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i .•-•.. :-:.::. -.• •-.:mss 5,.._. - �::-'.:.. -- r .a.. .. -- - ,.. .� . _ -=1 1 -- .. -- - :•-: .. .. -_r .;.n.. . , .-..._ i28April 20'0 .AGE-NC Project No.-98-0440 1 'Page 6 of 7 .y 4.0. SUNIlIIARY AND CONCLUSIONS - n, Based on the:'data collected.during`this monitoring period, AGE concludes: • -Damp to.saturated, gray to brown, silty sand was encountered at 10 feet and 20 feet bsg in borings MW7 and MW8.Dry to saturated,brown, silty clay and clay were encountered at all ' other depths in borings MW7 through MWl0. • Depths to'ground water ranged from 12.90 feet to 13.77 feet below the well casing tops. Ground water elevations at the site ranged from 26.33 feet above mean sea level (MSL) in 1' well MW8 to 27.34 feet above MSL in well MW 1: On 12 December 2002,the ground water ' at the site was inferred io be flowing toward the northeast under an average gradient of 0:006 ' feet/feet across the area monitored. • No analytes were detected at or above laboratory reporting limits in the soil samples . collected from wells MW7 through MW 10. The approximate extent of petroleum ' hydrocarbon-impacted soil is depicted on figures 5, 6 and 7. The highest concentration of dissolved Hydrocarbons quantified as TPH-g, TPH-d and BTEX ' . compounds extends northeast from,USTs No. 4 and 5. Dissolved hydrocarbons were encountered in the down-gradient direction from the former USTs in samples from monitoring wells MW2, MW5,"-MW7 and MW8. Dissolved hydrocarbon 'concentrations remained non-detectable in samples from,wells MW1, MW3 and MW4. Concentrations decreased to non-detectable' lever in well MW6. - It would appear that petroleum hydrocarbon-impacted ground water is migrating along the sand zone encountered at 10 to 15 feet bsg."A map showing the approximate extent of dissolved benzene and the areal ' location of the two sand zones encountered at 10 and 20 feet.bsg is presented in Figure 8. The concentrations of benzene detected-in the water:samples collected from wells MW2; MW5 and MW8 were above the State of California maximum contaminant level(MCL) of 1.0 µg/l for benzene in drinldng water. The lateral extent of petroleum hydrocarbon-impacted ground water has not been defined 'toward the northeast beyond MW7 and MW8.The hydrocarbon plume is fairly well defined, to'the south, east;west and northwest. ' 5.0. RECOMMENDATIONS Based on'the findings of this investigation,AGE recommends'continued*quarterlymonitoring of the t ground water monitoring wells. The next monitoring event was performed in Mauch 2003. In Addition, AGE recommends a corrective action plan(CAP) be prepared for the site. . Advanced GeoEnvironmental,Inc.