SITE INFORMATION AND CORRESPONDENCE_FILE 2

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Mr. Jack Badey - 2 - 26 September 2008 presents groundwater data from the monitoring wells. This letter reviews the work conducted to date and outlines the next phase of the investigation. Vapor Transport In the Soil Vapor Report, the data show that most of the sub-slab soil vapor concentrations exceed the California Human Health Screening Levels established by the California Environmental Protection Agency for indoor air at commercial/industrial facilities. AMEC Geomatrix estimated indoor air concentrations using these sub-slab soil vapor concentrations and knowledge of the building construction. In this site-specific soil vapor transport analysis, AMEC Geomatrix identifies the 10 mil thick vapor barrier placed beneath the concrete when the building was constructed in 2003 as a significant obstacle to vapor transport in the vertical plane. The transport assessment therefore relies on diffusive transport as the governing pathway between the subsurface and the interior of the building. The diffusive transport model estimates a vapor concentration within the building That is beiow levels of health concern. The diffusive transport model assumes 4 cm2 of defects per acre of liner. The liner covers 0.9 acres. If the puncture density were underestimated, advective transport would be the dominant transport pathway, which could put workers inside the building at risk. When the California Department of Toxic Substances Control version of the Johnson & Ettinger model is used with the site specific data, it estimates that the upper quartile of sub slab vapor samples could potentially pose a long-term threat to workers within the building. As a check on the Johnson & Ettinger model results, when considering attenuation factors of 0.01 and 0.001 through the concrete floor, the third and fourth quartile concentrations exceed screening levels. These estimates suggest that a vapor removal action should be conducted. Given the uncertainty in the results of the diffusive vapor transport estimate of vapor concentrations within the building, UniFirst should either collect indoor air samples to demonstrate that worker health is protected, or UniFirst may prepare a proposal to remove soil Vapors from beneath the building. Removal of vapors would additionally benefit UniFirst by removing this avenue of PCE migration to groundwater. In Table 1 of the Soil Vapor Report, the sampling date is recorded as December 2008, which should be changed to December 2007. Instrinsic Biodegradation In the Monitoring Well Installation Report, Geomatrix documents that biodegradation of PCE is occurring. The presence of the daughter products, TCE, cis- and trans-1,2-DCE in the vicinity of the Unifirst building along with the identification of the dechlorinating bacteria, Dehalococcoides ethenogenes, demonstrates that biological processes are degrading PCE in the highest concentration areas of the plume. The daughter products do not appear to be accumulating, and vinyl chloride is not detected. This further supports AMEC Geomatrix' assessment that conditions are conducive for complete dechlorination of the parent compound. The rate of degradation could be increased by identifying and providing the limiting nutrient.