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remove contamination that might be present below a depth of 15 feet, 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 PHSIEHD 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 bonngs were drilled at the site Bonngs B-10 and B- 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 bonngs 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 bonngs 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 (1310-40) Toluene was detected only in sample 1311-55, at the detection limit of ' 0 005 parts per million (ppm) Ethylbenzene and xylenes were detected in three samples at concentrations up to 0 018 ppm (MW1-45) No benzene or diesel were detected in any of ' the samples 2.6 Groundwater Monitoring ' Groundwater monitoring procedures were conducted on November 10, 1993, and May 27, 1994 TPH as diesel was detected in all three monitoring wells dunng both sampling events ' at concentrations from 90 µg/L (parts per billion, ppb) to 390 ppm TPH as gasoline was detected only in the sample from MW-3, at 53 ppb during the November 1993 monitoring Toluene was detected in the samples from MW-1 and MW-3 at concentrations of 199 ppb ' and 2 4 ppb, respectively during the November 1993 monitoring On November 10, 1993, the groundwater gradient was calculated to be 0 0063 ft/ft directed C;..bp�l A.&S.—Ix p_fi.o..va3,9-9=,3 1 r