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San Joaquin County <br /> Environmental Health Department DIRECTOR <br /> Donna Heran,REHS <br /> r` 600 East Main Street <br /> v. PROGRAM COORDINATORS <br /> Stockton, California 95202-3029 Robert McClellan, REHS <br /> Jeff Carruesco,REHS, RDI <br /> • Cq�iFO,Rij�P Website: www.sjgov.org/ehd Kasey Foley, REHS <br /> Phone: (209)468-3420 <br /> Fax: (209)464.0138 <br /> June 1, 2010 <br /> ConocoPhillips Company <br /> Mr. Terry L. Grayson <br /> 76 Broadway <br /> Sacramento, CA 95812 <br /> Subject: 76 Station #5886 AP#: 114-060-05 <br /> 2701 W. March Lane LOP Case#: 1176 <br /> Stockton, CA 95215 Global ID#: T0607700718 <br /> The San Joaquin County Environmental Health Department (EHD) has reviewed <br /> Request for Low-Risk Case Closure dated May 14, 2010, submitted by your consultant, <br /> Stantec Consulting Corporation. Methyl tertiary-butyl ether (MtBE) is the constituent of <br /> concern at this site. Historically elevated concentrations of MtBE dissolved in the <br /> shallow groundwater onsite have migrated southwesterly offsite across March Lane, <br /> showing the biggest impact in groundwater samples collected from monitoring well <br /> MW-9D. <br /> MW-91D is screened across the deepest depth interval monitored, 30 feet to 35 feet <br /> below ground surface ('bgs) and, as of the last groundwater sampling event (January 5, <br /> 2010) contained the highest concentration of MtBE (50 ug/1) for the site. Groundwater <br /> samples collected from MW-9D since January 2008, have shown an upward trend of <br /> MtBE concentrations from less than 1 ug/I, to a peak of 150 ug/I on April 1, 2009. Since <br /> this time, the MtBE groundwater concentrations in MW-91D have fallen. The most recent <br /> MtBE concentration, 50 ug/I on January 5, 2010 supports an interpretation that the worst <br /> of the impact offsite has passed and that the site may qualify for no further action if <br /> groundwater MtBE concentrations continue to drop. <br /> To verify this, the EHD directs ConocoPhillips to collect a groundwater sample from <br /> MW-91D and analyze it for MtBE by EPA Method 8260B. If the laboratory data <br /> demonstrates that MtBE concentrations in samples from MW-9D are continuing to drop, <br /> ConocoPhillips should submit a No Further Action Report (NFAR) to the EHD. <br /> Please include a soil vapor intrusion evaluation, based on residual soil and/or <br /> groundwater contamination concentrations to show whether or not the residual <br /> contaminants pose a health threat to the public through vapor intrusion, direct exposure, <br /> or gross contamination, when compared to the San Francisco Bay Regional Water <br /> Quality Control (SFBRWQCB) Environmental Screening Levels (ESLs). The NFAR is to <br /> include all of the items listed in Section 6.6 of the Tri-Regional Board Staff <br /> Recommendations for Preliminary Investigation and Evaluation of Underground Storage <br /> Tank Sites, Appendix A - April 2004 and a time line showing when MtBE concentrations <br /> will meet water quality objectives (WOOS). <br />