179 13 Miscellany and Summary This chapter contains topics that did not t 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 landll 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) landll cover was rst 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 landll remediation. The performance measurements for conventional-barrier landll covers cited in Chapter 3 were the result of relatively short eld 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 landll 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 landll cover is not harmful. Each landll has site-specic 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-specic allowable deep percolation amount. © 2009 by Taylor & Francis Group, LLC 180 Evapotranspiration Covers for Landfills and Waste Sites A criterion is proposed for landlls 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 eld measurements at numerous eld sites. It is conservative, yet allows latitude in design and performance. Where deep percolation should be greater (e.g., on a bioreactor landll), the ET cover is easily designed to meet the needs of the site. The ET landll cover can easily meet site requirements. 13.3 teChniCal resourCes The ET landll 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• Soil Science Society of America• Agronomy Society• Crop Science Society of America• U.S. Department of Agriculture• 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 landlls.• Landlls pass 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 landlls as rapid waste digesters. • Important work is underway on this topic now. The potential benet is large. Mechanisms for and the true magnitude of preferential ow.• Research shows evidence of preferential ow. However, available evidence from © 2009 by Taylor & Francis Group, LLC Miscellany and Summary 181 long-term eld measurements indicates that preferential ow is not a sig- nicant threat for ET landll covers. The evidence is conicting. Improved methods for soil mixing and placement.• The movement and cor- rect placement of soil on landlls for ET covers are difcult. Methods to create or preserve soil structure. • Poor soil structure dooms an ET cover to failure. 1 3 . 5 s u m m a r Y Chapter 3 describes alternative vegetative covers that do not meet the requirements for a landll 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 landll covers and alternative covers that do not satisfy relevant requirements; the primary difference is soil density. This book contains the philosophy for landll covers; proof of the ET landll cover concept; the technical basis for the ET cover, its design, construction, and mon- itoring; and a basis for dening the allowable leakage through an ET landll cover. The proof of the ET cover concept includes measurements representing decades and centuries of water movement. In addition, this book presents a unied engineering approach that describes the ET cover, how it meets requirements, its technical basis, modeling, and design. Although it focuses on landll covers, the principles are similar for application of the ET cover to other wastes. The ET landll 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. © 2009 by Taylor & Francis Group, LLC . conventional-barrier covers are adequate and provide a suitable “presumptive remedy” for landll remediation. The performance measurements for conventional-barrier landll covers cited in Chapter 3 were. 179 13 Miscellany and Summary This chapter contains topics that did not t in other chapters, sources for tech- nical data, and a closing summary statement. 13. 1 Design anD presCriptive. amount. © 2009 by Taylor & Francis Group, LLC 180 Evapotranspiration Covers for Landfills and Waste Sites A criterion is proposed for landlls that need a minimum deep percolation through