A comprehensive review of surface emissions, ambient air and raw landfill gas analytical data from 13 Southern California landfills was undertaken. A database was created which includes analytical determinations for 13 HAPs plus methane at these 13 landfills. There are over 13,000 HAPs entries, in addition to 16,000 methane entries, in this database. An analysis of this data implies that factors other than air dilution are operable in reducing landfill gas air pollutants. The most likely factor, based on very site-specific and/or bench-scale work by other researchers, is microbial degradation of the compounds. The findings herein suggest that microbial degradation of methane and HAPs in landfill soil covers is a common phenomenon. The above conclusion is offered with two caveats. Most of the landfills addressed in this investigation do not have final covers. Performance may have been better had all of the landfills had better cover. The landfills were all in a dry climate and the landfill gas collection systems at these landfills were, in general, being aggressively operated (i.e., high levels of air infiltration were being maintained). Landfills in other climates, and with a different philosophy of landfill gas collection system operation, may experience less dramatic performance in pollutant attenuation.
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