Laserfiche WebLink
September 8, 1999(p�� „! <br />To: Nanc y Olson - Martin r' <br />Santa Ana Regional Water Quality Control Board _ <br />3737 Main Street, #500 <br />Riverside, CA 92501 <br />Subject. Revised Health Risk Assessment <br />Old City Yard <br />730 East Warner Avenue <br />Santa Ana, CA 92707 <br />I have reviewed the most recently revised version of the vapor risk assessment for the Old <br />City Yard (dated September 8, 1999). The risk assessment was performed by Gradient <br />Engineers, The approach used is consistent with the protocol which is currently accepted <br />by the Orange County Health Care Agency for determining excess lifetime cancer risk <br />when carcinogenic materials are present in subsurface soils. This basic approach is also <br />consistent (at least for the site conditions in question) with the risk assessment <br />methodology used in the American Society for Testing Materials (ASTM) Risk Based <br />Corrective Action (RBCA) vapor risk assessment procedure. <br />The assessment estimates an excess lifetime cancer risk of about 1 in 660 million. It <br />conservatively assumes that the source contamination (gasoline) is present in the liquid <br />phase (non- aqueous phase liquid or NAPL). A 1% mole fraction (per cent fraction of <br />benzene in the residual gasoline matrix, based on molecular composition and on Raoult's <br />Law) was assumed. This assumption is quite conservative because (based on the data <br />which I reviewed) all soil samples exhibited empirical benzene mole fractions lower than <br />1 %. Numerous other "worst case" assumptions were made (including assumptions <br />concerning the degree of water saturation in the soil pathway) resulting in an appropriate <br />screening level application of the model which demonstrates that the residual gasoline <br />contamination does not present a significant risk (i.e. excess cancer risk greater than 1 in a <br />million) for potentially exposed individuals. Based on the available data, the excess <br />lifetime cancer risk posed by residual gasoline contamination is well below the significant <br />risk level of 1 in one million excess lifetime cancer cases. <br />Sincerely, <br />Peter Peuron <br />Project Scientist <br />Environmental Geoscience Services <br />(former Hazradous Waste Specialist with Orange County Health Care Agency) <br />