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Ms. Natalia Subbotnikova Page 2 November 9, 2023 (EW-1 and EW-2), and two gas-migration monitoring probes (VW-9, and VW-10) at the site (Figure 1) (URS 2005c). In December 2005, a vapor extraction pilot test was conducted to evaluate the feasibility of installing a permanent landfill gas extraction system. During this test, vapor samples were collected from the two extraction wells and analyzed for volatile organic compounds (VOCs). Analytical results of these samples confirmed the presence of VOCs;the total VOC concentration at each of the vapor extraction wells was below 5 ppmv. Based on these results, VOC constituents could be present in groundwater(URS 2006a). In September 2006, a passive soil vapor extraction trench and four associated passive vent pipes (VP-1 through VP-4) were installed to control methane from migrating off the site (URS 2006a). To assess the presence or absence of VOC constituents in groundwater, three groundwater monitoring wells were installed at the site on June 12, 2006 (URS 2006b). Following installation of these wells, groundwater samples were collected quarterly and analyzed for VOCs (URS 2007a). Ross Atkinson of Regional Water Quality Control Board, Central Valley Region (CVWB) requested the collection and analysis of groundwater samples for metals and general minerals (only total dissolved solids after the first event), beginning in the fourth quarter of 2006 (URS 2007a). During a meeting on May 24, 2007, Mr. Atkinson concurred with the recommendation in the Quarterly Monitoring Report dated April 30, 2007, to suspend groundwater monitoring activities for VOCs, as VOC concentrations were similar in magnitude over the previous four quarters, and these results were less than their respective established levels (URS 2007b). In the first quarter 2011 report to SJCEHD dated April 29, 2011, URS recommended reducing the monitoring frequency from quarterly to annually, as the methane gas concentration results at the monitoring probes had been less than 5% since December 2006 (following installation of the passive soil vapor extraction trench). SJCEHD agreed with URS' recommendation (URS 2011a). The annual monitoring activities are usually conducted during the fourth quarter of each year. In a letter to CVWB dated May 20, 2011, URS recommended no further action for groundwater activities (URS 2011 b). CVWB agreed that on-site groundwater issues had been resolved to its satisfaction (California Regional Water Quality Control Board 2011). URS decommissioned the groundwater monitoring wells on December 9,2011 (URS 2012). During the annual monitoring event in the fourth quarter of 2018, VW-8 was identified as damaged. URS completed repairs to VW-8 in the third quarter of 2019. The summary report documenting deviations from the Well Decommissioning and Installation Work Plan (URS 2019) and their compliance to both CalRecycle and San Joaquin well specifications were submitted to SJCEHD in the fourth quarter of 2019 (AECOM 2019). ANNUAL FIELD MONITORING ACTIVITIES On October 25, 2023, URS measured landfill gas concentrations (i.e., methane, oxygen, and carbon dioxide), at the gas migration monitoring probes (VW-1 through VW-10), vapor extraction wells (EW-1 and EW-2), and the passive vent pipes (VP-1 through VP-4). On November 9, 2023, URS