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Mr. Robert McClellon Page 3 <br /> October 31, 2023 <br /> (EW-1 and EW-2), and the passive vent pipes (VP-1 through VP-4) using a GEM 2000 hand-held <br /> portable gas monitor. The results are presented in Table 1 and discussed below. <br /> RESULTS <br /> At the gas migration monitoring probes (VW-1 through VW-10), methane was only detected at VW-4 <br /> and VW-5D (0.01% and 5.4%, respectively). Methane was not detected at either of the vapor <br /> extraction wells (EW-1 and EW-2). At the passive vent pipes (VP-1 through VP-4), methane was <br /> detected at concentrations ranging from 0% (VP-2) to 17.3% (VP-4). <br /> Oxygen concentrations at all sample locations ranged from 0.0% to 19.3%. Carbon dioxide <br /> concentrations at all sample locations ranged from 0.1% to 35.3%. <br /> CONCLUSIONS AND RECOMMENDATIONS <br /> Before installation of the passive soil vapor extraction trench, methane concentrations historically <br /> exceeded 5% at the following probes: VW-4, VW-5S, VW-5D, VW-6S, and VW-6D. Since the <br /> passive soil vapor extraction trench was installed in September 2006, methane concentrations <br /> decreased at these probes to less than 5% during the following quarter and have continued to be <br /> less than 5% during the subsequent monitoring events. In 4Q23, methane concentrations at VW-4 <br /> again exceeded 5%. <br /> Methane concentrations measured at all gas migration monitoring probes during 4Q23 were less <br /> than 5%, except VW-4 which measured 5.4%, Methane concentrations measured at the vapor <br /> extraction wells were both less than 5%. Methane concentrations at the passive vent pipes ranged <br /> from 0.0% to 17.3%. <br /> Methane concentrations at VW-4 have not exceeded 5% since December 2006 (following <br /> installation of the passive soil vapor extraction trench). As such, URS recommends that methane <br /> concentrations in VW-4 be monitored during two successive monthly events commencing in <br /> November 2023 to verify the methane concentration recorded at VW-4 during the 4Q23 monitoring <br /> event. <br /> If you have any questions regarding this report, please contact Scott Little at (916) 207-3053. <br /> Sincerely, <br /> Haley Meyrowitz Scott Little <br /> Geologist Project Manager <br />