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' 9 November 1992 <br /> (GeoAudit CV 42E1-1.8) <br />' Page 4 of 6 <br /> 4.2 Groundwater Sampling <br />' A disposable plastic bailer was used to obtain a sample of <br /> groundwater in boring B1, prior to completion as a vapor extraction <br /> well. The sample was obtained before the augers were removed from <br /> the hole, and immediately analyzed for TPH-g and BTE&X. <br /> 5.0 SITE GEOLOGY <br /> 5. 1 Subsurface Stratigraphy <br /> Based on the logs of the four soil borings drilled during this <br /> phase of investigation (see. Appendix B) , the stratigraphy at this <br /> site appears to be relatively uniform in comparison to many other <br /> sites in the Stockton area. All four borings encountered a three- <br /> layer sequence of deposits. The upper layer consists of light--brown <br /> to light-reddish-brown silt from the surface to at least 15 feet. <br /> This material is dry to slightly damp and slightly consolidated. <br /> Small nodules and stringers that have been weakly cemented by the <br /> precipitation of caliche are present below 10 feet. <br /> Sanples obtained from 20 to 35 feet record a downward increase in <br /> grain size from very-fine-grained sand to gravelly sand, ranging <br /> from medium brown to dark brown in color. The gravel ranges up to <br /> approximately 1 inch in diameter. This sandy layer is loose, <br /> unconsolidated, and ranges from slightly damp in the upper part to <br /> saturated in the gravelly portion at the base. This layer would <br /> I form a good aquifer, but at present it appears that the water is <br /> perched in a thin zone above the underlying silt layer. <br /> Below 35 feet, boring B1 and B2 penetrated a second silt layer. <br /> This layer was more gray in color than the upper silt bed, and <br /> noticeably more consolidated, requiring between 7 and 50 blow <br /> counts to advance the sampler 6 inches. Downward, there may be a <br /> slight increase in the amount of sand in this layer. <br /> Below 65 feet, B1 penetrated a layer of poorly-sorted, medium- <br /> grained, yellow sand. A sampled obtained at 70 feet was saturated. <br /> This layer may be the principal shallow aquifer beneath the site, <br /> although neither its thickness nor extent are known. <br /> 6.0 HYDROCARBON CONTAMINATION <br /> 6.1 Field Evidence <br /> All samples were field-screened with the PTD immediately after <br /> extrusion from the sampler, and the readings were recorded on the <br />