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v <br /> CONSULTING ENGINEERS & GEOLOGISTS, INC. <br /> 812 W.Wabash • Eureka,CA 95501-2138 • 7071441-8855 • FAX:7071441-8877 tdb@shn-eureka.com <br /> Memorandum <br /> Reference: 007152 <br /> Date: October 11,2010 <br /> To: Harlan Knoll " <br /> From John Aveggio <br /> Subject: Additional tooSSupport estern Oil and Spreading Closure Request <br /> In your October 5,2010 email you requested additional information regarding a residual <br /> contaminant mass estimate and a vapor intrusion evaluation. Based on the information we <br /> provided to you in the closure checklist I am providing this response to your request. <br /> Residual Mass Estimate <br /> Soil concentration data was collected at the site approximately 10 years ago. Concentrations of <br /> petroleum hydrocarbons have continued to be reduced through biodegradation over time. <br /> Therefore the estimate presented here is likely to greatly overstate the mass of petroleum <br /> hydrocarbons remaining in site soil. Furthermore the bulk of the residual petroleum hydrocarbons <br /> is between 10 and 30 feet below grade,is heavy end material,and is not impacting groundwater. <br /> Cross sections D-2 and D-3 were used to calculate the residual mass of petroleum hydrocarbons. <br /> The concentrations of total petroleum hydrocarbons were averaged across the 10 to 30 foot depth <br /> and then extrapolated to the volume of petroleum hydrocarbon impacted soil represented by the <br /> cross sections. This approach calculated a residual mass of approximately 84,000 kg of heavy end <br /> petroleum hydrocarbons. Again this is believed to be a conservatively high value based on <br /> ongoing bioremediation of this material and this material is not impacting sensitive receptors or <br /> human health and the environment due to its location and the site controls that are in place. <br /> Vapor Intrusion Evaluation <br /> The site is characterized as containing heavy end petroleum hydrocarbons associated with its use as <br /> an asphalt oil storage and application facility. Volatile compounds have been analyzed for in soil <br /> and groundwater over time. Benzene has never been detected in soil and the highest concentration <br /> of a BTEX compound was 6.9 ug/g of xylene at 30 feet below grade. BTEX has never been detected <br /> in any groundwater sample collected from a monitoring well at the site. Therefore our assessment <br /> is that vapor intrusion is not a viable pathway for exposure at this site due to the lack of volatile <br /> compounds and the depth at which the residual material remains. However we did run the DTSC <br /> vapor intrusion screening model using the 6.9 ug/g of m-xylene as the input. The soil <br /> concentration was converted to soil gas and input to the model. The risk associated with this <br /> concentration was well below any threshold of concern. Therefore based on the type of material <br /> present at the site and the results of the screening model we conclude that vapor intrusion is not a <br /> concern at this site. <br /> Thank you for your assistance in moving this site to closure. Please call me if you have any further <br /> questions. <br /> Civil • Environmental • Geotechnical • Surveying <br /> Construction Monitoring• Materials Testing <br /> Economic Development• Planning&Permitting <br /> H:\javeggio\job files\2007\(307152,ACAMPO FUND APPLICATION\Knoll-memo-mass&vi.doc <br />