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<br /> Request For Closure,Former Unocal Station#0123,Tracy,CA
<br /> April 19,2001
<br /> concluded that there were no USTs beneath the site However, hydrocarbon-impacted groundwater was -
<br /> detected in the northern portion of the site
<br /> In May 1995, nine exploratory soil borings were drilled and three groundwater monitoring wells (U-1
<br /> -through U-3) were installed by Pacific Environmental Group, Inc (PEG) at the subject site Exploratory '
<br /> test pits were dug and two abandoned USTs were encountered at the northern property boundary, beneath
<br /> the I Id' Street sidewalk In November 1996, two abandoned 550-gallon USTs were removed A third
<br /> UST that was encountered during the removal activities'was abandoned in place by grout injection One
<br /> groundwater monitoring well (U-4)was installed in December 1996 in the location of the removed USTs
<br /> to evaluate soil and groundwater conditions
<br /> On October 7, 1998;PEG drilled and installed two groundwater monitoring wells(U-5 and U-6) and one
<br /> soil boring (B-4) Soil samples collected from the borings had detectable concentrations of Total
<br /> Petroleum Hydrocarbons as gasoline (TPHg) of 220 parts per:million (ppm) at 11 feet below ground
<br /> surface (bgs) in well boring U-6, and Total Petroleum Hydrocarbons as diesel (TPHd) of 21 5 ppm at 10
<br /> feet bgs in boring B-4 and 12 3 ppm at L I feet bgs in well boring U-6 Benzene, methyl tert-butyl ether
<br /> (MtBE), lead, or volatile organic compounds (VOCs) were not detected in the soil samples Grab
<br /> groundwater samples had concentrations of TPHg (968 parts per billion, or ppb, in U-6) and TPHd (115
<br /> ppb in B-4 and 312 ppb in U-6) Benzene, MtBE, or lead were not detected in the groundwater samples
<br /> One soil boring B-5 was advanced at the northwest corner of I l" Street and Central Avenue on January
<br /> • 12, 2000 by GR A soil sample collected from the boring at 7 feet below ground surface (bgs) had a
<br /> detectable concentration of TPHd at 2 21 ppm TPHg or benzene, toluene, ethylbenzene, and xylenes
<br /> (BTEX) were not detected in the soil sample A grab groundwater sample from the boring had 66,700
<br /> ppb of TPHg, 29 6 ppb of TPHd, and 36 2 ppb of benzene Oxygenates were not detected is the soil or
<br /> grab groundwater samples A review of historic Sanborn maps suggests that boring B-5 was advanced in
<br /> the area of dispenser islands located at a former Shell service station that existed at the northwest corner
<br /> of I Ph Street and Central Avenue up to the mid 1960's
<br /> Two additional groundwater monitoring wells (U-7 and U-8) were installed downgradient of the site on
<br /> November 13, 2000 by GR TPHg, TPHd, BTEX, oxygenates except methanol, 1,2-dichloroethane
<br /> (1,2-DCA), ethylene dibromide (EDB), or lead were not detected in soil samples from the two well _ -
<br /> borings Methanol was detected in well boring U-8 at 21 feet bgs at a concentration of 195 ppm
<br /> Groundwater Monitoring and Sam lin
<br /> Groundwater monitoring and sampling has been conducted quarterly since June 2, 1995, in wells U-1,.
<br /> 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
<br /> most recent event on December 6, 2000, TPHg, BTEX, or MtBE were not detected in any of the wells
<br /> except for well U-4 TPHg and benzene were reported in well U-4 at concentrations of 149 and 3 68 ppb,
<br /> respectively Diesel range hydrocarbons were reported in all wells at the site ranging in concentrations
<br /> from 290 to 430 ppb, but the chemical analytical report (Appendix E) noted that those values were from
<br /> discrete peaks
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