U.S. EPA Contaminated Site Cleanup Information (CLU-IN)

U.S. Environmental Protection Agency
U.S. EPA Technology Innovation and Field Services Division

Evapotranspiration Covers

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In some states, alternative landfill cover technologies have not been officially covered by policy or regulation; hence, the regulators have no reference for the "equivalent alternative" of a conventional cover to have the correlation of the field data, performance assessment, modeling, and written regulations. For regulators to be comfortable approving alternative final landfill covers, they will need an understanding of the engineering of landfills and the science behind an ET cover to evaluate it and to apply the flexibility in the regulations associated with alternative landfill covers.

The resources that follow offer a variety of concepts, considerations, tools, procedures, and suggestions for ET cover design.

Darcian Flux Estimations in Evapotranspiration Landfill Covers.
Kelsey, J.A., J.T. Kay, M. Ankeny, and M. Plummer. Unsaturated Soils 2006: Proceedings of the 4th International Conference on Unsaturated Soils, 2-6 April 2006, Carefree, Arizona. ASCE, Reston, VA. Geotechnical Special Publication No. 147, ISBN: 0784408025, 2006

Prior to regulatory approval, ET covers are often required to pass a field test designed to demonstrate adequate performance. This paper evaluates the success of a model in predicting moisture flux in ET covers at different levels of saturation. Field demonstrations of ET covers have used a method that estimates cover flux based on daily soil matric potential values measured in situ. The method involves heat dissipation sensors, soil matric potential/soil moisture content relationships for site-specific soils, and Darcian flux calculations to estimate the rate, volume, and direction of soil moisture movement. Field results suggest that this method is valid for monitoring flux in covers where relatively low moisture conditions exist; however, it can be highly inaccurate under conditions approaching saturation or where extremely large gradients exist.

Design of Evapotranspirative (ET) Covers at the Rocky Mountain Arsenal.
Kiel, R., D. Chadwick Jr., J. Lowrey, C. Mackey, and L. Greer. Proceedings of the 7th Annual SWANA Landfill Symposium, 25-28 March 2002, Monterey, California. Solid Waste Assoc. of North America, Silver Spring, MD. 11 pp, 2002

This paper discusses design factors that influence the successful construction of ET covers in a semi arid climate. An alternative final cover (AFC) demonstration project was conducted consisting of four test covers built of two different soil types (based on grain-size distribution) with cover thickness of 42, 48, and 60 inches. Comparative analyses were performed to evaluate percolation and the water balance of the test cover soils using UNSAT-H. The percolation from the bottom of the cover is anticipated to be less than 1.3 (mm/yr). Along with soil type and thickness, vegetation plays a critical role in the performance of AFCs. The factors influencing plant species selection included soil type, height at maturity (to deter prairie dog invasion), persistence, leaf area index contribution, and seed availability. Both cool and warm season grass species were included in the final seed mix, plus a variety of native forbs. Other design issues included the development of soil type and compaction criteria. The growth-limiting bulk density concept was considered in selecting the upper density limit to ensure that the soils are not compacted to a degree that retards plant growth. For the AFCs, a preliminary soil acceptance zone was developed based on soil characteristics from the field demonstration, as well as on clay and silt limits based on vegetation considerations. Representative portions of the acceptable zone were validated by comparing test cover soils to planned borrow area soils through additional UNSAT-H modeling and evaluation. Once the model-predicted soil performance was determined, a final acceptance zone was established with limits defined on the USDA Soil Textural Triangle. Additional design elements include the use of recycled concrete from the former Denver Stapleton Airport as a biota barrier, stormwater controls, percolation monitoring and response action plans for the AFCs, and long-term maintenance plans.

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Design of Inclined Covers with Capillary Barrier Effect.
Parent, S.-E. and A. Cabral. Geotechnical and Geological Engineering 24(3):689-710(2006)

This paper proposes a design procedure to minimize water infiltration into landfills by optimizing the water diversion length of inclined covers with capillary barrier effect (CCBE). This design procedure is based on a conceptual, mathematical and numerical approach and aims at selecting materials and optimizing layer thickness. Selection among candidate materials is made based on their hydraulic conductivity functions and on a threshold infiltration rate imposed on the designer. The results obtained using the proposed design procedure were compared to those obtained from numerical simulations performed using a finite element unsaturated seepage software. The procedure was applied for two cover systems; one where deinking by-products were used as the moisture retention layer (MRL) and sand as the capillary break layer (CBL) and another where sand was used as MRL and gravel as CBL. Using this procedure, it has been shown that an infiltration control system composed of thin layers of sand over gravel is highly efficient in terms of diversion length and that its efficiency can be enhanced by placing a hydraulic barrier above the MRL.

