SITE INFORMATION AND CORRESPONDENCE FILE 1

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ENVIRONMENTAL HEALTH DEPARTMENT . SAN JOAQUIN COUNTY Unit Supervisors Donna K. Heran,. R.&H.S. 304 East Weber Avenue, Third Floor Carl Borgman, R.E.H.S. e Director Mike Huggins, R.E H.S., R.D.I. LL�SY.��. Al Olsen, R.E.Hs. Stockton, California 95202-2708 Douglas W. Wilson, R.E.H.S. • ��P Program Manager Telephone: (209) 468-3420 ' Margaret Lagorio, R.E.H.S . F oe Laurie A. Cotulla, R.E.H.S. Robert McClellon, R.E.H.S. Program Manager Fax: (209) 464-0138 Mark Barcellos, R.E.H.S. PAUL SUPPLE OCT 0 3 2003 ATLANTIC RICHFIELD COMPANY PO BOX 6549 MORAGA CA 94570 RE: Arco Station #2130 Site Code 1094 7906 N . EI Dorado St. RO#: 035 Stockton, CA, 95210 San Joaquin County Environmental Health Department (EHD) has reviewed Site Conceptual Model Report, (SCM) dated June 6 , 2003 as submitted by URS consultants on June 26, 2003. The report covers this site's lengthy investigation and provides a summary of the status of the site up to December 2002. EHD comments as follows : The approximately 20-foot rise in groundwater elevation at this site since the majority of monitoring and vapor wells were installed has seriously degraded effectiveness of the soil vapor extraction — air sparging (SVE AS) remediation system. The SVE has been non-operational since June 1998 , and since that time, only `low flow, ambient air injection' has been in operation at this site as the 'remedial action' . Conclusions in the SCM included restated findings and data from the report, an opinion that impacted ground water is unlikely to migrate vertically or laterally to a significant extent, and presented the interpretation that groundwater conditions at the site have benefited from the historical use of SVE and air injection. While the latter is intuitively probably correct, no pre- or post-remediation contaminant mass calculations were included in the SCM to support this statement and EHD is unable to concur at this time. The opinion that impacted ground water is unlikely to migrate significantly either vertically or laterally may also be correct, but should be demonstrated through use of a mathematical transport model . Default or reasonably estimated parameters can be used for such modeling for unknown variables such as hydraulic conductivity. Recent groundwater sampling events (June 11 , 2003) indicate that only VW-10 yields ground water samples containing significant concentrations of petroleum constituents of concern. (TPHg @14, 000 ug/l , benzene @ 1 ,600 ug/I , and MtBE @ 3,300 ug/l, the units were shown in Table 1 as "mg/f' , but are given on the laboratory report sheets as "ug/I") . Only VW-10 is located in the area of residual contamination and only VW-10 is screened across the current vadose/saturated soil interface (depth to water on June 11 , 2003 was 38.66 feet below ground surface). VWA0 has had only three groundwater sampling events conducted and all VW-10 ground water samples have contained elevated levels of contaminants of concern . No air injection well is located near VW-10 and no remedial activity is in operation or proposed at this area.