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review of the soil probe data, the RWQCB issued a March 4, 2021, letter recommending the installation of the MW-10 monitoring well cluster(RWQCB, 2021). The MW-10 monitoring well cluster was installed between August 27 and 31, 2021 (Wood, 2021 b). Elevated concentrations of PCE and TCE were detected in the three subsequent monitoring periods in well MW-10C, installed in the "C-Zone", the deepest zone monitored at this site. Discussions between UniFirst and the RWQCB concluded that a third revision to the 2019 Groundwater Interim Remedial Action Work Plan should include a new"C-Zone" monitoring well northwest of the current well MW-10C that would allow for the determination of a hydraulic gradient in the"C-Zone" and further delineate the plume of CVOCs beneath the site. 2 Site Description The UniFirst site is located in central downtown Stockton near the inland channels of the Port of Stockton (Figure 1). The City of Stockton is located at the eastern edge of the California Delta, a large estuary formed by the confluence of the Sacramento River and San Joaquin River before they empty into San Francisco Bay. The site is about 1.8 acres in size and occupies the city block bounded by East Poplar, East Flora, North Hunter, and North El Dorado streets (Figure 2). The site consists of a warehouse building on the northern half of the site and a paved parking lot occupying the southern portion of the site. This area of Stockton represents the older portion of the downtown area and has historically been an area of mixed residential and commercial use. The current site configuration was developed by UniFirst starting in 2002. Just prior to redevelopment, the site was occupied by residential units, a commercial laundry facility, a meeting hall, and other businesses. Previous tenants on the site may have included gas stations, auto repair, and auto sales businesses (EECI, 2004). The laundry facility occupied the southwest portion of site and operated for decades. CVOCs, used in the dry-cleaning process, were stored and used at the laundry facility from about 1948 to 1983 (EECI, 2001). 3 Groundwater Conditions Groundwater conditions beneath the UniFirst site and vicinity are described in the October 29, 2010, Groundwater Interim Remedial Action Plan (RAP; AMEC Geomatrix, 2010b) and its subsequent amendment(AMEC Geomatrix, 2011a), including the geologic setting, site-specific hydrogeology, and groundwater occurrence and direction. This information is briefly summarized in the following subsections. 3.1 Geologic Setting The City of Stockton is located on the lower flood plain of the San Joaquin River at the eastern edge of the California Delta, a large delta formed by the confluence of the Sacramento River and San Joaquin River. The shallow sediments beneath the Stockton area are a mix of predominantly fine- grained alluvial fan deposits and delta deposits that consist of unconsolidated interbedded clays, silts, sands, and gravels designated as Quaternary Flood Basin Deposits (Geomatrix, 2007a). 3.2 Site Geology Soils beneath the site and vicinity consist mostly of clays and silts that contain varying minor amounts of sand (Figures 3 and 4; Geomatrix, 2007b). Logging conducted at the MW-5 well cluster indicate silts and clays are present from the surface to approximately 100 feet below ground surface (bgs). However, a sand layer is also present from about 100 to 110 feet bgs, intersecting the lower portion of the screened interval of well MW-5C. 3.3 Groundwater Occurrence Groundwater occurs beneath the site and vicinity starting at depths from 14.29 to 21.42 feet bgs (March 2019). Groundwater elevations have varied over time but have generally been rising. Groundwater elevations in the on-site wells, the wells with the longest period of measurement, have 2019 Groundwater Interim Remedial Action Work Plan—Revision 3 Page 2 UniFirst Facility,Stockton,California Project No.0132902023 I:\13000s\13290\Arch ive\13290-177.docx