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1 <br /> ' Phase II Field <br /> ARCADIS Investigation and LNAPL <br /> t Mobility Evaluation Report <br /> Union Pacific Railroad Company <br /> Former Maintenance Facility <br /> ' Tracy, California <br /> ■ LNAPL removal and remediation activities have been conducted at the <br /> Site for over 20 years. Various LNAPL recovery methods have been <br /> attempted including dual-phase extraction, product skimmers, bailing, bilge <br /> bags, and hydrophobic absorbent socks. <br /> ■ The LNAPL is minimally recoverable. Bail-down tests recently conducted at <br /> ' the Site demonstrated that the LNAPL is minimally recoverable based on the <br /> slow rate that the LNAPL reentered monitoring wells after purging. Residual <br /> saturation tests performed on soil cores collected at the Site indicate that <br /> less than two percent of the total LNAPL present at the Site is recoverable. <br /> Only 90 gallons (650 pounds)of product were removed from the subsurface <br /> between 1995 and 2006 via multiple LNAPL recovery methods,which <br /> further reinforces the conclusion that LNAPL recoverability is limited(ERM <br /> 2006). The limited recoverability of the LNAPL renders significant additional <br /> product removal technically and economically infeasible. <br /> • The LNAPL is naturally being depleted overtime by dissolution and <br /> biodegradation. These natural source zone depletion (NSZD) mechanisms <br /> will restore the aquifer as described in the ITRC document Evaluating <br /> Natural Source Zone Depletion at Sites with LNAPL (ITRC 2009b). <br /> • Criteria 2—The Site has been adequately characterized. <br /> ■ The Site has been extensively characterized through numerous soil, soil-gas <br /> and groundwater investigations from 1987 through the present, as described <br /> ' in the RI Report(IC 1994). The extents of LNAPL and groundwater impacts <br /> were adequately delineated during the Phase II Investigation as shown on <br /> Figures 7 and 8. The Phase II Investigation data demonstrate that <br /> ' groundwater and LNAPL impacts are contained on site and are not <br /> migrating. <br /> • Criteria 3—The dissolved hydrocarbon plume is not migrating. <br /> ■ As discussed in Section 4.2.2, the results of Mann-Kendall statistical <br /> analysis of groundwater-quality trends for the near-source-area monitoring <br /> wells (MW-02A, MW-07A, MW-09A, MW-10A, MW-14A, MW-22A, and TW- <br /> 03)were that petroleum hydrocarbon concentrations in shallow groundwater <br /> are decreasing or stable.An increasing trend was noted at MW-02A located <br /> on the upgradient edge of the plume. This increasing trend has been <br /> attributed to a shift in groundwater flow direction following termination of <br /> treated groundwater reinjection related to former remedial system operations <br /> and not to a change in the footprint of the plume. <br /> ■ TEPH-d has limited mobility compared with lighter fractions of petroleum <br /> hydrocarbons and is not expected to migrate far from the source area as <br /> demonstrated at this Site. <br /> rpt-upmdrwyanapl mob fti 1-29.2810i.do 29 <br />