ARCHIVED REPORTS_RESULTS OF THE UPDATED HUMAN HEALTH RISK SCREENING EVALUATION

Home Browse Search

2101 Webster Street 12th Floor Oakland, CA 94612 (510) 665-4100 • FAX (51 O) 663-4141 GEOMATRIX July 20, 2001 Project 7243.002 n�� Mr. Robert Mihalovich JUL 2 7 2001 Chevron Environmental Management Company ENVIRONMENT HEALTH P.O. Box 6012 PERMIT/SERVICES San Ramon, California 94583 Subject: Results of the Updated Human Health Risk Screening Evaluation Ridgeway(former Burns)Property /F8/ 4'u.,.4r` /e1w, T� Tracy, California Dear Mr. Mihalovich: Geomatrix Consultants, Inc. (Geomatrix) has prepared this letter summarizing the results of the updated screening health risk evaluation for the Ridgeway(former Burns)Property (the site). In January 2000, Geomatrix conducted a human health risk screening evaluation to assess the potential human health risks associated with residual constituents detected in soil and groundwater. The results of the screening evaluation indicated that exposure to residual petroleum constituents at the site should not pose an unacceptable health risk to hypothetical future on-site residents under the conditions evaluated. The Department of Toxic Substance Control (DTSC) concurred with this conclusion in an April 27, 2000 letter and a March 29, 2000 Geomatrix response letter(Attachment 1). After completing this assessment, additional soil and groundwater samples were collected in May and October 2000, to satisfy recent data requirements of the California Regional Water Quality Control Board, Central Valley Region (RWQCB). The purpose of this letter is to update the screening evaluation with results from the additional investigations. This screening risk evaluation and the January 2000 screening risk evaluation were conducted based on the human health risk methodologies presented in the Preliminary Endangerment Assessment Guidance Manual (PEA; Cal-EPA, 1994). The PEA evaluation is based on the maximum concentration for each of the constituents detected in each medium at a site. This approach results in conservative estimates of the theoretical lifetime excess cancer risk and noncancer hazard index under a reasonable maximum exposure assuming a residential setting. Thus, if the estimated health risks associated with the maximum concentrations are below the acceptable risk range, as defined by U.S. EPA and Cal-EPA, it can be concluded that the constituents detected at a site should not present an unacceptable health risk to hypothetical residential receptors. The results of the May and October 2000 investigations are presented in the Geomatrix Report of Additional Soil and Groundwater Investigation, December 2000. Tables and figures from this report that summarize the results are included in Attachment 2. Total petroleum hydrocarbons quantified as crude oil (TPHc), benzene, toluene, ethylbenzene, • xylene (BTEX), and polynuclear aromatic hydrocarbons (PAHs)were not detected above the raeornatrixz Consultants, Nnc. Engineers, Geologists, and Environmental Scientists