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! • amec Mr. C. Dean Hubbard California Regional Water Quality Control Board February 1, 2012 Page 2 support possible closure with unrestricted use (i.e. suitable for either future commercial/industrial or residential use scenarios). A summary of the detections that exceeded risk-based screening levels for residential use are summarized below along with the rationale for further characterization. • Ethylbenzene in one sample from former boring GMX-13 in the northeast corner of the site. At this location, ethylbenzene was detected at 0.06 milligrams per kilogram (mg/kg) at 12 feet below ground surface (bgs) in a sample collected in 2001. The ethylbenzene concentration in this sample exceeds screening levels for the vapor intrusion pathway, for both a hypothetical future resident or commercial/industrial user. Ethylbenzene was not detected above laboratory reporting limits in 33 additional samples collected at the site. The sample with the elevated ethylbenzene was collected more than 10 years ago and volatile petroleum hydrocarbons are known to biodegrade in the unsaturated zone under aerobic conditions.2 Therefore, additional sampling at the same location and depth, in addition to sampling around this location, may demonstrate that the detection was isolated and has since degraded. • Total petroleum hydrocarbons quantified as crude (TPHc) in one sample from former boring GMX-02 in the northwest corner of the site. At this location, TPHc was detected at 21,000 mg/kg in a soil sample collected from 9.5 to 10 feet bgs. This concentration exceeds the risk-based screening level for direct contact with soil for a potential hypothetical future on-site resident. As a conservative practice, risk assessments assume that residents could be directly exposed to soil up to a depth of 10 feet bgs.3 No shallow soil samples were collected from boring GMX-02, however, the boring log indicates that soil above 7.5 feet bgs may not be impacted by petroleum. Since the TPHc detection from GMX-02 is at the maximum depth applicable for direct contact, more shallow soil sampling will be performed at the same location in addition to collecting adjacent samples at similar depths to characterize TPHc concentrations in soil up to a depth of 10 feet bgs in the vicinity of GMX-02. The concentrations of chemicals in soil will vary over space and time. However, a single estimate of an exposure point concentration is recommended for risk assessment calculations as currently required by U.S. Environmental Protection Agency ( U.S. EPA) guidance. This single value must be representative of the average concentration to which a person would be exposed over the duration of the exposure. The proposed additional soil sampling will better characterize potential 2 U.S. EPA, 2011, Petroleum Hydrocarbons and Chlorinated Hydrocarbons Differ in Their Potential for Vapor Intrusion, Office of Underground Storage Tanks,Washington, D.C., September. http-//www.epa.gov/oust/cat/pvi/pvicvi.pdf s Department of Toxic Substances Control (DTSC), 1996 , Supplemental Guidance for Human Health Multimedia Risk Assessments of Hazardous Waste Sites and Permitted Facilities (corrected and reprinted), Office of the Scientific Advisor, California Environmental Protection Agency(Cal-EPA), Sacramento, California.