SITE INFORMATION AND CORRESPONDENCE

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• • Mr. James Barton Page 3 of 4 Volatilization from groundwater should not be problem. Monitoring well MW-3 is the only well where groundwater is impacted with petroleum hydrocarbons. Table 1 lists groundwater contaminant concentrations in monitoring well MW-3 along with the most recent Groundwater Screening Levels for Vapor Intrusion Concerns — Commercial/Industrial Land Use (RWQCB, May, 2008). None of the groundwater contaminant concentrations even approach their respective ESLs and no further evaluation is necessary. Volatilization from contaminated soil should not result in significant risk. Table 2 lists all soil data for detected compounds, in the borings where they were detected, along with Soil Screening Levels for Vapor Intrusion Concerns—Commercial/Industrial Land Use (RWQCB, February, 2005). Note that the most recent versions of the RWQCB document do not include soil ESLs for vapor intrusion concerns. The table also summarizes the statistics for the contaminants. In calculating the statistics, non-detect values were treated as positive values at the detection limit concentrations. The 95th percentile upper confidence level concentrations (UCLs) for each contaminant can be compared to the ESLs to screen for risk. Of the detected contaminants, only benzene, tetrachloroethene (PCE) and naphthalene UCLs exceed their respective ESLs and only by one order of magnitude or less. Where concentrations exceed ESLs, a site specific evaluation is warranted. Such a"Tier IP' evaluation would be expected to conclude that these concentrations do not, in fact,pose a vapor intrusion health risk. The following site-specific factors differ substantially from the conservative, default model parameters used in deriving the ESLs. Modeling with these parameters would calculate indoor contaminant vapor concentrations much lower than the levels predicted by the default model: • The site building parameters are very dissimilar from the modeling parameters used to generate the ESLs. The area of the building is approximately 28,000 square feet(model uses 995 sq. ft.). The height of the ceiling is 20+ feet (model uses 8 feet). The indoor/outdoor air exchange rate is substantial due to the 6 large rollup doors that generally remain open during business (the model uses 1 air exchange per hour). • The open ventilation reduces or eliminates negative pressure differentials between the building interior and the underlying soil gas (model uses a negative differential pressure [vacuum] of 4 Pascals). • The permeability of the soil under the concrete foundation (clay/silt) is orders of magnitude lower than the permeability used in the model (sand). • Less than 3%of the structure overlies contaminated soil. The model assumes 100%. Taken together, these site-specific parameters would be expected to reduce the vapor flux and indoor air concentration by more than one order of magnitude from that calculated by RWQCB and indicates that significant health risk does not exist for building occupants. Since the excavation has been backfilled and paved with concrete, the only possibility for direct exposure to the soil contaminants would be during construction activities. Table 2 also lists ESIs for construction worker exposure. None of the hydrocarbon compounds or mixtures, except TRPH, exceeds the ESLs. G\GROUNDZE\GABBARD\repms\MONTHLY\Oct2008.dm