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Supplementary Site Assessment Report �r Former Mobil Bulk Plant 04-343 April 26, 1996 Onsite, soil hydrocarbons were excavated to groundwater, approximately 10 fbg, in July 1990. Greater than 6,000 cubic yards of soil were hauled to appropriate disposal facilities. ^ ' Approximately 7,300 cubic yards of clean fill were imported and compacted to grade. • Dissolved-phase hydrocarbons have historically been detected in Monitoring Well AW-3, ' located in the northwest corner of the site. i • Free product has not been observed in any monitoring well. ' In 1993, a human health risk assessment was performed for residual hydrocarbons present at the site (Alton Geoscience, 1993). Potential human health risks associated with the current site conditions are within acceptable levels as established by the State of California and the United States Environmental Protection Agency (EPA). 4.0 FIELD ACTIVITIES 4.1 DRILLING AND SOIL SAMPLING ' Between May 22 and June 1, 1995, four soil borings were drilled to depths ranging from 20 to 40 fbg. Soil samples were collected at 5-foot depth intervals where possible. Refer to Appendix A for details regarding general field procedures, drilling permits, and boring logs. ' Soil samples collected during drilling activities were submitted to a state-certified laboratory and analyzed for total petroleum hydrocarbons as gasoline (TPH-G) using EPA Method 8015 ' modified for gasoline, total petroleum hydrocarbons as diesel (TPH-D)using EPA Method 8015 modified for diesel, and benzene, toluene, ethylbenzene, and total xylenes (BTEX) using EPA Method 8020. The results of laboratory analysis of soil samples are listed in Table 1 and shown on Figure 3. Refer to Appendix B for copies of the official Laboratory Reports, Quality Assurance/Quality Control (QA/QC) Reports, and Chain of Custody Records. 4.2 FLUID-LEVEL MONITORING AND GROUNDWATER SAMPLING On August 16 and October 24, 1995, and January 26, 1996, fluid levels in Monitoring Wells AW-1 through AW-12 were measured and groundwater samples were collected in accordance with standard regulatory protocol. Refer to Appendix A for a description of fluid-level monitoring and groundwater sampling procedures. Compliance groundwater samples were submitted to a state-certified laboratory and analyzed for TPH-G, TPH-D, and BTEX. The results of groundwater sample analysis are listed in Table 2, and reported in the respective 1 I 3 I