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4.4 MODELING RESULTS 0 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 3x10-6 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 assumed to weather in response to the same conditions that would affect a prescriptive cover, the soil characteristics in the two model scenarios are the same. As a result, the most significant difference between the two models is the root distribution which is limited to the one -foot vegetative layer in the prescriptive case but extends to a three feet in the alternative case. 4.4.1.1 SOIL MOISTURE RESULTS Figure 2 depicts the modeled moisture content of the modeled degraded prescriptive final cover soils in percent by weight for six distinct depth intervals over a period of approximately ten years. As can be seen, in all cases for the constant potential boundary condition the shallow layers (3 -inch, 9 -inch, 15 -inch and 21 -inch) mimic seasonal precipitation patterns throughout the modeling period, while the deeper soil layers reflect only severe precipitation events. 0 -10- CA2003-033\EDOM HILL ALTERNATIVE COVER.DOC\6/16/2005 GeoLogic Associates