PRK terms estimated by a model. An appropriate model should continuously estimate the amount of soil-water in storage for all layers within the soil profile. The rate of plant water use and soil evaporation from a particular layer may be large or small depending on several interacting factors. A significant error in the amount of soil-water stored in one of the topsoil layers may have no effect on the value of PRK if lower layers were dry on that day. Errors in estimates of soil-water storage in each individual layer may or may not contribute to errors in PRK, depending on water content of each layer of the entire soil profile and other factors. The principles of water balance analysis are contained in Refs. 50 and 51, and water balance analysis for landfill covers is described in Refs. 9, 16, 42, 49, and 52-54.

25.5.2 Climate

The total amount of precipitation over a year, as well as its form and distribution, determines the total amount of water storage capacity needed for the cover system. The cover may need to accommodate a spring snowmelt event that causes the amount of water at the cover to be relatively high for a short period of time or conditions during cool winter weather with persistent, light precipitation. Storage capacity is particularly important if the event occurs when local vegetation is dormant, yielding less ET.

Regional climate should be the first consideration when evaluating the suitability of an alternative landfill cover for a site. If the regional climate appears to be compatible with the requirements of the alternative cover, then site characteristics should be examined to determine whether the site climate is also suitable. Site and regional climate may differ substantially for sites near mountains, in valleys, in the rain shadow of coastal mountains, or near the coast. The Air Force Center for Environmental Excellence commissioned a generic assessment of the suitability of the ET landfill cover based on regional climate for the continental United States.4

An adequate measurement of the climate at a site requires the longest available record and should contain a minimum of 20 years of data. The importance of long records can be illustrated by the annual precipitation from Coshocton, Ohio: While the 35-year average annual precipitation is 37 in., one 5-year period averaged 88% of the overall average (32.6 in.) and another averaged 115% (42.6 in.). Clearly, a short record may not accurately describe the climate at a site and should not be used for design.14

Site-specific climatic factors that are important to selection of landfill cover type and to design of ET landfill covers include daily measurements of precipitation, maximum and minimum temperature, relative humidity, total solar radiation, and wind run.6,45,55 If all of the data are not available, one can make useful—but less accurate—estimates of cover performance using only daily precipitation and maximum and minimum temperature measurements.

Any model used for ET cover design should, at a minimum, be able to utilize daily precipitation and temperature data and preferably should be able to utilize the other important climate factors as well in order to produce the most accurate estimates.

25.5.3 Evapotranspiration

ET is the evaporation of water from the soil surface and by plant transpiration (primarily through the stomata on the plant's leaves). ET should be carefully considered during all stages of design since it will be the largest mechanism of water removal in the water balance for an ET cover. With current knowledge, it is necessary to estimate potential evapotranspiration (PET) first and then using the PET estimate the actual evapotranspiration (AET) for the site.

PET is the maximum ET that can result from a set of climatic conditions. It is limited by the amount of energy available to evaporate water. The equivalent term "reference crop evaporation" is used by research workers who investigate the physics of ET. For the purposes of plant growth and production, PET is defined as the amount of water that would return to the atmosphere if abundant, freely transpiring plant leaves are available, and the water supply to the plants is abundant and unrestricted. The magnitude of PET is useful for preliminary planning to identify the maximum possible performance that might be expected from an ET cover.

Hauser and Gimon4 estimated the ratio of PET to precipitation for the continental United States; the results are summarized in Figure 25.5. They arbitrarily used a PET ratio of 1.2 or greater to indicate likely success for the ET cover because AET is likely to be less than PET. The ratio of PET to precipitation is >1 for almost all of the continental United States. The ET cover is likely to be appropriate for sites where the ratio is >1.2; but it may also be appropriate and should be evaluated for all sites where the ratio is >1.14

AET is less than the PET amount except for relatively short time periods during and after rainfall or snowmelt events. When modeling the performance of an ET landfill cover, the estimate of AET is very important. The accuracy with which a model estimates AET is the biggest controlling factor for hydrologic modeling accuracy because (1) AET is the largest term on the right-hand side of Equation 25.1 and (2) water removed from the soil by AET affects or controls the size of the other terms on the right-hand side of Equation 25.1.

Numerous factors control AET and thus control the hydrologic performance of an ET cover. Soil-water content, rate of root growth, and total root mass strongly affect the rate of AET. AET is also affected by whether wet soil is available in surface soil layers, deeper in the profile, or in

FIGURE 25.5 PET/precipitation ratio.14

TABLE 25.1

Factors Affecting Amount and Rate of Surface Runoff from ET Landfill Covers

Soil

Surface

Other Factors

Infiltration rate Water content Particle size distribution Frozen soil Bulk density Clay mineralogy Macro porosity

Surface crust and tilth

Plant type (sod or bunch grass, etc.)

