ARCHIVED REPORTS_SITE INVESTIGATION REPORT

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' Site Investigation Report for the Former UST Locations at Stockton AASF California Army National Guard 1.3 Physiographic Setting Approximately 90% of the land surface within the AASF is paved. Some grasses exist throughout the area; landscaping and maintenance is performed to reduce vegetation around aircraft locations. The ' site has been graded and modified to promote surface drainage into storm drain inlets(Figure 1-2). Catch basins collect rainwater and divert the runoff to the north through underground pipes into the adjacent airport/Stockton storm/sewer water system. The system drains approximately 1.5 miles ' north of the site into Little John Creek,then into French Camp Slough, and finally into the San Joaquin River(Versar, Inc., 2004). ' 1.4 Geology 1.4.1 The site lies within the Sacramento Valley which is composed of unconsolidated Quaternary sediments from alluvial and lake deposits. The Sierra Nevada Mountains to the east are a source of ' the alluvium deposits as a result of erosion. Also contributing to the deposits are the Sacramento Valley Coast Ranges to the west. Both ranges consist of a wide variety of rocks and granite,which are metamorphic (ranging widely in age and composition) and sedimentary. In addition,the Stockton ' Fault, which trends northeast-southwest, is located within the city limits, as well as the Midland Fault Zone (trending north), located approximately 25 miles northwest of the site(Versar, Inc., 2004). ' 1.4.2 Depth-to-groundwater in the vicinity of the site is estimated to be approximately 37 to 44 feet bgs. The groundwater gradient is 0.004 feet/foot(Versar,Inc., 2004),however,the gradient cannot be accurately measured based on data from only three closely spaced wells. Based on the limited ' data, groundwater flow is estimated to be toward the north; this conclusion is based on the historical groundwater measurements collected from the three existing AASF monitoring wells. ' 1.4.3 Based on the 12 soil borings drilled and sampled by URS in March 2008,vadose zone soils from beneath the surface fill to groundwater consist of interlayered silty clays,clayey silts, and silty and clayey sands (soil boring logs are shown in Appendix A). ' 2.0 AREAS OF POTENTIAL ENVIRONMENTAL CONCERN 2.1 The areas of potential environmental concern at the AASF, in regard to this site investigation, are the locations of the two former 20,000-gallon USTs and the three former 5,000-gallon USTs and associated piping and fuel dispensers. 2.2 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 the AASF). This site investigation is a continuation of those investigations, and is intended to determine the extent of contamination at the AASF. Contaminants of potential concern associated with this area include TPH-D,TPH-G, BTEX, oxygenates,and lead. VOCs were also detected in both soil and groundwater during the URS investigation; however,no potential source areas for VOCs have been identified during previous AASF site investigations. 3.0 CONCEPTUAL SITE MODEL ' This subsection presents details of the previous investigations of the area of the two former 20,000- gallon USTs at the Stockton AASF, the nature and extent of petroleum impacts, and migration potential.The following information describes the investigations nearby/adjacent to the three former KAWprocess\00251\CA ARNG\Stockton AASF SI\FINAL\SI Rpt.doc 3 June 2008