|
Technical Approach for Designing Alternative Landfill Covers at Edwards Air Force Base in California |
||||
|
|
Alternative Design Concept
A common alternative design is the evapotranspiration, or ET, concept that exploits water storage capacity of finer-textured soils and water removal capability of vegetation. The layers of finer-textured soil are designed to store infiltrating water with minimal drainage while vegetation is dormant or precipitation is excessive. Plant transpiration removes infiltrated water from the soil profile, which provides storage capacity for subsequent precipitation. This natural approach to isolating waste is distinctly different from the approach employed by conventional (or prescriptive) covers and is more likely to be effective over the long-term because it is congruent with nature. An added benefit is that alternative covers are often less costly than prescriptive covers. ET designs for waste-site closures in the western United States, particularly in the southwest, have a long history of validation. Large drainage lysimeters have provided the most reliable performance indicators of alternative landfill covers. Three lysimeter studies congruent with the environment at EAFB were completed in 1988–2003. In these field trials, various configurations of ET designs performed to acceptable regulatory standards and, in some cases, outperformed the recommended designs developed by the EPA. Some of these field trials involved side-by-side tests of alternative and conventional designs and demonstrated that properly designed alternative covers can meet equivalency requirements as specified in EPA regulations. ET covers require sufficient soil having acceptable water-storing characteristics. A preliminary site investigation at EAFB in September 2002 indicated that surface soils at several locations on the base may contain borrow material—earth material taken from one location and used as fill at another—to construct an ET cover. Results of soil sampling verified this and identified a common subsurface layer of decomposed granite called "grus" that could serve as the coarse layer in the capillary-barrier design. Ultimately, the ET concept was combined with the capillary-barrier feature in the alternative-cover design simulated for Edwards. |
||
