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For the prescriptive analyses presented herein, though the soils were assumed to yield similar performance under prescriptive and alternative cover conditions, the vegetation was assumed to be limited to the 1-foot thick vegetative layer. 4.3.4.5 Root Length As shown in Table 3, variations in the cumulative root length(within a 2-meter square area) do not significantly alter the performance of the cover system. As a result, this is not considered a sensitive parameter. A conservative 100-meter root length was selected for subsequent long-term analysis. 4.3.4.6 Root Flow Resistance As shown in Table 3, slight variations in root flow resistance yielded insignificant changes in cumulative system performance. As a result, this is not considered a sensitive parameter and a root flow resistance of 1.0 was assigned for the long- term analyses presented herein. 4.4 MODELING RESULTS Modeling the performance of the low hydraulic conductivity barrier layer of a prescriptive cover system constructed in an and and semi-arid climate is particularly difficult because environmental stresses (desiccation and settlement cracking) are likely to degrade the hydraulic properties of the low hydraulic conductivity barrier layer over the long term. Consequently, modeling results based on unstressed initial material properties will not be realistic. In fact, academic investigations indicate that the saturated hydraulic conductivity of a prescriptive cover system in a semi-arid to and climate can degrade by two to three orders of magnitude within a period of a few years (Dr. Craig Benson, personal communication). A similar condition is noted in the test results presented herein where the initial remolded saturated hydraulic conductivity of the clayey on-site soils is typically in the range of'10-7 to 10-8 cm/s but the conductivity of in-place interim cover soils (which exhibit otherwise similar characteristics) is on the order of 10-5 to 10-6 cm/s. Though perhaps the most important threat to cover performance, there is no methodology available to describe the degradation process in a way that would be realistic for input into an unsaturated flow analysis. In order to accommodate this difficulty,moisture flux modeling was performed based on material characteristics that are degraded in a fashion similar to that observed at the FSL. That is by degrading the hydraulic conductivity to about 3x106 cm/s and using the soil suction data associated with these weathered soils. 4.4.1 PRESCRIPTIVE FINAL COVER Modeling of a prescriptive final cover for the FSL was conducted using LEACHM in order to allow for performance comparison to the alternative final cover. It should be noted that since the proposed cover will be constructed to a prescriptive standard and is -9- C:\20050082\FOR WARD/ForwardCOVER.DOC\7/13/2005 Geologic Associates