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i_ r i Site Assessment Proposal Frontier Transportation Doc # 101501 October 15, 1997 hydrocarbons in the shallow subsurface soil from 0' to 9' bg. Subsurface soil from 9 . 5' to 11 ' bg where subsurface water was encountered identified maximum petroleum hydrocarbon concentrations . Subsurface water analysis identified significant attenuation in TPH-g, BTEX, and MTBE concentrations in monitoring wells MW- I and MW-2 since initial testing. An increase in the maximum benzene and TPH-g levels was identified in MW-3 in periodic monitoring performed initially in August 1996 and, subsequently, in April 1997. 5.0) CONCLUSIONS It is concluded that the lateral extent of dissolved petroleum impacting the subsurface water has not been adequately defined in all directions from monitoring well MW-3 where maximum and increasing dissolved benzene and MTBE were identified. Also, the vertical extent of petroleum hydrocarbons has not yet been defined below 11 ' bg. It is further concluded that increased benzene and MTBE concentrations in MW-3 have not caused significant migration of hthese dissolved petroleum constituents since concentrations of these constituents decreased in monitoring wells MW- 1 and MW-2 in that same time period. 6.0) RECOMMENDATIONS It is recommended that four additional monitoring wells be drilled to 20' bg and placed according to Figure 3 , Proposed Well Locations, enclosed . This testing is recommended to _. identify the lateral limits of petroleum impact in the shallow subsurface water. It is further recommended that, should these wells identify satisfactory attenuation, monitoring of continuing intrinsic attenuation and spatial stability be performed. In addition, it is recommended that risk analysis be completed identifying sources, pathways and potential receptors as remediation for this site. 7.0) GROUNDWATER MONITORING WELL INSTALLATION A hollow stem auger drill rig will be used to provide the soil borings for well installation. The auger size will be 6-inch outside diameter with a 2 . 25 -inch inside diameter. All well materials will be installed through the hollow stem auger to allow maximum control during placement. Drillers held a California State C-57 contractor's license. Permits will be obtained from the San Joaquin County Department of Environmental Health Services. Each well will be comprised of a central section composed of riser, screen and sump . The annular space around the sump and screen will be filled with sand to 5' bg. This will be overlain with a 5 ' seal of bentonite pellets, then grouted to near ground surface. A lockable flush-mount 4 l