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drop was typically from sixth to at least thirteenth or more. Four of the nine compounds dropped from the ranking and rank no more than 17`x. The average MSW landfill contribution per compound dropped from 13% to 1.5%: One of the more dramatic findings concerns U.S. EPA's original attribution of 84% of all 1,1,2,2 -Tetrachloroethane emissions to landfills; the WIAC findings show that the landfill emission level is about 2% of all sources. These findings indicate that municipal landfills have markedly less emissions, compared to other industrial sources, than U.S. EPA previously estimated. Table 3. Summary of changes to Urban Air Toxic (UAT) emission estimates based on changes from AR42 defaults to current compound levels measured by WIAC. Portion of UAT Number Annual Tons Inventory Rank of Comnound AP42 I WIAC I AP -42 I WIAC AP -42 I WIAC Sources I 1'1'2'2 216 1.0 84.08% 2.37% 1 5 16 compounds, the differences were larger yet. The overestimated AP -42 values may potentially Tetrachloroethane misdirect U.S. EPA's policy development. For example, the recently completed Urban Air Toxics Strategy appears to have substantially overestimated actual landfill emissions. Furthermore, the existing AP -42 default values may adversely impact individual landfills required to use these values. As a result, WIAC believes that the AP -42 defaults should be revised to reflect the decline in 23.6 3.0 3.59% 1.48% 6 8 12 Dichloropropane Acrylonitrile 389 2.2 .15.28% 0.10% 3 15 17 Benzene 173 87.9 3.86% 2.00% 11 13 17 Chloroform 4.17 1.3 4.94% 1.63% 6 9 17 Ethylene Dichloride 47 13.7 1.15% 0.34% 10 * 17 Methylene Chloride 1550 367 1.67% 0.40% 11 * 17 Tetrachloroethylene 717 229 0.59% 0.19% 6 * - 17 Trichloroethylene 429 104 0.64% 0.16% 13 * 17 Vinyl Chloride 531 77.9 19:65% 3.46% 2 4 17 Vinylidene 22:5. 10.3 1010% 3.45% 4 5 14 Chloride Landfill emissions are less than for other ranked sources. I Conclusions WIAC conducted a national survey of recent LFG analyses. Recent results from 75 landfills were analyzed using AP -42 methodologies. The AP -42 defaults were found to typically overestimate current levels by two to four hundred percent. For some of the more health.significant compounds, the differences were larger yet. The overestimated AP -42 values may potentially misdirect U.S. EPA's policy development. For example, the recently completed Urban Air Toxics Strategy appears to have substantially overestimated actual landfill emissions. Furthermore, the existing AP -42 default values may adversely impact individual landfills required to use these values. As a result, WIAC believes that the AP -42 defaults should be revised to reflect the decline in LFG constituents. The most recent AP -42 revision in 1995 added new data to older values and averaged the combined data sets. This approach is appropriate only for data that does not trend. It is recommended that older data be purged and replaced using current data presented in this paper. I