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• S E C 0 R Mr Mike Infurna April 7, 2003 Page 2 Groundwater samples collected from newly installed wells contained maximum concentrations of TPHg and MtBE of 29,000 ppb (MW-1) and 4,000 ppb (MW-2), respectively(PEG, 1999) In December 2000, IT drilled and installed five groundwater-monitoring wells (MW-5 through MW-9) to approximate depths of 24 feet bgs Laboratory analysis of sod samples reported maximum concentrations of TPHg and MtBE of 473 ppm (MW-9-10') and 0 119 ppm (MW-6-20'), respectively Groundwater samples collected from the newly installed wells reported maximum concentrations of TPHg and MtBE of 72,000 ppb (MW-6) and 750 ppb (MW-6), respectively(IT, 2001) In March 1999, IT performed a Sensitive Receptor Survey and reported 12 wells within a '/2-miles radius of the site The closest wells to the site are located approximately 0 15 mile west/southwest (upgradient) of the site The nearest downgradient wells are located approximately 0 3 northeast of the site (IT, 2001) x Between June 2002 and November 2002, SECOR performed three phases of off-site assessment at the site The initial phase was conducted on June 5, 2002 and consisted of the advancement of five geoprobe borings (GP-1 through GP-5) Based on the sod and groundwater analytical results from W GP-1 through GP-5, SECOR performed the second phase of offsite assessment on June 21, 2002 The second phase of off-site assessment consisted of four additional step-out geoprobe borings (GP- t 6 through GP-9) The final phase of offsite assessment, conducted in November 2002, consisted of the installation of six groundwater monitoring wells (MW-10 through MW-15) Results of these a investigations defined the petroleum hydrocarbon plume in all directions except northeast of well MW-12 which contained 1,300 ppb MtBE Groundwater monitoring has been performed at the site since August 1998 Groundwater flow has been to the east northeast at gradients of approximately 0 001 ft/ft v PROPOSED SCOPE OF WORK The scope of work for this off-site assessment includes the installation of one off-site groundwater monitoring well to delineate the lateral extent of MtBE in groundwater to the northeast of well MW-12 The proposed groundwater monitoring well location is shown on Figure 2 The specific scopes of work are discussed below Soil and Groundwater Assessment • Site Safety Pian As required by the Occupational Health and Safety Administration (OSHA) Standard "Hazardous Waste Operations and Emergency Response" guidelines (29 CFR 1910 120), and by California Occupational Health and Safety Administration (Cal-OSHA) "Hazardous Waste Operations and Emergency Response guidelines (CCR Title 8, Section 5192), SECOR has prepared a site-specific Health and Safety Plan (HASP) Field staff and contractors will review the HASP before field operations at the site begin • Permitting The applicable permits for groundwater monitoring wells will be obtained from San Joaquin County and encroachment permits for off-site wells will be obtained by the City of Lathrop prior to drilling activities • • Off-Site Monitoring Well Installation One groundwater monitoring well will be installed off- site as shown on Figure 2 The groundwater monitoring well will be drilled using 8-inch I IConocoPhiilips\Tosco Marketing\1205\Work Plans\W PLAN Off-Site Assmnt doc