13.2 alloWaBle leakage When the evapotranspiration ET landfill cover was first introduced to regulatory bodies, they rejected the concept because it does not follow the prescriptive rule
Trang 1This chapter contains topics that did not fit in other chapters, sources for tech-nical data, and a closing summary statement
13.1 Design anD presCriptive rules
Rules and regulations prescribe important parts of conventional-barrier cover design Such restrictive rules and their application allow limited freedom for the design engi-neer to create new and better landfill covers
Fortunately, the regulations also allow alternate designs; however, this important feature of regulations lay dormant and unused for many years At present, some regulators and engineers actively promote and examine alternative designs Thus, the situation is improving
13.2 alloWaBle leakage
When the evapotranspiration (ET) landfill cover was first introduced to regulatory bodies, they rejected the concept because it does not follow the prescriptive rules and regulations and also because of the widely held perception that conventional covers are “impermeable.” The myth of “impermeability” was part of the accepted notion that conventional-barrier covers are adequate and provide a suitable “presumptive remedy” for landfill remediation
The performance measurements for conventional-barrier landfill covers cited in Chapter 3 were the result of relatively short field tests None of the published mea-surements exceeded two decades in duration; most measurement periods were less than 5 years in length In some instances, the percolation rate through the cover was increasing at the end of the short test The duration of these tests is short when com-pared to an expected need for a cover that extends to multiple decades or centuries
In addition, future events are likely to increase the leakage through barrier covers; for example, waste settlement creates major stresses on the cover
Conventional-barrier landfill covers oppose natural forces; this is a major reason
to expect them to leak more, not less, in the future In spite of the strong evidence that they leak, correctly built barrier covers have protected human health and the environment; therefore, it is logical to conclude that some leakage through a landfill cover is not harmful
Each landfill has site-specific needs The allowable deep percolation through the cover at the site is perhaps the most important site requirement Average allowable leak-age estimates for ET covers are developed and contained in Chapter 8, Section 8.2; they provide guidance when setting the site-specific allowable deep percolation amount
Trang 2180 Evapotranspiration Covers for Landfills and Waste Sites
A criterion is proposed for landfills that need a minimum deep percolation through the cover:
The average allowable annual deep percolation rate through municipal
•
waste should not exceed 3% of average annual precipitation
Where waste decay or other factors require more water, the allowable
leak-•
age may be greater
The proposed criterion is based on field measurements at numerous field sites It is conservative, yet allows latitude in design and performance Where deep percolation should be greater (e.g., on a bioreactor landfill), the ET cover is easily designed to meet the needs of the site The ET landfill cover can easily meet site requirements
13.3 teChniCal resourCes
The ET landfill cover utilizes different technical resources than those needed for design and installation of conventional covers Because ET cover technology empha-sizes soil, plants, climate, hydrology, and their numerous interactions, the required engineering and science background includes several disciplines Useful information sources available to the design engineer are
American Society of Agricultural and Biological Engineers
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Soil Science Society of America
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Agronomy Society
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Crop Science Society of America
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U.S Department of Agriculture
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Agricultural engineers have used this science in their work for decades The stan-dard tests needed to implement ET covers are contained primarily in publications of these societies; Appendix A contains a list of selected publications and addresses
13.4 researCh neeDs
Because the quest for knowledge should be unending, the author expects new develop-ments in the future Concepts that appear worthy of research investigation include
A new concept for waste disposal that does not use landfills.
to future generations a large and unknown maintenance cost with little hope for cost recovery They consume vast amounts of capital, produce little that
is useful to humans, and consume capital that should be used to create new jobs and industries producing useful products
Continued development of landfills as rapid waste digesters
work is underway on this topic now The potential benefit is large
Mechanisms for and the true magnitude of preferential flow.
shows evidence of preferential flow However, available evidence from
Trang 3Miscellany and Summary 181
long-term field measurements indicates that preferential flow is not a sig-nificant threat for ET landfill covers The evidence is conflicting
Improved methods for soil mixing and placement.
cor-rect placement of soil on landfills for ET covers are difficult
Methods to create or preserve soil structure
ET cover to failure
13.5 summarY
Chapter 3 describes alternative vegetative covers that do not meet the requirements for a landfill cover or for an ET cover Section 3.5.5 (Chapter 3) describes common elements of failure for such vegetative covers It is important to understand the dif-ference between ET landfill covers and alternative covers that do not satisfy relevant requirements; the primary difference is soil density
This book contains the philosophy for landfill covers; proof of the ET landfill cover concept; the technical basis for the ET cover, its design, construction, and mon-itoring; and a basis for defining the allowable leakage through an ET landfill cover The proof of the ET cover concept includes measurements representing decades and centuries of water movement
In addition, this book presents a unified engineering approach that describes the ET cover, how it meets requirements, its technical basis, modeling, and design Although it focuses on landfill covers, the principles are similar for application of the
ET cover to other wastes
The ET landfill cover meets requirements for remediation of waste sites; it reduces remediation cost below that for conventional covers and provides a self-renewing cover suitable for centuries of use