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ENVIRON'-T IENTAL HEALTH 11EPARTMENT <br /> SAN JOAQUIN COUNTY <br /> Q '? Donna K.Heran,R.E.H.S. Unit Supervisors <br /> Director 304 East Weber Avenue, Third Floor Carl Borgman,R.E.H.S. <br /> Al Olsen,R.E.H.S. Stockton, California 95202-2708 Mike Huggins,R.E.H.S.,R.D.I. <br /> • 04.;„ ,. P • Program ManagerDouglas W.Wilson,R.E.H.S. <br /> �iFba� Laurie A.Cotulla,R.E.H.S. Telephone: (209) 468-3420 Margaret Lagorio,R.E.H.S. <br /> Fax: (209) 464-0138 Robert McClellon,R.E.H.S. <br /> Program Manager Mark Barcellos,R.E.H.S. <br /> SHELL OIL PRODUCTS US SEP 2 ° 2005 <br /> DENIS L BROWN <br /> 20945 S WILMINGTON AVENUE <br /> CARSON CA 90810 <br /> RE: .Ripon Shell Service Station SITE CODE 506186 <br /> 341 East Main Street RO: 445 <br /> Ripon, California <br /> San Joaquin County Environmental Health Department (EHD) has reviewed Case <br /> Closure Summary (CCS) prepared and submitted by Cambria Environmental <br /> Technology, Inc. (Cambria) for the above-referenced site. Based on site history and <br /> current site conditions, Cambria believes that this site meets the Central Valley Regional <br /> Water Quality Control Board (RWQCB-CVR) definition of a low-risk fuel site, as <br /> described in Appendix A of the Tri-Regional Board Staff Recommendations for <br /> Preliminary Investigation and Evaluation of Underground Tank Sites, dated April 16, <br /> 2004. <br /> EHD agrees with Cambria's assessment that this site may be a low-risk fuel site based on <br /> non-detect analytical groundwater results submitted recently from quarterly monitoring <br /> events. However, EHD is not convinced the same is true for impacted soil. Methyl <br /> tertiary butyl ether (MTBE)-impacted soil concentrations are documented in soil samples <br /> T-3 East, S-2, S-5, S-8, X-1, X-2, MW-1, CPT-1, and EW-1 in an area approximately 60 <br /> feet by 40 feet, and extending from approximately 5 feet below ground surface (bgs) in S- <br /> 5 to 35 feet bgs in MW-1 and CPT-1. As the soil lithology in this area tends to be <br /> primarily sand and silt, with very little clay, the released contaminants are more likely to <br /> descend through the subsurface to groundwater with minimal lateral diffusion in the <br /> vadose zone. However, confirmation borings have not been advanced to document a <br /> reduction of MTBE mass in the vadose zone soil and the issue of impacted soil was not <br /> addressed in the CCS. <br /> Although recent groundwater analytical data suggests that impacted vadose zone soil is <br /> not currently acting as a secondary source to impact ground*ater, EHD must take a <br /> conservative approach and require documentation that impacted soil will not impact <br /> groundwater on this site again. Submit to EHD either: <br /> • A mass estimate that includes calculations along with a fate and transport or <br /> .leaching model for residual sorbed MTBE utilizing existing data, which is <br /> assumed would present the more conservative case, or <br />