ARCHIVED REPORTS XR0008702 - Laserfiche WebLink

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Tosco Corporation A Subsidiary of Phillips Petroleum Company 2 Tosco Service Station No 4409, 1502 North El Dorado Street,Stockton,California May 29,2002 . activities included source removal of obviously impacted soil in the area beneath the former dispenser and product lines A concrete pad restricted the lateral extent of excavation During excavation of Impacted soil, unidentified fiberglass piping was found Additional excavation and sampling was completed on April 8, 1999 to remove the unidentified piping Samples were collected from the bottom and sidewalls of the excavated area Following removal of the former station canopy, canopy footings and kiosk, additional over-excavation was completed in the dispenser area Over-excavation was terminated when obviously impacted soil was removed, the structural integrity of the sidewalk was endangered, or groundwater was encountered A total of approximately 1,707 tons of soil was transported off-site for disposal at the Forward Landfill in Manteca, California On December 9 and 10, 1999, four groundwater monitoring wells (MW-1 through MW-4) were installed to investigate the extent of petroleum hydrocarbons in soil and groundwater beneath the site Well MW-1 was completed at 50 feet bgs, and wells MW-2 through MW-4 were completed at 45 feet bgs Petroleum hydrocarbons were not detected in any soil samples from beneath the site Groundwater sampling from the new wells was initiated on January 5, 2000 Concentrations of Total petroleum hydrocarbons as gasoline (TPHg), benzene, toluene, ethylbenzene, and xylenes (BTEX), and methyl tertiary butyl ether (MtBE) were generally not detected in the wells, with the exception of minor concentrations of benzene and/or MtBE (EPA Method 8260) detected in wells MW-3 and MW-4 Initial depth to water was approximately 28 to 29 feet bgs The direction of groundwater flow was established in the easterly direction, at an approximate gradient of 0 003 foot/foot According to IT's Site Assessment Report, dated February 25, 2000, IT performed a water supply well search to locate any water supply wells within 2,000 feet of the subject site Review of well logs at the California Department of Water Resources (DWR) indicated the presence of one water supply well within the 2,000-foot search radius, located at 1521 N Hunter Street Well construction details and usage of the well was not documented on the DWR well log The SJCPHD/EHD attempted to locate the well, however the well was not found On July 20 and 21, 2000, four Geoprobe borings (GP-1 through GP-4) were completed by IT to a depth of 60 feet bgs to investigate the vertical extent of petroleum hydrocarbons in soil and groundwater Petroleum hydrocarbon-impacted soil and groundwater were encountered at depths of approximately 50 to 60 feet bgs Based on the results of this investigation, IT recommended that an additional investigation be performed to determmne the vertical extent of petroleum hydrocarbons in soil beyond 60 feet bgs, and the installation of additional groundwater monitoring wells for assessment of deeper zone groundwater impacts On January 22, 2001, three deep on-site groundwater monitoring wells (MW-5, MW-6, and MW-7) were installed by IT to depths of 80 feet bgs to delineate the vertical extent of petroleum hydrocarbons in groundwater The highest concentrations of petroleum I\Tosco Markedng\4409\Reports\AsmtRpt doe