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KLEINFELDER <br /> evaporative zone is a full 32 inches. The evaporative zone for the Chapter 15 cover is only <br /> 12 inches because HELP assumes that surface vegetation will not extend roots into the clay <br /> barrier layer (roughly, the maximum root depth is equal to the evaporative zone depth). To <br /> test this explanation for the performance difference between the existing cover and the Chapter <br /> 15 cover, a third HELP modeling run was made (Run #3). In this run, the depth of the <br /> vegetative layer was set equal to the depth of the evaporative zone (32 inches). If ET is the <br /> controlling factor in this situation than the modified Chapter 15 cap should perform similarly <br /> to the existing cover because they will both have equivalent evaporative zones. As can be seen <br /> from Table 1, the modified Chapter 15 cover does perform similarly to the existing cover. <br /> Evapotranspiration, percolation from the bottom of the landfill and change in water storage are <br /> very similar. <br /> CONCLUSIONS <br /> Based on the three HELP modeling runs, the existing cover can be expected to perform as well <br /> as or better than a standard Chapter 15 cap. In fact, the presence of the compacted clay cap <br /> actually inhibits evapotranspiration and traps moisture in the landfill. Therefore, it appears that <br /> extending the Chapter 15 cap from the top deck of the landfill onto the steep, 2:1 sideslopes <br /> will not increase the overall performance of the cap and may actually cause moisture to buildup <br /> in the landfill. Remember that these conclusions are for the steep 2:1 sideslopes only. <br /> LII�MATIONS <br /> Our services were performed in accordance with the generally accepted standard of care which <br /> exists in Northern California at the time the investigation was performed. It should be <br /> recognized that definition and evaluation of geologic and environmental conditions are generally <br /> made with a limited knowledge of the subsurface conditions. The computer modeling <br /> presented in,this report is subject to the same uncertainty and is merely a simple simulation of <br /> complex physical and chemical processes. Modeling results are to be interpreted as estimates <br /> only. No warranty, expressed or implied, is made. <br /> File No.20-1008-49 <br /> April 6, 1994 <br /> Copyright 1994 Kleinfelder, Inc. Page 5 of 5 <br />