R 299.4425 Type II landfill design standards; final cover.

Rule 425. (1) The owner or operator of a type II landfill unit shall install a final cover system which is designed to minimize infiltration and erosion and which is comprised of an erosion layer underlain by an infiltration layer, as specified in this rule. (2) Except as provided for existing or preexisting units in subrules (3) and (4) of this rule, the owner or operator of a type II landfill shall install a final cover system that is comprised of all of the following components: (a) An infiltration layer that is comprised of a composite liner. The lower soil component of such a composite liner shall consist of either of the following: (i) A minimum of 18 inches of earthen material that has a permeability which is less than or equal to 1.0 x 10-5 cm/sec, as determined by test methods specified in R 299.4920. (ii) A bentonite geocomposite liner which is in compliance with R 299.4914 and which is underlain by not less than 18 inches of earthen material to protect the liner from waste and minimize the effect of settlement. (b) An erosion layer that consists of both of the following: (i) A soil layer which is not less than 2 feet thick, which is immediately above the composite cover liner, and which is designed to do all of the following: (A) Provide for the lateral drainage of precipitation off the cover of the landfill. The owner or operator may use permeable soil, geosynthetic drainage material, an alternative equivalent material approved by the director, or a combination to provide the lateral drainage. (B) Minimize frost penetration into the infiltration layer. (C) Protect the flexible membrane liner from root penetration, ultraviolet light, and other deleterious effects. (ii) A minimum of 6 inches of earthen material capable of sustaining native plant growth. (3) The owner or operator of an existing or preexisting type II landfill unit that does not contain a flexible membrane liner in all or portions of the bottom liner system may install a final cover system previously approved by the director over those portions if the final cover system contains both of the following: (a) An infiltration layer that is comprised of a minimum of 2 feet of earthen material which has a hydraulic conductivity that is less than or equal to 1.0 x 10-7 cm/sec, as determined by test methods specified in R 299.4920. The earthen material shall meet standards for soil liners specified in R 299.4913. (b) An erosion layer that consists of a minimum of 6 inches of earthen material which is capable of sustaining native plant growth. (4) The owner or operator of an existing or preexisting type II landfill unit that does not contain a flexible membrane liner in the bottom liner system may enhance the final cover specified in subrule (3) of this rule by adding a flexible membrane liner if the erosion layer specified in subrule (2)(b) of this rule is provided. The addition of the layer shall not constitute a vertical expansion. (5) The director shall approve an alternative final cover design if the owner or operator of the landfill units demonstrates that the cover design includes both of the following components: (a) An infiltration layer that achieves an equivalent reduction in infiltration as the infiltration layer specified in subrule (2) or (3) of this rule. (b) An erosion layer that provides equivalent protection from wind and water erosion as the erosion layer specified in subrules (2) and (3) of this rule. (6) The final cover of a type II landfill shall have either of the following to meet the gas control requirements of R 299.4433: (a) A permeable soil layer which is not less than 1 foot thick and which is located directly below the infiltration layer that vents landfill gas to gas risers. (b) Other means of assuring that gases cannot travel laterally from the site or accumulate in structures. (7) To prevent the ponding of water on completed fill surfaces, the grading contours shall be sufficient to prevent the development of local depressions due to postconstruction settlement. Slopes of the final cover shall not be less than 4% at any location. (8) Slopes of the final cover shall not exceed those necessary to prevent erosion and maintain slope stability. The final slope shall not be more than 1 vertical to 4 horizontal at any location, except where necessary to install berms for erosion control. If the final slope is more than 15%, then the slope shall include controls that the applicant demonstrates are sufficient to maintain slope stability, prevent erosion, and allow access. The controls shall be sufficient to limit erosion to not more than 2 tons per acre per year after vegetation is established based on the universal soil loss equation or other method approved by the director. The following ground cover estimates may be used in calculating erosion loss: (a) Up to 95%, if the closure and postclosure plan provides for all of the following: (i) Topsoil that has an organic matter content of more than 2.5%. (ii) Fertilization consistent with the natural resources conservation service critical area planting guide. (iii) Mowing twice annually until the required coverage is achieved. (b) Up to 90%, if the closure and postclosure plan for the unit provides for both of the following: (i) Topsoil that has an organic matter content of more than 1.25%. (ii) Mowing annually until the required coverage is achieved. (c) Up to 80%, if the organic content or mowing schedule is not specified. (d) Other estimates approved by the director, if the estimates are supported by measures to establish vegetation specified in the closure and postclosure plan. (9) All final covered areas shall be stabilized using appropriate shallow-rooted vegetation for the soil type, slope, and moisture conditions present. Seed and mulch rates shall, at a minimum, be consistent with recommendations contained in the United States department of agriculture document entitled "Natural Resources Conservation Service Critical Area Planting Guide." The natural resources conservation service critical area planting guide is adopted by reference in R 299.4141.