SITE INFORMATION AND CORRESPONDENCE

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C. Dean Hubbard October 13, 2006 Page 6 phenanthrene were considered volatiles (based on Henry's Law Constant greater than 10-5 atm- m3/mole and molecular weight less of 200 g/mole), while the rest were considered non-volatiles. Based on the above cited criteria three more PAHs should be considered volatiles, namely benzo(b)fluoranthene, chrysene, and pyrene. I have modified the list of PAHs accordingly and followed it in the exposure and risk calculations. Indoor Air Modeling 7. Geomatrix used the Johnson @ Ettinger model-based spreadsheet to calculate the indoor air concentrations resulting from migration of vapors from soil only. The rationale for many of the modeling assumptions is not provided in the report. The main deficiencies are discussed below: a) Geomatrix did not provide the rationale for selecting representative soil concentration (two sets—surface and surface plus subsurface) as input to the model (e.g., please refer to pyrene,Tables 2 and 3 and spreadsheet Iof 3 in Appendix B). As already stated, I used the concentrations from the surface plus subsurface set. b) As you are probably aware, using soil bulk concentrations will add additional uncertainty and is not recommended by CalEPA. Collection of soil-gas samples would result in much more realistic indoor air concentration modeling. In addition, soil contamination exists below 10 feet bgs at this site (higher soil concentrations might result in higher indoor air concentrations), but is not considered in this risk assessment. c) Geomatrix used unknown depth below grade to bottom of contamination. However, the groundwater(which is probably another source of vapors) is determined to be between 7 and 15 feet bgs. d) According to Geomatrix, VOCs were identified in previous site groundwater investigations and free product is currently being removed from ground water(see Site Conceptual Model section above). Despite these ongoing product removal activities, it remains unclear why the Johnson &Ettinger-based NAPL spreadsheet was not used to model the vapor migration indoors. e) Geomatrix used the default soil properties for loamy soil claiming it to be site-specific. However, my review of the boring logs provided as Appendix C of the "Soil Investigation and Groundwater Monitoring Report"shows a mix of sandy silt, silty sand to clayey sand. I am not in a position to determine whether this mix of different soil types may be conservatively(with respect to the soil vapor permeability) represented by a loamy soil. I have assumed that the above assumption is correct and used loamy soil type input parameters in my modeling. However, if you think that this assumption is incorrect, recalculation may be necessary.