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• 1.0 EXECUTIVE SUMMARY Stratus Environmental, Inc (Stratus), on behalf of Mr Kashman Singh and Mr Dalwinder Dhoot, has prepared the following Site Conceptual Model (SCM) for Joe's Truck Plaza, located at 15600 Harlan Road, Lathrop, California (Figure 1) This SCM was prepared pursuant to a letter from San Joaquin County Environmental Health Department (SJCEHD) dated May 13, 2003, and a telephone conversation with SJCEHD on July 11, 2003 The purpose of this SCM is to integrate the hydrogeologic and chemical analytical data gathered to date to better understand the extent of petroleum hydrocarbon impact to soil and groundwater The model identifies transport and exposure pathways, and data gaps that require further characterization This SCM consists primarily of tables and figures that have been generated from previous site assessment activities, and provides a platform to communicate activity and progress The SCM is summarized in Table 1 The subject site is an active service station located at the southeast corner of J Street and Harlan Road in Lathrop, California Assessment activities began at this site in January 1997 One gasoline and three diesel underground storage tanks (USTs) located in two separate UST pits (Figures 2 and 3) were removed and replaced during site renovation activities in February and • October 1998 To date, 18 shallow (between 4 and 10 5 feet below ground surface [bgs]) assessment or compliance soil samples have been taken, 25 soil borings have been advanced, and 10 groundwater monitoring wells have been installed at this site or in its vicinity Geology beneath the site consists of interbedded layers of sand and silty sand to approximately 13 to 18 feet bgs Underlying these sand layers is a hard clay layer extending to at least 22 feet bgs Laterally, this clay unit appears continuous underneath the site except in the vicinity of the former northern underground storage tanks, where the clay unit was not encountered to 20 feet bgs The lateral and vertical extent of the hard clay Iayer has not been completely evaluated Underlying the hard clay unit, another stratum of silty sand and another stratum of hard clay was encountered at depths of 22 to 28 and 28 to 30 feet bgs, respectively The vertical and lateral extents of these strata have not been evaluated beneath this site It appears that the sand units above and below the clay are in hydrologic communication Quarterly groundwater monitoring and sampling began in February 2000 Groundwater flow is generally northeast to east beneath the site at gradients ranging from 0 00083 to 0 0015 ft/ft Groundwater has been encountered from 6 to 10 feet bgs, has fluctuated 1 to 2 feet between monitoring events, and does not seem to exhibit seasonal fluctuation Total petroleum hydrocarbons as diesel (TPHD), benzene, and methyl tertiary butyl ether (MTBE) have been reported in soil samples collected during compliance sampling and assessment activities at the study site The highest concentration of TPHD (1,000 milligrams per kilogram [mg/Kg]) was reported in a preliminary assessment sample collected at a depth of P Voe's Truck Plaza\Reports\SCM_09-2003Uoes truck plaza SCM doc Page 1 STRATUO