COMPLIANCE INFO_FINAL WMU FU-23 DESIGN RPT & CONCURRENCE LTRS

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WMU FINAL WMU FU-23 DESIGN REPORT 2-18-23 2-1 SECTION 2 Base and Final Grading Plans 2.1 General The limits of WMU FU-23 were determined based on the site’s operational needs, the landfill site development plan (BAS et al., 2003), and the July 2022 site topographic map. The northern and eastern limits were established based on the site’s operational considerations for airspace. The western boundary of WMU FU-23 connects with WMU FU-17 and FU-19. The southern boundary of WMU FU-23 was established in accordance with the site development plan. The WMU FU-23 grading plan and liner and LCRS sections and details are included in the Construction Drawings for the development of WMU FU-23 (Appendix A). Permanent access roads and surface drainage facilities will be constructed as the WMU meets final grades. 2.2 Liner and LCRS Grades Design criteria and reasoning to develop the grading plan includes: • Forward’s current WDRs require a minimum 5-foot distance between waste in WMU FU-23 and the highest anticipated elevation of underlying groundwater. Based on the “Forward Landfill Expansion, Groundwater Elevation Evaluation” (GeoLogic Associates, 2007) (see Appendix B), the maximum historic high groundwater elevation underlying the Forward Landfill occurred in 1984 and is approximately minus 8-feet MSL in the WMU FU-23 area. It is highly unlikely, however, that these historic groundwater levels will ever re-occur given the significant amount of agricultural pumping in the area. As depicted on the 2022 quarterly groundwater contour elevation maps (see Appendix B), the lowest groundwater elevation beneath the WMU FU-23 area occurred in the fourth quarter and was approximately minus 37-ft MSL, while the highest groundwater elevation occurred in the first quarter and was approximately minus 26-ft MSL. The lowest elevation of waste in WMU FU-23 is approximately 4-ft MSL. This elevation is approximately 12-feet above the historic high groundwater elevation and 30-feet above current groundwater elevation and therefore meets the minimum 5-feet separation criteria. • WMU FU-23 will be graded to drain to east-west trending leachate collection and removal system (LCRS) pipes that lead to two temporary sumps. The maximum drainage distance to the LCRS pipe is approximately 150 feet (see the Construction Drawings in Appendix A). • As outlined in the Preliminary Closure and Postclosure Maintenance Plan (CH2M HILL, 1994e) and in the Geotechnical Report for the Forward Landfill Northeast Expansion (GLA, 2008), the soils at the site are firm and abrupt differential foundation settlement is not anticipated in the subgrade of the landfill cells. The estimated maximum differential foundation settlement ranges from approximately 0.05 to 0.36 percent. Because the net effective slope of the WMU base LCRS is approximately 2.24 percent, positive drainage grades would remain even if the estimated maximum differential settlement were to occur.