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Stockton Charter Way Commingled Plume - 3 - 7 November 2024 Staff has the following comments: 1. A re-evaluation of the current Site remediation strategy is needed. Stantec operated recent commingled plume remediation efforts in two phases. Phase 1 was soil vapor extraction (SVE) from well SLVE-2 and air-sparge (AS) in wells SLAS-1 through SLAS-3 on the west side of the Colombo/Toscano property. Phase 2, the current phase, is vapor extraction from soil and groundwater in the eight Advanced Remediation Technology (ART) wells on and east of the Former Texaco property and east of the Colombo/Toscano property. Stantec expanded the system for Phase 2 operation so that Phase 1 operations could be re-implemented if warranted. Recently, the Phase 2 system has operated only intermittently due to high water levels which limits available vadose zone soil for vapor extraction and proper functionality of the ART wells. Additionally, concentrations of benzene and methyl tert butyl ether (MTBE) in wells approximate in location and depth to the ART wells are all below Low Threat UST Case Closure Policy (LTCP) Groundwater Media- Specific Class 4 thresholds. Benzene remains above 1,000 micrograms per liter (ug/L) in three wells: SLVE-1, AMW-5, and AMW-6. Stantec concluded that hydrocarbons in wells AMW-5 and AMW-6 are from the nearby The Transmission Store UST case, and Staff concurred. Well SLVE-1 is not located within the vicinity of any ART wells. The only well with MTBE above 1,000 ug/L is SMW-19-81, which is screened from 71-81 feet below ground surface (bgs), though MTBE concentrations this well are generally increasing. ART-2 is the only ART well in the vicinity of SMW-19-81, is screened between 45 and 67 feet bgs, and is too shallow to address the deep MTBE concentrations in SMW-19-81. As such, Staff question the on-going effectiveness of the ART remediation. Additionally, as benzene concentrations in well SLVE-1 have rebounded above 1,000 ug/L, additional Phase 1 remediation may be warranted. Staff requests that by 22 November 2024 you schedule a meeting with Staff and Stantec to discuss how to address residual benzene and localized increasing MTBE concentrations above LTCP thresholds and the most cost-effective path to closure for this commingled plume. 2. California Code of Regulations (CCR) Title 23, Division 3, Chapter 30 requires the submittal of data, including survey data, in electronic format to the GeoTracker website in relation to Staff directives. Upon review of Site data on GeoTracker, electronic latitude and longitude (GEO_XY) and vertical (GEO_Z) survey data is missing for the following 33 Site wells: • CMW-8C • SLMW-10C/D, SLMW-11 D, SLMW-12A, SLMW-13B, SLVE-2 • SMW-24A/B, SMW-25A/B, SMW-26A/B, SMW-27C/D, SMW-28C/D, SMW- 29C/D, SMW-30A, SMW-31 B, SMW-32B, SMW-33B, SMW-35D, SMW-35E • ART-1 through ART-8 Staff requests that by 7 December 2024 you submit the electronic GEO_XY and GEO_Z survey data for these wells to GeoTracker and submit confirmation of upload to Staff via email.