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i i CLEARWATER ' G R O U P £nviro"Man Tal Services 1.3 Previous Investigations ' In letters dated September 18, 1992 and April 30, 1996, the SJCEHD indicated that additional investigation of this site was required because petroleum hydrocarbons were detected in soil samples collected from an underground storage tank (UST) excavation According to an April 1 30, 1996 SJCEHD letter, two USTs were removed from the site on June 15, 1992 Both USTs ' were 10,000-gallon capacity tanks that had been used to store diesel fuel Four soil samples were collected from the bottom of the excavation at a depth of approximately 16 feet below ground ' surface (bgs) Soil samples were analyzed for total petroleum hydrocarbons as gasoline (TPHg) as diesel fuel (TPHd), as kerosene (TPHk), and as motor oil (TPHmo) Samples were also ' analyzed for benzene, toluene, ethylbenzene, and xylenes (BTEX) The highest concentrations of TPHg, TPHd, and BTEX were detected at 5,700 milligrams per kilogram (mg/kg) TPHg, 50 ' mg/kg TPHd, and 48 mg/kg benzene TPHk, and TPHmo were not detected in any of the four samples On September 25, 1997, Clearwater Group (Clearwater) supervised the drilling of six soil ' borings to depths from 32 to 38 feet bgs Three of the borings were converted to groundwater ' monitoring wells (MW-1 through MW-3) Soil samples collected from four of the six borings contained detectable concentrations of petroleum hydrocarbons The highest concentrations of TPHg (570 mg/kg) and benzene (4 0 mg/kg) were detected in the sample from SB-2 at a depth of 25 feet bgs Groundwater samples collected September 29, 1997 from all three monitoring wells ' contained detectable concentrations of dissolved petroleum hydrocarbons The highest concentrations of TPHg (32,000 micrograms per liter [µg/L]) and benzene (2,200 µg/L) were ' dctected in the sample from monitoring well MW-1 Low concentrations of TPHd were detected in soil samples from SB-5 (11 mg/kg) and SB-6 (2 mg(kg), and in the groundwater sample from ' MW-1 (300 Ag/L), however, subsequent groundwater sample results have been non-detect for all three wells, and with approval from the SJCEHD, analysis for TPHd has been discontinued Results of these efforts were presented in Clearwater's Subsurface Investigation Report, dated • November 20, 1997 ZB I78C RAP 2 May 18,2004