ARCHIVED REPORTS FINAL SITE INVESTIGATION

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FMS #i24 Site Investigation Report California Army National Guard • 1.4.2 Depth to groundwater in the vicinity of the site is approximately 40 feet below ground surface (bgs), based on the soil borings drilled during the URS investigations. Groundwater gradient has been determined to be flat because the average hydraulic groundwater gradient of 0.004 foot/foot exists beneath the site (Versar, Inc., 2004). 2.0 AREAS OF POTENTIAL ENVIRONMENTAL CONCERN 2.1 The area of potential environmental concern at FMS #24, in regard to this report, is the former location of the diesel and gasoline USTs. 2.2 Two USTs, one 10,000 -gallon UST that contained gasoline and one 5,000 -gallon UST that contained diesel fuel, once existed at the site. The USTs, which were single -walled steel tanks, were installed sometime in the early 1950s, when the FMS began operation. Both of the USTs were removed in 1996 by Seward Schreder Construction (Versar, Inc., 2004). 2.3 Review of the preliminary assessment/site investigation update report indicates that Versar, Inc., has conducted a preliminary assessment and site investigations of the Stockton facility (including FMS #24). This report is a continuation of those investigations to determine the extent of detected contamination at the FMS #24 facility. Contaminants of potential concern associated with this area include TPHD, TPHG, BTEX, oxygenates, and lead. 3.0 NATURE AND EXTENT OF CONTAMINATION The following subsections present the details of previous investigations at FMS #24, the nature and • extent of contamination, and contaminant migration potential. 3.1 Previous Investigations Soil samples collected during the 1996 removal and excavation of the USTs and associated piping indicated that releases of TPHG, TPHD, and BTEX contaminants were present in soil. In March 2000, two soil borings were drilled and soil samples were collected at discrete depths (10, 15, and 20 feet bgs) and analyzed for hydrocarbons. Sampling results indicated TPHG and TPHD concentra- tions were present in soil up to 25.4 mg/kg and 209 mg/kg, respectively (Versar, Inc., 2004). In February 2004, three additional borings were drilled to groundwater and a total of eight soil samples and three groundwater samples were collected and analyzed for TPH, BTEX, oxygenates, and metals. Table 3-1 (Appendix B) presents the results of analyses of soil samples collected after the removal of the diesel and gasoline USTs and comparison criteria for each detected contaminant. Table 3-2 presents the results of analyses of groundwater samples collected after the removal of the diesel and gasoline USTs and compares them to water quality goal criteria for detected contaminants. Figure 3-1 shows the previous borehole locations at the former UST location. Soil and groundwater results from the Versar investigation are described in Section 3.2. 3.2 Versar, Inc., 2004 Site Investigation 3.2.1 Soil samples were collected from three boring locations around the vicinity of the previous USTs as follows: B30 located northeast of the UST excavation, B31 located southeast of the UST excavation, and B32 located northwest of the UST excavation (see Figure 3-1). Soil samples were analyzed for the following: TPHG, TPHD, BTEX and oxygenates, and lead by EPA Methods • SW8015G and D, SW8260B, and SW6010B, respectively. Soil sampling analysis results indicate that the maximum TPHD concentration was 260 mg/kg at a depth of 35 feet bgs in boring B31, December 2007