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I 1444 MARIPOSA ROAD WKA No 2307 02 December 15, 1993 Page 6 I I ' CONCLUSIONS AND RECOMMENDATIONS ' Based on field observations and laboratory analytical results for the soil samples collected during the construction of monitoring wells MW1 through MW3, and our previous work, it is our ' opinion that the majority of the significant gasoline contaminated soil appears to be confined beneath the central portion of the excavation at depths ranging from 36 to 50 feet below site ' grade ' Analytical results of the water samples collected from three monitoring wells MW1 through MW3 revealed no detectable concentrations of TPHg However, benzene was detected at a ' concentration of 110014glf in monitoring well MW2 It is also our opinion that these findings provide a basis for estimating the "zero-line" of soil and ground water contamination, and that further subsurface investigation is not necessary at this tune We recommend that sampling of the three monitoring wells continue on a quarterly basis for a period of one year to better evaluate the ground water flow direction and ground water quality Based on the above stockpile analytical results, Mr Michael Collins of San Joaquin County ' Public Health Services suggested that the stockpiled soil be used to backfill the excavation with the understanding that a soil venting system be installed in or adjacent to the excavation ' Based on discussions with Mr Collins and representatives of U S Rentals, we propose the ' installation of soil venting system within the existing excavation The system will consist of lateral piping along the bottom of the excavation attached to four perforated vertical risers These risers will then be connected to subsurface (approximately two feet deep) lateral piping ' which will lead to a common area where a future vapor extraction system will likely be placed o I `�W WALLACE KUHL Recycfedho . frASSGC[ATES IANC