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Mr. Marshall Cloud <br /> April 24, 2007 <br /> Page 2 <br /> considered the 95 percent Upper Confidence Level (95UCL) of the mean <br /> concentration as estimated by ProUCL 3.0. Using the 95UCL for dieldrin, DLA <br /> estimated the cancer risk at 5E-5 with the non-cancer hazard as 0.41. <br /> Conversely, using the maximum concentration of dieldrin in ground water would <br /> have resulted in a cancer risk of 2E-4 with a non-cancer hazard index of <br /> approximately 1.6. <br /> 3. The DLA 95UCL was calculated incorrectly because duplicate samples were <br /> included and non-detect samples were not properly accounted for. Using <br /> ProUCL 4.0, a non-parametric.Kaplan —Meier method that better accounts for <br /> duplicates in the analysis or non-detect samples produced a 95UCL of 0.041 <br /> pg/L. DTSC recommends using the ProUCL 4.0 95UCL result of 0.041 pg/L for <br /> any regulatory or risk management decisions. <br /> Conclusion <br /> The human health risk associated with exposure to dieldrin in ground water using the <br /> maximum concentrations resulted in a cancer risk of 2E-4, with a non-cancer hazard <br /> index of 1.6. Correcting the duplicate and non-detect errors, ProUCL version 4.0 <br /> produced a 95UCL exposure point concentration of 0.041 pg/L for dieldrin. Based on <br /> the agreed-upon decision logic outlined in the January 30, 2007 e-mail, dieldrin should <br /> be retained as a COC for DDJC-Tracy. <br /> Attached to this letter is a memorandum from HERD summarizing its analysis and <br /> conclusions. The HERD memorandum should be used to supplement this comment <br /> letter. <br /> If you have any questions or comments regarding this letter, please feel free to contact <br /> me at (916) 255-3713 or e-mail me at pmacnich(cDdtsc.ca.gov. <br /> Sinkely, _ <br /> �rel � <br /> Peter MacNicholl, P.E. <br /> Remedial Project Manager <br /> Federal Facilities Unit <br /> Attachment <br /> cc: See DDJC-Tracy Distribution List. <br />