COMPLIANCE INFO_EW-5 INSTALL PERMIT PACKAGE 2/14/25

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WELL INSTALLATION AND INTERIM REMEDIAL ACTION REPORT www.arcadis.com FINAL_Forward CHCF (CDCR) Well Installation Report_10182024 15 The TVOC plume is defined by groundwater monitoring wells screened in the shallow and intermediate WBZs, to the south by wells AMW-52S, AMW-52M, AMW-59S, and AMW-59M; to the west by wells AMW-51S and AMW- 51M; to the east by wells AMW-58S, AMW-58M, AMW-60S, and AW-60M; and to the north by wells AMW-50S and AMW-50M. Groundwater TVOC concentrations in monitoring well AMW-50M from December 2022 until the third quarter of 2024 fluctuated from 12.6 µg/L up to 36 µg/L, with an average concentration of approximately 27.8 µg/L. This indicates that this is the distal end of the TVOC plume with concentrations of 25 µg/L or greater. 5.4 CDCR Groundwater Capture Assessment Groundwater capture assessment was performed using the United stated Environmental Protection Agency ’s (USEPA’s) capture zone evaluation guidance (USEPA 2008). This method utilizes estimations of the hydraulic gradient, transmissivity, and pumping rate to calculate the capture zone, which is illustrated in the footnotes in Tables 15 and 16. The capture assessment focused on groundwater capture on the CDCR property of the TVOC plume with concentrations of 25 µg/L or greater. Groundwater capture assessment was performed individually for extraction wells CDCR-EW-2, CDCR-EW-3, and CDCR-EW-4. As part of the groundwater capture assessment, aquifer transmissivity values were derived from the hydraulic testing assessment (Tables 15 and 16), which included the aquifer thickness determined as the groundwater static water level to the base of the intermediate permeable unit, which is bounded by lower permeability strata in this area. This method of assessment was required to enable assessment of the dataset with AQTESOLV ™ and were then then used to calculate estimates of hydraulic conductivity. A hydraulic gradient of 0.002 ft/ft was used based on previous interpretations groundwater monitoring data (2016 to present), where the hydraulic gradient has varied from 0.001 up to 0.003 ft/ft depending on the season, with drier months typically having a lower gradient. A groundwater hydraulic gradient of 0.002 ft/ft represents the average. The regional groundwater flow pattern is orientated towards the northeast, but locally at the CDCR property, the TVOC plume is orientated towards the north-northeast and then migrates towards the northeast, along a zone with greater thickness of higher permeability materials. Four pumping rates were used for each of the well capture assessments, and results are presented in Tables 15 and 16 and on Figure 6. The pumping rate used on Figure 6, is based on the calculated maximum sustainable yield determined from the respective well step tests, which were 50 gpm for extraction well CDCR-EW-2 (Appendix F; Arcadis 2023b), 35 gpm for extraction well CDCR-EW-3, and 20 gpm for extraction well CDCR-EW- 4. With combined extraction from CDCR-EW-2, 3, and 4, adequate capture of the 25 ug/L or greater TVOC plume at the CDCR Property was not indicated to be achieved. Due to that, an additional extraction well for full coverage is described in the following section. 5.5 CDCR Additional Extraction Well Recommendation The combined capture assessment utilizing extractions wells CDCR-EW-2 through CDCR-EW-4 (Figure 6) indicated that a northeastern portion of the CDCR TVOC plume with concentrations of 25 µg/L or greater located near monitoring well AMW-50M will not be captured. Complete capture of the CDCR TVOC plume with concentrations of 25 µg/L or greater beneath the CDCR property will require an additional extraction well (CDCR- EW-5) located approximately 70 feet to the northeast of monitoring well AMW-50M.