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LHLVRON EMC 925842021303/29 '00 14:30 No.689 02/06 <br /> • 'Y�--11 UUI <br /> ' CALIFORNIA ENVIRONMENTAL PROTEc*OIY AGCNCY <br /> DEPARTMENT OF TOXIC SUBSTANCES CONTROL PETE WILSON, Gowmor <br /> 301 Capitol Mail,2nd Floor <br /> Sacramento, CA 95814 <br /> Mail: P. O. Box 806 <br /> Sacramento, CA 95812-0806 <br /> ' Voice: (916)327-2,517 <br /> Fax: (916)327-2509 <br /> MEMORANDUM <br /> TO: Wendy Wyels <br /> California regional Water Quality Control Board <br /> Central Valley Region <br /> 3443 Routier Road, Suite A <br /> ' Sacramento CA 95$27-3098 <br /> FROM: Stephen M. Di7io, Pb D. <br /> ' Humaa and Ecological Risk Division <br /> DATE; August 13, 1997 <br /> ' SUBJECT: RISK EVALUATION: CHEVRON ALDEN PARI{ <br /> PCA: 12185 Site: 900027 WP; 00 <br /> The Central Valley Regional Water Quality Control Board (CVRWQCB) has requested <br /> ' that the Department of Toxic Substances Control (DTSC) Human and Ecological Risk Division <br /> (HERD) review the documents Refined Risk Evaluation: Chevron Alden Park 7yacy <br /> California December 1996 and Supplemental Rlsk Evaluation, Chevron Alden Park, Tracy, <br /> California May 1992. These documents was prepared on behalf of Chevron Pipe Line by <br /> GEOMATRIX Consultants of San Francisco, California, <br /> BACKGROUND <br /> The document entitled Refined Risk Evaluation (RRE) was prepared after multiple <br /> meetings with technical staff representing Chevron and HERD in order to discuss and reach <br /> agreement on critical parameters for the risk evaluations. The analysis was stocsc, t ito <br /> say, probabilistic and relied not on point estimatesfor calculating human expohasure ti(e.gthas <br /> , the <br /> assumption that the adult body weight is 70 kilograms) but rather a distribution of values that <br /> ' spans a,range of possibilities. Random samples taken from this range represent the input <br /> parameters used in the risk calculations. As such, the output is not a single point (e.g. 1 x 10�) <br /> but a f. equency distribution which can graphically be depicted with a risk range on the x-axis and <br /> ' a probability on the y-axis. It is this distribution that is critically examined in the review of a ned above) axe <br /> agreement <br /> Stochastic assessment. Given that the input distributions (the ranges mentio <br /> critical to this output these values were extensively discussed in advance, w <br /> ' reached as described above. These agreements were reached verbally, and are documented as <br /> such in the RRF, This is not conventional procedure for the DTSC. If future iterations of the <br />