ARCHIVED REPORTS PHASE II FIELD INVESTIGATION AND LNAPL MOBILITY EVALUATION REPORT

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' Phase II Field ARCADIS Investigation and LNAPL ' Mobility Evaluation Report Union Pacific Railroad Company Former Maintenance Facility Tracy, California In summary, LNAPL near two of the site locations(LIF-02 and LIF-10) is mobile at the pore-scale, but LNAPL is not migrating at the plume-scale.As shown on the LNAPL mobility assessment logic flowchart, these two locations progressed through the flowchart to the determination of mobile LNAPL following the LNAPL pore velocity calculations. The lack of LNAPL accumulation in perimeter monitoring wells indicates that the plume is stable and not migrating.Also, the pore-entry pressure and dissolved- phase plume stability analysis confirms that the plume as a whole is stable and not migrating. LNAPL is mobile at the pore-scale, but the plume as a whole is not currently migrating and does not show the potential for migration. 6. Low-Risk Site Closure Evaluation The available site data collected for this Site was compared against the low-risk site closure guidance developed by the San Francisco Bay RWQCB and presented in the Interim Guidance on Required Cleanup at Low Risk Fuel Sites(SFRWQCB 1996). Although the Site receives oversight from the Central Valley RWQCB and the petroleum hydrocarbon impacts at the Site are a result of releases from an AST and locomotive fueling operations,this document may provide an appropriate framework for evaluating the Site for low-risk groundwater closure. There are six main criteria to assess whether low-risk fuel sites for soil or groundwater cases are candidates for closure. Summarized below are the site conditions and evaluation against the corresponding criteria. ' Criteria 1 —The leak has been stopped and ongoing sources, including free product, have been removed or remediated. ■ No fueling has been conducted at the Site since 1987. As described in Section 1, potential sources including ASTs and the former surface impoundments (or TPCA pond)have been remediated or removed. ' 0 Sludge from the former TPCA pond has been excavated and remediated. The biotreatment beds do not pose a risk to human health or groundwater as documented in the Response to Comments-Biotreatement Beds Post ' Remediation Investigation Report, Union Pacific Railroad Company, Former Tracy Maintenance Facility, prepared by ARCADIS and dated November 5, 2010(ARCADIS 2010d). ■ LNAPL accumulates in wells MW-06A, MW-08A, MW-11A, PZ-01, PZ-02, TW-01, TW-02, and TW-04. However,the LNAPL is functionally immobile and will not migrate outside of the current LNAPL plume footprint. "p prt4racy4naaimoNIPA11-29-2010).dm 28 1