SITE INFORMATION AND CORRESPONDENCE

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December 29,2009 Beacon Station No.3641 TechnicaE Justification for Na Soil Gas Investi ation Stockton California attenuation of soil gas concentrations. Additionally, subsurface bio-attenuation of petroleum hydrocarbon is not considered in the development of the ESL. Additional factors that tend to decrease the potential of soil vapor intrusion into the site building or structures on surrounding parcels include: • a portion of originally impacted soil in the UST basin area was remediated by excavation to a depth of approximately 18 feet bsg; • the extent of soil impacted by petroleum hydrocarbons did not extend into the subsurface of the station building or any other off-site structures;— old ? VW 3�{0 p-b • the predominantly fine-grained vadose zone soil types tend to retard the migration it"We }�,ial�tyay rate and mass flux of soil gas; • groundwater currently impacted with concentrations of petroleum hydrocarbons is limited to the UST basin area and is laterally separated from the site building; and • the current groundwater plrune is characterized by decreasing concentrations and positional stability. Evaluation of site data and utilization of appropriate ESLs indicates that it is unlikely that current site conditions pose a vapor intrusion health risk to workers in the site building or structures on adjacent parcels. On the basis of remedial actions conducted at the site, including excavation of impacted soil and soil vapor extraction, the limited, residual petroleum hydrocarbons that exist at depth in the UST basin do not pose a significant risk to { the public health and safety. l`• Studies have shown that the vapor intrusion threat from soil impacts is not a concern if the �* f top five feet of soil is clean and there is no free product on the water table. With respect to the low levels of hydrocarbons in groundwater, vapor intrusion is not a concern for the reason stated above. Horizon recommends that this technical justification appropriately addresses the risk of vapor intrusion and that a No Further Action Required Request be prepared for the site. Please call Horizon at 916-939-2170 if you have any questions or require additional information. Sincerely, AL HORI VIRONMENTAL Rexford K. Snaith, P.G. #5307 N'0' Pro' ct Manager � �], t( Gary D. arker Senior Project Manager TechJustfan Rpt Nc09 Project No. 1641.110 3 HORIZON ENVIRONMENTAL INC.