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Sierra-Pacific Groudwater Consultants, Inc. y y <br /> Geologists, Engineers, and Compliance Specialists <br /> March 27, 1998 <br /> Mr. David Stavarek,R.G. - Site Cleanup Unit <br /> Central Valley- Regional Water Quality Control Board <br /> 3443 Routier Road, Suite A <br /> San Ramon, California 95827-3098 <br /> Subject. Risk Assessment Workplan,FormerPureGroBrea Facility,1905North Broadway Avenue,Stockton, <br /> California,Sierra-Pacific Project No. 94-055.08 <br /> Dear Mr. Stavarek: <br /> On behalf of Unocal Corporation, Sierra-Pacific Groundwater Consultants,Inc. is pleased to submit the enclosed risk <br /> assessment workplan. In compliance with directives from your predecessor, Wendy Wyels,the primary guidance for <br /> this risk assessment will be the California Environmental Protection Agency Preliminary Endangerment Assessment <br /> Guidance Manual(PEA Manual), dated January 1994. The methods presented in that guidance document are based <br /> upon a deterministic approach to risk assessment. We are also, in addition,going to perform a probabilistic assessment <br /> of risk, in order to provide you with the most comprehensive risk assessment possible. This dual approach was <br /> discussed and agreed upon in our meeting on March 4, 1998 with Dr. Julio Salinas, toxicologist, of the State of <br /> California Office of Environmental Health Hazard Assessment (OEHHA), who we understand will be providing <br /> technical review of this workplan under contract to the Regional Water Quality Control Board. Dr. Salinas also advised <br /> us in that meeting to utilize the PEA Manual as a guidance document. <br /> We would like to acknowledge the authorship of Sections 3.0 through 11.0 of this workplan by Lee Shull, Ph.D., <br /> toxicologist,under contract with Sierra-Pacific Groundwater Consultants,Inc. After review of the workplan, we would <br /> like to meet with you to discuss it,and to verify the applicability of this risk assessment approach for this site. <br /> Please feel free to call either Nick Nickerson at Unocal or myself if you have any questions. <br /> Sincerely, <br /> Eri�SIERRA-PACIFIC GROUNDWATER CONSULTANTS, INC. jj�jT/ <br /> �e, CEG D �s++ L'( g <br /> Senior Engineering Geologist <br /> EDC/jb MAR 31 1998 <br /> Enclosure <br /> ENVIRONME$N�TAVLJ HEALTH <br /> C. Nick Nickerson,Unocal DBG/AMG <br /> Jan Wagoner, Unocal DBG/AMG <br /> Robert Sweeney, Unocal, Brea <br /> �Aike Infurna, San Joaquin County M -1998 <br /> Dr. Julio Salinas, OEHHA <br /> Chris Metzger, Sierra-Pacific ENVIRONMENTAL HEALTH <br /> Dr. Lee Shull PERMIUSERVICES <br /> (916) 93-1468 <br /> 4911 W indplay Drive,Suite 4 EI Dorado Hills,California 95762 (916)933-3197 FAX <br /> (F. <br />