Design Principles and Concepts for Enhancing Long-Term Cap Performance and Confidence.
Piet, S.J., R.P. Breckenridge, J.J. Jacobson, G.J. White, and H.I. Inyang. Practice Periodical of Hazardous, Toxic, and Radioactive Waste Management 9(4):210-222(2005)

Design principles are herein considered from the standpoint of long-term performance and management, including the ability to monitor and repair barriers. Design concepts that may be implemented on the basis of these principles, especially ET caps are discussed. Five design principles are recommended on the basis of considerations of infrastructure implementation experience and facility management experiences in other fields: establishment of a clear and defendable design basis; design for ease of monitoring and repair; analysis of barriers as dynamic systems; working with nature and not against it; and recognition that increased complexity can reduce, not enhance, net performance. The ET caps are an excellent embodiment of these design principles. The principles are applied to ET caps, as well as design variants such as erosion armor, capillary breaks, biointrusion layers, and low permeability material layers.

Evapotranspiration Covers for Landfills And Waste Sites.
Hauser, Victor L. CRC Press, Boca Raton, FL. ISBN: 978-1-420-08651-5, 216 pp, 2008

This book describes the design, construction, and maintenance of innovative ET covers for landfills and waste, discusses where ET covers can be used appropriately, explains why several vegetative covers have failed, and provides simple, inexpensive solutions. The design and construction of ET covers and other methods is examined, highlighting their differences and successful alternative construction methods. The text contains proposed performance measurements for conventional and innovative landfill covers based on the data collected at more than 55 sites. The data also provide the basis for a performance assessment of landfill cover performance and design.

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Findings of International Review of Soil Cover Design and Construction Practices for Mine Waste Closure.
Rykaart, M., D. Hockley, M. Noel, and M. Paul. The 7th International Conference on Acid Rock Drainage (ICARD), March 26-30, 2006, St. Louis MO. R.I. Barnhisel (ed.). American Society of Mining and Reclamation (ASMR), Lexington, KY. 1804-1822(2006)

Based on a review of over 200 case studies of soil covers for tailings impoundments, waste rock piles, backfilled pits, and heap leach pads, this paper provides some suggestions as to how operators, regulators, and practitioners could improve soil cover design and construction practices for better long-term cover performance.

Miscellaneous Methods for Measuring Matric or Water Potential.
Scanlon, B.R., B.J. Andraski, and J. Bilskie. Methods of Soil Analysis, Part 4: Physical Methods. Soil Science Society of America, Inc., Madison, Wisconsin. Section 3.2.4:643-670(2002)

Matric potential is determined when the sensor matrix is in direct contact with the soil, so salts are free to diffuse in or out of the sensor matrix, and the equilibrium measurement therefore reflects matric forces acting on the water. Water potential is determined when the sensor is separated from the soil by a vapor gap, so salts are not free to move in or out of the sensor, and the equilibrium measurement reflects the sum of the matric and osmotic forces acting on the water. Seven different techniques are described.

Modelling the Critical Interactions between Cover Systems and Vegetation
Natural Resources Canada, Mine Environmental Neutral Drainage Program, MEND Report 2.21.6, 66 pp, 2014

This report summarizes physical, physiological, and ecological properties of vegetation relevant to cover system design to help users determine the preferred approach for numerical simulation of vegetation in cover system design. A summarized compilation of current modeling codes and their deficiencies is followed by examples of how models that accurately capture plant processes are formulated. Recommendations and conclusions are included.

Postconstruction Changes in the Hydraulic Properties of Water Balance Cover Soils.
Benson, C.H., A. Sawangsuriya, B. Trzebiatowski, and W.H. Albright. Journal of Geotechnical and Geoenvironmental Engineering 133(4):349-359(2007)

Hydraulic properties of soils used for water balance covers measured at the time of construction and one to four years after construction are compared to assess how the hydraulic properties of cover soils change over time as a result of exposure to field conditions. Data are evaluated from 10 U.S. field sites that represent a broad range of environmental conditions. The comparison shows that the saturated hydraulic conductivity can increase by a factor of 10,000, saturated volumetric water content by a factor of 2.0, van Genuchten's parameter by a factor of 100, and van Genuchten's n parameter can decrease by a factor of 1.4. The data may be used to estimate changes in hydraulic properties for applications such as waste containment, where long-term maintenance of hydraulic properties in shallow engineered soil layers is important.

