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ENVIRONMENTAL HEALTH SEPARTMENT <br /> SAN JOAQUIN COUNTY <br /> Program Coordinators <br /> Donna K. Horan, R.E.xs. 304 East Weber Avenue Third Floor Carl Bor man,R.E.H.S. <br /> Director ' Borg <br /> man, <br /> A. Cotulla,R.E.H.S. Stockton,California 95202-2708 Mike Huggins,R.E.H.S.,R.D.I. <br /> o4c Foe " Assistant Director (Telephone: 209)468-3420 Margaret Lagorio,R.E.H.S. <br /> ll Robert McClellon, R.E.H.S. <br /> Fax: (209)464-0138 Jeff Can uesco,R.E.H.S <br /> Website:www.sjgov.org/ehd/ Kasey Foley, R.E.H.S. <br /> DON PRATT 'AUG 3 0 2006 <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 Quarterly Groundwater Monitoring Report, First Quarter 2006 (QR), <br /> dated 28 April 2006, which had been prepared and submitted by Stratus <br /> Environmental, Inc. (Stratus) for the above-referenced site. A well installation <br /> report for monitoring wells MW-16 through MW-20 has not yet been received. <br /> The QR indicates that measurable free product, or light non-aqueous phase <br /> liquid (LNAPL), is present in monitoring well MW-15 and LNAPL sheen is present <br /> in monitoring well MW-1 and vapor wells VW-1 and VW-4. During the first quarter <br /> 2006, the soil vapor extraction (SVE) system operating on the site removed and <br /> destroyed approximately 37,844 pounds of hydrocarbons, for an approximate <br /> total of 77,507 pounds removed and destroyed since initiation of the SVE <br /> system. <br /> LNAPL was first encountered in monitoring well MW-1 shortly after its installation <br /> in September 2000, thirteen years after the former underground storage tank <br /> (UST) system was replaced and site characterization had begun. Since LNAPL <br /> removal began, approximately 267 gallons of mixed LNAPL and water have been <br /> removed from the LNAPL-impacted wells, primarily MW-1. <br /> Cross sections previously prepared for the site (Subsurface Assessment Report <br /> and Site Conceptual Model Update, 03 September 2004) show that the sand unit <br /> yielding the sheen in VW-1 and VW-4 is not the same sand unit that yields <br /> LNAPL and sheen to MW-15 and MW-1. The groundwater has a reported <br /> gradient of 0.108 in the upper sand interval (VW-1 and V-4) and 0.175 in the <br /> lower sand interval (MW-1 and MW-10). <br /> Based on the site conceptual models (SCM) previously prepared by Stratus, Inc. <br /> for this site, the EHD believes that thin intervals or films of perched water are <br />