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3-2 The final cover should be designed to allow for minimal maintenance. The final grading design for areas flatter than 5:1 (horizontal:vertical) shall have a gradient of at least three percent, to prevent ponding and accommodate settlement. Federal Final Cover Prescriptive Design Standard The minimum final cover standards for the FL, as outlined in the closure criteria of 40 CFR, Subpart F, Section 258.60, include: ♦ A cover with a permeability less than or equal to the hydraulic-conductivity of any bottom liner system or natural sub-soils present, or a permeability no greater than 1 x 10-5 cm/sec, whichever is less. The barrier layer shall consist of a minimum 18 inches of earthen material. ♦ A cover which minimizes erosion of the final cover by the use of an erosion resistant layer that contains a minimum six inches of earthen material and is capable of sustaining native plant growth. 3.2.1.2 Final Cover Design Introduction Several factors were taken into consideration in determining the final cover design for the FL, including the geometry of the existing landfill, local climatic conditions (i.e., arid environment, low rainfall, and high evaporation rate), potential landfill settlement, available cover materials, erosion protection, vegetative growth, and the proposed end use after closure. Based on these factors, Forward, Inc. determined that an alternative final cover design utilizing an on-site soil engineered as a monolithic final cover design was the most appropriate cover system for the FL. Approval of a final cover design that is different than the prescriptive standard is allowed under 27 CCR, Section 20080(b) as an alternative cover s sy tin. The alternative design is allowed if the discharger demonstrates that: (1) The construction of prescriptive standard is not feasible as provided in subsection (c) of Section 20080, and (2) There is a specific engineered alternative that: (A) is consistent with the performance goal addressed by the particular construction or prescriptive standard, and (B) affords equivalent protection against water quality impairment. As stipulated in subsection (c) of Section 20080, to establish that the prescriptive standard is not feasible the discharger must demonstrate that the prescriptive standard: (1) is unreasonably and unnecessarily burdensome and will cost substantially more than alternatives which meet the criteria in subsection (b) of Section 20080; or (2) is impractical and will not promote attainment of applicable performance standards. The proposed alternative final cover design will meet or exceed the prescribed performance criteria and will be more economical for site closure than prescriptive standards. In 2005, GLA prepared a report in support of the alternative final cover design. The report evaluated the performance of the alternative final cover for the FL. The analysis preformed in support of the alternative final cover was done using local climatological data, soil data Forward Landfill Stage 16 Partial Final Closure/Post-Closure Maintenance Plan-June 2011 SWT Engineering z:\projects\allied waste\forward\11-1014 partial fnl clsr pin\partial final closure plan document\text\sec 3.doc