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Former Cheaper Store Number 37 Page 2
<br /> 15 East Grant Line Road July 2, 2010
<br /> Tracy, California 95376-2720
<br /> Once the temporary soil gas sampling points have been advanced and at least twenty
<br /> minutes have elapsed to allow subsurface conditions to equilibrate, ATC proposes to
<br /> conduct a purge volume test to ensure that the air inside the sampling system (sample
<br /> container, internal volume of tubing used, and annular space around the probe tip) is
<br /> removed and to assure that the samples collected are representative of subsurface
<br /> conditions. After purging one, three, and seven volumes at a rate of approximately 100 to
<br /> 200 milliliters per minute and analyzing air samples using an on-site mobile laboratory, a
<br /> purge volume will be determined based on the analytical results. ATC proposes to default
<br /> to purging three volumes prior to sampling if none of the chemicals of concern is detected
<br /> during the purge volume test or if a mobile laboratory is not employed at this site.
<br /> ATC proposes to conduct a shut-in test by applying a vacuum of approximately ten to
<br /> fifteen inches of mercury to the soil gas sampling apparatus for approximately ten minutes.
<br /> Once the vacuum has stabilized and the system has been purged, the soil gas sampling
<br /> apparatus will be used to collect confirmation samples in Summa TM canisters.
<br /> ATC proposes to apply the leak check compound 1,1-difluoroethane (1,1-DFA) at
<br /> appropriate sample-system locations during the purging and sampling process. A shroud
<br /> will be placed around the soil gas sampling system to contain the 1,1-DFA and a flame
<br /> ionization detector will be used to monitor the presence of 1,1-DFA.
<br /> ATC proposes to analyze the soil gas samples collected in Summa TM canisters for total
<br /> petroleum hydrocarbons as gasoline (TPHg) by Environmental Protection Agency (EPA)
<br /> method TO-3; benzene, toluene, ethylbenzene, and total xylenes (BTEX); methyl tertiary-
<br /> butyl ether (MTBE), ethyl tertiary-butyl ether (ETBE), di-isopropyl ether (DIPE), tertiary-
<br /> butyl alcohol (TBA), tertiary-amyl methyl ether (TAME), 1,2-dichloroethane (1,2-DCA) and
<br /> 1,2-dibromoethane (EDB) by EPA method TO-15; and selected soil gas samples will be
<br /> analyzed for methane by EPA method 80158, and oxygen and carbon dioxide by ASTM
<br /> method 1945-96. ATC proposes to have the laboratory report the compounds at detection
<br /> limits less than or equal to the residential screening levels provided in Table E,
<br /> Environmental Screening Levels (ESLs) for Indoor Air and Soil Gas (Vapor Intrusion
<br /> Concerns) Interim Final, November 2007, published by the San Francisco Bay Regional
<br /> Water Quality Control Board. In addition to the above, please analyze for the leak
<br /> detection compound 1,1-difluoroethane (1,1-DFA) and report this compound at ten
<br /> micrograms per liter (Ng/L).
<br /> If a mobile laboratory is on-site, ATC proposes that soil gas samples be collected from
<br /> each sampling point using a dedicated glass syringe following the purging of the system.
<br /> For screening purposes, the soil gas samples will be analyzed for TPHg, BTEX, MTBE,
<br /> ETBE, DIPE, TAME, TBA, 1,2-DCA, EDB, and select soil gas samples will be analyzed for
<br /> methane, oxygen, and carbon dioxide.
<br /> ATC states that soil gas samples will be collected in accordance with the Department of
<br /> Toxic Substances Control (DTSC) guidelines entitled Advisory-Active Soil Gas
<br /> Investigations dated January 28, 2003, and Interim Final — Guidance for the Evaluation
<br /> and Mitigation of Subsurface Vapor Intrusion to Indoor Air dated December 15, 2004,
<br /> revised February 7, 2005. In both documents the collection of one duplicate sample is
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