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1 <br /> ENVIRONMENTAL HEALA DEPARTMENT <br /> SAN JOAQUIN COUNTY <br /> Unit Supervisors <br /> Donna K.Heran,R.E.H.S. Carl Bergman,Q, 304 East Weber Avenue, Third Floor man,R.E.H.S. <br /> g <br /> Director Mike Huggins,R.E.H.S.,R.D.I. <br /> Al Olsen,R.E.H.S. Stockton, California 95202-2708 <br /> Douglas W.Wilson,R.E.H.S. <br /> • cq.. TP • Program Manager Telephone: (209) 468-3420 Margaret Lagorio,R.E.H.S. <br /> (iF59N Laurie A.Cotulla,R.E.H.S. Robert McClellon,R.E.H.S. <br /> Program Manager Fax: (209) 464-0138 Mark Barcellos,R.E.H.S, <br /> PAUL SUPPLE AUG 2 7 2DD3 <br /> ARCO PRODUCTS COMPANY <br /> P O BOX 6549 <br /> MORAGA CA 94570 <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 Site Conceptual Model, dated 03 October 2002, Remedial Action Plan <br /> (RAP), dated 01 July 2003, and Quarterly Groundwater Monitoring Report, <br /> Second Quarter 2003, dated 15 July 2003, all prepared for the above-referenced <br /> site by Delta Environmental Consultants, Inc. (Delta). <br /> As noted in the EHD letter dated 16 December 2002, the site conceptual model <br /> (SCM) defines the geological framework of the site as consisting of an upper silty <br /> unit, a sandy unit and a lower silty unit. The upper silty unit, approximately 55 to <br /> 60 feet thick, contains several transmissive, thin, discontinuous tabular sandy <br /> lenses. The sandy interval, 30 to 60 feet thick, consists of two laterally <br /> continuous sand strata separated by a silt unit. The upper sand stratum, 5 to 15 <br /> feet thick is identified as Sand 1, the lower sand stratum, Sand 2, is <br /> approximately 10 to 22 feet thick. The intervening silty stratum is 10 to 18 feet <br /> thick. The lower silty unit extends to the total depth explored, 202 feet below <br /> surface grade (bsg). The strata are inferred to dip 50 toward the southeast. <br /> The primary release of fuel hydrocarbons impacting the subsurface are inferred <br /> to have originated from the product lines and dispensers of the underground <br /> storage tank (UST) system; less significant releases occurred in the UST area. <br /> Regional ground water has not been encountered on the site, but perched <br /> ground water, believed to be of meteoric origin, has been encountered in Sand 1 <br /> and Sand 2, and is inferred to be migrating locally along the base of each sand <br /> unit. Delta believes it likely that Sand 1 and Sand 2 has some vertical <br /> communication. Impacted ground water in Sand 1 is believed to be laterally <br /> delineated. Impacted ground water (and free product) has not been delineated in <br /> Sand 2 toward the southeast. Approximately 56 gallons of free product had been <br /> recovered from MW-1, which is located structurally down-slope from the <br /> dispenser area. Borings VW-18, VW-19 and VW-20, advanced southeast of <br />