ARCHIVED REPORTS_2009_24 - Laserfiche WebLink

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r r8.0 DISCUSSION The monitoring data described above presents a soil moisture response that is similar to the pattern of moisture predicted by both the initial and revised alternative final cover models. That is,the final cover exhibits an upward soil moisture gradient over a majority of the year. A muted r response to moderate rainfall events and a more dramatic response (including short-term suction gradient reversals) are superimposed on the more consistent annual drying pattern during the winter. As noted above, the increases in soil moisture observed during the late summer(when no rainfall or other surface water application) has occurred is consistent with the introduction of water to the cover from the waste; a condition that will result in even better net long-term performance for the alternative final cover system. Data from the lysimeters did not record any tipping activity during this period, further supporting the hypothesis that,the alternative final cover system at the Forward Sanitary Landfill is performing as anticipated. r The results of moisture monitoring within the alternative final cover constructed over the FSL also demonstrates that final cover system performance is very sensitive to vegetative conditions on the ground surface. Given the fact that existing vegetation on the site has not fully matured, system performance is likely to improve in response to the development of a more robust soil/plant system. 9.0 RECOMMENDATIONS The results of moisture monitoring within the alternative final cover constructed over the FSL indicates that moisture movement within the final cover is occurring as expected. It also demonstrates that infiltration is very sensitive to vegetative conditions. As a result, and noting that vegetation development is inconsistent at the site, GLA recommends that Forward Landfill, r Inc. (An Allied Waste Company) apply for approval from the agencies that currently require the FSL site to be mowed. A request for mowing activities to cease for a period of one-year is recommended at the North Slope Stage I partial final closure footprint, so that the cover r system can be monitored with a more robust vegetative community. In addition, any areas where limited vegetation has been established should be evaluated and the soil should be enhanced if necessary, in order to facilitate development of the most robust native plant community. rSince the soil moisture monitoring completed to date suggests acceptable performance, even for a period when the plant communities are not fully developed, it is recommended that the r alternative final cover over the FSL be accepted and that soil moisture monitoring be discontinued. 1 r r 1 9 Geologic Associates 1