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UST closure activity and approximately 60 cy with subsequent excavation activities. the soil
<br /> was disposed at Forward Landfill in Stockton, California. Three new 10,000-gallon USTs
<br /> were installed in the former UST basin. New gasoline product dispensers were installed
<br /> within the former western pump island area; however, the southern pump island was
<br /> permanently abandoned.
<br /> In August 2000, John P. Cummings & Associates (JPCA) advanced one soil boring (SB1)
<br /> ,r via direct push technology at the north end of the western dispenser island. The soil boring
<br /> was advanced to 25 feet bgs before probe refusal was encountered. In September 2000,
<br /> JPCA advanced a soil boring using hollow-stem auger technology at the same location as
<br /> SB1 to a depth of 53 feet bgs. Analysis of the soil and groundwater samples from the re-
<br /> bored SB1 dissolved concentrations of MTBE and t-butanol (TBA), and the groundwater
<br /> sample contained detectable concentrations of MTBE, toluene, and total petroleum
<br /> hydrocarbons as gasoline (TPHg).
<br /> In May 2003, GHH Engineering, Inc. (GHH) supervised the advancement of three soil
<br /> borings (SB2, SB3, and S134). Soil samples from borings S133 and SB4 contained
<br /> detectable concentrations of MTBE. The soil samples from S133 also contained detectable
<br /> concentrations of benzene, ethylbenzene, and xylenes. Groundwater samples collected
<br /> from S133 and S134 contained detectable concentrations of TPHg, total petroleum
<br /> hydrocarbons as diesel (TPHd), and MTBE. The groundwater sample from boring SB3 also
<br /> �.► contained detectable concentrations of benzene, ethylbenzene, and xylenes- I)z i)GA— <--
<br /> On
<br /> —On January 12 and 13, 2004, ATC geologist supervised the advancement of three soil
<br /> borings (MW1, MW2, and MW3) to approximately 65 feet bgs. Petroleum hydrocarbon
<br /> constituents were not detected in soil samples from MW1, MW2, or MW3. Each soil boring
<br /> was then completed as a groundwater monitoring well.
<br /> In May 2006, ATC personnel supervised the advancement of six soil borings (ASB1 through
<br /> ASB6) at the Site to depths between 52 and 60 feet bgs to evaluate soil and groundwater for
<br /> the presence of petroleum hydrocarbons and fuel oxygenates across the Site. Groundwater
<br /> depths in each boring were measured at approximately 50 feet bgs. The soil samples
<br /> collected from borings ASB1 through ASB6 did not contain detectable concentrations of
<br /> TPHg; TPHd; benzene, toluene, ethylbenzene, or xylenes (BTEX); MTBE; ethyl tertiary butyl
<br /> ether (ETBE); di-isopropyl ether (DIPE); tertiary amyl methyl ether (TAME); ethyl dibromide
<br /> (EDB); 1,2-dichloroethane (1,2-DCA); or TBA. The groundwater samples collected from
<br /> ASB1 and AS134 through ASB6 did not contain detectable concentrations of TPHg, TPHd,
<br /> BTEX, MTBE, TAME, ETBE, DIPE, TBA, EDB, or 1,2-DCA.
<br /> Groundwater collected from boring ASB2 contained 1.0 micrograms per liter (}ag/1) of 1,2-
<br /> DCA. The groundwater sample collected from boring ASB3 contained 3.5 fag/I of MTBE and
<br /> 4.3 tag/I 1,2-DCA. Field activities and analytical results are detailed in ATC's Summary
<br /> Report of Additional Subsurface Investigation, dated January 21, 2007.
<br /> 4.0 REGIONAL AND SITE GEOLOGY-HYDROGEOLOGY
<br /> The Site is located in northeastern San Joaquin County in Stockton, California. The
<br /> Stockton area is underlain by the Alluvium and Modesto/Riverbank Formations (DWR,
<br /> Workplan to Conduct Subsurface Soil Vapor Intrusion Study CVVA%TC
<br /> ATC Project 54.26874.0001
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