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risen about 10 feet since 2005. Depth to groundwater and groundwater elevations are summarized in Table 1. The horizontal and vertical gradients of groundwater flow at the site have been fairly consistent over time. The horizontal direction of the groundwater gradient in October 2022 was typical for the site and was generally to the northeast though somewhat more easterly in the on-site wells than off-site (Figure 3). The vertical groundwater gradients at the site have been measured at the site at the MW-5 well cluster between wells MW-5A and MW-513 and between wells MW-513 and MW-5C. The vertical gradients between these wells have been consistently downwards. 3.4 Background Groundwater Quality The quality of groundwater beneath the western portion of the City of Stockton, where the site is located, is being impacted by seawater intrusion from the Sacramento and San Joaquin River Delta (USGS, 2006). Seawater intrusion results in elevated concentrations of total dissolved solids (TDS). The site is located within the eastern edge of the seawater intrusion front(defined by the 300 milligrams per liter TDS contour) (CDM, 2001). It has been estimated that the seawater intrusion front is migrating to the east at approximately 150 to 250 feet per year(CDM, 2001). 3.5 Beneficial Uses of Groundwater Shallow groundwater in the western downtown Stockton area, in which the site is located, is not used for domestic or municipal water supply. The groundwater beneath this area has elevated TDS and chloride concentrations as a result of seawater intrusion from the Sacramento and San Joaquin River Delta (to the west of the site). The poor groundwater quality of western Stockton has resulted in importation of drinking water from eastern Stockton through the municipal water distribution system (CDM, 2001). 4 Constituents of Concern in Groundwater The results of on-site and off-site groundwater investigations and groundwater monitoring indicate the presence of COCs in groundwater beneath the site and the off-site area (AMEC Geomatrix, 2010a). The COCs include CVOCs and trace amounts of petroleum hydrocarbons constituents. The nature and extent of the primary COCs, the CVOCs PCE and TCE, in groundwater beneath the on-site and off-site areas were described in the RAP and are summarized in the paragraphs below. CVOCs consisting of the historically common industrial solvent PCE and its transformation byproducts are the primary COCs in groundwater at this site. CVOC concentrations detected in samples collected during the October 2022 monitoring event are shown on Figure 4 and in Table 2. PCE is the most prevalent CVOC measured in groundwater samples collected from on-site and off- site monitoring wells (Figure 4). PCE degrades in the environment to TCE, then to either 1,1- dichloroethene or 1,2-dichloroethene, then to vinyl chloride, and finally to ethane. PCE has been detected in groundwater beneath the site and the off-site area northeast of the site. The groundwater samples containing the highest PCE concentrations in the site vicinity were collected from borings on North Hunter Street south of the intersection with East Poplar Street and from the MW-5 and MW-10 well clusters (Figure 4). TCE is the second-most prevalent CVOC measured in groundwater samples collected from on-site and off-site monitoring wells (Figure 4). TCE at the site is likely present in groundwater as a degradation product of PCE. TCE is in groundwater beneath the site and the off-site area northeast of the site. Groundwater samples containing the highest TCE concentrations in the site vicinity were collected from borings on North Hunter Street south of the intersection with East Poplar Street and from the MW-5 and MW-10 well clusters (Figure 4). Additional COCs, including cis-1,2-dichloroethene, trans-1,2-dichloroethene, 1,2-dichloroethene (1,2-DCA), chloroform, and vinyl chloride, and the petroleum hydrocarbon constituents benzene, toluene, ethylbenzene, and xylenes have been detected in groundwater beneath the on-site and the off-site area northeast of the site, typically at concentrations significantly less than PCE and TCE. In general, the additional CVOCs are found at the same locations as TCE and PCE, except for 2019 Groundwater Interim Remedial Action Work Plan—Revision 3 Page 3 UniFirst Facility,Stockton,California Project No.0132902023 IM 3000s\13290\Archive\13290-177.docx