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F. Amador <br /> March 1, 1993 <br /> Page 2 <br /> which constitute a fully acceptable document. These deficiencies <br /> are delineated in the following specific comments. <br /> SPECIFIC COMMENTS <br /> Section 7 , Human Health Risk Assessment <br /> 1. Page 7-6, Sec. 7 . 2 . Regarding the off-site samples for <br /> background chemical concentrations, please note that both <br /> USEPA (1989) and OSA (1992) state that the presence of <br /> detectable levels of chemicals off-site (e.g. , background or <br /> ambient levels) which are non-naturally occurring is not <br /> adequate justification to eliminate chemicals on-site from <br /> quantitative consideration. The better approach would be to <br /> include an assessment of the anthropogenic background to <br /> compare with the risk posed by the site. <br /> 2 . Page 7-9 , 2nd paragraph. The report indicates that <br /> groundwater data for inorganic compounds less than five <br /> times the concentration found in the method blanks were not <br /> . used in the risk assessment. This is not consistent with <br /> EPA (1989 , p. 5-17) guidance. The detection limits, as well <br /> as the results of the method blanks, should be reported. <br /> 3 . Page 7-12 . Some metals (As, Cd, Cr, Hg, Vn) were eliminated <br /> because concentrations detected in on-site soil samples were <br /> not statistically different from background concentrations. <br /> To determine whether the concentration of an inorganic <br /> compound detected in a single site sample differs <br /> significantly from the background concentration, the <br /> concentration in the site sample should be compared to the <br /> prediction interval for the background concentration. (Note <br /> that comparison to a prediction interval represents a less, <br /> stringent criterion than comprison to a confidence <br /> interval . ) If the average concentration detected in a <br /> population of site-related samples is compared to the <br /> average background concentration, a standard t;test should <br /> be used to demonstrate a statistically significant <br /> difference. <br /> 4 . Page 7-13, and Table 7-3 . Some organic compounds detected <br /> in groundwater were eliminated based on comparison to MCLS. <br /> Benzene was eliminated because the concentration was less <br /> than MCL. Arsenic was also eliminated on this basis. <br /> Elimination of chemicals based on comparison to MCLS is not <br /> acceptable since MCLS are not entirely health-based. <br />