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<br /> 1 Groundwater Investigation Report
<br /> Country Club Drain System Project
<br /> • City of Stockton Municipal Utilities Department
<br /> October 2 2003
<br /> Page 3
<br /> ' to collect monthly compliance samples from their SVE and groundwater treatment effluent prior to
<br /> discharge into the City of Stockton sanitary storm drain Effluent samples from the onsite groundwater
<br /> extraction system are required to be collected and analyzed for BTEX, TPH-G, and MTBE, but not TBA
<br /> or TAME, which are also present in the groundwater Results are summarized in monthly compliance
<br /> reports submitted to SJCEHD The discharge limit for MTBE is 0 5 µg/l- TBA and TAME analyses are
<br /> not required under the current wastewater discharge permit, issued on October 23, 2000 and revised on
<br /> June 28, 2002 Between March 1999 and the present, an estimated 2,321,429 gallons of groundwater
<br /> have been extracted, treated, and pumped into the sanitary sewer The groundwater treatment system
<br /> installed by Shell Oil at this site is not designed to treat TBA and TAME It is not known if this sewer
<br /> ' line also contains leaks
<br /> Monitoring wells at this site were historically sampled for the oxygenates TAME and TBA, but analysis
<br /> for these compounds appears to have been discontinued in October 1999 in all site wells except MW-5,
<br /> 1 MW-8, and RW-lA MTBE has historically been detected in almost all the shallow wells, and TBA and
<br /> TAME have only been detected in MW-5, MW-8, and RW-lA The highest concentrations of MTBE,
<br /> TBA, and TAME were detected in the area of the former tank pit The range of concentrations detected,
<br /> the frequency of detections, and the most recent concentrations of TBA, TAME, and MTBE in wells
<br /> MW-5, MW-8, and RW-lA are summarized below in Table 1 The TBA and TAME isoconcentration
<br /> contours for the most recent groundwater monitoring data are shown on Figures 3 and 4, Appendix A,
<br /> respectively MTBE was not contoured due to its widespread and diffuse distribution Copies of the
<br /> groundwater quality tables from the most recent groundwater monitoring report (April 2003) are located
<br /> in Appendix C
<br /> ' Table 1—Range,Frequency,and Recent Detections of TBA,TAME,and MTBE
<br /> in Groundwater at Shell Oil Site
<br /> ' Range Frequency* Recent
<br /> TBA TAME MTBE TBA TAME MTBE 'FBA TAME MTBE
<br /> Well ID pg/L ug/L pg/L µg/- Nth Pt/-
<br /> MW-5 4 900-30,000 120-5,100 17,000 2,200,000 6121 events 14/22 events 29129 events 14,000 220 17 000
<br /> MW-8 72-4,700 21-1,100 1 2 130,000 4/22 events 7123 events 26/27 events 4,700 12 1,600
<br /> RW-1A 1,700-14 DOD 380-5300 180-690000 5/20 events 14/21 events 27/27 events 3 700 540 32 000
<br /> *Elevated detection lirruts may result in under-reporting the number of"detected"events
<br /> Detection limits for TBA and TAME in MW-5, MW-8, and RW-lA are significantly elevated,
<br /> particularly in the samples collected between September 1997 and April 2000 This is likely the result of
<br /> interference in the samples from elevated concentrations of TPH-G These detection limits have
<br /> decreased over time since the groundwater extraction system has been in operation
<br /> 2 2 EXXON MOBIL AND 7-ELEVEN SITES
<br /> The Exxon Mobil site is located at 2705 Country Club Blvd , the northwest corner of the Country Club
<br /> Blvd and Plymouth Road In February 1991, Exxon retained Alton Geoscience to conduct a preliminary
<br /> soil assessment The preliminary soil assessment was conducted to obtain an estimate of the extent of
<br /> petroleum hydrocarbon impacted soil prior to replacing the site USTs The site soil assessment indicated
<br /> . that high levels of petroleum hydrocarbons were present in the soil Exxon subsequently installed SVE
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<br /> �� CONDOR
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