ARCHIVED REPORTS_REPORT ON SOIL VAPOR SAMPLING AND HUMAN HEALTH RISK ASSESSMENT DEC 2010

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AECOM Report on Soil Vapor Sampling and Human Health Risk Assessment 2-8 petroleum hydrocarbons of a group (like gasoline)are not the only chemical compounds that elute from a chromatograph within the retention time window assigned for each group and, as such,TPI-Ig values can be misleading with regard to vapor toxicity. The three most abundant of the identified C8 compounds are tetramethylbutane; 2,3,4- trimethylpentane; and 2,3,3-trimethylpentane.Their relative abundance in each of the samples collected is displayed in the attached chromatograms (Figure 4—May event, Figure 5—September event, and Figure 7—October event). On each of these figures, the chromatogram for the sample event is compared to that of a gasoline reference standard. Internal standard compounds (which are added by the laboratory for quality control)are marked in yellow and are at constant concentrations, while the measured components vary. It can be clearly seen in each comparison that the principal components (labeled in red as"1, 2, and 3") in each of the samples are not the principal components of gasoline. These chromatograms also confirm that three of the primary aromatic components of gasoline (toluene , ortho-xylene, and meta-&para-xylenes[benzene and ethylbenzene were not used in the comparison])are not present in the Samples. Therefore,while the May, September, and October samples have substantial quantities of material that elute within the time window defined for TPHg,that material is now clearly different from gasoline in its chemical composition. The potential health risks presented by the chemicals detected in the vapor samples are evaluated based on the detailed chemical composition data obtained. DTSC vapor intrusion guidance for TPH recommends the use of six groups of hydrocarbons for which chemical surrogates for toxicity have been selected (DTSC, 2009). [Although this guidance has recently been withdrawn, it is particularly relevant to this site.]These groups separate hydrocarbons into categories for aliphatics and aromatics in different carbon atom content ranges. Based on these categories,the composition observed at SV- 1 (during all three events)is in two groups: C5-C8 aliphatics and C9-C18 aliphatics,which are further discussed in Section 3.0,below. Based on the superior quality control performance (e.g., vacuum 510 inches Hg and helium analysis indicating no leaks)in the October sampling, data from that sampling event was carried forward into the indoor vapor intrusion risk analysis. Additionally, in order to exhaustively calculate potential risks, we further evaluated data(TICS and chromatograms)from the May and September events as part of a conservative uncertainty assessment, despite known data quality deficiencies associated with those data sets. The quantities of analytes detected as TICS (in the May and September data sets)were classified by their respective carbon chain lengths, and the totals for each chain length category(i.e., C5-C8)were then used in risk analysis, as discussed in Section 3.0, below. December 15,2010 60146576-M10