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1` <br /> San Joaquin County • DIRECTOR <br /> aQ i N Donna Heran, RENS <br /> Environmental Health Department ASSISTANT DIRECTOR <br /> 600 East Main Street Laurie Cotulla, REHS <br /> Stockton, California 95202-3029 PROGRAM COORDINATORS <br /> Carl Borgman, REHS <br /> i Mike Huggins, REHS,RDI <br /> Website: www.sjgov.org/ehd Margaret Lagorio,REHS <br /> Phone: (209) 468-3420 Robert McClellon,REHS <br /> Fax: (209) 464-0138 .teff Carruesw,REHS, RDI <br /> JUN 2 5 2007 Kasey Foley, REHS <br /> CONOCOPHILLIPS ARCO-BP <br /> SHELBY LATHROP DON PRATT <br /> 76 BROADWAY PO BOX 1257 <br /> SACRAMENTO CA 95812 SAN RAMON CA 94583 <br /> RE: Tosco-ConocoPhilips (former Unocal #4409) Site Code: 508431 <br /> 1502 N. EI Dorado St. RO#:0664 <br /> Stockton, CA 95204 <br /> RE: ARCO #4932 Site Code: 1136 <br /> 16 E. Harding Way RO#:0671 <br /> Stockton, CA 95204 <br /> The San Joaquin County Environmental Health Department (EHD) has reviewed the <br /> ConocoPhilips (CP) and ARCO soil and ground water data for the above referenced <br /> sites. Prior to July 2006 (third quarter), the EHD had concurred with CP consultant's <br /> interpretation that the limits of the site's dissolved gasoline constituent contamination <br /> were defined and that no additional investigation was necessary. Recent <br /> groundwater sample analytical data, particularly that for CP monitoring well MW-5, <br /> has raised the EHD concerns that the CP ground water plume has become unstable <br /> and may be migrating. <br /> In recent reports, CP has concluded that the substantial increase in MtBE, GRO, and <br /> TBA concentrations in the second zone below the CP site are "likely due to migration <br /> of dissolved concentrations from the upgradient ARCO gasoline service station site." <br /> To test this conclusion, the EHD has reevaluated both sites' soil and ground water <br /> data and comments as follows: <br /> The depth to water (DTW) at both sites commonly has been approximately 30 feet <br /> below ground surface (bgs). SECOR International Corporation (SECOR), on behalf of <br /> CP, has defined three hydrological zones based on the screened intervals of the <br /> groundwater monitoring wells associated with that site. The first ground water has <br /> been designated the "A" zone and is monitored by wells MW-1 through MW-4, <br /> screened to depths as great as 45 to 50 feet bsg. Two deeper permeable zones <br /> have been designated as "B" zone, generally screened between 78 and 83 feet bsg <br /> and the "C" zone is defined by three monitoring wells screened at approximately 110 <br /> to 120 feet bsg. CP has installed wells in the A, B, and C zones; the ARCO site wells <br /> are screened only in the A and B zones. <br />