Cover density

Growth rate

Stage of annual growth cycle Biomass production Roughness and storage

Rainfall intensity

Time of occurrence of high intensity

Storm duration

Interception by plants

Soil surface depressions

Litter on the soil surface

Land slope

Source: Hauser, V.L. and Gimon, D.M., Evaluating Evapotranspiration (ET) Landfill Cover Performance Using Hydro-logic Models, Air Force Center for Environmental Excellence (AFCEE), Brooks City-Base, TX, January 2004.

all layers. While root mass and root growth rate strongly affect AET, they are in turn controlled by other factors.

25.5.4 Surface Runoff

Surface runoff (Q) is the second-largest part of the hydrologic water balance for ET landfill covers at many sites in humid regions. Even at dry sites where surface runoff is small, errors in estimates of Q are important, and especially so if the model estimates significant Q on days with no runoff. Estimates of Q are therefore important to the design process at all sites.

Water leaving the site (Q) reduces the volume that must be stored within the cover. Errors in estimating daily Q will result in erroneous estimates of cover performance as measured by PRK of water below the cover. If the estimated Q is too low, the estimated PRK will be too high and vice versa.

Surface runoff can begin only after (1) rainfall or snowmelt fills storage by plant interception and surface ponding and (2) the rainfall or snowmelt rate exceeds the soil infiltration rate. Excellent sources for technical details include Chow et al.,50 ASCE Manual 28,53 and Linsley et al.56 Factors affecting Q are listed in Table 25.1.

Any model chosen for use in ET cover design should make reasonably accurate estimates of Q. There are several methods used to estimate Q. The ASCE Manual 2853 discusses 18 engineering design models that compute Q; some of them use infiltration equations to estimate Q. One of the models used the Richards equation to estimate infiltration. One used the Smith and Parlange infiltration equation, and two used an "index." Two models could use either the soil conservation service (SCS) curve number method or the Green-Ampt infiltration equation. Nine of the models used the SCS curve number method and six used the Green-Ampt infiltration equation. The data shown in ASCE Manual 2853 indicated that the SCS curve number method and the Green-Ampt infiltration equation are, by far, the most popular methods for estimating surface runoff in engineering design models.

25.5.5 Soil-Water Storage and Movement

ET landfill covers control the precipitation falling on the surface by providing adequate water storage capacity in the soil to contain the infiltrating precipitation. Total (potential) soil-water storage capacity is controlled by soil properties. The storage capacity available at any instant in time is controlled primarily by the balance between infiltration from precipitation and rate of water removal from the soil by ET. The majority of ET is the result of plant transpiration. ET covers perform best when the primary limitation to plant growth is soil-water content, thus assuring rapid soil drying.

The physics of water movement within the soil is important to the design of an ET cover. The modern understanding of water movement in unsaturated soils has been under development for about 150 years, and the development of new concepts continues in the modern era. Henri Darcy57

provided the earliest known quantitative description of water flow in porous media. Darcy developed an equation for water flow in saturated sand, and modern equations for both saturated and unsaturated flow are based on his early work.

Currently used equations for water flow in unsaturated soil are based on the assumption that soils are similar to a bundle of capillary tubes and that water flow can be approximated by the Hagen-Poiseuille equation.58 While it is obvious that the pore space in soil is not the same as a bundle of capillary tubes, the concept has proven highly useful and is currently used in mathematical descriptions of water flow in soil.

The Richards equation is widely used in research to estimate water flow in both saturated and unsaturated soils. It is also used in software proposed for use in evaluation of ET landfill covers.

Theoretical estimation of water flow in unsaturated soils is difficult and complex. The derivation of the versions of the Richards equation commonly solved in modern models required several assumptions. In addition, it is difficult to accurately estimate likely field values for unsaturated soil hydraulic conductivity on the scale of a complete ET cover. Nevertheless, the Richards equation provides useful estimates of flow of water within the soil where adequate estimates of soil hydraulic conductivity are available.

Other models successfully employ a simple water routing system. Each layer of soil is assumed to hold all water entering the layer up to the field capacity. When the water content of a soil layer exceeds the field capacity, water drains downward to the next layer at the rate specified by the hydraulic conductivity of the saturated soil in the layer.

