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1.0 INTRODUCTION Versar, Incorporated (Versar), on behalf of the Sacramento District Corps of Engineers (SD- COE) and the California Army National Guard (CARNG), has prepared this work plan for a Preliminary Assessment / Site Investigation Update (PA/SI Update) at the Stockton California Army National Guard Complex in Stockton, California (Site). The PA/SI Update is being performed to address residual environmental issues identified at the Site in the PA/SI report prepared by Versar on behalf of the CARNG in May 2000. The Site is divided into four divisions: 1) the Army Aviation and Support Facility (AASF); 2) the Combined Support and Maintenance Shop (CSMS); 3) the Operation and Maintenance Shop (OMS); and 4) the Armory. Environmental concerns were not identified at the Armory. Versar's 2000 PA/SI identified residual petroleum hydrocarbons in soil at two locations at the OMS and one location at the CSMS. Buried objects in the vehicle storage area east of the CSMS were also investigated. In the PA/SI Update, the petroleum hydrocarbons in soil at the OMS and CSMS will be further investigated, the buried objects east of the CSMS will be excavated and secured on Site, a recent fuel spill will be assessed at the AASF, and three monitoring wells at the AASF will be sampled. 1.1 SCOPE OF WORK The objective of the proposed work is to collect Site-specific information necessary for the evaluation of the residual and new areas of concern at the AASF, CSMS, and OMS. To meet this objective, soil borings will be drilled, soil and grab -groundwater samples collected and submitted for chemical analysis. Three existing groundwater monitoring wells will be sampled to evaluate groundwater conditions in the area of five former underground storage tanks (USTs). In addition, two areas of pavement upwelling will be excavated, and the reason for the upwelling identified and mitigated. A detailed discussion of the investigation activities is included in Section 3.0, Site Investigation. 1.2 GEOLOGY The Site is located in the central portion of the Sacramento Valley. The Sacramento Valley is composed of unconsolidated Quaternary sediments from alluvial and lake deposits. This alluvium is the result of the erosion of the foothills of the Sierra Nevada mountains to the east. The Sierras are composed of a wide variety of rocks, including granitic, metamorphic, and volcanic rocks, which range considerably in age and composition. To the west of the Sacramento Valley are the Coast Ranges which consist of the Franciscan Complex of sedimentary and metamorphic rocks. Structurally significant is the Stockton Fault which trends northeast -southwest and is located within Stockton city limits, and the north trending Midland Fault Zone which is located approximately 25 miles northwest of the Site. The soils beneath the AASF, characterized during drilling in February 1993 (Wallace -Kuhl), consisted of fine sands and silt/sand mixtures to approximately 50 feet below ground surface 3027-03/104700.4766.230/NL23' 03