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Page 2 excavated in 2005. Since that time no additional petroleum product has been stored or used in the tank excavation area. This means over 20 years have elapsed since petroleum products were released to the soil or ground water. In 2005, under the direction and at the behest of the SJCEHD, excavation of 1,308 tons of hydrocarbon-impacted soil and 326 tons of clean overburden were removed to a depth of 25 feet in the former UST area. Figure 2 depicts the area of the former UST and the excavation. As a result of this remedial activity, the groundwater contaminant concentrations on site were reduced by 98 to 99 percent. The remaining TPH diesel groundwater concentrations most recently reported were 3.5 mg/L in MW-3 and 1.5 mg/L in MW-4. Table 1 from the quarterly report is included for reference. 2. Vapor Intrusion 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 groundwater; 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. 3. 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 2 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. Figure 3 depicts the chromatograms of fresh and weathered gasoline and diesel, which clearly shows the loss of lower end or more volatile constituents. 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 have limited migration potential as their octanol partition coefficient is high, indicating greater adsorption to soils. A sample was collected on May 9,2008 to assess the mass fraction of the various carbon chains in groundwater associated with MW-3 and MW-4. Table 3 summarizes the mass fraction of the aliphatic and aromatic compounds in these two monitoring wells. The primary hydrocarbon R:\CONTGRAN\Stockton\2008 Correspondence\ltr to H Knoll 061108 Rev 7-21-08.doc