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Geospte& 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). Building 917 is located approximately 25 feet to the west-northwest and 20 feet to the south-southeast of the former UST excavation area. Building 917 is vacant and encloses a fenced yard with covered soil stockpiles from a prior remediation effort(DTSC 2020). 1.2 Summary of Previous Environmental Investigations Since the removal of the four USTs, multiple soil investigations, a soil vapor investigation, and numerous groundwater monitoring events have been performed to characterize the extent of residual contamination in the area. Figures 3 through 6 present soil, groundwater, and soil vapor sampling locations from these investigations, which occurred between 1998 and 2022. An initial soil and groundwater investigation was performed by the Navy in 1998 in response to the results of soil and water samples collected during tank removals. Twenty borings were advanced to determine the extent of petroleum contamination, and subsequently six shallow groundwater monitoring wells were installed (917-01 through 917-06) (Tetra Tech 2001). An additional investigation was performed in 1999 to further evaluate the vertical as well as horizontal extent of petroleum hydrocarbon contamination in soil and groundwater. Eight additional groundwater monitoring wells were installed(917-07 through 917-14). Subsequently, quarterly groundwater monitoring of the wells was performed between September 1999 through September 2000. Following the investigation effort, the Navy concluded that the soil and groundwater contamination had been delineated and evaluated. Several corrective action options to address the contamination including monitoring natural attenuation(MNA), enhanced MNA, and dual-phase extraction, were evaluated, with MNA being the recommended approach (Tetra Tech 2001). The CVRWQCB reviewed Tetra Tech's report and indicated that the contaminant concentrations in groundwater were stable; however, the CVRWQCB did not believe that MNA was viable and suggested proceeding with enhanced MNA (CVRWQCB 2002). In 2003, the RRI property was transferred from the Navy to the Port. At the time of the 2003 property transfer, no further corrective action had occurred at UST 81713, other than MNA (Finding of Suitability for Early Transfer [FOSET]; Tetra Tech 2003). The FOSET report stated that no additional investigation was required for UST 817B and that MNA was recommended. Groundwater monitoring of the wells associated with UST 817B resumed in 2007 (Environmental Risk Services [ERS] 2008). In 2008, ERS performed a soil investigation following the demolition of Building 817B. A total of 24 soil borings were advanced to depths ranging from 8 to 12 feet bgs. Based on PID screening of soil cores, soil samples were collected from 7 of the boring locations at 4 feet bgs. ERS concluded that the petroleum hydrocarbon concentrations in soil had decreased since the USTs were removed and did not pose a risk to groundwater quality and the environment, and that the institutional controls set forth in the Consent Agreement require that RRI, including UST 81713, be used solely for industrial and Port related activities (ERS 2008). The CVRWQCB reviewed the ERS report and concluded that additional characterization of the groundwater was Investigation Work Plan—UST 817B 2 December 2024