25.5.6 Deep Percolation

Estimates of water movement through the cover (PRK) are of particular concern for ET cover design and evaluation. The performance of ET covers should be estimated for large and critical climatic events expected during the life of the cover. Therefore, a major concern for ET cover performance is the determination of the greatest amount of water that the ET cover soil must store during a defined time period. Critical events causing maximum soil-water storage may result from a single-day storm, a multiple day storm, or other events.

The following example illustrates the concept.14 Model estimates are available for a landfill located on the western edge of the Central Great Plains; the cover soil was 0.6 m thick and composed of loam soil. Model estimates of soil water in storage for each day of a 100-year simulation period along with estimates of daily values of PRK are available. The estimates revealed that no water should be expected to move through the cover. Figure 25.6 presents the estimates of daily precipitation and daily soil-water content during the wettest year of the 100-year model estimate, and it includes

FIGURE 25.6 Critical event.14

the greatest single-day storage of soil-water during the 100-year period. In this example, the critical event was the result of several days with precipitation followed by a large, single-day precipitation event. The cover described could successfully control PRK.

Any model used for ET cover design should, at a bare minimum, demonstrate that the design being modeled will adequately control the projected critical event for the site being considered. Preferably, it should also estimate how much excess storage capacity would be available during that critical event so that an appropriate safety factor can be included in the final cover design. Additional details regarding factors that affect PRK may be found in Refs. 11 and 12.

25.5.7 Soil Type

Finer-grained materials such as silts and clayey silts are typically used for monolithic ET cover systems and the top layer of a capillary barrier ET cover system because they contain finer particles and provide a greater storage capacity than sandy soils. Sandy soils are typically used for the bottom layer of the capillary barrier cover system to provide a contrast in unsaturated hydraulic properties between the two layers. Many ET covers are constructed of soils that include clay loam, silty loam, silty sand, clays, and sandy loam.

The storage capacity of the soil varies among different types of soil, and depends on the quantity of fine particles and the bulk density of the soil. Compaction impacts bulk density, which in turn affects the storage capacity of the soil and the growth of roots. One key aspect of construction is minimizing the amount of compaction during placement. Higher bulk densities may reduce the storage capacity of the soil and inhibit growth of roots.659

25.5.8 Soil Thickness

The thickness of the soil layer(s) depends on the required storage capacity, which is determined by the water balance at the site. The soil layers need to accommodate extreme water conditions, such as snowmelts and summer thunderstorms, or periods of time during which ET rates are low and plants are dormant. Monolithic ET covers have been constructed with soil layers ranging from 2 to 10 ft. Capillary barrier ET covers have been constructed with finer-grained layers ranging from 1.5 to 5 ft, and coarser-grained layers ranging from 0.5 to 2 ft.

25.5.9 Vegetation Types

Vegetation for the cover system is used to promote transpiration and minimize erosion by stabilizing the surface of the cover. Grasses (wheatgrass and clover), shrubs (rabbitbrush and sagebrush), and trees (willow and hybrid poplar) have been used on ET covers.60,61 A mixture of native plants consisting of warm- and cool-season species is usually planted, because native vegetation is more tolerant than imported vegetation to regional conditions, such as extreme weather and disease. The combination of warm- and cool-season species provides water uptake throughout the entire growing season, which enhances transpiration. In addition, native vegetation is usually planted, because these species are less likely to disturb the natural ecosystem.43,45

Understanding important plant requirements is critical for correct selection of materials, design, construction, maintenance, and performance of an ET landfill cover. The success of an ET cover is ensured by optimizing all factors controlling plant growth except for soil-water supply. The goal is to make soil-water content a limiting factor to plant growth several times during each normal growing season.

The plant cover should have potential rooting depth greater than the thickness of the soil cover. Many native species have potential rooting depths of 2 m or more.14

Several plant parameters are important to the design of ET landfill covers. Among the most important are parameters describing: rooting depth, leaf-area-index (LAI), temperature requirements, time to maturity, and water requirements. Models that are suitable for use in design of ET covers will utilize these parameters. The quality of the plant model controls the quality of AET estimates.

25.5.10 Soil and Organic Properties

Soil provides the medium in which plants grow; it stores precipitation within the ET cover and provides nutrients for plant growth. Total (potential) soil-water storage capacity is controlled by soil properties. The storage capacity available at any instant in time is controlled primarily by the balance between infiltration from precipitation and rate of water removal from the soil by ET.

