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4 San Joaquin County DIRECTOR <br /> Dorina Heran,REHS <br /> Environmental Health Department F' ASSJSTANT DIRECTOR <br /> 2' y 600 East Main Street Laurie Cotulla,REHS <br /> Stockton, California 95202-3029 PROGRAM COORDINATORS <br /> Carl Borgman,REHS <br /> Mike Huggins,REHS, RDI <br /> 4tli ora Website: www.sjgov.org/ehd Margaret Lagorio,REHS <br /> Phone: (209) 468-3420 Robert McClellon,REHS <br /> 209 464-0138 ,teff Carruesco, REHS, RDI <br /> Fax: <br /> ( ) Kasey Foley, REHS <br /> DON PRATT (,SUN 0 7 W <br /> ATLANTIC RICHFIELD COMPANY <br /> P O BOX 1257 <br /> SAN RAMON CA 94583 <br /> RE: ARCO Service Station No. 2186 <br /> 3212 North California Street <br /> Stockton, California 94570 <br /> San Joaquin County Environmental Health Department (EHD) has reviewed analytical <br /> results for groundwater and vapor samples collected since August 30, 2004, when the soil <br /> vapor extraction (SVE) system was first operational as an interim remediation method at <br /> the above-referenced site. Concentrations for chemicals of concern (COC), including <br /> gasoline-range organics (GRO); benzene, toluene, ethylbenzene, and total xylenes <br /> (BTEX); 1,2-dichloroethane (1,2-DCA); and methyl-tertiary butyl ether (MTBE); have <br /> decreased significantly in groundwater samples collected from the two most-impacted <br /> monitoring wells MW-1 and MW-4. Also, vapor samples collected each month from the <br /> influent, intermediate vapor stream, and effluent of the SVE system and analyzed for <br /> GRO, BTEX, and MTBE have demonstrated non-detect concentrations for BTEX and <br /> MTBE since December 13, 2006; and low to non-detect concentrations for GRO in air <br /> samples collected from the influent. Based on these results, SECOR International <br /> Incorporated (SECOR) recommends suspending the SVE system operation for three <br /> months followed by the performance of a rebound test. EHD concurs with SECOR's <br /> recommendation to terminate the SVE operations for three months after which SECOR <br /> will perform a rebound test. <br /> Both the fourth quarter 2006 and first quarter 2007 groundwater monitoring reports <br /> indicate that the total well depth measured for MW-8 (the only deep-screened well) was <br /> approximately 131 feet below ground surface (bgs). This monitoring well has a screened <br /> interval from 129 to 134 feet bgs. EHD is concerned the results for groundwater samples <br /> collected from this well may be compromised because the more permeable silty sand in <br /> the screen interval is filled and only the less permeable clayey silt with sand is open to <br /> the screen. EHD directs you to redevelop MW-8 prior to the next sampling event. <br /> The Central Valley Regional Water Quality Control Board (CVRWQCB) has identified <br /> vapor intrusion of structures from underground storage tank sites as a potential health ' <br /> hazard; therefore, EHD directs you to evaluate the threat of vapor intrusion inside the <br /> building onsite and adjacent offsite structures. Please submit a work plan to EHD by <br /> August 10, 2007, to assess the potential for vapor intrusion into the existing onsitc and <br /> offsite buildings. <br />