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• Ms Devra Lewis Clayton Page 3 Regional Water Quality Control Board Clayton Project No. 70-03759.00 September 3, 2004 excavation at approximate 50 foot intervals. In addition to the excavation bottom samples, sidewall samples were taken at the bottom of the sidewall and at a location corresponding to a depth of 15 feet below the ground surface. A total of 16 soil samples were collected from the excavation. The soil samples were analyzed for the following constituents: • Total petroleum hydrocarbons as crude oil (TPHcr), • TPH as diesel (TPHd), • benzene, toluene, ethylbenzene, and xylene isomers (BTEX compounds). Twelve of these samples contained no detectable hydrocarbons, and the remainder contained less than 25 mg/kg TPH,with the exception of one sample (PB-50-CL), which contained 2,200 mg/kg TPHcr and 930 TPHd. This sample was collected from the bottom of the excavation just below the water table. The samples collected in the vicinity of the sample (PB-50-N29, 529,N15, and S 15) contained a maximum of 3.8 mg/kg TPH. Thus this elevated concentration appears to be an isolated occurrence. Figure 5 presents the confirmation sampling results from the main excavation area, and Figure 6 presents final soil analytical data for all locations within the area covered by this summary. • Table 1 presents the soil analytical results for the area covered by Figure 6. 3.0 GROUNDWATER INVESTIGATION Following completion of excavation activities in the main excavation area, two groundwater samples were collected from the accumulated groundwater within the excavation. Neither groundwater sample contained detectable petroleum hydrocarbons. Following completion of backfilling of the excavation, Clayton performed a groundwater investigation of the pipeline alignment within Neighborhood E. Clayton advanced soil borings along the alignment at locations where impacts were noted previously. Thus, soil borings were advanced at the location of Area 2, and at the location of Area 3/Area 4 (one boring was advanced at this location due to the proximity of the two areas). Boring B-31 was advanced at Area 3/Area 4, and Boring B-32 was advanced at Area 2. Grab groundwater samples were collected from these borings, and analyzed for petroleum hydrocarbons. The samples were analyzed for TPHd, TPHcr, and BTEX constituents. Low levels of hydrocarbons were detected in the grab groundwater samples, to a maximum of 0.14 milligrams per liter(mg/L) TPHcr in groundwater from B-32. BTEX constituents were not detected in either sample, with the exception of 0.00091 mg/L xylenes in B-32. As noted in Clayton's "Additional Subsurface Soil and Groundwater Investigation Report"dated August 13,2004, these detections are anomalous, as soil impact was not observed below a depth of 14 feet in this area. . Clayton installed six monitoring wells within and adjacent to the main excavation area, and collected groundwater samples from the six wells on three separate dates, June 17, 2004, July 8, 2004, and August 12, 2004. For the June and July events petroleum