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Plan Page 11 Corrective Action , r removal of all contaminated soil can be assured to adequately control the work so that e � Also, such excavations can not be backfilled to engineering specifications unless special, imported material, such as large-size, gap-graded crushed rock, is used and compacted by specialized equipment To control damage due to sand flowing into an excavation, it would be necessary to de- water the site so that the water table was lowered to a depth of at least 20 ft over essentially the entire area of the property and fully-interlocked, deeply-penetrating, steel, sheet piles would have to be installed around the whole perimeter before the contaminated soil was removed It is estimated that this would require continuous pumping at rates on the order of 300 gpm (gallons per minute) Management of such a quantity of rcontaminated water by temporary storage is not feasible and, even if were economically possible to continuously treat it so that it could be discharged directly to the sewers, such an operation would not be permissible under the terms of CRWQCB Order No 92-150 Even if the extraordinary measures to excavate contaminated soil from beneath the property that have been described above could be made feasible, such an operation would have only limited efficacy in remediating the affected groundwater This is because contaminated soil in inaccessible areas under the sidewalks and street, which include some 1 of the most severely affected zones of the subsurface, would not be included in the excavation The area that is accessible to excavation amounts to no more than 28% of the total area of the subsurface that is affected by gasoline undwatcr Recovery and TLcalm=L Groundwater affected by gasoline can be effectively remediated by pumping from recovery wells and treating the recovered water to remove contaminants before it is discharged as clean water Several water-treatment methods are available including air stripping and carbon adsorption, which are both well- proven and effective (Ref 1 ) The principal disadvantage of remediation by groundwater recovery and treatment methods is that they do not expeditiously remediate contaminated soil that serves as a source of contaminants that slowly desorb from the soil and dissolve into the water However, as water is drawn through the subsurface strata under the action of the pumps, contaminants are flushed from the affected soil and this effect can be accelerated by cyclically changing the pumping rate so that zones of affected soil at and beneath the static water table are repeatedly exposed and submerged This procedure also draws fresh supplies of oxygen into the vadose zone and stimulates microbial biodegradation of hydrocarbons into carbon dioxide and water The rate at which the contaminated soil is remediated by groundwater pumping depends upon the type and permeability of the soil and the contaminant concentrations Generally, permeable sands, such as those that are prevalent under the subject site are the most rapidly cleansed, while impermeable clay, which have a high adsorptive capacity, are the most resistant to remediation In the case of soils of intermediate permeability, such as the silty clays that occur together with the sands under the 152 East 11th Street site, the time