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San Joaquin County DIRECTOR <br /> Donna Heran,REHS <br /> °P'" Environmental Health Department <br /> F� ASSISTANT DIRECTOR <br /> 600 East Main Street Laurie Cotulla,REHS <br /> Stockton, California 95202-3029 PROGRAM COORDINATORS <br /> Carl Borgman,REHS <br /> Mike Huggins,REHS,RDI <br /> Website: www.s ov.or /ehd <br /> 9�rFO R� �g A Margaret Lagorio,REHS <br /> Phone: (209) 468-3420 Robert McClellon,REHS <br /> Fax: (209) 464-0138 .teff Carruesco, REHS,RDI <br /> Kasey Foley,REHS <br /> DON PRATT JUN 2 5 2007 <br /> ENVIRONMENTAL BUSINESS MANAGER <br /> ATLANTIC RICHFIELD COMPANY <br /> P O BOX 1257 <br /> SAN RAMON CA 94583 <br /> RE: ARCO #6100 SITE CODE: 1708 <br /> 25775 Patterson Pass Road <br /> Tracy, CA <br /> San Joaquin County Environmental Health Department (EHD) has received and <br /> reviewed Offsite Soil Vapor Extraction Testing Work Plan (WP), dated 5 April <br /> 2007, prepared and submitted by SECOR International Inc. (SECOR) for the <br /> above-referenced site. A copy of Summary of December 7, 2006 Meeting (letter) <br /> dated 19 December 2006, also prepared by SECOR, was included in WP as an <br /> appendix. <br /> The EHD approves the scope of work proposed in WP as necessary and <br /> adequate for this phase of work; however the EHD does not agree with the <br /> summary of points of agreement for this site noted in the 19 December 2006 <br /> letter. Specifically, the EHD does not recall and could not agree to waive the fate <br /> and transport evaluation of the light nonaqueous phase liquids (LNAPL). <br /> Based on the site characteristics and the known contaminant distribution noted in <br /> the EHD letter of 30 August 2006 the EHD is concerned that a very large mass of <br /> LNAPL has bypassed the wells currently impacted by LNAPL and may have <br /> been missed in the off-site investigation to date. Although the migrating LNAPL <br /> thickness is not thought to be great, over 18 years of migration of the LNAPL <br /> plume, potentially over 45 feet wide at gradients on the order of 0.108 to 0.175, <br /> can put a very large mass of LNAPL down-gradient of South Patterson Pass <br /> Road — a mass potentially too large for either you or the EHD to responsibly <br /> ignore. <br /> The EHD believes the misunderstanding regarding the LNAPL evaluation, as <br /> directed by EHD letter dated 30 August 2006, may have originated from EHD's <br /> concurrence with addressing the LNAPL through soil vapor extraction (SVE). <br /> While SVE may effectively address the LNAPL thought to occur in a thin layer on <br /> and near the site, it is necessary to have as complete an understanding of the <br /> LNAPL plume as can be practicably achieved to effectively and properly manage <br />