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Test Section Installation Instructions, Alternative Cover Assessment Program.
Benson, C.H., T. Abichou, X. Wang, G.W. Gee, and W.H. Albright. Environmental Geotechnics Report No. 99-3, 40 pp, 1999

This document describes installation procedures for test sections constructed for the Alternative Cover Assessment Program (ACAP). An ACAP test section is designed to simulate an earthen alternative cover for a waste containment facility. Each test section contains a lysimeter pan to collect percolation from the base of the cover; a collection system for surface runoff; and sensors to monitor hydrologic variables within the cover soils, percolation and runoff volumes, and meteorological data.

Water Balance Covers for Waste Containment Principles and Practice
Albright William H., Craig H. Benson, and W. Joseph Waugh
ASCE Press, 158 pp, 2010

This book presents the results of the latest research regarding water balance covers for landfill and other waste storage areas. Water balance covers (also known as evapotranspiration covers) can be effective and efficient alternative to conventional clay liner or geosynthetic barrier covers, especially in arid to semiarid regions such as parts of the Great Plains and southwest. Water balance covers are introduced and compared with conventional approaches to waste containment. A detailed analysis of the fundamentals of soil physics and design issues is provided along with a discussion of applicable ecological concepts and revegetation practices. The book also provides discussions on cover construction, modeling, and maintenance.�Case studies drawn from current field testing are included.

A Water Balance Study of Four Landfill Cover Designs Varying in Slope for Semiarid Regions.
Nyhan, J.W., T.G. Schofield, and R.H. Starmer. Journal of Environmental Quality 26:1385-1392(1997)

This paper evaluates the affect of slope on landfill cover evaporation rates. The goal of waste disposal in landfills is to reduce risk to human health by isolating contaminants until they no longer pose a hazard. To achieve this, the performance of a landfill cover design without an engineered barrier (Conventional Design) was compared with designs containing either a hydraulic barrier (USEPA Design) or two capillary barriers (Loam and Clay Loam Capillary Barrier Designs). Water balance parameters were measured at 6-h intervals for these designs in 1.0 by 10.0 m plots with downhill slopes of 5, 10, 15, and 25%. Whereas runoff generally accounted for only 2 to 3% of the precipitation losses on these designs from December 1991 through July 1995, similar values for evaporation ranged from 86 to 91%. Evaporation usually increased with increases in slope in our field plots; the Conventional Design at slopes of 5 and 25% exhibited 139 and 162 cm of evaporation, respectively. Consequently, interflow and seepage usually decreased with increasing slope: interflow decreased from 10.7 to 1.5 cm for the Clay Loam Capillary Barrier Design at slopes of 5 and 25%. Although seepage comprised up to 10% of the precipitation on the Conventional Design, seepage did not occur in either the USEPA design or the capillary designs at the larger slopes. This paper summarizes a 306-page report, A Water Balance Study of Four Landfill Cover Designs at Material Disposal Area B in Los Alamos, New Mexico (1997).

Water Balance of Two Earthen Landfill Caps in a Semi-Arid Climate.
Khire, M., C. Benson, and P. Bosscher. Journal of Land Contamination and Reclamation 5(3):195-202(1997)

Water balance data are presented that were obtained from two earthen cap test sections located in a semi-arid region. The test sections were constructed on a municipal solid waste landfill in East Wenatchee, Washington, USA. One test section represents a traditional resistive barrier, and is constructed with a compacted silty clay barrier 60 cm thick and a vegetated silty clay surface layer 15 cm thick. The other test section represents a capillary barrier and has a sand layer 75 cm thick overlain by a 15-cm-thick vegetated surface layer of silt. Extensive hydrological and meteorological data have been collected since November 1992. Unsaturated hydraulic properties of soils, hydrologic parameters, and vegetation have been extensively characterized. Results of the study show that capillary barriers can be effective caps in semi-arid and arid regions. They are also cheaper to construct and can perform better than traditional resistive barriers. This paper also can be found in the proceedings (p 252-261) of the 1997 International Containment Technology Conference [Warning: There are 1,164 pages in the proceedings!].

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State Guidance

Draft Landfill Guidance on Alternative Final Cover Demonstrations
Montana Dept. of Environmental Quality, Permitting & Compliance Division. 4 pp, 2011
Final Guidance Document: Water Balance Covers in Colorado
Colorado Department Public Health and Environment, 114 pp, 2013
Guidance for Requesting a Water Balance Alternative Final Cover for a Municipal Solid Waste Landfill
Texas Commission on Environmental Quality, Waste Permits Division. RG-494, 24 pp, 2012