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I plastic bag, examined visually for gas plant residues, and held for headspace analysis. In the event of poor sample recovery, the lead tube will be retained for possible analysis, and the remaining recovered soil will be examined as above. Visual and olfactory observations noted during the previous investigation at the Tracy site (PG&E 1987) suggest that gas plant residues exist in soil to a depth of approximately 5 feet in those areas investigated. It is therefore anticipated that soil borings will be typically advanced to a depth of 10 feet. ' Drilling augers will be steam cleaned between each boring to minimize the potential for cross -contamination. Runoff from steam -cleaning operations will be contained and stored for appropriate disposal. 3542a/BAV102 9 The initial determination of the presence of gas plant residues in soil for laboratory borings, and the selection of soil samples analysis, will be based on a qualitative screening process performed in the field. After ' heating each soil sample bagged for field analysis, headspace vapors will be analyzed with a Photovac TIP 1 Air Analyzer (PID). The PID will be calibrated several times each day, using a gas containing known hydrocarbon concentrations. The resulting data will provide a qualitative measure of the presence of ionizable hydrocarbons in soil samples. This information, along with visual and olfactory observations, will provide the basis for soil sample selection for laboratory analysis. The depth of each boring will be determined by the extent of any gas plant residues encountered in the field. At those locations where residues are encountered, soil borings will be advanced to a depth two sample intervals below the deepest apparent residues detected in the field, to a maximum depth within 5 feet of the bottom of the uppermost aquifer (estimated at approximately 50 feet below the ground surface). At those locations where no gas plant residues are detected in the field, borings will be advanced to ' a maximum depth of 10 feet. Section 4.2.1.3 provides the soil sampling interval rationale for this investigation. Visual and olfactory observations noted during the previous investigation at the Tracy site (PG&E 1987) suggest that gas plant residues exist in soil to a depth of approximately 5 feet in those areas investigated. It is therefore anticipated that soil borings will be typically advanced to a depth of 10 feet. ' Drilling augers will be steam cleaned between each boring to minimize the potential for cross -contamination. Runoff from steam -cleaning operations will be contained and stored for appropriate disposal. 3542a/BAV102 9