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Geosyntec° consultants According to the UST removal reports, the USTs were secured to a single continuous concrete pad located approximately 7 feet below ground surface (bgs). The concrete pad was reportedly left in place during the tank removal activities (PRC, 1993; Tetra Tech Environmental Management, Inc., 2001). The excavation was backfilled with imported fill. The excavation area is currently covered with asphalt. Buildings are located approximately 25 feet to the west-northwest and 20 feet to the south-southeast of the tank excavation. According to Mr. Jason Cashman of the Port, these buildings are vacant. Since the tank removal, several investigations have been conducted to characterize the extent of residual contamination in the area(Geosyntec, 2019b). In January 2019, Geosyntec conducted an investigation to evaluate the eligibility for closure of UST 817B under the State Water Resources Control Board's Low-Threat Underground Storage Tank Case Closure Policy(LTCP; State Water Resources Control Board, 2012). During this investigation, soil samples were collected to provide additional analytical data necessary to evaluate the UST for closure. Analytical results indicated that TPH-g was detected at a concentration of 1,100 milligrams per kilogram (mg/kg) in one boring. Based on this analytical result, the CVRWQCB requested that a soil vapor survey be conducted to evaluate potential soil vapor intrusion risks. Soil vapor samples were collected in June 2019, and the analytical results indicated the presence of elevated concentrations of TPH-g and BTEX. Based on this data, it was concluded that UST 817B is not currently eligible for closure under the LTCP, and that additional investigation was warranted to evaluate the source of the elevated concentrations (Geosyntec, 2019b). Historical soil and soil vapor data are shown in Figures 2 and 3, respectively. The Draft Soil and Soil Vapor Investigation— UST 817E (Geosyntec, 2019b), presenting the Site background, results of historical investigations, and the 2019 investigation, is included in Appendix C. 1.3 Site Hydrogeology Groundwater in the vicinity of the former UST 817B excavation area was sampled in September 2021 in accordance with the RRI 2018 Groundwater Monitoring Work Plan (Geosyntec, 2018) and Groundwater Monitoring Work Plan 2nd Addendum, 2020 Plan (Geosyntec, 2020). The results were documented in the 2021 Annual Groundwater Monitoring Report(Geosyntec, 2021). The wells sampled included five shallow groundwater monitoring wells (917-01, 917-04, 917-05, 917-06, and 917-10) and four deep groundwater monitoring wells (917-07, 917-08, 917-09, and 917-14). Prior to monitoring, the depth to groundwater was collected from the wells. The depth to groundwater was also obtained from shallow well 917-11, but this well was not sampled. The well locations are shown in Figures 4 and 5. In September 2021, depth to shallow groundwater in the vicinity of the former UST 817B area ranged from-7.01 feet below mean sea level (msl) in well 917-06 to -7.31 feet below msl in well 917-01 and is shown in Figure 4. The depth to deep groundwater in the vicinity of the former UST 817B area ranged from-7.02 feet below msl in well 917-08 to -9.48 feet below msl in well 917-14 and is shown in Figure 5. Based on this elevation data, the general groundwater flow is to the north-northeast in the shallow zone and to the southeast in the deep zone (Geosyntec, 2021). Final Investigation Work Plan Addendum—UST 817B 2 June 24,2022