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McDowell&Davis Towing Geoprobe Soil Boring&Revised Risk Assessment and Conceptual Site Model Report Page: 5 .0 high concentrations of benzene and MTBE suggesting a more recent spill. Very low contaminant concentrations were detected at a depth of 25 feet and contamination was not detected at depths of 35 and 45 feet. A revised hydrogeologic and contaminant cross-section illustrating the shallow geology groundwater occurrence, and contaminant distribution were prepared. A V revised estimate of the contaminant mass was calculated using the laboratory data from Boring B-5 and the previous laboratory analyses of tank removal and exploratory borings / monitoring well soil samples. A revised contaminant cross section is presented in Figure 4. A revised estimate of the remaining contaminant mass in the dispenser release is shown below: Revised Contaminate Mass Estimate - Dispenser Island cu ft Lbs. contamination. Gas 0.30615 13.372632 Benzene 0.000101265 0.004423255 Toluene 0.01413 0.6171984 .. Ethyl Benzene 0.00600525 0.26230932 Xylene 0.0471 2.057328 MTBE 0.010260264 0.448168332 �.. TBA 0.000317925 0.013886964 Groundwater contamination has been recognized at low concentrations in all three monitoring wells in quarterly groundwater monitoring events. The maximum contaminant level of 1.0 ugll for benzene was exceeded in the November 2002 .. monitoring event (MW2 - 9.6 ug/l, MW3 - 2.7 ug/l). Previous investigations determined that the low concentrations present do not pose a threat to potential groundwater receptors. Subsequent monitoring events have not detected the contaminants of concern in excess of the established water quality goals. The limited groundwater impact appears to be degrading and attenuating. Cumulative tables of laboratory analyses are presented in Appendix B. 4.0 RISK ASSESSMENT r. 4.1 ASTM—Risk Based Correction Action Evaluation The soil and groundwater contamination present in the dispenser area was evaluated through the use of a computer application, the "RBCA Tool Kit for Chemical Releases", prepared by Groundwater Services, Inc. Houston, Texas. The application is composed of the an EXCEL spread sheet that completes all calculations required for Tier 1 and Tier 2 Risk Based Corrective Action evaluation as defined in ASTM standard PS-104. The spreadsheet calculates baseline risk levels for soil and ,r groundwater remediation based upon the conceptual site model and measured contaminant concentrations. 1.. ha