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r. r« Monitoring Well Installation Report—Former Unocal Station#0123,Tracy,CA January 17,2001 parking lot to determine if any abandoned USTs existed beneath the site, and exploratory borings to collect soil and groundwater samples The underground survey detected no large metal anomalies, and concluded that there were no USTs beneath the site However, hydrocarbon-impacted groundwater was detected in the northern portion of the site In May 1995, nine exploratory soil borings were drilled and three groundwater monitoring wells (U-1 through U-3) were installed by Pacific Environmental Group, Inc (PEG) at the subject site Exploratory test pits were dug and two abandoned USTs were encountered at the northern property boundary, beneath the 11'h Street sidewalk In November 1996,the two abandoned 550-gallon USTs were removed A third UST that was encountered during the removal activities was abandoned in place by grout injection One groundwater monitoring well (U-4) was installed in December 1996 in the location of the removed USTs to evaluate soil and groundwater conditions On October 7, 1998, PEG drilled and installed two groundwater monitoring wells (U-5 and U-6) and one soil boring (B-4) Soil samples collected from the borings had detectable concentrations of total petroleum hydrocarbons as gasoline (TPHg) (220 parts per million (ppm) at l 1 feet below ground surface (bgs) in U-6)and total petroleum hydrocarbons as diesel (TPHd)(21 5 ppm at 10 feet bgs in B-4 and 12 3 ppm at 11 feet bgs in U-6) Benzene,methyl tert-butyl ether(MtBE), lead, or volatile organic compounds (VOCs) were not detected in the soil samples Grab groundwater samples had concentrations of TPHg (968 parts per billion (ppb) in U-6) and TPHd (115 ppb in B-4 and 312 ppb in U-6) Benzene, MtBE, or • lead were not detected in the groundwater samples One soil boring B-5 was advanced at the northwest corner of I Ith Street and Central Avenue on January 12, 2000 by GR A soil sample collected from the boring at 7 feet below ground surface (bgs) had a detectable concentration of TPHd at 2 21 ppm TPHg or benzene, toluene, ethylbenzene, and xylenes (BTEX) were not detected in the soil sample A grab groundwater sample from the boring had 66,700 ppb of TPHg, 29 6 ppb of TPHd, and 36 2 ppb of benzene Oxygenates were not detected is the soil or grab groundwater samples Groundwater Monitoring and Sampling Groundwater monitoring and sampling has been conducted quarterly since June 2, 1995, in wells U-1, U-2,and U-3, since March 3, 1997, in well U-4, and since October 22, 1998, in wells U-5 and U-6 In the most recent event on December 6, 2000, TPHg, BTEX, or MtBE were not detected in any of the wells except for well U-4 TPHg and benzene were reported in well U-4 at concentrations of 149 and 3 68 ppb, respectively Diesel range hydrocarbons were reported in all wells at the site ranging in concentrations from 290 to 430 ppb, but the chemical analytical report (Appendix F) noted that those values were from discrete peaks The chromatograms showing the discrete peaks are in Appendix G At the request of GR, Mr Alan Kemp of Sequoia Analytical (FLAP #1271) reviewed the chromatograms In a telephone call with Mr Kemp on January 10, 2001, he stated that the discrete peaks were not caused by diesel, nor did any other type of petroleum hydrocarbons cause them He could not determine from available information what caused the peaks 24007305-1 2