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15 June 2009 � AGE-NC Project No.05-1306 Page 27 of 33 Vapor intrusion is a potential pathway requiring evaluation to protect human health and prevent migration of vapors to building interiors. Screening for TPH subsurface mixtures for gasoline,middle distillates,and residual fuels were based primarily on nuisance odors rather than health concerns. See Table 1-2 of "Screening for Environmental Concerns at Sites with Contaminated Soil and Groundwater:" (California Regional Water Quality Control Board, San Francisco Region, November 1, 2007). A screening level of 5 mg/L was listed as a nuisance odor threshold for ground water; however, a value of 50 mg/L was listed as the limit for this screening level. Odors are typically associated with the more volatile constituents of petroleum mixtures and these volatile constituents are represented by the shorter hydrocarbon chains. 8.5. TPH CHARACTERISTICS �. Total petroleum hydrocarbons are a mixture of aliphatic and aromatic compounds with varying lengths of carbon chains. The physical properties of the TPH mixture vary according to the mass fractions of these various carbon compounds in the mixture. Table 8-1 presents the solubility, vapor pressure, and Henry's Law coefficient for various aliphatic and aromatic functions.As the carbon chain increases in length,the vapor pressure decreases.Vapor pressure is an important factor in determining the ability of a compound to migrate through soil in the pore spaces filled with air. '" As a hydrocarbon mixture ages, or "weathers", the lower molecular weight and more volatile constituents are degraded.Chromatograms of fresh and weathered gasoline and diesel,which clearly shows the loss of lower end or more volatile constituents are included in Appendix M. Chromatograms of TPH samples from MW-3 and MW-4 were reviewed and compared to the TPH &. diesel standard to evaluate the general composition of the residual remaining on site. It is evident in comparing the chromatograms of MW-3 and MW-4 to the TPH diesel standard that the higher boiling point, or lower volatility compounds,remain at the site. These lower volatility compounds to„ have limited migration potential as their octanol partition coefficient is high, indicating greater adsorption to soils. .. A sample was collected on 09 May 2008 to assess the mass fraction of the various carbon chains in ground water associated with MW-3 and MW-4. Table 8-2 summarizes the mass fraction of the aliphatic and aromatic compounds in these two monitoring wells.The primary hydrocarbon fractions �-- in both wells are aliphatic chains with 16 to 21 carbons(C16 to C21).The vapor pressure for the TPH residual remaining in MW-3 and MW-4 was calculated using information included in Table 2 and Advanced GeoEnvironmental,Inc. L.