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Additional Site Assessment Report and Human Health Risk Screening Evaluation www.arcadis.com 2 2.1 Site Location and Description The Site is located in Tracy, San Joaquin County, California (Figure 1). The Site includes historical easements for the former OVP and TAOC pipelines. The OVP was located on the south side of the UPRR ROW and Byron Road. It was installed between 1902 and 1904, and operated from 1903 until the early to mid-1930s. The OVP was again used during the early 1940s, primarily to convey Bunker C fuel oil from the Richmond Refinery to railroad hubs in Tracy. The OVP was reportedly abandoned in 1970. The former TAOC pipelines were located in the southern portion of the UPRR ROW. They were constructed in 1907 and transmitted heavy crude oil from the southern San Joaquin Valley to the San Francisco Bay Area. The TAOC pipelines operated until the 1970s. The UPRR ROW includes an active railroad track, but is otherwise undeveloped. The area north of the UPRR ROW is currently a residential development (Figure 2). A property between a portion of the Site and the residential development to the north is owned by the West Side Irrigation District and consists of a grassy area. Byron Road is located to the south of the UPRR ROW, with another residential neighborhood further south. 2.2 Site History Several historical soil and groundwater investigations have been conducted at the Site since 1998. Historical analytical results for these investigations were included in the Human Health Screening Evaluation (Science Applications International Corporation [SAIC] 2009), Human Health Screening Evaluation Addendum (SAIC 2012) and Subsurface Investigation Report and Health Risk Evaluation (Leidos 2019). 2.3 Geology and Hydrogeology The Site is located within the San Joaquin Valley Groundwater Basin. This basin is divided into nine sub-basins. The Site lies within the Tracy sub-basin (Groundwater Basin Number 5-22.15), which is defined by the areal extent of unconsolidated to semi consolidated sedimentary deposits that are bounded by the Diablo Range to the west, the Mokelumne and San Joaquin Rivers to the north, the San Joaquin River to the east, and the San Joaquin-Stanislaus County line to the south. The Tracy sub-basin is located adjacent to the eastern San Joaquin sub-basin to the east and the Delta-Mendota sub-basin to the south (California Department of Water Resources [DWR] 2006). The Tracy sub-basin comprises continental deposits of late Tertiary to Quaternary age, which includes the Tulare Formation, older alluvium, flood basin deposits, and younger alluvium. The thickness of these deposits ranges from a few hundred feet in the western foothills to approximately 3,000 feet near the western margin of the basin. The Tulare Formation consists of semi consolidated, poorly sorted, discontinuous deposits of clay, silt, and gravel. The Corcoran Clay is situated near the top of the Tulare Formation, separating groundwater in the basin in what has been reported as two primary aquifers. The upper aquifer is 15 to 250 feet below ground surface (bgs) and the lower aquifer is 600 feet bgs. Given the location of the Site within the sub-basin, the Site is likely immediately underlain by younger alluvium, and older alluvium an/or flood basin deposits. During the previous investigations, groundwater was encountered at depths between 5 and 7 feet bgs. Previous water level data from the monitoring wells indicate a north-northwesterly groundwater flow (SAIC 2006). Previous subsurface investigations at the Site have shown that the Site is underlain predominantly by silt and clay, with