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were developed from the combined datasets.WIAC surveyed five codisposal sites and 70 MSW- only sites.The WIAC toluene and benzene data were separately analyzed by disposal site type. No significant differences were found between types of disposal sites for other compounds with one exception.Carbon tetrachloride was detected at one codisposal site but at none of the MSW- only disposal sites.The WIAC value for carbon tetrachloride includes the codisposal sites as these had only a slight effect on the calculated value.The value is reported in Table 2 as a `nondetect'with a footnote indicating that it was found at one codisposal site. Data Summary The WIAC results are compared with AP-42 default concentrations in Table 2.WIAC 1 and 2 show the data prepared using past AP-42 and WIAC updated averaging methods,respectively (see Data Averaging above).The WIAC 1 and 2 concentrations are similarly reduced from AP-42 values by 761/6 and 80%,respectively.However simple alkane and alcohol compounds for which relatively few analyses were available disproportionately skewed the results.Omitting these compounds shows identical 56%overall reductions.Nearly identical reductions are also noted for aromatic(58%)and chlorinated(79%)compounds.Even though the AP-42 and WIAC averaging methods do not have any large overall effect,the two methods did lead to very significant differences for individual compounds(e.g.,note those for 1,1,2,2-Tetrachloroethane). Discussion AP-42 and WIAC Differences The differences between the AP-42 default values and the WIAC survey results may be,traced to , various factors.It was noted above that there are differences in the age of analyses between:the AP-42 and WIAC data sets.Trends in LFG constituents have been well documented an&ate addressed in the next section.Apart from differences in the age of analyses,it was found that procedures used in U.S.EPA's preparation of the AP-42 defaults departed from the AP-42 guidance6 in its use of nondetects and the minimum number of sources used for developing default values. The guidance specifies that nondetects should be used in the development of default values. However all nondetects were discarded in at least one AP-42 update.'Nondetects may be discarded under certain circumstances specified by the guidance where these are much greater in magnitude than detects(doing otherwise would bias the default values high).However,the AP-42 documentation does not identify which values are detects or nondetects making it impossible to implement this procedure.Finally,the guidance states that default values developed entirely from nondetects should be clearly identified as such.Since nondetects are not documented,this procedure cannot be carried out. 6"Procedures for Preparing Emission Factor Documents"Office of Air quality Planning and Standards, Office of Air and Radiation,U.S.Environmental Protection Agency,Research Triangle Park,NC, November 1997(EPA-454/R-95-015 REVISED). 7 Phone communication(June 2000)with Stephen Roe,U.S.EPA contractor for past AP-42 revisions. 4