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Union Pacific Railroad Company - 2 = 6 October 2005 Tracy, San Joaquin County an area of approximately 100 feet by 200 feet and the area where total HVOC concentrations exceed 10 ug/L is even smaller. The relatively small size of this plume appears to make this site an ideal candidate for enhancing remedial efforts using one of several in situ remedial technologies like chemical oxidation. While natural attenuation may be a viable remedy, the associated long-term monitoring costs may be more than the cost of enhancing natural degradation rates through in situ technologies and reducing the overall cleanup time. Regional Board staff recommends evaluating potential remedial alternatives that may enhance remediation of the HVOCs plume and achieve cleanup levels more quickly. 4. Regional Board staff does not concur with the proposed reductions in monitoring frequencies at this time. Our current policy on reducing monitoring frequencies requires approval by the Executive Officer and issuance of a new MRP. Regional Board staff may be issuing a revised Monitoring and Reporting Program (MRP) in the next 6 months and UPRC will have an opportunity to propose changes to the monitoring program before the revised MRP is issued. 5. It is difficult to assess the accuracy of the groundwater potentiometric surface maps in the Annual Report because the groundwater elevations measured at each well are not posted on the figures. In future monitoring reports please post all groundwater elevation data on groundwater potentiometric surface maps. 6. The annual reports do not provide a thorough assessment of contaminant concentrations over time. In future annual reports, add figures showing the full extent of the TEPH and HVOCs during the current reporting year and the previous four years. If necessary, Regional Board staff can provide examples of this graphical assessment of plume stability. 7. To facilitate our review of the monitoring program and future Annual Reports, please provide a CD with all groundwater monitoring data collected at the site since 1995 (or earlier). The data should be tabulated in Microsoft Excel Spreadsheet(s) to facilitate sorting and searches. Source Area Assessment 1. The Rebound and Remediation Effectiveness Evaluation (2003 Evaluation) showed rebound of VOCs in several portions of the source area. Furthermore, the 2003 Evaluation did not consider potential hazards caused by vapor migration into nearby buildings or future buildings constructed at the site. Concentrations of vinyl chloride in soil vapor are more than two orders of magnitude higher than California Human Health Screening Levels for vapor intrusion into indoor air. Regional Board staff request submittal of a work plan by 1 December 2005 to assess current concentrations of residual VOCs in the source area. This work plan should propose sampling of all previous hotspots and should include assessment of all exposure pathways including indoor air. If VOC concentrations in the vadose zone still exceed applicable human health screening levels, additional active remediation of the source area may be required.