SITE INFORMATION AND CORRESPONDENCE 1995-2004

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James L. Tjosvold, P.E. August 19, 1996 Page 17 concentrations of PNAs and petroleum hydrocarbons at 22 feet bgs,which correspond to the barrier encountered at 22 feet bgs. Comment: 11. Page 4-10, Section 4.2.1.3, Potential Source Area 3 (S3) The FS states that S3 is characterized by B-15, SB-19-95, SB-20-95,MW-3, and B-13. SB- 20-95 defines the lateral and vertical extent to the north. To the south, B-15 shows high contaminant concentrations between 3.5 and eight feet bgs and residual contaminants at about 17 and 21 feet bgs. The lateral extent has not been defined, but the vertical extent is defined at about 26 feet bgs. To the east, SB-19-95 defines the lateral extent but not the vertical extent since all the contaminants had the highest concentrations at the deepest sampling point of 16 feet bgs. There is no information to the west at or near the S3 boundary and, therefore, requires lateral and vertical definition. Response: 11. Source Area 3 (S3) is the location of a former tar pit and tar sump. Tar seeps have been observed near B-15 during hot weather. PG&E maintains a steel plate over the seeps to prevent direct contact with the tar. The lateral and vertical extent of S3 was determined by data from the following five soil borings: B-15, SB-19-95, SB-20-95, MW-3, and B-13. Chemical data for S3 are summarized in attached Table 4-4. Total PNAs detected in boring B-15 decrease with depth, with 1,551 mg/kg total PNAs at 3.5 feet bgs and 0.20 mg/kg at 24.0 feet bgs. The maximum total PNA concentration detected in SB-19-95 was 2.1 mg/kg at 15.5 feet bgs. In SB-20-95, total PNA concentrations in samples from 3.5, 11.5, and 23.5 feet bgs were all below 0.5 mg/kg. No PNAs were detected in the soil sample collected between 13 and 18 feet bgs from MW-3. In B-13, 251.7 mg/kg of total PNAs were detected in a sample collected at 0.7 feet bgs while four deeper samples (to 22 feet bgs) contained PNAs ranging from none detected to 5.19 mg/kg. TPH-gasoline was detected at concentrations greater than 100 mg/kg in two borings: B-15 at 7.5 feet bgs (370 mg/kg) and 20.5 feet bgs (398 mg/kg); and B-13 at 16 feet bgs (114 mg/kg). Elevated levels of TPH-diesel were detected in samples from B-15 at 3.5 and 7.5 feet bgs (502 and 6,602 mg/kg, respectively) and from B-13 at 0.7 feet bgs (209 mg/kg). In B-15, cyanide was detected in the samples from 4.5 feet bgs (59.75 mg/kg) and from 21 feet bgs (4.6 mg/kg). To summarize, S3 consists of a tar pit and tar sump that may have been closed in place by paving over the surfaces. Soil located immediately outside the footprint of