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Article 2. Closure and Post-Closure Maintenance Standards for Disposal Sites and Landfills §21090. SWRCB - Closure and Post-Closure Maintenance Requirements for Solid Waste Landfills. (C15: §2581 H T14: 117777, f17779) (Note: For SWRCB's final cover performance standard,see 120950(a)(2)(A);for related CIWMB requirements,see 121790 et seq.] (a)Final Cover Requirements—Final cover slopes shall not be steeper than a horizontal to vertical ratio of one and three quarters to one,and shall have a minimum of one fifteen-foot wide bench for every fifty feet of vertical height.Designs having any slopes steeper than a horizontal to vertical ratio of three to one,or having a geosynthetic component[under&(a)(2)],shall have these aspects of their design specifically supported in the slope stability report required under 121750(f)(5).The RWQCB can require flatter slopes or more benches where necessary to ensure preservation of the integrity of the final cover under static and dynamic conditions.The cost estimate,under 421769,for the final cover shall include a description of the type and estimated volume(or amount,as appropriate) of material needed for each component of the final cover based upon the assumption that all materials will need to be purchased;if on-site materials are to be used,the submittal shall include test results confirming the availability of such on-site materials and their suitability for such use.The RWQCB can allow any alternative final cover design that it finds will continue to isolate the waste in the Unit from precipitation and irrigation waters at least as well as would a final cover built in accordance with applicable prescriptive standards under&(a)(1-3). (1) Foundation Layer—Closed landfills shall be provided with not less than two feet of appropriate materials as a foundation layer for the final cover. These materials may be soil,contaminated soil,incinerator ash,or other waste materials,provided that such materials have appropriate engineering properties to be used for a foundation layer.The foundation layer shall be compacted to the maximum density obtainable at optimum moisture content using methods that are in accordance with accepted civil engineering practice.A lesser thickness may be allowed for Units if the RWQCB finds that differential settlement of waste,and ultimate land use will not affect the structural integrity of the final cover. (2) Low-Hydraulic-Conductivity Layer—In order to protect water quality by minimizing the generation of leachate and landfill gas,closed landfills shall be provided with a low-hydraulic-conductivity(or low through-flow rate)layer consisting of not less than one foot of soil containing no waste or leachate,that is placed on top of the foundation layer and compacted to attain an hydraulic conductivity of either 1x10 cm/sec(i.e., 1 ftlyr)or less,or equal to the hydraulic conductivity of any bottom liner system or underlying natural geologic materials,whichever is less permeable,or another design which provides a correspondingly low through-flow rate throughout the post- closure maintenance period.Hydraulic conductivity determinations for cover materials shall be as specified in Article 4,Subchapter 2,Chapter 3 of this subdivision[420310 et seq.],but using water as the permeant,and shall be appended to the closure and post-closure maintenance report.For landfills or portions thereof in which the final cover is installed after July 18, 1997,as part of the final closure plan for the Unit,the discharger shall provide a plan, as necessary[see&(a)(4)],for protecting the low-hydraulic-conductivity layer from foreseeable sources of damage that could impair its ability to prevent the throughflow of water(e.g., desiccation, burrowing rodents, or heavy equipment damage). (3) Erosion-Resistant Layer—The low-hydraulic-conductivity layer of&(a)(2)shall be directly overlain by an erosion-resistant layer,as follows. (A) Closed landfills shall be provided with an uppermost cover layer consisting of either: 1. Erosion-Resistance Via a Vegetative Layer—a vegetative layer consisting of not less than one foot of soil which: a. contains no waste(including leachate); b. is placed on top of all portions of the low-hydraulic-conductivity layer described in&(a)(2); c. is capable of sustaining native,or other suitable,plant growth; d. is initially planted—and is later replanted as needed to provide effective erosion resistance—with native or other suitable vegetation having a rooting depth not exceeding the depth to the top of the low-hydrauhc-