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2025 Annual Corrective Action Evaluation Report <br /> <br />www.arcadis.com <br />Forward Landfill 2025 Annual CA Eval Rpt_08012025(1) 2 <br />provided in the quarterly groundwater treatment system monitoring reports (Arcadis 2024a, 2025a, 2025c, <br />2025g) <br /> Residential groundwater treatment systems: Residents with wells impacted by low levels of VOCs wellhead <br />treatment systems installed to remove VOCs from the water supply and to provide treated water to the <br />residences. These systems will remain in place and sampled quarterly at 13 of the 14 residences, and <br />monthly at one residence, until the RWQCB agrees in writing that a specific domestic well is outside the <br />plume footprint and no longer requires sampling, as stipulated in the Cleanup and Abatement Order R5-2017- <br />0703 (CAO; RWQCB 2017). Details regarding the residential treatment systems are provided in the quarterly <br />residential groundwater treatment system monitoring reports (Arcadis 2024b, 2025b, 2025d, 2025h). <br /> Artificial groundwater recharge basin: This artificial basin introduces clean water into the shallow aquifer and <br />creates a clean groundwater mound that restricts plume migration northwards from the eastern portion of the <br />POC and directs impacted groundwater towards the POC capture wells on the western side of the POC. <br />3 Evaluation Procedures <br />To evaluate the effectiveness of corrective actions implemented at the Site, this report evaluates groundwater <br />monitoring results from samples collected from third quarter 2024 to second quarter 2025 (the reporting period) <br />using the three criteria listed below. Analytical data from all site groundwater monitoring wells were used in the <br />TVOC mass calculation. <br />The Evaluation Procedures Report (Herst 2002) specifies the following three exceedance criteria to evaluate the <br />need for the optimization of corrective action measures or additional corrective action measures: <br /> An exceedance of the TVOC mass threshold limit (TL); <br /> An increase in TVOC mass for four consecutive monitoring events; or <br /> An exceedance of any of the individual well TVOC concentration TLs. <br />The Evaluation Procedures Report (Herst 2002) specifies a TL for TVOC mass that was derived from modeling <br />data using historical groundwater monitoring data. The TVOC mass TL, therefore, represents a historical upper <br />bound of TVOC mass in impacted groundwater. An exceedance of the TVOC mass TL or four consecutive <br />increases in TVOC mass would be suggestive of potential increasing volatile organic compound (VOC) <br />concentrations on a plume-wide basis. Similarly, the Evaluation Procedures Report also specifies TLs for TVOC <br />concentrations at individual corrective action monitoring wells. An exceedance of a TVOC concentration TL at a <br />well would be suggestive of potentially increasing VOC concentrations at that well. The TVOC mass TL and <br />individual well TVOC concentration TLs are summarized in Table 1. <br />Tables 2 through 5 present a quarterly summary of the VOC groundwater monitoring results from the reporting <br />period, including the TVOC concentration for each well. Estimated (trace) detections of VOCs between the <br />laboratory reporting limit and method detection limit are also presented in the tables, but such trace detections are <br />not included in the calculation of TVOC concentrations.