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CHEVRON EMC 420213 03/29 '0014:31 NO.689 04/06 •1 �YJ uu9/006 Wendy Wyels August 13, 1997 1 Page 3 administrative record. Please request that Geomatrjx verify that the text of the document is 1 correct where it states on page 10 of Appendix A that 5000 trials were performed in the Stochastic analysis. The electronic file given to DTSC indicated that 500 trials were performed. The verification process performed by DTSC utilized runs of 10,000 trials, 1 ever, and essentially reproduced the printed results. 3. Use of the Refined Screening Levels 1 The document contains a comparison of individual chemicals detected in soils to the refined screening levels, V/Wle presenting no problem with regard to human health impact,please 1 be aware that such comparisons are potentially misleading and can result in public t, econcerto where none is warranted. 1 For DTSC projects, HERD is currently not approving use of any soil screening level calculations without some assurances as to how those levels are to be applied. It has been our experience that incorrect application of these levels (e.g., as target 1 performancc standards or, cleanup goals) has resulted in inconsistent and unn little if any benefit to public licalth. ecessary expense with 1 For residential settings, the levels should reflect exposures to humans over any 1000 f 2 Parcel which could be reasonably superimposed on a site. For comrnercial/jndustria] settings, they should reflect worker exposure over sections of a real or hypothetical 1 facility which could reasonably be accessed, including occasional intrusion into subsurface soils for normal activities (e.g.,trenching for underground utilities) expected over time. 1 For soils, these levels should also reflect the conditions under which risk is assessed. For exposure of residents or workers to soils, average concentrations are used in the exposure 1 estimates. For empirical data (e.g. analytical results from random sampling) to be used in this context, these averages arc expressed as the 95% upper ro dsamence limit to be% U d ) Of the estimated arithmetic mean calculated using those samples, or the 1 detected concenisatjon, whichever maximum is leas. We consider human health-based remedial levels to be attained if random sampling results confirm, that the 95% UCL of these samples arc at or below the calculated screening levels, and that isolated spots of 1 Contamination above these levels do not represent acute or subehrorric human health hazards. 1 If removal is deemed necessary, target concentrations presented for attainment of the remedial levels, depending on the distribution of site contaminant concentrations, may be above remedial levels. Therefore, OSA generally recommends that ncent a removal ' scheme is presented , regardless of the target concentration that isjustified, random confirmation sampling in order to assure that the 95% UCL of remaini concentrafions are at or below the health-based screening level. ng contaminant i i i