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Quik Stop No.39 June 30,2009 Stockton California Technical Justification for"No Further Action Re aired" the depth explored consist predominantly of sandy silts and clayey silts with subordinate silty sands. Quarterly groundwater monitoring has been conducted since 1999. Quarterly groundwater analytical data are summarized in Table 2. Monitoring data indicate the depth to groundwater has ranged from 43 to 52 feet throughout the monitoring period. For second quarter 2009, the depth to water was measured at approximately 47 feet and the groundwater gradient is shown on Figure 2. Groundwater analytical data for second quarter 2009 indicate low concentrations of total petroleum hydrocarbons as gasoline (TPHg) and 1,2-dichloroetliane (DCA) in wells MW-1, MW-2 and MW-3 located near the source area, the former dispenser island. The maximum concentrations of DCA in groundwater from wells MW-1, MW-2 and MW-3 over the past six quarters are 10, 80 and 59 micrograms per liter (ttg/L). Low concentrations of toluene, ethylbenzene and xylenes have been detected sporadically throughout the monitoring period, but benzene has never been detected in wells MW-1, MW-2 and MW-3. Monitoring data indicate wells MW-4 through MW-8 have not been impacted with petroleu rn hydrocarbons. Technical Justification for No Soil Gas Investigation The May 2008 document Screening for Emdroninental Concerns u1 Site with Contarninaled Soil and Groundwater published by the San Francisco Bay Regional Water Quality Control Board (SFBRWQCB) contains lookup tables of environmental screening levels (ESLs) for comparison of analyte concentrations in various media to evaluate the risk of vapor intrusion into buildings. Different screening levels are typically listed for residential and commercial sites. Conservative screening levels for groundwater, listed in Table E-1 (enclosed), are based on a depth to groundwater of 3 meters (9.8 feet). In the case of Quilt Stop No. 39, the depth to groundwater has been 43 feet or greater and the screening level can be considered extra- conservative. Groundwater screening levels for TPHg are referred to the soil gas screening levels. The Table E-1 groundwater screening level for DCA for a residential site setting is 200 µg/L, greater than DCA concentrations detected at the site since 2002. Additional site factors that tend to reduce the chance of soil gas migration from impacted soil and groundwater to the building foundation include: impacted soil was restricted to the immediate source area (dispenser Island); impacted soil and groundwater are laterally separated from the site building receptor; attenuation of soil gas concentrations due to biodegradation with increased distance from the source; and the predominantly fine-grained nature of vadose zone soils which retard the migration rate and mass flux of soil gas. The Site Conceptual Model presented in the Report indicates that the impacted groundwater plume is stable and does not pose a threat to the public or the beneficial use of groundwater. Evaluation of the risk of potential exposure to the public utilizing the SI=BRWQCB ESLs indicates that risk of vapor intrusion into site buildings or buildings on surrounding parcels is below the level of concern. Horizon recommends that the EHD evaluate the site for "no further action required" designation on the basis of current site data and this technical justification. Ted dusticui Apt Jun09 Project No.7039.16 2 HORIZON ENVIRONMENTAL INC.