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� EC ® R k. Circle K Store 1205 September 7, 2004 Page 2 reported maximum concentrations of 6,800 ppm TPHG in MW-3-16' and 7.2 ppm MtBE in MW-2-11'. 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 soil 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 b MW-6 and 750 b MW-6 , respectively IT, 2001). PP ( ) PP { ) p Y ( In March 1999, IT performed a Sensitive Receptor Survey and reported 12 wells within a '/-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). Groundwater monitoring has been performed at the site since August 1998. In the fourth Quarter 2002, groundwater samples contained concentrations of TPHg in excess of 10,000 ppb in wells MW- 1, MW-5, MW-7, and MW-9. The highest concentrations of benzene and MtBE reported at 240 ppb and 230 ppb, respectively. Groundwater flow was to the east northeast with a gradient of approximately 0.001 ft/ft. 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 soil and groundwater analytical results from 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- 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). In October 2003 SECOR arranged to sample the Lathrop Grammar School irrigation well, located approximately 1,600 feet northeast of the site and is the closest of three wells identified during a 2000-foot radius receptor survey conducted at the site. The well is used to irrigate the grammar school and operates from 7:00 pm to 5:10 am. The well was sampled at 4:40 am (under supervision of Manteca Unified School District staff), near the end of the well pumping cycle. The water sample was collected from a spigot at the well head. The sample was analyzed for total petroleum ` hydrocarbons calculated as gasoline (TPHg), benzene, toluene, ethylbenzene, xylenes (BTEX compounds), and fuel oxygenates MtBE, EtBE, TAME, TBA, DIPE, EDB, 1,2-DCA, and ethanol by y- EPA Method 8260, single run. All constituents were reported as not detected. :'p In October 2003, SECOR conducted a study to determine if the Lathrop Grammar School irrigation well was affecting groundwater flow or gradient beneath the site. Groundwater transducers were placed in site wells and recorded groundwater elevations for 4 days. Based on data collected from the transducers it was determined that the Lathrop Grammar School well was not influencing groundwater elevation and flow at the site. On June 12, 2004, SECOR supervised the advancement of five geoprobe borings (GP-10 through GP-14). The purpose of the work was to delineate the lateral extent of petroleum hydrocarbon impact in soil and groundwater downgradient of the site and to determine the placement of wells that will complete delineation of the plume. Soil samples at 10 and 15 feet bgs from geoprobe borings GP-10 through GP-14 were collected and submitted for laboratory analysis. Soil samples were I:1Con0coPhillips112051Work Plan For Well Installation 9-04.Doc 7N 4a