July 24, 1997 MEMORANDUM TO: RCRA Dockets F-97-FLXF-FFFFF & F-97-FLXP-FFFFF FROM: Allen J. Geswein, Environmental Engineer SUBJECT: Infiltration Layer Requirements for MSWLFs Under 40 CFR 258.60(a), the "owner or operator of all municipal solid waste landfill (MSWLF) units must install a final cover system that is designed to minimize infiltration and erosion. The final cover system must be comprised of an erosion layer underlain by an infiltration layer...The infiltration layer must be comprised of a minimum of 18 inches of earthen material that has a permeability less than or equal to the permeability of any bottom liner system or natural subsoils present, or a permeability no greater than 1 x 10-5 cm/sec, whichever is less." These requirements were promulgated under the Solid Waste Disposal Facility Criteria Final Rule on October 9, 1991, to protect human health and the environment. The rationale for permeability requirements are: REDUCTION OF WATER INFILTRATION. Establishing requirements for cover systems limits water infiltration into the waste (U.S. EPA, 1993). By limiting infiltration, the cover minimizes the creation of leachate that could possibly escape into ground water. PREVENTION OF THE "BATHTUB EFFECT." The regulations require that the permeability of the final cover should be less than or equal to that of the bottom liner system or natural subsoils present. The "bathtub effect" could occur if a landfill fills up with liquids because the final cover was more permeable than the bottom liner system or natural subsoils present. The liquids would increase the load on the bottom liner and the potential for the formation and migration of leachate from the landfill (U.S. EPA, 1991, 1993). CONSIDERATION OF LANDFILLS WITH POOR OR NONEXISTENT LINER SYSTEMS. In addition to requiring the permeability of the final cover to be less than or equal to that of the bottom liner system, EPA required a minimum permeability of 1 x 10-5 cm/sec. The Agency did not want to link cover permeability solely to a bottom liner's permeability. The minimum requirement prevents owners and operators of existing landfills with poor or nonexistent liner systems from installing very permeable final covers (U.S. EPA, 1991). The Agency's rationale behind requiring the 18 inches requirements is: PROTECTION OF HUMAN HEALTH AND THE ENVIRONMENT. An infiltration layer with a minimum of 18 inches of earthen material and a permeability no greater than 1 x 10-5 cm/sec reduces infiltration and the subsequent formation of leachate. An erosion layer consisting of at least 6 inches of earthen material must be installed above the infiltration layer. The erosion layer can be used to establish a vegetative cover and reduces degradation of the infiltration layer. The specific erosion and infiltration layer requirements are the minimum necessary to protect human health and the environment (U.S. EPA, 1991). These requirements are generally consistent with state programs. More than 40 states require two feet of final cover for MSWLFs, and many specifically require infiltration and erosion layers (U.S. EPA, 1991). Resources Used: U.S. EPA. 1991. 40 CFR Parts 257 and 258, Solid Waste Disposal Facility Criteria; Final Rule. 56 Federal Register 50978. Washington, DC. October 9. U.S. EPA. 1993. Solid waste disposal facility criteria: Technical manual. Washington, DC. November.