ARCHIVED REPORTS_XR0002255 - Laserfiche WebLink

Home Browse Search

i 1 Jonas & Associates Inc ' The survey involved driving a slotted sampling probe into the soil and using a vacuum pump to obtain samples of air from soil pores above the water table The ' air samples were collected from the drive tube with a microsyringe, and analyzed in the field for selected volatile components of gasoline with a portable gas chromatograph The gas chromatograph was field calibrated with 10 ppm ' standards of benzene and toluene (PG&E, 1988) Figure 3-2 shows the sample locations of the soil vapor survey Thirteen sample points were chosen in a grid pattern surrounding the tank area to provide the best estimate of potential migration pathways A vertical profile of the vadose zone was initially sampled at location V1, next to the fuel tank Vapor samples were extracted from the soil at 3, 5 5, 8, and 9 5 feet No detectable compounds were observed at V1, so the survey proceeded to V2 Hydrocarbon vapors were detected at V2 at a depth of 7 5 feet, and all remaining samples were taken at that depth This depth ' was slightly above the water table elevation found at that time (PG&E, 1988) Hydrocarbon vapors were detected at three sample locations The highest vapor ' concentrations (instrument response was 11,600 volt-seconds in the benzene range) were detected at point V2, directly to the north-northeast of the underground tank Much lower vapor concentrations were detected northeast of the tank at V7 (instrument response was 160 volt-seconds in the benzene range) and southwest of the tank at V5 (instrument response was 14 volt-seconds in the benzene range) ' Results of the soil vapor survey performed in June 1987 indicated that elevated hydrocarbon levels existed to the northeast (locations V2 and V7) and, to a lesser extent, the southwest (location V5) of the former tank site Elevated hydrocarbon ' levels were not detected to the east of the tank, nor further than 60 feet away from the tank in any direction (PG&E, 1988) ' To determine the groundwater flow direction and gradient, four wells were installed (OW-1, OW-2, OW-3, and OW-4) in December 1987 and January 1988 Figure 3-1 presents the well locations An attempt was made to locate at least one of these ' wells (OW-2) in a position hydraulically downgradient of the tank site Groundwater samples were subsequently collected from all of the wells for chemical analysis, as recommended by the Central Valley Regional Water Quality Control Board (PG&E, ' 1988) In 1988, two additional wells (see Figure 3-1) were then located, using results of the gradient analysis and the soil vapor survey, to provide more water quality sampling points downgradient (north-northeast) of the former UST site These wells (OW-5 and OW-6) were installed, developed and sampled in the manner described 9