ARCHIVED REPORTS_RESULTS OF THE UPDATED HUMAN HEALTH RISK SCREENING EVALUATION

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Ms. Dink Mather March 14, 2000 Page 2 • Document Reviewed We reviewed "Soil and Groundwater Investigation and Screening Health Risk Assessment of the Old Valley Pipeline Right-of-Way" Pombo Property (APN 232-170-07), Tracy, California. This document, dated September 1999, was prepared by Geomatrix Consultants, 2101 Webster Street, 12"' Floor, Oakland, California 94612, contractors to Chevron Environmental Management Com- pany. Scope of Review The Soil and Groundwater Investigation and Screen Health Risk Assessment of the Old Valley Pipeline Right-of-Way was reviewed for scientific content and sufficiency to support a risk-based clean closure of the site under the Voluntary Cleanup Program. Sampling details, analytical methodology, and technical approach were evaluated. General Comments A total of 15 soil samples and 1 grab groundwater sample were taken to define the vertical and horizontal extent of petroleum hydrocarbon affected soil and groundwater. Soil sample depth began at 8.5 feet bgs and advanced downward to 16- to 25 feet bgs. Surface soils were not affected and, presumably, the former pipeline was located approximately 5 feet below grade. The extent of petroleum hydrocarbon contamination in the soils and groundwater adjacent to the • former pipeline right-of-way is limited vertically and horizontally. Levels of petroleum-related hydrocarbons ranged from ND to 18,000 mg/kg for TPH crude in soils. BTEX in soils ranged from ND for benzene to 7.5 mg/kg for toluene. Low levels of PAHs in soils (commonly associ- ated with crude oils, fuel oils, and diesel) were detected up to 5.1 mg/kg for phenanthrene. Benz(a)anthracene and chrysene were the only PAHs detected at the site that were potential carcinogens (0.13 and 0.54 mg/kg). None of the constituents identified above were from sur- face soil samples. Specific Comments 1. The background information indicates that a former Mohawk gas station was located on the property. No location is provided in the text or Figure. From the sarnp!ing !C-2tin^s it is as- sumed that the former location was at or near the southeast corner of the property. Please clarify. 2. The text states that nine of fifteen samples were positive for petroleum related hydrocarbons based on staining while the results of the OVA screening indicates eight of fifteen samples were positive. Is this correct or there was staining without OVA positive readings? 3. All of the petroleum related hydrocarbons were identified in the "lean clays" horizon of the soil. Petroleum related hydrocarbons were identified at up to 18,000 mg/kg for crude oil and up to 6,800 mg/kg for diesel along the former pipeline. Gasoline was identified along the pipeline and where the suspected former Mohawk gas station was located. What is the source of gasoline along the pipeline if the pipeline only transported crude oil and heavy fuel • oils?