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Pm! I Introduction GHD is submitting this Well Installation Work Plan Addendum for the former Unocal Facility No. 4409 located at 1502 El Dorado Street in Stockton, California (site, Figures 1 and 2) on behalf of Phillips 66 Company (Phillips 66). The original work plan, dated November 20, 2018, was prepared by Arcadis and proposed installing two offsite groundwater monitoring wells. In November 2019, GHD installed B' Zone well MW-24 to the east of the site. B Zone well MW-23, planned to be installed to the north of well MW-13, was unable to be installed due to underground and overhead utility conflicts. On May 13, 2020, GHD staff and Central Valley Regional Water Quality Control Board (CVRWQCB) staff met via teleconference to discuss current site conditions and path to closure. During the meeting, CVRWQCB and GHD staff agreed on the following: Relocate proposed monitoring well MW-23 from it's originally proposed location further east on Sperry Avenue within the City of Stockton's right-of-way; Install additional offsite B and C Zone monitoring wells to the north of the site; Conduct an additional water well survey in the vicinity of the site; Review historical groundwater analytical data to determine if further analytical data are needed to define the extent of release in the A Zone; and Incorporate historic 1,2-Dichloroethane (1,2-DCA) analytical data into GHD analytical groundwater tables. In a letter dated May 21, 2020, CVRWQCB staff requested GHD submit a work plan addendum presenting the proposed installation of three offsite groundwater monitoring wells and addressing the other items mentioned above. Notes of the meeting and correspondence with CVRWQCB are included as Attachment A. The site description and background, previous environmental work, and GHD's proposed scope of work are presented below. 2. Site Background 2.1 Site Description The site is located on the northeast corner of North El Dorado Street and East Harding Way in Stockton, California. The site is a former 76 Unocal service station which dispensed gasoline and diesel fuel beginning in 1982. Currently, the site is a vacant lot and is bound to the north and east by vacant lots, to the south by Harding Way and an Autozone store, and to the west by El Dorado Street and a Taco Bell restaurant (Figure 3). 2.2 Regional and Site Geology The site is located near the center of San Joaquin County on the east side of California's Great Valley Province. Sediments encountered in the Great Valley include well sorted sands from the GHD I Well Installation Work Plan Addendum 111193145 (5) I Page 1