The cover technique actually uses to its advantage the high water storage capacity of fine grained soils to retain water in the soil above the waste and refuse. The water is "stored" in that layer until it is released back into the atmosphere either through evaporation from the soil or transpiration from the native vegetation that is planted on the surface (EPA 2). One of the only limitations of the evapotranspiration cover is its inability to function adequately outside of arid and semi-arid environments. Only there can the refuse be covered in a reasonable amount of soil with a storage capacity great enough to manage and store the water that will enter the soil at the landfill site. In humid climates, or ones with higher annual rainfalls, evapotranspiration is not as functional as a landfill cover (Albright et al. 75). Whereas evapotranspiration covers have not been as effective in wetter climates, geomembranes are nearly as effective as evapotranspiration covers, but can function in essentially any climate (Albright et al. 75). This is an important feature in a landfill cover, because obviously many landfills in the United States will be built in the humid Southeast or the rainy Northwest. Landfill cover systems must be developed for those regions as well in order to protect the environment from contamination and human health from pollutants. In those places that evapotranspiration covers would be ineffective, geomembranes offer a useful alternative. They consist, basically, of a layer of synthetic material -- generally a plastic of some kind -- that diverts water around and away from the refuse site. In combination with compacted clay and, sometimes, surface vegetation, geomembranes are functional alternatives to conventional landfill covers.

But, of course, these are not the only alternatives available. Research is being conducted into more exotic and unexpected landfill cover systems, whose effectiveness is less certain though nonetheless promising. Research into the use of anaerobically digested lime-stabilized wastewater sludge and soil as a cover for landfills produced surprisingly positive results. The researchers found that, in fact, the cover method had no negative effect on the landfill and produced no leached contaminants more significant than those produced from a clay compacted, conventional cover. The true potential of this cover type is in its extreme cost effectiveness, the fact that it could serve as an outlet for lime waste, and the sludge actually enhances and hastens natural decomposition of the landfill waste (Rhew and Barlaz 499). While it might seem a bit unconventional, perhaps too alternative, to simply dump sludge on top of a landfill as a cover method, there is every indication that it could be a successful remediation strategy as an alternative cover.

Another promising, albeit equally alternative method, is the use of cellulose material as a cover. Though actually forbidden by regulation in some places, there is research to suggest that cellulose covers could provide reasonable protection and sequestration of landfill contaminants. In the case of a recent study, researchers examined the effects of covering a landfill with waste cellulose material produced during the paper recycling process (Panarotto et al. 123). The researchers found, amazingly, that this cover method had no negative effects on the hydraulic or geomechanical requirements of a basic landfill cover. In other words, a cellulose cover of waste paper product performed as well as a landfill...

There was some concern that that cover material would not be a success because it is biodegradable and would settle in variable, and highly unpredictable, ways. But this proved not to be the case. After 400 days of close scrutiny by the researchers, it became clear that the cellulose cover was as functional as any conventional method for protecting landfills.
Conclusion

It is clear that modern landfills are moving towards a crisis point. Environmental damage and health threats caused by improperly contained landfills sites are increasing. The conventional techniques for completely isolating landfill waste form the surrounding environment is, quite simply, unrealistic. Worse, the damage that compacted clay barriers inevitably suffer means that an unfortunately high proportion of landfills using this cover technique are facing systemic failures. These threats add up to the need for new landfill cover strategies that are more cost-effective and more in-tune with the natural cycles and processes that consistently undermine the efficacy of conventional covers.

As this literature review has shown, however, alternative cover strategies for landfills have reached a significant stage of technical maturity and can be implemented in many sites without fear or compunction. Especially promising are methods like evapotranspiration, geomembranes, and anisotropic barriers that direct water away from the landfill site instead of through it. Together, these alternatives and others challenge the conventional wisdom that there is only one solution to the problem of waste remediation. Complete sequestration and "entombment" is not a viable option save perhaps in a few isolated incidences. To produce truly successful landfill covers, waste managers must attune themselves more closely to the local process and natural cycles that will affect the landfill site immediately and in the future. In this way, landfill covers can be implemented that are best suited for the specific locations in which they are needed.

Works Cited

Albright, William H., Benson, Craig H., Gee, Glendon W., Roesler, Arthur C., and Rock, Steven a. "Examining the Alternatives." Civil Engineering 73.5 (May 2003): 70-75.

Dwyer, Stephen F. "Alternative Landfill Covers Pass the Test." Civil Engineering 68.9 (Sept. 1998): 50-52.

Dwyer, Stephen F. "Finding a Better Cover." Civil Engineering 71.1 (Jan. 2001): 58-63.

Environmental Protection Agency. "Evapotranspiration Landfill Cover Systems Fact Sheet." EPA: Solid Waste and Emergency Response. Sept. 2003. Mar. 2007 http://www.epa.gov.

Hauser, Victor L. "Alternative Landfill Remediation -- Ready for Air Force Use." Proceedings, 2003 AFCEE Technology Transfer Workshop. 25 Feb. 2003. Mar. 2007 http://www.afcee.brooks.af.mil/products/techtrans/LandfillCovers/Alt_Cover_abst_final_AFCEE.pdf.

Koerner, Robert M. And Daniel, David E. "Better Cover-Ups." Civil Engineering 62.5 (Mar 1992): 55-57.

Panarotto, Claudia Teizeira, Cabral, Alexandre Rodrigues, and Lefebvre, Guy. "Environmental, Geotechnical, and Hydraulic Behavior of a Cellulose-Rich by-Product Used as Alternative Cover Material." Journal of Environmental Engineering and Science 4.2 (mar. 2005): 123-138.

Rhew, Rodney D., Barlaz, Morton a. "Effect of Lime-Stabilized Sludge as Landfill Cover on Refuse Decomposition." Journal of Environmental Engineering 121.7 (July 1995): 499-506.