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1 . Introduction GHD is submitting this Offsite Investigation Work Plan for the site located at 1502 N El Dorado Street in Stockton, CA(Figures 1 and 2)on behalf of Phillips 66 Remediation Management Company (Phillips 66). On June 15, 2022 GHD met with the Central Valley Regional Water Quality Control Board (RWQCB)to discuss the current status of the site. On August 11, 2022 the RWQCB sent a letter summarizing the meeting and requesting a Work Plan to address horizontal and vertical delineation of hydrocarbons in groundwater. The RWQCB also requested a plan to reduce the groundwater monitoring and sampling (GWM&S) program and requested additional construction and usage information for a domestic well located on North Hunter Street (Domestic Well 10). Agency correspondence is included in Appendix A. To address these concerns and further define the plume boundary, GHD proposes to install three new monitoring wells: one (1) B-zone monitoring well and one (1) C-zone monitoring well to the east- northeast of MW-23, located in North Hunter Street, and one (1) C-zone sentry well, located in East Alder Street, to monitor groundwater between the plume boundary and Domestic Well 10 (Figure 3). A reduced monitoring program is proposed in section 3 and additional construction and usage information for Domestic Well 10 is presented in section 4. 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. No fuel has been dispensed since the underground storage tanks (USTs)were removed and the service station was dismantled in 1999. Currently, the site is a vacant lot and is bound to the north by vacant lots, parking lots, and businesses, to the east the site is bound by vacant lots and residential areas, to the south by Harding Way, an AutoZone store, and other businesses, and to the west by El Dorado Street and a Taco Bell restaurant, parking lots, and residential areas. (Figure 2). 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 Sierra Nevada that interfinger with the poorly sorted clays and silts of the Coastal Ranges. Cretaceous and tertiary strata contain laterally continuous beds of mudstone, siltstone, conglomerate, sandstone, and limestone. Quaternary to recent units consist of lake and alluvium deposits. Soil encountered beneath the site generally consists of interbedded fine-grained sand and clay with silty sand and sand from ground surface to the total explored depth of approximately 115 feet below grade (fbg). The interbedded layers range in thickness from approximately 5 to 20 feet thick. 2.3 Regional and Site Hydrogeology The site lies within the Eastern San Joaquin Groundwater Basin. The basin is bound to the north by the Mokelumne River, to the west by the San Joaquin River, to the south by the Stanislaus River, and to the east by consolidated bedrock. The nearest major surface water body is Yosemite Lake, approximately 3,500 feet to the west of the site. Groundwater flow within the basin is dominated by a large cone of depression located east of Stockton due to agricultural pumping. Wells at the site have been installed in four water-bearing zones. The shallow A-zone extends from approximately 30 to 45 fbg, the B' and B-zones extend from approximately 55 to 70 fbg and 75 to 85 fbg, respectively, and the C-zone extends from approximately 100 to 120 fbg. The aquitards between each water-bearing zone primarily consist of silts, which allow some hydraulic communication between the water-bearing sand zones. GHD I Phillips 66 112594700 1 Offsite Investigation Work Plan 1