The cover design and construction should optimize soil conditions for water use by plants. This is an important tool and can be used to ensure success of the ET cover. Plant growth and water use are controlled by soil and air temperature, precipitation, solar radiation, wind, humidity, disease, and insect attack. Neither design nor construction practice can exert significant control over these factors; but they can be considered during design to assure success.

Other important soil properties of the ET landfill cover may be controlled by adequate design and good construction practices. The properties that govern root and plant growth and are important to design of ET landfill covers include soil density, aeration, pH, and nutrients. For a complete list of soil properties refer to Table 25.2.

Nutrient and salinity levels affect the ability of the soil to support vegetation. The soil layers need to be capable of providing nutrients to promote vegetation growth and maintain the vegetation system. Low nutrient or high salinity levels can be detrimental to vegetation growth, and if present, supplemental nutrients may need to be added to promote vegetation growth. For example, at Fort Carson, Colorado, biosolids were added to a monolithic ET cover to increase organic matter and provide a slow release of nitrogen to enhance vegetation growth. In addition, topsoil promotes growth of vegetation and reduces erosion. For ET covers, the topsoil layer is generally a minimum of 6 in. thick.62

After landfill cover completion, the plant cover may be changed but soil modification may be impractical. Therefore, good soil design and correct construction practices are of utmost importance to the success of the ET cover.

The U.S. Department of Agriculture (USDA) soil classification system was developed for use in describing soils in which plants grow.63-66 The USDA system is now universally accepted within the United States and it should be used to describe soils used in ET landfill covers.

By its very nature, construction of an ET landfill cover modifies the soil used to create the cover. Hence, the construction process offers the opportunity to either (1) place the soil so that it will

TABLE 25.2

Soil Properties That Govern Root and Plant Growth and Are Important to Design of ET Landfill Covers

Derived or Secondary

Soil Conditions/Factors Affecting Plant Growth

Temperature Water content Oxygen in soil air Toxic substances Ammonia

CO2 from decaying organics

Methane

Bacteria

Fungi

Basic Soil Properties

Particle size distribution Bulk density Clay mineral type pH

Total porosity Percentage large pores Soil salinity Soil sodium content Humus content

Soil strength Water-holding capacity Field capacity/wilting point Hydraulic conductivity Fertility

Available nutrient supply

Anions/salinity

Aeration properties/connection between pores

Tilth

Soil Properties

Source: Hauser, V.L. and Gimon, D.M., Evaluating Evapotranspiration (ET) Landfill Cover Performance Using Hydro-logic Models, Air Force Center for Environmental Excellence (AFCEE), Brooks City-Base, TX, January 2004.

perform better than before it was moved or (2) damage the soil and greatly reduce the opportunity for success in meeting the requirements for the cover. It is important to understand soil properties that control success and how they may be optimized during cover construction. An appropriate model can help the designer understand how changes in the properties of soils available at the site in question will affect the final design of an ET cover.

Agricultural interests have amended existing soil properties to improve productivity; their experience demonstrates the power of knowledge of soil properties and the ability to control them.14 A primary benefit of these amendment efforts was improvement in soil-water-holding capacity and increased rate of water removal from all soil layers by plants. The benefits of soil modification remain effective for decades. There is opportunity for similar improvements in soil during ET landfill cover design and construction. Control of ET cover soil properties has potential to enhance cover performance and should add little to construction cost.

The water-holding properties of ET cover soils are important to succeed. Soils that hold much water will achieve the desired water control with a thinner layer of soil than those with low waterholding capacity. The water-holding properties should be expressed as volumetric water content in keeping with model requirements and to facilitate understanding of design parameters. Important water-holding properties include the permanent wilting point, field capacity, and plant-available water content.

25.5.11 Control Layer Types

Control layers, such as those used to minimize animal intrusion, promote drainage, and control and collect landfill gas, are often included for conventional cover systems and may also be incorporated into ET cover system designs. For example, a proposed monolithic ET cover at Sandia National Laboratories in New Mexico will have a biointrusion fence with 1/4-in. squares between the topsoil layer and the native soil layer to prevent animals from creating preferential pathways, potentially resulting in percolation. The biointrusion layer, however, will not inhibit root growth to allow for transpiration. At another site, Monticello Uranium Mill Tailings Site in Utah, a capillary barrier ET design has a 12-in. soil/rock admixture as an animal intrusion layer located 44 in. below the surface, directly above the capillary barrier layer.

