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ERM Mike Quillid, CPESC, PMP, QSD 26 May 2023 Page 2 On 13 March 2023, PG&E submitted a letter, Discovery of Historic Petroleum Release at PG&E Turner Cut Station, 2121 NZukerman Road, Stockton, CA, to RWQCB describing the nature and extent of the historical release, detailing work performed to date,and included tabulated soil and water analytical data and laboratory data packages. The analytical data indicated the presence of low concentrations of volatile organic compounds(VOCs); total petroleum hydrocarbons (TPH) as gasoline(TPH-g), diesel (TPH-d), and motor oil (TPH-mo); and select metals in both soil and water. Analytical summary tables of the confirmation soil samples and water sample collected from within the excavation are included in Tables 1 through 6. Soil analytical results were compared against the San Francisco Bay Regional Water Quality Control Board Environmental Screening Levels (ESLs)for commercial/industrial shallow soil exposure,construction worker, and potential leaching to groundwater. Water analytical results were compared against Tier 1 and Direct Exposure, Tap Water Non-cancer Endpoint ESLs. The Site has an existing supply water well (Well 95215). Well 95215 is located approximately 250 feet northwest of gas well TC6S. This well is 6-inch diameter and is screened from 100 to 120 feet bgs(0.045 slot size) (Figure 2). The well construction includes gravel from 80 to 120 feet bgs and cement from surface to 80 feet bgs. 2. SCOPE OF WORK ERM will complete a focused groundwater investigation in the vicinity of the TC6S excavation area to identify potential groundwater contamination associated with the historical petroleum release. Note, during the well abandonment activities for TC6S, it was unclear if the water sampled within the excavation area was exposed groundwater or accumulated rainwater. According to the California State Water Resources Control Board GeoTracker website, the McDonald Island Compressor Station(approximately 1 mile northwestof the Site)groundwaterwas encountered at approximately 12 feet bgs in May 2013 during well installation activities. Based on this additional information, the water collected from the excavation area was likely precipitation. The approach forthe investigation is outlined in the sections below for the following main tasks: ■ One pilot soil boring will be completed nearthe TC6S excavation area to determine the depth of first encountered groundwater(expected to be between 10 to 15 feet bgs) using a direct- push drill rig. ■ Following installation of the pilot boring, up to three groundwater samples will be collected from first encountered groundwater using Hydropunch (or equivalent) sample equipment. ■ Sampling of existing Site water well 95215 will be performed to assess deep groundwater conditions at the Site. The sample collected from this well will supplement shallow grab groundwater samples (first encountered water) collected as part of this investigation. Figure 2 depicts the proposed boring locations. Borings will be located as close as feasible to the existing excavation area around TC6S in a triangular pattern to adequately delineate groundwater near the excavation area. The groundwater depth information obtained from the pilot soil boring will be used to determine sample depth for planned depth-discrete grab groundwater samples. 3. PRE-FIELD ACTIVITIES The following pre-field activities will be completed prior to the field investigation: