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------------------ Site Characterization and A 2nmediation Platt: 7500 West Eleventh Street, Trae.l. Page 23 hydrocarbons in a spread of treated 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 that were recovered from Stockpile No. 2 can be used 1 to estimate the number of discrete samples that must be taken from the spread of treated soil to statistically demonstrate, with a high degree of confidence, t �I that the total volume of treated soil contains no concentrations of total petroleum hydrocarbons above the established cleanup standards. The standard deviation of trace concentrations of TPH(d) in treated soil 1 containing an assumed mean concentration of 250 mg/Kg of that analyte with an assumed coefficient of variance of 1.01 would be 252 50 mg/Kg. This projected value for the standard deviation of concentrations of diesel in the F.-:I fully-treated soil can be applied according to the procedures defined in the EPA's guidance document for evaluating the attainment of cleanup standards in soils (U S EPA, 1989) to compute the statistical parameter z. Using Formula 6.7 on Page 6-7 of the above-referenced publication to compute 2 for a spread of treated soil for which a cleanup standard of 500 mg/Kg for j .3 THP(d) has been set, and in which the assumed mean concentration of TPH(d) is 250 mg/Kg (i.e., one half the proposed cleanup standard of 500 mg/Kg), with the previously estimated standard deviation of 252.50 mg/Kg yields z —(500 - 250)1252 50 — 0 99 Then, interpolating in Table A6 on Page A-6 of the above-cited U.S. EPA guidance document (Ref. 6), for a confidence level of 99% (i.e- a = 0.01) at a power of 0.80 (i.e. R = 0.20) that the complete volume of treated soil contains x; lower concentrations of the analytes of concern than those established by the cleanup standards, yields an estimate of 10.2 for the number of samples required. Rounding the above estimate up to the nearest whole number of samples yields a projected total of l I samples required from the spread and treated soil to demonstrate with 99% confidence that any residual concentrations of fuel hydrocarbons in the treated soil are lower than the established cleanup criteria. -� Selection of Sampling Locations. To implement the statistical sampling methodology, samples will be recovered from random locations within the area of the spread, treated soil. To select those random sampling locations, the spread of soil will be subdivided into a large number of uniformly-sized, rectangular sub-areas, which will be delineated by surveyor's flags inserted into the surface of the soil. Each sub- area will be assigned an identifying number. The total number of sub-areas set sic