In addition, a capillary barrier cover demonstration at Sandia National Laboratories has a drainage layer located above the capillary break. A drainage layer consisting of an upper layer of sand and a lower layer of gravel is located directly below the topsoil layer. The sand serves as a filter to prevent topsoil from clogging the drainage layer, while the gravel allows for lateral drainage of water that has infiltrated through the topsoil.39,67

In more recent applications, several types of ET cover designs also have incorporated synthetic materials, such as geomembranes, which are used to enhance the function of minimizing water into the waste. For example, the Operating Industries Inc. Landfill in California has incorporated a soil layer with a geosynthetic clay liner in the design. The cover system for this site will reduce surface gas emissions, prevent oxygen intrusion and percolation, and provide for erosion control.68

25.5.12 Safety Factor

As with any engineering design, the ET cover should be designed with safety factors because both design and construction introduce uncertainty regarding performance. Some safety factor concerns are similar between ET covers and conventional covers. However, control of water flow into the waste requires new safety factor considerations for the ET cover, including the following:

1. The size of the soil-water reservoir in the cover soil should be adequate to contain extreme or design storm events.

2. The time required to empty the soil-water reservoir is critical to success.

Live root mass

Depth

Extra soil

Live root mass

Depth

FIGURE 25.7 Root distribution in the cover.

One basis for providing a safety factor is to arbitrarily increase the soil thickness (e.g., build the soil 50% thicker than indicated as adequate by design). However, this intuitive approach may not produce the desired result.

Although the soil's total water-holding capacity is similar for all layers of a uniform soil, the distribution of roots and the rate and amount of water extraction are not. Consider the typical ET cover soil situation in which the soil has uniform properties from top to bottom. The distribution of living plant roots in soil controls the rate of drying of each soil layer. Figure 25.7 illustrates a normal root distribution pattern and an ET cover soil profile. Addition of extra soil to the thickness of the cover, where all soil is uniform, has the effect of adding soil to the bottom of the cover because the plant roots grow from the surface downward. The last increment of soil thickness results in relatively few roots growing in the newly added soil layer, which is effectively on the bottom of the cover. Plants remove water more slowly from deep soil layers than from near-surface soil layers. As a result, during one growing season, plant roots may not remove all plant-available water from the lower layers of the cover if the cover is thick.

As shown in Figure 25.8, an increase in soil thickness from the design thickness (A) by 50% to (B) may result in only a small increase in plant-available water-holding capacity during a single growing season.

A better way to provide a safety factor is to utilize hydrologic factors that are known to affect soil-water use and storage. They may be used in combination with a model to evaluate options and

A B Soil thickness ^^

FIGURE 25.8 Effective water storage capacity—one growing season.14

A B Soil thickness ^^

FIGURE 25.8 Effective water storage capacity—one growing season.14

select a good course of action. The model should estimate soil-water content for each soil layer on each day of evaluation. It should also maintain a balance of available soil-water storage space. Therefore, the model should indicate available storage for each day along with ET, Q, and PRK. Possible ways to introduce an adequate safety factor include14:

1. Base the design on reduced plant-available water-holding capacity (e.g., 10% reduction).

2. Base the design on increased daily precipitation (e.g., 110% of normal precipitation).

3. Increase surface runoff by replacing the second layer of soil—for example, 6-12 in.—with clay soil, or use clay soil for the top 6 in. of the cover; however, maintain the same soil thickness as required for a one-layer soil.

4. Design for either warm- or cool-season plants, but establish both to provide increased annual, total water use.

These possibilities may be used singly or in combination. Use of an appropriate model to simulate the effects of such design changes will enable the designer to add a suitable safety factor to the final design.

25.6 HYDROLOGIC MODELS

ET landfill cover performance is governed by a complex set of interacting processes. Mathematical models may describe individual processes, but because of interactions among processes, these more limited mathematical treatments should be integrated into a single working model. The development of a new computer model suitable for ET landfill cover design would be expensive and require several years of development and testing. An alternative is to evaluate currently available models to determine whether they are suitable for design and analysis of an ET landfill cover.

Engineering design and cover evaluation are best served by a model that incorporates all of the important elements of engineering design that are important to ET covers. Some models are good research or scientific investigation tools, but are not sufficiently complete to serve the practicing design engineer who must consider all aspects of landfill remediation during cover design. In practice, the design engineer must balance the need for high quality, input data against landfill remediation requirements, and available funds. The model should be sufficiently robust to provide reliable answers with less than optimum completeness or accuracy in the input data. It should also be capable of providing guidance to the design engineer regarding the consequences of incomplete input data.

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