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Union Pacific Railroad Company - 2 - 14 July 2023 Former Bunker C Vaults 1 . In the Report, Antea Group did not provide maps identifying the underground structures and sampling locations. Information on the locations of the underground structures and sampling points is needed to complete the conceptual site models and evaluate risks to potential sensitive receptors. 2. Composite soil samples are not appropriate to characterize a subsurface release. An analysis of individual soil samples is needed to evaluate the vertical and lateral extent of potential soil impacts. Due to the potential for releases to migrate vertically, an analysis of at least two soil samples from beneath each structure is recommended in the State Water Resources Control Board's Leaking Underground Fuel Tank Guidance Manual (LUFT Manual). 3. The previous investigations did not include an analysis of samples for benzene, toluene, ethylbenzene, and total xylenes (BTEX), as recommended in the State Water Resources Control Board's Low-Threat Underground Storage Tank Case Closure Policy (Low-Threat Closure Policy) and LUFT Manual. 4. In the Report, Antea Group evaluated concentrations of metals detected during the soil analyses against the maximum California background concentrations presented in the March 1996 Background Concentrations of Trace and Major Elements in California Soils report. Central Valley Water Board staff does not find it appropriate to compare soil data from each site to the maximum background concentrations, as regional background concentrations for different metals vary widely throughout the state. Instead, Water Board staff recommends comparing the soil analysis data from each site to the reported data for samples collected from the same county or from nearby regions. 5. If concentrations of petroleum and metals detected in the soil analysis indicate that an unauthorized release of fuel hydrocarbons has occurred from any of the underground structures, then additional investigation of the nature and extent of the release is required per the California Health and Safety Code 25296.10. Central Valley Water Board staff does not agree with the justification for recommending no further action based solely on the current site use and proximity to active rail lines. Based on the previous investigation results and the unique characteristics of each site property, Central Valley Water Board staff has determined that each site should be evaluated as an independent case. Central Valley Water Board staff has opened individual leaking underground storage tank (UST) cleanup cases for each of the site properties with corresponding GeoTracker ID numbers. Site-specific evaluations for each case are discussed below. Alturas Site (GeoTracker ID: T10000020828) On 11 July 2019, Antea Group conducted an investigation of the Alturas site. Antea Group collected and analyzed a water sample from within the metal UST to determine