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r Subsurface Investigation Report CHP Facility, 3330 North Ad Art Road, Stockton April 19 1999 The scope of this investigation includes only the uppermost deposits within the Central Valley. Continental deposits of Tertiary and Quaternary age compose a number of formations; in total they constitute the major aquifer of the Central Valley and yield large quantities of water to wells. Lithologically, these deposits are heterogeneous mixtures of poorly sorted clay, silt, sand, and gravel, and in places some beds of claystone, siltstone, sandstone, and conglomerate. The maximum thickness reported for these formations is 3,000 feet (USGS 1986). Continental deposits of Quaternary age outcrop primarily along the major rivers and streams of the valley, as well as along other low-lying areas; the deposits include river and flood-basin deposits, and sand dunes. River deposits, including channel and flood-plain deposits, are considered to be the most permeable deposits in the valley, however, they are generally not tapped by wells. Flood- basin deposits consist largely of fine-grained beds that restrict the vertical movement of water (USGS 1986). 3.0 SITE ASSESSMENT ACTIVITIES On March 25, 1999, four borings (B-1 through B4) were advanced onsite to approximately 25 fbg at the locations shown on Figure 2 using direct-push methods. Soil samples were collected continuosly in each boring using a 1.5-inch diameter macro-core sampler. Following completion of the sampling activities, each boring was sealed from total depth to the surface grade using neat cement. The boring logs and general field procedures are provided in Appendix A. Soil samples collected at 25 fbg from each boring were submitted to a state-certified laboratory and analyzed for total petroleum hydrocarbons as gasoline (TPH-G), benzene, toluene, ethylbenzene, and total xylenes (BTEX), and oxygenates by EPA Method 8260. The results of the laboratory analyses are listed in Table 1. The analytical results are also displayed relative to their collection location on Figure 3. Refer to Appendix B for a description of the analytical methods used and copies of the official laboratory reports, quality assurance/quality control (QA/QC) reports, and chain of custody records. 4.0 FINDINGS Soil types encountered during drilling generally consisted of silts and sands from surface to the total explored depth of 25 fbg. Groundwater was not encountered in any of the four borings. No significant hydrocarbon concentrations were detected in any of the samples analyzed. Trace concetrations of MTBE were detected in the samples collected from B-2 and B4, at concentrations of 8 and 7 ppb, respectively. All other gasoline-range hydrocabons were below detection limits.