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4.0 CONCLUSIONS AND RECOMAfENDATIONS �T The results of RES' soil investigation indicate that the native soils surrounding and beneath the abandoned UST pit have not been affected by petroleum hydrocarbons. TPHd and BTEX compounds were not detected by laboratory analyses of the selected soil samples obtained during the investigation. The analytical results of samples obtained from the excavated backfill material at the time of the tank removal, however, indicated concentrations of 17 and 230 ppm TPHd, 0.016 ppm ethylbenzene, and 0.047 ppm xylenes. The investigation detailed in this report was conducted r because samples of the native soils at the bottom of the UST pit could not be obtained at the time of the tank removal due to caving of the backfill gravel surrounding the newly installed 10,000-gallon UST. The materials removed from the abandoned UST pit were immediately { backfilled to avoid damage or rupture of the adjacent 10,000-gallon UST. f It is RES' opinion that the presence of up to 230 ppm of residual diesel fuel in the backfill materials contained in the abandoned UST pit should not pose a threat to ground water at the site. This opinion is based upon the following: The results of this investigation indicate that the native soils surrounding and beneath the abandoned UST pit have not been affected by petroleum hydrocarbons. The depth to ground water at the site is greater than 50 feet below 1 grade (as measured in the cooling water supply well in December 1993). The infiltration of precipitation or surface runoff through the backfill materials will be eliminated by the placement of pavement at the ground surface in the area of the abandoned UST pit. The subsurface soils at the site consist of low to medium permeability materials (the native soils encountered during this investigation consisted primarily of silts and silty sands to a depth of 21 feet below grade and the native soils encountered from 15 to 115 feet below grade during the November 1993 E water well installation were described as "clay and hardpan" by the drilling contractor). Based upon the conditions of the site outlined above, RFS concludes that no further investigation i or remediation of the UST site'is necessary. 1, 5.0 REPORTING REQUIREMENTS C A co of this report shall be submitted b RFS to Ms. Diane M. Hinson of the SJCPHS at the PY Po Y following address: Ms. Diane M. Hinson San Joaquin County Public Health Services E Environmental Health Division 1 . P.O. Box 388 Stockton, CA 95201-0388 ( 4195) BW"PRMi.RM7OCT% 5 1.