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i <br /> yt� _ <br /> PpU1N <br /> San Joaquin County DIRECTOR <br /> o• �''`=".•.c Donna Heran, RENS <br /> Environmental Health Department <br /> W.; :{ ASSISTANT DIRECTOR <br /> 600 East Main Street Laurie Cotulla, REHS <br /> Stockton, California 95202-3029 <br /> " PROGRAM COORDINATORS <br /> Carl Borgrnan, RENS <br /> c9�ffioa`a\P Mike Huggins, RENS,RDI <br /> Website: www.Si9ov.or9/ehd Margaret Lagorio, REHS <br /> Phone: (209)468-3420 Robert McCletlon,REHS <br /> Fax: (209) 464-0138 Jeff Carruesco,RENS, RDl <br /> Kasey Foley, REHS <br /> ERIC HETRICK <br /> CONOCOPHILLIPS COMPANY MAY 2 4 2007 <br /> 76 BROADWAY <br /> SACRAMENTO CA 95818 <br /> RE: 76 Service Station No. 11193 <br /> 3202 West Hammer Lane <br /> Stockton, California <br /> San Joaquin County Environmental Health Department (EHD) has reviewed analytical <br /> results for groundwater samples collected quarterly from both on- and off-site monitoring <br /> wells. Although an ozone sparge system has been operating for three years, reducing the <br /> concentrations for chemicals of concern onsite, high concentrations of methyl tertiary-butyl <br /> ether (MTBE) continue to be detected in groundwater samples collected from oft-site <br /> monitoring wells MW-11 and MW-12. Recent data show the groundwater flow direction <br /> to be toward the southwest; however, the groundwater flow direction rose diagram that <br /> includes data from the fourth quarter 2000 through first quarter 2007 indicates the flow <br /> direction has historically been to the northeast in the direction of MW-11 and MW-12. <br /> Further investigation of contamination in groundwater will be necessary to assess the <br /> lateral extent of impacted groundwater down gradient of MW-11 and MW-12. <br /> In July 2004, two cone penetrometer test (CPT) borings were advanced to approximately <br /> 120 feet below surface grade (bsg) onsite. Concentrations of diesel range organics (DRO) <br /> were detected in all six groundwater samples collected from these two borings, including <br /> DRO concentrations of 180 micrograms per liter (gg/L) at total depth of 115 feet bsg in <br /> CPT-1, and 120 pg/L at total depth of 105 feet bsg in CPT-2. EHD believes further <br /> groundwater investigation is necessary to assess the vertical extent of hydrocarbon <br /> contamination in the areas of CPT-1 and CPT-2. Based on soil lithology from borings <br /> CPT-1 and CPT-2, EHD recommends that a monitoring well screened in either the thin <br /> sand layer between 110 and 115 feet bsg near CPT-1 or a monitoring well screened in the <br /> thin sand Iayer between 100 and 105 feet bsg near CPT-2 be installed.. <br /> Please submit a work pian to EHD by July 31, 2007, to investigate the lateral extent of the <br /> MTBE groundwater plume down gradient of MW-I1 and MW-12, and to assess the <br /> vertical extent of groundwater contamination onsite. <br /> Monitoring well MW-12 has inadvertently been displayed on site maps submitted in <br /> quarterly groundwater monitoring reports as located in the sidewalk adjacent to West <br /> Hammer Lane. Please revise future site maps to indicate the correct location for MW-12 to <br />