Landfill Final Cover Systems

(1) General Performance Standards. The final cover system shall: (a) minimize the percolation of water through the final cover system into the landfill to the greatest extent practicable; (b) promote proper drainage of precipitation ; (c) minimize erosion of the final cover; (d) facilitate the venting and control of landfill gas; (e) ensure isolation of landfill wastes from the environment; and (f) accommodate settling and subsidence of the landfill such that the above performance standards will continue to be met. (2) General Design Standards. The final cover system shall: (a) have a final top slope of not less than 5% and side slopes no greater than three horizontal to one vertical (3:1); (b) be constructed of material(s) that are compatible with gases and leachate expected to be generated; (c) be constructed so as to minimize erosion of all layers of the final cover by using terraces or other appropriate stormwater controls; (d) be constructed so that the low permeability layer is protected from the adverse affects of frost and/or freeze/thaw cycles; (e) be constructed to maintain slope stability; (f) be sufficiently strong and stable enough to withstand the static and seismic loads at the site under all expected operating conditions; and (g) be designed with a factor of safety (FS) appropriate for the site-specific closure being evaluated. All factors of safety shall be identified and justified. (3) Final Cover System Components. Except as provided in 310 CMR 19.113: Alternative Landfill Final Cover System Design, or 19.114: Ground Water Protection System and Final Cover Waivers, the final cover system shall, at a minimum, consist of: (a) the subgrade layer; (b) a landfill gas venting layer unless the the subgrade layer is designed to function as a gas venting layer, or there is an active gas collection and extraction system and it is demonstrated that a gas venting layer is not needed; (c) a low permeability layer or layers; (d) a drainage layer; (e) filter material (when required); (f) a layer capable of supporting vegetation; (g) the vegetative cover; and (h) other components as may be required by the Department. (4) Subgrade Layer Standards. (a) Performance Standards. The subgrade layer shall provide adequate structural support for the final cover system and be capable of accommodating any anticipated subsidence or settling without impairing its ability to provide structural support; (b) Design Standards. The subgrade shall: 1. be free of materials that may damage or abrade the low permeability layer or venting layer; and 2. be of sufficient thickness to cover all solid waste. (5) Landfill Gas Venting Layer. (a) Performance Standard. A landfill gas venting layer shall provide for the free movement of landfill gas out of the landfill to gas control devices or vents. (b) Design Standard. Landfill gas venting layers shall: 1. be placed below the low permeability layer to facilitate the collection and control of landfill gases; 2. be of sufficient thickness and hydraulic conductivity to facilitate venting of landfill gases from below the low permeability layer; 3. be composed of either: a. soil material that has: i. a minimum thickness of six inches; and ii. a hydraulic conductivity equal to or greater than 1 x 10 cm/second; or -3 b. synthetic material (geonet) which shall: i. be of sufficient strength to prevent deformation and impairment of function by the weight of vehicles or the final cover; and ii. have sufficient flow capability. 4. be bound on its upper and/or lower surface with filter material where needed to prevent the infiltration of fine material and to maintain the integrity of the layer. (6) Low Permeability Layer Standards. (a) Performance Standard. A low permeability layer shall: 1. to the greatest extent practicable, minimize the movement or percolation of water into the landfill; 2. promote positive drainage of the landfill final cover system and prevent erosion; and 3. be designed and constructed to remain impervious for the expected life and post-closure period of the facility; and 4. be constructed in accordance with the quality assurance and quality control requirements of 310 CMR 19.106. (b) Design Standards. 1. Low Permeability Soil/Admixture Layer Standards. Compacted low permeability soil or admixture layers shall: a. have a minimum compacted thickness of 18 inches; b. be constructed using materials that have a maximum in-place saturated hydraulic conductivity of 1 x 10 cm/sec throughout the entire thickness of the layer; -7 c. be compacted to minimize void spaces; d. be capable of supporting the weight imposed by the post-closure use without settling or causing or contributing to the failure of the low permeability layer; and e. be free of materials that, because of their physical, chemical or biological characteristics, may cause or contribute to an increase in the permeability of the low permeability layer or otherwise cause a failure of the low permeability layer. 