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Work Plan—Site Sampling Page 11 West Weber Site-Stockton GPE Project 536.2 part, man-made and therefore there are no naturally occurring background levels. All soils, however, contain natural levels of heavy metals. In some cases, these naturally occurring metals may even exceed established threshold levels that would trigger a remedial investigation if the metals were artificially introduced. This is especially true with regard to arsenic due to the relatively high natural concentrations of this metal in California soils. It is therefore very important to collect and test a representative group of soil samples to determine what the background concentrations are for the various CAM-17 heavy metals that are being investigated. As described elsewhere, the areas of the site most likely to have been affected by heavy metals contamination include current, or former rail spurs, and sites that may have been impacted by petroleum releases. Ideally, background sample sites are well away from possibly impacted areas, and even off of the subject site altogether. In this case, the West Weber site is surrounded on three sides by water and on the fourth side by the 1-5 freeway. The background sample sites will therefore need to be collected at points on the west Weber peninsula. A group of 10 sample points will be selected from areas that have historically been little used. These will be located mostly along the waterfront. The background samples will be collected from a depth of 1 to 2 by use of the Geo-Probe. The following table provides the expected sample identifiers: Sample ID Sample Location Sample Analytes BG-1 Background Location 1 CAM-17 BG-2 Background Location 2 CAM-17 BG-3 Background Location 3 CAM-17 BG-4 Background Location 4 CAM-17 BG-5 Background Location 5 CAM-17 BG-6 Background Location 6 CAM-17 BG-7 Background Location 7 CAM-17 BG-8 Background Location 8 CAM-17 BG-9 Background Location 9 CAM-17 BG-10 Background Location 10 CAM-17 CAM-17=17 Priority Contaminant Heavy Metals The locations of these samples points are shown on Figure 3. 2.1.2 Soil Gas Sampling and Analysis The soil gas samples will be collected with the use of a Geo-Probe soil boring rig equipped with PRT soil gas sampling equipment. Borings of 1.5 inches diameter will be advanced to a depth of 5 feet. The Post-Run Tubing (PRT) system inserts a gas sampling probe to the desired depth. Once the point is reached, the protective sheath is retracted to expose the sampling tip. The annulus between the connective tubing and the soil boring is sealed with bentonite to prevent ambient air from entering the sample chamber. The subsurface vapors are allowed to equilibrate for 20 to 30 minutes and the atmospheric air will then be triple 2937 Veneman Avenue, Suite B240 Geo-Phase Environmental Inc. Phone: (209)569-0293 Modesto, Ca 95356 Fax: (209)569-0295