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ACE07-106-H Effective: April 00 This section provides a discussion of background information regarding the project site including a brief description of the site history. iAINII I XOTO7T11OI01 The DDJC-Sharpe Facility is located northeast of Lathrop, California in the San Joaquin Valley approximately 10 miles south of Stockton, California (Figure 2-1). The actual project site is a triangular shaped area (approximately 6,000 square feet) located immediately north of Building 199 in the northwest corner of the DDJC-Sharpe Facility (Figure 2-2). DDJC-Sharpe was established in 1941 during World War Il and consists of 724 acres. The present facility mission includes storage, handling, preservation, packaging, and shipment of general supplies and equipment. Until 1976, the facility mission included heavy equipment and aircraft maintenance. Maintenance activities included painting/paint stripping, metal refurbishing, engine overhauls, hydraulic and electrical repairs, airframe and body work, and component repair and refurbishing. The area requiring an interim soil remedial action is located north of Building 199 in an area formerly containing a maintenance shop (Building 180) and a 750 -gallon diesel tank (Tank #27). The areas of Buildings 180 and 199 have had a history of equipment maintenance. Building 199 is a 60 -foot by 80 - foot building used to house a wash rack facility. The building was constructed in 1991. The facility located at Building 199 prior to 1991 was also used for wash rack operations. The DDJC-Sharpe Facility is underlain by approximately 60 feet of interbedded, unconsolidated or discontinuous mixtures of sand, silt, and clay. These lithologic units are characteristic of the continental fan and interfan deposits of the Victor Formation (ESE 1992). The Victor Formation thins and blends into the Laguna Formation, which is composed of stream -laid units of discontinuous sand and silt with minor clay and gravel. In the vicinity of the DDJC-Sharpe Facility, the Victor Formation and underlying Laguna Formation are indistinguishable and may be overlain by recent stream deposits of similar composition. Subsurface lithology from approximately 60 feet to 300 feet below ground surface (bgs) is similar in composition to the Victor and Laguna Formations. These deposits are probably part of the upper Laguna Formation. Based on data collected during recent construction activities at the site, a hard silty clay to clayey silt layer was present at approximately two to six feet bgs. This hard clay -silt layer was overlain by mostly sandy silt to silty sand. A review of boring logs (MW 439-A and 472-A; performed by others) in the vicinity of the interim construction activities generally agree with recent construction observations with the exception that the shallow near -surface soils were classified to be more coarse grained. 2.2.2 Site Hydrogeology Four principal aquifer zones exist in the upper 300 feet of the DDJC-Sharpe stratigraphy. - These aquifer zones are termed A-, B-, C-, and D -zones and are located at approximately 25, 60, 140, and 270 feet bgs, respectively (ESE 1992). The A- and B -zones are encountered at varying depths and are often C:Ucg\Apr 00\PCRBLDG199.doe 2-1