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Corrective Action Plan Page 34 NAV 4 8 1600 ULT 9 9 2,3000 ULT 9 14 1100 These data have the following statistical properties NoofSamples 5 Degrees of Freedom 4 Mean Concentration of TVH 814 mg/Kg Standard Deviation 1,032 mg/Kg Coefficient of Variance 127 Experience has shown that, after treating contaminated soil by aeration to a condition where olfactory and visual indicators of the presence of gasoline have been eliminated, tests on samples of the treated soil typically show that remaining traces of hydrocarbons (if any) are close to or less than the routinely achieved analytical method detection limit (MDL) of 1 0 mg/Kg For such a condition, it is reasonable to assume for the purposes of statistical analysis that the mean concentration of hydrocarbons in the treated soil is 0 5 mg/Kg, one half of the MDL Experience also shows that the variability of trace concentrations of gasoline in spread aerated soil reflects the variability of the concentrations of hydrocarbons in the untreated material Using the assumptions stated above, the statistical data from the results of analyses of soil samples recovered from the site of the proposed remedial excavation can be used to estimate the number of discrete samples that must be taken from the spread of aerated soil to statistically demonstrate, with a high degree of confidence, that the total volume of treated soil contains no concentrations of total petroleum hydrocarbons, or any of its components of concern, greater than the treatment standard, which, for the soil excavated from the subject site, it is proposed to set at the MDL of the analytical procedure (i e 1 0 mg/Kg for total petroleum hydrocarbons as gasoline (TPHg) and 5 0 µg/Kg for benzene, toluene, ethyl benzene and total xylenes) The standard deviation of trace concentrations of TPHg in treated soil containing an assumed mean concentration of 0 5 mg/Kg of that analyte with an assumed coefficient of variance of 1 27 is, as computed above, 0 64 mg/Kg This value can be applied according to the procedures defined in the EPA's guidance document for evaluating the attainment of cleanup standards in soils (Ref 21) to compute the statistical parameter z Using Formula 6 7 on Page 6-7 of Reference 21 to compute z for a spread of treated soil for which a cleanup standard at the method detection limit (MDL) for TPHg (i e 10 mg/Kg) has been set and in which the mean actual concentration of TPHg is 0 5 mg/Kg 1