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1 <br /> Phase II Field <br /> ARCADES Investigation and LNAPL <br /> Mobility Evaluation Report <br /> Union Pacific Railroad Company <br /> Former Maintenance Facility <br /> ' Tracy, California <br /> 5.2.5 Dissolved-Phase Plume Stability <br /> The result of the Mann-Kendall statistical analysis utilizing MAROS is shown on <br /> Figure 10. Where the MAROS program returned a conclusive trend, groundwater <br /> concentrations of TEPH-d trends were stable to decreasing consistently across the <br /> Site. These results indicate that the groundwater plume is stable,which indicates that <br /> the LNAPL plume is also stable. <br /> ' 5.3 LNAPL Mobility Summary and Conclusions <br /> LNAPL at the Site is mobile at the pore-scale at specific site locations, but the LNAPL <br /> ' plume is stable and not migrating. These findings are supported by multiple LNAPL <br /> mobility and migration potential analyses. The conclusions of these analyses are <br /> presented on the LNAPL mobility assessment logic flowchart(Figure 15)and are <br /> ' described in the following bullets. <br /> • Current LNAPL thicknesses have increased marginally above historical maximums <br /> in the last year. However, LNAPL has not migrated into new areas. LNAPL has not <br /> accumulated in wells in areas where LNAPL has not been observed before, <br /> indicating that the LNAPL plume is stable. <br /> • Five undisturbed soil cores were collected from the Site.At four of these locations <br /> where RSWD was completed (LIF-02, LIF-10, LIF-06, and LIF-25), LNAPL field <br /> ' saturations exceed residual values in the vadose zone at several depths for each <br /> location. Therefore, LNAPL is mobile at the pore-scale near these locations. <br /> • LNAPL pore velocity near LIF-02 and LIF-10 exceeds the ASTM criterion for <br /> 1 LNAPL mobility of 1 x 10-6 cm/s. LNAPL velocities at these locations ranged from <br /> 7.83 x 10-7 to 6.46 x 10-6 cm/s. <br /> ' • Pore velocity calculations demonstrate that LNAPL is functionally immobile near <br /> LIF-06 and LIF-25. The LNAPL velocities at these locations ranged from 4.35 x <br /> 10-7 to 9.12 x 10-7 cm/s,which is less than ASTM criterion. <br /> • The LNAPL pore-entry calculations demonstrate that there is insufficient LNAPL <br /> head pressure (represented by critical thickness)to migrate into unaffected soils. <br /> ' The dissolved-phase plume of diesel fuel indicator compounds is stable or <br /> decreasing consistently across the Site,which indicates that the area affected by <br /> LNAPL is stable and not migrating. <br /> ryl-uRn{acyJnapl md6ilily(1h-28-2010)doc 27 <br />