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I 1 Groundwater investigation Report Country Club Drain System Project • City of Stockton Municipal Utilities Department October 2 2003 Page 4 ' wells and groundwater monitoring wells to monitor and remediate the impacted groundwater beneath the site The SVE wells have since filled with groundwater and are currently used as monitoring wells Currently, there are no remedial actions in place for plume control or cleanup at this site Currently, ' Exxon Mobil utilizes 12 shallow wells(four offsite and eight onsite)for monitoring Adjacent to the Exxon Mobil site is the 7-Eleven Store UST site (2725 Country Club Blvd) The USTs ' at this site were removed in 1998 During removal activities, soil and groundwater samples indicated high concentrations of benzene, TPH-G, and MTBE IT Corporation conducted soil and groundwater characterization activities at the site Currently, 7-Eleven utilizes seven shallow wells (two offsite and five onsite) for site monitoring To date, there are no remedial actions in place to control plume migration ' originating from this site Groundwater monitoring data collected on March 17, 2003 indicates that MTBE was detected in three wells at concentrations ranging from 1,300 to 10,000 }tg/L, TBA was detected in four wells at concentrations ranging from 4 8 to 200 �tg/L, and TAME was not detected in ' any of the wells The shallow subsurface geology encountered during soil boring installation activities at both sites ' consisted of clay, clayey silt, and clayey sand to approximately 25 feet bgs A geologic cross-section across the adjacent 7-Eleven Store UST site is included in Appendix C Groundwater was also first encountered at both sites between 10 and 16 feet bgs The groundwater flow direction ranges from northeast to southeast, with the gradient periodically to the northwest Overall, the groundwater gradient appears to be fairly flat and to the southeast The groundwater elevations at these sites fluctuates approximately three to four feet during a year, with the highest elevations in the spring and lowest elevations in the fall To date, there are no remedial actions in place to control plume migration ' originating from the 7-Eleven Store site In December 1995, site operations of the 7-Eleven Store site were transferred to Palisades Gas and Wash (Palisades) On February 24, 2003, S3CEHD issued a Notice of Responsibility to Palisades (the current operators of the facility) based on recent increased concentrations of MTBE and other contaminants that indicate a recent release Palisades subsequently filed an appeal to the State Water Resources Control ' Board (SWRCB) on March 25, 2003 As of September 2, 2003, the case is still being reviewed by the SWRCB ' Groundwater monitoring is currently being conducted quarterly at both sites Quarterly groundwater samples are analyzed for TPH-G, BTEX, MTBE, and other oxygenates (including TBA and TAME) The highest concentrations of MTBE, TBA, and TAME were detected in the area of the former tank pits at both sites The range of concentrations detected, the frequency of detections, and the most recent ' concentrations in Exxon Mobil wells with detections of TBA, TAME, and MTBE are summarized in Table 2, on the next page The TBA and TAME isoconcentration contours for the most recent groundwater monitoring data are shown on Figures 3 and 4, Appendix A, respectively MTBE was not contoured due to its widespread and diffuse distribution Copies of the groundwater monitoring tables included in the most recent groundwater quality report for the Exxon Mobil and 7-Eleven sites are located in Appendix C I 1 �� CONDOR