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S E C 0 R
<br /> . 7-Eleven Store #14117
<br /> Additional Site Assessment Report and Fate and Transport Model
<br /> February 27, 2004
<br /> Page 5
<br /> collected at 50 feet bgs in MW-8 Fuel oxygenates MtBE, TBA, DIPE, EtBE, and TAME were not
<br /> detected above laboratory reporting limits in MW-8
<br /> In boring SB-1, soil samples collected during drilling also did not contain TPHg, benzene, MtBE,
<br /> TAME, EtBE, or DIPE above laboratory reporting limits TBA was reported at concentrations ranging
<br /> from 0 026 mg/kg (10 feet bgs) to 2 0 mg/kg (20 feet bgs) (Figure 2 and Table 2) Methanol, ethanol,
<br /> 1,2-DCA, EDB, and acetone were not detected in soil samples collected from MW-8 and SB-1 above
<br /> laboratory reporting limits Copies of the analytical data are included in Attachment F
<br /> Groundwater Sample Analytical Results
<br /> Grab groundwater samples collected from SB-1 contained maximum concentrations of benzene
<br /> (0 87 micrograms per liter [pg/L], 52 feet bgs), TPHg (80 pg/L, 52 feet bgs), MtBE (1 1 pg/L, 52 feet
<br /> bgs), TBA (14,000 pg/L, 23 feet bgs), ethanol (16 pg/L, 33 feet bgs), and acetone (37 pg/L, 33 feet
<br /> bgs) (Figure 3 and Table 4) Methanol, 1,2-DCA, EDB, and fuel oxygenates DIPE, EtBE, and TAME
<br /> were not detected above laboratory reporting limits in grab groundwater samples collected from SB-
<br /> 1 Copies of the analytical data are Included in Attachment F
<br /> On September 18, 2003, groundwater samples were collected from MW-8 during third quarter 2003
<br /> monitoring and sampling Benzene, TPHg, and MtBE were reported at concentrations of 25 pg/L,
<br /> 3,200 pg/L, and 2 2 pg/L, respectively Other fuel oxygenates, and 1,2-DCA, EDB, methanol,
<br /> ethanol, and acetone, were not detected above laboratory reporting limits
<br /> FATE AND TRANSPORT MODEL
<br /> Groundwater and Transport Modeling
<br /> Groundwater flow and transport modeling using FLOWPATH II was conducted in the vicinity of the
<br /> site to assess the migration of contaminants in the site vicinity The FLOWPATH II model, which Is a
<br /> two-dimensional (x-y) finite-difference model, was used because it provides a straightforward
<br /> calculation of groundwater flow and Incorporates retardation, dispersivity, and first-order decay in the
<br /> calculations of mass transport
<br /> Groundwater flow direction in the site vicinity has historically fluctuated The Shaw Group Indicated
<br /> that during 13 recorded quarterly events, groundwater flow directions have been to the
<br /> south/southeast, east/northeast, and west (The Shaw Group, Workplan for Additional Offsite
<br /> Assessment, Fate and Transport Evaluation Addendum, Sensitive Receptor Survey, and Fourth
<br /> Quarter 2002 Monitoring and Sampling, 2003) Fate and transport modeling conducted by SECOR
<br /> for a site located approximately 1,000 feet east of the 7-Eleven site indicated the same time-
<br /> dependent variability in groundwater flow direction as the 7-Eleven site (SECOR, Conceptual Site
<br /> Model, Chevron 9-4054 2103 Country Club Boulevard, Stockton, California, May 14, 2003) These
<br /> variations are probably due 1) to the very gentle natural hydraulic gradient at the site, 2)
<br /> groundwater extraction in the area either by municipal wells or by groundwater remediation systems,
<br /> and 3) tidal influences Whatever the ultimate cause of the fluctuations in groundwater flow, these
<br /> fluctuations appear to result in a net zero gradient over an extended period of time
<br /> Model Setup
<br /> The FLOWPATH 11 model was established as a rectangular area with sides of 240 feet (E-W) and
<br /> 280 feet (N-S) and using 5-foot grid spacing with the 7-Eleven site at the approximate center of the
<br /> 14117 Vertical Assessment and Fate and Transport Model doc SECOR Intemational Incorporated
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