SITE INFORMATION AND CORRESPONDENCE

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(3 October 21, 2015 Reference: Response to California Regional Water Quality Control Board, Central Valley Regional Letter dated June 15, 2015 and Request for Additional Methane and Soil Gas VOC Investigation BRIEF BACKGROUND Site description The Site currently consists of relatively flat, fallow land and is mostly undeveloped except for Delta View located on the western and southern portion of the Site. A State Department of Motor Vehicles (DMV) building is located to the southeast of the Site. The Stockton Deep Water Channel (McLeod Lake), Tuleburg Levee, Weber Avenue, and an office building complex are located north of the Site. The Mormon Slough runs northwest to southeast of the site from the Stockton Deep Water Channel. The Work Net building is southeast of the Site across North Lincoln Street and there is a children's museum several 100 meters east of the Site across North Lincoln Street on the south side of West Weber Avenue. Environmental investigations began at the Site in 1989 and quarterly monitoring and sampling activities were initiated in 2002. SECOR International Inc.(now Stantec) was retained in late 2002 for environmental consulting services at the Site. A summary of previous investigations is included in Attachment 3. REGIONAL GEOLOGY/HYDROGEOLOGY The shallow groundwater bearing zone (A-Zone) primarily extends from the ground surface to depths ranging from approximately 25 ft below ground surface (bgs) to 40 ft bgs. Subsurface material in the shallow zone predominantly consists of silt, silty sand, and silty gravel from four to 10 ft bgs, and then organic clay, clay, high permeability silt, and lenses of silty sand and poorly graded sand to depths ranging from 25 to 40 ft bgs. The shallow zone is generally underlain by low permeability silt and clay (with an occasional, thin, silty sand or poorly graded sand lens) of highly variable thickness. A deeper groundwater bearing zone (B-Zone) is encountered at a depth of approximately 42 ft bgs and extends to approximately 50 ft bgs. Subsurface material in the B-zone consists of poorly graded sand and silty sand. Below 50 ft bgs, clay and silt with some imbedded sand is encountered and extends to a depth of approximately 64 ft bgs, underlain by sand and silty sand of varying moisture extending to a depth of approximately 73 ft bgs. Clay is encountered below the sands to an approximate depth of 84 ft bgs, underlain by a sand/silty sand layer extending to approximately 92 ft bgs. This stratum is a water-bearing zone identified as the C-Zone. Below this sand layer, imbedded clay and silt are encountered to a depth of approximately 115 ft bgs, underlain by a sand/silty sand layer with some imbedded clay to a 6