SITE INFORMATION AND CORRESPONDENCE

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�r I Beacon StatimNo.3641 December 29,20Q4 Stockto 1 C 6f rnia .. Soil.justificati6h for Soil Cas Investi anon from third quarter `1996 to fourth°quarter 2009.- Quarterly groundwater analytical data are summarized in Table-2. The depth to groundwater has ranged from 43 to 52 feet.bsg throughout the monitoring period. Fourth quarter 2009 depth to water was approximately 52 feet bsg. The fourth quarter 2009 groundwater elevation contouimap is shown on Figure 2. A soil vapor extraction system (VES) and an air sparge system operated at the site-from March 2005 to August 2009.' Over the period of soil and„groundwater reinediation, approximately 9,230 pounds of TPHg and 71 pounds of benzene were removed by the vapor extraction system.. VES performance data are summarized in Table 3, Technkfil Justification fot•No Soil Gas Investigation- The May 2008.San Francisco. Bay Regional Water.Quality Control Board (SFBRWQCB) document Screening fa• Environniental Concerns at Sites with Contaminated Soil and Groundwater (ESL document) contains lookup 'tables, of conservative environmental screening levels (ESLs) for comparison to analyte concentrations in various media to evaluate, among other parameters; the risk of vapor intrusion into buildings. Typically, separate ESLs for vapor intrusion concerns .are listed for residential and commercial land uses. Screening levels for residential sites are lower than those for commercial sites because of increased sensitivity,of potential residential receptors. Table E-'1 (enclosed) in the ESL document lists groundwater screening levels for, evaluation of potential vapor-intrusion . concerns based on a depth to groundwater of 3 meters(9.8 feet). Sources of residual volatile petroleum hydrocarbon constituents at the Beacon No. 3641 site that have the -potential to, cause vapor intrusion include residual,. impacted .soil and groundwater. In the case of Beacon Station No. 3641, the depth to groundwater throughout the monitoring period has been at least four times greater than the depth considered in setting the listed ESL. Results of fourth quarter 2009 monitoring indicate that the dissolved hydrocarbon constituents detected. (benzene, ethylbenzene, xylenes and MTBE) in groundwater were two to four orders.of magnitude.less than the appropriate residential ESL concentrations. No groundwater ESL is listed for-TPHg. According to performance calculations,approximately 9,230 pounds of TPHg were removed. by„the VES from vapor wells scre0iied from 21 ,to 55 feet bsg in the UST basin area. .The average calculated rate of TPHg removal,from vapor wells VW-1, VW-3; V W-S and VW-7 screened from approximately 40 feet bsg to below the water table over the last six months of VES operations was 0-4 ,pound per' day. -Field data indicate shallower vapor wells screened during the same period u 21 and 40 -feet bs °. roduced little to .no hydrocarbonsg p betwee g p '. indicating minimal residual impacts in that shallower vadose zone interval. The most recent (August 2009) analyticalresults for VES total influent vapor (combined from wells VW=l, VW-3 and VW�5) indicated a TPHg concentration of.16 parts per million by volume (the equivalent of 64,000 micrograms per cubic:meter [}tglm3]); no benzene was detected. The VES total influent TPHg concentration exceeds the soil gas ESL:of 10,000 ltg/m3 which is modeled for a soil gas.sample collected immediately beneath a slab-on-grade foundation. However; it is unlikely that concentrations of TPHg insub-slab soil would exceed the ESL because of the vertical and l'ate`ral 'distance to the source and the associated natural TechJUSlfon Rpt becO9 Project No. 164 110 2 HORIZON ENVIRONMENTAL INC: