ARCHIVED REPORTS PHASE II FIELD INVESTIGATION AND LNAPL MOBILITY EVALUATION REPORT

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' Phase II Field ARCADES Investigation and LNAPL Mobility Evaluation Report Union Pacific Railroad Company Former Maintenance Facility Tracy, California increased above historical maximums in the last year in several on-site wells (MW-08A, MW-11, PZ-01, TW-01, TW-02, and TW-04),which is due to a drop in groundwater elevation at these wells. LNAPL was observed recently in MW-08A during March 2009; however, LNAPL has not recently migrated into this area because 0.12 inch of LNAPL was observed once in March 2004.Also, LNAPL is not presently accumulating in wells with no historical LNAPL detections and plume fringe wells MW- 14A and MW-22A have not had any observable LNAPL since they were installed in 1991 These two findings indicate that the LNAPL plume is stable and not migrating. ' LNAPL mobility at the pore-scale is dependent upon the presence of a sufficient driving head and hydraulic gradient. Therefore, accumulation of LNAPL in monitoring wells is ' not a stand-alone indicator of LNAPL mobility. 5.2.2 LNAPL Field and Residual Saturations LNAPL saturation was assessed on soil cores collected from the Site through the petrophysical laboratory analyses PFS, FPM, and RSWD. Residual saturation values are generated through the latter two laboratory tests. LNAPL is immobile when field saturation equals residual saturation. LNAPL saturation data are presented in Table 7 and on Figures 12 and 13. LNAPL saturation data provided by PFS testing,which does not provide a residual saturation value,were compared to the residual saturation data from RSWD testing at the same ' site location. Field LNAPL saturation exceeded residual LNAPL saturation during the RSWD test at site locations LIF-02, LIF-06, LIF-10, and LIF-25. These results indicate that LNAPL is mobile at the pore-scale, as defined by ITRC, at these four site locations. LNAPL pore velocity calculations were performed when LNAPL field saturation exceeded the residual saturation. ' 5.2 3 LNAPL Pore Velocity Calculations There is potential for LNAPL movement in the subsurface at the pore-scale wherever measured field LNAPL saturations exceed residual saturation. LNAPL seepage velocity was calculated at each of the four locations (LIF-02, LIF-06, LIF-10, and LIF-25)where field saturation was greater than residual. LNAPL seepage velocity was ' calculated as a function of LNAPL saturation. Figure 14 presents LNAPL pore velocity for each location at all possible LNAPL saturation values(from residual LNAPL saturation on the left side of the figure to the irreducible water saturation on the right side). The measured field LNAPL saturation values from each soil core sample are rpt-upm_#acy�navi moUirty(91-29-201 o)dw 25