2. Flexible Membrane Liner Standards. Flexible membrane liners shall: a. be of sufficient thickness as determined by the Department; b. be constructed to ensure that the seams connecting FML panels are of equal or greater strength than the panels or manufacturers's seams within panels and are oriented parallel, as much as is practical, to the slope and not across the slope; c. have sufficient flexibility and strength for the proposed application, taking into consideration tensile strength, puncturability, stress cracking and chemical compatibility; and d. be capable of being seamed to produce leak-tight, high-strength seams that retain their integrity during final cover installation and the post-closure period. 3. Geosynthetic Clay Liners (GCLs) Standards. Geosynthetic clay liners shall: a. have sufficient bentonite (or similar low permeability clay) to provide an effective low permeability layer; b. be constructed to ensure that the bentonite (or similar low permeability clay) will be distributed and retained uniformly and securely throughout the GCL during the operating life and the post-closure period; c. have sufficient durability for the proposed application, taking into consideration hydration, desiccation and other considerations that may affect initial or long term performance; and d. be capable of being seamed or joined to produce leak-tight connections that retain their integrity during the operating life and the post-closure period. (7) Drainage Layers. (a) Performance Standard. Drainage layers shall provide continuous and freeflowing drainage over the entire low permeability layer. (b) Design Standards. The drainage layer or system shall: 1. be placed above the low permeability layer; 2. be of sufficient thickness and hydraulic conductivity to drain the immediate and up-gradient areas of the final cover; 3. be designed so that the head in the drainage layer will not exceed the thickness of the drainage layer; 4. be designed based on the hydraulic loadings resulting from the actual materials used in the final cover system, such as the vegetative support layer infiltration rate; 5. be composed of either: a. soil material that has: i. a minimum thickness of six inches; and ii. a hydraulic conductivity equal to or greater than 1 x 10 cm/second; or -3 b. synthetic drainage material (geonet) that shall: i. be of sufficient strength to prevent deformation and impairment of function by the weight of vehicles or the final cover; ii. have sufficient flow capability; iii. be designed and evaluated for its effective long-term flow capacity using reasonable and acceptable evaluation methods that consider factors that may reduce (reduction factors) the design flow capacity caused by, but not limited to, overburden forces, (deformation, creep, etc.),intrusion by overlying materials such as filter fabrics or soil, and chemical or biological clogging; and iv. be designed with an overall factor of safety appropriate for the effective long-term performance of the drainage layer. 6. be bound on its upper and/or lower surface with filter material where needed to prevent the infiltration of fine material and to maintain the integrity of the layer; and 7. be maintained to prevent conditions that could compromise the integrity of the landfill or cause erosion. (8) Filter Material Standards. Filter material, where placed, shall be capable of preventing the migration of fine soil particles into the drainage or venting layer. (9) Vegetative Support/Protection Layer Standards. (a) Performance Standards. The vegetative support/protection layer shall: 1. be of sufficient thickness and composition to support the selected vegetation; 2. protect underlying layers from the adverse effects of desiccation, extremes of temperature, including frost effects, and erosion; (b) Design Standards. There shall be at least 18 inches of soil material above the low permeability layer (310 CMR 19.112(6)). This 18 inches may be composed of soil in the vegetative support/protective layer (310 CMR 19.112(9)) and soil in the drainage layer (310 CMR 19.112(7)). The vegetative support/protection layer shall: 1. consist of a topsoil, loam or equivalent type of soil-based vegetative support material with a minimum organic content of 3%; 2. consist of at least 12 inches of soil capable of supporting the selected vegetation and any additional soil material needed to create the required total thickness; or 3. consist of at least eight-nine inches of soil with an organic content of 8-10% capable of supporting the selected vegetation in conjunction with a 12-inch thick drainage layer for a total vegetative/protection layer depth of 20-21 inches. (10) Vegetative Cover Standards. (a) Performance Standards. The vegetative cover shall: 1. provide complete coverage of the landfill; 2. minimize erosion of underlying material; 3. promote evapotranspiration of water to the maximum practicable extent; 4. provide for an effective and permanent cover compatible with the site; 5. have root systems that shall not compromise the drainage layer or low permeability layer; and 6. be composed of plants which shall be capable of self propagation.