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The proposed final cover should not present stability problems ~nen instal!cc at 25 percent slopes. A test plot of the cover will help verify these and other performance concerns and a backup cover system with a relatively impermeable clay barrier ~lll be used if necessary. A surface runoff drainage control system of ber~-s, spillways and sedimentation basins is proposed that has the capatillty to handle 2~-hour storTm of greater intensity than typically occur in a 1C-year period. The proposed cover and surface drainage system are estimated to reduce infiltration of rainfall into the site by 94 percent from what would occur under closure according to current permit conditions. The increase in slopes associated with the proposed expansion accounts for eight to ten percent of the improvement. Final cover installation is likely to be completed in 1991 regardless of ~hethe? the expansion is approved because of the requirement to test it for a year prior to beginning the two-year installation process. Extension of landfill operations beyond July of 1991 could correspondingly extend the period of tine the groundwater flow barrier may have to be operated. A net increase in the volume of leachate generated during the implementation of the expansion will occur due to the additional waste and the maintenance of an active landfill operation. The flow barrier system is designed to handle the flow from the !~ndfi!! while operations are taking place. The increased slopes will also provide natural protection in the event of a cover failure. Steeper slopes will re~uce infiltration cf water through any openings in the cover. AIR QUALITY The air emiss±cns of concern at !~ndfi!!s include dust, noise, odor and landfill gas. With the exception of landfill gas, these impacts are not likely to exceed levels associated with the c~-rent!y permitted operation. Removal cf cover in prevlcus!y finished areas areas where slopes will be increased during the expansion ~i!! have to be accomplished and monitored carefully to avoid odor problems. Landfill gas is projected to be generated at twice the rate necessary for cost- effective recovery, if methane acc'~.u!ates in concentrations between five and 15 percent in an enclosed space, an explosion may occur. Gas generation nay also result in unpleasant odors and have adverse effects on vegetaticn. _and,~__ is The~' network cf monitoring probes currently surrounding the ~ adequate to identify any =igratlcn cf gas from the site. Gas is currently co .... o_-.d by ~x~.ac~.on from 17 wells that is flared or used for electrical generation by a nearby ~ ..... ..... s~.~. An ~ ...... 1 12-we!! system is available as a backup. An additional gas system is proposeds..~.~"~o^' to modifications to produce more effective gas "~ "'~ - ~o_-e~..c... Together these systems should "- '~° -~--~' ~.o~ ...... =.=> three tines the discharge capacity needed to convey the volume cf gas production p. o3ec Vertical expansion construction costs are projected to fall ~ithin a range cf ~3.5 to S5 mi!!icn. Annual cperatirg costs are projected between $2 and fa million. Cover soils are available~ bu= some will have to be obtained off-site at substantial cost.