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owing to the increased risk of erosion and potential for gas migration with a thinner cover, it is unlikely to be accepted by the regulatory agencies. Since the negative flux potential of the final cover system is reduced as the cover is thickened however, a thin cover is preferred and the proposed final cover thickness for the specified bottom boundary condition is 4-feet. 4.3.2 SATURATED HYDRAULIC CONDUCTIVITY As shown in Table 3, net infiltration is calculated to vary only slightly over the range of weathered saturated hydraulic conductivities measured from the existing interim cover soils. In analyzing the sensitivity of other variables within this section and for long-term modeling of the alternative final cover system, the median saturated hydraulic conductivity of 3.3 x10"6 cm/sec was used. [It should be noted that while the available soils are expected to yield this performance over time, a prescriptive maximum hydraulic conductivity of less than 1x10"6 cm/s will be established as the minimum criteria for initial final cover construction.] 4.3.3 WATER APPLICATION RATE For modeling purposes the rainfall magnitude and duration (the application rate) was estimated in a fashion believed to conservatively represent typical anticipated conditions. Calculated net flux through several modeled profiles showed only minor variability when AML using application rates between 10 and 60 mm/day(Table 3). Recognizing the nature of typical storm events in the area, a reasonably conservative application rate of 60 mm/day was selected for long-term modeling of final cover performance. 4.3.4 VEGETATION CONDITIONS The sensitivity of model results to vegetation coefficients ranging from transpiration, root potentials, root distribution,root lengths and root flow resistance are discussed below and are also summarized in Table 3. 4.3.4.1 Maximum Actual Transpiration/Evaporation Sensitivity analyses for this parameter included stipulation of maximum actual transpiration/evaporation values slightly greater or less than 1.0 and these analyses resulted in only minor changes to the calculated net flux through the modeled soil profile. This suggests that with site soils in this environment this is not a critical parameter. However, since selection of values greater than 1.0 may not be conservative (in that it allows transpiration to compensate for an evaporative deficit), a value of 1.0 was selected for the long-term analyses completed as part of this study. Admilk -7- C:\20050082\FO R W ARD/Fonva rd CO V ER.DOC\7/13/2005 Geologic Associates