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I • 15 November 1995 AGE-NC Project No 95-0142 Page 10 of 19 I6 2 1 EXCAVATION I Z 1 All of the mentioned methods of ex-situ treatment require excavation of the impacted soil, which is ' generally the fastest and most effective method for soil remediation Impacted soil in the vadose and smear zones extend to a depth in excess of 45 feet Therefore, soil would have to excavated to ' depths greater than 45 feet below surface grade(bsg) Once excavated, the impacted soil is generally stockpiled on site for treatment or transportation for disposal If, soil is treated on-site and hydrocarbon concentrations can be reduced to non-detectable (ND) levels, the soil can be used to ' backfill the excavation, if soil is disposed off-site, backfill must be imported for the excavation ' 6 2 2 FEASIBILITY OF EXCAVATION Excavation of impacted soil, followed by ex-situ treatment and/or disposal is a very effective method I of remediation, non-detect (ND) can usually be achieved by excavation since, theoretically, all contaminated soil is removed However, excavation is usually too costly if the volume of impacted soil is too great or if the impacted soil is present at depths requiring special equipment or extensive shoring Both special excavation equipment and shonng would be required at the Stockton Plating site I Excavation at the Stockton Plating site is not feasible considering the depth of impacted soil, space limitations, current site usage and cost for disposal and backfill 6 2 3 ESTIMATED COSTS FOR EXCAVATION IAt the subject site, impacted soil could not be excavated with standard excavating equipment If disruption of site operations and space limitations force off-site disposal, the cost for excavation of impacted soil could exceed $200 per cubic yard Based upon the removal of at least 2,000 cubic yards of hydrocarbon-impacted soil, the costs would exceed $400,000 6 3 IN-SITU BIOREMEDIATION I Acceptance of this treatment technology by the regulatory community has grown in recent years, due to success achieved at sites in California and elsewhere I6 3 1 PRINCIPLES For this remediation method, hydrocarbon-degrading bacteria are propagated in a liquid medium, i