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ENVIRONMENTAL HEALTH DEPARTMENT <br /> o SAN JOAQUIN COUNTY <br /> UnitSupervisors <br /> 2` - ? Donna K.Heran,R.E.H.S' 304 East Weber Avenue, Third Floor Carl Borgman,R.E.H.S. <br /> Q: S <br /> Director Mike Huggins,R.E.H.S.,R.D.I. <br /> Al Olsen,R.E.H.S. Stockton, California 95202-2708 Douglas W.Wilson,R.E.H.S. <br /> ..cg4voe?gip.. Program Manager Telephone: (209) 468-3420 Margaret Lagorio,R.E.H.S. <br /> Laurie A.Corolla,R.E.H.S. Fax: (209) 464-0138 Robert McClellon,R.E.H.S. <br /> Program Manager Mark Barcellos,R.E.H.S. <br /> KAREN PETRYNA SEP 0 1 2961,4 <br /> SHELL OIL PRODUCTS US <br /> 20945 S WILMINGTON AVE <br /> CARSON CA 90810 <br /> RE: Shell Service Station #204-7524-4 Site Code:1883 <br /> 3011 W. Ben Holt Dr. RO#: 0000486 <br /> Stockton, CA., 95207 CUF#: 015906 <br /> San Joaquin County Environmental Health Department (SJCEHD) has reviewed <br /> Cambria's Site Investigation Report, Site Conceptual Model, and Ground Water <br /> Monitoring Report— Fourth Quarter 2003 & First Quarter 2004 dated August 16, 2004 <br /> and has the following comments. <br /> This large combination report included three new monitoring well installations from <br /> October 2003, ground water sampling events on November 26, 2003 and February 17, <br /> 2004, and a conceptualized site model (SCM). <br /> Based on analytical data collected at and around the site since the initial investigation <br /> during August 1988, Cambria has concluded that the soil and ground water plumes <br /> from this underground storage tank (UST) release have been delineated. SJCEHD has <br /> reviewed the data and concurs that the soil contamination has been adequately <br /> investigated and will not require any additional soil investigation at this time. <br /> With MtBE contamination (570 ug/1) in the most northwesterly monitoring well, MW-5, <br /> and in the most northerly, down gradient wells, MW-11 and MWA2, SJCEHD cannot <br /> concur that the full lateral extent of the MtBE impact is defined. Since May 16, 2002, <br /> ground water samples from MW-5 have consistently contained MtBE contamination <br /> ranging between 490 and 790 ug/1 and ground water samples from MW-11 and MW-12 <br /> have had MtBE concentrations as high as 5.3 ug/l and 94 ug/I, respectively. <br /> Cambria stated that Shell discontinued use of MtBE in their gasoline in January 2003 <br /> and that MtBE is attenuating as shown by decreasing MtBE concentrations and rising <br /> TBA levels. SJCEHD is unable to agree with the MtBE attenuation interpretation for the <br /> MW-5 data. <br /> MW-5 has been consistently sampled and analyzed for TBA since December 2002 <br /> which has been reported for all events "<50" (ug/1). It could also be suggested that a <br /> plume of fairly constant MtBE impacted water is migrating in this north-northwesterly <br /> direction that is not attenuating (forming TBA) as suggested. <br />