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# Comment Response to Comment vertical extent of the plume in the former source area by W-20 is monitored by W-004. 3c In the Central Area at UST 81713, there are data gaps to the The only VOCs that were detected in the UST 817B area during west and east.Additional grab groundwater samples are the third quarter 2018 monitoring event are associated with recommended near the buried well 917-013 and between wells petroleum hydrocarbons. Investigation in this area will be 917-06 and 917-11. evaluated and addressed as part of the UST Site closure process with the RWQCB. 3d At the Site 49 plume,there are data gaps to the north and There are two shallow wells 5-10 feet bgs in this area: 816-MW2 southeast.Additional grab groundwater samples are and W292, and one well screened from 25-30 feet bgs in this area, recommended near the missing/destroyed piezometer P-160 and that had detections of VOCs during the third quarter 2018 event. southeast of the plume. The groundwater flow direction in this area is to the southwest towards the drainage channel. Monitoring well W-293 is located to the northwest of the wells with VOC detections and defines the northern extent of VOCs, and well 816-MW1 is located to the east and defines the eastern extent. Proposed grab-groundwater samples GGW-I I and GGW-12 are located downgradient to the southwest of the three wells with VOC detections. Additional delineation of the upgradient extent is not warranted at this time. If additional investigation is needed for remedy evaluation purposes, sampling will be proposed and implemented as part of the RI/FS process. 3e The groundwater plume at Building 916 appears to be barely Well W-302 near building 916 had detections of 1,1-DCA at 1.5 defined. Delineation of this plume is recommended. µg/L, cis-1,2-DCE at 1.9 µg/L, and vinyl chloride at 1.7 µg/L during the third quarter 2018 monitoring event.No other VOCs were detected. Based on these low level detections, additional characterization in this area is not warranted. 3f At Site 5I, additional groundwater samples are recommended to Site 51 was granted no further action in 2012. The small plume the east,west and south of the plume. located near Site 51 West in Figure 13 was based on a HydropunchTM grab-groundwater sample collected in 2009. Vinyl chloride was detected at a low concentration of 0.8 µg/L in the • 2009 HydropunchTM sample. Based on this low level detection, additional characterization in this area is not warranted. 3 08.03.2019