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Mr. Alan Buehler, P.G., RWQCB Work Plan for Additional Site Investigation 2057 South El Dorado Street, Stockton, CA Page 2 May 18, 2018 Project No. 2092-2057-01 Assessment of subsurface conditions was initiated in March 2004, with the advancement of exploratory soil borings and groundwater monitoring wells. To date, approximately 24 exploratory soil borings (B1 through B8, CPT-1, CPT-2, MW-1 through MW-8, EW-1, EW-2, and VW-1 through VW-4) have been completed at the Site, with 14 of these borings converted to groundwater monitoring or remediation wells. The locations of the borings and wells are depicted on Figure 2, and Table 1 presents a summary of well construction information. In 2004, a groundwater monitoring and sampling program was initiated at the Site. Groundwater levels beneath the property have generally increased during the 13 year monitoring period, from approximately 32 feet below ground surface (bgs) in 2004 to a maximum of approximately 20 feet bgs in 2011 and 2013. An east groundwater flow direction, with variations to the southeast and northeast, has predominately been observed. Gasoline range organics (GRO), diesel range organics (DRO), benzene, toluene, ethylbenzene, and total xylenes (collectively BTEX), methyl tertiary butyl ether (MTBE), tertiary butyl alcohol (TBA), tertiary amyl methyl ether (TAME), di-isopropyl ether (DIPE), and 1,2-dichloroethane (1,2-DCA) have been detected in samples collected beneath the Site. In general, high levels of fuel contaminants are present in shallow groundwater onsite. Figures 4 through 7 illustrate concentrations and the approximate extent of GRO, benzene, MTBE, and TBA in shallow groundwater using data collected during the first quarter 2018. In May 2013, two CPT boring clusters (CPT-1 and CPT-2) were advanced onsite in order to conduct soil type profiling and to collect depth discrete groundwater samples. Fuel contaminant impact was detected in samples extending up to 120 feet bgs, the maximum depth explored. In December 2017, CPT-3 and CPT-4 boring clusters were advanced offsite to enable further assessment of the extent of impact offsite. Data from this work indicated that groundwater impact extends at least 300 feet offsite to the east-southeast to 2049 South San Joaquin Street. Groundwater samples were also collected adjacent to CPT-2 to assess potential reduction in concentrations in groundwater at this location since the 2013 subsurface investigation. Figure 8 presents a geologic cross section that incorporates both geologic information and analytical data from the CPT investigation work performed. Soil vapor extraction (SVE) was completed at the Site for approximately 1.5 years, from April 14, 2011 until October 1, 2012. During this time, approximately 155 pounds of GRO was removed from the subsurface in the vapor phase. The SVE system was shut down due to low contaminant mass extraction rates. In our opinion, further remediation of the vadose zone does not appear necessary, due to the combination of SVE and excavation remediation work. Increases in groundwater levels beneath the property in K:\Quick N Save #2 \Work Plans \Q-N-S#2-WP-2018.doc STRATUS