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CHEVRON EMC 9258 420213 03/29 '0014:31 NO.689 05/06 yy VV.]ruUfS Wady Wycls August 13, 1997 ' Page q ' 4• Background Concentrations of PAR Compounds The calculated refined screening levels have been developed for this site should also be ' used in the context of the known background levels of PAH compounds Sound in mete urban areas of California. The following information has been developed and is bet s y °g urttnttarized for the CVRWQCB in order to put P within the DTSC ' context, p the soil screening levels into As noted above, weighting factors (PEPS) can be applied to the carcinogenic PAH ' (cPAR)fraction of soil contaminants. With these PEPS total ePAli content of a soil sample can be expressed as a single number, the "total BaP equivalent", after the individual chemical approach, concentrations arc multiplied by the weighting factor. Using this ' we have examined twenty California.sires in which background sampling was conducted under DISC oversight (Final Remedial Motion Plan, Former Alhambra Manufactured Gas plaint) and have found that the sum of all CPAIor a ,41ho_a_Pyrene equivalents, fell within a lognormal distribution, with approximately 0.9 mg/kg zo- Sepresenting an upper toter mce limit(11TL) about the 95tb,percentile of the entire data, Set(95% coverage, 95% confidence), This allows the project manager to reliably determine whether site sampling data identifies soils impacted by site activities as Opposed to those that in all probability represent background levels of contamination. ' Large Sample sizes, even from background soils, will occasionally(approximately one out of every twenty samples) exceed the UTL. Recognizing this the compiled background data also allows an upper prediction limit to be identified, w ' dependent upon sample size. S mall sample sizes have lower prediction which is ones, bunco the increprobases lily of a sample being taken from the high end of the lognormal mits than larger distribution increases with sample number. The proximity of the sample with respect to ' others where the UTL was exceeded should be used to determine whether soils impacted by site activities have been identified. Please note that the UTL and the upper prediction limits were calculated assuming that carcinogenic PAHs, where not detected, existed at one-half the detection limit. Site data should, be treated in a similar manner in order to fairly use the above information. ' Given that tine final work product for the background study has not been completed, backgT recommends that the use of this information be restricted to Separating background soils from those contaminated from site activities in order to better delimit those areas where risk should be assessed and remediation considered. USL^Pq allows separating background boas site-impacted acted only at the discretion of the project manager. The HERDfor risk assessmentpurpose.to guidance decision but does not provide the decision itself. provides technical support to the