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2.4 Excavation and Backfilling On May 10 and 11, 1993, approximately 500 cubic yards of contaminated soil were removed from tank pit #1 Soil was removed from the excavation to a depth of 35 feet at the southern end of the excavation Field screening with a photo-ionization detector (PID) detected organic vapors at the floor of the enlarged excavation In subsequent verbal communication, the PHS/EHD notified Pacific Gas that, due to seasonal high groundwater ' of approximately 45 feet, a groundwater investigation involving the installation of three monitoring wells would be required On May 12 and 13, 1993, the excavation was partially backfilled Crushed rock and pea gravel were used to backfill the southern portion of the excavation The entire excavation was then backfilled and compacted to a depth of 5 feet below surface grade 2.5 Monitoring Well Installation and Assessment ' From November 3 to 5, 1993, five soil borings were drilled at the site Borings B-10 and 13- 11 were placed in the excavation (approximately 5 feet below surface grade), advanced to depths of 75 and 70 feet respectively and backfilled with portland cement Three other borings were advanced to 88 feet and completed as 2-inch groundwater monitoring wells (MW-1, MW-2 and MW-3) A total of 14 soil samples were analyzed for total petroleum hydrocarbons (TPH as gasoline) and volatile aromatics (BTE&X) Three samples were also analyzed for TPH as diesel Hydrocarbon odors were noted in soil samples from borings B-10, B-11 and MW-1 at depths ' between 45 and 65 feet Laboratory analysis detected TPH as gasoline in five of the 14 samples analyzed The highest concentration of gasoline was detected in B-10 at a depth of 40 feet (B-10-40) Toluene was detected only in sample B-11-55, at the detection limit of ' 0 005 parts per million (ppm) Ethylbenzene and xylenes were detected in three samples at concentrations as high as 0 018 ppm (MW1-45) No benzene or diesel were detected in any ' of the samples 2.6 Groundwater Monitoring ' On November 10, 1993, the water levels in all three wells were measured relative to the top of the casing Each well was then purged and samples collected Diesel was detected in ' groundwater samples at concentrations of 260 gg/L (parts per billion), 390 ppb and 140 ppb in monitoring wells MW-1, MW-2 and MW-3, respectively Gasoline was detected only in the sample from MW-3, at 53 ppb No benzene, ethylbenzene or xylenes were detected in ' the water samples Toluene was detected in the samples from MW-1 and MW-3 at concentrations of 199 ppb and 2 4 ppb, respectively On December 3, 1993, the elevation of the three monitoring well casings were surveyed relative to the fire hydrant at the southeast comer of Mariposa and Farmington Roads The 'A r..wbE.-L3 Mit Qerma l.c A P..f.0-A2 1691/552 3 4