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1 <br /> ' Phase II Field <br /> ARCADIS Investigation and LNAPL <br /> ' Mobility Evaluation Report <br /> Union Pacific Railroad Company <br /> Former Maintenance Facility <br /> ' Tracy,California <br /> Figure 10 presents the trend analysis results of dissolved-phase TEPH-d. Trend <br /> charts for site wells are included on Figures 4, 5, and 6 and in Appendix B. Stable and <br /> ' decreasing trends were observed at wells MW-10A, MW-07A,TW-03, and MWA4A. <br /> Sufficient data were present for wells MW-09A and MW-22A, but highly variable results <br /> between sampling events resulted in a 'no trend'determination from MAROS.An <br /> t increasing trend was observed at MW-02A likely because of a shift in groundwater <br /> gradient caused by changes in operation of remedial systems at the Site. <br /> ' 5. LNAPL Mobility Evaluation <br /> 5.1 Methodology <br /> A comprehensive LNAPL mobility assessment uses multiple lines of evidence to <br /> determine LNAPL mobility and the potential for future LNAPL migration. Immobile <br /> LNAPL is functionally locked in pore spaces. Mobile LNAPL is capable of moving <br /> laterally and vertically at the pore-scale. Migrating LNAPL has sufficient mobility at the <br /> pore-scale to cause expansion of the plume footprint. The following lines of evidence <br /> ' were used to assess LNAPL mobility at the Site: <br /> • LNAPL observations in monitoring wells <br /> ' • LNAPL field and residual saturations <br /> • LNAPL pore velocity calculations <br /> ' LNAPL pore-entry pressure calculations <br /> Dissolved-phase plume stability <br /> ' These complementary lines of evidence have an inherent logic that can be used to <br /> determine if LNAPL is immobile, mobile, or migrating. The flowchart shown on <br /> ' Figure 11 presents the logical approach underlying this mobility assessment.The <br /> following subsections describe each of the lines of evidence used in this assessment. <br /> ' 5.1.1 LNAPL Observations in Monitoring Wells <br /> LNAPL can be considered as potentially migrating when temporal fluid level gauging <br /> ' indicates the following: <br /> • Clear trend of increasing thickness of LNAPL in monitoring wells over time that is <br /> not attributable to seasonal water-table fluctuations <br /> Vq„ 4..y. MONW11aa200l aoc 17 <br />