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Corrective Action Plan Page 10 Due to the shallow depth to the groundwater beneath the site, the geotechnical and hydrogeologic properties of the soils, and the distribution of contaminated material in the ' subsurface, application of vanous technologies and combinations of technologies that rely on vacuum extraction or bio-venting to effect remediation of soil and groundwater were fudged to be impractical or not cost-effective at this site Remediation of contaminated soil by excavation and treatment by aeration was evaluated in detail as part of the engineering studies undertaken to develop the original corrective action plan that was issued in June 1994 (Ref 14) At that time, the effectiveness of soil excavation and treatment as a means of reducing the gross contaminant load in the subsurface and eliminating contaminated soil as a persistent source of components of gasoline affecting groundwater was recognized However, it was found that under the site-specific conditions it would not be possible to safely remove all contaminated soil from the sub-surface There was concern that the areal extent of excavation would be restricted by the public streets on the down-gradient and co-gradient boundaries of the site and further linnted over a significant portion of the affected area unless the exiting building ' on the property was demolished With respect to the depth of an excavation required to remove all traces of contaminated soil, it was found that the bottom of such an excavation would penetrate into the submerged, very loose, fine to medium-course sand that are present in an extensive stratum the top of which is at a depth of between 12 ft to 16 ft beneath the ground surface This would pose serious safety concerns because, when exposed in the bottom and walls of a pit, the submerged, loose sand would rapidly flow into the excavation and cause collapse of the sides of the pit with consequent damage to adjacent property, including the public streets, and potentially induce settlement of the ground surface over a wide area Pumping of groundwater from a pit that penetrated into the very loose sand would also likely cause migration of fine particulate matter from considerable distances in that stratum and threaten the stability of several old, un-reinforced masonry structures on neighboring sites That risk would be present even if shoring to prevent local collapse of the walls of the excavation could be cost-effectively constructed It was also recognized that shoring of an excavation bottomed in submerged loose sand can not prevent loss of stability by rbottom heave, sand-boils and the development of quick-sand conditions There was also concern about the ability to effectively control the bottom geometry of, and the removal of soft material from, an excavation that penetrates to any significant depth below the water table Such underwater excavation would be required at the 152 East 11 th Street site to remove all traces of components of hydrocarbons from the subsurface Furthermore, it is not generally possible to place and compact backfill beneath the water table to the engineering specifications that would be required to restore the property to use as a site for commercial buildings