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4 August 1995 AGE-NC-95-0120 Page 2 of 5 t2 3 EXCAVATION AND BACKFILLING On 10 and 11 May 1993, approximately 500 cubic yards of contaminated soil were removed from ' tank pit #1 Soil vwas 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 lugh ground water of approximately 45 feet, a ground water investigation involving the xnstaflation 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 t2 4 MONITORING WELL INSTALLATION AND ASSESSMENT From 3 to 5 November 1993, five soil bonngs were drilled at the site Borings 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 ground water momtonng wells (MW-1, MW-2 and MW-3) A total of ' 14 soil samples were analyzed for total petroleum hydrocarbons as gasoline (TPH-g) and volatile aromatics(benzene; toluene, ethyl benzene and xylenes, BTE&X) Three samples were also analyzed for TPH as diesel(TPH-d) ' p 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-g in five of the 14 samples analyzed The ' highest concentration of gasoline was detected in B-10 at a depth of 40 feet (B10-40) Toluene was detected only in sample B11-55, at the detection Wntt of 0 005 parts per mullion (ppm) Ethyl benzene and xylenes were detected in three samples at concentrations up to 0 018 ppm (MW 1-45) No ' benzene or diesel were detected in any of the samples ' 2 5 GROUND WATER MONITORING ' Ground water moanitonng was performed by GeoAudit on three occasions since November 1993 TPH-d was detected in all three monitoring wells during each sampling event at concentrations ranging from 90 pg/l (parts per billion, ppb)to 400 ppm TPH-g was detected only in samples from ' MW-3, at concentrations of 53 ppb and 63 ppb Analytical results are summarized on Table 2 The ground water gradient has ranged from 0 0033 ft/ft to 0 0063 ft/ft with a flow direction of N 35° E to N 54°E 1