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11 Groundwater Investigation Report Country Club Drain System Project City of Stockton Municipal Utilities Department October 2 2003 Page 8 1 infiltration of groundwater The only severe Infiltration was observed along a mne-foot section of the drain adjacent to the Shell Oil site At this section of the line, groundwater was observed rapidly infiltrating the drain Also, heavy sediment was observed in the section of the drain adjacent to the Shell ' Oil site, indicating groundwater infiltration through cracks in the drain The section of drain surveyed and the location of severe infiltration are shown on Figure 2, Appendix A The survey report for the video survey is included in Appendix E ' 7 0 CONCLUSIONS Elements of the evaluation to identify the most likely source(s) of contamination to the storm drain along Plymouth Road, north of Country Club Blvd , included the following • Groundwater investigations conducted by Condor along Country Club Blvd , ' • Historical groundwater monitoring data collected from the Shell Oil, Exxon Mobil, and 7-Eleven Stores UST sites, • Location of leaks in the storm drain, • Contarnmant plume dynamics for the Shell Oil and Exxon Mobil sites, and, • Most likely source(s)of contamination based on the above data On December 7, 1994, it was discovered that an estimated 8,500 gallons (approximately 51,000 pounds) of gasoline was discharged to the environment at the Shell Oil site According to a report prepared for the Shell Oil site, approximately 1,400 pounds of MTBE have been removed from the site up through the last quarterly monitoring event (April 2003) Subsequent groundwater monitoring indicated high concentrations of TPH-G, BTEX, and MTBE in the property's monitoring wells Releases at the Exxon lie Mobil and 7-Eleven Store sites were identified during tank removals, therefore the volumes of the releases could not be quantified ' The groundwater investigation conducted by Condor indicated that MTBE, TAME, and TBA have migrated off the Shell Oil and Exxon Mobil sites and that MTBE has likely also migrated off the 7- Eleven Store site MTBE was shown to be widely dispersed across the area, being detected in I1 of the 15 groundwater samples collected from GP-1 through GP-15 With the exception of the 3,400 µg/L MTBE detection in GP-4, the MTBE concentrations ranged from 1 1 to 48 gg/L Based on the Condor groundwater results and monitoring well groundwater monitoring data for each site, the MTBE plumes are diffuse and are possibly co-mangled The TAME and TBA plumes from each of these sites are smaller and not as laterally extensive as the MTBE plumes, and are not comingled The TBA and TAME plumes originating from the Shell Oil site appears to have n-agrated beneath the storm drain on Plymouth ' Road The TAME and TBA plumes originating from the Exxon Mobil site do not appear to have migrated beneath the storm drain and are contained within a small area on and immediately offsite TAME isoconcentration contours for the most recent groundwater monitoring data collected for all three ' sites are shown on Figures 3 and 4, Appendix A, respectively MTBE was not contoured due to its widespread and diffuse distribution Historic groundwater elevation data from the Shell Oil and Exxon Mobil sites shows that the water table is encountered between approximately 10 and 16 feet bgs, with southerly gradients, and fluctuates three to four feet during a year A review of the historic groundwater monitoring data for the Shell Oil site shows that MTBE has consistently been detected in a majority of their monitoring wells and at boring ' locations downgradient TAME and TBA have consistently been detected at elevated concentrations in MW-5, MW-8, and RW-lA Also, TAME and TBA have only been detected in wells that also contain �� CONDOR