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Groundwater Invesugation Report Country Club Drain System Prosect City of Stockton Municipal Utilities Department October 2 2003 Page 9 ' MTBE While TBA may also be a gasoline additive, under anaerobic conditions (low dissolved oxygen), MTBE also degrades to TBA as an intermediate compound TAME is not a degradation product of MTBE,but is a separate oxygenate compound Based on the storm drain video survey, the section of the storm drain along Country Club Blvd (west of Plymouth Road) and the section west of GP-1 (near Oxford Way) are dry The section of the storm drain along Plymouth Road and north of Country Club Blvd has a major infiltration between GP-3 and GPA This was confirmed on October 2, 2003 through the use of specially-equipped video cameras The location and depths of the storm drain along Country Club and Plymouth Road are shown on Figure 2, Appendix A Because of the shallow depth to groundwater, little fluctuation in the water table, depth of the storm drain, and confirmed infiltration in the storm drain along Plymouth Road, contaminated groundwater is likely infiltrating the storm drain I ' The Shell Oil site appears to be the source of contaminated groundwater that infiltrates the storm drain along Plymouth Road for the following reasons • The close proximity of the storm drain along Plymouth Road to the site (less than 10 feet), • The location of the severe infiltration in the storm drain with respect to the Shell Oil site, • The shallow depth to groundwater (approximately 10 feet bgs) and relatively constant water table elevation As a result, the storm drain along Plymouth Road is submerged beneath groundwater (up to 14 feet bgs), • The groundwater flow direction at the Shell Oil site is primarily southwest, towards the storm drain ' . line, • The downgradient well an the southwest edge of the Shell Oil site (MW-8), adjacent to the storm drain, had concentrations of MTBE at 1,600 ltg/L, TBA at 4,700 gg/L., and TAME at 12 µg/L in April 2003 • The lateral distribution and concentrations of TBA and TAME Based on the analytical results from the water samples collected from two manholes along Plymouth Road(MTBE [130 gg/L],TBA [890 µg/L], and TAME [0 7 µg/L])with respect to the TBA and TAME results for the Shell Oil site wells, Shell Oil is the most likely source • The extraction well (RW-IA) located on the Shell Oil property is not hydraulically controlling offsite migration of their MTBE, TBA, and TAME plumes This is based on the groundwater monitoring ' data collected, especially in monitoring well MW-8 Also, a data gap exists west of MW-7 without good lateral control of the plume extents • Severe infiltration of groundwater into the storm drain was identified near GP4 Samples collected from GP-4 had detections of MTBE at 3,400µg/L and TAME at 34µg/L 80 LIMITATIONS ' The stated purpose of tlus investigation was to identify potential sources of contamination (gasoline additives/oxygenates) that have impacted the City of Stockton MUD storm drain system along Country Club Blvd and Plymouth Road No other use of the results of this investigation is intended Condor developed the interpretations and conclusions presented herein in accordance with generally accepted principles and practice Conclusions presented in this report are professional opinions based on limited information obtained from the groundwater investigation conducted by Condor, public file reviews of the Shell Oil, Exxon Mobil, and 7-Eleven Store sites, and groundwater investigations conducted by ' Kleinfelder along Country Club Blvd on behalf of Kenko Contractor Inc , under contract with MUD Condor has endeavored to determine as much as practical about the site using conventional practices � I ' �� CONDOR