ARCHIVED REPORTS PHASE II FIELD INVESTIGATION AND LNAPL MOBILITY EVALUATION REPORT

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Phase II Field ARCAQIS Investigation and LNAPL Mobility Evaluation Report Union Pacific Railroad Company Former Maintenance Facility ' Tracy, California also plotted on Figure 14 for reference(field saturation values are indicated by the solid diamond-shaped markers along each curve). LNAPL velocities ranged from 3.89 1 x 10-7 to 6.46 x 10-6 cm/s as seen in Table 8.Appendix G contains the results in detailed spreadsheet format for each representative site location. Calculated LNAPL velocities were not calibrated using site-specific transmissivity results from the bail- down testing for reasons explained in Section 4.1.3. ASTM guidance indicates that LNAPL seepage velocities less than 1 x 10"6 cm/s indicate LNAPL in the formation is functionally immobile. LNAPL at LIF-02 and LIF-10 exceeds ASTM mobility criterion; hence, LNAPL at these locations is confirmed to be mobile at the pore-scale. These results indicate that LNAPL is able to move vertically or laterally within the formation at the pore-scale in these areas. This does not imply that there is plume-scale expansion of the LNAPL plume(s), only that LNAPL at these locations is capable of moving and redistributing itself within the footprint of the existing LNAPL plume. LNAPL at LIF-06 and LIF-25 is functionally immobile due to the calculated pore velocities at field saturations exceeding residual saturations being less than the ASTM criterion for mobility. 5.2.4 LNAPL Pore-Entry Pressure Calculations LNAPL migration into pristine soils occurs when there is sufficient LNAPL head pressure present in the subsurface at the fringe of the LNAPL plume to displace groundwater from the soil pores.The calculated critical LNAPL head pressures that are ' required for lateral LNAPL migration into saturated soils are shown in Table 9. These head pressures can be converted into critical LNAPL thickness,which is a more easily measured indicator than head pressure. The head pressure required for plume expansion, expressed in LNAPL thickness units, ranges from 4.2 to 8.6 feet. LNAPL accumulations of this magnitude have not been observed in any monitoring wells on site. Table 9 provides the maximum historical LNAPL thickness measurements in monitoring wells nearest to each LIF location, which are all less than half the critical thickness of each respective LIF location. Since LNAPL thicknesses of these magnitudes have not been observed across the area of affected groundwater, it is likely the LNAPL plume does not have sufficient LNAPL volume(and head pressure)to migrate into unaffected soils. ryt�pr-iracYanap6mgbilily(I i-242010}d c 26 1