SITE INFORMATION AND CORRESPONDENCE

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M. Scott Mansholt - 2 �- • 26 March 2008 990 Beechnut-Tracy depth of affected soil/groundwater) would preclude the shallow groundwater at the site from being a source of municipal water. An investigation was conducted in 2003 on- and offsite to investigate/delineate areas of elevated polynuclear aromatic hydrocarbons (PAHs) identified by earlier investigations. Fourteen soil borings completed off-site in the Greenbelt Parkway, were at locations lateral to earlier onsite borings where elevated PAH concentrations were previously reported. The Greenbelt Parkway, adjacent to the west of 990 Beechnut, is part of the former Alden Park project site. Analytical results of the 14 borings confirmed the lateral extent of PAHs was adequately delineated and primarily located in the vicinity of onsite boring GMX-09. Additional onsite borings were to assess previous locations reporting elevated TPH compounds. Consequently, Geomatrix utilized the existing data to conduct an exposure assessment and complete the Screening Level Risk Assessment to estimate the potential human exposure to chemicals of potential concern (COPCs) in soil, water, and air. At this time, Chevron does not have a formal plan for future use of the site. The City of Tracy General Plan map shows the site is zoned medium density residential, which allows uses other than residential development. According to the Geomatrix report the assessment considered future residents and commercial/industrial workers. In their report, Geomatrix used Environmental Screening Levels (ESLs) published in 2007 by the San Francisco Regional Water Quality Control Board for both residential and commercial/industrial receptors. Use of the ESLs is consistent with similar risk evaluations completed for other Chevron properties along the historical pipeline corridor; for the purposes of determining risk and hazard, Geomatrix considered subsurface conditions from the surface to 10-feet (3-meters) in addition to data from 10-feet and deeper. According to Geomatrix, risk-based ESLs represent the concentration of chemicals in soil associated with a target acceptable cancer risk level of 1X10-6 or a target non-cancer hazard index of less than 1.0. These values are based on standard U.S. EPA estimates of toxicity and default for residential or commercial/industrial exposure conditions. The soil ESLs Geomatrix used in their risk evaluation considers potential exposure to soil via direct contact (i.e., ingestion, dermal contact) and inhalation of particulates. However, since the ESLs do not provide screening values for soil based on the protection of indoor air quality, Geomatrix utilized an equivalent soil vapor concentration conservatively calculated assuming equilibrium partitioning. The onsite soil concentrations were used to estimate equivalent soil vapor concentrations following Department of Toxic Substances Control *1 (DTSC) guidelines. The equivalent soil vapor concentrations (derived from the DTSC vapor equivalency determinations) were compared to the soil vapor ESLs to evaluate potential exposures to COPCs as a result of subsurface vapor intrusion. *1 Department of Toxic Substances Control, 2005, Guidance for the Evaluation and Mitigation of Subsurface Vapor Intrusion to Indoor Air, Cal-EPA Interim Final, February 7 (revision).