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URS Mr Harlin Knoll Page 2 San Joaquin County Environmental Health Department May 19, 2003 City of Stockton Department of Municipal Utilities (CSDMU), well 3-2 was drilled to a total depth of approximately 625 feet Welt 3-2 is sealed with cement grout from the surface to 144 feet bgs and with bentonite from 376 to 386 feet bgs The well is screened from 144 feet bgs to 376 feet bgs, and from 386 feet bgs to 625 feet bgs Previous Investigations Eleven soil borings were advanced to depths of up to 80 feet below ground surface (bgs) during site investigations conducted at the site between 1992 and 1995 Soil analytical results for samples collected from these borings indicated up to 0 15 milligrams per kilogram (mg/kg) of benzene and up to 0 37 mg/kg of (suspected) methyl tent-butyl ether (MtBE) No hydrocarbons were reported in the deepest analyzed samples or at depth in adjacent borings (e g , borings B-4 and B-5) Based on these results,the extent of hydrocarbons in soil was considered defined In 1998, TOSCO replaced the underground storage tanks, dispensers, and associated piping at the site Soil analytical results for samples collected beneath the underground storage tanks reported up to 0 0083 mg/kg benzene, 24 mg/kg MtBE, and 120 mg/kg tert-butyl alcohol (TBA) • In December 2001, four monitoring wells were installed at the site During the installation of these wells, groundwater was first observed at depths ranging from 40 to 45 feet bgs Total petroleum hydrocarbons as gasoline (TPHg), MtBE, and TBA were reported in soil samples at maximum concentrations of 12, 0 97, and 2 66 mg/kg, respectively, in well MW-3 Concentrations of TPHg and MtBE were reported above 20,000 micrograms per liter (µg/L) in well MW-3 during the first quarter 2003 Based on historic monitoring data, the inferred direction of groundwater flow beneath the site fluctuates between northwest and southwest with a horizontal hydraulic gradient of 0 001 to 0 003 feet per foot ASSESSMENT ACIVITIES AND FINDINGS To monitor the down-gradient extent of groundwater impacts, two off-site groundwater monitoring wells were installed in the parking lot of the Union Safe Bank, located to the south of the site, across Hammer Lane One on-site well was installed to monitor the vertical extent of groundwater contamination The following paragraphs present a discussion of drilling and monitoring well installation, the subsurface conditions encountered during drilling, and monitoring well development and surveying i K 1WprocessU5 BP ARCOU 11911Well Installationlwell installation report03 doc