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0 Field Sampling Plan for the Former UST Location at Stockton FMS #24 California Army National Guard ■ 1.4.2 Depth to groundwater in the vicinity of the site is estimated to be approximately 37 to 44 feet below ground surface (bgs), and groundwater gradient has been determined to be flat because the average hydraulic groundwater gradient of 0.004 foot/foot (ft/ft) 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 FSP, 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 FSP is a continuation of those investigations to determine the extent of detected contamination at the FMS #24 facility. Contaminants of potential concern (COPCs) associated with this area include TPHD, TPHG, BTEX, oxygenates, and lead. 3.0 CONCEPTUAL SITE MODEL The following subsections present the details of any previous investigations pertaining to the UST investigation at the Stockton FMS, the nature and extent of contamination, and contaminant migra- tion potential. 3.1 Previous Investigations Two former USTs (one 5,000 -gallon diesel and one 10,000 -gallon gasoline) were removed in 1996 and soil samples collected during removal and excavation of the USTs and associated piping indicated that releases of TPHG, TPHD, and BTEX contaminants were detected in soil. In March 2000, two soil borings were drilled and soil samples were collected at discreet depths (10, 15, and 20 feet bgs) and analyzed for hydrocarbons. Sampling results indicated TPHG and TPHD concentrations were present in soil up to 25.4 mg/kg and 209 mg/kg, respectively (Vesar 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 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 comparison of water quality goal criteria for detected contaminants. Figure 3-1 shows the previous borehole locations at the former UST location. Soil and groundwater results are described in the below sections. 3.2 Nature and Extent of Contamination 3.2.1 No surface soil or stream sediment samples have been collected at the previous UST location for Stockton FMS. K:\Wprocess\00251\CA ARNOStockton FSP\FINAL\Stockton FSP.doc 4 August 2006