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owing to the increased risk of erosion and potential for gas migration with a thinner <br />cover, it is unlikely to be accepted by the regulatory agencies. Since the negative flux <br />potential of the final cover system is reduced as the cover is thickened however, a thin <br />cover is preferred and the proposed final cover thickness for the specified bottom <br />boundary condition is 4 -feet. <br />4.3.2 SATURATED HYDRAULIC CONDUCTIVITY <br />As shown in Table 3, net infiltration is calculated to vary only slightly over the range of <br />weathered saturated hydraulic conductivities measured from the existing interim cover <br />soils. In analyzing the sensitivity of other variables within this section and for long-term <br />modeling of the alternative final cover system, the median saturated hydraulic <br />conductivity of 3.3 x10'6 cm/sec was used. [It should be noted that while the available <br />soils are expected to yield this performance over time, a prescriptive maximum hydraulic <br />conductivity of less than 1x10b cm/s will be established as the minimum criteria for <br />initial final cover construction.] <br />4.3.3 WATER APPLICATION RATE <br />For modeling purposes the rainfall magnitude and duration (the application rate) was <br />estimated in a fashion believed to conservatively represent typical anticipated conditions. <br />Calculated net flux through several modeled profiles showed only minor variability when <br />using application rates between 10 and 60 mm/day (Table 3). Recognizing the nature of <br />typical storm events in the area, a reasonably conservative application rate of 60 day <br />IFwas selected for long-term modeling of final cover performance. <br />4.3.4 VEGETATION CONDITIONS <br />The sensitivity of model results to vegetation coefficients ranging from transpiration, root <br />potentials, root distribution, root lengths and root flow resistance are discussed below and <br />are also summarized in Table 3. <br />4.3.4.1 Maximum Actual Transpiration/Evaporation <br />Sensitivity analyses for this parameter included stipulation of maximum actual <br />transpiration/evaporation values slightly greater or less than 1.0 and these analyses <br />resulted in only minor changes to the calculated net flux through the modeled soil <br />profile. This suggests that with site soils in this environment this is not a critical <br />parameter. However, since selection of values greater than 1.0 may not be <br />conservative (in that it allows transpiration to compensate for an evaporative <br />deficit), a value of 1.0 was selected for the long-term analyses completed as part <br />of this study. <br />-7- <br />C:12005-0082\FOR WARD/ForwardCOPER.DOC17Fi3/2005 <br />Geologic Associates <br />