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5.2 LANDFILL GAS EMISSIONS Title 27 CCR indicates that another purpose for final covers is to control surface emissions of landfill gas. In fact, the placement of additional, similar cover soils as a part of final closure is expected to yield superior surface emissions performance relative to the existing interim cover. Since the landfill has not exhibited excessive surface emissions under interim closure conditions, landfill gas emissions are not expected be problematic after completion of the final cover. 5.3 MAINTENANCE As noted above, the ultimate performance of the proposed 4 -foot cover section will be very sensitive to the establishment and maintenance of an effective plant community over the closure area. As a result, routine inspection and as -needed repair (and reseeding) of the cover area should be anticipated. 5.4 COST The modeling completed to date and reported herein, indicates that a non- prescriptive final cover system constructed using available on-site soils would be consistent with the performance goal addressed by the prescriptive standard and would afford equivalent or superior protection against water quality impairment. In addition, while it is estimated that closure construction costs would be the same for the prescriptive and alternative covers discussed herein (i.e., the alternative will be constructed to a prescriptive as -built standard), post -closure maintenance of the prescriptive cover to it's as -built condition would be extremely expensive (likely requiring reconstruction every few years) and almost certainly ineffective since desiccation can occur in a matter of weeks or months. The results obtained from the LEACHM analysis (Figure 5) indicate that four feet of final cover material constructed and maintained as discussed herein will perform effectively at the site and this cover section is therefore recommended. On the basis of the soil conditions modeled, soils suitable for use in the FSL closure final cover can be generated by select excavation of soils from the proposed borrow source adjacent to the site. In order to generate adequate performance characteristics, the final cover materials should be mixed to exhibit a grain size distribution that generally excludes particles in excess of 3 inches. In addition, soil excavation and placement operations should be conducted so as to yield a minimum fines content (% by weight passing the No. 200 sieve) of 70, a minimum of at least 30% finer than 5 -microns and an as -built saturated hydraulic conductivity of less than 1x10-6 cm/s. As indicated above, sufficient borrow materials are likely to be available from the borrow source to meet the project specifications (Figure 1). A quality assurance plan describing the materials testing methods and procedures, testing frequencies, and materials -12- CA2003-033MOM HILL ALTERNATIVE COVER.D006116/2005 Geologic Associates