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' Phase 11 Field <br /> ARCADIS investigation and LNAPL <br /> ' (Mobility Evaluation Report <br /> Union Pacific Railroad Company <br /> Former Maintenance Facility <br /> ' Tracy,California <br /> b a0w - 0,00 •hd (Equation 10) <br /> nlcriCl— <br /> �1—P/ Yw)6aw Pruow <br /> ' Where, - <br /> b0(cdt) - critical LNAPL thickness in well, cm <br /> aaw = surface tension, dyne/cm <br /> aao air-oil interfacial tension, dyne/cm <br /> bow - oil-water interfacial tension, dyne/cm <br /> ' hd = air displacing water pressure head, cm <br /> 5.1.5 Dissolved-Phase Plume Stability <br /> Statistical trending of dissolved-phase constituent concentrations at locations where <br /> separate-phase LNAPL is not observed in the well can be used to assess the <br /> dissolved-phase plume stability. Stable or decreasing groundwater concentrations of <br /> dissolved LNAPL compounds indicate that the LNAPL plume is stable or decreasing in <br /> size. When an LNAPL plume is deemed unstable or migrating as a result of this <br /> analysis, a monitoring program needs to be developed to verify plume stability trends <br /> and determine whether plume instability is due to LNAPL migration or some other <br /> phenomena. <br /> 5.2 LNAPL Mobility Results and Discussion <br /> 1 The following sections provide a description of how the site data were used to support <br /> each LNAPL mobility line of evidence to assess whether the LNAPL at the Site is <br /> immobile or mobile at the pore-scale and migrating or stable at the plume-scale. <br /> 1 <br /> 5.2.1 LNAPL Observations in Monitoring Wells <br /> ' The hydrographs on Figures 4, 5, and to include wells with LNAPL_accumulations.An <br /> inverse relationship between the groundwater potentiometric surface elevation and <br /> LNAPL thickness (e.g., LNAPL thickness increases as groundwater elevations <br /> decrease)was observed in most wells, as is expected in unconfined aquifer systems. <br /> LNAPL observations in the monitoring well network at the Site indicate that the LNAPL <br /> plume is likely not migrating. The data show that LNAPL thickness has marginally <br /> i <br /> ipl-upn°wV4napl m0b1W1 i-28-29101-dm 24 <br />