BACKGROUND AND PROBLEM

Analytical results from soil samples collected from sources across EAFB were used in the DSA.

The U.S. Environmental Protection Agency (EPA) stringently controls landfill closures and construction of final covers. Federal regulations require the cover to eliminate water infiltration and minimize erosion. The EPA's prescribed cover includes a 6-inch erosion layer, designed to support native vegetation, directly above an 18-inch, low-permeability infiltration layer.

Traditional cover designs that use low-permeability materials (e.g., fine-grained soils or geomembranes) to impede percolation of water into waste largely avoid addressing performance by focusing on specific values such as saturated hydraulic conductivity of material used in the cover. Alternative designs, in contrast, rely on functional soil and plant systems to maintain a favorable water balance. The monolithic approach, a design comprised of locally obtained soil and native plants, is quickly gaining acceptance among government regulators, environmental engineers, and end users. Advantages include significant savings over the prescribed design and reduced operational expenditures for implementing and maintaining the cover.

Although research indicates that alternative designs are effective in protecting the environment, a standard methodology for developing effective alternative designs has not been devised. This is due, in part, to the lack of adequate site-specific field performance because the natural variability of site conditions (soils and plants) has not been factored into design sensitivity analyses (DSAs). A DSA systematically adjusts design parameters and predicts how those design changes influence the outcome, in this case, flux. Without a suitable DSA, design engineers and regulators face the difficult task of quantifying and constraining uncertainty in predicting how well an alternative cover will perform. Taking the design to practical application requires approval by government regulators. However, without acceptable standard designs, the regulatory community must first evaluate and approve a conceptual design and then tackle specific design features for each case. This process requires considerable time and can lead to inconsistency in reviewing permits and license applications.

DRI proposed to conduct a DSA that would objectively evaluate the effect of design changes on long-term water flux into the waste. Through the DSA, the natural variabilities of soil properties, climate, and plant species were considered and then evaluated against six different cover designs. This approach makes best use of onsite data thus reducing problems associated with using data collected offsite. Additionally, by including variabilities of input parameters—such as soil and plants—in the analyses, the results (or output) can be viewed in terms of variability.