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R10/71-51-1C While this may not promote the second objective ( i e , construction plans on site), passive remediation may be difficult to obtain This is due to the fact that this is a "no- action" alternative Regulatory agencies seldom agree that "no-action" is the appropriate remedial action Extensive site characterization and demonstration studies may be required to show that adverse environmental and human impacts will not occur 7 3 2 Remedial Action Option for Soil The "No-Acton" option was discussed in detail in section 7 3 1 No source control options were identified (see Table 14) The other options are described below 7 3 2 1 In Situ Containmgnt In situ containment of subsurface wastes or contaminated soils involves theP lacement of engineered barriers around the wastes to isolate them from groundwater and to minimize the infiltration of surface water into the waste containment The technology employed typically involves trenching around a site and into an impermeable subsurface layer The trench is filled with a grout or slurry for stability during excavation An impermeable backfill is then planned in the trench, dispersing the slurry and creating the cut-off wall A synthetic membrane and /or earthern cap is constructed over the site so that the cap perimeter extends beyond the cut-off wall thereby isolating the waste from the environment The actual construction is little different in principle from many large "earthwork" construction projects Caps and cut-off walls provide barriers to movement of wastes and contaminated groundwater (leacheate) off site, and to movement of groundwater into the containment Caps also prevent direct exposure to contaminated soil and prevent airborne contamination Depending on the waste constituents and site hydrogeological conditions,the subsurface impervious stratum (hydraulic conductivity<10- 7 cm/second) may be a native clay layer or bedrock The placement of an engineered bottom barrier may be considered if a natural barrier is not present at reasonable depths At Shop-10, the area above the locations where underground tanks were removed has a gravel fill and is very permeable The surrounding areas may however be considered to adequately capped so long as the asphalt pavement is kept in good repair 7322 Excavation and Disl2osallTreatment Excavation, as referred to in this document, is the process by which the affected soils are 27