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Date January 26, 2004 ARCO QUARTERLY GROUNDWATER MONITORING REPORT Facility No 6100 Address 25775 Patterson Pass Road, Tracy, CA ARCO Environmental Engineer Paul Supple Consulting Co(Contact Person Stratus Environmental, Inc 1 Jay R Johnson, R G Consultant Project No U6100-14-4 Primary Agency/Regulatory ID No Nuel Henderson 1 San Joaquin County Environmental Health Department 1 1708 BRIEF HISTORY Four gasoline UST's, one waste oil UST, and associated product piping were replaced in 1987 Between 1991 and 1995 a total of 31 soil borings were drilled to depths between 26 and 115 feet below ground surface (bgs) Twenty vapor extraction wells (VW-1 through VW-20) were installed in selected borings Pinnacle Environmental Solutions, Inc submitted a request for "No Further Action" to the RWQCB on February 1, 2000 The RWQCB denied the request, and requested a groundwater investigation One soil boring (B-32) was drilled at the site to a depth of approximately 200 feet bgs in August 2000 Groundwater monitoring well MW-1 was installed in this boring to a depth of approximately 115 feet bgs Fallowing construction of the well, free product was observed in the well Floating product has been • removed manually from the well on a weekly basis since September 2000 During August and September 2001, Delta advanced six onsite mud rotary soil borings (B-33 through B-38) to depths up to 140 feet bgs In each boring, soil samples were collected continuously and an open hole geophysical log completed An air rotary casing hammer rig was used to Install a well (B-2 through B-9) adjacent to each of the soil borings The dispensers and product piping were replaced during the first quarter 2002 Delta collected compliance soil samples between January 18 and February 11, 2002 lateral conveyance piping was installed to selected vapor and monitoring wells for future use in soil vapor extraction Piping was stubbed up in the northwest corner of the property which is the proposed future location of the treatment system compound WORK PERFORMED THIS QUARTER(Fourth 2004) 1 A Stratus technician conducted eleven site visits during fourth quarter 2003 to measure the thickness and remove floating product from well MW-1 When present, floating product in VW4 was also measured and bailed 2 Stratus monitored all vapor extraction and monitoring wells for the presence of water and free product on December 22, 2003 Stratus returned to the site on December 23, 2003 and attempted to collect a water sample from every well where the presence of water was detected Groundwater samples were collected from wells MW-2, VW-5, and the domestic well In most of the other wells, the thickness of water in the well (if present at all) was not great enough to enter the bailer 3 Stratus compiled and evaluated the fourth quarter 2003 monitoring and sampling data 4 Installed remediation piping manifold, system enclosure, and field tested all equipment P\Bp-Arco for URS161001Quarterly120031Arco_6100403 doc STleA7U5