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' SITE CONCEPTUAL MODEL r <br />' Based on the data collected to date from soil borings and ground water monitoring wells, AGE has <br /> developed a site conceptual model for the release,migration and distribution of the contaminants in <br />' the subsurface as described below <br /> Generally,the stratigraphy from surface grade to approximately 60 feet below surface grade(bsg)is <br />' composed of fine-grained material consisting primarily of silty�,clay, clayey silt and clay, with <br /> laterally discontinuous, coarser-grained intervals (siltyan <br /> sd, sandy sit and poorly graded sand) <br /> towards the west at approximately 30 to 35 feet bsg and 1n borings B-Zf(MW-1),MW-2 and MW-4 <br /> between 45 and 50 feet bsg In borings MW-1 through MW-4, a coarse-grained interval (sandy silt <br /> and poorly graded fine sand)was encountered between approximately 60 and 65 feet bsg,extending <br /> downwards to 75 feet bsg in boring MW-2, finer-grained silt and silty clay were encountered in the <br />' two other borings (MW-3 and MW-4) to 75 feet bsg Soil boring lPcatlons and geologic cross <br /> sections A-A', B-B' and C-C' are provided in Figures 1A throughA 4 tr\ s 0-t� <br /> jaw <br />' Since initiation of grouwater monitoring in November 1999,ground water elevations have ranged <br /> from approximately 25 Y.to 44 4 feet below mean sea level The ground water table underlying the <br /> site is generally flat and4oward the northeast with a low gradient of less than 0 01 fl/ft (Figure 3) <br /> Release/s of petroleum hydrocarbons occurred from the on-site USTs Once released to the <br /> t • subsurface,the conthuninants migrated into the upper clayey soil to a vertical depth of approximately <br /> 40 feet bsg (MW 1- b), target analytes w ge not detected above laboratory reporting limits in soil <br /> samples collected from soil boring B-I (former UST location) at 50, 60 and 64 feet bsg-Laterally, <br />' hydrocarbon-impacted soil is defined to the west by soil boring B-2,to the northwest by B-3,to the <br /> south by bonng MW t Yand the northeast by boring B-4 p rI 9 <br /> Laboratory analysis of groundwater samples detected petroleum hydrocarbon-impacted water at the <br /> GV <br />' former USTs T-1 and T-2 locations(at borings MW-1 and B-2),'—only MTBE at µg/1 was detected <br /> at the tester 'edge of the site (boring B-5) However, the detected concentrations in ground water <br />' samples collected from the former UST location (e g , MW-1) have remained stable since ground <br /> water monitoring began in November 1999, indicating that further migration of hydrocarbons from <br /> -w ire's. ✓«✓n/—.r <br /> the source area is not occurring Furthermore, analytes of concern have not been detected above <br />' laboratory reporting limits in ground water samples collected from perimeter monitoring wells <br /> MW-2, MW-3 and MW-4 during the six quarterly monitoring and sampling events conducted to ` <br /> date The lateral extent of ground water appears to be limited to the former UST area(MW-1) <br /> i r <br /> Typically, ground water would represent the migration medium for contaminants over significant <br /> horizontal distance°,H ever, sites located in up-gradient or cross-gradient directions from the <br />' subject property are not Iikely to be negatively impacted by the subject property In addition, first <br /> encountered ground water is not used for drinking or irrigation, although all ground water in the <br /> State of California is considered to be of potentia beneficial use <br /> • �� t� 1 irk <br /> �� , <br />