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 TPCA Pond Excavation area is fenced, there are no complete exposure pathways for current site workers. However, direct contact with groundwater by future site workers could be a complete exposure pathway in the TPCA Pond Excavation area if such controls do not remain in place. ■ Currently, the primary areas of concern at the Site are the TPCA Pond Excavation (groundwater)and the two Biotreatment Beds(soil),which were developed in 1994 for in situ bioremediation of soils excavated from the TPCA Pond. A recent human health risk evaluation of Biotreatment Bed soils found that TEPH-d concentrations remaining in soils within the Biotreatment Beds do not pose a risk to human health. Previous investigations have found that the concentrations of volatile organic compounds(VOCs)and polycyclic aromatic hydrocarbons (PAHs) in soil and groundwater at the Site were either not detected or were below site- specific cleanup goals or conservative screening levels. In June 2010, ARCADIS provided the RWQCB with the Biotreatment Beds Post Remediation Investigation Report, which concluded that current TEPH-d concentrations in Biotreatment Bed soils did not pose a human health risk. ■ The only area of concern at the Site that may pose an unacceptable risk to future site workers is the TPCA Pond Excavation where groundwater is encountered just below the ground surface.As shown in Table 3, elevated TEPH-d concentrations ranging up to 8,900,000 ug/L were detected in November 2009 in groundwater beneath the TPCA Pond Excavation at depths of 1 to 5 feet bgs.As these TEPH-d concentrations in groundwater exceed the site-specific TEPH-d cleanup level of 100 ugiL and occur in very shallow groundwater that may be encountered by site workers,TEPH-d in groundwater in this area does not pose a risk to current site workers because the area is fenced, but may pose a risk to future commercial I industrial workers, maintenance workers, and construction workers if access controls (e.g.,fencing)are not maintained. In the absence of any complete exposure pathways associated with the TPCA Excavation area,the Site ' overall poses only a minimal risk, if any,to human health. ■ LNAPL remaining in the subsurface does not currently present a risk to human health or the environment because access to the TPCA Excavation area is restricted and controlled by fencing.Therefore, exposure pathways are currently incomplete. LNAPL remaining in the subsurface would also not pose a risk to human health and the environment in the future if such controls are maintained. • Criteria 6—The Site presents no significant risk to the environment. ■ The Site has operated as a rail yard since the early 1960s and comprises only ruderal habitat that is not suitable for supporting viable populations of ecological receptors. There are no surface waters on the Site and the closest surface water is the Tom Paine Slough located 5.3 miles to the 1 %p pRtracyaaap1mob4�11-29-2010)eoc 31