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Union Pacific Railroad 2 23 September 2011 Maintenance Facility Tracy, California in a different direction than what the groundwater contours predict. Groundwater contours indicate a north-northeast flow, while the plume appears to be migrating to the east- northeast and possibly to the southeast. Please provide discussion about possible reasons why the two appear different. Please address this concern in the above requested work plan that is due by 28 October 2011. 3. Staff's original understanding was that the LNAPL plume at the Site has insufficient head pressure to continue to move into unaffected pore spaces and therefore is stable. However, after further review of the Site's groundwater data, the data indicate the LNAPL plume may not be stable. For example, MW-08A, which is located south-southeast of the LNAPL source area, was an unimpacted well (<50 ug/L TEPH) in 2001. However, over the next 8 years, the TEPH concentrations in this well gradually increased until LNAPL appeared in 2009. Since 2009, MW-08A has continued to contain LNAPL. The concern is that if the LNAPL migrated in this cross-gradient direction over 8 years with a pressure head that was significantly less than the calculated required pressure head of between 4 to 8 feet thick, then the reliability of the LNAPL mobility calculations presented in the Phase 11 Field Investigation and LNAPL Mobility Evaluation Report dated 26 November 2010 are questionable. Field observations indicate that the LNAPL plume may be migrating to south-southeast with a much lower pressure head than predicted. In addition, MW-23A, a recently installed well, was unimpacted in 2010; however, this well now has TEPH concentrations in the 60 to 70 ug/L range. Although these concentrations are low, they may be indicators of potential lateral plume movement. If the TEPH concentration trend continues to increase in MW-23A, a new well to the south-southeast may be warranted. 4. The TEPH contours as drawn on the TEPH contour maps (Figures 8 and 9) in the Report form a kidney shape around the known source area. The TEPH contours suggest that MW-12A, just south of the source area, is unimpacted with TEPH. Do to the close proximity of this well to the known source area, staff finds the lack of TEPH detections in this well anomalous. Please discuss possible reasons why MW-12A continues to be unimpacted. Please address these concerns in the requested work plan that is due by 28 October 2011. 5. Based on the review of the TEPH contours and the lack of delineation to the south- southeast and east northeast, there is concern that the TEPH plume may be migrating in these directions. In addition to delineating the plume in these directions, we request that UPRR research resent groundwater use activities in this area to determine if there is any groundwater gradient influence caused by local pumping (i.e. agricultural pumping). Please address this concern in the requested work plan that is due 28 October 2011. 6. On Figure 8 of the Report, TW-04's concentration is lined out making it difficult to read. Please correct this error in the next groundwater monitoring report. 7. On Figure 9 of the Report, well MW-27A was mislabeled as MW-22A on the March 2010 contour map. Please correct this error in the next groundwater monitoring report. Monitoring Well Network Reduction Recommendation 8. Staff has reviewed Arcadis' request to reduce the monitoring well network at the Site. Given the historical and current soil and groundwater conditions (dissolved TEPH and LNAPL) at the Site, staff concurs with the wells that are proposed for destruction, with the