SITE INFORMATION AND CORRESPONDENCE

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ENVIRONMENTAL HEALTH DEPARTMENT pP�'u'IN C SAN JOAQUIN COUNTY Unit Supervisors Donna K.Heran,R.E.H.S. 304 East Weber Avenue, Third Floor Carl Bergman,R.E.H.S. 4`< Director Mike Huggins,R.E.H.S.,R.D.I. At Olsen,R.E.H.S. Stockton, California 95202-2708 Douglas W.Wilson,R.E.H.S. Program Manager Telephone: (209) 468-3420 Margaret Lagorio,R.E.H.S. ��FoA Laurie A.Corolla,R.E.H.S. Fax: (209)464-0138 Robert McClellon,R.E.H.S. Program Manager Mark Barcellos,R.E.H.S. DEC 0 2 2002 PAUL SUPPLE GARY SCANNAVINO ATLANTIC RICHFIELD COMPANY SCANNAVINO PROPERTIES PO BOX 6549 5463 CHEROKEE LANE MORAGA CA 94570 STOCKTON CA 95215 RE: ARCO Station #595 SITE CODE: 0707 6100 N. Highway 99 Stockton CA 95212 San Joaquin County Environmental Health Department (SJC/EHD) has reviewed the "Site Assessment Report" dated September 13, 2002 that was prepared on your behalf by SECOR International Incorporated (SECOR) and has the following comments. The report documents additional site investigation that took place in June 2002. Three shallow monitoring wells and three deep monitoring wells were installed on and off site, to further investigate the lateral and vertical extent of the petroleum hydrocarbon plume migrating from this site. Soil samples for analysis were collected from all borings; results were non-detect for all tested constituents. This indicates that the vertical and lateral extent of the soil contamination has been defined. The new monitoring wells were developed on July 11-12, 2002, and sampled during a routine quarterly groundwater monitoring and sampling event on July 16-17, 2002. Analytical results from the new wells were all non-detect for total petroleum hydrocarbons as gasoline and benzene, but had minor concentrations of toluene, ethylbenzene and xylenes (BTEX), as well as methyl tertiary butyl ether, tertiary butyl alcohol and 1,2-dichloroethane. The quality control trip blank also had minor concentrations of the BTEX components. SECOR calculated three gradient flow directions from the depth to water data collected from the monitoring wells on July 11-12, 2002, one for the shallow zone onsite, one for the shallow zone offsite and one for the deep zone. The gradient direction in the deep zone was determined to be towards the southeast, which is nearly opposite of that in the shallow zones. However, the measured depth to water in DMW-2 was approximately 10 feet more that in all the other wells. Depth to water measurements taken on July 11-12, 2002, when the new wells were developed, did not show this discrepancy. SJC/EHD suspects that either a measurement error was made or that the deeper zone was significantly influenced by a pumping influence from the onsite domestic well or another nearby water well. This could be very significant to plume migration. SJC/EHD recommends conducting a pumping test utilizing the domestic well to extract groundwater and existing monitoring wells as observation points.