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* ENVIRONMENTAL HEALTH DEPARTMENT <br /> SAN JOAQUIN COUNTY Unit Supervisors <br /> ..0 <br /> Z. 2 Donna K.Heran,R.E.H.S. 304 East Weller Avenue, Third Floor Carl Borgman,R.E.H.S. <br /> Mike Huggins,R.E.H.S.,R.D.I. <br /> ���� < Director <br /> AI Olsen,R.E.H.S. Stockton, California 95202-2708 Douglas W.Wilson,R.E.H.S. <br /> • ., - p• Program Manager Telephone: (209) 468-3420 Margaret Lagorio,R.E.H.S. <br /> 4C]Fb'n�� 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 DEC 16 2o02 <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, and Work Plan for Additional <br /> Site Assessment, dated 15 October 2002, for the above-referenced site - both prepared <br /> by Delta Environmental Consultants, Inc. (Delta). <br /> The site conceptual model (SCM) defines the geological framework of the site as <br /> consisting of an upper silty unit, a sandy unit and a lower silty unit. The upper silty unit, <br /> approximately 55 to 60 feet thick, contains several transmissive, thin, discontinuous <br /> tabular sandy 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 feet <br /> thick is identified as Sand 1, the lower sand stratum, Sand 2, is approximately 10 to 22 <br /> feet thick. The intervening silty stratum is 10 to 18 feet thick. The lower silty unit extends <br /> to the total depth explored, 202 feet below surface grade (bsg). The strata are inferred to <br /> dip 50 toward the southeast. <br /> The primary release of fuel hydrocarbons impacting the subsurface are inferred to have <br /> originated from the product lines and dispensers of the underground storage tank (UST) <br /> system; less significant releases occurred in the UST area. <br /> Regional ground water has not been encountered on the site, but perched ground water, <br /> believed to be of meteoric origin, has been encountered in Sand 1 and Sand 2, and is <br /> inferred to be migrating as a thin film along the base of each sand. Delta believes it likely <br /> that Sand 1 and Sand 2 has some vertical communication. Impacted ground water in <br /> Sand 1 is believed to be laterally delineated. Impacted ground water (and free product) <br /> has not been delineated toward the southeast. Approximately 56 gallons of free product <br /> has been recovered from MWA, which is located structurally down-slope from the <br /> dispenser area. Borings VW-18, VW-19 and VW-20, advanced southeast of MW-1, <br /> encountered low hydrocarbon concentrations in Sand 1, suggesting continued down-dip <br /> migration toward the southeast, but did not go deep enough to encounter Sand 2. Delta <br /> concluded that additional investigation of Sand 2 toward the southeast is warranted; <br /> EHD concurs with that conclusion. <br />