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Working To Restore Nature 5.0 SPECIFIC INFORMATION FOR CLOSURE REQUEST L-11 Introduction ' This section provides information specifically required for closure review as per the October 7, 1993 Addendum to Appendix A to the Tri-Regional Recommendations 1 5�2 Rationale for Closure with Residual Petroleum Hydrocarbon Impacted Soil Removal of the remaining petroleum hydrocarbon impacted soil is not physically possible due to the existence of a structure located above the impacted area and relatively shallow structural footings at approximately 10 feet below surface grade Excavation beneath the sidewalk would threaten the structual integrity of the Police facility A majority of the impacted soil in the vicinity of the former underground storage tanks was removed during excavation activities in ' July 1992 The estimated volume of material removed is approximately 300 cubic yards The volume of remaining impacted soil is estimated to be approximately 57 to 63 cubic yards containing approximately 241 pounds of TPHg and 195 pounds of TPHd However, only an estimated 0 14 pounds of residual benzene (0 02 gallons) remain Figures 4 through 9 depict the excavated area as well as the area of residual soil contamination The residual soil contamination is not expected to impact groundwater with detectable concentrations of petroleum hydrocarbons This expectation is based on the high adsorbancy of the soil type (clay/silty clay with 2-5% organic matter, USDA, 1992), the minimization of infiltration due to the overlying concrete cap and the presence of very stiff, fat clay from approximately 7 to 10 feet below grade ' No impact to groundwater has occurred as a result of the unauthorized release of petroleum hydrocarbons The source of the unauthorized release has subsequently been removed and a potential continuing source of degradation has been minimized OW014MEPORTSTR-0694 FNL 8