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5-5 10 5.2.4 Final Cover 5.2.4.1 Regulatory Design Standards California Final Cover Prescriptive Design Standard The minimum final cover standards for Class II landfills contained in 27 CCR, Section 21090 include: ♦ Foundation Layer: A minimum two -foot thick layer of soil placed immediately over the entire surface of the last lift of refuse. This layer shall have the appropriate engineering properties, so as to provide a relatively unyielding surface upon which to place and compact the low -hydraulic -conductivity layer. ♦ Low -Hydraulic -Conductivity Layer: A minimum one -foot thick layer of clean low - hydraulic -conductivity soil containing no waste or leachate placed over the foundation layer. The low -hydraulic -conductivity (or low through -flow rate) soils shall be placed on top of the foundation layer and compacted to attain a hydraulic conductivity, which is the lesser of either; -1 x 10-6 cm/sec. -The hydraulic conductivity of any bottom liner system or underlying natural geologic materials. ♦ Erosion Resistant Layer: A minimum one -foot thick layer of soil containing no waste or leachate placed on top of all portions of the low -hydraulic conductivity layer. Vegetation root depths must not exceed the topsoil layer thickness. Vegetation is to be replanted, as needed, to provide effective erosion resistance. The final cover should be designed to allow for minimal maintenance. The final grading design for areas flatter than 5:1 (horizontal:vertical) will have a gradient of at least 3%, to prevent surficial slumping which may cause ponding and accommodate differential settlement during the post -closure maintenance period. Federal Final Cover Prescriptive Design Standard The minimum final cover standards, 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. Forward Landfill SWT Engineering Joint Technical Document - April 2014 z:\projects\allied waste\forward\five year permit rvw 2013\jtd-5 yr pr 2013\text\sec 5—final.doc