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. 17 April 1996 AGE-NC Project No 95-0120 E Page 2 of 7 contamination Low concentrations of hydrocarbons were detected in borings B7 and B9, near the former pipelines GeoAudit recommended further excavation of tank pit #1 to remove contamination that might be present below a depth of 15 feet r 2 3 EXCAVATION AND BACKFILLING On 10 and i 1 May 1993, approximately 500 cubic yards of contaminated soil were removed from ' tank pit 91 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 seasonally high ground water of approximately 45 feet, a ground water 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 24 SITE ASSESSMENT From 3 to 5 November 1993, five soil borings were drilled at the site Borings B 10 and B I 1 were ' placed in the excavation (approximately 5 feet below surface grade bsg), advanced to depths of 75 and 70 feet bsg, respectively and backfilled with Portland cement Three other borings were advanced to 88 feet bsg and completed as 2-mch ground water monitoring wells (MW 1, MW2 and MW3) 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) ' Hydrocarbon odors were noted in soil samples from borings B 10, B 11 and MW 1 at depths between 45 and 65 feet bsg 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 (B 10-40) Tbluene was detected only in sample B11-55, at the detection limit of 0 005 parts per million (ppm) Ethyl benzene 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 5 REGIONAL GROUND WATER DEPTH AND FLOW DIRECTION The surface of the water table occurs at a depth of approximately 70 feet bsg on the site The uppermost ground water flows in an unconfined aquifer With depth, the aquifer becomes semi- s